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Primary dolomite in the Late Triassic Travenanzes Formation, Dolomites, Northern Italy: Facies control and possible bacterial influence
Abstract
In the late Carnian (Late Triassic), a carbonate-clastic depositional system including a distal alluvial plain, flood basin and sabkha, tidal flat and shallow carbonate lagoon was established in the Dolomites (Northern Italy). The flood basin was a muddy supratidal environment where marine carbonates and continental siliciclastics interfingered. A dolomite phase made of sub-micrometre euhedral crystals with a mosaic microstructure of nanometre-scale domains was identified in stromatolitic laminae of the flood basin embedded in clay. This dolomite is interpreted here as primary and has a nearly stoichiometric composition, as opposed to younger early diagenetic (not primary) dolomite phases, which are commonly calcian. This primary dolomite was shielded from later diagenetic transformation by the clay. The stable isotopic composition of dolomite was analyzed along a depositional transect. The δ13C values range between ca -6‰ and +4‰, with the most 13C-depleted values in dolomites of the distal alluvial plain and flood basin, and the most 13C-enriched in dolomites of the tidal flat and lagoon. Uniform δ18O values ranging between 0‰ and +3‰ were found in all sedimentary facies. It is hypothesized that the primary dolomite with mosaic microstructure nucleated on Extracellular Polymeric Substance secreted by sulphate reducing bacteria. A multi-step process involving sabkha and reflux dolomitization led to partial replacement and overgrowth of the primary dolomite, but replacement and overgrowth were facies-dependant. Dolomites of the landward, clay-rich portion of the sedimentary system were only moderately overgrown during late dolomitization steps, and partly retain an isotopic signature consistent with bacterial sulphate reduction with δ13C as low as -6‰. By contrast, dolomites of the marine, clay-free part of the system were probably transformed through sabkha and reflux diagenetic processes into calcian varieties, and exhibit δ13C values of ca +3‰. Major shifts of δ13C values strictly follow the lateral migration of facies and thus mark transgressions and regressions.
... Large amounts of dolomite may have formed by pervasive dolomitization during burial (Machel, 2004), while the Holocene environments simply may not have existed long enough to produce the same large accumulations of dolomite as in some examples from the geological record. For marine dolomites, there is petrographic and geochemical indication that some Triassic dolomites formed as early diagenetic or penecontemporaneous dolomite (i.e., dolomite that formed during or soon after deposition; e.g., Iannace and Frisia, 1994;Meister et al., 2013;Preto et al., 2015;Meister and Frisia, 2019). Rieder et al. (2019) show sedimentary structures that are indicative of soft sediment deformation, suggesting that the sediment was not lithified yet and that it was deposited as fine authigenic carbonate mud. ...
... The recent findings of preserved nano-crystalline structures in the Carnian Travenanzes Formation (Venetian Alps; Preto et al., 2015) and the Dolomia Principale (Brenta Dolomites, northern Italy; Meister and Frisia, 2019) provide new evidence that support that some dolomites may indeed have formed as a primary precipitate. This interpretation is based on the assumption that structures in the range of a few nanometres are metastable and would not survive pervasive recrystallization. ...
... The primary formation of Mg/Ca-carbonates and dolomite have become an exciting field of research in recent time, as different pathways for nucleation and growth are being suggested. Observations of shallow marine dolomite from the Carnian Travenanzes Formation (northern Italy; Preto et al., 2015) showed nanocrystal structures where the few-nanometre-scale crystallites show lattice mismatch between each other. This finding was interpreted as indication that dolomite formed via the aggregation of nanocrystals. ...
The conditions in ancient evaporative environments conducive to authigenic carbonate (especially dolomite [CaMg(CO 3 ) 2 ]) formation are still insufficiently understood. Insights from microfacies analysis can help to constrain the conditions in these environments. We provide a brief overview of the microfacies association and carbon and oxygen isotope compositions of dolomite beds intercalated in a claystone-rich succession from the Norian Arnstadt Formation in Thuringia and Lower Saxony (Germany) in order to gain further insight into the depositional conditions and processes leading to the formation of authigenic Mg/Ca-carbonates in the Germanic Basin. The studied intervals are ascribed to lacustrine, partially evaporitic conditions, while the sedimentary structures were not obliterated by recrystallization. The microfacies of the dolomites is diverse, showing homogeneous micrite, mudclasts, lamination, and peloidal structures, and reflects a shallow to deeper water (below wave base) and episodically evaporative environment. The dolomites exhibit oxygen isotope values (δ ¹⁸ O) in the range from −5.21 to −0.36‰ VPDB and, hence, only represent a weak meteoric influence, suggesting that the authigenic carbonate generally formed under evaporative conditions. Carbon isotope values (δ ¹³ C) in the range of −4.28 to 1.39‰ VPDB indicate a small contribution of remineralized organic carbon, mainly in sediments that were presumably deposited in deeper water or under brackish conditions. Sedimentary structures, such as lamination with graded silt layers, reworked mudclasts embedded in a fine dolomicrite matrix, and peloids showing plastic deformation, indicate that the sediment was still unlithified. These observations would be consistent with an authigenic formation of Mg/Ca-carbonates directly from the lake water, and their deposition under variable conditions in a large playa-lake/perennial lake system.
... In contrast, a Mg-rich dolomite, which was not replacing a precursor aragonite and was characterized by strain contrast and "differently developed lamellar microstructures resembling agglomerates" (Wenk et al.), 17 was interpreted as a possible "primary" precipitate, that is, implying spontaneous precipitation from a fluid. Following the idea that strain contrast would suggest primary dolomite formation, Preto et al. 18 analyzed laminated dolomite from a Late Triassic, shallow marine evaporative setting, supersaturated with respect to dolomite. They observed dolomicrite laminae by HR-TEM and found dislocation-ridden dolomite crystals, similar to those described by Wenk et al. 17 The HR-TEM observations of the Late Triassic dolomite revealed that micrite grains consisted of an aggregate of several tens of nanometer-sized crystals with non-uniform orientation. ...
... They observed dolomicrite laminae by HR-TEM and found dislocation-ridden dolomite crystals, similar to those described by Wenk et al. 17 The HR-TEM observations of the Late Triassic dolomite revealed that micrite grains consisted of an aggregate of several tens of nanometer-sized crystals with non-uniform orientation. Preto et al. 18 then interpreted the strong contrast and the heterogeneous microstructure of the dolomicrite as the result of lattice mismatch between single nanograins and interpreted the dolomite as being of "primary" origin. ...
... In particular, the Dolomia Principale dolomicrite was interpreted as forming directly from an evaporative brine through a mechanism of poorly oriented attachment of primary dolomite nanocrystals (Meister and Frisia). 20 Preto et al. 18 pointed out that the concept of "primary" dolomite may not exclude its formation via a potential precursor. Indeed, using time-resolved X-ray diffraction in precipitation experiments, Rodriguez-Blanco et al. ...
Dolomite [CaMg(CO3)2] formation under Earth surface conditions is considered largely inhibited, yet protodolomite (with a composition similar to dolomite but lacking cation ordering), and in some cases also dolomite, was documented in modern shallow marine and lacustrine, evaporative environments. Authigenic carbonate mud from Lake Neusiedl, a shallow, episodically evaporative lake in Austria consists mainly of Mg-calcite with zoning of Mg-rich and Mg-poor regions in μm-sized crystals. Within the Mg-rich regions, high-resolution transmission electron microscopy revealed < 5-nm-sized domains with dolomitic ordering, i.e., alternating lattice planes of Ca and Mg, in coherent orientation with the surrounding protodolomite. The calcite with less abundant Mg does not show such domains but is characterized by pitted surfaces and voids as a sign of dissolution. These observations suggest that protodolomite may overgrow Mg-calcite as a result of the changing chemistry of the lake water. During this process, oscillating concentrations (in particular of Mg and Ca) at the recrystallization front may have induced dissolution of Mg-calcite and growth of nanoscale domains of dolomite, which subsequently became incorporated as ordered domains in coherent orientation within less ordered regions. It is suggested that this crystallization pathway is capable of overcoming, at least at the nanoscale, the kinetic barrier to dolomite formation.
... Although dolomites crystals with nonplanar-a(s) textures are generally interpreted to be formed above the critical roughening temperatures (50-60°C; Gregg and Sibley, 1984;Sibley and Gregg, 1987), they can also be precipitated from the fluids with high degree of supersaturation at lower temperatures (i.e., < 50-60°C) (Gregg and Sibley, 1984;Sibley and Gregg, 1987;Warren, 2000;Fu et al., 2006;Al-Helal et al., 2012). Dolomites with similar textures are extensively reported to be generated in the near-surface evaporative environments of different ages (e.g., McKenzie, 1981;Qing and Mountjoy, 1989;Qing, 1998;Whitaker et al., 2004;Rameil, 2008;Jiang et al., 2013Jiang et al., , 2014Preto et al., 2015). The Md1 dolomite, however, does not show compelling evidence for the occurrence of related evaporites (e.g., gypsum or anhydride), indicating the salinity of seawater during Md1 dolomite formation was likely below that of gypsum precipitation; that is, the parental fluids of this type of dolomite derived most likely from mesosaline to penesaline seawaters (e.g., Jones et al., 2000;Qing et al., 2001;Rott and Qing, 2013;Guo et al., 2016Guo et al., , 2017. ...
... indicating that the dolomitizing fluids in which Md1 dolomites were formed probably derived from the contemporaneous seawater (e.g., Land, 1985;Budd, 1997;Fu et al., 2006). In contrast to seawater-originated dolomites or its derivative ones, the δ 13 C values of BSR-mediated dolomites are usually reported to be more depleted (even up to −20‰ VPDB) (Vasconcelos and McKenzie, 1997;van Lith et al., 2003;Machel, 2004;Preto et al., 2015;Jiang et al., 2016). The δ 13 C values of Md1 dolomites, however, are more positive compared with BSR-mediated dolomites. ...
... The δ 13 C values of Md1 dolomites, however, are more positive compared with BSR-mediated dolomites. This discrepancy is interpreted to be the result of overprinting of the refluxing seawater at a very shallow burial depth (< 100 m; Choquette and Hiatt, 2008) (e.g., Machel, 2004;Wacey et al., 2007;Preto et al., 2015;Guo et al., 2016Guo et al., , 2017, or the limited involvement of BSR in the dolomite precipitation, or a combination of both. In the latter two cases, the BSR-mediation could have mainly contributed to the creation of tiny nuclei (or seed crystals; Warren, 2000;Machel, 2004) of Md1 dolomite crystals, and was then attenuated gradually with progressive burial. ...
... Stalagmites and flowstones from Italy and Australia, spanning the Holocene to the Early Pliocene, as well as Triassic dolomites and limestones from the Dolomites of Italy have all been the subject of previous research aimed at reconstruction of past climate and environmental changes in both continental and shallow marine settings (Frisia-Bruni and Wenk, 1985;Frisia and Wenk, 1993;Wenk et al., 1993;Frisia, 1994;Frisia et al., 2006Frisia et al., , 2012Borsato et al., 2008;Blyth and Frisia, 2008;Breda and Preto, 2011;Frisia and Borsato, 2010;Gattolin et al., 2013;Regattieri et al., 2014;Preto et al., 2015;Bajo et al., 2016Bajo et al., , 2017. ...
... Primary is often used in relation to aphanitic dolomite preserving original sedimentary structures and textures because diagenesis was hindered by the high impermeability of the sediment (Tucker, 1982(Tucker, , 1983Frisia, 1994;Preto et al., 2015). HR investigation of Triassic dolomite (Dolomia Principale, Northern Italy) initiated by Wenk et al. (1993) and Frisia and Wenk (1993) illustrated that there are different mechanisms for dolomite formation that pre-date the first stage of multi-step diagenesis, which can be considered primary in the sense of their being pre-diagenetic. ...
... Machel, 2004 andPetrash et al., 2017), and most of these likely follow the rules of inorganic chemistry . Preto et al. (2015) appraised the concept of primary dolomite through HR-TEM observation of aphanitic dolomite in supratidal laminated facies of the Triassic Travenanzes Formation (Dolomites, Northern Italy). They reported both assemblies of oriented nanocrystals, 50 to 200 nm in diameter and nanocrystals with non-oriented lattice, about 5 nm in diameter (cf. ...
... Although nonplanar dolomite crystals are generally favoured above the critical roughening temperature (50e60 C; Gregg and Sibley, 1984;Sibley and Gregg, 1987), they can also be formed lower than this temperature at high supersaturation levels (Gregg and Sibley, 1984;Sibley and Gregg, 1987;Warren, 2000;Meister et al., 2013). Numerous studies have reported that bacterial sulphate reduction (BSR) could facilitate the dolomitization by lowering the kinetic barrier that inhibits direct dolomite precipitation from Mgsupersaturated brines (Vasconcelos et al., 1995;Vasconcelos and McKenzie, 1997;Burns et al., 2000;Mazzullo, 2000;Van Lith et al., 2003;Loyd et al., 2012;Meister et al., 2013;Baldermann et al., 2015;Preto et al., 2015). The widespread presences of amorphous to crystalline pyrites scattering within intercrystalline pores of Md1 dolomite ( Fig. 4A and C) imply that BSR could have taken place in the reducing fluid media, promoting dolomite precipitation by removing the kinetic barrier preventing dolomite precipitation from near-surface, low-temperate fluid media (Vasconcelos et al., 1995;Vasconcelos and McKenzie, 1997;Van Lith et al., 2003). ...
... As documented above, d 13 C values (À2.7 to À1.1‰ VPDB) of Md1 dolomite are highly similar to those of the ambient limestone (À2.3 to À0.8‰ VPDB) and fall within the estimated d 13 C ranges (À3.0 to À0‰ VPDB) of calcite precipitated from the Late Cambrian-Early Ordovician seawater (Qing and Veizer, 1994;Veizer et al., 1999;Shields et al., 2003) (Fig. 10), suggestting that the Md1 dolomite as a whole was formed in the dolomitizing fluids contributed largely by progressive seawater reflux from penecontemporaneous to shallow burial conditions (Land, 1980;Banner and Hanson, 1990;Tucker and Wright, 1990;Budd, 1997). In this case, the carbon isotopic signatures (less than À20‰ VPDB) of the primeval BSRmediated dolomites (or dolomite nuclei) (Vasconcelos and McKenzie, 1997;Mazzullo, 2000;Warren, 2000;Van Lith et al., 2003;Machel, 2004) could have been overprinted by that of later refluxing seawater (Machel, 2004;Preto et al., 2015), accounting for the seawater-originated d 13 C values of Md1 dolomite. This scenario also agrees with their relatively depleted d 18 O values (À9.1 to À4.7% VPDB), which generally fall within the estimated d 13 C values of dolomites formed in equilibrium with contemporaneous seawater (Fig. 10), compared to the estimated values (À3.5eþ2.0‰ ...
... This indicates that, although the salinity of seawater may have increased to some extent due to evaporation, it was still lower than that of gypsum precipitation; that is, the platform interior was mainly occupied by mesosaline to penesaline seawater (Qing et al., 2001) under which an oxygen-deficient water column was favoured due to water restriction and tranquility. Under the sediment-water interface, the relatively reducing pore water in the fine-grained carbonate sediments (lime mud and/or peloid) deposited in lowenergy lagoon could have induced extensive BSR there in the presence of organic matter, mediating dolomite precipitation by overcoming the kinetic barrier of dolomite precipitation at low temperature in earth surface environments (Vasconcelos et al., 1995;Vasconcelos and McKenzie, 1997;Van Lith et al., 2003;Loyd et al., 2012;Meister et al., 2013;Baldermann et al., 2015;Preto et al., 2015) while vast amounts of fine-grained carbonate grains/particles further provided numerous nucleation sites for these primeval dolomites (seed crystals; Warren, 2000). As the host sediments were buried deeper (generally <100 m at depth) (Choquette and Hiatt, 2008), the chemically modified seawater could have percolated through underlying shallow carbonate sediments bearing primeval dolomites driven by increased brine density (Kaufman, 1994;Jones et al., 2003Jones et al., , 2004Jiang et al., 2013) and/or sea-level fluctuations (Sun, 1994; Qing et al., 2001), leading to pervasive dolomitization by dolomite maturation, precipitation and further overgrowth (Figs. 13 and 14) (Machel, 2004;Preto et al., 2015). ...
... The spheroidal Ca-rich dolomite crystals ( Fig. 17C and D) closely resemble the primary syngenetic dolomite precipitates formed by microbial activity in shallow alkaline lakes (Vasconcelos and McKenzie, 1997;García del Cura et al., 2001;Calvo et al., 2003;Bréhéret et al., 2008;Huerta et al., 2010;Preto et al., 2015). However, we note that similar spheroidal textures of dolomite are also considered to form by abiogenic recrystallization of primary micrite in closed, evaporative shallow alkaline lakes (Wanas, 2002;Armenteros, 2010). ...
... The association of spheroidal dolomite with Mg-rich clays could indicate that the dolomite formed in alkaline lacustrine environments as primary precipitates (Arakel et al., 1990;El-Sayed et al., 1991;Armenteros et al., 1995;Bustillo et al., 2002;Pimentel, 2002;Wanas, 2002;Wanas and Sallam, 2016). Moreover, Type A (spheroidal dolomite) is morphologically similar to microbially-induced primary dolomite that precipitates in shallow alkaline lakes Huerta et al., 2010;Preto et al., 2015). Conversely, the fine-crystalline (<20 μm) dolomite crystals (dolomicrite) may have an early diagenetic in origin (Tucker, 1991). ...
... The spheroidal Ca-rich dolomite crystals ( Fig. 17C and D) closely resemble the primary syngenetic dolomite precipitates formed by microbial activity in shallow alkaline lakes (Vasconcelos and McKenzie, 1997;García del Cura et al., 2001;Calvo et al., 2003;Bréhéret et al., 2008;Huerta et al., 2010;Preto et al., 2015). However, we note that similar spheroidal textures of dolomite are also considered to form by abiogenic recrystallization of primary micrite in closed, evaporative shallow alkaline lakes (Wanas, 2002;Armenteros, 2010). ...
... The association of spheroidal dolomite with Mg-rich clays could indicate that the dolomite formed in alkaline lacustrine environments as primary precipitates (Arakel et al., 1990;El-Sayed et al., 1991;Armenteros et al., 1995;Bustillo et al., 2002;Pimentel, 2002;Wanas, 2002;Wanas and Sallam, 2016). Moreover, Type A (spheroidal dolomite) is morphologically similar to microbially-induced primary dolomite that precipitates in shallow alkaline lakes Huerta et al., 2010;Preto et al., 2015). Conversely, the fine-crystalline (<20 μm) dolomite crystals (dolomicrite) may have an early diagenetic in origin (Tucker, 1991). ...
... When the ambient temperature is below this temperature, the formation of dolomite requires a supersaturated fluid with extremely high Mg-ion concentrations with respect to dolomite [7,36,50]. Moreover, bacterial sulfate reduction (BSR) has been reported to promote dolomite precipitation under the near-surface, low-temperature conditions by reducing the kinetic barrier [18,51,52]. BSR tends to occur in a reducing environment with the extensive presence of Fe 2+ , which is prone to forming pyrite [53]. ...
... Meanwhile, the δ 13 C values of D2 dolomite are highly coincident with the δ 13 C values of surrounding limestones, and within the δ 13 C range reported for the coeval marine calcites, indicating that the δ 13 C feature of D2 dolomite originates from the connate seawater. This is in line with the characteristics of the seepage-reflux dolomitization model, where the progressive refluxing seawater from tidal flats or lagoons derived from fluid density fluctuation or eustatic sea level changes seeps back into the porous precursor limestone, leading to pervasive dolomitization due to dolomite replacement, precipitation, and overgrowth [6,52]. ...
The dolomitization of carbonate rocks has always been a hot topic in the study of the dolomite reservoir. In this study, the genesis of Cambrian dolomite in the Bachu area, Tarim Basin, was assessed through petrographic examinations, isotope compositions (C, O, and Sr), trace and rare earth elements, and fluid inclusion microthermometry. Microscopic analysis revealed three types of dolomites: very fine-crystalline, nonplanar dolomite (D1); fine-crystalline, nonplanar to planar-s dolomite (D2); and medium- to coarse-crystalline, planar-e to planar-s dolomite (D3). D1 dolomite exhibits well-preserved original sedimentary features, such as algal laminae, stromatolite, and evaporite streak, and is characterized by the 87Sr/86Sr value and δ18O value in equilibrium with the coeval seawater, its high Sr and Na content, and its low Mn content. This indicates that D1 dolomite is primarily a penecontemporaneous dolomite in tidal flat or lagoon environments, and its dolomitizing fluid is mainly evaporated mesosaline to penesaline seawater. D2 dolomite shows ghosts of precursor particles; features δ13C values in equilibrium with the coeval seawater, high 87Sr/86Sr values, low Sr content, and positive Eu anomaly; and is widely distributed close to stylolite. This illustrates that D2 dolomite was principally formed by seepage–reflux dolomitization, and is closely related to hydrothermal activity and pressure dissolution. D3 dolomite displays a crystal texture with a cloudy core and compositional zoning, and the original sedimentary fabrics cannot be identified. It has similar δ13C values and REE patterns to the calcite precipitated from coeval seawater, high 87Sr/86Sr values, low Sr contents and high Mn/Sr ratios, which suggests that D3 dolomite is chiefly related to the recrystallization of the precursor dolomite during the deep burial stage, and the deep circular brine provides Mg ions through the fluid–rock reaction. This study shows that the Cambrian dolomite in the Bachu area is mainly formed in the coeval seawater environment during the penecontemporaneous and shallow burial stages, and has extensively suffered from recrystallization and burial diagenesis due to long-term deep burial, which was further strengthened in the fracture-enriched area.
... However, no evaporitic deposits have been found in the study area, suggesting the salinity levels of the Permian seawater may have increased only to mesosaline or penesaline, insufficient for the formation of evaporites (Qing et al., 2001;Vandeginste et al., 2009). Locally concentrated pyrites in the Md1 (Fig. 4B) imply microbial activity, such as bacterial sulphate reduction (BSR), may have occurred extensively in saturated Permian seawater (McKenzie and Vasconcelos, 2009), which can remove the kinetic barriers and promote dolomitization under nearsurface conditions characterized by low temperature (Vasconcelos et al., 1995;Van Lith et al., 2003;Meister et al., 2013;Preto et al., 2015). In the penecontemporaneous phase, under BSR modification, the brackish and saturated seawater provided a good opportunity for the nucleation of dolomite, facilitating the formation of seed crystals, which is of great significance for the dolomitization in the Md2 formation stages (Warren, 2000;Choquette and Hiatt, 2008). ...
... In the penecontemporaneous phase, under BSR modification, the brackish and saturated seawater provided a good opportunity for the nucleation of dolomite, facilitating the formation of seed crystals, which is of great significance for the dolomitization in the Md2 formation stages (Warren, 2000;Choquette and Hiatt, 2008). In the subsequent very shallow burial process, affected by seawater density fluctuations and/or eustatic sea-level changes (Kaufman, 1994;Jones et al., 2003), the concentrated seawater seeped and refluxed in the pores of the unconsolidated high-energy shoal carbonate rocks, resulting in the overgrowth of the original dolomite seeds, and the aggregation of crystals in Md1 (Machel, 2004;Preto et al., 2015). Therefore, we consider Md1 to be seepage reflux dolomitization on the periphery of high energy shoals near the platform margin (Fig. 12A). ...
... interpreted these dolomites to be an early diagenetic replacement of precursor carbonate. In a recent study, Preto et al. (2015) suggested that the dolomites of the Carnian Travenanzes Formation (Fm.) in the Venetian Alps are primary precipitates; i.e. they precipitated directly from solution in the sedimentary environment and not by the replacement of a precursor phase during burial. This interpretation is based on high-resolution transmission electron microscope (HR-TEM) analysis, which revealed that single micron-scale dolomite crystals consist of grains with incoherent crystallographic orientation at a scale of a few nanometres (see Meister and Frisia, 2019). ...
... But also, the observed trend in δ 18 O would be too steep to be explained by overprinting within a normal geothermal gradient, and no signs of any hydrothermal activity occur in this region. In any case, the oxygen isotope data do not imply any post-depositional overprint, while nanocrystalline structures observed by Preto et al. (2015) preclude a later pervasive recrystallization during burial diagenesis. Sedimentary structures indicate that most of the homogeneous dolomite and laminae containing aphanotopic dolomite was unlithified, and dolomite was therefore deposited as fine-grained mud. ...
The geochemical conditions conducive to dolomite formation in
shallow evaporitic environments along the Triassic Tethyan margin are still
poorly understood. Large parts of the Triassic dolomites in the Austroalpine
and the southern Alpine realm are affected by late diagenetic or
hydrothermal overprinting, but recent studies from the Carnian Travenanzes
Formation (southern Alps) provide evidence of primary dolomite. Here a
petrographic and geochemical study of dolomites intercalated in a
100 m thick Carnian sequence of distal alluvial plain deposits is presented
to gain better insight into the conditions and processes of dolomite
formation. The dolomites occur as 10 to 50 cm thick homogeneous beds,
millimetre-scale laminated beds, and nodules associated with palaeosols. The
dolomite is nearly stoichiometric with slightly attenuated ordering
reflections. Sedimentary structures indicate that the initial primary
dolomite or precursor phase consisted largely of unlithified mud. Strontium
isotope ratios (87Sr∕86Sr) of homogeneous and laminated dolomites
reflect Triassic seawater composition, suggesting precipitation in
evaporating seawater in a coastal ephemeral lake or sabkha system. However,
the setting differed from modern sabkha or coastal ephemeral lake systems by
being exposed to seasonally wet conditions with significant siliciclastic
input and the inhibition of significant lateral groundwater flow by
impermeable clay deposits. Thus, the ancient Tethyan margin was different
from modern analogues of primary dolomite formation.
... Thus their occurrence may represent the transition to extreme conditions during the Triassic-Jurassic boundary interval (Jenkyns, 1988;Olsen, 1999;. Also, the stromatolites generally occur in the shallow to very shallow water (Preto et al., 2015). In the present case, their stratigraphic position between the underlying shallow water, reworked, ferruginous carbonates and the overlying gypsum beds indicates near surface/at least partly emergent, intertidal to supratidal conditions. ...
... Frisia and Wenk, 1993;Iannace and Frisia, 1994;Meister et al., 2013). In northern Italy, the Travenanzes Formation (Preto et al., 2015) indicates deposition in a wide low-relief coastal area, dry land rivermuddy coastal plain and sabkha system (Breda and Preto, 2011). Seaward of this clay-rich environment, thick peritidal carbonates formed in a large-scale epicontinental platform. ...
The Triassic–Jurassic boundary interval was characterised by the change from warm, semiarid–arid to a hot and humid climate in the Tethyan domain linked to input of greenhouse gases from the Central Atlantic Magmatic Province (CAMP) activity and Pangaea breakup. This study provides the very first outcrop evidences of palaeoclimatic evolution during the Triassic–Jurassic boundary interval in the then southern hemisphere, along the eastern margin of Gondwana facing the western Tethys. In the Tethyan Salt Range of Pakistan a succession of Upper Triassic dolomites, green-black shales (Kingriali Formation) to overlying Lower Jurassic quartzose sandstones, shales, laterites and conglomerates (Datta Formation) represents the sedimentary archives of this critical time interval. Bulk and clay mineralogy of the Upper Triassic shales indicate the presence of mainly illite while kaolinite is a minor component. The kaolinite content, a reflection of the mature stage of chemical weathering and hence hot–humid conditions, increases up-section in the overlying shales and sandstone–shale succession. The following laterite–bauxite horizons lack illite and are entirely composed of kaolinite, boehmite and haematite. The bulk rock geochemistry of the succession confirms a similar trend. The Chemical Index of Alteration (CIAmolar) displays an increasing trend from the Upper Triassic (CIA 68–80) to the overlying Lower Jurassic strata (CIA 90–97). The overall results for the succession reveal an increasing chemical maturity trend from Rhaetian to Hettangian thereby supporting a change from warm-arid to a hot and humid palaeoclimate, probably extreme greenhouse conditions. Similar changes in the clay mineralogy and sediment geochemistry across the Triassic–Jurassic boundary have been reported from basins across Europe. Thus the Salt Range provides sections from the southern hemisphere for correlations across the Triassic–Jurassic boundary.
... Several studies have presented the significance of microbial impact on dolomitization of organic-rich peritidal carbonate deposits (Vasconcelos et al., 1995;Wright, 2000;Mastandrea et al., 2006;Bontognali et al., 2010;Preto et al., 2015;Haas et al., 2015;Hips et al., 2015). The presence of similar, microbially mediated primary dolomite or very high-Mg calcite is highly probable in the case of the studied stromatolitic samples. ...
... Such primary precipitates could play a significant role during subsequent dolomitization. However, distinction of organogenic dolomite from the mimetic replacements is extremely difficult in a completely dolomitized sediment but might be possible through circumstantial evidence (e.g., Mastandrea et al., 2006;Preto et al., 2015;Pertash et al., 2017). Thus, this question requires further investigations that fall outside the scope of this study. ...
The Middle Triassic shallow marine carbonates of the SE Pannonian Basin (Szeged Dolomite Formation) show evidence for multistage dolomitization and a complex diagenetic history. In first stage the whole sequence was completely dolomitized by reflux of slightly evaporated seawater. This process took place from the near surface till shallow burial realms and resulted in the formation of both fabric-preserving and fabric-destructive dolomite types.
In the following stage nonplanar matrix dolomite and saddle dolomite cement were formed in the intermediate and/or deep burial realm. These later dolomite phases are likely generated by invasion of exotic fluids at relatively high temperature evidenced from the fluid inclusion homogenization temperatures, and stable isotope compositions. Vuggy, fracture, and solution enhanced porosity are also related to this local hydrothermal event. Microthermometry performed on saddle dolomite-hosted primary fluid inclusions confirm the presence of hot (138–235 °C) and moderately saline brines (4.1–8.7 mass% NaCl equivalent). The calculated δ18Owater and the measured δDwater values of the fluid inclusions from the saddle dolomite cement together with the relatively low salinity values indicate a metamorphogenic (and/or magmatic) origin of the hydrothermal fluid that probably was channeled along the Upper Cretaceous subhorizontal overthrust zones.
The pores formed by the leaching effect of these hydrothermal fluids were subsequently partly occluded by meteoric calcite during the Paleogene–Middle Miocene subaerial exposure but a remarkable part was preserved, and currently serves as reservoir space.
Such an integrated study of the different dolomite and porosity types, the understanding of their genesis, and timing relative to hydrocarbon maturation and migration could aid in exploration and development.
... Comparing hydrothermal Dol 3 in Steltenberg Quarry to Late Triassic dolomites elsewhere (Geske et al., 2012;Gabellone et al., 2014;Preto et al., 2015;Hips et al., 2016;Mueller et al., 2020;Zou et al., 2023), it seems remarkable that most of these phases share a similar rockbuffered carbon isotopic composition, even after multiple tectonic, hydrothermal, and meteoric overprint (Fig. 14). It is at present unclear if this is an intrinsic pattern related to these fabrics or a more random coincidence related to a different set of mechanisms. ...
Carbonate archives record a brief snapshot of the ambient Earth’s surface conditions at their deposition. However, the geologically reasonable extraction and interpretation of geochemical proxy data from ancient, diagenetically altered rock archives is fraught with problems. Three issues stand out: the dichotomy between petrographic and geochemical alteration; the lack of quantitative age constraints for specific diagenetic phases resulting in a poorly constrained admixture of local, basin-wide and over-regional (far-field) features; and an often insufficient understanding of the temperatures and compositions of diagenetic fluids. Here, the archive of Devonian marine limestones exposed to multiple far-field diagenetic events is used as an example to explore the above-listed issues. Methods applied include petrography, U-Pb dating, micro XRF, fluid inclusion data, clumped isotopes, δ13C and δ18O isotopes, 87Sr/86Sr ratios and quartz trace element data. Devonian limestones studied here were overprinted by two cross-cutting regional fault zones (T ≈ 230 °C) by multiple events between the Variscan Orogeny and the late Paleogene. The following processes are recorded: (i) protolith deposition and partial dolomitisation during rapid burial in the Middle/Late Devonian (T ≈ 180 °C); (ii) deep burial to ca 6.5 km and tectonic/hydrothermal overprint during the Variscan Orogeny in the Carboniferous (T ≈ 90–230 °C); (iii) rapid uplift to 1–2 km burial depth at the end of the Variscan Orogeny and hypogene karstification (T ≈ 50 to 100 °C) initiated by regional geology in the Permian/Triassic; (iv) tectonic/hydrothermal overprint during the opening of the Proto-Atlantic Ocean between the Early Jurassic and the Early Cretaceous (T ≈ 50 to 130 °C); (v) tectonic/hydrothermal overprint including renewed hypogene karstification and hydrothermal calcite cement precipitation (T ≈ 50 to 180 °C) during Alpine Orogeny between the Late Cretaceous and late Paleogene. Despite this complex series of diagenetic events, the protolith limestones largely preserved their respective Middle/Late Devonian dissolved inorganic carbon (DIC) and 87Sr/86Sr signatures. This study documents that geochemical proxy data, placed into their petrographic, paleotemperature, and local to over-regional context, significantly increases the ability to extract quantitative information from ancient carbonate rock archives. Research shown here has wider relevance for carbonate archive research in general.
... In addition, these metastable proto-dolomites were proposed to be precipitated in specific settings, such as in evaporative lakes and sabkhas (e.g. Preto et al., 2015;Cheng et al., 2021;Meister et al., 2023), volcanic-hydrothermal lake (e.g. Jiao et al., 2023), or in lakes with high dissolved silica concentration sourced from freshwater input (e.g. ...
... Newly precipitated dolomite nanocrystals formed through epitaxial growth as they follow crystallographic orientations of bulk crystals demonstrated by SAED and TEM images (Figs 6, 10 and S12). Similar nucleation and growth of nanocrystal dolomites is also observed in modern sediments such as the Great Salt Lake with dissolved silica as a catalyst (Fang et al., 2023a), Lake Neusiedl Neuhuber et al., 2023), the Coorong region (Raudsepp et al., 2022) and Abu Dhabi sabkha (Wenk et al., 1993), as well as in older rock such as Dolomia Principale (Meister & Frisia, 2019), dolomite crust associated with deep sea modern methane seeps (Xu, 2010) and the eastern Dolomites, Northern Italy (Preto et al., 2015). The small misoriented nanocrystals of dolomite indicate that they formed directly and nucleated on bulk crystals implying a primary precipitation origin as opposed to replacement products. ...
In contrast to the prevalence of dolomite [CaMg(CO 3 ) 2 ] in the geological record, there are few instances of recent formation. This discrepancy occurs despite supersaturation with respect to dolomite in many modern marine and lacustrine environments. Additionally, laboratory experiments have struggled to precipitate dolomite at ambient temperatures (<40°C) even under highly saturated conditions. However, recent work has found dissolved silica to be an effective catalyst for the direct precipitation of dolomite. To test this hypothesis, the hydrology and mineralogy of Deep Springs Lake, a playa setting with primary dolomite precipitation has been explored. In this study, the central playa sites with the highest sedimentation rates were found to have dissolved silica concentrations close to saturation. Rietveld refinement showed that bulk mineral assemblages of core samples contain 20 to 40% fine‐grained, partially ordered dolomite along with a 10 to 20% poorly crystalline clay fraction, while transmission electron microscopy imaging found a co‐precipitation relationship between the fine‐grained dolomite and Mg‐rich smectite clays, where the Mg‐rich smectite clays act as a sink for the excess dissolved silica during evaporation. Transmission electron microscopy results indicate that the nanodolomite crystals formed through surface‐induced nucleation and growth processes in the presence of dissolved silica as a catalyst. The broad coincidence of conditions for the direct precipitation of dolomite and Mg‐rich smectite clays provides evidence that silica may be a key to the dolomite problem, as well as constraining palaeoenvironments such as the Pre‐salt Barra Velha Formation.
... For several of the examples cited above, it has been suggested that precipitation is penecontemporaneous, i.e. related to the evaporating surface water bodies (e.g. Meister et al., 2011a,b;McCormack et al., 2018;Fussmann et al., 2020;Fang et al., 2023), and this has also been proposed for some shallow-marine (Preto et al., 2015;Meister & Frisia, 2019;Rieder et al., 2019) and lacustrine dolomite occurrences (e.g. Reinhardt & Ricken, 2000;Hofbauer et al., 2021) in the geological record. ...
Authigenic Mg‐calcite and dolomite are frequently observed in restricted, evaporative environments, such as lagoon or lake systems, but their formation is difficult to capture due to slow growth rates. Lake Neusiedl, an alkaline and subhaline lake with a mean water depth of 0.7 m in Austria, offers a natural system to study the precipitation of Ca‐Mg‐carbonate phases, which occur as fine‐grained, unconsolidated and largely homogenized mud. To elucidate the timing and formation mechanisms of these authigenic carbonate phases, the mineralogical and isotopic composition and radiocarbon age of different sediment grain‐size fractions from <0.2 to >3.0 μm were analysed. X‐ray diffraction analyses show two broad peaks of Mg‐calcite and protodolomite (lacking ordering peaks), suggesting that the carbonates are authigenic rather than detrital in origin. Calibrated carbon‐14 ages range between 200 cal yr BP and 3700 cal yr BP. The linear correlation of age and grain size suggests a very slow growth rate of single crystals of 0.23 to 0.60 μm/ka. These rates suggest an extremely slow sedimentation rate in a shallow lake that existed during most of the Holocene. The higher abundance of protodolomite in older grain fractions, in contrast to the presence of high‐Mg calcite in the youngest fractions, suggests a growth succession where high‐Mg calcite develops first and subsequently transforms into protodolomite. Much higher ages of 6 cal ka BP to 15 cal ka BP are measured in carbonates of lake deposits exposed on land, in a section northwest of the recent lake, suggesting a growth rate of those carbonate minerals of 0.13 μm/ka. These time constraints further suggest that some carbonate grains could already have nucleated from lake water before or during the last glacial maximum, although under slightly different hydrochemical conditions.
... <50-60 • C; Gregg and Sibley, 1984;Al-Helal et al., 2012;Guo et al., 2020). Considering the occurrence of moldic pores by dissolution of gypsum/anhydrite nodules (Fig. 5d), Md1 seems to have likely formed through evaporative pumping effect at near-surface diagenetic environment during the penecontemporaneous period (e.g., McKenzie, 1981;Rameil, 2008;Preto et al., 2015). However, the absence of large-scale evaporate layers in Md1 and relatively low contents of Na and K (e.g., Liu et al., 2021, Table 1), compared with earlier studies (e.g., Ren et al., 2016;Sun et al., 2018), do not support an origin from brines of a highly evaporative sabkha environment but rather a mesosaline to penesaline seawater formed by modified seawater or seawater (possibly slightly evaporated) mixed with meteoric water. ...
... Contrarily to the oxygen signature, the carbon isotopic values of Lower Cangulo dolostones (D2, DC2 and D3) is highly depleted (− 6 to − 16‰; Fig. 11) compared with other upper sag carbonates from the South Atlantic basins Sabato-Ceraldi and Green, 2016). This shift is interpreted to result from the interaction of Aptian evaporated marine water with fluids derived from: i) microbial sulphate reduction (Mazzullo, 2000;Preto et al., 2015), ii) oxidation of organic matter (Moore, 2001) and/or iii) interaction with meteoric fluids (Bustillo et al., 2017). All three processes are feasible in the interpreted sabkha and tidal flat depositional setting and they potentially coexisted in the Cangulo palaeovalley. ...
The Aptian Pre-Salt sedimentary succession cropping out in Cangulo palaeovalley onshore Namibe Basin (Angola) was studied by a combination of field and analytical techniques to constrain the sedimentary and diagenetic evolution of the uppermost sag sequence of the South Atlantic passive margin. Field observations allows definition of four transgressive-regressive cycles characterised by fluvial to tidal-influenced mixed clasticcarbonate and carbonate-dominated deposits, that locally show evidence of evaporite dissolution; highlighting that evaporite deposition started earlier than deposition of the regional South Atlantic Loeme-Bambata evaporite formations. Two separate pre-salt carbonate units have been differentiated within the Cangulo Fm; i) a lower transitional to marginal marine, and ii) a younger upper non-marine freshwater travertine system, that is documented for the first time in the west African margin. Transgressive-regressive cycles control the early diagenesis of the tidal carbonates that include dolomitization due to mixing fluids during transgressions, and karstification due to evaporite dissolution by meteoric water circulation during regressive events. Clastic supply appears to have been completely shut down during carbonate deposition, suggesting major climatic change associated with carbonate deposition. During the lowstand between the two carbonate units, fluid flow through Cangulo palaeovalley was re-established resulting in extensive karstification and formation of a large-scale erosional unconformity that is interpreted to be time equivalent to an intra Chela-Cuvo Fm. event. The top of the studied succession corresponds to the transgressive deposits of the Bambata evaporites that are not preserved in the Cangulo palaeovalley due to its erosion but are regionally developed. The results of this study can be directly linked to along strike age equivalent Pre-Salt successions cropping out in the Namibe, Benguela and Kwanza basins, and directly offshore Angola and Brazil using well and seismic data. These new data shed important new light and constraints on the depositional and diagenetic evolution of the complex Pre-Salt reservoir systems of the South Atlantic, and the depositional and bathymetric setting at the time of onset of the main south Atlantic evaporite deposition.
... At the base of the loose soils, there is a 5-20 m thick layer of altered anhydrite and gypsum The local stratigraphy (Figure 5c) is composed of a shallow portion of loose debris, characterized by heterogeneous coarse materials mixed with a silty-clay fraction, which is overlapped by a second debris layer, constituted by a predominant clayey-silty matrix with intercalations of anhydrite and heterogeneous detrital sandy-gravelly-silty material. At the base of the loose soils, there is a 5-20 m thick layer of altered anhydrite and gypsum and, below, a stiffer bedrock whose lithological sequence consists of evaporitic facies related to the Travenanzes Formation [29]. The results of electrical tomography and piezometric measurements allowed the recognition of the following two aquifers: a shallow one, affecting the covering detrital layer and supported by the clayey-silty soil, and a deep one, affecting the fractured gypsum. ...
A procedure aimed at forecasting the velocity trend of a landslide for a period of some hours to one or two days is proposed here together with its MATLAB implementation. The method is based on continuous wavelet transform (CWT) and convolutional neural network (CNN) applied to rainfall and velocity time series provided by a real-time monitoring system. It is aimed at recognizing the conditions that induce a strong increase, or even a significant decrease, in the average velocity of the unstable slope. For each evaluation time, the rainfall and velocity scalograms related to the previous days (e.g., two weeks) are computed by means of CWT. A CNN recognizes the velocity trend defined in the training stage corresponds to these scalograms. In this way, forecasts about the start, persistence, and end of a critical event can be provided to the decision makers. An application of the toolbox to a landslide (Perarolo di Cadore landslide, Eastern Alps, Italy) is also briefly described to show how the parameters can be chosen in a real case and the corresponding performance.
... They were tentatively attributed to protodolomite (cf. [80]), which hypothetically may be related to microbial sulfate reduction [81,82]. Various modern demosponges (e.g., Chondrosia reniformis, Petrosia ficiformis and Geodia barretti) harbour abundant sulfate-reducing bacteria. ...
Following the end-Permian crisis, microbialites were ubiquitous worldwide. For instance, Triassic deposits in the Germanic Basin provide a rich record of stromatolites as well as of microbe-metazoan build-ups with nonspicular demosponges. Despite their palaeoecological significance, however, all of these microbialites have only rarely been studied. This study aims to fill this gap by examining and comparing microbialites from the Upper Buntsandstein (Olenekian, Lower Triassic) and the lower Middle Muschelkalk (Anisian, Middle Triassic) in Germany. By combining analytical petrography (optical microscopy, micro X-ray fluorescence, and Raman spectroscopy) and geochemistry (δ13Ccarb, δ18Ocarb), we show that all the studied microbialites formed in slightly evaporitic environments. Olenekian deposits in the Jena area and Anisian strata at Werbach contain stromatolites. Anisian successions at Hardheim, in contrast, host microbe-metazoan build-ups. Thus, the key difference is the absence or presence of nonspicular demosponges in microbialites. It is plausible that microbes and nonspicular demosponges had a mutualistic relationship, and it is tempting to speculate that the investigated microbial-metazoan build-ups reflect an ancient evolutionary and ecological association. The widespread occurrence of microbialites (e.g., stromatolites/microbe-metazoan build-ups) after the catastrophe may have resulted from suppressed ecological competition and the presence of vacant ecological niches. The distribution of stromatolites and/or microbe-metazoan build-ups might have been controlled by subtle differences in salinity and water depth, the latter influencing hydrodynamic processes and nutrient supply down to the microscale. To obtain a more complete picture of the distribution of such build-ups in the earth’s history, more fossil records need to be (re)investigated. For the time being, environmental and taphonomic studies of modern nonspicular demosponges are urgently required.
... However, some dolomite beds in the geological record have no evidence for secondary fluid flow and are hypothesized to have formed as a primary precipitate, or at least as a Ca-Mg carbonate with an approximate 1 : 1 Ca to Mg ratio that has since recrystallized into dolomite (Warren, 2000). In particular, primary dolomite is hypothesized to have precipitated in the Precambrian (Tucker, 1982), perhaps from aragonite-dolomite seas (Hood et al., 2011;Hood & Wallace, 2012) and in the Triassic in the Dolomites of Northern Italy (Preto et al., 2015;Meister & Frisia, 2019). ...
The Coorong Lakes, South Australia, are one of the models for unravelling the ‘Dolomite Problem’. Critically, today only a few modern environments remain where large quantities of very high magnesium calcite (Ca0.5Mg0.5CO3) (also described as protodolomite or disordered dolomite) and magnesite (MgCO3) precipitate. Previously conducted laboratory studies demonstrate that carbonate minerals can precipitate via classical and non‐classical crystallization pathways. This study uses the preserved crystal sizes, morphologies and microstructures of Ca–Mg carbonates in the Coorong Lakes (Milne Lake, Pellet Lake and North Stromatolite Lake) to evaluate which crystallization pathway most likely occurred. In the fine‐grained sediments of these lakes, very high magnesium calcite and magnesite occurs as aggregates particles of nanocrystals (<100 nm). Rietveld refinements using X‐ray diffraction data give modelled Lvol–IB crystallite size values of <120 nm for all carbonates. Transmission electron microscopy shows within very high magnesium calcite and magnesite particles, nanocrystals have an almost identical orientation of their crystal lattice fringes. This is morphologically similar to Ca–Mg carbonates formed via an amorphous carbonate precursor in non‐classical crystallization laboratory experiments. Precipitation of carbonate minerals via an amorphous‐to‐crystalline pathway requires the water to be supersaturated relative to both crystalline and amorphous phases. In the Coorong Lakes, surface water likely only becomes supersaturated relative to amorphous carbonate phases in the late summer after extensive evaporation. Observations suggest that very high magnesium calcite and dolomite do not directly precipitate from bulk modern seawater, despite oversaturation relative to the crystalline phases, because seawater is undersaturated with respect to amorphous calcium magnesium carbonate, thus limiting the precipitation through a non‐classical crystallization pathway.
... penetrated downward to the underlying strata, and altered the unconsolidated or semiconsolidated carbonate deposits, promoting the nucleation of D1 dolomite (Fig. 14). Small pyrites filled within the intercrystalline pores of D1 dolomite, implying that bacterial sulfate reduction (BSR) may have participated in the process of dolomite nucleation in the subsurface (Preto et al., 2015;Guo et al., 2016). At the very shallow burial depth (i.e., <100 m), the continuous penetration of dolomitizing fluids and increasing temperatures can promote dolomite precipitation and early-formed nuclei (Schmoker and Halley, 1982;Machel, 2001, Fig. 15). ...
The thick Upper Cambrian-Lower Ordovician carbonates of the Gucheng uplift (southeastern Tarim Basin, northwestern China) have been extensively dolomitized, leading to the formation of massive dolomite reservoirs. The origins of different dolomites and their original fluids were studied based on an integrated analysis of petrology, geochemical data (C, O, and Sr isotopes) and fluid inclusion assemblages (FIAs). The D1 (very finely to finely crystalline planar-e/s dolomite) and the D2 (finely to medium crystalline planar-s/a dolomite) dolomites were formed from the mesosaline to penesaline seawater in the subsurface to shallow burial stage. The increasing temperatures and changes of dolomitizing fluid properties led to the formation of different textures and distributions. Both D1 and D2 dolomites, compared to the matrix limestone, show no obvious improvements in porosity, which indicates that early dolomitizations did not create or preserve the primary pores. CD1 (finely to medium crystalline nonplanar-e/s dolomite) and CD2 (medium to coarsely crystalline nonplanar saddle dolomite) dolomites are dolomite cements of hydrothermal alteration at the deep burial stage. Due to changes in temperature and Mg²⁺ concentrations of the hydrothermal fluids, the precipitation of CD1 and CD2 dolomites exhibited different textures and filling sequences. In the limited space of matrix dolomite, hydrothermally altered dolomite was dominated by D3 (medium to coarsely crystalline nonplanar-a dolomite). The late calcite fillings were precipitated on account of decreasing Mg²⁺ concentration at the end of the hydrothermal alteration. Hydrothermal fluids, depleted in Mg²⁺ and enriched in silica, could be deep-basinal brines driven by the strongly transpressional faults from the Late Caledonian to Early Hercynian. Compared to D1 and D2 dolomites, the higher porosities in hydrothermally altered dolomite indicate that hydrothermal fluids are constructive to the formation of reservoirs. Hydrothermal alteration products showed obvious differences between the dolomite and limestone, and led to different performance in physical characteristics. Hence, the hydrothermally altered reservoirs in the eastern Tarim Basin were mostly distributed in the dolomite but were less developed in the limestone. This study provides new insights into the evolution of dolomitizing fluids that are responsible for multiple dolomitization events and the formation of the hydrtothermal dolomite in the Tarim Basin and elsewhere around the world.
... Originally thought as a restricted event, however, it is currently considered by various authors as a global event (Preto et al., 2010). At the end of the Carnian Pluvial Event, arid climatic conditions are again observed at the end of the Carnian, as well as during the Norian and Rhaetian in different regions of the world (Cleveland et al., 2008;Preto et al., 2010Preto et al., , 2015Sun et al., 2012;Iqbal et al., 2019;Tanner, 2018;Kovacs et al., 2020). This behavior has been widely reported in the rocks of the Chinle Group, in the Colorado Plateau region of the southwestern U.S.A. (Tanner and Lucas, 2006;Prochnow et al., 2006;Dubiel and Hastiotis, 2011;Whiteside et al., 2012;Tanner, 2018). ...
Geochemical (major oxides, trace and rare earth elements) studies were carried out on the siliciclastic rocks of
the Antimonio and Río Asunción formations to interpret the paleoweathering conditions and provenance. The
weathering indices like the Chemical Index of Alteration (CIA) and Plagioclase Index of Alterations (PIA) indicate
a low to moderate intensity of chemical weathering. The chondrite normalized REE patterns of the Antimonio
and Río Asunción formations have LREE enriched, flat HREE patterns with a negative Eu anomaly. However, a
few samples from these formations show a positive or an absence of the Eu anomaly, suggesting that the sediments
were dominantly derived from felsic source rocks with a minor contribution from mafic source rocks. The
vertical distribution of trace elements and their ratios indicate that sequence I of the Antimonio Formation
presents a signature for felsic source rocks, however, in sequence II the sediment source changed, i.e. mafic rocks
were recorded (SAAF06 and SAAF07). Sequences III to VI indicate a felsic source rock. The basal and upper parts
of sequence VII (samples SARAF02 and SARAF10) of the Río Asunción Formation indicate mafic source rocks,
whereas the remaining samples were largely contributed by felsic source rocks. The lower values of ferromagnesian
trace elements and their ratios in sequences VIII and IX reveal a felsic source for the upper part of the Río
Asunción Formation. The Antimonio and Río Asunción formations received sediments from two different sources:
Permo-Triassic granitoids of northwestern Sonora and Precambrian granitoids from Caborca and North American
blocks that contributed felsic rocks. Whereas mafic elements were derived from Precambrian amphibolites and
Cambrian basalts located north and south of the study area.
... Fabric-retentive dolomicrite and dolomicrosparite facies (Fig. 4b) have likely undergone little diagenetic change, with dolomicrite likely representing a primary precipitate, and dolomicrosparite precipitation associated with the upper sediment column (cf., Fallah-Bagtash et al., 2020). Their fabric, very fine to fine crystal size, lack of fossils and burrows, and the presence of scattered wind-blown silt-sized quartz grains are indicative of a supratidal to upper intertidal mesohaline environment where dolomite crystals precipitated under low temperature (e.g., Gregg and Shelton, 1990;Machel, 2004;Adabi, 2009;Preto et al., 2015;Husinec, 2016;Beigi et al., 2017). Furthermore, the locally developed fenestral fabric is suggestive of the upper intertidal zone processes likely associated with the organic matter decay (Shinn, 1983;Demicco and Hardie 1994;Flügel, 2010;Adabi et al., 2016). ...
The Lower Triassic Kangan Formation of the Persian Gulf Superbasin forms one of the largest reservoirs in the South Pars/North Field, the world's largest gas field. The upper Khuff-equivalent, gas-hosting carbonate-evaporite succession was formed on a gently-sloping, homoclinal carbonate ramp in a warm, tropical aragonite sea under a hot-and-arid greenhouse climate. This study explores the impact of vertical variations in depositional facies and diagenetic features on the reservoir quality to improve the prediction of the subsurface facies and hydraulic flow unit distribution.
Detailed core logging and petrographic analysis resulted in the recognition of twelve distinct carbonate-evaporite facies that are grouped into four major facies associations, from shallowest to deepest: supratidal to restricted subtidal, lagoon, carbonate shoal, and shallow mid-ramp. The facies associations are stacked into two long-term sequences KS2 (58 m) and KS1 (77 m) bounded by anhydrites and/or unconformities; three higher frequency sequences (KS1a, KS1b, and KS1c) can be identified within the KS1. Based on the petrophysical properties, sedimentary facies characteristics and their diagenetic modifications, five hydraulic flow units (HFUs) are identified, with the best porosity-permeability values and flow- and storage capacities in the late transgressive (TSTs) and the earliest highstand systems tracts (HSTs) of the two long-term sequences. The reservoir quality and the distribution of hydraulic flow units are a product of interactions between primary mineralogy, depositional facies and their stacking, and the early- and late-diagenetic alterations. The reservoir quality decreases in landward direction, being the lowest in the mud-supported textures of the lagoon and supratidal to restricted subtidal facies associations. The seaward improvement in reservoir quality reflects a change to grain-supported textures that formed along the tide-and wave-agitated ooid-bioclastic shoals, whereas the best reservoir quality characterizes the shallow mid-ramp open-marine facies association, where secondary porosity was improved by dissolution and dolomitization. Both the early and late diagenetic dolomitization overall improved the reservoir quality, with the dolomitization pathways controlled by the porosity and permeability heterogeneity in the original depositional facies. The results may be useful in reservoir modeling, recognition of the productive zones, and the further development of the South Pars/North Dome Field.
... Such finely crystalline dolomites can form as a result of slow crystallization, under evaporative condi tions, within highly concentrated Mg enriched brines (Folk & Land 1975). A recent study by Preto et al. (2015) revealed that mosaic dolomites consisting of aggregates of nanometer scale 'growth units' could be considered as indicative of a primary origin. The dolomites described from the Naokelekan Formation clearly formed in a near surface setting (see above). ...
Lithological, petrographic, and geochemical analysis of the Middle to upper Jurassic succession (i.e. Sargelu and Naokelekan formations) from northernmost Iraq were undertaken with the aim of providing an updated discussion for their sedimentary and diagenetic histories, as well as examining the evaporation proxies and paleoredox conditions under which these two formations were deposited. Lithologically, the Sargelu Formation comprises massive dolomites, interbedded with shales, rare cherts and one single limestone bed, whilst the Naokelekan Formation consists of shales overlain by limestones and one single dolomite bed. Petrographic analysis of both formations revealed the presence of rare ostracods, bioclastic fragments as well as calcispheres. Five main microfacies were recognized, including bioclastic wackestone, mudstone, dolorudite, dolarenite and dolomicrite microfacies.
... Such finely crystalline dolomites can form as a result of slow crystallization, under evaporative condi tions, within highly concentrated Mg enriched brines (Folk & Land 1975). A recent study by Preto et al. (2015) revealed that mosaic dolomites consisting of aggregates of nanometer scale 'growth units' could be considered as indicative of a primary origin. The dolomites described from the Naokelekan Formation clearly formed in a near surface setting (see above). ...
Lithological, petrographic, and geochemical analysis of the Middle to upper Jurassic succession (i.e. Sargelu and Naokelekan formations) from northernmost Iraq were undertaken with the aim of providing an updated discussion for their sedimentary and diagenetic histories, as well as examining the evaporation proxies and paleoredox conditions under which these two formations were deposited. Lithologically, the Sargelu Formation comprises massive dolomites, interbedded with shales, rare cherts and one single limestone bed, whilst the Naokelekan Formation consists of shales overlain by limestones and one single dolomite bed. Petrographic analysis of both formations revealed the presence of rare ostracods, bioclastic fragments as well as calcispheres. Five main microfacies were recognized, including bioclastic wackestone, mudstone, dolorudite, dolarenite and dolomicrite microfacies. The shales comprise clay minerals assemblages (illite/muscovite and kaolinite) with some quartz, alkali feldspar and rare pyrite. The Sargelu Formation was probably deposited in a shallow-marine environment. In contrast, the Naokelekan Formation is hypothesized to be deposited in a restricted shallow-lagoon environment. Palaeoredox indicators suggest that both formations were accumulated under anoxic conditions, most probably in silled basins where water circulation was restricted. Tectonic activity thus resulted in basin compartmentalization across the region, which also explains the marked differences which are often observed. 3
... As a result, lacustrine carbonates have received less attention than their marine counterparts. Nevertheless, many studies on modern and ancient lacustrine carbonates have been published in the last 40 years (Adams, 1980;Talbot, 1990;Platt and Wright, 1991;Wright et al., 1997;Alonso-Zarza, 2003;Heilbronn et al., 2015;Preto et al., 2015). Moreover, the recent formation of dolomite in coastal sabkha, inland lakes and playas has received attention since the discovery and documentation of the Coorong dolomites in southern Australia (Von Der Borch et al., 1964;Rosen et al., 1988Rosen et al., , 1989Sinha and Smykatz-Kloss, 2003;Bustillo et al., 2017;Al-Aasm et al., 2017), and the Deep Springs Lake dolomite (Meister et al., 2011). ...
... The origin of dolostone remains a controversial subject and is often referred to as the 'dolostone problem' (Fairbridge, 1957;Chilingar et al., 1979;Zenger et al., 1980;Baker and Kastner, 1981;Land, 1985;Tucker and Wright, 1990;McKenzie and Vasconcelos, 2009;Hendry et al., 2015). To date, no consensus regarding the formation of natural primary dolostone exists (Warren, 2000;Machel, 2004;McKenzie and Vasconcelos, 2009;Bontognali et al., 2010;Preto et al., 2015). ...
During the Eocene to Oligocene, lacustrine dolostone developed in the Anjihaihe Formation on the northern margin of the Tianshan Mountains in the southern Junggar Basin. Dolostone formation is usually restricted to specific environmental and geochemical conditions; therefore, dolostone contains considerable information on palaeoenvironment and palaeoclimate changes. Furthermore, this dolostone provides more information on the formation process and formation mechanisms of dolostone. The studied dolostone is white to greyish white and consists of dolomite crystals that are 5-10 µm in size, homogeneous, subhedral to euhedral and nearly stoichiometric (with average Mg/Ca molar ratios of 0.97). The high Sr content (average 1996 ppm), relatively high δ18O values (average -1.8‰) and co-occurrence with halite and gypsum reflect evaporative and saline conditions. The δ26Mg values of the dolostone range from -0.806 to -2.44‰, with a mean of -1.78‰. Strong positive correlations exist between the δ26Mg values and depth (r=0.76) and the Mg isotope Rayleigh distillation model results, which indicates that δ26Mg fractionation occurred in conjunction with evaporation and that most of the Anjihaihe lacustrine dolomite developed in a natural environment at 25 to 50°C. Moreover, evidence of microbes can be found covering the surface of the dolostone in the presence of a few framboids of pyrite. Additionally, clay layers in the dolostone are primarily composed of bentonite (40-60%) but also contain small amounts of montmorillonite (0.8-3.5%) and zeolite (0.1-0.5%). Detrital zircon ages of 40-44 Ma are presented for the first time for the Anjihaihe Formation. The results show that understanding the primary lacustrine dolostone is significant and essential to reconstructing the evolution of palaeoenvironments and synsedimentary volcanic activity in the northern Tianshan Mountains during the Eocene to Oligocene. This work also provides a possible model that explains the formation of the studied dolostone in an evaporative saline lacustrine environment.
... Complementary to microscopic methods, geochemical tools such as stable isotopes of oxygen, carbon or strontium were used to gain further insights in the environment or the "parental fluid" in which the mineral precipitated (e.g. Land, 1980;Wilson et al., 1990;Preto et al., 2015). ...
Dolomite is a very common carbonate mineral in ancient sediments, but is rarely found in modern environments. Because of the difficulties in precipitating dolomite in the laboratory at low temperatures, the controls on its formation are still debated after more than two centuries of research. Two important parameters to constrain the environment of dolomitization are the temperature of formation and the oxygen isotope composition of the fluid from which it precipitated. Carbonate clumped isotopes (expressed with the parameter Δ47) are increasingly becoming the method of choice to obtain this information. However, whereas many clumped isotope studies treated dolomites the same way as calcite, some recent studies observed a different phosphoric acid fractionation for Δ47 during acid digestion of dolomite compared to calcite. This causes additional uncertainties in the Δ47 temperature estimates for dolomites analyzed in different laboratories using different acid digestion temperatures. To tackle this problem we present here a (proto-)dolomite-specific Δ47-temperature calibration from 25 to 1100 °C for an acid reaction temperature of 70 °C and anchored to widely available calcite standards. For the temperature range 25 to 220 °C we obtain a linear Δ47-T relationship based on 289 individual measurements with R2 of 0.864: [Formula presented] Tin Kelvin When including two isotopically scrambled dolomites at 1100 °C, the best fit is obtained with a third order polynomial temperature relationship (R2 = 0.924): [Formula presented]. Applying a calcite Δ47-T relationship produced under identical laboratory conditions results in 3 to 16 °C colder calculated formation temperatures for dolomites (with formation temperature from 0 to 100 °C) than using the (proto-)dolomite specific calibration presented here. For the synthetic samples formed between 70 and 220 °C we also determined the temperature dependence of the oxygen isotope fractionation relative to the water. Based on the similarity between our results and two other recent studies (Vasconcelos et al., 2005 and Horita, 2014) we propose that a combination of the three datasets represents the most robust calibration for (proto-)dolomite formed in a wide temperature range from 25 to 350 °C. 103αCaMg−carbonates−Water=2.9923±0.0557×[Formula presented]−2.3592±0.4116 Because of the uncertainties in the phosphoric acid oxygen and clumped isotope fractionation for (proto-)dolomite, we promote the use of three samples that are available in large amounts as possible inter-laboratory reference material for oxygen and clumped isotope measurements. A sample of the middle Triassic San Salvatore dolomite from southern Switzerland, the NIST SRM 88b dolomite standard already reported in other Δ47 studies and a lacustrine Pliocene dolomite from La Roda (Spain). This study demonstrates the necessity to apply (proto-)dolomite specific Δ47-T relationships for accurate temperature estimates of dolomite formation, ideally done at identical acid digestion temperatures to avoid additional uncertainties introduced by acid digestion temperature corrections. In addition, the simultaneous analyses of dolomite reference material will enable a much better comparison of published dolomite clumped and oxygen isotope data amongst different laboratories.
... Experiments demonstrated the possibility of mineralizing the primary dolomite in the laboratory [2][3][4] and it was also observed in recent environments [94][95][96]. In the fossil record, similarly to the recent Ca:Mg mineral phases, stromatolitic laminae of dolomite were observed, and they were interpreted as primary phases, precipitated at surface settings [7,12,97,98]. Considering the observation on the studied Carnian samples, that allowed comparison of the pristine Ca-microbialite with isochronous re-mineralized Ca:Mg microbialite, particular attention should be paid to the interpretation of fossil autochthonous carbonates. In fact, organodiagenetic processes can induce the dolomitization of Ca-microbialite, forming a microfabric miming the primary texture. ...
In the present paper, we examine the influence of micrite types, autochthonous or allochthonous, on the dolomitization processes. The recrystallized and dolomitized Carnian samples from Rifugio Vallandro and Alpe di Specie erratic boulders (South Tyrol, Italy) offer a unique example for studying the relationship between microbialites and dolomitization processes. The comparison between the carbonates of the well-preserved erratic boulders of Alpe di Specie and the isochronous, recrystallized, and dolomitized, samples of Rifugio Vallandro, allows for hypothesizing the role of microbialites on dolomitization processes. The Rifugio Vallandro samples represent variously dolomitized boundstone (made of corals, sponges, and peloidal crusts) with a fine texture (aphanodolomite) which contain organic matter relics, suggesting microbial-mediated mineralization. Geomicrobiological characterization of the microbialites from Alpe di Specie indicates that they formed through microbial metabolic activity of sulfate-reducing bacteria, which thrive on organic matter accumulated in the suboxic to anoxic interspaces of the skeletal framework. Similar processes can be hypothesized for the microbialite precursor of Rifugio Vallandro. Extracellular polymeric substance (EPS) and other organic compounds trapped inside the fine crystal matrix can have a role in the dolomitization processes of the microbialites. High pH and high alkalinity, derived from the degradation of organic matter, may be critical in promoting the dolomitization of microbialites because the high pH increases the concentration and activity of the dissolved CO32−, thereby increasing the dolomite supersaturation and reaction rates. This process produces very fine dolomite (aphanodolomite) that replaces the original organic-rich micrite, while the fine crystalline dolomite forming larger euhedral crystals seems to derive from the allochthonous micrite due to the presence of a large amount of siliciclastics and the absence of organic remains.
... The CL characteristics of Md (Fig. 4c) point to the presence of Fe in the dolomitizing fluid (Pierson 1981;Machel and Burton 1991). A bacterial sulphate reduction model is generally considered to play a role in these kinds of CL characteristics, but the lack of primary pyrite within the intercrystalline pores of the Md may indicate that this mechanism is not applicable to the formation Md (Vasconcelos and McKenzie 1997;Preto et al. 2015). Furthermore, the petrographic characteristics of Fpd and Fdd show the absence of associated evaporites and pyrite minerals, and the negative shift in ␦ 18 O values of these dolomites imply that the hypersaline dolomitization model may also be ruled out. ...
The Upper Jurassic – Lower Cretaceous Berdiga Formation of the Eastern Pontides, Turkey, represents a carbonate platform succession composed of pervasively dolomitized intra-shelf to deep-shelf facies. In this area, polymetallic deposits occur as veins and lenses within the Berdiga Formation in close proximity to its upper contact with the overlying formation. Three different types of replacive dolomites occur in the formation: (i) microcrystalline dolomite, (ii) fabric-preserving dolomite, and (iii) fabric-destructive dolomite. Replacive dolomites are Ca rich and nonstoichiometric (Ca 56–58 Mg 42–44 ) and are characterized by a pronounced negative shift in oxygen (–11.38‰ to –4.05‰ Vienna Pee Dee Belemnite (VPDB)), δ ¹³ C values of 0.69‰ to 3.13‰ VPDB, radiogenic ⁸⁷ Sr/ ⁸⁶ Sr ratios (0.70753 to 0.70884), and extremely high Fe (2727–21 053 ppm) and Mn (1548–27 726 ppm) contents. All dolomite samples have low Y/Ho ratios (23–40), and they also contain highly variable contents of rare earth elements (REE) (7–41). REE patterns of dolomites normalized to Post-Archean Australian shale show a distinct positive Eu anomaly (1.3–2.1) and slightly flattened Ce anomalies (0.8–1.1). Integration of petrographic and geochemical studies reveals the history of a variety of diagenetic processes highly affected by hydrothermal alteration, which include dolomitization, recrystallization, dissolution, silicification, and pyrite mineralization associated with the emplacement of the polymetallic mineralization.
... The onset of the NCIE-3 is also visible in Hungary at the very top of the studied core (Zs-14), within the Sándorhegy Fm. (Fig. 6). In the Dolomites (Milieres-Dibona section), the colour alteration index of conodont apatite is very low (1; Preto et al., 2015), and previous biomarker analysis of organic matter showed the succession experienced negligible thermal maturation (Dal Corso et al., 2012). For the reasons outlined above, diagenesis cannot be considered the cause of the observed negative shift in the δ 13 C TOC . ...
The Carnian Pluvial Episode was a phase of global climatic change and biotic turnover that occurred during the early Late Triassic. In marine sedimentary basins, the arrival of huge amounts of siliciclastic sediments, the establishment of anoxic conditions, and a sudden change of the carbonate factory on platforms marked the Carnian Pluvial Episode. The sedimentary changes are closely associated with abrupt biological turnover among marine and terrestrial groups as, for example, an extinction among ammonoids and conodonts in the ocean, and a turnover of the vertebrate fauna and the flora on land. Multiple negative carbon-isotope excursions were recorded during the Carnian Pluvial Episode in both organic matter and marine carbonates suggesting repeated injection of 13C-depleted CO2 into the ocean–atmosphere system, but their temporal and causal links with the sedimentological and palaeontological changes are poorly understood. We here review the existing carbon-isotope records and present new data on the carbon-isotope composition of organic carbon in selected sections of the western Tethys realm that record the entire Carnian Pluvial Episode. New ammonoid, conodont and sporomorph biostratigraphic data were collected and coupled to an extensive review of the existing biostratigraphy to constrain the age of the sampled sections. The results provide biostratigraphically constrained composite organic carbon-isotope curves for the Carnian. This sheds light on the temporal and causal links between the main carbon-isotope perturbations, and the distinct environmental and biotic changes that mark the Carnian Pluvial Episode. The carbon-isotope records suggest that a series of carbon-cycle perturbations, possibly recording multiple phases of volcanic activity during the emplacement of the Wrangellia Large Igneous Province, disrupted Carnian environments and ecosystems repeatedly over a remarkably long time interval of about 1 million years.
... At the base of the dolomitic pinnacles the Travenanzes Formation (TVZ) is found (formerly referred to as Raibl beds). This unit consists of prevalent reddish mudstones with sandstone to conglomerate intercalations, and dolostones in decimetric beds (Breda and Preto, 2011;Preto et al., 2015). Below, the stratigraphic sequence is composed by the Heiligkreuz Formation (HKZ) and the Dolomia Cassiana Formation (DCS). ...
This study deals with the numerical modelling of large deep-seated instability processes affecting the Cinque Torri Group (Dolomites, Italy), which is a UNESCO World heritage site. The aim is to evaluate the role of intrinsic causal factors and to assess the failure mechanism proposed in previous investigations. The geological model is based on topographic, geological and geomechanical surveys, complemented with mineralogical, physical and geotechnical lab analyses for parameters determination. The numerical simulations have been carried out using a Distinct Element Method code on conceptual and site-specific slope models. A series of parametric analyses have been performed to characterize the role of the different predisposing factors potentially related with the ongoing rock spreading: i) load of the rocky pinnacles; ii) lithology, stratigraphy and attitude of the geological units; iii) discontinuity sets in the rock mass; iv) degradation of the mechanical properties. The shear zone initiation is controlled by stress concentration due to the loading of the dolomitic pinnacles, whereas its shape appears to be structurally controlled by the dip-slope attitude of bedding in clay-rich mudstones. The interpretation of the modelling results has provided a better understanding of the ongoing deformation process, which can help in targeting effective and low-impact landslide risk mitigation strategies in this iconic landscape.
... This phenomenon is likely ascribed to the limited involvement of BSR in the dolomite formation. Alternatively, type-1 dolomite was probably formed in an open system and its carbon features was overprinted by later refluxing seawater during shallow burial (Machel, 2004;Wacey et al., 2007;Preto et al., 2015;Guo et al., 2016a). The δ 18 O values (-4.97‰ to -4.04‰ VPDB) of type-1 dolomite largely overlap with the estimated δ 18 O range for dolomite precipitated from coeval seawater, with a positive trend (Figure 7), suggesting that evaporated seawater (salinity >3.5 wt %) was probably the major dolomitising fluids for type-1 dolomite. ...
... This phenomenon is likely ascribed to the limited involvement of BSR in the dolomite formation. Alternatively, type-1 dolomite was probably formed in an open system and its carbon features was overprinted by later refluxing seawater during shallow burial (Machel, 2004;Wacey et al., 2007;Preto et al., 2015;Guo et al., 2016a). The δ 18 O values (-4.97‰ to -4.04‰ VPDB) of type-1 dolomite largely overlap with the estimated δ 18 O range for dolomite precipitated from coeval seawater, with a positive trend (Figure 7), suggesting that evaporated seawater (salinity >3.5 wt %) was probably the major dolomitising fluids for type-1 dolomite. ...
High-frequency metre-scale cycles are present within the Lower-Middle Ordovician carbonate successions in northern Tarim Basin, NW China. These metre-scale cycles were variably dolomitised from top to bottom. Three types of replacive dolomites were recognised, including dololaminite (very finely to finely crystalline, planar-s to nonplanar-a dolomite; type-1), patterned dolomite (finely crystalline, planar-s dolomite; type-2), and mottled dolomite (finely to medium crystalline, nonplanar-a(s) dolomite; type-3). Petrographic evidence indicate these dolomites were primarily deposited in supratidal to restricted subtidal environments, and formed in near-surface to shallow burial realms. Geochemically, all types of dolomites have similar δ¹³C and ⁸⁷Sr/⁸⁶Sr ratios comparable to calcite precipitated in equilibrium with the Early-Middle Ordovician seawater. These geochemical attributes indicate that these dolomites were genetically associated and likely formed from connate seawater-derived brines. Of these, type-1 dolomite has δ¹⁸O values (‒4.97‰ to‒4.04‰ VPDB) slightly higher than those of normal seawater dolomite of the Early-Middle Ordovician age. Considering the absence of associated evaporites within type-1 dolomite, its parental fluids were likely represented by slightly evaporated (i.e., mesosaline to penesaline) seawater with salinity below that of gypsum precipitation. More depleted δ¹⁸O values (‒7.74‰ to‒5.20‰ VPDB) of type-2 dolomite and its stratigraphic position below type-1 dolomite indicate the generation of this dolomite from mesosaline to penesaline brines at higher temperatures in near-surface to shallow burial domains. Type-3 dolomite yields the most depleted δ¹⁸O values (–9.30‰ to–7.28‰ VPDB), pointing to that it was most likely formed from coeval seawater-derived brines at highest temperatures in a shallow burial setting. There is a downward decreasing trend in δ¹⁸O values from type-1 through type-2 to type-3 dolomites, and in abundance of dolomites, indicating that the dolomitising fluids probably migrated downward from above and persisted into shallow burial conditions.
... Mounting evidence shows that microorganisms may be involved in dolomite formation. For example, microbial fossils (Perri and Tucker, 2007;You et al., 2013), large amounts of organic matter (Baker and Burns, 1985;Brauchli et al., 2015), bio-minerals , and low carbon isotope values (Preto et al., 2015) are commonly observed in dolomite rocks, all of which suggest a microbial involvement in dolomite formation. Moreover, direct evidence came from the experiments in which dolomite was formed with the facilitation of microorganisms (Bontognali et al., 2012;Kenward et al., 2013;Sánchez-Román et al., 2009a;Vasconcelos et al., 1995;Warthmann et al., 2000). ...
Although most modern dolomites occur in hypersaline environments, the effects of elevated salinity on the microbial mediation of dolomite precipitation have not been fully evaluated. Here we report results of dolomite precipitation in association with a batch culture of Haloferax volcanii DS52, a halophilic archaeon, under various salinities (from 120‰ to 360‰) and the impact of salinity on microbe-mediated dolomite formation. The mineral phases, morphology and atomic arrangement of the precipitates were analyzed by XRD, SEM and TEM, respectively. The amount of amino acids on the archaeal cell surface was quantified by HPLC/MS. The XRD analysis indicated that disordered dolomite formed successfully with the facilitation of cells harvested from cultures with relatively high salinities (200‰ and 280‰) but was not observed in association with cells harvested from cultures with lower salinity (120‰) or the lysates of cells harvested from extremely high salinity (360‰). The TEM analysis demonstrated that the crystals from cultures with a salinity of 200‰ closely matched that of dolomite. Importantly, we found that more carboxyl groups were presented on the cell surface under high salinity conditions to resist the high osmotic pressure, which may result in the subsequent promotion of dolomite formation. Our finding suggests a link between variations in the hydro-chemical conditions and the formation of dolomite via microbial metabolic activity and enhances our understanding about the mechanism of microbially mediated dolomite formation under high salinity conditions.
... The observed depletion in the d 13 C may be attributed to the dilution of the seawater evaporated brine dolomitizing fluids by meteoric fresh water (Lu and Meyers 1998). Release of organogenic light carbon isotope from the oxidation of organic matter within the mother sediments of these dolostones via bacterial sulphate reduction may have caused the depletion of d 13 C (Irwin et al. 1977;Hudson 1977;Preto et al. 2015). The near positivity of the d 18 O reflects the relative increase of the sea water in the dolomitizing fluids (Wallace 1990). ...
Detailed petrographic and geochemical investigation of an exposed sequence of the Eocene Dammam Formation in Kuwait revealed significant gradational upward changes from friable chalky dolostone to hard dolocretic palaeoduricrust (karst carapace) passing through an extensively karstified dolostone enclosing cavity fill dolocretes. The dolostones were originally deposited as calcareous wackestone and mudstone within tidal flat and backshore lagoons that manifest cyclic sedimentation within minor sea regression and transgression cycles. Dolomitization of these sediments was early, pervasive, fabric selective and non-fabric destructive and it took place by Mg-rich sabkha brine mixed with meteoric fresh water. Regional sea regression was followed by chertification. Three sequential stages of dolocretization were identified within the karst dolostone and the karst carapace. Microscopic and SEM examination as well as stable isotopic analysis of Carbon and Oxygen indicate significant variation in the dolomite textures and genesis of the dolostone and the dolocretes. A suggested paragenesis model elaborated complex diagenetic processes. The results of this research may be applicable for analogues subsurface carbonate reservoir rocks.
... The fine-crystalline (b20 μm), subhedral to euhedral rhombohedral dolomite crystals can be interpreted as early diagenetic in origin (Tucker, 2001), whereas micro-spherical (rounded to sub-rounded) dolomite crystals (micritic-size, b20 μm) barren of marine fossils are similar to those formed as microbially-induced primary dolomite precipitates in shallow alkaline lakes (e.g., Bréhéret et al., 2008;Calvo et al., 2003;García del Cura et al., 2001;Preto et al., 2015;Vasconcelos and McKenzie, 1997). Primary origin of the recognized micro-spherical dolomite can be indicated by petrographic features such as homogeneous size, finely-crystalline texture (micritic-size, b20 μm; Fig. 7c, d), and absence of replacive calcite (Abdul Aziz et al., 2003;Bréhéret et al., 2008;García del Cura et al., 2001;Last, 1990;Sáez and Cabrera, 2002;Wanas, 2002 arid climate (García del Cura et al., 2001;Wanas and Abu El-Hassan, 2006). ...
This study aims to provide the depositional facies, sequence stratigraphic and paleoclimatic characteristics of the Mid-Eocene (Bartonian) continental succession exposed at Gebel El-Goza El-Hamra (Shabrawet Area, NE Eastern Desert, Egypt). The studied succession consists of siliciclastic rocks followed upward by carbonate rocks. Detailed field observation and petrographic investigation indicate accumulation in floodplain-dominated alluvial and shallow lacustrine systems. The floodplain-dominated alluvial facies (45mthick) is composed mainly of carbonate nodules-bearing, mottled mudrock with subordinate sandstone and conglomerate beds. The conglomerate and pebbly sandstone bodies interpreted as ephemeral braided channel deposits. The massive, laminated, planner cross-bedded, fine- to medium-grained sandstone bodies interlayered within mudstone reflect sheet flood deposits. The mudrocks associated with paleosols represent distal floodplain deposits. The shallow lacustrine facies (15 m thick) is made up of an alternation of marlstone, micritic limestone, dolostone and mudrock beds with charophytes and small gastropods. Both the alluvial and lacustrine facies show evidence of macro-and micro-pedogenic features. Pollen assemblages, stable δ18O and δ13C isotopes, and paleopedogenic features reflect prevalence of arid to semi-arid climatic conditions during the Bartonian.
The sequence stratigraphic framework shows an overall fining-upward depositional sequence, consisting of Lowand High-accommodation Systems Tracts (LAST, HAST), and is bounded by two sequence boundaries (SB-1, SB-2). Conglomerate and pebbly sandstone deposits (braided channel and sheet flood deposits) of the lower part of the alluvial facies reflect a LAST. Mudrock and silty claystone facies (distal floodplain deposits) of the upper part of alluvial facies and its overlying lacustrine facies correspond to a HAST. The LAST, HAST and SB were formed during different accommodation-to-sediment supply (A/S) ratio phases. The variation in A/S ratios was mainly controlled by sea-level change as well as by local tectonic subsidence and uplift of the basin coincident with the reactivation of the Syrian Arc System during the Bartonian.
Dolomite is widely developed in the shale sequences of saline lakes and has important significance in paleoenvironmental, paleoclimatic, and petroleum geology research. In this study, a new end-member type of dolomite is discovered from the Permian Lucaogou Formation in the Junggar Basin, northwest China, along with its geological significance. The microbial–hydrothermal origin of the bedded dolomite is evident from the development of hydrothermal minerals (including barite and cubic pyrite) and methanogenic micro-organisms. In terms of geochemistry, the bedded dolomite has high (Ba/La)N ratios (~4.3) and Sr contents (~594.9 ppm) and positive Sr anomalies (Sr/Sr* ~ 3.8). Chondrite-normalized rare earth element (REE) patterns exhibit light REE enrichment, heavy REE depletion, slightly positive Ce anomalies, and negative Eu anomalies. The bedded dolomite has relatively low ⁸⁷Sr/⁸⁶Sr ratios (~0.705283), high δ¹³C values (~6.86 ‰), and low δ¹⁸O values (~ −9.80 ‰). All these indicate that the dolomitizing fluid is a mixed source of hydrothermal fluids and lake waters, and the hydrothermal fluids and microbial methanogenesis provided the Mg²⁺ and C, respectively. The combination of microbial and hydrothermal activity overcame the limitation of dolomitization hydration shells around Mg²⁺. This is a new end-member type of dolomite in lacustrine shale sequence. In terms of geological significance, the studied Lucaogou Formation was deposited in an extensional tectonic setting under hot–arid climatic conditions, which controlled the high-frequency interbedding and meter-scale cyclicity of sediment deposition of dolomites and shales. The combined effects of tectonism and paleoclimate may be important factors for the formation of bedded dolomite, enrichment of organic matter and hydrocarbons in saline lacustrine shales.
Microbial-induced carbonate precipitation (MICP) has the potential to immobilize carbon durably. The carbon source of carbonate minerals is the crucial issue for understanding the fixation mechanism of CO2 by MICP. However, the carbon source and its temporal changes in microbial-induced carbonate minerals remain little explored. In this study, calcium carbonate (CaCO3) biomineralization experiments have been carried out using Bacillus cereus in the medium without additional dissolved inorganic carbon (DIC). X-ray diffraction (XRD), attenuated total reflection-infrared spectroscopy (ATR-IR) and scanning electron microscopy (SEM) indicated that the precipitate produced mainly consisted of rhombohedral and irregular calcite. The δ¹³C values of DIC, dissolved organic carbon (DOC), and calcite were the main parameters studied. Carbon isotope fractionation was characterized by carbon isotope offset (Δ¹³Ccalcite-DIC or Δ¹³Ccalcite-fluid). The Δ¹³Ccalcite-DIC values ranged from +8.2‰ to +21.5‰, indicating that strain LV-1 induced the accumulation of ¹³C in calcite. The Δ¹³Ccalcite-fluid values indicated that the calcite was up to +15.0‰ (on average) ¹³C-enriched relative to the fluid. Calculated chemical mass balance data showed that the proportion of CO2(g) derived from DOC on days 16 and 20 was negative, but it became positive after day 20. Meanwhile, the δ¹³CCO2(g) value calculated by isotope mass balance was −8.5‰, near to δ¹³C for air (−8.0‰) on day 16, and then shifted to −16.4‰, similar to δ¹³C for tryptone (−17.2‰) after day 20. These results suggested that the amount of CO2 arising from organic matter through bacterial respiration and the action of enzymes might determine whether the carbon in calcite is derived from organic carbon or atmospheric CO2. Our findings corroborate the potential utility of MICP for immobilization of CO2 to reduce net soil CO2 emissions and to mitigate the greenhouse effect. Microbial-driven carbon isotopic fractionation can also cause great carbon isotope shifts in calcite. Thus, carbon isotope value might be used as a marker to identify whether carbonate minerals/rocks are of microbial origin.
The sedimentary strata along the Permian–Triassic Boundary in the Western Salt Range of the sub-Himalayas were analysed to gain an insight of its geological character regarding palaeoenvironments, palaeoclimate, and sequence stratigraphic attributes. The Early Triassic Mianwali Formation is dominated by shallow marine facies with transgressive phases, differentiation in facial character, and palaeowater depth among different sections. The study suggests that compared to the siliciclastic-dominated Nammal Gorge Section, the Zaluch Nala Section is more carbonate-dominated, representing algal-laminated limestones, dolomitic marls, and siliciclastics – mainly restricted to the Mittiwali Member. There is a general decrease in ammonoids and increase in thickness from east to west, that is believed to be due to algal-induced rapid sedimentation in the shallow marine environments as compared to deeper marine in the east. The Mianwali Formation has been deposited in two transgressive systems tracts, two highstand systems tracts, and one falling-stage regressive systems tract. Two flooding surfaces, one each in the Mittiwali and Narmia members, have been recognized. The maximum regressive surfaces at the bottom (with the Chhidru Formation) and top (with the Tredian Formation) mark second-order eustatic falls. The study also advocates for existence of a low-energy platform in the temperate waters of the Palaeo-Tethys. The sediments have similarities with various sections of Gondwanaland having the same trend of shallow marine facies and presence of algal-laminated deposits. In the Middle Triassic, the gradual loss of sediment binding organisms shifted the low-energy meandering patterns to high-energy braided systems.
The elastic moduli estimated through geophysical studies carried out in wells (logging data) differ from those obtained from the triaxial tests conducted in laboratory on the available core samples. Terminologically former and latter are referred to as dynamic and static elastic moduli, respectively. Since the structural characteristics of rocks at the different scales, from micrometre to larger scales (tens of metre), are the controlling parameters of their dynamic and static moduli and their difference at the respective scale, in this study we aim to investigate the influence of the measurable (or quantifiable) parameters of the pore space on these elastic moduli. To do so, 19 dry carbonate samples of different structural characteristics were collected. Their basic petrophysical properties such as porosity and permeability were measured in laboratory. The ultra-sonic tomography was carried out to determine the heterogeneity degree, anisotropy system and average acoustic wave velocities for each core sample. SEM images were analysed to investigate the visual textural properties. The mineralogical composition of these samples was determined by the X-ray diffraction method. Based on the conducted experimental studies and using of the effective medium theory, a unique rock physics model (‘petroelastic model’) was constructed for each core sample. The average (effective) microstructural parameters characterizing the pore space of the studied carbonate samples, along with their elastic moduli were estimated through solving the inverse problem and the measured acoustic wave velocities. A multistage statistical approach, including computation of correlation coefficients, optimized regression analysis, factor analysis and bootstrap resampling, was suggested to investigate the effect of each microstructural parameters on the static and dynamic Young's moduli, ratio of dynamic to static Young's moduli (k-value), dynamic Poisson's ratio and mechanical properties (including unconfined compressive strength and internal friction angle). The obtained results show that the microstructural characteristics have different degrees of influence on the elastic moduli and can be successfully classified based on their physical nature. It was also concluded that the dynamic Poisson's ratio is independent of the studied, in this work, microstructural parameters.
Recent studies of continental carbonates revealed that carbonates with similar fabrics can be formed either by biotic, biologically‐induced, biologically‐influenced or purely abiotic processes, or a combination of all. The aim of this research is to advance knowledge on the formation of carbonates precipitated (or diagenetically altered) in extreme, continental environments by studying biotic versus abiotic mechanisms of crystallization, and to contribute to the astrobiology debate around terrestrial analogues of Martian extreme environments. Both fossil (upper Pleistocene to Holocene) and active carbonate spring mounds from the Great Artesian Basin (South Australia) have been investigated. These carbonates consist of low‐Mg to high‐Mg calcite tufa. Four facies have been described: (i) carbonate mudstone/wackestone; (ii) phytohermal framestone/boundstone; (iii) micrite boundstone; and (iv) coarsely crystalline boundstone. The presence of filaments encrusted by micrite, rich in organic compounds, including ultraviolet‐protectants, in phytohermal framestone/boundstone and micrite boundstone is clear evidence of the existence of microbial mats at the time of deposition. In contrast, peloidal micrite, despite commonly being considered a microbial precipitate, is not directly associated with filaments in the Great Artesian Basin mounds. It has probably formed from nano‐crystal aggregation on colloid particulate. Thus, where biofilms have been documented, it is likely that bacteria catalyzed the development of fabrics. It is less certain that microbes induced calcium carbonate precipitation elsewhere. Trace elements, including rare earth element distribution from laminated facies, highlight strongly evaporative settings (for example, high Li contents). Carbon dioxide degassing and evaporation are two of the main drivers for an increase in fluid alkalinity, resulting in precipitation of carbonates. Hence, although the growth of certain fabrics is fostered by the presence of microbial mats, the formation of carbonate crystals might be independent from it and mainly driven by extrinsic factors. More generally, biological processes may be responsible for fabric and facies development in micritic boundstone whilst micrite nucleation and growth are driven by abiotic factors. Non‐classical crystallization pathways (aggregation and fusion of nanoparticles from nucleation clusters) may be more common than previously thought in spring carbonate and this should be carefully considered to avoid misinterpretation of certain fabrics as by‐products of life. It is proposed here that the term ‘organic‐compound catalyzed mineralization’ should be used for crystal growth in the presence of organic compounds when dealing with astrobiological problems. This term would account for the possibility of multiple crystallization pathways (including non‐classical crystallization) that occurred directly from an aqueous solution without the direct influence of microbial mats.
Depositional texture, sedimentary structure and present fauna led to characterize twelve carbonate- evaporate facies in the Early Triassic Kangan Formation, South Pars gas field. These facies were classified in three facies belts including tidal flat, lagoon and carbonate shoal. Developpment of tidal zones together with evaporate deposits and stromatolite and thrombolite facies (signs to a shallow depositional environment), the absence of continuous reef‐frame builders, high production of carbonate mud, absents of calciturbidite, tempestites deposits and slump structures are evidences for a homoclinal carbonate ramp setting. The facies based on petrophysical characteristics which is the results of depositional texture and diagenesis are classified in seven reservoir rock types. In which, the first rock type (RT1) has weak reservoir property and toward RT7 reservoir quality will increase. Facies variation related to sea level fluctuations led to subdivide the whole strata into two 3rd order sequences. Facies stacking patterns in the sequences characterized by subtidal facies (lagoon and shoal) tend to have the most reservoir quality that covered by evaporative (Mf1) and peritidal facies (Mf2 to Mf4) with low-reservoir quality. The most reservoir quality in both K1 and K2 reservoir zones is coincident with late TST, maximum flooding surface (mfs) and late HST in identified depositional sequences which is settled in high-energy shoal facies intervals.
The 2.5 Ga stromatolitic dolostones from the Lower Transvaal Supergroup in the Kanye Basin (Botswana) pre-date the first iron deposits, recording conditions before the great oxidation event (GOE). These dolostones have been deposited within a shallow marine carbonate platform extending from Zimbabwe to South Africa. The Lower Transvaal Supergroup carbonates of the Kanye Basin have been affected by the circulation of metasomatic fluids related to emplacement of the Moshaneng Dolerites (1.9–2.1 Ga).
Here, geochemical and petrographic characterization of the Lower Transvaal Supergroup stromatolites is presented to shed light onto i) the effect of metasomatic fluids on the geochemistry of ancient carbonates, and ii) the environmental conditions prevailing before the onset of the GOE in the epeiric seas along the western margins of the Kaapvaal Craton.
The dolomites show high Fe an Mn contents (average 2000 ppm and 3500 ppm, respectively) and very low Na contents. The overall rare earth elements (REE) pattern of dolomite vary consistently across the different dolomite facies with a sensible increase of ∑REE in the altered dolomites. The overall REE patter lacks La, Ce and Gd anomalies (average 1, 0.91, 1.05 respectively) and shows an overall chondritic Y/Ho ratio. The Eu anomaly is slightly negative or absent in most of unaltered samples. A positive Eu anomaly (average 1.28) has been detected in samples altered by metasomatic fluids.
The dolomite samples from Ramonnedi Formation show Fe and Mn enrichment typical of Precambrian carbonates. Petrographic and geochemical analyses reveal that dolomitization and incipient diagenesis have probably affected the platform carbonates inducing neomorphism and recrystallization (i.e. xenotopic and syntaxial dolomite). The dolomitization processes have mobilized Y giving rise to a near chondritic Y/Ho ratio. Evidences for circulation of hydrothermal fluids related to the intrusion of Moshaneng Igneous Complex and Moshaneng Dolerites are shown only in one sample. Most samples of stromatolitic dolostone have negative Eu anomaly suggesting deposition in a closed basin system restricted from open marine circulation.
Wide carbonate platform environments developed on the western passive margin of the Tethys during the Late Triassic, after a major climate change (Carnian Pluvial Episode) that produced a crisis of high-relief microbial carbonate platforms. The peritidal succession of this epicontinental platform (Dolomia Principale/Hauptdolomit, Dachstein Limestone) is widespread in the Mediterranean region. However, the start-up stage is not fully understood. The original platform to basin depositional geometries of the system have been studied in the north-eastern Southern Alps, close to the Italian/Slovenian boundary where they are exceptionally preserved. Sedimentological features have been investigated in detail by measuring several stratigraphic sections cropping out along an ideal depositional profile. The analysis of the facies architecture allowed reconstruction of the paleoenvironments of the Dolomia Principale platform during its start-up and early growth stages in the late Carnian. The carbonate platform was characterized by an outer platform area, connected northward to steep slopes facing a relatively deep basin. Southward, the outer platform was connected to inner sheltered environments by a narrow, often emerged shelf crest. Behind this zone, carbonate sedimentation occurred in shallow lagoons and tidal flats, passing inward to a siliciclastic mudflat. The Dolomia Principale platform was initially aggrading and able to keep pace with a concomitant sea-level rise, and then prograding during the late Carnian. This stratigraphic interval was correlated to the Tuvalian succession of the Dolomites, allowing depiction of the depositional system on a wide scale of hundreds of kilometrew. This large-scale depositional system presents features in common with some Palaeozoic and Mesozoic carbonate buildups (fo example, the Permian Capitan Reef complex, Anisian Latemar platform), both in terms of architecture and prevailing carbonate producers. A microbial-dominated carbonate factory is testified in the outer platform and upper slope. The recovery of high relief microbial carbonate platforms marks the end of the Carnian Pluvial Episode in the Tuvalian of Tethys.
Laterally extensive beds of dolomitized carbonate are found interbedded with eolian to peritidal sandstones in the hydrocarbon-producing Pennsylvanian to earliest Permian successions of the Wyoming Shelf, USA. Subsurface and surface correlations often rely on these dolomite intervals yet their origin is poorly constrained. To characterize the nature of dolomitization, we integrate petrography, carbon and oxygen isotope data, and sedimentological characteristics of pervasively dolomitized shallow marine, supratidal, and pedogenic facies in the Amsden and Tensleep Formations of the Bighorn Basin (early to middle Pennsylvanian, northern Wyoming). Stable isotopic compositions are compared with the documented isotopic signature of protodolomite forming on present-day arid coastlines. The composition of fine- to medium-grained dolomitized matrix differs from that of late-stage calcite spars, suggesting that dolomites preserve a primary or early diagenetic signal. The δ18O values of dolomites (-1.2 to 7.6‰ VPDB) display a similar range to that of modern protodolomite forming in the tidal flats of the coast of Abu Dhabi. The δ13C values, however, are consistently lower than expected if dolomite had precipitated from sea-water. These relationships suggest that dolomite incorporated a considerable amount of isotopically light carbon during primary formation or later during overgrowth and/or recrystallization of the initial protodolomite. Pennsylvanian and earliest Permian successions in Wyoming, Montana, and northeastern Utah display very similar diagenetic modifications (i.e. pervasive dolomitization, evaporite replacement, silicification), suggesting that the models discussed here may be applicable to these contemporaneous formations.
In an effort to improve the calibration of the Late Triassic time scale, two stratigraphic successions in Italy were investigated for magnetostratigraphy: the Pignola-2 (Southern Apennines) and the Dibona (Dolomites) sections. These sections reveal a sequence of biostratigraphically calibrated (conodonts and palynomorphs) magnetic polarity zones encompassing the Julian/Tuvalian boundary (Carnian). A total of 63 samples have been collected from the Pignola-2 section that helped defining 3 magnetozones. These data are constrained by a published radiometric U/Pb age of 230.91 0.33 Ma from the Aglianico ash-bed (Furin et al. 2006). From the Dibona section, 81 samples have been collected, revealing 2 magnetozones in the Heiligkreuz Fm. (lower part of the succession) and 2 magnetozones in the Travenanzes Fm. (upper part). The Dibona section provided only few high-quality paleomagnetic data in spite of the sampling effort, so we considered only the Pignola-2 section as suitable for the correlation with other Carnian sections from the literature, in order to define the magnetostratigraphy around the Julian/Tuvalian boundary in the Tethys realm. The correlation between the Pignola-2 and the Newark Astrochronological Polarity Time Scale (APTS) provides independent insights on the age of the base of the Newark basin APTS. We found that the U-Pb zircon-dated Pignola-2 level, magnetostratigraphically projected on the Newark APTS, falls within Chrons E5–E4 at around 231 Ma, in agreement with the (extrapolated) astrochronological ages of the Newark APTS.
Due to difficulties in correlating aeolian deposits with coeval marine facies, sequence stratigraphic interpretations for arid coastal successions are debated and lack a unifying model. The Pennsylvanian record of northern Wyoming, USA, consisting of mixed siliciclastic–carbonate sequences deposited in arid, subtropical conditions, provides an ideal opportunity to study linkages between such environments. Detailed facies models and sequence stratigraphic frameworks were developed for the Ranchester Limestone Member (Amsden Formation) and Tensleep Formation by integrating data from 16 measured sections across the eastern side of the Bighorn Basin with new conodont biostratigraphic data. The basal Ranchester Limestone Member consists of dolomite interbedded with thin shale layers, interpreted to represent alternating deposition in shallow marine (fossiliferous dolomite) and supratidal (cherty dolomite) settings, interspersed with periods of exposure (pedogenically-modified dolomites and shales). The upper Ranchester Limestone Member consists of purple shales, siltstones, dolomicrites and bimodally cross-bedded sandstones in the northern part of the basin, interpreted as deposits of mixed siliciclastic–carbonate tidal flats. The Tensleep Formation is characterized by thick (3 to 15 m) aeolian sandstones interbedded with peritidal heteroliths and marine dolomites, indicating cycles of erg accumulation, preservation and flooding. Marine carbonates are unconformably overlain by peritidal deposits and/or aeolian sandstones interpreted as lowstand systems tract deposits. Marine transgression was often accompanied by the generation of sharp supersurfaces. Lags and peritidal heteroliths were deposited during early stages of transgression. Late transgressive systems tract fossiliferous carbonates overlie supersurfaces. highstand systems tract deposits are lacking, either due to non-deposition or post-depositional erosion. The magnitude of inferred relative sea-level fluctuations (>19 m), estimated by comparison with analogous modern settings, is similar to estimates from coeval palaeotropical records. This study demonstrates that sequence stratigraphic terminology can be extended to coastal ergs interacting with marine environments, and offers insights into the dynamics of subtropical environments. This article is protected by copyright. All rights reserved.
The pre-drift Wegenerian model of Pangea is almost universally accepted, but debate exists on its pre-Jurassic configurationsinceTedIrvingintroducedPangea‘B’byplacingGondwanafarthertotheeastbyV3000kmwith respect to Laurasia on the basis of paleomagnetic data. New paleomagnetic data from radiometrically dated Early Permian volcanic rocks from parts of Adria that are tectonically coherent with Africa (Gondwana), integrated with published coeval data from Gondwana and Laurasia, again only from igneous rocks, fully support a Pangea ‘B’ configuration in the Early Permian. The use of paleomagnetic data strictly from igneous rocks excludes artifacts from sedimentary inclination error as a contributing explanation for Pangea ‘B’. The ultimate option to reject Pangea ‘B’ is to abandon the geocentric axial dipole hypothesis by introducing a significant non-dipole (zonal octupole) component in the Late Paleozoic time-averaged geomagnetic field. We demonstrate, however, by using a dataset consisting entirely of paleomagnetic directions with low inclinations from sampling sites confined to one hemisphere from Gondwana as well as Laurasia that the effects of a zonal octupole field contribution would not explain away the paleomagnetic evidence for Pangea ‘B’ in the Early Permian. We therefore regard the paleomagnetic evidence for an Early Permian Pangea ‘B’ as robust. The transformation from Pangea ‘B’ to Pangea ‘A’ took place during the Permian because Late Permian paleomagnetic data allow a Pangea ‘A’ configuration. We therefore review geological evidence from the literature in support of an intra-Pangea dextral megashear system. The transformation occurred afterthecoolingoftheVariscanmega-sutureandlastedV20Myr.Inthisinterval,theNeotethysOceanopened between India/Arabia and the Cimmerian microcontinents in the east, while widespread lithospheric wrenching and magmatism took place in the west around the Adriatic promontory. The general distribution of plate boundaries and resulting driving forces are qualitatively consistent with a right-lateral shear couple between Gondwana and Laurasia during the Permian. Transcurrent plate boundaries associated with the Pangea transformation reactivated Variscan shearzonesandweresubsequentlyexploitedbytheopeningofwesternNeotethyanseawaysintheJurassic.
The origin of dolomite is a long-standing enigma in sedimentary geology. It has been proposed that microorganisms, especially
anaerobic microorganisms, can overcome kinetic barriers to facilitate dolomite precipitation, although their specific role
in dolomite formation is still unclear. Our experimental results demonstrate that disordered dolomite can be synthesized at
room temperature abiotically from solutions containing polysaccharides such as carboxymethyl cellulose or agar. We propose
that when dissolved in solution, polysaccharides can be strongly adsorbed on Ca-Mg carbonate surfaces through hydrogen bonding.
The adsorbed polysaccharides may help weaken the chemical bonding between surface Mg2+ ions and water molecules, which can lower the energy barrier to the desolvation of surface Mg2+-water complexes, enhance Mg2+ incorporation into the precipitating carbonate, and thereby promote disordered dolomite formation. In natural environments,
it is possible that polysaccharides produced by microorganisms, e.g., extracellular polysaccharides, may play a key role in
promoting disordered dolomite nucleation and crystallization. In marine sediments, the accumulated dissolved carbohydrates
produced from organic matter degradation during early diagenesis may also serve as catalysts for disordered dolomite formation.
A 55 m thick Pleistocene dolomite section was sampled in 33 borings in the Arabian Gulf, offshore of AI Jubayl, Saudi Arabia. The regional setting, suite of sedimentary structures, abundance of evaporites (crystals, nodules, and chickenwire horizons), and geochemistry of the dolomite are all consistent with accumulation as part of a series of vertically stacked sabkha depositional complexes. Analyses of the ordering, stoichiometry, and overall stable-isotope composition indicate that after dolomitization the sediment underwent diagenesis in contact with meteoric waters. Stable-isotope analyses of 385 samples have demonstrated that within this sequence there are four correctable horizons that display marked excursions to low S13C values; some shifts exceed 6%e between vertically adjacent samples. The 618O values show a similar, but less pronounced, shift to low values at the same stratigraphie horizons. The samples with low 5UC and 618O values have a greater admixture of fine-grained siliciclastics, are light gray (i.e., reducing conditions), and have higher concentrations of Fe, Mn, and Zn than other parts of the deposit. These diagenetic attributes, especially the marked change in 513C values, may be due to either diagenesis due to the descent of meteoric water beneath a soil horizon or diagenesis associated with bacterially mediated sulfate-réduction processes; sulfate reduction is the favored hypothesis. It is envisioned that after the initial dolomitization diagenesis associated with bacterially mediated sulfate reduction of laterally extensive microbial mats within the top parts of sabkha sequences produced the correctable horizons of low 6'3C and 6180 values.
Dolomite crystals in partially dolomitized limestone from the Platteville Formation are both compositionally and microstructurally
heterogeneous. A single dolomite crystal usually contains three phases: the host Ca-rich dolomite [Ca1.14Mg0.86(CO3)2], an Fe-bearing dolomite [Ca1.06Mg0.80Fe0.14(CO3)2], and calcite inclusions. These three phases show similar orientations. The Ca-rich dolomite exhibits modulated microstructures
with wavelength ranging from 7 to 30 nm. The modulated microstructures are not evident in Fe-bearing dolomite.
Modulations in the Ca-rich dolomite have three predominant orientation ranges in the studied sample: from (205) to (104),
from (001) to (1̄01), and ~(110), which are consistent with previous studies. Bright-field (BF) and high-angle annular dark-field
(HAADF) images confirm that these modulations are due to chemical variation rather than strain or diffraction contrast. The
Ca-rich lamellae are Mg-rich calcite with compositions ranging from Ca0.85Mg0.15CO3 to Ca0.70Mg0.30CO3. The observed results indicate that these Ca-rich exsolution lamellae formed during diagenesis. In this study, three kinds
of “c”-reflections, which are weak spots in the halfway position between the principal reflections along the (104)*, (1̄12)*, and
(110)* directions, have been found in the diffraction patterns of some Ca-rich dolomite. Mg-Ca ordering in x-y planes was not observed directly in Z-contrast images. FFT patterns from the Z-contrast images do not show “c”-reflections. STEM images confirm that the “c”-reflections could result from multiple diffraction between the host dolomite and twinned Mg-calcite nano-lamellae under
TEM imaging and diffraction modes.
Significance
Abundant in the geologic record, but scarce in modern environments below 50 ° C, the mineral dolomite is used to interpret ancient fluid chemistry, paleotemperature, and is a major hydrocarbon reservoir rock. Because laboratory synthesis of abiotic dolomite had been unsuccessful, chemical mechanisms for precipitation are poorly constrained, and limit interpretations of its occurrence. Here we report the abiotic synthesis of dolomite at 25 ° C, and demonstrate that carboxylated surfaces on organic matter catalyze precipitation through complexation between carboxyl groups and Mg ²⁺ , removing water to make Mg ²⁺ available for dolomite precipitation. This mechanism is consistent with dolomite formation in depositional environments rich in organic matter. Our experimental protocol provides opportunities for calibrating conditions of low-temperature dolomite formation throughout the geologic record.
Empirical Calibration for Dolomite Stoichiometry Calculation: Application on Triassic Muschelkalk-Lettenkohle Carbonates (French Jura) - This study concerns an approach for dolomite quantification and stoichiometry calculation by using X-ray diffractometry coupled with cell and Rietveld refinements and equipped with a newly substantial database of dolomite composition. A greater accuracy and precision are obtained for quantifying dolomite as well as other mineral phases and calculating dolomite stoichiometry compared to the classical "Lumsden line" and previous methods. The applicability of this approach is verified on dolomite reference material (Eugui) and on Triassic (Upper Muschelkalk-Lettenkohle) carbonates from the French Jura. The approach shown here is applicable to bulk dolostones as well as to specific dolomite cements and was combined with petrographical and isotopic analyses. Upper Muschelkalk dolomites were formed during burial dolomitization under fluids characterized by increased temperature and variable isotopic composition through burial. This is clear from their Ca content in dolomites which gradually approaches an ideal stoichiometry (from 53.16% to 51.19%) through increasing dolomitization. Lettenkohle dolostones consist of near-ideal stoichiometric (51.06%Ca) and well-ordered dolomites associated with anhydrite relicts. They originated through both sabkha and burial dolomitization. This contribution gives an improved method for the characterization of different dolomite types and their distinct traits in sedimentary rocks, which allows a better evaluation of their reservoir potential.
To study the process of microbial-mediated dolomite formation, growth experiments were carried out with selected bacterial cultures under anoxic environmental conditions simulating those found in Lagoa Vermelha, a hypersaline lagoon in Brazil where dolomite precipitation occurs. Specifically, we report the isolation of a particular strain of sulfate-reducing bacteria, LVform6, from Lagoa Vermelha sediment, which apparently promotes the formation of nonstoichiometric dolomite. Sulfate-reducing bacteria grown in a synthetic liquid medium produced dolomite during 30 days incubation at 30 °C. The precipitates have morphologies similar to those observed in Lagoa Vermelha sediment. Our results demonstrate that sulfate-reducing bacteria can influence dolomite precipitation under controlled low-temperature, anoxic conditions, and imply that anaerobic microorganisms can play an important role in carbonate sedimentation. They may have been particularly significant in Earth's earliest history when a more reducing atmosphere existed.
Facies analysis of the Durrenstein Formation, central-eastern Dolomites, northern Italy, indicates that this unit was deposited on a carbonate ramp, as evidenced by the lack of a shelf break, slope facies, or a reef margin, together with the occurrence of a ''molechfor'' biological association. Its deposition following the accumulation of rimmed carbonate platforms during the Ladinian and Early Carnian marks a major shift in growth mode of the Triassic shallow marine carbonates in the Dolomites. The Durrenstein Formation is characterized by a hierarchical cyclicity, with elements strongly suggestive of an allocyclic origin, including (a) subaerial exposure features directly above subtidal facies within meter-scale cyclothems, (b) purely subtidal carbonate cyclothems, (c) symmetric peritidal carbonate cyclothems, and (d) continuity of cyclothems of different orders through facies boundaries. The Durrenstein cyclothems are usually defined by transgressive and regressive successions, and so most of them probably originated from sea-level oscillations. Their allocyclic origin allows their use for high-resolution correlations over distances up to 30 km. A stratigraphic section in the Tre Cime di Lavaredo area, encompassing the upper part of the Durrenstein Formation and the lower part of the overlying Raibl Formation (Upper Carnian) was studied using time-frequency analysis. A strong Milankovitch signal appeared when interference arising from a variable sedimentation rate was estimated and removed by tuning the short precession line in a spectrogram. All of the principal periodicities related to the precession index and eccentricity, calculated for 220 Ma, are present: P1 (21.9 ky); P2 (17.8 ky); E1 (400 ky), E2 (95 ky), and E3 (125 ky), along with a peak at a frequency double that of the precession, which is a predicted feature of orbitally forced insolation at the equator. Components possibly related to Earth's obliquity at ca. 35 ky and ca. 46 ky are present as well. The recovery of Milankovitch periodicities allows reconstruction of a high-resolution timescale that is in good agreement with published durations of the Carnian based on radiometric ages. The recognition of a Milankovitch signal in the Durrenstein and lower Raibl formations, as well as in other Mesozoic carbonate platforms, strongly supports a deterministic and predictable-rather than stochastic-control on the formation of carbonate platforms. Carbonate platforms might thus be used in the future for the construction of an astronomical time scale for the Mesozoic.
Generally arid conditions that pervaded much of Europe and North America during the late Triassic were interrupted by a wet monsoonal climatic phase during Middle and Late Carnian times. Extensive fluviatile sandstones deposited at this time throughout the region, occur within a thick sequence of playa-lake mudstones. The sandstones occasionally contain kaolinite, suggesting a humid climate. An extreme δ ¹³ C depletion in a shallow marine sequence of this age in Israel has been interpreted as evidence for an influx of freshwater. A widespread change from carbonates to clastics in marine sequences at this time may also be climate-related. Water-course cave systems in limestone areas exposed during the late Triassic indicate high levels of runoff during the Middle and Late Carnian.
The marine invertebrate fauna shows a significant turnover at the end of the Early Carnian. The terrestrial fauna and flora were relatively unaffected at this time but subsequently diversified prior to a major biotic turnover at the Carnian-Norian boundary. These periods of biotic change appear to be synchronous with the onset and cessation of a Carnian humid phase.
The change to a monsoonal climate during this interval has been documented over more than 90° of longitude between 5° and 50° north of the Triassic equator. It may have been caused by rising atmospheric CO 2 levels due to volcanism associated with the incipient dispersal of Pangaea.
An ascending-brine model is proposed to address the observed isotope geochemistry, solute composition, and solute and water fluxes in the coastal sabkha of the Emirate of Abu Dhabi. Mass-balance measurements document that >95% of the solutes are derived from ascending continental brines; minor amounts are derived from rainfall and from groundwater entering from upgradient areas. Nearly 100% of the annual water loss is from evaporation and not lateral discharge. Direct rainfall on the sabkha and subsequent recharge to the underlying aquifer account for ~ 90% of the annual water input to the system; the remaining 10% comes from both lateral and ascending groundwater flow. Thus, the water and solutes in this system are from different sources. Solute concentrations of conservative (i.e., nonreactive) elements in the coastal, sabkha-covered aquifer are consistent with the fluid pore volumes of ascending brine calculated from hydrologic properties. Calcium to sulfate ratios and sulfur isotopes are consistent with this source of solute from the underlying Tertiary formations. Recharging rainwater dissolves halite and other soluble minerals on the surface, causing the solution to become more dense and sink to the bottom of the aquifer where it vertically mixes with less dense ascending brines. Solutes are returned to the surface by capillary forces and recycled or lost from the system by eolian or fluvial processes. Thus, the system becomes vertically mixed, consistent with the presence of tritium throughout the aquifer; but there is essentially no horizontal mixing of seawater with groundwater. The observed seawater solutes in the supratidal zone come from interstitial seawater trapped by the rapid progradation of the sediments into the Arabian Gulf and are not refluxed or laterally mixed. The ascending-brine model contrasts significantly with both the seawater-flooding and evaporative-pumping models previously proposed as a source of solutes to the coastal sabkha of the Emirate of Abu Dhabi. Use of these earlier models leads to incorrect conclusions and raises serious questions about their applicability in the evaluation of sabkhat in the geologic record.
Although the Late Triassic was a time of widespread aridity, evidence
exists for a significant increase in rainfall during the middle to late
Carnian. Upper Triassic playa-lake sediments were interrupted by late
Carnian fluviatile sandstones with erosive bases and high
kao-linite/illite ratios. There was also an increase in the clastic
component of marine sequences during this interval. Middle and upper
Carnian marine carbonates show an extreme depletion in
δ13C values, consistent with increased fresh-water
influx. Large-scale karstic phenomena in limestone areas subaerially
exposed during the Late Triassic are a further indication of increased
rainfall. Important faunal and floral changes occurred during the
Carnian-Norian interval; marine invertebrate turnover was greatest at
the lower/middle Carnian boundary, and terrestrial extinctions were
concentrated at the Carnian/Norian boundary. The cause of this Carnian
pluvial episode may have been related to the rifting of Pangea, through
disruption of atmospheric and oceanic circulation patterns, eustatic
changes, or the effects of volcanism associated with rifting. A change
in surface ocean temperature, salinity or pH, or habitat loss may have
caused the decline of many shallow-marine invertebrates at the start of
the middle Carnian; a return to arid conditions at the Carnian/Norian
boundary would account for the turnover among terrestrial vertebrates
and plants.
Introduction What happened at the end of the Early Carnian, some 235-230 million years ago? All over the Dolomites, the lower-upper Carnian transition is evident from the distance as a break between the majestic rock walls of the massive Cassian Dolomite and those of the well bedded Dolomia Principale. This morphological step is stri-kingly evident, for example, all around the Sella Platform, and locally evolved to extended plateaus, as below the Tre Cime di Lavaredo or at Lagazuoi, north of Passo Falzarego. Even the slopes of Col Gallina and Nuvolau, uniformly dipping northward toward Passo Falzarego, are structural surfaces representing the exhumed plat-form top of the demised lower Carnian Cassian Dolomite (Fig. 1). And here our excursion starts. The aim of this field trip is twofold. On the one hand, evidence will be shown of a climatic swing from arid, to humid, and back to arid climate in the Carnian of the Tofane area. We here denote the whole climatic episode, regardless of its polyphase nature, as the "Carnian Pluvial Event". On the other hand, the effects of this climatic event on sedimentation and biota will be illustrated, from the km scale of carbonate platform geometries to the smaller scale of facies associations and lithologies. The mor-phological features of famous mountain groups of the Dolomites, as depicted above, are a direct consequence of the sedimentary turnover triggered by the Carnian Pluvial Event.
A number of 3rd-order depositional sequences have been observed in the Southern Alps, far more than previously known: of these, 6 develop principally in the Scythian, 4 in the Anisian, 3 in the Ladinian, 4 in the Carnian, at least 2 in the Norian and finally 2 in the Rhaetian. Lower Anisian to Lower Carnian depositional sequences are best dated by ammonoids, while in the Scythian ammonoids and conodonts are scanty. In the Upper Carnian, Norian and Rhaetian deposits of the Southern Alps, very rare ammonoids and conodonts are available. Therefore Scythian, Upper Carnian, Norian and Rhaetian depositional sequences have been defined on the basis of stratal patterns and the evolution of facies, being their chronostratigraphical position inferred from sections elsewhere in the world. The contemporaneous analysis of platforms and basins and of carbonate, mixed and siliciclastic deposits has made a good comprehension of facies migration possible. This approach was crucial in the definition of 3rd-order depositional sequences. Moreover, on the basis of the previously defined 3rd-order sequences and systems tracts, a number of 2nd-order transgressive/regressive cycles have been pointed out. The sequence stratigraphic analysis compared with the tectonic history allowed the definition of the different phases of the basin evolution.
Dolomite is not a simple mineral; it can form as a primary precipitate, a diagenetic replacement, or as a hydrothermal/metamorphic phase, all that it requires is permeability, a mechanism that facilitates fluid flow, and a sufficient supply of magnesium. Dolomite can form in lakes, on or beneath the shallow seafloor, in zones of brine reflux, and in early to late burial settings. It may form from seawater, from continental waters, from the mixing of basinal brines, the mixing of hypersaline brine with seawater, or the mixing of seawater with meteoric water, or via the cooling of basinal brines. Bacterial metabolism may aid the process of precipitation in settings where sulfate-reducing species flourish and microbial action may control primary precipitation in some hypersaline anoxic lake settings.
Marine evaporites occurring in modern subaqueous (salina) settings and subaerial (sabkha) settings are different. Subaqueous Holocene evaporites occur as shoalingupward lacustrine sequences up to 10 m thick. They are evaporite dominated and are composed primarily of bottom-nucleated crystals that may be deposited as massive, laminated, or rippled units. Each coastal lake is dominated by laminated evaporites with subordinate carbonate sediments. In plan view, they show a well-developed bull's-eye pattern with a sulfate center and a carbonate rim. In contrast, subaerial (sabkha) evaporites occur as part of a laterally prograding, shoaling-upward, peritidal sequence in which the supratidal unit is usually no more than 1 m thick. Sabkha sequences are matrix dominated, not evaporite dominated, with the bulk of the sulfate phase occurring as diagenetic nodules, enteroliths, or diapirlike structures. These sulfates were formed during syndepositional diagenesis by replacement and displacement processes. The various facies of the sequence tend to accumulate in belts parallel with the shoreline. Relative to the sea level or the brine level, sabkhas tend to form over paleotopographic highs whereas salinas tend to occur in paleotopographic lows. Some of the characteristics that distinguish Holocene subaerial and subaqueous evaporite sequences can be used to do the same for similar ancient facies, even when gypsum has been converted to nodular anhydrite. The distinction is important for it can be used by explorationists in the oil industry to define the paleotopography of the associated underlying porous and nonporous carbonates.
Microsedimentary structures referred to as nanobacteria-like particles were described from modern carbonate environments, where they form in close spatial association with sulfate-reducing bacteria (SRB). However, the exact mechanism of their formation, as well as their paleontological signifi cance, remains controversial. Here we report on an investigation of microbe-mineral interactions in experimentally produced carbonate globules. The experi- ments were carried out under anoxic conditions at 30 °C with Desulfovibrio brasiliensis, a SRB known to mediate dolomite formation. We observed that extracellular polymeric substances (EPS) secreted by the microbial community play a key role in the mineralization process. Nanobacteria-like particles represent the early stage of carbonate nucleation within the EPS, which progressively evolve to larger globules displaying a grainy texture. We excluded the possibilities that these structures are fossils of nanobacteria, dissolution surfaces, or artifacts created during sample preparation. D. brasiliensis cells are predominantly located outside of the EPS aggregates where mineral growth takes place. As a result, they remain mobile and are rarely entombed within the mineral. This self-preservation behavior may not be limited to D. brasiliensis. Other microbes may produce, or may have produced during the geological past, biogenic minerals through a similar process. Mineralization within EPS explains why microbial relics are not necessarily present in biogenic carbonates.
Decades of various and numerous isotopic studies to interpret the environmental con- ditions of dolomite formation proved to be inconclusive because the temperature- dependent oxygen isotope fractionation factor between dolomite and the solution from which it precipitated could not be determined experimentally at low temperatures. With the discovery of bacteria that mediate the precipitation of dolomite, it is now possible to overcome kinetic barriers and precipitate dolomite under controlled temperature condi- tions in culture experiments. Herein we report on the results of microbial experiments that have enabled us to calibrate the dolomite-water oxygen isotope fractionation factor and provide a paleothermometer to evaluate conditions of ancient dolomite formation. The temperature (T) dependence of the fractionation is defined by the equation: 1000 ln adolomite-water 5 2.73 3 106T 22 1 0.26.
The Alpine Triassic units of Switzerland, Northern Italy and Western Austria
offer an extensive geological archive, in which the enigmatic process of dolomite
formation can be studied in a palaeoenvironmental context. Recent studies
clearly demonstrate that large amounts of the Alpine Triassic dolomites are
late diagenetic or hydrothermal. Nevertheless, as part of multiple generations
of diagenetic overprint, some generations of fine-crystalline, Ca-rich dolomite
appear strictly confined to their depositional facies and show signs of very
early formation at surface temperatures in specific ancient depositional environments.
In this review, three cases of Alpine Triassic dolomites are discussed,
where dolomite rocks may have formed during or soon after
sedimentation. The sedimentary facies indicate contrasting palaeoenvironmental
conditions and, hence, document three different possible processes of
dolomite formation: (i) In the Dolomite Mountains (Northern Italy), dolomite
beds of the partly isolated Middle Triassic (Anisian/Ladinian) Latemar Platform
are confined to the very top of shallowing-upward lagoonal facies cycles.
(ii) Dolomite beds of the San Giorgio Basin (Southern Switzerland), an intraplatform
basin that opened during the Anisian/Ladinian transition, are associated
with organic carbon-rich shales, which were deposited in a deeper water
environment under anoxic conditions. (iii) In the entirely dolomitized platform
facies of the Dolomia Principale (Hauptdolomit Formation), a very early
generation of fine-crystalline dolomite occurs in the shallowest part of evaporative
peritidal cycles. This platform extended over thousands of square kilometres
along the Tethys margin during the Late Triassic (Carnian and Norian)
and large amounts of carbonate were deposited under hypersaline sabkha-like
conditions. Representing three distinct depositional environments, these three
different Triassic systems show features in common with several dolomitization
models developed from the study of modern dolomite-forming environments;
for example, the sabkha model, the evaporative lagoon/lake model, the
organogenic model and the microbial model. Although these actualistic models
may be applicable to reconstruct the palaeoenvironmental conditions during
dolomite formation, dolomite-forming processes during the Triassic were
apparently quite different from the modern world in terms of distribution and
scale. Recent developments in stable-isotope geochemistry and high-resolution
geochemical probing offer the possibility to make better reconstructions of
Triassic palaeoceanographic conditions and suggest a non-actualistic
approach to better understand dolomite formation during the Triassic.
Major climate changes and mass extinctions are associated with carbon isotope anomalies
in the atmosphere-ocean system and have been shown to coincide with the onset of large igne- ous provinces (LIPs) and, by association, their emissions of greenhouse gases and aerosols. However, climatic and biological consequences of some known LIP eruptions have not yet been explored. During the Carnian (Late Triassic) large volumes of flood basalts were erupted to form the so-called Wrangellia LIP (western North America). This huge volcanic province is similar in age to a major climatic and biotic change, the Carnian Pluvial Event (CPE), but no evidence of a causal relationship exists other than timing. Here we report a sharp negative δ13C excursion at the onset of the CPE recorded in organic matter. An abrupt carbon isotope excursion of ~−4‰ occurs in terrestrial and marine fossil molecules, whereas total organic carbon records an ~−2‰ shift. We propose that this carbon isotope negative shift was caused by an injection of light carbon into the atmosphere-ocean system linked to the eruption of Wrangellia flood basalts. This carbon-cycle perturbation occurs slightly before two major evolutionary innovations: the origin of dinosaurs and calcareous nannoplankton.
Bedded dolomites in the Permian Basin were formed by the alteration of metastable limestones by hypersaline brines refluxing from evaporate lagoons. The hot, heavy, highly alkaline, carbon dioxide-free, magnesium-supercharged brines displaced connate waters to provide both a chemically favorable environment for magnesium-calcium exchange and a vehicle for removing displaced calcium. Fossil lagoonal brines and fillings of halite and anhydrite in the dolomite pores offer proof of the brine invasion. Sedimentary dolomites in other areas are commonly associated with evaporites, and for these dolomites a similar origin is postulated.
Microstructures, textures, and composition of dolomites from different facies and stratigraphic level of a carbonate platform sequence 1000-2000m thick in the Brenta Dolomites and Eastern Lombardy are interrelated. Matrix dolomite crystals with more abundant slightly calcian and stoichiometric areas have negative δ 18O values. Void-filling dolomites are stoichiometric and contain only a few dislocations. The onset of solid-state diffusion is recorded in some void-filling dolomites formed at temperatures above 60°C. Regionally extensive dolomitization was almost completed by the end of the Triassic. Late hydrothermal dolomitization did not cause neomorphism of early replacive dolomites. -from Authors
Surfaces of growing crystals of dolomite from Deep Springs Lake, Calif., are Ca- rich. The surfaces of the crystals have a Ca to Mg ratio in excess of 4; the surface layer is about 100A thick and approaches the composition of dolomite at its inner side. Successive fractions have been dated by C-14 and thus place a time parameter on the introduction of magnesium into the rhombohedral carbonate lattice. The crystals grow by the production of a transient surface layer which can be envisaged as a moving boundary through which the magnesium must move by solid-state diffusion with concomitant migration of calcium in the opposite direction. The formation of dolomite having a rudimentary R 3 space group takes place at the inner side of the surface layer. Crystal growth models are considered, along with implications concerning rates of growth and diffusion, solubility, and isotopic fractionations.
Two different styles of dolomitization pertain to these Late Triassic ages. The Rhaetian appears as a period of transition between the production of large bodies of early diagenetic dolomite of the Norian and the lack of such in the succeeding Jurassic/Cretaceous, at least in the Tethyan domain. The Rhaetian transition is probably related to a change from the very particular palaeogeographic/tectonic/climatic setting in which deposition and diagenesis of the Norian carbonates occurred. Norian carbonate shelves developed in the extensive Pangea Gulf, which was bordered landward by wide evaporitic deposits, indicating arid climatic conditions. The Rhaetian transgression, with terrigenous input and a more humid climate, by terminating these conditions, greatly hindered surface dolomitization. -from Authors
Dolomite, despite its thermodynamic stability and abundance in the ancient rock record, is rarely found forming in Holocene environments. This enigma is frequently called the Dolomite Problem. The recent discovery of modern dolomite formation in Lagoa Vermelha, a shallow-water isolated coastal lagoon east of Rio de Janeiro, Brazil, provides a new environment to investigate the factors promoting dolomite precipitation under earth surface conditions. Lagoa Vermelha serves as a natural laboratory in which the dolomite formation process was studied using an integrated hydrologic, geochemical, and sedimentological approach. The results of this study indicate that Ca-dolomite precipitation occurs under anoxic hypersaline conditions within a black sludge layer directly overlying the water/se iment interface. With deposition, the dolomite undergoes an "ageing" process, whereby increased ordering of the crystal structure occurs. Both the initial precipitation and subsequent early diagenesis are strongly mediated by microbial activity. In fact, using sulfate-reducing bacteria cultured from Lagoa Vermelha samples, a highly ordered dolomite has been produced in the laboratory at low temperatures. These experimental results combined with the study of the natural environment mandate that a microbial factor be added to the list of factors capable of causing dolomite precipitation. Considering the Lagoa Vermelha system, we propose a new actualistic model for dolomite formation, which we call the microbial dolomite model.
The origins of the waters (lagoon, open marine, continental and mixed) were defined by their K:Br ratios and stable isotope values. Downward movement of flood recharge waters was established by decreasing tritium contents with depth under the sabkha. The data were synthesized into a hydrologic model for the intermediate sabkha, the area where dolomitization occurs. A single hydrologic cycle was defined by 3 seqential stages; flood recharge, capillary evaporation and 'evaporative pumping.' The processes involved in the evolution of a dolomitizing solution and the driving forces required to move the solutions through the sediments being dolomitized are inherent in the model. -from Authors
Documents three carbonate phases: aragonite, ordered dolomite often showing modulated microstructure, and a mostly disordered calcium magnesium carbonate with submicrometer-sized ordered domains. Ordered and partially ordered dolomite formed mainly by direct precipitation from pore fluids, with changing temperature and chemistry in response to environmental parameters. -from Authors
At the northeastern extremity of the Abu Dhabi sabkha, less than 1 m below the surface, a lithified lenticular dolomitic body immediately overlies Holocene regressive mangrove deposits. In the lower part of the section, decay of the organic matter in the mangrove palaeosol results in an anoxic environment with slightly acidic conditions, probably responsible for the dissolution of aragonite observed in the palaeosol and overlying white mud. -from Authors
Terra Nova, 24, 248–254, 2012
The study of microbe-mineral interactions and the identification of biominerals in sedimentary rocks provide crucial information on the coevolution of life and Earth surface environments. Desulfovibrio brasiliensis, a sulfate-reducing bacterium isolated from Lagoa Vermelha (Brazil), is known for its ability to precipitate dolomite, a mineral that is common in the geological record, but difficult to obtain in laboratory experiments simulating Earth's surface conditions. Here, we report on a new bacterial strain capable of precipitating Mg-calcite and Ca-dolomite, isolated from a microbial mat in the sabkha of Abu Dhabi (UAE). The 16S rRNA gene sequence of the isolate revealed a 99.6% identity (i.e. same species) with Desulfovibrio brasiliensis. The presence of the same microbial species at two geographically distant dolomite-forming locations is difficult to explain in terms of pure coincidence. Rather, it suggests that the ability of precipitating dolomite may be a unique characteristic associated with specific strains.
The %Ca (molar Ca/(Ca + Mg) X 100) in sedimentary dolomite, which ranges from 48 %Ca to 62.5 %Ca, can be determined by atomic absorption spectrometry, electron microprobe analysis, or powder X-ray diffraction analysis. The latter procedure, which relies on the fact that the position of the d(104) reflection depends linearly on the Ca content, offers an easy and relatively inexpensive method of determining %Ca in dolomite. To date, such analyses have assumed that the dolostone is homogeneous with respect to the %Ca content of its constituent dolomite crystals. Herein, the XRD technique is extended to compositionally heterogeneous dolostones. A considerable amount of important information can be derived from X-ray diffractograms of dolostones if they are analyzed with peak-fitting techniques (PF-XRD). Specifically, this technique allows (1) determination of the %Ca with an accuracy of +/- 0.5 %Ca, (2) identification of heterogeneous dolostone samples formed of more than one population of dolomite crystals as defined by their %Ca content, and (3) derivation of the proportion of each population of dolomite crystals with an accuracy of +/- 10%. The PF-XRD technique is a powerful analytical tool that provides valuable information on the major-element composition of low-Fe dolomite. Compared to other techniques, the PF-XRD technique is accurate, rapid, relatively inexpensive, and easy to use.
Some species of sulphate-reducing bacteria (SRB) are known to mediate the formation of dolomite and Mg-calcite. However, their exact role in the mineralization process remains elusive. Here, we present the result of a laboratory experiment that was designed to test whether formation of carbonate minerals by SRB can occur in the absence of living cells, through passive mineralization of their exopolymeric substances (EPS). SRB capable of mediating dolomite were cultivated in the laboratory, allowing them to secrete EPS. Microbial activity within the cultures was subsequently inhibited with antibiotics. Only after this step, Ca2+ and Mg2+ were added to the solution and carbonate minerals could form. Mg-calcite and disordered Ca-dolomite precipitated in association with EPS. The mol.% of Mg2+ in the crystals increased with longer incubation times. This result demonstrates that organic compounds produced by SRB can mediate the formation of Ca-Mg carbonates in the absence of an active metabolism.
A dataset of 160 isotope δ13C and δ18O values from Anisian, Ladinian, Carnian and Rhaetian articulate brachiopod shells, complemented by 158 carbon and oxygen isotope values from whole rock carbonates, define the first continuous stable isotope baseline trends for the Triassic seawater. The carbon isotope data suggest the existence of several short-term high amplitude excursions in the Early Triassic, followed by a predominance of values around 0.5 ± 1‰ during the Middle Triassic, a rise to ∼ 3.5‰ during the Carnian, plateau at this level during the Late Carnian to Early Norian, and a 1.5‰ decline in the Middle Norian to values around 2‰ during the Late Norian–Rhaetian interval. The causation scenarios for these rapid oscillations are at present equivocal, but may in part reflect a biological instability of the carbon cycle following the recovery from the end-Permian extinction event and/or an input of “mantle”-derived CO2 from enhanced volcanic activity.The δ18O values from well-preserved brachiopods from the Tethyan realm range from − 3.9 to − 0.6‰ V-PDB. These values require open marine Triassic seawater δ18O values close to 0‰ V-SMOW for the calculated temperatures to be within the range of tolerance of the coexisting reef-building corals. A 2‰ δ18O increase within the uppermost Cordevolian and early Julian suggests either a distinct temperature decline in the Southern Alps during that time interval, an increase in seawater salinity, or their combination. Oxygen isotope values for the Muschelkalk brachiopods range between − 6.2 and − 2.0‰ and likely reflect a strong influx of meteoric waters into the Germanic Basin that shifted the oxygen isotopes to more negative values.
We propose that the so-called "terminal fan" facies model should be abandoned since it is flawed on several counts and it is leading to misunderstanding and poor communication. Rivers in drylands may experience excessive downstream discharge reduction such that they terminate subaerially rather than reach the sea or a lake. The facies model predicts that the distal reaches of such rivers form a network of bifurcating distributary channels producing a fan-shaped sediment body, with downstream thinning and fining of sedimentary units, ending in sand-filled ribbons encased in mud. Extensive review of modern rivers has failed to turn up convincing examples that fit the model. Rivers in drylands do not ubiquitously end in fans. Fan-shaped fluvial bodies are common wherever rivers are released from confinement and the discharge conditions promote frequent avulsion. Channels on such fans generally do not repeatedly bifurcate downstream. Where they are seen to do so, it can usually be shown they are lacustrine deltas inherited from wetter times. The term "distributary" is being used carelessly and is conveying incorrect understanding of sediment geometry and architecture. The proposed synonym of "fluvial distributary systems" is unsatisfactory as it perpetuates the same misunderstandings. Reliance on planform alone in analogue selection is highly risky. The fluvial fan is a composite sediment body resulting from frequent nodal avulsions in a setting without horizontal constraints. Channels on fans range in planform as much as any other river. The resultant sedimentary record differs little from that expected from non-fan fluvial systems except having a regionally radiating orientation when viewed over geological time scales. Contrary to the implications of the facies model, there is no distinctive "terminal fan" sedimentary succession.
Primary carbonates are a common feature of many modern and ancient lacustrine deposits. Carbonates from hydrologically open lakes show little or no correlation between δ13C and δ18O. In short-residence-time open lakes, carbonate oxygen isotopic composition is relatively invariant and typically is closely related to the bulk isotopic composition of inflow waters to the lake. Suites of carbonates which display covarying 13C and 18O compositions precipitate from waterbodies having relatively long residence times. Where the correlation between carbon and oxygen isotopic variations is high (r⪖ 0.7 ), the carbonates have normally precipitated from a closed lake. In addition, because of large changes in water balance, the δ18O of closed-lake carbonates usually varies over a range of several %o. Therefore, the combination of degree of covariance and spread of δ18O-values can be used to discriminate between carbonates produced in hydrologically open and closed basins. Within individual basins, covariant trends may have remarkable long-term persistence despite major environmental changes, indicating considerable stability in basin hydrology. Each closed lake has a unique isotopic identity defined by its covariant trend, which is a function of the basin's geographical and climatic setting, its hydrology, and the history of the waterbody. Any major interruption or realignment of this trend reflects a fundamental change in basin hydrology. Isotopic trends based upon the carbon and oxygen isotopic composition of primary lacustrine carbonates have several applications in palaeolimnology. The oxygen isotopic composition of open-lake carbonates may, with caution, be used as a proxy indicator of the composition of regional rainfall. Covariant trends can be used to trace the hydrological history of a basin, the evolution of individual water masses, and to correlate carbonate-bearing sediments from different parts of a basin.
The Upper Devonian Grosmont platform in the Western Canada sedimentary basin is a pervasively dolomitized giant heavy-oil reservoir with reserves of 317 billion bbl of bitumen. The principal type of Grosmont platform dolomite formed early and on the basis of stratigraphic and geochemical evidence is interpreted as early diagenetic reflux dolomite. We use a numerical ground-water flow model to investigate the viability of reflux to dolomitize the Grosmont platform. We simulate reflux at four key stages of platform evolution, incorporating the transient effects of changes in platform architecture, rock properties, and the salinity of platform-top waters. The pattern and magnitude of reflux is critically controlled by permeability and the distribution of platform-top brines, which are concentrated up to gypsum saturation. Reflux flow is focused in the relatively permeable carbonates of the Grosmont Formation and is from the platform interior toward the platform margin. The 120-m-thick shales of the Ireton Formation that separate the Grosmont and Cooking Lake formations restrict cross-formational flow and brine transport. During a 100-k.y. period of relative sea level rise and platform-top drowning, brines of reflux origin continue to sink and entrain platform-top waters (latent reflux). Where the intervening aquitards are thin or absent, reefs of the Leduc Formation capture reflux brines from the overlying Grosmont, platform and focus cross-formational brine transport. Lateral contrasts in salinity are sufficient to drive a series of free convection cells in the relatively permeable reefs of the Leduc Formation. Computed distributions of fluid flux in conjunction. with magnesium mass-balance calculations that incorporated the range of uncertainty, particularly in permeability, support the suggestion that the reflux of gypsum-saturated brines could have formed much if not most of the dolomite in the Grosmont Formation in the 1.6 m.y. available.
Despite intensive research over more than 200 years, the origin of dolomite, the mineral and the rock, remains subject to considerable controversy. This is partly because some of the chemical and/or hydrological conditions of dolomite formation are poorly understood, and because petrographic and geochemical data commonly permit more than one genetic interpretation. This paper is a summary and critical appraisal of the state of the art in dolomite research, highlighting its major advances and controversies, especially over the last 20–25 years.
The thermodynamic conditions of dolomite formation have been known quite well since the 1970s, and the latest experimental studies essentially confirm earlier results. The kinetics of dolomite formation are still relatively poorly understood, however. The role of sulphate as an inhibitor to dolomite formation has been overrated. Sulphate appears to be an inhibitor only in relatively low-sulphate aqueous solutions, and probably only indirectly. In sulphate-rich solutions it may actually promote dolomite formation.
Mass-balance calculations show that large water/rock ratios are required for extensive dolomitization and the formation of massive dolostones. This constraint necessitates advection, which is why all models for the genesis of massive dolostones are essentially hydrological models. The exceptions are environments where carbonate muds or limestones can be dolomitized via diffusion of magnesium from seawater rather than by advection.
Replacement of shallow-water limestones, the most common form of dolomitization, results in a series of distinctive textures that form in a sequential manner with progressive degrees of dolomitization, i.e. matrix-selective replacement, overdolomitization, formation of vugs and moulds, emplacement of up to 20 vol% calcium sulphate in the case of seawater dolomitization, formation of two dolomite populations, and — in the case of advanced burial — formation of saddle dolomite. In addition, dolomite dissolution, including karstification, is to be expected in cases of influx of formation waters that are dilute, acidic, or both.
Many dolostones, especially at greater depths, have higher porosities than limestones, and this may be the result of several processes, i.e. mole-per-mole replacement, dissolution of unreplaced calcite as part of the dolomitization process, dissolution of dolomite due to acidification of the pore waters, fluid mixing (mischungskorrosion), and thermochemical sulphate reduction. There also are several processes that destroy porosity, most commonly dolomite and calcium sulphate cementation. These processes vary in importance from place to place. For this reason, generalizations about the porosity and permeability development of dolostones are difficult, and these parameters have to be investigated on a case-by-case basis.
A wide range of geochemical methods may be used to characterize dolomites and dolostones, and to decipher their origin. The most widely used methods are the analysis and interpretation of stable isotopes (O, C), Sr isotopes, trace elements, and fluid inclusions. Under favourable circumstances some of these parameters can be used to determine the direction of fluid flow during dolomitization.
The extent of recrystallization in dolomites and dolostones is much disputed, yet extremely important for geochemical interpretations. Dolomites that originally form very close to the surface and from evaporitic brines tend to recrystallize with time and during burial. Those dolomites that originally form at several hundred to a few thousand metres depth commonly show little or no evidence of recrystallization.
Traditionally, dolomitization models in near-surface and shallow diagenetic settings are defined and/or based on water chemistry, but on hydrology in burial diagenetic settings. In this paper, however, the various dolomite models are placed into appropriate diagenetic settings.
Penecontemporaneous dolomites form almost syndepositionally as a normal consequence of the geochemical conditions prevailing in the environment of deposition. There are many such settings, and most commonly they form only a few per cent of microcrystalline dolomite(s). Many, if not most, penecontemporaneous dolomites appear to have formed through the mediation of microbes.
Virtually all volumetrically large, replacive dolostone bodies are post-depositional and formed during some degree of burial. The viability of the many models for dolomitization in such settings is variable. Massive dolomitization by freshwater-seawater mixing is a myth. Mixing zones tend to form caves without or, at best, with very small amounts of dolomite. The role of coastal mixing zones with respect to dolomitization may be that of a hydrological pump for seawater dolomitization. Reflux dolomitization, most commonly by mesohaline brines that originated from seawater evaporation, is capable of pervasively dolomitizing entire carbonate platforms. However, the extent of dolomitization varies strongly with the extent and duration of evaporation and flooding, and with the subsurface permeability distribution. Complete dolomitization of carbonate platforms appears possible only under favourable circumstances. Similarly, thermal convection in open half-cells (Kohout convection), most commonly by seawater or slightly modified seawater, can form massive dolostones under favourable circumstances, whereas thermal convection in closed cells cannot. Compaction flow cannot form massive dolostones, unless it is funnelled, which may be more common than generally recognized. Neither topography driven flow nor tectonically induced (‘squeegee-type’) flow is likely to form massive dolostones, except under unusual circumstances. Hydrothermal dolomitization may occur in a variety of subsurface diagenetic settings, but has been significantly overrated. It commonly forms massive dolostones that are localized around faults, but regional or basin-wide dolomitization is not hydrothermal.
The regionally extensive dolostones of the Bahamas (Cenozoic), western Canada and Ireland (Palaeozoic), and Israel (Mesozoic) probably formed from seawater that was ‘pumped’ through these sequences by thermal convection, reflux, funnelled compaction, or a combination thereof. For such platform settings flushed with seawater, geochemical data and numerical modelling suggest that most dolomites form(ed) at temperatures around 50–80 °C commensurate with depths of 500 to a maximum of 2000 m. The resulting dolostones can be classified both as seawater dolomites and as burial dolomites. This ambiguity is a consequence of the historical evolution of dolomite research.
Many sedimentary calcian dolomites commonly display a modulated microstructure when viewed with the transmission electron microscope. This structure has been attributed to a disordered incorporation of excess calcium in the dolomite structure. We describe here two more highly ordered metastable superstructures (gamma and delta dolomite) that are considered intermediate between disordered calcian dolomite as formed by dissolution-reprecipitation processes and---probably as a result of diagenesis---decomposition to stable stoichiometric dolomite and calcite. Small platelike regions of coherent calcite are observed in dolomite from the Devonian Lost Burro Formation in southeastern California and in heat-treated calcian dolomite. These observations document the evolution of sedimentary carbonates from metastable to stable phases under favorable kinetic conditions. *On leave from Department of Geology, Nanjing University, Nanjing, People's Republic of China
Thick dolomite-cemented horizons (dolocretes) occur within a fluvial sandstone-mudstone sequence of Late Triassic age in the western part of the Paris Basin, France. Two types of dolomites can be distinguished: (a) nodular dolomitic beds less than a few metres thick, which formed within mottled overbank siltstones and mudstones; and (b) massive dolomite up to 16 m thick, which occurs in coarse grained channel sandstones and conglomerates.
The majority of the dolomite consists of a finely crystalline groundmass of dolomicrospar and, less commonly, dolomicrite. Glaebules, irregular spar-filled cracks, spheroidal dolomite, silicification and vuggy porosity are locally abundant in the massive dolomite. In contrast, biologically induced micromorphological features such as rhizocretions and alveolar-septal fabrics were observed in the thin, nodular dolomite beds.
The dolomite is near stoichiometric, well ordered and non-ferroan. 18O values range from −7·7 to −0·4%o PDB and 18O values range from −5·1 to + 1·8%0 PDB and no obvious difference in the stable isotopic composition between both types of dolomites was observed. Sr isotope ratios range from 0·7101 to 0·7126 and are invariably higher than the contemporary Triassic sea water.
A vadose—pedogenic origin for the thin dolocrete layers is indicated by the occurrence of rhizocretions and other biological structures. Several features, however, argue against a pedogenic origin for the massive carbonates, most notably the absence of biologically induced structures, the occurrence in coarse grained channel (and not overbank) deposits, and the great thickness. These units are thus interpreted as groundwater in origin. Phreatic calcretes of Quaternary age, widespread in inland Australia, are regarded as a modern analogue for the Triassic Paris Basin dolocretes.
Petrographic observations argue in favour of primary (proto)dolomite precipitation, although early diagenetic replacement of calcite by (proto)dolomite cannot be ruled out. Strontium and carbon isotope data of early diagenetic dolocrete cements and oxygen isotope data of early diagenetic silica indicate an entirely non-marine, continental origin for the groundwaters. The poorly ordered and non-stoichiometric protodolomite probably underwent stabilization upon further burial resulting in a near-stoichiometric, well ordered dolomite that clearly lacks evidence for pervasive recrystallization.
The combination of high-resolution stratigraphy with high-resolution geochemistry allowed monitoring the cyclicity of a playa system in two dimensions, time and space. The playa cycles of the Middle Upper Triassic Steinmergel-Keuper, located in southern Germany, were studied both in terms of lateral facies shift variations and long-term cyclic patterns. The lateral variation was expressed by investigating a time equivalent transect that spanned the Kieselsandstein horizon within a 130km distance, from marginal to mid-playa position, whereas the long-term cyclic pattern was investigated in core Malschenberg, where a time record of ca. 5×106years was encountered. Mudstone/dolomite cycles, with dolomites representing the wet and mudstones the dry phases, reflect cyclic playa lake-level changes, indicating varying intensities of monsoon-like circulation. Moisture transport from the Tethys Ocean into the German Basin is implied by δ18O depletion trends of isochronous dolomite beds from the margin to mid-playa position, suggesting dominantly Rayleigh-like circulation patterns. Distinct covariance of δ18O/δ13C is used as indicator of lake-level stands. The K/Al curve, with inverse covariation compared to δ18O, reflects fluctuating weathering conditions on the fan and the Vindelician High. A hierarchic cycle stacking pattern composed of metre-scale to tens of metre cycles is interpreted to be a result of changing Pangaean monsoon-like intensities in the Milankovitch frequency band. Spectral analysis shows that the power spectra are dominated by precession and eccentricity.
Biozones of the Lake City Formation, Avon Park Formation and Ocala Limestone are characterized by interbedded, massive, fossiliferous carbonate rocks (wackestones to grainstones) and thinly bedded, peloidal and carbonaceous rocks (mudstones and wackestones). Several horizons have been partly or completely dolomitized.A wide range of early to late stages of dolomitic fabrics are recognized. The fabrics are classified descriptively as equigranular (unimodal) or inequigranular (multimodal). Fabrics composed of crystals < 0.002 mm in diameter (unresolvable) are termed aphanotopic. Equigranular fabrics include sutured mosaic and sieve mosaic fabrics, and a somewhat problematic peloidal fabric. Inequigranular fabrics include porphyrotopic, poikilotopic, fogged mosaic and spotted mosaic fabrics.Two processes of dolomitization are suggested: (a) homogeneous dolomitization resulting in a single-stage development of microtextured (groundmass crystals < 0.016 mm in diameter) aphanotopic, peloidal and mosaic fabrics; and (b) heterogeneous dolomitization resulting in multistage development of porphyrotopic and some mosaic fabrics. It is possible to deduce the original depositional fabric and lithofacies from the dolomitic fabric except where extensive neomorphism has occurred.
A structure refinement method is described which does not use integrated neutron powder intensities, single or overlapping, but employs directly the profile intensities obtained from step-scanning measurements of the powder diagram. Nuclear as well as magnetic structures can be refined, the latter only when their magnetic unit cell is equal to, or a multiple of, the nuclear cell. The least-squares refinement procedure allows, with a simple code, the introduction of linear or quadratic constraints between the parameters.
The diagenetic history of the Late Triassic Dolomia Principale tidal-flat complex is reconstructed utilizing transmission and analytical electron microscopic techniques and comparison with modern Abu Dhabi dolomite analogues. The Dolomia Principale, which completely dolomitized peritidal and subtidal cycles, shows five dolomite texture types with different stable isotope values. The early products of shallow subsurface dolomitization are preserved, and are calcian and characterized by fine, pervasive modulated microstructures. Texturally, microstructurally and geochemically similar dolomites are observed in subtidal facies of Abu Dhabi, generally replacing aragonites and Mg-calcites. The subsequent dolomitization history of the Dolomia Principale continues with the precipitation of progressively more ideal dolomites, with coarse modulated microstructures or ribbon structures. Ideal dolomites, generally characterizing void-filling crystals or the outer part of matrix-replacive as well as mimetic dolomites, show only a few dislocations. Dislocation networks characterize the highest-temperature dolomites. A considerable spread of δ18O values (+3.3‰ to below –4‰) characterizes ‘matrix’ dolomite crystals with different chemical compositions and microstructures coexisting in the same crystal. This spread is considered to be the result of an admixture of signals coming from calcian dolomites and ideal ones precipitated later in the diagenetic history. Calcian dolomites, slightly Ca dolomite, some ideal dolomite ‘overgrowths’ and void fillings with positive δ18O still form during the same cycle. δ18O-depleted ideal dolomites formed during burial. The dolomitizing fluids were provided by the cyclic subaerial exposures to which the Dolomia Principale was subjected, allowing saline fluid circulation through pores and fractures.
The Travenanzes Formation is a terrestrial to shallow-marine, siliciclastic–carbonate succession (200 m thick) that was deposited in the eastern Southern Alps during the Late Triassic. Sedimentary environments and depositional architecture have been reconstructed in the Dolomites, along a 60 km south–north transect. Facies alternations in the field suggest interfingering between alluvial-plain, flood-basin and shallow-lagoon deposits, with a transition from terrestrial to marine facies belts from south to north. The terrestrial portion of the Travenanzes Formation consists of a dryland river system, characterized by multicoloured floodplain mudstones with scattered conglomeratic fluvial channels, merging downslope into small ephemeral streams and sheet-flood sandstones, and losing their entire discharge subaerially before the shoreline. Calcic and vertic palaeosols indicate an arid/semi-arid climate with strong seasonality and intermittent discharge. The terrestrial/marine transition shows a coastal mudflat, the flood basin, which is usually exposed, but at times is inundated by both major river floods and sea-water storm surges. Locally coastal sabkha deposits occur. The marine portion of the Travenanzes Formation comprises carbonate tidal-flat and shallow-lagoon deposits, characterized by metre-scale shallowing-upward peritidal cycles and subordinate intercalations of dark clays from the continent. The depositional architecture of the Travenanzes Formation suggests an overall transgressive pattern organized in three carbonate–siliciclastic cycles, corresponding to transgressive–regressive sequences with internal higher-frequency sedimentary cycles. The metre-scale sedimentary cyclicity of the Travenanzes Formation continues without a break in sedimentation into the overlying Dolomia Principale. The onset of the Dolomia Principale epicontinental platform is marked by the exhaustion of continental sediment supply.
Dolomite [Ca,Mg(CO3)2] precipitation from supersaturated ionic solutions at Earth surface temperatures is considered kinetically inhibited because of the difficulties experienced in experimentally reproducing such a process. Nevertheless, recent dolomite is observed to form in hypersaline and alkaline environments. Such recent dolomite precipitation is commonly attributed to microbial mediation because dolomite has been demonstrated to form in vitro in microbial cultures. The mechanism of microbially mediated dolomite precipitation is, however, poorly understood and it remains unclear what role microbial mediation plays in natural environments. In the study presented here, simple geochemical methods were used to assess the limitations and controls of dolomite formation in Deep Springs Lake, a highly alkaline playa lake in eastern California showing ongoing dolomite authigenesis. The sediments of Deep Springs Lake consist of unlithified, clay-fraction dolomite ooze. Based on δ18O equilibria and textural observations, dolomite precipitates from oxygenated and agitated surface brine. The Na-SO4-dominated brine contains up to 500 mm dissolved inorganic carbon whereas Mg2+ and Ca2+ concentrations are ca 1 and 0·3 mm, respectively. Precipitation in the subsurface probably is not significant because of the lack of Ca2+ (below 0·01 mm). Under such highly alkaline conditions, the effect of microbial metabolism on supersaturation by pH and alkalinity increase is negligible. A putative microbial effect could, however, support dolomite nucleation or support crystal growth by overcoming a kinetic barrier. An essential limitation on crystal growth rates imposed by the low Ca2+ and Mg2+ concentrations could favour the thermodynamically more stable carbonate phase (which is dolomite) to precipitate. This mode of unlithified dolomite ooze formation showing δ13C values near to equilibrium with atmospheric CO2 (ca 3‰) contrasts the formation of isotopically light (organically derived), hard-lithified dolomite layers in the subsurface of some less alkaline environments. Inferred physicochemical controls on dolomite formation under highly alkaline conditions observed in Deep Springs Lake may shed light on conditions that favoured extensive dolomite formation in alkaline Precambrian oceans, as opposed to modern oceans where dolomites only form diagenetically in organic C-rich sediments.
The Coorong Region of South Australia is a classic location for the study of modern dolomite formation. A number of ephemeral lakes here contain dolomite, whilst other lakes in close proximity, contain aragonite with minor magnesite. Our previous field and experimental work has revealed that sulphate-reducing bacteria play a key role in overcoming kinetic inhibitors to dolomite formation in this area. We here use a combination of oxygen, carbon and sulphur isotopes to help to further constrain the processes leading to dolomite precipitation in modern hypersaline environments.