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Examples of encrustation relationships depicted in Fig. 5. Cincinnatian. & A. Encrusted mould wall. & B. Encrusted boring cast. & C. Encrusted face of calcite outer shell layer. & D. Revealed attachment face colonized by later encruster. Width of all views = 30 mm.
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The Ordovician was a time of extensive and pervasive low-magnesium calcite (LMC) precipitation on shallow marine sea floors. The evidence comes from field study (extensive hardgrounds and other early cementation fabrics in shallow-water carbonate sequences) and petrography (large volumes of marine calcite cement in grainstones). Contemporaneous sea...
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... If an aragonite shell was encrusted by a calcitic epifauna (usually bryozoans, which cover a large surface area), the attachment faces (undersides) of this epifauna became exposed after dissolution of the shell. The attachment face was in turn directly encrusted by post- dissolution encrusters, so that the pre-and post- dissolution generations of epifauna are directly ven- trally adpressed (Figs 5B5, 6D). ...
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Citations
... Other carbonate minerals, such as manganese carbonates, have been observed as precipitates on sedimentary foraminiferal surfaces (Boyle, 1983). Calcite precipitation triggered by aragonite dissolution may also contribute to preserving relics of sedimentary aragonite structures, such as skeletons of nautilids or ammonites, via the precipitation of molds (Bruni & Wenk, 1985;Janiszewska et al., 2018;Palmer & Wilson, 2004). As aragonite dissolves, dissolution products simultaneously recrystallize into more stable phases, constituting a multiphase dynamic equilibrium. ...
Carbon dioxide entering and acidifying the ocean can be neutralized by the dissolution of calcium carbonate, which is mainly found in two mineral forms. Calcite is the more stable form and is often found in deep‐sea sediments, whilst aragonite is more soluble and therefore rarely preserved. Recent research shows aragonite may account for a much larger portion of marine calcium carbonate export to the ocean interior via the biological pump than previously thought, and that aragonite does reach the deep sea and seafloor despite rarely being buried. If aragonite is present and dissolving at the seafloor it will raise local pH and calcium and carbonate concentrations, potentially enough to inhibit calcite dissolution, representing a deep‐sea, carbonate version of galvanization. Here, we test this hypothesis by simulating aragonite dissolution at the sediment‐water interface in the laboratory and measuring its effects on pH using microsensors. We show that the addition of aragonite to calcite sediment, overlain by seawater undersaturated with respect to both minerals, results in an unchanged alkalinity flux out of the dissolving sediment, suggesting a decrease the net dissolution rate of calcite. In combination with a diagenetic model, we show that aragonite dissolution can suppress calcite dissolution in the top millimeters of the seabed, locally leading to calcite precipitation within 1 day. Future research efforts should quantify this galvanization effect in situ, as this process may represent an important component of the marine carbon cycle, assigning a key role to aragonite producers in controlling ocean alkalinity and preserving climatic archives.
... The prevalence of hardgrounds during the Ordovician, which were less common in the preceding Cambrian times, may have been due to (i) the high sea levels and extensive cratonic flooding and (ii) the global cooling, which caused shallowing of the aragonite and calcite compensation depths (Taylor and Wilson 2003). The latter permitted dissolution of aragonite and extensive and pervasive precipitation of secondary low-magnesium calcite cements on shallow-water marine seafloors, as in the case of the studied hardgrounds Wilson and Palmer 1992;Taylor and Wilson 2003;Palmer and Wilson 2004). ...
The Early-Mid Ordovician Kheneg el Aatène Formation of the Ougarta Range is here divided into two members: (i) the Aatène Micaceous Sandstone Member and (ii) the Aatène Quartzite Member. A low diversity trace fossil assemblage is present in the upper part of the second member with abundant Skolithos linearis, Skolithos isp. and Rosselia erecta, common Arenicolites isp. and Diplocraterion isp. and rare Palaeophycus isp. and ?Rusophycus isp. This association is characteristic to the Skolithos ichnofacies and indicates high-energy conditions and opportunistic colonisation within a siliciclastic shallow marine environment. The ichnoassemblage is dominated by vertical traces which occur in great densities forming typical Skolithos pipe-rocks. The pipe-rock ichnofabric is characterised by burrows that are closely spaced and occur at 20 distinct levels. Possible predation has been detected in the same Member due to the presence of ?Rusophycus intersecting worm burrows. High densities of Trypanites are present in two hardgrounds composed of calcareous lithified sandstones developed during transgressive pulses that can be related to the brevis event. Trypanites borings are evidenced for the first time from inorganic hard substrates of the Gondwanan Ordovician. These borings can be related to beginning of the Ordovician Bioerosion Revolution in Gondwana.
... The latter features are commonly interpreted to be the result of incursion of undersaturated meteoric waters (e.g. Lohmann, 1982Lohmann, , 1987Palmer and Wilson, 2004). Rounded shapes of the moldic pores without the presence of skeletal structure suggests that the dissolved allochems were ooids (Morad et al., 2019b) or echinoderms (Scholle and Ulmer-Scholle, 2003), which are composed of HMC with high MgCO 3 solid solution (up to 3-43.5 mol%; Schroeder et al., 1969), i.e., more soluble. ...
Abstract
Petrography, petrophysics, and geochemistry of an Upper Cretaceous, foreland-basin carbonate reservoir, Abu Dhabi, United Arab Emirates, are used to constrain the spatio-temporal variations in the extent of dolomitization and its impact on reservoir quality. Dolomitization in highstand systems tracts (HST) is attributed to repeated tidal and evaporative pumping as well as seepage reflux of penesaline brines during restriction of the platform due to 4th and 5th order cycles of relative sea-level fall and particularly below parasequence boundaries. This interpretation is supported by the presence of rare poikilotopic gypsum cement and scattered laths and micro-nodules of calcitized gypsum in the dolomitized peritidal dolostones. Dolomitization along bioturbation sites, which is most common in the transgressive systems tracts (TST), is attributed to the development of suitable localized geochemical conditions (e.g., microbial sulfate reduction and related increase in carbonate alkalinity). Porosity and permeability of the dolostones are strongly controlled by depositional textures of precursor limestones and by subsequent diagenetic evolution. Dolomitization of mudstones, wackestones and matrix-rich packstones has resulted in the formation of micropore-dominated microcrystalline dolostones whereas dolomitization of the shoal grainstones resulted in the formation of coarse-crystalline dolostones with abundant well-connected intercrystalline and moldic macropores. Mesogenetic alterations of the dolostones, which are attributed to the flow of hot basinal brines along steep faults, include dolomite cementation and, subsequently, dissolution and calcitization of dolomite (dedolomitization) and calcite cementation. The lack of systematic differences in porosity and permeability of dolostones between the oil and water-saturated dolostones suggest that most diagenesis occurred prior to completion of oil emplacement and/or reflect the shallow maximum burial depths of the formation (around 1.3 km). This study demonstrates that variations in the distribution and extent of dolomitization within a sequence stratigraphic context across an oilfield should be considered as primary control on the spatio-temporal reservoir lithology and heterogeneity in carbonate successions.
... The steinkern of the endoceratid formed during early lithification in the sediment within the calcareous mud on the seafloor. The Ordovician was a time of extensive low-magnesium calcite precipitation on shallow sea floors (Palmer and Wilson 2004), and rapid sea-floor cementation enhanced early lithification of the nautiloid siphuncle. Later the lithified steinkern of the siphuncle was likely exhumed and stayed exposed on the seafloor for some time. ...
A steinkern of an endoceratid nautiloid siphuncle contains a Trypanites sozialis boring with a lingulate brachiopod Rowellella sp. shell inside. The steinkern of this endoceratid formed during early lithification of the sediment on the seafloor. The lithified steinkern of this siphuncle was either initially partially exposed to the seawater or was exhumed and stayed exposed on the seafloor, where it was colonized by boring organisms. This bioerosion resulted in numerous Trypanites borings in the siphuncle. After the death or exit of the Trypanites trace maker, a vacant boring was colonized by a small lingulate nestler Rowellella sp. This lingulate was likely preadapted to life in hard substrate borings when it first found its way into borings in living substrates in the Late Ordovician. The increased availability of hard substrate borings, combined with the increased predation pressure due to the GOBE, enhanced the colonization of hard substrate borings by lingulate brachiopods.
... The latter features are commonly interpreted to be the result of incursion of undersaturated meteoric waters (e.g. Lohmann, 1982Lohmann, , 1987Palmer and Wilson, 2004). Rounded shapes of the moldic pores without the presence of skeletal structure suggests that the dissolved allochems were ooids (Morad et al., 2019b) or echinoderms (Scholle and Ulmer-Scholle, 2003), which are composed of HMC with high MgCO 3 solid solution (up to 3-43.5 mol%; Schroeder et al., 1969), i.e., more soluble. ...
Petrography, petrophysics, and geochemistry of an Upper Cretaceous, foreland-basin carbonate reservoir, Abu Dhabi, United Arab Emirates, are used to constrain the spatio-temporal variations in the extent of dolomitization and its impact on reservoir quality. Dolomitization in highstand systems tracts (HST) is attributed to repeated tidal and evaporative pumping as well as seepage reflux of penesaline brines during restriction of the platform due to 4th and 5th order cycles of relative sea-level fall and particularly below parasequence boundaries. This interpretation is supported by the presence of rare poikilotopic gypsum cement and scattered laths and micro-nodules of calcitized gypsum in the dolomitized peritidal dolostones. Dolomitization along bioturbation sites, which is most common in the transgressive systems tracts (TST), is attributed to the development of suitable localized geochemical conditions (e.g., microbial sulfate reduction and related increase in carbonate alkalinity). Porosity and permeability of the dolostones are strongly controlled by depositional textures of precursor lime-stones and by subsequent diagenetic evolution. Dolomitization of mudstones, wackestones and matrix-rich pack-stones has resulted in the formation of micropore-dominated microcrystalline dolostones whereas dolomitization of the shoal grainstones resulted in the formation of coarse-crystalline dolostones with abundant well-connected intercrystalline and moldic macropores. Mesogenetic alterations of the dolostones, which are attributed to the flow of hot basinal brines along steep faults, include dolomite cementation and, subsequently, dissolution and calcitization of dolomite (dedolomitization) and calcite cementation. The lack of systematic differences in poros-ity and permeability of dolostones between the oil and water-saturated dolostones suggest that most diagenesis occurred prior to completion of oil emplacement and/or reflect the shallow maximum burial depths of the formation (around 1.3 km). This study demonstrates that variations in the distribution and extent of dolomitization within a sequence stratigraphic context across an oilfield should be considered as primary control on the spatio-temporal reservoir lithology and heterogeneity in carbonate successions.
... Instead, a diagenetic origin such as that reported by Jackson, Borradaile, et al. (1993) and Jackson, Rochette, et al. (1993) in Paleozoic carbonates is more plausible. In contrast, the presence of intraformational conglomerate in the undeformed footwall Snowy Range Formation (e.g., Palmer & Wilson, 2004) points to a more dynamic environment of deposition for which a detrital origin of pyrrhotite cannot be ruled out. A third possible origin for pyrrhotite through fluid flow-driven thermochemical sulfate reduction, as suggested by Zechmeister et al. (2012) in Mississipian carbonates in Alberta, is unlikely considering the lack of microstructural evidence for fluid flow. ...
The Heart Mountain Slide in Wyoming is one of the largest known terrestrial gravity slides (3,500 km²) formed ∼49 Ma ago by the nearly horizontal detachment of Paleozoic‐Eocene cover sliding on top of autochthonous formations. At the White Mountain locality, exposures offer an exceptional opportunity to investigate high strain rate/high velocity processes in carbonates. Here we use the anisotropy of magnetic susceptibility (AMS) of 274 samples to shed light on ultracataclastic deformation along this detachment. Contrary to predictions, the carbonate ultracataclasite displays a consistent AMS fabric, particularly in the upper ultracataclasite. The AMS in this unit is controlled primarily by magnetite formed through the breakdown of iron sulfides caused by frictional heating. Additional thermomagnetic experiments reveal that the new magnetic fabric began forming ∼250ºC and continued up to ∼400ºC when calcination of carbonate minerals caused a major drop in friction. The main cataclastic slip direction inferred from AMS is ∼N033°, at odds with the previously accepted NNW‐SSE direction. We validate these AMS fabrics through 3D shape preferred orientation analysis and micro X‐ray scanning of the same specimens. These results, however, may only represent cataclastic flow directions at the local scale as a result of synkinematic rotation of the White Mountain block. Alternatively, these results may call for a re‐evaluation of the large scale movement of the slide. Finally, this study demonstrates the usefulness of a magnetic approach in deciphering deformation processes in carbonates, particularly in high strain rate cases such as seismic faults.
... As argued above, the two principle controls on shallow marine porewater saturation states are: 1) the saturation state of the ocean, and 2) the degree of organic matter oxidation in the shallow-marine pore system. Undoubtedly, marine carbonate saturation states have fluctuated over time (e.g., Arvidson et al. 2011), as is borne out in petrographic studies showing high degrees of early dissolution in some ancient warm-water systems (e.g., Palmer and Wilson 2004;Cherns and Wright 2009). In warm-water settings of the modern icehouse system, with its relatively high marine carbonate saturation states, marine porewaters consistently remain supersaturated with respect to aragonite and calcite, based on the cements produced in these systems (see above). ...
... They interpreted that such deep-water deposits were less apt to be replaced by dolomite than shallow-water equivalents due to the inhibiting effects of lower seawater temperatures on dolomite precipitation. Significant formation of platform dolomite remains associated with greenhouse modes and calcite seas, which are broadly interpreted to have had lower carbonate saturation states relative to icehouse periods (Given and Wilkinson 1987;Wilkinson and Algeo 1989;Mackenzie and Morse 1992;Arvidson et al. 2011;Mackenzie and Andersson 2013), as supported by petrographically based diagenetic studies (Palmer and Wilson 2004;Cherns and Wright 2009), although it must be admitted that others have concluded the opposite (Riding and Liang 2005). ...
The “dolomite problem” is the product of two distinct observations. First, there are massive amounts of ancient marine limestone (CaCO3) deposits that have been replaced by the mineral dolomite (MgCa(CO3)2). However, recent (Holocene and Pleistocene) marine deposits contain relatively minuscule amounts of dolomite, although the occurrence of small quantities of dolomite is observed in many modern settings, from deep marine to supratidal. Second, low-temperature synthesis of dolomite in laboratory settings has been elusive, particularly in comparison to the ease with which common marine calcium carbonate minerals (aragonite and calcite) can be synthesized. Since low-temperature solid-state diffusion can be discounted as a method for Mg incorporation into calcium carbonate (as it operates on time scales too long to matter), the replacement of CaCO3 by dolomite is one of dissolution followed by precipitation. Therefore, an often overlooked but required factor in the replacement of limestone by dolomite is that of undersaturation regarding the original calcium carbonate mineral during replacement. Such conditions could conceivably be caused by rapid dolomite growth relative to aragonite and calcite dissolution–precipitation reactions, but laboratory studies, modern systems analyses, and observations of ancient deposits all point to this possibility being uncommon because dolomite growth is kinetically inhibited at low temperature. Pressure solution by force of dolomite crystallization is a second possible driver for CaCO3 undersaturation, but requires a confining stress most likely attained through burial. However, based on petrographic observations, significant amounts of ancient dolomite replaced limestone before burial (synsedimentary dolomite), and many such platforms have not suffered any significant burial. Because these possibilities of undersaturation caused by dolomite precipitation and crystal growth can be largely discounted, the undersaturation required for “dolomitization” to proceed is most likely to be externally forced. In modern natural systems, undersaturation and selective CaCO3 dissolution in marine porewaters is very common, even in warm-water environments, being forced by the breakdown of organic matter. Such dissolution is frequently attended, to varying degrees, by precipitation of a kinetically-less-favored but thermodynamically more stable phase of CaCO3. Laboratory studies as well as observations of modern systems show that when undersaturation is reached with respect to all common marine CaCO3 phases, dolomite assumes the role of this kinetically-less-favored precipitate. This degree of undersaturation is uncommon in modern shallow marine pore systems in warm-water settings, but it was more common during times of elevated atmospheric CO2, and ocean acidification. Furthermore, because oxidation of organic matter drives dolomite formation, near-surface organic-rich deposits such as the remains of microbial mat communities, were more predisposed to dolomite replacement in the acidified oceans of the ancient past relative to contemporaneous deposits that contained less organic matter. These observations lend to a more harmonious explanation for the abundance and occurrence of dolomite through time.
... The Ordovician was a period of extensive early cementation occurring near the sediment-seawater interface on shallow-marine seafloors (Palmer and Wilson 2004). Early cemen tation depends on several prerequisites. ...
... Aragonite shells are considered as rapidly dissolved in Jurassic calcite seas after the death of skeletal organisms (Palmer et al., 1988;Palmer and Wilson, 2004 The neomorphic replacement of skeletal aragonite was considered as occurred during meteoric or burial diagenesis and characterized by a one-step process of low-Mg calcite crystallization (Sandberg and Hudson, 1983;Martin et al., 1986;Maliva and Dickson, 1992) or multiple stages of neomorphic replacement (Al-Aasm and Veizer, 1986b;Hendry et al., 1995). ...
Dans un contexte de changement climatique et de pression croissante sur la ressource en eau des aquifères carbonatés, une meilleure compréhension du fonctionnement et de la recharge des réservoirs est nécessaire afin d'optimiser leur exploitation. Une caractérisation sédimentaire et diagénétique des calcaires s'avère indispensable. Ce travail pluridisciplinaire se concentre sur les roches carbonatées du Jurassique moyen et de l'Oxfordien de la bordure nord-est du Bassin Aquitain. Le premier objectif est de quantifier l'influence couplée des fluctuations climatiques à long terme et de la création d'accommodation sur les producteurs carbonatés et l'accumulation de carbonates. Le second objectif est de reconstruire l'histoire diagénétique et paléohydrologique du bassin et de proposer une méthode pour dater les phases de dissolution et les périodes de création de porosité dans les calcaires. Les études menées dans le Bassin Aquitain et l'ouest de la France montrent que l'augmentation du taux de création d'accommodation provoque (1) l'augmentation du taux d'accumulation de carbonates et (2) des changements de producteurs carbonatés, quel que soit le climat. En période de climat aride, la création d'espace disponible est comblée par une production micritique-microbienne. En climat humide, les taux d'accumulation de carbonate diminuent drastiquement et provoque le déclin des producteurs oolithiques, photozoan et micrite-microbien. La combinaison d'observation pétrographique et de la méthode géochronologique U-Pb permet de dater les phases de dissolution et de création de pores, mais également d'établir une paragenèse à grande échelle. Les processus de néomorphisme et dolomitisation sont datés au cours de la période Jurassique supérieur à Crétacé inférieur. Les processus de dissolution postérieure affectent préférentiellement les corps dolomitiques bajociens. Les âges U-Pb mettent en évidences une succession de phases de dissolution-recristallisation au Paléocène-Éocène. Les phases de cimentation calcitique sont synchrones des processus de dissolution lors de périodes de karstification et peuvent se produire en profondeur formant et comblant des pores vacuolaires de taille millimétriques à métriques.
... The aragonite loss is interpreted to 324 be related to organic decomposition and oxidation of H2S in oxic conditions, which produce 325 CO2 and H2SO4 that reduce porewater pH for aragonite dissolution(Walter et al., 1993; Hu 326 and Burdige, 2008;Jordan et al., 2015). Furthermore, the Cretaceous period corresponds to a 327 calcite sea with lower carbonate saturation, which favors precipitation of Mg calcite and 328 aragonite dissolution(Sandberg, 1985;Palmer and Wilson, 2004).329 In the study, the aragonitic grains (green algae and aragonitic bivalves) were of shallowmarine origin (Aqrawi et al., 2010; Mahdi and Aqrawi, 2014), and their increased abundance 331 in Mi-4 and Mi-3 may be explained by enhanced transportation and re-deposition of 332 shallow-marine aragonitic grains towards deep-marine slope settings. ...
