Article

Stratigraphic shifts in carbon isotopes from Proterozoic stromatolitic carbonates (Mauritania): influences of primary mineralogy and diagenesis

January 1990
American Journal of Science 290:46-79

January 1990
290:46-79

Authors:

Ian Fairchild

University of Birmingham

Jim Marshall

University of Liverpool

In the Atar Group of Mauritania (about 900 Ma), a shift from positive (+0.3 to +2.8 permil) to variably negative (-0.2 to -3.3 permil) carbon isotope values of carbonate carbon occurs between Formation I-5 with its Conophyton-Jacutophyton biostromes and Formation I-6 in which the stromatolites display tussocky microstructure. No biologically-related 13C fractionations can be recognized by comparison of stromatolitic micrites with marine cements so in principle stromatolites seem suitable for studies of secular change in δ13C. Diagenesis of the initially metastable marine Proterozoic precipitates poses the main problem for such studies. A suggested strategy for isotope stratigraphy is to seek out deliberately the diagenetic patterns of alteration and to correct for them. -from Authors

Palaeoproterozoic magnesite from playa/sabkha environments

Chapter

Apr 2022

Stromatolites in Precambrian Carbonates: Evolutionary Mileposts or Environmental Dipsticks?

Article

May 1999

Stromatolites are attached, lithified sedimentary growth structures, accretionary away from a point or limited surface of initiation. Though the accretion process is commonly regarded to result from the sediment trapping or precipitation-inducing activities of microbial mats, little evidence of this process is preserved in most Precambrian stromatolites. The successful study and interpretation of stromatolites requires a process-based approach, oriented toward deconvolving the replacement textures of ancient stromatolites. The effects of diagenetic recrystallization first must be accounted for, followed by analysis of lamination textures and deduction of possible accretion mechanisms. Accretion hypotheses can be tested using numerical simulations based on modern stromatolite growth processes. Application of this approach has shown that stromatolites were originally formed largely through in situ precipitation of laminae during Archean and older Proterozoic times, but that younger Proterozoic stromatolites grew largely through the accretion of carbonate sediments, most likely through the physical process of microbial trapping and binding. This trend most likely reflects long-term evolution of the earth’s environment rather than microbial communities.

??13C stratigraphy of the Proterozoic Bylot Supergroup, Baffin Island, Canada: Implications for regional lithostratigraphic correlations

Article

Mar 1999

The Bylot Supergroup, northern Baffin Island, contains >1500 m of platform, shelf, and slope carbonates deposited between ~1270 and ~723 Ma. Limited chronostratigraphic data have led to the broad correlation of the Bylot Supergroup with predominantly Neoproterozoic successions in northern and western Laurentia; yet, detailed correlation has been impossible given biostratigraphic and lithostratigraphic limitations. Carbon-isotope chemostratigraphy represents a potential dataset to constrain such interregional correlations. Carbon isotopic data from the Bylot Supergroup and broadly coeval successions from Somerset Island and northwest Greenland reveal distinct stratigraphic trends in δ13C, with intervals of moderate 13C enrichment (+3.5 ± 1‰) punctuated by excursions to slightly negative values (-1.0 ± 1‰). Although the scale of the observed variation is muted relative to Neoproterozoic standards, the dissimilarity of values to those recorded in northwestern Laurentia suggests that these strata delineate a discrete depositional interval. Comparison of isotopic values with published data indicates that δ13C values between approximately -1.0 and +4.0‰ are characteristic of the interval between ~1300 and ~800 Ma. This pattern is distinct from that of younger Neoproterozoic successions, which typically record values >+5‰, and older Mesoproterozoic successions, which record values near 0‰, and suggests that these moderately positive values may be useful for broad time correlation. Compilation of new and published data permits the tentative reconstruction of a global Mesoproterozoic carbon isotopic curve.

Geochemical and stable isotope signatures of Proterozoic stromatolitic carbonates from the Vempalle and Tadpatri Formations, Cuddapah Supergroup, India: Implications on Paleoenvironment and depositional conditions

Article

Jun 2017
PRECAMBRIAN RES

Spectacular stromatolitic dolomites with varied morphological features are preserved in the Vempalle and Tadpatri Formations of the Proterozoic Cuddapah basin, eastern Dharwar Craton. They exhibit stratifera, columnar, conical, domal and spheroidal/lobate morphologies in which columns are branched at some places (in Tadpatri Formation). Based on their trace elements such as REY and stable isotope compositions (carbon and oxygen) the Vempalle stromatolites have been classified as three types. Vempalle I stromatolites exhibit pronounced positive Eu anomalies with small scale positive Ce anomalies whereas Vempalle II counterparts have slight negative to positive Eu, feeble positive Ce and positive Gd anomalies. Vemaplle III stromatolites display positive Ce, Eu and Gd anomalies. The Tadpatri Formation stromatolites have pronounced Gd anomalies with low order positive Ce and Eu anomalies. Significant depletion in δ¹⁸O is observed in Vempalle ranging from – 10.70‰ to -7.07‰ and 20.73‰ to -11.87‰ in Tadpatri Formations. The Tadpatri stromatolites display negative δ¹³C varying from -4. 56‰ to -1.26‰. Contrarily, the Vempalle stromatolites recorded both positive and negative δ¹³C ranging from -0.24‰ to 2.00‰ thereby reflecting their deviation from the popular belief of the boring billion concept of displaying Midproterozoic δ¹³C stasis. The flat REE patterns along with Ce, Eu and Gd anomalies, chondritic to superchondritic Y/Ho ratios collectively reflect on marine water conditions for their deposition with variable inputs of siliciclastic/terrigenous materials and hydrothermal signatures. These cherty dolomitic stromatolites were deposited in subtidal-intertidal-supratidal zones of a shallow shelf of Proterozoic open ocean under warm, alkaline and anoxic conditions.

Sedimentological perspectives on climatic, atmospheric and environmental change in the Neoproterozoic Era

Article

Full-text available

Feb 2016
SEDIMENTOLOGY

Sedimentological insights underpin many of the important recent advances in understanding of Earth system behaviour in the Neoproterozoic Era. This article reviews three main areas: (i) chemical proxies and their preservation, with emphasis on carbonate facies; (ii) glacial and post-glacial facies, including their age constraints; III) sedimentary evidence for biotic innovations and responses. Chemostratigraphy plays an important role in ordering Neoproterozoic events and defining disturbances to the carbon cycle. There is increasing attention being paid to assessing the role of diagenetic origination or modification of chemostratigraphic signals. Alongside this, new criteria for identifying primary dolomite and precursor metastable phases such as ikaite have been developed. In respect of oxygenation, geochemical proxies substantiate the concept of a Neoproterozoic Oxygenation Event as a very gradual transition, the ocean being at any one time a heterogeneous assemblage of ferruginous, sulphidic and oxic conditions, with some evidence of increasing deep-sea oxidation through the Ediacaran Period. Techniques such as Fe-speciation need to be supplemented with proxies sensitive to suboxic conditions. More generally, it is predicted that petrographically constrained microanalytical studies will also become more important in reconstructing palaeoenvironmental conditions. The global distribution of Neoproterozoic glacial deposits combined with palaeomagnetic evidence supports the concept of panglaciations in which ice sheets reached sea-level in the tropics. Advances in radiometric dating have demonstrated the synchronous onset of global (Sturtian) glaciation at 717 Ma and the demise of a second (Marinoan) glaciation at 635 Ma and plausibly indicate long durations for each (55 Ma for Sturtian and 5 to 15 My for Marinoan). However, a compilation of radiometric dates indicates ambiguities indicating the need for further improvements to the radiometric and Sr-isotope database to understand events within the Sturtian time frame, the timing of onset of Marinoan glaciations, and age and synchroneity of individual negative δ13C anomalies. Sturtian deposits are typically thick, rift-related successions containing a range of environments influenced or dominated by dynamic glaciers, as well as ice-free marine intervals. Marinoan glacial deposits, by contrast, tend to be thin and continental. During the latter interval, oxygen isotope systematics of sulphate demonstrate that atmospheric CO2 was high as predicted by Snowball Earth theory, and that sedimentation was influenced by orbital forcing. The Sturtian record, by comparison, needs to be searched for evidence of cold-climate hiatuses on the one hand and orbital forcing on the other. Cap carbonate formation appears to have coincided with rising sea-levels following panglaciations. Snowball theory considers that they formed rapidly in the postglacial greenhouse, but an alternative model of slower formation with clastic sediment starvation during transgression may prove to be consistent with new data and models showing extensive glacier terminations on land. For all facies, but especially caps, the use of microanalytical techniques and holistic studies of petrogenesis are future priorities. There are important discrepancies between molecular clock predictions of early metazoan origination and the hard evidence from the sedimentary records which largely depend on local exceptional preservation by early diagenesis. A variety of life survived panglaciation, and there is little evidence that glaciations directly caused oxygenation or stimulated evolution. This article is protected by copyright. All rights reserved.

The Yudoma group of stratotype area: C-isotope chemostratigraphic correlations and Yudomian-Vendian relation

Article

Full-text available

Sep 2004
STRATIGR GEO CORREL+

Representative C-isotope characteristics are first obtained for the Lower Yudomian carbonates (YuKanda Formation) from the Belaya and Sakhara sections in the Yudomian stratotype area. The least altered carbonate samples that elucidate further C-isotope chemostratigraphy of formerly studied Upper Yudomian interval were selected based on strict geochemical criteria (Mn/Sr ≤ 4, Fe/Sr ≤ 10.0, δ 18O > -10‰ in limestones and Mn/Sr ≤ 6. Fe /Sr <15δ18O > -10‰ in dolostones). In the studied samples, δ13C values demonstrate the in-phase variations of comparable amplitude at the correlative levels of both sections. They are used to plot the generalized curve of δ 13C variations in the Lower Yudomian succession that is divisible, based on respective trends, into intervals corresponding to three subformations of the Yukanda Formation. Characteristic of the Yalan Subformation, the lower one, are prevailing high positive δ 13C values. In the Mal Subformation (middle one), this parameter declines down to moderate positive values with two short negative excursions complicating the general trend. Low negative δ 13C values are typical of the Tokur Subformation that is upper in the succession. By means of C-isotope chemostratigraphy, biostratigraphy and isotopic-geochronological data, the Yudomian type sections are correlated with terminal Precambrian successions of North Siberia, Namibia, and Oman, and the resulting inferences are as follows. (1) The pre-Nemakit-Daldynian intervals of Vendian successions in Siberia are compatible in their C-isotope characteristics with the reference chemostratigraphic curve (Saylor et al., 1998) and add to it from below the interval with high positive δ13C values, the lower boundary of which is at the level of inversion from positive to negative values in the cap carbonates of the lower Varangerian tillites. (2) In the stratotype area, the Lower and Upper Yudomian deposits are separated by the regional hiatus (time span of 4 to 4.5 m.y.). (3) Judging from δ13C values, basal horizons of Yudomian carbonates are stratigraphically higher than the cap dolomites of the lower Varangerian tillites, and consequently the Yudomian carbonate succession is less than Vendian in stratigraphic range. (4) The direct evidence of age relation between the Vendian and basal Yudomian terrigenous deposits is unknown, but indirect considerations suggest that the latter accumulated during the Early Vendian deglaciation. (5) Negative excursions of δ13C values, which are recorded in the Mal Formation of Uchur-Maya region and near the Kufai Formation top in Oman, presumably reflect events related to the late Varangerian glaciation.

Continental carbonate facies of a Neoproterozoic panglaciation, north-east Svalbard

Article

Full-text available

Nov 2015
SEDIMENTOLOGY

The Marinoan panglaciation (ca 650 to 635 Ma) is represented in north-east Svalbard by the 130 to 175 m thick Wilsonbreen Formation which contains syn-glacial carbonates in its upper 100 m. These sediments are now known to have been deposited under a CO2-rich atmosphere, late in the glaciation, and global climate models facilitate testing of proposed analogues. Precipitated carbonates occur in four of the seven facies associations identified: Fluvial Channel (including stromatolitic and intraclastic limestones in ephemeral stream deposits); Dolomitic Floodplain (dolomite-cemented sand and siltstones, and microbial dolomites); Calcareous Lake Margin (intraclastic dolomite and wave-rippled or aeolian siliciclastic facies); and Calcareous Lake (slump-folded and locally re-sedimented rhythmic/stromatolitic limestones and dolomites associated with ice-rafted sediment). There is no strong cyclicity, and modern analogues suggest that sudden changes in lake level may exert a strong control on facies geometry. Both calcite and dolomite in stromatolites and rhythmites display either primary or early diagenetic replacive growth. Oxygen isotope values (-12 to +15‰VPDB) broadly covary with δ13C. High δ13C values of +3·5 to +4·5‰ correspond to equilibration with an atmosphere dominated by volcanically degassed CO2 with δ13C of -6 to -7‰. Limestones have consistently negative δ18O values, while rhythmic and playa dolomites preserve intermediate compositions, and dolocretes possess slightly negative to strongly positive δ18O signatures, reflecting significant evaporation under hyperarid conditions. Inferred meltwater compositions (-8 to -15·5‰) could reflect smaller Rayleigh fractionation related to more limited cooling than in modern polar regions. A common pseudomorph morphology is interpreted as a replacement of ikaite (CaCO3·H2O), which may also have been the precursor for widespread replacive calcite mosaics. Local dolomitization of lacustrine facies is interpreted to reflect microenvironments with fluctuating redox conditions. Although differing in (palaeo)latitude and carbonate abundance, the Wilsonbreen carbonates provide strong parallels with the McMurdo Dry Valleys of Antarctica.

Carbonate platform architecture and carbon isotope stratigraphy from the neoproterozoic Narji limestone formation, Kurnool Group, India: Decoding clues of relative sea level fluctuations

Article

Dec 2022
MAR PETROL GEOL

From a detail process-based facies and facies association analysis in a spatio-temporal framework, a rimmed shelf platform geometry is proposed for the Neoproterozoic Narji Limestone Formation, Kurnool Group, India. Ten carbonate facies types were delineated and clubbed under seven facies associations belonging to four different paleogeographic settings viz. Forereef (intertidal to shallow subtidal and shallow to deep subtidal), peritidal reef and back-reef lagoon. Whereas crinkly microbial laminites with subordinate domal stromatolites constitute the reef part, the fore slope deposition in the platform largely remained tide-and storm-dominated. Grey to black colored pyrite bearing bedded limestone-shale alternation without any current or wave imprint represents the back-reef lagoon deposition. A high carbonate saturation and productivity in the Narji inner and mid shelf is suggested from an overall dominance of microalgal laminite and microdigitate stromatolites over columnar stromatolites. From documentation of decimeter-to meter-thick shallowing-upward depositional hemicycles, embedded within an overall aggradational Narji lithosuccession, a combined control of sea level and carbonate productivity is inferred in course of Narji sedimentation. A short-duration medium-to large-scale rise in the base level led to the aggradation in shallow water rim of the Narji platform margin; interruption of this overall motif by short thickening- and shallowing-upward progradational hemicycles bear testimony for higher productivity in the Narji mid-to inner shelf areas. The present study intends to correlate shallowing-upward hemicycles with keep-up to catch-up depositional cycles with signature of apparent sea level fall. The control of relative base level variation in Narji depositional history is strengthened with multiple signatures including secular δ13C composition, total organic carbon (TOC) content and siliciclastic content documented from the entire Narji litho-succession.

Carbonate Platform Architecture and Carbon Isotope Stratigraphy from the Neoproterozoic Narji Limestone Formation, Kurnool Group, India: Decoding Clues of Relative Sea Level Fluctuations

Article

Dec 2022
MAR PETROL GEOL

From a detail process-based facies and facies association analysis in a spatio-temporal framework, a rimmed shelf platform geometry is proposed for the Neoproterozoic Narji Limestone Formation, Kurnool Group, India. Ten carbonate facies types were delineated and clubbed under seven facies associations belonging to four different paleogeographic settings viz. forereef (intertidal to shallow subtidal and shallow to deep subtidal), peritidal reef and back-reef lagoon. Whereas crinkly microbial laminites with subordinate domal stromatolites constitute the reef part, the fore slope deposition in the platform largely remained tide-and storm-dominated. Grey to black colored pyrite bearing bedded limestone-shale alternation without any current or wave imprint represents the back-reef lagoon deposition. A high carbonate saturation and productivity in the Narji inner and mid shelf is suggested from an overall dominance of microalgal laminite and microdigitate stromatolites over columnar stromatolites. From documentation of decimeter- to meter-thick shallowing-upward depositional hemicycles, embedded within an overall aggradational Narji lithosuccession, a combined control of sea level and carbonate productivity is inferred in course of Narji sedimentation. A short-duration medium- to large-scale rise in the base level led to the aggradation in shallow water rim of the Narji platform margin; interruption of this overall motif by short thickening- and shallowing-upward progradational hemicycles bear testimony for higher productivity in the Narji mid-to inner shelf areas. The present study intends to correlate shallowing upward hemicycles with keep-up to catch-up depositional cycles with signature of apparent sea level fall. The control of relative base level variation in Narji depositional history is strengthened with multiple signatures including secular δ13C composition, total organic carbon (TOC) content and siliciclastic content documented from the entire Narji litho-succession.

Del13C and 87Sr/86Sr signatures from carbonates of the Morar Formation, Gwalior Group: Implications towards depositional setting and Paleoproterozoic seawater chemistry

Article

Full-text available

Oct 2022
J EARTH SYST SCI

Pritam Paul

The Paleoproterozoic Gwalior Group of central India holds carbonates at two stratigraphic levels within the Morar Formation, exposed at the Bela-Ki-Bauri and Utilla sections. Limestones are micritic/sucrosic in grain size and dominantly calcitic in mineralogy. Preservation of primary sedimentary structures, poor correlation between δ13C and δ18O values and a narrow range in isotopic values suggest preservation of near-pristine sea water signature. δ13C values for Bela-Ki-Bauri and Utilla carbonates vary between –1.1 and –2.8‰ and 0.5 to 0.1‰, respectively, with an average 2.37‰ depletion in value in Bela-Ki-Bauri carbonates. Without any significant difference in total organic carbon content between the two studied sections, the depletion in δ13C value within Bela-Ki-Bauri carbonates is correlated with influx of 12C enriched meteoric water at a shallow marine setting dominated by stromatolites and algal laminites. The 87Sr/86Sr values obtained from Gwalior carbonates (0.707403–0.711283) are well consistent with Proterozoic carbonates and a value of 0.707403 is considered composition of Gwalior seawater. For the first time Strontium isotopes data are reported from Paleoproterozoic Gwalior Basin.87Sr/86Sr values from Gwalior carbonates are well consistent with other Proterozoic carbonates.Positive shift in δ13C in the Gwalior carbonate helps to infer the paleo bathymetry. For the first time Strontium isotopes data are reported from Paleoproterozoic Gwalior Basin. 87Sr/86Sr values from Gwalior carbonates are well consistent with other Proterozoic carbonates. Positive shift in δ13C in the Gwalior carbonate helps to infer the paleo bathymetry.

A newly discovered Neoproterozoic diamictite-cap carbonate couplet from the Western Himalaya: The expansion of the Marinoan snowball Earth glaciation to the northwestern margin of the Indian Plate in North Pakistan

Article

Aug 2022
PRECAMBRIAN RES

One of the central tenets of the snowball Earth hypothesis is the global recognition of the glacial diamictite-cap carbonate couplets in the Cryogenian sedimentary record. However, the scarcity of such peculiar stratigraphic elements in the Neoproterozoic successions of the Himalaya brings into question the severity and global extent of the snowball Earth event and likewise hampered glacio-stratigraphic correlation within the Himalaya and elsewhere. This study provides the first convincing evidence of the glacial diamictite-cap carbonate couplet from the Tanakki Member of the basal Kakul Formation, previously unknown from the Western Himalaya in North Pakistan. Detailed sedimentological analysis of the diamictite from the Tanakki Member reveals deposition in glacially-influenced proximal to distal subaqueous debris apron. The presence of glaciogenic clasts (striated, facetted and bullet-shaped) together with evidence of the ice-rafted dropstones in pervasive facies association provides credence to the glaciogenic affinity. The thin cap carbonate (herein referred to as Tanakki-cap dolomite; TCD) overlying the glacial diamictite record deposition in a deeper shelf (offshore) setting. The lithological, depositional and persistent negative C-isotope characteristics (ca. −3.2 to −5.8‰) combined with regional stratigraphic and available geochronological data allow us to interpret TCD as a ‘Marinoan’ cap carbonate and the underlying diamictite as an expansion of the terminal-Cryogenian (Marinoan) glaciation in the Western Himalaya. Moreover, the analyses of the tectonic and depositional history of the Tanakki Member coupled with the Neoproterozoic paleogeographic evolution of the northern margin of the Indian Plate argue against a previous interpretation of the culminating foreland basin orogeny and instead support deposition in an extensional fault-controlled rift basin. Finally, this study permits us to revise the Neoproterozoic stratigraphic framework of the Western Himalaya by describing the Cryogenian-Ediacaran boundary interval in the region that ultimately helps to overcome the previous glacio-stratigraphic discrepancies in the Neoproterozoic record of the Himalaya.

Article

Jun 2022
PRECAMBRIAN RES

Los mármoles cálcicos de El Escorial (Complejo Metamórfico Cushamen, Macizo Norpatagónico): caracterización isotópica de 87Sr-86Sr y edad de sedimentación

Article

Full-text available

Nov 2021

The El Escorial marbles (Cushamen Metamorphic Complex) along with amphibolites form metamorphic septa within the permian granitoids of the Mamil Choique Formation (261-286 Ma). The metamorphism, determined in granulite facies migmatic gneisses septa cropping out 120 km southwest of El Escorial, occurs at 311 ± 27 Ma (CHIME method in monazite). The marbles are calcitic (calcite > 95%, R.I.: 0.5 to 2.5%) and show 87Sr/86Sr values between 0.70768 and 0.70825 (n = 10). The data provided in this work, added to previous contributions, allow to constraints the sedimentation age of the silicic-carbonate successions of the Cushamen Metamorphic Complex between ca. 385 and 335 Ma. This suggests the existence of a mixed carbonate-siliciclastic platform at least in the southernmost portion of southwestern Gondwana between Middle Devonian and early Carboniferous (Middle Mississippian).

Stable carbon and oxygen isotope data of Late Ediacaran stromatolites from a hypersaline environment in the Tarim Basin (NW China) and their reservoir potential

Article

Full-text available

Jul 2021

The occurrence of Precambrian stromatolites is closely related to the ancient seawater composition and the evolution of life. It is also a potential oil and gas reservoir. In what kind of environment the stromatolites of the upper Ediacaran Qigebulak Formation in the Tarim Basin developed, and whether they constitute potential hydrocarbon reservoirs, remained unclear. Stromatolites occur in 1–5 m thick layers, interbedded with thrombolites and dolostones. Distinct stromatolite morphologies were observed, including columnar, sinuous, short columnar, domal, and conical shapes. The δ13C values of the stromatolites (6.1‰ on average) are slightly lighter than those of the dolostones, and the δ18O values (− 1.4‰ on average) are significantly heavier than that of the dolostones. The stromatolites have a relatively high content of rare-earth elements and a minor Ce anomaly. The geochemical results imply that the stromatolites formed in an evaporative hypersaline lagoon environment. The presence of barrier dams near the coast led to the formation of lagoons, where hypersalinity was achieved when evaporation was greater than marine or freshwater input. High salinity conditions inhibited the growth of Ediacaran metazoans, allowing the buildup of stromatolites in the restricted lagoons. The stromatolites are rich in primary fenestral pores and sheet-like cavities along the laminae, and the secondary dissolution pores and vugs are related to meteoric karstification. The stromatolites, together with dolostones and thrombolites, constitute the majority of the hydrocarbon reservoirs in the Upper Ediacaran in the Tarim Basin. The results clarify the environment where the stromatolites could still flourish and be well preserved whereas they significantly declined globally throughout the Neoproterozoic elsewhere. The results imply that extensive stromatolites in the Proterozoic strata are potentially important reservoir rocks of Precambrian petroleum systems.

Carbonate fabric diversity and environmental heterogeneity in the late Mesoproterozoic Era

Article

Full-text available

May 2021

Time-distinctive features within carbonate rocks, specifically the presence, abundance and distribution of stromatolites, molar-tooth fabric and specific morphologies of marine cement, have been identified as potential indicators of global-scale changes in the chemistry of marine environments. Recently, Cantine et al. (2019) introduced a database approach seeking to quantify spatial and temporal patterns in these carbonate features through the Precambrian. Despite the coarse temporal scale, results support earlier inferences of temporal change in carbonate sedimentation. Here, we use original field notes to dissect late Mesoproterozoic (˜1.3 to 1.0 Ga) carbonate strata at a high resolution, analyse time-distinctive carbonate fabrics within a database context and compare sedimentological patterns within this narrow time range to observations of the Proterozoic as a whole. Late Mesoproterozoic strata contain a variety of features (e.g. stromatolites, seafloor precipitates, herringbone carbonate, molar-tooth carbonate), often in close spatial and temporal proximity, that are commonly considered to be temporally restricted during the Precambrian. The spatial distribution of such features within Mesoproterozoic basins demonstrates the importance of recognizing even rare occurrences of time-distinctive facies and permits inference of environmental drivers that may have interacted to affect carbonate precipitation. Such spatial variability reflects a subtle division of Mesoproterozoic carbonate platform environments driven by globally high sea level, elevated carbonate saturation and a low-oxygen water column. The heterogeneous, mosaic nature of environments appears to be a hallmark of Mesoproterozoic carbonate sedimentation and emphasizes the importance of these basins in understanding longer-term trends in carbonate deposition.

Tectonically conditioned record of continental interior paleoclimate during the Carnian Pluvial Episode: The Upper Triassic Los Rastros Formation, Argentina

Article

Full-text available

Mar 2021

Discerning paleoclimate parameters in depositional systems of the continental interior is challenging because the system response and stratigraphic record of climate are controlled by tectonic processes and are mediated through landscape and hydrological evolution of fluvial lacustrine systems. Climate and tectonic signals cannot be deconvolved from stratigraphic patterns alone but require additional information or data sets that directly record climate or tectonic influence. The Carnian Los Rastros Formation in northwest Argentina provides an excellent case study that integrates an appropriate range of information in a system with strong climate and tectonic signals, being deposited in part during the Carnian Pluvial Episode and spanning the active rift phase of the Ischigualasto−Villa Unión Basin. We examined the stratigraphic and spatial patterns of carbon (C) and oxygen (O) stable isotopes in lacustrine carbonates from the Los Rastros Formation in multiple parts of the basin to constrain paleohydrological conditions and paleotemperatures. Practically all C and O isotope values are characterized by negative values: δ18Ocarb −11.6‰ and −15.7‰ (χ average −13.1‰; 1σ = 1.6) and δ13Ccarb −2.6‰ to −8.0‰ (χ average −5.1‰; 1σ = 2.1), reflecting the latitude, altitude, and continentality of the lake system and its vegetated and humid catchment area. Stratigraphic patterns of stable isotope data from two different localities (Cerro Bola North and Cerro Bola South) show a change from short water-residence time to long residence time and back to short residence time. This contrasts with sedimentologic, organic geochemical, and small-scale stratigraphic patterns that indicate an overfilled lake basin, which is expected to contain a completely open-hydrology isotopic signature. Paleotemperatures calculated from marginal lacustrine carbonates show a warm and quite variable paleothermal range consonant with their continental interior position and with Global Climate Model estimates for high paleolatitudes. Warmer paleotemperatures (linked to aridity, probably smaller lake size, and less thermal mass) precede the Carnian Pluvial Episode, whereas relatively cooler paleotemperatures coincide with the Carnian Pluvial Episode (linked to humidity, probably larger lake size, and more thermal mass). Carbon and oxygen stable isotope signatures integrated with sedimentologic and physiographic information allow us to propose that tectonics, specifically, half-graben tilting during the active synrift phase, dominated over climate effects as the cause of hydrological fluctuations of this system, even during the Carnian Pluvial Episode. Without appropriate stratigraphic-tectonic context, single-proxy reconstructions of continental-interior paleoclimate can be misleading. A robust interpretation of climate effects requires characterization of tectonic effects, geomorphology, paleohydrology, and sedimentary system responses.

Revised stratigraphic framework for the lower Anti-Atlas Supergroup based on U–Pb geochronology of magmatic and detrital zircons (Zenaga and Bou Azzer-El Graara inliers, Anti-Atlas Belt, Morocco)

Article

Jul 2020
J AFR EARTH SCI

U–Pb geochronology of magmatic and detrital zircons (Zenaga and Bou Azzer-El Graara inliers, Anti-Atlas Belt, Morocco) and a reassessment of the published constraints suggest a revised stratigraphic framework for the lower Anti-Atlas Supergroup. Five, major unconformity-bounded lithostratigraphic packages are here distinguished: the two lower units of Paleoproterozoic age are named the Tasserda-Taghatine Group (2030–1706 Ma) and the Oumoula (Mimount) Formation (ca. 1745–1650 Ma); the third unit of Paleoproterozoic to Neoproterozoic age (ca. 1650 to >883 Ma) is the Tizi n’Taghatine Group; the fourth and fifth units of Neoproterozoic age are the ca. 883 Ma Tachdamt and the ca. 700 Ma Bleida formations. Implications of this revised stratigraphic framework include: 1) the Tasserda-Taghatine Group might be linked to the post-orogenic collapse after the Eburnean Orogeny; 2) the Tizi n’Taghatine Group might be ca. 1.1 Ga in age based on proposed correlation with the Taoudeni Basin succession in Mauritania; 3) the Bleida Formation likely reflects deposition in the foreland basin at the early stage of the Pan-African Orogeny; 4) the Oumoula (Mimount) Formation, Tizi n’Taghatine Group, and Tachdamt Formation potentially record extensional events within the Nuna/Columbia and Rodinia supercontinents; 5) the provenance of the lower Anti-Atlas Supergroup (based on our new detrital zircon dating) is mainly from the West African craton along with possible contributions from other cratons such as Amazonia and the Sahara Metacraton; 6) the flood basalt sequence of the Tachdamt Formation likely belongs to the ca. 885-883 Ma intraplate Iguerda-Taïfast Large Igneous Province (LIP) event defined by previously dated dykes in the Iguerda and Taïfast inliers; and 7) the 1650 Ma Zenaga LIP can be potentially linked with LIP magmatism in Baltica and Laurentia.

A MULTIPROXY APPROACH TO PALEOHYDROLOGIC AND PALEOCLIMATE RECONSTRUCTION OF TRIASSIC LACUSTRINE BASINS OF ARGENTINA

Conference Paper

Jan 2019

Can crystal morphology indicate different generations of dolomites? Evidence from magnesium isotopes

Article

Apr 2019
CHEM GEOL

Among various geochemical and petrographic approaches, dolomite crystal morphology and dolostone fabric have been widely applied in the study of ancient dolostones. It is proposed that dolomite crystal morphologies and the rock fabric may reflect the formation temperature, and thus can be used to distinguish different generations of dolomite. However, this scenario has also been challenged by some researchers. In order to test whether the dolomite crystal morphology can be used to differentiate different generations of dolomite, in this study, we measured the Mg isotopic compositions (δ26Mg) of dolomite with different crystal morphologies. δ26Mg of dolomite is controlled by a variety of factors, including temperature, magnesium isotopic composition of dolomitization fluids, and the flow rate of dolomitization fluids. If dolomite with distinct crystal morphologies were derived from different dolomitization processes, it is highly plausible that they would have different δ26Mg. Five types of dolomite with distinct crystal morphologies and rock fabric were recognized from three sampling intervals (S1, S2, and S3) in the middle Ordovician Majiagou Formation in North China. Different types of dolomite in the same sampling interval have similar δ26Mg values, suggesting that these dolomites might have derived during the same dolomitization event. Our study indicates that the crystal morphology alone may not unambiguously differentiate the generations of dolomites. We propose the following reasons: (1) the dolomite crystal morphology might be controlled by various factors rather than the formation temperature alone, or (2) the dolomite crystal morphology might be modified in diagenesis, but δ26Mg remains unchanged.

Sedimentological and geochemical evolution of an Ediacaran Mixed Carbonate-Siliciclastic Continental Slope System, Windermere Supergroup, southern Canadian Cordillera, British Columbia, Canada

Article

Feb 2019
PRECAMBRIAN RES

In this study a 165–265 m-thick siliciclastic-carbonate (mixed) base-of-slope succession within the Ediacaran Isaac Formation (hereafter termed FIC) in the deep-marine Windermere Supergroup (WSG), southern Canadian Cordillera was analyzed. Excellent outcrop exposure at three study areas (Castle Creek, Hill Section and Milk River), spaced over a distance of 20 km parallel to depositional dip provided the opportunity to observe the stratigraphic and inorganic carbon (δ ¹³ C carb ) isotopic evolution of an ancient mixed, passive margin deep-water system. Based on detailed sedimentological analysis six stratal elements make-up the FIC (channel complexes, levees, siliciclastic-to-carbonate tripartite deposits, scour dominated units, debrites and slides). Within these strata the most voluminous carbonate phase (C1) represents neomorphism of primary aragonite on the slope and during early burial, and thus preserves the primary shallow-marine isotopic signature even after downslope transport. At the base of the FIC δ ¹³ C carb is negative (−5.2‰) and then progressively increases to 2–3‰ in the middle of the succession, after which it plateaus for about 65 m before declining to a nadir of −6.3‰ at its top. Coincident with these isotopic changes are systematic changes in the architecture and mineralogical composition of the component stratal elements that makeup the FIC, which collectively suggest long-term transgressive followed by regressive conditions that quite possibly are tied to rising and then falling sea-level, and accordingly the state of the continental shelf carbonate factory. Additionally, the upward change in δ ¹³ C carb closely resembles the trend reported in the upper June beds-Gametrail Formation of the WSG in the northern Canadian Cordillera, but the interpreted association of the latter with the Shuram-Wonoka anomaly (570–551 Ma) makes its correlation with the FIC unlikely. Instead, the FIC excursion potentially correlates with the EN2 excursion in the Doushantuo Formation of South China (and its potential global correlatives), which has been interpreted to be associated with the Gaskiers glaciation (580 Ma). This suggests that the FIC might be a stratigraphic and δ ¹³ C carb record of conditions that prevailed in the deep sea preceding the Gaskiers, which previously has been unreported from strata of the WSG.

Sponge-microbial build-ups from the lowermost Triassic Chanakhchi section in southern Armenia: Microfacies and stable carbon isotopes

Article

Nov 2017
PALAEOGEOGR PALAEOCL

The end-Permian mass extinction was the most severe biotic crisis in Earth's history. In its direct aftermath microbial communities colonized some of the space left vacant after the severe decline of skeletal metazoans. The Permian-Triassic boundary microbialites were peculiarly abundant on low-latitude shallow-marine carbonate shelves of central Tethyan continents. Armenia features particularly well preserved and diverse basal Triassic sponge-microbial build-ups (BTSMBs), which were not studied in detail to date. Here, the Chanakhchi section in southern Armenia is described petrographically and by means of stable isotope analyses. The Armenian BTSMBs formed in a distally open marine setting on a pelagic carbonate ramp in the course of two phases of microbial growth during the Induan (Lower Triassic). The BTSMBs are represented by predominantly thrombolitic but also dendrolitic and digitate stromatolite biostromes and mounds that vary in height between 5 cm to 12 m. The digitate stromatolites are associated with calcium carbonate crystal fans (CCFs). Microfacies analyses revealed that the BTSMBs exhibit a number of different growth forms and internal fabrics. The formation of CCFs was apparently not devoid of biological influence and took place above the sediment surface. The abundance of sponges in the BTSMBs reveals that ecologically complex metazoan-microbial reefs have been present already early after the end-Permian mass extinction. However, the formation of biostromes and mounds did not depend on sponges or other metazoans. BTSMBs that formed during the second microbial growth phase revealed similar δ13C-values like the surrounding sediment. In contrast, the δ13Cmicrobialite and δ13Csediment values from the BTSMBs and CCFs of the first growth phase show a difference of up to + 2.3‰, suggesting a significant influence of photoautotrophy during microbially induced carbonate precipitation.

Trace element and stable (C, O) and radiogenic (Sr) isotope geochemistry of stromatolitic carbonate rocks of the Mesoarchaean Pongola Supergroup: Implications for seawater composition

Article

Jan 2018
CHEM GEOL

https://www.sciencedirect.com/science/article/pii/S0009254117306666

Tonian-Cryogenian boundary sections of Argyll, Scotland

Article

Sep 2017
PRECAMBRIAN RES

The Tonian-Cryogenian System boundary is to be defined at a GSSP (Global Boundary Stratigraphic Section and Point) beneath the first evidence of widespread glaciation. A candidate lies within the Dalradian Supergroup of Scotland and Ireland, which is least deformed and metamorphosed in Argyll, western Scotland. We present new stratigraphic profiles and interpretations from the Isle of Islay and the Garvellach Islands, update the chemostratigraphy of the Appin Group Tonian carbonates underlying the thick (ca. 1. km) glacigenic Port Askaig Formation (PAF) and demonstrate an environmental transition at the contact.The Appin Group forms a regionally extensive, >4km-thick, succession of limestones, shales and sandstones deposited on a marine shelf. On Islay, the upper part of the lithostratigraphy has been clarified by measuring and correlating two sections containing distinctive stratigraphic levels including molar tooth structure, oolite, stromatolitic dolomite and intraclastic microbial mounds. Significantly deeper erosion at the unconformity at the base of the overlying PAF is demonstrated in the southern section. Carbonate facies show a gradual decline in δ¹³CVPDB from +5 to +2‰ upwards.In NE Garbh Eileach (Garvellach Islands), a continuously exposed section of Appin Group carbonates, 70m thick, here designated the Garbh Eileach Formation (GEF), lies conformably beneath the PAF. The GEF and the GEF-PAF boundary relationships are re-described with new sedimentological logs, petrological and stable isotope data. Interstratified limestone and dolomicrosparite with δ¹³C of -4 to -7‰ (a feature named the Garvellach anomaly, replacing the term Islay anomaly) are overlain by dolomite in which the isotope signature becomes weakly positive (up to +1‰) upwards. Shallow subtidal conditions become peritidal upwards, with evidence of wave and storm activity. Gypsum pseudomorphs and subaerial exposure surfaces are common near the top of the GEF. The basal diamictite (D1) of the PAF is rich in carbonate clasts similar to slightly deeper-water parts of the underlying succession. D1 is typically several metres thick with interstratified sandstone and conglomerate, but dies out laterally. Scattered siliciclastic coarse sandstone to pebble conglomerate with dropstones associated with soft-sediment deformation is interbedded with carbonate below and above D1. Dolomite beds with derived intraclasts and gypsum pseudomorphs are found above D1 (or equivalent position, where D1 is absent).Published and new Sr isotope studies, including successive leach data, demonstrate primary Tonian ⁸⁷Sr/⁸⁶Sr values of 0.7066-0.7069 on Islay, decreasing to 0.7064-0.7066 in the younger GEF limestones on the Garvellachs, with 1700-2700ppmSr. Other typically Tonian characteristics of the carbonates are the Sr-rich nature of limestones, molar tooth structure, and dolomitized peritidal facies with evidence of aridity. Seabed surveys suggesting uniformly-dipping strata and shallow borehole core material illustrate the potential for extending the Tonian record offshore of the Garvellachs.A candidate Tonian-Cryogenian GSSP is proposed on Garbh Eileach within the smooth δ¹³C profile at the cross-over to positive δ¹³C signatures, 4m below the first occurrence of ice-rafted sediment and 9m below the first diamictite. Although lacking radiometric constraints or stratigraphically significant biotas or biomarkers, the Scottish succession has a thick and relatively complete sedimentary record of glaciation, coherent carbon and strontium chemostratigraphy, lateral continuity of outcrops and 100% exposure at the proposed boundary interval.

Unradiogenic strontium and moderate-amplitude carbon isotope variations in early Tonian seawater after the assembly of Rodinia and before the Bitter Springs Excursion

Article

Jun 2017
PRECAMBRIAN RES

Paleo-climatic and paleo-environmental evolution of the Neoproterozoic basal sedimentary cover on the Río de La Plata Craton, Argentina: Insights from the δ13C chemostratigraphy

Article

Full-text available

Apr 2017
SEDIMENT GEOL

The Sierras Bayas Group of the Tandilia System constitutes the Neoproterozoic sedimentary cover of the Río de La Plata Craton in Argentina that accumulated amid the breakup of the Rodinia supercontinent and subsequent assembly of Gondwanaland. Evidence for glaciation in the Villa Mónica Formation (VMF) at the base of the successioncomes in the form of iron-rich laminated sediments containing dropstones composed predominantly of basement crystalline rocks and quartzites that, are sequentially overlain by a phosphatic mudstone and a ~40 m thickstromatolitic dolomite. Subtidal facies preserve columnar forms similar to post-glacial tubestone stromatolitesseen in the Neoproterozoic records. Thesemorphologies suggest rapid growth associated with elevated seawater alkalinityand high rates of carbonate accumulation records. The VMF dolomites in our four studied sections near Olavarría-Sierras Bayas area reveal a pronounced negative-to-positive δ13C up section that is similarly to these capcarbonates and others worldwide. Our sedimentological and geochemical study of the VMF sections reveal consistentcarbon and oxygen isotope trends that may be useful for detailed intra-basinal correlations. Samples of the VMFfabric-retentive dolomite preserve an unusually narrow range of non-radiogenic strontium isotopic compositions(0.7068 to 0.7070) that are consistent with Cryogenian limestone facies in the potential Namibian and Brazilianequivalents. Exceptional preservation of 87Sr/86Sr compositions suggest the possibility of primary dolomite precipitation in post-glacial seawater, and furthermore that REE patterns and distributions may yield similar insights to redox conditions in the depositional basin. In particular, the VMF dolomites reveal depleted LREE abundances, a negative Ce anomaly, positive La and Gd anomalies, and low Y/Ho values. As a whole, these observations suggest oxidizing post-glacial seawater conditions associated with significant freshwater inputs into the basin. Global warming and increases in primary productivity and organic carbon burial linked to the buildup of oxygen, the positive δ13C trend and the increase in stromatolite biodiversity in the VMF.

Paleo-climatic and paleo-environmental evolution of the Neoproterozoic basal sedimentary cover on the Río de La Plata Craton, Argentina: Insights from the δ 13 C chemostratigraphy

Article

Mar 2017
SEDIMENT GEOL

The Sierras Bayas Group of the Tandilia System constitutes the Neoproterozoic sedimentary cover of the Río de La Plata Craton in Argentina that accumulated amid the breakup of the Rodinia supercontinent and subsequent assembly of Gondwanaland. Evidence for glaciation at the base of the succession in the Villa Mónica Formation (VMF) comes in the form of iron-rich laminated sediments containing dropstones composed predominantly of basement crystalline rocks and quartzites, which are sequentially overlain by a phosphatic mudstone and a ~ 40 m thick stromatolitic dolomite. Subtidal facies preserve columnar forms similar to the tubestone stromatolites seen in the Neoproterozoic records. The VMF dolomites in four of our studied sections near Olavarría-Sierras Bayas area reveal a pronounced negative-to-positive δ¹³C up section trend suggestive of post-glacial seawater variations as recorded in Neoproterozoic marine proxies worldwide. Our sedimentological and geochemical study of the VMF sections reveal consistent carbon and oxygen isotope trends that may be useful for detailed intra-basinal correlations. Samples of the VMF fabric-retentive dolomite preserve an unusually narrow range of non-radiogenic strontium isotopic compositions (0.7068 to 0.7070) that are consistent with Cryogenian limestone facies in the potential Namibian and Brazilian equivalents. Exceptional preservation of ⁸⁷Sr/⁸⁶Sr compositions suggest the possibility of primary dolomite precipitation in post-glacial seawater, and furthermore that REE patterns and distributions may yield similar insights to redox conditions in the depositional basin. In particular, the VMF dolomites reveal depleted LREE abundances, a negative Ce anomaly, positive La and Gd anomalies, and depleted Y/Ho. As a whole, these observations suggest oxidizing post-glacial seawater conditions associated with significant freshwater inputs into the basin. Subsequent warming and increases in primary productivity and organic carbon burial are associated with the positive δ¹³C trend and the increase in stromatolite biodiversity in the VMF.

Sr isotopic composition in Riphean, Vendian, and Lower Cambrian carbonates from Siberia

Article

Full-text available

Jan 1995
STRATIGR GEO CORREL+

Microfossils from the late Mesoproterozoic – early Neoproterozoic Atar/El Mreïti Group, Taoudeni Basin, Mauritania, northwestern Africa

Article

Full-text available

Jan 2017
PRECAMBRIAN RES

The well-preserved Meso-Neoproterozoic shallow marine succession of the Atar/El Mreïti Group, in the Taoudeni Basin, Mauritania, offers a unique opportunity to investigate the mid-Proterozoic eukaryotic record in Western Africa. Previous investigations focused on stromatolites, biomarkers, chemostratigraphy and palaeoredox conditions. However, only a very modest diversity of organic-walled microfossils (acritarchs) has been documented. Here, we present a new, exquisitely well-preserved and morphologically diverse assemblage of organic-walled microfossils from three cores drilled through the Atar/El Mreïti Group. A total of 48 distinct entities including 11 unambiguous eukaryotes (ornamented and process-bearing acritarchs), and 37 taxonomically unresolved taxa (including 9 possible eukaryotes, 6 probable prokaryotes, and 22 other prokaryotic or eukaryotic taxa) were observed. Black shales preserve locally abundant fragments of benthic microbial mats. We also document one of the oldest records of Leiosphaeridia kulgunica, a species showing a pylome interpreted as a sophisticated circular excystment structure, and one of the oldest records of Trachyhystrichosphaera aimika and T. botula, two distinctive process-bearing acritarchs present in well-dated 1.1 Ga formations at the base of the succession. The general assemblage composition and the presence of three possible index fossils (A. tetragonala, S. segmentata and T. aimika) support a late Mesoproterozoic to early Neoproterozoic (Tonian) age for the Atar/El Mreïti Group, consistent with published lithostratigraphy, chemostratigraphy and geochronology. This study provides the first evidence for a moderately diverse eukaryotic life, at least 1.1 billion years ago in Western Africa. Comparison with coeval worldwide assemblages indicate that a broadly similar microbial biosphere inhabited (generally redox-stratified) oceans, placing better time constraints on early eukaryote palaeogeography and biostratigraphy.

Isotope (Sr, C) and U-Pb SHRIMP zircon geochronology of marble-bearing sedimentary series in the Eastern Sierras Pampeanas, Argentina. Constraining the SW Gondwana margin in Ediacaran to early Cambrian times

Article

Jun 2016
PRECAMBRIAN RES

The Sierras de Córdoba Metasedimentary Series consists of marbles and metasiliciclastic rocks of Ediacaran to early Cambrian age (ca. 630 and 540 Ma) that underwent high-grade metamorphism during the collisional Pampean orogeny in the early Cambrian. The ages of the marbles were determined from the Sr-isotope composition (blind dating) of screened samples of almost pure calcite marble and were further constrained with C- and O-isotope data and U-Pb SHRIMP detrital zircon ages of an interbedded paragneiss. Two groups of samples are recognized with Sr-isotope composition ca. 0.7075 and 0.7085 that are considered stratigraphically significant. The first is inferred early Ediacaran, the second late Ediacaran to early Cambrian. The Sierras de Córdoba Metasedimentary Series is correlated for the first time with marble-bearing metasedimentary series in several sierras to the west and north of Sierras de Córdoba (e.g., the Difunta Correa Sedimentary Sequence and the Ancaján Series), implying that all were probably parts of an originally extensive sedimentary cover. These series bear evidence of sedimentary sources in the Mesoproterozoic (and Paleoproterozoic) basement of the Western Sierras Pampeanas (part of the large MARA continental block) and farther west (Laurentia?). In terms of the age of limestones/marbles and detrital zircon patterns, the Sierras de Córdoba Metasedimentary Series differs strongly from the older section of late Ediacaran to early Cambrian Puncoviscana Formation of northwestern Argentina, which outcrops in northern Sierra Chica and Sierra Norte, with sedimentary input from western Gondwana sources. The Sierras de Córdoba Metasedimentary Series and the Puncoviscana Formation were probably juxtaposed during the Pampean orogeny along a complex suture zone that was further folded and/or imbricated at mid-crustal depths. The peak of metamorphism was attained at 527 ± 2 Ma. According to the evidence found here most of the Sierras Pampeanas to the west of the Sierras de Córdoba were part of the lower colliding plate during the final amalgamation of SW Gondwana.

Low Sr-87/Sr-86 ratios, in Seawater of the Grenville and post-Grenville time: Determining factors

Article

Full-text available

Jan 2002

The results obtained contribute much to the reconstruction of secular Sr isotope variations in seawater of the late Middle-early Late Riphean and elucidate factors that determined low values of the Sr-87/Sr-86 ratios in the World Ocean at the formation time of Rodinia (Grenville orogenic cycle). The new curve that depicts the Sr isotope variations in seawater 1050-1000 Ma ago is much more detailed than others characterizing the Precambrian. It is plotted after investigation of limestones collected from the Kerpyl' and Lakhanda groups of the East Siberian Uchur-Maya region and of correlative limestones from the Turukhansk region of Central Siberia. In order to obtain the confident analytical data determining the curve configuration, the selection of limestone samples was controlled by petrographic and strict geochemical criteria (Mn/Sr less than or equal to0.20, Fe/Sr less than or equal to5.0, Mg/Ca less than or equal to0.024) that ensure the least alteration degree of rocks. In addition, all the selected samples were preliminary treated in 1N solution of ammonium acetate for the partial removal of epigenetic carbonate phases. We took into consideration the lowest Sr-87/Sr-86, ratios obtained for dolomites only as evaluating the maximum limit of this parameter in the sedimentation medium. After generalization of all Sr-isotopic data available for the Middle-early Late Riphean, we established that the Sr-87/Sr-86 ratios in seawater did not exceed 0.70490 about 1300 Ma ago and began to rise afterward to become as high as 0.70518-0.70549 approximately 1200 Ma ago. At the end of the Middle Riphean (1050-1030 Ma), the ratios reached a relative peak of values corresponding to 0.70563-0.70592, after which they decreased. During the earliest Late Riphean (1030-1000 Ma), they varied between 0.70519 and 0.70569, generally decreasing with time, and became equal to 0.70523-0.70527 1000 Ma ago. Within the time span of 900-800 Ma, the Sr-87/Sr-86 ratios were likely not greater than 0.70525-0.70585. The following factors and events appear to be responsible for the persistently low Sr-87/Sr-86 ratios in seawater of the Grenville and post-Grenville time: a high proportion of the pre-Grenvillide mantle rocks in the crust of Grenvillides; peculiar features of metamorphism, exhumation, and unroofing of different lithotectonic units in the Grenville orogens; an active influx of fresh juvenile material into oceans of the Grenville time; the Late Grenville sea-level rise; and, possibly, the partial locking up of the continental runoff in inner continental depressions at the beginning of the Late Riphean.

The Pre-Sturtian Negative δ 13 C Excursion of the Dajiangbian Formation Deposited on the Western Margin of Cathaysia Block in South China

Article

May 2016

The Dajiangbian Formation in South China is a siliciclastic-dominated sedimentary succession with low-grade metamorphism deposited on the western margin of the Cathaysia Block, capped by a glaciaogenic diamictite (the Sizhoushan Formation). The Sizhoushan glaciogenic strata can be attributed to the Jiangkou glacial (Sturtian glacial) episode as they share stratigraphic and lithological similarities with Jiangkou strata in South China. Some carbonate, chert and shale units throughout the upper part of the Dajiangbian Formations were sampled for carbonate carbon isotope (13Ccarb) and organic carbon isotope (13Corg) analyses. A range of geochemical indices including oxygen isotopes (18O) and Mn/Sr (Fe/Sr) ratios suggest that primary carbon isotope values were preserved in the upper Dajiangbian Formation. The upper Dajiangbian Formation shows 13Ccarb of -0.1‰, upward decreasing towards to -5.4‰. We suggest that the negative 13C excursion beneath the Sizhoushan diamictite is correlative with the pre-Sturtian Islay 13Ccarb anomaly and allows correlation with the global Neoproterozoic isotope stratigraphy. We find that carbonate and organic carbon isotope data of the upper Dajiangbian Formation are coupled, consistent with the 13Ccarb-13Corg pattern observed on multiple continents.

Sr Isotopic Composition in Riphean, Vendian, and Lower Cambrian Carbonates from Siberia

Article

Full-text available

Jan 1995

The Sr Isotopic Composition in Carbonate Rocks of the Karatau Group (Southern Urals) and the Standard Variation Plot of the 87Sr/86Sr Ratio in the Late Riphean Ocean

Article

Full-text available

Jan 2003

Reinterpreting a Proterozoic Enigma: Conophyton-Jacutophyton Stromatolites of the Mesoproterozoic Atar Group, Mauritania

Article

Jan 2009

Chemostratigraphic constraints on the time of deposition of carbonate rocks in the Karasjok Greenstone Belt, northern Norway

Article

Dec 2015

The time of deposition of the rocks of the Karasjok Greenstone Belt, the most continuous and longest, linear Precambrian terrane in northern Norway, remains poorly dated. The belt is composed of several volcanic and sedimentary formations including a continuous unit of calcite and dolomite marbles. Most rocks have been strongly deformed and altered under amphibolite-facies metamorphism.Carbon isotope chemostrati-graphy has been applied to provide an apparent depositional age of the carbonate unit and associated sedimentary and volcanic rocks. Thirty-eight samples representing marbles in four different locations in the northern part of the belt show high δ 13 C carb values, hence recording the Palae-oproterozoic Lomagundi–Jatuli isotopic event. The least-altered δ 13 C carb values suggest the presence of two isotopically different assemblages (+8 to +9‰ and +12 to +14‰) whose apparent time of deposition can be constrained between 2220 and 2140 Ma, and between 2220 and 2110 Ma, respectively.

The Sr isotope chemostratigraphy as a tool for solving stratigraphic problems of the Upper Proterozoic (Riphean and Vendian)

Article

Full-text available

Nov 2014

Published and original data on the Sr isotopic characterization of carbonates from the Riphean and Vendian key sections of the Southern Urals, Siberia, Asia, Africa, Australia, and North America are considered in compliance with the suggested principles of reconstructing the Sr isotopic composition of the Proterozoic seawater. The suggested methodic approach is used to plot the reference curve of the 87Sr/86Sr variations in the Riphean and Vendian oceans. During the time span of 1600–1250 Ma, the 87Sr/86Sr variations were in a narrow range corresponding to 0.70456–0.70494, but approaching the date of about 1030 Ma, the 87Sr/86Sr ratio rose to 0.70601–0.70611 and then quickly declined to 0.70519–0.70523 near the date of 1000 Ma. In the second half of the late Riphean and in the Vendian, the ratio grew almost steadily from 0.70521–0.70535 to values of 0.70874–0.70885 characteristic of the Late Vendian time. The subsequent regular growth of that ratio in seawater lasted from 840 to 550 Ma, though there were short-term epochs when the ratio noticeably dropped to 0.70561–0.70575 at approximately 760 Ma and to 0.70533–0.70538 at 670–660 Ma. After the mid-Late Vendian maximum, it declined to 0.70812–0.70823 at the end of the Nemakit-Daldynian Age and decreased to 0.70806–0.70812 during the Tommotian Age of the Early Cambrian. As is shown, the Sr isotopic variations in the Riphean and Vendian oceans were interrelated with global tectonic events in geospheres and formation stages of the Rodinia and Gondwana supercontinents. The Baikalian Complex of Siberia is considered in the work as a case in point illustrating advantages of the expounded approach with respect to age substantiation of particular stratigraphic subdivisions.

Sr isotope composition in carbonates of the Karatau Group, southern Urals, and standard curve of 87Sr/86Sr variations in the Late Riphean Ocean

Article

Full-text available

Sep 2003
STRATIGR GEO CORREL+

The presented Rb-Sr systematics of carbonates from the Karatau Group, the Upper Riphean stratotype of southern Urals, elucidates important details of secular variations of 87Sr/86Sr ratios in the Late Riphean seawater, which have been formerly unknown. Samples selected for analysis satisfy all the strict geochemical criteria characterizing the least altered carbonate rocks (Mn/Sr ≤ 0.2, Fe/Sr ≤ 5.0, Mg/Ca ≤ 0.024 in limestones and Mn/Sr ≤ 1.2, Fe/Sr ≤ 3.0, Mg/Ca ≥ 0.608 in dolostones), and they all have been preliminary treated in IN solution of ammonium acetate for a partial removal of epigenetic carbonate phases. A verified curve of secular variations of 87Sr/86Sr ratio in the Late Riphean ocean is plotted based on new data and Sr isotope parameters formerly known for carbonates of the Upper Riphean key sections. As is established, that ratio was nearly constant, ranging from 0.70519 to 0.70566, within the time span of 1030-810 Ma and next rose up to 0.70611 about 775 Ma ago. Afterward, between 765 and 740 Ma, it decreased down to 0.70561-0.70575 and then, within the time span of 740-690 Ma, it ranged from 0.70646 to 0.70686 with a short-term drop down to 0.70620 about 720 Ma ago. At the end of the Late Riphean (660-640 Ma), the ratio lowered to 0.70538-0.70580 to become growing up to 0.70840-0.70860 during the Vendian and initial Cambrian. The established variations of the 87Sr/86Sr ratio in the Late Riphean ocean have been controlled by a combination of geodynamic factors, magmatic events, sea-level oscillations, and compositional changes in provenances, which were subjected to erosion at that time. An additional influence of climatic fluctuations over supercontinent and its fragments is also admissible.

Sedimentary-volcanic successions of the Alta –Kvænangen Tectonic Window in the northern Norwegian Caledonides: Multiple constraints on deposition and correlation with complexes on the Fennoscandian Shield

Article

Full-text available

Jun 2015

Sr, C and O isotope composition of marbles from the Sierra de Ancasti, Eastern Sierras Pampeanas, Argentina: Age and constraints for the neoproterozoic-lower Paleozoic evolution of the Proto-Gondwana margin

Article

Full-text available

Mar 2011
GEOL ACTA

The Sierra Brava Complex on the eastern flank of the Sierra de Ancasti consists of marbles, metabasites, calc-silicate rocks, psammo-pelitic schists and gneisses. In the central part of this sierra a thick succession of banded schists (Ancasti Formation) crops out. Regional metamorphism of these rocks is attributed to the Famatinian orogeny (Ordovician), metamorphic grade increasing westwards and southwards and culminating in a migmatite complex on the western side of the Sierra. The meta-carbonate rocks are subdivided into a northeastern group (low-grade calcite marbles), and a southeastern group (high-grade calcite and calcite-dolomite marbles). Twenty-three marble samples were analysed for Sr isotope composition and Rb, Mn, Mg and Ca contents, and six for C and O isotope composition. An Ediacaran depositional age of 570 -590Ma is inferred by reference to the trend of 87Sr/86Sr in Neoproterozoic seawater. Thus the metacarbonates are probably slightly older than the Ancasti Formation (equivalent to the Puncoviscana Formation of northern Argentina), which has a maximum sedimentation age of ca. 570Ma. Ediacaran depositional ages have also been reported for metacarbonates elsewhere in Argentina, Uruguay and Brazil. We propose that the Sierra de Ancasti carbonates on one hand, and those in the Western Sierras Pampeanas (Difunta Correa Sequence) and -tentativelythe Corumbá Group of Brazil on the other, represent platforms on opposite margins of the extinct Clymene Ocean, whereas Neoproterozoic carbonate successions such as the Loma Negra Formation (Tandilia, southern Argentina) and the Arroyo del Soldado Group (Uruguay) were deposited on the eastern side (present coordinates) of the Río de la Plata craton, which at the time occupied a position farther to the north.

Chapter 4 Chemostratigraphy and the Neoproterozoic glaciations

Article

Nov 2011

Although the pre-glacial Proterozoic isotopic record is poorly constrained, it is apparent that the chemical and isotopic composition of the oceans began to change during the early to mid-Neoproterozoic and experienced considerable fluctuations alongside climatic instability during much of the subsequent Cryogenian and Ediacaran periods. The earliest known large negative δ 13C excursion appears to post-date 811 Ma and fluctuations became progressively more extreme, culminating in the late-Ediacaran 'Shuram-Wonoka' anomaly. The negative excursions are commonly associated with pre-glacial and post-glacial times, while extremely high δ 13C values are characteristic of strata between glaciations. The precise causal mechanism for these excursions is subject to debate. Seawater 87Sr/ 86Sr rose during the Neoproterozoic, with abrupt increases following deglaciation consistent with enhanced weathering rates. Reported marine sulphate and pyrite δ 34S data exhibit marked variation through this interval, although the changes are not always consistent within or between sedimentary successions of equivalent age. Iron-speciation studies indicate that much of this variation was caused by fluctuating and low sulphate concentrations in seawater, which at times led to the build-up of ferruginous conditions in the ocean. The application of chemostratigraphy to understanding and correlating the Neoproterozoic glaciations evokes considerable controversy, and many questions persist regarding the reliability and calibration of the δ 13C, 87Sr/ 86Sr and δ 34S record. Nevertheless, the individual glaciations appear to be characterized by distinct combined chemostratigraphic signatures, in large part due to the generally increasing strontium isotope composition of seawater through the Neoproterozoic Era.

Carbon and oxygen isotopes of the Lianglitage Formation in the Tazhong area, Tarim Basin: Implications for sea‐level changes and palaeomarine conditions

Article

Nov 2022

Carbon isotopes can be used to interpret sea‐level changes during deposition, but the accuracy of sea‐level changes indicated by shallow‐water carbonates remains unclear. We carried out sedimentary microfacies and stable isotope analysis of carbonate rocks in Upper Ordovician Lianglitage Formation in Tazhong area, Tarim Basin, to examine the response of isotopes to high‐frequency cycles of shallow‐water carbonate rocks. The Lianglitage limestones can be divided into four types of microfacies that were deposited in reef and shoal environments on a platform margin. A total of sixty‐one (61) selected limestone samples have the δ13C value varying from 0.5993‰ to 1.6228‰ (average 1.1364‰). The extremely low correlation coefficient of carbon and oxygen isotopes indicate that the samples can represent the deposition seawater, thus the estimated Z value and temperature show that the Lianglitage Formation was deposited in the normal marine environment in a tropical to subtropical zone. The Lianglitage Formation shows a clear deposition trend from tidal flat to reef shoal and then to an open platform. The sedimentary environment controls the difference in carbon and oxygen isotopes of shallow‐water carbonates through the rate of burial of organic carbon, and thus the δ13C tend to increase when sea level rise. Hence the carbon isotopic composition of shallow‐water carbonate rocks can reflect sea‐level changes. The δ13C of Lianglitage Formation in well TZ72 shows four sedimentary cycles (20–40‐meters‐thick) controlled by sea‐level changes, which were also recorded in this Formation from other parts of the Tarim Basin. Sedimentary environment controls the difference in carbon isotopes of shallow‐water carbonates through the rate of burial of organic carbon, thus the δ13C tend to increase when sea level rise. Carbon isotopes in shallow‐water carbonate rocks can reflect sea‐level changes.

Microbialites right under our noses: Miocene and modern lakes near Las Vegas, Nevada, USA

Chapter

Mar 2022

Prepared in conjunction with the 2022 GSA Cordilleran/Rocky Mountain Sections Joint Meeting, this Field Guide showcases trips to geologically interesting areas in Arizona, Nevada, and California. Enjoy a three-day trip to the Buckskin-Rawhide and northern Plomosa Mountains metamorphic core complexes in Arizona. In Nevada, learn about the geology of Frenchman Mountain and Rainbow Gardens and landslide deposits and mechanisms in the eastern Spring Mountains. Or learn about microbialites in Miocene and modern lakes near Las Vegas. When weather permits, unravel the geological history of southern Death Valley, and explore vertebrate paleontology and Cenozoic depositional environments in Death Valley, California.

Stable isotope and rare earth element geochemistry of the Baluti carbonates (Upper Triassic), Northern Iraq

Article

Mar 2018

Faraj Tobia

Stable isotope ratios of ¹⁸O/¹⁶O and ¹³C/¹²C and rare earth elements geochemistry of the Upper Triassic carbonates from the Baluti Formation in Kurdistan Region of Northern Iraq were studied in two areas, Sararu and Sarki. The aim of the study is to quantify the possible diagenetic processes that postdated deposition and the paleoenvironment of the Baluti Formation. The replacement products of the skeletal grains by selective dissolution and neomorphism probably by meteoric water preserved the original marine isotopic signatures possibly due to the closed system. The petrographic study revealed the existence of foraminifers, echinoderms, gastropods, crinoids, nodosaria and ostracods as major framework constituents. The carbonates have micritic matrix with microsparite and sparry calcite filling the pores and voids. The range and average values for twelve carbonate rocks of δ¹⁸O and δ¹³C in Sararu section were –5.3‰ to –3.16‰ (–4.12‰) and –2.94‰ to –0.96‰ (–1.75‰), respectively; while the corresponding values for the Sarki section were –3.69‰ to –0.39‰ (–2.08‰) and –5.34‰ to –2.70‰ (–4.02‰), respectively. The bivariate plot of δ¹⁸O and δ¹³C suggests that most of these carbonates are warm-water skeletons and have meteoric cement. The average ΣREE content and Eu-anomaly of the carbonates of Sararu sections were 44.26 ppm and 1.03, respectively, corresponding to 22.30 ppm and 0.93 for the Sarki section. The normalized patterns for the carbonate rocks exhibit: (1) non-seawater-like REE patterns, (2) positive Gd anomalies (average = 1.112 for Sararu and 1.114 for Sarki), (3) super chondritic Y/Ho ratio is 31.48 for Sararu and 31.73 for Sarki which are less than the value of seawater. The presence of sparry calcite cement, negative ¹³C and ¹⁸O isotope values, the positive Eu anomaly in the REE patterns (particularly for Sararu), eliminated Ce anomaly (Ce/Ce*: 0.916–1.167, average = 0.994 and 0.950–1.010, average = 0.964, respectively), and Er/Nd values propose that these carbonates have undergone meteoric diagenesis. The REE patterns suggest that the terrigenous materials of the Baluti were derived from felsic to intermediate rocks.

Neoproterozoic marine carbonates and their paleoceanographic significance

Article

Nov 2017
GLOBAL PLANET CHANGE

The primary mineralogy of marine carbonate precipitates has been a crucial factor in constraining the major element composition of ancient oceans. Secular changes in Phanerozoic marine chemistry, including Mg/Ca, have been well-documented using the original carbonate mineralogy of ooids, marine cements and biominerals. However, the history of Precambrian seawater chemistry is not as well constrained, partially due to the prevalence of dolomitisation in the Precambrian geological record. The Neoproterozoic (~ 1000 Ma to ~ 541 Ma) record of primary carbonate mineralogy is documented here using a combination of literature data and new analysis of marine carbonate precipitates from the Otavi Fold Belt, Namibia, the Death Valley succession, USA and the Adelaide Fold Belt, Australia. These data suggest that the last ~ 460 million years of the Proterozoic were dominated by aragonite and high-Mg calcite precipitation in shallow marine settings. In contrast, low-Mg calcite has only been recognised in a small number of formations. In addition to aragonite and calcite precipitation, marine dolomite precipitation was widespread in Neoproterozoic oceans, including mimetic (syn-sedimentary) dolomitisation and primary dolomite marine cementation. The combination of marine aragonite, high Mg-calcite and dolomite precipitation during the Neoproterozoic suggests extremely high seawater Mg/Ca conditions relative to Phanerozoic oceans. Marine dolomite precipitation may also be linked to widespread marine anoxia during this time.

The carbon and oxygen isotopic compositions and their evolution of the Xixiangchi Group carbonate rocks in Sichuan Basin and their geological implications

Article

Nov 2016

Carbon and oxygen isotope analysis of Xixiangchi Group shows that the bottom boundary of the Upper Cambrian is defined for the first time. The Xixiangchi Group formed in a high salinity on shore marine sedimentary environment with a warm to dry climate and underwent a slow regression after a rapid transgression in the early and middle stages, In the late stage it entered a rapid regression after a slow transgression.

Proterozoic Stromatolite Taxonomy and Biostratigraphy

Chapter

Jan 2000

Precambrian stromatolites are unique objects in Earth history. The predominance of microbes in ecosystems that they document, their specific global environments, and the scale of their evolution have no counterparts in the Phanerozoic. Among several, basically different stromatolite classifications known in the literature, the current version of the traditional system is most extensively employed in the study of Precambrian buildups. It is artificial in nature but follows conventional rules of paleontological classification and requires definition of a hierarchy of taxa: forms (form-species), groups (form-genera), and types. At present, there is a common understanding of stromatolite characteristics, providing the most efficient basis for definition and identification of the traditional system taxa. The types are based on the most general features of the buildup’s morphology. Groups are based on particular combinations of morphological characteristics defined by the mode of accretion and shape of stromatolite laminae (plus some general features of the microstructure in several cases). Forms are predominantly or exclusively based on microstructure. The stratigraphic potential of Precambrian stromatolites, revealed by empirical time-and-space distribution data of the distinctive assemblages, is evident. Stromatolites are not suitable for the subdivision of the Proterozoic, but provide paleontological characterization of units which have been defined by other methods and significantly contribute to their correlation especially within the limits of particular stromatolite provinces. Interprovincial stromatolite-based correlations are of lower reliability and time-resolution due to strong variations in the taxonomic composition of coeval stromatolite assemblages across provincial boundaries. Precambrian stromatolites demonstrate distinctive directional secular changes in taxonomic composition and diversity which were defined by the evolution of both global environmental and biological factors relevant to the construction and habitat restrictions of these biolites.

Tussocky Microstructure, a Biological Event in Upper Proterozoic Stromatolites; Comparisons with Modern Freshwater Stromatolite Builders

Chapter

Jan 1992

An unusual type of microstructure occurs in stromatolites of the Upper Proterozoic cover of the West African Craton. These stromatolites bearing the same microstructure are reported from diverse sections along the northern edge of the Taoudenni basin and from the Pan-African fold belt of the Hoggar (Bertrand-Sarfati 1972). Their microstructure has been defined as “tussocky” (Bertrand-Sarfati 1976). The stromatolites are entirely built by tussocks, absent from the interspaces between columns. They comprise hemispherical bodies, juxtaposed and overgrowing, roughly superimposed to compose irregular but obvious laminae. The tussocks are made of rods which are interpreted as the remnants of previous filaments. Most of the time they are straight, rarely branched. A common feature of the tussocks is the banding, concentrically arranged bands of brown pigmentation. According to Fairchild et al. (1990), the carbonate of the tussock was originally aragonitic and recrystallization into calcite appeared during burial. In order to compare the tussocks with modern stromatolite builders, a range of measurements were made on Rivularia, Inzeria, and Serizia to clarify their structural relationships. Among 11 characters listed, 7 are shared by Rivularia and the tussocks. The four differences are heliotropism, branching and curvature of the filaments and the interface between adjacent colonies. The observations, however, point to a biogenic origin for the tussocks, but we must accept that they have no modern counterparts. The microorganisms building the tussocks may have been filamentous, weakly heliotropic cyanobacteria with a radiating structure.

Microstructures in Tertiary Nonmarine Stromatolites (France). Comparison with Proterozoic

Chapter

Jan 1994

Late Cretaceous to Early Miocene series of Southern France and northeastern Spain display an almost continuous record of nonmarine stromatolites. Well preserved organic remains, mainly microbial or algal filaments, are tentatively used as a base for the description of stromatolite microstructures of fluviatile, lacustrine and brackish origins. Four terms can be described as follows: a) microstructures with well preserved filaments are subdivided according to filament size and disposition within the lamina, b) microstructures with dubious filaments, c) zoned structures composed of fibro-radial sparite, d) associated non-microbial biogenic microstructures. Recent petrological works and biological studies on modern fluviatile and lacustrine stromatolites afforded terms of comparisons whereas paleoenvironmental considerations suggest comments on salinity requirements for several of the microstructures. This descriptive approach should allow for the addition of new categories according to similar criterion: nature of the organic remains, size and habit of filaments and spheres. It furthermore facilitates comparisons with some Proterozoic stromatolite microstructures.

Carbon and oxygen isotopic constraint on the sedimentary environment and formation mechanism of the Longhu potash deposit, Laos

Article

Apr 2015

Carbon and oxygen isotopes have been used widely to indicate ancient sedimentary environment, sources of ore deposits and water-rock reactions. In this paper, characteristics of carbon and oxygen isotopic composition of carbonates in the mudstone from the borehole ZK309, ZK301, ZK311 and ZK004 were studied to discuss the geological evolution and sedimentary environment of the Nong Bok Formation in Laos. It is concluded that the carbonates are primarily authigenic according to analysis of mineral composition and carbon and oxygen isotopic composition of the mudstones. The result shows that the stable carbon isotope composition (δ13C) slightly higher than -5.69‰ and the stable oxygen isotope composition (δ18O) varying between -2.71‰ and -10.8‰, and the average value of δ13C and δ18O of the carbonates are -5.1‰ and -4.6‰ respectively in the mudstone. The multiple correlation coefficients R between δ13C and δ18O values for samples from the borehole ZK309 and ZK301 are lower than 0.7. Based on the carbon and oxygen isotopic compositions and correlations of δ13C and δ18O value, we conclude that the Nong Bok Formation is terrestrial facies. The average value of δ13C are slightly higher than the threshold value of terrestrial carbonates (-5.69‰), it may due to the mudstones were reformed by brine after deposited. The brine might be generated from dissolution of the underlying salt rocks by meteoric water. This could decipher the abnormal sequence of salt rock deposition (sylvite overlies carnallite). Therefore the meteoric water played an important role in the changes of oxygen and carbon isotopic compositions of the carbonates in mudstones in the study area.

Petrology and stable isotope (S, C, O) studies of selected sedimenthosted basemetal ore deposits in the proterozoic Aravalli-Delhi Fold Belt, Rajasthan

Article

Feb 2014

An integrated mineralogical-geochemical and stable isotopic study of Pb-Zn deposits located at Kayar-Ghugra (Zn-Pb ± Ag), Rampura-Agucha (Zn-Pb, Ag), Dariba-Bethumni (Zn-Pb) and Zawar (Pb-Zn ± Cd, Ag) in Rajasthan is presented in this paper. The Kayar Zn-Pb deposit hosted by (i) phlogopite-tremolite bearing dolomitic carbonates and (ii) scapolite bearing calc-silicates, both belonging to Mesoproterozoic Delhi Supergroup exhibit distinctly different δ13C signatures being close to zero permil for the former reflecting deposition in pristine marine environment and much depleted isotopic values for the latter possibly related to post-depositional alterations. The Zn-Pb sulphides of Agucha, hosted in amphibolite facies to lower granulite facies metasedimentary units belonging to the Bhilwara Supergroup have δ34S values that indicate (i) H2S dominated regime characterized by low fO2, low pH, wherein the δ34S(fluid) responsible for mineralisation approximates the δ34S(sulphide); (ii) the role of seawater in the generation of Agucha ores; (iii) the process of a low temperature oxidation of sulphides in the hydrothermal fluids resulting in the formation of sulphate, by the interaction of ground water; (iv) isotopic disequilibrium in sulphatesulphide pairs that explain oxidation of H2S by acid groundwater (low pH) and deposition of sulphides at higher temperatures and (v) equilibrium isotopic fractionation of the coexisting sulphides reflecting in a higher concentration of H2S (>10−5m) in relation to the total metal content in the hydrothermal fluid \(\left( {m_{H_2 S} \geqslant mS_{_{metals} } } \right)\) . Accordingly the concentration of sulphide-sulphate in the hydrothermal solution responsible for the mineralization in Agucha exceeds that of total metals. The sulphides of Bethumni-Rajpura-Dariba belt hosted in low to medium grade siliceous carbonates has a marginally positive (mean of +1.5‰) δ13C values. At Sindeswar, broad and widely scattered δ34S values indicate a polymodal sedimentary source of sulphur that recrystallised at rather low temperature of < 50°C possibly during the processes of low temperature bacterial reduction. The C and O-isotopic studies on mineralized and non-mineralized carbonates reveal (i) normal marine depositional signatures for non-mineralized carbonates with possible minor influence of biogenic carbon during deposition and (ii) ore zone carbonates exhibit depleted δ13C values presumably due either to the deeper mantle-like source of carbonates or due to post-depositional equilibration with isotopically light meteoric waters. In Zawar belt, sulphides hosted in dolomitic carbonate indicated (i) near identical δ34S values of disseminated galena and pyrite veinlets and depleted values of −4.6 ‰ for late veins of massive galena of Zawar Mala (ii) pyritepyrrhotite veinlet having enhanced δ34S values when compared to the PbS-ZnS veinlet in Morchia-Magra, Balaria and Baroi mines. The carbon isotopic values for carbonates of Zawar Mala mine area are mostly depleted and those from Balaria and Baroi mines exhibit values of 13C close to zero. The generally depleted δ 18O clustering around −15 ‰ tally well with the reported Paleoproterozoic carbonates and is attributed to the post-depositional equilibration reactions with isotopically light meteoric waters. It is summarized that the host carbonates for Zn-Pb deposits occurring in different tectono-stratigraphic units in Rajasthan have largely similar but bimodal distribution of δ 18O and δ13C isotopic ratios that suggest normal marine values and much depleted values. Whereas the former seems to be in general agreement with the nature of distribution in the Palaeoproterozoic carbonates the latter is attributed to (i) depositional conditions of the basins that includes absence or presence of biogenic activity (ii) isotopic re-equilibration under different metamorphic recrystallization events and/or (iii) interaction with isotopically lighter meteoric waters. In contrast to the uniformity in the C and O distribution pattern, the S-isotopic distribution in the deposits of Rampura-Agucha, Bethumni-Rajpura-Darbia and Zawar mine areas show marked variations reflecting complex deposit-specific ore-forming processes in the said deposits.

Primary Submarine Cements and Neomorphic Spar in a Stromatolitic-Bound Phylloid Algal Bioherm, Laborcita Formation (Wolfcampian), Sacramento Mountains, New Mexico, U.S.A.

Chapter

Full-text available

Jan 1981

Several mound-shaped bioherms with a maximum depositional relief of 12 m are developed within and upon a tabular limestone stratum in the middle of the Laborcita Formation (Lower Permian; Wolfcampian) in the Sacramento Mountains, near Tularosa, New Mexico, U.S.A. The bioherms are constructed by a restricted biota of erect codiacean (?) phylloid algae encrusted by wavy-laminar, digitate, and mammillary stromatolites preserved in living habit. Mound morphology, biotic constituents and stratigraphie relations are described elsewhere (Cross and Klosterman, this vol.). Primary void space was created as sheltered cavities within and between phylloid algal fronds during development of the bioherms. Although the algal constituents supported weight and created an interlocking organic constructional framework, penecontemporaneous submarine botryoidal cements precipitated within sheltered cavities contributed significantly to mechanical rigidity and prevented the collapse of the primary biotic framework. In this respect, these cements had a more important mechanical function than similar cement fabrics described from other reef and biohermal structures of Paleozoic through Holocene age, in which precipitation occurs within already rigid framework-supported void space.

Chemical analysis of sedimentary rocks

Article

Full-text available

Jan 1988

Describes important principles of sedimentary geochemistry: concentrations and activities, equilibrium, adsorption, incorporation of trace elements and partition coefficients, and stable isotope fractionation. The techniques covered in this chapter are X-ray fluorescence, atomic absorption spectrometry, inductively-coupled plasma optical emission and mass spectrometry, electron microbeam analysis, neutron activation analysis and stable isotope (C.O.S.) analysis. Applications are described to the study of provenance and weathering, the deduction of environmental parameters, diagenesis and pore fluid chemistry, and elemental cycling. -from Editor

Les corrélations du Protérozoïque au Cambrien en Afrique de l'Ouest: Nouvelle interprétation géodynamique

Article

Full-text available

Sep 1987

The Role of Skeletal Porosity in Aragonite Neomorphism--Strombus and Montastrea from the Pleistocene Key Largo Limestone, Florida

Article

Full-text available

Jan 1986

The neomorphism of aragonite to calcite involves complex textural and chemical alterations that are in part controlled by the degree of communication between diagenetic and ambient formation fluids. Two taxa were examined in order to evaluate the effect of initial skeletal porosity on system closure during neomorphism. Fabrics of replacement calcite in both taxa are similar, consisting of blocky crystals largely restricted to areas of former skeletal aragonite. Oriented aragonite and organic inclusions result in the preservation of relic fabrics in both taxa. The chemical signatures of coralline and molluscan aragonite and calcite are strongly dependent on initial skeletal porosity. Differences in the thickness of replacement calcite rims between taxa reflect variable skeletal porosities and rates of migration of neomorphic fronts. Thus, skeletal porosity influences degrees of system openness during aragonite neomorphism.-from Authors

Derivation of the Original Isotopic Composition of Permian Marine Cements

Article

Full-text available

Jan 1985
J Sediment Petrol

Former aragonite and magnesian calcite marine cements from the Permian Reef Complex may contain a secondary calcite phase which has inherited the original isotopic composition of the precursor marine cement. Derives the isotopic composition of this secondary calcite phase in the Permian Reef cements in order to test the proposition that marine isotopic signatures are indeed preserved.-from Authors

Geochemical Patterns of Meteoric Diagenetic Systems and Their Application to Studies of Paleokarst

Chapter

Full-text available

Jan 1988

Kyger C. Lohmann

The isotopic and cation chemistry of meteoric waters changes in response to the effects of rock—water interaction, uptake of organically derived CO2, and primary mineralogic differences among carbonate terranes. Moreover, variations in the dominance of these factors produce diverse chemical conditions within the meteoric systems which allow the sub- environments of vadose-phreatic, mixed-water, and spelean diagenesis to be distinguished. Therefore, geochemical patterns within the meteoric water system are examined to provide criteria for recognition of these subenvironments of meteoric diagenesis in ancient carbonate sequences.

The Nd and Sr isotopic evolution of Proterozoic seawater

Article

Full-text available

Apr 1988

Nd and Sr isotopic measurements from a variety of marine chemical precipitates provide insight into processes controlling the chemistry of seawater during the Proterozoic. Nd isotope measurements on banded iron formations and phosphorites, and Sr isotope measurements on carbonates indicate that during both the Early and Late Proterozoic, hydrothermal input to the oceans was a significant fraction of the total input to ocean chemistry. Isotopic data from Early Proterozoic clastic sediments show systematic differences from coeval chemical sediments. These differences become less marked toward the end of the Proterozoic. This implies a higher hydrothermal water to river water flux ratio during the Early Proterozoic. The significant changes in seawater isotopic composition during the Proterozoic reflect the transition from mantle dominated Archean oceans to a typically modern system. Isotopic and mass balance constraints from banded iron formations indicate that oxidation of hydrothermal Fe2+ in seawater could have been an important control on the redox state of Proterozoic ocean-atmosphere system. The present isotopic balances of carbon and sulfur could have been decoupled as a result.

Sedimentation of organic-rich limestones in the late Precambrian of Southern Norway

Article

Full-text available

Oct 1983
PRECAMBRIAN RES

Maurice Tucker

Organic-rich laminated limestones are a major facies of the Late Precambrian Biri Formation, deposited around the margins of the Sparagmite basin of southern Norway. The laminated limestones consist of an alternation of calcitic laminae and clay laminae. Trace element and stable isotope analyses confirm a marine origin for the carbonate and suggest that the original precipitate was aragonite. The organic matter reaches 3% (weight loss on ignition) and apparently is finely dispersed within the sediment. Both cm-thick limestone beds of probable storm origin, and thin, disrupted and folded horizons of slump origin occur within the laminated facies. The organic-rich laminated limestones occur at several horizons and are interbedded with carbonate grainstones of shallow-water origin. Basinwards, they pass into light to dark grey to green, organic-deficient mudrocks with resedimented and slumped units.The facies context of the organic-rich laminated limestones suggests deposition in an outer shelf setting at times of higher sea-level stands. The transgressive events, possibly induced by changes in glacial ice cover of near or distant landmasses, led to increased organic productivity through seasonal blooms of phytoplankton. Preservation of the organic matter in the sediments resulted from the development of anoxic or at least oxygen-depleted bottom waters. Some physical constriction of water circulation in the Sparagmite basin may have contributed towards the anoxic events.The development of Precambrian organic-rich ‘black shales’ in the Sparagmite basin of southern Norway does show that potential hydrocarbon source rocks were formed in the pre-Phanerozoic, in situations comparable to some Mesozoic and Tertiary oil source rocks.

87Sr86Sr in Late Proterozoic carbonates: evidence for a "mantle" event at ∼900 Ma ago

Article

Full-text available

Feb 1983
GEOCHIM COSMOCHIM AC

Strontium isotopic measurements were made on Late Proterozoic carbonates from West African Craton. Comparison of samples with acceptable trace element patterns with coeval data from southern Africa and with the published Australian results suggests that the ratio of the Late Proterozoic sea water evolved in the following manner about 0.7075 at 1000 ± 50 Ma, 0.7056 to 0.7074 at 900 ± 50 Ma, 0.7068 to 0.7091(0.7106) at 800 ± 50 Ma, 0.7074 to 0.7077 at 700 ± 50 Ma, and 0.7076 to 0.7089(0.7096) at 600 ± 50 Ma ago. The variations are comparable in magnitude and frequency to those described previously for the Phanerozoic. Strontium isotopic values in the radiogenic part of this range suggest that the continental river flux of Sr into Late Proterozoic oceans was of comparable isotopic composition to its present day counterpart (∼0.711). Consequently, the non-radiogenic value at ∼900 ± 50 Ma ago signifies a large flux of “mantle” strontium into the ocean at this time. Because the present time resolution is only about 75 ± 25Ma, additional sampling as well as better stratigraphie resolution and more definite selection criteria are required for construction of a more detailed Late Proterozoic sea water curve.

∂18O and ∂13C variations in Late Devonian marine cements from the Golden Spike and Nevis Reefs, Alberta, Canada

Article

Full-text available

Sep 1989

Detailed chemical and petrographic analysis indicates that diagenetic alteration is not the cause of the low δ 18O values. In addition, such a difference cannot be fully explained by warmer Late Devonian oceans. These data suggest that Late Devonian seawater had a δ 18O value lower than modern oceans. This in turn, suggests that the balance of low temperature silicate weathering and high temperature seawater-basalt exchange reactions must have been different from today's. -Authors

Diagenesis in limestones‐3. The deep burial environment

Article

Jan 1987

Calcification of Encrusting Aragonitic Algae (Peyssonneliaceae): Implications for the Origin of Late Paleozoic Reefs and Cements

Article

Jan 1988

Although the confirmed fossil range of this family extends only to the Early Cretaceous, striking similarities between these aragonitic forms and some late Paleozoic phylloid algae suggest that they may be closely related. Their ability to grow on soft mud substrates and form structures composed of irregular arched sheets with extensive pore space, as well as their brittle nature, so susceptible to fragmentation, are all characteristics of mound-forming fossil phylloid algae. -from Authors

Sea-level changes: An integrated approach: Society of economic paleontologists and mineralogists, special publication, no. 42

Article

Waterrock interaction history of regionally extensive dolomites of the Burlington-Keokuk Formation (Mississippian): isotopic evidence

Article

Jan 1988

Two sequences of pervasive dolomitization are preserved in the Mississippian Burlington-Keokuk Formation of Iowa, Illinois, and Missouri. Cathodoluminescent petrography reveals 1) an early, post-depositional, dolomite-forming episode (dolomite I), and 2) a later dolomite (dolomite II), which replaced the first generation. These texturally and temporally distinct dolomites are correlative over 100 000 km2 of outcrop and subsurface and have distinguishing isotopic and trace-element characteristics. Calculation of the simultaneous isotopic variations that occur during water-rock interaction demonstrates important differences in the relative rates at which the O, C, Sr, and Nd isotopic compositions of diagenetic carbonates are altered. These quantatitve models are used to place constraints on the water-rock interaction history of the Burlington-Keokuk dolomites. -from Authors

Isotopic evidence for source of diagenetic carbonates formed during burial of organic-rich sediments

Article

Jan 1977

Radiaxial fibrous calcite: a reappraisal ( Western Australia).

Article

Jan 1985

Alan Kendall

New observations upon cavity-filling radiaxial calcites from Western Australia suggest that most of the features originally considered of neomorphic origin are instead primary.-from Author

Diagenesis of the Upper Proterozoic Siliciclastic Sediments of the Taoudeni Basin (West Africa) and Relation to Diabase Emplacement

Article

Jan 1989

In spite of a thin sedimentary pile, the Taoudeni basin experienced a high-temperature diagenesis. This resulted from circulation of hot fluids, generated during the diabase emplacement, over extensive areas within the basin. -Authors

Fluid-flow in the crust - an example from a Pyrenean Thrust Ramp

Article

Mar 1988

En-echelon calcite filled vein arrays and accompanying stylolites are developed in the carbonate unts of a ramp section in the upper plate of the Pineta Thrust Complex in the Cinca Valley of the Spanish Pyrenees. Non-fibrous euhedral calcite infillings often crosscut the pre-existing fiber patterns in discrete zones of late dilation. Carbon and oxygen isotope data were obtained from chronologically placed micro-samples of calcite both from rock matrix and sigmoidal veins. We interpret these data to indicate that the matrix calcite was massively exchanged with an externally derived fluid during an early porous flow regime prior to the development of the vein-stylolite system. -from Authors

Chapter 5.2 An Attempt to Classify Late Precambrian Stromatolite Microstructures

Chapter

Dec 1976

J. Bertrand-Sarfati

This chapter presents an attempt to classify late Precambrian stromatolite microstructures. The stromatolite primary lamination reflects the growth pattern of the algal coenose and the habit of the carbonate precipitated or trapped within the filament framework, among other things. The fossil stromatolite microstructure includes furthermore the imprint of subsequent diagenesis. A carefully detailed description of the actual fabric of the microstructure mixing up original and diagenetic features leads to an excessive number of microstructural patterns, and a very artificial classification. The need of modern models is great because of the excessive attention paid by the geologists to the sedimentary and physical processes. Well-preserved microstructures were proposed for the classification of stromatolites that were well adapted: (1) simple microstructures where the laminations or the dominant fabrics follow the rhythmical growth pattern of the coenose; and (2) complex microstructures where the historical succession of laminations presents microstructural changes due to seasonal differentiation of the algal coenose.

Kinetic Control of Morphology, Composition, and Mineralogy of Abiotic Sedimentary Carbonates

Article

Jan 1985

Conventional thinking has long held that the abiotic precipitation of calcium carbonate occurs with a causal relationship between fluid Mg/Ca ratios and crystal morphology, crystal composition, and carbonate mineralogy, resulting in the formation of meteoric, equant, low-magnesium calcite and marine, acicular, high-magnesium calcite and aragonite. Problematically, calcites with varying amounts of incorporated magnesium occur either as equant or acicular crystals, and aragonite may coexist with calcite in either environment. Commonly, however, a systematic relation exists between crystal morphology, composition, mineralogy, and rates of reactant supply to growing crystal surfaces. Such relations suggest that crystal morphology, composition, and mineralogy are controlled by the kinetics of surface nucleation and the amount of reactants, principally carbonate ions, at growth sites. Precipitating phases are the ones which can best accommodate such excess reactants; ambient Mg/Ca ratios only indirectly control the nature of inorganically precipitated carbonate phases.-from Authors

Proterozoic stromatolite zonation

Article

Nov 1969

The zonation of the Proterozoic of the USSR is reviewed and extension to other places is attempted. Preliminary results are: some of the columnar stromatolites of northern Eurasia occur in other continents and in the same stratigraphic ranges; some of the forms ('species') of the distinctive columnar types have intercontinental distribution, such as Gruneria biwabikia from Australia and North America. Pre-upper Proterozoic (pre-Riphean) strata also contain distinctive stromatolites of intercontinental distribution not known in younger strata, such as the new groups ('genera') Gruneria and Katerina. The above, together with progress in microbiota studies offers hope that at least a gross worldwide biological subdivision of Proterozoic will be practicable. The Belt Series seem to represent only one of the Soviet stromatolite zones.

Kinetic control of morphology, composition, and mineralogy of abiotic sedimentary carbonates; discussion and reply

Article

Nov 1985

Calcification of cyanobacterial mats in Solar Lake, Sinai

Article

Oct 1984
GEOLOGY

Pore-water samples were obtained from the shallow-water part of Solar Lake (Sinai) where luxurious cyanobacterial mats grow. These samples were analyzed for Ca2+, Mg2+, Sr2+, Cl-, and titration alkalinity (TA) to determine the role of cyanobacterial growth and degradation on the calcification of the mats. The data are modeled thermodynamically to predict mineral pore-water equilibria. Our data support earlier bacterial and sedimentological studies suggesting that the degradation of the cyanobacterial mat via sulfate reduction is of major importance in the calcification process.

Preserved stable isotopic signature of subaerial diagenesis in the 1.2-b.y. Mescal Limestone, central Arizona: Implications for the timing and development of a terrestrial plant cover

Article

Jan 1985

A Precambrian exposure surface in the 1.2-b.y. Mescal Limestone has been isotopically examined for indications of a carbon isotopic signature that might indicate the presence of a subaerial vegetative cover in the middle Proterozoic. delta 18O values of the Mescal carbonates show two distinct data sets: 1) dolomites from an unaltered zone which were unaffected by subaerial diagenesis have delta 18O values ranging from +19.9 to +25.6per mille Standard Mean Ocean Water (SMOW) with cherts averaging +30per mille, and 2) dolomites from a dissolution zone subaerially exposed in the Precambrian with delta 180 values ranging from +13.9 to +22.4per mille and cherts averaging +25per mille. delta 13C of dolomite ranges from +3.7per mille Pee Dee Belemnite (PDB) in the unaltered zone to 0per mille in the dissolution zone. The dissolution zone consists of a karst breccia of recemented dolomite and chert fragments with numerous clastic solution dikes. Isotopic and field data indicate that the delta 18O of the unaltered dolomite was fixed during early meteroic-water diagenesis, including dissolution-silicification of evaporites and dolomitization. During a later subaerial exposure event, a large flux of meteoric water flushed through the dissolution zone and produced the same isotopic patterns that Allan and Matthews documented for younger examples as indicative of vegetatively covered land surface. Alternative explanations for producing the observed delta 13C variations in the absence of vegetation do not seem feasible. We therfore suggest that the subaerial environment 1.2 b.y. ago was a biologically active zone. -Authors

40Ar/39Ar polyorogenic mineral age record within the central Mauritanide orogen, West Africa

Article

Jan 1989

The central Mauritanide orogen in the vicinity of Magta Lahjar is characterized by several internally imbricated, polydeformed and variably metamorphosed infrastructural allochthons. 40Ar/39Ar incremental-release ages have been determined for 10 hornblende and 22 muscovite concentrates prepared from various units of the infrastructural and suprastructural allochthons. Results from these analyses are interpreted to show that the early tectonic history recorded within the central Mauritanide orogen appears to have been related to restricted rifting of a western crustal block from the West African craton and formation of a limited, intervening oceanic domain. Subsequent closure resulted in compression and imbrication of continental crust with resultant development of an ensialic volcanic arc (calc-alkaline sequences within the infrastructural allochthons) together with widespread metamorphism and deformation between ca. 700 and 650 Ma. Later orogenesis at ca. 550 Ma could have reflected distal effects of collisional events which dominate the Rokelide orogen in Sierra Leone and Guinea. Late Paleozoic tectonothermal activity within the central Mauritanides is interpreted to have developed as a result of a renewed translation between the western continental block and the West African craton during final collision of Laurentia and Gondwana. -from Authors

Carbon and oxygen isotopic composition of Holocene reefal carbonates

Article

Jan 1985
GEOLOGY

Uses samples from Enewetak, Bikini, Bermuda, and Belize to identify components that exhibit systematic isotopic fractionation relationships. Skeletal components exhibit inconsistent fractionation relative to inorganically precipitated marine cements, suggesting that marine cement compositions are more reliable indicators of the physicochemical conditions of precipitation than co-occurring skeletal carbonates. -from Authors

Diagenesis of Late Proterozoic Carbonates: The Beck Spring Dolomite of Eastern California

Article

Jan 1988

Petrographic and compositional attributes of limestone and dolostone indicate that late Proterozoic mineralogies, isotopic compositions, and diagenetic processes were similar to those inferred for younger Phanerozoic marine carbonate sequences. -from Authors

New information on the probable isotopic age of the late Proterozoic glaciation in West Africa

Article

Sep 1987
PRECAMBRIAN RES

New RbSr age determinations on smectitic clay fractions from a formation of the late Proterozoic glaciation in West Africa constrain the occurrence of that major event to between 630 and 595 Ma.

Changes in marine isotopic composition and the late Ordovician glaciation

Article

Feb 1990

Well-preserved brachiopod shells and marine cements from limestone coquinas which cap carbonate mudmounds in the Siljan area of central Sweden have tube heaviest stable oxygen and carbon isotope values yet reported for Lower Palaeozoic marine sediments. be coquinas formed during the eustatic regression at the onset of the late Ordovician glaciation: the isotopic compositions reflect simultaneous shifts in both carbon and oxygen (more than 5% δ13C and up to 2% δ18O) away from more normal Lower Pplneozoic values in similar carbonates from within the mounds. Oxygen isotope data are consistent with a change in the isotopic composition of the sea water probably accompanied by a decrease in temperature. The changes in carbon values suggest enhanced deposition of organic carbon, a process which would have decreased pC02 in the ocean and the atmosphere and thus contributed to rapid global cooling.

Palaeoenvironment and carbon isotope stratigraphy of Upper Proterozoic carbonates of the Yangtze Platform

Article

Jan 1987

The δ13C values for Phanerozoic marine carbonates display secular trends, mainly in the range –1 to 2‰1. These have been interpreted as primary features reflecting changes in the carbon isotope composition of dissolved bicarbonate in seawater, brought about largely by variations in the proportions of carbon being sequestered in the oxidized and reduced carbon reservoirs1,2. Carbon isotope trends are not as well established for the Precambrian, but there are indications that Upper Proterozoic sequences contain carbonates with unusually high δ13C values3–9. Here we present the results of the first carbon isotopic study of the Upper Proterozoic carbonate strata from the type locality of the Sinian System in the Yangtze Gorges sections near Yichang, South China, and discuss their implications for accumulation of organic matter. We also compare the results for these Chinese strata with δ13C trends in other Upper Proterozoic–Cambrian sequences, and assess the extent to which such isotopic data can be used to make stratigraphic correlations between widely spaced sequences.

Late Proterozoic glacial carbonates in northeast Spitsbergen: new insights into the carbonate–tillite association

Article

Sep 1989
GEOL MAG

Carbonate-rich glacial deposits from two discrete Vendian glacial periods are described. The older is represented by the 24–40 m thick Petrovbreen Member (E2) of the Elbobreen Formation which contains abundant detrital dolomite. Clasts in E2 and their possible source rocks have positive δ ¹³ C and negative δ ¹⁸ O PDB values. In contrast the carbonate mud-fraction of E2 sediments has different cathodoluminescence characteristics from clasts, slightly negative δ ¹³ C values, and higher Fe and Mn concentrations than clasts. Oxygen isotopes vary from −2.5 to +4.5 ‰PDB , thought to be related to various seawater–meltwater mixtures in the depositional environment. Preservation of information about glacial sedimentary environments is attributed to early diagenetic recrystallization forced by excess surface free energy (Ostwald's ripening) and coinciding with sulphate reduction. The younger glaciation is represented by the Wilsonbreen Formation (170 m) which has a distinctive glaciolacustrine Middle Carbonate Member (W2). W2 contains precipitated periglacial carbonates (with high Mn/Fe ratio): both limestone (rhythmitic and stromatolitic) and dolostone (rhythmites, stromatolites and dolomite-rich sandstones) which are compared with carbonates in modern Antarctic lakes. Evaporitic environments for dolomitic sandstones and stromatolitic dolostones are indicated by heavy oxygen isotope values (up to + 10.5 ‰PDB ), high Na concentration and evidence for dissolved evaporites. High Mn concentration in detrital dolostones in W2 is suggestive of syn-sedimentary dolomite recrystallization in freshwater diamictites and haematitic siltstones. The carbonate–tillite association ultimately arises from the erosion of underlying carbonates which originated (in this case) under radically different climatic conditions. Glacial depositional waters then became carbonate-saturated as a result of dissolution of detrital carbonate. Massive recrystallization of glacially transported carbonate is proposed as a geologically significant process with considerable potential for palaeoenvironmental analysis. In glacial lakes carbonate precipitated in response to evaporation or photosynthesis. Carbonate precipitation as the result of seawater freezing, or in warm interglacial conditions, is not yet established. Oxygen isotope value are inconsistent (too heavy) with the presence of high-latitude meltwaters, implying that glaciation extended to low latitudes as proposed by Harland.

87Sr/86Sr in Late Proterozoic carbonates: evidence for a "mantle" event at ca. 900 m.y. ago

Article

Jan 1983
GEOCHIM COSMOCHIM AC

Did major changes in the stable-isotope composition of Proterozoic seawater occur?

Article

Jan 1990
GEOLOGY

Early Proterozoic marine carbonates of the Rocknest Formation (1.93-1.89 Ga) have very depleted delta18O values (about 80/00) relative to younger, Late Proterozoic marine carbonates that formed in similar depositional environments. Two isotopic trends are superimposed on the data for open-marine components. The first involves stabilization of tidal-flat sediments during early, possibly reflux-type dolomitization by evaporative pore fluids enriched in delta18O The second trend toward isotopically light delta18O values was established during dolomitization of open-marine facies in contact either with meteoric waters (mixing zone) or under conditions of higher temperatures during burial. This resulted in precipitation of blocky, pore-occluding cements. The isotopically most enriched ooids are the best preserved normal marine components and may suggest that the delta18O of seawater was about -9.750/00 ±1.00/00 (SMOW) at 1.9 Ga. This composition would require a major change in the balance of high-temperature oxygen isotopic exchange between seawater and basalt and low-temperature weathering in order to explain the 80/00 positive shift in inferred seawater delta18O between 1.9 and 1.0 Ga. Alternatively, the depleted delta18O values represent an approximately 30-35 °C higher temperature of surface waters at 1.9 Ga. The heaviest carbonate delta13C values are +1.75permil;, more enriched than previously reported for the Early Proterozoic on the basis of bulk-rock data.

The Chemical Evolution of the Atmosphere and Oceans

Book

Dec 1984

Heinrich D. Holland

Is cratonic sedimentation consistent with available models? An example from the Upper Proterozoic of the West African craton

Article

Aug 1988
SEDIMENT GEOL

The Atar Group, part of the Upper Proterozoic sequence covering the West African craton, stable since 2000 Ma, is characterized by an alternation of extensive carbonate beds and mixed siliciclastic and carbonate facies. The carbonate beds comprise essentially columnar stromatolite biostromes and bioherms which reflect sublittoral environments. The mixed facies contain a variety of laterally discontinuous facies which imply more variable environmental conditions. The settings of the mixed facies are not always clear but they do not contain thick sequences of high-energy facies.

Biogeochemical cycles of carbon and sulfur and their effect on atmospheric oxygen over Phanerozoic time

Article

Jan 1989
GLOBAL PLANET CHANGE

Robert A. Berner

The biogechemical cycles of carbon and sulfur contain within them the principles processes that control the level of atmospheric oxygen over Phanerozoic time. These are the production, deposition, burial and eventual oxidative weathering of organic matter and the diagenetic formation, burial and eventual oxidative weathering of sedimentary pyrite. Observations of modern sedimentary environments indicate that: (1) most organic matter, on a worldwide basis, is deposited in marine shelf-deltaic sediments, rather than in high-productivity upwelling regions or anoxic basins; (2) burial rate and the degree of preservation of organic matter are positively correlated with total sedimentation rate; (3) the rate of decomposition of organic matter in natural waters and sediments is highly dependent on the source and degree of aging of the organic matter itself, and is largely unaffected by the concentration of the principal oxidants, O2 and SO42-; (4) pyrite sulfur concentration in normal marine environments correlates well with organic carbon concentration due to the limitation of bacterial sulfate reduction by reactive organic matter that escapes aerobic destruction during deposition and bioturbation; (5) depth-integrated sulfate reduction in normal marine sediments correlates positively with total sedimentation rate; (6) in euxinic sediments excess pyrite is formed from sulfidic bottomwaters leading to a lack of correlation between organic carbon and pyrite sulfur, a good correlation between pyrite sulfur and total iron, and high degrees of pyritization of iron; and (7) very low concentrations of pyrite are found in continental fresh water sediments, regardless of organic matter or iron content, due to limitation of pyrite formation by the low concentration of dissolved sulfate in fresh water.

14C and Th/U Dating of Pleistocene and Holocene Stromatolites From East African Paleolakes

Article

May 1986

During recent humid episodes, stromatolites were built along paleolake margins, some 60 m above the modern water level of Lakes Natron and Magadi (southern Gregory Rift Valley). Three generations of stromatolites are observed, the more recent ones frequently covering pebbles and boulders eroded from the older ones. The youngest one yielded 14C ages ranging from approximately 12,000 to 10,000 yr B.P. Their δ13C values (≥2.6%) suggest isotopic equilibrium between the paleolake total inorganic dissolved carbon and the atmospheric CO2, thereby lending credence to the reliability of the 14C. An initial ratio in the detrital component was determined by measurements on the 14C dated stromatolites. Using this value a chronology for the older stromatolites was calculated. Ages of ≥240,000 and 135,000 ± 10,000 yr were obtained for the first and second generations, respectively. A humid episode apparently characterized eastern Africa during each glacial-interglacial transition. 18O and 13C measurements on stromatolites, when compared to values on modern waters and carbonates, provide paleohydrological information. Long residence time of the paleolake waters and less seasonally contrasted regimes are inferred.

Precambrian sedimentary carbonates: carbon and oxygen isotope geochemistry and implications for the terrestrial oxygen budget

Article

Feb 1975
PRECAMBRIAN RES

Carbon isotope values of 260 Precambrian limestones and dolomites (most of them being substantially unaltered) have yielded an overall mean of vs. PDB; the corresponding oxygen values average at vs. SMOW. Like the overall mean, the δ 13C values furnished by individual carbonate occurrences are, as a rule, fairly “modern” and almost constant as from the very beginning of the sedimentary record. A remarkable exception are the “heavy” dolomites of the Middle Precambrian Lomagundi Group, Rhodesia, with vs. PDB. As a result of our measurements, the sporadic occurrence in the geological past of anomalously heavy carbonates seems to be established.

Precambrian organic geochemistry - Preservation of the record

Article

Feb 1983

A review of earlier studies is presented, and new results in Precambrian organic geochemistry are discussed. It is pointed out that two lines of evidence can be developed. One is based on structural organic chemistry, while the other is based on isotopic analyses. In the present investigation, the results of both structural and isotopic investigations of Precambrian organic matter are discussed. Processes and products related to organic geochemistry are examined, taking into account the carbon cycle, an approximate view of the principal pathways of carbon cycling associated with organic matter in the present global ecosystem, processes affecting sedimentary organic matter, and distribution and types of organic matter. Attention is given to chemical fossils in Precambrian sediments, kerogen analyses, the determination of the structural characteristics of kerogen, and data concerning the preservation of the Precambrian organic geochemical record.

Sr isotope variations in Upper Proterozoic carbonates from Svalbard and East Greenland

Article

Oct 1989
GEOCHIM COSMOCHIM AC

Precambrian Sr isotope stratigraphy was investigated by determining variations in Sr-87/Sr-86 ratios in the Upper Proterozoic carbonate succession from Svalbard and East Greenland. Data from this study were combined with those from literature to construct a curve of Sr-87/Sr-86 versus time for Upper Proterozoic seawater. The curve for the Upper Riphean-Vandian showed that the isotopic composition of Sr in seawater was low (Delta Sr-87 of about -500) between 900 and 650 Ma but rose rapidly to about +30 by 600 Ma (this range of long-term variation exceeds the total Phanerozoic variation). The very low values of Delta Sr-87 inferred for the Riphean require that, for this time, the submarine hydrothermal water flux was a large fraction of the Sr input to the oceans, while the rise in Delta Sr-87 in the Upper Proterozoic seawater reflects both a change in the ratio of hydrothermal and continental fluxes of Sr to the oceans, and a change in the isotopic composition of Sr from continental sources.

Carbonate deposition during the Late Proterozoic Era: an example from Spitsbergen

Article

Feb 1990

The Upper Proterozoic (approx 800-700 Ma) Akademikerbreen Group, Spitsbergen, comprises 2000 m of carbonates, with only minor intercalations of quartz arenite and shale and about 45 percent limestone. Stromatolites are conspicuous in outcrop but constitute only 25 percent of the total section. Micrites and coarser intraclastic carbonates derived mainly from micritic precursors comprise 60 percent of the group, while oolites make up the remaining 15 percent. Distinctive sedimentary features are discussed. Carbonate sedimentology reinforces data from other sources which indicate the last 200 to 300 Ma of the Proterozoic Eon was a distinctive interval of Earth history. -from Authors

Isotopic compositions of carbonates and organic carbon from upper Proterozoic successions in Namibia: stratigraphic variation and the effects of diagenesis and metamorphism

Article

Feb 1991
PRECAMBRIAN RES

The carbon isotope geochemistry of carbonates and organic carbon in the late Proterozoic Damara Supergroup of Namibia, including the Nama, Witvlei, and Gariep groups on the Kalahari Craton and the Mulden and Otavi groups on the Congo Craton, has been investigated as an extension of previous studies of secular variations in the isotopic composition of late Proterozoic seawater. Subsamples of microspar and dolomicrospar were determined, through petrographic and cathodoluminescence examination, to represent the “least-altered” portions of the rock. Carbon-isotopic abundances in these phases are nearly equal to those in total carbonate, suggesting that 13C abundances of late Proterozoic fine-grained carbonates have not been significantly altered by meteoric diagenesis, although 18O abundances often differ significantly. Reduced and variable carbon-isotopic differences between carbonates and organic carbon in these sediments indicate that isotopic compositions of organic carbon have been altered significantly by thermal and deformational processes, likely associated with the Pan-African Orogeny.

Secular variation in carbon isotope ratios from Upper Proterozoic successions of Svalbard and East Greenland

Article

Jul 1986

Analyses of stratigraphically continuous suites of samples from Upper Proterozoic sedimentary successions of East Greenland, Spitsbergen and Nordaustlandet (Svalbard) provide an approximation to the secular variation in carbon isotope ratios during a geologically and biologically important period of change from around 900 million years ago to the beginning of the Cambrian period. Late Riphean carbonates and organic material show a stratigraphically useful pattern of enrichment in 13C relative to Phanerozoic or earlier Proterozoic samples. Isotopic compositions of isolated samples from other localities are consistent with a worldwide extended interval of enhanced organic burial and consequent net survival of oxidized material, probably O2, just before the initial radiation of metazoans.

Evaporitic sediments of early Archaean age from the Warrawoona Group

Jan 1990
247-277

S. R., 1990, Evaporitic sediments of early Archaean age from the Warrawoona Group, North Pole, Western Australia: Sedimentology. v. 37, p. 247-277.

Diagenesis of skeletal carbonates

Jan 1967
J Sediment Petrol
914-930

S. 1967, Diagenesis of skeletal carbonates: Journal of Sedimentary Petrology, v. 37,p. 914-930.

Magnesian calcites: low temperature occurrence, solubihty and solid-solution behaviour

Jan 1983
97-144

M Loijens
J Schoonmaker
R Wollast

Loijens, M., Schoonmaker, J.. and Wollast, R., 1983, Magnesian calcites: low temperature occurrence, solubihty and solid-solution behaviour, in Reeder, R.j., ed., Carbonates: Mineralogy and chemistry: Mineralogical Society of America, Reviews in Mineralogy, v. II, p. 97-144.

Jan 1988
57-75

A Moussine-Pouchkine
J Bertrand-Sarfati
E Ball
R Caby

Moussine-Pouchkine, A., Bertrand-Sarfati, j., Ball, E.• and Caby, R., 1988, Les series scdimentaires et volcaniques anorogeniques proterozoi'ques impliquees dans la chainee pan africaine: la region de I'Adrar Ahnet (NW Hoggar, Algerie): Journal of African Earth Sciences, v. 7, p. 57-75.

Measurement of 018/016 ratios of total oxygen and of carbonates: Geochimica et Cosmochimica Acta

Jan 1965
1347-1353

Clayton T R N Sharma

Sharma. T.• and Clayton. R. N.. 1965. Measurement of 018/016 ratios of total oxygen and of carbonates: Geochimica et Cosmochimica Acta. v. 29. p. 1347-1353.

Why the oxygen isotopic composition of seawater changes with time: Geophysical Research Letters, v. 16

Jan 1989
323-326

J C G Walker
K Lohmann

Walker, J. C. G.. and Lohmann. K. c.. 1989. Why the oxygen isotopic composition of seawater changes with time: Geophysical Research Letters, v. 16. p. 323-326.

Stratigraphic shifts in carbon isotopes from Proterozoic stromatolitic carbonates (Mauritania): influences of primary mineralogy and diagenesis

Abstract

No full-text available

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