Geochemistry of Marine and Nonmarine Environments of a Neoproterozoic Cratonic Carbonate/Evaporite: The Bitter Springs Formation, Central Australia
A. C. Hill, K. Arouri, P. Gorjan, M. R. Walter, 2000. "Geochemistry of Marine and Nonmarine Environments of a Neoproterozoic Cratonic Carbonate/Evaporite: The Bitter Springs Formation, Central Australia", Carbonate Sedimentation and Diagenesis in the Evolving Precambrian World, John P. Grotzinger, Noel P. James
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The Bitter Springs Formation constitutes the upper part of Supersequence 1, an 830 Ma carbonate succession deposited in the intracratonic Centralian Superbasin. The Superbasin covered 2 million km2 of the Australian continent during Neoproterozoic times. We have integrated new carbon, oxygen, sulfur, and strontium isotopic information, and new biomarker information, with existing results, to test and extend previous studies of the formation.
The Gillen Member of the Bitter Springs Formation was deposited in a marine environment. The evidence for this interpretation includes isotopes of strontium, carbon, and sulfur, and biomarkers. 87Sr/86Sr ratios in the Gillen Member are comparable to the lowest ever recorded from the Shaler Supergroup of Canada, of similar age. These low strontium isotopic ratios are associated with comparable secular change in seawater δ13Ocarb. Steroidal hydrocarbon biomarker assemblages, seawater δ34Ssulfate values, and low δ34Spyrite values further indicate a marine origin.
The lower part of the Loves Creek Member (Units 1 and 2) was also deposited in a marine environment. Facies-independent secular change in seawater δ13Ocarb and low 87Sr/86Sr ratios are comparable to those in the upper Shaler Supergroup of Canada. Seawater δ34Ssulfate, values, low δ34Spyrite, and evidence for algal biomarkers are also consistent with marine deposition.
The upper part of the Loves Creek Member (Unit 3), and lower part of Unit 2, is nonmarine. Rare pyrite, heavy δ34Spyrite, abundant irregularly branched C25 and C30 isoprenoids from methanogens, and abundant pseudomorphs after halite indicate carbonate precipitation in sulfate-poor hypersaline lakes. Heavy and variable δ13Scarb and δ18O indicate that continental groundwater brines, which were low in dissolved carbonate, drained into the lakes. Evaporation of the lake waters increased the concentration of dissolved carbonate, and increased still more the δ18O values, and may have had a small positive effect on δ13Ccarb. Dolomitization of the continental redbeds occurred by the evaporative reflux of groundwater. Cyanobacteria formed mats on the lake margins, where they were quickly silicified. Domical and irregular stromatolites formed within the lakes. The occasional presence of C30 desmethyl steranes and 4-methylsteranes in the lower parts of Unit 3 indicates that either there were marine incursions or that marine algae were transported to the lakes, possibly by winds. Nonmarine Neoproterozoic algae may have synthesized these sterols.
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Carbonate Sedimentation and Diagenesis in the Evolving Precambrian World - Precambrian carbonates are usually regarded at the simple cousins of the sedimentary realm, composed of stromatolites and dolostones, texturally not challenging and commonly altered beyond recognition by the vagaries of time, diagenesis and metamorphism. However, these carbonates that formed deep in time are commonly exquisitely preserved and contain within them a record of the evolving young earth. SEPM Special Publication 67 explores these aspects. Resulting from a 1997 SEPM/CSPG symposium entitled? Precambrian Carbonates,? these 18 papers demonstrate the importance of understanding these rocks, since within them is contained a record of the early ocean, atmosphere, and biosphere.