Depositional δ18O Signatures in Proterozoic Dolostones: Constraints on Seawater Chemistry and Early Diagenesis
Linda C. Kah, 2000. "Depositional δ18O Signatures in Proterozoic Dolostones: Constraints on Seawater Chemistry and Early Diagenesis", Carbonate Sedimentation and Diagenesis in the Evolving Precambrian World, John P. Grotzinger, Noel P. James
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The Mesoproterozoic Society Cliffs Formation consists of approximately 700 meters of cyclic, peritjdal dolostones that were deposited on an extensive shallow-marine platform. Stratigraphic analysis allows the reconstruction of broad hydrologie and paleoenvironmental conditions on the platform (Kali, in press), and sedimentological and paleobiological information help constrain such aspects of the depositional environment as the origin of precipitated carbonate, degree of evaporation, temperature, and timing and environment of early diagenesis. Together, these observations provide an independent framework within which to examine the geochemical record of carbonate deposition and early diagenesis.
During evolution of the Society Cliffs platform, shoaling and subaerial exposure of a mid-ramp intraclastic/oolitic grainstone shoal restricted circulation of marine waters within an areally extensive microbial flat. Evaporation during periods of restriction resulted in widespread precipitation of abiotic, seafloor carbonate precipitates and marine sulfates. Early dolomitization of these primary phases is evidenced by a close correspondence of petrograpic fabrics preserved in dolomite and early diagenetic chert, and by excellent fabric retention during dolomitization of highly-soluble evaporite phases.
Dolomitic rocks of the Society Cliffs Formation retain a pattern of oxygen isotopic variation that reflects regional depositional environments. Whereas mid-ramp dolostones record the isotopically lightest values (δ18O ~ −6‰), inner ramp dolostones vary from — 5‰ to — 3‰, values consistent with the isotopic enrichment of marine waters during periods of restriction and evaporation. An additional 2–3‰ enrichment of δ18O in subaerially exposed cycle caps (to – 3‰ in mid-ramp facies and – l‰ in inner ramp facies) suggests a continued evaporative influence during widespread exposure of the platform. Data require that depositional, as well as diagenetic, variation be accounted for when interpreting the sedimentary record of δ18O. Furthermore, evidence that significant primary paleoenvironmental variation in δ18O can be retained in Proterozoic dolostones suggests first, that isotopic compositions were imparted during early dolomitization, and second, that samples containing the most enriched 18O abundances cannot a priori be presumed to record “least-altered” isotopic signatures. Therefore, despite a recognized propensity for alteration, when evaluated within an independently derived depositional framework, oxygen isotopic compositions of shallow marine dolostone facies may provide important constraints on both the geochemical evolution of marine systems and processes of early diagenesis.
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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.