Environmental and diagenetic variations in carbonate associated sulfate: An investigation of CAS in the Lower Triassic of the western USA

An integrated stable isotope, elemental and petrographic analysis of Early Triassic (Spathian) carbonates and evaporites along a proximal to deep environmental transect reveals significant variations in delta S-34 composition of carbonate associated sulfate (CAS). The variations in the delta S-34 of CAS are strongly correlated with the Ca/Mg composition of carbonates, suggesting that the variations are driven by the degree of dolomitization. The delta S-34 of dolostones and evaporites are similar to one another and exhibit lower delta S-34 values than limestones from all localities. Three hypotheses may explain the differences in delta S-34 between proximal dolostones/evaporites and inner/middle shelf limestones: (1) limestones experienced anaerobic sulfate reduction and subsequent incorporation of S-34-enriched sulfate into CAS during diagenesis, while dolostones did not-this is unlikely because of the lack of correlation between delta S-34(CAS) and TOC, as well as other indicators of diagenesis, (2) dolomitization controlled the delta S-34(CAS) in proximal palcoenvironments, where the source of the S-34 depleted fluids was either continentally-derived or the result of Rayleigh distillation during evaporite formation, and (3) a delta S-34 depth gradient existed during the Early Triassic such that limestones formed in distal waters are more enriched in S-34 versus evaporites and dolostones formed in proximal settings-we do not favor this hypothesis because the strong correlation between Ca/Mg and delta S-34(CAS) implies that dolomitization controls the delta S-34(CAS) in these samples. Results from subtidal, well-preserved (non-dolomitized) limestones suggest that the delta S-34 of Spathian seawater sulfate may have been heavier than previously suggested from analyses of evaporite deposits alone. (C) 2008 Elsevier Ltd. All rights reserved.