ABSTRACT

Petrographic and compositional attributes of limestone and dolostone in the Beck Spring Dolomite of eastern California indicate that late Proterozoic mineralogies, isotopic compositions, and diagenetic processes were similar to those inferred for younger Phanerozoic marine carbonate sequences. Allochems in limestones include strontium-rich ooids with well-preserved concentric banding and cortices of equant neomorphic calcite that commonly exhibit dissolution and compaction features; cements include microdolomite-rich columnar marine calcite and equant spar. Calcites are depleted relative to dolomites and become enriched with stratigraphic depth.

Acicular and columnar marine cement and allochems in dolostones display fabrics identical to those in limestones but are isotopically distinct. Dolomite ooids, columnar and acicular cement, crystal silt, and early luminescent spar cement from four stratigraphic sections define overlapping positive oxygen-carbon covariant fields that trend toward depleted calcite compositions. Early luminescent dolomite spar cement is unconformably overlain by depleted ferroan dolospar.

Fabric data suggest that marine cement and ooids were precipitated as magnesian calcite and aragonite. Isotopic data suggest that metastable components locally stabilized to calcite during subaerial exposure while laterally, dolomitization took place in mixed marine-meteoric waters. Early dolospar precipitation began during replacive dolomitization, and late ferroan dolospar precipitated during burial. Lack of evidence for hypersaline conditions suggests that late Proterozoic normal marine carbonate isotopic composition was -0.5 ± 2.0¹⁸O and +5.3 ± 0.59¹³C.