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Deep-Marine Origin of Equant Spar Cements in Bahama Escarpment Limestones
R. P. Freeman-Lynde, K. Fulker Whitley, K. C. Lohmann
Early and middle Cretaceous shallow-water limestones exposed on the Bahama Escarpment by late Cretaceous and Tertiary erosional processes have abundant petrographic evidence of shallow diagenesis, particularly cloudy, fine-coarse crystalline, isopachous, bladed, fibrous, and radiaxial fibrous cements. Similar petrographic evidence is observed in coeval rocks buried in eastern Texas. Isotopic and Sr compositions of micrite and isopachous, radiaxial fibrous cement separates support petrographic observations. 18O values range from -3.1 to +0.1 and 13C from +0.6 to +3.4, similar to those of original marine components of coeval strata in eastern Texas. Original shallow-marine components in Bahama Escarpment limestones also have relatively high Sr/Ca values (2.0-10.1 10-4 = 177-884 ppm). Significant cementation in shallow environments was, however, limited to the shallow-marine phreatic zone. Cementation in vadose and meteoric-phreatic environments was rare.
Cements from shallow diagenetic environments only partially fill syndepositional primary porosity (fenestrae, vugs, molds, and intra- and interparticle pores) and are completely absent in secondary porosity, principally fractures resulting from erosion of the Bahama Escarpment. Abundant clear, medium-coarse crystalline equant spar in both primary and secondary porosity apparently precipitated, either at the seafloor, or in deep-burial environments, from fluids with compositions similar to deep-marine waters adjacent to the Bahamas.
Isotopic compositions of equant spar in both primary and secondary pores trend from -0.618O and +1.913C to +2.018O and +3.613C. Despite partial overlap of the original shallow-marine field, equant spar is generally 18O-enriched relative to original shallow-marine components, consistent with precipitation in equilibrium with colder, deep-marine waters. 18O values for Bahama Escarpment equant spar are also 18O-enriched relative to equant spar in coeval strata in eastern Texas, reflecting substantially different deep-burial fluids. Bahamian deep-burial environments have remained open, continually invaded by deep-marine water. Coeval strata in eastern Texas, however, were overlain by impermeable beds as the Gulf Coast shelf prograded and subsurface fluids evolved in restricted diagenetic environments.
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