Abstract

Rates of burial diagenesis and subsurface distribution of chalk and limestone in modern deep-water carbonate slopes were little known prior to 1985 when the Ocean Drilling Program established two drill transects in the northern Bahamas: north of Little Bahama Bank and in southwestern Exuma Sound. From these transects, different shallow-burial (<250 m) patterns of diagenesis have been determined for each slope through examination of mineralogy, geochemistry, and texture of bulk carbonate and dolomite, and integrated with pore-water chemistry. In these carbonate-rich and organic-poor sediments, diagenesis is largely controlled by metastability of bank-derived aragonite and magnesian calcite relative to the ambient temperature and saturation state of marine-derived pore fluids. Dissolution of aragonite elevates alkalinity of pore waters promoting precipitation of calcite and incipient (<15%) dolomitization. Across-slope heterogeneity in subsurface distribution of ooze, chalk, and limestone and rate of bulk carbonate calcitization is controlled by bank-margin sedimentation patterns, which influence burial rates of metastable minerals, differential dissolution rates, and relative proportions of fine- and coarser-grained aragonite. Most of the dolomite precipitates after initial aragonite dissolution, typically in the upper 70 m, with Mg derived from alteration of magnesian calcite and diffusion from overlying sea water. Given an initial abundant volume of fine-grained aragonite, with additional coarser aragonitic grains, the diagenetic end member of diagenesis is a rapidly formed, biomoldic dolomitic limestone found today locally at shallow (113 m) subsurface depths.

Along the accretionary north margin of Little Bahama Bank, the amount of lithified strata decreases seaward because of a decreasing volume of bank-derived aragonite. The rate of bulk carbonate calcitization increases seaward as the rate of diagenesis begins to match, then exceed, the rate of burial. Mineral-controlled diagenesis is of little consequence below about 20 m subsurface only 30 km seaward of the bank margin. Along the bypass margin in Exuma Sound, the extent of lithification increases downslope toward preferential accumulation of bank-derived aragonite; much higher accumulation rates and high percentages of aragonite along this margin sustain a still-present, mineral-controlled diagenetic potential in limestone at 300 m.

From analogy of modern Bahama slopes, considerable diagenetic heterogeneity within the shallow-burial realm may be expected along some ancient carbonate margins. Our results serve to illustrate carbonate-mineral stabilization and early limestone formation with secondary porosity in a sea-water-mediated environment, and they further highlight the high diagenetic potential of periplatform carbonates.

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