Abstract

Well-lithified, deep-water, periplatform limestones of late Miocene age occur at subsurface depths from 113 m to total depth of 283 m at Ocean Drilling Program (ODP) Site 632 in Exuma Sound, Bahamas. This occurrence, marked by a dramatic increase in compressional wave velocities up to 5.0 km/s, represents the most rapid transformation of ooze to limestone yet discovered in either periplatform or open-ocean settings. The burial diagenetic pathway is similar to those for meteoric and deep-burial diagenetic environments: aragonite dissolves, magnesian calcites exsolve or dissolve, low-Mg calcite precipitates as cement, and secondary moldic/vuggy porosity develops. Stable isotope ratios suggest precipitation of low-Mg calcite from cold, marine-derived pore waters. Accelerated diagenesis is a response to the influx of large quantities (up to 80%) of bank-derived, metastable carbonates and rapid burial via high accumulation rates (up to 210 m/m.y.). Our findings (1) document carbonate mineral stabilization and lithification in a seawater-mediated, shallow-burial diagenetic environment; (2) highlight the very high diagenetic potential of periplatform carbonates; and (3) call into question the use of seismic velocities as support for the Bahama megabank hypothesis.

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