Deep-Marine Origin of Equant Spar Cements in Oligocene-Miocene Perireef Boundstones, Leg 101, Site 635, Northeast Providence Channel, Bahamas

Abstract

Oligocene-Miocene perireef boundstone clasts buried in slumped canyon-fill sediments drilled at Hole 635B in Northeast Providence Channel during Leg 101 contain petrographic evidence of shallow-marin e diagenesis, including abundant cloudy, isopachous, very fine to medium, radiaxial fibrous cement in shelter and intraparticle porosity. A sin­ gle microsample of this cement has a stable oxygen and carbon isotopic composition (- 1. l%o S1 O, + 2.8%o 5 C) that is consistent with precipitation from warm, shallow-marine water. This isotopic composition also lies in the isotopic field defined for petrographically similar cement in bank-interior limestones exposed nearby on the Bahama Escarp­ ment. Effects of vadose and meteoric phreatic diagenesis are absent. Following shallow-marine diagenesis, these boundstone clasts were transported to the deep ( — 3500 m) axis of Northeast Providence Channel and exposed to deep-marine waters, which appear to have initiated deep-marine diagen­ esis. The principal results of deep-marine diagenesis were dissolution of unstable, presumably aragonitic, grains and reprecipitation of carbonate into both newly opened molds and partially filled primary pores as clear, fine to medium, equant calcite spar. Stable oxygen and carbon isotopic compositions ( + 2.0%o 5180, +3.6%o 513C, and +2.6%o 6 O, + 3.9%o 513C) of two microsamples of this equant spar are significantly 180-enriched (> 3%o) relative to shallow-marine cements in the same samples, which is consistent with precipitation in equilibrium with colder, deep-marine waters. Iso­ topic composition of these equant spars is similar to that of deep-marine hardgrounds in Tongue of the Ocean and Northwest Providence Channel, and to Pleistocene equant spars in Early and middle Cretaceous shallow-water lime­ stones exposed on the Bahama Escarpment. that early diagenesis of these clasts occurred syndepositionally in shallow-marin e water, we will demonstrate that precipitation of equant calcite spar cements took place during late diagenesis when these clasts were immersed in cold, deep-marine water.