Nodular Carbonate Sediment on Bahamian Slopes: Possible Precursors to Nodular Limestones

Abstract

ABSTRACT Lithified nodules up to 6 cm across floating in a muddy matrix have been recovered by piston coring from carbonate slopes in the northern Bahamas. Petrographic data indicate that most nodules are multi-generation, deep-water, grain-supported intramicrites to intramicrudites cemented by pelloidal high-Mg calcite. Carbon-14 activities indicate that the nodules are at least 4-5 103 years younger than the surrounding sediment. Carbon and oxygen isotope data indicate that the nodules were submarine cemented and that cementation was accompanied by increases in O18 and C13 values. Nodular units were found predominately in association with peri-platform oozes. Thickness of these nodular units is variable (9-345 cm) but is usually less than 130 cm, and upper and lower contacts are commonly indistinct. These units appear to have formed essentially in situ by nodular submarine cementation of carbonate sediment. Cementation occurred in the shallow subsurface, yet near the sediment-water interface at water depths above the permanent thermocline ( The extent to which carbonate slope sediments are lithified is controlled by a combination of physical, biological and chemical processes that act concomitantly. Bottom currents tend to enhance synsedimentary cementation if they either: 1) increase the permeability of the sediment by winnowing muds which allows for increased interstitial circulation of pore fluids; 2) create a pause in sedimentation and allow sediment to be exposed near the sediment-water interface for extended periods of time; or 3) provide large volumes of seawater that contain the necessary ions (Ca2+, Mg2+, CO32-)for cementation. Burrowing organisms may also enhance submarine cementation by irrigating the shallow subsurface which may also increase interstitial circulat on. On the slope north of Great Bahama Bank, the degree of submarine cementation decreases downslope from well-lithified hardgrounds at depths These modern nodular deposits on Bahamian slopes may serve as analogs for some deeper-water nodular limestones in the rock record. Recognition of similar facies in the ancient may be of great use for paleo-environmental interpretations. End_Page 117------------------------