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 x 10 3 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 delta O 18 and delta C 13 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 (<800-1000 m). The sequence of events leading to the formation of these modern nodules includes: 1) an initial predominantly grain-supported forampteropod sediment; 2) submarine cementation of this sediment by amorphous to vaguely pelloidal high-Mg calcite; 3) break-up of this cemented material by currents and benthic organisms; 4) reworking and winnowing by bottom currents to produce a grain-supported sediment of silt to gravel-sized intraclasts, reworked and infilled grains plus new forums and pteropods; 5) cementation of this reworked sediment by pelloidal high-Mg calcite to produce nodules; 6) decreased current velocities and the deposition of pelagic muds; and 7) vertical mixing by burrowing organisms. 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 (Ca (super 2+) , Mg (super 2+) , CO 3 (super 2-) )for cementation. Burrowing organisms may also enhance submarine cementation by "irrigating" the shallow subsurface which may also increase interstitial circulation. On the slope north of Great Bahama Bank, the degree of submarine cementation decreases downslope from well-lithified hardgrounds at depths <375 m to nodules from 375-500 m, and finally to soft, unlithified oozes at greater depths. This transition in the degree of submarine cementation correlates well with a gradual downslope decrease in the strength of contour following bottom currents as observed in bottom photographs. The current action appears to control the degree of lithification by controlling the permeability of the near-surface sediments and thus, the degree of interstitial circulation. 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.

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