Nodular Submarine Cementation on Bahamian Slopes--Possible Model for Origin of Some Nodular Limestones: ABSTRACT

Abstract

Submarine cemented nodules (< 6 cm across) floating in a muddy matrix have been recovered by piston coring from carbonate slopes (depths 300 to 800 m) in the northern Bahamas. These nodular units are usually associated with peri-platform oozes, and range in thickness from 9 to 345 cm. Petrographic data indicate that the 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 4 to 5 × 103 years younger than surrounding sediment. Carbon and oxygen isotopes plus paleotemperature data suggest that the nodules were cemented in situ near the sediment-water interface. The origin of these nodular units involves a complex interplay of physical, biologic, and chemical processes that act concomitantly above the permanent thermocline. Bottom currents play a key role in the cementation of the nodules by controlling the permeability of the sediment via winnowing and thus the degree of interstitial circulation. Burrowing organisms may enhance submarine cementation by irrigating the shallow subsurface, and may be responsible for the vertical mixing of nodules. On the slope north of Great Bahama Bank facies transitions from hardgrounds at depths < 375 m, to nodular sediment at depths of 375 to 500 m, to soft, unlithified peri-platform oozes at greater depths correlate with a downslope decrease of bottom current strength. These observations suggest that ottom currents in conjunction with bioturbation may have major influences on the degree of early diagenesis and thus facies relations. Recognition of similar facies in the rock record may be useful in interpretation of open-marine carbonate slope deposition, synsedimentary submarine cementation, deposition above the permanent thermocline, and the existence of contour-following currents. End_of_Article - Last_Page 755------------