ABSTRACT

Coarse-grained, leeward-margin sand shoals, developed during the late Quaternary along the western edges of Little and Great Bahama Bank, have been deposited and preserved in response to regional sediment-transport processes and local physicochemical conditions. These sand bodies are fundamental depositional sequences, chronostratigraphically bounded by subaerial exposure crusts, and thus are of major importance in determining rates of bank-margin growth and in understanding the dynamics of carbonate margin buildup. In the Holocene sediments, these sand bodies are 1-2 km wide, up to 35 m thick, and are present along 400 km of the western Bahamas. Surficial sediment is composed of coarse sand and granules dominated by composite-ooid grains. Submarine cementation is active and has led to the development of coarser grain sizes during the depositional interval. Fine to medium-sized grains are cemented with fibrous aragonite and surficially coated at the tidally active margin. The resultant composite-ooid sediment is more hydrodynamically stable and is rapidly cemented into hardgrounds characterized by a smooth (tidally abraded) upper surface. Lithification is gradational through a thickness of 50-100 mm to an irregular lower boundary transitional with uncemented material. These hardgrounds are submarine discontinuity surfaces developed during inter-storm conditions of winnowing and bypass sedimentation along a depositional profile of equilibrium. Rock cores into the Pleistocene section have recovered sediments and submarine discontinuity surfaces identical to those in the Holocene. These are present in the two latest Pleistocene sequences representing the last major interglacial intervals. Both the Holocene and Pleistocene sequences have one or two cemented zones per meter in the upper section. The presence of these deposits throughout the preserved stratigraphic package indicates the persistence of characteristic leeward depositional processes during the late Quaternary and their importance in bank-margin growth. A combination of characteristics including typical microfacies, cements, and discontinuity surfaces (if seen in the ancient) should be good indicators of leeward-margin settings.

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