Sedimentary Models of Pattern, Process, and Succession Derived from Bahamian Carbonates: ABSTRACT

Abstract

Three decades of research on the Cenozoic carbonate platforms of the Bahamas have produced several models of deposition and diagenesis. The models are sufficiently varied and numerous that they can be categorized and their interactions evaluated. The models can be divided into three types: two-dimensional patterns of depositional facies; process models of sediment formation, deposition, and diagenesis; and concepts of facies succession. Patterns of depositional facies range in scale from a kilometer or less in the bar and channel morphology of ooid sand spreads, to a few kilometers in channeled tidal flats, to as much as 100 km (62 mi) in the platform to basin zonation of reefs and sediments. The spectrum of process models extends from grain formation (peloids and ooids), to major modes of accumulation (net shoreward movement of lime mud with attendant seaward progradation of shoaling cycles), to sea-floor cementation that yields aggregate grains and hardgrounds. Vertical facies successions on the scale of several meters are seen in the shoaling deposits of tidal flats and in the coarsening-upward sequences of sand shoals. Larger scale changes in facies, from skeletal to nonskeletal deposits that are tens of meters thick, occur in the subsurface Pliocene-Pleistocene carbonates. These three kinds of sedimentary models interact in various ways. An example of the influence of pattern on process is seen in the channels between tidal bars of ooid sand. The growth of tidal bars of ooid sand leads to accelerated tidal currents in the intervening channels; the result is a channel lag of coarser sediments that limits traction movement of lime sands and especially the nuclei for continued formation of ooids. Deceleration of ooid formation can in time lead to stabilization of the moving sands with attendant alterations of grains and sedimentary structures. An example of the effect of process on pattern is seen in the formation of extensive hardgrounds on lime sands of the platform interior by submarine cementation. These hardgrounds are rapidly colonized by attached or anisms, including reef-building corals; the end result is a major transformation of sediment type and the appearance of the preferred substrate for reefs. An example of the interaction between facies is seen in two aspects of reefal deposits: the preferential development of some reefs seaward of islands or growing sand shoals and the burial of other reefs that fringe leeward margins of the platforms by off-bank transport of lime sands from the interiors. End_of_Article - Last_Page 1836------------