Modern Carbonate Shelf-Slope Boundaries: ABSTRACT

Abstract

The shelf-slope boundary along modern carbonate buildups on a worldwide basis demonstrates a high variability in morphology, structure, biogenic barriers, sediments, depth of occurrence, dominant processes, and general geologic history. This boundary is defined as the zone of maximum gradient change between the nearly horizontal shallow-water shelf End_Page 938------------------------------ margin or reef flat and the seaward, more steeply inclined upper slope or marginal escarpment. The carbonate margins fronting detached platforms and attached shelves can be grouped as either open (Campeche, West Florida) or rimmed (Bahamas, Belize, Great Barrier Reef). Insular margin fringing/barrier reefs (New Guinea, Tahiti) and atolls (Eniwetok, Bikini) form a third major group. The shelf-slope boundary within these highly variable margins spans from shallow, abrupt, and distinct to deep, broad, and dimly defined. The regional geologic setting, basement structure, and tectonic history are the primary controls determining carbonate buildup type and hence the general nature and location of the shelf break. Once established, carbonate buildups are profoundly influenced by the available physical-energy flux. Where winds and waves are dominantly unidirectional, the margins of carbonate buildups acquire significant windward and leeward characteristics. Where the tidal range is elevated, tidal currents control sedimentation. Acting in conjunction with tectonic movement and the physical processes are geo- and glacio-eustatic-induced sea-level fluctuations. Other important factors influencing the carbonate shelf break are antecedent topography, fluvial-terrigenous sediment input, and oceanic circulation/upwelling. The dominance of reefs and sand bodies at the carbonate shelf-slope boundary produces rocks with initially high porosites/permeabilities which may form good reservoir rocks. This boundary is also sensitive to climate change and sea-level fluctuation and therefore may contain detailed data on the geologic history of the entire carbonate buildup. End_of_Article - Last_Page 939------------