Carbonate Platform Facies Models

ABSTRACT Various types of carbonate platforms are characterized by distinctive profiles, facies, and evolutionary sequences. Ramps may be homoclinal or distally steepened, and may have fringing or barrier shoal-water complexes of ooid-pellet sands or skeletal banks. Homoclinal ramps pass seaward into deeper water without major break in slope, and lack deep-water breccias. Distally steepened ramps may below energy, and characterized by widespread, shallow, subwave-base mud blankets, or high energy with coastal beach/dune complexes and widespread skeletal sand blankets. Slope facies may contain abundant breccias of slope-derived clasts. Rimmed shelves have relatively flat tops, and marked break in slopes at the high energy, shallow-shelf edge where they pass into deep water. Such shelves may be aggraded with peritidal facies extending over much of the shelf, or incipiently drowned, depending on magnitude of sea level fluctuations. They may be accretionary, or bypass types that include gullied slope, escarpment, and high-relief erosional forms. Intrashelf basins occur on some shelves, controlling distribution of reservoir and source beds. Isolated platforms are surrounded by deeper water and may be located on rifted continental margins, or on submarine volcanoes. Most have high-relief rimmed margins. Platforms that have been subjected to rapid sea level rise may be incipiently drowned, and characterized by raised rims, elevated patch or pinnacle reefs, and widespread subwave-base carbonate or fine clastic blankets. Completely drowned shelves develop where the shelf is submerged to subphotic depths, terminating shallow water deposition, and commonly resulting in blanketing of the shelf by deeper water facies. Some margins show extensive down-to-basin faulting that is contemporaneous with carbonate deposition, or associated with thick prograding clastic sequences. The various types of platforms change in response to variations in sedimentation, subsidence or sea level rise, and may form distinctive evolutionary sequences. The relatively few models presented appear to accommodate most geological examples, some of which contain major reservoir facies.