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Texas A&M University, Department of Geology and Geophysics, College Station, TX 77843-3115, USA
The carbonate ramp as a type of sedimentary depositional platform was conceived after failed attempts to fit some carbonate facies patterns to shelf models. Ramps were first defined as two-dimensional (2D) depositional surfaces along which facies variations reflect differences in the oceanographic environment from shore to basin. Shelves and ramps were placed at opposite poles without definitions of intermediate platform configurations or environmental subdivisions between shore and basin. The 2D ramp is of limited use in determining slope and surface configuration in the rock record where stacked depositional surfaces are separated by beds of different age and thickness, and the original definition did not anticipate the complexities that would develop as high- v. low-energy and tropical v. temperate carbonate systems became better known. Though the original definition is flexible, modifications were inevitable. Refinements of the concept introduced distal steepening, among other intermediate styles of platform geometry. Environmental subdivisions were proposed for shallow, intermediate, and deeper-water ramp segments. Further studies of ramps and shelves brought to light differences in sediment production and retention on ramps, response of ramps to sea-level change, and the variety of tectonic settings in which ramps occur. Sequence stratigraphic analysis of ramps resolved most of the problems in reconstructing depositional surfaces from 3D sequences in the rock record, high- v. low-energy and temperate v. tropical ramps became well known, and studies on hydrocarbon reservoir characterization focused on diagenesis in a carbonate ramp sequence stratigraphic context. From a paper in a rather obscure regional journal, the ‘carbonate ramp’ has grown to become a world-wide standard.
