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The Carbonate Ramp: An Alternative to the Shelf Model
Wayne M. Ahr (1)
One of the common depositional models for carbonate rocks is the "shallow shelf." This model is typically constructed to show a nearly flat platform and a clearly defined "shelf-slope break." Typically, there are detrital carbonates on the platform, reefs or banks at the "shelf-margin" and basinal rocks seaward of the reefs. The modern analogs are the Florida-Bahamas (recent), the Cretaceous of Texas and Northern Mexico (Edwards-El Abra), and the Permian of West Texas-New Mexico (Capitan model).
Less common--but very important--is the ramp depositional model. The ramp is an inclined platform that extends basinward without a pronounced break in slope. Carbonate facies, therefore, are not necessarily protected by a shelf-margin barrier, reefs and facies patterns of the detrital carbonates tend to be distributed in bands which (1) parallel the coastline and (2) reflect the greater wave and current activity near the mainland shore.
A recent example of the ramp model is the Campeche Bank. The detrital carbonates of the Campeche Bank are arranged in concentric bands which range from grainstones and boundstones in shallow, agitated water to mudstones and wackestones on the seaward reaches of the ramp. Coral-algal reefs are common, but they do not occupy a position at the "margin" of the ramp. An ancient example of the ramp model is the Jurassic (Smackover and Cotton Valley) around the ancestral Gulf of Mexico. This example of the ramp is modified by salt tectonics.
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