Page 339 - Carbonate Facies in Geologic History

326 The Rise of Rudists; Middle Cretaceous Facies in Mexico and the Middle East

areas on some of the platforms became the sites of evaporite deposition when
periodically cut off from the open Gulf. These shelf areas were separated from the
Gulf by narrow carbonate rims reaching nearly to sea level and composed of
accumulations of rudists and biogenic lime sand. Slopes off these rims into the
basin tend to be from 2 to 5 degrees around the Gulf and steeper along some of
the offshore banks in Mexico. Bathymetric relief could have been as much as
1000 m. Thickness of shallow-water Cretaceous carbonate in these great plat-
forms is even greater. In the Gulf and Atlantic, Lower and Middle Cretaceous