Abstract

Two thousand feet of lower Cretaceous carbonates, exposed in Sierra de la Paila, southern Coahuila, are divisible into three interbedded lithofacies, all rich in carbonate mud: Lithofacies I (= Cuchillo Formation, dolomite); Lithofacies II (= “Aurora” Formation, fossiliferous wackestone and packstone); and Lithofacies III (= Cuesta del Cura Formation, lime mudstone). The sediments are all subtidal and were deposited on a gently-sloping carbonate ramp as the Mexican Sea transgressed from the southeast onto the paleogeographic Coahuila Peninsula. Lithofacies III was deposited below wave base under deep slope or basinal conditions; Lithofacies II in normal marine, shallow water, generally above wave base; and Lithofacies I still higher on the ramp in waters restricted by a series of disconnected low rudist biostromes. The shoals and organic buildups broke up already gentle waves and effected the complex intertonguing between Lithofacies I and II. There was no appreciable break in depositional slope.

There is no hydrocarbon potential in Sierra de la Paila because of post - depositional cementation. Beds similar to Lithofacies I and II could be excellent reservoirs elsewhere under favorable conditions.