Stratigraphy of Sierra de Picachos and Vicinity, Nuevo Leon, Mexico

Abstract

The section exposed in the Sierra de Picachos is approximately 4,900 ft (1,490 m) thick and is divisible into seven formations, which range in age from Neocomian to Campanian and possibly to Maestrichtian (Early to Late Cretaceous). These formations are, from oldest to youngest, Cupido, La Pena, Tamaulipas, Sombreretillo, Cuesta del Cura, San Felipe, and Mendez. The Sombreretillo is newly named, and the San Felipe has been redefined to include beds formerly assigned to the Agua Nueva Formation. The section is composed mainly of limestone with lesser amounts of dolomite (confined to the Cupido Limestone), chert, and terrigenous clay. The limestone is of one type, lithified carbonate mud (micrite). The micrite is classified into types based on the contained organic constituents, which consist principally of pelagic forms: foraminifers, calcispheres, tintinnids, and radiolarians. With the exception of the Cupido Limestone, the micrite is interpreted as representing basinal deposits. The depth of the water during deposition is unknown but may have been relatively shallow. The nature of the micrite in the Sierra de Picachos indicates that the sea floor was below wave base throughout deposition. The sea floor in the Sierra de Papagayos was above wave base during part of Tamaulipas deposition. Some of the carbonate mud may have been precipitated in the shallower shelf waters on the west, and then winnowed into the lower energy basinal environment. Intermittent rains of the calcareous tests of pelagic microorganisms contributed substantially to the mud. Chert occurs as lenses, stringers, and irregular nodules in the Cupido, La Pena, Tamaulipas, and Cuesta del Cura formations. The chert formed during diagenesis and the silica is thought to have come from siliceous organisms. The deposition of carbonate mud was interrupted several times (La Pena, Sombreretillo, San Felipe, and Mendez) by the influx of terrigenous clay resulting from climatic changes and/or tectonism in the source area. The final flood of terrigenous sediment (Mendez Shale), brought about by the Laramide orogeny, terminated carbonate deposition.