Stratigraphy and carbonate petrography of the Sierra de Picachos and vicinity, Nuevo Leon, Mexico

The stratigraphic section in the Sierra de Picachos is approximately 4,900 feet thick, is divisible into seven formations, and ranges in age from Neocomian to Campanian and possibly Maestrichtian (Early and Late Cretaceous). The formations are, from oldest to youngest: Cupido, La Peña, Tamaulipas, Sombreretillo, Cuesta del Cura, San Felipe, and Méndez. The Sombreretillo Formation is introduced in this paper. The Cretaceous section is composed predominantly of limestone, with minor amounts of dolomite, chert, and terrigenous clay. The limestone is of one type: lithified carbonate mud (micrite). Pore-filling sparry calcite is completely absent, which is unusual for such a thick section of limestone. The micrite differs mainly in the contained organic constituents, which consist principally of pelagic protistans (Foraminifera, calcispheres, tintinnids, and radiolarians). The micrite averages about 15 percent organic constituents by volume; some of the micrite in the Tamaulipas and San Felipe contains more than 50 percent organic constituents by volume. Subdivision of the micrite into petrographic types is based principally on the kinds and relative abundance of allochemical constituents. Petrographic types include micrite, dolomitized micrite (restricted to the Cupido), intramicrite, ostracod-bearing micrite, calcisphere biomicrite, Foraminifera biomicrite, and others. The study of electron micrographs suggests that recrystallization is the process by which an unconsolidated carbonate (aragonite) mud becomes a hard aphanitic limestone. The recrystallization, which involves the inversion of aragonite to calcite, results in crystal growth and a welding together of the calcite crystals to produce a lithified micrite. Sedimentary silica, thought to be of both organic and inorganic origin, was deposited contemporaneously with the lime mud. The silica was reconstituted and redistributed during early diagenesis, probably as a response to changes in pH, to form lenses, stringers, and irregular nodules of chert. Chert occurs in the Cupido, La Peña, Tamaulipas, and Cuesta del Cura; it is especially abundant in the Cuesta del Cura. The Cretaceous section is divided into seven zones, based principally on the identification of pelagic microfossils in thin section. The three best developed zones are: 1) Colomiella (a tintinnid) zone, which is restricted to the lower 290 feet of the Tamaulipas, 2) thick-walled calcisphere zone (150 to 180 feet thick), which is restricted to the upper part of the Tamaulipas and the overlying Sombreretillo Formation, and 3) Globigerina-Globotruncana zone (greater than 1,150 feet thick), which is restricted principally to the San Felipe. The Early Cretaceous (middle Albian to late Cenomanian) Stuart City reef trend of southwest Texas is postulated to extend into northeast Mexico and to pass west of the Picachos region. The reef trend developed along the outer margin of a broad shelf (undaform). The lime mud is a basin facies and is interpreted to have been deposited on the clinoform, in front of the postulated undaform-edge Stuart City reef trend. The depositional interface was below wave base throughout deposition of most of the sediment. The rate of accumulation of lime mud during the Comanchean Epoch is calculated to be 3.0 to 3.5 cm per 1,000 years. Most of the lime mud is thought to have been precipitated from the relatively shallower and warmer waters that bathed the undaform, and then transported seaward by currents and deposited on the clinoform. Both algally (as suggested by abundant algae in contemporaneous shelf limestones) and physicochemically precipitated aragonite needles were probably important constituents of the lime mud. Intermittent rains of the calcareous tests of pelagic microorganisms contributed substantially to the mud. Some of the Upper Cretaceous micrite (San Felipe) probably originated during the passage of shell fragments through the digestive tracts of burrowing organisms (possibly annelid worms). Carbonate sedimentation was terminated by the widespread influx of terrigenous clay (Méndez Shale) during the late Cretaceous, as a response to increasing tectonism in the source area.Geological Science