Sedimentological-stratigraphical study of Sierra Las Lilas (Paleozoic) and Sierra El Cartucho (Mesozoic) in northwestern Chihuahua State, Mexico (Bismark mine district)

Abstract

Sedimentary rocks belonging to two different stratigraphic sequences of Paleozoic and Mesozoic age are exposed in the vicinity of the Bismark mining district. The regressive Paleozoic sequence of Early Permian age is represented by La Concha Formation, composed of thin to massive dolomites, and the Santa Rita Formation, which includes interbedded sandstones and dolomites. The fauna and the lithology of both formations suggest an initial marine shallow environment in a carbonate ramp-type platform, which changed transitionally to an environment with clear continental siliclastic influence. The Permian deposits represent the sedimentary cover deposited on the North American craton of the Chihuahua terrane. The Mesozoic sedimentary sequence consists exclusively of Lower Cretaceous rocks and includes (1) a sequence with Neocomian fauna; (2) the Aptian – lower Albian Cuchillo Formation; (3) the Albian Estudio sedimentológico-estratigráfico de las sierras Las Lilas y El Cartucho, Chihuahua, México 299 Benigno Formation, which correlates with the middle part of the U-Bar Formation from New Mexico; and (4) the upper part of the U-Bar Formation. These formations demonstrate a transgressive trend ranging from marginal-marine sedimentation containing fine grained terrigenous detritus (Neocomian–Aptian) to a shallow marine water environment with deposition of floatstones and boundstones in a carbonate platform with local reef buildups (lower Albian). This carbonate domain changed at the end of the Albian age and was replaced by sedimentation in a marginal marine-continental environment. The fauna and the lithology of the Mesozoic sedimentary sequences show that the marine deposition began in this part of northwestern Chihuahua in the Neocomian in a paleogeographic zone related to the Chihuahua trough and the northern margin of the Aldama platform.