Abstract

FERRUSQUIA-VILLAFRANCA, Ismael et al. Una nueva formación miocénica en el relleno del graben de Peotillos-Tolentino, oeste de la Sierra Madre Oriental de San Luis Potosí, México. Parte 1: Geología. Bol. Soc. Geol. Mex [online]. 2016, vol.68, n.2, pp.247-282. ISSN 1405-3322.

In spite of ~130 years of geologic research in Mexico, much remains pending e.g. only 1/3 of this country's territory is mapped in detail [i.e., scale 1:50000], the formal stratigraphic differentiation of the continental sedimentary Cenozoic is barely started, discriminating the depositional systems that make up basic units is even less pursued. Towards filling this gap, the detailed Cenozoic lithostratigraphy of a graben fill in central-east Mexico is reviewed, a new Miocene formation is proposed, and its importance in understanding the regional geologic evolution is discussed.

The study area lies in San Luis Potosí State, within the Sierra Madre Oriental Morphotectonic Province [SMO], between 22°11'-22°19' N Lat. and 100°30'-100°39' W Long., and 1295 - 2025 masl. The ~1200 m thick Tertiary sedimentary sequence is preserved in the N-S trending Peotillos-Tolentino Graben, which is bound by horsts of folded and faulted Cretaceous carbonate units; it includes a Paleogene volcanic succession formed by an andesitic-basaltic lavic stack intertongued/overlain by a rhyolitic ash flow-tuff sheet, which non-conformably underlies the new unit, a Late Miocene, ~1100 m thick, fluvio-lacustrine, largely calcilithitic, 15° - 20° NE dipping, peneplained sequence, in turn unconformably overlain by a Quaternary, ~40 m thick, calcilithitic-volcarenitic blanket; locally overlain by mafic lavic/pyroclastic rocks.

The new formation was deposited in a subsiding basin, under humid to subhumid conditions allowing erosion of large clastics volumes, and the development of an axial fluvial network, capable of transporting/depositing such volumes. Nevertheless, floodplain calcisols and calcretes indicate periods of dry conditions. The ~1100 m thick, largely fluvial stratal pile thus formed, records cyclic superimposing of such network's systems during Late Miocene time; this in turn, denotes stability of the tectono-sedimentary-climatic conditions. Subsequently, faulting, erosion and finally peneplanation took place. In the Quaternary, clastic sedimentation was reestablished.

The holistic approach used to study the Peotillos-Tolentino Graben and its sedimentary fill, could be applied to the other post-orogenic basins of the Sierra Madre Oriental that bear Cenozoic clastic sequences, probably resulting from similar processes, to identify and characterize such processes, thus contributing to a better understanding of the Cenozoic geologic makeup and evolution of this province.

Keywords : Mexico; Miocene; continental lithostratigraphy; depositional systems; tectono-sedimentary evolution; paleosols.