Boletín de la Sociedad Geológica Mexicana
Volumen 68, núm. 2, 2016, p. 283-304
A new Miocene Formation from The Peotillos-Tolentino Graben fill, Western Sierra Madre Oriental at San Luis Potosí, Mexico: Part 2, Depositional environments and regional significance
Ismael Ferrusquía-Villafranca1, José E. Ruiz-González1, José Ramón Torres-Hernández2, Enrique Martínez-Hernández1, Jorge Gama-Castro1
1 Instituto de Geología, Universidad Nacional Autónoma de México, Ciudad Universitaria, C.P. 04510, México, Distrito Federal, México.
2 Instituto de Geología, Universidad Autónoma de San Luis Potosí, Ciudad Universitaria, C.P. 78000, San Luis Potosí, S.L.P., México.
The discrimination and characterization of the depositional systems registered in the formal lithostratigraphic units of our country, is practically a pending task of Mexican Geology, notwithstanding their academical and applied importance in the investigation of placer mineral deposits and of aquifers. As a contribution to fill this gap, the depositional systems of the Late Miocene San Nicolás Formation, the major sedimentary fill of the Peotillos-Tolentino Graben, are described and characterized.
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 an altitude of 1295 – 2025 masl. Such unit is an ~1100 m thick, fluvio-lacustrine, largely calcilithitic, 15° – 20° NE dipping, peneplained sequence, unconformably overlain by a Quaternary, ~40 m thick, calcilithitic-volcarenitic blanket, locally covered by mafic lavic/pyroclastic rocks.
San Nicolás Formation includes these systems: Fluvial, lacustrine, debries flow and pyroclastic [the latter two are volumetrically negligible]. The first consists of these lithofacies: (1) Gm, Gh, Gp: Channel lag deposits. (2) Gm, Gp, Sp, St: Bars of various types. (3) Sr, Sl, Fl; Floodplain deposits; it bears the Late Miocene Paso del Águila local fauna and palynoflora, which discloses the presence of pine-oak forest with a well developed understory component, and savanna, as well as a much more humid climate regime then than that prevailing at present. (4) Gh, Sp: Thin channel conglomerates and/or sandstone deposits. (5) Sl, Fl: levee deposits. (6) Sp, Sr, F1: Crevasse splay deposits. Paleosols [associated to floodplain deposits]: Fluvisol, Calcisol, Gleysol and Luvisol. The lake system includes these lithofacies: (1) Lacustrine-carbonate, (2) -fine clastics, (3) -shore, and (4) –mud flats. The debries flow includes lithofacies Gms and Sm.
The great thickness of this continental unit largely records the cyclic superposition of fluvial systems that took place in a basin subsiding pari passuwith sedimentary accumulation, under humid to subhumid conditions interrupted by others of [?seasonal] dryness. Finally, the use of plant taxa as paleoaltitude indicators, and the discovery of pre-Pleistocene paleosols are discussed.
Keywords: Mexico, Miocene, continental lithostratigraphy, depositional systems, tectono-sedimentary evolution, paleosols.