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, calcilithiticvolcarenitic 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 passu with 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.
