DEPOSITIONAL SYSTEMS OF THE SAN NICOLÁS UNIT, PEOTILLOS GRABEN, SIERRA MADRE ORIENTAL AT SAN LUIS POTOSÍ, CENTRAL-EAST MEXICO: A FIRST APPROACH

The area lies in central San Luis Potosí State, within the Sierra Madre Oriental Morphotectonic Province, between 22^o10’-22^o22’ N Lat. and 100^o28’-100^o40’ W Long. The ~220 m thick Tertiary sequence preserved in the Peotillos Graben [a major N-S structure in western SMO bound by horsts of Cretaceous carbonate units] is best exposed in the southern part; it includes the San Nicolás unit, an Oligo-Miocene, meso- and fine-grained, fluvio-lacustrine sequence interbedded in the lower part by basaltic lava flows and rhyolite ignimbrite sheets, seemingly of Oligocene age, and by a complexly intertongued, coarse-grained and volcanic sequence of Plio-Quaternary age, that unconformably overlies it.

The San Nicolás lake facies are: Lake shore and mud flats [thinly bedded, poorly indurated, locally planar cross-bedded fine sand to silty clay, grading to mud], carbonatic nearshore [well lithified, thinly bedded, plant-bearing silty biomicrite; it locally shows terrestrial influx], and clastic offshore [thinly bedded to laminar, poorly indurated, varv-looking silt or clay].

The fluvial facies are: Channel [medium to thickly stratified, moderately lithified, usually cross-bedded conglomerate to gravelly sand bodies of complex architecture –bars and channel lag], levee [medium to thinly bedded and planar cross-bedded fine sand to silt], crevasse splay and swamps [small, isolated thinly and cross-bedded clay and silt bodies -discordant over levees, and/or intercalated in mud strata], flood plain [thin bedded, poorly consolidated fine grained sand, silt and clay; locally bearing Miocene mammal remains], and alluvial fan [medium to thickly bedded conglomerate].

The lake facies record several shallow ephemeral lakes or ponds fed both by carbonate-rich spring water and pluvial water; these and the fluvial facies are complexly intertongued, though the fluvial ones dominate in the upper part; some Oligocene tuff sheets are present largely in the lower part. The San Nicolás unit records a long period of clastic deposition in an episodically sinking basin, perhaps associated to pulses of volcanic activity, within an extensional tectonic setting.