NEW PETROLOGIC AND STRATIGRAPHIC CONSTRAINTS FOR THE TIANGUISTENGO FORMATION IN SOUTHERN MEXICO: A NEW RECORD OF A TRIASSIC RIVER DRAINING EQUATORIAL PANGEA?

During Triassic time, the Pacific margin of Mexico was the site of accumulation of large submarine fans that were inferred to be supplied by major fluvial systems draining equatorial Pangea. Unfortunately, fluvial successions that have been solidly recognized to be deposited during Triassic time are very few in Mexico and are restricted to a limited number of exposures in the northeast. Fluvial beds of the Tianguistengo Formation are exposed in southern Mexico and were previously inferred to be deposited during Jurassic time by most authors. However, at present, the age of this unit remains unconstrained. We present here new sedimentological data and a detailed sandstone provenance analysis that show that the Tianguistengo Formation is composed of fluvial deposits supplied by a regional drainage system interbedded with local alluvial-fan-deposits that were developed adjacent to the exhuming Carboniferous-Permian Totoltepec pluton. Preliminary fission tracks data on apatite grains extracted from the Totoltepec pluton indicate that this intrusive body experienced a complex thermal history characterized by distinct exhumation and burial events from Triassic to Cretaceous time. Previous authors documented that the Jurassic exhumation history of the Totoltepec pluton is recorded by the Jurassic stratigraphic units that overlay the Tianguistengo Formation. Therefore, we discuss in this work the possibility that the alluvial-fan-deposits of the Tianguistengo Formation were deposited as a consequence of rapid exhumation of the Totoltepec pluton during Triassic time. According to this scenario, the Tianguistengo Formation could be reinterpreted as a stratigraphic unit developed by the interaction of local alluvial fans with a regional-scale fluvial system, which may represent one the major rivers that drained equatorial Pangea during Triassic time.