Depositional systems of the late Eocene Yolomécatl Formation, northwestern Oaxaca, southeastern Mexico: a first approach

The discrimination/characterization of depositional systems recorded in formal lithostratigraphic, Cenozoic, continental, epiclastic units of Mexico, is largely a pending task, notwithstanding its great academic and economic importance. Contributing to fill this gap, we present a first approach to discriminate and characterize the depositional systems of the late Eocene Yolomecatl Formation, which fills the namesake, triangle-shaped tectonic basin, and carries the southernmost Paleogene vertebrate fauna of North America. The study area lies between 17 degrees 25'-17 degrees 30' North Latitude and 97 degrees 29'-97 degrees 36' West Longitude, and between 2000-2500 ma.s.l.; the federal highway 125 traverses it; Santiago Yolomecatl is the main town. The Yolomecatl Formation is at least 650 m thick, although the continuous thickness measured in the principal reference section is much less (similar to 250 m); it includes fluvial and lake depositional systems sparsely interbedded by tuff sheets (a system per se). The fluvial system consists of 1) Gmm, Gmg, Gcm lithofacies: alluvial fans and related deposits; 2) Gm, Gh, Gp, Gt, St, Sp lithofacies: channel lag deposits; 3) Sr, Sh, Sl, Fl lithofacies: floodplain deposits. The (clastic) lacustrine system includes: 1) Ll, Lsm, Lm lithofacies: offshore deposits; 2) Ll, Lsm lithofacies: playa lake and mud flats deposits. This large thickness of this unit indicates superposition of fluvial/lacustrine systems, and sparse pyroclastic emplacements that took place in a basin gradually subsiding with sedimentation, under humid to subhumid conditions, probably interrupted by seasonal dryness. Finally, the tectonic setting of the Yolomecatl Basin, indicates that its genesis and development are related to the dynamics of the regional, left-lateral strike-slip Tamazulapam fault, which is also regarded as the boundary between the Mixteca (west) and Oaxaca (east) terranes.