DEPOSITIONAL SYSTEMS OF THE YOLOMÉCATL FORMATION, LATE EOCENE OF NORTHWESTERN OAXACA, SOUTHEASTERN MEXICO: A FIRST APPROACH

The Cenozoic stratigraphy of the Sierra Madre del Sur is largely known, but characterizing the depositional systems recorded in lithostratigraphic units largely remains to be done. As a contribution to start filling this gap, we describe such systems in the Yolomécatl Formation, a unit that carries the southernmost mammal fauna of North America. The study area lies in northwestern Oaxaca, Southeastern México between 17°25’00”-17°30’10” N Lat. and 97°30’50”-97°37’00” W Long. The Cenozoic sequence unconformably overlies Mesozoic rocks, and consists of an andesitic succession (Unit A), unconformably overlain by the ~275 m thick, 40.7 Ma, vertebrate-bearing, largely epiclastic Yolomécatl Formation [correlative with another epiclastic body barely exposed in the area]; another andesitic succession (Unit B, 27.0 Ma) unconformably overlies Yolomécatl and interfingers with the lacustrine/tuffaceous Chilapa Formation; a dacitic dome intrudes the Unit A. Quaternary deposits and soils crown the Tertiary.

The Yolomécatl Formation includes fluvial, lacustrine and pyroclastic systems. The first lies in the east, consists of volcarenitic, thickly-bedded conglomerate that forms alluvial fans, slope and channel lag deposits; clast composition proves that Unit A was the source. The lake system wraps around the first, and consists of reddish, very fine-grained sandstone, siltstone and claystone chiefly set in laminar to medium thick strata; it alternates both horizontally and vertically with thin, conglomerate lenses or beds, which indicate that the bottom lake was intermittently exposed and traversed by creeks. The presence of thick paleosols indicates the same phenomenon. To the west, the lake beds are intermittently interbedded by thin sheets of felsic tuffs, some fully silicified, that record extra-basinal volcanism. The large thickness of this unit records the cyclic superposition of fluvial and lake systems that took place in a basin subsiding pari pasu with sedimentary accumulation, and intermittently receiving the products of felsic volcanism.