SEDIMENTARY ENVIRONMENTS AND TECTONIC CONTROL OF THE VALLE DE OAXACA GRABEN MIOCENE CONTINENTAL SUCCESSION, SOUTHEASTERN MEXICO

The Miocene fill of the inverted Y-shaped Valle de Oaxaca Graben [16°50'-17°18' N Lat. and 96°21'-96°54' W Long.]; includes the Suchilquitongo Fm. in the north, and the Matatlán Fm. in the east. The first unit's lower part chiefly consists of laminated to thin and medium bedded tuffaceous mudstone and fine-grained sandstone, sparsely intercalated by medium to thickly bedded silicified limestone and ignimbrite sheets; the upper part is composed of medium to thickly bedded, polymictic, pebble-cobble, matrix- supported conglomerate.

Facies analysis shows that Suchilquitongo deposition largely occurred as mud flats in a playa lake; occasional lake level rise and/o lesser terrigenous influx allowed silicified limestone to be formed. Flood plain and channel facies are scarce, suggesting that drainage was limited. Volcanism emplaced ignimbrite sheets. A sudden horst uplift induced debris flow, which formed the conglomerate body [coalescing tallus fans]. Afterwards, deposition ceased. K-Ar data and mammal taxa date the unit as Early Miocene [~20 Ma].

The Matatlán Fm. consists of laminated to thinly bedded tuffaceous mudstone, siltstone and fine grained sandstone, partly intertongued by coarser-grained, cross-bedded sandstone and granule-pebble conglomerate, covered by an alternation of tabular to cross bedded sandstone and pebble-cobble, matrix supported polymictic conglomerate; tuff interbeds are present. The Mitla Tuff partly intertongues this formation. Matatlán facies include mud flats, offshore lake sheets, channel beds, point bars, crevasse splays and extensive flood plain deposits, which suggest frequent avulsion. The sand/conglomerate alternation indicates successive debris flow and stream flow pulses related to climate and/or tectonic instability. Synsedimentary volcanism emplaced ignimbrite sheets, furnished abundant fine clastics; the Mitla Tuff confined the basin to the north and east. Both sedimentation and volcanic activity ceased afterwards. K-Ar data and mammal taxa date this formation as Middle Miocene [~15 Ma].

The similarities and differences and age of the sedimentary fill in the north and east, suggests that the development of the Valle de Oaxaca Graben started in the north, and involved two tectonic episodes, which afforded the chief control to sedimentation there.