Tectono-Sedimentary Evolution of Southeastern Mexico's Valle De Oaxaca Graben

This is a major structure of southeastern Mexico, extending from northern Puebla to central Oaxaca; there it is an inverted Y-shaped depression centered on Oaxaca City, bounded to the east by crystalline basement units, and to the west by Albian-Cenomanian marine carbonate units. The graben fill includes an Early Paleogene-Quaternary continental sequence, which records the graben Cenozoic sedimentary and tectonic evolution. North of Oaxaca, the fill includes an unnamed polymictic conglomerate unconformably overlain by the Early Miocene Suchilquitongo Formation: a lacustrine, fluvial and volcanoclastic succession [K-Ar tuffs dated ~20 Ma] bearing mammals, in turn overlain by Quaternary deposits. Southeast of Oaxaca, the fill includes only Middle Miocene units: the K-Ar dated Mitla Tuff and the mammal-bearing Matatlán Formation, which partly intertongues the former, and is sedimentologically alike to the Suchilquitongo Fm. Quaternary deposits also crown Matatlán. Both are extensively faulted. There are of course, some differences.

Facies analysis shows that Suchilquitongo/Matatlán 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 conglomerate bodies [coalescing tallus fans]. Afterwards, Tertiary deposition ceased. K-Ar data and mammal taxa date Suchilquitongo Fm as Early Miocene [~20 Ma], and the Matatlán Formation as Middle Miocene [~15 Ma].

It is interpreted that the similarities and differences and age of the sedimentary/volcanic fill in the north and southeast, suggests that the development of the Valle de Oaxaca Graben started in the north, involving a pre-Miocene and an Early Miocene tectonic episodes, and a Middle Miocene episode, only recorded in the South. These episodes largely controlled sedimentation and volcanism in the graben.