CONTRIBUTION TO THE CENOZOIC GEOLOGY OF THE YOLOMéCATL-TLAXIACO AREA, CENTRAL SIERRA MADRE DEL SUR, SOUTHEASTERN MEXICO, AND ITS REGIONAL SIGNIFICANCE

The Sierra Madre del Sur [= SMS] intricate, rugged relief and knotty makeup denotes a complex geologic evolution not fully understood, as bear out by the numerous existing interpretations, usually based on regional studies. We believe that there is ample room to improve the cognition

on this phenomenon by studying in detail an area, and discuss how it fits in the current orthodoxy. So, we selected the Yolomécatl-Tlaxiaco Area, Western Oaxaca, a key SMS area, because it includes the boundary between the Mixteco and Oaxaca Terranes, and exhibits the full geologic column and deformation record.

The basement includes the Late Paleozoic Acatlán Complex, uncorfomably covered by clastic and carbonate Middle to Late Jurassic units [Zorrillo/Taberna associated Formations, San Juan and Las Lajas Limestones], unconformably overlain by the Late Cretaceous Teposcolula Limestone. The Cenozoic sequence includes the Tamazulapam Conglomerate [Early Tertiary, uncorformably overlies the Teposcolula Ls.], Nduayaco Unit [new taxon, coeveal with the Conglomerate, intermediate to mafic lava flow stack], Yolomécatl Formation [new taxon, Late Eocene (40 Ma), fine-clastics], Tayata and Reforma Units [new taxa, coeval to Yolomécatl Fm., fine-clastics], Nundiche Group [new taxon, Early Oligocene (~33 Ma), felsic pyroclastic sheet], Nicananduta Group [new taxon, early Late Oligocene, intermediate to mafic lava flows stack], ?Chilapa Formation [Early Oligocene, intertongues the Nicananduta Grp., largely lacustrine], and Quaternary deposits.

The deformation record includes folds [Teposcolula Anticlinorium and Tayata open anticline] and fractures/faults referred to four geographic sectors [Southwestern, Northwestern, Southern and Eastern], which disclose these pulses of activity: Pre-Late Eocene Tamazulapam Fault left lateral transtension [Nduayaco magmatic outpour], Tamazulapam left lateral transpression [Teposcolula Ls. Block thrusted over Nduayaco]; post-Late Eocene Tayata open folding and Nundiche Grp. emplacement], Extensional deformation [in Nundiche Group] and uplift [of Southwestern and Jurassic ls. Blocks. Magmatism and deposition ended not earlier than 27 Ma. Our results [three magmatic events four linked-deformations events] afford more precise data on the makeup and evolution of the SMS.