THE FIRST DOCUMENTED EOCENE VOLCANIC AVALANCHE (NDUAYACO V. A.) OF MEXICO, EVIDENCE OF PARTIAL COLLAPSE SECTOR OF A VOLCANO ADJACENT TO THE TAMAZULAPAM FAULT IN NORTHWESTERN OAXACA

The Nduayaco Avalanche lies in the Yolomécatl area, which includes some 400 km² of rugged terrain within the Mixteca Region, NW Oaxaca, Sierra Madre del Sur Morphotectonic Province, SE Mexico, between 17°25’-17°36’ N. Lat. N and 97°28’-97°36’ W Long. The Cenozoic sequence unconformably overlies carbonate rock units of Late Jurassic to Late Cretaceous ages. The area also includes the Mixteco/Oaxaca Terrane boundary, namely the Tamazulapam fault.

The Tertiary sequence consists of five lithostratigraphic units: Pre-Middle Eocene Nduayaco “Group” [a stack of andesitic flows], late Middle Eocene Yolomécatl Formation [an ~650 m thick, fluviolacustrine succession, its clast composition is equal to that of the previous unit; it is interbeded by silicic tuff sheets, one such yielded an ³⁹Ar-⁴⁰Ar age of 40.7 ± 1.0 Ma], Nundiche “Group” [a sequence of felsic tuffs dated as of ~33.0 Ma], Nicananduta “Group” [an andesitic lava flow succession dated as of ~27.0 Ma], Chilapa Formation [a tuffaceous sequence intertonguing this Group], and Ticu Dome [a small, rhyodactic intrusion]; finally Quaternary deposits overlie the previous units The structural record chiefly includes folds in the Mesozoic units, and faults in the Tertiary ones.

The Nduayaco Avalanche is part of the name sake unit, lengthwise (N-S) is ~10 km ~2 km wide, and ~45 km²; it is located between Ixtapa and San Miguel Achiutla. The morphology of this avalanche is hummocky, and consists of blocks with a jigsaw structure set in a sandy matrix; the blocks facies largely lies between Santa María Nduayaco and San Juan Achiutla, whereas the matrix facies is best exposed between Ixtapa and San José de Gracia. This avalanche is westernly confined by the left-lateral Tamazulapam fault, whose activity (concentration of compressive stresses in the north) thrusted a Teposcolula Limestone tectonic scale over the avalanche (northern part). The volcanic structure where the collapse occurred lies ~1.0 km east of San Juan Achiutla. Finally given that the clasts of the Yolomécatl Formation (~40.7 Ma) were derived from the Nduayaco “Group,” the age of the latter must at least be somewhat older (~42 Ma), which makes it the first documented Eocene volcanic avalanche in Mexico.