The Tertiary sequence in Northwestern Oaxaca is preserved in four elongated basins trending NW-SE, they are from NE to SW: I Tehuacan-Cuicatlan, II Tlaxiaco-Huajuapan-Coixtlahuaca-Nochixtlan, III Mixtepec-Mariscala, and IV Coycoyan. The basin-fill is broadly similar, it includes Early Paleogene syntectonic calclithitic conglomerates intertongued and/or overlain by fluviolacustrine fine-grained clastics, which are overlain by Oligocene rhyodacitic to andesitic pyroclastic sheets, covered by andesitic lava flows and pierced by hypabyssal andesitic intrusions.The basins are bound by horsts composed of Pre-Cenozoic rocks: Paleozoic metamorphics to the W and NW, Precambrian metamorphics to the SE, and Jurassic clastics and Cretaceous carbonate platforms elsewhere.

Basin II is the largest, best known, and is considered typical. Here, the Paleogene fine clastics (Yanhuitlan Formation) include numerous silicic tuffs. A small stock close to Tamazulapan, yielded an age of 40 Ma. The lava flows range from 35 to 25 Ma. Besides other hypabyssal bodies, there is a 60 km long belt of some100 bodies (diameter >400 m), oriented N10°W, which marks a major fault, just east of Nochixtlan. The lack of volcanic flows, hypabyssals and Paleogene fine clastics, and the presence of Hemingfordian (Early Miocene) clastics intertongued by silicic tuffs, and covered by coarse clastics, sets Basin I apart from the others.

Thus in NW Oaxaca magmatism occurred in the Eocene (~40 Ma), the latest Eocene-Oligocene (~35-25 Ma), which is the most widespread, and the Early Miocene, only recorded in Basin I. Miocene magmatism is better represented in central and eastern Oaxaca.

The similar structural and lithostratigraphic pattern of Basins II-IV, strongly suggest that they originated during the Early Paleogene in a transtensional stress field, which lasted till the Oligocene, probably related to oblique subduction of the Cocos Plate, as the Chortis Block migrated eastward. The development of Basin I, is related to rejuvenation of the Oaxaca Fault.