The revision of the available data on the geology of southern Mexico, south of the 20th Parallel, indicates that the region falls into six structural provinces which are the Tehuantepec-southern Chiapas, Sierra Madre del Sur, Sierra Madre Oriental, southern Gulf Coastal Plain, Yucatán Peninsula, and the Trans-Mexico volcanic belt.
The basement of this region is formed by Paleozoic sediments which were deformed and became cratonal in the Tehuantepec-southern Chiapas and Sierra Madre del Sur during middle Paleozoic, and in areas occupied at present by the Sierra Madre Oriental, southern Gulf Coastal Plain and the Yucatán Peninsula during late Paleozoic time.
The Mesozoic sediments were deposited, in general, in the Mexican geosyncline, a trough developed over the marginal zone between the early and late Paleozoic orogenic belts. The sequence is a molasse at its base (Jurassic), normal marine in its middle part (Lower Cretaceous), and a Flysch on its top (Upper Cretaceous). The main zone of late Cretaceous-early Tertiary orogeny was off the present Pacific Coast and folded the Mesozoic sediments making up the Sierra Madre Oriental during early Tertiary.
The Teritary section which covers the southern Gulf Coastal Plain and the Yucatán Peninsula is a molasse at its base (Eocene-Oligocene) grading into normal marine upward (Miocene). Deformation resulting in randomly oriented folds and flexures in the Coatzacoalcos region is recorded during the Miocene. The east-west-trending belt of Pliocene-Recent volcanoes which form the Trans-Mexico volcanic belt is considered to have formed along en échelon tension fractures over a deep-seated shear zone belonging to the Central American-Caribbean fracture system.