The oldest salt basin of southeastern Mexico, the Isthmian salt basin, usually is referred by most authors to the Triassic-Jurassic. However, it is now possible to date the oldest salt basin as pre-Kimeridgian and not older than early Oxfordian (Divesian). This conclusion is based on new data from wells and from studies of the Todos Santos Formation, a unit which has been the object of many studies.

A second basin, with salt beds between Late Jurassic strata, below, and Early Cretaceous strata, above, was penetrated in the Turipache No. 1 well near Tuxtla Gutierrez, Chiapas. Information from the La Pita No. 1 and Tortugas No. 1 wells in Guatemala indicates that this basin extended through southern and central Guatemala--perhaps into western Belice (British Honduras).

The development of two salt basins of different ages--one on the northwest and the other on the southeast--suggests a northwest to southeast marine transgression along an arcuate line of embayments marginal to the Gulf of Mexico. The marginal position of the basins supports the concept that, before Cretaceous time, the Gulf was a land (shield) area and that the salt basins formed before the foundering of the Gulf was completed at the end of the Mesozoic.

The tectonic history of the area includes both Mesozoic and Tertiary crustal movements. Transcurrent faults are present south and southwest of the Yucatan platform; tension or gravity faults characterize the Tertiary basins of the Gulf coastal plain and the eastern side of the Yucatan Peninsula. The transcurrent movement phase is a Cretaceous-early Tertiary event; the tension-movement phase is a middle to late Tertiary event.