Preliminary Geologic Map of Southernmost Texas, United States, and parts of Tamaulipas and Nuevo Leon, Mexico

Rocks in central part of the map area are Tertiary units consisting mostly of sandstone, shale, and claystone units deposited in mixed marine and continental environments. These Paleocene to Pliocene rocks dip east and formed as a result of rapid deposition and progradation of sediments across the continental margin into the Gulf of Mexico. The rapid infilling resulted in the development of syndepositional growth faults which formed episodically from Paleocene to the Pliocene.

The growth faults are concealed beneath Pliocene and Quaternary deposits and form a structurally complex Gulf Coast Tertiary basin. Tertiary bedrock units are important because they contain oil and gas resources in south Texas and in Tamaulipas, Mexico, and uranium deposits in the South Texas Uranium District. In addition, some of these rocks also are aquifers for ground water in the region.

Rocks bordering the Gulf of Mexico are the youngest units in the map area consisting mostly of Quaternary and modern coastal deposits in deltaic, tidal flat, beach, barrier island, lagoonal, esturary, and dune environments. [Summary provided by the USGS.]