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

The geologic map covers the southernmost part of Texas (parts of Jim Hogg,
Brooks, Starr, Hidalgo, Zapata, Willacy, Kenedy, and Cameron Counties), and
parts of Nuevo Leon and Tamaulipas, Mexico. Geologic units become younger from
west to east across the map area. The oldest rocks are in the western part of
the map area in the states of Nuevo Leon and Tamaulipas, Mexico, and include
mostly Cretaceous and Jurassic marine limestone, sandstone, shale, and
evaporite units exposed in a series of north to northwest-trending ranges.
These rocks are highly folded and are part of the Sabinas foldbelt, a
compressional terrain which formed during the Laramide (Cretaceous to
Paleocene) due to complex interactions related to subduction of the Pacific
plate beneath North America. The Mesozoic rocks were intruded by Miocene and
Oligocene igneous rocks.

The central and eastern portions of the map cover the Gulf Coastal Plain
province extending across southern Texas and Tamalupias 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.]