USGS Open-File Report 2005-1231
The study area of the U.S.-Mexico Border Environmental Health Project (BEHP) (refer to index map) encompasses 10,240 square miles of which 6,155 are in Mexico and 4,085 are in the United States. The elevations above sea level range from a maximum of 1511 m in the western part of the area to 0 m at the coast. A color-shaded-relief image of digital elevation data shows the effects of erosion and dissection caused by the Rio Grande River and its tributaries. The general trend of the terrain is that elevations increase more rapidly as the distance from the coast increases (refer to profiles of the digital elevation data.
A broad representation of the geology (based on the map by Page, VanSistine, and Turner, 2005 shows that the BEHP study area includes limited areas of Cretaceous rocks with the major portion of the area underlain by Tertiary and Quaternary sediments. A table of geologic units (Page, VanSistine, and Turner, 2005) lists those that occur within the BEHP study area. The oldest rocks outcrop in the western part of the BEHP study area. The age of the geologic units decreases from west to east. The oldest rocks are mostly Cretaceous and Jurassic marine limestone, sandstone, shale, and evaporite units with a complex history of folding. The Tertiary rocks consist mostly of sandstone, shale, and claystone units deposited in mixed marine and continental environments. The Tertiary sediments were deposited rapidly which resulted in the development of syndepositional growth faults that formed episodically from Paleocene to the Pliocene. The growth faults (Ewing, 1986) are mostly concealed and form a structurally complex Gulf Coast Tertiary basin. The Quaternary units are sediments deposited in deltaic, tidal flat, beach, barrier island, lagoonal, estuary, and dune environments (Brown and others, 1980).
The aerial survey that is the focus of this report covers a small part of the BEHP study area (refer to the index map of the aerial survey). A geologic map taken from Page, VanSistine, and Turner (2005) shows that the Tertiary Goliad Formation outcrops over most of the northern part of the survey area. Weeks (1945) states that "The Goliad Formation is composed of red, pink, orange, and gray clays, white to gray sugary sandstones, and conglomerate beds containing pebbles of limestone, gray and black chert, and fossil wood all commonly interbedded with sand or intermixed with clay balls. A characteristic feature of the Goliad [Formation] is the large amount of calcareous material which accounts for the thick beds of caliche ordinarily found in the upper part of the Goliad section exposed in cuts and pits in southwest Texas." The Pleistocene Lissie Formation overlies the Goliad and consists of gravel, coarse red sand, and some lenses of red gravelly clay. The gravel consists of rounded pebbles and cobbles (Deussen, 1924). The Pleistocene Beaumont Formation overlies the Lissie and includes calcareous clay, sand that is calcareous in places, and sandy clay (Deussen, 1924). Recent sediments include sand dunes and floodplain deposits of the Rio Grande.
U.S. Department of the Interior |
U.S. Geological Survey
URL: http://pubsdata.usgs.gov/pubs/of/2005/1231/geology.htm
Page Contact Information: Stephen Snyder
Page Last Modified: Saturday, 12-Jan-2013 21:28:00 EST
