Geoenvironmental Map of the Matagorda–Matagorda SW Quadrangles, Texas Gulf Coast

Thomas A. Tremblay, Edward W. Collins, and Jeffrey G. Paine
Bureau of Economic Geology, University of Texas at Austin, University Station, Box X, Austin, Texas 78713–8924

The Matagorda−Matagorda SW quadrangles include the delta of the Colorado River system that has prograded across Matagorda Bay, connecting the coastal plain with Matagorda Peninsula. This map will be used as a source of basic geologic information for managing land resources and studying shoreline and land-cover change, depositional environments, and active faulting. Geology of the area consists of Pleistocene Beaumont Formation and Holocene to Modern deposits. The area lies within fluvialdeltaic, bay/estuary, and strandplain settings having sensitive geologic environments that are under stress from natural processes and human activities. Extensive estuarine emergent wetlands (salt and brackish marshes) occur on the Colorado River delta (6082 acres [2346 ha]).

The most significant wetland trend on the Colorado River delta from the 1950’s through 2001 was an increase in estuarine marsh (White et al., 2002). The estuarine marsh habitat, which makes up most of the delta, increased in total area from 5,219 acres (2112 ha) in the 1950s to 6086 acres (2463 ha) in 2001. The net increase in marsh of 870 acres (352 ha) reflects a gain of about 17 percent. The principal gain in estuarine marsh since the 1950s has occurred in the southwest corner of the delta on the Tiger Island subdelta that formed at the mouth of an artificial channel. Since the 1950s when the channel was cut until 1979, the subdelta has prograded about 2625 ft (800 m), creating 222 acres (90 ha) of marsh. The subdelta continued to prograde until the early 1990s, extending the edge of the marsh another 1640 ft (500 m) into the bay and increasing the area of marsh by an additional 198 acres (80 ha). In 1991, as part of the Colorado River Diversion Project (Wilber and Bass, 1998), a dam was constructed across Tiger Island Channel, causing progradation and marsh development at this subdelta to cease. Additional small marsh areas have developed near the mouth of the Colorado River diversion channel at the upper west side of the delta. New marshes in this area appear to have developed primarily along islands and shoals resulting from disposal of material dredged from the channel. Total additional estuarine marsh in this area is about 272 acres (110 ha). Countering the growth of marshes on the west side of the delta was marsh erosion on the eastside. Erosion of the delta margin in this area occurred at rates averaging about 5 ft/yr from 1956 through mid-1995, accounting for a marsh loss of about 124 acres (50 ha).

AAPG Search and Discovery Article #90158©2012 GCAGS and GC-SEPM 6nd Annual Convention, Austin, Texas, 21-24 October 2012