Submerged Lands of Texas, Brownsville-Harlingen Area: Sediments, Geochemistry, Benthic Macroinvertebrates..

Surface sediment textures, sediment geochemistry, and benthic fauna of the State-owned submerged lands were mapped and described using bottom samples collected at 1-mi (1.6-km) intervals from bays, estuaries, and lagoons, and the inner continental shelf. In one area of Laguna Madre samples were collected at 0.5-mi (0.8-km) intervals. In addition, the distribution of wetlands in adjacent areas was mapped using color infrared photographs taken primarily in 1979. Textural maps of the Brownsville-Harlingen area show that sand and muddy sand, having a mean grain size of between 2.5 and 5, are the dominant sediment types in bay-estuary-lagoon and inner-shelf areas. Generally, in Laguna Madre sands occur on the barrier island side of the lagoon, whereas muddier sediments are more abundant along the mainland side and in deeper areas. Muddy sand is dominant in the relatively wide southern end of Laguna Madre, and sand is dominant along the narrower, northern two-thirds of the lagoon where broad sandy wind-tidal flats on Padre Island grade into shallow subaqueous lagoon sands. Dominantly sand-sized sediments blanket most of the inner shelf and extend about 10 mi (16 km) offshore from Padre Island. The greatest extent of sand is associated with marine reworked late Pleistocene fluvial-deltaic deposits that underlie much of the inner shelf. Water depths average about 90 ft (30 m) at the outer limits of this sand-rich area. A nearshore patch of mud occurs near the mouth of the Rio Grande and represents the most recent deposition of the river. To the east and north the mud grades into muddy sand that represents a mixture of relict shelf sands and more modern fluvial muds.