Gulf Coast physiographic setting

The US Gulf Coast physiographic region extends from Brownsville, Texas to the Florida Keys and encompasses the coastal plain, low hills, barrier islands, estuaries and river deltas of the northern Gulf of Mexico. Sediments consist of coastal plain deposits and thick land-derived sediments on the outer continental shelf that, ultimately, transition to evaporite and carbonate deposits in deep offshore waters. While the coastal margin is considered tectonically stable, its geomorphic features are subject to natural processes that alter the landscape at time scales ranging from millennia to days.

At the eastern end of the coastline in Florida, sediments are underlain by limestone at varying depths from the surface. Sinkholes, caves and freshwater springs are common features where the limestone is nearest the surface, particularly in the coastal plain from Tallahassee southward to Tampa Bay. Quartz sands, clayey sands and clays in this region occur as a surface veneer about 10 m thick or as elongated ridges that may be over 30 m thick. The ridges are relict shorelines created over the past several million years by the Pliocene-Pleistocene sea level cycles (Cooke, 1945; White, 1970). Most of the Florida landscape, however, is characteristically flat (Scott, 2001).

Sediments in the central Gulf Coast are dominated by thick fluvial deposits of the Mississippi and smaller rivers that deliver eroded sediments from the mainland. Deposited in coalescing river floodplains, the sediments of the Mississippi, Alabama and Louisiana coastal zone are dominated by marshes, floodplain forests, natural river levees and barrier islands. The barrier islands in Mississippi and Alabama are relatively recent features (less than 5000 years old) that are nurtured by sand carried alongshore by wave transport from the Florida panhandle and Alabama. They have all diminished in size since the mid-1880s with the greatest erosion on their east end (Morton, 2007). The barrier islands of Louisiana are remnants of Mississippi River deltas created and abandoned over the past 7000 years (Coleman et al, 1998; Gosselink et al, 1998). The Louisiana barrier islands are all retreating and diminishing in size, with the most significant breaching and retreat occurring during storms and frontal passages.

The Mississippi River has had a pronounced influence on the development of the central Gulf of Mexico coast. The Mississippi River delta covers an area of roughly 30,000 km2 in Louisiana and accounted for 41 per cent of the coastal wetlands in the US in 1998 (Coleman et al, 1998). Large volumes of sediment transported by the Mississippi River during the Tertiary period (from about 2—65 million years before present) created a major basin with prolific oil and gas reservoirs (Gosselink, 1984). The current landscape of south Louisiana was shaped by the Mississippi River as it built a series of overlapping delta lobes during the Holocene (see Figure 8.1). Offshore sand shoals and barrier islands are erosional features of the oldest delta lobes. The formation of the present Mississippi Delta plain and most of the other large deltas of the world during the past 7000 years has been linked with low rates of sea level rise (Nicholls et al, 2007). For millions of years sea level has determined whether the Mississippi River delta front was transgressing or regressing across the continental shelf (Fisk, 1944; Winker, 1991). The current combination of storms and high rates of relative sea level rise are

Bernard Ramsey Statue
Figure 8.1 Location of the six major deltas of the Mississippi River that have developed during the past 9000 years

Note: In order from oldest to youngest, the six deltas are the (1) Maringouin, (2) Teche, (3) St. Bernard, (4) Lafourche, (5) Modern (Plaquemines—Balize), and (6) Atchafalaya lobes.

Source: Draut et al, 2005, modified from Penland and Ramsey, 1990, based on radiocarbon dating work of Frazier, 1967. Reprinted from Continental Shelf Research 25, A. E. Draut, G. C. Kineke, D. W. Velasco, M. A. Allison and R. J. Prime, Influence of the Atchafalaya River on recent evolution of the chenier-plain inner continental shelf, northern Gulf of Mexico, 2005, pp91-112, with permission from Elsevier.

responsible for the high rate of shoreline retreat and barrier island disintegration in the Mississippi River deltaic plain (Coleman et al, 1998).

The Chenier Plain of southwest Louisiana and southeast Texas is characterized by recessional beach ridges ('cheniers') that represent former positions of the Gulf of Mexico shoreline. Reworked sediments from Mississippi River deltas are thought to be the main source of sediments of the sand and shell fragment ridges that parallel the shoreline in this region between Vermillion Bay and the tip of Texas Bolivar Peninsula. The westward drift and deposition of sediments along this part of the Gulf of Mexico shoreline coincides with periods when Mississippi River deltas were being constructed or abandoned (and eroded) in the western delta plain. Lake, marsh and bay deposits in the Chenier Plain are highly interconnected, and small changes in rates of sea level rise can convert a marsh to a bay within a relatively short time (Gosselink et al, 1979).

West of the Chenier Plain, the Gulf coastline is characterized by long, sandy barrier islands (such as Mustang and Padre Islands), sandy mainland beaches and dunes, river mouth accretions, sheltered bays and lagoons, and fringing wetlands. The barrier islands between Aransas Pass and Mansfield Channel formed several thousand years ago and they have continued to enlarge as a result of abundant sand supplied by the along-shore currents (Morton, 1994). Because the tide range

Figure 8.2 Estuarine systems located along the US coastline of the Gulf of Mexico

Source: NOAA, 1997.

Figure 8.2 Estuarine systems located along the US coastline of the Gulf of Mexico

Source: NOAA, 1997.

in the western Gulf of Mexico is so low and sand supply in the littoral system is so high, there are only a few natural tidal inlet positions (for example, Aransas Pass and Packery Channel) along the Texas coast. In some areas, such as the northern part of Padre Island National Seashore, sand dunes have migrated across the barrier island and into adjacent Laguna Madre (Morton and Peterson, 2006). Estuaries (formed when the fresh water from rivers flows into coastal embayments and mixes with the salt water from the ocean) are common along the entire Gulf Coast. Salinity in Gulf Coast estuaries generally grades from marine to fresh water along a continuum beginning at the mouth of the estuary and extending landward from the Gulf shoreline. The morphology of tidal inlets and barrier islands, water levels and tidal flushing rates, and fresh water inflows determine the environmental gradients and vegetation zonation in the estuaries and wetlands of the region. The 30 large estuaries in the Gulf Coast region account for 42 per cent of US total estuarine acreage (NOAA, 1997) (see Figure 8.2). The Laguna Madre is a unique feature of the western Gulf Coast that extends southward into Mexico (see Figure 8.3). The hypersalinity of the Laguna Madre (averaging 50-100 parts per 1000) is dependent upon the balance of evaporation, fresh water runoff from the mainland, and inflows from the Gulf of Mexico, the salinity of which averages 35 parts per 1000.

Was this article helpful?

0 0

Post a comment