41. 



GULF COASTAL PLAIN 



GENERAL CHARACTERISTICS* 



Topography 



The Gulf Coastal Plain is coextensive with the Atlantic Coastal Plain 

 (discussed in Chapter 10) and, together, from Tampico, Mexico, to 

 Cape Cod, Massachusetts, they are 3000 miles long. The Gulf Coastal 

 Plain averages 250 miles wide, and the Mississippi embayment from the 

 delta to Cairo is 575 miles long. The peninsula of Florida is 400 miles 

 long. See Fig. 41.1. This vast plain rises very gently from the sea, and 



For an up-to-date detailed account, see Grover Murray's Gulf Coastal Plain 

 (Harper & Brothers, New York, 1961). 



in parts of Texas attains an elevation somewhat more than 1000 feet. 

 Beyond the Rio Grande in Mexico, the country that can properly be 

 classed as coastal plain narrows toward the south and becomes struc- 

 turally more complex than in the United States. At Tampico, it is very 

 narrow and continues so to Yucatan, where the plain broadens to include 

 most of the peninsula. 



Geologically, the coastal plain extends out under the sea to the outer 

 margin of the continental shelf. 



Sedimentary Rocks 



The Gulf Coastal Plain is underlain by a series of sedimentary forma- 

 tions composed chiefly of sand, clay, marl, limestone, and chalk, with 

 subordinate amounts of salt, diatomaceous earth, volcanic tuff, and gravel. 

 The calcareous deposits are more abundant in lower formations and 

 along the seaward margin. The various sediments range in age from 

 Late Jurassic to Recent and are mainly unconsolidated, though some in- 

 durated layers are intercalated from place to place. All the beds are 

 lenticular and interfingered with others, and no two columnar sections 

 are similar unless close together. This diversity in succession poses a 

 constant problem for the stratigrapher, and microfossils have proved in- 

 valuable in correlation. 



The Gulf Coastal Plain sediments were deposited in seas that invaded 

 the margin of the continent. Several rivers draining the central part of the 

 continent deposited vast amounts of sand, silt, and clay in the sea 

 as the crust along the invaded margin subsided, and large amounts of 

 chemical precipitates from the sea water were added. As a result, a great 

 wedge was built up that thickens seaward. Along the site of the present 

 coast of Mississippi, Louisiana, and Texas, the wedge of sediments is 

 estimated to range from 20,000 to 30,000 feet thick. In spite of the great 

 thickness, the wedge is very thin in relation to its length in cross section; 

 and if it is laid out to true scale, one is impressed with the very small 

 angle of tilt imparted to the beds by the subsiding of the land. 



The stratigraphy of the central part of the Gulf Coastal Plain (Texas, 

 Arkansas, Louisiana, Mississippi, and Alabama) is summarized in Figs. 

 41.2 and 41.3. 



650 



