76 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



The Tertiary outcrops in the Gulf Coastal 

 Plain include thick continental sandy and lignitic 

 deposits and thinner marine sands and clays. 

 Down-dip from the outcrops, drilling has shown 

 that the Tertiary continental deposits pass into 

 brackish water and near-shore marine deposits. 

 According to Lowman (1949, p. 1941), rapid 

 transgressions and slow regressions produced 

 cyclical effects in the sediments with most of the 

 sediments deposited during the regressive phases 

 of the cycles. Farther down-dip or seaward the 

 sediments change to a succession of offshore 

 marine clays. 



In general, the Gulf coastal area may be di- 

 vided into intergrading depositional areas as fol- 

 lows: Rio Grande Embayment, East Texas Basin, 

 Mississippi Embayment, the Gulf coastal region 

 of Alabama, Georgia, and North Florida, and 

 South Florida. The amount of rainfall on the 

 land area surrounding the ancient Gulf may have 

 been the chief factor in determining the contem- 

 poraneous deposition of many sedimentary de- 

 posits ranging from anhydrite and salt to shales 

 and limestones. Rolshausen (1947, p. 5) sug- 

 gested that during pre-Eagle Ford Cretaceous 

 time, west of the Appalachian Mountains, rivers 

 entering the Gulf from the north and northeast 

 supplied the major load of sediments. East of the 

 mountains the rivers entered the Gulf from the 

 northwest and west. After Eagle Ford time, 

 rivers entering the Gulf from the west, and prob- 

 ably draining the western part of the present 

 Mississippi basin, were the chief source of sedi- 

 ments. The Rio Grande may have been the major 

 source of sediments from the late Cretaceous 

 through early Miocene time with the Mississippi 

 River contributing little sediment during that 



time. 



PLACE OF DEPOSITION 



The sediments brought to the Gulf of Mexico 

 are probably not carried far from shore. Parr 

 (1935, p. 62) showed that at a point only 70 miles 

 out in front of the mouth of the Mississippi River 

 the water has "transparency practically equal to 

 the clearest ocean water known." It is a gen- 

 erally accepted fact that water discharged from 

 the Mississippi River is carried almost entirely to 

 the west and that it stays relatively close to the 

 shore. Clarke (1938, p. 91) found that measure- 

 ments of transparency supported this conclusion. 

 Geyer (1950b, p. 100) noted that the salinity of 



the offshore coastal waters of Louisiana west of 

 the delta was largely controlled by the discharge 

 of fresh water from the Mississippi River and the 

 westward moving littoral current. The observa- 

 tions of the writer between 1948 and 1951 confirm 

 the westward movement of the fresh water enter- 

 ing the Gulf from the Mississippi River. 



Cogen (1940, p. 2101) examined samples of sed- 

 iments taken from the bottom of the Gulf near 

 the mouth of the Rio Grande and concluded that 

 the present bottom sediments of this region were 

 carried into the Gulf by the Rio Grande. 



Bullard (1942, pp. 1021-1043) showed that each 

 of the principal rivers carries a distinct suite of 

 heavy minerals. The Rio Grande sand shows its 

 primary source by the predominance of basaltic 

 hornblende and pyroxene and only 30 percent of 

 the stable minerals such as garnet, rutile, zircon, 

 tourmaline, and staurolite in the heavy mineral 

 residue. The Nueces, San Antonio, Brazos, 

 Trinity, and Sabine Rivers, draining areas of sed- 

 imentary rocks, have little hornblende and pyrox- 

 ene and a high content of stable minerals. Since 

 the Colorado River derives its load from both 

 primary and secondary rocks, its suite of heavy 

 minerals is over half green hornblende. North- 

 ward from the Rio Grande the beach of Padre 

 Island contains the Rio Grande suite of heavy 

 minerals, but the influence of the other rivers is 

 clearly shown by an increased ratio of more stable 

 minerals in the samples farther north in Texas. 



The sediments of the Coastal Plain do not end 

 at the shore but extend out under the sea, and "if 

 the basement surface on which they rest con- 

 tinues to slope uniformly, the mass of sediments 

 must increase in thickness at least as far as the 

 edge of the continental shelf, beyond which they 

 should thin out rapidly as they merge into the 

 oozes of the ocean depths" (Stephenson 1926, p. 

 463). 



Land derived sediments are not being moved in 

 a "continuous sheet of detritus all the way from 

 the beach to the continental slope" (Daly 1942, 

 p. 100). If this were true, much of the con- 

 tinental shelf would be some fathoms shallower 

 than at present. With continuing deposition the 

 sea would become more shallow, and wave and 

 current action would push the sediments nearer 

 the edge of the shelf. When the sediments 

 reached the edge of the continental shelf and a 

 profile of equilibrium was attained, the shelf sur- 



