GULF OF MEXICO 



47 



may come in very close to the mouth of a deltaic 

 river that drains a large basin. Tiie chief excep- 

 tions to the outward banding of sediments (Emery 

 1952) are any coai-se sediments of local organic 

 or chemical origin, or, along the northwestern 

 shelf of the Gulf, sediments on mounds believed 

 to lie above buried intrusive salt dunes (Shepard 

 1937 b). 



The alluvial sectors of the Gulf of Mexico (Sec- 

 tors Nos. 1.11, 1.12, and 1.2, fig. 14) have smooth 

 shorelines with sandy beaches on the mainland, 

 or on barrier islands (Price 1951 a). The beaches 

 may be more or less interrupted by deltas of vary- 

 ing degrees of protuberance and shoreline irregu- 

 larity (Russell 1940; Bates 1953). Offshore, the 

 alluvial sectors have broad, smooth continental 

 shelves, 130 miles wide at the maximum, with 

 relatively steep inshore shelf-bottom profiles (fig. 

 15, Sector VII) and a rather uniform gradation of 

 sediment from sand (generally inside the 5- or 10- 

 fathom depth contour) to sand-and-mud, with 

 mud at the outer margins. The elevated mounds 

 on some outer parts of the northwestern shelf have 

 nodular algal limestone on their tops and possibly 

 some coral. 



Subsectors, alluvial coast: terraced detaic plain. — 

 Sector 1.2, Alabama, Mississippi, and western 

 Florida (fig. 14), has a fairly steep coastal plain," 

 with two Pleistocene-and-Recent deltas (Apala- 

 chicola, Pascagoula, and Pearl), a minor amount 

 of embayment of drowned stream valleys and a 

 reported series of low, parallel elevated shoreline 

 scarps (Carlston 1950). In places, the younger 

 two of these have roughly parallel Pleistocene 

 barrier islands and coastal lagoons (MacNeil 1950) 

 in part entrenched by drainage and embayed. 

 This coast is like that of the southern Atlantic 

 coastal plain of the United States, with which it 

 has a common geologic history. These similari- 

 ties exist because of the position of the old (Pale- 

 ozic), almost entirely quiescent Appalachian 

 mountains fairly close (90 to 150 miles) to the 

 coast hut not in a bordering position. Drainage 

 basins extending from the mountain front across 

 the coastal plain are small in relation to those of 

 the deltaic 1.12 alluvial coast. The large cuspate 

 Pleistocene-Recent Apalachicola delta and the 

 long, broad, and shallow Mobile Bay are striking 

 features of this coast 



• Eight feet per mile nenr the coast in some places. 



Broadly embayed deltaic coastal plain. — ^Sector 

 1.12 (fig. 14), the coast of Louisiana, Texas, and 

 part of Tamaulipas, receives the drainage of some 

 ten major rivers. Three major Recent deltas now 

 reach the Gulf; the Mississippi-Red, Brazos- 

 Colorado, and Rio Grande deltas. A very broad, 

 gently sloping deltaic coastal plain (Barton 1930) 

 has been built, forming a fully concordant coast 

 (Suess 1888). Coastal plain deposits form a new 

 structiu'al (monoclinal) trend in front of the abrupt 

 southwestern ends of Appalachian folds once 

 projected into the broad Mississippi embayment. 



Sector 1.12 (fig. 14) is deltaic except between 

 arcuate delta fronts where the active barrier and 

 the Pleistocene Ingleside barrier island (Price 

 1933) with their parallel, active and entrenched 

 coastal lagoons form a diversified inner coast tran- 

 sected by many broadly drowned and embayed 

 stream valleys (Price 1947). There are, thus, 

 intermittent terraced riverine plains between ad- 

 jacent protuberant Recent deltas. Behind the 

 terraced belt are continuously overlapping and 

 coalescing Pleistocene deltas with their surfaces 

 slightly up-warped inland. The great protuber- 

 ant Mississippi-Red delta (Russell 1936; 1940; 

 Bates 1953) dominates the eastern part of this 

 sector both at the shoreline and on the shelf where 

 large shoals seem to indicate submerged deltas. 

 A minor feature of the deltaic coast is the saline 

 marsh (paralic) environment described on a later 

 page with the biogenous environments. 



Saline plain of Rio Grande delta. — A broad, 

 treeless, saline plain, the Jackass Prairie of 

 Cameron County, dominated in the native state 

 by coarse, bunchy Spartina salt grass (sacahuista), 

 stretches inland across the Recent delta north of 

 the natural levees of the present Rio Grande 

 course for a maximum distance of 10 miles. The 

 Gulf ward edge of the plain is honeycombed by 

 saline lagoons lined on their lee (N., NW., W., 

 and SW.) sides by clay dunes (Coffey 1909, Price 

 1933, Huffman and Price 1949). The soil of the 

 low deltaic plain is made heavily saline by wind- 

 blown (cyclic) salt contained in clay pellets and 

 dust blown from the saline tidal flats of the la- 

 goons. These flats undergo strong deflation dur- 

 ing the warm months. Sand-sized pellets of 

 flocculated saline clay accumulate on lee shores 

 to build the dunes, while saline dust passes over 

 the 30-foot-high dunes under the strong steady 

 hot winds of the warm months. 



