GULF OF MEXICO 



73 



EASTERN GULF AREA 



The Florida Plateau includes not only the State 

 of Florida but an equally great or greater area 

 that lies submerged beneath water less than 50 

 fathoms deep and forms the Florida shelf (H. 

 Gunter, 1929, p. 41). This plateau has been in 

 existence since ancient time and is a part of the 

 Gulf of Mexico "plate" of Suess and Schuchert. 

 Its history includes submergence during Upper 

 Cretaceous, part of Oligocene, and Upper Miocene. 

 Since Miocene time uplift has continued, and 

 erosion has removed much of the once continuous 

 cover of Miocene sandy limestone. The Florida 

 Peninsula now has very little relief. It has a 

 wide continental shelf off its west coast, thus 

 demonstrating the physiographic similarity be- 

 tween the coastal plain and the adjacent con- 

 tinental shelf. 



The 1947 expedition of the United States Coast 

 and Geodetic Survey ship Hydrographer in the 

 waters on the continental slope southwest of the 

 Apalachicola River, Florida, has been reported, 

 in part, by Jordan (1951, pp. 1978-1993). Many 

 new and interesting data have been secured in the 

 25,000-square-mile area of this report. 



The greater part of the continental shelf west 

 of the peninsula of Florida is covered by about 

 40 fathoms of water, and the slope out to the 

 100-fathom contour is for the most part gradual. 

 The westward slope varies from 1° at the north 

 to 5° at the south end of the shelf. 



In the 25- to 80-fathom depths, domes, ridges, 

 and troughs were discovered; escarpments and 

 knobs with a relief of more than 300 feet were 

 found in the 70- to 90-fathom depths. Most of 

 these features occur along the shelf margin. 



Within the 400- to 1,760-fathom zone the con- 

 tinental slope contains a deep escarpment, faults, 

 and the terminus of the De Soto Canyon, as well 

 as domes and depressed areas. 



The continental slope escarpment is of special 

 interest since it may materially aid in the ultimate 

 solution of the origin of the Gulf of Mexico. 

 Jordan (op. cit., p. 1991) noted a 35° gi-adient on 

 a 4,000-foot drop, contrasting with 1° gradients 

 or less above and below the escarpment. A ridge 

 30 miles long parallels the escarpment at 700 to 

 800 fathoms, and ridges and troughs with relief 

 up to 600 feet occur along the bottom of the 

 escarpment. The main escarpment undoubtedly 



represents faulting, and some of the minor troughs 

 and ridges may have a like origin. 



There can be little doubt that the Florida 

 Plateau has been faulted along its western edge, 

 but the faulting is difficult to date. Schuchert 

 believed this faulting was due to the inbreaking 

 of the Gulf of Mexico "plate" and that it probably 

 began in the Upper Cretaceous. However, Weaver 

 (1950, p. 359) beheved "that the Gulf of Mexico 

 as a deep sea is young" and therefore the faulting 

 must have occurred at a much more recent date. 



MISSISSIPPI DELTA AREA 



The Mississippi River brings to its mouth a 

 daily load of sediment in the order of 2 million 

 tons. This material has permitted the Mississippi 

 to build its delta out on the continental shelf with 

 the overlapping delta reaching within some 10 

 miles of the landward edge of the continental 

 slope. It might be expected that a deep trough 

 would exist in the outer edge of the continental 

 shelf in front of the Mississippi River, but such 

 is not the case. 



An ancient, deeply buried channel is found 

 about 30 miles southwest of the passes of the 

 Mississippi River. Shepard (1948, p. 213) stated 

 that this trough, which has a depth of 1,800 feet, 

 is the only major indentation in the shelf margin 

 in the Gulf of Mexico and that the trough-head 

 penetrates the shelf for nearly 30 miles. The 

 sides are steep, and the flat floor is filled with 

 loosely consolidated sediments. The canyon has 

 been traced out on the continental slope to a depth 

 of 900 fathoms before it becomes merged in the 

 irregularities of the slope. 



A second trough, called De Soto Canyon, has 

 been discovered off the Apalachicola River of 

 southwestern Florida. Shepard (1948, p. 179, 

 fig. 65) reproduced a map of this trough or canyon 

 as contoured by H. W. Murray of the United 

 States Coast and Geodetic Survey. This map 

 shows a series of depressions, some with relief 

 exceeding 20 fathoms, along the bottom of the 

 trough and a few depressions along the sides of 

 the trough. This canyon is shown in Jordan's 

 map (1951, p. 1982, fig. 2) of the continental 

 slope. The canyon has a relief of about 600 feet, 

 heads near the 240-fathom contour, and terminates 

 near the 500-fathom contour. Stetson (1951, 

 p. 1993) stated that cores of the steepest walls of 

 the canyon showed sediment and no bed rock. 



