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FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



was ever an area of shallow seas over a flat "plate." 

 This is shown by his statement that 



The isostatic equilibrium of the Gulf is inconsistent 

 with the conditions that should result if a continental mass 

 had sunk ... I, myself, regard the Gulf as representing 

 a mass of basalt which was erupted in Pre-Canibrian 

 time, either before or soon after the eruptions of the 

 granitic nuclei of North America. If so, it has been a 

 basin ever since . . . The Caribbean, Yucatan Deep, 

 and the Gulf of Mexico are, from the point of view of 

 actual isostatic equilibrium, all of the same nature. They 

 are, I think, all of them basins of great antiquity. 



Van der Gracht (1931, p. 121) discussed the 

 origin of the Gulf of Mexico and the downbreaking 

 of Llanoria. He believed the coastal plain 

 "represents a sunken basin over old central chains" 

 and that both the Caribbean Sea and the Gulf of 

 Mexico were part of a great geosyncline and a 

 "very complicated system of anticlinoria, ridges 

 and chains . . . must now fill the original geosyn- 

 cline, generated by its late-Paleozoic compression 

 stage. Since then, complete abrasion and renewed 

 sedimentation . . . have obscured the original 

 structure." 



Fifty years after Suess, Schuchert (1935, p. 340) 

 confirmed the conclusion of Suess as to the Gulf 

 of Mexico "plate" and described it as extending 

 from Tabasco northward so as to include part of 

 Texas, Ai-kansas, the southern tip of Illinois, 

 Alabama, the peninsula of Florida, and the 

 northern Bahama Banks, as well as other Mis- 

 sissippi embayment States. 



The Gulf of Mexico and the Caribbean were 

 separated, according to Schuchert, by a Central 

 American-Antillean anticlinorium until Jurassic 

 time. By mid-Cretaceous, the Gulf of Mexico 

 area responded to crustal movements in Mexico 

 and the Antillean geanticline and began to sub- 

 side; this downward movement continued until 

 great depths were reached. Thus, the Gulf of 

 Mexico was a shallow sea probably from Pro- 

 terozoic to mid-Mesozoic time, and by late 

 Cenozoic time the depth had changed from 

 possibly less than 1,000 to over 12,000 feet. 

 Schuchert believed the cause of the inbreaking of 

 the "plate" and the subsequent subsidence was 

 related to "the geologic structures of the Central 

 American-Antillean region, those of northern 

 South America, and those of the present Caribbean 

 sea bottom" and that all were "due to subcrustal 

 flowage, to the rising of plutonic masses into the 



various arches, and to the subsequent cooling of 

 these masses." He also believed that — 



The present depth of 12,000 feet was surely exceeded 

 during Cenozoic time, since in the course of this era sedi- 

 ments thought to be many thousands of feet thick ac- 

 cumulated upon it ... In the latitude of South Lou- 

 isiana, the ancient Gulf bottom has subsided over 25,000 

 feet, about twice the depth of the present Mexican Basin 

 (Sigsbee Deep). Therefore we may say that the greater 

 part of the Gulf of Mexico has sunk since Middle Cretaceous 

 time at least 20,000 feet. These are striking facts, in- 

 dicating slow, but in the end enormous, loading and 

 isostatic adjustment, accompanied by subcrustal move- 

 ments and rock fiowage toward the rising geanticlines of 

 Mexico and the Central American-Antillean arch, a move- 

 ment that is not yet completed. 



GULF COAST GEOSYNCLINE 



Barton, Ritz, and Hickey (1933, pp. 1446- 

 1458) were among the first to publish concerning 

 the Gulf coast geosyncline, and they presented 

 both stratigraphic and geophysical evidence for 

 the existence of a geosyncline in the Gulf coast of 

 Texas and Louisiana. They showed geophysical 

 calculations to indicate a horizontal increase in 

 density of the basement rocks from the Sabine 

 uplift to near the middle of the Gulf of Mexico, 

 and they concluded that a geosyncline must occur 

 in the basement surface with its trough axes 

 slightly landward from the present coast line 

 (op. cit., p. 1456). They also showed the great 

 thickening of the Upper Cretaceous and Tertiary 

 beds as they dip Gulf ward, with the Tertiary 

 beds reaching a stratigraphic thickness of more 

 than 25,000 feet near the coast. Knowing that 

 the deepest part of the Gulf of Mexico is 12,500 

 feet and assuming that the thickness of the Upper 

 Cretaceous-Tertiary sedimentary deposits in the 

 great depths of the Gulf are 10 percent or less of 

 their thickness in the Gidf coast, it was concluded 

 that "the basement of the Upper Cretaceous- 

 Tertiary beds must be down-warped 6,000 to 

 16,000 feet in reference to the depth of that 

 basement under the Sigsbee Deep." 



The geosynclinal trough is a well-marked feature 

 indicating considerable subsidence. Its westward 

 limit is not definitely known, but some thinning 

 of formations is noted in the longitude of Mata- 

 gorda County, Texas. It is further complicated 

 by transverse structures such as the Rio Grande 

 syncline, the San Marcos arch, the Houston 

 syncline, the Sabine uplift, and the Mississippi 

 River syncline. 



