70 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



from the land masses on the northwest and north, 

 particularly during Midway-Wilcox time. 

 Bornhauser (op. cit., p. 709) stated: 



In order to explain the progressive enlargement of the 

 southeast Mississippi plateau and the corresponding 

 shifting toward the north and northwest of its frontal 

 synclinal zones during the Eocene, the theory is advanced 

 that this plateau, together with the Gulf of Mexico 

 "plate," drifted in successive stages to the north as a 

 result of Tertiary orogenic movements in the Antilles. 

 A maximum penetration of the' plateau into the Missis- 

 sippi embayment was reached at the close of the Eocene 

 and early Oligocene periods, when it touched the northern 

 land masses. A breakdown of the southern part of this 

 plateau and a large part of the Gulf of Mexico followed 

 during the Oligocene and Miocene, forming the present 

 Gulf of Mexico. This downbreaking in connection with 

 the emergence of the embayment probably caused a change 

 in direction of the Gulf Coast geosyncline in south Loui- 

 siana. During the Eocene, the axis of this syncline 

 followed a southwest-northeast trend, with the Missis- 

 sippi embayment syncline forming its northeastern 

 extension. With the formation of the present Gulf of 

 Mexico during Oligocene and Miocene time, this axis was 

 diverted to a west-east trend. 



Trask, Phlcger, and Stetson (1947, pp. 460-461) 

 obtained sediments from the northwestern part of 

 the Gulf of Mexico during the 1946 expedition of 

 the Atlantis. In the central part of the Gulf, 

 where the depth of water exceeds 11,000 feet, two 

 distinctly different layers of sediment were found. 

 A thin top zone of globigerina was underlain, in 

 most cores, abruptly, by alternating clay and very 

 fine, well-sorted silt containing a cold water fauna. 

 In other cores, from the same depth, ripple marks 

 and crossbedding were found. Such conditions 

 suggest shallow-water deposition; and, to get such 

 conditions, it is necessary to assume either a rather 

 recent great depressing of the Gulf floor or an 

 equally great lowering of sea level. The other 

 alternate is to assume sufficient currents at depth 

 to cause sorting, ripple marks, and crossbedding. 



Lowman (1949, pp. 1986-1993) believed that 

 the central part of the Gulf of Mexico might have 

 been epicontinental in character during Eocene 

 time. The evidence cited includes the wide extent 

 of the Eocene into the transverse embayments, the 

 gentle depositional slopes, the dominance of con- 

 tinental shelf faunas, and the character of the 

 sediments of the southeast Mississippi platform. 

 In contrast to the Eocene, the Upper Tertiary is 

 absent from the transverse embayments and has 

 continental-slope facies on relatively steep deposi- 

 tional slopes. Therefore, the Upper Tertiary sup- 



ports a deep hole in the central part of the Gulf of 

 Mexico, as it is today, though not necessarily in 

 the same location. 



Lowman did not believe the stratigraphic evi- 

 dence was conclusive that the Mississippi River 

 syncline subsided in response to load. He believed 

 some workers have used facies criteria instead of 

 planes of stratification in the isopach maps which 

 find "maxima under the delta in the Quaternary 

 and the Pliocene-Miocene" (op. cit., p. 1991). 



Weaver (1950, p. 359) studied the continental 

 shelves of the Gulf of Mexico and decided that a 

 significant tectonic zone is at the outer edge of the 

 continental shelf. He concluded that the topo- 

 graphic contours on the continental slope are 

 really structural contours and that they exist in 

 sufficient number to indicate active tectonic 

 regional features. He proposed "the theory that 

 the Gulf of Mexico as a deep sea is young, and 

 that its present central great depth is due to 

 downfaulting." The most intense faulting is 

 indicated along the outer margin of the continental 

 shelf west of Florida and near Yucatan, but even 

 the more gentle continental slopes are considered 

 fault zones. No definite time of faulting was 

 given by Weaver. 



Moody (1950) favored a single salt mass as the 

 source of the Gulf coast and Mexican domes and 

 suggested that it may extend across the Gulf of 

 Mexico into the Isthmus of Tehuantepec. If this 

 is true, the Gulf of Mexico was shallow enough to 

 allow salt deposition beyond the present continent 

 during the time of the deposition of the Eagle Mills 

 salt, which is Jurassic in the opinion of Moody, 

 although some writers place it in the Triassic or 

 Permian. He believed the Gulf of Mexico had 

 some downwarping during Upper Cretaceous ; that 

 it began to take shape at the end of the Laramide 

 Revolution; and that it subsided, and maybe 

 formed the Mexican Basin, in post-Reynosa 

 (Pliocene) diastrophic movements. The finding 

 of Reynosa gravels in Florida at an elevation of 

 360 feet suggests a great change in sea level to 

 allow these gravels to be transported there. This 

 means a great post-Reynosa diastrophic movement 

 during which the west Florida shelf scarp and 

 possibly the Mexican Basin came into existence. 



Eardley (1951b, p. 2236) stated that "the Gulf 

 of Mexico came into existence after the Appa- 

 lachian orogeny by subsidence. ' ' Much of the Gulf 

 is surrounded by the belt of late Paleozoic orogenj^. 



