Crustal studies in the Gulf of Mexico

Abstract The eastern margins of the Gulf of Mexico are areas of carbonate buildup. They are represented by the south Florida and Yucatan platforms which are similar in respect to topography, sediment type, seismic velocities, depth to equivalent age horizons, and bordering buried reefs. The existence of the Lower Cretaceous reefs is of particular importance as they have been the major factor in controlling the sedimentary history during the Mesozoic and Cenozoic in the eastern Gulf of Mexico. The areal extent of the Lower Cretaceous reef is best known on the Florida Platform but its existence on the edge of the Yucatan Platform suggests the possibility of a Lower Cretaceous reef complex that nearly encircles the Gulf. In this case these buried offshore reefs would be part of a system including the Golden Lane (Faja de Oro) of eastern Mexico and the well known oil producing Lower Cretaceous reef trends of Texas and Louisiana. Reef material recovered from topographic highs in the Straits of Florida suggests that the bordering reefs of the west Florida Platform continue to northern Cuba. These similarities between the Yucatan and Florida platforms allude to a geographical connection between these areas in the geologic past. If this is true, the mode of separation of the platforms suggested by seismic reflection data is erosion (controlled mainly by the Gulf Stream) and faulting. The western margins of the Gulf of Mexico are characterized by thick deposits of terrigenous elastics. The main structural element is in the north; the Gulf Coast geosyncline. Structure within the geosyncline is very complex, structural mobility being caused by the presence of extensive buried Triassic-Jurassic salt. Similar conditions exist in the southwestern Gulf (Bay of Campeche) where salt structures dominate the topography. Although there is no conclusive proof, there is much evidence that indicates the north-south trending ridges of the western Gulf of Mexico are also related to buried Mesozic salt. If these folds off eastern Mexico prove to be salt anticlines at depth, the post-Paleozic development of the western margins of the Gulf of Mexico becomes clear; 1. (1) during Mesozoic, salt was deposited on the western margins of the Gulf of Mexico; 2. (2) the salt was covered by pelagic and continental derived sediments; 3. (3) the salt was folded into a series of linear ridges parallel with the seaward extent of the salt; and 4. (4) as sedimentation over the anticlines continued the salt became more mobile, more complex structures developed and diapirism became the dominant mechanism. Both phases three (3) and four (4) are evident in the western Gulf. The linear features on the central portion of the eastern Mexican slope are examples of the relatively undeformed salt anticlines and the structures of the Bay of Campeche represent an intermediate stage where the ridge outlines can still be detected but secondary growth features are prevalent. The continental slope off Texas and Louisiana represents the final stage of development i.e., there is no preferred alignment of structures and diapirism predominates. The crustal structure of the central basin is quasi-oceanic. Earthquake seismology studies and gravity measurements both indicate that the oceanic type crust extends some distance inland on the northern margin of the gulf coastal plain. The northern extent of the oceanic crust and the great thickness of unconsolidated sediments present across the central basin suggest a long history of filling. This is consistent with a hypothesis which considers the Gulf of Mexico as an old ocean basin which is being flooded by sediments. The effect of the filling has been to decrease the water depth in the basin and to increase the depth to the Mohorovicic discontinuity. In terms of the new global tectonics, this concept of the Gulf representing a remnant of an old ocean with the characteristics mentioned above, it appears that the Gulf of Mexico moved passively with the North American continent.