ABSTRACT

The eastern and western carbonate platforms of the Gulf of Mexico have subsided about 5,000 m since the middle part of Late Jurassic time; about 7,000 m beneath the Isthmian (Isthmus of Tehuantepec) Cenozoic terrigenous clastic sequence south of the Bay of Campeche (in the Northern Central American orogen); and 15,000-16,000 m beneath the Jurassic through Holocene Gulf Coast geosyncline of the northern Gulf rim.

Carbonate-platform sequences are present in eastern Mexico (Tamaulipas platform; includes Tampico-Tuxpan and Veracruz basins) and in Yucatan and Florida (Yucatan and Florida platforms), because terrigenous clastic provenances either were not available, or because barriers prevented the transport of terrigenous materials into these regions. In contrast, the terrigenous sedimentary piles of the Isthmian basin and Gulf Coast geosyncline regions had important provenances for terrigenous debris. The Isthmian section differs from that of the Gulf Coast geosyncline because (1) the provenance and, therefore, the sediment supply was smaller, and (2) the Cenozoic basin is part of an orogenic belt. In contrast to the Isthmian region, the provenance area for the Gulf Coast geosyncline is huge, and the basin is on a stable continental margin, well removed from active orogenic belts.

The central oceanic plate of the Gulf of Mexico sank at the same time and at about the same rate as its margins. This conclusion is substantiated by several observations. (1) Seismic data suggest that the Mohorovicic; discontinuity is at least 17-18 km below sea level. (2) The same seismic data show that a minimum thickness of 5,000 m of sedimentary strata is present beneath the Gulf of Mexico floor. This amount is 5 to 10 times the normal thickness of sediments on oceanic crust. (3) The "normal" depth of an oceanic abyssal plain is 5,000 m, yet the depth of the Sigsbee Abyssal Plain is only 3,500 m below s.l. Thus the base of the sedimentary column at the Gulf center is about 8,500 m below s.l. If the "normal" abyssal plain depth is assumed to have prevailed once in the Gulf, one may subtract the figure of 5,000 m from 8,500 m and arrive at a minimum of about 3,500 m subsidence for the center of the Gulf. However, seismic data suggest that the average sediment thickness beneath the Gulf exceeds 6,500 m, and locally reaches 9,000 m. If the 6,500-m figure is used, the Gulf has subsided at least 5,000 m, or about the same amount as the carbonate platform areas on the eastern and western margins of the deep basin.

This amount of subsidence shows that the Gulf was an oceanic basin during Late Jurassic time. One must accept this choice, assume that the Gulf was 8,500-10,000 m deep during Late Jurassic time, or adopt the position that its subsidence rate has been about twice that of the surrounding land areas. The simple geometric argument presented here to show that the Gulf most probably has been oceanic from Late Jurassic to the present time is supported strongly by the geology of the surrounding continental areas. Subsurface and surface data indicate that the Gulf is at least as old as Late Mississippian-Early Pennsylvanian. New data just obtained from the deep Gulf support this minimum date. The writers believe that the Gulf basin has been an oceanic plate since the beginning of earth history.

The much greater subsidence of the Gulf's northern margin resulted from the huge accumulation of sediments along that margin. This much greater subsidence beneath the Gulf Coast geosyncline imparts to the Gulf oceanic plate and the Mohorovicic discontinuity an overall northward tilt. Yet the flanking Yucatan and Florida platforms on the east and Tamaulipas platform on the west show no conspicuous northward tilt. This fact indicates that a hinge-type, north-south-oriented, scissorslike, differential movement has taken place along the western and eastern flanks of the deep Gulf basin. This movement presumably is manifested now by the Florida scarp, the western Campeche scarp, DeSoto and Campeche Canyons, and the north-south belt of salt-anticline ridges along the western side of the Gulf. This same hypothesis and the predominance of clastic deposition explain the markedly different topography of the northern margin of the Gulf and along the southern side of the Bay of Campeche--the two prominent "hummocky" topography zones of the Gulf of Mexico.

Very large and substantial petroleum reserves remain to be found and exploited in this region. Some of them are not exploitable at present, but ultimately the technology to drill commercial wells in very deep water will be developed, and the petroleum reserves of the continental slopes will be produced. Among the many trends which can be drilled and which remain to be tested and exploited are the post-Ouachita facies Paleozoic marine strata of the northern Gulf and the area north of the Northern Central American orogen. Mesozoic reefs, atolls, and carbonate banks still are only partly tested. Late Tertiary and Quaternary reserves of the northern Gulf and the southern part of the Bay of Campeche still must be drilled and exploited. The two "hummocky" zones of the continental slope offer particular promise. Finally, the diapiric structures of the deep Gulf and the salt ridges of the western Gulf must be tested thoroughly, though at present they are not economic objectives.

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