Abstract

The early geologic history of the DeSoto Canyon Salt Basin (DSCSB) in the northeastern Gulf of Mexico can be explained by Mesozoic extensional processes associated with a rifted, passively subsiding continental margin. Interpretation of over 10,000 km of multifold seismic reflection data integrated with computed isostatic relations based on a lithospheric buoyancy, Airy-type model, provide the basis for quantification of the crustal structure and recognition of a Late Triassic-Early Jurassic graben complex beneath the northwest Florida shelf, and evidence for NW-SE rift-stage extension during the early evolution of the Gulf region.

Thermo-tectonic subsidence trends, crustal extension $\beta$-values and calculated crustal thicknesses suggest moderately attenuated continental crust underlies a thick succession of postrift Mesozoic and Cenozoic strata in the DSCSB. Thinning and onlap of Callovian-age autochthonous salt coincident with a topographic ramp in the regionally unconformable basement surface, a decrease in crustal extension and an increase in crustal thickness indicates stretching and attenuation of crust occurred either before or during deposition of salt and that the basic architecture of the basin was established by Middle Jurassic time. Deposition of overlying Upper Jurassic and thick Lower Cretaceous sediments in the central part of the basin, underlain by crust attenuated as much as 45%, was enhanced primarily by local subsidence due to sediment loading. In contrast, Late Cretaceous and Cenozoic strata reflect uniform deposition in a tectonic setting characterized by subsidence through lithospheric cooling.

Widespread distribution of dipping intrabasement or sub-salt seismic events supports the presence of a Late Triassic-Early Jurassic graben system trending NE-SW which is probably the offshore extension of the South Georgia basin, an Early Mesozoic graben complex in northern Florida and southern Georgia. The asymmetric graben-like structure, approximately 125 km wide, contains a basinward thickening section of rift-fill strata, at least 2.7 km thick. A supra-rift lacustrine sequence may be present within the rift-fill succession.

The overall geometry of the graben system and observed regional structural trends suggests the DSCSB occurs within an oblique-shear NW-SE extensional terrane, the Northeast Gulf corridor, bounded by NW-SE oriented transfer faults. Northwest-southeast rift-stage extension is consistent with the inferred direction of separation of the North American plate from the South American and African plates during the breakup of Pangea and the early evolution of the Gulf of Mexico. Extension accommodated by transfer faults probably followed pre-existing structural weaknesses.