--> --> Testing the Two-Stage Triassic-Jurassic Opening Model for the Gulf of Mexico From Faults Bounding Shallowly Buried Rifts in the Southeastern Gulf of Mexico

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Testing the Two-Stage Triassic-Jurassic Opening Model for the Gulf of Mexico From Faults Bounding Shallowly Buried Rifts in the Southeastern Gulf of Mexico

Abstract

Most workers agree that the opening of the Gulf of Mexico (GOM) basin began in a southeastward direction during the Triassic-early Jurassic and is recorded by NE-SW-striking continental rifts across a broad zone. These earlier Phase 1 rifts, assumed to be filled by coarse clastics of the Triassic Eagle Mills Formation, are deeply buried, rarely drilled, and poorly imaged along most GOM margins. Overlying these Phase 1 rifts is a large, post-rift sag basin, filled by a thick salt layer. A second phase of more north-south-directed rifting began in the late Jurassic and formed Phase 2 rifts along with an arcuate area of oceanic crust, separating the post-Phase 1 sag basin into two unequal parts: the larger Louann salt of the northern GOM in the USA and the smaller Campeche salt in Mexico. In this study, we focused on testing the two-stage, two-direction rifting process in the southeastern GOM because: 1) two sets of orthogonal rifts are known from previous mapping and are consistent with the proposed orientations of Phase 1 and 2 rifting, and 2) rift structure and stratigraphy can be mapped in detail and dated from DSDP wells because rifts are not buried by thick salt or clastic sedimentary rocks as in the northern GOM. Five tectonostratigraphic sequences mapped from seismic data and wells include: 1) early Paleozoic metamorphic and igneous basement; 2) pre-rift late-Paleozoic dolomite; 3) syn-rift Jurassic-Early Cretaceous sandstone, conglomerate, shallow carbonate, and shales; 4) early to Mid-Cretaceous hemipelagic and platform carbonates; and 5) late Cretaceous-Cenozoic pelagic carbonate, ooze, chalk, clay and mud. Four 2D seismic lines were decompacted, backstripped, and structurally restored, with one line in the Phase 1 rift direction and the other three in the Phase 2 rift and seafloor spreading direction. The results showed that total subsidence of the area exceeds the sum of the tectonic and sediment load subsidence; stretching factors range from 1.07 to 1.24 and are consistent with similar failed rifts in continental crust worldwide. To explain the discrepancy between the total subsidence, load subsidence, and tectonic subsidence, we propose that Phase 1 rifting is a likely mechanism because structural restorations of Phase 2 rifts show that significant Phase 1, rift-related basement relief existed prior to Phase 2 rifting. Less likely mechanisms include lower crustal extension and regional effects related to the Cenozoic thrust deformation of Cuba.