ABSTRACT

Major events in the early Mesozoic development of the northern Gulf Coast region are related to corresponding stages in the opening of the central North Atlantic Ocean from its southern end, as interpreted from three prominent lineaments on aeromagnetic maps of the southeastern United States and its offshore region. The Brunswick Magnetic Anomaly, which marks the edge of continental crust in the Carolina Trough, comes ashore in southern Georgia and follows the subsurface belt of Triassic rocks into southwestern Alabama. This belt continues into eastern Texas, occupying a rift system along which North America was then drawing away from South America.

East of the Brunswick Anomaly (and therefore younger) is the East Coast Magnetic Anomaly, which is generally regarded as marking the eastern edge of the North American continent northward from Norfolk, Virginia. South of there the East Coast Anomaly separates from the Brunswick Anomaly and diverges from it to a distance of almost 50 miles (80 km) off South Carolina, as a result of restricted clockwise rotation of North America accompanied by translation of its concave central eastern edge around the projecting bulge of northwestern Africa. Termination of the East Coast Anomaly southward at the Blake Spur Fracture Zone suggests that spreading had started in the Blake Plateau Basin south of it by the time the East Coast Anomaly began to form along the Carolina Trough to the north.

The start of spreading in the Blake Plateau Basin signaled the final separation from South America of the sliver of more-or-less continental material south of the Triassic rifts, and therefore the opening of the present Gulf of Mexico. All this was powered by a mantle plume located in the Blake Plateau Basin, which was also responsible for continuing Jurassic compressional and right-lateral deformation in the southeastern states. Considering the rotations involved, the sequence of initial openings must have been: Gulf of Mexico, Blake Plateau Basin, Carolina Trough, and finally the northern central North Atlantic Ocean.

The Triassic rift system localized the Interior Salt Basins of Jurassic age, whose southern rim is a continuous, gently-curving trend of positive features that begins on the east with the Florida-Bahama Platform and continues west along the Wiggins Arch to the Caldwell-Angelina Flexure in southeastern Texas. This composite feature, which defines the northern margin of the Gulf of Mexico structural basin, functioned as an outer basement high that determined the seaward edge of the Early Cretaceous carbonate platform.

The western limit of the Interior rift-basin system is a right-lateral wrench fault, which transferred the extensional movement south-southwest to a rift beneath the present Rio Grande Embayment. The edges of this rift are marked by the Chittim Anticline on the north and by Mexico's Salado Anticline farther south, which face each other with their steeper flanks, and converge to the northwest as they approach the Sierra Madre Oriental. They are inferred to have developed along the contact of rift and pre-rift crust as a result of the subduction-related compression from the west that deformed the Sierra.