TRIASSIC/JURASSIC RIFT STRUCTURES AND THE EARLY OPENING OF THE GULF OF MEXICO

The initial stages of the rifting along the northern Gulf of Mexico margin are poorly understood. No Triassic, Jurassic, or Early Cretaceous rocks crop out on the northern Gulf of Mexico coastal plain so the geologic history of the area from this period is sparsely documented. In this study we examine the subsurface of Drew County, Arkansas. Drew County is north of the northern extent of the Luann Salt (so salt tectonism is not a factor) and is thought to be at the edge of the North American craton. Drew County lies over an undescribed Triassic age rift system that represents the initial opening of the Gulf of Mexico. We interpreted seismic profiles, and produced structural contour and isopachus maps, and cross sections of the subsurface. These data show a graben structure in the subsurface that has a general NW-SE strike and is approximately 15 miles wide and could be as much as approximately 25 miles wide in places. The structure was created during late Triassic/ early Jurassic incipient rifting before the predominate rifting in the area moved to the south along the Texas coast. During middle Cretaceous the area was uplifted by local igneous activity creating a large angular unconformity over the area. There is evidence for reactivation of the structure after Cretaceous into Paleocene and possibly into early Eocene. The structure under Drew County could be considered a good example of the early stages of continental rifting before advanced extension gives rise to an ocean margin.