Gravity and Magnetic Modeling of Basement Beneath Alabama Gulf Coastal Plain

Abstract

The southeastern United States has experienced two complete successions of Wilson cycles: (1) the assembly and break up of Rodinia and the opening of the Iapetus ocean; and (2) the closing of Iapetus ocean, the assembly of supercontinent Pangaea and its subsequent break up, and the opening of modern Atlantic Ocean. Evidence of these supercontinent cycles are recorded in the rocks of Alabama and adjacent areas, but in the southern portion of the state, these rocks are covered by as much as 7 km of Coastal Plain sediments. The Grenville-aged basement rocks beneath the Plateaus, the Valley and Ridge, and most of the Piedmont provinces have Laurentian origin, but the Uchee terrane in the Southern Piedmont province to the south is interpreted to have exotic Peri-Gondwanan origin. Also to the south, rocks of the Suwannee terrane are thought to have a Gondwanan origin based on faunal assemblages found in well cores. Triassic to Upper Jurassic sedimentary rocks of South Georgia basin onlap the northern limit of Gondwanan-affiliated Suwannee terrane rocks and obscure the suture between Laurentia and Gondwana. In this study, I use airborne gridded gravity and magnetic data to develop crustal models along three transects that cross major tectonic structures, geophysical anomalies, and the ancient North American (Laurentian) margin. Models derived from gravity and magnetic data are constrained by well-log information, geologic mapping, and previous geophysical studies in Alabama and nearby areas. Results show that a pronounced east-west trending gravity low observed in southern Alabama can be interpreted as the suture between relict Gondwanan crust and Peri-Gondwanan/Laurentian crust. The denser crystalline rocks of the Piedmont and Valley and Ridge provinces correspond to minor gravity highs. Based on its distinctive gravity and magnetic properties, the Wiggins terrane in southwestern Alabama is interpreted as a unique tectonic terrane. The eastern boundary of Wiggins terrane with Gondwanan crust is delineated by another prominent gravity low. Based on its magnetic expression, Laurentian crust is thought to continue beneath the Coastal Plain sediments until it is truncated by the tectonic suture with Gondwanan-affiliated crust. This truncation is marked by the Brunswick magnetic anomaly (BMA). Sharp magnetic gradients and long-wavelength gravity gradients along faults such as the Towaliga fault, Alexander City fault, and Bartletts Ferry fault suggest these structures are major, crust-penetrating features. Gravity and magnetic modeling reveal the thickening of crust from south to north, and a change in crustal thickness near the suture zone. Results suggest that the crust beneath the Wiggins Arch, in western to southwestern Alabama, is similar to that of Mississippi Gulf coast and most closely resembles a transform margin. Unlike the Texas Gulf coast, however, there is no evidence of a volcanic rifted margin in Alabama. The gravity and magnetic data are consistent with the presence of Mesozoic rift basins.