Establishing Tighter Constraints on the Structural Configuration of Basement Rocks beneath the Alabama Gulf Coastal Plain through Modeling

Currently, our knowledge of the continental crust lying beneath nearly six kilometers of Gulf Coastal Plain sediments in southern Alabama is limited. The little that we do know is from exploratory oil wells that predate 1992 and geophysical surveys that exist at a variety of scales and detail. Many new wells have been drilled since 1992 and we now have available newly compiled gravity and magnetic data. New information from these sources has yet to be incorporated into models for crustal evolution in southern Alabama. By synthesizing the geologic and geophysical information on the crustal structure beneath the Alabama Gulf Coastal Plain, a gravity model was created that is consistent with several interpretations: (1) a regional trend of a southern gravity high and a northern gravity low is created by shallow depths to the Moho of ~37 km, which gradually increase northward to depths of ~43 km; (2) a southern gravity high is attributed to a series of Mesozoic rift-related mafic rocks (diabase and granophyre) within grabens south of the Gondwanan Suwannee terrane boundary fault; (3) a wedge of higher density material (diorite) between the terrane boundary fault and the Coastal Plain onlap is attributed to Peri-Gondwanan arcs accreted to Laurentia during the Alleghanian orogeny; and (4) several minor high gravity anomalies north of the Coastal Plain onlap are attributed to the denser, predominately crystalline rocks thrust above the Laurentian basement, which include sheets of Piedmont and Valley and Ridge rocks. This modeling provides an improved understanding of the late Paleozoic suturing of proto-Africa (Gondwana) onto proto-North America (Laurentia) during the Appalachian continent-continent collision, as well as the subsequent Mesozoic rifting that led to the opening of the modern Atlantic Ocean and the Gulf of Mexico.