Gravity and magnetic modeling of basement beneath the Appalachian Plateau, Valley and Ridge, and Piedmont provinces, Alabama

Abstract

Despite the significance of regional geophysical anomalies and the economic potential of the area, central Alabama has lacked detailed crustal gravity and magnetic models. The southeastern United States has undergone at least two complete successions of Wilson cycles, making the geology of the region complex. This study focuses on creating tectonic and geologic models of this region in Alabama, specifically targeting the New York – Alabama lineament and what is interpreted as the southern continuation of the Amish Anomaly. These anomalies and the basement rock beneath Alabama are difficult to study due to the presence of overlying Appalachian rocks. In northwest Alabama, the Appalachian/Cumberland Plateau is comprised mainly of the Black Warrior basin, a thick package of Paleozoic sedimentary rocks that display minor evidence of Alleghanian deformation. In central Alabama, Paleozoic sedimentary rocks of the Valley and Ridge province cover the basement rocks to even greater depths. To the southeast, sedimentary rocks abruptly transition to metamorphic rocks of the Talladega slate belt and the Piedmont province. These provinces were sutured onto Laurentia during the Acadian orogeny, further obscuring the basement from study. In this study, airborne-gridded gravity and magnetic data were used to produce a detailed crustal model for this region of Alabama. These transects cross major tectonic boundaries, geophysical anomalies, and Grenville-aged structures. Geologic maps, well-logs, and interpreted cross-sections were used to constrain these models to make them as accurate as possible. However, these constraints only apply to the upper few kilometers of the crust, and much is unknown about the basement beneath Alabama. Previous geophysical studies from adjacent areas in Mississippi, Georgia, and Tennessee, and Alabama further constrain this study’s models and help add to their validity. Results support interpretations that the New York – Alabama lineament and Amish Anomaly mark the northwestern and southeastern boundaries of a westward dipping terrane with relatively high density and magnetic susceptibility. This finding is consistent with other studies done on the New York – Alabama lineament in Tennessee and Ohio, and suggests that a previously unknown Grenville-aged tectonic terrane may be present in the basement beneath Alabama.