A GRAVIMETRIC INVESTIGATION OF THE SUBSURFACE STRUCTURAL GEOLOGY OF NORTHERN MOBILE BAY

Many of the regional structural features of the northern Gulf of Mexico are attributed to migration of the Jurassic aged Louann Salt formation. Of these features, the Mobile graben is a north-south oriented graben bounded on the easternmost flank by the Jackson fault. The Jackson fault has been mapped on the surface and in cores as far south as McIntosh, Alabama; however, past researchers have suggested that movement along this fault system may be associated with the present location of Mobile Bay located 70 km to the south. Due to a lack of subsurface data in the northern Mobile Bay and Mobile-Tensaw Delta regions, the lower extent of the Mobile graben remains unknown. North-south oriented bluffs located on the eastern side of the Bay near Spanish Fort could be attributed to the easternmost flank of the Mobile graben or related en echelon faults, but more data are needed to detail the subsurface geology. Therefore, the objective of this study was to determine if the gravity anomaly in the northern Mobile Bay region supports the presence of a graben or related faulting. A relative gravimetric survey was conducted along an east-west transect in the vicinity of the bluffs near Spanish Fort using a Worden gravimeter. Gravity measurements were recorded every 250 meters moving west away from the bluffs along a causeway towards the upper reaches of the Bay. A total of 27 average gravity measurements spanning approximately 6.5 km were recorded, and a relative gravity anomaly profile was plotted for the study area. The data indicate a steady decline in gravity of 4 mGal from the bluffs westward towards the Bay. The average decline in gravity from the foot of the bluffs to the Bay is approximately 0.6 mGal per km. The results of this study are consistent with the hypothesized subsurface faulting. Two explanations for the gravity low over the northern reaches of Mobile Bay are offered from regional structural observations: (1) normal faulting along the present location of the Spanish Fort bluffs and subsidence in the Mobile Bay and/or (2) salt migration from the Louann Salt underneath the Bay towards Spanish Fort and subsequent uplift resulting in bluffs. Although this study offers insight into events leading to the present location of Mobile Bay, additional observations and geophysical constraints are needed to shed more light on the subsurface geology.