Wetumpka Impact Structre, Alabama: Gravity Survey

Introduction: The Wetumpka Structure is an ~7-8 km diameter impact in Alabama of Late Cretaceous (80-83 Ma) age. The structure was originally reported by [1-2]; subsequent work including drilling has been discussed by [3-10]. An earlier gravity survey [11] indicated that the structure exhibited an annular gravity high gravity over the crystalline rock ridge and low gravity within the interior and total gravity relief of about 10 mGal. Here, a more comprehensive gravity survey of the structure is reported. Geology: Presently, the structure consists of a horseshoe-shaped topographic ridge composed of Appalachian piedmont schist and gneiss that is open to the southwest west, and an inner zone of undulating topography composed of deformed Upper Cretaceous sedimentary rocks [1,3]. The target geology at the time of the impact included weakly consolidated Upper Cretaceous strata overlying pre-Cretaceous crystalline basement. Two cores have been drilled into the eastern flank of the structure [3-4]. Both holes were drilled to about 190 m and both penetrated steeply dipping Upper Cretaceous units in the upper 64 m and then interbedded impactite units below 64 m to the bottom of the hole (>130 m thick). PDF features were observed in shocked quartz grains recovered from the core, confirming the impact origin of the feature [3]. Previous Work: Gravity data were collected in 1994 along an approximately east-west profile across the Wetumpka structure. Analysis of those data was reported by [11] and the profile is illustrated in Figure 1. The field exhibits an eastward decreasing gravity of about 1.0 to 1.5 mGal km. A central low and flanking highs are observed on the profile with a maximum gravity relief of about 10 mGal. The high gravity corresponds to the circular ridge of crystalline rock and the low gravity is associated with the interior area of deformed sedimentary rocks. The western side of the interior low exhibits more complex gravity in the profile because the westerns portion of the profile is composed of three offset segments; thus, different portions of the structure are being sampled by the profile. New Data: In order to more completely define the gravity field associated with the Wetumpka impact structure additional gravity data were collected in 1999 and 2006. Gravity readings were made using a LaCoste Romberg gravity meter and are tied to the absolute gravity base at Auburn University. A local base station was reoccupied every few hours to correct for meter drift. Position information was derived from both differentially corrected GPS and from the USGS 1:24,000 map sheets. For the GPS established positions, the signal was integrated for 10 minutes; vertical precision is typically of the order 10 cm (except in a few cases of poor satellite distribution). A base station set up within the structure was used for the differential correction. For stations whose locations were derived from the 1:24,000 map sheets, only locations with defined elevations (e.g., intersections, spot elevations) were used. These stations have vertical precision of ~30 cm. Bouguer reduction was done using a density of 2.67 g cm. Terrain corrections have not been applied to the data.