The gravity field over the Bane Dome in Giles County, Virginia

The Bouguer gravity field, deterrmined from 395 measurements in Giles County, Virginia exhibits a broad positive anomaly approximately 12 mgal in amplitude situated over the Bane Dome, and several smaller anomalies of a few milligals amplitude. For the most part they are produced by the distribution of relatively high density carbonate rocks and lower density clastic rocks within the dome.

These anomalies can be explained by two contrasting interpretations of the structure of the Bane Dome. One interpretation, represented by the geologic cross sections of Woodward and Gray (Woodward, 1985) and Bartholomew (personal communications, 1987) suggests that the dome is cored by a relative abundance of high density carbonate rocks transported by overthrusting within the Narrows thrust sheet. Because the gravity anomalies can be entirely explained by sources confined to the Narrows thrust sheet, this interpolation precludes the existence of significant lateral density contrasts associated with deeper structure beneath the decollement zone in the Rome Formation.

The contrasting interpretation represented by the cross section of Gresko (1985), suggests a smaller proportion of carbonate rocks in the core of the dome due to duplex structures in the lower density clastic rocks. This interpretation proposes high angle faults with associated lateral density contrasts in the deeper rocks underlying the decollement. Because sources within the Narrows thrust sheet are insufficient to completely account for the gravity anomalies, the density contrasts associated with deeper structure are required.

The Bouguer gravity field can be separated into regional and residual parts. The regional field is caused by changes in crustal thickness known independently from the seismic measurements of James, Smith, and Steinhart (1968). The remaining residual field can be explained in terms of anomaly sources within the upper 10 km of the crust.