Deep Structure of the Western Border Faults and Basin Margins

The deep structure of the western Culpeper basin margins, and the western border fault  is, at best, poorly understood. Knowledge of this structure and it's geometry at depth, however, is critically required before hydrologic analyses of regional flow in 2-D cross-section can proceed. Geologic mapping by Lee and Froelich (1989) and modifications by Burton and others (1992) indicate that the fault lies at the foot of Catoctin mountain in the Leesburg, Virginia area, and essentially separates the Proterozoic rocks of the Catoctin Formation from their Mesozoic neighbors within the Culpeper basin towards the east. In addition, the mapped distribution of the Leesburg member conglomerate of the Balls Bluff Siltstone, clearly indicates that this rock type underlies the western basin margins north of the town of Leesburg.

A pair of Audio-Magnetotelluric experiments were designed to explore the deep structure of the western basin margin. These experiments were conducted at the Selma plantation north of Leesburg, and at the Morven Park estate near the northeastern city limits of Leesburg. In both cases, the traverses began on the eastern slope of Catoctin mountain, and then proceeded east, towards U.S. Route 15. Along the traverses, AMT soundings were conducted with and without a controlled frequency source. Both the Selma and the Morven Park traverses crossed the bounding western fault, and both then crossed into the Leesburg member conglomerate. Both traverses were subparallel, and were generally orthogonal to regional structure.  These two traverses were designed to test "along border-fault strike" reproducibility of the lithology and structure at depth. What is clear from the paired AMT results, is that there is significant variability in the border fault and basin rocks in this northern portion of the Culpeper Basin.

|| Culpeper Basin AMT || USGS - Eastern Earth Surface Processes Team || USGS - Geology || USGS ||

Contact: Herbert A. Pierce
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