GEOPHYSICAL INVESTIGATIONS OF THE CAMDEN FAULT: POST CONIACIAN-TURONIAN (LATE CRETACEOUS) FAULTING OF ATLANTIC COASTAL PLAIN SEDIMENTS, SOUTH CAROLINA

The Camden fault is a northeast striking fault within the Carolina terrane that separates crystalline rocks of different deformational fabrics associated with the Alleghanian Modoc shear zone. In the study area the Camden fault is covered by Upper Cretaceous kaolinitic sands that have been displaced by post-Cretaceous brittle faulting coincident with the fault trace. Previous auger drilling and field mapping studies suggest northwest-side up vertical separation of the basal unconformity of approximately 25 m along one or more steeply dipping fault planes. Along strike the kaolinitic sands are overlain by the Upland unit (Late Eocene ?), which does not appear to be deformed by faulting.

Coincident seismic reflection, seismic refraction, and gravity surveys were conducted across the Camden fault where a previous auger section indicated offset of the kaolinitic sand unit. The seismic reflection data were collected using a hammer source, 0.61 m receiver spacing, and 1.22 m source spacing to yield 30 CMP fold data. After processing the dominant frequency of the data were greater than 200 Hz. Gravity stations were taken at 3.05 m intervals along the profile.

The results of the multiple geophysical surveys yield complementary results. The outlier of kaolinitic sand northwest of the fault is variable in thickness, but is as much as 6 m thick. The Camden fault is a zone of deformation approximately 30 m wide. Seismic reflection data show subhorizontal Coastal Plain sediments offset step-wise within the deformed zone. Most of the offset occurs along one steeply dipping (vertical ?) fault causing an abrupt lateral discontinuity. Gravity and seismic data indicate greater than 10 m offset. Additional faults are interpreted to exist within the zone of deformation. The aggregate throw within the deformed zone is as much as 16-24 m up to the northwest confirmed by auger data. The attitude of the fault(s) is unknown. Seismic events emanating from the deformed zone are ambiguous as to indication of fault dip.