EVIDENCE FOR AND IMPLICATIONS OF A SUB-HORIZONTAL STRATIGRAPHIC SEQUENCE BELOW THE POST-RIFT UNCONFORMITY IN THE VICINITY OF THE 1886 CHARLESTON, SC EARTHQUAKE

The ~7 Mw 1886 Charleston, SC earthquake was the largest recorded earthquake in the southeastern U.S. and the epicentral tract has been subject to extensive studies to understand the geology and seismotectonics in the last 40-50 years. The region is known for its complex geology of the shallow crust in the region is due to Mesozoic rifting and volcanism. More recently there have been smaller earthquakes recorded in the study area. It is not clear whether these earthquakes are due to the shallow Mesozoic faulting, or due to faults that are deeper than previously thought. Re-analysis of profiles from three seismic reflection surveys (total of 18 profiles), collected in the late 1970’s and early 1980’s in and near the epicentral region in Summerville, SC image the shallow crust. The first of two reflectors is seen at 745-800m depth, present on all three surveys, and is the unconformity between Cretaceous Coastal Plain sediments and underlying units. Well data from the three Clubhouse Crossroads, Norris Lightsey #1, Summerville oil well, and DOR-211 indicate that the lithologies directly below the unconformity consist of redbeds of inferred Triassic age and mafic flows and sills of Jurassic age. Only the Clubhouse Crossroads #3 penetrate the unconformity in the vicinity of Summerville. Previous studies concluded that the sequence is lower Mesozoic in age and is considered to be basement and is identified as the B reflector. However reinterpretation of these surveys show another strong relatively flat lying reflector beneath the supposed basement at an estimated 900-1300m depth, with thickness varying from 200-350m depth and a lateral extent of ~800km². The new reflector appears to coincide with the base of the B sequence. This potentially new sedimentary sequence, which appears to be unbroken by faults with the exception on profile VT5, may provide additional insight into the regional geology and the structure of the 1886 Charleston earthquake.