GROUND PENETRATING RADAR ANALYSIS OF THE ALLUVIAL SEDIMENT FILL IN PROVIDENCE CANYON STATE PARK, SOUTHWEST GEORGIA

Providence Canyon State Park in southwest Georgia contains spectacular examples of human-induced erosion. The canyons cut through poorly consolidated Cretaceous sandstone (Providence Fm) and into underlying marine mudstone (Perot Fm). Ground penetrating radar (GPR) data were collected to examine the record of deposition and erosion preserved in the sediment that fills the canyon bottoms. GPR data were collected both along a 650m longitudinal transect down the canyon with several cross-sections, and as a closely spaced 3-D grid in a small area near the mouth of one of the canyons. Three GRP facies, defined based on reflector characteristics, were tied to sediment cores collected in the canyon. A low amplitude facies (LA) is characterized by reflectors that absorb the GPR signal and has been interpreted to represent the fine-grained, clay-rich Perot Fm. A moderate amplitude semi-continuous facies (MASC) has been interpreted as modern sandy alluvium. The sand is coarse-grained, and contains abundant cut and fill features and kaolinite rip-up clasts. The Providence Fm is also characterized by the MASC facies where it is penetrated by the GPR near the canyon head. A high amplitude continuous GPR facies (HAC) is interpreted to be fine-grained, vertically-accreting floodplain and slope wash deposits.

The LA facies represents Cretaceous strata beneath the canyon and is used to identify the base of the channel fill. Based on GRR data, total sediment fill thicknesses in the canyon range from 1m at the canyon head to 6m in the furthest downstream site examined. The HAC and MASC facies are interpreted to reflect canyon fill. They show 2 distinct canyon fill phases. The lower fill (0-4m thick) contains HAC and MASC facies that represent channels and associated floodplain sediments deposited before the landscape was modified by humans. The upper portion of sediment fill (1-4m thick) is interpreted as braided channel sediments and colluvium related to rapid human-induced headword erosion of the canyons. The 3-D GPR survey data permits estimations of the volume of total sediment fill in the canyon, and of the two individual sediment fill phases.