ANALYSIS OF MODERN ALLUVIAL ARCHITECTURE IN PROVIDENCE CANYON STATE PARK, GA, USING 2D AND 3D GROUND PENETRATING RADAR

Ground Penetrating Radar (GPR) data were collected in modern alluvium at the bottom of Providence Canyon State Park, GA, to assess the depositional nature of the valley fill. 2D data were collected along one canyon axis (~650m) and at several cross-canyon locations. Two detailed 3D surveys each built from ~15 GRP lines at 0.5m spacing were collected at an actively eroding headwall and where the canyons open into a wider valley. These data were used to contrast the sedimentary architecture at each location. Shallow vibracores were used to constrain the interpretations.

Three GPR facies were identified based on visual comparisons of the amplitude and continuity of GPR reflectors. The LA facies (low amplitude reflectors) occurs outside of the modern valley fill and is interpreted to represent the clay-rich Cretaceous Ripley Fm into which the canyons erode. The HAC facies (high amplitude, continuous reflectors) is confined to valley fill and is interpreted to represent finer grained floodplain deposits and colluvium washing in from the canyon sides. Finally, the MASC facies (moderate amplitude, semi-continuous reflectors) represents coarse braided stream sand (when within the canyon fill) and trough cross-bedded sand of the Cretaceous Providence Fm (when outside the canyon fill).

Canyon fill at the eroding headwall is dominated by MASC facies representing braided stream sediment and colluvium derived from the Providence Fm into which the canyon headlands erode. The surface separating canyon fill from the Providence Fm is difficult to recognize because both sediment types are unconsolidated, cross-bedded sand. In the downstream location, where the valley begins to widen, valley fill is characterized by both MASC (braided stream) and HAC (floodplain) facies. The uppermost few meters of sediment at this location is composed of the MASC facies that ¹⁴C dates from vibracores indicate reflects accelerated erosion that resulted from settlement and land use changes beginning in the early 1800’s. GPR data will provide the first estimates of the volume of sediment stored within the canyons, permitting an interpretation of erosion and sediment transportation rates in one of the most extreme examples of human-induced erosion within the country.