VIBRACORE RECORDS OF ALLUVIAL AGGRADATION FOLLOWING MASSIVE HUMAN-INDUCED EROSION, PROVIDENCE CANYON, GEORGIA

Providence Canyon in Southwest Georgia is often cited as among the most extreme examples of human-induced soil erosion on the continent. Anecdotal accounts suggest initiation of the canyons, now >60m deep, following European settlement in the early 1800’s. Sediments eroded from canyon headwalls have aggraded the valley floor downstream resulting in thick deposits of alluvium above pre-existing lithologies. Previous studies have examined rates of fluvial recovery and lacustrine sedimentary records of landscape change, but there have been very few attempts to identify either the quantity of cross-valley fill or the date of initial alluviation.

This study describes seven vibra-cores (to 4.7 m in length) recovered from transects across the valley floor down gradient from the canyons. Three stratigraphic units are recognized. Unit I is interpreted as pedogenically altered Cretaceous-aged clays and sands of the Ripley and Providence Formations that contain large 0.05 m diameter mottles. These sediments are sharply overlain by medium to coarse grained sands of Unit II which contain limited soil mottles, zones of altered colors (perhaps related to leaching) and capping organic rich horizons. This sequence is interpreted as buried soil profiles (cores PC-01, PC-02) and/or stream deposit (PC-04) indicating a period of stable surface conditions. This sequence is in turn sharply overlain by in excess of 4.0m of medium to coarse-grained sands that contain abundant clay balls, organic debris, and iron stones eroded from the canyon headlands and similar in appearance to modern valley bottom alluvium. These cores indicate 4 meters or more alluvial aggradation since initial deposition of coarse canyon-derived sediments. Abundant organic matter occurs at well defined contacts between soil surfaces (upper Unit II) and fluvial sediments (Unit III). Organic samples are being submitted for radiometric carbon dating to determine whether buried soil horizons are within the range of 14C dating.