CUT-FILL-DRAPE: UNRAVELLING THE HOLOCENE ALLUVIAL DEPOSITS OF ONION CREEK, AUSTIN TEXAS

Cut and fill alluvial sequences are common in central Texas, and delineating different age deposits is usually straight forward using a combination of lithologic and pedogenic criteria. But in the vicinity of Austin, Texas, where streams emerge from bedrock confinement of the Edwards Plateau onto more open and freely meandering landscapes of the Gulf Coastal Plain, delineating discrete alluvial deposits may be a bit more challenging. The lower reaches of Onion Creek, a tributary of the Colorado River, has been the site of considerable archeological and geoarchaeological work in the last several years owing to the construction of State Highway 130, and distinguishing different age alluvial deposits in this area has been complicated in part by hydraulic damming of Onion Creek at its confluence with the Colorado River. This process has resulted in a thick cumulic veneer of overbank deposits which mantle discrete age alluvial fills at depth beneath the first terrace. At least four Holocene aggradational phases appear to be present: 1) 10,000-8,000 years BP, 2) 6000-4500 years BP, 3) 3800-2300 years BP, and 4) post-1300 years BP. This paper examines the alluvial stratigraphy of Onion Creek in the vicinity of its confluence with the Colorado River, compares the timing of alluvial deposition with the Colorado River and other central Texas streams, and examines some of the physical manifestations of slackwater sedimentation near the confluence of these two streams.