Stratigraphy, paleohydrology, and soil variability in late Quaternary river valleys of the southeastern Atlantic Coastal Plain, USA
Suther, Bradley Edward
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Southeastern Atlantic Coastal Plain river valleys contain a diverse assemblage of late Quaternary fluvial landforms and deposits that provide important information about past river behavior and constitute geomorphic frameworks for chronosequence studies that evaluate changes in soils over time. However, most geologic maps fail to resolve the complexity of surficial deposits in Coastal Plain river valleys, and the hydrologic implications of large meandering paleochannels preserved in these settings remain poorly understood. Furthermore, previous chronsequence studies have not evaluated the extent of soil variability on fluvial landforms in these environments. To address these problems, three separate but inter-related studies were conducted. In the first study, the Quaternary sediments of the lower Oconee River valley were mapped using allostratigraphy. The Oconee valley was found to contain eight Oconee River-derived alloformations ranging from early-middle Pleistocene to Holocene age. The Oconee River had a braided morphology during the late Wisconsin interval (35-17 ka), a large meandering morphology during the terminal Pleistocene (17-11 ka), and modern-sized meanders during the Holocene (11 ka-present). These distinct morphological phases produced unique alloformations. In the second study, the bankfull discharge of large, terminal Pleistocene meandering paleochannels (“mega-meanders”) along six Coastal Plain rivers was estimated using channel boundaries determined from stratigraphy and hydraulic modeling. Mega-meander bankfull discharge was found to be similar to the discharge of present-day Coastal Plain rivers. The modest discharge of mega-meanders results from their lower hydraulic efficiency relative to modern rivers, which is due to a high width-to-depth ratio and a correspondingly reduced hydraulic radius. In the third study, soil variability and development were evaluated across a chronosequence of optically-dated meandering channel scroll bars in the Pee Dee River valley in South Carolina that range in age from Holocene to 119±30 ka. Soil morphology and chemistry show increasing development with time across the chronosequence, but soil variability resulting from intra-landform variations in parent material and relief is considerable, indicating the importance of understanding of soil variability within individual geomorphic surfaces when evaluating chronosequences in these settings. Together, these studies improve understanding of fluvial behavior, paleohydrology, and soil evolution within Coastal Plain valleys during the late Quaternary.