128-19 Bedload Textural Characteristics of Late Quaternary Braided Versus Meandering Phases of the Lower Oconee River, Atlantic Coastal Plain, Georgia, USA

Abstract:

Braided terraces dating to 30-17 ka occur along many rivers of the southeastern Atlantic Coastal Plain that reflect regional channel response to high sediment loads and bank instability associated with the colder, drier paleoclimatic conditions of the Late Pleistocene.  Although these terraces are well-documented in Georgia and the Carolinas, few studies provide quantitative data on the sedimentology of braided channel deposits and its relationship to that of other fluvial sediments in the region.  Thus, this research uses particle size analysis and field morphological descriptions to evaluate textural differences in the bedload of late Wisconsin (35-17 ka) braided versus terminal Pleistocene (17-11 ka) and Holocene (11-0 ka) meandering channel deposits of the lower Oconee River valley in southeast Georgia, a locality with a wide array of late Quaternary fluvial landforms that is representative of other alluvial valleys in the region. 

Along the Oconee, braided river bedload facies are 8 to >10 m thick and comprised of gravelly coarse sand that was deposited in lower braid bar and channel bed environments.  In their upper 1 to 3 m, these deposits grade to medium sand that reflects sedimentation in middle and upper braid bar positions.  In contrast, meandering channel bedload deposits are 2 to >6 m thick and predominantly composed of coarse sand associated with channel lag and lower point bar settings.  Somewhat similarly, these sediments fine upward in their upper meter to medium sand that represents deposition in upper point bar positions.  Although the gravel contents of uppermost bed sediments for braided and meandering channels are comparable (2-3% by weight), the gravelly sand that constitutes most of the braided river bedload averages more gravel than the channel lag and lower point bar deposits of meandering channels (16 vs. 10%).  Greater gravel content in basal bedload sediments of braided versus meandering channel deposits is consistent with the higher bed sediment loads and possibly higher stream powers (relative to later meandering forms) inferred by previous studies for Late Pleistocene braided rivers in the region.

Work is underway to identify additional grain size parameters useful for differentiating braided versus meandering channel deposits along the Oconee River.  Results should help to better inform lithofacies interpretations and geologic mapping efforts in late Quaternary alluvial valley fills of the southeastern Coastal Plain. 

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