Mineralogical indicators of alluvial sediment sources in the Cape Fear River basin, North Carolina

Rivers of the southeastern coast of the United States transport sediment from the Piedmont to the Coastal Plain, where it is deposited on floodplains or delivered to estuaries. This study examines recent floodplain deposits throughout the Cape Fear River basin in North Carolina to address downstream changes in mineralogy and the signature of alluvium from Piedmont and Coastal Plain sources. Sand in Piedmont alluvium is dominated by quartz, but includes significant amounts of feldspar. The fine silt and clay mineralogy of Piedmont alluvium includes quartz, kaolinite, illite, vermiculite, and gibb-site, which are also present in the main-stem Cape Fear River floodplain throughout the Coastal Plain. Coastal Plain tributaries are dominated by quartz sand and contain few clay minerals. Weatherable minerals decrease downstream across the Coastal Plain due to weathering, floodplain storage, and dilution by local inputs. Sand in the estuary is dominated by quartz, but fine sediment contains a fairly diverse mineralogy resembling the Piedmont-draining rivers. Our analysis indicates that sediment transport in the Coastal Plain is highly dependent on particle size. Sand in the estuary is derived from local "blackwater" sources, but much of the fine silt and clay is delivered by "brownwater" rivers that drain from Piedmont sources.