AN EVALUATION OF SOURCE-TO-SINK SEDIMENT DYNAMICS IN THE SANTEE RIVER BASIN AND SOUTH CAROLINA CONTINENTAL SHELF USING DETRITAL-ZIRCON GEOCHRONOLOGY

Understanding the Source-To-Sink dynamics of sediment transport is critical to accurate sediment provenance analysis. It is largely assumed that sediments are eroded, exported and deposited down-dip on short geologic timescales, and that these deposits are representative of the cumulative geology of their source region. This study tests these assumptions by comparing (1) the known or inferred zircon age distribution of Carolina Zone, Piedmont Zone and Blue Ridge bedrock units within the Santee River basin, with (2) the detrital-zircon ages of Recent inner continental shelf deposits down-dip. In so doing, we compiled the known or inferred crystallization ages of igneous rocks and the detrital-zircon age distribution of metasedimentary rocks upstream of the Fall Zone in the Santee River basin. We then determined the surface area of these units from regional geologic maps, using source unit age distributions to predict the expected zircon composition of sediments delivered by the Santee River system to the shelf. Significant discrepancies between observed and expected zircon chronofacies of bedrock and shelf data require explanation. Most strikingly, approximately 40% of the Santee River basin upstream of the Fall Zone is composed of rocks interpreted to primarily contain Pan-African aged (Neoproterozoic to early Paleozoic) zircons; however, zircons of this age constitute only ~7% of shelf sample zircons. When the expected proportion of Pan-African zircons in the source region is reduced to 7% with the remainder being proportionally distributed to other age populations in the source region, the expected age distribution nearly matches the observed age distribution of shelf zircons. This suggests that units containing Pan-African zircons were disproportionately excluded from the sedimentary system when delivered to the shelf. Possible explanations include (now-eroded) sedimentary overlap by coastal plain strata, preferential erosion from upstream reaches in the Piedmont Zone and Blue Ridge, dramatic watershed reorganization, or a ~6x lower zircon fertility of Carolina Zone units. Alternative explanations include a dilution of shelf Pan-African zircons with zircons of other ages by longshore drift or coastal plain sediment recycling, or poorly constrained knowledge of the geochronology of the basin.