Mud in sandy riverbed deposits as a proxy for ancient fine-sediment supply

The amount of silt and clay available to rivers reflects source-terrain composition and weathering and can be a primary control on the form and dynamics of channel networks. Fine sediment also affects the permeability of buried fluvial reservoirs. Despite this significance, there is currently a lack of methods for reconstructing how much fine sediment was transported by ancient rivers. Mud accumulations in sandy river deposits are often interpreted as indicators of variable flow conditions; however, these deposits may present an opportunity to constrain how much fine sediment was transported through ancient rivers. We report results from a series of experiments designed to evaluate how much clay and silt are preserved in sandy riverbed deposits under constant and variable discharge conditions. Our results demonstrate that (1) mud deposits, including drapes and lenses, form readily under constant, high-discharge conditions, (2) the amount of fine sediment recovered from bed-material deposits increases as fine-sediment supply increases, and (3) fine-sediment retention is higher during bed aggradation than during bypass conditions. These results indicate that the net retention of clay and silt in sandy riverbed deposits may be a simple but powerful proxy for comparing the overall amount of fine sediment supplied to ancient rivers.