Abstract
AbstractThe 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.
Highlights
The amount of fine sediment in sand-bed rivers significantly influences channel form and movement and the architecture of fluvial deposits at a wide range of scales (e.g., Peakall et al, 2007; Hampson et al, 2014; Ghinassi et al, 2016; Lapôtre et al, 2019; Dunne and Jerolmack, 2020)
The ability to interpret the sedimentary archive of fluvial landscape dynamics and predict subsurface reservoir and aquifer quality is currently limited by a lack of constraints on fine-sediment flux to ancient rivers
Significant mud transport and deposition can occur during high-energy, high-concentration flows, which can be common in tidal or highly seasonal channels (e.g., Dalrymple and Choi, 2007; PlinkBjörklund, 2015)
Summary
The amount of fine sediment (silt and clay) in sand-bed rivers significantly influences channel form and movement and the architecture of fluvial deposits at a wide range of scales (e.g., Peakall et al, 2007; Hampson et al, 2014; Ghinassi et al, 2016; Lapôtre et al, 2019; Dunne and Jerolmack, 2020). Based strictly on particle size, silt and clay have slow settling velocities; mud deposits in sand-bed channels are commonly attributed to periods of slow or stagnant flow (e.g., Martin, 2000). Flocculation and mud aggregates allow silt and clay to behave like larger particles and interact with the channel bed (e.g., Rust and Nanson, 1989; Lamb et al, 2020), and advective pumping through bedforms can inject fine sediment into bed material (Packman and MacKay, 2003). Large channel-bed features like bars create locally variable flow conditions, which can promote suspended-sediment deposition (e.g., Szupiany et al, 2012) and enhance bed-deposit preservation (Ganti et al, 2020)
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