Upper-Flow-Regime Mud Floodplains, Lower-Flow-Regime Sand Channels: Sediment Transport and Deposition in a Drylands Mud-Aggregate River

Abstract

Abstract Dryland rivers in which fine sediments travel as aggregates are increasingly recognized in modern and ancient fluvial systems. Fowlers Creek, Australia, is an ephemeral dryland mud-aggregate river whose sediments provide insights into the dynamics of mud-aggregate floodplains, the origin of massive mudrocks from arid depositional environments, and the nature of planar bedding. Fowlers Creek's flow conditions were inferred from relationships between landforms and the sediment texture, bedforms, and sedimentary structures remaming after flow ceases. Floodplain muds, consisting of fine sand and sand-size mud aggregates, are distributed over the floodplain in suspension. As flow decelerates they are deposited as bedload. The shallow depth, high sediment load, and low aggregate particle density promote flow conditions ranging from high in the lower flow regime to upper flow regime, producing coexisting ripples, scours, flat beds, and clay layers. With time, visible signs of aggregate structure are lost, leaving a massive cohesive mud; consolidation is not achieved by burial. Aggregates reappear when muds reenter fluvial transport. In unchanneled reaches, sheetflows deposit sediment with a pervasive horizontal fabric. Channel sediments (coarse sands and gravels) are a minor component of Fowlers Creek's deposits. Widespread lower-flow-regime conditions produce planar beds and 2-D dunes, usually deposited without internal stratification or in horizontal laminae. Lower-flow-regime planar bedding is also observed in fine silty sands. Near the close of flow, rapid shallowing may move channel conditions from lower to upper flow regime, or from lower to higher positions in the lower flow regime bedform stability spectrum, leading to unusual bedform associations. In the rock record sediments from a river like Fowlers Creek would be characterized by structureless gravelly sands (channel facies), massive red mudstones (floodplain facies), and sediments with horizontal fabric but poorly expressed bedding (sheetflow facies).