Journal Article

The Role of Fluvial Morphodynamic Hierarchy in Shaping Bedform Deposits

Ganti, Vamsi, Reesink, Arjan, Das, Debsmita

2024

Overview

Fluvial cross strata are fundamental sedimentary structures that record past flow and sediment transport conditions. Bedform preservation can be significantly influenced by the presence of larger‐scale topographic features that cause spatial gradients in flow. However, our understanding of the controls on cross strata preservation in the presence of a morphodynamic hierarchy is limited. Here, using high‐resolution bathymetry from a physical experiment, we quantify bedform evolution and cross strata preservation in a zone of flow expansion and deceleration. Results show that the size and celerity of superimposed bedforms decreases along the host‐bedform lee slope, leading to a systematic downstream increase in the sediment accumulation rate relative to bedform celerity. This increase in local bedform climb angle results in the preservation of a larger fraction of formative bedforms. Our results highlight the need to revise current paleohydraulic reconstruction models, and demonstrates that fluvial morphodynamic hierarchy is a fundamental determinant of sedimentary strata. Plain Language Summary Dune evolution in rivers creates inclined layers of sediment, called cross strata, that are an integral part of the rock record on Earth and Mars. The thickness distribution of cross strata is the primary means of estimating ancient flow and sediment transport conditions. Dunes exist with larger‐scale features, such as bars and larger dunes, in rivers, where a train of dunes responds to flow steering by larger‐scale features through changes in dune size and speed. However, we currently lack data to assess the influence of larger‐scale features on dune evolution and cross strata. Here, we studied dune evolution on the lee side (downstream facing slope) of a larger bedform in an experimental channel, where flow expands and slows down. Using high‐resolution data, we show that the dune size and speed decrease with downstream distance along the host‐bedform lee side. The rate of sediment build‐up relative to dune speed increases downstream, which leads to the preservation of a larger fraction of dunes in cross strata. Results suggest that cross strata preserved in the presence of larger‐scale features are common in the rock record, and we need to revise our current models for estimating past flow conditions from cross strata. Key Points We characterize bedform evolution and cross strata preservation in a zone of flow expansion and deceleration in a physical experiment Bedform size and celerity decrease along the host‐bedform lee slope, causing an increase in aggradation rate relative to bedform celerity A larger fraction of the formative bedforms is preserved as cross strata than typically assumed by paleohydraulic reconstruction models

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: Bathymetry / Bed forms / bedform / Bedforms / cross strata / Dunes / Evolution / Flow / Fluvial deposits / morphodynamic hierarchy / paleohydraulics / Physiographic features / Preservation / River beds / Rivers / rock record / Rocks / Sediment / Sediment deposits / Sediment transport / Sedimentary structures / Steering / Strata / Thickness