Recognizing and characterizing Froude supercritical flow deposits in the sedimentary record

Extreme depositional events in ancient sedimentary systems are often overlooked because it is assumed that the sedimentary record is dominated by average conditions. Yet where high-energy events are preserved, their analysis is essential for reconstructing past climates; without it, we risk underestimating the true magnitude of environmental change possible in future warm-climate scenarios.

Despite their importance, bedform-scale interpretation of Froude-supercritical flow deposits is challenging due to rapid deposition, abundant soft-sediment deformation, and morphological overlap among sedimentary structures. Here we examine supercritical-flow bedforms in variable-discharge fluvial deposits of the Green River Formation, Utah, USA, using systematic comparison with experimental literature.

Nested scour features interpreted as cyclic-step deposits show depths (x˜=2.45 m, min = 0.73 m, max = 14.72 m, n = 66) that are not statistically distinct from paleo-channel depths measured in the same interval (x˜=2.75 m, min = 0.95 m, max = 21.00 m, n = 22; Kruskal–Wallis test, p = 0.2513). A continuous train of nearly to fully preserved scour fills overlying partially preserved, nested basal scour complexes indicates that large-scale cyclic steps formed during and persisted throughout a catastrophic flood, acting as the dominant sediment-transport mechanism.

Rivers dominated by sediment transport during extreme events operate under fundamentally different sediment transport and flow mechanisms than those represented in most fluvial models. Such systems remain underrecognized and require greater study to improve reconstructions of Earth’s climate history and assessments of future hazards.