Beyond boundaries: Depositional environment controls on erodibility, process, and form in rivers incising sedimentary bedrock

Bedrock rivers adjust to the properties of the rock into which they incise, imprinting the geologic past on Earth’s surface. We compared rock properties and channel form along the Dry Fork in the Allegheny Mountains, West Virginia, as it crosses between Mississippian sandstone and carbonate rock units, to investigate how the depositional history of channel-margin bedrock influences modern channel form. We used thin-section petrography to interpret site-specific depositional environments. We quantified rock strength with point-load testing, discontinuity spacing by measuring bed and fracture spacing, and channel form through cross-section surveys. Petrography indicates that the sandstone was likely deposited in an alluvial fan, while the carbonate formed in a shallow-marine environment. The sandstone has modestly higher point-load strength than the carbonate, but the units differ more dramatically in their discontinuity spacing. The sandstone is thinly (3–10 cm) bedded and densely (50–100 cm) fractured; the carbonate has thicker (45 cm) beds and sparser (180–300 cm) fractures. Sandstone channel cross sections are wider, shallower, and rougher, whereas carbonate cross sections are narrower, deeper, and smoother. Results suggest that a transition from plucking-dominated erosion in the discontinuity-rich sandstone to abrasion- and/or dissolution-dominated erosion in the discontinuity-poor carbonate, rather than differences in rock strength, drives observed morphologic differences. Differences in discontinuity spacing might arise from differential bed thickness between the two units, both because bed boundaries are discontinuities and because thinner beds lead to more densely spaced fractures. We hypothesize that depositional dynamics—the unsteady deposition of an alluvial fan resulting in thin beds versus steady, shallow-marine deposition that deposited thicker beds—explain the observed differences in bed thickness, discontinuity spacing, and modern erosion process dominance and channel form, emphasizing how modern Earth-surface processes are contingent on the geologic past.