LITHOLOGIC CONTROLS ON BEDROCK CHANNEL FORM FROM THE MICRON TO METER SCALE

The relative resistance of different rock types to river incision varies widely and the properties governing bedrock erodibility are poorly understood. Lithologic properties influencing erodibility are the culmination of a rock’s depositional, diagenetic, and exhumation history. In this study, we used petrographic and survey methods to investigate lithologic influences on rock erodibility and channel geometry in the Dry Fork, WV. Petrographic analysis reveals six distinct litho-stratigraphic units, each displaying unique mineralogic and textural features. At the outcrop scale, we measured tensile strength as well as discontinuity spacing and orientation to understand the physical properties of each identified lithology. We then compared micro- and macro-scale rock properties to surveyed channel cross-section geometry. By examining depositional and diagenetic features in light of channel geometry and observed erosion processes, we assessed the influence of inherited lithologic characteristics on modern channel form. We found that, in our study sites, rock tensile strength cannot act as a primary control on channel shape due to broadly overlapping values among the identified lithologies. Rather, the degree of lithologic homogeneity–that is, the spacing of discontinuities and variations in mineralogic composition–seems to exert the greatest control on channel form and process. We suggest that lithologic homogeneity, or lack thereof, influences the density and orientation of fractures induced during tectonic compression. The greater degree of fracturing in the more chemically heterogeneous clastic units results in a reach more susceptible to fluvial plucking compared to the carbonate unit. The blocks defined by vertical joining in the carbonate unit are too large to be moved by even the largest modern flows and are, therefore, subject only to erosion by abrasion and dissolution, which are less effective at removing material and result in a narrower channel which focuses more energy on the channel bed. This illustrates the effects of depositional environment and burial history on modern channel form and the contingency of the modern landscape on historical processes.