Transient response of bedrock channel networks to sea-level forcing in the Oregon Coast Range, USA

Abstract:

Although sea level has fluctuated repeatedly over the Pleistocene by up to 120 meters, how (or whether) this cyclic base level forcing impacts the development of bedrock river profiles in tectonically active settings is poorly understood. A major reason for this uncertainty is that bedrock river channels in unglaciated locations are typically buried at their mouths under sediment resulting from the Holocene marine transgression, making direct observations impossible. Here we present a novel approach to constraining the influence of cyclical sea-level forcing on the development of bedrock river profiles in the Oregon Coast Range.

Using a 1 m LiDAR DEM, we estimate the depth to the buried bedrock longitudinal profile using measurements of valley width, elevation, wall slope, and present day river width for the Smith River and its tributaries where the river is currently buried by Holocene fluvial and marine sediments. Assuming the current river width is spatially and temporally constant, we calculate the depth to bedrock based on a trapezoidal geometry. In an effort to reduce the noise in our signal, we calculate linear regressions for valley and river width and hold the valley wall slope constant at its average value (~ 30 degrees) for all our calculations.

Several important observations stem from this analysis. First, the bedrock profile of the Smith River projects to the same elevation (~ -110 m) as the bench cut into the continental shelf by the sea-level low stand of the last glacial maximum, suggesting the river is currently graded to the low-stand elevation. Second, almost all of the tributary bedrock profiles plot at the same elevation (~ 40 m below sea-level), which is much higher than the mainstem bedrock profile. One key exception is the significantly larger North Fork of the Smith River, which falls directly on the projected bedrock curve for the Smith. These observations suggest that sea-level low stands force incision of the entire bedrock river network, but that smaller tributaries, which are buried and/or flooded during highstands, cannot keep pace with the mainstem during these periods of brief down-cutting. Our results imply that difference in erosion rates between tributaries and the mainstem results in the formation of fluvial hanging valleys that are buried during marine transgressions.