B. L. Mullenbach

Evidence for superelevation, channel incision, and formation of cyclic steps by turbidity currents in Eel Canyon, California

We performed a multibeam survey of Eel Canyon, offshore northern California. The survey revealed a significant bend in the canyon that appears to be due to the oblique compressional tectonics of the region. A series of steps within a linear depression, similar to 280 m above the canyon floor, extends from the canyon rim at this bend to the subduction zone and a distinct fan-like topographic rise. We hypothesize that the linear depression is a distributary channel and the steps are cyclic-step bedforms created by turbidity currents. Our interpretation indicates that turbidity currents are able to run up and overspill the 280-m-high canyon wall, resulting in a partial avulsion of the canyon and the construction of a fan lobe that is offset from the canyon mouth. Simple hydraulic calculations show that turbidity currents generated in the canyon head from failure of 2-3 m of material would be capable of partially overflowing the canyon at this bend, assuming steady-uniform flow, full conversion of the failed mass into a turbidity current, and a range of friction coefficients. These estimates are consistent with analyses of sediment cores collected in the head of Eel Canyon, which suggest that 2-3 m of material fails on decadal time scales. Our calculations show that the overflowing parts of the currents would have large shear velocities (>10 cm/s) and supercritical Froude numbers, consistent with erosion of the distributary channel and formation of cyclic steps by turbidity currents.