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

摘要

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,~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 sug-gest 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.