MULTIPLE SCOURS AND UPWARD FINING CAUSED BY HYDRAULIC JUMPS: IMPLICATIONS FOR THE RECOGNITION OF CYCLIC STEPS IN THE DEEPWATER STRATIGRAPHIC RECORD

摘要

Hydraulic jumps control the bypass, erosion, and depositional processes of Froude-supercritical turbidity currents, so they represent a significant process for understanding the development of submarine geomorphology. Hydraulic jumps actively occur from submarine canyons to fans, where the seafloor slope is relatively steep. Turbidites in such areas comprise large-scale bedforms called cyclic steps, and they exhibit complex internal structures, including localized erosion and the accumulation of coarse-grained fining-upward sequences. However, it is unclear which turbidity-current properties are reflected in the heterogeneous depositional characteristics and grain -size sorting of these deposits. To this end, we conducted flume experiments to reproduce deposits associated with the hydraulic jumps of surge-type flows. Turbidity-current surges were repeatedly generated in an experimental flume with a knickpoint that transitioned from a steep to a gentle slope, resulting in cyclic steps. Overall, the upstream migration of the cyclic steps produced a downstream-upward-fining succession of turbidites. However, hydraulic jumps occurred at several places over the trough to the stoss side of the step in a single flow due to the non-uniform and unsteady flow state of the surge-type turbidite succession. As a result, the reproduced succession exhibited multiple local scours and coarse-grained fill in the lower parts of the turbidites. This suggests that multiple local scours and fining-upward trends are discriminant characteristics of cyclic-step deposits formed by surge-type supercritical turbidity currents.