Morphological diversity of cyclic steps at submarine canyon-fan transition zone: A two dimensional numerical modeling approach

Abstract:

Recent high-resolution geophysical surveys indicated that sediment waves are common in submarine canyon-fan transition zones and levees. They exhibit laterally continuous crests that migrates upstream, and they have been interpreted as cyclic steps. On the other hand, crescent-shaped bedforms can be observed in submarine canyons that are relatively small in wavelength and height. The origin of these morphological features has not been clarified yet. From results of numerical experiments, we propose that both of sediment waves are cyclic steps accompanied with hydraulic jumps of turbidity currents. The model used in this study is based on two-dimensional shallow water equation of turbidity currents, and it was solved by CIP-CUP method that is the third-order-accurate scheme. We set a submarine canyon and a flat basin plain in calculation domain, and two types of experimental series were conducted: experiments with continuous and surge-like turbidity currents. The experimental turbidity currents are injected from the upstream end of the canyon. The canyon was 5% in slope, 9 km long and was 200 m wide at the transition to the basin. The basin was 4 km long and wide. The turbidity currents were 120 m in width and were ca. 5 m in maximum depth. Initial sediment concentration was set to be 1 %, and grain size was 180 micron meters. As a result of numerical experiments, upstream-migrating bedforms were formed both inside submarine canyon and on the canyon-fan transition zone. All bedforms are associate with hydraulic jumps, so that they can be classified to cyclic steps. Bedforms inside the canyon show linear or crescent shapes with relatively short wavelength, while those developed on the canyon-fan transition zone were longer in wavelength and their crests were more continuous. The crescent shaped bedforms in canyons produced in this study resemble bedforms observed in modern submarine canyons, and the laterally continuous cyclic steps are common features of lobate submarine fans at the upstream end. Thus, cyclic steps can be a unified explanation for sediment waves in the proximal region of submarine fans.