Cyclic steps and related supercritical bedforms: Building blocks of deep-water depositional systems, western North America

Abstract

Cyclic steps are long-wave (the ratio of wavelength to height is ≫1), upstream-migrating, upper-flow-regime bedforms bounded by internal hydraulic jumps (i.e., transition from densimetric Froude supercritical to subcritical flow) in turbidity currents. They commonly occur in regions with high gradients and slope breaks. Here we review the morphodynamic evolution and depositional products of cyclic steps and related supercritical bedforms (e.g., antidunes). We present examples from high-resolution geophysical surveys and monitoring of continental margins in western North America integrated with physically based numerical modeling of turbidity currents and associated bedforms. We compare numerical modeling results to direct monitoring of turbidity currents in the Squamish prodelta, British Columbia, Canada. Cyclic steps and antidunes influence phases of canyon-channel evolution, levee-overbank deposition, and channel-lobe-transition-zone sedimentation, thereby advancing channels and lobes into the basin. Bedforms range from relatively small cyclic steps and antidunes within active submarine canyons and channels (~101m wavelength; ~100m height) to large cyclic steps in less confined levee-overbank environments and the channel-lobe transition zone (~103m wavelength; ~102m height). Cyclic steps and related supercritical bedforms are important to the morphodynamic evolution of architectural elements of some deep-water depositional systems, especially those located along tectonically active margins with high gradients and slope breaks that can promote internal hydraulic jumps in turbidity currents.