3D anatomy and flow dynamics of net-depositional cyclic steps on the world\u2019s largest submarine fan: a joint 3D seismic and numerical approach

Abstract

Supercritical flows are ubiquitous in natural environments; however, there is rare 3D anatomy of their deposits. This study uses high-quality 3D seismic datasets from the world’s largest submarine fan, Bengal Fan, to interpret 3D architectures and flow processes of Pliocene undulating bedforms that were related to supercritical flows. Bengal undulating bedforms as documented in this study were developed in unconfined settings, and are seismically imaged as strike-elongated, crescentic bedforms in plan view and as rhythmically undulating, upstream migrating, erosive, discontinuous reflections in section view. Their lee sides are overall 3 to 4 times steeper (0.28° to 1.19° in slope) and 3 to 4 times shorter (117 to 419 m in length) than their stoss flanks and were ascribed to faster (high flow velocities of 2.70 to 3.98 m/s) supercritical flows (Froude numbers of 1.53 to 2.27). Their stoss sides, in contrast, are overall 3 to 4 times gentler (0.12° to 0.27° in slope) and 3 to 4 times longer (410 to 1139 m in length) than their lee flanks and were related to slower (low velocities of 2.35 to 3.05 m/s) subcritical flows (Froude numbers of 0.58 to 0.97). Bengal wave-like features were, thus, created by supercritical-to-subcritical flow transformations through internal hydraulic jumps (i.e., cyclic steps). They have crests that are positive relative to the surrounding region of the seafloor, suggesting the predominant deposition of draping sediments associated with net-depositional cyclic steps. Turbidity currents forming Bengal wave-like features were, thus, dominated by deposition, resulting in net-depositional cyclic steps. Sandy deposits associated with Bengal net-depositional cyclic steps are imaged themselves as closely spaced, strike-elongated high RMS-attribute patches, thereby showing closely spaced, long and linear, strike-elongated distribution patterns.