3D anatomy and flow dynamics of net-depositional cyclic steps on the world’s 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.

Knickpoints and crescentic bedform interactions in submarine channels

“…Prior studies have recognized the prevalence of crescentic bedforms over a range of subaqueous depositional settings, including delta slopes (Hughes Clarke et al, 2012;Turmel et al, 2015;Hage et al, 2018), the axis of shallow-water submarine channels (Smith et al, 2005;Hughes Clarke et al, 2014;Normandeau et al, 2014Normandeau et al, , 2015, the axis of deep-water channels (Babonneau et al, 2013;Heijnen et al, 2020) and across unconfined submarine fans (Normark et al, 2002;Shao et al, 2021). Recent bathymetric mapping has revealed that the upper sections of submarine channels are often dominated by upslopemigrating crescentic bedforms (Hughes Clarke, 2016;Symons et al, 2016).…”

“…, 2002; Shao et al . , 2021). Recent bathymetric mapping has revealed that the upper sections of submarine channels are often dominated by upslope‐migrating crescentic bedforms (Hughes Clarke, 2016; Symons et al .…”

Supercritical‐flow processes and depositional products: Introduction to thematic issue

“…Fielding, 2006; Allen et al ., 2014; Fricke et al ., 2015; Normandeau et al ., 2016; Gall et al ., 2017; Wang & Plink‐Björklund, 2020) to coastal and shallow‐marine (Ventra et al ., 2015; Hughes Clarke, 2016; Kostic et al ., 2019; Slootman et al ., 2019; Di Celma et al ., 2020), to more distal marine settings where sediments are dominantly transported by turbidity currents (e.g. Postma et al ., 2009; Covault et al ., 2014; Symons et al ., 2016; Lang et al ., 2017; Shao et al ., 2021). For a long time, antidunes and corresponding sedimentary structures captured the most attention in supercritical sedimentology, owing to their (as yet partly unresolved) morphodynamic complexities, their potential for palaeohydraulic estimates, and their occurrence in successions originating from both continental and marine environments (e.g.…”