How to recognize crescentic bedforms formed by supercritical turbidity currents in the geologic record: Insights from active submarine channels

Abstract

Submarine channels have been important throughout geologic time for feeding globally significant volumes of sediment from land to the deep sea. Modern observations show that submarine channels can be sculpted by supercritical turbidity currents (seafloor sediment flows) that can generate upstream-migrating bedforms with a crescentic planform. In order to accurately interpret supercritical flows and depositional environments in the geologic record, it is important to be able to recognize the depositional signature of crescentic bedforms. Field geologists commonly link scour fills containing massive sands to crescentic bedforms, whereas models of turbidity currents produce deposits dominated by back-stepping beds. Here we reconcile this apparent contradiction by presenting the most detailed study yet that combines direct flow observations, time-lapse seabed mapping, and sediment cores, thus providing the link from flow process to depositional product. These data were collected within the proximal part of a submarine channel on the Squamish Delta, Canada. We demonstrate that bedform migration initially produces back-stepping beds of sand. However, these back-stepping beds are partially eroded by further bedform migration during subsequent flows, resulting in scour fills containing massive sand. As a result, our observations better match the depositional architecture of upstream-migrating bedforms produced by fluvial models, despite the fact that they formed beneath turbidity currents.

Supercritical flows overspilling from bypass‐dominated submarine channels and the development of overbank bedforms

“…Coarse-grained sediment waves and bars have also been described from studies of the modern seafloor (Piper et al, 1985;Wynn et al, 2002;Paull et al, 2011;Gamberi et al, 2013;Kostic, 2014;Covault et al, 2017;Hage et al, 2018). These are typically larger than the bedforms described here, with wavelengths on the order of tens to hundreds of metres and heights from 1 to 10 m. These features are generally interpreted as antidunes or cyclic-steps formed under Froudesupercritical flow Postma and Cartigny, 2014).…”

“…These have been attributed to formation under Froude-subcritical flow conditions (densimetric Froude number <1, but see Huang et al, 2009) as internal lee waves (Flood, 1988;Kneller and Buckee, 2000;Lewis and Pantin, 2002), or to Froude-supercritical flow (Froude number >1), forming as antidunes (Normark et al, 1980;Wynn et al, 2000;Ercilla et al, 2002) or cyclic-steps (Cartigny et al, 2011;Kostic, 2011;Zhong et al, 2015). Beneath typical seismic resolution, smaller, coarser-grained overbank bedforms attributed to Froude-supercritical flow have been reported from seafloor channel-levee systems Wynn et al, 2002;Hughes Clarke, 2016;Hage et al, 2018) but only a few examples are known from outcrop studies (Winn and Dott, 1977;Ito and Saito, 2006;Lang et al, 2017), where high-resolution studies of the preserved sediments are possible.…”

How turbidity current frequency and character varies down a fjord‐delta system: Combining direct monitoring, deposits and seismic data

“…Deposition of massive sand intervals within the channel thalweg also suggests elevated near‐bed sediment concentrations (Talling et al ., ), and this is consistent with imaging of denser near‐bed layers (Hughes Clarke, ). These channelized flows are supercritical and produce trains of up‐slope migrating bedforms, or cyclic steps (Cartigny et al ., ; Hughes Clarke et al ., , Hughes Clarke, ; Hage et al ., ). Some of the most powerful and longest runout events are caused by sediment settling from surface plumes, rather than slope failure, at least over sub‐annual timescales (Hizzett et al ., ).…”

“…The river discharge does not plunge directly to form hyperpycnal flows (Hughes Clarke, ). The sea floor sediment for the first few hundred metres seaward of the delta lip is characterized by fine to medium sands and silt (Hage et al ., ), while the mid‐slope and basin are characterized by silt and clay, respectively (Syvitski & MacDonald, ).…”

Pyroclastic dune bedforms: macroscale structures and lateral variations. Examples from the 2006 pyroclastic currents at Tungurahua (Ecuador)

“…Alternatively, pulsating Froude-jumps might produce similar features, yet natural observations of these processes are almost exclusively linked with lee side erosion (e.g. Dietrich et al 2016, Hage et al 2018 and no argument allows to solve this question. The steep truncations and backset lineations are similar to what is generally interpreted as chute and pools (e.g.…”