Knickpoints and crescentic bedform interactions in submarine channels

Abstract

Submarine channels deliver globally important volumes of sediments, nutrients, contaminants and organic carbon into the deep sea. Knickpoints are significant topographic features found within numerous submarine channels, which most likely play an important role in channel evolution and the behaviour of the submarine sediment‐laden flows (turbidity currents) that traverse them. Although prior research has linked supercritical turbidity currents to the formation of both knickpoints and smaller crescentic bedforms, the relationship between flows and the dynamics of these seafloor features remains poorly constrained at field‐scale. This study investigates the distribution, variation and interaction of knickpoints and crescentic bedforms along the 44 km long submarine channel system in Bute Inlet, British Columbia. Wavelet analyses on a series of repeated bathymetric surveys reveal that the floor of the submarine channel is composed of a series of knickpoints that have superimposed, higher‐frequency, crescentic bedforms. Individual knickpoints are separated by hundreds to thousands of metres, with the smaller superimposed crescentic bedforms varying in wavelengths from ca 16 m to ca 128 m through the channel system. Knickpoint migration is driven by the passage of frequent turbidity currents, and acts to redistribute and reorganize the crescentic bedforms. Direct measurements of turbidity currents indicate the seafloor reorganization caused by knickpoint migration can modify the flow field and, in turn, control the location and morphometry of crescentic bedforms. A transect of sediment cores obtained across one of the knickpoints show sand–mud laminations of deposits with higher aggradation rates in regions just downstream of the knickpoint. The interactions between flows, knickpoints and bedforms that are documented here are important because they likely dominate the character of preserved submarine channel‐bed deposits.

The formation and evolution of submarine headless channels

“…1B, C, D), where slope gradients are generally below 0.5°. The submarine channel becomes less entrenched downstream and is also dominated by large knickpoints (Heijnen et al, 2020;Chen et al, 2021). Sediment waves are also present in the overbank areas along the lower part of the channel (Fig.…”

“…A sandy floored submarine channel is incised into Holocene fjord-bottom silts and clays, created by turbidity currents from the Homathko and Southgate River deltas at the fjord head (Zeng et al, 1991;Heijnen et al, 2020). The submarine channel extends over 44 km and reaches water depths of >600 m (Prior et al, 1987;Chen et al, 2021).…”

Channel incision into a submarine landslide on a Carboniferous basin margin, San Juan, Argentina: Evidence for the role of knickpoints

“…Here, the sedimentological record of these episodic knickpoint periods is restricted to the stepped (terraced) and narrow conduit, the bedload lag deposits stranded on these terrace surface, and the abrupt facies change on top of the lag deposits. This contrasts with studies of modern systems that have also recognised extensive aggradational deposits, including bedforms, across and downstream of knickpoints (Chen et al, 2021;Guiastrennec-Faugas et al, 2021). Such deposits have been identified from flows cutting into previously deposited sand-rich deposits, rather than in cases such as this where the erosion is into mud-rich MTDs.…”

Longest sediment flows yet measured show how major rivers connect efficiently to deep sea

“…6 ). In particular, deep (20–40 m) erosion may be associated with knickpoints 58 , 63 , 64 , defined as zones of locally steeper gradients along the canyon or channel floor (Fig. 6 ), and such localised deep erosion will undermine cables and cause breaks 9 .…”