New Insights on Gravity Flow Dynamics During Submarine Canyon Flushing Events

Millions of tons of material are flushed through submarine canyons during infrequent high-magnitude events, transporting coastal sediment to the deep ocean. However, observations related to individual canyon flushing events are challenging due to the destructive nature and infrequency of flushing events. The impacts of one of the largest gravity flows in the past decade were documented in Kaikōura Canyon, Aotearoa–New Zealand, where >1 km3 of sediment was mobilized by the 2016 CE Kaikōura earthquake (Mw 7.8). We present new high-resolution (<1 m) multibeam data collected with an autonomous underwater vehicle (AUV) along the Kaikōura Canyon axis, together with side-scan sonar and seafloor video imagery. These data sets reveal a wide range of erosional and depositional features that were not previously identified. Eroded bedrock and deep erosional structures are found in the upper canyon, including linear grooves, and rockfall debris (>5-m-diameter boulders). This erosional area transitions downcanyon to coarse-grained depositional bedforms, including cyclic steps and gravel waves (average wavelengths of 250 m and wave heights of ∼20 m), covering the mid- and lower canyon. Our observations provide high-resolution field-based evidence of (1) flow transformation, from a debris flow to a high-density turbidity current; and (2) variations of flow dynamics within turbidity currents both across- and downcanyon, during an infrequent, high-energy canyon flushing event. This research offers new insights into the processes that create and shape nearshore bedrock-incising submarine canyons.