Submarine channel initiation, filling and maintenance from sea-floor geomorphology and morphodynamic modelling of cyclic steps

摘要

Advances in acoustic imaging of submarine canyons and channels have provided accurate renderings of sea-floor geomorphology. Still, a fundamental understanding of channel inception, evolution, sediment transport and the nature of the currents traversing these channels remains elusive. Herein, Autonomous Underwater Vehicle technology developed by the Monterey Bay Aquarium Research Institute provides high-resolution perspectives of the geomorphology and shallow stratigraphy of the San Mateo canyon-channel system, which is located on a tectonically active slope offshore of southern California. The channel comprises a series of crescent-shaped bedforms in its thalweg. Numerical modelling is combined with interpretations of sea-floor and shallow subsurface stratigraphic imagery to demonstrate that these bedforms are likely to be cyclic steps. Submarine cyclic steps compose a morphodynamic feature characterized by a cyclic series of long-wave, upstream-migrating bedforms. The bedforms are cyclic steps if each bedform in the series is bounded by a hydraulic jump in an overriding turbidity current, which is Froude-supercritical over the lee side of the bedform and Froude-subcritical over the stoss side. Numerical modelling and seismic-reflection imagery support an interpretation of weakly asymmetrical to near-symmetrical aggradation of predominantly fine-grained net-depositional cyclic steps. The dominant mode of San Mateo channel maintenance during the Holocene is interpreted to be thalweg reworking into aggrading cyclic steps by dilute turbidity currents. Numerical modelling also suggests that an incipient, proto-San Mateo channel comprises a series of relatively coarse-grained net-erosional cyclic steps, which nucleated out of sea-floor perturbations across the tectonically active lower slope. Thus, the interaction between turbiditycurrent processes and sea-floor perturbations appears to be fundamentally important to channel initiation, particularly in high-gradient systems. Offshore of southern California, and in analogous deep-water basins, channel inception, filling and maintenance are hypothesized to be strongly linked to the development of morphodynamic instability manifested as cyclic steps.