Recent observations of gravity flows in submarine canyons
Charles A. Nittrouer, School of Oceanography, University of Washington, Seattle, WA,
USA
The existing paradigms regarding sedimentation in submarine canyons indicate that they
experienced extensive gravity flows (e.g., turbidity currents) during low stands of sea
level, and, other than hemipelagic sedimentation (e.g., from nepheloid layers), most
canyons are undergoing relatively little transport and accumulation of sediment during
the present high stand. However, recent studies, especially on tectonically active
continental margins, have documented massive sediment transport through some
submarine canyons (e.g., Eel, Sepik). These observations suggest that in mountainous
coastal areas where continental shelves are narrow and steep, gravity flows can be active
today. These conditions might be more prevalent than previously predicted. In addition,
cold-water gravity flows have recently been documented as an efficient means of
sediment transport through submarine canyons (Cap de Creus).
Most submarine canyons have complex morphology, and developments with multibeam
surveying, ROV and submersible operations, and observatory science provide
opportunities to observe processes with respect to known morphological elements.
Among important recent observations is a range of shelf-to-canyon supply mechanisms,
which can be initiated by large storms, river discharge, and dense-water formation.
Proximity of the canyon head to a river mouth is probably an important characteristic for
modern sedimentation. In some cases, hyperpycnal flows with freshwater might be the
supply mechanism. Not all processes flush sediment through entire canyon systems, and
commonly sediment is stored in canyon heads for decades – before being dispersed
farther down the canyon. The heads of some canyons are shallow enough to be within
the zone of intense wave reworking. In these cases, resuspension or even liquefaction
can initiate down-slope transport mechanisms. Much of the sediment transported to and
through canyons can be sandy and carbon-rich (both terrestrial and marine). Because
canyons are ultimately erosional features, a better understanding is needed to determine
whether the storage of sediment in canyon heads is unique to high-stand conditions, or
whether episodic transport is a general characteristic of canyon sedimentation. Canyons
on tectonically active margins may be good analogues for sedimentary processes that
were more common during low stands of sea level.