Submarine Canyon and Fan Deposits
Dispersal of sediment across a submarine fan is controlled by a distributary system of migrating fan valleys, which are generally contiguous with a feeding canyon that provides a point source for the sediments moving onto the sea floor. Under conditions of fan growth, one active, leveed fan valley on the upper fan leads to a suprafan, a depositional bulge with an irregular surface that is probably the site of most rapid aggradation on the fan. Below the suprafan, the fan surface is relatively smooth and apparently free of distributary channels. Fan-building processes are sensitive to changes in the rate of sediment supply, grain-size distribution within the sediments, and tectonic disturbances within fan and source areas, but they are relatively insensitive to the shape of the depositional basin or to scale factors (ultimate size of the deposit). Specific deep-sea fans included in this discussion are listed in order of decreasing radial dimensions: Bengal Fan (3000 km), Monterey Fan (300 km), La Jolla Fan (80 km), San Lucas Fan (60 km), Navy Fan (60 km), Coronado Fan (50 km), and a small fan in western Lake Superior (5 km).
Submarine canyons allow coarse sediment to bypass shelf environments. To remain active, a canyon must maintain its head in or near the surf zone to intercept the littoral drift; a rapid rise in sea level can cut off its major source of sediment. Ascension Canyon, one of several canyons leading to the Monterey Fan, is now relatively inactive as its head lies near the outer shelf. In the California Continental Borderland, Coronado Canyon likewise has been inactive during the present interglacial period, and reflection-profiling data suggest that it was probably inactive during earlier interglacial periods. The rapid sea-level fluctuations of greater than 100 m during the Pleistocene Epoch may be considered extreme, but similar effects may occur when submarine canyons experience large, apparent sea-level changes due to tectonic tilting. In either event, sudden changes affecting sediment supply to canyon heads cause a marked shift in the locus of turbidite deposition.
Depositional sites on a deep-sea fan also may change in response to fluctuating sea levels even when the canyon continues to receive sediment. If headward erosion within a canyon head keeps pace with transgression, incision of the fan valley on the upper and middle fan results; the La Jolla fan valley has cut across the entire fan area, and deposition is now confined to a suprafan farther down the San Diego Trough.