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Recent Sediments of the Monterey Deep-Sea Fan


The Monterey deep-sea fan is an arcuate wedge of sediment that occupies 100, 000 square kilometers of the floor of the Pacific Ocean at the base of the continental shelf off the coast of central : .:. California. The slope of the fan surface gently decreases radially from an axerage 6f 28' at the apex of the fan, at a depth of 3000 meters, to an average of 07' at the outer edge of the fan, at depths about 4500 meters. Two parallel submarine channels (Ascension and Monterey east), which flow respectively out of the mouths of the Ascension and Monterey canyons, cut into the smooth surface of the fan and extend approximately 300 kilometers to the outer edge of the fan. Hydraulic functions (Leopold and Maddock, 1953) calculated for these channels O. 38, (2) D = 0. 39 Q 0. 34 are: (1) W = 17. 3 Q 0. 26 , and (3) V = 0.19 Q . The hydraulic functions indicate that the energy of the current, which ' forms the channel, is concentrated at the base of the current. Bankfull mean velocities calculated from a modified Chezy-Manning equation (Hurley, 1964) decrease downstream from about 8. 5 meters per second, near the apex of the fan, to 1. 0 meters per second at the outer margin of the fan.

Turbidity currents, which flow down the submarine canyons and out the submarine channels, seem to be the major agency for transporting 'material to, and distributing material on the fan. The hydraulic functions suggest that thin- dense currents (Stoneley, 1957) carve the channels. Thick, relatively les s dense currents (Plapp and Mitchell, 1960) that are subject to lateral spreading may distribute sediment to regions away from the submarine channels. Repeated migrations of the submarine channels, analogous to the migrations of stream channels on alluvial fans, resulting in the shift of the areas of maximum deposition probably produce the half- cone shape of the Monterey fan. Low velocity bottom currents of undetermined origin probably play a minor role in redistributing sediments and modifying the shape and surface of the fan.

Forty- six gravity cores and three piston cores of sediments from the upper surface of the Monterey fan and adjacent regions are the primary sources of data for lithologic and mineralogic studies of the fan. The sediments from the surface of the fan are chiefly green-gray mud (silt and finer material) occasionally interbedded with thin (approximately one to two centimeters) dark very fine sand layers, which have a muddy matrix. Cores taken near the submarine channels generally have sediment that is coarser and contains a higher percentage of sand than sediment taken at a distance from the channels. BP 10, a core from the Monterey east channel 274 kilometers from the head of the Monterey canyon, has a 1. 8 meter thfck. unit of a poorly sorted mixture of pebbles, granules, sand and mud, with pebbles up to 50 millimeters long.

The light mineral suite of the sand fraction is characterized by a low quartz-feldspar ratio, and high mica and altered rock fragment content. The heavy mineral suite of the sand fraction consists chiefly of green and brown hornblende, "cockscomb" pyroxene, chlorite, and manganese coated rock fragments. The composition and immature aspect of the sand fraction and the high content of mud matrix in the sand layers indicate that most of the sands on the Monterey deep-sea fan are the modern equivalents of graywackes.

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