Recent sediments of the Monterey deep-sea fan

Title: Recent sediments of the Monterey deep-sea fan
Abstract:: "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 average of 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 are(1) W = 17.3 Q exp 0.38, (2) D = 0.39 Q exp 0.34, and (3) V = 0.19 Q exp 0.26. The hydraulic functions indicate that the energy of the current, which forms the channel, is concentrated at the base of the current. Bank-full 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 less 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 muddy matrix. Cores taken near the submarine channels generally have sediment that is coarser and contains a higher percentage of sand the 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 thick 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 the mud matrix in the sand layers indicate that most of the sands on the Monterey deep-sea fan are the modern equivalents of graywackes. The principal source of the sand-sized material on the Monterey fan are the quartz diorite plutons in the Salinian geologic province near the heads of the Ascension and Monterey canyons. The similarity between the mineralogy of the sand fraction of the fan and that of the quartz diorite bodies indicate that short fluvial transport and short underwater transport by longshore drift and in turbidity flows does not obscure the provenance. The sand layers show a gradual downslope decrease in diagnostic hydraulically heavy grains, such as hornblende and pyroxene, with an increase in hydraulically lighter grains, such as the micas, until the sands on the outer margin of the fan contain over 90 per cent mica flakes. Thus the provenance of turbidite sands deposited from flows that have traveled long distances (more than 300 kilometers for the case of the Monterey fan) would be difficult to determine as most of the minerals grains characteristic of the source would have been deposited upslope. The sources of silt and finer material on the fan, or the bulk of the fan sediment, are (1) the Great Valley, which contributes 5 x 10 exp 5 cubic meters per year, that amount which escapes from San Francisco Bay through the Golden Gate, and (2) Salinia and local Coast Range drainage basins, which contribute 5 x 10 exp 5 cubic meters per year. The estimated annual supply of sediment to the heads of the submarine canyons that debouche onto the fan is 1 x 10 exp 6 cubic meters per year. Ages calculated from rates of deposition for the upper five meters of sediment, or the approximate thickness of the interval sampled by the cores, are on the order of 500,000 years and agree with the radiolarian ages of Pleistocene to Recent for sediment in the cores. The maximum age of the oldest fan sediment is about 30 to 40 million years, which means an average rate of deposition on the fan of 1.0 centimeters per thousand years. Sediment on the fan probably is not withdrawn permanently from continental cycles of erosion because eventual uplift of the fan is implied by (1) the suggested common origin for deep-sea fans and certain eugeosynclinal deposits exposed on land that have similar facies, current structures, and areal extent, and (2) the continental aspect of the crust underlying the Monterey fan as indicated by gravity and seismic evidence (Woolard and Strange, 1962). Thus the study of the Monterey and other deep-sea fans may give valuable clues to the perplexing problems of the relationship between the continents and the ocean basins."
Supplemental_Information:: Report duplicates P. Wilde's PhD. dissertation. Submitted under contract DA-49-055-CIV-ENG-63-4 with the Coastal Engineering Research Center, U.S. Army. Financial support from Institute of Marine Resources of the University of California (Scripps Institution of Oceanography), the National Science Foundation Summer Fellowship Program, and the Summer Field Support Program of the Department of Geological Sciences, Harvard University. Data are from five Scripps IO cruises: CUSP 1954 (CUSP); Fanfare 1959 (HMS/BG/BP); Mendocino 1960 (Men); Marezine 1960 (Mare); Leapfrog 1961 (LFGS). Some data may be duplicated (or be similar) in NGDC Deck 41 and NGDC's MGG15005001 (Mendocino); NGDC's MGG15095001; (Leapfrog); NGDC's MGG15995003 (Fanfare); NGDC's MGG15995001 (CUSP). Data digitized by the USGS for inclusion into usSEABED (<http://walrus.wr.usgs.gov/usseabed>)

Range of values
Minimum:	68
Maximum:	4770
Units:	Meters
Resolution:	5.0

Range of values

Minimum:

Maximum:

4770

Units:

Meters

Resolution:

5.0

Recent sediments of the Monterey deep-sea fan

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