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STRUCTURAL GEOLOGY OF NORTH AMERICA 



Shepard ( Shepard and Emery, 1941 ) to be a fault scarp. In other places it 

 is not so steep and does not appear to be due to faulting. 



One of the most fascinating discoveries of echo sounding is canyons 

 that gash the shelf and its outer slope. Some of them are veritable gorges. 

 A V shape is characteristic. There are about 66 of these submarine gorges 

 or canyons along the California coast, and they are spaced irregularly at 

 distances of 10 to over 50 miles. Most of the large canyons head within 

 3 to 5 miles of the present shore, but a few extend to within half a mile. 

 Some of the smaller ones head 30 miles out. The longitudinal gradients 

 are high and compare closely with stream gradients whose canyons have 

 been cut in fault scarps. The gradients average about 4 degrees, are 

 steeper near their heads, and gentler in the lower reaches and the longest 

 canyons. The canyon bottoms are as continuous down hill as those of 

 typical mountain canyons, at least out to depths of 6000 to 9000 feet, 

 where the gentler outer slope may in places have suggestions of shallow 

 basins. 



The depth of the canyons is variable. The long Arguello Canyon west 

 of the Santa Rarbara basin starts in four tributaries, each only 300 feet 

 deep. These shallow gorges trench the shelf slope out to where it is 3000 

 feet deep. Each of the tributaries is about 15 miles long. They converge 

 into a single canyon which, in another 15 miles, is nearly 2000 feet deep. 

 At about the 5000-foot depth contour the V widens, although the canyon 

 is over 1000 feet deep at the point. The canyon turns southward, and 

 may be followed down to 11,700 feet below sea level. 



Another great submarine canyon, the Monterey, begins in tributaries 

 in the Ray of Monterey which are 2000 feet deep a mile below their heads. 

 The main canyon is 3000 to 4000 feet deep, and it trenches the shelf 

 margin as a narrow V-shaped valley to a depth of 9000 feet, where it 

 widens and shallows. It turns southward at this point and may be traced 

 clearly still deeper to 11,000 feet below sea level. 



Long stretches of the outer slope of the continental shelf are not dis- 

 sected by submarine canyons. One stretch is north of Arguello Canyon 

 between latitudes 34° and 35° 40', and another is between Eel Canyon, 

 off Cape Mendocino, and the Columbia River. Gentle slopes are in 

 part characteristic of these margins, and Shepard points out that 



canyons are not so common on gentle offshore slopes as on steep ones. 

 The continental shelf north of the Aleutian trench, quoting from 

 Murray (1945), is: 



. . . approximately defined by the 100-fathom contour. The maximum width 

 of the shelf, 120 miles, is in the vicinity of Kodiak Island. To the northeast 

 and southwest, the shelf narrows to a few miles as it converges with the major 

 land features. The coast line is generally irregular and precipitous, although 

 there are interspersed occasional areas of low relief. Only two principal rivers, 

 the Susitna emptying into Cook Inlet and the Copper northwest of Cape St. 

 Elias, discharge sediment onto the shelf or into the inland waters. 



Deep-Sea Fans. Turbidity currents debouching from the mouths of 

 submarine canyons have built large cone-shaped deposits called deep- 

 sea fans. See Fig. 32.2. Their volume is usually many times the volume 

 of material that could have been eroded from the canyons, so it is pre- 

 sumed that much sediment is contributed by shoreline processes to the 

 heads of the submarine canyons (H. W. Menard, Jr., 1955), which then 

 moves down the canyons to the fans below. The fans bury much or all 

 of the previous relief on the deep-sea floor and produce smooth gentle 

 slopes. 



Origin of Submarine Canyons. The submarine canyons of the Cali- 

 fornia shelf were postulated to be drowned subaerial valleys, smothered 

 by sediment, and excavated by glacial and recent turbidity currents (Daly, 

 1936). Shepard (1952) contends that turbidity currents are not potent 

 enough to erode the canyons and suggests that drowned river valleys 

 have been kept permanently open by the turbidity currents during the 

 process of submergence. Kuenen (1953) counters that this process does 

 not explain all types of submarine canyons. Figure 32.3 is a reproduction 

 of his conception of the different kinds of submarine canyons off the Cali- 

 fornia coast, and he comments as follows about their origin: 



Instead of assuming that drowned valleys were perpetuated by sliding and 

 turbidity currents, which have no ability to erode, it is suggested that the 

 ancient land surface was first smothered; later the poorly consolidated covering 

 materials were eroded during the Ice Age, and to some extent in postglacial 

 times to form the submarine canyons. 



Some localities were particularly favorable to the generation of turbidity 

 currents because of incompletely buried topographic depressions, local supply 



