NO. 1 EMERY AND HULSEMANN: SUBMARINE CANYONS 7 



floors. The width is uncertain but it is believed to commonly range up 

 to 200 meters. 



Basin-floor Portion: — At the foot of the basin slopes both the gen- 

 eral bottom topography and the canyons exhibit a change. The general 

 steepness is much less and both contours and samples show that the 

 basin slope is bordered by a broad concave fan or apron built up of sedi- 

 ments carried through the submarine canyons (Gorsline and Emery, 

 1959; Emery, 1960b). Fans from adjacent canyons may coalesce to form 

 a general bajada-like feature whose steepness ranges downward from 

 about 1.5°. Beyond the fans are basin plains which are so flat that the 

 depth may change only 1 meter in 6 km. 



Extensions of the submarine canyons have been recognized only 

 across the fans, where they take the form of low winding channels. 

 These channels are bordered by natural levees which often cause the 

 floor of the channel to be higher than the surface of the adjacent fan. 

 Such levees are shown by profiles for Mugu, Dume, Santa Monica, 

 Redondo, San Pedro, Newport, La Jolla, Coronado, Santa Cruz, and 

 Santa Catalina canyons and they may occur at others. The first recog- 

 nition of levees in the region appears to have been by Buffington (1952) 

 for San Pedro, Newport and La Jolla canyons. Heights of the levees 

 above the channels range up to about 50 meters, but 25 meters is prob- 

 ably a better average height. The channels are probably less than 200 

 meters wide and their axial slopes range from 3° to 0.4°, as shown by 

 the data of Figure 16. 



Lithology and Age 



Rocks have been dredged from the walls of many of the canyons. 

 Most common are sedimentary and volcanic rocks of Miocene age 

 (Fig. 17). Pliocene shales were obtained at San Pedro Sea Valley, San 

 Gabriel Canyon (about 20 km east of San Pedro Sea Valley), and 

 Coronado Canyon. Landward extensions of canyons have been filled with 

 Recent sediments. Therefore, the age of the canyons is pre-Recent and 

 at least parts of some of them are post-Pliocene. The strata which crop 

 out on the walls represent seaward extensions of the same strata en- 

 countered in outcrops or in wells on the adjacent land, but not enough 

 samples are available to reveal the tops and bottoms of individual beds 

 or to show whether the beds dip seaward or have structural peculiarities. 



