Abyssal Sea Floor 



59 



STATUTE MILES 



V X 5 



Figure 56. Profiles of continental slope: heavy line, off southern California (lat. 33°) showing shallow trench at base 

 of slope; light line, off northern California (lat. 38°) showing apron at base of slope. 



slope off southern California and the com- 

 plex nature of the topography inshore of it 

 indicates that this particular continental 

 slope cannot be a depositional feature. Law- 

 son (1950), mistakenly impressed by the ap- 

 parent smoothness of the continental slope 

 shown by generalized contours of that area, 

 concluded that it is a warped peneplain; this 

 behef may be supported by the rarity of 

 seismic activity along the continental slope 

 as compared to that of areas farther east of 

 it. However, the en echelon offsets of trend 

 off middle and northern California and the 

 straightness and general uniformity of its 

 profile (except for trench and apron at its 

 base) are more suggestive of some type of 

 faulting. In the absence of more than mere 

 scraps of factual information, further specu- 

 lation about its origin seems worthless. 



Abyssal Sea Floor 



Although it has been known for many 

 years that the abyssal sea floor is not the 

 featureless plain it once was supposed to be, 

 the recent work of Menard (1955, 1956) has 

 given system to the irregularities present in 

 the northwestern Pacific. He has shown a 

 close correlation of smoothness of the bot- 

 tom with supply of clastic sediment from the 

 mainland. Extensive smooth flat plains exist 

 in the Gulf of Alaska (Fig. 57) where the 

 blanket of sediment has buried the lower 



flanks of many seamounts (Menard and 

 Dietz, 1951). Off Vancouver Island, how- 

 ever, a trough has intercepted the flow of 

 sediment carried presumably by turbidity 

 currents, so that the sea floor west of the 

 trough is irregular. An alternate but less 

 likely explanation is that diastrophism there 

 has been so recent that sediments have not 

 had time to bury the irregular topography 

 as they have farther north. Off northern 

 California there is again a deep plain com- 

 posed of sediment contributed from shore. 

 This smoothness ceases at the Murray Frac- 

 ture Zone off Point Conception. South of 

 this zone the sediment from land is inter- 

 cepted by the basins of the continental bor- 

 derland and by the peninsula of Baja Cali- 

 fornia so that the topography there is highly 

 irregular. 



Crossing parts of the abyssal plains of 

 sediment are ten or more deep-sea channels 

 which appear to be the routes followed by 

 turbidity currents, most of which appear to 

 have started at the mouths of submarine 

 canyons. These channels are similar to those 

 previously described on the floors of basins 

 of the continental borderland. 



Menard (1956) has shown the presence of 

 four major fracture zones which extend 1400 

 to 3300 miles, mostly west of the shoreline 

 (Fig. 58). One of these, the Murray Frac- 

 ture Zone, appears to be a westward exten- 

 sion of the Transverse Ranges on land. It 

 is separated from the Transverse Ranges by 



