PACIFIC SUBMARINE PROVINCES 



519 



Fig. 32.3. Possible constitution of different submarine canyons off California. After Kuenen, 

 1953. 



of sediment by rivers, and coastal configuration. Some narrow rocky land 

 valleys were thus swept out (Carmel, Scripps, etc.), but the majority of old 

 valleys may still lie buried in the terrace beneath sediments. 



In some cases the turbidity currents only cleaned parts of the old valleys 

 where these happened to offer small resistance. But other parts of these valleys 

 did not conform to the requirement of following the present slope. Such parts 

 remained buried. 



Elsewhere a new valley cleaned off along its wall some small part of an 

 ancient mountain slope, without conforming to the original drainage pattern. 

 This may be the case for Monterey Canyon, which has granite overlain by 

 sedimentary rock on one wall opposite a wall which has yielded only mud or 

 soft sedimentary rock; or for Dume Canyon with basalt on the east side and 

 mud with calcareous shale on the west. 



Origin of the Continental Shelf Slope. The imposing slope has been 

 ascribed to faulting, and the shelf itself primarily to wave cutting 

 (Shepard, 1948). The Atlantic terrace, however, has been described as 

 developed by sedimentation and isostatic subsidence caused initially by 

 i the sedimentary load (Kuenen, 1950). This theory of origin is amply 

 attested locally, for instance, by the Mississippi delta building and con- 

 sequent subsidence in the Gulf of Mexico. See Chapter 36. 



We have to deal primarily with the consequences of orogeny in the 

 marginal belts of the continent and then secondarily, with the processes of 

 erosion, sedimentation, and epeirogeny in explaining the existing con- 

 : tinental shelf and shelf slope. It is not clear yet what an orogeny such as 

 the folding of the strata of the Coast Ranges of Oregon and Washington 

 does to the continental shelf slope, or in what condition it is left, but in 



any consideration, the gradation from continental crust to oceanic crust 

 will result isostatically in a suxficial (submarine) slope toward the ocean. 

 This may then be altered by erosional, depositional, and epeirogenic proc- 

 esses. In the previous discussions of submarine canyons and slope aprons 

 or fans, and in subsequent discussions of the Aleutian and Middle Amer- 

 ica trenches and the possible faulting off Oregon and northern California 

 the nature of the secondary processes is illustrated. 



ALEUTIAN TRENCH 



The Aleutian trench is a narrow depression in the ocean floor parallel- 

 ing the convex side of the Kenai and Alaska peninsulas and the Aleutian 

 volcanic island archipelago. See Figs. 32.1, 32.4, and 39.1. It extends from 

 Yakutat Bay in the Gulf of Alaska westward to Attu Island, a distance of 

 over 2200 statute miles. It has a maximum depth of 25,000 feet. Accord- 

 ing to Murray ( 1945 ) : 



The vertical relationship between the crest of the conspicuous mountain 

 features and the floor of the trench is shown in Fig. 32.4. An approximate 

 difference of 28,000 feet exists throughout most of the region. The greatest 

 single known difference throughout the entire arc exists slightly east of the 

 mid-section and is centered at Unimak Island, where Shishaldin Volcano (9372 

 feet) rises 32,472 feet above the floor of the trench (about 110 miles dis- 

 tant). . . . 



\ 



BERING SEA 





X / 



2545, /I 



STATUTE MILES 





\ 



f ft • KtSKA I 



/ AGATTU I A^ * 



■/-. I ' ^ 



Fig. 32.4. Profiles of the Aleutian trench in the vicinity of Attu and Agattu Islands, western end 

 of the Aleutians. After Murray, 1945. 



