SECT. 2J ABYSSAL PLAINS 325 



The Cascadia Abyssal Plain is connected to the Tufts Abyssal Plain via the 

 Cascadia Channel. The Cascadia Channel starts on the continental rise west of 

 the Washington-Oregon boundary as a seaward extension of the Columbia 

 River submarine canyon. The canyon runs south to the southern edge of the 

 Cascadia Abyssal Plain, where it enters a tectonic trough through the ridge and 

 trough province. When it emerges from this tectonic trough, it forms a large 

 fan which gradually merges with the Tufts Abyssal Plain. 



The actual area of the northeast Pacific occupied by abyssal plains is subject 

 to some controversy, not only owing to inadequate data but also to the manner 

 in which workers on that area have chosen to define abyssal plains. Hurley 

 recognizes no continental rise and applies the term "abyssal plain" to all the 

 relatively smooth sea floor which lies between the base of the continental slope 

 and the abyssal hills. Less than a quarter of the area included by Hurley in 

 the northeast Pacific abyssal plains classifies as abyssal plains according to the 

 original definition. Unfortunately, those central portions of the Alaskan Abyssal 

 Plain, the Tufts Abyssal Plain and the Aleutian Abyssal Plain which appear 

 to be true abyssal plains having gradients less than 1 : 1000 are poorly surveyed ; 

 therefore, it is impossible to give accurate figures for the gradients or smoothness 

 of any of the northeast Pacific abyssal plains. For this reason, the labels in 

 Fig. 8 merely indicate the approximate positions of the various abyssal plains 

 without indicating their boundaries. Those smooth portions of the northeast 

 Pacific which have been investigated with the aid of a Precision Depth Re- 

 corder have proved not to be as smooth as the Atlantic and Indian Ocean 

 abyssal plains. Minor irregularities of a few fathoms are common and the topo- 

 graphy of most of the areas so far explored resembles more the continental rise 

 than the abyssal plains of the Atlantic. 



The area mapped as the "Aleutian Abyssal Plain" (Fig. 8) presents some 

 very important problems. The slope indicated by Hurley on the basis of 

 numerous Coast and Geodetic Survey soundings is from north to south, with 

 the minimum soundings found along the crest of the outer ridge which bounds 

 the Aleutian Trench on the south. The few Precision Depth Recorder echo- 

 grams obtained in the northern part of the Aleutian Abyssal Plain certainly do 

 not indicate as smooth a bottom as that found in what would normally be 

 referred to as an abyssal plain. This is not surprising when one considers the 

 possible source of sediments for the smoothing of the Aleutian Abyssal Plain. 

 The northern portion of the Aleutian Abyssal Plain is cut off from turbidity 

 currents originating on the Aleutian Arc by the deep Aleutian Trench. It is cut 

 off from the Alaskan Panhandle by an oceanic rise. If the sediment which 

 smoothed this area came downhill by turbidity currents or other bottom 

 transjiortation mechanisms, it must have come before the Aleutian Trench was 

 formed or before the oceanic rise to the east w^as uplifted. The northern part of 

 the Aleutian Abyssal Plain must, therefore, be a relic plain and must no longer 

 be receiving sediments of the type which probably smoothed the plain. The 

 securing of long cores in this relic plain should stand high on any list of 

 important deep-sea research projects. 



