SYMPOSIUM ON PROBLEMS OF EUSTATISM IN 

 THE PACIFIC BASIN AND RIM 



SUBMARINE CANYONS IN THE PACIFIC AND THEIR BEARING ON 



EUSTATISM 



By Francis P. Shepard, Scripps Institution of Oceanography, 

 University of California 



[A bstracf] 



The submarine slopes along the margins of the Pacific are creased 

 with numerous submarine canyons. Recently the canyons have also 

 been discovered outside the deeply eroded portions of the Hawaiian 

 Islands. Depths of half a mile or more are found in the outer portions 

 of these canyons. Investigations of typical submarine canyons in the 

 California area have been made by bottom sampling, submarine 

 photography, and by diving operations. The results of these studies 

 have shown that the canyons have chatacteristics which are exact 

 duplications of typical land canyons. Many of the submarine canyons 

 are known to be direct continuations of river valleys on land. 



Since all oceanographic data obtained to date serve to show the 

 virtual impossibility that the canyons could have been formed by any 

 other process than river erosion it becomes evident that widespread 

 submergence must have taken place. If this has been the case it should 

 also be indicated by other lines of evidence. Recent soundings show 

 that there are flat-topped seamounts in many parts of the Pacific. These 

 have depths varying from a few hundred to more than a thousand 

 fathoms. The flatness is indicative of wave-bevelling when the submerged 

 mountains stood much higher in relation to sea-level. The discovery 

 of rounded cobbles on various elevations of the sea-floor gives more 

 evidence of submergence. Finally, the borings into four coral atolls 

 ■have all shown that shallow-water deposits extend for at least a thousand 

 feet below sea-level in all cases ; in fact, the submergence of atolls in 

 general seems probable. 



The explanation of the widespread submergence is not clear, but the 

 possibility should be considered that it has been due to a change of 

 sea-level rather than of the lands. An alternative which seems 

 somewhat more likely is that diastrophic submergence has been 

 proceeding over a long period of time, but that sea-level changes have 

 also been significant in producing the present conditions. 



Summary of Discussion 



Dr. At wood asked if there really had been any careful research into 

 the quantity of water required to produce the ice of the Pleistocene 

 glaciation. Dr. Shepard did not think so, but felt that some geologists 

 rather underestimated the thickness of the ice. In view of the evidence 

 of Tertiary submergence, he was of the opinion that it played the chief 

 part in the formation of the canyons rather than lowering of sea level 

 due to glaciation. 



Dr. Atwood stated that in his judgment these submarine canyons 

 represent a short period of time and were formed by extremely quick 

 action of running water at some period of emergence, and felt that the 

 amount of water held as ice should be considered. - 



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