SECT. 3] ORGANIC TRANSPORTATION OF MARINE SEDIMENTS 789 



(Gorsline and Emery, 1959) clearly show that the coarser materials there are 

 concentrated at the- mouths of submarine canyons. Presumably the sediments 

 were transported by turbidity currents flowing down submarine canyons or 

 down steep slopes where they were initiated by slumping. After deposition of 

 the coarser parts of their loads, the turbidity currents can carry fine sands and 

 silts for additional hundreds of kilometers. Where ridges cross their paths the 

 turbidity currents are blocked, but the areas of their effective deposition are well 

 indicated by smooth, gently sloping topography even at great depths (Elmen- 

 dorf and Heezen, 1957). Unlike ice-rafted sediments, the turbidity-current 

 deposits can occur at all latitudes, but are largely in strips adjacent to con- 

 tinental areas (Fig. 2). 



Possibly the bulk of sea-floor sediment is transported in suspension from shore 

 areas. Transportation of sediment coarser than sand probably does not occur by 

 suspension. The grain size appears to decrease outward to a point beyond which 

 further decrease owing to limitation of transportation is masked by changes 

 caused by diagenetic alteration of clay minerals. 



9. Criteria for Recognition of Rafting Agent 



The criteria to be used for possible recognition of the agent which rafted a 

 given deposit of sand or gravel in recent, and especially in ancient, sediments 

 must necessarily be somewhat elastic. However, the following are suggested 

 for possible use: 



(a) Kelp: within 500 km of shore, between 20° and 60° of latitude, often 

 rounded, pholad-bored and encrusted by shallow-water organisms, in clusters. 



(6) Driftwood: anywhere in ocean, angular, weathered, isolated. 



(c) Sea mammals: on beach or in shallow water at high latitudes and pos- 

 sibly in deep water below margin of floe ice, less than 10 cm in diameter, 

 rounded, slightly polished, in clusters. 



{d) Fishes: less than 2 cm in diameter, many of coral, much fine-grained 

 material if calcareous. 



(e) Birds: less than 1 cm in diameter, varied lithology, in clusters. 



(/) Wind: sand or finer, angular shards of silt to well-rounded sands, in 

 layers. 



(g) Ice: at high latitudes, clay to boulders, angular, un weathered, some 

 striated, in layers. 



{h) Turbidity currents: on flat areas of sea floor, silt to gravel, often graded 

 upward from coarse to fine, gravel usually rounded, in layers. 



References 



Aleem, A. A., 1955. Structure and evolution of the sea grass communities Posidonia and 

 Cymodocea in the southeastern Mediterranean. Essays in the Natural Sciences in 

 Honor of Captain Allan Hancock. Univ. Southern CaUfornia Press, 279-298. 



Andr6e, K., 1920. Geologic des Meereshodens. Gebriider Borntraeger, Leipzig, 1-689. 



