SECT. 3] PELAGIC SEDIMENTS 659 



high-speed turbidity currents, invading the unprotected areas of the deep- 

 ocean floor adjacent to the shelf slope, and smoothing the original topography 

 to almost level abyssal plains (Kuenen, 1950; Heezen et at., 1959; Menard, 1959; 

 Hurley, 1960). Rises or trenches protect areas separated from the continental 

 shelf from invasion by turbidity currents; in the absence of topographic 

 barriers such as in the Gulf of Alaska and in large parts of the Atlantic Ocean, 

 detrital sediments are, or were once, spread over extensive areas of the deep 

 ocean by this mechanism (Fig. 2). On the other hand, some pelagic deposits 

 accumulate close to the coast in areas where river discharge is low, where 

 topographic protection is provided and where currents prevent fine-grained 

 elastics from accumulating, such as on banks and rises, or where the clastic 

 erosion products are efficiently funneled into deep catchment basins by sub- 

 marine canyons (Shepard, 1948; Kuenen, 1950; Emery, 1960; Inman and Cham- 

 berlain, 1960). Iron and manganese oxide rocks, phosphorite and glauconite 

 deposits, coral reefs, pteropod and foraminiferal oozes are thus frequently 

 found in local areas close to the continents. 



Although few measurements exist which permit quantitative estimates of 

 the rate of deposition of the terrigenous component, i.e. the parameter suggested 

 as a basis for division of marine sediments into pelagic and rapidly accumulating 

 ones, the large differences in accumulation rate between these two sediment 

 types often permit their recognition on the basis of a number of easily observed 

 features. One of these is evidence of reworking of the sediment by organisms. 

 Benthic animals appear to be distributed over all areas of the ocean floor where 

 free oxygen is available, even at the greatest depths. Studies of the mixing of 

 sediments across unconformities demonstrate that although single worm bur- 

 rows might occasionally penetrate as deep as 20-30 cm, the mean mixing depth, 

 above which 50% of the extraneous material is located, is of the order of 4-5 cm. 

 In pelagic sediments the time required for burial of such a layer under another 

 equally thick one varies between 10^ and 10^ years. The longer time is tjrpical 

 of areas with a low rate of deposition of organic remains, where a correspondingly 

 low population-density of benthic animals is sustained. The total amount of 

 reworking of a given stratum before ultimate burial might, therefore, not vary 

 as much as the total rate of deposition within the area of pelagic sedimentation. 

 When adjacent strata have different colors or shades, the mixing process 

 causes a typical mottled appearance (Fig. 3). In pelagic sediments without a 

 color stratification, the mud-eating animals leave less conspicuous traces, but 

 their presence is indicated by fecal pellets, annelid jaws, and other fossil 

 remains including chemical reduction structures. Non-pelagic deep-sea sedi- 

 ments, on the other hand, are deposited so rapidly that the sparse benthic 

 population does not have enough time to distiu-b the strata as extensively as in 

 pelagic deposits with similar population densities, and the original stratification 

 is preserved, often in minute detail. Examples are the thin laminations often 

 present in graded beds, deposited by turbidity cm-rents, and laminae of volcanic 



ash. 



Another effect of the low rate of detrital deposition, characteristic of pelagic 



