SECT. 3] PELAGIC SEDIMENTS 721 



Pacific might well be due to such a stratigraphically determined preferred level 

 of intrusion. 



Probably because of the uplift, sedimentation appears in many cases to have 

 ceased on the topographic highs for varying lengths of time, except for halmeic 

 minerals accreting by precipitation. In some cases erosion seems to be or has 

 been active on the crests and slopes of the hills, while accumulation continues 

 around them (Fig. 41). The older deposits on the topographic highs, outcropping 

 or covered by relatively thin layers of younger sediments, appear to range in age 

 between Oligocene and Pliocene (Arrhenius, 1952; Riedel, Chap. 33 of this 

 volume). If it is correct to postulate control of the tojDography by volcanic 

 intrusions, it would then seem that such events have occurred frequently 

 during the Cenozoic era. 



Quaternary ^^^^ ,, , . 



, , I , . „ ^-^ Unconformity 



conformably overlying ^^^^^ , 



■r«^t'°^y hiatus 



Fig. 43. Hypothetical schematic profile illustrating topographic control of pelagic sedi- 

 mentation in low, hilly topography. The order of magnitude size of the hills is a few 

 kilometers horizontally and up to a few hundred meters vertically. The thickness 

 scale of the Quaternary formation is exaggerated; the maximum thickness found in 

 pelagic clay facies is a few meters. 



Scouring even by weak currents might prevent accumulation of fine-grained 

 clay sediment from taking place on hill-tops and slopes and may account for 

 lateral displacement of resuspended sediment, but is unlikely to be responsible 

 for the resuspension which appears to affect Pleistocene and older sediments 

 with comparatively high shearing strengths. The dispersion of Tertiary Radio- 

 laria through the Pleistocene sediments adjacent to the eroded hills (Riedel, 

 Chapter 33 of this volume) indicates a continuous process of resuspension and 

 transport downhill of the deposit from the topographic highs. This is probably 

 caused by the churning action of benthic animals, some of which have been 

 observed forming a suspension cloud (Fig. 42), which may be displaced by any 

 horizontal movement of the near-bottom water. Repetition of this process 

 seems to result in slow abrasion of the elevated areas and movement of the 

 resusjDended sediments downhill, until a level is reached where the streamlines 

 are parallel to the sediment surface. The accumulating sediment, mostly new 

 material and including some redeposited sediment, appears gradually to invade 

 the elevated areas of non-deposition, as schematically indicated in Fig. 43. The 

 upper Cenozoic clay sediments thus form a blanket with frequent holes and thin 



