place in all sediments inhabited by benthonic organisms. 

 Sometimes it is localized, but normally it exists over great 

 areas and often with very little variation from environment 

 to environment. 



V. PHYSICAL DISTURBANCES OF BOTTOM SEDIMENTS 



Waves and currents, together with earthquake jarr- 

 ing, faulting, gravitational slumping, turbidity flows, and 

 tsunamis, all affect the sediment-water interface. Study 

 of the types, numbers, and distribution patterns of the 

 marks left on the interface helps determine the origin, 

 strength, and mode of operation of the physical forces 

 involved. Normally, the latter are active in canyons, on 

 seamounts, on topographic highs, on marine terraces, in 

 channels, on beaches, and in coarse sediments around 

 rocky outcrops. 



Symmetrical ripple profiles denote oscillatory 

 wave motion, whereas asymmetrical profiles are indicative 

 of currents coming strongly from one general direction. 

 Visible ripple marks are evidence of the water velocities 

 mentioned in a previous section, above and below which 

 sediment grains move only in sheet flow without rippling. 

 Clays do not ripple at any velocity because of the coherency 

 of the fine grains, but gravels can be rippled by strong 

 currents under certain conditions. Many ripple marks 

 have been preserved in place by the cementing of sand 

 grains after rippling. Water motion, sand grain size, and 

 chemical makeup, rather than depth, are the limiting 

 factors in the formation of ripple marks. The water veloc- 

 ities needed to initiate rippling are directly proportional to 

 the density of the sand grains. Coarse sediments are most 

 often detected where rocky outcrops or boulders are exposed, 

 that is, usually on shoaler topographical features. 



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