Microrelief Resulting from Physical Processes 



Major processes, other than biological, responsible 

 for the formation and disruption of microstructures on and 

 within the sea floor are gravitational downslope movements, 

 waterwaves and currents, and turbidity flows . The magnitude 

 of these forces is attested to by an ever-growing library of 

 bottom photos showing wave- and current-derived ripple 

 marks in sandy sediments (fig. 13), grain sorting (fig. 14), 

 gravel beds or lag deposits (fig. 11), scour around bottom 

 objects (fig. 10), slumping on unstable slopes (fig. 15), and 

 current-distorted benthonic animals. Even weather factors 

 including surface winds have an effect. The author 24 has 

 described the formation of cross ripples by the action of 

 currents on submerged seamounts when underwater obstruc- 

 tions are present (fig. 14). Menard 36 regarded internal 

 waves as a possible source of oscillations in deep water. 

 Inman 26 pointed out that symmetrical ripple marks require 

 oscillatory currents for formation. Busby 27 considered 

 that, at a depth of 1929 meters in the Tongue of the Ocean, 

 Bahamas, ripples were probably caused by tidal oscillations 

 rather than by internal waves. Laughton stated that, in 

 the light of our present knowledge of deep currents, it may 

 be possible to explain the formation of deep ripple marks 

 in terms of steady currents. Further, variations in the 

 symmetry of the ripples may be ascribed to local current 

 and tidal fluctuations, rather than to short-period oscillatory 

 motion of the water. Deep-seated tidal-wave motion plays 

 an important part in the movement and formation of sedi- 



19 



ments. 



An increasing amount of information as to its effects 

 on bottom sedimentation indicates the importance of turbid- 

 ity flow. The irregular and heterogeneous sediment surfaces 

 often attributed to turbidity flow are difficult to detect in 

 bottom photos, and physical and biological processes tend 

 to modify near-surface microstructures in a short period 

 of time anyway. 



Biological activity, especially burrowing on unstable 

 slopes, could initiate and accelerate turbidity flows and 

 resultant slumping and disruption of microstructures. 

 Marks, pits, mounds, grooves, scratches, and impressions 

 are short-lived and ever- changing. The bottom surfaces 



36 



