Camera lowering Station 4, although presenting the most featureless bottom for 

 the first 1,500 meters of track, showed the most unexpected features of all the photo- 

 graphs. At 24° 4T49"N, 77° 35'01"W the camera system traversed a well indurated 

 limestone outcrop approximately 24 feet across and terminating in a 3-foot vertical 

 to concave scarp striking northeast (Plate VIM). Stereographic examination of the 

 outcrop reveals cavities and depressions in the exposure which range from 5 to 60 

 centimeters in both width and depth, and, in many instances, unconsolidated sediment 

 covers the base of the depression. A number of the cavities are interconnected to form 

 a network of channels, and almost all display sharp angular rims (Plate IX). A micro- 

 topographic contour map of the edge of the outcrop is presented in Plate X. 



Busby (1962) discussed this outcrop and the possible origin of the features, and 

 concluded that the depressions are solution basins of subaerlal or littoral zone origin 

 that were formed when the outcrop or the floor of the channel was at an elevation of 

 about 1,900 meters higher than at the present. 



Bottom Currents 



Twenty-four meters northwest of the outcrop observed In the photographs from 

 Station 4, pebble and cobble-sized debris is present, and immediately adjacent to 

 this material are well developed oscillatory ripple marks facing northeast (Plate XI). 

 The ripple marks at this location appear symmetrical and average 13 centimeters from 

 crest to crest. 



Utilizing various sources of data, a rough estimate of the minimum current veloc- 

 ity necessary to produce these ripples can be calculated. The average median diam- 

 eter of the surface sediments In the area of ripple mark formation is 15 microns, and, 

 according to Hjulstrom (in Trask, 1955), a mean water velocity of 28 to 43 centimeters 

 per second is required to instigate movement of particles of this diameter. Ripple 

 marks disappear or are obliterated when water velocity exceeds a critical value, which 

 in the instance of very coarse sands is 90 centimeters per second (Shipek, 1961). Con- 

 sequently, a current of minimum velocity of 28 to 43 centimeters per second and maxi- 

 mum velocity of 90 centimeters per second is necessary for formation and maintenance 

 of the ripple marks observed in Plate XI . The maximum velocity Is probably much 

 higher than that necessary to obliterate the ripples observed in this area; however, as 

 no data are available concerning ripple marks in domlnantly silt-sized sediments, this 

 value Is taken as the maximum in lieu of further information. 



Menard (1952) attributed symmetrical ripple-mark development at 4,500 feet in 

 the Pacific Ocean to short-period water oscillations perhaps caused by tides, tsuanamis, 

 or internal waves. Inman (1957) pointed out that symmetrical ripple marks require 

 oscillatory currents for formation, since an unidirectional current produces asymmetrical 

 ripples with one slope at the angle of repose of the sediment and the other more or less 



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