ABSTRACT 



Seventy-three sediment cores, 6 grab samples, and 4 stereographic camera 

 tracks were taken on the bottom and flanks of the Tongue of the Ocean, Bahamas. 

 The Tongue of the Ocean is a long, deep re-entrant or channel into the Great 

 Bahama Bank. It is oriented northwest-southeast, is about 700 fathoms deep in 

 its southern portion (cul-de-sac), and gradually descends northward to 1,300 fath- 

 oms over a distance exceeding 100 miles. The flanks or walls of the channel are 

 precipitous and average 15° to 20° slope above 250 to 300 fathoms depth; how- 

 ever, the slope below this depth range to the bottom is more gradual. Incised into 

 the flanks are deep gullies trending at right angles to the surrounding bank edges; 

 the gullies are more prevalent In the long, narrow northern portion of the channel 

 than in the southern cul-de-sac area. 



Laboratory analyses show the bottom sediment to be predominantly silt-sized 

 skeletal and nonskeletal carbonate particles of both deep and shallow water origin. 

 Organic carbon content of the sediment is low, averaging between 1.0 and 2.0 

 percent. Water content, void ratio, and porosity decrease with depth in the sedi- 

 ment, while conversely, density and cohesion increase. 



Sediment accumulation in the channel can be attributed to slow, continuous 

 particle- by- particle deposition from the overlying water column and turbidity 

 current type deposition originating on the upper walls and bank edges of the 

 channel. The latter type accumulation accounts for over 50 percent of the sedi- 

 ment column sampled. Rate of sediment accumulation is extremely high along the 

 flanks and central reaches of the southern portion in the channel. Radiological 

 dating shows accumulation as high as 640 cm /1, 000 years at selected areas. 

 Sediment accumulation In the northern, central area of the Tongue of the Ocean 

 is much less and is measured at between 3 to 5 cm/ 1,000 years. 



Stereographic photographs of the channel bottom show a paucity of benthic 

 animal or plant life, and, in general, an almost featureless unconsolidated ooze is 

 pictured. In the central, northern portion of the channel at 1,000- fathoms depth, 

 a limestone outcrop is present containing cavities or basins suggestive of su baerial 

 erosion at some earlier geologic time. Adjacent to the outcrop are well-developed 

 symmetrical ripple marks probably caused by tidal oscillations or internal waves, 

 and, on the basis of ripple form and dominant sediment grain size, bottom cur- 

 rents of between 0.3 to 1.0 knot are calculated. 



ROSWELL F. BUSBY 

 Oceanograpb'ic Development Division 



