640 



GUILCHER 



[chap. 24 



tons i)er day, the present-day sediments do not settle farther east than some 

 forty statute miles off the eastern passes. This part of the shelf, 20 to 40 fm 

 deep, is occupied by old sediments, much coarser than the present bottomset 

 shelf deposits nearer shore, since sand on the average constitutes almost 60% 

 of these older sediments compared with only 2% in the latter. They include 

 macro-organisms representing an older environment characteristic of a shallow 

 or even estuarine water, in which a i'*C determination on some of the shells 

 showed an age of about 7000 years. They seem thus to have been related 



62° 30' 



62° 



61° 30 



Noutical miles 



Fig. 18. Distribution of terrigenous sediments on the shelf off Peninsula de Paria, Vene- 

 zuela, and to the north of Trinidad Island, showing an increase of coarse particles 

 near the edge of the shelf. (Simplified from Koldewijn, 1958.) 



Legend — 1: percentage of pelites (particles smaller than 50 y.) exceeding 30; 

 2: percentage of pelites between 30 and 5; 3: percentage of sand particles between 

 5 and 30; 4: percentage of sand particles exceeding 30. Contours in fathoms. 



to a lower sea-level at the end of the Pleistocene or the beginning of the Holo- 

 cene. Off West Louisiana and Texas, however, Greenman and LeBlanc (1956) 

 assume that Recent clastic deposits have accumulated on the shelf (although 

 they do not give figures for the thickness of accumulation on the shelf, but only 

 on the slope). The causes would be the discharge from the Mississippi and Texas 

 Rivers and the prevailing east-to-west current. But "Recent" is defined by 

 these authors as including the last 20,000 years, that is, a large part of the 

 Wisconsin or Wiirm glacial stage (see Woldstedt, 1958), so that it cannot be 

 said that these sediments have been deposited in Holocene conditions. 



