(depositional environment), currents exceed 20 cm/s only about 5 percent of 

 the time, and are less than 5 cm/s 40 percent of the time (Figure 7). On the 

 slope, the 300- to 500-m isobaths are an approximate transition zone between 

 an erosional and a depositional environment. In Lydonia Canyon and 

 Oceanographer Canyon, the currents at 300 and 500 m exceeded 20 cm/s at least 

 30 percent of the time. For these canyons, at the depths observed, currents 

 are much stronger than on the adjacent slope. Profiles of beam attenuation 

 compared sediment concentrations in the canyon and the adjacent shelf and 

 slope. In the canyons, suspended sediments were always greater than on the 

 slope at comparable depth. Suspended sediments increased near the bottom, 

 especially near the canyon head, presumably due to bottom resuspension. 



LYDONIA AND OCEANOGRAPHER CANYONS COMPARED 



Both Lydonia Canyon and Oceanographer Canyon are complex environments 

 where there are rapid changes (over a few hours) in suspended sediment 

 concentration, temperature, current velocity, and direction. The two canyons 

 differ in sediment texture, current strength, and direction of net flow. In 

 Oceanographer Canyon a net down-canyon flow was observed at 300 and 500 m. 

 Above about 500 m in Lydonia Canyon, currents exceed 20 cm/s 20 to 30 percent 

 of the time; in Oceanographer Canyon, they exceed this rate 40 to 60 percent 

 of the time (Figures 8 and 9). Based on the coarse-grained sediments and 

 stronger currents, the head of Oceanographer Canyon is primarily erosional, 

 whereas Lydonia Canyon has areas both of erosion and deposition. Both canyons 

 are much more energetic than the adjacent slope. Though the erosional/ 

 depositional classification of canyons is important in analyzing the 

 hypotheses of this workshop, it is important to recognize that this 

 classification is an extreme simplification. 



17 



