SLOPE ENVIRONMENT 



The upper slope east of Oceanographer Canyon, to a depth of 200 m 

 demonstrates normal downslope textural gradients related to bottom current 

 energies. By contrast, the upper slope between depths of 200 to 300 m is 

 erosional in nature with little silt or clay present. Below 300 m, silt and 

 clay content increases steadily downslope to the heads of small gullies found 

 at 500 m. The interval below 300 m is depositional and includes the floors of 

 small slope canyons such as Filebottom Canyon, which is covered with silty 

 sand. The lack of deposition in the 200- to 300-m interval is attributed to 

 strong, northeastward-flowing bottom currents of Gulf Stream warm-core rings 

 that intersect the upper slope environment. Nineteen warm-core rings impinged 

 on the Georges Bank margin in the Oceanographer-Lydonia Canyon region between 

 1976 and 1983. The upper slope was influenced by the clockwise flow of these 

 eddies for a total of 17 months during this 8-year period. Current velocities 

 of 50 to 60 cm/s (about 1 kt) effectively removed most fine sediment during 

 this interval . 



CANYON ENVIRONMENTS 



Sedimentary facies related to bottom currents were investigated in and 

 around several canyons on the southern margin of Georges Bank. These facies 

 are related to the overall morphology and energetics of the two canyons and 

 the sediment sources. The most extensive observations have been made in 

 Oceanographer and Lydonia Canyons. Gravel deposits and mobile sands are found 

 along the eastern rims of both canyons, and they are best developed along the 

 northeastern side of Oceanographer Canyon. A strong bottom current of unknown 

 origin flows westward at about 50 cm/s across the rims of both canyons at 

 depths of 150 to 200 m, transporting shelf sand across deposits of ice-rafted 

 gravel onto the east canyon walls. 



Oceanographer Canyon walls are steep in places, with extensive bioeroded 

 outcrops of Pleistocene silt. Sediment that is spilled over the canyon rim 

 combines with silt eroded from the canyon walls and is transported downslope 



- 24 



