SEDIMENTARY ENVIRONMENTS IN SUBMARINE CANYONS AND 

 ON THE OUTER SHELF-UPPER SLOPE OF GEORGES BANK 



Page C. Valentine 

 U.S. Geological Survey 

 Woods Hole, MA 02543 



Sedimentary environments have been identified on the southern margin 

 of Georges Bank at depths of 150-600 m on the basis of texture and 

 bottom current patterns. At the shelf edge, rippled sand gives way to 

 smoother and finer-grained sand deposits as water depth increases on the 

 upper slope, which is dissected by steep-walled gullies. Submarine 

 canyons of varying size and wall steepness incise the Georges Bank Shelf 

 and contain such sedimentary facies as pavements of ice -rafted gravel, 

 bioeroded and collapsed Pleistocene silt outcrops, sand accumulations 

 surfaced with large and small bedf orms , and almost featureless silty 

 sand. 



Patterns of strong bottom currents are attributed to Gulf Stream 

 warm-core rings, tidal currents, and an unknown source. (1) Gulf Stream 

 warm-core-ring currents flow northeastward at about 50 cm/s , winnowing 

 very fine sand, silt, and clay from the upper slope between 200 and 300 

 m water depth. (2) Regional tidal currents are weak on the upper slope. 

 However, canyon morphology appears to affect tidal flow. At depths of 

 150 to 600 m in large canyons having steep walls (Oceanographer Canyon) , 

 tidal currents are strong, attaining speeds of 75 to 100 cm/s. The 

 canyon floor is covered by coarse sediment and large bed forms. By 

 contrast, small, shallow canyons (Heel Tapper Canyon) are relatively 

 tranquil, and, like the upper slope, canyon walls and floors are covered 

 by silty sand. (3) A strong bottom current of unknown origin flows 

 westward at about 50 cm/s across the rims of Oceanographer and Lydonia 

 Canyons at depths of 150 to 200 m, transporting shelf sand across 

 deposits of ice-rafted gravel onto the east canyon walls. 



Sediment movement is more rapid in a narrow band northeastward along 

 the upper slope, along the lower walls and floors of large, high- energy 

 canyons, and from the shelf westward across the east rims of large and 

 medium canyons. Sediment movement is less rapid on the outer shelf, on 

 most of the upper slope, including the gullies, on the floors of medium 

 and small canyons, and around some canyon heads. A moderately energetic 

 canyon of medium size such as Lydonia may be accumulating sediment most 

 rapidly. It traps a large volume of shelf sand as well as bioeroded 

 silt from outcrops on lower canyon walls. 



B-6 



