THE LYDONIA CANYON EXPERIMENT: 

 CIRCULATION, HYDROGRAPHY, AND SEDIMENT TRANSPORT 



by 



Bradford Butman 



U.S. Geological Survey 



Branch of Atlantic Marine Geology 



Woods Hole, MA 02543 



A field program was conducted to study the circulation and sediment 

 dynamics in Lydonia canyon, located on the southern flank of Georges Bank, 

 and on the adjacent continental shelf and slope. The program included (1) 

 insitu measurements by an array of moored current meters, bottom tripods, 

 and sediment traps maintained between November 1980 and November 1982; (2) 

 synoptic observations of the hydrography and suspended sediments; (3) 

 sidescan-sonar and high-resolution seismic reflection surveys; (4) samples 

 of the surficial sediments; and (5) direct observations of the sea floor 

 from the submersible ALVIN. 



The distribution of surficial sediment and the high- resolution 

 seismic reflection data suggest that very fine sand and silts and clays 

 accumulate in the head of the canyon and on an area of the shelf adjacent 

 to the canyon. However, the current measurements show that the surficial 

 sediments are reworked and resuspended along the canyon axis to a water 

 depth of at least 600 m. Thus, although fine sediments may be 

 accumulating, the axis is not tranquil. Maximum hour-averaged current 

 speeds 5 meters above bottom (mab) were greater than 60 cm/s at about 300 

 and 600 m in the canyon axis. No evidence of sediment movement was 

 observed at 1,380 m. The current observations suggest down-canyon 

 transport of sediment along the axis near the head at a water depth of 

 about 300 m and up-canyon transport at about 600 m, implying a convergence 

 in the transport of sand as bedload toward the head. Qualitatively, the 

 sediment distribution along the axis mirrors the strength of the near- 

 bottom currents (finer sediments in areas of weaker currents). 



The mean Eulerian (measured at a fixed point) current was 

 southwestward on the shelf adjacent to Lydonia Canyon and above the level 

 of the canyon rim which is consistent with previous studies of the mean 

 circulation on Georges Bank. On the continental slope, the mean flow was 

 strongly influenced by Gulf Stream warm core rings. Several rings passed 

 to the south of Lydonia Canyon during the observation period; the strong 

 clockwise flow around them caused eastward flow along the edge of the 

 shelf as strong as 80 cm/s. On the slope, the influence of the rings in 

 the water column extended to at least 250 m, but not to 500 m. The 

 influence of the rings did not extend onto the Continental Shelf to water 

 depths of 125 m. Over the slope, there was a persistent off -shelf and 

 downslope component of flow near the bottom of a few centimeters per 

 second. There is some evidence Chat the warm core rings affect flow in 

 the canyon by generating packets of high-frequency current fluctuations. 



Within the canyon, the mean Eulerian flow near the bottom was 

 complex. Near the head of the canyon, net Eulerian flow 5 mab was down- 

 canyon, at about 3 cra/s , and weak at 50 mab. At 550 m, the near-bottom 

 flow was up-canyon. At 600 m the near-bottom flow was weak; the flow 100 



B-4 



