Date of Award
Master of Science in Oceanography
Robert L. McMaster
Time lapse bottom photographs, current measurements 1 m off bottom, sonographs and high resolution seismic reflection profiles were collected to define the origin and present modes of maintenance of sand waves adjacent to the head of Wilmington Canyon on the Middle Atlantic outer continental shelf. The sand waves have crest-lines all oriented northwestward, steeper sides facing southwest, 100-200 m wavelengths and rounded crests reaching 1-9 m in height. They occur in three major fields, two associated with the seaward flanks of low ridges comprised of sand waves less than 4 m high and the third field, immediately north of the canyon head comprised of sand waves mostly 4-8 m in height. Sediment distribution across the sand waves is systematic with outcropping subbottom horizons in the troughs and wave formed megaripples on the crests. Internally a few southwest dipping foreset beds and occasionally a faint reflector paralleling the sea floor are seen. Underlying the sand waves north of the canyon head is a 8-12 m thick zone of south and east dipping foreset beds. Present sediment activity mostly coincides with the northeast passage of low pressure storm systems. It occurs both by suspension and by traction only during 4 percent of the winter observation period and less than 0.5 percent of the summer period. The resulting transport is to the northeast and southwest parallel of the shelf edge.
The presences of wave formed megaripples only on the crests of these sand waves, the infrequent and small amount of sediment presently being transported, and the reflectors paralleling the sea floor all suggest that these bed forms are static with only their surface skin being reworked by present processes. These sand waves are thought to have originated during the initial stages of the Holocene transgression when a deltaic system was submerged and subjected to a tide dominated estuary mouth environment. The sand waves formed under ebb dominated tidal conditions and built the two low ridges west of the canyon head. However, as the transgression continued the estuary mouth environment moved leaving the sand waves behind, but their proximity to the shelf edge may have helped preserve bed forms which otherwise are absent from the Middle Atlantic shelf.
Twichell, David Cushman Jr., "The Origin and Maintenance of Sand Waves Near the Head of Wilmington Canyon, Middle Atlantic Shelf, United States" (1978). Open Access Master's Theses. Paper 2041.