SEDIMENTARY PATTERNS AT RIDGE AND SWALE BATHYMETRY, INNER CONTINENTAL SHELF, VIRGINIA, U.S.A.

Abstract

Abstract A computer generated three-dimensional plot of 220 sq km of the Virginia inner continental shelf revealed three distinct bathymetric zones, not detectable on navigation charts. The survey area is characterized by broad low-relief plains, “ridge and swale” regions, and “dome and swale” regions. Low-relief plains are featureless areas of the sea floor that gently dip seaward at 0.03 to 0.06 degrees. Ridge and swale regions have linear trends in bathymetry with up to 6 m of local relief and 2 to 3 degree slopes. The dome and swale regions are differentiated from the ridge and swale areas by the lack of well-defined parallel ridges. Bathymetrically high areas stand out as individual mounds or domes. Detailed studies were made at a ridge and swale area located 30 km east of the Chesapeake Bay entrance. Three ridges in a 3.4 sq km survey area are spaced approximately 0.75 km apart and have a maximum relief of 5.5 m. Each of the ridges is transected by shallow submarine depressions that are subnormal to the trends of the ridges. As a result, the ridge crests are undulating surfaces formed by linear trending domes and the shallow depressions. Spatial distribution of five lithosomes in the study area corresponds with bathymetrically distinct portions of the ridge and swale system. The five areas are described as dome crests (along the ridge axis), cross-ridge depressions and upper-ridge margins, lower-ridge margins, swale floors, and scour troughs. The characteristics of these lithosomes suggest that the surface deposits are not relict, but are due to active marine processes. Grain size increases between the swale floor and the ridge crest. The only exceptions to this trend are coarse deposits at scour troughs on the swale floors. Surface sediments on domes along the ridge crests contain primary sedimentary structures in a coarse sand lag. Cross-ridge depressions contain primary sedimentary structures in fine sand, indicative of both bedform migration and storm-related sand drapes. The presence of bioturbated fine sand along the adjacent ridge-face slope, between the cross-ridge depressions, suggests that these areas are relatively stable.