SAND-WAVE MORHOLOGY AND SEDIMENTARY PROCESSES IN THE VICINITY OF SIX MILE REEF, EASTERN LONG ISLAND SOUND

Digital terrain models from multibeam bathymetric data provide detailed base maps that yield new geologic perspectives and reveal unknown aspects of the dynamic seafloor. Interpretations presented here are based on NOAA hydrographic surveys H11361 and H11252, which cover approximately 156 sq. km around Six Mile Reef in eastern Long Island Sound. Six Mile Reef, a sandy east-west trending shoal located about 7.8 km off the Connecticut coast, has a core of post-glacial marine deltaic deposits mantled by tidally-reworked modern sediments.

Sedimentary environments off the eastern end of the shoal are characterized by processes associated with long-term erosion or nondeposition, a mobile-sediment-limited seafloor armored by gravelly sand, and scattered elongate fields of barchanoid sand waves. The barchanoid waves reach amplitudes of 20 m, are concave westward, and occur in individual and coalesced forms that progressively become more complex westward within the fields. The seafloor on and adjacent to the shoal is characterized by processes associated with coarse bedload transport and covered primarily with asymmetrical transverse sand waves. The transverse waves exceed 8 m in amplitude, have slip faces predominantly oriented to the west and southwest, and have straight, slightly sinuous, or curved crests. Megaripples, which mimic the asymmetry of the transverse waves, are commonly present on stoss slopes and in troughs; current ripples are ubiquitous. Amplitude and abundance of large bedforms decrease rapidly westward of the shoal. The seabed there is covered with small degraded ripples, reflecting lower energy environments and processes associated with sorting and reworking.

Megaripples and current ripples on the sand waves suggest that transport is active and that the waves are propagating under the present hydraulic regime. Net sediment transport is primarily to the west as evidenced by textural trends of surficial sediments, orientation of the barchanoid waves, and asymmetry of the transverse waves and of scour marks around bedrock outcrops, boulders, and shipwrecks. One exception occurs at the western tip of the shoal where bedform morphology indicates long-term transport is to the east, suggesting counter currents in this area help shape the shoal and are important to its maintenance.