GEOLOGICAL INTERPRETATIONS OF A LOW-BACKSCATTER ANOMALY FOUND IN 12-KHZ MULTIBEAM DATA ON THE U.S. MID-ATLANTIC LOWER CONTINENTAL SLOPE

An anomalous low-backscatter area on the mid-Atlantic lower continental slope between the Hudson and Baltimore channels was recently mapped with a 12-kHz multibeam echo-sounder (MBES). The low-backscatter area is approximately 5000 km2 in size and is located at water depths ranging from 3500 to 4000 m. Analysis of compiled MBES data, 3.5 kHz chirp profiles and sediment cores suggest several working hypotheses for the change in backscatter intensity in this region. One possible hypothesis is that the low-backscatter anomaly results from sediment erosion and redistribution due to deep water currents. Truncated subsurface reflectors in 3.5-kHz seismic profiles collected across the low-backscatter region show surficial erosion and sub-bottom unconformities. MBES bathymetry also shows sediment piling and removal at the base of Knauss Seamount, a bathymetric feature located nearby the low-backscatter anomaly. Another explanation for the low-backscatter anomaly is the presence of high gas content in the subsurface sediments. Chirp profiles collected outside of the low-backscatter area show well-stratified sub-bottom reflectors up to 60 m beneath the surface. However, within the anomaly, seismic profiles show very little stratification and weakly reflective layers, possibly indicative of seismic blanking due to gas. A third working hypothesis is that the low-backscatter area is related to downslope sediment deposition. Several small submarine channels located directly upslope of the low-backscatter area are shown by MBES bathymetry. Furthermore, a linear, high-backscatter feature that transects the low-backscatter region aligns with a disruptive, acoustically transparent reflector in several seismic profiles, suggesting a gravity-driven failure. A piston core located in the high-backscatter linear feature indicates silty-clay sediments with intermittent coarse-silt and fine sand layers. However, future sampling efforts are still needed to further ground-truth the sonar.