Geologic Implications of Abyssal Bed Forms along the East Coast Continental Margin

Submarine bed forms in abyssal regions were identified shortly after deep-water cameras obtained photographs in the deep ocean in the late 1940´s and 50´s; larger bed forms were recognized shortly after the advent of precision acoustic echo sounding in the 1950´s. However, it was not until a series of landmark papers by Bruce Heezen and his students and colleagues at the Lamont-Doherty Geological Observatory that a general framework was developed to explain the distribution of abyssal bed forms: deep currents flowing along the boundaries of the major ocean basins were Inferred to transport fine-grained sediments long distances throughout the abyssal ocean and to sculpt the sediment surface. Subsequent identification of near-bottom maxima in suspended concentration and direct measurement of abyssal currents generally supported the initial hypothesis, although it rapidly became apparent that flow near the bottom was neither always steady nor simply predicted. During the 1970´s our understanding of the types of bed forms on the abyssal sea floor and their distribution expanded dramatically as a result of detailed studies utilizing near-bottom towed acoustic sensors, extensive photography, and direct observation from deep-diving submersibles. We are now aware of a series of bed forms ranging in size from a few centimeters (ripples, lineations, streamlined obstacle marks) to a few meters (longitudinal triangular ripples, furrows), a few kilometers (large sediment waves - both asymmetric and symmetric), and tens to hundreds of kilometers (sediment drifts or ridges which parallel the major current path- ways). However, our understanding of physical controls on bed form development and the time scales involved is still in a preliminary stage. Extensive studies will be required to develop and verify models which accurately relate flow characteristics to bed form types. The first comprehensive approach to this problem is the High Energy Benthic Boundary Layer Experiment (- - HEBBLE) which has brought together a diverse group of engineers, geologists, physical oceanographers, and biologists to examine in detail the relationship between fine- scale surface morphology and boundary-layer flow in a region of strong bottom currents. Our increased knowledge and understanding of abyssal bed forms will ultimately allow us to interpret the fine- scale structural/stratigraphic relationships in continental margin sequences as well as the types and variability of present-day sedimentary processes in the deep sea.