SAND RIDGE DEPOSITS ON EASTERN NORTH AMERICAN CONTINENTAL SHELF

A series of bedforms, such as subaqueous sand ridges, sand ribbons between ridges, sand dunes and sand waves is well developed on the inner and outer shelves along the east coast of the United States from Long Island of New York, via Maryland, New Jersey to Florida, as a part of the lagoonal shore. Statistics show that there are 35 ridges at the water depth ranging from 20 to 80 meters off New Jersey. The ridges are about 2 to 11 km long, 1 to 4 km wide, with a length/width ratio between 2:1-3:1. Comparing to length/width ratio of 40:1 of the typical sand ridges in the world,they belong to the type of wave-dominated sand ridges with short axes. They are usually 1-3 m in height and stretching in the direction of NE 10°-30°. The ridges are asymmetric and the upstream slope is relatively gentle whereas the downstream slope is relatively steep with a dipping around 2.5-7.0 According to the data from more than 20 drilling vibracores and ¹⁴C measurements on the "Fortune Ridge" at the water depth from 34 to 48 meters, the sand ridges have a three-layered structure from bottom to top:(1) continental plain facies before 9-11 ka BP;(2)lower ridge sand unit in 9-5 ka BP;(3) upper ridge sand unit since 5 ka BP. The upper layer of sand ridges now keep moving downstream toward SE direction with a speed of 1-2 m/a determined by the local positioning observation.