The Continental Margin off the East Coast of North America: A Discussion

Abstract

ABSTRACT The original continental margin off the east coast of North America was formed mainly by the welding of a marginal orogen against the continental block and partially by rifting. Jurassic and post-Jurassic sediment upbuilding and outbuilding, and carbonate accretion modified the original continental margin into its present form. The continental rise north of Cape Hatteras and the Blake Outer Ridge south of the Cape are Cenozoic features developed by turbidity currents and bottom currents flowing parallel to the contours. Gravity tectonics have greatly altered the sedimentary framework of the rise north of Cape Hatteras. INTRODUCTION Continental margin9 are of interest to geologists because they form a transitional zone between continents and ocean basins, and as such are a key to our understanding of the origin and evolution of continents and oceans. Much of the marine sediment in the geologic column appears to have been deposited in environments similar to those existing at present in continental margins. Thus, data on processes active in the margins today could be used to decipher the history of ancient sediments. Of all the continental margins of the world the one off the east coast of North America has received more attention from geologists than any other comparable area in the world. The east coast margin has served as a laboratory to test many of the ideas as to the origin and evolution of margins, just as the Appalachians has served to test many of the concepts of geosynclinal development. SEDIMENTS AND TOPOGRAPHY Sedimentologically and topographically the continental margin off the east coast of North America can be divided into two segments. North of Cape Hatteras, N. C., bottom sediments are predominantly silicate terrigenous detritus. Sediments south of the cape are mainly carbonate in composition. The continental margin within the terrigenous province consists of a broad shelf flanked by a continental slope deeply dissected by submarine canyons [Fig.1]. East of New York the shelf is unusually deep and two U-shaped troughs, Northeast and Laurentian Channels, cut across the shelf to the top of the continental slope. The irregular topography of this section of the shelf is believed to be due to fluvial erosion followed by glacial erosion.33 Seaward of the continental slope is a broad sedimentary prism known as the continental rise. Most of the slope submarine canyons appear to have seaward extensions that almost cut the width of the rise. Off New England a series of seamounts known as the New England Seamount Chain extends across the rise to the abyssal plain and beyond. The continental margin within the carbonate province is much more complex than the margin within the terrigeneous province. The shelf immediately off the mainland coast is smooth and relatively narrow. Off southern Florida the shelf is separated from its seaward extension, the Bahama Platform, by the Straits of Florida. Off northern Florida, Georgia and the Carolinas the shelf is separated from the continental slop by an intermediate surface, the Blake Plateau, with an average depth of slightly less than 900 meters.28