“Basement” to East Coast Continental Margin of North America

A reference surface is defined as the top of seismic layers of velocity greater than 4.5 km/sec., and referred to as “basement.” The configuration of this surface of the East Coast continental margin of North America is a series of basins, linear arches, and escarpments which have controlled the thickness and facies distribution of the lower velocity Cenozoic and later Mesozoic sediments. Velocities within basement cluster in two groups, less than and greater than about 5.6 km/sec. High-velocity basement can be equated with certainty with crystalline bedrock only in some nearshore areas of the inner shelf. Much of the Lower Cretaceous and Jurassic section beneath the outer margin has velocities in both the high-and the low-velocity ranges, and in many areas these deeper strata have been mistaken for true crystalline basement. Low-velocity basement represents a great range of lithologies and ages, including low-grade meta-morphic rocks, Mississippian strata, lower Paleozoic platform cover, Lower Cretaceous-Jurassic sedimentary rocks, and layer 2 of the ocean basin. High-velocity basement under the continental rise between the Scotian shelf and the Carolinas has velocities mostly in the range of oceanic layer 3. Few measurements are in the range of normal upper continental crust. Between the Blake Plateau and the eastern Grand Banks, a broad basement arch underlying the outer shelf and a secondary, more subdued, arch beneath the upper continental rise are formed on low-velocity basement; both are probably sedimentary features. Basement underlying the secondary arch, and seaward of it, has velocities characteristic of layer 2, but is much thicker and may be a complex mixture of sediments and lavas which has subsided several kilometers since the region was the shallow margin to a young riff ocean. North of the New England seamounts the secondary arch appears to underlie the “ridge complex” observed by seismic profiler, which in turn follows the magnetic “slope anomaly.” A series of deep basins beneath the shallow-water areas along the length of the margin has similar maximum thicknesses of sediment fill, consistently 7–9 km (23,000–30,000 ft) of section.