STRUCTURE OF THE CONTINENTAL SLOPE OFF THE EASTERN UNITED STATES

Abstract

The continental slope off eastern United States is built across a zone of deeply buried 6 12 km fault blocks that formed during the rifting of continents Associated with these blocks are intrusives salt diapirs and reef complexes all are underlain by a zone of thinned continental transitional and oceanic crust beneath the shelf slope and upper continental rise Four basic sedimentary units appear to underlie much of the slope I a probable terrigenous c1astic evaporite volcanic sequence associated with fault blocks 2 a carbonate platform reef sequence which probably formed along the ancestral shelf slope break 3 a marine and non marine sequence built over the carbonate rocks north of the Blake Plateau as the shelf in some areas prograded seaward during the Cretaceous and 4 a thick rise prism of sediments of Tertiary and younger age that laps up on the base of the slope north of the Blake Spur Much of the present slope is an erosional surface cut into lower Tertiary and Upper Cretaceous strata during Cenozoic marine regressions as sediment moved downslope to build the rise prism the shelf slope break has retreated 5 30 km from the break that existed during the Late Cretaceous Over the Blake Plateau periodic scour by currents associated with the ancestral Gulf Stream caused a 300 km landward jump of the shelf break at about the end of Paleocene as the plateau continued to subside Laterally the slope varies in the distance that it retreated maximum to the south and in the degree to which reeflike masses are developed Continuous well developed carbonate banks or reefs controlled location of the shelf edge of the southern Blake Plateau they were terminated both there and to the north near the end of the Early Cretaceous although reefformation may have shifted to locations beneath the present continental shelf Local progradation of the outer shelf and slope occurred during parts of the Jurassic and Cretaceous depending on the sediment supply and the effects of transgression and regression of sea level During the Tertiary however erosion and retreat of the shelf edge was dominant