Richard N. Benson
Delaware Geological Survey, University of Delaware, Newark, Delaware
Seaward-dipping reflectors (SDRs) on offshore seismic reflection profiles of the Atlantic continental margin of North America are assumed to represent volcanic and volcaniclastic rocks that accumulated within a few m.y., and along with underlying intrusive and underplated rocks they are considered to be the major source of the East Coast Magnetic Anomaly (ECMA). SDRs are associated with the final breakup of Pangea and have been considered nearly contemporaneous with the formation of the earliest oceanic crust imaged on seismic reflection profiles as the J3 escarpment. Extrapolations of seafloor-spreading rate lines for the western Atlantic basin based on the most recent Jurassic time scales give Bathonian-age estimates of 160-165 Ma for J3 and 161-166 Ma for the ECMA. The only eastern North American offshore sedimentary basins in which nearly the entire stratigraphic sections have been drilled and dated paleontologically are the Scotian and Georges Bank basins. Three groups of rocks have been recognized in these basins: two before continental breakup are rocks of the rifting stage separated by the postrift unconformity (PRU) from rocks of the postrifting stage; the third group of rocks was deposited during the drifting stage following continental breakup and is separated from the previous two by the breakup unconformity (BU). CAMP basalts of early Hettangian age (~200 Ma) are related in time to the PRU whereas SDRs of Bajocican-Bathonian age are associated with the BU. On seismic profiles across the Baltimore and Carolina troughs and the Blake Plateau Basin, these two unconformities may be coincident beneath the rocks of the drifting stage. Seaward near the continental edge, the SDRs overlap the PRU and synrift rocks of the rift basins and are separated from the overlying drifting stage rocks by the BU.