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Examination of approximately 3,000 km of multichannel seismic-reflection profiles collected over the U.S. Middle Atlantic Shelf (Cape Hatteras to Long Island) shows that the margin was rifted prior to the separation of Africa and North America and that it broadly subsided after the separation. Major accumulation of sediment is centered in the Baltimore Canyon Trough, where more than 15 km of Triassic(?) and younger sedimentary rocks has been deposited. The trough is asymmetric in that the thickest sedimentary section is beneath the outer part of the shelf seaward of a hinge zone; further, the trough is wider on the north off New Jersey than it is seaward of Virginia. Flanking the trough are two block-fault platforms (Long Island and Carolina) where thickness of the sedime tary cover is 1 to 4 km.
Eight depositional sequences have been delineated in analysis of the profiles by use of the techniques Vail and others have presented in AAPG Memoir 26. Marked changes in reflection characteristics and unconformities are used to bound the sequences. The oldest sequence (Late Triassic-Early Jurassic) is thought to be nonmarine and restricted-marine sediments associated with a complexly faulted crust. The changing seismic facies of younger sequences chronicle the transition to more open marine shelf and slope paleoenvironments during the Cretaceous and Tertiary. The thickest units accumulated during a phase of rapid basin subsidence in the Jurassic; sediment accumulation decreased substantially during the Late Cretaceous and Cenozoic so that the sequences are thinner and unconformities re more evident. The most conspicuous unconformities seismically are correlated with the Albian, Turonian-Coniacian, Late Cretaceous-Paleocene, Oligocene, and late Miocene-early Pliocene, by correlation with COST (Continental Offshore Stratigraphic Test) and coastline wells. Several of these breaks coincide with pronounced fluctuations in the Vail curve of relative sea level, particularly during the Tertiary, when indications of falling sea level were widespread. Lastly, the records confirm the presence of a carbonate platform along the seaward edge of the trough during the Jurassic and Early Cretaceous. The stages of formation of the continental margin here are similar to those of the margin off the Canadian Maritime Provinces.
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