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Geophysical studies and deep drill-hole data define the major structural elements and seismic stratigraphy for the U.S. Atlantic margin north of Cape Hatteras. Two basins, the Baltimore Canyon Trough (BCT) and Georges Bank Basin (GBB), and three adjacent platforms (the Long Island, Carolina, and La Have platforms) were formed during rifting of Africa from North America in the Early Jurassic. Postrift sediments, that is, those deposited after sea-floor spreading began, can commonly be distinguished from prerift and synrift sediments by a conspicuous "breakup" unconformity. Postrift sediments are as thick as 7 km in GBB and 13 km in the BCT. Prerift and synrift sediments within grabens are 5 to 9 km thick and are characterized by tilted reflection sequences and diffractions
The East Coast magnetic anomaly (ECMA) marks the landward edge of oceanic crust and a distinctive hinge zone in crystalline basement separates the continental platforms from the major basins. This hinge zone is mirrored by an abrupt shallowing seaward in the depth to the Moho; a further shoaling in depth to Moho also coincides with the ECMA. The rift-stage crust underlying the marginal basins is 12 km thick in BCT and 25 km thick in GBB.
The synrift sequence includes red beds and evaporites (including salt); the postrift sequence grades upward from massive Jurassic dolomites, limestones, and anhydrites into shale and clastic facies of Cretaceous and Tertiary age. Approximately 70% of the postrift sediment was deposited in the Jurassic during a period of rapid thermal subsidence and sediment loading.
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