Structure of the North American Atlantic Continental Margin
The use of multichannel seismic-reflection profiles to study oceanic areas has advanced our understanding of deep crustal structure and the history of its development. Off eastern North America, where the structure of the continental margin is essentially constructional, seismic profiles have approximated geologic cross sections up to 10–15 km below the sea floor and revealed major structural and stratigraphic features that have regional hydrocarbon potential. These features include (a) a block-faulted basement hinge zone; (b) a deep, broad, rifted basement filled with clastic sediment and salt; and (c) a buried paleoshelf-edge complex that has many forms. The mapping of seismo-stratigraphic units over the continental shelf, slope, and rise has shown that the margin's developmental state included infilling of a rifted margin, buildup of a carbonate platform, and construction of an onlapping continental-rise wedge that was accompanied by eroison of the slope. Correlation of seismic stratigraphy with well-log biostratigraphy reveals that the area experienced a gradual rise in sea level during the Mesozoic and an episodic sea-level fall during the Cenozoic. These long-term changes formed a subdued continental shelf-slope-rise transition in the early Tertiary and a steepened one in the Quaternary. Comparison of seismic and drill-hole data has also shown margin-wide unconformities similar to those shown by the Vail curve of coastal onlap and eustatic sea-level change, i.e., unconformities that provide markers to subdivide the sequences of reflections on the seismic-reflection profiles. Further, the termination of older key reflectors against oceanic basement gives us a corroborative check on the age of oceanic crust as determined from the magnetic anomalies produced by sea-floor-spreading.