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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
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Recently available multichannel-seismic data have provided a detailed look at many Atlantic passive margins. DSDP holes and COST wells have provided geologic calibration. Reefal-carbonate-bank underpinnings provided a mechanism for slope migration seaward of the original continental edge, especially in the Jurassic-Early Cretaceous. Tertiary defacement by erosion and mass-wasting has caused large landward retreats of the slope. These events are nearly coeval on both sides of the Atlantic Ocean, indicating their significance.
Deeper crustal layers are identified on the modern reflection and refraction data. Intermediate seismic-velocity layers, 7.1 km/sec, near the continental edge on both sides of the Atlantic might be characteristic of transitional-type crusts, or merely continuations of layer 3b under the slope and shelf.
Deep-cutting, listric normal faults are observed where the soles of the faults merge into a lower crustal layer (6.3 km/sec velocity). Thinning with listric faulting of apparent continental crust has brought the mantle (8.2 km/sec) to within 14 km depths. Viscous creep in the lower continental crust appears necessary to account for the measured thinning.
Detailed analysis of the multichannel data permits sequence identification within the thick margin sediments. Sea-level cycles can be identified, and correlations reveal the configuration of genetically related stratigraphic units. Such analyses define the subsidence history and paleobathymetry of the margins. Some passive margins start with an uplift and rifting phase, whereas others are rifted through previous deep basins without uplift or volcanism. Other margins are dominated by volcanism in the early stages, and outer-ridge structures have formed.
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