Recent Research on Passive Continental Margins
Recently available multichannel-seismic data have provided a detailed look at many Atlantic passive margins. DSDP holes and COST (Continental Offshore Stratigraphic Test) wells have provided geologic calibration. Reefal-carbonate bank underpinnings were involved in continental slope migration seaward of the original continental edge, especially in the Jurassic-Early Cretaceous. Tertiary erosion has caused a large landward retreat of the continental slope. These erosional events are nearly coeval on both sides of the Atlantic Ocean, indicating that they were caused by some basin- wide oceanographic change
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 3 under the slope and shelf.
Deep, 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 of the surface. Viscous creep in the lower continental crust appears necessary to account for the measured crustal thinning.
Detailed analysis of the multichannel reflection data permits sequence identification within the thick margin sediments. Sealevel 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 deepbasins without uplift or volcanism. Other margins are dominated by volcanism in the early stages, and outer ridge structures have formed. Others have been involved in ridge-jump early in the seafloor spreading history which isolated transitional crusts of dispersed volcanism and continental slivers.
Figures & Tables
At the present the Glomar Challenger has drilled over 500 holes over the world ocean, involving hundreds of scientists from dozens of countries. This volume is intended as a review of some of theimportant results from the most comprehensive, ambitious and successful earth-bound geologic project ever undertaken. The symposium upon which this volume originated was held April 4, 1979 at the SEPM/AAPG Annual Meeting in Houston. No comprehensive synthesis of all aspects of the DSDP has appeared, and the topic coverage in this volume is biased towards the sediments and fossils, and their significance for certain aspects of earth history – paleogeography, bathymetry, climatology, oceanography, ecology, environments – all in keeping with the audience of sedimentary geologists.