Canada's passive continental margins ? A review

Abstract

L. King (GSC II-10) renews the inspiring discussion initiated by Drake et al. (1959) which compares modern passive margins with ancient geosynclines. The comparison still remains very attractive despite the fact that classical geosynclinal nomenclature appears to become more and more untenable in the light of geophysical and subsurface mapping of presently preserved basins. This ought to present a challenge to students of ancient folded belts! If the comparison is valid, most ancient geosynclines should reveal a drifting and rifting phase of formation, as indeed has been shown by recent studies of Alpine geologists. The shelf margins of supposed miogeosynclines should be characterized by slope unconformities, growth faulting and conceivably by diapiric structures. A comparison of various types of unconformities and hiatuses seen on passive margins with those seen in reconstructed fold belt sequences could be most intriguing. Obviously modern oceanographic results combined with the results of offshore exploration demand that textbooks concerned with stratigraphic principles have to be rewritten. Maybe the oceanographers should search for explanations of the shelf margin anomalies by looking into ancient folded belts. For instance, what would the magnetic anomalies look like which are generated by the Proterozoic sills of the Purcell in the southern Canadian Cordillera if modeled on an adequate reconstruction? Little, if any, work has been done to compare and contrast the western Canadian Cordilleran geosyncline with the Atlantic shelf. Members of the same Canadian Survey and Canadian academic institutions work on both ancient and modern geosynclines. The time appears to be ripe to intensify the dialogue between continentologists and oceanographers. Returning to the two books here reviewed, we should congratulate our Canadian colleagues for a job well done, speedily published and with very substantial documentation. Obviously many problems remain, some of which were suggested, and we trust that they will pursue these and other problems with due diligence, vigor and joy.