The evolution of the Canadian Cordilleran eugeosyncline can be understood in terms of five rock sequences or assemblages. Three of these, Laberge, Bowser, and Sustut: and Georgia Assemblages, fulfill the requirements of epieugeosynclines (or successor basins) as defined by Marshall Kay very closely.
The Laberge Assemblage (Lower and Middle Jurassic) was derived from an island complex having a granitoid core and was probably deposited within a mobile, marginal oceanic basin.
The Bowser Assemblage (Upper Jurassic and Lower Cretaceous) was derived from a mobile orogenic core made up of rocks as old as Precambrian and as young as the Laberge Assemblage. The Bowser sediments were probably deposited within a marginal basin underlain by crust transitional in character between oceanic and continental.
The Sustul: and Georgia Assemblages (Upper Cretaceous and Paleogene) are characterized by extensive continental clastics that were deposited in structurally controlled basins underlain by incompletely cratonized continental crust.
Intracrustal spreading of sialic material is invoked to explain the progressive continentalization during the evolution of the successor basins. The transition from predominantly marine deposition (Bowser Assemblage) to predominantly nonmarine deposition (Sustut Assemblage), seems to coincide with a time of peripheral cooling of metamorphic complexes and the emplacement of large discordant plutons. The transition from nonmarine deposition to regional erosion coincides with shallow-level plutonism, explosive volcanism, and uplift of the high-grade metamorphic core zones. It is suggested that the rate of these transitions is directly related to the magnitude of shortening of sialic crust and to the magnitude of the isostatic anomaly resulting therefrom.
Figures & Tables
Modern and Ancient Geosynclinal Sedimentation
The Kay Conference was held in Madison, Wisconsin, November 1972. This symposium volume contains the texts of papers presented at Madison. It is organized in a topical manner, and in most areas of discussion, modern analogues and ancient examples together provide a comparative basis for evaluating sedimentary models for geosynclines. In the 1970s students of both modern and ancient sediments have compiled an immense body of knowledge relevant to the geosynclinals concept. Moreover, the new theory of plate tectonics has required a complete reassessment of the geosynclines as well as orogenesis. The purpose of this volume is to evaluate by comparison of modern and ancient sediments a number of depositional models applicable to the great variety of strata seen in orogenic belts also called geosynclinal.