The molasse facies of the Columbian orogen, Canadian Cordillera

Abstract

The authors present a tentative model for the relationship between deformation, uplift and molasse sedimentation in the Columbian Orogen of the Canadian Cordillera. Data are mainly drawn from the north-central part of the Columbian Orogen. Two distinct deformational episodes are recognized. The first, probably of mid-Jurassic age, was accompanied by regional metamorphism and resulted in southwesterly directed folds and related faults. This episode of crustal shortening appears to have been the precursor of uplift in the western Columbian Orogen (Omineca Crystalline Belt), causing the progradation of a primogenic non-marine molasse over marine shale and flysch. The second deformation, of probable late Cretaceous to Eocene age, caused overprinting of earlier structures by steep faults in the west and led to the growth of the fold-and-thrust belt of the Rocky Mountains. This episode created a morphogenic drainage system with molasse deposition in intramontane fault basins and along both flanks of the Columbian Orogen. Modal analyses from fluviatile sandstones within the primogenic and morphogenic molasse illustrate the progressive stripping of sedimentary and volcanic units along both sides of the orogenic core zone. Post-orogenic drainage from Miocene to Recent was strongly modified by the development of high-gradient drainage towards the Pacific Ocean.