IMPLICATIONS OF BOWSER BASIN AND SKEENA FOLD BELT GEOLOGY FOR EVOLUTION OF THE MESOZOIC-CENOZOIC COAST MOUNTAINS OROGEN, NORTH AMERICAN CORDILLERA: THE VIEW FROM THE EAST

The deposition and deformation of basinal strata bounding core zones of orogenic belts are vital records of an orogen's evolution. The Bowser Basin and Skeena Fold Belt occur immediately east of the central Coast Mountains orogen (CMO). They formed in the Jurassic to earliest Cretaceous and the Early to latest Cretaceous, respectfully, and therefore provide insight on the Mesozoic evolution of the CMO.

The Bowser Basin succession is at least 5 km thick. It stratigraphically overlies Stikinia, recording marine to nonmarine clastic deposition in an overall regressive sequence where depositional facies more proximal to source migrated west through time. Deposition was on the west coast of North America from late Middle Jurassic through earliest Cretaceous. The distribution and migration of facies belts, paleocurrents, and clast types indicate north, northeast, east, and southeast sources. Cumulative probability plots of over 750 zircon grains from samples across the temporal, stratigraphic, and geographic limits of the basin are consistent with sources in Stikinia, Cache Creek, and Quesnellia. The depositional history of Bowser Basin does not support or negate models that require the presence of a volcanic arc associated with the CMO west of Bowser Basin in late Middle Jurassic to earliest Cretaceous time, but does require that if an arc was present, it was distant enough to not directly influence deposition in the basin.

Bowser Basin strata were deformed in the Cretaceous to form the thin skinned Skeena Fold Belt. The fold belt is mainly northeast vergent and has an associated synorogenic clastic succession on its northeast side. Structures accommodated at least 50% horizontal shortening, involve at least the upper parts of Stikinia, and are interpreted to have rooted in the CMO. Domains of north northeast-trending structures in the western fold belt resulted from Early Cretaceous sinistral plate convergence in the vicinity of the CMO.

The Skeena Fold Belt is the most widespread expression of a belt of east vergent structures that probably bounded the length of the eastern CMO. These structures are interpreted to be kinematically associated with structures that accommodated crustal thickening in the core of the CMO during the mid Cretaceous, coeval with the formation of west vergent structures on the west side of the CMO.