NEW CONSTRAINTS PROVIDED BY PROVENANCE STUDIES ON THE DEPOSITIONAL HISTORIES OF THE BOWSER AND SUSTUT BASINS, AND THEIR IMPLICATIONS FOR TECTONIC EVOLUTION OF THE NORTHERN CANADIAN CORDILLERA

The Bowser and Sustut basins are located in north-central British Columbia, in the Intermontane Belt of the Canadian Cordillera and overlie Devonian to early Middle Jurassic strata of allochthonous Stikinia. The Bowser Basin records Middle Jurassic to Jura-Cretaceous marine to nonmarine deposition on the western margin of North America and the Sustut Basin records mid- to latest Cretaceous nonmarine deposition. Provenance studies have been initiated to constrain the age and sources of basin strata, in order to understand the depositional histories of the basins. The Bowser Basin, which consists of a huge volume of clastic rocks, has been previously divided into lithofacies assemblages that interfinger and overlap in age. A suite of sandstone to siltstone samples from nonmarine to distal marine lithofacies assemblages was collected for U-Pb detrital zircon geochronology. The samples, which range from Callovian to early Cretaceous in age, were collected from localities where the age of the unit is narrowly and confidently constrained based on macrofossils. The youngest population of detrital grains in each sample dominates the detrital zircon dataset from that rock. The age of the youngest population of zircons in each sample is within error of the depositional age of that rock as determined by good macrofossil control. Thus, there are shifts in the dominant-age populations corresponding to the depositional ages of the basin rocks. This dataset suggests the presence of volcanism of late Middle Jurassic to earliest Cretaceous age depositing detritus into the eastern basin as it was being erupted. The older detritus in all of the Bowser samples is of similar age, Triassic through Middle Jurassic, and is most likely derived from Stikinia and/or Quesnellia, in addition to the Cache Creek source recognized by the presence of radiolarian chert clasts. Detrital zircon analyses from the Cretaceous Sustut Basin range in age from Archean to Cretaceous, suggesting derivation from numerous sources, including the Omineca belt, in contrast to the Bowser basin rocks which show no evidence of an older Omineca belt source. Constraints on the depositional histories of these basins provide insight on the tectonic evolution of the northern Canadian Cordillera.