Detrital zircon U–Pb ages and Hf-isotopes from Eocene intermontane basin deposits of the southern Canadian Cordillera

Abstract

Lower Eocene strata exposed in the hinterland of the southern Canadian Cordillera were deposited in extensional–transtensional basins and provide insights into the evolution of the orogen. We measured stratigraphic sections and collected 22 sandstone samples for detrital zircon uranium–lead (U–Pb) geochronology (n = 2995) and hafnium isotope (εHf) values (n = 67) in order to reconstruct the Eocene physiography and sedimentary history of the region. Early Eocene sedimentation in the interior of the southern Canadian Cordillera occurred in alluvial fan, fluvial, lacustrine and paludal settings in multiple compartmentalized basins. In most locations, detrital zircons have ages of ca. 50 Ma, which are interpreted to be derived from the syndepositional Challis–Kamloops volcanics. Several locations have dominant detrital zircon populations with ages between ca. 145 and 200 Ma, suggesting distinct sediment sources and paleodrainages within the southern Canadian Cordillera. Maximum depositional ages record coeval sedimentation across the southern Canadian Cordillera hinterland, supporting the hypothesis that upper crustal deformation and basin subsidence occurred at the same time across the orogen interior. εHf(t) values of ca. 50 Ma and 145–200 Ma detrital zircons vary between −16 and +12, with more evolved εHf(t) values in the eastern portion of the study area and more juvenile εHf(t) values in the west. Differences in εHf(t) are interpreted to reflect crustal changes across the region, likely related to underlying Laurentian basement rock. Detrital zircons from intermontane basins may provide an additional tool for examining crustal heterogeneity in cordilleran orogens.