SEDIMENTARY AND DETRITAL ZIRCON DATA FROM EOCENE STRATA EXPOSED IN THE HINTERLAND OF THE SOUTHERN CANADIAN CORDILLERA: IMPLICATIONS FOR BASIN EVOLUTION IN A COLLAPSING OROGEN

Eocene sedimentary strata in southern British Columbia (BC) and northern Washington contain a record of orogenic collapse within the southern Canadian Cordillera hinterland. These sedimentary units consist primarily of clast- and matrix-supported cobble and pebble conglomerates, sandstones, and mudstones, which were deposited in fluvial and lacustrine environments within one or multiple intermontane basins. Here, we present new sedimentary and detrital zircon (DZ) uranium-lead (U-Pb) and Hafnium (Hf) data from Eocene strata exposed in the southern Canadian Cordillera in an effort to better understand sediment provenance, reconstruct the degree of interconnectedness between basins, and determine areas of exhumation within the orogen. Conglomerate and sandstone exposed in Princeton and Merritt, BC contain detrital zircons with U-Pb ages between ca. 50 to 400 Ma, with large populations of ca. 155 Ma grains. Detrital zircon U-Pb ages from Eocene strata exposed in Republic, Washington, and Kelowna, BC contain large populations of ca. 50 Ma grains. Zircons in the Princeton and Merritt area of BC are interpreted to have been derived from Mesozoic plutons that were exposed in local fault-bounded ranges, whereas the ca. 50 Ma zircons in the Republic and Kelowna areas are interpreted to be associated with widespread Paleogene volcanism. Different U-Pb populations between the areas suggest localized sediment provenance and the presence of multiple, discrete basins during the Eocene. Hafnium isotopes (εHf) of detrital zircons vary between 14 and -16 across the southern Canadian Cordillera: Merrit contains detrital zircons with εHf of 14 to -3, suggesting a juvenile sources; Republic contains detrital zircons with εHf values of -7 to -16, suggesting derivation from evolved sources; and Kelowna contains detrital zircons with εHf values of 14 to -10, indicating both evolved and juvenile sources. The εHf values are controlled by their location with regards to the boundary between North American crust and accreted terranes. The differences in εHf values between depocenters further supports a hinterland occupied by multiple, isolated basins.