MULTI-METHOD SEDIMENT PROVENANCE ANALYSIS OF MID-CRETACEOUS STRATA IN THE METHOW BASIN, NORTHERN WASHINGTON STATE AND SOUTHERN BRITISH COLUMBIA

Numerous accretion events and abundant Cenozoic deformation complicate interpretation of Cretaceous Cordilleran paleogeography. The Methow basin is situated between the Insular and Intermontane superterranes in northern Washington State and southern British Columbia, and its Cretaceous strata contain a record of terrane accretion that may help better constrain the Cretaceous paleogeography of the Pacific northwest region. Integrated results from detrital zircon U-Pb age dating and Hf analysis, sandstone petrography, and whole-rock geochemistry indicate that the source of east-derived sediment in the Methow basin was characterized by Middle to Late Jurassic and Early Cretaceous magmatic arcs of intermediate composition and juvenile isotopic character.

Turbidites of the Harts Pass Formation and partially-correlative Jackass Mountain Group record a consistent provenance signal that suggests homogenization of sediment derived from an unchanging source region. The Harts Pass Formation samples show little variability on major element, trace element, and incompatible/compatible element plots. Sandstone framework grain composition analysis of the Harts Pass Formation also shows little compositional variability (Q_55-70F_30-45L₀). Samples from the overlying fluvial Winthrop Formation are broadly distributed across major element, trace element, and incompatible/compatible element plots, and exhibit a greater range of QFL values. These data, coupled with detrital zircon U-Pb dating and Hf analysis, suggest a changing source area for the Winthrop Formation and/or little homogenization during sediment transport.

Although a unique source area cannot be unequivocally determined, the southern Canadian Cordillera provides the best match for these provenance results, given the close correlation in the age and available epsilon Hf values between Jurassic and Cretaceous detrital zircon in the Methow Basin and plutons found in the southern Canadian Cordillera. In addition, the geochemical and petrographic character of east-derived Methow rocks is similar to terranes currently located east of the basin in the Intermontane superterrane. These provenance results suggest that the Methow Basin underwent limited displacement (<1000 km) since deposition near the southern Canadian Cordillera.