EVIDENCE FOR MINOR POST-DEPOSITIONAL DISPLACEMENT OF THE METHOW BASIN, NORTHERN WASHINGTON: DETAILED PROVENANCE ANALYSIS OF THE HARTS PASS FORMATION

Detrital zircon U-Pb age and Hf analysis of the Cretaceous Methow Basin of northern Washington, combined with sandstone petrography, characterizes the basin’s sources and permits evaluation of the extent of post-depositional basin translation. The Harts Pass Formation, a mid-Albian west–directed marine turbidite sequence within the Methow basin, has a remarkably uniform provenance signature throughout deposition. Detrital zircon U-Pb ages from nine samples (844 grains) range from 180 to 105 Ma with peaks at 160 Ma and 116 Ma, and detrital zircon epsilon Hafnium values range from +5 to +17 with a weighted average of 8.5 ± 0.6 for Jurassic grains, and +3 to +16 with a weighted average of 9.0 ± 0.7 for Cretaceous grains. Sandstone petrography records a basement uplift provenance (Q₃₉F₆₀L₁) with no evidence of an active volcanic arc shedding sediment into the basin.

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 age and available epsilon Hf values between Jurassic and Cretaceous detrital zircon in the Harts Pass and plutons found in southern Canadian Cordillera. Other possible source regions include a combination of the southern Canadian Cordillera, Klamath Mountains, and Blue Mountains, or a combination of the Peninsular Range and Sonoran plutons in Mexico. Well-documented west-directed paleocurrent indicators in the Harts Pass Formation would require the basin to be west of a Klamath Mountains source, putting it a significant distance removed from additional sources in the southern Canadian Cordillera and the Blue Mountains. If the Harts Pass provenance included the Peninsular Range, then it should contain more volcanic lithic grains similar to the Early Cretaceous Alisitos Formation that was shed off the active volcanic arc of the Peninsular Range during the Early Cretaceous. Therefore, provenance results from the Harts Pass Formation suggest that the Methow Basin has not undergone a significant amount of post-depositional translation, and that the most likely sources for the Harts Pass are in the southern Canadian Cordillera, perhaps in combination with the Blue Mountains.