Stratigraphy and Sedimentology of the Pipestone Canyon Formation, North Central Washington

Since Mesozoic time, the western margin of North America has grown through the addition of tectonic terranes, transported atop the subducting paleo-Pacific oceanic plate. The “Baja British Columbia” terrane model postulates accretion of the Insular superterrane at the latitude of southern Mexico and subsequent, margin-parallel translation north to juxtaposition with the western portion of the Intermontane superterrane. After amalgamation, the composite block continued north to its current location. The timing of the proposed translation has not bear well constrained. In the study area, the Okanogan Range batholith of the Intermontane superterrane’s Quesnellia terrane is juxtaposed with the sedimentary and volcanic deposits of the Insular superterrane’s Methow terrane across the Pasayten fault, the superterrane boundary. Deposited upon basement rocks of both superterranes are the Late Cretaceous non-marine conglomerate, sandstone and subordinate volcaniclastics of the Pipestone Canyon Formation. Based on geochemical comparisons between samples from the batholith and plutonic clasts from the formation, a significant proportion of the plutonic debris within Pipestone Canyon Formation strata is inferred to have been derived from the Okanogan Range batholith. The age of the formation, determined by palynology and a zircon fission-track date, is latest Campanian to early Maastrichtian. Therefore, the Pipestone Canyon Formation links the superterranes by 70 Ma and constrains the timing of any northward translation of the Insular superterrane and subsequent, post-amalgamation, northward movement of the Insular-Intermontane superterrane composite unit