Abstract

Basin analysis of Albian–Cenomanian rocks of the Tyaughton and Methow basins suggests that the two basins were filled by three principle petrofacies during a regionally significant contractional event. The Volcanic petrofacies, which occurs only in the Tyaughton basin, comprises west-derived volcaniclastic strata. This petrofacies is dominated by intermediate volcanic clasts and minor metavolcanic, sedimentary, and plutonic clasts. This petrofacies and correlatives to the west (Taylor Creek volcanics and the Gambier Group) are inferred to have been a volcanic cover and volcaniclastic apron to the Insular terrane.The Cherry petrofacies occurs in both the Tyaughton and the Methow basins. Paleocurrents suggest that these chert-rich sediments were shed both east and west off a topographic high that separated the two basins. The Cherry petrofacies is dominated by chert-lithic detritus with subordinate sedimentary and volcanic rock fragments. Locally, the petrofacies is characterized by clasts of chert, greenstone, serpentinite, and blueschist; all these lithologies are common in the unconformably underlying Bridge River terrane, which is inferred to have been the dominant source terrane.Rocks that contain the Arkosic petrofacies are 3–8 km thick in the Methow basin, but only a thin unit is present in the Tyaughton basin. This petrofacies is rich in quartz and feldspar, with lesser quantities of volcanic lithic and metamorphic lithic fragments. Detrital muscovite, which is the hallmark of this petrofacies, is interpreted to have been derived from granitic and metasedimentary rocks to the east. This petrofacies is interpreted to have been largely derived from the Omineca Crystalline Belt, which was rapidly uplifting during this time. The three petrofacies in the two basins provide the first provenance link between the Insular terrane to the west to what was then North America (Omineca Crystalline Belt and the Intermontane Belt) to the east; juxtaposition is inferred to have occurred in a contractional setting.

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