Abstract

The Toroda Creek half graben is one of several north-northeast–trending Eocene extensional basins associated in time and space with the Okanogan and Kettle metamorphic core complexes in the southern Omineca crystalline belt of Washington and British Columbia. Sedimentary rocks of the middle Eocene Klondike Mountain Formation are the youngest stratified, nonvolcanic rocks in the Toroda Creek half graben. These sedimentary rocks preserve details of the character of Toroda Creek half-graben basin development in a humid temperate setting, help constrain the timing of brittle faulting in the basin, and provide insight into approximate rates of Okanogan core-complex uplift. Facies distributions and provenance determinations indicate that the Klondike Mountain Formation accumulated in a localized basin. There is no evidence that the Klondike Mountain Formation strata in the Toroda Creek half graben were once part of a more widespread deposit that later was segmented by faulting into its present configuration. Klondike Mountain sedimentary strata consist of sedimentary, gravity-dominated rock-avalanche, slide, and debris-flow deposits that accumulated on alluvial fans that delivered their sediments into either a single elongate lake or a series of possibly interconnected lakes. Provenance determinations and facies distribution patterns suggest that Klondike Mountain detritus primarily was derived from steep, unstable highlands exposed in the footwall of the Bodie Mountain fault on the east side of the basin. Sediments at the southern end of the basin were shed from an upland created by a west-northwest–trending transfer fault (Granite Creek fault). Rates of footwall uplift (and/or basin subsidence) estimated from unroofing of plutonic and presumed mylonitic rocks suggest rapid and significant basin margin uplift.

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