Sedimentary and stratigraphic evidence of Eocene extension in the Okanogan Highlands, Washington

Middle Eocene sedimentary and volcanic rocks in the Okanoogan Highlands, north-central Washington and southernmost British Columbia are preserved in a series of extensional basins that formed during emplacement of the Okanogan and Kettle core complexes. The middle Eocene basin-fill is divided into three stratigraphic units which are, from oldest to youngest, the O'Brien Creek Formation, the Sanpoij Volcanics, and the Klondike Mountain Formation. Field and laboratory data gathered from the Klondike Mountain Formation in the Toroda Creek and First Thought half-grabens and the Republic graben suggest these strata originated in isolated basins rather than in a single, widespread downsag which subsequently was dissected by extensional faulting, Infilling of these basins was controlled by regional extensional faulting and local volcanism. Sedimentary, stratigraphic, and sedimentary-tectonic evidence permit interpretation of middle Eocene basin evolution and paleogeography. Analysis of sedimentary units in the Klondike Mountain Formation reveal lithofacies assemblages of dominantly mixed lacustrine, debris flow, and rock fall (slide) origins. Lacustrine lithofacies in the Republic graben thicken towards the western graben-margin fault suggesting synchroneity between basin development and infilling. Petrographic and whole-rock geochemical analyses of coarse-grained Klondike Mountain sediments demonstrate varied source terrains for each basin. Klondike Mountain strata in the Republic graben weremore » derived from source rocks within the graben as well as from horst rocks peripheral to the graben. Compositional trends in the strata from the Toroda Creek half-graben document unroofing of 50 Ma epizonal plutons and high-grade metamorphic rocks of the eastern margin of the Okanogan core complex.« less