Abstract
Our geologic mapping in concert with petrography, geochronology, geochemistry, geophysics, and stratigraphy of ten contiguous 7.5' quadrangles from North Bend to Granite Falls indicate that most Pleistocene nonglacial strata (PNS) have a distinct Cascade Provenance (CP) fluvial origin. Our correlation of this CP PNS with the Olympia beds (20-60 ka), Whidbey Formation (75-130 ka), Hamm Creek unit (178-243 ka), and pre-Hamm Creek PNS units are supported by 42 14C and 44 OSL/IRSL ages and by stratigraphic relations. The CP PNS is distinctly compositionally and stratigraphically similar to Holocene Snoqualmie, Skykomish and Pilchuck River (SSPR) alluvium.Point count petrography of CP PNS sands reveals a granitic source containing mostly monocrystalline quartz, plagioclase, K-spar, hornblende, pyroxene, mica and granitic lithic grains. The granitic CP is largely ascribed to Quaternary erosion of the Eocene-Miocene Snoqualmie, Index and Mount Pilchuck batholiths in the upper SSPR basins and is supported by detrital zircon age data for the PNS. Higher Pb/Yb ratios for the CP PNS and Holocene SSPR sediments also distinguish glacial and CP sediments. Most CP PNS basin sediments (e.g. Monroe basin) have a distinctive modest magnetic susceptibility due to the inclusion of granite source oxides such as magnetite, which we use to track basin axes and extent. Fault and fold geometry indicate local Quaternary structural control of the SSPR basins by the Monroe, Carpenter Creek, and southern Whidbey Island fault (SWIF) zones. Local thick sequences of CP PNS were deposited in actively subsiding tectonic basins such as the Monroe synclinal basin north of the Monroe reverse fault. In the SWIF, the CP PNS were deposited in small, strike-slip basins then inverted into active growth folds such as the Tolt River anticline. Some extensional PNS basins, such as the Explorer Falls basin, which is bounded by the active Carpenter Creek reverse fault, were later uplifted to form inverted basins. Regional CP PNS sand petrography reveals Pleistocene SSPR alluviation into the lowlands as far W to NW as central Whidbey Island and Lofall/Quilcene areas, respectively. The apparent restriction of the CP PNS north of the Seattle fault may reflect structurally controlled, Pleistocene SSPR alluviation into the Seattle basin north of the Seattle uplift.