PLIOCENE ISOLATION OF THE KITTITAS VALLEY FROM THE COLUMBIA BASIN AND IMPLICATIONS FOR UPLIFT OF THE HOG RANCH-NANEUM ANTICLINE

The eruption and widespread emplacement of the mid Miocene Columbia River Basalt Group filled paleo-topography in the Columbia River basin, effectively reset much of the landscape and established a uniform baselevel by ~15 Ma. Tectonic and fluvial processes have since reshaped the landscape dramatically. In central Washington state, the north-south trending Hog Ranch-Naneum anticline (HRNA) is a broad, high amplitude structure that bisects the Columbia Basin and forms the drainage divide between the Yakima and Columbia Rivers. The HRNA intersects and is orthogonal to several Yakima fold structures, which have been mainly active since 3-2.5 Ma. To constrain the timing of uplift along the HRNA, we collected terrestrial sandstone samples interbedded and overlying the Columbia River Basalt Group within the Kittitas Valley watershed, located west of the HRNA. We analyzed U-Pb detrital zircon age spectra for these samples and compared them with coeval stratigraphic horizons east of the HRNA, older locally exposed sandstones, and known sediment sources that deliver sediment into the Yakima and Columbia Rivers. Our provenance results identify a distinct change in sediment source into the Kittitas Valley between mid-Miocene and Pliocene time. Sedimentary horizons interbedded in the Grande Ronde member of the Columbia River Basalt Group (16 – 15.6 Ma) include zircon sources that are distally derived from the Idaho-Wyoming thrust belt and exposed in the modern upper Snake River watershed. Several of the distinctive distally sourced zircon populations are absent from locally exposed Cenozoic strata and thus are not recycled from older units. Suprabasaltic (post-5 Ma) sedimentary horizons, on the other hand, only contain older zircon age populations that are recycled from Eocene Swauk and Chumstick formations, units that are located within the Kittitas Valley watershed. These results, along with depositional age estimates from sampled strata, indicate that the Kittitas Valley was isolated from the Columbia River basin before ~3.7 Ma. Thus, the HRNA became an agent of drainage division by the Pliocene, prior to the onset of major uplift along the nearby Yakima folds.