OXYGEN ISOTOPE EVIDENCE FOR THE RAPID LATE MIOCENE DEVELOPMENT OF THE RAIN SHADOW ON THE EAST SIDE OF THE WASHINGTON CASCADE RANGE

The Cascade Range now forms an extensive climate boundary in the Pacific Northwest. A mild and humid climate predominates west of the range, and a cooler and drier climate prevails to the east due to a massive orographic rain shadow effect. Distinct pedogeneses between forest soils on the west and desert and grassland soils on the east are obviously associated with the disparate vegetation on each side. However, geological and paleofloral evidences indicate that a temperate and humid climate, similar to that in the southeastern U.S. today, extended to the east side of the Washington Cascades in Miocene time. The timing of climate change in eastern Washington is poorly understood. Therefore, we have examined the climate change and the timing of the development of the rain shadow effect in eastern Washington using the oxygen isotopic composition of authigenic clay minerals from paleosols preserved in the series of stratigraphic sediments interbedded within the Miocene Columbia River Basalt (~ 9-16 Ma) and in the Neogene Ringold Formation (3.4-8.5 Ma?). Smectite d¹⁸O values are 11.5 ± 0.13 per mill for the Vantage Member (15.6 Ma), 11.3 ± 0.22 per mill for the Squaw Creek Member (15 Ma), 11.9 ± 0.24 per mill for the Ellensberg Formation (12-15 Ma), and 10.9 ± 0.22 per mill for the Rattlesnake Ridge Member (11 Ma). Also, kaolinite d¹⁸O values are 11.0 ± 0.22 per mill for the Squaw Creek Member (15 Ma), 11.0 ± 0.07 per mill for the Ellensberg Formation (~ 11 Ma), and 9.5 ± 0.16 per mill for the Ringold Formation (~ 4 Ma). These data show a systematic and temporal isotopic decrease of ~ 3.5 per mill in these authigenic clay minerals. Therefore, the climate in eastern Washington probably started to transform from a sub-tropical to a semi-arid/desert climate regime in late Miocene time (8-11 Ma). This isotopic shift is attributed to variation over time of the stable isotopic composition in meteoric water due to significant uplift of the Washington Cascades in late Miocene time, producing an orographic barrier to Pacific-derived precipitation. The uplift history of the Washington Cascades examined by the oxygen isotope evidence from authigenic clay minerals is synchronously correlated to the timing of their rapid uplift as determined by apatite and zircon (U-Th)/He and apatite fission-track ages.