PALEOSOLS IN 16.5 -15.3 MA GRANDE RONDE AND WANAPUM FORMATIONS, EASTERN COLUMBIA RIVER BASALT PROVINCE, USA: CORRELATIONS WITH THE MID-MIOCENE CLIMATIC OPTIMUM

Paleosols preserved in the upper Grande Ronde (16.5–15.6 Ma) and lower Wanapum (15.6-15.3 Ma) Formations of the eastern Columbia River Basalt Province (CRBP) record hiatuses in basalt emplacement and subaerial physical and chemical weathering. Nine paleosols studied thus far range from 58cm to 5m in thickness but show similar mineralogy. These weathering horizons formed on the tops of tholeiitic flood basalt flows during the Mid-Miocene Climatic Optimum, a brief (<2 million years) warming spike during the general global cooling of the Cenozoic. Ped formation, pseudoslickenlines, and in situ roots in these horizons are consistent with a pedogenic origin. The complete alteration of primary basalt minerals to smectite clays is a common feature of these paleosols, indicating that they were mineralogically similar to modern vertisols. The presence of kaolinite in some paleosol horizons suggests ultisol-like mineralogy and that mean annual precipitation varied spatially and temporally in the CRBP in the mid-Miocene. Presence of hematite throughout the weathering horizons and concretions of goethite in some horizons also suggest ultisol-like origins. The presence of both alfisol-like and ultisol-like properties in these horizons presents a possible record of a changing Miocene climate, from sub-humid temperate to humid warm. Although these horizons formed before the existence of high-elevation Cascade Mountains and the related orographic precipitation effects, potential climate and precipitation effects of the extremely low-albedo properties of the widespread flood basalts in the Miocene CRBP are considered.