Eocene-Oligocene proto-Cascades topography revealed by clumped (Δ₄₇) and oxygen isotope (δ¹⁸O) geochemistry (Chumstick Basin, WA, USA)

The topography of the present-day Washington Cascades impacts atmospheric circulation and precipitation patterns in the Pacific Northwest, introducing a pronounced orographic rain shadow in the lee of the mountain range. The temporal development of Cascade topography, however, remains largely unconstrained for the early Cenozoic. Based on coupled carbonate clumped isotope (Δ₄₇) and oxygen isotope (δ¹⁸O) measurements we reconstruct δ¹⁸O values of Eocene groundwater (δ¹⁸O_water) in the Chumstick basin (central Washington), today located in the Cascade rain shadow. Δ₄₇ (paleo)thermometry indicates a systematic change in basin burial temperatures from 110°C to 70°C depending on burial depth in the basin. These data are in good agreement with low-T thermochronological and vitrinite reflectance data, and further constrain the basin burial and exhumation history. In concert with field observations, microstructural analysis, and δ¹⁸O values of the analyzed carbonates, we suggest that the Δ₄₇ temperatures and δ¹⁸O values reflect open-system carbonate cement recrystallization in meteoric-derived groundwaters during early burial diagenesis. Assuming open-system behavior, reconstructed mean δ¹⁸O_water values of ~ -7‰ (middle Eocene) to -9‰ (late Eocene/early Oligocene) are consistent with a low-elevation origin of the corresponding meteoric waters that permeated the sandstone/conglomerate members of the Eocene sedimentary units. In light of the paleogeographic setting of the Chumstick basin, the reconstructed δ¹⁸O_water values agree well with Pacific-derived moisture that did not experience strong rainout. The absence of a rain shadow effect therefore permits only moderate Eocene/Oligocene elevations at least for the southern part of the Washington proto-Cascades.