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

Abstract

AbstractThe 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 (δ18Owater) 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 δ18Owater 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 δ18Owater 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.