EO-/OLIGOCENE CASCADE TOPOGRAPHY REVEALED BY THE CLUMPED (Δ47) AND OXYGEN ISOTOPE (δ18O) RECORD OF THE CHUMSTICK BASIN (WA, USA)

The topographic evolution of mountain ranges is a key part of (paleo-)climate and tectonic studies of mountain building due to the fact that the surface elevation of a mountain range is a direct expression of the underlying lithospheric-scale processes and climate driven erosion. At the same time, topography exerts first-order control on precipitation patterns, for example, when orographic barriers divert or block atmospheric circulation trajectories and introduce orographic precipitation on the windward and rain shadow conditions on the leeward side of mountain ranges, thereby affecting hinterland exhumation rates and sediment fluxes to adjacent basins. Thus, understanding the long-term topographic evolution of the Pacific Northwest may be instrumental in determining the rate and timing of orogeny, as well as the geodynamic processes acting at depth.

Here we provide an oxygen isotope record of groundwater (δ¹⁸O_water) as preserved in terrestrial sedimentary sequences of the Eocene Chumstick basin (central Washington) and investigate the early Cenozoic development of Cascade topography and relief at the western margin of the North American Cordillera. A combination of our carbonate Δ₄₇-thernometry results (T(Δ₄₇)), field observations and microstructural analysis of the analyzed carbonates with vitrinite reflectance and low-T thermochronology data constrain the basin burial history and lead us to suggest that the T(Δ₄₇) and δ¹⁸O_carbonate values reflect open-system carbonate cement recrystallization in meteoric-derived groundwater during early burial diagenesis in the Eocene/Oligocene. Reconstructed mean δ¹⁸O_water values (-9 to -7 ‰) (based on T(Δ₄₇) and δ¹⁸O_carbonate data, assuming open-system behavior) thus reflect a time and area-integrated groundwater signal derived from local recharge and are consistent with low-elevation precipitation supplying the Chumstick basin groundwater system.

Evaluating the δ¹⁸O_water values in light of the paleogeographic setting of the basin, we suggest that the δ¹⁸O_water values reflect near-coastal Pacific-derived moisture that did not experience strong rainout across an orographic barrier. The absence of a rain shadow effect permits only moderate Eocene/Oligocene elevations at least for the southern part of the ancient Washington Cascades.