Finding the Plateau: Paleogene Topography and Basin Formation in the Cordilleran Hinterland

Abstract:

In the US Cordilleran hinterland, Paleogene terrestrial strata independently constrain relative topography and provide information on the local depositional environment and regional drainage morphology necessary for interpreting stable isotope paleoaltimetry data. Volcanic glass hydration waters from fluvially interbedded ignimbrites record increasing δD values eastward from the paleo-coast, indicating that the region maintained demonstrably higher than modern elevations (3.5 ^+.7/_-.3 km) through the Paleogene. This orogen had a distinct crest and continuous westward-draining slope extending to central California. To the east of this crest, Eocene strata consist of basal fluvial gravel conglomerate ≤ 200 m thick, overlain by interbedded fine-grained lacustrine strata up to 1 km thick, which record hydrologic closure. Conglomerates and sandstones display structures indicative of deposition primarily in gravel-bed braided rivers. Overlying interbedded laminated marl, siltstone, and carbonaceous mudstone containing abundant fossil flora, gastropods, and ostracods indicate a deep freshwater lacustrine depositional environment. Up section, alternating laminated kerogen-rich marl and calcareous mudstone, mud-cracked siltstone, stromatolites, and an absence of bioturbation record anoxic, saline bottom waters and fluctuating profundal to evaporative conditions. Tephras interbedded with lake strata have highly enriched δD values due to hydration by evaporative lake waters, supporting the interpretations that depositional environment controls δD values and that fully hydrated glass resists alteration on geologic timescales. Eocene stratigraphy indicates increased accommodation, continental drainage ponding, and basin closure. It is unlikely that local geomorphic damming provided the regionally extensive driver to create these deep, high elevation closed basins. Instead, migrating regional construction of topographic barriers resulting from the onset of extension and/or broad dynamic and thermal uplift associated with Farallon slab rollback likely drove basin formation. Late Oligocene δD values from fluvially interbedded tephras suggest < 500-900 m of uplift following rollback, while stratigraphy indicates a coincident westward shift in the locus of deposition.