Diachronous isotopic and sedimentary responses to topographic change as indicators of mid‐Eocene hydrologic reorganization in the western United States

Abstract

ABSTRACTEarly Cenozoic terrestrial deposits in the western United States represent well‐preserved archives of climatic and tectonic processes that together shaped the Earth's surface during the demise of a large continental plateau. This study examines a Cenozoic terrestrial sedimentary sequence in the central part of the Cordilleran orogen (Montana) using sedimentologic and geochemical techniques. At ∼49 Ma, we observe rapid major shifts in oxygen, carbon and strontium isotope records that are too large to directly reflect changes in meteoric water composition due to simple orographic rainout. The transition to low‐δ18O values in pedogenic carbonate in concert with changes in the composition of clastic material at ∼49 Ma points to the input of evolved meteoric water to the hydrological cycle due to a change in the source of waters reaching Cordilleran intermontane regions in southwestern Montana. This drainage reorganization coincides with the initiation of magmatism and extension to the west in what is now Montana and Idaho. The sedimentological record shows evidence that depositional gradients increased in the study area ∼46 Ma, ∼3 Myr after the drainage reorganization occurred. This interval is most likely the time it took for extensional deformation to propagate to the study area itself. Evidence of freshening events in Laramide Basins to the southeast suggests that this drainage reorganization diverted waters to progressively fill these basins and highlights the impact of post‐plateau extension‐related landscape reorganization on river networks and lake dynamics. This study also emphasizes the importance of using multiple tools in deciphering topographic history through the study of terrestrial basin deposits, in that interpretation based on any single method employed would have compromised our ability to successfully identify the regional evolution of topography and drainage networks.