Diachronous isotopic and sedimentary responses to topographic change as indicators of mid-Eocene hydrologic reorganization in the western United States Malinda L. Kent-Corson, n Andreas Mulch, w Stephan A. Graham, z Alan R. Carroll, ‰ Bradley D. Ritts z and C. Page Chamberlain k n Department of Earth Sciences, Bridgewater State College, Bridgewater, MA, USA wInstitut fur Geologie, Universitat Hannover, Hannover, Germany zDepartment of Geological and Environmental Sciences, Stanford University, Stanford, CA, USA ‰Department of Geology and Geophysics, University of Wisconsin, Madison,WI, USA zChevron EnergyTechnology Company, San Ramon, CA, USA k Department of Environmental Earth System Science, Stanford University, Stanford, CA, USA ABSTRACT Early 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 re£ect changes in meteoric water composition due to simple orographic rainout.The transition to low-d 18 O 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 ¢ll 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. INTRODUCTION Over the past decade, many studies have demonstrated the utility of stable isotopes of terrestrial sedimentary de- posits in reconstructing the paleoelevation history of mountain belts and plateaus (e.g. Chamberlain & Poage, 2000; Garzione et al., 2000, 2006, 2008; Rowley et al., 2001; Horton et al., 2004; Graham et al., 2005; Horton & Chamberlain, 2006; Kent-Corson et al., 2009). Although these studies have met with some success, they often lack the constraints in chronology and sedimentary evolution to fully relate elevational histories to the topographic, cli- matic and tectonic evolution of the associated orogens. These problems can partly be overcome in the intermon- tane basins of the western North American Cordillera, where there are robust chronologic constraints (e.g.Tab- rum et al., 1996; Smith et al., 2003, 2008) and detailed stu- dies of basin evolution (e.g. Surdam & Stanley, 1980; Dickinson et al.,1988; Carroll et al., 2006) that allow a more complete understanding of what drives stable isotopic changes in lacustrine and £uvial environments. EAGE Correspondence: Malinda L. Kent-Corson, Department of Earth Sciences, Bridgewater State College, Bridgewater, MA 02325, USA. E-mail: mkentcorson@bridgew.edu Basin Research (2010) 22, 829–845, doi: 10.1111/j.1365-2117.2009.00456.x r 2010 The Authors Basin Research r 2010 Blackwell Publishing Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists 829