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Paleogene landscape evolution of the central North American Cordillera: Developing topography and hydrology in the Laramide foreland Steven J. Davis † Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA Andreas Mulch Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA Institut fur Geologie, Universitat Hannover, 30167 Hannover, Germany Alan R. Carroll Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706, USA Travis W. Horton Department of Geological Sciences, University of Canterbury, Private Bag 4800, Christchurch, New Zealand C. Page Chamberlain Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA ABSTRACT Isotopic and elemental records of authi- genic calcite from lacustrine deposits in the intraforeland basins of Utah were analyzed in an effort to reconstruct the regional paleocli- mate, paleohydrology, and paleotopography of the early Cenozoic central North Ameri- can Cordillera. Isotopic pro les for Paleo- gene Lakes Uinta, Flagstaff, and Claron show relatively large oxygen isotopic shifts that are diachronous among basins with an ~7‰ decrease in δ 18 O calcite values at ca. 45 Ma in Lake Flagstaff, an ~5‰ decrease in δ 18 O calcite values between ca. 42 and 35 Ma in Lake Cla- ron, and an ~6‰ decrease in δ 18 O calcite values between ca. 44 and 43 Ma in Lake Uinta. We interpret these negative oxygen isotopic shifts to be the combined result of increased hypsometric mean elevation of basin catch- ments and related freshening associated with basin in lling. The basins studied also have undergone periods of intense evaporation during periods of hydrologic closure, which, for example, produced an ~7‰ increase in δ 18 O calcite values in Lake Uinta beginning at ca. 51 Ma. Hydrologic closure in the Uinta Basin probably resulted from growth of local topography that diverted previously substan- tial in ows from low elevation regions within the foreland. This study adds to the growing body of E-mail: sjdavis@stanford.edu evidence that suggests a pattern of along- strike variations in the timing of topographic development and dissection of the Cordille- ran landscape during the early Cenozoic. We favor an interpretation that calls for the mid- dle Eocene rearrangement of regional drain- age patterns such that intraforeland basins that once received waters from far- ung foreland river systems became dominated by in ows of low δ 18 O waters from catchments with higher hypsometric mean elevations that drained the adjacent hinterland and/or basin-bounding uplifts. This drainage reor- ganization is analogous but subsequent to the large-scale integration of catchments in the northern Cordillera that has been recog- nized on the basis of isotopic and sedimen- tological evidence in Montana and Idaho at ca. 50–47 Ma, with rivers  owing southeast into Lake Gosiute at ca. 49 Ma and then for a time reaching Lake Uinta, causing a promi- nent highstand in that lake at 48.6 Ma. The negative oxygen isotopic shifts presented herein, which occurred between ca. 45 and ca. 35 Ma in the intraforeland basins of Utah, may re ect the north-to-south progression of drainage integration in the Cordillera as mag- matism and related topography swept south- ward through the hinterland and increased the hypsometric mean of catchments that fed subjacent intraforeland basins. Keywords: isotope ratios, lacustrine sediments, Paleogene, paleotopography, paleolimnology, Laramide orogeny, Sevier hinterland, foreland basin, North American Cordillera. INTRODUCTION Despite profound relationships and feedbacks among climate, topography, and tectonics (e.g., Molnar and England, 1990; Ruddiman and Kutz- bach, 1990), paleoenvironmental reconstructions of evolving orogens remain dif cult and contro- versial. Patterns in the distribution of stable oxy- gen and hydrogen isotopes in precipitation and surface waters have long been used as a tool for the study of hydrological and surface processes, but studies of geological proxies have generally focused on isotopic variation caused by changes in atmospheric circulation and/or surface eleva- tion (e.g., Chamberlain et al., 1999; Chamberlain and Poage, 2000; Garzione et al., 2000; Garzione et al., 2006; Rowley and Currie, 2006). In this study, we reconstruct trends in the oxygen iso- topic composition of lake and river water (δ 18 O lw and δ 18 O rw , respectively) of the Uinta, Flagstaff, and Claron Basins of Paleogene Utah (Fig. 1), and make use of several geochemical techniques and proxies to argue that the observed trends were controlled by changing drainage patterns in the evolving late Laramide Cordillera and atten- dant changes in basin hydrology. Precipitation is progressively depleted in 18 O and D during cooling of air masses that travel along surface-temperature gradients or over orographic barriers (Dansguard, 1964; Ambach et al., 1968; Rozanski et al., 1993). Empirically GSA Bulletin; January/February 2009; v. 121; no. 1/2; p. 100–116; doi: 10.1130/B26308.1; 8  gures; Data Repository item 2008140. For permission to copy, contact editing@geosociety.org © 2008 Geological Society of America