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Geochemistry and basin analysis of Laramide Rocky Mountain basins

ProQuest Dissertations and Theses, 2009
Dissertation
Author: Majie Fan
Abstract:
The Laramide Rocky Mountains in western U.S.A is an important topographic feature in the continental interior, yet its formation and evolution are poorly constrained. This study uses the oxygen and strontium isotope geochemistry of freshwater bivalve fossils from six Laramide basins in order to reconstruct the spatial evolution of the paleotopography and Precambrian basement erosion in late Cretaceous-early Eocene. In addition it uses the sedimentology, detrital zircon U-Pb geochronology, and isotope paleoaltimetry of early Eocene sedimentary strata to constrain the tectonic setting, paleogeography and paleoclimate of the Wind River basin. Annual and seasonal variation in ancient riverwater δ 18 O reconstructed from shell fossils shows that the Canadian Rocky Mountains was 4.5±1.0 km high in late Cretaceous-early Paleocene, and the Laramide ranges in eastern Wyoming reached 4.5±1.3 km high, while the ranges in western Wyoming were 1-2 km high in late Paleocene. The 87 Sr/86 Sr ratios of riverwaters reconstructed from the same fossils show that Proterozoic metamorphic carbonates in the Belt-Purcell Supergroup were not exposed in the Canadian Rocky Mountains during Late Cretaceous-early Paleocene, but that Precambrian silicate basement rock was exposed and eroded in the Laramide ranges during late Paleocene-early Eocene. The sedimentary environment of the early Eocene Wind River basin changed from gravelly fluvial and/or stream-dominated alluvial fan to low-sinuosity fluvial systems. Tectonic uplift of the Washakie and Wind River Range in early Eocene formed the modern paleodrainage system, although the elevation of the basin floor was only ~500 m high at that time, and early Eocene paleoclimate is more humid than modern climate.

6 TABLE OF CONTENTS LIST OF FIGURES………………………………................................………………...8 LIST OF TABLES……………………………............................……....................…...10 ABSTRACT...............................................................................................................…...11 CHAPTER 1: INTRODUCTION.........................................................................…......12 CHAPTER 2: LATE PALEOCENE HIGH LARAMIDE RANGES IN NORTHEAST WYOMING: OXYGEN ISOTOPE STUDY OF ANCIENT RIVER WATER............................................................................................................................17 ABSTRACT ..........................................................................…….............................17 INTRODUCTION ...........................................................…….................................18 REGIONAL SETTING.................................................................…........................20 ANALYTICAL METHODS...........................................…......................................21 OXYGEN ISOTOPE RESULTS....................................………………………......22 CALCULATION OF ANCIENT RIVER WATER δ δδ δ 18 O VALUES.....................23 CONSTRAINTS, CORRECTIONS, AND APPLICATION TO PALEOALTIMETRY..........................................................…………………….....24 SEASONAL δ δδ δ 18 O VARIATION IN ANCIENT AND MODERN RIVER WATERS.......................................................…………………………………....….29 DISCUSSION.............................................................……………………………....33 High Canadian Rocky Mountains in Late Cretaceous………………...…..…33 High Surface Elevation of Eastern Laramide Ranges During Paleocene.......35 Lower Surface Elevation of the Western Laramide Province before Early Eocene...................................................................................................................37 Implications..........................................................................................................38 CONCLUSIONS........................................................................................................40

CHAPTER 3: WIDESPREAD BASEMENT EROSION IN LATE PALEOCENE- EARLY EOCENE IN THE LARAMIDE ROCKY MOUNTAINS INFERRED FROM 87 SR/ 86 SR RATIO OF BIVALVE FOSSILS....................................................71 ABSTRACT................................................................................................................71 INTRODUCTION.....................................................................................................72 GEOLOGICAL SETTING AND STRONTIUM SOURCE TERRANES...........74 STUDY AREA AND FIELD SAMPLING..............................................................76 ANALYTICAL METHODS.....................................................................................77 RESULTS...................................................................................................................78 Modern River Water and Shell..........................................................................78 Fossil Shell ......................................................................................................... .80 DISCUSSION.............................................................................................................81 Modern River System..........................................................................................81

7 TABLE OF CONTENTS - continued

Ancient River System and Basement Erosion...................................................83 Powder River Basin ........................................................................................83 Washakie Basin................................................................................................85 Other Basins.....................................................................................................87 The Cause of Positively Correlated 87 Sr/ 86 Sr ratios and δ 18 O values..............88 Implications......................................................................................................90 CONCLUSIONS........................................................................................................92

CHAPTER 4: SEDIMENTOLOGY, DETRITAL ZIRCON GEOCHRONOLOGY, STABLE ISOTOPE GEOCHEMISTRY OF THE LOWER EOCENE STRATA IN THE WIND RIVER BASIN, CENTRAL WYOMING..............................................106 ABSTRACT..............................................................................................................106 INTRODUCTION ..................................................................................................107 REGIONAL GEOLOGY .......................................................................................109 Stratigraphy and Age Control..........................................................................110 Tectonic Setting..................................................................................................112 SEDIMENTOLOGY...............................................................................................113 Indian Meadows Formation: Alluvial Fan Association..................................114 Wind River Formation: Braided River Association.......................................115 SANDSTONE PETROGRAPHY AND PROVENANCE ...................................117 Methods and description...................................................................................117 Interpretation.....................................................................................................118 DETRITAL ZIRCON U-Pb GEOCHRONOLOGY............................................119 Samples and Methods............................... ........................................................119 Results.................................................................................................................121 Interpretation.....................................................................................................122 STABLE ISOTOPE GEOCHEMISTRY..............................................................124 Methods...............................................................................................................124 Results.................................................................................................................125 Evaluation of Diagenesis...................................................................................126 Oxygen Isotopes and Paleoaltimetry................................................................127 Carbon Isotopes, Paleoclimate and pCO 2 ........................................................131

REGIONAL PALEOGEOGRAPHY.....................................................................133 IMPLICATIONS FOR TECTONICS....................................................................135 Rapid Late Paleocene-Early Eocene Uplift of Basement-Cored Ranges......135 Post-Early Eocene Regional Uplift...................................................................137 CONCLUSIONS......................................................................................................139

WORKS CITED............................................................................................................178

8 LIST OF FIGURES

Figure 2.1. Shaded relief map of central and Canadian Rocky Mountains.......................42 Figure 2.2. Generalized stratigraphic columns of sedimentary successions......................43 Figure 2.3. Measured and modeled δ 18 O values of ancient river water............................44 Figure 2.4. Seasonal δ 18 O variation of representative fossil shells....................................45 Figure 2.5. Seasonal δ 18 O variation of three kinds of modern river to the east of Rocky Mountains..........................................................................................................................47 Figure 2.6. Regression for sampling station latitude and the δ 18 O values of river water in low-elevation stations within the USA..............................................................................48 Figure 2.7. Estimated paleoelevation of the Canadian and Laramide Rocky Mountains..49 Figure 2.8. Paleodrainage reconstruction of the studied area............................................50 Figure 2.9. Schematic cross-sections showing the mechanism of forming high Laramide ranges.................................................................................................................................51 Figure 3.1. Simplified geological map of the Laramide Rocky Mountains showing Sr source terranes. .................................................................................................................94 Figure 3.2. Generalized lithostratigraphic columns of studied sedimentary successions in studied basins. ...................................................................................................................95 Figure 3.3. Maps of modern river watershed and Geology...............................................96 Figure 3.4. Diagrams of 87 Sr/ 86 Sr ratio vs. Sr concentration of river water.......................98 Figure 3.5. Seasonal variation of the δ 18 O values and 87 Sr/ 86 Sr ratios of modern bivalve Unionids collected in the Tongue River............................................................................99 Figure 3.6. Diagrams of 87 Sr/ 86 Sr ratio vs. δ 18 O value.....................................................100 Figure 3.7. Diagram of 87 Sr/ 86 Sr ratio vs. Sr/Ca of modern and ancient rivers...............101 Figure 3.8. Inferred drainage patterns and Precambrian basement exposure in late Cretaceous-early Paleocene, and late Paleocene-early Eocene....………………...........102 Figure 4.1. General map of the United States, Wyoming and northwestern Wind River basin............... .................................................................................................................142 Figure 4.2. Chronostratigraphic chart for the northwestern Wind River basin...............143 Figure 4.3. Measured sections.........................................................................................144 Figure 4.4. Photographs of northwestern Wind River basin outcrops.............................147 Figure 4.5. Ternary diagrams showing sandstones compositions....................................148 Figure 4.6. Photos of petrographic thin sections of sandstone .......................................149 Figure 4.7. Photos of the zircon grains............................................................................150 Figure 4.8. U/Pb concordia diagrams..............................................................................151 Figure 4.9. U-Pb age-probability diagrams......................................................................152 Figure 4.10. U/Pb concordia diagrams for a granite pebble............................................154 Figure 4.11. Field photos and images of thin sections of the early Eocene carbonate nodules.............................................................................................................................155 Figure 4.12. Results of the stable isotope analyses in this study.....................................156 Figure 4.13. Simplified stratigraphy, carbon and oxygen isotope values, percentage of granite clasts, Achaean zircons, and feldspar through section.........................................157 Figure 4.14. Oxygen isotope data of Early Eocene paleosol carbonate...........................158

9 Figure 4.15. Paleogeographic sketch maps of the northwestern Wind River basin .......159

10 LIST OF TABLES

Table 2.1. Age constraints for the study intervals in each basin........................................52 Table 2.2. Paleomagnetic data and paleolatitude...............................................................53 Table 2.3. δ 13 C and δ 18 O values of bulk individual shells and sample locations..............54 Table 2.4. δ 13 C and δ 18 O values of micromilled representative shells..............................61 Table 3.1. Element concentration, and corrected strontium isotope ratios for selected fossil shells.......................................................................................................................103 Table 3.2. Sampling location, isotope ratios, element concentration data for modern rivers................................................................................................................................104 Table 3.3. Isotope data for modern and fossil shell.........................................................105 Table 4.1. Lithofacies and interpretations used in this study .........................................160 Table 4.2. Modal petrographic point-counting parameters.............................................161 Table 4.3. Modal petrographic data................................................................................162 Table 4.4 U-Pb (zircon) geochronologic analyses by laser-ablation multicollector ICP mass spectrometer ...........................................................................................................163 Table 4.5 Major age populations of detrital zircons in modern river sand and the early Eocene Eediment.............................................................................................................175 Table 4.6 Isotope results.................................................................................................176

11 ABSTRACT

The Laramide Rocky Mountains in western U.S.A is an important topographic feature in the continental interior, yet its formation and evolution are poorly constrained. This study uses the oxygen and strontium isotope geochemistry of freshwater bivalve fossils from six Laramide basins in order to reconstruct the spatial evolution of the paleotopography and Precambrian basement erosion in late Cretaceous-early Eocene. In addition it uses the sedimentology, detrital zircon U-Pb geochronology, and isotope paleoaltimetry of early Eocene sedimentary strata to constrain the tectonic setting, paleogeography and paleoclimate of the Wind River basin. Annual and seasonal variation in ancient riverwater δ 18 O reconstructed from shell fossils shows that the Canadian Rocky Mountains was 4.5±1.0 km high in late Cretaceous-early Paleocene, and the Laramide ranges in eastern Wyoming reached 4.5±1.3 km high, while the ranges in western Wyoming were 1-2 km high in late Paleocene. The 87 Sr/ 86 Sr ratios of riverwaters reconstructed from the same fossils show that Proterozoic metamorphic carbonates in the Belt-Purcell

Supergroup were not exposed in the Canadian Rocky Mountains during Late Cretaceous-early Paleocene, but that Precambrian silicate basement rock was exposed and eroded in the Laramide ranges during late Paleocene-early Eocene. The sedimentary environment of the early Eocene Wind River basin changed from gravelly fluvial and/or stream-dominated alluvial fan to low-sinuosity fluvial systems. Tectonic uplift of the Washakie and Wind River Range in early Eocene formed the modern paleodrainage system, although the elevation of the basin floor was only ~500 m high at that time, and early Eocene paleoclimate is more humid than modern climate.

12 CHAPTER 1: INTRODUCTION The eastern portion of the Cordilleran orogenic belt in the western interior U.S.A. consists of the Laramide structural province, a region of Precambrian basement-cored uplifts and intervening sedimentary basins that developed during late Cretaceous-Eocene time (e.g., Dickinson and Snyder, 1978; Bird, 1988; Snoke, 1997; DeCelles, 2004). The deformed region partitioned what had been a continental scale foreland basin that developed east of the Cordilleran thrust belt ~1,000-1,500 km inland from the subduction zone. Modern regional elevation of the Laramide province is ~1.5 km with the range summits at >4 km. By analogy, a similar landscape is present in the Sierras Pampeanas in South America (e.g., Jordan et al., 1983; Wagner et al., 2005). Shallow subduction of the Farallon Plate underneath North America is commonly accepted as the tectonic mechanism for the Laramide deformation (e.g., Coney and Reynolds, 1977; Dickinson and Snyder, 1978; Bird, 1988; Constenius, 1996; Saleeby, 2003; Sigloch et al., 2008). However, it remains unclear how shallow subduction would produce the individual Laramide structures and the extent to which Laramide deformation may be viewed as an eastward propagation of the greater Cordilleran strain front (Erslev, 1993; DeCelles, 2004). The timing of individual Laramide uplifts, their paleoelevation at the time of uplift, and the temporal relationships among Laramide uplifts are essential to the reconstruction of the regional deformation pattern, tectonic modification after deformation, and the evaluation of tectonic models. The temporal and spatial patterns of erosional exhumation of the Precambrian basement cores in Laramide uplifts are critical to understanding the

13 processes leading to the modern landscape. Tectonic models explaining the deformation mechanism during flat slab subduction include basal shear traction (Bird, 1988), lateral injection of intracrustal flow from the overthickened Sevier orogenic hinterland (McQuarrie and Chase, 2001), and lithospheric buckling in response to horizontal endload of the North American plate (Tikoff and Maxson, 2001). Post-Laramide modification of the lithosphere in the Laramide region may have played a vital role in shaping the modern landscape. Mechanisms of modification include thermal uplift caused by removing the subducted slab or thickened mantle lithosphere in in the western U.S.A. (Dickinson and Snyder, 1978; Humphreys, 1995; Sonder and Jones, 1999), subcontinental-scale subsidence by induced asthenospheric counterflow above the subducted slab (McMillan et al, 2002; Heller et al., 2003; McMillan et al., 2006), and regional uplift caused by isostatic rebound of lithosphere due to climate driven erosion and/or thermal upwelling associated with the initiation of Rio Grande Rift (Heller et al., 2003; McMillan et al, 2006). Therefore, sufficient quantitative data including paleoelevation, source terrane unroofing, basement exhumation, and paleogeography within a precise chronological context are required to 1) reconstruct the uplift and basement erosion pattern during and after Laramide deformation, and 2) evaluate existing competing hypotheses. This investigation focuses on oxygen isotope and 87 Sr/ 86 Sr ratios of Late Cretaceous- early Eocene river water reconstructed from Unionid shell fossils in six Rocky Mountain basins and sedimentary strata of the Wind River basin in early Eocene, central Wyoming. The goals are to: 1) provide the paleoelevation and Precambrian basement erosion history

14 of several Laramide uplifts in Wyoming during late Cretaceous-early Eocene time; and 2) reconstruct the early Eocene paleogeography and tectonic setting of the Wind River basin. The data within this dissertation include, but are not limited to, >2000 stable isotope analyses of carbonate and water; >80 87 Sr/ 86 Sr ratio analyses of carbonate and water; ~750 m of detailed measured stratigraphic sections; 211 paleocurrent measurements collected at 18 locations; 1108 clast counts from 11 locations; 16 point counted petrographic thin sections and >700 U-Pb analyses of zircon grains collected from six early Eocene sandstones, two modern river sands, and one Precambrian granite cobble. The following chapters represent three manuscripts that are in various stages of publication. Chapter 2, Late Paleocene High Laramide Ranges in Northeast Wyoming: Oxygen Isotope Study of Ancient River Water, presents the timing and distribution of high elevation in the Rocky Mountains during Late Cretaceous - early Eocene by tracking the oxygen isotope ratios of ancient river waters. The oxygen isotope ratios of ancient river waters are extracted from the δ 18 O values of well-preserved shell aragonite of the Unionid family. Both mean annual and seasonal stable isotope record of ancient river water in the Alberta foreland basin, and five other Laramide basins in Wyoming are presented. The patterns of low oxygen isotope values in river waters show that the Canadian Rocky Montanans were at high elevations in late Cretaceous-early Paleocene, and Laramide ranges reached high elevation in the eastern Laramide province by late Paleocene. Interestingly the Laramide ranges in the western province seem to achieve

15 high elevation later than the eastern ones. This manuscript is submitted to Earth and Planetary Science Letters. Chapter 3, Widespread Basement Erosion in Late Paleocene-early Eocene in the Laramide Rocky Mountains inferred from 87 Sr/ 86 Sr ratio of bivalve fossil, confirms that the 87 Sr/ 86 Sr ratio of modern river water in the Rocky Mountains are controlled by river bedrock lithology by examining the modern river water strontium geochemistry in the Wind River and Powder River tributaries. Weathering of Precambrian silicate rocks in the cores of Laramide ranges produce high 87 Sr/ 86 Sr ratios of highland rivers. Therefore, I use reconstructions of the 87 Sr/ 86 Sr ratios of late Cretaceous-early Cenozoic river water from fossil shells in six basins of the Rocky Mountains to trace the erosion of Precambrian basement cores in the Laramide ranges. The results show that Proterozoic low-grade metamorphic carbonates in the Belt-Purcell

Supergroup were not exposed in the Canadian Rocky Mountains during late Cretaceous-early Paleocene, and that Precambrian silicate basement rock was extensively exposed and eroded during late Paleocene-early Eocene in the Laramide Rocky Mountains. The widespread basement erosion in late Paleocene-early Eocene is mainly a result of tectonic exhumation of Laramide ranges, and may have been intensified by the wet and warm global climate. Chapter 4, Sedimentology , Detrital Zircon Geochronology, Stable Isotope Geochemistry of the Lower Eocene Strata in the Wind River Basin, Central Wyoming, is conducted in order to reconstruct basin evolution, source terrane unroofing, and changes in paleoelevation and paleoclimate. The early Eocene depositional environment changed from from alluvial fan to low-sinuosity fluvial systems. The paleogeographic

16 reconstruction based on paleocurrent directions, sandstone petrography, and detrital zircon geochronology shows that rapid unroofing of the Washakie and Wind River Ranges formed a paleodrainage similar to present in central Wyoming by early Eocene. Oxygen isotope paleoaltimetry shows that the paleoelevation of the Wind River basin was ~500 m, and that local relief between the Washakie and Wind River Ranges and the basin floor was 2.3±0.8 km in early Eocene. Up to 1 km of post-Laramide regional net uplift is required to form the present landscape in central Wyoming.

17 CHAPTER 2: LATE PALEOCENE HIGH LARAMIDE RANGES IN NORTHEAST WYOMING: OXYGEN ISOTOPE STUDY OF ANCIENT RIVER WATER

ABSTRACT The distribution and initial timing of the establishment of high surface elevations in the Rocky Mountains during the Early Cenozoic remain controversial despite the importance of these data in testing tectonic models for this region. We track the timing and distribution of high elevation in the Rocky Mountains during Late Cretaceous – Early Eocene by examining the annual and seasonal δ 18 O values of the ancient river water, which are extracted from the δ 18 O values of well-preserved shell aragonite of the Unionid family. In the Powder River basin of the eastern Laramide province, the δ 18 O values of the ancient river water vary between -23.0‰ and -8.0‰ SMOW in both seasonal and annual records in Late Paleocene-Early Eocene. The large variation suggests that the ancient rivers were fed yearly or seasonally by snowmelt from highland of 4.5±1.3 km. This can be explained by the existence of the Bighorn Mountains and Black Hills with a drainage pattern similar to the present in northeast Wyoming. The δ 18 O values of the ancient river water along the front of the Sevier thrust belt generally follow a trend from lower values in north, -14.2±1.4‰ in the Crazy Mountains basin in Early Paleocene, to higher values in south, ~-11.1±0.8‰ in the Bighorn basin in Late Paleocene, and -7.1±1.6‰ in the Washakie basin in Early Eocene. The variations within each basin are relatively small. These rivers most likely rise in the Sevier thrust belt, and may reflect highland elevation of 1-2 km. The δ 18 O values in the Alberta foreland and Williston basin are very low (-

18 20.5‰) in Late Cretaceous, indicating the rivers water were fed by snowmelt from the Canadian Rocky Mountains of 4.5 ±1.0 km high. The attainment of high elevation in the eastern Laramide province prior to the western province could be explained by southwestward progression of back-thrusts soled into an earlier east-directed master detachment, which may be formed by the westward rollback of subducted shallow slab. Keywords: Rocky Mountains; Paleoelevation; Oxygen isotope ratios; Laramide; freshwater bivalve INTRODUCTION The Laramide orogeny of the Rocky Mountains is a system of basement-cored uplifts and intervening basins that formed ~80-40 Ma in the foreland basin of the Sevier thrust belt in the western United States (Dickinson and Snyder, 1978; Bird, 1998; DeCelles, 2004). Analogous to modern flat-slab subduction in western South America (Jordan and Allmendinger, 1986), Laramide uplifts are the result of the NE-SW compression due to shallow subduction of the Farallon Plate beneath the North American Plate (e.g., Dickinson and Snyder, 1978; Bird, 1998; Saleeby, 2003; DeCelles, 2004). Although this region is very well studied, some fundamental questions remain unanswered: how does shallow subduction thicken foreland crust and produce a landscape with intervening basins and ranges, and how is basement-involved deformation connected to the thin- skinned Sevier fold and thrust belt. A range of tectonic models has been proposed to explain the mechanisms of Laramide deformation: basal traction (Bird 1998), lithospheric buckling (Tikoff and Maxson, 2001), intracrustal flow (McQuarrie and Chase, 2001), and

19 thrust, back-thrust and crustal detachment (Erslev, 1993; 2005). The timing, magnitude, and pattern of high surface topography in the Laramide province need to be examined to help answer these basic questions and to test tectonic models. Oxygen isotope ratios of precipitation derived from authigenic minerals (e.g., paleosol carbonate, biogenic apatite and aragonite) have been applied as paleoaltimeters in numerous studies (e.g., Dettman and Lohmann, 2000; Garzione et al., 2006; DeCelles et al., 2007; Quade et al., 2007). This approach is based on the decrease in oxygen isotope ratios of precipitation as elevation increases, which is controlled by progressive condensation of atmospheric water vapor due to cooling or adiabatic expansion as the vapor mass ascends, leading to Rayleigh isotope fractionation (Dansgaard, 1953; Rowley, 2007). At present, there have been only a few studies addressing the paleoelevation of the Laramide Rocky Mountains (Gregory and Chase 1992; Norris et al., 1996; Wolfe et al. 1998; Dettman and Lohmann, 2000; Fricke 2003; Sewall and Sloan, 2006). Even though these studies have been mostly focused on Eocene elevations, they have yielded highly varying results. By using leaf margin analyses, Wolfe et al. (1998), and Gregory and Chase (1992) suggested that the Eocene southern Rocky Mountains were more than 3 km high, similar to today’s elevation. This agreed with the δ 18 O values of lake microbial carbonates in the Green River basin (Norris et al., 1996), and the δ 18 O values of unaltered freshwater bivalves and regional GCM modeling of Early Paleogene Laramide foreland (Dettman and Lohmann, 2000; Sewall and Sloan, 2006). However, Morrill and Koch (2002) concluded that diagenesis may have altered the δ 18 O of lacustrine microbial carbonates in the Green River basin. Moreover, Fricke (2003) argued that Early Eocene

20 Laramide range elevations were ~500 m based similar δ 18 O values of mammal teeth from three Wyoming basins. These conflicting conclusions may arise from a number of factors: 1). diagenetic overprinting of original isotopic patterns; 2). not accounting for all the non-altitude factors that can affect the δ 18 O of surface water (e.g., latitude, temperature); 3). small sample numbers failing to document a regional pattern in surface water δ 18 O values. In this study we survey the oxygen isotope composition of fossil freshwater bivalves (Unioniacea superfamily) of Late Cretaceous-Early Eocene age collected from six basins in the Laramide tectonic province. Unionids have relatively thick growth increments aiding the study of seasonal isotopic variation. Our study examines both the seasonal and annual δ 18 O values of ancient river waters as recorded in fossil shells. We compare these to δ 18 O values of modern precipitation and river water in our discussion of the paleoelevation of ancient river catchments. We then track the timing and spatial patterns of high elevation regions in Laramide ranges. REGIONAL SETTING Fossil shells were collected from the Alberta foreland basin, western Williston basin, Crazy Mountains basin, northern Bighorn basin, Powder River basin and southern Washakie basin (Fig. 2.1, 2.2). Prior to Laramide deformation, these regions were a broad foreland basin of the thin-skinned Sevier fold-thrust belt. Laramide deformation partitioned the central Rocky Mountain foreland into discrete local basins separated by basement-cored uplifts with NE-SW to E-W orientations (Dickinson et al., 1988). The

21 uplifts are bounded by moderately dipping to high-angle faults or are broadly anticlinal. The Crazy Mountains basin, Bighorn basin, Powder River basin and Washakie basin are of this type. Late Cretaceous through Miocene age sediments derived from the Sevier thrust belt and Laramide uplifts were deposited in these basins. Fossil shells from the Washakie basin are from the Luman Tongue Member of the Green River Formation, a mix of fluvial and lake facies in the early stages of Lake Gosiute (Sklenar and Anderson, 1985). Shell samples in the other three basins are from Paleocene to Early Eocene fluvial sediments. In the Alberta foreland basin, Laramide age deformation is an eastward continuation of thin-skinned Sevier fold and thrust, which overthrusted Mesozoic shale and molasse, but did not generate any basement uplifts (Obsborn et al., 2006). Shells from the Alberta foreland basin were collected from sediments of Late Campanian and Maastrichtian age. In the Williston basin, a depression in the Canadian Shield, fossil shells were collected from Late Cretaceous and Early Paleocene fluvial sediments. Sample ages are based on radiometric ages, paleomagnetic ages and intrabasin mammalian biostratigraphic correlation with magnetostratigraphy (Table 1). ANALYTICAL METHODS Aragonite shells used in this study are all unaltered as judged by physical appearance, cathodoluminescence microscopy and X-ray diffraction for a subset of the samples. X-ray diffraction was performed with a Bruker D8 Advance Diffractometer using Cu Kα r adiation. When possible, ten shells and shell fragments from each stratigraphic horizon were analyzed for stable isotopes. The bulk shell δ 18 O values were mostly presented in Dettman and Lohmann, 2000, with new analyses added (supplementary data). Total of

22 881 individual shell fossil analyses are included in this paper. Shell Aragonites were collected by drilling through the shell body, integrating isotopic variation and yeilding a growth amount-weighted average δ 18 O value. The bulk δ 18 O value presented in this paper refers to the average values of all analyzed bulk shell samples in stratigraphic horizon except the Powder River basin, where some data represent single shell analyses. In addition, we micro-milled 12 selected samples in order to study the seasonal isotopic variation of the Early Cenozoic rivers. Shells were sectioned along the axis of maximum growth, mounted as thick sections (~1 mm). Growth bands in cross-sectioned shells were subsampled using a computer-controlled micro-mill with a 30 µm sampling resolution (Dettman and Lohmann, 1995). The δ 18 O and δ 13 C values of aragonites were measured using an automated carbonate preparation device (KIEL-III) coupled to a gas-ratio mass spectrometer (Finnigan MAT 252). Samples (20 to 150 µg) were reacted with dehydrated p hosphoric acid under vacuum at 70°C. The isotope ratioa are calibrated based on measurements of NBS-19 and NBS-18; precision is ±0.1‰ for δ 18 O and ±0.06‰ for δ 13 C (1σ). OXYGEN ISOTOPE RESULTS The δ 18 O values of bulk shells from basins along the front of the Sevier thrust belt follow a trend from lower values in north (-13.7±1.3‰ PDB , Crazy Mountains basin) to higher values in south (-10.7±0.8‰ PDB , Bighorn basin, and -7.3±1.5‰ PDB , Washakie basin), and the within-basin variation is small. In the Powder River basin and western Williston basin, shell δ 18 O values show large variation between -22.9‰ and -7.9‰ PDB , and -21.8‰ and -9.3‰ PDB , respectively. Our results in the western Williston basin are

23 consistent with values from the eastern Williston basin (Carpenter et al., 2003; Cochran et al., 2003). The δ 18 O values of shells in the Alberta foreland basin vary between - 19.3‰ and -13.3‰ PDB (Fig. 2.3). Seasonal isotopic variation of selected shells from each basin are used to compare ancient and modern river seasonal variability in the Laramide region (Fig. 2.4, 2.5). Oxygen isotope ratios of modern river water are from Coplen and Kendall (2000), modern precipitation are from the United States Network for Isotopes in Precipitation (USNIP) (www.uaa.alaska.edu/enri/usnip), Canadian network for Isotopes in Precipitation (CNIP) (www.science.uwaterloo.ca/~twdedwar/cnip ), and Dutton et al. (2005). CALCULATION OF ANCIENT RIVER WATER δ δδ δ 18 O VALUES The oxygen isotope composition of Unionid shell aragonite is controlled by the temperature and δ 18 O value of the river water in which it grew (Grossman and Ku, 1986; Dettman et al., 1999). Here, we use the empirically determined relationship between the bulk δ 18 O values of Unionid shell and mean annual river water in temperate climates from Kohn and Dettman (2007): δ 18 O (shell,PDB) =(0.892±0.024) δ 18 O (riverwater,SMOW) -0.978±0.240 (R 2 =0.98) (1) to calculate the mean annual δ 18 O values of ancient river water. Because these freshwater bivalves stop growing below approximately 10-12°C and their growth is heavily biased to late spring and early summer temperatures, a large majority of shell aragonite is produced in a limited range of temperatures (20-25°C) (Dettman et al., 1999). This growth temperature bias leads to a good linear correlation between mean annual river

24 water and bulk shell δ 18 O values, which can be used to calculate the mean δ 18 O value of ancient river water if the climate is temperate and seasonal (Kohn and Dettman, 2007). If temperatures were not seasonal, but remained at one temperature extreme (e.g. 12° or 30°) throughout the year, then calculated river water δ 18 O values could be as much as 2‰ too high or too low, but this seems extremely unlikely given prominent growth bands in the shells, indicating seasonal growth cessation, and the botanical and modeling evidence for moderate seasonality (Wilf; 2000, Sewall and Sloan, 2006). Our calculated mean annual δ 18 O values of ancient river waters range from -23‰ to -5‰ SMOW (Fig. 2.3). Each shell sample represents an average of several years of growth in these river systems. Although the average river water δ 18 O values calculated from a single shell could be affected by a few years of anomalous precipitation patterns, the chances of this are reduced by averaging ten shells from each stratigraphic level. CONSTRAINTS, CORRECTIONS, AND APPLICATION TO PALEOALTIMETRY Knowing the δ 18 O values of ancient river water provides some insight into the paleoelevation of ancient river catchments, but many other factors can affect the δ 18 O of rainfall. Many of these effects (e.g. temperature, continentality) are combined into the strong relationship between the δ 18 O of rainfall and latitude (Dutton et al., 2005). Therefore we will attempt to remove the latitudinal effect on δ 18 O values by comparing the ancient river δ 18 O values to modern low-elevation rivers (representing low-elevation rainfall δ 18 O values) across a range of latitudes. We use river water δ 18 O data because it is much more abundant than precipitation data and it comes closer to a weighted average

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Abstract: The Laramide Rocky Mountains in western U.S.A is an important topographic feature in the continental interior, yet its formation and evolution are poorly constrained. This study uses the oxygen and strontium isotope geochemistry of freshwater bivalve fossils from six Laramide basins in order to reconstruct the spatial evolution of the paleotopography and Precambrian basement erosion in late Cretaceous-early Eocene. In addition it uses the sedimentology, detrital zircon U-Pb geochronology, and isotope paleoaltimetry of early Eocene sedimentary strata to constrain the tectonic setting, paleogeography and paleoclimate of the Wind River basin. Annual and seasonal variation in ancient riverwater δ 18 O reconstructed from shell fossils shows that the Canadian Rocky Mountains was 4.5±1.0 km high in late Cretaceous-early Paleocene, and the Laramide ranges in eastern Wyoming reached 4.5±1.3 km high, while the ranges in western Wyoming were 1-2 km high in late Paleocene. The 87 Sr/86 Sr ratios of riverwaters reconstructed from the same fossils show that Proterozoic metamorphic carbonates in the Belt-Purcell Supergroup were not exposed in the Canadian Rocky Mountains during Late Cretaceous-early Paleocene, but that Precambrian silicate basement rock was exposed and eroded in the Laramide ranges during late Paleocene-early Eocene. The sedimentary environment of the early Eocene Wind River basin changed from gravelly fluvial and/or stream-dominated alluvial fan to low-sinuosity fluvial systems. Tectonic uplift of the Washakie and Wind River Range in early Eocene formed the modern paleodrainage system, although the elevation of the basin floor was only ~500 m high at that time, and early Eocene paleoclimate is more humid than modern climate.