The Pennsylvania State University The Graduate School Department of Geosciences LATE MIOCENE EROSION AND EVOLUTION OF TOPOGRAPHY ALONG THE WESTERN SLOPE OF THE COLORADO ROCKIES A Thesis in Geosciences by Russell H. Rosenberg 2013 Russell H. Rosenberg Submitted in Partial Fulfillment of the Requirements for the Degree of Master of Science May 2013 The thesis of Russell H. Rosenberg was reviewed and approved* by the following: Eric Kirby Associate Professor of Geosciences Thesis Advisor Kevin Furlong Professor of Geosciences Rudy Slingerland Professor of Geology Chris Marone Professor of Geosciences Associate Department Head of Graduate Programs *Signatures are on file in the Graduate School ii ABSTRACT It is increasingly apparent that dynamic effects associated with changes in mantle flow and buoyancy can influence the evolution of surface topography. In the Rocky Mountain province of the western United States, recent high-resolution seismic imaging of the crust and upper mantle reveals intriguing correlations between mantle velocity anomalies and regions of high topography. To determine whether these regions of low-seismic wavespeed are associated with recent changes in buoyancy structure of the lithosphere, I explore the relationship between the longitudinal profiles of tributaries draining the western slope of the northern Colorado Rockies and the history of Late Cenozoic fluvial incision and exhumation across this region. Major tributaries of the upper Colorado River, including the Gunnison and Dolores Rivers, that drain high topography in central and western Colorado overlie upper mantle with slow seismic wave velocities; these drainages exhibit relatively steep longitudinal profiles (normalized for differences in drainage area and discharge) and are associated with ~1000-1500 m of incision over the past 10 Ma. In contrast, tributaries of the Green River that drain the western slope in northern Colorado (White, Yampa, and Little Snake Rivers) overlie mantle of progressively lower seismic wave velocities to the north. River profiles in northern Colorado are two to three times less steep along reaches within comparable bedrock lithologies. New 40Ar/39Ar ages on basalt flows capping the Tertiary Browns Park Formation in this region range in age from ~11-6 Ma, and provide local datums from which I reconstruct ~500-900 m of incision along tributaries of the Green River. The correspondence of steep river profiles in regions of greater incision and lower gradient profiles in regions of less incision suggests that the fluvial systems are dynamically adjusting to an external forcing. Moreover, spatial differences in the pattern and magnitude of incision are not readily explained by a putative increase in erosivity associated with late Cenozoic climate change. Rather, fluvial incision appears to reflect relative base level fall along the iii western slope. Given the correspondence of steep channels, deep incision and regions of low seismic velocity mantle, I suggest that differential rock uplift driven, in part, by differences in the buoyancy and/or convective flow of the mantle beneath western Colorado is the likely driver for Neogene incision. iv TABLE OF CONTENTS LIST OF FIGURES ................................................................................................................. vii LIST OF TABLES ................................................................................................................... x ACKNOWLEDGEMENTS ..................................................................................................... xi 1 Introduction ........................................................................................................................ 1 2 Background ........................................................................................................................ 4 2.1 Support of High Topography in the Colorado Rockies ............................................. 5 2.2 Timing, Magnitude and Rates of Incision along the Western Slope ......................... 8 2.2.1 Colorado and Gunnison Rivers ......................................................................... 10 2.2.2 White, Yampa, and Little Snake Rivers ............................................................ 17 2.3 Analysis of River Longitudinal Profiles ................................................................... 22 2.4 Controls on Channel Profile Form along the Western Slope of the Rocky Mountains ................................................................................................................. 24 3 Analysis of Channel Profiles along the Western Slope ..................................................... 26 3.1 Measuring Normalized Channel Steepness Index ..................................................... 26 3.2 Evaluating the Effect of Substrate Lithology on Channel Steepness ........................ 28 3.3 Evaluating Relationships between Discharge and Drainage Area ............................ 31 4 Results of Channel Profile Analysis .................................................................................. 32 4.1 Relationships Between Discharge and Drainage Area .............................................. 32 4.2 Colorado, Gunnison, and Dolores Rivers ................................................................. 32 4.3 Yampa, White, and Little Snake Rivers .................................................................... 36 5 New Constraints on Late Miocene Exhumation ................................................................ 41 5.1 Reconstructing Exhumation in the Elkhead Mountains ............................................ 44 5.1.1 Battle Mountain, Squaw Mountain, and Bible Back Mountain ........................ 46 5.1.2 Black Mountain and Mt. Welba ........................................................................ 47 5.2 Reconstructing Exhumation in the Flattops .............................................................. 49 5.3 Reconstructing Exhumation in the Yampa River Valley .......................................... 51 5.3.1 Woodchuck Hill ................................................................................................ 52 5.3.2 Lone Spring Butte ............................................................................................. 52 5.4 Summary of Constraints on the Timing and Magnitude of Incision ......................... 55 6 Discussion .......................................................................................................................... 60 6.1 Extensional Faulting in the Sand Wash Basin .......................................................... 60 6.2 Regional Correspondence between Late Miocene Incision and Channel Steepness .................................................................................................................. 62 v 7 Potential Drivers of Late Miocene Incision ....................................................................... 65 7.1 Enhanced Fluvial Incision in the Late Miocene ........................................................ 65 7.2 Base-level Fall and Transient Incision Associated with Basin Integration ............... 67 7.3 Differential Rock Uplift and Tilting along the Western Slope ................................. 69 8 Conclusions ........................................................................................................................ 72 References ................................................................................................................................ 74 Appendix: 40Ar/39Ar Analytical Methods and Results ............................................................. 89 vi LIST OF FIGURES Figure 1: Modern topography of the Rocky Mountain physiographic province (left pannel) compared to differential P-wave velocity at 100 km depth (right pannel). Geographic points for reference: GJ--Grand Junction, CO; R--Rifle, CO; SB--Steamboat Springs, CO; NP--North Park, CO; SP--South Park, CO; GM--Grand Mesa; BC--Book Cliffs; FT--Flat Tops. Tomographic data from Schmandt and Humphreys (2010). ............... 7 Figure 2: Interpolated normalized channel steepness (ksn) for the entire Colorado River watershed; values calculated with 10 km channel segments and a reference concavity of 0.45 (modified from Karlstrom et al., 2012). The study area for this work, and the extent of Figure 1 (above) is shown by black inset. .................................................... 7 Figure 3: Simplified geologic map showing the locations of previously dated landforms which provide constraints on the timing and magnitude of incicision along the Colorado River (modified from Green, 1992; Tweto, 1979). The location of evaporite collapse centers along the Colorado River (from Kunk et al., 2002) are also shwon. Date for prveiously published incision markers along the Colorado River are given in Table 1. ......... 11 Figure 4: Simplified geologic map showing the extent of the Browns Park Formation (modified from Green, 1992; Tweto, 1979; Green and Drouillard, 1994; Love and Christiansen, 1985; Hintze et al., 2000; Hintze, 1980). The extent of detailed study areas for this work (Figure 11: Elkhead Mountains, Figure: 13 Flat Tops, Figure 14: Yampa River Valley) are shown above by white boxes. Locaities constraining the age of the Browns Park Formation (see text): 1 – Dead Mexican Park; 2 – west bank of Little Snake River; 3 – City Mountain; 4 – Vermillion Creek.. ................................................................... 18 Figure 5: