Joel L. Pederson

A geologically constrained Monte Carlo approach to modeling exposure ages from profiles of cosmogenic nuclides: An example from Lees Ferry, Arizona

We present a user-friendly and versatile Monte Carlo simulator for modeling profiles of in situ terrestrial cosmogenic nuclides (TCNs). Our program (available online at http://geochronology.earthsciences.dal.ca/downloads-models.html) permits the incorporation of site-specific geologic knowledge to calculate most probable values for exposure age, erosion rate, and inherited nuclide concentration while providing a rigorous treatment of their uncertainties. The simulator is demonstrated with 10Be data from a fluvial terrace at Lees Ferry, Arizona. Interpreted constraints on erosion, based on local soil properties and terrace morphology, yield a most probable exposure age and inheritance of 83.9−14.1+19.1 ka, and 9.49−2.52+1.21 × 104 atoms g−1, respectively (2σ). Without the ability to apply some constraint to either erosion rate or age, shallow depth profiles of any cosmogenic nuclide (except for nuclides produced via thermal and epithermal neutron capture, e.g., 36Cl) cannot be optimized to resolve either parameter. Contrasting simulations of 10Be data from both sand- and pebble-sized clasts within the same deposit indicate grain size can significantly affect the ability to model ages with TCN depth profiles and, when possible, sand—not pebbles—should be used for depth profile exposure dating.

Colorado Plateau magmatism and uplift by warming of heterogeneous lithosphere

The forces that drove rock uplift of the low-relief, high-elevation, tectonically stable Colorado Plateau are the subject of long-standing debate. While the adjacent Basin and Range province and Rio Grande rift province underwent Cenozoic shortening followed by extension, the plateau experienced ∼2 km of rock uplift without significant internal deformation. Here we propose that warming of the thicker, more iron-depleted Colorado Plateau lithosphere over 35–40 Myr following mid-Cenozoic removal of the Farallon plate from beneath North America is the primary mechanism driving rock uplift. In our model, conductive re-equilibration not only explains the rock uplift of the plateau, but also provides a robust geodynamic interpretation of observed contrasts between the Colorado Plateau margins and the plateau interior. In particular, the model matches the encroachment of Cenozoic magmatism from the margins towards the plateau interior at rates of 3–6 km Myr-1 and is consistent with lower seismic velocities and more negative Bouguer gravity at the margins than in the plateau interior. We suggest that warming of heterogeneous lithosphere is a powerful mechanism for driving epeirogenic rock uplift of the Colorado Plateau and may be of general importance in plate-interior settings.

Colorado Plateau uplift and erosion evaluated using GIS

Study of the interaction between uplift and erosion is a major theme of our science, but our understanding of their interplay is often limited by a lack of quantitative data. A classic example is the Colorado Plateau, for which the starting and ending points are well known: The region was at sea level in the Late Cretaceous, and now, the deeply eroded land surface is at ~2 km. The path of the landscape between these endpoints is less clear, and there has been longstanding debate on the mechanisms, amounts, and timing of uplift and erosion. We use a geographic information system to map, interpolate, and calculate the Cenozoic rock uplift and erosional exhumation of the Colorado Plateau and gain insight into its landscape development through time. Initial results indicate uplift and erosion are highly spatially variable with mean values of 2117 m for rock uplift and 406 m for net erosional exhumation since Late Cretaceous coastal sandstones were deposited. We estimate 843 m of erosion since ca. 30 Ma (a larger value because of net deposition on the plateau over the early Cenozoic), which can account for 639 m of post-Laramide rock uplift by isostatic processes. Aside from this isostatic source of rock uplift, paleobotanical and fission-track data from the larger region suggest the early Cenozoic Laramide orogeny alone should have caused more than the remaining rock uplift, and geophysical studies suggest mantle sources for additional Cenozoic uplift. There is, in fact, less uplift on the plateau than proposed sources can supply. This suggests Laramide uplift of the plateau was significantly less than that of the Rocky Mountains, consistent with its prevalent sedimentary basins, and/or that there has been little or no post-Laramide uplift beyond erosional isostasy.

Differential incision of the Grand Canyon related to Quaternary faulting—Constraints from U-series and Ar/Ar dating

Incision of the Colorado River in the Grand Canyon, widely thought to have happened between ca. 6 and 1.2 Ma, has continued at variable rates along the canyon over the past ;500 k.y., based on measurements of bedrock incision combined with U-series and 40 Ar/ 39 Ar ages. River incision rates downstream of the Toroweap fault in the western Grand Canyon are about half the ;140 m/m.y. incision rate calculated for a distance of at least 200 km upstream of the fault. We hypothesize that this differential incision is due to westdown slip on the Toroweap fault of 94 6 6 m/m.y. based on measured offset of the newly dated Upper Prospect basalt flow, which is the major middle-late Quaternary slip evident along the river. Regional incision has been driven mostly by base-level fall related to drainage reversal off the Colorado Plateau ca. 6 Ma. Because local normal faulting is lower in rate than this regional incision and is likely an expression of Basin and Range extension and subsidence rather than uplift, this is a case where active faulting diminishes, but does not drive, incision. Quaternary incision rates are insufficient to have carved the Grand Canyon in 6 m.y., suggesting either that rates have decreased through time as the original base-level signal has attenuated, or that some component of the canyon relief we see today existed prior to Colorado River integration.

Forecasting the response of Earth's surface to future climatic and land use changes: A review of methods and research needs

In the future, Earth will be warmer, precipitation events will be more extreme, global mean sea level will rise, and many arid and semiarid regions will be drier. Human modifications of landscapes will also occur at an accelerated rate as developed areas increase in size and population density. We now have gridded global forecasts, being continually improved, of the climatic and land use changes (C&LUC) that are likely to occur in the coming decades. However, besides a few exceptions, consensus forecasts do not exist for how these C&LUC will likely impact Earth-surface processes and hazards. In some cases, we have the tools to forecast the geomorphic responses to likely future C&LUC. Fully exploiting these models and utilizing these tools will require close collaboration among Earth-surface scientists and Earth-system modelers. This paper assesses the state-of-the-art tools and data that are being used or could be used to forecast changes in the state of Earths surface as a result of likely future C&LUC. We also propose strategies for filling key knowledge gaps, emphasizing where additional basic research and/or collaboration across disciplines are necessary. The main body of the paper addresses cross-cutting issues, including the importance of nonlinear/threshold-dominated interactions among topography, vegetation, and sediment transport, as well as the importance of alternate stable states and extreme, rare events for understanding and forecasting Earth-surface response to C&LUC. Five supplements delve into different scales or process zones (global-scale assessments and fluvial, aeolian, glacial/periglacial, and coastal process zones) in detail.

Colorado River chronostratigraphy at Lee’s Ferry, Arizona, and the Colorado Plateau bull’s-eye of incision

ABSTRACTLee’s Ferry (Arizona, United States) lies at an important geo-logic transition between the Grand Canyon margin and the Canyon-lands center of the Colorado Plateau. It marks a knickpoint along the Colorado River at the top of the steep Grand Canyon, and it is central to debate about the patterns of erosion and sources of uplift in this famous landscape. New chronostratigraphic data from the suite of fi ll terraces here indicate a strong fl uvial response to climate driv-ers superimposed upon an integrated mid-to-late Pleistocene incision rate of ~350 m/m.y. A regional compilation of well-constrained results over the same timescale reveals that this is intermediate between slower rates downstream in Grand Canyon and even faster rates in the central Colorado Plateau, which taper off again farther upstream near the plateau’s eastern edge. This bull’s-eye pattern of rapid inci-sion in the central Colorado Plateau does not match proposed sources of uplift from mantle dynamics at the south and west fl ank of the pla-teau, nor patterns of river steepness and energy. Instead we suggest that this incision pattern is primarily driven by transient response to drainage integration and isostatic feedback from the deep exhuma-tion of weak rocks in the central plateau.INTRODUCTION

New thermochronometric constraints on the Tertiary landscape evolution of the central and eastern Grand Canyon, Arizona

Thermal histories are modeled from new apatite (U-Th)/He and apatite fission-track data in order to quantitatively constrain the landscape evolution of the Grand Canyon region. Fifty new samples and their associated thermochronometric ages are presented here. Samples span from Lee’s Ferry in the east to Quartermaster Canyon in the west and include four age-elevation transects into Grand Canyon and borehole samples from the Coconino Plateau. Twenty-seven samples are inversely modeled to provide continuous thermal histories. This represents the most extensive and complete dataset on patterns of long-term exhumation in the Grand Canyon region, and it enables us to constrain the timing and magnitude of erosion and also discriminate between canyon incision and broader planation. The new data suggest that the early Cenozoic landscape in eastern Grand Canyon was low in relief and does not indicate the presence of an early Cenozoic precursor to the modern Grand Canyon. However, there is evidence for the incision of a smaller-scale canyon across the Kaibab Uplift at 28–20 Ma. This middle-Cenozoic denudation event was accompanied by the removal of a majority of remaining Mesozoic strata west of the Kaibab Uplift. In contrast, just upstream in the area of Lee’s Ferry, ∼2 km of Mesozoic strata remained over the middle Cenozoic and were removed after 10 Ma.

The mystery of the pre-Grand Canyon Colorado River— Results from the Muddy Creek Formation

The Colorado River’s integration off the Colorado Plateau remains a classic mystery in geology, despite its pivotal role in the cutting of Grand Canyon and the region’s landscape evolution. The upper paleodrainage apparently reached the southern plateau in the Miocene, and recent work supports the longstanding idea that the river was superimposed over the Kaibab uplift by this time. Once off the plateau, the lower river integrated to the Gulf of California by downstream basin spillover from ca. 6–5 Ma. An unknown link remains: the history of the river in the western Grand Canyon region in Miocene time. One of the viable hypotheses put forward by previous workers—that the late Miocene Muddy Creek Formation represents the terminal deposits of the paleo–Colorado River in the Basin and Range northwest of Grand Canyon—is tested in this paper. Results indicate instead that local drainages along with the paleo–Virgin River are the likely sources of this sediment. The remaining hypothesis—that the paleo-upper Colorado River dissipated and infiltrated in the central-western Grand Canyon area—has modern analogs, provides a potential source for extensive Miocene spring and evaporite deposits adjacent to the southwestern plateau, and implies a groundwater-driven mechanism for capture of the upper drainage.

Optically stimulated luminescence (OSL) as a chronometer for surface exposure dating

[1] We pioneer a technique of surface-exposure dating based upon the characteristic form of an optically stimulated luminescence (OSL) bleaching profile beneath a rock surface; this evolves as a function of depth and time. As a field illustration of this new method, the maximum age of a premier example of Barrier Canyon Style (BCS) rock art in Canyonlands National Park, Utah, USA, is constrained. The natural OSL signal from quartz grains is measured from the surface to a depth of >10 mm in three different rock samples of the Jurassic Navajo Sandstone. Two samples are from talus with unknown daylight exposure histories; one of these samples was exposed at the time of sampling and one was buried and no longer light exposed. A third sample is known to have been first exposed 80 years ago and was still exposed at the time of sampling. First, the OSL-depth profile of the known-age sample is modeled to estimate material-dependent and environmental parameters. These parameters are then used to fit the model to the corresponding data for the samples of unknown exposure history. From these fits we calculate that the buried sample was light exposed for � 700 years before burial and that the unburied sample has been exposed for � 120 years. The shielded surface of the buried talus sample is decorated with rock art; this rock fell from the adjacent Great Gallery panel. Related research using conventional OSL dating suggests that this rockfall event occurred � 900 years ago, and so we deduce that the rock art must have been created between � 1600 and 900 years ago. Our results are the first credible estimates of exposure ages based on luminescence bleaching profiles. The strength of this novel OSL method is its ability to establish both ongoing and prior exposure times, at decadal to millennial timescales or perhaps longer (depending on the environmental dose rate) even for material subsequently buried. This has considerable potential in many archeological, geological and geo-hazard applications.

RECOGNITION OF THE HEMPHILLIAN/BLANCAN BOUNDARY IN NEVADA

Abstract The boundary between Hemphillian and Blancan North American Land Mammal Ages has been elusive and equivocal. Our work identifies that boundary at about 4.9 to 5.0 Ma, based on paleomagnetic and radioisotopic dating of strata producing Blancan and Hemphillian mammals in Meadow and Spring Valleys of Lincoln County, eastern Nevada, and in the Pine Nut Mountains of western Nevada. Magnetostratigraphic study of the Panaca Formation in Lincoln County yielded a composite section with six magnetozones. The Healdsburg Tephra was identified near the top of the section in Meadow Valley, thereby placing the Lincoln County composite section relative to the Geomagnetic Polarity Time Scale (GPTS). The arvicolid rodent Mimomys, an indicator of the Blancan LMA, appears near the base of magnetic chron C3n.3r, about 4.98 Ma, in the Panaca Formation. Both Blancan and Hemphillian mammals have been reported from a stratigraphic sequence near Buckeye Creek in the Pine Nut Mountains of western Nevada. Magnetostratigraphic study of this area produced a thick (280 m) section with six magnetozones. A pumice layer near the top of magnetozone D+ yielded an 40Ar/39Ar date of 4.96 Ma, thereby correlating magnetozone D+ with Chron C3n.4n (Thvera subchron) of the GPTS. A Hemphillian rhino is recorded below the pumice layer and a Blancan bear is recorded above it. Correlation of the Lincoln County and Pine Nut Mountain sections in Nevada is consistent with placement of the Hemphillian/Blancan boundary in chron C3n.3r or the top of chron C3n.4n of the GPTS, about 4.9 to 5.0 Ma.