Unsteady late Pleistocene incision of streams ... - Wiley Online Library

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117, F01023, doi:10.1029/2011JF002232, 2012

Unsteady late Pleistocene incision of streams bounding the Colorado Front Range from measurements of meteoric and in situ 10Be Miriam Dühnforth,1,2 Robert S. Anderson,1,3 Dylan J. Ward,4 and Alex Blum5 Received 4 October 2011; revised 4 January 2012; accepted 10 January 2012; published 15 March 2012.

[1] Dating of gravel-capped strath terraces in basins adjacent to western U.S. Laramide Ranges is one approach to document the history of late Cenozoic fluvial exhumation. We use in situ 10Be measurements to date the broad surfaces adjacent to the eastern edge of the Rocky Mountains in Colorado, and compare these calculated ages with results from meteoric 10Be measurements. We analyze three sites near Boulder, Colorado (Gunbarrel Hill, Table Mountain, and Pioneer) that have been mapped as the oldest terrace surfaces with suggested ages ranging from 640 ka to the Plio-Pleistocene transition. Our in situ 10Be results reveal abandonment ages of 95 912 ka at Table Mountain, 175 27 ka at Pioneer, and ages of 251 10 ka and 307 15 ka at Gunbarrel Hill. All are far younger than previously thought. Inventories of meteoric 10Be support this interpretation, yielding ages that are comparable to Table Mountain and 20% lower than Pioneer in situ ages. We argue that lateral beveling by rivers dominated during protracted times of even moderate glacial climate, and that vertical incision rates of several mm/yr likely occurred during times of very low sediment supply during the few interglacials that were characterized by particularly warm climate conditions. In contrast to the traditional age chronology in the area, our ages suggest that the deep exhumation of the western edge the High Plains occurred relatively recently and at an unsteady pace. Citation: Dühnforth, M., R. S. Anderson, D. J. Ward, and A. Blum (2012), Unsteady late Pleistocene incision of streams bounding the Colorado Front Range from measurements of meteoric and in situ 10Be, J. Geophys. Res., 117, F01023, doi:10.1029/2011JF002232.

1. Introduction [2] Variations in either tectonic or climate conditions can greatly perturb the fluvial system of a landscape, resulting in the alternating pattern of incision and aggradation that leads to the formation of fluvial terraces. Examples of both fluvial strath and fill terraces can be found within and adjacent to many mountain ranges worldwide. While fluvial terraces are common, the particular mechanisms that drove the cyclic variation in sediment deposition and incision in a particular setting are not always known. For example, base level changes, variations in rainfall and runoff, or altered sediment supply can change the erosional behavior of a stream. Absolute dating of sediment from terraces constrains the timing of surface abandonment and can potentially 1 INSTAAR, University of Colorado at Boulder, Boulder, Colorado, USA. 2 Now at Department of Earth and Environmental Sciences, LudwigMaximilian-University, Munich, Germany. 3 Department of Geological Sciences, University of Colorado at Boulder, Boulder, Colorado, USA. 4 Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico, USA. 5 U.S. Geological Survey, Boulder, Colorado, USA.

Copyright 2012 by the American Geophysical Union. 0148-0227/12/2011JF002232

help to understand the transient catchment dynamics that cause terrace formation. Here, we explore the mechanisms that drive strath terrace formation in the Denver Basin, a range-bounding sedimentary basin directly adjacent to the Colorado Front Range at the edge of the Laramide age Rocky Mountains. The temporal pattern of the exhumation history in the Denver Basin is poorly constrained. Even though a few relevant absolute dates exist, these do not reveal when exactly the fluvial system along the Colorado Front Range switched from sediment aggradation to incision and removal of the basin sediments. [3] By improving our understanding of the absolute timing of surface abandonment, absolute ages ideally provide insights into possible mechanisms that control the dynamics of strath terrace formation in the Denver Basin, and by analogy in other Laramide Range-Basin pairs. Possible controls on fluvial incision of the High Plains (which includes the Denver and Arkansas basins; see Figure 1) include climate-induced variations in sediment supply [Wobus et al., 2010], a decline in sediment supply due to a drop in the erodibility of the bedrock being exhumed in the headwaters [Carroll et al., 2006], or late Cenozoic tectonic tilting [e.g., Leonard, 2002; McMillan et al., 2002]. However, even the assertion that climate controls the sediment supply lacks sufficient specificity, as climate can modulate sediment

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DÜHNFORTH ET AL.: FLUVIAL INCISION OF COLORADO FRONT RANGE

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total meteoric 10Be flux [Baumgartner et al., 1997; Graham et al., 2003]. Because meteoric 10Be is adsorbed on mineral particles in the near-surface, its inventory represents the residence time of these materials in the near-surface. Meteoric 10Be has been shown to be particularly useful in studies that address the residence times and erosion rates of soils and sediments in various environmental settings [e.g., Pavich et al., 1985; Pavich et al., 1986; Brown et al., 1992; Harden et al., 2002; Reusser and Bierman, 2010, Graly et al., 2010]. One of the great advantages of meteoric 10 Be over in situ 10Be is its independence from quartzbearing rocks, allowing the user to apply meteoric 10Be in a wider range of lithologic settings. In this study, we used it to date the soil matrix associated with the quartz-bearing clasts that we sampled for in situ 10Be measurements.

2. Regional Setting of the Denver Basin Figure 1. Shaded relief image showing the topography in Colorado. The Colorado Front Range delineates the eastern edge of the Rocky Mountains and forms a sharp boundary to the High Plains in the eastern part of the state. The Palmer divide separates the two major drainage systems, the South Platte and the Arkansas, that have cut 500 m deep wedges into upper Cenozoic sediments and Cretaceous shales, clay-, and siltstones. White box delineates location of Figure 2. supply via changes in the extent of glaciation [Hancock and Anderson, 2002], precipitation intensity [Tucker, 2004], the phase of precipitation [Pelletier, 2009], vegetation cover [Istanbulluoglu and Bras, 2006], and temperature, which can vary sediment production by frost-cracking processes [e.g., Walder and Hallet, 1986; Anderson, 1998; Hales and Roering, 2005, 2007]. The detailed timing of terrace formation and abandonment, in conjunction with a locally relevant paleoclimate record, can potentially serve to rule out one or more of these scenarios. With such process-based knowledge as our ultimate goal, we use in situ 10Be measurements, coupled with chemical and mineralogical measures of soil development, to constrain the absolute timing of surface deposition and abandonment. We then compare these data with the global stacked benthic d 18O data set [Lisiecki and Raymo, 2005] to evaluate when within the last several glacial cycles deposition last occurred on these surfaces. [4] We also measure meteoric 10Be for comparison with the in situ 10Be-based ages. While the in situ methods we employ are now common practice, the meteoric 10Be method has only recently seen more widespread use; we therefore provide a brief introduction. Motivated by the goal to use meteoric 10Be as a geomorphic dating tool, its use has recently been expanded by improved understanding of the modern spatial patterns of meteoric 10Be delivery rates and their variations through time [Monaghan et al., 1986; Graham et al., 2003; Field et al., 2006; Heikkilä, 2007]. Meteoric 10Be is produced in the atmosphere by spallation and is then primarily delivered to the Earth’s surface by precipitation of rain and snow and by dry deposition [e.g., Monaghan et al., 1986]. A secondary ‘recycled’ component of delivery to the surface arises from 10Be adsorbed onto dust particles. Outside of regions with loess accumulation, and during the Holocene, this recycled component of 10Be is generally thought to represent a negligible fraction of the

2.1. Geologic and Geomorphic Setting [5] The Colorado Front Range delineates the easternmost edge of the Rocky Mountains in the western United States. At present, the two main river systems draining the eastern edge of Colorado’s mountains, the South Platte and the Arkansas and their tributaries, have incised the western edge of the Colorado Piedmont (High Plains) to create two great wedge-shaped topographic basins [Wobus et al., 2010] (Figure 1). In most areas, these rivers have etched downward into easily eroded Upper Cretaceous shales, claystones, and siltstones [Scott, 1960, 1962; Madole, 1991; Birkeland et al., 1999] and have exported the eroded syn- and post-Laramide Cenozoic sedimentary units. Each basin displays high smooth fluvially bevelled surfaces that stand up to 100 m above the local streams (Figure 1). These broad mesas and ridges are common, and represent bedrock (usually Cretaceous shale) surfaces that are thinly mantled by severalmeter thick gravel deposits. The gravels are comprised of clasts of Precambrian igneous and metamorphic rocks derived from the crystalline headwater basins [Madole, 1991] and were deposited by fluvial processes. These surfaces can be described as pediments or strath terraces. [6] The exhumation of these range-bounding basins of the Colorado Piedmont served as a baselevel drop that initiated a wave of incision upstream into the crystalline rocks of the Colorado Front Range [Wobus et al., 2010; Anderson et al., 2006]. The prominent convexities in longitudinal profiles of most rivers and streams draining eastward from the continental divide toward range-bounding sedimentary basins such as the Denver Basin have been inferred to represent the present position of this wave of incision that likely initiated during the Pliocene after the abandonment of the Ogallala surface [Anderson et al., 2006]. [7] Importantly, the picture painted above is not unique to the Colorado Front Range and the adjacent South Platte and Arkansas basins. This situation is mirrored in other Laramide Ranges and their bounding basins. The basins have been incised in late Cenozoic and are characterized by gravel-capped strath surfaces. The streams within the crystalline cores of the ranges display convexities that represent the slow-moving wave of ongoing incision into harder rock [Anderson et al., 2006]. [8] Our study sites focus on the eastern edge of the Denver Basin, immediately adjacent to the Colorado Front Range

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Figure 2. Shaded relief image of the Colorado Front Range near Boulder as a backdrop for the mapped sequence of strath terraces along the Front Range after Scott [1962, 1963]. The expected, mapped ages of these surfaces range from the Plio-Pleistocene boundary to the latest Pleistocene [Madole, 1991]. The geologic names of each terrace unit are shown together with the expected surface age from the classic correlation and the height (both in red) above the modern local base level on which the classic relative age correlation is based. White circles and triangles indicate the locations of our study sites: Table Mountain, Gunbarrel Hill, and the Pioneer site, as well as additional sampling localities with absolute age constraints by Riihimaki et al. [2006] and Schildgen et al. [2002]. (Figure 1). The basin has been incised by the South Platte River system and its tributaries, including Boulder Creek and St. Vrain Creek, which drain the glaciated parts of the Front Range, and smaller streams that fail to tap the glaciated crest. Water discharge in all streams is dominated by late spring and early summer snowmelt. In the Denver Basin, at least five pediment surfaces stand at different elevations above the modern rivers (Figure 2) [Scott, 1960, 1962; Madole, 1991].

In order of decreasing age, they are named as the pre-Rocky Flats (sometimes called Nussbaum), Rocky Flats, Verdos, Slocum, Louviers, and Broadway surfaces. As these preserved surfaces stand at different elevations above the modern riverbed, they represent an excellent archive for the reconstruction of the incision history. [9] Direct age control on the surfaces themselves exists only for isolated locations scattered throughout the Denver

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Basin. At present, ages of surfaces elsewhere in the basin, including in the Boulder area that we target here, are obtained by correlation, primarily using the height of the surface above the local modern stream (Figure 1) [Scott, 1960; Izett and Wilcox, 1982]. The only available absolute age constraints on the youngest fluvial history in the Boulder area come from fluvial fill terraces in Boulder Canyon. At least four flights of fluvial terraces were mapped by Schildgen et al. [2002], recording a history of sediment aggradation and scour by Boulder Creek over approximately the last 100 ka. Boulder Creek drains headwaters that have experienced repeated glaciation [e.g., Madole, 1976; Madole et al., 1998; Ward et al., 2009]. While the majority of 10Be ages from clasts on alluvial fill terraces range between 12 and 15 ka, Schildgen and her colleagues report two terrace samples with ages around 26 ka and one terrace location with an age of 105 ka [Schildgen et al., 2002] (Boulder Canyon 10Be ages are recalculated using the most recent production and decay rates to allow comparison with our 10 Be data). The key conclusion to be drawn from these data is that during the last glacial cycle this glacially fed river system was supplied with sufficient sediment to aggrade its bed by tens of meters. We suspect that this and other similar glacially sourced rivers draining the Front Range remained in a state of aggradation for much of the glacial cycle. It is therefore reasonable to assume that deposition on and lateral beveling of the pediment surfaces has occurred during periods characterized by glacial climate conditions. We emphasize that not all of the streams associated with these surfaces have glaciated headwaters. In these areas, the glacial climates may instead lead to an increase in frost-cracking and periglacial hillslope processes that in turn lead to enhanced sediment transport rates and higher sediment supply to the range-bounding basin [Anderson et al., 2011]. 2.2. Existing Age Control on Terrace Surfaces [10] The Rocky Flats pediment to the south of Boulder, which lies between our sample sites, is mapped as middle to late Pliocene in age and is the only surface on which absolute dating methods have been intensively used (Figure 2). Rocky Flats is fed by Coal Creek, a small stream that does not tap the glacial headwaters of the range crest. Riihimaki et al. [2006] reported measurements from four boulder samples with ages of 380 41 ka, 469 52, 546 62 ka, and 780 94 ka. One amalgamated clast sample yielded an age of 649 76 ka. They also sampled two depth profiles on the Rocky Flats surface. These reveal a complex history of exposure that entails deposition, stripping, and re-deposition. The base of the oldest profile site suggests that the bedrock surface there was cut and capped by gravel by 2.4 Ma. The top of the youngest site suggests abandonment at 400 ka. These ages, and the evidence for nonconformities in the gravels within the profile sites, imply that Coal Creek, the stream responsible for both deposition and incision, remained at roughly the same elevation for many hundreds of thousands of years, allowing reoccupation of the same surface at least twice. The locations of the sample sites, and the present profile of the stream responsible for formation of the Rocky Flats, led Riihimaki et al. [2006] to propose that the surface is being progressively abandoned from toe to tip, by headward incision of Coal Creek. Their interpretation is supported by the spatial distribution of ages: the

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younger ages of last deposition are found near the mountain front, where Coal Creek is incised only a few meters into the surface, whereas the oldest ages are located much further from the mountain front, adjacent to which Coal Creek is now incised by many tens of meters. Riihimaki et al. [2006] point out that in contrast to other pediment surfaces bounding the Front Range, Rocky Flats remains effectively “attached” to its local stream at its apex. All other surfaces, including those we focus on in our study, are strongly disconnected from their bounding streams. This likely reflects the small drainage area of Coal Creek, giving it much less stream power to incise into the underlying rock [Riihimaki et al., 2006]. Whereas the knickpoint reflecting the present position of the late Cenozoic incision wave from the South Platte is well within the mountain front for most of its tributary streams, including the streams associated with our three sampling sites [Anderson et al., 2006], the Coal Creek knickpoint has not yet reached the mountain front. [11] In contrast to the Rocky Flats surface, no other local surfaces have been subjected to such detailed age scrutiny with modern methods. The next youngest surface is the Verdos (Figure 2) [Hunt, 1954; Scott, 1963], which is mapped as early Pleistocene. This age is based on the occurrence of Lava Creek B ash (age = 640 ka) [Izett and Wilcox, 1982; Sarna-Wojcicki et al., 1987; Lanphere et al., 1999] within the terrace cover deposit at the type locality at Verdos Ranch, southwest of Denver [Scott, 1960]. The height of this surface above the local stream has since been used to correlate other Verdos surfaces throughout Denver Basin, including the one that we target in one of our sample localities (our Pioneer surface). Using 10Be, Riihimaki et al. [2006] dated three isolated boulders from a Verdos surface just south of the type locality of Rocky Flats (and just west of our Pioneer sampling site), and report a wide spread of ages: 273 29 ka, 347 38 ka, and 391 43 ka. [12] The Slocum strath terrace is mapped as middle Pleistocene and is thought to be closer in age to the next lowest surface, Louviers, than to the older Verdos pediment [Scott, 1960; Szabo, 1980; Madole, 1991]. The expected age of the Slocum terrace is around 190 50 ka (original age 160 60 by Scott and Lindvall [1970]; later corrected by Szabo [1980] and Madole [1991]) and is based on U/Th dating of a Bison horn in the Arkansas Basin in southern Colorado. Riihimaki et al. [2006] report results from 10Be exposure dating of two boulder ages from a Slocum terrace south of their Verdos site: the ages are 296 32 ka and 258 27 ka. [13] The next lower surface (named Louviers) has been associated with the Bull Lake glaciation during marine isotope stage (MIS) 6 [Hunt, 1954; Scott, 1960; Szabo, 1980; Madole, 1991]; this age is based on fossil mammals and mollusks at the type locality south of Denver [Scott, 1960]. The lowest surface (Broadway) is accordingly correlated with the last glaciation, the Pinedale glaciation of MIS 2 [Hunt, 1954; Scott, 1960; Birkeland et al., 2003].

3. Sampling Sites and Samples [14] We selected three locations for absolute dating, corresponding to the three oldest mapped terrace units: Gunbarrel Hill (pre-Rocky Flats), Table Mountain (Rocky

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Figure 3. Shaded relief image of Table Mountain area adjacent to the mountain front, based on 1 m resolution LiDAR data. The smooth low relief surface of Table Mountain is evident. A 1 km2 gravel pit on the north end of the surface is the only interruption of the surface. White dashed lines delineate the cross-section of the profiles shown in Figure 4; white star shows location of the Table Mountain CRN depth profile. Flats), and Pioneer (Verdos), which we discussed. We sampled these locations with the goal of using in situ cosmogenic 10Be measurements to determine the timing of strath terrace occupation and to constrain the subsequent onset of incision and sediment evacuation in the northern part of the Denver Basin. We collected surface clasts and boulders from Table Mountain and Gunbarrel Hill to constrain surface exposure ages using their in situ 10Be concentration. In addition, at Table Mountain and Pioneer, we sampled collections of clasts in depth profiles in pits within the cover gravels for measurement of in situ-produced 10Be and its inherited component [e.g., Anderson et al., 1996; Repka et al., 1997]. Finally, at the same localities, we sampled sediment matrix for meteoric 10Be concentrations, as a quasi-independent measure of the timing of last deposition on the terrace site. 3.1. Table Mountain [15] Table Mountain is an isolated gravel-capped mesa or strath terrace located about 3 km away from the catchment mouth of Lefthand Canyon (Figures 2 and 3). Table Mountain is mapped as correlative to Rocky Flats [Cole and Braddock, 2009], with an expected age of about 1 Ma. Several terrace surfaces at different heights above Lefthand Creek have been mapped in the vicinity of Table Mountain, but Table Mountain is clearly the highest mapped surface in this area. In great contrast with Rocky Flats to the south, Table Mountain is completely disconnected from its bounding stream. Lefthand Creek does not tap the glaciated crest of the range, but its 145 km2 catchment drains instead the broad rolling montane surface. A thin fluvial gravel layer, with thicknesses ranging from 5–10 m, caps the bedrock of the upper Cretaceous Pierre Shale Formation

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[Winester, 2009]. The Table Mountain surface extends 3 km in north-south direction, about 2 km in east-west direction, and tilts less than 1° to the north/northwest. Its cross section appears to be a truncated arch from a broad alluvial fan, with the southerly portion of the arch cut away. The southern edge of Table Mountain is about 85 m above Lefthand Creek. The surface of Table Mountain has very low relief: along a 100 m long cross-section, the short wavelengthrelief is of order 10 cm (Figure 4; see also photograph in Figure 5). The low relief suggests that the surface is quite stable, and that clasts and boulders were not affected by post-depositional reworking. Therefore, Table Mountain is ideal for the application of cosmogenic dating. [16] For the measurements of in situ and meteoric cosmogenic 10Be, we sampled a 170 cm soil pit (Figure 5). This pit is dominated by an unsorted, very uniform gravel deposit with a sediment matrix consisting of a mixture of clay, silt and sands. The coarse fraction is dominated by sandstones, granites, granodiorites, gneisses, amphibolites, and schists with grain sizes ranging from a few to about 30 cm in diameter [Cole and Braddock, 2009]. No obvious soil horizonation exists; we identified no well-defined A- and B-horizons. Many of the clasts are too large to have been moved about by rodents or other subsurface disturbance mechanisms. The profile appears to be quite weathered, based on the red-brown soil color throughout the profile and the ease of disintegration of some of the crystalline clasts, especially those with significant mica. 3.2. Sampling Details of Table Mountain [17] On the Table Mountain surface, we collected six samples from pure quartz surface boulders for in situ cosmogenic 10Be exposure dating. These boulders had diameters between 20 and 100 cm. We chiseled 2–3 cm thick rock pieces from the tops of the boulders. The boulder surfaces look intact and smooth and show only minor erosion by ventifaction. There is no evidence of surface disruption; where sampled, the ventifacted faces pointed toward the west winds. [18] From the soil pit exposure, we took four amalgamated clast samples for the in situ-produced 10Be depth profile. These samples were taken at about 35 cm, 60 cm, 120 cm, and 170 cm depths, and consisted of five to 10 sandstonequartzite clasts that ranged in size from 5–10 cm. Due to the limited wall exposure in the 200 300 cm soil pit, we were limited to collecting samples of only 10 clasts each. [19] For the measurements of meteoric 10Be, we took seven sediment samples along the 170 cm deep profile starting at 15 cm depth. The deepest sample was taken at 170 cm depth. Each sample was comprised of 100 g of soil matrix. We also measured the dry bulk soil density of the matrix at each sample depth using a container of known volume. The sampled material was dehydrated in a drying oven before being weighed. 3.3. Pioneer Site [20] The Pioneer surface is here named for the Pioneer sand and gravel pit, a commercially operated gravel mine that is located about 3 km from the catchment mouth of Ralston Canyon (Figure 2). The Pioneer sand and gravel pit occupies a surface with a maximum north-south extent of about 2 km and east-west extent of about 5 km. The Pioneer

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Figure 4. Topographic profiles across Table Mountain. (top) Topographic transect across Table Mountain through sample site (line AB on Figure 3) showing the smooth low relief surface, and its slight tilt to the north. Note the small depth of the gravel pit at the northern end of the profile, which serves as an indicator of the 5–10 m thickness of the gravel cap on the Table Mountain surface. (middle) Topographic profile normal to line AB (line CD on Figure 3, intersecting at the sample site) showing gentle uniform tilt away from the mountain front. Note abrupt edges of the surface on all sides, and that these edges have little to no affect on the broader surface at distances of even a few meters away from the edge. (bottom) Detail of profile near sample site covering 150 m with much greater vertical exaggeration revealing the 10–30 cm local relief of the surface. While the total elevation difference is less than 50 cm, the profile demonstrates that the amplitude of short-wavelength topography is on the order of only 10–20 cm over 10 m distances. surface is about 65 m above Ralston Creek. The Pioneer site is mapped as Verdos in age [Kellogg et al., 2008], which is inferred to correspond to an age of about 640 ka. Near our Pioneer site, closer to the mountain front, Lava Creek B ash has been reported within the gravel deposits that cap the Verdos surface [Izett and Wilcox, 1982; Van Horn, 1976; Scott, 1962]. This ash locality is about 2 km to the west of our sample location [Machette et al., 1976], but the stratigraphic and topographic relationship between the ash locality and our sampling site is unclear. Two other ash deposits have been described to the south of our Pioneer sampling site. These sites are in Golden, on surfaces that are presumably related to the evolution of Clear Creek. Urban development in Golden has led to the removal of these

outcrops. We also failed to find ash deposits exposed in either the walls in the Pioneer pit or elsewhere near our sampling site. Several other terrace surfaces, mapped as Verdos and Slocum, are located east and south of the Pioneer location (Figure 2). [21] We have no data available on the thickness of the gravel layer at this location, but based on the depth of the gravel pit in this area, we suggest that gravel thicknesses are comparable to the 5–10 m thickness on Table Mountain. The Pioneer pit site exposes an excellent section of fluvial gravels with interbedded sands. For the measurements of in situ cosmogenic 10Be, we sampled one profile of 370 cm depth in the edge of an inactive portion of the gravel pit (Figure 5). Here, the stratigraphic units were easily distinguished, with a

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depths, and that mining activities had not altered the local surface outside of the pit. 3.5. Gunbarrel Hill [23] Gunbarrel Hill is located about 12 km from the catchment mouth of Boulder Canyon (Figure 2). The surface area is only about a quarter of that of Table Mountain, making it the smallest among our three study sites. The southern edge of Gunbarrel Hill is about 100 m above the modern riverbed of Boulder Creek. We have no measurements of the gravel thickness at our sample site. However, gravel exposure on the south side of Gunbarrel Hill, where Boulder Creek undercuts the vertical cliff face of Gunbarrel Hill, shows that the gravel is only a few meters thick there. The surface of Gunbarrel Hill is neither as smooth nor as planar as the Table Mountain surface. Gunbarrel Hill is mapped as pre-Rocky Flats in age [Cole and Braddock, 2009]. Based on the traditional correlation of terrace heights above the local riverbed and the age of the Rocky Flats surface from Riihimaki et al. [2006], the expected age of Gunbarrel would be older than 2.4 Ma.

Figure 5. Photographs showing (a) the surface of Table Mountain, and soil profiles (b) on Table Mountain and (c) at the Pioneer location. Note the low relief of the Table Mountain surface. The Table Mountain profile is 170 cm deep; the yellow tape in Figure 5c is 250 cm long. sand lens extending from 1 to 3 m depth separating coarser units. For the meteoric 10Be, we were not able to sample the same vertical wall, as continuing mining operations had removed our initial profile between our visits to the pit. We therefore sampled a 240 cm vertical wall about 100 m horizontally from our first sample location. In this exposure, the boundaries between the sediment layers were generally not very well-defined. The coarser units were poorly sorted and the gravels were poorly imbricated. The clasts in the profiles were weathered, although they appeared to be less weathered than those at the Table Mountain site. The coarse fraction was dominated by granites, granodiorites, gneisses, amphibolites, schists, and quartzites [Kellogg et al., 2008], with grain sizes ranging from a few cm to about 30 cm in diameter; sizes comparable to those that dominate the Table Mountain site. 3.4. Sampling Details of Pioneer Site [22] In the pit wall from which we extracted samples for in situ 10Be, we took six amalgamated clast samples from about 30–370 cm depth, consisting of 30 rounded to subangular clasts with diameters between 2 and 3 cm. For meteoric 10 Be, as on Table Mountain, we collected about 100 g of soil at six different depths, starting at 30 cm below the surface. The surfaces of both profiles showed little disturbance, suggesting that the sampled depths represent the original

3.6. Sampling Details of Gunbarrel Hill [24] We collected two amalgamated samples from the Gunbarrel Hill crest, with 50 rounded quartzite clasts per sample. The clasts were collected on the highest part of Gunbarrel Hill, where the surface compared to most other parts of this terrace is relatively planar and smooth with little surface relief. The average clast diameter is 3 cm in sample GB-A, and 5 cm in sample GB-B.

4. Sample Preparation, Measurement, and Data Processing 4.1. In Situ 10Be [25] The 10Be samples, both in situ-produced and meteoric, were processed at the University of Colorado cosmogenic isotope laboratory. We used procedures according to Kohl and Nishiizumi [1992], Ivy-Ochs [1996], and Ochs and Ivy-Ochs [1997] for the preparation of in situproduced 10Be samples. The AMS measurements were performed at Purdue University’s PRIME Laboratory. The ratios of our samples were determined using an ICN revised 10 Be standard [07KNSTD; Nishiizumi et al., 2007]. We ran five process blanks altogether and used their average ratio of 1.34 0.47 1014 for correction of the measured isotopic ratios. Exposure ages were calculated using the CRONUSEarth online age calculator (version 2.2; http://hess.ess. washington.edu) [Balco et al., 2008]. At the time of our calculations, the CRONUS calculator used an updated value of the 10Be half-life of 1.387 0.012 Ma, measured by Korschinek et al. [2010] and Chmeleff et al. [2010]. The scaling of production rate to sample latitude and elevation follows Stone [2000]. We report ages that are based on the time-dependent reference 10Be production rate model, abbreviated “Lm” in Balco et al. [2008]. The used sea level, high latitude spallation production rate is 4.39 0.37 10Be atoms (g quartz)1 yr1 [Balco et al., 2008; updated values are available in the CRONUS calculator documentation on http://hess.ess.washington.edu]. The ages are based on an attenuation length scale of 160 g/cm2.

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[26] For the depth profile age estimates, we swept through a suite of models and assessed the model misfit using the c2 statistic to constrain the best fit solutions for our measured data: c2 ¼

N 1X Ccalc ðiÞ Cmeas ðiÞ 2 N i¼1 1smeas

ð1Þ

where N is the number of samples in the profile, Ccalc is the calculated concentration of 10Be, Cmeas is the measured 10Be concentration in the sample, and smeas represents the average measurement uncertainty. [27] The calculated 10Be concentration at a particular depth and age includes the role of inheritance, decay, and production: C ðz; t Þ ¼ Cin elt þ

P0 ez=ð160=rÞ 1 elt l

ð2Þ

where l is the decay constant for 10Be, Cin is inherited concentration (assumed to be uniform with depth), t the depositional age, z the depth of the sample, and P0 is the production rate from nucleon spallation. We ignore the importance of muogenic processes due to the fact that both our depth profiles are so shallow that the muogenic contribution to the total production rate is negligible. In accord with several characteristics of these surfaces, to be discussed in the discussion section, we ignore surface erosion in these calculations. For a prescribed bulk density, we sweep through a suite of ages and Cin to generate a matrix of model misfits, from which the best fitting age and Cin is reported. We then sweep through all possible densities. [28] This strategy follows similar approaches for the treatment of cosmogenic depth profiles by Siame et al. [2004], Wolkowinsky and Granger [2004], Riihimaki et al. [2006], [Braucher et al., 2009], and Schaller et al. [2009]. In our c2 analysis, we explore ranges of three parameters: depositional age, 10Be inheritance, and soil density. For both profiles, the tested ages range from 0 to 300 ka and the model age step is 1 ka. We explore the inheritance from zero to twice the 10 Be concentration of the deepest sample, using a step size of 1 103 10Be atoms (g quartz)1. Densities are tested from 2200 (saturated) to 1495 kg m3 (dry, as constrained by our measurements) with a step resolution of 35 kg m3. Soil density is allowed to vary between runs. While we measured soil densities in the meteoric 10Be profiles, we do not have these data available for either in situ 10Be profile. The best fit profile solution is presented as the combination of parameter values that minimize the sum of the c2 misfit. We used the mean s error instead of the individually measured uncertainty of each sample in order to increase the strength of the control of the fit by our uppermost sample; in the Table Mountain profile, this sample has relatively high uncertainty compared to the other samples, which therefore leads to a poor fit between the measured and calculated uppermost sample concentration if we use individual errors in our model scoring. 4.2. Meteoric 10Be [29] Meteoric 10Be was extracted from 1.0 g of soil that had been subsampled and ground from 100 g of each field sample. For the preparation of meteoric 10Be samples, we followed the revised instructions (http://depts.washington.

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edu/cosmolab/chem/fusionmethod.pdf) reported by Balco [2004] based on Stone [1998]. As for the in situ-produced 10 Be, the AMS measurements were performed at PRIME Laboratory. We referenced our samples to KNSTD07 assuming a 10Be half-life of 1.387 0.012 Ma [Chmeleff et al., 2010; Korschinek et al., 2010], and used a 10Be decay constant of 4.998 0.043 107 yr1. The age of a sampled soil section can be calculated from the total inventory of meteoric 10Be in the profile. The total meteoric 10Be inventory is calculated according to I10Be ¼

n X

ðNmeas Ninh Þrsoil z

ð3Þ

i¼1

where I10Be is the total 10Be inventory in the soil profile attributable to post-depositional accumulation, Nmeas is the measured 10Be concentration, Ninh is the inherited 10Be concentration, r is the soil density, and z is thickness of each sampled soil layer. As in the application of in situ 10Be, it is important to determine the amount of inheritance in a profile that is contributing to the apparent age. In this study, we assess inheritance of meteoric 10Be by subtracting the lowest measured concentration in the bottom part of the profiles from each measured sample and then integrate all concentrations over the entire depth of the profile. [30] To calculate ages from these concentrations, we use the only available empirical short-term meteoric 10Be delivery rate of 5.2 105 atoms cm2 yr1 from Salt Lake City [Monaghan et al., 1986] as being representative of the long-term delivery rate. As there are no contemporary meteoric 10Be flux measurements available for Colorado, we believe that our use of the Salt Lake City measurements as reference value can be justified. Our field site is 700 km to the east of Salt Lake City, with a similar latitude and climatic setting (mean annual precipitation Boulder: 50 cm; mean annual precipitation Salt Lake City: 40 cm; http://www. wrcc.dri.edu). While the modification is small, we do apply a correction of +3% to account for the lower intensity of the geomagnetic field when measured over periods greater than 20 ka, which leads to increased long-term average 10Be production rates [Masarik and Beer, 2009]. Our final rate of delivery used in our calculations is therefore 5.0 105 10Be atoms cm2 yr1. 4.3. Soil Properties Analysis [31] We also analyzed soil, mineralogy and grain sizes along the two depth profiles to characterize the soil properties and the state of soil development. For grain size analysis, we measured the size distribution of the