Critical zone evolution: Climate and exhumation in the Colorado Front Range
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Suzanne P. Anderson, Robert S. Anderson, Gregory E. Tucker, David P. Dethier, 2013. "Critical zone evolution: Climate and exhumation in the Colorado Front Range", Classic Concepts and New Directions: Exploring 125 Years of GSA Discoveries in the Rocky Mountain Region, Lon D. Abbott, Gregory S. Hancock
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The architecture of the critical zone—the distribution of mobile regolith, the thickness of weathered rock, and their characteristics, as well as the topography of the land surface—is shaped by erosion and weathering processes that depend upon both lithology and climate. In this trip we explore the Boulder Creek watershed, a landscape that juxtaposes uplifted Precambrian crystalline rocks of Colorado’s Front Range against Mesozoic marine sedimentary rocks underpinning the western edge of the High Plains. The landscape is strongly shaped by Quaternary climate cycles operating on this template inherited from the Laramide orogeny. Stop 1 will provide an overview of the abrupt topographic step at the Front Range–High Plains join, where we will discuss fluvial strath terraces on the Plains. At Stop 2 in Betasso Preserve, we will discuss the impact of the canyon cutting set off by late Cenozoic exhumation of the High Plains on the hillslopes and groundwater systems lining the master stream. At Stop 3, we will hike 2 miles down Gordon Gulch, a focus site in the Boulder Creek Critical Zone Observatory. At stops on the hike, we will discuss exhumation rates, climate-modulated weathering, hillslope hydrology and hillslope sediment transport, and the influence of slope aspect on these processes. Our goal is to focus on the history of climate-driven erosion and weathering processes, and how to incorporate these processes into quantitative models of landscape evolution.
The Front Range is one of a number of mountain ranges formed in the compressional tectonics of the Tertiary Laramide orogeny (e.g., Bird, 1988, 1998; Dickinson et al., 1988). At this location, starting at roughly 65 Ma, the orogeny thrust Paleoproterozoic crystalline rocks up against the marine sediments from the Cretaceous Western Interior Seaway. The first-order topographic feature of the watershed arises from this lithologic contrast set by tectonic history: the crystalline rocks of the Front Range tower above the soft sedimentary rocks of the Plains, separated by an abrupt topographic front. As the name suggests, the Plains comprise smooth topography with broad fluvial terraces standing in low relief above the modern rivers. The Front Range is characterized by more rugged terrain cut into crystalline rocks, notably deep canyons gashed upvalley from the Plains, and steepsided glacial valleys hugging the range crest (see R.S. Anderson et al., 2006; S.P. Anderson et al., 2012).
The canyons cut along the eastern edge of the Front Range reflect an acceleration in regional erosion, including erosion of the proximal part of the Denver Basin, beginning in the Pliocene (Fig. 1). During this time, east of the Front Range, the South Platte River and its tributaries cut through the Miocene Ogallala Group sediments, as well as older Tertiary sediments (Fig. 2).
The headwaters of Boulder Creek were glaciated during the Pleistocene (Fig. 3), which is manifested in U-shaped valleys and areas of bare, abraded bedrock surfaces in the valley floors and walls. Below the glacier limit is the Rocky Mountain Surface, a region of relatively low relief mantled by thin soils and dotted with tors (outcrops). This area is a product of weathering and erosion since the end of the last phase of uplift in the Laramide orogeny. At its outer, eastern margin, the Rocky Mountain Surface is dissected by river canyons, each with a prominent knickpoint or knick zone at its head (R.S. Anderson et al., 2006). Hillslopes lining the canyons are stripped of weathering profiles, although the mechanism is base-level lowering rather than glacial abrasion (Fig. 3). Hillslopes are steepest and show the least weathering in the knick zone, and become progressively less steep and more weathered with distance downstream from the knick zone, a progression that reflects the evolution of the critical zone following the perturbation of rapid river incision. Finally, on the Plains, topography reflects a bimodal erosion history, in which the rivers either broaden their footprint by lateral planation, or cut into bedrock and abandon the alluvium-mantled strath terraces (see Fig. 3). The long periods of lateral planation, punctuated by periods of fluvial incision on the Plains (Dühnforth et al., 2012), are coincident with aggradation and removal of fluvial fill terraces in the canyons. Headward progress of the knick zone up the canyon accelerates when canyon fill terraces are removed, and decelerates when fill aggrades.
Understanding how erosion and weathering together shape the architecture of the critical zone forms the core question of the Boulder Creek Critical Zone Observatory (S.P. Anderson et al., 2008). The question unites exploration of topographic signatures of erosion, understanding distributions of surface features such as soil, outcrops, and vegetation, and deciphering the subsurface reach of weathering and movement of water. In each of the varied erosional settings of the Front Range and adjacent Plains, the architecture of the critical zone represents the integrated effects of weathering front propagation and erosional exhumation over time operating on the particular rocks with their particular tectonic histories as they are brought to the surface by exhumation. In the alpine regions of the Front Range, rapid erosion and low chemical weathering front propagation rates under glaciers explains the truncated weathered profiles. Soils in the alpine areas are found only on deposits, rather than from in situ weathering of the bedrock. For instance, moraines, glacial till, talus and colluvial slopes, and very old diamicton (poorly sorted sediment of unspecified origin) on the ridge tops above recent glacial limits are deposits in which weathering processes produce soils on preexisting sediments, augmented by eolian inputs (Muhs and Benedict, 2006). The deepest weathering profiles are found on the Rocky Mountain Surface, where, although spatially variable, they average 7 m in depth (Dethier and Lazarus, 2006). In contrast, on hillslopes lining the fluvial canyons, bare rock slopes dominate. Slope angles, extent of mobile regolith cover, and rock weathering vary owing to differences in time since perturbation by fluvial incision. The weathered profile is thinnest and least developed along the river channel and near the knick zone, places where the erosion rates exceed weathering front propagation rates. One can assess development of the critical zone in any of these locations as a one-dimensional problem of weathering processes generating the critical zone, and erosion removing material from the critical zone. Because rates of river incision respond to sediment flux, the production and transport of sediment upstream can influence process rates downstream.
Stop 1: Ncar Table Mesa Laboratory Parking Lot
The eastern edge of the Front Range is abrupt, forming the scenic backdrop to the Denver metropolitan area. This results from the juxtaposition of easily eroded rocks of the High Plains (largely Cretaceous shales) against Precambrian crystalline rocks of the core of the Front Range. The topographic step and the change from smooth to rugged local relief at the mountain front reflect not active faulting, but a difference in the erodibility of the rocks.
A short walk downhill from the parking lot of the National Center for Atmospheric Research (NCAR) laboratory, built in the 1960s to the design of I.M. Pei, provides a view of the prominent fluvial strath terraces cut into the soft bedrock of the Plains by rivers draining the Front Range (Fig. 4). To the south, the most extensive and oldest of these is the Rocky Flats surface, now host to the large wind turbines being tested at the National Renewable Energy Laboratory (NREL) facility seen on the horizon. Between the Rocky Flats surface and the viewpoint runs South Boulder Creek, which is bounded by several lower, younger surfaces. Each of these surfaces consists of a beveled bedrock terrace (strath terrace) etched across Laramide-warped Cretaceous rocks, and capped with several meters of alluvial gravel sourced from the crystalline rocks of the range. At the viewpoint itself, locally sourced sedimentary rocks, comprised of sediments shed from the Ancestral Rockies, cap the surface. These sedimentary units, including the Pennsylvanian-age Fountain Formation, were uplifted and tilted in the Laramide orogeny and subsequently exhumed to create the Flatirons that form the dramatic backdrop to the NCAR site and the city of Boulder.
The terraces have long been mapped (e.g., Scott, 1960, 1962; Madole, 1991; Cole and Braddock, 2009), and are given names reflecting correlations based largely upon height above the adjacent streams and degree of soil development (Fig. 5). Dating of these surfaces can constrain long-term exhumation of the edge of the Plains. Recent research employing cosmogenic radionuclides 10Be and 26Al has revealed comparatively young ages of several of these surfaces (Riihimaki et al., 2006; Dühnforth et al., 2012). These ages, best interpreted as the timing of last deposition on the surface, support a conceptual model of the formation of strath terraces in which sediment supply governs switching between lateral widening of valley walls and vertical incision of channel floors (Hancock and Anderson, 2002). The widths and times of last deposition on these surfaces suggest that they are occupied and widened for long periods of time corresponding to glacial climates, and are abandoned in rapid pulses of vertical incision into the Cretaceous shale during deep interglacials. As these surfaces occur at the outlets of both drainage basins that drain the glaciated crest and those that do not access the glaciated crest (green and red channels in Fig. 5, respectively), access to glacially derived sediment is not required. We hypothesize that modulation of the sediment supply from nonglaciated basins is also slaved to climate, as the landscape becomes more productive of sediment when periglacial processes reign during glacial times. Rates of incision into the Cretaceous Pierre Shale on the Plains during times of very low sediment supply appear to exceed 1 mm/yr (Dühnforth et al., 2012). This argument suggests a strong role for critical zone processes in governing the topographic form of the western edge of the High Plains.
Key Points at Stop 1
The Laramide orogeny juxtaposed hard crystalline rocks against soft sedimentary rocks, setting up the framework of bedrock with contrasting erodibility in the present landscape.
Alluvium-mantled, fluvial strath terraces record exhumation on the Plains during the Quaternary. While onset of exhumation is not well established, cosmogenic dating of these surfaces constrains the pace of incision in the past few hundred thousand years.
The terraces appear to be occupied and widened during glacial periods and abandoned due to vertical channel incision during deep interglacials.
Stop 2: Betasso Preserve
Betasso Preserve, part of the Boulder County Parks and Open Space system, includes areas of rolling terrain of the Rocky Mountain Surface and steep slopes slaved to the incision of Boulder Creek. Within the crystalline rocks of the Front Range, exhumation takes the form of canyon cutting, rather than terrace planation and abandonment seen at Stop 1 on the Plains (Fig. 3). Each river draining eastward in the Front Range has a knickpoint or knick zone indicating the headward progress of exhumation. The steep bedrock walls lining the canyons at knick zones give way to shallower slopes with varying degrees of mobile regolith cover with increasing distance downstream (Fig. 6). Boulder Creek Critical Zone Observatory selected a steep watershed within Betasso Preserve for monitoring to represent areas that have undergone transient erosion in response to base-level lowering (S.P. Anderson et al., 2008). The informally named ephemeral Betasso creek discharges into Boulder Creek ~7.5 km downstream from the knick zone. The watershed therefore represents the state of evolution several million years after initial rapid base-level lowering associated with passage of the knick zone in Boulder Creek, assuming knickpoint celerity of 2.5–5 mm/yr. Base level for this watershed, set by Boulder Creek, has probably varied over time due to alluviation and scour over glacial-interglacial cycles, and possibly due to multiple waves of incision from lowering on the Plains.
Just above its confluence with Boulder Creek, Betasso catchment is steep and flanked by steep slopes (Fig. 7). Bedrock outcrops in the lower catchment are visible on the light detection and ranging (LiDAR)-derived shaded relief map, especially on the western slopes. In the bowl-shaped upper catchment, however, a colluvial fill that in places exceeds 4 m in thickness can be discerned from the smooth topography etched with several deep gullies. Bedrock ribs are again visible around the upper catchment divide. We will examine only one weathered profile in the watershed, in a location near the catchment divide and above the thick colluvium. The impression given by patterns of weathering, the topography, and the distribution of colluvium in the catchment is that base-level lowering has impacted the lower catchment but has had little influence on the upper catchment.
A borrow pit located at the top of the Betasso Link bike trail, near its intersection with Betasso Road (the access to the City of Boulder water filtration plant), provides an exposure of a weathered profile near the top of the watershed in the low-relief surface. The profile comprises a 15-30-cm-thick layer of mobile regolith over several meters of saprolite (Fig. 8) developed in the early Proterozoic Boulder Creek granodiorite (Gable, 1980). The presence of highly erodible saprolite at this location shows that hillslope adjustment associated with the base-level lowering along Boulder Creek has not propagated all the way to the catchment divide.
The profile shows minor enrichments in kaolinite and Feoxides in the top 50 cm, and a far greater enrichment in smectite (Fig. 8). Smectite and Fe-oxides are found in hydrothermally altered bedrock throughout the Boulder Creek batholith (Dethier and Bove, 2011). However, the lack of Fe-oxides below ~50 cm depth, and the strong bulge in smectite concentrations at the top of the saprolite suggests that in this profile these are neoformed minerals due to weathering of the bedrock. The smectite bulge coincides with a drop in plagioclase concentration, further supporting this interpretation. However, several lines of evidence suggest that more is going on here. First, the Fe-oxides are dominated by magnetite, a primary mineral in the granodiorite parent rock. Second, the large amount of smectite throughout the profile is unusual, and more than is easily explained by pedogenesis alone. Clay fills microfractures in quartz and plagioclase grains in thin sections from the profile, even at 150 cm depth (Fig. 9). The drop in plagioclase is associated with a change in parent material lithology suggested by a change in Ca/Na ratios in bulk samples (not shown).
These features may arise from eolian inputs of smectite-rich material, or from a layer of hydrothermally altered parent material. The domination of smectite over kaolinite in the clay phases suggests that leaching of cations is incomplete, as one might expect in the semi-arid climate of the Front Range.
Monitoring of soil moisture in saprolite at two sites in the Betasso watershed reveals that saprolite below about half a meter in depth tends to stay relatively dry (below 5% volumetric water content) throughout the year. One of these sites is at the borrow pit exposure (Fig. 10). During 2011, the deeper saprolite moisture sensors responded to late spring snow (April), early summer rain, and to October rains, but did not respond to other events during the year. These data support the hypothesis that the availability of water limits the rate of chemical weathering in saprolite in this semi-arid environment.
Understanding the deep flow system and full weathered profile were targets of a project to drill the ridge at Betasso in January 2013. Situated at the drainage divide above the borrow pit, drilling reached 124 m (406 ft). Core was recovered over about half of the borehole, primarily the top of the boring (Fig. 11). Extensive sections were air-hammered without core recovery owing to friable rock, apparently due to hydrothermal alteration of the granodiorite. The fracture density increased dramatically just above the first air-hammered section at ~53 m (175 ft). The completed well is 121 m deep (398 ft), due to debris filling in the bottom 3 m (8 ft). It is cased to a depth of 60 m (198 ft), and screened from 60 to 121 m (198-398 ft). During drilling, a prominent fracture encountered at 43 m (143 ft) was flagged as possibly water filled based on dramatic drop in the rate of loss of drilling water during drilling, although no water was detected in the borehole after sitting overnight. Water inflow at a low rate (estimated ~1.9 L/min, or ~0.5 gal/min) was detected at 101 m (331 ft), which caused the water level in the borehole to rise 17 m (56 ft) overnight. In the months since drilling was completed, the water level measured manually in the well has hovered around 29 m (95 ft) depth, and rose to 28 m (91.6 ft) following heavy snows in April. An automated water-level sensor was installed in May 2013.
In addition to providing data on deep-water dynamics, the core and samples collected during air hammering will be used to analyze alteration products, fractures, and rock characteristics. We hypothesize that the vertical pattern of rock alteration from the borehole may reveal evidence of dropping of base level associated with incision of Boulder canyon (Fig. 6B).
Key Points at Stop 2
The 1.7 Ga granitic rocks of the Boulder Creek batholith exposed here are weathered to soil and saprolite. In places, local hydrothermal fluids, probably during and after the Laramide orogeny, altered rock at depths well below the top-down weathering profile.
The strong soil here is smectite-rich, suggesting both incomplete leaching and additions to the soil, perhaps from eolian activity.
Hydrologic monitoring suggests that the saprolite below a depth of 0.5 m remains dry most of the time and that the water table is >28 m below the surface at the divide.
Extensive areas of soft saprolite and local colluvium >4 m thick in the upper watershed show that base-level lowering associated with deepening of Boulder Canyon has not strongly impacted the upper part of the Betasso catchment.
Drive Up Boulder Canyon Between Stops 2 and 3
Between Stops 2 and 3, we drive up Boulder Canyon. In the lowermost canyon, the canyon walls are steep and rocky. Block sizes and outcrop patterns are governed largely by fracture patterns that reflect the Laramide stress field through which these crystalline rocks moved tens of millions of years ago. Trees pry blocks off the hillslopes by root growth in cracks (Fig. 12). We drive past Boulder Falls, a discrete knickpoint in North Boulder Creek near its confluence with Middle Boulder Creek. The road follows Middle Boulder Creek. For the next few kilometers upstream, the creek remains very steep (signs advise drivers going downhill of 10% grades), and the rocky walls are steepest, providing many rock-climbing challenges. A few kilometers above the confluence of Middle and North Boulder Creeks, the slope of the river declines, and the valley walls relax to much less rocky tree-cloaked hillslopes. We have passed through the knick zone of Boulder Creek, which represents the position in the landscape upstream of which the stream and hence the adjacent landscape has not “felt” the exhumation of the High Plains (Fig. 6A).
A few fluvial fill terrace remnants, rare fluvial strath terrace remnants, and isolated bedrock potholes are found within Boulder Canyon, most ~7-10 m above the present channel. These are dated to 12-20 ka based on 10Be exposure ages (Schildgen et al., 2002), and they record aggradation and degradation of alluvial fill terraces within the canyon during and immediately following the end of the last glaciation. These erosional and depositional features presumably record temporal variations in the production and exhumation of glacial sediment in the headwaters. Exhumation within the crystalline rock of the mountains differs in style and form from exhumation on the softer bedrock of the Plains. Rather than forming broad strath steps in the landscape, incision of rock by Boulder Creek is accomplished by a knick zone working its way upstream in the hard rock, casting off rock slopes that slowly recover following this passage (Fig. 6A). Downcutting in both environments appears to be associated with interglacial climates, when sediment supply declines; planation of straths on the Plains and aggradation of alluvium in the canyon occurs during glacial climates.
After passing Barker Meadow Dam, impounding Barker Reservoir, we obtain views of the crest of the Front Range. The Last Glacial Maximum (LGM) and earlier glaciers extended almost to the western edge of the town of Nederland on the west side of Barker Reservoir. The moraines there and in Green Lakes valley provide our best constraints on the timing of the maximum extents of the LGM and the penultimate glaciations (Schildgen, 2000; Schildgen et al., 2002).
For lunch, we will drive to the University of Colorado Mountain Research Station (MRS), which is situated on the Arapaho Moraine from glaciers in Green Lakes valley. The MRS is a research facility of the Institute of Arctic and Alpine Research devoted to advancement of study of mountain ecosystems. The MRS runs the Mountain Climate Program, which has measured meteorological data from four stations since 1952 (Pepin and Losleben, 2002).
Stop 3: Gordon Gulch Watershed
Starting from the drainage divide a few km east of the Peak-to-Peak highway, we will hike down the axis of the 2.7 km2 Gordon Gulch drainage basin, exploring the evolution of hillslopes that bound the creek.
First, look west to see the summits of the Indian Peaks that grace the Continental Divide (Fig. 13). The valleys tuck little scraps of glaciers, namely the Arapaho, Isabelle, and Arikaree, into their head cirques; these bodies of ice are only a few tens of meters thick. In the LGM, and in many glacials before that, the glaciers fully occupied valley heads scooped into the range crest (Figs. 3 and 5). The advances and retreats of these glaciers paced the delivery of sediment to the High Plains in the major streams. The timing of the LGM and penultimate glacial advances has been established by 10Be concentrations in morainal boulders near Nederland (Schildgen, 2000; Schildgen et al., 2002). The timing of retreat of glaciers in Green Lakes valley and in Fourth of July valley has been established using 10Be concentrations in glacially polished rock knobs (Ward et al., 2009; Dühnforth and Anderson, 2011). This history in Green Lakes valley records a detachment of the glaciers from their morainal belts by 18 ka, stalling of retreat ca. 14 ka, and rapid final deglaciation that was completed prior to 12 ka.
Gordon Gulch (Figs. 5, 14) lies beyond the glacial limit, however, and so has evolved in the absence of direct influence of glaciers. The east-west-trending watershed provides a natural experiment in which the influence of the surface energy balance imposed by differing slope aspects on critical zone development can be explored. The bedrock here is primarily early Proterozoic migmatitic sillimanite-biotite gneiss intruded by small pods of Proterozoic granites (Gable, 1996, 2000). We find differences in critical zone architecture on opposing slopes that are more than skin deep (S.P. Anderson et al., 2010, 2011). Contrasts in vegetation and snow cover on the north-facing and south-facing hillslopes are striking and obvious, even to the casual observer (Fig. 15). North-facing slopes are dominated by Lodgepole pine (Pinus contorta); south-facing slopes are dominated by open stands of Ponderosa pine (Pinus ponderosa), with much wider swaths of meadow. The tree cover and slope-related differences in incoming solar radiation generate great differences in the delivery of snowmelt to the subsurface, and in the ground surface temperature histories. Seismic refraction profiling (Fig. 16) revealed that weathered rock is ~8-14 m in thickness, and extends up to 2 m deeper on north-facing slopes than on south-facing slopes (Befus et al., 2011). The geophysical transects imply that differences in weathering and erosion rates have persisted for ~100 thousand years, based on local cosmogenic-radionuclide denudation rates (Dethier and Lazarus, 2006; Shea et al., 2012; Foster et al., 2012). The present critical zone architecture therefore contains a legacy of processes that have operated during Quaternary climate cycles (Leopold et al., 2013).
Although the depth to fresh rock varies systematically with slope aspect (Fig. 16), the character of weathering on the two slopes displays only subtle differences. Measurement of rock strength shows that rock in tors and rock beneath mobile regolith is significantly more weathered (weaker) on north-facing slopes than on south-facing slopes, although differences in chemical analyses were insignificant (Kelly et al., 2011; Kelly, 2012). Mobile regolith is slightly thinner on south-facing slopes than north-facing slopes, and there is a downslope thickening evident on steeper slopes (Shea et al., 2012; Shea, 2013). Microbial community diversity correlates with soil pH and C:N, which themselves are structured by slope aspect (Eilers, 2011). Microbial communities showed greater diversity with depth in soil profiles than across slope aspects and vegetation communities (Eilers et al., 2012). Organic matter leached from the same profiles yielded complementary findings (Gabor and McKnight, 2011). In fact, these two studies show that the variability that is so obvious at the surface (as reflected in the macroflora in Fig. 15) is much less apparent at depth in the mobile regolith. However, the organic matter becomes increasingly microbial in origin with depth, and the humic material in the upper saprolite sampled appears to be used as an electron shuttle by microbes involved in bedrock weathering (Gabor and McKnight, 2011).
The variations in thickness of the critical zone (defined here as depth to unweathered rock) on opposing slopes are produced over timespans of ~105 years and require contrasting conditions at depths of more than 10 m. Hillslope hydrology is strongly modulated by surface energy balance, and is affected by contrasting conditions on different slope aspects that persist through climate cycles. We therefore turn our attention to water and its movement in the subsurface as a cause of the spatial patterns in critical zone architecture. The annual hydrograph shows a snowmelt-dominated system, with peak discharge and peak water table height generally corresponding to the timing of loss of the snowpack on north-facing slopes (Fig. 17). As the sandy soils have very high infiltration rates, all water that does not evaporate enters the subsurface. This region is on the margins of the areas that develop a seasonal snowpack in the Front Range: a snowpack is sustained on north-facing slopes but is transient on south-facing slopes. Tracer experiments (Hinckley et al., 2012) revealed rapid delivery of water through soil during snow melt on north-facing slopes, while water movement on south-facing slopes was episodic, and was dominated by preferential flow. Calculations with VS2DI, a two-dimensional numerical model of vadose-zone dynamics, imply that aspect-controlled differences in the timing of snowmelt alone, independent of any variation in evapotranspiration (ET), are sufficient to account for the surprisingly strong contrast in soil moisture between the opposing slopes (Fig. 18). The differences in snow-pack evolution on north-and south-facing slopes (Jepsen et al., 2012) would therefore be expected to produce greater water delivery to underlying rock on north-facing slopes where sustained input of water during melt of an accumulated snowpack is possible (Langston et al., 2011). The conceptual model emerging (Fig. 19) is that water flux downward through variably saturated mobile regolith is controlled by delivery rate and duration, and losses through evapotranspiration, all of which vary greatly with slope aspect. At times of rapid delivery of water, relatively impermeable rock diverts water laterally. Fractures dominate the deep flow system, and the low effective porosity of this system allows large swings in the depth to groundwater, as seen in preliminary well data from Betasso.
Several lines of evidence point to differing rates of sediment transport and sediment residence time on north-versus south-facing slopes. Analysis of clay and iron accumulation rates on stable sites in the Colorado Front Range provides a basis to assess residence time for mobile regolith in Gordon Gulch catenas (Wyshnytzky and McCarthy, 2011; Dethier et al., 2012). We have also explored the use of both in situ and meteoric 10Be in the Gordon Gulch catenas (Fig. 20).
In situ and meteoric 10Be analyses yield steady-state lowering rates that range between 45 mm/yr (N-facing) and 30 mm/yr (S-facing) in Gordon Gulch. In addition, mobile regolith residence times on the north-facing slope (16 ka) appear to be slightly higher than those on the south-facing slope (15 ka) (Wyshnytzky and McCarthy, 2011; Shea et al., 2012; Shea, 2013; Foster et al., 2012). These conform well with the average residence times derived from iron and clay contents at the same sites (Dethier et al., 2012).
The thermal state of the landscape strongly governs the rates of physical weathering by frost cracking, and of downslope transport of mobile regolith. Modern mean annual ground surface temperatures in Gordon Gulch are only a few degrees above 0 °C. Because LGM conditions were likely 4-7 °C colder (e.g., Dühn-forth and Anderson, 2011), it is likely that frost-related processes of variable strength have dominated the geomorphic system here over the Quaternary. R.S. Anderson et al. (2012) put forward a climate-driven model of weathering and sediment transport in which they attempt to reproduce the critical zone structure and erosion rate differences described above. The model focuses on frost cracking as a climate-modulated weathering process and frost creep as a climate-controlled transport process. Because frost-related transport rates are slower on warmer south-facing slopes, steeper slopes evolve to drive the mobile regolith transport required to balance soil production by frost cracking. The interfluve—the divide between channels—in the model therefore evolves toward an asymmetrical profile in which the divide migrates toward the south-facing slope (Fig. 21). In addition, the frost damage is shallower on the warmer south-facing slope, yielding a shallower damage zone, in accord with seismically sensed depth to fresh rock.
Elements of reality observed on slopes of both Gordon Gulch and Betasso catchments that are not captured in this model include the prominence of rocky outcrops (see Fig. 14) and the associated spatial variation in soil thickness (Shea et al., 2012; Shea, 2013). In Gordon Gulch, these reflect in part the heterogeneity of the Precambrian bedrock (interfingering of Boulder Creek granodiorite and schists), and in part variations in the fracture density.
Key Points at Stop 3
The importance of hillslope aspect is readily apparent in the contrasting macro vegetation and snow cover on the north- and south-facing slopes.
Microbial communities and organic matter characteristics vary with slope aspect; they vary more strongly with depth, and point to a microbial role in rock weathering.
Shallow seismic refraction surveys show that weathered rocks extend 8-14 m below the surface and that the depth to fresh rock is greater on north-facing slopes.
Erosion rates measured by a variety of cosmogenic techniques are 20-45 mm/yr, and the average residence time for mobile regolith is 10-20 ka.
Although the catchment is on the margins of the seasonal snowpack, its hydrology is dominated by snowmelt and/or late spring storms.
North-facing slopes build a seasonal snowpack, and transfer water rapidly and deeply into the subsurface during snow melt. South-facing slopes lose snow cover repeatedly in winter, and are dominated by brief preferential flow events. Hence there is less water flux into rock on south-facing slopes.
Because the modern mean annual temperatures in Gordon Gulch are only slightly above 0 °C, it is likely that frost-related (periglacial) processes have dominated weathering and mobile regolith transport over the Quaternary.
We thank Kamini Singha and Will Ouimet for careful reviews, and send kudos to Lon Abbott and Greg Hancock for their editing. The Boulder Creek Critical Zone Observatory, funded by National Science Foundation grants EAR 0724960 and 1239281, supported this work. We thank Eric Parrish, Nate Rock and Colin Millar and the entire field crew for assistance with the work.
Figures & Tables
Classic Concepts and New Directions: Exploring 125 Years of GSA Discoveries in the Rocky Mountain Region
- climate change
- crystalline rocks
- drainage basins
- field trips
- fluvial features
- Front Range
- glacial features
- Great Plains
- landform evolution
- Laramide Orogeny
- North America
- road log
- sediment transport
- sedimentary rocks
- United States
- Boulder Creek watershed
- Gordon Gulch