Lowering rates in the crystalline core of the Front Range

As the timescale of surface processes is often 10³-10⁵ years, cosmogenic radionuclides (CRNs) are a powerful tool in constraining the timing of landscape evolution. In situ and meteoric ¹⁰Be are the most-widely used CRNs in geomorphic studies of regolith. In situ ¹⁰Be is produced within the silicon and oxygen located inside the crystal lattice of quartz; meteoric ¹⁰Be is produced in the atmosphere and then deposited onto the outside of particle surfaces via via rainfall or dust accumulation. Because the accumulation mechanisms for in situ and meteoric ¹⁰Be are fundamentally different, using the two approaches together provides independent constraints on lowering rates and residence times.

We use both in situ and meteoric ¹⁰Be to calculate residence times and production rates for mobile regolith on hillslopes in a non-glaciated catchment within the Colorado Front Range. We also develop analytical models of ¹⁰Be concentrations to test the steady-state assumption underlying our calculations. Close agreement between meteoric and in situ results indicates that upper and middle hillslope positions are eroding both steadily and uniformly; mobile regolith residence times are 10-20 ka and production rates are 3.1 cm/ka. Comparison with our steady state analytical model indicates that these numbers are likely no closer than ± 25% of the geomorphic reality. Our data also reveal that large quantities of meteoric and in situ ¹⁰Be are stored not only in mobile regolith but in the underlying saprolite, highlighting the importance of careful field identification of the mobile regolith-saprolite boundary.

Generation and occupation of strath terraces along the western High Plains

New Quaternary dating methods, such as cosmogenic radionuclide dating, allow us to constrain existing relative stratigraphy with absolute ages. However, such absolute ages can differ between methods and sometimes disagree with established relative chronologies. In this study, we investigate gravel-capped strath terraces along the western High Plains of Colorado in an attempt to reconcile disparate age estimates. We use ¹⁰Be and 26Al cosmogenic radionuclides (CRNs), optically stimulated luminescence (OSL), and thermal-transfer OSL (TT-OSL), to date three strath terraces, all cut and deposited by Lefthand Creek, draining the crystalline core of the Front Range. Our study reveals: (1) a long history of fluvial occupation (0.5-1.5 Myr duration), with recent (<120 ka) fluvial abandonment of the high Table Mountain terrace; (2) a brief occupation of a narrow spillway at 96 ± 4 ka; and (3) a long period of fluvial occupation of a lower terrace, marked by the deposition of a lower soil between 50 and 75 ka, followed by deposition of an upper soil at 40 ± 5 ka. Our interpretation of burial and erosion at Table Mountain reconciles both the young CRN abandonment ages previously reported (Dühnforth et al., 2012) and the old soil-profile ages reported by soil geomorphologists (e.g., Madole, 1963). In conjunction with other recent CRN studies of strath terraces along the Colorado Front Range, our data reveal long periods of aggradation and lateral planation, sometimes lasting several glacial-interglacial cycles, punctuated by brief episodes of rapid incision. These data call into question what a singular terrace “age” represents, as the strath may be cut at one time (“cutting-age”) and the terrace may be abandoned at a much later time (“abandonment age”). These interpretations would not have been possible without the combination of multiple dating techniques. Our study therefore both provides a more nuanced and complex history of fluvial incision of the western edge of the High Plains, and serves as a cautionary tale for the extraction of climate and tectonic information from strath terraces based on a single dating method.