122 From the Cosmos and Back: Quantifying Processes and Rates of Landscape Change

This session explores how methods such as cosmogenic nuclides, apatite (U-Th)/He thermochronometry, U-series geochemistry, luminescence, and chemical mass balances quantify relationships between climate, tectonics, and erosion.

Monday, 20 October 2025
1:30 PM - 5:30 PM
HBGCC, 212AB

In situ Produced 10Be and 26Al in Shale Quartz: Structural and Lithologic Influence on Erosion Rates in the Appalachian Valley and Ridge

Paper number: 122-2
Program: Technical Sessions
Time: 1:55 PM – 2:15 PM

Authors

Abstract

Cosmogenic nuclides are routinely measured in sand-sized quartz to learn about exposure ages and erosion rates, but there have been few applications to very-fine-grained rocks.  Shale contains abundant quartz with grain sizes ranging from single microns to tens of microns.  Because this grain size is similar to the recoil length for cosmogenic nuclide production, it is important to test the reliability of cosmogenic nuclide measurements.  We have developed methods to purify fine-grained quartz, and show that 10Be and 26Al can be reproducibly measured with concentrations equivalent to those in larger grains.  This holds true even for grain sizes <5 microns, implying that recoil losses of 10Be and 26Al are balanced by implantation.  

We measured 10Be and 26Al in samples from shale and sandstone in the Valley and Ridge of the Appalachian Mountains in southwestern Virginia.  In this landscape, mountain ridges are typically held up by resistant sandstones, while more erodible shales and limestones form the anti-dip slopes and valley bottoms.  Many of the ridges are characterized by steep upper slopes and gentler, often dissected, lower slopes mantled by weathered colluvium.  

We sampled transects across four different mountains: two synclines and two monoclines.  Our measurements indicate that mountains with strong caprocks and boulder-mantled slopes are eroding slowly, typically at rates of ~10 m/My along the transects.  In contrast, those with weaker caprocks and minimal armoring by boulders exhibit variable erosion rates.  The upper portions of these slope transects are marked by more rapid erosion (~30 m/My), while the lower portions feature gradual erosion of dissected colluvial surfaces or shale slopes (<5-10 m/My).  

Together, these results point to significant variability across the modern landscape that is not apparent from measurements of sandstone alone.  We see that sandstone-capped synclinal mountains are eroding slowly relative to the surrounding landscape, growing their relief over time.  In contrast, steep monoclinal ridges tend to erode faster than the valley bottoms, lowering their relief over time.  This points to the dynamic nature of the landscape in which relief grows and shrinks over time and space, depending on geologic structure and long-term trends in river incision and climate.  

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-412775

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