Mapping erosion with glacial and fluvial sediment cooling ages in Garnet Canyon

Mountains are shaped by ice and water moving over rock surfaces and transporting sands, gravel, cobbles, and boulders away from their original source. Glaciers and streams erode rock most efficiently at different elevations. If we determine where glaciers and streams most effectively remove bedrock, we can better understand how mountain landscapes change over time.

The erosional efficiency of glaciers and streams can be studied in the Teton Range by tracing the source of apatite grains removed from igneous and metamorphic rocks in Garnet Canyon and deposited in stream and moraine sediments (Figure 1). Apatite grains work well as tracers because they record bedrock cooling history. Igneous and metamorphic rocks buried deep below the surface get hotter with increasing depth. When tectonic uplift and erosion cause rocks to move toward the surface, they begin to cool. When rock temperatures cool below ~70°C, apatite grains trap helium produced by the radioactive decay of uranium and thorium within the crystal (Ehlers and Farley, 2003). The concentration of helium increases with time, so the rocks that cooled first are older and contain more helium than rocks the cooled recently (Figure 2). This produces older rocks at high elevations and younger rocks at low elevations. When the bedrock is broken down by erosional processes, the apatite grains recording the cooling history of the bedrock are carried to the end of the catchment. Looking at the age of individual apatite grains in the glacial and fluvial sediments indicate where the grain originated in the catchment (Stock et al., 2006).

The rocks in the Teton Range were uplifted by Basin and Range extension and migration of the Yellowstone Hotspot (Love et al., 2003). The rocks at the high elevations cooled 65-70 million years ago (Ma). The youngest rocks found at low elevations in Garnet Canyon cooled ~10 Ma (Figure 1). Approximately 14,000 years ago, alpine glaciers formed at high elevations in the Teton Mountains, filled the east draining catchments, and deposited sediments in moraines (Licciardi et al., 2008). Glaciers have since retreated, and mountain streams formed by melting snow and ice currently flow out of the canyons. One sediment sample was collected from the moraine and one sediment sample was collected from the stream at the base of Garnet Canyon.

Cooling ages from 78 moraine and 60 stream apatite grains were determined to identify the most frequent ages found in both types of sediments. Apatite grains from the stream showed the highest concentration of ages between 40-45 Ma. The most frequent age in the glacial apatite grains was between 30-35 Ma (Figure 3). The source for glacial sediments is bedrock at the base of steep walls. The stream sediments are sourced from higher elevations in the upper ridges and the floor of the south fork of the canyon (Figure 4). Since the stream sediments are sourced from higher elevations than the glacial sediments, it is possible that the stream is transporting material deposited within the canyon by rock falls, rather than incising into the bedrock floor.

When glaciers filled Garnet Canyon, they carved into rock at the base of the ice to create steep walls and U-shaped valleys. Removing rock from the lower elevations along these walls destabilized rock along the ridges at higher elevations. Since the glaciers retreated, many large and small boulders broke away from the steep walls and ridges to form deposits of talus, which now cover the floor of the canyon. Streams flowing over these deposits currently transport the sediments which originated from higher elevations. In Garnet Canyon the apatite grains deposited in the stream and glacial moraine sediments record the shift of effective erosional processes from glacial incision to rock falls from steep canyon walls.

• Ehlers, T.A., and K.A. Farley, 2003, Apatite (U-Th)/He thermochronometry: Methods and applications to problems in tectonic and surface processes: Earth and Planetary Science Letters, v. 206, p. 1-14.
  
• Licciardi, J. M., and Pierce, K. L., 2008, Cosmogenic exposure-age chronologies of Pinedale and Bull Lake glaciations in greater Yellowstone and the Teton Range, USA: Quaternary Science Reviews, v. 27, p. 814-831.
  
• Love, J. D., Reed, J. C., and Pierce, K. L., 2003, Creation of the Teton Landscape, a Geologic Chronicle of Jackson Hole and the Teton Range: Grand Teton National History Association, Moose, WY.
  
• Stock, G. M., Ehlers, T. A., and Farley, K. A., 2006, Where does sediment come from? Quantifying catchment erosion with detrital apatite (U-Th)/He thermochronometry: Geology, v. 34, no. 9, p. 725-728.