From the mountains to the plains: Constraining the evolution of Front Range surfaces through Quaternary dating
PhD: University of Colorado Boulder, 2016.
At the Colorado Front Range, the low-relief High Plains meet the crystalline rocks of the Rocky Mountains. The western High Plains record a long period of post-Laramide deposition with late Cenozoic exhumation. Variation of sediment production within the Front Range, associated with glacial-interglacial climate cycles, likely influenced the pattern and pace of this exhumation. Here, I use the cosmogenic radionuclides 10Be and 26Al to document both the rates of sediment production in the crystalline core of the Front Range and the ages of Quaternary strath terraces recording High Plains exhumation.
Working on hillslopes developed on crystalline rocks of the Boulder Creek Critical Zone Observatory, I documented mobile regolith (soil) production rates using 10Be. It takes ~10-25 kyr to produce roughly 50 cm of soil from the underlying weathered rock. The implied ~2-5 cm/kyr rate of soil production also appears to be in balance with the rate of lateral motion of soil toward the streams.
Sampling stream sediment, I documented the rates of basin-averaged denudation in several major basins. Streams with glaciated headwaters are denuding at rates >5.4 cm/kyr, and contrast with unglaciated stream basins eroding at rates <3.5 cm/kyr. I also assessed whether the huge September 2013 storm alters basin-averaged denudation rates deduced from 10Be. I find that (1) such an event does not significantly affect the concentration of 10Be in stream sediments, and (2) repeat measurements yield basin-averaged denudation rates that are typically reproducible within 15%.
Following stream sediments outboard of the Front Range, combined dating techniques on a terrace sequence beside Lefthand Creek reinforces the picture that incision into the western High Plains is accomplished by long periods of lateral planation interrupted by short periods of vertical incision. These are likely basin-specific, rather than synchronous across all large streams draining the Front Range. Pairing of 26Al and 10Be profiles revealed ~ 1 million years of fluvial occupation of the km-wide Table Mountain strath terrace, spanning several glacial-interglacial cycles. Although CRN dating reveals that the link between climate and geomorphic process is actually quite complicated, this method allows us to refine the questions we can ask about the landscape.