Ongoing bedrock incision of the Fortymile River driven by Pliocene–Pleistocene Yukon River capture, eastern Alaska, USA, and Yukon, Canada
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- Data Releases:
- USGS data release - Charley River Cosmogenic Isotope Data Collected 2019-2021
- USGS data release - Fortymile River Cosmogenic Isotope and Luminescence Data Collected 2016-2019
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Abstract
Quantification of river incision via process rate laws represents a key goal of geomorphic research, but such models often fail to reproduce traits of natural rivers responding to base-level lowering. The Fortymile River flows from eastern Alaska in the United States to the Yukon River in Canada across a tectonically quiescent region with near-uniform precipitation and bedrock erosivity. We exploit these stable boundary conditions to quantify bedrock incision evident in a gravel-capped strath terrace that flanks the lower ∼175 km of the river and grades to the minimally incised headwaters. The terrace gravel yields a cosmogenic isochron burial age of 2.44 ± 0.24 Ma, consistent with abandonment triggered by late Pliocene–early Pleistocene Yukon River headwater capture. The deeply incised reach forms a linear knickzone where basin relief nearly doubles and inferred bedrock incision rates (∼19–110 m/m.y.) averaged since ca. 2.44 Ma increase downstream toward the Fortymile–Yukon River confluence. Basin-scale 10Be-based erosion rates of tributaries to the Fortymile River trunk nearly double from the headwaters (∼9 mm/k.y.) to the knickzone (average ∼16 mm/k.y.), revealing the pace of ongoing landscape response to knickzone incision over 104 yr. Our observations calibrate a stream-power model (erosion coefficient K ∼ 1.1 × 10–6 m0.2) that closely reproduces the knickzone profile and thus implies long-term (104–106 yr) efficacy of a simple stream-power bedrock incision law.
Suggested Citation
Bender, A., Lease, R.O., Corbett, L.B., Bierman, P., Caffee, M., 2018, Ongoing bedrock incision of the Fortymile River driven by Pliocene–Pleistocene Yukon River capture, eastern Alaska, USA, and Yukon, Canada: Geology, v. 46, no. 7, p. 635-638, https://doi.org/10.1130/G40203.1.
Study Area
| Publication type | Article |
|---|---|
| Publication Subtype | Journal Article |
| Title | Ongoing bedrock incision of the Fortymile River driven by Pliocene–Pleistocene Yukon River capture, eastern Alaska, USA, and Yukon, Canada |
| Series title | Geology |
| DOI | 10.1130/G40203.1 |
| Volume | 46 |
| Issue | 7 |
| Publication Date | June 07, 2018 |
| Year Published | 2018 |
| Language | English |
| Publisher | Geological Society of America |
| Contributing office(s) | Alaska Science Center Geology Minerals |
| Description | 4 p. |
| First page | 635 |
| Last page | 638 |
| Country | Canada, United States |
| State | Alaska |
| Other Geospatial | Yukon, Yukon River |