Ongoing bedrock incision of the Fortymile River driven by Pliocene–Pleistocene Yukon River capture, eastern Alaska, USA, and Yukon, Canada

Geology
By: , and 

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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.

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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
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
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