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Published January 7, 2014 | Supplemental Material + Published
Journal Article Open

Amphitheater-headed canyons formed by megaflooding at Malad Gorge, Idaho


Many bedrock canyons on Earth and Mars were eroded by upstream propagating headwalls, and a prominent goal in geomorphology and planetary science is to determine formation processes from canyon morphology. A diagnostic link between process and form remains highly controversial, however, and field investigations that isolate controls on canyon morphology are needed. Here we investigate the origin of Malad Gorge, Idaho, a canyon system cut into basalt with three remarkably distinct heads: two with amphitheater headwalls and the third housing the active Wood River and ending in a 7% grade knickzone. Scoured rims of the headwalls, relict plunge pools, sediment-transport constraints, and cosmogenic (^3He) exposure ages indicate formation of the amphitheater-headed canyons by large-scale flooding ∼46 ka, coeval with formation of Box Canyon 18 km to the south as well as the eruption of McKinney Butte Basalt, suggesting widespread canyon formation following lava-flow diversion of the paleo-Wood River. Exposure ages within the knickzone-headed canyon indicate progressive upstream younging of strath terraces and a knickzone propagation rate of 2.5 cm/y over at least the past 33 ka. Results point to a potential diagnostic link between vertical amphitheater headwalls in basalt and rapid erosion during megaflooding due to the onset of block toppling, rather than previous interpretations of seepage erosion, with implications for quantifying the early hydrosphere of Mars.

Additional Information

© 2013 National Academy of Sciences. Edited by Thure E. Cerling, University of Utah, Salt Lake City, UT, and approved November 25, 2013 (received for review June 27, 2013). Published online before print December 16, 2013. We thank Joel Scheingross, Mathieu Lapotre, and Jim McKean for field assistance; Willy Amidon for sample preparation; and Bill Phillips for regional comparisons and mapping. This work was supported by NSF Grant 1147381 and NASA Grant PGG12-0107 to M.P.L. Comments from two reviewers strengthened the final version of this paper. Author contributions: M.P.L. and B.H.M. designed research; M.P.L. and B.H.M. performed research; K.A.F. contributed new reagents/analytic tools; M.P.L., B.H.M., and K.A.F. analyzed data; and M.P.L., B.H.M., and K.A.F. wrote the paper. The authors declare no conflict of interest. This article is a PNAS Direct Submission.

Attached Files

Published - PNAS-2014-Lamb-57-62.pdf

Supplemental Material - pnas.201312251SI.pdf


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