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Edited by Thure E. Cerling, University of Utah, Salt Lake City, UT, and approved November 25, 2013 (received for review June 27, 2013)
Significance
The shapes of bedrock canyons offer clues to the history of surface water on Earth and Mars. Using field examples in Idaho, we found that canyons with amphitheater-shaped heads were likely carved rapidly by outburst flooding about 46,000 y ago and that canyons with more pointed heads evolved progressively by river erosion over tens of thousands of years. Our study suggests that the many amphitheater-headed canyons in fractured basalt on Mars, long inferred to be carved by groundwater seepage, may owe their origins instead to megafloods.
Abstract
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.
Footnotes
- ↵1To whom correspondence should be addressed. E-mail: mpl{at}gps.caltech.edu.
↵2Present address: Department of Geological Sciences, University of Canterbury, Christchurch, New Zealand.
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.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1312251111/-/DCSupplemental.
Amphitheater-headed canyons formed by megaflooding
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