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

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, sedimenttransport constraints, and cosmogenic ( 3 He) 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.megaflood | knickpoint | sapping | waterfall

Erosion during extreme flood events dominates Holocene canyon evolution in northeast Iceland

“…Large canyons can be formed during extreme flood events through the toppling and removal of blocks in heavily jointed basaltic bedrock (18,19), once a threshold flow depth has been exceeded (27). Bedrock erosion during extreme floods in such settings is therefore dominated by plucking rather than abrasion, resulting in the formation and upstream propagation of large vertical knickpoints [e.g., Box Canyon and Stubby Canyon, Idaho (18,19)].…”

“…Bedrock erosion during extreme floods in such settings is therefore dominated by plucking rather than abrasion, resulting in the formation and upstream propagation of large vertical knickpoints [e.g., Box Canyon and Stubby Canyon, Idaho (18,19)]. As the knickpoints propagate upstream, rock is removed typically over the thickness of one or more lava flows, exposing pristine rock surfaces to cosmic rays and initiating the accumulation of cosmogenic nuclides.…”

Origin and development of theater-headed valleys in the Atacama Desert, northern Chile: Morphological analogs to martian valley networks

“…In addition to groundwater sapping and basal attack by waterfalls, a third mechanism for headscarp retreat is toppling of columns in vertically jointed rock by surface floods that cascade over the headscarp (Lamb and Dietrich, 2009;Lamb et al, 2014). This process could occur independently of enhanced weathering or lithologic weakness at the base of the headscarp, providing that the jointing is continuous and dips nearly vertically.…”

“…This configuration facilitates headscarp retreat as shear and drag during floods topple the jointed columns and remove the resulting debris. This process is also not expected to yield valley floors that are much wider than the stream, unless the valley was carved by a past flood that was much larger than the present stream (e.g., Lamb et al, 2008aLamb et al, , 2014.…”

“…In addition, significant aquifer recharge would have been needed to transport the weathered volume of debris equivalent to martian valley volumes (Howard, 1988;Gulick and Baker, 1990;Goldspiel and Squyres, 1991;Goldspiel et al, 1993;Grant, 2000;Gulick, 2001;Craddock and Howard, 2002). Alternative explanations for steep or vertical valley headscarps in terrestrial bedrock have been noted, including surface flood erosion of strong-over-weak stratigraphy or vertically jointed rock, although very large floods ('megafloods') may be required to topple columns of jointed rock (Lamb et al, 2008a(Lamb et al, , 2014Lamb and Dietrich, 2009). Flood erosion of strong-over-weak stratigraphy can involve undercutting of the headscarp caprock at or around waterfall plunge pools (e.g., Lamb et al, 2006Lamb et al, , 2007.…”