Amphitheater-Headed Valley (Mars, Earth)
KeywordsPlunge Pool Florida Panhandle Runoff Erosion Glen Canyon Vertical Drilling
Valley characterized by theater-shaped alcoves.
Characteristic features include smooth steep-sided valley walls containing sharp wall-floor junction angles, smooth valley floors with wide U-shaped profiles, abrupt theater-like head terminations (Kochel and Howard 1985), and short stubby tributaries.
Sapping: Martian amphitheater-headed valleys are apparently similar to channels that form by headward erosion of groundwater seepage. On Mars, a possible source of water for sapping could be the melting of ice or infiltration of surface water (Mangold et al. 2008). Lamb et al. (2006) suggested that groundwater-sapping origin for the Martian landforms is based primarily on the assumption that seepage erosion creates a distinctive morphology, but this assumption may be wrong. Morphological features alone may not be diagnostic, and the same features that characterize sapping valleys can be produced by runoff- or glacier-eroded valleys (Howard 1988). Sapping is effective in cohesionless sediments (Howard and McLane 1988), whereas the role of sapping remains unclear for valleys carved into bedrock (Lamb et al. 2006).
Since heavily cratered terrains are probably well permeable because of heavy fracturing resulted from impacts and due to brecciated and coarse-grained (silt or clay-sized) property of impact debris, it might provide sufficient conditions for sapping. Infiltration could have been supported by enclosed depressions and craters on the impact terrain (Howard 1988).
Sapping is a much slower process in consolidated sediments and needs 105 times more water than for unconsolidated rocks (Howard 1988), and at least occasional direct runoff may be required to transport the collapsed materials produced by sapping (Lamb et al. 2006; Luo and Howard 2008; Mangold et al. 2008). Impacts (Brackenridge et al. 1982) or volcanic processes (Gulick 1998) result in the heating of groundwater (that becomes hydrothermal groundwater), which may lead to sapping processes and valley formation that can supply the large water demand of sapping valleys in cohesive materials:
A combination sapping and runoff erosion: These valleys may have developed through a range of combinations of runoff erosion and seepage weathering (Luo and Howard 2008). In certain cases strong evidence exists for only surface runoff while there are no observable signatures of sapping process (Gulick and Baker 1989), and in other cases like Tinto Vallis (Erkeling et al. 2011), precipitation-recharged groundwater sapping is inferred (Grant and Parker 2002). Analysis of some of the prominent drainage systems on Mars suggests that sapping occurred after the surface runoff had produced the original channels (Aharonson et al. 2001), e.g., when weakening of surface runoff left sapping as the dominant valley forming process, reactivating previously formed fluvial valleys (Baker and Partridge 1986).
- (3)Catastrophic water discharge: Kraal et al. (2008) suggested that at least some of the amphitheater-headed channels on Mars were produced by short timescale catastrophic processes. Stepping (terraced) deltas associated with short, amphitheater-headed valleys are indicative of a “sudden release of water from subsurface water reservoirs, massive discharges, and short timescales” (Kraal et al. 2008), for which water may have been provided from melting ground ice (Hauber et al. 2013).
Toppling of jointed rock during megafloods. Headwall retreat by block toppling may be the dominant process in producing amphitheater valley heads, e.g., in basaltic landscapes that lack strong-over-weak stratigraphy (e.g., Lamb et al. 2008, 2014; Lamb and Dietrich 2009). Based on investigations on the formation of Box Canyon, Idaho (Lamb et al. 2006, 2008), and Malad Gorge, Idaho (Lamb et al. 2014), these amphitheater-headed canyons were suggested to have been carved by catastrophic flash floods which may provide an analog for similar Martian features.
- (4)Waterfall erosion:
Plunge pool undercutting in strong-over-weak stratigraphy where plunge pool erosion and mass wasting drive headwall retreat in upstream propagating waterfalls (e.g., Lamb et al. 2006). Examples: Box Canyon, Idaho (Fig. 4), Niagara Falls, Kohala valleys in Hawaii.
Prominent Examples on Mars
Prominent Examples on Earth
In Glen Canyon, Colorado Plateau, USA, where water emerges from bedding plane boundaries, between permeable, relatively unjointed (hence collapse-resistant) but easily weathered sandstones overlying impermeable, but mechanically weak strata, was proposed to be formed by sapping (Laity and Malin 1985) or by overland flow through plunge pool erosion and transport of collapsed debris (Lamb et al. 2006).
Malad Gorge, Idaho, is proposed to have formed by rapid megaflood erosion (e.g., Lamb et al. 2014).
Valleys in basaltic bedrock in Hawaii, which has similar lithology to where some of the amphitheater-headed valleys occur on Mars, were proposed to have formed by sapping (Kochel and Piper 1986), but Lamb et al. (2006, 2007) found that these are associated with waterfall plunge pools.
IAU Descriptor Term
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