Formation of amphitheater-headed valleys by waterfall erosion after large-scale slumping on Hawai'i

Abstract

Amphitheater-headed valleys are common on the surfaces of Earth and Mars. The abrupt terminations of these valleys at their headwalls have been used extensively to argue for valley erosion from springs (i.e., seepage erosion or groundwater sapping) rather than surface runoff. This interpretation has significant implications for Martian hydrology and the associated prospects for life. A connection between channel form and the erosion processes induced by groundwater, however, has not been demonstrated in resistant rock. Perhaps the most widely cited terrestrial analogs for Martian amphitheater-headed valleys in basalt are the spectacular canyons of Kohala, Hawai'i. Here we present new fi eld observations and topographic analyses of the amphitheater-headed Kohala valleys. We found no evidence for intensively weathered rocks or alcoves around springs at valley headwalls. Instead, valley-head erosion appears to be dominated by waterfall plunge pools. Stream fl ow from peak annual precipitation events exceeds spring discharge by more than an order of magnitude, and such fl ow is responsible for evacuation of the coarse sediment that lines the streams. Bathymetric surveys along the Kohala coast have revealed a large submarine landslide, the Pololū Slump, directly offshore of the Kohala valleys. We propose that the headscarp of this massive landslide is expressed as the present-day ~400 m Kohala sea cliffs. As dominant streams poured over this headscarp as waterfalls, vertical plunge pool erosion and undercutting caused upstream propagation of knickpoints, eventually producing amphitheater-headed valleys. Island subsidence rates and the ages of volcanic eruptions and submarine terraces indicate that the average rate of valley headwall advance is as high as 60 mm/yr. We propose a simple expression for upslope headwall propagation by vertical waterfall erosion based on abrasion by impacting sediment particles in plunge pools. This model indicates that headwall propagation depends nonlinearly on the sediment fl ux passing over the waterfall and linearly on the ratio of kinetic versus potential energy of sediment impacts. After the PololūSlump, many streams did not form upslopepropagating waterfalls because they had smaller discharges due to a radial drainage pattern and fault-bounded drainage divides, which prevented runoff from the wetter summit of the volcano. A threshold for headwall propagation due to sediment supply or sediment-transport capacity is consistent with the model. Island subsidence following valley formation has resulted in alluviation of the valley fl oors, which has created the observed U-shaped valley cross sections. Our interpretation implies that surface runoff can carve amphitheater-headed valleys and that seepage erosion cannot be inferred based solely on valley form on Earth, Mars, or other planets.

Chemical weathering fluxes from volcanic islands and the importance of groundwater: The Hawaiian example

“…Finally, on Kohala volcano deeply incised amphitheaterheaded canyons dissect the windward escarpment (Figure 4c). The Kohala escarpment was initiated by the Pololu slump, a giant landslide constrained to have occurred between 385 and 173 ka, and possibly between 250 and 230 ka (Lamb et al, 2007). The escarpment would have been considerably higher relative to sea level at the time, given continual subsidence of the northern part of the island of ca.…”

“…contour. Some of these valleys, most notably Pololu and Waipi'o Valleys were cut deeper during periods of Pleistocene low sea level stands and have since been filled with alluvial sediments as base levels rose (see Lamb et al, 2007 and references therein).…”

“…The island retains considerable constructional volcanic topography, with the exception of the NE-coast of the Kohala peninsula where a large flank collapse occurred around 250 ka (Lamb et al, 2007). The leeward side of Hawai'i receives much less precipitation than the wet windward side and has no permanent streams.…”

“…Kohala volcano is the oldest part of the island of Hawai'i. It is the site of a large landslide that most likely accelerated valley formation on the windward side of the peninsula (Lamb et al, 2007). Thick soils mantle the high plateau, which is the remnant shield surface.…”

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

“…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. The latter studies suggest that a groundwater sapping origin of bedrock valleys may not have been uniquely demonstrated anywhere on Earth, and that a positive relationship between spring discharge and weathering rate similarly lacks empirical support.…”