HR: 1340h
AN: H33C-1404    [Abstracts]
TI: Hawaiian Analog for Martian Amphitheatre-headed Valleys
AU: * Lamb, M P
EM: mpl@berkeley.edu
AF: University of California - Berkeley, Earth and Planetary Science 307 McCone Hall, Berkeley, CA 94720-4767
AU: Howard, A D
EM: alanh@virginia.edu
AF: University of Virginia, Department of Environmental Sciences 291 McCormick Rd, P.O. Box 400123, Charlottesville, VA 22904-4123
AU: Dietrich, W E
EM: bill@geomorph.berkeley.edu
AF: University of California - Berkeley, Earth and Planetary Science 307 McCone Hall, Berkeley, CA 94720-4767
AU: Perron, J T
EM: perron@eps.berkeley.edu
AF: University of California - Berkeley, Earth and Planetary Science 307 McCone Hall, Berkeley, CA 94720-4767
AB: Stubby, amphitheatre-headed valleys are common on the surface of 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 and groundwater sapping) rather than precipitation runoff. This interpretation has significant implications for Martian hydrology and the associated prospects for life. However, such a connection between channel form and the erosion processes induced by groundwater has only been demonstrated on Earth for sediment with little to no cohesion. Martian valleys have most likely been carved into basalt, and the extension of previous work to bedrock erosion is unclear. Perhaps the most widely cited terrestrial analogs for Martian amphitheatre-headed valleys in basalt are the spectacular canyons of Kohala, on the big island of Hawaii. Based on their amphitheatre heads and U-shaped cross sections, previous workers have suggested that these valleys were formed by enhanced chemical weathering and erosion induced by dike-impounded groundwater. Neighboring smaller streams that do not have amphitheatre heads and have more V-shaped cross sections were thought to represent streams where seepage erosion did not occur. These smaller streams run along side of and often drain into the larger canyons, or they end in waterfalls spilling over the ~350 m vertical sea cliffs into the ocean. We propose that the Hawaiian amphitheatre-headed valleys formed by upstream propagation of huge headwalls induced by a large deep-seated landslide. Recent bathymetric surveys have revealed that the large sea cliff at the mouth of the Kohala valleys is likely the headscarp of a huge rotational slump, the Pololu slump. Dominant streams cascaded over the cliffs forming waterfalls which, through plunge pool erosion and mass wasting, induced upstream propagation of headwalls eventually forming deep amphitheatre-headed valleys. We propose headwall retreat by waterfall erosion rather than seepage erosion because, at least under current conditions, this region receives over 4 m of annual precipitation, waterfalls are eroding plunge pools into the rock at most valley heads, and the streams are able to move the large boulders on the bed during storms. While springs do occur in some valley heads, we have not observed weathering, erosion, or transport of rock by seepage water, nor has it been documented by others to our knowledge. Following valley formation, island subsidence has resulted in alluviation of the valley floors creating the observed U-shaped valley cross sections. Our interpretation implies that deep amphitheatre-headed valleys can result from precipitation runoff with very little landscape dissection upstream of the valley head. If Martian valleys formed by similar processes, the Martian climate must once have been capable of supporting precipitation.
DE: 1625 Geomorphology and weathering (0790, 1824, 1825, 1826, 1886)
DE: 1825 Geomorphology: fluvial (1625)
DE: 5415 Erosion and weathering
DE: 5419 Hydrology and fluvial processes
SC: Hydrology [H]
MN: Fall Meeting 2005