Mathematics – Logic
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p13b1375i&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #P13B-1375
Mathematics
Logic
[1825] Hydrology / Geomorphology: Fluvial, [1856] Hydrology / River Channels, [5415] Planetary Sciences: Solid Surface Planets / Erosion And Weathering, [5419] Planetary Sciences: Solid Surface Planets / Hydrology And Fluvial Processes
Scientific paper
Hoke et al. (2004, Geology 32, 605-608) published one of the more prominent studies suggesting a groundwater-sapping origin for theater-headed valleys (box canyons), focusing on sites in the hyperarid Atacama Desert of northern Chile. Their study was based on remote-sensing datasets, digital topography, geologic maps, and regional hydrology rather than field investigations. Similarly, groundwater sapping has often been invoked to explain theater-headed tributaries to valley networks on Mars. Our recent field work in the Atacama Desert provided a valuable test of the ability to interpret a groundwater-sapping origin for valleys using only remote datasets. At two sites described by Hoke et al. (2004), we found that the box canyon headwalls include strong ignimbrite cap rocks (Schmidt hammer R-value ~ 55) overlying weakly cemented, erodible sandstones (R-value ~ 25) within the upper Oligocene/lower Miocene Oxaya Formation and its southern lateral equivalent, the Altos de Pica Formation. This stratigraphy allows headward retreat of a near-vertical knickpoint without preparatory seepage weathering of the headscarp base. We found no springs, vegetation, salt weathering, alcove development, or other evidence of present or past seepage weathering at these headwalls. In contrast, ephemeral overland channels supplied by infrequent runoff from the Andean slope to the east are well-developed, and these streams are geomorphically effective even in the hyperarid climate, although they flow too rarely to support dense vegetation in this generally barren landscape. Ephemeral channels have widths of >10 m and depths of >1 m, based on recent high-water markers. The streams have transported rocks tens of centimeters in diameter, and deeply sculpted bedrock is exposed on some channel beds. Similar interpretations of past groundwater sapping in Martian theater-headed valleys may be viewed as equivocal, pending corroboration by in situ investigation of the headwall stratigraphy and past weathering processes. Figure: Box canyon headwall near Pica, Chile, with ignimbrite over weak fluvial sandstone. Relief is 55 m.
Baptista A.
Craddock Robert A.
Howard Alan D.
Irwin Rossman P.
Tooth Stephen
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