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Theater-headed Valleys and Landscape Evolution in Terrestrial Arid to Hyperarid Deserts: Analogs to the Noachian Highlands of Mars

Wednesday, July 29, 2009
Ross
Irwin (Naltional Air and Space Museum, Smithsonian Institute)

Field studies of theater-headed valleys and their contributing upland surfaces provide insight into the development of similar landscapes on Mars. Throughout the Middle to Late Noachian Epochs, impact crater degradation was concurrent with development of an intercrater geomorphic surface, which is less rugged than the lunar highland equivalent. Most of the relict valley networks and related alluvial deposits formed near the Noachian/Hesperian transition. A relative change in the weathering and fluvial transport capacities in the Noachian highlands may have been responsible for dissection of the older geomorphic surface. Morphologically similar theater-headed valleys with poorly dissected interfluves were examined in two arid and hyperarid settings on Earth. In a massive Navajo sandstone aquifer in Glen Canyon, Utah, seepage weathering is responsible for maintaining a vertical knickpoint at tributary canyon headwalls, but seepage erosion has a negligible role in sediment transport. At similar valley headwalls attributed to groundwater sapping in the Atacama Desert, strong ignimbrites overlie weakly cemented sedimentary rocks, and no spring discharge or past seepage weathering is evident, so seepage is neither indicated nor required for headwall retreat. Ephemeral floods are responsible for nearly all sediment transport in both cases and for headwall retreat in the latter, and the topography of the contributing watershed rather than the hydrologic gradient controls the network planform. Noachian Mars may have varied from a hyperarid climate, where weathering outpaced limited fluvial erosion, to discrete epochs of arid to semiarid conditions, in which fluvial valleys were incised largely by overland streams. An abrupt decline of the water cycle and rock weathering occurred around the Noachian/Hesperian transition.

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