Other
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p42c..03h&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P42C-03
Other
[5415] Planetary Sciences: Solid Surface Planets / Erosion And Weathering, [5419] Planetary Sciences: Solid Surface Planets / Hydrology And Fluvial Processes
Scientific paper
Martian alluvial fans have now been recognized in many craters around the equatorial latitudes of Mars. HiRISE imagery reveals that their surfaces often show a radial pattern of narrow (10s to 100s of meters), long (km), low relief (<10 m), flat topped ridges. These may crosscut and superpose on each other, branch (especially at their downslope ends), and in nighttime thermal infrared imaging are brighter than the surrounding material, indicating a comparatively higher thermal inertia. These features among others suggest that these ridges are fluvial distributary channels, coarser grained than their surrounding material,now inverted by the postdepositional removal of many of these fines. This study focuses on a suite of alluvial fans in the Pampa de Tamarugal, part of the northern Atacama Desert. This site is an ideal analogue for the Martian examples, both in terms of the fan sedimentology and also the postdepositional inversion of flow deposits. The scales of both the fans and the channels associated with the flows approximate the Martian equivalents. The sediment sourced to the fans is a bimodal mixture of coarser (gravel to cobble) and finer (silt and mud) grain sizes, variable across both space and time and controlled by release from source rocks of highly variable lithology upstream. Such a bimodal grain size distribution is a match to the inferred sizes present in the Martian fans. We use WorldView-2 satellite visual imagery (0.6 m resolution) first to assess depositional processes and likely sediment grain sizes, then to reconstruct a history of activity across the fan surface and to assess the subsequent degradation of the deposits of each age. We infer the flow processes and grain size distribution of the deposits visually based on color, texture, and form in the images, groundtruthed with field reconnaissance. Active flow on the fan is strongly channelized, and near the rangefront commonly reoccupies earlier abandoned courses. Where unconfined by existing structures, the channels are slightly meandering and largely single thread, with some anastomosing reaches. Almost all flows transport abundant muds as well as a coarser load of cobbles and larger clasts. Occasionally this mud fraction may instead be replaced by sandier material, but this contrast makes little difference to the bulk channel form. Low natural levees are present around many channels, but lobate sheets of fine sediment are ubiquitous beyond these, indicating very frequent overtopping of the levees by hyperconcentrated flows. The main channel course also avulses frequently. Once inactive, channels subsequently begin to be inverted by wind erosion and winnowing of fines, and the second order surface texture of the eroded surfaces and the orientation of some channel segments are both controlled by the wind direction. The first order degradation of the individual channel lobes however is not well predicted by the deposit age and hence duration of exposure. Instead, the original grain size distribution of the deposit and the channel form are of comparable importance. We are developing a numerical model of concurrent fan deposition and inversion to further understand and quantify these processes.
Hobley D. E.
Howard Alan D.
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