Mathematics – Logic
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufm.p22a..05m&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #P22A-05
Mathematics
Logic
1039 Alteration And Weathering Processes (3617), 3672 Planetary Mineralogy And Petrology (5410), 3675 Sedimentary Petrology, 5470 Surface Materials And Properties
Scientific paper
Aluminous and ferruginous clay minerals detected in the Mawrth Vallis area [Poulet et al., 2005] correspond to ancient, light-toned (TES albedo = ~0.22), layered bedrock. We conclude that the layered surface materials are indeed bedrock based on 1) their weathering style, they form scarps, steep cliffs, buttes, and mesas; and 2) their thermal inertia which, based on THEMIS data, is > 500 J K-1 m-2 s-1/2. Because the rocks are nearly flat-lying and morphologically unique, it is possible to make stratigraphic correlations throughout the Mawrth Vallis area. The clay mineral deposits all correspond to a complex section of rock present throughout an area similar in size to the Colorado Plateau. Through stratigraphic correlations and basic structural interpretations, we conclude that the layered clay mineral-bearing bedrock is >1500 m thick and contains 100s to 1000s of individual layers, indicating a dynamic environment during deposition. At least some of the light-toned layers are phyllosilicate-poor, making origin of the clay minerals by regional-scale, subsurface replacement an unlikely scenario. Geomorphic evidence shows that the light-toned rock unit was eroded and incised after lithification and it is disconformably overlain by a clay mineral-poor, dark-toned (TES albedo = ~0.12), layered sedimentary rock that occurs at all elevations throughout the area. This dark toned unit is interpreted as sedimentary rock because it contains abundant evidence for inverted topography in the form of meandering streams that may have fed Mawrth Vallis. The light-toned, clay mineral-bearing unit ends abruptly at the present day erosional remnant of the dichotomy boundary, though remnants of it are observed in buttes within the northern plains. Therefore, either the light-toned, clay mineral-bearing unit is older than the dichotomy (possibly > 4 Ga), or the basin in which these units were deposited was previously bounded to the north by a topographic barrier that no longer exists. It is unclear whether the upper, dark-toned units could also be older than the dichotomy. It is impossible to assign a unique interpretation to the light- toned clay-bearing rock unit, as it could ultimately be sedimentary rock or altered pyroclastic rock. However, its preservation implies the former existence of a depositional basin in this area. If the layered rocks are interpreted as volcanic and erosion erased the evidence of associated volcanic structures, then the erosion must have generated sediments that would have been deposited along with volcanic materials in the basin. Regardless of the interpretation of volcanoclastic + sedimentary versus purely sedimentary origins, these rock units are seemingly a window into early geologic processes in the Solar System, in a time period not observable on Earth. Indeed the evidence for ancient orogenic systems on Earth lies in ancient sedimentary sequences. This package of ancient clastic rocks may represent ancient fluvial, lacustrine, and/or marine deposits, and could have implications for early orogenic processes on Mars.
Fergason Robin
Michalski Joseph R.
Noe Eldar
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