Analysis of two sections of Shalbatana Vallis’ tributary channels

Six tributaries of Shalbatana outflow channel were analyzed based on visible imagery by THEMIS, MOC, HRSC, and HiRISE, as well as on topographic data by MOLA. Two different sections could be identified on the tributaries: the upper and the lower reaches. The upper reaches are shallow, showing U-shaped cross-sectional profiles. Their bottom level is filled with sediments, forming a plain. They have nearly horizontal longitudinal profiles, suggesting either the formation under long term fluvial erosion, or their bottom level represents a strong subsurface layer, until which the valley could cut itself downward into the rocks. Lower reaches show wider and deeper cross-sectional V-shaped profiles, with a narrow, infilled central bottom level. They show steep and at some occasion convex longitudinal profiles, indicating other erosional processes than the upper reaches, and/or that the erosion did not have time to form an equilibrium-like profile. They were formed after the Shalbatana outflow channel, probably by a process different from the formation of the upper reaches that may not involve fluvial erosion, or under different environmental conditions. ssible scenarios are proposed to explain the formation of these two classes of reaches: (1) They may have formed under different climatic conditions on Mars, and show examples for surface forming forces that changed during the planetary evolution. Such different valley sections may point to differences both in lithology and differences in the Martian surface conditions if they formed at different periods. (2) The other possibility is that they formed roughly during the same period, and the two different sections represent differences only in lithology and erodibility. The incut of the upper reach happened in a harder rock unit, forming a nearly horizontal longitudinal profile, and was not deep enough to reach the deeper, more erodible rock level. At the lower reach a different type of erosion took place because of different lithology, and formed the deeper, steeper lower reaches there.