Abstract
The extensive light-toned deposits in canyons and troughs in Valles Marineris provide evidence of formation through water-related processes. As such, these deposits offer a window to past conditions on Mars. We study a small outcrop of light-toned deposits in a closed trough in Coprates Catena, a chain of collapse pits to the south-east of the main Valles Marineris system. A well-exposed sequence of deposits on the base of the north wall of the trough offers a 220m section for geochemical and morphologic analysis. Using CRISM data we identify the presence of both phyllosilicates and sulfates and/or opaline silica in the light toned deposits, which vary in relative strength with elevation. We observe a trend in the dominant mineralogical signal, with Al phyllosilicates occurring near the base of the deposits, both below and above a band of Fe/Mg phyllosilicates, before a transition to more sulfate- or opaline silica-rich material near the top of the section. This trend likely reflects a change in the chemistry of the water in which the deposits formed. Using a HiRISE Digital Elevation Model, we find that the layers in the light-toned deposits on both sides of the trough dip gently towards the center of the trough, with a dip direction that aligns with the strike of the trough, suggesting that the light-toned deposits formed after the trough. Our general morphologic and mineralogical observations fit well with significant amounts of water in the trough. The deposits are too small to be dated using crater counting techniques, however, our crater analysis suggests that the plains in which the trough formed are probably Late Hesperian in age. If the chemistry of the light-toned deposits reflects the primary depositional mineralogy, then this and other small troughs in Coprates Catena might provide evidence of limited phyllosilicate formation in this region towards the end of the Hesperian era on Mars.
Highlights
The canyons and troughs of Valles Marineris often contain distinctive, layered deposits of light-toned material (e.g. McCauley et al, 1972; Nedell et al, 1987; Lucchitta et al, 1992, 1994)
The spectral parameter summary images (Fig. 4c and d) show that there is an apparent variation in absorption wavelength that depends on elevation, which we investigate by taking spectral profiles through the most well-defined outcrop sequence of the light-toned deposits (LTDs) in the western side of the northern base of the trough (Fig. 3a)
Using Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) data we have identified material containing phyllosilicates, and sulfates and/or opaline silica associated with the LTDs that appear to represent a sequence of dominant mineralogies
Summary
The canyons and troughs of Valles Marineris often contain distinctive, layered deposits of light-toned material (e.g. McCauley et al, 1972; Nedell et al, 1987; Lucchitta et al, 1992, 1994). The canyons and troughs of Valles Marineris often contain distinctive, layered deposits of light-toned material McCauley et al, 1972; Nedell et al, 1987; Lucchitta et al, 1992, 1994) These light-toned deposits (LTDs) are most often found towards the center of chasmata, and despite sometimes having heights almost as large as the surrounding canyon walls, LTDs usually show evidence of having undergone significant erosion (Lucchitta et al, 1992). The study trough contains a distinctive fan deposit, suggesting the occurrence of water at some point (Di Achille et al, 2006; Weitz et al, 2006), and we address the question of whether the LTDs are evidence of a lacustrine environment (see Lucchitta (2009a,b), for recent summaries). The implications from the results of this study are placed into a broader chronologic and geologic context, with the aim of contributing towards our understanding of the evolution of water on Mars
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