Abstract
Stepped fan deposits and phyllosilicate mineralogies are relatively common features on Mars but have not previously been found in association with each other. Both of these features are widely accepted to be the result of aqueous processes, but the assumed role and nature of any water varies. In this study we have investigated two stepped fan deposits in Coprates Catena, Mars, which have a genetic link to light-toned material that is rich in Fe–Mg phyllosilicate phases. Although of different sizes and in separate, but adjacent, trough-like depressions, we identify similar features at these stepped fans and phyllosilicates that are indicative of similar formation conditions and processes. Our observations of the overall geomorphology, mineralogy and chronology of these features are consistent with a two stage formation process, whereby deposition in the troughs first occurs into shallow standing water or playas, forming fluvial or alluvial fans that terminate in delta deposits and interfinger with interpreted lacustrine facies, with a later period of deposition under sub-aerial conditions, forming alluvial fan deposits. We suggest that the distinctive stepped appearance of these fans is the result of aeolian erosion, and is not a primary depositional feature. This combined formation framework for stepped fans and phyllosilicates can also explain other similar features on Mars, and adds to the growing evidence of fluvial activity in the equatorial region of Mars during the Hesperian and Amazonian.
Highlights
Evidence of the history of water on Mars is recorded in the sedimentary characteristics and stratigraphy of sedimentary rock exposed at the surface
We propose a two-stage sequence for formation of the fans and phyllosilicates in this region, involving (1) early shallow standing water with associated fluvial-deltaic processes in the trough, and (2) a later period with little to no deposition in the trough but alluvial processes active on the fan (Fig. 14)
Mineralogical and chronologic constraints may suggest that sedimentary fans and related phyllosilicate deposits preserved in adjacent troughs in the Coprates Catena region of south east Valles Marineris formed by similar processes
Summary
Evidence of the history of water on Mars is recorded in the sedimentary characteristics and stratigraphy of sedimentary rock exposed at the surface. Of particular interest are sedimentary fans associated with channel features, as they can in principle be used to determine whether subaerial or subaqueous conditions were prevalent at the time of formation Moore and Howard, 2005; Fassett and Head, 2008; Di Achille and Hynek, 2010). Difficulties remain in determining whether a particular fan deposit on Mars is a subaerial alluvial fan Jerolmack et al, 2004; Moore and Howard, 2005; Di Achille et al, 2006; Williams et al, 2006, 2011) or a subaqueous (lacustrine) delta fan
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