Presentation #503.02 in the session Mars Surface Mapping and Geology.
Surface-exposed hydrated minerals offer an important opportunity to investigate and understand the aqueous alteration history of Mars. Distinct channel-exposed phyllosilicate-bearing layers have been initially characterized with the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) within the Her Desher and Nirgal Valles systems in northwestern Noachis Terra (Buczkowki et al., 2010; 2014); however, the full extent and geologic context of the clays are unconstrained and their origin remains open to several possible interpretations, including both wetter and drier conditions in the past. In this study, exposures of the clay layer and the associated stratigraphic units are identified throughout the Her Desher and Nirgal Valles and are mapped in further detail. This investigation combines hyperspectral data from CRISM and multispectral images from the Color and Stereo Surface Imaging System (CaSSIS) to develop an archetypal stratigraphy based on exposures in the valley systems to place further constraints on the origin of the clays. Typically, at the base of the valley is a bluish, unaltered ferrous-bearing unit, likely igneous in origin. It is often obscured by talus and aeolian bedforms. This is followed up-section by a discontinuous aqua-colored unit consistent with the presence of low-calcium pyroxene (LCP). Above this is the thick, orange Mg-Fe phyllosilicate unit. It is capped by rare, discontinuous lenses of another LCP-bearing aqua-colored unit and/or a more continuous yellow-buff unit, which thins eastward and may be consistent with a sulfate- and hematite-bearing lithology (Buczkowki et al., 2021). The highest stratigraphic level, which continues out on the surrounding plains, is another bluish ferrous-bearing unit. Exposures of this exact stratigraphy across both valleys and in distal excavated craters indicates that clay and some of its associated stratigraphic units are more extensive throughout the region. As such, any origin for the clays will need to account for a regionally extensive clay layer that may even extend as far north as Valles Marineris. The defined stratigraphy gives some credibility to the theory of groundwater sapping as the origin for the Valles; terrestrial sapping valleys tend to form due to a permeable layer overlain by a more coherent lithology (Baker et al., 1990). Continued mapping and analysis of the clays and their associated units may provide a better understanding of past alteration conditions in the region and may inform future Martian missions in their search for where extinct life forms may have thrived.