Morphologic and mineralogic analysis of Nirgal and Her Desher Valles, Mars: Evidence of groundwater flow in northwest Noachis Terra

A widespread phyllosilicate-bearing layer has been identified in a distinct region in northwest Noachis Terra. This layer is exposed along the length of two valley systems in the region, Nirgal and Her Desher Valles, as well as in nearby crater rims and walls. It has been suggested that the phyllosilicates were produced by pedogenic weathering fed by meteoric water or melted snow. However, the geomorphology and stratigraphy of the valley systems is more consistent with formation due to groundwater sapping, not fluvial erosion. Geomorphic analysis of the valleys shows that they are structurally controlled and are consistent with regions of groundwater trapping by fracture and fault systems on Earth. Identification of hematite/sulfate intermixed with the phyllosilicates in the Her Desher and Nirgal valleys, suggests that pre-existing phyllosilicates were altered as acidic, sulfur-rich groundwater sourced by the Tharsis uplift flowed through the region. While groundwater seepage is unlikely to be able to erode resistant bedrock, basalt previously altered into pedogenic soils would be more conducive to channel formation due to groundwater seepage.