Anomalous Phyllosilicate‐Bearing Outcrops South of Coprates Chasma: A Study of Possible Emplacement Mechanisms. Issue 1 (7th January 2020) - Buczkowski, Debra L.
Abstract: The formation of widespread phyllosilicate‐bearing near‐surface layers on Mars has often been attributed to pedogenesis, a process of weathering basaltic soils by continued exposure to meteoric water percolating down from the surface, which can result in layers of aluminum phyllosilicates forming over layers of iron‐magnesium phyllosilicates. We present evidence of an Fe/Mg‐smectite bearing layer stratigraphically above Al‐phyllosilicates in three circular features to the south of Coprates Chasma, suggesting that some process other than, or in addition to, a single pedogenic sequence must have been involved. A review of several formation mechanisms shows that all models require multiple episodes of aqueous alteration. In addition, only by invoking groundwater alteration in conjunction with pedogenesis can we reconcile the stratigraphic pattern of altered material exposed by these features. Plain Language Summary: The layered surface composition of many places on Mars is thought to have formed by top‐down weathering due to rainfall or the melting of snow. We present evidence from specific locations along the southern rim of Valles Marineris that suggests that there must have been at least two distinct periods of weathering. In addition, the characteristics of these weathered materials and their relationship to craters and other landforms suggest that some of the weathering is better explained by groundwater flow, not water percolating down from the surface. Key Points: Three circular features south of Coprates Chasma display an Fe‐/Mg‐smectite bearing layer stratigraphically above an Al‐phyllosilicates bearing layer Multiple episodes of aqueous alteration are required to model the stratigraphic pattern of altered material exposed by these features Groundwater alteration in addition to pedogenesis is required to model the stratigraphic pattern of altered material exposed by the features