Anomalous Phyllosilicate‐Bearing Outcrops South of Coprates Chasma: A Study of Possible Emplacement Mechanisms

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.