ANOMALOUS CIRCULAR PHYLLOSILICATE-BEARING OUTCROPS SOUTH OF COPRATES CHASMA: A STUDY OF POSSIBLE EMPLACEMENT MECHANISMS

A widespread phyllosilicate-bearing layer identified in NW Noachis Terra has been proposed to be formed by pedogenesis [LeDeit et al., 2012], a process of weathering basaltic soils by continued exposure to meteoric water percolating down from the surface. However, just south of Coprates Chasma there are three outcrops in which Al-smectites appear to be below, not above, Fe/Mg-smectites, suggesting that some process other than, or in addition to, pedogenesis must be at work.

HiRISE imagery shows a low-albedo circular feature surrounded by a ring of high-albedo material at 16°S, 305.6°E. The circular shape suggests that it was an impact structure, but MOLA topography shows that it is now effectively flat. CRISM hyperspectral targeted observations (20-40 m/pixel) indicate that the bright ring is Al-smectite, while the central material is Fe/Mg-smectite located below a cap material. HiRISE observations show that the polygonally fractured Al-smectites are just below the Fe/Mg smectites

A second high-albedo ring structure has been identified in CTX imagery at 15.5°S, 300.2°E. This ring is crossed by one of the Coprates Catena, allowing a view of the stratigraphy of the altered rock and showing Fe/Mg-smectites in the ring below the Al-phyllosilicates exposed at the surface. However, in the interior of the ring the catenae exposes only Fe/Mg-smectites, with no overlying Al-phyllosilicates, suggesting a similar pattern as at the first ring.

Unlike the first two rings of Al-phyllosilicates, CRISM mapping data shows that the semi-circular structure at 16.65°S, 307.9°E is encircled by an Fe/Mg-smectite “ring” just below a spectrally bland cap rock, which is then surrounded by an Al-phyllosilicate plains deposit. A cross-section of the structure shows that its stratigraphy includes a Fe/Mg-smectite layer just below the cap rock, an underlying Al-phyllosilicate layer, and a second Fe/Mg-layer under the Al-materials.

A review of several potential 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.