Morphology, stratigraphy, and mineralogical composition of a layered formation covering the plateaus around Valles Marineris, Mars: Implications for its geological history

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An extensive layered formation covers the high plateaus around Valles
Marineris. Mapping based on HiRISE, CTX and HRSC images reveals these
layered deposits (LDs) crop out north of Tithonium Chasma, south of Ius
Chasma, around West Candor Chasma, and southwest of Juventae Chasma and
Ganges Chasma. The estimated area covered by LDs is similar to 42,300
km(2). They consist of a series of alternating light and dark beds, a
100 m in total thickness that is covered by a dark unconsolidated mantle
possibly resulting from their erosion. Their stratigraphic relationships
with the plateaus and the Valles Marineris chasmata indicate that the
LDs were deposited during the Early- to Late Hesperian, and possibly
later depending on the region, before the end of the backwasting of the
walls near Juventae Chasma, and probably before Louros Valles sapping
near Ius Chasma. Their large spatial coverage and their location mainly
on highly elevated plateaus lead us to conclude that LDs correspond to
airfall dust and/or volcanic ash. The surface of LDs is characterized by
various morphological features, including lobate ejecta and pedestal
craters, polygonal fractures, valleys and sinuous ridges, and a pitted
surface, which are all consistent with liquid water and/or water ice
filling the pores of LDs. LDs were episodically eroded by fluvial
processes and were possibly modified by sublimation processes.
Considering that LDs correspond to dust and/or ash possibly mixed with
ice particles in the past, LDs may be compared to Dissected Mantle
Terrains currently observed in mid- to high latitudes on Mars, which
correspond to a mantle of mixed dust and ice that is partially or
totally dissected by sublimation. The analysis of CRISM and OMEGA
hyperspectral data indicates that the basal layer of LDs near Ganges
Chasma exhibits spectra with absorption bands at similar to 1.4 mu m,
and similar to 1.9 mu m and a large deep band between similar to 2.21
and 2.26 mu m that are consistent with previous spectral analysis in
other regions of LDs. We interpret these spectral characteristics as an
enrichment of LDs in opaline silica or by Al-phyllosilicate-rich layers
being overlain by hydroxylated ferric sulfate-rich layers. These
alteration minerals are consistent with the aqueous alteration of LDs at
low temperatures. (C) 2010 Elsevier Inc. All rights reserved.