Abstract
Abstract The presence of abundant phyllosilicate minerals in Noachian (>3.7 Ga) rocks on Mars has been taken as evidence that liquid water was stable at or near the surface early in martian history. This study investigates some of these clay-rich strata exposed in crater rim and inverted terrain settings in the Mawrth Vallis region of Mars. In Muara crater the 200-m-thick, clay-rich Mawrth Vallis Group (MVG) is subdivided into five informal units numbered 1 (base) to 5 (top). Unit 1 consists of interbedded sedimentary and volcanic or volcaniclastic units showing weak Fe/Mg-smectite alteration deposited in a range of subaerial depositional settings. Above a major unconformity eroded on Unit 1, the dark-toned sediments of Unit 2 and lower Unit 3 are inferred to represent mainly wind-blown sand. These are widely interlayered with and draped by thin layers of light-toned sediment representing fine suspended-load aeolian silt and clay. These sediments show extensive Fe/Mg-smectite alteration, probably reflecting subaerial weathering. Upper Unit 3 and units 4 and 5 are composed of well-layered, fine-grained sediment dominated by Al-phyllosilicates, kaolinite, and hydrated silica. Deposition occurred in a large lake or arm of a martian sea. In the inverted terrain 100 km to the NE, Unit 4 shows very young slope failures suggesting that the clay-rich sediments today retain a significant component of water ice. The MVG provides evidence for the presence of large, persistent standing bodies of water on early Mars as well as a complex association of flanking shoreline, alluvial, and aeolian systems. Some of the clays, especially the Fe/Mg smectites in upper units 1 and 2 appear to have formed through subaerial weathering whereas the aluminosilicates, kaolinite, and hydrated silica of units 3, 4, and 5 formed mainly through alteration of fine sediment in subaqueous environments.
Highlights
The Mawrth Vallis region of Mars is known both for the prominent outflow valley from which it gets its name (Fig. 1) and for the wide presence of phyllosilicate-rich rocks formed during the Noachian Period, 4.1–3.7 Ga (Michalski and Noe Dobrea, 2007; Loizeau et al 2012)
The abundance of phyllosilicate minerals in Noachian rocks across the surface of Mars was revealed by OMEGA (Observatoire pour la Minéralogie, l’Eau, les Glaces et l’Activité) on board Mars Express (Poulet et al, 2005; Bibring et al, 2006) and by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) on board the Mars Reconnaissance Orbiter (e.g., Murchie et al, 2009; McKeown et al, 2009; Bishop et al, 2008a, 2011, 2013a; Bishop and Rampe, 2016)
The present study examines the Mawrth Vallis Group (MVG) in two areas in the Mawrth Vallis region (Fig. 1): (1) around the walls of Muara crater (Figs. 2 and 3) just west of Mawrth Vallis and (2) in an area of so-called inverted terrain ~100 km northeast of Muara crater and east of Mawrth Vallis
Summary
The Mawrth Vallis region of Mars is known both for the prominent outflow valley from which it gets its name (Fig. 1) and for the wide presence of phyllosilicate-rich rocks formed during the Noachian Period, 4.1–3.7 Ga (Michalski and Noe Dobrea, 2007; Loizeau et al 2012). The abundance of phyllosilicate minerals in Noachian rocks across the surface of Mars was revealed by OMEGA (Observatoire pour la Minéralogie, l’Eau, les Glaces et l’Activité) on board Mars Express (Poulet et al, 2005; Bibring et al, 2006) and by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) on board the Mars Reconnaissance Orbiter (e.g., Murchie et al, 2009; McKeown et al, 2009; Bishop et al, 2008a, 2011, 2013a; Bishop and Rampe, 2016).
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