Clay-rich strata on Mars Deposition of >3.7 Ga clay-rich strata of the Mawrth Vallis Group, Mars, in lacustrine, alluvial, and aeolian environments Donald R. Lowe1,†, Janice L. Bishop2, Damien Loizeau3,4, James J. Wray5, and Ross A. Beyer2 1Department of Geological Sciences, Stanford University, Stanford, California 94305-2115, USA 2SETI & NASA-Ames Research Center, Mountain View, California, USA 3Université Claude Bernard Lyon 1, Ens de Lyon, CNRS, UMR 5276 LGL-TPE, F-69622, Villeurbanne, France 4Institut d’Astrophysique Spatiale, Université Paris Sud, F-91405 Orsay, France 5School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia 30332-0340, USA ABSTRACT pear to have formed through subaerial weath- rocks have been widely discussed in terms of ering whereas the aluminosilicates, kaolinite, the climate and surface conditions of early Mars The presence of abundant phyllosilicate and hydrated silica of units 3, 4, and 5 formed and/or hydrologic and diagenetic conditions in minerals in Noachian (>3.7 Ga) rocks on mainly through alteration of fine sediment in the martian subsurface (e.g., Poulet et al., 2005; Mars has been taken as evidence that liquid subaqueous environments. Loizeau et al., 2007, 2010; Wray et al., 2008, water was stable at or near the surface early 2009, 2010; Bishop et al., 2008a; Mustard et al., in martian history. This study investigates INTRODUCTION 2008; McKeown et al., 2009; Michalski et al., some of these clay-rich strata exposed in cra- 2010; Noe Dobrea et al., 2011; Gou et al., 2015). ter rim and inverted terrain settings in the The Mawrth Vallis region of Mars is known In the Mawrth Vallis region (Fig. 1), the Mawrth Vallis region of Mars. In Muara cra- both for the prominent outflow valley from phyllo silicate-rich rocks, here informally termed ter the 200-m-thick, clay-rich Mawrth Vallis which it gets its name (Fig. 1) and for the wide the Mawrth Vallis Group (MVG), are well dis- Group (MVG) is subdivided into five informal presence of phyllosilicate-rich rocks formed played on the walls of a small crater, Muara, just units numbered 1 (base) to 5 (top). Unit 1 con- during the Noachian Period, 4.1–3.7 Ga west of Mawrth Vallis (Figs. 2 and 3). The old- sists of interbedded sedimentary and volcanic ( Michalski and Noe Dobrea, 2007; Loizeau est part of the MVG (Fig. 4) is characterized by or volcaniclastic units showing weak Fe/Mg- et al. 2012). The abundance of phyllosilicate Fe/Mg smectite alteration (Figs. 5A, 5B). It is smectite alteration deposited in a range of minerals in Noachian rocks across the surface subaerial depositional settings. Above a ma- of Mars was revealed by OMEGA (Observa- jor unconformity eroded on Unit 1, the dark- toire pour la Minéralogie, l’Eau, les Glaces et 20°W 15°W toned sediments of Unit 2 and lower Unit 3 l’Activité) on board Mars Express (Poulet et al., are inferred to represent mainly wind-blown 2005; Bibring et al., 2006) and by the Compact Inverted Terrain sand. These are widely interlayered with and Reconnaissance Imaging Spectrometer for Mars draped by thin layers of light-toned sediment (CRISM) on board the Mars Reconnaissance 25°N representing fine suspended-load aeolian silt Orbiter (e.g., Murchie et al., 2009; McKeown and clay. These sediments show extensive et al., 2009; Bishop et al., 2008a, 2011, 2013a; Muara crater Fe/Mg-smectite alteration, probably reflecting Bishop and Rampe, 2016). These studies have subaerial weathering. Upper Unit 3 and units shown that phyllosilicate-bearing rocks include 4 and 5 are composed of well-layered, fine- two contrasting assemblages of phyllosilicate 0 100 grained sediment dominated by Al-phyllo- minerals: a basal sequence over 100 m thick of kilometers silicates, kaolinite, and hydrated silica. De- Fe/Mg smectite-bearing rocks and an overlying position occurred in a large lake or arm of a unit variously estimated at 50–60 m thick con- martian sea. In the inverted terrain 100 km to taining Al-bearing phyllosilicates and hydrated the NE, Unit 4 shows very young slope fail- silica (Bishop et al., 2008a; Loizeau et al., 2010; ures suggesting that the clay-rich sediments Noe Dobrea et al., 2010, 2011). Because the today retain a significant component of wa- formation of phyllosilicate minerals requires the 20°N ter ice. The MVG provides evidence for the presence of water (e.g., Chamley, 1989), their –4000 –3000 –2000 –1000 presence of large, persistent standing bodies abundance in Noachian rocks has been taken by Elevation (meters) of water on early Mars as well as a complex some to indicate the existence of a more clement association of flanking shoreline, alluvial, and early climate, including the presence of liquid Figure 1. Shaded relief map of the Mawrth aeolian systems. Some of the clays, especially water at the surface (e.g., Poulet et al., 2005; Vallis region, Mars, showing the location of the Fe/Mg smectites in upper units 1 and 2 ap- Mustard et al., 2008; Bristow and Milliken, the study areas at Muara crater and the in- 2011; Carter et. al., 2015; Bishop et al., 2018). verted terrain. MOLA (Mars Orbiter Laser †drlowe@ stanford .edu The origin(s) of these phyllosilicate-bearing Altimeter) Science Team, NASA. GSA Bulletin; January/February 2020; v. 132; no. 1/2; p. 17–30; https://doi.org/10.1130/B35185.1; 19 figures; published online 2 May 2019. Geological Society of America Bulletin, v. 132, no. 1/2 17 © 2019 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/132/1-2/17/4906812/17.pdf by guest on 29 September 2021 Lowe et al. east of Muara crater and east of Mawrth Vallis. N The objective of this study is to better charac- terize the internal stratigraphy of the phyllo sili- cate-rich layers, to evaluate the processes and Area of Fig. 3 Figure 2. Muara crater, Mars, conditions under which they formed, and to as- showing the field of windblown sess whether they represent a wetter early Mars dunes covering the crater floor or some other combination of conditions less (medium to dark) and bed- removed from those prevailing today. rock outcrops of the Mawrth Vallis Group (medium to METHODOLOGY very light) around the crater walls. NASA HiRISE image The geologic interpretations developed here PSP_004052_2045. utilize grayscale images of the martian surface provided by the High Resolution Imaging Sci- ence Experiment (HiRISE) camera (McEwen 1000 m et al., 2010). The images have been enlarged and examined using a range of brightness, contrast, and sharpening options, but otherwise, unless specified, individual figures are unenhanced. overlain across a transition zone by 50–70 m of Bishop and Rampe, 2016), as detrital phyllosili- Both study areas have stereo images available rocks rich in Al-phyllosilicates and hydrated silica cate sediments deposited in a large martian sedi- and the anaglyph images have been used as well and a complex variety of other minerals (Bishop mentary basin (Wray et al., 2008), or alteration as available digital terrain maps. All references et al., 2013a, 2016). The Al-phyllosilicate-rich products formed through widespread hydro- to the relative tone of outcrops (e.g., light-toned, unit is overlain in most areas by a thin, younger thermal and/or diagenetic processes (Ehlmann dark-toned, etc.) are to the grayscale tones as caprock of largely unaltered pyroxene-bearing et al., 2011; Noe Dobrea et al., 2010; Sun and seen in the HiRISE images and the specific mafic volcanic rocks (Loizeau et al., 2007). Milliken, 2015). They have also been attributed figures included in this paper. The distance and The implications of these rocks toward the to precipitation from magmatic fluids moving length scales used are those provided on the nature of the early martian climate is compli- upwards during the last-stage degassing of the HiRISE images. Single pixels on the HiRISE cated by the uncertain origins of the rocks them- martian interior (Meunier et al., 2012). images are 25–50 cm across; CRISM spectral selves and of the included phyllosilicates. These The present study examines the MVG in images have a resolution of 18 m/pixel or more. layered clay-rich rocks have been variously two areas in the Mawrth Vallis region (Fig. 1): Determining the strike and dip of layering in interpreted as weathering products of crustal (1) around the walls of Muara crater (Figs. 2 the walls of Muara crater has been problematic. rocks or sediments under surface conditions and 3) just west of Mawrth Vallis and (2) in an Bedding attitudes along the north and northwest (Noe Dobrea et al., 2010; Farrand et al., 2014; area of so-called inverted terrain ~100 km north- walls, where the present study is focused, have been discussed by Wilhelm et al. (2013). Their conclusion was that the structural complexity in this area makes it difficult to evaluate dip mag- nitudes and directions but their figure 2 shows dips averaging ~20–25 degrees, mostly toward Fig. 12 N the crater center. However, this is also roughly Fig. 12 Fig. 13 the magnitude and direction of the topographic slope in this area and the more linear, slope- Fig. 10A perpendicular outcrop pattern of units is not Fig. 10B consistent with rocks dipping parallel or nearly Fig. 8 parallel to the topographic slope. Most small, Fig. 11 Fig. 9A 5 Unit simple craters show nearly flat to outward dips of crater-wall strata (Melosh, 1989). We have Unit 4 3 Unit attempted to estimate dips from bed deflec- Unit 2 Unit 1 tion where the beds cross topography and from Fig.