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 phyllosilicate-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.
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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 phyllosili 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. 7 HiRISE images adjusted to show views along the crater walls. These results suggest roughly flat-lying strata. Our estimates of unit thickness 500 meters are based on the strata being horizontal. The spectral approaches and methodolo- Figure 3. Northwest wall of Muara crater, Mars, enlarged from HiRISE image gies are summarized by Murchie et al. (2009), PSP_004052_2045, showing outcrops of the Mawrth Vallis Group subdivided Loizeau et al. (2010), and Bishop et al. (2013a). into units 1 through 5, a thin regional caprock of what is probably basalt, and The present study has involved spectral analy- the locations of other figures. The prominent light-toned band is the alumino- sis of CRISM image FRT000094F6 (Figs. 5 silicate-, clay-rich Unit 4. The dark-toned areas to the lower right and lower and 6), which has been calibrated with the center are covered by windblown sand. Map-projected Targeted Reduced Data Records
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ABC SPECTRAL ANALYSIS Figure 4. Stratigraphy of Caprock Allophane + the Mawrth Vallis Group, Spectral images of Muara crater and their in- Unit 5 imogolite 80 Mars. (A) Stratigraphic di- terpretation in terms of the main phyllosilicate visions of (A) Loizeau et al. minerals present are shown in Figures 5 and 6. Al/Si 60 (2010) and (B) Bishop et al. Examples of CRISM spectra of the dominant Unit 4 rich Al/Si-rich (2013b). (C) Stratigraphic three compositional units are shown in Figure 6.
70 m subdivisions recognized in Mapped in green (Fig. 5B) are spectra consistent 40 the present study in Muara with short-range ordered aluminosilicates such Al/Si-rich + Unit 3 crater. Correlations be- as allophane and imogolite containing an OH Fe/Mg-smectite 20 tween the spectral divisions overtone band near 1.39–1.40 µm, a broad H O Fe/Mg Fe/Mg-smectite 2 smectite + Al/Si Unit 2 of (A) and (B) and the litho- combination band centered near 1.92 µm, and a 0 logic divisions of the present broad OH combination band near 2.19–2.20 µm m Fe/Mg-smectite Unit 1 study are approximate. (Bishop et al., 2013b; Bishop and Rampe, 2016). This unit is observed at the top of the phyllo- silicate-bearing units as shown in Figure 5C. Mapped in blue (Fig. 5B) are spectra consistent (MTRDR) technique (Seelos, 2011). Spectral et al., 2018). The main shortcoming in these with Al-phyllosilicates such as montmorillonite parameters (Viviano-Beck et al., 2014) were comparisons is that rovers are confined to low- and halloysite/kaolinite or opal. These spectra used to identify regions of specific compositions relief terrain whereas the best stratigraphic and are characterized by an OH overtone band at
and spectra (0.4–4 µm) were acquired from sedimentological details are generally provided 1.41 µm, a H2O combination band at 1.91 µm several sites in each region to confirm phyllo by higher-relief areas where cliffs show internal that is narrower than in the upper unit, and an silicate identifications as in previous studies stratigraphic details and erosional topography OH combination band centered near 2.20–2.21 (e.g., Bishop et al., 2008a, 2013a; Bishop and can reflect internal architecture. In general, µm (Fig. 6). This blue unit is present above the Rampe, 2016). many larger-scale sedimentologically useful de- red unit and below the green unit (Fig. 5C). Fi- The accuracy of orbital interpretations of mar- tails can be seen in high-quality orbital images nally, mapped in red is a larger portion of the tian geology can be problematic but have been but finer, more detailed analyses at scales below phyllosilicate-bearing outcrop (Fig. 5C) that is tested where rovers have explored the same por- 1 or 2 m require ground-based images (Stack characteristic of Mg-bearing nontronite or Fe- tion of the surface (Stack et al., 2016; Williams et al., 2016). bearing saponite because the spectral features fall
Figure 5. Phyllosilicate zones in and around Muara crater, Mars, 3.8 km in di- A B ameter. (A) 3-D view from CRISM image FRT000094F6 highlighting the phyllosilicates in Muara (far) and Tarrafal (close) craters. CRISM image with 5× vertical exaggera- tion is draped over a high resolution stereo camera image (HRSC). (credit: C. Gross, Freie Universität-Berlin, Germany). The colors are from CRISM parameters (Red = bd2290, Green = bd2190, Blue = bd2210_2) as defined by Viviano-Beck et al. (2014). The reddish areas are spectrally dominated by C Fe/Mg smectite, the bluish areas by Al-phyl- losilicates, and the greenish areas by poorly D crystalline aluminosilicates similar to allo- phane. (B) 3-D view of Muara crater showing distribution of phyllosilicate-rich units (Red = R600, Green = R530, Blue = R440). CRISM FRT000096F6 image with 5× vertical exag- geration is draped over MOLA topographic image. (C) 3-D CRISM view of phyllosilicate- rich units with colors as in A. CRISM image with 5× vertical exaggeration is draped over MOLA topographic image. (D) Oblique image showing a horizontal view of the NW wall of Muara crater with colors as in A (after Bishop et al., 2018). The phyllosilicate zones from CRISM are superimposed on HiRISE image PSP_004052_2045 in grayscale overlain on a HiRISE digital terrain model. The jagged edge of the red/blue contact reflects the pixelation due to the lower resolution of the CRISM image. Note that the lower part of the Fe/Mg smectite zone lies 50–60 m below the contact of units 1 and 2 (arrows). Lower parts of Unit 1 show little alteration. The uppermost part of the crater wall above the blue Al-phyllosilicate zone corresponds to Unit 5 and is character- ized by allophane and imogolite. CRISM parameters colorizing each image are as defined by Viviano-Beck et al. (2014).
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Figure 6. I/F CRISM spectra not exposed. These rocks are well stratified in from image FRT000094F6 rep- most exposures and in the CRISM image show resenting the units colored green, a weak Fe/Mg-smectite alteration except for the blue, and red in Figure 5C. Spec- uppermost 30–50 m, which show an intense tra 1 and 2 are dominated by Fe/Mg smectite alteration (Fig. 5C). Stratifica- short-range ordered aluminosili- tion in Unit 1 ranges from thin layers 1–2 m or cates, spectra 3 and 4 include Al- less in thickness (Fig. 9A) to crude layering at rich phyllosilicates, and spectra 5 a scale of 5–10 m thick (Figs. 9B, 9C). Over- and 6 include Fe-rich smectites. all, the lower half of Unit 1 is more finely lay- Lab spectra of minerals/materi- ered than the upper half. On the lower part of als are shown for comparison: the northern and NW crater wall, fine layering allophane and imogolite from is locally well developed (Fig. 9A). These rela- Bishop et al. (2013b), montmoril- tively thin, even, tabular layers are continuous lonite and halloysite from Bishop for at least 100 m along strike with no evidence et al. (2008b), and nontronite of internal scour or erosion. In other areas, there and Fe/Mg smectite from Bishop appears to be abundant large-scale cross-strati- et al. (2008a, 2008b). Grey verti- fication (Fig. 9A). There is common large-scale cal lines mark the spectral fea- thickening, thinning, and wedging of units and
tures due to H2O and metal-OH there are a number of internal erosion surfaces in the crystal structure that are (Figs. 9A, 9C, and 10A). The upper third to half used to characterize the phyl- of Unit 1 around most of Muara Crater crops out losilicates and related materials as massive to thickly bedded, intensively frac- present in the regions associated tured rock (McKeown et al., 2013) broken into with these spectra. blocks from <1 m to ~20 m across (Figs. 8 and 10B). Crude layering is often visible through the fracturing (Fig. 10B) and it is possible that the between those of nontronite and saponite (Bishop north side of the crater by a fringe of post-crater rock itself is more finely layered. et al., 2008a). Example spectra from this red unit ing sedimentary deposits interspersed with out- (Fig. 6) exhibit features similar to nontronite and crops of the MVG (Fig. 7). The sedimentary Interpretation ferrosaponite including an OH overtone band at deposits in this fringe include patches of dark The upper 30–50 m of Unit 1 appear to repre-
1.42 µm, a H2O combination band at 1.91 µm, windblown sand, chaotic landslide deposits, and sent the lower part of the Fe/Mg smectite zone an OH combination band centered near 2.29 µm, water-worked deposits that include what appear of previous investigators (e.g., Bishop et al., and an additional OH band near 2.39 µm. to be well-developed, contour-parallel gravel 2013a). However, the bulk of Unit 1 shows only beach ridges (Fig. 7). A variety of small chan- weak Fe/Mg smectite alteration (Fig. 5). Based MUARA CRATER nels is present including some eroded into MVG on the presence of plagioclase and pyroxene in bedrock and others cutting across the older land- this and other dark units in Muara crater and vi- Muara is a small, roughly circular impact slides (Fig. 7). These features attest to an early cinity (Poulet et al., 2008; Viviano and Moersch, crater ~3.8 km in diameter centered at ~24.3°N history of wall collapse, water runoff, and ac- 2013), the widely developed stratification, and and 340.7°W (19.3°E) (Figs. 1 and 2). It is cov- cumulation of debris in a lake on the crater floor the alteration of the rock to Fe/Mg smectite, we ered by HiRISE images ESP_012873_2045 following crater formation. would infer that the more crudely layered parts and PSP_004052_2045, which form a stereo Illumination at the time HiRISE images of Unit 1, such as much of the section on the pair. The crater is a simple, bowl-shaped crater were collected was from the SSW and the best east crater wall, may represent altered mafic or with a rim that reaches ~300 m above the crater exposed and structurally least complicated sec- ultramafic volcanic flows and/or volcaniclastic floor. The HiRISE images show surface features tion of the MVG along the northwestern and units. Previous studies of alteration and clay for- strongly suggesting the presence of layered sedi northern crater wall is taken as the type section mation in this area have also assumed that these mentary and/or volcanic rocks around the crater (Fig. 8). Based on tone, texture, and structur- rocks represent a basaltic sequence (e.g., Zolo- walls (Figs. 3) and spectral features are present ing, the 200–250 m of exposed section are here tov and Mironenko, 2016). The fine, even layer- in CRISM image FRT000094F6 indicating the divided into five stratigraphic units, numbered ing in the lower part of Unit 1 (Fig. 9A) suggests presence of phyllosilicates (Fig. 5). units 1 through 5 from base upward (Figs. 4 that some parts may have been deposited in The floor of Muara is covered by a field and 8). The dark caprock that overlies the MVG standing bodies of water and the association of of wind-generated sand dunes ~2.5 km across contains pyroxene (Loizeau et al., 2007), lacks these layers with large-scale cross-stratification (Figs. 2 and 3). Spectral images show no phyllo phyllosilicates, and probably represents un (Fig. 10A) and channel-like features (Fig. 9A) silicates in the dunes. Similar low albedo, altered volcanic and/or volcaniclastic rocks. may indicate that overall the lower part of Unit 1 pyroxene-bearing deposits throughout the region represents a complex of interfingering alluvial, have been interpreted as aeolian or pyroclastic Unit 1 fluvial, aeolian, and/or lacustrine environments. deposits composed of volcanic sand (Loizeau The more crudely stratified upper half of Unit 1 et al., 2007). The dune crests are elongate gener- Description that shows undulating layering and some large- ally ENE-WSW. Dune geometry suggests wind Unit 1, making up the lower half of the walls scale cross-stratification on the northern and from the SSE toward the NNW. The dune field of Muara crater, is a succession of medium- NW wall of the crater may represent largely has an irregular edge and is surrounded on the toned rocks at least 100 m thick with the base windblown sediments.
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Figure 7. Lower part of the NW crater wall and adjacent crater floor of Muara crater, Mars. (A) With features unlabeled. (B) With features labeled showing large landslides, runoff channels, beach ridges marking old shorelines around crater lake, and later windblown sand. This part of the wall is composed mainly of bedrock Unit 1 of the Mawrth Vallis Group. Lower on the slope, bedrock is overlain by landslides, some of N which probably represent the edge of the breccia lens formed at the time of cratering. Water runoff subsequently cut local channels across the Unit 1 outcrops and the landslide breccia. This runoff resulted in the forma- tion of a transient crater lake that gradually dried, leaving a set of very coarse-grained 100 m beach ridges as the shoreline retreated. A Bedrock
Bedrock l Unit 2 Channe Landslide
Description Windblown Above Unit 1 across an irregular and probably Landslide Sand unconformable contact is up to 35 m of medium- Windblown Sand to dark-toned, well stratified rock of Unit 2 Channel Complex (Figs. 3, 8, and 11). The base forms a distinct break around the crater wall between underlying, Landslide Lacustrine Beach Ridges massive, fractured rock of Unit 1 and overlying, dark-toned, stratified rock of Unit 2 (Fig. 8). Unit 2 varies in thickness from 0, where Unit 3 rests directly on Unit 1, to ~35 m. These variations in thickness probably reflect in large part the highly Windblown Sand Windblown Sand irregular surface on which Unit 2 was deposited. B This contact may be the regional unconformity identified by Loizeau et al. (2012) as a “paleo- surface” within the MVG sequence. The best ex- 0100 200 posures of Unit 2 in Muara crater occur along the NW crater wall, although there is some young meters dark windblown sediment masking portions of N Unit 5 the rock in this area and layering in the lower m Unit 4 i part of Unit 2 may be better developed than can R a t e r Unit 3 be seen in the HiRISE images (Fig. 11). C r Along the NW wall, the outcrop of Unit 2 shows a series of parallel, sharp-crested ridges, Unit 2 20–50 m wide and a few meters high, that run up-and-down the crater wall, roughly perpen- dicular to contours and the strike of bedding (Fig. 11). The bottoms of the round-bottomed Unit 1 swales between ridges are widely coated by dark windblown sand (Fig. 11). The ridges are elongated parallel to the wind direction up the crater wall, show bedrock layering throughout, and were apparently cut into bedrock by the pre- vailing wind. Overall, Unit 2 consists of well-stratified rock made up of lenticular, interfingering light-, medium-, and dark-toned layers from <0.5 to Figure 8. Northwest wall of Muara crater, Mars, showing main subdivisions of the Mawrth perhaps 2 or 3 m thick. Locally, the lowest few Vallis Group.
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Figure 9. Unit 1 of the Mawrth Vallis Group b (MVG) in Muara crater, Mars. (A) Lower A A N c part of Unit 1 of the MVG on the northwest b crater wall showing the fine layering parts of this section (a), a possible erosional chan- b N nel (b), and cross-sets that may be aeolian b beds (c). (B) A portion of the Mawrth Vallis a Group on the eastern wall of Muara crater. a The upper part of Unit 1 occupies most of the photo. It shows crude layering and the c 50 m interstratification of resistant and weaker 50 m units. Many areas are covered by patches B a of younger windblown sand (a) and the area B is crossed by irregular wind-sculpted ridges c (b). There is some divergence of bedding in e b Unit 1 but no conspicuous erosional chan- d c a nels or steeply dipping layering. Overlying b MVG units 2 (c), 3 (d), and 4 (e) lie along the right side of the photo. Details have been aab c sharpened. (C) Lower part of NE crater wall showing major truncation surface (dashed 50 m 50 m line) in the lower part of Unit 1. Figure 10. Unit 1 on north wall of Muara C crater, Mars. (A) Middle to upper part of Unit 1 showing thicker, crude layering that includes large cross sets (a) and undulat- meters of Unit 2 are very dark-toned, massive to ing layering (b). This sequence may consist weakly stratified rock (Fig. 11). Upward, well- largely of windblown sand. The thinner lay- developed stratification is defined by complexly ering of the lower part of Unit 1 can be seen interfingering lighter streaks and lenses up to landslide at the bottom of the photo (c). (B) Massive ~1 m thick, a few discontinuous blocky layers, fractured rock at the top of Unit 1 (a). Some medium-toned layers up to ~2 m thick, and thin, areas of well-defined layering are laterally very dark layers mostly <1 m thick. The thin dark 50 m equivalent to fractured rock (b) and some layers are mostly overlain or underlain by thin layering can be seen through fractures (c). light-toned beds (Fig. 11). Most layers persist for See Figure 3 for location. less than 100 m along the outcrop (Fig. 11). or steeply inclined layering. The abundance of lenticular, dark-, and light-toned layers and Interpretation couplets is consistent with an aeolian origin, es- NW crater wall consists of prominently banded Unit 2 shows an intense Fe/Mg-smectite pecially toward the top where the dark-light al- units showing either post-depositional deforma- spectral signature (Fig. 5C). Modeling estimates ternation is most prominent. There may also be tion or syndepositional stratigraphic complexity of the unit containing spectral features near a component of water-worked sediments in the (Fig. 11). Dark-toned layers in this zone, where 2.3 µm indicate 30–70 vol% Fe/Mg-smectite medium-toned units, but there is no clear evi- thicker, are commonly lenticular or show pinch- (Poulet et al., 2014). We infer that Unit 2 rep- dence for aqueous transport, such as erosional and-swell (Fig. 11). They also widely exist as resents a sedimentary sequence, although some channels, and all layers could be composed of thin, very continuous layers probably mostly blocky-weathering units may be thin mafic lava windblown components. <50 cm thick that coat both unconformities and flows. Interbedded layers probably include dark, the boundaries of virtually all light-toned units. reworked volcaniclastic debris derived in large Unit 3 Dark-toned layers lack evidence for internal part by erosion of the underlying rocks of Unit layering, although their low albedo may simply 1 and possibly pyroclastic materials. The thin, Description make internal stratification difficult to see. Inter- lenticular dark layers overlain by light layers Unit 3 is a transitional unit, 20–30 m thick, of bedded light-toned layers are generally thicker, are more prominent in Unit 3 and will be dis- well-stratified, mainly medium- to light-toned more continuous, and often underlie or drape cussed there, but are thought to represent wind- rock between the top of the predominantly the dark layers (Fig. 11). The light layers com- blown sand layers (darkest) capped by thin very dark-toned, layered rocks of Unit 2 and the very monly show even finer internal layering down to fine-grained layers of windblown dust (light). light-toned rocks of Unit 4 (Figs. 8, 11, and 12). the resolution of the HiRISE images. A few peaked, lenticular, very dark layers may Much of Unit 3 shows well-developed layering The boundary between the Fe/Mg smectite represent dunes of windblown sand (Fig. 11). down to the resolution of the HiRISE images, zone and the overlying Al-phyllosilicate layer All bed-scale units are highly lenticular and at indicating that it is stratified at scales to or be- appears to fall within Unit 3 (Fig. 5D), although least one surface within Unit 2 along the north- low ~50 cm. The base of Unit 3 is locally an the lower resolution of the CRISM images and ern wall appears to cut down through a few erosional unconformity (Fig. 11) along which as uncertainties in correlating HiRISE and CRISM meters of underlying material, but overall there much as 5 m of strata of the underlying Unit 2 images would also allow the boundary to coin- is no evidence for large-scale scour, erosion, are truncated. The lower half of Unit 3 in the cide roughly with the Unit 2–3 contact.
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Figure 11. Uppermost Unit 1 N zone of evenly stratified light- or medium-toned (light, lower right), Unit 2 (dark c rock (Fig. 12). There is a rather cyclic inter- band, center), and lower Unit 3 layering of light layers ~1–3 m thick and thin, d a (light banded, upper left) of the <50-cm-thick dark- or medium-toned layers. Mawrth Vallis Group, north- b The light layers locally show a very fine, even, west wall of Muara crater, flat internal layering at the resolution of the Mars. The crater wall slopes HiRISE images. The thinness and evenness of from upper left to lower right. the layering, the predominance of phyllosilicate Low, sharp-crested ridges (a), a minerals and the implied fine grain size, and ab- illuminated on their SW sides, sence of scour features suggest deposition under extend up and down the slope. low energy, possibly subaqueous conditions. In The blocky weathering of some areas, flat, even bedding passes laterally Unit 1 is overlain with a sharp a 25 m into curved, upward-peaked layering that re- contact by the more massive, sembles inferred dunes lower in Unit 3 (Fig. 12). dark layer of lowest Unit 2 (solid line). The bulk of Unit 2 shows medium- to light-toned lay- ers, <1 to ~2 m thick, interbedded with slightly darker layers and very dark thin layers. Most Unit 4 layers are relatively flat, with apparent undulations reflecting topographic irregularities, but some show low peaks (immediately right of b) and swales (c). The complex stratigraphy of Description the lower part of Unit 3 is evident, with an unconformity between units 2 and 3 (dashed line) Unit 4 is the prominent 20–25-m-thick light- and internal erosional unconformities (d), discontinuous strata, pinch and swell features, and toned band around the upper part of the crater draping units. Contrast and sharpness enhanced. wall. This part of the crater wall is a zone of ex- tensive fracturing, including both closely spaced Interpretation hydraulically equivalent dark sand grains and sets of more-or-less orthogonal fractures that We would suggest that the dark layers in light clay aggregates. sweep across the crater wall (Fig. 13), probably the lower half of Unit 3 represent windblown It is not clear whether the undulating layers related to the impact that formed Muara crater, sand that was commonly swept into dunes but and truncated units in the lower part of Unit 3 and large, cross-cutting fractures and faults. is widely distributed and/or preserved as thin, represent deformation in response to an external There also appears to be a change in northern- more tabular layers. If the dark layers repre- event, such as an impact, or whether it is entirely crater-wall slope roughly coincident with the sent sand-sized windblown bed-load sediment, of depositional origin involving erosion, drap- Unit 3–4 contact and there may be fractures at the draping light-toned caps are probably silt to ing of irregular surfaces, deposition of lenticular this point formed during downslope sliding and clay-sized debris deposited during the waning layers, and perhaps minor sliding. However, we slumping. Fractures at all scales are marked by stages of dust storms as well as during periods suspect that all of these features are sedimen- accumulations of medium- to dark-toned ma- of reduced wind activity between storm events. tary in origin because no clearly deformational terial and the smaller fracture sets subdivide An especially good example of this close asso features are present, such as over-steepened the rock into small rectangular blocks a few ciation of lenticular, dark-toned dune sands beds (dips greater than the angle of repose), meters across that are separated by thin septa and light fine-sediment drapes can be seen on overturned or recumbent layers, isolated blocks of dark material. We infer that the fractures the north crater wall in the lower part of Unit 3 (slides), or truncation of overlying layers by are marked by more easily erodible materials (Fig. 12). Mixed medium-toned layers may underlying ones. and/or were open and have been widely filled represent either stacks of very thin alternating Above this lower zone of dunes, drapes, and by dark windblown sand. There is also a host light- and dark-toned layers or of mixtures of complex bedding, the upper part of Unit 3 is a of irregular lines and features across the crater wall at all scales that could be fractures, joints, Figure 12. Part of the northern small faults, layering, or other features within wall of Muara crater showing Unit 3 c the rock, but for which, in many cases, a specific the upper part of Unit 2 and origin remains elusive. most of Unit 3 of the Mawrth b The intersection of bedding and fracture sets Vallis Group, Mars. The lower has widely resulted in the downslope slippage part of Unit 3 shows a train a of fracture-bounded blocks, forming a jagged, of well-developed, lenticular, a saw-tooth geometry (Figs. 12 and 13). Wind- dune-like features (a) composed N blown dark sand has filled the fractures and a of dark sandy(?) sediment that more horizontal surfaces formed by block slid- are draped by light-toned ma- ing (Fig. 13). terial. Above the zones of dunes and lenticular dark layers, Interpretation Unit 2 50 m Unit 3 consists of tabular light- Overall, Unit 4 is characterized by bedded, toned layers separated by very thin, continuous layers of dark material (b), many of which light-toned rock interrupted by very thin lami- appear thicker than they actually are because of younger, dark, windblown sand accumulat- nations of darker material. This unit correlates ing on small benches marking bedding surfaces. A set of closely spaced joints produces saw- with the Al-phyllosilicate-rich layer observed tooth like fractures of the light-toned layers that are filled by dark windblown(?) sediment. in CRISM images (Fig. 5C). As with the upper The tabular light layers traced to the right show swales and features suggesting possible part of Unit 3, it appears to have been deposited dunes (c). For location of this area, see Figure 3. mainly as fine-grained sediment.
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today’s valley floors (Loizeau et al., 2007; Noe N Dobrea et al., 2010). The present study lies within an area of in- a Figure 13. Joint set (a) in the up- verted terrain at ~26°N and 18.5°W, Oxia Pallis per wall of Muara crater, Mars. Quadrangle, and is covered by the stereo pair This view is immediately higher of HiRISE images ESP_013361_2060 and on the crater slope than the view ESP_013084_2060. It is in the northwest part in Figure 12 and includes Unit 4 of a large valley bounded by channel-like pla- (light toned) and overlying teaus to the north and west (Figs. 14, 15, and Unit 5, which is largely covered 16). These are capped by flat-lying, medium- to by dark debris. The joints and dark-toned, well-cratered rock, probably basalt, bedding form saw-tooth frac- and local patches of dark windblown dunes. tures and displaced blocks (b). b The elevation difference between the valley bot- tom and flat-topped plateau is ~25–30 m. The 50 m light-toned valley floor is made up of essentially flat-lying, light material. The cliff-forming unit, which is composed of interbedded dark- and Bedding can be difficult to distinguish in toned units was interrupted by volcanic episodes light-toned units with a dark caprock forming Unit 4 because of the uniformly light-toned or floods of volcaniclastic sediment, although the plateau surface, appears to be roughly equiv- character and paucity of layers of contrasting details of the internal makeup of these layers alent to Unit 5 of the MVG and the caprock at tone or color. The spaced dark laminations seen cannot be resolved. CRISM images indicate that Muara crater and the underlying light-toned, on the crater wall may be sedimentary layers but Unit 5 contains poorly crystalline aluminosili- valley-floor unit to Unit 4, although precise cor- others may be accumulations of modern wind- cate phases similar to allophane and imogolite relation across this distance is uncertain. The blown sand along crevices and on ledges follow- (Fig. 5D). base of Unit 4 in the inverted terrain is not ex- ing bedding. These suggest layering on the scale posed within the HiRISE images used. of 1–3 m thick. Layers appear to be tabular with INVERTED TERRAIN The valley floor (Fig. 16) slopes down from no clear undulations, dune-shaped features, or west to east at ~4° to 6° and appears to be under scours/channels. Finer-scale layering has been Surface Features lain by light-toned Unit 4 rock, possibly inter- widely observed in other areas (Loizeau et al., bedded with thin layers of dark-toned material: 2010) and, if the light-toned layers in Unit 4 are About 100 km north-northeast of Muara dark layers are interbedded with light-toned similar to those in Unit 3, then the thick light crater (Fig. 1) is an area of irregular terrain in layers in the valley wall, but these are not ap- layers are also most likely made up of much which dark, sinuous features that resemble parent on the valley floor. Part of the valley thinner layers. We would also infer deposition channels cross broad, lighter areas (Fig. 14). floor is marked by step-like elongate zones of of the fine silts and clays of Unit 4 in a subaque- When examined closely, it is seen that the rubble up to ~40 m wide separated by areas ous environment. “channels” are topographically high plateaus of smoother light topography (Fig. 16). The and the intervening lighter areas are broad, open smooth light-toned surfaces are covered either Unit 5 valleys. This topography has been referred to as by very fine, featureless material, possibly bed- inverted topography because one hypothesis for rock or a regolith cover, or by finely patterned Description its origin is that it started as a flatter landscape ground (Fig. 16). Adjacent to the dark debris Unit 5, the uppermost member of the MVG crossed by channels that were later filled with aprons at the bases of the bounding cliffs, dark in the crater-wall sequence, is laterally hetero resistant volcanic rock or sedimentary materi- material has been swept into small dunes. In geneous and the outcrop widely disrupted by im- als. Subsequent erosion left the more resistant contrast, dunes of light-toned material are es- pact processes and crater-wall collapse. On the “channel fill” standing in relief above the more sentially absent. The polygons making up the northwestern wall (Fig. 8), there is a reasonably easily eroded surrounding materials that form patterned ground are mostly 1–4 m across, al- well-exposed section. It includes a basal zone of dark rock that widely produces debris that covers and obscures the Unit 4–5 contact. Above this N zone, Unit 5 consists of interfingering and inter- Figure 14. Location photo for layered dark and medium- to light-toned layers the inverted terrain study area, for a total Unit 5 thickness of ~25–30 m. Mars. The area is characterized by topographically high, sinu- Interpretation ous plateaus capped by dark The dark, more friable layers that yield abun- rock, probably basalt, and low, dant slope-mantling debris most probably rep- Area of Fig. 15 light- to medium-toned valleys resent mafic flows, mafic volcaniclastic layers, underlain by phyllosilicate- and/or windblown sediment composed of mafic bearing rocks of the Mawrth sand grains. The light-toned layers are thought Vallis Group. to be layers of phyllosilicate-rich clays and silts and poorly crystalline materials. This interlayer- 1000 m ing suggests that the sedimentation of the light-
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N a c 50 m
b e a c d N b c gh e b d Area of Fig. 16 c e d k k i k d e
Area of Fig. 18 k f
100 m
Figure 15. Study area in the inverted ter- i rain, Mars. A large, low, essentially un- cratered area of light-toned rock of Unit 4 of the Mawrth Vallis Group is bounded to the Figure 16. Area in the inverted terrain showing zones of rubble and brecciated rock sepa- north and west by a plateau capped by dark- rated by zones of smooth, intact rock. The rubble zones have developed by failure, sliding, toned rock, probably basalt (a). The plateau and brecciation of surface layers composed mainly of light-toned sediments. Some of these is flanked by cliffs of exposed Unit 5 of the failures display a distinct downslope zonation including locally a headward fracture (a), a Mawrth Vallis Group (b) and, lower on the head scarp (b), and an upper zone of large, little displaced blocks (c) grading downslope slope, by accumulations of dark sandy or into finer rubble zones to a front of small, jumbled blocks probably representing debris- gravelly debris (c). Unit 4 outcrops on the flow material (d). The front of the slides are distinct, shadowed scarps (e). Where most of valley floor (light toned) show zones of heav- the light-toned layers have been removed there are accumulations of darker sediment (f). ily broken rock (d) separated by smooth ar- On the right are a series of fractures (g and h) parallel to the slide front. In some areas, eas of unbroken light-toned rock (e). brecciation involved fracturing along polygon boundaries in the strongly patterned ground (i). A few through-going fractures are present (k). Enhanced contrast and sharpened. See Figure 15 for location. though smaller polygons may be present below the resolution of the images. The rubbly zones bear a strong resemblance cates a terminal drop-off. There are many areas terned ground, and slid only a short distance to many terrestrial landslides and debris flows that do not display these exact size trends and downslope, forming a distinct rubble front formed by slope failure (Fig. 17). In the study some slides have coarser material toward the along the downslope side. There is an upslope area on Mars, their formation appears to have tips but these general trends are widespread. scarp and larger proximal blocks but no clear involved failure of the surface layers of the These features suggest that the rubble areas downslope zone of mixed, debris-flow material. light-toned rock, collapse and brecciation, but formed by decoupling of the light-toned rock The central part of this rubble has further col- with only minor downslope movement. Most layers, possibly along dark layers, and some rubbly areas show a distinct, arcuate head scarp downslope sliding of the detached layers, with or series of arcuate scarps on the upslope side. normal faulting and fragmentation of the upslope Down-dropped blocks immediately below the parts of the detached mass of material and in- head scarp are often large and broken by addi- creasing amounts of flow and brecciation toward tional fractures along which the downslope side the fronts of the slides. The frontal zones of the a has been further down-dropped. Above the head slides often mixed and became debris flows. scarp in some areas are arcuate dark lines par- These features are well developed in many mod- allel to the head scarp that appear to represent ern terrestrial landslides (Fig. 17). In some areas, fractures that have not yet evolved into failures the slides appear to have collapsed, thinned, and (Fig. 16). Within the rubble zones, brecciation lost much of their volume (Fig. 16, point f). Such b tends to become more chaotic from the head areas include abundant dark sediment. scarp to the downslope tip: the blocks tend to In several areas, localized failure has formed become smaller, more jumbled, and do not fit irregular, disconnected patches of rubble with- Figure 17. Landslide triggered by the 27 together. The distal 5–10 m of the slides widely out major downslope movement of the frac- March 1964, earthquake in Anchorage, consist of blocks only 1–3 m across embedded tured debris. In the small rubbly patch shown Alaska, USA. Note head scarp (a), down- in a matrix of fine material, probably represent- in Figure 18, a detached mass of light-toned dropped blocks in upper part of slide, and ing debris flows (Fig. 16). Shadowing of the material has fragmented internally, in part along debris-flow-like toe of slide (b). Photo by A. sharp downslope ends of the chaotic zones indi- the boundaries of polygons in the surficial pat- Grantz, U.S. Geological Survey Circular 491.
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Figure 18. Small slide area of brecciated N contain abundant water ice and probably perma- Unit 4 surrounded by largely intact, strongly frost at a shallow depth. The polygons formed patterned ground. The slide shows a head- before the mass movement as shown by the ward scarp (a), a downslope end composed g fact that in many places the polygon boundaries of larger blocks (b), and a middle area that g served as the loci of fracturing and breakage of includes two topographic low areas covered e the light layers into blocks during collapse and with dark, finer, probably sandy sediment b sliding (Figs. 16 and 18). Because exposed ice (c and d). (d) is at the head of a low area cannot survive at the martian surface, the sur- with abundant dark sediment that extends d face of the light-toned “bedrock” and permafrost as a low-channel-like feature to the NW. It c layers is probably mantled by non-icy debris. is characterized by dark sediment and iso- f A similar debris and vegetative layer character- lated blocks of Unit 4. One small debris-flow a izes most terrestrial permafrost terrains. Because lobe extends 8–10 m out onto the adjacent these light-toned areas and the light regolith at flat area of patterned ground (e). The fine 25 m the surface are widely characterized by alumi- fractures characterizing the mid-part of the nous clays, we would suggest that the light-toned failure have formed along the sides of polygons in the original patterned ground. Older lobes layers consist mainly of a mixture of phyllosili- of failed and brecciated rock (f) appear to be covered by a thin layer of fine light-toned, pos- cates and water ice. At the surface, the water sibly windblown dust and silt. There are irregular ribbons of darker material and isolated has been lost through sublimation until a solid, larger blocks (g) extending downslope from the tips of the failures that appear to mark a protective layer of light clay, phyllosilicates, and runoff channel. See Figure 15 for location. other rocky debris has formed. We infer that the rubble zones formed as a lapsed in two areas (Fig. 18), each marked by those in the study area are very uniform in size result of partial melting of the permafrost at a a distinct, dark-toned topographic low area that and shape, most 0.5–4 m across, and include only shallow depth (Fig. 19). As the ice melted, the extends to the edge of the failure. One shows a one size of visible crack and bounding joints, not overlying sedimentary and regolith layers were small debris-flow tongue extending beyond the a hierarchy of crack types common to desicca- destabilized, decoupled from the underlying rubble zone onto the edge of the adjacent light, tion fractures (El-Maarry et al., 2014). The pres- layers, fractured, and locally slid. Water appears patterned ground (Fig. 18). The dark areas are ence of expandable smectite clays in Unit 4 upon to have run out through more porous layers, per- interpreted to expose or be partially covered by which polygons are widely developed may sug- haps the dark layers, and/or via the fractures, dark sediment, possibly from dark layers, and the gest that desiccation as well as thermal contrac- carrying some of the fine dark sand and silt. light collapsed layer is greatly thinned or miss- tion, ice wedging, and sublimation influenced Fracturing associated with collapse exposed ad- ing in these low areas. Many of the rubbly areas their formation. ditional permafrost to sublimation and/or melt- have narrow, dark, topographically low zones ing, promoting continued fracturing, downslope along their fronts and extending downslope Controls on Failure and Mass Movement movement, and head scarp retreat. (Fig. 18). These locally show the accumulation Within many of the rubbly areas, the frac- of dark sediment along their courses, small dark The wide distribution of patterned ground, the tured light-toned layers have thinned and largely debris cones right up against the rubble fronts, collapse and thinning of the light-toned layers disappeared during collapse, probably through and isolated light-toned blocks. Some of these within the slides, and the apparent evolution of increased sublimation of water ice within the topographically low areas appear to represent water during sliding suggest that the light layers more intensely fractured rock. The latitude of small runoff channels.
Origin of the Patterned Ground Failure Zone Slide Debris Flow
While impact-generated joints are widespread in the areas studied, such as along the upper dark layers walls of Muara (Fig. 13), the small polygons that light-toned sediment r e g o l i t h top of permafrost widely cover areas of flatter ground underlain by Unit 4 in both study areas are most likely either thermal contraction fractures (Mellon, 1997; Mellon et al., 2008), like those formed widely Figure 19. Inferred origin of the breccia zones in the inverted terrain study in terrestrial glacial and peri-glacial settings, or area, Mars. Outcropping rock includes thicker light-toned layers of Unit 4 desiccation contraction fractures. Based on cri- composed of fine silty and clayey sediments and possibly thinner, probably teria discussed by Levy et al. (2009), evidence granular, dark layers. Both are saturated with water ice. There is a permafrost for the wholesale collapse of the light layers, and substrate (surface indicated by dashed line) overlain by the lowest light-toned features suggesting that collapse was locally ac- sediment and a capping layer of debris and regolith. Partial melting at the top companied by flowing water, we would infer that of the permafrost results in destabilization of the overlying bedrock and surface the patterned ground in the study area formed debris, failure, and sliding. Progressive upslope stepping of the headward frac- mainly by thermal contraction associated with tures exposes additional rocks to sublimation and possible melting, promoting a layer of permafrost. Others have interpreted additional downslope sliding. Downslope the size and proportion of light blocks these structures as desiccation cracks (El-Maarry diminishes due to block fracturing, melting, and sublimation. Displaced blocks et al., 2014) but unlike most desiccation cracks, are shown stippled; the finer matrix in gray.
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the inverted terrain study area (26°N) is close representing reworked and eroded volcanic ma- Unit 3 and Unit 4. Both scenarios are consis- to the lower latitude of 30°N for permafrost terial. Most appear to represent windblown sand. tent with the light layers having been composed and polygon development (Head et al., 2003) Wind-generated sand dunes are among the largely of fine-grained clay, silt, and very fine- and ground ice may be periodically subject to most widespread sedimentological features grained sand. The thin, even, fine layering down freeze-thaw cycles during martian summers. on Mars (Grotzinger et al., 2005), both at the to the resolution of the HiRISE images in the This would be consistent with the inferred run- surface and in the geologic record, so their de- light-toned layers is consistent with deposition off features seen associated with Unit 4 areas of velopment in Muara crater and the MVG is not under low-energy conditions with little scour fracturing and collapse and perhaps even more unexpected. There are many studies of martian or erosion. However, loess deposits are typi- likely under different, recent climatic conditions aeolian deposits, most focused on dunes that cally massive, homogeneous, and accumulate at higher obliquity (e.g., Forget et al., 2006). provide 3-D outcrops, including smaller, rover- to thicknesses of many meters with little or no Our results suggest that the light-toned layers based exposures, than the MVG studied here internal layering (e.g., Pye, 1995). We would are mixtures of phyllosilicates, silt, and water (e.g., Grotzinger et al., 2005; Milliken et al., suggest that the features of the light-toned lay- ice. This interpretation is consistent with the 2014; Banham et al., 2018). ers of upper Unit 3, Unit 4, and parts of Unit 5 observation that the light layers yield little or (2) Upper Unit 3 in Muara crater and Unit 4 are most consistent with sedimentation by the no coarse debris to form windblown dunes or in both study areas are composed primarily of settling of silt and clay out of a water column talus slopes: they tend to break into large blocks light-toned materials. Because these light-toned in a large lake or sea. With the possible excep- during fracturing and then break down rapidly layers correspond to the distribution of alumi- tion of the stratigraphically complex zone at the through sublimation and melting of the included nosilicate clays, they are probably composed base of Unit 3, deposition took place under very water into very fine-grained silt and clay. largely of clay and silt- to fine sand-sized phyllo quiet, subaqueous conditions. The presence of silicate particles. The common draping of dark- some sweeping, dune-like strata in upper Unit 3 Age of Surface Features in toned dune-like bedforms by thin light-toned suggests that the lake/sea floor may have been the Inverted Terrain layers and fine interlayering of the two sedi- periodically exposed with fluctuations in water ment types in units 2 and 3 are consistent with level and that the exposed lake margins were The rubble zones and patterned ground in the interpretation that the dark layers represent sites of wind activity and the construction of the inverted terrain study area show evidence of windblown sand and the light layers are com- small dunes or deposition of fine suspended a very young origin. There are very few craters posed of sediment deposited out of suspension, silt and clay eroded from the exposed clay-rich across the valley surface. The patterned ground often forming drapes, during periods of reduced lake beds. is sharp and fresh in most areas and probably wind or current activity. Higher in Unit 3 and The stratigraphically complex interval at the formed relatively recently perhaps over the last in parts of Unit 4, light-toned sediment shows base of Unit 3 includes erosive surfaces, trunca- few million years during periods of climatic in- widespread fine layering and tabular geometries tions, onlap surfaces, and a complex interlayer- stability (Head et al., 2003). The patterned ground consistent with deposition of very fine silt and ing of dark and light sediments. This stratigraph- polygons are cut and disturbed by the processes clay under quiet, subaqueous conditions. How- ically complex deposit may mark a coastal zone of failure, sliding, and mass movement to form ever, ripped up silt- and sand-sized chunks of where the largely terrestrial sequence of Unit the rubbly zones, further indicating an even more light-toned sediment may have formed sparse 2 interfingers with the largely lacustrine/ocean recent origin of the mass transport events. dunes that are largely indistinguishable in the sequence of upper Unit 3. Wave and current uniformly light sediment as seen in upper Unit 3 activity could have locally been at a maximum DISCUSSION (Fig. 12). Medium-toned layers may be mix- within this zone and it could also have been a tures of both light- and dark-toned particles. zone of local erosion during falls in water level. Composition of the Mawrth Vallis Group Studies in the inverted terrain suggest that today This zone is also roughly coincident with the the light-toned layers are mixtures of fine sedi- contact between the lower Fe/Mg-smectite zone We interpret the Mawrth Vallis Group as a ment and water ice, a conclusion consistent with and upper Al-phyllosilicate zone in CRISM im- stratigraphic sequence within which the layers HiRISE observations of near surface ice in mid- ages. This narrow contact zone is locally char- are primarily of sedimentary origin. The bulk latitude regions (Dundas et al., 2018). acterized by a spectral signature indicating an of units 2 through 5 can be described in terms abundance of Fe+2-bearing components (Bishop of two main end-member sediment types rep- Depositional Evolution of the MVG et al., 2013a) in contrast to the phyllosilicate- resented by the dark- and light-toned materials. rich units on either side. We suggest that this (1) The dark-toned sediment is well repre- If the dark-toned layers in units 2 and 3 are zone includes shoreface sediments composed sented in Muara crater where dark sand makes composed mostly of windblown sand, their en- mainly of winnowed, unaltered mafic minerals up the modern dune field on the crater floor vironment of deposition was mainly subaerial. worked and deposited within the coastal zone and sand patches on the crater walls, and where The upward increase in the proportion of light- represented by lower Unit 3. The presence of dark beds in the Noachian MVG show com- toned layers through units 2, 3, and 4 in Muara jarosite along this horizon (Bishop et al., 2016) mon pinch-and-swell and dune-like features and crater offers two possibilities: (1) upper Unit 3 is also consistent with the development of local geometries in cross section. Deposits at the bases and Unit 4 are parts of a gradually expanding coastal evaporitic ponds. of cliffs are composed almost exclusively of regional subaerial loess plain, perhaps along the fine, dark-toned material, locally swept up into margins of a drying ocean or large lake that oc- Origin of the Phyllosilicates in the MVG small dark dunes. The similarity of all of these cupied the lowlands along the eastern edge of dark-toned sedimentary layers, including their Chryse Planitia, or (2) there is a transition from The presence of pyroxene and plagioclase is apparent fine granularity and presence of dune- subaerially deposited windblown sediment in consistent with Unit 1 being composed largely like bedforms, suggests that most are composed Unit 2 and the lower part of Unit 3 to subaque- of basalt or a similar mafic or ultramafic vol- of sand- to fine gravel-sized material, probably ously deposited sediment in the upper part of canic rock and associated volcaniclastic sedi-
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ments. There is a low level of Fe/Mg-smectite The presence of layers of dark, sand-sized sedi- increasingly interrupted by thin lenses of light- alteration throughout the middle and lower parts ment interlayered with light, Al-phyllosilicate toned sediment, which often drape undulat- of Unit 1. Alteration in the bulk of Unit 1 does rocks in Unit 3 confirms that wholesale regional ing surfaces. We suggest that this association not seem to form discrete layers. Sun and Mil- hydrothermal alteration did not form the phyllo of dark lenticular units draped by light-toned liken (2015) have argued that Fe/Mg smectite silicates in situ from a single, compositionally layers represents windblown layers composed alteration is more widespread in rocks exposed uniform protolith since adjacent layers would of mafic sand-sized sediment that are overlain in central peak uplifts of craters in Noachian have suffered similar degrees of alteration. and/or underlain by light-toned layers deposited terrain across Mars than Al-phyllosilicate rocks The interstratification of contrasting sediment as finer, suspended windblown sediment. Depo- and that this Fe/Mg smectite alteration appears types further argues against the formation of sition of upper Unit 2 and lower Unit 3 occurred to have originated at depth within the mafic vol- the entire MVG as an extraordinarily thick soil on the lower parts of an alluvial surface with canic crust and was brought to the surface dur- zone during weathering at the top of Unit 4 or increasing proximity to water and to potential ing uplift of the central peaks. In the absence of Unit 5. Unit 4 can be traced over wide areas in sources of windblown silt and clay eroded from any visible clay stratigraphy in the middle and the Mawrth Vallis region and appears to have a exposed basinal sediments. lower parts of Unit 1 and the widespread and sheet-like geometry, parallel in general to lay- In Muara crater, the thickness and proportion deep record of Fe/Mg smectite alteration across ering in other MVG rocks, suggesting that all of light-toned layers increases upward through broad areas of Noachian terrain, we would represent depositional stratigraphic units and Unit 2 and into Unit 3. The accompanying up- suggest that the Fe/Mg smectite alteration of not hydrothermal alteration zones. ward thinning of the dark layers and thickening rocks of the middle and lower parts of Unit 1 Our results suggest that the Al-phyllosilicates of the light layers suggests a transition from occurred under the influence of deep diagenetic in units 2, 3, 4, and 5 of the MVG could have mainly aeolian sand deposition (dark) to the de- or hydrothermal fluids. However, the absence formed (1) elsewhere and were transported to position of fine-grained dust and clays (light). of minerals characteristic of high-temperature their present sites of deposition as fine wind- This transition is locally marked by a zone of alteration, such as Mg-rich clays, chlorite, ser- blown grains, (2) through water runoff from stratigraphic complexity in lower Unit 3, in- pentine, prehnite, and chlorite (e.g., Ehlmann surrounding land areas, or (3) in situ through cluding channels eroded several meters into et al., 2011) implies that the unit was not altered the alteration of fine silt- to clay-sized clastic underlying strata, and widespread interlayer- at elevated temperatures (Michalski et al., 2015; grains transported into a body of water by wind, ing of dark windblown sand and thicker, finely Bishop et al., 2018). as would be the case for volcanic dust and ash. laminated dust deposits. There is a zone of Fe+2- The upper part of Unit 1, Unit 2, and lower All of these processes may have been involved rich deposits that may mark a zone of hydraulic Unit 3 show intense Fe/Mg smectite altera- in forming the final Mawrth Vallis Group sedi- concentration of more resistant, heavy mineral tion (Fig. 5C). All of these units are made up in ments, as occurs in and around modern terres- phases, such as occurs on terrestrial beaches and large part of layers inferred to represent detrital, trial oceans and lakes (e.g., Bristow and Mil- coastal zones. sand-sized, mafic volcaniclastic debris, but their liken, 2011). Most of the light-toned layers in the upper greater degree of alteration in comparison to part of Unit 3 and lower Unit 4 are tabular, with lower parts of Unit 1 suggest either that they CONCLUSIONS only a few, very thin dark layers. The abundance were affected by different processes of alteration of phyllosilicates suggested by CRISM data, the and/or that the processes were similar but more The Mawrth Vallis Group is here subdivided continuity and evenness of layering, the wide intense. Our results suggest that this intense al- into five informal stratigraphic units numbered presence of fine flat layering down to the reso teration originated through subaerial weathering 1 (base) through 5 (top). Unit 1 consists of what lution of the HiRISE images, and paucity of associated with the unconformity at the top of we infer to be interlayered and interfingering evidence for coarse sediments and features sug- Unit 1 and, subsequently, of debris derived from volcaniclastic strata and locally volcanic flows. gesting high levels of current activity, such as erosion of Unit 1 during deposition of Unit 2. The sedimentary rocks show evidence of depo- visible erosional features, all suggest deposition In Unit 2 and lower Unit 3 mafic dark-toned sition under a range of conditions that included in a quiet subaqueous setting. This could have sediments are interbedded with layers of light- aeolian, fluvial/alluvial, and quiet, possibly sub- been a large lake or arm of a sea. The interlayer- toned material. Based on spectral data, light- aqueous environments. The surface of Unit 1, ing of thick dark and light layers in Unit 5 sug- toned layers in upper Unit 3 and Unit 4 are below Unit 2, is irregular and in some places gests the possible alternation of mafic volcanic interpreted to be Al-phyllosilicate-rich layers. Unit 2 appears to be absent, suggesting that the and volcaniclastic events with continuing sub- The resolution of the CRISM images is lower top of Unit 1 is an erosional unconformity and aqueous deposition of clay-rich sediments. than that of the HiRISE images and we cannot that Unit 2 has locally accumulated as valley Our results suggest that the MVG phyllo- determine at this stage whether the thin, light- fill between highs of Unit 1 rock. This surface silicates formed in three main settings: (1) the toned layers in Unit 2 and lower Unit 3 are also may represent a major paleosol characterized by weak, widely distributed Fe/Mg smectite altera- composed of Al-phyllosilicates interbedded Fe/Mg smectite alteration that extends several tion of middle and lower Unit 1 is thought to with dark Fe/Mg-smectite layers, as we suspect, tens of meters into the top of Unit 1. reflect low-temperature diagenetic to hydro- or whether they are light toned but also show Unit 2 and lower Unit 3 form a transition thermal alteration. (2) The intense Fe/Mg smec- Fe/Mg smectite alteration. from the dark bedrock of Unit 1 to the flat-lay- tite alteration in uppermost Unit 1, Unit 2, and The light-toned rocks of upper Unit 3 and ered, fine-grained, muddy, subaqueous deposi- lower Unit 3 formed by alteration at the martian units 4 and 5 are dominated by aluminous clays tional settings represented by sedimentary rocks surface during accumulation of the sequence, and hydrated silica that must have formed be- of upper Unit 3 and Unit 4. Unit 2 appears to probably through subaerial weathering of mafic fore the fresh volcanic caprock was emplaced. be dominated by mafic volcaniclastic debris materials and lithic grains. (3) The Al-phyllo Much of the layering in these units is flat, even, derived from Unit 1 and perhaps some thin, silicates of upper Unit 3 and Unit 4 may include and tabular, consistent with the deposition of late-stage volcanic flows. Upward in Unit 2, some clay blown into a sea or lake by the wind fine sediment under subaqueous conditions. its initially massive, dark-toned character is and some material carried across fluvial systems
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(HiRISE) during MRO’s Primary Science Phase (PSP): National Aeronautics and Space Administra- Bristow, T.F., and Milliken, R.E., 2011, Terrestrial perspec- Icarus, v. 205, p. 2–37, https://doi.org/10.1016/j.icarus tion (NASA) Mars Data Analysis Program grant tive on authigenic clay mineral production in ancient .2009.04.023. no. NNX12AJ33G provided support for this project. Martian lakes: Clays and Clay Minerals, v. 59, p. 339– McKeown, N.K., Bishop, J.L., Noe Dobrea, E.Z., Ehl- D. Lowe received additional support from Stanford 358, https://doi.org/10.1346/CCMN.2011.0590401. mann, B.L., Parente, M., Mustard, J.F., Murchie, S.L., University, Stanford, California, USA. J. Bishop and Carter, J., Loizeau, D., Mangold, N., Poulet, F., and Bibring, Swayze, G.A., Bibring, J.-P., and Silver, E., 2009, J. Wray also received support from the NASA Astro- J.-P., 2015, Widespread surface weathering on early Characterization of phyllosilicates observed in the biology Institute. D. Loizeau received support from Mars: A case for a warmer and wetter climate: Icarus, central Mawrth Vallis region, Mars, their potential for- the European Research Council under the European v. 248, p. 373–382, https://doi.org/10.1016/j.icarus mational processes, and implications for past climate: Union’s Seventh Framework Program (FP7/2007– .2014.11.011. Journal of Geophysical Research. Planets, v. 114, Chamley, H., 1989, Clay Sedimentology: New York, USA, https://doi.org/10.1029/2008JE003301. 2013)/ERC grant agreement no. 280168 and under the Springer-Verlag, 623 p., https://doi.org/10.1007/978-3 McKeown, N.K., Bishop, J.L., and Silver, E.A., 2013, Vari- H2020-COMPET-2015 program (grant 687302). The -642-85916-8. ability of rock texture and morphology correlated with authors thank C. Gross for assistance with Figure 5 Dundas, C.M., Bramson, A.M., Ojha, L., Wray, J.J., Mellon, the clay-bearing units at Mawrth Vallis, Mars: Journal and M. 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Revised Manuscript Received 29 January 2019 ous mineralogy and stratigraphy at and around the Viviano-Beck, C.E., Seelos, F.P., Murchie, S.L., Kahn, Manuscript Accepted 18 March 2019 proposed Mawrth Vallis landing site: New insights into E.G., Seelos, K.D., Taylor, H.W., Taylor, K., Ehlmann, the aqueous history of the region: Mars, v. 6, p. 32–46. B.L., Wisemann, S.M., Mustard, J.F., and Morgan, Printed in the USA
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