The aim of this paper is to reconstruct the sedimentary processes and palaeomorphology of the Stimson depositional system using observations from the suite of cameras onboard Curiosity . The specific objectives are to: (i) characterize sedimentary structures, facies and architecture of the Stimson formation; (ii) describe the relationship of the Stimson to the sub‐Stimson unconformity; (iii) reconstruct the dune‐field morphology and the mechanism of deposition and sediment accumulation; (iv) determine palaeo‐sediment transport patterns; and (v) discuss the broader palaeoclimatic significance of aeolian strata preserved in Gale crater. The overarching goal is to use the stratigraphy of the Stimson formation to constrain Martian sedimentary processes and palaeoenvironmental conditions, and to place first‐order bounds on the ancient Martian climate at Gale crater.

Since 6 August 2012, the Mars Science Laboratory (MSL) science team has been using the Curiosity rover to explore the sedimentary archive preserved in the ca 3·7 ± 0·1 Ga crater, ‘Gale’. The team has documented a rich array of clastic sedimentary rocks in lower Aeolis Mons (a 5 km high mountain of stratified rock informally called Mount Sharp) and Aeolis Palus (the valley between the north wall of Gale and Aeolis Mons) (Williams et al ., 2013 ; Grotzinger et al ., 2014 , 2015 ; Vasavada et al ., 2014 ). The overall aim of the MSL mission is to determine the origin of the accessible layered sedimentary rocks within the crater and the potential for ancient habitable environments. Through detailed sedimentary, stratigraphic and geochemical investigations using the rover's instruments, a robust model for the sedimentary evolution of potentially habitable environments in Gale at a time chrono‐correlative with Earth's early Archean has been derived. Field observations enable reconstruction of a first‐order stratigraphy for these Martian rocks and interpretation of various facies that range from alluvial fan deposits, cross‐bedded fluvial sandstones, deltaic sandstones and lacustrine mudstones (Williams et al ., 2013 ; Grotzinger et al ., 2014 , 2015 ; Vasavada et al ., 2014 ; Stack et al ., 2016 ; Edgar et al ., 2017 ; Hurowitz et al ., 2017 ; Rampe et al ., 2017 ; Rice et al ., 2017 ). This sedimentary record indicates a climate with sufficient warmth and humidity to sustain river systems and long‐lived lakes in the crater. During the rover's traverse across the lacustrine Murray formation, which has represented a focus of the search for a habitable environment, the rover encountered a stratigraphic unit overlying the Murray formation mudstones. Analysis of rover‐derived images indicates that this unit unconformably overlies the Murray formation and was informally designated the Stimson formation by the MSL team. The Stimson potentially represents one of the younger stratigraphic units preserved in the crater.

However, despite the differences in boundary conditions and the additional bedforms which arise from this, Ewing et al . ( 2017 ) argue that the aeolian stratigraphic record is largely controlled by the evolution of bedform patterns, and that pattern evolution is similar on Earth and Mars, potentially suggesting that their stratigraphic records will be similar. Such hypotheses require testing against high‐quality outcrop exposures of aeolian stratigraphy. This paper addresses the following questions: what is the range of aeolian sedimentary structures preserved in Martian sedimentary rocks; what do these structures tell us about aeolian sedimentary processes on Mars; what does the sedimentary architecture of these aeolian rocks tell us about the spatial and temporal evolution of aeolian sedimentary environments and preservation of aeolian geomorphology; did the presence of water play a role in the accumulation of aeolian strata and generation of its sedimentary architecture; and what can the strata reveal about ancient sediment transport pathways?

Aeolian sedimentary systems have been documented on many planetary bodies besides Earth within the solar system: Venus (Greeley et al ., 1992 ); Mars (Bourke et al ., 2008 ); Titan (Radebaugh, 2013 ); Triton (Sagan & Chyba, 1990 ); Pluto (Telfer et al ., 2017 ); and comet 67P/Churyumov–Gerasimenko (Jia et al ., 2017 ); however, aeolian strata – the preserved expression of ancient wind‐blown sediments – have only been documented on Earth and Mars (Grotzinger et al ., 2005 ; Metz et al ., 2009 ; Hayes et al ., 2011 ; Edgar et al ., 2012 ; Milliken et al ., 2014 ). Present‐day Mars has fundamentally different boundary conditions relative to Earth, with lower atmospheric density and gravity. These differences in boundary conditions are manifest in a number of ways, notably in sediment transport mechanics and bedform evolution. Sullivan & Kok ( 2017 ) observed that even though the fluid friction threshold speed ( u *tf ) is much higher on Mars than on Earth, the impact threshold ( u *ti ) is lower and that mobile grains form saltation clusters, allowing sediment transport with relatively low flux. The low density atmosphere gives rise to an intermediate class of bedform described by Lapotre et al . ( 2016 ) as ‘wind‐drag ripples’ which are not observed on Earth. Additional complexities arise when considering present‐day Mars as an analogue for past processes on Mars: Slipski & Jakosky ( 2016 ) show that two‐thirds of the Martian atmosphere has been lost to space in the last 4·4 Ga, meaning that ancient aeolian processes would probably have occurred under different atmospheric conditions to those of modern processes.

One goal associated with most recent robotic missions to the surface of Mars is the exploration for rocks with potential to contain evidence of biosignatures. Central to this endeavour is the robust reconstruction of the palaeoenvironmental context of sedimentary rock strata investigated during rover traverses and identification of those rock successions that best record ancient habitable environments. Although aeolian strata are not considered to be the best geological materials in which to preserve records of ancient habitable environments (Summons et al ., 2011 ), understanding the processes responsible for generating extra‐terrestrial aeolian stratigraphy is important for reconstructing the context of Martian sedimentary successions and for determining the regional palaeoclimate history. The current understanding of aeolian stratigraphic records on Earth is relatively advanced, such that analogues derived from Earth provide invaluable templates to reading Martian aeolian archives. Here an ancient aeolian sedimentary rock unit in Gale crater, Mars, is analysed based on observations derived from images acquired by the mobile platform (rover), Curiosity , in concert with images obtained by a high spatial resolution ( ca 30 cm per pixel) orbiting camera system. This paper aims to evaluate how this rock unit places bounds on variations in palaeoenvironmental conditions in Gale crater.

Here, the sedimentology and stratigraphy of the Stimson formation in the area where the Stimson was first identified, between Logan Pass and the East Glacier location, is described and interpreted (Fig. 2 A). The MSL team investigated these sites using the Curiosity rover between Sols 987 and 1055. The study area encompasses an elevation gain from −4455 m at Logan's Run to −4432 m at East Glacier.

When viewed in ‘Mastcam’ mosaics, the Stimson formation consists of distinctive metre‐scale cross‐bedded sandstone, which typically has a blocky grey‐colour. The unconformity on which the Stimson formation accumulated has been mapped in detail along the traverse by Watkins et al . ( 2016 ). These authors show that the unconformity exhibits metre‐scale palaeotopography across the area traversed by Curiosity . The unconformity dips generally towards the north indicating a northward palaeotopographic slope at the time of Stimson deposition (Fig. 2 B).

Stratigraphic context of the Stimson formation. Stratigraphic column depicts key units encountered by the rover during the mission. The column shows units associated with the vertical elevation at which they were observed along the traverse and does not account for total stratigraphic thickness. The Stimson formation was the first observed unit within the Siccar Point group and unconformably overlies the Murray formation of the Mount Sharp group. For this study, the Stimson formation was studied between elevation −4455 m and −4437 m.

Detailed maps of the Marias Pass to East Glacier region of the rover traverse depicting key locations and the distribution of Stimson formation outcrops. (A) Digital elevation model (1 m resolution) overlain on a colourized High Resolution Imaging Science Experiment (HiRISE) mosaic, depicting key locations (green circles), and their relationship to the rover traverse and end‐of‐Sol positions for the rover (white triangles). Note that elevation rises from the north‐east towards the south‐west, in the direction the rover traversed. Contours (black) represent 2 m increases in vertical elevation. The inset is a context map showing the location of the study area along's traverse to Aeolis Mons. (B) Geological map showing outcrop of Stimson formation superimposed on a colourized HiRISE mosaic (from Calef & Parker,). Coloured line indicates sub‐Stimson unconformity, which is coloured by elevation (Watkins.,). The elevation of the unconformity increases to the south‐east onto the flank of Aeolis Mons Image credits: NASA/JPL/University of Arizona.

Mount Sharp group strata are truncated and overlain by a younger stratigraphic unit named the Siccar Point group (Fraeman et al ., 2016 ) which is inferred to have been deposited following an episode of erosion and exposure of the Mount Sharp group strata (Fig. 1 ). Along ca 500 m of the rover traverse described in this paper, the Murray formation strata are unconformably overlain by the Stimson formation which is placed within the Siccar Point group (Figs 2 and 3 ). This stratal unit was first identified in orbital images and termed ‘draping strata’ or ‘washboard unit’ by Milliken et al . ( 2014 ) and Rubin et al . ( 2014 ). Fraeman et al . ( 2016 ) note that the Stimson formation is characterized by high thermal inertia in orbital data and link it to several other units higher up on the slope of Aeolis Mons (High Thermal Inertia Units 1 and 2) that appear to stratigraphically truncate Mount Sharp group rocks (Fig. 1 ). However, a correlation based on ground‐based data between the Stimson and these other units characterized by high thermal is currently not possible because Curiosity has not traversed these strata.

During the August 2012 to August 2017 period, the MSL team explored a portion of the lowermost exposed ca 300 m of the stratigraphy exposed on the lower north‐facing slopes of Aeolis Mons (Fig. 1 ). The first ca 100 m of the succession (Bradbury group and lower Murray formation through to Pahrump Hills outcrop) are interpreted as interfingered fluvial, deltaic and lacustrine sedimentary rocks (conglomerates, sandstones and mudstones) with minor aeolian interbeds (Williams et al ., 2013 ; Grotzinger et al ., 2014 , 2015 ; Stack et al ., 2016 ; Edgar et al ., 2017 ; Rice et al ., 2017 ; Schieber et al ., 2017 ). The rocks ca 200 m stratigraphically higher which comprise the Murray formation (Mount Sharp group) are predominantly laminated mudstones with minor interfingered sandstones and represent accumulation of lake‐bed sediments (Grotzinger et al ., 2015 ). Both the Bradbury group and Murray formation were buried to a substantial depth (hundreds to thousands of metres) and then exhumed by aeolian deflation (Malin & Edgett, 2000 ; Day et al ., 2016 ). These rocks have been affected by burial diagenesis, including cementation, fracturing and vein mineralization (Frydenvang et al ., 2017 ; Yen et al ., 2017 ).

High Resolution Imaging Science Experiment (HiRISE) satellite mosaic showing the region of therover traverse within Gale crater, and the relation of the traverse with major Mount Sharp units mapped from orbit (Fraeman.,) on the northern flank of Aeolis Mons. The study area is focused on the area of the traverse between Marias Pass (Sol 992) to East Glacier (Sol 1154), where the Stimson formation was first observed. The inset map displays the position of the regional map within the context of Gale crater. Basemap, MSL Gale Merged Orthophoto Mosaic 25 m was generated by Calef & Parker,. Image credit: NASA/JPL/University of Arizona.

Gale crater is a 155 km diameter impact crater located ca 5° south of the Martian equator (5·4°S, 137·8°E) (Fig. 1 ). Gale occurs on the hemispherical topographic boundary between the cratered southern highlands and the much less‐cratered northern lowland plains of Mars, known as the Martian crustal dichotomy. Crater counts suggest that Gale crater formed at ca 3·8 to 3·6 Ga (Thomson et al ., 2011 ; Le Deit et al ., 2013 ) – a time that is considered to be the transition between the Noachian and Hesperian epochs (Tanaka et al ., 2014 ) – and that the crater‐filling strata were deposited over a 200 to 500 Myr period thereafter (Thomson et al ., 2011 ; Grant et al ., 2014 ; Palucis et al ., 2014 ; Grotzinger et al ., 2015 ). The central portion of the crater contains stratified rocks, interpreted to be sedimentary, that form the 5 km high mountain called Aeolis Mons (Malin & Edgett, 2000 ; Anderson & Bell, 2010 ; Thomson et al ., 2011 ).

Formally, the International Astronomical Union (IAU) determines formal nomenclature for features identified on Mars. In this paper, Earth, Mars, Gale, Aeolis Mons and Aeolis Palus are the only IAU designated features. All other locations, landmarks, landforms and targets of observation were informally named by the MSL Team during the course of surface operations. Internationally, Mars has no formally recognized stratigraphic code and all geological unit names (outcrop, member, formation or group) are thus informal and are not capitalized. The MSL Science team informally names geological units, landforms, features and rock targets of interest that are investigated using a scheme described by Vasavada et al . ( 2014 ).

Planetary Robotics 3D viewer (PRo3D) (Paar & Consortium, 2016 ; Traxler et al ., 2018 ) was used to visualize stereo‐images collected by the Navcam and Mastcam instruments. Ordered Point Clouds are generated from processed Navcam and Mastcam frames, which are viewed and analysed in PRo3D (Paar et al ., 2017 ). Using this software, dimensions and geometries of features in the landscape, such as set thickness or dip azimuths of cross‐lamina can be measured, as described by (Barnes et al ., 2015 ). A more detailed description of this software is given in Data S3 .

Attributes related to each cross‐bed dip measurement were recorded in a database. These include rover position coordinates, measurement coordinates, bearing and azimuth of measurement, interpreted facies, mosaic examined, elevation, dip orientation relative to the observer, observation notes, estimated dip direction, and compass quadrant. From these data dip directions coordinates and direction were plotted on a HiRISE base map using ESRI ArcGIS ® . Statistics related to dip‐azimuth measurements such as vector mean and vector magnitude were calculated using methods described by Lindholm ( 1987 ) to determine the mean bearing of the palaeotransport direction, and the dispersion of the dip‐azimuth values from the mean. All cited compass orientations are given in present day coordinates. The dip‐azimuth data are provided as a GIS shapefile in Data S2 .

Palaeotransport analysis was performed by visual estimates of cross‐bedding foreset dip directions and azimuths of cross‐set trough axes using stereo ranging and bearings of points selected on Navcam or Mastcam mosaics, plus the Cartesian coordinates of the point relative to the rover, allowing the point to be projected onto HiRISE images. Cross‐set dip azimuths were estimated by identifying outcrop sections approximately perpendicular to the foreset dip direction and recording the bearing of this observation relative to the rover (Fig. 4 ). Depending on the orientation of the foreset dip direction, 90° is either added (sets dip to the right of the image) or subtracted (sets dip to the left of the image) from the observation bearing to obtain the foreset dip azimuth. Additional refinements to the dip azimuth were applied where set tops or the front of the sets are visible, allowing refinement of the azimuth value depending on whether the sets are dipping left and slightly away from (small addition) or left and slightly towards (small subtraction) the viewer, or right and slightly away from (small subtraction) or right and slightly towards (small addition). Where trough axis orientations were estimated, outcrop sections perpendicular to the trough dips were sought, and the bearing of the observation relative to the rover was recorded. Again, additional refinements to the azimuth value were estimated depending on whether troughs dip towards and left (add) or towards and right (subtract). Where sets in troughs dip away from the rover, the orientation of the axis was difficult to estimate, unless laminations were visible in the upper surface of the outcrop.

The MAHLI images of targets of interest were used to determine the texture and grain size of facies within the Stimson formation. Working distance and pixel scale for images can be calculated using the focus motor position value (Edgett et al ., 2015 ), allowing measurement of features in the sensor plane. Once working distance and pixel scale were calculated, measurement of grain size was carried out using ESRI ArcGIS ® software. To negate grain selection bias, a fishnet grid was created and superimposed on the image to randomize grain sampling. The long axis of the grain beneath a grid intersection was measured in pixels; this measurement was then multiplied by the calculated pixel scale to convert the grain length in micrometres. Grain length values were recorded in a shapefile and were exported to Microsoft Excel ® for analysis using gradistat software (Blott & Pye, 2001 ). Grain roundness was determined visually using the scale of Powers ( 1953 ). Additional details of these methods are given in Data S3 .

Mastcam images of significant outcrops were processed to remove the effects of surface radiation and lens distortion, merged into a single mosaic, and adjusted to preserve geometric relationships within the projection. Stereopair mosaics taken by Navigation cameras are used to triangulate distances and generate a 3D mesh of the environment surrounding the rover (Alexander et al ., 2006 ). Points and distances between points observed in Navcam and Mastcam mosaics were measured in Navcam stereo‐mesh to determine the dimensions of geological features, such as bedding thicknesses, lamination lengths and lateral extents of surfaces or architectural elements. The spatial resolution of the mesh permitted accurate measurements to the decimetre‐scale, but the resolution was insufficient to measure at the millimetre‐scale. Decimetre‐scale cross‐sets in close proximity to the rover were measurable; however, the thicknesses of laminations or grains were not measureable. Mosaics were interpreted using vector graphic software to highlight facies, geometric relationships between depositional surfaces, unconformities and locations of measurements, such as bed thicknesses, lengths and the coordinates of dip‐azimuth measurements.

The most complete suite of images that cover the terrain investigated using Curiosity is provided by 12 monochrome engineering cameras that are used principally for navigation, hazard avoidance and science target planning (Maki et al ., 2012 ). In this study, data from the Navcams were used primarily, which are mounted on the pan‐tilt mast adjacent to the Mastcam cameras. Navcams offer a field of view of 45° and a pixel scale of 0·82 mrad/pixel (Maki et al ., 2012 ). Mosaics produced by these images are used primarily for: navigation; production of three‐dimensional terrain models to plan rover drives and robotic arm deployment, and for targeting observations using Mastcam and MAHLI cameras. The Navcams are capable of 360° azimuth panoramic and stereopair imaging (360° stereo mosaics composed of 45 frames are approximately 26 Mbits) out to 100 m from the rover. These mosaics and three‐dimensional meshes are used for supplementing Mastcam observations where coverage is sparse and for taking scientific measurements of geological features.

The MAHLI is a two‐megapixel colour camera with a focusable macrolens attached to the instrument turret on Curiosity 's robotic arm. At its highest spatial resolution, MAHLI can acquire images of up to 14 μm/pixel at a 21 mm working distance (Edgett et al ., 2012 ). Typical imaging on Mars is performed in suites of nested images of 16 to 21 μm/pixel, 31 μm/pixel and 100 μm/pixel (Yingst et al ., 2016 ). The primary science application of this instrument is for observation of sediments and sedimentary rock textures, and hand‐lens scale sedimentary structures. Image coverage obtained by MAHLI is limited due to planning constraints associated with deployment of the arm, a process which typically takes 2 Sols to complete. Other constraints of deploying MAHLI are described by Yingst et al . ( 2016 ).

The Mastcam instrument suite consists of two focusable cameras with a charge‐coupled device capable of acquiring full RGB colour images. The left Mastcam (Mastcam Left, or M34) has a fixed focal length of 34 mm and a 15° field of view; the right Mastcam (Mastcam Right, or M100) has a fixed focal length of 100 mm and field of view of 5° (Bell et al ., 2017 ; Malin et al ., 2017 ). The Mastcams are mounted on a pan‐tilt mast ca 1·9 m above the surface (Maki et al ., 2012 ; Warner et al ., 2016 ). These cameras are the primary scientific cameras aboard Curiosity , and are typically used for targeted imaging of outcrops for identifying sedimentary structures, textures, stratal geometries and depositional architecture. Targets are identified during the rover traverse using the Navcams (described below). The spatial resolution provided by the M34 camera is 450 μm/pixel at 2 m distance from the target to 22 cm/pixel at 1 km distance, whereas the M100 camera provides a resolution of 150 μm/pixel at 2 m and 7·4 cm/pixel at 1 km from the target (Malin et al ., 2017 ). Providing sufficient grain–grain colour contrast, the M100 is capable of resolving medium sand grains (250 to 500 μm) at a distance of ca 3·5 m from the rover. Where grains share similar colours or are dust covered, the ability to resolve grains is diminished. Coverage obtained by the Mastcams limited by data volumes that can be stored onboard (8 Gb per camera) (Bell et al ., 2017 ) and bandwidth limitations associated with interplanetary communications. A full 360° azimuth × 80° elevation mosaic captured by the M100 camera is ca 6·6 Gb of data, which is 14 times larger than the typical daily data downlink capabilities.

Data generated during this study is provided online as an additional resource on the Sedimentology website. A slide pack containing figures, supplementary data and supporting images is provided as a teaching resource (Data S0). A list of all Navcam, Mastcam and MAHLI data can be found (Data S1 ), along with GIS shapefiles (Data S2 ), detailed methodologies (Data S3 ), grain‐size data (Data S4 ), lamination thickness measurements (Data S5 ), maps depicting the relative positions of mosaics used in this study (Data S6 ), a list of supporting images and URLs (Data S7 ), measurements made using Pro3D software (Data S8 ) and a high resolution map of visually estimated palaeocurrents (Data S9 ).

For this study, images of Martian sedimentary rock outcrops were collected using three separate instrument suites on the MSL rover Curiosity , the Mast Cameras (Mastcams), Mars Hand Lens Imager (MAHLI) and Navigation cameras (Navcams). In addition, satellite images (with resolution down to 25 cm/pixel) obtained by the High Resolution Imaging Science Experiment (HiRISE) aboard the Mars Reconnaissance Orbiter (McEwen et al ., 2007 ), plus digital elevation model products derived from the HiRISE stereo pair and Mars Orbiter Laser Altimeter, were used as a base map for photogeological mapping, localization of the rover, and for strategic planning of the rover drives.

Fence diagram depicting the stratigraphic relationship between the Stimson formation and the underlying Murray formation. The vertical thickness of the Stimson at each key location is depicted by sedimentary logs that are positioned according to distance along traverse and vertical elevation. Solid red lines indicate direct observations of the sub‐Stimson unconformity; dashed lines indicate locations where the unconformity is inferred at depth. The fence diagram illustrates the likely onlap of the Stimson formation onto palaeotopography formed at the erosion surface cut into the Murray formation.

The Stimson formation is designated as a distinct sandstone unit that unconformably overlies the mudstone‐dominated Murray formation. It forms a series of isolated erosion‐resistant outcrops that are distributed along the break in slope between the Aeolis Palus plains and the foothills of Aeolis Mons, and are mostly confined between the −4460 m and −4290 m elevation relative to the Martian datum (Figs 1 , 2 and 3 ). In areas where outcrops were encountered along the rover traverse, the upper boundary of the Stimson is a modern erosion surface and, consequently, the true stratigraphic thickness of the Stimson is currently unknown. Locally, some mesas show up to 4 m thickness of Stimson sandstone preserved above the unconformity. Geological mapping of the Stimson formation on HiRISE satellite images by the Gale Mapping Work Group (Calef et al ., 2013 ) shows the Stimson outcrops to cover a contiguous area of ca 1·4 km 2 , with isolated outcrops distributed across an area of ca 17 km 2 . The sub‐Stimson unconformity was mapped along the traverse using rover‐derived data, augmented by analysis of HiRISE images to extend mapping coverage beyond the areas traversed by the rover (Watkins et al ., 2016 ). Watkins et al . ( 2016 ) show that the unconformity forms an undulating palaeo‐surface with relief of up to 20 m over a 275 m distance at Logan Pass. Regionally the unconformity shows a ca 140 m rise in elevation traced southward towards Aeolis Mons across a 2 km long north–south oriented profile, with the unconformity dipping north towards the crater rim (Fig. 3 B). Traced southward, Stimson formation sandstones overlie Murray formation strata that occur at progressively higher elevations. Since within the Mount Sharp group strata elevation is a proxy for stratigraphic height, this means that Stimson strata overlie progressively younger Murray formation strata traced southward. Between the outcrops at Logan's Run (−4454 m) and East Glacier (−4429 m), the unconformity rises in elevation by ca 25 m (Figs 4 and 5 ). The absence of distinct marker beds or surfaces within the Stimson renders correlation of stratal packages within the formation impossible between adjacent outcrops. Set boundaries, although common, are commonly truncated by overlying bounding surfaces, or obscured by regolith, preventing correlation between outcrops. Superbounding surfaces (as defined by Havholm & Kocurek, 1994 ) that could potentially be used for correlation purposes have not been identified. As a result, for this study, vertical elevation is used for relative positioning of the outcrops, with the sub‐Stimson unconformity acting as an additional datum.

Reactivation surfaces, referred to as third‐order surfaces (Brookfield, 1977 ; Schenk et al ., 1993 ), form by fluctuations in air flow. These surfaces have been demonstrated to arise by changes to the height and shape of bedforms, or speed and direction of dune migration, where the bedform generating the cross‐strata could not fully respond (Brookfield, 1977 ; Kocurek, 1991 ). Causes of these changes are interpreted to arise from wind reversal or seasonal variations in wind direction and velocity (Mountney, 2006 ), or by modification of the local wind field caused by migration of adjacent dunes (Brookfield, 1977 ).

Within cross‐bed sets, arcuate and inclined bounding surfaces, originally described as intraset surfaces, can be observed. These commonly truncate cross‐laminations obliquely, are discordant to underlying cross‐laminations and are overlain by cross‐laminations concordant to the bounding surface. Within the cross‐set, these bounding surfaces downlap asymptotically to the lower set‐bounding surface, and are truncated by the upper set‐bounding surface.

Superposition surfaces, described as second‐order surfaces, represent the migration of smaller bedforms across a larger main dune, giving rise to the observed compound cross‐strata (Brookfield, 1977 ; Kocurek, 1991 ). Compound dunes, with small dunes migrating across larger dunes are common in Earth‐based aeolian systems (Breed & Grow, 1979 ; Lancaster, 1988 ) and frequently have crestlines oblique to that of the main dune crestline on which they are superimposed. As these superimposed bedforms migrate across the lee slope of the main dune, scour pits preceding their advance will scour the main dune lee slope generating the superposition surface, with the cross‐laminations within the decimetre‐scale cross‐sets representing the lee slope advance on the superimposed bedform. The associated first‐order bounding surfaces represent scouring in advance of the main dune.

These surfaces are identified within cosets containing decimetre‐scale cross‐sets, where the coset bounding surfaces are defined as dune migration (first‐order) surfaces. Decimetre‐scale cross‐sets within these cosets will share a common foreset dip azimuth, whereas adjacent cosets typically demonstrate a different dip azimuth common to the sets within it. These superposition surfaces can be traced laterally for several metres, but do not have the same lateral extent as the dune bounding surfaces. These superposition surfaces are frequently observed to have a scalloped, or arcuate geometry, with a convex down geometry. Sets bound by superposition surfaces are typically on the order of a few tens of centimetres thick, whereas the cosets containing them have thicknesses similar to that of a metre‐scale cross‐set.

The geometry of these bounding surfaces and their relation with laminasets leads to the interpretation that they represent the erosional passage of a scour pit preceding the advance of an aeolian dune. These surfaces are similar to first‐order bounding surfaces (Brookfield, 1977 ; Kocurek, 1981b ) or interdune surfaces (Mountney & Thompson, 2002 ). The migration vector of the depression and ensuing angle of climb of the dune is controlled by the rate of sediment accumulation, with net sediment accumulation causing a positive angle of climb and sediment preservation, and net sediment entrainment causing a negative angle of dune climb and scouring of previously accumulated sediment. The angle of climb, while not measured here, is typically measured as a few tenths of a degree to a couple of degrees in ancient aeolian systems on Earth (Rubin & Hunter, 1982 ; Mountney et al ., 1999 ; Mountney, 2006 ; Rodríguez‐López et al ., 2014 ). Where bounding surfaces downcut, this potentially represents autocyclic processes within the dune field, where fluctuating wind strength and direction causes the dune to change shape, height or depth of scour, or an allogenic control where a deficit in sediment supply results in re‐entrainment of sediment into the flow (Paola & Borgman, 1991 ; Ganti et al ., 2013 ; Brothers et al ., 2017 ). Where the dune‐bounding surface climbs, this may represent fluctuations in wind strength favourable to sediment deposition or a net increase in sediment flux to allow sediment accumulation.

Cross‐bed sets are bounded by truncation surfaces which are ubiquitous within the Stimson formation and are an easily distinguishable feature in outcrops across the traverse. An example of such a bounding surface is illustrated in Fig. 7 which shows cross‐laminations terminating onto the lower bounding surface, and the truncation of cross‐laminations in the upper part of the set by an upper bounding surface. Throughout the Stimson, these surfaces are expressed at a gross scale as largely sub‐horizontal, sub‐parallel surfaces that can be traced laterally across outcrops for distances of up to 30 m in the best exposed outcrops. Locally, they are observed to undulate by a few tens of centimetres along their length, causing variations in set thickness where they downcut into the underlying set. In some cases, these surfaces downcut sufficiently to truncate underlying sets, resulting in the pinch‐out of cross‐bed sets.

The mudstone clasts observed in this facies are interpreted to have been derived from the Murray formation, because of their similar visual expression. They were probably eroded from the underlying Murray formation mudstones and incorporated into the base of the Stimson formation by aeolian processes, and the lens probably formed contemporaneously with erosion occurring on the unconformity. Clasts are interpreted to have formed by exfoliation and abrasion in an interdune area and were incorporated into the base of a migrating dune. This facies is restricted to the immediate vicinity of the unconformity, because after a period of dune accumulation, the unconformity – the source of the clasts – became buried, preventing further supply of mudstone clasts. However, the absence of additional evidence for deposits clearly formed by sub‐aqueous processes favours aeolian incorporation.

This facies consists of medium to coarse‐grained sandstone containing matrix‐supported clasts that are up to 3·5 mm in diameter. Where visible, sand grains are polymict, well‐rounded and commonly show a polished surface, although some examples show surface pitting. Intraclasts are rounded, oblate to sub‐spherical in shape, and exhibit a similar colour to the underlying Murray formation. Some examples of these clasts exhibit a weathering‐inward profile, where the centre of the clast weathered inward relative to the rim of the clast, suggesting that the clast was made of a relatively soft material. The sandstone is crudely planar laminated, with laminations being 3 to 5 mm thick. The facies only occurs as a 0·5 m wide and 30 mm thick lens in the lowermost few centimetres of Stimson strata in the Missoula area of Marias Pass (Newsom et al ., 2016 ). The lens onlaps and pinches out onto an inclined section of the unconformity, which shows local relief of 20 cm.

Facies 3 is interpreted to be the product of post‐depositional diagenesis, in which preferential cementation of the sandstone generated concretions which, when exposed at the surface, better resist erosion than the surrounding sandstone. Evidence that these protrusive features represent concretions rather than nodules comes from the identification of grains and laminations within them and the absence of deformation of laminations and beds surrounding the concretion, which would be typical of incipient nodule growth. On Earth, preferential cementation of sandstone concretions occurs by pervasive growth of a crystal through the pore space surrounding sand grains, from a single nucleating centre point (Clarke & Stoker, 2011 ), where the cement will probably grow by precipitation of elements from chemically saturated pore fluids. The exact mineralogy of the cement generating these concretions is presently unknown; however Alpha Particle X‐ray Spectrometer (APXS) analysis of concretions indicates that the bulk chemistry is similar to the bulk of the Stimson formation (Siebach et al ., 2017 ). The dominant control on the density of concretions formed at any location is uncertain and probably involved a number of factors, including the presence of minerals which can act as a nucleus for cement precipitation, pore water chemistry providing ions to sustain concretion growth, and a source of ions, potentially derived from the host rock, ground water movement or infiltrating meteoric waters (Clarke & Stoker, 2011 ; Arribas et al ., 2012 ).

Facies 3 – Concretion‐rich sandstone facies observed in Marias Pass. Amalgamated concretions are observed to overprint cross‐bedded sandstones in the upper part of the mosaic, with the lower part of the outcrop appearing to be largely devoid of concretions. The origin of this sharp boundary is uncertain, but may represent a difference in permeability. Mastcam‐100 mosaic mcam04719 was taken on Sol 1073 at the end‐of‐Sol 1067 drive position. Image credit: NASA/JPL‐Caltech/MSSS.

Facies 3 comprises sandstones of the metre‐scale and decimetre‐scale cross‐bedded facies but with abundant spheroidal concretions (Fig. 9 ). From a distance, these protrusions give the Stimson a characteristic ‘knobbly’ surface texture. Although not ubiquitous across the Stimson, Facies 3 is typically distributed in Stimson sandstones immediately overlying the basal unconformity, and extends upward from the unconformity into the Stimson by up to a few metres. Concretions are typically between 20 mm and 40 mm in diameter, and have sub‐spherical to oblate shapes. Sand‐sized grains are observed within concretions, and laminations are seen to pass from the surrounding sandstone through them with no evidence of deformation of laminae. Concretions occur either sporadically distributed through the facies, or densely packed with concretions becoming locally amalgamated and overprinting primary sedimentary structures.

The thin cross‐bed sets that characterize this facies are interpreted to result from: (i) a small preserved section of a large‐scale aeolian bedform, for example the edge of a trough; (ii) the preserved expression of small sinuous‐crested aeolian dunes migrating in the lee of larger bedforms, where the local wind‐field is influenced by larger bedforms (Rubin, 1987 ; Langford & Chan, 1993 ); or (iii) the preserved expression of large ripples or wind‐drag ripples (Lapotre et al ., 2016 , 2017 ; Ewing et al ., 2017 ). The latter represent an intermediate class of bedform identified only on Mars that fall between impact ripples and dunes in size. These may be preserved either by ripple climbing processes or may be preserved when they are buried by larger migrating dune‐scale bedforms.

This facies comprises cross‐bedded sandstones that consist of sets typically between 0·05 m and 0·2 m thick, but occasionally up to 0·4 m thick, with cosets that are typically less than 1 m thick (Fig. 8 ). Individual sets comprise cross‐laminations that are similar in appearance to those in metre‐scale cross‐beds. These cross‐laminations can be traced laterally within the set for a few tens of centimetres, and typically no more than 0·6 m. Cross‐laminations terminate at the base of the set with either an asymptotic, or a tangential profile. Lower set boundaries associated with this facies can be traced laterally for no more than 1 to 2 m before they are truncated by a successive set, or are obscured by regolith.

Metre‐scale planar cross‐bedding is interpreted to represent the preserved lower lee slopes of migrating aeolian dunes (the dune plinth). Where sediment supply into the depositional environment is in surplus, the spacing between dunes migrating across a depositional surface ( sensu Kocurek & Havholm, 1994 ) decreases until the interdune area is limited to scour pits between dunes. As scour pits migrate across the stoss slope of the down wind dune, the stoss slope and upper part of the lee slope are eroded, with the remaining lower portion of the lee slope being subsequently buried by advance of the following dune. The erosion surface formed by migration of the scour pit is referred to as a set bounding surface (Brookfield, 1977 ; Rubin & Hunter, 1982 ; Kocurek, 1991 ). Where these sets are associated with horizontal and laterally extensive scour surfaces, they most probably represent the migration of simple main dunes within the dune field. The cross‐lamination dip direction represents the azimuth of net sediment transport at the time of deposition. Troughs present in sections perpendicular to sediment transport direction are diagnostic of the migration of 3D or sinuous‐crested bedforms.

Within cross‐sets, there is no clear evidence for the preservation of grainflow strata, wedge‐shaped sediment packages that thin and terminate down‐dip towards the set base and are formed by lee‐slope avalanching. Where viewed in planform section, laminasets are parallel, and largely straight with uniform thicknesses over distances of up to 3 m. Beyond 3 m distance, lengths are difficult to trace due to the presence of regolith, fractures or cross‐cutting bounding surfaces. Where laminasets are exposed in plan‐view, they can appear straight or slightly curved with a concave curvature in the direction of lamination dip. Where set‐bounding surfaces truncate underlying sets, the underlying cross‐laminations show a toplap relationship with the bounding surface, and exhibit an angular discordance typically between 15° and 20° (Fig. 7 ). Where viewed in sections perpendicular to sediment transport direction, sets of cross‐strata form scallop‐shape or cylindrical‐scour troughs (similar to those described by Collinson et al ., 2006 ), with observed widths in excess of 6 m in the plateau between Marias Pass and Williams outcrop.

Within the set, laminations are planar in the upper part but when traced to the base of the set they show an asymptotic relationship with the lower bounding surface. Analysis of dips from a digital outcrop model (DOM) indicate a maximum average dip of 22° in Set 2, with a maximum recorded dip of 27° and ca 15° in the centre of the set (Fig. 7 ).

Facies 1 – Metre‐scale planar and trough cross‐bedding, recorded at the Williams outcrop (M100 Mastcam mosaic acquired on Sol 1092, mcam04793, at the end‐of‐Sol 1087 rover position). (A) Uninterpreted example of a single cross‐bed set (Coset 2, Figure 14 ). (B) Interpretation of same cross‐bed set, showing geometries of key features of laminations and bounding surfaces. (C) Close up of cross‐laminations within set observed in (A) and (B). Laminations are typically 2 to 4 mm in thickness, and show uniform thickness along their length. (D) Close‐up of bounding surface truncating cross‐laminations at the Williams outcrop. Image credit: NASA/JPL‐Caltech/MSSS.

A close‐range mosaic illustrates the main features of cross‐stratification within the Stimson (Fig. 7 ). The set is composed of repetitively arranged, sub‐parallel, relatively uniform thickness millimetre‐scale laminations that can be traced across the entire cross‐set. The thickness of laminations ranges from 2·8 to 8·5 mm, with an average of 4·1 mm ± 1·9 mm (measurements are provided in Data S5 ). These laminasets are characterized by nearly uniform thickness, exhibiting only sub‐millimetre variations in thickness along much of their length (Fig. 7 C and D). It was not possible to determine grain‐size variations within laminae or laminasets; at the closest proximity to the Williams outcrop (6·5 m), resolution was 0·5 mm/pixel, which would only allow identification of grains >1·0 to 1·5 mm in diameter. Despite these limitations, a millimetre‐scale rough surface texture was observed on the surfaces of laminations, even though individual grains cannot be identified. Cross‐lamination lengths range from tens of centimetres to 2 m.

The Stimson formation is dominated by metre‐scale cross‐bedded sandstone facies. This facies consists predominantly of medium‐grained sandstones; however, in some locations the sandstones contain up to 5% coarse to very coarse sand grains with diameters up to 2 mm. At a distance, this facies is composed of cross‐stratified sandstones with cross‐sets arranged as repetitively stacked sets typically ranging between 0·3 m and 1·0 m in thickness. Stacked sets which share a common cross‐lamination dip azimuth are defined as cosets and have thicknesses ranging from 0·5 m thick to greater than 1·5 m in thickness. The sets are bounded by sub‐horizontal bounding surfaces, which are described below. Cross‐strata within sets comprise inclined parallel laminations, and typically show an arcuate, concave‐up profile.

The bimodal grain‐size distribution observed is characteristic of wind‐blown sediments (Sharp, 1963 ; Greeley & Iversen, 1985 ; Jerolmack et al ., 2006 ), in which saltating particles can facilitate transport of grains too large to saltate by surface creep. The high degree of grain rounding observed in the MAHLI targets is characteristic of grains transported by aeolian processes (Folk, 1980 ; Garzanti et al ., 2015 ; Garzanti, 2017 ). Such a high degree of rounding is typically not achieved by sub‐aqueous transport, which is relatively ineffective at rounding sand grains (Pettijohn, 1957 ).

The Ledger target (Fig. 6 D) is an unaltered section of the Stimson formation, which was brushed by the Dust Removal Tool (DRT) before MAHLI imaging was undertaken. The target is an approximately bedding‐parallel section, which cuts across four cross‐lamination surfaces. Grain‐size measurement of 259 grains confirmed a normal grain‐size distribution, with a dominant peak at 250 to 355 μm ( ϕ 1·5 to ϕ 2·0) (medium–fine grained) and a coarse skew. The mean grain size for this sample was 398 μm, with a minimum grain size of 132 μm and maximum recorded as 1604 μm (see Data S4 C). Cementation of the sandstone at Ledger appears to be different from that at the other two targets, in that the grains appear less pronounced. The larger very coarse grains are highly spherical and very well‐rounded, and have extremely smooth surfaces, with little or no pitting observed. Within the very coarse fraction of grains, several examples of tan, light grey and dark grey‐coloured grains were observed, suggesting variable grain mineralogy.

The Ivanhoe target (Fig. 6 B) is on a broken rock surface with no bedding surface visible. The Stimson here has undergone post‐depositional alteration (Yen et al ., 2017 ) as indicated by lighter toned colour by comparison to the Lebo target. Ivanhoe is also characterized by a bimodal grain distribution. Abundant coarse grains were observed and these are well‐rounded and spherical in shape. On closer inspection, these coarse grains exhibit pronounced surface pitting compared to grains in Lebo. Grain‐size measurements of 253 randomly selected grains indicates two peaks centred on the 180 to 250 μm ( ϕ 2·5 to ϕ 2·0) and >710 to 1000 μm ( ϕ 0·5 to ϕ 0) distributions (see Data S4 C). The mean grain size for this sample is 406 μm, with a minimum of 85 μm and maximum of 1297 μm.

The Lebo target (Fig. 6 C) is on a bedding‐parallel face that is eroded through a number of lamination surfaces in a cross‐bed foreset. Lebo is characterized by abundant randomly distributed very coarse‐grained sand grains, which weather proud from the matrix. Grains in Lebo show a bimodal grain‐size distribution (see Data S4 C). Coarser grains are generally well‐rounded, with some highly spherical grains observed. On closer inspection, some of these coarse sand grains exhibit surface pitting, or a surface rugosity. The matrix is characterized by medium‐grained sand infilling the space between the larger grains. Grain‐size analysis of 236 grains yields a bimodal grain‐size distribution characterized by two peaks, one at the 355 to 500 μm ( ϕ 1·0 to ϕ 1·5) and the second at the 710 to 1400 μm ( ϕ 0·5 to ϕ −0·5) range. The minimum grain size was 182 μm, a maximum of 2596 μm, and an average size of 691 μm was recorded. The colour of the coarse grains is varied, with white, tan and grey‐coloured particles, suggesting varied mineralogy of the constituent grains.

Mars Hand Lens Imager images taken at the base of Williams outcrop on Sol 1092. (A) Overview Navcam mosaic depicting location of MAHLI targets (Sol 1087 end‐of‐drive position). Field of view is 2·5 m. (B) Ivanhoe Target, 2 cm stand‐off. Target was located in alteration halo. Grain‐surface texture and bimodal sorting can be observed. Note pitting on grain surfaces. (C) Lebo Target, 5 cm stand‐off: MAHLI image recorded the top surface of a lamination, characterized by bimodally sorted rounded grains. Surface exhibits a light dust covering. (D) Ledger target, 2 cm stand‐off: MAHLI imaged area cleared by dust removal tool, revealing bimodally‐sorted, well‐rounded, medium‐grained sandstone. Grain surfaces are smooth and have a polished appearance. The MAHLI images are available on the NASA PDS website. Image credit: NASA/JPL‐Caltech/MSSS.

At the Williams outcrop (Fig. 6 A), three targets, all within metre‐scale cross‐stratified sandstones (Facies 1) were imaged using MAHLI. These targets, Ivanhoe (Fig. 6 B), Lebo (Fig. 6 C) and Ledger (Fig. 6 D), were documented on Sol 1092. These targets were associated with an alteration halo indicated by lightened tone of bedrock (Frydenvang et al ., 2017 ; Yen et al ., 2017 ). The Ivanhoe target was located in an area of intense alteration, Lebo is located 0·65 m away from Ivanhoe where alteration was less intense and Ledger was located largely outside of the area of alteration, 1·2 m from Lebo. Grain‐size measurements are provided in Data S4 A, along with a histogram of all grain‐size measurements in Data S4 B.

The MAHLI instrument was used to investigate the sedimentary texture, which provides insight into sediment transport processes responsible for depositing the Stimson formation. Owing to the complexity of mission planning, positioning of the rover in a suitable and safe location to deploy the arm and outcrop quality, the sedimentary texture of the Stimson was best resolved at the Williams outcrop (Fig. 2 ). Grain‐size histograms and measurements of grain size can be found in Data S4 .

In order to establish the sedimentary processes by which the Stimson formation was deposited and to reconstruct its depositional setting, the sedimentology of distinctive lithofacies within the formation and stratal bounding surfaces were studied and are described in this section.

Secondary wind currents are the inferred cause of strata with dip directions deviating significantly from the general transport direction. Spurs and defects are crestline elements that deviate from dominant crestline trend and may be nearly orthogonal to the dominant crestline trend (Kocurek et al ., 2010 ; Swanson et al ., 2018 ). Migration of these elements can produce cross‐stratification that deviates from the overall trend (Brothers et al ., 2017 ). The presence of pre‐existing large bedforms can modify the local wind field, creating eddy currents that generate subordinate bedforms travelling obliquely to the general transport direction (Rubin & Hunter, 1982 ; Kocurek, 1991 ; Rubin & Carter, 2006 ). Cross‐sets in palaeotopographic lows on the sub‐Stimson unconformity were observed with cross‐lamination dip azimuths oblique to the general transport direction. This divergence is inferred to arise as a result of interaction between the wind and the palaeotopography, which modifies the local wind field, and the resulting bedform migration direction.

Palaeotransport analysis based on measurements of dip azimuths from sets of cross‐strata using a visual estimation method, together with analysis of cross‐strata in selected stereo‐reconstructed outcrops indicate that the general direction of dune migration was towards the north‐east. The average dip direction of cross‐strata however does not necessarily reflect the average wind direction. Wind direction can only be loosely inferred, because bimodal or polymodal wind regimes can generate bedforms with crests oblique to both the prevailing wind direction, and the resultant transport direction (Rubin & Hunter, 1987 ; Rubin & Ikeda, 1990 ; Eastwood et al ., 2012 ). Thus dunes could be migrating obliquely in response to two different wind directions. Although Curiosity's traverse provided only a limited view of the entire Stimson sand body, the presence of more than one wind acting on the dunes is supported by the abundance of wind‐ripple stratification and reactivation surfaces. These occur where the angle between the instantaneous wind direction and the local crestline orientation is oblique to longitudinal (Sweet & Kocurek, 1990 ; Eastwood et al ., 2012 ). Additionally, the absence of grainflow stratification in Stimson cross‐sets indicates that, at least in the areas of the Stimson surveyed, crestline transverse flows were not frequent. Although the wind regime at the time of Stimson deposition remains difficult to reconstruct, it can be inferred that the wind regime drove dune migration, and hence net sediment transport, from the south‐west to the north‐east. The absence of any evidence for large‐scale horizontal and vertical deformation of the Stimson formation outcrops (except for fractures) indicates that the dip directions have not been altered by post‐depositional displacement of the outcrops.

The outcrop at East Glacier shows broad variation in cross‐bed dip directions within the study area. In the lowermost parts of the succession, both within the Stimson outlier and the lower parts of the main outcrop, foreset dips are aligned along palaeotopographic lows at the sub‐Stimson unconformity (Figs 20 C and 21 C) and show dips towards the south‐east. Slightly higher up in the stratigraphy exposed at the main outcrop (Fig. 20 B), several cross‐sets show dip azimuths towards the south‐west – the opposite direction to the north‐east dominant transport direction.

Maps showing cross‐strata dip directions as determined by visual inspection of cross‐stratified units in Navcam and Mastcam mosaics. Foreset dip azimuths indicate general sediment transport direction within the Stimson formation. (A) Palaeotransport directions observed in the Marias Pass area. (B) Palaeotransport directions observed at Williams and Bridger Basin. (C) Palaeotransport directions observed at East Glacier. (D) Rose diagram depicting dominant transport direction within the Stimson formation, across the study area ( n = 139). The vector mean is towards 052°, with a vector magnitude of 0·65. Image credit: NASA/JPL/University of Arizona.

Visual estimation of 117 cross‐strata dip azimuths across the study area showed a dominantly unimodal dip azimuth, and hence general dune migration direction, towards the north‐east in present‐day geographic coordinates (Fig. 21 ). Trigonometric calculation of vector mean and vector magnitude from these results indicates a vector mean of 052°, with a vector magnitude of 0·65 (using the method described by Lindholm, 1987 ). Although the dominant transport direction was towards the north‐east, cross‐sets commonly show deviation from this general transport direction at a local level. At Marias Pass, cross‐set dip azimuths at the base of Apikuni Mountain (Fig. 21 A) show deviation of up to 70° from the dominant transport direction, with small decimetre‐scale sets dipping towards the south‐east. On the plateau between Marias Pass and Williams outcrop, cross‐bed dip azimuths show a general trend towards the north‐east, although a broad range of dip azimuths with a spread of almost 180° were observed (Fig. 21 B), with three examples of decimetre‐scale cross‐sets showing dip azimuths either to the south‐east or north‐west. If these sets are excluded from the dataset, the range of dip azimuths are spread across a 70° arc. Visual estimation of palaeocurrents at Williams is in agreement with DOM‐derived data, with the general transport direction towards the north‐east (stereo derived vector mean: 067°; Fig. 16 ). Observations at the Bridger Basin outcrop were limited due to the distance from which the outcrop was observed; however measurements of sets show a north‐easterly dip azimuth, with a spread of ca 20° (Fig. 21 C).

Aeolian dune cross‐strata provide the most direct record of patterns of dune migration and the palaeowind regime. The orientation of cross‐strata as measured by their dip and plunge‐azimuth provide information about the local and regional pattern of dune migration direction, and have been used to infer the wind regime at time of deposition. The average dip direction of cross‐strata within a set indicates the overall dune migration direction. Palaeotransport direction was determined using a visual estimation method on outcrops of the Stimson formation across the study area (Fig. 4 ), and in more detail at specific outcrops where high‐quality stereo image data permitted construction of DOMs from which cross‐bed dip azimuths could be accurately measured. The visually estimated palaeocurrent measurements are provided as a shapefile and as a high‐resolution map in Data S2 and S9 .

Observations show that palaeotopography exerted a control on the transport direction of the aeolian dunes recorded by the Stimson formation. Below the uppermost level of the unconformity, cross‐sets are aligned broadly parallel to the palaeo‐ridges at the unconformity – towards the south‐east – whereas above this level cross‐sets are aligned with the regional transport direction – towards the north‐east. This suggests that palaeotopography influenced local transport direction by modification of the wind regime. Once sufficient Stimson accumulated to bury the palaeotopography, it no longer modified the local wind regime and dune migration reverted to that of the dominant regional transport trend.

Onlap relationships along the undulating sub‐Stimson unconformity at this location indicate that the unconformity represents a relict palaeotopographic surface. The origin of the palaeo‐ridges at the unconformity is uncertain, but could have been a consequence of aeolian deflation of Murray bedrock before deposition of the Stimson. The absence of gravel lags or fluvial deposits above the unconformity suggests that fluvial processes did not create this palaeotopography. The Stimson formation at this location shows comparable set sizes and set characteristics (cross‐lamination thickness, uniformity and geometry) to the outcrops at Williams, Bridger Basin and McLeod, suggesting deposition by migrating dunes. The concretions that are concentrated near the base of the outcrop, particularly in palaeotopographic lows, are interpreted to be post‐depositional diagenetic features. The mechanism to explain their origin and distribution is uncertain; one possibility is that their formation represents the presence of a perched aquifer at the base of the Stimson, or an increased concentration of minerals which acted as a nucleus for concretion growth (Berner, 1968 ).

Above the unconformity, the Stimson at this location comprises repeated sets of sub‐metre‐scale cross‐bedded sandstones, with eight sets observed at the tallest part of the outcrop (Fig. 20 B). Individual sets are broadly uniform in thickness along their length, however Sets 5, 6 and 7 pinch out towards the north‐east. The thickest set (Set 6) is ca 0·7 m thick, while other sets are thinner, with some measuring ca 0·5 m in thickness. Within sets, cross‐laminations dip predominantly to the north‐east above the level of unconformity relief. Horizontal bounding surfaces separating the sets are largely sub‐parallel and sub‐horizontal, and undulate by a few centimetres, leading to little variation in set thickness. Intraset surfaces are common in Sets 2 to 8, mirroring the dips of the cross‐laminations, but typically with shallower dip angles.

At the northern edge of the amphitheatre, an outlier of Stimson eroded into Murray formation bedrock is present in a topographic low (Fig. 20 C). Within the outlier, two trough cross‐sets in planform section were observed, with the eastern set truncating the western set. These sets are eroded into Murray formation bedrock. The eastern set has a width of 3·5 m, while the partially truncated western set has a width of 4·2 m. In both sets, uniform millimetre‐thick laminations define inclined troughs that dip towards the south‐east. The fully preserved eastern trough allowed complete measurement of the foreset dip direction, which is towards 130°.

At the base of the amphitheatre, the upper modern surface of bedrock of the Murray formation forms a series of ridges 0·6 to 1·3 m high, that trend north‐east/south‐west, and are spaced ca 15 m apart. These ridges can be traced to the southern side of the amphitheatre where they are unconformably overlain by a 6·5 m thick section of Stimson formation. On the south side of the amphitheatre (Fig. 20 ), Stimson sandstones onlap onto both sides of a small 1·3 m high ridge formed of Murray formation mudstones. Sub‐horizontal bounding surfaces within the Stimson truncate against the ridge on both sides. Sets defined by these bounding surfaces appear to thin as they onlap onto the unconformity. This ridge is completely overlapped and buried by Stimson sandstone. Two smaller ridges west of this ridge (Fig. 20 ) show similar relationships with the overlying Stimson. These ridges have heights of ca 0·6 m, and the Stimson infills the lows between these ridges and onlaps and thins onto their flanks. The undulatory character of the unconformity surface and the onlap of Stimson sandstones onto this surface indicates that the unconformity shows metre‐scale palaeotopography. The palaeotopographic ridges have a relief of ca 1 m in amplitude, with a wavelength of ca 15 m between the individual ridges (Fig. 20 D). The axes of the palaeo‐ridges are aligned approximately NNW–SSE and can be traced for ca 30 m along their axis before they are occluded by regolith or the overlying Stimson formation. Within the palaeotopographic lows, cross‐bed troughs show axes aligned parallel to the ridge crests. Above the crest of the ridges, cross‐bed sets dip towards the north‐east. Stimson sandstones in the lows between the Murray ridges contain abundant concretions, showing a rough bulbous surface texture. Here, the Stimson is densely packed with concretions to the point where they are commonly amalgamated. The concentration of concretions decreases with increasing stratigraphic height above the tops of the palaeo‐highs, becoming more isolated and sporadic in their distribution towards the top of the outcrop.

East Glacier area and key stratigraphic observations at this location. (A) Mastcam mosaic showing overview of East Glacier area and demonstrating the relationship of key locations at the outcrop. Mastcam‐34 mcam05216. (B) East Glacier main outcrop. Architecture at this location is representative of the Stimson formation, with dune migration surfaces (first‐order surfaces) clearly identifiable (yellow) Cross‐laminations dip predominantly towards the left of the outcrop, which is towards the north‐east. Mastcam‐100 mcam05219. (C) East Glacier outlier. Outcrop consists of two trough cross‐bed sets exposed in plan‐view, surrounded by Murray formation. Palaeotransport direction is towards the south‐east, which is at a tangent to the regional transport direction. Mastcam‐100 mcam05214. (D) Detailed view of the Stimson formation onlapping onto a palaeotopographic surface expressed in the upper surface of the Murray formation. Mastcam‐34 mcam05216. Image credit: NASA/JPL‐Caltech/MSSS.

To further understand the relationship of the Stimson formation to the underlying Murray formation, these stratigraphic relationships were observed in detail at the East Glacier outcrop located 100 m south of Bridger basin (Figs 2 and 20 , and the context map in Data S4 ). Here, excellent exposures of the Stimson and underlying Murray formation, together with the unconformity separating the two, enable investigation of the stratigraphic relationship between the two formations at a decimetre‐scale and reconstruction of the palaeotopography at the unconformity. The East Glacier area forms a ca 30 m wide alcove oriented approximately east–west that exposes Murray formation mudstones at its core. Stimson formation sandstones form the topographic boundaries of the depression.

The McLeod outcrop exhibits a similar texture to that observed at the MAHLI target Lumprey (Fig. 7 ) at the Missoula outcrop at Marias Pass. In both locations, cross‐bedded sandstones contained very coarse to granule size grains interspersed in an apparent fine to medium‐grained sandstone matrix. The origin of the larger white grains is uncertain, but they are likely to have been moved by aeolian surface creep, caused by grain collisions. The abundance of intra‐set surfaces within this set suggests either the superimposition of smaller bedforms migrating across the lee slope of a larger dune, variations in wind direction and strength, or the merger of two dunes. The oblique trough at the top of the outcrop suggests either a brief episode during which sediment transport was discordant to the main transport direction, or that the trough was created by a subordinate bedform migrating across the lee of a larger bedform. Laminations with smooth surface textures are interpreted as possible grainfall strata, in which finer grains capable of saltation were segregated and concentrated away from coarser particles moved by creep.

McLeod outcrop (A) Overview of McLeod outcrop, which is composed of a single cross‐bed set. Yellow lines denote inclined intraset surfaces within the set. (B) Close‐up of smooth laminations interstratified with ‘rough laminations’. (C) Close‐up of laminations within the set. Note that 2 mm diameter grains are resolvable. Mastcam‐100 mosaic mcam05194 of McLeod outcrop was taken on Sol 1148, from Sol 1144 end‐of‐drive position. Image credit: NASA/JPL‐Caltech/MSSS.

The McLeod outcrop allowed for close‐range imaging of Stimson sandstone with Mastcam, capturing subtle facies variations normally only seen in MAHLI images. The outcrop (Figs 2 and 19 ) is a cross‐bedded sandstone which forms a small, 1 m high ridge. The close proximity (<6 m) of the rover to the outcrop resulted in images with sub‐millimetre resolution, capturing grain‐scale texture preserved within the outcrop. The McLeod outcrop is a single ca 1 m thick set of metre‐scale cross‐bedded sandstone. It exhibits a rough speckled surface texture, caused by the presence of white, 1 to 2 mm diameter grains that represent 2 to 5% of grains present (Fig 19 B and C). The remainder of the sandstone consists of grey coloured grains expressed as a rough surface texture; although grain boundaries cannot be resolved due to a lack of contrast between grains. Cross‐laminations in the outcrop are largely planar, lack curvature and exhibit uniform thicknesses of a few millimetres (3 to 7 mm; Fig. 19 B and C). Laminations dip to the south‐east and have lengths of up to 1 m. Horizontal bounding surfaces have not been identified at this outcrop. Intraset bounding surfaces which truncate cross‐laminations at a shallow angle are common, and typically dip in a similar direction to the cross‐laminations. Dip directions above and below these intraset surfaces demonstrate discordances of a few degrees. A single partially preserved ca 2 m wide trough cross‐set is observed to cut obliquely across the underlying set at the top of the outcrop, and is composed of laminations that dip approximately north (Fig. 19 ). Locally several interstratified bed sets characterized by a smooth surface texture and lacking resolvable grains were observed.

The outcrop is composed of stacked cross‐bed sets and is interpreted to record accumulation of sediment by migration of aeolian dunes along the axis of dominant sediment transport. Set thickness in this outcrop is variable, particularly within a single set. Dune migration surfaces undulate along their length where they can pinch‐out or downcut into certain sets (surfaces BB5 and BB7) or climb, resulting in a downwind increase of set thickness (BB2 and Set 1, BB5 and Set 5a, BB7 and Set 6). These variations in set thickness may have been caused by autogenic processes related to internal processes within the dune field, by allogenic controls representing a regional change in processes affecting the dune field, or a combination of both factors. One cause of these thickness variations could arise internally to the dune field from the complex interrelation of migrating scour pits and spurs in sinuous‐crested transverse dunes. As scour pits migrate along their dominant transport vector, they can shift laterally, resulting in spatial variations of scouring, causing these variations of set thickness. Alternatively, this may have been controlled by variations in sediment flux, where the wind temporarily became sediment undersaturated, allowing entrainment of previously accumulated sediment, resulting in temporary downcutting of the dune migration surface. The horizontal bounding surfaces identified are unlikely to represent water‐table controlled deflation surfaces, as described by Kocurek & Day ( 2017 ) or supersurfaces (Rodriguez‐Lopez et al ., 2013 ), because the surfaces undulate by several tens of centimetres over a distance a few decametres and microtopography associated with cohesion on that surface has not been observed. It is more likely that these surface undulations are associated with autogenic processes within the dune field, such as variations of sediment supply within the dune field, or interaction between dunes.

Sets at Bridger Basin vary in thickness greatly across the outcrop, in some places exhibiting complex relationships (Fig. 18 B). The lowermost sets, 1 and 2, show a pinching out relationship, where Set 1 increases in thickness in the direction of foreset dip to a maximum thickness of 0·4 m, having been partially truncated by surface BB2 and overlain by Set 2. Set 2 also pinches out in the direction of foreset dip, where it is truncated by surface BB3, which is the lower bounding surface for Sets 3 and 4. A similar pinching out relationship was observed in Sets 3 and 4. Bounding surface BB5 also shows relief, and downcuts the entire thickness of Set 5 by ca 0·4 m, scouring into the top of Set 4 over a distance of 4 m. The surface then rises to the north‐west, bounding the top of Set 5a. Set 6 thickens gradually from left to right of the outcrop, from a thickness of 0·3 to 0·64 m. Surfaces BB5 and BB6 which bound Set 6 are relatively parallel, but diverge towards the north‐east over a distance of 20 m. Surface BB7 completely cuts out Set 7, truncating surface BB6 and the top of Set 6, and downcutting ca 20 to 30 cm. This scour is infilled by Set 8, which exhibits a lens shape, thickening to 0·4 m in the centre of the depression, then thinning in the direction of foreset dip. Set 9 has a uniform thickness of <0·2 m for 7 m laterally, before thickening towards the right of the outcrop, to a maximum thickness of 0·3 m. The upper surface of Set 10 is poorly defined, and has limited stereo coverage, preventing accurate measurements of thickness. Across the outcrop, several intraset surfaces were identified with prominent examples in Set 6.

The unconformity was observed in the west of Bridger Basin, and can be traced from the western edge of the outcrop south towards the East Glacier outcrop (Fig. 2 ). The elevation of the unconformity at the west edge of the Bridger Basin outcrop is −4438 m. At the tallest part of the escarpment (Fig. 18 ), the Stimson section comprises ca 12 cosets of sub‐metre‐scale planar cross‐bedding, in which the cross‐laminations dip towards the north‐east. Planar sets are defined by bounding surfaces that are largely sub‐parallel and sub‐horizontal to one another, and can be traced across the outcrop for distances in excess of 30 m. Locally, they show relief of several tens of centimetres resulting in variable set thicknesses along the outcrop. Maximum set thicknesses are typically between 0·3 m and 0·7 m. Some bounding surfaces are truncated by overlying downcutting bounding surfaces.

Bridger Basin area. (A) Uninterpreted overview of Bridger Basin outcrop, situated on the south side of the ridge where the Williams outcrop is located. The outcrop is ca 4 m high, and the wall is ca 90 m long. (B) Interpreted close‐up of Bridger Basin architecture. Large‐scale architecture is exposed across the outcrop, and dune migration surfaces were traced for more than 30 m, where they are not truncated by overlying surfaces. Cross‐laminations dip predominantly towards the right of the outcrop, towards the north‐east. Mastcam‐100 mosaic mcam04872 was taken on Sol 1100 from the Sol 1099 end‐of‐drive position. Image credit: NASA/JPL‐Caltech/MSSS.

The large‐scale sedimentary architecture of the Stimson formation is best exposed at the Bridger Basin outcrop, a natural amphitheatre located on the south‐west side of the Bob Marshall Ridge (Figs 2 and 18 , and context map in Data S4 ). Here, the inter‐relation of bounding surfaces and set geometries is exposed in a ca 4 m high, south‐east‐facing escarpment that demarcates the north‐western side of the basin. The outcrop extends for 90 m, and trends 060° to 240° with an average slope 14° towards the south‐east. The outcrop and the adjacent basin floor are composed of cross‐bedded Stimson sandstones; however a small inlier of Murray formation occurs at the open end of the amphitheatre at its south‐western edge. The exact contact between the Stimson and the Murray is occluded by regolith at this location; however the contact appears to be undulatory because the Stimson is inferred to onlap onto the contact beneath this cover.

Cross‐bed dip data derived from the DOM show that the dominant transport direction of the Williams outcrop was similar to the regional transport direction, towards the north‐east. Where dip‐azimuth measurements are grouped by set, variability of these vector means are spread across an arc of ca 95°; however the two largest sets show a dominant transport direction broadly towards the regional transport direction. These variations could have arisen from autogenic processes, where dune migration direction was altered by turbulent flow around adjacent bedforms (see (Kocurek & Ewing, 2017 ) or changes in sediment availability (Courrech du Pont et al ., 2014 ), or by allogenic processes during which the regional wind regime shifts as a result of seasonal or longer‐term climate cycles (Hunter & Rubin, 1983 ; Mountney, 2006 ).

Cross‐sets and their underlying bounding surfaces are interpreted to represent the preserved basal section of migrating aeolian dunes, which were truncated and buried by successive migrating aeolian dunes during episodes of net sediment accumulation. Coset 2, which is cross‐cut by an adjacent set out of the plane of the outcrop can be interpreted as a cross‐bed trough, and is interpreted to have formed as a result of the migration of a sinuous‐crested out of phase transverse dune. Some dune migration surfaces downcut in the same direction as foreset dip (i.e. WOS 3.4), which may represent fluctuations in scour depth arising from a change in dune shape or an episode where the wind became undersaturated with sediment, resulting in entrainment of previously accumulated sediment. Intraset surfaces observed within cross‐sets at this outcrop correspond to reactivation (third‐order) surfaces.

Interpretation of Williams outcrop using Pro3D software. (A) Williams outcrop rendered and interpreted in 3D. Bounding surfaces and sets mirror those interpreted in Fig. 14 . Bounding surfaces are interpreted in yellow, with laminations interpreted in white. Red discs represent plane of best‐fit interpolated on cross‐laminations, which give dip‐azimuth and palaeotransport direction. (B) Close‐up view of cross‐set. For details, see Fig. 6 . (C) Detailed view of bounding surface, and the difference in dip azimuth of cross‐laminations above and below the bounding surface. Rose diagrams: The general trend of cross‐lamination dip azimuths follow the regional trend towards the north‐east. When grouped by set, dip azimuths show a significant divergence from the regional trend.

Using a DOM, dip azimuths from 347 measurements at this location show a dominant palaeotransport direction between 040° and 050° (‘All cross‐laminations’, Fig. 17 ). The dip and strike measurements were divided into sets described above to ascertain variability up the section. Set 0 shows dip values from 27° at the top of the set to 6° at the bottom of the set and a dominant palaeotransport direction of 070° to 080°. Dip values in Set 1 vary from 20° to 3°, with a dip direction towards 110° to 120°. The majority of palaeotransport readings were taken in Coset 2 ( n = 141), and the asymptotic geometry was reflected in dip values of up to 30° (purple disks in Fig. 17 ) at the top of the set, and decreasing to 10° to 12° in the middle, with minimum values of 4° at the base (Fig. 17 A). The dip azimuths agreed with the overall values measured, dominantly towards 050° to 060°. A small amount of northerly dipping laminations were recorded in the discontinuous sets in the western part of the outcrop (Fig. 17 C). Set 3 also showed a dominant palaeotransport to 050° to 060°, with less scatter apparent (Set 3 in Figs 16 or 17 ). The DOM quality deteriorates above Set 3, so fewer dip and strike values were taken from Sets 4 and 5 (21 and 12, respectively, Fig. 17 ). Comparable dominant palaeotransport directions were recorded in Sets 4 and 5, with Set 4 showing a dominance to the north, between 350° and 020°, and minor peaks towards 310° and 050°. Set 5 showed peaks towards the north (350° to 000°) and north‐east (040° to 060°). Both sets also had lower maximum dips of laminations (14°). Bed set boundaries dip between 2° and 14° striking towards 062° ( n = 15). Measurements made using the DOM can be found in Data S8 .

The highest defined set in the outcrop, Set 5, is characterized by a discontinuous upper bounding surface, where the outcrop had degraded. Surface 3.5.1, which demarcates the upper left side of the set dips towards the north‐east, in the same direction that the laminations dip. The maximum thickness in this set is 0·55 m, where the upper bounding surface is visible.

Set 4 is defined at its base by a scour surface 3.4. Laminations within this set are distinct in that they are largely sub‐horizontal, with little curvature to the laminations. Laminations have measured distances of up to 2·5 m within this set. Set thickness increases to the left of the outcrop, from 0·27 to 0·74 m, indicating a thickness increase of ca 0·5 m over a 10 m distance. The upper surface is defined by a subtle angular discordance caused by an apparent change in lamination orientation in the set above.

The base of Set 3 is occluded by regolith across part of the outcrop; however, surface 3.3 can be delineated by the angular discordance of cross‐laminations above and below it. The upper surface of Set 3 is defined by the angular discordance between cross‐laminations in Set 3 and Set 4 which defines surface 3.4. The thickness of this set ranges between 0·9 m on the right of the outcrop and 0·7 m on the left, indicating that the set thins by 0·2 m over a distance of 4.7 m. Within this set, there are ca 10 inclined intraset surfaces, showing arcuate geometries similar to cross‐laminations.

Coset 2 is a composite set consisting of one main set (left side of coset) and several smaller discontinuous sets (on the right of the outcrop). These small discontinuous sets may represent the preserved edge of adjacent troughs which cut into the main set. The base of the main set is visible across the outcrop, and is defined by the asymptotic downlap of cross‐laminations onto surface 3.2. Surface 3.2 can be traced across the outcrop, with a measured length exceeding 10 m. The upper surface of the coset is defined by the laterally continuous surface 3.3, which can also be traced across the outcrop. Cross‐laminations in this set have measured lengths in excess of 2 m in the centre of the set, and on closer inspection have a uniform thickness across their length, with no measurable variation in thickness (Fig. 7 ). Slight variations in cementation between laminations are discernible, which causes alternating prominent and recessed laminations. Numerous intraset surfaces are present within this set, with maximum dips of ca 10°. The overall thickness of Coset 2 ranges between 0·8 m and 0·87 m, with the overall thickness increasing towards the left of the outcrop, to the north‐east. Internally, bounding surfaces 3.2.1 and 3.2.2 can be traced laterally for 2·5 m and 1·7 m, respectively. These surfaces appear to cut in and out of the exposed section of the outcrop, giving them a discontinuous appearance.

The main Williams outcrop exhibits six cross‐bed sets with maximum thicknesses varying between 0·2 m and 0·9 m (Fig. 16 ). Cross‐laminations within these sets dip on average to the north‐east. At the base of the outcrop, the lowermost set (Set 0) (Fig. 16 ) consists of millimetre‐scale repetitive cross‐laminations that are truncated at their apex by a sub‐horizontal bounding surface WOS3.1 that can be traced across the outcrop. Set 0 is overlain by Set 1 which is characterized by sweeping asymptotic laminations that downlap onto surface 3.1. The set shows marked variation in thickness (0·19 to 0·33 m), which is attributed to undulations of the overlying dune migration surface bounding the set. Within the set, cross‐laminations have an apparent low angle inclination, when compared with other sets.

Williams outcrop, at the south end of the Emerson plateau. A full‐size version of this mosaic is available at https://mars.nasa.gov/resources/7468/ . (A) An uninterpreted view of Williams outcrop. Height of outcrop is 5 m, and slopes towards the point of observation with an average angle of 15°. (B) Interpretation of Williams outcrop mosaic. Cross‐laminations are interpreted in white and dune migration surfaces are yellow. Set thicknesses are indicated in blue. Cross‐laminations dip towards the left of the outcrop which is towards the north‐east. Mastcam‐100 mosaic mcam04777, was taken on Sol 1087, at the Sol 1085 end‐of‐drive position. Image credit: NASA/JPL‐Caltech/MSSS.

The preserved geometry of dune cross‐bedding in the Stimson formation is best illustrated at the Williams outcrop (Figs 2 and 16 , and context map in Data S4 ) where relatively close‐range M100 imaging of sedimentary structures permitted excellent characterization of the sedimentology in a vertical section. The Williams outcrop is located along a 100 m long, ca 5 m high, north‐east/south‐west trending ridge named the Bob Marshall Ridge. The main outcrop investigated (Fig. 16 ) is at the northern end of this ridge and slopes towards the north‐west at ca 15°. The basal contact between the Stimson and Murray formation is not exposed at this location, but is inferred to subcrop at a relatively shallow depth beneath the plateau (Watkins et al ., 2016 ).

The Stimson formation at both Apikuni Mountain and Mount Shields is characterized by small‐scale (0·05 to 0·4 m thick) trough cross‐sets, which are interpreted to have formed by the migration of small‐scale sinuous‐crested aeolian dune forms, with smaller sets interpreted to potentially represent large sinuous‐crested ripples. At Mount Shields, bounding surfaces defining decimetre‐scale cross‐sets are interpreted to be superposition surfaces, while the laterally extensive coset bounding surfaces are interpreted to be interdune surfaces (Brookfield, 1977 ). Interdune (coset) surfaces correspond to scour pits migrating in advance of a lee slope of a migrating compound dune, with lower‐order (superposition) surfaces within the cosets corresponding to scour pits generated by migration of smaller‐scale superimposed bedforms. Cosets at this outcrop show thicknesses similar to large‐scale cross‐set thicknesses at Williams outcrop, Bridger Basin and East Glacier, later in the traverse, where set thickness corresponds to dune‐scale bedform migration. It is uncertain as to whether the small‐scale cross‐sets represent small‐scale dunes (tens of metres wavelengths) or large ripples (metre‐scale wavelengths). Within coset C, decimetre‐scale cross‐sets have maximum thicknesses of ca 0·1 m, which would represent the preservation of about one‐third of a typical large ripple observed within the present‐day Bagnold dune field (Ewing et al ., 2017 ); whether this proportion of preservation is feasible is uncertain.

Mudstone clasts within the basal sandstone lens probably represent products of erosion from the underlying Murray formation, reworked into the base of migrating aeolian dunes as dune‐field construction ensued. Later, after the dune field was established, the supply of Murray mudstone clasts was eliminated as the sub‐Stimson palaeo‐surface became buried.

The north‐east face of Mount Shields exposes a 3·3 m thick section of Stimson formation (Fig. 15 A). The outcrop consists of 0·1 to 0·3 m thick cross‐sets, some of which have characteristic trough shapes, bounded by dune migration surfaces which can be traced for distances of ca 1 to 2 m. Analysis of the bounding surfaces reveals that some surfaces are more laterally extensive, and can be traced across the outcrop. These surfaces bound groups of sets, forming cosets between 0·5 m and 0·8 m thick. Within these cosets, sets dip predominantly in a common direction (Fig. 15 A): Coset A dips predominantly towards the east; Coset B dips predominantly north and east; Coset C dips towards the north and west; and Coset D shows a general apparent dip towards the west. Coset bounding surfaces can be traced from the north‐east side of the outcrop, onto the south‐east side of Mount Shields, where they are otherwise indistinct from set‐bounding surfaces (Fig. 15 B).

Mount Shields, Marias Pass. (A) North‐east face of Mount Shields. This face of Mount Shields is perpendicular to the dominant transport direction, and exposes trough‐shaped cross‐sets. Cross‐set bounding surfaces can be identified (yellow), and groups of cross‐sets (cosets) can be observed to dip in similar directions. Coset bounding surfaces represent major bounding surfaces indicating variation in palaeotransport direction. Four major cosets, separated by three coset bounding surfaces are identified. The general transport direction is towards the observer (north‐east). Mastcam‐100 mosaic mcam04617 was taken on Sol 1049 from end‐of‐drive position for Sol 1042. (B) South‐east face of Mount Shields. Dune migration surfaces are readily identifiable; however because the dip azimuth of cross‐laminations are all in a similar direction, coset bounding surfaces are difficult to distinguish. In this example, coset bounding surfaces have been traced from the north‐east facing slope, and inferred along the length of horizontal bounding surfaces at the same elevation. Cross‐laminations dip towards the right of the outcrop – towards the north‐east. Mastcam‐100 mosaic mcam04521 was taken on Sol 1033 from Sol 1030 end‐of‐drive position. Image credit: NASA/JPL‐Caltech/MSSS.

At the Mount Shields outcrop, on the western side of Marias Pass, the north‐east (trending 160°) and south‐east (trending 260°) faces of the outcrop are exposed (Figs 11 and 15 ). The sub‐Stimson unconformity is located towards the base of the outcrop; however it is mostly occluded by regolith. The south‐east facing outcrop has a stepped profile and exposes a 3·6 m thick succession of cross‐bedded sandstones (Fig. 15 ). Sets show a similar range of thicknesses to those observed at Apikuni Mountain, with sets typically ranging between 0·1 m and 0·40 m in thickness. Set thickness increases from the base of the outcrop, where sets are typically 0·1 to 0·2 m thick, to ca 0·4 m thick near the top of the outcrop. The largest set measured was 47 cm thick. Cross‐bed foresets in this outcrop exhibit variable dip directions, with a slight bias to sets dipping towards the north‐east, suggesting that the outcrop surface is oblique to the axis of sediment transport.

The north‐west‐facing slope of Apikuni Mountain forms an outcrop (trending 60 to 240°) ca 30 m long and exposes a vertical section of Stimson ca 5·5 m thick above the sub‐Stimson unconformity (Fig. 13 A). The outcrop slopes towards the centre of the pass with an average angle of 14°. The Stimson at Apikuni Mountain is largely composed of decimetre‐scale cross‐bedded sandstones (Facies 2). At the base of Apikuni Mountain (Fig. 13 B) a 0·7 m high step exposes decimetre‐scale cross‐sets, with set thickness ranging between 0·05 m and 0·2 m, and cross‐laminations exhibiting an apparent uniform thickness. Cross‐sets at the base of the succession exhibit variable dip directions, with apparent dips to the north‐east and south‐west of the outcrop. In some cases, troughs are discernible, with their axes aligned north‐west/south‐east. Above the basal step, the outcrop is sloped and contains larger cross‐sets, with thicknesses of up to 0·3 m and lamination lengths of up to 2·5 m. Bounding surfaces in the upper section of the outcrop can be traced for distances of more than 3·5 m before occlusion by superficial material. Cross‐lamination dip directions are less variable than the lower section, and a dominant transport direction towards the north‐east was recorded. The outcrop is cross‐cut by sub‐vertical fractures with spacings of >5 m and characterized by distinct light grey alteration halos a few tens of centimetres wide to either side of the fracture. Similar fracture‐associated halos in the Stimson are described in more detail by Yen et al . ( 2017 ) and Frydenvang et al . ( 2017 ).

Missoula area, Marias Pass. A context Mastcam mosaic of the target area is available at https://mars.nasa.gov/resources/7312/ . (A) Mastcam overview of Missoula area, which straddles the unconformity. Mastcam‐100 mosaic mcam04406, Taken Sol 997, at end‐of‐Sol 995 drive position. (B) MAHLI image of Clarke target, which consists of Sandstones of Murray formation mudstone clasts (Facies 4). White arrows denote mudclasts, yellow arrows denote sand grains. (C) MAHLI image of Lumpry target, which consists of cross‐bedded Stimson facies. Yellow arrows highlight millimetre‐scale sand grains. See Supporting Information for table of MAHLI images used. Image credit: NASA/JPL‐Caltech/MSSS.

At the south‐western end of the Apikuni Mountain outcrop, the sub‐Stimson unconformity is exposed in a 0·3 m thick vertical section at the target Missoula (Fig. 14 ). Here, the basal part of the Stimson weathers proud forming an overhang, whereas the underlying Murray formation forms a recess. The unconformity shows ca 0·2 m of relief over ca 0·7 m lateral distance, with an apparent dip towards the north‐west. The Murray formation at this location is a mudstone. A polygonal fracture network infilled by calcium sulphate veins cross‐cuts the laminations in the Murray. Directly above the unconformity, a lens of sandstone, 0·5 m wide, and 30 mm thick, is present that contains clasts of Murray mudstone (Facies 4) (Fig. 14 B). MAHLI images (Fig. 14 C) show that the lens consists of poorly sorted, medium to coarse‐grained sandstone, with grains up to 2 mm in diameter, with cross‐laminations that are 2 to 4 mm in thickness. The lens is overlain by a cross‐bedded sandstone outcrop, which forms the basal part of the Apikuni Mountain outcrop.

Marias Pass area. A context Navcam mosaic is available at https://mars.nasa.gov/resources/7600/ . (A) Mastcam‐100 mosaic of Marias Pass area, covering a field of view between 090° and 240°, showing the location of Apikuni Mountain, Mount Shields and Missoula area (mcam04395). The unconformity is observed to run along the base of Apikuni Mountain, through the Missoula area and back around to the base of Mount Shields (see map, Fig. 10 ). (B) Mastcam‐100 close‐up image of Apikuni Mountain, with interpretation of stratigraphic architecture preserved. Cross‐laminations are interpreted in white, and bounding surfaces are highlighted as yellow surfaces. Mcam04395 was taken from end‐of‐Sol 992 drive position, on Sol 993. Image credit: NASA/JPL‐Caltech/MSSS.

The Stimson formation was first studied in detail at Marias Pass, a col that separates two outcrops of Stimson sandstone – Mount Shields and Apikuni Mountain (Figs 11 and 13 A; for mosaic context see Data S4 ). The Murray formation is well‐exposed in topographic lows in the central and eastern areas of the pass (Fig. 13 ). The sub‐Stimson unconformity is clearly visible on both sides of the pass (Fig. 13 ); it is exposed at the base of Mount Shields and can be traced across the pass to the base of Apikuni Mountain. From the base of Apikuni Mountain, in the Missoula area, the unconformity was traced around to the north side of the outcrop, and towards Mount Stimson (as viewed from Logan's Pass; Fig. 12 ). The unconformity at Marias Pass undulates with local palaeo‐relief of up to 0·6 m vertically. The elevation of the unconformity at the base of Apikuni Mountain is approximately −4446 m. The Murray formation exposed at the base of Marias Pass is a mudstone, exhibiting a smooth surface texture, tan colour and millimetre‐scale planar laminations.

The boundary between the Stimson and Murray formations observed at Logan's Run and Mount Stimson is a sharp contact and exhibits marked vertical relief over a short distance. The geometries of bounding surfaces in the Stimson suggest that the Stimson has not been tilted or deformed since deposition, and that it was probably formed by deposition of the sandstone directly against the inclined surface. The contact is interpreted to represent an unconformity along which the Murray formation experienced substantial erosion before deposition of the Stimson (Watkins et al ., 2016 ). The observation of onlap of Stimson formation sandstones onto an inclined unconformity surface indicates that the unconformity here is characterized by local vertical palaeo‐relief on the order of several metres.

At Logan's Run, a north‐facing ramp composed of Murray formation mudstone is observed, with outcrops of Stimson sandstone overlying it across the height of the slope (Fig. 12 B). The slope shows ca 7 m of vertical relief over 26 m distance, with the observed stratigraphic thickness of the Stimson overlying it measuring ca 10 m thick from the base to the lower part of Mount Shields. The thickness of Stimson here is likely to be slightly higher as the top of Mount Shields is occluded in the mosaics. The Stimson formation exposed on the slope is blocky, with cross‐set troughs present in most parts of the outcrop (Fig. 12 B and C). Bounding surfaces can be identified, with a mixture of sub‐horizontal and inclined surfaces. The sub‐horizontal bounding surfaces appear to truncate against the sub‐Stimson unconformity when traced to the south (Fig. 12 C). Sub‐horizontal bed sets of Stimson strata terminate against the unconformity, suggesting an onlap relationship.

Mount Stimson and Logan's Run A context image giving an overview of the area can be found here: https://mars.nasa.gov/resources/7180/ . (A) Mosaic showing the unconformity observed at Mount Stimson – the first location where the sub‐Stimson unconformity was clearly identified Mastcam‐100 mosaic mcam04330 was captured on Sol 980 from Sol 978 end‐of‐drive position. (B) Mosaic shows the Stimson formation onlapping onto the Murray formation, the elevation difference between the base and top of the slope is8 m. At this location, over 10 m of vertical exposure of Stimson is observed. Mastcam‐34 mosaic 04273 was captured on Sol 964 from Sol 963 end‐of‐drive position. (C) Close‐up of Stimson formation on west side of ramp at Logan's Run. Cross‐bedded sandstones are clearly visible above the unconformity. Mastcam‐100 mosaic mcam04380 was captured on Sol 911 from the end‐of‐drive location on Sol 990. Image credits: NASA/JPL‐Caltech/MSSS.

Overview map of Marias Pass area depicting the locations of Logan's Run, Mount Shields, Apikuni Mountain and the Missoula area. The position of the rover relative to these locations is demarcated by a white triangle. The red line indicates the outcrop of the Murray–Stimson unconformity. Contours (black lines) represent 2 m increases in vertical elevation. Image credits: NASA/JPL/University of Arizona.

Evidence for a stratigraphic unit overlying the Murray formation was first observed after leaving the Pahrump Hills area (Sol 957, Figs 2 and 11 , for mosaic context see Data S4 ); however, neither the exact nature of this unit nor the relationship with the underlying Murray formation were established until the rover entered the Logan's Run area after Sol 964. At Mount Stimson (Figs 2 A and 12 A), a sharp contact was observed to separate dark‐toned, bedded and blocky ca 6 m thick sandstones from the underlying relatively light‐toned, apparently homogeneous and smooth‐surfaced Murray formation. The Stimson formation was named after this peak where the contact was first clearly recorded.

The architecture of the Stimson observed on the traverse can be synthesized into four generic architectural elements, which contain facies in predictable vertical and lateral arrangements with distinctive geometric properties. These relationships are summarized in Fig. 10 . To aid the reader with understanding the complex relationships, a description of the sedimentology at key locations along the traverse is provided in the following section. The map positions of mosaics used in this section are documented in Data S6 , along with additional online materials in Data S7 .

To reconstruct the palaeomorphology and stratigraphic evolution of the Stimson depositional system, the following key features were analysed: (i) the outcrop‐scale architecture and the geometric relations of the facies; and (ii) the relationship of these outcrops to the underlying sub‐Stimson unconformity. Here the study focuses on the section traversed by the rover between Logan's Run and East Glacier. The stratigraphic architecture provides a record of the style of deposition and sediment accumulation, the dominant transport direction, and the relationship between the Stimson formation and the underlying Murray formation.

Discussion

Evidence for aeolian deposition of the Stimson sandstone Taken together, the grain‐scale texture, sedimentary structures and facies presented by the Stimson formation sandstone lead to its interpretation as an ancient aeolian sand deposit. This section focuses on the sedimentary texture and the facies as the primary evidence for this aeolian interpretation. Grain‐scale texture observed in MAHLI images indicates that the Stimson sandstone has a bimodal grain‐size distribution and well‐rounded grains (Fig. 6; Data S4). These are common attributes of wind‐sorted sediments on Earth (Sharp, 1963; Ellwood et al., 1975; Yizhaq & Katra, 2015). This study demonstrates that the geometric mean grain size for the Stimson formation is 406 μm (long‐axis measurements). This is compared to grain sizes measured in the Bagnold dune field which were 120 μm (modal) (Ehlmann et al., 2017), ca 112 μm (modal) (Sullivan & Kok, 2017) and 113 μm (intermediate axis) or 135 μm (long axis) (Ewing et al., 2017). As a comparison, the grain size of aeolian systems on Earth typically range between 150 μm and 300 μm (Bagnold, 1941), with example mean grain sizes of: 156 μm for White Sands National Monument; 243 μm for the Page Sandstone, Arizona; 285 μm for Great Sand Dunes New Mexico, USA; and in extreme cases, 660 μm observed on seif dunes in the Sabha Desert, Libya (Ahlbrandt, 1979). The major modal peak obser