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The overview Evidence for water and aqueous processes during the initial infilling phase of Gale crater are abundant. Since 2012, the Mars Science Laboratory (MSL) rover Curiosity has investigated a variety of fluviolacustrine environments represented by the Bradbury and Mount Sharp groups. Both units accumulated as a result of sediment transport as bedload by fluvial processes, or by sediment settling from suspension in a lake. What is unclear is the duration that water persisted within Gale crater after the crater infilling was complete. In this contribution, we present evidence of water present within the base of the aeolian Stimson formation, which is recorded in the stratigraphic record by fluidization structures. We will describe the morphology of these structures, the mechanism of emplacement and implications for the history of aqueous processes in Gale crater. The Background The Stimson formation is the portion of a broader mound skirting unit which has been directly investigated by the MSL rover. This mound skirting unit unconformably drapes the northern flank of the present-day Mount Sharp. Where encountered on the traverse, the Stimson formation represents the preserved expression of an ancient aeolian dune field. Specifically, the stratigraphic record and the sedimentary architecture indicates that it was a dry dune field: water played no role in the accumulation of this sediment body. This fact is betrayed by the absence of fine-grained interdune deposits, microtopography, or other facies which are characteristic of water at or near the depositional surface. At the time the Stimson formation accumulated, Gale was devoid of surface water. The Observations Around Sol 3434, as Curiosity traverse onto the east side of the Greenheugh pediment, it encountered prominent erosion-resistant bedrock ridges which formed a polygonal (boxwork) network. These structures were observed in the basal portion of the Stimson formation, just a few metres above the base Stimson formation unconformity. Internally, these structures consisted of convolute, folded and warped laminations. Laterally, these structures were juxtapose against undeformed Stimson formation: large-scale cross-bedding composed of well-sorted medium-grained sandstones. Figure1: Polygonal (boxwork) ridges observed at Feòrachas Closer inspection of these internally-deformed ridge structures yielded evidence of both ductile displacement and destruction of primary stratification in a vertical orientation. The displacement and destruction are localised to the polygonal, linear ridges. Where stratification was displaced, laminations were folded into upright closed antiforms, with a tight bend radi on the hinge. Often, these are adjacent to open-folded synforms. Folding of laminations become tighter and more prominent with increasing vertical distance through the antiforms. Some original cross-laminations exhibited complex, recumbent folding where the underlying bounding surface exhibited open-folded synforms. In some ridges, such as at the Lamington sandstone target, stratification was destroyed, and was replaced by continuous, vertically-oriented laminations that cross-cut horizontally-orientated laminations adjacent to the ridge (Figure 2). These laminations are oriented parallel to the long axis of the ridges, occasionally displaying vertical fold structures. In some locations these erosion-resistant ridges contained vertically oriented fractures, with a few subordinate horizontal fractures, which were found to cross-cut some of the ductile deformation. Figure 2: Close-up view of deformation structures within the Feorachas structure Figure 3: Lamington Sandstone: example of vertical laminations within a sand dyke The Interpretation These structures are interpreted to represent fluidization of sediment caused by water escaping through unconsolidated sediment, under pressure. The linear ridges such as the Lamington structure are interpreted to represent well-cemented sand dykes. The result is the partial or complete destruction of primary sedimentary structures (e.g. cross-bedding) and replacement with secondary sedimentary structures related with fluidization. The tight antiforms are interpreted to be cusps while the vertical laminations are interpreted as pipes. Synforms, cusps and pipes exhibit pervasive vertical shear. The recumbent folding was also interpreted as evidence of vertical shear due to the displacement of the underlying bounding surfaces into vertical synforms. The vertical shear seen in cusp and synforms, and the vertical laminations observed in the dykes are evidence for the fluidization of sediment caused by upward fluid drag supporting the grain weight as water escapes through an open system under pressure. The source of the fluid may include groundwater upwelling or pore water from liquefaction occurring below the Stimson. As brittle deformation overprinted the soft sediment deformation, the trigger or cause of the fluidization may have been cyclic and caused brittle failure after the sediment regained its strength. The implications Regardless of the origin of the water, these fluidization structures provide evidence of persistent water within Gale crater after the exhumation of Mount Sharp, and the accumulation of the Stimson dune field. Presence of this water indicates: Water percolated into the base of the Stimson formation, either from fractures within the Mount Sharp group, from strata higher up on Mount Sharp, or from Meteoric sources. Water was present at the time which the Stimson formation was unconsolidated: which would be penecontemporaneous with dune field formation. This may represent a secondary habitable environment within the lithosphere.

The geologic origin of the ancient, phyllosilicate-rich bedrock at Oxia Planum, Mars, the landing site of ESA’s ExoMars rover, is unknown. The phyllosilicates record ancient aqueous processes, but the processes that formed the host bedrock remain elusive. Here, we use high-resolution orbital image and topographic datasets (HiRISE, CTX, CaSSIS) to investigate and characterize fluvial sinuous ridges (FSRs), found across the Oxia Planum region. The FSRs form segments up to 70 km long, with sub-horizontal layering commonly exposed in ridge margins. Some FSRs comprise multi-story ridge systems; many are embedded within and are being exhumed from the phyllosilicate-rich bedrock. We interpret the FSRs as deposits of ancient, episodically active, alluvial river systems (channel-belt and overbank deposits) at Oxia Planum. Thus, the phyllosilicate-rich bedrock was formed at least partly by ancient alluvial rivers, active across the wider region. Our alluvial hypothesis does not exclude other sources of sediment playing a role in building the terrain as well. Future exploration by ExoMars can verify this interpretation and provides an opportunity to investigate some of the oldest river deposits in the Solar System.

The stratigraphy preserved within Aeolis Mons (Mount Sharp) in Gale crater (Mars) shows a major transition from mudstone-rich strata (with subordinate sandstones) recording deposition in lacustrine to fluvial settings into a major sulphate-bearing unit (the Layered Sulphate-bearing unit (LSu)) [1, 2]. This transition is interpreted to represent a major environmental change from wetter conditions toward a more arid palaeoclimate on early Mars. A stratigraphic section over this transition constructed along Curiosity’s traverse shows a vertical change from mudstones with interstratified sandstones of the Glasgow and Mercou members of the Carolyn Shoemaker formation into strata of the Pontours member which have a strong diagenetic overprint, and thence into large-scale cross-stratified sandstones of the Mirador formation that are interpreted to be the deposits of large, migrating aeolian dunes. The lower section of the LSu is dominated by stacked, cross-bedded facies with variable diagenetic overprint, that likely records a purely dry aeolian dune environment [3]. However, higher up in the studied section, approximately 80 m above the base of the Mirador formation, there is a transition into a succession still dominated by large-scale cross-beds but with interstratified lenses of a different sandstone facies. The presence of these lenses within large-scale cross bedded rocks is denoted by a transition to the Contigo member of the Mirador formation. Whilst a number of lenses are visible in the stratigraphy of the Contigo member in cliffs along Curiosity’s traverse, the Mars Science Laboratory science team selected one lens, informally named The Prow to investigate in detail. Here, we describe the sedimentology of The Prow at a range of scales using Navcam, Mastcam, ChemCam Remote Micro-Imager(RMI), and Mars Hand Lens Imager (MAHLI) images with a focus on characterizing sedimentary structures, their depositional process interpretation and comparison to Earth analogs. The 3D geometries of The Prow’s sedimentary structures are analysed by [4]. The Prow was investigated during sols 3349 and 3379 in early 2022 and had been identified as a target of interest from long distance observations suggesting that it formed constrasting facies to surrounding rocks. The Prow is an ~18-m-long, ~0.5-1-m-thick lenticular sedimentary body that is interbedded with large-scale cross-stratified facies interpreted to be aeolian dune deposits. The nature of the lower contact of The Prow is unclear. The lens pinches out to the south. The Prow shows a range of sedimentary structures suggestive of deposition under predominantly aqueous conditions. These structures differ from structures in surrounding bedrock indicating contrasting environmental conditions. The lowermost part of The Prow section appears to comprise decimetre-scale cross-beds indicative of deposition from subaqueous dune migration. The scale of these cross-beds is quite different from the large metres-scale trough cross-beds in rocks surrounding the lenses. The upper sections of The Prow are dominated by cm-scale ripple structures well observed in cross-section. In particular, lenticular and flaser geometries are observed. Lenticular ripple forms with convex upper surfaces are common with concave lower surfaces where they overlie underlying ripple forms. The ripple forms occur vertically stacked and laterally offset. The individual lenses are commonly interconnected forming complex interwoven structures. The crests generally show rounded symmetric profiles. Locally, symmetric vertical accretion over ripple crests is observed implying rapid sediment aggradation. Many ripple forms do not appear to show internal lamination, although this may be due to a lack of grain size variation. In a few examples where internal lamination is observed, preserved foresets are suggestive of unidirectional flow. RMI and MAHLI images reveal that ripple forms appear to have a finer-grained drape overlying ripple crests and extending into ripple troughs. MAHLI data confirm initial ChemCam observations and show that ripple cores comprise a sandstone and draping laminations are finer grained and likely below MAHLI-resolution (~60 microns). The drapes are provisionally interpreted as mud drapes formed from suspension fallout onto ripple topography during low energy quiescent episodes. The overall sedimentary geometry resembles flaser- to lenticular bedding common in Earth examples. The symmetric form of many of the ripple structures with preservation of form sets is suggestive of formation by oscillatory flow by wave action. Locally, there is evidence of asymmetric accretion which is likely indicative of combined flow ripples. The presence of drapes of finer-grained material superimposed on coarser-grained ripple forms is interpreted to record episodes of higher-energy sand transport separated by recurrent intervals of low-energy conditions during which sand grains could not be mobilised. Wave and current activity caused bedload transport of sand constructing symmetric and asymmetric ripples. Between these episodes, there were quiescent periods where active flow ceased and finer-than-sand grains deposited out of suspension onto temporarily fossilised ripple forms. The preservation of stacked well-preserved ripple forms with crests intact suggests conditions of rapid deposition. Locally, planar laminated beds with laterally continuous laminae are present; these maybe interpreted as either wind-ripple laminations formed by aeolian transport or as upper flow regime plane beds. The presence of mudstone-draped wave and current ripple forms in the upper section of The Prow is strongly indicative of deposition from aqueous flows and moreover suggests the existence of a likely aerially small, shallow standing body of water in which the sediments were deposited. We compare the observed structures to forms observed in 1 Ga lake deposits from the Diabaig Formation of the Torridon Group (NW Scotland). We tentatively infer that The Prow and by inference the other lenses observed in the Contigo member may record the episodic interdune or scour-fill presence of transient small standing bodies of water in an otherwise dominantly dry aeolian dune-dominated environment. The vertical transition from the underlying Dunnidear and Port Logan members of the Mirador formation that are dominated purely by large-scale trough cross bedding indicative of dry aeolian conditions indicates a change to a more mixed environmental setting with episodic fluvial/lacustrine activity perhaps from transient snow-melt or precipitation events occurring in otherwise arid conditions. Lenses such as The Prow demonstrate fluvio-lacustrine intervals punctuated dominant aeolian environment in the layered sulphate-bearing unit. References: [1] Milliken et al., 2010, Geophys. Res. Lett. 37, L04201; [2] Rapin et al., 2021, Geology 49; [3] Rapin et al., 2022, EPSC, this meeting; [4] Caravaca et al., 2022, EPSC, this meeting.

International audience; The accumulated sediments and erosional unconformities in the deposits of Mt. Sharp of Gale Crater record multiple periods of wet and dry cycles similar to the episodic climate aridification and later brief wet periods found across Mars [1]. Early post impact (~3.6 billion), an extended period of lacustrine conditions prevailed, leading to over 300 m of lake sediments exposed at the base of Mt. Sharp [2]. This was followed by eolian sediment accumulation perhaps with fluvial deposition interludes [3]. Accumulation ceased and extensive wind erosion shaped Mt. Sharp to nearly its present topography. This erosional event is recorded as the Siccar Point unconformity, which is preserved under a “mound skirting unit”, which locally has been documented to be the remnants of an extensive, large-scale dune field – the Stimson formation [4].The Stimson formation and the Siccar Point unconformity are predominantly exposed along the Greenheugh pediment, a planar sloping topographicfeature that lies downslope of Gediz Vallis [5].Overlying the Stimson formation on the Greenheugh pediment is the remnant of a fan deposit [5] that may have advanced into transient deep lakes in Gale crater that postdated Stimson fm. deposition [6]. Investigation of the northern exposure of this pediment revealed that the sediments underlying the Siccar Point unconformity (the Carolyn Shoemaker formation) had undergone extensive diagenetic alteration. Nodular alteration was also noted in the basal sediments of the Stimson. These water-driven alterations led to hypotheses about the source and pathways of water which included the possibility that groundwater (derived from runoff from Mt. Sharp) may have developed in the overlying Stimson on the much less permeable fine-grained underlying lake sediments [7].Here we report that along a southern exposure of the pediment, 0.9 km upslope from the northern area previously investigated (Tower butte area), adiscontinuous thin massive unit lies at the boundary between the distinctive eolian dune bedding of Stimson and the underlying strongly diagenetically altered Carolyn Shoemaker formation. We propose that this unit is a result of soft-sediment deformation, a condition favored by groundwater saturation of this basal unit, and possibly driven by impact events before lithification of the Stimson.