Search

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.

International audience; Curiosity’s southward traverse up the lower north slope of Mt. Sharp (in Gale crater, Mars) and across Glen Torridon has brought it in contact with the organics-bearing sediments of the Knockfarril Hill member, the light-toned, nodule-rich strata found at Western, Central, and Tower buttes, and to the abrupt truncation of these altered Murray sediments at the Greenheugh pediment. Curiosity ascended the steep northern edge of the Greenheugh pediment near Tower butte, thus accessing strata identified ~20 years ago as recording a major environmental transition in the history of Gale. The basal truncation surface of the pediment is the Siccar Point group (SPg) basal unconformity, which extends past Vera Rubin ridge (VRR) to the north. Above the unconformity lies a

International audience; The basal Siccar Point group unconformity observed at the Greenheugh pediment is a major regional break in sedimentation within Gale crater. Below the unconformity, mudstones and sandstones of the Murray formation records a predominantly lacustrine palaeoenvironment that was likely habitable. Overlying the unconformity is a distinctive fan-shaped, erosion-resistant, tabular stratal unit (~2-6 m thick) informally called the Greenheugh pediment capping unit. This unit has been identified as part of the Stimson formation, which encodes the signature of surfaces processes and the climate in Gale crater postdating the formation of the unconformity.Between Sols 2633 and 2781, the Curiosity rover was used to investigate the pediment capping unit, where detailed observations of sedimentary architecture, facies and sediment texture were obtained. Textural and grain-size analyses of the sandstone show that it comprises a well-sorted, medium-grained, well-rounded sandstone with an average grainsize of ~425 μm, which suggests aeolian transport processes The dominant sedimentary structure are cross-bedded sandstones composed of 2-4 mm thick, uniform-thickness cross-laminations, that are typical of wind ripple stratification. Architectural observations reveal that the unit is composed of compound cross-sets, with sets that are 0.2-0.7 m thick, that form cosets up to 2.5 m thick. These observations lead us to conclude that the pediment capping unit accumulated as the result of aeolian processes, where wind-blown dunes migrated across the unconformity, leaving preserved aeolian cross strata.Further to this, our observations of sedimentary texture, facies and architecture are consistent with Stimson formation sandstones observed in the Emerson plateau, Naukluft plateau and Murray buttes areas. We conclude that the pediment-capping unit is an up-slope equivalent of the Stimson formation, and that the Stimson formation drapes the unconformity over a ~500 m elevation range. The Stimson formation is the preserved expression of a dry aeolian dune field, that accumulated in a climate of extreme aridity, when Gale crater would likely have been devoid of surface water. The juxtaposition of desert deposits overlying lake deposits indicates a major change in palaeoclimate in Gale crater.

International audience; The Stimson formation (Siccar Point group) is recognized to represent a major change in depositional style and climate within Gale crater during the Hesperian period [1,2,3,4]. Deposition in Gale was dominated by aqueous processes in what is interpreted to be a relatively humid climate prior toStimson formation accumulation [5]. A depositional hiatus and some duration of erosion encapsulated by the base Siccar Point Group unconformity followed [6]. The deposition of the Stimson formation (currently the lowermost unit of the Siccar Point group) commenced in conditions apparently dominated by aeolian processes in an arid environment [1], subsequently the Stimson sandstones were lithified by an episode involving the flow of water within the subsurface [1,2,5,8].Internally, the Stimson formation records temporal changes of aeolian dune morphologies associated with changing dune size and sediment availability common across an evolving dune field, and changes of sediment transport directions, associated with fluctuating wind strength and direction at multiple temporal scales [3].Since Sol 2600, the Curiosity rover has been circumnavigating the Greenheugh pediment capping unit. This is a distinctive body of rock that covers an area of ~2.6 km2 (Fig.1) and occurs at the mouth of Gediz Vallis, a prominent canyon eroded into the lower slopes of Mt. Sharp. The capping unit is formed of Stimson formation strata [4]. The base of the capping unit is interpreted to form part of the base Siccar Point Group unconformity, which separates the strata from the underlying Mount Sharp group and the overlying Siccar Point group. Here we document changes in the stratigraphy and internal architecture of Stimson formation between the northern and southeastern flanks of the pediment capping unit to characterize variability in sedimentary geometries, facies, and sediment transport directions. Our observations provide constraints on the geologicand paleoclimate history of Gale crater.