Reconstruction of the palaeoenvironmental context of Martian sedimentary rocks is central to studies of ancient Martian habitability and regional palaeoclimate history. This paper reports the analysis of a distinct aeolian deposit preserved in Gale crater, Mars, and evaluates its palaeomorphology, the processes responsible for its deposition, and its implications for Gale crater geological history and regional palaeoclimate. Whilst exploring the sedimentary succession cropping out on the northern flank of Aeolis Mons, Gale crater, the Mars Science Laboratory rover Curiosity encountered a decametre‐thick sandstone succession, named the Stimson formation, unconformably overlying lacustrine deposits of the Murray formation. The sandstone contains sand grains characterized by high roundness and sphericity, and cross‐bedding on the order of 1 m in thickness, separated by sub‐horizontal bounding surfaces traceable for tens of metres across outcrops. The cross‐beds are composed of uniform thickness cross‐laminations interpreted as wind‐ripple strata. Cross‐sets are separated by sub‐horizontal bounding surfaces traceable for tens of metres across outcrops that are interpreted as dune migration surfaces. Grain characteristics and presence of wind‐ripple strata indicate deposition of the Stimson formation by aeolian processes. The absence of features characteristic of damp or wet aeolian sediment accumulation indicate deposition in a dry aeolian system. Reconstruction of the palaeogeomorphology suggests that the Stimson dune field was composed largely of simple sinuous crescentic dunes with a height of ca 10 m, and wavelengths of ca 150 m, with local development of complex dunes. Analysis of cross‐strata dip azimuths indicates that the general dune migration direction and hence net sediment transport was towards the north‐east. The juxtaposition of a dry aeolian system unconformably above the lacustrine Murray formation represents starkly contrasting palaeoenvironmental and palaeoclimatic conditions. Stratigraphic relationships indicate that this transition records a significant break in time, with the Stimson formation being deposited after the Murray formation and stratigraphically higher Mount Sharp group rocks had been buried, lithified and subsequently eroded.

Thrust imbrication of Ordovician and Silurian submarine fan sequences overlapping pelagic deposits in the Southern Uplands has been interpreted in terms of an accretionary prism formed above a NW-directed subduction zone. Structural features invoked to support accretion are not definitive and could be explained in terms of a thin-skinned thrusting model. New palaeocurrent and compositional evidence from Llandeilo to mid-Llandovery age turbidites in the northern part of the Southern Uplands proves interdigitation of sediments with strongly contrasting petrography. Turbidites derived from the south contain significant quantities of fresh andesitic detritus whereas those from the north form more mature quartz-rich formations. This implies a back-arc situation; the turbidites being deposited in a basin with a relatively mature continental landmass to the north and a rifted continental fragment containing an active volcanic arc to the south. Oblique collision of the opposing continental margins of the Iapetus Ocean during the Llandovery caused the cessation of subduction. Underthrusting of the southern margin initiated a SE-propagating thrust stack which deformed the back-arc basin sequence and may eventually have ramped over the eroded and faulted remains of the volcanic arc. A southward-migrating foreland basin formed ahead of the rising thrust stack and is now represented by the late Llandovery Hawick Group and Wenlock sequences. Mid- to end-Silurian sinistral strike-slip resulted from oblique collision and produced a transpressional regime during which reactivation of deep-seated structures allowed the intrusion of lamprophyre dykes and granites.

Lithified aeolian strata encode information about ancient planetary surface processes and the climate during deposition. Decoding these strata provides insight regarding past sediment transport processes, bedform kinematics, depositional landscape, and the prevailing climate. Deciphering these signatures requires a detailed analysis of sedimentary architecture to reconstruct dune morphology, motion, and the conditions that enabled their formation. Here, we show that a distinct sandstone unit exposed in the foothills of Mount Sharp, Gale crater, Mars, records the preserved expression of compound aeolian bedforms that accumulated in a large dune field. Analysis of Mastcam images of the Stimson formation shows that it consists of cross-stratified sandstone beds separated by a hierarchy of erosive bounding surfaces formed during dune migration. The presence of two orders of surfaces with distinct geometrical relations reveals that the Stimson-era landscape consisted of large dunes (draas) with smaller, superimposed dunes migrating across their lee slopes. Analysis of cross-lamination and subset bounding surface geometries indicate a complex wind regime that transported sediment toward the north, constructing oblique dunes. This dune field was a direct product of the regional climate and the surface processes active in Gale crater during the fraction of the Hesperian Period recorded by the Stimson formation. The environment was arid, supporting a large aeolian dune field; this setting contrasts with earlier humid depositional episodes, recorded by the lacustrine sediments of the Murray formation (also Hesperian). Such fine-scale reconstruction of landscapes on the ancient surface of Mars is important to understanding the planet's past climate and habitability.Plain Language Summary Sedimentary rocks formed from sediments transported and deposited by the wind provide valuable information about the ancient environment, such as climate, wind direction, and the types of desert landforms that were present. This study focuses on the windblown sediments, now sandstone, imaged using the Mastcam instrument onboard the Mars Science Laboratory rover, Curiosity, at the Murray buttes inside Gale crater, between the 1383rd and 1455th Martian days (sols) of the mission. Analysis of the sedimentary structures in images shows that the Stimson formation at the Murray buttes was deposited by the wind in the form of compound sand dunes. These were large dunes with smaller dunes migrating across their surfaces. Analysis of the sedimentary structures generated by the complex interaction of these two scales of dune indicates that the large dunes migrated north, and that the smaller superimposed dunes migrated across the faces of the large dunes toward the northeast. The presence of large, wind-driven dunes indicates that the region was extremely arid, and that-at the time the Stimson dune field existed-the interior of Gale crater was devoid of surface water, unlike the setting recorded by the older, underlying lake sediments of the Murray f...

Temporal fluctuations of wind strength and direction can influence aeolian bedform morphology and orientation, which can be encoded into the architecture of aeolian deposits. These strata represent a direct record of atmospheric processes and can be used to understand ancient Martian atmospheric processes as well as those on Earth. The strata can: give insight to ancient atmospheric circulation, how the atmosphere evolved in response to global changes in habitability, and how ancient processes differ from modern processes. The Stimson formation at the Greenheugh pediment (Gale crater) records evidence of fluctuating wind across multiple temporal scales. The strata can be subdivided into three intervals-Gleann Beag, Ladder, and Edinburgh intervals. Internally, the intervals record changes of dune morphology and orientation, correlatable to wind fluctuations at multiple temporal scales. The basal Gleann Beag interval comprises compound cross-strata, deposited by oblique compound dunes. These dunes record a bimodal wind regime, resulting in net sediment transport toward the north. The Ladder interval records a reversal of sediment transport to the south, where straight-crested simple-dunes shaped by a seasonally variable winds formed. Finally, the Edinburgh interval records sediment transport to the west, where a unimodal wind formed sinuous-crested simple dunes. These observations demonstrate active and variable atmospheric circulation in Gale crater during the accumulation of the Stimson dune field, at multiple temporal scales from seasonally driven winds to much longer time-frames, during the Hesperian. These observations can be used to further understand ancient atmospheric conditions and processes, at a high temporal resolution on Mars.

The timing of deformation in the Southern Uplands is constrained by tectonostratigraphical and intrusive relationships. Deformation migrated diachronously southwards, affecting the southern part of the Southern Uplands in the mid-Silurian. In contrast deformation in the Lake District cacan be shown by the same criteria to be Early Devonian. Thus, since the possibility of post-Emsian strike-slip along the Iapetus suture is remote, the regional tectonism cannot be related to a climactic episode due simply to closure of the Iapetus Ocean. Instead a southeast migrating deformation front is envisaged linking Silurian closure of Iapetus and the Southern Uplands backarc basin with Early Devonian collision farther south. The timing and geometry of the main cleavage-producing deformation within the paratectonic Caledonides of northern Britain is generally debated in terms of closure of the Iapetus Ocean. Structures developed are related to accretion of a collage of terranes onto the southern margin of Laurentia, within a sinistral transpressive stress regime, during the collision of the three plates Laurentia, Baltica and Eastern Avalonia (Soper & Hutton 1984). However, within this overall model there is a considerable divergence of opinion as to the chronology and correlation of the main episode of deformation across the Iapetus suture collision zone. In north-west England and Wales, recent regional syntheses propose that the main structures were formed during an end-Silurian/Early Devonian collision and subsequent NW-directed underthrusting. This climaxed with the main cleavage-producing episode in the Emsian ( Soper et al. 1987 ; McKerrow 1988 ) which has been correlated with the Acadian Orogeny of the