Geologic overview of the Mars Science Laboratory rover mission at the Kimberley, Gale crater, Mars

The Mars Science Laboratory (MSL) Curiosity rover completed a detailed investigation at the Kimberley waypoint within Gale crater from sols 571–634 using its full science instrument payload. From orbital images examined early in the Curiosity mission, the Kimberley region had been identified as a high‐priority science target based on its clear stratigraphic relationships in a layered sedimentary sequence that had been exposed by differential erosion. Observations of the stratigraphic sequence at the Kimberley made by Curiosity are consistent with deposition in a prograding, fluvio‐deltaic system during the late Noachian to early Hesperian, prior to the existence of most of Mount Sharp. Geochemical and mineralogic analyses suggest that sediment deposition likely took place under cold conditions with relatively low water‐to‐rock ratios. Based on elevated K2O abundances throughout the Kimberley formation, an alkali feldspar protolith is likely one of several igneous sources from which the sediments were derived. After deposition, the rocks underwent multiple episodes of diagenetic alteration with different aqueous chemistries and redox conditions, as evidenced by the presence of Ca‐sulfate veins, Mn‐oxide fracture fills, and erosion‐resistant nodules. More recently, the Kimberley has been subject to significant aeolian abrasion and removal of sediments to create modern topography that slopes away from Mount Sharp, a process that has continued to the present day.


Introduction
The search for habitable environments on Mars requires integrated observations of stratigraphy, sedimentology, geochemistry, mineralogy, geomorphology, isotopic ratios, and organic compounds to unravel past environmental conditions. The Mars Science Laboratory (MSL) Curiosity payload [Grotzinger et al., 2012] is uniquely qualified to perform such an integrative investigation. Early in the mission the rover successfully characterized the habitability of an ancient, shallow lacustrine system in Gale crater at Yellowknife Bay . At this location, Curiosity detected organic molecules preserved in the Cumberland drill sample within the Sheepbed mudstone [Freissinet et al., 2015], which had been deposited in moderate to neutral-pH waters with low salinity and variable redox states, indicating an environment that could have been habitable to chemoautotrophic microorganisms . (The names Yellowknife Bay, Cumberland, Sheepbed, and other names assigned to rock targets and waypoints are informal [e.g., Vasavada et al., 2014]). Geochemical observations show that Yellowknife Bay rocks were derived from basaltic sources and that postdepositional aqueous alteration took place under arid and possibly cold paleoclimates [McLennan et al., 2014]. The young cosmogenic exposure age of the Sheepbed mudstone, combined with its location at the base of a retreating scarp in the overlying Gillespie sandstone, suggests that the organics in the Cumberland sample had been protected from degradation by high-energy radiation until the overlying outcrop was removed relatively recently (~80 Ma) by erosion [Farley et al., 2014[Farley et al., ]. et al., 2014. After leaving Yellowknife Bay, Curiosity drove southwest toward Aeolis Mons (informally called "Mount Sharp," a~5 km high mound of sedimentary rock) across the Bradbury Rise area of Aeolis Palus (the plains surrounding Aeolis Mons), along a path called the "Rapid Transit Route" (RTR) that offered the fastest progress across safely traversable terrain [Vasavada et al., 2014]. Along the RTR, Curiosity stopped to perform in situ analyses at three "waypoints," informally named Darwin, Cooperstown, and the Kimberley (Figure 1a). These waypoints were selected from orbital images as locations near the planned traverse that had clear exposures of Aeolis Palus stratigraphy [Vasavada et al., 2014;Stack et al., 2016].
To expedite the arrival at Mount Sharp, the MSL team agreed to consider drilling at only one of these three waypoints. The Kimberley was selected for a drill campaign based on its clear stratigraphic relationships in a layered sedimentary sequence that had been exposed by differential erosion (Figure 1c). In images from the High-Resolution Imaging Science Experiment (HiRISE) camera on board the Mars Reconnaissance Orbiter, some outcrops exposed at the Kimberley and distributed in patches over an~8 km 2 area of the crater floor were observed to contain regularly spaced, northeast-southwest oriented striations that had not been encountered previously on Aeolis Palus [Stack et al., 2016]. This enigmatic unit has been termed the Orbital Striated Outcrop (OSO), and the striations show a remarkable consistency in their strike direction across the~8 km 2 area over which the unit is exposed . At the Kimberley, an exposure of OSO crops out beneath a light-toned, scarp-and butte-forming unit termed the Rugged Unit (RT), which was in contact with a darker-toned unit with undulating topography termed the Hummocky Plains Unit (HP) [Stack et al., 2016] (Table 1). A prominent scarp in the RT unit was observed in the same orientation as the active Sheepbed-Gillespie scarp in Yellowknife Bay (~152°clockwise from north; Figure 1b), and the underlying OSO appeared less dusty than surrounding outcrops (as indicated by relatively bluer hues in the HiRISE infrared-red-blue (IRB) false-color images [Delamere et al., 2010]); together, these observations suggested that scarps in the RT could be actively eroding and exposing relatively fresh surfaces. Therefore, the Kimberley offered an excellent opportunity to test the sampling strategy developed at Yellowknife Bay and to continue the characterization of habitability and search for organic molecules within sedimentary rocks at Gale crater.
Curiosity approached the Kimberley waypoint and performed initial reconnaissance during sols 535-570, surveyed and characterized the units at the Kimberley during sols 571-609, completed a drilling and sampling campaign at the target Windjana during sols 610-629, and departed the Kimberley during sols 630-634. Key results of the rover's geologic investigation during these periods are presented in this special section of Journal of Geophysical Research-Planets [Le Deit et al., 2016;Lasue et al., 2016;Litvak et al., 2016;Mangold et al., 2016;Thompson et al., 2016;Treiman et al., 2016;Vasconcelos et al., 2016] and elsewhere [e.g., Grotzinger et al., 2015;Lanza et al., 2016]. This introduction provides the context for specific findings at the Kimberley and integrates them together into a broader understanding of the geologic history and past habitability of Gale crater. In the following sections, we present an overview of the science campaign at the Kimberley and summary of key observations (section 2); implications for depositional environments (section 3) and sediment provenance (section 4); the history of diagenetic alteration (section 5); ongoing erosion and landscape evolution (section 6); and conclusions and implications for habitability (section 7). Although Curiosity also performed a suite of environmental monitoring and atmospheric characterization experiments at the Kimberley, the work presented here only includes results from the geological investigation.   [Vasavada et al., 2014].

Goals for the Kimberley Waypoint Campaign
A major goal of Curiosity's investigation at the Kimberley was to document the stratigraphy and sedimentology with remote sensing (Mast Cameras (Mastcam) and Chemical Camera (ChemCam)) and contact science (Alpha-Particle X-ray Spectrometer (APXS) and Mars Hand Lens Imager (MAHLI)) observations to determine depositional environments, stratigraphic relationships, diagenesis, and erosional history. In particular, determining the origin of the enigmatic OSO and understanding its connection to other Aeolis Palus strata was a high priority. Another key goal was to identify the sedimentary facies with the highest probability for preserving organic molecules and to drill a sample to deliver to Curiosity's analytical instruments (Chemistry and Mineralogy (CheMin) and Sample Analysis at Mars (SAM)). A drill target was sought in a fine-grained sedimentary unit that might have been deposited in a habitable environment and also was at the base of a retreating scarp indicating recent exposure from beneath several meters of overburden. The site also needed to meet the engineering criteria for drilling, which included a rover placement with all six wheels on a solid surface with no loose rocks, a rover tilt <7°, a rover geometry with respect to local topography that would allow for good illumination conditions within the workspace in front of the rover, and a clear line of sight to the orbital spacecraft used for communication.
From the perspective of mission operations, an additional goal was to execute the drilling and sampling activities as efficiently as possible in a constrained number of sols, making use of orbital imagery, mapping, and strategic planning to guide the selection of a drill target. Previously, Curiosity had collected only two drill samples-John Klein and Cumberland-adjacent targets in the Sheepbed mudstone in Yellowknife Bay; the Kimberley would be the first drill campaign acquired "on the road," and the only planned drill location before reaching lower Mount Sharp. Therefore, the Kimberley campaign was treated as a "dress rehearsal" for efficient sampling and characterization of habitability at Mount Sharp. Key observations to be made by each instrument at the Kimberley were prioritized by the science team, in discussions led by the campaign  [Vasavada et al., 2014], prior to arrival at the waypoint (Table 2). These preapproved observations were used to guide the tactical planning process during the campaign, but additional science observations were added to each sol's plan opportunistically as time, power, data volume, and other planning constraints allowed.

Approach to the Kimberley Waypoint, Sols 535-570
On the approach to the Kimberley, bedrock exposures became more frequent in a set of shallow valleys, providing the first opportunities to observe the OSO and other units exposed at the Kimberley. During this approach, the team established a goal of commanding a drive on every possible planning cycle. To  accommodate this schedule, several planning sols were designated as "drive-only sols," with minimal staffing from the science team and instrument observations mostly focused on documenting the traverse with Navcam, Mastcam, MAHLI (from its stowed position), and MARDI. During these sols, Mastcam mosaics were typically the only targeted observations added to support the rover's geologic investigation. When not in the "drive-only" cadence, regular documentation activities were performed with the ChemCam and Dynamic Albedo of Neutrons (DAN) instruments to provide constraints on elemental chemistry without having to deploy the arm. Brief opportunistic science observations with the arm instruments (APXS and MAHLI) were sometimes possible during the approach period, especially within 3 sol weekend plans. Along the traverse, the rover also acquired systematic images of its wheels using MAHLI, in order to document ongoing damage to the wheels that had been documented since sol~400 [Vasavada et al., 2014]. Details of drive distances and  Table 3, and details of geologic observations from each instrument are given in Table S1.
While Curiosity encountered the OSO briefly at the Balmville outcrop on sol 513, the rover's approach was not close enough to interpret the sedimentary structures [Edgar et al., 2014]. A short time later, on sol 535 (6 February 2014), Curiosity crossed over a transverse aeolian ridge at Dingo Gap (Figures 2 and 3a) in order to enter a narrow valley. Curiosity acquired several large Mastcam mosaics of the sedimentary section exposed in the valley walls as well as in situ observations of the sandstone outcrop targets Halls and Fitzroy on sol 537. Following the RTR, Curiosity continued through Moonlight Valley and made a close approach to the OSO at the Junda outcrop on sol 547, where it first was observed that the regular striations observed from orbit could be attributed to individual beds in a sandstone facies dipping 10-20°to the south. From this point forward, the OSO is referred to as "clinoform sandstones" where observed on the ground . Kylie had previously been considered as an alternate waypoint to the Kimberley, but the science team determined that the stratigraphic contacts at the Kimberley were more clearly exposed, and the outcrops in general were less dust covered. The rover drove toward Kylie on sol 552, and over the subsequent 3 sols captured large Mastcam mosaics of outcrop exposed in the valley walls. These images revealed clinoform sandstones that could not be observed from orbit and a complex interfingering of sandstone and conglomerate facies . At the rover's closest approach to Kylie on sol 554, the first images of the RT unit overlying the clinoform sandstones were acquired, revealing subhorizontal bedding and the presence of cm scale nodules ( Figure 3c).
The traverse curved to the southeast after Kylie to avoid the steep cliffs directly between Kylie and the Kimberley. On sols 560 and 564, Curiosity acquired contact science observations of dark, vuggy float rocks (Secure and Monkey_Yard) that might have been remnants of the formerly more extensive capping unit at the tops of cliffs and mesas in the vicinity. On the final approach to the Kimberley, during a series of driveonly sols, Mastcam acquired mosaics of the strata exposed in cliffs to the north of the Kimberley and at Jurgurra, a large outcrop of clinoform sandstones ( Figure 3d).

Geologic Characterization at the Kimberley, Sols 571-609
Curiosity acquired its first long-range reconnaissance imaging toward the Kimberley on sol 571. The rover arrived at the north side of the Kimberley on sol 574, approaching a prominent rock ridge that had been identified from orbit as clearly exposed and relatively dust free (as indicated by the blue hues in the HiRISE falsecolor images; Figure 1c). Near the target Square_Top along this ridge, the rover experienced a remote sensing mast fault on sol 575 and an arm fault on sol 578, which kept Curiosity at this location for four additional sols, during which extended imaging of the Kimberley from the north was possible (e.g., Figures 4a and 4b).
Following fault recovery and diagnostics, Curiosity bumped forward on sol 581 to perform an extended contact science campaign to characterize the Square_Top strata (Square_Top and Virgin_Hills targets) and the underlying granule conglomerates (Panadus_Yard). After this target, the clinoform sandstone at the Kimberley was named the Square_Top member of the Kimberley formation Le Deit et al., 2016]. These observations from sols 583 to 585 show that the clinoform sandstones are composed of fine sands with dispersed coarser grains (from 0.5 to 3.1 mm in diameter) . After leaving the Square_Top location, Curiosity traversed east and imaged a pebble conglomerate outcrop named Point_Coulomb. These conglomerates are consistently observed at the base of the stratigraphic section and have been collectively termed the Point_Coulomb member of the Kimberley formation [Mangold et al., 2016;Le Deit et al., 2016].
On sol 588, the team celebrated the "arrival" at the Kimberley, after rounding the eastern side of the Mount Joseph butte (the team had designated this spot as the "entrance" to the Kimberley outcrops; however, in this paper we designate sol 574's drive to Square_Top as the arrival at the Kimberley, as this was Curiosity's first  Table 3) to the side of the rover to document variations in texture, grain size, and bedding geometries.
On sol 601, Curiosity examined the unit underlying the clinoform sandstones at the Liga target, where MAHLI observations revealed inclined, interstratified granule-and pebble-rich beds and sandstone beds with poorly sorted, subangular to subrounded grains as large as 4.7 mm in diameter . This unit, which lies stratigraphically between the Point_Coulomb and Square_Top members, has been termed the Liga Member of the Kimberley formation Le Deit et al., 2016]. From this location, Curiosity also acquired its first close look at the unit overlying the clinoform sandstones, which had been previously glimpsed at Kylie and mapped from orbit as the RT unit. Mastcam mosaics indicated that this unit was thinly bedded, fine-grained, internally laminated, and contained dark-toned, erosionally resistant fracture fills. The apparently structureless material of the overlying butte, Mount Remarkable, was observed to have a small scarp at its base and dark-toned, erosionally resistant boulders on its flanks that are likely remnants of a former more resistant unit that is now eroded from the top of Mount Remarkable. Taken together, these observations suggest that Mount Remarkable may represent a former mesa in its final stages of erosion. The morphologic similarity of the Mount Remarkable scarps to the actively eroding scarp studied previously at Yellowknife Bay [Farley et al., 2014] suggests that the underlying outcrops may have been exposed on comparably recent timeline (<100 Ma).
On sol 603, the team discussed options for drilling at the Kimberley and decided that the RT unit beneath Mount Remarkable would be the best candidate, given its fine grain size and hypothesized recent exposure. However, the closest RT unit to the rover at this point would have required a two-sol traverse across a stretch of fractured clinoform sandstones in order to position the rover for drilling, which raised concerns about ongoing wheel damage [Vasavada et al., 2014]. The science team opted instead to traverse~75 m across smoother terrain to an alternate drilling location beneath the southeastern flank of Mount Remarkable (Figures 1c and 4c). Curiosity made this traverse in two drives on sols 603 and 606, acquiring several Mastcam mosaics for context. On sol 608, ChemCam acquired the first analyses in the vicinity of the drill target, marking the start of an extended campaign to characterize the RT unit. The RT unit has since been termed the Dillinger member of the Kimberley formation after one of the ChemCam targets.

Activities at the Windjana Drill Site, Sols 610-629
Along an outcrop of the Dillinger member on the southeast side of Mount Remarkable, the target Windjana was selected as having the best geometry for drilling. Predrill contact science observations (the dust-removal tool: DRT, MAHLI, and APXS) were performed on sol 612 to vet the chemistry and texture of the desired drill target, and a self-portrait of Curiosity at the drill site was taken with MAHLI on sol 613 to document the geologic context and the complete sampling activities at a drill site. A 20 mm deep test drill ("mini-drill") was successfully performed on sol 615 to assess the suitability of the target for drilling as well as sample delivery to  The 65 mm full drill hole ( Figure 4d) produced a sample of powdered rock at Windjana on sol 621 that was delivered to the CheMin instrument for analysis on sol 623. The CheMin X-Ray Diffraction (XRD) pattern constrained the mineralogy of the K-rich sediment component as sanidine and provided little evidence for low-temperature aqueous alteration, as described by Treiman et al. [2016]. The Windjana sample was delivered to the SAM instrument for an evolved gas analysis (EGA) experiment on sol 624. The drive away from the Windjana location was initially planned for sol 629, but in response to the first ChemCam and APXS measurements of the fracture-fill target Stephen, which revealed surprisingly high Mn concentrations, revealing apparent Mn oxides [Lanza et al., 2016], the departure was delayed to allow for an additional sol of contact science of this target.

Stratigraphy and Depositional Environments at the Kimberley
The outcrops encountered at the Kimberley appear to show a fining-upward succession: pebble conglomerates at the base of the Kimberley formation (Point_Coulomb member), granule conglomerates and The south dipping Square_Top member sandstones have been interpreted as small-scale delta clinoforms . The presence of large grains (up to 3.1 mm in diameter) observed in MAHLI observations at Square_Top requires transport by water rather than by wind. The underlying conglomerates of the Liga and Point_Coulomb members are also interpreted as being formed by migrating subaqueous bed forms or barforms in a fluvial environment, similar to the interpretation of conglomerates observed early in Curiosity's mission prior Yellowknife Bay [Williams et al., 2013]. The rounding of clasts within the conglomerates suggests transport distances of tens of kilometers [Szabó et al., 2015], which is consistent with deposition in an alluvial fan environment and transport from Gale crater's northern rim (section 4).
The Dillinger member forms a subhorizontal stratal unit of very fine-grained sandstone or siltstone (with grains at or below the grain size detection limit of MAHLI ) that is approximately 0.5 m thick and overlies the Square_Top member across a distinct truncation surface . Analysis of multiple Mastcam mosaics demonstrates that the Dillinger member is characterized by the presence of undulose lamination-forming mounded bed geometries; similar bedset geometries have been described from a number of ancient terrestrial fluvial examples and linked to conditions characterized by abrupt discharge variability [Gupta et al., 2014].
Multiple sediment transport directions have been reconstructed from cross-bed dip directions that suggest an ancient fluvial (or possibly mixed fluvial-aeolian) system with a primarily southwesterly flow . Some observations of sets of cross beds in the Dillinger show a reverse in dip between north and south, and these beds may have been deposited by small aeolian dunes when lake levels were low [Rubin et al., 2015], indicating that lake levels would have fluctuated over the period of sediment deposition at the Kimberley, allowing for intermittent periods of subaerial exposure.
Similar conglomerate, clinoform sandstone, and cross-bedded sandstone facies to those of the Kimberley formation have been observed in the valley walls near Dingo Gap [Edgar et al., 2014], at Kylie , and in cliff faces immediately north of the Kimberley. The same facies observed at multiple locations across Curiosity's~1.5 km traverse during the Kimberley campaign, over an elevation gain of~12 m, do not represent the same stratal units, but rather distinct units that were stacked during fluvial progradation, which is commonly the case in terrestrial fluvial-lacustrine deposits . Where observed in cross section in cliffs and valley walls, the clinoform sandstones are 1 to 4 m thick, suggesting water depths shallower than a few tens of meters .
The interpretation of the clinoform sandstones as having been primarily transported from the north and deposited in small deltas implies a transition to lacustrine facies within the basin was likely to occur farther to the south. Curiosity tested this hypothesis along its continued traverse southwest at Pahrump Hills, where thinly laminated lacustrine mudstones were indeed encountered within the Murray formation , further validating a fluvio-lacustrine interpretation for the depositional environment at the Kimberley.
The geochemistry of the Kimberley formation, as revealed by analyses of APXS [Thompson et al., 2016] and ChemCam [Le Deit et al., 2016] observations, shows that little alteration of the primary igneous minerals in the sediments occurred during deposition. The CheMin analyses described by Treiman et al. [2016] show that the Windjana drill sample contains only igneous minerals and their low-temperature alteration products: sanidine (21% weight,~Or 95 ); augite (20%); magnetite (12%); pigeonite; olivine; plagioclase; amorphous and smectitic material (~25%); and percent levels of others including ilmenite, fluorapatite, and bassanite. From mass balance on the APXS analysis of the Windjana drill tailings, the amorphous component is Fe rich with almost no other cations (e.g., ferrihydrite). There are no detectable concentrations of minerals observed by CheMin that would suggest metamorphism or hydrothermal alteration, and no minerals that are characteristic of most chemical sediments (e.g., carbonates or silica) except iron oxyhydroxides . The Dillinger member sandstone, therefore, is likely cemented by magnetite and ferrihydrite. The low-albedo and shallow blue-to-red spectral slope of the Windjana drill tailings, as observed in quantitative visible/nearinfrared reflectance spectra from ChemCam passive and Mastcam multispectral observations, are consistent with the presence of these mineral phases [Johnson et al., 2016;Wellington et al., 2016].

10.1002/2016JE005200
Observations from the DAN instrument on the lower members of the Kimberley formation (Dillinger, Square_Top, Liga and Point_Coulomb members) confirm that there is very little water-equivalent hydrogen in these outcrops, consistent with a lack of hydrated minerals from aqueous alteration. Litvak et al. [2016] describe how DAN observations at the Kimberley are best described by a two-layer model with~2.3% water in the top 15 cm and 1.4% water in the bottom layer. This inferred hydration in the upper layer is consistent with estimates from SAM EGA analyses of the Windjana sample [Litvak et al., 2016]. DAN also measured variations in neutron-absorbing elements throughout the stratigraphy of the Kimberley formation, observing a general decrease in a modeled parameter called the chlorine-equivalent concentration with increasing stratigraphic position, consistent with measurements of Cl made by APXS at the Kimberley [Litvak et al., 2016].
Collectively, these data suggest that weathering, transport, and diagenesis of the sediment did not occur in a warm and wet environment, but under low temperatures and low water-to-rock ratios and/or low oxygen fugacity. Thompson et al. [2016] argue further that water-to-rock ratios may have been too low to explain the entire deposition of the Kimberley formation by fluvio-deltaic processes, and they suggest some sediment may have been transported into Gale crater via glacial/periglacial processes in a cold, icy environment [e.g., Le Deit et al., 2013;Fairén et al., 2014;Oehler et al., 2016]. While some of the conglomerates observed at the Kimberley (targets Bungle_Bungle and Jum_Jum) show particle characteristics (sizes, shapes, fabrics, compositions, and sorting) consistent of supraglacial till, no clear sedimentological evidence for deposition in a glacial environment (e.g., dropstones, frost wedges, or tills) has been observed at the Kimberley or elsewhere along the rover traverse.

Sediment Provenance
The interpretation of the south dipping sandstones at the Kimberley as having been deposited in small, southward prograding deltas requires that sediment was transported from higher elevations in the north, likely from the north rim and walls of Gale crater. The sedimentary rocks that Curiosity had previously examined at Yellowknife Bay and on Bradbury Rise were also sourced from the north crater rim [e.g., McLennan et al., 2014;Palucis et al., 2014], but the composition of rocks at the Kimberley suggests differences in provenance. As described by Mangold et al. [2016], the conglomerate outcrops observed by ChemCam on Bradbury rise (prior to sol 540) have a felsic alkali-rich composition (with a Na 2 O/K 2 O > 5), whereas those observed during the Kimberley campaign have an alkali-rich potassic composition with Na 2 O/K 2 O < 2. ChemCam observations of the other members of the Kimberley formation are described in detail by Le Deit et al. [2016] and suggest an overall mean K 2 O abundance that is significantly greater than had been previously observed by Curiosity, and more than 5 times higher than that of the average Martian crustal value of 0.45 wt % [Taylor and McLennan, 2009]. This K 2 O abundance generally increases upward through the stratigraphic section or is more enriched in finer-grained sediments ( Figure 5) [Le Deit et al., 2016;Thompson et al., 2016]. APXS observations show that high potassium content is associated with elevated abundances of iron, magnesium, manganese, and zinc and diminished abundances of sodium, aluminum and silicon, relative to average Mars [Thompson et al., 2016].
CheMin analyses show that the abundant potassium in the Windjana sample is contained nearly entirely in the K-rich feldspar sanidine . Although alkali-rich igneous rocks have been previously encountered in several places in Gale crater [Sautter et al., 2015] and are present in the Martian meteorite NWA 7034 [e.g., Santos et al., 2015;Wittmann et al., 2015], felsic rocks on Mars appear so far to be rare [e.g., Sautter et al., 2016]. Alkali feldspars are notably difficult to observe in the infrared bands, essentially only appearing when Fe substitutes for Ca [e.g., Serventi et al., 2013]. To date, no alkali feldspars have been confirmed from orbit; the limited identification of feldspars made by the orbital Compact Reconnaissance Imaging Spectrometer for Mars instrument is most consistent with anorthitic plagioclase [Carter and Poulet, 2013;Wray et al., 2013], and no alkali feldspars have been uniquely detected from Mars Global Surveyor Thermal Emission Spectrometer observations [e.g., Bandfield, 2002;Rogers and Hamilton, 2015;Rogers and Nekvasil, 2015]. Therefore, CheMin's analysis at Windjana is the first mineralogical detection of an alkali feldspar on Mars and is an opportunity to gain new insights into the planet's magmatic evolution. Treiman et al. [2016] argue that the alkaline magmas could have been low volume partial melts of metasomatized mantle, in which case Mars would have had a differentiating and (probably) fluid-bearing mantle before the melting, eruption, and crystallization of igneous rocks on Gale crater's north rim, which Journal of Geophysical Research: Planets 10.1002/2016JE005200 has been dated to 4.21 AE 0.35 Ga [Farley et al., 2014]. Few alkaline igneous rocks of this age are known on Earth, suggesting that the Martian mantle had differentiated far earlier than the Earth, which is consistent with thermochemical models of Mars' early mantle [Baratoux et al., 2013;Filiberto and Dasgupta, 2015].
The chemistry at the Kimberley contained additional surprises based on ChemCam data, including the detection of fluorine in a number of observations [Forni et al., 2015a]. A large fraction of these observations come from the Kimberley, where F abundances were observed in the range of 0.2 to 0.8 wt %, and were correlated with K, Al, Si, and Mg, in decreasing order [Forni et al., 2015b]. Investigation of the chemical trends of the targets with the highest potassium yielded predictions of phlogopite [Treiman and Medard, 2016], consistent with the fluorine observations.
Unfortunately, attempts to refine the crystallization ages of the Kimberley sediments using SAM data from Windjana were unsuccessful, as described by Vasconcelos et al. [2016]. An aliquot heated to~915°C yielded a K-Ar age of 627 AE 50 Ma, and reheating the same aliquot to verify complete extraction yielded no additional Ar. A second aliquot heated in the same way yielded a much higher K-Ar age of 1710 AE 110 Ma; these discordant data are most readily understood as arising from incomplete Ar extraction from a rock with a K-Ar age older than 1710 Ma. Most likely, however, the protoliths for the Kimberley sedimentary rocks have similar crystallization ages as those elsewhere on the north rim of Gale crater, which are Noachian (~4.2 Ga) [Farley et al., 2014;Le Deit et al., 2013;Thomson et al., 2011].
In addition to the potassium-rich alkali feldspar source, ChemCam observations suggest at least two additional bedrock petroliths for the Kimberley sediments: (1) a component is rich in mafic minerals, with little feldspar (similar to a shergottite); and (2) a component is richer in plagioclase and in Na 2 O, likely to be basaltic . The presence of significant percentages of easily weathered primary igneous minerals such as olivine, pyroxene, and pyrrhotite, and the absence of any hydrous silicate minerals indicates that significant water-based alteration did not occur at the sediment source or during cementation. Instead, the mixtures of distinct igneous petroliths are the likely origin of observed chemical variations through the stratigraphic section [Le Deit et al., 2016;Mangold et al., 2016;Thompson et al., 2016;Treiman et al., 2016]. Physical sorting of fine potassium feldspar grains into the finest-grained sediments may have also played a role in the enrichment in K 2 O in the Kimberley formation, although this still requires a distinct igneous source of potassium feldspar since other mineral sorting trends in the Bradbury group segregate coarse feldspar grains from fine-grained mafic minerals [Le Deit et al., 2016;Mangold et al., 2016;Siebach et al., 2015;Thompson et al., 2016]. The presence of sediment from multiple igneous sources, in concert with Curiosity's previous identifications of other igneous materials (e.g., the mugearite rock Jake Matijevic [Sautter et al., 2014;Stolper et al., 2013]), implies that the northern rim of Gale crater exposes a diverse igneous complex, at least as diverse as those found in similar-age provinces on Earth [Mangold et al., 2016;Treiman et al., 2016].
Elevated trace element concentrations in the Dillinger unit relative to surrounding units imply a mechanism for concentrating these elements, whether by integration of particles from a concentrated source rock (sediment provenance) or by later fluid flow through the Kimberley units (diagenesis). Thompson et al. [2016] describe elevated Zn, Ge, and Cu detected by APXS in the Windjana drill tailings, and Lasue et al. [2016] describe the confirmation by ChemCam of elevated Zn (averaging about 0.2 wt % but present up to 8.4 wt %) in up to 20% of targets within the Dillinger member, independent of rock texture. Lasue et al. [2016] propose that the Zn could be in the form of Zn-oxides; however, Lanza et al. [2016] show that Zn is not correlated with the occurrences of Mn-oxides observed at the Kimberley (section 5). A lack of sulfur suggests the Zn phase is not sphalerite, and correlation with Na 2 O suggests it could be in an amorphous clay such as sauconite [Lasue et al., 2016]. Lasue et al. [2016] and Thompson et al. [2016] suggest that localized hydrothermal alteration in the source rocks at the northern rim of Gale could create trace-metal-enriched sediment that then contributed to the Dillinger formation, although no hydrothermal minerals are present at the detection limit of CheMin in the analysis of Windjana. Thompson et al. [2016] and Lanza et al. [2016] discuss very significant trace-metal enrichment along diagenetic fractures within the Dillinger sandstone, so it is also possible that the trace metals were concentrated in the Dillinger during cementation and later diagenetic processes.

Diagenetic Alteration
Although the Windjana sample likely experienced only limited chemical alteration or weathering, given its abundance of easily altered minerals like plagioclase, olivine, pyrrhotite, and sanidine ,   and are observed near Windjana (Figure 4f). Similar lighttoned veins were observed in Yellowknife Bay and near the Kimberley and beyond [e.g., Kronyak et al., 2015;Nachon et al., 2014]. The Windjana sample contains small amounts (near detection limits) of the calcium sulfates bassanite and anhydrite, which are likely related to the Ca-sulfate veins in the Windjana area, although no veins are observed within MAHLI observations of the drill hole walls.
In addition to the light-toned veins, the Dillinger member also contains dark-toned, erosionally resistant fracture fills with high-Mn abundances (>25 wt % MnO), as observed at the ChemCam targets Stephen, Neil, and Mondooma (e.g., Figure 4d). The Mn in these targets correlates with trace metal abundances but not with other elements such as C, Cl, and S, which suggests that these deposits are Mn oxides rather than other salts or evaporites [Lanza et al., 2016]. This indicates that after the strata were deposited, lithified, and fractured, highly oxidizing fluids moved through the fractures and precipitated the Mn-oxide veins. Based on the strong association between Mn-oxide deposition and changing atmospheric O 2 levels on Earth, Lanza et al. [2016] argue that the presence of these Mn phases at the Kimberley means that there were higher concentrations of molecular oxygen within the atmosphere and some groundwaters of Mars in the past. No cross-cutting relationships have been observed between the light-toned Ca-sulfate veins and the Mn-oxide materials, and thus we are unable to place constraints on the relative timing of the two fracture-filling diagenetic events. However, it is clear that at least two distinct fluid-flow events with different aqueous chemistries had occurred within the Dillinger member.
At some exposures of the Dillinger and Mount Remarkable members at the Kimberley (Figure 4f), and in similar facies at Kylie (Figure 3c inset), Mastcam images of the outcrops show spherical protrusions that are relatively uniform in size and dispersed with varying spatial distributions . These nodules appear similar to those seen previously in Yellowknife Bay, although they are significantly larger (cm scale at the Kimberley, as opposed to < 5 mm in the Sheepbed mudstone). The Sheepbed nodules have a higher Fe content than the surrounding outcrops and have been interpreted as concretions formed during early diagenesis [Stack et al., 2014], possibly by the alteration of olivine and/or glass [Schieber et al., 2016]. Unfortunately, compositional data for the nodules at the Kimberley could not be acquired as the rover did not come within the~5 m distance to these concretions required for obtaining high-quality ChemCam data, and a definitive origin for these features cannot be determined.

Landscape Evolution
The interpretation of the Kimberley formation as having formed at the margins of a rising lake implies that the downhill direction must have been to the south during the time of sediment deposition. The current topography slopes to the north, in the direction away from Mount Sharp; therefore, there was once a basin where there is now a mountain, and deposition of the Kimberley formation would have to predate the deposition of the bulk of Mount Sharp. In this model, a substantial amount of erosion and removal of material from Gale crater must have subsequently occurred in order to explain the modern topography. Although the exact process by which this occurred is unknown, the mechanical breakdown of crater-filling bedrock by aeolian abrasion and removal of sediment by winds has been previously suggested for Gale crater [e.g., Malin and Edgett, 2000] and other craters with sedimentary mounds Bennett and Bell, 2016]. Extensive evidence for aeolian abrasion has been observed along Curiosity's traverse in the form of ventifacts [Bridges et al., 2014] and actively retreating scarps [Farley et al., 2014], and the rover has encountered the full spectrum of progressive surface denudation from fractured bedrock, to retreating bedrock-capped mesas, to remnant hills capped by bedrock rubble, and to desert pavement plains . The geomorphology at the Kimberley suggests a landscape that has undergone-or is still undergoing-this process of denudation, with three buttes of the Mount Remarkable member covered with boulders of the Beagle member that are interpreted to be the remnants of former mesa-capping bedrock. Crater statistics of Aeolis Palus and the base of Mount Sharp indicate that the exhumation and exposure of most of the Gale crater floor had occurred by~3.3 billion to 3.1 billion years ago [Palucis et al., 2014;Grant et al., 2014]. Therefore, the landscape at the Kimberley may have acquired most of its present expression by the middle Hesperian Period, with a long epoch of slow aeolian erosion continuing to the present.

10.1002/2016JE005200
At the Windjana drill site, Curiosity was able to test hypotheses about ongoing aeolian abrasion and deflation leading to the recent exposure of the lower members of the Kimberley formation. Specifically, as described by Vasconcelos et al. [2016], the error-weighted mean surface exposure age from 3 He and 21 Ne in SAM analyses of the Windjana sample is 46 AE 16 Ma, which is a minimum exposure age given the possibility of incomplete extraction (however, the concordance between the 3 He (30 AE 27 Ma) and 21 Ne (54 AE 19 Ma) ages provides no evidence for underextraction). The exposure of the Windjana target from beneath the Mount Remarkable member may have occurred via scarp retreat at~46 Ma, via vertical erosion occurring at a mean rate of~1.5 cm Ma À1 , or some combination of erosional processes [Vasconcelos et al., 2016]. This inferred erosion rate at the Kimberley is similar to that reported previously for the Sheepbed-Gillespie scarp at Yellowknife Bay [Farley et al., 2014], implying that rates of erosion by aeolian abrasion may be similar across Aeolis Palus.

Conclusions and Remaining Questions
The results from Curiosity's campaign at the Kimberley, summarized here and in this special section of Journal of Geophysical Research-Planets, suggest the following sequence of events for the geologic evolution of Gale crater.
1. Potassium feldspars formed in alkaline igneous rocks, emplaced on what is now the north rim and walls of Gale crater, suggesting that the Martian mantle had differentiated significantly by the late Noachian, far earlier than similar differentiation occurred on Earth. 2. Sediments eroded from this alkali feldspar protolith and other diverse igneous sources were transported to the floor of Gale crater from the northern rim and deposited at the Kimberley in a prograding, fluviodeltaic system during the late Noachian to early Hesperian, prior to the existence of most of Mount Sharp. 3. Lake levels fluctuated during the period of deposition, and the Kimberley was at times covered by up to tens of meters of water and at times dry. 4. Sediment deposition likely took place under cold conditions with relatively low water-to-rock ratios. 5. After deposition, the rocks at the Kimberley were buried and underwent multiple episodes of diagenetic alteration with different aqueous chemistries and redox conditions to deposit Ca-sulfate veins, Mn-oxide fracture-fills, and erosion-resistant nodules. 6. Starting in the middle Hesperian, significant aeolian abrasion and removal of sediments occurred within Gale crater, creating the modern topography that slopes away from Mount Sharp. 7. Aeolian erosion has continued to the present day, leading to the exposure of the Kimberley formation at Windjana from beneath a few meters of overburden at only~46 Ma.
The geological history implied here has important implications for the past habitability of Gale crater. The Kimberley sediments were deposited near the margins of an ancient, crater-filling lake during the later part of the period when Mars is thought to have had the most favorable conditions for life. The 1 to 4 m stratigraphic thickness of each delta deposit implies a standing body of water of at least that depth for long enough to accumulate the sediment. Based on terrestrial analog rates, Grotzinger et al. [2015] estimated that each lake was present for a time span on the order of 100 to 10,000 years, and even as individual lakes came and went, they could have been connected in time through a common groundwater table. Therefore, water may have been continually present within Gale crater for a biologically relevant period of time, albeit under cold climate conditions.
Although detection of organic compounds from the Windjana sample has not been reported, its relatively recent exposure age suggests that, had organics been concentrated during the deposition of the Dillinger member at the Kimberley, they would have been shielded from cosmic ray degradation by a few meters of overburden and likely would have be preserved until~46 Ma. The absence of organic compounds, however, perhaps is not surprising given that the Dillinger member is a fine-grained sandstone interpreted as a fluvial (or mixed fluvial-aeolian) deposit; organic matter is much more likely to be concentrated in finer-grained facies [e.g., Summons et al., 2011], such as the lacustrine mudstones where organics were detected at Yellowknife Bay [Freissinet et al., 2015], which are not present in the Kimberley formation. During Curiosity's ongoing traverse south through the lower stratigraphy of Mount Sharp, including its current exploration of Murray formation, the rover will characterize mudstones that were deposited in deeper portions of the basin-filling lakes. If ongoing erosion rates at Mount Sharp are similar to those measured at Aeolis Palus, there is a strong possibility that Curiosity can sample a recently exposed outcrop of lacustrine mudstone where organic molecules may have been concentrated and preserved.