Mineralogy and Fluvial History of the Watersheds of Gale, Knobel

PUBLICATIONS

Geophysical Research Letters

RESEARCH LETTER Mineralogy and fluvial history of the watersheds 10.1002/2014GL062553 of Gale, Knobel, and Sharp craters: A regional Key Points: context for the Mars Science Laboratory • Olivine- and Fe/Mg phyllosilicate-bearing bedrock throughout the watersheds Curiosity’s exploration • Watershed Fe/Mg phyllosilicates different from Mount Sharp Bethany L. Ehlmann1,2 and Jennifer Buz1 • Intermittent, increasingly localized fl Hesperian/Amazonian uvial activity 1Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California, USA, 2Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA Supporting Information: • Figure S1 Abstract A 500 km long network of valleys extends from Herschel crater to Gale, Knobel, and Sharp craters. Correspondence to: The mineralogy and timing of fluvial activity in these watersheds provide a regional framework for deciphering B. L. Ehlmann, the origin of sediments of Gale crater’s Mount Sharp, an exploration target for the Curiosity rover. Olivine-bearing [email protected] bedrock is exposed throughout the region, and its erosion contributed to widespread olivine-bearing sand dunes. Fe/Mg phyllosilicates are found in both bedrock and sediments, implying that materials deposited in Citation: Gale crater may have inherited clay minerals, transported from the watershed. While some topographic lows Ehlmann, B. L., and J. Buz (2015), Mineralogy and fluvial history of the watersheds of of the Sharp-Knobel watershed host chloride salts, the only salts detected in the Gale watershed are sulfates Gale, Knobel, and Sharp craters: A regional within Mount Sharp, implying regional or temporal differences in water chemistry. Crater counts indicate context for the Mars Science Laboratory progressively more spatially localized aqueous activity: large-scale valley network activity ceased by the early ’ Curiositysexploration,Geophys. Res. Lett., fl 42,264–273, doi:10.1002/2014GL062553. Hesperian, though later Hesperian/Amazonian uvial activity continued near Gale and Sharp craters.

Received 19 NOV 2014 Accepted 18 DEC 2014 Accepted article online 22 DEC 2014 1. Introduction Published online 29 JAN 2015 Mount Sharp within Gale crater is the target of exploration for the Mars Science Laboratory (MSL) Curiosity rover. Various volcanic (airfall ash), aeolian, lacustrine, and diagenetic processes have been proposed to explain its sedimentary units, which contain hematite, sulfates, and nontronite in discrete stratigraphic units [Malin and Edgett, 2000; Pelkey et al., 2004; Milliken et al., 2010, 2014; Anderson and Bell, 2010; Thomson et al., 2011; Kite et al., 2013; Fraeman et al., 2013; Le Deit et al., 2013]. Each sedimentary system records a selective sample of geologic history; thus, knowledge of regional processes is necessary for time ordering the source-to-sink record of environmental change preserved in any given basin [e.g., Allen, 2008]. The composition of the bedrock and the nature and timing of transport and depositional processes in the greater Gale-Knobel-Sharp (GKS) watersheds are the major focus of this work. Key driving questions are the following: How do the mineralogy of sands and rock units sampled by Curiosity compare to regional bedrock? What was the timing of regional fluvial activity, sedimentary transport, aqueous alteration, and deposition? After briefly reviewing the GKS watershed geologic context, we describe methods for our investigations using high-resolution imagery and mineralogy from Mars Reconnaissance Orbiter (MRO) data sets. We then describe the mineralogy of the watershed region, implications for water chemistry, and results from crater counts and superposition relationships to establish a history of fluvial activity. Finally, we discuss broader implications for the composition of sediments delivered to Gale crater’s Mount Sharp, accessible to the Curiosity rover.

2. Geologic Context Gale, Knobel, and Sharp craters are located along a topographic gradient at the northern lowlands-southern highlands dichotomy. Between 300 km diameter Herschel crater (15°S, 129°E), 155 km diameter Gale crater This is an open access article under the (5°S, 138°E), and 150 km Sharp crater lies a >150,000 km2 valley system that emanates from highlands terms of the Creative Commons ﬂ Attribution-NonCommercial-NoDerivs immediately north of Herschel crater and drains northward (Figure 1). The last large-scale uvial activity License, which permits use and distri- within the watershed was in the late Noachian/early Hesperian around 3.5–3.7 billion years ago [Irwin et al., bution in any medium, provided the 2005; Fassett and Head, 2008]. Dissection of terrain over the 7 km elevation range has eroded the bedrock original work is properly cited, the use is non-commercial and no modiﬁcations and transported sediments toward Gale, Sharp, and Knobel craters at the distal end of the valleys, possibly or adaptations are made. contributing to the sedimentary materials being explored by the Curiosity rover.

Figure 1. THEMIS daytime infrared image with colorized Mars Orbiter Laser Altimeter elevation showing valley and channel networks, sedimentary deposits, and mineralogy of the watersheds of Gale, Knobel, and Sharp craters. Outlines of targeted CRISM shortwave infrared data (L channel) are shown, colored red when olivine ± high-calcium pyroxene is present in rocks. Colored dots indicate secondary minerals found in the image(s).

Early Viking imagery of the GKS watersheds showed Noachian bedrock, with Hesperian volcanic plains infilling some topographic lows and craters [Greeley and Guest, 1987]. Gale crater formed during the late Noachian/early Hesperian [e.g., Thomson et al., 2011, Le Deit et al., 2013]. The age of Sharp crater is not well constrained, but superposition relations constrain it to be older than Gale. Gale crater has a degraded northern rim, a significantly degraded ejecta blanket, and the enigmatic 5 km tall Mount Sharp whose high point rises above the northern rim. Sharp crater has a fully degraded northern rim, no apparent ejecta blanket, and interior plateaus of later emplaced materials that have been heavily dissected into “chaos terrain” (Figure 1). Small valleys incise the rims of both Sharp and Gale craters. Knobel crater has a flat floor with wrinkle ridges, indicating volcanic fill, and no incised rim. The GKS watersheds are within one of the dustier regions on Mars [Ruff and Christensen, 2002], confounding orbital infrared studies of mineralogy. Nevertheless, portions with lower dust cover have been examined as part of the Thermal Emission Spectrometer’s global mapping. The GKS region has elevated olivine and high-calcium pyroxene, relative to other Martian terrains [Rogers and Christensen, 2007], and dunes within Gale crater are enriched in 60Fo olivine [Lane and Christensen, 2013]. Mars Express/Observatoire pour la Minéralogie, l’Eau, les Glaces et l’Activité (OMEGA) data corroborate the above, showing aeolian materials emanating from Gale Crater that are enriched in mafic minerals [Anderson and Bell, 2010]. Mapping of chemical elements by the coarser spatial scale Gamma Ray Spectrometer shows chlorine enrichment [Taylor et al., 2010], and chloride-bearing sedimentary units were discovered by Mars Odyssey/Thermal Emission Imaging System (THEMIS) [Osterloo et al., 2010] and one described by Reusch et al. [2012] (Figure 1).

3. Methods Mineralogy of the region was assessed with the highest spatial resolution data sets to ﬁnd less dust-covered exposures of bedrock and sediments. MRO’s Compact Reconnaissance Imaging Spectrometer for Mars (CRISM)

Figure 2. (a) Overview of the south Sharp crater watershed. (b) Mineral maps from CRISM-targeted images overlain on CTX show olivine (red) in the crater walls and knobs within the crater and Fe/Mg phyllosilicates (green) in both crater walls and SF3 fan sediments. Area with Al phyllosilicates (cyan) is outlined and shown in Figure 2e. A white arrow indicates a valley incised on the fan surface. HiRISE image (ESP_019975_1750) and ratio spectra from CRISM image (FRT0001963E) show (c) Fe/Mg phyllosilicates in layered sediments within the fan and multiple episodes of sedimentation indicated by distinct populations of unﬁlled (white arrows) and ﬁlled craters (black arrows) on the fan surface (see also Figure 4b); (d) knobs within Sharp crater host olivine within rocky deposits that shed boulders; and (e) bright toned layered units atop one knob host Al phyllosilicates, indicated by the arrow. (f) Chlorides are found as bright toned materials in the watershed, overlain by crater ejecta and an inverted channel with Fe/Mg phyllosilicate-bearing sediments as well as olivine. White arrow indicates an inverted channel. (g,h)HiRISE PSP_008780_1735 false color data show that the chlorides underlie mantling materials.

targeted data are acquired at 18–40 m per pixel over 0.4–4.0 μm and were used to detect Fe-bearing minerals and secondary minerals. Standard data processing procedures described in Murchie et al.[2009]andEhlmann et al. [2009] were employed to generate calibrated spectra with ﬁrst-order atmospheric and photometric corrections as well as mineral parameter maps. Spectra were compared with laboratory data sets [Clark et al., 2007]. THEMIS nighttime infrared data were also used to qualitatively assess thermal inertia differences between geologic units to aid in mapping of units and channels. For morphologic analysis, a 6 m per pixel orthorectiﬁed and equalized basemap was constructed from MRO/Context Imager (CTX) data [Malin et al., 2007]. In select locations, MRO/High Resolution Imaging Science Experiment (HiRISE) data at 0.25 m per pixel were available. Crater counts and diameter measurements were

Figure 3. (a) Gale crater, including the SW inlet valley. Olivine-bearing materials are found within several images on the Gale wall as well as within the sands. Fe/Mg phyllosilicates are detected at a range of elevations in CRISM data, some of which were previously reported by Wray [2013]. Shown for context are CRISM detections of nontronite [Milliken et al., 2010] and hydrated silica [Seelos et al., 2014] as well as Curiosity’s detection of Mg-smectite [Vaniman et al., 2014]. (b) CRISM spectra show some compositional heterogeneity among the Fe/Mg phyllosilicates. Mount Sharp spectra indicate Al-substituted nontronite, while those around the walls of Gale and Sharp craters appear to be intermediate Fe/Mg-smectites, e.g., ferrosaponite or Mg-nontronite. (c) Olivine spectra from the walls of Gale and Sharp craters compared to sand dunes within Gale.

made over deﬁned geologic regions in these high-resolution data sets. Craters were pseudolog binned, and surface ages were estimated with the production function from Ivanov [2001] and the chronology of Hartmann and Neukum [2001], using the Craterstats2 program [Michael and Neukum, 2010] (see supporting information for additional details).

4. Results 4.1. Regional Mineralogy and Bedrock Composition Three dozen CRISM images within the study area have infrared spectra in which minerals can be identified from diagnostic absorptions. Most detections are within small impact craters that expose underlying materials from beneath dust; scarps in fluvial channels and erosion of sedimentary features also permit remote sensing of composition. Throughout the Gale-Sharp-Knobel region, much of the bedrock is olivine-bearing (Figure 1), and the presence of high-calcium pyroxene is also sometimes discernible in the spectra. Outcrops in the walls of Gale and Sharp craters have olivine-bearing rocks (e.g., Figure 2b). Elsewhere, sands appear to be the main hosts of mafic minerals. Fe/Mg phyllosilicates are found regionally associated with small impact craters, large crater walls, and fluvial landforms. They are detected within the southwestern wall of Sharp crater (e.g., Figure 2b) and at multiple elevations on the walls of Gale crater within ejecta of small craters (Figure 3a). Other alteration minerals have also been identified, including kaolin family aluminum phyllosilicates, silica, and hydrated features that lack metal-OH features (Figure 1). A hydroxylated phase within Sharp, identified by Carter et al. [2014] as akaganeite, is found in a localized depression. Chlorides are found within the watershed of Sharp crater, though the adjacent Gale watershed shows no such detections. 4.2. Mineralogy of Intracrater Sedimentary Deposits Sharp crater has three sedimentary fans that emanate from the crater walls. The western fan (SF1, 83 km2) appears sourced only in the immediate watershed around Sharp crater; there is little rim dissection. The southern fan has two lobes: one is sourced from near the crater rim (SF2, 33 km2) and the second (SF3, 147 km2) emanates from a hundreds of kilometers long channel, though this entire reach may not have contributed to the formation of the fan (Figures 2a and 2b). The fans are mantled by sands and later units; however,

their eroded fronts reveal meter-scale bedding (Figure 2c). The sediments of fan SF3 have Fe/Mg phyllosilicates. SF3 embays a set of knobs within the crater that are olivine bearing and blocky (Figure 2d); atop one knob are layered materials with aluminum phyllosilicates (Figure 2e). In Gale crater, in addition to the sedimentary deposits of Mount Sharp, multiple small fluvial features (20–110 km2), emanating from the rim, have previously been identified, mapped, and crater counted [Grant et al., 2014]. A southwestern set of fans is sourced from a valley emanating from the main valley network. Smaller fluvial features, such as Peace Vallis near the Curiosity landing site, are sourced near the crater rim. To date, no sedimentary fans within Gale have hydrated silicates or mafic minerals identified in CRISM data, though two of the southern fans are unimaged. 4.3. Intercrater Chloride-Bearing Depressions Of the eight chloride deposits in the Sharp-Knobel watershed originally cataloged in Osterloo et al. [2010], two have been imaged by MRO-CRISM and HiRISE. The largest is 100 km to the southwest of the valley entrance to Sharp crater and west of Knobel crater. The chlorides are located in a local depression where the feeder valley becomes indistinct, and an incised valley leads from it into Sharp crater. The chlorides are within distinctively bright toned, polygonally fractured, lithified materials (blue-toned in Figures 2g and 2h) while the olivine is within bed forms atop these units. The Fe/Mg phyllosilicate is in lithified materials (tan-colored in Figure 2g). In several cases phyllosilicate-bearing materials are stratigraphically above the chlorides (e.g., Figure 2g), while in other cases the relationship is indeterminate. 4.4. Comparative Compositional Analysis Orbital data show that Sharp and Gale crater walls are of similar composition, with olivine present in bedrock units, though changes in continuum slope with location indicate minor compositional variation (Figure 3c). Sands in Gale and Sharp also have olivine and, depending on the portion examined, sometimes high-calcium pyroxene (e.g., Figure 2d) [Seelos et al., 2014]. In many instances, wall rock units are also Fe/Mg phyllosilicate bearing. The spectra of the Fe/Mg phyllosilicates are consistent with a composition intermediate between end-member smectites nontronite and saponite; the wavelengths of metal-OH absorption minima near 1.4 μm and 2.3 μm are not simultaneously consistent with a single end-member (Figure 3b). The phyllosilicates in Gale and Sharp walls and in the sediments within Sharp and the chloride unit do not have the Fe,Al-OH absorption near 2.24 μm[Bishop et al., 2002] that is a characteristic of nontronite-bearing materials previously identified in Mount Sharp [Milliken et al., 2010]. 4.5. Surface Ages Sedimentary fans within Gale crater show evidence for late fluvial activity, possibly extending into the Amazonian, and some small segments appear superposed on middle Hesperian crater fill [Grant et al., 2014]. Similarly, fans within Sharp crater superpose Hesperian-aged floor materials (Figure 4b) and have distinct crater count-derived surface ages that are Amazonian but are derived from small areas and are therefore somewhat uncertain (Figures 4a–4d). The Sharp fan SF3 is itself incised by a small valley (Figure 2b) and has both filled craters and unfilled craters, pointing to multiple episodes of sediment deposition and erosion, including at least one episode of fluvial reworking (Figures 2b, 2c, and 4b). The population of the smallest craters has an isochron of 4 Ma, indicating a possible recent age for the last episode of resurfacing. Considering both filled and unfilled craters together gives an SF3 surface age of 1.2 ± 0.3 Ga., The upper phyllosilicate-bearing unit near the chloride deposits (Figure 4e) in the Sharp crater feeder valley has a surface age of 1.1 ± 0.3 Ga, similar to that of the fan (Figure 4f). This may be a lower bound on the age of the chlorides, which appear to be stratigraphically beneath this cap but are above a late Noachian/early Hesperian floor unit. The interpretation of the fan and chloride cap unit surface ages above may, however, be complicated by the small surface areas available for crater counting, and these surfaces may be older than their crater count-based surface ages imply (see section 5.4). Farther from Gale and Sharp craters, we sought additional constraints on the timing of last fluvial activity within the larger, extended valley network system. There are two large craters within the Sharp and Gale watersheds that have been filled by lava or sediments, and this fill shows no evidence of fluvial dissection (context for Figures 4g and 4i in Figure 1). The crater fill surface ages are late Noachian/early Hesperian for the Sharp watershed and late Hesperian/early Amazonian for the Gale watershed (Figures 4g–4j).

Figure 4. Crater counts establish surface age estimates that constrain the relative timing of events in the GKS watersheds (see supporting information for incremental plots). Red lines indicate those where crater count statistics are more uncertain because of small count areas. (a, b) Sharp fan SF3 (Figure 2) has an Amazonian surface age and is atop a floor with a surface age near the early Hesperian/late Noachian boundary. In the legend, the data set used is indicated, and BLE and JB indicate counts by the authors. The smallest craters point to a resurfacing (sedimentation) event at ~4 Ma. (c, d) The SF1 fan on the western wall of Sharp crater has an Amazonian surface age. (e, f) The unit capping the underlying chloride unit from Figure 2g and 2h shows an Amazonian surface age. (g, h) A crater in the watershed feeding Sharp at (129°E, À8.3°N) was resurfaced in the early Hesperian/late Noachian but not subsequently dissected by fluvial activity. (i, j) Late Hesperian/early Amazonian surface age of an undissected crater floor unit (135°E, À9.4°N) in the watershed feeding Gale.

5. Discussion 5.1. Mineralogy of the Regional Bedrock Detection of regionally widespread olivine-bearing materials corroborates previous studies from Thermal Emission Spectrometer [Rogers and Christensen, 2007] and OMEGA [Anderson and Bell, 2010]. However, more specifically, it is now apparent that olivine is within bedrock throughout the GKS watersheds, including olivine-bearing rocks in the upper wall rocks of Gale crater itself. These are potential sources for the olivine- and high-calcium pyroxene sands within both Gale and Sharp craters. It remains for further study to understand controls on the observed spectral variability in dune sand olivine and pyroxene content [Seelos et al., 2014] and whether this reflects variability in olivine versus pyroxene in the bedrock or sorting by aeolian processes. Our analyses to date do not indicate the presence of any mafic-poor, feldspar-rich lithologies within the wall rock, though these have been reported by the Curiosity rover [e.g., Sautter et al., 2014]. However, feldspars are

detectable in visible/shortwave-infrared (VSWIR) data only when the feldspar has iron, so lack of detection does not uniquely imply its absence in the region. Nevertheless, the suite of igneous bedrock in the region clearly includes olivine-bearing lithologies; these provided at least one of the protoliths for aqueous alteration and were a source for sediments within the Gale, Sharp, and Knobel valley systems. 5.2. Secondary Minerals, Aqueous Alteration, and Water Chemistry Widespread detections of Fe/Mg phyllosilicates indicate sustained chemical interactions with near-neutral to alkaline waters throughout the region. These are within scarps in the walls of Sharp crater and other small craters throughout the region as well as within valley and crater sediments. As previously reported for two craters by Wray [2013] and including a new crater detection reported here, small craters on the Gale crater walls with elevations ranging from À2170 m to À4255 m host Fe/Mg phyllosilicates. Al phyllosilicates are also within the watersheds, found in two locations in the uppermost reaches near Herschel crater and in one <1km2 set of layered materials atop a knob in Sharp crater. Elsewhere on Mars, Al phyllosilicates are thought to involve near-surface weathering [e.g., Ehlmann et al., 2011; Carter et al., 2013]. Their paucity in the GKS watersheds might imply that the duration, quantity, or acidity of waters in the GKS watersheds were less than those at other locations. Alternatively, poorer exposures combined with less data coverage may be responsible, pointing to the need for continued orbiter data acquisition of new targets within the region. Multiple occurrences of chloride salts are within the Sharp-Knobel watershed but have to date not been found by the Curiosity rover in substantial amounts. Mount Sharp contains magnesium sulfate minerals detectable from orbit [Milliken et al., 2010], and gypsum occurs within smectite-bearing sediments at Yellowknife Bay [Vaniman et al., 2014]; however, sulfates have not been observed in orbital data of the GKS watersheds. This suggests either a unique air fall source for Mount Sharp sulfate-bearing materials or a significant difference in water chemistry or a chemical divide between the waters that flowed through the valley systems to deposit chlorides and those brines that deposited sulfates within and near Mount Sharp. 5.3. Implications for the Sediments Explored by Curiosity The position of the 2.3 μm absorption in Fe/Mg phyllosilicate is a sensitive indicator of the relative proportions of Mg versus Fe cations in the octahedral site, permitting discrimination between trioctahedral and Mg-rich saponites, and dioctahedral and Fe-rich nontronites [e.g., Bishop et al., 2002]. The composition of Fe/Mg phyllosilicate in the GKS watersheds, based on this absorption position, is intermediate and similar from locality to locality, including Sharp crater wall, Sharp SF3 fan, Sharp valley chloride cap rocks, and wall rock and products of mass wasting in Gale exhumed by impact. The similarity of the sedimentary clay minerals to the bedrock clay minerals allows the simplest explanation that the sedimentary clay minerals are simply detrital. This is the case in most sedimentary systems on Earth, with postdepositional modification substituting interlayer cations or forming mixed layer clays [e.g., Weaver, 1958]. However, formation in place, associated with a fluviolacustrine system or later groundwater-supplied diagenesis, may have formed similar clay compositions by reactions of similar protolith materials under similar water chemistries, such as alteration of olivine proposed for Yellowknife Bay [Vaniman et al., 2014]. Indeed, early diagenetic textures indicating fluid flow through lithifying sediments are found in the smectite-bearing mudstones [Grotzinger et al., 2014], possibly related to late fluvial activity within Gale [Grant et al., 2014]. To what extent are Mount Sharp sediments endogenous to Gale crater versus transported from longer distances in the watershed versus sourced from air fall? In the Noachian materials surrounding Gale, olivine-bearing and Fe/Mg phyllosilicate-bearing materials are common. These are likely to comprise part of the protolith for sedimentary materials within the lower portion of Mount Sharp, transported northward during late Noachian/early Hesperian fluvial activity. Interestingly, the spectral signature of the Fe/Mg phyllosilicates in the GKS watersheds is consistent with a trioctahedral Mg-smectite with some Fe, similar to that reported by Curiosity at Yellowknife Bay [Vaniman et al., 2014]. However, the GKS Fe,Mg-smectites are distinctly different spectrally from the clay minerals in Mount Sharp, which have absorptions indicating dioctahedral, Al-substituted nontronite [Milliken et al., 2010] (Figure 3). This suggests a distinct process formed and/or modified Mount Sharp’s clays. 5.4. Liquid Water Availability Through Time Large-scale activity within the valley has previously been dated to terminate in the late Noachian/early Hesperian [Irwin et al., 2005; Fassett and Head, 2008]. Our measurements of the surface ages of large crater

interiors whose fill superposes the GKS valley systems are consistent with this prior work, within model uncertainties. Interestingly, the fluvial evolution of Sharp crater appears to parallel that of Gale to the east, where several small fans show evidence of late resurfacing, during the Hesperian and possibly as recently as the Amazonian (Figures 4 and 5) [Grant et al., 2014; Palucis et al., 2014]. The surfaces of small fans within Gale and Sharp and the chloride cap rock have model ages of ~1 Ga to hundreds of Ma (Figure 5). Because of small counting areas and the possibility of destruction of the oldest craters, these ages may be underestimated (see for discussion Warner et al.[2015]).However,superposition Figure 5. Timeline of major events in the watersheds. The center dark relationships and counts demonstrate that line indicates the estimate from crater counts, shading indicates the fans and chlorides are younger than uncertainties, and an arrow indicates that the data are an upper or underlying materials, constraining their lower bound only; dashed lines indicate counts from small areas where uncertainties are greater. Estimates for Gale crater formation formation to Hesperian or after. SF3 is itself are from Thomson et al. [2011], and estimates for Gale fan and Aeolis incised by a later valley and shows evidence Palus surface ages are from Grant et al. [2014]. Fan ages are last formultipleepisodesofresurfacing resurfacing ages. (Figures 2b, 2c, and 4b). Thus, a consistent regional picture emerges: (1) alteration of the olivine-bearing bedrock to Fe/Mg phyllosilicate-bearing compositions prior to the late Noachian/early Hesperian and perhaps continuing even after Sharp and Gale craters’ formations; (2) large-scale valley network activity and erosion and transport of sediments into Sharp and Gale craters that ended in the late Noachian/early Hesperian; (3) continued Hesperian/Amazonian fluvial activity, becoming progressively more spatially localized, that deposited chlorides in local basins and Fe/Mg phyllosilicates in fans; and (4) intermittent Amazonian resurfacing. There are two implications specifically relevant to interpreting the sedimentary explored by the Curiosity rover: the progressive localization of source regions of fluvial sediments over time and the long-lasting, albeit intermittent, water availability for diagenesis and secondary mineral formation.

6. Conclusions Analyses of highest resolution MRO composition and imaging data sets within the watersheds of Gale, Knobel, and Sharp craters show the widespread occurrence of olivine-bearing bedrock as well as the common occurrence of Fe/Mg phyllosilicate alteration minerals. Al phyllosilicates, silica, and other hydroxylated phases are also found but are less common. The watershed of Sharp crater hosts at least eight deposits of chloride, but no sulfates were identified elsewhere in the watersheds, contrasting with the sulfate found within Mount Sharp, Gale crater. Like Gale crater, Sharp crater hosts a number of small alluvial fans within the crater. These fans have Fe/Mg phyllosilicate-bearing sediments. Crater counting within the regional watersheds agrees with previous estimates, constraining large-scale regional valley network activity to the late Noachian/early Hesperian. Progressively more localized and intermittent aqueous activity continued into the Hesperian and perhaps later. Olivine- and intermediate Fe/Mg phyllosilicate-bearing materials would have been transported into Gale crater and may have contributed to the sediments found by Curiosity at Yellowknife Bay. They may also have contributed to the provenance of lower Mount Sharp sediments; however, the compositions of clay minerals within the watersheds are distinct from the Al-nontronite found within Sharp. Such data point to a complex aqueous history for Gale crater with changing sediment sources or diagenetic fluid compositions sometimes distinct from the surroundings. Geologic mapping coupled with in situ compositional measurements by the Curiosity rover will unravel the sedimentary history of the watershed and the geochemistry of fluids participating in the aqueous alteration of Mount Sharp.

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