Sedimentological evidence for a deltaic origin of the western fan deposit in Jezero crater, Mars and implications for future exploration

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Sedimentological evidence for a deltaic origin of the western fan deposit in Jezero crater, Mars and implications for future exploration Timothy A. Goudge a,b,∗ , Ralph E. Milliken a , James W. Head a , John F. Mustard a , Caleb I. Fassett c a b c

Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, RI, USA Jackson School of Geosciences, The University of Texas at Austin, Austin, TX 78712-1722, USA Department of Astronomy, Mount Holyoke College, South Hadley, MA, USA

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Article history: Received 10 December 2015 Received in revised form 27 April 2016 Accepted 27 October 2016 Available online xxxx Editor: C. Sotin Keywords: Mars Jezero crater paleolake delta deposit

a b s t r a c t We examine the stratigraphic architecture and mineralogy of the western fan deposit in the Jezero crater paleolake on Mars to reassess whether this fan formed as a delta in a standing body of water, as opposed to by alluvial or debris flow processes. Analysis of topography and images reveals that the stratigraphically lowest layers within the fan have shallow dips (<2◦ ), consistent with deltaic bottomsets, whereas overlying strata exhibit steeper dips (∼2–9◦ ) and downlap, consistent with delta foresets. Strong clay mineral signatures (Fe/Mg-smectite) are identified in the inferred bottomsets, as would be expected in the distal fine-grained facies of a delta. We conclude that the Jezero crater western fan deposit is deltaic in origin based on the exposed stratal geometries and mineralogy, and we emphasize the importance of examining the stratigraphic architecture of sedimentary fan deposits on Mars to confidently distinguish between alluvial fans and deltas. Our results indicate that Jezero crater contains exceptionally well-preserved fluvio-deltaic stratigraphy, including strata interpreted as fine-grained deltaic bottomsets that would have had a high potential to concentrate and preserve organic matter. Future exploration of this site is both geologically and astrobiologically compelling, and in situ analyses would be complementary to the ongoing in situ characterization of fluvio-lacustrine sediment in the Gale crater paleolake basin by the Curiosity rover. © 2016 Elsevier B.V. All rights reserved.

1. Introduction The martian sedimentary rock record offers unique insight into ancient surface processes that have occurred on the planet by providing information on past sediment transport, deposition, and accumulation in a variety of distinct environments (e.g., Ori et al., 2000; Malin and Edgett, 2003; Fassett and Head, 2005; Howard et al., 2005; Irwin et al., 2005; Lewis and Aharonson, 2006; Di Achille and Hynek, 2010; Ansan et al., 2011; Grotzinger and Milliken, 2012; DiBiase et al., 2013; Grotzinger et al., 2015). Constraining the origin of a given sedimentary deposit requires an understanding of the deposit’s facies and stratigraphic architecture (i.e., the geometry of discrete bundles of exposed strata and bounding unconformities), which together record how the processes responsible for its formation evolved in both space and time. This record,

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Corresponding author at: Jackson School of Geosciences, The University of Texas at Austin, 2275 Speedway, Stop C9000, Austin, TX 78712-1722, USA. E-mail address: [email protected] (T.A. Goudge). http://dx.doi.org/10.1016/j.epsl.2016.10.056 0012-821X/© 2016 Elsevier B.V. All rights reserved.

in turn, can provide important information about the depositional environment of the sediment, such as the relative contributions of fluvial, submarine, and aeolian transport and deposition (e.g., Mitchum et al., 1977). Understanding the stratigraphic architecture of terrestrial sedimentary deposits requires the analysis and reconstruction of threedimensional structures within the deposit, which is commonly achieved by studying well exposed outcrops and seismic reflection imaging (e.g., Mitchum et al., 1977). In a similar manner to the study of stratal geometries exposed in outcrop on Earth, highresolution orbital topography and orthorectified images of Mars allow for detailed observations of the large-scale stratigraphic architecture of martian sedimentary deposits that have been exhumed, exposing their three-dimensional structures (e.g., Lewis and Aharonson, 2006; Ansan et al., 2011; DiBiase et al., 2013). Many sedimentary fan-shaped deposits have been identified on Mars (e.g., Ori et al., 2000; Malin and Edgett, 2003; Fassett and Head, 2005; Howard et al., 2005; Irwin et al., 2005; Di Achille and Hynek, 2010; DiBiase et al., 2013), including landforms interpreted as both deltas and alluvial fans. The formation of a delta deposit

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requires deposition into a standing body of liquid water, whereas an alluvial fan forms subaerially, a distinction that implies significantly different environmental conditions. Delta deposits also have a much higher potential for concentration and preservation of organic matter in fine-grained intervals when compared with alluvial fan deposits, making them more attractive candidates for future in situ exploration of organic compounds (e.g., Summons et al., 2011). Although modern terrestrial deltas and alluvial fans show distinct differences in morphology and surface topography (e.g., Blair and McPherson, 1994), interpreting a given sedimentary fan deposit on Mars as a delta on the basis of morphology and geologic context alone is not necessarily conclusive. Morphology and geologic setting are certainly important aspects to consider and can provide compelling arguments for a deltaic over alluvial fan origin (e.g., Fassett and Head, 2005; Schon et al., 2012); however, the complication with relying exclusively on these characteristics is that sedimentary fan deposits on Mars represent exposed portions of the rock record (e.g., Grotzinger and Milliken, 2012), and not perfectly preserved depositional surfaces. Therefore, analysis of modern surface morphology and geologic setting of a fan deposit cannot be uniquely linked to the original depositional surface or geologic setting of that landform, because such analyses often cannot account for all of the effects of potential post-depositional processes (e.g., burial, erosion, compaction, etc.). It is thus crucial to also examine the stratigraphic architecture of sedimentary fan deposits on Mars in order to differentiate deltas from alluvial fans, as the different processes responsible for deposition and accumulation of strata within these depositional environments result in distinct stratal geometries (e.g., Rich, 1951; Bull, 1977; Orton and Reading, 1993; Blair and McPherson, 1994). A characteristic sedimentary structure of delta deposits is a clinoform, which comprises two prominent slope breaks going from approximately flat-lying topset strata to steeper dipping foresets to shallowly dipping bottomsets (Mitchum et al., 1977; Rich, 1951; Orton and Reading, 1993; Pirmez et al., 1998). Deltaic clinoforms also commonly exhibit downlap, where more steeply dipping foresets transition to shallower dips and terminate (lapout) against underlying strata that have shallower dips (Rich, 1951; Mitchum et al., 1977; Pirmez et al., 1998). In contrast, alluvial fans more commonly show approximately sub-parallel layering and a distinct concave radial profile, indicative of their subaerial formation by rapidly decelerating river channels (Bull, 1977; Blair and McPherson, 1994). These distinctions in stratigraphic architecture provide a key basis upon which deltas can be distinguished from alluvial fans on Mars through studies of exposed stratal geometries using remotely sensed data. However, only in a few instances have the stratal geometries of sedimentary fan deposits on Mars been closely examined and shown to be consistent with a deltaic origin (e.g., Ansan et al., 2011; DiBiase et al., 2013; Grotzinger et al., 2015). One specific site of interest is the ∼45 km diameter Jezero impact crater that is breached by two inlet valleys and an outlet valley, and once hosted an open-basin lake (Fig. 1; Fassett and Head, 2005). Jezero crater itself is located in terrain mapped as the Hesperian and Noachian transition unit (HNt) by Tanaka et al. (2014), and the fluvial activity that fed the Jezero crater paleolake ceased near the Noachian–Hesperian boundary based on buffered crater counting of the associated valley network system by Fassett and Head (2008). Jezero crater contains two sedimentary fan deposits, one at the mouth of each inlet valley (Fassett and Head, 2005), which have sampled a large portion of ancient martian crust and contain Fe/Mg-smectite and Mg-rich carbonate (Ehlmann et al., 2008a, 2008b, 2009; Goudge et al., 2015). The Jezero fan deposits have been interpreted as deltas on the basis of their geologic setting (i.e., within a hydrologically open pa-

Fig. 1. Jezero crater paleolake basin, located at ∼18.4◦ N, ∼77.7◦ E (Fassett and Head, 2005). North is up in both images. (a) Overview of the Jezero crater paleolake showing the two inlet valleys and the outlet valley (labeled thin black lines). Thick black outline is the −2395 m MOLA contour, the minimum level to which water must have ponded in the basin (Fassett and Head, 2005). Location of part (b) is indicated by white box. Red box indicates the extent of the HiRISE DEM used in this work. HRSC-derived DEM h5270_0000 overlain on a mosaic of Context Camera (CTX; Malin et al., 2007) images P04_002664_1988, P17_007714_2001, P15_007068_1971, P06_003521_1971, and P06_003376_1987. (b) Overview of the Jezero crater western fan deposit. Locations of Figs. 2a and 2b are indicated in white boxes. HiRISE-derived DEM from stereo-pair images PSP_003798_1985 and PSP_002387_1985 overlain on a mosaic of HiRISE image PSP_003798_1985 and CTX image P04_002664_1988. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

leolake) and exposed distributary channels (e.g., Fassett and Head, 2005; Ehlmann et al., 2008a; Schon et al., 2012); however, there has yet to be a detailed analysis of the exposed stratal geometries of these deposits. Here we examine the stratigraphic architecture and mineralogy of the western fan deposit (Fig. 1b), which is the better preserved and exposed of the two fans (Fassett and Head, 2005; Goudge et al., 2015), to further understand the depositional environment and relative importance of alluvial versus fluvio-deltaic processes for the formation of this deposit. 2. Methods To explore the origin of the western fan deposit we have employed two complementary approaches: (1) qualitative and quan-

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titative analyses of the stratigraphic architecture of the fan, and (2) detailed analysis of the mineralogy of the exposed portions of the fan. 2.1. Stratigraphic architecture Prominent sedimentary structures of the western fan deposit are exposed along the erosional front of the deposit (Fig. 2; Schon et al., 2012), and were investigated using a high-resolution digital elevation model (DEM; 1 m/pixel) and orthorectified image (∼0.25 m/pixel) from the High Resolution Imaging Science Experiment (HiRISE; McEwen et al., 2007). The HiRISE-derived DEM was produced from stereo-pair images PSP_003798_1985 and PSP_002387_1985 using the NASA Ames Stereo Pipeline (ASP) stereogrammetry software (Broxton and Edwards, 2008; Moratto et al., 2010; Beyer et al., 2014). To account for errors in regional-scale topography, the HiRISE DEM was tied to Mars Orbiter Laser Altimeter (MOLA) point shot data (Smith et al., 2001) using the ASP pc_align function (Beyer et al., 2014). This algorithm allows for translation and rotation of the HiRISE DEM in three-dimensional space to find the minimum error between the input and reference elevation point clouds (Beyer et al., 2014). While any one HiRISE DEM has only minimal coverage by MOLA point shots, we chained this process whereby we first tied the High Resolution Stereo Camera (HRSC) DEM h5270_0000 (Neukum et al., 2004; Gwinner et al., 2010) to MOLA point shot data, and then tied the HiRISE DEM to the corrected HRSC DEM. Once the HiRISE DEM was corrected, the left HiRISE image (PSP_003798_1985) was orthorectified to the final DEM topography using the ASP. For qualitative analysis of exposed sedimentary structures the HiRISE DEM and orthorectified image were examined using ESRI’s ArcScene 3D visualization software. Quantitative analyses were performed on exposed strata, which were identified as differentially eroded units that protrude from the surrounding material or form clear topographic benches (e.g., Fig. 2). Surfaces of exposed layers are interpreted to be representative of bedding planes based on their contiguous nature and similarity to other examples of mapped bedding in martian sedimentary deposits (e.g., Lewis and Aharonson, 2006; Ansan et al., 2011; DiBiase et al., 2013). Strata that are laterally contiguous over >50 m were mapped in ESRI’s ArcMap geographic information system (GIS) software, and location information (in three dimensions) was extracted at ∼5 m intervals. Assuming an approximately planar surface at the mapping scale, planes were fit to the extracted points using a linear least squares method (e.g., Lewis and Aharonson, 2006; Lewis et al., 2008; DiBiase et al., 2013). From these plane fits, strike and dip values were calculated for the exposed layers, as well as 95% confidence limits on these values based on the linear least squares plane misfit. Following the methods of Lewis et al. (2008), a principal components analysis (PCA) was used to evaluate the quality of plane fits and to ensure that the points for a given mapped layer span an appropriate range in x, y, and z dimensions to produce a meaningful strike and dip; if the points along a mapped layer are too co-linear (i.e., fall approximately on a straight line in plan view), then the strike and dip of that layer will be poorly constrained. A PCA was completed for all the points mapped along each layer, and the fit was deemed robust and used for final analysis only if the PCA results satisfied two criteria: (1) the variance explained by the first principal component was <99.5%, and (2) the ratio of the variance explained by the second principal component to the third principal component was >15 (Lewis et al., 2008).

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2.2. Mineralogy The mineralogy of the exposed portions of the fan deposit was investigated using visible to near-infrared (VNIR) hyperspectral data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM; Murchie et al., 2007). We analyzed a targeted CRISM image over the fan deposit (ID: HRL000040FF) alongside co-registered images and topographic data to determine the context and stratigraphic position of identified mineral phases. The analyzed CRISM image has a spatial resolution of ∼36 m/pixel and covers the wavelength range ∼0.36–3.9 μm at a spectral sampling of ∼6.5 nm/channel. Calibrated I/F data were downloaded from the Planetary Data System (PDS) and corrected for the cosine of the incidence angle. Next, a standard atmospheric correction method was applied, which relies on scaling atmospheric gas absorptions retrieved from an independent CRISM observation over a portion of the Olympus Mons volcano (the so-called ‘volcano scan correction’; e.g., Ehlmann et al., 2009; McGuire et al., 2009). The CRISM image was then map projected (Murchie et al., 2007, 2009), and manually georeferenced to the orthorectified HiRISE image in ArcMap. CRISM data analysis was guided by spectral parameter maps that highlight spectral diversity within the image (Pelkey et al., 2007; Ehlmann et al., 2009; Salvatore et al., 2010). CRISM spectra from units of interest were divided by spectra from a spectrally bland region within the same detector column in the CRISM image to emphasize the unique spectral signature of the unit of interest within the ratioed spectrum (e.g., Ehlmann et al., 2008a, 2008b, 2009). 3. Results We mapped 26 exposed layers within the Jezero crater western fan deposit that satisfy our criteria for robust plane fits (Fig. 2; Table 1). These layers each represent a discrete bedding plane exposure along the erosional front of the fan deposit. While it is possible that some of these exposures represent laterally continuous layers within the deposit, confidently identifying disconnected strata across the deposit is extremely difficult. Therefore, we have taken the conservative approach of only fitting layers where they are visibly continuous at the HiRISE scale. The mapped layers all dip approximately towards the basin depocenter (i.e., to the east; Figs. 2a, b), as expected for a prograding sedimentary fan deposit building out from the edge of a basin (Rich, 1951; Mitchum et al., 1977). Dips range from ∼0.5–9◦ (Fig. 3a; Table 1) and there is a clear trend of increasing layer dip with increasing elevation along the eroded front of the fan deposit (Fig. 3a). We also identify several locations where exposed layers downlap onto underlying strata (Fig. 2). CRISM data analysis reveals spectral signatures of both Fe/Mgsmectite and Mg-rich carbonate within the Jezero crater western fan deposit (Fig. 4), consistent with previous analyses (Ehlmann et al., 2008a, 2008b, 2009; Goudge et al., 2015). Fe/Mg-smectite is identified in many locations across the deposit, including strong spectral signatures in the deposit’s lower stratigraphic units (Figs. 4a, c). In contrast, Mg-rich carbonate is identified in only a few isolated locations across the deposit, and all of these identifications correspond to stratigraphically higher light-toned materials (Figs. 4a, d) interpreted to be fluvial channel point bar deposits (Schon et al., 2012). Fe/Mg-smectite is identified based on the presence of diagnostic vibrational absorptions at ∼1.4, 1.9, and 2.3 μm (Fig. 4b; Clark et al., 1990; Bishop et al., 2002). The ∼1.4 μm absorption is due to the first overtone of OH stretch, the ∼1.9 μm

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Fig. 2. Examples of exposed layering within the Jezero crater western fan deposit. (a, b) Red and cyan lines show mapped layers, with corresponding strike and dip directions (yellow symbols). Red lines show layers with dips >2◦ that are interpreted as foresets. Cyan lines show layers with dips <2◦ that are interpreted as bottomsets. Note that the layers all dip approximately toward the basin depocenter, and that foresets always overlie bottomsets. Portions of HiRISE image PSP_003798_1985. White ‘<’ symbols show the approximate look directions for the perspective views in parts (c, d). North is up in both images. (c, d) Three-dimensional perspective views of the exposed layering in parts (a, b). Red arrows show stratigraphically higher layers, interpreted as foresets, downlapping onto approximately flat-lying stratigraphically lower layers, interpreted as bottomsets. Portions of HiRISE image PSP_003798_1985 draped over a HiRISE-derived DEM from stereo-pair images PSP_003798_1985 and PSP_002387_1985. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Table 1 Attributes of layers mapped within the Jezero crater western fan deposit. Layers are sorted by mean elevation in descending order. Strikea (◦ )

Dipa (◦ )

Mean elevationb (m)

Latitudec (◦ N)

Longitudec (◦ E)

Inferred origin of stratad

338.2 (±2.7) 344.4 (±3.5) 356.4 (±17.2) 14.8 (±17.9) 302.4 (±4.8) 325.3 (±17.1) 18.5 (±19.6) 314.7 (±8.9) 27.3 (±6.7) 323.8 (±10.8) 31.7 (±16.9) 359.7 (±3.5) 21.6 (±24.4) 334.6 (±11.1) 18.7 (±7.2) 327.8 (±49.6) 0.9 (±11.3) 0.3 (±8.7) 13.8 (±31.3) 22.3 (±36.8) 4.5 (±28.8) 354.2 (±6.2) 346.8 (±14.7) 32.8 (±5.9) 294.7 (±32.2) 351.4 (±45.8)

8.5 5.7 7.6 4.4 9.4 8.6 4.0 4.7 7.9 3.4 2.1 9.3 5.6 2.9 3.1 2.9 3.8 2.5 0.5 0.7 1.8 1.6 1.5 0.7 0.9 0.6

−2470.8 −2472.0 −2474.0 −2475.0 −2475.4 −2476.3 −2480.8 −2480.8 −2481.5 −2483.7 −2484.6 −2486.2 −2488.8 −2493.6 −2495.5 −2495.7 −2497.7 −2499.4 −2503.9 −2508.6 −2510.0 −2513.1 −2515.3 −2523.8 −2526.5 −2527.5

18.532 18.513 18.523 18.522 18.512 18.503 18.524 18.527 18.521 18.528 18.530 18.501 18.521 18.529 18.510 18.522 18.511 18.521 18.520 18.516 18.515 18.503 18.516 18.491 18.501 18.488

77.413 77.415 77.417 77.416 77.417 77.427 77.418 77.418 77.417 77.418 77.417 77.435 77.417 77.421 77.424 77.420 77.423 77.419 77.419 77.419 77.422 77.437 77.422 77.428 77.438 77.429

Foreset Foreset Foreset Foreset Foreset Foreset Foreset Foreset Foreset Foreset Foreset Foreset Foreset Foreset Foreset Foreset Foreset Foreset Bottomset Bottomset Bottomset Bottomset Bottomset Bottomset Bottomset Bottomset

(±0.5) (±0.3) (±1.8) (±1.8) (±1.2) (±1.8) (±1.4) (±0.7) (±1.0) (±0.5) (±0.4) (±0.3) (±2.3) (±0.3) (±0.2) (±2.1) (±0.3) (±0.4) (±0.6) (±0.6) (±0.3) (±0.2) (±0.2) (±0.1) (±0.2) (±0.6)

(±2.4) (±5.0) (±2.4) (±1.9) (±1.8) (±8.6) (±0.9) (±2.7) (±1.8) (±2.3) (±2.0) (±9.2) (±1.3) (±2.7) (±2.9) (±1.3) (±3.1) (±1.4) (±0.4) (±0.7) (±1.2) (±2.6) (±2.3) (±0.8) (±1.0) (±1.7)

a

Values listed in parentheses are errors from 95% confidence limits from the linear least squares plane misfit. Values listed in parentheses are 1 standard deviation on extracted elevation values along the layer. Latitude and longitude values are from center points of the mapped layers, which correspond to the locations of the strike and dip symbols in Figs. 2a, b. b c

d Steeply dipping (>2◦ ) downlapping strata are interpreted as delta foresets. Shallowly dipping (<2◦ ) strata are interpreted as delta bottomsets.

absorption is due to a combination tone of OH stretch and H– O–H bend, and the ∼2.3 μm absorption is due to a combination tone of (Mg,Fe)–OH bend and OH stretch (Clark et al., 1990; Bishop et al., 2002). Mg-rich carbonate is identified based on the presence of diagnostic paired absorptions at ∼2.3 and 2.5 μm (Fig. 4b) that are due to an overtone of a fundamental vibrational mode of CO3 (Schroeder et al., 1962; Hunt and Salisbury, 1971). The Mg-rich carbonate-bearing material also has a broad absorption centered at ∼1 μm and a narrower vibrational absorption centered at ∼1.9 μm (Fig. 4b), consistent with previous identifications of Mg-rich carbonate in this region (Ehlmann et al., 2008b, 2009; Goudge et al., 2015). The ∼1 μm absorption is due to an electronic crystal field transition from octahedrally coordinated Fe2+ in olivine admixed with the Mg-rich carbonate (King and Ridley, 1987; Burns, 1993), and the ∼1.9 μm absorption is due to structural H2 O in either a hydrated carbonate (e.g., Calvin et al., 1994) or an additional hydrated mineral, such as smectite (e.g., Clark et al., 1990). 4. Discussion 4.1. Stratigraphic architecture of the Jezero crater western fan deposit Our observations of the stratigraphic architecture of the Jezero crater western fan deposit are diagnostic of clinoform structures within a delta deposit rather than sub-parallel layering in an alluvial fan system (Rich, 1951; Bull, 1977; Mitchum et al., 1977; Orton and Reading, 1993; Blair and McPherson, 1994). Downlapping stratal geometries, such as those observed here (Fig. 2), are consistent with layer terminations in the distal portions of deltaic clinoforms and are common in delta deposits on Earth (Rich, 1951; Mitchum et al., 1977; Pirmez et al., 1998). Additional evidence of clinoforms within the deposit is the trend in layer dip versus elevation (Fig. 3). An idealized progra-

dational delta will accumulate sediment in clinoforms as shown schematically in Fig. 3b (e.g., Rich, 1951; Mitchum et al., 1977; Pirmez et al., 1998). If this deposit is eroded and the stratigraphic architecture exposed, as is the case at the Jezero crater western fan deposit erosional front (Figs. 1b, 2; Schon et al., 2012), a trend of increasing layer dip with increasing elevation (and stratigraphic position) along the erosional front is expected (e.g., Fig. 3b). This is the trend observed for the western fan deposit (Fig. 3a). We interpret the more steeply dipping (>2◦ ) downlapping strata as delta foresets and the underlying, shallowly dipping (<2◦ ) strata as delta bottomsets. These dips are consistent with observations of foreset and bottomset dips in both terrestrial delta deposits (Rich, 1951; Orton and Reading, 1993) and other martian fan deposits interpreted as deltaic in origin (e.g., Lewis and Aharonson, 2006; Ansan et al., 2011; DiBiase et al., 2013). Therefore, based on the exposed stratal geometries of the Jezero crater western fan, we conclude that this deposit is deltaic in origin. This conclusion is consistent with previous interpretations based on the morphology and geologic setting of this deposit (Fassett and Head, 2005; Ehlmann et al., 2008a; Schon et al., 2012). However, we again note that it is only the results of an analysis of the stratigraphic architecture of this deposit (Figs. 2, 3) that allow us to uniquely constrain the depositional environment for this fan. We suggest that future studies of sedimentary fan deposits on Mars should not only focus on morphology and geologic context, but also on observations of exposed stratal geometries in order to definitively classify a deposit as a delta as opposed to an alluvial fan. The stratal geometries of the Jezero crater western delta, and in particular the interpreted foreset dips (∼2–9◦ ), can provide further insight into the sedimentology of the deposit. Based on a comparison to delta foreset slopes from a compilation of terrestrial data, the Jezero delta foreset dip angles fall within the range of values expected for deposits dominated by sand- or silt-sized grains

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to foreset transition (e.g., Pirmez et al., 1998); however, it is also possible that this observation records something fundamental about how the Jezero crater western delta deposit was constructed. Future studies that focus on examining the deposits of the topset distributary channels (e.g., Fassett and Head, 2005; Schon et al., 2012) and their relationship to the exposed foresets and bottomsets analyzed here may be able to further reconstruct the paleohydraulics and sediment transport conditions of the flows associated with the formation of this deposit. Our conclusion that the Jezero crater western fan deposit is a delta has important implications for understanding the geologic history of this deposit and inferring the paleoenvironmental information that may be preserved within these rocks. The western delta deposit must have formed when a lake existed within the Jezero crater basin. It has been shown that the fluvial activity associated with the formation of this paleolake occurred during the major era of valley network formation (Fassett and Head, 2005, 2008), which is hypothesized to represent an era of intense fluvial activity early in martian history that ceased at approximately the Noachian–Hesperian boundary, ∼3.7 Ga (Howard et al., 2005; Irwin et al., 2005; Fassett and Head, 2008). Therefore, the Jezero crater western delta deposit is likely to record important information about ancient environmental conditions during this early period of widespread fluvial activity on Mars. 4.2. Mineralogy of the Jezero crater western fan deposit

Fig. 3. (a) Mean layer elevation versus dip for the 26 mapped layers in the Jezero crater western fan deposit. Note the trend of increasing dip with increasing elevation (i.e., stratigraphic position). Error bars for mean elevations are 1 standard deviation on extracted elevation values along the layer. Error bars for dips are the 95% confidence limits from the linear least squares plane misfit. (b) Idealized sketch of clinoforms within a prograding delta showing the expected trend of increasing layer dip with increasing elevation along an erosional front of the deposit.

(Orton and Reading, 1993). However, a direct comparison to empirical observations of terrestrial delta slopes is highly unlikely to be sufficient to accurately characterize any given delta deposit on Mars. Although this is the case, the foreset angles are large enough to exclude the possibility of the Jezero crater western delta being built into a body of water that was shallow compared with the source channel depths, which would act to suppress the formation of steeper foreset strata (e.g., Orton and Reading, 1993). This conclusion is consistent with the absolute elevations of these strata, as they sit ∼100 m below the minimum water level of the Jezero crater paleolake (Fig. 1; Fassett and Head, 2005). The measured foreset angles for the Jezero crater western delta are also well below the angle of repose. This indicates that the studied portion of the deposit must have been primarily built by deposition of material out of suspension, as opposed to by discrete, gravity-driven mass wasting events that are common on steeply sloping terrestrial delta fronts (e.g., Orton and Reading, 1993; Pirmez et al., 1998). This interpretation may simply be the result of evaluating the portions of the delta that have remained preserved and exposed, as we have examined only the foreset to bottomset transition of this deposit, and clinoforms are commonly asymmetric with steeper angles expected near the topset

The spatial distribution of mineral identifications within the Jezero crater western delta deposit is also consistent with a deltaic origin. Of particular importance is the identification of strong signatures of Fe/Mg-smectite, likely a detrital clay mineral in this system (Ehlmann et al., 2008a, 2009; Goudge et al., 2015), in the stratigraphically lowest units of the delta deposit (Figs. 4a, c), which we interpret as bottomsets. This observation is consistent with terrestrial facies models of delta deposits in which finegrained materials transported in suspension, such as clay minerals, are concentrated in bottomsets in the most distal margins of the delta (Rich, 1951; Orton and Reading, 1993). Interestingly, the few exposures of Mg-rich carbonate within the western delta deposit are concentrated in point bar deposits (Figs. 4a, d) of the topset distributary channels of the delta (Schon et al., 2012). This observation is consistent with previous conclusions that the Mg-rich carbonate in this deposit is detrital (Ehlmann et al., 2008a, 2008b, 2009; Goudge et al., 2015), which would also imply the carbonate is of a coarser grain size than the Fe/Mg-smectite. However, it is also possible that the carbonate signature in these coarser-grained facies is a cement precipitated from carbonate-saturated fluids, possibly as a result of the dissolution of carbonate in the watershed and interaction with atmospheric CO2 . In this latter case, the carbonates may record information about ancient climate conditions during the period of lacustrine activity. Regardless of the specific origin, the provenance of sediments within this deposit can be traced to regional Fe/Mg-smectite- and Mg-rich carbonate-bearing units within the ∼12,000 km2 watershed of the Jezero western delta deposit (Goudge et al., 2015). This deposit has thus incorporated sediment from a large region of the adjacent martian crust that was primarily aqueously altered prior to the fluvial activity that fed the paleolake within the Jezero crater basin during delta formation (Goudge et al., 2015). 4.3. Implications for future exploration of Jezero crater Our observations and conclusions add to the suggestion that the Jezero crater paleolake basin is a compelling site for fu-

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Fig. 4. Mineralogy of the Jezero crater western fan deposit from CRISM data. North is up in all images. (a) Overview CRISM parameter map of the Jezero crater western fan deposit. Locations of parts (c) and (d) are shown by white boxes. CRISM false-color map created using the parameters OLINDEX2 (Salvatore et al., 2010), BD2500 (Ehlmann et al., 2009), and D2300 (Pelkey et al., 2007), overlain on a mosaic of HiRISE image PSP_003798_1985 and CTX image P04_002664_1988. Interpretive legend based on CRISM parameter map is shown in upper right corner. Outline of fan deposit is from Goudge et al. (2015), and the parameter map is highlighted in this region. (b) CRISM ratioed spectra of Mg-rich carbonate (green spectra C1 and C2) and Fe/Mg-smectite (blue spectra S1, S2, and S3) detections from the fan deposit. Numerator spectra are extracted from CRISM image HRL000040FF at the locations shown by the labeled circles in part (a). Dashed lines are located at ∼1.4, 1.92, 2.3, and 2.5 μm. (c) Example Fe/Mg-smectite detection within the stratigraphically lowest units of the fan deposit, interpreted to be delta bottomsets. Image shows the same CRISM false-color parameter map as in part (a) overlain on a portion of HiRISE image PSP_003798_1985. (d) Example Mg-rich carbonate detection within light-toned material interpreted to be fluvial channel point bar deposits (Schon et al., 2012). Image shows the same CRISM false-color parameter map as in part (a) overlain on a portion of HiRISE image PSP_003798_1985. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

ture in situ exploration (e.g., Ehlmann et al., 2008a; Schon et al., 2012), such as by the upcoming Mars 2020 rover (Mustard et al., 2013). This site provides an exceptionally exposed delta deposit with a well-studied geologic history (Fassett and Head, 2005, 2008; Ehlmann et al., 2008a, 2008b, 2009; Schon et al., 2012; Goudge et al., 2015). The erosional front and isolated erosional remnants of the delta deposit (Figs. 1b, 2; Schon et al., 2012; Goudge et al., 2015) are accessible to a rover that lands on the Jezero crater floor and would provide a wealth of potential targets for in situ exploration, as has been seen with the sedimentary deposits within Gale crater currently being explored by the Mars Science Laboratory Curiosity rover (e.g., Freissinet et al., 2015; Grotzinger et al., 2015). In situ exploration of deltaic sediment within Jezero crater would be highly complementary to the ongoing analyses of fluviolacustrine and deltaic sedimentary deposits at Gale crater by Cu-

riosity (Grotzinger et al., 2015), as the Gale crater paleolake is hydrologically closed (Irwin et al., 2005), whereas the Jezero crater paleolake is hydrologically open (Fassett and Head, 2005). Hydrologically open and closed paleolake basins should be expected to have water columns with a significantly different aqueous geochemistry: hydrologically open lakes have a continual throughput of water and dissolved ions, whereas hydrologically closed lakes only lose water to evaporation and infiltration, and so are able to continually build up large concentrations of dissolved ions. Comparing results from in situ analyses of the mineralogy and geochemistry of deltaic sediment deposited in paleolake basins with distinct hydrologic settings would provide an exciting opportunity to further understand a more complete range of ancient martian aqueous surface environments than could be inferred from in situ analyses at only one type of paleolake basin.

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Furthermore, exposures of Fe/Mg-smectite-rich deltaic bottomsets (Figs. 2, 4a, c) that would be accessible to a rover provide an excellent opportunity for in situ analyses from an astrobiology perspective. Quiescent depositional environments dominated by fine-grained sedimentation, such as deltaic bottomsets, have a high concentration and preservation potential for organic matter (e.g., Rich, 1951; Blair and Aller, 2012), making them attractive locations for in situ exploration on Mars (Summons et al., 2011). As originally suggested by Ehlmann et al. (2008a), the presence of Fe/Mg-smectite within the delta deposit, and particularly within the bottomsets of the deposit (Figs. 4a, c), is of further importance as clay minerals have the ability to bind organic matter (e.g., Kennedy et al., 2002). Indeed, it has recently been shown that claybearing lacustrine mudstones observed by Curiosity in Gale crater exhibit evidence for preserved organic molecules (Freissinet et al., 2015). Aside from the delta deposit itself, Jezero crater also contains exposures of a widespread, regional Mg-rich carbonate-bearing unit that underlies the delta (Ehlmann et al., 2008b, 2009; Goudge et al., 2015) and a volcanic unit on the floor of the basin (Schon et al., 2012; Goudge et al., 2015). While the volcanic unit within Jezero crater is likely to bury any non-deltaic distal sediment, our observations indicate that there are readily accessible exposures of deltaic bottomsets within the basin (Figs. 2, 4a, c), which are equally as compelling for in situ exploration from an astrobiology perspective (Summons et al., 2011). Therefore, the presence of a dateable volcanic unit and a regional carbonate-bearing unit, in addition to the well-exposed deltaic sediment, constitute a diverse array of scientifically interesting geologic units within Jezero crater that we suggest make this site highly compelling for future exploration. 5. Conclusions Based on both the stratigraphic architecture and mineralogy of the Jezero crater western fan deposit, we conclude that this deposit is deltaic in origin and formed when the basin hosted a stable body of standing water during the era of valley network formation early in martian history. We stress the importance of examining the stratigraphic architecture of sedimentary fan deposits on Mars, in addition to their morphology and geologic setting, for definitively constraining a depositional environment (e.g., delta versus alluvial fan). The Jezero crater western delta deposit displays well-exposed strata that are likely to preserve paleoenvironmental information from early in Mars’ history and have a high concentration and preservation potential for organic matter. This includes strata we interpret as Fe/Mg-smectite-rich bottomsets that would be accessible to exploration by a landed mission, such as the upcoming Mars 2020 rover. We suggest that in situ analysis of the remarkably wellpreserved stratigraphy of the western delta deposit shown here has the potential to address both geologically and astrobiologically compelling questions, and that Jezero crater is a prime candidate for exploration by future landed missions to Mars. Acknowledgements We thank J.L. Dickson for help with image and topographic data processing, and B.L. Ehlmann and D. Mohrig for helpful discussions. Two anonymous reviewers are thanked for helpful comments that improved the quality of this manuscript, and C. Sotin is thanked for editorial handling. We express our appreciation for the superb work of the NASA MRO project team and the CRISM Science Operations Center (SOC). TAG gratefully acknowledges support for this work from the Natural Sciences and Engineering Research Council of Canada (NSERC) Postgraduate Scholarships Pro-

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