# APXS Derived Geochemistry of Shallow Water Lens Bodies Within the Mirador Formation, Gale Crater, Mars-Evidence for Intermittent Wet Periods and Implications for the Water Record

C D O'connell-cooper, L M Thompson, J G Spray, S J Vanbommel, J A Berger, R. Gellert, Gwénaël Caravaca, D. Fey, M A Mccraig, N I Boyd, et al.

► To cite this version:

C D O'connell-cooper, L M Thompson, J G Spray, S J Vanbommel, J A Berger, et al. APXS Derived Geochemistry of Shallow Water Lens Bodies Within the Mirador Formation, Gale Crater, Mars-Evidence for Intermittent Wet Periods and Implications for the Water Record. Journal of Geophysical Research. Planets, 2025, 130 (11), pp.e2025JE009001. 10.1029/2025JE009001. hal-05346659

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<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# JGR Planets

## RESEARCH ARTICLE

10.1029/2025JE009001

**Key Points:**

* We present Alpha Particle X-ray Spectrometer data for interdune bodies in the Contigo member of the Mirador formation, Gale crater

* We interpret the lenses to represent a mixing between the host MtSharp group and an influx of relatively unaltered basaltic units

* The interdune lenses highlight locally changing environmental conditions through the periodic presence of water in a dry environment

**Supporting Information:**

Supporting Information may be found in the online version of this article.

**Correspondence to:**

C. D. O'Connell-Cooper,
oconnell.cooper@unb.ca

**Citation:**

O'Connell-Cooper, C. D., Thompson, L. M., Spray, J. G., VanBommel, S. J., Berger, J. A., Gellert, R., et al. (2025). APXS derived geochemistry of shallow water lens bodies within the Mirador formation, Gale crater, Mars—Evidence for intermittent wet periods and implications for the water record. *Journal of Geophysical Research: Planets*, 130, e2025JE009001. https://doi.org/10.1029/2025JE009001

Received 19 FEB 2025

Accepted 10 SEP 2025

**Author Contributions:**

**Data curation:** S. J. VanBommel, R. Gellert, D. Fey, M. A. McCraig, N. I. Boyd

**Funding acquisition:** R. Gellert

**Resources:** D. Fey

**Writing - review & editing:**

L. M. Thompson, J. G. Spray, G. Caravaca

# APXS Derived Geochemistry of Shallow Water Lens Bodies Within the Mirador Formation, Gale Crater, Mars— Evidence for Intermittent Wet Periods and Implications for the Water Record

<figure><img src="image_2.png" /></figure>

C. D. O'Connell-Cooper¹ ID, L. M. Thompson¹ ID, J. G. Spray¹ ID, S. J. VanBommel² ID, J. A. Berger³, R. Gellert⁴, G. Caravaca⁵,⁶ ID, D. Fey⁷ ID, M. A. McCraig⁴, N. I. Boyd⁴, A. B. Bryk⁸ ID, W. E. Dietrich⁸, S. Gupta⁹, A. Roberts⁹ ID, and A. Yen¹⁰ ID

¹Planetary and Space Science Centre, University of New Brunswick, Fredericton, NB, Canada, ²Washington University in St. Louis, St. Louis, MO, USA, ³Amentum, NASA Johnson Space Center, Houston, TX, USA, ⁴University of Guelph, Guelph, ON, Canada, ⁵Géosciences Environnement Toulouse (GET), UMR 5563, Université de Toulouse, CNRS, CNES, IRD, Toulouse, France, ⁶Institut de Recherche en Astrophysique et Planétologie (IRAP), UMR 5277, Université de Toulouse, CNRS, CNES, Toulouse, France, ⁷Malin Space Science Systems, San Diego, CA, USA, ⁸Earth and Planetary Science, University of California, Berkeley, CA, USA, ⁹Department of Earth Science and Engineering, Imperial College London, London, England, ¹⁰Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA

**Abstract** The Mount Sharp group in Gale crater is typically interpreted to record a progressive change (with increasing elevation) from wetter fluvio-lacustrine environments to dry, eolian environments. This shift has been linked to orbital evidence for a global change in environmental and depositional conditions leading to an overall drying out of Mars. The Mars Science Laboratory Curiosity rover has documented evidence at Gale for the periodic continuation of aqueous depositional processes within the Mount Sharp group within the Clay Sulfate Transition, suggesting that, in Gale crater, it was not a simple, unidirectional drying out pattern. We present data on a series of interdune lenses at Gale crater, providing a window into locally changing environmental conditions. They represent wet (fluvial to lacustrine) deposits in the otherwise dry eolian Contigo dune field. Alpha Particle X-Ray Spectrometer geochemical data reveal compositional similarities between the interdune lenses and both the host Contigo member and unaltered basaltic sediments. Modeling indicates that the lenses may contain up to 52% basaltic sands/soils mixed with local sediment. A high degree of homogeneity in composition and sedimentary structures are identified across the lenses. This suggests a common source of influx material and a repeated process (wet episodes in a dry environment over several cycles of seasonal variations). Furthermore, differences in composition between the interdune lenses and the host Contigo member help to elucidate the timing of alteration within the Mount Sharp group.

**Plain Language Summary** Although Mars is a dry planet today, there is abundant evidence for the presence of water earlier in the planet's history. At some point, however, the climate is interpreted to have changed from one with abundant water to the dry planet we see today. The Mars Science Laboratory Curiosity rover in Gale crater has found evidence of local climatic fluctuations between wetter and drier conditions. This research highlights one aspect of this evidence. Within the dry dune fields of the Contigo member, a series of interdune lenses were identified. These are interpreted to have formed in a combination of running water (in river channels), which then accumulated in shallow pools under lower energy conditions. Modeling using Alpha Particle X-Ray Spectrometer geochemical data indicates that the lenses may contain up to 52% basaltic sand mixed with the sediments that make up the host Contigo member. The interdunes repeat over at least 50 m of elevation, indicating a process that took place more than once.

## 1. Introduction

The Mars Science Laboratory (MSL) Curiosity rover has been exploring the Gale impact crater on Mars since landing on 6 August 2012. Gale is a ~150 km diameter impact structure, the central structure of which is largely obscured by an ~5 km high mound of stratified rock, Aeolis Mons (informally known as Mount Sharp) (Anderson & Bell, 2010; Golombek et al., 2012; Grotzinger et al., 2012; Grotzinger, Crisp, et al., 2015; Grotzinger, Gupta, et al., 2015; Milliken et al., 2010). The impact structure formed close to the Noachian-Hesperian transition, c. 3.8 to 3.55 Ga (Le Deit et al., 2013; Thomson et al., 2011). Based on orbital spectroscopic data, this period has been

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interpreted to have experienced a fundamental global shift in environmental and depositional conditions, from a wetter climate, distinguished in part by the presence of phyllosilicates, to a drier climate that favored the formation of sulfates and other salts, and oxides (Bibring et al., 2006; Carr & Head, 2010; Ehlmann et al., 2011, 2013). There is some evidence to suggest transient wet events continuing into the Hesperian and Amazonian, suggesting that it was not a simple, unidirectional drying out pattern (for example, Gupta et al., 2022, 2023; Morgan et al., 2022; Ramirez & Craddock, 2018).

At Gale crater, a corresponding zone of “sulfate-clay mixture” or “clay-sulfate transition” zone (CST) (Figure 1) has been defined within the Mount Sharp group (MtSgrp), primarily through orbital observations. This represents a transition from phyllosilicate-rich strata to the layered Mg-sulfate bearing unit (LSu), interpreted as marking a change from a wetter to drier environment (for example, Bennett et al., 2023; Edgar et al., 2024; Fraeman et al., 2016; Milliken et al., 2010; Rapin et al., 2021; Sheppard et al., 2021). Curiosity's traverse across the CST has provided a unique opportunity to study this proposed global climatic transition to a drier climate.

There are several lines of evidence within the CST for continued aqueous activity. The sand sheet depositional environment within the LSu proposed by Edgar et al. (2024), Gupta et al. (2023), Rapin et al. (2021, 2022), Roberts et al. (2024, 2025), indicates that the water table was closer to the surface, hindering the ability to form dunes (Kocurek & Nielson, 1986; Runyon et al., 2017). The lateral extent (~75 km) of the shallow lacustrine deposits of the Amapari Marker Band (AMB) may indicate a more extensive wet episode (Gupta et al., 2023; Milliken et al., 2010; Mondro, Fedo, et al., 2025; Mondro, Grotzinger, et al., 2025; Weitz et al., 2022). Identification of Ca + S (within the CST) and Mg + S (in the overlying LSu) enriched targets in APXS data (Sections 4, 5) (Berger et al., 2022, 2023; O'Connell-Cooper et al., 2024; Thompson, Berger, Boyd, et al., 2020; Thompson et al., 2024) indicates continuing fluid activity, as do the “boxwork structures” higher in the stratigraphy (average elevation of ~3,620 ± 50 m), which are interpreted as a fracture system potentially resulting from groundwater diagenesis (Siebach & Grotzinger, 2014).

Interstratified lenticular bodies, typically 0.5–1 m thick and up to 20 m in length, repeating periodically over a 50 m elevation range, were identified in the Contigo member (mbr) within the CST (Figure 1, Figure S2 in Supporting Information S1) (Caravaca, Le Mouélic, et al., 2022; Caravaca et al., 2025; Gupta et al., 2022). Sedimentary structures and textures within the Contigo lenticular bodies suggest that they were deposited in interdune environments (Section 2.2) (Caravaca, Le Mouélic, et al., 2022; Caravaca et al., 2025; Gupta et al., 2022). Interdune lenses form under a diverse range of environmental conditions, from dry to damp to wet (for example, Ahlbrandt & Fryberger, 1981; Kocurek, 1981; Lancaster & Teller, 1988; Mountney, 2012). As interdune lenses can therefore be important indicators of localized environments and changes, the Contigo lenses are of importance in understanding the local hydrogeologic and climatic record in Gale crater.

In early 2022, in situ analysis was undertaken by Curiosity at two interdune sites. (a) An intensive 22 sol (sol = martian solar day; mean period of 24 hr, 39 min, and 35.244 s) campaign concentrated on the informally named “Prow” and “Panari” outcrops located at the base of the Mirador butte (elevation -3,955 m; Figure 1). The Prow is a well preserved, ~18 m long, ≈0.5–1 m thick lens (Figures 2a and 2b; Figures S2-S15 in Supporting Information S1). The adjacent Panari lens is smaller (~1 m long, ≈0.5–1 m thick; ~6 m from the center of the Prow) and less well-preserved (Figures 2a and 2c; Figures S4, S5, S16-S19 in Supporting Information S1) (Gupta et al., 2022). (b) The “Issano” outcrop is the highest stratigraphic lenticular body identified (elevation -3,916 m) and is separated from the Prow and Panari by 200 m of rover traverse and 40 m of elevation gain (Figures 1 and 2d; Figures S20 and S21 in Supporting Information S1). In situ investigation allowed for closeup imaging using Mastcam (Malin et al., 2017), the Chemistry Camera (ChemCam) Remote Micro Imager (Maurice et al., 2012), and the Mars Analysis Hand Lens Imager (MAHLI) (Edgett et al., 2012) instruments. Images acquired were used in Digital Outcrop Model image reconstruction for the Prow and Panari outcrops (Caravaca, Mangold, et al., 2022). These complemented geochemical analysis by the APXS (Campbell et al., 2012; Gellert & Clark, 2015; Gellert et al., 2006) and the ChemCam Laser Induced Breakdown Spectroscopy (LIBS) instrument (Wiens et al., 2012).

We present and discuss the results of the geochemical investigation of the Contigo interdune lenses and the host Contigo mbr by the APXS instrument. The results are compared with previous MtSgrp bedrock analyses, the CST in general, and other Gale crater APXS analyses. The APXS-derived geochemistry helps to elucidate the origin of the interdune lenses, their relationship to the eolian host Contigo mbr and questions regarding provenance as well as timing of alteration within the MtSgrp.
<figure><img src="image_3.png" /><figcaption>Figure 1.</figcaption></figure>
## 2. Geological Context

### 2.1. Sedimentology of the Mount Sharp Group and the Clay Sulfate Transition

The MtSgrp encompasses a diverse range of depositional settings, broadly interpreted to represent a progression from wetter to drier conditions (see references below). Layered sedimentary strata of the MtSgrp are almost horizontal with a shallow dip (2-7°) (Turner & Lewis, 2023), thus elevation is an acceptable proxy for strati-graphic position (Grotzinger, Crisp, et al., 2015; Grotzinger, Gupta, et al., 2015). With increasing elevation, the depositional setting moves from fluvio-lacustrine in the Murray formation (fm) (for example, Edgar et al., 2020; Grotzinger et al., 2014; Gwizd et al., 2022; Stack et al., 2019; Vasavada, 2022) through more marginal lake environments and river bar deposits in the Carolyn Shoemaker fm (e.g., Caravaca, Mangold, et al., 2022; Car-denas et al., 2022; Fedo et al., 2022), to dry, eolian environments in the Mirador fm (for example, Edgar et al., 2024; Gupta et al., 2023; Rapin et al., 2021; Roberts et al., 2024, 2025). Alternative interpretations posit that the entire sequence may be predominantly lacustrine or, alternatively, eolian, the latter without the need for any large-scale aqueous processes (for example, Kite et al., 2013, 2016; Liu et al., 2021; Schieber et al., 2024, 2025; Yen et al., 2024, 2025).

Investigation of the Clay Sulfate Transition (CST) (sols 3,110 to 3,637) spanned 150 m of elevation gain (–4,070 m to –3,640 m), and approximately 2.7 km drive distance. The CST is herein defined as encompassing the Pontours mbr (Carolyn Shoemaker fm), and the Dunnideer, Port Logan, Contigo and Catrimani mbrs (Mirador fm), terminating at the AMB (Amapari mbr) (AMB) (Figure 1; Table 1; Table S1). At the base of the CST sequence, millimeter scale parallel planar laminations in the lowermost Pontours mbr suggest a continuation of marginal lake environments previously identified in the underlying Glasgow mbr (Edgar et al., 2024; Fedo et al., 2022; Meyer et al., 2024). Primary sedimentary structures (such as laminations) and grain size for much of the Pontours and Dunnideer mbrs are obscured by a strong diagenetic overprint (typically in the form of pervasive millimeter-scale nodules), which decreases in intensity with elevation (Edgar et al., 2024; Meyer et al., 2024). In the Port Logan and Contigo mbrs, large cross-stratification structures (decimeter to multiple meters) indicate the presence of well developed eolian dunes (Edgar et al., 2024; Gupta et al., 2022; Meyer et al., 2024; Rapin et al., 2021, 2022; Roberts et al., 2024, 2025).

The upper 30 m (m) of the CST lie within the “Marker Band Valley” (MBV) (Figure 1), which coincides with the start of the Layered Sulfate unit (LSu) as identified orbitally (e.g., Fraeman et al., 2016). This is detected in APXS targets as a consistent, significant enrichment in correlated Mg + S (e.g., Berger et al., 2023; O’Connell-Cooper et al., 2024). The MBV drill target (Canaima; analyzed by APXS on sol 3,609) marked the first identification of crystalline magnesium sulfate [starkeyite; MgSO₄·4H₂O], by CheMin (Chipera et al., 2023). Cross stratification is replaced by thin finely laminated layers (pinstripe lamination—Fryberger & Schenk, 1988) interpreted as ev-idence for an eolian sand sheet depositional environment (e.g., Edgar et al., 2024; Rapin et al., 2021; Roberts et al., 2024, 2025).

Mineralogy, as determined by the Chemistry and Mineralogy (CheMin) instrument using X-ray diffraction (Blake et al., 2012), reveals that phyllosilicates are not present in CST drill samples above the Pontours drill target (–4,041 m) (Rampe et al., 2023). Additionally, evolved gas analyses by the sample Analysis at Mars instrument (Mahaffy et al., 2012) show changes in high-temperature releases for S, indicative of a shift from predominantly Fe-sulfates in samples below the Pontours drill target to increasing contributions from Mg-sulfates in drill targets above Pontours (Franz et al., 2023).

The CST terminates at the AMB, a dark toned, indurated, 1–3 m thick unit, traceable (in non-continuous exposure) across roughly 75 km of the northwestern, southwestern, and southeastern margins of Mount Sharp in Gale crater (Milliken et al., 2010; Thompson et al., 2024; Weitz et al., 2022). Overlying the AMB, the Mg-sulfate bearing LSu (including the pinstripe laminated Chenapau mbr immediately above the AMB) (Edgar

**Figure 1.** Stratigraphic column and localization maps for the Contigo lensoid bodies, the Clay Sulfate Transition (CST) and the lowermost 100 m of the Chenapau mbr (Layered Sulfate unit). (a) Stratigraphic column after Edgar et al. (2024) and the Mars Science Laboratory Sedimentology-Stratigraphy Working Group.(b) Gale crater, showing Mount Sharp Ascent Route since landing in gray, highlighting the Clay Sulfate Transition and overlying Chenapau mbr (Mirador fm). (c) Traverse Sols 3,330–3,700. Member boundaries show individual members as encountered on the traverse only and do not imply any interpretation of lateral extent outside the traverse path. Base maps for images 1b-1c: High Resolution Stereo Camera (HiRise; on board the European Space Agency Mars Express orbiter) orbital images of Gale crater, Mars. Image credits: NASA/JPL-Caltech.
<caption>**Table 1**</caption>

APXS Compositional Data for Contigo Interdune Targets and Select Averages

<table><thead><tr><th></th><th>Unit/target</th><th>SiO<sub>2</sub></th><th>TiO<sub>2</sub></th><th>Al<sub>2</sub>O<sub>3</sub></th><th>Cr<sub>2</sub>O<sub>3</sub></th><th>MnO</th><th>FeO</th><th>MgO</th><th>CaO</th><th>Na<sub>2</sub>O</th><th>K<sub>2</sub>O</th><th>P<sub>2</sub>O<sub>5</sub></th><th>SO<sub>3</sub></th><th>Cl</th><th>Ni</th><th>Zn</th><th>Br</th><th>Fe/Mn</th></tr></thead><tbody><tr><td colspan="20">Contigo Interdune targets, Mirador formation (errors in Table S1 Data Repository)</td></tr><tr><td colspan="20">Issano outcrop</td></tr><tr><td>3493</td><td>Issano_DRT</td><td>37.41</td><td>1.01</td><td>7.30</td><td>0.31</td><td>0.42</td><td>17.79</td><td>7.56</td><td>8.63</td><td>2.50</td><td>0.61</td><td>1.04</td><td>12.53</td><td>2.45</td><td>757</td><td>1,493</td><td>851</td><td>42</td></tr><tr><td colspan="20">Panari outcrop</td></tr><tr><td>3362</td><td>Morok</td><td>39.53</td><td>1.05</td><td>7.74</td><td>0.24</td><td>0.38</td><td>17.04</td><td>7.07</td><td>8.16</td><td>2.29</td><td>0.66</td><td>0.91</td><td>12.57</td><td>1.89</td><td>944</td><td>1,474</td><td>1,531</td><td>45</td></tr><tr><td>3362</td><td>Coati</td><td>41.21</td><td>0.94</td><td>7.94</td><td>0.23</td><td>0.43</td><td>18.69</td><td>8.17</td><td>6.61</td><td>2.65</td><td>0.66</td><td>1.05</td><td>9.27</td><td>1.64</td><td>933</td><td>1,456</td><td>1,901</td><td>43</td></tr><tr><td colspan="20">Prow outcrop</td></tr><tr><td>3355</td><td>Kamuda</td><td>42.76</td><td>1.07</td><td>8.15</td><td>0.30</td><td>0.36</td><td>21.04</td><td>8.08</td><td>5.30</td><td>2.60</td><td>0.56</td><td>0.97</td><td>6.93</td><td>1.40</td><td>1,131</td><td>1,553</td><td>1,306</td><td>58</td></tr><tr><td>3355</td><td>Angasima</td><td>42.21</td><td>1.03</td><td>8.38</td><td>0.29</td><td>0.30</td><td>20.47</td><td>7.35</td><td>5.75</td><td>2.48</td><td>0.65</td><td>1.02</td><td>8.41</td><td>1.21</td><td>1,235</td><td>1,949</td><td>509</td><td>68</td></tr><tr><td>3374</td><td>Surumu</td><td>43.21</td><td>1.01</td><td>8.93</td><td>0.22</td><td>0.44</td><td>19.64</td><td>7.70</td><td>4.78</td><td>2.72</td><td>0.69</td><td>0.97</td><td>6.93</td><td>2.30</td><td>1,164</td><td>1,954</td><td>849</td><td>45</td></tr><tr><td>3374</td><td>Surumu_offset</td><td>42.79</td><td>0.98</td><td>8.70</td><td>0.22</td><td>0.53</td><td>18.25</td><td>7.81</td><td>5.35</td><td>2.58</td><td>0.69</td><td>0.90</td><td>8.67</td><td>2.01</td><td>1,065</td><td>1,915</td><td>884</td><td>34</td></tr><tr><td>3376</td><td>Kokadai</td><td>42.13</td><td>0.98</td><td>8.92</td><td>0.19</td><td>0.36</td><td>15.55</td><td>7.96</td><td>6.30</td><td>2.71</td><td>0.57</td><td>1.01</td><td>11.01</td><td>1.95</td><td>806</td><td>1,421</td><td>527</td><td>43</td></tr><tr><td colspan="20">Prow outcrop—float blocks, origin unresolvable</td></tr><tr><td>3364</td><td>Mazaruni</td><td>42.60</td><td>1.01</td><td>8.66</td><td>0.21</td><td>0.48</td><td>19.21</td><td>7.84</td><td>5.57</td><td>2.51</td><td>0.76</td><td>0.92</td><td>7.43</td><td>2.29</td><td>1,171</td><td>2,432</td><td>924</td><td>40</td></tr><tr><td>3354</td><td>Ilu</td><td>43.36</td><td>0.83</td><td>7.05</td><td>0.29</td><td>0.27</td><td>21.33</td><td>7.67</td><td>5.19</td><td>2.40</td><td>0.55</td><td>0.75</td><td>7.48</td><td>1.56</td><td>1,229</td><td>2,438</td><td>2,671</td><td>79</td></tr><tr><td colspan="20">Averages and standard deviations (1σ):</td></tr><tr><td rowspan="2">10</td><td>Contigo interdune lenses</td><td>41.72</td><td>0.99</td><td>8.18</td><td>0.25</td><td>0.40</td><td>18.90</td><td>7.72</td><td>6.16</td><td>2.54</td><td>0.64</td><td>0.95</td><td>9.12</td><td>1.87</td><td>1,044</td><td>1,809</td><td>1,195</td><td>50</td></tr><tr><td>St.Dev</td><td>1.79</td><td>0.06</td><td>0.63</td><td>0.04</td><td>0.08</td><td>1.73</td><td>0.32</td><td>1.23</td><td>0.13</td><td>0.06</td><td>0.08</td><td>2.07</td><td>0.39</td><td>164</td><td>373</td><td>642</td><td>13</td></tr><tr><td rowspan="2">220</td><td>Mount Sharp group (MtSGrp)</td><td>47.98</td><td>1.05</td><td>9.01</td><td>0.31</td><td>0.23</td><td>18.05</td><td>5.35</td><td>4.83</td><td>2.47</td><td>0.88</td><td>1.01</td><td>7.29</td><td>1.18</td><td>850</td><td>1,299</td><td>349</td><td>94</td></tr><tr><td>St.Dev</td><td>3.56</td><td>0.08</td><td>0.90</td><td>0.03</td><td>0.11</td><td>2.37</td><td>1.14</td><td>1.37</td><td>0.26</td><td>0.14</td><td>0.26</td><td>2.90</td><td>0.49</td><td>176</td><td>566</td><td>295</td><td>47</td></tr><tr><td rowspan="2">44</td><td>Clay Sulfate Transition (CST)</td><td>46.31</td><td>1.05</td><td>8.44</td><td>0.30</td><td>0.24</td><td>18.80</td><td>4.73</td><td>5.74</td><td>2.35</td><td>0.80</td><td>1.13</td><td>8.86</td><td>0.92</td><td>877</td><td>1,186</td><td>256</td><td>83</td></tr><tr><td>St.Dev</td><td>4.05</td><td>0.07</td><td>0.66</td><td>0.03</td><td>0.07</td><td>1.65</td><td>1.23</td><td>1.50</td><td>0.19</td><td>0.10</td><td>0.11</td><td>3.20</td><td>0.34</td><td>116</td><td>346</td><td>164</td><td>22</td></tr><tr><td rowspan="2">35</td><td>Mirador formation (fm)</td><td>46.57</td><td>1.06</td><td>8.49</td><td>0.30</td><td>0.24</td><td>18.93</td><td>4.58</td><td>5.78</td><td>2.38</td><td>0.81</td><td>1.13</td><td>8.50</td><td>0.89</td><td>895</td><td>1,088</td><td>244</td><td>84</td></tr><tr><td>St.Dev</td><td>3.99</td><td>0.07</td><td>0.65</td><td>0.02</td><td>0.07</td><td>1.67</td><td>1.19</td><td>1.45</td><td>0.19</td><td>0.10</td><td>0.09</td><td>2.98</td><td>0.37</td><td>113</td><td>252</td><td>162</td><td>23</td></tr><tr><td rowspan="2">68</td><td>Carolyn Shoemaker fm</td><td>47.14</td><td>1.06</td><td>8.68</td><td>0.31</td><td>0.25</td><td>18.34</td><td>5.73</td><td>4.84</td><td>2.40</td><td>0.91</td><td>1.01</td><td>7.64</td><td>1.30</td><td>868</td><td>1,557</td><td>517</td><td>92</td></tr><tr><td>St.Dev</td><td>3.13</td><td>0.08</td><td>0.51</td><td>0.03</td><td>0.15</td><td>1.50</td><td>1.15</td><td>1.32</td><td>0.23</td><td>0.14</td><td>0.16</td><td>2.97</td><td>0.35</td><td>141</td><td>611</td><td>298</td><td>40</td></tr><tr><td rowspan="2">117</td><td>Murray fm</td><td>48.90</td><td>1.04</td><td>9.35</td><td>0.32</td><td>0.22</td><td>17.62</td><td>5.36</td><td>4.54</td><td>2.55</td><td>0.88</td><td>0.98</td><td>6.72</td><td>1.20</td><td>826</td><td>1,213</td><td>283</td><td>98</td></tr><tr><td>St.Dev</td><td>3.39</td><td>0.08</td><td>0.99</td><td>0.03</td><td>0.09</td><td>2.83</td><td>1.00</td><td>1.25</td><td>0.27</td><td>0.15</td><td>0.31</td><td>2.68</td><td>0.55</td><td>203</td><td>553</td><td>284</td><td>55</td></tr><tr><td rowspan="2">13</td><td>Contigo mbr, pre_MBV</td><td>46.25</td><td>1.08</td><td>8.39</td><td>0.30</td><td>0.28</td><td>19.17</td><td>4.48</td><td>5.88</td><td>2.30</td><td>0.81</td><td>1.15</td><td>8.63</td><td>0.90</td><td>904</td><td>1,225</td><td>383</td><td>75</td></tr><tr><td>St.Dev</td><td>4.18</td><td>0.08</td><td>0.68</td><td>0.02</td><td>0.08</td><td>1.29</td><td>1.36</td><td>1.79</td><td>0.18</td><td>0.08</td><td>0.10</td><td>3.17</td><td>0.48</td><td>94</td><td>182</td><td>176</td><td>25</td></tr><tr><td rowspan="2">46</td><td>Chenapau mbr, LSu above AMB</td><td>38.08</td><td>0.77</td><td>6.96</td><td>0.36</td><td>0.31</td><td>17.94</td><td>8.84</td><td>5.99</td><td>2.33</td><td>0.52</td><td>0.86</td><td>15.24</td><td>1.48</td><td>606</td><td>790</td><td>699</td><td>58</td></tr><tr><td>St.Dev</td><td>2.07</td><td>0.05</td><td>0.42</td><td>0.05</td><td>0.05</td><td>1.48</td><td>0.91</td><td>1.09</td><td>0.27</td><td>0.05</td><td>0.10</td><td>2.22</td><td>0.58</td><td>96</td><td>158</td><td>533</td><td>8</td></tr><tr><td rowspan="2">18</td><td>Emerson & Naukluft Plateaus</td><td>46.70</td><td>0.94</td><td>8.39</td><td>0.38</td><td>0.31</td><td>16.31</td><td>7.79</td><td>6.37</td><td>2.70</td><td>0.44</td><td>1.00</td><td>7.07</td><td>1.44</td><td>399</td><td>298</td><td>237</td><td>53</td></tr><tr><td>St.Dev</td><td>7.13</td><td>0.08</td><td>1.98</td><td>0.05</td><td>0.12</td><td>5.02</td><td>2.03</td><td>0.76</td><td>0.15</td><td>0.05</td><td>0.20</td><td>2.10</td><td>0.53</td><td>167</td><td>162</td><td>102</td><td>16</td></tr><tr><td rowspan="2">41</td><td>Active Gale sands</td><td>43.95</td><td>0.96</td><td>8.98</td><td>0.51</td><td>0.42</td><td>20.52</td><td>9.90</td><td>7.15</td><td>2.50</td><td>0.41</td><td>0.79</td><td>3.27</td><td>0.51</td><td>636</td><td>205</td><td>34</td><td>49</td></tr><tr><td>St.Dev</td><td>1.31</td><td>0.09</td><td>0.58</td><td>0.13</td><td>0.03</td><td>1.30</td><td>1.00</td><td>0.20</td><td>0.15</td><td>0.06</td><td>0.08</td><td>0.65</td><td>0.08</td><td>222</td><td>28</td><td>13</td><td>3</td></tr><tr><td rowspan="2">11</td><td>Inactive Gale soils</td><td>42.93</td><td>1.07</td><td>9.09</td><td>0.52</td><td>0.43</td><td>20.01</td><td>8.78</td><td>7.06</td><td>2.66</td><td>0.46</td><td>0.86</td><td>5.29</td><td>0.68</td><td>525</td><td>311</td><td>40</td><td>47</td></tr><tr><td>St.Dev</td><td>0.74</td><td>0.07</td><td>0.22</td><td>0.08</td><td>0.01</td><td>1.07</td><td>0.21</td><td>0.13</td><td>0.06</td><td>0.03</td><td>0.06</td><td>0.55</td><td>0.08</td><td>62</td><td>39</td><td>11</td><td>2</td></tr></tbody></table>

Note. All data reported as wt.% except Ni, Zn, and Br (ppm). Full APXS data set from sol 3,078 to 4,028 (including errors and localization data) is provided in Table S1 in Data Repository (O'Connell-Cooper, 2025). Averages and 1σ standard deviations (italicized) shown for: Contigo interdune lenses, Mount Sharp group, pre MBV (MtSgrp); Clay Sulfate Transition (CST); Mirador formation (fm); Carolyn Shoemaker fm; Murray fm, Contigo mbr (pre MBV); Chenapau mbr (above AMB); Active Gale sands; Inactive Gale soils. Averages based on data available in the PDS (Gellert, 2012, 2013) and previously reported (Berger et al., 2020; O'Connell-Cooper et al., 2107, 2018, 2022; Thompson, Berger, Spray, et al., 2020; Thompson et al., 2022, 2024). Calculations shown in Table S2 in Data Repository (O'Connell-Cooper, 2025).
et al., 2024; Roberts et al., 2024, 2025) (Figure 1) continues for at least several hundred meters (for example, Milliken et al., 2010; Fraeman et al., 2016).

## 2.2. Sedimentology of the Contigo Interdune Lens Unit

Interdune lenses occur in the Mirador fm as low as ~4,080 m elevation, repeating periodically over a 50 m elevation range (Figure 1) but are concentrated in the Contigo mbr, which contains several examples (Figure 2a, Figure S2 in Supporting Information S1). The lens bodies do not appear to be connected, and do not all occur in the same horizon, indicating multiple formation episodes (Caravaca, Mangold, et al., 2022; Caravaca et al., 2025; Gupta et al., 2022). Interdune lens bodies have previously been identified on Mars in Meridiani Planum (Grotzinger et al., 2005; Metz et al., 2009); however, the Contigo lenses are better preserved (Caravaca et al., 2025; Gupta et al., 2022).

The term “interdune” was defined by Ahlbrandt and Fryberger (1981), in the context of an interdune depression, as a surface enclosed or at least partially bounded by eolian deposits such as sand sheets or dunes. Interdunes can be classified as dry, damp, or wet and can be deposited by both eolian and fluvial processes (either exclusively by one or through a combination of both), unlike dune sands that are deposited exclusively through eolian processes (Kocurek, 1981; Lancaster & Teller, 1988; Liang et al., 2020). The variability in transport mechanisms is a key factor, as it permits different sources for the dunes and the interdunes (for example, Liang et al., 2020). Additionally, the sediments characteristic of a given interdune may accumulate on adjacent dunes (Folk, 1971; Langford et al., 2016). The extent and shape of the interdune areas are strongly restricted by the surrounding dunes (Lancaster & Teller, 1988), but they can range from as small as the trough between individual dunes or as large as playa lakes, up to tens of km in diameter (Ahlbrandt & Fryberger, 1981).

In situ analysis was concentrated on the Prow and Panari lenses. Their sedimentary structures and textures are interpreted as evidence of deposition in a wet interdune environment within the dry dune fields of the Contigo mbr (Figures 1 and 2, Figures S2–S5 in Supporting Information S1) (Caravaca, Le Mouélic, et al., 2022; Caravaca et al., 2025; Gupta et al., 2022). An overall fining up sequence is identified, moving from a dynamic fluvial environment at the base to quiescent periods when water pooled in shallow ponds toward the top. With evolving conditions, the sediment supply for the lenses could incorporate both fluvially transported sediments and an airfall component, such as eolian blown sand or dust. The base of the unit comprises medium to coarse grained sandstones with cross beds (10–50 cm scale), interpreted to have formed in a dynamic, high-energy environment, such as a river channel (Caravaca, Le Mouélic, et al., 2022; Caravaca et al., 2025; Gupta et al., 2022). The upper part of the unit is finer grained, containing fewer cross-bedding structures but with preserved symmetrical ripple forms at the top of the unit, superimposed by finer grained mud drapes (mm-scale siltstone to mudstone). These are suggestive of formation by oscillatory flow via wave action in a lower energy, potentially lacustrine environment (Caravaca, Le Mouélic, et al., 2022; Caravaca et al., 2025; Gupta et al., 2022; Mondro, Fedo, et al., 2025). Changes in grain size within the Contigo lens targets are typically in the range of sub cm scale layering. For example, the Surumu target (Figure 2f) has well defined layers of finer grained material (typically ≤0.25 cm width) and coarser grained layers (typically 0.25–0.5 cm width).

The Contigo interdune APXS data set comprises one brushed sample (at the Issano outcrop) and nine unbrushed samples (at the Prow and Panari outcrops) (Table 1; Figure 2, Figures S2–S5 in Supporting Information S1). Sedimentological analysis interprets the Prow (and other lenses) to represent a fining up sequence (Caravaca, Le Mouélic, et al., 2022; Caravaca et al., 2025; Gupta et al., 2022); however, APXS targeting at the base of any of the lens outcrops was precluded by rover and arm placement constraints. APXS investigated along the length of the Prow and compared results to the adjacent Panari outcrop. APXS targets also profile across the finer grained material, ranging from the top layer of the lenses (e.g., the Kamuda and Issano targets) toward the middle of the finer-grained material (e.g., the Surumu, Kokadai, Coati and Morok targets) (Figure 2). Homogeneity with changing elevation was investigated by comparing the Prow and Panari outcrops with the Issano outcrop, separated from the Prow and Panari outcrops by 200 m of rover traverse and 40 m of elevation gain. The data set includes two float blocks (Ilu, Mazaruni) at the Prow outcrop, whose origins are not definitively resolvable.

Supplementary Figures S2–S4 in Supporting Information S1 show contextual Mastcam images focusing on the Prow and Panari outcrops. Figure S5 in Supporting Information S1 is a Navam mosaic of the Prow and Panari areas showing the location of targets. Figures S6–S25 in Supporting Information S1 focus on individual
<figure><img src="image_4.png" /><figcaption>Figure 2.</figcaption></figure>
workspaces and targets (Mastcam and MAHLI images with full target specifications in Tables S5 and S6 in Supporting Information S1.

### 3. APXS Instrumentation, Target Classification, Data Sets and Statistical Methodology

Curiosity's Canadian built and funded APXS instrument determines the elemental composition of both rock and unconsolidated materials, through a combination of X-ray fluorescence and particle-induced X-ray emission spectroscopy (Text S1 in Supporting Information S1 for detailed information) (for example, Campbell et al., 2012; Gellert et al., 2006; Gellert & Clark, 2015; Grotzinger et al., 2012; VanBommel et al., 2016, 2017). APXS routinely reports major and minor elements with atomic number (Z) 11 through 26 (from Na to Fe, and select trace elements (Ni, Zn, Br). Data are presented as elemental concentrations (wt% oxide, wt% element, or µg/g), alongside 2σ statistical precision uncertainties (Table 1; Table S1) (Gellert et al., 2006). APXS targets are categorized as bedrock (brushed or unbrushed), drill fines, diagenetic, or modern unconsolidated surficial sed- iments (Table 1; Tables S1 and S2; Text S2 in Supporting Information S1 for more details).

We compare the Contigo interdune lens unit to the Clay Sulfate Transition (CST), Gale basaltic units (uncon- solidated and lithified), the Chenapau mbr above the AMB and the MtSgrp as a whole (Figures 3 and 4; Table 1; Tables S1–S3). Modern unconsolidated basaltic Gale crater sediment is broadly divided into sand (active sedi- ment) and soil (dusty, less active sediment) (after O’Connell-Cooper et al., 2018 and references therein). The lithified basaltic deposits of the Stimson fm (Siccar Point grp) are interpreted as eolian dunes (Banham et al., 2018); we focus here on the sandstones of the Emerson and Naukluft Plateaus (sols 842–1,360).

Table 1 presents elemental concentrations and uncertainties for the Contigo interdune data set and averages for MtSgrp (sols 722–3,532; pre MBV targets only), CST (sols 3,085–3,712; excluding Mg + S enriched MBV targets), select formations, members and the sand/soil data sets. Full compositional data for targets (sols 3,078–4,028) can be found in Table S1, with average calculations shown in Table S2. Variance analysis was used to determine if there were statistically significant differences between units (Table S3; Text S3 in Supporting Information S1). Potential endmembers and their respective proportions within the lenses were explored through an iterative least-squares mass balance approach (Section 4.2; Table S4; Text S3 in Supporting Information S1). Further details on the APXS instrumentation, data sets, statistical methods and data treatment can be found in Text S1–S3 in Supporting Information S1.

## 4. Results—APXS Geochemical Analyses

### 4.1. Comparison to the Mount Sharp Group and Basaltic Units

With the exception of correlated Mg-SO₃ and Na-Cl enrichment in the MBV and overlying LSu, the APXS data set for the lower MtSgrp (up to the contact with the AMB) displays a remarkable degree of overall geochemical homogeneity across ~600 m (Berger et al., 2022, 2023; O’Connell-Cooper et al., 2024; Thompson, Berger, Boyd, et al., 2020; Thompson et al., 2024). Gale crater basaltic composition lithologies, such as the unaltered active sands of the Bagnold dunes, are compositionally comparable to average basaltic martian crust (for example, O’Connell-Cooper et al., 2017, 2018; Taylor & McLennan, 2010). Relative to this unaltered material, persistent depletions in Mn, Mg, Ca and enrichment in Si, K, P, S, Zn, and Ge are identified throughout the MtSgrp below the AMB (Figure 3; Figure S1 in Supporting Information S1; Table S1). This is in contrast to the Chenapau mbr bedrock above the AMB, which has consistent enrichment in Cr, Ca, Cl, Mn, Mg, Br, SO₃ and depletions in Ti, P, Ni, K, Zn (O’Connell-Cooper et al., 2024; Thompson et al., 2024).

**Figure 2.** (a) Context image for the “Prow” outcrop showing the Mirador butte in the background. Alpha Particle X-Ray Spectrometer lens targets are shown in yellow; host Contigo bedrock targets are shown in white. Distal lensoid structures, covering a range of elevations along Mirador butte, are shown in cyan blue. Scale shown is approximate. Image: Navigation camera mosaic N_R000_3372_ILT093CYL_S_0000_UNCORM2, sol 3,372. Image credit: NASA/JPL-Caltech. (b) The “Prow” outcrop. Image: Mastcam ID: mcam101385, sol 3,371. Image credit: NASA/Caltech-JPL/MSSS. (c) The “Panari” outcrop (finer grained, upper section). Mastcam ID: mcam101307, sol 3,357. (d) The “Issano” outcrop (finer grained, upper section). Mastcam ID: mcam102164, sol 3,494. Image credit: NASA/Caltech-JPL/MSSS. (e). 25 cm standoff view of lens target “Coati”, (sol 3,362) with 2 cm APXS Field of View (FOV) shown in yellow. MAHLI ID: MH0007060011200002C00. (f). 52 mm standoff view of lens target “Surumu offset” (sol 3,375) with 2 cm APXS (FOV) shown in yellow. MAHLI ID: MH0004400001200449R00. Image credits (2b-2f): NASA/Caltech-JPL/MSSS. Mastcam mosaics processed by the Mastcam Team at MSSS. MAHLI images were processed by D. Fey, at MSSS.
<figure><img src="image_5.png" /><figcaption>**Figure 3.** Comparison of Contigo interdune lenses with MtSgrp units. (a) Averages for all units (Table 1; Tables S1 and S2) ratioed to mean MtSgrp. Logarithmic scale. (b and c) Percentage change relative to average pre MBV MtSgrp (Table S1) is used to compare the Contigo interdune lenses to MtSgrp units. Similarities are observed between lenses and host bedrock for elements including Ti, Cr, Fe, whilst the lenses show similarity to the basaltic units for elements including Mn, Mg, K, Ca. Volatiles are highly enriched in the lenses over other units.</figcaption></figure>

APXS derived geochemistry exhibited by brushed bedrock samples of the Mirador fm (excluding MBV) shows a high degree of similarity to the Murray and Carolyn Shoemaker fms, with few points of statistically significant variance (Figures 3 and 4; Figure S1 in Supporting Information S1; Table 1; Tables S1–S3). Minor changes
<figure><img src="image_6.png" /><figcaption>**Figure 4.** Select oxide x-y graphs showing relationship of the Contigo interdune lenses to MtSgrp units. All data in wt. % or ppm. Dashed line shows average pre-MBV MtSgrp. Bedrock targets shown include brushed samples only, except the Contigo interdune lens unit. (a and b) Show addition of pure (a) CaSO₄ and (b) MgSO₄ rich fluids, with fluid addition lines drawn through origin. (c) (Cr₂O₃ vs. TiO₂) shows similarities between the lenses and host bedrock, whilst (d) (FeO/MnO vs. MnO) highlights similarities to basaltic units. (e) Places the lenses on a mixing line between host bedrock and basaltic units. MgO versus K₂O. (f) (Br vs. Cl) and (g) (Ni vs. Zn) highlight volatile enrichment in the lenses. LSu = Layered Sulfate unit.</figcaption></figure>

($\le \pm$10% relative to mean MtSgrp) are identified between MtSgrp and the Contigo mbr (excluding MBV) for all elements except Mn, Ca, S (1.2x) and Mg, Cl (0.8x) (Table S1b). Within MBV, the Contigo mbr shows consistent enrichment in Mn, Cl, Br, Mg, S, and depletions in Cr, K, Zn, whilst the overlying Catrimani mbr (in contact with
the AMB) shows more moderate enrichment in Mg, S, and a depletion in Ni (Figures 3a–3c; Figure S1 in Supporting Information S1; Table S1b).

The Contigo interdune lenses exhibit a geochemistry distinct from that of the host CST with a high degree of homogeneity between the three workspaces (Prow, Panari, Issano). Enrichment are identified in Mn, Cl, Mg, Zn, Ca, S, Ni, Br and depletions in Si, Cr, K relative to MtSgrp (Table S1; Figures 3a–3c; Figure S1 in Supporting Information S1). Minor changes (≤±10% relative to MtSgrp) are identified for elements Ti, Al, Fe, Na and P. Within the lens data set, the greatest degree of differences to the mean values are identified in the float target “Ilu” whose origin on the immediately adjacent Prow outcrop was not definitively resolvable (Figure 2a). Ilu exhibits the highest Si, Fe, Zn, Br and lowest Mn, K, Ti, P, of the Prow targets (Figure 4). Additionally, both Ilu and the float “Mazaruni” (origin also not definitively resolved) exhibited the highest Zn concentrations.

Mn is routinely depleted within MtSgrp bedrock below the AMB (including MBV), with concentrations typically half that of unaltered basaltic sand (Table 1; Tables S1 and S2) (e.g., Berger et al., 2022; O’Connell-Cooper et al., 2024; Thompson et al., 2022, 2024). However, the Contigo lenses show no sign of such depletion or evidence of Mn-rich fluid movement. A slight spread is identified in Mn (from 0.27 wt. % to 0.53 wt. %) (Figure 4d; Table 1), but average Mn concentrations (0.40 ± 0.08 wt.%) and Fe/Mn ratio (50 ± 13) are comparable to Gale basaltic sands (41 ± 7), Stimson fm (53 ± 16) and Chenapau mbr (57 ± 5) (O’Connell-Cooper et al., 2024; Thompson et al., 2024).

Mg concentrations for both the Contigo lenses (enriched 1.4x relative to MtSgrp) and the Stimson fm lie midway between those of MtSgrp and the Gale sands, Contigo_MBV and Chenapau mbr (Figures 3a-b, 4e). Unlike the LSu and MBV targets, Mg and S are not correlated with the Contigo lenses. However, a correlated enrichment is identified in Ca and S (≈1.3x; *r* = 0.94), similar to that of the host CST bedrock (below MBV), which has higher concentrations of Ca + S than the pre_CST MtSgrp (Figures 3b, 3c, 4a, and 4b; Table 1). Relative to MtSgrp, the lenses show a depletion in K (0.7x), with mean K values lying midway between the enriched MtSgrp and the basaltic sands, Stimson fm and Chenapau mbr (Figures 3a, 3b, 4d and 4e Table 1).

Both Ti and Cr are relatively consistent across the 600 m of elevation gain in the MtSgrp below the AMB (including MBV) (Berger et al., 2023; O’Connell-Cooper et al., 2022, 2024; Thompson et al., 2024). Whilst Ti in the Contigo lenses is comparable to MtSgrp, Cr₂O₃ shows a moderate depletion (0.8x) (Figures 3a, 3b, and 4c, Figure S1 in Supporting Information S1). In contrast, relative to MtSgrp, Cr shows a trend of enrichment in the Chenapau mbr and Stimson fm (1.1x to 1.2x) and Gale sand (1.6x), whilst Ti is depleted in sand and the Stimson fm (≈0.90x) and Chenapau mbr (0.8x).

Volatile concentrations in the lenses are enriched relative to MtSgrp (Cl 1.6x; Ni 1.2x; Zn 1.4x; Br 3.4x) (Figures 3a, 3b, 4e, and 4f; Figure S1c in Supporting Information S1; Table S1). The host Contigo bedrock and CST trend lower than MtSgrp for Cl, Zn and Br (excluding Br in the Contigo: 1.1x). The Mg + S enriched MBV units and Chenapau mbr exhibit moderate enrichment in Cl (1.0–1.2x), Br (1.2–1.8x) and significant depletions in Zn and Ni (0.6–0.7x; except Ni in the Contigo_MBV: 1.0x). Enrichment in the lenses are even higher when compared to modern Gale sands, where lower levels of S, Cl and Zn are associated with higher levels of motion and active saltation (for example, O’Connell-Cooper et al., 2017, 2018; Yen et al., 2005). The lithified sandstones of the Stimson fm also show enrichment (relative to modern saltating sands) in S, Cl, Zn and Br (2x to 7x) but a decrease in Ni (0.6x).

## 4.2. End Member Modeling Results

We explored potential endmembers and their respective proportions within the lenses through an iterative least-squares mass balance approach (Figure 5; Table S4; Text S3 in Supporting Information S1). Varying proportions of basaltic sediment—(Component A) active modern Gale sand (Table 1) (e.g., O’Connell-Cooper et al., 2017, 2018) and (Component B) dust (Berger et al., 2016)—were added to Component C–baseline MtSgrp bedrock [average Mirador fm or average Clay Sulfate transition (CST)]. Proportions for components A + B + C are varied systematically, resulting in a range of modeled compositions using all 16 reported elements. The best fit model is determined by finding the lowest “goodness of fit”. Goodness of fit values are relative, account for degrees of freedom, and are based on the sum of errors, normalized by their uncertainties for the elements in the modeled compositions. Lens compositions are reasonably modeled by adding 35% modern Gale sand and 15% dust to 50% Clay Sulfate Transition bedrock (Figure 5). Similar results are obtained when Mirador fm bedrock is used in lieu of CST due to their closely aligned bulk composition. Full modeling runs can be found in Table S4.
<table><thead><tr><th>Potential end members</th><th>Figure</th><th>Clay Sulfate Transition</th><th>Active Gale sands</th><th>Dust</th><th>Naukluft Plateau</th><th>Emerson Plateau</th><th>Relative Goodness of fit</th></tr></thead><tbody><tr><td rowspan="3">End member proportions %</td><td>5a</td><td>50</td><td>35</td><td>15</td><td>-</td><td>-</td><td>1</td></tr><tr><td>5b</td><td>34</td><td>-</td><td>-</td><td>66</td><td>-</td><td>1.32</td></tr><tr><td>5c</td><td>55</td><td>-</td><td>-</td><td>-</td><td>45</td><td>2.08</td></tr></tbody></table>

<figure><img src="image_7.png" /><figcaption>**Figure 5.** Results of iterative geochemical endmember modeling for average lens composition. Geochemical endmember mixing was conducted by testing two- and three-endmember models. Endmembers not utilized in a model run are denoted by a dash. Goodness of fit values are relative and are based on the sum of errors for the elements in the modeled compositions. The best modeled fit is determined by finding the lowest “goodness of fit”. Best fit: obtained by adding ≈35% (modern) active Gale sand and ≈15% dust to ≈50% MtSgrp (CST or Mirador fm). Similar results are obtained when Mirador fm is used in lieu of CST due to their closely aligned bulk composition. Full modeling can be found in Table S4.</figcaption></figure>

We also conducted two component endmember modeling to explore the effects of adding ancient basaltic sediments (Component A: average Emerson Plateau or Naukluft Plateau; both Stimson fm; Figure 5, Table S4) to the baseline MtSgrp bedrock (Component B: average Mirador fm or Clay Sulfate transition). Incorporating ancient sediments as endmembers results in fits of poorer quality, particularly for the model incorporating Emerson plateau data.

# 5. Discussion

## 5.1. Geochemical Composition of the Lenses-Interpretation

The Contigo interdune lenses represent a distinct unit within the dry Contigo aeolian dune fields. APXS data show that the composition of the lenses differs from that of the host bedrock by >±10% for several elements (>+10%: Mn, Mg, Zn, Ca, S, Ni, Cl Br; >-10%: Si, Cr, K) (Section 4.1).

Relative to pre MBV MtSgrp sediments, Mg is enriched in the interdune lenses, Stimson fm, soil, sand, Contigo_MBV (below the AMB), and Chenapau mbr (Figures 3 and 4e, Figure S1b in Supporting Information S1). As higher concentrations of both Na and Mg in dust cover on unbrushed targets can complicate the compositional assessment of bedrock (for example, Berger et al., 2016; VanBommel et al., 2024), bedrock averages are generally based on brushed samples only (Text S2 in Supporting Information S1). Due to difficulties accessing DRT'able targets within the Contigo interdunes, all but one interdune target was unbrushed. Variable proportions of dust cover are apparent in MAHLI images (Figures S6–S25 in Supporting Information S1), for example, targets Ilu (Figures S6, S7 in Supporting Information S1), Angamsima and Kamuda (Figures S8, S9 in Supporting Information S1) appear to have a higher proportion of dust cover than Surumu (Figure 2f; Figures S13–S11 in
Supporting Information S1) or Kokadai (Figure S15 in Supporting Information S1). However, the small standard
deviations for average Na and Mg (Table 1) for the interdune samples indicate a high degree of homogeneity
between the targets. When compared to the single brushed target (Issano_DRT), Na and Mg concentrations for the
unbrushed targets all lie within 10% of Issano_DRT. Additionally, average Na within the lenses shows a
negligible variation from MtSgrp (1.03x). We interpret this as evidence that the enriched Mg is not explainable
only by modern day dust cover.

Enriched Mg (1.7x) within the MBV and LSu targets is strongly correlated with enriched S (2.1x) (Pearson
correlation coefficient, r = 0.94), indicating the addition of MgSO₄ in these units (Berger et al., 2023; O'Connell-
Cooper et al., 2024). However, within the lenses (and the pre LSu MtSgrp) (Figures 4a and 4b) S is correlated with
Ca rather than Mg, for example, both the brushed Issano_DRT and unbrushed Morok targets are enriched in
Ca + S (Table 1). The Ca and S enrichment are interpreted here as a secondary feature resulting from the
movement of the same CaSO₄-rich fluids identified throughout the pre-LSu MtSgrp (Figure 4a). We interpret the
Mg enrichment within the lenses to be a primary compositional feature, similar to that in the active sands and
Stimson fm basaltic sandstones, rather than correlated with the later addition of S and by inference MgSO₄-rich
fluids.

APXS targets represent a vertical profile through the finer grained material (from the top layer of the Prow outcrop to the middle of the finer-grained material), a lateral profile (from the Prow outcrop to the Panari outcrop) and a profile with changing elevation (from the Prow and Panari outcrops to the Issano outcrop). Compositional differences between targets are minimal, showing instead a high degree of geochemical homogeneity, both between targets within each outcrop and between outcrops. Compositional gradients reflecting internal sorting, such as those in the Glen Torridon region (resulting in a high Mg, low K coarser grained sandstone and a low Mg, high K finer grained mudstone) (Caravaca, Le Mouélic, et al., 2022; Dehouck et al., 2022; O'Connell-Cooper et al., 2022) are not identified in the APXS data set. However, some internal sorting is suggested by ChemCam results, which identify higher than average K in some LIBS spots at the Prow (Caravaca et al., 2025). The identification of lenses at several different stratigraphic levels (e.g., the 40 m elevation gap between the Prow and Issano outcrops) indicates a cyclicity or repeated process (for example, Mountney & Jagger, 2004). The high degree of geochemical homogeneity identified by APXS within the upper lens unit, both internally and across a 40 m elevation gain, suggests repeated sediment input from a common source.

The persistent Mn depletion (with concentrations routinely half those of basaltic sands) in the MtSgrp below the
AMB, coupled with localized enrichments, is interpreted as evidence of the mobility of Mn-rich fluids in the
MtSgrp protolith (e.g., Berger et al., 2022; Thompson, Berger, Boyd, et al., 2020; Thompson et al., 2022, 2024;
VanBommel et al., 2022, 2023). The Contigo lenses show no sign of such depletion or evidence of Mn-rich fluid
movement. Mn concentrations and Fe/Mn ratios for the lenses are comparable to those of modern basaltic sand
and soil, Stimson fm, and the Chenapau mbr (MtSgrp above the AMB), suggesting that the Mn concentrations
within the lenses represent an unaltered component, and are a primary compositional feature (Figure 4d).

We believe that the majority of the compositional trends (e.g., higher Mg, Mn and lower K in the lenses than the
host bedrock) could be achieved through the addition of relatively unaltered basaltic material to the Contigo mbr,
and later alteration (e.g., CaSO₄ enrichment; Figures 3a, 3c and 4a) common to both the lenses and the
contemporaneous Contigo dune fields. As such, we interpret the lenses to comprise a mix of a relatively unaltered
basaltic component such as sand, not locally sourced from the altered host Mirador eolian dune field but
incorporated as a fluvial component or airfall, to an average CST or Mirador composition. This interpretation is
supported by endmember modeling (Section 4.2), which indicates that the current composition of the lenses can
be accounted for mathematically by adding 35% modern Gale sand and 15% dust to 50% CST bedrock.

However, enrichment in some of the volatiles and metals in the lenses cannot be explained using the same process.
Concentrations for Cl, Zn, Br, and for Ni are higher in the lenses than either modern sand or CST bedrock (sand
concentrations < CST), whilst Cr is lower in the lenses than both active sand and CST (sand > CST) (Figures 3
and 4, Figure S1 in Supporting Information S1).

In the saltating sands of the Bagnold dunes (Gale crater), a correlation between high Mg + Ni values concentrated
along active sand ripple crests was interpreted as a primary sorting process (O'Connell-Cooper et al., 2018).
However, although both Mg and Ni are enriched in the lenses, they are not correlated. Correlated enrichment in
Na + Cl (Na ≤ 13 wt.%; Cl ≤ 20 wt.%; Pearson correlation coefficient *r* ≤ 0.97), interpreted as halite, are
identified in a few CST targets and throughout the Mg + S enriched units (Berger et al., 2023; Meslin et al., 2024; O'Connell-Cooper et al., 2025). However, although Cl is enriched in the interdune lenses, Na concentrations mirror those of the host Contigo bedrock, indicating that the source of the elevated Cl of the interdunes is similar to that of the Mg + S enriched units.

The volatile enrichment within the interdunes may represent an episode of volcanic activity, such as that suggested for the halogen (Cl, Br) and metal (Cr, Mn, Fe, Zn) enriched AMB (Thompson et al., 2024). An airfall deposit at the time of interdune emplacement deposition should be identifiable within the penecontemporaneous host Contigo bedrock; however, no such enrichment is identified in the Contigo or other CST units (Section 4.1). A volcanic enrichment in the interdunes could represent an enrichment at the sediment source, transported by airfall or fluvial processes into the lenses, suggesting again a sediment source beyond the local environment.

## 5.2. Broader Implications

Although the Mirador fm below the AMB (including the MBV) is interpreted as an eolian dune field (for example, Edgar et al., 2024; Rapin et al., 2021, 2022; Roberts et al., 2024, 2025), it shows a broad range of statistically significant differences from the lithified sandstones of the Stimson fm (Ti, Cr, Mn, Mg, Na, K, P, Ni, Zn, Ni) and unconsolidated modern active sands (all reported elements except Fe, Na, K) (Figures 3 and 4; Figure S1 in Supporting Information S1; Table S3). It shows greater similarity to the fluvio-lacustrine Carolyn Shoemaker and Murray fms (Table S3).

All three formations show a similar, consistent deviation in average composition from a more typical basaltic composition (persistent depletion in Mn, Mg, Ca; enrichment in Si, K, P, Zn, Ge) and a striking lack of geochemical variability across the lower MtSgrp (up to the AMB) (Berger et al., 2022, 2023; O'Connell-Cooper et al., 2022, 2024; Thompson, Berger, Boyd, et al., 2020; Thompson et al., 2022, 2024). Recent work (Nekvasil et al., 2024; Zimmerman & Nekvasil, 2024) raises the possibility that deviations from typical basaltic compositions may have resulted, in part, from volcanic gas-solid reactions, where degassing of magmatic gas could have played a role in the evolution of the martian surface, without the need for a major aqueous component. Alternatively, the deviation has been interpreted as evidence for open system alteration of a basaltic precursor, for example, through open-system leaching of Mn from olivine and pyroxene (Berger et al., 2022, 2023). This may indicate that all three fms have been subjected to a similar alteration regime (e.g., Berger et al., 2022, 2023; Thompson, Berger, Boyd, et al., 2020).

However, as the Contigo interdune lenses do not exhibit this distinctive signature apparent in the bulk chemical composition of the MtSgrp below the AMB and host Mirador fm, we can use the composition of the lenses to help determine whether the alteration occurred prior to deposition in Gale crater or in-place via post-depositional fluids.

Interdunes are emplaced penecontemporaneously with their host dune field rather than as a later feature (Lancaster & Teller, 1988). We would therefore expect the Contigo interdunes to reflect any alteration via post-depositional fluids undergone by the host Mirador dune fields. Although terrestrial interdune strata typically have lower porosities and permeability than surrounding eolian dune strata (Krystinik, 1990), which may inhibit fluid movement, the Contigo lenses show evidence of Ca-sulfate cement at concentrations similar to the host Contigo bedrock, indicating that fluids were able to move through both dune and interdune strata (Section 4.1). The lack of evidence within the lenses for a shared episode of alteration via post-depositional fluids (e.g., the characteristic Mn depletion) suggests that the sediments of the pre-MBV MtSgrp were altered prior to deposition. All MtSgrp strata below the AMB may have been sourced from a common, previously altered, protolith; alternatively, alteration may have been confined to the Murray and Carolyn Shoemaker fms, which were then reworked to form the eolian deposits of the Mirador fm (e.g., Berger et al., 2022; Meyer et al., 2024).

# 6. Conclusions

APXS geochemical data reveal homogeneity within the lenses but significant differences compared with the host Contigo bedrock. Geochemical data and end-member modeling indicate that the lenses represent the addition of unaltered basaltic material to the Contigo mbr.

We interpret the lenses to represent a mixing between the host MtSgrp sediments and relatively unaltered basaltic units. Adding unaltered basaltic sediment (45%-52%) to a previously altered protolith (i.e., Mirador fm or CST)
accounts for the current composition of the lenses and points to a basaltic sediment source beyond the host Contigo dune fields. Volatile enrichment in the interdunes, above that seen in the local bedrock or in typical basaltic soil, may suggest the incorporation of a volcanic component at the source of the interdune basaltic sediment.

The same depositional process is posited both for the three outcrops investigated in situ (Prow, Panari, Issano) and for other lenses with similar sedimentary structures (too far from the rover for APXS analysis). This, along with the geochemical homogeneity of the lenses analyzed, suggests both a common source of influx material and process (wetter episodes in a drier eolian environment) over several cycles of seasonal variations. The fluvial Contigo interdune lenses represent a window into locally changing environmental conditions in Gale crater, indicating the periodic presence of water in a predominantly dry environment.

The Contigo interdune lenses represent fluvial to shallow lacustrine deposits in the otherwise dry eolian Contigo dune field. Whilst APXS geochemical data reveal compositional similarities to both the host Contigo mbr and to unaltered basaltic sediments, the lenses lack the alteration signature (e.g., consistent Mn depletion) apparent in the bulk chemical composition of the MtSgrp below the AMB, helping to elucidate the timing of the proposed alteration.

## Conflict of Interest

The authors declare no conflicts of interest relevant to this study.

## Data Availability Statement

All raw and reduced APXS data are available at the Planetary Data System (https://pds-geosciences.wustl.edu/missions/msl/apxs.htm). The MSL APXS RDR data set has the https://doi.org/10.17189/1518757 (Gellert, 2012, 2013).

APXS data and calculations in this article can be found in Tables S1–S4 (O’Connell-Cooper, 2025) and accessed online at https://doi.org/10.5281/zenodo.14893713.

Table S1 lists all APXS compositional data for target sols 3,078–4,028; Tables S2–S4 show calculations (detailed in Sections 3-4). Data processing was completed in Microsoft Excel. Figures 1 and 2 were created using Microsoft Powerpoint. Figure 3 plots were constructed using IoGas Geochemistry Software (https://www.imdex.com/software/iogas). Figures 4 and 5 plots were graphed and processed in Microsoft Excel.

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## Acknowledgments

MSL APXS is managed & financed by the Canadian Space Agency (CSA). We appreciate & acknowledge the support of engineers at JPL, the MSL science team during operations and the Mastcam and MAHLI teams at Malin Space Science Systems (NASA/JPL-Caltech/MSSS). We would like to thank our two reviewers for their time and very constructive feedback, who helped make this more robust paper. Science team member funding for O’Connell-Cooper, Thompson, Spray, Boyd, Gellert, and McCraig is provided by the CSA. VanBommel was supported by NASA Mars Science Laboratory Participating Scientist Grant 80NSSC22K0650. Caravaca’s work was supported by the French Space Agency CNES under convention CNES 180027. Yen acknowledges that a portion of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.
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