# Stratigraphy of Carbonate-Bearing Rocks at the Margin of Jezero Crater, Mars: Evidence for Shoreline Processes?

Alexander J Jones, Sanjeev Gupta, Robert Barnes, Briony H N Horgan, Kenneth H Williford, Gerhard Paar, Kathryn M Stack, Bradley J Garzcynski, Eleni Ravanis, Candice C Bedford, et al.

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Alexander J Jones, Sanjeev Gupta, Robert Barnes, Briony H N Horgan, Kenneth H Williford, et al. Stratigraphy of Carbonate-Bearing Rocks at the Margin of Jezero Crater, Mars: Evidence for Shoreline Processes?. Journal of Geophysical Research. Planets, 2026, 131 (1), pp.e2025JE009111. 10.1029/2025je009111. hal-05451044

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# JGR Planets

## RESEARCH ARTICLE

10.1029/2025JE009111

**Special Collection:**

The Mars 2020 Perseverance
Rover's Exploration of the Jezero
Crater Margin and Neretva Vallis

**Key Points:**

* The Margin unit underlies the Jezero Western fan, drapes the inner crater rim and may record both igneous and sedimentary lithofacies

* Most of the unit comprises structureless to planar-layered rocks, which most likely preserve an aqueously altered olivine cumulate

* In the east of the unit, rounded grains and sedimentary structures indicate reworking by waves along the shoreline of the Jezero paleolake

**Supporting Information:**

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

**Correspondence to:**

A. J. Jones,
alexander.jones19@imperial.ac.uk

**Citation:**

Jones, A. J., Gupta, S., Barnes, R., Horgan, B. H. N., Williford, K. H., Paar, G., et al. (2026). Stratigraphy of carbonate-bearing rocks at the margin of Jezero crater, Mars: Evidence for shoreline processes? *Journal of Geophysical Research: Planets*, 131, e2025JE009111. https://doi.org/10.1029/2025JE009111

Received 11 APR 2025

Accepted 16 DEC 2025

**Author Contributions:**

**Conceptualization:** Alexander J. Jones, Sanjeev Gupta, Robert Barnes

**Data curation:** James F. Bell, Justin Maki

**Formal analysis:** Alexander J. Jones

**Investigation:** Alexander J. Jones, Sanjeev Gupta, Robert Barnes, Briony H. N. Horgan, Kenneth H. Williford, Kathryn. M. Stack, Bradley J. Garzczynski, Eleni Ravanis, Candice C. Bedford, Justin I. Simon, Athanasios Klidas, Linda

© 2026. The Author(s).
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properly cited.

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# Stratigraphy of Carbonate-Bearing Rocks at the Margin of Jezero Crater, Mars: Evidence for Shoreline Processes?

Alexander J. Jones¹, Sanjeev Gupta¹, Robert Barnes¹, Briony H. N. Horgan²,
Kenneth H. Williford³, Gerhard Paar⁴, Kathryn. M. Stack⁵, Bradley J. Garzczynski⁶,
Eleni Ravanis⁷, Candice C. Bedford², Justin I. Simon⁸, Athanasios Klidas²,
Linda C. Kah⁹, James F. Bell¹⁰, Justin Maki⁵, Stephen W. Lokier¹¹, Adrian Brown¹²,
Nathan R. Williams⁵, Sanna Alwmark¹³, Jorge I. Núñez¹⁴, Fred Calef⁵, Larry Crumpler¹⁵,
Alicia Vaughan¹⁶, Libby R. W. Ives⁵,¹⁷, Sam Gwizd⁵, Ken Farley¹⁸, Christian Tate¹⁹,
Nicolas Randazzo²⁰, Olivier Beyssac²¹, and Gwénaël Caravaca²²,²³

¹Department of Earth Science & Engineering, Imperial College London, London, UK, ²Purdue University, West Lafayette, IN, USA, ³Blue Marble Space Institute of Science, Seattle, WA, USA, ⁴Joanneum Research, Institute for Digital Technologies, Graz, Austria, ⁵Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA, ⁶Geology Department, Western Washington University, Bellingham, WA, USA, ⁷University of Hawaiʻi at Mānoa, Honolulu, HI, USA, ⁸Astromaterials Research and Exploration Science Division, NASA Johnson Space Center, Houston, TX, USA, ⁹University of Tennessee, Knoxville, TN, USA, ¹⁰School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA, ¹¹School of Science, University of Derby, Derby, UK, ¹²Plancius Research, Severna Park, MD, USA, ¹³Department of Geology, Lund University, Lund, Sweden, ¹⁴Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA, ¹⁵New Mexico Museum of Natural History & Science, Albuquerque, NM, USA, ¹⁶Apogee Engineering, LLC, Flagstaff, AZ, USA, ¹⁷Michigan Geological Survey, Department of Geological and Environmental Sciences, Western Michigan University, Gwinn, MI, USA, ¹⁸Division of Geological and Planteary Sciences, Caltech, Pasadena, CA, USA, ¹⁹Cornell University, Ithaca, NY, USA, ²⁰University of Alberta, Edmonton, AB, Canada, ²¹IMPMC, Paris, France, ²²Laboratoire Géosciences Environnement Toulouse (GET), UMR 5563 Université Paul Sabatier Toulouse III, CNRS, IRD, CNES, Toulouse, France, ²³Institut de Recherche en Astrophysique et Planétologie (IRAP), UMR 5277, Université Paul Sabatier Toulouse III, CNRS, CNES, Toulouse, France

**Abstract** Martian carbonate-bearing rocks are compelling targets for exploration because they preserve detailed records of past aqueous processes, climate, and habitability. The Margin unit in Jezero crater is a distinct olivine- and carbonate-bearing unit stratigraphically underlying the western fan, lining the inner margin of the western crater rim and has a contested origin. *Perseverance* spent ~350 sols investigating the unit as part of its fourth mission campaign, aiming to constrain its origin, alteration history and biosignature preservation potential. This study reports on the lithofacies and stratigraphy of the unit by analyzing Mastcam-Z mosaics and derived 3D outcrop models, supplemented by long-distance SuperCam observations and detailed textural analyses from SHERLOC WATSON and ACI images. We find that the Margin unit comprises two distinct sub-units. The Eastern Margin Unit (EMU) comprises well-stratified, low-angle basinward-, rimward- and sub-horizontally inclined medium-grained sandstones which preserve angular to rounded grains, occasional cross-stratification, convex-up bedding, and erosion surfaces. The Western Margin Unit (WMU) comprises distinctly structureless to decimeter-scale parallel-layered rocks which drape the crater rim and are inclined into the crater. The origin of the WMU is uncertain but may be most consistent with a variably carbonated olivine cumulate. The favored depositional model for the EMU is a lacustrine shore zone environment where sediments derived from the adjacent WMU have been locally reworked by wave action along a paleoshoreline at around -2,400 m elevation. These observations suggest that the Margin unit preserves diverse subsurface and surface aqueous environments and further extends the habitability window at Jezero crater.

**Plain Language Summary** Carbonate minerals are rare on Mars and are important targets because they preserve information about ancient climates, water availability, and habitable environments. NASA's Mars 2020 *Perseverance* rover spent around one Earth year exploring the “Margin unit”, a geologic unit lining the inner crater rim west of the Jezero delta, rich in olivine and carbonate-minerals. Here, we use images acquired by *Perseverance* to describe the Margin unit rocks and consider how they formed. The Margin unit primarily consists of structureless to thick-layered rocks with compositions and textures suggesting they have experienced significant alteration by subsurface circulating water. This alteration makes determining their origin challenging, but their overall structure, texture, composition and distribution appear most consistent with an igneous origin, formed by crystal settling in a cooling body of molten rock. At lower elevations in the east, the
C. Kah, Stephen W. Lokier, Adrian Brown, Sanna Alwmark, Jorge I. Núñez, Fred Calef, Larry Crumpler, Alicia Vaughan, Libby R. W. Ives, Sam Gwizd, Ken Farley, Christian Tate, Nicolas Randazzo

**Methodology:** Alexander J. Jones, Robert Barnes, James F. Bell, Justin Maki, Nathan R. Williams, Christian Tate

**Project administration:** Alexander J. Jones, Sanjeev Gupta

**Software:** Gerhard Paar, Christian Tate

**Supervision:** Sanjeev Gupta, Robert Barnes, James F. Bell, Justin Maki

**Validation:** Robert Barnes, Kenneth H. Williford

**Visualization:** Alexander J. Jones, Nathan R. Williams

**Writing – original draft:** Alexander J. Jones

**Writing – review & editing:** Alexander J. Jones, Sanjeev Gupta, Robert Barnes, Briony H. N. Horgan, Kenneth H. Williford, Eleni Ravanis, Candice

C. Bedford, Justin I. Simon, Athanasios Klidaras, Linda C. Kah, Stephen W. Lokier, Adrian Brown, Jorge I. Núñez, Nicolas Randazzo, Olivier Beyssac, Gwénaël Caravaca

Margin unit displays rounded grains and sedimentary structures consistent with a lake shore environment. These rocks are interpreted as a sandy veneer reworked from the bulk of the Margin unit by waves at the Jezero lake shoreline. These observations show that the Margin unit preserves multiple ancient, and potentially habitable, settings, including water-rich surface and subsurface environments.

# 1. Introduction

Martian carbonate-bearing rocks are key indicators of past water-rock-atmosphere interactions, preserving detailed information about ancient climate and habitable environments, and as such have been a priority for exploration by landed missions to Mars (Bridges et al., 2019; Burtt et al., 2024; Horgan et al., 2020; Hu et al., 2015; Wray et al., 2016). Fe- and Mg-carbonate is often linked to habitable environments on Earth, forming either by subsurface carbonation of ultramafic rocks by CO₂-rich fluids, or via authigenic precipitation from fluids which have interacted with ultramafic rock, such as in soils or alkaline lakes (Scheller et al., 2021). Such environments may provide habitable niches for microbial life, and the resulting carbonate precipitation has been shown to preserve molecular biosignatures on Earth (Newman et al., 2020; Rempfert et al., 2017; Robles-Fernández et al., 2022), making Mg-carbonate rocks on Mars of astrobiological interest.

Jezero crater, the site of the Mars 2020 *Perseverance* rover mission, is located in one of the most olivine- and carbonate-rich terrains identified on Mars (Ehlmann, Mustard, Murchie, et al., 2008; Hoefen et al., 2003). The association of carbonate phases with geomorphic evidence of an open basin paleolake (Fassett and Head III, 2005; Goudge et al., 2015) was a key factor during landing-site selection, and made Jezero a compelling astrobiological target for in situ and returned sample science (Grant et al., 2018). The Margin unit is a light-toned, fractured exposure of olivine- and carbonate-bearing rock lining the inner margin of the western crater rim and extending to the north and south of the Jezero inlet channel, Neretva Vallis (Horgan et al., 2020; Stack et al., 2020; Sun & Stack, 2020). The unit unconformably underlies the deposits of the Jezero western delta (Williford et al., 2025), and thus may record a distinct time period and paleoenvironmental setting in the geologic evolution of Jezero crater. Prior to exploration by the rover, the observed distribution of the unit around the inner margin of the crater rim across a narrow elevation range led Horgan et al. (2020) to propose a hypothesis that the unit may have formed in a nearshore lacustrine setting. By contrast, other studies have suggested that the unit shares an origin with a regional olivine- and carbonate-bearing unit draping much of the Jezero watershed and wider northwest Isidis region, and which has been interpreted to have formed by carbonation of an igneous protolith (Brown et al., 2020; Mandon et al., 2020; Tarnas et al., 2021). Thus, the origin of the carbonate-bearing Margin unit was contested prior to the landing of *Perseverance*. To resolve this, on-ground investigation of the lithologic texture and architecture of the unit is required. Initial observations of the Margin unit by the *Perseverance* Science Team support an affinity with these regional olivine-rich rocks, but may also indicate potential lacustrine influence at lower elevations (Williford et al., 2025). To develop robust models for the processes of emplacement of the Margin unit and evaluate its context and evolution, detailed characterization of its surface geologic expression is necessary.

Here, we report on the lithology, stratigraphy and surface structure of the Margin unit explored by the *Perseverance* Science Team between sols 910 and 1266. Our purpose is to reconstruct the emplacement mechanism and geologic context of the Margin unit, and consider implications for Jezero's geologic history, lake evolution, past habitability and climate. Furthermore, our study provides context for the three samples of the Margin unit collected by *Perseverance* for future return to Earth as part of the Mars Sample Return (MSR) mission.

# 2. Geologic Context

## 2.1. Regional Geology of the Nili Fossae Region and Jezero Watershed

Jezero crater is located at the edge of a 1,350 km diameter, Noachian-age impact basin known as Isidis (Figure 1a). The Isidis basin is flanked to the north-west by a series of curvilinear features known as the Nili Fossae, interpreted as circumferential tectonic relaxation graben associated with the Isidis impact (Ritzer & Hauck, 2009) and onlapped in the south by the younger (~3.5 Ga) lava flows of the Syrtis Major volcanic province (Hiesinger & Head, 2004). Much of this area, including large volumes of the Jezero watershed, is
<figure><img src="image_4.png" /><figcaption>**Figure 1.** Context maps for *Perseverance*'s investigation of the Margin unit, explored between sols 910 and 1266. (a) Regional geologic context of Jezero crater showing relationships to the Isidis impact basin, Syrtis Major volcanic province, Nili Fossae and the Regional Olivine-bearing Unit (ROU) as mapped by Kremer et al. (2019), displayed on a Mars Orbiter Laser Altimeter (MOLA) basemap. (b) Mars 2020 mission context, showing the rover's traverse of key units to date since its landing at the Octavia E. Butler (OEB) landing site. The location of the Margin unit, Jezero western fan and distribution of the ROU west of the Jezero rim (as mapped by Mandon et al. (2020)), are displayed on a High Resolution Imaging Experiment (HiRISE) basemap with MOLA topography overlain. Elevation contours of the outlet channel (–2,395 m) and a break in slope in the crater rim (–2,260 m), which bound the Margin unit, are also plotted after Fassett and Head (2005). The dashed box indicates the location of the lower panel (c). (d) HiRISE basemap with *Perseverance*'s traverse of the Margin unit shown. Sol numbers, names of key locations and core and abrasion sites are annotated. Locations of detailed traverse route maps Figures 6 and 17 are indicated by dashed boxes.</figcaption></figure>

variably draped by the regional olivine-bearing unit (Brown et al., 2020; Goudge et al., 2015; Kremer et al., 2019; Mandon et al., 2020).

Orbital investigations of the regional olivine-bearing unit surrounding Jezero crater have led to its interpretation as igneous in origin, perhaps with an emplacement related to crustal thinning associated with the Isidis impact event (Hoefen et al., 2003). Hypotheses range from the unit being emplaced as lava flows or shallow intrusions (Hamilton & Christensen, 2005; Hoefen et al., 2003; Tornabene et al., 2008), to pyroclastic deposits resulting from explosive volcanism (Kremer et al., 2019; Mandon et al., 2020; Ruff et al., 2022) or impact melt (Mustard et al., 2007). Carbonate phases have also been identified within these rocks, and always co-occur with olivine, and often with Fe/Mg-phylosilicate phases. These carbonates are therefore interpreted to have formed as a result of
and subsequently the Pelican Point sample was collected on sol 923. The proceeding northerly traverse then allowed the Science Team to investigate a gentle topographic ridge in the Margin unit known as "Turquoise Bay," between sols 932 and 950. This region was identified from orbit as possessing a particularly strong spectral signature of carbonate (Williford et al., 2025), and was where the Bill's Bay abrasion patch and Lefroy Bay sample were acquired. A further ~400 m drive north to Gnaraloo Bay, an erosional window through the apex of the Jezero western fan and the underlying Margin unit, allowed investigation of the western fan-Margin unit unconformity between sols 952 and 999. Following this, *Perseverance* began a ~2.5 km westward ascent toward the crater rim, initially observing a cross-section through the Margin unit at "Muiron Island," a topographic ridge striking parallel to Neretva Vallis and the rover traverse. En route to the crater rim, three further abrasion proximity science sites were selected. "Bunsen Peak," a high standing block, was the location of the Castle Geyser abrasion and was the site of the final Margin unit sample, Comet Geyser, on sol 1088. The Margin unit was also abraded at Old Faithful Geyser, a low-lying outcrop at the flank of a mound named "Overlook Mountain," on sol 1151. *Perseverance* then descended into Neretva Vallis on sol 1158 to investigate the channel stratigraphy and Margin unit outcrops exposed in the northern channel walls. Following this, the rover returned to the Margin unit south of the channel on sol 1244, allowing the Science Team to complete its exploration campaign of this unit with a final abrasion patch at Eremita Mesa on sol 1257, near its basal contact with the rocks of the crater rim. Following the end of the Margin campaign, further olivine- and carbonate-bearing lithologies were observed continuing up onto the crater rim (Willford et al., 2025), but are not reported in detail in this study.

# 3. Data and Methods

## 3.1. Lithofacies Characterization

Outcrop-scale lithologic facies of the Margin unit were characterized across the rover's traverse using image data primarily acquired by Mastcam-Z, a pair of cameras providing multispectral images and stereoscopic landscape mosaics (Bell et al., 2021). These images were supplemented by lower-resolution image mosaics from the Navigation cameras (Navcam, Maki et al., 2020), and high-resolution telescopic images acquired by the SuperCam Remote Micro-Imager (RMI, Maurice et al., 2021; Wiens et al., 2020). Each of these instruments is mounted ~2 m above the surface on the rover's Remote Sensing Mast. Assuming 3 pixels are required for confident identification of geologic features, at a distance of 5 m, Navcam can resolve features of ~5 mm, Mastcam-Z can resolve features of ~1 mm (at its highest 110 mm focal length), and RMI can resolve features of ~0.15 mm. At a 100 m distance from the rover, this decreases to ~10 cm for Navcam, ~2 cm for Mastcam-Z, and ~3 mm for RMI (Bell et al., 2021; Maki et al., 2020; Maurice et al., 2021; Wiens et al., 2020). Mastcam-Z landscape images were white-balanced, contrast-stretched and gamma-corrected for improved viewability (Bell et al., 2021), and analyzed to characterize textures, structures and lithologies at the outcrop scale. Where Mastcam-Z data were absent, for instance between end-of-drive locations, Navcam images were used for lithologic characterization and mapping facies extents and boundaries. SuperCam RMI images were analyzed for characterization of long-distance targets, and in this study were color-enhanced using a gaussian stretch to better resolve textural and tonal variability. Additionally, where available, images acquired by the *Ingenuity* helicopter (Balaram et al., 2021) during its multiple flights across the Margin unit were used to characterize lithologic facies and structures in planform view.

These outcrop-scale analyses were supplemented by textural analyses of high-resolution images acquired on select rock targets by the Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals (SHERLOC) Wide Angle Telegraphic Sensor for Operations and eNgineering (WATSON) and Autofocus Context Imager (ACI) cameras (Bhartia et al., 2021), both positioned on the rover's robotic arm. WATSON acquires color images, achieving resolutions of up to 13.3 µm/pixel at a 1.8 cm standoff distance (Bhartia et al., 2021); targets analyzed in this study had resolutions of ~21–23 µm/pixel, allowing for the resolution of materials coarser than very fine sand (>62 µm) under favorable lighting. The ACI can acquire higher resolution, greyscale images at 10.1 µm/pixel (Bhartia et al., 2021), allowing for the resolution of grains larger than ~30 µm (silt-sized grains). These data products were used for grain-scale lithologic characterization including particle shapes, sizes, and distributions. A combination of daytime (i.e., under natural solar illumination) and nighttime (i.e., under artificial white light LED illumination) WATSON images was used for analysis, with the former primarily providing a sense of surface topography, and the latter reducing shadowing to aid identification of grain contacts (e.g., Bhartia et al., 2021; Wogsland et al., 2023). Grain long-axes were measured from nighttime WATSON images using the ImageJ program (Schneider et al., 2012). Grains were selected for
measurement using a modified grid method: a millimeter-square grid was placed over the WATSON image and the most readily resolvable grain near each intersection was measured. As noted by Stack et al. (2024), this method will be inherently biased toward larger grains and those with higher tonal contrast. Grain sizes were classified according to the Wentworth scale (Wentworth, 1922), and the degree of grain roundness was determined by visual comparison to the scale described by Powers (1953).

## 3.2. Structural Mapping

Measurements of strike and dip were collected across the rover traverse of the Margin unit to constrain its overall structure using Mastcam-Z derived 3D data products generated by the Planetary Robotics processing and viewing tools PRoViP and PRo3D (Paar et al., 2023). The resulting structural data and the mapped extents of lithofacies were localized and mapped on a HiRISE basemap. Three methods of structural measurement were employed:

1. For outcrops close to the rover (typically within ~50 m), Ordered Point Clouds (OPCs) were generated from Mastcam-Z stereoscopic landscape mosaic data, and rendered in the PRo3D software (Figure S1 in Supporting Information S1; Paar et al., 2023). To calculate a strike/dip, points are picked along the surface of a geologic feature, and a least squares regression method is used to calculate a best-fit plane through the points (Barnes et al., 2018; Quinn & Ehlmann, 2019), from which trigonometric calculations provide the strike and dip values relative to north and horizontal. The radial error for each point relative to the best-fit plane was calculated (Paar et al., 2023) and is recorded for each measurement in Table S1. The radial error (i.e., the distance of a point picked along a geologic feature from the calculated best-fit plane) for all measurements using this method ranged between 0 and 50 mm, with a mean of 4 mm. This method was only applied to outcrops with sufficient surface topography to reliably calculate a best-fit plane in this way and minimize the radial error. Measurements were verified by ensuring that they accurately intersected the desired geologic feature in the OPC.

2. For larger outcrops which were too far from the rover to employ the OPC method, structural features were mapped onto the HiRISE Digital Terrain Model (DTM) (McEwen et al., 2007), with the guidance of Mastcam-Z landscape mosaics. These mosaics were projected as panoramic photospheres and georeferenced to topographic and geomorphic features identifiable in the HiRISE DTM (Figure S2 in Supporting Information S1). This method was employed uniquely for the major stratigraphic contacts observed near the basal contact of the Margin unit with the crater rim (discussed in Section 6.1.5), where close-proximity 3D data were not available to measure bedding orientation.

3. For outcrops which were too far from the rover for reliable OPC stereoscopic data, and too small to map on to HiRISE using the photosphere method, the dip azimuth was estimated directly from a Mastcam-Z landscape mosaic using the camera viewing geometry, described by Banham et al. (2018). Following this method, the dip azimuth of the feature is estimated by taking the known viewing azimuth and applying a correction depending on whether the feature is dipping to the right (+90°) or left (−90°). Additional corrections were made to refine this further, depending on whether strata are dipping toward or away from the camera. This method was additionally applied to each feature measured by methods (a) and (b) to quantify the associated error; the mean discrepancy between the dip azimuth estimation and the “true” values determined by (a) and (b) was 14° and did not exceed 30°. This method was not deemed sufficiently accurate to quantitatively estimate dip magnitude. Data acquired using this method were used to assist in geologic mapping, but not used in the construction of cross sections or calculation of mean bedding orientations.

Measurements of bedding were only recorded when the feature of interest was convincingly in place, typically forming part of a laterally continuous bedrock exposure. Due to the highly fractured terrain of the Margin unit, there were several instances of dust obscuring contacts between large blocks, precluding absolute determination of whether they were in place. In such instances, measurements were only recorded for large blocks (>2 m), where stratification was parallel to adjacent outcrops which were convincingly in situ. The acquired structural data for each target, the associated error, as well as the data set and method used for each measurement are provided in Table S1.

## 4. Margin Unit Facies Associations

The Margin unit comprises a highly fractured terrain with most outcrops occurring as discontinuous slabby, low-lying polygonal outcrops (e.g., Figures 2a and 3a), or blocky, wind-abraded ridges (e.g., Figures 4a and 5a). *Perseverance* observations reveal that the unit has distinct lithofacies, structures and stratigraphic successions in
<figure><img src="image_5.png" /><figcaption>**Figure 2.** Grain-scale characteristics of the Eastern Margin Unit at the Hans Amundsen memorial workspace. (a) Sol 911 Mastcam-Z view of the low-lying polygonal bedrock of the Hans Amundsen memorial workspace (zcam08917; image credit: NASA/JPL-Caltech/ASU/MSSS/L. Duflot). Location of (b) is annotated. (c) Sol 923 Navcam view of the workspace providing context for the Amherst Point abrasion (A) and Pelican Point sample (S), (d) Daytime illumination WATSON image of the Amherst Point abrasion patch at 7 cm standoff, acquired on sol 915 and (e) Annotated nighttime WATSON close-up of the region highlighted in (c) showing a clastic texture and multiple examples angular to sub-rounded sand-sized grains (yellow arrows) both in the abrasion patch, and on the adjacent natural surface. Several grains are annotated with their size: FS, fine sand; MS, medium sand; CS, coarse sand. Image credit: NASA/JPL-Caltech/MSSS/A. Pascuzzo.</figcaption></figure>

the Eastern Margin Unit (EMU), explored between Sols 910 and 1007, compared to the subsequently explored Western Margin Unit (WMU), investigated between Sols 1007 and 1266. The EMU largely comprises low-lying dusty polygonal outcrops of low-angle, cm-scale planar bedded sandstones typically inclined either basinwards or rimwards and exhibits multiple candidate sedimentary structures (Figures 7–15). Conversely, the WMU comprises a series of blocky topographic ridges and depressions exposing structureless to locally dm-scale planar layered rocks inclined east and apparently lacking sedimentary structure (Figures 18–22).
<figure><img src="image_6.png" /><figcaption>**Figure 3.** Grain-scale characteristics of the Eastern Margin Unit at Turquoise Bay. (a) Sol 933 Mastcam-Z view of the low-lying polygonally-fractured terrain comprising the Turquoise Bay outcrop (zcam08945; image credit: NASA/JPL-Caltech/ASU/MSSS/L. Duflot). Location of (b) is annotated. (b) Sol 942 Navcam view of the workspace providing context for the Bill's Bay abrasion patch (A) and the Lefroy Bay sample (S), (c) Daytime illumination WATSON image of the Bill's Bay abrasion at 7 cm standoff, acquired on sol 935 and (d) Close-up annotated nighttime WATSON of the region highlighted in (c) showing a clastic texture and multiple examples of sub-rounded to rounded sand-sized grains (yellow arrows), acquired on sol 935. Several grains are annotated according to their size (MS, medium sand; CS, coarse sand; VCS, very coarse sand). Image credit: NASA/JPL-Caltech/MSSS/J. Huggett.</figcaption></figure>

Here, we distinguish the rock types observed in the Margin unit into five primary structural-lithologic facies (Table 1) based on (a) stratal thickness and orientation, (b) presence or absence of sedimentary structure and (c) fine-scale depositional and diagenetic textural characteristics of abraded rock surfaces. A brief overview of these
<figure><img src="image_7.png" /></figure>

**Figure 4.** The Bunsen Peak workspace in the Western Margin Unit. (a) Sol 1064 Mastcam-Z view of the Bunsen Peak outcrop and surrounding terrain (zcam09083; image credit: NASA/JPL-Caltech/ASU/MSSS/K. Powell). Location of (b) is indicated. (b) Sol 1092 Mastcam-Z view of the Bunsen Peak workspace, with the Castle Geyser abrasion (A) and Comet Geyser sample (S) annotated (zcam09108, credit: NASA/JPL-Caltech/ASU/MSSS/S. Larriva), and (c) Stretched nighttime full illumination WATSON of the Castle Geyser abrasion, acquired at 7 cm standoff on sol 1080. Close up panel in (d) has been rotated 90° clockwise, and shows mm-scale sub-angular to sub-rounded light-toned masses (dashed black outlines) with occasional translucent green cores. Image credit: NASA/JPL-Caltech/MSSS/P. Barr/A. Pascuzzo.

facies is presented below, followed by a tour of key outcrops in the EMU (Section 5) and WMU (Section 6), demonstrating facies relationships, structures and successions critical for their interpretation.

## 4.1. Facies of the Eastern Margin Unit (EMU)

Proximity science observations were acquired at two sites in the EMU, both on abraded rock surfaces. First, at the Amherst Point abrasion near the Hans Amundsen memorial workspace on sol 913 of the mission (Figure 2). SHERLOC WATSON and ACI image analyses reveal an apparently clastic texture dominated by angular to sub-
<figure><img src="image_8.png" /><figcaption>**Figure 5.** Images of the Eremita Mesa workspace in the Western Margin Unit near its contact with the crater rim. (a) Sol 1252 Mastcam-Z view looking south toward the Eremita Mesa workspace (zcam09314; image credit: NASA/JPL-Caltech/ASU/MSSS/L. Mehall). Location of (b) is annotated. (b) Sol 1257 Navcam context view of the Eremita Mesa workspace and abrasion patch, (c) Stretched, full illumination nighttime WATSON of the Eremita Mesa abrasion acquired at 7 cm standoff on sol 1257, and (d) close up nighttime WATSON at 4 cm standoff of the area marked in (c) acquired on sol 1262, and showing several regions of possible interlocking crystalline textures (yellow arrows). Image credit: NASA/JPL-Caltech/MSSS/A. Pascuzzo.</figcaption></figure>

rounded grains ranging from 0.03 mm (the approximate imaging resolution) to 1.17 mm in diameter (Figure 2d). The mean and standard deviation was 0.2 mm ± 0.14 mm, consistent with a medium sandstone. Compositional analyses by the Planetary Instrument for X-ray Lithochemistry (PIXL, Allwood et al., 2020) indicate that these grains are composed of olivine and/or carbonate locally rimmed by serpentine in a matrix of silica (Williford et al., 2025). The Bill's Bay abrasion acquired on sol 934, ~350 m northwest and ~15 m higher in elevation at Turquoise Bay reveals a similar clastic texture, with more examples of coarser and sub-rounded to well-rounded grains (Figure 3). Grains in the Bill's Bay abrasion range between 0.03 and 2.3 mm, with a mean and standard deviation of 0.45 mm ± 0.31 mm, which is also consistent with a medium sandstone. Williams et al. (2024) also identified a gray isopachous phase (likely amorphous silica or carbonate) coating the rims of grains and surrounding void spaces at both Amherst Point and Bill's Bay, and which can be seen in the Lefroy Bay core (Figure S3 in Supporting Information S1).
<table><caption>**Table 1**
Structural-Lithologic Facies of the Margin Unit</caption><thead><tr><th>Facies</th><th>Sub-unit</th><th>Texture</th><th>Structures</th><th>Key outcrops</th><th>Abrasions</th><th>Samples</th><th>Interpreted emplacement setting</th></tr></thead><tbody><tr><td>Basinward-inclined sandstone (S<sub>B</sub>)</td><td>Eastern Margin Unit</td><td>Moderately sorted, medium sandstones with angular to well-rounded olivine- and carbonate-grains. Grains range from silt to very coarse sand, with rare granules observed at Bills Bay (e.g. Figures 2 and 3)</td><td>Plane parallel lamination, candidate low-angle cross bedding (e.g. Figures 7, 9, 10, 13 and 14)<br>Cross stratification, truncation surfaces (e.g. Figures 10–12)</td><td>Hans Amundsen memorial workspace, Turquoise Bay, Gnaraloo Bay<br>Turquoise Bay</td><td>Amherst Point, Bills Bay</td><td>Pelican Point, Lefroy Bay</td><td>Shoreline sedimentary complex</td></tr><tr><td>Rimward-inclined sandstone (S<sub>R</sub>)</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Sub-horizontal, undulate-stratified sandstone (S<sub>H</sub>)</td><td></td><td></td><td>Low-angle convex-up stratification, trough cross stratification, plane parallel lamination, erosion surfaces, concentric features (e.g. Figures 7, 8 and 10)</td><td>Hans Amundsen memorial workspace, Turquoise Bay, Gnaraloo Bay</td><td></td><td></td><td></td></tr><tr><td>Structureless facies (F<sub>S</sub>)</td><td>Western Margin Unit</td><td>Millimeter-scale, concentrically arranged masses of carbonate, with occasional olivine cores and rimmed by serpentine-group minerals, in a silica-rich matrix (Williford et al., 2025; Figure 4d). Near contact with crater rim displays millimeter-scale olivine with candidate interlocking, crystalline textures (e.g. Figure 5d)</td><td>None observed (e.g. Figure 18)<br>Decimeter-scale, plane parallel layering inclined east (e.g. Figures 19–21)</td><td>Bunsen Peak, Overlook Mountain, Dunn Butte Muir Island, Bunsen Peak</td><td>Castle Geyser, Old Faithful Geyser, Eremita Mesa</td><td>Comet Geyser N/A</td><td>Crystal settling in a molten igneous body</td></tr><tr><td>Planar layered facies (F<sub>L</sub>)</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>
The EMU sandstones represent the lowest explored elevations of the Margin unit, between -2,405 and 2,421 m, and comprise massive to well-stratified planar beds, typically 1-70 cm thick, and occasionally preserving sub-cm scale plane parallel lamination. Three facies are further distinguished based on their outcrop scale textures, structures and 3D geometries: basinwards-inclined sandstones ($S_B$), rimwards-inclined sandstones ($S_R$) and sub-horizontal, undulate-stratified sandstones ($S_H$).

$S_B$ comprises low-angle planar stratified to cross-bedded sandstones typically inclined east toward the center of the crater basin (ranging 005°-167°, mean 073°) and are predominantly low-angle inclined (7°-15°). Fewer examples are steeper (19°-25°) and appear to display a down-dip shallowing which may represent poorly exposed cross stratification. $S_R$ similarly comprises low-angle inclined (7°-9°), planar to sigmoidal, parallel stratified sandstone horizons dipping in the opposite direction to $S_B$, toward the west (216°-329°, mean 274°). Candidate cross strata are better exposed, showing steeper rimward-inclined dips (19°-27°) and asymptotic bases which truncate the underlying strata. $S_H$ is ascribed to sub-horizontal to especially low-angle (<7°), planar-to undulate-stratified sandstones. Stratification in $S_H$ typically shows gentle dip inversions with wavelengths of 3-5 m. This facies also contains several sedimentary structures including possible erosion surfaces, and planar and trough cross stratification (shown and discussed in Section 5). Due to the discontinuous, fractured outcrop of the Margin unit, it is likely $S_H$ is a shallow-dipping subset of $S_B$ and/or $S_R$ that cannot be ascribed confidently to either.

## 4.2. Facies of the Western Margin Unit (WMU)

The WMU lithologies are exposed at higher elevations than the EMU and occur closer to the crater rim, stratigraphically underlying the EMU between -2,410 and 2,270 m. The rocks of the WMU lack evidence for plane-parallel lamination, layer truncations and sedimentary structure, distinguishing them from the facies observed in the EMU. Furthermore, close-proximity investigations of the Castle Geyser abrasion patch in the WMU revealed mm-scale sub-angular to rounded masses consisting of carbonate, occasionally containing greenish olivine cores, and rimmed with serpentine, in a matrix of silica (Williford et al., 2025; Figures 4c and 4d). This intense alteration overprint obscures grain-to-grain contacts, precluding the confident identification of a clastic or crystalline protolith texture, and inhibiting a detailed lithofacies reconstruction on the basis of grain-scale textural data. WATSON and ACI image analyses show that the whiteish masses range 0.18-2.4 mm in diameter, with a mean and standard deviation of 0.7 mm ± 0.36 mm. Similar textures and compositions were observed in the Old Faithful Geyser abrasion, acquired from the WMU ~750 m further west toward the crater rim.

In the absence of diagnostic grain-scale data, two facies are identified in the WMU based on outcrop-scale characteristics. The first, and most pervasive lithology in the WMU is a structureless facies, $F_S$, which typically comprises many of the high standing ridges of the WMU. $F_S$ does not show any evidence for stratification at the outcrop scale and regularly exhibits abundant polygonal fractures forming blocks typically 0.2-5 m across. Both the Castle Geyser and Old Faithful Geyser abrasion patches, and the Comet Geyser sample, were acquired from $F_S$. A second, planar layered facies, $F_L$, is also observed in the WMU. Layering in $F_L$ is sub-horizontal to gently inclined eastwards into the crater (with a mean dip of 13° toward 057°). Layering is plane-parallel, ranging ~2-100 cm thick, and, where well exposed, shows significant lateral continuity with layer surfaces remaining parallel over distances of >50 m. $F_L$ appears periodically interstratified with $F_S$ throughout the WMU. No proximity science investigations were conducted on $F_L$, but given its similarity at outcrop and close spatial and stratigraphic association to $F_S$, we infer that it likely preserves a similarly altered rock with ambiguous grain-scale texture.

A third and final abrasion proximity science investigation was conducted near the basal contact of the WMU with the crater rim on sol 1257 (Figure 5). The abrasion patch, informally named "Eremita Mesa", shows a greater abundance of olivine crystals compared to Castle Geyser and Old Faithful Geyser. SHERLOC WATSON images of the abraded surface reveal regions of high particle angularity and interlocking, crystalline textures (Figure 5d). PIXL X-ray diffraction and elemental maps evidence a higher crystallinity and absence of an interstitial high-silica phase at Eremita Mesa compared to previous Margin unit targets, characteristics that are also shared with olivine- and carbonate-bearing exposures explored at higher elevations on the crater rim (Williford et al., 2025). Planar layering was observed near the Eremita Mesa abrasion site, but the outcrop itself appeared structureless (Figures 5a and 5b). As such, it is unclear whether this abrasion patch sampled $F_S$, or a poorly exposed instance of $F_L$.
<figure><img src="image_9.png" /><figcaption>**Figure 6.** *Perseverance's* traverse of the Eastern Margin Unit. (a) Full traverse route of the Eastern Margin Unit from the rover's initial descent of Mandu Wall on sol 909 to enter the Margin unit terrain, to its exploration of the Hans Amundsen memorial workspace, Turquoise Bay and Gnaraloo Bay. Close-up maps are presented showing *Perseverance's* exploration of Gnaraloo Bay (b) and Turquoise Bay (c). Sol numbers, locations of abrasion/core sites and viewing geometries of other key figures are annotated.</figcaption></figure>

**5. Sedimentology and Stratigraphy of the Eastern Margin Unit**

The following section provides an outcrop-by-outcrop documentation and analysis of key exposures observed by *Perseverance* in the EMU between sols 910 and 1007 (Section 5.1), followed by a discussion of potential depositional environments for this sub-unit (Section 5.2). We document the observed facies distributions, structures and stratigraphic relationships critical for the interpretation of their depositional environment (Figures 6–16). Figure 6 presents maps of *Perseverance's* traverse route through the EMU, and viewsheds for subsequent Figures 7–15 are presented in the Figures S4–S13 of Supporting Information S1.
<figure><img src="image_10.png" /><figcaption>**Figure 7.** Outcrop views of the Hans Amundsen memorial workspace in the Eastern Margin Unit. (a) Mastcam-Z landscape mosaic of the low-lying fractured pavers of the Margin unit exposed at the Hans Amundsen memorial workspace, acquired on sol 916 (ZCAM08923, credit: NASA/JPL-Caltech/ASU/MSSS/S. Larriva). Yellow arrows indicate the rim of a small crater. (b) Close-up of (a) showing several blocks of the basinward-inclined sandstone facies ($S_B$) exposing 5–15 cm thick layering, (c) Close-up of (a) showing lamination-scale stratification in another exposure of $S_B$, (d) Close-up of a small crater exposed west of the Hans Amundsen memorial workspace; yellow arrows indicate the crater rim. Blocks around the rim of the small crater lack evidence for stratification and may represent the structureless facies ($F_S$) excavated from the underlying Western Margin Unit. Foreground pavers also reveal several concentric features, shown close-up in Figure 8. A viewshed map is presented in Figure S4 of Supporting Information S1.</figcaption></figure>

## 5.1. Observations

### 5.1.1. Structures at the Hans Amundsen Memorial Workspace

The rover first encountered the Margin unit after descending the Western fan at Mandu Wall (Figure 6) and performed its first remote and proximity science campaign at the Hans Amundsen memorial workspace. This was the lowest elevation outcrop observed throughout the Margin unit campaign and where the Amherst Point abrasion and Pelican Point sample (Figure 2) were acquired. Figure 7a shows a Mastcam-Z view of the fractured, low-lying polygonal terrain in this area. In the left of the viewshed, much of the terrain has been disturbed by a small, 25 m diameter impact crater. Variably exposed low-angle layering is present throughout the view across multiple blocks and continuously across low-lying pavers. Figure 7b shows an example where 5–15 cm thick layers are exposed across three blocks and appear plane-parallel with no truncations of layering observed. While the apparent orientations of these layers appear sub-horizontal to east-inclined, the individual blocks cannot be confidently assessed as in situ. A more continuous outcrop is present in the right of the image looking north from the Hans Amundsen memorial workspace (Figure 7c), and exposes plane-parallel, laterally continuous, low-angle (<5°) basinwards-inclined strata attributed to the $S_B$ facies. Stratification here reaches lamination-scale; resolvable horizons are 6–8 mm thick. This contrasts with many of the blocks immediately surrounding the rim of the small crater, which appear to lack stratification and may represent excavated $F_S$ facies rocks attributed to the WMU.

Nearfield observations of the Hans Amundsen memorial workspace reveal several closely packed, concentric features, 1–3 m across, extending across the undisturbed polygonal outcrops east of the small impact crater (Figure 8). 3D analyses of these structures reveal they are low-angle (<8°) dome and hummock structures in the layering of the Margin unit bedrock. Locally, these show potential evidence of flaking, which may be attributed to exfoliation. Around the margins of some of these structures are smaller-scale, 3–20 cm wide, irregular quasi-circular pits (Figure 8c). The concentric features are radially cross-cut by 1–2 cm wide resistant fracture fills which protrude from the surrounding bedrock. These features typically have a planar to arcuate morphology to some showing gentle undulations with amplitudes of a few centimeters. These fracture fill features are sporadically observed throughout every facies in the EMU and WMU.
<figure><img src="image_11.png" /></figure>

**Figure 8.** Concentric and pitted features exposed in pavers of the Eastern Margin Unit at the Hans Amundsen memorial workspace. (a) Sol 911 Mastcam-Z landscape view of several meter-scale closely packed concentric features (zcam08917, credit: NASA/JPL-Caltech/ASU/MSSS/S. Larriva), (b) Sol 926 Mastcam-Z close-up mosaic view of one of these concentric structures, showing concentric lamination patterns forming an apparent hummock-like structure. Several smaller irregular pits (yellow lines) flank the concentric structure (zcam08394, credit: NASA/JPL-Caltech/ASU/MSSS/L. Duflot), (c) Zoom of (b) highlighting these smaller-scale irregular pit structures (yellow arrows), (d) Image of another concentric structure acquired on sol 925, this time showing apparently domed layering and possible exfoliation flaking. The structure is radially crosscut by undulating 1–2 cm wide fracture fills (zcam08932, credit: NASA/JPL-Caltech/ASU/MSSS/L. Duflot). A viewshed map is presented in Figure S5 of Supporting Information S1.

5.1.2. Sedimentary Architecture of the Turquoise Bay Ridge

Following its exploration of the Hans Amundsen memorial workspace, *Perseverance* began traversing north toward Turquoise Bay, acquiring its first view of this area on Sol 929 (Figures 6 and 9a). In this view looking northwest toward the crater rim and Nerevta Vallis, the Margin unit is exposed along a N–S striking ridge, flanked by a low lying polygonal outcrop similar to that observed at the Hans Amundsen memorial workspace. Observations of Mastcam-Z images and comparisons to the HiRISE DTM of the region suggest that this ridge extends ~250 m to the north, rising to 6 m above the surrounding Margin unit terrain. The low-lying outcrop to the east of the ridge displays plane-parallel, laterally continuous stratification 0.8–5 cm thick (Figure 9b). While individual grains cannot be resolved, much of the low-lying outcrop expresses a rougher and more granular surface texture than the Hans Amundsen memorial workspace, potentially indicating coarser grain sizes. Strata here, which consist of $S_B$ facies, are inclined east (with a mean dip of 12° toward 063°). Clear basinward-inclined stratification is also observed along the east-facing flank of the Turquoise Bay ridge; stratification here also shows a potential arcuate geometry and down-dip shallowing (Figure 9c).
<figure><img src="image_12.png" /><figcaption>**Figure 9.** Outcrop observations of the Turquoise Bay ridge. (a) Mastcam-Z landscape mosaic view looking northwest toward the Turquoise Bay ridge (indicated by the yellow triangles) acquired on sol 929, with dashed boxes labeled (b) and (c) showing locations of subsequent zoom panels (ZCAM08936, credit: NASA/JPL-Caltech/ASU/MSSS/K. Powell). (b) Zoom of (a) showing pervasive plane-parallel layering inclined basinward ($S_B$), east of the ridge. Dashed boxes provide close-up views of the basinward inclined stratification. Several outcrops east of the ridge display a coarse, granular texture, although individual grains cannot be resolved at this distance. (c) Zoom of (a) showing meter-scale basinwards-inclined layers (white arrows), which show a potential down-dip shallowing and arcuate geometry. A viewshed map is presented in Figure S6 of Supporting Information S1.</figcaption></figure>

*Perseverance* subsequently traversed around the western edge of this ridge, and acquired images of the Margin unit close to the center of the ridge axis (Figure 6c). A view looking east across the central axis of the Turquoise Bay ridge is shown in Figure 10a. In this view, multiple discontinuous large blocks and low-lying outcrops expose more low-angle layering and several potential sedimentary structures. At the center of the ridge, stratification is sub-horizontal to inclined at low angles, and strata near the ridge crest express convex-up stratification with 3–4 m wavelengths (Figure 10b). Structureless sandstone is juxtaposed above well-stratified sandstone along an angular surface in an adjacent block (Figure 10c). This irregular truncation is exposed throughout the block for ~2 m, and may represent an erosional scour surface. Another nearby block exposes potential planar and trough cross stratification (Figures 10d and 10f). On the well-lit face of this block, variations in dust cover highlight strata 2–5 cm thick, forming a low-angle, ~2 m wide trough cross set. Stratification at the base of this well-lit face is harder to resolve, but appears to be inclined away from and truncated by, the overlying trough. These strata show down-dip shallowing and may represent partially exposed lower trough cross sets. Trough axes broadly trend ENE-WSW. At the southern end of this same block, stratification is observed downlapping at a low angle against a potential bounding surface, representing partially exposed cross strata with an amplitude of ~3 cm (Figure 10e). Larger-scale basinward-inclined cross stratification may be exposed on the far (eastern) side of the Turquoise Bay ridge in this view, with at least 30 cm amplitude candidate foresets exposed here (Figure 10f). The Bill's Bay abrasion and the corresponding Lefroy Bay sample (Figure 3b), were collected from a nearby low-lying outcrop at this location, likely within $S_R$ facies bedrock.
<figure><img src="image_13.png" /><figcaption>**Figure 10.** Outcrop-scale observations of the center of the Turquoise Bay ridge. (a) Mastcam-Z landscape mosaic view looking east across the crest of the Turquoise Bay ridge on sol 934, with close-up views (b–f, indicated by lettered arrows) revealing several candidate sedimentary structures (zcam08949, credit: NASA/JPL-Caltech/ ASU/MSSS/S. Larriva). (b) Low-angle, convex-up stratification, (c) Irregular, undulating candidate erosion surface (red dashes), where poorly stratified Eastern Margin Unit (EMU) materials are emplaced above more thinly layered, potentially cross stratified EMU sandstones (d) Apparent trough cross stratification exposed in a paver near the ridge crest. The location of a potential bounding surface between an upper set of concave-up trough cross strata and the underlying cross strata is annotated in red. Low-angle cross stratification is also exposed in the southern end of this paver (e) where cross beds (black lines) terminate down against a bounding surface (red dashes). (f) Potential larger scale, basinward-inclined cross stratification with amplitude of >30 cm. A viewshed map is presented in Figure S7 of Supporting Information S1.</figcaption></figure>

The view west from the Bill's Bay abrasion site is shown in Figure 11. Here, several distinct facies are observed. Foreground exposures, and those in the right of the viewshed display clear rimwards-inclined stratification typical of $S_R$. These beds occupy gentle topographic highs and exhibit ~1 m amplitude, sigmoidal bedding which shallows down-dip laterally over ~10 m (Figure 11b). Another ridge to the north shows $S_R$ juxtaposed against $S_B$ (Figure 11c). It is not clear whether $S_B$ truncates $S_R$ here (or vice versa), or whether this structure preserves a gentle convex-up inflexion of bedding. In the mid-to far-field, no clear stratification or sedimentary structure is observed; instead, the Margin unit is present as structureless blocks, which may represent the first observed in situ
<figure><img src="image_14.png" /><figcaption>**Figure 11.** Outcrop views looking west from Turquoise Bay. (a) Mastcam-Z landscape mosaic view looking west of Turquoise Bay toward the Western Margin Unit (WMU) and crater rim, acquired on sol 936 (zcam08950, credit: NASA/JPL-Caltech/ASU/MSSS/L. Mehall). Foreground exposures and topographic highs preserve well-stratified and granular-textured lithologies attributed to the Eastern Margin Unit (EMU), while the more distant exposures appear structureless and are attributed to the WMU. The transition zone is poorly exposed but can be broadly constrained to strike north-south (yellow dashed line). Locations of lower zoom panels are by dashed boxes labeled (b-d) respectively. (b) Zoom of (a) showing well stratified, rimwards inclined sandstone beds ($S_R$) with sigmoidal bedding geometry in cross section, occupying a local topographic high. (c) Zoom of (a) showing juxtaposed beds of basinward- ($S_B$) and rimward- ($S_R$) inclined sandstones; it is unclear if this forms a continuous convex up structure, or whether one facies truncates and overlies the other. (d) Zoom of (a) showing an exposure of structureless material west of Turquoise Bay, attributed to $F_S$ of the WMU. A viewshed map is presented in Figure S8 of Supporting Information S1.</figcaption></figure>

occurrence of $F_S$ (Figure 11d). The transition zone between $F_S$ in the WMU and exposures of $S_R$ and $S_B$ in the EMU is poorly exposed, and no abrupt contact is observed.

A final view of Turquoise Bay was acquired on Sol 952 looking southwest back toward the northernmost portion of the ridge (Figure 12). This view confirms the presence of $S_B$ on the eastern flank and $S_R$ on the western flank of the ridge, however from this view, the nature of the contact can be more readily observed. $S_R$ appears juxtaposed above $S_B$ along a truncation surface which is exposed striking north-south for ~50 m at the surface (Figure 12b). These strata appear to correlate with truncating layer sets observed in subsurface radargrams acquired by the Radar Imager for Mars' Subsurface Experiment (RIMFAX; Hamran et al., 2020) during its traverse of the EMU (Williford et al., 2025). The $S_R$ strata immediately overlying $S_B$ at the central ridge axis appear particularly low-angle, and further along strike appear to asymptote down onto more steeply inclined $S_R$ strata (Figure 12c), which may represent meter-scale rimward-inclined cross stratification. The sub-horizontal orientation of the bounding surface here (Figure 12c) supports that these strata have not been rotated, implying that the rimward-inclined dips in this area are primary depositional dips rather than the product of post-depositional deformation. Additionally, no evidence for deformation (e.g., fault breccias) was observed along the Turquoise Bay ridge to imply that bedding had been tilted in this area. An *Ingenuity* Return-To-Earth (RTE) image acquired along-strike from the central ridge axis to the north revealed a complex, concentric pattern of laminations cropping out across several
<figure><img src="image_15.png" /><figcaption>**Figure 12.** Sedimentary structures and architectures exposed at the northern end of the Turquoise Bay ridge. (a) Sol 952 Mastcam-Z view southwest, back toward the Turquoise Bay ridge (yellow triangles), foreground rocks show well-exposed basinward-inclined stratification indicated by white arrows (zcam08969, credit: NASA/JPL-Caltech/ASU/MSSS/L. Mehall). Boxes (b, c) show the locations of the lower zoom panels. (b) Zoom of (a) showing rimward-inclined sandstones ($S_R$) appearing to truncate and overlie basinward-inclined sandstones ($S_B$) along a north-south striking truncation surface (yellow dashed line), with white box showing zoom of well-stratified outcrop of $S_B$. (c) Zoom of (a) showing rimward-inclined cross stratification. Overlying beds appear to asymptote down onto a sub-horizontal bounding surface (yellow dashed line), which abruptly overlies a set of rimward-inclined strata ($S_R$). (d) Concentric pattern of laminations observed by Ingenuity's Return-To-Earth (RTE) camera during its 60th flight (sol 924), acquired approximately 125 m along strike from the ridge axis truncation surface shown in (b). A viewshed map for (a) is presented in Figure S9 of Supporting Information S1 and (d) in Figure S10 of Supporting Information S1.</figcaption></figure>

low-lying polygonal outcrops in this area (Figure 12d). Given the generally flat topography in this area, these laminations are inferred to preserve the internal structure of a convex-up, 3D lobe of material with internal dips in multiple directions, varying from a gentle westward, to northward, to eastward dip azimuth.

### 5.1.3. Sedimentary Architectures and Regional Stratigraphic Relations at Gnaraloo Bay

Following its exploration of Turquoise Bay, Perseverance drove a loop of Gnaraloo Bay between sols 959 and 999 of the mission (Figure 6b). The area is an erosional window through the apex of the Jezero western fan and into the underlying Margin unit, ~500 m north-northeast of Turquoise Bay. A view looking northeast into Gnaraloo Bay is presented in Figure 13a, showing the light-toned fractured Margin unit extending across the floor of the area and exposed in the western scarp at a location named “Burnt Island.” The steeper slopes surrounding Gnaraloo Bay are comprised of well-bedded buff-colored sandstones and cobble-conglomerates, attributed to deposits of the Jezero Upper Fan group, capped by a succession of unconsolidated olivine and low-calcium pyroxene rich boulders (Beyssac et al., 2024; Vaughan, Rice, et al., 2025). The sandstones of the Jezero Western fan are observed unconformably overlying the Margin unit in the east of Gnaraloo Bay, at a location named Airey Hill (Figure 13b). The contact at this location is abrupt; no evidence for interfingering of facies between the fan and Margin unit was observed, supporting an unconformable relationship. At this location, the Margin unit underlying the fan is well-stratified, with well-exposed resolvable stratification reaching the sub-cm scale. Southeast inclined horizons of $S_B$ are observed downlapping onto laterally continuous plane parallel layers of $S_H$. Looking west across the floor of Gnaraloo Bay from Airey Hill (Figure 13c), Margin unit rocks appear dominated by beds of $S_B$ with stratification potentially reaching lamination scale. The surfaces of these rocks generally show
<figure><img src="image_16.png" /></figure>

**Figure 13.** The Eastern Margin Unit (EMU) exposed underlying the Jezero western fan at its apex, at Gnaraloo Bay. (a) Sol 959 Mastcam-Z view looking northeast into Gnaraloo Bay, with key locations annotated, including the most complete stratigraphic section of the EMU observed by *Perseverance* in the west of Gnaraloo Bay, at a location named Burnt Island (zcam08981, credit: NASA/JPL-Caltech/ASU/MSSS/K. Powell). Location of panel (b) is annotated. (b) Mastcam-Z view of Airey Hill on sol 996, where the deposits of the Jezero western fan are observed unconformably overlying the EMU (red dashed line). Foreground exposures of the EMU expose basinwards-inclined sandstones ($S_B$) terminating against sub-horizonal EMU sandstones ($S_H$) along a downlapping surface (yellow dashed line) (zcam09005, credit: NASA/JPL-Caltech/ASU/MSSS/S. Larriva). (c) Mastcam-Z view northwest from Airey Hill toward the Burnt Island stratigraphic section, acquired on sol 962. Close up shows finely layered, planar-stratified EMU bedrock (zcam08986, credit: NASA/JPL-Caltech/ASU/MSSS/L. Mehall). Viewshed maps are presented in Figure S11 of Supporting Information S1.

smoother textures, similar to those observed near the Hans Amundsen memorial workspace (e.g., Figure 7), whilst the granular textures observed near Turquoise Bay (e.g., Figures 9b and 11a) were largely absent.

The most complete stratigraphic section through the EMU is exposed in the slope west and southwest of Gnaraloo Bay, around Burnt Island. The lower elevation slopes expose gently inclined beds of $S_B$, which when traced appear to laterally pinch out forming lenticular packages of sandstone with internal stratification 1–2 cm thick (Figure 14). The best exposed of these is a ~5 m-wide, ~30 cm-thick package, with mean internal stratification oriented 11° toward 086°. The upper ~3 m of stratigraphy at Burnt Island was imaged from multiple orientations, providing an excellent three-dimensional view of the overall structure. Viewed looking west on Sol 994, strata appear apparently sub-horizontal, exposed as planar, laterally continuous beds which extend along strike for tens of meters (Figure 15a). However, an orthogonal view acquired on Sol 959 looking north at this section reveals that these beds are low-angle inclined both basinward ($S_B$) and rimward ($S_R$), forming a series of stacked, convex-up structures (Figure 15b).

### 5.1.4. Integrated Stratigraphic Sections of the Eastern Margin Unit

The observations reported above are placed in stratigraphic context in Figure 16 as several east-west cross sections through the EMU at the Hans Amundsen memorial workspace, Turquoise Bay and Gnaraloo Bay, to aid interpretation of the depositional environment. These cross sections suggest that the higher elevation exposures of the EMU (–2,400 to –2,410 m), are dominated by shallowly inclined beds of $S_B$ and $S_R$, and exhibit layer
<figure><img src="image_17.png" /><figcaption>**Figure 14.** (a) Unannotated and (b) annotated view of low-angle sandstone packages exposed at the base of the Burnt Island stratigraphy, imaged by Mastcam-Z on sol 998 (ZCAM09011, credit: NASA/JPL-Caltech/ASU/MSSS/L. Duflot). Strata are annotated with black lines in (b), and potential package boundaries in yellow dashed lines. Stratification and candidate package bounding surfaces are all low-angle and occur in the basinward-inclined sandstone facies ($S_B$). Individual packages appear to pinch out laterally over 10's of meters. A viewshed map is presented in Figure S12 of Supporting Information S1.</figcaption></figure>

truncations, erosion surfaces and localized planar cross stratification and trough cross stratification. Low-angle convex up stratification with 3–5 m wavelengths is also observed at the center of the Turquoise Bay ridge and potentially at Burnt Island. The Bill's Bay abrasion conducted at Turquoise Bay suggests that these rocks contain

<figure><img src="image_18.png" /><figcaption>**Figure 15.** Orthogonal views of the upper Burnt Island stratigraphic section exposed in the west of Gnaraloo Bay. (a) Sol 994 Mastcam-Z view looking west toward the Eastern Margin Unit (EMU) strata exposed at Burnt Island, which from this angle appear planar and laterally continuous with apparent sub-horizontal dip (yellow arrows) along strike to the north and south (ZCAM09001, credit: NASA/JPL-Caltech/ASU/MSSS/K. Powell). (b) Orthogonal view acquired by Mastcam-Z on sol 959 looking north at the same stratigraphic succession, now revealing a complex succession of partly exposed, basinward- and rimward-, low-angle inclined layers (yellow arrows) (ZCAM08981, credit: NASA/JPL-Caltech/ASU/MSSS/K. Powell). A viewshed map is presented in Figure S13 of Supporting Information S1.</figcaption></figure>
<figure><img src="image_19.png" /><figcaption>**Figure 16.** 3D-offset synthetic cross sections through the Eastern Margin Unit (EMU) at the Hans Amundsen memorial workspace, Turquoise Bay and Gnaraloo Bay. Sections use the same vertical and lateral scales and are offset in 3D according to their along-strike occurrence in the EMU during *Perseverance*'s northward traverse. Red dots indicate the relative locations of the Bill's Bay and Amherst Point proximity science investigation sites, and where the Lefroy Bay and Pelican Point samples were acquired. At the lowest elevations, at the Hans Amundsen memorial workspace and lower Gnaraloo Bay, planar stratified beds of basinward-inclined ($S_B$) to sub-horizontally inclined ($S_H$) sandstones are the dominant facies. At higher elevations, in the Burnt Island stratigraphy and at Turquoise Bay, both basinward- ($S_B$) and rimward- ($S_R$) inclined sandstone beds are present, and $S_H$ typically exhibits broad convex up features. At Turquoise Bay, a large north-south striking, low-angle, convex-up architecture appears present in the mapped stratigraphy, and the majority of observed candidate sedimentary structures occur near the center of this feature.</figcaption></figure>

sub-angular to well-rounded coarse sand grains. Conversely, at lower elevations (–2,415 to –2,425 m), the EMU is dominated by low-angle stratification (typically < 5°) either basinward-inclined ($S_B$) or sub-horizontal ($S_H$). As previously noted, observations of the Amherst Point abrasion suggest that these rocks display a finer texture with more angular grains.

While the boundary of the EMU and WMU is not clearly observed at outcrop, the potential occurrence of $F_S$ in the excavated materials of the small crater near the Hans Amundsen memorial workspace (Figure 7d) and exposed due west of Turquoise Bay (Figure 11a) supports the suggestion that the EMU overlies $F_S$ facies rocks of the WMU, along a broadly east-dipping contact. The nature of this boundary remains unclear, although the subtlety of the transition at outcrop may support a gradational nature rather than an abrupt planar contact.

## 5.2. Interpretations

In this section, we explore several hypotheses for the depositional setting of the EMU, assessing their consistency with the outcrop- and grain-scale observations reported in this work. Hypotheses are evaluated by employing a hypothesis testing matrix, presented in Table 2.

### 5.2.1. Hypothesis 1: The EMU Was Emplaced by Non-sedimentary Processes

As described by Williford et al. (2025), the apparent clastic texture observed in the Bill's Bay and Amherst Point abrasion patches are not unambiguously sedimentary in origin, given the possibility of carbonation reactions generating “pseudoclastic” textures, and/or obscuring primary crystallinity or pyroclastic textures (Williford et al., 2025). Hence, an igneous intrusion, lava flow, or pyroclastic origin must be considered for the EMU. Lamination scale stratification and occasional sedimentary structures including rare cross-bedding and erosional features can occur in crystalline igneous deposits, generated by flow within a large magma body (such as a lava lake, thick lava flow or igneous intrusion) as well as by flow within a pyroclastic density current (Fisher & Schmincke, 2012; Namur et al., 2015; Parsons, 2013). Cooling and contraction of hot igneous intrusions, lava flows and pyroclastic density current deposits on Earth also typically result in them displaying polygonal fracture
<table><caption>**Table 2**
Hypothesis Testing Matrix for the Deposition of the Eastern Margin Unit (EMU)</caption><thead><tr><th>Key observations</th><th>Tephra fall</th><th>Pyroclastic density current</th><th>Cumulate (sill-like intrusion or lava lake)</th><th>Lava flows</th><th>Shoreline</th><th>Fluvial/alluvial</th><th>Deltaic</th><th>Aeolian</th></tr></thead><tbody><tr><td>Clastic textures consistent with moderately sorted medium sandstones (e.g. Figures 2 and 3)</td><td>✓</td><td>✓</td><td>✗</td><td>✗</td><td>✓✓</td><td>✓✓</td><td>✓✓</td><td>✓✓</td></tr><tr><td>Angular to rounded grains (e.g. Figures 2 and 3)</td><td>✗✗</td><td>✓</td><td>✗</td><td>✗</td><td>✓✓</td><td>✓✓</td><td>✓✓</td><td>✗</td></tr><tr><td>Sedimentary structures including lamination (e.g. Figures 7c and 9b), cross stratification (e.g. Figures 10d–10f and 12c) and erosion surfaces (e.g. Figures 10c and 14)</td><td>✗✗</td><td>✓</td><td>✓</td><td>✗✗</td><td>✓✓</td><td>✓✓</td><td>✓✓</td><td>✓✓</td></tr><tr><td>Strata are inclined at low angles (~12°)</td><td>✓</td><td>✓</td><td>✓</td><td>✓</td><td>✓✓</td><td>✓</td><td>✓</td><td>✗</td></tr><tr><td>Bimodal bedding orientations inclined toward and away from the basin (e.g. Figures 11c, 12b and 15b)</td><td>✗</td><td>✗</td><td>✗</td><td>✗</td><td>✓✓</td><td>✗</td><td>✗</td><td>✓</td></tr><tr><td>Low-angle, convex-up (domed) stratification (e.g. Figures 10b and 15b)</td><td>✗</td><td>✗</td><td>✗</td><td>✗</td><td>✓✓</td><td>✗</td><td>✗</td><td>✗</td></tr><tr><td>Extremely low Al<sub>2</sub>O<sub>3</sub> and CaO (Williford et al., 2025)</td><td>✗✗</td><td>✗✗</td><td>✓</td><td>✗✗</td><td>✓</td><td>✓</td><td>✓</td><td>✓</td></tr><tr><td>SCORE</td><td>-6</td><td>0</td><td>-1</td><td>-7</td><td>13</td><td>6</td><td>6</td><td>3</td></tr></tbody></table>

Note. Key observations are tested for their consistency with several possible depositional models. ✓✓ = likely (+2 points), ✓ = possible (+1 point), ✗ = unlikely (-1 point), ✗✗ = highly unlikely (-2 points).

networks (Hetényi et al., 2012; Tibaldi & Bonali, 2018), such as are pervasive throughout the Margin unit. Such fracture networks are observed in siliciclastic rocks at Gale crater, however (Kronyak et al., 2019), and the absence of glass or its alteration products challenges an extrusive igneous or pyroclastic origin. Furthermore, Williford et al. (2025) reported extremely low bulk CaO and Al₂O₃ compositions for the Margin unit, which are atypical of any known terrestrial pyroclastic or extrusive igneous rocks. It is also unclear how an igneous intrusion, lava flow, or pyroclastic density current or fall deposit could generate bimodal bedding dip orientations toward and away from the crater rim, or complex bedding architectures such as those outlined in Figure 16 without sedimentary reworking. Therefore, the co-occurrence of clastic textures, diverse grain compositions and multiple candidate sedimentary structures and complex bedding architectures favors a sedimentary emplacement for the EMU.

### 5.2.2. Hypothesis 2: The EMU Was Deposited as an Aeolian Sandstone

The presence of medium to very coarse sand-sized grains and candidate cross stratification may support an aeolian interpretation for the EMU, where the inclined sandstone beds could be interpreted as variably preserved aeolian cross strata (Fryberger & Schenk, 1981). The mean dip magnitude of the strata measured in the EMU is ~10°, however, which is significantly lower than the angle-of-repose inclination expected for aeolian cross-stratification. The Stimson formation at Gale crater observed by the Mars Science Laboratory rover *Curiosity*, displays aeolian cross-stratification with mean dips in excess of 20° (Banham et al., 2018, 2021), and terrestrial aeolian cross bedding shows similar dip magnitudes (e.g., Dias & Scherer, 2008). Furthermore, low-angle convex-up structures such as those observed in the S<sub>H</sub> facies (e.g., Figure 10b) are not typical of aeolian deposits. Pinstripe laminations typical of wind ripple migration (e.g., Fryberger & Schenk, 1988) were also not observed in the EMU. Additionally, the moderately sorted texture and numerous examples of high angularity grains in the Amherst Point and Bill's Bay abrasion patches appear inconsistent with the high degree of grain sorting and rounding typically expressed by aeolian deposits (Kapui et al., 2018).

### 5.2.3. Hypothesis 3: The EMU Was Deposited in a Fluvial/Alluvial Environment

Given the geologic context of the EMU immediately adjacent to a well-preserved delta deposit (Caravaca et al., 2024; Mangold et al., 2021, 2024), a more proximal, fluvial, or alluvial origin for the EMU is also considered. The typically sub-mm scale grains and relatively narrow range of grain sizes in the EMU significantly contrast with the medium- to coarse-grained sandstones and coarse-conglomeratic textures of the adjacent fluvial and deltaic deposits of the Jezero western fan (Caravaca et al., 2024; Gwizd et al., 2024; Mangold et al., 2024;
Stack et al., 2024). Given that the EMU is located closer to the crater rim than the Jezero western fan deposits, a more proximal fluvial or alluvial setting would be expected to display greater textural and compositional immaturity, however the opposite is observed in the EMU, which instead comprises relatively well-sorted olivine- and carbonate-sandstones. The occurrence of rimward inclined stratification west of Turquoise Bay (Figures 11 and 12), and in the stratigraphy exposed at Burnt Island (Figure 15) is also challenging to justify in a fluvial scenario, where the prevailing dip azimuth of cross strata would be expected to be toward the crater interior.

### 5.2.4. Hypothesis 4: The EMU Was Deposited in a Shore Zone Complex

The EMU is distributed just below the -2,395 m elevation contour, a critical elevation of the Jezero paleoshoreline inferred from orbit (Fassett & Head, 2005), thus warranting consideration that the EMU preserves a lacustrine shoreline setting.

Lacustrine shoreface environments typically preserve a diverse range of facies, with structures often varying significantly throughout a single stratigraphic section owing to complex swash-backwash processes that redistribute sediment and modify the shoreface (Andrews & Hartley, 2015). Compared to coastal shoreface settings, the absence of tidal influence in lake margin environments means that their morphosedimentary characteristics are largely determined by wind fetch and lake level fluctuations (Gracia et al., 2021). Lacustrine shore zone facies (in clastic- and carbonate-dominated environments) are most often identified by their characteristic low-angle stratification which rarely exceeds 10–15° and may be accompanied by planar to hummocky bedding and small scour surfaces (Andrews & Guo, 2024; Andrews & Hartley, 2015), characteristics which are all observed in the EMU. Wave- and current-induced distribution of sediment can also generate larger scale sedimentary architectures in lake shoreface settings, including beach bars, sand spits and shoals (Andrews & Guo, 2024; Andrews & Hartley, 2015; Cohen & Thouin, 1987; Gracia et al., 2021; Nordstrom & Jackson, 2012).

In an upper shoreface model for the EMU, the plane-parallel bedded, low-angle basinward inclined strata ($S_B$) are interpreted to form by sedimentation in the swash zone during fairweather conditions. These strata may be locally modified in the foreshore-backshore setting into sub-horizontal (resembling $S_H$ in the study area), or low-angle planar strata inclined both lakeward (preserved as $S_B$) and landward (as $S_R$) by swash-backwash processes, forming complex stratigraphic successions of variably dipping beds (e.g., Andrews & Hartley, 2015; Karabiyikoğlu et al., 1999). This is similar to the sequence observed in the Burnt Island stratigraphy (Figure 15b). The east-west trending axes of the trough cross strata preserved at Turquoise Bay (Figure 10d) also support an easterly or westerly paleoflow direction, consistent with onshore-offshore swash-backwash processes. Additionally, the presence of a candidate scour surface at Turquoise Bay (Figure 10c) supports sufficient flow energies for sediment transport and incision.

Basinward-inclined swash deposits preserved in terrestrial lake shoreface successions typically show high lateral continuity, and occasional low-angle pinch outs (Karabiyikoğlu et al., 1999), like the strata observed in the EMU at Gnaraloo Bay (Figures 14 and 15). Low-angle planar stratified to steeper cross-stratified beds of $S_R$ such as those preserved west of Turquoise Bay (e.g., Figures 11 and 12) may be interpreted as washover features formed during periods of sediment reflux up the shoreface (Lazarus & Armstrong, 2015). The planar to sigmoidal bedding geometries of some of the rimward inclined strata west of Turquoise Bay (e.g., Figures 11b and 12c) share significant morphologic similarities to ancient terrestrial examples of landward-prograding foresets of lacustrine washover fans and lobes (e.g., Schuster & Nutz, 2018).

Where such swash and washover processes are temporally sustained, larger-scale beach bars may be generated striking parallel to the lake coastline (in this case, parallel to the crater rim), which on Earth are characterized by broad, convex-up stratification and bimodal lakeward and landward dip orientations (Fraser & Hester, 1977; Gracia et al., 2021; Schuster & Nutz, 2018; Zhang et al., 2018). Such broad convex-up stratification may be preserved at Turquoise Bay (e.g., Figure 10b), which itself may preserve the center of a potentially much larger scale (>50 m wide) bar-like structure striking north-south and bound by lakeward and landward inclined strata (Figure 16). Where such structures are emergent, aeolian modification and early cementation may lead to the generation of cross-bedded aeolianite in the upper reaches of the ridge (Dalrymple & Rivers, 2023; Rowe & Bristow, 2015), which may resemble some of the steeper-inclined cross strata observed in the study area (e.g., Figures 10f and 12c). Sand spits also share numerous morphological similarities to beach bars, but predominantly migrate along their long axis owing to longshore currents or angular wave approaches transporting sediment along their strike (Blair, 1999; Isla et al., 2023). The concentric lamination pattern observed by
Ingenuity (Figure 12d) occurs directly along strike from the long axis of the candidate barrier structure, and shares morphologic similarity to the interior laminations often exposed at the lobate terminus of a sand spit arm (e.g., Bezzi et al., 2019).

Shoal complexes are similar coast-parallel, subaqueous mound-like architectures of loose sediment, but are usually constructed by complex tide-wave interactions (Dalrymple & Rivers, 2023). Examples have been observed in lakes, however, where sustained wave action in shallow waters can act to redistribute sediment to form longshore shoal complexes, even in the absence of tides (e.g., Andrews & Guo, 2024; Cohen & Thouin, 1987; Milroy & Wright, 2000). Lacustrine shoals also display similar low-angle bedding and cross stratification (e.g., Yang et al., 2017) but may lack internal erosion surfaces (e.g., Keighley et al., 2003), such as those observed in the EMU.

Overall, the consistency of observations in the EMU with a shorezone sedimentary model leads it to score most highly in the hypothesis testing matrix (Table 2). While the bimodal orientations of low-angle stratification, as well as erosion surfaces and cross stratification observed in the EMU are non-diagnostic of a particular shore zone environment, we note that each of the settings described above (i.e., foreshore, beach bars, spits and shoals) all occur in shallow, littoral settings where persistent wave action can penetrate to the sediment-water interface. As such, each of these settings is used as a paleoshoreline indicator in the terrestrial rock record (Lebrec et al., 2022). Given that each of these environments produces similar, often non-diagnostic sedimentary and stratigraphic features, we suggest that the EMU most likely preserves a shore zone complex, including a littoral bar-like structure at Turquoise Bay (Figure 16), but further distinction of specific shoreline environments would require additional exploration and data collection.

A shore zone model would also be consistent with the grain scale observations of the Bill's Bay and Amherst Point abrasion patches. Terrestrial coastal shoreline deposits typically display consistently well-rounded grains owing to persistent wave action. In lacustrine settings however, with more limited fetch leading to lower amplitude waves, sediments are typically not as well rounded as coastal beach environments, with angular and rounded grains often co-occurring in the same location (Gracia et al., 2021; Nordstrom & Jackson, 2012). Furthermore, the isopachous phase and rimmed grains reported by Williams et al. (2024) implies formation in a phreatic environment with fluid filled pore spaces, consistent with lacustrine deposition. Similar coated grains are commonly observed in the shallow, agitated waters of terrestrial lacustrine shoreline sediments (Platt & Wright, 1991).

Evidence for shore zone reworking (i.e., the presence of sedimentary structure, rounded grains and erosional features) is only present in the upper 10 m of the EMU section observed by Perseverance (Figure 16). These strata represent shallower water depths at the lake margin where the effects of surface water waves could penetrate to the bed surface. One hypothesis is that at lower elevations such as in lower Gnaraloo Bay or at the Hans Amundsen memorial workspace, planar-stratified sandstones inclined basinwards may represent the less modified, mid-to lower-shoreface environment where wave energies at the bed were lower and unable to significantly rework lake margin sands. The concentric features at the Hans Amundsen memorial workspace may be interpreted as gentle dome and hummock features, akin to those that are observed in lower lake shoreface successions on Earth (e.g., Andrews & Guo, 2024; Andrews & Hartley, 2015) or, alternatively, as exfoliated layers associated with spheroidal weathering (Ollier, 1971). A lower energy setting is also supported by the finer grain size and lower degree of grain rounding observed at the Amherst Point abrasion.

## 6. Stratigraphy of the Western Margin Unit

Evidence of fine-scale layering and sedimentary structures typical of the S<sub>B</sub>, S<sub>R</sub> and S<sub>H</sub> facies were no longer observed shortly after Perseverance began its westward traverse of the Margin unit on Sol 1007 (Figure 17). This transition correlated with a geomorphic change from low-lying polygonal outcrops to a higher standing rugged terrain comprising a series of steep, fractured ridges and troughs. Additionally, a geochemical transition from a relatively broad range of Fe/Mg compositions in the EMU, to a much narrower range of values in the WMU was observed by SuperCam Laser Induced Breakdown Spectroscopy (LIBS) and PIXL X-ray fluorescence measurements (Clavé et al., 2024; Williford et al., 2025). Prominent east-dipping subsurface radar reflectors identified by the RIMFAX instrument during this traverse suggest that as the river traversed west, it descended through the stratigraphy (Williford et al., 2025). Key outcrops revealing lithologic and stratigraphic characteristics of F<sub>S</sub> and F<sub>L</sub> observed during this traverse are presented in detail below (Section 6.1) and inform a
<figure><img src="image_20.png" /><figcaption>**Figure 17.** *Perseverance's* traverse of the Western Margin Unit. (a) Full traverse route of the Western Margin Unit from the rover's initial departure from the Eastern Margin Unit around sol 1007, to its exploration of Muiron Island, Bunsen Peak and Overlook Mountain before descending into Neretva Vallis. *Perseverance* subsequently returned to the Margin unit on sol 1230, before conducting a final abrasion at Eremita Mesa. Close-up maps are presented showing *Perseverance's* exploration of the sol 1021 EOD location area, Jenabuddi, Muiron Island and Bunsen Peak (b) and Overlook Mountain, Neretva Vallis, Dunn Butte and Eremita Mesa (c). Sol numbers, locations of abrasion/core sites and viewing geometries of other key figures are annotated.</figcaption></figure>
discussion of interpretations for the WMU (Section 6.2). Viewsheds for outcrop images shown in Figures 18–22 are presented in the Figures S14–S19 of Supporting Information S1.

**6.1. Observations**

**6.1.1. F<sub>S</sub> Facies Observations Near Jenabuddi**

Upon entering the rougher terrain of the WMU, the Science Team spent ~20 sols conducting a thorough imaging campaign of the F<sub>S</sub> facies from the Sol 1021 end-of-drive location (Figure 17b). A Mastcam-Z view from this location is presented in Figure 18a, where the Margin unit rocks show a remarkable absence of bedding structure at the outcrop scale. This lack of structure is also observed in *Ingenuity* RTE images over this area (Figure 18b), showing the terrain pervasively jointed into polygonal blocky pavers and ridges 0.2–5 m across. Both the Mastcam-Z outcrop view and *Ingenuity* RTE planform view of this area are in stark contrast to observations of the EMU, where the Margin unit is characterized by common cm- to mm-scale stratification. One similarity to the EMU is that F<sub>S</sub> here is cross-cut by gently undulating, often arcuate fracture fills, typically 1–2 cm wide, protruding from the bedrock. These features often have an asymmetric, wavy geometry, and are occasionally present as stacked sheets, forming larger structures which may be mistaken for primary layering (Figure 18c). Fracture fills have highly variable strike and dip orientations and often cannot be traced along strike for more than a few meters, suggesting they are a later stage diagenetic feature rather than primary depositional fabric. Locally, F<sub>S</sub> is observed to have small, mm-sized pits persisting across wind-abraded surfaces (Figure 18d), likely resulting from mm-sized constituent grains or crystals winnowing out of the bedrock.

A cross-sectional view through F<sub>S</sub> is exposed to the north at a Margin unit ridge named Jenabuddi (Figures 17, 18e and 18f). Jenabuddi displays a similar lack of bedding structure to the Sol 1021 end-of-drive area and is pervasively jointed into meter-scale polygonal blocks. In places, these joints appear lenticular to lobate in geometry (Figure 18e).

**6.1.2. Stratigraphy and Structure at Muiron Island and Bunsen Peak**

As *Perseverance* continued its westward traverse past Jenabuddi, it began to encounter decimeter-scale planar-layered, F<sub>L</sub> facies rocks on Sol 1045.

The best exposed section of the F<sub>L</sub> facies is at Muiron Island, a ~10 m tall, 350 m long topographic ridge striking east-west, parallel to the margins of the Jezero inlet channel Neretva Vallis, and parallel to the rover traverse route (Figures 17b, 19a and 19b). The base of the section is exposed at the western end of the outcrop, and comprises a structureless succession of F<sub>S</sub> facies rocks, of which up to ~5 m is exposed here. This is overlain by a 25 m thick succession of F<sub>L</sub>, which possesses internal layering a few centimeters to a meter thick and is laterally continuous across the outcrop. The transition between these facies is subtle but can be picked out in a SuperCam RMI image of the contact (Figure 19c). F<sub>L</sub> facies strata are parallel to the upper and lower contact of this layered interval and can be traced laterally through the Muiron Island outcrop for >50 m without any observed stratal thickening, thinning, or presence of sedimentary structures. This layered succession is overlain by a ~3 m thick, poorly exposed and variably regolith covered structureless succession of F<sub>S</sub>, exposed in the easternmost portion of Murion Island. This interval is likely stratigraphically equivalent to the structureless rocks exposed at Jenabuddi and around the sol 1021 end-of-drive location (Figure 18) and shows hints of possible layering across a few isolated blocks in its lowest meter. The orientations of these layers are parallel and conform to the basal contact of this interval, which forms a resistant, planar surface inclined east, protruding from the outcrop and which can be traced laterally for >10 m (Figure 19d). The transition from the lower F<sub>L</sub> interval into the overlying F<sub>S</sub> interval is also associated with a geomorphic change from a recessive and concave to resistant and convex hillslope morphology. These stratigraphic intervals are denoted as F<sub>S2</sub> (the lowermost structureless interval), F<sub>L1</sub> (the overlying layered interval) and F<sub>S1</sub> (the uppermost structureless interval). This naming convention (F<sub>S1</sub>, F<sub>S2</sub>... F<sub>S4</sub>/F<sub>L1</sub>, F<sub>L2</sub>... F<sub>L4</sub>) is employed for the rest of the stratigraphic descent through the Margin unit as *Perseverance* traversed westwards, and is labeled on the cross-section of the unit presented in Figure 23.

Continuing westward along the traverse, the base of F<sub>S2</sub> observed in the Muiron Island section cropped out closer to the rover traverse, near the Bunsen Peak outcrop (Figure 17) where the Castle Geyser abrasion and Comet Geyser sample were collected (Figure 4b). Looking east toward Bunsen Peak (Figure 20), the base of F<sub>S2</sub> is exposed overlying a distinct interval of layered rocks, here termed F<sub>L2</sub>. Similar to the contact of F<sub>S1</sub> on F<sub>L1</sub> at
<figure><img src="image_21.png" /><figcaption>**Figure 18.** Structureless facies ($F_S$) exposed in the Western Margin Unit (WMU). (a) Mastcam-Z view of the sol 1021 end-of-drive location marking one of the first occurrences of the $F_S$ facies observed by *Perseverance* (Sol 1028, zcam09033, credit: NASA/JPL-Caltech/ASU/MSSS/L. Duflot). Lettered arrows show locations of zoom panels (c, d). (b) *Ingenuity* RTE view of the same area, acquired during its 63rd flight on sol 947, showing the 0.2–5 m scale fractured paver network across this area. The location of Jenabuddi (f) in this view is also labeled. (c) Zoom of (a) showing closely packed resistant fracture fill features (white arrows) protruding from the bedrock. (d) Zoom of (a) showing the pitted surface texture of the $F_S$ facies exposed at this location. (e–f) Mastcam-Z views of Jenabuddi, a nearby ridge providing a cross-section through the $F_S$ facies in this area acquired on sol 1041 (zcam09039, credit: NASA/JPL-Caltech/ASU/MSSS/S. Larriva). This view reveals abundant jointing, which is occasionally lobate (e) and presents no indication of internal stratification across several tens of meters of exposure. A viewshed map is for (a, f) is presented in Figure S14 of Supporting Information S1, and a viewshed for the *Ingenuity* RTE image shown in (b) is presented in Figure S15 of Supporting Information S1.</figcaption></figure>
<figure><img src="image_22.png" /><figcaption>**Figure 19.** Stratigraphic succession in the Western Margin Unit (WMU) at Muiron Island, a linear ridge running parallel to the walls of the Jezero inlet, Neretva Vallis. (a) Unannotated and (b) annotated Mastcam-Z view from sol 1045 looking north toward Muiron Island (zcam09025, credit: NASA/JPL-Caltech/ASU/MSSS/L. Duflot). Letters (c, d) show the locations of the lower panels. Three distinct stratigraphic intervals are exposed inclined east in this view: a structureless lithologic association at the base ($F_{S2}$), overlain along a planar contact (yellow dashed line) by a decimeter-scale planar layered interval ($F_{L1}$), with layers parallel to the basal contact, which itself is overlain by another structureless interval, $F_{S1}$, along a resistant planar surface inclined east (yellow line). (c) SuperCam Remote Micro Imager (RMI) image of the basal contact of $F_{L1}$ on $F_{S2}$, showing the transition from a structureless lithology ($F_{S2}$) to an overlying plane-parallel layered lithology ($F_{L1}$), acquired on sol 1047 (SCAM05047). (d) Zoom of (a) showing the transition from a layered interval ($F_{L1}$) to an overlying structureless interval ($F_{S1}$) along a resistant planar surface (yellow arrows) at the eastern edge of Muiron Island. Close-up panels show the planar-layered and structureless nature of $F_{L1}$ and $F_{S1}$, respectively. A viewshed map is presented in Figure S16 of Supporting Information S1.</figcaption></figure>

Muiron Island, the basal contact of $F_{S2}$ is associated with a geomorphic change from a resistant ridge-forming material to a low-lying, variably dust-covered, recessive $F_{L2}$ interval. This contact was driven over by the rover during its Sol 1052 traverse (Figure 17), and the contact was imaged as a very prominent, apparently east-dipping radar reflector in the subsurface by the RIMFAX instrument (Williford et al., 2025). Planar layers within $F_{L2}$ ranged between 3 and 15 cm thick and were not observed to possess any internal structure or lamination; additionally, no layer truncations, thickening or thinning were observed. The basal contact of $F_{L2}$ was not well exposed at the surface nor associated with a significant geomorphic change, but the appearance of an additional structureless zone at Bunsen Peak ($F_{S3}$) permitted the stratigraphic thickness of $F_{L2}$ to be constrained to ~7 m.

### 6.1.3. Stratigraphy in the Walls of the Neretva Vallis Channel

*Perseverance* continued traversing through $F_{S3}$ for ~600 m, until it reached Overlook Mountain (Figure 17), where the second WMU abrasion, Old Faithful Geyser, was acquired from nearby. During the traverse to this location, occasional blocks with well-developed decimeter-scale planar parallel layering characteristic of $F_L$ were observed, but the terrain was so highly fractured that it was not possible to conclude with certainty whether these blocks were in situ. As a result, $F_{S3}$ may contain additional layered horizons, but these could not be confidently mapped. *Perseverance*'s subsequent descent into Neretva Vallis on Sol 1158 revealed spectacular views of the Margin unit exposed in the walls of the Jezero inlet channel, including further repeated stratigraphic intervals of $F_S$ and $F_L$ approaching the crater rim (Figure 21).

Imaged from the Sol 1162 end-of-drive location, the southern wall of Neretva Vallis provides a cross-sectional view through the Margin unit the rover had been traversing prior to its descent into the channel (Figure 21a). In this view, Overlook Mountain forms a resistant mound exposed in the valley walls. The outcrop here is highly jointed and discontinuous, with no clear stratification observed east of Overlook Mountain (attributed to the $F_{S3}$
<figure><img src="image_23.png" /><figcaption>**Figure 20.** Western Margin Unit (WMU) stratigraphy at Bunsen Peak. (a) Mastcam-Z view of the stratigraphic succession in the WMU near Bunsen Peak acquired on sol 1058 (ZCAM09079, credit: NASA/JPL-Caltech/ASU/MSSS/S. Larriva). The yellow circle shows the location of the Comet Geyser sample site, and the red circle indicates the location of the Castle Geyser abrasion patch (shown in Figure 4b). This view shows the base of a structureless facies association ($F_{S2}$), exposed as a resistant ridge overlying a planar layered facies ($F_{L2}$) in the midground. Foreground rocks, including Bunsen Peak, mark the transition to a new structureless interval, $F_{S3}$. (b) Sol 1085 view of the contact (yellow dashed line) of $F_{S2}$ on $F_{L2}$. The location of the layered outcrop in panel (c) is indicated. (c) close-up view of the planar, decimeter-scale layering observed in $F_{L2}$ (ZCAM09103, credit: NASA/JPL-Caltech/ASU/MSSS/L. Mehall). A viewshed map is presented in Figure S17 of Supporting Information S1.</figcaption></figure>

stratigraphic interval). West of Overlook Mountain, however, a new layered interval is observed ($F_{L3}$) displaying decimeter-scale planar-parallel layering inclined east. Layering displays a consistent orientation across multiple fractured blocks (Figure 21b), suggesting that they are either in situ or have experienced only minor transport or rotation. One outcrop of $F_{L3}$, Saddle Mountain, a low-lying exposure near the base of the southern wall, was imaged within 5 m of the rover during its descent into the channel (Figure 21c) and measurements from Mastcam-Z stereoscopic data support an easterly dip (inclined 11° toward 103°). Although poorly constrained due to the fractured outcrop, layering of $F_{L3}$ gradually stopped being observed ~50 m west of its upper contact, where a further structureless interval ($F_{S4}$) was observed. This implies an approximate true stratigraphic thickness of ~30 m for $F_{L3}$.
<figure><img src="image_24.png" /><figcaption>**Figure 21.** Annotated Mastcam-Z views of the Western Margin Unit (WMU) exposed in the southern wall of Neretva Vallis, imaged on sols 1167–1168 (zcam09202, zcam09204, credit: NASA/JPL-Caltech/ASU/MSSS/K. Powell). (a) Southern wall of Neretva Vallis observed by *Perseverance*, with the rover traverse annotated (white dotted line) and locations of panels (b, c) indicated by lettered arrows. Three stratigraphic intervals of the WMU are recognised in this section, a lower structureless interval (*F*<sub>S4</sub>), which is overlain by a planar layered interval (*F*<sub>L3</sub>), which itself is overlain by another structureless interval (*F*<sub>S3</sub>), where the Old Faithful Geyser (OFG) abrasion was conducted. (b) Close up of (a) showing that the orientation of planar layering in *F*<sub>L3</sub> is continuous in orientation across several blocks. (c) View of layering (white arrows) in *F*<sub>L3</sub> at Saddle Mountain observed by *Perseverance* on sol 1159 as it descended into Neretva Vallis (zcam09197, credit: NASA/JPL-Caltech/ASU/MSSS/K. Powell). A viewshed map for (a) is presented in Figure S18 of Supporting Information S1.</figcaption></figure>

### 6.1.4. The Basal Margin Unit Exposed Near Dunn Butte

From within Neretva Vallis, *Perseverance* imaged the lowermost exposed stratigraphy of the Margin unit and its contact with the crater rim rocks looking southwest on Sol 1229 (Figure 22a), before ascending the crater rim to end the Margin unit campaign. To the east of this viewshed, the Margin unit is expressed as the typical blocky, fractured geomorphic expression previously encountered throughout the WMU. This blocky material is observed overlying smoother, low-relief Margin unit terrain along a NE-inclined surface at the base of a high standing ridge informally named “Dunn Butte” (Figure 22). This surface forms a resistant, topographic bench and is mapped as the base of *F*<sub>S4</sub>, and has a dip of 14° toward 036°. The smoother, underlying light-toned terrain continues to the west where it terminates against the regolith-covered crater rim slope.

Figure 22b shows a second view looking east across this smooth terrain toward Dunn Butte, acquired from the Margin unit-crater rim contact on Sol 1248. The smooth terrain in this mosaic is highly fractured and has low relief, but while poorly exposed, occasional planar layering can be observed inclined to the northeast. This smooth terrain represents the stratigraphically lowest exposed example of the *F*<sub>L</sub> facies (*F*<sub>L4</sub>), where it is observed overlying the crater rim. *F*<sub>L4</sub> also exhibits multiple strata which are more regionally continuous, forming darker-toned resistant layers protruding from the smooth terrain and extending laterally for tens of meters. Structural measurements of these horizons measured from the HiRISE DTM (see Section 3.2; Figure S2 in Supporting Information S1) suggest they are also inclined to the northeast, on average dipping 14° toward 038°, sub-parallel to the basal resistant layer of *F*<sub>S4</sub> in Dunn Butte. The Eremita Mesa abrasion patch was acquired from this smooth terrain and revealed a more crystalline and less altered texture than previous Margin unit abrasions (Figure 5d).

### 6.1.5. Stratigraphic Relationship to the Crater Rim

To assess the relationship between the Margin unit and the crater rim, stratal orientations within the lowermost WMU near Dunn Butte were compared to the apparent orientation of the contact between the Margin unit and the crater rim, and to the modern slope orientation of the crater rim just above the contact. Given that these features were
<figure><img src="image_25.png" /><figcaption>**Figure 22.** Views of the apparent Margin unit contact with the crater rim (yellow dashed lines). (a) Sol 1229 Mastcam-Z view south from Neretva Vallis looking toward Dunn Butte and the western crater rim (zcam09279, credit: NASA/JPL-Caltech/ASU/MSSS/Cornell/J. Joseph). Numerical values show dip magnitude and dip directions (e.g., 14/036 = dipping 14° toward 036°). Dunn Butte appears to mark the base (solid yellow line) of a structureless facies interval (F<sub>S4</sub>), where it overlies a comparatively recessive interval with poorly exposed planar layering (F<sub>L4</sub>). Strata observed here are inclined into the crater, at a comparable angle to the modern topographic slope of the crater rim (18/040), supporting a draping relationship. One outcrop, higher up the rim, also appears to drape the underlying topography and has a similar blocky outcrop expression to the Margin unit. (b) Sol 1248 Mastcam-Z view south east from the Margin unit/crater rim contact showing the same stratigraphic relationships, as well as the site of the Eremita Mesa (EM) abrasion patch, shown in Figure 5 (zcam09310, credit: NASA/JPL-Caltech/ASU/MSSS/K. Powell). A viewshed map is presented in Figure S19 of Supporting Information S1.</figcaption></figure>

not imaged at sufficiently close proximity by Mastcam-Z for reliable 3D measurement, measurements were instead acquired by mapping these features onto the HiRISE DTM with the guidance of Mastcam-Z photospheres (see Section 3.2; Figure S2 in Supporting Information S1). The mean dip of the contact was 17° toward 038°, sub-parallel to layering in F<sub>L4</sub> (inclined 14° toward 038°), the base of F<sub>S3</sub> (inclined 14° toward 036°) and to the mean modern topographic slope of the crater rim above the contact (inclined 18° toward 040°). This implies that the WMU is draping the crater rim and is consistent with the apparent draping outcrop geometry of the Margin unit on the rim shown in Figure 22. An integrated structural lithologic map and measured cross-section of the Margin unit, as well as stereonets comparing the structural measurements for the WMU and EMU are presented in Figure 23.

## 6.2. Interpretations

Following the observation of multiple candidate sedimentary architectures, structures and clastic textures in the EMU, the presence of mm-scale sub-angular to rounded Fe/Mg-carbonate particles in the Castle Geyser abrasion at Bunsen Peak initially suggested that the WMU was also a sedimentary deposit. However, as previously discussed, the assemblages observed in the Castle Geyser and Old Faithful Geyser abrasion patches are also consistent with pseudoclastic textures generated by carbonation of an ultramafic igneous unit, where the reaction of olivine with CO₂-rich fluids leads to the production of carbonate and silica (Williford et al., 2025). Circulation of carbonating fluids around olivine crystal boundaries progressively alters them inwards (e.g., Boskabadi et al., 2020; Decrausaz et al., 2023), destroying grain contacts and thus overprinting primary crystalline textures. This origin would be supported by the concentric arrangement of mineral phases observed in the Castle Geyser and Old Faithful Geyser abrasion patches, and with the higher crystallinity of the Eremita Mesa abrasion and additional olivine-rich exposures exposed at higher elevations on the crater rim, which may represent a less-altered, crystalline protolith. Unlike in the EMU, SuperCam LIBS and PIXL X-ray fluorescence measurements show a much narrower range of Fe/Mg ratios in the WMU (Clavé et al., 2024; Williford et al., 2025), implying in situ carbonation of an ultramafic protolith may be a feasible means of generating carbonate here. Alternatively, it is possible that carbonation of an ultramafic sedimentary protolith, or silicification of a carbonate-cemented sedimentary rock during early diagenesis may yield similar assemblages (e.g., Bustillo, 2010). Assisted by a hypothesis testing matrix (Table 3), the following section evaluates the feasibility of generating the WMU stratigraphy by various sedimentary or igneous emplacement mechanisms.
<figure><img src="image_26.png" /></figure>

**Figure 23.** Structural lithologic map and west-east cross-section of the Margin unit explored by *Perseverance*, constructed from Mastcam-Z landscape mosaics and stereoscopic image measurements. Abrasion and core sample sites are annotated. Sol numbers are annotated along the traverse in white. Facies ($S_B/S_R$...) and specific stratigraphic intervals ($F_{S1}$, $F_{L1}$, $F_{S2}$...) are indicated on the cross-section. Solid color is used within 85 m of the rover traverse, marking the distance at which sub-centimeter scale features such as sedimentary lamination are no longer resolvable to Mastcam-Z its 63 mm focal length. Dip ticks are provided for all 3D structural measurements, and dip azimuth arrows are provided where dip azimuths were estimated directly from Mastcam-Z mosaics. An additional radius of 150 m is also annotated, where sub-cm scale resolution is lost at the highest 110 mm Mastcam-Z focal length. Stereonets are also presented for the Western Margin Unit (WMU), showing a consistent shallow E/NE dip into the basin and sub-parallel to the modern topography of the crater rim (red line), and for the Eastern Margin Unit (EMU), showing more variable east (basinward) and west (rimward) shallow dip orientations.

### 6.2.1. Hypothesis 1: Sedimentary Emplacement

Unlike in the EMU, no distinct evidence for sedimentary processes acting in the WMU is present at the outcrop scale. Across more than 2.5 km of traverse distance, clear evidence for cross stratification, sedimentary lamination, or other sedimentary structures was lacking. Therefore, any sedimentary interpretation for the WMU would require either deposition of uniquely structureless beds, or the obfuscation of sedimentary structures by later alteration processes. Sedimentary structures are still readily observable in fluvio-lacustrine and aeolian strata at Gale crater that show significant diagenetic overprinting (e.g., Seeger & Grotzinger, 2024). Nevertheless, it is not clear how well sedimentary structures may be preserved following olivine carbonation reactions. Extensive carbonation of ultramafic rocks is largely restricted to igneous protoliths on Earth, exposed in Archean greenstone belts or ophiolite complexes (e.g., Decrausaz et al., 2023; Shibuya et al., 2012), and as such, the effects of carbonation on sedimentary rocks are poorly understood. However, given that planar parallel layering is preserved in the $F_L$ facies, it is unclear how cross bedding or sedimentary structure could be preferentially obscured whilst these planar layers are well retained following carbonation. Intense ventifaction atop the Greenheugh pediment, Gale crater, has been shown to locally obscure sedimentary structure in the aeolian Stimson formation, generating a terrain of wind-sculpted, polygonal sandstone blocks with an outcrop style similar to the WMU (Bryk et al., 2025). This effect is largely restricted to the low-lying outcrop atop the pediment, however, with more vertical exposures of the Stimson formation displaying well preserved cross-stratification (e.g., Banham et al., 2018, 2021). Sedimentary structure is not only absent in the low-lying, wind-blasted outcrop of the WMU, but is also lacking in the multiple extensive vertical exposures through the unit encountered by
<table><caption>**Table 3**
Hypothesis Testing Matrix for the Emplacement of the Western Margin Unit (WMU)</caption><thead><tr><th>Key observations</th><th>Tephra fall</th><th>Pyroclastic density current</th><th>Cumulate (sill-like intrusion or lava lake)</th><th>Lava flows</th><th>Shoreline</th><th>Fluvial/alluvial</th><th>Deltaic</th><th>Aeolian</th></tr></thead><tbody><tr><td>Structureless to dm-scale planar layered rocks</td><td>✓✓</td><td>✓</td><td>✓✓</td><td>✓</td><td>✓</td><td>✗</td><td>✗</td><td>✗✗</td></tr><tr><td>Plane parallel layer boundaries with high lateral continuity</td><td>✓✓</td><td>✓</td><td>✓✓</td><td>✓</td><td>✓</td><td>✓</td><td>✓</td><td>✓</td></tr><tr><td>Layering inclined basinwards</td><td>✓✓</td><td>✓</td><td>✓</td><td>✓✓</td><td>✓✓</td><td>✓✓</td><td>✓✓</td><td>✓</td></tr><tr><td>Inclined at ~12°</td><td>✓✓</td><td>✓✓</td><td>✓</td><td>✗</td><td>✓</td><td>✗</td><td>✓</td><td>✓</td></tr><tr><td>Absence of sedimentary structures</td><td>✓✓</td><td>✓✓</td><td>✓✓</td><td>✓✓</td><td>✗</td><td>✗✗</td><td>✗✗</td><td>✗✗</td></tr><tr><td>Relatively homogeneous ~ mm-scale “grains”, absence of larger clasts/facies variations</td><td>✓</td><td>✗</td><td>✓✓</td><td>✓✓</td><td>✗</td><td>✗✗</td><td>✗✗</td><td>✓</td></tr><tr><td>Draping crater rim</td><td>✓✓</td><td>✓✓</td><td>✓</td><td>✓</td><td>✓</td><td>✓</td><td>✓</td><td>✓✓</td></tr><tr><td>Candidate crystalline textures</td><td>✓</td><td>✓</td><td>✓✓</td><td>✓✓</td><td>✗</td><td>✗</td><td>✗</td><td>✗</td></tr><tr><td>Absence of glass</td><td>✗✗</td><td>✗✗</td><td>✓✓</td><td>✗✗</td><td>✓✓</td><td>✓✓</td><td>✓✓</td><td>✓✓</td></tr><tr><td>Extremely low Al<sub>2</sub>O<sub>3</sub> and CaO (Williford et al., 2025)</td><td>✗✗</td><td>✗✗</td><td>✓</td><td>✗✗</td><td>✓</td><td>✓</td><td>✓</td><td>✓</td></tr><tr><td>SCORE</td><td>10</td><td>5</td><td>16</td><td>6</td><td>6</td><td>0</td><td>2</td><td>4</td></tr></tbody></table>

Note. Key observations are tested for their consistency with several possible pyroclastic, crystalline igneous and sedimentary models. ✓✓ = likely (+2 points), ✓ = possible (+1 point), ✗ = unlikely (−1 point), ✗✗ = highly unlikely (−2 points).

Perseverance (e.g., Figures 18f, 19 and 21a). Consequently, aeolian or fluvial deposition associated with abundant cross stratification, is unlikely. Furthermore, the average basinward dip of 13° is likely too steep for an alluvial fan deposit, particularly in the absence of coarser grained conglomeratic horizons (Le Hooke & Rohrer, 1979; Stock et al., 2008).

The grain scale characteristics of the WMU also challenge a sedimentary interpretation. It is not clear how detrital sedimentary processes could generate such limited grain diversity immediately adjacent to the crater rim. The Jezero western rim exposes diverse lithologies including low-Ca pyroxene-rich Noachian basement, Al- and kaolinite-rich megabreccia blocks, as well as extensive olivine-rich exposures (Deahn et al., 2024; Mayhew et al., 2024). The sedimentary deposits of the Jezero western fan preserve similarly diverse sediments derived from the crater rim and beyond (Horgan, Duflot, et al., 2023). The predominantly olivine- and carbonate-dominated nature of the WMU would require it to be derived almost uniquely from an olivine-rich terrain, perhaps deposited shortly after emplacement of the regional olivine-bearing unit when much of Jezero rim was covered in olivine-rich material. Furthermore, the WMU is dominated by mm-scale and finer grains, with no clear examples of conglomeratic horizons which would be expected at such proximity to a crater rim source.

Deposition of the WMU as a lake shoreline deposit scores best among the sedimentary emplacement mechanisms in the hypothesis testing matrix (Table 3), and may imply a shared emplacement mechanism with the EMU. Foreshore progradation in carbonate shoreline settings can lead to the deposition of predominantly massive, basinward-inclined strata (Dominic et al., 1995; Karabiyikoğlu et al., 1999; Nayanthara et al., 2021; Tamura, 2012). Widespread, early carbonate cementation is common in such environments, typically leading to the formation of beachrock during periods of changing sea or lake level (e.g., Benson, 1994; Binkley et al., 1980; Vousdoukas et al., 2007). This early cementation often enhances the preservation of the original sedimentary structure (e.g., García-Hidalgo & Elorza, 2022; Kelly et al., 2014; Wiles et al., 2018), however multiple examples have been reported of beachrock displaying decimeter-scale, planar bedding similar to that observed in the WMU (e.g., Evren Erginal et al., 2012; Nayanthara et al., 2021). In a sedimentary model, it is also possible that the high silica interstitial phase observed in the Castle Geyser and Old Faithful Geyser abrasion patches is the product of silicification of original carbonate cements, rather than the product of olivine carbonation reactions. Such silicification reactions can occur due to minor pH changes in alkaline lakes, and are known to destroy sedimentary structure in the surrounding bedrock (Bustillo, 2010). As a result, intense, early diagenesis in the shoreline environment of an alkaline lake may contribute to the obfuscation of sedimentary structures.
A shoreline sedimentary model may be consistent with the stratigraphy and distribution of the unit along the inner margin of the western crater rim between a possible lake highstand at -2,260 m and the Jezero breach elevation at -2,395 m (Horgan et al., 2020). In this case, the exposures of olivine- and carbonate-bearing rocks, which lack a high silica phase and display possible crystalline textures at Eremita Mesa (and at higher elevations on the crater rim), may represent the sediment source. The shoreline model is challenged, however, by the absence of any lateral facies transitions across the WMU; terrestrial shorefaces display rapid lateral facies transitions as water deepens lakeward (Pemberton et al., 2012; Talbot & Allen, 1996). No such lateral variation in grain size, stratal thickness, or internal structure was confidently observed in the WMU.

### 6.2.2. Hypothesis 2: Volcaniclastic Emplacement

While explosive volcanism capable of forming ignimbrites and extensive tephra fall deposits on Earth is typically silicic in composition, explosive mafic or ultramafic volcanism may have been more likely on Mars (Bedford et al., 2024; Ruff et al., 2022; Wilson and Head III, 1994). The few terrestrial examples of ultramafic pyroclastic deposits show variable degrees of olivine alteration to carbonate and serpentine (e.g., Pittari et al., 2015), and the clastic nature and increased permeability of pyroclastic rocks may permit enhanced carbonation compared to crystalline igneous units (Ruff et al., 2022). There are no known terrestrial examples, however, of volcanic rocks with such extremely low CaO and Al₂O₃ bulk compositions comparable to those reported for the Margin unit (Williford et al., 2025), and as such, if the Margin unit was emplaced by pyroclastic processes, they would have been compositionally exceptional. The absence of angular juvenile grains, vesicularity and glass phases in the WMU further challenges a pyroclastic model.

A pyroclastic origin has however been suggested as an emplacement mechanism for the regional olivine-bearing unit, primarily reflecting the observation that it appears to mantle and drape pre-existing topography (Kremer et al., 2019; Mandon et al., 2020; Ruff et al., 2022). Massive to planar layered rocks inclined basinward and draping the crater rim could be consistent with a pyroclastic density current or a pyroclastic fall deposit. In these models, variations between structureless and layered stratigraphy inclined basinward would be attributed to either (a) transitions between turbulent- ($F_S$) and tractional- ($F_L$) dominated flow regimes during the emplacement of a granular ignimbrite (Branney & Kokelaar, 2003) or (b) fluctuations in discharge rate during an explosive eruption emplacing fallout tephra (Fisher & Schmincke, 2012). Given the repeated stratigraphic succession of $F_S$ and $F_L$ in the WMU, this may imply multiple eruptions or eruptive phases during its emplacement.

Ignimbrites are the favored mechanism of Ruff et al. (2022) to explain the origin of the regional olivine-bearing unit. They suggest that the abundant polygonal fractures pervasive throughout the regional unit (and the Margin unit) share similarities to the surfaces of terrestrial ignimbrites associated with cooling following emplacement. They argue that such features would be less likely to occur in pyroclastic fall deposits which are emplaced at a lower temperature following atmospheric cooling. Low aspect-ratio terrestrial ignimbrites can mantle topography much like pyroclastic fall deposits (Walker et al., 1980) and have massive to internally layered stratification (Fisher & Schmincke, 2012). Although terrestrial ignimbrites are typically poorly sorted and often contain larger angular clasts, modeling of pyroclastic processes under a thinner Mars atmosphere has suggested larger clasts would only be deposited in regions highly proximal to the source vent (Wilson & Head, 2007). Furthermore, enhanced volatile exsolution caused by the lower atmospheric pressures would lead to more thorough magma fragmentation such that, beyond only the most proximal regions, most pyroclasts may not exceed a few millimeters (Wilson & Head, 2007). Consequently, it is possible that the deposits of martian ignimbrites may resemble the mm-scale grains observed in the WMU. No evidence was observed in the WMU, however, for features typical of pyroclastic density current deposits such as graded bedding, truncation surfaces caused by flows scouring underlying deposits, or variation in layer thickness and/or degree of welding (Fisher & Schmincke, 2012).

The pyroclastic fall hypothesis scores more highly than the pyroclastic density current model in the hypothesis testing matrix (Table 3). Conversely to pyroclastic density current deposits, pyroclastic fall deposits exhibit planar layering, which is highly laterally continuous, and characteristically drapes pre-existing topography (Fisher & Schmincke, 2012), much like the geometry of the WMU stratigraphy on the crater rim. Atmospheric modeling of ash dispersal suggests that finer grains can travel much larger distances in a thin martian atmosphere, but that the mm-scale grains such as those observed in the WMU would not travel further than ~20 km (Kerber et al., 2012; Ravanis et al., 2022; Wilson & Head, 2007). Horgan, Udry, et al. (2023) have identified several candidate edifices at Jezero within this radius, suggesting that explosive volcanism in the Jezero region could emplace ash into the
crater. However, the lack of significant lateral grain size variations, which would be expected at increasing distances from the vent for a tephra fall deposit, along with the absence of glass (or its alteration products) also challenges an eruptive volcanic origin (Fisher & Schmincke, 2012).

### 6.2.3. Hypothesis 3: Extrusive or Intrusive Igneous Emplacement

Given the distinct absence of sedimentary structures or lateral facies variations in the WMU, and the possibility that the apparent granular textures observed in abrasion patches may represent an altered crystalline rock (Williford et al., 2025), the emplacement of the unit as an igneous intrusion, lava lake, or lava flow(s) is a viable hypothesis.

No obvious lava flow morphologies or textures (e.g., Alwmark et al., 2023) were observed in the WMU. The ultramafic composition inferred for the Margin unit protolith suggests that any flows would have extremely low viscosity, resembling terrestrial komatiites. Calculations of the viscosity of the Séitah formation revealed similarly low viscosities when it was emplaced (Brown et al., 2024). Such low viscosity lava would have likely had flow fronts only a few centimeters thick, and their emplacement onto even shallow slopes has been shown to reduce this thickness even further (Cas et al., 1999). As such, komatiite flows are unlikely to generate the dm-to m-scale inclined layers observed in the WMU. Cooling rates would also be exceptionally high for such thin, high temperature flows exposed at the martian surface, which would be inconsistent with an apparent lack of glass, or other supercooling related features such as spinifex texture (Arndt, 1986; Arndt & Fowler, 2004; Shore & Fowler, 1999). Furthermore, the extremely low bulk $Al_2O_3$ and CaO in Margin unit targets reported by Williford et al. (2025) are significantly lower than terrestrial komatiites, and do not seem to represent any known bulk silicate melt composition on Earth or Mars. Williford et al. (2025) note that these abundances are more akin to what would be expected for mineral accumulation by sedimentary or igneous processes, for instance by crystal settling in a large igneous body, or hydraulic sorting of grains by sedimentary processes.

The observed structureless to planar parallel layered facies of the WMU may be generated by gravitationally driven crystal settling (e.g., Schmidt et al., 2012) in an igneous intrusion (or lava lake, if exposed at the surface). Such a cumulate origin has been proposed for the Séitah formation (Beyssac et al., 2023; Farley et al., 2022; Liu et al., 2022), which appears continuous with the Margin unit as observed from orbit and, hence, may represent the same deposit with a different alteration history. Such an interpretation would also be consistent with the candidate crystalline textures observed in the Eremita Mesa abrasion patch (Figure 5d). Additionally, Williford et al. (2025) report observations of rocks with similar compositions and outcrop characteristics to the Margin unit at higher elevations atop the crater rim. These rocks display similar degrees of crystallinity to Eremita Mesa, with one example named “Goat Island Rock”, displaying a candidate intercumulus texture (Williford et al., 2025). Highly laterally continuous, decimeter-scale planar layers such as those reported in the $F_L$ facies of the WMU are observed in many terrestrial large igneous intrusions (e.g., Jenkins & Mungall, 2018; McCallum, 1996; Miller & Ripley, 1996; Namur et al., 2015). Igneous cumulate layering is typically deposited at low angles within an intrusion, but steeper angles (>30°) have been observed, attributed to low melt/crystal density contrasts, or rapid melt congelation acting to stabilize the slope (Upton et al., 1996). Inclined cumulate layering is typically formed sub-parallel to the margins of igneous bodies (Cawthorn, 1996; Donaldson et al., 2003; Irvine, 1974; Pêcher et al., 2013). Given the context of the unit draping the rim, a deeply buried igneous intrusion is unlikely. Therefore, in a cumulate model for the WMU, the basinward-inclined, planar layering draping the crater rim may represent olivine accumulation at the base of a lava lake, or shallow sill-like intrusion, juxtaposed against the crater rim.

Terrestrial olivine cumulates typically possess some internal compositional variability in layering, however, including the presence of chromite bands, and pyroxene or plagioclase rich horizons in the upper reaches of the cumulate (Namur et al., 2015). There is no clear correlation between composition and layering in the WMU stratigraphy. Furthermore, terrestrial olivine cumulates are typically overlain by complementary gabbros or pyroxene-rich cumulates owing to fractional crystallisation (McCallum, 1996; Miller & Ripley, 1996; Namur et al., 2015); no such complementary components have been observed for the Margin unit. Notably, the absence of a less-mafic component was also raised for the olivine cumulate of the Séitah formation (Farley et al., 2022). One possible explanation is that these have been removed by erosion following exposure of the unit, potentially during the period of open basin lacustrine conditions occurring within Jezero, which led to the development of the western fan. Additionally, recent developments in crystal fractionation models have shown that repeated magma
injection could lead to repeated phases of olivine settling, while the complementary components can migrate away from the system prior to crystallisation, thus requiring a much less extensive complementary felsic component (Jenkins & Mungall, 2018). Furthermore, early investigations of the Jezero crater rim have revealed exposures of pyroxenite and gabbro, although their stratigraphic context to the Margin unit is unclear (Udry et al., 2025; Vaughan, Horgan, et al., 2025).

### 6.2.4. Summary of WMU Emplacement Mechanisms

While it is not possible to conclude with certainty the origin of the WMU from our current rover outcrop observations alone, an igneous origin as an olivine cumulate (by crystal settling in a sill-like igneous intrusion or lava lake) is the most parsimonious interpretation, and scores most highly in the hypothesis testing matrix (Table 3). This is consistent with the finding of Williford et al. (2025), that the composition of the WMU is consistent with terrestrial olivine cumulates, as well as the Chassigny martian meteorite, which is also interpreted as an olivine cumulate (Floran et al., 1978; Udry et al., 2020). The repeated stratigraphic sequence of structureless into layered successions is attributed to changes in crystal growth rate, convective regime, or injection of fresh melt into a cooling body of molten rock (McBirney & Nicolas, 1997). In this model, the transition from more convincingly crystalline exposures of olivine- and carbonate-bearing rocks at Eremita Mesa and at higher elevations on the crater rim (e.g., Williford et al., 2025), to more pervasively altered rocks with a high-silica matrix at lower elevations of the WMU (e.g., at Castle Geyser and Old Faithful Geyser), would reflect more extensive olivine carbonation reactions at lower elevations in the unit. A pyroclastic origin, as the result of olivine-rich pyroclastic density currents or tephra fall may also be a feasible means of emplacing the WMU stratigraphy, and while this would not necessarily require a complementary, less mafic component or compositional layering as in the cumulate hypothesis, it is challenged by its distinct composition indicative of mineral accumulation by igneous or sedimentary processes (Williford et al., 2025). If the material had explosively erupted directly from the lower reaches of an already-differentiated magmatic body, this may explain the anomalous composition. The absence of glass, vesicularity, welding textures, or variations in facies character typical of terrestrial pyroclastic deposits, however, renders a pyroclastic origin unlikely. The hypothesis testing matrix implies a sedimentary origin also appears to be inconsistent with the observations reported here (Table 3), primarily due to the lack of any discernible sedimentary structures or variability in facies character across the traverse, including the absence of conglomeratic horizons immediately adjacent to the crater rim. A sedimentary origin, most likely as a shoreline deposit dating to an older lake highstand cannot be entirely excluded, however, due to the possibility that extensive alteration has obscured sedimentary structures and hindered the confident identification of facies variability.

## 7. Discussion

### 7.1. What Is the Relationship Between the Eastern and Western Margin Unit?

The observations and interpretations of the EMU and WMU described above imply disparate origins for these sub-units of the Margin unit as sedimentary and igneous deposits, respectively. As such, this implies the presence of an unconformity between the EMU and WMU. Given their compositional similarity (Williford et al., 2025), this raises the possibility that the EMU is a sedimentary deposit derived directly from a lithified WMU.

The moderate to high degree of angularity of many grains in the Bill's Bay abrasion and more so in the Amherst Point abrasion suggests a relatively local sediment provenance, potentially supporting local sourcing of grains from the WMU. This could be through a combination of rivers draining from the crater rim across the WMU, or through highly localized, in situ reworking of WMU material along a paleoshoreline. This would be consistent with the similarity in grain sizes of the EMU and WMU; limited longshore sediment transport in terrestrial lacustrine beach settings means that shoreline sediments typically closely resemble local source materials (Nordstrom & Jackson, 2012). Alternatively, olivine- and carbonate-rich sediments may have been transported into the crater from the regional olivine-bearing unit exposed in the Jezero crater watershed. However, Neretva Vallis, which is the major supplier of sediment from outside the crater to the western fan, incises the Margin unit, suggesting that it at least mostly post-dates Margin unit emplacement. If sediment were supplied from outside the crater, this would therefore be via a more juvenile Neretva Vallis.

Williford et al. (2025) identified small (<300 µm) chromite grains as a minor component of the Margin unit rocks, reporting that PIXL elemental maps of abrasion patches in the EMU and WMU show distinct populations of
chromite Ti/Cr ratios. This implies that the chromite grains in the EMU cannot be sourced from the WMU at Bunsen Peak or further west, suggesting that fluvial routing of sediment from higher elevation portions of the Margin unit is unlikely. However, the data points between the two closest PIXL targets in the western (at Bunsen Peak) and eastern margin (at Turquoise Bay), are separated laterally by >1 km and vertically by >25 m (Figure 23). As such, the chromite compositions of the WMU near the contact with the EMU are unconstrained. Consequently, the EMU may represent a shoreline sedimentary complex where sediment is sourced locally from the adjacent WMU bedrock. Local sourcing of sediment is common in terrestrial lacustrine shoreline complexes (Gracia et al., 2021; Nordstrom & Jackson, 2012), and is further supported by the moderate to high angularity grains in the EMU abrasion patches, indicating limited sediment transport distances.

While the abundance of olivine grains suggests a largely detrital origin for the EMU, the origin of carbonate in the EMU remains uncertain. Carbonate phases in the EMU may be (a) detrital (likely from the same source terrain as the olivine grains), (b) formed as authigenic precipitates in the Jezero paleolake and reworked along the shoreline, or (c) formed by post-depositional carbonation of detrital olivine. The range of Fe- and Mg-compositions of carbonate is broader than those of primary olivine compositions in the EMU (Clavé et al., 2024; Williford et al., 2025), suggesting in situ olivine carbonation may be unable to explain the presence of carbonate in the EMU alone. This would support some component of detrital and/or authigenic carbonate input in the EMU. Intriguingly, the interpreted shallow, reworked strata around Turquoise Bay correspond to the strongest CRISM carbonate signatures across the Margin unit explored by *Perseverance*, supported by in situ SHERLOC carbonate detections at Bill's Bay (Williford et al., 2025). These observations would also be consistent with the prediction of Horgan et al. (2020), that any authigenic carbonate input at Jezero would be concentrated in the littoral zone, where agitated waters and high levels of dissolved ions supplied from the watershed may facilitate carbonate precipitation. Carbonate precipitation and resulting early cementation of sediments is also a primary means of preserving relict shoreline features such as those interpreted in the EMU (Lebrec et al., 2022).

## 7.2. Implications of a Relict Shoreline at Jezero Crater

The shore zone model favored here for the EMU requires an ice-free Jezero paleolake at the time of deposition, and ancient atmospheric conditions suitable for the generation of waves on the surface of the lake. The development of a shoreline is dependent on atmospheric pressure and wind speeds being above a certain threshold to exert sufficient shear stress on the water surface to generate waves (Banfield et al., 2015). Observations of aeolian deposits at Gale crater have shown that regardless of atmospheric density, the atmospheric conditions required to form the observed bedforms would be capable of creating surface water waves on an ice-free lake (Rubin et al., 2022). The recent discovery of symmetrical ripple marks in the Amapari member of the Mirador formation at Gale provides compelling evidence for surface water waves on a shallow, ice-free lake (Mondro et al., 2025). The east-west paleoflow orientations implied by the basinward- and rimward-inclined stratification and trough cross strata at Turquoise Bay (e.g., Figure 16) imply broadly onshore or offshore paleo-wind directions acting on the lake surface. This is consistent with modern day east-west wind directions and orientations of ancient winds implied by ventifact orientations, which are attributed to strong slope flows developing next to the western crater rim (Herkenhoff et al., 2023; Viúdez-Moreiras et al., 2022).

The candidate relict shoreline features reported here for the EMU form in water depths sufficiently shallow for wave action to rework and redistribute sediment. Coast-parallel nearshore bars typically form at water depths approximately equal to their overall thickness (Jiang et al., 2014), and given that lower shear stresses are required to induce grain motion under Mars gravity, this would place the structures reported here in no more than a few meters of water depth. This interpretation would imply a Jezero paleoshoreline around -2,400 to -2,405 m. Generation of a lacustrine shore zone complex depends on numerous variables including wind fetch and direction, lake margin bathymetry and lake level stability; however, the latter is often considered the most important factor given the time required for the development of a shore zone facies (Andrews & Hartley, 2015; Liao et al., 2024). A sustained lake level of -2,400 m could either be achieved by balancing water inflow with evaporation at Jezero, or by the formation of an outlet channel, regulating lake levels. Given that the Jezero outlet breach occurs within 5 m of the estimated lake level required for the relict shoreline features interpreted here (Fassett and Head III, 2005), the formation of the EMU following the onset of open basin conditions may be more likely.

The unconformable contact of the Margin unit with the Jezero Western fan (Figure 13) also extends the window of surface aqueous conditions and habitability at Jezero, and enhances our understanding of changing water
availability in the crater lake prior to deposition of the fan. Given that the Shenandoah formation, at the base of the Western fan, is interpreted to record the onset of alluvial and subsequently deltaic deposition at Jezero (Stack et al., 2024), the presence of a shoreline deposit stratigraphically and unconformably below this would support an additional, sustained period of lacustrine conditions at Jezero, pre-dating the deposition of the Western fan. In this scenario, the unconformity between the EMU and Western fan would represent a period of non-deposition occurring between a period of shoreline sedimentation at ~2,400 m, and alluvial deposition occurring at elevations ~125 m lower in the crater, thus representing a major lake level regression pre-dating the fan. An alternative model is also possible, however, given that the lower portion of the Shenandoah formation may preserve a distal deltaic, rather than alluvial, environment (Tebolt et al., 2025). In this scenario, it is possible that the EMU represents a shoreline facies developed following breach of the eastern crater rim at ~-2,400 m, shortly prior to the main episode of fan deposition. Subsequent lake level fall may have exposed these deposits, where they were incised by prograding river delta deposits of the western fan. Ongoing lake level fall below the stable shoreline and outlet channel elevation at ~-2,400 m led to sedimentation in closed basin conditions. This regressive phase of lake activity is recorded by the progressive downstepping of delta foresets preserved in the Tenby formation of the Jezero western fan, and at lower elevations at Kodiak butte (Caravaca et al., 2024; Mangold et al., 2024).

### 7.3. Integrated Geologic History of the Margin Unit

Integrating lithofacies analyses with 3D structural data allows us to reconstruct a potential emplacement history for the Margin unit, starting with an initial igneous emplacement as an olivine cumulate (potentially contem poraneous with the emplacement of the regional olivine-bearing unit and/or the Séítah formation), followed by localized sedimentary reworking of these materials into a thin veneer of shore zone deposits along the Jezero paleoshoreline (Figure 24).

In the model proposed here, following the Isidis impact at ~3.96 Ga (Werner, 2008) and the subsequent Jezero-forming impact, igneous activity commenced in and around Jezero crater. This may have occurred on both the local scale, perhaps due to Jezero impact-triggered magmatism (Michaut & Pinel, 2018), or on a regional scale, due to widespread magmatic activity following the Isidis impact, but pre-dating the emplacement of Syrtis Major lava flows (Bramble et al., 2017; Mandon et al., 2020). This igneous activity most likely emplaced the Margin unit as a molten body of rock, either as a sill-like intrusion or thick lava flow/lava lake draping the crater rim. Olivine accumulation by crystal settling within this molten body generated an ultramafic composition akin to terrestrial and martian olivine cumulates, while the complementary, less mafic materials may have been subsequently eroded away or been transported elsewhere in the magmatic plumbing system (Williford et al., 2025). The Séítah formation and the regional olivine-bearing unit outside Jezero may also have been emplaced during this period of igneous activity following the Jezero impact, perhaps contemporaneously with the Margin unit. Post-emplacement cooling may have led to the generation of abundant polygonal fracturing throughout the unit. Following (or during) its emplacement, the Margin unit underwent intense alteration by interacting with CO₂-rich fluids, variably altering primary olivine into carbonate + serpentine + silica (e.g., Beinlich et al., 2020). These fluids may have been primary magmatic fluids rich in CO₂ sourced from the mantle (Brown et al., 2010) or derived from meteoric or groundwater systems (Scheller et al., 2021), potentially associated with the Jezero paleolake.

Then, during the onset of open water conditions which persisted at Jezero between ~3.8 and 3.6 Ga (Fassett and Head III, 2005; Goudge et al., 2015, 2017), we suggest that a sustained shoreline developed around ~2,400 m elevation, following the breach of the eastern crater rim. Here, persistent wave action locally mobilized and reworked sediments derived from the Margin unit. Repeated swash processes from waves incident on the shoreline may have formed basinward-inclined sands derived from adjacent WMU materials. Swash-backwash processes or increased lake levels may have modified the shoreface, potentially into small-scale beach bars, sand spits, or shoal-like structures (e.g., Figure 16). The shallowest portions of the shoreface experienced the greatest wave energies leading to increased grain rounding and the formation of erosion surfaces and trough cross strata, as observed at Turquoise Bay. Following this, the Western fan was deposited atop the Margin unit. The abrupt nature of the contact from orbit and rover observations, as well as the distinct change in sedimentary facies and absence of any facies interfingering between the Western fan and EMU at Gnaraloo Bay (e.g., Figure 13) supports an unconformable relationship between these units. This unconformity may represent a period of major lake regression prior to alluvial deposition of the Shenandoah formation (e.g., Stack et al., 2024). Alternatively, if the lower Shenandoah formation preserves a pro-delta setting (e.g., Tebolt et al., 2025), the unconformity may represent a transition from
<figure><img src="image_27.png" /></figure>

**Figure 24.** 3D schematic block diagrams illustrating a potential geologic history for the emplacement of the Margin unit at Jezero crater. (a) Jezero forming impact, followed by (b) emplacement of the Western Margin Unit (WMU) as an olivine-rich igneous cumulate formed in a lava lake or sill-like intrusion, potentially contemporaneously with the Séítah formation and/or Regional Olivine-bearing Unit (ROU) outside of Jezero and other olivine- and carbonate-bearing exposures observed atop the crater rim (Williford et al., 2025). Reaction of primary olivine with circulating CO₂ rich fluids during or following its emplacement leads to the generation of Fe/Mg carbonate masses. (c) The onset of lacustrine conditions at Jezero leads to the localized mobilization and reworking of the WMU by waves at the lake shoreline to form the deposits of the Eastern Margin Unit. This is followed by (d) ongoing incision of the crater rim and Margin unit by Neretva Vallis and deposition of the western fan unconformably atop the Margin unit.

an open-basin, crater-filling lake with a wave-dominated shoreline to a closed-basin lake where the progradation of river deltas was the dominant process, without the need for a major regressive episode.

**8. Implications for Aqueous Activity and Past Habitability at Jezero Crater**

Our favored model for the emplacement of the Margin unit (Figure 24) records a temporal shift from igneous to sedimentary emplacement of the unit. The transition from extensive carbonation reactions inferred to have occurred in the WMU (Williford et al., 2025) to the onset of open water conditions and the generation of a
shoreline also tracks the progression from subsurface to surface aqueous environments and habitability at Jezero. Carbonation of ultramafic rocks has been shown to provide habitable subsurface niches for microbial communities on Earth (e.g., Rempfert et al., 2017) and the resulting precipitation of carbonate provides a means to preserve molecular biosignatures (Newman et al., 2020; Robles-Fernández et al., 2022). Furthermore, the repeated agitation of sediment by waves can lead to the formation of carbonate-coated grains in lake margin settings (Platt & Wright, 1991). Carbonate precipitation has been shown to bound lipids within the cortices of coated grains, and as such, these environments can be excellent preservers of molecular biosignatures (O'Reilly et al., 2017). As such, both the WMU and EMU remain of significant astrobiological interest for returned sample science investigations.

Our model supports the hypothesis proposed by Horgan et al. (2020) that near-shore lacustrine processes may be recorded in the Margin unit, and although we do not see evidence for shoreline sedimentation beyond the lowest elevations in the east of the unit, this does not preclude that lake levels were also once much higher at Jezero (Salese et al., 2020). The increased crystallinity and absence of a high-silica matrix at Eremita Mesa and in olivine-rich rocks exposed higher up the Jezero rim, implies significantly lower extents of olivine carbonation than in lower elevations of the WMU (e.g., at Old Faithful Geyser and Castle Geyser). One possibility is that this increased carbonation was facilitated by more sustained interaction with fluids with high dissolved CO₂ concentrations (Williford et al., 2025), perhaps facilitated by an earlier Jezero lake level, situated up to ~100 m higher in elevation than the level implied by the Jezero outlet channel (Fassett & Head, 2005) and the proposed shoreline deposits of the EMU. These lake levels may have been too short-lived for shoreline features to develop, or any sedimentary remnants may have subsequently been eroded.

When returned to Earth, laboratory investigations of the Pelican Point, Lefroy Bay and Comet Geyser samples will also help test the model proposed here for the geologic evolution of the Margin unit. Absolute and relative ages of primary igneous olivine crystals, carbonation reaction products and detrital grains and cements, if present, will be critical in building a complete picture of the emplacement and diagenetic history of the unit, and the relative timing of surface and subsurface aqueous processes. If igneous in origin, a firm geochronology may also be able to resolve whether the carbonation of the WMU occurred during, or following, its emplacement. A contemporaneous igneous-carbonation model would have significant implications for the volatile content of Mars' early mantle, while a post-emplacement model may require an early Mars greenhouse atmosphere (Brown et al., 2020). Stable isotopic investigations of ¹³C/¹²C and ¹⁸O/¹⁶O of carbonate phases contained within the samples could also be used to reconstruct paleoenvironmental conditions including temperature, pH and pCO₂ at the time of their formation (Kim & O'Neil, 1997; Royer et al., 2001; Sosdian et al., 2018), which are essential in understanding Mars' ancient climate and propensity to have supported life.

## 9. Conclusions

Between sols 909 and 1266, the Mars 2020 Perseverance rover conducted its fourth science campaign, exploring the olivine- and carbonate-bearing rocks of the Margin unit, Jezero crater, which unconformably underlie the deposits of the Jezero western fan. This study documents the lithofacies of the Margin unit by analyzing images collected by the rover and provides stratigraphic context for the three samples (Pelican Point, Lefroy Bay and Comet Geyser) collected from the unit during the campaign. We generate a structural-lithologic map and cross-section through the unit (Figure 23) and propose a potential geologic history (Figure 24) for its emplacement. From this work, we conclude:

1. The Margin unit shows distinct lithofacies, structures and outcrop expressions in the east (Sols 910–1007) and west (Sols 1007–1266), which likely reflects a temporal change in the emplacement mechanism of the unit from predominantly igneous to sedimentary processes.

2. Five lithofacies are recognised in the Margin unit. In the east these include low-angle, well-stratified basin-ward- ($S_B$) and rimward- ($S_R$) inclined sandstones, and sub-horizontal, planar-to convex-up-stratified sandstones ($S_H$), which preserve sedimentary structures including cross-stratification and erosion surfaces. In the west, grain-scale relations are overprinted by significant alteration, precluding confident reconstruction of the protolith texture. Lithofacies are assigned based on outcrop characteristics, and include a structureless facies association ($F_S$) and a decimeter-scale planar-layered facies association ($F_L$), which both lack sedimentary structure.
3. The most parsimonious emplacement mechanism for the WMU is an aqueously altered olivine cumulate formed by crystal settling in an igneous body which draped (or intruded against) the rim, forming repeated sequences of structureless ($F_S$) and layered ($F_L$) rocks inclined into the basin.

4. Grain-scale textures and outcrop-scale structures observed in the EMU are most consistent with a lake shore zone depositional environment, where wave action at the margin of the Jezero paleolake has reworked grains from the WMU to generate localized, relict shoreline deposits. These structures are consistent with a paleo-shoreline at Jezero at ~2,400 m elevation.

## Conflict of Interest

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

## Data Availability Statement

The data used in this publication are from the Mastcam-Z (Bell et al., 2021), Navcam (Maki et al., 2020), SuperCam RMI (Maurice et al., 2021) and SHERLOC WATSON (Bhartia et al., 2021) instruments on the Mars2020 *Perseverance* rover, the RTE camera on board the *Ingenuity* helicopter (Balaram et al., 2021), and the HiRISE instrument on the Mars Reconnaissance Orbiter (McEwen et al., 2007). All *Perseverance* data products used here are archived in the Planetary Data System Imaging node (https://pds-imaging.jpl.nasa.gov/volumes/ma rs2020.html) and the Geosciences node (https://pds-geosciences.wustl.edu/missions/mars2020/). Mastcam-Z data used in this study are available via this database: Bell and Maki (2021 [Data] https://doi.org/10.17189/ 1522843), or via the Mastcam-Z public website mosaic page (https://mastcamz.asu.edu/mars-images/panoramamas mosaics/). Navcam image data is available via Maki (2020 [Data] https://doi.org/10.17189/d3nm-pp09), SuperCam RMI image data is available via Maurice and Wiens (2021 [Data] https://doi.org/10.17189/1522646). SHERLOC WATSON image data is available via Beegle and Bhartia (2021 [Data] https://doi.org/10.17189/ 1522643), and *Ingenuity* data is available via Balaram (2021 [Data] https://doi.org/10.17189/1522845). 3D measurements were made from Mastcam-Z stereoscopic data in the PRo3D software, which is available at: https://pro3d.space/. All the 3D OPC data files used in this study, as well as unannotated versions of all image data used in this study (and relevant links to the Planetary Data System) are available for download from this Zenodo data repository: (Jones et al., 2025 [Data]), https://doi.org/10.5281/zenodo.17256475.

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## Acknowledgments

The authors wish to thank all members of the Mars 2020 science and engineering teams for their hard work in tactical and strategic science operations and for making this paper possible. We also wish to pay particular thanks to the Mars 2020 project management and campaign science leads for their efforts in making the mission a success, as well as the Mastcam-Z, SuperCam and SHERLOC operations teams for their hard work and investment in the mission and in acquiring the data products used here. We would also like to thank Dr Michelle Tebolt and one anonymous reviewer for their thorough feedback and suggestions during the review process. A. J. Jones was funded by the Imperial College London President's PhD Scholarship. UK Space Agency Grants ST/Y000153/1 and ST/X002373/1 funded S. Gupta and R. Barnes. G. Paar was supported by FFG ASAP Mars 4-D Grant 911911. B. Horgan, B. Garczynski, A. Klidas and J. Bell III were funded by NASA's Mars 2020 Project via a subcontract from the California Institute of Technology/Jet Propulsion Laboratory to Arizona State University (Subcontract 1511125). Jet Propulsion Laboratory subcontract 16769020 funded K. H. Williford. K. Stack, J. Maki, N. R. Williams, F. J. Calef, L. R. W. Ives and S. Gwizd carried out their research contributions at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (80NM0018D0004). J. I. Simon was funded by the Return Sample Science Participating Scientist Program supported by the NASA Mars Exploration Program. E. Ravanis was funded by NASA's Mars 2020 project via ASU subcontract 15–711. L. C. Kah was funded by the SHERLOC-WATSON instrument team. O. Beyssac and G. Caravaca were supported by the French Space Agency CNES.
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