Sedimentology and Stratigraphy of the Shenandoah Formation, Western Fan, Jezero Crater, Mars

Abstract

Sedimentary fans are key targets of exploration on Mars because they record the history of surface aqueous activity and habitability. The sedimentary fan extending from the Neretva Vallis breach of Jezero crater's western rim is one of the Mars 2020 Perseverance rover's main exploration targets. Perseverance spent ∼250 sols exploring and collecting seven rock cores from the lower ∼25 m of sedimentary rock exposed within the fan's eastern scarp, a sequence informally named the “Shenandoah” formation. This study describes the sedimentology and stratigraphy of the Shenandoah formation at two areas, “Cape Nukshak” and “Hawksbill Gap,” including a characterization, interpretation, and depositional framework for the facies that comprise it. The five main facies of the Shenandoah formation include: laminated mudstone, laminated sandstone, low‐angle cross stratified sandstone, thin‐bedded granule sandstone, and thick‐bedded granule‐pebble sandstone and conglomerate. These facies are organized into three facies associations (FA): FA1, comprised of laminated and soft sediment‐deformed sandstone interbedded with broad, unconfined coarser‐grained granule and pebbly sandstone intervals; FA2, comprised predominantly of laterally extensive, soft‐sediment deformed laminated, sulfate‐bearing mudstone with lenses of low‐angle cross‐stratified and scoured sandstone; and FA3, comprised of dipping planar, thin‐bedded sand‐gravel couplets. The depositional model favored for the Shenandoah formation involves the transition from a sand‐dominated distal alluvial fan setting (FA1) to a stable, widespread saline lake (FA2), followed by the progradation of a river delta system (FA3) into the lake basin. This sequence records the initiation of a relatively long‐lived, habitable lacustrine and deltaic environment within Jezero crater.

Assessing Radar Attenuation in RIMFAX Soundings at the Jezero Western Fan Front, Mars

“…Surface rocks analyzed by the Perseverance rover at JWF, indicate distinct formation compared to on the Crater Floor. Whereas the latter was mainly concluded to have magmatic origin (Farley et al, 2022), JWF is found to represent a sedimentary fan deposit (Stack et al, 2024). The difference in radar propagation velocities between the Crater Floor and JWF, is consistent with change from magmatic lithologies to sedimentary.…”

Characterizing Hydrated Sulfates and Altered Phases in Jezero Crater Fan and Floor Geologic Units With SHERLOC on Mars 2020

“…These results indicate that the mineralogy of the fan front and the upper fan are distinct: the fan front is dominated by sulfates while the upper fan has almost two times more carbonate detections than sulfate detections. PIXL analyses of elemental composition report ∼20 wt.% SO 3 in the sulfatedominated targets Berry Hollow and Uganik Island in the fan front, which is ∼25× greater than in the sulfatecontaining crater floor target Dourbes (Liu et al, 2022;Stack et al, 2024). SuperCam observations also report the upper fan to be dominated by carbonates with no sulfate detections (Royer, Wiens, et al, 2023).…”

“…We conduct a statistical analysis of the deep-UV Raman spectra of all stratigraphically in-place rock targets measured by SHERLOC at the Séítah and Máaz formations of the crater floor, the fan front (informally named the Shenandoah formation, Stack et al, 2024), and the upper fan deposits (informally named the Tenby formation, e.g., Siebach et al, 2023) of the sedimentary western fan. We first analyze Raman spectral features in SHERLOC scans at 100 μm/point spatial scale over 800-1,200 cm 1 to identify the alteration minerals present.…”

Variable Iron Mineralogy and Redox Conditions Recorded in Ancient Rocks Measured by In Situ Visible/Near‐Infrared Spectroscopy at Jezero Crater, Mars

“…The base of the Hawksbill Gap stratigraphy corresponds to the Devils Tanyard member and overlying Hughes River Gap strata (Figure 1), which consist of planar laminated sandstones of variable exposure. Here, the outcrops share morphological and spectral features with the Knife Creek member (located right above Amalik) and hence have been interpreted as its lateral equivalent in the Cape Nukshak section (Dehouck et al, 2023;Stack et al, 2024). Abrasion of a Devils Tanyard rock (target Rose River Falls) shows the strongest oxidation of Fe features: the rock is redder than the other abraded members, has narrower reflectance peaks in the visible, and deeper 0.53 and 0.86 μm absorptions, indicative of fine-grained crystalline hematite (Figures 3 and 5).…”

“…We compare their spectral signatures to those of igneous rocks present on the crater floor (below the sedimentary units) as well as similar data from prior Mars landing sites to understand the changes in weathering and redox state. This work is conducted in parallel with others that define the fan stratigraphy (Stack et al, 2024), present the overviews of SuperCam (Dehouck et al, 2023) and Mastcam-Z (Núñez et al, 2023) results from Jezero's western fan, or use other rover instruments to interrogate composition (e.g., Hurowitz et al, 2023;Phua et al, 2024).…”