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

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 similar to 250 sols exploring and collecting seven rock cores from the lower similar to 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. Fan-shaped deposits are important exploration targets on Mars because they record the activity of water and conditions suitable for life on the surface. The Mars 2020 Perseverance rover has been exploring an ancient impact crater, named Jezero, since landing on Mars in February 2021. One of the rover's main exploration targets is a fan-shaped deposit of sedimentary rock extending into the crater from the western rim. One Earth year into the rover's mission, Perseverance arrived at the eroded edge of this fan, exploring several outcrops of sedimentary rock, and collecting seven rock samples. This study uses images and data from Perseverance to describe these rocks, called the "Shenandoah" formation, and to determine how they formed. The oldest rocks of the Shenandoah formation contain sand and pebbles that were likely laid down at the edge of an alluvial fan. Next, very fine-grained rocks were deposited in a lake setting. Dipping layers of alternating sand and pebbles represent a change in the local setting and the growth of a delta into a relatively long-lived lake. The rover's observations of the Shenandoah formation show that past conditions in Jezero crater were capable of hosting and preserving signs of ancient life. The Shenandoah formation is a sand-dominated, clastic, sedimentary sequence comprising the lower 25 m of Jezero's western fan This sequence records the transition from an alluvial fan setting to a lacustrine setting, followed by progradation of a river delta This sequence records evidence for a warm, wet, habitable depositional setting with the potential to preserve biosignatures