Reassessing the Sedimentary Depositional Origin of the Jezero Crater Western Fan’s Curvilinear Unit

The Mars 2020 Perseverance rover spent 118 mission sols observing an orbitally-defined sedimentary unit in the Jezero crater western fan informally known as the “curvilinear unit”. Works prior to the mission formed a consensus that it was deposited as laterally accreting bars in a meandering fluvial system, but this hypothesis is inconsistent with the observations made by Perseverance.

The curvilinear unit has an orbital expression of decimeter-scale sets of alternating light- and dark-toned strata that occur in amalgamated and stacked sets that are 4–500 m across and 4–30 m tall. Rover observations from the Mastcam-Z, SuperCam, WATSON cameras, and the RIMFAX ground-penetrating radar have shown that the banded appearance of the curvilinear unit in orbital images is due to alternating lithologies of steeply-dipping (25°–40°) sedimentary rock. The light-toned bands are an erosion-resistant facies comprised of structureless, thin beds of a medium-grained sandstone that is relatively well cemented. The dark-toned bands are composed of coarse-grained sandstone to conglomerate that is relatively poorly cemented, weathers recessively and is generally regolith-covered. The contacts between these facies are consistently sharp, planar, and parallel to the stratal dip.

These observations of steeply-dipping, structureless, sandstone beds suggest that the curvilinear unit was most likely deposited through downstream accretion via grain flow in a subaqueous setting. Grain size variations are likely due to changes in flow transport capacity. This depositional mechanism likely precludes lateral-accretion bars as a depositional setting but is potentially consistent with downstream accreting fluvial bars, delta mouth bars, and/or delta foresets.