Depositional History of the Blocky Unit at Jezero Crater, Mars

The “blocky” unit is the youngest orbitally defined and mapped unit of Jezero crater’s western fan. This unit, characterized by its high density of boulder-sized clasts, can be subdivided into individual fan-shape to elongate deposits, and placed into a stratigraphic framework based on cross-cutting relationships. Mapped boulder deposits have sharp, topographically variable contacts that suggest an unconformable relationship with the underlying stratigraphy. The Mars 2020 Perseverance rover encountered exposures of these blocky deposits beginning on Sol 755. Here, we assess the stratigraphic context, lithology, and clast characteristics of mapped deposits observed from the ground along Perseverance’s traverse to determine the depositional history of this unit and assess implications for the hydrologic evolution of Jezero crater.

Rover-derived images reveal that boulders mantle the surface of mapped deposits, which are predominantly composed of a finer-grained lithology. Boulders are primarily sub-rounded to rounded to ventifacted, and some boulders appear to be sourced from erosion of underlying stratigraphy. Different morphologies broadly correlate with compositional signatures of olivine-bearing (rounded) and pyroxene-bearing (variable) boulders. This study considers multiple possible origins for the blocky unit boulders: (1) a series of small alluvial fans, (2) fluvial splay deposits, and/or (3) an erosional lag of overlying coherent layers. Contrasting with the published interpretation of Goudge et al., 2018, this study suggests that the blocky deposits represent an interval of deposition at Jezero crater that is possibly distinct in time and depositional process from the underlying fan sequence.