The Boulder-Rich Blocky Unit of the Western Jezero Upper Fan: Discriminating Olivine and Pyroxene Compositions and Constraining Provenance

Abstract

The boulder-rich unit of the Upper Western Fan in Jezero crater, Mars is the youngest preserved unit of the fan stratigraphy. Understanding boulder compositions, distribution, and textures is critical for constraining their emplacement, provenance, and the depositional history of the Western fan. We use Mastcam-Z multispectral images to identify and discriminate between two compositions of boulders: olivine- and pyroxene-bearing. Spectral parameters distinguish endmembers from other geologic units investigated by the Perseverance rover within Jezero crater and indicate a distinct provenance. We map the distribution of each type across distinct lobes of the fan, characterize their morphology, and use WATSON images to interpret grain-scale textures. Differences in type-distribution among lobes of the fan supports the distinction of those lobes as previously mapped. The olivine-bearing boulders are dominant across the unit with rounded shapes consistent with fluvial deposition sourced from the regional olivine unit. Less populous and rougher pyroxene-bearing boulders may have been emplaced through a shorter fluvial transport distance from the pyroxene-bearing crater rim deposits, or as possible impact ejecta. This work supports earlier claims that high-energy fluvial activity played a role in the later stages of the Western Fan formation in Jezero crater. Ongoing and future exploration of the Jezero crater rim will allow further study and evaluation of potential source regions for the boulders of the Western Jezero fan top.