A HEMISPHERICALLY INTEGRATED SEDIMENTARY GEOLOGICAL SYSTEM AT NILI FOSSAE, MARS

On Earth, study of the sedimentary record of denudation, transport, and deposition has allowed reconstruction of the planet’s ancient environments, tectonic activity, and biological evolution. Meanwhile, on Mars, despite extensive study of several exposures of ancient sedimentary rocks by satellites and rovers, the crustal provenance, transport histories, and depositional context of most martian sediments, many over 3.5 Gyr in age, remain unclear. The Nili Fossae region of Mars fits this description with its orbital-identified outcrops that record multiple iterations of basin formation, impact gardening, explosive volcanism, fluvial activity and denudation. These processes led to the formation of the ancient crust exposed at NE Syrtis and led to deposition of similar materials to form the Jezero crater deltaic system, both sites considered for the Mars 2020 rover. We synthesize recent work to propose that the lithologically diverse stratigraphy of Nili Fossae integrates not only a billion years of regional geological processes, but also records several generations of sedimentary activity and thereby serves as a reference section for sediments of similar compositions and textures across the planet.

The most complete outcrop sections of the Nili Fossae stratigraphy occur in the NE Syrtis region that lies adjacent to Jezero crater, exposing many of the regionally observed deposits of diverse hydrous alteration minerals, megabreccia, layered sulfates, and Hesperian lavas. Sedimentological work on Jezero crater and recent work suggesting a pyroclastic origin for the regional olivine-rich unit shows well-understood sedimentary processes are recorded in Nili Fossae. The sedimentary processes of impact gardening facilitated three or more generations of crustal recycling. Basement megabreccia may therefore record formation of the Borealis and Isidis basins as well as Jezero’s excavatation of deep Noachian crustal materials and their subsequent deposition in the vicinity of NE Syrtis. Collectively, the Nili Fossae stratigraphy may act as a key reference section for lithologies distributed across the planet’s surface. In situ examination of the two above Mars 2020 sites, or of a site midway between them, would provide a well-chracterized tiepoint for this sedimentary reference section.