Distinguishing the geochemical effects of sedimentary processes and source region characteristics in Gale Crater, Mars

Abstract

The NASA Curiosity rover has investigated a diverse stratigraphic record at Gale crater, Mars, ranging from poorly-sorted conglomerates to cross-bedded sandstones and thinly-laminated mudstones. This stratigraphic record is indicative of deposition in multiple paleoenvironments (fluvial, lacustrine and aeolian), within which variable degrees of mineral sorting, chemical weathering and post-depositional aqueous alteration occurred. This study delineates the geochemical effects of these different sedimentary processes using major element data from the Chemistry and Camera (ChemCam) instrument suite and mineralogical data provided by the Chemistry and Mineralogy (CheMin) instrument suite to identify source region characteristics, mineral sorting regimes, variations in chemical weathering, and the impact of diagenesis on the geochemistry of the host rock. Delineating the different geochemical effects of these sedimentary processes on the lithified units will better constrain our understanding of the geological record and paleoenvironmental conditions in Gale crater, along with the extent of geochemical variation in the surrounding Martian crust. Here, I show that four previously identified compositional endmembers (subalkaline basalt, trachybasalt, trachyte and rhyolite), and one new endmember (silica-rich basalt) contributed to Gale crater’s stratigraphic groups. The previously identified endmembers’ influence extends throughout Gale crater’s stratigraphic record beyond where they were initially identified. The silica-rich basalt source region is the main contributor to the lower Mt Sharp Group, including the Vera Rubin Ridge, whose hardness likely results from a later diagenetic event precipitating additional hematite in sedimentary pore spaces. This thesis further supports that Martian fluviolacustrine deposits are more enriched in felsic material relative to mafic material with distance from the sediment source. Meanwhile, aeolian deposits become preferentially enriched in mafic materials. Geochemically identifying the ancient aeolian mineral sorting trend has also constrained the prevailing wind direction at the time of deposition as SW-NE, which is the reverse of the current prevailing wind direction within Gale crater today.