Figure 17.

(A) Schematic regional stratigraphy of the Noachian Basement based on key locations in Nili Fossae andNE Syrtis. The lowermost units/features include Blue Fractured Unit (BFU),Stratified Basement Unit (SBU), megabreccia, Fe/Mg-Smectite Mounds Unit (SmMU) and Mixed Lithology Plains Unit (MLPU). The LCP-bearing Plateaus Unit (LPU) appear to be stratigraphically above these units. Ridges are observed to cross-cut all of the lowermost units. Kaolinite-bearing bright materials (KBM) has been observed to form on SmMU and MLPU. Question marks denote unresolved questions on the nature of individual contacts through lack of unambiguous contact exposures. These include the contact between MLPU and SBU, whether MLPU occurs underneath SBU in the Nili Fossae graben, megabreccia and SBU, SmMU and MLPU, and the diffuse transition between LCP-dominated and Fe/Mg-smectite-dominated parts of the MLPU. (B) Schematic of the preferred interpretation of the geological history of the Noachian Basement Group. Pre-Isidis lithologies include SBU, BFU, and other lithologies recorded within megabreccia blocks (e.g., beige and purple blocks). We neglect SmMU among these, as we do not understand its exact stratigraphic position. During Isidis basin formation mass-wasting and faulting leads to graben and megabreccia formation. Near-contemporaneous, excavation and ejection forms the MLPU that includes entrained deposits of pre-Isidis lithologies and megabreccia. Fluid flow within this system causes mineralization of fractures within these deposits. Erosion creates topography, and depressions are then infilled by the unknown LPU-forming process. Last, surface fluids are the cause for KBM-formation, and erosion leads to the modern plateaus/mounds/ridges expression of the LPU/SmMU/ridges.