UNDERSTANDING THE SEDIMENTARY RECORD OF MARS THROUGH GLOBAL STRATIGRAPHY AND ORBITAL FACIES

Our understanding of Mars’ composition and stratigraphy has fundamentally changed over the past two decades due to a wealth of high resolution orbital and rover-based observations. Importantly, it is now recognized that various regions on Mars preserve a rich history of sedimentary and aqueous processes. As on Earth, sedimentary rocks on Mars are likely to record important information about the interaction between sediment, water, and the atmosphere over local, regional, or global scales. Indeed, current paradigms suggest the clay, sulfate, and Fe-oxide minerals that dominate the modern surface of Mars are the mineralogical signatures of global climate evolution. However, to further test this hypothesis and to provide a more detailed understanding of how environmental conditions are preserved in the Martian stratigraphic record, it is necessary to identify key stratigraphic sections and apply an objective classification scheme to those sections.

We have developed a suite of ‘orbital facies’ that are defined by specific morphologic and mineralogical characteristics as observed from orbital data. They include Massive Breccia (MBR), Complexly Stratified Clays (CSC), Laterally Continuous Sulfates (LCS), Laterally Continuous Heterolithic (LCH), Distributary Networks (DNW), and Rhythmite (RHY) facies. We have also identified a number of stratigraphic sections across the planet that may be useful reference sections for global scale correlation. The most ancient terrains are dominated by MBR and CSC facies, with impact cratering being an important process for some occurrences of these facies. In contrast, Hesperian terrains are characterized by LCS, DNW, and, to a lesser extent, the CSC facies. RHY occurrences are commonly associated with post-Noachian terrains and may record sediment accumulation linked to orbital forcing. These results indicate that transitions in mineralogy and any associated changes in climate may have been slower-paced or more spatially restricted than previously recognized. The data largely support a gradual desiccation of Mars, but the timing, duration, and mineralogical record of this process may be better constrained through global-scale application of an objective classification scheme that is ultimately evaluated in multiple locations by future landed missions.