BUILDING A GLOBAL STRATIGRAPHY OF MARS: NEW APPROACHES, KEY STRATIGRAPHIC SECTIONS, AND THE CONCEPT OF ORBITAL FACIES

The past several decades of successful missions have revolutionized our view of geologic processes on Mars. It is recognized that various regions and stratigraphic exposures on Mars preserve distinct records of sedimentary, igneous, and possibly metamorphic processes, as well as variability in post-depositional processes and local, if not regional or global, environmental conditions. Relationships between minerals exposed at the surface and ages of those terrains based on crater counting has led to the hypothesis that global-scale climatic changes are recorded as a mineralogical ‘stratigraphy’: clay minerals are associated with ancient Noachian terrains, sulfate salts with younger Hesperian terrains, and anhydrous Fe-oxides with Amazonian terrains. 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 observation-based analysis to those sections. We have developed a suite of ‘orbital facies’, defined by distinct morphologic and mineralogic characteristics, primarily as observed from orbit. They include Massive Breccia (MBR), Complexly Stratified Clays (CSC), Laterally Continuous Sulfates (LCS), Laterally Continuous Heterolithic (LCH), Distributary Networks (DNW), and Rhythmite (RHY) facies. Key stratigraphic sections across the planet were defined and the most appropriate facies were assigned to those strata. Comparing facies within and between sections and integrating with the relative age of these strata reveals that ancient terrains are dominated by MBR and CSC facies. In contrast, Hesperian terrains are characterized by LCS, DNW, and several examples of CSC facies. Occurrences of RHY are commonly associated with Hesperian or Amazonian terrains. Together, these results indicate that transitions in mineralogy and any associated changes in climatic conditions may have been slower-paced or more spatially restricted than previously recognized. Although the data support a gradual ‘drying out’ of Mars, understanding the timing, duration, and spatial variability of this process could be greatly improved by identifying additional key sections and applying this style of objective classification.