Understanding Martian Macrostratigraphy

Session: Geomorphology and Surface Processes Across the Solar System

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Abstract:

Understanding the sedimentary rock record of Mars is an essential tool in understanding the planet’s evolving climate. Mars’ sedimentary record is globally distributed, and broadly categorized into three units that were identified from orbit; the Clays, Laterally Continuous Sulfates, and Rhythmites (Grotzinger and Milliken, 2012). These units are hypothesized to span from warmer, wetter periods early in Mars’ history (Clays), to colder, dryer ones (Rhythmites) (Bibring et al., 2006). However, this simple model for Mars’ planetary evolution hosts many uncertainties centered around the post-3.5 Gyr portion of Mars’ history where it was less water rich, but still sustainably forming sedimentary rocks. Initial hypotheses about Mars’ transition from warm and wet to cold and dry propose a unidirectional and time-correlative transition across the surface (Bibring et al., 2006). Although alternative hypotheses suggesting diachronous, localized shifts have been proposed (McLennan, 2019), we have as yet been unable to resolve the spatial and temporal shifts recorded within and between Mars’ major geologic units. Furthermore, though many global hypotheses have been proposed as formation mechanisms for Mars’ sedimentary rocks (e.g. volcanic ash (Kerber et al., 2012), groundwater (Andrews-Hanna and Lewis, 2011), snowmelt (Kite et al., 2013)), thus far there have been no global-scale quantitative tests for these mechanisms against the Martian rock record. 

We address these unknowns by analyzing the post-3.5 Ga sedimentary record. Specifically, we analyze the global transition from warm and wet to cold and dry by quantifying Mars’ sedimentary rock production using deposit volumes and ages. We test against different global formation hypotheses using layer thickness trends to monitor sedimentary depocenters. Lastly, we calculate the detailed aggradation rate of deposits to better understand when Mars was actively producing sedimentary rocks, how that evolved over time, and how it relates back to Mars’ climate and stratigraphic evolution. We found that Mars’ global sedimentary record shows intra-regional, rather than global, trends in layer thickness, disfavoring global formation mechanisms despite globally consistent stratigraphy. Furthermore, we show that Mars’ global stratigraphic transitions are diachronous. We also find evidence that aggradation during different time periods was relatively constant. Together, these data provide a time integrated view of the macrostratigraphy of Mars’ young (post 3.5 Ga) sedimentary rocks.