GALE CRATER AS A KEY REFERENCE SECTION IN UNDERSTANDING THE GLOBAL STRATIGRAPHY OF MARS

Using orbiter and rover data to build and understand the stratigraphic record of Mars at a global scale is critical to understanding the evolution of that planet. Moderate to high spatial resolution observations from the MGS, Mars Odyssey, Mars Express, and MRO orbiters, combined with in situ outcrop scale observations from the Spirit, Opportunity, and Curiosity rovers, provide data for this task at a level of detail that was not achievable during the Viking era. Here we focus on sedimentary deposits and outline a classification scheme based on gross morphological properties of purported sedimentary strata. In addition, we define several distinct ‘orbital facies’ that combine these morphologic attributes with mineralogy derived from orbital data [1]. The goal of the orbital facies concept is to provide a framework to objectively classify various stratigraphic sequences on Mars in order to identify distinct changes within a given sequence as well as allow for correlation between sequences.

Gale Crater is a key reference section on Mars and is considered an ‘overfilled crater’ in our classification. The mound in Gale Crater post-dates the crater formation and has a likely maximum age along the Noachian-Hesperian boundary, and it exhibits transitions between several common orbital facies upsection. Clay and sulfate-bearing strata are found in the Lower formation of the mound, examples of the Complexly Stratified Clay (CSC) and Laterally Continuous Sulfate (LCS) orbital facies. In contrast, the Upper formation of the Gale mound lacks orbital signatures of hydrated phases, exhibits beds of consistent thickness and is thus classified as Rhythmite (RHY) orbital facies [1]. The lowermost strata in Gale will be those explored by Curiosity, providing detailed observations at outcrop scale to further constrain the true nature of these orbital facies and to determine how the LCS facies in Gale compare to the LCS facies in Meridiani Planum [1]. The rover traverse region will also allow for the exploration of several major geologic units identified in Gale Crater from orbital data, making it possible to test the hypothesis that the mound in Gale Crater records distinct climatic changes and the ‘drying out’ of Mars, events suggested to be global in scale.

[1] Milliken, R.E. and J.P. Grotzinger (2012), SEPM Special Pub. 102, pp. 1-48.