Composition, Stratigraphy, and Geological History of the Noachian Basement Surrounding the Isidis Impact Basin

1Eva L. Scheller,1,2Bethany L. Ehlmann
Journal of Geophysical Research (Planets) (In Press) Link to Article [https://doi.org/10.1029/2019JE006190]
1Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California, USA
2Jet Propulsion Laboratory, Pasadena, California Institute of Technology, Pasadena, California, USA
Published by arrangement with John Wiley & Sons

The western part of the Isidis basin structure hosts a well‐characterized Early Noachian to Amazonian stratigraphy. The Noachian Basement comprises its oldest exposed rocks (Early to Mid‐Noachian), and was previously considered a single LCP‐ and Fe/Mg‐smectite‐bearing unit. Here, we divide the Noachian Basement Group into 5 distinct geological units (Stratified Basement Unit, Blue Fractured Unit, Mixed Lithology Plains Unit, LCP‐bearing Plateaus Unit, Fe/Mg‐smectite‐bearing Mounds Unit), 2 geomorphological features (megabreccia and ridges), and a mineral deposit (kaolinite‐bearing bright materials), based on geomorphology, spectral characteristics, and stratigraphic relationships. Megabreccia contain four different pre‐Isidis lithologies, possibly including deeper crust or mantle materials, formed through mass‐wasting associated with transient crater collapse during Isidis basin formation. The Fe/Mg‐smectite‐bearing Stratified Basement Unit and LCP‐bearing Blue Fractured Unit likewise represent pre‐Isidis units within the Noachian Basement Group. Multiple Fe/Mg‐smectite‐bearing geological units with different stratigraphic positions and younger kaolinite‐bearing bright materials indicate several aqueous alteration episodes of different ages and styles. Units with slight changes in pyroxene spectral properties suggest a transition from low‐Ca pyroxene‐containing materials to those with higher proportions of pyroxenes higher in Ca and/or glass that could be related to different impact‐ and/or igneous processes, or provenance. This long history of Noachian and potentially Pre‐Noachian geological processes, including impact basin formation, aqueous alteration, and multiple igneous and sedimentary petrogeneses, records changing ancient Mars environmental conditions. All units defined by this study are available 20 km outside of Jezero crater for in‐situ analysis and sampling during a potential extended mission scenario for the Mars 2020 rover.

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