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

Eva L Scheller,Bethany L Ehlmann

doi:10.1029/2019je006190

Eva L Scheller, Bethany L Ehlmann

Open Access

https://doi.org/10.1029/2019je006190

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Abstract

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 low-Ca pyroxenes (LCP)- and Fe/Mg-smectite-bearing unit. Here, we divide the Noachian Basement Group into five distinct geological units (Stratified Basement Unit, Blue Fractured Unit, Mixed Lithology Plains Unit, LCP-bearing Plateaus Unit, and Fe/Mg-smectite-bearing Mounds Unit), two 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.

Highlights

Understanding the geological history of the ancient Pre‐Noachian to Mid‐Noachian crust on Mars is imperative as it includes processes such as impact basin formation, igneous petrogenesis, climate evolution, and ancient aqueous environments that are essential for understanding the origin, early evolution, and habitability of terrestrial planets
We searched south and east of the Isidis basin structure, but due to thick, fine‐grained covers within this region, it remains indeterminate whether megabreccia are present
We propose that the Noachian Basement Group records at least four events of hydrated mineral formation: (1) pre‐Isidis Fe/Mg‐smectite formation in target rock that are megabreccia blocks; (2) at least one and possibly several episodes of Fe/Mg‐smectite formation within potential syn‐Isidis impact deposits (MLPU); (3) contemporaneous or subsequent Fe/Mg‐smectite formation in crosscutting fractures that form ridges; and (4) kaolinite formation

Summary

Introduction

Understanding the geological history of the ancient Pre‐Noachian to Mid‐Noachian crust on Mars is imperative as it includes processes such as impact basin formation, igneous petrogenesis, climate evolution, and ancient aqueous environments that are essential for understanding the origin, early evolution, and habitability of terrestrial planets. This time period encompasses rocks formed >3.82 Ga The Noachian Basement includes a variety of geomorphological features such as ridges (Pascuzzo et al, 2019; Saper & Mustard, 2013), smooth plains, knobby plains, mounds, and megabreccia (Bramble et al, 2017)

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