POSSIBLE (OR NOT) MICROBIAL STRUCTURES IN THE < 3.7GA GILLESPIE LAKE MEMBER, MARS

Sandstone beds of the < 3.7 Ga Gillespie Lake Member on Mars have been interpreted as lake and river deposits that over time fell dry. On Earth, such settings are colonized by microbial mats that may cause microbially induced sedimentary structures (MISS). Because of the similar ages of both the Gillespie Lake rocks on Mars and the oldest MISS-bearing rocks on Earth, the 3.48 Ga Dresser Formation, aim of this study was to search for possible MISS in the Martian deposits.

Three lithofacies of the Gillespie Lake Member sandstone display cm- to m- scale structures that are similar to terrestrial MISS. These MISS-like structures appear to be arranged into spatial associations and temporal successions. Such associations and their temporal successions are known from terrestrial MISS. One site in the Gillespie Lake Member displays a fossil surface with a morphological relief similar to that of the exposed depositional surfaces.

Terrestrial MISS are distinct and rarely masked by abiotic processes. However, on Mars the surfaces might have experienced post-depositional erosion and alteration causing abiotic sedimentary structures include pits, clasts, and cracks. However, abiotic processes that may have shaped the surface morphologies on Mars during a time that long ago are still widely unknown. More so, the site displaying a possible ancient morphology may indicate that the modern surface relief is original. On Earth, the MISS associations would record the development of a microbial mat ecosystem, which then was subject to desiccation of the setting.

The conclusion of the study is the hypothesis that the sedimentary structures in the Gillespie Lake Member are fossil MISS. In order to verify or falsify this hypothesis, the ‘Criteria of Biogenicity of MISS’ must be fulfilled. A critical amount of sedimentary structures of comparable morphology must be found to have a statistical base of morphological comparisons allowing identification as MISS; petrological and geochemical analyses on thin-sections obtained from returned samples must be conducted allowing confirmation that the candidate structures are indeed biological.