Diverse hydrothermal and sedimentary habitats in the 3.5 Ga Dresser Formation, Pilbara Craton, Western Australia

Abstract

<p>Our understanding of habitable early Earth environments is limited by the preservation potential of Archean rock deposits. Sedimentary intervals within the relatively homogeneous volcanic sequences of the Barberton Greenstone Belt (BGB), South Africa and Swaziland, and the eastern Pilbara Craton (PC) in Western Australia provide the best windows into interpreting variability in early Earth environments due to their low strain and low metamorphic grade. These deposits host Earth's oldest, most convincing, evidence of life (~3.3-3.5 Ga), and contribute to a better understanding of both the setting and the origin of life (OoL) and the search for life on other planets, such as Mars.</p><p>The 3481 ± 2 Ma Dresser Formation, PC contains one of these preservation windows. Previous outcrop studies indicate changes in dominant volcanism composition (mafic to felsic), transitions from subaqueous to subaerial sedimentary facies, and large regional tectonic events that promoted circulation of hydrothermal fluids and secondary alteration. Here we present detailed lithologic and mineralogic details of this early Earth surface deposit, obtained from a recent drilling campaign.</p><p>Three fresh drillcores obtained through the lower chert member of the Dresser Formation (~15 m thickness) were drilled at ~75 m below the weathered surface via HQ diamond drilling. Core sites were selected to provide a 3D perspective of geologic variability. Results show a high degree of complex lithology consisting of repeating bedded black carbonaceous cherts (TOC of up to 0.16 wt %), jaspilitic cherts, bedded carbonates, volcanogenic sandstones, beds of sulfidized stromatolites, edgewise conglomerates, laminated siliceous deposits that are texturally similar to hot spring sinter and directly overlie putative feeder veins, and  thin spherulitic beds with petrographic similarities (spherical shapes, quenched features, radial crystal splays, and off-centered vesicles) to younger impact spherule beds. In addition, veins of carbonate, chert, barite and sulfides were identified throughout much of the lower chert member. Lithologic variability occurs on scales of millimetres-decimetres vertically, and from metres to decametres laterally, while mapping of regional scale (kms) stratigraphic sections indicate repetition of specific lithologic successions identified in the cores.</p><p>The marked heterogeneity and variability both across stratigraphy and along strike contrasts sharply with marine settings, which are relatively homogeneous and consistent over 10’s-100’s of kilometres laterally, and across tens of metres vertically. Younger geologic systems that show similar depositional associations and sharp lateral facies changes include geothermal environments. The presence of sulfidized stromatolites, bedded black cherts, and hot spring deposit indicate that there were a diverse range of habitats (both marine and terrestrial) that hosted life. If spherulitic beds are impactor in origin, they would represent the oldest evidence of a bolide impact in the geologic record. The Dresser Formation provides insights into complex, dynamic, early Earth environments that host apparently already diverse microbial communities adapted to a range of habitats.</p><p> </p>