Microfossil assemblage from the 3400 Ma Strelley Pool Formation in the Pilbara Craton, Western Australia: Results form a new locality

A new microfossil locality for the ca. 3400 Ma Strelley Pool Formation (SPF), in the Pilbara Craton, Western Australia, was discovered in the Panorama greenstone belt. Large and morphologically diverse microfossils (>20 μm, up to 100 μm, along the major dimension) are abundant in a carbonaceous laminated black chert of shallow water origin that was originally collected in 2010 and relocated in 2011 just south of the Trendall Reserve (State Geoheritage Reserve, R50149). Over 300 small, spherical and lenticular (flanged and non-flanged lenses) carbonaceous cellular microfossils are present in each replicated thin section measuring 2.5 cm × 3.4 cm. In addition, film-like structures and tubular filaments were discovered. Narrow carbonaceous filaments of several types are also present in and around the secondary cavity-fill cherts, although their biogenicity is uncertain. Colonies of spherical and lenticular structures are often observed. Colonies composed of small spheroids can be classified into at least two groups based on the sizes of the individuals. Their mode of clustering is variable, ranging from tight to loose and from spherical to random. Colonies of lenticular structures are often composed of a mixture of flanged and non-flanged lenses, in addition to those solely composed of either flanged or non-flanged lenses. The size ranges of these three types are not statistically distinguishable. Therefore, it is currently unknown whether flanged and non-flanged lenses represent different species or different life stages of the same species. Although more research will be needed to establish the taxonomic framework for these ancient microfossils, they provide evidence that the coastal region of the 3.4 Ga “Strelley Pool Sea” was inhabited by various microbial communities, reflecting the early stage diversification of life on Earth.

► Microfossils were discovered from a new locality of the Strelley Pool Formation. ► New morphological types that have never been reported are described. ► We collected over 600 size data and performed statistical analyses. ► The Palaeoarchen coast has been inhabited with various microbial communities. ► The early stage diversification of life was suggested.