Earth’s oldest putative fossil assemblage from the ~3.5 Ga Apex chert, Chinaman Creek, Western Australia: a field and petrographic guide

This Record is based on the results of field and laboratory investigations that commenced in 1999 and were largely completed by 2004. In 1999, the initial researchers were Prof. Martin Brasier in the Department of Earth Sciences, University of Oxford, England, together with his son Dr Alexander Brasier of Edinburgh University, and Mr Crispin Stoakes, a Western Australian geologist. In 2001, their laboratory work was followed up with further field work by Dr John Lindsay of the Research School of Earth Sciences, Australian National University, Canberra, ACT, together with Dr Martin Van Kranendonk of the Geological Survey of Western Australia. In 2002, the group was joined by Nicola McLoughlin, PhD student, Department of Earth Sciences, University of Oxford. Between 2003 and 2004, additional team members were Mr Owen Green, Technical Officer, Department of Earth Sciences, University of Oxford, and Dr David Wacey, postdoctoral researcher, Department of Earth Sciences, University
of Oxford. The project investigated the geological setting and biogenicity of microscopic structures in the early Archean ‘Apex chert’, widely regarded as representing Earth’s oldest microfossils. The Geological Survey of Western Australia (GSWA) assisted parts of the field program by supplying a vehicle and field assistant, and by providing geological advice. However, the observations and interpretations in this Record are those of the authors, and are not necessarily the same as interpretations by staff of GSWA. Limited editing to conform to GSWA house style has been undertaken by GSWA.

Between 1999 and 2004, we undertook detailed field and petrographic studies to critically assess the biogenicity of microfossil-like structures in c. 3460 Ma chert of the Apex Basalt near Marble Bar. For almost 20 years the scientific
community had generally accepted that these carbonaceous structures were Earth’s oldest known microfossils. However, the main conclusions from our study, first published about a decade ago, were that the structures interpreted to be microfossils failed to meet various key criteria required for the acceptance of a biological origin. We now present additional information on the geological origin of the host rocks, plus petrographic descriptions
of the thin sections that were originally used to argue for the presence of microfossils. Important field observations include: a) evidence that the previously reported carbonaceous microfossil-like structures are hosted not by bedded sedimentary chert (informally referred to as the ‘Apex chert’), but by a
hydrothermal silica vein system 50−100 m beneath the paleosurface; b) that hydrothermal silica veins are locally common beneath the ‘Apex chert’, and occupy upward-thickening and upward-bifurcating fissures; c) that these
hydrothermal silica veins underplate and intrude the ‘Apex chert’, but do not cut through it; and d) that the silica veins are concentrated along two growth faults that divide the lower part of the Apex Basalt into three structural
blocks. Our study of petrographic microfabrics and integrated geochemistry reveal that complex, septate, and filamentous carbonaceous and hematitic structures, including previously reported microfossil holotypes, are considerably more chaotic than previously described. Carbonaceous and hematitic structures are extremely common in the silica, but most do not resemble microfossils and appear to intergrade with abiogenic morphologies probably formed by devitrification of carbonaceous microquartz and volcanic glass.