˜3,426–3,350 Ma, Strelley Pool Formation, East Pilbara, Western Australia
The Strelley Pool Formation is the rock unit that I myself have studied in most detail. I will now take this opportunity to summarise the various claims for life from this Formation; some are widely accessible in the literature, whilst others are new discoveries. Beginning at the base of the Formation, the Strelley Pool sandstone contains well rounded detrital grains of pyrite, chromite, rutile, and zircon, occurring as a heavy mineral placer-type deposit within a beach (or plausibly even river) setting. Many of the pyrite grains contain microborings (Fig. B43). These microborings typically comprise meandering cylindrical tunnels (microtubes) or channels, or spherical to elliptical surface pits. They have a near constant diameter of 1–5 μm and can penetrate into the substrate for several tens of microns. They are often concentrated in clumps along one side of a mineral grain. Some microborings show slight constrictions along their margins suggestive of colonisation by individual cells (Fig. B43). The microborings do not extend into adjacent grains nor into the microcrystalline silica cement. Nano-scale geochemical mapping shows enrichments of carbon and nitrogen within and/or along the edges of the microborings, consistent with some preservation of biological material (Wacey et al., 2008c).
Non-biological features which closely resemble microborings include microfractures and physico-chemical erosion and corrosion marks (Fig. B102). Microfracturing could be rejected as a formation mechanism for these microborings because they lack a jigsaw puzzle fit, are not sheet-like, and show a nearly constant diameter. Their origin from erosion and corrosion structures was also rejected because the microtubes penetrate far into the substrate and most are an order of magnitude smaller than co-occurring pit marks. Ambient inclusion trails can also result in microtubular structures but these would be expected to have striations along their length, polygonal cross sections and show evidence for a propelled crystal at their terminations (see Fig. B45).
KeywordsGreenstone Belt Trace Fossil Constant Diameter Geological Context Doushantuo Formation
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