Pilbara craton timing

Rather interestingly, the oldest usable biomarkers in carbonaceous shales date from the Neoarchean. Molecular fossils extracted from 2.5 Ga to 2.7 Ga shales of the Fortescue and Hamersley groups in the Pilbara Craton, Western Australia, indicate that the photic zone of the water column in the areas where these shales were deposited was probably weakly oxygenated, and that cyanobacteria were part of the microbial biota (Brocks et al., 1999, 2002 Summons et al., 1999). The similarity of the timing of the rise in the range of in sediments and the earliest evidence for the presence of cyanobacteria may, however, be coincidental, because to date no sediments older than 2.7 Ga have been found that contain usable biomarker molecules (Brocks, personal communication, 2002). 

Van Kranendonk, M. J. Collins, W. J. 1998. Timing and tectonic significance of late Archaean, sinistral strike-slip deformation in the central Pilbara structural corridor, Pilbara Craton, Western Australia. Precambrian Research, 88, 207-232. 

The Archean clastic rocks are also often enriched in chromium, nickel, and cobalt (Danchin, 1967 Condie, 1993), but this feature may only be of regional significance, mostly for the Kaapval Craton and the Pilbara block, where it may reflect the ubiquity of komatiites in the source regions of the sediments. Nevertheless, considering the frequent discrepancies in the anticipated Ni/Cr ratios, in lower than expected MgO content and, at times, high abundance of incompatible... 

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