Fortescue group

Macfarlane, A. W., Danielson, A., Holland, H. D. Jacobsen, S. B. 1994. REE chemistry and Sm-Nd systematics of late Archean weathering profiles in the Fortescue Group, Western Australia. Geochimica et Cosmochimica Acta, 58, 1777-1794. 

Figure 4 Distribution of triaromatic steroids (68) in GC-MS m/z = 231 selected ion chromatograms in (a) a Phanerozoic oil of low thermal maturity, (b) a mature Phanerozoic oil, and (c) an overmature bitumen from the late Archaean Fortescue Group in Western Australia. The inset in (c) is a 20 X magnification of the elution range of C26 to C28 triaromatic steroids (68b) (Brocks et ah, 2003a,b) (reproduced by permission of Elsevier...

The relatively small number of late Archaean basins may reflect a low preservation potential, but may also indicate that relatively few cratonic nuclei had stabilized before the end of the Archaean. Examples of late Archaean basins include the (3.1-2.7 Ga) Witwatersrand basin in South Africa (Robb Meyer, 1995), and its correlative the 3.0-2.67 Ga Pongola Supergroup in Swaziland (Walraven. Pape, 1994 Gold von Veh, 1995), both unconformable upon the Kaapvaal Craton. In western Australia the 2.77-2.71 Ga Fortescue Group is unconformable upon older Archaean rocks of the Pilbara Craton (Blake et al., 2004). 

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). 

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