A LATE ARCHEAN PALEOSOL WITH MICROFOSSILS, RAINDROP STRUCTURES AND EPHEMERAL PONDS?

The Mount Roe #2 paleosol is a Late Archean weathering horizon derived in situ from an underlying basaltic flow (Macfarlene et al., 1994; Rye and Holland, 2000). The 2.765 ± 0.01 Ga age of the Mount Roe #2 paleosol is well constrained by U-Pb age dates, obtained from zircons, from both the underlying and overlying basaltic flows (Arndt, et al., 1991). Geochemical work by Rye and Holland (2000) on the sericite and chlorite zones yielded mircrobial mat-like structures and d13C values similar to those of stromatolitic sediments of the same age, suggesting Archean organisms may have had a terrestrial footing as early as 2.76 Ga . Recently, a variety of microstructures have been found within the sericite zone of the Mount Roe #2 paleosol. Some of these structures support a subaerial, terrestrial environment, however, minor shales, sandstones and microlaminated sediments, preserved within an ancient stream channel in the sericite zone strongly indicate that surface water conditions are more than ephemeral.

References:

Arndt, N.T., Nelson, D.R., Compston, W., Trendall, A.F., and Throne, A.M., (1991). The age of the Fortescue Group, Hamersley Basin, Western Australia, from ion microprobe results. Australian Journal of Earth Sciences. Vol. 38, p. 261-281.

Macfarlene, A.W., Danielson, A. and Holland H.D. (1994). Geology and major trace element chemistry of Late Archean weathering profiles in the Fortescue Group, Western Australia: Implications for atmospheric pCO2. Precambrisn Research. Vol.65. p. 297-317.

Rye, R. and Holland, H.D. (2000). Life associated with a 2.76 Ga ephemeral pond?: Evidence from Mount Roe #2 paleosol. Geology: Vol. 28, No. 6, pp. 483–486.