Sedimentology, paleontology, and stable isotope geochemistry of selected formations in the 2.7-billion-year-old Fortescue Group, western Australia.

摘要

The environment of deposition for the Meentheena Carbonate Member of the Tumbiana Formation was sub- to supralittoral, based on the association of sedimentary structures.; Sedimentary structures and detrital content indicate that chert from the lowermost Jerrinah Formation (Woodiana Sandstone Member) was formed by silica replacement of clastic and carbonate rocks. Ladder ripples and probable pseudomorphs after gypsum suggest a shallow-water paleoenvironment. These cherts have five subfacies, which are laterally discontinuous and brecciated to various degrees. Lateral variation in lithology is due to: sediment deposition in shallow water, undulating surface topography of underlying basalt, and limited areal extent of silica replacement. Brecciation of these rocks may have been the result of silica replacement (which may have been hydrothermal) and/or tectonic deformation.; Based on the description of six distinct stromatolite types (A-F) from the Meentheena Carbonate and the Woodiana Sandstone, a diverse assemblage of microorganisms was extant 2.7 Ga, although their affinities remain uncertain.; Stable isotopes from the Meentheena Carbonate, Woodiana Sandstone chert, and Jerrinah Formation shales reflect biological, post-depositional, and sedimentological influences. Kerogen values ({dollar}deltasp{lcub}13{rcub}{dollar}C{dollar}sb{lcub}rm PDB{rcub}{dollar}) range between {dollar}-{dollar}39.16 and {dollar}-{dollar}59.38 and average {dollar}-{dollar}47.11 per mil.. These light values are primary, and support the theory that methanotrophs may have been part of the Archean biosphere. Variations in kerogen {dollar}deltasp{lcub}13{rcub}{dollar}C values among lithologies (carbonate, chert, and shale) reflect a diagenetic (and catagenetic) overprint, which masks most of the original biological signature. Mineral matrix probably influenced the degree of isotopic shift toward more positive values (specifically carbonate, illite and montmorillonite). Carbonate {dollar}deltasp{lcub}13{rcub}{dollar}C{dollar}sb{lcub}rm PDB{rcub}{dollar} values for the Meentheena Carbonate average 0.03 with a standard deviation of 1.22; they show no biological influence. Values either represent primary marine values or a secondary homogenization by a large reservoir of water.; Sulfur-isotope values ({dollar}deltasp{lcub}34{rcub}{dollar}S) from pyrite average {dollar}-{dollar}0.54 per mil., with a standard deviation of 1.83, supporting post-2.7 Ga emergence for dissimilatory sulfate reduction.