A giant, Late Archean lake system: The Meentheena Member (Tumbiana Formation; Fortescue Group), Western Australia

Abstract The ∼2720 Ma-old Meentheena Member of the Tumbiana Formation, Fortescue Group, is a 30–50 m-thick succession of alternating siliciclastic and limestone beds that crop out intermittently over a distance of ∼680 km across the Pilbara Craton, Western Australia. Limestones of the member consist of flat-pebble (intraformational) conglomerate, oolite, grainstone, calcisiltite, and calcilutite. Siliciclastics consist of shale, siltstone, and sandstone, and many of these are tuffaceous. Sedimentary structures include abundant symmetrical ripples, ripple cross-stratification, desiccation cracks, teepee structures, and planar to wavy lamination. The member is superficially similar in appearance wherever it crops out; however, the correlation of individual beds and units over distances greater than 15 km is problematic. There are no regional marker beds. Radiometric ages from tuffaceous units young to the west by a few million years. Four lithofacies are defined: (a) flat-pebble conglomerate, (b) ripple cross-laminated oolite/grainstone, (c) planar to wavy-laminated calcilutite/calcisiltite, and (d) shale/siltstone. Stromatolites are found in all three of the limestone facies. Evaporites are rare and include halite pseudomorphs. The four lithofacies are generally found in a vertical succession that characterizes a lithofacies association and reflects a deepening upward succession. Associations may not have all four lithofacies, but do reflect the same deepening upward pattern. The uppermost portions of shale/siltstone facies commonly contain symmetrical ripples and desiccation cracks indicating subaerial exposure. Measured sections contain as many as 25 of these parasequences indicating fluctuating water levels. Limestone lithology and facies commonly change abruptly laterally (at the meter and greater scale) and vertically (centimeter- to meter-scale). The sedimentary structures, differences and abrupt changes in lithofacies patterns, the inability to correlate confidently between regions, and the differences in age from east to west suggest a complex, diachronous, depositional system of fluctuating water levels in separate bodies of water. The limestone is dominated by sedimentary structures indicative of shallow water within wave base; however, it is devoid of evidence of any tidal influence. Possibilities for depositional environments include a tideless sea or a lacustrine system. 87Sr/86Sr values from limestone differ from known Archean marine carbonates. REE patterns are decidedly non-marine as well. A lacustrine system is favored for the Meentheena Member based on sedimentary structures, the abruptness of lithologic and facies changes, the facies association, lateral gradation into fluvial deposits, the inability to correlate many units and beds for more than a few to several kilometers, geochemical data, stromatolites, and comparisons with younger lacustrine basins as well as marine successions. There is no single line of evidence that unequivocally establishes a lacustrine origin; however, multiple lines of evidence are collectively consistent with a lacustrine origin. The Meentheena Member would therefore represent the oldest known carbonate-rich, lacustrine system, which we call the Meentheena lakes system.