Late Archaean crustal extension, sedimentary basin formation, flood basalt volcanism and continental rifting: the Nullagine and Mount Jope Supersequences, Western Australia

Abstract

The Mount Bruce Megasequence Set (formerly the Mount Bruce Supergroup) is a succession of volcanic and sedimentary rocks that was deposited on the Pilbara Craton of Western Australia in the late Archaean to Early Proterozoic. Its history can be described and explained in terms of modern-style sequence stratigraphy and of Phanerozoic-style plate tectonics. The Megasequence Set comprises the Chichester Range Megasequence, that was deposited on a rifted to divergent continental margin, and the overlying Hamersley Range Megasequence, that was deposited on a convergent continental margin. The Chichester Range Megasequence comprises, in order of superposition, the Nullagine and Mount Jope Supersequences, and the Marra Mamba Supersequence Package. The Nullagine and Mount Jope Supersequences are each the rock record of one phase of a protracted, two-phase late Archaean (∼ 2770-2700 Ma) continental break-up. The Nullagine Supersequence consists of the Mount Roe Sequence, which comprises mostly subaerial basalt, and the overlying Hardey Sequence Package, which comprises mostly terrigenous sedimentary rocks that were deposited in extensional sedimentary basins. The Mount Jope Supersequence consists of (in order of superposition): the Kylena Sequence, which comprises mostly basalt; the Tumbiana Sequence, which comprises mostly tuff and tuffaceous rocks; and the Maddina Sequence Package which comprises mostly basalt. The Nullagine Supersequence was deposited during an episode of WNW-ESE directed crustal extension that possibly led to the development of a rift to the west of the present Pilbara Craton. The Mount Jope Supersequence was deposited during the subsequent development of a WNW-ESE trending rift, the position of which approximates the present southern Craton margin. Regional-scale uplift was associated with the development of both Supersequences, particularly with the Nullagine Supersequence. Basement strike-slip faults were reactivated as dip-slip structures during crustal extension and controlled the locations of some Hardey Sequence Package extensional sedimentary basins. Nullagine Supersequence and Mount Jope Supersequence rifting led to a divergent continental margin bordering an ocean along the south and probably the far west of the Craton. Post-rift thermal subsidence was accompanied by the accumulation of mudrock, carbonate sedimentary rock and iron formation of the Marra Mamba Supersequence Package in a shelf setting. The implications of these interpretations are that at ∼ 2800 Ma the Pilbara Craton was significantly larger than at present, and that it had a sufficiently thick subcontinental lithosphere that divided it in a manner analogous to the break-up of Phanerozoic continents. The Nullagine and Mount Jope Supersequences contain three stacked flood basalt successions which were erupted over a time period of between of 55 and 80 m.y. Stacked flood basalt successions covering such a long time period are not known in the Phanerozoic and are tangible evidence of a hotter late Archaean mantle.