Deep-marine depositional setting of banded iron formation: sedimentological evidence from interbedded clastic sedimentary rocks in the early Palaeoproterozoic Dales Gorge Member of Western Australia
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Sedimentary rocks in S macrobands of the Dales Gorge Member are the key to identifying the depositional setting of bandediron formation, because they can be described and interpreted by analogy to modern marine sediments. S macrobands comprise mostly shelf-derived epiclastic and volcaniclastic mud-rich turbidites, carbonate-rich turbidites and hemipelagites. Ash-fall tuffs occur in S macrobands and in their BIF equivalents, but are not diagnostic of the depositional setting other than it being below wave base and coeval with distant volcanism. Sedimentological content and stratigraphic geometry define the sedimentary rocks in S macrobands as being the deposits of distal low stand fans in a sediment-starved basin-plain setting. Sedimentation was pulsed, with gaps between sedimentation units being recorded by province-scale seafloor silicification that produced bedded chert. BIF occurs in some S macrobands, but sedimentary rocks characteristic of S macrobands do not occur in BIF macrobands. Mudrock-chert couplets in S macrobands are remarkably similar in internal structure, areal extent and thickness to BIF-chert couplets in BIF macrobands, suggesting that the precursor sediments to BIF were also deposited from episodic density currents. In contrast to the shelf source of sedimentary rocks in S macrobands, the precursor sediments to BIF were probably iron-rich granular muds derived from the flanks of submarine volcanoes, and brought to the depositional site by bottom currents. Basin filling therefore reflects competitive deposition between shelf-derived and intrabasinally derived sediments. Sedimentation units capped by bedded chert in S and BIF macrobands may record depositional rhythms at the scale of Milankovitch Cycles, whereas depositional sequences are composed of an underlying S macroband and an overlying BIF macroband.
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