Stable Isotopic Compositions of Carbonates from the Mesoproterozoic Bangemall Group, Northwestern Australia

Abstract

Marine carbonate rocks from the Mesoproterozoic Bangemall Group of northwestern Australia show little deviation (+/- 1.3 parts per thousand) in whole-rock delta(13)C(carb)-values about a mean of -0.5 parts per thousand. This narrow range persists despite close sampling (every 10-20 m) through long sections (up to 2500 m) that are geographically widespread (up to 250 km apart), over many depositional environments (supralittoral to outer shelf), sediment sources (stromatolitic bioherms to detrital calcilutites) and rock types (pure limestones to dolomitic shales). The only major excursions from the norm seem related to unusual environmental or postdepositional processes, as they are correlated with large enrichments (to - 3 parts per thousand) or depletions (to - 16 parts per thousand) in O-18. Relatively heavy delta(13)C-values, up to + 2.5 parts per thousand, occur in a single bed of brecciated ferruginous dolostone at a single locality; these abnormal values may result from local evaporitic conditions. Limey and shaley nodular dolostones have delta(13)C-values as low as -4.3 parts per thousand, probably caused by remineralization of organic matter during late and patchy dolomitization. Most notably, sharp negative excursions in delta(13)C, up to -8.4 parts per thousand, occur in bleached kerogen-free rocks with mineral assemblages of dolomite + quartz + calcite + tremolite + talc, reflecting isotopic re-equilibration in thick metamorphic aureoles around dolerite intrusions. General environmental variations are minor, with delta(13)C-values of peritidal facies tending to be slightly positive whereas those of subtidal facies are slightly negative. There are no strong secular trends, but subtle fluctuations within the range -2 to +1 parts per thousand can be correlated along the northwestern margin of the basin. This resembles the pattern seen in other Mesoproterozoic successions, but is markedly unlike the heavy background (> + 5 parts per thousand) and extreme variations (up to 10 parts per thousand) in delta(13)C evident in Neoproterozoic successions of similar thickness and environmental setting. Hence, in contrast to the Neoproterozoic, the global rate of organic carbon burial was probably fairly constant during deposition of the Bangemall Group, and perhaps generally during the Mesoproterozoic, as was the redox state of the atmosphere and hydrosphere.