HETEROGENEITY IN THE MESOPROTEROZOIC RECORD OF CARBONATE ROCKS

Mesoproterozoic sedimentary basins have long been considered enigmatic. Despite relative stability in the biological realm, Mesoproterozoic strata preserve an incredible diversity of stromatolite morphology. Carbonate facies similarly display a broad range of carbonate fabrics, including water-column micrite, sea-floor precipitates, molar-tooth microspar, and herringbone carbonate, each of which had been described previously as time-distinctive Precambrian facies. Late Mesoproterozoic sedimentary basins are also increasingly likely to preserve deposits of bedded marine gypsum compared to older sedimentary successions, despite geochemical evidence for extremely low marine sulfate concentration. And although early data suggested prolonged geochemical stasis, a growing body of evidence indicates variably oxic, euxinic, and ferruginous marine conditions.

At first glance, the diversity of sedimentary facies and geochemistry preserved in these basins hampers efforts to define long-term changes in the Earth’s global biosphere. Do differences between basins reflect local overprinting of a global signal? Do differences between basins reflect secular change at rates unresolvable by our current geochronology? Or do differences between basins reflect an underlying heterogeneity of the late Mesoproterozoic ocean system? Here, we review sedimentary fabrics and geochemistry of Mesoproterozoic basins and suggest that much of the heterogeneity observed in the Mesoproterozoic can be explained by the dynamic interaction of the marine substrate, sea level, and a chemically stratified water column. Such a scenario highlights that heterogeneity of both facies and geochemistry is a natural consequence of protracted biospheric oxygenation.