Sedimentary and Geochemical Heterogeneity in the Mesoproterozoic: A Natural Consequence of Protracted Oxygenation?
Linda C Kah, University of Tennessee, Department of Earth and Planetary Sciences, Knoxville, TN, United States, Ashley R Manning-Berg, University of Tennessee at Chattanooga, Biology, Geology, and Environmental Science, Chattanooga, United States, Geoffrey J Gilleaudeau, George Mason University, Department of Atmospheric, Oceanic, and Earth Sciences, Fairfax, United States and Julie K Bartley, Gustavus Adolphus College, St. Peter, MN, United States
Contact First Author: Linda C Kah; lckah@utk.edu
English Abstract:
Late Mesoproterozoic (1.3 to 1.0 Ga) sedimentary basins have long been considered enigmatic. Despite relative stability in the biological realm, late 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. Finally, although early data suggested prolonged geochemical stasis, a growing body of evidence from black shale facies 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 the 1.12 Ga Atar Group, Mauritania, West Africa. The Atar Group preserves nearly the full range of sedimentary facies that is observed in other Late Mesoproterozoic, carbonate-dominated basins. The arrangement of these facies, both spatially and stratigraphically, in combination with geochemical evidence, suggests that much of the heterogeneity preserved across late Mesoproterozoic basins can be explained by the dynamic interaction of the marine substrate, sea level, and a chemically stratified water column. We highlight a scenario wherein heterogeneity of both facies and geochemistry is a natural consequence of protracted biospheric oxygenation.
