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Abstract:

Thick sulfate evaporite accumulations are absent from Proterozoic strata between ca. 2000 and ca. 1000 Ma, and detailed sedimentologic, stratigraphic, and geochemical data for the oldest Neoproterozoic thick marine sulfate evaporite successions are largely lacking. The middle Neoproterozoic Ten Stone Formation (Little Dal Group, Northwest Territories, Canada) consists of ∼500 m of pelagic lagoonal gypsite and anhydritite (rocks consisting of the minerals gypsum and anhydrite) deposited shortly before the ca. 811 Ma Bitter Springs carbon isotope anomaly in an intracratonic basin that developed prior to breakup of Rodinia. The thickness of regional stratigraphic subdivisions of this formation, defined by subtle silt- and carbonate-bearing intervals, indicates a minor terrigenous source in the southeast and a silled connection to the open ocean in the northwest. Deposition of the Ten Stone Formation began with abrupt, tectonically triggered subsidence and restriction, and ended equally abruptly, as shown by stratigraphic contacts across which lithofacies corresponding to strikingly different paleoenvironments change sharply, with no evidence for hiatus or erosion. Stratigraphic cyclicity in the evaporite succession is minimal owing to isolation of bottom-hugging, dense lagoonal brine from overlying waters. Deposition of the Ten Stone Formation in a basin that experienced intermittent, basin-scale tectonic adjustments, as recorded by details of its stratigraphy, supports the interpretation that the Mackenzie Mountains Supergroup accumulated in an extensional, tectonically active intracratonic basin whose structure resembled a lower-plate extensional system. The absence of halite from the Ten Stone Formation contrasts with its abundance in the stratigraphically lower, gypsum-free Dodo Creek Formation, suggesting that deposition of the lower to middle Little Dal Group spanned a major oxygenation event, during which the sequence of evaporite mineral precipitation from seawater changed from halite-first to sulfate-first in response to rapid accumulation of atmospheric oxygen and concomitant increase in the global marine sulfate reservoir. The limited range of sulfur isotope values in a new data set spanning hundreds of meters of gypsite indicates a strongly and persistently oxidizing mid-Neoproterozoic atmosphere, an abundance of sulfate in seawater, and marine oxygenation extending below storm wave base. The mineralogy, sedimentology, stratigraphy, and geochemistry of the Ten Stone Formation are virtually indistinguishable from those of thick, Phanerozoic “deep-water” (below wave-base) evaporite successions, and indicate that the tectonic, climatic, and geochemical conditions required for deposition of thick successions of marine sulfate evaporites were well established prior to ca. 811 Ma. Thick sulfate evaporite successions in equivalent stratigraphic positions just below the Bitter Springs carbon isotope excursion elsewhere in Laurentia, as well as on the Congo craton, and in South Australia attest to the global impact of the rapidly increased seawater sulfate reservoir prior to Rodinia’s breakup. High relative burial rates of organic matter prevailed before the breakup of Rodinia and led to oxygenation of the atmosphere-ocean system, growth of the seawater sulfate reservoir, and, in association with a warm and arid climate, deposition for the first time in Earth’s history of thick sulfate evaporites in the middle Neoproterozoic, ∼100 m.y. before the first Cryogenian glacial episode. The Neoproterozoic Oxygenation Event may have taken place in several steps, the first of which preceded the Bitter Springs anomaly.<br/>