Rhenium-Osmium Age Determinations of Glaciogenic Shales from the Mesoproterozoic Vazante Formation, Brazil

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2006-12-12

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The Vazante Group is a sedimentary package of carbonate interbedded with diamictite and organic-rich shale, potentially representing a Pleistocene-style glacial cycle of Proterozoic age in Brazil. Lacking volcanic ashes, the age of the Vazante strata has eluded researchers. This study applied the Re-Os geochronometer to successive organic-rich horizons, revealing pre- and synglacial ages of ca. 1.35 and 1.13 Ga. Time-series stable isotopic studies were conducted throughout the succession to investigate active metabolisms and depositional redox conditions. Given the radiometric constraints, the Vazante Group contains the only recognized glacial deposit of Mesoproterozoic age, but may be similar to Neoproterozoic events. Vazante strata were deposited on a low-latitude passive margin formed during the diachronous break-up of the supercontinent Rodinia, with local ice-sheets potentially developing on uplifted rift margins. A low-latitude Mesoproterozoic glaciation suggests that the "climate paradox" of the Neoproterozoic extends back further than previously believed.

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