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The Proterozoic Baraboo Quartzite was laid down by braided rivers and in near-shore marine environments on the southern edge of Paleo–North America during a period of tectonic stability following a prolonged interval of orogenic activity. The exceptional chemical maturity of the Quartzite points to warm, wet, tropical climate conditions, and its distinctive maroon to pink and purple color marks it as one of the earliest “red beds” on Earth. Detrital zircons from the base of the Quartzite constrain its depositional age to be younger than ca. 1710 Ma. The Quartzite is overlain by two other units known only from subsurface exploration: the Seeley Slate, interpreted as a shallow marine deposit, and the Freedom Formation, which has the physical and mineralogical characteristics of classic Superior-type iron formations but is younger than any of these by at least 150 m.y. The folding event at Baraboo was previously thought to have occurred at ca. 1630 Ma, based on indirect regional arguments. However, a growing number of 40Ar/39Ar ages in the range of 1450–1480 Ma have been obtained from occurrences of muscovite in hydrothermal veins in the Quartzite and from tectonic cleavage surfaces in the Seeley Slate, and these suggest a possible connection with the Wolf River Batholith igneous interval. The remarkable topographic relief that existed in the Baraboo Range at the time of the late Cambrian marine transgression, one billion years after the folding event, is another aspect of the regional geologic history that remains incompletely understood.

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