Redox history and preservation of the 1.71 – 1.47 Ga Freedom formation, Baraboo, Wisconsin

Abstract

The Freedom Formation (<1.71 Ga and >1.47 Ga) is a subsurface unit preserved in historic drill cores near Baraboo, WI, USA that may record information about Earth’s surface conditions in the Late Paleoproterozoic, Early Mesoproterozoic. As an iron-rich chemical sedimentary unit, the Freedom Formation has the potential to provide valuable insight into redox conditions for Earth’s surface at a time and place where such rock type is poorly preserved in surface outcrops. Drill cores preserve units that document a transition from low energy (Seeley Slate, Freedom Formation) to high energy environments (Dake Quartzite), and the lithologic gap of the Great Unconformity (Cambrian sandstone overlying Baraboo Interval units). Newly identified lateritic horizons preserved in the Freedom Formation indicate significant or long duration weathering associated with the time gap. Core observations, mineralogy, and petrographic relationships in thin section document a distinct transition from reduced (chamosite, siderite) to oxidized (hematite) minerals towards the top of the section in the Freedom Formation. Additionally, magnetite crystals have oxidized hematite rims. I interpret this transition to reflect post-depositional oxidation, potentially driven by surface exposure and infiltration of oxygenated fluids sometime in the Late Proterozoic to Early Paleozoic.