REVIVING HISTORICAL DATA >100 YEARS LATER: A FRESH LOOK AT THE DEPOSITIONAL SETTING AND PETROGENESIS OF THE <1.71 GA FREEDOM FORMATION, BARABOO, WI

The Freedom Formation (FF) is an unusually young (<1.71 Ga) non-Snowball Earth iron formation preserved in drill core extracted ca. 1905 from the Baraboo Hills, WI. Iron formation, a chemical sedimentary rock type almost exclusive to the Precambrian, is thought to archive the timing and rate of ocean-atmosphere oxygenation. Primary, diagenetic, and metamorphic mineral assemblages of the FF both inform the depositional setting of this unit and provide an important young constraint on Precambrian ocean chemistry. We present quantitative sub-micron scale elemental maps and complimentary geochemistry of 43 samples of the FF and underlying Seeley Slate preserved in recently recovered, rediscovered and re-catalogued historic core, logs, and maps. The FF conformably overlies the Seeley Slate, which is conformable with the underlying Baraboo quartzite. The Baraboo quartzite nonconformably overlies 1749 ± 12 Ma granite and rhyolite and contains a population of 1710 Ma detrital zircons, providing a maximum age constraint on the overlying FF. A lower iron-rich and overlying carbonate-rich unit comprise the FF. Sedimentary structures include mm- to cm-scale planar to wavy bedding, convolute bedding and locally, a ~20cm-thick granule interbed. FF mineralogy includes chamosite, stilpnomolene, quartz, magnetite, hematite, dolomite, and siderite with interbedded detrital quartz and clays. Quantitative high-spatial resolution element maps obtained using the EPMA at low (8) keV document micron-scale reactions between primary reduced iron phases (chamosite) and secondary mixed Fe²⁺-Fe³⁺ minerals (e.g. stilpnomelane) of ranging composition. The presence of primary Fe²⁺ mineral phases implies reducing conditions at or near the time of deposition. Anoxic conditions may have developed within shallow, isolated transtensional (pull-apart) basins, a setting recently suggested for the FF and underlying sedimentary rocks based on regional structure and stratigraphic relationships. Broadly, the presence of mixed Fe²⁺-Fe³⁺ minerals affirms recent work that documents protracted iron-rich marine conditions and highlights the importance of reevaluating primary mineral assemblages of iron formations. Further, this work demonstrates the importance of preserving, archiving, and re-evaluating historical samples and records.