STRATIGRAPHIC REVISIONS TO THE PROTEROZOIC BARABOO INTERVAL IN THE BARABOO HILLS, WISCONSIN, AND ITS IMPACT ON TECTONIC HISTORY

New 1:24,000-scale geologic mapping of Proterozoic Baraboo interval rocks across the Baraboo Hills of south-central Wisconsin requires three revisions to the Precambrian stratigraphic section, each of which has implications for the regional tectonic history. First, the Baraboo Quartzite now contains four informal members: a lower conglomerate and quartzite member, a lower quartzite member, an upper conglomerate and quartzite member, and an upper quartzite and phyllite member. Contacts between these new informal members act as marker horizons within the previously undivided Baraboo Quartzite, and allow thrust faults to be mapped for the first time. Folds in the western Baraboo Hills are linked to the mapped thrusts, and both are part of a south vergent fold-thrust belt. Second, we re-establish the Dake Quartzite and Rowley Creek Slate at the top of the Baraboo interval section in the Baraboo Hills area. Consistent with new geologic map relations, detrital zircon analyses document maximum depositional ages of 1714 ± 17 Ma for the Baraboo Quartzite and 1630 ± 9 Ma for the younger Dake Quartzite. Mapping also shows the base of the Dake Quartzite was deposited over an angular unconformity with an unknown amount of missing time, supporting two separate quartzite intervals. Third, regional mapping and overlapping detrital zircon ages indicate the Dake Quartzite is correlative with the base of the Waterloo Quartzite, a Baraboo interval quartzite that outcrops about 75 km to the southeast of the Baraboo Hills. Previous work assumed the Waterloo Quartzite to be correlative with the older Baraboo Quartzite. Our revised stratigraphic correlation indicates deposition of the Baraboo Quartzite could have pre-dated the 1650-1630 Ma Mazatzal Orogeny, allowing deformation in the Baraboo Quartzite to occur during both the Mazatzal Orogeny and the Baraboo/Picuris Orogeny.