NEW INSIGHTS INTO THE CAMBRIAN-ORDOVICIAN BOUNDARY INTERVAL SEA-LEVEL HISTORY ON THE WISCONSIN ARCH

Cambrian-Ordovician strata of the North American Upper Mississippi River Valley (UMV) are a classic reference succession for shallow-water, inner detrital belt and carbonate bank depositional environments. However, given the paucity and poor preservation of biostratigraphically useful fossils near the boundary in this region, relatively few high-resolution chronostratigraphic studies have been undertaken. Previous studies focused primarily on sedimentology and lithostratigraphy resulted in the boundary interval described as either conformable or (to varying degrees) unconformable. This is in part due to the interbedded siliciclastic and carbonate succession of the lowest member of the Oneota Formation, and, therefore, the artificial appearance of a conformable and gradual transition from siliciclastic-dominated Upper Cambrian Jordan Formation to carbonate-dominated Lower Ordovician Oneota Formation deposits. We integrate litho-, bio-, sequence-, and chemostratigraphic data from outcrop and core to reconstruct the sea level history at the Cambrian-Ordovician boundary on a transect in central Wisconsin arrayed slightly oblique to depositional strike, alongside and onto the positive-paleorelief Wisconsin Arch. In contrast to previous studies, we focus on the shallowest marine depositional environments of the UMV. Trilobites have not been recovered from these strata, but preliminary conodont data suggests that the ~10m thick study interval spans the Eoconodontus through Cordylodus angulatus zones; it is not clear if intervening zones are present. Silcrete-formation, chert pebble conglomerates, and karst features associated with subaerial exposure are in some cases sharply overlain by stromatolitic (digitate and domal) hardgrounds indicating marine floodings. We recognize numerous decimeter to meter-scale sedimentary packages bound by unconformities, some of which can be traced regionally up to 160 km. We tentatively place the Cambrian-Ordovician boundary at the lowest of these regional unconformities pending additional biostratigraphic data. Ongoing work will focus on determining whether the recognized regional unconformities represent a local sea-level signal, or are related to the Red Tops, Lange Range, Acerocare, and Black Mountain eustatic events.