SEA-LEVEL HISTORY OF CAMBRIAN-ORDOVICIAN BOUNDARY INTERVAL STRATA ON THE WISCONSIN ARCH

Cambrian-Ordovician strata on the Wisconsin Arch are a classic reference for shallow-water inner detrital belt and carbonate bank depositional environments. Early studies of the boundary interval described it as both conformable or, to varying degrees, unconformable. This confusion was due to the interbedded siliciclastic and carbonate strata of the lowermost Oneota Formation, and, therefore, the artificial appearance of a conformable and gradual transition from the siliciclastic-dominated Upper Cambrian Jordan Formation to the carbonate-dominated Lower Ordovician Oneota Formation. Subsequently, Smith, Byers, and Dott (1993), using mostly outcrops, interpreted the Jordan-Oneota contact as a sequence boundary and noted a conglomerate of silicified oolitic-grainstone clasts in the boundary interval.

Building on this work, we recognize two meter-scale, unconformity-bound successions that are observed in both recently acquired core and classic outcrops. These are traceable regionally (over 100 km) on the Wisconsin Arch. We place the Cambrian-Ordovician boundary at an unconformity which sedimentological and stratigraphic relationships suggest is significant both in duration and spatial extent. At this contact, >1m deep fractures within the silicified and brecciated Jordan sandstone are infilled with friable quartz sandstone of the overlying Oneota. This unconformity is overlain by interbedded friable quartz sandstone, conodont-bearing green mudstone, and stromatolitic (digitate and domal) and oolitic sandy dolostone. This is capped by a vuggy chert pebble conglomerate (informally termed ‘Bubble Rock’ by Bob Dott) comprised of chalcedony, which is re-worked and overlain by another interbedded succession of sandstone, mudstone, and dolostone. We interpret the silicified and brecciated uppermost Jordan and the chert pebble conglomerate within the lower Oneota to be silcrete features that formed at sequence boundaries. The silcretes and brecciation, along with early diagenetic silicification of oolites and stromatolites, tepee structures, and anhydrite molds all suggest an arid shallow subtidal to subaerial depositional setting. Ongoing work will focus on determining whether the recognized regional unconformities represent a local sea-level signal or eustatic events.