ABSTRACT New data and review of classic sections from the Middle and Upper Ordovician North American Midcontinent in the Upper Mississippi Valley provide a refined picture of the age, stable isotope geochemistry, faunal composition, and—ultimately—origin of this epeiric ramp succession. Sequence stratigraphic analysis reveals a series of unconformity-bounded, genetically related facies packages. Shallowing and deepening trends are sometimes difficult to resolve due to a paucity of hydrodynamic indicators, yet unconformity surfaces are well marked by hardgrounds and confirmed by negative C-isotope spikes. Recent conodont biostratigraphy, new U-Pb radioisotopic ages for K-bentonites, and correlation of C-isotope profiles to global trends suggest that the succession spans the Darriwilian to Hirnantian epochs. Focus on Platteville to lower Galena Group strata (Sandbian to early Katian) provides a temporally high-resolution look at the onset and evolution of a long-term (>2 m.y.) positive carbon-isotope excursion, short-term perturbations in that record, and relationship to the preservation and diversity of the enclosed fauna and strata. Major changes in authigenic mineral suites and organic carbon content throughout the Upper Ordovician Upper Mississippi Valley suggest at least three major redox cycles. The combined evidence for globally recognized, positive carbon-isotope excursions coincident with these redox cycles, as well as high-frequency, sea-level fluctuations and successive faunal turnover events, suggests far-field responses to multiple global oceanic anoxic events.

ABSTRACT This one-day trip provides an overview of the hydrostratigraphic attributes of the Platteville aquitard in the Twin Cities Metropolitan area. As a shallowly buried, extensively fractured carbonate rock in an urban setting, vulnerable to contaminants, the Platteville has been the subject of a wide variety of geomechanical and hydrogeologic studies over the past few decades. This work, combined with our own borehole geophysics and outcrop observations, has led to a more comprehensive understanding of the Platteville. The field trip will provide examples of what we have learned from these many different data sources, which collectively lead to a characterization of the Platteville as a complex “hybrid” hydrogeologic unit. Under certain conditions, and from one perspective, it can serve as an important aquitard that limits vertical flow, whereas in other conditions, and from another perspective, it is best considered a karstic aquifer with bedding-plane parallel conduits of very high hydraulic conductivity that permit rapid flow of large volumes of water. One particular focus of the trip will be demonstration of what appears to be predictability in both vertical and bedding-plane fracture patterns that in turn provides some degree of predictability of flow paths in three dimensions. These relationships appear to be operative for the Platteville in other parts of the Upper Midwest where the Platteville is shallowly buried. We will demonstrate that effective management of such complex, karst, “hybrid,” hydrogeologic units requires a sophisticated, nuanced understanding of their heterogeneous behavior.