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 The Middle to Upper Ordovician Decorah and Galena Formations in Wisconsin are composed of stratigraphically condensed sequences(net accumulation rate : 0.35--0.9 cm/k .y., characterized by extreme lateral cont inuity of facie s and dominated by fine-grained carbonate with numer ousomission surfaces. Depositional sequences are defined by relatively conformable successions bounded by identified or interpreted exposure surfaces across which siliciclastics and skeletal grains increase abruptly. Depositional sequences are subdivided in a nested hierarchy into cycle sets,cycles, and omission-surface-bounded packages. The lower sequence, sequence D, corresponds to the Decorah Formation and has a siliciclastic-rich fining-upward succession followed by a coarsening-upward carbonate succession. The phosphate-enriched condensed section above the fining-upward succession represents maximum transgression .Cycles in seq uence D are represented by the upward increases in ske letal-grain and tempestite abundance. The middle sequence, sequence G 1, corresponds to the Galena Formation (lower to middle Dunleith Member) and shows an overall transition from a shale- and skeletal-grain-rich base to a carbonate-mud-dominated top. Both shale content and the abundance of skel etal-grain-supported rocks increase northward in eastern Wisconsin and westward in southern Wisconsin toward the inferred paleoshore line. The upper sequence, sequence G2, corresponds to the Galena Formation (upper part of Dunleith, Wise Lake, and Dubuque Members) and consists of a skeletal-grain-rich base with minor shale content grading upward into mudstones and into a skeletal-grain-rich top. Cycle sets and cycles in the sequences G I and G2 are characterized by relatively sharp boundaries across which there is an abrupt increase in shale content and skeletal-grain-rich facies grading upward to a carbonate-mud-dominated facies. The asymmetry of cycles-represented by the abrupt increase followed by gradual decrease in shale content and skeletal grains-is incon sistent with progradation of high-energy shale-rich facies over low-energy mudstone. This facies stacking pattern is better explained by reciprocal sedimentation in which skeletal-grain-rich shale deposition and carbonate-mud deposition are disassociated from each other both in time and space. The omission-surface-bounded pack ages form the smallest recognized sedimentary units within the sequences Gland G2 and represent alternations of periods of sediment deposition and starvation. The Decorah and Galena Formations are dominated by mudstone-rich lithofacies and abundant omission surfaces, and their contained fauna consists predominantly of heterotrophs. These characteristics are in contrast with tho se of the Lower Ordovician and Silurian "Bahama-like" carbonate strata. Apparently, in addition to tectonics, antecedent topography, and changes in eu stasy, siliciclastic influx, and storm transport, the paleoclimatic and paleoceanographic conditions in the Late Ordovician epeiric sea had important controls on depositional models, sequence development, and facies stacking patterns.

Abstract Gutenberg is a picturesque community nestled in the Mississippi River Valley, where local relief approaches 400 ft (120 m). Lock and Dam No. 10 at Gutenbergis a major conduit for upper Mississippi barge traffic. Visitors can view the lock in operation at close range from an observation platform. Excellent roadcuts and many additional exposures of Middle Ordovician strata are easily accessible in the Gutenberg area, where virtually every bed is available in continuous fresh exposures over a stratigraphic interval of more than 250 ft (75 m).Three reference sections (Fig. 1B) enable the sequence of Ordovician formations (St. Peter, Glenwood, Platteville, Decorah, Dunleith, Wise Lake) to be composite for general discussion (Fig. 2) :1) Gutenberg South Roadcut, US. 52 (SW¼Sec.29, T.92N., R.2W.), a superb continuous section that offers a spectacular view of the Mississippi River (Delgado, 1983); 2) Gutenberg North Roadcut, U.S. 52 (SW¼SW¼Sec.5 and NW¼Sec.8, T.92N., R.2W.), a well-studied but now partly overgrown sequence (Templeton and Willman, 1963; Bakush, 1985); and 3) X56 Roadcut, Great River Road (NW¼NW¼Sec.32 and SW¼SW¼Sec.29, T.93N., R2W.), a similar sequence to Gutenberg North, but better exposed by recent road construction. Excellent exposures of Lower and Middle Ordovician rocks are also accessible at Clayton (especially private barge terminal road) and the Pikes Peak State Park area (Fig. 1A). Good exposures of Jordan Sandstone (Cambrian) can be seen along the river from McGregor north to Effigy Mounds (Fig. 1A). Ordovician stratigraphic terminology in