ABSTRACT Lithologies, depositional environments, stratigraphic architecture, and conodont biostratigraphy of Lower to Middle Mississippian rocks in the western Ozarks comprise five depositional sequences in ramps on the southern Burlington shelf. Aggradational ramps in the Kinderhookian to early Osagean St. Joe group were relatively strongly overprinted by Ouachita-related tectonism involving inferred recurrent passage of fore-bulge highs and associated basins across central and southern parts of the outcrop area. Significant effects of tectonism are southward facies shallowing onto the broad Kanoka ridge paleotopographic high associated with locally extensive marine and lesser subaerial erosion, sediment thickening and deposition of generally northward down-lapping, resedimented wedges with dislodged reef blocks and conglomerates into relatively rapidly subsiding basins, and formation of a regionally extensive paleosol at the top of the group. Back-stepping subsidence due to middle Osagean foundering of the Kanoka ridge was followed by rapid, long-distance progradation of middle- and outer-ramp facies in the Bentonville and Reeds Spring limestones. Tectonism at this time resulted variously in local folding, uplift, marine and subaerial erosion, and reversal of shelf bathymetry. Southward erosion of the Reeds Spring and Bentonville occurred at least in Oklahoma on rejuvenated segments of the Kanoka ridge. Overlying lower Meramecian limestones are shallow-water deposits truncated by a major unconformity.

Abstract Drilling and geophysical data demonstrate that the Mississippi Valley graben, the Rough Creek graben, and the Rome trough are fault-bounded graben structures filled with as much as 27,000 feet of Cambrian sediments. Data including stratigraphic tops from 1,764 wells, 106 seismic profiles, aeromagnetic and gravity surveys, and mapped surface geology at a 1:24,000 scale have been used to study seven stratigraphic packages resolvable in both wells and seismic profiles across parts of Kentucky, Ohio, Indiana, Illinois, Missouri, and Tennessee. Detailed analyses of the thickness patterns of these stratigraphic packages have been used to interpret the locations and timing of movements along major faults systems in the study area. Active rifting of the Precambrian crystalline bedrock began by the Early Cambrian, and resulted in thick, sand-rich deposits of the Reelfoot Arkose in the Mississippi Valley graben and Rough Creek graben, and the Rome Formation in the Rome trough. Subsidence continued in these grabens during the Middle to Late Cambrian, leading to an alternating succession of shales and carbonates being deposited (Eau Claire Formation of the Illinois basin and Conasauga Group of the Appalachian basin) on top of the coarse clastic Reelfoot Arkose and Rome Formation. Although the tectonic extension that formed these features ended by the Late Cambrian, fault zone reactivation during the Taconic, Acadian, and Alleghenian Orogenies altered fault block orientations and produced areas of basin inversion, creating the possibility of numerous deep structural traps for hydrocarbons sourced by the Cambrian shales of the Eau Claire Formation and Conasauga Group.

Abstract The Reelfoot rift is one segment of a late Proterozoic(?) to early Paleozoic intracontinental rift complex in the south-central United States. The rift complex is situated beneath Mesozoic to Cenozoic strata of the Mississippi embayment of southeastern Missouri, northeastern Arkansas, and western Tennessee and Kentucky. The rift portion of the stratigraphic section consists primarily of synrift Cambrian and Ordovician strata, capped by a postrift sag succession of Late Ordovician to Cenozoic age. Potential synrift source rocks have been identified in the Cambrian Elvins Shale. Thermal maturity of Paleozoic strata within the rift ranges from the oil window to the dry gas window. Petroleum generation in Elvins source rocks likely occurred during the middle to late Paleozoic. Upper Cretaceous sedimentary rocks unconformably overlie various Paleozoic units and define the likely upper boundary of the petroleum system. No production has been established in the Reelfoot rift. However, at least nine of 22 exploratory wells have reported petroleum shows, mainly gas shows with some asphalt or solid hydrocarbon residue. Regional seismic profiling shows the presence of two large inversion structures (Blytheville arch and Pascola arch). The Blytheville arch is marked by a core of structurally thickened Elvins Shale, whereas the Pascola arch reflects the structural uplift of a portion of the entire rift basin. Structural uplift and faulting within the Reelfoot rift since the late Paleozoic appear to have disrupted older conventional hydrocarbon traps and likely spilled any potential conventional petroleum accumulations. The remaining potential resources within the Reelfoot rift are likely shale gas accumulations within the Elvins Shale; however, reservoir continuity and porosity as well as pervasive faulting appear to be significant future challenges for explorers and drillers.

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.