An integrated analysis of lateral trends in the faunal and sedimentologic character of meter-scale limestone-mudrock cycles in the Kope Formation of the Cincinnati region (Ohio).

摘要

Extension of faunal marker horizons, thin, well-preserved fossil deposit of regional extent, from outcrops into cores has permitted the construction of a new stratigraphic framework for the Kope Formation (Cincinnatian Series, Upper Ordovician), a mudrock-carbonate unit deposited in southwest Ohio and northern Kentucky in an epeiric sea frequently swept by major storms. This framework at once opens up previously unknown medial and distal Kope facies and achieves a stratigraphic resolution of less than one meter.; The purpose of this study is to present the technique of faunal marker correlation as applied in the middle Kope Formation, and the results of semiquantitative and quantitative analyses carried out on data collected in the context of the new subsurface stratigraphy. These analyses reveal a more complete picture of the Ordovician sea floor in southwest Ohio during Kope time than has been available previously, since they incorporate a large portion of the unit not available to earlier workers. Examination of lateral stratigraphic, faunal, and taphonomic variability in a single "meter-scale cycle," composed of a mudrock and limestone couplet, across a large region shows initial evidence that the fossil-rich Kope limestones may ultimately be the result of siliciclastic sediment starvation of the seafloor. Abundance patterns of three environmentally diagnostic taxa and changes in limestone:shale proportions, as observed in six meter-scale cycles across southwest Ohio, refine the paleogeography of this area by suggesting a non-uniform sea floor paleogradient, with extensive areas having little or no slope and narrow regions having steeper slopes. Finally, multivariate analyses (detrended correspondence analysis, cluster analysis, and two-way cluster analysis) performed on quantitative faunal abundance data from the same six middle Kope cycles confirms the existence of a paleogradient in the region. These results further show that paleoenvironmental conditions were more variable between localities at any one time than they were throughout all of middle Kope time in one place.