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Effects of climate, tectonics, and sea-level changes on rhythmic bedding patterns in the Niobrara Formation (Upper Cretaceous), U.S. Western Interior

¹ Department of Geology, Indiana University, Bloomington, Indiana 47405
² AZTeL, P.O. Box 2563, Bloomington, Indiana 47402

Regional correlation of interbedded pelagic/hemipelagic limestones and calcareous shales of the Fort Hays Limestone Member of the Niobrara Formation confirms development of at least two levels of cyclicity. Occurrence of time-parallel rhythmically deposited beds that can be correlated over distances exceeding 800 km lends strong support to the hypothesis that these cycles were induced by Milankovitch-type climatic forcing. However, visual observation and Fourier analysis of variations in shale and limestone thickness demonstrate that the pattern of Fort Hays cyclicity lacks regional uniformity. Complications in the cyclic pattern may have resulted from the influence of orbital parameters having differing periodicities. Additionally, departures from simple cyclical patterns resulted from sedimentary effects of the Sevier orogeny and from erosional events associated with sea-level changes. Documentation of thickness variations within the regionally persistent Fort Hays bedding sequence furnishes a basis for fine-scale analysis of Cretaceous crustal movements within the Western Interior seaway.