Cyclostratigraphy of the Upper Cretaceous Niobrara Formation, Western Interior, U.S.A.: A Coniacian–Santonian orbital timescale

The Turonian–Campanian Niobrara Formation of the Western Interior basin, U.S.A., is characterized by decimeter-scale rhythmic alternations of chalk and marl beds and decameter-scale oscillations of chalky and marly facies. This study applies recent advances in quantitative cyclostratigraphic analysis to high-resolution geophysical records in order to test for an orbital signal. We studied records of the entire formation (~ 85 m) from two wells in Colorado (40° 17′ N, 104° 38′ W; 40° 14′ N, 104° 41′ W). The study utilized high-resolution time series of borehole resistivity measured from Formation MicroImager tools (FMI). Advanced spectral techniques indicate the presence of eccentricity, obliquity, and precession periodicities throughout the study interval. Deviations of bundling ratios in the lower Fort Hays Member from those predicted by simple eccentricity modulation of precession, previously reported in the literature, are likely due to the influence of obliquity. Deconvolution and frequency tracking of orbital components were used to reconstruct sedimentation rates for the Niobrara Formation and thus develop a high-resolution orbital timescale, which permits calculations of accumulation rates for sedimentary components and the duration of inclusive stages, substages, and, in some cases, biozones. The Niobrara Formation in the studied cores appears to be hiatus-free and therefore provides a continuous record of rhythmic deposition for astronomical timescale development. The relationship between carbonate content and sedimentation rate throughout the Niobrara Formation indicates that the decameter-scale oscillations of chalk and marl are dominantly driven by variations in siliciclastic flux (dilution). ration, derived from the orbital timescale, between the Turonian–Coniacian and Coniacian–Santonian boundaries is 3.40 ± 0.13 myr, which is generally consistent with radiometric estimates for the Coniacian. The orbitally derived duration of the Santonian Stage is 2.39 ± 0.15 myr, which is very similar to estimates based on curve fitting between radiometric dates. Development of high-resolution orbital chronometers provides excellent time-resolution in intervals that lack radiometric-time control or where control points are widely spaced.