USE OF SPECTRAL TECHNIQUES FOR EVALUATION OF ORBITAL FORCING OF SEDIMENTATION IN THE UPPER CRETACEOUS (TURONIAN-CAMPANIAN) NIOBRARA FORMATION, WESTERN INTERIOR, USA

Hemi-pelagic facies are commonly characterized by decimeter-scale alternations of chalks/limestones and marls/shales and have been interpreted to reflect depositional oscillations related to Milankovitch-scale orbital forcing. Such lithologic oscillations have been attributed to climatic forcing associated with variations in one or more orbital parameter, such as precession (20 kyr), obliquity (40 kyr), and eccentricity (100 kyr). The Fort Hays Limestone Member and intervals of the Smoky Hill Chalk Member of the Niobrara Formation exhibit such decimeter-scale alternation of carbonate-rich and carbonate-poor beds and have also been hypothesized to reflect orbital forcing of sedimentation. Most cyclostratigraphic studies of the Niobrara Formation have relied upon analysis of bedding thicknesses and bundling patterns in relation to time-averaged sedimentation rates. Results typically show 2.5-3 couplets per meter with bundling ratios of 4-6:1. Although consistent with hypothesized precessional forcing of sedimentation and eccentricity modulation of precession (5:1), these results do not unequivocally substantiate orbital forcing of sedimentation in this sequence. Nor do these results provide quantitative assessment of potential cycle interference resulting from obliquity forcing that may contribute to observed variations in bedding frequencies and bundling ratios. Quantitative evaluation of hypothesized orbital frequencies in the Niobrara is possible using a number of spectral techniques, however, published radiometric control in the interval is sparse, increasing the level of uncertainty of analytical results. This study employs some recently developed methods to improve the confidence of spectral estimates in strata with limited time control. These include AM/FM analyses of high-resolution grayscale and geochemical data series generated from cores that penetrated the Niobrara in Colorado and Kansas. Results of these analyses indicate that sedimentary oscillations within the studied intervals of the Niobrara are characterized by periodicities within the range of both eccentricity and precession. Confirmation of the orbital chronometer in the Niobrara is a critical step toward deducing the mechanism by which climatic changes were translated into the sedimentary record.