Multiple Milankovitch Cycles in the Bridge Creek Limestone (cenomanian-turonian), Western Interior Basin

Spectral analyses of complementary lithologic and paleoecologic data sets from the Cenomanian-Turonian Bridge Creek Limestone are used to test for Milankovitch periodicities. Because the analyses quantitatively assess variance in the data sets through the study interval, they also offer a new method for evaluating relationships between different components of the depositional system. The analysis was made possible by highresolution sampling for geochemistry, ichnology, and microfossils from a complete section of the Bridge Creek Limestone Member in the USGS #1 Portland core, coupled with a detailed chronostratigraphic framework established for the interval in a recently published compilation ofnew radiometric dates and biozones. The analyzed data included weight percent carbonate (% CaCO3) and organic carbon (% OC), grayscale pixel values, ichnologic measures such as maximum burrow diameter and ichnocoenosis rank, and relative abundance values for selected nannofossil taxa. The lithologic parameters produced significant spectral responses for all three major orbital cycles (eccentricity, obliquity, and precession) in the upper 6 meters of the study interval. Spectra for ichnologic data are similar to those for % OC, possibly because both of these variables are dominantly controlled by benthic redox conditions. Spectra for some nannofossil taxa show results similar to % OC or % CaCO3 but are less definitive due to preservational effects. A new model to explain the Bridge Creek cycles is developed based on the spectral results. The model combines dilution and productivity mechanisms by suggesting that obliquity predominantly forces dilution through its effect on high-latitude precipitation, whereas precession predominantly forces carbonate productivity through its effect on evaporation and nutrient upvvelling in the Tethyan realm to the south. The two influences mix in the shallow Western Interior epicontinental basin, where they result in constructive and destructive interference (because of the different frequencies of obliquity and precession) to produce the complex bedding pattern observed in the Bridge Creek Limestone.