GRAND CYCLES OF THE NIOBRARA FORMATION: FROM GILBERT TO CHAOS

The rhythmic strata of the Cretaceous Western Interior Basin (WIB) have served as an essential foundation, and testing ground, for the pursuit of deep-time cyclostratigraphy and astrochronology. Indeed, the notion of deep-time astrochronology was first introduced in G.K. Gilbert’s (1895) seminal study of these strata, where he applied the decimeter-scale lithologic rhythms to develop the first pre-Pleistocene astronomical time scale. Subsequent multi-proxy studies and modeling work on the WIB cycles helped to inform our understanding of the complex climatic, depositional, and diagenetic processes that underlie the manifestation of bedding rhythms and their integrity. In addition, the extraordinary abundance of intercalated radioisotopically-dated ashes within the WIB strata allowed the independent confirmation of Milankovitch-forced rhythms, and spurred development of new approaches for integrating astrochronologic and radioisotopic data to derive accurate and precise geologic time scales. This century-long foundation of cyclostratigraphic research, along with recent advances in radioisotopic geochronology and quantitative statistical analysis, provides a remarkable opportunity to use the geologic record of astronomical forcing preserved within the WIB to test the theoretical astronomical solutions themselves. Here, we demonstrate how the ‘grand cycles’ (≥ 1 Myr) of the Niobrara Formation preserve the first unequivocal geologic evidence for a chaotic secular resonance transition, which occurred during the Coniacian (85-87 Ma). This result confirms the chaotic behavior of the Solar System that is predicted by theoretical models, and provides a new constraint for refining insolation solutions, which will result in a more precise and accurate Phanerozoic time scale.