ALBERT

Description: The development of integrated astronomical and radioisotopic time scales from rhythmic strata of the Western Interior Basin (WIB) has played a fundamental role in the refinement of Late Cretaceous chronostratigraphy. In this study, X-ray fluorescence (XRF) core scanning is utilized to develop a new elemental data set for cyclostratigraphic investigation of Cenomanian-Turonian strata in the WIB, using material from the Aristocrat-Angus-12-8 core (north-central Colorado). The XRF data set yields the first continuous 5-mm-resolution analysis of lithogenic, biogenic, and syngenetic-authigenic proxies through the uppermost Lincoln Limestone Member, the Hartland Shale Member, and the Bridge Creek Limestone Member, including oceanic anoxic event 2 (OAE 2). The 40 Ar/ 39 Ar ages from ashes in three biozones, including a new age from the Dunveganoceras pondi biozone (uppermost Lincoln Limestone Member), provide geochronologic constraints for the cyclostratigraphic analysis. Astrochronologic testing of the 5-mm-resolution XRF weight percent CaCO 3 data via average spectral misfit analysis yields strong evidence for astronomical influence on climate and sedimentation. Results from the Bridge Creek Limestone Member are consistent with the previously published astrochronology from the U.S. Geological Survey #1 Portland core (central Colorado), and identification of an astronomical signal in the underlying Hartland Shale Member now permits extension of the WIB astrochronology into the earlier Cenomanian, prior to OAE 2. High rates of sedimentation in the Angus core during the interval of OAE 2 initiation, as compared to the Portland core, allow recognition of a strong precessional control on bedding development. As a consequence, the new results provide a rare high-resolution chronometer for the onset of OAE 2, and the timing of proposed hydrothermal trace metal enrichment as observed in the 5 mm XRF data.

Description: This study revises and improves the chronostratigraphic framework for late Turonian through early Campanian time based on work in the Western Interior U.S. and introduces new methods to better quantify uncertainties associated with the development of such time scales. Building on the unique attributes of the Western Interior Basin, which contains abundant volcanic ash beds and rhythmic strata interpreted to record orbital cycles, we integrate new radioisotopic data of improved accuracy with a recently published astrochronologic framework for the Niobrara Formation. New 40 Ar/ 39 Ar laser fusion ages corresponding to eight different ammonite biozones are determined by analysis of legacy samples, as well as newly collected material. These results are complemented by new U-Pb (zircon) chemical abrasion–isotope dilution–thermal ionization mass spectrometry ages from four biozones in the study interval. When combined with published radioisotopic data from the Cenomanian-Turonian boundary, paired 206 Pb/ 238 U and 40 Ar/ 39 Ar ages spanning Cenomanian to Campanian time support an astronomically calibrated Fish Canyon sanidine standard age of 28.201 Ma. Stage boundary ages are estimated via integration of new radioisotopic data with the floating astrochronology for the Niobrara Formation. The ages are determined by anchoring the long eccentricity bandpass from spectral analysis of the Niobrara Formation to radioisotopic ages with the lowest uncertainty proximal to the boundary, and adding or subtracting time by parsing the 405 k.y. cycles. The new stage boundary age determinations are: 89.75 ± 0.38 Ma for the Turonian-Coniacian, 86.49 ± 0.44 Ma for the Coniacian-Santonian, and 84.19 ± 0.38 Ma for the Santonian-Campanian boundary. The 2 uncertainties for these estimates include systematic contributions from the radioisotopic measurements, astrochronologic methods, and geologic uncertainties (related to stratigraphic correlation and the presence of hiatuses). The latter geologic uncertainties have not been directly addressed in prior time scale studies and their determination was made possible by critical biostratigraphic observations. Each methodological approach employed in this study—new radioisotopic analysis, stratigraphic correlation, astrochronology, and ammonite and inoceramid biostratigraphy—was critical for achieving the final result.

Description: The fluviolacustrine Wilkins Peak Member of the Eocene Green River Formation preserves repetitive sedimentary facies that have been interpreted as an orbitally induced climate signal. However, previous quantitative studies of cyclicity in this member have used oil-yield data derived from single locations. Here, macrostratigraphy is used to quantitatively describe the spatiotemporal patterns of three different lithofacies associations from 8 to 12 localities that span much of the basin. Macrostratigraphic time series demonstrate that there is a reciprocal basin-scale relationship between carbonate-rich lacustrine facies and siliciclastic-rich alluvial facies. Spectral analyses identify statistically significant periods (≥90% confidence level) in basin-scale sedimentation that are consistent with Milankovitch-predicted orbital periodicities, with a particularly strong ~100 k.y. cycle expressed in all lithofacies associations. Numerous non-Milankovitch periods are also recognized, indicating complex depositional responses to orbital forcing, autocyclic controls on sedimentation, or harmonic artifacts. Although fluctuations in Lake Gosiute water level did affect basin-scale patterns of sedimentation, they are not directly related to the 100 k.y. short-eccentricity cycle, as previously supposed. Instead, 100 k.y. cycles are principally recorded by the recurrence of alluvial environments, which exerted a dominant control on basin-scale patterns of sedimentation generally. Thus, the hydrologic controls on lake level that have been classically linked to short-eccentricity actually occurred at finer temporal scales (〈100 k.y.). Understanding the complex links between orbital forcing and sedimentation in the Wilkins Peak Member is facilitated by analysis of time series that reflect spatial as well as temporal variability in stratigraphic data. Macrostratigraphy is, therefore, promising as an analytical tool for basin-scale cyclostratigraphy.