Spectral Analysis of the Lower Eocene Wilkins Peak Member, Green River Formation, Wyoming: Support for Milankovitch Cyclicity

This study is the first to employ spectral analysis to examine meter-scale sedimentary cyclicity in the Wilkins Peak Member of the lower Eocene Green River Formation of Wyoming. Generally regarded as the classic example for orbital forcing of lacustrine sediments at eccentricity and precession time scales, this long-standing interpretation was recently contested, with a much shorter duration (≤ 10 ky) inferred for the dominant cyclicity. Earlier work lacked adequate age control or spectral analysis or both. Our analysis is based upon an evaluation in the frequency domain of oil-yield values from four boreholes, accuracy estimation for suggested orbital interpretations, and comparison to independent geochronology. Cored intervals 266–364 m thick represent a span of 1.2–1.7 m.y., with temporal resolution of ∼ 3–5 ky (∼ 1 m) for oil-yield values. Variations in spectral power with depth within the original records are interpreted to reflect changes in the rate of sediment accumulation. These changes are corrected prior to testing the orbital forcing hypothesis by using two methods: 1) a minimal adjustment (three segments) accounting for the dominant changes of spectral frequency with depth; and 2) correlating the published definitions of precessional cycles in these records to a 21 ky cosine curve. Orbital age models resulting from the two tuning methods are compared to available chronology and the tuned records are tested for the expected spectral peaks from orbitally forced records. We conclude that the dominant cyclicity of the Wilkins Peak Member is orbitally forced. Orbital age models overlap 40Ar/39Ar ages and inferred periods include long and short eccentricity, weak obliquity and precession. Eccentricity is resolved in the analyzed records but the expected ∼ 95 and ∼ 125 ky periods are not resolved, controlling the range of possible tuning periods and the accuracy of orbital age models. Sub-Milankovitch variability exists and can be resolved to a minimum period of ∼ 3–5 ky by the analyzed records. However, it cannot be characterized fully with the available chronology or by the previously calculated mean cycle duration

Eocene Calibration of Geomagnetic Polarity Time Scale Reevaluated: Evidence from the Green River Formation of Wyoming

We reevaluate the Eocene geomagnetic polarity time scale on the basis of single-crystal 40Ar/39Ar ages for air-fall tuffs from the Wilkins Peak Member of the Green River Formation of Wyoming. Tuff 6 is dated as 49.1 ± 0.2 Ma, and tuff 3 is dated as 50.4 ± 0.3 Ma (maximum estimate). When combined with published magnetostratigraphic constraints, these age determinations suggest that the currently accepted age of chron C22r is 1.5–2.5 m.y. too old, which supports a significantly longer duration for the early Eocene, for the early Eocene climatic optimum, and the Wasatchian North American Land Mammal Age

A strategy for cross-calibrating U–Pb chronology and astrochronology of sedimentary sequences: An example from the Green River Formation, Wyoming, USA

Astronomical calibration of the geological timescale has been limited until recently by the precision and accuracy of radioisotopic dates, especially for pre-Neogene records. Uncertainties for radioisotopic dates of older strata were typically much larger than a single precessional cycle, and dates were often sparse, leading to the practice of orbital tuning of cyclic strata in order to astronomically calibrate the desired interval. Ideally, in order to test the assumptions of astronomical calibration with geochronology, it is necessary that the precision of radioisotopic dates be comparable to the period of the cycle being tested. The new U–Pb CA-TIMS (chemical abrasion–thermal ionization mass spectrometry) zircon dates reported here conform to this precision requirement, with 2σ analytical uncertainties from ±11000 to ±52 000 years for seven volcanic ashes from the Wilkins Peak Member of the Green River Formation. The zircon dates have simple distributions with few outliers and allow accurate estimations of the eruption ages with potential inaccuracies of less than precessional cycle. The Eocene Green River Formation (Wyoming, USA) has long been recognized as a record of cyclicly- deposited lacustrine sediments, and the abundant intercalated volcanic ashes make it a suitable place to test new approaches to astronomical calibration of cyclic strata. The abundance of different types of marker beds, including tuffs that are intercalated with the sedimentary cycles, guarantee an unambiguous correlation between sampling locations of dated tuffs on the margins of the basin and the basin center where the cyclicity is best developed, thus reducing any stratigraphic uncertainties to a fraction of (hypothesized) precession cycle. Tuning-based orbital age models, accepted by the previous geochronology, significantly deviate from the new geochronology, whereas a previously rejected model that assumes a short eccentricity period of 125 ky is now allowed. In order to test possible explanations for the apparent 125 ky period, such as changes in orbital periods, or gaps in the sedimentary record, we present an iterative strategy to select future ashes for dating such that the astronomical calibration/testing is optimized. We iteratively contrast two ad-hoc age models that bracket the linear interpolation between the dated ashes. The optimal intervals for further dating are located where the deviations between the models exceed our reported uncertainties. We propose that the iterative approach described here should become the standard for establishing a rigorous orbital calibration of the stratigraphic record where sufficient ashes exist

An assessment of sanidine from the Fire Clay tonstein as a Carboniferous 40Ar/39Ar monitor standard and for inter-method comparison to U-Pb zircon geochronology

Radioisotopic geochronology applied to the high-resolution calibration of Earth history requires a set of syn- thetic and natural reference materials for both 40Ar/39Ar and U-Pb techniques that permit both inter-laboratory and inter-technique comparisons. The sanidine- and zircon-bearing Carboniferous Fire Clay tonstein provides a potential natural Paleozoic reference for these two widely used radioisotopic systems. Here we report results for both radioisotopic systems, examining the suitability of this tonstein as a geochronologic reference. Sanidine crystals from the Fire Clay and co-irradiated monitors from eight irradiation positions were divided into eleven 40Ar/39Ar experiments. Single-grain sanidine 40Ar/39Ar analyses (n = 263) of the simplest 9 experiments have internal 2σ uncertainties at the ± 1 Myr level ( ± 0.3%), with a range of dates between ~315 and ~317 Ma (~1% precision), similar to the observed dispersion in the Fish Canyon sanidine monitor dates. Forty-one U-Pb analyses by the CA-ID-TIMS method on carefully selected single Fire Clay tonstein zircons have produced 206Pb/238U dates with an average 2σ precision of ± 0.23 Myr (0.14%). Our Fire Clay preferred mean 40Ar/39Ar date of 315.36 ± 0.10 Ma ( ± 1.10 Ma: fully propagated 2σ uncertainty, relative to a Fish Canyon age of 28.201 Ma) is consistent with our weighted mean 206Pb/238U zircon date of 314.629 ± 0.039 Ma ( ± 0.35 Ma: fully propagated 2σ uncertainty; n = 27). The good single-crystal reproducibility of the sanidine data and the overall consistency between the two chronometers suggest that the tonstein holds promise as a Paleozoic age reference material

Expedition 381 Summary

The primary objective of International Ocean Discovery Program Expedition 381 was to retrieve a record of early continental rifting and basin evolution from the Corinth rift, central Greece. Continental rifting is fundamental for the formation of ocean basins, and active rift zones are dynamic regions of high geohazard potential. However, the detailed spatial and temporal evolution of a complete rift system needed to understand rift development from the fault to plate scale is poorly resolved. In the active Corinth rift, deformation rates are high, the recent synrift succession is preserved and complete offshore, and earlier rift phases are preserved onshore. Additionally, a dense seismic database provides high-resolution imaging of the fault network and seismic stratigraphy around the basin. As the basin has subsided, its depositional environment has been affected by fluctuating global sea level and its absolute position relative to sea level, and the basin sediments record this changing environment through time. In Corinth, we can therefore achieve an unprecedented precision of timing and spatial complexity of rift-fault system development, rift-controlled drainage system evolution, and basin fill in the first few million years of rift history. The following are the expedition themes: High-resolution fault slip and rift evolution history, Surface processes in active rifts, High-resolution late Quaternary Eastern Mediterranean paleoclimate and paleoenvironment of a developing rift basin, and Geohazard assessment in an active rift. These objectives were and will be accomplished as a result of successful drilling, coring, and logging at three sites in the Gulf of Corinth, which collectively yielded 1645 m of recovered core over a 1905 m cored interval. Together, these cores provide (1) a long rift history (Sites M0078 and M0080), (2) a high-resolution record of the most recent phase of rifting (Site M0079), and (3) the spatial variation of rift evolution (comparison of sites in the central and eastern rift). The sediments contain a rich and complex record of changing sedimentation, sediment and pore water geochemistry, and environmental conditions from micropaleontological assemblages. The preliminary chronology developed by shipboard analyses will be refined and improved during postexpedition research, providing a high-resolution chronostratigraphy down to the orbital timescale for a range of tectonic, sedimentological, and paleoenvironmental studies. This chronology will provide absolute timing of key rift events, rates of fault movement, rift extension and subsidence, and the spatial variations of these parameters. The core data will also allow us to investigate the relative roles of and feedbacks between tectonics, climate, and eustasy in sediment flux, basin evolution, and basin environment. Finally, the Corinth rift boreholes will provide the first long Quaternary record of Mediterranean-type climate in the region. The potential range of scientific applications for this unique data set is very large, encompassing tectonics, sedimentary processes, paleoenvironment, paleoclimate, paleoecology, geochemistry, and geohazards