ALBERT

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.