Macrostratigraphy of the Lacustrine Wilkins Peak Member, Green River Formation, Wyoming, USA

The Wilkins Peak Member of the Eocene Green River Formation consists of littoral, palustrine, evaporitic, profundal, and alluvial sediments that exhibit prominent cyclicity. The Wilkins Peak has been the subject of several recent studies that test for Milankovitch-band periodicity, in part because interbedded tuffs have yielded high-precision 40Ar/39Ar ages. Most cyclostratigraphic studies have focused on proxy data derived from a single location, but eleven high-resolution stratigraphic sections, including measured sections and cores, spread across the basin reveal considerable spatial variability. Here we use a new quantitative approach called macrostratigraphy to characterize the basin-wide spatio-temporal pattern of Wilkins Peak sedimentation. Sediments were divided into three depth-related lithofacies: alluvial, marginal lacustrine, and basinal lacustrine. Five dated tuffs and the upper and lower contacts of the member were used to establish time-equivalent surfaces and to determine the numerical ages of intervening sediments. For this study, eleven stratigraphic sections were sampled at 10 cm intervals to determine the temporal durations of continuous lithofacies packages at each sampling location. Macrostratigraphic results were expressed in three ways: total number of packages, which reflects the area of sedimentation, and rates of stratigraphic initiation and truncation, which reflect area-weighted rates of expansion and truncation of environments. These three time series were calculated for each lithofacies and then subject to fast Fourier transform, Lomb-Scargle normalized periodogram, and evolutionary powerspectrum analysis.

Preliminary results show power at periods from <1 - 1000 k.y. Longer periods (100 – 1000 k.y.) may reflect tectonics or unknown climatic controls. Shorter periods (<1 – 100 k.y.) may represent a mixture of internal and external controls, including short eccentricity and obliquity. Our results indicate that spatial and temporal variability in the Wilkins Peak is considerable and that macrostratigraphy may be a useful tool for incorporating basin-wide patterns of sedimentation into cyclostratigraphic analyses.