INFLUENCE OF LAKE BASIN MORPHOLOGY ON CLIMATE-SEDIMENT TRANSFER FUNCTIONS: EARLY EOCENE WILKINS PEAK MEMBER, GREEN RIVER FORMATION, WYOMING

Lacustrine strata are among the most detailed and integrative of terrestrial paleoclimate archives. It has long been recognized, however, that not all lake systems are equally sensitive to such forcing due to differences in the processes by which climate signals are transferred into their sedimentary records. This transfer function plays a critical role in filtering which signals are preserved, but has received little attention in deep time lacustrine systems. We use high-resolution X-ray fluorescence (XRF) core scanning of the Wilkins Peak Member (WPM) to address this question.

Our analysis identifies a previously unrecognized shift in the frequency distribution of Milankovitch-band variance in temporally-calibrated Si, S, K, Ca, and Fe between the lower- and upper-WPM. Spectral analysis of the lower-WPM shows power and amplitude concentrated in frequencies associated with short eccentricity, obliquity, precession, and sub-Milankovitch cyclicity. In the upper-WPM, power and amplitude are concentrated in frequencies associated with long and short eccentricity, with reduced power at higher frequencies. This transition coincides with a dramatic decline in the number and volume of bedded evaporite.

We attribute this altered transfer function to a change in the morphology of the Bridger basin, which directly influenced the preservation of bedded evaporite. Initially, a structural foredeep near the southern basin margin accommodated a relatively deep, meromictic lake with a well-defined thermocline. The high degree of lithologic contrast afforded by evaporite beds in alternation with carbonate mudstone and siliciclastic mudstone, siltstone, and sandstone results in enhanced spectral power at higher frequencies (obliquity, precession, sub-precession). As the depocenter became broader and flatter, however, the lake evolved to be shallower and holomictic. Shallow lake waters resulted in the dissolution and recycling of bedded evaporite and the preferential growth of displacive phases beneath the sediment-water interface instead. The loss of bedded evaporite, combined with obscuration of other primary lithologies by secondary evaporite growth, results in reduced obliquity and precession band power, and enhanced eccentricity band power.