DIRECT MEASUREMENT OF DEPOSITIONAL CYCLICITY USING 40AR/39AR DATING: A 430 THOUSAND YEAR RECORD FROM EOCENE LAKE GOSIUTE

One of the most recognizable features of lacustrine deposits is the repetitive succession of facies that record lake expansion and contraction. Expansion-contraction cycles within the Green River Formation have been attributed to climate change at orbital timescales. Our study provides a direct test of precessional forcing of climate on the middle portion of the Wilkins Peak Member using ⁴⁰Ar/³⁹Ar dating. The Wilkins Peak Member provides an optimal setting to record even the most subtle variations in basin hydrology, because lake level is not strongly governed by sill levels.

Cycles in the Wilkins Peak Member are asymmetrical, with a thin transgressive facies succession overlain by a thicker regressive succession, and typically range from 0.1 to 2.0 m in total thickness. Cycles are defined by 6 lithofacies, that we have related to specific stages of flooding and desiccation, and are correlated along a 50 km basin to margin transect at an unprecedented level of sedimentologic detail. The number of expansion-contraction cycles recorded between two regionally correlated time markers, the Grey and Main tuffs, increases southward from 13 to 42 within our cross-section, reflecting the interplay between varying magnitudes of lake expansion and a south-dipping depositional gradient. Cycles terminate due to gradual facies transitions from discrete transgressive and profundal intervals into undifferentiated lake margin facies. At the 95% confidence level, ⁴⁰Ar/³⁹Ar age determinations for the Grey (50.39 ± 0.13 Ma) and Main (49.96 ± 0.65 Ma) tuffs constrain the duration of the enclosed interval of the middle Wilkins Peak Member to 430 ± 150 kyr. Therefore, the apparent average cycle duration ranges from 33.1 ± 11.5 to 10.2 ± 3.6 kyr, depending on position in the basin. Our estimate of 10.2 ± 3.6 kyr represents the maximum average duration for the most strongly expressed cycles near the basin depocenter. This study indicates that these cycles can not be directly correlated to 19-23 ky precessional periods. However, this result does not exclude the possibility that precession or other orbitally induced periodicity might be expressed in the overall stacking of these beds.