ECCENTRICITY-DRIVEN CARBONATE-SILICICLASTIC ALTERNATION WITHIN THE EARLY EOCENE WILKINS PEAK MEMBER OF THE GREEN RIVER FORMATION IN WYOMING

The Wilkins Peak Member of the Green River Formation in Wyoming exhibits prominent basin-wide eccentricity-scale alternations between carbonate-rich lacustrine facies and siliciclastic alluvial facies. Lacustrine facies can be further subdivided into highstand and lowstand end members that together define 0.1-6.0 m lake expansion-contraction cycles. Highstand facies include laterally-extensive, kerogen-rich micritic carbonate deposited in a relatively deep lake. Lowstand facies include displacive and bedded evaporite in the central Bridger subbasin, and mudcracked micritic carbonate and marl that accumulated in a shallow lake and on fringing mudflats and marshy areas. Dominantly alluvial facies interrupt the carbonate-rich lacustrine facies at intervals of approximately 15-30 meters, recording episodic first-order shifts to siliciclastic deposition. Abundant tractive and pedogenic structures within alluvial intervals indicate deposition within a broad network of generally west-directed fluvial channels and crevasse splays onto vegetated, bioturbated floodplains. Changes in δ¹³C values preserved in micrite generally parallel first-order facies shifts, ranging from a high of about 5 per mil (PDB) in lacustrine intervals to a low of about 0 per mil in alluvial intervals.

We infer that carbonate-rich lacustrine facies were deposited during maxima in 100 ky eccentricity, during which evaporation and monsoonal precipitation fluctuated markedly in response to enhanced insolation variation. Lakes expanded during wet periods, which also promoted high rates of chemical weathering and vegetative stabilization of uplands and floodplains. Clastic sediment was stored within upland soil profiles or trapped near highstand lake margins. Dry periods in contrast caused lakes to shrink and solutes to become concentrated, but reduced runoff hindered the capacity of rivers to transport clastic sediment. During eccentricity minima, which appear to have coincided with low d¹³C Early Eocene hyperthermals, precipitation was generally inadequate to substantially raise lake levels. It was however very effective at transporting sediment basinward, which we attribute to a combination of aridity-induced vegetative overturn and rare, but large-magnitude monsoon events that mobilized clastic sediment.