EOCENE CYCLOSTRATIGRAPHY AND ASTROCHRONOLOGY: THE GREEN RIVER FORMATION (BRIDGER BASIN, WYOMING)

The Eocene Green River Formation, deposited in intermontane basins of the Rocky Mountains, displays rhythmic alternations of lacustrine sapropelic oil shales with playa and floodplain deposits. Early attribution of the rhythmic bedding in the Wilkins Peak Member to precessional forcing [1-3] was later corroborated by radioisotope dating with ages from 51.6 Ma to 49.7 Ma [4-7], and objective testing for Milankovitch cycles [8]. In-depth analysis of an oil-yield time series from the Blacks Fork core reveals a precessional pattern with modulations matching the La2004 precession index [9], interpreted as follows. In the winter perihelion phase of the precession cycle, high precipitation and low evaporation and sedimentation rates allowed the water table to flood the basin, leading to highstands. In the summer perihelion phase, high evaporation and high sedimentation rates buried the water table, leading to lowstands. This is consistent with a precession-forced climate model for Eocene North America [10]. Carbonate strontium isotopes [11] indicate less radiogenic strontium in highstand facies; surplus precipitation from the Pacific Ocean contributed to annual snowpack to the west of the basin, reacting with less radiogenic marine carbonate before entering the lake. More radiogenic strontium occurs in lowstand facies; precipitation from the Gulf of Mexico associated with summer monsoons reacted with highly radiogenic Precambrian rocks to the east of the basin before entering the lake. In the mid-portion of the member, excessive alluviation appears to have bridged from one cycle to the next without recording a sapropel, leading to a discrepancy between the record of precession and orbital eccentricity. References: [1] W.H. Bradley, USGS Prof Paper 158E, 110, 1929 [2] A.G. Fischer, A.G. and Roberts, L.T., J Sed Pet 61, 1146-1154, 1991 [3] H.W. Roehler, USGS Prof Paper 1506-F, F1-F74, 1993 [4] M.E. Smith et al., Geol Soc Am Bull 115, 549-565, 2003 [5] M. Machlus et al., Earth Planet Sci Lett 268, 64–75, 2008 [6] M.E. Smith et al., Geology 38, 527-530, 2010 [7] M. Machlus et al., Earth Planet Sci Lett 413, 70–78, 2015 [8] S.R. Meyers, Geology, 36, 319–322, 2008 [9] J. Laskar et al., Astron Astrophys 428, 261, 2004 [10] K.T. Lawrence et al., Spec Pap Geol Soc Am 369, 65-77, 2003 [11] M. Baddouh et al., Earth Planet Sci Lett 448, 62-68, 2016