HIGH-RESOLUTION PHYSICAL STRATIGRAPHY OF THE LACUSTRINE AND EVAPORITIC GREEN RIVER FORMATION, WYOMING: IMPLICATIONS FOR CORRELATION, CYCLICITY AND PALEOENVIRONMENTS

The transition between the Wilkins Peak and Laney members of the Green River Formation (Eocene) of Wyoming records a change from evaporitic to fresh-water lacustrine deposition. An interpretation of the physical stratigraphy of this interval, based on abundant ash layers and laterally persistent turbidites 2 mm to 30 cm thick, leads to a revision of the correlation between the Washakie and the Green River basins and to new perspectives on the paleoenvironmental reconstruction and cyclicity.

The facies distribution between each pair of consecutive time markers is used to reconstruct depositional environments. In both members, the shallowest-water facies consists, among others, of stromatolitic carbonate and micrite with desiccation cracks and bird footprints. The deepest-water facies is composed of microlaminated, black, gray-weathering oil shale, with abundant fish fossils. In the evaporitic Wilkins Peak Member, an intermediate-depth facies is laminated micrite and platy limestone; in the comparatively fresh-water Laney Member, this paleoenvironment is represented by laminated to thickly laminated micrite.

Vertical facies changes in the Laney are abrupt, so that in the deeper parts of the basin it is composed mainly of alternating oil shales and stromatolitic carbonate, with no obvious cyclic pattern. In contrast, the trona-rich Wilkins Peak Member is strongly cyclic (based on spectral analysis). High relief deltas prograded mostly from the west and the north during deposition of the Laney. These deltas are associated with large-scale soft sediment deformation, including the development of tight folds, overturned bedding and, in distal locations, the brecciation of oil shale.

Based on this new physical stratigraphy and the distribution of water-depth-sensitive facies, the transition from saline, evaporite-depositing lake to a fresh-water lake requires climate change, and can not be attributed to tectonics alone. The facies distribution and arrangement of deltaic sandstone bodies in the Laney suggest deposition in a deep-water, chemically stratified lake. A fluctuating but mostly shallow lake with low-relief deltas is suggested for the evaporitic phase of the Wilkins Peak Member.