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Cyclicity in the Green River Formation (Lacustrine Eocene) of Wyoming

1991; Society for Sedimentary Geology; Volume: Vol. 61; Linguagem: Inglês

10.1306/d4267852-2b26-11d7-8648000102c1865d

1938-3681

Alfred G. Fischer, Lillian T. Roberts,

Paleontology and Stratigraphy of Fossils

ABSTRACT Basinal facies of the Green River Formation have two main modes, lacustrine and playa. The lacustrine mode (Tipton and Laney members) accumulated mainly varved oil shale. Here annual cycles are recorded as varves. Variations in verve thickness demonstrate El Nino (ENSO)-type and sunspot cycles (Ripepe et al., this volume). Milankovitch-scale cycles are not obvious in lithic variations, but gamma ray logs record 1) precessional variations with a mean period (varve-timed) of 19.5 ka, and 2) a bundling of these in the ca. 100 ka eccentricity cycle. In the playa mode (Wilkins Peak Member), the lithic succession oil shale-trona-dolomitic marlstone records the precessional drying up of a lake and is again bundled in sets of 5, by the 100 ka eccentricity rhythm. The Tipton Member persisted f r 450 ka, the Wilkins Peak Member for ca. 1 Ma.