The Climate of Eocene Fossil Lake (Green River Formation, Wyoming) as Determined from Vertical and Lateral Facies Trends

H. P. Buchheim¹ and M. A. Loewen^2
1Laboratory of Limnogeology, Department of Natural Sciences, Loma Linda University, Loma Linda, CA
²Utah Museum of Natural History, Department of Geology and Geophysics, University of Utah, Salt Lake City, UT

The Green River Formation of Fossil Basin contains a varied sequence of carbonates that provide insights into climatic changes through time. While a wetter climate and fresh-water conditions dominated most of its early history, Fossil Lake exhibited a complicated climate pattern (alternating wet and arid conditions) during significant periods of its late history. However, the climate became increasingly arid through time, culminating in an extended evaporitic stage. Fresher conditions at the lake margins, due to localized fresh-water inflow, provided a sanctuary where fish survived until wetter climate periods resumed.

The Green River Formation in Fossil Basin is informally divided into the lower, middle, and upper units, each representing a significantly different climatic phase of Fossil Lake. Siliciclastics, bioturbated micrites, and kerogen-poor calcimicrites containing fossil fish dominate the lower unit. The wet climate and high precipitationevaporation ratio resulted in a fresh-water lake in an over-filled lake basin.

The middle unit is composed of kerogen-rich laminated-micrite and contains abundant fossil fish. Fossil Lake was relatively fresh during middle unit time, but experienced some periods of dryer climate as indicated by interbeds of dolomicrite.

The vertical trends in the dolomite-dominated upper unit are complicated by lateral facies trends that indicate a lateral salinitygradient. The facies trends can be identified by calcite-dolomite ratios, stable-isotope ratios, TOC, evaporite content, and paleontologic variations within single time-synchronous beds. In some beds, laminated dolomicrites and evaporites grade shoreward into laminated and bioturbated calcimicrite containing fossil fish. Carbonate d¹⁸O values decrease shoreward. These features clearly demonstrate a shoreward freshening within Fossil Lake. This demonstrates that both the lake-center and shore facies must be studied in order to develop a complete climate model.

AAPG Search and Discovery Article #90904©2001 AAPG Pacific Section Meeting, Universal City, California