Salinity Gradients in Eocene Fossil Lake (Green River Formation)
H. Paul Buchheim
Eocene Fossil Lake exhibited an unusual pattern of salinity during significant periods of its history. Lake margins were fresh compared with more saline alkaline, and at times hypersaline lake centers. Abundant evidence of a salinity gradient is suggested by lateral variations in carbonate and tuff bed mineralogy, evaporite content, oxygen isotope values and paleontology.
Laminated dolomicrites containing calcite psuedomorphs after saline minerals (sodium carbonates) grade shore-ward into either bioturbated or laminated calcimicrite. Dolomite is evidence of hypersaline-alkaline waters, while calcite shows fresher waters.
Tuff beds composed of authigenic potassium feldspar at lake center grade shore-ward into analcime and clay in that order. This pattern is evidence of a geochemical gradient of pore water within a single tuff bed, inherited from the overlying water column or produced via evaporative pumping and capillary draw on exposed lake mudflats.
Stable oxygen isotope (^dgr18O or 18O/16O ratio) compositions of carbonates become more negative shoreward within a single time synchronous bed showing fresher water conditions toward the lake margins. And finally, the presence of fossil fish in near-shore laminated calcimicrite and absence in lake center dolomite facies is evidence of a saline-fresh water transition. Fossil fish are rarely found in dolomite in Fossil Lake, but occur frequently in calcimicrite.
Gradual lake-bottom gradients within a closed hydrographic basin and low but perennial fresh water inflow probably set up the physical conditions neccesary to allow the development of a fresh water apron at the lake margins, resulting in the unique facies transistions.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California