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Paleoenvironmental Constraints Controlling Deposition of Oil Shale in Eocene Fossil Lake, Wyoming

H. Paul Buchheim, Hans P. Eugster

The shoreward increase in sedimentation of precipitated calcite with its consequent dilution of organics was the primary paleoenvironmental constraint controlling the deposition and richness of oil shales in Eocene Fossil Lake (Fossil Butte Member of the Green River Formation).

Kerogen-rich laminated calcimicrite (oil shale) was deposited as a result of a continuous rain of organic matter that was periodically interrupted by calcite precipitation. Precipitation was in response to the inflow of calcium-rich waters that also transported varying amounts of detrital mud-flat dolomite and fluvial siliciclastics. Organic input significantly exceeded sediment deposition, resulting in dark reducing sediments.

Kerogen-poor laminated calcimicrite formed midway toward the basin margins because higher sedimentation rates closer to inflow areas diluted the organic matter. The higher sedimentation also resulted in less reducing conditions, thus allowing bioturbation. Massive or bedded calcimicrites were deposited near the lake margins where organic input was most diluted and bioturbators were uninhibited.

Lateral variations in sedimentation rates seem to be a primary constraint on oil shale deposition. Evidence for this constraint includes the shoreward decrease in kerogen content matched by a shoreward thickening of micrite between tuff beds (time markers), a shoreward increase in bioturbation structures, a shoreward increase in siliciclastics, and an increase in probable eolian quartz in the richest oil shales.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.