Stable oxygen isotope ratios and calcite-dolomite ratios were determined for 40 samples of various lacustrine carbonate rocks from the Green River Formation (Eocene) of the eastern Uinta basin and the central Piceance Creek basin. Petrographic studies indicate there are five main carbonate rock types: (1) micrite and dolomicrite; (2) algal biolithite; (3) oolite and pisolite; (4) structureless microcrystalline carbonate aggregate (pellet); and (5) kerogen-rich dolomitic claystone (oil shale). The respective ^dgr O¹⁸_PDB isotopic ranges in per mille for these rocks are: -3.27 to -15.85; -2.43 to -7.19; +2.73 to -4.54; +2.60 to -3.43; and +0.67 to -9.51. The percent dolomite in the carbonate fraction is from 0 to 100.

These isotopic values, which are similar to values obtained by other workers for lacustrine carbonate of various ages, suggest that the oxygen isotopes in the carbonate material comprising the algal biolithite, oolite, pisolite, and oil shale were biologically fractionated to isotopically heavier values relative to the inorganically precipitated micrite. The similarity in isotopic values between the structureless microcrystalline carbonate (+2.60 to -3.43) and the oolite (+2.73 to -4.54) also suggests that the former may be a dolomitized and recrystalized form of the latter. No correlation between percent dolomite in the carbonate fraction and the oxygen isotopic composition was found for the oolite, pisolite, and algal biolithite rocks, suggesting that the dolomite in these samples f rmed by diagenetic replacement of primarily precipitated calcite. A positive correlation was found for the oil shale and some of the micrite, suggesting that the dolomite in these samples was primary.

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