Carbonate petrology of the Green River Formation (Eocene), Uinta Basin, Utah and Colorado

The Green River Formation (Eocene) shows a diversity of lacus­trine carbonate lithologies comparable to carbonate formations of marine origin. Petrographic analyses of nearly 100 thin sections show that fossils (calcareous algae, ostracodes, gastropods, pelecypods), coated grains, microcrystalline carbonate aggregates, sparry carbonate, microcrystalline carbonate and terrigenous grains are the main rock-forming components of the lacustrine carbonate rocks. The most abundant allochemical constituents are polygenetic microcrys­ talline carbonate aggregates (intraclasts, pellets) and fragmental algal "plates." Coated grains (ooliths, pisoliths, circumcrusts) are also common and probably are biochemical (algal ?) precipitates. Pore-filling sparry calcite and neomorphic sparry calcite are the dominant orthochemical constituents in some rocks, however microcrystalline carbonate clearly is the most abundant orthochemical constituent in the formation. Terrigenous constituents are present in nearly all carbonate rocks; they compose up to 50 percent of some carbonate rocks. Micrite and sandy and silty carbonate rocks are the most, abundant carbonate lithologies in the Green River Formation. Sparites and low- terrigenous carbonate rocks are less common. The similarity of lacustrine carbonate rock components and lithologies with marine carbonate components and lithologies leads to the conclusion that lacustrine carbonate rocks can not be differentiated from marine carbonate rocks solely on the basis of petrographic relationships. Subenvironments of deposition are recognized on the basis of lithologic variations, sedimentary structures, stratification, color and lithologic associations within the formation. Nearshore and shore­ line environments are represented by cleanly washed allochem-sparite, sandy and silty sparite, and biohermal and biostromal algal biolithite whereas lagoonal, mudflat and offshore deeper water deposition is char­ acterized by micrite, allochem-micrite and sandy, silty and clayey micrite. Diagenesis has caused substantial changes in the carbonate rocks, including neomorphism of microcyrstalline carbonate mud to coarser microcrystalline calcite and sparry calcite. Dolomicrite is ubiquitous in the formation and probably formed during early diagenesis prior to lithification as a replacement product of calcium carbonate micrite. Stable carbon and oxygen isotopes were analyzed in an effort to evaluate the effects of biological precipitation of calcium carbonate and to determine whether dolomicrite formed as a primary precipitate or by replacement of calcium carbonate. Preliminary results suggest a biogenic fractionation for the microcrystalline carbonate of ooliths, pisoliths and certain microcrystalline carbonate aggregates; an inorganic origin for most micrite not incorporated in allochems; and an early diagenetic replacement origin for dolomicrite.