Petrology of carbonate rocks of the Green River Formation (Eocene)

ABSTRACT Lacustrine carbonate rocks of the Green River Formation contain a diversity of lithologic components and sedimentary features that are comparable to carbonate formations of marine origin. Fossils (algal plates, ostracodes, molluscs), coated grains (ooliths, pisoliths, circumcrusts), and polygenetic microcrystalline carbonate aggregates (intraclasts, peloids) are the main rock-forming carbonate grains. No genetic difference between ooliths and pisoliths was found. Both grain types display probable algal cell molds and are believed to be algal precipitates. Microcrystalline carbonate is more abundant than sparry carbonate in most rocks, although well-washed, grain-supported sparites are not uncommon. Dolomicrite is ubiquitous in the formation and petrographic evidence indicates that it ormed as a selective replacement product of calcium carbonate mud during early diagenesis prior to lithification. Admixed terrigenous constituents are an important component in most of the carbonate rocks; siltstone and sandstone are transitional with silty and sandy carbonate rocks. The similarity of lacustrine and marine carbonate rocks indicates that the two types can not be differentiated solely on the basis of petrographic relations. Sedimentary structures and stratification types are especially well developed in many of the carbonate rocks and greatly aid in the interpretation of subenvironments of deposition. The use of structures and stratification types in conjunction with lithologic variations and associations permits the recognition of several lacustrine subenvironments: mudflat, beachbar, lagoonal, shoal, transitional and offshore. Algal stromatolite bioherms locally are present in rocks of the shoal facies.