Abstract

A comparative study of the relative distribution of analcime, quartz, K-feldspar, Na-plagioclase, calcite, dolomite, and ankerite shows variable partitioning of these minerals between the marginal-lacustrine and open-lacustrine lithofacies of the Parachute Creek Member of the Green River Formation in the southern and western Piceance Creek Basin, Colorado, and the eastern Uinta Basin, Utah. The variations correlate with changes in the depositional environments of ancient Lake Uinta.

Four individual, but gradational, environments of deposition are recognized in the Parachute Creek Member: (1) deltaic lacustrine; (2) carbonate flat; (3) proximal open lacustrine; and (4) distal open lacustrine. Rocks from these depositional facies range from sandstone and algal limestone in the first two marginal-lacustrine environments to oil shale, tuff, and saline-mineral beds in the latter two open-lacustrine environments.

Detrital quartz, K-feldspar, and Na-plagioclase are most abundant in channel-form sandstone beds in the deltaic-lacustrine facies. In the open-lacustrine facies where oil shale is dominant, these silicates are still abundant but are authigenic or diagenetic in origin. Analcime is found in fine-grained rocks in all environments but is least abundant in oil shale of the distal open-lacustrine environment where saline minerals such as nahcolite and halite are present.

Calcite, dolomite, and ankerite show the most complex facies distribution. Calcite is apparently most abundant in the finegrained calcareous rocks intercalated with sandstone and algal limestone in the marginal-lacustrine facies, and it is least abundant in the distal oil shale. Dolomite is abundant throughout the Parachute Creek Member but is most abundant in the distal oil shale. Ankerite is also most abundant in the distal oil shale but is rare in marginal-lacustrine rocks. The overall zonation of the carbonate suggests that the more chemically complex types are found in the more basinward environments. Thus, the pattern for the carbonate distribution is a reflection of local biogeochemical and geochemical conditions and not the result of transportation of carbonate phases from a fringing mud-flat (playa fringe) environment.

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