Comparative petrographic analyses of channel-form and tabular sandstone bodies in the upper Wasatch Formation (Paleocene-Eocene) and lower Green River Formation (Eocene) in the southern Piceance Creek basin show that compositional and textural variability is primarily a reflection of provenance and the environment of deposition.

Sandstones from the upper Wasatch Formation and lower Green River Formation are generally similar in texture and composition and have varying concentrations of (1) angular to well-rounded monocrystalline quartz grains, some with abraded overgrowths; (2) fresh and slightly altered potassic and sodic feldspars; and (3) volcanic lithic fragments, mostly andesite. Wasatch sandstones contain slightly more lithic fragments than those of the lower Green River, which are more quartzose. This difference is attributed to the fluvial mode of deposition of the Wasatch in contrast to the marginal-lacustrine nature of the Green River sandstones. Lacustrine sandstone also commonly contains accessory analcime and pyrite.

The sampled intervals of the Green River Formation permit an evaluation of the source terrane and its evolution during development of Lake Uinta. Paleocurrent data suggest that the sources for most of the sediment were on the south, southwest, and southeast. Petrographic similarities among the samples imply a relatively constant source terrane during deposition of the Green River Formation that was composed of Mesozoic and Paleozoic sedimentary rocks and late Mesozoic and early Cenozoic silicic volcanic and plutonic rocks.

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