Stratigraphy of Black Shale Facies of Green River Formation (Eocene), Uinta Basin, Utah: ABSTRACT

In the Uinta basin, Utah, the black-shale facies of the Green River Formation (Eocene) is divided into 5 rock units: 4 lacustrine units designated by the letters A-D, and a fluvial unit, the Wasatch tongue of the black-shale facies. The Wasatch tongue occupies the same stratigraphic position as unit C. Unit A contains the oldest lacustrine rocks of the black-shale facies, and is transgressive on the underlying fluvial Wasatch Formation (Paleocene-Eocene). All of the lacustrine units contain black fine-grained clastic rocks. Units B and D contain more carbonate rocks than do other units. Units B and D are also the most extensive of the lacustrine units. Units A, B, and C range in thickness from 100 to 300 ft, whereas the Wasatch tongue is 100-400 ft thick. Unit D has the g eatest thickness range, from about 100 to 500 ft. Rocks contained in the 4 lacustrine units vary in composition depending upon where they were deposited in relation to the center of the basin. The central lake environment of deposition produced mostly dark-gray to black, fine-grained clastic rocks and finely crystalline, brown to dark-brown carbonate rocks. The total clastic content of the lacustrine rocks and their model grain size increase toward the peripheries of the depositional basin and sandstone and siltstone become more abundant. Near the edges of the basin carbonate rocks are more saccharoidal in texture and contain larger amounts of silt- and sand-sized grains, oolite, pisolite, and shell fragments. The depositional axis trends east-west and the depositional center of the lacustrine units is south of Duchesne, Utah, except for unit D where the center is farther south. Well control is sparse in the western part of the Uinta basin. Lake Uinta was initiated by the coalescing of several small freshwater lakes on a broad alluvial plain. Downwarping led to the formation of the first moderately deep Green River Lake and the deposition of the black fine clastics and other rocks in unit A. The lake was thermally or chemically stratified, which is suggested by the preserved organic material and the presence of pyrite and salt crystals. As the lake transgressed over the fluvial sediments and became larger, the sediments of unit B were deposited. Unit B contains abundant carbonate rocks that were deposited over the entire lake, but particularly in the shallower part where the temperature was highest. A change in climate and/or increasing downwarping caused the lake to diminish its total area and unit C was deposited. Duri g the deposition of unit C, the fluvial sediments of the Wasatch tongue of the black-shale facies were deposited along the southern part of the basin. The lake then transgressed back across the fluvial sediments and deposited unit D. The dominant rock types of unit D are very similar to those of unit B, indicating a similar environment of deposition. Much of the oil and gas production in the Green End_Page 543------------------------------ River Formation in the Uinta basin is from the black-shale facies or equivalent units. End_of_Article - Last_Page 544------------