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Recent stratigraphic studies have greatly improved our knowledge of the relation between the facies of the Green River Formation in the Piceance Creek basin, thus allowing for a more precise interpretation of the development of Eocene Lake Uinta through time.
In general, the evolution of Lake Uinta can be divided into six main stages. During the first stage, which is represented by almost half of the preserved Green River section in the central part of the Piceance Creek basin, there were two lakes, one located in the Uinta basin and one in the Piceance Creek basin. Freshwater mollusks occur throughout the stratigraphic section representing this period of time, suggesting that the lake was at least periodically fresh. These two lakes should probably not be rightfully called Lake Uinta, since a single lake did not exist. The second stage begins with the Long Point transgression in which the lake in the Piceance Creek basin transgressed across the Douglas Creek arch and connected with the lake in the Uinta basin. The area under water was qua rupled, and Lake Uinta, as envisioned by Bradley, came into being. During the following stages, Lake Uinta extended unbroken between the two basins. Low-grade, clay-rich oil shale is the dominant lithology from this stage, with the exception of some nearshore areas where shallow shelves began to form. Freshwater mollusks are found in rocks of the second stage, but are not common in rocks of later stages of Lake Uinta in the Piceance Creek basin. The third stage began with an abrupt increase in the kerogen content of the offshore oil shales. In the marginal lacustrine areas, however, there was no noticeable change. Here, marginal shelves, which began to form immediately after maximum Long Point transgression, continued to prograde into the lake. Large fluctuations in water level are sugge ted by rapid changes in facies on the marginal shelves. Thick, ripple-laminated sandstones were deposited during rising water, and deep meandering channels formed when water level dropped.
The water level appears to have been much more stable during the fourth stage. Thick stromatolites and tufa mounds interlayered with laminated carbonate-rich mudstone are the dominant lithologies found in the marginal shelf deposits. Laminated, kerogen-rich, dolomitic oil shale was deposited in the center of the lake. Carbonate content increased in all Lake Uinta sediments during this stage; and for the first time, the saline mineral nahcolite is found associated with oil shale. At the beginning of the fifth stage, water level gradually rose, bringing intermitted oil-shale deposition over about the outer half of the marginal shelves. Nahcolite deposition in the offshore oil shales ceased during transgression but began again once water level stabilized. In fact, most of the nahcolite a d halite in Lake Uinta sediments were deposited during this comparatively long stage. This higher lake level brought some peculiar changes to the marginal shelves. Oil shale is commonly interlayed with ripple-laminated siltstones and fine sandstones, ranging in thickness from a few inches to as much as 70 ft (21 m). These clastic sequences can be traced toward the center of the lake where they form lean zones in the oil-shale section.
The final stage of Lake Uinta in the Piceance Creek basin begins with a major transgression, represented approximately by the base of the Mahogany Ledge, a rich oil-shale sequence. Lake Uinta expanded to its maximum extent in the early part of this stage, possibly expanding to near the limits of the sedimentary basin. Infilling of the lake began at maximum transgression when a rapidly prograding shelf complex, composed largely of volcanoclastic sediments, started at the north shore of Lake Uinta and reached the southwest corner of the basin before halting. Lake Uinta evidently persisted in this limited area considerably longer than elsewhere in the basin.
The stratigraphic model presented here demonstrates that Lake Uinta evolved with time, and that each succeeding stage represented an accumulation of characteristics acquired during the preceding stages. Geochemical models that have been proposed to explain the unique oil shale and saline deposits from Lake Uinta should be reexamined in light of this more complete stratigraphic picture.
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