| Description |
Lake Uinta achieved its maximum extent durinq the deposition of the Mahogany bed, the richest oilshale unit in the Mahogany zone. Filling of the lake with stream-borne debris began immediately after deposition of the Mahogany bed. The lake gradually constricted from about 20,000 square miles in areal extent to one approximately a tenth this size just before its demise. This was accomplished through many expansions and contractions of the lake that are recorded as a series of tan, dominantly clastic units alternating with light-gray to white marlstone or oil-shale units. This intertonguing is easily observed along the drainage of Piceance Creek and Yellow Creek and is particularly well depicted along the drainage of Big Duck Creek. Essentially all lacustrine deposition in the Piceance Creek Basin and the eastern part of the Uinta Basin ended shortly after an extensive period of volcanism resulted in the deposition, throughout the lake, of a thick bed of volcanic ash locally called the Porcupine Creek tuff. In the Piceance Creek Basin in the area between the Colorado and White Rivers, stream encroachment into the lake began in the north and gradually extended southward. The thickness of the interval between the Mahogany zone and the lowest Uinta tongue is shown on figure 1. This ranges in thickness from 0 feet, just south of the White River where Uinta tongues are developed in the upper part of or just above the Mahogany zone, to more than 500 feet just north of the Colorado River where a continuous sequence of oil shale above the Mahogany zone contains no Uinta tongues. Development of the Tonguing Concept Thick zones of marlstone and oil shale form prominent white bands on dominantly tan slopes along Piceance Creek, Yellow Creek, and their tributaries. These were thought, by early workers, to be deposits formed in dying phases of Lake Uinta resulting from lake rejuvenation following periods of complete dessication. In the mapping of structure in the basin, they were thought to be Parallel or subparallel to the Mahogany zone. The recognition of these white bands as tongues of the main body of the Green River Formation developed through detailed geologic mapping on accurate topographic base maps in conjunction with detailed surface and subsurface stratigraphic studies. Duncan, Hail, Pipirinqos and O'Sullivan (1974) named and described the characteristics of and the qeneral areal extent of the Yellow Creek, Dry Fork, Thirteen Mile Creek, and Black Sulfur tonques. These are the conspicuous white bands that are well exposed in much of the northern part of the Piceance Creek Basin. Additional studies showed that, in various parts of the basin, intertonguing exists throughout the entire sequence between upper part of the Mahogany zone and the beds just above the Porcupine Creek tuff. The intertonquing is diagrammatically portrayed on figure 2. This shows the approximate position in section of the major tonques with relation to key horizons and zones in a histogram of a continuous sequence of oil shale from the Triangulation Station Shale corehole (C-37). Most of the more prominent Green River tongues in the interval between the Tgy (Yellow Creek) and the Tgsk (Skinner Ridge) have been either formally or informally named in U.S. Geological Survey publications. The tongues below the Yellow Creek are informally named in this publication for ease of discussion. The names of the tonques and their letter designation, as shown on figure 2, are listed below. |
