Field guide to lacustrine and related nonmarine depositional environments in tertiary rocks, Uinta Basin, Utah

The lower part of the Tertiary System in the Uinta Basin of northeastern Utah and northwestern Colorado (fig. 1) reflects a series of very complex and variable forms of continental sedimentation in a basin of internal drainage. These complex forms represent the disruption of the Cretaceous depositional regime and the initiation of lacustrine sedimentation that began in a freshwater environment, evolved into a more saline regime and returned to a fresher water stage before the lake became extinct in the Tertiary. Figures 2 and 3 illustrate the relation of rock facies, stratigraphic nomenclature, and many of the important stratigraphic marker units of the upper part of the Cretaceous and the lower part of the Tertiary Systems. A complete discussion of the characteristics and evolution of the environments in which the rocks were deposited is presented by Ryder, Fouch, and Elison (1976), and terms used to identify and designate depositional environments are adopted from their work and from Cashion (1967) . Oil, gas, and bitumen-bearing (including "tar sands") siliciclastic and carbonate units are a product of this nonmarine depositional regime as are oil shale and certain saline minerals. In addition, coal is present in rocks of both alluvial and lacustrine origin. Many of the rocks deposited in fresh water contain abundant and diverse mollusk and charophyte assemblages, and palynomorphs and ostracodes are recovered from much of the sequence. J. H. Hanley and R. H. Tschudy of the U.S. Geological Survey identified the mollusk and palynomorph specimens respectively, and contributed insight into paleoenvironmental interpretation.