Late Middle Eocene time marks one of the most dynamic periods of the Paleogene in the western interior of North America. Analysis of an extensive, new collection of plant, invertebrate, and vertebrae fossils from the Uinta Formation in the Uinta Basin, south of Myton, Utah, USA, provides evidence of environmental change. Paleobotanical specimens are preserved in late stage Uinta Lake sediments and coarse-grained fluvial sediments which are stratigraphically 650 to 660 m above the Green River Formation. Deposition rates estimates of 18 to 55 cm/kyr, for Uinta Lake sediments in the Uinta Basin suggest a period of 1.18 Ma to 3.69 Ma to deposit the thick section of lacustrine and fluvial sediments that separates the well-documented Green River Flora from this new fossil leaf assemblage, the Wells Draw flora. Prolific invertebrate trace fossils and invertebrates have a preference for areas with high to fluctuating water tables and soil moistures. The trace fossils are similar to traces of extant invertebrates found in temperate to tropical climates. A variety of reptiles, namely eight species of turtles/tortoise, one lizard (first report of Saniwa from the Uinta Formation), and at least two crocodilian species, are indicative of warm-temperate to subtropical climatic zones. Flow direction data derived from sedimentary structures in sandstone channel-fills confirms previous studies which indicate the major sediment source area was to the east with a prevailing westward to northwestward flow direction. The presence of higher elevation plant material within the flora, however, implies possible hydraulic transport from the nearby Uinta Mountain highlands, located north of the basin. The first report of Palmoxylon from the Uinta Formation, included in this report, corroborates the faunal indicators of a tropical climate. Physiognomical analysis of the flora yields a mean annual temperature (MAT) of 16.1◦C and a mean annual precipitation (MAP) of 56 cm. Compared with older Green River flora, these new data suggest a slight drying and cooling trend toward the end of the Eocene. Leaf types also indicate possible seasonality with seasonally dry periods and an overall warm-temperate environment with extreme minima temperatures not much below freezing.

The present work is a general survey of the vegetation of the Utah portion of the Dinosaur National Monument with special emphasis on some of the ecological aspects of the area. The vegetation ranges from xerophytic plant communities of the lower elevations to mesophytic communities of the higher elevations. Hydrophytic plant communities are also present to a limited extent along the Green River and where seeps or springs arise. Soils vary from fine clay to coarse gravel. The dividing lines between formations is often marked by extreme differences in soil type. During the author's first collecting trip into the Monument in April 1955 the possibility of correlations between geological formations and vegetation was suggested by the occurrence of certain plant species on some of the formations. The presence of a well defined geological section in a relatively short distance makes it possible to determine if such correlations exist.

Outcrop and petrographic studies of the Eocene Green River and Colton formations in the Uinta basin, Utah, document the facies architecture and heterogeneity characteristic of lacustrine reservoirs. A southwest-northeast transect of Eocene strata in the Uinta basin records three main marginal lacustrine depositional environments: fluvial, deltaic, and wave-dominated. Heterogeneity exists between and within individual depositional systems. Reservoir rocks of Outcrops One and Two (the flu vial facies of the Colton Formation and the deltaic facies of the Green River Formation, respectively) consist of 2 to 18 m thick lenticular, tabular, or undulatory channel-fill, distributary channel, and distributary mouth bar deposits that are partially to entirely compartmentalized, or encased, by mudstone units. These reservoir analog intervals are dominated by large­ scale heterogeneity, in that sand bodies show a variety of connectivity and lateral continuity. Small-scale heterogeneity exists within these sand bodies in the form of mud chip lag surfaces, large mud clast horizons, and discontinuous finer-grained beds. These features add complexity to the systems by reducing flow transmissibility or acting as flow baffles. The complex heterogeneity characteristic of these reservoir analogs confirms the need for detailed reservoir characterization studies on all scales in order to improve exploration and production efficiency in such systems. Outcrop Three (the wave-dominated facies of the Green River Formation) is dominated by thinner (2 to 4 m) tabular and laterally extensive offshore bar deposits that are compartmentalized by mudstone units. Large-scale heterogeneity is minimized in these reservoir analogs, in that sand bodies exhibit excellent lateral continuity and less complex amalgamation. Therefore, documentation of the smaller-scale heterogeneities (similar to those mentioned in the previous two outcrops) is necessary to better address exploration and production potential in these types of reservoirs. Data collected in this study were utilized in geostatistical simulations and fluid flow models in an attempt to document the effects of reservoir heterogeneity on hydrocarbon exploration and production efficiency in lacustrine basins. Further studies of this type are necessary if predictable classification systems and hierarchies of bounding surfaces are to be derived for lacustrine reservoirs. (152 pages)

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