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.



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Physical and Mathematical Sciences; Geological Sciences



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paleobotany, Eocene, paleoclimate, Uinta Basin, Uinta Formation, geology, paleontology



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