EVOLUTION OF THE GREAT LAKES SYSTEM OF THE GREEN RIVER FORMATION: TECTONICALLY & TOPOGRAPHICALLY CONDITIONED RECORDS OF CLIMATE CHANGE

Lakes of the Eocene Green River Formation system covered more than 110,000 km² at maximum extent in the segmented foreland of the Sevier orogen. A wide range of lake types occupied a series of basins between 37 and 41°N paleolatitude under a secular climate change from warm and wet to cool and seasonally dry. Although they ultimately formed an integrated hydrologic system, the stratigraphic record of each basin is distinct because of the influences of local conditions and upstream hydrology. Similar changes in precipitation were variously recorded as a function of evolving basin tectonics and sedimentary fill along with secular climate changes. Sedimentation rates and completeness of stratigraphic record change significantly with lake-basin type and stratigraphic and lateral position. Overfilled lake phases have the most complete record (Luman Tongue, Douglas Creek member) whereas underfilled phases record the largest range of short-term sedimentation rates from prolonged exposure of profundal strata to 5-m thick sheetflood deposits (Wilkins Peak, mid-Parachute Creek members). Depositional sequences are thicker and more complete near steep margins and thinner and more punctuated with hiatuses at gradual margins.

In the Greater Green River basin, these strata show a predictable evolution of lake-basin type, similar to many other lake basins. The various members record changes from fluvial to overfilled to balanced fill to underfilled and back through balanced fill to overfilled to fluvial. This records increasing then decreasing ratios of potential accommodation to sediment+water supply as the basin formed and filled. Closely analogous evolutions occur in neighboring Fossil, Piceance Creek, and Uinta basins, but are not necessarily coeval or physically connected. The major control on the underfilled lake phases appear to have been active deformation at the margins of the hydrologic basin and intrabasinal sills. The ultimate demise of the system progressed from north to south due to both decreasing tectonic accommodation and increasing supply of sediment and water.

This system contains a wide variety of depositional environments useful as analogues for different lake settings. More than one model is necessary to describe the Green-River lakes systems as they evolved through different stratigraphic levels in different basins.