FLUVIAL - LACUSTRINE FACIES AND ARCHITECTURE OF THE TERTIARY DUCHESNE RIVER FORMATION, NORTHERN UINTA BASIN, UTAH: LATE-STAGE FILLING OF LAKE UINTA

Globally, many lacustrine basins with prolific hydrocarbon resources show a common upward-coarsening sequence of: deep lacustrine, deltaic and fluvial facies. The juxtaposition of lower organic-rich lacustrine source rock and the overlying coarse-grained fluvial sandstone reservoir creates favorable conditions for upward hydrocarbon migration and accumulation. Eocene to earliest Oligocene strata of the Uinta Basin exhibit this same upward-coarsening or shallowing sequence preserved in the ascending order of Green River, Uinta and Duchesne River formations. The Duchesne River Formation represents the late basin filling of Lake Uinta, with significant tar sands in Asphalt Ridge of the eastern Uinta Basin, likely sourced from organic-rich basinal Green River shales. The Duchesne River Formation is subdivided into four members. The lower three members comprise an upward-fining sequence of alluvial and fluvial channels, flood plain, and shallow to marginal lacustrine deposits. Internally this upper member has numerous tuff/tuffaceous beds that are important stratigraphic markers. The uppermost contact of this member indicates the onset of another cycle with a sequence boundary overlain by alluvial and fluvial deposits. The sequence stratigraphy at these member scales is likely tectonic-driven by uplift of the Uinta Mountains which is similar to but smaller than the Laramide events that produced the series of Paleogene lacustrine basins. There are multiple, stacked fluvial and lacustrine facies architectures affected by: accommodation space, multiple source terrains and distance from the source, sediment supply, discharge, slope, and climate. Lacustrine deposits are rare in the basal member. Small to medium lakes are commonly interbedded with meandering stream and floodplain deposits in the middle member. The upper member records thick and extensive lacustrine deposits, reflecting periods of differential subsidence in the basin. These stacked fluvial-lacustrine architecture patterns were responsive to tectonic uplift and are valuable records of the late basin filling history in the intermontane lacustrine basins.