Autogenic and Allogenic Fluvial-lacustrine Interactions: Revisiting Sunnyside Delta Interval, Green River Formation (Eocene), Uinta Basin, Utah

Abstract

Eocene-aged fluvial and lake deposits in the Sunnyside Delta Interval in the Nine Mile Canyon provide a great opportunity to investigate autogenic and allogenic controls. The basin formed in response to Cretaceous-Paleocene tectonic loading by the Uinta uplift, whereas deposition occurred during the post-tectonic phase of intense climate changes during the Early Eocene Climatic Optimum. This dataset includes detailed measured sections, cliff-face photomosaics, a GPS survey and thin-section analyses. Three main types of facies are identified: lacustrine, fluvial, and mixed fluvial-lacustrine facies. Fluvial-dominated intervals vertically alternate with lacustrine or mixed lacustrine-fluvial intervals. Some lake intervals are continuous across the study area of 125 km², although they display internal facies changes; whereas others pinch out and are laterally replaced by fluvial facies across distances of less than 100 m. The fluvial succession is recognized as a fluvial fan, based on the large lateral extent across more than 150 km, and lateral changes in the nature of channel and floodplain deposits. In the axial and most proximal part of the system, towards the eastern end of the Nine Mile canyon, channels are largest, most amalgamated, and channel fills are dominated by sandstones. Basinward, proportion of floodplain deposits increases and channel amalgamation decreases, and floodplains are splay rich. Further basinward, proportion of floodplain sands decreases, channel size decreases, and channel fills become heterolithic. In some stratigraphic intervals, there are no mixed facies observed and fluvial and lacustrine environments do not meet, whereas in others different styles of mixing occur, including progradation of relatively small deltas into the lake, and mixing of fluvial-derived siliciclastic sediment with lake carbonates. Lake intervals that are correlatable across the dataset are interpreted as major lake transgressions that coincide with fluvial fan inactivity. Intervals where mixed facies occur are interpreted as intervals with overall rising or falling lake levels that coincide with fluvial fan progradation. Yet, other intervals where no mixing occurs are interpreted as intervals of lake level fall and fluvial fan stagnation. Moreover, lake intervals that only correlate across parts of the dataset are interpreted to form during overall rising lake level that coincides with fan progradation as distinct lobe avulsions.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016