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The Outflow of Eocene Lake Gosiute Into Lake Uinta and its Affects on Sedimentation in Lake Uinta in the Piceance Basin of Western Colorado

Johnson, Ronald C.
U.S. Geological Survey, Denver, CO

     The Eocene Green River Formation, where exposed near the mouth of Yellow Creek in western Colorado, provides evidence of outflow from in Lake Gosiute of western Wyoming and northwestern Colorado into Lake Uinta of western Colorado and eastern Utah. Throughout much of their histories, the two lakes were separated by the Axial Arch, however, Lake Gosiute, apparently drained southward across the arch into Lake Uinta late in its history, entering Lake Uinta near the present-day mouth of Yellow Creek. This outflow, which caused Lake Gosiute to evolve from saline to fresh water, caused a major expansion of Lake Uinta represented by the base of the Mahogany oil shale zone. Because of this outflow, the normally rich Mahogany oil shale zone grades into organic-poor rocks in the vicinity of Yellow Creek, and cannot be recognized. Coarse volcaniclastic sediments, sourced from the Absaroka volcanic field in central Wyoming, flooded into Lake Uinta starting about 150 ft above the base of the Mahogany zone at Yellow Creek, implying that Lake Gosiute was filled in by that time. Many of these volcanoclastic wedges grade into thin airfall tuffs toward the center of Lake Uinta.
     The Yellow Creek area was a locus of sand and silt deposition starting much earlier than Mahogany time, implying that outflow from Lake Gosiute followed a previously established drainage system that flowed southward into Lake Uinta. Outflow must have reached the upper part of this drainage through a low somewhere along the Axial Arch, possibly near Cross Mountain.


AAPG Search and Discovery Article #90071 © 2007 AAPG Rocky Mountain Meeting, Snowbird, Utah