--> --> Connections between Eocene Lakes Uinta and Gosiute preceding and during the infilling stage of Lake Uinta, Piceance Basin

AAPG Rocky Mountain Section Meeting

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Connections between Eocene Lakes Uinta and Gosiute preceding and during the infilling stage of Lake Uinta, Piceance Basin

Abstract

Late in its history, Eocene saline Lake Gosiute in the Greater Green River Basin of Wyoming and Colorado was filled from north to south with coarse volcaniclastic sediments. During the infilling, Lake Gosiute began to drain southward across the Axial Arch into saline Lake Uinta in the Piceance and Uinta Basins of Colorado and Utah (about 49 Ma) causing Lake Gosiute to freshen. Once Lake Gosiute was filled entirely (about 48 Ma), volcanoclastic sediments spilled over into Lake Uinta. The first coarse volcanic sediments entered the north part of Lake Uinta in the Piceance Basin near the present-day mouth of Yellow Creek 15 miles south of the Axial Arch during deposition of the Mahogany oil shale zone. There is evidence that a south-flowing river entered Lake Uinta from the Axial Arch starting early in the history of the lake and prior to substantial outflow from Lake Gosiute began. A petrographic study of sandstones from this period is consistent with an Axial Arch source. Thus, the outflow channel likely occupied this pre-existing drainage. Determining when outflow from Lake Gosiute began to move through this preexisting channel is difficult as reliable dates for most of the strata deposited in Lake Uinta are lacking. A partial section of Lake Uinta strata is preserved at Deep Channel Creek about 10 mi south of the Axial Arch. Here the R-6 oil shale zone, below the Mahogany zone, has graded into fluvial strata–the only place in the basin where this zone is not lacustrine. In addition, the underlying L-5 zone is atypically sandy. We propose that Lake Gosiute began to drain into Lake Uinta starting at about the beginning of deposition of the L-5 oil shale zone increasing the input of sediments into the northern part of Lake Uinta. Mudsized sediments could have come from Lake Gosiute, but the coarser sediments likely came from the Axial Arch. Volcaniclastic sediments produced a rapidly prograding deltaic complex that ultimately filled much if not all of the eastern part of Lake Uinta. The first volcanic sediments to reach the deep depocenter were mainly fine-grained turbidites but ultimately the depocenter was largely filled by slumps off the over-steepened delta front. A petrographic study of the volcanoclastic sandstones indicates that the Absaroka volcanic field in northwest Wyoming was the likely source of the volcanic fraction.