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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 66 (1982)

Issue: 10. (October)

First Page: 1698

Last Page: 1699

Title: Physical Evidence for Saline Cycles of Deposition in Eocene Lake Gosiute in Southwest Wyoming: ABSTRACT

Author(s): Henry W. Roehler

Article Type: Meeting abstract

Abstract:

The Wilkins Peak Member, the saline unit of the Green River Formation in southwest Wyoming, is more than 985 ft (300 m) thick and contains more than 35 beds of trona or trona with halite. The trona and halite were deposited in the deepest part of the basin of Lake Gosiute, during arid periods of the Eocene Epoch, by the periodic evaporation and contraction of the lake waters. Alternating with the arid periods were more humid periods, when the lake expanded and less saline sediments were deposited across and beyond the previously deposited salt beds. The water-level fluctuations resulted in a concentric pattern of

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lithofacies within the Wilkins Peak Member.

The innermost of the cyclic lithofacies, which includes bedded evaporites, occupies the depositional center of the ancient lake basin and consists of repetitious ascending sequences of (1) dark-brown oil shale, (2) white to brown trona and halite, and (3) gray or green dolomitic mudstone. A second lithofacies that encircles the first, but does not contain bedded evaporites, shows in vertical section cyclic sequences of (1) gray siltstone, (2) dark-brown oil shale, (3) tan or light-brown oil shale, and (4) gray or green dolomitic mudstone. The oil shale and siltstone in these cycles were deposited as lake-bottom muds and along ephemeral shorelines during periods of lake expansion and water-freshening; the trona and halite and gray and green mudstone were deposited in salt pans and on m d flats during periods of lake contraction and water-salting. A third lithofacies is present at the outermost margins of the lake basin, but the cyclic sequences there, consisting mostly of gray and green mudstone and some thin interbedded tan and brown oil shale, are largely obscured by irregularly interbedded tan algal limestone, oolites, and thin beds of dolomite and sandstone. Floodplain deposits of interbedded red and gray sandstone and mudstone are present nearly everywhere between the outer edges of the lake basin and surrounding mountains.

The number of major contractions of Lake Gosiute during deposition of the Wilkins Peak Member can be determined by counting the number of oil shale beds involved in cyclic sequences near the depocenter of the basin. Seventy-three oil shale beds, and hence 73 saline cycles, were counted in a hole cored in the Blacks Fork area by the U.S. Energy Research and Development Administration. Histograms of oil shale beds in drill holes and outcrops in other parts of the basin support 70 to 75 as the probable total number of saline cycles.

The time intervals of the saline cycles can be roughly estimated by using potassium-argon dating methods for biotites in tuff, and by counting oil-shale varves. From these data, the shortest cycle in which salines were deposited is believed to have lasted less than 1,500 years and the longest cycle more than 100,000 years.

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Copyright 1997 American Association of Petroleum Geologists