Contributions to Geology 15.1

Geologic implications of trans-basin water diversion in southwestern Wyoming

JAMES D. MURPHY and RONALD C. SURDAM Department of Geology, The University of Wyoming, Laramie, Wyoming 82071

Pages
1-16

Keywords
Wyoming, chemical, Plains Reservoir, Flaming Gorge, Sweetwater River

Abstract
The chemical history of Flaming Gorge Reservoir in southwestern Wyoming is used as a model to predict the water chemistry of Plains Reservoir, a proposed impoundment of the Green River east of Fontenelle Dam. The impounded water would be pumped over South Pass into the Sweetwater River for ultimate removal and use in the Powder River Basin in northeastern Wyoming. This diversion would increase the annual average flow of the Sweetwater River from 64.8 cfs to 272 cfs, causing increased bank erosion in the Sweetwater channel, increased suspended sediment load, and increased sedimentation in Pathfinder Reservoir.

The chemistry of the water in Plains Reservoir would be governed by the same geologic and climatologic constraints that exist at Flaming Gorge Reservoir. The estimate of water quality for Plains Reservoir, five years after impoundment, is 510 mg/l total dissolved solids. The water would be supersaturated with respect to calcite and only slightly undersaturated with respect to gypsum. Using alternatives suggested by planners in the diversion proposal, the quality of the Sweetwater River water is estimated to change from its present 121 mg/1 total dissolved solids to values varying from 386 mg/1 to 681 mg/1.

It is shown that the chemical model is relatively insensitive to anomalously high or low volume water years. Construction of Plains Reservoir would consistently degrade the quality of water in both the Sweetwater and Green River systems.

K-Ar ages of biotites from tuffs in Eocene rocks of the Green River, Washakie, and Uinta Basins, Utah, Wyoming, and Colorado

RICHARD L. MAUGER Department of Geology, East Carolina University, Greenville, North Carolina 27834

Pages
17-42

Keywords
Green River, Washakie, Uinta, Utah, Wyoming, Colorado, tuff, Eocene, Lake Gosiute, biotites

Abstract
K-Ar dating of biotites in air-fall tuffs and some reworked tuffs in the Eocene lacustrine basins of Utah, Wyoming, and Colorado, met with mixed success. Some biotite populations were bimodal and gave dates that were too old; degraded biotites with evidence for loss of magnesium, iron, or potassium, generally gave young ages, depending upon the time of alteration as compared to the time of deposition. Unimodal, non-degraded samples gave reliable ages. Tuffs, composed of ash that fell directly into the lakes, were least likely to have been mixed with extraneous, older micas. Tuffs that were reworked by fluvial processes and tuffaceous clastic sediments were very likely to have included extraneous micas that were derived from older rocks in the drainage basin. Microprobe data for Fe and Mg from biotite separates formed the rational basis for identifying samples with "reliable" ages. Highly degraded or altered biotites generally contained more than a normal complement of atmospheric argon and there is a gross correlation between the amount of atmospheric argon and the extent of degradation. The "extra" atmospheric argon was acquired through chemical interaction of the biotite with groundwaters that contained small amounts of atmospheric noble gases in solution.

In the Green River Basin, Wilkins Peak saline deposition was in progress 49 m.y. ago and ended about 48 m.y. ago. Saline facies sediments in the Piceance Basin beneath the Mahogany zone are probably correlative with the Wilkins Peak. Expansion of Lake Gosinte, associated with deposition of the Laney Shale Member in the Green River and Washakie basins, started between 48 m.y. and 47 m.y. ago. A fairly rapid expansion of the lake is postulated to account for the lack of intertonguing between oil shale-bearing Laney and underlying sediments. As time passed, deposition of clastic fluvial material caused the lake to shrink along its margins while oil shales were being deposited in the interior of the lake basin. In the Washakie Basin the transition from lower Laney oil shales to the coarser, clastic, upper Laney took place about 45.5 m.y. ago. This transition also coincided with the widespread appearance of "detrital" (as opposed to air-fall) volcanic material in the Washakie Basin. This influx of detrital volcanic material indicates that a drainage gap had opened across the Wind River-Granite mountains uplift, allowing rivers from the Wind River Basin to transport Absaroka volcanic debris into the Great Divide Basin and eventually into the Washakie Basin.

The similar FeO/MgO ratios and ages of tuffs in the extreme upper part of the lower Laney (Washakie Basin) and in the tuffaceous part of the Parachute Creek Member (including the Mahogany) in the Uinta Basin, suggest a time correlation of the two units. Lacustrine deposition ceased in the Washakie Basin between 45 and 44 m.y. ago, but persisted in the central and western Uinta Basin to as late as about 41 m.y. ago. The correlation (Roehler, 1973) of the lower brown sandstones (Washakie Fm., Adobe Town Member, Washakie Basin) and the Lower Uinta A sandstones (Uinta Basin) is supported by the reliable dates. In the eastern Uinta Basin, deposition of the sparsely fossiliferous Uinta A sandstones began 44 m.y. ago. This data and Roehler's (1973) redefinition of the Uintan Mammalian Age combine to place the Uintan-Bridgerian boundary at between 44 and 43 m.y. ago.

Summary of known occurrences of terrestrial vertebrates from Eocene strata of southern California

DAVID J. GOLZ Section of Vertebrate Paleontology, Natural History Museum of Los Angeles County, Los Angeles, California 90007
JASON A. LILLEGRAVEN Department of Geology, The University of Wyoming, Laramie, Wyoming 82071

Pages
43-66

Keywords
Eocene, vertebrate, California, fauna, assemblages

Abstract
Recent work has greatly expanded the known Eocene terrestrial vertebrate assemblages of southern California. We present here an updated summarizing list of all known taxa, the localities from which they are known, the physical correlations of the fossil-bearing formations, and a bibliography of original research. Although the extent of species-level commonality between the West Coast and Rocky Mountain region was high in the early Uintan (middle to early late Eocene), endemism of the southem Californian West Coast land vertebrate fauna apparently increased markedly in the later Uintan to Duchesnean (late Eocene proper).

Radiocarbon dates from carbonates of soils on Bull Lake and Pinedale tills of the Libby Creek area, Medicine Bow Range, Wyoming

RICHARD G. REIDER Department of Geography, The University of Wyoming, Laramie, Wyoming 82071

Pages
67-72

Keywords
till, Medicine Bow, Wyoming, Pinedale, soils

Abstract
Radiocarbon dates from soil carbonates on Bull Lake and lower and middle Pinedale (Pinedale I and 2) tills of the Libby Creek area of the Medicine Bow Mountains of southern Wyoming range from greater than 37,000 to 3550 + 130 C14 years B. P. The oldest date comes from the soil on middle Pinedale (2) till at an elevation of 3048 m and represents carbonates locally incorporated into the till and soils from an underlying Tertiary conglomerate cemented by calcium carbonate.

At low and intermediate elevations (2500 to 2900 m), dates from soil carbonates range from 6670 + 190 to 3550 + 130 C14 years B. P. and suggest an Altithermal age of soil carbonates and thus a general lack of correlation between till and soil ages in the Libby Creek area. The range of dates from older to younger tills in an upvalley progression, in addition to the polygenetic character of soils on middle Pinedale till at intermediate elevations, probably represents marked vegetational shifting in response to climatic changes along the mountain slopes during Altithermal and Neoglacial times. Accordingly, a grassland-sagebrush ecotone, confirmed by phytolith analyses, prevailed up to elevations of approximately 2900 m during the Altithermal, in which case calcification of the soils dominated. At the same time, coniferous forests were confined above 2900 m where podzolization was the dominant pedogenic process. This was followed by encroachment of the coniferous forests downslope to about 2500 m during the Neoglacial, resulting in partial podzolization of the soils at intermediate elevations.