Contributions to Geology 6.1

Investigation of pavement performance in relation to geology

JAMES 0. DUGUID and DONALD R. LAMB Civil Engineering Department, University of Wyoming, Laramie, Wyoming

Pages
1-4

Keywords
asphalt, pavement, failures, highway, soil, Wyoming

Abstract
Pavement performance is controlled by several factors and varies as a function of these parameters. Probably the parameter that has the greatest effect on asphalt pavement behavior is the type of soil on which the pavement is constructed. Since the soils of Wyoming are primarily residual soils, it would be suspected that pavement performance in this state is related to geologic rock units.

By applying statistical methods to pavement performance data which were taken from some 2500 miles of highway, it was indicated that certain pavement failures are related to soils formed from rocks of a particular geologic period.

Methods of petrologic calculation and the relationship between mineral and bulk chemical composition

KENNETH PERRY, JR. Department of Geology, University of Wyoming, Laramie, Wyoming

Pages
5-38

Keywords
mineral, composition, phases, molecular

Abstract
An application of the equations of mass balance...to rock systems provides a useful set of petrologic calculations. The requirement that the molecular members (i.e., the column vectors of the matrix aij) within each mineral phase be linearly independent provides a base for classification of mlnerals according to their molecular members and for computation of the percents of molecular members from mineral chemical analyses.

Methods are also described for calculating the stoichiometric coefficients for both systems of mass action equations necessary for equilibrium and for ordinary conservative and non-conservative chemical reactions.

A comparison of the equations of material balance with those of the normative calculation clarifies the difference between the mode and the norm. In addition, it suggests a method for computing the compositions of mineral phases of variable composition where only bulk composition and modal data in addition to the compositions of a limited number of mineral phases are known.

Finally, algebraic methods are used to show that the effect of bulk chemical composition on the distribution of major chemical species between coexisting phases can be separated from that of temperature in systems where the number of degrees of freedom is greater than two.

Stratigraphy and depositional environments of the Red Peak Member of the Chugwater Formation (Triassic), west-central Wyoming

M. DANE PICARD Department of Geology, University of Nebraska, Lincoln, Nebraska

Pages
39-68

Keywords
Red Peak, Chugwater, Wyoming, Triassic, tectonic, deposition, source area

Abstract
Based on differences in rock and bedding types, the red beds of the Red Peak Member of the Chugwater Formation (Triassic) in west-central Wyoming are divided into the following regional facies (in ascending order): silty claystone, lower platy, alternating, upper platy, and sandy. The silty claystone facies is 145-265 feet thick and dominantly silty claystone. The lower platy facies is 125-375 feet thick; the upper platy facies is 20-75 feet thick. There is about 50-70 percent well-sorted and poorly sorted siltstone, 25-30 percent silty claystone, and 5-15 percent very fine grained sandstone in the two platy facies. The alternating facies is 200-425 feet thick. It is named for alternations of: massive siltstone (8 feet); and poorly sorted, fissile and non-fissile siltstone (2 feet), which commonly contains interbeds of silty claystone, well-sorted siltstone, and very fine grained sandstone. The sandy facies is 4-30 feet thick and dominantly composed of massive-appearing, very fine grained sandstone and sandy siltstone.

Average depositional environments are distinguished for each facies by associations of sedimentary structures and bedding types, in conjunction with the petrography and geometric configuration of the beds. These criteria indicate that the Red Peak contains facies that were deposited in a paralic and near-shore marine complex that bordered the Early Triassic miogeosyncline on the west. The silty claystone facies is assigned to the transitional environment. The lower and upper platy facies and the platy intervals in the alternating facies accumulated in a tidal flat complex where gently fluctuating conditions probably prevailed. The massive siltstones of the alternating facies and the sandy facies probably originated in a near-shore, marine environment on the shelf. Sub-environments that existed within the dominant, average environments are not distinguished.

It is interpreted that moderate tectonic uplift in the source areas, or the emergence of new source areas, initiated a flood of poorly sorted, fine-grained material (silty claystone facies) onto a slowly subsiding craton at the beginning of Red Peak time. Generally quiescent conditions persisted for millions of years (lower platy, alternating and upper platy facies) and there was probably little uplift in source areas or subsidence on the shelf. Depositional rates were slow. A change in the source areas, either tectonic uplift or the emergence of new source areas, occurred near the end of deposition (sandy facies) of the Red Peak.

Quartz-feldspar separation by flotation method

K. PUNDAREEKAKSHUDU Department of Geology, University of Wyoming, Laramie, Wyo.

Pages
69-70

Keywords
quartz, feldspar, flotation, microfossils

Abstract
Quartz can be separated from feldspars, particularly from oligoclase and andesine, by using an inexpensive flotation method. The apparatus and chemicals needed for this method are described. This method is simple, rapid, non-hazardous, and less expensive than specific gravity methods. This method might also be useful for segregating microfossils.

Geology of the Stinkingwater mining region, Park Co., Wyoming, Part I: general geology and petrology of the Tertiary intrusive rocks

FREDERICK S. FISHER University of Wyoming, Laramie, Wyoming

Pages
71-86

Keywords
Wiggins, basalt, Stinkingwater, volcanic, Tertiary, Wyoming, Park, mining, breccia, Eocene, Oligocene, dioritic

Abstract
Multiple intrusions of dioritic to granodioritic rocks have domed, fractured, and metamorphosed rocks of the Early Basic Breccia (middle Eocene), Early Basalt Flows (late Eocene), and Wiggins Formation (late Eocene or early Oligocene), in the Stinkingwater region. The oldest and largest intrusive body is composed of fine- to medium-grained granodiorite to diorite. Smaller plutons of porphyritic rhyodacite and dacite have intruded the older material in an irregular pattern. Abundant andesitic and dacitic apophyses from the plutons cut the surrounding country rocks in a roughly radial pattern. Numerous dikes have been intruded into the layered rocks and also into the larger plutons. At least three periods of emplacement of intrusive material can be demonstrated by cross-cutting relationships.

Faulting and fracturing, stoping, assimilation, and doming all played a role in the process of emplacement of the plutons. However, no single process is considered to have been dominant.

The cumulative thickness of the volcanic sediments and extrusive material in the Stinkingwater region is approximately 5,000 feet, of which the Early Basic Breccia makes up about 300-400 feet, the Early Basalt Flows about 800-1,000 feet, and the Wiggins Formation about 3,600-3,900 feet.

The Wiggins Formation and the Early Basalt Flows are offset a few hundred feet along several high angle faults. In two localities small blocks of the intrusive rocks have been faulted upward into the Wiggins Formation. These faults are probably related to the igneous activity in the area.

The intrusive rocks in the Stinkingwater area were probably derived from the same parent magma as patrt of a calc-alkalic series. The area may have been at one time a center for extrusive volcanic material and the present intrusives may represent plugs which are filling an old vent area.