Contributions to Geology 3.2

The Lower Devonian Water Canyon Formation of northern Utah

J. STEWART WILLIAMS and MICHAEL E. TAYLOR: Department of Geology, Utah State University, Logan, Utah

Pages
38-53

Keywords
Devonian, Water Canyon, Bear River Range, Utah, Card member, Grassy Flat

Abstract
Nine measured and described sections of the Lower Devonian Water Canyon formation in the Bear River Range and adjacent areas, northeastern Utah, are reported. The formation consiats of two members, the lower, called the Card member, composed mostly of thin-bedded white-weathering argillaceous dolostone; and the upper, called the Grassy Flat member, composed of sandy dolostone, sandstone, shale, and intraformational sandstone-dolostone and sandstone-limestone breccias. The formation is marine, deposited in the shallow neritic and littoral zones. It contains some invertebrate fossils, plant remains, and an important fish fauna.

Flexural folding of inactive surfaces

RONALD B. PARKER Department of Geology, University of Wyoming, Laramie

Pages
54-59

Keywords
flexural, folding, rotation axis

Abstract
In most folding by simple flexure the axis of external rotation is parallel to the folded surface and is parallel to the b axis defined by the folded surface. If, however, a surface undergoes external rotation about an axis that is not parallel or perpendicular to that surface, the b axis defined by the folded surface is inclined to the rotation axis. In a rock mass containing two planar systems of surfaces the folding may result in two b maxima, one for each system. In a rock mass containing a folded surface system and a planar surface system, refolding with the planar system containing the rotation axis will spread the old b maximum in a characteristic fashion.

Geologic reconnaissance of the southern Absaroka Mountains, northwest Wyoming: Part I - the Wood River-Greybull River area

WILLIAM H. WILSON Geological Survey of Wyoming, Laramie

Pages
60-77

Keywords
volcanic, Wood River, Greybull River, Wyoming, Wiggins, Absaroka

Abstract
Calc-alkalic volcanic rocks, aggregating more than 5,000 feet in thickness crop out in a 280 square mile area in the Southern Absaroka Mountains. The Pitchfork formation (middle Eocene) consists of interbedded volcanic siltstones, sandstones, and conglomerates that have been intruded by many small irregular plug-like bodies of basalt and basaltic breccia. The overlying Wiggins formation (upper Eocene and Oligocene) is composed of andesite flows and breccias which grade laterally into massive volcanic conglomerates and sandstones, lenticular flows and breccias. These rocks have been intruded by andesite, dacite, granodiorite, and rhyolite plugs as well as andesite and dacite dikes. Some of these intrusive bodies are believed to be in the position of former vents.

Both the Pitchfork and Wiggins formation, in the northeast part of the area, have been displaced by vertical normal (?) faults. A rhyolite stock, in the southwest part of the area, has intensely deformed the Wiggins formation and uplifted a large block of Paleozoic strata.

The primary source of these volcanic rocks is believed to have been an andesitic magma which was extruded with progressive differentiation from mafic to felsic andesite up to middle Wiggins time. The upper part of the Wiggins formation is characterized by more mafic andesitic eruptives. Contemporaneous with volcanism, or slightly later, the igneous rocks were reworked and redeposited by mud flows and streams as volcanic sandstones and conglomerates.

Cataclastic migmatites of the Medicine Bow Mountains, Wyoming

M. E. McCALLUM Colorado State University, Fort Collins, Colorado

Pages
78-89

Keywords
cataclastic, migmatites, Medicine Bow Mountains, Wyoming, Mullen Creek, Nash Fork

Abstract
A diverse sequence of migmatitic rocks occurs as a slice within the northeast-trending Mullen Creek-Nash Fork shear zone in the east-central part of the Medicine Bow Mountains. The slice, which extends several miles along strike and attains a maximum width of about 22 miles, consists of a dominantly felsic rock assemblage (granodioritic to quartz dioritic in composition) that appears to reflect variable granulation intensities and degree of metamorphic differentiation and recrystallization. Cataclastic augen gneisses, the least deformed rocks of the sequence, grade into crudely layered gneisses which, in turn, merge into finely laminated or "banded" gneisses.

The "granitic" material characteristic of these rocks occurs in layers, lenses, dikes, veinlets, and irregular masses and is believed to be largely a product of deformationally induced metamorphic segregation and recrystallization, accompanied by partial melting(?), and possibly some minor metasomatic potash enrichment.

The effects of weak to moderate cataclasis are evident in many small resistant bodies of ortho-amphibolite, metagabbro, and granite dispersed randomly throughout the sequence. Occasional thin, tabular bodies of submylonite and mylonite apparently represent local zones of intense cataclasis related to a second, less extensive episode of kinetometamorphism (late? Precambrian).