Contributions to Geology 8.1

Oriented, linear-shrinkage cracks in Alcova Limestone Member (Triassic), southeastern Wyoming

M. DANE PICARD Department of Geology, University of Utah, Salt Lake City, Utah

Pages
1-8

Keywords
cracks, Green River, Alcova, limestone, Wyoming, shrinkage, oriented, shoreline

Abstract
Oriented, linear-shrinkage cracks, similar to those in the Green River Formation (Eocene) and several other units, occur in the Alcova Limestone Member of the Crow Mountain Formation (Triassic) in southeastern Wyoming. They are developed in limestone (microsparite and pseudosparite), possibly extending the range in depositional environments wherein these cracks have formed.

Linear-shrinkage cracks are mainly straight (or slightly curved) single cracks, which show preferential orientation. Minor two- and three-segment cracks are associated with the linear-shrinkage cracks. Lengths and widths of 35 single cracks were measured. Lengths range up to 55 mm. with a mean and median of 28 mm. and a standard deviation of 12 mm. Widths range up to 6 mm, with both a mean and median of 3.3 mm., and a standard deviation of 1.3 mm. As the length of the cracks increases the width increases. The cracks penetrate beds from 2 to 8 mm.; crack-casts rise above the upper surface of beds from 1 to 2 mm.

Where the linear-shrinkage cracks are associated with ripple mark horizons they are parallel to the strikes of the crests. The incompleteness of the cracks is attributed to an overriding directional stress that was sufficiently intense to overcome the normal tendency of homogeneous, fine-grained material, in a suitable setting, to form polygons through capillarity. The cracks are believed to have formed subaqueously. The general parallelism of the cracks to the strike of asymmetrical ripple marks suggests that they may have opened parallel to the shoreline. My interpretation is that the linear-shrinkage cracks opened parallel to paleostrike because of very slight, down-slope, gravity movement of homogeneous material (microcrystalline ooze). The same origin was suggested for the Green River cracks.

A computer program which computes the mode

KENNETH PERRY, JR. Department of Geology, University of Wyoming, Laramie 82070

Pages
9-18

Keywords
chemical, mineral assemblage, compositions, phases, computer, mode

Abstract
The normative calculation assigns to each rock bulk chemical analysis a unique, simplified mineral assemblage. In experimental and natural rock systems, on the other hand, it is possible for two or more mineral assemblages to crystallize from the same bulk chemical analysis. This program is designed to compute this many to one correspondence between mineral assemblage and bulk chemical analysis under the assumption that the chemical compositions of the mineral phases in each assemblage are, a priori, known.

Technique for the recovery of heavy liquid separates after centrifugation

JAMES R. STEIDTMANN Department of Geology, University of Wyoming, Laramie

Pages
19-20

Keywords
centrifuge, heavy liquid, glass, tubes, separates

Abstract
Previously described techniques for the recovery of heavy minerals from centrifuge tubes can be categorized as: (1) those in which specifically designed glass tubes are used to facilitate the removal of the light and heavy separates (examples described by Krumbein and Pettijohn, 1938, p. 341; Bertholf, 1940; and Ramesam, 1966) and (2) those in which standard centrifuge tubes are used in conjunction with various methods of removing the light and heavy minerals separately (examples described by Fessenden, 1959; Scull, 1960; Barsdate, 1962; and Pollack, 1962). The difficulty and expense encountered in obtaining specifically designed tubes and the problem with leakage of their glass joints during centrifugation makes the use of standard tubes most desirable.

Methods for the removal of the separates from standard centrifuge tubes include (1) simple pouring off of the light minerals, (2) inserting a plug in the bottom of a tapered tube before pouring, (3) freezing the lower portion of the tube with dry ice or liquid nitrogen before pouring, and (4) drawing off the heavy minerals by inserting a hypodermic needle through the light mineral clot to the bottom of the tube. None of these methods is entirely satisfactory. The first three do not prevent contamination and often require a high degree of digital dexterity and additional washing with solvent with attendant loss of heavy liquid. The hypodermic needle is not suitable for quantitative studies.

Differential thermal contractions and compressibilities as a cause for mineral fracturing and annealing

GEORGE W. DEVORE Department of Geology, Florida State University, Tallahassee, Florida

Pages
21-36

Keywords
strain, mineral, annealing, thermal, fracture, stress, elastic

Abstract
Natural mineral fracturing in rocks is evaluated in terms of the anisotropic thermal and elastic properties of the crystals. Models for calculation of anisotropic thermal stresses are proposed. The computed thermal stresses that result from differential thermal contractions between different coexisting minerals or between the same minerals of different crystallographic orientations from reasonable temperature changes in the system are adequate to cause brittle rupture of the crystals or provide the elastic strain energies for annealing recrystallization. Brittle fracture of the crystals under tensile strains would effectively relieve the thermal strains whereas complex solution -- reprecipitation processes of the crystals under compressive strains are required to relieve the strains or to equilibrate the strain energies. Thermal strains seem to be relieved by brittle rupture in the ferromagnesian minerals whereas annealing recrystallization seems to be the mechanism involved in the quartz-feldspar system.

Fracture stress is evaluated in terms of the theoretical cohesive strength of the minerals. The modulus of rigidity for the cleavage directions in minerals is found to be less than half the value of Young's modulus. It is proposed that the modulus of rigidity be substituted for Young's modulus in the fracture stress equations for an improved description of fracture stress.

Basement response to the Laramide Orogeny at Coad Mountain, Wyoming

CHARLES W. BARNES and ROBERT S. HOUSTON Department of Geology, Northern Arizona University; Department of Geology, University of Wyoming

Pages
37-42

Keywords
Paleozoic, Mesozoic, Tertiary, basement, Laramide, Coad Mountain, Wyoming, structure, orogeny, Medicine Bow

Abstract
The manner in which rocks of the Precambrian basement have been folded along with overlying sedimentary rocks during the Laramide orogeny has been a controversial question among students of Rocky Mountain geology for many years. One of the major problems has been that of understanding the mechanism whereby seemingly brittle gneisses of the Precambrian basement take almost the exact configuration of folds developed in overlying sedimentary cover.

The Medicine Bow Mountains of Wyoming are a complex anticlinal uplift of Laramide age exposing a number of excellent examples of folded basement. These examples of folded basement were first recognized by Beckwith (1941) who studied Laramide structure along the northwest and southeast margins of the mountain area. Since 1957 geologists of the Geological Survey of Wyoming and the Department of Geology of the University of Wyoming have studied the Medicine Bow Mountains with emphasis on rocks of Precambrian age (Houston and others, in press). Rocks of Paleozoic, Mesozoic, and Tertiary age have also been mapped to clarify the relationship between Precambrian and Laramide structure. In the course of regional mapping a number of well-exposed areas were selected for detailed study, where basement rocks of Precambrian age appear to be folded during the Laramide orogeny.

The first area studied is Coad Mountain in the northwest part of the Medicine Bow uplift where regional studies indicated a relationship between a lineation in the gneiss of the Precambrian core of Coad Mountain and Laramide fold structure.

Scanning electron microscope analysis of the homeomorphs Melonis pompilioides and Melonis soldani

WILLIAM E. FRERICHS Department of Geology, University of Wyoming, Laramie

Pages
43-46

Keywords
micropaleontologists, Melonis pompiliodes, Neogene, deep water, indicator

Abstract
Melonis pompiliodes (Fichtel and Moll) is generally recognized as a reliable deep-water (>1800 meters) indicator in Neogene sediments of the world. Studies of the distribution of the species in different water masses (Bandy and Chierici, 1966) have substantiated its isobathyal nature and many North American micropaleontologists consider this species to be the most reliable of all deep-water indices. In the eastern hemisphere, however, M. pompilioides has been reported from Neogene sections which based on other evidence appear to have been deposited in the shelf or upper bathyal zones. In these sections M. pomilioides is the only species reported which is normally considered indicative of deep water, and there is no indication that its occurrence is not due to reworking from an older section or that the shallow water assemblage with which it is associated is not in place.

Reconnaissance petrology of Precambrian rocks in the Bighorn Mountains, Wyoming

RICHARD A. HEIMLICH Department of Geology, Kent State University, Kent, Ohio

Pages
47-62

Keywords
Bald Mountain, granitic, Precambrian, Bighorn Mountains, Wyoming, metamorphism, gneiss, foliation

Abstract
Regional mapping of the 1100 square-mile exposure of Precambrian rocks in the Bighorn Mountains has led to the recognition of seven mappable lithologic units. A southern gneiss terrain is composed of plagioclase-quartz gneiss, augen gneiss, amphibolite, and a body of quartz diorite. A northern granitic terrain is composed of intergradational, scattered bodies of quartz monzonite, quartz diorite, and agmatite.

The strike of foliation is primarily east-west, east-northeast, and northeast-southwest in the area; dips are commonly steep. Regional synformal structures are defined by foliation in both the northern granitic terrain and the southern gneiss terrain. Although geologic contacts between the major rock units are broadly parallel to the regional strike of foliation, notable exceptions occur locally throughout the area. Many of the rock units are transgressed by shear zones of relatively limited length and width marked by retrograde metamorphism. Several well-defined faults are present as well.

A synthesis of the data suggests that the northern granitic complex evolved essentially contemporaneously with the southern gneiss complex during a major episode of regional metamorphism and metasomatism. Gneiss and associated rocks were formed by metamorphism at the level of the almandine amphibolite facies, staurolite-quartz subfacies. Introduction of potash at the northern end of the region, later in the metamorphic episode, converted the gneiss there to granitic rocks. The isolated Bald Mountain area may represent the center of such metasomatic activity which appears to die out generally to the east and southeast.