Contributions to Geology 4.2

Genesis of recent gypsum in the Stinkingwater mining region, Park Co., Wyoming

FREDERICK S. FISHER Department of Geology, University of Wyoming, Laramie, Wyoming

Pages
45-50

Keywords
Stinkingwater, gypsum, Park, Wyoming, lake, calcium sulfate

Abstract
A bedded deposit of pure gypsum occurs in association with a mineralized area within an intrusive complex in the Absaroka Mountains. The gypsum unit covers approximately one-quarter square mile and is irregularly shaped in plan. It is flat lying and ranges in thickness from a few feet near its edges to approximately 15 feet in its thickest parts. In hand specimen it is a grayish white to buff yellow, massive, poorly indurated rock.

Interplanar "d" spacings and intensities obtained by X-ray diffraction of samples from the unit compare very closely with those given for gypsum in the ASTM Powder Diffraction File. Analyses by X-ray fluorescence methods indicate only the presence of strontium and minor amounts of iron. Gypsum from the Jurassic Gypsum Spring Formation near Cody, which was chemically analyzed as 99% pure, gives an X-ray fluorescence pattern almost identical to that of gypsum from the Stinkingwater Mining Region. Chemical analysis of gypsum from the Stinkingwater region indicates a purity of 93.3%.

It is tentatively suggested that the unit was deposited in a small pond or lake formed by local disruption of drainage patterns by landslides from Crater Mountain. The waters of this pond or lake may have been enriched in calcium sulfate by four possible means: leaching of the mineralized zone by surface waters; hydrothermal leaching of the mineralized and altered zone; upward percolating ground water or hydrothermal solutions enriched in calcium sulfate derived from the Gypsum Spring Formation; or primary hydrothermal solutions.

Multiple metamorphic episodes in Precambrian rocks near South Pass City, Wyoming

DENNIS S. HODGE and RONALD G. WORL Department of Geology, University of Wyoming, Laramie

Pages
51-58

Keywords
metamorphic, South Pass City, Wyoming, granite, greenschist

Abstract
The petrology of Precambrian igneous and metamorphic rocks in the southwestern Wind River Mountains, Wyoming, was investigated by field and laboratory study. A granite stock and related pegmatite and quartz veins have been emplaced in pelitic and quartzo-feldspathic metasedimentary rocks. The metasedimenary rocks are mainly quartz-plagioclase-hornblende and quartz-biotite-plagioclase porphyroblastic schists belonging to the almandine-amphibolite regional metamorphic facies. A contact aureole belonging to the hornblende-hornfels contact metamorphic facies surrounds the granite stock. Recrystallization of these metamorphics and the granitic rocks during a retrogressive metamorphism produced greenschist facies mineral assemblages.

There have been at least three phases of metamorphic adjustment in the area. The first phase is represented by an almandine-amphibolite regional metamorphism. Plagioclase (An24-25), amphiboles, almandine, and possibly andalusite were developed during this phase.

The second phase, contact metamorphism, coincided with the granite stock emplacement and probably occurred after the peak of amphibolite facies regional metamorphism. An increase in twinned plagioclase, average grain size, interstitial quartz, with a decrease in biotite, and the presence of interstitial poikiloblastic microcline characterize this contact zone.

Extensive replacement of the regional metamorphic mineral assemblage (especially plagioclase and biotite) by felted mica porphyroblasts, chlorite, and quartz suggests a retrogressive period of metamorphism, the third phase. These minerals probably crystallized sometime after the peak of regional metamorphism.

Random orientation of fossils and its significance

HEINRICH TOOTS Department of Geology, University of Wyoming, Laramie

Pages
59-62

Keywords
orientation, fossil, random,

Abstract
It has been taken for granted that random orientation of fossils is the normal result of undisturbed sedimentation or life processes. Neither process is capable of producing random orientation. Primary random orientation can originate only in sediment traps where the movement of organic remains is stopped before they can assume a stable orientation. Most cases of random orientation of fossils are secondary. This can be caused by flow of the whole sediment mass. More commonly it is the cumulative effect of many small disturbances of the sediment produced by the activity of burrowing animals.

Oriented plagioclase grains in K-feldspar porphyroblasts

SCOTT B. SMITHSON Department of Geology, University of Wyoming, Laramie

Pages
63-68

Keywords
plagioclase, oriented, porphyroblasts, Norwegian, Precambrian, rotate, K-feldspar

Abstract
A striking alignment of plagioclase grains within K-feldspar megacrysts in granitic rocks has been interpreted as indicative of magmatic origin and constitutes an important textural feature of these D rocks. In augen gneisses from the southern Norwegian Precambrian, undoubted K-feldspar porphyroblasts contain oriented plagioclase grains as do K-feldspar porphyroblasts from inclusions within granitic plutons. Euhedral-to-subhedral K-feldspar porphyroblasts are apparently able to rotate adjacent plagioclase grains into concordance with possible crystal faces and incorporate them. This process occurs in the solid state but may indicate beginning anatexis. Oriented plagioclase grains in K-feldspar megacrysts are more likely indicative of growth in a restricted space or in the solid state rather than growing crystals freely floating in a magma.

Persistence of variegated claystone in Red Peak Member, Chugwater (Triassic) Formation, Big Horn basin, Wyo.

M. DANE PICARD Department of Geology, University of Nebraska, Lincoln, Nebraska
SAMUEL S. WELLMAN Department of Geology, Princeton University, Princeton, N. J.

Pages
69-74

Keywords
Wyoming, Bighorn Basin, Chugwater, Red Peak, claystone, miogeosyncline

Abstract
We have traced a variegated claystone in the alternating facies of the Red Peak Member of the Chugwater (Triassic) Formation in an east-west direction for more than 73 miles in the southern part of the Bighorn Basin, Wyoming. It averages 5.5 feet in thickness, ranging from 2 to 9 feet. The north-south extent is probably more than 110 miles. The base of the variegated claystone is from 98 to 138 feet below the base of the Alcova Limestone Member of the Chugwater Formation along a generally east-west cross section, possibly reflecting a thickening of the interval above the base of the variegated claystone toward the depositional axis of the former miogeosyncline. Contrary to published views, other persistent beds and sequences of regional extent occur in outcrops of the Chugwatel Formation, as well as the widely recognized subsurface horizons.

Tower sandstone lenses at Green River, Wyoming

RICHARD J. EBENS Department of Geology, University of Wyoming, Laramie

Pages
75-80

Keywords
Tower, sandstone, Green River, Wyoming, channel fill, Laney Shale, volcanic

Abstract
The Tower sandstone at Green River is represented by lenses and channel fillings at several stratigraphic levels in the lower part of the Laney Shale Member of the Green River Formation (Eocene). The detrital fraction of the sandstone is composed of approximately 40% quartz, 25% feldspar, 6% biotite, 2% hornblende, and 25% lithic fragments; approximately 50% of this detrital material is of volcanic origin. Most of the volcanic material, along with varying amounts of grains or other origin, was reworked and redeposited by streams. Soon after burial, the sand and silt detritus was well cemented by calcite and iron carbonate. The cement comprises approximately 25% of the rock.

Pleistocene convolutions near City Springs of Laramie, Wyoming

MICHAEL D. FENTON and M. R. VOORHIES Department of Geology, University of Wyoming, Laramie

Pages
81-84

Keywords
convolutions, City Springs, Wyoming, Pleistocene, deformation

Abstract
Highly contorted siltstone strata of the Satanka Formation (Permian) are exposed in a stream bank northeast of Laramie, Wyoming. Overlying the Satanka at this locality are Pleistocene fluviatile sediments (several lenses of sand and a bed of cobbles) which are also involved in the convolutions.

In contrast with most previously described Pleistocene convolutions, those discussed herein show a pronounced directionality. The folds are asymmetrical with their steep limbs dipping southwest and many are overturned in the same direction.

Although these features may have formed under periglacial conditions a simple freeze-thaw mechanism is insufficient to account for their configuration. Deposition of the overlying load of cobbles on an unstable slope may well have induced the deformation.