Contributions to Geology 11.2

Relations among pH, carbon dioxide pressure, alkalinity, and calcium concentration in waters saturated with respect to calcite at 25 degrees C and one atmosphere total pressure

JAMES I. DREVER Department of Geology, University of Wyoming, Laramie, Wyoming 82070

Pages
41-42

Keywords
calcite, carbonate, natural waters, pH, alkalinity, pressure

Abstract
Relations among pH, alkalinity, carbon dioxide pressure and calcium concentration for calcite saturated water at 25 degrees C have been calculated by computer and presented in the form of a graph. This graph allows various problems involving the carbonate system in natural waters to be solved by inspection rather than by time consuming hand calculation.

Field relations and gravity interpretation in the Laramie anorthosite complex (with field trip guide)

SCOTT B. SMITHSON Department of Geology, University of Wyoming, Laramie, Wyoming 82071
DENNIS S. HODGE Department of Geological Sciences, State University of New York, Buffalo, New York

Pages
43-60

Keywords
magma, Laramie, anorthosite, gravity, metamorphic, metasomatic

Abstract
This report presents the status of data on the Laramie Anorthosite complex and a field trip guide that was prepared for the 1972 Rocky Mountain Section Meeting of the Geological Society of America. Most geological data are from the northern part of the complex; gravity interpretation covers the entire complex. In places, the anorthosite exhibits layering on a scale of 1 to 10 meters in width, and some lenticular layers suggest possible cut-and-fill structures. Contact relations of syenite, which borders anorthosite on the northwest, show that syenite is intrusive. Contact metamorphic and metasomatic effects are found at the contacts between syenite and gneisses and marbles in the country rocks. Country rocks consist of amphibolite, biotite schist, quartzo-feldspathic gneiss, quartzite, calc-silicate gneiss, and marble. Biotite gneiss contains Al-silicates which show a zonation into an andalusite-sillimanite zone, an andalusite sillimenite-kyanite zone, and a kyanite-sillimanite zone. Gravity interpretation suggests that the anorthosite is about 4 km thick. Syenites are 1-2 km thick and are underlain by a greater thickness of mafic rock which is interpreted to be norite. The most important result of gravity interpretation is that no large amount of mafic accumulate is associated with the Laramie anorthosite at its present level in the crust. Based on mass estimates from gravity interpretation, the average composition of the anorthosite complex approximates a noritic anorthosite; therefore, noritic anorthosite would be the best estimate for the parent magma that formed the Laramie anorthosite complex.

Oncolites from the Green River Formation of western Wyoming

CLAUDIA WOLFBAUER Department of Geology, University of Wyoming, Laramie, Wyoming 820701

Pages
61-62

Keywords
oncolites, Wyoming, Laney Shale, Green River, Lake Gosiute

Abstract
Oncolites have recently been discovered near Carter, Wyoming in the Laney Shale Member of the Green River Formation. They occur locally in a widespread lacustrine carbonate unit. Algal laminae are developed around the recrystallized shells of Goniobasis tenera. These oncolites are analogous to those described from the Flagstaff (Paleocene to Eocene) and North Horn (Late Cretaceous to Paleocene) Formations of Utah (Weiss, 1969) and from a modern stream in Meade County, Kentucky (Minckley, 1963). The oncolites formed in a bicarbonate-rich stream and were subsequently deposited in Eocene Lake Gosiute.

A Late Wisconsin-recent vertebrate fauna from the Chimney Rock animal trap, Larimer County, Colorado

MICHAEL W. HAGER Department of Geology, University of Wyoming, Laramie, Wyoming 82070

Pages
63-72

Keywords
Chimney Rock, Colorado, Wisconsin, fauna, Little Box Elder, Hell Gap

Abstract
The Chimney Rock animal trap is a circular depression in the Casper Sandstone approximately 65 feet in diameter and 10 feet deep with an overhang of 4 to 25 feet. Four to 6 feet of fine sand in the depression contain a late Pleistocene to recent fauna. The fauna consists of 33 vertebrate species, 7 of which show boreal affinities and whose extant representatives presently inhabit higher altitudes and latitudes. Three extinct species are present, Martes nobilis, Panthera atrox, and Neogyps errans. The absence of a small cusp on the M1 of Vulpes vulpes is tentatively correlated with a cold climate in contrast to the presence of the cusp in the same species of temperate zones. Bone from the 48 inch level yielded a date of 11,980 + 180C14 years.

The fauna and inferred late Wisconsin environment correlates with Little Box Elder Cave, Jaguar Cave, and part of the Hell Gap Site.

The bones were not vertically stratified in life zone assemblages and extinct species were found throughout the deposit. The animal trap had formed and major sedimentation occurred by the late Wisconsin. More recent faunas were added to and mixed in the deposit.

Dendroid graptolites from the Bighorn Dolomite (Ordovician), southeastern Idaho

ROBERT W. SCOTT The University of Texas at Arlington, Arlington, Texas
DONALD O. ASQUITH F. Beach Leighton and Associates, La Habra, California

Pages
73-82

Keywords
dendroid, graptolites, Bighorn, Idaho, Palisades Creek, biostratigraphic

Abstract
Nine dendroid species, two of them new, and one species of the inarticulate brachiopod, Lingulepis, were discovered near the top of the "lower unit" of the Bighorn Dolomite on Palisades Creek in southeastern Idaho. This faunule has some affinities with the more diverse dendroid fauna of Lower and Middle Ordovician rocks in the Appalachian Geosyncline. The biostratigraphic aspects of this faunule are not inconsistent with a previously established latest Medial Ordovician (Barnveld) age for the lower part of the Bighorn. The discovery of these dendroids occasioned a re-evaluation of the correlation of the Bighorn in western Wyoming and Idaho. A persistent unconformity between the "lower" and "upper" units can serve as a marker surface in this area to revise previous correlations that have been based on similar but homotaxial lithofacies.

Geology of the Sage Creek nephrite deposit, Wyoming

RICHARD L. SHERER Minerals Exploration Company, 1708 W. Grant Road, Tucson, Arizona 85705

Pages
83-87

Keywords
Sage Creek, nephrite, Wyoming, dike, metasomatic

Abstract
The Sage Creek nephrite deposit is related to amphibolite xenoliths in a quartz diorite dike. Nephrite and epidote-rich rocks have formed by metasomatic alteration of amphibolite. The chemical environment of nephrite formation was characterized by relatively high concentrations of calcium and magnesium, and relatively lower concentrations of sodium, iron, aluminum and silicon. Shearing stress did not play an important role in the development of nephrite.