Contributions to Geology 9.1

Boys (geologists) be careful or Probable Tertiary landslide in western Washakie Basin, Wyoming

E. R. McAUSLAN Geologist, Special Projects, Union Pacific Railroad Company, Natural Resources Division, Rock Springs, Wyoming

Pages
1-6

Keywords
Tertiary, Washakie, Green River, Laney, Cathedral Bluffs

Abstract
Geological phenomena occasionally exhibit features not apparent from casual observations. A seemingly normal succession of Tertiary rocks in the western Washakie Basin, when measured, proved to be Green River-Laney beds lying unconformably on Wasatch Cathedral Bluffs as a result of an ancient landslide. Therefore, be careful when drawing conclusions based on brief geological examinations.

An unusual burrow of a Tertiary vertebrate

M. R. VOORHIES Department of Geology, University of Georgia, Athens, Georgia
HEINRICH TOOTS Department of Geology and Geography, C. W. Post College, Greenvale, N.Y.

Pages
7-8

Keywords
fossil, burrow, Ash Hollow, Nebraska, carnivore

Abstract
A large fossil burrow (perhaps the largest recorded) occurs in the unconformity between the late Tertiary Ash Hollow Formation and the Cretaceous Niobrara Formation at Medicine Creek Dam, Frontier County, Nebraska. A large fossorial rodent (e.g. Epigautus) or a medium-size carnivore (e.g. Pliotaxidea, Leptocyon) was probably responsible for digging it.

The petrology and chemistry of the Karmutsen Group volcanic rocks

RONALD C. SURDAM Department of Geology, University of Wyoming, Laramie, Wyoming 82070

Pages
9-12

Keywords
mineral, Karmutsen, volcanic, Triassic, Vancouver

Abstract
The primary mineral chemistry of the Triassic Karmutsen Group volcanic rocks of central Vancouver Island, British Columbia, is characterized by striking compositional variations. These variations reflect inhomogeneities in the original bulk composition. It is suggested that the Karmutsen Group volcanics, a classical suite of eugeosynclinalrocks, were originally theoleiitic.

An Eocene puffbird from Wyoming

PIERCE BRODKORB Department of Zoology, University of Florida, Gainesville, Florida

Pages
13-16

Keywords
puffbird, Piciformes, Eocene, Wyoming, fossil, Green River, Bucconidae

Abstract
A new genus and species of Bucconidae, Primobucco mcgrewi, is described from Lincoln County, Wyoming, in Lower Eocene beds of the Green River Formation (late Wasatchian age). The type specimen is a wing. This fossil provides the earliest record of the family, and of the order Piciformes, of which the puffbirds are the structurally most primitive group. Today the Bucconidae are restricted to the American tropics.

Another fossil bird, Uintornis lucaris Marsh, was described on a tarsometatarsus from the Middle Eocene in the upper part of the Bridger Formation at Henry's Fork, Wyoming. It has been considered as of uncertain systematic position but is here referred to the Bucconidae. It is a larger bird than the Green River species.

Interpretation of gravity anomalies in Precambrian rocks of the central Laramie Mountains, Wyoming

ROBERT D. WOODFILL Department of Geology, University of Wyoming, Laramie, Wyoming 82070

Pages
17-26

Keywords
Slate Creek, Precambrian, Laramie Mountains, Wyoming, anomaly, Squaw Peak, granite, gneiss

Abstract
About 300 gravity stations were established one-half mile apart in the Squaw Rock area in order to determine geometric relationships in Precambrian crystalline rocks. These rocks consist of granite gneiss, mica schist, and hornblende schist folded into large plunging antiforms and synforms, a domal granite gneiss complex, and a syenite intrusion. A surveying technique based on resection and trigonometric leveling was used to determine horizontal and vertical positions for gravity stations. The accuracy for vertical position was 1.0 foot for 22 determinations. Estimates to determine the error of the gravity survey were made. The deviation of the Bouguer gravity values were found to range between + 0.47 mgal of the calculated Bouguer gravity value with a 95 per cent confidence.

A sampling program was used to determine a density value for each major Precambrian rock unit in the area. Density values with a standard deviation less than 0.04 gm/cc were assigned to the major lithologic units.

The principal features of the Bouguer gravity anomaly map are a northeast-southwest trend of the regional gradient, a gravity low over the Squaw Peak granite gneiss, and a gravity high near the granite geniss-hornblende syenite contact.

Most of the residual anomalies in the area studied have a correlation with geology. The Squaw Peak granite gneiss has an anomaly of -3 mgal which is attributed to the mass deficiency of the granite gneiss. A +5 mgal anomaly located near the granite gneiss-hornblende syenite contact is attributed to a subsurface disturbing mass underneath the syenite.

A gravity model of the Slate Creek area is composed of synformal dense mafic schists. This gravity model suggests the hornblende schist unit of the George Creek synform thins with depth, and the Slate Creek synform is approximately 0.6 km deep. A subsurface disturbing mass is used to interpret the anomaly across the Squaw Peak granite gneiss-hornblende schist contact in the Slate Creek area. The hornblende schist west of the Squaw Peak granite gneiss complex has an anomaly of +2 mgal and a calculated thickness of 0.4 km. This rock unit is thought to continue beneath the hornblende syenite and cause the positive anomaly south of the Squaw Peak granite gneiss complex. A regional gradient at approximately 90¡ to the large-scale regional gradient of the Laramie Mountains was found in the Squaw Rock area. This small-scale regional could be caused by a sloping interface of more dense material below the surface.

Geology and geophysics of syenites associated with Laramie anorthosite, Wyoming

BRUCE D. SMITH Dept. of Geology, University of Wyoming, Laramie, Wyoming
DENNIS S. HODGE Dept. of Geological Sciences, State University of New York, Buffalo, New York
SCOTT B. SMITHSON Dept. of Geology, University of Wyoming, Laramie, Wyoming

Pages
27-38

Keywords
syenites, anorthosite, Wyoming, Laramie, gravity, differentiation

Abstract
Pyroxene and hornblende syenites comprise the northern border of the Laramie anorthosite complex. The syenites intrude regional gneisses that are metamorphosed in the middle to upper amphibolite facies. Locally the syenites have caused thermal metamorphism in country rocks. Chemical and mineralogical composition of these syenites closely resembles composition of syenites from other anorthosite complexes. Syenite has been partially emplaced by forcible intrusion. Positive gravity anomalies follow the syenites and peak over norite exposed within hypersthene syenite; however, densities of syenites are lower than densities of most other rocks in the area. Gravity interpretation suggests that syenites are underlain at shallow depth by dense rocks such as norite. The mass of the dense rocks is several times greater than the mass of the syenite. This interpretation indicates that syenites were formed by differentiation from mafic rock associated with anorthosite.

The Late Miocene Trail Creek mammalian fauna

ANN FORSTEN Institute of Zoology, N. Jarnvagsgatan 13, Helsinki, Finland

Pages
39-52

Keywords
carnivora, Trail Creek, Miocene, fauna

Abstract
Voorhies (1965) recently described the Carnivora from Trail Creek, Wyoming. The rest of the fauna is described below. The specimens are from the Trail Creek quarry in NE 1/4, SEc. 29, T. 17 N., R. 66 W., about 20 miles NE of Cheyenne, Wyoming.