Contributions to Geology 8.2

The northern Gallatin Range, Montana: northwestern part of the Absaroka-Gallatin volcanic field

ROBERT A. CHADWICK Department of Earth Sciences, Montana State University, Bozeman, Mont.

Pages
150-167

Keywords
breccia, stock, Gallatin Range, Montana, Absaroka, volcanic, dikes, Hyalite Peak, Golmeyer Creek, metamorphic

Abstract
The northern Gallatin Range of southwestern Montana is largely underlain by Eocene volcanic rocks which unconformably overlie Precambrian metamorphic rocks and Paleozoic-Mesozoic sedimentary beds. These volcanics are at the northwestern end of the Absaroka-Gallatin volcanic province. A belt of eruptive centers extending from the eastern Absaroka Range through the northern Gallatin Range evidently furnished lava flows and breccias for much of the volcanic field.

The Golmeyer Creek Volcanics, of restricted extent, are more than 2,000 feet thick. Lava flows, which predominate over breccias, are moderately to extensively chloritized and propylitized augite, hornblende, and hypersthene-bearing andesites. The breccias are andesitic to dacitic, well-consolidated, highly altered, and probably of epiclastic origin.

The more widespread Hyalite Peak Volcanics unconformably overlie the Golmeyer Creek Volcanics, early Tertiary conglomerates, or pre-Tertiary rocks. This sequence is at least 2,800 feet and probably over 4,000 feet thick and consists of lava flows and subordinate intercalated epiclastic and flow breccias. The flows are augite-hypersthene andesites which are in places oxidized but less chloritized or propylitized than the Golmeyer Creek Volcanics. Breccias range from monolithologic, unstratified flow breccias to heterolithologic, poorly to well stratified laharic and fluvial types.

Biotite and biotite-hornblende dacite stocks, plugs, and dikes intrude the volcanic rocks in and between eruptive centers. Dacite from one stock is dated at 49.5 m.y.

Igneous centers of the northern Gallatin Range erupted principally mafic andesite; in contrast, centers along the belt further southeast, in the Yellowstone Valley, western Beartooth Range, and eastern Absaroka Range, extruded a greater variety of lavas and breccias.

Environmental reconstruction from cross-stratification: an example from the Pass Peak Formation (Eocene), western Wyoming

JAMES R. STEIDTMANN Department of Geology, University of Wyoming, Laramie 82070

Pages
168-171

Keywords
sigmoidal, cross-stratification, Pass Peak, point bar, stream

Abstract
An association of large scale, sigmoidal cross-stratification overlain by trough cross-stratification in the Pass Peak Formation is interpreted as a point bar deposit. Comparison of the characteristics of these structures with observations on modern point bars, streams and flumes allows interpretations regarding flow regime, sediment supply, sinuosity, width and depth of the stream depositing the sediment.

Preliminary gravity study of the Precambrian Sherman Granite, Albany and Laramie Counties, Wyoming

WALLACE A. BOTHNER Dept. of Geology, University of New Hampshire, Durham, New Hampshire 03824

Pages
172-177

Keywords
Sherman, granite, Wyoming, Precambrian, gravity, batholith, Laramie Mountains

Abstract
The Precambrian Sherman granite of southeastern Wyoming is a homogeneous-appearing, coarse-grained, porphyritic granite over much of its areal extent. Strong gravity relief over the batholith is lacking (maximum relief is less than 6 mgal) which suggests the absence of major lateral inhomogeneities within the granite. An elongate north-south trending positive Bouguer anomaly of low magnitude is found over the anticlinal crest of the southern Laramie Mountains. Gravity models support the idea that more dense material may occupy a shallow position beneath the crest as a result of arching and subsequent erosion.

Pseudobrecciation in the Valentine Formation (Late Tertiary of Nebraska)

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

Pages
178-183

Keywords
sedimentary, structures, brecciation, Valentine, Nebraska, Tertiary, Burge, Devils Canyon, quicksand

Abstract
Sedimentary structures resembling brecciation were found in the Burge Sand Member of the Valentine Formation in Devils Canyon, Brown Co., Nebraska. Layers of cohesive greenish sand are disrupted by tubular and tabular bodies of less cohesive gray sand. In the majority of cases evidence indicates that parts of the greenish layers have not been moved out of their original position.

These structures were probably formed by injection of mobilized sand (quicksand) into burrows and along planes of weakness in layers of the greenish sand.

Biostratigraphy of fluvial sediments of the Upper Wasatch Formation in the Northern Green River Basin, Wyoming

ROBERT M. WEST Department of Biology, Adelphi University, Garden City, New York 11530

Pages
184-197

Keywords
Bridger, fauna, Wasatch, Green River, Cathedral Bluffs, New Fork, Great Divide, lacustrine

Abstract
Three distinct subunits of the flavial Eocene Wasatch Formation are present in the northern Green River Basin: the arkosic facies and western facies of the New Fork Tongue, and the Cathedral Bluffs Tongue. These tongues extend toward the center of the basin from various source areas and terminate by facies change into lacustrine sediments of the Green River Formation.

The arkosic facies of the New Fork Tongue is an informally named variegated mudstone unit along the southwestern flank of the Wind River Mountains, the source of its detritus. It contains a late Early Eocene mammalian fauna. The western facies of the New Fork Tongue, fine-grained sandy mudstone variegated in more pastel tones than the arkosic facies, intergrades laterally with the arkosic facies in the northern Green River Basin. Western facies detritus was derived mainly from the western overthrust ranges. Its fauna is essentially the same as that of the arkosic facies. The Cathedral Bluffs Tongue, which consists mostly of drab green to brown mudstones and arkoses with some tuffaceous material, overlies the arkosic facies in the northern Green River Basin. The Cathedral Bluffs Tongue fauna is younger than that of the New Fork Tongue, as it lacks the representative Lost Cabinian taxa, and contains many elements of the Bridger B fauna; in this area it may actually represent the poorly fossiliferous Bridger A horizon.

The age of the much thicker Cathedral Bluffs Tongue sequence in the Washakie and Great Divide Basins is difficult to determine because of the paucity of fossils. Available evidence indicates a post-Lost Cabinian, probably earliest Bridgerian age, for at least the upper part of the unit.