Contributions to Geology 22.2

Precambrian iron-rich pods and uranium mineralization near Warm Spring Creek, Fremont County, Wyoming

EDWIN ULLMER Rocky Mountain Energy Company, Broomfield, Colorado 80020

Pages
75-82

Keywords
Little Warm Spring, uranium, Wind River, Wyoming, Precambrian, lineament, iron

Abstract
The mineralization of the Little Warm Spring uranium prospect, located in the Archean gneiss terrain of the northern Wind River Mountains, is hosted by an iron-rich pod of micaceous schist. The schist is probably an alteration product of a rock type similar to a small, granular iron-rich pod about five kilometers north-northwest of the prospect that contains an unusual primary mineralogic assemblage of quartz, olivine, hypersthene, ferroaugite, and magnetite. The mafic minerals are high-iron varieties. The northern pod contains no uranium, but is partly altered by crosscutting granite dikes. Uranium is a late introduction in the Warm Spring pod and occurs as uraninite and secondaries within a brittlely deformed zone. Elevated quantities of Co, Ag, Mo, Ni, Cu, Pb, and V correspond with the uranium mineralization. From high altitude photography, a single lineament can be discerned passing through both pods. Float from other altered and unaltered pods is also found along the lineament. Comparative studies suggest that the unaltered pods are thermally metamorphosed slatey and cherty iron formation. Similar mineralogic assemblages are found as products of contact metamorphism in portions of the banded iron formations of Minnesota and Wisconsin. More extensive iron formation is also found about 16 km (10 mi.) south of the Little Warm Spring prospect in similar geologic terrain. The uranium mineralization is probably genetically related to one of the granite intrusions found in the region.

Revision of type Lower Triassic Dinwoody Formation, Wyoming, and designation of principal reference section

RACHEL K. PAULL and RICHARD A. PAULL Department of Geological and Geophysical Sciences, The University of Wisconsin-Milwaukee, Milwaukee, Wisconsin 53201

Pages
83-90

Keywords
Dinwoody, Wyoming, Triassic, Red Peak, Chugwater

Abstract
A revision of the upper boundary of the Lower Triassic Dinwoody Formation in the type area of west-central Wyoming is proposed to eliminate an anomalous situation. The upper contact should be reestablished at the color change from greenish gray to red lithologies of the overlying Red Peak Formation of the Chugwater Group, rather than at the top of a locally developed, relatively resistant siltstone unit near the middle of the formation as initially described. This revision is in agreement with the original definitions for both the Dinwoody and Red Peak Formations. It also is consistent with the placement of the upper contact of the Dinwoody in northern Utah, western Wyoming, easternmost Idaho, and southwestern Montana where it is overlain by red beds of the Chugwater Group or the Woodside Formation.

Since the type section of the Dinwoody is in an area of the Wind River Indian Reservation where geologic field work is prohibited, a principal reference section is designated. This section is located on public lands in proximity to the type section, and it is similar to the type section in thickness, lithology, and age.

Currant Creek Formation: record of tectonism in Sevier-Laramide orogenic belt, north-central Utah

JOHN S. ISBY and M. DANE PICARD Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112

Pages
91-108

Keywords
Currant Creek, Uinta, Utah, Sevier, paleocurrent, uplift

Abstract
The Currant Creek Formation is a sequence of conglomerate, sandstone, and siltstone that crops out in a narrow band along the northwestern margin of the Uinta Basin in north-central Utah. At its base, where it unconformably overlies the Upper Cretaceous Mesaverde Formation, its age is Maestrichtian; the age of the upper part is unknown. The Duchesne River Formation (Eocene-Oligocene?) unconformably overlies the Currant Creek Formation.

Conglomerate is petromict. Clasts range from boulder to granule in size and decrease in maximum diameter from west to east. Sandstone is primarily quartz arenite, sublitharenite and litharenite. Grains are moderately well to poorly sorted.

Paleocurrent directions measured from cross-stratification in sandstone give a dominant southerly transport direction and a minor component to the east. A strong east-southeasterly direction measured in imbricated clasts and sandstone lenses within conglomerate intervals may indicate multiple source areas for the detritus. Coarse-grained material was probably derived mainly from the west and fine-grained material dominantly from the north.

The formation was deposited in a fluvial setting. Lateral gradations in grain size define proximal, medial, and distal parts of alluvial fans. Fan facies range from coarse-grained proximal deposits that represent extensive channel lag and bar deposition to distal beds that form an intricate braided-stream network.

Formerly, there was a much more laterally extensive clastic wedge that extended eastward from the Wasatch Range in latest Cretaceous time. This sedimentary body graded on its southern edge into continental alluvial-plain facies and on its northern edge into sandstone facies that were eroded from the newly risen western part of the Uinta arch. This synorogenic deposit therefore records major tectonism within the Wasatch Range and, moreover, may document initial uplift of the Uinta Mountains.

Megaclasts in alluvial fills from the Ogallala Group (Miocene), Banner, Kimball, and Morrill counties, Nebraska

R. F. DIFFENDAL, JR. Conservation and Survey Division, IANR, University of Nebraska-Lincoln, Lincoln, Nebraska 68588-0517

Pages
109-116

Keywords
Ogallala, Nebraska, Miocene, megaclasts, mass wasting

Abstract
Locally derived blocks and boulders (megaclasts) occur in conglomerate and in sand and gravel fills of channels in the Ogallala Group (Miocene) at localities in Banner, Kimball, and Morrill counties in western Nebraska. Some of the megaclasts are up to one or more orders of magnitude larger than the largest distantly derived grains in the sediments surrounding them. A number of these megaclasts probably moved downslope by mass wasting from outcrop sites on paleovalley sides and were later transported by running water. Other blocks and boulders were eroded by streams from bedrock along channel sides and then were transported by these bodies of water to their depositional sites.

The compositions and source formations of the blocks and boulders vary. Clasts of sandstone, caliche, and cemented volcanic ash were derived from erosion of older beds of the Ogallala Group. Interbedded sandstone and concretions were eroded from rocks of the Arikaree Group, and masses of siltstone were part of the Brule Formation.