Contributions to Geology 7.2

Precambrian stromatolites, bioherms, and reefs in the lower half of the Nash Formation, Medicine Bow Mountains, Wyoming

S. H. KNIGHT Professor Emeritus, Department of Geology, University of Wyoming, Laramie

Pages
73-116

Keywords
metamorphism, Nash Formation, Medicine Bow Mountains, Wyoming, stromatolites, bioherms, reefs, metasediments, Proterozoic, metadolomite

Abstract
Early Proterozoic stromatolitic metadolomite bioherms and reefs occur in the Nash Formation, Medicine Bow Mountains, Wyoming. The Nash Formation (6,500 feet thick) is part of a 25,000 foot sequence of metasediments which forms the core of the range. The sequence ranges in age from 1,650 m.y. to less than 2,340 m.y. The Nash Formation is believed to be not less than 1,700 m.y. old.

Some 150 bioherms and three reefs were mapped. The bioherms range from four to one hundred feet thick and from ten to one hundred feet long. The reefs are two hundred feet thick and several thousand feet long. The structural forms and growth-patterns are described in physical (geometric) terms. Descriptions are based upon field observations and on one hundred large (6x9 cms) thin sections having a total area of 5,000 square cms. Three types of stromatolites are recognized: (1) digitate type -- characterized by cyclic developmental units consisting of digital growth-patterns alternating with fine-grained metadolomitic fill; (2) undulate type -- composed of alternating successions of undulating graphitic sheets (where best preserved) separated by bands of metadolomite; (3) micro-oncolite type consisting of very small (0.5-1.0 mm.) ovate forms with enclosed nuceli. Structural forms include: single and laterally-linked domes; spheroidal, ellipsoidal and oblong-shaped heads; and combinations.

Changes attributable to metamorphism are discussed.

Tertiary faults with reversed movements near Copper Mountain, central Wyoming

C. S. FERRIS, JR. Xerr-McGee Corporation, Box 98, Arvada, Colorado 80002

Pages
117-128

Keywords
Owl Creek Mountains, faults, Copper Mountain, Wyoming, Cedar Ridge, Wind River, Teepee Trail, Steffen Hill

Abstract
The eastern Owl Creek Mountains of central Wyoming are part of a block of Precambrian and younger rocks which moved upward and southward during very early Eocene time on a moderately high angle thrust fault. Subsequently the leading edge or toe of the thrust was extensively broken by normal faulting, and the part of the thrust block north of the toe rose and became the arch known as Copper Mountain. The southernmost visible structure in the faulted toe area is the Cedar Ridge fault scarp which is composed of post-thrust arkoses of the Lower Eocene Wind River Formation. Most of the area of the toe between Copper Mountain and Cedar Ridge is directly underlain by mudstone and muddy conglomerate of the Middle and Upper Eocene Tepee Trail Formations. However, within this central region there are horst blocks of Wind River Formation and the Steffen Hill complex of Paleozoic strata and Precambrian igneous and metamorphic rocks.

Drill hole data suggest that arkose of the Wind River Formation exists in small outcrops and more extensively in subsurface a short distance north of the Cedar Ridge fault. Interestingly, detrital glauconite occurs in this arkose. In most of the toe which was drilled, however, the Tepee Trail is not underlain by the Wind River Formation but by Paleozoic and Precambrian rocks, indicating extensive erosion of Wind River sediment on the toe of the thrust prior to Tepee Trail time. It is postulated that the Wind River horst blocks, and very possibly the Steffen Hill complex as well, were grabens at the time of this erosion and were thus protected. They were faulted upward into their present positions after Tepee Trail time.

Nyctitheriidae (Mammalia, Insectivora) from the Bridger Formation of Wyoming

PETER ROBINSON University of Colorado Museum, Boulder, Colorado

Pages
129-138

Keywords
insectivores, nyctitheriidae, Bridger, Wyoming, Eocene

Abstract
In 1872, Othniel Charles Marsh described several genera of small insectivores from the Bridger Formation of southwestern Wyoming. Marsh did not review these animals and the small insectivores were overlooked until Matthew's classic work on the Bridger Carnivora and Insectivora appeared in 1909. Several people have studied individual specimens of Marsh's Bridger insectivores in relation to general studies on particular insectivore groups or in relation to Eocene or Paleocene insectivores, but the entire, unexcelled, collection of small, middle Eocene (Bridger) insectivores at Yale has not been available until recently. Indeed, many of the specimens discussed here were collected for Marsh but were never seen by him, and Matthew only saw a small fraction of the collection.