Contributions to Geology 24.1

Paleocene bird and amphibian tracks from the Fort Union Formation, Bighorn Basin, Wyoming

KIRK R. JOHNSON Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06511

Pages
1-10

Keywords
fossil, bird, amphibian, tracks, Fort Union, Bighorn, Wyoming, Hunt Creek, Paleocene

Abstract
This paper documents fossil bird and amphibian tracks from the lowest Paleocene (Puercan) Hunt Creek member (informal name) of the Fort Union Formation in the Bighorn Basin of Wyoming. This is the first record of trace fossils of these taxa at this stratigraphic interval in the Bighorn Basin. A paleoenvironmental reconstruction suggests that the tracks were made on sandy shores of two streams that flowed through a wooded lowland.

Paleontology and biostratigraphy of lower part of Polecat Bench Formation, southern Bighorn Basin, Wyoming

JANE E. HARTMAN Department of Geology and Geophysics, The University of Wyoming, Laramie, Wyoming 82071-3006

Pages
11-64

Keywords
Bighorn, Polecat Bench, Wyoming, fossil, mammals, Paleocene

Abstract
Fossil mammals were studied from 47 localities in the lower 350 m of the Polecat Bench Formation of early Tertiary age in adjacent parts of Washakie and Hot Springs counties, Cedar Mountain area, southern Bighorn Basin, Wyoming. A few specimens of fossil mammals tentatively show the section from 0-62 m above the base of the formation to be Puercan ("early" Paleocene) in age. The section from 62-185 m above the base was unfossiliferous. The section from 185-300 m above the base was richly fossiliferous, with a fauna of latest Torrejonian ("middle" Paleocene) age (Pronothodectes jepi Zone). This fauna is similar in composition and diversity to other northern localities representative of the late Torrejonian (e.g., Rock Bench and Gidley quarries). The section from 300-338 m was found to be of earliest Tiffanian ("late" Paleocene) age (Plesiadapis praecursor Zone). The included fauna has a strong Torrejonian element, with transition to the Tiffanian marked by the presence of Plesiadapis praecursor and Nannodectes intermedius. The remaining 550 m of the Polecat Bench Formation, though unstudied paleontologically, presumably represents all or part of the later Paleocene and possibly earliest Eocene. Detailed dating of strata established in this report will allow further studies on all aspects of geologic history of the southern Bighorn Basin for precise increments of early and middle Paleocene time.

Dental anatomy and ontogeny of Lambdopsalis bulla (Mammalia, Multituberculata)

MIAO DESUI Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, P. O. Box 643, Beijing, Peoples Republic of China

Pages
65-76

Keywords
Paleogene, multituberculate, Lambdopsalis bulla

Abstract
The early Paleogene taeniolabidoid multituberculate Lambdopsalis bulla was named by Chow and Qi in 1978, but its taxonomic validity was soon challenged (Kielan-Jaworowska and Sloan, 1979). Examination of new material from another locality (Bayn Ulan in Inner Mongolia) shows the genus and species to be valid. Cladistic analysis clearly documents phylogenetic unity of the Asian Tertiary multituberculates Prionessus, Sphenopsalis, and Lambdopsalis. Dental anatomy and ontogeny of Lambdopsalis bulla are treated in detail.

The South Fork detachment fault, Park County, Wyoming: discussion and reply

WILLIAM G. PIERCE U.S. Geological Survey, Geologic Division; Branch of Western Regional Geology, 345 Middlefield Road, Menlo Park, California 94025

Pages
77-90

Keywords
South Fork, fault, Wyoming, Heart Mountain, Hardpan

Abstract
Blackstone (1985) published an interpretation of South Fork detachment fault and related features. His interpretation of the area between Castle and Hardpan transverse faults is identical to mine of 1941. Subsequent detailed mapping has shown that the structure between the transverse faults is more complicated than originally envisioned and resurrected by Blackstone. The present paper describes and discusses geologic features that are the basis for my interpretations; also discussed are differences between my interpretations and those of Blackstone. Most data are shown on the geologic map of the Wapiti Quadrangle (Pierce and Nelson, 1969). Blackstone's "allochthonous" masses are part of the South Fork fault. Occurrences of Sundance Formation, which he interpreted as the upper plate of his "North Fork fault," are related to Heart Mountain fault. Volcaniclastic rocks south of Jim Mountain mapped as Aycross Formation by Torres and Gingerich may be Cathedral Cliffs Formation, emplaced by movement of the Heart Mountain fault.

The late Cenozoic Camas Prairie Rift, south-central Idaho

J. KELLY CLUER Department of Geosciences, The University of Arizona, Tucson, Arizona 85721
BRIAN L. CLUER Department of Geology, Idaho State University, Pocatello, Idaho 832

Pages
91-102

Keywords
Camas Prairie Rift, Idaho, Cenozoic, extensional, faults, flows

Abstract
A new structural interpretation of the Camas Prairie in south-central Idaho is described and the name "Camas Prairie Rift" is proposed for this prominent topographic feature. Evidence of rifting is prevalent and several geological phenomena are described, each of which independently suggests that north-south extensional tectonics dominated the late Cenozoic structural evolution of the area. Relationships of normal faults, intrabasinal basalt flows, relict base levels, and provenance studies of Miocene gravel deposits all reveal the presence of rift structure development, and constrain extension to sometime between about 5 m.y. and 1.8 m.y. ago. Geomorphological relationships and a general lack of present seismicity are used to infer that rifting began and ended quite suddenly, lasting only a relatively short
period.

Deformational shortening of the Snake River Plain is developed into a model of marginal rifting. Plio-Pleistocene tectono-isotatic downwarping of the Snake River Plain has effectively shortened the Miocene volcanic surface of the proto Snake River Plain. Shortening has been compensated for by extension at the plain's margins where graben have developed that trend parallel to the axis of the Snake River Plain. The Camas Prairie Rift is one of these graben which occupy a structurally weak geologic transition zone between the late Cenozoic Snake River Plain volcanics and the Cretaceous Idaho batholith.

The Camas Prairie Rift model successfully integrates many geologic features into a unified geological interpretation. More detailed studies such as geothermal and ground water exploration now can be carried out with an efficient model of extensional tectonics as a beginning framework.