Contributions to Geology 20.2

Implications of basement trends and features of eastern Wyoming and adjacent areas observed from National Uranium Resource Evaluation program (NURE) aeromagnetic data

GREGORY J. INDELICATO Kerr-McGee, Oklahoma City, Oklahoma 73125
KENNETH E. KARP Bendix Field Engineering Corporation, Grand Junction, Colorado 81502

Pages
85-90

Keywords
Wyoming, Mullen Creek, Nash Fork, shear zone, magnetic, structure, uranium, Archean

Abstract
Presented here is a study of regional magnetic basement trends and features for the eastern Wyoming Archean Province and surrounding areas. The data have been obtained from 14 National Uranium Resource Evaluation aeromagnetic surveys. Inferences have been made from only those magnetic trends and features which can be distinguished clearly on the map.

Magnetic trends in southeast Wyoming can be directly correlated with the Mullen Creek-Nash Fork shear zone. An apparent parallel, northeast-trending structure can be recognized in Colorado.

The aeromagnetic data reveal a contact between the eastern Wyoming Archean Province and the Proterozoic Churchill Province. The Black Hills are characterized by northwest magnetic trends of the Churchill Province. This suggests that the Archean rocks in the Black Hills may not be directly rooted to the Wyoming Province.

The role of seismic modeling in deep crustal reflection interpretation. Part I

YUN K. WONG, SCOTT B. SMITHSON and RONALD L. ZAWISLAK Department of Geology and Geophysics, Program for Crustal Studies, The University of Wyoming, Laramie, Wyoming 82071-3006

Pages
91-110

Keywords
reflection, seismic, crustal, diapirs, Moho

Abstract
Because crustal reflection interpretation is a new field, it requires special techniques. One of these techniques is seismic modeling, which constitutes a major constraint and control on deep crustal interpretations. Synthetic seismograms have, therefore, been generated on a computer for models of multiple-reflection generating systems, overthrust basement, a sheet-like granite batholith, a granite diapir, a layered mafic intrusion, a greenstone belt, a subduction complex, an island arc, a ductile deformation zone (mylonite), and the Helvetic nappes of the Alps. Modeling of multiple reflections with a sedimentary section demonstrates that short-path (peg-leg) multiples may constructively interfere to mask primary reflections from the crystalline crust. Reflection patterns from the different crustal structures are complex, showing abundant dipping events. Some dipping events follow raypaths related to reflectors at large lateral offsets. As a result, dipping reflections may be recorded where none would be expected so that a diapiric intrusion appears to be floored at a position where there are no actual reflectors. Mylonite zones may give a strong and distinctive reflection pattern. Reflections associated with the island-arc model may be very complex and the Moho reflection is broken up by the overlying velocity structure. The Helvetic-nappes model has distinctive reflection patterns associated with the nappes and with the root zone.

Depositional environments of high-level sand and gravel deposits, western Bighorn Mountains, Wyoming

R. CRAIG KOCHEL Department of Geology, State University College, Fredonia, New York 14063
DALE F. RITTER Department of Geology, Southern Illinois University at Carbondale, Carbondale, Illinois 62901

Pages
111-122

Keywords
fossils, Bighorn, depositional, Arikaree

Abstract
High-level sand and gravel deposits on the western flank of the Bighorn Mountains of Wyoming were examined to determine their environment of deposition. These deposits at 2750 m elevation have previously been interpreted as remnants of late Tertiary fluvial aggradation of the Bighorn Basin.

Gravel deposits at two localities contain a basal conglomerate which rests unconformably on Precambrian schist. Inversely graded beds, laterally and vertically discontinuous strata, channel-fill deposits, and textural data suggest that these sediments were deposited on an alluvial fan by debris flow and braided stream processes. Conformably above the conglomerate is a 25 m thick section of stream-deposited tuffaceous sandstone, the basal part of which contains numerous channels filled with angular crystalline debris. Our thin-section data and early Miocene terrestrial fossils described by previous workers suggest that this sandstone is correlative with the Arikaree Formation. Above the sandstone is 20 m of unconsolidated sand and gravel. High-level gravel deposits at four other localities are not alluvial, but resulted from glacial and colluvial processes.

Our sedimentological and geomorphological analyses indicate that the Bighorn Mountain high-level gravels accumulated in a variety of depositional environments. Therefore, they should not be considered collectively in interpretations of regional geologic history.

Baby dinosaurs from the Late Cretaceous Lance and Hell Creek formations and a description of a new species of theropod

KENNETH CARPENTER University of Colorado Museum, Boulder, Colorado 80309

Pages
123-134

Keywords
dinosaurs, Lance, Hell Creek, Wyoming, Montana, Cretaceous, egg

Abstract
Published accounts of very young or baby dinosaurs suggest that such remains are rare in the fossil record. The use of screen-washing in the Late Cretaceous Lance and Hell Creek formations in Wyoming and Montana, however, has produced small teeth and cranial elements which are identified as from baby dinosaurs. At least eleven taxa from nine localities are represented: a dromaeosaurid, an unidentifiable theropod, a tyrannosaurid, a ceratopsian, a hadrosaur, Saurornithoides inequalis, Pectinodon bakkeri new genus and species, Aublysodon mirandus, Paronychodon lacustris, Thescelosaurus sp., and Ankylosaurus magniventris. The abundance and diversity of remains of baby dinosaurs evidenced by this study suggest that they are not as rare as previously thought. Nevertheless, they are not as abundant as the remains of adults. The present global distribution of remains of baby dinosaurs and dinosaur egg shells suggests paleoenvironmental controls. The two most important controls are believed to be soil drainage and soil pH.

Diacodon alticuspis, an erinaceomorph insectivore from the early Eocene of northern New Mexico

MICHAEL J. NOVACEK Department of Zoology, San Diego State University, San Diego, California 92182-0063

Pages
135-149

Keywords
San Juan, New Mexico, Eocene, Diacodon, insectivore, teeth, erinaceids

Abstract
Undescribed fossils referable to Diacodon alticuspis from the Wasatchian San Juan Basin fauna of northern New Mexico, provide new evidence for transferral of Diacodon from the Leptictidae to the Erinaceomorpha. With this allocation for the type species, the original concept of Diacodon has changed considerably; in recent years five of the twelve named species of the genus have been transferred from the Leptictidae to early groups of lipotyphlous insectivores and the remaining seven species have been either invalidated or referred to other leptictid genera. Diacodon has a primitive erinaceomorph dentition resembling Mckennatherium in the lower teeth and Scenopagus in the upper teeth. Several derived dental features support the monophyletic concept of the Erinaceomorpha. This suborder is recognized here to include the Erinaceidae (late Paleocene-Recent) and the Dormaaliidae (early Eocene-middle Oligocene). In agreement with several previous studies, Adapisorex and Creotarsus are not regarded as valid erinaceomorphs, and thus erinaceomorph families or subfamilies based on these taxa are inappropriately named. Diacodon and Mckennatherium (and the possibly congeneric Adunator and "Diacodon" minutus) are Erinaceomorpha incertae sedis. Paleocene and Eocene erinaceids do not clearly relate to any of the recognized erinaceid subfamilies. Suggested affinities between the earliest erinaceids and galericines seem largely based on the common retention of primitive traits; the Galericinae requires a clearer definition to support a concept of its monophyly.