Contributions to Geology 25.2

Sequential development of Dry Fork Ridge anticline, northeastern Bighorn Mountains, Wyoming and Montana

PETER H. HENNINGS, JOHN H. SPANG Center for Tectonophysics, Department of Geology, Texas A&M University, College Station, Texas 77843-3113

Pages
73-94

Keywords
Dry Fork Ridge, uplift, Bighorn, Wyoming, Montana, Tongue River, fractures, fault

Abstract
Dry Fork Ridge is a crystalline basement-cored uplift that forms the northeastern margin of the Bighorn Mountains between the Tongue River Lineament, which trends N60E, and a parallel linear feature to the north. The controlling basement fault (reverse) trends N45W and dips 76 degrees to the northeast. Analysis of fractures in basement indicates that pre-existing fractures controlled the geometry of the uplift during Laramide deformation. An anticline/syncline pair in the cover rocks has been tightly folded to conform to the basement hanging wall block that has been uplifted and rotated to the northeast.

Structural relief on the basement fault decreases from 915 m (3,000 ft) to 0 m toward the northern termination. It is probable that the controlling basement fault terminates against a transverse fault or zone of faults coincident with a major linear feature that transects the Bighorn Mountains. Intersecting balanced cross sections provide a palinspastically controlled, three-dimensional model that predicts basement geometry and yields a kinematic model of the development of the basement uplift and the overlying fold pair in the cover rocks. The basement fault has been projected to depth using a displacement vector (slip line) method which yields a listric fault surface that flattens at 8,800 m (29,000 ft) below sea level.

Geology and mammalian paleontology of the Wind River Formation, Laramie Basin, southeastern Wyoming

JEANNE R. DAVIDSON Department of Geology and Geophysics, The University of Wyoming, Laramie, Wyoming 82071-3006

Pages
103-132

Keywords
fossil, mammals, Wind River, Wyoming, Sciuravus, fauna

Abstract
Fossil mammals were studied from the Wind River Formation in Albany County, Cooper Lake area, of the Laramie Basin. Thirteen species are newly recorded and described, including the earliest occurrence of a species of Sciuravus. The fauna is similar to the early Wasatchian Four Mile and No Water faunas. It overlaps the upper and lower parts of the Haplomylus-Ectocion Range-Zone and straddles Biohorizon A. All major structural elements of the Laramie Basin were present prior to the early Graybullian when the Wind River Formation was deposited. Use of the term "Wind River Formation" for rocks in the Laramie Basin is questionable.

Paleogeomorphic significance of two paleosols in the Dakota Formation (Cretaceous), southeastern Nebraska

R. M. JOECKEL Department of Geology, University of Nebraska-Lincoln, Lincoln, Nebraska 68588

Pages
95-102

Keywords
paleosol, Dakota, Nebraska, Cretaceous, channel sandstone

Abstract
Much of the Dakota Formation in southeastern Nebraska consists of channel sediments from high-sinuosity streams and associated paleosol-containing floodplain siltstones. Two paleosols, one in Lancaster County, Nebraska and another in Jefferson County, are overlain by extensive channel sandstones; the contact of the paleosol-bearing units and the sandstones represents an abrupt facies change and a significant intraformational diastem. The paleosols have discernible B horizons; one paleosol is particularly thick and has well-differentiated A, E, and C horizons. These paleosols are probably the products of long periods (perhaps 10,000 years in the case of the thick paleosol) of local non-deposition controlled by eustatic changes in sea level.

Middle Blancan vertebrate assemblage from the Verde Formation, Arizona

NICHOLAS J. CZAPLEWSKI Ralph M. Bilby Research Center and Department of Biological Sciences, Northern Arizona University, Flagstaff, Arizona 86011

Pages
133-157

Keywords
vertebrate, Verde, Arizona, Blancan, Clarkdale, fossils

Abstract
The Verde Formation in its northern portion consists primarily of lacustrine and fluvial sediments with a 4.5 million year old (mya) interbedded lava flow. Few vertebrate fossils were previously known from the formation. An assemblage of vertebrate species occurring in some of the uppermost beds of the northern Verde Formation is named the Clarkdale local fauna. The list of vertebrates includes a fish, an amphibian, four reptiles, a bird and 21 mammals. Most of the specimens were wet-screened from a small amount of green clay matrix removed from a gomphothere skull. Members of the local fauna represent probable inhabitants of a diversity of paleocommunities including aquatic, lake margin, and terrestrial environments. Biochronologic correlation suggests a middle Blancan (middle Pliocene) age for the Clarkdale local fauna.

Airflow-terrain and hydrologic controls on eolian sedimentation and Holocene paleoclimatic fluctuations in Wyoming

DAVID R. GAYLORD Department of Geology, Washington State University, Pullman, Washington 99164-2812

Pages
157-166

Keywords
eolian, Holocene, Wyoming, dunes, paleoclimatic, sedimentation

Abstract
Eolian deposits, and particularly sand dunes, are important repositories of paleoclimatically-sensitive data. Sand dunes and related interdune deposits preserve records of both the timing and character of paleoclimatic fluctuations; they bury datable material rapidly, and record prevailing depositional conditions in primary sedimentary structures.

Interbedded dune and interdune deposits exposed along the Clear Creek valley of south-central Wyoming offer insight into paleoclimatic fluctuations that affected that area during at least the last 8000 yrs. An unusual combination of airflow-terrain interactions and hydrologic factors promoted net sediment accumulation near Clear Creek, even when climatically-induced stresses favored deflation over much of the remainder of the dune field. Recognition of the manner in which airflow-terrain interactions and hydrology have dictated the styles and modes of sedimentation at Clear Creek is essential to paleoclimatic interpretation there. Recognition of the manner in which these controlling factors interacted to promote sedimentation also is an important factor in locating other similarly complete and informative eolian sedimentary sequences.