Contributions to Geology 31.2

A pre-Lancian regional unconformity and its relationship to Hell Creek paleogeography in south-eastern Montana

EDWARD S. BELT Department of Geology, Amherst College, Amherst, MA 01002; JASON F. HICKS, Department of Geology & Geophysics, Yale University, P.O. Box 208109, New Haven, CT 06520-8109; DAVID A. MURPHY, Department of Geological Sciences, San Diego State

Pages
1-26

Keywords
A pre-Lancian regional unconformity and its relationship to Hell Creek paleogeography in south-eastern Montana

Abstract
A regional unconformity lies beneath Harebell-Lance-Hell Creek nonmarine facies and above much older Meeteetse and Fox Hills marginal marine facies. This pre-Lancian unconformity (U1) extends eastward from the Sevier overthrust belt in northwestern Wyoming to the southwest margin of the Williston cratonic basin in the tri-state area of Montana and the two Dakotas. Chronostratigraphic data from five areas scattered from west to east across 600 km of the Western Interior Basin bracket the ages of this unconformity with precision that is based on isotopic ages, magnetozone boundaries, and ammonite range zones. Deposition on top of this unconformable surface occurred 3.5 Myrs earlier in NW Wyoming than in SE Montana.The erosion represented by the unconformity migrated eastward with time, generally removing 2 Myrs of strata as it went.

A tectonic cause is more probable than a eustatic one for the U1 unconformity. If an erosional gap resulted from lower-ing global sea level, a larger time gap would have been produced on the eastern side of the Western Interior Basin than on the western side. Our data show the opposite pattern of ages: older and wider time gaps occur in the Jackson Hole Basin of NW Wyoming than are found in north-central Wyoming or in SE Montana.

Sedimentological studies in the tri-state area in this and previous research shows that at the beginning of Lancian time, neither the Big Horn Mountains nor the Black Hills were uplifted source areas. Detailed sedimentological investigation has identified a trunk paleodrainage of the Hell Creek Formation in southeastern Montana (67 Ma to 65 Ma) that flowed to the east and southeast. Hell Creek and Lance drainage in central Montana and central northern Wyoming flowed northeast to east (Connor, 1992; Lillegraven and Ostresh, 1988) and represented tributary drainage to the trunk system of eastern Montana. The position of this trunk drainage follows the axis of the foreland basin, which by 67 Ma had shifted eastward into eastern Montana, half way between the Sevier overthrust belt and the North American craton.

The Big Horn Mountains were uplifted approximately at the time of the K/T boundary, but this uplift lay sufficiently to the west that it did not divert southeast flowing Puercan drainages of the Tullock and Lower Ludlow members of the Fort Union Formation. At some time after the Puercan and prior to the mid-Torrejonian, the Black Hills were uplifted. This uplift produced a local unconformity (U2) along the Miles City Arch, a structure that is co-axial with the Black Hills. This uplift diverted the regional paleodrainages of eastern Montana and western North Dakota from southeast to northeast.

Early Paleocene Cannonball oscillatory, but generally westward transgressing changes in sea level seem to have been independent of the Black Hills uplift event that produced the U2 unconformity along the Miles City Arch. Transgressions and regressions of the Cannonball sea were also apparently independent of the U3 unconformity, and its correlative conformity that regionally lies beneath the Tongue River Member of the Fort Union Formation, and above nonmarine facies of the Lebo and Ludlow members in eastern Montana and western North Dakota.

Marine fossils from Permian redbeds (Satanka Shale) at Laramie, Wyoming

XUNHONG CHEN Conservation and Survey Division, University of Nebraska, Lincoln, Nebraska 68588-0517 DONALD W.BOYD Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071-3006

Pages
27-32

Keywords
Marine fossils from Permian redbeds (Satanka Shale) at Laramie, Wyoming

Abstract
Foraminifera and marine mollusks were found in the upper 1.5 m of the Satanka Shale, a Permian unit, at Laramie, Wyoming. This fossiliferous interval was divided into three distinct units based on lithology and fauna. The lower unit is a porous sandstone bed characterized by pelecypods, gastropods, and scaphopods. The middle limestone unit contains abundant foraminifera and small pelecypods. The upper unit is mudstone containing foraminifera and scattered small pelecypods.

The fossiliferous interval probably represents the early stage of the Wordian transgression that resulted in the overlying stromatolitic Forelle Limestone. The fossils decrease in diversity upward in the fossiliferous interval, suggesting that salinity changed as deposition proceeded. The irregularly coiled foraminifera belong to a single species whose tests accumulated at times in sufficient abundance to form individual laminae.

Silicified gastropods from the Permian Phosphoria rock complex of Wyoming

HELEN A. KULAS 412 Melrose St., unit 104, Glen Ellyn, Illinois 60137 ROGER L. BATTEN 77 E. Missouri, Phoenix, Arizona 85012

Pages
33-58

Keywords
Silicified gastropods from the Permian Phosphoria rock complex of Wyoming

Abstract
This is a study of silicified gastropods from carbonate strata of the Phosphoria rock complex of Lower to Middle Permian (Leonardian-Wordian) age in Wyoming. Twenty-five collections from 13 locations in western and central Wyoming were studied. Collections are from the Grandeur, Franson and Ervay Members of the Park City Formation, the lower member of the Shedhorn Sandstone and limestone tongues of the Phosphoria Formation. Gastropods are a minor but relatively diverse com-ponent of the faunas sampled. One new genus, Shedhornia, is recognized. Seven of the 55 species are described as new: Euphemites fremontensis, Bellerophon (Pharkidonotus) altitropis, Apachella boydi, Lamellospira alveozona, ?Gyronema clausepeakensis, Platyceras (Platyceras) yochelsoni, and Shedhornia ornata. Many taxa are indeterminate at particular taxo-nomic levels due to poor preservation. Several unexpected resemblances to Pennsylvanian faunas are noted.

Species diversity, tooth size, and shape of Haplomylus (Condylarthra, Hyopsodontidae) from the Powder River Basin, northeastern Wyoming

PETER ROBINSON University of Colorado Museum, Boulder, CO 80309-0315 BLYTHE ANNE WILLIAMS Department of Biological Anthropology and Anatomy, Duke University Medical Center, Box 3170, Durham, NC 27710

Pages
59-78

Keywords
Species diversity, tooth size, and shape of Haplomylus (Condylarthra, Hyopsodontidae) from the Powder River Basin, northeastern Wyoming

Abstract
Haplomylus specimens from Lower Eocene rocks of the Powder River Basin can be assigned to four species based on morphology. One of these is the generotypic species, H. speirianus, and a second is a similar-sized, but morphologically dis-tinct new species, H. bozemanesnsis. These two species occur together, in apparent stasis, for over 120 meters of section. Two other species replace them after an interval represented by approximately 20 meters of deposits. These latter species are larg-er, but each resembles one of the earlier forms.

The co-occurrence of two closely related species with total overlap in metric characteristics shows the need for caution in using an exclusively measurement-based stratophenetic approach to lineage evaluation through time.