Contributions to Geology 31.1

Sauropod dinosaurs of the Morrison Formation (Upper Jurassic), Black Hills, South Dakota and Wyoming

JOHN R. FOSTER Museum, University of Colorado, Box 315, Boulder, Colorado 80309

Pages
1-26

Keywords
sauropod, Morrison Formation, Apatosaurus, Barosaurus, Camarasaurus

Abstract
Sauropod dinosaurs known from the Morrison Formation of the Black Hills of South Dakota and Wyoming include Apatosaurus, Barosaurus, and Camarasaurus. Diplodocus is present but only one specimen clearly distinguishable from Barosaurus has been found. A partially articulated forelimb, which may belong to Barosaurus, appears to indicate that this genus had a single carpal and metacarpals that were elongate and slender relative to Diplodocus.

Jumpoff Joe Formation, a new Mississippian formation near Springdale, Washington

MEHEMMED S. GHEDDIDA Department of Geology, Eastern Washington University, Cheney, Washington 99004
ERNEST H. GILMOUR and BRUCE R. WARDLAW U.S. Geological Survey, Reston, Virginia 22092

Pages
27-34

Keywords
Jumpoff Joe Formation, Mississippian, conodont

Abstract
A sequence of Mississippian dolomite and dolomitized limestone exposed near Springdale, northeastern Washington, is hereby named the Jumpoff Joe Forrnation. Two major dolomite lithofacies constitute most of the formation; a lower dolomite lithofacies with equant to nonequant idiotopic texture, and an upper dolomite lithofacies with equant to nonequant idiotopic to xenotopic textures. Both lithofacies have undergone dolomitization with partial to complete obliteration of primary depositional fabric. Remnants of crinoids, bryozoans, ostracods, corals, and brachiopods are found throughout the formation.

Based on the conodont microfauna, the age of the Jumpoff Joe Formation is Late Mississippian (Early Meramecian to Early Chesterian). Species of Hindeodus dominate the fauna and have a color alteration index (CAI) of 5.5 to 7. The high CAI probably results from the high heat flow associated with nearby plutons. Carbonate lithologies and associated faunas indicate deposition in a shallow near-shore marine environment with local variations in energy conditions reflected by lateral and vertical lithologic changes.

The giant mustelid Megalictis from the Early Miocene carnivore dens at Agate Fossil Beds National Monument, Nebraska: earliest evidence of dimorphism in New World Mustelidae (Carnivora, Mammalia)

ROBERT M. HUNT, JR. and ROBERT SKOLNICK Division of Vertebrate Paleontology, University of Nebraska, Lincoln, Nebraska 68588-0514

Pages
35-48

Keywords
Megalictis, Miocene, Agate Fossil Beds National Monument, dimorphism, Mustelidae

Abstract
University of Nebraska paleontologists excavating from 1981 to 1990 at the carnivore den site, Beardog Hill, Agate Fossil Beds National Monument, Nebraska, discovered rare fossil remains of the giant terrestrial mustelid Megalictis. Found only in the Early Miocene of North America, the approximately 19 known individuals can be grouped into a single chronospecies, M. ferox Matthew, 1907. Size dimorphic postcranial bones interpreted as small females and large robust males provide the earliest evidence of sexual dimorphism in New World Mustelidae. Many living species of mustelids display a high degree of sexual dimorphism but the reason for its often profound expression in these carnivorans remains unclear.

Geomyoid rodents (Mammalia) from the Bijou Hills Local Fauna (Barstovian), South Dakota

WILLIAM W. KORTH Rochester Institute of Vertebrate Paleontology, 928 Whalen Road, Penfield, NY 14526

Pages
49-56

Keywords
Geomyoid, Bijou Hills Local Fauna, South Dakota

Abstract
Geomyoid rodents from the medial Barstovian Bijou Hills Local Fauna include one eomyid and six heteromyids. Of the heteromyids three are new, Cupidinimus smaragdinus, "Diprionomys" arcarius, and Lignimus austridakotensis. The geomyoid fauna from Bijou Hills shows greatest homotaxial similarity to the Norden Bridge Fauna of Nebraska and the Wood Mountain Fauna of Saskatchewan.

Dolomitization of the "C" zone, Red River Formation (Upper Ordovician) in a deep core, Williston basin, Richland County, eastern Montana

DONALD H. ZENGER Department of Geology, Pomona College, Claremont, California 91711-6339

Pages
57-75

Keywords
Red River Formation, Dolomitization, Williston Basin, Montana

Abstract
The Superior #1 Vanderhoof Unit-C 97-foot (29.6 m) core consists of 63 feet (19.2 m) of the upper part of the "C" burrowed member (BM) overlain by 34 feet (10.4 m) of the "C" laminated member (LM) of the productive Red River Formation. The LM is mainly laminated, anhydritic, stylolitic, essentially unfossiliferous dolomudstone to calcareous dolomudstone, with minor intervals of dolomitic lime mudstone, and represents a restricted, hypersaline, inner shelf environment. The BM is burrow-mottled (Thalassinoides?), skeletal, dolomitic wackestone to grainstone (matrix) to calcareous dolowackestone (burrow fills), and represents near-normal salinity, inner shelf conditions below fair-weather wave base.

Dolomite is primarily replacive and in the LM occurs as 25-50 mm rhombs floating in lime mudstone or associated with stylolites, and as nonplanar, bimodal (5-50 mm) crystals in totally dolomitized intervals. Most BM dolomite consists of 20-160 mm, primarily nonplanar crystals in the largely replaced burrow fills; it also occurs as crystals that are disseminated or focused along stylolites (as in LM; both pre- and post-stylolitization), as large crystals selectively and pseudomorphically replacing echinoderm fragments, and more rarely as late-stage, void-filling saddle dolomite. Although there are significant differences in geochemistry between the LM and BM because of the higher dolomite content of the former, the geochemical data for "C" zone dolomites are remarkably similar, particularly with regard to stable isotopes; d13C and d18O averages are 0.29 and -5.84 % PDB, respectively. The latter represents relatively depleted values compared with those relatively light values reported for late Ordovician seawater. There is significant positive covariance between dolomite content and both Fe and Mn, and negative covariance with Sr.

Based primarily on the downward decrease in dolomitization in the "C" zone, most previous theories have invoked descending (i.e., refluxing), Mg-rich brines for relatively shallow dolomitization that could account for the small rhombs of disseminated dolomite, as well as pre-stylolitization dolomite. However, characteristics such as post-stylolitization dolomite, mottled CL zoning, saddle dolomite, and particular geochemical attributes such as depleted d18O, high Fe content, relatively high Mn, and low traces of Na and Sr, strongly suggest some degree of burial neomorphism, with associated replacement, and stabilization of earlier-formed dolomite. Any additional Mg required for the later dolomitization would have a ready source provided by pressure solution of the earlier dolomite.