Contributions to Geology 29.2

Behavior of hadrosaurs as interpreted from footprints in the "Mesaverde" Group (Campanian) of Colorado, Utah, and Wyoming

KENNETH CARPENTER Department of Earth Sciences, Denver Museum of Natural History, City Park, Denver, E Colorado 80205

Pages
81-96

Keywords
dinosaur, footprints, Mesaverde, Colorado, Utah, Wyoming, hadrosaur

Abstract
Dinosaur footprints are common in the ceiling rocks of many coal mines in the "Mesaverde" Group of Colorado, Utah, and Wyoming. Bones from the "Mesaverde" Group provide a clue as to the possible identity of the footprint makers. The bone collections are dominated by an unidentifiable genus of lambeosaurid hadrosaur. The hadrosaurid Maisaura is also present, although it is rare. Ceratopsids also occur, and a skull is tentatively referable to Anchiceratops. Theropods are rare and include dromaeosaurids and tyrannosaurids. Thescelosaurids, ankylosaurids and nodosaurids are extremely rare and are known only from the shed teeth.

A comparison of the "Mesaverde" Group footprints with the foot bones of representatives of the families known from the "Mesaverde" bone assemblages identify the footprints as being made by hadrosaurs, a tyrannosaurid, and a ceratopsid. As yet no dromaeosaurid, thescelosaurid, ankylosaurid, or nodosaurid footprints are know from the ''Mesaverde" Group, suggesting that they rarely, if ever ventured into the swamps.

The abundance of hadrosaur footprints at two localities, one near Price, Utah and the other near Grand Mesa, Colorado, are interpreted as evidence of herding for group protection. Such conclusions appear to be supported by mass accumulations of skeletons that have been interpreted as evidence of herding. Azimuth plots of hadrosaur footprints from the two localities show that most of the animals were traveling south, and, based on footprint speeds from one of the mines, were not traveling rapidly. Because so many animals from two separate sites were traveling in the same direction, it is suggested that these animals might have been migrating. The shallowness of the footprints, even of twenty ton animals, the incompleteness of trackways due to missing footprints, the presence of hatchling hadrosaur footprints, and charcoal due to fires indicate that the hadrosaurs ventured into the swamps late in the dry season, and probably during a drought when the substrate was dried out.

The presence of hatchling footprints suggests that egg hatching in hadrosaurs coincided with the start of the rainy season when new plant growth would be available. That the rainy season during the Campanian did not always occur on schedule may explain why the swamps were not inundated at the time the tiny footprints were made (otherwise the hatchlings would have floated). The lack of rain and absence of new plant growth may also explain the high mortality rates for juvenile hadrosaurs in upland nesting sites. This suggests the possibility that regions affected by droughts were occasionally widespread during the Campanian just as they are today.

Stable isotope composition of calcareous paleosols and ground-water cements from the Ogallala Group (Neogene), western Nebraska

LEONARD ROBERT GARDNER Department of Geological Sciences, University of South
Carolina, Columbia, SC 29208
R. F. DIFFENDAL, JR. Conservation and Survey Division, IANR, University of
Nebraska-Lincoln, Lincoln, NE 68588-0517
DOUGLAS F. WILLIAMS Departme

Pages
97-110

Keywords
isotope, Ogallala, Nebraska, carbonates, paleosols

Abstract
A preliminary survey has been made of the stable isotope composition of secondary carbonates in the Ogallala Group of western Nebraska. The purpose of the study was to ascertain whether the isotopic signatures of calcareous paleosols in the fine grained facies of the Ogallala and groundwater cements in gravel facies might provide a basis for stratigraphic correlation and/or paleoclimatic interpretation.

The groundwater cemented channel gravels are modestly depleted in 18O (d18O = -10%) as compared to calcareous paleosols (d18O = - 8.0 to -9.0%). Gravels of different age, however, have essentially identical isotopic signatures. The isotopic variability within paleosols is about as great as the variability between paleosols. As a result, the mean isotopic values for individual paleosols are not significantly different from one another. Thus, stratigraphic correlation of paleosols based on isotopic signatures will not be possible unless intensive vertical and lateral sampling capable of distinguishing small differences among paleosols is undertaken. Nevertheless, when the mean d18O values for individual paleosols are arranged in the stratigraphic order proposed by Lugn (1939), they increase upsection from about -9.0% to -8.0%, suggesting that the pedogenic environment possibly became more arid through time. Unfortunately, this trend is not confirmed by a clear upsection increase in d13C values.

A probable new Lava Creek ash locality: implications for Quaternary geologic studies in the western Wind River Basin, Wyoming, USA

CHERYL JAWOROWSKI Department of Geology and Geophysics, University of Wyoming, P.O. Box 3006, Laramie, Wyoming 82071-3006

Pages
111-118

Keywords
incision, Quaternary, Wind River, Wyoming, Lava Creek, ash, geochronologic, Kinnear

Abstract
Geochemical and field evidence indicate that a recently discovered volcanic ash bed is the 620 ka Lava Creek ash. The correlation of the volcanic ash with Lava Creek ash is not yet confirmed by radiometric dating, but geochronologic studies are ongoing. This paper discusses the significance and implications of this new ash locality.

The interbedded ash and mid-Pleistocene Wind River gravels provide a crucial link between two previously published Lava Creek ash localities in central Wyoming. The Kinnear ash locality is the second ash site along the paleo-Wind River. It confirms previous terrace mapping and correlation of mid-Pleistocene terraces (Morris and others, 1959). The Kinnear ash locality also has significant implications for the correlation of Quaternary glacial and fluvial deposits in the western Wind River Basin.

Four known Lava Creek ash localities allow estimation of average incision rates for the western Wind River Basin. Calculation of local average incision rates for the Dinwoody Lake area yields rates of 0.21-0.23 m/kyr. Lower average rates of local incision (0.15-0.16 m/kyr) are calculated for the central basin ash localities near Kinnear and Muddy Creek. The Lander ash locality yields an anomalous average rate of incision (0.07 m/kyr). The spatial variation of average incision rates in the western Wind River Basin may reflect adjustments of the Wind River and its tributaries to varying sediment and flows during Quaternary time, tectonic influences, or combined fluvial and tectonic influences.

Stratigraphy and sedimentology of the Libby Formation, Belt Supergroup (Middle Proterozoic) of Montana and Idaho, U.S.A.

DAVID L. KIDDER Department of Geological Sciences, Ohio University, Athens, Ohio 45701

Pages
119-132

Keywords
Belt Supergroup, Montana, Idaho, Libby, sedimentology, sandstone, McNamara, Garnet Range

Abstract
Shallow-water mudrocks give way to overlying hummocky cross-stratified sandstone during a major shift in sedimentation in the upper 20% preserved in the Belt Supergroup. The Libby Formation is > 1700 m thick and is capped by a regional unconformity. The formation has been subdivided into 8 informal members. The lower four members correlate to the McNamara Formation southward and eastward of the region of Libby, Montana, whereas the upper four members correlate to the Garnet Range Formation.

In the lower Libby Formation, member A represents an anoxic, submerged environment that existed prior to development of mud flats. Members B and C represent the buildup and zenith of the flats, and member D marks the beginning of their submergence. Sedimentological characteristics of these mudrocks support either a marine or non-marine origin; sediment types and vertical and lateral facies relations are consistent with either a lacustrine or shallow marine muddy tidal flat. Possible tidal channels support a marine origin, and a mixed assemblage of originally calcitic and aragonitic ooids suggests that the environment was at least somewhat restricted.

In the upper Libby Formation, member D' reflects either waning of clastics that maintained the mudflat, and/or a rise in water level that completed their submergence. Member E was deposited during an interval of anoxic, subaqueous sedimentstarvation followed by an influx of silt and fine-grained sand that was deposited and reworked as member F under storm conditions. Clastic influx waned yet again during the last known phase of Libby Formation deposition (member G), which was another anoxic phase. The hummocky cross-stratified strata of the upper part of the Libby Formation were deposited in deeper water than the mud flat deposits, but the hummocky facies could have either a marine or non-marine origin.

Burrowing and dolomitization patterns in the Steamboat Point Member, Bighorn Dolomite (Upper Ordovician), northwest Wyoming

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

Pages
133-142

Keywords
Bighorn, Steamboat Point, Wyoming, Ordovician, dolomite, Wind River Gorge, Thalassinoides

Abstract
The widespread Bighorn Dolomite in this area represents pervasive yet shallow burial dolomitization. The most ubiquitous and conspicuous of its members, the massive, cliff-forming Steamboat Point Member, ranges in thickness from about 90 m in the Tetons to 31 m along Wind River Gorge (Owl Creek Mountains) before pinching out a short distance to the southeast by loss from the top. The characteristic fretted weathering surface represents a Thalassinoides burrow-mottled fabric in predominantly decimicron-size, low acid-insoluble (x = 1.5%) and locally vuggy and/or cherty dolomite. A marine fauna, dominated by echinoderm debris, and the bedding parallel burrowing suggest normal subtidal deposition. Except for some stratigraphic intervals in the Bighorn Mountains, essentially all precursor carbonates in the member have been completely dolomitized; the original lithology was an echinoderm wackestone to packstone. The dolomite in both burrow fills and matrix is near-stoichiometric and has normal marine d13C ofÑ0.44% (PDB) and a slightly depleted (?) d18O ofÑ3.57% (PDB). Average trace element content in ppm is: Fe = 1460; Na = 471; Mn = 182; Sr = 49.

There is collective evidence against the tidal flat, mixed-water, low-sulfate and deep burial models for origin of this dolomite. Fine crystallinity, relatively well-preserved fossils, slightly depleted d18O and low conodont color alteration indices suggest dolomitization by only slightly modified seawater in the very shallow burial environment. Coeval units, with similar fauna and burrow-mottled fabric extend across much of the western craton although generally in these formations the burrow fills are preferentially dolomitized.