Methodology

1. Geological Framework

Recently completed studies at Dinosaur National Monument and elsewhere on the Colorado Plateau provide a detailed geological framework for the southern part of the Rocky Mountain Region. Several National Park Service units on the Colorado Plateau, including Arches NP, Black Canyon of the Gunnison NM, Capitol Reef NP, Colorado NM, Curecanti NRA, and Glen Canyon NRA, have significant Morrison exposures and these need to be correlated to the existing framework. The existing geologic framework is restricted to the southern part of the Western Interior and needs to be extended northward into Montana and Wyoming. This correlation is necessary because the northern exposures contain habitats and environments (such as coal swamps) that are preserved nowhere else in the Morrison and are important to the overall interpretation of the ecosystem. Several National Park Service units, including Bighorn NRA, Glacier NP, and Yellowstone NP, contain Morrison exposures and are included in this project.

Correlation within the Colorado Plateau and between the Plateau and northern exposures of the Morrison will be accomplished by:

• A. Measuring detailed stratigraphic sections of the Morrison in or near each park to determine the extent and thickness of the Morrison, what subdivisions of the Morrison are present, and the contacts and boundaries between the Morrison and overlying and underlying rock units.
• B. Identifying marker beds (volcanic ashes, welded cherts, calcretes, silcretes, etc.) within these sections. Such beds are useful for establishing lateral correlations.
• C. Where the boundary of the Morrison Formation with overlying Cretaceous rocks is unclear or poorly defined, stratigraphic study and petrographic analysis of the recently discovered extensive calcrete horizon at or near the top of the Morrison will help clarify the nature of this boundary.

2. Age Relations

• A. Isotopic Dating Methodology:

Recently completed work at Dinosaur NM and elsewhere on the Colorado Plateau has provided isotopic dates from volcanic ashes for the lower and upper parts of the Morrison sufficient to bracket the time interval represented by Morrison deposition. However, no reliable isotopic dates exist for the Morrison in the northern part of the region in Wyoming and Montana. Previous attempts at dating the Morrison in the northern area have relied on techniques that have subsequently proven to be inaccurate (i.e. fission-track dating of zircon crystals). Single crystal laser-fusion Ar/Ar dating techniques on feldspar grains from volcanic ash beds have been used to establish a chronology for the Morrison in Dinosaur NM and on the Colorado Plateau. This methodology will be extended into the northern Morrison exposures if possible. Because of the lack of dates from the northern region, all volcanic ashes will be evaluated for their dating potential, and selected ones will dated.
• B. Biostratigraphic Dating Methodology:

Recent micropaleontological work at Dinosaur NM and elsewhere on the Colorado Plateau has yielded preliminary paleontological age determinations. This work has focused on fossil pollen, spores, charopytes and ostracodes. Reconnaissance work has revealed that these microfossils are also present in the northern part of the Morrison basin. Therefore, favorable horizons in this area will be sampled for similar analyses.

3. Paleoclimate:

• evaporite deposits (gypsum)
• wind-blown sand deposits (fossil dune fields)
• alkaline-saline lake deposits
• paleosols (ancient soils)
• coals (ancient peat swamps)

Another approach to paleoclimate is through reconstruction of the ancient landscape on a large scale. The paleotopography (mountains, basins, oceans, etc.) had a great influence on the development and evolution of the climate. Paleolatitude, determined by paleomagnetic studies already available in the literature, can be combined with paleotopographic reconstructions, drainage patterns, and wind directions, to determine such things as rain shadow effects and latitudinal variations in paleoclimate.

The reconstruction of paleoclimate will be accomplished by the following methodologies:

• A. Within the regional geologic framework, map the distribution of climatically diagnostic rock types throughout the Morrison Formation. Determine vertical trends that may indicate changes in overall climate through time. Also determine lateral variations that may reflect climatic gradients at any given time.
• B. From available studies of Morrison paleomagnetic pole positions, determine approximate paleolatitude of the Western Interior during the time of deposition of the Morrison Formation. This will provide information on large-scale atmospheric circulation. These broad-scale studies will be supplemented by detailed field measurements of paleowind directions obtained by study of the orientation of ancient wind-blown sand dunes.
• C. Evaluate the effect of mountain belts on rainfall and drainage patterns in the depositional basin of the Morrison Formation. This will be accomplished by integrating atmospheric circulation patterns inferred from paleolatitude reconstructions with paleogeographic and paleotopographic reconstructions derived from the plate tectonic setting.
• D. Construct lateral profiles and determine alluvial architecture for Morrison stream deposits. Determine the perennial, intermittent, or ephemeral nature of the streams. Useful indicators include presence of abundant unionid clams and extensive coalesced carbonate nodules beneath stream deposits

4. Ecosystem Reconstruction

The Morrison Formation has yielded numerous plant and animal fossils, but, for the most part, they have only been studied in their local context. This project will take a broader approach, identifyng plant and animal communities, and integrating this information into the geological framework to understand patterns of biogeography and community evolution.

A. Physical Component

1. An analysis of paleosols (ancient soils) developed in overbank floodplain and certain other Morrison depositional environments will provide clues to water-depth and the degree of water-table fluctuation. In addition, the type of paleosol is often an indicator of paleoclimate. Detailed analyses of paleosols will be made and integrated into the geologic framework. Carbon, and to some extent oxygen, isotopes from soil carbonate and organic matter can potentially yield paleoecological information on the nature of the plants and, indirectly, paleotemperatures.

2. Coal beds are present in the Morrison Formation, chiefly in Montana, and are an important indicator of the local environment. Examination of coal samples by microscopic techniques (coal petrography and palynology) can yield information on the specific type of coal deposit, which, in turn, is an indicator of the local environment. Samples from Morrison coal beds will be collected and analyzed by coal petrographic and palynologic techniques to give us additional information on the Morrison ecosystem.

 

3. Reconstruction of regional paleogeography from sedimentologic studies on the numerous rock types in the formation will yield important data on the Morrison ecosystem. Information on the lateral and vertical distribution of the mosaic of depositional environments, especially in the northern part of the region where such information is scant or unknown, will be collected while gathering information for the geologic framework studies. The sedimentologic studies will be directed toward the preparation of maps showing the distribution of the various depositional environments and how they varied through time.
 

B. Biological Component

1. Paleoecology
Some fossils are important because of the information they provide about their habitat. These environmentally sensitive fossils will be studied in detail to elucidate local habitat. Such fossils include:

• Palynomorphs (spores and pollen), because of their abundance, provide information on environmentally sensitive plants that are often not preserved as body fossils. Over 200 palynomorph samples have already been collected and processed from the Morrison for biostratigraphic studies; these samples will be analyzed for environmental implications.
• Charophytes and ostracodes are indicators of the chemistry (alkalinity/salinity) of freshwater lakes and ponds, water depth, clarity, and other parameters. Approximately 100 samples containing charophytes and ostracodes have already been collected and processed from the Morrison for biostratigraphic studies; these samples will be analyzed for environmental implications.
• Molluscs can provide information about stream permanence and the amount of suspended sediments in rivers and ponds.
• Conchostracans are indicators of ephemeral water bodies.
• Some vertebrate and invertebrate trace fossils can provide information on water-table fluctuation, including those made by insects, crayfish, and lungfish.

2. Isotope Geochemistry:
Newly developed techniques using amino acid analyses and isotopes of carbon, nitrogen, and oxygen (Bocherens and others, 1991; Ostrom and others, 1990) preserved in fossil teeth, eggshell, and bone have yielded details about ancient aquatic and terrestrial foodwebs which have previously been impossible to study in the fossil record. These new techniques will be applied to vertebrate fossils from Dinosaur NM where good community samples already exist for aquatic and terrestrial habitats.

3. Plant Taphonomy:
Newly developed techniques in plant taphonomy, coupled with data from palynomorphs, will be used to identify and define the plant communities of the Morrison. These techniques offer greatly improved ecological characterization of Morrison plant communities because they emphasize features that reflect adaptations to the environment.

 4. Biodiversity and Biogeography:
Over the last decade, many new localities have been discovered that significantly increase our knowledge of plant and animal diversity in the Morrison (Britt, 1991; Callison, 1987; Chure, 1992a,b,c; Chure and Engelmann, 1989; Chure and others, 1989, 1991, 1992; Engelmann and others, 1990; Henrici, 1992; Tidwell, 1990; Tidwell and Medlyn, 1992). This published and unpublished data, plus new information gained from collections study and field fossil assessments in selected units, will be used to assemble a comprehensive floral and faunal list for the Morrison and will be integrated into the geological framework in order to analyze it for patterns of community and biogeographic evolution.