Northern dinosaurs discovered:

 Geology and paleoenvironment of the Upper Jurassic Morrison Formation, central Montana
stegfade2

 

 

Introduction

Two previously undescribed sauropod dinosaurs have been discovered in the Upper Jurassic Morrison Formation of central Montana. This area may also have been home to a breeding herd of stegosaurs (Saitta, 2015).  These relatively new discoveries were made in the northernmost portion of the Morrison Formation foreland basin. A paleontological study of the dinosaurs and a comprehensive geologic study of the dinosaur quarries and the surrounding region is presently being undertaken. The scope of this study is to understand the geologic structure, stratigraphy, sedimentology, depositional facies, vertebrate taphonomy and paleoclimate of the Morrison Formation in central Montana.

Morrison Basin_Update

 

Location – Study Area

The research area is located south of the town of Grass Range in the southeastern portion of Fergus County, Montana. The dinosaur quarries are located on the southern flank of Spindletop Dome, a small anticline on the east flank of the Big Snowy Mountains at the convergence of the Rocky Mountains and the Great Plains provinces. The rolling hills of central Montana are covered by Ponderosa pine (Pinus ponderosa) and Great Plains mixed prairie grass, predominantly western wheatgrass (Pascopyeum smithii). The pine and grass canopy conceals the underlying geology. Within the study area, there are three dinosaur quarries. In Quarry 1 (Ralph Quarry), an exceptionally preserved “Montanasaurus” (a large unidentified camarasaurid) was found.  Several stegosaurs and a second “Montanasaurus” were excavated from Quarry 2 (Ava/Stegosaur Quarry). Quarry 3 (Wrecking Yard Quarry) is the excavation site of an unusually large Haplocathosaurus. Morphological features of the bones suggest this Haplocathosaurus may be a new species.

 

Structure

Owing to the lack of outcrop exposures a review of the structure is essential to the stratigraphic interpretation. The Big Snowy Mountains and Spindletop Dome formed during the Laramide orogenic event of the Late Cretaceous and Early Tertiary. The structure of the region is very complex (Porter et al., 2002) and displays components of two orthogonal lineaments present in the Rocky Mountains and the adjacent plains (Maughan and Perry, 1986).

 

Stratigraphy

The stratigraphic position of a dinosaur excavation is a critical element of any paleontological study. To fully recognize and comprehend the stratigraphy and depositional environment of the Morrison Formation, an understanding of the bounding formations is necessary. The Upper Jurassic marine Swift Formation, which underlies the terrestrial Morrison Formation, recorded the third and final transgressive-regressive sequence of the Sundance Sea (Imlay, 1954; Khalid,morrison-outcrop 1990). The Swift Formation in central Montana is equivalent to the Sundance Formation of Wyoming (Uhlir et al., 1988), the Williston Basin in North Dakota (Francis, 1956; 1957) and South Dakota (Imlay, 1947), the Stump Formation of Idaho (Pipiringos and Imlay, 1979), and the Curtis Formation of Utah (Eicher, 1955; Currie and Reeder, 2002). The J-5 unconformity (Pipiringos and O’Sullivan, 1978), recorded at the base of the Morrison Formation in more southern regions, is absent in central Montana (This study; Imlay, 1948; Uhlir et al., 1988; Khalid, 1990). Overlying the Morrison Formation in central Montana is the Lower Cretaceous Kootenai Formation, marked by the K-1 unconformity (Pipiringos and O’Sullivan, 1978). These fluvial sandstones and mudstones are the formational equivalent of the Cloverly (Wyoming), the Lakota (South Dakota), the Cedar Mountain (Utah), and the Burro Canyon (Colorado) (Stokes, 1952; Heller and Paola, 1989).

Twelve stratigraphic sections have been measured throughout the research area. These sections provide insight to the stratigraphy and how the depositional systems changed during the Upper Jurassic. The structure of the area has prevented a complete measurement from the Swift through the Morrison to the Kootenai Formation. Composite sections were measured and surveyed to verify data.

Recognizing the importance of this research, the TGS-NOPEC Geophysical Company donated a digital well log data set to the project geologist. This will be used to determine the thickness and net sand of the Morrison Formation in Fergus and surrounding counties. The digital log data has been imported into a LMKR Geographix Discovery project. Log tops have been picked by the project geologist for the Swift, Morrison and Kootenai Formations in all relevant well logs. The gamma-ray (GR) curve will be normalized in the petrophysical module (Prizm) using curve histogram analysis. Using the normalized GR curves, a sandstone/mudstone cutoff will be determined and calculated. Isopach and net sand maps of the Morrison Formation will be created.

 

Thin section of Morrison sandstone outcropping directly above quarry. Red is calcium carbonate cement. White and grey are quartz grains.

Thin section of Morrison sandstone outcropping directly above Quarry 2. Red is calcium carbonate cement. White and grey are quartz grains.

Sedimentology

The sedimentology is currently being evaluated for the Swift, Morrison, and Kootenai Formations. The sandstones are typically calcite cemented. Forty-two thin sections were prepared and were reviewed for mineralogy, grain-size distribution, sorting, and matrix percentage. For each thin section, a 300 point count was performed to evaluate mineralogy. JMicroVision software was utilized to determine grain size and sorting.  Matrix percentage and grain rounding were also determined. These values establish the mineralogical variance, the sandstone classification (Pettijohn et al., 1987), provenance (Dickinson and Suczek, 1979), and maturity (Song, 1991) of each formation. An additional 21 sandstone samples have been collected are awaiting thin section preparation and evaluation.

A total of sixteen samples from the three formations were analyzed using XRF at the University of Texas at Austin (UT Austin) to geochemically classify the sandstones (Herron, 1988) and determine matrix percentage. Mudstones of the Morrison Formation were analyzed using XRF and XRD by UT Austin. Of particular interest is the geochemical data from Quarry 2 (Ava/Stegosaur Quarry) which provides insight to the depositional environment. The XRD and XRF data has been sent to a clay mineralogist from the Geology Laboratory of Ecole Normale Supérieure in Paris, France for an additional interpretation.

 

Depositional Environment

The depositional environment of the uppermost Swift, the Morrison, and the Kootenai formations will be determined using field observations, stratigraphy, sedimentology, vertebrate taphonomy, invertebrate paleontology, and geochemistry. The project will present the transitions and variations in the depositional environment for each of these formations. A microstratigraphic section was measured on Quarry 2 (Ava/Stegosaur Quarry) and each unit was analyzed geochemically at UT Austin to facilitate the interpretation of depositional environment.

 

Vertebrate Taphonomy

The taphonomy of each of the three quarries will be studied to determine the likely cause of death of the dinosaurs.  Determining the cause of death for these dinosaurs is much like solving a 155 million year old murder mystery.  It is important to integrate the available geologic and paleontologic clues to propose a reasonable hypothesis.  To reference a few previous Morrison Formation taphonomic studies, read Richmond and Morris (1996 and 1998).

Detrital zircon from a Kootenai sandstone.

Detrital zircon from a Kootenai sandstone within the study area.

 

Age

Mudstone samples from Quarry 2 (Ava/Stegosaur Quarry) were sent to Brigham Young University, Provo, Utah for zircon dating. The samples were disaggregated and searched, but no zircons were found. Detrital zircons have been noted in the Morrison and Kootenai sandstone thin sections. The most recent zircon age date analyses for the Swift, Morrison, and Kootenai formations was completed by Fuentes et al. (2011).

 

 Invertebrates

Freshwater bivalves were collected from a mudstone splay deposit in the Morrison Formation. A few of the bivalves have been identified by a paleontologist at the University of West Chester, Pennsylvania. Additional bivalves were collected from the splay bed and, based upon morphological characteristics, there are several different species present. The researchbivalve team is currently attempting to identify them. Some partial bivalves were thin-sectioned to reveal growth patterns. Other partial specimens have been collected and are waiting to be thin sectioned and studied. The species collected and the information from the shell growth patterns will help interpret the paleoenvironment and paleoclimate (Good, 2004).

 A mudstone sample from Quarry 2 (Ava/Stegosaur Quarry) was disaggregated and searched for ostracods by the project geologist. None were found in the sample. A similar sample sent to a paleontologist at the University of Vienna also yielded no ostracods. Several additional mudstone samples from the formation were sent to the University of Vienna paleontologist and are awaiting processing for ostracods.

 A mudstone sample from Quarry 2 (Ava/Stegosaur Quarry) yielded two charophyte genera identified by a student from Plymouth University (UK).  The charophytes are Kimmeridgian age (157 – 152 Mya; Schudack et al., 1998). There is a possibility of a new genus of charophyte among the collected specimens.

 

Flora

Petrified wood has been found throughout the stratigraphic section of the Morrison Formation.  In one case, a petrified log was discovered. A series of thin sections were prepared from the log using cross-sectional, axial and radial views for identification. An additional 33 petrified wood samples are awaiting thin section preparation. The study will include identification of all samples and a study of the paleodendrochronology (tree growth rings) for climatic variation.

A carbonaceous shale from the Morrison Formation displays plant impressions which are being reviewed for identification. Fossil pollen was extracted from the shale and the genera are currently being identified. A second carbonaceous shale was found and will also be studied for palynology.

A large petrified log from the Kootenai Formation was sampled, thin-sectioned, and is being studied for identification. A second Kootenai petrified wood sample has been discovered and will be identified. All samples are being studied by the lead geologist and a paleobotanist from the University of Oklahoma.

Jurassic dinosaur bone section

Bone thin section of a possible theropod dinosaur found within the study area, but not associated with the quarries.

 

Bone Histology

An isolated unidentified dinosaur bone fragment was collected from the Morrison Formation. It was thin-sectioned and photographed and the images sent to a bone histologist from the University of Cape Town, South Africa. She reviewed the bone growth pattern and proposed some interesting insight into the animal and the climate in which it lived (Chinsamy-Turan, 2005).

 

Tufa Deposits

Several tufas (fresh water spring deposits) have been discovered within the study area in the Morrison Formation. They may be the oldest documented tufa deposits in North America. Tufas are often very helpful in determining paleoclimate and are often dated using the Ur/Pb half-life. If these deposits can be dated, it would provide the first tufa derived date for the formation and the first date for the formation in Montana.

 

Paleoclimate

Nine samples of paleosols and freshwater limestones from the Morrison Formation were sent to geochemists at the University of Washington, Seattle, to determine temperature using clumped isotope analysis (Huntington et al., 2011). Unfortunately, diagenetic alteration has prevented the analysis of eight samples. The one viable sample was collected from the Quarry 2 (Ava/Stegosaur Quarry). The measured temperature provides an interesting data point. A tufa sample will be sent to the University of Washington for temperature analysis.

 

Summary

Prior to this discovery, no Upper Jurassic dinosaurs had been found in central Montana. These newly discovered dinosaurs provide a fresh look at the Jurassic past in the northernmost portion of the foreland basin. This study proposes to use varying branches of science to achieve an integrated hypothesis. The aim is to accurately portray the paleoenvironment and paleoclimate of the Morrison Formation in central Montana in which these important new dinosaurs lived and died

We are currently seeking funding for this project.  If you would like to contribute funds, please go to our GoFundMe page.

 

Selected References

Chinsamy-Turan, A., 2005, The microstructure of dinosaur bone, The John Hopkins University Press, 195 p.

Currie, B. S., and Reeder, M. D., 2002, Sequence stratigraphy of the Jurassic Curtis, Summerville, and Stump Formation, Utah-Colorado, The Geological Society of America, Abstract with Programs, 124-9.

Dickinson, W. R., and Suczek C. A., 1979, Plate Tectonics and Sandstone Composition, The American Association of Petroleum Geologists Bulletin v. 63, p. 2164-2182.

Eicher, D. L., 1955, Microfossil of the Curtis Formation, eastern Uinta Mountains, Utah-Colorado, Guidebook to the geology of northwest Colorado, p. 27-31.

Francis, D. R., 1956, Some aspects of Jurassic stratigraphy in the Williston Basin area, North Dakota Geological Society and Saskatchewan Geological Society First International Williston Basin Symposium, p. 179 – 185.

Francis, D. R., 1957, Jurassic stratigraphy of the Williston Basin area, American Association of Petroleum Geologists Bulletin, v. 41, p. 367-398.

Fuentes, F., DeCelles, P. G., Constenius, K. N., and Gehrels, G. E., 2011, Evolution of the Cordilleran foreland basin system in northwestern Montana, U.S.A., Geological Society of America Bulletin, v. 123, p. 507-533.

Good, S. C., 2004, Paleoenvironmental and paleoclimatic significance of freshwater bivalves in the Upper Jurassic Morrison Formation, Western Interior, USA, Sedimentary Geology, v. 167, 176, 163-176

Heller, P. L. and Paola, C., 1989, The paradox of Lower Cretaceous gravels and the initiation of thrusting in the Sevier Orogenic Belt, United States Western Interior, Geological Society of America Bulletin, v. 101, p. 864-875.

Herron, M. A., 1988, Geochemical classification of terrigenous sand and shales from core or log data, Journal of Sedimentary Petrology, v. 58, p. 820-829.

Huntington, K. W., Budd, D. A., Wernicke, B. P., and Eiler, J. M., 2011, Use of clumped-isotope thermometry to constrain the crystallizations temperature of diagenetic calcite, Journal of Sedimentary Research, v. 81, p. 656-669.

Imlay, R. W., 1947, Marine Jurassic of Black Hills area, South Dakota and Wyoming, American Association of Petroleum Geologists Bulletin v. 31 p. 227-273.

Imlay, R. W., Gardner, L. S., Rogers, C. P. Jr, and Hadley, H. D., 1948, Marine Jurassic Formations of Montana, U.S. Geological Survey Oil and Gas Investigations Chart OC-32.

Imlay, R. W., 1954, Marine Jurassic formations in the Pryor Mountains and the northern Bighorn Mountains, Montana, Billings Geological Society Guidebook, p. 54-64.

Khalid, M. E. A., 1990, Sedimentology of the Swift Formation (Jurassic) in the Little Rocky Mountains of Montana, unpublished Master’s Thesis, University of Saskatchewan, 107 p.

Maughan, E. K. and Perry, W. J. Jr., 1986, Lineaments and their tectonic implications in the Rocky Mountain and adjacent Plains region: Part I. Regional Overview, Paleotectonics and Sedimentation in the Rocky Mountain region, United States, p. 41-53.

Pettijohn, F. J., Potter, P. E., and Siever, R., 1987, Sand and Sandstone, Springer-Verlag, Berlin, 553 p.

Pipiringos, G. N., and Imlay, R. W., 1979, Lithology and subdivisions of the Jurassic Stump Formation in southeastern Idaho and adjoining areas, U.S. Geological Survey Professional Paper 1035-C, 25 p.

Pipiringos, G. N., and O’Sullivan, R. B., 1978, Principal unconformities in Triassic and Jurassic rocks, Western Interior United States-A preliminary survey, U.S. Geological Survey Professional Paper 1035-A, 29 p.

Porter, K., Wheaton, J., and Miller, M., 2002, Potential for a public water supply from the Madison Limestone in the eastern Big Snowy Mountains and Little Snowy Mountains, Montana, Montana Bureau of Mines and Geology Open File Report 449, 25 p.

Richmond, D. R. and Morris, T. H., 1996, The dinosaur death trap of the Cleveland-Lloyd Dinosaur Quarry, Emery County, Utah, in Morales, M., ed., The Continental Jurassic: Museum of Northern Arizona Bulletin 60, p. 533-545.

Richmond, D. R. and Morris, T. H., 1998, Stratigraphy and cataclysmic deposition of the Dry Mesa Dinosaur Quarry, Mesa County, Colorado, Modern Geology, v. 22, p. 121-143.

Saitta, E. T, 2015, Evidence for sexual dimorphism in the plated dinosaur Stegosaurus mjosi (Ornithischia, Stegosauria) from the Morrison Formation (Upper Jurassic) of Western USA, PLOS One.

Schudack, M. E., Turner, C. E., and Peterson, F., 1998, Biostratigraphy, paleoecology and biogeography of charophytes and ostracodes from the Upper Jurassic Morrison Formation, Western Interior, USA, Modern Geology, v. 22, p. 379-414.

Song, T., 1991, Textural maturity of arenaceous rocks derived by microscope grain size analysis in this section, analysis in Syvitski, J., ed., Principles, methods and application of particle size,  p. 163-173.

Stokes, W. L., 1952, Lower Cretaceous in Colorado Plateau, American Association of Petroleum Geologists Bulletin, v. 36, p. 1766-1776.

Uhlir, D. A., Akers, A., Vondra, C. F., 1988, Tidal inlet sequence, Sundance Formation (Upper Jurassic), north-central Wyoming, Sedimentology, v. 35, p. 739-752.