Presentation on theme: "Analysis of Freshwater Limestones at the Cleveland- Lloyd Dinosaur Quarry, Emery County, Utah By Marina Suarez Trinity University, San Antonio, TX Temple."— Presentation transcript:
Analysis of Freshwater Limestones at the Cleveland- Lloyd Dinosaur Quarry, Emery County, Utah By Marina Suarez Trinity University, San Antonio, TX Temple University, Philadelphia, PA
General Information Northwestern flank of the San Rafael Swell Brushy Basin Member of the Morrison Formation Dinosaur bones are in a matrix of gray calcareous mudstone, which is overlain by limestone Over 10,000 bones Greatest concentration of Allosaurus fragilis Predator to Prey ratio of 3:1
Limestone outcrops - Cleveland-Lloyd Study Area Determine stratigraphic relationships of the limestone unit(s) and bones contained within them Interpret a possible depositional environment
Methods Field Determine Extent of the limestone Measure limestone thicknesses (32 measurements) Describe stratigraphic sections (14) Collect rock and fossil samples (102 collected) Lab Compiled stratigraphic info Identified fossils Analyzed thin sections Analyzed REE concentrations in vertebrate fossils
Thin Sections A lower unit shows greater amounts of siliciclastic material (approx. 5-7%) An upper unit shows less amounts of siliciclastic material (approx. 1-2%) [note: figures removed to reduce file size]
Gastropod Paleoecology Fossaria sp.- Small bodies of water…at home on sticks, stones, and other debris in the water or on its edge. Gyralus sp.- Found adhering to sticks in standing water. Lymnaea sp.- In more or less stagnant parts of ponds, lakes or rivers, about vegetation, floating among weeds and algae. (Baker, 1928) Gastropods present in the upper limestone unit and in the limestone northeast of the quarry
1mm Present in all limestone units Theriosynoecum sp. Paleoecology: Freshwater to brackish water (Van Morkhoven, 1963) Ostracodes
Charophytes Paleoecology: Shallow lakes or ponds (less than 15 meters), in muddy substrates (Tucker and Wright, 1990) Present in all limestone outcrops.
Plant fossils Identified by: Dr. William D. Tidwell, Dr. Brooks Britt, and Kirk Johnson. 1 cm Gingko and conifers Present only in upper limestone unit
Assemblages of invertebrate fossils in different units Lower Units Ostracodes Charophytes Upper Units Ostracodes Charophytes Gastropods Plant Fragments Northeast Unit Ostracodes Charophytes Gastropods
Rare Earth Elements Become incorporated into fossil bones quickly through adsorption and substitution The REE geochemistry in bones records the pore water REE chemistry at the time of deposition/fossilization REE concentration of pore water are a reflection of aqueous conditions (pH, redox) of the depositional environment (Trueman and Tuross, 2002)
Bones northeast of the quarry
There is no significant difference between bones in the mudstone and bones in the limestone within the quarry Bones in the quarry are LREE and MREE-enriched Therefore: bones within the mudstone and limestone within the quarry were deposited in similar water chemistries (near neutral pH) Gd N Yb N Nd N
Bones in limestone northeast of the quarry are HREE-enriched Therefore, the two sites do not correlate Bones in the limestone NE of the quarry were deposited in a more alkaline, higher pH environment than those of the quarry. This is consistent with drier climate. Gd N Yb N Nd N
Generalized cross section looking south, toward the quarry and visitor center. Not to scale
Conclusions Limestone units are lacustrine in origin There are two to three limestone units in the immediate vicinity of the CLDQ. One (lower limestone unit) that directly overlies the bone-bearing mudstone. One (upper limestone unit) that overlies the lower limestone unit, separated by mudstone. A third unit exists NE of the quarry that is distinct from the quarry based on REE geochemistry. This unit can neither be correlated with or distinguished from the upper limestone unit.
Conclusions (cont.) Based on REE, dinosaur bones in the quarry were deposited in neutral pH water. Also, the deposition of calcareous mudstone suggests fluvial influence to the body of water. As conditions became drier, reduced recharge, coupled with aquatic plant growth, resulted in increased alkaline condition and carbonate deposition. The data are consistent with other studies suggesting alternating wet/dry environments for the Morrison Formation.
Acknowledgments Numerous persons from the University of Utah, College of Eastern Utah, BYU, and Bureau of Land Management Faculty of the Department of Geosciences at Trinity University, especially my thesis advisor, Dr. Edward C. Roy Geoscience Department Secretaries: Linda Hyatt and Blanca Kirkman Additional advice, guidance, critiques provided by: Dr. Clive Trueman, Dr. Elizabeth Gierlowski-Kordesh, Dr. Perry Roehl, Dr. David Grandstaff, Doreena Patrick, Patricia Jannett, Kirk Johnson, Celina Suarez REE analysis by XRAL Laboratories, Canada Thin section preparation by National Petrographics, Houston, TX Financial support from Trinity University, GSA-South Central Region, BLM, parents
References Baker, F.C., 1928, The Freshwater Mollusca of Wisconsin. Madison: Wisconsin Academy of Sciences, Arts, and Letters, part 1, 507p. Gates, T.A., 2002, The Cleveland-Lloyd Dinosaur Quarry as a drought-induced assemblage: Late Jurassic Morrison Formation, central Utah [Master of Science Thesis]: Salt Lake City, Utah, University of Utah, 57p. Tucker, M.E., and Wright. V.P. 1990, Carbonate sedimentology. Boston: Blackwell Scientific Publications: p Trueman, C.N. and Tuross, N. 2002, Trace elements in recent and fossil bone apatite, in Kohn, M.J., Rakovan, J.F., Hughes, J.J., eds, Reviews in mineralogy and geochemistry: Phosphates: geochemical, geobiological, and materials: Washington D.C.: The Mineralogical Society of America, v. 48, p Van Morkhoven, F.P.C.M., 1963, Post-Paleozoic Ostracoda. New York: Elsevier Publishing Co. vol. 2, 478p.