STABLE ISOTOPE STRATIGRAPHY OF A THERIZINOSAUR-BEARING SECTION OF THE TROPIC SHALE NEAR BIG WATER, UTAH

In 2000 and 2001, a team of paleontologists from Northern Arizona University/Museum of Northern Arizona discovered a nearly complete therizinosaurid fossil, a rare terrestrial dinosaur, in the Tropic Shale near Big Water, Utah. In an attempt to better constrain the environment of deposition for this specimen, stable isotope stratigraphies of the complete lithostratigraphic sequence, the therizinosaur site, and nearby plesiosaur fossil-bearing sections were constructed. Samples were collected approximately every three meters from the entire thickness of the Tropic, in addition to samples taken every 0.1m from 1.5m thick stratigraphic intervals at each fossil locality investigated. To determine sample mineralogy we performed XRD analysis on preferred orientation glass-slide mounts of the clay-sized (<0.2µm) fraction for a representative suite of samples. The generated XRD spectra indicate that the clay-sized fraction of all fossil-bearing intervals is composed of kaolinite, illite, smectite, calcite, and quartz. The presence of a mixture of clay minerals (kaolinite, illite, and smectite) and quartz in the clay-sized fraction prevented direct isotopic analysis of primary authigenic clays (i.e. smectite) from the fossil localities. δ (super 18) O and δ (super 13) C values for calcite cements from both the therizinosaur and plesiosaur locations show minor variation, ranging between ~ -6.5‰ and -8‰ (PDB). Calcite stable isotope ratios from the entire formation show more variability, and can be broken down into three isotopically distinct units: a highly variable lower section ranging between -9‰ and -6‰ for δ (super 18) O and -2‰ to +4‰ for δ (super 13) C, a stable middle section trending towards more positive δ (super 18) O values and more negative δ (super 13) C values, and a highly variable upper section ranging between -12‰ and -7 ‰ for δ (super 18) O and -5‰ to 0‰ for δ (super 13) C. We suggest that these three units represent the initial transgression of the Greenhorn Cyclothem, stabilization of a marginal marine depositional system, and subsequent regression, respectively. The fossil-bearing stratigraphic intervals are all located in the lower third of the Tropic, suggesting that burial of the marine reptiles and terrestrial dinosaur occurred during the initial transgression of the Greenhorn Cyclothem on a broad, shallow plain.