D’Emic et al.—Revision of the sauropod dinosaur Sonorasaurus


the mass spectrometer, carbonate powders were roasted at 200°C for 1.5 hours and reacted with anhydrous phosphoric acid at ~25°C. Isotope values were normalized to internal and external standards (VPDB and NBS-19), and reported VPDB in


105 standard per mil notation. Precision is better than 0.1‰for both


minor silt and fine sand component within the otherwise sandstone-dominated fluvial deposit within the holotypic quarry were sampled. Such fine-grained beds are more likely to preserve ash fall zircons in greater abundance (Trujillo and Chamberlain, 2013). Sample preparation followed standard techniques of physical disaggregation into individual grains followed by water table,magnetic, and heavy liquids separations (Gehrels et al., 2008). Zircons were analyzed at the University of Arizona’s LaserChron facility using a laser-ablation multi- collector inductively coupled plasma mass spectrometer with a measurement error of 1%–2% (2-sigma level). Euhedral grains that appeared clear in scanning electronmicroscope images were specifically targeted for age determinations, as these tend to be the youngest grains (Gehrels, 2012). A total of 136 age determina- tions were made of unknown samples and 59 from in-house standards (Fig. 3.1).We applied a discordance filter of 15%to the age determinations yielding 132 dates. Recovered U/Pb ages range from 129.2±9.9 (2σ)to 2105.9±4.9 (2σ) Mawithpeaks in the frequency distributions at ~167Ma, 227Ma, ~1407Ma and ~1675Ma (Fig. 3.2; Supplemental Data 1). Phanerozoic ages (<310Ma) show additional young peaks at ~136 and 151Ma


δ18O and δ13C values. For radiometric dating, two fine-grained mudstones with a


(Fig. 3.3). Asmentioned above, the weightedmean average of the five youngest grains is 136.3±4.1 (2σ) (Fig. 3.3–3.4), but we regard this age as too old to represent the Sonorasaurus holotype quarry based on other geological and biostratigraphic evidence (see above).


Sedimentology of the Turney Ranch Formation.—In the Whetstone Mountains the Turney Ranch Formation varies from


900 to 1420m thick and is composed of repetitive successions of fluvial and overbank deposits (Archibald, 1987). In outcrop fluvial units form regularly spaced ridgeswith floodplain units occupying low-lying area in between (Fig. 4.1). Fluvial sand-bodies are typically 2–10m thick and can extend laterally for well over a kilometer (Archibald, 1982; this study). The base of these fluvial sand-bodies is commonly a sharp scour surface while the top can be sharp to gradational (Archibald, 1982; this study; Fig. 4.2, 4.3). Internally they are composed of a basal coarse- to medium- grained sandstone with siltstone rip-up clasts, rounded carbonate nodules, and chert pebbles (Archibald, 1982; this study).


Grain-size fines up-sectionwithin the sand-bodies to medium- and fine-grained sandstones and large scale trough-bedding transitions to ripple cross lamination (Archibald, 1982; this study). Planar cross-bedding, climbing ripples, and planar laminations with


Figure 3. Detrital zircon U-Pb data. 1, and 2, Pb*/U Concordia plots for the two detrital zircon samples from the Turney Ranch Formation in and near the Sonorasaurus holotype quarry. Gray circles represent 2-sigma uncertainty for each age determination on a single grain and black dots concordant ages between the two isotope decay systems. Grains whose ages were greater than 15% discordant were eliminated from further study; 2, Normalized probability spectra for all detrital zircon U/Pb age determinations and peaks identified; 3, normalized probability spectra for detrital zircon U/Pb ages for the Phanerozoic only. Note that mostly euhedral grains were targeted during analysis (i.e., grain selection was non- random), and peak height should not be interpreted as indicative of provenance signatures; 4, Youngest ages obtained from single grain analyses for the two sediment samples at the fossil site. Vertical grey brackets are uncertainty on single grain analyses and black line denotes weighted mean of the single grain analyses. Bold text provides average and represents a maximum depositional age for the site.


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