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Journal of Paleontology 90(1):102–132
sauropods before their disappearance from the North American record.
We revisited the Sonorasaurus holotype quarry and
surrounding exposures of the Turney Ranch Formation in 2013 and 2014 with five goals. First, we re-opened the holotype quarry in an effort to find more material. The quarry has been tectonically uplifted into an essentially vertical position, and Ratkevitch (1998:79) noted that: “Further skeletal elements are predicted to be contained in the downstream (now vertically down) unexcavated portion of the sandstone ridge.” Second, we prospected the surrounding exposures for more vertebrate remains. Third, we obtained geological data to evaluate previous interpretations of the depositional environment of the quarry as well as sedimentological data to establish the paleoenvironment in which Sonorasaurus lived. Fourth, we assessed the plant macroflora of the Turney Ranch Formation as a complement to our paleoenvironmental data. Fifth, we obtained detrital zircon samples fromthe quarry and a nearby mudstone for the purposes of precisely dating the age of Sonorasaurus.
Institutional Abbreviations.—ASDM, Arizona-Sonora Desert Museum, Tucson, USA; DMNH, Denver Museum of Natural History, Denver, USA; FMNH PR, Field Museum of Natural History, Chicago, USA; NMST-PV, National Science Museum, Tokyo, Japan; YPM, Yale Peabody Museum, New Haven, USA; MCF-PHV, Museo Municipal Carmen Funes, Plaza Huincul, Argentina; MCS, Museo Cinco Saltos, Cipoletti, Argentina. USNM, National Museum of Natural History, Smithsonian Institution, Washington, D.C., USA.
Geology of the Turney Ranch Formation
Below we present new lithologic descriptions of the strata within the Turney Ranch Formation, the first stable isotope records from the Whetstone Mountains exposures of the formation, and radiometric dates from detrital zircons sampled from the holotype Sonorasaurus quarry. The Turney Ranch Formation, underlain by the fluviodeltaic-to-braided fluvial Shellenberger Canyon Formation and overlain by Quaternary pediment gravels, contains alternating river and floodplain deposits that are about a kilometer in thickness (Fig. 2). Short stratigraphic sections were trenched and measured through sedimentary successions and corrected for local dip of strata using a Jacob’s staff. In order to gain insights into the paleoen- vironment of Sonorasaurus, pedogenic carbonate nodules were obtained from the B-horizons of paleosols at five locations for stable carbon and oxygen isotope analysis. Carbonate nodules were also obtained from a conglomeratic layer (inter- preted as flood deposit), which also hosts petrified tree remains. The exact stratigraphic position of each stable isotope sample was not determined, but all sample sites are approximately from within the 200m below and 200m above the Sonorasaurus site. In all cases pedogenic carbonate nodules were cut and polished and their micritic cores sampled with a dental drill. Spar was also sampled at two localities for comparison to micrite values. δ18O and δ13C values were obtained using a Thermo Finnigan DeltaPLUS XP Continuous Flow Stable Isotope Ratio Mass Spectrometer housed at the University of Wyoming Stable Isotope Facility. Prior to introduction to
Figure 2. Generalized stratigraphic section and interpretations of depositional setting in the study area in the southwestern Whetstone Mountains. Approximate stratigraphic position of Sonorasaurus thompsoni and detrital zircon sample marked with black star. Sand bodies are shown schematically, in reality each is about 2–10 meters thick and spaced at roughly regular 5–10 meter intervals within the Turney Ranch Formation. Pedogenic carbonates were obtained from strata within ~200 meters above and below fossil site. Information for the Turney Ranch based on personal observation (BZF) and Archibald (1987).
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