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


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Figure 5. Representative exposure of floodplain and fluvial deposits within the Turney Ranch Formation; (1) Photograph of outcrop within modern ravine; (2) Designation of stratigraphic units and general interpretation (Jacob’s staff is 1.5 meters tall); (3) stratigraphic section through the outcrop. Numbers on stratigraphic section correspond to units mentioned in the text.


mudstone and siltstone units that represent floodplain or overbank deposition. Gray, green, maroon, and red mudstones and siltstones represent floodplain low-lying areas, ephemeral ponds, and well-drained soils (Archibald, 1982; this study). Ephemeral pond deposits are uncommon, but are characterized by green gray claystones containing carbonaceous matter, laminations, gastropod shell fragments and are overlain by purple mottled siltstones occasionally with pedogenic carbonate nodules. Red and maroon mottled siltstones contain weak ped structures and varying degrees of pedogenic carbonate devel- opment, from isolated nodules a few millimeters in diameter to thick hardpan layers several decimeters thick (Figs. 4, 5). One other significant facies occurs within the floodplain sequences, thin (<0.4 m), but laterally extensive (several hundred meters) coarse-grained sandstones to conglomerates with abundant


this facies to large-scale bedform migration, however, we view them as more consistent with deposition on point andmid-channel bars within a river channel (Mohrig et al., 2000). Additionally, some sand-bodies are completely devoid of primary sedimentary structure and are instead intensely bioturbated. Sub-vertical and sub-horizontal burrows (Fig. 4.4, 4.5) are ubiquitous in these sandstones and are typically a few millimeters wide and <5cm long; some are reminiscent of marine trace fossils Skolithus and Planolites (possibly suggested as such by Archibald, 1982; 1987) but are not definitively identifiable as such. These fluvial sand-bodies are overlain and underlain by


carbonate nodules. Typically, these deposits contain a basal unit of crude cross-bedding followed by a zone rich in carbonate and chert pebbles followed by a ripple laminated fine- to medium-grained sandstone. These beds host the most concen- trated zones of petrified wood fragments and logs in the Turney Ranch Formation and are interpreted as large, destructive breach or splay deposits from fluvial channels. A detailed stratigraphic section through a representative


overbank–fluvial transition is shown in Figure 5. From bottom to top, units 1 and 2 are grey green siltstone and mudstone with grayish red (10R 4/2) and dark yellowish orange (10YR 6/6) mottles, weak burrow structures, blocky ped structures, and carbonate nodules that range in diameters from 1mm to 3 cm. These units represent B-horizons within a paleosol sequence. Unit 3 is a massive, pale greenish yellow (10Y 8/2) fine-grained sandstone with a significant silt component that likely represents an illuviated A-horizon within the paleosol. Unit 4 fines upwards from a grayish yellow green (5GY 7/2) medium- grained sandstone to a siltstone. The basal contact is sharp with siltstone rip-up clasts and there is a transition from multiple trough cross-bed sets to ripple cross-lamination, to fine laminations up-section coincident with grain-size changes (Fig. 5). The very top is weakly mottled dark yellowish orange (10YR 6/6). Unit 5 if a medium- to fine-grained sandstone with a sharp lower contact and is composed of a set of repetitive sigmoidal clinoforms dipping down and to the left in the


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