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The AAS occurs within the Lewisville Member of the Woodbine Formation based on lithostratigraphic and biostratigraphic criteria. The presence of the invertebrates Anchura and Gyrodes place the AAS in the lower to middle strata of the Lewisville Member (Stephenson, 1952). The palynomorphs Dichastopollenites and Stellatopollis, recovered from the AAS, affirm a mid-Cenomanian age (May, 1975).

The sediments of the AAS suggest sedimentation in a low-lying coastal plain setting. All beds dip slightly to the east. The lowermost part of the section consists of rippled shallow marine sandstones that are heavily marked by traces of Diplocraterion, Thalassinoides, and Arenicolites (Skolithos ichnofacies [Seilacher, 2007]). Overlying these nearshore sandstones is a thin (50–60 cm thick) peat that has locally high claystone, sulphur, and pyrite content (Main, 2009). The peat is laterally continuous, cropping out along the base of adjacent hillsides. It is also highly fossiliferous, containing the mixed remains of numerous vertebrates, and carbonized wood, branches, twigs, and fallen logs (Main, 2009: Main et al., in press). Overlying the peat is a claystone-rich, heavily rooted histic gleysol with numerous calcareous concretions (Main, 2009; Fig. 5.2). The uppermost part of the AAS section is distinguished by a lag deposit overlain by heterolithic mudstones and, subsequently, finely laminated and rippled sandstones.

We interpret the AAS section as recording fine-grained, organic-rich sediments that were deposited successively on a flood-inundated coastal plain. Within the peat, claystones, and mudstones, numerous coprolites and fossil wood have been recovered that can be used to study the ecosystem of the region. The large amount of fossil plant and wood at the site indicates a plant-rich, coastal wetland, such as a fen or mangrove forest. Several large (>3 m) and numerous partial (<2 m) logs were excavated from the peat bed immediately underlying the primary dinosaur bonebed. The ornithopod fossil-bearing horizon occurs within the paleosol, in association with calcareous concretions. Within the concretions are charcoal fragments and several large (28–36 cm long) charcoal tree stumps and roots (Fig. 5.2). We propose that concretion formation indicates seasonal dryness, and that the charcoal tree stumps and roots are possible evidence of wildfires. The charcoal (wildfire) horizons occur below, within, and above the vertebrate fossil horizons. The charcoal-rich conglomerates contain numerous charcoal and fossil wood fragments (1–4 cm long) bound in an iron-rich sandstone. We speculate that the charcoal and other clasts were transported and deposited during distributary-channel flooding.