OF DELTAS AND DINOSAURS: CRETACEOUS (CENOMANIAN) SEQUENCE STRATIGRAPHY AND PALEOECOLOGY OF THE WOODBINE FORMATION AT THE ARLINGTON ARCHOSAUR SITE, TX

The Arlington Archosaur Site (AAS) of North Central Texas is a diverse fossil locality that occurs within the Cretaceous (Cenomanian: 95 Mya) Woodbine Formation. The paleoenvironmental setting is a delta plain that was situated along the southeastern interior seaway coastline. The AAS preserves multiple components of a delta plain ecosystem and several sequence boundaries within a 4 m section. The AAS section consists of near shore sands, as evidenced by Skolithos facies trace fossil burrows (Rhizocorallium) and a prominent ripple bed. The near shore facies is overlain by a bed of a peat bed containing numerous fossil turtles, crocodyliforms, dinosaurs and logs. The logs are carbonized and vary from 2m - 4m in length. The contact of the basal Skolithos sands and the peat bed is considered as sequence boundary one. Overlying the peat is a fossil horizon containing mixed remains of vertebrates in a clay rich inundated delta plain paleosol. The paleosol is a well developed, rooted Histic Gleysol that contains calcareous concretions. Within the concretions occur charcoal fragments and several large burned tree stumps and roots. Concretion formation is indicative of seasonal dryness, and the burned tree stumps are evidence of wildfires. Also of interest, within this horizon are numerous fresh water invertebrates, principally gastropods. The vertebrates occurring within this horizon include dinosaur, crocodyliform, turtle and lungfish. The dinosaurs recovered include a large herbivorous ornithopod and several small theropods.

Overlying the dinosaur bone bearing, wildfire- paleosol horizon are a series of thin, lenticular sand beds that are interpreted as crevasse splay deposits. The thickest of these beds contains numerous rounded to sub-rounded gravel sized clasts, and brackish to marine fossils. This bed is interpreted as a transgressive lag deposit and is used to denote a 2^nd sequence boundary at the site. Overlying the lag is a thick bed of finely ripple laminated mudstones interpreted as tidal flat rhythmites. Invertebrates sampled in this upper most section include brackish to marine taxa that support this model. Thus the AAS represents a coastal ecosystem from a migrating Cenomanian coastline, with multiple faunal components present over time, an excellent record of geological and environmental change.