Читать книгу Hadrosaurs - David A. Eberth - Страница 89

R. T. Hill was the first geologist to map and describe the Cretaceous of Texas, which extends from North Texas to the Big Bend region (Hill, 1901; Alexander, 1976), and named the Woodbine Formation of North Texas for the village of Woodbine in Cooke County. Woodbine exposures extend as an irregular and narrow, north–south-oriented band from Cooke to Johnson counties (Johnson, 1974; Fig. 5.1). Woodbine sediments were sourced from the Ouachita Mountains in southern Oklahoma and were deposited in fluvial, deltaic, and shelf environments in coastal plain and shoreline settings of the subsiding East Texas Basin (Dodge, 1952; Oliver, 1971; Trudel, 1994). The Woodbine Formation unconformably overlies the Grayson Marl of the Washita Group and, in turn, is overlain conformably by the Eagle Ford Group (Dodge, 1952, Oliver, 1971; Fig. 5.2). A minimum age for the Woodbine Formation has been determined using the ammonite biostratigraphy of the overlying Eagle Ford Group, which lies within the Conlinoceras tarrantense zone at the base of the middle Cenomanian. This approach indicates that the Woodbine is no younger than 95 Ma (Kennedy and Cobban, 1990; Head, 1998).

5.2. North Texas Cretaceous stratigraphy, with the Woodbine Formation highlighted in gray. The Arlington Archosaur Site lies in the lower to middle Lewisville Member of the Woodbine Formation. The site consists of ~1.7 m of sediments representing a delta-plain environment. The adult ornithopod material discussed here was discovered in a paleosol overlying a carbonaceous peat bed containing crocodyliform, turtle, and juvenile ornithopod remains (not discussed herein). North Texas stratigraphic column modified after Jacobs and Winkler (1998).

The Woodbine Formation comprises four members. In ascending order these are Rush Creek, Dexter, Lewisville, and Arlington (Dodge, 1968, 1969; Fig. 5.2). The Rush Creek Member consists of a southern province distinguished by offshore bars and lagoonal deposits, and a northern province distinguished by lower shoreface mudstones and sandstones, and estuarine sandstones (Bergquist, 1949; Johnson, 1974). The Dexter Member consists of coastal-plain distributary and meander belt facies (Oliver, 1971). The Lewisville Member consists of deltaic sandstones and mudstones, and marine mudstones and shales deposited adjacent to active deltas (Oliver, 1971; Trudel, 1994; Main, 2005). The Lewisville Member is highly fossiliferous, containing mollusks, ammonites, and foraminifers (Trudel, 1994), and the material described here. A basal unit of lenticular, cross-bedded sandstones with localized lignite beds was designated as the Red Branch Member by Bergquist (1949), but subsequently was included as part of the Lewisville Member (Beall, 1964; Dodge, 1969; Trudel, 1994). The Arlington Member comprises nonmarine delta plain deposits dominated by fluvial sandstones (Murlin, 1975). The type locality of Protohadros occurs within this member (Head, 1998).