The Geological Society of America Field Guide 30 2013 Late Cretaceous strata and vertebrate fossils of North Texas Louis L. Jacobs* Michael J. Polcyn Dale A. Winkler Timothy S. Myers Jamell G. Kennedy John B. Wagner Roy M. Huffi ngton Department of Earth Sciences, Southern Methodist University, Dallas, Texas 75275, USA ABSTRACT Outcrops of Late Cretaceous Gulf Series strata (Woodbine, Eagle Ford, and Aus- tin) in the Dallas area expose middle Cenomanian to the early Campanian (96 to ~83 Ma) rocks, which are well known in the subsurface of the oil-rich East Texas Basin. Together with the underlying Comanche Series and overlying younger Gulf Series, this set of strata provides a record of the last 50 million years of the Creta- ceous. Although both marine and terrestrial vertebrates are known in this interval, the Late Cretaceous record is primarily marine. On this fi eld trip, sites are visited that have yielded sharks, bony fi sh, turtles, dinosaurs, crocodiles, pterosaurs, mam- mals, long- and short-necked plesiosaurs, and a classic record of mosasaur evolution. INTRODUCTION of the Gulf Series (Woodbine, Eagle Ford, Austin, Taylor, and Navarro) are generally considered third-order cycles (e.g., Adams The stratigraphic section (Fig. 1) between Lake Grapevine and Carr, 2010; Liro et al., 1994), ranging from ~2 million years west of the Dallas–Fort Worth International Airport and the North in duration (Woodbine) to ~12 million (Taylor). Because of time Sulphur River (Fig. 2), 130 km to the east of Dallas, records the and logistical constraints, this fi eld trip will focus on the Wood- past 30 million years of Cretaceous time and displays in out- bine, Eagle Ford, and Austin units between Lake Grapevine and crop the highly petroliferous rock units that produce from the downtown Dallas. subsurface in the East Texas Basin. The rocks comprise the Gulf Structurally, the rocks visited on this fi eld trip lie on the west Series of Hill (1887, 1901; Jacobs et al., 2012). Hill divided the side of the East Texas Basin, which extends between the San Mar- Gulf Series into the Woodbine Formation, Eagle Ford Formation, cos Arch and the Sabine Uplift (Fig. 3), both of which are Late Austin Chalk, Taylor Formation, and Navarro Formation. Each Cretaceous topographic highs (Adams 2006, 2009). The western of these units was subdivided further by Hill and other authors. boundary of the basin from Dallas toward the east is marked by The placement of Hill’s original units and their subdivisions into two parallel fault systems (Fig. 2). In Dallas, the Balcones fault a lithostratigraphic hierarchy is utilitarian but inconsistent among zone of central Texas extends along the White Rock escarpment various authors. More importantly, however, the major units (Reaser, 1961) capped by the Austin Chalk, and according to *[email protected] Jacobs, L.L., Polcyn, M.J., Winkler, D.A., Myers, T.S., Kennedy, J.G., and Wagner, J.B., 2013, Late Cretaceous strata and vertebrate fossils of North Texas, in Hunt, B.B., and Catlos, E.J., eds., Late Cretaceous to Quaternary Strata and Fossils of Texas: Field Excursions Celebrating 125 Years of GSA and Texas Geology, GSA South-Central Section Meeting, Austin, Texas, April 2013: Geological Society of America Field Guide 30, p. 1–13, doi:10.1130/2013.0030(01). For permission to copy, contact [email protected]. © 2013 The Geological Society of America. All rights reserved. 1 2 Jacobs et al. Reaser (2002), behaves as a true hinge. [These faults are not to be confused with Paleozoic faults of the Ouachita trend slightly to the west in the Fort Worth Basin that may have been activated by fl uid injection related to Barnett Shale production (Frohlich, 2012).] East of Dallas lies the Mexia fault zone, which is a pull- apart structure that formed at the edge of the mobile salt deposits of the Jurassic Luann Formation, which underlies the East Texas Basin (Jackson, 1982). Faults of the Mexia fault zone were active during the Cretaceous and into the Paleogene. The regressive Woodbine Formation at the base of the Gulf Series represents fl uvial, strand plain, deltaic, and paludal envi- ronments (Oliver, 1971). In the Dallas area the Woodbine was emergent as shown by its angiosperm fl ora, including leaves and logs up to 30 cm or more in diameter, bones, teeth, and foot- prints of terrestrial vertebrates, trace fossils of intertidal crabs, and occasional Protosols and Histosols (sensu Mack et al., 1993). The remainder of the Gulf Series is marine. The Wood- bine and the Eagle Ford are completely truncated by erosion at the Sabine Uplift on the eastern margin of the East Texas Basin, indicating post-Woodbine uplift followed by subsidence prior to the deposition of the Austin Chalk, which is continuous across the Basin (Adams and Carr, 2010; Fig. 3). The structural setting of the East Texas Basin indicates that depositional systems of the Late Cretaceous were not solely controlled by eustatic sea level (Adams, 2009). Chronological control of the Gulf Series is provided primar- ily through biostratigraphy, although tuffaceous units and ben- tonites have long been reported in the Woodbine, Eagle Ford, and Austin (e.g., Ross et al., 1929). Radiometric dates of 86 Ma have been determined for some Austin bentonites and have been recorded in unpublished theses (e.g., Collins, 1997). A δ13C curve based on plant remains spanning the Comanche Series through the base of the Austin Chalk has been published, but it is impre- cise (Jacobs et al., 2005). A more precise biostratigraphic and chemostratigraphic study of the Austin Chalk in Dallas County is provided by Gale et al. (2007) who propose the section as a global stratotype for the base of the Santonian Stage. Chalk-marl couplets in the lower Austin may record Milankovitch signals, possibly the 412 Ka eccentricity cycle and the 25 Ka precession cycle (Larson et al., 1997). The transition from the Comanche to the Gulf Series involves major reorganization of marine vertebrate communities, includ- ing the mid-Cretaceous extinction of ichthyosaurs, the decline of pliosaurs and the ascension of elasmosaur and polycotylid plesiosaurs, the fi rst radiation of marine snakes, and the evolu- tion of lizard-like marine dolichosaurs and mosasaurs, the latter of which rapidly diversify and become the apex predators of the Upper Cretaceous oceans. Marine rocks in north Texas preserve a nearly continuous record of these marine predators throughout Figure 1. Late Cretaceous stratigraphic section of north Texas. Hutchins formation (*) of the Austin Group has not been formally designated. Late Cretaceous strata and vertebrate fossils of North Texas 3 Figure 2. Geologic map of Upper Cre- taceous strata of north Texas (data from Stoeser et al., 2005). Figure 3. Cross section of East Texas Basin Mississippi Embayment after Adams and Carr (2010). This section represents strata to the south of the Dallas–Fort Worth area, across the San Marcos Swell. 4 Jacobs et al. the remainder of the Austin, Taylor and Navarro series, or essen- and Carr (2010) recognized two major units in the Woodbine, tially all of the Late Cretaceous. the Dexter and the Lewisville. Locally, Reaser (2002) recog- On occasion, terrestrial taxa are preserved in marine beds. nized the Rush Creek, Dexter, Lewisville, and Arlington forma- When these isolated occurrences are combined with terrestrial tions, and the Tarrant and Templeton facies within the Wood- fossils reported from marginal marine and terrestrial deposits bine. The Woodbine is considered to be middle Cenomanian in lower in the sequence, the Upper Cretaceous record of terres- age, or ~96 Ma. trial ecosystems in the Dallas–Fort Worth (DFW) area reveals At Stop 1, Woodbine foreshore deposits include cross- evidence of thriving terrestrial communities comprising a bedded and bioturbated sands. There is an extensive bioturbated variety of dinosaurs, pterosaurs, crocodiles, turtles, birds, and unit characterized by crab feeding balls and burrows (e.g., Allen mammals. Discoveries of new material in the 14 years since the and Curran, 1974). A channel is seen cutting across dune sands, last review of the north Texas fossil record (Jacobs and Win- with its margin continuing into the bioturbated unit. Rare frag- kler, 1998) have signifi cantly expanded our understanding of ments of fossil wood are found here. the taxonomic composition of Late Cretaceous terrestrial com- Hadrosaur (duckbill) dinosaur bones and several spectacular munities in the DFW area. trackways are also found in Murrell Park (Lee, 1997a, 1997b). Our objectives on this fi eld trip (Fig. 4) are to review the The holotype of the early hadrosaur Protohadros byrdi was found stratigraphic section from the Woodbine through the Austin to within a few kilometers of here (Head, 1998). Footprint ichno- become better informed as to the stratigraphy and to recognize taxa from Murrell Park have been attributed to theropod (Fusci- the truly exceptional sequence of marine amniote vertebrate fos- napedis woodbinensis) and hadrosaur (Caririchnium protohad- sils—the marine reptiles—found here. The Gulf Series as a whole rosaurichnos) dinosaurs and to a large bird (Magnoavipes lowei) documents the past 30 million years of life in Cretaceous seas. (Lee, 1997b). The major trackways are now covered (Fig. 6). No other section in North America can boast better coverage, and The only marine amniote described thus far from the Wood- taken with the underlying Comanche Series, it is unique in the bine is the pholidosaurid crocodile Terminonaris cf. T. robusta record it provides for the past 50 million years of the Cretaceous (Adams et al., 2011) from the uppermost part of the Woodbine period. Time and interest permitting, we will close this fi eld trip Formation. The ammonite Conlinoceras tarrantense is found by a visit to the fossil hall of the Perot Museum of Nature and within the Lewisville member and in the Tarrant member of the Science, which presents a magnifi cent portrayal of the fossil ver- overlying Eagle Ford.