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Home , Buda Limestone, Dallasaurus, South Dallas, Terminonaris, Woodbine Formation

The Geological Society of America Field Guide 30 2013

Late strata and vertebrate of North

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, , Texas 75275, USA

ABSTRACT

Outcrops of Gulf Series strata (Woodbine, Eagle Ford, and Aus- tin) in the Dallas area expose middle 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 , bony fi sh, , dinosaurs, , , mam- mals, long- and short-necked plesiosaurs, and a classic record of 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 . 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 , Eagle Ford Formation, cos Arch and the Sabine Uplift (Fig. 3), both of which are Late , 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 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 , 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 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 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 —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 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 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. That ammonite marks the base of tebrates of this interval and from this region. the middle Cenomanian (Kennedy and Cobban, 1990), and the Terminonaris fossil was collected just a couple of meters below STOP 1. WOODBINE FORMATION; GRAPEVINE the base of the Eagle Ford, suggesting it is no younger than early LAKE, MURRELL PARK middle Cenomanian (~96 Ma). There are currently no reports of (33° 0′1.22″N; 97° 5′44.75″W) plesiosaurs, dolichosaurs, or mosasaurs from the Woodbine.

Grapevine Lake was impounded in 1952 by the U.S. Army STOP 2. WOODBINE–EAGLE FORD CONTACT AT Corps of Engineers by damming Denton Creek, a tributary of BEAR CREEK the Elm Fork of the . The purpose of the damming (32°50′33.75″N; 97° 2′31.77″W) was fl ood control and to provide water for the Dallas–Fort Worth Metroplex. The closest natural lake to the Dallas–Fort Worth area This stop is on DFW airport land and it provides the best is Caddo Lake on the Texas- border. exposure of the Woodbine–Eagle Ford contact, as most com- The porous Woodbine Formation naturally supports forests monly accepted, in this region. Here, the Tarrant unconformably of oak and associated , remnants of which can be seen overlies the upper Woodbine (but note that the Tarrant is consid- around Grapevine Lake (Fig. 5). This belt of woodlands is called ered the upper unit of the Woodbine Group by Reaser, 2002). The the Eastern Cross Timbers. Deep tree roots can obtain water year contact is characterized here by Woodbine clinoforms truncated round from Woodbine soils, but grassland prairies are more fi tted by horizontal strata of the Tarrant (Fig. 7). for soils developed on and shale. The trace of the cross- Adams and Carr (2010) term this contact the Eagle Ford timber forest follows the Woodbine outcrop across the prairies, Unconformity, and they consider it to vary in age and duration splitting it into the Black Prairie to the east and the Grand Prai- across the East Texas Basin. This is supported by ammonite dis- rie to the west. In 1832, Washington Irving referred to the Cross tributions in the upper Woodbine and lower Eagle Ford. They con- Timbers as a “forest of cast iron” (Francaviglia, 2000). The Fort sider the erosion associated with the unconformity to be related Worth Prairie (a component of the Grand Prairie) lies to the west to tilting and uplift of the San Marcos Arch and the Sabine Uplift, of the Eastern Cross Timbers and is delimited on its western edge where it has eroded into the Lower Cretaceous Glen Rose and the by the Western Cross Timbers, which grows on Trinity Group Eagle Ford is completely missing. The Eagle Ford Unconformity (Comanche Series) rocks for the same reason. merges with the unconformity at the base of the Austin in the The Woodbine lies unconformably on the east, but is well below a distinct disconformable condensed zone and Grayson Marl (Dodge, 1969). In their regional study, Adams at the Eagle Ford–Austin contact in the DFW area. Late Cretaceous strata and vertebrate fossils of North Texas 5

Figure 4. Road map showing fi eld trip itinerary (based on Google Maps).

Figure 5. Woodbine outcrops at Grapevine Lake. (A) Cross-bedded sands capped with dark bioturbated sand. (B) Cross- bedded beach sand with distributary channel shown above and behind teacher. (C) Sand bioturbated by fi ddler crabs. (D) Dinosaur track in boulder (http://www.smu.edu/isem/oceandallas.html). 6 Jacobs et al.

Figure 6. Dinosaur footprints at Grapevine Lake, now covered. Trackway trails (A); Caririchnium protohadrosaurichnos (B); Magnoavipes lowei (C); Fuscinapedis woodbinensis (D); Caririchnium protohadrosaurichnos (E); dinoturbated surface (F) (courtesy Yuong-Nam Lee; http://www.smu.edu/isem/oceandallas.html).

The early middle Cenomanian ammonite Conlinoceras tar- The oldest occurrences of plesiosaurs in the Eagle Ford rantense is found in the Tarrant and indicates an age of 96 Ma at are isolated teeth of the large pliosaurid (short-necked) Bra- Bear Creek. Fossiliferous gravel lags are common near the con- chauchenius recovered from the Tarrant. Low in the Britton, tact, and teeth, dinosaur bones, and fossil wood are relatively the remains of three plesiosaurs were recovered from Lewis- common. The oldest marsupial found in Texas, a single tooth, was ville Lake, near Little Elm, Texas. A skeleton of Brachauche- discovered here (Jacobs and Winkler, 1998; Winkler et al., 1995). nius was recovered during excavation for Terminal B (then

Figure 7. (A) Contact of Woodbine and overlying Tarrant at Bear Creek. This is the Eagle Ford Unconformity of Adams and Carr (2010). (B) Detail of the Eagle Ford Unconformity at Bear Creek, key for scale (from Trudel, 1994). Late Cretaceous strata and vertebrate fossils of North Texas 7

known as Terminal 2W, the Braniff terminal) at DFW Interna- STOP 3. EAGLE FORD, ARCADIA PARK, tional Airport, near the base of the Britton (Fig. 8). Additional KAMP RANCH, AND AUSTIN CHALK AT specimens of Brachauchenius have been recovered higher PINNACLE PARK (FIG. 10) in the section in the lower middle Collingnoceras (Arcadia Park: 32°45′1.06″N; 96°52′51.71″W) woollgari zone at Cedar Hill. The elasmosaurid plesiosaur, (Austin Chalk: 32°45′40.32″N; 96°53′18.22″W) Libonectes morgani, originally reported by Welles and Slaugh- ter (1963), was recovered from Eagle Ford rocks at Cedar Hill, The Eagle Ford in the Dallas area is generally subdivided Texas, and is probably from the Britton (Storrs, 1981) (Fig. 9). into Tarrant, Britton, Kamp Ranch, and Arcadia Park units. Nor- The skull of this specimen was fossilized in a concretion and is ton (1965) suggested a possible disconformity in the upper part uncrushed, making it one of the best preserved mid-Cretaceous of the Britton, below the Kamp Ranch, based on the correla- elasmosaurs in existence. The Tarrant member throughout the tion of concretions from two localities relative to the overlying DFW area yields specimens of the marine squamate, Coni- Kamp Ranch. In the fi rst, a concretionary mass occurred 4–6 m asaurus cf. crassidens. below the Kamp Ranch and in the other 3 m below. Pessagno The earliest mosasaurs to appear in the fossil record of (1969) recognized a paraconformity based on missing planktonic north Texas are found in the C. woollgari Zone of the Arca- foramineral zones within the Eagle Ford “4–14 feet below the dia Park. coheni was named by Polcyn and Britton-Arcadia Park contact” in the Socony Mobil core from Bell (2005) on the basis of a specimen collected just above south Dallas. However, Valentine (1984) did not recognize an the Kamp Ranch Limestone. This mosasaur occupies a basal unconformity within the Eagle Ford based on his study of calcar- position in a lineage that gave rise to more derived forms such eous nannofossils. Kennedy (1988), in his study of ammonites as . turneri, a primitive mosasaurine from northeast and central Texas, stated that he had observed an mosasaur, was described and named by Bell and Polcyn (2005) unconformity in a temporary exposure 2.5 m below the Kamp from material collected about a kilometer from the type local- ity of Russellosaurus coheni, and approximately the same horizon. Dallasaurus is a primitive mosasaur, about one meter in length and retains elongate lizard-like limbs, but is clearly related to more derived forms such as , known from the Coniacian (Bell and Polcyn, 2005). These early mosasaurs are amongst the oldest in North America and have been pivotal in understanding the basal relationships of the group and also in elucidating patterns of evolution within secondarily adapted marine amniotes (Lindgren et al., 2011). Fragmentary remains of more derived tylosaurine and plioplatecaprine mosasaurs have been found in the late middle Turonian of north Texas and equivalent horizons near Waco and Austin, indicating subfa- milial identity was attained early in the evolution of mosasaurs (Polcyn et al., 2008). No marine crocodiles have been reported from the Eagle Ford.

Figure 8. Short-necked (pliosaur) plesiosaur Brachauchenius excavat- Figure 9. First long-necked elasmosaur plesiosaur from the lower Ea- ed at Dallas–Fort Worth Airport on display at the old Braniff terminal gle Ford (Britton) at Cedar Hill (from Shuler, 1950; see also Welles, (http://www.smu.edu/isem/oceandallas.html). 1949). This became the holotype of Libonectes morgani. 8 Jacobs et al.

Figure 10. (A) Eagle Ford School (cup on Kamp Ranch). (B) Teacher kneeling on Kamp Ranch, Britton below, Arcadia Park above. (C) crystals in septarian nodule; (D) Crab Homolopsis pikeae from the Eagle Ford Formation (scale bar = 1 cm). (E) Impression Collignoniceras woollgari in the Kamp Ranch (http://www.smu.edu/isem/oceandallas.html). Late Cretaceous strata and vertebrate fossils of North Texas 9

Ranch (which he considered the mappable base of the Arcadia woollgari, the aragonitic shells dissolved away, are common and Park). He further suggested his unconformity in the upper Brit- indicate an age of ~92.5 Ma. The Kamp Ranch occurs consis- ton, below the Arcadia Park, may vary in magnitude or there may tently ~30 m below the Eagle Ford–Austin contact from Ellis be more than one unconformity in that interval. County in the south, across Dallas County, to Denton County in The community of Eagle Ford, and the type section of Eagle the north. The mosasaurs Dallasaurus turneri and Russellosau- Ford rocks, is north of I-30 on Chalk Hill Road, which runs along rus coheni occur within a few centimeters of the top of the Kamp the western margin of Pinnacle Park. The lower Eagle Ford, or Ranch (Fig. 12). The upper Eagle Ford (Arcadia Park) exposed in Britton, was exposed in the original ford of the Trinity River, but Pinnacle Park is mainly represented by rocks of the Prionocylcus there are currently no exposures. Arcadia Park, which gives its hyatti Zone of the lower middle Turonian. name to the upper Eagle Ford in this region, is the community to The highest rocks in Pinnacle Park form the former quarry the southwest of Pinnacle Park. Pinnacle Park is built in an old walls, and tool marks can still be seen. They comprise the Aus- industrial area known as Cement City. Most of the buildings along tin Group, also referred to as the Austin Chalk or Austin Forma- I-30 in Pinnacle Park are built on the Kamp Ranch Limestone, as tion, Upper Cretaceous (Coniacian to lower Campanian) marine is the old pink Eagle Ford School, where Bonnie Parker of Bon- carbonates that underlie much of the city of Dallas. The unit is nie and Clyde notoriety, went to school. The two met in this area divided into formations (or members) with differing names in of old . Britton rocks are named for the village of different parts of Texas. In the Dallas area, the Austin Group is the same name, now being swallowed by the town of Mansfi eld commonly divided into three units. The basal Atco Formation (or south of Fort Worth. Britton holds a place in Bonnie-and-Clyde member) has been widely used in the literature, unlike the various lore because it boasted the only bank in the area not knocked off names given to the middle and upper parts of the Austin Group. by the gangsters. Clyde’s mother kept her money there. The Atco Formation is well exposed along a cut behind the We drive through the lower Eagle Ford (Britton) on the way warehouse district at Rock Quarry Road near Pinnacle Point to Pinnacle Park, and we catch a glimpse of it in the pit near Boulevard and at the Bulk Mail Center on I-30 (Figs. 13 and 15). Chalk Hill Road and I-30. The pit is famous for the beautiful fos- Light-colored carbonates alternate rhythmically with darker gray sil ammonites collected there before it was closed to collectors. It units commonly referred to as marls. Bedding is very continuous has been considered to be in the Arcadia Park (upper Eagle Ford; for the most part, interrupted by numerous small normal faults Reaser, 2002), although it clearly lies below the Kamp Ranch and made obvious by the clear bedding and steep walls of the quar- therefore lies in the upper Britton. ries. The elevation of the Atco, and that of the Kamp Ranch, both The Kamp Ranch Limestone (Reid, 1952) is a packstone traceable units, demonstrates that the cumulative throw on the composed predominately of prisms, lying between faults is ~70 m between Pinnacle Park and the channel of the the gray shales of the Britton below and Arcadia Park above (Fig. Trinity River near downtown Dallas, where the Kamp Ranch is 11). It is generally interpreted to represent a lowering of sea level again exposed. The faults are not obvious in the shales of the or wave base, but Hancock and Walaszczyk (2004) interpret it to Eagle Ford, but they are a factor that should be considered in any be a highstand condensed zone. Impressions of Collignoniceras detailed stratigraphic studies of the area. Most have interpreted the coccolith-rich pelagic chalk of the Austin Group (Fig. 14) as a deepening of the sea in north-central Texas compared to the (Hancock and Walaszc- zyk, 2004). Dawson et al. (1983) believed that strata of the lower Austin Group represent outer shelf environments. In contrast, Daw- son and Reaser (1990) interpreted the trace fossil assemblage from the Austin Group just south of the Dallas area as inner to middle shelf with a water depth of 15–25 m below normal wave base. Large valves of inoceramid bivalves emblemize the macro- invertebrate fauna of the Austin Group here and elsewhere around north-central Texas. Their large fl at shells are suggested to have allowed them to colonize the soft muddy sea fl oor. Fragments of inoceramid shell are abundant in some units. Dispersed fi sh and shark remains are common in this outcrop as well as mollusks other than Inoceramus and relatives. Germane to the depth interpretation of the Austin Group is the occurrence of relatively shallow channel forms carved into otherwise uniformly parallel bedding (Lock, 1984; Hovo- Figure 11. Scanning electron microscope image of Kamp Ranch Lime- rka and Nance, 1994). The channels are not uncommon in the stone showing prisms of Inoceramus shell (http://www.smu.edu/isem/ lower Austin Group. Formation of these channels has been oceandallas.html). attributed to deep-water turbidites in West Texas (Lock, 1984), 10 Jacobs et al.

Figure 12. Dallasaurus turneri, a primitive-limbed mosasaur from the Eagle Ford immediately above the Kamp Ranch (http://www.smu .edu/isem/oceandallas.html). or to reworking of deep-water shelf sediments by either slope were not accepted as the source of any of the fi ll in the channels processes or fan formation in Ellis County (Hovorka and Nance, (Hovorka and Nance, 1994). 1994). Hovorka and Nance (1994) argued that during deposition At Rock Quarry Drive, several small channel forms present of the deep-water units in the lower Austin Group, benthic envi- an opportunity to test these models. One of us (Kennedy) has ronments lacked the hydraulic energy necessary to rework sedi- mapped and sampled the channels and examined their lithology ment into channels. Carbonized and pyritized wood, along with and sedimentary structures. X-ray diffraction was used to assess lags of shell material, were found in Ellis County channels (Hov- mineralogy and element composition. The shallow channels here orka and Nance, 1994). In spite of this, shallow shelf sources are less than one meter thick and only a few meters in width. Other nearby channels within the Atco Formation are larger. The channels show signs of soft sediment deformation and burrowing at the base. Preliminary results demonstrate that the color differ- ences between the channels and the surrounding rock are related to the presence of abundant siderite in the channels. Organic matter is also abundant within the channel fi ll. Pyrite is common throughout the Atco Formation and within this outcrop, but it is not present in the channel fi ll, in contrast to the channel deposits described by Hovorka and Nance (1994).

STOP 4. CHALK HILL ROAD, EAGLE FORD–AUSTIN CONTACT (32°45′0.19″N; 96°54′4.94″W)

Figure 13. Outcrop of the Atco Formation (Austin Group) near Rock At Chalk Hill and West Davis Street, shales of the upper Quarry Road showing small-scale normal faults. View to the south. Eagle Ford form a covered slope, and lowermost Austin Group Late Cretaceous strata and vertebrate fossils of North Texas 11

Figure 16. Outcrop of the basal Austin Group at Chalk Hill Road. View to the northeast. Figure 14. Scanning electron microscope image of coccoliths from the Austin Chalk (http://www.smu.edu/isem/oceandallas.html).

Dawson (2000), among others, interprets the phosphatic carbonates cap the ridge (Fig. 16; also see stop in Hancock and intervals in the Eagle Ford Group as representing condensed Walaszczyk, 2004). Throughout much of Texas, the base of the sections formed by slow sedimentation. Condensed sections are Austin Group has been recognized as unconformable on the commonly interpreted, in a sequence stratigraphic sense, as times underlying Eagle Ford Group (Stephenson, 1929; Sellards et al., of low sedimentation rates during maximum fl ooding or maxi- 1932; Forgotson, 1958). The basal contact with the Eagle Ford mum transgression (Loutit et al., 1988). Hancock and Walaszc- Group is marked in the Dallas area by a glauconitic, phosphatic zyk (2004) state that the disconformity at the base of the Austin contact zone rich in marine vertebrate fossils. In northern Texas, Group represents a lowstand in sea level. The condensed bed this phosphatic unit has come to be referred to as the “fi sh bed.” with phosphatic nodules, they argue, is uppermost Turonian and What Taff and Leverett (1893) originally dubbed the “fi sh bed represents a strong rise in sea level at the beginning of Austin conglomerate” was a bed within the upper portions of the Eagle Group sedimentation. Ford Group in northern Texas, below the contact with the Austin Within the condensed section at Chalk Hill Drive, shark and Group (McNulty, 1954). They also noted other phosphatic beds bony fi sh remains are abundant, as are . Coprolites rich in fi sh remains in the upper Eagle Ford Group. McNulty and pyrite also occur within this unit. The contact zone here is not (1965) described a “fi sh bed” at the basal Austin Group contact as dark colored, nor as well developed, as exposures to the south in addition to the others previously known within the upper Eagle in Dallas along Kiest Boulevard (Fig. 17). Ford Group. Such a fi sh bed is present at the Chalk Hill outcrop. Hamm and Cicimurri (2011) summarized the selachian fauna from the basal Atco Formation in Dallas. The vertebrate fauna is diverse and includes 29 species of sharks and rays includ- ing Squalicorax, Scapanorhynchus, Dallasiella, Ischyrhiza, and

Figure 17. Typical phosphatic zone at the base of the Atco Forma- Figure 15. Channel form within the Atco Formation at Rock Quarry. tion (Kiest Boulevard) showing fi sh debris and shark tooth. Scale Jamell Kennedy and 1.5 m stick in view. in centimeters. 12 Jacobs et al.

several species of Ptychodus (Hamm and Cicimurri, 2011). Many guide was supported by a Ray Marr Scholarship from the Institute of these can be found in the Chalk Hill contact zone. for the Study of Earth and Man at Southern Methodist University. In contrast to the rich and unique marine amniote fauna of the underlying Eagle Ford, the Austin in north Texas produces REFERENCES CITED a sequence of mosasaurs taxonomically comparable to those known from the Smoky Hill Chalk of Kansas (Everhart, 2001), Adams, R.L., 2006, Basement tectonics and origin of the Sabine Uplift, in the Mooreville Chalk of Alabama (Kiernan, 2002), and the upper Turner, R., ed., The Gulf Coast Mesozoic Gas Province: Tyler, East Texas Geological Society, p. 1-1–1-31. part of the of the Big Bend area of west Adams, R.L., 2009, Basement tectonics and origin of the Sabine Uplift: Gulf Texas. The condensed phosphatic zone at the base of the Atco Coast Association of Geological Societies Transactions, v. 59, p. 3–19. yields occasional fragmentary remains of plioplateacarpine and Adams, R.L., and Carr, J.P., 2010, Regional depositional systems of the Wood- bine, Eagle Ford, and Tuscaloosa of the U.S. Gulf Coast: Gulf Coast tylosaurine mosasaurs, but none of this material can be confi - Association of Geological Societies Transactions, v. 60, p. 3–27. dently referred to a specifi c . The Atco in the Dallas area Adams, T.L., Polcyn, M.J., Mateus, O., Winkler, D.A., and Jacobs, L.L., 2011, has produced Platecarpus planifrons and nepaeoli- First occurrence of the long-snouted crocodyliform Terminonaris (Pholi- dosauridae) from the Woodbine Formation (Cenomanian) of Texas: Jour- cus. The Bruceville has produced Tylosaurus proriger and Cli- nal of Vertebrate , v. 31, no. 3, p. 712–716, doi:10.1080/ dastes cf. C. propython (Thurmond, 1969), and the Hutchins 02724634.2011.572938. has produced specimens of Tylosaurus proriger. Echols (1972) Allen, E.A., and Curran, H.A., 1974, Biogenic sedimentary structures produced by crabs in lagoon margin and salt marsh environments near Beaufort, reported Clidastes propython, Clidastes sp., Platecarpus sp., North Carolina: Journal of Sedimentary Petrology, v. 44, no. 2, p. 538–548. and Tylosaurus proriger from the Roxton member of the Gober Bell, G.L., and Polcyn, M.J., 2005, Dallasaurus turneri, a new primitive mosa- Chalk, northeast of Dallas. The Gober is laterally equivalent to sauroid from the middle Turonian of Texas and comments on the phylog- eny of Mosasauridae (): Netherlands Journal of Geosciences, the Hutchins in Dallas. Echols (1972) reported both Polycotylus v. 84, no. 3, p. 177–194. latipinnis and an indeterminate elasmosaur plesiosaur from the Collins, J.G., 1997, Characteristics and origin of the Cedar Hill bentonite bed, Roxton member of the Gober Chalk, northeast of Dallas. lower Austin Chalk, Dallas County vicinity [M.S. thesis]: The University of Texas at Arlington, 102 p. Dawson, W.C., 2000, Shale microfacies; Eagle Ford Group (Cenomanian–Turonian) OPTIONAL STOPS 5 AND 6. DOWNTOWN DALLAS, north-central Texas outcrops and subsurface equivalents: Transactions, Gulf PEROT MUSEUM OF NATURE AND SCIENCE, Coast Association of Geological Societies, v. 50, p. 607–621. Dawson, W.C., and Reaser, D.F., 1990, Trace fossils and paleoenvironments of KLYDE WARREN PARK (TIME AND lower and middle Austin Chalk (Upper Cretaceous), north-central Texas: INTEREST DEPENDENT) Gulf Coast Association of Geological Societies Transactions, v. 40, (Perot Museum: 32°47′12.44″N; 96°48′23.89″W) p. 161–173. Dawson, W.C., McNulty, C.L., Jr., and Reaser, D.F., 1983, Stratigraphic and structural overview of Upper Cretaceous rocks exposed in the Dallas Located at 2201 North Field Street (corner of Field and vicinity: Dallas Geological Society Guidebook for SEPM (Society for Woodall Rodgers Freeway), the Perot Museum of Nature and Sedimentary Geology) Annual Meeting, 99 p. Dodge, C.F., 1969, Stratigraphic nomenclature of the Woodbine Formation, Tar- Science opened on 1 December 2012. The Perot Museum has rant County, Texas: The Texas Journal of Science, v. 21, no. 1, p. 43–62. four fl oors of interactive exhibits housed in an iconic building. Echols, J., 1972, Biostratigraphy and Faunas of the Upper Austin and This pace-setting museum was built with $185 million in private Taylor Groups (Upper Cretaceous) of Texas, With Special Reference to Hunt, Fannin, Lamar and Delta Counties, Texas [Ph.D. thesis]: University funds and completed without debt. The reaction of the people of of Oklahoma at Norman, Oklahoma, 240 p. Dallas, of Texas, and of visitors from all over has demonstrated Everhart, M.J., 2001, Revisions to the biostratigraphy of the Mosasauridae (Squa- an overwhelming interest, thirst for knowledge, and desire for mata) in the Smoky Hill Chalk Member of the Niobrara Chalk (Late Creta- ceous) of Kansas: Transactions of the Kansas Academy of Science, v. 104, education in science and technology. It is a museum for the curi- no. 1, p. 59–78, doi:10.1660/0022-8443(2001)104[0059:RTTBOT]2.0.CO;2. ous of all ages. Its galleries are comprehensive. Fossils are a clear Forgotson, J.M., 1958, The basal sediments of the Austin Group and the strati- draw to the Perot Museum. Of particular interest to this fi eld trip graphic position of the Tuscaloosa Formation of central Louisiana: Gulf Coast Association of Geological Societies Transactions, v. 8, p. 117–125. are the exhibits on late Cretaceous marine reptiles, a spectacular Francaviglia, R.V., 2000, The Cast Iron Forest: A Natural and Cultural History representation of mosasaur evolution (including Dallasaurus), of the North American Cross Timbers: Austin, University of Texas Press. dinosaurs, fossil plants, rocks and minerals, Earth processes, Frohlich, C., 2012, Two-year survey comparing earthquake activity and injection-well locations in the Barnett Shale, Texas: Proceedings of the and energy. Its exhibits explain the story revealed in the strata on National Academy of Sciences of the of America, v. 109, which this region is built. p. 13,934–13,938, doi:10.1073/pnas.1207728109. Gale, A.S., Kennedy, W.J., Lees, J.A., Petrizzo, M.R., and Walaszczyk, I., 2007, An integrated study (inoceramid bivalves, ammonites, calcareous nanno- ACKNOWLEDGMENTS fossils, planktonic foraminifera, stable carbon isotopes) of the Ten Mile Creek section, Lancaster, Dallas County, north Texas, a candidate Global We are grateful to the Dallas–Fort Worth Airport Board for boundary Stratotype Section and Point for the base of the Santonian Stage: Acta Geologica Polonica, v. 57, p. 113–160. allowing access to Bear Creek. We thank Anthony Fiorillo (Perot Hamm, S.A., and Cicimurri, D.J., 2011, Early Coniacian (Late Cretaceous) Museum of Nature and Science), George Maxey (University of selachian fauna from the basal Atco Formation, lower Austin Group, north North Texas), and Diana Vineyard (Southern Methodist Univer- central Texas: Paludicola, v. 8, no. 3, p. 107–127. Hancock, J.M., and Walaszczyk, I., 2004, Mid-Turonian to Coniacian changes sity) for their reviews. Leslie Reeder-Myers drafted Figure 2. The of sea level around Dallas, Texas: Cretaceous Research, v. 25, p. 459–471, work of J.G. Kennedy on Austin channels included in this fi eld doi:10.1016/j.cretres.2004.03.003. Late Cretaceous strata and vertebrate fossils of North Texas 13

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