1

The Geological Society of AmericaField Guide 30

2013

Late Cretaceous strata and vertebrate fossils of North Texas

Louis L. Jacobs*Michael J. PolcynDale A. Winkler

Timothy S. MyersJamell G. Kennedy

John B. WagnerRoy 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.

*jacobs@smu.edu

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 editing@geosociety.org. © 2013 The Geological Society of America. All rights reserved.

INTRODUCTION

The stratigraphic section (Fig. 1) between Lake Grapevine west of the Dallas–Fort Worth International Airport and the North Sulphur River (Fig. 2), 130 km to the east of Dallas, records the past 30 million years of Cretaceous time and displays in out-crop the highly petroliferous rock units that produce from the subsurface in the East Texas Basin. The rocks comprise the Gulf Series of Hill (1887, 1901; Jacobs et al., 2012). Hill divided the Gulf Series into the Woodbine Formation, Eagle Ford Formation, Austin Chalk, Taylor Formation, and Navarro Formation. Each of these units was subdivided further by Hill and other authors. The placement of Hill’s original units and their subdivisions into a lithostratigraphic hierarchy is utilitarian but inconsistent among various authors. More importantly, however, the major units

of the Gulf Series (Woodbine, Eagle Ford, Austin, Taylor, and Navarro) are generally considered third-order cycles (e.g., Adams and Carr, 2010; Liro et al., 1994), ranging from ~2 million years in duration (Woodbine) to ~12 million (Taylor). Because of time and logistical constraints, this fi eld trip will focus on the Wood-bine, Eagle Ford, and Austin units between Lake Grapevine and downtown Dallas.

Structurally, the rocks visited on this fi eld trip lie on the west side of the East Texas Basin, which extends between the San Mar-cos Arch and the Sabine Uplift (Fig. 3), both of which are Late Cretaceous topographic highs (Adams 2006, 2009). The western boundary of the basin from Dallas toward the east is marked by two parallel fault systems (Fig. 2). In Dallas, the Balcones fault zone of central Texas extends along the White Rock escarpment (Reaser, 1961) capped by the Austin Chalk, and according to

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-tially all of the Late Cretaceous.

On occasion, terrestrial taxa are preserved in marine beds. When these isolated occurrences are combined with terrestrial fossils reported from marginal marine and terrestrial deposits lower in the sequence, the Upper Cretaceous record of terres-trial ecosystems in the Dallas–Fort Worth (DFW) area reveals evidence of thriving terrestrial communities comprising a variety of dinosaurs, pterosaurs, crocodiles, turtles, birds, and mammals. Discoveries of new material in the 14 years since the last review of the north Texas fossil record (Jacobs and Win-kler, 1998) have signifi cantly expanded our understanding of the taxonomic composition of Late Cretaceous terrestrial com-munities in the DFW area.

Our objectives on this fi eld trip (Fig. 4) are to review the stratigraphic section from the Woodbine through the Austin to become better informed as to the stratigraphy and to recognize the truly exceptional sequence of marine amniote vertebrate fos-sils—the marine reptiles—found here. The Gulf Series as a whole documents the past 30 million years of life in Cretaceous seas. No other section in North America can boast better coverage, and taken with the underlying Comanche Series, it is unique in the record it provides for the past 50 million years of the Cretaceous period. Time and interest permitting, we will close this fi eld trip by a visit to the fossil hall of the Perot Museum of Nature and Science, which presents a magnifi cent portrayal of the fossil ver-tebrates of this interval and from this region.

STOP 1. WOODBINE FORMATION; GRAPEVINE LAKE, MURRELL PARK(33° 0′1.22″N; 97° 5′44.75″W)

Grapevine Lake was impounded in 1952 by the U.S. Army Corps of Engineers by damming Denton Creek, a tributary of the Elm Fork of the Trinity River. The purpose of the damming was fl ood control and to provide water for the Dallas–Fort Worth Metroplex. The closest natural lake to the Dallas–Fort Worth area is Caddo Lake on the Texas-Louisiana border.

The porous Woodbine Formation naturally supports forests of oak and associated species, remnants of which can be seen around Grapevine Lake (Fig. 5). This belt of woodlands is called the Eastern Cross Timbers. Deep tree roots can obtain water year round from Woodbine soils, but grassland prairies are more fi tted for soils developed on limestone and shale. The trace of the cross-timber forest follows the Woodbine outcrop across the prairies, splitting it into the Black Prairie to the east and the Grand Prai-rie to the west. In 1832, Washington Irving referred to the Cross Timbers as a “forest of cast iron” (Francaviglia, 2000). The Fort Worth Prairie (a component of the Grand Prairie) lies to the west of the Eastern Cross Timbers and is delimited on its western edge by the Western Cross Timbers, which grows on Trinity Group (Comanche Series) rocks for the same reason.

The Woodbine lies unconformably on the Buda Limestone and Grayson Marl (Dodge, 1969). In their regional study, Adams

and Carr (2010) recognized two major units in the Woodbine, the Dexter and the Lewisville. Locally, Reaser (2002) recog-nized the Rush Creek, Dexter, Lewisville, and Arlington forma-tions, and the Tarrant and Templeton facies within the Wood-bine. The Woodbine is considered to be middle Cenomanian in age, or ~96 Ma.

At Stop 1, Woodbine foreshore deposits include cross- bedded and bioturbated sands. There is an extensive bioturbated unit characterized by crab feeding balls and burrows (e.g., Allen and Curran, 1974). A channel is seen cutting across dune sands, with its margin continuing into the bioturbated unit. Rare frag-ments of fossil wood are found here.

Hadrosaur (duckbill) dinosaur bones and several spectacular trackways are also found in Murrell Park (Lee, 1997a, 1997b). The holotype of the early hadrosaur Protohadros byrdi was found within a few kilometers of here (Head, 1998). Footprint ichno-taxa from Murrell Park have been attributed to theropod (Fusci-napedis woodbinensis) and hadrosaur (Caririchnium protohad-rosaurichnos) dinosaurs and to a large bird (Magnoavipes lowei) (Lee, 1997b). The major trackways are now covered (Fig. 6).

The only marine amniote described thus far from the Wood-bine is the pholidosaurid crocodile Terminonaris cf. T. robusta (Adams et al., 2011) from the uppermost part of the Woodbine Formation. The ammonite Conlinoceras tarrantense is found within the Lewisville member and in the Tarrant member of the overlying Eagle Ford. That ammonite taxon marks the base of the middle Cenomanian (Kennedy and Cobban, 1990), and the Terminonaris fossil was collected just a couple of meters below the base of the Eagle Ford, suggesting it is no younger than early middle Cenomanian (~96 Ma). There are currently no reports of plesiosaurs, dolichosaurs, or mosasaurs from the Woodbine.

STOP 2. WOODBINE–EAGLE FORD CONTACT AT BEAR CREEK(32°50′33.75″N; 97° 2′31.77″W)

This stop is on DFW airport land and it provides the best exposure of the Woodbine–Eagle Ford contact, as most com-monly accepted, in this region. Here, the Tarrant unconformably overlies the upper Woodbine (but note that the Tarrant is consid-ered the upper unit of the Woodbine Group by Reaser, 2002). The contact is characterized here by Woodbine clinoforms truncated by horizontal strata of the Tarrant (Fig. 7).

Adams and Carr (2010) term this contact the Eagle Ford Unconformity, and they consider it to vary in age and duration across the East Texas Basin. This is supported by ammonite dis-tributions in the upper Woodbine and lower Eagle Ford. They con-sider the erosion associated with the unconformity to be related to tilting and uplift of the San Marcos Arch and the Sabine Uplift, where it has eroded into the Lower Cretaceous Glen Rose and the Eagle Ford is completely missing. The Eagle Ford Unconformity merges with the unconformity at the base of the Austin in the east, but is well below a distinct disconformable condensed zone 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).

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).

The early middle Cenomanian ammonite Conlinoceras tar-rantense is found in the Tarrant and indicates an age of 96 Ma at Bear Creek. Fossiliferous gravel lags are common near the con-tact, and shark teeth, dinosaur bones, and fossil wood are relatively common. The oldest marsupial found in Texas, a single tooth, was discovered here (Jacobs and Winkler, 1998; Winkler et al., 1995).

The oldest occurrences of plesiosaurs in the Eagle Ford are isolated teeth of the large pliosaurid (short-necked) Bra-chauchenius recovered from the Tarrant. Low in the Britton, the remains of three plesiosaurs were recovered from Lewis-ville Lake, near Little Elm, Texas. A skeleton of Brachauche-nius was recovered during excavation for Terminal B (then

Late Cretaceous strata and vertebrate fossils of North Texas 7

known as Terminal 2W, the Braniff terminal) at DFW Interna-tional Airport, near the base of the Britton (Fig. 8). Additional specimens of Brachauchenius have been recovered higher in the section in the lower middle Turonian Collingnoceras woollgari zone at Cedar Hill. The elasmosaurid plesiosaur, Libonectes morgani, originally reported by Welles and Slaugh-ter (1963), was recovered from Eagle Ford rocks at Cedar Hill, Texas, and is probably from the Britton (Storrs, 1981) (Fig. 9). The skull of this specimen was fossilized in a concretion and is uncrushed, making it one of the best preserved mid-Cretaceous elasmosaurs in existence. The Tarrant member throughout the DFW area yields specimens of the marine squamate, Coni-asaurus cf. crassidens.

The earliest mosasaurs to appear in the fossil record of north Texas are found in the C. woollgari Zone of the Arca-dia Park. Russellosaurus coheni was named by Polcyn and Bell (2005) on the basis of a specimen collected just above the Kamp Ranch Limestone. This mosasaur occupies a basal position in a lineage that gave rise to more derived forms such as Platecarpus. Dallasaurus turneri, a primitive mosasaurine mosasaur, was described and named by Bell and Polcyn (2005) 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 Clidastes, 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-ed at Dallas–Fort Worth Airport on display at the old Braniff terminal (http://www.smu.edu/isem/oceandallas.html).

Figure 9. First long-necked elasmosaur plesiosaur from the lower Ea-gle Ford (Britton) at Cedar Hill (from Shuler, 1950; see also Welles, 1949). This became the holotype of Libonectes morgani.

STOP 3. EAGLE FORD, ARCADIA PARK, KAMP RANCH, AND AUSTIN CHALK AT PINNACLE PARK (FIG. 10)(Arcadia Park: 32°45′1.06″N; 96°52′51.71″W)(Austin Chalk: 32°45′40.32″N; 96°53′18.22″W)

The Eagle Ford in the Dallas area is generally subdivided into Tarrant, Britton, Kamp Ranch, and Arcadia Park units. Nor-ton (1965) suggested a possible disconformity in the upper part of the Britton, below the Kamp Ranch, based on the correla-tion of concretions from two localities relative to the overlying Kamp Ranch. In the fi rst, a concretionary mass occurred 4–6 m below the Kamp Ranch and in the other 3 m below. Pessagno (1969) recognized a paraconformity based on missing planktonic foramineral zones within the Eagle Ford “4–14 feet below the Britton-Arcadia Park contact” in the Socony Mobil core from south Dallas. However, Valentine (1984) did not recognize an unconformity within the Eagle Ford based on his study of calcar-eous nannofossils. Kennedy (1988), in his study of ammonites from northeast and central Texas, stated that he had observed an unconformity in a temporary exposure 2.5 m below the Kamp

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) Calcite 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 Park). He further suggested his unconformity in the upper Brit-ton, below the Arcadia Park, may vary in magnitude or there may be more than one unconformity in that interval.

The community of Eagle Ford, and the type section of Eagle Ford rocks, is north of I-30 on Chalk Hill Road, which runs along the western margin of Pinnacle Park. The lower Eagle Ford, or Britton, was exposed in the original ford of the Trinity River, but there are currently no exposures. Arcadia Park, which gives its name to the upper Eagle Ford in this region, is the community to the southwest of Pinnacle Park. Pinnacle Park is built in an old industrial area known as Cement City. Most of the buildings along I-30 in Pinnacle Park are built on the Kamp Ranch Limestone, as is the old pink Eagle Ford School, where Bonnie Parker of Bon-nie and Clyde notoriety, went to school. The two met in this area of old West Dallas. Britton rocks are named for the village of the same name, now being swallowed by the town of Mansfi eld south of Fort Worth. Britton holds a place in Bonnie-and-Clyde lore because it boasted the only bank in the area not knocked off by the gangsters. Clyde’s mother kept her money there.

We drive through the lower Eagle Ford (Britton) on the way to Pinnacle Park, and we catch a glimpse of it in the pit near Chalk Hill Road and I-30. The pit is famous for the beautiful fos-sil ammonites collected there before it was closed to collectors. It has been considered to be in the Arcadia Park (upper Eagle Ford; Reaser, 2002), although it clearly lies below the Kamp Ranch and therefore lies in the upper Britton.

The Kamp Ranch Limestone (Reid, 1952) is a packstone composed predominately of Inoceramus prisms, lying between the gray shales of the Britton below and Arcadia Park above (Fig. 11). It is generally interpreted to represent a lowering of sea level or wave base, but Hancock and Walaszczyk (2004) interpret it to be a highstand condensed zone. Impressions of Collignoniceras

woollgari, the aragonitic shells dissolved away, are common and indicate an age of ~92.5 Ma. The Kamp Ranch occurs consis-tently ~30 m below the Eagle Ford–Austin contact from Ellis County in the south, across Dallas County, to Denton County in the north. The mosasaurs Dallasaurus turneri and Russellosau-rus coheni occur within a few centimeters of the top of the Kamp Ranch (Fig. 12). The upper Eagle Ford (Arcadia Park) exposed in Pinnacle Park is mainly represented by rocks of the Prionocylcus hyatti Zone of the lower middle Turonian.

The highest rocks in Pinnacle Park form the former quarry walls, and tool marks can still be seen. They comprise the Aus-tin Group, also referred to as the Austin Chalk or Austin Forma-tion, Upper Cretaceous (Coniacian to lower Campanian) marine carbonates that underlie much of the city of Dallas. The unit is divided into formations (or members) with differing names in different parts of Texas. In the Dallas area, the Austin Group is commonly divided into three units. The basal Atco Formation (or member) has been widely used in the literature, unlike the various names given to the middle and upper parts of the Austin Group.

The Atco Formation is well exposed along a cut behind the warehouse district at Rock Quarry Road near Pinnacle Point Boulevard and at the Bulk Mail Center on I-30 (Figs. 13 and 15). Light-colored carbonates alternate rhythmically with darker gray units commonly referred to as marls. Bedding is very continuous for the most part, interrupted by numerous small normal faults made obvious by the clear bedding and steep walls of the quar-ries. The elevation of the Atco, and that of the Kamp Ranch, both traceable units, demonstrates that the cumulative throw on the faults is ~70 m between Pinnacle Park and the channel of the Trinity River near downtown Dallas, where the Kamp Ranch is again exposed. The faults are not obvious in the shales of the Eagle Ford, but they are a factor that should be considered in any 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 Eagle Ford Group (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-rka and Nance, 1994). The channels are not uncommon in the lower Austin Group. Formation of these channels has been attributed to deep-water turbidites in West Texas (Lock, 1984),

Figure 11. Scanning electron microscope image of Kamp Ranch Lime-stone showing prisms of Inoceramus shell (http://www.smu.edu/isem/oceandallas.html).

10 Jacobs et al.

or to reworking of deep-water shelf sediments by either slope processes or fan formation in Ellis County (Hovorka and Nance, 1994). Hovorka and Nance (1994) argued that during deposition of the deep-water units in the lower Austin Group, benthic envi-ronments lacked the hydraulic energy necessary to rework sedi-ment into channels. Carbonized and pyritized wood, along with lags of shell material, were found in Ellis County channels (Hov-orka and Nance, 1994). In spite of this, shallow shelf sources

Figure 12. Dallasaurus turneri, a primitive-limbed mosasaur from the Eagle Ford immediately above the Kamp Ranch (http://www.smu.edu/isem/oceandallas.html).

Figure 13. Outcrop of the Atco Formation (Austin Group) near Rock Quarry Road showing small-scale normal faults. View to the south.

were not accepted as the source of any of the fi ll in the channels (Hovorka and Nance, 1994).

At Rock Quarry Drive, several small channel forms present an opportunity to test these models. One of us (Kennedy) has mapped and sampled the channels and examined their lithology and sedimentary structures. X-ray diffraction was used to assess mineralogy and element composition. The shallow channels here 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)

At Chalk Hill and West Davis Street, shales of the upper Eagle Ford form a covered slope, and lowermost Austin Group

Late Cretaceous strata and vertebrate fossils of North Texas 11

carbonates cap the ridge (Fig. 16; also see stop in Hancock and Walaszczyk, 2004). Throughout much of Texas, the base of the Austin Group has been recognized as unconformable on the underlying Eagle Ford Group (Stephenson, 1929; Sellards et al., 1932; Forgotson, 1958). The basal contact with the Eagle Ford Group is marked in the Dallas area by a glauconitic, phosphatic contact zone rich in marine vertebrate fossils. In northern Texas, this phosphatic unit has come to be referred to as the “fi sh bed.” What Taff and Leverett (1893) originally dubbed the “fi sh bed conglomerate” was a bed within the upper portions of the Eagle Ford Group in northern Texas, below the contact with the Austin Group (McNulty, 1954). They also noted other phosphatic beds rich in fi sh remains in the upper Eagle Ford Group. McNulty (1965) described a “fi sh bed” at the basal Austin Group contact in addition to the others previously known within the upper Eagle Ford Group. Such a fi sh bed is present at the Chalk Hill outcrop.

Figure 14. Scanning electron microscope image of coccoliths from the Austin Chalk (http://www.smu.edu/isem/oceandallas.html).

Figure 15. Channel form within the Atco Formation at Rock Quarry. Jamell Kennedy and 1.5 m stick in view.

Figure 16. Outcrop of the basal Austin Group at Chalk Hill Road. View to the northeast.

Figure 17. Typical phosphatic zone at the base of the Atco Forma-tion (Kiest Boulevard) showing fi sh debris and shark tooth. Scale in centimeters.

Dawson (2000), among others, interprets the phosphatic intervals in the Eagle Ford Group as representing condensed sections formed by slow sedimentation. Condensed sections are commonly interpreted, in a sequence stratigraphic sense, as times of low sedimentation rates during maximum fl ooding or maxi-mum transgression (Loutit et al., 1988). Hancock and Walaszc-zyk (2004) state that the disconformity at the base of the Austin Group represents a lowstand in sea level. The condensed bed with phosphatic nodules, they argue, is uppermost Turonian and represents a strong rise in sea level at the beginning of Austin Group sedimentation.

Within the condensed section at Chalk Hill Drive, shark and bony fi sh remains are abundant, as are foraminifera. Coprolites and pyrite also occur within this unit. The contact zone here is not as dark colored, nor as well developed, as exposures to the south in Dallas along Kiest Boulevard (Fig. 17).

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

12 Jacobs et al.

several species of Ptychodus (Hamm and Cicimurri, 2011). Many of these can be found in the Chalk Hill contact zone.

In contrast to the rich and unique marine amniote fauna of the underlying Eagle Ford, the Austin in north Texas produces a sequence of mosasaurs taxonomically comparable to those known from the Smoky Hill Chalk of Kansas (Everhart, 2001), the Mooreville Chalk of Alabama (Kiernan, 2002), and the upper part of the Boquillas Formation of the Big Bend area of west Texas. The condensed phosphatic zone at the base of the Atco yields occasional fragmentary remains of plioplateacarpine and tylosaurine mosasaurs, but none of this material can be confi -dently referred to a specifi c genus. The Atco in the Dallas area has produced Platecarpus planifrons and Tylosaurus nepaeoli-cus. The Bruceville has produced Tylosaurus proriger and Cli-dastes cf. C. propython (Thurmond, 1969), and the Hutchins has produced specimens of Tylosaurus proriger. Echols (1972) reported Clidastes propython, Clidastes sp., Platecarpus sp., and Tylosaurus proriger from the Roxton member of the Gober Chalk, northeast of Dallas. The Gober is laterally equivalent to the Hutchins in Dallas. Echols (1972) reported both Polycotylus latipinnis and an indeterminate elasmosaur plesiosaur from the Roxton member of the Gober Chalk, northeast of Dallas.

OPTIONAL STOPS 5 AND 6. DOWNTOWN DALLAS, PEROT MUSEUM OF NATURE AND SCIENCE, KLYDE WARREN PARK (TIME AND INTEREST DEPENDENT)(Perot Museum: 32°47′12.44″N; 96°48′23.89″W)

Located at 2201 North Field Street (corner of Field and Woodall Rodgers Freeway), the Perot Museum of Nature and Science opened on 1 December 2012. The Perot Museum has four fl oors of interactive exhibits housed in an iconic building. This pace-setting museum was built with $185 million in private funds and completed without debt. The reaction of the people of Dallas, of Texas, and of visitors from all over has demonstrated an overwhelming interest, thirst for knowledge, and desire for education in science and technology. It is a museum for the curi-ous of all ages. Its galleries are comprehensive. Fossils are a clear draw to the Perot Museum. Of particular interest to this fi eld trip are the exhibits on late Cretaceous marine reptiles, a spectacular representation of mosasaur evolution (including Dallasaurus), dinosaurs, fossil plants, rocks and minerals, Earth processes, and energy. Its exhibits explain the story revealed in the strata on which this region is built.

ACKNOWLEDGMENTS

We are grateful to the Dallas–Fort Worth Airport Board for allowing access to Bear Creek. We thank Anthony Fiorillo (Perot Museum of Nature and Science), George Maxey (University of North Texas), and Diana Vineyard (Southern Methodist Univer-sity) for their reviews. Leslie Reeder-Myers drafted Figure 2. The work of J.G. Kennedy on Austin channels included in this fi eld

guide was supported by a Ray Marr Scholarship from the Institute for the Study of Earth and Man at Southern Methodist University.

REFERENCES CITED

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