Glen Rose Formation Research Articles

Carbon isotope chemostratigraphy of the Yucca Formation from the Solitario, Big Bend Ranch State Park, Texas

Introduction: The Yucca Formation is a Lower Cretaceous sedimentary unit present in West Texas. Based on its relative stratigraphic position in the Cretaceous succession of West Texas, it is expected that the Yucca Formation is of Albian and/or Aptian age. It is also expected that the carbon isotope excursions associated with OAE 1a and OAE 1b should be identified in the Yucca Formation. The goals of this project are to 1. construct a carbon isotope chemostratigraphic record of the Yucca Formation, and 2. correlate the Yucca Formation with strata of similar age using chemostratigraphy.Methods: 163 samples were collected from Big Bend Ranch State Park (BBRSP) to determine the δ13C value of bulk sedimentary organic matter.Results: C-isotope values range from −27.02‰ to −18.42‰.Discussion: Carbon isotope excursions (CIEs) that are associated with the Aptian-Albian Boundary are identified as well as CIEs associated with Oceanic Anoxic Events (1a and 1b). This allows us to conclude that the Aptian-Albian boundary is recorded within the Yucca Formation strata at about 71 m above the base of the section exposed in the Lower Shutup of the Solitario in Big Bend Ranch State Park. Regional correlation of the Yucca Formation to other chemostratigraphic records from other Cretaceous strata suggests that the Yucca Formation in BBRSP is time equivalent to the Sligo, Pine Island, James, Bexar, and a portion of the lower Glen Rose Formation on the Comanche Platform and to a portion of the lower Glen Rose Formation in Big Bend National Park.

Paleoenvironmental constraints on shallow-marine carbonate production in central and West Texas during the Albian (Early Cretaceous)

The Albian Glen Rose Formation in central and west Texas records carbonate sedimentation in a broad epicontinental sea partly contemporaneous with Oceanic Anoxic Event 1b. The paleogeographic setting of the Comanche Platform facilitated the deposition of carbonate sediments that resulted in carbonate matrix-rich facies, with various amounts of allochems, which permitted the interpretation of shallow lagoon depositional settings. Geochemical proxies for environmental conditions (nutrient supply, detrital input, redox conditions) were calculated from the elemental geochemical analysis of samples, while their carbon and oxygen isotope composition was used to evaluate the impact of diagenesis. The statistical analysis of this dataset helps identify three intervals within the Glen Rose Formation where the matrix-rich carbonate sedimentation was modulated by environmental parameters. First, the strong correlation between MgO concentration and δ18O vectors on a Principal Component Analysis for the whole dataset confirms the deposition of dolomitic series with stromatolites in a restricted environment. Second, moderately high nutrient input and low detrital flux suggest that part of the sedimentation in west Texas was affected by upwelling. Third, mesotrophic conditions associated with a patch reef made of rudist bivalves support their suspension feeder ecological behavior. Conclusions drawn for this research may apply to other shallow-marine series deposited in broad epicontinental seas affected by greenhouse conditions.

Deeper-water deposition in intrashelf basins: Example from the Lower Cretaceous (Albian) upper Glen Rose Formation in the Houston trough, eastern Texas

In easternTexas, a thick (150-200 ft) interval of Lower Cretaceous upper Glen Rose D strata was deposited in the Houston trough intrashelf basin landward of the Stuart City reef margin and seaward of the main eastern Texas intrashelf basin. The stratigraphic section in this intrashelf basin forms a primarily oil-bearing hydrocarbon system where porous grainstones in the Alabama Ferry and Fort Trinidad fields were deposited coevally with organic-rich calcareous argillaceous mudstones. Here, we present evidence to suggest that the depositional setting within the Houston trough and the contained ooid grainstone beds were of deeper-water origin (400-500 ft), below storm wave base. Further, the water column was stratified, allowing dysaerobic to anaerobic conditions to develop, fostering accumulation and preservation of organic matter. Grain-rich gravity flows consisting of ooids, intraclasts, and thin-walled mollusks, derived from the shallower shelf to the northwest, were deposited in the basin, forming the Alabama Ferry and Fort Trinidad fields. The upper Glen Rose interval is composed of lithofacies that vary laterally within the Houston trough. Five lithofacies are recognized on the basis of sedimentary features, texture, fabric, biota, and organic-matter content: (1) laminated to nonlaminated argillaceous lime mudstone to lime wackestone; (2) laminated to nonlaminated calcareous siliciclastic mudstone to siltstone; (3) intraclast–ooid skeletal lime packstone; (4) intraclast–bivalve–ooid lime grainstone; and (5) oyster lime floatstone and rudstone. Grain-rich lithofacies laterally adjacent to and interbedded with organic-rich argillaceous lime mudstones and calcareous siliciclastic mudstones were previously interpreted as being deposited in a tidally influenced, high-energy, shallow-water shoal environment; however, reconstruction of the depositional setting and interpretation of the vertical and lateral distribution of lithofacies suggest that these lithofacies were deposited as grain-rich gravity flows into the deeper-water Houston trough. This interpretation suggests a new conceptual framework integrating facies models from submarine channel and fan complexes to explore for grainstone reservoirs similar to Alabama Ferry and Fort Trinidad fields.

Influence of mechanical layering and natural fractures on undercutting and rapid headward erosion (recession) at Canyon Lake spillway, Texas, U.S.A

This study investigates the role of mechanical layering and fractures on flood-related erosional undercutting and resulting rapid spillway recession. In the summer of 2002, 86 cm of rain fell in an 8-day period across the Guadalupe River drainage basin in central Texas, causing Canyon Lake reservoir to completely fill and overtop the emergency spillway for the first time. The resulting flood incised a gorge into the mechanically layered Glen Rose Formation and caused headward erosion (recession) at the downstream edge of the emergency spillway. Comparison of pre- and post-flood imagery and assessment of flood records indicates that maximum recession localized at the northern end of the emergency spillway where 28 m recession occurred. This recession occurred at an estimated rate of up to 10 m/day during the first ~3 days of the flood, which is among the highest rates of recorded bedrock recession. Analysis of historical photographs, field observations and measurement of erosional undercutting, along with measurements of fracture orientation, fracture spacing, and mechanical rebound are used to understand rock mass characteristics that influenced erosional undercutting and rapid recession of the spillway. Evidence of significant undercutting was observed where incompetent argillaceous wackestone (marl) underlies competent limestone. These results reveal that the greatest amount and rate of recession of the spillway was associated with undercutting and toppling collapse of fracture-bounded limestone blocks. Block size may be a factor in continuation of the process, in that large blocks may accumulate at the base of the scarp and inhibit continued erosional undercutting, whereas in other areas smaller eroded blocks can be carried away by the floodwaters and undercutting may continue, facilitating recession. The combination of mechanical contrast between layers and natural fractures in competent layers together contributed to exceptionally high rates of headward erosion. Observed rock mass erodibility behavior was in the range of medium to high erodibility in limestone with widely spaced fractures that would normally be expected to have very low erodibility. Bulk rock mass erodibility in this situation was similar to the most erodibile layer, specifically, the marl at base of spillway pour-off cliff.

FischerLab: An Application for Generating Fischer Plots and Dynamic Fischer Plots from Wireline Well-Logs and Stratigraphic Data

Fischer plots are a technique that is used to graph changes in accommodation in cyclic carbonate successions. They typically depict the cumulative departure from the average cycle thickness as a function of the cycle number or stratigraphic depth. Many applications of Fischer plots focus on their construction from exposed cyclic carbonate successions. No published programs allow the direct construction of Fischer plots from digital wireline well-logs or dynamic presentations of Fischer plots. Here, we introduce a program known as FischerLab, which facilitates the generation and analysis of Fischer plots. In addition to accepting interpreted stratigraphic data input, FischerLab facilitates the interpretation of digital wireline logs for the generation of Fischer plots in cycle and depth domains, as well as in a dynamic evolving cycle and relative depth domain from an easy-to-use interface. The dynamic construction facilitates the correlation of specific stratigraphic packages to parts of the accommodation cycle while simultaneously tracking the locus of the mean subsidence vector. We demonstrate the use of FischerLab on data derived from the carbonate succession outcrops of the Al-Athrun Formation, Libya, and the Glen Rose Formation, Central Texas, USA, as well as on wireline well-log data from the Western Great Bahama Bank, the Bahamas.

Thunder lizard handstands: Manus-only sauropod trackways from the Glen Rose Formation (Lower Cretaceous, Kendall County, Texas)

Three parallel, manus-only sauropod trackways from the Coffee Hollow A-Male tracksite (Glen Rose Formation, Kendall County, Texas) were studied separately by researchers from the Heritage Museum of the Texas Hill Country and the Houston Museum of Natural Sciences. Footprint and trackway measurements generally show good agreement between the two groups’ data sets. Footprints appear to be shallowly impressed true tracks rather than undertracks. One of the Coffee Hollow trackways shows marked asymmetry in the lengths of paces that begin with the left as opposed to the right forefoot, and two of the Coffee Hollow trackways are unusually broad. The Coffee Hollow trackways differ enough from the manus portions of other Glen Rose Formation sauropod trackways to suggest that they were made by a different kind of sauropod. Greater differential pressure exerted on the substrate by the forefeet than the hindfeet probably explains the Coffee Hollow trackways, like other manus-only sauropod trackways, but the possibility that they indicate unusual locomotion cannot at present be ruled out.

Early Cretaceous cyclostome bryozoans from the early to middle Albian of the Glen Rose and Walnut formations of Texas, USA

AbstractThe Glen Rose and Walnut formations of southcentral and northcentral Texas comprise shallow-water carbonates deposited during the late Aptian to middle Albian on a carbonate platform. The formations are famous for their rich fossil faunas. Although bryozoans are absent in late Aptian sediments, they are frequently found encrusting bivalve shells from the early to middle Albian parts of these formations. Here, we describe the cyclostome bryozoan fauna, which includes six species;Stomatoporasp.,Oncousoecia khirarn. sp.,Reptomultisparsa mclemoreaen. sp.,Hyporosopora keeran. sp.,Mesonopora bernardwalterin. sp., and ?Unicaveasp. Most cyclostomes are found encrusting rudist shells from Unit 2 of the Lower Member of the Glen Rose Formation and units 3 and 6 of the Upper Member of the Glen Rose Formation.UUID:http://zoobank.org/4380dcb5-63b2-4aa9-959c-09eb6b03831f

LIDAR-based characterization and conservation of the first theropod dinosaur trackways from Arkansas, USA.

LIDAR-based analyses of the first theropod dinosaur trackways known from the state of Arkansas, USA are reported. The trackways were found on a limestone bedding plane in the Albian De Queen Formation in an active gypsum quarry. Because limited access precluded thorough field study, fieldwork focused on preserving the entire site digitally with ground-based LIDAR, and detailed measurements were later taken digitally from point cloud data. The site contains eight tridactyl trackways associated with sauropod trackways and numerous isolated tracks. Although there appear to be two different tridactyl morphotypes, we show that the tracks are all likely from a single species of trackmaker. We apply a simple method of estimating substrate consistency by comparing the differences between true track dimensions and apparent track dimensions. The tridactyl tracks at the southern end of the site are preserved with significantly greater differences in true vs. apparent dimensions and are shallower than the rest of the tridactyl tracks at the site, which we interpret as the result of outward expansion of the soft tissues of the foot upon contact with a firm substrate. We interpret the firm substrate as having high bulk density and high shear strength, which also explain associated manus-only sauropod tracks. We show that the tridactyl tracks are likely from theropod trackmakers and that footprint lengths, trackway paces, stride lengths, and pace angulations of the De Queen trackways are statistically indistinguishable from equivalent measurements of theropod trackways in the Glen Rose Formation. The Glen Rose tracks are attributed to the large-bodied theropod, Acrocanthosaurus and we likewise attribute the De Queen tracks to Acrocanthosaurus, which is known from skeletal remains in temporally equivalent units and from the mine itself.

Early Cretaceous (Albian) spores and pollen from the Glen Rose Formation of Texas and their significance for correlation of the Potomac Group

ABSTRACTBecause the Gulf Coast Lower Cretaceous is dated by ammonites, it has great potential as a reference section for correlating continental sequences such as the Potomac Group and dating events in the rise of angiosperms. Middle Albian terrestrial palynofloras from Oklahoma have been described in detail, but the early Albian interval, represented in the Glen Rose Formation of Texas, is less well known. Samples from two localities on the Paluxy River, which correlate with horizons containing late early Albian ammonites, are dominated by Classopollis and Exesipollenites, but angiosperm pollen is the next most common terrestrial element, and there are several index spore species for Zone II in the Potomac Group. Among the angiosperms, reticulate monosulcates are most common and diverse, but there are also several tricolpate species. Stratigraphically important angiosperms include the Clavatipollenites rotundus group and reticulate tricolpates, which appear in the upper part of Potomac Zone I and the dated earliest Albian of England and Portugal. However, there are also tricolpates with striate-reticulate sculpture, a pollen type that is not known from upper Zone I but appears in the late early Albian of Portugal. This assemblage contrasts with floras from Potomac Zone II and the middle Albian (lower Fredericksburg Group) of Oklahoma, where tricolpates overtook monosulcate angiosperms in species diversity. These results confirm arguments based on the Portuguese section that there is a significant hiatus between Potomac Zones I and II, and that this gap is at least partly late early Albian. The dominance of Classopollis and Exesipollenites and the occurrence of isolated Northern Gondwanan elements (Sergipea, Tucanopollis) suggest that Texas lay in a transition zone between Southern Laurasia and the hotter and drier Northern Gondwana province, but regional studies are needed to disentangle geographic and climatic factors from effects of the lagoonal local environment.

Geophysical mapping of Mount Bonnell fault of Balcones fault zone and its implications on Trinity-Edwards Aquifer interconnection, central Texas, USA

Geophysical surveys (resistivity, natural potential [self-potential], conductivity, magnetic, and ground penetrating radar) were conducted at three locations across the Mount Bonnell fault in the Balcones fault zone of central Texas. The normal fault has hundreds of meters of throw and is the primary boundary between two major aquifers in Texas, the Trinity and Edwards aquifers. In the near surface, the fault juxtaposes the Upper Glen Rose Formation on the Edwards Plateau, consisting of interbedded limestone and marly limestone, against the Edwards Group, which is mostly limestone, on the eastern down-thrown side (coastal plain). The Upper Glen Rose member is considered to be the Upper Trinity Aquifer and also a confining zone underlying the Edwards Aquifer. However, recent studies have documented a hydraulic connection between the Edwards and Upper Trinity aquifers. The uppermost portions of the Upper Trinity and the Edwards aquifers, in some places, operate as a single aquifer system, while the lowermost units of the Upper Glen Rose are confining layers between the Edwards Aquifer and the Middle Trinity Aquifer. Geophysical data, which include resistivity, natural potential, magnetic, conductivity, and ground penetrating radar, indicate not only the location of the fault but additional karstic features on the west side (Upper Glen Rose Fm.) and on the east side (Edwards Group) of the Mount Bonnell fault. Resistivity values of the Glen Rose and Edwards Group do not appear to have significant lateral variations across the fault. In other words, the Mount Bonnell fault does not appear to juxtapose different resistivity units of the Edwards Group and Upper Glen Rose member. Thus, the fault may not be a barrier for groundwater flow. With the abundant karstic features (caves, sinkholes, fractures, collapsed areas) on both sides of the fault, determined by the geophysical data, one can conclude that lateral groundwater flow (intra-aquifer) between the Edwards Aquifer and Upper Trinity is likely.

The first South American sandownid turtle from the Lower Cretaceous of Colombia.

Sandownids are a group of Early Cretaceous-Paleocene turtles that for several decades have been only known by cranial and very fragmentary postcranial elements. Here I report and describe the most complete sandownid turtle known so far, including articulated skull, lower jaw and postcranial elements, from the Early Cretaceous (upper Barremian-lower Aptian, >120 Ma), Paja Formation, Villa de Leyva town, Colombia. The new Colombian sandownid is defined here as Leyvachelys cipadi new genus, new species and because of its almost identical skull morphology with a previously reported turtle from the Glen Rose Formation, Texas, USA, both are grouped in a single and officially (ICNZ rules) defined taxon. Phylogenetic analysis including L. cipadi supports once again the monophyly of Sandownidae, as belonging to the large and recently redefined Pan-Chelonioidea clade. The morphology of L. cipadi indicates that sandownids were not open marine turtles, but instead littoral to shallow marine durophagous dwellers. Leyvachelys cipadi not only constitutes the first record of sandowinds in South America, but also the earliest global record for the group.

The ups and downs of Diplocraterion in the Glen Rose Formation (Lower Cretaceous), Dinosaur Valley State Park, Texas (USA)

Diplocraterion, a U-shaped burrow attributed to infaunal invertebrates, is normally a shallow-marine trace fossil and not part of a continental vertebrate ichnoassemblage. Hence, the Glen Rose Formation (Aptian–Albian) of Texas (USA) presents an opportunity to study Diplocraterion associated with a world-class dinosaur tracksite. Most Diplocraterion are in a bioclastic wackestone–packstone bed just above the Taylor Tracklayer, a significant dinosaur track horizon. Diplocraterion are consistently sized, but with variable depths; most have protrusive spreiten and northeast–southwest trends. Smaller Arenicolites co-occur with Diplocraterion, and other trace fossils include Rhizocorallium and a large theropod trackway. Based on our analysis, a sea-level rise buried the Taylor Tracklayer, with a shallow-marine carbonate mud colonised by Diplocraterion and Arenicolites tracemakers. Protrusive Diplocraterion, eroded burrow tops, Rhizocorallium, and other criteria point towards firming and net erosion of the bed caused by a stillstand. The depositional environment of the Diplocraterion bed was possibly a subtidal lagoon that covered shoreward sediments impacted by large theropods. Burrow orientations suggest bidirectional currents consistent with trends of theropod trackways, implying each were controlled by a shoreline. The results of our study demonstrate how marine invertebrate and continental vertebrate trace fossils can be used together to define fine-scale changes in former carbonate shorelines.

Morphometric Analysis of Dinosaur Tracks from Southwest Arkansas

Dinosaur trackways were discovered in Cretaceous De Queen Limestone strata in Howard County, Arkansas, in June 2011. Multiple trackways with variably sized tridactyl tracks were exposed in a commercial quarry, suggesting multiple theropod species or adult and juvenile tracks of a single species. Results of morphometric analyses of 32 plaster casts from selected trackways are reported in an effort to identify the specific track-making dinosaurs and differentiate large and small tracks. Track measurements included length and width of each track, the lengths and widths of each digit impression, and the angular spread (divarication) between digit impressions. Twenty-nine plaster casts were of tridactyl theropod tracks whereas three casts were of poorly preserved tracks of a presumed but unknown tetradactyl (and possibly tetrapod) organism. Plaster casts of tridactyl theropod tracks ranged from 0.36 to 0.61 m long and 0.22 to 0.54 m wide. The longest digit impression on each track was the second, or middle, digit (range = 0.15 – 0.35 m long) with total digit divarication ranging from 31 - 57 degrees. The Arkansas track measurements were compared to tracks (Eubrontes glenrosensis Shuler 1935) preserved in the correlative Glen Rose Formation, Texas and attributed to the large Early Cretaceous carnosaur, Acrocanthosaurus atokensis. The E. glenrosensis track measurements from Texas plotted within the Arkansas data range, suggesting affinity of the Arkansas tracks to E. glenrosensis. Relatively poor preservation of tetradactyl tracks precluded morphometric analysis, but visual comparison to known Cretaceous crocodilian tracks is suggestive of affinity to such organisms.

Early eusuchia crocodylomorpha from the vertebrate-rich Plattenkalk of Pietraroia (Lower Albian, southern Apennines, Italy)

The locality of Pietraroia (Lower Albian, southern Apennines, Italy) has provided two fully articulated crocodylomorphs, exposed in ventral and in dorsal aspect, which are described here as representing a new species of a new genus, Pietraroiasuchus ormezzanoi gen. nov, sp. nov. The new taxon is found to be the sister taxon of Pachycheilosuchus trinquei from the Albian of the Glen Rose Formation, Texas. Pietraroiasuchus ormezzanoi resolves the phylogenetic position of the controversial P. trinquei, and is crucial in enabling an extensive understanding of the family Hylaeochampsidae. Phylogenetic analysis places Hylaeochampsa vectiana as a sister group of Iharkutosuchus makadii plus Pachycheilosuchus and Pietraroiasuchus. The phylogenetic result reveals the presence of an evolutionary mosaicism within non-crocodylian eusuchians. Mosaic features in Pietraroiasuchus are denoted by the combination of primitive character states, such as the position of the choana with its anterior margin formed by an inverted V-shaped palatine processeses, and the presumed presence of a tiny antorbital fossa, in conjunction with derived states involving the slight vertebral procoelia, the presence of tetraserial segmented dermal armour with an accessory lateral row, and isolated nuchal osteoderms. The disjoint occurrence between Pachycheilosuchus and Pietraroiasuchus species suggests that Pietraroia was a refuge island inhabited by endemic forms. © 2011 The Linnean Society of London, Zoological Journal of the Linnean Society, 2011, 163, S199–S227.

Remarkable preservation of a new genus and species of limuline horseshoe crab from the Cretaceous of Texas, USA

Abstract: A single specimen, part and counterpart of a carapace, of a horseshoe crab from the Lower Cretaceous (Albian) Glen Rose Formation in north‐central Texas, forms the basis for the definition of a new genus and species, Crenatolimulus paluxyensis. The discovery represents only the fifth limuline known from the Cretaceous. Its preservational style is remarkable in that the carapace exterior is faithfully replicated by a massive overgrowth of serpulid worms.

Fault zone deformation and displacement partitioning in mechanically layered carbonates: The Hidden Valley fault, central Texas

The Hidden Valley fault is exposed in Canyon Lake Gorge (central Texas) and cuts the Cretaceous Glen Rose Formation. This exposure provides an opportunity to explore the relationship between deformation mechanisms and fault displacement along 830 m (2723 ft) of a normal fault typical of those in carbonate reservoirs and aquifers around the world. The fault zone has five domains: gently deformed footwall damage zone, intensely deformed footwall damage zone, fault core, intensely deformed hanging-wall damage zone, and gently deformed hanging-wall damage zone. Footwall deformation is more intense and laterally extensive than hanging-wall deformation, and the intensely deformed hanging-wall damage zone is narrow and locally absent. The fault core contains thin clay-rich gouge or smear in most places but is locally represented by only a slickensided surface between limestone layers. The 55- to 63-m (180–207-ft) fault throw across a 43- to 98-m (141- to 322-ft)-wide fault zone is accommodated by slip along the fault core, layer tilting (synthetic dip development) in footwall and hanging-wall damage zones, and distributed faulting in footwall and hanging-wall damage zones. Total offset across the fault overestimates actual stratigraphic offset by 8 to 12 m (26–39 ft) or about 14 to 21%. In our interpretation, the Hidden Valley fault zone records both early extensional folding of the Glen Rose Formation and subsequent normal faulting that propagated downward from the overlying competent Edwards Group. The damage zone width is thus established before fault breakthrough.

The Oldest African Eucryptodiran Turtle from the Cretaceous of Angola

A new Late Cretaceous turtle, Angolachelys mbaxi gen. et sp. nov., from the Turonian (90 Mya) of Angola, represents the oldest eucryptodire from Africa. Phylogenetic analysis recovers Angolachelys mbaxi as the sister taxon of Sandownia harrisi from the Aptian of Isle of Wight, England. An unnamed turtle from the Albian Glen Rose Formation of Texas (USA) and the Kimmeridgian turtle Solnhofia parsonsi (Germany), are successively more distant sister taxa. Bootstrap analysis suggests those four taxa together form a previously unrecognized monophyletic clade of marine turtles, herein named Angolachelonia clade nov., supported by the following synapomorphies: mandibular articulation of quadrate aligned with or posterior to the occiput, and basisphenoid not visible or visibility greatly reduced in ventral view. Basal eucryptodires and angolachelonians originated in the northern hemisphere, thus Angolachelys represents one of the first marine amniote lineages to have invaded the South Atlantic after separation of Africa and South America.

Echinoid diversity, preservation potential and sequence stratigraphical cycles in the Glen Rose Formation (early Albian, Early Cretaceous), Texas, USA

The correlation between echinoid species diversity and second- and third-order sequence stratigraphical cycles is investigated in the Lower Cretaceous carbonate platform sequences of central Texas. Echinoid faunas from the early Albian Glen Rose Formation have been collected and placed into a sequence stratigraphical framework for the region. There are three discrete communities, each characterising a different part of a second order transgression–regression cycle within the Coahuila Group. A peri-reefal community is found in the Transgressive Systems Tracts (TST) characterised by Pygopyrina, Balanocidaris, Hyposalenia and Pseudodiadema, an open shelf level-bottom community of spatangoids and saleniids is developed at the Maximum Fooding Surface, and a more coastal algal meadow community characterized by Loriolia is preserved in small flooding events in the Highstand Systems Tracts. Beds yielding echinoid faunas are restricted to early TST stages of small-scale cycles within the Glen Rose Formation. This, we argue, reflects changing preservation potential, with echinoid tests only entering the fossil record during periods when accommodation space on the carbonate platform temporarily increased, promoting sediment accumulation rates to rise. The following new taxa are recognized: Parorthopsis gen. nov., Goniopygus whitneyi sp. nov., Tetragramma tenerum sp. nov., Polydiadema travisensis sp. nov., Codiopsis (Hemicodiopsis) pulchella sp. nov. and Plagiochasma texanum sp. nov.

Soil and topographic controls on runoff generation from stepped landforms in the Edwards Plateau of Central Texas

The rugged Hill Country of Central Texas is part of the extensive Edwards Plateau region. Significant portions of the Texas Hill Country overlie the Glen Rose Formation, which is characterized by a stair‐step topography formed by the weathering of interbedded carbonate materials having different weathering susceptibilities. This process has sculpted the strata into a series of “risers” and “treads” that mimic stairways. In this paper, we document the soil hydrology within the riser–tread catena. Our results are counterintuitive in that we find the highest infiltration and deepest soils on the steep riser slopes. In addition, we find that the riser subsoils are saturated or very wet for extended periods. On the basis of these findings, we suggest that (1) groundwater recharge on these hillslopes is minimal and occurs only in highly fractured zones; (2) the water‐holding capacity of the subsoils is sufficient for supporting the woody vegetation; and (3) runoff generation occurs as a combination of surface and subsurface flow, with the risers serving as sinks or recharge zones and the treads as source areas.

Strontium isotope stratigraphy of the Comanchean Series in north Texas and southern Oklahoma

Pioneering studies in north Texas and adjacent Oklahoma played an important role in the paleontologic and stratigraphic development of the Cretaceous in North America. We have used the results from 135 strontium isotope analyses from fossils and rocks collected from 43 sites to define the Cretaceous path of seawater 8 7 Sr/ 8 6 Sr during Comanchean time. The section represents an estimated duration of 19.3 m.y., from late Aptian through late Cenomanian. The 8 7 Sr/ 8 6 Sr of seawater rises from 0.707318 ′ 13 (errors given x 10 - 6 ) in the basal Glen Rose Formation to reach a peak value of 0.707522 ′ 10 in the Walnut Clay, then gradually declines to 0.707468 ′ 6 in the middle Grayson Marl. The upper Grayson Marl and lower Buda Limestone 8 7 Sr/ 8 6 Sr values decline to 0.707422 ′ 9, then rise to near 0.707473 by the middle Buda Limestone. The 8 7 Sr/ 8 6 Sr mean of the basal Woodbine Formation, in the overlying Gulfian Series, is in agreement with the middle Buda value. Thirty of the analyzed samples yielded ratios higher than the mean determined for corresponding stratigraphic units. The 8 7 Sr/ 8 6 Sr ratios from whole-rock limestones and spar fillings of shells are inconsistent and more radiogenic than the best shell material. All but two samples yielding aberrant ratios could be identified by using appearance and trace element content. Isotopic results from oyster and pectin shells with low Mn and Fe concentrations are remarkably consistent. The agreement of results among samples collected from a stratigraphic interval at a single site with results from geographically removed sites in the same interval is strong evidence that these samples retain the original open-marine 8 7 Sr/ 8 6 Sr of Comanchean seawater. The seawater variation defined here shows fine structure not apparent in previous work. This definition can be used to better understand the complex relationship and timing between seawater 8 7 Sr/ 8 6 Sr variation, ocean anoxic events, mid-ocean-ridge production, and subsequent climate and ocean-productivity changes.