Cretaceous Interior Research Articles

Sedimentological and geochemical insights into the opening of the Cretaceous interior seaway: The Lower Cretaceous Skull Creek Formation, Colorado

Sedimentological and geochemical insights into the opening of the Cretaceous interior seaway: The Lower Cretaceous Skull Creek Formation, Colorado

Application of μXRF analysis on the Upper Cretaceous Mancos Shale: A comparison with ICP-OES/MS

This study investigates the elemental signature of fine-grained sedimentary rock (mudstone) obtained using energy-dispersive micro-X-ray fluorescence (μXRF). A short section (28.5 cm) of Upper Cretaceous Mancos Shale was analysed with an Itrax μXRF core scanner at 0.2 mm intervals for comparison to elemental and petrographic composition. Fifteen subsamples were collected at 2 cm intervals for ICP-OES/MS analysis to obtain absolute concentrations of elements and oxides, and thin sections were used for petrographic and microfacies analysis. The data obtained from the Itrax core scanner show a high level of reproducibility between separate scans and moderate to strong Spearman-Rho correlation (R2 > 0.6) to ICP-derived concentrations for several oxides: Fe2O3, SiO2, Al2O3, K2O, TiO2, and MnO. Common elemental ratios used in palaeoenvironmental studies (e.g., Zr/Rb, Ca/Ti, Mn/Ti) also showed good correlation to ICP-derived data when the trends were compared. The elemental signature obtained from the Itrax also showed good relationships to the petrography and lithology, with the Si/Ti ratio accurately identifying quartz-rich layers, and the Ca/Ti and Mn/Ti ratios corresponding to increased carbonate content, which are of importance in the planning of hydraulic fracturing for unconventional resources. At core scale, the elemental trends in Zr/Rb, Ti, and Si were useful for detecting short-term, rapid patterns of sedimentary events, likely resulting from storm activity (e.g., tempestites, wave-enhanced sediment gravity flows). The high-sampling resolution data from μXRF core scanners can provide advantages over traditional geochemical techniques since data aliasing is minimized. Analysing these elements and ratios in longer core sections from the Mancos Shale may serve as a proxy for determining high-frequency climatic changes in the Cretaceous Interior Seaway and allow for improved correlations of intra- and inter-basinal events in other Cretaceous basins.

Cenozoic uplift of the Fátima Basin, NE Brazil

The Fátima Basin is a small Cretaceous interior basin that was formed within the continental Borborema Province in northeastern Brazil. Its formation was related to the opening of the southern Atlantic Ocean. This study investigates the hypothesis of post-rift tectonic reactivation within this basin through an integrated set of geophysical (aeromagnetic), surface (outcrops), and subsurface (boreholes) data to model the tectono-stratigraphic framework of the basin. An analysis of several morphometric parameters, including the slope index, relief aspect, stream profiles, asymmetry factor (AF), stream-gradient index (SL), and valley floor width-to-height ratio (VF) was used to identify possible tectonic adjustments related to post-rift stress fields. The integration of geophysical and subsurface data showed that the basin possesses a half-graben geometry with the main basin axis trending NE-SW. Two families of topographic lineaments were identified: 1) NE-SW, corresponding to the main shear zones and faults of the Pre-Cambrian basement, and 2) NW-SE, representing faults and fractures in the adjacent basement and the sedimentary cover. A digital elevation model revealed that the maximum topographic elevation was 950 m, associated with deposits of the Paleozoic Tacaratu Formation located over the hinge line of the flexural margin of the basin. The AF values in the basin ranged between 25 and 76, and first- and second-order SL index anomalies were frequent, especially in the faulted border, the southeast sector, and the northeast sector. Analyses of the main river channels revealed VF values of around 1, which indicate active incisions in the river valleys. We suggest that the basin was subjected to differential uplift during the Cenozoic, which strongly affected the southeast and northeast sectors. Based on the exhumation of the Tacaratu Formation deposits, we estimate that the basin experienced approximately 500 m of uplift that influenced the erosion of Cretaceous deposits and adjacent basement rocks.

Crocodylian swim tracks from the Gates Formation (Albian), British Columbia, Canada: comparisons with Cretaceous crocodylian ichnology in western USA

Numerous crocodylian swim tracks occur in the late Early Cretaceous and earliest Late Cretaceous of western North America, in coastal plain deposits marginal to the Cretaceous interior seaways. New assemblages from the lower Albian Gates Formation, within the Tumbler Ridge UNESCO Global Geopark Quintette site (TRUGG-Q) in British Columbia, Canada, resemble those from Albian-Cenomanian Dakota Sandstone sites in Colorado, Kansas and Utah, and facilitate comparative analysis of how such assemblages are registered and preserved. The Gates Formation track assemblage formed on the southwest margin of the ‘Moosebar Sea’ that filled the foredeep of the Western Canada Foreland Basin in the Early Albian. Northeast-prograding sandy deltas were backed by low-lying and heavily-vegetated coastal plains, inhabited by track-making dinosaurs, birds, and crocodylians. The base of a tidal channel-fill at the TRUGG-Q site preserves crocodylian swim traces indicating progression largely parallel to the channel axis, as also inferred for some USA sites. Swim track lengths can be categorized on a short–long spectrum which reflects swimming behavior, with short, wide toe tip traces indicating firm digging-in of toes (‘punting’ or ‘poling’ behavior), and elongate scrape marks indicating light touch-down (or substrate ‘raking’) by buoyant animals making intermittent contact with a subaqueous substrate. The body lengths size of crocodylian trackmakers can be calculated from rare complete ‘walking’ track footprints or from the widths, not lengths, in the case of ‘swim’ traces. The mean size of the Canadian tracks indicates individuals with body lengths in the range of ~1.0 m, with larger trackmakers up to ~4.0 m more common at the USA sites.

Nonmarine turtles from the Aguja Formation (Late Cretaceous, Campanian) of Chihuahua, Mexico

Fragments of turtle shell are known from several Upper Cretaceous formations in Mexico, including Corral de Enmedio and Packard Shale, Sonora; Aguja and Cerro del Pueblo, Coahuila; San Carlos, Chihuahua; El Gallo, Baja California; and Ocozocoautla, Chiapas. Turtles are important members of Upper Cretaceous vertebrate assemblages throughout North America and are considered a useful tool to define biogeographic patterns. The Upper Cretaceous sedimentary rocks exposed in the Aguja Formation record the final transgressive/regressive sequence of the Western Interior Cretaceous Seaway and the subsequent transition from a marine to terrestrial environment. The total area of outcrops is small compared to correlative exposures of these strata elsewhere in North America, in spite of this, numerous invertebrate and vertebrate fossils have been collected. The Aguja Formation in Texas preserves one of the southernmost well-studied Upper Cretaceous (Campanian) terrestrial vertebrate faunas in North America; contrary to what happens with the outcrops in northern Chihuahua, where the record of terrestrial vertebrates is scarce and especially those related to Testudines. Non-marine strata of the Aguja Formation do not appear to be present farther south in Mexico, and this is the reason why the Aguja fauna is relevant to documenting latitudinal variation in Campanian continental faunal associations. In this paper, the richness of Upper Cretaceous turtles collected from three localities within the Aguja Formation in Chihuahua is reviewed. Six taxa are recognized based on shell fragments with distinctive sculpture patterns: stem cryptodires, including cf. Baenidae, c.f. Denazinemys nodosa and cf. Compsemys victa, and crown group cryptodires including trionychians (trionchids and Basilemys sp.) and a kinosternoid (cf. Yelmochelys rosarioae). With the recognition of these taxa, new records are reported for the Aguja Formation (e.g. cf. Yelmochelys rosarioae) and Mexico (e.g. c.f. Denazinemys nodosa). The knowledge of Cretaceous turtle richness of the Aguja Formation and Mexico is increased and confirms the proposal of the variability richness along the North American localities. This latitudinal variability suggests that at any time turtles would have had a climatically controlled northern limit of distribution, and the richness of turtles would have decreased as this limit was reached.

P.817 The effects of different length of displacement on the war-related stress induced health outcomes in internally displaced persons in Georgia

Fragments of turtle shell are known from several Upper Cretaceous formations in Mexico, including Corral de Enmedio and Packard Shale, Sonora; Aguja and Cerro del Pueblo, Coahuila; San Carlos, Chihuahua; El Gallo, Baja California; and Ocozocoautla, Chiapas. Turtles are important members of Upper Cretaceous vertebrate assemblages throughout North America and are considered a useful tool to define biogeographic patterns. The Upper Cretaceous sedimentary rocks exposed in the Aguja Formation record the final transgressive/regressive sequence of the Western Interior Cretaceous Seaway and the subsequent transition from a marine to terrestrial environment. The total area of outcrops is small compared to correlative exposures of these strata elsewhere in North America, in spite of this, numerous invertebrate and vertebrate fossils have been collected. The Aguja Formation in Texas preserves one of the southernmost well-studied Upper Cretaceous (Campanian) terrestrial vertebrate faunas in North America; contrary to what happens with the outcrops in northern Chihuahua, where the record of terrestrial vertebrates is scarce and especially those related to Testudines. Non-marine strata of the Aguja Formation do not appear to be present farther south in Mexico, and this is the reason why the Aguja fauna is relevant to documenting latitudinal variation in Campanian continental faunal associations. In this paper, the richness of Upper Cretaceous turtles collected from three localities within the Aguja Formation in Chihuahua is reviewed. Six taxa are recognized based on shell fragments with distinctive sculpture patterns: stem cryptodires, including cf. Baenidae, c.f. Denazinemys nodosa and cf. Compsemys victa, and crown group cryptodires including trionychians (trionchids and Basilemys sp.) and a kinosternoid (cf. Yelmochelys rosarioae). With the recognition of these taxa, new records are reported for the Aguja Formation (e.g. cf. Yelmochelys rosarioae) and Mexico (e.g. c.f. Denazinemys nodosa). The knowledge of Cretaceous turtle richness of the Aguja Formation and Mexico is increased and confirms the proposal of the variability richness along the North American localities. This latitudinal variability suggests that at any time turtles would have had a climatically controlled northern limit of distribution, and the richness of turtles would have decreased as this limit was reached.

Microbialite fields developed in a protected rocky coastline: The shallow carbonate ramp of the Aptian Romualdo Formation (Araripe Basin, NE Brazil)

The Aptian Romualdo Formation is a siliciclastic-dominated sedimentary unit recording the last marine ingression within the Cretaceous interior basins of northeastern Brazil. At the western margin of the Araripe Basin, rocks of the Romualdo Formation are mainly represented by carbonates resting abruptly over the Proterozoic crystalline basement. Detailed mapping and stratigraphic analysis revealed two stromatolite fields that were described and discussed for the first time. Several bioherms, biostromes and isolated stromatolites characterized by distinct microbialite morphologies associated with echinoid-rich strata have been identified, suggesting that hypersalinity, water depth and hydraulic conditions were the main factors controlling stromatolite morphogenesis. A cm-thick amalgamated bivalve rudstone, resting directly on the basement and representing a shell concentration formed above the fair-weather wave base was also recorded. Based on the regional distribution of the stromatolite types and associated sedimentary facies, we interpret the depositional environment as a local low-gradient carbonate ramp deepening to the east. Our data robustly indicated that the western rocky shorelines of the Araripe Basin during the Aptian were populated by microbial mats and stromatolites in a condition analogous to the modern world-famous Hamelin Pool, Shark Bay, Western Australia. Finally, the development of stromatolites and echinoid-bearing microbialites at the western margin of the basin may be correlated with the formation of bakevelliid- and cassiopid-rich shell beds in the upper part of the Romualdo Formation at the eastern margin.

Geochemical and micropaleontological evidence of paleoenvironmental change in the Late Cretaceous Western Interior Seaway from the Niobrara to the Pierre Shale Formation

Deposition of the Upper Cretaceous Niobrara Formation and overlying Sharon Springs Member of the Pierre Shale occurred during transgressive phases within the Western Interior Cretaceous (WIC) Seaway, however their respective geochemical and biological differences are significant. The Niobrara Formation is an alternating calcareous chalk/marl system, biologically defined by fecal pellets and robust foraminifers including Heterohelix globulosa, Globigerinelloides ultramicrus, Hedbergella, Giimbelina and two Archaeoglobigerina species. However, the Sharon Springs is primarily argillaceous in composition with lesser amounts of biogenic calcite and silica. Larger foraminifers within the Sharon Springs are primarily arenaceous whereas foraminifers of the same species found in the Niobrara are dwarfed in the Sharon Springs. Dwarfism of foraminifers species and the presence of dispersed diatoms, which flourish in cold waters, indicate a shift in paleocurrents. Deposition of the Niobrara occurred during times when warm Gulfian currents allowed for carbonate production through the Early Campanian. Paleogeographic maps of the Middle Campanian indicate a southerly restriction of the WIC during the time of the southerly Claggett Transgression. The influx of cold, southerly waters resulted in an environment conducive to diatoms and environmentally stressful to foraminifers. Lithological and geochemical evidence from the Sharon Springs indicates that the cold-water Claggett Transgression resulted in a stratified water column in the middle of the WIC as well as increased organic matter production. Petrographic analysis of preserved organic matter show clumped floccules or flattened amalgamations of floccules in laminated facies. While dispersed amorphous organic matter is also present, flocculation had a major influence on organic matter preservation. Two possible depositional mechanisms, each dependent on water density, probably produced these water conditions. If density contrasts existed between the cold water influx and the warm waters present during the deposition of the Niobrara, the cold waters would have progressed along the bottom of the basin leading to upwelling along the basin margin and basin stratification. If no significant density contrast existed, caballing would occur at the mixing front, leading to downwelling, rapid transport of organic matter to the sea floor, better organic matter preservation and basin stratification.

Detailed facies analysis of the Upper Cretaceous Tununk Shale Member, Henry Mountains Region, Utah: Implications for mudstone depositional models in epicontinental seas

Lower-Middle Turonian strata of the Tununk Shale Member of the greater Mancos Shale were deposited along the western margin of the Cretaceous Western Interior Seaway during the Greenhorn second-order sea level cycle. In order to examine depositional controls on facies development in this mudstone-rich succession, this study delineates temporal and spatial relationships in a process-sedimentologic-based approach. The 3-dimensional expression of mudstone facies associations and their stratal architecture is assessed through a fully integrative physical and biologic characterization as exposed in outcrops in south-central Utah. Sedimentologic characteristics from the millimeter- to kilometer-scale are documented in order to fully address the complex nature of sediment transport mechanisms observed in this shelf muddy environment.The resulting facies model developed from this characterization consists of a stack of four lithofacies packages including: 1) carbonate-bearing, silty and sandy mudstone (CSSM), 2) silt-bearing, calcareous mudstone (SCM), 3) carbonate-bearing, silty mudstone to muddy siltstone (CMS), and 4) non-calcareous, silty and sandy mudstone (SSM). Spatial and temporal variations in lithofacies type and sedimentary facies characteristics indicate that the depositional environments of the Tununk Shale shifted in response to the 2nd-order Greenhorn transgressive-regressive sea-level cycle. During this eustatic event, the Tununk shows a characteristic vertical shift from distal middle shelf to outer shelf (CSSM to SCM facies), then from outer shelf to inner shelf environment (SCM to CMS, and to SSM facies). Shifting depositional environments, as well as changes in dominant paleocurrent direction throughout this succession, indicate multiple source areas and transport mechanisms (i.e. longshore currents, offshore-directed underflows, storm reworking). This study provides a rare documentation of the Greenhorn cycle as exposed across the entire shelf setting. High-resolution mapping of genetically-related packages facilitate the development of process-based depositional models that can be utilized for lateral correlations into the equivalent foredeep strata of the Cretaceous Interior.

Sequence stratigraphy of the lower Pierre Shale of the southern Powder River Basin: A ramp margin sequence that terminates Niobrara Formation carbonate deposition

The Upper Cretaceous section of the southern Powder River Basin of the Rocky Mountain region includes the Niobrara Formation, which is one of the most significant source rocks of the Western Interior Cretaceous Seaway, and it is overlain by the lower Pierre Shale interval. The lower Pierre Shale is divided into eight members, which are the progradational highstand deposits of the Gammon Ferruginous Member; lowstand prograding wedge deposits of the Shannon Sandstone, the Unnamed Member, and the Sussex Sandstone; transgressional Ardmore Pedro Bentonite Beds and Sharon Springs members; and highstand deposits of the Mitten Black Shale and Red Bird Silty members. The Shannon and Sussex Sandstone members are known targets for oil production. Based on integrated research of 1490 well logs with two cores, this study demonstrates that the lower Pierre Shale interval is a Type 1 Ramp Margin Sequence. Although previous studies primarily focused on individual sandstone members or parts of the lower Pierre Shale section in relatively limited areas, this study provides an in-depth sequence stratigraphic analysis of the lower Pierre Shale interval. A depositional model was created, which demonstrates that the Shannon Sandstone, Unnamed, and Sussex Sandstone Members were deposited as an encased lowstand prograding wedge between the progradational Gammon Ferruginous Member and the transgressional Ardmore Pedro Bentonite Beds and Sharon Springs members hundreds of miles basinward from the stratigraphically equal Mesaverde lowstand sandstones of the Bighorn Basin. The shift of facies across long distances is explained with a forced regression that has taken place during the deposition of the uppermost Gammon Ferruginous Member and the following deposition of the Shannon and Sussex sandstone members. Rapid eastward progradation of the clastic sediments of the lower Pierre Shale and the following rapid forced regression effectively terminate the deposition of the Niobrara Formation carbonates in the Western Interior Seaway.

The Coffee Sands: A Cretaceous Beach Deposit in the Mississippi Embayment of Western Tennessee

The Coffee Sands represent a beach deposited associated with a vast network of rivers draining the highlands of the Pascola Arch to the west along the margin of the Mississippi Embayment of the Cretaceous Interior Seaway in western Tennessee. The Coffee Sands occur only as ephemeral, discontinuous deposits capping ridges within the Western Highland Rim in western Tennessee and Kentucky. This paper describes an outcrop of the Coffee Sands along U.S. Hwy 140 just east of Buchanan, TN.

Detrital zircon geochronology from the Bighorn Basin, Wyoming, USA: Implications for tectonostratigraphic evolution and paleogeography

Tectonostratigraphic assemblages record phases of basin history during which the fundamental controls of tectonic setting, subsidence style, and basin geometry are relatively similar. Because these fundamental controls, in combination with climate and eustasy, influence paleogeography and sediment-dispersal patterns, they should also yield similar patterns, or facies, of detrital zircon age spectra. Such age-distribution patterns should be documented on the craton in order to make meaningful comparisons to sedimentary rocks from suspect terranes along continental margins. The Rocky Mountains of western North America provide excellent outcrops of sedimentary rocks that record >500 m.y. of tectonostratigraphic evolution. One such Phanerozoic section is exposed along the margins of the Bighorn Basin in northwest Wyoming, from which we report over 4000 U/Th/Pb detrital zircon ages from 48 samples that span a stratigraphic interval from the Middle Cambrian Flathead Sandstone through the Eocene Willwood Formation. These data provide one of the most complete records of detrital zircon age patterns from this part of cratonic North America. The stratigraphic record of the Bighorn Basin is subdivided into four tectonostratigraphic assemblages (TSA1–TSA4). These assemblages record an initial passive margin, followed by a transition to a convergent margin, followed by a marine-dominated retroarc foreland basin, followed by a retroarc foreland segmented by local basement uplifts. This tectonostratigraphic architecture is expressed as four, first-order patterns within the detrital zircon age distributions. TSA1 represents a Paleozoic–Triassic proximal continental margin assemblage dominated by Proterozoic zircons with abundant grains in the 1600–1950 Ma range, a Grenville population at ca. 1100 Ma, and a Phanerozoic population at ca. 420 Ma. TSA2 is a transitional assemblage associated with the Jurassic–Early Cretaceous organization of a west-facing convergent margin and Cordilleran orogen. The TSA2 detrital zircon age distribution is characterized by the appearance of Mesozoic grains, age peaks at ca. 420 and 600 Ma, and a dominant population of Grenville (1.0–1.1 Ga) grains with a suite of Proterozoic grains diminishing in abundance as age increases to 1.9 Ga. TSA3 sedimentary rocks were deposited in the Cretaceous Interior Seaway in a retroarc foreland basin and are dominated by zircons for which ages are close to the depositional age of the strata, reflecting input from the active Idaho Batholith and Sierran segments of the Cordilleran magmatic arc. The older zircon fractions from TSA3 sedimentary rocks are characterized by a dominant detrital zircon age peak at 1.7–1.8 Ga, which probably reflects reworking of Belt Supergroup metasedimentary rocks from the northwest into the Cretaceous foreland, based on regional paleogeographic patterns. TSA4 reflects the phase of basin fill associated with Paleogene structural segmentation of the retroarc foreland during the Laramide orogeny. Detrital zircon age spectra from this assemblage record erosion and redeposition of all previous sedimentary rocks from surrounding basement uplifts. Patterns of detrital zircon ages reflect fundamental changes in paleogeography and sediment dispersal at the 10–100 m.y. time scale and are clearly related to major tectonic events or phases. Detrital zircon ages also provide evidence for linkages between convergent margin processes such as arc magmatism and sedimentation in the retroarc foreland. During these times of strong arc-retroarc linkage, detrital zircon geochronology provides a potentially useful tool for high-resolution chronostratigraphy.

Sequence Stratigraphy of the Upper Cretaceous Sego Sandstone Member Reveals Spatio-Temporal Changes In Depositional Processes, Northwest Colorado, U.S.A

Abstract The Upper Cretaceous Sego Sandstone Member of the Mesaverde Group has been extensively studied in the Book Cliffs area of Utah and Colorado, and has been the focus of stratigraphic reconstruction aimed at developing an understanding of the evolution of the Western Cretaceous Interior Seaway. The Sego Sandstone Member was deposited in a marginal marine, tide-influenced environment of the Cretaceous Seaway. This study documents the sequence stratigraphy of the Sego Sandstone Member in northwestern Colorado, just north of Rangely, and compares and contrasts it with equivalent strata in the Book Cliffs area in Utah. The Sego Sandstone Member in the study area contains three sequences characterized by progradational and aggradational stacking patterns. The stratigraphically lowest sequence consists of a prograding, tide-influenced delta overlain by marine mudstones, which represents a retrogradation and flooding surface. The second sequence is composed of multiple parasequences and consists of an incised valley filled with stacked tidal bars which then pass into a largely aggradational stacking pattern, composed of barrier-island deposits with back-barrier, flood-tidal-delta deposits, and wave-dominated-shoreface deposits. The third sequence is a broad, tide-dominated distributary-mouth system with a sharp, incisional basal contact. The three sequence boundaries documented in northwestern Colorado are consistent with the three main sequence boundaries identified in the Book Cliffs. However, whereas barrier islands and flood-tidal deltas are characteristic of the Sego Sandstone Member in northwestern Colorado, similar deposits are not as prevalent in the Book Cliffs of Utah, suggesting different depositional processes and paleogeography. Tidal and fluvio-deltaic processes are the dominant controls on deposition of the Sego Sandstone Member north of Rangely, Colorado. The transition from a tide-dominated fluvio-deltaic system to a mixed wave–tide-influenced coastline indicates a fundamental change in processes and depositional environment in the upper part of Sequence 2. Such change from a prograding fluvio-deltaic system to a more passive tide-modified coastline is not observed in the Book Cliffs, and may be the result either of large scale transgression or of relocation of the river system through large-scale avulsion, which is not observed in the Book Cliffs. Our study shows significant stratigraphic variability between rocks exposed in the Book Cliffs versus time-equivalent rocks exposed in northwestern Colorado in the Upper Cretaceous, which has implications for the regional basin architecture and stratigraphic correlations.

Practical problems in the application of the sequence stratigraphic method and key surfaces: integrating observations from ancient fluvial–deltaic wedges with Quaternary and modelling studies

AbstractSequence stratigraphy solved the basic problem that genetically related, but different, lithofacies were routinely assigned to different lithostratigraphic units defined by arbitrary vertical and horizontal cutoffs. Stratigraphically important stratal relationships, and especially stratal terminations (for example, lapout) can be observed in seismic data and can be documented in continuous outcrops, such as in the deserts of the Western Interior of North America and in closely spaced well‐log data sets. In sparse data sets, interpolation of lapout patterns introduces uncertainty in correlation and designation of sequences and systems tracts and their associated surfaces. Diachroneity of fluvial terrace deposits is documented using detailed facies architectural studies, and calls into question the assumed chronostratigraphic significance of many so‐called sequence boundaries identified in the rocks of the Cretaceous Interior Seaway of North America. Sequence boundaries, formed during prolonged sea‐level falls, may be less chronostratigraphically significant than transgressive surfaces formed during rapid rises. As a consequence, flooding surfaces are theoretically more significant, and also have greater utility as allostratigraphic boundaries, although they may also exhibit diachroneity. Tectonic unconformities produce differential lithospheric deformation, which results in angular unconformities, although the angle of discordance may be very small. In the Cretaceous Interior Seaway of North America, such unconformities may be expressed by marine erosion in distal basin settings. Regional isochronous bentonites provide useful marker beds that clearly illustrate angular discordance. In the fluvial realm, such tectonic discontinuities may be indicated by changes in palaeocurrent orientations, as well as by provenance changes. Although sequence stratigraphy provides a powerful methodology and theoretical framework for correlating and understanding the evolution of stratigraphic successions in the context of changing accommodation, allostratigraphy remains the only accepted scheme for the formal naming of stratigraphic units based on bounding discontinuities. Whatever type of sequence stratigraphy or allostratigraphy is preferred, in all cases it is key to recognize that sequence stratigraphy, at its heart, is the re‐ordering, correlation, and sometimes renaming of stratigraphic units on the basis of bounding discontinuities and their correlative surfaces, as opposed to the arbitrary lithofacies‐oriented approach using broad facies zones (for example, ‘shazam lines’) or arbitrary cutoffs, such as is used in traditional lithostratigraphy.

Ocean overturning since the Late Cretaceous: Inferences from a new benthic foraminiferal isotope compilation

Benthic foraminiferal oxygen isotopic (δ18O) and carbon isotopic (δ13C) trends, constructed from compilations of data series from multiple ocean sites, provide one of the primary means of reconstructing changes in the ocean interior. These records are also widely used as a general climate indicator for comparison with local and more specific marine and terrestrial climate proxy records. We present new benthic foraminiferal δ18O and δ13C compilations for individual ocean basins that provide a robust estimate of benthic foraminiferal stable isotopic variations to ∼80 Ma and tentatively to ∼110 Ma. First‐order variations in interbasinal isotopic gradients delineate transitions from interior ocean heterogeneity during the Late Cretaceous (>∼65 Ma) to early Paleogene (35–65 Ma) homogeneity and a return to heterogeneity in the late Paleogene–early Neogene (35–0 Ma). We propose that these transitions reflect alterations in a first‐order characteristic of ocean circulation: the ability of winds to make water in the deep ocean circulate. We document the initiation of large interbasinal δ18O gradients in the early Oligocene and link the variations in interbasinal δ18O gradients from the middle Eocene to Oligocene with the increasing influence of wind‐driven mixing due to the gradual tectonic opening of Southern Ocean passages and initiation and strengthening of the Antarctic Circumpolar Current. The role of wind‐driven upwelling, possibly associated with a Tethyan Circumequatorial Current, in controlling Late Cretaceous interior ocean heterogeneity should be the subject of further research.

Soot and palynologic analysis of Manson impact-related strata (Upper Cretaceous) of Iowa and South Dakota, USA

The Campanian Manson impact structure of Iowa represents the best-preserved, large-diameter complex crater within the continental U.S. To assess the timing and potential mode of crater infilling and the possible presence of an impact event horizon, we analyzed samples from both within and distal to the impact structure for their elemental carbon, soot and palynomorphs. Within the impact structure, identifiable soot occurred in fragmented impact breccia and suevite but not in lower impact-melt breccia. Although most of this soot likely represents reworking of material from older Cretaceous marine shales, one high soot concentration occurs with melt material in a Keweenawan Shale–Phanerozoic clast breccia mix. This represents the first association of soot and impact-generated materials within an impact structure and the best sample candidate for Manson impact-generated soot. No palynomorphs occurred in the impact melt breccia. Overlying suevite (Keweenawan Shale clast breccia) of the central peak yielded sparse and thermally altered palynomorphs, indicating deposition prior to full cooling of the crater debris. Presence of easily degraded soot also argues for rapid backfilling of the crater. Distal samples from South Dakota represent the Sharon Springs and Crow Creek members of the Pierre Shale 230 km northwest of the Manson impact structure. Although containing shocked grains, the Crow Creek preserves no soot. In contrast, the Sharon Springs, generally considered as predating the Manson impact, has significant soot quantities. Palynomorphs differ markedly across the unconformity separating the two members with the Crow Creek containing more terrestrial forms, normapolles, and older reworked palynomorphs, consistent with a terrestrial impact to the east. Origin of the Sharon Springs soot remains unclear. Given soot occurrence within four of the five Cretaceous marine units sampled, the relatively shallow, anoxic bottom conditions of the Western Interior Cretaceous Seaway may have simply favored soot preservation. Until a better understanding of the broader occurrence and preservation of soot is achieved, some soot-impact associations will remain ambiguous.

Estuarine-type foraminifera from Australia's Permian to Cretaceous interior seas

Abstract

MAPA LITOESTRATIGRÁFICO DA PARTE ORIENTAL DA BACIA BAURU (PR, SP, MG), ESCALA 1:1.000.000

A Bacia Bauru é uma bacia continental interior do Neocretáceo (Coniaciano-Maastrichtiano), desenvolvida no centro-sul da Plataforma Sul-Americana. Acumulou uma seqüência sedimentar essencialmente arenosa, que ocorre em área de cerca de 370.000 km2. Tem por substrato rochas vulcânicas, sobretudo basaltos da Formação Serra Geral (Cretáceo Inferior). A área do mapa corresponde à parte oriental da bacia, com cerca de 180.000 km2. A seqüência neocretácea é composta por dois grupos cronocorrelatos: Caiuá (formações Rio Paraná, Goio Erê e Santo Anastácio) e Bauru (formações Uberaba, Vale do Rio do Peixe, Araçatuba, São José do Rio Preto, Presidente Prudente e Marília, incluindo os Analcimitos Taiúva). LITHOSTRATIGRAPHIC MAP OF THE BAURU BASIN EASTERN PART (PR, SP, MG), SCALE 1:1.000.000 Extended Abstract The Bauru Basin is a Late Cretaceous (Coniacian-Maastrichtian) interior continental basin developed as a result of subsidence of the central-southern part of the South-American Platform. It spreads over an area of approximately 370,000 km2 and is filled by a sandy sequence with a preserved maximum thickness of 300 meters. The substratum is made up of volcanic rocks (mainly basalts) of the Serra Geral Formation (Early Cretaceous). The map area corresponds to the eastern part of the basin, with about 180,000 km2 (west of São Paulo, southwest of Minas Gerais, northwest of Paraná), located between the coordinates 18°S to 25° S and 47° W to 54° W (Figure 1). The Upper Cretaceous sequence is composed by two partially contemporaneous groups (Figure 2): Caiuá (Rio Paraná, Goio Erê and Santo Anastácio formations) and Bauru (Uberaba, Vale do Rio do Peixe, Araçatuba, São José do Rio Preto, Presidente Prudente, and Marília formations, including the Taiúva analcimites). The intensification of the tectonic activities at the north-northeastern and eastern margins, and probably also at the northwestern margin, caused significant changes in the paleogeographic scenario, promoting progradation of alluvial fans. At the same time, climatic changes increased the humidity in the marginal zones. These changes mark the beginning of the sedimentary phase during which important fluvial systems from northeast to southwest were established between the marginal zone and the inland Caiuá Desert. During the uplift of the margins, the intrusion of several alkaline magmatic bodies cause extrusive magmatism near the eastern border (Taiúva analcimites), as well as localized silicification in the southern border. All known fossil occurrences are located in the eastern part of the basin and were paleogeographically controlled. They comprise bone remains, mainly from reptiles -crocodiles, turtles, dinossaurs -, as well as amphibians, fishes, mollusks, crustaceans, and oogones of charophytic algae. The main occurrences are from fluvial flood plain deposits and ephemeral lagoons in the interior of the basin, as well as from those of braided water flows and lagoons located in zones of marginal alluvial fans. In the interior of the basin the hot desertic condition prevailed during the two phases of sedimentation.

TERLINGUACHELYS FISCHBECKI, A NEW GENUS AND SPECIES OF SEA TURTLE (CHELONIOIDEA: PROTOSTEGIDAE) FROM THE UPPER CRETACEOUS OF TEXAS

EA TURTLES (superfamily Chelonioidea) were diverse and abundant during Late Cretaceous time, and likely reached their peak diversity during the Campanian stage (Weems, 1988; Hirayama, 1997). They were the only major group of marine reptiles to survive the terminal Cretaceous mass extinction relatively unscathed. Remains of sea turtles are commonly preserved in the pelagic marine shale and chalk deposits that accumulated in the Late Cretaceous interior epeiric sea of North America. Most of these remains pertain to the extinct family Protostegidae. It is therefore of interest to report an unusual and previously unknown sea turtle from paralic Upper Cretaceous (Campanian) strata that accumulated along the western shore of the epeiric sea in Texas. The single and as yet only known specimen of this turtle is here designated the type of Terlinguachelys fischbeckin. gen. and sp. Recognition of this new species further increases the known diversity of Cretaceous sea turtles, and is of interest in examining the phylogeny of sea turtles. T. fischbeckiis likely a primitive protostegid but has several features generally thought to be synapomorphic for more derived lineages, making its systematic placement uncertain. A brief description of this specimen was given previously by Tomlinson (1997). Together, most marine turtles comprise a natural group (superfamily Chelonioidea). Of the six extant genera of sea turtles, five (Caretta, Chelonia, Eretmochelys, Lepidochelys, Natator) are allied together in family Cheloniidae, and one (Dermochelys )i s the sole living representative of family Dermochelyidae. The extinct family Protostegidae is known only from Cretaceous strata. These three families of sea turtles are recognized by most authors and each comprises a relatively distinct and well-defined clade. However, some extinct taxa are hypothesized to represent near relatives or sister groups of one or several of these three families, and authors have interpreted such taxa alternatively as either constituting separate families (e.g., Toxochelyidae, Osteopygidae, Desmatochelyidae) or have included some or all within three more expansively defined conventional families. Retention of plesiomorphic features among their early representatives and parallel evolution in the separate lineages presents a problem of recognizing ‘‘grades’’ rather than clades, particularly among the least derived sea turtles. Nevertheless, in the following description and discussion, comparisons are made with other sea turtles, using the terms cheloniid, dermochelyid, and protostegid generally in a manner consistent with the phylogenetic hypothesis of Hirayama (1998), recognizing, however, that taxa included in each family vary among authors (see discussion of sea turtle relationships below). GEOLOGIC SETTING

Microscale δ18O and δ13C isotopic analysis of an ontogenetic series of the hadrosaurid dinosaur Edmontosaurus: implications for physiology and ecology

Stable isotope analysis of vertebrate biominerals, primarily in mammals, has been used to address questions of paleodiet, paleoclimate, trophic level, migration, foraging zone, and thermophysiology with varying degrees of success. Isotopes have been used less commonly to study physiology and ecology through ontogeny in dinosaurs, generally due to (1) the lack of modern analogs as a basis for comparison of observed fossil isotope values, and (2) difficulty in sampling very thin tooth enamel (a non-remodeled biomineral). By utilizing a relatively new technique in microsampling, this study addresses the following questions: Do microscale analyses of oxygen and carbon isotopes from mineralized tissues of hadrosaurid dinosaurs record temporal variation? If so, is the cause of the variation physiological or ecological? Isotope values from the carbonate component of enamel ( δ 18O ec and δ 13C e) were obtained by microsampling multiple teeth in a temporal series from the dental batteries of a juvenile, sub-adult, and adult Edmontosaurus from the Late Cretaceous Maastrichtian Hell Creek Formation of South Dakota. To establish isotope variability in an extant archosaur, consecutive teeth in a temporal series from an extant Alligator mississippiensis specimen were microsampled for isotopic analysis and compared to those of Edmontosaurus. To test for diagenesis, bulk samples from the phosphate component ( δ 18O p) of modern and fossil tooth enamel, bone, and dentine from Edmontosaurus, A. mississippiensis, and extant ratites were analyzed and compared. Edmontosaurus bone and dentine indicate a greater degree of alteration than does enamel, and while absolute δ 18O ec values may be altered, the pattern of seasonal cycles appears to be preserved and can provide detailed information on hadrosaur physiology (tooth mineralization times, rates, and seasons) and ecology (dietary information). δ 18O ec seasonal patterns are preserved in Edmontosaurus specimens, and are interpreted to correlate with annual δ 18O variation of local meteoric waters rather than thermophysiology, changes in drinking water sources, or migration. All teeth were mineralized in <0.65 year with no consistent season of mineralization. Mean tooth mineralization times are shorter in the juvenile and sub-adult than the adult. Enamel mineralization rates are estimated to be ∼38 mm/year in Edmontosaurus and ∼36 mm/year in Alligator (consistent with mineralization rates for modern ungulates), although the length of time for tooth formation is shorter in the archosaurs compared with mammals. Heavier than predicted δ 13C e values are hypothesized to result from (1) enrichment of δ 13C in ingested plant material due to higher atmospheric δ 13C ( δ 13C atm) in the Late Cretaceous; (2) taxon-specific δ 13C effects of ingested plants (primarily gymnosperms); (3) isotopic enrichment of ingested plant material ( δ 13C p) due to osmotic stress from proximity to the Western Cretaceous Interior Seaway, (4) taxon-specific δ 13C diet– δ 13C e fractionation factors for Edmontosaurus that vary from those observed in modern mammals, and/or (5) diagenesis. Microsampling provides a detailed perspective on the physiological (tooth mineralization times, rates, and seasons) and ecological (dietary) mechanisms of oxygen and carbon isotope incorporation in dinosaur biominerals that is not obtainable through bulk sampling alone.