Western Interior United States Research Articles

Reconstructing paleosinuosity and sedimentary mass balance in the Upper Triassic Shinarump paleoriver in Utah and Arizona, U.S.A.

ABSTRACT The Upper Triassic Shinarump Member forms the basal part of the Chinle Formation in the western interior United States and was deposited by a continental-scale fluvial system which ran approximately 2,500 km from the Ouachita Orogen in the east into the Auld Lang Syne marine basin in the west. Previous studies of the Shinarump Member have concluded that the deposits represent a braided-river system but have not produced estimates for paleo-sinuosity and paleo-discharge. Recent advances in sedimentology allow detailed morphometric assessment of the nature of the river system that deposited the Shinarump Member enabling us to produce quantitative estimates for these parameters. We therefore present architectural data from the Shinarump Member in northern Arizona and Utah, supported by lithofacies data and 39 sandstone petrographic analyses, and a dataset of 4,298 paleocurrent measurements from trough cross-strata. Lithofacies and architectural analysis supports previous interpretations of the Shinarump and equivalent strata as braided-river deposits. Petrographic analysis shows that the Shinarump is dominated by monocrystalline quartz and exhibits low spatial variation in composition, ranging from 85.4% to 99.8% total quartz. Paleocurrent measurements are used to calculate the channel sinuosity of the fluvial system as varying between 1.02 and 1.77, with a median value of 1.33 (compared to the Yangtze River, ranging from 1.05 to 1.50 and the Ganges–Brahmaputra, ranging from 1.05 to 1.13 in their lower 250 km). Paleohydrological estimates using data from the architectural surveys produce slope estimates from 2.01 × 10–4 to 6.51 × 10–4 and bankfull discharge estimates from 4.36 × 103 m3 s–1 to 2.38 × 104 m3 s–1 for individual channels, comparable to extant continental-scale fluvial systems. Estimates of lifetime sediment transport volume range from 7.75 × 104 km3 to 6.09 × 105 km3, which are in order-of-magnitude agreement with estimates for the volume of the depositional sink (1.35 × 105 km3 to 4.17 × 105 km3). These results demonstrate the potential for paleohydrologic estimates to provide new avenues for analysis of heavily studied units.

Habitat Climate Change Vulnerability Index Applied to Major Vegetation Types of the Western Interior United States

We applied a framework to assess climate change vulnerability of 52 major vegetation types in the Western United States to provide a spatially explicit input to adaptive management decisions. The framework addressed climate exposure and ecosystem resilience; the latter derived from analyses of ecosystem sensitivity and adaptive capacity. Measures of climate change exposure used observed climate change (1981–2014) and then climate projections for the mid-21st century (2040–2069 RCP 4.5). Measures of resilience included (under ecosystem sensitivity) landscape intactness, invasive species, fire regime alteration, and forest insect and disease risk, and (under adaptive capacity), measures for topo-climate variability, diversity within functional species groups, and vulnerability of any keystone species. Outputs are generated per 100 km2 hexagonal area for each type. As of 2014, moderate climate change vulnerability was indicated for >50% of the area of 50 of 52 types. By the mid-21st century, all but 19 types face high or very high vulnerability with >50% of the area scoring in these categories. Measures for resilience explain most components of vulnerability as of 2014, with most targeted vegetation scoring low in adaptive capacity measures and variably for specific sensitivity measures. Elevated climate exposure explains increases in vulnerability between the current and mid-century time periods.

Coilopoceras inflatum Cobban and Hook, 1980, a United States Western Interior ammonite from the Upper Turonian of the southern Corbières, Aude, France

Abstract A newly discovered ammonite faunule from the Padern region of the southern Corbières in southern France includes representatives of typical northwest European Upper Turonian species Subprionocyclus cf. neptuni (Geinitz, 1850) and Lewesiceras cf. woodi Wright 1979, tethyan/ northwestern Pacific species Phyllopachyceras cf. ezoense (Yokoyama, 1890), Anagaudryceras involvulum (Stoliczka, 1865) and, Desmoceras (Pseudouhligella) sp., together with Coilopoceras inflatum Cobban and Hook, 1980, a species previously known only from New Mexico in the United States, where it is regarded as Middle Turonian. The faunule occurs above one with Romaniceras (R.) mexicanum Jones, 1938 and Coilopoceras springeri Hyatt, 1903, also originally described from New Mexico and northern Mexico, and recently described from the Uchaux massif in Vaucluse in southern France. The records suggest that the base of the Upper Turonian may be drawn at different, higher level in the United States Western Interior than in Europe. The coming together of these mixed faunal elements may be a result of high sea levels, and changing oceanic circulation patterns.

Early Cenozoic drainage reorganization of the United States Western Interior–Gulf of Mexico sediment routing system

Abstract Continental-scale drainages host the world’s largest rivers and offshore sediment accumulations, many of which contain significant petroleum reserves. Rate of sediment supply in these settings may be a signal of external controls (e.g., tectonics) on landscape evolution, yet deciphering these controls remains a major challenge in interpreting the ancient stratigraphic record. Integration of new and published detrital zircon U-Pb ages from the United States Rocky Mountain region and Gulf of Mexico (GOM) sedimentary basin demonstrates profound changes in the U.S. continental drainage divide that controlled the rate of sediment delivery to the northern GOM during Paleocene–Eocene time. Sedimentation rate increased dramatically during deposition of the lower Wilcox Group, reaching approximately three times the Cenozoic average, accompanied by pronounced shoreline regression and delivery of a large volume of sand to the basin floor. We hypothesize that this increase in sediment delivery to the GOM resulted from drainage capture of a significant portion of the Sevier-Laramide structural province (∼900,000 km2) that included the headwaters of the California and Idaho Rivers. Capture of the California River drainage may have occurred in the vicinity of the Hanna Basin of eastern Wyoming that previously emptied northward into a shallow seaway, but was subsequently diverted southward to the Rockdale delta, which accumulated within the Houston embayment during the time of deposition of the lower Wilcox Group. Detrital zircon U-Pb ages from Wilcox samples within the Rockdale delta show a remarkable similarity with contemporaneous Laramide synorogenic units, including enrichment in detritus derived from the Cordilleran arc and basement terranes of western North America relative to older and younger units in the Houston embayment. A subsequent order of magnitude decline in sedimentation rate to the GOM can be partly attributed to well-documented drainage closure (∼800,000 km2) that accompanied lake formation in interior Laramide basins (ca. 53–51.8 Ma). Our results demonstrate that tectonically induced drainage migration in the high-relief segments of continental-scale drainages can have a pronounced effect on the rate of sediment transfer to continental margins.

A survey of the Cretaceous ammonite Placenticeras Meek, 1876, in the United States Western Interior, with notes on the earliest species from Texas

Abstract This contribution documents the record of the late Cretaceous ammonite Placenticeras Meek, 1876, from the late Cenomanian of Texas and the southern part of the U. S. Western Interior up to the late Middle Campanian zone of Baculites scotti, reconstructed and updated from an incomplete manuscript by the late W. A. Cobban based on the collections of the U. S. Geological Survey. The original manuscript dates from the late 1980’s, and there is now additional information on the occurrence of the genus that is incorporate here; much of this comes from Neal Larson of Hill City, South Dakota, to whom I am indebted for his help in preparing Bill’s manuscript for publication. It now provides an objective documentation of the distribution of Placenticeras in space and time on which any subsequent analysis of the evolution of the genus will depend.

Inoceramid bivalves and biostratigraphy of the upper Albian and lower Cenomanian of the United States Western Interior Basin

Inoceramid bivalves of the upper Albian and lower Cenomanian of the United States Western Interior are revised, Eleven species-level taxa and three genera are described. Two new species, Gnesioceramus mowriensis, characterizing the Mowry Shale of the early, but not the earliest, Cenomanian, and Posidonioceramus merewetheri, of the lower Cenomanian, and on new genus, Posidonioceramus, are recognised. The Western Interior inoceramid species from this interval are strongly endemic and are not good tools for long-distance correlations, although they are very effective in regional dating.In terms of the inoceramid biostratigraphy, middle and upper parts of the upper Albian can be referred to the Gnesioceramus Biozone, represented by G. comancheanus (Cragin) and G. bellvuensis (Reeside). These taxa are endemic to the Western Interior and some adjacent areas (Gulf Coast; Greenland?), but are closely allied to the cosmopolitan species, Gnesioceramus anglicus (Woods). At approximately the Albian-Cenomanian boundary, the endemic clade of ‘Inoceramus’ nahwisi appears, now referred to the newly erected Posidonioceramus, resulting in a distinct P. nahwisi biozone. This zone corresponds to the lower part of the ammonite Neogastroplites’ stratigraphic range. Gnesioceramids re-appear in the early Cenomanian. Close to base of the Cenomanian, for the first in the Western Interior, the genus Inoceramus, represented by Inoceramus irenensis Warren and Stelck, 1958, apparently immigrated into the Western Interior Basin.The Western Interior inoceramids do not allow for direct correlation to chronostratigraphic standard subdivision. The Albian-Cenomanian boundary, as earlier recognized on geochronologic correlations and confirmed, to some extent, based on ammonites, may approximately be located close to the appearance level of the genus Posidonioceramus.

Migration of dynamic subsidence across the Late Cretaceous United States Western Interior Basin in response to Farallon plate subduction

Backstripped cross sections of the Late Cretaceous succession across central Utah, Colorado, and southern Wyoming in the Western Interior Basin, United States, reveal a component of continuously evolving long-wavelength residual subsidence, in addition to subsidence driven by the Sevier thrust belt and associated sediment loads. The loci of maximum rates of this residual subsidence moved eastward from ca. 98 to 74 Ma in phase with the west to east passage of the Farallon slab, as reconstructed from tomography based on quantitative inverse models. These new subsidence data allow testing of existing subduction models and confi rm the dynamic subsidence origin of the Western Interior Basin. Furthermore, regional variations in subsidence rates suggest a possible defi cit of negative buoyancy (mantle loading) inside the slab beneath Colorado, supporting the hypothesis that the thickened slab represents a subducted oceanic plateau. This paper documents how the Cretaceous stratigraphy records the timing, patterns, and position of underlying mantle processes during Farallon slab subduction. The new data also reveal patterns indicative of the commencement of the Laramide orogeny in the western United States.

Stratigraphy of the upper Vijlen Member (Gulpen Formation; Maastrichtian) in northeast Belgium, the southeast Netherlands and the Aachen area (Germany), with special reference to belemnitellid cephalopods

AbstractRecent studies have demonstrated the complex depositional history of the Vijlen Member (Gulpen Formation) in the Maastricht-Aachen-Liège area, on which synsedimentary tectonics had a profound impact, in conjunction with regressive and transgressive episodes, the oldest portion of this member being preserved only locally, within channel-like structures. Our reinterpretation of the Vijlen Member, here presented, is at variance with current belemnite stratigraphy. To account for incongruities, reworking of belemnites is here proposed as a typical feature during Vijlen Member deposition. A renewed study of belemnites on the basis of these new data suggests that intervals 4–6 of the Vijlen Member are of late Maastrichtian age. With respect to the mean values of populations ofBelemnitella juniorNowak andBelemnitella IwowensisNaidin, Vijlen Member intervals 4–5 are coeval with theSpyridoceramus tegulatus/Belemnitella juniorZone (sensu germanico), whereas at least the upper portion of interval 6 is correlatable with the basalTenuipteria argentea/Belemnitella juniorZone (sensu germanico). Although reworking of belemnites appears to have been a common feature, this cannot be properly substantiated without additional geochemical analyses of individual guards. For this reason, we refrain from proposing a biostratigraphic zonation of the Vijlen Member based on belemnites. Alternatively, scaphitid ammonites might be used to subdivide this member, because species such asAcanthoscaphites(Acanthoscaphites)tridens(Kner),A.(Euroscaphites)varians blaszkiewicziJagt, Kennedy & Machalski,Hoploscaphites constrictus(J. Sowerby),H. tenuistriatus(Kner) andHoploscaphitessp. have been recorded. The last-named taxon represents an interspecies hybrid between the North AmericanJeletzkytes dorfiLandman & Waage and an unidentified subspecies of the endemic European lineage ofH. constrictus, allowing correlation of the lower portion of Vijlen Member interval 6 with theHoploscaphites birkelundaeZone of the United States Western Interior.

Increasing eolian dust deposition in the western United States linked to human activity

Mineral aerosols from dust are an important influence on climate and on marine and terrestrial biogeochemical cycles. These aerosols are generated from wind erosion of surface soils. The amount of dust emission can therefore be affected by human activities that alter surface sediments. However, changes in regional- and global-scale dust fluxes following the rapid expansion of human populations and settlements over the past two centuries are not well understood. Here we determine the accumulation rates and geochemical properties of alpine lake sediments from the western interior United States for the past 5,000 years. We find that dust load levels increased by 500% above the late Holocene average following the increased western settlement of the United States during the nineteenth century. We suggest that the increased dust deposition is caused by the expansion of livestock grazing in the early twentieth century. The larger dust flux, which persists into the early twenty-first century, results in a more than fivefold increase in inputs of K, Mg, Ca, N and P to the alpine ecosystems, with implications for surface-water alkalinity, aquatic productivity and terrestrial nutrient cycling. Dust input to alpine lakes in the western United States has risen dramatically following westward expansion of human settlements and increased livestock grazing over the past two centuries. The increased dust flux deposits additional nutrients and minerals to the lakes, with important implications for water chemistry, productivity and nutrient cycling.

Decoupled Plant and Insect Diversity After the End-Cretaceous Extinction

Food web recovery from mass extinction is poorly understood. We analyzed insect-feeding damage on 14,999 angiosperm leaves from 14 latest Cretaceous, Paleocene, and early Eocene sites in the western interior United States. Most Paleocene floras have low richness of plants and of insect damage. However, a low-diversity 64.4-million-year-old flora from southeastern Montana shows extremely high insect damage richness, especially of leaf mining, whereas an anomalously diverse 63.8-million-year-old flora from the Denver Basin shows little damage and virtually no specialized feeding. These findings reveal severely unbalanced food webs 1 to 2 million years after the end-Cretaceous extinction 65.5 million years ago.

Stratigraphy of the Upper Cenomanian–Lower Turonian Chalk succession at Eastbourne, Sussex, UK: ammonites, inoceramid bivalves and stable carbon isotopes

Thirty-one ammonite, and eight inoceramid bivalve taxa are recorded from the Cenomanian-Turonian boundary interval at Eastbourne, Sussex, in southern England, and correlated with the δ 13C isotope record in carbonate and organic carbon. New ammonite records from Western Europe include the North American species Cunningtoniceras arizonense Kirkland and Cobban, 1986, Tarrantoceras ( Sumitomoceras) conlini Wright and Kennedy, 1981, Puebloites corrugatus ( Stanton, 1894) and Euomphaloceras costatum Cobban, Hook and Kennedy, 1989. The ammonites allow definition of three Late Cenomanian ( C. guerangeri, M. geslinianum, N. juddii) and three Turonian ( W. devonense, F. catinus, M. nodosoides) zones. The inoceramid study enables recognition of the Mytiloides puebloensis Zone, and allows us to demonstrate the absence of the Mytiloides hattini Zone at Eastbourne. The first occurrence of M. puebloensis Walaszczyk and Cobban, 2000, is coincident with that of the ammonite genus Watinoceras, as in the United States Western Interior, and allows precise placement of the Cenomanian-Turonian boundary in the Anglo-Paris Basin for the first time. We present stable carbon isotope data through the succession, including the first δ 13C org record for the UK Chalk. The positive Cenomanian-Turonian boundary carbon isotope excursion at Eastbourne is reflected in the δ 13C carb record by an overall stepwise increase in values, whereas the δ 13C org record displays distinct minima at times of δ 13C carb plateaux. Each plateau precedes the following δ 13C increase recorded in both the carbonate and the organic carbon curves. The distinct pattern in the organic and carbonate δ 13C records results in three intervals that show marked increases in the isotopic fractionation between organic and inorganic carbon. We use a combination of carbon isotope, ammonite and inoceramid data to develop a detailed correlation with the proposed C–T stratotype at Pueblo, Colorado, which has a resolution close to that of Milankovitch precession cycles.

Perspectives of ammonite paleobiology from shell abnormalities in the genus Baculites

Many Baculites specimens from the Upper Cretaceous of the United States Western Interior show exceptional preservation of the original aragonitic shell and its fine-scale surface ornamentation. Growth lines are ubiquitous, with two orders of these structures represented on some shells, and reflect the incremental addition of new shell at the apertural margin. Growth line interruption in the form of repair of minor shell damage at the aperture, commonplace in contemporary Nautilus, is essentially absent in Baculites, suggesting that its members fed on small prey in the water column. As typical of Mesozoic ammonites in general, and in striking contrast to contemporary Nautilus, no in vivo epizoans have been recognized on specimens of Baculites. It is inferred that the shell of Baculites was covered in periostracum to eliminate epizoic colonization. By analogy with Nautilus, a distinctive micro-ornament oriented at right angles to growth lines and visible on parts of some specimens was probably associated with periostracal attachment. A small proportion of Baculites specimens show abnormalities in shell growth categorized as v-shaped indentations of growth lines, shell grooves, fine-scale folds on the surface of growth lines and feather structures. We view this entire set of structures as due to abnormalities in mantle growth induced at the leading edge, impressed into the periostracum during its fabrication, and then in turn into the shell surface. As many of the Baculites with shell abnormalities are smooth, the proposal by Checa linking homologous structures recognized on other ammonites to the formation of comarginal ribs is rejected. A case of sutural inversion, in which the form of minor divisions of the major saddles and lobes are transposed, is recognized in a specimen of Baculites codyensis Reeside. We consider sutural pattern in ammonites, an expression of septal fluting, as replicating the genetically specified standing form of an elastic adapical visceral mass. The inverted sutural pattern, and by implication the style of septal fluting, was transcribed exactly in the three successive septa preserved on the specimen. The abnormality appears to be a case of homeotic mutation in which the plan for one body region becomes translocated to another. The conservatism of major elements of sutural (=septal) patterns for Mesozoic ammonites in their evolutionary spectrum suggests that a homeobox of conserved DNA sequence, with the transcription factors encoded in homeotic genes, is likely to have been involved.

Ammonoid faunas from the Cardium Formation (Turonian–Coniacian; Upper Cretaceous) and contiguous units, Alberta, Canada: II. Collignoniceratidae and Placenticeratidae

Ammonite species of the genera Prionocyclus, Subprionocyclus, and Collignoniceras described here from the Cardium Formation, in combination with scaphitid ammonoids and inoceramid bivalves, allow recognition of all biozones for the Turonian Stage (Upper Cretaceous) established farther south in the United States Western Interior Basin. Although many of the species used in this zonation were endemic to the Western Interior seaway, more cosmopolitan forms at some levels do allow correlation with western European sequences. The presence of Prionocyclus germari Reuss is firmly established in the lower parts of the Leyland Member, confirming the position of the Turonian-Coniacian boundary and earlier proposed constraints on the duration of the Cardinal–Leyland hiatus.

Ammonoid faunas from the Cardium Formation (Turonian-Coniacian, Upper Cretaceous) and contiguous units, Alberta, Canada: I. Scaphitidae

While the Cardium Formation (Turonian–Coniacian, Upper Cretaceous) is one of the major hydrocarbon sources, and consequently one of the most intensely studied siliciclastic formations in the Western Canada Sedimentary Basin, it remains one of the more controversial units in terms of understanding the depositional environments and processes it represents, and correlations between subsurface and outcrop. Proposed subsurface subdivisions based on sequence stratigraphic concepts, and relation of these allomembers to long-established members of the outcrop belt, have provoked further disagreements. The general lack of biostratigraphic data within the Cardium Formation makes it difficult to test different models and to resolve conflicting proposals. This paper provides stratigraphic and taxonomic information on all known scaphitid ammonoids from the Cardium Formation and correlation of these faunas with the Turonian and Coniacian zonal scheme established in the United States Western Interior; future publications will provide similar treatments of the collignoniceratid ammonites and inoceramid bivalves.

Additions to the Ammonite Fauna of the Upper Cretaceous Navesink Formation of New Jersey

New fossil collections provide additional information about the late Campanian and Maastrichtian ammonites from the Navesink Formation of New Jersey. Late Campanian ammonites include Pseudophyllites indra (Forbes, 1846), Nostoceras (N.) approximans (Conrad, 1855) (of which Nostoceras (N.) stantoni Hyatt, 1894, is a synonym), Nostoceras (N.) hyatti Stephenson, 1941, Nostoceras (N.) pauper (Whitfield, 1892), Didymoceras cf. D. draconis (Stephenson, 1941), Exiteloceras rude n. sp., Hoploscaphites pumilus (Stephenson, 1941), and Jeletzkytes cf. J. nodosus (Owen, 1852). Maastrichtian ammonites from the Navesink Formation include Pachydiscus (P.) neubergicus neubergicus (Hauer, 1858), Kitchinites sp., Nostoceras (N.) alternatum (Tuomey, 1854), Baculites sp., Eubaculites cf. E. labyrinthicus (Morton, 1834), Eubaculites sp.?, Jeletzkytes cf. J. plenus (Meek, 1876), Jeletzkytes criptonodosus Riccardi, 1983, and Discoscaphites gulosus (Morton, 1834).These faunas are correlated with those of Western Europe, the Gulf Coast, and the Western Interior of the United States. The older fauna from the basal phosphatic beds of the Navesink Formation at the classic Atlantic Highlands locality is referred to the Nostoceras (N.) hyatti zone. It is late Campanian in age and equivalent to the Nostoceras (N.) pozaryskii/Belemnella langei zone in Europe and the Baculites jenseni zone in the United States Western Interior. In addition, these beds contain ammonites that range into the early Maastrichtian, as well as Pachydiscus (P.) neubergicus, whose appearance marks the base of the Maastrichtian. Thus, these phosphatic beds represent a condensed sequence that spans the late Campanian to early Maastrichtian. Ammonites also occur at other localities in the Navesink Formation in New Jersey, and correspond to higher levels in the Maastrichtian. The youngest ammonite known from the Navesink Formation, Discoscaphites gulosus, from Sewell, New Jersey, indicates a correlation with the Hoploscaphites nicolletii or Jeletzkytes nebrascensis zone of the Western Interior.

Mississippian coral latitudinal diversity gradients (western interior United States): Testing the limits of high resolution diversity data

Analysis of high resolution diversity data for Mississippian corals in the western interior United States yielded mild latitudinal diversity gradients despite the small geographic area covered by samples and a large influence on diversity patterns by geographic sampling intensity (sample bias). Three competing plate tectonic reconstructions were tested using the diversity patterns. Although none could be forcefully rejected, one reconstruction proved less consistent with diversity patterns than the other two and additional coral diversity data from farther north in Canada would better discriminate the two equivalent reconstructions.Despite the relatively high sampling intensity represented by the analyzed database, diversity patterns were greatly affected by sample abundance and distribution. Hence, some effort at recognizing and accounting for sample bias should be undertaken in any study of latitudinal diversity gradients. Small-scale geographic lumping of sample localities had only small effects on geographic diversity patterns. However, large-scale (e.g., regional) geographic lumping of diversity data may not yield latitudinally sensitive diversity patterns. Temporal changes in coral diversity in this region reflect changes in eustacy, local tectonism, and terrigenous sediment flux, far more than they do shifting latitude. Highest regional diversity occurred during the interval when the studied region occupied the highest latitude. Therefore, diversity data from different regions may not be comparable, in terms of latitudinal inference. Small-scale stratigraphic lumping of the data caused a nearly complete loss of the latitudinal diversity patterns apparent prior to lumping. Hence, the narrowest possible stratigraphic resolution should be maintained in analyzing latitudinal diversity gradients.

Didymoceras puebloense,a new species of heteromorph ammonite from the Upper Campanian of Colorado and Wyoming

Didymoceras puebloense nov. sp. is described from the upper Campanian zones of Didymoceras nebrascenseand Didymoceras stevensoni in Pueblo and Pitkin Counties, Colorado, and Weston County, Wyoming in the United States Western Interior. The species is characterized by a middle helical growth stage in which the succesive whorls are widely separated followed by a terminal pendant U-shaped sector of the adult body chamber. Ornament is of distant, narrow bituberculate ribs usually separated by a nontuberculate rib. Widely separated poorly-defined constrictions with associated collar ribs are occasionally present. The style of coiling in middle and later growth resembles that of those growth stages in upper Campanian Didymoceras, whereas the ornament recalls that of middle Campanian representatives of the genus in the Western Interior.

The skull and upper dentition ofHeliscomys senexWood (Heliscomyidae: Rodentia)

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Ammonites from the Saratoga Chalk (Upper Cretaceous), Arkansas

The Saratoga Chalk of Arkansas yields a rich ammonite fauna of 17 species, referred to theNostoceras(N.)hyattizone. Recognition that the northwest EuropeanN.(N.)pozaryskiiBlaszkiewicz, 1980, is a synonym ofN.(N.)hyattiStephenson, 1941, andN.(N.)helicinum(Shumard, 1861) dates the zone as latest Campanian on the basis of co-occurrence withBelemnitella langeiJeletzky, 1948, in Poland. Most previous estimates of the Campanian–Maastrichtian boundary in the Gulf Coast have been drawn at too low a level, at least in terms of ammonite faunas. Elements of theN.(N.)hyattizone fauna occur in the United States Western Interior, and show the base of the Maastrichtian there to lie above theBaculites jensenizone.

Regional isotopic variations in the Fort Hays Member of the Niobrara Formation, United States Western Interior: Primary signals and diagenetic overprinting in a Cretaceous pelagic rhythmite

High-resolution stratigraphic correlation of rhythmically interbedded shale-limestone couplets in the Fort Hays Member of the Niobrara Formation made possible a detailed comparative analysis of individual shale and limestone beds over a large part of the United States Western Interior region. Petrographic and isotopic data suggest that many of the differences observed between shale and limestone beds in the Fort Hays Member are attributable to diagenetic enhancement of originally subtle lithologic variations. Similar oxygen isotopic values from relatively unaltered inoceramid shells, and similar faunal content in limestone and shale beds, indicate that periods of increased runoff and detrital influx into the Western Interior Sea were not accompanied by extensive salinity variations. Inoceramid bivalves in the Fort Hays Member that have been less affected by diagenesis than surrounding matrix provide a sensitive indicator of environmental conditions. Cathodoluminescence and elemental data indicate that elevated levels of manganese occur in generally unaltered inoceramid bivalve remains, mostly in the eastern part of the study area. Manganese enrichment of these inoceramids is associated with increased 18O and decreased 13C contents and is interpreted as primary in origin. Regional east-west gradients in 18O, 13C, and manganese content suggest that the composition of bottom water varied across the Western Interior Sea, possibly in response to a north-ward influx of Tethyan water into the eastern part of the seaway. Similar gradients are observed in shale and limestone beds, indicating that these regional isotopic variations in the Fort Hays Member occurred independently of episodes of increased runoff and detrital influx into the seaway.