Late Cretaceous Western Interior Seaway Research Articles

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.

Molybdenum speciation as a paleo-redox proxy: A case study from Late Cretaceous Western Interior Seaway black shales

Molybdenum speciation as a paleo-redox proxy: A case study from Late Cretaceous Western Interior Seaway black shales

Local versus seaway-wide trends in deoxygenation in the Late Cretaceous Western Interior Seaway

Local versus seaway-wide trends in deoxygenation in the Late Cretaceous Western Interior Seaway

Iron fertilization of primary productivity by volcanic ash in the Late Cretaceous (Cenomanian) Western Interior Seaway

Iron fertilization of primary productivity by volcanic ash in the Late Cretaceous (Cenomanian) Western Interior Seaway

Bringing Sternberg Museum Fossils into the 21st Century

The Sternberg Museum of Natural History (FHSM) has a rich paleontology history extending back to the late 1800s and early 1900s. From the 1902 to 1992, the museum was housed on the Fort Hays State University campus. FHSM outgrew the campus capacity and moved to a new location that afforded the museum significant space for collections and exhibits. Just as the museum had to change locations with its growing collections, so too must the means of care for the collections change and be updated. In order to improve the state of the paleontology collections and make them more accessible, the FHSM’s Chief Curator pursued grants to achieve these goals. Two grants later, FHSM is well on its way to a digitized paleontology collection. One National Science Foundation (NSF) grant focuses on basic collections improvement: transcription of hand-written records, cataloging the specimen backlog, and imaging specimens. One of the most important updates is the addition of the relational database, CollectiveAccess. This database enables FHSM to have a public-facing, searchable database that can show not only specimen data but also images and 3D scans of fossils. The second grant is an NSF funded Integrated Digitized Biocollection (iDigBio) collaborative research project. This grant focuses on fossils collected from the Late Cretaceous Western Interior Seaway fossils. The digitization goals of the collections improvement grant work synergistically with the iDigBio grant. When preparing to start work on these two projects, workflows and how-to guides were developed to fit the needs of both grants. This synergy increased efficiency for training student workers and aided quality control. In regards to the new relational database, many considerations had to be made: what is the nature of the data, with whom are we sharing data, what are the data standards, what controls need to be in place to increase ease of use. Digitization of the collection started with transcription of hand-written records into a spreadsheet. The data from those record books and ledgers was cross referenced with the specimen cards to check for accuracy. Between the two grants, FHSM has two photography stations, four undergraduate students, and two graduate students. This small army of students, along with the Collections Manager, have succeeded in digitizing the invertebrate paleontology collection and have made significant progress on the vertebrate collection. Once the database is finalized, the images produced by these students will be uploaded and shared with aggregators such as iDigBio and accessible via the FHSM website.

Isotope sclerochronology of ammonites (Baculites Compressus) from methane seep and non-seep sites in the Late Cretaceous Western Interior Seaway, USA: Implications for ammonite habitat and mode of life

Ammonites, as well as other fauna, were common in methane seeps of the Late Cretaceous Western Interior Seaway (WIS) of North America. Biogeochemical processes at the seeps, in particular the anaerobic oxidation of methane, produced a dissolved inorganic carbon reservoir with a low δ13C, manifested in the carbon isotope composition of the inorganic calcium carbonate concretions associated with the seeps and recorded in well-preserved shells of ammonites documented at the sites. Detailed sclerochronological sampling of six well-preserved specimens of Baculites compressus collected at seep sites in the Pierre Shale of South Dakota reveals three patterns that can be explained by reference to two specimens of the same species collected at age-equivalent non-seep sites. Three of the specimens exhibit uniformly low values of δ13C that are significantly different (unpaired t-test, p

The first report of <em>Toxochelys latiremis</em> Cope, 1873 (Testudines: Panchelonioidea) from the early Campanian of Alabama, USA

Toxochelys latiremis Cope, 1873 is currently thought to be one of the oldest members of the clade originating from the last common ancestor of all extant species of marine-adapted turtles (Chelonioidea). Fossil material of this species has been reported from numerous lower Campanian marine formations across North America; however, reported occurrences have been conspicuously absent from the upper Santonian-to-lower Campanian Mooreville Chalk of Alabama and Mississippi, USA, the type stratum for the only other valid species within the genus, Toxochelys moorevillensis Zangerl, 1953. The apparent absence of T. latiremis from the Mooreville Chalk, and from the southern expanse of the Mississippi Embayment, has made T. latiremis one of the few outliers in previously proposed paleobiogeographic models for marine turtles in the Late Cretaceous Western Interior Seaway. This absence also confounded attempts at reconciling the distribution and phylogeny of these taxa. Here we report the first material of T. latiremis identified from the Mooreville Chalk of Alabama, which represents the southern-most occurrence of this taxon. The discovery of this species in the Mooreville Chalk of Alabama helps to reconcile the previously hypothesized paleobiogeography of North American Late Cretaceous chelonioids with their fossil occurrence and provides the first evidence for overlapping ranges of the only two currently recognized species of Toxochelys.

A Late Cretaceous polygonal fault system in central North America

Research Article| May 01, 2017 A Late Cretaceous polygonal fault system in central North America Andy St-Onge Andy St-Onge 1PFS Interpretations Ltd., 427 28 Avenue NW, Calgary, Alberta T2M 2K7, Canada Search for other works by this author on: GSW Google Scholar Author and Article Information Andy St-Onge 1PFS Interpretations Ltd., 427 28 Avenue NW, Calgary, Alberta T2M 2K7, Canada Publisher: Geological Society of America Received: 23 May 2016 Revision Received: 28 Oct 2016 Accepted: 04 Dec 2016 First Online: 06 Jul 2017 Online Issn: 1943-2674 Print Issn: 0016-7606 © 2017 Geological Society of America GSA Bulletin (2017) 129 (5-6): 582–593. https://doi.org/10.1130/B31582.1 Article history Received: 23 May 2016 Revision Received: 28 Oct 2016 Accepted: 04 Dec 2016 First Online: 06 Jul 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Andy St-Onge; A Late Cretaceous polygonal fault system in central North America. GSA Bulletin 2017;; 129 (5-6): 582–593. doi: https://doi.org/10.1130/B31582.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Polygonal fault systems (PFS) have been interpreted worldwide using seismic data imaging sedimentary strata. Normal faults initiate over a large area in fine-grained subaqueous strata soon after deposition. As they continue to grow laterally and vertically, their fault traces intersect to form polygons in plan view. These polygons are difficult to image without three-dimensional (3-D) seismic data. The faulting is initiated during sediment dewatering and mud particle consolidation that can be independent of external stresses. In the past 20 years, hundreds of basins worldwide have been interpreted to contain polygonal faults.This paper presents a PFS interpretation for fine-grained sediments deposited in the Late Cretaceous Western Interior Seaway of the Great Plains of North America. The faulted strata have been observed as a PFS at depths ranging from ∼2750 m subsurface to outcrop. Seismic dataset interpretations and borehole analyses corroborate previously published outcrop analyses and seismic interpretations. The larger observed faults are mesoscale in size, with throws up to 80 m, and strike lengths up to ∼1.5 km. Potentially encompassing over 2,000,000 km2, observational averages imply 107 or more mesoscale-size faults, with an order of magnitude greater number of smaller faults. At shallow depth and outcrop, the PFS model of extensive normal faulting could help to explain Late Cretaceous shale faulting attributed to other causes such as deeper sediment dissolution or glacial processes. In the subsurface, faulting and fracturing consistent with a PFS model can help to explain fault geometries observed in well control and 3-D seismic data. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Temperature and salinity of the Late Cretaceous Western Interior Seaway

Abstract The Western Interior Seaway (WIS) was a shallow and expansive body of water that covered the central United States during the Late Cretaceous. Attempts to reconstruct temperatures in the seaway using the oxygen isotopic composition of biogenic carbonates have suffered from uncertainty in the oxygen isotopic composition of seawater (δ18Ow) in the semi-restricted basin. We present new reconstructed temperature and δ18Ow data from marine and estuarine environments in the WIS and freshwater environments in WIS source rivers, derived from clumped isotope analyses of bivalve and gastropod shells. We find temperatures of 5–21 °C, δ18Ow values below contemporaneous Gulf of Mexico marine sites, and a strong correlation between δ18Ow and environmental setting. We propose that decreasing δ18Ow values reflect decreasing salinity driven by an increasing contribution of continental runoff. Using a two-end-member salinity-δ18Ow mixing model, we estimate salinities of 29–35 psu (practical salinity units) for the deep marine, 20–32 psu for the shallow marine, and 11–26 psu for the estuarine environments of the WIS. New climate model simulations agree with reconstructed temperatures and salinities and suggest the presence of salinity driven stratification within the seaway.

Redox conditions of the Late Cretaceous Western Interior Seaway recorded by rare earth elements of Bearpaw molluscan fossils

Understanding the causes of the low-biodiversity faunas in the Western Interior Seaway (WIS), overwhelmingly dominated by ammonite and bivalve molluscs, will help us to understand paleoenvironmental conditions in the WIS. In this study, we examined rare earth elements (REEs) in well-preserved molluscs from the Bearpaw Formation to reconstruct WIS redox conditions. Both nektonic and benthic molluscs share similar REE patterns with enrichment in light REEs. There is only a slight Ce depletion in both types of molluscs, indicating no significant fractionation of Ce from the other REEs. A lack of significant Ce anomalies in molluscs points to oxygen-deficient (probably dysoxic) conditions in the middle to bottom part of the water column where the molluscs lived. Given the general lack of significant Ce anomalies also in molluscs from older formations, oxygen deficiency was likely prevalent in the Late Cretaceous in the WIS. A warmer climate in the Cretaceous is probably a driver of such conditions, as predicted by ocean models. Long-term oxygen deficiency and stratification, inferred from the heterogeneity in δ18O and δ13C values of molluscs, may also imply weak circulation, reducing the exchange of surface water and subsurface water, and the transport of oxygen into the WIS. The oxygen deficiency and weak circulation of the seaway is also expressed in the general characteristics of faunas and possibly Bearpaw sedimentary rocks. Oxygen deficiency rather than the brackish water conditions may have been largely responsible, therefore, for the low diversity of the WIS fauna.

Deciphering the origin of cyclical gravel front and shoreline progradation and retrogradation in the stratigraphic record

AbstractNearly all successions of the near‐shore strata exhibit cyclical movements of the shoreline, which have commonly been attributed to cyclical oscillations in relative sea level (combining eustasy and subsidence) or, more rarely, to cyclical variations in sediment supply. It has become accepted that cyclical change in sediment delivery from source catchments may lead to cyclical movement of boundaries such as the gravel front, particularly in the proximal segments of sediment‐routing systems. In order to quantitatively assess how variations in sediment transport as a consequence of change in relative sea‐level and surface run‐off control stratigraphic architecture, we develop a simple numerical model of sediment transport and explore the sensitivity of moving boundaries within the sediment‐routing system to change in upstream (sediment flux, precipitation rate) and downstream (sea level) controls. We find that downstream controls impact the shoreline and sand front, while the upstream controls can impact the whole system depending on the amplitude of change in sediment flux and precipitation rate. The model implies that under certain conditions, the relative movement of the gravel front and shoreline is a diagnostic marker of whether the sediment‐routing system experienced oscillations in sea level or climatic conditions. The model is then used to assess the controls on stratigraphic architecture in a well‐documented palaeo‐sediment‐routing system in the Late Cretaceous Western Interior Seaway of North America. Model results suggest that significant movement of the gravel front is forced by pronounced (±50%) oscillations in precipitation rate. The absence of such movement in gravel front position in the studied strata implies that time‐equivalent movement of the shoreline was driven by relative sea‐level change. We suggest that tracking the relative trajectories of internal boundaries such as the gravel front and shoreline is a powerful tool in constraining the interpretation of stratigraphic sequences.

Lakotacrinus brezinain. gen. n. sp., a new stalked crinoid from cold methane seeps in the Upper Cretaceous (Campanian) Pierre Shale, South Dakota, United States

AbstractDespite a rich and varied record, Mesozoic stalked crinoids are relatively rare in the Western Interior Seaway of North America compared to those found in Northern Europe. A unique example of Mesozoic stalked crinoid is described from cold methane seeps (hydrocarbon seep mounds also called “tepee buttes”) from the Upper Cretaceous (upper Campanian) of the Northern Great Plains of the United States; the first crinoids to be described from such an environment. The Late Cretaceous Western Interior Seaway has never before yielded any identifiable stalked crinoid remains. Nevertheless, there have been significant studies on both free living and stalked crinoids from other locations in the Upper Cretaceous of North America that provide a good basis for comparison.Lakotacrinus brezinain. gen. n. sp. is characterized by a tapering homeomorphic column with through-going tubuli, lacking any attachment disc. The arms are unbranched and pinnulate, with muscular and syzygial articulations. The unique morphology of the column justifies the establishment of Lakotacrinidae new family. A new suborder Lakotacrinina n. subord., is also proposed as there exists no corresponding taxon within the Articulata that can accommodate all the characteristics of this new genus. This new crinoid shares many features with other members of the articulates, including bathycrinids, bourgueticrinids and guillecrinids within the Order Comatulida, as currently defined in the revised Treatise of Invertebrate Paleontology. Reconstructing the entire crinoid using hundreds of semi-articulated and disarticulated (well preserved) fossils, reveals a unique paleoecology and functional morphology specifically adapted to living within this hydrocarbon seep environment.

Fossil Marine Vertebrates from the Blue Hill Shale Member (Middle Turonian) of the Upper Cretaceous Carlile Shale in Northeastern Nebraska

The Blue Hill Shale Member of the Carlile Shale is a Middle Turonian (ca. 90 Ma) nearshore deposit formed during the regressive phase of the Late Cretaceous Western Interior Seaway of North America. Over 900 identifiable specimens of fossil vertebrates were recovered from a locality in northeastern Nebraska that include at least 40 taxa, comprising a minimum of 23 chondrichthyan fishes, 16 osteichthyan fishes, and one aquatic squamate reptile. The fauna includes taxa such as Meristodonoides, Cantioscyllium, Scapanorhynchus, Cretodus, Protoplatyrhina, Pseudohypolophus, and Ptychotrygon, that are commonly represented in Middle Turonian nearshore deposits of North America. The vertebrate fauna described here is composed of nearly 50% non-durophagous and 50% durophagous fishes by number of taxa. Among the nondurophagous fishes are a large carnivorous Cretodus crassidens and medium-sized scavengers Squalicorax spp., but others are primarily piscivores.

Applications in Paleohistology: Osteohistology of Protostega gigas

Protostega gigas was a large sea turtle that lived in the Late Cretaceous Western Interior Seaway of North America. Protostega fossils are found in the Smoky Hill Member of the Niobrara Formation in Kansas. Despite their abundance, little is known about how these turtles grew and aged. The purpose of this study is to expand the knowledge of sea turtles osteohistology and determine the ontogenetic stage of the specimen at the time of death. Osteohistology is the study of bone microstructure, and is used to study bone growth rates. Microstructure and growth rates directly relate to phylogeny (evolutionary relationships), ontogeny (growth stage), biomechanics (how a bone is used), and environment. The femur of the Protostega gigas specimen analyzed is spongiose, with large networks of vascular canals and little or no open medullary cavity. Vascular canal orientation is dominated by longitudinal and circumferential canals organized in concentric layers. Vascular canals increase in circumferential organization towards the outer cortex. Overall, the Protostega bone analyzed in this study has a similar microstructure to that that described in the leatherback sea turtle. Because there are vascular canals open to the periosteal surface of the bone, with no indications of reduced vascularity and cessation in bone growth, this specimen is interpreted to have been a juvenile at the time of death. ; Histologic results contradict previous research interpreting this specimen as an adult.

Identification of a New Hesperornithiform from the Cretaceous Niobrara Chalk and Implications for Ecologic Diversity among Early Diving Birds.

The Smoky Hill Member of the Niobrara Chalk in Kansas (USA) has yielded the remains of numerous members of the Hesperornithiformes, toothed diving birds from the late Early to Late Cretaceous. This study presents a new taxon of hesperornithiform from the Smoky Hill Member, Fumicollis hoffmani, the holotype of which is among the more complete hesperornithiform skeletons. Fumicollis has a unique combination of primitive (e.g. proximal and distal ends of femur not expanded, elongate pre-acetabular ilium, small and pyramidal patella) and derived (e.g. dorsal ridge on metatarsal IV, plantarly-projected curve in the distal shaft of phalanx III:1) hesperornithiform characters, suggesting it was more specialized than small hesperornithiforms like Baptornis advenus but not as highly derived as the larger Hesperornis regalis. The identification of Fumicollis highlights once again the significant diversity of hesperornithiforms that existed in the Late Cretaceous Western Interior Seaway. This diversity points to the existence of a complex ecosystem, perhaps with a high degree of niche partitioning, as indicated by the varying degrees of diving specializations among these birds.

Geochemical evidence (C and Sr isotopes) for methane seeps as ammonite habitats in the Late Cretaceous (Campanian) Western Interior Seaway

Methane seeps in the Upper Cretaceous Pierre Shale of the U.S. Western interior contain a rich fauna including ammonites (Baculites, Hoploscaphites, Didymoceras, Placenticeras, Solenoceras), bivalves (Lucina), gastropods, sponges, and crinoids. Occasionally, the shell material in the seeps is very well preserved, retaining the original mineralogy and microstructure. We explored two such seeps from the upper Campanian Didymoceras cheyennense and overlying Baculites compressus Zones (74.7–73.5 Ma) in southwestern South Dakota. Light values of δ13C in the micritic limestones (–11 to –47 ‰) confirm the impact of anaerobic oxidation of methane on the isotopic composition of the dissolved inorganic carbon reservoir. At the seep from the D. cheyennense Zone, δ13C values in well-preserved specimens of Hoploscaphites and Baculites are significantly lighter than those measured in specimens from approximately age-equivalent non-seep deposits. In addition, the 87Sr/86Sr ratio is elevated in the authigenic carbonates and ammonite shells compared with the coeval marine value. This suggests that seep fluids imprinted with a radiogenic Sr signature, perhaps derived from isotopic exchange with granitic deposits at depth associated with the Black Hills uplift, are transported through the surficial sediments into the overlying water. The persistence of these isotopic tracers of seep fluids in the ammonite shells suggests that these mobile animals were likely demersal and were living in close proximity to the seep. A more restricted data set on a single baculite and nautilid from the seep in the B. compressus Zone shows less divergence of δ13C and 87Sr/86Sr relative to non-seep values, suggesting that fluid transport was not as strong at that seep.

Ischyodus rayhaasi(Chimaeroidei; Callorhynchidae) from the Campanian-Maastrichtian Fox Hills Formation of Northeastern Colorado, USA

Modern ratfish (chimaeroids) are examples of a poorly documented lineage of cartilaginous fishes that extends back to Triassic time. Here we report on the discovery of abundant remains of the callorhynchid chimaeroid Ischyodus rayhaasi Hoganson and Erickson 2005 from a Campanian-Maastrichtian age locality in the Fox Hills Formation of northeastern Colorado. A second, distinct group of chimaeroid specimens, designated Ischyodus sp., occurs at the same locality and may represent a species separate from I. rayhaasi, an earlier ontogenetic stage, or sexual dimorphism within I. rayhaasi. The presence of chimaeroid fishes is discussed in regard to the paleoecology and paleogeography of the Late Cretaceous Western Interior Seaway.

Durophagous predation on scaphitid ammonoids in the Late Cretaceous Western Interior Seaway of North America

This study is the first to report a trend of predation intensity on scaphitid ammonoids from the Turonian to the Maastrichtian (Late Cretaceous) on the basis of analysis of ventral shell breakage in large samples from the US Western Interior Province. Analysis of 835 adult specimens revealed ventral shell breakage in 50 specimens. In most of the damaged specimens, the breakage occurred in a preferred position at the rear part of the body chamber. Ventral breakage is rare in the Turonian specimens, whereas it is common in the Campanian and Maastrichtian specimens. The shell diameter of adult scaphitid ammonoids tends to increase with time. The position of the breakage and the absence of repairs indicate that the ventral breakage resulted from lethal predation. Based on the incidence of breakage and the size and shape of the breaks, possible predators include fish, reptiles and cephalopods such as Placenticeras, Eutrephoceras and coleoids. Our statistical analysis of ventral shell breakage indicates that the incidence of lethal predation increased in conjunction with an increase in adult shell size, suggesting that the body size of the prey was an important factor in predator–prey interactions. In addition, the predatory damage is more extensive in larger adults.

Comparative osteohistology of Hesperornis with reference to pygoscelid penguins: the effects of climate and behaviour on avian bone microstructure.

The broad biogeographic distribution of Hesperornis fossils in Late Cretaceous Western Interior Seaway deposits has prompted questions about whether they endured polar winters or migrated between mid- and high latitudes. Here, we compare microstructures of hesperornithiform long bones from Kansas and the Arctic to investigate whether migration or Late Cretaceous polar climate affected bone growth. We also examine modern penguin bones to determine how migration and climate may influence bone growth in birds with known behaviours. Histological analysis of hesperornithiform samples reveals continuous bone deposition throughout the cortex, plus an outer circumferential layer in adults. No cyclic growth marks, zonation or differences in vasculature are apparent in the Hesperornis specimens. Comparatively, migratory Adélie and chinstrap penguin bones show no zonation or changes in microstructure, suggesting that migration is not necessarily recorded in avian bone microstructure. Non-migratory gentoos show evidence of rapid bone growth possibly associated with increased chick growth rates in high-latitude populations and large body size. The absence of histological evidence for migration in extinct Hesperornis and extant pygoscelid penguins may reflect that these birds reached skeletal maturity before migration or overwintering. This underscores the challenges of using bone microstructure to infer the effects of behaviour and climate on avian growth.

A quantitative analysis of calcareous nannofossil bioevents of the Late Cretaceous (Late Cenomanian–Coniacian) Western Interior Seaway and their reliability in established zonation schemes

Abstract Calcareous nannofossil assemblages from 8 localities in the Upper Cenomanian–Coniacian strata of central (Colorado and Kansas) and southern (Texas) Western Interior Seaway, USA were analyzed to produce a new high resolution biostratigraphic framework for the Greenhorn Formation, Eagle Ford Formation, and basal Austin Chalk. Eighteen datasets from coeval successions around the world, including the Global Stratotype Section and Point (GSSP) at Pueblo, CO and proposed reference sections for the base of the Turonian and candidate GSSPs for the base of the Coniacian in Germany, Poland, and New Mexico, were incorporated from literature sources. Additional discussion is devoted to the basal Turonian and Coniacian boundaries as they reflect paleoceanographic phenomena (e.g. OAE2, widespread Late Turonian regression) where high-resolution biostratigraphy has academic, as well as industrial, application. Ranking and Scaling of 48 selected bioevents produces a quantitatively derived optimum sequence of first and last occurrences. Scaling of events along a relative time-scale reveals an 8–9% evolutionary turnover during OAE2. Comparison with the qualitatively developed nannofossil zonations known as the CC zonation of Perch-Nielsen and UC zonation of Burnett reveals inconsistencies, largely due to the recognition of new/amended species concepts. Discrepancies near the Turonian/Coniacian boundary reflect changes to its placement in recent revisions to the geologic timescale. A recently documented sequence of bioevents provides an accurate proxy for the boundary in the absence of the inoceramid index fossil Cremnoceramus deformis erectus (Meek). Fifteen bioevents from the optimum sequence are given greater priority based on their interpreted reliability between the sections examined and the CC and UC zonations.