Cretaceous Stage Boundaries Research Articles

Estimation of the ages of Devonian and Cretaceous stage boundaries in the Geologic Time Scale

Estimation of the ages of period and stage boundaries of the Geologic Time Scale (GTS) has a long history that commenced over a century ago with the pioneering work of Arthur Holmes. Frequencies, precision and accuracy of radiogenic isotope age determinations used for time scale construction continue to increase steadily. Later stage boundary age estimates are accompanied by error bars based on 2-sigma age dating errors with incorporation of stratigraphic uncertainty. Most GTS2004 and GTS2012 results involved spline-curve fitting. In GTS2012, Milankovitch-type orbital climate cyclicity was used to tune the Neogene geologic time scale while seafloor spreading was combined with sedimentary cycle scaling to construct the Paleogene time scale, and it also contributed to the construction of the Cretaceous and Jurassic time scales. Geomathematical procedures continue to be refined for the next GTS which is in Gradstein et al. (2020). In this study smoothing splines are used to construct Devonian and Late Jurassic - Early Cretaceous time scales. This methodology and its results are described and some estimates are refined by incorporating Milankovitch cycle durations.

An analysis of the impacts of Cretaceous oceanic anoxic events on global molluscan diversity dynamics

AbstractOceanic anoxic events (OAEs) are contemporaneous with 11 of the 18 largest Phanerozoic extinction events, but the magnitude and selectivity of their paleoecological impact remains disputed. OAEs are associated with abrupt, rapid warming and increased CO2flux to the atmosphere; thus, insights from this study may clarify the impact of current anthropogenic climate change on the biosphere. We investigated the influence of the Late Cretaceous Bonarelli event (OAE2; Cenomanian/Turonian stage boundary; ~94 Ma) on generic- and species-level molluscan diversity, extinction rates, and ecological turnover. Cenomanian/Turonian results were compared with changes across all Cretaceous stage boundaries, some of which are coincident with less severe OAEs. We found increased generic turnover, but not species-level turnover, associated with several Cretaceous OAEs. The absence of a species-level pattern may reflect species occurrence data that are too temporally coarse to robustly detect patterns. Five hypotheses of ecological selectivity relating anoxia to survivorship were tested across stage boundaries with respect to faunality, mobility, and diet using generalized linear models. Interestingly, benthic taxa were consistently selected against throughout the Cretaceous regardless of the presence or absence of OAEs. These results suggest that: (1) the Cenomanian/Turonian boundary (OAE2) was associated with a decline in molluscan diversity and increase in extinction rate that were significantly more severe than Cretaceous background levels; and (2) no differential ecological selectivity was associated with OAE-related diversity declines among the variables tested here.

Constraining the depositional age of an Upper Cretaceous non-marine and shallow marine siliciclastic succession, Kuji Group, northeastern Japan, based on carbon isotope stratigraphy and U−Pb radiometric dating

Determining detailed geologic ages for pre-Quaternary non-marine and shallow marine siliciclastic successions has generally been difficult due to the rare occurrence of age-diagnostic fossils. In this study, we present the first record of stable carbon isotope stratigraphy derived from detrital woods with U–Pb radiometric dating for felsic tuffs in the Upper Cretaceous Kuji Group, northeast Japan, for which the geologic age has been poorly constrained to date. The radiometric ages for each tuff, obtained by laser ablation–inductively coupled plasma mass spectrometry (LA–ICPMS) analyses, range from 92.6 ± 0.6 to 79.1 ± 1.2 Ma (95% confidence interval). The composite δ13Cwood curve in the Kuji Group is correlated to the Upper Cretaceous reference sections in the Northwest Pacific, North America and Northwest Europe. Based on our correlations, several global carbon isotope excursion events in the late Turonian–early Campanian interval were recognized, and the Upper Cretaceous stage boundaries in the Kuji Group were partially identified, which agree well with previous biostratigraphic assumptions. Further, the depositional age of the dinosaur-bearing bone bed in the Kuji Group was estimated to be restricted to the early–middle Santonian. Our detailed chronostratigraphic framework provides significant implications for sequence stratigraphy, regional tectonic reconstruction and further exploration into the terrestrial ecosystem of the Late Cretaceous East Asian continental margin.

The Chalk Group (Upper Cretaceous) of the Northern Province, eastern England – a review

A review of the Chalk of the Northern Province recognizes six formations, five of which (Hunstanton, Ferriby, Welton, Burnham and Flamborough formations) crop out in northern Norfolk, Lincolnshire and Yorkshire, and a sixth (Rowe Formation) is buried beneath the drift of Holderness. The Hunstanton, Ferriby and Flamborough formations are largely devoid of flints, whereas the Welton (other than its two basal members) and Burnham formations have nodular and tabular flints, respectively. Previous work on the lithostratigraphy and marker beds is presented, and an overview of the distribution of the more important macrofossils is provided. Especial attention is given to the succession at Speeton through the Hunstanton, Ferriby and lower Welton formations with details of the oxygen and carbon isotope signatures. The Speeton section is internationally important because of its expanded Albian-Mid Cenomanian succession. Small sections of the Burnham Formation in Speeton–Buckton cliffs are also presented for the first time. The appendix reviews the relevant Cretaceous stage boundaries with reference to the Chalk of the Northern Province together with details of the fossil zones that are used in this paper.

A new biostratigraphic correlation for Late Cretaceous–Paleocene strata of the Gulf of Guinea: Evidence from dinoflagellate cysts

A new biostratigraphic correlation for Late Cretaceous and Palaeocene strata of the Côte d’Ivoire–Ghana continental margin has been developed from the identification of significant dinoflagellate cyst events in ODP Hole 959D. The Late Cretaceous stage boundaries are mostly consistent with previous studies. However, the Maastrichtian/Danian boundary is placed much lower than previously recognized on the basis of the first occurrences of Carpatella cornuta and Damassadinium californicum. The base of the Selandian is recognized from the last occurrence of Cerodinium diebelii and the first occurrence of Adnatosphaeridium multispinosum. The base of the Thanetian is recognized from the first occurrence of Areoligera gippingensis. The rarity of the age-marker taxa is the main reason for different age determinations among studies of the same section.

Coniacian – Maastrichtian calcareous nannofossil biostratigraphy and carbon-isotope stratigraphy in the Zagros Basin (Iran): consequences for the correlation of Late Cretaceous Stage Boundaries between the Tethyan and Boreal realms

Coniacian – Maastrichtian calcareous nannofossil biostratigraphy and carbon-isotope stratigraphy in the Zagros Basin (Iran): consequences for the correlation of Late Cretaceous Stage Boundaries between the Tethyan and Boreal realms

An integrated study (geochemistry, stable oxygen and carbon isotopes, nannofossils, planktonic foraminifera, inoceramid bivalves, ammonites and crinoids) of the Waxahachie Dam Spillway section, north Texas: a possible boundary stratotype for the base of the Campanian Stage

The spillway of Lake Waxahachie, Ellis County (Texas), exposes a >17 m section of the Hutchins Member of the Austin Chalk Group, unconformably overlain by Taylor Clay. The Austin sequence was regarded as a potential Global Stratotype Section for the base of the Campanian Stage at the 1995 Brussels meeting on Cretaceous Stage boundaries, with the last occurrence of the crinoid Marsupites testudinarius (von Schlotheim, 1820) as the potential boundary marker. An integrated study of the geochemistry, stable carbon and oxgen isotopes, nannofossils, planktonic foraminifera, inoceramid bivalves, ammonites and crinoids of this section place the last occurrence of M. testudinarius in a matrix of eighteen ancillary biostratigraphic markers, while the boundary can also be recognised on the basis of a δ 13C excursion that can, in principle, be detected globally in marine sediments. A new forma of the crinoid Marsupites testudinarius is introduced. The Waxahachie section fulfils sufficient geological criteria as to be an excellent candidate GSSP for the base of the Campanian Stage, if problems of ownership and access to the section can be resolved.

The Vöhrum section (northwest Germany) and the Aptian/Albian boundary

Prior to 1947, the Aptian/Albian boundary was placed at the base of the Nolaniceras nolani ammonite Zone. From (Trav. Lab. Géol. Fac. Sci. 26 (1947) 17), until (Cret. Res. 20 (1999) 609), the base of the Albian stage has been taken at the first appearance of the ammonite Leymeriella ( Proleymeriella) schrammeni, a species known only from a limited area of northwest Germany. The early succession of Leymeriella and its relationship with ammonite taxa from the underlying Hypacanthoplites jacobi ammonite Zone can only be determined in the northwest German sections, all of which are ephemeral clay pits. Thus, the definition of the base of the Albian on these criteria cannot be used in a global context. The positioning of the boundary has been the subject of intense discussion in the last years. After the second International Symposium on Cretaceous Stage Boundaries in 1995 (Bull. Inst. R. Sci. Nat. Belgique, Sci. Terre 66 (1996) 45). (Cret. Res. 20 (1999) 609) lowered the boundary to the base of his Hypacanthoplites rubricosus ammonite Subzone without describing a global boundary stratotype section. (Cret. Res. 21 (2000) 591) raised the boundary to the alleged first occurrence (FO) of Leymeriella tardefurcata in a permanent section, but neither proposals can be accepted globally (Cret. Res. 23 (2002) 1). (Cret. Res. 22 (2001) 677) supported by (Cret. Res. 23 (2002) 1), suggested a radical solution by raising the boundary to the base of the geographically widespread Lyelliceras lyelli ammonite Subzone, currently the base of the Middle Albian. In order to document accurately the classical northwest German succession of the Aptian/Albian boundary, a detailed multidisciplinary study has been made of a new section at Vöhrum (clay pit no. 4) with respect to its content of calcareous nannofossils, benthic foraminifera, planktonic foraminifera, ostracods, ammonites, as well as its clay mineralogy, gamma ray signature and tuff geochemistry. The ammonite succession, in particular the Callizoniceras–Leymeriella lineage, is discussed in detail. The FO of Leymeriella ( Proleymeriella) schrammeni anterior, 0.65 m below a tuff horizon near the base of the section is taken as the Aptian/Albian boundary in the classical German sense. Sections in France are discussed and the Leymeriella lineage is reviewed.

Magnetostratigraphic correlation of the Upper Cretaceous System in the North Pacific

Magnetostratigraphic data for Upper Cretaceous sedimentary strata in widely separated regions of the North Pacific, including Japan, Far East Russia and western North America, are reviewed in terms of calibration to the geomagnetic polarity time scale and regional correlation. A series of normal and reversed polarity zones are recognized in the Upper Cretaceous strata in Shikoku and Hokkaido, Japan, and South Sakhalin, Far East Russia. Combined magnetostratigraphic and biostratigraphic correlation has assigned these zones to the Late Cretaceous geomagnetic polarity chrons including C31r through C33r and the Cretaceous long normal interval. Corresponding geomagnetic reversals have been documented from the Upper Cretaceous successions of the Western Interior of North America, in combination with high-resolution ammonite biostratigraphy and radiometric age dates. Biostratigraphy of the Great Valley Sequence in California is also well-defined, but there is only one reversed interval that can be correlated with polarity chron C33r. The most complete record of polarity reversals in South Sakhalin would provide an integrated reference scheme which can be of significant use not only to correlate dissimilar faunal assemblages of disparate regions in the North Pacific, but also to contribute to a global definition of Upper Cretaceous stage boundaries.

The base of the Maastrichtian

At the Brussels Symposium on Cretaceous Stage Boundaries in 1995, the Maastrichtian Working Group decided to recommend the first occurrence of the ammonite Pachydiscus neubergicus in the Tercis quarry near Dax in the Landes, southwest France as the boundary stratotype for the base of the Maastrichtian stage. On the basis of the echinoid genus Echinocorys the Campanian-Maastrichtian boundary beds of Tercis are correlated with the succession in north Norfolk, England, which in turn is correlated with the succession at Kronsmoor, northwest Germany on the basis of belemnites and brachiopods. If the possible correlation between Tercis and northwest Germany is true, the P. neubergicus and Belemnella lanceolata standards for the base of the Maastrichtian stage are not separated by more than 0.2 m.y.

Definition of Late Cretaceous Stage Boundaries in Antarctica Using Strontium Isotope Stratigraphy

New 87Sr/86Sr analyses of macrofossils from 13 key marker horizons on James Ross and Vega Islands, Antarctica, allow the integration of the Antarctic Late Cretaceous succession into the standard biostratigraphic zonation schemes of the Northern Hemisphere. The 87Sr/86Sr data enable Late Cretaceous stage boundaries to be physically located with accuracy for the first time in a composite Southern Hemisphere reference section and so make the area one of global importance for documenting Late Cretaceous biotic evolution, particularly radiation and extinction events. The 87Sr/86Sr values allow the stage boundaries of the Turonian/Coniacian, Coniacian/Santonian, Santonian/Campanian, and Campanian/Maastrichtian, as well as other levels, to be correlated with both the United Kingdom and United States. These correlations show that current stratigraphic ages in Antarctica are too young by as much as a stage. Immediate implications of our new ages include the fact that Inoceramus madagascariensis, a useful fossil for regional austral correlation, is shown to be Turonian (probably Late Turonian) in age; the “Mytiloides” africanus species complex is exclusively Late Coniacian in age; both Baculites bailyi and Inoceramus cf. expansus have a Late Coniacian/Early Santonian age range; an important heteromorph ammonite assemblage comprising species of Eubostrychoceras, Pseudoxybeloceras, Ainoceras, and Ryugasella is confirmed as ranging from latest Coniacian to very earliest Campanian. An important new early angiosperm flora is shown to be unequivocally Coniacian in age. Our strontium isotopic recalibration of ages strengthens the suggestion that inoceramid bivalves became extinct at southern high latitudes much earlier than they did in the Northern Hemisphere and provides confirmation that, in Antarctica, belemnites did not persist beyond the Early Maastrichtian.

Integrated stratigraphy across the Aptian-Albian boundary in the Marnes Bleues, at the Col de Pré-Guittard, Arnayon (Drôme), and at Tartonne (Alpes-de-Haute-Provence), France: a candidate Global Boundary Stratotype Section and Boundary Point for the base of the Albian Stage

The history of definition of the Aptian-Albian boundary is reviewed with particular reference to the ammonite schemes of Brinkmann (1937), Breistroffer (1947), Casey (1961a, 1996, 1999), Kemper (1982), Owen (1996, 1999), and Ruffell & Owen (1995). The classic sequence in the Hannover area of Germany described by Brinkmann (1937) is reviewed in the light of subsequent work. The definition of the base of the Albian Stage at the first occurrence of the ammonite Proleymeriella schrammeni (Jacob, 1907) is rejected as it can only be recognised over a limited area near Hannover; no more widely recognised secondary markers have been documented, and there is no permanent section at the present time.An alternative Aptian-Albian boundary, defined by the first occurrence of the ammonite Leymeriella (L.) tardefurcata (d'Orbigny, 1841) in the expanded Marnes Bleues section at Tartonne, Alpes-de-Haute-Provence, France, is suggested.The palynomorph, coccolith, planktonic foraminiferan, ammonite, and inoceramid bivalve sequence, organic and inorganic carbon, trace, rare earth, and major element record, oxygen and carbon isotope sequence, and strontium isotope data are presented for sections at the Col de Pré-Guittard, Arnayon (Drôme), and Tartonne (Alpes-de-Haute-Provence) and described in detail. The Pré-Guittard section, considered as a candidate Global Boundary Stratotype Section (GSS) for the base of the Albian Stage at the Second International Symposium on Cretaceous Stage Boundaries held in Brussels in September 1995, provides a standard section for the boundary interval in SE France. It is, however, unsuitable as a GSS, as there is a hiatus at the critical level in the section. In contrast, the Tartonne section is a potential GSS. The Boundary Point for the base of the Albian Stage suggested here is the first criterion proposed at Brussels: the first appearance of the ammonite Leymeriella (L.) tardefurcata at the base of the Niveau Paquier within the expanded Marnes Bleues sequence. The first appearance is only 60 cm above the last record of its presumed ancestor, L. (L.) germanica Casey, 1957. So defined, the boundary lies within thePrediscosphaera columnata Nannofossil (NF) Zone NC/CC8, and the planktonic foraminiferal Hedbergella planispira Partial Range Zone.There is no major or trace element event associated with the proposed boundary, nor is there any distinctive oxygen or carbon isotopic signal. Strontium isotope data from the Tartonne section are compatible with those elsewhere in the basin, and show that the base of the Albian, defined by the first appearance of L. (L.)tardefurcata corresponds to an87Sr/86Sr ratio of 0.707339±2.Systematic sections deal with a new palynomorph, calcareous nannofossils and ammonites; full range data and taxonomic indices are given for the first two groups.

Magnetostratigraphy of Upper Cretaceous strata in South Sakhalin, Russian Far East

A stratigraphic sequence of magnetic polarity reversals consisting of 13 magnetozones is recognized in Upper Cretaceous sedimentary strata in the Naiba area of South Sakhalin, Russian Far East. Combined with local biostratigraphic age assignments using ammonites and other molluscs that apparently range from Coniacian to Maastrichtian, the geomagnetic polarity sequence can be correlated with polarity chrons from C34n, the Cretaceous long normal interval, through C30n in the upper Maastrichtian. The recognition of these polarity chrons and their correlation with regional faunal assemblages that commonly occur in Japan and other areas in the Far East provide an integrated reference that should be of value for linking local biostratigraphic zonations in the North Pacific and serve as another basis for calibration in the quest to establish a global definition of Upper Cretaceous stage boundaries.

The Late Albian to Early Cenomanian succession at Mont Risou near Rosans (Drôme, SE France): an integrated study (ammonites, inoceramids, planktonic foraminifera, nannofossils, oxygen and carbon isotopes)

The Second International Symposium on Cretaceous stage boundaries held in Brussels from September 8-14 1995 reached a preliminary agreement on the definition of the base of the Cenomanian stage as the first occurrence of the planktonic foraminiferan Rotalipora globotruncanoidesSigal, with a section at Mont Risou near Rosans, Drôme, SE France as the global reference section, Sigal, with a section at Mont Risou near Rosans, Drome, SE France as the global reference section, where the first occurrence of R. globotruncanoideslies 36 m below the top of the Marnes Bleues as defined by a zero datum limestone at the base of the succeeding unit. The succession yields ammonites, inoceramid bivalves, nannofossils and planktonic foraminifera that provide a series of marker levels across the boundary, while an ornate δ 1 3C curve provides a geochemical marker that can also be recognized in NE England and Italy. Proxies for R. globotruncanoidesare: the first appearance of Mantelliceras mantelliand other ammonites of the mantelliZone at -30 m, the first occurrence of the nannofossil Calculites anfractus, the first occurrence of the planktonic foraminiferan Rotalipora gandolfiiand the last occurrence of the planktonic foraminiferan Rotalipora ticinensisat -40 m, and the last occurrence of the nannofossil Staurolithites glabraat -12 m. Ammonite, inoceramid, nannofossil and planktonic foraminiferan occurrences are documented, a systematic account of the ammonites, and full range data and nomenclature of the nannofossils provided. Previous alternative definitions of, and boundary stratotypes proposed for the base of the Cenomanian are critically reviewed and compared with the Risou section.

Uintacrinus anglicus Rasmussen from the Upper Cretaceous Flamborough Chalk Formation of Yorkshire: implications for the position of the Santonian-Campanian boundary

Calyx plates of the crinoid Uintacrinus anglicus are recorded from the top and bottom of a 5 m interval, 4 m above the disappearance of calyx plates of the crinoid Marsupites testudinarius within the Flamborough Chalk Formation at Danes Dyke in North Yorkshire, UK. The U. anglicus Zone, as used here, comprises the interval from the disappearance of M. testudinarius to the disappearance of U. anglicus calyx plates, and therefore includes, but does not comprise, the total range of the index species. Two subzones are recognized in the M. testudinarius Zone; a lower subzone characterized by smooth Marsupites calyx plates and an upper subzone characterized by variably ornamented Marsupites calyx plates. A provisional study of the belemnite indicates the Gonioteuthis granulataquadrata appears in the highest part of the M. testudinarius Zone. The 1983 Copenhagen symposium on Cretaceous stage boundaries proposed that the base of the Campanian Stage should be drawn at a level close to the appearance of G. granulataquadrata. Consequently, the extinction of M. testudinarius is used to define the base of the Campanian here and U. anglicus Zone is placed in the basal Lower Campanian.

Short report on the 2nd International Symposium on Cretaceous Stage Boundaries

Short report on the 2nd International Symposium on Cretaceous Stage Boundaries

Stratigraphy of the upper cretaceous white chalk of Sussex

The Middle/Upper Chalk boundary has been removed from the geological maps in the current survey of the Chalk of Sussex by the British Geological Survey leaving an undifferentiated Middle-Upper Chalk. In place of this undivided sequence, six new Members are introduced using Rowe's (1900–1908) term ‘White Chalk’ for the Formation with stratotypes nominated in the thickest sequences along the Sussex coast between Beachy Head and Brighton and around Lewes. 1. The Ranscombe Chalk Member extends from the base of the Melbourn Rock, through a succession 70–100 m thick, to the top of a massively bedded sequence containing conspicuous marl seams that occurs within the Terebratulina lata Zone, terminating with the Glynde Marl. The basal marker is taken as the Foyle Marl at Eastbourne (boundary stratotype), and Gun Gardens Beachy Head is nominated as the holostratotype. 2. The Ranscombe Chalk is succeeded by a sequence containing regular seams of nodular chalks and the first regular seams of flint. This sequence of nodular chalks with flints is named the Lewes Chalk Member and persists for some 80 m at Lewes, terminated by the upper of the Shoreham Marls at the traditional boundary between the Micraster cortestudinarium and Micraster coranquinum Zones. The basal marker is the Glynde Marl 1 in Caburn Pit, Lewes, boundary stratotype. Caburn Pit, South Street to Southerham Cliffs Lewes, and Beachy Head are nominated as holostratotype sections. 3. Overlying the Shoreham Marls is a thick (60–80 m) sequence of soft, flint bearing, relatively featureless chalks containing seven conspicuous semi-tabular courses of flint; the Seaford Chalk Member. The basal marker is the Shoreham Marl 2 at Seaford Head, the boundary and holostratotype section. 4. Flint bearing chalks with marl seams return in the crinoid and Offaster pilula Zones and characterise the 60–80 m thick Newhaven Chalk Member. The basal marker is the Buckle Marl 1 at Seaford Head, boundary stratotype and holostratotype section. 5. Marl seams cease to be a significant lithology in the Gonioteuthis quadrata Zone where the chalk is generally very soft and featureless but contains several conspicuous courses of flint. This part of the succession is named the Culver Chalk Member and is between 60–100 m thick. The basal marker is the upper Castle Hill Marl at Seaford Head (boundary stratotype) and the only reasonably accessible, complete section through this chalk at Whitecliff Isle of Wight, is nominated as the holostratotype section. 6. Marl seams return in the uppermost part of the G. quadrata Zone and persist into the lower part of the Belemnitella mucronata Zone. These beds are found in the highest chalk on the Isle of Wight, Portsdown and the Sussex coast east of Bognor Regis and form the Portsdown Chalk Member. The basal marker is the Portsdown Marl at the Farlington British Gas Store and Whitecliff (boundary and holostratotype sections). These six members are divided into groups of beds within which numerous lithological marker horizons are identified. Conspicuous marl seams are chosen as the key markers dividing the members and beds and these seams can be recognised on geophysical borehole logs as well as in the field. The relationship of this stratigraphy to geophysical borehole logs, to the key fossil marker bands and to Upper Cretaceous Stage boundaries is shown.

Cretaceous stage boundaries - Introduction

In October 1983, the Subcommission on Cre­taceous Stratigraphy arranged a symposium on Cretaceous Stage Boundaries in Copenhagen to fulfil the Subcommission's primary responsibility: the defining of stage boundaries...

Cretaceous stage boundaries - Proposals

This paper is concerned with the chronostratigraphy of the Cretaceous System. It represents a compilation of the proposals that have resulted from the joint efforts of the three working groups of the Subcommis­sion on Cretaceous Stratigraphy, as presented at the symposium in Copenhagen 1983. The working groups are those on: the Pre-Albian, the Albian-Turonian, and the Coniacian-Maastrichtian.

Cretaceous stage boundaries in the southern Interior Plains of Canada

Marine Cretaceous rocks of Berriasian to Aptian age are restricted to northwestern Alberta and north­eastern British Columbia where many stage and substage boundaries may be drawn only tentatively on the basis of limited molluscan faunas. A complete marine Albian succession is similarly restricted, although the middle and upper divisions of the stage are much more widespread. The complete Albian succession of the Peace River district contains a refined and well-documented sequence of ammonite and foraminiferal zones and would be a most suitable continental standard. The Albian-Cenomanian (Lower-Upper Cretaceous) boundary has been reliably established in the continuous foraminifer- and mollusc-bearing sequences of the northwestern plains, but eastward it becomes enclosed in a hiatus. The same is true of successive stage boundaries from the Cenomanian­Turonian to the Santonian-Campanian. The bases for establishment of these boundaries, therefore, can be fully considered only on the western flank of the basin. The Campanian-Maastrichtian boundary may be precisely drawn in western Saskatchewan and traced westward to the Rocky Mountain front. Zones based on ammonites and inoceramid bivalves form· the cornerstone of the biostratigraphy and chronostratigraphy of the southern Interior Plains and hold the key to the stage boundaries. Zones based on assemblages of benthonic foraminifers, rarely of planktonic foraminifers, supplement the molluscan zones. The foraminiferal zones are less reliable and less useful, however, because some benthonic assemblages are weakly diachronous, most foraminiferal zones span several molluscan zones, and many stage boundaries fall within individual foraminifer zones.