Ammonite Succession Research Articles

Late Cenomanian–early Turonian ammonite successions of north Eastern Desert, Egypt: biostratigraphy and paleobiogeographic implications

The upper Cenomanian-lower Turonian sequence of Egypt yield a remarkably diverse and plentiful ammonite fauna. Three stratigraphic sections located on Wadi Araba, north of the Southern Galal Plateau were investigated. Lithostratigraphically, the measured sections are subdivided from base to top into the Malha (pre-Cenomanian), Galala (Cenomanian) and Maghra El Hadida (Turonian) formations. Over a hundred well-preserved and nearly complete ammonite internal molds were collected. The studied ammonites are systematically classified as Vascoceratidae, Pseudotissotiidae, Acanthoceratidae and Engonoceratidae. Vascoceratidae and Pseudotissotiidae comprise 75% of the identified ammonites. Twelve ammonite species have been identified. Six late Cenomanian-early Turonian ammonite biozones are proposed herein: Neolobites vibrayeanus, Vascoceras gamai, Vascoceras cauvini, Vascoceras proprium, Choffaticeras segne, and Wrightoceras munieri. The identified ammonite biozones are correlated with NW European standard zonation and the Tethyan biostratigraphical schemes was performed. The first occurrences of Vascoceras proprium, as well as the first occurrences of the inoceramids Mytiloides kosmati and Mytiloides puebloensis define the Cenomanian-Turonian boundary. The diversity and abundance of studied fossil groups varied dramatically over the C/T transition. In the latest Cenomanian, all identified Cenomanian species had become extinct. Only three species have reached the Turonian time. The paleobiogeographic implications of the descried ammonites are thoroughly discussed. The paleogeographic distribution of the studied ammonite fauna shows that the majority of the described species are restricted to the shallow shelf of the southern Tethys.

Taxonomy, phylogeny and biostratigraphy of the upper Aptian–lower Albian ammonites of the Chott area, southern Central Tunisia

Four sections were measured and studied in the uppermost Aptian and lower Albian of the Chott Basin (southern Central Tunisia), providing an abundant, bed-by-bed collected paleontological material. The ammonite succession allows precise correlations with the ammonite biozonation established in Central Tunisia.The phylogeny of knemiceratids and acanthohoplitinids of the Chott Basin is investigated and the alleged extreme variability of knemiceratids is questioned.Five new ammonite species are herein described: Knemiceras sagetae sp. nov., Knemiceras lanceolatum sp. nov., Knemiceras multicostatum sp. nov., Knemiceras tunisiense sp. nov., Parengonoceras chottensis sp. nov., highlighting the highly endemic character of the Chott Basin ammonite faunas.

Sr-isotope stratigraphy (87Sr/86Sr) of the lowermost Toarcian of Peniche, Portugal, and its relation to ammonite zonations

At Peniche, Portugal, the GSSP for the base of the Toarcian and in a Submediterranean province, a value of 0.707 073 ± 0.000 002 (2s.e., n = 13) is determined for 87Sr/86Sr at the base of the Toarcian. In Yorkshire, UK, a Subboreal province, the base of the Toarcian has an 87Sr/86Sr value of0.707 072 ± 0.000 002 (2s.e., n = 23). The similarity of the values shows that, within analytical uncertainty, these levels are exactly equivalent in time. At Peniche, the base of the Serpentinum (Falciferum/Levisoni) Chronozone has an 87Sr/86Sr value of 0.707 096 ± 0.000 003 (2.s.e). The value for this boundary in Yorkshire is 0.707 094 ± 0.000 002 (2s.e., n = 27). The similarity of the values shows that, within analytical uncertainty, these levels are exactly equivalent in time. The same ammonite successions occur through the lowermost Toarcian successions at Peniche and in Yorkshire. Between Peniche and Yorkshire, there is therefore an exact coincidence, within measurement uncertainty, between the Lower Toarcian, Tethyan Realm, Tenuicostatum (Polymorphum) [ammonite] Chronozone of Peniche and the Lower Toarcian, Boreal Realm, Tenuicostatum [ammonite] Chronozone of Yorkshire. The 87Sr/86Sr profile through the Tenuicostatum Subchronozone at Peniche reveals large variations in sedimentation rate, with lowest rates in a condensed basal 3 m of the zone, highest rates in mid-zone, with rates decreasing upward through the upper half of the zone. Apportioning time in proportion to 87Sr/86Sr shows that around half of Tenuicostatum time is condensed into the basal 3 m of the section.

The Bajocian to Kimmeridgian (Middle to Upper Jurassic) ammonite succession at Sentralbanken High (core 7533/3-U-1), Barents Sea, and its stratigraphical and palaeobiogeographical significance

Ammonites recovered from Upper Bajocian to Upper Kimmeridgian strata in a core drilled at Sentralbanken High in the northern Barents Sea are described and chronostratigraphically interpreted. The lower part of the core comprises Upper Bajocian to Upper Callovian deposits of the Fuglen Formation with ammonites of the genus Cranocephalites near the base, and Longaeviceras in the upper part. The overlying Oxfordian to Kimmeridgian deposits of the Hekkingen Formation are thin and condensed. The Hekkingen Formation is dominated by Boreal cardioceratid ammonites (Cardioceras, Amoebites, Euprionoceras, Hoplocardioceras), with numerous Subboreal aulacostephanid ammonites (Rasenia, Zenostephanus) at two levels. The occurrences of Boreal and Subboreal ammonites are discussed in relation to the palaeogeography and fluctuations of ammonite faunas within the Boreal Realm during the Kimmeridgian with special attention to levels rich in Subboreal ammonites whose appearance has been controlled by tectonic and climatic factors. The uppermost part of the succession deposited during a time of maximum flooding in the Late Kimmeridgian is dominated by Boreal ammonites.

Rare and little-known ammonites from the Lower and Middle Jurassic of the Western and Central Balkan Mts (Bulgaria)

This paper presents insufficiently or previously unknown Jurassic ammonites from 21 localities and sections of the Western and Central Balkan Mts (Bulgaria). The total stratigraphical range of the studied fauna corresponds to a broad interval, from the Pliensbachian (Lower Jurassic) to the Callovian (Middle Jurassic), with emphasis on the ammonites from the Bajocian–Callovian interval. These ammonites belong to 36 species and 23 genera that are affiliated to 12 ammonite families: Polymorphitidae, Acanthopleuroceratidae, Amaltheidae, Hildoceratidae, Graphoceratidae, Strigoceratidae, Sonniniidae, Stephanoceratidae, Oppeliidae, Tulitidae, Parkinsoniidae, and Perisphinctidae. Of them, 13 species have been left in open nomenclature. This account comprises both newly recorded ammonite taxa and redefined ammonite species from older collections. The new evidence also includes the upper Bathonian species Siemiradzkia mangoldi sp. n., which was recorded from south-eastern France as well. Although being represented mainly by single specimens, often with no other supporting material, the newly discovered taxa have high stratigraphical significance, being both valuable additions to well-known Bulgarian ammonite zonal associations and new indications of ammonite successions that have quite poorly been identified in Bulgaria to date. In this regard, they are useful for better dating of the sedimentary successions in Bulgaria and are valuable for the purposes of correlation outside Bulgaria.

A new Griesbachian–Dienerian (Induan, Early Triassic) ammonoid fauna from Gujiao, South China

AbstractBed-by-bed sampling of the lower portion of the Daye Formation at Gujiao, Guizhou Province, South China, yielded new Griesbachian–Dienerian (Induan, Early Triassic) ammonoid faunas showing a new regional Induan ammonoid succession. This biostratigraphic scheme includes in chronological order the late GriesbachianOphiceras mediumandJieshaniceras guizhouensebeds, and the middle DienerianAmbites radiatusbed. The latter is recognized for the first time as a separate biozone in South China. Eight genera and 13 species are identified, including one new species,Mullericeras gujiaoensen. sp. The new data show that a relatively high level of ammonoid taxonomic richness occurred rather rapidly after the Permian/Triassic mass extinction in the late Griesbachian, echoing similar observations in other basins, such as in the Northern Indian Margin.UUID:http://zoobank.org/a24a3387-f3dd-4da4-a134-84372352a63d

Палинологическая характеристика среднеюрских отложений из местонахождения Кулинда (Забайкалье): биостратиграфический и биофациальный анализ

The Kulinda section belongs to the base of the Ukurey Formation in the Orlov Depression (Transbaikalia). The site has yielded numerous bones and associated integumentary structures belonging to the primitive ornithischian dinosaur. However, the age of the Kulinda deposits has not been investigated in detail so far. Palynological studies discussed in this paper clarify this question and provide the data on the palaeoenvironments. The Kulinda section is composed of siltstone and sandstone with admixture of volcanogenic material. Ten samples were collected from the bone-bearing deposits as well as the underlying and overlying sediments. The samples were treated by 15% hydrochloric and 30% hydrofluoric acids. Mineral and organic matters were separated by centrifugation in cadmium heavy liquid with specific gravity 2.25. Spore-pollen assemblage contains the species, which are important for Middle Jurassic stratigraphy: Podocarpidites rousei Pocockand, Eboraciato rosa (Sachanova et Iljina) Timochina. Their lowermost occurrences are revealed in the Bathonian in Western Siberia (Palynozone 10) and Kansk-Achinsk basin. The domination of Pseudopicea variabiliformis Bolchovitina and abundant Cyathidites are also characteristic features of the Bathonian of Western Siberia. Palynological zones in this region are calibrated against ammonite Boreal standard and foraminiferal successions that give the reliable age constraints. Spore-pollen assemblage from the Kulinda section shows that neighboring areas were occupied by coniferous forests with an admixture of pteridosperms, podocarpaceans and ginkgoaleans. More humid habitats near the lake were mostly occupied by cyatheaceous/dipteridaceous ferns. The climate was temperate and rater humid. The water genesis of deposits is confirmed by the presence of green algae Botryococcus and Leiosphaeridia. The age of the Ukurey Formation was broadly regarded as the Late Jurassic according to palaeontological data, isotope dating and its general position in the section. New palynological data show that its lower stratigraphic extend is wider, than it is was assumed earlier, and changed over the territory. The Bathonian age of the Ukurey Formation does not contradict to the general geological situation in Transbaikalia. Biofacial analysis confirms lacustrine genesis of the Ukurey Formation and evidences temperate warm and rather humid conditions on the nearby land areas.

The ammonite zones of the Bathonian in Bulgaria

A scheme of nine ammonite zones for the Bathonian in Bulgaria is described herein. The stratigraphical distribution of the Bathonian ammonites has been revised through new collecting and re-examination of older collections and fields. It has become evident that the criteria for zonal discrimination applied in NW Europe are relevant for Bulgaria to a limited extent, and we cannot compare directly our successions with those from the NW European areas as done in earlier work. Also, many traits of typically Submediterranean dispersal have been found to take a considerable part of the ammonite associations. Therefore, a composite Submediterranean–NW European zonation seems to be applicable to the Bathonian ammonite successions in Bulgaria. The ammonite zonal scale used in previous Bulgarian accounts has been retained, but several changes have been made to attain a more balanced sequence. This scale can be subject to further confirmation and modification, as not enough biostratigraphically well-constrained specimens have yet become available to characterize the ranges and variability of the zones.

Cenomanian–Turonian ammonite successions in the Tinrhert Basin (Southeastern Algeria): Revision and new data

The Cenomanian–Turonian succession (C–T) outcropping in the Tinrhert Basin, in the southeastern part of Algeria, has been the focus of many studies since the turn of the 19th century. In the present research, new C–T sections have been scrutinized over a distance of approximately 300 km, from Gour Ben Houilet in the west to Oued In Adaoui in the east. More than 1.700 ammonites and nautilids were collected bed-by-bed and 30 taxa subsequently identified; these are systematically described and illustrated herein. Thirteen of them [Placenticeras cf. kaffrarium Etheridge, Metengonoceras dumbli (Cragin), Calycoceras (Proeucalycoceras) sp., Pseudaspidoceras pseudonodosoides (Choffat), Vascoceras aff. glabrum (Barber), Rubroceras burroense Cobban, Hook and Kennedy, Fagesia peroni Pervinquière, Eotissotia simplex Barber, Hoplitoides wohltmanni (Von Koenen), Kamerunoceras turoniense (d'Orbigny), Choffaticeras sp., Choffaticeras meslei (Peron), and Choffaticeras sinaiticum Douvillé] are recorded from the Tinrhert area for the first time. This study also evidences that both subspecies Neolobites vibrayeanus vibrayeanus (d'Orbigny) and Neolobites vibrayeanus brancai Eck are associated in the same section and in the same level.In the Tinrhert area, the biostratigraphic ammonite successions are subdivided into nine bio-events that are easily correlatable with the upper Cenomanian–lower Turonian standard zonation. The paleobiogeographic affinities of the recorded taxa characterize these ammonoid assemblages as part of the southern Tethyan Province during the early Late Cretaceous.

The late Famennian ammonoid succession in the vicinity of Béni Abbès (Saoura Valley, Algeria)

The late Famennian ammonoid succession in the vicinity of Béni Abbès (Saoura Valley, Algeria)

EARLY JURASSIC (SINEMURIAN TO BASAL TOARCIAN) AMMONITES OF THE BRESCIAN PREALPS (SOUTHERN ALPS, ITALY)

This work provides a more precise up-to-date biostratigraphical framework of the Early Jurassic ammonite succession of the Brescian Prealps with more than seventy taxa for the Sinemurian, Pliensbachian and basal Toarcian corresponding to a about 30 horizons or faunal assemblages rather well correlable with the NW European standard zonation. These results are supported with already published data, new data from recent field investigations, revised contributions of different authors and with the study of the historical collection of the Museum of Natural Sciences of Brescia (Northern Italy). The biohorizons are referred to the Lower Jurassic carbonate series of the Brescian Prealps, cropping out between the eastern surroundings of Brescia (Botticino-Serle), to the East, and the Lake Iseo, to the West. This area was located in the eastern border of the wide Lombardian Basin, a part of the southern continental passive margin subjected to the Jurassic rifting, preceding the Neo-Tethys opening.

High-resolution biochronology and diversity dynamics of the Early Triassic ammonoid recovery: The Dienerian faunas of the Northern Indian Margin

A new high resolution biozonation based on the Unitary Association (UA) method is constructed for the Dienerian ammonoid succession of the Northern Indian Margin. It includes 12 UA-zones and leads to the subdivision of the Dienerian into three parts (early, middle and late). The corresponding diversity analyses, coupled with results previously obtained for the early Smithian ammonoids of the same regions, highlight the four following phases: (1) a first modest peak of diversity in the early Dienerian; (2) a very low diversity persisting throughout the middle Dienerian; (3) a slow increase of diversity during the late Dienerian, and (4) a marked diversification in the early Smithian. Turnover rates are very high during this entire time interval, and the boundaries between early–middle and middle–late Dienerian are emphasised by complete renewals of the ammonoid faunas. The low diversity values in the middle and early late Dienerian are concomitant with an anoxic event on outer continental shelves and coincide with warmer temperatures than those of the early Dienerian and early Smithian. This diversity pattern stands in strong contrast with the credo of a protracted or stepwise recovery following the end-Permian mass extinction. Together with the end-Smithian extinction, the middle and early late Dienerian diversity crises were likely both radical setbacks in the recovery of Early Triassic ammonoids. However, these two diversity crises do not necessarily imply identical environmental triggers that ultimately led to anoxic bottom waters on outer continental platforms in both cases.

Smithian ammonoid faunas from Utah: implications for Early Triassic biostratigraphy, correlation and basinal paleogeography

Intensive sampling of the lower portion of the Thaynes and Moenkopi Groups (Lower Triassic) at separate localities within the Confusion Range, Pahvant Range, Mineral Mountains, Star Range, Kanarraville, Cedar City, Torrey and San Rafael Swell areas (mainly central and southern Utah, USA) leads to the recognition of a new key regional Smithian ammonoid succession. The new biostratigraphical sequence, which is more precise than the long-recognized Meekoceras gracilitatis and Anasibirites kingianus Zones, comprises twelve subdivisions, thus resulting in a sequence with much higher resolution that can be correlated not only with other western USA sites, but also with major worldwide localities as well. Middle and late Smithian faunas contain many taxa with wide geographic distribution, thus enabling long-distance correlation with faunal successions from other regions (e.g., British Columbia, Canadian Arctic, South China, Spiti and Oman). New assemblages from the lowermost beds are the least diversified and poorest preserved; they represent the earliest early/middle Smithian ammonoid faunas reported from the western North American basin. They highlight (a) the sudden Smithian advancement of the marine transgression within this epicontinental sea, (b) that this event is diachronous, and (c) that the paleotopography of the basin most likely was highly irregular. The newly obtained ammonoid succession also allows us to date and follow the transgression from the northern and central part of the basin to the southwesternmost and southeasternmost parts, which were reached during the late Smithian (Anasibirites kingianus beds). In addition, we briefly discuss the now-limited previous regional biozonation in the light of these new results. One new genus (Minersvillites) and nine new species (Kashmirites utahensis, Kashmirites confusionensis, Kashmirites stepheni, ?Xiaoqiaoceras americanum, Minersvillites farai, Inyoites beaverensis, Meekoceras olivieri, Meekoceras millardense, Vercherites undulatus) are also described.

Pennsylvanian (Desmoinesian–Virgilian) ammonoid zonation for Midcontinent North America

The most complete, abundant and diverse Middle–Upper Pennsylvanian ammonoid succession in the world occurs in the North American Midcontinent, where more than 40,000 ammonoids representing 52 genera from at least 42 successive stratigraphic levels have been recovered. The Desmoinesian–Virgilian succession comprises a series of seven substage-scale ammonoid genus zones, in ascending order: Wellerites, Eothalassoceras, Pennoceras, Preshumardites, Pseudaktubites, Shumardites and Subperrinites (new), which are subdivided into 14 ammonoid species- or genus-based subzones. The base of the Eothalassoceras Zone is near the current base of the global Kasimovian Stage, the base of the Pennoceras Zone coincides with the base of the regional Missourian Stage (and likely will be close to the base of a revised Kasimovian Stage when its official base is formally selected), the base of the Pseudaktubites stainbrooki Subzone nearly coincides with the base of the regional Virgilian Stage, and the base of the Shumardites Zone coincides with the base of the global Gzhelian Stage, illustrating the value of this zonation for regional and global correlation.

Lower Aptian ammonite biostratigraphy and potential for further studies of OAE 1a in Bulgaria

Sediments of Early Aptian age in Bulgaria can be assigned to four different facies: platform carbonates (Urgonian complex), shallow-water siliciclastics, hemipelagic and flyschoid siliciclastics. The taxonomic analysis of the ammonite faunas of 18 sections from these four different facies resulted in a revision of the existing ammonite zonation scheme so far applied in Bulgaria and adjoining areas. A new biostratigraphic scheme, which bridges the western and eastern Tethys, is thereby proposed for the Lower Aptian of Bulgaria.The Upper Barremian Martelites sarasini Zone is characterized in its upper part by the Pseudocrioceras waagenoides Subzone in the shallow-water sections and by a horizon with Turkmeniceras turkmenicum in the deep-water settings. The Upper Barremian/Lower Aptian boundary is fixed by the first appearance of Paradeshayesites oglanlensis. For the Lower Aptian the following ammonite zones were established (from bottom to top): The Paradeshayesites oglanlensis Zone, the Deshayesites forbesi Zone (= formerly Paradeshayesites weissi Zone) including the Roloboceras hambrovi Subzone in the upper part, the Deshayesites deshayesi Zone including the Paradeshayesites grandis Subzone in the upper part and the Dufrenoyia furcata Zone. The Lower–Middle Aptian boundary has been defined by the appearance of species belonging to the genera Epicheloniceras and Colombiceras.The Lower Aptian ammonite faunas of Bulgaria, allow an interregional correlation with other areas of the Tethyan Realm. The presence of Turkmeniceras in the Upper Barremian enables a correlation with the Transcaspian region, whereas Roloboceras, Koeneniceras and Volgoceratoides found in the middle part of the Lower Aptian are more typical representatives of the ammonite faunas in northern Europe (England, Germany, Volga region).The analysis of the ammonite successions in combination with sedimentological observations enable us to conclude that the marls and marly limestones of the Lower Aptian studied here also cover the interval of the Oceanic Anoxic Event 1a. An interval of thin-laminated clays, rich in organic matter, was identified in the upper part of the D. forbesi Zone (Roloboceras hambrovi Subzone). This interval is characterized by a total lack of benthic faunas.

The Pliensbachian ammonites of the Algarve Basin (Portugal) and their palaeobiogeographical significance for the “Iberia-Newfoundland” conjugate margins

A review of the Pliensbachian ammonite faunas of the Algarve Basin is given covering their taxonomy, stratigraphy and palaeobiogeography (palaeobiodiversity). This review is based on both an extensive revision of the previously collected and/or published material and on new finds precisely located within the ammonite succession. This new material allows us to clarify the variations within the most abundant species—a new species Reynesocoeloceras elmii nov. sp. is described—and to improve our understanding (palaeobiodiversity, palaeobiogeography and bio- or chrono-stratigraphy) of the two single known Pliensbachian fossiliferous assemblages in the Algarve Basin. The first of these assemblages is relatively diverse and is ascribed to the upper half of the Luridum Subchronozone. The second assemblage is less diversified and probably partly condensed. It is broadly attributed to the upper half of the Stokesi Subchronozone. Despite the newly collected ammonites and extensive prospecting, our knowledge of the Pliensbachian ammonite faunas of the Algarve remains fragmentary. Therefore, it is difficult to propose an incontrovertible extensive palaeobiogeographical interpretation for these faunas, but it seems that Tethyan (Mediterranean) affinities were of major importance whereas there were probably no NW European influences via the Lusitanian Basin.

Smithian (Early Triassic) ammonoids from Tulong, South Tibet

Intensive sampling of the Tulong Formation in South Tibet has facilitated the construction of a highly-resolved middle and late Smithian ammonoid succession. The new biostratigraphical sequence comprises the middle Smithian Brayardites compressus beds, Nammalites pilatoides beds, and the Nyalamites angustecostatus beds followed by the late Smithian Wasatchites distractus beds and Glyptophiceras sinuatum beds. This faunal succession correlates very well with that of other Tethyan sequences such as the Salt Range (Pakistan), Spiti (India), Oman and South China. The Smithian faunal sequence from Tulong contains several taxa with broad geographic distribution (e.g., Owenites, Paranannites spathi, Shigetaceras, Wasatchites), thus enabling correlation with faunal successions from areas outside the Tethys (e.g., USA, British Columbia, Arctic Canada, South Primorye, Siberia). Early Smithian ammonoid faunas are almost absent in Tulong because of a preservation bias (absence of carbonate rocks). Five new ammonoid genera ( Brayardites, Nammalites, Nyalamites, Shigetaceras, Tulongites) and six new species ( Brayardites crassus, Brayardites compressus, Prionites involutus, ? Subflemingites compressus, Tulongites xiaoqiaoi, Urdyceras tulongensis) are described.

Ammonoid succession across the Wuchiapingian/Changhsingian boundary of the northern Penglaitan Section in the Laibin area, Guangxi, South China

AbstractA detailed examination has been made of the Lopingian (Late Permian) ammonoid succession across the Wuchiapingian/Changhsingian boundary of the northern Penglaitan Section in the Laibin area, South China. Five ammonoid interval zones were identified via bed‐by‐bed collection: the Prototoceras–Konglingites, Etoushanoceras–Jinjiangoceras, Shevyrevites, Penglaites and Pleuronodoceras–Rotodiscoceras zones, in ascending order. The lower three zones, located in the upper part of the Heshan Formation, are all of Wuchiapingian age in terms of the associated conodonts, whereas the Penglaites Zone, which crosses the lithologic boundary between the Heshan and Talung formations, and the uppermost zone in the lower part of the Talung Formation are of Changhsingian age. Ammonoid faunal turnover, from otoceratoid‐dominated to Xenodiscoid‐dominated faunas, occurred between the Etoushanoceras–Jinjiangoceras and Shevyrevites zones. This horizon is slightly lower than the Wuchiapingian/Changhsingian boundary, which is located between the Shevyrevites Zone and the Penglaites Zone of the northern Penglaitan Section in the Laibin area, South China. Copyright © 2010 John Wiley & Sons, Ltd.

Direct correlation of radiolarian Kilinora spiralis Zone with the Late Jurassic ammonite faunal succession in the Kurisaka Formation, Kurosegawa Terrane, SW Japan

The last appearance datum of the radiolarian Kilinora spiralis is recorded above the first appearance datum of the ammonite Ataxioceras (A.) kurisakense in the Todoro Section of the Kurisaka Formation, Southern Kurosegawa Terrane, Shikoku, SW Japan. The constraint by ammonite age prolongs the range of the Kilinora spiralis Zone, a remarkable Jurassic radiolarian zone in Japan-NW Pacific region, into the lower Kimmeridgian. The direct correlation of the Kilinora spiralis zone with the Late Jurassic ammonite faunal succession in the Kurisaka Formation will provide a clue to the still pending chronological difference between European and North American radiolarian zones.

Numerical age calibration of the Albian/Cenomanian boundary

New biostratigraphic and sequence stratigraphic correlations of the U.S. Western Interior Albian/Cenomanian sections with North Texas and European sections calibrate the Albian/Cenomanian boundary at 97.2 Ma, which supports previous correlations in the Western Interior. This age, however, is 2.4 myr younger than the recent calibration at 99.6 Ma derived from radiometric ages of volcanic beds with diagnostic marine fossils in northern Japan. This discrepancy suggests that additional analyses are required to test the age calibration of the geologic time scale. New data of cosmopolitan dinoflagellates enable direct correlation with the European reference sections of the Albian and Cenomanian. Previous correlations in the Western Interior were based on endemic or Boreal ammonites. The first and last occurrences of dinoflagellate species bracket the Albian/Cenomanian boundary in Europe and many of these species also bracket the Clay Spur Bentonite bed previously dated 97.17±0.69 Ma. The high number of radiometrically dated bentonites in the Western Interior provides a numeric age control for the ranges of many dinoflagellates. Sequence stratigraphic correlation of the North Texas section with the U.S. Western Interior sections also demonstrates that the Clay Spur Bentonite bed correlates with the Ablian/Cenomanian boundary in Texas defined by the ammonite succession. The flooding contact SB WA6 in Texas at the Albian/Cenomanian zonal boundary of Stoliczkaia dispar and Mantelliceras mantelli correlates into the Western Interior with an erosional sequence boundary SB4 at the base of the Romeroville Sandstone in New Mexico and the Dakota Sandstone in Colorado. This contact correlates with the base of the Belle Fourche Shale that overlies the Clay Spur and the Mowry Shale in Montana. The revised correlation of the Western Interior poses two questions. 1) What is the accurate age of the boundary? Both cannot be correct. 2) What were the durations of the Albian and Cenomanian stages? Cyclostratigraphic analyses estimate that the Albian was 11.9 to 11.6 myr in duration but the Western Interior data project the duration at about 15 myr.