Calcareous Nannofossil Bioevents Research Articles

Age Assignment, Palaeoecology and Sequence Stratigraphic Framework of the Nukhul Formation at Wadi Baba, West-Central Sinai (Egypt)

The Nukhul Formation has been a point of controversy until now due to a lack of index fossils, varied facies modifications, and different time gaps. As a result, the calcareous nannofossils of the exposed Miocene rocks at Wadi Baba, west-central Sinai have been studied quantitatively for the first time. An important sequence of calcareous nannofossil bioevents has been recorded in the Nukhul Formation, including the FOs of Discoaster druggii, Sphenolithus dissimilis, Helicosphaera ampilaperta, and Sphenolithus disbelemnos, which correspond to the Aquitanian age of upper NN1 and NN2 zones. The Nukhul Formation contains four intervals that reflect eutrophic conditions in the nearshore habitats with substantial nutrient input from the terrigenous influx, as shown by the dominance of cool-water taxa, Coccoliths pelagicus and Reticulofenestra minuta. The lithofacies, microfacies types, abundance, dominance, and diversity of nannofossil assemblages classified the studied interval into two depositional sequences with four system tracts. These tracts refer to fluctuations in relative sea level changes in response to the eustatic sea-level curve.

Calcareous nannofossils from the Poggio le Guaine core (Umbria-Marche Basin, central Italy): Biostratigraphy and discussions on the bioevents of the Aptian–Albian interval

The calcareous nannofossil biostratigraphy of the Aptian–Albian interval has been continuously applied and developed along the last decades. The shortage of complete and continuous outcropping marine sections and even marine subsurface sections is one of the challenges of the stratigraphic studies of this interval. Taxonomic ambiguities, diachronism and reworking are constantly pointed as challenging aspects for the calcareous nannofossil biostratigraphic studies of this interval. Amid this scenario, the Poggio le Guaine section of the Umbria-Marche Basin (central Italy) stands out by presenting a complete and undisturbed pelagic to hemipelagic marine sedimentary succession of the Aptian–Albian interval. The present work is aimed to provide a detailed biostratigraphic zonation based on calcareous nannofossils, to calibrate the results to other stratigraphic tools of previous published works and to discuss some important bioevents of the Aptian–Albian interval along the PLG core. The following biozones of the calcareous nannofossil reference zonation for the Aptian–Albian interval were recognized: the Chiastozygus litterarius Zone (NC6 Zone), the Rhagodiscus angustus Zone (NC7 Zone), the Prediscosphaera columnata Zone (NC8∗ and NC8 zones), the Axopodorhabdus albianus Zone (NC9 Zone) and the Eiffellithus turriseiffelii Zone (NC10 Zone). The PLG core encompasses the chronostratigraphic interval from the Barremian/Aptian through the Albian/Cenomanian boundaries. Finally, detailed discussions on some important calcareous nannofossil bioevents of this interval were performed, among them: the first occurrence (FO) of Hayesites irregularis, the FO of Hayesites albiensis, the last occurrence (LO) of Conusphaera rothii, the LO of Micrantholithus hoschulzii/Micrantholithus obtusus, the FO of Rhagodiscus achlyostaurion, the FO of Prediscosphaera columnata, the FO of Tranolithus orionatus and the LOs of Assipetra spp.

Astronomical calibration of the latest Aptian to middle Albian in the South Atlantic Ocean

Carbon cycle perturbations dramatically affected the ocean-climate system during the Aptian–Albian transition. However, the chronology of this interval is still intensively debated. Here we present a comprehensive astronomical calibration of planktonic foraminiferal and calcareous nannofossil bioevents along a complete marine sedimentary succession, comprising the latest Aptian to middle Albian interval at drilling Core SER-03, located in the Sergipe-Alagoas Basin. The SER-03 astrochronology is based on the tuning of a high-resolution gamma ray (GR) record to the long eccentricity (405 kyr) metronome, and is integrated with high-resolution paleomagnetic records. Our results span the interval between ~114.7 and 109.5 Ma, within magnetochron C34n, and our age estimates for biostratigraphic markers agree within one long eccentricity cycle with previous estimates. We also present geochemical records, such as stable carbon isotopes (δ13C), organic carbon and carbonate contents, as well as redox-sensitive elemental ratios, measured on bulk sediments at Core SER-03. Most of these proxy records depict marked changes across the Aptian–Albian boundary, likely characterizing the local expression of Oceanic Anoxic Event 1b (OAE 1b), filling the gap for low-latitude records of this event.

Calcareous nannofossil biostratigraphy of the Eocene succession in West Central Sinai, Egypt

This work aims to put an end to the controversies about the occurrences of the upper Eocene in west-central Sinai by using the calcareous nannofossils. The Eocene calcareous nannofossil zones NP15 to NP18 were identified in Wadi Thal, west-central Sinai. Although the late Eocene Isthmolithus recurvus is recorded here for the first time, the base of NP19-20 zones is not delineated because the first occurrence of this taxon is considered a diachronous bioevent. The Zone NP18 is unconformably overlain by the lower marine Miocene NN2 Zone. Many calcareous nannofossil bioevents such as the last occurrence of Chiasmolithus grandis, the acme beginning of Cribrocentrum erbae, and the last occurrence of Neococcolithes dubius are used to approximate the Priabonian in west-central Sinai. It is recorded in the Tanka and Tayiba formations and probably in the uppermost part of the Thal Formation. Sea shallowness during the late Eocene due to uplifting in west-central Sinai might be responsible for the absence of many microfauna characterizing this stage/age. This suggestion is supported by the increased abundance of the nearshore nannofossil indicators, Lanternithus minutus, Zygrhablithus bijugatus and the shallow water Micrantholithus and Braarudosphaera from the upper part of Thal Formation through the overlain Tanka and Tayiba formations.

Integrated biostratigraphy for the Santonian/Campanian boundary interval, the Kurdistan Region, northeast Iraq

The first high-resolution integrated biostratigraphic study for Santonian/Campanian sediments of the Tabin section in the Kurdistan Region, northeast Iraq is provided. The study, based on 28 closely spaced samples, combines data from planktic foraminifers (25 species), calcareous nannofossils (32 species) and two ammonite genera in the Kometan Formation, marking the Santonian/Campanian boundary (S/C boundary) in the Kurdistan Region. In the absence of the crinoid Marsupites testudinarius, the proposed boundary marker, secondary markers such as calcareous nannofossils, planktic foraminifers and ammonites, have been used to establish a multi-stratigraphic biozonation for the late Santonian–early Campanian duration. Based on the occurrences of calcareous nannofossils, three biozones are identified — Lucianorhabdus cayeuxii (late Santonian), Calculites obscurus (latest Santonian–earliest Campanian), and Broinsonia parca parca (early Campanian). Seven calcareous nannofossil bioevents and three planktic foraminiferal bioevents are also identified. The Santonian/Campanian boundary is marked by: (a) the LO (Last Occurrence) of the planktic foraminifera D. asymetrica, (b) the FOs (First Occurrence) of the calcareous nannofossil species B. parca parca and B. parca constricta, (c) the extinction of several planktic foraminiferal species of Dicarinella and Marginotruncana, (d) the abundance and diversification of the planktic foraminifera genera, Globotruncana and Globotruncanita at the beginning of the Campanian, and (e) the disappearance of the ammonite genus Texanites, 0.5 m below (i.e., at 19 m) the disappearance of all Dicarinella and Marginotruncana species in the study section. Similar to several other Tethyan sections, the FO of B. parca parca is above the LOs of D. concavata and D. asymetrica; the LO of D. asymetrica is used here to mark the S/C boundary

The Berriasian–Valanginian boundary in the Mediterranean Province of the Tethyan Realm: Ammonite and calcareous nannofossil biostratigraphy of the Vergol section (Montbrun-les-Bains, SE France), candidate for the Valanginian GSSP

The Vergol locality (Montbrun-les-Bains, South-East France), a candidate section for the Valanginian GSSP (Global Stratotype Section and Point), was studied in detail for ammonites and calcareous nannofossils. A bed-by-bed sampling and a quantitative approach were carried out on this expanded succession presenting a continuous record of calcareous-marl alternations covering the uppermost Berriasian and lowermost Valanginian. The ammonite distribution observed allows a precise characterization of the Berriasian–Valanginian boundary with the recognition of the Tirnovella alpillensis (latest zone of the Berriasian) and “Thurmanniceras” pertransiens (earliest zone of the Valanginian) standard zones. The base of the Valanginian is here defined by the First Appearance Datum of “T.” pertransiens; this boundary can be well-characterized by ammonite assemblages and by using various calcareous nannofossil bioevents, like the First Occurrence of Calcicalathina oblongata. A tidy comparison with previous works on the Vergol section is made in order to integrate data on calpionellids. A discussion on the choice of the primary marker (ammonites versus calpionellids) for the Valanginian GSSP is proposed.

Upper Paleocene-lower Eocene calcareous nannofossil stratigraphy at the Misheiti Section, Central Sinai, Egypt

Calcareous nannofossil, δ13C, δ18O, and carbonate content data were used to define and reveal the variations across the latest Paleocene-earliest Eocene interval at the Misheiti Section, Central Sinai, Egypt. The upper part of Tarawan Formation, the Esna Formation, and the lowermost Thebes Formation were included in this study. In this study, the Esna Formation is informally divided into Hanadi/Mahmiya and Abu Had members. Four calcareous nannofossil zones (NP9, NP10, NP11, and NP12) and four subzones (NP9a, NP9b, NP10a, and NP10b) were recognized. The biostratigraphic importance of significant calcareous nannofossil bioevents associated with the P/E boundary was discussed. The LOs of Rhomboaster spp., Discoaster araneus, and/or D. anartios were used to delineate the base of earliest Eocene Subzone NP9b within the lower part of the Esna Formation. The global decreases of δ13C, δ18O, and carbonate contents at the P/E boundary were documented in the study section.

A review of basin-wide calcareous nannofossil bioevents in the Mediterranean at the onset of the Messinian salinity crisis

During the Messinian (7.2–5.3 Ma) the Mediterranean area experienced fast and deep climatic and eustatic structural changes. The stratigraphic framework for this interval is relatively well constrained and the beginning of the Messinian salinity crisis (MSC) dated at 5.971 Ma suggests a duration of at least 1.2 Ma for the pre-evaporitic Messinian that is the object of this study. A number of sites (Faneromeni, Pissouri, Polemi Basin, Kalamaki, Falconara, Fanantello, Lemme, Pollenzo, Govone, Moncalvo: Blanc-Valleron et al., 2002; Wade and Bown, 2006, Kouwenhoven et al., 2006; Morigi et al., 2007; Lozar et al., 2010, 2018; Dela Pierre et al., 2011, Karakitsios et al., 2017) show similar behaviour of the calcareous nannofossil record where several peaks of Sphenolithus spp. are recognised at different levels in each of the section. This paper compares the calcareous nannofossil data from six different sections across the Northern and Eastern Mediterranean area encompassing the onset of the MSC. Interestingly, a tight succession of bioevents (Sphenolithus abies, Helicosphaera carteri, Umbilicosphaera rotula, Rhabdosphaera spp.) is recorded in all the sections analysed and appears to correlate precisely among the investigated sites, approximating the onset of the Messinian salinity crisis, thus offering the possibility to use these as bioevents for regional correlation.

Identification of the Oligocene-Miocene boundary in the Central Iran Basin (Qom Formation): calcareous nannofossil evidences

A diverse Late Oligocene to Early Miocene calcareous nannofossil assemblage was examined from the Qom Formation in the Central Iran Basin, and the Oligocene-Miocene boundary was identified based on the quantitative analysis of the assemblages in 303 smear slides. Eleven well-established calcareous nannofossil bio-events are delineated in the Upper Oligocene through Lower Miocene. The results clearly show that the Highest Occurrence (HO) of Sphenolithus delphix is the closest bio-event to the boundary as traditionally delineated on the lithostratigraphic criteria, and provides a distinct biohorizon below it. The Lowest Occurrence (LO) of the species Discoaster druggii is the oldest Miocene bio-event that is observed shortly after the HO of S. delphix , showing that calcareous nannofossils are well suited for approximating the Oligocene-Miocene boundary in the Qom Formation. The Oligocene-Miocene boundary is placed in the upper part of Sub-member “c1” in all three sections studied here and it is traceable throughout the Central Iran Basin, which makes a potentially reliable marker horizon for sequence stratigraphic and hydrocarbon studies in the area.

Barremian-Aptian Calcareous Nannofossil Biostratigraphy in Zagros Basin (west Iran), Tethyan Realm

The calcareous nannofossils from the upper part of Garau Formation were studied in the NW Zagros Basin, western Iran (Kabir-Kuh section). The ca. 510 m thick studied interval is mainly composed of interlayered marls, marly limestones, marly shales, laminated black shales and limestones. The succession of nannofossil bioevents defines the stratigraphic range of the studied part of section spanning the interval from the early Barremian (CC5b/NC5C) to the late Aptian (CC7b/NC7B). Six primary calcareous nannofossil bioevents and eight secondary bioevents are recorded within the studied interval. The comparison of the recorded bioevents with those referred from other parts of the Tethyan and the Boreal realms indicates some diachroneity between these localities.

Upper Hauterivian-upper Barremian Planktonic Foraminiferal Assemblages from the Arroyo Gilico Section (southern Spain)

AbstractLate Hauterivian−earliest late Barremian planktonic foraminifera from the Arroyo Gilico section (Subbetic domain, Betic Cordillera) have been investigated to correlate their distribution to ammonite zonation and calcareous nannofossil bioevents. The 72-m-thick section spans the interval between the upper Hauterivian ammonite Crioceratites binelli Subzone, Crioceratites baleraris Zone and the lowermost upper Barremian ammonite Barrancyloceras barremense Subzone, Toxancyloceras vandenheckii Zone. Planktonic foraminiferal assemblages are rich and relatively diverse. At the base of the section in the C. binelli Subzone, as already known, Hedbergella sigali, H. infracretacea, H. daminiae, and Lilliputianella semielongata are recorded. New findings include common Hedbergella praetrocoidea and H. aptiana along with rare specimens resembling H. excelsa based on their long and elevated spire. Early appearances of taxa continue up-section with the lowest occurrences (LOs) of H. ventriosa, then Lilliputianella eocretacea in the overlying Crioceratites krenkeli Subzone, followed by H. gorbachikae, while the LO of the ‘clavate’ Lilliputianella pauliani occurs just before the end of the Hauterivian within the topmost Pseudothurmannia picteti Subzone. The lower Barremian is punctuated by a number of successive appearances, including the typical Globigerinelloides blowi recorded in the upper lower Barremian Kotetishvilia compressissima Zone. The acme of the ‘clavate’ morphotypes occurs in the Moutoniceras moutonianum Zone across the Mid-Barremian Event (MBE); besides common L. semielongata, the taxa consistently present are L. pauliani with both typical and transitional morphologies, L. eocretacea, and to a minor extent “Globigerinelloides” sigali.

Calcareous nannofossil bioevents at the Paleocene/Eocene boundary in Kharga Oasis, Western Desert of Egypt

Two upper Palaeocene – lower Eocene stratigraphic sequences at the Kharga Oasis (Umm El Ghanayim and Naqb Assiut sections) were studied biostratigraphically on the basis of their calcareous nannofossil content. The investigated interval includes the upper part of the Tarawan Formation, the Tarawan Chalk, and the Esna formations. A total number of sixty-seven different taxa have been identified. The lowest occurrence (LO) of Discoaster araneus was used to place the base of the NP9b Subzone (base of Eocene) at the Gabal Umm El Ghanayim section. The lowest occurrences (LOs) of Rhomboaster bitrifida, Discoaster araneus and D. anartios are used to define the NP9a/NP9b subzonal boundary at the Gabal Naqb Assiut section. In this section,the P/E boundary is marked by a minor lithologic hiatus as indicated by the absence of the basal part of the Dababiya Member.At the studied two sections, a major turnover in calcareous nannofossil assemblages across the P/E transition was documented. The abundance of warm water Ericsonia subpertusa, Fasculithusspp., Coccolithus eopelagicus, Discoaster spp., Rhomboaster bitrifida and Tribrachiatus bramlettei characterize the Palaeocene-Eocene transition and suggest global warming and the Palaeocene – Eocene Thermal Maximum (PETM).

An integrated stratigraphic study across the Santonian/Campanian boundary at Bocieniec, southern Poland: A new boundary stratotype candidate

An integrated biostratigraphic (foraminifera, calcareous nannofossils, crinoids), chemostratigraphic (stable carbon isotopes) and magnetostratigraphic study of the Bocieniec section (southern Poland) is presented here. The section presents a continuous and lithologically monotonous sedimentary record across the Santonian–Campanian boundary transition. A large number of macrofossil, foraminiferal and calcareous nannofossil bioevents along with several well-identified carbon-isotope excursions of the upper Santonian and lowermost Campanian are documented. The base of the Campanian is well-constrained by the last occurrence (LO) of the crinoid Marsupites testudinarius, and correlates to the onset of the first δ13C positive peak of the Santonian–Campanian Boundary Event (SCBE peak a). A presumable primary Cretaceous paleomagnetic signal highlights the potential presence of the C34N/C33R magnetic reversal although its exact position remains uncertain between peaks a and b of the SCBE. The planktic foraminifer Dicarinella asymetrica is very rare at Bocieniec but a potential LO of this important marker may be recorded in coincidence with peak b of the SCBE. The first occurrence (FO) of calcareous nannofossil Broinsonia parca parca coincides with the lower part of chron C33R and with the early Campanian pilula zone event. A large set of additional nannofossil events and benthic foraminifer events further constrain the stratigraphy of the section and along with the carbon isotopes, allows for correlation with other important sections of the Boreal realm. Although the Bocieniec section is relatively thin and condensed (5.5 m), the successive order of events and presence of all past proposed stratigraphic criteria for the Santonian-Campanian boundary makes it the most complete reference section for this interval at the European and at the global scale. Moreover, this section allows for a precise correlation of the Tethyan and Boreal domains. The Bocieniec section fulfils the geological criteria to be a potential boundary stratotype candidate for the base of the Campanian Stage.

High‐resolution integrated calcareous plankton biostratigraphy and magnetostratigraphy at the Oligocene–Miocene transition in Southwestern Atlantic Ocean

After the formalization of the base of the Miocene in the Lemme‐Carrosio section (Italy) at the base of Subchron C6Cn.2n, the calcareous plankton biostratigraphy was refined in several open ocean Deep Sea Drilling Project/Ocean Drilling Program sites. However, high‐resolution quantitative biostratigraphic studies, integrating planktonic foraminifera and calcareous nannofossils, are still lacking for the time interval spanning the Oligocene–Miocene transition. Here, we present a reinvestigation of Deep Sea Drilling Project Hole 516F (Rio Grande Rise) and 4 oil wells drilled by Petróbras Brasileiro SA in the Campos Basin (SW Atlantic Ocean). We identified 12 planktonic foraminiferal and 18 calcareous nannofossil bioevents that have been integrated with an updated magnetostratigraphy of Hole 516F allowing the correlation with the GPTS and the identification of the Oligocene/Miocene boundary (base of Subchron C6Cn.2n) between the Top of Sphenolithus delphix and the Base of common Paragloborotalia kugleri. Furthermore, our results give new insights on the reliability of major calcareous plankton events across the Oligocene–Miocene transition: (a) the Sphenolithus ciperoensis Top, the S. delphix Base and Top, and the Sphenolithus cometa Base are reliable events at a global scale; (b) the Bases of Globoquadrina dehiscens and Sphenolithus disbelemnos > 4 μm are correlatable events only within the study sector of the SW Atlantic Ocean; and (c) the Globoturborotalita ciperoensis Top, Globoturborotalita angulisuturalis Top, and Sphenolithus procerus Base are diachronous. Finally, previously unreported biostratigraphic data, such as the distribution range of S. disbelemnos < 4 μm and Sphenolithus cf. S. pseudoheteromorphus, the Tenuitellinata praestainforthi acme interval, and the Top of common Globigerinoides primordius were identified in the Campos Basin.

A high precision U–Pb radioisotopic age for the Agrio Formation, Neuquén Basin, Argentina: Implications for the chronology of the Hauterivian Stage

A new CA-ID TIMS U–Pb age of 130.39 ± 0.16 Ma is presented here from the Pilmatué Member of the Agrio Formation, lower Hauterivian of the Neuquén Basin in west-central Argentina. This high precision radioisotopic new age, together with the two former ones from the upper Hauterivian Agua de la Mula Member of the Agrio Formation and modern cyclostratigraphic studies in the classical sections of the Mediterranean Province of the Tethys indicate that the Hauterivian Stage spans some 6 Ma, starting ca. 132 Ma and ending ca. 126 Ma. These radioisotopic ages are tied to ammonite biostratigraphy and calcareous nannofossil bioevents and biozones recognized in the Neuquén Basin which in turn are correlated with the Mediterranean standard zones. A new geological time scale for the Valanginian–Hauterivian stages in the Mediterranean region integrating astrochronological and radiochronological data differs with the current official geological time scale which still maintains poorly constraint absolute ages for the Berriasian-Aptian interval.

The Pliocene Mediterranean infilling of the Messinian Erosional Surface: New biostratigraphic data based on calcareous nannofossils (Bajo Segura Basin, SE Spain)

The Bajo Segura Basin (eastern Betic Cordillera) is a Mediterranean marginal basin where the Messinian Erosional Surface (MES), formed during the Messinian Salinity Crisis sea-level fall, is well developed. Overlying this major discontinuity the lower Pliocene transgressive sediments record the reflooding of the Mediterranean and the return to an open marine environment, the continental shelf being rebuilt after the Messinian erosion. The stratigraphic and biostratigraphic study of seven sections allow two transgressive-regressive sequences filling the MES to be distinguished, correlated with the previously distinguished Mediterranean offshore seismic units. Ten calcareous nannofossil bioevents have been identified. The lower sequence can be dated according to nannofossil biozones NN12 to NN14 and the upper sequence by NN15 to NN16. The boundary between both lower Pliocene sedimentary sequences is dated by the First Common Occurrence (FCO) of Discoaster asymmetricus in the uppermost sediment of the lower sequence and the First Occurrence (FO) of Discoaster tamalis in the lowermost part of the upper sequence. This discontinuity can be calibrated at between 4.1 and 4.0 Ma ago.

The mid/late Eocene transition in the Eastern Carpathians (Romania) based on foraminifera and calcareous nannofossil assemblages

Foraminiferal and calcareous nannofossil assemblages from the Eastern Carpathians (northern part of the Tarcau Nappe, Romania) were documented and correlated in order to reconstruct the palaeonvironmental settings and provide a biostratigraphic framework of the Plopu Formation. The benthic foraminiferal assemblages are dominated by flysch-type agglutinated taxa, suggesting a bathyal palaeodepth with frequent oscillations of the carbon compensation depth. The agglutinated foraminifera morphogroup analyses suggest different levels of organic matter influx and oxygenation. Both the foraminifera and calcareous nannofossil assemblages suggest a shift between the warm mid Eocene to the cooler late Eocene climate. Biostratigraphic data based on calcareous nannofossils (NP15–NP19/NNTe8–NNTe12 biozones) helped to establish the age of the formation. Four assemblages of benthic agglutinated foraminifera ( Psammosiphonella cylindrica – Nothia excelsa ; Paratrochamminoides spp. – Trochamminoides spp.; Karrerulina spp.– Reticulophragmium amplectens ; Spiroplectammina spectabilis ) correlated with calcareous nannofossil bioevents supported the placement of the mid to late Eocene transition within the Plopu Formation.

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.

Late Hauterivian–early Barremian calcareous nannofossil biostratigraphy, palaeoceanography, and stable isotope record in the Subbetic domain (southern Spain)

Two sections in the Subbetic domain of the Betic Cordillera (Río Argos and Arroyo Gilico), encompassing the latest Hauterivian–earliest late Barremian interval, and including the Faraoni level equivalent (FLE) and the ‘Mid-Barremian event’ (MBE), were selected for stable isotope analyses and calcareous nannofossil investigation. Six calcareous nannofossil bioevents were identified throughout the study interval, which were used in the identification of NC5B, NC5C, NC5D and NC5E subzones. Calcareous nannofossil bioevents and subzones have been directly correlated to Tethyan ammonite biostratigraphical zonation and with respect to a detailed carbon and oxygen stable isotope stratigraphy. An integrated analysis of calcareous nannofossil assemblages and chemostratigraphic data (carbon and oxygen isotopes) enabled the reconstruction of the surface water conditions throughout the study interval. Calcareous nannofossil assemblages record several episodes of relative eutrophication of surface water that correlate with relative minimum values in the stable oxygen isotope curves. These eutrophication episodes are interpreted as the result of probably warmer and wetter climatic conditions in the continental hinterlands adjacent to the Subbetic Basin, which led to an enhanced runoff and increased freshwater input to the sea. During these episodes, surface waters were enriched in nutrients and probably had a lower salinity, favouring the development of a high nutricline and, occasionally, water stratification and development of localized dysoxic/anoxic bottom sedimentary environments. Most of these episodes are of minor importance, and only two of them (those correlating with FLE and MBE), had sufficient impact on the marine environments to generate deviations in the stable carbon isotope record and localized deposition of organic-rich sediments. This suggests that global dysoxic/anoxic episodes, such as those linked to OAEs, probably do not represent singular events in a normally undisturbed environment, but are part of long-lasting periods of environmental change.

Eocene biogeochronology and magnetostratigraphic revision of ODP Hole 762C, Exmouth Plateau (northwest Australian Shelf)

We present a chronostratigraphic model for the Eocene from ODP Leg 122 Hole 762C (northwest Australian shelf), integrating calcareous nannofossil data with previously published planktonic foraminiferal data, magnetostratigraphy and stable isotopic data. This ~240m thick interval extends from magnetic polarity Chron C25r to Chron C15r, and nannofossil zones CP6/7 to CP16a (NP7/8-NP21). Examination of the calcareous nannofossil biostratigraphy showed several potential hiatuses within this stratigraphic section at Hole 762C. The presence of these hiatuses was supported by cross-correlation of planktonic foraminiferal P-zones, magnetostratigraphic reversals and δ13C and δ18O isotopic excursions. These biostratigraphic and isotopic data were used to re-evaluate the original magnetostratigraphy, and this revised framework was integrated into a new age model. The original depositional model from Leg 122 proposed a continuous section with relatively low sedimentation rates in the Lutetian and basal Ypresian (< 1.0cm/ky); however, our results suggest that sedimentation rates were relatively high but that the section is interrupted by four hiatuses, each ~1-2 myr in duration. This new age model allowed revision of sedimentation rates at Hole 762C. These revised rates are used to estimate the ages of calcareous nannofossil bioevents, which are compared to several additional, globally distributed localities.