Turonian Boundary Interval Research Articles

Chemostratigraphy of the lower Danubian Cretaceous Group (Cenomanian–lower Turonian, Bavaria, SE Germany)—A new carbon isotope reference curve and inter-basinal correlation

Core section KB 06-2020, drilled north of Regensburg (SE Germany), covering the lower Danubian Cretaceous Group, has been logged in detail and sampled for chemostratigraphic investigation throughout the Cenomanian–lower Turonian. Resting unconformably on Upper Jurassic carbonates, the succession includes the Regensburg (lower Cenomanian–middle upper Cenomanian), the Eibrunn (Cenomanian–Turonian boundary interval), the Winzerberg (lower Turonian; Reinhausen and Knollensand members) and the Kagerhöh formations (middle Turonian, Eisbuckel and Pulverturm members only). The correlation of carbon stable-isotope trends from core section KB 06-2020 with the expanded Dover section (southern England) and the Bad Abbach section (south of Regensburg) provides evidence for a diachronous, NE-directed onlap of the Cretaceous strata onto the Bohemian Massif. Several Cenomanian carbon isotope events have been recognised (Mid-dixoni, P/B Break, Monument as well as the b1 and b2 peaks) while the selective absence of events above conspicuous sedimentary unconformities provides evidence for stratigraphic gaps associated with sea-level falls. A significant positive carbon isotope excursion in the Cenomanian–Turonian boundary interval, Oceanic Anoxic Event 2 (OAE 2), has been recorded in the Eibrunn Formation. Correlation with the European reference section from Eastbourne (southern England), Pont d'Issole (southern France), and the regional Grub section provides evidence of a rather complete development of OAE 2 in the Danubian Cretaceous Basin. The δ13C curve obtained from core section KB 06-2020 is proposed as a regional reference curve. Additionally, the Plenus Cold Event (PCE) is recognisable in the oxygen stable-isotope data.

The Cenomanian–Turonian boundary event in Egypt: Foraminiferal turnover and carbon isotopic change

This study deals with the integration between geochemical data (carbon isotope and, total organic carbon) and foraminiferal turnover to track the Oceanic Anoxic Event 2 (OAE2) from the OCT-A8st1 well, which located in October field, central part of Gulf of Suez, Egypt. The studied well is subdivided into two formations, namely the Galala (Early–Late Cenomanian) and Abu Qada (Late Cenomanian–Early Turonian). Concerning the foraminiferal assemblage, agglutinated foraminifera dominate the Galala Formation, while planktonic taxa only occur within a few levels in the Abu Qada Formation. Due to the absence of keeled planktonic forms, the biostratigraphic framework is established on the integration between benthonic and planktonic foraminifera. Three biozones are recognized, including Thomasinella aegyptia–Merlingina cretacea Interval Zone (Early–Middle Cenomanian), Thomasinella fragmentaria–Praebulimina aspera Interval Zone (Middle–Late Cenomanian), and Whiteinella archaeocretacea Taxon Range Zone (Late Cenomanian–Early Turonian). Therefore, the Cenomanian–Turonian boundary interval (CTBI) is placed within the lower part of the Abu Qada Formation, which is characterized by the presence of black shales and low richness of both foraminifera and ostracods. A remarkable increase in TOC values is observed close to the CTBI. Also, the carbon isotope curve shows a distinctive positive δ13C anomaly at the same level of the maximum TOC. The comparison between the recorded δ13C data and those obtained from other sections worldwide, indicates that the base of the Turonian is placed directly above the δ13C excursion. Furthermore, the recorded foraminiferal community shows evidence for the deepening of the environment and extension of the oxygen minimum zone during the CTBI. This conclusion is based on a sequence of bioevents starting with the disappearance of some agglutinated benthic foraminifera, followed by the dominance of opportunistic groups including buliminids, whiteinellids and heterohelicids. Consequently, the studied well may be considered as a local reference section for OAE2.

An Albian–Turonian shallow-marine carbonate succession of the Bey Dağları (Western Taurides, Turkey): biostratigraphy and a new benthic foraminifera Fleuryana gediki sp. nov.

The studied Cretaceous succession is exposed at the Toçak Mountain in the southeastern part of the Bey Dağları Carbonate Platform (BDCP). The Alakır outcrop section presents seemingly continuous shallow-marine carbonate sedimentation during the Albian–Turonian times which is known from very few parts of the peri-Mediterranean platforms. Approximately 550 m thick platform carbonate succession is unconformably overlain by carbonate breccia/conglomerate and pelagic limestones of Campanian age. A benthic foraminiferal biostratigraphic zonation scheme is presented. The biozones are tentatively placed into the stages, without chronostratigraphic calibration, based on stratigraphic distribution of common benthic foraminifera in the peri-Mediterranean platforms. Protochrysalidina elongata–Cuneolina pavonia assemblage zone and Coskinolinella bariensis taxon range subzone are assigned to the upper Albian. In the conformably overlying limestones, Sellialveolina gr. viallii taxon range zone (lower–middle Cenomanian), Pseudorhipidionina casertana assemblage zone (upper Cenomanian), Pseudorhapydionina dubia–Pseudolituonella reicheli assemblage zone (uppermost Cenomanian) and Pseudocyclammina sphaeroidea assemblage zone (Turonian) have been distinguished. Two-step pattern of extinction of benthic foraminifera across the Cenomanian–Turonian boundary interval, which was first recorded from the Apennine Carbonate Platform, has been documented also from the BDCP. The tentative Cenomanian–Turonian Boundary is constrained into an interval of one meter between the last occurrence of the Cenomanian larger foraminifera and the first occurrence of the Turonian benthic foraminiferal assemblage comprising morphologically-simple and less-known taxa and other taxa left in open nomenclature. Fleuryana gediki sp. nov. is described from the Turonian.

Ichnological analysis of the Cenomanian–Turonian boundary interval in a collapsing slope setting: A case from the Rio Fardes section, southern Spain

The Cenomanian–Turonian Boundary interval (CTB) at the Rio Fardes section (Betic Cordillera, southern Spain) is characterized by pelagic and hemipelagic deposition mixed with mass-flow conglomerates and sandstones. The Rio Fardes section lends itself to high-resolution ichnological analysis of the late Cenomanian Oceanic Anoxic Event (OAE-2) on a collapsing slope affected by gravity-flow sedimentation. The trace fossils include Alcyonidiopsis isp. A., Alcyonidiopsis isp. B., Chondrites intricatus, C. stellaris, C. targionii, Planolites isp., ?Pilichnus isp., Thalassinoides isp., and Trichichnus isp. This ichnoassemblage differs from most CTB sections given the presence of small Alcyonidiopsis isp., Chondrites stellaris, and ?Pilichnus isp., and the absence of Zoophycos. The particular depositional setting, affected by coarse clastic gravity flow sedimentation, may be responsible for these differences by affecting palaeoenvironmental conditions. The lower part of the Rio Fardes section shows a more or less continuous record of diverse and abundant trace fossils, generally produced under aerobic/low dysaerobic conditions. The middle part is characterized by a diminution in trace fossil diversity and abundance, related to moderately/highly? dysaerobic conditions disrupted by short intervals of anoxia. Both the lower and middle parts represent pre-Bonarelli event conditions. The upper part reflects the most significant change in ichnological characteristics. The absence of discernible trace fossils caused by decreasing oxygenation corresponds to the Bonarelli Level. In comparison with previously studied CTB sections, the lower number and thickness of anoxic layers in the Rio Fardes section point to generally better oxygenation on the sea-floor. A variable influence of factors limiting the macrobenthic trace maker community during the OAE-2 can be related to depositional or palaeogeographic contexts. Oxygenated water in the Rio Fardes section was delivered from shallower areas along with gravity-flow deposits. The usually bioturbated background pelagic or hemipelagic deposits above the coarse gravity-flow deposits suggest rapid re-establishment of the macrobenthic trace maker communities following each mass depositional event.

Astronomical pacing of relative sea level during Oceanic Anoxic Event 2: Preliminary studies of the expanded SH#1 Core, Utah, USA

AbstractProximal marine strata of the North American Western Interior Basin (WIB) preserve a rich record of biotic turnover during Oceanic Anoxic Event 2 (OAE2; ca. 94 Ma), a pronounced Late Cretaceous carbon cycle perturbation interpreted to reflect global warming, widespread hypoxia, and possible ocean acidification. To develop a more robust synthesis of paleobiologic and geochemical data sets spanning this Earth-life transition, we drilled the 131 m Smoky Hollow #1 Core (SH#1), on the Kaiparowits Plateau of southern Utah, USA, recovering the Cenomanian–Turonian Boundary (CTB) interval in the Tropic Shale Formation. A 17.5 m positive excursion in high-resolution bulk carbon isotope chemostratigraphy (δ13Corg) of SH#1 characterizes the most expanded OAE2 record recovered from the mid-latitudes of the WIB.Depleted values in a paired carbonate carbon isotope (δ13Ccarb) chemostratigraphy cyclically punctuate the OAE2 excursion. These depletions correspond to intervals in the core with a higher degree of carbonate diagenesis and correlate well to an existing sequence stratigraphic framework of flooding surfaces in the shoreface facies of the Markagunt Plateau (∼100 km west). We detect statistically significant evidence for astronomical cycles in the δ13Ccarb data set, imparted by diagenesis at flooding surfaces, and develop a floating astronomical time scale (ATS) for the study interval. Stable eccentricity cycles (405 k.y.) align with stratigraphic sequences and associated trends in sedimentation rate, and short eccentricity cycles (∼100 k.y.) pace nested parasequences. These results confirm an astronomical signal and, therefore, climatic forcing of relative sea level during OAE2 in the WIB. Furthermore, cross-basin correlation of the ATS and expanded δ13C chemostratigraphy of SH#1 suggests that these transgressive-regressive parasequences modulated siliciclastic sediment delivery in the seaway and contributed to deposition of prominent rhythmically bedded CTB units across the WIB, including the Bridge Creek Limestone. The presented approach to analysis of these proximal offshore siliciclastic facies links early diagenetic influences on chemostratigraphy to astronomically modulated sequence stratigraphic horizons, and helps to resolve rates of paleobiologic and paleoenvironmental change during a significant Mesozoic carbon cycle perturbation.

A new marine palynomorph from the Turonian (Upper Cretaceous) in the USA

This paper documents a previously undescribed marine palynomorph, Doricodinium obscurum gen. et sp. nov., from Upper Cretaceous sections in the central and southern Western Interior of the USA. Although an archaeopyle has not been confirmed, a case is made here for a probable dinoflagellate cyst, possible peridinioid affinity. The type locality of D. obscurum is the Global boundary Stratotype Section and Point (GSSP) for the base of Turonian Stage, at Pueblo, Colorado, where it consistently occurs in intra-Lower to middle Turonian deposits. The taxon also occurs consistently in intra-Lower Turonian strata from the Iona-1 core, SW Texas, where astronomical age modelling gives an age range of 93.92–93.25 ± 0.12 Ma in that section. Outside the USA, it has until now only been recorded from the Cenomanian–Turonian boundary interval in Pakistan.

Age and synchronicity of planktonic foraminiferal bioevents across the Cenomanian – Turonian boundary interval (Late Cretaceous)

Age and synchronicity of planktonic foraminiferal bioevents across the Cenomanian – Turonian boundary interval (Late Cretaceous)

Ammonites and the mid-Cretaceous saga

Ever since the introduction of the Cretaceous System, nearly two centuries ago, the terms ‘mid-Cretaceous’ and ‘middle Cretaceous’ keep appearing in the literature, implying a three-fold division rather than the more generally accepted two-fold division of the system/period. The frequent and persistent use of these informal terms proves that they fulfil a need in stratigraphic communication; consequently, formalisation of a middle series/epoch of the Cretaceous System/Period is justified and desirable and will put an end to the present terminological confusion with respect to division of the Cretaceous. According to long-established practice, biotic turnover events (major extinctions and/or radiation events) of biostratigraphically significant fossil groups provide convenient, correlatable horizons for positioning chronostratigraphic boundaries. As a first step towards establishing a solid database for a formal proposal for a three-fold division of the Cretaceous, taxonomic turnover events at the level of family- and genus-group taxa (families/subfamilies and genera/subgenera) for the historically most important fossil group, the ammonites, have been analysed for the broad ‘mid-Cretaceous’ interval, ranging from base Barremian to base Santonian. The results provide little support for a series boundary at the traditional Lower–Upper Cretaceous boundary at the base of the Cenomanian. Instead, the most significant ammonite turnover event occurred at the Aptian–Albian boundary interval, which then emerges as a potential lower boundary of a middle Cretaceous series. Higher in the Cretaceous, no similarly distinctive turnover event is recorded for the ammonites, although the turnover at the Cenomanian–Turonian boundary interval is slightly more pronounced. For a formal proposal for a three-fold division of the Cretaceous, the ammonite data need to be integrated with data on other chronostratigraphically significant fossil groups, such as inoceramid bivalves, foraminifers, calcareous nannofossils and dinoflagellates, and with magnetostratigraphy, chemostratigraphy and cyclostratigraphy.

A Southern Hemisphere record of global trace‐metal drawdown and orbital modulation of organic‐matter burial across the Cenomanian–Turonian boundary (Ocean Drilling Program Site 1138, Kerguelen Plateau)

AbstractDespite its assumed global nature, there are very few detailed stratigraphic records of the late Cenomanian to the early Turonian Oceanic Anoxic Event 2 from the Southern Hemisphere. A highly resolved record of environmental changes across the Cenomanian–Turonian boundary interval is presented from Ocean Drilling Program Site 1138 on the central Kerguelen Plateau (southern Indian Ocean). The new data lead to three key observations. Firstly, detailed biostratigraphy and chemostratigraphy indicate that the record of Oceanic Anoxic Event 2 is not complete, with a hiatus spanning the onset of the event. A decrease in glauconite and highly weathered clays after the onset of Oceanic Anoxic Event 2 marks the end of the hiatus interval, which can be explained by a relative sea‐level rise that increased sediment accommodation space on the Kerguelen Plateau margin. This change in depositional environment controlled the timing of the delayed peak in organic‐matter burial during Oceanic Anoxic Event 2 at Site 1138 compared with other Oceanic Anoxic Event 2 locations worldwide. A second key observation is the presence of cyclic fluctuations in the quantity and composition of organic matter being buried on the central Kerguelen Plateau throughout the latter stages of Oceanic Anoxic Event 2 and the early Turonian. A close correspondence between organic matter, sedimentary elemental compositions and sediments recording sea‐floor oxygenation suggests that the cycles were mainly productivity‐driven phenomena. Available age‐control points constrain the periodicity of the coupled changes in sedimentary parameters to ca 20 to 70 ka, suggesting a link between carbon burial and astronomically forced climatic variations (precession or obliquity) in the Southern Hemisphere mid‐latitudes both during, and after, Oceanic Anoxic Event 2: fluctuations that were superimposed on the impact of global‐scale processes. Finally, trace‐metal data from the black‐shale unit at Site 1138 provide the first evidence from outside of the proto‐North Atlantic region for a global drawdown of seawater trace‐metal (Mo) inventories during Oceanic Anoxic Event 2.

A new proxy for Cretaceous paleoceanographic and paleoclimatic reconstructions: Coiling direction changes in the planktonic foraminifera Muricohedbergella delrioensis

Among some modern and recent fossil species of planktonic foraminifera, the proportion of left- to right-coiled shells in a population appears to be temperature-dependent; the relative abundance of each morphotype reflecting ecological preferences. A similar relationship is identified among Muricohedbergella delrioensis (Carsey, 1926) at the Cenomanian–Turonian stage boundary in mid-latitude sites of the Western Interior Seaway, including the Pueblo type section. The increase of sinistral M. delrioensis in the assemblage is related to higher δ18O carb values and decrease in inner porosity which suggests that changes in the coiling direction in this morphospecies could represent a new proxy for constraining Sea Surface Temperature (SST) variations. As surface dwellers, muricohedbergellids were not affected by Oceanic Anoxic Event (OAE2) and provide a continuous paleoclimatic signal throughout the Cenomanian–Turonian boundary interval. Furthermore, genetic evidence obtained from extant foraminifera indicates that shifts of coiling ratios in planktonic foraminifera species can express the signature of distinct genetic types, which are revealed through their opposite coiling directions. Coiling direction could be a genetic trait, implying that cryptic species may occur in the Mesozoic.

Planktonic foraminiferal biostratigraphy and assemblage composition across the Cenomanian–Turonian boundary interval at Clot Chevalier (Vocontian Basin, SE France)

The Cenomanian–Turonian boundary interval is generally considered a critical time for planktonic foraminifera due to the environmental perturbations associated with Oceanic Anoxic Event 2. However, only the rotaliporids became extinct at the onset of the event, whilst several lineages evolved and/or diversified. This remarkable morphologic plasticity is often overlooked in the literature, partly because a number of stratigraphic sections have only been studied in thin-section due to the degree of lithification of the samples. Improved documentation of the morphological variability of planktonic foraminifera and better defined species concepts are required in order to improve biostratigraphy, particularly as Helvetoglobotruncana helvetica is an unreliable marker for the base of the Turonian. At the same time, detailed study of the planktonic foraminiferal response to OAE 2 demands a more profound knowledge of the assemblage composition.We present new biostratigraphic, taxonomic, and quantitative data for planktonic foraminiferal species from the Clot Chevalier section (Vocontian Basin, SE France), with the aim of (1) providing a detailed biostratigraphic analysis of the section, (2) documenting the morphological plasticity of specimens in this time interval and stabilizing species concepts, and (3) identifying promising markers to improve the resolution of the present biozonation and allow regional correlation. Samples were processed with acetic acid to extract isolated planktonic foraminifera. Assemblages were assigned to the upper Cenomanian Rotalipora cushmani Zone and to the uppermost Cenomanian–lowermost Turonian Whiteinella archaeocretacea Zone. Planktonic foraminiferal bioevents and assemblage composition identified at Clot Chevalier are compared with the well-studied Pont d'Issole section located ca. 15 km to the NE, highlighting similarities and differences in the species occurrences that may complicate the stratigraphic correlation between the two sections.The results of our study support the validity and common occurrence of species that have been misidentified and/or overlooked in the literature (i.e., Dicarinella roddai, Praeglobotruncana oraviensis, Marginotruncana caronae) and indicate that primitive marginotruncanids evolved before the onset of OAE 2, although species diversification occurred only after the event. Moreover, we believe that the first appearance of P. oraviensis might represent a promising bioevent for approximating the Cenomanian/Turonian boundary, after calibration with bio- and chemostratigraphically well-constrained sections. Finally, we describe three new trochospiral species, named “Pseudoclavihedbergella” chevaliensis, Praeglobotruncana pseudoalgeriana and Praeglobotruncana clotensis.

Pacific 187Os/188Os isotope chemistry and U–Pb geochronology: Synchroneity of global Os isotope change across OAE 2

Studies of OAE 2 sections beyond the Atlantic Ocean, Western Interior Seaway (WIS) and European pelagic shelf are limited. Here, we present initial osmium isotope stratigraphy (187Os/188Os–Osi) from two proto-Pacific sites that span the Cenomanian–Turonian boundary interval (CTBI): the Yezo Group (YG) section, Hokkaido, Japan, and the Great Valley Sequence (GVS), California, USA; to evaluate the 187Os/188Os seawater chemistry of the proto-Pacific. Additionally we combine new 206Pb/238U zircon CA-ID-TIMS geochronology from five volcanic tuff horizons of the Yezo Group section to test and facilitate inter-basinal integration with the WIS using radio-isotopically constrained age–depth models for both sections, and quantitatively constrain the absolute timing and duration of events across the CTBI.The YG shows an almost identical Osi profile to that of the WIS, and very similar to that of other sites of the proto-Atlantic and European pelagic oceans (Turgeon and Creaser, 2008; Du Vivier et al., 2014). The characteristics of the Osi profile are radiogenic and heterogeneous (∼0.55–0.85) prior to the OAE 2, and synchronous with the inferred OAE 2 onset the Osi abruptly become unradiogenic and remain relatively homogeneous (∼0.20–0.30) before showing a gradual return to more radiogenic Osi(∼0.70) throughout the middle to late OAE 2. A206Pb/238U zircon age of an interbedded tuff (HK017) in the adjacent horizon to the first unradiogenic Osi value constrains the age of the Osi inflection at 94.44±0.14 Ma. This age, including uncertainty, agrees with the interpolated age of the same point in the Osi profile (94.28±0.25 Ma) in the only other dated OAE 2 section, the WIS; indicating a coeval shift in seawater chemistry associated with volcanism at the OAE 2 onset at the levels of temporal resolution (ca. 0.1 Myr). Further, prior to the onset of OAE 2 an enhanced radiogenic inflection in the Osi profile of the YG is correlative, within uncertainty, with a similar trend in the WIS based on the U–Pb age–depth model. The interpolated ages, 94.78±0.12 Ma and 94.66±0.25 Ma for this Osi inflection in the YG and WIS, respectively, indicate that palaeocirculation was sufficient to simultaneously influence transbasinal seawater chemistry. In contrast, the pre-OAE 2 Osi profile for the GVS is disparate to that of the YG and those of the proto-Atlantic and European pelagic shelf locations. We interpret the pre OAE 2 heterogeneous Osi values (0.30–0.95) to record a palaeobasin that was regionally influenced interchangeably by both unradiogenic (hydrothermal flux) and radiogenic (continental flux) Os.We conclude that the Osi profiles from the proto-Pacific sections record both trends that are consistent globally (OAE 2 onset, syn and post OAE 2), but also show regional differences (pre OAE 2) between OAE 2 sections worldwide. As such the Osi profiles coupled with U–Pb geochronology facilitate the correlation of OAE 2 stratigraphy, and demonstrate both regional and global ocean dynamics.

Inter-regional sequence-stratigraphical synthesis of the Plänerkalk, Elbtal and Danubian Cretaceous groups (Germany): Cenomanian–Turonian correlations around the Mid-European Island

Sequence-stratigraphical case studies of lower Upper Cretaceous shelf successions deposited in the periphery of the Mid-European Island (MEI) have been undertaken to precisely date sedimentary unconformities and corresponding depositional sequences, to test their inter-regional correlation, and to relate the observed stratigraphical architectures to a tectonic or eustatic control. In total, ten 3rd-order sequence-bounding unconformities (SB) have been identified in Cenomanian–Turonian sections from the Münsterland (MCB), Lower Saxony (LSB), Saxonian (SCB) and Danubian Cretaceous basins (DCB). SB Ce 1–3 are of early, mid- and late early Cenomanian age, while SB Ce 4 has been recognized in the uppermost middle and SB Ce 5 in the mid-upper Cenomanian strata. SB Tu 1 has been placed in the early–middle Turonian boundary interval and SB Tu 2 is a mid-middle Turonian unconformity. Sequence boundaries SB Tu 3–5 are of earliest late, mid-late and latest Turonian age, respectively. These inter-regional unconformities define depositional sequences DS Ce 1–5, DS Ce–Tu 1 and DS Tu 2–4 that are stacked into a 2nd-order cycle with a major onlap phase during the Cenomanian and early Turonian, culminating in a late middle Turonian 2nd-order maximum flooding. SB Ce 3 and Ce 5 as well as SB Tu 2 and Tu 4 represent major unconformities associated with eurybatic sea-level falls and subsequent rises of significant amplitude based on their stratigraphical and sedimentological signatures. Plate margin deflection or plate buckling are rather unlikely causes of the inferred sea-level changes given the considerable distance (>500 km) between the investigated basins and the long-distance persistence of the unconformities in identical stratigraphical positions. Inversion tectonics, however, influenced sedimentation from the mid-Turonian, increasing subsidence of marginal troughs along basin-bounding future thrust faults. However, careful intra- and inter-basinal correlations allow recognition of these tectonic signals. The present sequence-stratigraphic synthesis is in good agreement with recently published Cretaceous eustatic charts and thus provides a strong case for the inter-regional nature of early Late Cretaceous unconformities and corresponding depositional sequences in Europe, supporting a predominant eustatic control on Cenomanian–Turonian sequence architectures.

Detrital zircon ages and provenance data from the Early–Middle Turonian boundary interval of Amberg (upper Winzerberg Formation, Danubian Cretaceous Group, Bavaria)

Detrital zircon ages and provenance data from the Early–Middle Turonian boundary interval of Amberg (upper Winzerberg Formation, Danubian Cretaceous Group, Bavaria)

Stable-isotope stratigraphy of the Cenomanian–Turonian (Upper Cretaceous) boundary event (CTBE) in Wadi Qena, Eastern Desert, Egypt

A high-resolution δ13C isotope record from Cenomanian–Turonian boundary interval of shallow marine successions in Egypt is presented. The δ13C curves show the typical features of the globally documented Cenomanian–Turonian positive excursion, including three of the main positive isotope peaks defining the Cenomanian–Turonian Boundary Event (CTBE). Based on high-resolution ammonite biostratigraphy, the CTBE started in the study area above the Late Cenomanian Neolobites vibrayeanus Zone within the Galala Formation, directly above the global sequence boundary Cenomanian 5 (SB Ce 5). A stratigraphic gap at that level cuts out the lower a-peak of the CTBE. The Cenomanian–Turonian boundary is located within the upper part of the positive excursion between carbon excursion peaks c and d, coinciding with the boundary between the Late Cenomanian Vascoceras cauvini and the Early Turonian Vascoceratid zones. The CTBE ended up-section of peak d, at the base of the Choffaticeras spp. Zone. The amplitude of the positive δ13C excursion in Egypt is very high (reaching 6.5‰ vs. V-PDB) and largely matches curves of European standard sections and others localities from different basins. Furthermore, the Lower Turonian Holywell Isotope Event, an important marker within the lowermost Turonian, has tentatively been recognized. The positive carbon stable isotope curves presented herein represent the outreach of the oceanic anoxic event (OAE) 2 in shallow-water nearshore sequences.

Marine 187Os/188Os isotope stratigraphy reveals the interaction of volcanism and ocean circulation during Oceanic Anoxic Event 2

High-resolution osmium (Os) isotope stratigraphy across the Cenomanian–Turonian Boundary Interval from 6 sections for four transcontinental settings has produced a record of seawater chemistry that demonstrates regional variability as a function of terrestrial and hydrothermal inputs, revealing the impact of palaeoenvironmental processes. In every section the 187Os/188Os profiles show a comparable trend; radiogenic values in the lead up to Oceanic Anoxic Event 2 (OAE 2); an abrupt unradiogenic trend at the onset of OAE 2; an unradiogenic interval during the first part of OAE 2; and a return to radiogenic values towards the end of the event, above the Cenomanian–Turonian boundary. The unradiogenic trend in 187Os/188Os is synchronous in all sections. Previous work suggests that activity of the Caribbean LIP (Large Igneous Province) was the source of unradiogenic Os across the OAE 2 and possibly an instigator of anoxia in the oceans. Here we assess this hypothesis and consider the influence of activity from other LIPs; such as the High Arctic LIP.A brief shift to high radiogenic 187Os/188Os values occurred in the Western Interior Seaway before the onset of OAE 2. We evaluate this trend and suggest that a combination of factors collectively played critical roles in the initiation of OAE 2; differential input of nutrients from continental and volcanogenic sources, coupled with efficient palaeocirculation of the global ocean and epeiric seas, enhanced productivity due to higher nutrient availability, which permitted penecontemporaneous transport of continental and LIP-derived nutrients to trans-equatorial basins.

Broeckina gassoensis sp. nov., a larger foraminiferal index fossil for the middle Coniacian shallow-water deposits of the Pyrenean Basin (NE Spain)

The Upper Cretaceous shallow-water carbonates of the Pyrenean Basin (NE Spain) host rich and diverse larger foraminiferal associations which witness the recovery of this group of protozoans after the dramatic extinction of the Cenomanian–Turonian boundary interval. In this paper a new, large discoidal porcelaneous foraminifer, Broeckina gassoensis sp. nov., is described from the middle Coniacian shallow-water deposits of the Collada Gassó Formation, in the Bóixols Thrust Sheet. This is the first complex porcelaneous larger foraminifer of the Late Cretaceous global community maturation cycle recorded in the Pyrenean bioprovince. It differs from the late Santonian–early Campanian B. dufrenoyi for its smaller size in A and B generations and the less developed endoskeleton, which shows short septula. Broeckina gassoensis sp. nov. has been widely employed as a stratigraphic marker in the regional geological literature, under the name of “Broeckina”, but its age was so far controversial. Its middle Coniacian age (lowermost part of the Peroniceras tridorsatum ammonite zone), established in this paper by strontium isotope stratigraphy, indicates that it took about 5 My after the Cenomanian–Turonian boundary crisis to re-evolve the complex test architecture of larger foraminifera, which is functional to their relation with photosymbiotic algae and K-strategy.

Cretaceous Oceanic Anoxic Event 2 in the Arobes section, northern Spain: nannofossil fluctuations and isotope events

Abstract The Cenomanian–Turonian boundary interval of the Arobes section, northern Spain, represents the maximum depth of a relatively shallow succession. The investigated section extends within the Rotalipora cushmani and Whiteinella archaeocretacea planktonic foraminiferal zones, and from UC3 up to UC8 nannofloral zones, respectively. The Oceanic Anoxic Event 2 (OAE2) is about 16 m thick and includes a positive δ 13 C excursion, from 3 up to 5.5‰. The first peak of δ 13 C is situated towards the upper part of the Rotalipora cushmani planktonic foraminiferal zone, while the second peak of δ 13 C is situated in the lower part of the Whiteinella archaeocretacea planktonic foraminiferal zone. The plateau, the youngest phase of OAE2, ends slightly above the first occurrence (FO) of the nannofossil Quadrum gartneri . Based on nannofloral fluctuation, an unstable environment is recognized from the last occurrence (LO) of the nannofossil Axopodorhabdus albianus up to the FO of the nannofossil Quadrum gartneri . Mesotrophic, eutrophic and oligotrophic nannofossils have successive peaks throughout the OAE2. In the lower part of OAE2, especially in the trough phase and second build-up, productivity increased. The calcareous dinoflagellate Thoracosphaera spp. peaks up to almost 30% in the lower part of the second build-up phase. The critical nannofloral turnover episode is characterized by impoverished calcareous nannofossil assemblages and temporary disappearances of high-fertility taxa, such as Biscutum constans and Zeugrhabdotus erectus . This shift in nannofloral assemblages starts in the last stages of OAE2; that is, towards the top of the second build-up phase and also covers the main part of the plateau phase of δ 13 C.

The expression of the Cenomanian–Turonian oceanic anoxic event in Tibet

The Gongzha section of Tibet, China is located at the northern margin of the Indian Plate (SE Tethys) and is characterized by hemipelagic grey marls and marly limestones, light grey limestones and silty limestones, but no organic-rich sediments. High-resolution biostratigraphy reveals an expanded Cenomanian–Turonian (CT) boundary interval and the δ13C record includes the main features of the classical positive carbon-isotope excursion that characterizes the CT oceanic anoxic event. The biotic response inferred from the foraminifera suggests that oxic to dysoxic conditions prevailed, except for a short interval marked by peak abundance of Heterohelix that indicates a significantly dysoxic environment during the δ13C “b” peak excursion. The overall decreasing trend in redox-sensitive trace elements (RSTE) during the maximum δ13C excursion confirms the absence of significant longer-lasting anoxia in the Gongzha section. Enrichments in RSTE are linked to phases of increased detrital input. Chemical weathering indices suggest that the upper Cenomanian sediments accumulated under an increasingly hot and humid climate that culminated near the CT boundary. In the early Turonian lower weathering indices suggest a warm, drier climatic regime with reduced continental runoff. Phosphorus mass-accumulation rates show a significant peak at the onset of the positive δ13C excursion, followed by a decrease up to the basal Turonian. This pattern is positively correlated with the long-term decrease in detrital index as also observed in numerous other CT boundary sections (e.g., Eastbourne, Pueblo, and Whadi El Ghaib, Sinaï). Long-term phosphorus accumulation in the Gongzha section is therefore associated with changes in detrital input. The overall decreased detrital input can be explained by the increasingly remote continental sources due to the major transgression at the end of Cenomanian, coupled with changes in continental weathering intensity linked to increasingly more arid climate conditions.

Late Cenomanian–Early Turonian facies development and sea-level changes in the Bodenwöhrer Senke (Danubian Cretaceous Group, Bavaria, Germany)

The Upper Cenomanian–Lower Turonian litho-stratigraphic units of the Danubian Cretaceous Group of the proximal Bodenwohrer Senke (Regensburg, Eibrunn and Winzerberg formations, the latter consisting of a lower Reinhausen Member and an upper Knollensand Member), have been investigated with a focus on facies analysis and sequence stratigraphy. Analyses of litho-, bio-, and microfacies resulted in the recognition of 12 predominantly marine facies types for the Eibrunn and Winzerberg formations. Petrographic and paleontological properties as well as gradual transitions in the sections suggest that their depositional environment was a texturally graded, predominantly siliciclastic, storm-dominated shelf. The muddy–siliceous facies types FT 1–3 have been deposited below the storm wave-base in an outer shelf setting. Mid-shelf deposits are represented by fine- to medium-grained, bioturbated, partly glauconitic sandstones (FT 4–6). Coarse-grained, gravelly and/or shell-bearing sandstones (FT 7–10) developed in the inner shelf zone. Highly immature, arkosic coarse-grained sandstones and conglomerates (FT 11 and 12) characterize an incised, high-gradient braided river system. The Winzerberg Formation with its general coarsening- and thickening-upward trend reflects a regressive cycle culminating in a subaerial unconformity associated with a coarse-grained, gravelly unit of marine to fluvial origin known as the “Hornsand” which is demonstrably diachronous. The overlying Altenkreith Member of the Roding Formation signifies the onset of a new transgressive cycle in the early Middle Turonian. The sequence stratigraphic analysis suggests that the deposition of the Upper Cenomanian and Lower Turonian strata of the Bodenwohrer Senke took place in a single cycle of third-order eustatic sea-level change between the major sequence boundaries SB Ce 5 (mid-Late Cenomanian) and SB Tu 1 (Early–Middle Turonian boundary interval). The southeastern part of the Bodenwohrer Senke was flooded in the mid-Late Cenomanian (Praeactinocamax plenus transgression) and a second transgressive event occurred in the earliest Turonian. In the central and northwestern parts of the Bodenwohrer Senke, however, the initial transgression occurred during the earliest Turonian, related to pre-transgression topography. Thus, the Regensburg and Eibrunn formations are increasingly condensed here and cannot be separated anymore. Following an earliest Turonian maximum flooding, the Lower Turonian Winzerberg Formation filled the available accommodation space, explaining its constant thickness of 35–40 m across the Bodenwohrer Senke and excluding tectonic activity during this interval. Rapid sea-level fall at SB Tu 1 terminated this depositional sequence. This study shows that Late Cenomanian–Early Turonian deposition in the Bodenwohrer Senke was governed by eustatic sea-level changes.