Middle Late Cenomanian Research Articles

Palaeoecology and facies analysis of benthic communities in mid-Cretaceous successions in the Sinai platform, Egypt

Within the Aptian-Cenomanian formations of Sinai successions in Egypt, lateral and vertical changes occur in the numerous benthic communities. Microbenthos such as Orbitolina foraminifera and calcareous algae are widespread in the Aptian-Albian deposits in North Sinai, whereas oysters and gastropods are common in the Cenomanian siliciclastic deposits in South Sinai. In order to establish the biostratigraphy, 90 benthic and planktonic foraminifera species have been identified from the studied succession. Based on the stratigraphic distribution of the identified species the studied succession could be subdivided into three planktonic foraminiferal zones: Pseudothalmanninella ticinensis zone (Late Albian), Rotalipora cushmani zone (Middle-Late Cenomanian), and Whiteinella archaeocretacea zone (Late Cenomanian-Early Turonian). This work discusses the first record of the Albian dasycladalean alga Holosporella sugdeni (Elliott) from Sinai and reports on Montiella elitzae (Bakalova) from the Albian Risan Aneiza Formation. The occurrence of Orbitolina (Conicorbitolina) conica distinguished the Albian-Cenomanian boundary in the North Sinai sequences. The benthic communities of the studied successions indicated a faunal shift from an algal-orbitolinid foraminifera community around the Aptian-Early Albian to a rudist community during the Middle Albian and an oyster-gastropod community during the Cenomanian. This faunal shift coincided with the major sea transgression that covered North Sinai until the Middle Albian and reached South Sinai during the Cenomanian. The identified Cenomanian gastropods included Neoptyxis olisiponensis, Nerinea gemmifera, and Tylostoma bussoni. Many oyster species distinguished the Cenomanian deposits, such as Ceratostreon flabellatum, Rhynchostreon mermeti, Rhynchostreon suborbiculatum, and Costagyra olisiponensis. Identifying the benthic fauna and facies indicated a shallow inner to middle ramp environment. Stable carbon and oxygen isotope data from the Cenomanian oyster shells indicate an inner ramp environment. Comparing the carbon isotope values of the oysters with those previously recorded from the rudists may explain the different trophic levels of both groups.

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.

A Cenomanian–Santonian rudist–bearing carbonate platform on the northern Arabian Plate, Turkey: facies and sequence stratigraphy

Recent studies have shown that Cenomanian–Santonian carbonate sedimentary rocks rich in rudists are widespread throughout southeastern Turkey. The Derdere and Karababa formations have been analysed in the Sabunsuyu section (Kilis Province). On a field scale, we can distinguish rudist–rich beds that rhythmically alternate with bioclastic levels composed of bivalves, gastropods and roveacrinids in these formations. Although pelagic faunal elements (predominantly planktic foraminifera and pithonellids) are documented in the lowermost part of the series, the rudist–rich facies are represented mainly in the Derdere and Karababa formations. Rudists, benthic and planktic foraminifera indicate middle–late Cenomanian and Turonian–Santonian ages for the Derdere and Karababa formations, respectively. Based on studied facies, five microfacies have been recognised and categorised in three facies groups: inner ramp, mid ramp and outer ramp. Evidence such as gradual changes in facies and absence of extensive uninterrupted barrier reefs indicate that the Derdere Formation was deposited in a ramp depositional system. The sedimentology and taphonomic signature of the rudist shell beds have been described in order to obtain a better understanding of the depositional environment and the physical processes that controlled Cenomanian–Santonian sedimentation. Monospecific tabular beds characterise mainly the upper part of the series (topmost part of Derdere Formation and Karababa Formation); more complex rudist concentrations, characterised by moderate species diversity, increase upsection. The rudist levels are associated with highstand systems tract deposits because of the suitability of trophic conditions in the rudist–dominated ramp.

Calcareous nannoplankton proxies for palaeoenvironmental reconstruction of the Albian–Cenomanian succession in North-western Israel (Mount Carmel Region)

Abstract We present data on the calcareous nannofossil distribution in the Albian–Cenomanian borehole CT8 obtained from the Carmel area, NW Israel. The nannofloral assemblage consists of 113 species, with Watznaueria spp. dominating the entire succession. The detailed calcareous nannofossil biostratigraphy of this section is established, and seven nannofossil (sub)zones are recognised. For the first time the age of the Tavasim Tuff (V2) is biostratigraphically identified as Late Albian. The quantitative analysis of the calcareous nannofossils suggests warm, open marine or coastal, mainly oligotrophic conditions throughout the entire succession of borehole CT8, with evident fluctuations of temperatures and fertility and a general trend towards cooling in the uppermost part of the sequence (Middle–Late Cenomanian). Temperature and nutrient variations correspond to those in the western Tethys, thus suggesting at least a supra-regional extent of processes detected in the Carmel Region. The biostratigraphy shows no evidence of hiatuses in the studied section, but the absence of positive carbon isotope anomalies associated with the Oceanic Anoxic Event (OAE) 1d and Mid-Cenomanian Event elsewhere suggests that the OAE 1d is possibly missing in the studied section due to a hiatus around the Isfiye Fm./Arqan Fm. boundary.

Planktonic Foraminiferal Biostratigraphy of the Upper part of Balambo Formation (Lower-Upper Cretaceous) in Azmer Anticline, Northeastern Iraq

The present work is a part of detailed study of biostratigraphy of Balambo Formation (Lower - Upper Cretaceous) from Azmer anticline - northeastern Iraq. Lithologically, the upper part of the studied section (Balambo Formation) consists of dark gray, reddish brown shale and marl intercalation with amonitic and marly limestone. Based on planktonic foraminifera, the studied section is divided into five main biozones and two subzones. It is correlated with collection of global regional and local previous biozones. These biozones led to conclude that the age of the upper part of Balambo Formation extends over the Latest Aptian to Middle-Late Cenomanian. Moreover, the identified biozones are from older (at bottom) to younger (at top): 5- Rotalipora cushmani Taxon range Zone. 4- Thalmanninella globotruncanoides Interval Zone. 3- Biticinella breggiensis Interval Zon. Ticinella praeticinensis Interval Subzone. Pseudothalmanninella subticinensis Interval Subzone. 2- Ticinella primula Interval Zone. 1- Paraticinella eubejaouensis Taxon range Zone.یتضمن البحث الحالی د ا رسة طباقیة حیاتیة لمفو ا رمنیف ا ر الطافیة لتکوین بالامبو )الکریتاسی الأسفل الأعمى( فی طیة أزمر شمال شرقی الع ا رق. یتألف ىذا الجزء من تتابعات لطبقات - - الطفل الرصاصی الغامق المون والطفل البنی المحمر وطبقات من المارل متداخمة مع طبقات الحجر الجیری الامونایتی وطبقات الحجر الجیری المارلی. تم تقسیم ىذا المقطع الطباقی الى خمسة أنطقة حیاتیة رئیسة واثنین من الأنطقة الحیاتیة الثانویة. وتمت مضاىاة ىذه الأنطقة مع مجموعة من الأنطقة الطباقیة الحیاتیة العالمیة والإقمیمیة والمحمیة، وبیذا إتضح أن عمر ىذا الجزء من التکوین یمتد من أواخر الأبتیان المتأخر الى السینومینیان الأوسط المتأخر. وىذه الأنطقة من الأقدم )فی الأسفل( الى الأحدث )فی الأعمى(: - 5- Rotalipora cushmani Taxon range Zone. 4- Thalmanninella globotruncanoides Interval Zone. 3- Biticinella breggiensis Interval Zone. . Ticinella praeticinensis Interval Subzone. Pseudothalmanninella subticinensis Interval Subzone. 2- Ticinella primula Interval Zone. 1-Paraticinella eubejaouensis Taxon range Zone.

Cretaceous rudist-bearing platform carbonates from the Lycian Nappes (SW Turkey): Rudist associations and depositional setting

Lycian Nappes (in SW Turkey) lie between the Menderes Massif and Bey Dağları carbonates and comprise thrust sheets (nappes piles) of Paleozoic-Cenozoic rocks, ophiolitic and tectonic mélanges and serpentinized peridodites. This study focuses on identification of rudists and their palaeoenvironmental features observed within the Cretaceous low grade metamorphic successions (dominated by recrystallized limestones) from the Tavas and Bodrum nappes. The study is based on fifteen stratigraphic sections measured from Tavas, Fethiye, Köyceğiz, Bodrum, Ören and Bozburun areas. The Lower Cretaceous successions with rudists are very sparse in the Lycian Nappes and a unique locality including a Berriasian epidiceratid-requieniid assemblage is reported so far. A new requieniid-radiolitid assemblage was found within the pre-Turonian (?Albian-?Cenomanian) limestones. Four different Late Cretaceous rudist assemblages were firstly identified as well: 1) Caprinid-Ichthyosarcolitid assemblage (middle-late Cenomanian); 2) Distefanellid assemblage (late Turonian); 3) Hippuritid-Radiolitid assemblage (late Coniacian-Santonian-Campanian); 4) Radiolitid-Hippuritid assemblage (‘middle’-late Maastrichtian). Microfacies data and field observations indicate that the rudists lived in the inner and outer shelves of the Cretaceous carbonate platform(s) in this critical part of the Neotethys Ocean. Rudists formed isolated patchy aggregations in very shallow palaeoenvironments and deposited as shell fragments particularly on the outer shelf environment, which is characterized by higher energy and platform slope characteristics.

RADIOLARIAN AGE AND LITHOSTRATIGRAPHY OF LATE CRETACEOUS PELAGIC SEDIMENTS OVERLYING BASALTIC EXTRUSIVE ROCKS, NORTHERN OMAN MOUNTAINS

A detailed radiolarian biostratigraphy of pelagic sediments overlying basaltic extrusive lavas of the Oman Ophiolite is reported for the type locality of the Suhaylah Formation near the Wadi Jizzi River, northern Oman Mountains. The extrusive lavas in the studied section are classified as V1 lava produced by mid-ocean-ridge magmatism. The sedimentary succession in the type section (ca. 18 m thick) is subdivided into three units (in stratigraphic order): metalliferous sediments (Unit 1), red mudstone intercalated with radiolarian chert (Unit 2), and micritic limestone (Unit 3). The recovery of well-preserved radiolarians from all units led to the recognition of three distinctive assemblages: Assemblage A, obtained from metalliferous sediments and chert intercalations within red mudstone, could be correlated with the middle-late Cenomanian fauna; Assemblage B, recovered from red mudstone and chert, is assigned to the early Turonian, based on correlation with radiolarian occurrences across the Cenomanian-Turonian boundary; Assemblage C, recognized in micritic limestone, is broadly identified as Turonian-Coniacian. The age of the Suhaylah Formation has previously been interpreted as early Cenomanian to Coniacian-Santonian; however, the revised age could be from late Cenomanian to Turonian-Coniacian. The estimated possible maximum age of metalliferous sediments (96.5-93.9 Ma) is in good agreement with high-precision U-Pb zircon dates of ophiolite crust attributed to ridge magmatism. The depositional age of the pelagic sediments clearly indicates that the ridge magmatism had ended by ca. 95.5 Ma.

Successive stages of calcitization and silicification of Cenomanian spicule-bearing turbidites based on microfacies analysis, Polish Outer Carpathians

Mid-Cretaceous turbidites with large proportions of sponge spicules are widely distributed in the Silesian Nappe of the Outer Carpathians, giving rise to diversified types of sediments, from spiculites to spicule-bearing siliciclastics and calcarenites. Part of this succession, Middle–Late Cenomanian in age, was transformed into cherts. A microfacies study showed that these turbidite sediments underwent several stages of calcitization and silicification, which took place during Mid-Cretaceous times in different sedimentary environments, i.e., on a northern shelf bordering the Silesian Basin and on a deep sea floor. The first diagenetic changes were related to changes to the biotic components of the turbidite layers, dominated by siliceous sponge spicules. This process, which took place in the spiculitic carbonate mud on the shelves, was related to the calcitization of sponge spicules. Calcareous clasts and calcified skeletal elements also were corroded by bacteria. After transportation down the slope, the biogenic and siliciclastic particles were deposited below the carbonate compensation depth. Taphonomic processes on the basin floor and alternating phases of carbonate and silica cementations, recrystallization and dissolution occurred in these sediments and were related to the diversification in composition of successive turbidite layers. Silicification was related to the formation of quartz precipitates as fibrous chalcedony or microcrystalline quartz, which were derived from the earlier dissolution of amorphous silica, originating mostly from siliceous sponge spicules and radiolarian skeletons. However, a source of silica from hydrothermal vents was also possible. The initial silica precipitation could have taken place in a slightly acidic environment, where calcite was simultaneously dissolved. A number of silicification stages, visible as different forms of silica precipitate inside moulds after bioclasts, occur in the particular turbidite layers. They were related to changes in various elements of the pore-water profile after descending turbidity-current flows. A very low sedimentation rate during the Middle–Late Cenomanian in the Silesian Basin may have favoured the sequence of initial calcitization and silicification stages of the turbidite sediments.

Stratigraphy and structure of the central East Sakhalin accretionary wedge (Eastern Russia)

The East Sakhalin accretionary wedge is a part of the Cretaceous-Paleogene accretionary system, which developed on the eastern Asian margin in response to subduction of the Pacific oceanic plates. Its for� mation was related to the evolution of the Early Cretaceous Kem-Samarga island volcanic arc and Late Cre� taceous-Paleogene East Sikhote Alin continentalmargin volcanic belt. The structure, litho�, and biostratig� raphy of the accretionary wedge were investigated in the central part of the East Sakhalin Mountains along two profiles approximately 40 km long crossing the Nabil and Rymnik zones. The general structure of the examined part of the accretionary wedge represents a system of numerous eastvergent tectonic slices. These tectonic slices. tens to hundreds of meters thick. are composed of various siliciclastic rocks, which were formed at the convergent plate boundary, and subordinate oceanic pelagic cherts and basalts, and hemipelagic siliceous and tuffaceous-siliceous mudstones. The siliciclastic deposits include trenchfill mudstones and turbidites and draping sediments. The structure of the accretionary wedge was presumably formed owing to offscraping and tectonic underplating. The offscrape d and tectonically underplated fragments were proba� bly tectonically juxtaposed along outofsequence thrusts with draping deposits. The radiolarian fauna was used to constrain the ages of rocks and time of the accretion episodes in different parts of the accretionary wedge. The defined radiolarian assemblages were correlated with the radiolarian scale for the Tethyan region using the method of unitary associations. In the Nabil zone, the age of pelagic sediments is estimated to have lasted from the Late Jurassic to Early Cretaceous (Barremian); that of hemipelagic sediments, from the early Aptian to middle Albian; and trenchfill and draping depo sits of the accretionary complex date back to the middle-late Albian. In the Rymnik zone, the respective ages of cherts, hemipelagic sediments, and trench facies with draping deposits have been determined as Late Jurassic to Early Cretaceous (middle Albian), mid� dle Aptian-middle Cenomanian, and middle-late Cenomanian. East of the rear toward the frontal parts of the accretionary wedge, stratigraphic boundaries between sediments of different lithology become succes� sively younger. Timing of accretion episodes is based on the age of trenchfill and draping sediments of the accretionary wedge. The accretion occurred in a period lasting from the terminal Aptian to the middle Albian in the western part of the Nabil zone and in the middle Cenomanian in the eastern part of the Rymnik zone. The western part of the Nabil zone accreted synchronously with the Kiselevka-Manoma accretionary wedge located westerward on the continent. These accretionary wedges presumably formed along a single conver� gent plate margin, with the Sakhalin accretionary system located to the south of the Kiselevka-Manoma ter� rane in the Albian.

Migration and stratigraphic fill of an underfilled foreland basin: Middle–Late Cenomanian Belle Fourche Formation in southern Alberta, Canada

The geometry of a foreland basin is mainly a product of a complex dynamic balance between the orogenic loading, erosion and sedimentation, and lithospheric flexural response. To improve our understanding of the relationships between these processes and the foreland basin stratigraphy, a detailed stratigraphic and sedimentologic study of the Middle–Late Cenomanian Belle Fourche Formation in the subsurface of southern Alberta is presented in this paper. The Middle Cenomanian lower Belle Fourche Formation and the Late Cenomanian upper Belle Fourche Formation were deposited in underfilled conditions, showing prominent forebulge zones. It is demonstrated that during the Late Cenomanian with the cratonward migration of the uplifting forebulge zones, the axes of backbulge zones also migrated cratonwards and a wide uplifted forebulge zone was formed. During the last stage of the underfilled condition, the foreland basin had a relatively flat topography and fine sands were deposited along the flank of the uplifting forebulge. A qualitative model for the migration and stratigraphic fill of an underfilled foreland basin is proposed. This model provides a qualitative pattern of the cratonward migration of the forebulge zone and backbulge zone within a several-million-year tectonically-driven cycle. It is suggested that during the early underfilled period, the basin geometry is mainly controlled by the orogenic loading, and a prominent forebulge zone with an approximately fixed location is formed. During the late underfilled period, the basin geometry is mainly controlled by the sediment loading, and the forebulge zone move very fast along with the rapid cratonward shift of the sediment loading center. Over a longer time period and between different tectonically-driven cycles, along with the cratonward propagation of the thrusting wedge, the orogenic loading plays a more important role than the sediment loading for the migration of the forebulge zone.

Upper Cretaceous rudist biostratigraphy of the Bey Dağları Carbonate Platform, Western Taurides, SW Turkey

The Upper Cretaceous (Middle Cenomanian-Coniacian) successions of the Bey Dağları Carbonate Platform (Western Taurides, SW Turkey) are represented by rudist-bearing shallow-water limestones. Four rudist lithosomes are distinguished for the first time from the Eastern, Northern and Southern Areas of the Bey Dağları Autochthon. The oldest rudist assemblages dominated by caprinids are observed in the Eastern (Katran Dağ) Area (caprinid lithosomes) and suggest a Middle-Late Cenomanian age. The uppermost part of the platform carbonates in the Northern Area is characterized by an association of hippuritid and radiolitid rudist bivalves dominated by Vaccinites praegiganteus (Toucas) (hippuritid lithosomes). The rudist fauna indicates the Late Turonian age, which is confirmed by the previously obtained 87Sr/ 86Sr values of well-preserved low-Mg calcite of Vaccinites praegiganteus (Toucas) shells. The rudist associations of the Southern (Susuzdağ) Area are represented by two rudist formations. The lower lithosomes are mainly made up of hippuritids and radiolitids (hippuritid-radiolitid lithosomes). The stratigraphical distributions of the species of the assemblage indicate a Santonian-Early Campanian age. The rudist associations of the upper lithosomes are dominated by species of Joufia and Gorjanovicia ( Joufia-Gorjanovicia lithosomes), which suggest a Late Campanian-Maastrichtian age. Identification of the rudist lithosomes yields information on the palaeobiogeographic distribution of the rudist species in the eastern Mediterranean region and also on the biostratigraphic frame of the Upper Cretaceous successions of the Bey Dağları Carbonate Platform.

Evolution of the northern Cordilleran foreland basin during the middle Cretaceous

A detailed study of the middle–late Cenomanian Belle Fourche Formation in southern Alberta helps to elucidate the evolution of the Cordilleran foreland basin during the middle Cretaceous. By combining isopach data from the study area with previously published research on middle Cretaceous stratigraphy in adjacent areas of northern Alberta and Montana, it is possible to defi ne the position of the proximal foredeep, forebulge, and backbulge depozones from late Albian to Cenomanian time. The foredeep probably lay to the west of the present fold-thrust belt in western Montana from the late Albian to middle Cenomanian, with the forebulge extending northwestward through the northwest corner of Wyoming and curving northeastward through the present-day location of Calgary. Thick Dunvegan deposits (middle Cenomanian) of northwestern Alberta are interpreted as foredeep deposits. Palinspastic reconstruction of the southern Rocky Mountains of Alberta is consistent with a location of the foredeep within the area of the present fold-thrust belt. At the end of the middle Cenomanian the forebulge retreated westward within Alberta, defi ning a nearly straight northwest-southeast trend through the present position of the Foothills and Front Ranges of Alberta and northwestern Montana. The change in trend of the foreland basin at the end of the middle Cenomanian may refl ect the change of convergence vectors along the western margin of North America during the middle Cretaceous.

Cenomanian-lower coniacian radiolarian assemblages from the Naiba reference section (Sakhalin)

Reexamination of radiolarians from the Naiba and Bykovo formations of the Naiba reference section (West Sakhalin Mountains) is carried out. Distinguished in the section are the Cromyomma (?) nodosa-Amphipyndax sp. A (terminal early Cenomanian), Cuboctostylus kasinzovae-C. sakhalinensis (Middle Cenomanian), Cuboctostylus trifurcatus-Cassideus yoloensis (terminal middle-late Cenomanian), and Spongodiscus concentricus-Multastrum robustum (late Turonian to early Coniacian probably) assemblages. According to results of comparative analysis for North Pacific regions, concurrent radiolarian assemblages from the Naiba section and California contain up to 40% of species in common, whereas taxonomic similarity with assemblages from Japan is insignifican in contrast.

Le Cénomanien et le Turonien de la Plate-forme Préafricaine du Maroc

The Moroccan Cretaceous North Saharian Platform is classically divided into three formations: Ifezouane (Albian?), Aoufous (Albo-Cenomanian) and Akrabou (Cenomano-Turonian). These deposits reflect a fluviatile or deltaic environment in the Albian?, evolving to a lagoonal environment or a paralic sabkha in the Albian and Middle–Late Cenomanian, and to an open marine platform during the Late Cenomanian and Turonian. This paper deals mainly with the Akrabou Formation, which consists of massive carbonates, divided into four lithostratigraphic units (C1, C2, T1 and T2) bounded by regional unconformities, and organized in four cyclic sequences respectively. Units C1 and C2 are dated as Late Cenomanian, based on foraminifera and ostracodes. Units T1 and T2 are attributed to the Turonian. The Cenomanian/Turonian boundary is a sedimentological and ecological unconformity. The correlations between the sections studied indicate that considerable subsidence took place in the Goulmima-Tadighoust area, at the centre of the basin. This study has also revealed that the Moroccan Preafrican Platform was connected to the Tethyan realm in Late Cenomanian and remained as such through Turonian. However, a minor connection also existed with the Central Atlantic.

Signature of hydrodynamic activity caused by rapid sea level changes in pelagic organic-rich sediments, Tarfaya basin (southern Morocco)

Abundant beds with calcareous tempestites, hummocky cross-stratifications ‘HCS’, bioclastic limestones ‘BL’ and limestone channels ‘LCh’ occur in the Late Cretaceous sequence (Middle-Late Cenomanian / Early Turonian) of the Tarfaya basin (SW Morocco) with a strongly erosive base. Results of the study have significant implications for sea level fluctuations in fine-grained basinal facies. According to the palaeoenvironmental reconstructions, the strata were deposited in the offshore area dominated by an upwelling system. The regular distribution of the ‘BL’, ‘HCS’ and ‘LCh’ throughout the section provides a constraint on palaeobathymetry. The water depth during deposition fluctuated frequently and ranged from below, but near, storm wave base.

Cenomanian events in the deep western Basque Basin: the Leioa section

Analysis of the microfaunas (foraminifera, ostracods) and the stable isotope values (δ13C, δ18O) of the Leioa section, as representative of the deep Basque Basin, has allowed us to propose a detailed palaeoenvironmental reconstruction of this region during the Cenomanian, as well as to register global chronostratigraphic reference levels to facilitate interregional correlations. During the Cenomanian, part of the basin, the Plentzia Trough, was occupied by intermediate water masses, as deduced by the relative percentages of planktonic (Rotalipora) and benthonic foraminifera. A noticeable change is observed at the middle-late Cenomanian transition: the replacement of the dominance of keeled (rota-liporids) by incipiently-keeled (dicarinellids, praeglobotruncanids) planktonic foraminifera, indicative of the new influence of the upper intermediate waters. The temporary effect of shallow waters is deduced in one interval of the latest early Cenomanian and two more of the middle Cenomanian, as indicated by the dominance of globular planktonic foraminifera (hedbergellids). These water masses were moderately to strongly hypoxic (<4 to <2 ml/l of disolved oxygen) after the ostracod platycopid signal and benthonic foraminiferal hypoxic indicators. The dysaerobia may have been particulary strong (almost anoxia?) during part of the middle Cenomanian. Micronutrient availability was also restricted during several intervals of the middle Cenomanian, as indicated by the sudden decrease in the species diversity of the calcitic benthonics during the period when increased trends of the δ13C isotopic signals are observed. The combination of both hypoxia and nutrient depletion produced drastic changes in the microfaunal assemblages, with emigrations and local extinctions, showing benthonic perturbations from the time indicated by the base of theRotalipora reicheliZone onwards. From the beginning until the end of the middle Cenomanian, eleven of these perturbations are recorded as regional bioevents, using as bioevent-markers, intervals where microfauna was absent (including benthic-free intervals, B-FI; benthonic calcitic-free intervals, BC-FI; and ostracod-free intervals, O-FI). These changes led to the renewal of the microfaunas; benthonic foraminifera renewed their specific stocks during the early to early middle Cenomanian, with planktonic foraminifera and ostracods undergoing renewal at the end of the middle Cenomanian. Isotope values of δ18O and δ13C are consistent with the palaeoenvironmental changes detected by the microfaunas; their maximum and minimum shifts coincide with the bioevents. The double-peaked positive shift of δ13C for the mid-Cenomanian of northwest Europe (Jenkynset al., 1994; Paulet al., 1994a) has been recognized in this series of the Basque Basin. The palaeoenvironmental perturbations deduced in the Cenomanian of the Leioa section are attributed essentially to palaeoceanographic changes, where intermediate water masses profoundly influenced the planktonic and benthonic ecosystems. The influence of other local causes, such as volcanic activity at that time, or tectonics between the Iberian and European plates, are more difficult to prove. Several of the bioevents defined in the middle Cenomanian of this basin could probably be global in nature, and thus may be useful for establishing interregional correlations.

Bioistratigraphical and sedimentological study of Upper Senonian–Lower Eocene sediments of Tripolitza Platform in central Crete (Greece)

The stratigraphic and palaeoenvironmental implications of a section in the Upper Senonian to Lower Eocene carbonates of the Tripolitza Platform in central Crete are discussed in this paper. The lower part (upper Campanian–lower Maastrichtian) of the succession consists of about 75 m of thickly bedded, light to dark grey limestones, dolomitic limestones and dolomites that were deposited on an inner carbonate platform. It is characterized by stratified bioclastic rudist facies (shelly limestone) associated with foraminifera and especially with species of the Rhapydioninidae family. These are overlain by 35 m of crystalline light grey dolomites that were deposited on a very restricted internal platform, characterized by intertidal-supratidal facies. The overlying 75-m-thick light grey dolomitic limestones and dolomites are characterized by the presence ofRhapydionina liburnicaStache, and in the uppermost part byNeobalkhania bignotiCherchi, Radoicic & Schroeder, dating it as Late Maastrichtian. The facies, cryptalgal laminites with fenestrae of varying dimensions, suggest relatively extensive subaerial exposure. Possible pedogenic textures are common in this sequence and especially in the uppermost part, which coincides with the K/T boundary. The presence ofPseudonummoloculina heimi(Bonet) at two levels in the Maastrichtian succession suggests transportation of Middle-Late Cenomanian sediments from emergent blocks of the platform during this period. A gap is suspected, for regional stratigraphic reasons, between this horizon and the next which containsSpirolinasp. and “Pseudochrysalidina” sp., dating it as Early-Middle Eocene.

Depositional model of a Cenomanian-Turonian extensional basin (Sopeira Basin, NE Spain): interplay between tectonics, eustasy and biological productivity

Abstract The Sopeira Basin is a small extensional basin with a continuous record of sedimentation from the Late Albian to the Late Turonian. In its early stages (Late Albian-Middle Cenomanian), it formed a half-graben, bounded to the south by a low-angle normal fault. During this time, sedimentation was limited to the downfaulted block until the emerged footwall was flooded, resulting in the deposition of shallow-water limestones over a vast platform. The platform was exposed during the Middle-Late Cenomanian by a relative sea-level fall. The sedimentation up to this point was controlled by local factors (extensional tectonics), and influenced by eustatic sea-level change. Subsequently, the entire area was flooded by the Late Cenomanian transgression. During the Cenomanian-Turonian boundary interval, the basinal sedimentation was characterized by calcisphaerid limestones with black cherts and organic-rich shales, while on the platform a hiatus was produced. These sedimentary features are interpreted as having been caused by global mechanisms: changes in atmospheric CO 2 and temperature, along with an increased rate of precipitation that caused an intense primary productivity at the water surface. In the basinal sea, eutrophication led initially to an unusual development of calcisphaerids and later, to the development of opportunistic planktonic foraminifera, whereas in the platform, it affected the life of carbonate-producing organisms dramatically and sedimentation ceased. Eutrophication of marine water is thus interpreted to be the main cause of the extinction of K-strategist foraminifera and associated fauna at the end of the Cenomanian.

La sédimentation sur la plateforme de Tripolitza au Crétacé inférieur-Cénomanien en Crète centrale (Grèce)

The Tripolitza zone of the Hellenic orogen is closely related to the south Tethys passive margin. It represents the internal part of the Gavrovo-Tripolitza platform which crops out in the central-south Peloponnese, Crete and the south-east Aegean islands. From bottom to top, the Tripolitza sequence is composed of Triassic clastic sediments (Tyros beds), Triassic to upper Eocene platform carbonates and a clastic flysch series. In the present paper the sedimentation and stratigraphy of the Lower Cretaceous to Cenomanian carbonate succession of the Tripolitza zone in central Crete, are investigated. The basal beds consist of 100 m of laminated to brecciated dolomites with some intercalations of micritic limestones which are interpreted as peritidal deposits. They are of Early Cretaceous, probable Barremian age. They are overlain by 150 m of carbonates, which represent a shelf-lagoonal facies with sea-level fluctuations from upper subtidal to upper intertidal conditions. Influence of the open sea is observed towards the top of this series, dated as early Aptian on the basis of a fairly rich microfauna. The Aptian continues with 130 m of stromatolitic dolomites and dolomite breccia of intertidal to supratidal environments and 45 m of limestones reflecting a subtidal environment. A shallow water, subtidal limestone facies 15 m thick containing gastropods and rudists probably constitute the base of the Albian formation. It is followed by a regressive series 240 m thick. This is composed of laminated limestones and dolomites, gradually becoming stromatolic dolomites with dolomite breccia intercalations. This regressive carbonate sedimentation took place during the Aptian p.p.(?)-Albian-early Cenomanian. Sedimentation during the middle-late Cenomanian had a transgressive character. The succession is characterized by the presence of rudist-reef facies as well as of low-energy subtidal-lagoonal facies. The latter are expressed by limestones with intercalations of dolomites. Locally, the microfauna and microflora are rich and of high diversity.