Turonian Transition Research Articles

Tectono‐sedimentary history of the upper Cedar Mountain Formation, Central Utah, USA

ABSTRACTCurrent investigations into the Albian–Cenomanian sedimentary record within the Western Interior have identified multiple complex tectono‐sedimentary process–response systems during the ongoing evolution of North America. One key sedimentary succession, the upper Cedar Mountain Formation (Short Canyon Member and Mussentuchit Member), has historically been linked to various regionally and continentally significant tectonic events, including Sevier fold‐and‐thrust deformation. However, the linkage between the Short Canyon Member and active Sevier tectonism has been unclear due to a lack of high‐precision age constraints. To establish temporal context, this study compares maximum depositional ages from detrital zircons recovered from the Short Canyon Member with that of a modified Bayesian age stratigraphic model (top‐down) to infer that the Short Canyon Member was deposited at ca 100 Ma, penecontemporaneous with rejuvenated thrusting across Utah [Pavant (Pahvant), Iron Springs and Nebo thrusts]. These also indicate a short depositional hiatus with the lowermost portion of the overlying Mussentuchit Member. The Short Canyon Member and Mussentuchit Member preserve markedly different sedimentary successions, with the Short Canyon Member interpreted to be composed of para‐autochthonous orogen–transverse (across the Sevier highlands) clastics deposited within a series of stacked distributive fluvial fans. Meanwhile, the muddy paralic Mussentuchit Member was a mix of orogen–transverse (Sevier highlands and Cordilleran Arc) and orogen–parallel basinal sediments and suspension settling fines within the developing collisional foredeep. However, the informally named last chance sandstone (middle sandstone of the Mussentuchit Member) is identified as an orogen–transverse sandy debris flow originating from the Sevier highlands, similar to the underlying Short Canyon Member. During this phase of landscape evolution, the Short Canyon Member – Mussentuchit Member depocentre was a sedimentary conduit system that would fertilize the Western Interior Seaway with ash‐rich sediments. These volcaniclastic contributions, along with penecontemporaneous deposits across the western coastal margin of the Western Interior Seaway, eventually would have lowered oxygen content and resulted in a contributing antecedent trigger for the Cenomanian–Turonian transition Oceanic Anoxic Event 2.

Paleoenvironmental significance of the carbon isotope record across the Cenomanian–Turonian transition and the Oceanic Anoxic Event 2 (OAE2) in the southeastern Neotethys, Zagros, Iran

A high–resolution carbon isotope record of pelagic carbonates (δ13Ccarb) from the Zagros Mountains, Iran, documents a 1.8‰ positive carbon isotope excursion (CIE) in the southeastern Neotethys during the Cenomanian–Turonian transition, corresponding to Ocean Anoxic Event (OAE2). The succession is controlled by biostratigraphy that includes the Rotalipora cushmani, Whiteinella archaeocretacea, and Helvetoglobotruncana helvetica biozones. The CIE is characterized by a positive shift culminating in a first positive peak (at 3.3‰), followed by a 1‰ negative excursion, accompanied by the highest δ18Ocarb values that can be interpreted as the result of a short–lived global cooling, the Plenus Cold Event. A subsequent second positive peak and a plateau of higher δ18Ocarb values are followed by relatively low values in the earliest Turonian akin to pre–excursion values. The biostratigraphic positions of the three diagnostic maximum points, referred to as peaks A, B, and C, within the generally positive CIE indicate a synchronous δ13Ccarb variability worldwide. The average δ13Ccarb values in southeastern Neotethys are generally lower compared to the open–ocean records of the same period. We attribute this offset to a substantial regional regression combined with recycling of isotopically light organic carbon. The diachronous deposition of black shales/mudstones after the onset of positive CIE suggests that its formation strongly depends on local paleogeography and paleoenvironmental factors.

The Cretaceous (early Albian to early Campanian) biostratigraphy and palaeotemperature reconstruction of the eastern Tethys: Calcareous nannofossil evidence from southern Tibet, China

Studies of geochemical proxies, such as carbonate δ18O and TEX86, show that the Earth's climate has changed significantly during the Cretaceous Period. However, knowledge about the sea surface temperature (SST) history of the eastern Tethys is limited. Based on the calcareous nannofossil record of the Nirang section, Tethys Himalayas, this paper attempts to establish the biostratigraphic framework and SST evolution of the Cretaceous Period in the southern Tibet. A total of 131 species were identified from the studied succession. Twelve bioevents were recognized, allowing for early Albian to the early Campanian (CC8 to CC18 biozones) in the section. Meanwhile, a disconformity within 81–71 Ma was also identified. The SST evolution was analysed by the calcareous nannofossils modified temperature index (mTI), and the results reveal that the long-term SST changes in the Tethys Himalayas of southern Tibet experienced progressive warming from the early Albian to the early Turonian and the warm interval persisted until the early Campanian. Short-term warming and cooling events, e.g., rapid warming in the early and late Albian, subsequent cooling in the latest Albian and peak warmth at the Cenomanian–Turonian transition, are also recorded in the study area. The SST history of the Albian-Cenomanian in the eastern Tethys can be well correlated to the western Tethys (central Italy). However, the compilation of TEX86 data indicates substantial global cooling during the Coniacian to Campanian. This study suggests that the northward drift of the Indian continent towards the equator is likely to be responsible for the sustained warmth in the Tethys Himalayas from the middle Turonian to the early Campanian.

The Cenomanian–Turonian Boundary Event (CTBE) as recorded in the northern margin of Africa: palaeoceanography of the Oceanic Anoxic Event (OAE-2), North-Central Tunisia

ABSTRACT Samples spanning the Cenomanian–Turonian transition at Guern Halfaya Section of North-Central Tunisia were analysed for elemental and isotopic chemostratigraphic characterization of the OAE-2 global anoxic event. Comparison with other published sections of the same age was achieved. These geochemical analyses, which cover the local Bahloul Formation, add new insights into the palaeoceanographic evolution of the southern Tethyan margin during the Cenomanian–Turonian transition. The use of discrete redox-sensitive and primary productivity-sensitive trace element data coupled to isotopic and biostratigraphic events is shown as leading proxies for the determination of prevailing warming palaeoceanographic conditions, in terms of productivity and climate, during the OAE-2 in the northern margin of Africa. Such event is believed to have generated one among the most significant source rocks of the region.

Plagioptychus (Hippuritida) dominated assemblage from northern Peri-Tethys margin (Bohemian Cretaceous Basin, Cenomanian–Turonian transition)

Rudist bivalves are frequently used as an important tool for stratigraphy and palaeoecological reconstructions in Cretaceous ecosystems. In this study, we partly refine stratigraphic ranges of rudist genera and species reported from the Bohemian Cretaceous Basin (BCB). The first occurrences of Plagioptychus haueri (Teller) and consequently, the entire genus Plagioptychus might be dated to the upper Cenomanian. The research focused on two sites exposing near-shore/shallow-water deposits, located in the north-western part of BCB. This area represents palaeobiogeographical boundary between Tethyan and Boreal Realms. Here, the Tethyan rudist fauna reaches its northern limits within the Peri-Tethyan shelfs in Central Europe. Revision resulted in detection of the following rudist species: Plagioptychus haueri, Radiolites bohemicus (both already recognised), Petalodontia aculeodentata, Radiolites sanctaebarbarae and Radiolites saxoniae. Furthermore, large numbers of rudist fragments assignable to Hippuritidina suborder, corals and other macrofauna have been detected. Localities described herein are possibly of upper Cenomanian age, even though lower Turonian origin of some of the material deposited in studied localities cannot be excluded. We assign this taphocenosis with co-occurrence of rudist and coral fauna to ‘Coral-Rudist Lithosomes’ with two distinctive rudist eco-morphotypes typical of different substrate types and/or environments. Additionally, we discuss stratigraphical position of several taxa within the context of the entire Bohemian Cretaceous Basin.

Late Cretaceous marine reptiles from Malyy Prolom in Ryazan Oblast, Central Russia

During the Cenomanian–Turonian transition (~94 Ma), what is today Central Russia formed part of the northern epicontinental margin of the Tethys Ocean. Diverse marine vertebrate faunas inhabited these palaeoenvironments, but their fossils are incompletely documented. Here, we report the discovery of marine reptile remains, recovered together with pterosaur, chondrichthyan, and actinopterygian fish material from a basal-most glauconitic sand and gravel layer of the Dmitrov Formation. These strata are exposed in an active quarry near the village of Malyy Prolom in the Shatsky District of Ryazan Oblast, Central Russia. The Dmitrov Formation deposits are middle–upper Santonian, but unconformably contact the underlying lower–middle Cenomanian Yakhroma Formation via a condensed boundary horizon that contains the vertebrate fossils with bivalve shell fragments and siliceous and phosphatic clasts. Such sedimentary characteristics indicate a high-energy shoreface setting where the vertebrate teeth and bones were likely reworked during cyclical regressions commencing in the latest Cenomanian–early Turonian. Time-averaging is also evidenced by the mixed occurrences of brachauchenine pliosaurids, elasmosaurid and polycotylid plesiosauroids, ophthalmosaurid ichthyosaurians similar to Pervushovisaurus, and a possible yaguarasaurine mosasauroid. These taxa are typical of Cenomanian–Turonian assemblages from across the northern peri-Tethys, and represent components of what were probably palaeobiogeographically widespread marine reptile faunas.

Cenomanian–Turonian facies, sequence stratigraphy and sea–level changes from east–central Sinai (Egypt)

The widely exposed Cenomanian–Turonian successions in east–central Sinai display well–marked vertical and lateral changes in lithology, biofacies and depositional sequences. Three sections were studied (from northeast to southwest: Gebel Themed, Sheikh Attiya and Gebel Shiti, respectively) to document sequence stratigraphy. Twenty–four microfacies types grouped into nine lithofacies associations ranging from peritidal to mid– to outer ramp settings were identified. These microfacies reveal a general shallowing trend towards the southwest (Gebel Shiti section) in a mixed siliciclastic/carbonate ramp, whereas the northwest (Gebel Themed section) is marked by a deeper carbonate–dominated regime. The Cenomanian–Turonian succession is divided into six third–order depositional sequences (SD1 to SD6) that are bracketed by basin–wide sequence boundaries (SB AIB to SB Tu3) of varied time span and marked by vertical changes in lithofacies, microfacies and faunal content. Within these depositional sequences, six maximum flooding surfaces are also recorded. In most depositional sequences, the absence of lowstand and falling systems tract are marked, possibly as a result of the lack of accommodation space due to the proximal position of the studied sections. The identified sequence boundaries are also compared with those identified in Egypt, Jordan, Tunisia and the Arabian Plate. The present sequence boundaries match with those documented from the Arabian and African plates as also with the eustatic sea–level falls. However, for those noted across the Cenomanian/Turonian boundary and during the early–middle Turonian transition, there is discordance, possibly due to the influence of regional tectonics, that caused major hiatuses and amalgamated several of them. The chronostratigraphic assignment followed in the present study is based on ammonites, enabling the recognition of five zones, upper Cenomanian Neolobites vibrayeanus and Vascoceras cauvini, lower Turonian Choffaticeras (Choffaticeras) segne and Wrightoceras munier and the middle–upper Turonian Coilopoceras requienianum Zone.

Assessing the contribution of the La Luna Sea to the global sink of organic carbon during the Cenomanian-Turonian Oceanic Anoxic Event 2 (OAE2)

Epicontinental seas were important features of the paleogeographic landscape during the Cretaceous; however, the role they played as sinks of organic carbon is still poorly understood. The La Luna Formation (Albian-Coniacian) is a series of organic-rich limestones deposited in northwestern South America on an epicontinental sea (the La Luna Sea). This formation offers a forty-million-year continuous record of environmental change characterized by periods of oceanic anoxia in an epicontinental sea. The La Luna Sea, may have played an important role –although so far unexplored– in carbon cycling through the ocean during the Cretaceous, specifically during short-term, global-scale disruptions in the carbon cycle known as oceanic anoxic events (OAEs). To evaluate the role of the La Luna Sea in global carbon cycle perturbations, we conducted a detailed lithological and chemostratigraphic analysis of two stratigraphic sections from the Upper Magdalena Basin of Colombia, both of which encompass the Oceanic Anoxic Event 2 (OAE2) at the Cenomanian–Turonian boundary. Compared to deposits in the modern ocean, the La Luna Formation has high total organic carbon (TOC) before, during, and after OAE2. Foraminifera and nannoplankton assemblages also imply a stressed upper water column during OAE2. Geochemical and paleontological evidence suggests that the sediment-water interface was anoxic across the late Cenomanian and early Turonian. Strata deposited just after OAE2, however, contain inoceramid bivalves, consistent with short-lived re‑oxygenation of the benthic layer. Estimates of primary productivity, the covariation of Mo and U enrichment factors, and relations between Cd, Mo, Co, and Mn also reveal that the La Luna Sea was biogeochemically similar to the modern Cariaco Basin. Despite high concentrations of organic carbon found in the La Luna Formation, mass-accumulation rates of organic carbon are low, a finding that can be explained by a reduction in the accumulation rate of sediments caused by the peak of sea-level transgression that took place at the Cenomanian–Turonian transition. Based on the areal extent of the La Luna Sea and mass-accumulation rates of organic carbon, 1.7 Eg of C were removed from the ocean over 500 ky and deposited in the La Luna Sea. Interestingly, although the La Luna Sea was one-third the size of the Western Interior Seaway (WIS), the amount of organic carbon buried in the WIS during OAE2 was similar (1.4 Eg of C). In these two epicontinental seas, 3.1 Eg of C were removed from the ocean during OAE2, accounting only for 3.4% of the total C needed to cause a perturbation of the carbon cycle similar to that observed during OAE2. The low amount of organic carbon buried in the La Luna Sea and the WIS suggests that neither of these inland seas were responsible for the efficient removal of organic carbon from the ocean during OAE2. This conclusion challenges the explanation that epicontinental seas were major sinks of organic carbon—and therefore they did not play a significant role in the carbon cycle during the Mesozoic OAEs and other disruptions of the carbon cycle in Earth's history.

A mid-Cretaceous petroleum source-rock in the North Atlantic region? Implications of the Nanok-1 fully cored borehole, Hold with Hope, northeast Greenland

The Cenomanian–Turonian transition includes the youngest of the two so-called Cretaceous Oceanic Anoxic Events, sometimes also referred to as the “Bonarelli Event”. This is an interval of global deposition of organic-rich oceanic deposits, including some of the most prolific petroleum source rocks of the World. However, in the greater North Atlantic realm, little direct evidence of notable volumes of rich Cenomanian–Turonian petroleum source rocks exists. As part of a major study of the Cretaceous succession in northeast Greenland between 2009 and 2012, a fully cored borehole (Nanok-1) on eastern Hold with Hope was drilled to penetrate a poorly or non-exposed stratigraphic succession including the Cenomanian–Turonian transition. The Nanok-1 penetrated 160.5 m of Cenomanian to Campanian clastic deposits and terminated at a TD of 168.35 m in an igneous intrusion. Although none of the deposits penetrated qualify as a petroleum source rock, a downwards increasing trend in petroleum source potential starting at c. 100m hint at the existence of such rocks at levels deeper than the TD of the well. The downwards increasing trend in petroleum generation potential is curtailed by maturation from c. 140m, but assuming the trend would have prevailed, were it not for the intrusion, the presence of at least a marginal source rock would be expected at the level of the sill and deeper. The presumed existence of petroleum source rocks below the TD of the borehole is supported by the presence of marked oil staining in sandstones (i.e. migrated petroleum) at shallower levels of the drilled succession. The hopane and sterane distributions and stable carbon isotopic compositions and age-specific nordiacholestane ratios of the oil stains are compatible with an origin from the Cretaceous succession, while the Jurassic can be ruled out. The oil stains recorded were probably generated from a marine shale source rock of Cretaceous age, containing a significant proportion of terrigenous organic matter. The possible presence of at least one marginal petroleum source rock in the mid-Cretaceous succession at the Nanok-1 location has implications for the exploration potential of the conjugate margin. Hence, comparison of sediment accumulation histories of the Møre and Vøring basins and northeast Greenland suggests that the presumed source rock was deposited before the accommodation space at the Nanok-1 location was exhausted. Thus, coeval sediment starvation and deposition of more condensed and richer petroleum source rocks may have prevailed in the offshore basins to the east during the mid-late Cenomanian. This potential source rock will thus be slightly older than Cenomanian–Turonian source rocks that are important worldwide.

Oxygen as a factor controlling palynological record: An example from the Cenomanian–Turonian transition in the Rybie section, Polish Carpathians

Abstract Palynology constitutes a useful tool for biostratigraphical and palaeoenvironmental analyses and is commonly used in academic and applied research. However, the palynological signal can be significantly influenced by both, intra- and post-depositional oxidizing processes. To estimate the dependence of the palynological record on the oxygen level near the sediment-water interface, the palynofacies have been analysed from shale and marlstone of the Cenomanian-Turonian (C-T) transition in the Rybie section (Carpathians, Poland) and correlated to previously obtained ichnological record and interpretation. The results show abrupt and intense palynofacies fluctuations from samples rich in organic matter to almost barren. Only the short, middle to upper Cenomanian interval yields well preserved and diversified organic-walled dinoflagellate cysts. The whole section reflects palaeoenvironmental fluctuations within a deep-water basin, including: mesotrophic conditions and environment deterioration (interval 1); extreme oligotrophic conditions (hyperoligotrophy) influencing oxydation of organic matter (interval 2); reduction of oxygen content, enhanced organic productivity, increased production of organic carbon and deposition of amorphous organic matter (Bonarelli Level comprising the Late Cenomanian Oceanic Anoxic Event OAE-2); rapid increase in oxygenation above the Bonarelli Level (interval 3). The hyperoligotrophy recorded within the interval 2 is likely associated with the Plenus Cold Event and coeval enhanced oxygenation of bottom waters prior to the OAE-2. The results yield the enhanced environmental fluctuations near the C–T transition and emphasise the sensibility of palynological record to near-bottom oxygen content.

Marine reptiles and climates of the Jurassic and Cretaceous of Siberia

All available data on the Jurassic and Cretaceous climates of Siberia, based on isotope, palaeontological and lithological markers are summarized. Late Pliensbachian cooling, early Toarcian warming, followed by late Toarcian to Middle Jurassic cooling and long-term Late Jurassic warming are well-recognized. Gradual cooling started since the late Ryazanian and continued during the whole Early Cretaceous except the short early Aptian warming event. At the beginning of the Late Cretaceous climate became warmer with warming peak at the Cenomanian–Turonian transition. During the middle and late Turonian climate became colder. During the Coniacian–Campanian time interval climate became warmer, but at the end of the Campanian new cooling event occurred. New records of marine reptiles from the Toarcian, Kimmeridgian, Volgian and Santonian–Campanian of the north of Eastern Siberia are described. All data concerning marine reptile occurrences in the Jurassic and Cretaceous of Siberia are reviewed; these records (from 51 localities) are mostly located at high palaeolatitudes. The analysis has revealed that most of the localities containing fossil reptile remains were llocated in the Transpolar palaeolatitudes (70°–87°). There are no direct relationship between climate oscillations and distribution of these animals. Taking into account recent data arguing that nearly all groups of the Jurassic and Cretaceous big marine reptiles were able to maintain constant body temperature and also were capable make long-range seasonal migrations, any conclusions concerning usage of these animals as markers of warm climate should be treated with a caution.

The Cenomanian–Turonian transition in the carbonate platform facies of the Western Saharan Atlas (Rhoundjaïa Formation, Algeria)

Microfacies, fossil macroinvertebrates and microfossil assemblages (foraminifera and ostracods) and δ13C from the Rhoundjaia Formation (Ksour Mountains, Western Saharan Atlas, Algeria) were studied. During the Late Cenomanian–Early Turonian, the Ksour Basin was an isolated platform at mid to outer shelf depths. The neritic conditions were interrupted by pelagic conditions in transgressive intervals. This deepening of the palaeoenvironment is reflected by changes in the foraminiferal assemblages. The composition of benthic and planktic foraminiferal assemblages, characterized by low diversity and abundance of low oxygen tolerant species, reflects a carbonate platform with biotic stress conditions (oxygen and nutrient fluctuations and sea-level changes). Planktic foraminiferal assemblages are dominated by opportunists such as Muricohedbergella and Planoheterohelix, which colonized new ecospace on the shelf during the transgression. The Oceanic Anoxic Event 2 (OAE2) is not represented by organic rich facies but does coincide with the successive proliferation of different opportunist organisms such as microgastropods and small echinoid (hemiasterids) and the heterohelicid shift. The continuous presence of trace fossils (Thalassinoides and Planolites-like) and benthic macroinvertebrates allows us to exclude anoxic conditions during the Whiteinella archaeocretacea Zone. The proliferation of Planoheterohelix indicates the record of eutrophic conditions and the transition from oxygenated to poorly oxygenated waters during stratified open marine settings at the time of maximum flooding. The subsequent increase in thin-shelled bivalves and cytherellid ostracods would confirm the stress conditions at the time of the δ13C isotopic excursion in the upper part of the W. archaeocretacea Zone. The Lower Turonian is represented by a shallowing upward trend in the Ksour Basin. Comparison of the taxonomic composition of the planktic foraminiferal assemblages (phenogram and class diagram) indicates the high degree of relationship of the Ksour Basin with the Northeast Sicily Basin, Central Tunisia Basin and Egypt. This work sheds light on the impact of the OAE2-related environmental perturbation in environments where anoxic conditions were not reached, as well as the response of the fossil assemblages to the biotic crisis.

Integrated foraminifera and δ13C stratigraphy across the Cenomanian–Turonian event interval in the eastern Baltic (Lithuania)

The Cenomanian–Turonian transition marks one of the most important extinction episodes of the Mesozoic era. This extinction event was associated with the development of widespread oceanic anoxia and pronounced stable carbon isotopic excursion. Despite its importance, the effects of the perturbation on higher latitude biotas, and from the Baltic region in particular, are currently underexplored. Therefore, in this contribution we present the fossil record of a foraminifera succession integrated with δ13C trends from two deep cores: Bliūdsukiai-19 from western Lithuania and Baltasiskė-267 from southern Lithuania. Two foraminiferal zones were distinguished: Rotalipora cushmani from the upper Cenomanian and Whiteinella archaeocretacea from the boundary strata between the Cenomanian and Turonian in the Baltasiskė-267 core section, and a W. archaeocretacea Zone in the Bliūdsukiai-19 core section. A chemostratigraphical analysis of the stable carbon isotopes revealed a positive Cenomanian–Turonian δ13C anomaly, with maximum values reaching 3.57‰ in the upper part of the Bliūdsukiai-19 core section. A non-metric multidimensional scaling analysis of the foraminifera communities revealed that the major changes in their assemblages were strongly temporally organized and associated with the changes in the stable carbon isotopic ratios. This fact points to the significant effects of the C–T extinction event on the northern Neotethys paleocommunities.

The architecture and associated fauna of Perouvianella peruviana, an endemic larger benthic foraminifera from the Cenomanian–Turonian transition interval of central Peru

During the Cenomanian–Turonian transition interval, larger benthic foraminifera became extinct or suffered decreases in terms of their diversity. Anoxic/dysoxic conditions of oceanic bottom water masses and potentially seawater acidification during this pivotal time also affected ecological niches in the shallow epeiric domain and resulted in the demise of many taxa of benthic foraminifera worldwide. Nevertheless, exceptions to this pattern have been reported too. Here, evidence is presented for mass occurrences of larger benthic foraminifera (Perouvianella peruviana) from the Western Platform in Peru that were not affected by environmental conditions related to the Cenomanian–Turonian transition. A new architectural analysis of P. peruviana tests found in abundance in central Peru has permitted a detailed update on its systematic and functional morphology. The paleoenvironment associated with the P. peruviana mass occurrence was characterized by mesotrophic conditions, perhaps in one way or another similar to the environmental parameters that lead to Aptian and Albian larger benthic foraminifera mass occurrences at the southern Tethyan margin (‘Orbitolina levels’). Algae and small, cosmopolitan r-strategist foraminifera coexisted with P. peruviana. Moreover, a first description of the soritid taxa Pseudopeneroplis oyonensis n. gen., n. sp. is provided. Interestingly, the superposition of local and global environmental patterns onto the Western Peruvian shelf water masses triggered a mass development of the Perouvianella population coincident with Oceanic Anoxic Event 2. Conversely, the migration of foreign complex-shelled K-strategist larger benthic foraminifera from neighboring realms was inhibited.

Water-mass evolution in the Cretaceous Western Interior Seaway of North America and equatorial Atlantic

Abstract. The Late Cretaceous Epoch was characterized by major global perturbations in the carbon cycle, the most prominent occurring near the Cenomanian–Turonian (CT) transition marked by Oceanic Anoxic Event 2 (OAE-2) at 94.9–93.7 Ma. The Cretaceous Western Interior Seaway (KWIS) was one of several epicontinental seas in which a complex water-mass evolution was recorded in widespread sedimentary successions. This contribution integrates new data on the main components of organic matter, geochemistry, and stable isotopes along a north–south transect from the KWIS to the equatorial western Atlantic and Southern Ocean. In particular, cored sedimentary rocks from the Eagle Ford Group of west Texas (∼ 90–98 Ma) demonstrate subtle temporal and spatial variations in palaeoenvironmental conditions and provide an important geographic constraint for interpreting water-mass evolution. High-latitude (boreal–austral), equatorial Atlantic Tethyan and locally sourced Western Interior Seaway water masses are distinguished by distinct palynological assemblages and geochemical signatures. The northward migration of an equatorial Atlantic Tethyan water mass into the KWIS occurred during the early–middle Cenomanian (98–95 Ma) followed by a major re-organization during the latest Cenomanian–Turonian (95–94 Ma) as a full connection with a northerly boreal water mass was established during peak transgression. This oceanographic change promoted de-stratification of the water column and improved oxygenation throughout the KWIS and as far south as the Demerara Rise off Suriname. In addition, the recorded decline in redox-sensitive trace metals during the onset of OAE-2 likely reflects a genuine oxygenation event related to open water-mass exchange and may have been complicated by variable contribution of organic matter from different sources (e.g. refractory/terrigenous material), requiring further investigation.

The Cenomanian–Turonian boundary in Jordan: Ammonite biostratigraphy and faunal turnover

Well-exposed fossiliferous Upper Cenomanian–Lower Turonian marine sedimentary rocks are present in west-central Jordan. Ammonites serve as an important faunal marker for this interval and can be used to subdivide the Cenomanian–Turonian transition into two upper Cenomanian biozones (Neolobites vibrayeanus and Vascoceras cauvini) and two lower Turonian biozones (Vascoceras proprium and Choffaticeras segne). A revised stratigraphic range of the Vascoceras cauvini Zone in the study area is proposed, consisting of the Metoicoceras geslinianum and Neocardioceras juddii zones of the standard zonation. Based on intercontinental biostratigraphic correlation, a minor unconformity appears to be present around the Cenomanian–Turonian boundary, and a part of the lower Turonian is probably missing. In addition, a faunal turnover is recorded in the uppermost Cenomanian, marked by the disappearance of most of the Cenomanian taxa, including Costagyra olisiponensis (Sharpe), Ceratostreon flabellatum (Goldfuss), Ilymatogyra africana (Lamarck), Rhynchostreon suborbiculatum (Lamarck), Harpagodes nodosus (Sowerby), and Heterodiadema libycum (Desor). This bioevent is thought to be an effect of the Oceanic Anoxic Event OAE 2; the dramatic shifts in species richness and diversity spanning the Cenomanian–Turonian boundary in the study area occurred in response to the major paleoclimatic and paleoenvironmental perturbations prevailing at that time. The stratigraphic and paleontological patterns studied in Jordan are very similar to those recorded in Egypt in terms of litho- and biostratigraphy, event stratigraphy, and macroinvertebrate content, suggesting the presence of uniform triggering mechanisms and bio-sedimentary responses in the Upper Cretaceous basins of the Middle East and providing clues for a high-resolution correlation between the two areas.

Orbital control on the timing of oceanic anoxia in the Late Cretaceous

Abstract. The oceans at the time of the Cenomanian–Turonian transition were abruptly perturbed by a period of bottom-water anoxia. This led to the brief but widespread deposition of black organic-rich shales, such as the Livello Bonarelli in the Umbria–Marche Basin (Italy). Despite intensive studies, the origin and exact timing of this event are still debated. In this study, we assess leading hypotheses about the inception of oceanic anoxia in the Late Cretaceous greenhouse world by providing a 6 Myr long astronomically tuned timescale across the Cenomanian–Turonian boundary. We procure insights into the relationship between orbital forcing and the Late Cretaceous carbon cycle by deciphering the imprint of astronomical cycles on lithologic, physical properties, and stable isotope records, obtained from the Bottaccione, Contessa and Furlo sections in the Umbria–Marche Basin. The deposition of black shales and cherts, as well as the onset of oceanic anoxia, is related to maxima in the 405 kyr cycle of eccentricity-modulated precession. Correlation to radioisotopic ages from the Western Interior (USA) provides unprecedented age control for the studied Italian successions. The most likely tuned age for the base of the Livello Bonarelli is 94.17 ± 0.15 Ma (tuning 1); however, a 405 kyr older age cannot be excluded (tuning 2) due to uncertainties in stratigraphic correlation, radioisotopic dating, and orbital configuration. Our cyclostratigraphic framework suggests that the exact timing of major carbon cycle perturbations during the Cretaceous may be linked to increased variability in seasonality (i.e. a 405 kyr eccentricity maximum) after the prolonged avoidance of seasonal extremes (i.e. a 2.4 Myr eccentricity minimum). Volcanism is probably the ultimate driver of oceanic anoxia, but orbital periodicities determine the exact timing of carbon cycle perturbations in the Late Cretaceous. This unites two leading hypotheses about the inception of oceanic anoxia in the Late Cretaceous greenhouse world.

Bioevents and redox conditions around the Cenomanian–Turonian anoxic event in Central Mexico

The Xilitla section of central Mexico (western margin of the proto-North Atlantic) is characterized by pelagic sediments enriched in marine organic matter. Using biostratigraphic and radiometric data, it was dated at the latest Cenomanian–earliest Turonian transition. We identified an interval coeval with the faunal turnover associated with the Oceanic Anoxic Event 2 (OAE 2), recording the Heterohelix shift and the “filament event” for the first time in Mexico. An integral analysis of sedimentary facies, pyrite and geochemical proxies reveals vertically variable redox conditions, with prevailing anoxic to dysoxic bottom waters. Along with phosphorous and manganese depletion, the highest content of total organic carbon and of certain redox-sensitive trace elements (RSTEs) is found during part of the anoxic event, confirming more uniform and constant oxygen-depleted conditions. This interval is also characterized by a significant enrichment in biogenic barium and elevated TOC/NTOT ratios, suggesting a link between productivity and anoxia. Sulfur isotope fractionation has a maximum value within the anoxic event, favored by the increase in the flux of organic matter and intensified through sulfur recycling. Highly bioturbated beds representing short-lived episodes of oxic conditions are intermittent within the OAE 2 and become more frequent in the early Turonian. This study proposes a model similar to that of modern upwelling regions. High marine productivity controlled organic matter burial and oxygenation at the seafloor, varying between anoxic (laminated facies with small pyrite framboids) and dysoxic conditions (bedding-parallel bioturbated facies with inoceramid bivalves and large pyrite framboids), interrupted by short-term well-oxygenated episodes (thoroughly bioturbated facies with common benthic foraminifera). General low-oxygen conditions led to the formation of glauconite and pyrite (bacterially mediated); the enrichment of redox-sensitive trace elements in sediments (Cd, Zn, V and Cr scavenged by organic matter and Ni, Mo, Pb, Co and Re by pyrite) and resulted in Mn and P depletion.

The biotic crisis across the Oceanic Anoxic Event 2: Palaeoenvironmental inferences based on foraminifera and geochemical proxies from the South Iberian Palaeomargin

Open marine sediments deposited during the Cenomanian–Turonian transition are well exposed in the Spanish Baños de la Hedionda section (Betic Cordillera, South Iberian Palaeomargin). Analysis of foraminiferal assemblages and geochemical proxies allow inferences on the impact of the Oceanic Anoxic Event 2 (OAE2) in this area of the western Tethys. Three main intervals have been identified corresponding to different lithological units and biozones. (1) The top of the Capas Blancas Member (Rotalipora cushmani Biozone) represents the pre-extinction phase with diverse foraminiferal assemblages and well developed water-column tiering, well-oxygenated, oligotrophic deep-waters and oxygenated to poorly oxygenated, mesotrophic surface-waters. Foraminiferal opportunist species point to a minor event with dysoxic conditions preceding the OAE2. (2) The black radiolaritic shales (Whiteinella archaeocretacea Biozone) consist of a foraminiferal-barren interval, except for the lowermost centimetres where planktic surface-dweller opportunists are common. Redox sensitive elements (Cr/Al, V/Al, U/Th, MoEF, Moaut, UEF and Uaut) and increased TOC values reflect oxygen depleted conditions related to the OAE2. The increase in P/Ti values at the base of this stratigraphic interval indicates an abrupt increase in productivity. High concentrations of radiolarians are congruent with high surface productivity probably related to changes in oceanic circulation and enhanced upwelling currents, as well as subsequent shallowing of the oxygen-minimum zone. The increase in MoEF and Moaut towards the top of the black radiolaritic shales indicates temporal euxinic conditions. (3) A slow, bottom-up recovery of foraminiferal assemblages is inferred at the base of the Boquerón Member (Helvetoglobotruncana helvetica Biozone), with seafloor recolonization by benthic foraminifera being recorded previous to the water column colonization by planktic forms, mainly by intermediate-dwellers typical of mesotrophic waters. The subsequent proliferation of surface-dweller opportunists and deep-dweller opportunists adapted to mesotrophic to eutrophic conditions, and the decrease in planktic foraminiferal diversity, may indicate the persistence of poorly oxygenated conditions in the water column towards the lower-middle part of the H. helvetica Biozone.

First microbialites associated to organic-rich facies of the Oceanic Anoxic Event 2 (Northern Tunisia, Cenomanian–Turonian transition)

From northern Tunisia, small-scale well-preserved microbialites, contemporaneous to the global oceanic anoxic event 2 (OAE 2) are first reported on the southern Tethyan Margin. These microbialites are encased within the pelagic organic-rich black shales of the Bahloul Formation (Cenomanian–Turonian transition). Biostratigraphic, petrographic, and geochemical investigations carried out to constrain their biogenicity and genesis character led to consider them as thrombolites and stromatolites occurring in lenticular bioherms/biostromes and columnar bodies co-relatable to the global ‘filament event’ of the authors, close to the base of the Watinoceras ammonite zone. Abundant clotted micrite, cyanobacterial filaments, and algal tissues point to the key process of microbial carbonate precipitation and to a major role played by microbes in the stabilisation and subsequent lithification, which in turn favoured the preservation of the original structure of the microbialites. These microbially induced carbonate formations are considered as favoured by chemosynthetic fauna of bivalve molluscs and lithistid sponges which were able to host symbiotic microbial communities. The latter contributed to the precipitation of authigenic calcite and non-carbonate minerals (e.g. pyrite) fuelled by microbial activity under sulphate-reducing conditions. The carbonate body onset is considered to be initiated by seafloor instability due to syndepositional fault acting that induced the appraisal of uplifted tilted blocks within oxygenated waters but near the anoxic water masses. Generated depressions allowed the development of chemosynthetic-based communities. Deep faults related to Triassic salt domes acted as conducts for hydrocarbon and salt brine expulsion to the seafloor and the microbialite growth was enhanced by an abrupt uprising sea level under warmer conditions.