Turonian Coniacian Research Articles

Palaeoenvironment and bio-events of the Cretaceous sediments of the Cauvery Basin, India

The Cauvery Basin is an important rift margin basin on the east coast of India. Its long research history began in the mid-nineteenth century with the pioneering work of H. F. Blanford. While much of the Cretaceous succession in the basin is fault-controlled, some of the recorded events represent global sea level changes, especially in the mid-Cretaceous. Macrofossils (ammonites, bivalves, etc.) and foraminifera are abundant throughout, and there is an important occurrence of fossil wood and ‘log-grounds’ in the Turonian–Coniacian. The basin is subdivided into a series of sub-basins (known as depressions in earlier literature), which, in places, have their own distinctive depositional history. The results of our collective fieldwork have provided a re-assessment of the lithostratigraphy, biostratigraphy and the tectono-stratigraphical history of the Ariyalur outcrop. Three sedimentary units have been identified: the syn-rift Gondwana Group (of early Cretaceous age), the syn-rift Uttatur Group (of Albian to Coniacian age) and the post-rift Ariyalur Group (of Santonian to Maastrichtian age). Both microfossil and macrofaunal information have been integrated in order to construct a biostratigraphical framework for the basin and develop a tectono-stratigraphical model. Structures exposed onshore, which have occasionally been interpreted as Albian reefs, are thought to be irregularly shaped limestone olistoliths and olistostromes produced by significant intra-Cretaceous faulting and slumping within the basin.

Unraveling sauropod diversity in the Portezuelo Formation of Patagonia through a comprehensive analysis of new and existing material

The Portezuelo Formation preserves an outstanding record of the upper Turonian – lower Coniacian. Despite the discovery of a significant quantity of sauropod fossil material from the formation, only two species have been formally described to date: Malarguesaurus florenciae and Futalognkosaurus dukei. Here we present new sauropod material mostly composed of non-articulated caudal vertebrae (MCF-PVPH 916 and 917) that belong to two titanosauriforms on the basis of the following features: anterior caudal vertebrae with procoelous-opisthoplatyan articulations, transverse processes that reach the posterior articular face of the centrum and neural spines with a transverse width of around 50% of their anteroposterior length; anterior and middle caudal vertebrae with the neural arch restricted to the anterior half of the centrum; middle caudal centra with circular cross-section. Phylogenetic analysis recovers the new material in close relation to Malarguesaurus within a monophyletic clade at the base of Somphospondyli. This clade shares large pedicel height with a vertical anterior border on the middle caudal vertebrae, a vertical orientation of the neural spines on the distalmost middle caudal vertebrae and proximalmost posterior caudal vertebrae, and subequal relative lengths of the proximal ulnar condylar processes. The specimens presented here are distinct not only from Futalognkosaurus, but also from other indeterminate titanosaurian remains from the same formation. However, there are no significant differences between the specimen MCF-PVPH 917 and Malarguesaurus, but there are differences between the posterior caudal vertebrae of MCF-PVPH 916 and Malarguesaurus, so they could be considered different species. Whilst we err on the side of caution in not naming new taxa here, the two specimens significantly expand what we know about sauropods in the Turonian–Coniacian ecosystems of Patagonia, which will continue to do so as more material is discovered.

Diuqin lechiguanae gen. et sp. nov., a new unenlagiine (Theropoda: Paraves) from the Bajo de la Carpa Formation (Neuquén Group, Upper Cretaceous) of Neuquén Province, Patagonia, Argentina

BackgroundUnenlagiine paravians are among the most relevant Gondwanan theropod dinosaur clades for understanding the origin of birds, yet their fossil record remains incomplete, with most taxa being represented by fragmentary material and/or separated by lengthy temporal gaps, frustrating attempts to characterize unenlagiine evolution.Results and conclusionsHere we describe Diuqin lechiguanae gen. et sp. nov., a new unenlagiine taxon from the Upper Cretaceous (Santonian) Bajo de la Carpa Formation of the Neuquén Basin of Neuquén Province in northern Patagonia, Argentina that fills a substantial stratigraphic gap in the fossil record of these theropods. Although known only from a very incomplete postcranial skeleton, the preserved bones of Diuqin differ from corresponding elements in other unenlagiines, justifying the erection of the new taxon. Moreover, in several morphological aspects, the humerus of Diuqin appears intermediate between those of geologically older unenlagiines from the Neuquén Basin (e.g., Unenlagia spp. from the Turonian–Coniacian Portezuelo Formation) and that of the stratigraphically younger, larger-bodied Austroraptor cabazai from the Campanian–Maastrichtian Allen Formation. Consequently, the morphology of the new taxon appears to indicate a transitional stage in unenlagiine evolution. Phylogenetic analysis recovers Diuqin as a paravian with multiple plausible systematic positions, but the strongest affinity is with Unenlagiinae. The humerus of the new form exhibits subcircular punctures near its distal end that are interpreted as feeding traces most likely left by a conical-toothed crocodyliform, mammal, or theropod, the latter potentially corresponding to a megaraptorid or another unenlagiine individual. Thus, in addition to filling important morphological and temporal gaps in unenlagiine evolutionary history, the new taxon also offers information relating to the paleoecology of these theropods.

A 40 Ar/ 39 Ar and U–Pb timescale for the Cretaceous Western Interior Basin, North America

Abstract Improvements in analytical procedures in parallel with intercalibration of 40 Ar/ 39 Ar and U–Pb methods and laboratories, spurred since 2003 by the EarthTime geochronology community initiative, have led to ±2 σ uncertainties of the order of 50–100 ka, or better, for Cretaceous ash beds. Assembled here are 57 40 Ar/ 39 Ar ages and 17 238 U– 206 Pb ages of volcanic ash beds in strata of the Western Interior Basin of North America determined during the last 15 years since these improvements have been made. These age determinations span from 108 Ma in the middle Albian to 66 Ma in the latest Maastrichtian. Five of the 40 Ar/ 39 Ar ages from Campanian and Maastrichtian strata are newly reported here, whereas the remainder are from the literature. Building on the pioneering work of John Obradovich and Bill Cobban, where possible these age determinations are tied to ammonite and inoceramid biostratigraphy. For most ash beds, the temporal uncertainties, unlike earlier timescales for the Western Interior Basin, are much shorter than the duration of fossil biozones. Proposed ages for stage boundaries based on this review of the radioisotopic ages include: Maastrichtian–Danian, 66.02 ± 0.08 Ma; Campanian–Maastrichtian, 72.20 ± 0.20 Ma; Santonian–Campanian, 84.19 ± 0.38 Ma; Coniacian–Santonian, 86.49 ± 0.44 Ma; Turonian–Coniacian, 89.75 ± 0.38 Ma; Cenomanian–Turonian, 93.95 ± 0.05 Ma; Albian–Cenomanian, 100.00 ± 0.40 Ma. Six bentonites that occur within the Vascoceras diartianum, Neocardiocerus juddi, Prionocylus macombi, Scaphites preventricosus, Scaphites depressus and Desmoscaphites bassleri ammonite zones, dated using both 40 Ar/ 39 Ar and U–Pb methods, yield ages in agreement to within 150 ka and form the backbone of the Western Interior Basin timescale. In parallel, improvements in the taxonomy of ammonites, inoceramids and foraminifera, and recent field work, are better establishing the biostratigraphic framework for these age determinations. Each of these efforts contributes to the progressive refinement of the chronostratigraphic framework of the Western Interior Basin, and enhances its utility for global correlation.

Bio-Magnetostratigraphy of the Turonian–Coniacian Deposits of the Lower Bannovka Section, South-East of the Russian Plate

Bio-Magnetostratigraphy of the Turonian–Coniacian Deposits of the Lower Bannovka Section, South-East of the Russian Plate

Agglutinated foraminifera from the Turonian–Coniacian boundary interval in Europe – paleoenvironmental remarks and stratigraphy

Abstract. Agglutinated foraminiferal assemblages of the Turonian–Coniacian from the GSSP (Global Boundary Stratotype Section and Point) of Salzgitter–Salder (Subhercynian Cretaceous Basin, Germany) and other sections, including Bielefeld–Ostwestfalendamm (Münsterland Cretaceous Basin, Germany) and the Dover–Langdon Stairs (Anglo-Paris Basin, England), from the temperate European shelf realm were studied in order to collect additional stratigraphic and paleoenvironmental information. Stable carbon isotopes were measured for the Bielefeld–Ostwestfalendamm section to establish a reliable stratigraphic correlation with other sections. Highly diverse agglutinated foraminiferal assemblages were obtained from sections in the German basins, whereas the fauna from Dover is less rich in taxa and less abundant. In the German basinal sections, a morphogroup analysis of agglutinated foraminifera and the calculated diversities imply normal marine settings and oligotrophic to mesotrophic bottom-water conditions. Furthermore, acmes of agglutinated foraminifera correlate between different sections and can be used for paleoenvironmental analysis. Three acmes of the species Ammolagena contorta are recorded for the Turonian–Coniacian (perplexus to lower striatoconcentricus zones, lower scupini Zone, and hannovrensis Zone) and likely imply a shift to more oligotrophic bottom-water conditions. In the upper scupini Zone below the Turonian–Coniacian boundary, an acme of Bulbobaculites problematicus likely indicates enhanced nutrient availability. In general, agglutinated foraminiferal morphogroups display a gradual shift from Turonian oligotrophic environments towards more mesotrophic conditions in the latest Turonian and Coniacian.

Late Albian–Early Turonian Grebenka Flora of the North Pacific: Systematic Composition, Age, Distibution

Starting from the mid-Cretaceous and in the Late Cretaceous, the landscape features of the North Pacific make it possible to divide this region into a number of territories called subregions. The earliest Cenophytic (with a significant number and diversity of angiosperms) Late Albian–Early Turonian Grebenka flora and its analogs are known only in three subregions of the North Pacific: Anadyr-Koryak, Northern Alaska and Yuk-on-Koyukuk. In the middle of the Cretaceous these subregions were represented by coastal plains and lowlands periodically flooded by the sea. Cenophytic floras populated the area of terrestrial volcanism of the Okhotsk–Chukotka subregion and the Asian continental interiors of the Verkhoyansk–Chukotka subregion later, in the Turonian–Coniacian, but Mesophytic vegetation with the predominance of Early Cretaceous ferns and gymnosperms continued to exist there at least until the Coniacian. Consequently, the invasion of evolutionarily new Cenophytic vegetation into the continental interiors of North-Eastern Asia was gradual and extended over time. This should be taken into account when studying the Cretaceous nonmarine phytostratigraphy of the North Pacific region.

Import of building stone to the northern Black Sea region in the Middle of the 1st mil. BC according to micropaleontological analysis

The paper presents the results of micropaleontological studies of a series of stones found during the study of the pier of the ancient Greek city of Phanagoria. The samples contain Upper Turonian–Coniacian, Campanian, and Maastrichtian foraminifers characteristic of the Central Tethys zones. This makes it possible to exclude the Crimea and the North Caucasus from the possible regions of origin of the samples. It has been established that Upper Cretaceous carbonate strata similar in genesis and chronology are located on the site of the southern coast of the Black Sea near the ancient cities of Heraclea and Sinop.

The expression of late Cenomanian–Coniacian episodes of accelerated global change in the sedimentary record of the Mexican Interior Basin

Climate and oceanographic changes during the Oceanic Anoxic Event 2 (OAE 2) and the Late Turonian–Coniacian Event (LTCE) are reported from a succession deposited in the central part of the Mexican Interior Basin, based on sedimentological, microfacies, mineralogical, and geochemical analysis. Typical δ13C positive excursions that characterize both the OAE 2 and the LTCE were identified. Organic-rich sediments during the initial stage of OAE 2 (before and around peak “A”, late Cenomanian) and the middle stage of LTCE (around the Hitchwood event) accumulated under increasingly warm and humid conditions, as evidenced by high chemical index of alteration values. Elevated concentrations of detrital proxies coupled with high phosphorus mass-accumulation rates suggest that this scenario led to a rise in detrital and nutrients fluxes that induced eutrophication and anoxic/dysoxic bottom waters. Eutrophic-anoxic/dysoxic marine conditions are further supported by the highest TOC values, coinciding with significant enrichments in redox- and productivity-sensitive trace elements. Moreover, these conditions can also be inferred from the abundant presence of radiolarians and filaments in the OAE 2 interval, and the occurrence of opportunistic foraminifera in the LTCE interval. Oxygen-depleted bottom waters are also indicated by Mo–U systematics. The onset of the Mexican Orogen tectonic uplift and increased upwelling strengthened the transference of nutrients, enhancing organic matter burial during the initial stage of OAE 2. In the mid-OAE 2 δ13C trough interval (between peaks “A” and “B”) equivalent to the Plenus Cold Event, bioturbated sediments with low TOC values accumulated during a short episode of cold climate conditions probably reflecting the southward flow of boreal water throughout the Mexican Interior Basin. The minimum δ34Spy value occurring within the OAE 2 interval in the Mexican Interior Basin is lower than elsewhere because of a local increase in sulfate concentrations.

Elemgasem nubilus: a new brachyrostran abelisaurid (Theropoda, Ceratosauria) from the Portezuelo Formation (Upper Cretaceous) of Patagonia, Argentina

AbstractAbelisaurids are medium–large‐sized theropod dinosaurs that were predominant in the carnivorous fauna during the Late Cretaceous of Gondwana. These predators are abundant in the Cretaceous fossil strata of Patagonia, which yield the best record for this group. In the Late Cretaceous, abelisaurids appear in almost all regions of Gondwana and in all stages, except for the Coniacian, in which they are globally unknown. Here we describe a new abelisaurid,Elemgasem nubilusgen. et sp. nov., from the Portezuelo Formation (Turonian–Coniacian), Patagonia, Argentina. The palaeohistology of the appendicular bones ofElemgasemshows that the holotype was a subadult individual, but had achieved sexual maturity. This taxon is based on several axial and appendicular elements, and is diagnosed by the presence of a marked pattern of rugosity on the lateral surface of the fibula and a dorsoventrally deep lateral wall of the calcaneum. Moreover, the posterior caudal vertebrae have a morphology slightly different from any other abelisaurid.Elemgasem nubilusis recovered as an unstable taxon within Brachyrostra, given that it was recovered as sister taxon of Furileusauria or in several positions within this clade. Despite the problematic phylogenetic relationships ofElemgasem nubilus, it is important because it is the first abelisaurid from the Turonian–Coniacian interval and it increases the diversity of this theropod family at a time of marked turnover in the tetrapod fauna of South America, global climate change, and mass extinction events recorded worldwide in the marine realm.

Climatic Fluctuations and Sedimentation Conditions of the Turonian–Coniacian Sediments of the Northwest Caucasus

Climatic Fluctuations and Sedimentation Conditions of the Turonian–Coniacian Sediments of the Northwest Caucasus

Lateral changes of organic matter preservation in the lacustrine Qingshankou Formation (Cretaceous Songliao Basin, NE China): Evidence for basin segmentation

Organic matter-rich intervals in the Cretaceous succession of the large non-marine Songliao Basin in northeastern China (covered area ~ 87 × 103 km2), including the Turonian–Coniacian Qingshankou Formation (K2qn), offer unique records to study the role of global (e.g., climate, “limno-eustasy”) and regional (tectonics, basin geometry) controls on organic matter accumulation. A wealth of scientific papers concentrate on the vertical variations in organic matter (OM) preservation along the Southeastern Uplift of the basin, which hosts its most prolific source rock intervals. However, the corresponding K2qn interval in the Central Depression is less well studied, and no lateral correlation between both parts of the basin based on direct information from well logs and drill cores exists to date. Hence, the dominant influencing factors causing lateral changes organic-richness and source potential are not yet fully understood. Basin geometry effects in particular could not be evaluated in detail, as highly resolved seismic data is lacking for the transitional zone between the Southeastern Uplift and Central Depression. This paper uses Integrated Prediction Error Filter Analysis of wireline logs together with petrographical and geochemical data from drill cores of two key wells in the Central Depression and Southeastern Uplift areas for a detailed correlation of the K2qn1 interval and its three sub-units (Sq1 to Sq3). Bulk geochemical (e.g., total organic carbon vs. total sulphur), biomarker (ratios of pristane vs. phytane, dibenzothiophene vs. phenanthrene, and total steroid vs. hopanoid compounds; Gammacerane Index; abundance of 4-methylsteranes and C29 steranes), and petrographical (e.g., maceral composition; framboidal pyrite size) data to trace changes in the depositional conditions. All data sources support a higher degree of salinity stratification and more intense anoxia at the Southeastern Uplift compared to the Central Depression particularly during accumulation of the lowermost and most organic-rich Sq1 sub-unit. These observations indicate that two depocenters may have existed during times of Sq1 deposition, and that the pronounced basin separation inhibited marine incursions from the Southeastern Uplift to the Central Depression. During deposition of the overlying Sq2 and Sq3 sub-units, a low relative lake level and dry climatic conditions likely prevailed. The geochemical data suggest similar dysoxic brackish-fresh water conditions in both depocenters. Moreover, the well log correlation indicates that a gradually connected basin may have formed at this time, reducing the effects of two separate depocenters on sediment accumulation and organic matter preservation. Thus, this study provides the first direct evidence that apart from global factors, basin segmentation strongly controlled the distribution of organic matter-rich rocks within the Qingshankou Formation.

Lithological and geochemical characteristics and paleoclimatic conditions of the origin of Upper Cretaceous deposits of the epicontinental basin of the Russian plate in the region of the Ulyanovsk-Saratov foredeep

The results of a geochemical study and paleogeographic, paleoclimatic interpretation for a cyclically constructed section of upper Cretaceous deposits near Volsk city, Saratov region, are presented. Elementary formation cyclites and cyclic variations of a number of certain parameters were associated with the Milankovich astronomical-climatic cycles. The curves of changes of paleotemperature, humidity, paleobathymetry were compiled. The results obtained give an idea of the migration of the arid belt boundaries in the upper Cretaceous and the overall climatic zonation, which is important for regional and global paleoclimatic reconstructions, as well as the history of the development of shelf seas that covered the Russian plate (especially Ulyanovsk-Saratov trough). Paleotemperatures of the land surface in the denudation areas are obtained from the chemical index of alteration (CIA). In the Turonian-Campanian interval selected climatic cyclicity, including period of relative cooling (Turonian–Coniacian) with paleotemperature about 20 °С, the period of relative warming in the mid-late Campanian (20–24 °С), the cooling time at the end of the late Campanian (19–21 °С) and the period of warming at the turn of the Campanian and Maastrichtian and in the early Maastrichtian time. In Maastrichtian age, there are two climatic cycles, beginning with a time of relative cooling (about 19 °С) and ending with a time of relative warming (about 20 С, at the end of Maastrichtian to 25 °С). The cycles of climate humidity change are also determined: two cycles in Campanian time, three cycles in early Maastrichtian, and one cycle in late Maastrichtian. The boundary of the early and late Maastrichtian corresponds to the change of arid conditions to humid ones. The paleobatimetry curves show transgressive-regressive cycles: one in the late Turonian-Coniacian time, two in the late Campanian time, five in the early Maastrichtian time, and one in the late Maastrichtian time. Depth variations were estimated: in the Turonian-Coniacian time in the range of 70–80 m, in the Campanian-Maastrichtian time, the paleobatimetry consistently increased and changed from 100 to 200 m (on average about 150 m). The results obtaired give and idea of the migration of the boundaries of the arid belt in the Late Creataceous and main features of the climatic zonation, which is important for regional and global paleoclimatic reconstructions, as well as for the history of the development the Russian plate in the Ulyanovsk-Saratov region.

A brackish to non-marine aquatic and terrestrial fossil assemblage with vertebrates from the lower Coniacian (Upper Cretaceous) Gosau Group of the Tiefengraben locality near St. Wolfgang im Salzkammergut, Austria

The Turonian–Coniacian continental fossil record in Europe is scarce. Here we present a new fossil assemblage of early Coniacian age that was systematically collected from the coal-bearing Gosau Group of the Tiefengraben locality near St. Wolfgang, Austria. The diverse assemblage is composed of at least 60 taxa including sporomorphs and Normapolles-related pollen, seeds and leaves of angiosperms and gymnosperms, charophytes, gastropods, bivalves, ostracods, termites, fishes, crocodiles and dinosaurs. Concerning charophytes, ostracods, gastropods, crocodiles and dinosaurs, the discovered specimens either extend the temporal and spatial range of specific groups (in some cases as possible relict forms) or suggest the occurrence of new taxa. The discovered remains of algae, molluscs, ostracods, calcareous nannofossils and lepisosteid fish represent a mixed faunal assemblage from different palaeohabitats, from marginal marine to low salinity and freshwater or terrestrial environments. As Normapolles-related angiosperm plants dominate the flora with a relatively high number of dentate leaves, a slightly cooler microenvironment compared to other Turonian–Coniacian Central European localities is indicated. The characteristically grooved crocodylian teeth of Tiefengraben differ from the previously known Upper Cretaceous European crocodyliform teeth and suggest a more diverse crocodyliform fauna in the region. Dinosaurs are represented by teeth of at least three different theropods, the largest of which is referred here to as basal tetanurans. The fossil assemblage of this early Gosau Group occurrence is of great importance for our understanding of the continental floristic and faunistic composition of the western Tethyan archipelago during the Cenomanian–Campanian gap.

Upper Cretaceous (Coniacian -Campanian) siliciclastic/carbonate ramp depositional sequences and relative sea-level changes in Sinai, Egypt: Implications from facies analysis

Through the analysis of biostratigraphy, facies types and distribution, lithofacies associations, and bounding surfaces, we derived high-resolution sequence stratigraphy of the mixed siliciclastic–carbonate rocks for four sections in the Coniacian–Campanian succession in Sinai (Egypt). These stratigraphic sections provide a framework for studying the sedimentary-facies evolution, the paleogeography, and the role of eustatic and tectonic processes in Egypt and surrounding countries. Twenty-six foraminiferal species and four foraminiferal zones (the Dicarinella concavata Interval Zone in the upper Coniacian, Dicarinella asymetrica Total Zone in the Santonian, Globotruncanita elevata Partial Range Zone in the lower–middle Campanian, and Globotruncanita ventricosa Partial Range Zone in the middle Campanian) are noted. Twenty-three lithofacies and microfacies types into six facies associations ranging from progradational shoreface to outer ramp are detected and categorized. These facies associations exhibit lateral variation and deepening toward the north from siliciclastic–carbonate (Matulla Formation) to dominant carbonate (Themed Formation). Three sequences separated by four sequence boundaries have been determined in the field and/or from strong vertical facies changes associated with short time gaps representing the Turonian–Coniacian; the lower–middle and upper Coniacian; the upper Coniacian–Santonian; and the Santonian–Campanian stage boundaries. Comparisons of the examined sequence boundaries in Egypt and in some parts of Europe, within the Arabian Plate, and in global charts suggest that eustatic and tectonic effects were simultaneous.

Fossil fishes from a lag deposit within the Upper Cretaceous Mancos Shale in New Mexico, USA, with comments on correlative Turonian–Coniacian time-transgressive lags in the Western Interior Seaway of North America

A lag deposit between the Tocito Sandstone and Mulatto Tongue of the Upper Cretaceous Mancos Shale in Sandoval County, New Mexico, USA, contains a fossil assemblage of late Turonian–early Coniacian chondrichthyans and osteichthyans. This assemblage consists primarily of isolated teeth that derive from at least 26 taxa including: Meristodonoides sp.; Ptychodus mortoni; P. mammillaris; Scapanorhyncus raphiodon; Protolamna sp.; Cretodus cf. C. semiplicatus; Cretodus sp.; Cretalamna “appendiculata”; cf. Eostriatolamia tenuiplicatus; Archaeolamna cf. A. kopingensis; Squalicorax cf. S. falcatus; S. deckeri; Squalicorax sp.; Paranomotodon(?) sp.; Rhinobatos lobatus; Ptychotrygon triangularis; Texatrygon hooveri; Pseudohypolophus mcnultyi; Ischyrhiza mira; Chondrichthyes indet.; Micropycnodon cf. M. kansasensis; Pycnodontiformes indet.; Aspidorhynchidae indet.; Protosphyraena sp.; and Enchodus cf. E. gladiolus. The lag deposit formed along a series of outer shoreface and discontinuous sandbars in the southeastern corner of the San Juan Basin during eustatic sea-level fluctuation herein referred to as the Turonian–Coniacian Time Transgressive (TCTT) Event. This sea-level event and the concentration of fish remains into a lag deposit is also recorded at several other states within the Western Interior Seaway of North America. These stratigraphic properties have correlative potential across basins and states and provide a framework by which regional and eustatic sea-level events can be interpreted. Differences in coeval faunas found within these TCTT lags are bathymetrically controlled, related to the degree of taphonomic reworking, and proximity of the ancestral shoreline.

УСЛОВИЯ ФОРМИРОВАНИЯ ВТОРИЧНЫХ ДОЛОМИТОВ НИЖНЕГО СИЛУРА И ОСОБЕННОСТИ РАСПРЕДЕЛЕНИЯ В НИХ ПУСТОТНОГО ПРОСТРАНСТВА (СЕВЕРО-ВОСТОК ТУНГУССКОЙ СИНЕКЛИЗЫ)

New materials on the microfauna and lithology of the Kuznetsovskaya and Sedelnikovskaya Formations of the same horizons are presented. These data have allowed us to clarify the age of the boundary Turonian–Coniacian deposits of the Middle and Upper series of the Cretaceous system. Based on a comparison with geological units of marginal regions, where finds of secretion-calcareous foraminiferal shells are known, the Сoniaсian age of the Sedelnikovskaya Formation was established. The time interval of the regional break between the Turonian and Coniacian stages was determined, apparently, corresponding to the Early Сoniacian. The studied sections of the northern region are characterized by the absence of foraminiferal assemblage known in the sections of Kamyshlovskiye wells of the Omsk depression and in some places of the Trans-Urals.

ПОГРАНИЧНЫЕ ОТЛОЖЕНИЯ ТУРОНА – КОНЬЯКА ЗАПАДНОЙ СИБИРИ (ПО ДАННЫМ ИЗУЧЕНИЯ ФОРАМИНИФЕР)

New materials on the microfauna and lithology of the Kuznetsovskaya and Sedelnikovskaya Formations of the same horizons are presented. These data have allowed us to clarify the age of the boundary Turonian–Coniacian deposits of the Middle and Upper series of the Cretaceous system. Based on a comparison with geological units of marginal regions, where finds of secretion-calcareous foraminiferal shells are known, the Сoniaсian age of the Sedelnikovskaya Formation was established. The time interval of the regional break between the Turonian and Coniacian stages was determined, apparently, corresponding to the Early Сoniacian. The studied sections of the northern region are characterized by the absence of foraminiferal assemblage known in the sections of Kamyshlovskiye wells of the Omsk depression and in some places of the Trans-Urals.

Elemental geochemistry of the Upper Cretaceous reservoir and surrounding formations applied in geosteering of horizontal wells, Lebăda Field – Western Black Sea

The precise landing and steering of horizontal wells using conventional mudlogging and Logging While Drilling (LWD) data is a particular challenge for the Lebăda Field, offshore Romania. The use of a new technique of elemental geochemistry analysis (or chemosteering) became an option for the identification of Cenomanian, Turonian–Coniacian–Santonian, Campanian and Eocene strata. This has enabled more accurate placement of the horizontal development wells within the desired reservoir target interval. Geochemical data enabled the identification of chemostratigraphic zones C1, C2, C3 and zone R that correspond to the reservoir section. The application is a result of the geochemical zonation performed using elements and ratios that are sensitive to depositional environment, sea level change, heavy mineral concentrations and siliciclastic input namely: Sr/Ca, Zr/Th, Si/Zr and Si/K. In ascending stratigraphic order, the ratio thresholds of zone C3 are Zr/Th > 11, Sr/Ca > 1.1, Si/Zr < 22 and Si/K < 19, while zone R corresponds to 5.5 < Zr/Th < 11, Sr/Ca < 1.1, Si/Zr > 22 and Si/K > 19. C2 zone is defined by Zr/Th < 5.5, Sr/Ca > 1.1, Si/Zr < 22 and Si/K < 19 and C1 zone is characterized by Si/Zr > 22 and Si/K > 19. The selected geochemical ratios indicate a strong geochemical zonation. In the case of offset wells, 85.9% of the data confirmed the proposed classification and 89.4% for the real-time application case. The zone R shows a strong contrast with the surrounding formations facilitating critical decisions during well placement and geosteering, increasing the reservoir exposure by 28%. The quantitative approach delivered very valuable results, providing a solid foundation to define correlation and well landing intervals. Simultaneously, the cost of the method represents a fraction of the LWD cost and 0.15% of the total project cost, making it very cost effective and a standard approach for future projects.

Integrated chemostratigraphy (δ13C-δ34S-δ15N) constrains Cretaceous lacustrine anoxic events triggered by marine sulfate input

During the Late Cretaceous period (~100.5–65 million years ago), two episodes of expansive lake anoxia occurred in the Songliao Basin (~3× the size of Lake Superior), NE China, either lasting less than one million years. These events, known as lacustrine anoxic events (LAEs), were the factors driving the development of the main source rocks for the most productive oilfields in China. The details underpinning the initiation of the LAEs, however, remain problematic. As a test of this scientific issue, we present time series of bulk organic carbon isotopes, pyrite sulfur isotopes, nitrogen isotopes and trace element concentrations from the Turonian–Coniacian Qingshankou Formation in the Songliao Basin. A notable δ34S excursion (~−10‰) and a spike in pyrite sulfur content are recorded in the basal Qingshankou Formation when the basin reached the maximum expansion. We attributed these changes to marine transgression, which would have brought high sulfate concentrations to the lake, thereby promoting microbial sulfate reduction (MSR). Subsequent sulfate limited by MSR and pyrite burial associated with isolation of the lake from the seawater drove a 20‰ positive δ34S shift. Note that the modes of the δ34S that fluctuations we describe from the LAEs have also been recognized in marine black shale during Oceanic Anoxic Event (OAE) 2, implying a common signature of anoxia. The carbon isotope data suggest that the maximal fractionation found in Qingshankou Member I was likely a consequence of the increased [CO2]aq in lake water and inputs from chemoautotrophs. Increased sedimentary δ15N during the interval of Qingshankou Member I implies intensified denitrification in the oxygen minimum zone, which lowered the water-column N/P ratios, thereby stimulating bacterial nitrogen fixation in Qingshankou Members II-III. By comparing and summarizing the sulfur, carbon and nitrogen cycles of the LAEs during deposition of the Qingshankou and Nenjiang formations, we argue that the forcing function underlying LAEs was enhanced sulfate levels from marine transgression, which might have intensified nutrient recycling and further regulated the lacustrine carbon and nitrogen cycles.