First evidence for the Cenomanian–Turonian oceanic anoxic event (OAE2, ‘Bonarelli’ event) from the Ionian Zone,western continental Greece

Integrated biostratigraphic (planktonic foraminifera, calcareous nannofossils), chemostratigraphic (bulk C and O isotopes) and compound-specific organic geochemical studies of a mid-Cretaceous pelagic carbonate—black shale succession of the Ionian Zone (western Greece), provide the first evidence for the Cenomanian–Turonian oceanic anoxic event (OAE2, ‘Bonarelli’ event) in mainland Greece. The event is manifested by the occurrence of a relatively thin (35 cm), yet exceptionally organic carbon-rich (44.5 wt% TOC), carbonate-free black shale, near the Cenomanian–Turonian boundary within the Vigla limestone formation (Berriasian–Turonian). Compared to the ‘Bonarelli’ black-shale interval from the type locality of OAE2 in Marche–Umbria, Italy, this black shale exhibits greatly reduced stratigraphic thickness, coupled with a considerable relative enrichment in TOC. Isotopically, enriched δ¹³C values for both bulk organic matter (−22.2‰) and specific organic compounds are up to 5‰ higher than those of underlying organic-rich strata of the Aptian-lower Albian Vigla Shale member, and thus compare very well with similar values of Cenomanian–Turonian black shale occurrences elsewhere. The relative predominance of bacterial hopanoids in the saturated, apolar lipid fraction of the OAE2 black shale of the Ionian Zone supports recent findings suggesting the abundance of N₂-fixing cyanobacteria in Cretaceous oceans during the Cenomanian–Turonian and early Aptian oceanic anoxic events.     

Calcareous nannofossils from late Jurassic sediments of the Volga Basin (Russian Platform): evidence for productivity-controlled black shale deposition

The Jurassic/Cretaceous boundary interval in the northern hemisphere is characterized by the widespread occurrence of black shales. About 60% of all petroleum source rocks comprise sediments of late Jurassic and early Cretaceous age with the origin of such black shales still under discussion. In order to better understand the factors that controlled black shale sedimentation, 78 samples were analyzed for calcareous nannofossils from two sections (Gorodische, Kashpir) of the Volga Basin (NE Russia). Calcareous nannofossils are ideal proxies for deciphering nutrient, temperature and salinity fluctuations. Additionally 58 samples from both sections were also analyzed for clay mineralogy, ¹³C_org , TOC and CaCO₃ composition. Both sections contain calcareous claystones and intercalated organic rich shales overlain by phosphorite beds. The presence of the calcareous nannofossil species Stephanolithion atmetros throughout both successions allows a biostratigraphic assignment to the S. atmetros Nannofossil Biozone (NJ 17), which corresponds to the Dorsoplanites panderi Ammonite Biozone of the Middle Volgian. The marlstones of the Kashpir section yield a well-preserved rich and diverse nannoflora, whereas all black shale beds are essentially barren of calcareous nannofossils. Only the uppermost black shale layers yield an impoverished assemblage of low diversity and abundance. Geochemical data suggest an early diagenetic nannofossil dissolution in the black shales of the Kashpir section. This is supported by the occurrence of coccoliths in black shale horizons of the Gorodische section. The assemblages in both sections are dominated by coccoliths of the Watznaueriaceae group (Watznaueria barnesae, Watznaueria fossacincta, Watznaueria britannica, Watznaueria communis), Biscutum constans and Zeugrhabdotus erectus. In Kashpir rare specimens of Crucibiscutum salebrosum occur in the higher part of the section. These taxa indicate boreal affinities. B. constans and Z. erectus are considered to be taxa indicative of a higher productive environment, while C. salebrosum is a cool-water species. From base to top of the Kashpir section, consecutive mass occurrences of different taxa/groups were observed: W. barnesae–W. fossacincta acme, W. britannica–W. communis acme, Z. erectus acme, B. constans acme (including sparse occurrences of C. salebrosum).The observed distribution patterns have been interpreted as characterizing a transition from a low productive, oligotrophic setting with high abundances of K-selected cosmopolitan species (Watznaueriaceae) and predominating marlstone sedimentation to a higher productive, mesotrophic setting. Cooler water temperatures marked by r-selection and acmes of opportunistic species (Z. erectus, B. constans) are coincident with the deposition of black shales and phosphorites in the higher part of the section. Interpretation of clay mineral distribution indicates that black shale deposition occurred under semi-arid hinterland climatic conditions concomitant with a sea level rise. This induced dysoxic conditions in the deeper parts of the Volga Basin, favoring the preservation of organic matter. The cause of the nutrient enrichment in the surface water is still unclear, but possible river water input from the continents does not seem to have been the controlling factor under a semi-arid climate. The occurrence of phosphorites in the upper part of both sections presumably indicates that enhanced productivity may be better explained by the upwelling of nutrient-rich bottom water and thereby causing the recycling of nutrients from oxidized phytoplankton back into the photic zone. This recycling effect finally may have led to an intensified phytoplankton growth which seemed to be a sufficient source for the enrichment of organic matter. This is well correlated with the increase in black shale horizons in the upper part of the Kashpir section.     

Mercury enrichment and Hg isotopes in Cretaceous–Paleogene boundary successions: Links to volcanism and palaeoenvironmental impacts

We investigate the use of Hg as a proxy for volcanism by studying four distal and two proximal sections in relation to the Deccan volcanic center, straddling the Cretaceous–Paleogene (KPg) boundary at (a) Højerup (Denmark), Bottaccione and Padriciano (Italy), (b) Meghalaya and Jhilmili (India), and (c) Bajada del Jagüel (Argentina). Hg sequestration by organic matter results in constant Hg/TOC ratio and linear correlation between Hg content of the sediments and total organic carbon (TOC).Elevated Hg concentrations that deviate from this linear relationship represent most likely true Hg anomalies and these notable Hg/TOC spikes (all TOC <1%) are found in the Meghalaya, Bottaccione and Højerup sections within the CF2 planktic foraminiferal biozone (spike I), at the KPg boundary (spike II), and within the P1a planktic foraminiferal subzone (spike III). Spike III occurs also in the Jhilmili section. No clear correlation between Hg/TOC and Al₂O₃ exists in any of the studied sections. The Hg anomalies probably result from strong volcanic episodes of the Deccan phase-2 (started 250 kyr before the KPg boundary and lasted for 750 kyr) that exhaled sulfuric aerosols, carbon dioxide and other toxic agents which reached a critical threshold, represented in true Hg enrichments in the paleoenvironments. The possibility that Hg enrichments resulted from anoxia scavenging on the seafloor and penetration downward into sediments is not supported in the stratigraphic record of Mo/Al ratios redox proxy.Hg isotopes were analyzed in samples from all KPg boundary sections in this study and from Bidart, France, the latter for comparison. Hg isotopes yielded δ²⁰²Hg values ranging from −1 to −2‰ and Δ²⁰¹Hg signatures from 0 to 0.05‰ (spike II in Højerup, Bottaccione and Meghalaya KPg boundary layers) consistent with volcanic emission of Hg (0 to −2‰). The δ²⁰²Hg in spike I in Meghalaya and Padriciano and spike III in Jhilmili is consistent with volcanic emission of Hg. Two samples from Bajada del Jagüel and four from Bidart, however, display isotope signals compatible with volcanic emission/chondrite Hg. The results of three other samples are characteristic for reworked sediment, soil and/or peat. Most of the data show small positive Δ²⁰¹Hg, in favor of long-term atmospheric transport prior to deposition, supporting a volcanic origin for the Hg. The present study broadens, therefore, the potential use of Hg as stratigraphic marker and, moreover, confirms that in the critical KPg transition, Hg was enriched in paleoenvironments at three distinct stages during the Deccan phase-2.     

Regional record of a global oceanic anoxic event: OAE1a on the Apulia Platform margin, Gargano Promontory, southern Italy

The Gargano Promontory of southern Italy, located on the eastern margin of the Apulia Platform, represents a peculiar Tethyan area where the transition between carbonate platform and adjacent basins is exposed on land. The Aptian stratigraphic record, represented in shallow-water, slope and deep-water deposits, provides a good opportunity to investigate the regional response to the worldwide documented climatic, biotic and palaeoceanographic changes related to Oceanic Anoxic Event 1a (OAE1a). A synthesis of data previously published (Ischitella and Coppitella sections), together with original data (Val Carbonara and Coppa della Guardia sections), from four stratigraphic sections from different depositional settings (proximal to distal) is provided, using an integrated, high-resolution micropalaeontological (planktonic foraminifera and calcareous nannofossils) and, for one section, geochemical (stable carbon and oxygen isotopes) approach. Organic matter preservation is confined to the more distal areas and consists of two thin intervals of black shales in the Aptian portion of the Marne a Fucoidi Formation. Biostratigraphic data assign the older black shale (5 cm thick) to the Selli Level equivalent (OAE1a, Lower Aptian); this carbon-rich interval is immediately followed by another black shale (7–10 cm thick) of early Late Aptian age.OAE1a is generally interpreted as a high-productivity event during a warming interval, followed by a cooling trend. In the Gargano Promontory, although the oxygen isotope curve indicates the above-mentioned climatic evolution, the micropalaeontological data do not support high fertility of the surface water, whereas micropalaeontological and geochemical data for the younger black shale do record high productivity (radiolarian increase in overall abundance, low nannofossil and foraminiferal species richness, increase in abudance of nannofossil fertility indices) associated with a cooling trend. The carbon and oxygen isotope record is in line with evidence from the curves documented elsewhere, whereas, among the biotic events, only the “nannoconid crisis” preceding OAE1a is revealed to be globally correlable.Environmental models for the two episodes of organic matter preservation are proposed, taking into account both global and local controlling factors.     

Exceptionally favourable life conditions for macrobenthos during the Late Cenomanian OAE-2 event: Ichnological record from the Bonarelli Level in the Grajcarek Unit,Polish Carpathians

Pelagic and hemipelagic sediments of the Bonarelli Level (uppermost Cenomanian) in the Sztolnia section (Grajcarek Unit, Polish Carpathians) contain trace fossils of the Zoophycos ichnofacies, including (in descending order of abundance): Chondrites (smaller and larger forms), Planolites, Thalassinoides, Palaeophycus, Taenidium, Teichichnus, and Zoophycos. They occur in thick bioturbated layers, which are interbedded with rare, thin layers of unbioturbated black shales. The black shale layers mark the Bonarelli Level and are interpreted as a record of anoxia or dysoxia. Coeval sections in the Western Tethys contain similar trace fossils but they are less abundant and these sections are characterized by thicker unbioturbated black shale layers and thinner bioturbated layers. This confirms the exceptionally favourable life conditions in sediments of the Sztolnia section, which do not record strong global anoxia during the OAE-2 event. Such favourable conditions were probably caused by effective oxygenation of pore waters and deep burial of organic matter, which are a consequence of high rates of accumulation and the palaeogeographical location of the section on a flank of a submarine high, under strong circulation.     

A new approach to interpreting Rock-Eval S2 and TOC data for kerogen quality assessment

A simple method for application in source potential mapping is used to assess the original oil and gas potentials in source rock horizons based upon Rock-Eval potential (S₂) and total organic carbon (TOC) values. The method assumes that kerogens consist of mixtures of end-members with assigned hydrogen index values. Based on suggested algorithms, the average amounts of oil-prone, gas-prone and inert organic material over source rock intervals are determined in TOC units. The method uses regression lines from plots of remaining hydrocarbon potentials (S₂) versus total organic carbon (TOC), and “quick-look” transparent overlays are used to read the appropriate kerogen mixture.Mineral matrix effects during pyrolysis, when strong, can cause erroneous results. This effect which occurs for oil-prone kerogens and adsorptive minerals can cause problems particularly for lean samples (S₂ = 0–3 mg HC/g rock) whilst the errors for richer samples are less.The method is applied on three sections of Upper Jurassic organic-rich rocks from the Danish North Sea sector, which are at different maturity stages. One of these sections is dominated by gas-prone material, one is dominated by oil-prone material and the third section contains a mixture of oil- and gas-prone material.The method has been compared with other methods that split kerogens in oil and gas generating potential and has given reasonable results.Experience using the method and a presented example suggest that sedimentological, system tract information may be derived from S₂ to TOC cross-plots. A constructed modelling example suggests that the end-member concept used in this approach may be used in forward type source rock prediction models when combined with sedimentological models. The resulting S₂–TOC plots can be used in order to check the forward modelling results against observed values.     

Lower Eocene carbonate-cemented “chimney” structures (Varna, Bulgaria) — Control of seepage rates on their formation and stable isotopic signature

The genesis of Lower Eocene calcite-cemented columns, “pisoid”-covered structures and horizontal interbeds, clustered in dispersed outcrops in the Pobiti Kamani area (Varna, Bulgaria) is related to fossil processes of hydrocarbon migration. Field observations, petrography and stable isotope geochemistry of the cemented structures and associated early-diagenetic veins, revealed that varying seepage rates of a single, warm hydrocarbon-bearing fluid, probably ascending along active faults, controlled the type of structure formed and its geochemical signature. Slow seepage allowed methane to oxidize within the sediment under ambient seafloor conditions (δ¹⁸O = − 1 ± 0.5‰ V-PDB), explaining columns' depleted δ¹³C ratios of − 43‰. Increasing seepage rates caused methane to emanate into the water column (δ¹³C = − 8‰) and raised precipitation temperatures (δ¹⁸O = − 8‰). Calcite-cemented conduits formed and upward migrating fluids also affected interbed cementation. Even higher-energy fluid flow and temperatures likely controlled the formation of “pisoids”, whereby sediment was whirled up and cemented.     

蓟县下马岭组富菱铁矿地层的基本地质特征

天津蓟县铁岭子村附近新出露的下马岭组下部黑色岩系中富含菱铁矿,本文从野外产出特征、岩石学特征及常量元素特征等3个方面对这一富菱铁矿地层的基本地质特征进行了报道.该剖面地层主要以黑色页岩、粉砂质富铁层/菱铁矿结核层互层产出为特征,夹有少量粉砂岩.部分富铁层由于风化严重,野外露头以褐铁矿层出现.菱铁矿为地层中主要的铁矿物相,可以形成菱铁矿结核,显微镜下具泥晶或微晶结构;也可以与含量相当的石英粉砂一起,构成致密粉砂质富铁层;亦或呈颗粒状零星分布于黑色页岩、粉砂岩中;另外褐铁矿层中亦存在极少量的菱铁矿残余.地层的常量元素特征整体表现为富含SiO2、TFe及有机质,而贫MnO、CaO、MgO、P2O5及S元素,且除风化层位中的铁多呈三价外,其余多呈二价.总有机碳(TOC)含量由高到低依次为菱铁矿结核、粉砂质富铁层、黑色页岩、粉砂岩.TFe与Al2O3含量的相关性图解显示,在富铁地层中,二者呈现出很好的负相关关系,而在正常的黑色页岩和粉砂岩中,二者则呈现一定的正相关关系,表明富铁地层中的铁主要为海洋自身铁的化学沉积,而正常的黑色页岩和粉砂岩中的铁主要源自陆源碎屑.同时新鲜样品中FeO与TOC含量呈现出很好的正相关关系,表明菱铁矿的形成可能与有机质有关.     

Paleoceanographic evolution and chronostratigraphy of the Aptian Oceanic Anoxic Event 1a (OAE1a) to oceanic red bed 1 (ORB1) in the Gorgo a Cerbara section (central Italy)

We performed a detailed study of the stratigraphic transition of Oceanic Anoxic Event 1a (OAE1a) to oceanic red bed 1 (ORB1) from the classic Gorgo a Cerbara section in the Umbria region of central Italy. We focused on a 25.5-m-thick stratigraphic succession, from which we analyzed 305 samples for total organic carbon (TOC), CaCO₃, magnetic susceptibility, diffuse reflectance spectrophotometry and the stable carbon and oxygen isotopic composition of both bulk samples and organic matter. In the Gorgo a Cerbara section, the Selli Level of OAE1a (∼1.81 m thick) consists of laminated to bioturbated dark gray to black mudstones and shales with medium to dark gray radiolarian-rich silty to sandy layers and a maximum TOC content of 20.22%. The carbon isotopic values show a negative excursion (C3 stage, ∼0.14 m) at the base of the Selli Level, followed by a stepwise positive excursion (C4–C6 stages, ∼1.67 m) in the upper part of the Selli Level. The transition from OAE1a to ORB1 (∼3.19 m thick) is characterized by bioturbated greenish gray cherty limestones and marly limestones with subordinate marls, corresponding to stable carbon isotopic C7 stage and lasts for ∼0.75 Ma. The ORB1 interval (∼13.15 m) consists of reddish marly claystones, dark-red marlstones, red marly limestones and red calcareous shales which indicate a highly oxic environment. Our results reveal a stepwise transition from a predominantly mesotrophic and dysoxic to anoxic environment at the time that the OAE1a black shales were deposited to an oligotrophic and oxic environment during the transitional interval and finally to highly oxic conditions during the ORB1 interval. The nannoconid crisis occurs at the top of the C2 stage, just 0.34 m below the negative excursion in δ¹³C isotopic values. The massive CaCO₃ dissolution phase occurs 0.25 m above the negative excursion; it persisted for 0.85 Ma and probably resulted from excess CO₂, ocean acidification, and carbonate compensation depth (CCD) shoaling. Deposition of massive black shales occurs at the base of the C6 stage and lasted for 0.4 Ma.     

Origins and accumulation of organic matter in expanded Albian to Santonian black shale sequences on the Demerara Rise, South American margin

Ocean Drilling Program Leg 207 recovered thick sequences of Albian to Santonian organic-carbon-rich claystones at five drill-sites on the Demerara Rise in the western equatorial Atlantic Ocean. Dark-colored, finely laminated, Cenomanian–Santonian black shale sequences contain between 2% and 15% organic carbon and encompass Oceanic Anoxic Events 2 and 3. High Rock-Eval hydrogen indices signify that the bulk of the organic matter in these sequences is marine in origin. However, δ¹³C_org values lie mostly between −30‰ and −27‰, and TOC/TN ratios range from 15 to 42, which both mimic the source signatures of modern C₃ land plants. The contradictions in organic matter source indicators provide important implications about the depositional conditions leading to the black shale accumulations. The low δ¹³C_org values, which are actually common in mid-Cretaceous marine organic matter, are consequences of the greenhouse climate prevailing at that time and an associated accelerated hydrologic cycle. The elevated C/N ratios, which are also typical of black shales, indicate depressed organic matter degradation associated with low-oxygen conditions in the water column that favored preservation of carbon-rich forms of marine organic matter over nitrogen-rich components. Underlying the laminated Cenomanian–Santonian sequences are homogeneous, dark-colored, lower to middle Albian siltstones that contain between 0.2% and 9% organic carbon. The organic matter in these rocks is mostly marine in origin, but it occasionally includes large proportions of land-derived material.     

Analysis of geological effects on methane adsorption capacity of continental shale: a case study of the Jurassic shale in the Tarim Basin,northwestern China

The Jurassic shale is an important source rock for the found gas reservoirs in the Tarim Basin, northwestern China, but has never been researched for shale gas potential. The geological effects on methane adsorption capacity for the gas shale have been investigated in this paper through the geochemical, mineralogical and adsorption analyses on samples from wells and sections. The methane adsorption capacity ranges from 0.58 to 16.57 cm³/g, and the total organic carbon (TOC) content is between 0.5 and 13.5 wt%. The organic maturity measured by T_max is between 410 °C (immature) and 499 °C (overmature). The methane adsorption capacity of the Jurassic continental shale in the Tarim Basin is affected by many geological factors, including the TOC content, organic matter maturity, mineral composition, surface area and pore size distribution. The TOC content is the most significant factor with a positive effect on the adsorption capacity of the Jurassic shale, and the influence varies piecewise according to the TOC content. The TOC content contributes much more to the methane adsorption capacity of organic‐rich shale samples (TOC content > 0.7 wt%) than to the organic‐lean samples (TOC content < 0.7 wt%). The mineral composition is a secondary factor, and the abundance of clay content has a positive effect on the methane adsorption capacity despite its relatively weaker adsorption ability compared to TOC. The pore size distribution has different effects on surface area and pore volume. Mesopores and micropores provide the major surface area and are mainly derived from TOC and illite, which has a positive influence on the adsorption capacity. Mesopores and macropores offer the major pore volume and are mainly formed by illite, which is the major contributor for pore volume rather than surface area. In addition, the TOC and illite contents of the Jurassic shale in the Tarim Basin are closely related to the origin, maturity and diagenesis evolution of the shale: (1) both TOC and illite content variations are related to the different provenances and depositional environments of shale; (2) the decrease of TOC content with increasing maturity is also partly attributed to hydrocarbon generation; and (3) the increase of illite content with increasing maturity is due to illitization in the diagenesis of shale. Copyright © 2015 John Wiley & Sons, Ltd.     

The use of bisnorhopane as a stratigraphic marker in the Oseberg Back Basin, North Viking Graben, Norwegian North Sea

This paper discusses the occurrence of 28,30-dinor-17α,18α,21β-hopane (bisnorhopane) in stratigraphically, fairly well preserved Viking Group sections from wells in local depressions in the North Viking Graben Area. The results show the presence of high relative amounts of bisnorhopane in the “Syn-rift sections”, whilst the “Post-rift sections” contain little or no bisnorhopane. Since most exploration wells are drilled on structural highs, primarily penetrating the “Post-rift Draupne”, this may explain why many analyzed source rock sections in this area contain little bisnorhopane.As a correlation of Draupne sections using the vertical, relative bisnorhopane distributions, it is suggested to be a potential stratigraphic marker for the area, indicating the presence of “Syn-rift Draupne” sediments.The relative bisnorhopane amounts follow a logarithmic reduction with depth and thermal maturity. The bisnorhopane signal is nearly extinguished at 3700 m depth at a maturity of R_o = 0.9–1.0%.     

A study of the kinetic parameters of individual macerals from Upper Permian coals in South China via open-system pyrolysis

A unique Upper Permian coal, Leping coal, is widely distributed in South China. The coal samples studied in the paper were collected from two mines in the Shuicheng coalfield of Guizhou Province, southwest China. The geochemical works including coal petrography, maceral content, Rock–Eval pyrolysis, and kinetic modelling of hydrocarbon-generating have been carried out on whole coal and individual macerals. The higher contents of volatile matter, elemental hydrogen, and tar yield, and the high hydrocarbon generation potential of the Leping coals are attributed to their high content of “barkinite”, a special liptinite maceral.The hydrocarbon generation potential of “barkinite” (S₂=287 mg/g, hydrogen index (HI)=491 mg/g TOC) is greater than that of vitrinite (S₂=180 mg/g, HI=249 mg/g TOC), and much higher than that of fusinite (S₂=24 mg/g, HI=35 mg/g TOC). At the same experimental conditions, “barkinite” has a higher threshold and a narrower “oil window” than those of vitrinite and fusinite, and consequently, can generate more hydrocarbons in higher coalification temperature and shorter geological duration. Data from the activation energy distributions indicate that “barkinite” has a more homogenous chemical structure than that of vitrinite and fusinite. The above-mentioned characteristics are extremely important for exploring hydrocarbon derived from the Leping coals in South China.     

Tephra layers from Holocene lake sediments of the Sulmona Basin, central Italy: implications for volcanic activity in Peninsular Italy and tephrostratigraphy in the central Mediterranean area

We present a new tephrostratigraphic record from the Holocene lake sediments of the Sulmona basin, central Italy. The Holocene succession is represented by whitish calcareous mud that is divided into two units, SUL2 (ca 32 m thick) and SUL1 (ca 8 m thick), for a total thickness of ca 40 m. These units correspond to the youngest two out of six sedimentary cycles recognised in the Sulmona basin that are related to the lake sedimentation since the Middle Pleistocene. Height concordant U series age determinations and additional chronological data constrain the whole Holocene succession to between ca 8000 and 1000 yrs BP. This includes a sedimentary hiatus that separates the SUL2 and SUL1 units, which is roughly dated between <2800 and ca 2000 yrs BP. A total of 31 and 6 tephra layers were identified within the SUL2 and SUL1 units, respectively. However, only 28 tephra layers yielded fresh micro-pumices or glass shards suitable for chemical analyses using a microprobe wavelength dispersive spectrometer. Chronological and compositional constraints suggest that 27 ash layers probably derive from the Mt. Somma-Vesuvius Holocene volcanic activity, and one to the Ischia Island eruption of the Cannavale tephra (2920 ± 450 cal yrs BP). The 27 ash layers compatible with Mt. Somma-Vesuvius activity are clustered in three different time intervals: from ca 2000 to >1000; from 3600 to 3100; and from 7600 to 4700 yrs BP. The first, youngest cluster, comprises six layers and correlates with the intense explosive activity of Mt. Somma-Vesuvius that occurred after the prominent AD 79 Pompeii eruption, but only the near-Plinian event of AD 472 has been tentatively recognised. The intermediate cluster (3600–3100 yrs BP) starts with tephra that chemically and chronologically matches the products from the “Pomici di Avellino” eruption (ca 3800 ± 200 yrs BP). This is followed by eight further layers, where the glasses exhibit chemical features that are similar in composition to the products from the so-called “Protohistoric” or AP eruptions; however, only the distal equivalents of three AP events (AP3, AP4 and AP6) are tentatively designated. Finally, the early cluster (7600–4700 yrs BP) comprises 12 layers that contain evidence of a surprising, previously unrecognised, activity of the Mt. Somma-Vesuvius volcano during its supposed period of quiescence, between the major Plinian “Pomici di Mercato” (ca 9000 yrs BP) and “Pomici di Avellino” eruptions. Alternatively, since at present there is no evidence of a similar significant activity in the proximal area of this well-known volcano, a hitherto unknown origin of these tephras cannot be role out. The results of the present study provide new data that enrich our previous knowledge of the Holocene tephrostratigraphy and tephrochronology in central Italy, and a new model for the recent explosive activity of the Peninsular Italy volcanoes and the dispersal of the related pyroclastic deposits.     

The Daduhe gold field at the eastern margin of the Tibetan Plateau: He, Ar, S, O, and H isotopic data and their metallogenic implications

The Daduhe gold field comprises several shear-zone-controlled Tertiary lode gold deposits distributed at the eastern margin of the Tibetan Plateau. The deposits are hosted in a Precambrian granite–greenstone terrane within the Yangtze Craton. The gold mineralization occurs mainly as auriferous quartz veins with minor sulphide minerals. Fluid inclusions in pyrite have ³He/⁴He ratios of 0.16 to 0.86 R_a, whereas their ⁴⁰Ar/³⁶Ar ratios range from 298 to 3288, indicating a mixing of fluids of mantle and crust origins. The δ³⁴S values of pyrite are of 0.7–4.2‰ (n = 12), suggesting a mantle source or leaching from the mafic country rocks. δ¹⁸O values calculated from hydrothermal quartz are between − 1.5‰ and + 6.0‰ and δD values of the fluids in the fluid inclusions in quartz are − 39‰ and − 108‰. These ranges demonstrate a mixing of magmatic/metamorphic and meteoric fluids. The noble gas isotopic data, along with the stable isotopic data suggest that the ore-forming fluids have a dominantly crustal source with a significant mantle component.     

Environmental control of carbon isotope variations in Pennsylvania black-shale sequences, Midcontinent, U.S.A.

A reversal of the conventional carbon isotope relationship, “terrestrial-lighter-than-marine” organic matter, has been documented for two Pennsylvanian (Desmoinesian) cyclothemic sequence cores from the Midcontinent craton of the central United States. “Deep” water organic-rich phosphatic black shales contain a significant proportion of algal-derived marine organic matter (as indicated by organic petrography, Rock-Eval hydrogen index and ratios) and display the lightest δ¹³C-values (max −27.80‰ for kerogen) while shallower water, more oxic facies (e.g. fossiliferous shales and limestones) contain dominantly terrestrial organic matter and have heavier δ¹³C_kerogen-values (to −22.87‰ for a stratigraphically adjacent coal). δ¹³C-values for extract fractions were relatively homogeneous for the organic-rich black shales with the lightest fraction (often the aromatics) being only 1‰, or less, more negative than the kerogen. Differences between extract fractions and kerogens were much greater for oxic facies and coals (e.g. saturates nearly 5‰ lighter than the kerogen).A proposed depositional model for the black shales calls upon a large influx of nutrients and humic detritus to the marine environment from the laterally adjacent, extremely widespread Pennsylvanian (peat) swamps which were rapidly submerged by transgression of the epicontinental seas. In this setting marine organisms drew upon a CO₂-reservoir which was in a state of disequilibrium with the atmosphere, being affected by isotopically light “recycled-CO₂” derived from the decomposition of peaty material in the water column and possibly from the anoxic diagenesis of organic matter in the sediments.     

贵州遵义板桥剖面奥陶系湄潭组黑色页岩沉积环境条件判识及烃源意义*

华南上扬子地区下奥陶统湄潭组是一套厚度较大的泥页岩沉积,黑色页岩十分发育,但对其沉积环境的认识争议较大。文中以贵州遵义板桥剖面为例,通过对黑色页岩及其共生岩石的沉积特征及地球化学特征的综合研究,着重分析湄潭组黑色页岩沉积期的相对水深、沉积水动力条件以及古海水物理化学性质等沉积环境条件。结果表明: (1)板桥剖面湄潭组黑色页岩以颜色暗黑和水平纹理发育良好为显著特征,不产出宏体生物化石,主要以与生屑灰岩或钙质粉砂岩不等厚频繁互层的共生形式产出,说明湄潭组黑色页岩应该不是深水环境的沉积产物,而是在混积台地内低能的浅水泥质潟湖环境中沉积的;(2)与黑色页岩互层的生屑灰岩层段多发育下细上粗的逆粒序结构,顶部可见铁质氧化物,钙质粉砂岩则多发育浪成交错层理和生物钻孔,均呈现出浅水沉积特征;(3)黑色页岩的V/(V+Ni)、Th/U、V/Cr以及Ni/Co的平均值分别为0.71、5.52、1.07和2.04,均指示次氧化—氧化环境;Sr/Ba值介于0.11~2.20之间,平均值为0.55,指示淡水—半咸水环境;(4)生屑灰岩的部分层段可见明显的侵蚀面,镜下可见岩溶发育,同时其REE配分形态趋于水平,∑REE值较高、Y/Ho值较低,具暴露岩溶特征;(5)黑色页岩TOC值介于0.09%~1.05%之间,平均值为0.39%,恢复的原始TOC含量介于0.28%~3.16%之间,平均值为1.25;R_O平均值为2.095,有机质类型主要为腐泥型。综合上述分析结果,认为遵义板桥剖面湄潭组黑色页岩为一套形成于浅水、次氧化-氧化环境的有效烃源岩,高的古生产力是有机质富集的主要原因。研究结果为判识川南地区湄潭组黑色岩系沉积环境和生烃潜力提供有力的证据,也为浅水成因黑色页岩沉积环境条件判识研究提供了一个新的实例。     

Physils and organic matter-base palaeoenvironmental records of the K/Pg boundary transition from the late Cretaceous-early Palaeogene succession of the Um-Sohryngkew River section of Meghalaya,India

A continuous, shallow marine succession of the Um-Sohryngkew River section is distinct as it contains late Maastrichtian-early Danian planktonic foraminiferal zones and the K/Pg boundary, although K/Pg transition events remain inconclusive. Physils divide entire succession in to lower, middle and upper sub-divisions and represent anomalous values of redox-sensitive elements (δCe, La/Yb and Gd/Yb) in biozone CF3. Organic matter when analyzed show TOC and C spikes in biozone CF3. Illite thermometry also revealed sudden increase in the palaeo-temperature (>140 °C) for yellowish brown 1–2 mm thick organic rich clay layer in biozone CF3. Interestingly, conspicuous increase in the short chain n-alkanes and fatty acids is observed in the biozone CF3, although, long chain n-alkanes (C₂₇–C₃₃) derived from terrestrial plants show low abundance throughout the succession.High amount of combustion derived fluoranthene, pyrene, chrysene, benzo(a) anthracene PAH compounds found in the biozone CF3 are analogous to those reported from the K/T boundary sections of Stevns Klint, Gubbio, Woodside Creek and Arroyo el Mimbral. The pyrolitic signatures of these organic macro-molecules reflect global fire, caused distress to biota (during the deposition of biozone CF3 layer) which is coincidental with the well documented Ce anomaly layer, but, preceded by planktonic foraminiferal change in biozone P0 and PGE anomaly bearing layer in the biozone CF2. These organic macro-molecules reflect global fire, induced by the heat supplied by the late Cretaceous Abor and/or Deccan extrusions perhaps linked with the K/T transition events as later initiated prior to the K/Pg boundary, however, the main episode of Deccan volcanic activity occurred ∼300 ky earlier or at the K/Pg boundary itself. The deposition of 1–2 mm thick, yellowish brown, smooth (with conchoidal fracture) pyrite nodules and micro-spherules bearing organic rich clay layer marked with the decrease in the carbonate content (2.43%) that lies at the contact between the silty mudstone and grey calcareous shale located in the biozone CF3 of this succession coincides with the first appearance of Pseudoguembelina hariaensis representing age of 66.83–65.45 Ma is also related to the India’s collision with the Eurasia and Burma andextrusion of Abor volcanic.These events also endorse succeeding events such as anomalous concentration of platinum group of elements and concentration of spherules during biozone CF2, which are other end Cretaceous events before the advent of the K/Pg boundary.     

Cenomanian-Turonian high-resolution biostratigraphy of north-eastern Mexico and its correlation with the GSSP and Europe

Thin-bedded and millimetrically laminated platy marly limestone quarried near Vallecillo, north-eastern Mexico, contains abundant excellently preserved marine fossils. Planktic foraminifers, inoceramids, and ammonites occur throughout the 7.7-m-thick section of this plattenkalk and provide a precise and detailed biostratigraphic zonation from the uppermost Cenomanian to early Turonian, with a mixed assemblage of Tethyan and Western Interior Seaway faunal elements. Five species of inoceramids are present and described herein: Inoceramus pictus pictus, Mytiloides hattini, M. puebloensis, M. goppelnensis, and M. kossmati. The faunal characteristics of the Vallecillo fossil assemblage combined with the monotonous lithology are favourable attributes for correlation with the GSSP at Pueblo, Colorado, and the Eastbourne section in southern England. The first appearances (FAs) of Watinoceras and Mammites nodosoides are considered approximately isochronous and thus suited for long-distance correlation. In contrast, the FAs of Pseudaspidoceras flexuosum, Fagesia catinus, and Helvetoglobotruncana helvetica are clearly diachronous. The range of M. kossmati needs further evaluation.     

A Combination of N2 and CO2 Adsorption to Characterize Nanopore Structure of Organic-Rich Lower Silurian Shale in the Upper Yangtze Platform, South China: Implications for Shale Gas Sorption Capacity

The pores in shales are mainly of nanometer-scale, and their pore size distribution is very important for the preservation and exploitation of shale gas. This study focused on the organic-rich Lower Silurian black shale from four wells in the Upper Yangtze Platform, and their TOC, mineralogical composition and pore characterization were investigated. Low pressure N2 and CO2 adsorption were conducted at 77.35 K and 273.15 K, respectively, and the pore structures were characterized by modified Brunauer-Emmett-Teller (BET), Dubinin-Radushkevich (DR), t-plot, Barrett-Joyner-Halenda (BJH) and density functional theory (DFT) methods and then the relationship between pore structure and shale gas sorption capacity was discussed. The results indicate that (1) The Lower Silurian shale has high TOC content of 0.92%–4.96%, high quartz content of 30.6%–69.5%, and high clays content of 24.1%–51.2%. The total specific surface area varies from 7.56 m2/g to 25.86 m2/g. Both the total specific surface area and quartz content are positively associated with the TOC content. (2) Shale samples with higher TOC content have more micropores, which results in more complex nanopore structure. Micropore volumes/surface areas and non-micropore surface areas all increase with the increasing TOC content. (3) A combination of N2 and CO2 adsorption provides the most suitable detection range (~0.3–60 nm) and has high reliability and accuracy for nanopore structure characterization. (4) The TOC content is the key factor to control the gas sorption capacity of the Lower Silurian shale in the Upper Yangtze Platform.