Black Shale Facies Research Articles

A New Epistemological Insight of the Coniacian-Santonian Oceanic Anoxic Event (OAE3)

Eighteen planktic and eleven benthic foraminiferal species were recorded from the dark grey to black shale facies of the Matulla Formation in Abu Zeneima area, West-Central Sinai, Egypt. The faunal assemblage is dominated by cosmopolitan whiteinellids, marginotruncanids, Dicarinellids, Contusotruncanids and Heterohelicids. The planktic species with high taxonomic diversity were used to zone the Coniacian and Santonian stages, as well as define the Coniacian/ Santonian boundary, while benthic foraminifera is of minor contribution in age assignment. The stratigraphic analysis of the relations and ranges of these fauna led to the recognition of five biozones<i>; Dicarinella primitiva</i> or <i>Huberella huberi</i> or <i>Marginotruncana</i> <i>sinuosa</i> for the Coniacian, while <i>Dicarinella concavata</i> and <i>Dicarinella asymetrica</i> for the Santonian. Also, the Coniacian/Santonian boundary was delineated, considering the appearance of <i>Dicarinella</i> <i>concavata</i> and disappearance of <i>Huberella huberi</i>, as well as the<i> </i>increase of Marginotruncanids (<i>M. renzi, M. sigali, M. marginata, M. pseudolinneiana</i>….etc.). Furthermore, the black shales found in the middle part of the Matulla Formation were attributed to the Coniacian-Santonian Oceanic Anoxic Event (OAE3). The occurrence of black shales with planktic foraminifera during the Coniacian–Santonian interval in several countries belonging to five continents, was the main impetus to render this event a global event.

Facies association and Paleo depositional environment of the exposed Neogene succession in the Sitapahar Anticline of Rangamati Area, Chittagong Tripura Folded Belt (CTFB) region, Bengal Basin, Bangladesh

Neogene sedimentary rocks are exposed in the Chittagong Tripura Folded Belt (CTFB) Region in Bengal Basin. Sitapahar anticline is situated in the middle part of the CTFB Region. In this structure, eleven litho-facies have been identified in fluvial, overbank, shallow marine, deltaic, and deep marine depositional environments. Surma Group is composed of Bhuban and Boka Bil formation during Miocene time and was deposited in shallow marine, deltaic, and deep marine environments and characterized by the identification of interbedded shale facies (ShI), fine sandstone facies (SFs), lenticular laminated silty shale to shale facies (Zw), wavy bedded sandstone siltstone facies (Slsh), flaserbedded sandstone siltstone facies (Sxhs), ripple laminated sandstone facies (Sr), black Shale facies (Shb) and nodular shale facies (Zn). While, Pliocene Tipam group and Plio-Pleistocene Dupi Tila Formations were deposited in fluvial intervention and characterized by crossbedded sandstone facies (Sxbt), mudstone facies (Mf), interbedded mudstone and sandstone facies (MI).

Sedimentary Facies Types and Their Control of Reservoirs in the Lower Jurassic Lacustrine Facies Shale of the Lianggaoshan Formation, Northeastern Sichuan Basin, China

In recent years, new breakthroughs have been made in the field of shale oil and gas exploration in the Lower Jurassic Lianggaoshan Formation in Sichuan Basin. At present, there is a lack of systematic studies on reservoir properties and sedimentary facies of the Lianggaoshan Formation shale. Therefore, in this study, taking the Lianggaoshan Formation in Sichuan Basin as an example, the sedimentary facies types of shale reservoirs and their control over shale oil and gas are systematically studied, based on a large number of outcrops, experimental testing, logging, and seismic interpretation methods. The results show that five sedimentary microfacies are developed in the Lianggaoshan Formation in the study area, namely, semi-deep lake mud, shallow lake mud, wave-influenced shallow lake mud, delta-influenced shallow lake mud, and underwater interbranch bay microfacies. The stratum thickness of the Lianggaoshan Formation is in the range of 26–315 m, and mainly distributed in the eastern region, but rapidly thinned in the northwestern region. The sedimentary sequence framework of the Lianggaoshan Formation has been constructed. Moreover, the lithology of the Lianggaoshan Formation shale has been divided into three types, including shale, massive mudstone and silty mudstone. The brittleness index and total organic carbon (TOC) value of three types of shale show a negative correlation. Silty mudstone has the highest brittleness, while that of black shale is the lowest. For porosity and permeability, massive mudstone is better than silty mudstone, and silty mudstone is better than black shale. There are many kinds of matrix pores in the Lianggaoshan Formation shale, and the development degree of inorganic pores is higher than that of organic pores. Finally, based on the analysis of oil-bearing, pore types, physical properties and productivity, it is considered that black shale facies is the most favorable lithofacies type. The deep–semi-deep lacustrine facies belt obviously controls the shale oil enrichment of the Lianggaoshan Formation.

Coupled sedimentary and δ13C records of western Thailand and South China from Middle to Late Ordovician: Sea-level and climate changes prior to the Hirnantian glaciation

The Middle to Late Ordovician sedimentary records are of great interest because they potentially contain critical signatures about triggers of the Great Ordovician Biodiversification Event and the Late Ordovician Mass Extinction. A long climate cooling has been proposed by some scholars, which may have initiated in Middle Ordovician and eventually culminated in the Late Ordovician (Hirnantian) glaciation; however, the origin, frequency, magnitude and duration of this cooling-glaciation event are still in debate. A sedimentary and multi-stratigraphic study of the Middle to Late Ordovician transition records may help reveal the evolutionary history of this great climate change. Here we document and correlate the sedimentary facies, conodont biostratigraphy and carbon isotope chemostratigraphy of latest Darriwilian through early Katian limestone successions in the Kanchanaburi area of western Thailand and in South China. This correlation reveals two remarkably cross-regional transgression events and their following sea-level highstand periods. The first transgression happened in the latest Darriwilian, leading to the deposition of the Tha Manao Formation of restricted carbonate facies at the Nautiloid Site Geopark in Thailand, Sibumasu Terrane, and the Miaopo and chert-rich Hulo formations of black shale facies in South China. The second transgression was initiated in the earliest Katian, forming the extensively distributed open-marine carbonates of the Pa Kae and Pagoda formations in platform settings, and the debrite-rich, deep-water carbonate of the Yenwashan Formation in slope settings. We also document coupled changes in depositional facies and carbon isotopic compositions, showing that the rising limb of the Guttenberg Isotopic Carbon Excursion (GICE) co-occurred with the second transgression period. The South China and Thailand Sibumasu records are comparable to those of many North American successions, indicating a presumably global sea-level rise in the early Katian, which may have been caused by the waning of the potential Darriwilian–Sandbian icecaps.

A First Record of Lower Jurassic (Toarcian) Coleoid Jaws in Siberia

In the present paper, we describe several coleoid jaws discovered in the lower Toarcian black shales, cropping out along the Vilyui River (Yakutia, Russia). This is the first record of a Lower Jurassic coleoid jaw outside Europe and the first report of such a finding from the Mesozoic of Siberia. The described coleoid jaws demonstrate the same mode of preservation and morphology as the coeval jaws previously reported from Europe. Their preservation in Siberia became possible due to the widespread occurrence of black shale facies associated with the early Toarcian oceanic anoxic event (TOAE).

Stratigraphy and Paleoenvironment(s) of the Late Cretaceous deposits in Ohafia area, Afikpo Basin, Southeastern Nigeria

Lithofacies and biofacies data extracted from three outcropping sections of Nkporo and Mamu Formations’ sediments in Ohafia area were used for paleoenvironmental and sequence stratigraphic interpretations. The five (5) lithofacies distinguished from the sections were Dark-Grey to Black Shale Facies (A), Oolitic Limestone Facies (B), Calcareous Siltstone Facies (C), Sandstone Facies (D) and Heterolithic Facies (E). The attributes of these lithofacies, including lithotype, rootlets, bioturbation, reducing conditions and microfossil contents, were used to discriminate the units into various environments of deposition and paleodepths. The oolithic limestone facies for instance was interpreted as deposits in shallow coastal areas with high turbulence, especially, in warm shallow marine environments. Paleobathymetry of the lithofacies ranged from non-marine through coastal deltaic, shallow inner neritic, inner neritic to middle neritic paleodepths. Micropalaeontological analysis of the sediments revealed a complete absence of planktonic foraminifera while agglutinated benthic assemblages (dominated by Haplophragmoides spp., Textularia spp. and Anomalinoides spp.), calcareous benthic foraminifera (Gavelinella guineana and Planulina nacatochensis), Gastropods, Ostracods, and Arenaceous indeterminate were recovered. The recognition of Bolivina afra-Haplophragmoides talokaense assemblage zone in the Mamu Formation conferred Maastrichtian age to the deposits. The presence of palynological assemblages like the Adallussiella polymorpha were used to assign Late Campanian age for the Nkporo Formation. Sequence stratigraphic analysis using the integration of foraminiferal, paleontological and lithofacies data identified two incomplete depositional sequences. The first sequence identified in the Nkporo Formation, comprised a Highstand systems tract (HST) bounded at the top by a sequence boundary (SB) dated 75 Ma. The SB also marked the beginning of the second depositional sequence that occurred in the successions of the Mamu Formation and comprised of Transgressive systems tract (TST), bounded at the top by a maximum flooding surface (MFS) dated 69.8 Ma. The results of this study have indicated the presence marker beds and constrained stratigraphic surfaces which can be used to map the identified play elements (source, reservoirs, traps and seals) in the study area. The surfaces are thus essential ingredients in basin-wide correlations for improved hydrocarbon exploration and development in the Afikpo and other adjoining inland basins of Nigeria.

Paleoenvironment, organic matter maturity and the hydrocarbon potential of Menilite shales (Silesian Unit, Polish Outer Carpathians) – Organic and inorganic geochemical proxies

The Oligocene Menilite shales (Silesian Unit, Polish Outer Carpathians), an example of the organic-rich black shale facies of the Paratethys, were sampled in the two sections of Znamirowice and Kobielnik, representing proximal to the land and open-marine environments, respectively. Terrestrial organic matter (OM) imprinted in maceral and biomarker composition was probably supplied by the fresh-water runoff, as suggested by the total organic carbon (TOC)–total sulfur (TS) values and low Sr/Ba ratio in the Znamirowice section. In contrast, normal marine-salinity conditions and the scarcity of higher plant-derived biomarkers or macerals in the Kobielnik section suggest the deposition in a more distal environment. Abundant alginite and bituminite, high TOC content and EF P2O5 values, together with biogenic silica and the occurrence of triaromatic dinosteroids indicate a medium-to-high primary productivity, represented by algal–bacterial communities including silica-secreting organisms.The inorganic proxies show that the analysed rocks were mainly deposited in unrestricted basin probably affected by temporary upwelling. Only a small number of samples approach the restricted basin model. Concentrations of redox-sensitive elements (e.g. Mo, U, V, Ni, Co) corroborating with biomarker proxies (homohopane index, pristane/phytane) allow us to suppose oxic conditions within the water column with periods of oxygen reduction in the bottom waters and mainly dysoxia/anoxia in the sediment during the Menilite shale formation.The studied Menilite shales are source rocks with good-to-excellent hydrocarbon potential, characterised by mostly immature OM. However, the hopane and sterane isomerisation ratios show a more advanced stage of thermal maturation in the Kobielnik section, reaching the threshold of the oil window. In the Znamirowice section the OM is still immature.

Characterization of Cenomanian-Turonian oxic facies in Tunisia: sequence stratigraphy and correlation of recorded bioevents

On the southwestern Tethyan Margin of Tunisia, the well-known anoxic black shale facies of the Cenomanian-Turonian transition contrasts with oxic fossiliferous carbonates, first characterized here as lateral equivalents. In terms of sequence stratigraphy, the analysis of four wisely sampled sections in Central and Southern Tunisia led to interpret these deposits as a transgressive interval (TST). This interval spans the Whiteinella archaeocretacea Zone of foraminifera and is capped with a glauconite-rich highly bioturbated maximum flooding surface (MFS). In the reference section of Oued Bahloul and the Kalaat Senan area of North-Central Tunisia, these deposits overlay a Shelf Margin Wedge made of conglomeratic and bioclastic limestones. In South-Central and Southern Tunisia, the TST is characterized by the onset of oxic facies relaying laminated carbonates with local emergence surfaces. The analysis of both oxic and anoxic facies from the Cenomanian-Turonian transition allows identifying five successive bioevent markers, known elsewhere within the Tethyan domain: the extinction of the foraminifera genus Rotalipora (or Thomasinella), the Heterohelix shift, the Whiteinella proliferation, the filament event, and the appearance of Helvetotruncana helvetica. Among these bioevents, the Heterohelix shift coincides with the transgressive surface, while the filament event announces the maximal flooding surface. These Cenomanian-Turonian transition bioevents are of a particular relevance for regional and long-distance high-resolution correlations.

Bottom- and pore-water oxygenation during the early Toarcian Oceanic Anoxic Event (T-OAE) in the Asturian Basin (N Spain): Ichnological information to improve facies analysis

Detailed bed-by-bed ichnological and sedimentological analysis of the early Toarcian Oceanic Anoxic Event (T-OAE) in two sections from the Asturian Basin – Rodiles and Lastres – reveals changes in the macrobenthic tracemaker community mainly induced by oxygen fluctuations. Ichnological analysis enabled us to document an abundant and diverse trace fossil assemblage, with ten ichnogenera, probably corresponding to alternations between the Zoophycos ichnofacies and the distal expression of the Glossifungites ichnofacies: Arenicolites, Chondrites, Diplocraterion, Halimedides, Palaeophycus, Planolites, Rhizocorallium, Thalassinoides, Trichichnus and Zoophycos. Six ichnofabrics were differentiated: laminated?, Diplocraterion–Thalassinoides, Chondrites-mottled, Chondrites–Palaeophycus–Planolites, Chondrites–Halimedides–Planolites and Chondrites-laminated. Ichnological features (including ichnodiversity, distribution of traces and cross-cutting relationships) allowed us to characterise a well-developed multi-tiered macrobenthic tracemaker community, as well as distinguish different tiering models associated with variations in bottom- and pore-water oxygenation. Anoxic conditions associated with the T-OAE corresponded mainly, but not exclusively, to black shale facies. Minor-order oxygen fluctuations from suboxic/anoxic to disoxic/oxic conditions developed during the middle part of the T-OAE. Laminated intra-T-OAE dark and light grey marls show lithological features similar to those of bioturbated pre- and post-event marls; thus, the ichnological information provides evidence that the T-OAE had a larger effect on biota than on depositional features.

Characteristics of Chang 7 shale in Qingyang area, Ordos Basin

Chang-7 shale is the most important source rock of Mesozoic in Ordos Basin. The characteristics of Chang 7 shale in Longdong area of Ordos Basin are analyzed from the aspects of shale thickness, distribution, sedimentation, petrology and organic geochemistry. The results show that the shale in the study area is mainly distributed in the middle and lower part of Chang-7 member. The shale is mainly composed of deep lake and semi deep lake facies, with a thickness of 10m-40m and good horizontal continuity. The lithofacies are mainly black shale facies, laminated mudstone facies and silty mudstone or silty mudstone facies. The organic matter abundance is high, and the types of organic matter are mainly type I and type II, and they are in the mature evolution stage It has good oil-bearing property. In tight oil accumulation, shale is not only an important material source, but also provides an important reservoir forming power. Only with the existence of shale, can oil form industrial reservoir.

Copper isotope evidence of particulate shuttle dynamics in the Late Pennsylvanian North American Midcontinent Sea, with implications for glacio-eustatic magnitude

The North American Midcontinent Sea (NAMS) covered a vast area during Late Pennsylvanian glacio-eustatic highstands, providing a laboratory for analysis of the internal watermass dynamics of large cratonic seas (of which few exist in the Recent). In this study, a novel proxy, copper (Cu) isotopes, was used to gain a better understanding of NAMS watermass dynamics. We analyzed the 63-cm-thick Stark Shale (Dennis Formation, Missourian Stage, Upper Pennsylvanian) in the Iowa Riverton core (IRC) at a centimeter scale in order to reconstruct secular variations in Cu-isotopic compositions (δ65Cu) and other geochemical proxies. The gray shale facies yielded δ65Cu of +0.02 ± 0.06‰ (2σ, n = 14), similar to detrital Cu in modern marine sediments (δ65Cu = +0.08 ± 0.20‰; 2σ, n = 42). In contrast, the black shale facies, in which the proportion of authigenic Cu hosted by the organic fraction is 50–100% of total Cu content, exhibits heavier δ65Cuauth values, mostly between +0.09 and +0.43‰. In modern marine systems, one of the main processes leading to 65Cu-enriched sediment compositions is adsorption of aqueous Cu onto Fe-Mn particulates. The black shale facies of the Stark Shale exhibits four peaks in Mo/U ratios that are indicative of an active particulate shuttle because of the tendency of Mo to adsorb more strongly than U to Fe-Mn particulates. The Sr/Ba ratio, a paleosalinity proxy, shows correlated variations, with high values (indicative of more fully marine conditions) linked to low Mo/U and light δ65Cu, and low values (indicative of brackish conditions) linked to high Mo/U and heavy δ65Cu. These considerations suggest that the flux of isotopically heavy Cu to the sediment–water interface via Fe-Mn cycling was enhanced during brackish intervals, with subsequent reductive dissolution of Fe-Mn particulates allowing transfer of hydrogenous Cu to the organic fraction of the sediment. Control of δ65Cu by Fe-Mn cycling is supported by the results of a sequential extraction experiment showing that organic δ65Cu is positively correlated with Mo/U. These findings provide evidence of large fluctuations in watermass salinity and sea-level elevation within the NAMS at timescales of ∼104 yr during core shale deposition, as previously inferred but not conclusively demonstrated until now. The dynamics of particulate shuttle operation suggest that sea-level fluctuations must have had amplitudes of at least a few tens of meters. These fluctuations ended during the final stages of black shale deposition, probably owing to overall sea-level fall and permanent shallowing of the study site into the oxic, low-salinity surface layer of the NAMS.

Glacial origin of the Cryogenian Nantuo Formation in eastern Shennongjia area (South China): Implications for macroalgal survival

The Cryogenian Period was characterized by global glaciations, imposing extreme environmental stresses on early multicellular organisms. Globally distributed diamictite deposits are the major evidence for these glaciation events, but the glacial origin of at least some of these diamictite deposits has been disputed, complicating interpretations of the environmental conditions influencing the evolution of life in the Cryogenian (~720–635 Ma). In this study, we conducted a systematic sedimentological investigation of the Marinoan-age (~650–635 Ma) Nantuo Formation in six sections representing a ~20-km-long, east–west transect in the eastern Shennongjia area of South China. The Nantuo Formation, which ranges in thickness from ~2 m in the east to ~300 m in the west, comprises diamictite, conglomerate, lonestone-bearing sandstone, green siltstone, and black shale facies. The eastern sections (e.g., Longxi and Wushanhu) consist mainly of diamictite facies exhibiting micro-scaled rotation structures, microshear, grain stacking, grain-to-grain contacts and water-escape structures, supporting their deposition in a subglacial setting. In contrast, the western sections (Shijiahe, Liziping and Songluo) consist of massive and graded diamictite and conglomerate facies, consistent with an origin as glacially induced submarine debris flows. Furthermore, the lonestone-bearing sandstone facies in these sections contain abundant dropstones and mud pellets within finely-laminated turbidites, indicating an ice-rafted origin with locally reworking by turbidity currents. The green siltstone and black shale facies lack glacigenic features and may represent late deglacial deposits. Based on thickness and facies variations among the study sections, we infer that the Nantuo Formation records glacial sedimentation on a complex paleotopographic surface that was incised into the underlying bedrock, with subglacial deposits to the east transitioning into glaciomarine deposits to the west of study area. The distal Songluo section likely records three cycles of glacial advance and retreat in the form of a facies alternation between diamictite and black shale, consistent with a dynamically fluctuating ice margin. In summary, our sedimentological investigation supports a glacial origin for the Nantuo Formation in the eastern Shennongjia area. The occurrence of the benthic macroalgal-dominated Songluo Biota in the black shale facies at Songluo suggests that strong glacial retreats provided niches for macroalgal survival in the Cryogenian glacial ocean.

Evidence for elevated and variable atmospheric oxygen in the Precambrian

Oxygen concentration in the Precambrian atmosphere-ocean system is currently estimated from a series of geochemical proxies that depend on the concentration of redox-sensitive trace elements or their isotopic ratios measured in organic-bearing marine shales. This research has indicated that atmospheric oxygen content throughout the Precambrian was very low, <0.01% of the present atmospheric level (PAL; <0.002% O2). However, a rise in oxygen, known as the Great Oxygenation Event (GOE), is thought to have occurred around 2400 to 2300 Ma. Here we combine the results from two independent methods to estimate atmospheric oxygen concentrations during the Precambrian. Firstly, measurement of oxygen concentrations in sedimentary halite and ooids at four times – around 800, 1440, 2000 and 2660 Ma. Secondly, measurement of the Se/Co ratio in 2,037 sedimentary pyrite grains from 310 black shale samples spread throughout the Precambrian. By combining these two approaches we have derived the following relationship: AtmosphericO2%=30×P/1+PWhereP=100.89×logSe/Co+0.07,r2;r2=0.85The oxygen curve revealed by this relationship suggests a broad first-order trend of increasing oxygen of ~0.25 to 27% from 3500 to 1850 Ma, followed by a general decrease to around ~1.0% at ~1000 Ma, with a spike around 1400 Ma. Oxygen then rose gradually through the Ediacaran to reach a maximum of 20 to 29% in the early Cambrian. Detailed analysis of sedimentary pyrite in sixteen separate black shale formations, spaced throughout the Precambrian, demonstrates consistent results using the marine pyrite proxy, with standard deviations of the predicted oxygen for each formation of between 0.4 and 7%. In seven formations, we show that each formation demonstrates an increase in oxygen passing up stratigraphy from the base of the black shale facies toward the top.

Integration of Darriwilian (Middle Ordovician) δ13Corg chemostratigraphy with graptolite biostratigraphy in the classical Röstånga area in northwestern Scania (southern Sweden)

The largely covered Middle Ordovician succession in the classic geological Rostanga area in northwestern Scania has not been studied for some 80 years. A new drill core through a succession ranging from the lower–middle Darriwilian to the lower Sandbian has provided a unique opportunity to investigate the graptolite biostratigraphy and the δ13Corg chemostratigraphy, and clarify their stratigraphic relations, through this ~90 m thick interval, which is developed within a black shale facies. The lithology, biostratigraphy and chemostratigraphy are closely similar to those of the coeval strata in the Fagelsang area, south­central Scania, including the presence of the Fagelsang Phosphorite, which was previously unrecorded in the Rostanga area. The new data are particularly important in providing evidence of the relations between graptolite biostratigraphy and δ13Corg chemostratigraphy. The Fagelsang­3 and Rostanga­2 drill core successions are currently the only Darriwilian sequences in the world where these relations have been well established. (Less)

Geochemistry of aggradational - Progradational sequence sets of the Upper Jurassic – Lower Cretaceous Vaca Muerta shales (Añelo area, Neuquén Basin, Argentina): Relation to changes in accommodation and marine anoxia

Unconventional shale plays have received marked attention over the last years because of their economic potential for hydrocarbon generation. The size and geology of the Neuquén Basin of western Argentina lends to large volumes of unconventional shale gas, particularly within the marine shales of the Upper Tithonian -- Valanginian Vaca Muerta Formation. This unit is characterized by a well-developed lithological transition from Tithonian monotonous organic-rich marly shales of several hundred metres thickness, to Lower Cretaceous increasingly carbonate-rich marls and marly limestones. Samples from four wells located in the central sector of the Neuquén Basin (Añelo area) were selected for petrographic, X-ray fluorescence and TOC analysis. Based on seismic interpretation of the strata geometry, these data were grouped according to their location in aggradational (AAS) and progradational accommodation sets (PAS). The AAS is composed of basinal black shale facies showing a planar stacking pattern, whereas the PAS is characterized by low-angle sigmoidal strata in which bottomset, foreset and topset deposits (and coeval basinal deposits) are recognized. Petrographic studies allowed to distinguish lithological variations between basinal (organic-rich, laminated unbioturbated shales and marly shales), bottomset (dark gray, diffuse laminated, slightly bioturbated shaly marls and marls), and foreset and topset deposits (gray, massive, bioturbated marls and calcareous marls with a wacky texture). Basinal deposits show a clear tendency towards compositions rich in silica. These distal deposits, together with the distalmost bottomset facies of the clinoforms, are relatively enriched in Al2O3. In turn, the content of CaO, MgO and P2O5 supplied by carbonate biogenic activity are higher in the shallower topset and foreset deposits. Distal (basinal and bottomset) deposits are significantly enriched in redox-sensitive elements (Mo, V, Co, S, Cr) and in organic productivity indicators (Zn, Ba, Cu and TOC).Basinal deposits accumulated under conditions of maximum sediment starvation, restricted circulation and low dissolved oxygen levels. These conditions partially persisted for the more distal progradational (bottomset) deposits, together with a progressive increase in carbonate intrabasinal productivity. Conversely, the foreset and topset deposits of the PAS formed under normal seawater oxygenation. These conditions favoured the increase of carbonate productivity associated with an ostensible decline of organic matter productivity and preservation. This approach to geochemical analysis in the sequence stratigraphic framework of the Vaca Muerta Formation suggests that redox conditions, biogenic carbonate productivity, detrital siliciclastic fluxes and carbon accumulation rates, were closely related to basin isolation, space available for sediment accumulation, and changes in water depth and marine water circulation.

The Devonian Horn River Group and the basal Imperial Formation of the central Mackenzie Plain, N.W.T., Canada: multiproxy stratigraphic framework of a black shale basin

This study proposes six new lithostratigraphic units and redefines two others within the Horn River Group (HRG), a shale-dominated succession recently prospected for shale hydrocarbons. Its lower Hare Indian Formation rests on a drowning unconformity. In the SOB area (the area between Norman Wells and the Keele tectonic zone), this formation consists of the Bluefish black shale, Francis Creek grey shale, and Prohibition Creek black shale members. The latter two are lateral equivalents of the grey shale Bell Creek Member (updated definition) in the northern part of the study area. The overlying Canol Formation in the SOB area is divided into the Vermillion Creek and Dodo Canyon members. The Prohibition – Dodo Canyon package is ≥100 m thick and composed of brittle siliceous mudrocks with median SiO2 68%–79%, Al2O3 6%–9%, varying amount of calcite and dolomite, and type I–II kerogen with median TOC 4.5%. The thin Canol Formation blanketing Kee Scarp carbonate banks is identified as its Dodo Canyon Member. The latter is a “sweet bed” for fracking with median SiO2 75%–79%, TOC 5.1%, and trace-metal signatures indicating strongest anoxia in the section. The Mirror Lake Member in the base of the overlying Imperial Formation consists of soft illite-rich grey shales with minor siderite and dolomite. The overlying Loon Creek Member is composed of Canol-like black shale facies in its basal part and less organic-rich, lighter-colored shales and siltstones above. The Loon Creek Member is partly coeval with an overlying succession of sandstones and siltstones of the Canyon Member in its updated definition. Conodont data do not indicate significant time gaps between lithostratigraphic units. The majority of standard conodont zones from the uppermost Eifelian (ensensis Zone) up to the middle Frasnian (punctata Zone) are recognized. Assemblages of the hemiansatus and semialternans/latifossatus zones are missing probably owing to scarcity of conodont samples. The base of the HRG approximates the Eifelian–Givetian boundary, and the Hare Indian Formation is mostly Givetian. The Vermillion Creek Member spans the Givetian–Frasnian boundary. In places where thick Kee Scarp carbonates aggrade on thick sections of the Bell Creek Member, the Kee Scarp – Canol contact is found within the transitans Zone and the Middle Frasnian punctata Zone conodont assemblage is found in the basal part of the Dodo Canyon Member. The main part of the Dodo Canyon Member and basal beds of the Imperial Formation remain undated.

The hydrocarbon source rocks of the Pliensbachian (Early Jurassic) in the Asturian Basin (Northern Spain): Their relationship with the palaeoclimatic oscillations and gamma-ray response.

Deposition of black shale facies, one of the main contributors for hydrocarbon production, is commonly assumed to be linked to Oceanic Anoxic Events (OAEs), which are supposed to be generated during warm palaeoclimats. This assumption could bias the exploration for hydrocarbon source rocks preferentially towards sediments deposited under warm palaoenvironments, as a preferential guide for hydrocarbon exploration. As a consequence, the establishment of the links between palaeotemperature and the formation of organic-rich deposits is of primary importance to find arguments on this subject. For this purpose, the Upper Sinemurian, Pliensbachian and Lower Toarcian (Lower Jurassic) deposits of the Asturian Basin in Northern Spain, including more than 100 m thick succession containing organic-rich and black shale deposits, has been studied. Correlation between palaeoclimatic data, previously obtained from oxygen isotopes, and the TOC content reveals that black shales and organic-rich sediments were deposited not only during warming intervals but also during a prominent cooling event that occurred at the Late Pliensbachian. This cooling interval has been pointed out as one of the main candidates to have developed ice caps in the poles during the Jurassic. On the contrary, no black shales were generated during the postulated Early Toarcian OAE, which coincides with a superwarming interval. Additionally, the study of the facies cycles and the measurement of gamma-ray in outcrops and its correlation with TOC content provides data on the use of natural radioactivity and sea level changes as a proxy for preliminary organic matter richness evaluation.

Synsedimentary, Diagenetic, and Metamorphic Pyrite, Pyrrhotite, and Marcasite at the Homestake BIF-Hosted Gold Deposit, South Dakota, USA: Insights on Au-As Ore Genesis from Textural and LA-ICP-MS Trace Element Studies

The Paleoproterozoic Homestake deposit, northern Black Hills, South Dakota, is the largest banded iron-formation (BIF)-hosted gold deposit in the world and one of the largest single gold deposits globally (~1,300 t Au mined at an average grade of 8.4 g/t). The origin of the deposit has remained in dispute from its discovery, with views broadly falling into two groups: (1) the gold and associated elements were externally sourced and deposited in a suitable structural and/or chemical trap (e.g., iron formation), and (2) gold was indigenous to the host iron formation, from which it was remobilized into dilatant structural zones during deformation and metamorphism. In recent times, most workers have favored the former model, appealing to the broad synchroneity of Paleoproterozoic metamorphism, felsic magmatic activity, and gold event timing. LA-ICP-MS analysis of synsedimentary to early diagenetic sulfides at Homestake and surrounding area indicates that the Au in the orebodies may have originally had a significant syngenetic component. In particular, LA-ICP-MS mapping of multistage pyrites from carbonaceous shale in the upper Poorman Formation shows that Au and a suite of trace elements (Co, Ni, Cu, Zn, Se, Mo, Ag, Sb, Te, Au, Hg, Tl, Pb, and Bi), similar to those in diagenetic pyrite from around the globe, are contained in anhedral, sponge-textured cores and nodules surrounded by relatively trace element barren, Au-poor, euhedral pyrite rims. Furthermore, interelement ratios (e.g., Co/Ni, Ni/Ag) of the trace element-rich spongy and nodular pyrites are also similar to those of typical diagenetic pyrite. LA-ICP-MS imaging of pyrrhotite in the same rocks reveals multistage growth in this mineral as well, with the earlier generation having higher concentrations of Ag, Sb, Pb, Tl, and Bi than its presumed metamorphic counterpart. Imaging of marcasite in the shales of the upper Poorman Formation demonstrates this mineral’s high abundance of W and Tl relative to all generations of pyrite and pyrrhotite. Mass-balance calculations indicate that the volume of upper Poorman Formation in the mine area (approx. 12 km3) could potentially yield ~4,500 t of Au, a value greater than the total mined resource by more than a factor of three. These geochemical data and calculations suggest that a significant portion of the gold at Homestake may have been sourced from the relatively thick carbonaceous and sulfidic black shale facies in the stratigraphic footwall to the host iron formation, in a manner similar to other sediment-hosted gold districts.

Early Jurassic carbon-isotope excursion in the Qiangtang Basin (Tibet), the eastern Tethys: Implications for the Toarcian Oceanic anoxic event

The Early Jurassic was characterized by a global disturbance of the carbon cycle known as the Toarcian oceanic anoxic event (T-OAE). This event is recorded by a large negative carbon-isotope excursion (CIE) in marine carbonates and marine organic matter, as well as fossil wood. Here, we present new high-resolution carbonate (δ13Ccarb) and organic (δ13Corg) carbon-isotope records and total organic content from the Bilong Co section in the Qiangtang Basin, the eastern Tethys. This section is high in organic carbon, and black shale facies are also identified. The Bilong Co section shows heavy δ13Corg values compared to other reported marine organic carbon-isotope values for the T-OAE. This is explained by variable mixing of terrestrial and marine organic matter. Compared to European sections, the magnitude of the negative CIE in the Bilong Co section is slightly different, which could be explained by different local paleoenvironmental conditions and strata missing. Although these, the carbon-isotope profile from the Bilong Co area displays a similar trend to those in contemporaneous strata in the western Tethyan and Boreal realms of NW Europe, which is characterized by a distinct negative excursion in the Early Toarcian. Our new data from the Bilong Co section in N Tibet provide the first high-resolution record of the Early Toarcian Oceanic Anoxic Event in the eastern Tethys.

The Silurian of south-western margin of the East European Platform (Ukraine, Moldova and Romania): lithofacies and palaeoenvironments

Silurian strata, stretching along the western margin of the East European Platform from the Baltic to the Black Sea, represent a potential target for both conventional and unconventional hydrocarbon exploration. Distribution of the black shale facies, prospective for shale gas, and the reef facies, prospective for oil, has been studied in respect of palaeoenvironments. The Silurian sequence has been investigated in the territory of Ukraine (Volyn-Podillyan Plate, Dobrogean Foredeep) and correlated with the data on Moldova and Romania (Moldovian Platform). The occurrence of Silurian strata, their thickness, and petrographic and lithological characteristics allowed reconstructing the distribution of open-shelf, reef and lagoonal facies. The reef facies migrated during the Wenlock–Middle Pridoli, shifting towards the open sea and back towards the shore, and therefore has been termed a migrating reef facies. Correspondingly, the boundary between the open-shelf and reef facies was shifting. The facies distribution was controlled by the transgressive-regressive cycles, which caused the fluctuations of the shelf water depth in different time intervals of the Silurian. The shelf water depth of about 100 m, where the top of the oxygen-minimum layer impinged on the sea bottom, was the boundary between the open-shelf facies, represented by organic-rich sediments, and the reef buildups.