Devonian Black Shales Research Articles

Comparative organic geochemistry of shale deposits of northern Appalachian Basin

The organic-rich black shales in the Appalachian basin are a vital producer of natural gas. In this study, we present new multiproxy geochemical data from the Ordovician and Devonian black shales in New York (NY) and Pennsylvania (PA). The samples include outcrop samples collected in NY (Utica Group and Marcellus Formation) and core samples from PA (Marcellus Formation, Skaneateles Formation, and Genesee Group). We combined organic geochemical data (% total organic carbon or %TOC, δ13Corg, C/N ratio, and lipid n-alkane distribution) with trace element (TE) data to identify the organic matter (OM) sources and depositional conditions. The TE analysis data shows that water conditions were variable during the deposition of these black shales, fluctuating between oxic and dysoxic conditions with occasional anoxia. There was probably a change from an open water condition (Co∗Mn = 0.2) during the deposition of the Flat Creek Formation to a more restricted exchange later during the deposition of the Indian Castle Formation (Co∗Mn = 2.9) in the Ordovician. Basin circulation likely remained restricted during the deposition of the Devonian black shales (Co∗Mn ranges from 0.4 to 1.3). Based on δ13Corg values (−32.9‰ and −29.6‰) that are more depleted than marine OM δ13Corg, C/N ratios (11.2 and 9.2) higher than marine OM, and the presence of longer chain n-alkanes in the range of C25 to C33, we suggest that bryophytes were possibly a significant organic source to the Ordovician Utica Group in NY. The kerogen type in Utica Group samples is type III, mostly terrestrial OM. The Devonian NY and PA samples show mostly bimodal distributions. In some samples, a secondary, though sometimes dominant, shorter chain peak in the range of C14 to C20 is present, in addition to the long chain peak. We suggest the bimodal n-alkane distribution signifies a mixed OM source consisting of terrestrial and marine contributions with differing degrees of thermal maturation compared to the samples with a unimodal distribution. Our results suggest that samples from the NY Marcellus Group are composed of type III kerogen, while samples from the PA Marcellus Formation, Skaneateles Formation, and Genesee Group contain both type II and type III kerogens.

Influence of Organic Matter Thermal Maturity on Rare Earth Element Distribution: A Study of Middle Devonian Black Shales from the Appalachian Basin, USA

This study focuses on understanding the association of rare earth elements (REE; lanthanides + yttrium + scandium) with organic matter from the Middle Devonian black shales of the Appalachian Basin. Developing a better understanding of the role of organic matter (OM) and thermal maturity in REE partitioning may help improve current geochemical models of REE enrichment in a wide range of black shales. We studied relationships between whole rock REE content and total organic carbon (TOC) and compared the correlations with a suite of global oil shales that contain TOC as high as 60 wt.%. The sequential leaching of the Appalachian shale samples was conducted to evaluate the REE content associated with carbonates, Fe–Mn oxyhydroxides, sulfides, and organics. Finally, the residue from the leaching experiment was analyzed to assess the mineralogical changes and REE extraction efficiency. Our results show that heavier REE (HREE) have a positive correlation with TOC in our Appalachian core samples. However, data from the global oil shales display an opposite trend. We propose that although TOC controls REE enrichment, thermal maturation likely plays a critical role in HREE partitioning into refractory organic phases, such as pyrobitumen. The REE inventory from a core in the Appalachian Basin shows that (1) the total REE ranges between 180 and 270 ppm and the OM-rich samples tend to contain more REE than the calcareous shales; (2) there is a relatively higher abundance of middle REE (MREE) to HREE than lighter REE (LREE); (3) there is a disproportionate increase in Y and Tb with TOC likely due to the rocks being over-mature; and (4) the REE extraction demonstrates that although the OM has higher HREE concentration, the organic leachates contain more LREE, suggesting it is more challenging to extract HREE from OM than using traditional leaching techniques.

Pedostratigraphic age model for Devonian red beds of NW China

Detrital zircon geochronology has been used to determine provenance, but also gives maximum depositional ages that can constrain other forms of geochronology, such as magnetostratigraphy and pedostratigraphy. Pedostratigraphy and magnetostratigraphy correlate a series of global events by spacing or intensity in a fashion comparable with a commercial barcode. The Late Devonian Shaliushui Formation of NW China is poorly fossiliferous, but shows spikes in magnetic susceptibility, Al/Si ratios, Rb/Sr ratios and the depth to carbonate nodules (Bk horizon) corresponding with the timing of named Late Devonian black shale horizons. By contrast, the Ti/Al ratios show only small variations, indicating little long-term change in the source terrain or in the hydraulic sorting of heavy minerals. Most of the detrital zircon ages are older than an age model from the pedostratigraphic correlation of deep calcic horizons corresponding to global atmospheric CO 2 spikes at times of black shale expansion, but one detrital zircon age converges with the pedostratigraphic depositional ages near the top of the section. Pedostratigraphy is widely used in Quaternary sequences, but can also be a basis for age models of rocks in deep time if supported by detrital zircon ages. Supplementary material: Paleosol and geochemical data, and youngest zircon ages for a section of Devonian rocks near Ciyao, Gansu, China are available at https://doi.org/10.6084/m9.figshare.c.7008192

The model of the oil and gas system of the Devonian sedimentary complex of the Biloliskyy block of the Dobrudja foredeep

The purpose of the article is to find out the main principal features of the ontogenesis of the oil and gas system of the Devonian deposits of the Dobrudja Foredeep. Methods and methodology - generalization of data on the lithology of Devonian deposits, analysis of materials on the development of paragenic facies associations in evaporite basins, construction of a principled model of the hydrodynamics of the Dobrudja basin on this basis and predicting the distribution of various lithogenetic types in the Devonian period. Historical-catagenetic reconstructions were carried out on the basis of the provisions of the fluid-dynamic concept of catagenesis. Results. On the basis of the reconstruction of the dynamics of the sea basin and the nature of the lithofacies zonation, the conditions of Middle-Late Devonian sedimentation were modeled. A wider development of sediments enriched in organic matter, both in the section and laterally, is argued. First of all, this applies to the Middle Devonian deposits, which, together with the Lower Devonian black shale formations, formed the body of oil and gas formation centers, that were localized in the depocenters of the Tuzliv and Alibey depressions. The Devonian sediments were affected by two major cycles of catagenesis, with active substages (so-called critical moments) associated with large-scale fluid migration. The time of manifestation of the active substage of the first cycle (Middle-Late Carboniferous) was the main one in the migration of hydrocarbons, which is associated with a sufficiently high degree of post-diagenesis of the generating strata: gradation of catagenesis MK5 (Lower Devonian), MK3 - MK4 (Eifelian). Hydrocarbons moved through the zones of regional decompaction, which were developed at three hypsometric levels. In this case, flows of hydrocarbons of different phase composition were formed. The source of the gas was the Lower Devonian deposits, and the main volume of oil was generated in the Eifelian layers. During the second cycle of catagenesis, due to a decrease in reservoir temperatures and a significant depletion of the potential of organic matter, only processes of reformation of previously formed hydrocarbon deposits are predicted. Due to the fact that the generation centers were confined to deposits of different ages, differed in temperature regime, ways of fluid migration, and were separated from each other, there are reasons to assume the existence of three oil and gas systems. Scientific novelty and practical significance. For the first time, a wider spatial and age range of development of oil and gas source strata, localized centers of generation, discrete paths and time of large-scale migration of hydrocarbons have been scientifically substantiated. The studies carried out provide good grounds for predicting the possibility of the formation of large volumes of hydrocarbons that could be accumulated in the Devonian deposits of the Dobrudja Foredeep.

Depositional Environment and Hydrocarbon Distribution in the Silurian–Devonian Black Shales of Western Peninsular Malaysia Using Spectroscopic Characterization

The present study aimed to evaluate the hydrocarbon functional groups, aromaticity degree, and depositional environment in the Silurian–Devonian Kroh black shales of western peninsular Malaysia. Fourier transform infrared spectroscopy (FTIR) was applied to measure the hydrocarbon functional groups in the sedimentary succession and associated organic matter of the black shale samples. The results showed that aromatic C=C stretching, aromatic C-H out-of-plane, aromatic C-H in-plane, and aliphatic =C–H bending are the major hydrocarbon functional groups in the Kroh shales. Also, ultraviolet-visible spectroscopy (UV-Vis) was used to evaluate the type of humic substance and analyze the sample extract ratios of E4/E6. It was revealed that the methanol-treated Kroh shale samples ranged from 0.00048 to 0.12 for E4 and 0.0040 to 0.99 for E6. The lower E4/E6 ratio (>5) indicates the dominance of humic acid over fulvic acid in the Kroh shales. The Kroh shale samples’ total organic carbon content (TOC) ranges from 0.33 to 8.5 wt.%, analyzed by a multi-N/C 3100 TOC/TNb analyzer. The comparison study revealed that the TOC content of the Kroh shale has close obtainable values for the Montney shales of Canada. Furthermore, both hydrocarbon functional groups from FTIR, and the E4/E6 ratio from UV-Vis show no correlation with TOC content. It is revealed that humic acid, aromatic, and aliphatic hydrocarbons are not the controlling factors of the enrichment of organic matter in the Kroh shales. Conversely, a positive correlation between aliphatic and aromatic hydrocarbons in the Kroh shales indicated that organic matter is thermally overmatured. The presence of humic acid and enrichment of aromatic hydrocarbons in the Kroh shales demonstrated that the organic matter in these shales contains plant-derived hydrophilic minerals, i.e., terrestrial in origin. These findings may provide clues on the depositional and thermal maturation of organic matter for the exploration efforts into the pre-Tertiary sedimentary successions of the peninsular.

Reconstructing the paleoceanographic and redox conditions responsible for variations in uranium content in North American Devonian black shales

The uranium (U) content, and more recently, the ratio between 238 U and 235 U in black shales are commonly applied as a proxy to determine redox conditions and infer organic-richness. Uranium contents typically display a linear relationship with total organic carbon (TOC) in shales. This relationship is due to the processes and mechanisms responsible for the incorporation of U into the sediment during the deposition and remineralization of organic matter. This U/TOC relationship can vary, however, and some shales display uncharacteristically low U content despite having high TOC content, while others show large enrichments of U relative to TOC. Here we examine the U to TOC ratios and U-isotope compositions of three Upper Devonian-Lower Mississippian shales: the Woodford Shale, the Cleveland Shale, and the Bakken Shale, with two study sites in Oklahoma, one site in eastern Kentucky, and three sites in eastern Montana and western North Dakota, respectively. The U/TOC ratios of each shale are distinct from one another exhibiting average ratios ranging from 3 in the Cleveland Shale, to over 10 in the Bakken Shale. The distinct geochemical composition of the three shales suggests that, although lithologically similar, each study site represents a markedly different and dynamic depositional environment. The low average U/TOC (~3) along with the relatively high δ 238 U values (~0.03‰) of the Cleveland Shale core suggests deposition along the basin margin under normal marine conditions with periods of reduced bottom water oxygenation, likely due to fluctuations in the location of the pycnocline. The Woodford Shale on the other hand, shows higher U/TOC ratios (~4, George core, ~9, Poe core) and δ 238 U (~0.02‰ average, George core, ~0.06‰ average, Poe core), which suggests an unrestricted setting with intermittent euxinic conditions. In contrast, high U/TOC ratios (2–15), and very high δ 238 U values (up to 0.55‰) in the Bakken Shale cores indicate intense metal draw-down into sediments under sulfidic waters. The results show that when the U/TOC ratios and U-isotopic compositions of each studied shale are compared to modern anoxic basins and upwelling areas, it allows for an enhanced understanding of the paleoenvironmental conditions such as basin restriction and redox state of waters within the Late Devonian epicontinental seas of North America. • U/TOC ratios of Late Devonian shales vary due to basin restriction. • Bottom water euxinia does not require strong basin restriction. • δ 238 U values provide new insight into the paleogeography of N. American basins.

The discovery of widespread agrichnia traces in Devonian black shales of North America: another chapter in the evolving understanding of a “not so anoxic” ancient sea

The discovery of widespread agrichnia traces in Devonian black shales of North America: another chapter in the evolving understanding of a “not so anoxic” ancient sea

Paleoecology and Predominance Facies of Late Devonian Foraminifera in Successions of the Catskill Delta Complex, Western New York, U.S.A.

ABSTRACTBeyond yielding signals of extinction or stressed ecological conditions, modern and ancient foraminiferal assemblages reflect specific marine depositional environments and depths. Foraminiferal predominance facies and benthic foraminiferal depth zonation has been successfully used to identify specific marine environments dating back to the Carboniferous. Using insights from modern equivalents, correlative assemblages allow for paleoecological analysis and insights. Middle to Late Devonian (Frasnian) black and gray shale beds of western New York contain hundreds of diminutive calcareous and agglutinated foraminifera. The genera within these beds are reminiscent of shallow modern predominance facies. These foraminiferal assemblages and their associated predominance facies correlate well with prior lithologic and geochemical investigations that establish this portion of the Appalachian Basin as a deltaic setting but suggest it is likely inner neritic zone. Dominant genera include several species of Ammobaculites and Saccammina which suggest that paleodepths did not exceed 50 m throughout the Frasnian. Opportunistic genera reflect a muted crisis associated with the punctata isotopic event (Rhinestreet Event) and Lower Kellwasser (Pipe Creek) events. While there are definite shifts in the diversity of assemblages between gray and black shale, the foraminiferal type and feeding mode, indicative of depth and oxygen availability respectively, there is little variation between the distinct shale units. No significance was found between total organic carbon and foraminiferal type of feeding mode. Identification at the species level is problematic but assemblages at the genus-level suggest that the depositional environment was stressed. However, the effects of these marine crisis events were not significant for these foraminifera in comparison to those frequently reported; we found no local extinction for foraminifera at least through the lower Hanover Shale, just prior to the Hangenberg marine crisis event, within this deltaic complex of the Appalachian Basin in western New York.

Petrographic and chemical structure characteristics of amorphous organic matter in marine black shales: Insights from Pennsylvanian and Devonian black shales in the Illinois Basin

Amorphous organic matter (AOM or bituminite) is a major oil-prone liptinite maceral in black shales that can be classified into two main types: low-reflectance AOM and micrinized AOM. In this study, we investigated the petrographic and chemical structure characteristics of these two types of AOM and discussed their origin based on 15 marine black shale samples from the Illinois Basin, including 5 samples from the Pennsylvanian shales and 10 samples from the Upper Devonian New Albany Shale. The studied samples are at early mature stage, with mean vitrinite reflectance ranging from 0.64 to 0.74%. Low-reflectance AOM is the dominant organic matter (OM) in all samples and micrinized AOM is common only in the Pennsylvanian strata but is very rare in the New Albany Shale. There are significant differences in reflectance, petrographic characteristics, and chemical structure characteristics between low-reflectance and micrinized AOM. Low-reflectance AOM has a reflectance of 0.30 to 0.57%, whereas the reflectance of micrinized AOM is typically >1.0%, which makes micrinized AOM brighter than low-reflectance AOM under an optical microscope. Both types of AOM have heterogeneous texture, occur as structureless organic streaks or lenses, and have abundant clay-sized mineral inclusions. Compared to micrinized AOM, low-reflectance AOM has lower aromaticity and higher aliphatic chain length (inferred from the CH2/CH3 ratio). Based on the petrographic and chemical functional groups characteristics, we proposed evolutionary pathways for the formation of low-reflectance and micrinized AOM in marine black shales during early diagenesis. AOM is originally derived from microbially degraded phytoplankton. Low-reflectance AOM formed in the fermentation zone through microbial degradation of phytoplankton by anaerobic bacteria. In comparison, micrinized AOM is a product of an advanced degree of microbial degradation of phytoplankton by aerobic bacteria that preferentially consume aliphatic functional groups over aromatics in the oxidation zone. Different depositional environments and subsequent diagenetic alterations resulted in the difference in micrinized AOM content between the New Albany Shale and Pennsylvanian shales. This study provides a better understanding of the origin, formation, and thermal maturation of dispersed AOM in black shale successions.

The type section of the Canol Formation (Devonian black shale) at Powell Creek: Critical assessment and correlation in the northern Cordillera, NWT, Canada

Abstract The Canol Formation is only 24.5 m thick at its historic type section at Powell Creek, northern Mackenzie Mountains, whereas in the off-bank sections of the Mackenzie Plain subsurface, where it is considered a high-quality shale hydrocarbon prospect, it thickens to 60–120 m. This paper reviews available lithological and conodont biostratigraphic information from the type section, discusses choices of contacts and subdivisions, and explores the limits of regional correlation using gamma spectrometry proxies. We position the base of the Canol Formation at the top of the lower resistant unit of the “allochthonous limestone beds”, the thick off-reef debris package present in this outcrop but absent in other well-known Canol sections. The base of the formation can be of a latest Givetian age as suggested by the norrisi zone conodont fauna from the “allochthonous limestone”. The top of the Canol Formation is placed at the base of a distinct, 2.1 m thick horizon with concretionary carbonate beds within the thick shale transition between the Canol and the Imperial formations. Limestone nodules from this horizon produced a conodont fauna that can occur in the jamieae to Upper rhenana zones (Frasnian zones 11–12) thereby suggesting a middle to earliest Late Frasnian age for the Canol top. The cross-section tying several outcrop and well sections across the regional facies zonation reveals that the Dodo Canyon Member, a unit erected in thick off-bank Canol sections, is traceable at Powell Creek. In this correlation, the Vermillion Creek Member, which is the lower portion of the Canol Formation in thick off-bank sections, finds its counterpart in the allochthonous limestone beds sensu MacKenzie (1970). This cross-section is the first correlation of the Canol stratotype at member level available in published sources. Thinness of the Canol Formation at Powell Creek, as well as its location in the carbonate bank toe-of-slope setting, are factors impairing its reference value and calling for more representative sections to act as reference sections and constitute a composite-stratotype for the Canol Formation.

A Geologically Based Indoor-Radon Potential Map of Kentucky.

We combined 71,930 short‐term (median duration 4 days) home radon test results with 1:24,000‐scale bedrock geologic map coverage of Kentucky to produce a statewide geologically based indoor‐radon potential map. The test results were positively skewed with a mean of 266 Bq/m3, median of 122 Bq/m3, and 75th percentile of 289 Bq/m3. We identified 106 formations with ≥10 test results. Analysis of results from 20 predominantly monolithologic formations showed indoor‐radon concentrations to be positively skewed on a formation‐by‐formation basis, with a proportional relationship between sample means and standard deviations. Limestone (median 170 Bq/m3) and dolostone (median 130 Bq/m3) tended to have higher indoor‐radon concentrations than siltstones and sandstones (median 67 Bq/m3) or unlithified surficial deposits (median 63 Bq/m3). Individual shales had median values ranging from 67 to 189 Bq/m3; the median value for all shale values was 85 Bq/m3. Percentages of values falling above the U.S. Environmental Protection Agency (EPA) action level of 148 Bq/m3 were sandstone and siltstone: 24%, unlithified clastic: 21%, dolostone: 46%, limestone: 55%, and shale: 34%. Mississippian limestones, Ordovician limestones, and Devonian black shales had the highest indoor‐radon potential values in Kentucky. Indoor‐radon test mean values for the selected formations were also weakly, but statistically significantly, correlated with mean aeroradiometric uranium concentrations. To produce a map useful to nonspecialists, we classified each of the 106 formations into five radon‐geologic classes on the basis of their 75th percentile radon concentrations. The statewide map is freely available through an interactive internet map service.

From source to sink: Glacially eroded, Late Devonian algal “cysts” (Tasmanites) delivered to the Gulf of Mexico during the Last Glacial Maximum

Abstract The source of reworked Devonian algal “cysts” in last glacial maximum (LGM) sediment in the Gulf of Mexico is traced to their host black shales, which ring the southwestern Great Lakes. The source-to-sink pathway includes intermediate storage in fine-grained LGM glacial lacustrine sediment and till. The “cysts” are pelagic chlorophyllous algae (Tasmanites and Leiosphaeridia), collectively referred to herein as tasmanitids. Radiocarbon dates of syndepositional Gulf of Mexico foraminifera, derived from accelerator mass spectrometry, bracket the Gulf of Mexico sediment age with common tasmanitids from 28.5 ± 0.6–17.8 ± 0.2 cal kyr B.P. Approximately 1400 km north of the Gulf of Mexico, tasmanitids are abundant in Upper Devonian black shales (New Albany, Antrim, and Ohio Shales) that ring the Michigan, Illinois, and Appalachian intracratonic basins. Tasmanitids were eroded from bedrock and incorporated in glacial sediment dating from ca. 28.0–17.6 cal kyr B.P. by the Lake Michigan, and Huron-Erie lobes of the Laurentide Ice Sheet. The physical characteristics of tasmanitids are ideal for long-distance transport as suspended sediment (density: 1.1–1.3 g/cc, size ranging from 63 µm to 300 µm), and these sand-sized tasmanitids traveled with the silt-clay fraction. Thus, the source-to-sink journey of tasmanitids was initiated by subglacial erosion by water or friction, sequestering in till or glaciolacustrine sediment, re-entrainment and suspension in meltwater, and final delivery in meltwater plumes to the Gulf of Mexico. River routes included the Mississippi, Illinois, Ohio, Wabash, Kaskaskia, and many of their tributaries. Reworked Devonian tasmanitids are a previously unrecognized link between their occurrence in deep-water deposits of the Gulf of Mexico and the late Wisconsin glacial history of the Upper Mississippi Valley. We propose that tracking occurrences of tasmanitid concentrations from the source area to sink, along with adjunct proxies such as clay minerals, will facilitate a more refined analysis of the timing and duration of megafloods. This study also demonstrates that isotopically dead carbon, from reworked Devonian tasmanitid “cysts,” can contaminate radiocarbon dating of LGM bulk sediment samples toward older ages.

Mineralogical and lithological properties of Domanikites from the south-east of Tatarstan Republic

The article was described main types of rocks – mudstones, carbonate-siliceous rocks, containing organic matter and carbonate breccia, based on detailed analysis of rock samples from Semiluksky horizons of Upper Devonian black shale formations, Volga-Ural region, Russian Federation. Different structures and texture features, mineral composition, and content of organic matter characterized selected types of rocks. Only in the carbonate-siliceous type of rocks is the high content of organic matter, about 9%. It was found direct dependence between the content organic matter and content in rocks silica minerals (quartz and chalcedony). Such dependence caused by joint precipitation silica minerals and accumulation of organic matter in the seafloor in conditions of active seepage of hydrothermal fluids. The presence of an abundance of biophilic elements in ascending fluids led to an explosion of biota and widespread of silica organisms. This hypothesis doesn’t disagree with any authors but considers the significant contribution of fluid factor to the deposition of Semiluksky formations.

Assessing the thermal maturity of black shales using vitrinite reflectance: Insights from Devonian black shales in the eastern United States

Thermal maturity of source rocks indicated by vitrinite reflectance (Ro) is an important and reliable parameter to determine the petroleum potential of sedimentary basins. Originally, Ro was used to characterize the degree of coalification of coals and thus works well for coal seams, but it has limitations when used to determine the thermal maturity of black shale successions. A suite of six coal-shale pairs from the Upper Devonian black shale formations in the eastern United States (New Albany Shale, Ohio Shale, Dunkirk Shale, and Rhinestreet Shale) was selected to study the applicability of Ro in assessing the thermal maturity of black shale successions. The results show that vitrinite in the studied coal samples is dominated by collotelinite, whereas vitrinite in black shales occurs as small dispersed particles (~5 μm) in the mineral matrix. When comparing the size and morphology of dispersed vitrinite particles in shales and collotelinite fragments in coals, vitrinite in shales and adjacent coals should have the same origin. The measured mean random Ro of vitrinite in coals ranges from 0.51–0.68%, and is 0.04–0.11% (average 0.07%) lower than that of dispersed vitrinite particles in enclosing shales. This observation contrasts with previously reported Ro suppression in liptinite-rich black shales. A high standard deviation of Ro measurements indicates a highly heterogeneous nature of vitrinite in black shales.Three mechanisms can be envisioned to contribute to the higher Ro of dispersed vitrinite particles in black shales relative to enclosed coals. First, small vitrinite particles may become more oxidized because small particles are more frequently suspended during transport to the site of deposition and reworking of bottom sediments after deposition than large pieces of driftwood that will turn into enclosed coal lenses during burial diagenesis. Second, misidentification of zooclast (e.g., chitinozoan) fragments as vitrinite due to loss of diagnostic morphology. Third, various macerals in the vitrinite group (e.g., corpogelinite and collotelinite) may have had different original reflectance values. Based on the petrographic characteristics of vitrinite and vitrinite-like particles, caution should be applied when using the Ro values of dispersed vitrinite and vitrinite-like particles in black shales as indicators of thermal maturity.

Inverse correlation between the molybdenum and uranium isotope compositions of Upper Devonian black shales caused by changes in local depositional conditions rather than global ocean redox variations

Coupled Mo-U isotope data from the Upper Devonian Kettle Point Formation (Ontario, Canada) provide a cautionary tale regarding interpretation of global ocean redox conditions using data from euxinic black shales. In the Gore of Chatham core, the Kettle Point black shales have high Mo concentrations (48–473 μg/g) and consistently high Mo/U ratios (≥3 times the Mo/U ratio of modern seawater), suggesting a euxinic depositional environment. These shales yield an inverse correlation (r = 0.89, p < 0.001) between authigenic U isotope (δ238U = −0.3‰ to +0.6‰ relative to CRM145 = 0‰) and Mo isotope (δ98Mo = +0.5‰ to +2.0‰ relative to NIST SRM 3134 = 0.25‰) compositions and a stratigraphic trend upsection towards lower δ98Mo and higher δ238U. These stratigraphic trends were probably not caused by global ocean redox variations, which should shift both isotope systems in the same direction. Instead, the inverse correlation between the δ98Mo and δ238U of the black shales indicates that changes to local depositional conditions in the Chatham Sag affected both isotope systems, consistent with the lithological and elemental evidence for variable paleohydrographic conditions (paleosalinity and sea-level). Black shales with the highest δ98Mo and lowest δ238U likely capture the most quantitative removal of Mo and U from more intensely sulfidic bottom waters, and thus most closely approximate global seawater isotope compositions. Lower δ98Mo and higher δ238U in the black shales likely record less quantitative Mo and U removal from weakly euxinic bottom waters and thus capture greater sediment-seawater isotope fractionation (lighter Mo isotopes and heavier 238U were preferentially removed from seawater to the sediments). Some samples with low δ98Mo (0.5–1.0‰) have high V and Mo enrichments, suggesting delivery of V and isotopically light Mo to the sediments by Fe-Mn particulates, but this process does not significantly affect U isotopes (because of weak adsorption of U onto Fe-Mn particulates) and is thus not the major driver of the inverse correlation. The highest δ98Mo (2.0‰) and lowest δ238U (−0.3‰) from the lower Kettle Point Formation represent minimum and maximum estimates of the Mo and U isotope compositions, respectively, of early Famennian global seawater. The slope of the inverse Mo-U isotope correlation for all Kettle Point black shales broadly parallels data from modern euxinic basins but is shifted to slightly lower isotopic compositions. Hence, Famennian seawater likely had δ98Mo (about 2.0–2.2‰) and δ238U (about −0.7‰ to −0.4‰) values that were slightly lower than modern global seawater (δ98Mo = 2.3‰; δ238U = −0.4‰), suggesting a mildly greater extent of euxinia (up to 5% of the seafloor) in the Famennian oceans. The opposite stratigraphic trends in δ98Mo and δ238U through the Kettle Point Formation may have caused misinterpretation of global ocean redox conditions if either isotope system was used individually. Hence, paired Mo and U isotope analyses are recommended to determine if stratigraphic trends through euxinic black shales are caused by local depositional changes or global ocean redox variations.

The pore structural evolution of the Marcellus and Mahantango shales, Appalachian Basin

Abstract The generation and primary migration of hydrocarbons in organic-rich shale leaves void space in organic matter, which is the porosity associated with organic matter commonly observed under scanning electron microscope (SEM). In this study, Middle Devonian black shale core samples were collected from three wells penetrating the organic-rich Marcellus Shale and the organic-lean Mahantango Formation in Pennsylvania and West Virginia. Pyrolysis, ion milled SEM and low-pressure nitrogen adsorption analysis were conducted to investigate the organic richness and the properties of the pore system. Vitrinite reflectance (Ro) values in the range of 1.36%–2.89% represent a maturity spectrum covering the wet-gas to post-mature zones. In general, the pore system is composed of organic matter-hosted pores and mineral-hosted pores. However, the dominant pore types and pore sizes vary stratigraphically across lithology and abundance of organic matter. All the organic matter observed in this study shows an amorphous occurrence. Pore space between mineral grains (both silt-size and clay-size) can be filled by organic matter, which contains secondary porosity generated by thermal cracking of kerogen. Mineral-hosted pores are concentrated in organic-lean samples in which secondary organic matter could not fill most of the primary pore space. The destruction of primary mineral-hosted pores and the generation of secondary organic matter-hosted pores were observed. TOC values show positive correlations with the porosity, specific surface area, and the abundance of micropores. Increasing thermal maturity correlates with a significant decrease of pore volume and surface area, primarily through diminishing or vanishing of micropores. The richness and thermal maturity of organic matter in organic-rich Devonian shale can be effective parameters for evaluation of reservoir quality and upscaling the appraisal.

Geochemical Evidence of First Forestation in the Southernmost Euramerica from Upper Devonian (Famennian) Black Shales

The global dispersal of forests and soils has been proposed as a cause for the Late Devonian mass extinctions of marine organisms, but detailed spatiotemporal records of forests and soils at that time remain lacking. We present data from microscopic and geochemical analyses of the Upper Devonian Chattanooga Shale (Famennian Stage). Plant residues (microfossils, vitrinite and inertinite) and biomarkers derived from terrestrial plants and wildfire occur throughout the stratigraphic section, suggesting widespread forest in the southern Appalachian Basin, a region with no macro plant fossil record during the Famennian. Inorganic geochemical results, as shown by increasing values of SiO2/Al2O3, Ti/Al, Zr/Al, and the Chemical Index of Alteration (CIA) upon time sequence, suggest enhanced continental weathering that may be attributed to the invasion of barren lands by rooted land plants. Our geochemical data collectively provide the oldest evidence of the influences of land plants from the southernmost Appalachian Basin. Our synthesis of vascular plant fossil record shows a more rapid process of afforestation and pedogenesis across south-central Euramerica during the Frasnian and Famennian than previously documented. Together, these results lead us to propose a new hypothesis that global floral dispersal had progressed southward along the Acadian landmass rapidly during the Late Devonian.

New Devonian plant fossil occurrences on the New South Wales South Coast: geological implications

Devonian fossil localities are summarised for coastal southeastern Australia, including both marine invertebrates representing a major late Frasnian (Late Devonian) transgression, and plant remains of Middle–Late Devonian age in associated non-marine strata. Lycopsid plant specimens (Leptophloeum) from near Majors Creek, NSW, first documented by Edgeworth David and E. F. Pittman in association with marine fossils to demonstrate a Late Devonian age, are illustrated for the first time. Lycopsid plants of presumed Middle Devonian age are illustrated from three new localities: Yowrie River inland from Cape Dromedary, Tuross River northwest of Narooma and Charlies Forest Road northeast of Braidwood. The Yowrie River occurrence (in float) indicates probable Middle Devonian strata upstream that are not shown on published maps. The Tuross River occurrence is in sheared and folded strata previously mapped as Ordovician, and the Braidwood occurrence demonstrates Devonian black shales adjacent to previously mapped Ordovician. Evidence supporting alternative thrust or simple fold interpretations for the Budawang Synclinorium is discussed, and relevant Ordovician fossil localities are listed.

Rinistachya hilleri gen. et sp. nov. (Sphenophyllales), from the upper Devonian of South Africa

A rich and diverse plant assemblage has been excavated from latest Devonian (Famennian) black shales of the Witpoort Formation (Witteberg Group) at Waterloo Farm, close to the city of Grahamstown (South Africa). Several specimens of a new sphenopsid have been collected. The description of this as a new taxon, here named Rinistachya hilleri, gen. et sp. nov., provides an important addition to the scarce early record of the group. Rinistachya hilleri presents a novel architecture that include apparently plesiomorphic characters, reminiscent of the organisation of the Iridopteridales (including the production of two types of laterals at one node, the location of fertile parts in loose whorls on lateral branches and an organisation of the fertile parts in which they branch several times before bearing distally elongate sporangia). Other characters unambiguously nest Rinistachya within the Sphenopsida (including presence of planate and slightly webbed ultimate appendages and lateral strobili made of successive whorls of fertile leaves with fertile parts located at their axil). This provides strong support for a close relationship between Sphenopsida and Iridopteridales. Rinistachya furthermore represents the first record of a Devonian sphenopsid from Gondwana and extends the known distribution of the Sphenopsida from the tropics to very high palaeolatitudes. It is a new sphenopsid with a peculiar organisation. The new taxon allows better characterization of the initial evolutionary radiation at the base of the group.

Vanadian and chromian garnet- and epidote-supergroup minerals in metamorphosed Paleozoic black shales from Čierna Lehota, Strážovské vrchy Mountains, Slovakia: crystal chemistry and evolution

AbstractSilicate minerals enriched in V, Cr and Mn including garnets and epidote-supergroup members, in association with amphiboles, albite, hyalophane, titanite, chamosite, sulfides and other minerals occur in Devonian black shales near Čierna Lehota in the Strážovské vrchy Mountains, Slovakia. The garnets have high concentrations of V, Cr and Mn (up to 17 wt.% V2O3, ≤11 wt.% Cr2O3and ≤ 21 wt.% MnO) and several compositional types. Vanadian-chromian grossular (Grs 1) usually preserves primary metamorphic oscillatory zoning, whereas solid solutions between goldmanite (Gld 2A,B), V- and Cr-rich grossular and spessartine (Grs 2A,B, Sps 2) form irregular domains or crystals with variable zoning. Dominant substitutions in the garnets include CaMn–1and (V,Cr)Al–1, resulting in coupled Ca(V,Cr)Mn–1Al–1. Epidote-supergroup minerals occur as abundant anhedral crystals with variable compositional zoning. Nearly all crystals have a complete zoning sequence beginning withREE-rich allanite-(La), followed by mukhinite and by V- and Cr-rich clinozoisite to mukhinite and V- and Cr-poor clinozoisite. In common with garnets, the epidote-supergroup minerals are enriched in V, Cr and Mn (&lt;7 wt.% V2O3, &lt;5 wt.% Cr2O3and &lt;3 wt.% MnO). Lanthanum is the dominantREE(up to 11.5 wt.% La2O3) in allanite-(La). The composition of epidote-supergroup minerals is controlled byREEFe2+(CaAl)–1,REEMg(CaAl)–1,REEMn2+(CaAl)–1andREEFe2+(CaFe3+)–1substitutions introducingREE, together with VAl–1and CrAl–1substitutions. The negative Ce and slightly positive Eu anomalies displayed in chondrite-normalized patterns and enrichment in V, Cr and Mn are ascribed to the geochemical properties of the protolith. The minerals investigated exhibit multi-stage evolution: (1) presumed low-grade greenschist-facies metamorphism; and (2) development of V- and Cr-rich zones in both garnet- and epidote-supergroup minerals which result from late-Variscan contact metamorphism due to granitic intrusion of the Suchý Massif. Decreased temperature following the metamorphic peak probably resulted in the formation ofREE-, V- and Cr-poor clinozoisite and secondary garnet.