Devonian Carbonate Research Articles

Recurring cryptic ecosystems in Lower to Middle Devonian carbonate mounds of Hamar Laghdad (Anti-Atlas, Morocco)

Exceptionally well-preserved fossil cryptic communities have been discovered in submarine cavities of the Lower and Middle Devonian (Emsian to Givetian) mud mounds in the Hamar Laghdad area (Anti-Atlas, Morocco). The cryptic biota encrusted the roofs of the cavities and grew predominantly oriented upside down. These unique coelobiotic communities were dominated by various solitary rugose corals, which very often displayed a “calice-in-calice” growth pattern. Apart from the rugosans, the cavities were inhabited by other sessile invertebrates: tabulate corals, cladochonids, crinoids and lithistid sponges. Many of the skeletons were encrusted by microbial structures. The high density of organisms overgrowing each other points to intensive competition for space in all studied Devonian submarine crypts. The late Emsian to latest Givetian cavities were colonised by various sets of coelobiotic inhabitants mostly originating from the surrounding deep-water marine environments. It is suggested that the local species pool was a decisive factor in determining the ecological succession and taxonomic structure of cryptic communities. The changes in taxonomic composition and diversity, over time and space, were primarily related to variations in local physical environment. Consequently, the communities in crypts affected by venting of high-temperature fluids or hydrocarbon seepage were of low diversity and included endemic elements and characteristically small specimens.

Three-dimensional interpretation of tectono-sedimentary evolution and hydrocarbon prospectivity by the integration of airborne gravity gradiometer, regional gravity, magnetic, and two-dimensional seismic data in the Canning Basin, Western Australia

The Canning Basin is a largely unexposed and underexplored frontier basin, formed mostly in the Paleozoic. Geological knowledge of this basin is based predominantly on sparse regional “vintage” two-dimensional seismic and small three-dimensional (3-D) seismic surveys and less than 230 exploration wells. Following seismic interpretation, an integrated interpretation was completed on airborne gravity gradiometer (AGG), magnetic, seismic, well, and complementary data along the southwestern margin of the Fitzroy trough and Gregory subbasin. Seismic data were reinterpreted using AGG data to produce a better constrained geological model. A basement structure map, two intrasedimentary structure maps, and a formation distribution map were produced. The interpretation of seismic profiles, validated through 2.5-dimensional gravity gradiometer modeling, is essential to this workflow. Repeatedly reactivated west–northwest and northwest structural trends, inherited from Proterozoic orogenies, respectively delineate the Fitzroy trough and the Gregory subbasin with its northwestern structural extension into the Fitzroy trough, the Gregory subbasin trend. Subsidence occurred during two periods of extension. An asymmetric extensional system of the Fitzroy trough controlled Ordovician–Silurian deposition of the Carribuddy Group. Devonian–Carboniferous subsidence defines the Gregory subbasin trend. This Pillara extension reactivated structures in the east of the Fitzroy trough. Simultaneous activity of both extensional fault systems and growth faulting controlled the facies and thickness distribution of carbonates and clastics of the early Carboniferous Fairfield Group. The Meda and Fitzroy transpressional phases inverted faults of the Gregory subbasin trend and Fitzroy trough, producing prospects by structural interference. The improved understanding of tectono-stratigraphic relationships, including the 3-D distribution of carbonate reservoirs, benefited the planning of seismic surveys, prospect evaluation, drilling, and acreage relinquishment.

Fluid compartmentalization of Devonian and Mississippian dolostones, Western Canada Sedimentary Basin: petrologic and geochemical evidence from fracture mineralization

Integrated petrographic, geochemical, and fluid inclusion study of fracture mineralization and associated host rock in selected Mississippian and Devonian carbonates extending from southeastern Alberta to northwestern British Columbia, Canada, aims to quantify the type and nature of fluid precipitated saddle dolomite and late calcite cement and their origin. Petrographic and isotopic evidence from both the Devonian and Mississippian fracture-filling carbonates indicate the presence of a hydrothermal fluid source. The δ18O isotopic values for the Devonian saddle dolomite (−14.62‰ to −3.75‰ VPDB, average −11.12‰) combined with enriched 87Sr/86Sr isotopic ratios (0.70827–0.71599, average 0.71006) and higher homogenization temperatures (Th = 74–194.6 °C, average 126.8 °C) and salinity values (7.7–26.6 wt.% NaCl, average 16.2 wt.% NaCl) show significant differences from the Mississippian saddle dolomite, which is characterized by less negative δ18O isotopic values (−12.53‰ to −7.82‰ VPDB, average −9.14‰), less radiogenic 87Sr/86Sr isotopic ratios (0.70859–0.70943, average 0.70887), and lower homogenization temperatures (Th) and salinity values of fluid inclusions (87.6–214.2 °C, average 136.3 °C; 2.0–13.2 wt.% NaCl, average 9.6 wt.% NaCl). Later fracture- and vug-rimming blocky calcite cement records comparable or slightly lower values of δ18O (−16.31‰ to −4.08‰ VPDB, average −9.76‰) and 87Sr/86Sr (0.70784–0.709743, average 0.70868) and much lower salinity values (0–22.5 wt.% NaCl, average 2.86 wt.% NaCl) for samples mostly from the Mississippian age group. These results possibly suggest two different hydrothermal episodes related to early (Antler) and late (Laramide) tectonic events that affected the Western Canada Sedimentary Basin with possible compartmentalization of hydrothermal systems and their associated brines in the basin.

Anoxia and fluctuating climate recorded from the Devonian–Carboniferous black shales, Tethys Himalaya, India: a multi-proxy approach

Due to inaccessibility and tough conditions (high-altitude low-oxygen region) very less information is available about the Indian Tethys Himalayan sequences, particularly geochemical data are scarce. An attempt is made in the present study to evaluate the sediment geochemistry and paleo-redox conditions of Devonian–Carboniferous black shales from the Spiti region, Tethys Himalaya, using multi proxy approach. Besides black shales, this paper also provides the first in-depth study of the geochemistry of phosphatic nodules (hosted by the black shales) that will allow primary and secondary processes to be understood. Geochemical investigations, particularly redox-sensitive trace element ratios (for e.g. V/(V + Ni) > 0.5; Ni/Co = 4.3–7.2; V/Cr > 2 and positive Ce anomalies), suggest deposition of these black shales under sub-oxic to anoxic marine conditions, thus recording an ocean anoxic event in the region. These ocean anoxic events of Devonian–Carboniferous Spiti black shales, which possess distinctive geochemical signatures, can very well be correlated with global events such as Hangenberg Events and the Lower Alum Shale Event. The span of the chemical index of alteration (CIA) values (54–78), variation of ∆4Si% parameter (82–3) and C values (0.1–0.6) recorded in the black shales strongly indicate varying degrees of weathering conditions in the source area, which in turn reflect fluctuating climatic conditions prevailing during the deposition of these sediments in Devonian–Carboniferous period in the Spiti region. The distinct geochemical characteristics such as Y, Sr and rare earth element (REE) enrichments (particularly middle REE enrichments with a “hat-shaped” pattern) with high P2O5 contents (up to 14 wt%) distinguish the nodules from the host rocks (black shales). SEM (scanning electron microscope) results, supplemented by XRD (X-ray diffraction) studies of the nodules, show the dominance of calcium, silica and phosphatic phases. The diversity of clay minerals, significant variations in mineralogical composition and lack of any continuous vertical trend in the nodules suggest a relatively low impact on the primary environmental signal by burial diagenesis.

From intrabasinal volcanism to far-field tectonics: causes of abrupt shifts in sediment provenance in the Devonian–Carboniferous Drummond Basin, Queensland

The Drummond Basin of central Queensland preserves a large-volume succession of little studied, predominantly fluviatile, coarse-grained sedimentary rocks of mid-Mississippian age. The stratigraphy of the basin has been subdivided into three sedimentary cycles. The Cycle 1/Cycle 2 boundary records a distinct, but poorly understood change in provenance from a volcanic-dominated succession related to initial basin rifting (Cycle 1) to a quartz-rich, craton-derived succession (Cycle 2). Cycle 3 has been thought to mark a resumption of intrabasinal volcanism and related sedimentation. The purpose of this study was to enhance the understanding of the basin-wide siliciclastic sedimentation of Cycles 2 and 3, and causes for the changes in sediment provenance. This objective was achieved by constraining large-scale spatial and temporal depositional trends and investigating sediment transport pathways into and through the basin. Petrographic, QFL, paleocurrent and conglomerate clast analyses were undertaken. The observations presented here have several implications relevant to understanding the stratigraphy of the Drummond Basin and regional tectonic events at this time. Cycle 3 is revised here primarily to be a continuation of Cycle 2-style basement-derived sedimentation, rather than recording a resumption of volcanism in the area, as per prevailing models. Quartz-rich sedimentation in the Drummond Basin was, therefore, more long-lived than previously envisaged, and once established, was not significantly disrupted by volcanism. Cycle 2 formation thicknesses appear highly variable across the basin. This is unlikely to be a result of pre-existing rift-related topography as suggested in previous models. The thickness variations are more likely related to sediment bypassing and post-depositional deformation in the area. The distinctive coarse-grained, relatively quartz-rich sedimentation of Cycles 2 and 3 is unusual in its volume and extent. The sediment was transported into the basin from its southern/southwestern margin, implying long-distance transport and extrabasinal sediment supply. While the specific source terrain(s) remain unknown, one plausible tectonic driver was far-field influence of the intraplate Alice Springs Orogeny.

Earliest Cretaceous Transgression of North America Recorded in Thick Coals: McMurray Sub-Basin, Canada

Strata of the Lower Cretaceous McMurray Formation in the McMurray Sub-Basin (MSB) of Alberta and Saskatchewan, Canada, comprise marginal marine, tidal-fluvial, fluvial and interfluve deposits. In the Firebag Tributary located in the northeastern portion of the sub-basin, a thick coal seam (up to >20 m) forms a semi-continuous bed at the top of the lowermost McMurray Fm deposits (herein referred to as Lower McMurray). In coastal plain settings, peat-forming mires are highly sensitive archives of base-level. We investigate the stratigraphic significance of coals in the Firebag Tributary by analyzing coal petrography (4 cores), coal geochemistry (2 cores), and coal distribution in ~4500 wells. Additionally, we record the first absolute age for the McMurray Fm through the dating of an ash layer within the coal seam.The petrographic character and distribution of coals in the northeastern part of the MSB suggest a complex history of base-level rise at the end of Lower McMurray deposition. Wetting-upward character coal seams record initial transgression of the Boreal Sea during a 4th order sea level rise (Milankovitch-scale) within lowstand to early transgressive systems tract conditions. Sea-level rise during Lower McMurray times was estimated based on temperate climate peat formation rates, as well as peat-to-coal compaction factors, and probably occurred on a scale of 0.5–3 mm a−1. Seam-internal wetting-upward cycles suggest that base-level rise was punctuated. Locally, compound and drying-upward character coals represent zones of high accommodation, wherein peat accumulation in mires was influenced by phases of syn-depositional subsidence and karst collapse. The thickest coals in the study area form an arc on a coastal plain as the Boreal Sea inundated east- to southeastward into the Firebag Tributary. Within the arc of thick coals, a north-south decrease in coal thickness is interpreted to reflect the general southward inundation of the Boreal Sea across the MSB. Exceptionally thick coals (>10 m) likely formed where basement processes (e.g., karsting in Devonian carbonates) provided preferential groundwater pathways.Chemical abrasion thermal ionization mass spectrometry dating of 5 sharply faceted zircons recovered from an ash bed within the coal provides the first absolute age (121.39 ± 0.2 Ma – Aptian) to the southward transgression of the Boreal Sea. We hypothesize that initial inundation of the MSB was related to tectonic activity in the Canadian Cordillera and the onset of magmatic flow in the western Coast Plutonic Complex.

Rare earth elements and radiogenic strontium isotopes in carbonate minerals reveal diagenetic influence in shales and limestones in the Appalachian Basin

Rare earth element (REE) signatures are often applied to interpret paleoenvironmental conditions in sedimentary basins, however the complicated diagenetic histories in dynamic depositional environments can affect interpretation of measured REE signatures. Prior studies on REE content in shales and limestones indicated that REE in specific mineral phases may provide unique information concerning diagenetic reactions occurring in sedimentary rocks during burial and compaction. Application of sequential extraction techniques to target REE signatures in specific mineral fractions may provide greater insight into the complex processes that occurred during diagenesis and catagenesis. Thus, using this technique, this study provides a detailed account for REE, 87Sr/86Sr, and δ13C in the carbonate fraction of the Marcellus Shale and adjacent formations in the Appalachian Basin. A suite of 49 rock samples collected from two cores recovered from Monongalia County, West Virginia, USA was analyzed. The results showed that the carbonate concretions in calcareous shales and carbonate cements in black shales were very distinct. The REE plus yttrium (REY) were more concentrated in the carbonate concretions than in the carbonate cements. The carbonate concretions displayed REY patterns that are closely similar to modern seawater, while MREE-enrichment was observed in the carbonate cements. Likewise, the 87Sr/86Sr values in the carbonate concretions were similar to those measured for unaltered Middle Devonian carbonates, while the 87Sr/86Sr in the carbonate cements were much more radiogenic. This observation indicates that the carbonate cement could have been inherited radiogenic 87Sr expulsed from clays during illite-smectite transition. Overall, this study demonstrated two distinct processes involved with controlling the carbonate geochemistry within Appalachian Basin shales and limestones: one fraction displaying minimal diagenetic alteration relative to depositional conditions (carbonate concretions in calcareous shales and limestone carbonate in limestones), and another fraction displaying the evidence for extensive chemical alteration during illite-smectite transition and catagenesis (carbonate cements in black shales). In addition, highly variable and significant enrichments of U and Mo demonstrate that the Union Spring member and the Lower Oatka Creek member of the Marcellus Shale in the southwestern Appalachian Basin (MSEEL site) were deposited under mainly anoxic environment whereas intermitted episodes of dysoxic to perhaps oxic conditions occurred during the deposition of the Upper Oatka Creek member.

Geology, Geochronology, and Hf Isotopic Composition of the Pha Lek Fe Deposit, Northern Laos: Implications for Early Permian Subduction-Related Skarn Fe Mineralization in the Truong Son Belt

The Truong Son metallogenic belt in central Laos and Vietnam is an important Fe-Cu-Sn-Au polymetallic ore district. The Pha Lek Fe deposit is closely related to Late Carboniferous- Early Permian I-type granitic magmatism, and contains >50 Mt @ 45% to 50% of Fe ore. Ore minerals occur mainly as magnetite and hematite in the skarn alteration zone between a granitic pluton and metamorphosed Middle-Upper Devonian carbonates. The granitic pluton comprises granodiorite and granite, with zircon U-Pb dating indicating synchronous emplacement at 288.2±1.3 and 284.9±1.2 Ma, respectively. Zircons from these granitoids have eHf(t) values of 2.9-11.2 and relatively young TDM2 ages (<1.0 Ga), indicating an origin by partial melting of depleted mafic crust or magma mixing. Previous studies have shown that these granitoids have high Y, Yb, and K2O contents, and low Sr and Na2O contents, which are interpreted as the melting of mafic continental crust. Pyrite of the main mineralization stage yields an 187Re/188Os-187Os/188Os isochron age of 287±17 Ma, indicating that mineralization is associated with Pha Lek granitic magmatism. A Late Carboniferous-Early Permian subduction-related skarn-type Fe mineralization model is proposed for the Pha Lek deposit. More evidence is needed to verify a hypothesis of volcanic overprinting during Late Triassic post-collisional extension.

Chronology of the Saxothuringian subduction in the West Sudetes (Bohemian Massif, Czech Republic and Poland)

Isotopic dating of monazite and garnet from high-pressure metamorphic rocks exposed in the northern part of the Saxothuringian palaeo-suture in the Bohemian Massif revealed a diachronous metamorphism of various rock types that are now closely associated within allochthonous units representing the Devonian–Carboniferous subduction–accretionary complex. Mafic blueschists of the middle unit yielded a Lu–Hf garnet age of 363.9 ± 1.3 Ma. The blueschists occur within high-pressure, garnet-free phyllites. Monazite extracted from this rock type yielded a U–Pb isotope dilution thermal ionization mass spectrometry age of 336.5 ± 0.5 Ma. Garnet-bearing micaschist of the lower unit contains monazite with a U–Pb secondary ion mass spectrometry age of 341 ± 3 Ma, consistent with Lu–Hf garnet–whole-rock ages of 344.5 ± 1.3 and 342 ± 7 Ma obtained from the same rock type. Existing tectonic models of the Bohemian Massif, and particularly of its northern part, assume that the period of oceanic subduction was terminated at c. 380–375 Ma by the arrival of an attenuated Saxothuringian continental crust, which was partly subducted and partly relaminated underneath the overriding Teplá–Barrandian Domain. However, our data, as well as data from mafic high-pressure rocks in the southern part of the Saxothuringian Domain, suggest that the initial collision was probably caused by the arrival of a smaller crustal block present within the Saxothuringian oceanic domain. After its subduction and relamination, the oceanic subduction was re-established and terminated by continental subduction and later collision of the Saxothuringian passive margin at c. 345–335 Ma.

Is seawater geochemical composition recorded in marine carbonate? Evidence from iron and manganese contents in Late Devonian carbonate rocks

Iron and manganese are the important redox-sensitive elements in the ocean. Previous studies have established a series of paleo-depositional redox proxies based on the form and content of iron in sedimentary rocks (e.g., degree of pyritization, FeHR/FeT, Fe/Al). These proxies were developed and applied on siliciclastic-rich marine sediments. Although marine carbonate rocks are generally considered to preserve the geochemical signals of ancient seawater, neither Fe nor Mn content in marine carbonate rocks (Fecarb, Mncarb) has been independently used as a proxy to quantify environmental cues in paleo-oceans. Both Fe and Mn are insoluble in oxic conditions (Fe2O3, Fe(OH)3, MnO2), while their reduced forms (Fe2+ and Mn2+) are soluble. Therefore, oxic seawater should have low concentrations of dissolved Fe2+ and Mn2+, and accordingly carbonate rocks precipitated from oxic seawater should have low Fecarb and Mncarb, and vice versa. To evaluate whether Fecarb and Mncarb can be used to quantify oxygen fugacity in seawater, we measured Fecarb and Mncarb of Upper Devonian marine carbonate rocks collected from nine sections in South China. Fecarb of intraplatform basin samples was significantly higher than that of shelf samples, while shelf and basin samples had comparable Mncarb. The modeling result indicates that the dramatic difference in Fecarb cannot be explained by variation in oxygen fugacity between the shelf and basin seawater. Instead, both Fecarb and Mncarb appear to be more sensitive to benthic flux from sediment porewater that is enriched in Fe2+ and Mn2+. Porewater Fe2+ and Mn2+ derive from bacterial iron and manganese reduction; flux was controlled by sedimentation rate and the depth of the Fe(Mn) reduction zone in sediments, the latter of which is determined by oxygen fugacity at the water–sediment interface. Thus, high Fecarb of the basin samples might be attributed to low sedimentation rate and/or low oxygen fugacity at the seafloor. However, invariant Mncarb of the shelf and basin samples might be the consequence of complete reduction of Mn in sediments. Our study indicates that marine carbonate rocks may not necessarily record seawater composition, particularly for benthic carbonate rocks. The influence of benthic flux might cause carbonate rocks’ geochemical signals to deviate significantly from seawater values. Our study suggests that interpretation of geochemical data from carbonate rocks, including carbonate carbon isotopes, should consider the process of carbonate formation.

Burial and thermal history of the Lower Palaeozoic petroleum source rocks at the SW margin of the East European Craton (Poland)

Thermal maturity modelling was carried out in over sixty wells along the SW margin of the East European Craton (EEC). The burial and thermal history modelling of the EEC, using thermochronological data, allowed the construction of burial history maps showing its geological development in the Phanerozoic. These results have proved that the Ordovician and Silurian source rocks occurring at the SW margin of the EEC reached a maximum palaeotemperature in the Palaeozoic, mainly during Devonian–Carboniferous time and at the latest during the Silurian in the most westerly part of this margin, along the Teisseyre-Tornquist Zone. In Mesozoic and Cainozoic time, the Ordovician and Silurian strata generally were subjected to cooling or to very minor heating, certainly below the Variscan level. The maximum burial and maximum temperature of the Ediacaran–Lower Palaeozoic strata were reached during the Early Carboniferous in the Baltic Basin and during the Late Carboniferous in the Lublin area, and even in the Early Permian in the SE corner of the Lublin Basin. Thus, the main period of maturation of organic matter and hydrocarbon generation in the Ordovician and Silurian source rocks was in the Late Palaeozoic (mainly Devonian–Carboniferous) and in the westernmost zone along the Teisseyre-Tornquist line at the end of the Silurian.

Late Paleozoic extensional reactivation of the Rheic–Rhenohercynian suture zone in SW England, the English Channel and Western Approaches

Abstract The Rheic Ocean is a persistent feature of Paleozoic palaeogeographies whose closure contributed to the development of the Variscan Orogen and the formation of Pangaea. Geological and geophysical data indicate repeated episodes of Paleozoic rifting and plate convergence around SW England and the adjacent offshore areas. SW England occupied a lower plate position during the Devonian–Carboniferous, on the northern passive margin of the short-lived Rhenohercynian Ocean that had formed near a recently closed segment of the Rheic Ocean. Variscan plate convergence resulted in the development of the composite southwards-dipping Rheic–Rhenohercynian suture zone by the latest Devonian and inversion of the lower plate basins during the Carboniferous. Early Permian NNW–SSE extensional reactivation of this suture zone controlled the development of the Western Approaches basins in its hanging wall and provides an excellent example of Wilson cycle structural inheritance. The onshore expression of this episode includes shear zones and detachment faults consistent with top-to-the-SSE extensional reactivation of Variscan thrust faults. There is a progression to higher-angle brittle extensional faults that cut out earlier structures. Exhumation of the lower plate was accompanied by Early Permian mantle and concomitant crustal partial melting, the construction of the Cornubian Batholith and W–Sn–Cu fracture-hosted mineralization.

Девонские и раннекаменноугольные угли и эволюция болотных фаций

Девонские и раннекаменноугольные угли и эволюция болотных фаций

Brachiopod faunas from the basinal facies of southeastern Thuringia (Germany) before and after the Hangenberg Crisis (Devonian–Carboniferous boundary)

Brachiopod faunas from the basinal facies of southeastern Thuringia (Germany) before and after the Hangenberg Crisis (Devonian–Carboniferous boundary)

From widespread Mississippian to localized Pennsylvanian extension in central Spitsbergen, Svalbard

Abstract. In the Devonian–Carboniferous, a rapid succession of clustered extensional and contractional tectonic events is thought to have affected sedimentary rocks in central Spitsbergen, Svalbard. These events include Caledonian post-orogenic extensional collapse associated with the formation of thick Early–Middle Devonian basins, Late Devonian–Mississippian Ellesmerian contraction, and Early–Middle Pennsylvanian rifting, which resulted in the deposition of thick sedimentary units in Carboniferous basins like the Billefjorden Trough. The clustering of these varied tectonic settings sometimes makes it difficult to resolve the tectono-sedimentary history of individual stratigraphic units. Notably, the context of deposition of Mississippian clastic and coal-bearing sedimentary rocks of the Billefjorden Group is still debated, especially in central Spitsbergen. We present field evidence (e.g., growth strata and slickensides) from the northern part of the Billefjorden Trough, in Odellfjellet, suggesting that tilted Mississippian sedimentary strata of the Billefjorden Group deposited during active (Late/latest?) Mississippian extension. WNW–ESE-striking basin-oblique faults showing Mississippian growth strata systematically die out upwards within Mississippian to lowermost Pennsylvanian strata, thus suggesting a period of widespread WNW–ESE-directed extension in the Mississippian and an episode of localized extension in Early–Middle Pennsylvanian times. In addition, the presence of abundant basin-oblique faults in basement rocks adjacent to the Billefjorden Trough suggests that the formation of Mississippian normal faults was partly controlled by reactivation of preexisting Neoproterozoic (Timanian?) basement-seated fault zones. We propose that these preexisting faults reactivated as transverse or accommodation cross faults in or near the crest of transverse folds reflecting differential displacement along the Billefjorden Fault Zone. In Cenozoic times, a few margin-oblique faults (e.g., the Overgangshytta fault) may have mildly reactivated as oblique thrusts during transpression–contraction, but shallow-dipping, bedding-parallel, duplex-shaped décollements in shales of the Billefjorden Group possibly prevented substantial movement along these faults.

Formation of the Diversity of the Brachiopod Order Spiriferida at the Devonian–Carboniferous Boundary

The diversity of the brachiopod order Spiriferida at the Devonian–Carboniferous boundary possibly formed due to the changes in the sedimentation that took place during this period and influenced the composition and hardness of the seafloor substrates and bottom-water hydrodynamics in the marine basins. These characteristics are considered among the main factors, both stimulating and restricting, that determined the composition and ethological trophic type of the main spiriferide taxa at this stage.

The Kellwasser events in the Upper Devonian Frasnian to Famennian transition in the Toc Tat Formation, northern Vietnam

AbstractUpper Devonian carbonates of the Toc Tat Formation in the Si Phai Pass area of Dong Van District, northern Vietnam were deposited in carbonate platform, slope, and basin environments. These carbonates yield abundant conodonts indicative of the Palmatolepis nasuta, Pa. linguiformis and Pa. triangularis zones, the Frasnian–Famennian stage boundary being identified by the first occurrence of Pa. triangularis. Two positive carbon isotope excursions are recognized, the lower excursion peaking in the interval of the lower to middle Pa. nasuta Zone, whilst the upper excursion peaks just above the local Frasnian–Famennian boundary. Based on the biostratigraphy, these excursions equate to the Lower and Upper Kellwasser events. Locally, tentaculitoid taxa (Nowakia, Styliolina, Homoctenus, and Metastyliolina?) are abundant in the interval of the Pa. nasuta Zone, but show a drastic decline in abundance before the Lower Kellwasser Event, and only two taxa survived into the Famennian.

A fish and tetrapod fauna from Romer's Gap preserved in Scottish Tournaisian floodplain deposits

AbstractThe end‐Devonian mass extinction has been framed as a turning point in vertebrate evolution, enabling the radiation of tetrapods, chondrichthyans and actinopterygians. Until very recently ‘Romer's Gap’ rendered the Early Carboniferous a black box standing between the Devonian and the later Carboniferous, but now new Tournaisian localities are filling this interval. Recent work has recovered unexpected tetrapod and lungfish diversity. However, the composition of Tournaisian faunas remains poorly understood. Here we report on a Tournaisian vertebrate fauna from a well‐characterized, narrow stratigraphic interval from the Ballagan Formation exposed at Burnmouth, Scotland. Microfossils suggest brackish conditions and the sedimentology indicates a low‐energy debris flow on a vegetated floodplain. A range of vertebrate bone sizes are preserved. Rhizodonts are represented by the most material, which can be assigned to two taxa. Lungfish are represented by several species, almost all of which are currently endemic to the Ballagan Formation. There are two named tetrapods, Aytonerpeton and Diploradus, with at least two others also represented. Gyracanths, holocephalans, and actinopterygian fishes are represented by rarer fossils. This material compares well with vertebrate fossils from other Ballagan deposits. Faunal similarity analysis using an updated dataset of Devonian–Carboniferous (Givetian–Serpukhovian) sites corroborates a persistent Devonian/Carboniferous split. Separation of the data into marine and non‐marine partitions indicates more Devonian–Carboniferous faunal continuity in non‐marine settings compared to marine settings. These results agree with the latest fossil discoveries and suggest that the Devonian–Carboniferous transition proceeded differently in different environments and among different taxonomic groups.

Identification of the genetic heterogeneity of Tatarstan oils based on the composition of alkyl-substituted aromatic compounds

This study presents data on concentrations of n-alkylbenzenes, n-alkylnaphthalenes, phytanylnaphthalene, and methylphytanylnaphthalene in representative crude oils of Tatarstan. The results of the study reveal the elevated concentrations of C19, C21, and C23 homologues of n-alkylbenzenes and n-alkylnaphthalenes, which can be considered as biomarkers. The proposed procedure for comprehensive quantification of this group of biomarkers can be used as an efficient tool to study oils from the major petroleum basins of Russia. Based on the results of the study, four genetic groups of oils in Tatarstan have been distinguished: (1) oils from the north and northwest (Bir saddle, Lower Kama system of linear faults, and Saraily saddle), (2) oils from Devonian terrigenous reservoirs within the South Tatar arch and Melekes depression, (3) oils from Carboniferous reservoirs, and (4) oils from Devonian carbonate reservoirs. All these oils belong to the same genetic macrotype. Based on the results of this study, the sedimentary sections of the Melekes depression cannot be regarded as potential source rocks. It is assumed that oil has migrated to the northern part of the region from the north or east. Some of the possible migration routes for oils from the remaining part of Tatarstan are from the southeast and/or south.

Reef Formation during Mass Extinction Events: Frasnian—Famennian and Devonian—Carboniferous Boundaries

An analysis of the development of Late Devonian reefs in a number of regions worldwide shows that reef formation decreased gradually and discontinuously during the second half of the Frasnian and was completely terminated in some places. These events were associated with regression stages. The final Frasnian regression and Kellwasser event, which led to the biotic crisis, resulted, first, in the quantitative reduction of reef formation in the Famennian and, second, in the change of the frame reef formation by the formation of microbial mud mounds in the Famennian.