Late Devonian Research Articles

Depositional setting and organic matter characterization of the Upper Devonian Antrim Shale, Michigan Basin: Implications for hydrocarbon potential

During the Late Devonian, the Michigan Basin in the Eastern Interior Seaway was characterized by phases of well-developed bottom water anoxia, triggering extensive accumulation of organic matter-rich black shales. Palynological analysis for paleoenvironmental reconstruction and biostratigraphy of the Upper Devonian rocks has been previously performed in other basins of eastern North America, but it is lacking in the Michigan Basin. Here, the Upper Devonian Antrim Shale Formation is investigated for biostratigraphic, paleoenvironmental, and sea level reconstructions, organic matter characterization, and source rock potential. 35 core samples from three drill holes across the Michigan Basin spanning the Norwood, Paxton, Lachine, and Upper members. Palynological analysis showed a moderately diverse assemblage of 31 genera represented by 53 species of relatively well-preserved prasinophyte phycomata and acritarchs, with sparse records of plant spores and freshwater algae. Based on marker prasinophytes and acritarchs, an age of late Frasnian was assigned to the Norwood and Paxton members, while an early Famennian age was proposed to the Lachine and Upper members of the Antrim Shale. Two palynofacies assemblages (PFA) were identified from the variation in the particulate organic matter (POM) composition, both of which indicate deposition in a distal inner neritic shelf environment, but the PFA-2 took place in slightly shallower conditions than the PFA-1. Geochemical screening indicated high organic matter content (up to 25 wt%, avg. 8.5 wt%) and excellent hydrocarbon generating potential of kerogen Type II for the Norwood, Lachine, and Upper members. Only the Paxton Member was dominated by poor to fair organic richness (<0.6 wt%) and low hydrocarbon potential with a mixed Type II/III kerogen. Organic petrography indicated that the dominant organic matter assemblage consists mainly of unicellular marine Tasmanites telalginite and Leiosphaeridia, followed by solid bitumen of the initial oil type. Thermal maturity, determined from VRo-eq and Tmax, indicated that all samples are in the oil window, except for some intervals in the Paxton Member.

Keratose sponge fabrics from the lowermost Triassic microbialites in South China: Geobiologic features and Phanerozoic evolution

The Permian-Triassic mass extinction (PTME) strongly devastated marine ecosystems, and, consequently, sponges, especially the reef-building clades, suffered dramatic losses in biodiversity. The Early Triassic therefore was believed to be an evolutionary gap for sponges. Microbialites spread over shallow marine carbonate settings across the entire low-latitude Tethys region following the PTME and occupied the ecospace that the pre-extinction metazoan reefs left. Here, we report putative keratose sponge consortia from the microbialites near the Permian-Triassic boundary in the Xiushui, Laolongdong, and Dongwan sections, South China. The putative keratose sponges exhibit vermiform, filamentous textures forming maze-like networks. Within the keratose sponge-microbial fabrics, the calcified sponge skeleton might firm the overall framework of microbialite, promoting construction of the sponge-microbial build-ups. The coeval occurrence of putative keratose sponges in both eastern Palaeotethys and central Neotethys regions indicates that sponges may have widely spread and played important roles in constructing metazoan-microbial reefs in earliest Triassic. Besides, several characteristics are conducive to keratose sponge surviving the stressful environments after the PTME. Global dataset shows that keratose sponges mostly confined to tropic and temperate zones. Keratose sponges overall flourished in coincidence with occurrence abundance peaks of microbial reefs during the Phanerozoic history, and they seem to be particularly abundant and widespread in the Cambrian-Ordovician, Late Devonian and the aftermath of the end-Permian mass extinction.

Multi-proxy provenance of the lower Pennsylvanian Pottsville sandstone of the northern Appalachian basin in Pennsylvania, U.S.A: Paleodrainage, sources, and detrital history

ABSTRACT The lower Pennsylvanian Pottsville Formation, in the Appalachian foreland basin, constitutes a late Paleozoic clastic wedge formed close to the Appalachian orogenic belt. In this study, we analyzed Pottsville sandstone from the western bituminous and eastern anthracite fields in Pennsylvania to evaluate the detrital history of the sediments. Petrographic modal analyses show that sandstone in the western bituminous field ranges from quartzarenite to sublitharenite, with mean composition of Qt84F1L15; sampled sandstone from the eastern anthracite field is dominated by sublitharenite to litharenite with mean composition of Qt70F2L29. Heavy-mineral assemblages from both fields are dominated by ultrastable minerals (zircon, rutile, and tourmaline), apatite, sphene, spinel, siderite, and abundant opaques. Almost all the studied sandstone samples are garnet-depleted except one from the eastern anthracite field. The chemical composition of chromium- and zinc-rich spinel in samples from both fields might suggest exhumation of an arc terrane and ophiolitic belt with ultramafic igneous rocks. Particularly, the ternary plot of Fe3+–Cr3+–Al3+ end members for the chrome spinels possibly suggest a derivation from an alpine-type peridotite complex. Laser 40Ar/39Ar analyses of detrital muscovite from eastern anthracite fields and western bituminous fields record separate ages, with the former characterized by prominent Middle to Late Ordovician Taconic and Middle Devonian Acadian ages with two discrete modes at 463 and 369 Ma, and the latter dominated by Late Ordovician Taconic, Middle Devonian Acadian, and Late Devonian Neoacadian ages with discrete modes at 445, 397, 360, and 351 Ma. The new data suggest that early Pennsylvanian sedimentation in the Appalachian foreland basin was controlled by southward, southwestward, and westward drainage systems that originated in the Appalachian orogenic belt to the east and northeast.

Fluid Inclusion, Isotopic, and Elemental Geochemistry Studies of Cave-Filling Calcite in the Lower–Middle Ordovician Yingshan Formation of Tahe Oilfield, NW China: Implication for Karstification in Non-exposed Limestone

Improving the recognition of paleo-fluid circulation history is of great significance to reconstruct pore evolution during carbonate diagenesis. Integrated petrography, fluid inclusion, isotopic and elemental geochemistry (laser ablation inductively coupled plasma mass spectrometry) studies, calcites generated in paleocaves, and fractures were investigated. This study aims to reveal the paleo-fluid origin and karstification event within the nonexposed limestone of the Lower–Middle Ordovician Yingshan Formation in the Tarim Basin. The only generation of blocky calcite growing along the karst paleocave and fracture walls [cave-filling calcite (CFC)] crosscuts burial stylolites. The secondary fluid inclusions obtained from CFC are characterized by the coexistence of liquid-only and liquid-dominated aqueous inclusions with low salinities values (0–2.4 wt%), suggesting that the CFC has experienced a low-temperature environment (&amp;lt;50°C). The depleted δ18O values (−15.32‰ to 12.45‰), seawater-like yttrium and rare earth element patterns, and low ΣREE (&amp;lt;0.65 ppm) have recorded the major episode of meteoric water leaching the Yingshan limestone. This view was further confirmed by the calculated δ18Owater values of parent fluids (−14.3‰ to 2.2‰). The relatively higher 87Sr/86Sr ratios (0.70942–0.70994) are interpreted as the result of meteoric water interacting with the overlying Silurian detrital rock when percolated downward. The geochemical evidence recorded by CFC indicates that the karstification event for Yingshan nonexposed limestone possibly took place in the Early Hercynian period during the late Devonian. Therefore, meteoric water percolating downward along the fractures penetrating insoluble strata and/or migrating laterally along the permeable strata is deemed responsible for the karstification event, although the overlying thick insoluble strata in the coverage area.

New U[sbnd]Pb zircon and geochemical constraints on Late Devonian Back-arc basin origin of eclogite protoliths from northeastern Hainan Island, South China

Geochronological, geochemical, and SrNd isotopic data are used to constrain the age, origin, and tectonic setting of the protoliths of newly discovered eclogites from Chaotanbi, northeastern Hainan Island, South China. New sensitive high-resolution ion microprobe (SHRIMP) UPb zircon dating reveals that the protoliths of the Chaotanbi eclogites formed at 364 ± 2 Ma and were metamorphosed at 297 ± 1 Ma. All samples collected from a coherent area of ~1.8 km2 in the intertidal and subtidal zones on the island exhibit tholeiitic compositions, with initial 87Sr/86Sr and εNd values of 0.705–0.707 and − 1.1 to +7.4, respectively. The samples can be further categorized into three groups based on incompatible trace elements. Group #1 samples have a normal mid-ocean ridge basalt (N-MORB) affinity and were originated from relatively high-degree partial melting of a depleted spinel lherzolite mantle source slightly modified by slab-derived fluids. Group #2 samples have an enriched mid-ocean ridge basalt (E-MORB) affinity and were derived from an enriched spinel and spinel–garnet lherzolite transition mantle source at depths greater than that of Group #1. Group #3 samples have an island arc basalt (IAB) affinity and were generated by partial melting of an enriched spinel–garnet lherzolite mantle source modified by fluid- and sediment-related recycled components. The association of these diverse basalt types, with MORB-type samples accounting for ~90% of all samples, suggest a Late Devonian mature back-arc basin existed on northeastern Hainan Island, as a part of the Jinshajiang–Ailaoshan–Song Ma–Hainan back-arc basin system. Therefore, the Chaotanbi eclogites are a fingerprint of the opening and closure of the eastern Paleo-Tethyan Ocean.

The tornoceratid ammonoids from the Roteisenstein Formation of Dillenburg (Cephalopoda, Ammonoidea)

The ammonoids of the suborder Tornoceratina from the Middle to Late Devonian Red Ironstone Formation of the area around Dillenburg (eastern Rhenish Mountains) are revised, mainly based on historical collections stored in the Museum für Naturkunde, Berlin. The species Tornoceras typus (Sandberger &amp; Sandberger, 1851), Tornoceras frechi Wedekind, 1918 and Epitornoceras mithracoides (Frech, 1888) are re-described based on the original material from the Red Ironstone. The new genus Lentitornoceras gen. nov. is proposed for the new species L. materni gen. et sp. nov.; the new genus Paucitornoceras gen. nov. is proposed with the type species Goniatites paucistriatus. Epitornoceras transmediterraneum sp. nov. is described from the Anti-Atlas of Morocco and compared with E. mithracoides.

Together but separate: decoupled Variscan (late Carboniferous) and Alpine (Late Cretaceous–Paleogene) inversion tectonics in NW Poland

Abstract. In Europe, formation of the Palaeozoic Variscan orogenic belt, and the Mesozoic–Cenozoic Alpine–Carpathian orogenic belt led to a widespread inversion events within forelands of both orogenic domains. We used legacy 2-D seismic data together with the newly acquired 3-D seismic data that, for the first time, precisely imaged sub-Zechstein (i.e. sub-evaporitic) upper Palaeozoic successions in NW Poland in order to develop a quantitative, balanced 2-D model of the late Palaeozoic–recent evolution of this area, characterised by a complex pattern of repeated extension and inversion. Four main tectonic phases have been determined: (1) Late Devonian–early Carboniferous extension and subsidence possibly related to extensional reactivation of Caledonian thrusts, (2) late Carboniferous inversion caused by the Variscan orogeny, (3) Permo-Mesozoic subsidence related to the development of the Polish Basin and (4) its Late Cretaceous–Paleogene inversion. Variscan and Alpine structures form a superimposed multilayer inversion system, mechanically decoupled by the Zechstein evaporites.

Late Devonian–early Carboniferous ostracods (Crustacea) from South China: taxonomy, diversity and implications

The impact of the late Devonian Hangenberg Event on ostracods is quantified for the first time from newly acquired data from the Blue Snake section, Guizhou Province, South China. Ninety–eight species belonging to 31 genera are identified and figured. Four new species are described: Clavofabella? lanshella sp. nov., Sansabella gelaohensis sp. nov., Cytherellina caerulea sp. nov., Sulcella baisuzhena sp. nov. The ostracod associations from the Blue Snake section document a significant drop in specific diversity as well as major changes in taxonomic composition through the Hangenberg Event. We here report the reduction of the proportion of Palaeocopida and the increase in that of Podocopida between the latest Famennian and the Tournaisian. The specific extinction and renewal rates are estimated at 44% and 62%, respectively. The main factor of the post–crisis renewal of ostracod faunas in the Blue Snake section appears to be the progressive diversification of the family Bairdiidae and was probably related to palaeoenvironmental changes. The characteristics of associations point to an increase of the water depth over the studied area, from a near–shore shallow environment during the latest Famennian to a deeper and more open offshore environment during the Tournaisian.

Zircon Th/U ratios suggest a post-collision extensional setting for the Permian Ni-Cu sulfide deposits in the Eastern Tianshan, NW China

Magmatic Ni-Cu sulfide deposits have been discovered in orogenic belts worldwide. However, the optimal period for extensive basaltic magmatism generating Ni-Cu sulfide mineralization during the development of orogenic belts has not been well addressed. Zircon Th/U may be correlated with end-member tectonic stress regimes (continental extension vs. compression) that correlate with distinct magmatic conditions. Here, zircon Th/U ratios from the Permian Huangshan-Jingerquan Ni-Cu metallogenic belt in the Eastern Tianshan are revisited, including magmatic zircons from mafic and felsic rocks and detrital zircons from sedimentary rocks. These zircons are filtered to minimize the effect of amagmatic conditions (e.g., metamorphism, alteration, inclusion and metamictization). After filtration, zircon Th/U ratios in the Huangshan-Jingerquan belt are likely linked to the silica content of the host melt and crystallization temperature under zircon growth with fractionating melts. They display high Th/U ratios (up to 2.5) and δEu values (up to 1.35) during the early Permian. In contrast, the others with low Th/U (most < 1.0) ratios and δEu (<0.8) appear in the pre-Permian except the late Devonian. This indicates that the crust in the region was thickened induced by collision at the end of late Carboniferous and then extended during the early Permian. Conclusively, we suggest that the post-collision extensional setting is the preferred stage for the Permian short-lived (<5 Ma) Ni-Cu sulfide deposits during the development of the Tianshan Orogenic Belt.

Molybdenum isotope-based redox deviation driven by continental margin euxinia during the early Cambrian

Oceanic redox changes may be a key environmental trigger for the “Cambrian explosion”. However, a hypothesized predominantly oxygenated early Cambrian ocean, inferred from high sedimentary molybdenum (Mo) isotope ratios, conflicts with other evidence for euxinic continental margins. Here, we demonstrate that seawater Mo isotope ratios like those of today’s predominantly well-oxygenated oceans can also occur in less oxygenated early Cambrian oceans. New and existing Mo isotope data from early Cambrian black shales in South China span a range of greater than 3‰, which can only be explained by multi-stage fractionation of Mo isotopes. The lowest Mo isotopes ratios likely reflect a Fe-Mn (oxyhydr)oxide shuttle that preferentially transferred the lighter Mo isotopes into weakly euxinic bottom waters where Fe-Mn (oxyhydr)oxide dissolution released isotopically light Mo that was then scavenged by organic matters and sulfide minerals. We use a novel isotope mass balance model to show that Fe-Mn (oxyhydr)oxide shuttle operating on euxinic continental margins would preferentially bury lighter-mass Mo isotopes to such an extent that early Cambrian seawater can become enriched in heavier Mo isotopes at levels comparable to modern seawater. Hence, widespread ocean oxygenation may not have occurred during the early Cambrian as previously thought. Based on this paradigm shift, reconsideration of sedimentary Mo isotope temporal trends suggests a stepwise and protracted oxidation of the oceans until the Late Devonian. Sporadic intervals with Mo isotope ratios spanning over 3‰ and often accompanied by dramatic change in biodiversity (e.g., Ediacaran-Cambrian and Permian-Triassic transitions) probably had euxinic continental margins. Links between fossil records and marine redox structures disclose the importance of spatiotemporal fluctuations in continental margin euxinia on biodiversity changes.

Carbonate shutdown, phosphogenesis and the variable style of marine anoxia in the late Famennian (Late Devonian) in western Laurentia

The shutdown of carbonate productivity and its replacement by black shale deposition is often observed in the geological record and yet the factors driving such a change are debated. The latest Famennian of western Laurentia (present day North America) provides a good example of a limestone – black shale transition that is roughly contemporaneous with the global Hangenberg Crisis – a series of environmental and biotic changes associated with the widespread development of black shale deposition. However, in western Laurentia limestone deposition ceased and black shale deposition began prior to this crisis. Examining the type section of the Exshaw Formation at Jura Creek (Alberta) reveals that declining seafloor oxygen levels were likely responsible for the loss of carbonate productivity, followed by phosphogenesis of the youngest carbonates. The top surface of the limestone is a hard ground that was reworked, possibly by internal wave action, producing a lag of phosphatic clasts and pyrite grains including giant framboids (reaching 100 μm diameter) and polyframboids. The subsequent redox history of the lower Exshaw Formation shows substantial variations. Initially, the occurrence of small framboids, and elevated FeHR/FeT and Fepy/FeHR combined with enhanced U, Mo and Re concentrations indicates intense euxinia in the water column. These conditions were replaced by ferruginous anoxic conditions recorded in radiolarian-rich black shales that have moderate concentrations of trace metals, and low pyrite content, as confirmed both by low Fepy/FeHR ratios and scanning electron microscope (SEM) observation. Mo-U covariation trends suggest that Mo enrichment under ferruginous conditions was promoted by drawdown in association with Fe minerals precipitated in the water column. The return of better ventilated conditions around the Devonian/Carboniferous boundary is marked by a decrease in trace metal content and the loss of syngenetic pyrite. The Hangenberg Crisis is thus marked by an improvement in ventilation, from euxinic to ferruginous conditions in our study, and evidence from other basins in western North America suggests an even greater increase in oxygenation at this time.

Structural styles and linkage evolution in the middle segment of a strike-slip fault: A case from the Tarim Basin, NW China

The structural geometric and kinematic analysis of the Shunbei No. 5 fault zone (fault SB5) in the Tarim Basin was conducted based on the fault interpretation on the three-dimensional (3D) seismic sections and coherence slices of several seismic reflecting surfaces in the study area. The fault SB5 evolution model was recovered using the fault pattern analyses and palaeostress reconstruction, which required factors like the width of the damage zone, maximum throw of the main fault, vertical separation, and the shear crack angle along the fault zone. The results show that: (1) Four tectonic layers in the vertical direction are identified according to the difference of structure style, divided by the top and bottom of the Cambrian salt rocks, and the top surface of the Ordovician carbonate. (2) Strike-slip faults in the study area are the middle segment of the large strike-slip fault zone composed of major faults and overlap zones; 11 stepovers in the study area can be classified into three types according to their geometric characteristics. Strain concentration appears on the overlap zone under the continuous shear stress field and shows high activity. Fault SB5 has undergone three major evolutionary stages: the transpressional stage in the Middle Ordovician, the dextral shearing stage in the Early Silurian, and the final evolutionary sinistral process in the Late Devonian.

Zircon reveals multistage, magmatic and hydrothermal Rare Earth Element mineralization at Debert Lake, Nova Scotia, Canada

Rare Earth Element (REE) mineralization at Debert Lake (Nova Scotia, Canada) is hosted by the anorogenic Hart Lake-Byers Lake granites which are associated with the Late Devonian to Early Carboniferous Wentworth plutonic complex in the Cobequid Highlands. Mineralized zones contain up to 1.2 wt% REE2O3, 3.4 wt% ZrO2, and 0.4 wt% Nb2O5 produced by multiple, separate mineralizing events. We use a multi-faceted approach combining whole-rock and mineral geochemistry, together with U-Pb radiometric dates to uncover complex formative processes and resolve the timing for REE mineralization.Episodic mineralization yielded distinct REE assemblages including Nb-oxide (pyrochlore and fergusonite) phases, chevkinite, zircon, thorite, monazite, and titanite, formed during multiple magmatic-to-hydrothermal stages in the rare-metal bearing granites. Zircon, as a persistent phase of the REE assemblages, occurs with variable texture, morphology, and composition in all granites. Zircon grains, displaying intergranular to interstitial textures in REE rich granite, have spongy cores in which Y, Th, and Zr were redistributed by magmatic fluids. U-Pb dating of these zircons yields an age of 355 to 360 Ma, approximately coeval with igneous monazite crystallization, dated at 366 ± 5 Ma. These findings document a late-magmatic REE mineralization event. A subsequent pulse of post-magmatic hydrothermal mineralization associated with Zr-F-rich fluid complexes produced two other zircon populations, dated at ca. 309 and 318 Ma, with unusual habits and REE patterns that are interpreted to be of hydrothermal origin. Principal Component Analysis (PCA) separates zircon populations based on element concentrations and element associations. The LREE-rich hydrothermal zircon types with their distinct geochemical patterns revealed by PCA are characterized by the relative enrichment of either Na or Ca, reflecting differences in the composition of hydrothermal fluids driving sodic and calcic alteration processes, respectively

Mercury isotope evidence for regional volcanism during the Frasnian-Famennian transition

Mercury (Hg) concentrations and isotopes are widely used as proxies for volcanic inputs to paleodepositional systems. Enhanced volcanism during the Late Devonian was previously mainly inferred in the western Paleo-Tethys Ocean region from high Hg enrichments around the Upper Kellwasser horizon (UKW) and Frasnian-Famennian Boundary (FFB). In this study, we generated high-resolution Hg concentration data for four UKW-FFB sections (Walnut Creek and Dupont-GHS in the U.S.A., and Yangdi and Nandong in South China) and Hg isotope data for two of them (Walnut Creek and Nandong) in order to test the geographic distribution and stratigraphic range of volcanic inputs during the Frasnian-Famennian transition. Within the uppermost Frasnian Palmatolepis linguiformis Zone and the lowermost Famennian Lower Pa. triangularis Zone, Nandong (Paleo-Tethys Ocean) exhibits enrichment factors (HgEF > 2) and mass independent fractionations (MIF) of odd isotopes (Δ199Hg ∼0 ‰) that are consistent with significant volcanic inputs. Positive Δ199Hg values and an absence of Hg enrichments in the equivalent stratigraphic intervals at Walnut Creek and Dupont-GHS (peri-Panthalassic region) indicate that Late Devonian volcanism was a regional rather than a global phenomenon, and that it may have been associated with subduction zones and/or a large igneous province (LIP) of limited size in the Paleo-Tethyan region rather than with a major continental LIP of potentially global significance.

Palaeoecological reconstruction of the Late Devonian Lebedjan biota

ABSTRACT Palaeozoic fossil-bearing deposits in Russia record important and rare insights into pre-Mesozoic palaeoecological systems. Among the known formations, the Late Devonian (Famennian) deposits exposed in the central part of Russian platform have been under-documented. To expand the record of these formations, some of which show evidence of soft-bodied preservation, we revisit the Famennian-aged Lebedjan Formation from the Lipetsk region, central European Russia. We examine and record three outcrops lithologically and stratigraphically. In re-considering the Lebedjan Formation, we illustrate novel evidence for macrophytic algae, crustaceans, fishes, stromatolites, and xiphosurans preserved within the deposit. In doing so, we present a more complete reconstruction of this Late Devonian palaeoecosystem and demonstrate that sections of the Lebedjan Formation record low energy, lagoonal palaeoenvironments that had limited influence from the open ocean.

Report of Sublepidodendron (Lycopsida) from the Upper Devonian borehole core of the CSDP-2 Well, South Yellow Sea, China

Borehole core provides a series of working materials for research in paleontology, stratigraphy, and tectonics. This study demonstrates a tree-like lycopsid species of Sublepidodendron from the Upper Devonian Wutong Formation, 2063.8 to 2057.0 m depth of the CSDP-2 Well, Yellow Sea, China. The new plant consists of a slender leafy stem, persistent leaves, fusiform leaf bases, spoon-like shaped sporophylls, and megaspores. Morphology and distribution of the genus Sublepidodendron is discussed. Sublepidodendron species are widely distributed from the Upper Devonian to Lower Carboniferous all around the world. Morphological discrimination of its dozens of species is needed to uncover its disposal mysteries during the Late Devonian.

The Formation of Highly Positive δ34S Values in Late Devonian Mudstones: Microscale Analysis of Pyrite (δ34S) and Barite (δ34S, δ18O) in the Canol Formation (Selwyn Basin, Canada)

The sulfur isotope composition of pyrite in marine sedimentary rocks is often difficult to interpret due to a lack of precise isotopic constraints for coeval sulfate. This study examines pyrite and barite in the Late Devonian Canol Formation (Selwyn Basin, Canada), which provides an archive of δ34S and δ18O values during diagenesis. Scanning electron microscopy (SEM) has been combined with microscale secondary ion mass spectrometry (SIMS) analysis (n = 1,032) of pyrite (δ34S) and barite (δ34S and δ18O) on samples collected from nine stratigraphic sections of the Canol Formation. Two paragenetic stages of pyrite and barite formation have been distinguished, both replaced by barium carbonate and feldspar. The δ34Sbarite and δ18Obarite values from all sections overlap, between +37.1‰ and +67.9‰ (median = +45.7‰) and +8.8‰ and +23.9‰ (median = +20.0‰), respectively. Barite morphologies and isotopic values are consistent with precipitation from diagenetically modified porewater sulfate (sulfate resupply &amp;lt;&amp;lt; sulfate depletion) during early diagenesis. The two pyrite generations (Py-1 and Py-2) preserve distinct textures and end-member isotopic records. There is a large offset from coeval Late Devonian seawater sulfate in the δ34Spyrite values of framboidal pyrite (-29.4‰ to -9.3‰), consistent with dissimilatory microbial sulfate reduction (MSR) during early diagenesis. The Py-2 is in textural equilibrium with barite generation 2 (Brt-2) and records a broad range of more positive δ34SPy-2 values (+9.4‰ to + 44.5‰). The distinctive highly positive δ34Spyrite values developed from sulfate limited conditions around the sulfate methane transition zone (SMTZ). We propose that a combination of factors, including low sulfate concentrations, MSR, and sulfate reduction coupled to anaerobic oxidation of methane (SR-AOM), led to the formation of highly positive δ34Spyrite and δ34Sbarite values in the Canol Formation. The presence of highly positive δ34Spyrite values in other Late Devonian sedimentary units indicate that diagenetic pyrite formation at the SMTZ may be a more general feature of other Lower Paleozoic basins.

A New Radiolarian Genus Nestelliana gen. nov. and New Species from the Late Devonian of the Polar Urals and the Early Carboniferous of the Volga–Ural Basin of Russia

A New Radiolarian Genus Nestelliana gen. nov. and New Species from the Late Devonian of the Polar Urals and the Early Carboniferous of the Volga–Ural Basin of Russia

Geochronology and Geochemistry of Late Devonian I- and A-Type Granites from the Xing’an Block, NE China: Implications for Slab Break-off during Subduction of the Hegenshan-Heihe Ocean

Geochronology and Geochemistry of Late Devonian I- and A-Type Granites from the Xing’an Block, NE China: Implications for Slab Break-off during Subduction of the Hegenshan-Heihe Ocean

INSIGHTS FROM FOSSIL FISH TAPHONOMY INTO THE DEPOSITIONAL ENVIRONMENT OF THE LOWER MILWAUKEE FORMATION (BERTHELET MEMBER, LATEST MIDDLE DEVONIAN)

ABSTRACT The fossiliferous, argillaceous dolostone of the latest Middle Devonian (Givetian) Milwaukee Formation contains abundant disarticulated fish skeletal elements. This study refines depositional environment reconstruction of the Milwaukee Formation through taphonomic analysis of this fish assemblage. Robust skeletal elements of placoderm grinding teeth, fin spines, and armored plating dominate the assemblage. Specimens display variation in taphonomic attributes including color, luster, corrasion (chemical corrosion and/or physical abrasion), presence of pyrite and phosphate permineralization, and epibionts. Rare occurrences of teeth belonging to osteichthyan fish are also taphonomically variable. Associated invertebrates are diverse and include abundant cephalopod internal molds and pyritized brachiopods often randomly oriented in shell hashes. In sum, these observations suggest deposition on a marine shelf between normal and storm wave base. During low-energy background periods, fish plates exposed on the seafloor would undergo initial disarticulation, epibiont attachment, and color- and luster-altering corrasion and phosphatization. Buried skeletal elements were ‘protected' from epibiont attachment and corrasion, but occasional low oxygen conditions in the substrate led to early diagenetic pyrite permineralization. Periodic high-energy storm events were responsible for complete disarticulation as well as reworking and reexposure of buried skeletal elements (and burial of others), resulting in a time-averaged assemblage with a wide range of taphonomic attributes. Robust placoderm skeletal elements were best suited to survive fossilization in this setting and therefore the placoderm-dominated fossil assemblage appears to be an artifact of taphonomic processes and energy of the depositional environment.