Permian Boundary Research Articles

Multiple enrichment mechanisms of organic matter in the Fengcheng Formation of Mahu Sag, Junggar Basin, NW China

Based on core and thin section data, the source rock samples from the Fengcheng Formation in the Mahu Sag of the Junggar Basin were analyzed in terms of zircon SIMS U-Pb geochronology, organic carbon isotopic composition, major and trace element contents, as well as petrology. Two zircon U-Pb ages of (306.0±5.2) Ma and (303.5±3.7) Ma were obtained from the first member of the Fengcheng Formation. Combined with carbon isotopic stratigraphy, it is inferred that the depositional age of the Fengcheng Formation is about 297–306 Ma, spanning the Carboniferous–Permian boundary and corresponding to the interglacial period between C4 and P1 glacial events. Multiple increases in Hg/TOC ratios and altered volcanic ash were found in the shale rocks of the Fengcheng Formation, indicating that multiple phases of volcanic activity occurred during its deposition. An interval with a high B/Ga ratio was found in the middle of the second member of the Fengcheng Formation, associated with the occurrence of evaporite minerals and reedmergnerite, indicating that the high salinity of the water mass was related to hydrothermal activity. Comprehensive analysis suggests that the warm and humid climate during the deposition of Fengcheng Formation is conducive to the growth of organic matter such as algae and bacteria in the lake, and accelerates the continental weathering, driving the input of nutrients. Volcanic activities supply a large amount of nutrients and stimulate primary productivity. The warm climate and high salinity are conducive to water stratification, leading to water anoxia that benefits organic matter preservation. The above factors interact and jointly control the enrichment of organic matter in the Fengcheng Formation of Mahu Sag.

Two pulsed activities of the Emeishan large igneous province in southwestern China inferred from dolomite U-Pb geochronology and significance

Abstract The Emeishan large igneous province (ELIP) was a major geological event that took place around the Middle–Late Permian boundary. The major episode of the ELIP is generally considered to have occurred at 260–259 Ma, although an increasing number of geological data have suggested that multiple episodes of magmatic activity might have occurred. Here we report a study of a Lower Permian dolomite whose formation was influenced by the ELIP in the Sichuan Basin of southwestern China. Results show two new pulsed activities at 273.2 ± 9.7 Ma to 269.1 ± 12.0 Ma (ELIP-I) and 263.2 ± 10.3 Ma to 258.7 ± 8.1 Ma (ELIP-II). The two stages of the ELIP magmatism and thermal activity controlled the regional tectonic, sedimentary, and diagenetic evolution and exerted a profound influence on the oil and gas reservoirs, source rocks, and lithofacies paleogeography. This study is the first to interpret the timing of the ELIP from analysis of hydrothermal dolomite and provides new clues and data for improving our understanding of the dynamic formation mechanism and Earth system of the ELIP.

Jake Seller Draw impact structure, Bighorn Basin, Wyoming, USA: The deepest known buried impact structure on Earth and its possible relation to the Wyoming crater field

We provide evidence demonstrating the impact origin of a structure, here named the Jake Seller Draw impact structure, which is buried below the Jake Seller Draw drainage basin, in the Bighorn Basin of Wyoming, western United States. The 4.3-km-diameter structure was first recognized as a seismic disturbance at a depth of ∼6.5 km in two- and three-dimensional seismic profiles. Microstructural analysis of drill cuttings situated in the center and outside of Jake Seller Draw revealed the presence of multiple sets of planar deformation features and planar fractures in nine quartz grains, thereby confirming the hypervelocity impact origin of the structure. The seismic data show that Jake Seller Draw is a complex impact structure containing a 1-km-wide central uplift. The geologic and seismic data suggest that Jake Seller Draw is the most deeply buried impact structure known on Earth to date. The stratigraphic framework suggests that the crater was formed in a nearshore environment at the Pennsylvanian−Permian boundary, ∼280 m.y. ago. This age coincides with the age of the Wyoming crater field 300 km southeast of Jake Seller Draw and may suggest a common origin.

Palynological analysis (biochronostratigraphy and paleoenvironments) from a fossil-Lagerstätte section within the Late Paleozoic Ice Age unit of the Paraná Basin (Itararé Group), Western Gondwana

A diverse and exceptionally preserved fossil association occurs in Campáleo Outcrop black shales of the upper Itararé Group, correlated to the Lontras Member, considered representative of the Pennsylvanian–Permian transition in the Paraná Basin, Brazil. In this paper, we present the palynological records from 20 stratigraphic horizons of these shales and related strata, which revealed 21 taxa of spores, 33 of pollen grains, two species of fungi, one species of Chlorophyceae and two species of Prasinophyceae. The section is correlated to the Vittatina costabilis Zone, due to the records of certain guide species, such as Converrucosisporites confluens, Illinites unicus and several species of Vittatina. An additional subsurface section (TC-4 core) partially correlated with the Campáleo Outcrop and comprising the passage of the Crucisaccites monoletus and Vittatina costabilis palynozones was also studied in order to support the stratigraphic positioning. Once combined with the stratigraphic positioning in the basal portion to the Lontras Shale, it allows us to consider it around the Pennsylvanian–Permian boundary, probably in the Gzhelian to age of the Campáleo Outcrop. We observed distinct palynofloristic compositions from (i) the rhythmic shale with dropstones (basal portion) and from (ii) the fossiliferous black shales (upper portion). The former presents a large dominance (about 90 %) of pollen grains (xerophylous elements) and low frequency (about 7 %) of spores (meso-hygrophylous), while the results of the latter are conversely. Marine palynormorphs were recorded herein only from the rhythmic shale. The palynological assemblages on the base of the Campáleo Outcrop demonstrated a more intense sediment supply, bringing sporomorphs from more distant regions being represented mainly by xerophylic flora, indicating the existence of regions devoid of ice cover, where forests could develop, as occur in seasonal climate regions. The upper level of the Campáleo Outcrop characterizes one of the predominance of spores (hygrophylic communities) in these palynological associations supported by the large number of insects found that indicates proximity to the coastline, little transport (spore tetrads), poor contribution of more distant continental sediments (low expressiveness of pollen grains) and low salinity water, due to the absence of marine palynomorphs, representing a reducing restricted marine environment. This data integrated with the stratigraphic context and other geological data evidenced changes in sea level due to the climatic oscillations in the course of the LPIA that influenced the sediment supply and the distance of the coastline in this region of the Paraná Basin.

A new eryopid temnospondyl from the Carboniferous–Permian boundary of Germany

AbstractA new eryopid temnospondyl, Stenokranio boldi n. gen. n. sp. is described based on well-preserved cranial and postcranial material from fluvio-lacustrine deposits of the Permo-Carboniferous (Gzhelian/Asselian) Remigiusberg Formation at the Remigiusberg quarry near Kusel, Saar–Nahe Basin, southwest Germany. The new taxon is characterized by three autapomorphies within the Eryopidae: (1) the relatively narrow posterior skull table, therefore nearly parallel lateral margins of the skull; (2) the short postparietals and tabulars; and (3) the wide ectopterygoid. Phylogenetic analysis reveals a monophyletic Eryopidae with the basal taxa Osteophorus, Glaukerpeton, and Onchiodon labyrinthicus forming a polytomy. Actinodon may be either a basal eryopid or a stereospondylomorph, and the genus Onchiodon is not monophyletic. Stenokranio n. gen. is found as a more derived eryopid forming the sister taxon to Eryops. Stenokranio n. gen. was among the largest predators of the Saar–Nahe Basin. Its semiaquatic lifestyle enabled Stenokranio n. gen. to browse riverbanks and lake shorelines for prey, but most likely it fed on aquatic vertebrates. Stenokranio n. gen. was part of a faunal assemblage of aquatic, semiaquatic, and fully terrestrial vertebrates, such as sarcopterygian and actinopterygian fishes, xenacanthid sharks, a dvinosaurian temnospondyl, different “lepospondyls”, diadectomorphs, and synapsids. This is in general accordance with the vertebrate community from the Permo-Carboniferous of North America and from the early Permian localities of Manebach (Thuringian Forest Basin) and Niederhäslich (Döhlen Basin). It is notable that the occurrence of Stenokranio n. gen. and other eryopids in these localities excluded the presence of other large temnospondyls such as Sclerocephalus. However, a previously described isolated eryopid mandible from the Remigiusberg locality differs from that of Stenokranio n. gen. in several characters, implying that probably two different eryopid taxa lived at the same locality.UUID: www.zoobank.org/88a52547-d6fc-40af-965c-a6786c252ed5

Conodont biostratigraphy of a Carboniferous–Permian boundary section in siliceous successions of pelagic Panthalassa revealed by X-ray computed microtomography

Pelagic deep-sea siliceous successions in accretionary complexes preserve precious records of a vast deep seafloor that is now lost due to plate subduction. Microfossils are the key means of age assignment of these successions, but poor preservation due to tectonic deformation and metamorphism at the subduction zone hamper biostratigraphic records. X-ray computed microtomography, while not widely used in biostratigraphic studies until now, allows us to visualize fossils that are impossible or difficult to extract from host rocks due to poor preservation. In this study, we applied this method on conodonts from a pelagic chert–claystone succession in Okoshizawa, Iwaizumi Town, Northeast Japan, using a laboratory-based X-ray microscope. This work is a first close look at conodont biostratigraphy across the Carboniferous–Permian boundary in pelagic deep Panthalassa. We identified conodonts including ten species that are used as zonal markers in intensely studied areas such as around the East European Platform and Midcontinent United States. Based on the occurrence of conodonts, the studied section in Okoshizawa was correlated to the lower Moscovian to middle Artinskian. Confirmation of Moscovian to Artinskian age diagnostic conodonts from pelagic Panthalassa strengthens their role as global biostratigraphic indicators. By identifying more numerous specimens compared to the conventional hydrofluoric acid dissolution method, we were able to obtain information about conodont faunal characteristics around the Carboniferous–Permian boundary in pelagic deep areas of Panthalassa. The dominant taxa changed from Streptognathodus to Mesogondolella in the middle Asselian, probably reflecting an ecological takeover by the latter in the deep waters.

Sedimentary features and sequence stratigraphy of the successions around the Carboniferous–Permian boundary in the Ordos Basin: links to glacial and volcanic impacts

The sedimentary successions around the Carboniferous–Permian boundary (CPB) in the Ordos Basin were investigated using extensive outcrop, borehole, well logging, thin section, and geochemistry data to study sedimentary and sequence stratigraphic responses to glaciation and volcanism in paleotropical transitional strata. Within the studied interval, five distinct lithofacies have been identified, including bauxite, coal, and carbonaceous shale (No. 8 + 9 coal seams), sandstone (Qiaotou), limestone (Baode), and mudstone, which can be classified into three lithofacies associations. The most complete lithofacies association is composed of bauxite, coal, carbonaceous shale, sandstone, limestone/mudstone or their combinations from the bottom to the top, while coal and carbonaceous shale, as well as sandstone, are absent locally, resulting in the formation of the other two types of lithofacies associations. The occurrence of bauxite indicates shelf exposure and weathering, the occurrence of coal and carbonaceous shale indicates swampiness of the shelf, and the occurrence of sandstone reveals river rejuvenation; all of these are thought to be sedimentary responses to the transcontinental glacier expansion in Gondwana around the CPB. The presence of limestone and mudstone indicates carbonate platform and lagoon deposition, respectively, in the context of the earliest Asselian transgression caused by volcanism-induced glacier melting. The lithofacies associations record the regressive–transgressive cycles that occurred because of glaciation and volcanism near the CPB. The top surface of bauxite can be used as a sequence boundary, while the lowstand systems tract consists of the No. 8 + 9 coal seams and the Qiaotou sandstone, and the transgressive systems tract consists of the overlying Baode limestone and laterally equivalent mudstone. The lowstand systems tract, which contains source rock and hydrocarbon reservoirs, and the overlying transgressive systems tract, which serves as cap rock, form an excellent source-reservoir-seal combination.

Early Permian zircon ages from the P. confluens and P. pseudoreticulata spore-pollen zones in the southern Bonaparte and Canning basins, Northwestern Australia

The Pseudoreticulatispora confluens–P. pseudoreticulata spore-pollen zonal datum typically coincides with the end of widespread Permian glacial deposits in Western Australia. Although previously attributed to the mid-Sakmarian, chemical abrasion isotope dilution thermal ionisation mass spectrometry (TIMS) dating of zircons from volcanic tuffs in the Ditji Formation of the Bonaparte Basin and the Grant Group in the Canning Basin point to an Asselian age of about 295.25 Ma for this datum. All dated zircons from the Ditji Formation came from petroleum well cuttings but the accompanying palynology was mostly from sidewall cores; however, all Grant Group samples were from conventional core. TIMS dates from the Ditji Formation range in age from 295.2 to 292.7 Ma whereas the only productive tuff from the Grant Group yielded a 296.26 Ma date. By comparison, there are no zircon dates to constrain the onset of glacial deposition in Australia. The Bonaparte Basin ages overlap with those for the Edie Tuff (296.1–294.5 Ma) in Queensland’s Galilee Basin, approximately 2000 km to the southeast, which also lies close to the base of the P. pseudoreticulata Zone. To date the only fossil group within the P. confluens Zone in Western Australia to provide independent age control, albeit loosely, are goniatites from the northern Perth Basin (Uraloceras irwinense and Juresanites jacksoni) that have consistently been attributed to the Sakmarian; these require a reassessment of their affinity with Russian faunas and therefore to global stratotypes. The position of the Carboniferous–Permian boundary is elusive in Australia and will remain so until additional volcanic tuffs containing young datable zircons are found; however, spore-pollen and zircon dates from Namibia place this boundary within the P. confluens Zone.

Astrochronology and carbon-isotope stratigraphy of the Fengcheng Formation, Junggar Basin: Terrestrial evidence for the Carboniferous-Permian Boundary

The Carboniferous–Permian Boundary (CPB) is a major stratigraphic boundary and a marker for global stratigraphic correlation. However, the delimitation and global correlation of the CPB is hindered by the erosion during the Late Paleozoic Ice Age. Several carbonate platforms show a negative carbon isotope excursion (NCIE) in response to the CPB, which has great potential for the delimitation of this boundary. However, the response of terrestrial profiles to the CPB is unclear, and the global synchroneity of this NCIE is controversial. We studied a high paleolatitude section to evaluating the feasibility of the NCIE for delimitating CPB. We used astrochronological analysis of the 340 m Gamma Ray log of the Fengcheng Formation in Mahu Sag, Junggar Basin, northwest China, where the CPB may have been recorded. We constructed a high-resolution chronostratigraphic framework for the Fengcheng Formation, which precisely defines the chronostratigraphic attribution of the Fengcheng Formation as the Gzhelian–Asselian, and confirms that the CPB is recorded in the Fengcheng Formation. In addition, a global correlation of carbon isotope profiles revealed a negative excursion in terrestrial carbon isotope profiles across the CPB, which is consistent with the characteristics of NCIEs in several marine profiles. The similarity of the NCIE records supports their global synchrony and confirms that they can be used to delimit the CPB in terrestrial sections. Combining the astronomical time scale and the carbon isotope profile, we constrain the position of the CPB to the depth range of 4694.84–4703.56 m, with 4698.9 m as a preliminary estimate of the CPB. This new astronomical timescale and the stratigraphic position of the CPB enable the revision of the chronostratigraphic attribution of the terrestrial strata in the Junggar Basin. Overall, our results contribute to studies of the Carboniferous–Permian global carbon cycle, high-latitude glaciation, eustatic fluctuations, global volcanism, and their potential relationships.

Mixed eolian–longshore sediment transport in the late Paleozoic Arizona shelf and Pedregosa basin, U.S.A.: A case study in grain-size analysis of detrital-zircon datasets

ABSTRACT Detrital-zircon U–Pb geochronology has revolutionized sediment provenance studies over the last two decades, and zircon has been successfully analyzed from nearly all sedimentary lithologies, depositional environments, and sediment grain sizes. However, despite the ubiquity of this method and the far-reaching interpretations supported by detrital-zircon data, few studies have investigated the potential role of zircon grain size on age spectra and provenance interpretation. In this study, we investigate the connections between sample grain size, zircon grain size, U–Pb age spectra, and interpreted provenance using 18 detrital-zircon samples (4999 individual grains) collected from Pennsylvanian–Permian strata in central and southern Arizona, USA. In these samples, there is no clear correlation between sample grain size and zircon grain size and no clear correlation between sample grain size and age spectra. However, when all grains are grouped by zircon minimum long-axis dimension, the abundance of some age groups is correlated to zircon grain size. In Pennsylvanian samples, < 400 Ma grains and 2500–3000 Ma zircons are more abundant in the finer fractions, and 1400–1900 Ma zircons are more abundant in coarser fractions of both Pennsylvanian and Permian samples. In Permian samples, 500–800 Ma zircons are most abundant in the finer fractions, and 2500–3000 Ma grains are concentrated in the coarser fractions. Based on changes in abundance and grain-size distribution of 500–800 Ma grains, we interpret a change in zircon provenance across the Pennsylvanian–Permian boundary that reflects regional climate and paleogeographic changes driven in part by the northward drift of Laurentia across the equator. Specifically, we interpret the concentration of 500–800 Ma zircons in Permian samples in central and southern Arizona to indicate that these grains were: 1) sourced from Gondwana, 2) deposited in, and subsequently eroded (recycled) from, Mississippian–Pennsylvanian strata in the Arkoma, Anadarko, and Fort Worth basins at the margins of Laurentia, and 3) finally transported into the Arizona study area as loess by easterly trade winds. This study serves as a case study in the value and interpretive power of basic grain-size characterization of detrital-geochronology datasets.

Reexamination of a Permian Tentaculites-Like Fossil Iwakiella ichiroi Hatai, Kotaka and Noda, 1972, as an Orthocerid Cephalopod

The holotype of Iwakiella ichiroi Hatai, Kotaka and Noda, 1972, from the Permian Kashiwadaira Member of Northeast Japan is reexamined. Although previously placed within the Tentaculitoidea or retained in the Problematica, the possession of the septum and the spherical shape of the initial chamber suggest that this specimen belongs to sphaerorthoceratid orthocerid of the Cephalopoda. The present result confirms that the Tentaculitoidea probably became extinct near the Carboniferous – Permian boundary with Hidagaienites arcuatus Niko, 2000, as the latest representative.

New Insight into the Depositional Age of No. 6 Coal in Heidaigou Mine, Late Paleozoic Jungar Coalfield, Inner Mongolia, China

Coal deposits can provide novel stratigraphic markers for reconstructing the evolution history of a sedimentary basin and correlating sedimentary successions. Age dating was conducted on zircons harvested from the No. 6 coal seam within the Heidaigou Mine, Inner Mongolia. Two-kilogram samples were taken, and the recovered zircons were analyzed for U–Pb isotopic and rare earth elements (REE). The REE results of the zircon grains showed that all the zircon grains were enriched in heavy rare earth elements (HREE) but depleted in light rare earth elements (LREE). In addition, zircons from the No. 6 coal seam had strongly positive Ce (Ce/Ce* = 2.4–224.6) and strongly negative Eu anomalies (Eu/Eu* = 0.1–0.6). Combined with the clear oscillatory zones in the cathodoluminescence images, all the zircon grains of the No. 6 coal were characteristic of zircons with magmatic origins. The 206Pb/238U ages of 34 zircon grains produced a narrow age population of 303–286 Ma, with a weighted average age of 293.0 ± 1.5 Ma (mean-squared weighted deviation = 1.5). Therefore, we infer that the No. 6 coal in the Heidaigou Mine was deposited during the Early Permian, and the Carboniferous–Permian boundary should be located stratigraphically lower than the No. 6 coal. The zircon U–Pb geochronology is a useful tool to determine the depositional ages of non-marine-influenced coal.

Tracing southern Gondwanan sedimentary paths: A case study of northern Namibian late Palaeozoic sedimentary rocks

AbstractThe northern Namibian Karoo‐aged successions are part of a Gondwana‐wide sedimentary system emerging at the Carboniferous–Permian boundary and existing for more than 50 Ma. The Karoo Supergroup sedimentary successions are of importance in understanding the evolution of the Karoo rift system. This study presents new whole‐rock geochemical data combined with detrital zircon morphology as well as U−Pb ages and Lu−Hf composition of late Palaeozoic siliciclastic rocks of the Namibian Huab Basin and Kunene area (south‐west Africa). Inferred by youngest detrital zircon U−Pb ages the Verbrande Berg Formation (lower Ecca Group) yields a Sakmarian to Asselian maximum depositional age, whereas the overlying Tsarabis Formation yields an Artinskian maximum depositional age. These ages coincide with the end of the Dwyka ice age and an overall warming and a contemporaneous evolution of the Karoo I rift system across southern Gondwana. The zircon age distribution of the investigated samples yields clusters ranging between ca 500 to 650 Ma (Cambrian–late Neoproterozoic), ca 950 to 1200 Ma (early Neoproterozoic–Mesoproterozoic) and ca 1800 to 1900 Ma (Palaeoproterozoic). Their rounded shapes characterize the zircon grains of the Kunene area and the lower Huab Basin section, whereas upper Huab Basin strata yield mostly unrounded grains. The rounded nature of zircon grains with a diverse U−Pb age spectrum putatively points towards sediment homogenization and multiple recycling stages during the deposition of the sediments and large catchment areas of the depositing rivers. As suggested by zircon grains with a low roundness value and a single Palaeoproterozoic age cluster, the upper Huab Basin successions were probably deposited under drier climatic conditions, small catchment areas and limited sedimentary homogenization. Therefore, the southern Gondwana sedimentary transport and homogenization system may change over time and is dependent on the climate prevailing during deposition. This study shows that the laws of detrital zircon are very complex and are yet to be explored.

Revised age of the Fengcheng Formation, Junggar Basin, China: Global implications for the late Paleozoic ice age

The age of the Fengcheng Formation in the Junggar Basin, China, is key to understanding the effects of the late Paleozoic ice age (LPIA) on the Northern Hemisphere. Here we present zircon UPb age and Hf isotope data, along with stable CO isotoope data, and discuss the implications of these for the LPIA. The age of the Fengcheng Formation is constrained to ca. 300 Ma (i.e., Kasimovian–Asselian), which is at the Carboniferous–Permian boundary. Most of the Fengcheng Formation was deposited during the late Carboniferous warming period and partly during the LPIA Glacial III/P1. The C isotope data and occurrence of evaporites sensitively record the LPIA. The Hf isotope data record the complete closure of the paleo-Junggar Ocean at ca. 300 Ma. The hydrologically restricted Junggar Basin was likely affected by the LPIA due to an atmosphere–hydrological response. The complete closure of the paleo-Junggar Ocean coincided broadly with the beginning of early Permian glaciation and hints at a subtle relationship between continental amalgamation and glaciation.

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The Wabunsee (Carboniferous: Virgilian) through Sumner (Permian: Leonardian) Groups of Oklahoma preserve a record of terrestrial and nearshore marine environments during glacio-eustatic fluctuations caused by southern hemisphere glaciation. This section, with the Pennsylvanian–Permian Boundary firmly established in the Red Eagle Formation on the basis of conodonts, permitted study of vegetational change across the boundary interval. The palynology of 43 productive samples from five cores in Kay and Osage counties in Oklahoma was investigated. Palynomorphs were assigned to eight morphologic groups that also reflect climate preferences. Four distinct palynofloras are recognized. The three transitions between them—in the lower Council Grove, middle Chase and lower Sumner groups—are abrupt and, in succession, consistent with an interpretation of increasing seasonality/aridity from latest Pennsylvanian into the Early Permian. None of these floristic transitions align with the conodont-based boundary, or with any lithostratigraphic “group” boundary. These palynological data from Oklahoma are in accord with previous data from Kansas that also demonstrated no palynological shift at the boundary and increased seasonality/aridity through this period. Comparison of the palynofloras from Oklahoma, Kansas, and the boundary interval of the Pennsylvanian–Permian Boundary type section in Aidaralash Creek in Kazakhstan reveals differences in composition that suggest local to regional, including climatically induced, heterogeneities that may present an obstacle to confident palynologically based, global correlations in fully terrestrial sections.

The Duration and Geodynamics of Formation of the Angara–Vitim Batholith: Results of U–Pb Isotope (LA-ICP-MS) Dating of Magmatic and Detrital Zircons

Abstract —We present results of U–Pb (LA-ICP-MS) dating of detrital zircons from the alluvial deposits of the Angarakan River (North Muya Ridge, northern Baikal region), whose drainage basin is composed mainly of granitoids of the Barguzin Complex, typomorphic for the late Paleozoic Angara–Vitim batholith (AVB). Three age clusters with peaks at 728, 423, and 314 Ma have been identified in the studied population of detrital zircons. It is shown that small outliers of igneous and metamorphic rocks, probably similar to the large AVB roof pendants mapped beyond the drainage basin, are the source of Neoproterozoic and early Paleozoic zircons. The late Paleozoic cluster comprises two close peaks at 314 and 28 Ma, which totally “overlap” with the time of the AVB formation and mark a granitoid source of the zircons. The results of detrital-zircon geochronology, together with the data on bedrocks, point to the prolonged (~40 Myr) formation of the AVB, but the intensity of magmatism during this period calls for additional study. Based on the analysis of published geological, geochemical, and geochronological data, we assume that the AVB resulted from the plume–lithosphere interaction that began in the compression setting and gave way to extension 305–300 Ma (the Carboniferous–Permian boundary), which caused replacement of “crustal” granitoids by granitoids formed from a mixed mantle–crustal source.

High-resolution late Paleozoic cyclostratigraphy and tectonic evolution of the Keeler Basin, California, southwest Laurentia

Abstract Cyclic strata exposed in the Inyo Mountains of eastern California contain a continuous 6 m.y. record of deep marine deposition that spans the Pennsylvanian–Permian boundary. To better understand the geologic evolution of southwest Laurentia and the role of glacially driven eustasy in upper Paleozoic stratigraphy, we measured two detailed ∼600 m composite stratigraphic sections of the Keeler Canyon Formation and collected a handheld spectral gamma ray log. Post-depositional deformation complicates field relationships, but 1:5000 scale mapping of faults and folds permits assembly of two continuous sections. Measured strata alternate at the 5–20 m scale between intervals of fine-grained laminated marlstone and intervals of mixed carbonate and siliciclastic turbidites and debrites. Based on facies characteristics and a prominent marker horizon, we reassign the Pennsylvanian-Permian age upper Salt Tram unit of the upper Keeler Canyon Formation to a new Estelle Member. We estimate sediment accumulation rates within the Keeler Canyon Formation using published conodont biostratigraphy and correlative U-Pb geochronology from Eastern Europe combined with spectral analysis and timescale optimization using the Astrochron R package. Evolutive harmonic analysis of gamma ray-derived element concentrations reveals prominent bundled periodicities that are consistent with both long and short eccentricity cycles. Average sediment accumulation rates calculated using the time scale optimization function of Astrochron suggest a gradual increase from 40–60 m/m.y. to ∼120 m/m.y. during the late Pennsylvanian and then a minima of ∼50 m/m.y. near the Pennsylvanian–Permian boundary, which is followed by an increase to ∼175 m/m.y. into the Early Permian. This trend in sediment accumulation rates and subsequent Permian contractile deformation are compatible with flexural subsidence in a SW-migrating foreland basin related to the southern part of the late Antler orogenic system.

Organic petrology, geochemistry, and hydrocarbon generation capacity of Permo–Carboniferous source rocks in the Mahu Sag, northwestern Junggar Basin, China

The Mahu Sag in the Junggar Basin, China, is a hydrocarbon-rich sag. Abundant hydrocarbon source rocks were developed in the late Paleozoic there across the Carboniferous–Permian boundary. However, studies of the source rocks have focused mainly on the lower Permian Fengcheng Formation. Here we compare the Fengcheng Formation with other Carboniferous rocks and those of the lower Permian Jiamuhe and middle Permian Lower-Wuerhe formations. Based on organic petrological and geochemical data, the organic matter precursors, sedimentary facies, and resource potential of these source rocks were investigated. The bio-precursors of Carboniferous, Jiamuhe, and Lower-Wuerhe Formations were dominantly estuarine higher plants, Nematothallus -higher plants, and spores, respectively. The bio-precursors of Fengcheng Formation were mainly bacteria and algae, and the organic matter is abundant, with a high hydrocarbon-generating capacity and significant shale oil potential. In contrast, the other three formations contain poor-quality source rocks, although the Lower-Wuerhe Formation has a higher organic matter abundance than the Jiamuhe Formation. The Carboniferous organic matter contains mainly type II kerogen, whereas the Jiamuhe and Lower-Wuerhe formations contain type III kerogen. The thermal maturities determined from Tmax values is larger than those indicated by biomarkers. The biomarkers show that the three studied formations contain a mixture of terrestrial higher plants and bacteria–algae, with the contribution of green algae being higher than that of bacteria in most samples. However, the ratio of algae to bacteria is lower than that of the Fengcheng Formation. The Fengcheng Formation was deposited in a strongly reducing, high-salinity, and water-stratified sedimentary environment. The other three formations were deposited in an oxidizing–reducing, low-salinity, and water-unstratified environment. They are characterized by the predominance of mudstone over carbonate rocks and the descending distribution type of tricyclic terpenes. Our results provide a basis for research on upper Paleozoic source rocks in western China, and useful information for oil and gas exploration in the Mahu Sag.

New information on the dissorophid Conjunctio (Temnospondyli) based on a specimen from the Cutler Formation of Colorado, U.S.A

ABSTRACT The dissorophid genus Conjunctio (Temnospondyli) is poorly characterized, known only from two incomplete specimens from the upper El Cobre Canyon Formation (lower Permian), Cutler Group, New Mexico, U.S.A. Nonetheless, the taxon’s conserved morphology and stratigraphic occurrence near the Carboniferous–Permian boundary (ca. 299 million years ago) make it an important datum to resolve the early diversification of dissorophids. We report the first occurrence of Conjunctio cf. C. multidens in the adjacent undivided Cutler Formation of San Miguel County, Colorado, which also represents only the second dissorophid from southwestern Colorado’s historic Placerville assemblage. The new specimen highlights the plesiomorphic anatomy of Conjunctio, with newly described mandibular and postcranial data, and provides further evidence for a relationship to the Eucacopinae. We performed a phylogenetic analysis of 34 temnospondyl taxa by modifying a previously published matrix of 102 craniodental and postcranial characters, scoring Conjunctio at the specimen-level, and found a monophyletic Conjunctio at the base of Eucacopinae. The clade also included the earliest Permian Reiszerpeton and Scapanops in relatively basal positions, and an unresolved polytomy among the later Permian Cacops, Kamacops, Zygosaurus, and Anakamacops. Geographically, the discovery of Conjunctio among the Placerville assemblage is consistent with a broader southwestern U.S. Wolfcampian fauna, correlative to that of the upper El Cobre Canyon Formation in northern New Mexico, and may underscore previously proposed regional provincialism among early Permian tetrapod assemblages.

Upper Paleozoic stratigraphy and detrital zircon geochronology along the northwest margin of the Sverdrup Basin, Arctic Canada: insight into the paleogeographic and tectonic evolution of Crockerland

The upper Paleozoic succession along the northwest margin of the Canadian Arctic Sverdrup Basin is little studied and poorly understood yet has the potential to yield insights into the paleogeographic and tectonic evolution of the Arctic regions including Crockerland. Carboniferous and Permian drill cuttings were collected from five exploration wells on Brock, Mackenzie King, and Ellef Ringnes islands. Seven unconformity-bounded sequences were identified and correlated. Reflection seismic interpreted on Ellef Ringnes Island indicates that a major syn-sedimentary fault offsets the Mississippian succession bounding a down-to-the-north half-graben. Late Pennsylvanian (Gzhelian) fault reactivation, associated with the Melvillian Disturbance, created a depression that extended northward and was bordered to the south by a structural high. Episodic minor fault reactivation occurred until the Early–Middle Permian boundary. During the latest Early Permian (Kungurian), sand derived from Crockerland prograded southward onto the Sverdrup Basin’s northwest margin and continued into the Roadian. After a lull during the Wordian, clastic progradation resumed in the Capitanian. Detrital zircon U–Pb ages recovered from Kungurian and Roadian samples on Brock and Ellef Ringnes islands display Devonian Clastic Wedge (DCW) signatures. A Moscovian–Artinskian carbonate blanket likely covered Crockerland and sheltered DCW material from erosion, implying it was a subsiding, carbonate bank throughout most of the Pennsylvanian – Early Permian. Base level fall in the Kungurian, associated with a transition to a more humid climate, breached these carbonate rocks to allow erosion and transportation of DCW material. Recycling of the DCW started earlier (Artinskian) and peaked later (Wordian) along the southern margin of the basin.