Conodont Assemblages Research Articles

Ordovician conodont biostratigraphy and redefinition of the age of lithostratigraphic units on northeastern Melville Peninsula, Nunavut

Northeastern Melville Peninsula, Nunavut, Canada, preserves the stratigraphic record of the northwestern margin of the Foxe Basin. The Ordovician sequence on the peninsula includes the Lower Ordovician Ship Point Formation and Upper Ordovician Frobisher Bay, Amadjuak, Akpatok, and Foster Bay formations. Their biostratigraphic ages and correlations are poorly understood; in particular it is unclear whether the organic-rich “Boas River Formation” exists on the peninsula. Following extensive sampling of these stratigraphic units, studies of numerous conodont elements from both outcrops and rubble at about 60 localities have established five conodont assemblages through the five lithostratigraphic units on the peninsula. Oepikodus communis – Jumudontus gananda Assemblage in the Ship Point Formation is correlated to the Reutterodus andinus Zone in the uppermost Ibexian, Lower Ordovician. The other four assemblages from the Upper Ordovician are as follows: Appalachignathus delicatulus – Polyplacognathus ramosus – Belodina confluens in the Frobisher Bay Formation correlated to lower B. confluens Zone in the upper Chatfieldian; Belodina confluens – Periodon grandis in the Amadjuak Formation to the upper B. confluens, Oulodus velicuspis, O. robustus, and lower Aphelognathus grandis zones from Edenian to lowest Richmondian; Amorphognathus ordovicicus – Plegagnathus and Rhipidognathus symmetricus – Aphelognathus cf. A. divergens in the Akpatok and Foster Bay formations to the lower and upper Richmondian. The biostratigraphy is combined with geographical information systems (GIS) and Google Earth technologies in estimating the thickness of Paleozoic strata, which reduces the likelihood of “Boas River Formation” existing on Melville Peninsula to minimum.

Middle and Upper Ordovician conodonts of the Moscow Syneclise: New data on stratigraphy of the Borehole Gavrilov Yam-1 section

On the basis of the first data on the taxonomic composition and stratigraphic distribution of conodont assemblages in the Borehole Gavrilov Yam-1 section (central part of the Moscow Syneclise), its biostratigraphic subdivision is proposed with specification of some formations previously substantiated by finds of acritarchs and benthic faunal assemblages. The Nefedovo Formation, which was previously attributed to the Volkhov Horizon (Dapingian Stage), is correlated by conodonts with the Kunda Horizon (basal part of the Darriwilian Stage). The middle part of the overlying Veksa Formation, which was formerly considered to be of Kunda age, yielded conodonts that mark the lower boundary of the Azeri Horizon. Conodonts found in the Mitino Formation with highly ambiguous age allow it to be correlated with the Johvi and Keila horizons of the Sandbian Stage (Upper Ordovician). The lower part of the Varlyga Formation, which is attributed to the Lower Silurian in the regional stratigraphic scale, is correlated by conodonts with the Oandu and Rakvere regional stages (lower part of the Katian Stage, Upper Ordovician). In total, 44 conodont species were found in 79 samples; most of them are illustrated. A brief description is given for representatives of the genera Pectinognathus, Phragmodus, and Semiacontiodus. The conodont assemblage includes species distributed in the northwestern part of the East European Platform (Baltoscandian paleobasin) and taxa characteristic of the Timan-Pechora region.

Foraminifers and conodonts from the late Viséan to early Bashkirian succession in the Saharan Tindouf Basin (southern Morocco): biostratigraphic refinements and implications for correlations in the western Palaeotethys

The Carboniferous succession in the Tindouf Basin of southern Morocco, North Africa, displays Mississippian to Early Pennsylvanian marine beds, followed by Pennsylvanian continental deposits. The marine beds comprise a shallow water cyclic platform sequence, dominated by shales and fine-grained sandstones with thin but laterally persistent limestone/dolostone beds. Foraminiferal assemblages have been studied in the limestone beds in several sections from the Djebel Ouarkziz range in the northern limb of the Tindouf Syncline; they indicate that the age of the limestones range from late Asbian (late Viséan) to Krasnopolyanian (early Bashkirian). The foraminiferal assemblages are abundant and diverse, and much richer in diversity than those suggested by previous studies in the region, as well as for other areas of the western Palaeotethys. The richest assemblages are recorded in the Serpukhovian but, unusually, they contain several taxa which appear much earlier in Western European basins (in the latest Viséan). In contrast, conodont assemblages are scarce due to the shallow-water facies, although some important taxa are recorded in the youngest limestones. Copyright © 2013 John Wiley & Sons, Ltd.

Trilobites Tremadocianos en Facies de Lutitas Oscuras del Miembro Alfarcito (Formación Santa Rosita), Quebrada de Moya, Jujuy, Argentina

TREMADOCIAN TRILOBITES FROM DARK SHALE FACIES OF THE ALFARCITO MEMBER (SANTA ROSITA FORMATION), QUEBRADA DE MOYA, JUJUY, ARGENTINA. The Quebrada de Moya constitutes a distinctive locality of the Santa Rosita Formation (uppermost Furongian—lowermost Ordovician) in the Quebrada de Humahuaca (Sierra de Tilcara, Cordillera Oriental of Jujuy, Argentina). The trilobites Leptoplastides marianus (Hoek), Parabolinella sp., Hapalopleura acantha Malanca and Brandán, and Asaphellus sp. are described from the dark shale facies of the upper part of the Alfarcito Member. A grainstone interstratified in the mudstones contains a low-diversity conodont assemblage of the Cordylodus angulatus Zone (lower Tremadocian). The Ordovician age of the section is confirmed, in addition, by the record of the graptolite Rhabdinopora flabelliformis ssp. in a sandstone underlying the studied levels. The dark shales accumulated by suspension fall-out during periods of maximum flooding, in a reducing, distal offshore marine environment. The trilobite fauna is largely dominated by L. marianus. The upper part of the Alfarcito Member does not show dark facies in other sectors of the Sierra de Tilcara. The sedimentologic characters of the section described, the low diversity of the conodont assemblage, and the scarcity of agnostoid trilobites may be indicative of a partial isolation of the basin.

A Viséan (Brigantian) conodont assemblage preserved in the dissolution residue component of a breccia matrix at Bullslaughter Bay, Pembrokeshire, South Wales

A Viséan (Brigantian) conodont assemblage preserved in the dissolution residue component of a breccia matrix at Bullslaughter Bay, Pembrokeshire, South Wales

The Carboniferous-Permian boundary at Carrizo Arroyo, Central New Mexico, USA

At Carrizo Arroyo southwest of Albuquerque, New Mexico,USA, an approximately 105-m-thick section of upper Paleozoic clastic and carbonate rocks yields extensive fossil assemblages of marine and nonmarine origin. Most of the section at Carrizo Arroyo belongs to the Red Tanks Member of the Bursum Formation, ~ 100m thick and mostly variegated shale, mudstone and siltstone of nonmarine origin, intercalated with some beds of limestone and shale of marine origin. Red Tanks Member fossils include palynomorphs, charophytes, plant megafossils, non-fusulinid foraminifers, fusulinids, bryozoans, brachiopods, gastropods, bivalves, nautiloids, eurypterids, ostracods, syncarid crustaceans, conchostracans, insects and some other arthropods, echinoids, crinoids, conodonts, fish ichthyoliths and bones of amphibians and reptiles. At stratigraphic levels 43m and 68m above the base of the section are Lagerstätten of plants, insects, crustaceans, eurypterids and other fossils that form unique Late Paleozoic nearshore arthropod assemblages. Most of the fossil groups from the Red Tanks Member have been used to support diverse placements of the Pennsylvanian-Permian boundary at Carrizo Arroyo. We review previous placement of the Pennsylvanian-Permian boundary in the Carrizo Arroyo section and present newly collected insect and conodont data. The insects indicate that the two Lagerstätten in the Red Tanks Member are of early Asselian age. The new conodont data include the presence of Streptognathodus virgilicus in the uppermost part of the Atrasado Formation, which constrains its age to the middle to upper part of the Virgilian and to a comparable position in the Gzhelian. The only biostratigraphically-significant conodont assemblage in the Red Tanks Member comes from a marine horizon near the middle of the member, and the assemblage is probably equivalent in age to the Midcontinent Streptognathodus nevaensis Zone, of early to middle Asselian in age. Asignificant amount of latest Pennsylvanian to earliest Permian time apparently is not represented by rock record at the Carrizo Arroyo section, most likely at a major disconformity at the top of the Atrasado Formation and smaller ones at the bases of depositional sequences in the lower part of the Red Tanks Member. Conodont biostratigraphy provides compelling evidence that Bursum Formation deposition was not simply driven by glacio-eustatic cyclicity, but in this area it was partly overprinted by local tectonics.

CONODONT DISTRIBUTION ACROSS THE MID-CARBONIFEROUS BOUNDARY IN THECENTRAL TAURIDES, TURKEY

The Mid-Carboniferous boundary in the Aladað Unit of the Central Taurides, previously delineated on the basis of foraminifers, is now further recognizable on the basis of conodonts. Latest Serpukhovian conodonts are assigned to the Rhachistognathus muricatus Zone, whereas the early Bashkirian forms correspond to the Declinognathodus inaequalis - Declinognathodus noduliferus Zone. Conodont assemblages discovered in the Taurides are comparable to those described from Mid-Carboniferous boundary sections in North America, Japan, southern Urals, southern China, Ukraine and Uzbekistan. The Mid-Carboniferous boundary is located at a horizon marked by the first appearance of Declinognathodus inaequalis , just above the last appearance of the conodont Gnathodus girtyi simplex . This horizon coincides with the boundary indicated by foraminiferal data.

The “Vaginaten”: the dominant cephalopods of the Baltoscandian Mid Ordovician endocerid limestone

The lower part of the Baltoscandic Orthoceratite Limestone (Kunda Stage, Darriwilian) is characterized by abundant and often large orthoconic endocerid cephalopods. The endocerids of this limestone, known historically as the “Vaginaten”, are among the historically earliest figured Swedish fossils. The taxonomic history of the two most common and most characteristic of these endocerids is complicated by the use of wastebasket taxa and an extensive oversplitting. Here, the status of the name Anthoceras vaginatum (Schlotheim) is clarified for an annulated endocerid with high variability in ornamentation and conch form. The large, smooth Proterovaginoceras incognitum (Schröder) is the predominant endocerid of the lower parts of the Orthoceratite Limestone. Together, more than 90% of the cephalopods of the endocerid limestones can be assigned to these two species. Massive changes in the cephalopod composition throughout the Kunda Stage at Hälludden, Öland, Sweden, indicate significant changes in the ecosystem during this time interval. These changes were formerly unrecognized and coincide with drastic changes in conodont assemblages and benthic communities.

Stratigraphy of Upper Permian and Lower Triassic Strata of the Žiri Area (Slovenia)

The paper deals with the stratigraphy of Late Permian and Early Triassic strata of the Lukač section in the Žiri area of western Slovenia. This is the only section presently known in the External Dinarides where the Permian- Triassic boundary is defined following international criteria based on the first appearance of the conodont Hindeodus parvus. The following lithostratigraphic units have been formalized: the Bellerophon Limestone and Evaporite-dolomite Members of the Bellerophon Formation and the Lukač Formation with the three members, the Transitional Beds, Streaky Limestone and Carbonate-clastic Member. The paper presents the results of micropaleontological study based on foraminifers and conodonts as well as petrographic and sedimentologic research results. The investigation of conodont assemblages enabled the conodont biozonation of the Permian-Triassic interval of the studied Lukač section.

Ordovician radiolaria and conodonts from the peri-Gondwanan Karatau-Naryn microplate (Sarydzhaz, eastern Kyrgyzstan)

Cambrian-Ordovician sedimentary sequences crop out extensively in the area surrounding the Sarydzhaz River, a remote mountainous region situated in the eastern part of Kyrgyzstan. These sequences, composed essentially of fine siliciclastic lithologies, were deposited on a passive margin of the peri-Gondwanan Karatau-Naryn microplate. Palaeontological constraints for the age of these sequences are rare. Recent efforts of geological mapping in the area discovered reasonably well-preserved radiolarian and conodont faunas. The occurrence of conodonts was mentioned previously, but no taxonomic details were ever reported. Two conodont assemblages are identified from the Oldzhobai Formation; the first is dominated by Paracordylodus gracilis, which can be correlated with the upper Tremadocian to lower Floian; the second assemblage is more diverse and characterized by Prioniodus elegans, suggesting a mid Floian age. This is the first discovery of Ordovician Radiolaria from Kyrgyzstan. In spite of its rather moderately good to poor preservation, the fauna is interesting in many respects, especially due to the presence of a new radiolarian species ( Inanigutta (?) kyrgyza nov. sp.). The latter is easily identifiable by the distinctive pore pattern of its cortical shell and has therefore the potential to become in the future a biostratigraphic marker species.

Tribes Hill–Rochdale formations in east Laurentia: proxies for Early Ordovician (Tremadocian) eustasy on a tropical passive margin (New York and west Vermont)

AbstractSlow subsidence and tectonic quiescence along the New York Promontory margin of Laurentia mean that the carbonate-dominated Tribes Hill and overlying Rochdale formations serve as proxies for the magnitude and timing of Tremadocian eustatic changes. Both formations are unconformity-bound, deepening–shoaling, depositional sequences that double in thickness from the craton into the parautochthonous, western Appalachian Mountains. A consistent, ‘layer cake’ succession of member-level units of the formations persists through this region. The Tribes Hill Formation (late early Tremadocian, late Skullrockian, late Fauna B–Rossodus manitouensisChron) unconformably overlies the terminal Cambrian Little Falls Formation as the lowest Ordovician unit on the New York Promontory. It was deposited during the strong early Tremadocian, or Stonehenge, transgression that inundated Laurentia, brought dysoxic/anoxic (d/a) slope water onto the shelf and led to deposition of the Schaghticoke d/a interval (black mudstone and ‘ribbon limestone’) on the Laurentian continental slope. The uniform lithofacies succession of the Tribes Hill includes a lower sand-rich member; a middle, dark grey to black mudstone that records d/a in eastern exposures; and an upper, shoaling-up carbonate highstand facies. A widespread (12000+ km2) thrombolitic interval in the highstand carbonate suggests the New York Promontory was rimmed by thrombolites during deposition of the Tribes Hill. Offlap and erosion of the Tribes Hill was followed by the relatively feeble sea-level rise of the Rochdale transgression (new) in Laurentia, and deposition of the Rochdale Formation. The Rochdale transgression, correlated with theKierograptusDrowning Interval in Baltica, marks a eustatic rise. The Rochdale Formation represents a short Early Ordovician interval (early late Tremadocian, middle–late Stairsian,Macerodus dianaeChron). It correlates with a depositional sequence that forms the middle Boat Harbour Formation in west Newfoundland and with the Rte 299 d/a interval on the east Laurentian slope. The Rochdale has a lower carbonate with abundant quartz silt (Comstock Member, new) and an upper, thrombolitic (Hawk Member, new) high-stand facies. Tribes Hill and Rochdale faunas are mollusc-rich, generally trilobite-poor, and have low diversity, Laurentian faunal province conodonts.Ulrichodina rutnikaLanding n. sp. is rare in Rochdale conodont assemblages. Trilobites are also low in diversity, but locally form coquinas in the middle Tribes Hill. The poorly preserved Rochdale trilobites include the bathyuridRandaynia, at least two hystricurid species andLeiostegium.

Reassessment of Lochkovian Pragian conodont faunas from the Rade de Brest and the Presqu'ile de Crozon (Massif Armoricain, France)

Four informal Lochkovian-Pragian conodont faunal intervals, FI 1 to FI 4 have been recognized in Lower Devonian sections yielding in the westernmost part of the Massif Armoricain. The conodont assemblages are in general not abundant, poorly diversified, and largely dominated by icriodids. The structural constraints induced poor preservation and fragmentation of conodont elements. Most conodont taxa are endemic and show close affinities with those of the Lower Devonian of Spain, mainly Celtiberia and the Guadarrama. One of the rare, widespread conodont species, is Masaraella pandora. A new morphotype, angustoides angustoides beta, is described. The vertical range of conodont taxa and the biostratigraphic interpretation of the four faunal intervals strenghten the position for the Lochkovian-Pragian boundary as previously proposed, and allows complementary correlations between the studied sections.

Conodonts and radiolarians through the Cisuralian–Guadalupian boundary from the Pingxiang and Dachongling sections, Guangxi region, South China

Serrated gondolellid conodonts (Jinogondolella) are used to define the Cisuralian–Guadalupian (C–G) or Kungurian–Roadian (K–R) boundary in the stratotype sections of West Texas and in some sections of South China, but in other sections Jinogondolella is absent. This absence has been interpreted to be the result of strong provincialism during this interval and geographic clines have been demonstrated between an Equatorial Warm Water Province (EWWP) and a North Cool Water Province (NCWP). Results from a succession of deep-water limestones in the Pingxiang section, southwestern Guangxi, China, suggest that the distribution of warm-water and cool-water lineages may not in all cases be separated over large geographic scales, but may be segregated in some areas by local changes of water-mass temperature or depth and, in some cases, fossil assemblages from these lineages may be mixed. Two contemporaneous gondolellid evolutionary lineages are represented within warm-water and cool-water conodont assemblages from limestones spanning the Cisuralian–Guadalupian (C–G) boundary in the Pingxiang section, southwestern Guangxi, China. The warm-water conodont lineage begins with the first occurrence (FO) of Mesogondolella saraciniensis, followed by Mesogondolella siciliensis, then M. idahoensis lamberti and finally Mesogondolella pingxiangensis sp. nov. The latter new species lacks serration, but has a platform outline and lateral profile similar to J. nankingensis nankingensis found elsewhere in South China and they may be closely related. Although less common in the samples, a coeval cool-water lineage begins with M. idahoensis idahoensis and develops successively with the evolution of J. nankingensis gracilis; serration is also lacking in the latter species. The lack of serration on these forms is ascribed to some form of environmental control. The contemporaneous appearance of Mesogondolella pingxiangensis and Jinogondolella nankingensis gracilis is correlated with the C–G boundary. This correlation is supported by the co-occurrence of the zonal indices of the cool-water lineage with important zonal radiolarians in a C–G boundary section at Dachongling, southeastern Guangxi. Here the radiolarian assemblages are assigned to the Albaillella foremanae Zone, Pseudoalbaillella globosa Zone and P. fusiformis Zone in ascending order. The first appearance of J. nankingensis gracilis within this succession defines the base of the Guadalupian to be at the base of the P. globosa Zone. Conodont correlation between carbonate ramps of platform margins and radiolarian-bearing pelagic environments has determined that the first appearance of J. nankingensis gracilis precisely defines the base-Guadalupian boundary in the cool-water settings.

Basin evolution of the Anisian Peri-Tethys: implications from conodont assemblages of Lower Muschelkalk key sections (Central Europe)

Basin evolution of the Anisian Peri-Tethys: implications from conodont assemblages of Lower Muschelkalk key sections (Central Europe)

Three Jinogondolella apparatuses from a single bed of the Bell Canyon Formation in the Apache Mountains, West Texas

Complete apparatuses are described for Jinogondolella aserrata, J. errata (new species), and J. postserrata (transitional from J. aserrata) from a single bed in a section of the Bell Canyon Formation exposed in the Apache Mountains, West Texas. The conodont assemblage precisely equates this bed in age to the upper part of the Pinery Limestone Member of the Bell Canyon Formation and the base of the Capitanian in its type locality in the Guadalupe Mountains. The apparatus of Jinogondolella is different from most other gondolellid genera in that it has dimorphic to polymorphic elements in the S2 position and three pairs of platform elements.

Conodonts from the upper Ordovician siliceous rocks of Central Kazakhstan

Several conodont localities of the upper Sandbian Stage are known in siliceous deposits of Central Kazakhstan. All of them produced similar assemblages overwhelmingly dominated by Periodon grandis with insignificant admixture of Scabbardella altipes, Hamarodus europaeus, Pygodus anserinus, Protopanderodus sp., and Drepanodus sp. The main feature of this fauna is in the co-occurrence of H. europaeus and P. grandis, forms characteristic for deep-water facies at shelf or microcontinents margins of temperate and warm-water paleobiogeographic provinces. The Ordovician paleo-oceanic basin of Kazakhstan and southern Urals were parts of the uniform biogeographic area as indicated by similarity of Ordovician conodont assemblages in siliceous deposits of these regions.

Generic turnovers of Carnian/Norian conodonts: Climatic control or competition?

Pizzo Mondello section (western Sicily, Italy) yields a very rich conodont record, ranging from Late Carnian to Rhaetian in age, shedding new light into the evolution of conodonts across the Carnian/Norian boundary. Conodont biostratigraphy around the Carnian/Norian boundary is still problematic due to an inferred provincialism that affects most of the conodont species and to the occurrence of a great number of transitional forms, linked to the fast recovery of conodonts after the Middle Carnian crisis. The lower 144 m of the succession, Tuvalian to Lacian in age, were sampled in detail for a biostratigraphic and ecological study of five Upper Triassic conodont genera: Paragondolella, Carnepigondolella, Metapolygnathus, Epigondolella, and Norigondolella. After a taxonomic revision of the genera studied, meant to define the most significant morphological features for their classification, a statistical approach was applied to the study of conodont populations. Quantitative curves of the absolute abundances for each genus show potential ecological competition between Paragondolella– Carnepigondolella and later between Metapolygnathus– Epigondolella (and partially between Epigondolella– Norigondolella). Recognition of morphoclines among species, integrated by a similar ecological behaviour, supports the phylogenetic derivation of Norigondolella and Metapolygnathus from Paragondolella and that of Epigondolella from Carnepigondolella. Furthermore, cross checks of the quantitative curves evidenced the presence of three major assemblage changes: at metre 64.76 (named event T1) Carnepigondolella is replaced by its descendant Epigondolella in an evolutionary step; at metre 80 (event T2) Epigondolella is substituted by the mass occurrence of Metapolygnathus and at metre 95 (event T3) Metapolygnathus is succeeded by advanced Epigondolellae species and by Norigondolella. In looking for environmental explanations to these biological events, the conodont assemblages are compared to coeval δ 18O and δ 13C isotopic curves, based on new data from Pizzo Mondello. From the comparison, a correspondence appears between higher 13C/ 12C ratios and the interval between events T2–T3, but not with event T1. This is in accordance with the interpretation of event T1 as an evolutionary turnover. In more detail, we observe the possible influence of environmental conditions on the absolute abundances of all the studied genera: while Epigondolella proliferate when seawater δ 13C ranges between 2.1‰ and 2.5‰ , Carnepigondolella proliferate in the range between 1.6‰ and 2.1‰; Metapolygnathus instead appears to be limited to environmental conditions related to higher δ 13C values in the seawater. We explain this behaviour by interpreting the genus Metapolygnathus as an opportunist taxon, exactly as its forerunner, the genus Paragondolella.

Late Permian Sedimentation in the Sverdrup Basin, Canadian Arctic: The Lindstrom and Black Stripe Formations

Abstract Contrary to a long-held belief, Upper Permian strata are present in the Sverdrup Basin of Arctic Canada, and a nearly continuous conformable record with the overlying Blind Fiord Formation occurs in the basin central area. Upper Permian strata are dominated by spiculitic chert, the bulk of which is part of a Wuchiapingian unconformity-bounded transgressive-regressive sequence, one of four sequences that encompass the Middle Permian to Lower Triassic interval. Two new stratigraphic units are here formally erected to better account for Upper Permian strata in the Sverdrup Basin. The Lindstrom Formation is a unit of white to pale grey spiculitic chert that represents sedimentation on a shallow, open marine, wave- and storm-dominated ramp. The Black Stripe Formation, which is in part correlative with the Lindstrom Formation, comprises black to dark grey spiculitic chert associated with siliceous shale and siltstone of slope to basinal origin. The upper part of the Black Stripe Formation comprises a deepening-upward succession of black cherty siltstone and siliceous shale that recorded Changhsingian sedimentation. The basal Blind Fiord contact which coincides with the Latest Permian Extinction event in the Sverdrup Basin is marked by an unusual abundance of pyrite, a hallmark of the extinction horizon around the world. A hiatus associated with maximum flooding and condensation exist across the Black Stripe-Blind Fiord contact in distal settings. However, a seemingly rapid, yet progressive carbon isotopic shift suggests minimal stratigraphic interruption. The Late Permian age of the Lindstrom and Black Stripe formations is substantiated by conodont biostratigraphy and carbon isotope chemostratigraphy. Late Changhsingian conodonts were recovered from the base of the Blind Fiord Formation, while Hindeodus parvus, the species that marks the base of the Triassic at the GSSP in China, and associated conodont assemblages, occurs a few tens of metres above the base of the Blind Fiord Formation. Black shale in the Lindstrom and correlative Black Stripe Formation may have a source potential as shown by high TOC values and preliminary RockEval analysis.

New data on Ordovician–Silurian conodonts and stratigraphy from the Severnaya Zemlya Archipelago, Russian Arctic

AbstractThirty samples from 22 sections collected by the SWEDARCTIC international expedition to Severnaya Zemlya in 1999 contained Ordovician and Silurian conodont faunas. Several taxa, includingApsidognathuscf.milleri,Aulacognathuscf.kuehni,Nudibelodina sensitiva,Ozarkodina broenlundiandPterospathodus eopennatus, allow precise dating of the strata in this region for the first time. The occurrence ofAphelognathus pyramidalisandRhipidognathusaff.R. symmetricusin samples from the Strojnaya Formation fits well with the earlier dating of these strata as latest Ordovician. However,Aphelognathussp. in sample BG-99/14-a, collected from the upper Ushakov Formation, indicates that at least in the lower reaches of the Ushakov River the top of this formation is considerably younger than considered earlier: the sampled strata are Late, not Early Ordovician in age. In the Ordovician and Silurian the present-day Severnaya Zemlya region was dominated by extensive shallow-water, mainly semi-restricted basin environments with habitat specific faunas. The occurrence ofRiphidognathusaff.R. symmetricusat some levels in the Upper Ordovician suggests extreme shallowing episodes in the basin. On Severnaya Zemlya, ‘normal-marine’ faunas (includingPt. eopennatus) invaded the distal peripheral regions of the wide shallow-water platform at times of maximum sea-level rise only. The occurrence ofOz. broenlundiandN. sensitivaindicates that in the early Silurian the Severnaya Zemlya basin was quite well connected to the basins over modern North Greenland as well as to the Baltic Palaeobasin. The lower Silurian conodont assemblages in the Vodopad to Samojlovich formations are most similar to those described from the eastern Timan–northern Ural region.

“Sub-Triticites Beds” of the Donskaya Luka and lower boundary of the Kasimovian Stage

Micro- and macrofauna remains were studied from transitional deposits of Moscovian and Kasimovian Stages in the Donskaya Luka (Volgograd Region). The preliminary analysis of microfauna showed that “sub-Triticites Beds” of the Donskaya Luka contain fusulinid and conodont assemblages enabling correlation of the Middle and Upper Carboniferous deposits in the study region with the type sections of Moscow area and Donbass. Conodonts from the “sub-Triticites Beds” stratotype were studied for the first time. As is established, upper part of the Sukhov Fm. and the base of the Seleznev Fm. belong to the Protriticites pseudomontiparus-Obsoletes obsoletus Zone. Based on fusulinids, higher parts of the Seleznev Fm. belong to the Montiparus Zone of the Khamovnikian Substage, whereas conodonts suggest their correlation with lower part of the Khamovnikian Substage, i.e., with the Ratmirovo Fm. or a basal part of the Neverovo Fm. Middle part of the Seleznev Fm. is correlated to middle cycle of the Neverovo Fm. of the Khamovnikian Substage in Moscow area. The Middle-Upper Carboniferous boundary deposits of the Donskaya Luka are represented by deposits of extremely shallow-water settings and contain only sporadic microfauna. These sections cannot be considered as possible candidates for the GSSP of the Kasimovian Stage base.