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We present a δ13Ccarb chemostratigraphy for the Late Ordovician Hirnantian Stage based on 208 whole-rock samples from six outcrops in the Oslo–Asker district, southern Norway. Our data include the Norwegian type section for the Hirnantian Stage and Ordovician–Silurian boundary at Hovedøya Island. The most complete record of the Hirnantian Isotope Carbon Excursion (HICE) is identified in a coastal exposure at Konglungø locality where the preserved part of the anomaly spans a c. 24 m thick, mixed carbonate–siliciclastic succession belonging to the upper Husbergøya, Langåra and Langøyene formations and where δ13Ccarb peak values reach c. +6 ‰. Almost the entire HICE occurs above beds containing the Hirnantia Fauna, suggesting a latest Hirnantian age for the peak of the excursion. The temporal development of the HICE in southern Norway is associated with substantial shallowing of depositional environments. Sedimentary facies and erosional unconformities suggest four inferably fourth-order glacio-eustatically controlled sea-level lowstands with successively increased exposure and erosion to the succession. The youngest erosional unconformity is related to the development of incised valleys and resulted in cut-out of at least the falling limb of the HICE throughout most of the Oslo–Asker district. The fill of the valleys contains the falling limb of the HICE, and the postglacial transgression therefore can be assigned to the latest part of the Hirnantian Age. We address the recent findings of the chitinozoan Belonechitina gamachiana in the study area and its relationship to the first occurrence of Hirnantia Fauna in the studied sections, challenging identification of the base of the Hirnantian Stage.

The Cambrian succession of the Tingskullen drill core from northern Ãland comprises Cambrian Series 2 and Miaolingian (Wuliuan Stage) siliciclastic strata. The major portion of the succession is represented by the Miaolingian Borgholm Formation, which, in ascending order, is subdivided into the Mossberga, BÃ¥rstad and Ãleklinta members. The Ãleklinta Member is barren of body fossils, whereas the Mossberga and BÃ¥rstad members are moderately to highly fossiliferous and biostratigraphically reasonably well constrained. Trilobites and agnostoids from the BÃ¥rstad Member are indicative of the Acadoparadoxides pinus Zone. The Mossberga Member has not yielded any zonal guide fossils but is tentatively assigned to the Eccaparadoxides insularis Zone. A δ13Corg curve throughout the Borgholm Formation shows a general positive trend up-section without any distinctive excursion, suggesting that the Wuliuan Acadoparadoxides (Baltoparadoxides) oelandicus Superzone (the âOelandicus bedsâ) of Ãland is younger than the negative RedlichiidâOlenellid Extinction Carbon isotope Excursion (ROECE), which is known from near the top of Stage 4 and close to the traditional âLowerâMiddle Cambrian boundaryâ in several parts of the world.

The Lower and Middle Ordovician of the Yangtze Platform, China, is characterized by a sedimentary succession dominated by carbonate rocks. Three sections spanning the Nantsinkuan/Lunshan, Fenhsiang, Hunghuayuan, and Dawan/Zitai formations, corresponding to the Tremadocian–Dapingian in age, have been sampled for high-resolution δ13C chemostratigraphy (542 samples in total). Our new δ13C data reveal five tie-points with the potential for global correlation: (1) a positive δ13C excursion in the lower Nantsinkuan Formation within the Tremadocian Rossodus manitouensis Zone; (2) an excursion with two peaks roughly within the late Tremadocian Paltodus ‘deltifer’ Zone; (3) a positive δ13C shift in the lower Hunghuayuan Formation, within the early Floian Serratognathus diversus Zone; (4) a gradual positive δ13C shift in the late Floian, ranging from the uppermost S. diversus Zone to the basal Oepikodus evae Zone; (5) a minor negative shift in the lower Dawan/Zitai Formation, within the early Dapingian Baltoniodus triangularis Zone. These excursions are herein used for correlation of the Yangtze Platform strata with successions from South China, North China, the Argentine Precordillera, North America and Baltica. From a palaeogeographical perspective the Gudongkou, Xiangshuidong and Daling sections represent depositional environments along an inner to outer ramp profile. δ13C data from these sections show successively heavier (higher) δ13C values with increasing depositional depth. This is interpreted as due to remineralization of organic carbon within the carbonate rocks. Supplementary material: Carbon and oxygen isotope data are available at: https://doi.org/10.6084/m9.figshare.c.4767080

Introduction Early Ordovician (Floian) lithistid sponge-Calathium-microbial reefs were discovered in the lower part of the Lower Setul Limestone in Perlis, northwestern Malaysia Peninsula. Results With a thickness of more than 30 m, massive microbial boundstone is surrounded by thick-bedded oncoid-intraclast rudstone/grainstone. The reefs were constructed of cylindrical stromatolites and maze-like thrombolites, accompanying a small proportion of lithistid sponges (anthaspidellids) and hypercalcified sponges (Calathium). The earliest Floian age is based on a conodont fauna including the zonal index species Serratognathus bilobatus. Conclusions The microbialites might have grown in shallow subtidal environments, indicated by the high aspect ratio of the stromatolites and the associated coarse-grained sediments. The initial growth of stromatolites was sufficient to inhibit hydrodynamic removal of sediment and to create microhabitats in which environmental conditions were favourable for later accretion of thrombolites and colonization by anthaspidellids and calathids. These sponge-bearing microbialites represent the initial rise of metazoans in reefs at the dawn of the Ordovician Radiation, providing crucial information for understanding the transition from microbial- to metazoan-dominant reefs during this unique interval.

The Swedish island of Gotland is located within the Baltic Basin. During the Late Ordovician the region around Gotland was part of a shallow epicratonic basin in the southern subtropics. In these w ...

La Formación San Juan es la unidad terminal de una secuencia que comprende rocas carbonáticas depositadas desde el Cámbrico al Ordovícico temprano, predominantemente en ambiente de plataforma somera. Se estudiaron los conodontes del perfil de Niquivil, que abarcan asociaciones desde la Zona de P. elegans hasta posiblemente la Subzona de Eoplacognathus? variabilis/Microzarkodina jlabellum, la Subzona inferior de la Zona de A. variabilis de las sucesiones baltoescandinavas, extendiéndose desde el Arenigiano Inferior hasta el Arenigiano Superior. El material estudiado fue correlacionado con las zonas baltoescandinavas, las zonas de asociación de la Cuenca Canning (Australia), de la provincia del Continente Medio (Norte América), del Sur de China y con las faunas de Ny Friesland/Spitzbergen.

COSC investigations and drilling activities are focused in the Åre-Mörsil area (Sweden) of central Scandinavia. COSC-2 was drilled with nearly 100% core recovery in 2020 to 2.276 km depth with drilling ongoing from mid-April to early August. Drilling targets for COSC-2 included (1) the highly conductive Alum shale, (2) the Caledonian décollement, the major detachment that separates the Caledonian allochthons from the autochthonous basement of the Fennoscandian Shield, and (3) the strong seismic reflectors in the Precambrian basement.Combined seismic, magnetotelluric (MT) and magnetic data were used to site the COSC-2 borehole about 20 km east-southeast of COSC-1. Based on these data it was predicted that the uppermost, tectonic occurrence of Cambrian Alum shale would be penetrated at about 800 m, the main décollement in Alum shale at its stratigraphic level at about 1200 m and the uppermost high amplitude basement reflector at about 1600 m. Paleozoic turbidites and greywackes were expected to be drilled down to 800 m depth. Below this depth, Ordovician limestone and shale with imbricates of Alum shale were interpreted to be present. Directly below the main décollement, magnetite rich Precambrian basement was expected to be encountered with a composition similar to that of magnetic granitic rocks found east of the Caledonian Front. The actual depths of the main contacts turned out to agree very well with the predictions based on the geophysical data. However, the geology below the uppermost occurrence of Alum shale is quite different from the expected model. Alum shale was only clearly encountered as a highly deformed, about 30 m thick unit, starting at about 790 m. Between about 820 and 1200 m, preliminary interpretations are that the rocks mainly consist of Neo-Proterozoic to Early Cambrian tuffs. Further below, Precambrian porphyries are present. The high amplitude reflections within the Precambrian sequence appear to be generated by dolerite sheets with the uppermost top penetrated at about 1600 m. Several deformed sheets of dolerite may be present down to about 1930 m. Below this depth the rocks are again porphyries.A preliminary conclusion concerning the tectonic model is that the main décollement is at about 800 m and not at 1200 m. Also the thickness of the lowermost Cambrian/uppermost Neoproterozoic sediments on top of the basement is much greater than expected (hundreds of meters instead of tens of meters) and likely to have been thickened tectonically. Detailed studies are required to assess the actual importance of the “main décollement” and the degree, type and age of deformation in its footwall. We can also conclude that the Precambrian basement is very similar to the Dala porphyries succession that are typically present farther south.An extensive set of downhole logging data was acquired directly after drilling. Borehole seismic measurements in 2021 will help to define and correlate seismic boundaries with lithology and structures in the core. Unfortunately, work for describing the geology of the drill core in detail is still on hold due to Covid-19.

The Komstad Limestone forms a distal part of the widespread ‘orthoceratite limestone’ of the Baltoscandian continent. In this paper, we present an integrated conodont biostratigraphy and carbon isotope stratigraphy for a major part of this formation and address its significance for evaluating Middle Ordovician correlation and overall stratigraphy. Four conodont zones are distinguished, including the Lenodus antivariabilis Zone, Lenodus variabilis Zone, the Yangtzeplacognathus crassus Zone, and the Eoplacognathus pseudoplanus Zone. Carbon isotopes have previously not been published from the Komstad Limestone. The data herein show that carbonate strata may retain a comprehensive pristine δ13C signal despite relatively strong thermal influence during diagenesis (Conodont Color Alteration Index 4–5). (Less)

The present study reviews the main geochemical, sedimentological, and paleontological events associated with one of the largest perturbations in the Phanerozoic carbon cycle, the Mid-Ludfordian Carbon Isotope Excursion (MLCIE), which is globally documented in late Silurian marine successions. Climate changes associated with the MLCIE are not yet well documented in contrast to most other Silurian CIEs. We report seawater surface temperature changes across the entire MLCIE using δ18Oapatite records of conodonts originating from different regions located in temperate as well as tropical paleolatitudes. Significant positive δ18Oapatite excursions (up to 3.8‰) measured in temperate-water areas of the Prague Basin and Carnic Alps (peri-Gondwana) as well as in the tropical areas of Baltica (Laurussia) and Australia (Gondwana) provide evidence for significant global cooling during the MLCIE. The marked cooling of sea surface temperatures, coupled with a significant eustatic sea-level fall recorded on all corresponding paleocontinents, point to a major glaciation in polar and subpolar Gondwana which is denominated here as the “Mid-Ludfordian Glaciation”. The onset of rapid cooling in the Prague Basin (peri-Gondwana) is preceded by the deposition of shales enriched in redox-sensitive trace metals suggesting that anoxic/euxinic waters expanded on the deeper carbonate shelf just before the MLCIE. The coincidence of culmination of the anoxic event (named here as the Siluricus Ocean Anoxic Event) with the Lau/Kozlowskii extinction events is documented for the first time outside of the Laurussian epeiric basins. Widespread oxygen-deficient conditions are interpreted to have been stressful to shelfal ecosystems and may have been a major cause of the extinctions. The mid-Ludfordian glacio-eustatic sea-level fall caused a reduction of shelf habitats culminating in a lower faunal diversity and a change in the structure of faunal communities. The observed positive δ18Oapatite excursion is tightly coupled to δ13C and δ34S anomalies documented from temperate (peri-Gondwana) as well as tropical (Laurussia) paleolatitudes. The δ18Oapatite data complete the Silurian seawater temperature curve and show evidence that one of the largest positive carbon isotope anomalies (MLCIE) of the Phanerozoic was preceded by a widespread anoxic event and tightly linked with a climate cooling event as documented as well for most other Silurian CIEs.

Results of conodont-based δ18Ophos studies from two core sections in Estonia (Mehikoorma-421 and Valga-10) indicate that the pre-Hirnantian Late Ordovician was characterized by a general cooling trend: a rapid climate cooling in the early Sandbian and a less intense cooling in the late Sandbian through Katian. During this time interval, sea-surface temperatures probably decreased by up to 9o C. This fits with earlier ideas of a colder Late Ordovician climate before the Hirnantian. On the background of general cooling trend in the pre-Hirnantian Late Ordovician, tendencies in the temperature changes were quite variable. Several episodes of SST decrease and increase of different magnitudes and durations are evident from the δ18Ophos curves in the studied sections. In the interval sampled in detail, 7 cooling events are recognized and tentatively called (in stratigraphical order): Early Kukruse, Haljala, Keila, Early Nabala, Vormsi, Early Pirgu and Middle Pirgu CE-s. Duration of these events and amplitudes of changes in seawater temperatures increased from Sandbian to late Katian. All cooling events may have triggered the waxing of polar and subpolar Gondwana icecaps and are reflected in the sedimentary succession by major sea-level lowstands recognized also outside the study region. Results of our research do not support a simple, straightforward correlation between temperature trends inferred from δ18Ophos measurements and changes in the δ13C record. Comparison of δ18Ophos and δ13C curves reveal that these two variables behave quite independently: in some intervals their trends fit quite well, in others they might show completely the opposite. For this comparison we provide also a new, detailed δ13C curve from the Valga-10 core section.

The Ludfordian (late Silurian) records one of the largest perturbations in the Phanerozoic marine carbon cycling, the mid-Ludfordian positive δ13Ccarb excursion (MLCIEcarb), which occurred in the aftermath of the Lau/Kozlowskii Bioevent (LKB). There are still controversies regarding origin of the MLCIE and its possible causal link(s) with respect to the preceding LKB. Surprisingly, the isotope fractionation of carbon between inorganic and organic carbon pools during the MLCIE was not investigated in detail. To constrain better this parameter of marine C cycle, its linkage to the LKB, and local sulfur cycle, we analysed high-resolution δ13Ccarb, δ13Corg, and δ34Spy records in the Prague Basin (peri-Gondwana) representing one of the most complete records of the MLCIEcarb in the world. Evaluation of these high-resolution δ13Corg and δ13Ccarb records reveals their tight coupling, which in turn indicates the photosynthetic origin of the MLCIEorg via links to primary production and changing rates of organic carbon burial. On the other hand, our data also reveal a short-term decoupling of the δ13Corg and δ13Ccarb records preceding the MLCIE which may have been related to a purported increase in atmospheric pCO2 levels or anoxic event. The peak of the Δ13C anomaly exactly coincides with the LKB and, therefore, for the first time suggests a tight link between the change in marine carbon cycling and the bioevent. Subsequent rapid drop of the Δ13C values (and by inference also atmospheric pCO2) fits well with a rapid cooling and globally documented regression reported for this time interval. Finally, the documented strong coupling of δ13Ccarb and δ34Spy records during the MLCIE points to an intensive microbial sulfate reduction which should increase carbonate alkalinity input, thus contributing to formation of authigenic carbonates and an increase in δ13C values of DIC (and anticipated seawater carbonate saturation). The coupling of δ13Corg and δ34Spy values suggests a global enhancement of organic carbon and pyrite burial likely driven by an expansion of euxinic (anoxic and sulfidic water column) conditions in the oceans. Present study of C and S isotope variations during the MLCIE reveals for the first time a tight link among carbon and sulfur cycling, global cooling, and reorganization of faunal communities and fits well with purported seawater carbonate hypersaturation at the onset and during MLCIE.

The positive δ13C excursion associated with the Late Silurian Lau Event is recognized globally and is the most prominent carbon isotope excursion of the Phanerozoic. We present new δ13C data (195 samples) and the first continuous carbon isotope stratigraphy across the stratigraphic range of this bioevent based on two drill cores from Gotland, the informal type area for the Lau Event. The positive δ13C anomaly reaches a peak value of 9.01‰ in an interval correlated to the Icriodontid conodont Zone in the upper Eke Formation, an interval with generally impoverished fauna and strongly dominated by one single conodont species, Panderodus equicostatus, and a mass-occurrence of oncoids. The distinct morphology of this Late Ludfordian Carbon Isotope Excursion, including a well-defined rising limb, a plateau and a falling limb can be used for intercontinental correlation of the strata.

The widespread growth of reefs formed by a framework of biogenic constructors and frame-lacking carbonate mounds began on Baltica during Ordovician time. Previously, Ordovician reef and mound development on Baltica was considered to be sporadic and local. A review of all known bioherm localities across the Baltic Basin reveals a more consistent pattern. Ordovician bioherms grew in a wide E–W-aligned belt across the Baltic Basin and occur in several places in Norway. Substantial reef development began simultaneously across the region during the late Sandbian – early Katian interval and climaxed during the late Katian Pirgu age. The current spatiotemporal distribution of bioherms is a result of interdependent factors that involve original drivers of reef development such as relative sea level, climate during the time of deposition and effects of post-depositional erosion. Oceanographic conditions were likely more favourable during times of cooler global climates, low sea level and glacial episodes. At the same time, the likelihood that bioherms are preserved from long-term erosion is higher when deposited during low sea level in deeper parts of the basin. A main factor controlling the timing of the reef and mound evolution was Baltica's shift toward palaeotropical latitudes during Late Ordovician time. The time equivalence between initial reef growth and the Guttenberg isotope carbon excursion (GICE) suggests that global climatic conditions were important.

Late Ordovician mud-mounds in the Boda Limestone, Siljan area, central Sweden, consist mainly of massive spiculitic stromatactis limestone similar to other well-known Paleozoic mud-mounds. The mound cores are predominantly formed by spiculite and ostracod-rich mudstone and/or wackestone, and networks of spar-filled syndepositional cavities. The mounds of the Boda Limestone are specific in two ways: (1) large parts of the lower mound cores are formed by a Palaeoporella framestone and/or bafflestone with signs of early recrystallization and cementation; and (2) the stromatactis facies is accompanied by masses of limestone with non-stromatactis cryptocrystalline patches. Cryptocrystalline patches are filled with massive, fibrous low-Mg calcite, have cloudy, transitional margins, contain randomly spaced skeletal grains and dolomitic micro-inclusions, and are interpreted as recrystallized patches of the original host lime mud. Models of formation of Paleozoic mud-mounds generally hypothesize accretionary processes that are based on sponge biomass degradation and associated diagenetic processes. In the Boda Limestone Palaeoporella skeleton disintegration, early recrystallization, and additionally, cementation were essential for the mound formation. The Boda mounds exemplify that formation of Paleozoic mud-mounds reflects a spectrum of different essentially diagenetic accretionary mechanisms. This diversity should be taken into account in future discussions about Paleozoic mud-mound formation. The new name Osmundsberget Formation is given for a Hirnantian succession of brachiopod-rich limestone above the Boda Limestone. The Osmundsberget Formation occurs only in direct geographical vicinity to the mud-mounds.

One of the few and most complete records of the MDICE (Middle Darriwilian Isotope Carbon Excursion) is herein documented from Baltoscandia. Based on a core section penetrating the condensed Lower–Middle Ordovician succession (~46 m) on the island of Öland, southeastern Sweden, we provide an integrated scheme for carbon isotope chemostratigraphy (313 samples) and conodont biostratigraphy (29 samples) for this period. The carbonate succession in the Tingskullen core records 12 conodont zones and 6 subzones, including theOepikodus evae, Trapezognathus diprion, Baltoniodus triangularis, B. navis, B. norrlandicus, Lenodus antivariabilis, L. variabilis, Yangtzeplacognathus crassus, Eoplacognathus pseudoplanus(Microzarkodina hagetianaandMicrozarkodina ozarkodellasubzones),E. suecicus, Pygodus serra(E. foliaceus, E. reclinatus, E. robustusandE. lindstroemisubzones) andPygodus anserinuszones in ascending order. The δ13Ccarbrecord reveals an apparently complete record of the MDICE, including a rising limb, a well-defined peak and a falling limb. The anomaly covers a thickness ofc. 27 m in the core and spans theEoplacognathus pseudoplanus, E. suecicus, Pygodus serraandP. anserinusconodont zones. Combined with the new, detailed conodont biostratigraphy, the MDICE in the Tingskullen core can be used for detailed correlation with successions from Baltica, North America, the Argentine Precordillera, South China and North China.

Carbonate mud mounds developed within the Baltoscandian Basin, an epicratonic basin on the Baltica palaeocontinent, during the Ordovician. In the Upper Ordovician succession of the Baltoscandian Basin, a large number of mud mounds are present at three stratigraphic levels namely, the Kullsbergs mounds (late Sandbian-early Katian), Nabala and Rakvere mounds (middle Katian), and the Boda mounds (late Katian). These formed in a subtropical-tropical carbonate platform environment, covered by a shallow epicontinental sea. The mud mounds at these stratigraphic levels beneath and around Gotland have been characterized using a comprehensive seismic and well dataset acquired during a period of hydrocarbon exploration and exploitation which began more than 30 years ago. Interpretation of the largely unpublished seismic data in this study provides details on the distribution of mound complexes in the basin and constraints on the geometry of the mounds. Detailed structure contour maps of the top and the base of the Ordovician succession beneath Gotland based on the seismic interpretation are presented. The results give a comprehensive characterization of carbonate mud mound generation on Gotland which may help in understanding the distribution patterns of similar mound complexes in other parts of the Ordovician world formed in similar environments.

The Fezouata Lagerstätte, discovered in the Lower Ordovician rocks of Morocco, is a Konservat‐Lagerstätte of prime scientific importance. It provides access not only to the ‘shelly’ (skeletonized) part of its fossil assemblages, but also to non‐biomineralized to lightly sclerotized organisms and to exceptionally preserved soft tissues of a complex ecosystem, mixing elements of both the ‘Cambrian Explosion’ and the ‘Great Ordovician Biodiversification Event’ (GOBE). The Fezouata Lagerstätte occurs at different intervals in the Fezouata Shale, a formation ranging from the lower Tremadocian to the upper Floian (Lower Ordovician). In spite of recent major advances in the detailed biostratigraphy of the Fezouata Shale, there is currently no consensus on the precise age of the fossiliferous levels yielding exceptionally preserved assemblages. Consequently, all available biostratigraphical evidence based on several fossil groups is here critically reviewed and discussed. It can be concluded that exceptional preservation is restricted to a few thin, discontinuous, lens‐shaped horizons occurring in two distinct parts of the Fezouata Shale: a lower interval (260–330 m above the base of the formation) and an upper interval (570–620 m). Integrated biostratigraphical studies, essentially based on investigations of graptolites, acritarchs and conodonts, indicate that the lower interval can be correlated with the upper Tremadocian, whereas the upper interval corresponds to the middle Floian., This work is a contribution to IGCP project 653 ‘the Onset of the Great Ordovician Biodiversification Event (GOBE)’, and an outcome of the RALI (Rise of Animal LIfe) project, funded by the ANR (Agence Nationale de la Recherche, France; grant number ANR‐11‐BS56‐0025). This study is also part of the TelluS‐SYSTER project ‘Vers de nouvelles découvertes de gisements à préservation exceptionnelle dans l'Ordovicien du Maroc’, funded by the INSU (Institut National des Sciences de l'Univers, France), CNRS. JCGM acknowledges the support of the Spanish Project CGL2012‐39471/BTE. This study is also a contribution to the CPER research project CLIMIBIO, and HN and TS acknowledge the French Ministère de l'Enseignement Supérieur et de la Recherche, the Hauts de France Region and the European Funds for Regional Economical Development for their financial support to this project. OL contributed to the study during a stay at the Czech University of Life Sciences in Prague supported by the Alexander von Humboldt Foundation (Germany) and during his guest professorship at NIGPAS (China).

The Siljan impact structure is the largest known impact structure in Europe, the result of a Late Devonian meteorite impact (380.9 +/- 4.6 Ma). It is outlined mainly by a ring of lakes and Palaeozoic sedimentary rocks of Ordovician to Devonian age. The Palaeozoic successions are generally poorly exposed, but often well preserved with clear stratigraphy. At some locations they are strongly tectonised with sharply inclined or nearly overturned packages of crystalline basement and/or sediments. Down-hole logging data were acquired in the western part of the Siljan impact structure to determine some of the physical properties of the Palaeozoic sedimentary rocks in the area. Boreholes Mora 001 (356 m logged depth), Vattumyra Production (420 m logged depth), Mora VM 2 (94 m logged depth) and Mobillyft (437 m logged depth) were logged for temperature, sonic velocity and electrical resistivity. Logging data were compared to the lithology in the Mora 001 core, which had been mapped in detail, and information from other cores in the area. Good agreement between the natural gamma log and the core lithology was found. The sonic log shows a marked difference in velocity for the more clastic Silurian succession compared to the Ordovician succession and the Precambrian basement. A synthetic seismogram shows that a high amplitude reflection is expected at the Silurian-Ordovician boundary, raising some questions concerning interpretation of a seismic profile located about 6-7 km north of the study area. Correlation of the borehole logs shows that the thickness of the Silurian succession varies rapidly in the area and that its composition differs over distances of less than 1 km. These rapid variations suggest that the study area may be located in a megablock zone that was highly influenced by the impact Caledonian tectonics and changing depositional environments may also play a role in explaining the present-day borehole lithologies. Even though the boreholes are relatively far from the seismic profile and the geology is complex, the new data confirm that the Silurian has significant thickness along parts of the seismic profile. Potentially, the Silurian can be up to 450 m thick on parts of the profile. Further geophysical investigations in the area, including seismic surveying and gravity measurements, may help in mapping the complex structures away from the boreholes and discriminating between possible geological models. (C) 2015 Elsevier B.V. All rights reserved. (Less)

The limestones of the upper Katian Boda mud mounds (Ordovician) of the Siljan district in central Sweden are deeply fractured. The fissures were partly synsedimentary and are often lined with stromatolite-like crusts. These crusts thus far are the only known subaerial Ordovician speleothems. They reach depths of up to 30 m below the former mound top. Macroscopically the crusts form decimetre-sized, cone-shaped domal aggregates, stalactites and stalagmites. Microfabric and morphology identify them as microbially mediated speleothems in a dark environment. Combined Sr and C isotope values indicate a formation of the speleothems from meteoric waters without influence of a significant soil horizon. For the first time the age of the speleothems can be precisely constrained by δ 13 C whole-rock and brachiopod shell isotope data to the mid-Hirnantian. Repeated and/or prolonged subaerial exposure of the Boda mud mounds during the Hirnantian is evident from karst surfaces and early cements in the mound capping carbonates. The speleothems and the karst surfaces record an estimated sea-level fall in the range of 80–130 m within the time window of the Hirnantian Isotopic Carbon Excursion. This massive regression coincides with maximum ice sheet extent inferred from sections in West Gondwana. Supplementary material: 87 Sr/ 86 Sr isotope ratio of selected brachiopod shells and results of Energy-dispersive-X-ray spectroscopy are available from http://www.geolsoc.org.uk/SUP18809

The province of Jamtland, central Sweden, records a well-preserved but very thin suite of the Lower-Middle Ordovician cool-water carbonates. In this study we present the first carbon isotope profile for these strata on the basis of whole-rock samples from the Brunflo 2 core. The studied succession is 41.55 m thick and includes, in ascending order, the Bjorkasholmen Formation, Latorp Limestone, Toyen Shale, and the Lanna, Holen, and Segerstad limestones. We identify two distinct positive delta C-13 excursions that can be used for intra-as well as inter-continental correlations. A positive excursion in the Toyen Shale probably correlates to the mid-late Floian and the Oepikodus evae Zone in Baltica and Precordillera, and the Reutterodus andinus Zone in North America. A continuous increase in delta C-13 values through the upper Holen Limestone and the preserved part of the overlying Segerstad Limestone is interpreted as the rising limb of the Middle Darriwilian Isotope Carbon Excursion (MDICE), an important tie-point for the global correlation of the Jamtland strata. Negative delta C-13 values associated with the Latorp Limestone may correlate with similar low values in the late Tremadocian and early Floian of the Argentine Precordillera and the Shingle Pass and Ibex sections in North America. (C) 2015 Elsevier B.V. and Nanjing Institute of Geology and Palaeontology, CAS. All rights reserved. (Less)

The Late Devonian Siljan Ring structure in Sweden is the largest known impact structure in Europe. The present-day structure comprises a central dome that is about 20–30 km in diameter, which is surrounded by a ring-shaped depression. In this study, we focus on the southwestern part of the Siljan Ring with the aim to map the structure of the Paleozoic sedimentary rocks. Four 2D high-resolution seismic lines with a total length of about 3 km were acquired in the Mora area. A three component eighty-unit land streamer, combined with wireless recorders, was used for data acquisition along with a weight drop source. Processing of the data shows that clear reflections are present, but results are less distinct where external noise was present during acquisition or the maximum source-receiver offset was too short. Petrophysical measurements on core samples, core log data and a density model along one line were used to guide the interpretation of the seismic sections. These data demonstrate that fault blocks are present in the study area and that the individual blocks have been affected differently by impact-related tectonics.

The Vasalemma Formation (early Katian, Late Ordovician) of northern Estonia consists of a succession of biodetrital grainstones up to 15 m thick with numerous intercalated reef bodies, which reach diameters of more than 50 m. Four dominant facies types are distinguished within the reef core limestones: (1) a bryozoan framestone—bindstone, (2) an echinoderm bindstone, (3) a receptaculitid-bryozoan-microbial framestone, and (4) a tabulate bafflestone. A linking theme between the different reef-core limestones is the presence of clotted microbial bindstone, which in some places contains spicules. Except for the tabulate bafflestone, all facies types occur in the youngest and oldest intervals of reef growth. Generally, a tendency can be observed with a dominance of echinoderm framestone low in the formation and at the base of individual reefs, towards a more receptaculitid dominated facies at the top of the formation. The reefs developed in a narrow, ca. 20-km-long and max. 5-km-wide band on a shallow NE–SW-directed platform in the central part of the North Estonian Confacies Belt. Reef growth can be constrained toward the latest Keila age, representing the rising limb and the peak interval of the Guttenberg Isotopic Carbon Excursion (GICE). Reef termination falls within a second-order sea-level lowstand, the Frognerkilen Lowstand Event, which led to partial subaerial exposure of the reefs. The dead reefs subsequently and rapidly drowned during the Nakkholm Drowning Event at the Oandu/Rakvere Stage. This timing is nearly equivalent to a phase of enhanced reef development elsewhere in Baltica and probably is related to locally increased nutrient availability during the GICE interval.

Organic geochemical assessment of seep oil from the Upper Ordovician (Hirnantian) Boda Limestone and solid bitumens from the Middle Ordovician (Darriwilian) limestone formations in the Siljan Crater, Sweden indicates their generation primarily from peak oil window maturity, clay-rich marine source rocks that contained aquatic algal organic matter and were deposited in an anoxic environment. Biomarkers indicate some significant differences between the seep oil and solid bitumens and their alteration by moderate to severe biodegradation. The Solberga quarry seep oil from the Boda Limestone and Solberga-1 solid bitumen from the Skarlov-Seby-Folkslunda Limestone equivalents do not show signs of mixing and the similarity in their biomarker distributions indicates a genetic relationship. However, solid bitumens recovered from cavities in the underlying Segerstad and Holen limestones were derived predominantly from clay-rich source rocks that contained mixed marine and lacustrine algal organic matter and minor inputs of prokaryotic organic matter, possibly from thin beds of calcareous rocks interbedded within the main clay-rich source rocks. It is inferred that the Siljan Crater seep oil and solid bitumens were generated from the organic rich Upper Ordovician Fjacka Shale, a source rock for economic accumulation of oil in the eastern Baltic region, and/or from Silurian shale units. The presence of various compounds of known biological origin and existence of organic rich Fjacka Shale/Silurian Shale units are consistent with the biogenic origin of Siljan Crater petroleum. Although these seep oil and solid bitumens were generated at peak oil window maturities, the intense heat generated by meteorite impact caused secondary reactions resulting in unusual distributions of maturity sensitive aromatic hydrocarbon isomers and conflicting molecular maturities. Crown Copyright (C) 2014 Published by Elsevier Ltd. All rights reserved. (Less)

Based on the Tingskullen drillcore, we present the first continuous carbon isotope stratigraphy from the Lower–Middle Ordovician “orthoceratite limestone” of Oland, Sweden. The extremely condensed Tremadocian and Floian stages include large gaps as well as the Ceratopyge Regressive Event and the widespread Evae transgression accompanied by prominent shifts in the δ 13C record. The Dapingian and Darriwilian stages are characterised by low sedimentation rates and a relatively complete sedimentary record. A total of 99 whole-rock samples were analysed for carbon isotope geochemistry from the Ordovician part of the succession (46 m thick). The most striking anomaly detected is the middle Darriwilian isotope carbon excursion (MDICE) that appears unusually well developed and complete for the region, and thus forms an important proxy for intercontinental correlation of the succession.

The Late Ordovician is a period of major global environmental changes. Its understanding requires an integrated stratigraphical approach. Here, chitinozoan assemblages are described from the Borenshult-1 drillcore from Motala (Sweden) and related to stable carbon isotope event stratigraphy. Fifty-six rock samples, corresponding to a stratigraphic interval ranging from the upper Sandbian to Llandovery, were dissolved and analysed for chitinozoans. This stratigraphic interval covers the Sandbian to Llandovery and includes, in ascending order, the Freberga, Slandrom, Fjacka, Jonstorp, Loka and Motala formations. This study resulted in identification of two biozones the Spinachitina cervicomis Biozone (lower part of the Freberga Formation), as well as its Angochitina multiplex Subzone, and the Fungochitina spinifera Biozone (upper part of the Freberga Formation). The spinifera Biozone is characterized by the presence of Lagenochitina baltica, whereas the cervicomis Biozone is defined by the presence of its index species. Despite a low recovery of chitinozoans from Borenshult-1, we suggest that the Guttenberg Isotopic Carbon Excursion (GICE), as observed in the Borenshult-1 drillcore, correlates with the S. cervicomis Biozone. This fits well with chitinozoan and carbon isotope data from the Fjacka section in central Sweden (Dalarna, Siljan region) and from the Smedsby Gard drillcore (Ostergotland). The A. multiplex Subzone which is recognised in the Borenshult-1 and nearby Smedsby Gard drillcores, predates the GICE at both localities. (C) 2013 Elsevier B.V. All rights reserved. (Less)

The Stumsnas 1 core, drilled in 2011 in the southern part of the Siljan Ring, provides new insights into the stratigraphy of the Ordovician succession in central Sweden. The core section shows evidence of the structural complications caused by the late Devonian meteorite impact in the area. In the core, about 90m of Tremadocian to Darriwilian strata are sandwiched between Proterozoic igneous basement rocks. At the lower contact, the sedimentary succession starts with glauconitic, fine-grained siliciclastics and carbonates directly resting on the weathered basement. The basal siliciclastic unit is overlain by a ca. 19-m-thick limestone succession, well-known from this part of the Baltoscandian Basin, includes strata corresponding to the Latorp Limestone through the upper Holen Formation. This succession is covered by a limestone-marl alternating sequence, representing some yet unknown deeper water facies, presumably coeval to the topmost Holen Formation through upper Dalby Limestone. The upper third of the sedimentary succession in the core includes numerous slices of partly overturned strata, including relatively thick successions of the Slandrom Formation, Fjacka Shale and Jonstorp Formation, as well as carbonate and siliciclastic units of yet unknown stratigraphic origin.

The Stumsnas 1 core, drilled in 2011 in the southern part of the Siljan Ring, represents a structurally complex section and provides new insights into the impact-induced local structure in central Sweden. The Siljan Ring was formed around the central uplift of a Late Devonian meteorite crater, the largest known impact structure in Europe. The Stumsnas 1 core section reveals that about 90 m of Palaeozoic (Lower to Upper Ordovician) sedimentary rocks are sandwiched between Proterozoic igneous basement rocks. The sedimentary contact to the underlying ∼260 m of Proterozoic basement is a prominent unconformity. The contact to the overlying ∼190-m-thick slab of Proterozoic basement is a few metres wide complex fault zone, comprising alternating thin slices of sedimentary and granitic rocks together with fault breccia and gouge. The tectonic emplacement of basement rocks over the Palaeozoic sedimentary succession is apparently impact-related and caused folding and faulting of the underlying sediments, some of wh...

The remotely located Cambro-Ordovician strata in Ny Friesland, NE Spitsbergen, Svalbard, are referred to the Oslobreen Group, which is composed of the Tokammane, Kirtonryggen and Valhallfonna formations. Herein we record the conodont succession of the Kirtonryggen and Valhallfonna formations. Collections of well-preserved conodonts (CAI 1) have been recovered from the Lower to lower Middle Ordovician interval in the region. Eight regional conodont zones are established in this preliminary study of the Oslobreen Group exposed at Hinlopenstretet, NE Svalbard. In ascending order these zones are: Rossodus manitouensis, Macerodus dianae, Oneotodus costatus, Oepikodus communis, O. evae, O. intermedius, Periodon aff. flabellum-Periodon hankensis and Periodon macrodentatus. The yield is relatively poor for the Kirtonryggen Formation, but the oldest Ordovician conodont faunas span the Tremadocian Rossodus manitouesis through Macerodus dianae zones followed by the Floian Oepikodus communis Zone. The taxa are typical of the tropical North American Midcontinent faunal province that dominated the region until open marine associations invaded the shallow shelf (i.e. Evae transgression). The pandemic Oepikodus evae becomes frequent and is succeeded by O. intermedius in abundance in the lower Valhallfonna Formation (Olenidsletta Member). The upper strata are characterised by the Periodon-Paroistodus assemblage characteristic for outer shelf and slope environments around Laurentia. In total, this formation comprises the Floian and Dapingian stages and ends in the early Darriwilian Periodon macrodentatus Zone in this region. The Ny Friesland faunas show affinities to coeval assemblages from Northeast Greenland and West Newfoundland. (Less)

Two new reflection seismic profiles over the Paleozoic successions of the western part of the Siljan Ring impact structure show a contrasting seismic signature. The more southerly c. 10 km long Mora profile reveals a highly disturbed structure, with only a few kilometers of relatively horizontally layered structures observed. However, interpretations of refracted arrivals in the data, that can be correlated to reflections, indicate the Silurian clastic rocks to be about 200 m thick in the central part of the profile. Weak reflections from about 600 m depth suggest a 400 m thick Ordovician limestone sequence to be present. Cores from the area show a mainly shale lithology for the Silurian and only a thin sequence of Ordovician strata, suggesting a rapid thickening of the Ordovician towards the north. On the more northern c. 12 km Orsa profile clear reflections from the Paleozoic successions are seen along the entire profile, except on the southernmost few kilometers. Based on interpretations of refracted arrivals, the Silurian succession appears to be considerably thinner here, and possibly absent at some locations. The Ordovician is also interpreted to be thinner in this area, with a maximum thickness of about 200–300 m along most of the profile. A deeper reflection from about 2 km within the crystalline basement may represent a dolerite sill. The lack of clear basement reflections on the Mora profile can be attributed to near-surface conditions and the acquisition geometry. The seismic data and recent coring in the area suggest the presence of a deeper paleo-basin towards the southwest with significantly more shales being deposited and the Paleozoic successions being severely disturbed. The shallow coring and seismic data will help form the basis for locating future boreholes for deeper drilling to study impact processes and the Paleozoic evolution of central Sweden. peerReviewed

This study documents conodont faunas of the Fezouata Formation, recovered from the AZ-1 borehole at Adrar Zouggar Mountain and from outcrops near Zagora in south-eastern Morocco. The Fezouata Formation was deposited on the peri-Gondwanan shelf near the South Pole during Early Ordovician times. It is composed of mostly fine-grained siliciclastics and is well known for including beds displaying the exceptionally well preserved Fezouata Biota. Studies on different microfossil groups, including conodonts, increase the diversity recorded from this exceptional ecosystem. Strongly recrystallised conodont elements were extracted from the fine-grained siliciclastic sediments by diluted hydrofluoric acid (HF), which was used for isolating palynomorphs. The material is dominated by simple cone taxa such as Parapaltodus, Semiacontiodus, Scolopodus, Scalpellodus, Drepanoistodus, Acodus, Paltodus, and Cornuodus. In addition, a few elements of early Prioniodus are recovered. This composition allows an estimation of uppermost Tremadocian through basal Floian ages for the fragmented elements because characteristic elements of younger assemblages, including ramiform (e.g., Oepikodus, Baltoniodus) or coniform (e.g., Tropodus, Protopanderodus) apparatuses, have not been recorded. The association is devoid of any warm/tropical and temperate water taxa and is typical for faunas in the cold water environments of the subpolar siliciclastic shelves during Early Ordovician times. The new term of ‘subpolar faunal domain’ is proposed for these conodont associations of low diversity that occur at high southern latitudes during the Early Ordovician. Ecological implications and palaeobiogeographical relationships of the conodont faunas are discussed., The research was supported as part of the RALI project (Rise of Animal Life), funded by the ANR (Agence Nationale de la Recherche, France; grant number ANR-11-BS56-0025). G.N. Sarmiento and J.C. Gutiérrez-Marco were financed by the project CGL2012-39471/BTE of the Spanisch Ministry of Economy and Competitiveness.

New drill cores from the largest known impact structure in Europe, the relict of the Siljan meteorite crater, provide new possibilities to reconstruct Early Palaeozoic marine environments and ecosystems, and to document changes in sedimentary facies, sea level and palaeoclimate in Baltoscandia. The impact crater is an important target of the project "Concentric Impact Structures in the Palaeozoic" within the framework of the "Swedish Deep Drilling Program". Two core sections, Mora 001 and Solberga 1, have been analysed. The sedimentary successions of these core sections include strata of late Tremadocian through late Wenlock ages. Our preliminary studies show not only that several of the classical Palaeozoic units of Sweden are represented in the area, but also that other significantly different facies are preserved in the Siljan district. An erosional unconformity representing a substantial hiatus occurs between Middle Ordovician limestone and a Llandovery-Wenlock (Silurian) shale succession in the western part of the Siljan structure and suggests an extended period of uplift and erosion. This may be related to forebulge migration due to flexural loading by the Caledonian thrust sheet to the west. Thus, this part of Sweden, previously regarded as a stable cratonic area, presumably was affected by the Caledonian collision between Baltica and Laurentia.

A total of 239 isotope samples are used for establishing the δ13C chemostratigraphy in the upper Middle Ordovician to Lower Silurian succession in the approximately 70 m long Borenshult drillcore. The study interval starts in the upper Darriwilian Furudal Limestone and ends in the Rhuddanian Motala Formation. Four named δ13C excursions are recognized in 3–4 formations, namely the Guttenberg isotope carbon excursion (GICE) in the lower-middle Freberga Formation, the Kope (Rakvere) excursion in the uppermost Freberga Formation and possibly the Slandrom Formation, the Whitewater (Moe) excursion in the Lower Member of the Jonstorp Formation and the Hirnantian isotope carbon excursion (HICE) in the Loka Formation. The Middle Darriwilian isotope carbon excursion (MDICE) is missing in the drillcore and it is suggested that the study succession starts just above the interval of this excursion, which is in agreement with the range of MDICE in other Baltoscandic successions. The widespread Waynesville (Saunja) excu...

A recent drilling at Borenshult near Motala resulted in discovery of the stratigraphically most complete succession through the upper Darriwilian–Rhuddanian interval known in Ostergotland. The approximately 70 m long drillcore succession is subdivided into eight formations, the oldest being the late Darriwilian Furudal Limestone and the youngest being the Rhuddanian Motala Formation. Conodonts are used for a detailed biostratigraphic classification of the Borenshult drillcore into three subzones of the Pygodus serra Zone, two subzones of the Pygodus anserinus Zone, and three subzones of the Amorphognathus tvaerensis Zone. The base of the Amorphognathus superbus Zone is taken to be ∼10 m above the Kinnekulle K-bentonite, that of the Amorphognathus ordovicicus just below the Fjacka Shale, and that of the Ozarkodina hassi Zone at the base of the Middle Member of the Loka Formation. Because of its unique lithology and paleontology and its wide geographic occurrence, this member is formally named herein the Sk...

Mass wasting at continental margins on a global scale during the Middle Ordovician has recently been related to high meteorite influx. Although a high meteorite influx during the Ordovician should not be neglected, we challenge the idea that mass wasting was mainly produced by meteorite impacts over a period of almost 10 Ma. Having strong arguments against the impact-related hypothesis, we propose an alternative explanation, which is based on a re-evaluation of the mass wasting sites, considering their plate-tectonic distribution and the global sea level curve. A striking and important feature is the distribution of most of the mass wasting sites along continental margins characterised by periods of magmatism, terrane accretion and continental or back-arc rifting, respectively, related to subduction of oceanic lithosphere. Such processes are commonly connected with seismic activity causing earthquakes, which can cause downslope movement of sediment and rock. Considering all that, it seems more likely that most of this mass wasting was triggered by earthquakes related to plate-tectonic processes, which caused destabilisation of continental margins resulting in megabreccias and debris flows. Moreover, the period of mass wasting coincides with sea level drops during global sea level lowstand. In some cases, sea level drops can release pore-water overpressure reducing sediment strength and hence promoting instability of sediment at continental margins. Reduced pore-water overpressure can also destabilise gas hydrate-bearing sediment, causing slope failure, and thus resulting in submarine mass wasting. Overall, the global mass wasting during the Middle Ordovician does not need meteoritic trigger.

In western Laurentia a thick Paleozoic succession is exposed that shows repeated paleokarst development. Karst events culminate within the Ordovician towards the top of the Sauk megasequence. Concomitantly, there is a systematic increase in depth of erosion and intensity of weathering from the base of the system towards the Mohawkian. After deposition of the Ninemile Shale, the amount of fine-grained detrital material is drastically reduced at the expense of quartz sand. Sand deposition culminated with the formation of the Eureka quartzite (Mohawkian/Cincinnatian). In the southern Great Basin, it is the only prominent (∼ 150 m thick) pure quartz sand unit within a thick (several 1000 m) Cambrian through Devonian dominantly carbonate platform succession. This paper discusses the reasons for the drastic changes in karst typology and the origin of the Eureka quartz sands. It is argued that Ordovician volcanism was the ultimate trigger for increasing kartsification and increasing quartz sand production through the formation of acid rain.

Telychian to Sheinwoodian conodonts (Pterospathodus eopennatus ssp. n. 1 Zone through the Ozarkodina sagitta rhenana Superzone) were studied in detail for their oxygen isotope composition. The Upper Kockelella ranuliformis and Ozarkodina sagitta rhenana Zones represent the peak interval of the Sheinwoodian positive δ18Oapatite excursion reflecting the coolest conditions with marine sea–water temperatures decreased by more than 6 °C in the subtropics. The δ18Oapatite data cover pre- through post-‘Ireviken Event’ strata. The study includes also material from younger levels up to the late Wenlock Ctenognathodus murchisoni Zone. However this record is incomplete with one major hiatus reflecting the regression associated with the Middle Silurian Mulde Event. The stratigraphic levels of the analyzed conodont samples from Estonia can be correlated with high precision to the datum points of the ‘Ireviken Event’ strata in the Lower and Upper Visby formations on Gotland, Sweden. Inferred short-term climatic changes in the upper Telychian part of the studied interval may have been caused by phases of major volcanic activity along the Caledonian front. Across the ‘Ireviken Event’ we observe variations in oxygen isotope ratios which allow a direct comparison to the stepwise extinctions and faunal reorganisations in different groups. Our data suggest that faunal extinctions are connected to time intervals of warming before the establishment of more stable and cooler conditions during the main Sheinwoodian glacial. The most severe extinctions and faunal turnovers at datum points 2 and 4 occurred during the early, warmer interval of the event. After the shift into the Sheinwoodian icehouse in the Lower Kockelella ranuliformis Zone, reflected by rapidly decreasing sea–water temperatures, faunas restabilized and reefal communities started to flourish. Sedimentary evidence for the glaciation is discussed for different palaeolatitudes. (Less)

We report on widespread and contemporary palaeokarst in the middle Katian (Upper Ordovician) of Sweden, Estonia and Latvia, the first major palaeokarst horizon to be reported from the Ordovician of Baltoscandia. The solution features occurwithin a fewmetres of thick zone of limestone immediately belowthe widespread Fjacka Shale and are related to an unconformity with preserved palaeorelief in the Slandrom Limestone in Sweden and the time-equivalent Saunja Formation in Estonia and Latvia. Facies evidence for a karst origin comes fromseveral outcrops and core sections and includes 1) frequent karren-likemorphologies interpreted as ‘Swiss-cheese’ karst [sensu Baceta et al., 2001], 2) local occurrences of solution/collapse breccia, 3) presence of bladed pseudospar crystals in solution cavities, and 4) carbon isotope values indicating meteoric influence to the succession. These findings are herein put in context with previously reported, large-scale erosional channels that may cut down several tens of metres below the Fjacka Shale in the subsurface Baltic Sea area, and with regional, anomalous thickness variations in the Slandrom Limestone and Saunja Formation, all together forming strong support for regional exposure of the Baltoscandian continent in the middle Katian. High-resolution stable isotopic data show that the regression and lowstand of sea-level overlap with theWaynesville carbon isotope excursion. It resulted in basin-wide cessation of carbonate production near the Amorphognathus superbus and A. ordovicicus conodont zonal boundary. The contemporary development of palaeokarst in different confacies belts of the basin suggests that this was an extraordinary sea-level lowstand, herein interpreted as reflecting a middle Katian glaciation.

Phosphatic microfossils, such as conodonts and fish microremains (dermal scales) from the upper Silurian(Pridoli) of Lithuania have been studied for their oxygen isotope composition. The conodont c ...

Oxygen isotopes measured on Late Ordovician conodonts from Minnesota and Kentucky (United States) were studied to reconstruct the paleotemperature history during late Sandbian to Katian (Mohawkian–Cincinnatian) time. This time interval was characterized by intense volcanism, as shown by the prominent Deicke, Millbrig, and other K-bentonite beds. A prominent carbon isotope excursion (Guttenberg δ 13 C excursion, GICE) postdates the Millbrig volcanic eruptions, and has been interpreted to reflect a drawdown of atmospheric carbon dioxide and climatic cooling. The oxygen isotope record in conodont apatite contradicts this earlier interpretation. An increase in δ 18 O of 1.5‰ (Vienna standard mean ocean water) just above the Deicke K-bentonite suggests an abrupt and short-lived cooling that possibly initiated a first short-term glacial episode well before the major Hirnantian glaciation. The decrease in δ 18 O immediately after the mega-eruptions indicates warming before the GICE, and no cooling is shown in the GICE interval. The coincidence of the Deicke mega-eruption with a cooling event suggests that this major volcanic event had a profound effect on Late Ordovician (late Mohawkian) climate.

The Katian (Upper Ordovician) facies succession of the Siljan district, central Sweden, records some of the most prominent environmental changes in the Ordovician of Baltoscandia. These changes include two separate phases of major sea-level drawdown that were of basinwide and presumably global importance. The first regression and lowstand terminated an entire generation of carbonate mud mounds (the Kullsberg Limestone) and resulted in the formation of polymict carbonate conglomerates (SkAyenlberg Limestone) belonging to the Amorphognathus superbus Zone. New stable isotope data from the Amtjarn quarry shows that this is immediately after the peak of the Guttenberg Carbon Isotope Excursion (GICE), which reaches a delta C-13 peak value at 3.3aEuro degrees in the uppermost Amorphognathus tvaerensis Conodont Zone. A second major regression and sea-level lowstand is manifested by palaeokarst morphologies in the Slandrom Limestone, which formed close in time to the comparably minor Waynesville positive carbon excursion in the basal Amorphognathus ordovicicus Conodont Zone. The widespread exposure associated with this latter lowstand terminated carbonate production in much of the basin, and, during the subsequent flooding, organic-rich, graptolitic shale formed across most of Baltoscandia. The two corresponding sequence boundaries are amalgamated at the top of truncated carbonate mud mounds in the Siljan district, resulting in a pronounced Middle Katian hiatus in the immediate mound areas.

Detrital zircon geochronology and analysis of fossiliferous olistoliths from Chios Island, Greece, are used here to constrain terrane accretion processes and the provenance of crustal sources for sediments during the subduction of (a branch of) the Palaeotethys Ocean. U/Pb ages obtained by ion microprobe (SHRIMP-II) analyses of detrital zircons from a Carboniferous greywacke belonging to the tectonostratigraphic Lower Unit of Chios gave major age groups of 2150–1890 Ma, 640–540 Ma, 505–475 Ma and 365–322 Ma. Detrital zircons from a Permian–Triassic sandstone yielded prominent age clusters of 2200–1840 Ma, 1100–910 Ma, 625–560 Ma and 385–370 Ma. The lack of zircon ages between 1.8 and 1.1 Ga in both samples, coupled with the occurrence of ca. 2 Ga-old zircons, imply a northern Gondwana (NW Africa) source. The conodont fauna recovered from an ‘Orthoceras’-bearing limestone lens from the Carboniferous succession of the Lower Unit indicates a Late Silurian age. The fauna is typical of the Ludfordian Polygnathoides siluricus conodont zone. The material has a conodont colour alteration index (CAI) of about 1–2, indicating very low-grade thermal alteration of less than 100 °C. The closest localities with similar conodont-bearing limestones are to be found in the Balkan region and in the Istanbul Zone of northern Turkey. The occurrence of ‘Orthoceras Limestone’ can be used as an indicator of palaeosource reconstruction. Our new zircon ages in conjunction with provenance analysis of Silurian to Lower Carboniferous olistoliths strongly suggest that the clastic succession of Chios received its detritus from basement rocks of the Sakarya microcontinent in western Turkey and time and facies equivalents of Palaeozoic units from the Istanbul Zone in northern Turkey and the Balkan region due to subduction of a branch of Palaeotethys close to the southern active margin of Eurasia in Late Palaeozoic times. The multidisciplinary approach of this study underlines the importance of terrane accretion during stepwise closure of Palaeotethys.

Based on the study of Early Paleozoic protoconchs, we (Nutzel et al. 2006) concluded that planktotrophic mollusc larvae are unknown from the Cambrian but became common in the Ordovician. Therefore, we assumed that planktotrophy itself evolved at the Cambrian/Ordovician transition and that it is unlikely that planktotrophy represents the original state in Metazoa. This is in agreement with other authors who followed different lines of evidence (see Nutzel et al. 2006). Freeman and Lundelius (FL Riedel 2000). However, such gastropods are known from the Cretaceous onwards only. Scissurellids and some other Vetigastropoda represent another important exception. They are nonplanktotrophic (as all vetigastropods) but have a small initial shell (scissurellids are known from the Triassic onwards). However, all these exceptions do not change the general picture that at a shell length of about 100mm, most planktotrophic molluscs are smaller than most nonplanktotrophic molluscs. Even in recent gastropods, it is commonly hard or impossible to determine the boundary between embryonic shell (protoconch I) and larval shell (protoconch II) and it becomes even more difficult in fossil material that is almost always recrystallized so that fine structures are obscured. Generally, protoconch preservation is rare in the Paleozoic. Most Late Paleozoic to extant gastropods have a tightly coiled initial whorl so that it is possible to measure the diameter of the first whorl if it is preserved. Most planktotrophic gastropods hatch with about one whorl (Figs. 1A and 2J) (e.g., Bandel 1975) and therefore the diameter of the initial whorl can be used to infer the approximate size of the hatchling even if the transition from protoconch I to II is not visible (Nutzel 1998). The oldest gastropods with well-preserved larval shells have been EVOLUTION & DEVELOPMENT 9:4, 313 –318 (2007)

During the late Silurian the Prague Basin was located in middle southern latitudes. In contrast to palaeocontinents positioned in tropical and subtropical latitudes like Baltica, no reefs are developed, which is in accordance with the predicted cooler water. The Prague Basin represents a relatively restricted and shallow rift basin with a complex tectonic history. Sections in different palaeoenvironments have been studied to document the most prominent Silurian stable carbon isotope excursion recorded during the late Silurian (Ludfordian) Lau Event from this part of peri-Gondwana. Deeper water deposits of the Kopanina Formation investigated in the present study were deposited on the slope-to-basin transition near the Kosov volcanic centre in the western part of Prague Basin. The sediments are developed as an alternation of dark, partly laminated limestones and marls with an increase of the limestone–marl ratio in the upper part of the succession. A pronounced positive carbon isotope excursion starts in the Neocullograptus kozlowskii graptolite and in the upper Polygnathoides siluricus conodont zone. The maximum of the shift is observed in the lower part of an interval characterised by the Ananaspis fecunda–Cyrthia postera community. The maximum values scatter around 8‰, which represent the highest values reported hitherto from the Prague Basin. In low latitudes, often a decrease of δ13C values towards deeper water settings is reported. In contrast, in the present study the δ13C values of about 8‰ are much higher than those recorded from the contemporaneous shallow-water sections studied in the classical Muslovka and Požary quarries. The most reasonable explanation is the presence of stratigraphical gaps in the shallow parts of the basin. As indicated by karstification these gaps were caused by a sea-level drop. Another effect of this sea-level fall was a strongly reduced sedimentation of the cephalopod limestone facies around volcanic and tectonic elevations.

Cruziana ichnospecies have been repeatedly reported to have biostratigraphic significance. This study presents a re-evaluation of the arthropod ichnotaxa of the Cruziana rugosa Group from bio- and/or lithostratigraphically well-defined Lower to Upper Ordovician siliciclastic sections of southern and central Bolivia. With the exception of Cruziana rouaulti, the ichnofaunas contain all the members of the Cruziana rugosa Group throughout the Ordovician (Arenig to Caradoc) successions in Bolivia. The Bolivian material therefore indicates that these arthropod ichnofossil assemblages are suitable for recognizing Ordovician strata in Bolivia. These findings cast doubt on their use as reliable indicators for a global intra-Ordovician (Arenig to Caradoc) biozonation of Peri-Gondwanan sedimentary successions.

Two lineages exist for Cahabagnathus. Lineage 1 is C. friendsvillensis–C. chazyensis–C. sweeti–C. carnesi. Three new species are proposed in lineage 2, which consists of C. baueri n. sp.–C. directus–C. cooperi n. sp. and C. craigi n. sp. The distribution pattern of Cahabagnathus was strongly influenced by Chazyan sea-level changes. The wide distributions of C. friendsvillensis and C. sweeti correspond to two large transgressions and the more narrow distribution of C. chazyensis and C. carnesi correspond to regression events. A similar relationship exists between sea-level change and the distribution of species in lineage 2. Evolution of the cahabagnathids was apparently influenced by the rise of sea level, which widely distributed taxa, and the fall of sea level, which isolated taxa. We propose that it was from peripheral isolates of widely distributed taxa that endemic Cahabagnathus taxa (C. baueri, C. cooperi, C. craigi, C. chazyensis, and C. carnesi) evolved.

A more than 2000-m-thick Cambrian–Ordovician carbonate platform succession developed on the exotic Argentine Precordillera terrane during rifting from Laurentia and drifting towards Gondwana. On base of these carbonates, a carbon isotope curve could be developed for the Cambrian–Ordovician. We measured δ13Ccarb and δ13Corg values on bulk rocks, selected components and diagenetic cements. Whereas the carbon isotope signals of intertidal and supratidal rocks are altered by diagenesis, most subtidal carbonates exhibit primary marine values. We can report reliable curves for the Middle–Late Cambrian transition and for the latest Cambrian to earliest Middle Ordovician. The δ13C curve matches well with the published data from global Cambrian–Ordovician boundary sections. Excursions and shifts in the carbon isotope curve correspond to events in sequence stratigraphy. This indicates the interdependence on sea level which rules the productivity and/or preservation of organic carbon and therefore the partition between Corg and Ccarb burial.

The Mid-Silurian (Wenlock) Mulde Event is a globally recognised faunal turnover and extinction event affecting pelagic fauna as well as shallow marine benthos. Time-equivalent strata in the carbonate platform succession of Gotland, Sweden reveal marked changes in the compositions of reefs and the formation of chemical and microbial carbonates in the early aftermath of the extinctions. New high-resolution carbon isotope data from north-eastern Gotland provide information useful for global correlation of this event.

Collections from upper Ibexian (Tulean Stage) rocks of western United States, from the Canning Basin in western Australia, and from the Argentine Precordillera contain a seximembrate apparatus of multidenticulate conodonts whose elements have been included by authors in species of Prioniodus Pander, 1986, and Reutterodus Serpagli, 1974. The individual elements as well as the complete apparatus are not consistent with assignment of the species to either of these genera or to any other extant genus. A new generic name, Stiptognathus Ethington, Lehnert, and Repetski, is proposed with Reutterodus borealis Repetski, 1982, as type species. The apparatus consists of Pa, Pb, Sa-c, and M elements; the genus represents either the Prioniodontidae or the Periodontidae.