Sandbian Stage Research Articles

Late Middle to Late Ordovician Phosphate Accumulation of the Moyero River Section (Siberia): A Record of Upwelling and Global Cooling

The phosphate-bearing rocks (phosphate rocks and phosphorites) were studied over a 45-meter interval of the Moyero river section, covering the upper part of the Darriwilian and the lower part of the Sandbian stages of the Ordovician. These rocks were investigated by field observations and laboratory methods, including optic and scanning electron microscopy, X-ray diffraction analyses. The accumulation of phosphatic matter is manifested in carbonates, sandstones, and aleurolites in the form of grains (ooids and peloids), intraclasts, phosphatic and phosphatized shells. In argillites, it is represented by cryptocrystalline matter. Phosphate matter consists of fluorapatite. The peaks of phosphate accumulation are associated with the formation of layers of physically reworked granular phosphorites (condensation horizons) directly above the depositional sequence boundaries. The formation of the studied phosphate-bearing rocks during the Darriwilian-Sandbian transition was influenced by a combination of global (Great Ordovician Biodiversification Event (GOBE), reduction in sea surface temperature, atmospheric CO2, high sea level, flooding of craton margins) and regional (equatorial position of Siberia, arid climate, facies) factors. Global conditions led to the enrichment of seawater with phosphorus and the effect of upwelling. Regional conditions determined the characteristics of phosphate formation. Studied phosphate-bearing rocks can be considered as a record of upwelling on the Siberian craton during the Middle-Late Ordovician transition and one of the manifestations of long-term global cooling started early in the Middle Ordovician.

The “Guttenberg” positive δ13C excursion (GICE) interval of Latest Sandbian–early Katian (Late Ordovician) spans ca. 1.2 Myr in Asian reference sections

The boundary between the Sandbian and Katian stages of the Late Ordovician is directly followed by the onset of the Guttenberg positive isotope carbon excursion (GICE), which is potentially a means to aid in global correlation of this stage boundary. The sedimentary record of the GICE in mid-continent North American sections often spans only a few meters and has been estimated to span <400 kyr. In contrast, a major sustained positive excursion that has been considered equivalent to the GICE spans over 100 m within some other sections in North America and in the Qilang Formation of the Tarim Basin in northwest China. In addition to enhancing the carbon-isotope records of this expanded record of the GICE, we applied cyclostratigraphy analysis to detailed natural-gamma logging from the Dawangou section, Tarim Basin, which is the auxiliary GSSP for the Sandbian Stage, and from the nearby Sishichang section. The average proportion of carbonate to marl in the outer shelf facies of these sections exhibits cycles of ca. 30 m. Superimposed on these large-scale cycles are ca. 1.5-m oscillations. We interpret these cycles as a sedimentary record of long-eccentricity (405 kyr) and precession cycles. Three secondary low-amplitude oscillations in δ13C are superimposed on the broad GICE plateau and are in phase with the long-eccentricity cycles. The astronomical-tuned sedimentary stratigraphy of these sections implies that the main GICE interval has a duration of ca. 1.2 Myr, with an uncertainty of ca. 200 kyr depending upon how the onset and termination of the GICE is defined. Similar secondary peaks in δ13C on the main GICE excursion reported from other regions have been termed G1, G2 and G3. In the Dawangou section, the estimated placement of the Sandbian-Katian stage boundary is midway between G1 and G2. The GICE record in the composite δ13C stratigraphy from the Cincinnati region of Ohio-Kentucky exhibits an initial “Logana” positive excursion followed by a “Macedonia” positive excursion, and we suggest that these are correlative to the G2 and G3 secondary peaks on the broad GICE as identified in Asia. In both the Tarim region and in the Cincinnati composite, these two secondary peaks in δ13C occur during intervals of greater clay content relative to carbonate. If verified, this astronomically tuned reference scale of the δ13C modulations will enable high-resolution global correlation of biozones and other events near the Sandbian-Katian stage boundary and during the Early Katian.

Emended Sandbian (Ordovician) conodont biostratigraphy in Baltoscandia and a new species ofAmorphognathus

AbstractConodonts are an important biostratigraphic tool for many Phanerozoic stages. Along with graptolites, they define all global Ordovician Stage boundaries. Within the Upper Ordovician interval, a known species ofAmorphognathus tvaerensis(Bergström) is present in both Sandbian and Katian stratotype sections. Study of changes in the succession ofA. tvaerensisrevealed that elements in the upper part of its range differ morphologically quite distinctly from those in its lower part. Here, they are described as a new conodont species,A. viiraesp. nov. This new species is recognized in several Estonian and Swedish sections, with apparent occurrence also in Mójcza Quarry, Holy Cross Mountains, Poland and Black Knob Ridge, Oklahoma, USA. Detailed analysis of earlyAmorphognathuselements from Estonian and Swedish sections revealed the absence ofA. inaequalis(Rhodes) in both regions, although a conodont subzone based on this species was identified earlier by some authors. Both the absence ofA. inaequalis(Rhodes) and recognition of the new speciesA. viiraesp. nov. resulted in the revision of the conodont zonation, and a new version of it is proposed for the Sandbian Stage in the Atlantic Realm. The new zonation includes (from below)Pygodus anserinus,Baltoniodus variabilis,A. tvaerensis,B. gerdae,A. viiraeandB. alobatusConodont zones.

Ostracods of the Gryazno Formation (Upper Ordovician, Sandbian Stage and Idavere Regional Stage) in the West of Leningrad Region

Ostracods of the Gryazno Formation (Upper Ordovician, Sandbian Stage and Idavere Regional Stage) in the West of Leningrad Region

GRAPTOLITES FROM THE UPPER ORDOVICIAN SECTION «VERKHNYAYA KARASU» IN THE CENTRAL PART OF GORNY ALTAI

Taxonomic complex of graptolites from the «Verkhnyaya Karasu» section have been studied. This section is located in the central part of Gorny Altai Mountains. The section covers a large stratigraphic interval: from upper Ordovician (Sandbian Stage) to Middle Silurian (Telychian Stage). For the Ordovician, the Bugryshikha, Khankhara and Tekhten' formations are distinguished. For the Silurian, the Vtorye Utyosy, Syrovaty, Polaty, Chesnokovka and Chesnokovka formations are distinguished. Previously, in this section no graptolites were found in the upper part of the Bugryshikha formation. There is a large and diverse complex of taxa collected in 2020. The graptolites assemblage from the upper part of the Bugryshikha formation consists of Reteograptus uniformis Mu et Zhang, Reteograptus geinitzianus Hall, Eoglyptograptus euglyphus Mitchell, Orthograptus apiculatus Elles et Wood, Orthograptus whitfieldi (Hall), Rectograptus truncatus (Lapworth), Rectograptus ex.gr. truncatus (Lapworth), Hustedograptus sp., Callograptus sp.

A New Bryozoan of the Genus Dianulites Eichwald from the Ordovician of Gorny Altai and the Russian Arctic

A new bryozoan species Dianulites altaicus sp. nov. (order Esthonioporata) is described from the Sandbian Stage of the Upper Ordovician of the environs of Lake Teletskoe in Gorny Altai. A collection of bryozoans of the genus Dianulites Eichwald, 1829 from the Lower and Middle Ordovician of the Novaya Zemlya archipelago and Vaygach Island in the Russian Arctic has been revised. Bryozoans from the Lower and Middle Ordovician of the Novaya Zemlya archipelago previously classified as D. janischevskyi Modzalevskaya, 1953 were shown to belong to D. altaicus sp. nov. Two groups of species of the genus Dianulites have been identified in the Ordovician of the Russian Arctic: the representatives of the first group had mesozooecia, whereas the representatives of the second group, which includes the new species, did not have mesozooecia, but some of them had minutopores.

Tectono‐magmatic division of the Late Ordovician (Sandbian) volcanism at the south‐western margin of Baltica using immobile trace elements: Relations to the plate movements in the Iapetus Palaeo‐Ocean

Immobile trace elements in K‐bentonites indicate three volcanic stages during the Sandbian Stage of the Late Ordovician: Early Grefsen is characterised by high‐Ti source magmas alternating with a few high‐Zr layers. Middle Grefsen is characterised by overall low‐Ti source magmas with multiple high‐Zr layers alternating with less frequent low‐Zr layers. Late Grefsen to Early Katian is characterised by a low content of all studied immobile trace elements (Ti, Zr, Nb, and Th). In all K‐bentonites, the Nb/Al2O3 ratio indicates subalkaline volcanism dominated by rhyolites. In the Early Grefsen stage, andesites and dacites occur. The TiO2/Al2O3 versus Zr/Al2O3 discrimination diagram suggests low‐ and high‐temperature magmatism. Low‐temperature magmatism may have occurred during subduction of water‐rich oceanic sediments in the frontal arc of Avalonia and high‐temperature magmatism during the overriding of a mid‐ocean ridge or hot spot. The coeval end of the volcanic record in Avalonia and Baltica may indicate the collision of Avalonia with a hypothetical continental crust within the Iapetus Ocean.

The δ13C chemostratigraphy of Ordovician global stage stratotypes: geochemical data from the Floian and Sandbian GSSPs in Sweden

ABSTRACTThe δ13C chemostratigraphy of five of the seven Ordovician global stages has been published previously but no such data have been available from the Floian GSSP and most of the Sandbian GSSP in Sweden. This lack of information has now been remedied by isotope data obtained from series of closely spaced shale samples collected from the Floian stratotype at Diabasbrottet in Västergötland and the Sandbian stratotype at Fågelsång in Scania. Although the bases of these stages cannot be precisely tied to levels of conspicuous δ13C excursions, that of the Floian Stage, which is marked by the appearance of the graptolite Tetragraptus approximatus, is between the closely spaced excursions named LTNICE and BFICE. The base of the Sandbian Stage, which is defined as the appearance level of the graptolite Nemagraptus gracilis, is just below a negative excursion previously known as the “Upper Kukruse Low”, which is nowadays known as the LSNICE. The relations between chemostratigraphy and graptolite and conodont biostratigraphy in the Swedish GSSPs and some coeval key sections in Baltoscandia, China, and America are briefly discussed. It is concluded that the data at hand indicate that there is good regional agreement in these relations.

Did early land plants produce a stepwise change in atmospheric oxygen during the Late Ordovician (Sandbian ~458 Ma)?

A stepwise change in atmospheric oxygen (O2) levels during the Ordovician has been attributed to the emergence of land plants. This phenomenon is tied to a major baseline shift in the stable carbon isotope (δ13C) curve and inferred increase in nutrient delivery and enhanced primary productivity in nearshore settings, which led to high organic carbon burial. The timing and magnitude of this baseline shift, however, is still elusive in part because of the lack of high-resolution δ13C data that span this period. Much of the existing Ordovician δ13C literature is focused on isotopic excursions with less emphasis on identifying long-term shifts in baseline (pre- and post-excursion) values.This study presents new high resolution δ13C data from stratigraphic sections at Germany Valley (West Virginia) and Union Furnace (Pennsylvania) in the Central Appalachian Basin. These sections span the entire Sandbian Stage and continue into the lower Katian Stage. The δ13C data from both sections are characterized by relative stability carbon isotope values (mean = −0.61‰) in the lower Sandbian, followed by a + 1.2‰ shift in the upper Sandbian (mean = +0.62‰). Herein, we propose that the positive shift represents a long-term global shift in baseline δ13C values of dissolved inorganic carbon. The timing of this positive shift coinciding with the diversification of early land plants (i.e., bryophytes) supports earlier models that suggested enhanced organic carbon burial rates served as a mechanism for the stepwise oxygenation of the atmosphere during the Late Ordovician.

Geochemistry and U–Pb geochronology of K‐bentonites from the Pingliang Formation of the Upper Ordovician in Gansu, North China, and their tectonic implications

The Late Ordovician Pingliang Formation in the Guanzhuang section, located in the south‐western Ordos Basin, North China, contains several K‐bentonite layers, which are important for understanding the tectonic evolution of the south‐western Ordos Basin. This study provides new geochemical and SIMS U–Pb geochronological data for the K‐bentonites to establish the Ordovician high‐resolution chronostratigraphic framework for the Ordos Basin and to provide temporal constraints of major volcanic–magmatic activity. K‐bentonites in the succession yield U–Pb zircon ages of 457.6 ± 3.8 Ma and 454.8 ± 3.8 Ma (2σ analytical uncertainty), showing a depositional age in the Sandbian Stage (Late Ordovician). The result is consistent with the biostratigraphic and astrochronologic framework, achieving the intercalibration of different dating methods. The discrimination diagrams of whole‐rock Na2O+K2O against SiO2 and Zr/TiO2 against Nb/Y and Th/Yb against Nb/Yb and Th‐Hf‐Ta indicate that the K‐bentonites have obvious arc affinities. Based on the geological background, we conclude that the K‐bentonites in the south‐western Ordos Basin were mainly deposited during the development of the North Qilian‐North Qinling volcanic arc in the Sandbian.

Middle–Upper Ordovician (Darriwilian–Sandbian) paired carbon and sulfur isotope stratigraphy from the Appalachian Basin, USA: Implications for dynamic redox conditions spanning the peak of the Great Ordovician Biodiversification Event

The Evans Ferry section from the Appalachian Basin of eastern North America has been analyzed for chemostratigraphic trends to elucidate possible causal mechanisms facilitating the Great Ordovician Biodiversification Event (GOBE). Paired stable isotope (δ13C and δ34S) analyses were used in this carbonate-dominated locality from the Appalachian Basin to reconstruct the marine redox states during this key period of rapid biodiversification. This succession is one of the most expanded Sandbian Stage (Upper Ordovician) deposits known from North America, and it allows new high-resolution reconstructions of long-term, global carbon and sulfur cycle fluctuations. The integrated geochemical and sequence-stratigraphic investigation presented here from the Laurentian epeiric seaway allows for possible identification of the contraction and expansion of reducing water masses during the Middle–Late Ordovician that may have been linked to global-marine paleoredox dynamics. Utilizing new conodont-based 87Sr/86Sr isotope stratigraphy along with previous conodont biostratigraphy, we can confidently correlate our stable isotope profiles to other carbonate successions globally. Carbonate facies indicate that this area experienced partial restriction from open-marine conditions during certain intervals in the Sandbian. These semi-restricted environments record carbon (δ13Ccarb) isotope trends that are secular in nature, as they can be correlated to other successions across Laurentia and Baltica. We identify several intervals when δ13Ccarb and δ34SCAS (carbonate-associated sulfate) trends are decoupled. These inverse stratigraphic trends are, at times, followed by parallel positive shifts in δ13Ccarb and δ34SCAS. Causal mechanisms for the observed decoupled δ13C and δ34S trends may include a wide variety of factors such as more closed-system, local biogeochemical processes associated in part with diagenesis, or they could instead reflect variations in the global fluxes of organic matter and pyrite burial linked to changing marine paleoredox conditions. The positive covariation in trends presented here likely represents transient increases in organic carbon and pyrite burial in response to expansion of reducing marine environments.

Structure, Age, and Settings of Formation of Ordovician Complexes of the Northwestern Frame of the Kokchetav Massif, Northern Kazakhstan

This work presents the data on the structure, geochronology, and formation settings of the Ordovician sedimentary and volcanogenic-sedimentary complexes of the Sterlitamak, Mariev, and Imanburluk structural and formational zones located in the western and northwestern frames of the Kokchetav massif (Northern Kazakhstan). In addition, the results of detailed stratigraphic, geochemical, and geochronological studies of the reference section of the Ordovician deposits of the Mariev Zone are given. The studied section is composed of carbonate, terrigenous, and less commonly volcanogenic-sedimentary deposits, confined to a wide stratigraphic interval from Tremadocian Stage of the Lower Ordovician to the lower Sandbian Stage of the Upper Ordovician. For the first time, the study of conodont assemblages made it possible to establish the Early to Middle Ordovician age of the most ancient limestone–dolomite sequence, which was previously conventionally attributed to the Cambrian. The above-lying tuffaceous–terrigenous Kupriyanovka Formation is now attributed to the Middle Ordovician. On the basis of compositional features of the lithoclastic tuffs composing the middle part of the formation, we assume that it was formed within the island arc zone. Limestones from the base of the youngest terrigenous–carbonate Kreshchenovka Formation are attributed to the lower part of the Sandbian Stage of the Upper Ordovician. The study of the geochronology of detrital zircons from terrigenous rocks of the limestone–dolomite sequence has shown that the Early Neoproterozoic quartzite–schist sequences of the Kokchetav massif were the most probable provenance area during its deposition. It was established that there was the change of sedimentation environments from closed lagoons to a relatively deep sea basin with normal salinity and intense circulation of water masses in the northwestern frame of the Kokchetav massif during the Ordovician. During this period of time, there was a sufficiently high level of erosion of provenance areas that resulted in the deposition of thick strata of terrigenous material. A general tendency of the deepening of sedimentation environments from the Early to Late Ordovician was interrupted by sea level rises in the Dapingian and early Darriwilian ages.

Local and trans-Atlantic chemostratigraphic significance of new δ13Ccarb data from the Sandbian and Katian Stages (Middle–Upper Ordovician) of the Oslo region, Norway

Late Sandbian to early Katian δ13Ccarb chemostratigraphy has in Norway been described in only two previous reports that dealt with two geographically rather widely separated areas, namely the Oslo-Asker district and the Nes-Hamar district. No data have been available from the Ordovician outcrop areas between these districts that could help clarify the partly unclear regional stratigraphic relations across the Oslo region. A chemostratigraphic study of the classical road section at Tønnerud in the northwestern Hadeland district about halfway between the previously investigated districts resulted in the recognition of two δ13C excursions, namely one in the uppermost Furuberget Formation tentatively identified as the Guttenberg Isotopic Carbon Excursion and one in the lower Solvang Formation that is classified as the KOPE (RAKVERE) excursion. Based on these results, the Furuberget and Solvang formations are interpreted to be separated by a significant gap in the study area corresponding to the Oandu and lower Rakvere regional Baltoscandic stages. This suggests that the Tønnerud succession is less complete stratigraphically than those in the Oslo-Asker district. Similar gaps are not uncommon in this interval in Baltoscandia and in North America and probably reflect both eustatic and local epeirogenetic movements that make it difficult to establish a regionally applicable sequence stratigraphy.

To the revision of the Upper Ordovician Bilobia Cooper (Strophomenida, Brachiopoda)

The diagnosis and species composition of Bilobia Cooper, 1956 (family Leptestiidae, Strophomenida, Brachiopoda) are specified based on literary data and collection from the Leningrad Region. In particular, the cardinal process with cleft shaft and trifid myophore and denticles along the area of ventral valve are adduced to the diagnosis. A new species, B. alichovae sp. nov., from the Sandbian Stage of the Leningrad Region is established.

Synchronous diversification of Laurentian and Baltic rhynchonelliform brachiopods: Implications for regional versus global triggers of the Great Ordovician Biodiversification Event

Abstract The profound global impact of marine radiations during the Great Ordovician Biodiversification Event (GOBE) is widely appreciated; however, diversification varied among paleocontinents and these individual trajectories are less understood. Here we present a new species-level diversity curve for rhynchonelliform brachiopods from midcontinental Laurentia based on bed-by-bed analysis of the Simpson Group of Oklahoma (USA). Diversity and abundance data span the Dapingian through Sandbian Stages, which encompass the interval of maximum global diversification. A rapid, statistically significant increase in brachiopod diversity was observed in the early Darriwilian Histiodella holodentata Biozone. We interpret this as a biological signal because the increase cannot be explained by sampling intensity, facies types, or position along depositional gradient. Diversifications on Laurentia and Baltica were temporally synchronous at the biozone level, and cumulative diversity curves for the regions show similar patterns, suggesting a global driver for the radiations. The taxonomic composition of the brachiopod faunas, however, differs substantially, highlighting the importance of regional controls on diversification. Thus both global and local factors controlled diversity increase during the GOBE.

Upper Middle to lower Upper Ordovician chitinozoans and conodonts from the Bliudziai-150 core, southern Lithuania

Integration of chitinozoan and conodont biozones is based on detailed collections from the Bliudziai-150 well, southern Lithuania, that penetrated upper Darriwilian – lower Sandbian sedimentary carbonate rocks. The strata accumulated at the southeastern side of the Livonian Tongue and represent the transition from shallow shelf facies of the Lithuanian shallow-water shelf facies belt to the deeper-water shelf facies belt. Systematically collected and closely spaced samples of the carbonates from the Bliudziai-150 core yielded both chitinozoans and conodonts in abundance. The chitinozoans are assigned to the Laufeldochitina striata, Laufeldochitina stentor, Angochitina curvata – Armoricochitina granulifera, Lagenochitina dalbyensis, Belonechitina hirsuta, and Spinachitina cervicornis chitinozoan biozones, and the conodonts are assigned to the Pygodus serra Zone (with three subzones), Pygodus anserinus Zone (with two subzones), and the three subzones of the Amorphognathus tvaerensis Zone. The fossils are clearly of Baltoscandian affinity, and the integrated results support existing knowledge concerning the biostratigraphy of the two groups in Baltoscandia. Thus, the studied interval is confidently assigned to the Lasnamägi, Uhaku, Kukruse, and Haljala regional stages of the East Baltic regional chronostratigraphical scheme, corresponding to the global upper Darriwilian – lower Sandbian stages (Middle to Upper Ordovician). International correlation of the chitinozoan and conodont succession is presented, and the position of the base of the international Sandbian Stage is placed at 1380.00 m in the Bliudziai-150 core. A horizon marked by multiple discontinuity surfaces represents the base of Kegel depositional sequence and coincides with the appearance of Armoricochitina granulifera, Angochitina curvata, Lagenochitina dalbyensis, and Baltoniodus gerdae in the Bliudziai-150 core.

Strontium isotope (87Sr/86Sr) stratigraphy of Ordovician bulk carbonate: Implications for preservation of primary seawater values

The present study on bulk carbonate 87 Sr/ 86 Sr stratigraphy represents a companion work to earlier research that presented a conodont apatite-based Ordovician seawater 87 Sr/ 86 Sr curve for the Tremadocian–Katian Stages (485–445 Ma). Here, we directly compare the curve based on conodont apatite (including some new data not published in earlier work) with a new curve based on 87 Sr/ 86 Sr results from bulk carbonate from the Tremadocian–Sandbian Stages. We sampled eight Lower to Upper Ordovician carbonate successions in North America to assess the reliability of bulk carbonate to preserve seawater 87 Sr/ 86 Sr and its utility for 87 Sr/ 86 Sr chemostratigraphy. A high-resolution 87 Sr/ 86 Sr curve based on 137 measurements of bulk conodont apatite is used as a proxy for seawater 87 Sr/ 86 Sr ( 87 Sr/ 86 Sr seawater ). In total, 230 bulk carbonate samples that are paired to conodont samples were measured for 87 Sr/ 86 Sr in order to determine the conditions under which 87 Sr/ 86 Sr seawater is preserved in bulk carbonate. Results indicate that well-preserved bulk carbonate can faithfully record the 87 Sr/ 86 Sr seawater trend, but that its 87 Sr/ 86 Sr values are commonly more variable than those of conodont apatite. On average, bulk carbonate samples of the same age vary by 10–20 × 10 −5 , compared to 5–10 × 10 −5 for conodont apatite. The amount of isotopic alteration of bulk carbonate from seawater 87 Sr/ 86 Sr (Δ 87 Sr/ 86 Sr) was determined by taking the difference between 87 Sr/ 86 Sr values of bulk carbonate and the approximated seawater trend based on the least radiogenic conodont 87 Sr/ 86 Sr values. Cross plots comparing Δ 87 Sr/ 86 Sr values to bulk carbonate Sr concentration ([Sr]) and conodont color alteration indices (CAI; an estimate of the thermal history of a rock body) indicate that bulk carbonate is most likely to preserve 87 Sr/ 86 Sr seawater (minimally altered) when either: (1) bulk carbonate [Sr] is greater than 300 ppm, or (2) carbonate rocks experienced minimal thermal alteration, with burial temperatures less than ~150 °C. Carbonates with intermediate [Sr] (e.g., between 130 and 300 ppm) can also yield 87 Sr/ 86 Sr seawater values, but results are less predictable, and local diagenetic conditions may play a greater role. Modeling results support the argument that seawater 87 Sr/ 86 Sr can be preserved in bulk carbonates with low [Sr] if pore water:rock ratios are low ( 87 Sr/ 86 Sr is similar to the seawater 87 Sr/ 86 Sr value preserved in limestone. Bulk carbonate samples that meet these criteria can be useful for high-resolution measurements of 87 Sr/ 86 Sr seawater , with a sample variation on par with fossil materials ( −5 ), particularly for successions where well-preserved fossil material (i.e., conodonts or brachiopods) is not available, such as Precambrian strata, sequences recording mass extinction events, or otherwise fossil-barren facies. These criteria and model predictions based on bulk carbonate [Sr] must be considered in the context of whether a limestone accumulated under calcite seas (e.g., Ordovician), with relatively high seawater Sr/Ca, or aragonite seas, in which case the diagenetic transformation of aragonite to calcite may result in incorporation of non-seawater Sr.

Biostratigraphy and palaeoecology of Middle–Late Ordovician conodont and graptolite faunas of the Las Chacritas River section, Precordillera of San Juan, Argentina

AbstractA conodont-graptolite biostratigraphic study was carried out on the top strata of the San Juan, Las Chacritas and Las Aguaditas formations in the La Trampa Range, Precordillera of San Juan in western Argentina. Significant conodont records in the San Juan and Las Chacritas formations allow for the recognition of theYangtzeplacognathus crassus, Eoplacognathus pseudoplanus(Microzarkodina hagetianaandM. ozarkodellasubzones) andEoplacognathus suecicuszones of Darriwilian age. Index species and co-occurrences of graptolites and conodonts were recorded in the Las Aguaditas Formation allowing the identification of theNemagraptus gracilisand thePygodus anserinuszones, which represent the Sandbian Stage. These data indicate a hiatus between the Las Chacritas and the Las Aguaditas formations, corresponding to thePygodus serraZone and thePterograptus elegansandHustedograptus teretiusculuszones (upper Darriwilian). A total of 7287 identifiable conodont elements were recorded from the study section. The species frequency registered for each zone shows thatPeriodonandParoistodusare the most abundant taxa, which are indicative of open marine environments. The records of particular conodont taxa, such asHistiodella, Periodon, Microzarkodina, EoplacognathusandBaltoniodus, allow a precise global correlation with other regions such as south-central China, Baltoscandia, North America, Great Britain, Southern Australia and New Zealand. The graptolite fauna identified here are recognized worldwide in equivalent strata in the Baltic region, Great Britain, North America, China, southern Australia and New Zealand. The presence of graptolites in the ribbon limestones of the Las Chacritas Formation is documented for the first time.

Metabentonites from the Sandbian Stage, Upper Ordovician, in Scotland – a geochemical comparison with their equivalents in Baltoscandia

Synopsis Four metabentonites, including three new discoveries, are described from the Sandbian Stage, Upper Ordovician, of southern Scotland. They are compared geochemically with suites of more numerous contemporary metabentonites from Scandinavia. Two correlations are made with the Scandinavian suites that confirm relative stratigraphical age relationships. The Scottish examples represent distal ash falls from a volcanic source between Avalonia and Baltica.

Geochemistry of Upper Ordovician metabentonites and their cognate apatite microphenocrysts from Norway and Sweden

A suite of metabentonites (altered volcanic ashes or K-bentonites) from the Sandbian Stage, Upper Ordovician, from Scandinavia was subjected to a detailed geochemical study which highlighted specific correlations and magmatic processes. A bimodal distribution in elemental compositions for one suite of metabentonites suggested that it was generated by magma mixing processes. There is strong evidence to correlate three Grefsen samples from Fjäcka, Sweden, with the lowermost Grefsen samples from Sinsen, Oslo, Norway. New data for rare earth element (REE) concentrations in cognate apatite microphenocrysts support the bulk trace element scenarios and confirm the role of plagioclase feldspar and pyroxene ± hornblende fractionation in the formation of eruptive magmas of intermediate composition formed during the closure of the Iapetus Ocean and Tornquist Sea in Upper Ordovician times. New REE data from apatite microphenocrysts can assist in correlating these metabentonites with contemporary beds elsewhere in Northern Europe.