Early Caradoc Research Articles

Systematics of Shumardiidae (Trilobita), with new species from the Ordovician of Argentina

New shumardiid trilobites from Arenig to Early Caradoc deposits of the Argentine Precordillera include species of a revised Kweichowilla (K. salasae, K. sterrenae, and K. piojensis) and the new genus Changchowilla (C. sanjuanina and C. gracielae, and C.? carrerai). Conophrys fabiani new species occurs in the Early Tremadoc of the western Puna region, northwestern Argentina. Leoforteyia new genus is based on L. hintzei new species from the Ibexian of Utah. Published taxa recognized as new species are Conophrys rushtoni, Conophrys wrighti, Leoforteyia ludvigseni and Shumardia whittingtoni. Phylogenetic analysis suggests the differentiation of Shumardia Billings, 1862; Kweichowilla Chang, 1964; Changchowilla new genus; and Leoforteyia new genus as separate taxa and indicates that Conophrys Callaway, 1877, is a grade group. Silicified material provides a nearly complete growth series for Kweichowilla salasae, including the first protaspides known with confidence for the Shumardiidae. Additionally, ontogenetic material for K. sterrenae, C. sanjuanina, C. riojana (Benedetto and Cañas) and C.? carrerai is described.Leoforteyia is restricted to the Ibexian-Whiterockian of western Laurentia; species of Shumardia are widespread in the Arenig to Early Ashgill of western Europe, Australia, Laurentia, China and Russia. Changchowilla is recorded in the mid-Late Arenig-Llanvirn of the Precordillera and in the Llanvirn-middle Caradoc of China. Argentine species of Kweichowilla are closely allied to Australian species, suggesting biogeographic affinities of the Precordillera to the eastern margin of Gondwana by the Llanvirn and Caradoc.Functional morphology, occurrence in fine grained sediments, and association mainly with atheloptic and pelagic trilobites suggest that shumardiids were deposit-feeders that inhabited low light levels in low energy settings, with a limited ability to burrow in soft sediments.

Quantitative evaluation of microplankton palaeobiogeography in the Ordovician–Early Silurian of the northern Trans European Suture Zone: implications for the timing of the Avalonia–Baltica collision

Quantitative analysis of assemblage similarity among chitinozoan and acritarch associations recovered from various sedimentary sequences across the Trans European Suture Zone (TESZ; southern Baltic Sea and northern Germany region), permits evaluation of changes in microplankton palaeobiogeography during the Ordovician in the study area. The present data confirm strong palaeobiogeographic differences between the lower Ordovician of the Rügen area, and the coeval domains of the East European Platform (EEP), corroborating the idea that the subsurface of Rügen should be considered palaeogeographically as the eastern extension of Avalonia. Cluster analysis of chitinozoan assemblages from numerous wells in the Rügen area, and one well from the southern margin of the EEP indicates that chitinozoan bioprovincialism reached its maximum during the Llanvirn; during this period, the Rügen microplankton communities were clearly Gondwanan in character. Calculations using the coefficient of similarity support the conclusion of a high similarity between Llanvirn acritarch assemblages from the Rügen subsurface and from coeval Perigondwana localities (e.g. Tunisia). Since the early Caradoc, this Gondwanan affinity of the Rügen microfossils starts to lessen, and becomes negligible during the late Caradoc. During latest Caradoc–early Ashgill through Llandovery times the chitinozoan assemblages from either side of the TESZ are undistinguishable. If palaeobiogeographical differentiation is primarily related to palaeolatitudinal distance, then the present data support closure of the Tornquist Ocean during late Caradoc–Ashgill times. The presence of reworked Llanvirn acritarchs of Perigondwanan affinity in middle Ashgill sedimentary sequences at the southern margin of the EEP, clearly shows that by this time erosion of an uplifted area was taking place. Accordingly, the closure of the Tornquist Ocean, and consequent Avalonia–Baltica collision must have taken place during the time interval between the middle Caradoc and the early middle Ashgill (Rawtheyan). New chitinozoan data from boreholes H 2 and K 5, offshore of Rügen Island, northern Germany, allow for precise biostratigraphic dating. In the pre-Devonian part of H 2, Siphonochitina formosa indicates an Abereiddian (Early Llanvirn) age, facilitating correlation with boreholes Binz 1/73 and Lohme 2/70 of Rügen. In borehole K 5, the occurrence of Belonechitina robusta and Tanuchitina bergstroemi permits attribution of the investigated pre-Carboniferous sequence to the middle-late Caradoc, and, possibly, early Ashgill.

Chitinozoans from the Ordovician Los Azules Formation, Central Precordillera, Argentina

Other| April 01, 2001 Chitinozoans from the Ordovician Los Azules Formation, Central Precordillera, Argentina E. G. Ottone; E. G. Ottone Universidad de Buenos Aires, Departamento de Ciencias Geologicas, Buenos Aires, Argentina Search for other works by this author on: GSW Google Scholar G. D. Holfeltz; G. D. Holfeltz Search for other works by this author on: GSW Google Scholar G. L. Albanesi; G. L. Albanesi Search for other works by this author on: GSW Google Scholar G. Ortega G. Ortega Search for other works by this author on: GSW Google Scholar Author and Article Information E. G. Ottone Universidad de Buenos Aires, Departamento de Ciencias Geologicas, Buenos Aires, Argentina G. D. Holfeltz G. L. Albanesi G. Ortega Publisher: Micropaleontology Press First Online: 03 Mar 2017 Online ISSN: 0026-2803 Print ISSN: 1937-2795 GeoRef, Copyright 2004, American Geological Institute. Micropaleontology (2001) 47 (2): 97–110. https://doi.org/10.2113/47.2.97 Article history First Online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation E. G. Ottone, G. D. Holfeltz, G. L. Albanesi, G. Ortega; Chitinozoans from the Ordovician Los Azules Formation, Central Precordillera, Argentina. Micropaleontology 2001;; 47 (2): 97–110. doi: https://doi.org/10.2113/47.2.97 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyMicropaleontology Search Advanced Search Abstract Chitinozoans from the Los Azules Formation, central Precordillera of Argentina are documented. The formation is divided into three members. Two chitinozoan assemblages are recognized: the late Llanvirn Lower Los Azules Palynological Assemblage, in the middle member includes smooth forms, while the early Caradoc Upper Los Azules Palynological Assemblage, from the upper member, is mainly composed of ornamented specimens. Two new species are described: Eisenackitina yolei Ottone and Holfeltz and Hercochitina volkheimerii Ottone and Holfeltz. The Los Azules chitinozoan faunas have species in common with contemporaneous assemblages from Laurentia but also northern Gondwana and Baltica. This content is PDF only. Please click on the PDF icon to access. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Llanvirn-early Caradoc trilobite biofacies of western Hubei and Hunan, China

On the basis of representative collections made from seven measured sections in western Hubei and Hunan, four early Llanvirn and five late Llanvirn-early Caradoc trilobite biofacies are distinguished along a bathymetric gradient from shallow to deep outer shelf. The diversity and relative abundance of mesopelagic cyclopygids gradually increased southwards or seawards. According to the facies association exhibited in the region, the upper limit of living depth for different forms is inferred as follows: 1) more than 70 m (M. (Microparia)); 2) less than 100 m (M. (Quadratapyge), Pricyclopyge, Aspidaeglina); 3) at approximately 100 m (Cyclopyge); and 4) more than 100 m (Symphysops, Psilacella). Benthic forms were absent in the deep outer-shelf basin because of stagnant conditions. In shallower sites of the outer shelf, genera of nileids in the carbonate facies and raphiophorids in the clastic facies were relatively abundant, but some endemic forms, such as the asaphid Birmanites and remopleuridid Hexacopyge, became dominant during the late Llanvirn-early Caradoc. Rorringtoniids are mainly associated with deeper outer-shelf facies. Panderia is common in an early Llanvirn nileid fauna at Yichongqiao, Cili area, and may well indicate development of shelf carbonate buildups. A transgressive event is suggested by the sudden appearance of rich Cyclopyge in the early Caradoc at Jiuxi, Changde.

Rogercooperia, a new genus of Ordovician glossograptid graptolite from southern Scotland and New South Wales, Australia

Synopsis Rogercooperia phylloides (Elles & Wood 1908) gen. nov. and R. paucispinosus (Sherwin & Rickards) form the basis of a new genus evolutionarily related to the last of the glossograptids in the wilsoni Biozone (early Caradoc) of Scotland, UK and the calcaratus Biozone (highest Gisbornian) of New South Wales, Australia. The similar genus Nanograptus Hadding (1915), from the early Caradoc of Scandinavia, is reviewed, and is considered a late evolutionary derivative of Cryptograptus Lapworth (1880). Rogercooperia and Nanograptus are convergent genera. Although of different evolutionary lineages both show loss of proximal spinosity, loss of rhabdosomal spinosity, and lessening of thecal spinosity, resulting in an overall similar appearance. The loss of spinosity is typical of many lineages of biserial graptolites in the late Caradoc and Ashgill. The early development of the colonies, in both genera, includes a low metasicular origin of thl 1 , again a late evolutionary feature.

Low-grade metamorphism in the Scottish Southern Uplands terrane: deciphering the patterns of accretionary burial, shearing and cryptic aureoles

ABSTRACTSystematic studies of metapelitic grade linked with the geological re-survey of the Southern Uplands have been used to generate a contoured metamorphic map currently covering nearly two-thirds of the terrane. These studies, based on approximately one pelite sample per 2·5 km2, have used XRD measurements of clay mineral reaction progress, particularly illite crystallinity, to delineate zones of diagenesis and low-grade metamorphism in the imbricated Ordovician and Silurian strata. The regional pattern revealed by metapelitic zonal sequences does not agree with earlier observations that grade increases across the strike, from SE to NW. Instead, the map shows considerable variations in metamorphic trends, including patterns of grade increasing from older into younger strata, indicative of accretionary burial. Other patterns identified include those generated by high strain rates in the Moniaive Shear Zone (MSZ), and by extensive low-temperature cryptic aureoles associated with late granitic intrusions. The present pattern is the result of uplift that generated normal movement on reactivated thrust faults and differential block movement on NW-trending faults.Regional metamorphic patterns were generated by burial and underplating in an accretionary thrust stack. Subduction was initiated in the early Caradoc and probably ceased in the early Wenlock. Metapelitic patterns suggest that two levels of accretion are exposed in the terrane. Strata accreted to the toe of the prism and stacked above the décollement zone are typically at late diagenetic grade. Underplated strata below the décollement are typically at anchizonal grades with moderate to well-developed slaty cleavage. Coherent thrust-bounded tracts of strata at both levels were rotated and buried to produce a syntectonic depth-controlled pattern of meta-morphism. Shear zone metamorphism at depths of 12 km or more was probably confined to the underplated lower level of the thrust stack, and Devonian granitic intrusions were also emplaced mainly within the underplated strata.

Late Ordovician brachiopod biofacies of the Girvan district, SW Scotland

ABSTRACTDiverse and abundant brachiopod faunas, associated with unstable outer shelf and slope environments, occur through the Upper Ardmillan Group (upper Caradoc–upper Ashgill) in the Girvan district of SW Scotland. Representatives of the deep-waterFoliomenafauna occur intermittently throughout the group, appearing in both the Whitehouse and Drummuck subgroups. This distinctive assemblage of small, thin-shelled brachiopods, includingDedzetina, Christiania, CyclospiraandFoliomenaitself, first appeared in South China during the early Caradoc but had colonised the Laurentian margins by the late Caradoc. Within the upper Caradoc–lower Ashgill Whitehouse Subgroup, theFoliomenafauna is interbedded with a variety of other less cosmopolitan deep-water assemblages including theOnniella–SkenidioidesandLingulella–Trimurellinaassociations. Shallower-water environments in the middle Ashgill Lower Drummuck Subgroup hosted theFardenia–Eopholidostrophiaassociation in sands, and theChristiania-Leptaenaassociation in muds and silts. The remarkable Lady Burn Starfish Beds in the upper part of the group contain a variety of brachiopod-dominated assemblages including theEochonetesandPlaesiomys-Schizophorellaassociations, transported from various shelf locations, within a very diverse mid-Ashgill biota. Nevertheless, elements of theFoliomenafauna persisted to near the top of the Drummuck Subgroup, occurring as rare assemblages in more muddy and silty facies. The upper Ashgill High Mains Formation contains abundant elements of the terminal OrdovicianHirnantiafauna includingEostropheodonta, HindellaandHirnantiaitself, but also some taxa more typical of the Laurentian Edgewood Province. As a whole, the changing brachiopod biofacies monitor environmental fluctuations, on part of the Laurentian margin, driven by mainly eustatic and tectonic events.

Ordovician pliomerid and prosopiscid trilobites from Argentina

Pliomerina Chugaeva, 1958, a characteristic element of the peri-Gondwanan Eokosovopeltis-Pliomerina Province, occurs in Llanvirn and Caradoc strata of the Argentine Precordillera. Three protaspid instars are present in the ontogeny of the Early Caradoc Pliomerina peripata new species. A species possibly representing Pliomerella is a rare element in Early Caradoc strata of the Las Aguaditas Formation. Prosopiscus is represented by a Llanvirn species with a first record of protaspid larvae for the enigmatic Family Prosopiscidae. Early growth stages support closest relations of prosopiscids to Phacopina. The presence of Pliomerina and Prosopiscus, along with the cheirurid Pateraspis, demonstrates a peri-Gondwanan influence on the Precordilleran fauna by the Llanvirn, and supports the proximity of the Precordillera terrane to the South American margin by that time.

Late Ordovician (Caradoc-Ashgill) Brachiopod Faunas with Foliomena Based on Data from China

New investigations of mid-Ashgill brachiopod faunas with Foliomena in an onshore to offshore transect in the border region of Zhejiang-Jiangxi provinces, East China, indicate a remarkable increase in numbers of Foliomena folium (Barrande) with significant variations of faunal composition, diversity, and abundance from shallow-water to deep-water environments. Based on data from China and elsewhere, faunas with Foliomena are partitioned into typical and atypical associations. The typical Foliomena fauna is characterized by Foliomena along with some distinctive constituents (such as Dedzetina, Leptestiina, Christiania, and Cyclospira), and is essentially located in deep-water environments (mainly BA 5-6). The atypical Foliomena fauna is represented by those assemblages in which rare Foliomena is associated with various genera such as Kassinella at different localities representing shallower environments (outer BA 3-BA 4). The Foliomena fauna previously was reported as latest Caradoc to early Ashgill in age. New Chinese data demonstrate that it ranges from the early Caradoc (Nemagraptus gracilis Biozone) to the mid-Ashgill. It reached its evolutionary acme in the early Ashgill and became extinct before the Hirnantian. Paleobiogeographically, it is associated with eight paleoplates within the Kosov Province, where the shallow-water Hirnantia fauna was widely distributed later in the Hirnantian. Continuous and gradual changes in the taxonomic composition and ecological niches of the Foliomena associations across a range of depths may indicate that the distribution of faunas with Foliomena may not be controlled always by the position of the thermocline. Distribution of the typical Foliomena fauna in time and space verifies a successful early Paleozoic ecological experiment in the marine benthos: brachiopods expanded their ranges into vacant deep-water benthic niches to exploit soft substrates associated with dark, possibly oligotrophic conditions.

Rare earth geochemistry of Arenig cherts from the Ballantrae Ophiolite and Leadhills Imbricate Zone, southern Scotland: implications for origin and significance to the Caledonian Orogeny

Rare earth element (REE) data from low to mid-Arenig cherts are used to test competing models for the early Ordovician evolution of the Laurentian margin in the northern British Isles. Cherts from the Ballantrae Ophiolite Complex have chondrite-normalized REE patterns typical of continental margin settings with LREE enrichment, a slight negative Eu anom and shale and chondrite-normalized La/Yb values of 0.97–1.41 and 7.78–11.4 respectively. This pattern, together with a large positive chondrite-normalized Ce anom (1.44–1.70), is virtually identical to that found in radiolarian chert of the Gascoyne Abyssal Plain, in the Timor Sea. Cherts from the Raven Gill Formation within the Leadhills Imbricate Zone, Northern Belt, Southern Uplands have typical continental margin REE patterns, chondrite-normalized Ce anom (0.9–1.21) and Eu anom (0.61–0.79) values indicating that they formed closer to the continental margin than those from Ballantrae. Shale and chondrite-normalized and La/Yb values of 0.95–1.27 and 4.92–13.88 respectively confirm this interpretation. It is concluded that the Ballantrae ophiolite formed in a rifted–arc basin above a northwards dipping, intra-oceanic subduction zone. The modest depth of burial of the Raven Gill Formation precludes it being part of a marginal basin which was subsequently trapped as the Ballantrae Ophiolite was obducted in the late Arenig. The Arenig rocks of the Leadhills Imbricate Zone represent an allochthonous terrane accreted to the western extension of the Midland Valley in Ireland in pre-Caradoc times. Here it formed the basement to the Southern Uplands basin. Palaeontological evidence places this basin adjacent to Pomeroy, Co. Tyrone in the early Caradoc. Sinistral strike-slip faulting, from the late Ashgill transported the Southern Uplands Terrane to its present location, a distance of less than 250 km.

Palynomorphs, Conodonts and Associated Graptolites from the Ordovician Los Azules Formation, Central Precordillera, Argentina

Palynomorphs and conodonts are reported from the Ordovician Los Azules Formation, Argentine Precordillera. The formation is mainly made up of argillites, shales and mudstones, and is divided into three informal members. Fourteen species of acritarchs including Gorgonisphaeridiumm granulobrachium Ottone, Lophosphaeridium ruidum Ottone, and Polygonium echinatus Ottone that are proposed as new; two species of prasinophytes, one of cryptospores and three different morphotypes of phytodebris are identified throughout the middle and upper members of the formation. Two distinct palynomorph assemblages are recognized: the Lower Los Azules Palynological Assemblage (middle member) and the Upper Los Azules Palynological Assemblage (upper member). Eight species of conodonts are preserved as casts on bedding planes of the lower shales of the middle member. This conodont association is correlated with the upper division of the Eoplacognathus suecicus Zone from the late early Llanvirn. The upper part of the middle member contains the eponymous species of the Pygodus serra Zone of Llanvirn age. A calcirudite sample from the upper member yielded a rich and diverse microfauna that includes ten conodont species representing the lower part of the Amorphognathus tvaerensis Zone (early Caradoc). Conodonts and palynomorphs confirm previous graptolite-based dating of the Los Azules Formation (early Llanvirn-early Caradoc). The palynological assemblage exhibits similarities with coeval assemblages from the Northern Hemisphere and Gondwanaland.

Caradoc bivalves from the Argentine precordillera: A local radiation-extinction event

Pebbles with a diverse shelly fauna, including bivalves, brachiopods, gastropods, and bryozoans havebeen found in the basal diamictite of the Don Braulio Formation, cropping out in the Sierra de Villicum, Argentine Precordillera. The Don Braulio Formation is of Ashgill age, but according to the brachiopods the fauna of the pebbles is of early Caradoc age. On the basis of this age and some elements in common, pebbles probably come from the underlying La Cantera Formation. Bivalves include nine genera, seven of which are new. The relatively high diversity and endemism of this fauna contrasts with the low diverse and widespread Caradocian associations from other regions of South America. The life span of this association is short. The Ashgillian beds of the Don Braulio Formation have yielded a different, low diverse association of bivalves. Causes of this brief radiation-extinction event affecting the bivalves are not clear. According to the recently proposed palaeobiogeographic reconstructions, during the early Caradoc the Precordillera attained the “pre-accretion stage”, and was placed near to the Gondwana continent. Consequently, a geographic isolation cannot be advocated to account for the high endemism of the bivalves. The principal cause of the endemism could be linked to climatic conditions. According to the global and local temperature curves, during the Caradoc an increase in water temperature occurred. Caradoc units of the Precordillera (Las Plantas and Las Aguaditas formations) include some carbonate deposits suggesting temperate waters. This could be the reason for the rapid diversification of the bivalves. Conversely, the dominantly clastic sedimentation of other Gondwanan areas such as the Central Andean basin of NW Argentina and Bolivia suggests that water temperature could have been cooler than in the Precordillera basin and that cold waters could have been a climatic barrier precluding expansion. The rapid extinction of the fauna could be linked to the progressive cooling during the Ashgill which culminates with the latest Ordovician glacial event.

New Late Ordovician (Early Caradoc) bivalves from the Sierra de Villicum (Argentine Precordillera)

A new and unusually well preserved bivalve fauna of Late Ordovician age was recovered from pebbles within the Late Ashgill (Hirnantian) glacigenic diamictite of the Don Braulio Formation, in the Argentine Precordillera. According to the associated brachiopods and the presence of graptolites of the bicornis Zone in the matrix of the diamictite, an early Caradoc age is accepted for the bivalve fauna. The clasts contain bivalve molluscs representing nine genera, of which seven are new. Identified taxa are the following: Praenucula sp., Cuyopsis symmetricus new genus and species, Villicumia canteraensis new genus and species, Trigonoconcha acuta new genus and species, Concavodonta ovalis new species, Hemiconcavodonta minuta new genus and species, Hemiconcavodonta sp., Emiliania cuerdae new genus and species, and Concavoleda braulense new genus and species. Two new subfamilies are proposed: Praenuculinae and Concavodontinae. The genera Praenucula Pfab, Cuyopsis new genus, Villicumia new genus, and Trigonoconcha new genus are included in the new subfamily Praenuculinae. The genera Concavodonta Babin and Melou, Hemiconcavodonta new genus, and Emiliania new genus are included in the new subfamily Concavodontinae.

Biostratigraphy of the Currarie Formation on the northwestern edge of the Southern Uplands: implications for the cessation of basic volcanism

Synopsis The Currarie Formation forms part of the deep water succession near the northern margin of the Southern Uplands. It comprises red and green mudstones (distal turbidites), chert and interbedded lava and lava breccia and overlies basalt lavas. The age of the Currarie Formation is critical in providing a minimum age for the end of basic volcanism in the northernmost part of the Southern Uplands and a maximum age for the onset of northerly derived turbidite flysch. It also allows biostratigraphical correlations to be made across the Southern Upland Fault. Graptolites indicative of the lower Nemagraptus gracilis Biozone and the first record of the conodont species Protopanderodus varicostatus from the formation indicate an earliest Caradoc (early Aurelucian) age for the Currarie Formation. This dates the cessation of basic volcanism in the northernmost part of the Southern Uplands as earliest Caradoc. However, the waning of volcanism may have been diachronous, continuing into the upper TV. gracilis Biozone in the area between the Southern Upland–Glen App and Carcow faults in a succession thought to have been developed further to the south than the Currarie Formation. The onset of coarse turbidite deposition followed within the same graptolite zone marking the onset of a major phase in uplift of the Midland Valley terrane and subsidence in the Southern Uplands basin.

Chitinozoan biostratigraphy of the early Caradocian Lockne impact structure, Jämtland, Sweden

Abstract— The chitinozoan biostratigraphy in seven outcrops and four drilling cores in connection with the Lockne impact structure has been investigated. The impact event took place in early Caradoc (i.e., ∼460.4 Ma ago) and in beds corresponding to the lower part of the Lagenochitina dalbyensis Zone (upper Dalby Limestone). The contact between the impact‐related rocks and the secular postimpact sediments can be traced all over the impact structure, and up to a distance of 50 km away from the presumed crater center. The youngest postimpact sediments in the Lockne impact structure correspond to the lower Örå Shale (Belonechitina hirsuta Zone).

Late Ordovician and early Silurian Trimerellide brachiopods from Kazakhstan

All previously known late Ordovician and Silurian trimerellides of Kazakhstan are reviewed and reillustrated, together with some recently discovered taxa, including the late Caradoc-Ashgill Palaeotrimerella medojevi new species from the Chu-Ili Range, the late middle Ashgill Gasconsia cf. worsleyi Hanken and Harper, and Silurian Dinobolus cf. hubeiensis Rong and Yang, both from Central Kazakhstan. Kazakhstanian terranes appear to contain the oldest-known trimerellide faunas from late Llandeilo to early Caradoc. The observed geographic distribution of Ordovician trimerellides may be explained as the result of two major migrations from the Kazakhstanian terranes: 1) during the early Caradoc, to Gondwana and North America; 2) during the late Caradoc, to Baltica and Angaria. All local trimerellide lineages in Kazakhstan became extinct just before the Hirnantian, and were replaced in the Llandovery by new forms, probably of Chinese origin.

Ontogeny of the proetoid trilobiteStenoblepharum,and relationships of a new species from the Upper Ordovician of Argentina

Silicified material from the Early Caradoc part of the Las Aguaditas Formation in San Juan Province, Argentina, includes a nearly complete growth series for a new species of the tropidocoryphidStenoblepharumOwens, 1973. Cladistic analysis ofStenoblepharumspecies indicates thatS. astiniinew species is most closely allied to the Early CaradocS. strasburgense(Cooper, 1953) from Virginia. Chinese species ofStenoblepharumare sister group to a Baltic/Laurentian clade. A single adult-like protaspid stage occurs in the life cycle ofS. astinii,closely resembling the protaspis ofDecoroproetus.It is preceded by a non-adult-like first protaspid instar that appears to be characteristic of Proetoidea in general but contrasts markedly with the early larval stages of other taxa in Proetida.

Precise chitinozoan dating of Ordovician impact events in Baltoscandia

Abstract. The chitinozoan biostratigraphy of four Ordovician impact craters has been investigated. Three of these (Tvären, Kärdla and Lockne) contain complete sequences of early Caradoc age, while the Granby crater contains rocks of late Arenig age yielding two bentonitic horizons at their top. Chitinozoans, together with graptolites and other planktic organisms, were the first to invade the craters after the impact event. It has therefore been possible to date the impact events with a precision of less than one million years. An immigration of graptolites from Australia during the late Arenig corresponds to an immigration of chitinozoans from Gondwana at this time. Two stratigraphically important taxa, not previously described or discussed, Lagenochitina sp. A aff. capax and Laufeldochitina sp. A aff. striata are commented upon. Three species. Cyathochitina hunderumensis, Spinachitina tvaerenensis and Tartuchitina granbyensis, are described as new.

The Llandeilo (Ordovician) series in the core of the Tywi Anticline, Llanwrtyd, Powys, UK

The Llandeilo and Caradoc volcanic rocks that form an inlier near Llanwrtyd, south-west of Builth, Powys, consist of acidic tuffs and basaltic rocks. They are interbedded with marine mudstones deposited in conditions of low oxygenation, and turbidites, slumps and debris flows. The cored Gilfach Farm No. 1 Borehole shows that this igneous activity is of Llandeilo age, extending from the teretiusculus Biozone to the gracilis Biozone. Evidence from exposures suggests that this igneous activity ceased at about the end of gracilis Biozone times (early Caradoc) and was probably extrusive; but evidence from the core, especially the presence of peperites towards its base, also indicates intrusion into wet sediments. Evidence is given for an early Caradoc basin-wide sedimentary event.

Evolution of the Northern Belt, Southern Uplands: implications for the Southern Uplands controversy

Conodont and graptolite dating provide a high resolution biostratigraphical framework for correlation across the Southern Upland Fault. The establishment of an early Caradoc age for the red chert and volcanic sequences within the Northern Belt of the Southern Uplands confounds the accretionary prism model. and our data support the view that following the obduction of the Ballantrae ophiolite, fan complexes prograded southeastwards across a narrow, fault-bounded shelf into the deep basin of the Northern Belt. The fan complexes were initiated by uplift in the hinterland, caused by the intrusion of plutons into the Midland Valley Terrane. Sedimentation was controlled by regional extension, subsidence and eustasy through the Llanvirn and Caradoc. Basic lavas, with attenuated within-plate geochemical signatures, were erupted during the early Caradoc peak of extension and may have just post-dated the initiation of southwards subduction beneath the Lake District.