Late Arenig Research Articles

Reply to Discussion on ‘The sole of an ophiolite: the Ordovician Bay of Islands Complex, Newfoundland’ Journal , 170, 2013, pp. 715–722

Zagorevski & van Staal (2014) question our model (Dewey & Casey 2013) in which we invoked forearc suprasubduction zone (SSZ) spreading to form the Bay of Islands Ophiolite Complex (BOIC) as part of the Notre Dame arc of the Appalachian–Caledonian Orogen. We interpret the formation of the BOIC to have occurred by spreading proximal to a ridge–trench–trench (R–TR–TR) triple junction. Our model explains (1) the formation of the BOIC within 15–20 myr of obduction, (2) the associated subcretion of the metamorphic sole shortly after formation during subduction of the ocean lithosphere bordering Laurentia, (3) subsequent oblique ophiolite encroachment to the Laurentian passive margin above a west-facing subduction zone as part of the Notre Dame forearc–arc system, and (4) the final obduction with the high-angle palaeo-ridge geometry relative to the Laurentian margin, with the emplacement initiated in the Late Arenig (Dapingian) during the Taconic Orogeny. We respond to the statements and questions posed by Zagorevski and van Staal that contest our model, which we further clarify and develop. Zagorevski and van Staal conclude, mistakenly, that we stated that the formation of the c . 489–484 Ma BOIC was associated with ‘intra-oceanic subduction initiation’ by the conversion of a ‘ridge–transform boundary’ in the Early Ordovician. They imply that we proposed a typical mid-ocean ‘intra-oceanic’ spreading centre environment for the origin of the BOIC that is linked to subduction initiation on a transform. On the contrary, in our fig. 1, p. 716, we proposed that the BOIC formed by sea-floor spreading adjacent to a pre-existing older lithosphere (Karson & Dewey 1978; Casey & Dewey 1984; Cawood & Suhr 1992; Kurth-Velz et al . 2004), represented by the earlier Middle–Late Cambrian Coastal Complex (CC) and Lush’s Bight (LB) oceanic tract, that we interpret as SSZ forearc. The CC–LB …

A Yapeenian (late Arenig) graptolite fauna from the Ordovician rocks of the Charlestown Inlier, Co. Mayo, Ireland

A Yapeenian (late Arenig) graptolite fauna from the Ordovician rocks of the Charlestown Inlier, Co. Mayo, Ireland

A Yapeenian (late Arenig) graptolite fauna from the Ordovician rocks of the Charlestown Inlier, Co. Mayo, Ireland

A Yapeenian (late Arenig) graptolite fauna from the Ordovician rocks of the Charlestown Inlier, Co. Mayo, Ireland

Ophiolite obduction in the Quebec Appalachians, Canada — 40Ar/39Ar age constraints and evidence for syn-tectonic erosion and sedimentation1Geological Survey of Canada Contribution 20100430.2GEOTOP (Centre de recherche en géochimie et géodynamique) Contribution 2011-0002.3This article is one of a series of papers published in this CJES Special Issue: In honour of Ward Neale on the theme of Appalachian and Grenvillian geology.

Detailed field work conducted in the Dunnage zone of the Quebec Appalachians, is herein combined with 40Ar/39Ar dating on a series of ophiolitic massifs, crosscutting granites, and associated metamorphic rocks occurring along the Baie Verte–Brompton line, the Taconian suture between Laurentia and Lower Paleozoic peri-Laurentian oceanic terranes. Studied massifs are the Lac-Brompton ophiolite and the Rivière-des-Plante Ultramafic Complex in southern Quebec, and the Nadeau Ophiolitic Mélange in the Gaspé Peninsula. Our work suggests that these massifs form remnants of eroded ophiolitic nappes, which are unconformably overlain by the Saint-Daniel and Rivière-Port-Daniel mélanges, and correlate with the Thetford-Mines and Mont-Albert ophiolitic complexes. Our 40Ar/39Ar data and compiled regional age constraints indicate that ophiolite obduction was diachronous along the strike of the orogen. The timing of obduction and mélange formation varies according to the irregular geometry of the Early Paleozoic Laurentian margin, with earlier collision occurring along, or at the margins of promontories. Obduction was initiated with the formation of infraophiolitic metamorphic soles between ca. 479 and 472 Ma in southern Quebec and the Nadeau Ophiolitic Mélange, and possibly as late as ca. 470–466 Ma for the Mont-Albert Complex. These sole rocks were later exhumed and translated onto the Laurentian margin with the overlying ophiolites between 475 and 460–457 Ma. The uplifting and erosion of the orogenic wedge during the waning stages of obduction, has resulted in the sedimentation of olistostromal mélanges and onlapping flysch units above the ophiolitic nappes, as well as foredeep flysch successions during the latest Arenig(?) to earliest Caradoc.

The Middle Ordovician litho- and biostratigraphy of northwestern Gorny Altai

The structure of the Lower–Middle Ordovician Tuloi and Karasa Formations, which occur widely in northeastern Gorny Altai, has been studied. A bed-by-bed description of a new section of the boundary sediments between these stratigraphic units has been given. The graptolite, trilobite, and brachiopod assemblages typical of the Tuloi and Karasa Formations have been analyzed. Graptolite zones and subzones have been recognized, which permit correlating the stratotype and key sections of these local stratigraphic units with the stages of the International Stratigraphic Chart (ISC). The Karasa Formation occupies a considerably larger stratigraphic volume than presumed: It spans the late Arenig and the entire Llanvirn (including the Llandeilo) of the British standard, which is used in Russia (=Upper Dapingian–Darriwilian of the new Ordovician standard in the ISC). Finally, the paper discusses the suggested changes in the names of regional stratigraphic units (horizons) in the Ordovician stratigraphic chart of the western Altai–Sayan fold belt.

Trilobite biofacies and palaeogeographic development in the Arenig (Ordovician) of the Yangtze Block, China

Cluster analysis was conducted using Pearson's coefficient based on the relative abundance data of 73 trilobite genera and subgenera in 72 collections, and fourteen Arenig (Floian–early Darriwilian) trilobite biofacies are recognized from the inner shelf to deep outer shelf of the Yangtze Block. The biofacies patterns and faunal dynamics across the eight successive time intervals suggest that the progressive changes in generic composition, diversity, and dominant components may have been affected by bathymetric gradients and lithofacies transition along the palaeoramp of the block, and by temporal constraints on the different trilobite clades. On the whole, the total faunal diversity reached its peak in the shallow outer shelf, from which it decreased both seawards and shorewards. The benthic forms generally exhibited increased endemicity shorewards, but atheloptic taxa were confined largely to the outer shelf, with the generic diversity increased seawards, while the mesopelagic trilobites were associated mainly with the deep-outer-shelf biofacies. The progressive alternations of trilobite biofacies spectra indicate that continual transgressions may have played an important role in a consecutive palaeogeographic development in the Yangtze Block, including mainly the drowning of the huge platform in the early Arenig (early Floian) (TI 2), the shoreward expansion of the deep outer shelf (TI 5) and the ongoing shoreward retreats of the inner shelf (TIs 6–7) in the Yangtze Sea, and the successive deepening of the southern Shaanxi sea especially in the late Arenig (Dapingian–early Darriwilian) (TIs 5–8) since its taking shape in TI 3.

TRIARTHRINID TRILOBITES (OLENIDAE) FROM THE MIDDLE AND UPPER ORDOVICIAN, PRECORDILLERA OF ARGENTINA

Middle and Upper Ordovician Triarthrinae from the Precordillera of San Juan, Argentina, include the Whiterock/Llanvirn Porterfieldia turneri (Baldis and Pöthe, 1995) and P. acava new species, and the early Caradoc Triarthrus jachalensis (Harrington and Leanza, 1957). Each of these species is described based on silicified material, including ontogenies, from the Las Aguaditas Formation. Porterfieldia maanssonae new species from the lower member of the Gualcamayo Formation (late Arenig) at Río Gualcamayo, San Juan Province, and Río Guandacol, La Rioja Province, is closely allied to P. turneri. Two protaspid stages are present in Porterfieldia, whereas mineralized protaspid stages are apparently lacking in Triarthrus jachalensis. Triarthrus jachalensis was capable of sphaeroidal enrollment by meraspid degree 2. Precordilleran species bear closest comparison to others from Spitsbergen (Porterfieldia acava n. sp. and P. parapunctata) and Sweden/Norway (Triarthrus jachalensis and T. linnarssoni).

Reply to discussion Reply to discussion by Maletz on: Stone, P. & Rushton, A.W.A.2003. A late Arenig (early Yapeenian) graptolite fauna and the coeval tectonic development of the Ballantrae Complex ophiolite, SW Scotland. Scottish Journal of Geology, 39, 29–40.

Reply to discussion by Maletz on: P. Stone & A.W.A. Rushton 2003. A late Arenig (early Yapeenian) graptolite fauna and the coeval tectonic development of the Ballantrae Complex ophiolite, SW Scotland. Scottish Journal of Geology, 39, 29–40. We thank Dr Maletz for his comments on the graptolite fauna from the North Ballaird Borehole. In identifying the horizon as Yapeenian, we relied partly on the identification of Yutagraptusl v-deflexus. Williams & Stevens thought that their X. declinatus was, like Y.? v-deflexus, a Yapeenian fossil; they, like us, laid more weight on the presence of a supposed index for that unit than on the absence of other such local indices as Oncograptus. We agree that, if one accepts that Xiphograptus declinatus is the same as Yutagraptus? v-defiexus and that both occur in the immediately underlying Castlemainian 4 interval, our case for the presence of a Yapeenian fauna is weakened. We are grateful to him for drawing our attention to Pseudisograptus initialis and accept the suggestion that the specimens we identified as Arienigraptus gracilis (our fig. 4) are better referred …

Early Ordovician bivalves from Dali, West Yunnan, China

A bivalve fauna of Early Ordovician (late Arenig) age is described from the Hsiangyang Formation of the eastern part of West Yunnan, China. The fauna contains elements in common with Early Ordovician faunas of southern Gondwanan areas such as the Montagne Noire and Morocco and with those of Avalonia, together with several previously undescribed taxa. The following taxa are new: Biseriodonta simplex gen. et sp. nov.; Glyptarca sinensis sp. nov.; Trigonoglyptarca magna gen. et sp. nov.; Erhaiconcha xiangyangensis gen. et sp. nov.; Fasciculodonta impressa gen. et sp. nov.; Yunnanoredonia laevis gen. et sp. nov.; Daliella gen. nov.; Goniophorina (Goniophorina) contracta sp. nov.; Haidongoconcha radialis gen. et sp. nov.; Eopterinea aequiconcha gen. et sp. nov. The bivalve fauna includes the most diverse glyptarcid fauna and the earliest nuculanid and paracyclid known hitherto. Eastern West Yunnan belonged to the Indochina terrane and it is concluded that the bivalve fauna represents a high‐latitude assemblage and that the Indochina terrane should be considered a component part of the Peri‐Gondwanan continent in the Ordovician; its bivalve faunas contrast with those of neighbouring terranes which have affinities with those of lower latitudes.

Palaeobiography and Biodiversity Change: the Ordovician and Mesozoic-Cenozoic Radiations

J. Alistair Crame and Alan W. Owen, 2002, Geological Society of London Special Publication 194, Geological Society, London, 206 p. (Hardcover, US $108.00) ISBN: 1-86239-106-8. This book resulted from the 2001 Lyell Meeting where Alistair Crame and Alan Owen brought together two groups of paleontologists whose interests are so widely separated in geologic time that they would seldom rub elbows. As study of the great radiations of life begins to compete with the extinction debates as a major focus, paleontology will lose some of the attention it has received because of the appeal of catastrophes to the general public. However, the future will be no less contentious within the paleontological community. Even sorting out what radiations are and how they proceeded will take some time. This book takes a look at two radiations—a straightforward task at first sight, but the radiations are so different that comparisons are no easy matter. The Ordovician Radiation was abrupt, taking place, for the most part, in the Arenig, with a sudden pulse in the Late Arenig at about the base of the Laurentian Whiterock. In terms of biogeography the primary phase of the Ordovician radiation took place in such a short interval that the continental positions were essentially stable. The Mesozoic–Cenozoic radiation took place during such a long interval that plate motions provided a means of distribution and interrupted pathways of dissemination. Entire climatic regimes changed. Even the initiation of the radiations differed. As yet no unusual biotic event has been recognized preceding the Ordovician radiation (although one is suggested here) while the largest mass extinction in earth history set the stage for the Mesozoic radiation. Most contributions are detailed examinations of specific biotic changes that took place during one or the other of the radiations, with little space devoted to comparisons of groups …

Provenance analysis and tectonic setting of the Ordovician clastic deposits in the southern Puna Basin, NW Argentina

AbstractProvenance studies on Early to Middle Ordovician clastic formations of the southern Puna basin in north‐western Argentina indicate that the sedimentary detritus is generally composed of reworked crustal material. Tremadoc quartz‐rich turbidites (Tolar Chico Formation, mean composition Qt89 F7 L4) are followed by volcaniclastic rocks and greywackes (Tolillar Formation, mean Qt33 F42 L25). These are in turn overlain by volcaniclastic deposits (mean Qt24 F30 L46) of the Diablo Formation (late Arenig–early Llanvirn) that are intercalated by lava flows. All units were deformed in the Oclóyic Orogeny during the Middle and Late Ordovician. Sandstones of the Tolar Chico Formation are characterized by Th/Sc ratios > 1, La/Sc ratios ≈ 10, whereas associated fine‐grained wackes show slightly lower values for both ratios. LREE (light rare earth elements) enrichment of the arenites is ≈ 50× chondrite, Eu/Eu* values are between 0·72 and 0·92, and flat HREE (heavy rare earth elements) patterns indicate a derivation from mostly felsic rocks of typical upper crustal composition. The εNd(t = sed) values scatter around −11 to −9. The calculated Nd‐TDM residence ages vary between 1·8 and 2·0 Ga indicating contribution by a Palaeoproterozoic crustal component. The Th/Sc and La/Sc ratios of the Tolillar Formation are lower than those of the Tolar Chico Formation. Normalized REE (rare earth elements) patterns display a similar shape to PAAS (post‐Archaean average Australian shale) but with higher abundances of HREEs. Eu/Eu* values range between 0·44 and 1·17, where the higher values reflect the abundance of plagioclase and feldspar‐bearing volcanic lithoclasts. Average εNd(t = sed) values are less negative at −5·1, and Nd‐TDM are lower at 1·6 Ga. This is consistent with characteristics of regional rocks of upper continental crust composition, which most probably represent the sources of the studied detritus. The rocks of the Diablo Formation have the lowest Th/Sc and La/Sc ratios, lower LREE abundances than the average continental crust and are slightly enriched in HREEs. Eu/Eu* values are between 0·63 and 1·17. The Nd isotopes (εNd(t = sed) = −3 to −1; TDM = 1·2 Ga) indicate that one source component was less fractionated than both the underlying Early Ordovician and the overlying Middle Ordovician units. Synsedimentary vulcanites in the Diablo Formation show the same isotopic composition. Our data indicate that the sedimentary detritus is generally composed of reworked crustal material, but that the Diablo Formation appears to contain ≈ 80% of a less fractionated component, derived from a contemporaneous continental volcanic arc. There are no data indicating an exotic detrital source or the accretion of an exotic block at this part of the Gondwana margin during the Ordovician.

A Late Arenig (early Yapeenian) graptolite fauna and the coeval tectonic development of the Ballantrae Complex ophiolite, SW Scotland

Synopsis A Late Arenig graptolite fauna is known from one isolated occurrence within the Ballantrae Complex ophiolite; a borehole core taken near North Ballaird farm. Reassessment of that fauna shows it to be of early Yapeenian (Yal) (just possibly late Castlemainian; Ca4) age and confirms that it is significantly younger than all other dated components of the complex. Of particular importance are the records of Arienigraptus gracilis (Ruedemann), Isograptus australis Cooper and Yutagraptus ? v-deflexus (Harris). The graptolitic sequence conformably underlies a microconglomerate bed in which the clasts are coated by algal encrustation. Some of those clasts are of serpentinite, which was therefore clearly available for erosion into shallow water during the early Yapeenian stage. It seems most likely that the serpentinite was structurally elevated during obduction of the ophiolite, a process driven by arc-continent collision. On a broad, regional scale this can be associated with the collisional event initiating the Grampian Orogeny, for the timing of which the early Yapeenian age of the North Ballaird graptolite fauna provides an additional control.

MIDDLE ORDOVICIAN LITHISTID SPONGES FROM THE BACHU-KALPIN AREA, XINJIANG, NORTHWESTERN CHINA

Moderately diverse Middle Ordovician sponge faunas of Darriwilian (late Arenig to early Llanvirn) age have been discovered in outcrops in the Bachu-Kalpin area of the Tarim platform of Xinjiang, China. These are the first reported occurrences of early Paleozoic sponges from Xinjiang, China. These sponges occur in the Yijianfang and Dawangou Formations, which are of nearly the same age, but crop out in different areas of the platform margin. Fossil sponges discovered to date are the anthaspidellid lithistid sponges Archaeoscyphia minganensis (Billings, 1859); Hudsonospongia cyclostoma Raymond and Okulitch, 1940; Calycocoelia profunda sichuanensis Deng, 1990; Calycocoelia sp.; Pseudopalmatohindia sp.; Aulocopium sp.; Annulospongia tarimensis n. gen. and sp.; Rhopalocoelia kalpinensis n. sp.; Rhopalocoelia bachuensis n. sp.; and Zittelella xinjiangensis n. sp. They are preserved in reefal limestones and are associated with Calathium, cyanobacteria, and bryozoans, like in many North American assemblages.

Middle Ordovician lithistid sponges from the Bachu-Kalpin area, Xinjiang, northwestern China

Moderately diverse Middle Ordovician sponge faunas of Darriwilian (late Arenig to early Llanvirn) age have been discovered in outcrops in the Bachu-Kalpin area of the Tarim platform of Xinjiang, China. These are the first reported occurrences of early Paleozoic sponges from Xinjiang, China. These sponges occur in the Yijianfang and Dawangou Formations, which are of nearly the same age, but crop out in different areas of the platform margin. Fossil sponges discovered to date are the anthaspidellid lithistid sponges Archaeoscyphia minganensis (Billings, 1859); Hudsonospongia cyclostoma Raymond and Okulitch, 1940; Calycocoelia profunda sichuanensis Deng, 1990; Calycocoelia sp.; Pseudopalmatohindia sp.; Aulocopium sp.; Annulospongia tarimensis n. gen. and sp.; Rhopalocoelia kalpinensis n. sp.; Rhopalocoelia bachuensis n. sp.; and Zittelella xinjiangensis n. sp. They are preserved in reefal limestones and are associated with Calathium, cyanobacteria, and bryozoans, like in many North American assemblages.

The Tornquist Sea and Baltica–Avalonia docking

Early Ordovician (Late Arenig) limestones from the SW margin of Baltica (Scania–Bornholm) have multicomponent magnetic signatures, but high unblocking components predating folding, and the corresponding palaeomagnetic pole (latitude=19°N, longitude=051°E) compares well with Arenig reference poles from Baltica. Collectively, the Arenig poles demonstrate a midsoutherly latitudinal position for Baltica, then separated from Avalonia by the Tornquist Sea. Tornquist Sea closure and the Baltica–Avalonia convergence history are evidenced from faunal mixing and increased resemblance in palaeomagnetically determined palaeolatitudes for Avalonia and Baltica during the Mid-Late Ordovician. By the Caradoc, Avalonia had drifted to palaeolatitudes compatible with those of SW Baltica, and subduction beneath Eastern Avalonia was taking place. We propose that explosive vents associated with this subduction and related to Andean-type magmatism in Avalonia were the source for the gigantic Mid-Caradoc (c. 455 Ma) ash fall in Baltica (i.e. the Kinnekulle bentonite). Avalonia was located south of the subtropical high during most of the Ordovician, and this would have provided an optimum palaeoposition to supply Baltica with large ash falls governed by westerly winds. In Scania, we observe a persistent palaeomagnetic overprint of Late Ordovician (Ashgill) age (pole: latitude=4°S, longitude=012°E). The remagnetisation was probably spurred by tectonic-derived fluids since burial alone is inadequate to explain this remagnetisation event. This is the first record of a Late Ordovician event in Scania, but it is comparable with the Shelveian event in Avalonia, low-grade metamorphism in the North Sea basement of NE Germany (440–450 Ma), and sheds new light on the Baltica–Avalonia docking.

Assessing trilobite biodiversity change in the Ordovician of the British Isles

AbstractThe Ordovician Period saw the most sustained, steep rise in marine biodiversity in the history of life on Earth and set the ecological pattern for the rest of the Palaeozoic Era. The long history of research, wide variety of depositional settings and juxtaposition of terranes with very different palaeobiogeographical histories makes the Ordovician of the British Isles an excellent laboratory in which to study this change. A database approach to analysis of trilobite biodiversity change is described using a simple compilation of stratigraphical ranges of 663 species of Avalonian trilobites, and a much more sophisticated information‐rich relational database based on the fundamental information of the fossil record–species at localities. The latter includes 2001 occurrence records of 617 trilobite species at 508 localities in the Welsh Basin. The first ever species‐level diversity curves for the whole Ordovician of an entire region are presented. Both databases reveal an overall increase in species‐ and genus‐level diversity through the Ordovician. Random resampling tests and environmental information are used to remove sampling effects from the Welsh Basin diversity curves. The main features of trilobite biodiversity change in Avalonia are the following: a late Arenig–early Llanvirn increase, during which time the highest species‐to‐genus ratios occur, is contemporaneous with a rise in global diversity and rifting of Avalonia from Gondwana; a late Abereiddian dip is followed by recovery during the Llandeilian–early Caradoc; decline during the late Caradoc–early Ashgill is at least partly attributable to lack of preserved rock and restriction of most of the preserved faunas to deep‐water environments. The greatest diversity occurs in the palaeoenvironmentally differentiated Rawtheyan and is followed by the diversity crash seen in the Hirnantian throughout the world. Copyright © 2001 John Wiley & Sons, Ltd.

Reconstruction of the Grampian episode in Scotland: its place in the Caledonian Orogeny

A study of the composition of detrital garnets from Ordovician siliciclastics from the Midland Valley and Southern Uplands terranes of Scotland reveals characteristics of the metamorphic sources very similar to the Grampian terrane plus a ‘lost’ blueschist–eclogite terrane. The radiometric ages of detrital muscovite from the Southern Uplands overlaps the ages of the metamorphic muscovite from the Grampian terrane. The depositional age of the earliest Midland Valley detritus is Llanvirn; in the Southern Uplands it is Caradoc. These observations support the hypothesis that flysch was formed when the Grampian terrane was suddenly uplifted into mountains which were immediately eroded into neighboring basins and trenches. Thus the Grampian, Midland Valley and Southern Uplands terranes were not exotic to each other in the Upper Ordovician. Therefore the main compressional orogeny (i.e. Peri-Laurentian island arc versus Laurentian continental margin collision) in the Grampian terrane in Scotland was post-Cambrian and began in the late Arenig. New radiometric age dates for Grampian terrane syn-metamorphic granites and gabbros and post-metamorphic granites in Scotland and Ireland plus mineral cooling ages support the case for a relatively short <15Ma catastrophic arc-continent collisional orogeny between ∼480 and ∼465Ma. Thus the Caledonian Orogeny is an example of one which was episodic with a short catastrophic collisional tectonometamorphic and magmatic (Grampian) episode followed by a long period of isostatic adjustment, decompression melting, erosion, subduction and batholith formation that lasted 70Ma. Orogeny stopped when Avalonia docked (i.e. the Scottish Scandian episode).

A new assemblage of graptolites, rhabdopleuran hemichordates and chitinous hydroids from the late Arenig (Ordovician) of the Banestan area, east-central Iran

A rich benthic and planktonic graptolite fauna is associated with encrusting rhabdopleuran hemichordates and chitinous hydrozoans in the late Arenig (Ordovician) part of the Katkoyeh Formation at Banestan, east-central Iran. New taxa described here are the dendroid graptolites Callograptus huckriedei sp. nov. and Dictyonema bitubulata sp. nov. Other dendroid graptolites Dendrograptus sp. cf. D. flexuosus J. Hall, 1865; Acanthograptus divergens Skevington, 1963; and Thallograptus ?succulentus Ruedemann, 1904 are described. The tuboid graptolite Galeograptus sp., the rhabdopleuran hemichordates Rhabdopleura sp. aff. R. primaevus (Kozlowski, 1967) and Kystodendron sp., and the chitinous hydroid Palaeotuba sp. are also described. Graptoloid graptolites described here are Aulograptus? sp., Didymograptus incertus Perner, 1895, Undulograptus formosus (Mu & Lee, 1958), and Yutagraptus sp. cf. Y. mantuanus Riva, 1994. All but two species-level taxa are described from Iran for the first time. The fauna is confirmed as being late Arenig or, less likely, early Llanvirn and thus probably correlates with the early part of the Darriwilian stage. It resembles the Atlantic cold water faunas in lacking isograptids and sinograptids.

Lower Palaeozoic stratigraphy and palaeontology, Karadere–Zirze area, Pontus Mountains, northern Turkey

Lower Palaeozoic rocks in the Karadere-Zirze area, east of Safranbolu (Pontides, northern Turkey), range from Early Ordovician to Silurian. Overlying the probably Tremadoc Bakacak Formation are Aydos Formation quartzites, followed conformably by the Karadere Formation, dated as Early Arenig to Early Llanvirn by means of graptolites which are assigned to seventeen genera and include three new forms: Eoglyptograptus bouceki, Prolasiograptus haplus praecursor and Undulograptus? mui. Late Arenig trilobites from the Karadere Formation include Bergamia, Cyclopyge, Dionidella?, Leioshumardia and Seleneceme. In the Limestone Member of the overlying Ketencikdere Formation, uncommon trilobites suggest only a mid- to late Ordovician age, but conodonts with Colour Alteration Index 5–6 indicate the Amorphognathus tvaerensis Biozone (early Caradoc). Macrofossils are rare in the Siltstone Member, but conodonts from the middle of the unit suggest the highest subzone of the A. tvaerensis Biozone; the youngest visible strata are, on acritarch evidence, at least as high as Caradoc, but the Ashgill is not confirmed and the contact with overlying Silurian rocks is unexposed. The Findikli Formation comprises: a Lower Member, black argillites with Llandovery graptolites and acritarchs; and an Upper Member, grey shales with late Wenlock graptolites, overlain unconformably by Devonian rocks. The succession differs significantly from contemporaneous deposits in southern Turkey and its affinities lie with western Europe, including the Welsh Basin.

Palaeoenvironmental interpretation of microphytoplankton diversity trends in the Cambrian–Ordovician of the northern Sahara Platform

Analysis of vertical fluctuations in assemblage diversity and relative abundances of microphytoplankton morphological classes permits palaeoenvironmental reconstructions of the Cambrian–Ordovician stratal sequences in the Hassi-R'Mel area (north-central Algeria) and northern Rhadames Basin (southern Tunisia). Abundance fluctuations of acritarch morphological classes and assemblage diversity appear to be related to changes in depositional facies. The present palaeoenvironmental interpretation is primarily based on a comparison with microphytoplankton distributional trends in modern depositional environments. Where possible, the observations are tested against palaeoenvironmental information derived independently from previous lithostratigraphical, macropalaeontological, and petrographical studies of the formations investigated. The present data support the following palaeoenvironmental scenario: during Late Cambrian times, relatively shallow water, neritic conditions prevailed throughout the study area, with evidence for a more proximal environment in southern Tunisia. In the early Tremadoc, open marine, offshore conditions characterized the central Algerian region, but a shallower water setting is indicated for southern Tunisia. During the late Arenig–Llanvirn interval, an open marine, offshore setting is envisaged for the southern Tunisian region, with more proximal conditions prevailing in the Algerian Hassi-R'Mel area. During the entire Ashgill, an essentially restricted marine environment was established throughout the study area. Some stratigraphically important acritarch species (e.g. Acanthodiacrodium angustum) appear to be facies-sensitive; the recognition of palaeoecological control on microphytoplankton distribution has important consequences on the biostratigraphic application of these microfossils.