The palaeobiogeographical spread of the acritarch Veryhachium in the Early and Middle Ordovician and its impact on biostratigraphical applications

The genus Veryhachium Deunff, 1954, is one of the most frequently documented acritarch genera, being recorded from the Early Ordovician to the Neogene. Detailed investigations show that Veryhachium species first appeared near the South Pole in the earliest part of the Tremadocian (Early Ordovician). The genus was present at high palaeolatitudes (generally>60° S) on the Gondwanan margin during the Tremadocian before spreading to lower palaeolatitudes on the Gondwanan margin and other palaeocontinents (Avalonia and Baltica) during the Floian. It became cosmopolitan in the Middle and Late Ordovician. Although useful for distinguishing Ordovician from Cambrian strata, the diachronous first appearance data of Veryhachium morphotypes mean that they should be used with caution for long-distance correlation

Middle Ordovician acritarchs and problematic organic-walled microfossils from the Saq-Hanadir transitional beds in the QSIM-801 well, Saudi Arabia

Core samples from the QSIM-801 water well, drilled in central Saudi Arabia, cover a 93-foot interval spanning the transition between the Sajir Member of the Saq Formation, that consists mainly of sandstones of tidal sand flat environments, and the Hanadir Member of the Qasim Formation, characterized by argillaceous graptolitic mudstones, corresponding to a tidal delta front. The samples contain abundant, exceptionally well-preserved and diverse palynomorphs, which include cryptospores, acritarchs and chitinozoans, other problematic organic-walled microfossils as well as other organic particles such as cuticle-like fragments. The studied interval is biostratigraphically well constrained by the presence of chitinozoans of the formosa and pissotensis Zones of late-early to late Darriwilian age (Middle Ordovician) in the uppermost Saq Formation and Hanadir Member. The biostratigraphic age of the Sajir Member considered to span the Dapingian–Darriwilian boundary, is re-discussed based on the results herein. The uppermost part of the Sajir Member yielded the ichnofossil, Phycodes fusiforme. Acritarch assemblages from the Sajir Member of the Saq Formation are poorly diversified and dominated by sphaeromorphs. More diverse assemblages of acritarchs, associated with enigmatic forms, occur in the Hanadir Member of the Qasim Formation. The contact between the two formations and the transition between the palynomorph assemblages are sharp, suggesting a stratigraphic hiatus. A quantitative analysis allows us to discuss the paleoenvironmental changes and possibly climatic changes associated with an hypothesis of ice house conditions during this period. Among the diagnostic acritarch taxa observed are Frankea breviuscula, F. longiuscula, Baltisphaeridium ternatum, Dasydorus cirritus, Dicrodiacrodium ancoriforme, Poikilofusa ciliaris, Pterospermopsis colbathii and Uncinisphaera fusticula. These are associated with other typical forms known to range across the Lower–Middle Ordovician boundary, such as Aremoricanium rigaudae, Aureotesta clathrata, Barakella fortunata, B. rara, Baltisphaeridium klabavense, Glaucotesta latiramosa and Striatotheca spp. Galeate and peteinoid acritarchs are also well represented, as well as tiny forms of ultraplanctonic size. Three new species of acritarchs are proposed: Frankea longiuscula var. darriwilense var. nov, Micrhystridium regulum sp. nov, and Tyrannus proteus sp. nov. Repeated occurrences throughout the section of cryptospores, problematic microfossils such as organic filaments, cuticle-like tissues, striated and pigmented leiospheres frequently in clusters, are interpreted to reflect recurrent terrestrial and freshwater inputs in the depositional environment. Single-specimen, high-resolution analyses using Confocal Laser Scanning Microscopy on the enigmatic form Tyrannus proteus sp. nov. show fluorescence emission spectra and microstructural properties significantly different from those of typical marine acritarchs from the same levels

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Fossil microphytoplankton dynamics across the Ordovician-Silurian boundary

A critical review of all available data on acritarch biostratigraphy and diversity dynamics across the Late Ordovician through the early Silurian, permits a better appreciation of the potential of acritarchs for the recognition of the systemic boundary. This analysis also reveals the response of marine microphytoplankton populations to the Late Ordovician palaeoenvironmental crisis (Hirnantian glaciation). Previous zonal schemes are improved, and an update acritarch biostratigraphic chart is proposed, plotted against the most recent chronostratigraphic subdivisions. Sections from Anticosti Island (Québec, Canada), Algeria, Morocco, and Estonia preserve the best palynological record for the investigated interval. The present analysis shows that no true mass-extinction event occurred in latest Ordovician times in connection with the well known glacial event. "Pre-glacial" Ashgill acritarch suites are dominated by species of Baltisphaeridium, Multiplicisphaeridium, Ordovicidium, Orthosphaeridium, and netromorph acritarchs. An important proportion of these taxa (excluding Ordovicidium and Orthosphaeridium) survive the onset of glacial conditions in Hirnantian (latest Ordovician) times and continues through the early Silurian. The development of morphological polymorphism appears as a response (a survival strategy?) to the establishment of glacial conditions. In glacial-related sediments of Hirnantian age in North Africa, acritarch assemblages display a burst of relative abundance and intra-specific morphological variability (polymorphism) of long-ranging taxa such as Veryhachium, Multiplicisphaeridium, Dactylofusa, Poikilofusa, and Evittia. The extinction of several species characteristic of Upper to uppermont Ordovician strata occurs near the boundary, in "post-glacial" Ashgill (uppermost Hirnantian). This extinction event is counterbalanced by the almost contemporaneous (within the limits of stratigraphic resolution) appearance of several new taxa showing already a clear "Silurian affinity", e.g., Tylotopalla, Cymbosphaeridium, and Visbysphaera. This origination event is observable in, and correlatable between the North African, the Bohemian and the Anticosti sections, making it global in extent. The completion of the palynological turnover and the establishment of a diverse Silurian acritarch suite occurs well above the base of the Silurian, during Aeronian times. The strong survival capability of the oceanic plankton through periods of palaeoenvironmental crisis in latest Ordovician times (but also throughout the Phanerozoic) could have played an important role in the post-extinction rebounds of metazoan clades, by assuring the continuity of marine trophic resources to consumers and avoiding irreversible disruptions of the trophic chains

New sedimentological and biostratigraphic data in the Kwahu Group (Meso- to Neo-Proterozoic), southern margin of the Volta Basin, Ghana: Stratigraphic constraints and implications on regional lithostratigraphic correlations

Since several decades, geologists disagree about the lithostratigraphic subdivision of the Volta Basin sedimentary infilling. Correlations at the scale of the Volta Basin were up to now largely limited by the discontinuity of outcrops along the basin margin and by the scarcity of stratigraphic constraints within the sedimentary succession. Even if a subdivision of the Volta Basin infilling into three main groups (Bombouaka, Oti, and Obosum Groups) is now generally accepted, there is no agreement for lithostratigraphic subdivision at the formation scale. The Bombouaka Group, which represents first deposits of the Volta Basin sedimentary infilling, is particularly concerned. During mapping project in Ghana, authors proceeded to a sedimentological study of the Kwahu Group deposits on the Kwahu Plateau area. Some samples collected during field mapping were analyzed on a palynological point of view and revealed several layers rich in palynomorphs (acritarchs). Siliciclastic sediments composing the Kwahu Group are distinguished in 12 main facies associations, which are interpreted to be representative of both marine and continental environments. Sedimentological study of these facies associations leads to identify a cyclic vertical evolution of depositional settings at different scales, which are interpreted as sequences. The Kwahu Group is thus composed of three distinct macrosequences (about 300-500 m thick) interpreted as deltaic progradational sequences. Each macrosequence has been used to define formations for mapping, sequence stratigraphy key surfaces being considered as limits for cartographic units. The acritarch content of the analyzed samples allows rather coarse but nonetheless useful biostratigraphic constraints which, in association with data from sedimentological and sequential analysis, are used to improve correlations into the Bombouaka and Kwahu Groups at the scale of the Volta basin. The present data complemented with the few recently published studies on Voltaian deposits, lead to a new regional lithostratigraphic synthesis of the Bombouaka and Kwahu Groups. based on stratigraphic constrains and sequence stratigraphy. (C) 2011 Elsevier B.V. All rights reserved

Reworked acritarchs as provenance indicators in the Lower Palaeozoic of Denmark

Profuse and well-preserved acritarchs were recovered from subsurface Lower Palaeozoic successions cored by the boreholes Slagelse-1 and Pernille-1 (Danish-North German Basin). Together with Llandovery in situ microphytoplankton, reworked Cambrian and Ordovician species occur. The reworked Ordovician acritarchs show a clear Perigondwanan palaeobiogeographic affinity and indicate elastic sedimentary input from a Perigondwanan-related terrane located south of the East European Platform. Microfloral similarity enables identification of the detrital source area with the Avalonia Terrane. The present data also suggest that development of a foreland basin marginal to the Caledonian Deformation Front in the Danish-North German Basin started in Early Silurian times. (C) 2001 Academie des sciences / Editions scientifiques et medicales Elsevier SAS

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 Rugen area, and the coeval domains of the East European Platform (EEP), corroborating the idea that the subsurface of Rugen should be considered palaeogeographically as the eastern extension of Avalonia. Cluster analysis of chitinozoan assemblages from numerous wells in the Rugen 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 Rugen 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 Rugen subsurface and from coeval Perigondwana localities (e.g. Tunisia). Since the early Caradoc, this Gondwanan affinity of the Rugen microfossils starts to lessen, and becomes negligible during the late Caradoc. During latest Caradoc-early Ashgilll 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 Rugen 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 Rugen. 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. (C) 2001 Elsevier Science B.V. All rights reserved