First Appearance Datums (FADs) of selected acritarch taxa and correlation between Lower and Middle Ordovician stages

First Appearance Datums (FADs) of selected, easily recognizable acritarch morphotypes are assessed to determine their potential contribution to correlation between Lower and Middle Ordovician stages and substage divisions along the Gondwanan margin (Perigondwana) and between Perigondwana and other palaeocontinents. The FADs for 19 genera, species and species groups are recorded throughout their biogeographical ranges. The taxa investigated fall into three groups. Some have FADs at about the same level throughout their biogeographical ranges and are useful for long‐distance and intercontinental correlation. Among these are Coryphidium, Dactylofusa velifera, Peteinosphaeridium and Rhopaliophora in the upper Tremadocian Stage; Arbusculidium filamentosum, Aureotesta clathrata simplex and Coryphidium bohemicum in the lower–middle Floian Stage; Dicrodiacrodium in the upper Floian Stage; Frankea in the Dapingian–lower Darriwilian stages; and Orthosphaeridium spp., with FADs in the Dapingian–lower Darriwilian stages of Perigondwanan regions and at about the same level in Baltica. Other taxa, however, have diachronous (or apparently diachronous) FADs, and this needs to be taken into account when using them for correlation. A second group of genera and species, comprising Striatotheca, the Veryhachium lairdii group and the V. trispinosum group, have a recurring pattern of FADs in the Tremadocian Stage on Avalonia and in South Gondwana and West Gondwana, but in the Floian Stage of South China and East Gondwana. The third group, consisting of Arkonia, Ampullula, Barakella, Dasydorus, Liliosphaeridium and Sacculidium, have FADs that are markedly diachronous throughout their biogeographical ranges, although the global FADs of Arkonia, Ampullula, Liliosphaeridium and Sacculidium are apparently in South China and/or East Gondwana. It is possible that diachronous FADs are only apparent and an artefact of sampling. Nevertheless, an alternative interpretation, suggested by recurring patterns, is that some as yet undetermined factor controlled a slower biogeographical spread over time, resulting in diachroneity

Structure de la paroi de Aremocanium Rigaudau Deunff, 1955 (acritarche, ordovicien)

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Présence de chitino-zoaires ordoviciens et dévoniens dans la boutonnière des Zekkara, région d'Oujda (Maroc oriental). Occurrence of Ordovician and Devonian chitinozoa in the Zekkara's inlier, Oujda area (Eastern Morocco)

Paleozoic rocks are represented from the Oujda area (NE Morocco). These strata, surrounded by Cenozoic deposits, occur in the Zekkara's inlier. The investigated section corresponds to a complex succession of shales and mudstones including several chaotic levels which have been interpreted as gravity settled deposits (olistostroms). The age of the whole sequence has long been debated. The aim of the present study, based on chitinozoa, is to improve recent data provided by acritarchs. A fairly closely spaced sampling is realised exclusively in the shales and siltstones in order to get an accurate distribution of the chitinozoa. Most of the samples yield Early Llanvirn taxa but a few Upper Ordovician and Givetian chitinozoan assemblages are also recorded. Obviously, no reworked taxa occur in our material. Consequently, the chitinozoa provide the age of the sedimentation.L'analyse palynologique du matériel argilo-silteux de l'unité sud de la boutonnière des Zekkara (Maroc oriental) a révélé l'existence de chitinozoaires bien conservés. La plupart des assemblages observés appartiennent à l'Ordovicien inférieur (Llanvirn). D'autres échantillons ont livré des formes de l'Ordovicien supérieur et un prélèvement contient des chitinozoaires du Dévonien moyen. La juxtaposition de matériel d'âge différent résulte de processus de sédimentation par gravité dans un complexe ultérieurement perturbé par des écaillages.Elaouad-Debbaj Zohra, Desteucq Christian, Fournier-Vinas Chantal. Présence de chitino-zoaires ordoviciens et dévoniens dans la boutonnière des Zekkara, région d'Oujda (Maroc oriental). Occurrence of Ordovician and Devonian chitinozoa in the Zekkara's inlier, Oujda area (Eastern Morocco). In: Sciences Géologiques. Bulletin, tome 40, n°4, 1987. pp. 363-379

The Late Ordovician glacio-eustatic record from a high-latitude storm-dominated shelf succession: The Bou Ingarf section (Anti-Atlas, Southern Morocco)

Evidences of glaciation at the end of the Ordovician are widespread in western Gondwana Some authors consider the glaciation was restricted to the Hirnantian time, but occurrences of glacial deposits in the Lower Silurian strata of South America indicate that the Gondwana glaciers did not completely disappear after the Hirnantian glaciation. In addition, numerous studies based on palaeoecology, sedimentology, sequence stratigraphy and stable isotopes have suggested that ice sheets formed as soon as the Early/Middle Katian The objective of this work is to investigate the high-frequency eustatic signals in a stratigraphic succession corresponding to the Katian and the Hirnantian (similar to 10 My comprising the uppermost part of the Ordovician) The studied section (Bou Ingarf section >600 m) is located in the Central Anti-Atlas, southern Morocco It shows an almost continuous succession of siliciclastic platform deposits. The lower package, Katian to early Hirnantian in age, corresponds to a shelf succession, within which storm dynamics prevailed The upper package, middle to upper Hirnantian in age, comprises a glaciation-related succession made up of a suite of coastal to flood-dominated fluvio-glacial deposits The latter have essentially filled in large palaeochannels interpreted as subglacial tunnel valleys From high-resolution facies and sequential analysis, a curve of variation of depositional environments was established at very high, high and low frequencies Time calibration was performed based on a high-quality biostratigraphic control mainly derived from chitinozoan biozones through the whole succession. Assuming a constant tectonic subsidence and a bathymetric model (shoreface/upper offshore boundary 30 m, upper offshore/lower offshore boundary: 120 m), changes in facies-based water depths are converted into an eustatic sea-level curve using a one-dimensional backstripping procedure The eustatic sea-level curve shows that the stratigraphic succession is dominated by the stacking of sequences at very high- (40 m) and coeval forced regressions were identified during the Katian They represent glacial episodes of significant extent prior to the Hirnantian, but essentially lacking a glacial record Strata reflecting the Hirnantian glaciation include two differentiated events and an intermediate but important transgression. The second Hirnantian glacial event, the only one associated with subglacial erosion in the Bou Ingarf area, corresponds to the Late Ordovician glacial climax characterised by a continental-scale ice sheet Ice sheets permanently occupying the centre of the Gondwana landmass throughout the Late Ordovician may reconcile moderate Hirnantian eustatic sea-level fall amplitude (4080 m) and palaeoglacial reconstructions that show a Hirnantian ice sheet covering the main part of western Gondwana. (C) 2010 Elsevier B V All rights reserved