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International audience; The QUSAIBA-1 shallow core drilled in NW Saudi Arabia penetrated the Qalibah, Sarah and Qasim formations. The Silurian graptolites and chitinozoans from the Qusaiba Member of the Qalibah Formation have been investigated previously. The present study focuses on the Upper Ordovician part of the core hole. A total of 39 core samples from the glacio-marine Sarah Formation and from the upper part of the Qasim Formation were processed for chitinozoans. Part of the sample set yielded abundant and well-preserved specimens associated with graptolite remains, scolecodonts, acritarchs and cryptospores. Other samples from glacially related sediments contained only a few fragmented chitinozoan vesicles of early Late Ordovician species suggesting reworking. Four different chitinozoan assemblages are identified. The youngest one is restricted to two samples located very close to the gamma ray peak and to the highest evidence of glacial sediments. This assemblage is dominated by Cyathochitina species including C. caputoi, associated with subordinate numbers of Ancyrochitina, Euconochitina and Belonechitina species. This assemblage does not contain the classical Late Ordovician chitinozoan taxa and an early Rhuddanian age is considered possible. The underlying interval is less productive and sometimes virtually barren. This interval yields a second assemblage mainly composed of broken specimens of Tanuchitina, Cyathochitina, and Euconochitina. Most of these forms seem reworked, probably through glacial erosion. The composition changes drastically in the older core samples where abundant Armoricochitina nigerica and Ancyrochitina species are associated with e.g., Euconochitina lepta, Calpichitina lenticularis, Acanthochitina barbata and Desmochitina minor. This constitutes the third recovered chitinozoan assemblage. In addition to the classical components, new species have been observed. This assemblage occurs in pre-glacial strata and is assigned to the late Katian (Ashgill). The oldest processed sample contains Belonechitina, Hercochitina, Conochitina and Euconochitina species. This fourth assemblage, however, lacks A. nigerica and suggests an older age assignment (e.g., late Caradoc of the British chronostratigraphy).

International audience; A total of 58 core samples from three shallow core holes (Qusaiba-1, Baq'a-3 and Baq'a-4) penetrating the Qalibah Formation in Central Saudi Arabia have been investigated. The core samples are grey-greenish hemipelagic shale or silty-shale and yield very well-preserved and abundant Early Silurian chitinozoans. Most of the recorded chitinozoan assemblages are highly diverse and include several new taxa coexisting with well-known species. Five chitinozoan biozones ranging from the Aeronian to the Telychian, and considered of regional value at the scale of Northern Gondwana regions, are identified. From the oldest to the youngest they are the: Conochitina alargada, Angochitina hemeri, Angochitina maclurei, Tanuchitina obtusa and Euconochitina silurica biozones. The three older regional biozones have been already reported in Saudi Arabia (Paris et al. 1995) whereas the two youngest ones are documented for the first time in the Arabian Peninsula. Their eponymous index species were described from the subsurface of the Algerian Sahara. Several sub-biozones of at least local application are also defined. The fairly closely spaced and regular sampling permits the total range of several highly diagnostic forms to be better constrained and thus the interval of uncertainty between biozones is reduced. Beside abundant chitinozoans and acritarchs, sporadic occurrences of scolecodonts and of eurypterid cuticle fragments are also noted in the organic residues. The presence of eurypterids suggests a shallowing trend in the sedimentary record, which is consistent with the distribution of the graptolites sicula and/or rhabdosomes. Indeed, the graptolites are frequent through the Qusaiba Member, except in the youngest examined samples where shallower environments like those in the Sharawra Member begin. Recently, Zalasiewicz et al. (2007) studied in detail the range of the graptolites in the three shallow cores. These graptolite data provide independent chronostratigraphical control refining previous age assignments. The alargada and the hemeri chitinozoan biozones correspond to the mid-Aeronian convolutus graptolite biozone whereas the maclurei chitinozoan biozone extends through the early Telychian guerichi and turriculatus graptolite biozones. The obtusa and the silurica chitinozoan biozones occur in an interval devoid of graptolites are referred to the mid-late Telychian as they do not contain yet typical Sheinwoodian species.

International audience; Core samples from the QASIM-801 (QSIM-801) water well in the Qasim Region of central Saudi Arabia were investigated palynologically. The interval studied contains the transgressive succession from the Sajir Member of the Saq Formation to the Hanadir Member of the Qasim Formation, and yields well-preserved and abundant cryptospores, microphytoplankton (acritarchs and chlorophycean algae), and chitinozoans, indicative of a Darriwilian (Llanvirn) age. The lowermost portion of the cored interval from the upper Sajir Member consists of fine-grained sandstones deposited in tidal flat, and shallow-marine settings. It is characterized by abundant and diverse cryptospores (permanent tetrads, dyads, monads), cuticle-like phytoclasts, and a low diversity assemblage of marine palynomorphs. The upper part of the cored interval from the Hanadir Member is composed of hemipelagic to pelagic, organic-rich, graptolitic shales, and micaceous shales, of middle- to outer-shelf depositional settings, which yield abundant marine palynomorphs (acritarchs and chitinozoans). Cryptospore-bearing horizons are interspersed in the succession. Changes in the composition of palynomorph assemblages reflect higher frequency environmental changes within an overall transgressive succession. Four new acritarch species are described in open nomenclature, Tyrannus sp. A, Clypeolus sp. A,?Pulvinosphaeridium sp. A, and?Tinacula sp. A. Une étude palynologique a été réalisée sur des échantillons de carottes du forage aquifère QASIM-801 près de Qasim, dans la partie centrale de l'Arabie Saoudite. L'intervalle étudié contient la succession transgressive du Membre de Sajir de la Formation de Saq et le Membre de Hanadir de la Formation Qasim. Cet intervalle a livré des cryptospores, abondantes et bien préservées, un microphytoplancton (acritarches et algues chlorophycées) et des chitinozoaires, d'âge Darriwilien (Llanvirn). La base de l'intervalle carotté, appartenant au sommet du Membre de Sajir, consiste en un grès à grain fin déposé sur l'estran ou dans la zone côtière. Il est caractérisé par l'abondance et la diversité des cryptospores (tétrades permanentes, dyades et monades), des phytoclastes ressemblant à des cuticules et une faible diversité de l'assemblage de palynomorphes marins. La partie supérieure de l'intervalle carotté dans le Membre de Hanadir est composée d'hémipélagites à pélagites, riches en matière organique, d'argilites à graptolites et d'argilites micacées, de la plate-forme externe, où les palynomorphes marins (acritarches et chitinozoaires) sont abondants. Les horizons à cryptospores sont intercalés dans la succession. Les changements dans la composition des assemblages de palynomorphes reflètent des changements environnementaux à haute fréquence dans une tendance transgressive globale de la succession. Quatre nouvelles espèces d'acritarches sont décrites en nomenclature ouverte, Tyrannus sp. A, Clypeolus sp. A,?Pulvinosphaeridium sp. A et?Tinacula sp. A.