Biostratigraphy of Late Ordovician chitinozoans from the QUSAIBA shallow core (NW Saudi Arabia). In : IPC, Bonn, September 2008
Résumé
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).