Late Ordovician–earliest Silurian chitinozoans from the Qusaiba-1 core hole (North Central Saudi Arabia) and their relation to the Hirnantian glaciation
Résumé
The continuously cored Qusaiba-1 drilled in North Central Saudi Arabia penetrated successively the Qalibah, Sarah and Qasim formations. Silurian graptolites and chitinozoans from the Qusaiba Member of the Qalibah Formation were previously investigated. The present study focuses on the Upper Ordovician and lowermost Silurian parts of the core hole. Part of the sample set yielded abundant and well-preserved specimens associated with eurypterid remains, scolecodonts, acritarchs and cryptospores. Other samples from glacially derived shaly sediments contain only a few fragmented chitinozoan vesicles of Middle and Late Ordovician species indicating reworking through glacial processes. Four different chitinozoan assemblages are identified. The first recorded chitinozoan assemblage is restricted to the deepest processed sample (core 56). It contains Belonechitina cf. robusta, Hercochitina sp. A, and Spinachitina cf. kourneidaensis, and taxa also represented in the second assemblage, e.g., Fungochitina spinifera and Euconochitina species. This first assemblage, however, lacks some of the diagnostic species of the second one (e.g., Acanthochitina barbata, Tanuchitina cf. elongata) and suggests a slightly older late Katian age. The second recovered chitinozoan assemblage is documented in the upper part of the Qasim Formation (cores 49–56) and in the deeper core samples referred to the lower part of the Sarah Formation (cores 45–47 from the disrupted facies of the Sarah Sandstone Member). It contains abundant and well-preserved Armoricochitina nigerica and Ancyrochitina merga associated with e.g., Euconochitina lepta, Calpichitina lenticularis, A. barbata and Desmochitina typica. In addition to these classical components, new species have been observed. This assemblage occurring in pre-glacial as well as in glacially related strata is assigned to the late Katian–earliest Hirnantian (Ashgill). The overlying productive interval, corresponding to the Baq'a Shale Member, is less productive and sometimes virtually barren of chitinozoans. This interval yields a third assemblage mainly composed of broken specimens of Angochitina cf. curvata, C. lenticularis, Cyathochitina sp., D. typica, F. spinifera, Tanuchitina fistulosa, Euconochitina gr. lepta, and even Siphonochitina formosa. Most of these forms are interpreted as glacially reworked. The youngest assemblage is restricted to three samples (core 27), which are located very close to a Silurian gamma ray peak and above the highest evidence of glacial sediments represented by the Sarah Formation. This forth assemblage contains extremely abundant chitinozoans coexisting with a few graptolite remains and inarticulate brachiopods. This chitinozoan assemblage is dominated by Cyathochitina caputoi, which is associated with Belonechitina pseudarabiensis and subordinate numbers of Ancyrochitina and Euconochitina species. The classical Late Ordovician chitinozoan taxa are no longer present and an earliest Rhuddanian age is proposed.Carbon stable isotope (δ13Cchit.) values have been measured on picked chitinozoan vesicles from the upper part of the Quwarah Member (upper Qasim Formation) and from the basal part of the Qusaiba Member (Qalibah Formation) where they register a shift towards more negative values close to the Gamma ray peak. The timing of the Late Ordovician glaciation and correlation with Hirnantian glacial events recorded in Northern Gondwana regions are briefly discussed based on the recovered chitinozoans.Four new species, i.e., Conochitina rotundata sp. nov., Belonechitina tenuicomata sp. nov., Hercochitina multiansata sp. nov., and Calpichitina bernardae sp. nov. are described, discussed and illustrated. Biometric data are provided for Acanthochitina barbata and for Armoricochitina nigerica.