Middle to Late Ordovician graptolite and chitinozoan biostratigraphy of the Kandava-25 drill core in western Latvia

摘要

This paper presents integrated graptolite and chitinozoan biostratigraphical data for the upper Middle and lower Upper Ordovician of the Kandava-25 drill core in Latvia, a core that overwhelmingly contains the most diverse graptoloid fauna in the East Baltic carbonate-dominated sections. Graptolites are abundant in two intervals in the Kandava-25 core. The older strata (Stirnas and Furudal formations) contain the graptolites Pseudamplexograptusdistichus, Proclimacograptusangustatus, Archiclimacograptuscaelatus, Archiclimacograptus sebyensis, Archiclimacograptusosloensis n. sp., Dicaulograptushystrix, Haddingograptuseurystoma, Haddingograptus luperus, Haddingograptus maennili n. sp., Hustedograptusteretiusculus, Hustedograptus vikarbyensis, Gymnograptuslinnarssoni, Dicellograptusvagus, Dicellograptus intortus, Dicellograptus gurleyi, Dicranograptusirregularis and Nemagraptussubtilis. These species indicate that the lower interval belongs to the P. distichus and D. vagus zones, zones that correlate with the more widely recognizable Didymograptusmurchisoni (upper part) and H.teretiusculus zones. Thus, the lower interval is late Dariwillian (Lasnamagi and Uhaku regional stages) in age. A second graptolite-rich interval begins in the middle of the Dreimani Formation and contains Nemagraptusgracilis, Hustedograptusuplandicus, Orthograptuswhitfieldi, Orthograptus calcaratusincisus, Oepikograptusbekkeri, Dicellograptussextans, Dicellograptus salopiensis, Normalograptusrotundatus and Archiclimacograptusbalticus n. sp. This fauna is Sandbian (Kukruse and Haljala regional stages) in age. Chitinozoans are abundant and well preserved throughout the Kandava-25 core. Six chitinozoan biozones and five subzones can be recognized in the upper Middle and lower Upper Ordovician strata of the core. The co-occurrence of biostratigraphically important graptolites and chitinozoans in the Kandava-25 core provides direct tie-points for Ordovician graptolite and chitinozoan biozonations. http://zoobank.org/urn:lsid:zoobank.org:pub:CC9E068C-E56C-4197-89B8-E30C067B69CB