Llanvirnian Stage

(also Llanvirn stage; named after the population center of Llanvirn, Wales, Great Britain), the third stage from the bottom of the Ordovician system. It was identified in 1881 by the English geologist H. Hicks in the county of Pembrokeshire (Great Britain). The type section is composed of clay shales and ash tuffs and, based on graptolites, it is divided into two zones: Didymograptus bifidus and D. murchisoni. The upper part of the latter contains conodonts of the Pygodus serrus zone. In the United States, Australia, and the Northeast of the USSR the Llanvirnian stage is divided into three zones: (1) Didymograptus bifidus, (2) Isograptus caduceus, and (3) Paraglossograptus etheridgei. Deposits of the Llanvirnian stage are distributed in numerous Western European countries (Great Britain, France, Sweden, Norway, Poland, and Czechoslovakia), in the USSR (Estonian SSR, Leningrad Oblast, Kazakh SSR, Kirghiz SSR, the Urals, the Northeast, the Altai-Saian region), the United States, South America (Argentina and elsewhere), the People’s Republic of China, Burma, and Australia.