Biostratigraphy and trilobite fauna of the lower silurian Tegart formation, southeastern British Columbia

Abstract

In the Western Ranges of the Rocky Mountains, the Tegart Formation is preserved locally beneath the sub-Devonian unconformity, with a maximum thickness of 66 m. Argillaceous limestone and shale of the lower 55 m contain graptolites, conodonts and benthic macrofossils of late Telychian (Late Llandovery) age. These different groups of fossils allow correlation of the Tegart Formation within the three most effective biostratigraphic schemes that are used to zone the Silurian System. The stratigraphic association of the groups of fossils aids precise calibration between these three biostratigraphic schemes. The graptolite faunas and the conodont faunas are useful for correlation between continents and within North America; the trilobites and other benthic fossils provide correlation with carbonate rocks within North America. The graptolite fauna represents the upper part of the Monograptus spiralis Zone, which is well known in central Europe, Asia, the Arctic Islands and the Yukon. The conodont fauna indicates the Pterospathodus amorphognathoides Zone of central Europe, Great Britain, southeastern Alaska, the Mackenzie Mountains, the Arctic Islands, Gaspe, and Anticosti Island. The lower boundary of the Pterospathodus amorphognathoides Zone with the underlying Pterospathodus celloni Zone is just above the base of the Tegart Formation. Trilobites and brachiopods dominate the shelly benthic fauna, which also has rare corals, gastropods, cephalopods and conulariids. The most precise correlation is with the faunas of trilobite assemblages e and f (ivithin the lower amorphognathoides Conodont Zone) in the Whittaker-Road River transitional rocks of the Avalanche Lake region, western District of Mackenzie. Three new species of trilobites are erected: Paracybantyx tegartensis, Podowrinella rossi and Wallacia hendersoni.