The Bogo Shale graptolite fauna from the Trondheim Region, Norway, has been restudied. As it is closely similar to North American Ordovician graptolite faunas, its age is determined in terms of the North American Ordovician graptolite succession. The Bogo Shale graptolite fauna has an age in the span of the Isogroptus and Paraglossograptus etheridgei zones. Similar faunas from western lreland have the same faunal affinities and age. Trilobites from the same stratigraphic position as the graptolites in both the Trondheim Region and western lreland also have North American affinity and indicate the same age as the graptolites. The Bogo Shale graptolites were collected from near the base of the Lower Hovin Series. That group of rocks unconformably overlies Cambro-Ordovician rocks that were deformed prior to its deposition. The western Ireland faunas of the same age are also from rocks that unconformably overlie rocks that were deformed prior to their deposition. The deformation that resulted in the unconformity beneath the Lower Hovin Series has been termed the Trondheim Disturbance (Holtedahl 1920). Skjeseth (1952) suggested that the Orthoceras Limestone, a limestone unit that intervenes between the Lower and Upper Didymograptus Shales in the Oslo Region, may have been deposited during the Trondheim Disturbance. The Orthoceras Limestone may be correlated with the North American Didymograptus bifidus zone (the zone just prior to the Isogroptus zone) and with the latter part of the Didymograptus hirundo and at least the early part of the D. bifidus zone in Britain. The Trondheim Disturbance is suggested to have occurred in that interval. If the deformation and metamorphism of the rocks beneath the lsograptus- P. etheridgei zone rocks in western lreland took place at the same time as the Trondheim Disturbance in Norway, then that deformation and metamorphism was going on during the latter part of the D. hirundo zone and the early part of the D. bi.fidus zone in terms of the British Ordovician graptolite zones. If, as many authors have suggested, the Dalradian Schists in Scotland were deformed and metamorphosed at the same time as those beneath the lsograptus-P. etheridgei zone rocks in western Ireland, then that deformation and metamorphism is also dated as pre-late British D. bifidus zone age.
Age of Bogo Shale and western Ireland grapholite faunas and their bearing on dating Early Ordovician deformation and metamorphism in Norway and Britain