Late Wenlock graptolite-bearing tuffaceous sandstone from Bomholm, Denmark

Abstract

Late Wenlock (Silurian) graptoliferous dark grey mudstone and tuffaceous sandstone lithofacies occur, mainly as loose boulders, at the southeast coast of Bomholm. The mudstone may be estimated to repre­sent a 25 m thick sequence, and contains a graptolite fauna indicating the Cyrtograptus lundgreni Zone. The fauna is correlated with sequences of similar age from other areas, specially from Scania. Monograp-tus flemingi and Pristiograptus dubius pseudodubius are virtually the only species in the tuffaceous sandstone, some of which were deposited by turbidity currents. There is evidence from the taphonomy and diversity of the sandstone graptolites to suggest that the two species lived at a lower level in the water column than other, coexisting species. Size-frequency analyses are inconclusive as to whether or not the fauna in the tuffaceous sandstone represents a rapidly buried life-assemblage. The tuffaceous sandstone samples represent at least 3 airfall water deposited tuff beds and one bed deposited by turbidity current action. Mineral composition and grain-size of the tuffaceous sandstone indicate an origin from plinian eruptions at an estimated distance of about 300 km from Bomholm, and deposition at a depth of 1000 m ± 300 m, on the outer rim of the Fennoscandian platform. The volcanoes were probably situated on a continent south of Bomholm. The eruption centre later may either have disappeared within the Caledo­nian foldbelt or have been displaced by early Palaeozoic large-scale strike-slip faults towards the SE, along the SW border of the East-European craton.