Fossils from the Ordovician "Upper Hovin Group" (Caradoc-Ashgill), Trondheim Region, Norway

Abstract

The 'Upper Hovin Group' on the downthrown side of the Horg Fault between the Orkla and Gaula rivers contains rocks of both Late Caradoc and Early Ashgill age, determined from fossils from eight localities. The older part of the sequence may represent a shelf to basin, transition from a carbonate build-up at the western edge (Kalstad Limestone) to a deeper water equivalent (shale and the limestone at Svartsætra) towards the east. Brachiopods and trilobites from these limestones indicate a Late Caradoc-Ashgill age, the former and associated coonodonts showing clear North American (Chatfieldian) affinities. The siltstone matrix of a limestone-bearing conglomerate near the Gaula river contains brachiopods that do not permit precise age determination, but black shale that underlies it is like the Upper Caradoc 'Dicranograptus Shales' at Gyllan, on the east side of the Gaula river. Brachiopods from the volcanogenic sandstone of the Espehaug boulder, together with similar brachiopods and pele- cypod molluscs from an outcrop of identical rock nearby, are of Late Ordovician age.