Some stratigraphical and sedimentological studies on the Devonian of the Trondheimsled, Norway

Abstract

The Devonian rocks considered here, outcrop over a distance of 100 kms., along the northern side of a strip of water, known as the Trondheimsled, near Trondheim, Norway. Since time did not permit complete examination, two areas were selected for detailed study: 1) the western end of the outcrop on the large island of Hitra, where fossils suggesting an Upper Silurian or Downtonian age had been found; and 2) the outcrops on the island complex south of the neighbouring island of Smola. The precise age of these beds was unknown. An attempt was made to establish the stratigraphy of both areas, and to deduce the environment of deposition, the nature of the source area, and the direction of derivation of the sediments. Hitra. The succession (1055 m. thick) has been divided into a number of stratigraphical units on the basis of lithology. Much of it is made up of thick sandstones, often pebbly or conglomeratic, and alternating sandstone/siltstone sequences. Study of the sedimentary structures and size distributions of these sequences suggests that the cycles have been produced in an inshore estuarine environment, as a result of the lateral migration of channels. The petrography of the sandstones and conglomerates shown thot the sediments were derived from a source area composed mainly of plutonic rocks, while current direction analysis shows derivation from the northwest. Smola: The Smola Series, composed of 3745 m. of conglomerate, has been divided petrographically into four units. The oldest conglomerate lies unconformably on the beds below and is made up of large boulders of locally derived material. The younger conglomerates are composed of abundant pebbles of green sandstone (possibly the Ordovician Hovin Sandstone), and various igneous and metamorphic PEBBLES which are difficult to match with rocks now exposed in Norway. The sediments wore derived from the north and were probably deposited in a piedmont environment. Relative age of the Hitra and Smola series Consideration of the stratigraphical, petrographical, and sedimentological evidence shows that the Hitra sediments are older than those Smola. A reconsideration of the paleontological evidence suggests that the Hitra beds could be of Upper Welockian or early Ludlovian age, while the Smola beds are probably Lower or Middle Devonian.