Stratigraphy and major structures of the Grong District, Nord- Trøndelag.

Abstract

The rock sequence of the Grong district, which occurs in a depression within the Norwegian Caledonides between two Precambrian massifs, Børgefjell to the north and the Grong culmination in the south, can be divided into three main tectonostratigraphical units: an Eastern and lower, a Central, and a Western and upper Complex. The Eastern Complex consists of the Seve Nappe of probable Precambrian (?to Cambrian) age, and a Lower Køli Nappe above with Ordovician and Silurian rocks. The Central Complex of the Rantser Nappe is thought to be inverted and of Cambrian to Ordovician age. The Western Complex is inverted and comprises the Limingen Nappe, composed of Ordovician rocks, and the overlying Helgeland Nappe consisting of rocks of assumed Precambrian (?to Cambrian) age. In the southwestern part of the district, near Grong, it is difficult to detect whether thrusting of younger rocks of Precambrian basement has taken place. Likewise it is here difficult to confirm the presence of the Helgeland Nappe. The Precambrian (?to Cambrian) rocks of the Seve and the Helgeland Nappe have striking lithological similarities and also have a higher metamorphic grade than the centrally situated younger rocks. They are rhought to have been deposited in the same basin. In contrast, all the groups of the Lower Køli, Rantser and Limingen Nappes are markedly different in lithology and none of them can be correlated.