A summary of Caledonian front stratigraphy, northern Jämtland/southern Västerbotten, central Swedish Caledonides

Abstract

The sedimentary rocks of the Caledonian front in northern Jämtland and southernmost Västerbotten described here include late Pre-Cambrian, Cambrian and Ordovician sequences occurring beneath the metamorphic allochthon of the Seve-Köli Nappe Complex. Three groups are recognized. The lowest, the Risbäck Group (c. 700 m) includes four formations in the type area, dominated by alluvial sandstones. Tillites (Varangian) and a quartzite-shale formation compose the overlying Sjoutälven Group (c. 300 m), which ranges into the Middle Cambrian. The overlying Tåsjön Group (c. 250 m) contains a lower black shale and an upper greywacke-dominated formation, ranging in age from the Middle Cambrian into the Middle Ordovician. The full succession, represented in the west, thins eastwards, and in the autochthon only the uppermost part of the Sjoutälven and the Tåsjön Group are present. The paper briefly discusses the correlation of the three groups elsewhere in the Caledonides.