Sedimentary and tectonic environment of the Ordovician Föllinge Greywacke, Storsjön area, Swedish Caledonides

Abstract

The Middle to Upper Ordovician Föllinge Greywacke occupies about 4 500 km2 of the easternmost overthrust terrain in the Caledonides of central Sweden. Its outcrop area around lake Storsjön can be divided into two realms in which turbidites are preserved from the two principal stages of development of a basin that was characterized by a western facies of sand-rich fans and an eastern basin plain with dark mud. The eastern, Föllinge Realm, represents an early stage of the basin, with lithic greywackes, with feldspar and detrital carbonate among the clasts. The western, Mörsil Realm is characterized by quartz wackes to lithic greywackes with a high clay content and little carbonate or feldspar, and transport from the southwest and west. The heavy-mineral suite of both realms is dominated by zircon and minerals derived either from old granitic terrains or reworked sediments derived from the erosion of such terrains. The Cr and Ni content is relatively high and increased in late stages of basin evolution, as did the chlorite content. The basin was at first situated between a western tectonic rise, deformed between the Middle Cambrian and the Middle Ordovician, and an eastern starved carbonate facies to the east. Subsidence of the rise in late Middle Ordovician to early Late Ordovician led to westward migration of the deepest part of the basin, which was subsequently fed from an elevated terrain still farther to the west, but also from rises to the north and south.