Stratigraphical section through Lower Cambrian at Söderfjärden, Vaasa, western Finland

Abstract

The Lower Cambrian sedimentary rocks form a doughnut-shaped deposit on the bottom of the Söderfjärden depression. The sedimentary pile was first established and has now been penetrated by drilling. The remaining thickness of sedimentary rocks at the drilling site is 244 m under 74 m of glacial overburden. Sediments in the lower half of the sequence are clayey and continue in the upper half mixed with sandy and silty beds. Thin phosphoritic conglomerates intercalate with any other rocks. Syndepositionary sedimentary structures and penecontemporaneous soft-state deformations are well preserved in these completely unmetamorphosed rocks. Indications of rapid basin filling are met with throughout the sequence. A volcanic origin of Söderfjärden basin, Lower Cambrian in age, is claimed to have implications with coeval alkaline magmatism in the Fennoscandian Shield.