The Cambrian–Ordovician succession at Lanna, Sweden: stratigraphy and depositional environments

A ca 20 m thick succession of upper Furongian (Cambrian Stage 10) through Middle Ordovician (Darriwilian) strata exposed at Lanna, in the province of Närke, south-central Sweden, is described. The upper Furongian is represented by the Alum Shale Formation and reflects an overall shallowing trend that ultimately resulted in emergence above sea level and subaerial conditions. Hence, as in most other areas in south-central Sweden, the boundary between the Cambrian and the Ordovician is marked by a prominent disconformity and significant hiatus. In Närke, the hiatus spans the middle Stage 10 through the uppermost Tremadocian or lowermost Floian. The presence of stromatolites indicates quite shallow marine conditions during the latest Cambrian. The Ordovician succession is characterized by flatly bedded ‘orthoceratite limestone’, belonging to the ‘Latorp’, ‘Lanna’ and ‘Holen’ limestones (‘topoformations’). Widely varying microfacies characteristics in the ‘orthoceratite limestone’ suggest that the depositional environment underwent substantial changes through time, largely due to changes in sea level. A long-term trend of coarsening carbonate textures and more diverse fossil assemblages is seen upwards through the Ordovician succession. Cyclic microfacies patterns probably reflect high-frequency sea-level changes. Comparisons to other parts of Sweden and Baltoscandia reveal consistent patterns in the sedimentary development across a wide geographical area.