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

A Lindskog,A M Lindskog,J V Johansson,M E Eriksson,P Ahlberg

doi:10.3176/earth.2018.10

Abstract

A ca 20 m thick succession of upper Furongian (Cambrian Stage 10) through Middle Ordovician (Darriwilian) strata exposed at Lanna, in the province of Narke, 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 Narke, 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. (Less)

Highlights

The Baltoscandian region hosts extensive areas with lower Palaeozoic sedimentary rocks, which are largely preserved in place, undisturbed and often richly fossiliferous
The collective results of this study, combined with previously published data, reveal details about the palaeoenvironmental development throughout the late Cambrian (Furongian)–mid Ordovician. Both the macroscopic and microscopic characteristics of the strata at Lanna show that significant changes occurred in the local depositional environment during this time interval, and many distinct changes and ‘events’ in the local rocks correspond to palaeoenvironmental changes recorded in other parts of Sweden and Baltoscandia
Sea level appears to have risen in the latest Tremadocian and/or earliest Floian, resulting in the glauconite-rich bed that is commonly found at the base of the Ordovician succession in Närke

Summary

Introduction

The Baltoscandian region hosts extensive areas with lower Palaeozoic sedimentary rocks, which are largely preserved in place, undisturbed and often richly fossiliferous. The regional sedimentary succession has become an important archive for our understanding of environmental and biotic changes, including the evolution of life, during the early Phanerozoic (e.g., Lindström 1971). Lower Palaeozoic rocks are widely distributed in several areas in the province of Närke, south-central Sweden (e.g., Linnarsson 1875a). Närke has long been considered a key area for Ordovician stratigraphy and palaeontology. This is evident not least because several regional stages and other stratigraphic units have been named after important outcrop areas in the province. The interval spanning the Hunneberg and Billingen regional stages was formerly referred to as the Latorp Stage (Jaanusson 1960a; Männil 1966) and the term Lanna Stage was long a contender for the interval that is the Volkhov Regional Stage

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Conclusion