Cambrian to basal Ordovician lithostratigraphy in southern Scandinavia
Authors: Nielsen Arne Thorshøj , Schovsbo Niels 
Published: February 2021

The lithostratigraphic subdivision of the Cambrian successions in Scania-Bornholm, Östergötland, Västergötland, Närke, Öland-Gotland, the southern Bothnian Sea and the Mjøsa District is reviewed and revised.

The review includes the Tremadocian part of the Alum Shale Formation. The Cambrian of Scania-Bornholmcomprises the Nexø, Hardeberga, Læså, Gislöv and Alum Shale formations. The Nexø Formation of Bornholm is subdivided into the new Gadeby and Langeskanse members, which are c. 40 and 50 m thick, respectively.

The 1–15 m thick arkosic basal part of the sandstone succession in Scania, previously treated as part of the Hardeberga Sandstone, is allocated to the Nexø Formation. The ‘Balka Sandstone’ of Bornholm is considered an integrated part of the Hardeberga Formation and the designation Balka Sandstone Formation is abandoned. The Haddeberga Formation, which is 109 m thick on Bornholm and c. 105–110 m in Scania, comprises the Hadeborg (new), Lunkaberg (Scania only), Vik, Brantevik and Tobisvik members. The overlying Læså Formation contains the Norretorp and Rispebjerg members; the former is regarded a senior synonym of the Broens Odde member of Bornholm. The Norretorp Member is thicker in Scania than previously estimated (> 25 m, rather likely even > 30 m); on Bornholm it is 103 m thick. The Rispebjerg Member is 1–3.7 m thick.

The Cambrian of the Öland-Gotland area, the southern Bothnian Sea and the districts of south central Sweden comprises the File Haidar, Borgholm and Alum Shale formations. The File Haidar Formation of the Öland-Gotland area, which is up to 127 m thick, includes the Viklau, När Shale and När Sandstone members; the Grötlingbo Member is transferred to the Borgholm Formation. The validity of the Kalmarsund Member is questioned; its lithological characteristics probably reflect diagenesis. The Lingulid and Mickwitzia sandstone members constitute the File Haidar Formation in south central Sweden, where the unit is up to 37 m thick. New names and to some extent new definitions are introduced for the members of the Borgholm Formation, viz. Kvarntorp Member (new name for the thin glauconitic sandstone overlying the Lingulid Sandstone Member in central Sweden), Mossberga Member (new name for the coarse part of the Eccaparadoxides oelandicus Shale sensu Hagenfeldt 1994), Bårstad Member (new name for the fine-grained part of the Eccaparadoxides oelandicus Shale), Äleklinta Member (new name for theParadoxides paradoxissimus Siltstone) and Tornby Member (new name for the Paradoxides paradoxissimus Shale). The Granulata Conglomerate (= Acrothele Conglomerate of previous literature) is formally ranked as a bed at the base of the Äleklinta Member. The informal designation Söderfjärden formation is abandoned. The Borgholm Formation locally exceeds 150 m in the Öland-Gotland area; it is significantly thinner in south central Sweden.

The Cambrian of the Mjøsa District comprises the Vangsås, Ringstrand and Alum Shale formations. Of these, the new Ringstrand Formation encompasses the strata between the Vangsås and Alum Shale formations, previously referred to as the ‘Holmia Series’. The Ringstrand Formation includes the Brennsætersag (new), Redalen (new), Tømten (new), Evjevik and Skyberg (new) members. Thickness estimates are rendered difficult due to tectonic overprinting, but the Ringstrand Formation is probably about 50-60 m thick in the Lower Allochthon around Lake Mjøsa.

The Scandinavian Alum Shale Formation, which is up to 100 m thick in Scania and even thicker subsurface of Kattegat, is restricted to encompass only kerogeneous mudstones/shales with subordinate limestones and very rare sandstone beds. It is proposed abandoning the Kläppe Shale and Fjällbränna Formation of the Lower Allochthon of Jämtland and to regard these units as part of the Alum Shale Formation. Several widespread thin units are formally ranked as beds within the Alum Shale Formation, including the Forsemölla Limestone Bed (new name for the ‘fragment limestone’at or near the base of the Alum Shale Formation in Scania; this unit is also developed in Östergötland and Närke), the Exsulans Limestone Bed, the Hyolithes Limestone Bed, the Andrarum Limestone Bed, the Exporrecta Conglomerate Bed, the Kakeled Limestone Bed (new name for the ‘Great Orsten Bank’ of south central Sweden), the Skåningstorp Sandstone Bed (new name for the thin sandstoneintercalation at the base of the Ordovician in Östergötland) and the Incipiens Limestone Bed.