Biochronology of the autochthonous Lower Cambrian in the Laisvall–Storuman area, Swedish Caledonides

Abstract

New records of phytoplankton (acritarchs), ichnofossils and olenellid trilobites have been studied from the autochthonous upper Neoproterozoic–Lower Cambrian successions along the Caledonian Thrust Front in the Laisvall–Storuman region of northern Sweden. The fossils are from a newly examined natural outcrop at Bergmyrhobben near Lake Storuman, and from previously described fossiliferous outcrops at Delliknäs and Mt. Assjatj, the Laisvall mine and the Maiva borehole successions in the Laisvall area. Acritarch assemblages are recorded throughout the Grammajukku Formation. They are age-diagnostic for the Skiagia–Fimbriaglomerella acritarch Zone, time-equivalent to the Schmidtiellus mickwitzi trilobite Zone (the lower part of the formation), and the Heliosphaeridium–Skiagia acritarch Zone corresponding to the Holmia kjerulfi trilobite Zone (the upper part of the formation). The acritarch record from the Storuman area documents the presence of strata contemporaneous to the Schmidtiellus mickwitzi Zone for the first time in the Scandinavian Caledonides. This zone was previously only recognized in the platform regions of the Baltica palaeocontinent. The ichnofossils from the upper Såvvovare Formation, including ?Harlaniella, Phycodes, Gyrolithes and Palaeophycus ichnogenera, allowed the base of the Cambrian System to be determined within the Maiva Member and the coeval Kautsky Ore Member in the subsurface successions, and to attribute this part of the formation to the Lower Cambrian Platysolenites antiquissimus faunal Zone of Baltica. The trilobite fauna from the Storuman area, attributed tentatively to Holmia sp., occurs at the lowermost stratigraphic level among olenellids in the Caledonides. The range of this species, estimated from the concurrent acritarch biostratigraphy, is within the Schmidtiellus mickwitzi Zone. The stratigraphic significance of the acritarch assemblages and ichnofossils is analysed and the biochronology of the Grammajukku Formation and the upper Såvvovare Formation is discussed in detail in the context of Lower Cambrian zonation in Baltica.