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. ‡Author for correspondence: Malgo.Vidal@pal.uu.se

The oldest rocks of the Holy Cross Mountains, Poland – biostratigraphy of the Cambrian Czarna Shale Formation in the vicinity of Kotuszów

“…Key studies for the development of global lower Cambrian acritarch biostratigraphy include those conducted on the Lublin Slope of the EEC (Volkova 1969;Volkova et al 1979Volkova et al , 1983Moczydłowska and Vidal 1986;Moczydłowska 1989). Acritarchs with a similar age are known from other parts of the EEC in Poland (e.g., Moczydłowska 1981;Jankauskas and Lendzion 1992;Szczepanik 2000), eastern Baltic region (e.g., Volkova 1968;Yankauskas 1972Yankauskas , 1975Yankauskas and Posti 1976;Paskeviciene 1980), and Scandinavia, both from the margin of the Baltic Shield and from the Caledonides (e.g., Vidal 1981;Tynni 1982;Vidal 1986, 1992;Hagenfeldt 1989;Eklund 1990;Vidal and Nystuen 1990;Vidal and Moczydłowska 1996;Moczydłowska et al 2001;Högström et al 2013). Comparable assemblages are known from the Cambrian of Volhynia (Kiryanov 1974;Volkova et al 1979Volkova et al , 1983) and the Moscow Syneclise (Volkova 1996).…”

Zn-Pb Ores of Mississippi Valley Type in the Lycksele-Storuman District, Northern Sweden: A Possible Rift-Related Cambrian Mineralisation Event

“…These rocks are largely eroded now and preserved only as a narrow rim along the Caledonian margin ( Figure 1). This sequence comprises sandstones and shales [10,11], deposited in response to opening of the Iapetus Ocean. These units are overlain by the Alum Shale Formation, which comprises organic-rich (10%-20% by weight) and pyrite-bearing carbonaceous shale [12].…”

“…These units are overlain by the Alum Shale Formation, which comprises organic-rich (10%-20% by weight) and pyrite-bearing carbonaceous shale [12]. This shale represents strongly anoxic conditions, dominated by clay deposition and accumulation of limestones [11,12].…”

Life cycle of Early Cambrian microalgae from theSkiagia-plexus acritarchs

“…Acritarchs in these collections are in various states of preservation because of different geological and thermal histories of the successions and their palaeogeographic settings. The successions are broadly of early Cambrian age (Moczydłowska, 1991;Vidal and Peel, 1993;Moczydłowska et al, 2001;Moczydłowska and Zang, 2006;Zang et al, 2007).…”

“…Numerous specimens without the endocyst or with the excystment structure only are preserved in the same stratigraphically thin interval (Moczydłowska et al, 2001). Remarkably, the Australian and Swedish assemblages contain microfossils with the endocyst together with numerous Skiagia specimens in different developmental stages (immature and closed to emptied cysts with the excystment structure).…”