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Trilobite Biostratigraphy and Carbon Isotope Chemostratigraphy in the Tingskullen Drill Core, Öland, Sweden

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Trilobite Biostratigraphy and Carbon Isotope Chemostratigraphy in the Tingskullen Drill Core, Öland, Sweden Estonian Journal of Earth Sciences, 2021, 70, 1, 18–35 https://doi.org/10.3176/earth.2021.03 Miaolingian (Cambrian) trilobite biostratigraphy and carbon isotope chemostratigraphy in the Tingskullen drill core, Öland, Sweden Per Ahlberga, Frans Lundberga, Mikael Calnera, Oliver Lehnertb–d, Peter Dahlqviste and Michael M. Joachimskib a Department of Geology, Lund University, Sölvegatan 12, SE­223 62 Lund, Sweden; [email protected], [email protected], [email protected] b GeoZentrum Nordbayern, Friedrich­Alexander University of Erlangen­Nürnberg (FAU), Schloßgarten 5, D­91054 Erlangen, Germany; [email protected], [email protected] c Key Laboratory of Economic Stratigraphy and Palaeogeography, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing, China d Faculty of Environmental Sciences, Czech University of Life Sciences Prague, Praha 6 – Suchdol, Czech Republic e Geological Survey of Sweden, Kiliansgatan 10, SE­223 50 Lund, Sweden; [email protected] Received 19 May 2020, accepted 5 September 2020, available online 14 January 2021 Abstract. The Cambrian succession of the Tingskullen drill core from northern Öland comprises Cambrian Series 2 and Miaolingian (Wuliuan Stage) siliciclastic strata. The major portion of the succession is represented by the Miaolingian Borgholm Formation, which, in ascending order, is subdivided into the Mossberga, Bårstad and Äleklinta members. The Äleklinta Member is barren of body fossils, whereas the Mossberga and Bårstad members are moderately to highly fossiliferous and biostratigraphically reasonably well constrained. Trilobites and agnostoids from the Bårstad Member are indicative of the Acadoparadoxides pinus Zone. The Mossberga Member has not yielded any zonal guide fossils but is tentatively assigned to the Eccaparadoxides insularis Zone. 13 A δ Corg curve throughout the Borgholm Formation shows a general positive trend up­section without any distinctive excursion, suggesting that the Wuliuan Acadoparadoxides (Baltoparadoxides) oelandicus Superzone (the ‘Oelandicus beds’) of Öland is younger than the negative Redlichiid–Olenellid Extinction Carbon isotope Excursion (ROECE), which is known from near the top of Stage 4 and close to the traditional ‘Lower–Middle Cambrian boundary’ in several parts of the world. Key words: biostratigraphy, carbon isotope stratigraphy, Miaolingian Series, Wuliuan Stage, Cambrian, Öland, Sweden. INTRODUCTION brief review). Each new global chronostratigraphic unit in the current conceptual model of the Cambrian System In most parts of the world, the Cambrian has traditionally will have definitions based on the boundary­stratotype been divided into lower, middle and upper parts concept (Salvador 1994), in which a Global boundary (corresponding to series/epochs). The boundary of each Stratotype Section and Point (GSSP) is used to define the series (epoch) was, however, placed at a chronostrati­ base of a series or stage. graphic level that varied from region to region (e.g. So far, six Cambrian stages (Fortunian, Wuliuan, Robison et al. 1977; Geyer & Shergold 2000). In order to Drumian, Guzhangian, Paibian and Jiangshanian) and resolve this problem, the International Subcommission on three series (Terreneuvian, Miaolingian and Furongian) Cambrian Stratigraphy is working towards a global have been defined and formally named (Peng et al. 2004, subdivision of the Cambrian System into four series and 2009, 2012; Babcock et al. 2007; Landing et al. 2007; ten stages (e.g. Babcock et al. 2005), as opposed to the Zhao et al. 2012, 2019; Geyer 2019). The remaining traditional tripartite subdivision into three series. Hence, undefined series and stages have received provisional the chronostratigraphic subdivision of the Cambrian numerical designations. The Miaolingian Series was System is currently undergoing substantial changes and ratified in 2018 and is divided into three global stages, in the traditional series ‘Lower’, ‘Middle’ and ‘Upper’ ascending order, the Wuliuan, Drumian and Guzhangian Cambrian will be abandoned (see Peng et al. 2012 for a stages. Although of considerably greater stratigraphic © 2021 Authors. This is an Open Access article distributed under the terms and conditions of the Creative Commons Attribution 4.0 International Licence (http://creativecommons.org/licenses/by/4.0). 18 P. Ahlberg et al.: Miaolingian trilobites in the Tingskullen drill core thickness, it replaces and includes the traditional Middle stratigraphic succession has to be pieced together using Cambrian of Baltica and Avalonia. The conterminous base information from several outcrops and available drill of the Miaolingian Series and the Wuliuan Stage is cores. This paper focuses on the biostratigraphy and 13 defined by a GSSP coinciding with the first appearance carbon isotope chemostratigraphy (δ Corg) of the datum of the oryctocephalid trilobite Oryctocephalus Cambrian succession in the Tingskullen drill core from indicus in the Kaili Formation of Jianhe County, eastern the island of Öland, southern Sweden. It is one of the 13 Guizhou, China (Zhao et al. 2019). very few δ Corg investigations in the Miaolingian of 13 In Scandinavia, the boundary between provisional Baltoscandia and also one of the few δ Corg studies Cambrian Stage 4 (traditional upper Lower Cambrian) and dealing with this interval in the world. the Wuliuan Stage (lower Miaolingian Series) is generally marked by a prominent unconformity ascribed to non­ deposition and erosion during a eustatic sea­level fall that GEOLOGICAL SETTING correlates partly with the regressive ‘Hawke Bay Event’ (Nielsen & Schovsbo 2015; Nielsen & Ahlberg 2019; Fig. During much of the early Palaeozoic, the Baltic (or 1 herein). The Wuliuan Stage of southeastern and south­ Baltoscandian) Basin was submerged under a shallow to central Sweden comprises the silt­ and mudstone­ moderately deep epeiric sea that covered most of present­ dominated Borgholm Formation (sensu Nielsen & day Scandinavia, the East Baltic area and northeastern Schovsbo 2007), followed by dark grey to black, organic­ Poland. The island of Öland is located in the western part rich siliciclastic mudstones of the Drumian through the of the Baltic Basin (Fig. 2A) and a classical geological Lower Ordovician (Tremadocian) portion of the Alum outcrop area for Cambrian and Ordovician research in Shale Formation. In southern and south­central Sweden, Baltoscandia (for reviews, see Martinsson 1974; Stouge the upper or topmost part of the Wuliuan Stage is 2004). The sedimentary rock succession rests uncon­ generally developed as a bioclastic limestone, the formably on a Proterozoic crystalline basement and Exsulans Limestone Bed, assigned to the Ptychagnostus consists of Cambrian–Ordovician strata, which dip slightly, gibbus Zone. The Borgholm Formation is well developed generally less than 2º, towards the east. Thus, the oldest in the Öland–Gotland area, southeastern Sweden, and in formations are exposed in the west­facing cliffs along the Närke and Östergötland, south­central Sweden (Fig. 1). western shore, and the youngest ones in the east. Wuliuan strata are widely distributed in Scandinavia, The Cambrian succession of Öland consists mainly of but exposures are relatively few and small. Hence, the siliciclastic rocks and has a thickness of 125–230 m Trilobite zonation N S Superzones Zones Scania Västergötland Öland Gotland Närke Ptychagnostus atavus Drumian Alum Shale Fm. Alum Shale Fm. Paradoxides paradoxissimus Ptychagnostus gibbus Kva rnt . M. B.F. Äleklinta Mbr Tornby Mbr Acado- Faludden Mbr Miaolingian Bårstad Mbr paradoxides Wuliuan Acado- pinus / Kvarntorp Mbr paradoxides Ptychagnostus Bårstad Mbr Borgholm Fm. oelandicus praecurrens ? Eccaparadoxides Mossberga Mbr insularis Borgholm Fm. Mossberga Mbr Borgholm Fm. ? ‘Hawke Bay unconformity’ B.F. Grötlingbo Mbr Dellingia scanica– Gislöv När Sst. Mbr När Sst. Mbr Kingaspidoides Lingulid Sand- Series 2 Stage 4 Fm. Lingulid Sand- lunatus stone Mbr F.H.F. F.H.F. stone Mbr F.H.F. F.H.F. När Shale Mbr När Shale Mbr Fig. 1. Stratigraphic diagram showing the lithostratigraphic terminology and correlation of the upper provisional Cambrian Series 2 through the lower Miaolingian succession (traditional ‘Lower–Middle’ Cambrian boundary interval) in some regions of Scandinavia. B.F., Borgholm Formation; F.H.F., File Haidar Formation. Based on Nielsen & Schovsbo (2007, 2015). 19 Estonian Journal of Earth Sciences, 2021, 70, 1, 18–35 Ordovician: limestone, shale A B Cambrian: sandstone, siltstone, Jämtland Cambrian: sandstone, siltstone, shale, stinkstone, limestone Böda Hamn Precambrian crystalline basement Finland Thickness Alum Shale Formation 2 m Norway Dalarna Tingskullen Drill core locations Siljan Helsinki Oslo Sweden 6 m d Närke Stockholm Tallinn n Borgholm la Baltic Sea Estonia Ö ? Västergötland Östergötland Mossberga-1 Russia Gotland 10 m Kalmar North Sea Riga Latvia Öland C a le d t o Baltic Sea i n ia a Baltic Sea n Skåne fro Denmark tr n Copenhagen S t Lithuania r 16 m a Bornholm m l Vilnius a K Minsk Segerstads Fyr 18 m Belarus Kvinnsgröta-1 Poland 22 m Berlin Amsterdam Germany Warsaw N Netherlands 0 200 km 01020 km Ukraine Fig. 2. A, map of southern Sweden and the surrounding Baltoscandian region showing the distribution of lower Palaeozoic rocks (green shading). Modified from Lindskog & Eriksson (2017, fig. 1). B, simplified geological map of Öland, southern Sweden, showing the location of the Tingskullen drill site and the location of other core drillings
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