Sandbian) K-Bentonites in Oslo, Norway T ⁎ Eirik G
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Palaeogeography, Palaeoclimatology, Palaeoecology 520 (2019) 203–213 Contents lists available at ScienceDirect Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo A new age model for the Ordovician (Sandbian) K-bentonites in Oslo, Norway T ⁎ Eirik G. Balloa, , Lars Eivind Auglanda, Øyvind Hammerb, Henrik H. Svensena a Centre for Earth Evolution and Dynamics (CEED), University of Oslo, Pb. 1028, 0316 Oslo, Norway b Natural History Museum, University of Oslo, Pb. 1172, 0318 Oslo, Norway ARTICLE INFO ABSTRACT Keywords: During the Late Ordovician, large explosive volcanic eruptions deposited worldwide K-bentonites, including the Chronostratigraphy Millbrig and Deicke K-bentonites in North America and the Kinnekulle K-bentonite in Scandinavia. We have Sandbian-Katian boundary studied a classical locality in Oslo containing one of the most complete sections of K-bentonites in Europe. U-Pb dating In a 53 m section of Sandbian age, we discovered 33 individual K-bentonite beds, the most notable beds being Milankovitch the Kinnekulle and the upper Grimstorp K-bentonite. Magnetic susceptibility (MS) measurements on two in- Age model tervals show significant periodicity peaks interpreted as Milankovitch cycles and thus astronomically forced Kinnekulle changes in sediment supply and composition. These cycles fit remarkably well with both the expected Milankovitch periodicities for the Ordovician as well as the radiometric ages presented in this study and may represent one of the most convincing demonstrations of Milankovitch cycles from the lower Paleozoic so far. Five of the K-bentonites have been dated by high-precision chemical abrasion-thermal ionization mass spectrometry (CA-TIMS) U-Pb zircon geochronology, where the Kinnekulle K-bentonite gives an age of 454.06 ± 0.43 Ma. We have integrated the new data and calculated an age model showing the sedimentation rates through the section and thereby the ages of each of the 33 K-bentonites. Using the age model, we further present a new age for the Sandbian-Katian stage boundary. The section in Oslo provides the highest resolution window into the Upper Ordovician K-bentonite succession so far and helps shed more light on the chronology of one of the most intense volcanic periods of the Paleozoic and the relationship with the global carbon cycle changes that followed. 1. Introduction recent advances in geochronological methods provide the possibility to obtain precise age constraints (Condon et al., 2007; Mattinson, 2005; The Ordovician was one of the most environmentally, biologically, Schmitz and Schoene, 2007). and climatologically dynamic periods in Earth history with events such Subduction-related island arc volcanism during the Ordovician de- as the first occurrence of land plants (Lenton et al., 2012), and the posited volcanic tephras, now preserved as K-bentonites (Huff, 2016; extensive Great Ordovician Biodiversification Event (GOBE) that led to Huff et al., 2010). K-bentonites represent excellent stratigraphic mar- a surge of marine life (Servais et al., 2009; Webby et al., 2004). kers of the Phanerozoic, as they cover widespread areas (up to Moreover, a general long-term cooling trend is documented through the 2.2 × 106 km2 in Europe and 6.9 × 105 km2 in North America; Huff entire period (Nardin et al., 2011), culminating in the Hirnantian gla- et al., 2010). Sedimentary successions can record environmental, bio- ciation at the end of the period—accumulating ice sheets on the logical, and climatological data—providing a timeline of geochemical Gondwanan continent and consequently lowering of the global sea level information on these events, with interlayered K-bentonite beds (Armstrong, 2007; Brenchley et al., 1994; Herrmann et al., 2004). The working as age markers. Radiometric dating of well-preserved zircons first of the ‘Big Five’ mass extinctions of the Phanerozoic, killing about in K-bentonites is therefore an important tool in refining the geological 85% of marine species, ended the Ordovician Period (Sheehan, 2001). timescale and defining large-scale geochemical perturbations. Still, the causes of these events remain elusive (Munnecke et al., 2010). Milankovitch cycles are associated with astronomically forced The general lack of precise age determinations is a key factor in un- changes in sediment supply, composition and climate change (Elrick certainties in the cause-and-effect relationships responsible for these et al., 2013; Fang et al., 2016; Sutcliffe et al., 2000; Williams, 1991). To intervals of significant global change (Cramer et al., 2015). However, identify astronomically forced cycles, magnetic susceptibility (MS) is ⁎ Corresponding author. E-mail address: [email protected] (E.G. Ballo). https://doi.org/10.1016/j.palaeo.2019.01.016 Received 1 October 2018; Received in revised form 10 January 2019; Accepted 10 January 2019 Available online 01 February 2019 0031-0182/ © 2019 Elsevier B.V. All rights reserved. E.G. Ballo, et al. Palaeogeography, Palaeoclimatology, Palaeoecology 520 (2019) 203–213 Fig. 1. (A) Southern Scandinavia with box showing the Oslo Region and logs from the Vollen, Sinsen and Kinnekulle localities show the Kinnekulle, lower and upper Grimstorp K-bentonite beds illustrating an apparent westward increase in sedimentation rate. The map is adapted from Svensen et al. (2015). (B) Simplified geological map of the Oslo Region showing the studied localities at Sinsen (including the Storoveien locality; 59°56′13″ N, 10°47′13″ E), Carl Berner (59°55′39″ N, 10°46′48″ E) and Vollen (59°48′11.4″ N, 10°29′12.6″ E). The map is modified from Lutro and Nordgulen (2008). (C) Satellite overview of the Sinsen area showing the main Sinsen section (black box), the stratigraphic up direction (arrow) and strike and dip of the bedding. The image is adapted from Google Maps (2017). frequently used, as it is a low-cost logging parameter that allows for 2. Upper Ordovician stratigraphy of the Oslo Region field measurements (Svensen et al., 2015; Fang et al., 2016; Plado et al., 2016). Climate-driven changes in weathering, erosion and sea level lead In the Oslo Region, the Ordovician successions are characterized by to a variation in sediment influx, reflected in the concentration of for epicontinental, subtidal marine depositional environments, as de- instance iron (Ellwood et al., 2000). Combining high-precision radio- scribed in classical contributions by Kjerulf (1857) and Kiær (1897), metric ages with cyclostratigraphic spectral analyses may test whether and summarized by Nielsen (2004), Owen et al. (1990), and Worsley sediments provide a record of astronomical forcing. and Nakrem (2008). The Middle Ordovician stratigraphy is dominated The first combined radiometric dating and cyclostratigraphic study by shales of the Tøyen and Elnes formations separated by the massive of K-bentonites in Norway was conducted by Svensen et al. (2015) at limestones of the Huk Formation (Bruton et al., 2010; Fig. 2). The Vollen south of Oslo (Fig. 1B). Svensen et al. (2015) dated two of the Upper Ordovician is characterized by the alternation of shales (e.g. key K-bentonites (the Kinnekulle and the upper Grimstorp K-bentonite) Arnestad and Nakkholmen formations) and nodular and tabular lime- and identified cyclicities interpreted to represent the short eccentricity stones (e.g. Vollen and Frognerkilen formations; Bjørlykke, 1974; (100 kyr) and obliquity (30.3 kyr) components. However, as the out- Bruton et al., 2010; Möller and Kvingan, 1988; Fig. 2). The Ordovician crop at Vollen is limited to three K-bentonites, a high-resolution age successions in Oslo are found within the Oslo Graben, a Carboniferous model was not calculated. to Permian rift structure extending about 200 km from the Skien-Lan- The aim of this study is to provide a new age model for this part of gesund area in the south to the Mjøsa district in the north (e.g. the Ordovician stratigraphy, based on a section in Oslo. We have re- Neumann et al., 1992; Størmer, 1953; Sundvoll and Larsen, 1994). visited the classic locality of Hagemann and Spjeldnæs (1955), re- The K-bentonites in this study are all located in the Arnestad vealing 33 K-bentonite beds of which 11 are not previously reported. Formation, predominantly consisting of thick shale beds (30–40 cm), New high-precision ages of the K-bentonites may help constrain the interlayered by thin bedded or nodular horizons of early diagenetic relationships between large explosive eruptions and environmental calcareous limestone rarely exceeding 10–20 cm in thickness. In the changes. upper part of the formation, the limestone horizons develop from 204 E.G. Ballo, et al. Palaeogeography, Palaeoclimatology, Palaeoecology 520 (2019) 203–213 Fig. 2. Global distribution of Middle and Upper Ordovician K-bentonites modified from Huff (2008) using ages from Gradstein et al. (2012). nodular to massive (Owen et al., 1990). A westward shallowing en- two beds originated from different arc settings. The large magnitude of vironment is suggested by faunal and sedimentological indicators these Ordovician volcanic eruptions has brought up questions about (Hansen and Harper, 2006), which is reflected in a westward increase their potential climatic and environmental effects. Perrier et al. (2012) in sedimentation rate (Fig. 1A). suggested that the Kinnekulle K-bentonite was associated with re- The position of the Sandbian-Katian boundary in the Oslo Region organization of ostracod assemblages and strong, long-term perturba- has been discussed by several authors