The 13C Chemostratigraphy of the Upper

NORWEGIAN JOURNAL OF GEOLOGY The δ13C chemostratigraphy of the Upper Ordovician Mjøsa Formation 65 The 13C chemostratigraphy of the Upper Ordovician Mjøsa Formation at Furuberget near Hamar, southeastern Norway: Baltic, Trans-Atlantic, and Chinese relations Stig M. Bergström, Birger Schmitz, Seth A. Young & David L. Bruton Bergström, S.M., Schmitz, B., Young, S.A. & Bruton, D.L.: The δ13C chemostratigraphy of the Upper Ordovician Mjøsa Formation at Furuberget near Hamar, southeastern Norway: Baltic, Trans-Atlantic, and Chinese relations. Norwegian Journal of Geology, Vol 90, pp. 65-78. Trondheim 2010, ISSN 029-196X. Whereas no studies have previously been carried out on the δ13C chemostratigraphy of the Sandbian-Katian (Upper Ordovician) succession any- where in Norway, such investigations in Sweden and the East Baltic region have made the δ13C chemostratigraphy of that interval well known. In an attempt to document for the first time the presence of the globally distributed and stratigraphically important Guttenberg Carbon Isotope Excursion (GICE), 45 samples were collected at 2 m intervals through the Furnesfjorden and Gålås members of the Mjøsa Formation from two sections at Furuberget in the Nes-Hamar District 70 km north of the City of Oslo. Relatively high δ13C values, which are interpreted to represent the GICE, were obtained throughout the Gålås Member. The presence of this δ13C excursion in the middle part of the Mjøsa Formation, combined with biostratigraphic and other evidence, are used for detailed correlations of the Mjøsa Formation with other successions in Baltoscandia, North Ame- rica, and on the Yangtze Platform in southern China. Stig M. Bergström, School of Earth Sciences, Division of Geological Sciences, The Ohio State University, Columbus, Ohio 43210, USA. E-mail: stig@ geology.ohio-state.edu. Birger Schmitz, GeoBiosphere Science Centre, Department of Geology, Lund University, Sölvegatan 12, Lund, SE-223 62 Lund, Sweden. E-mail: [email protected]. Seth A. Young, Department of Geological Sciences, Indiana University, Bloomington, Indiana 47405, USA. E-mail: [email protected]. David L. Bruton, Naturhistorisk Museum, University of Oslo, Sars gate 1, Oslo, Norway. E-mail: [email protected] Introduction unique, position among the many Ordovician major stratigraphic units in the Oslo region. This prominent Although a large amount of work has been carried out formation, which reaches a thickness of approximately in recent years on the Sandbian and Katian δ13C che- 100 m or slightly more in many sections, is widely dis- mostratigraphy in the East Baltic region and Sweden (e.g. tributed round the northern part of Lake Mjøsa (Opal- Saltzman et al. 2003; Tobin et al. 2005; Kaljo et al. 2007; inski & Harland 1981) about 70 km north of the City Schmitz & Bergström 2007; Calner et al. 2010; Bergström of Oslo (Fig. 1). The Mjøsa Formation consists mainly et al. in press), no chemostratigraphic data have previously of relatively pure shallow-water limestones of baham- been published from the coeval successions in the Oslo itic type (Jaanusson 1973) containing locally Solenopora Region, or elsewhere in Norway. Indeed, the only δ13C bioherms. Its rather diverse, but unevenly distributed, chemostratigraphic information currently available from fossil fauna, part of which is still undescribed, includes the Ordovician of Norway is some data from the Hirnan- brachiopods, trilobites, corals and other shelly fossils tian succession in the Oslo area (e.g. Brenchley et al. 1997; along with stromatoporoids, algae, and microfossils. The Brenchley & Marshall 1999; Kaljo et al. 2004b; Bergström microfossils include relatively common ostracodes and et al. 2006). The present study is the first attempt to intro- conodonts, two groups that have not yet been subjected duce δ13C chemostratigraphy to a pre-Hirnantian forma- to detailed published studies. For faunal references, see tion in the classic Oslo Region Ordovician succession. Owen et al. (1990). As noted in several recent papers, since the 1990s, δ13C chemostratigraphy has become a very important tool for A significant number of the fossils present in the Mjøsa clarifying stratigraphic relationships at both the local and Formation differ markedly from those in most con- global scale. However, Ordovician chemostratigraphic temporaneous Baltoscandian faunas but have affinities studies are still in the pioneer stage with no such work yet to those in warm-water deposits in Laurentia (Jaanus- having been carried out in many, if not most, key succes- son 1979; Bergström 1997; Bergström et al. 1998). This sions round the world as is the case also in Norway. applies particularly to the conodont fauna that differs strikingly from that present in most coeval Baltoscan- In terms of lithology, fauna, and also economic impor- dic deposits but shows a remarkable similarity to that in tance, the Mjøsa Formation occupies a special, if not some formations in the North American Midcontinent, 66 S. M. Bergström et al. NORWEGIAN JOURNAL OF GEOLOGY Fig. 1. Sketch-map of parts of the Nes-Hamar-Toten districts showing the location of the Furuberget Quarry northwest of Hamar (modi- fied after Opalinski & Har- land 1981, fig. 2). Inset map (after Owen et al. 1990) illus- trates the distribution of Ord- ovician rocks (black) in the several districts in the Oslo region distinguished by Stør- mer (1953). Arrow indicates the location of the study area in the Lake Mjøsa region. such as the Lexington Limestone of Kentucky and coeval Spjeldnæs 1982) but the quarry succession, which is now strata in adjacent states (Bergström & Sweet 1966; Rich- exposed more extensively stratigraphically than in the ardson & Bergström 2003). past due to recent quarrying, has never been described in detail. The lower-middle part of the Lexington Limestone dis- plays the chemostratigraphically important Guttenberg δ13C excursion (GICE), one of the seven named Ordo- Distribution and geology of the Mjøsa vician δ13C excursions (Bergström et al. 2009a). The Formation GICE has been recorded at many localities in Lauren- tia (e.g. Ludvigson et al. 2004; Young et al. 2005; Barta In the region round the City of Oslo Ordovician rocks et al. 2007), the East Baltic region (Kaljo et al. 2007) and occur in a number of geographically more or less sepa- Sweden (e.g. Saltzman et al. 2003; Bergström et al. 2004) rated areas, some of which have their own lihologically and recently also in China (Bergström et al. 2009b). The and faunally specific Ordovician succession. The latter close similarity between the conodont faunas from the applies to our study area in the Lake Mjøsa region. For Lexington Limestone and the Mjøsa Formation, and the convenience, when discussing various parts of the Oslo fact that the latter is a shallow-water carbonate unit of Region below, we follow the standard district designa- the type that tends to preserve particularly well perturba- tions introduced by Størmer (1953) and subsequently tions in the marine carbon cycle, suggested to us that this used by many authors (Fig. 1). unit would be ideal for an attempt to establish the presence of the GICE for the first time in Norway. The pur- Occurrence and lithostratigraphy pose of this report is to document the δ13C chemostratigraphy through most of the Mjøsa Formation, and to The Mjøsa Formation is widely distributed in the Toten- describe the discovery of the GICE, in the well-known Nes-Hamar districts and can be studied in numerous, in outcrops at the Furuberget Quarry about 4 km northwest most cases now inactive, quarries as well as in road cuts of the centre of Hamar (Fig. 1). These outcrops have been and natural outcrops. Because of its resistant nature, it discussed repeatedly in the literature (see, e.g. Holte- tends to be better exposed than underlying shaly strata dahl 1909; Skjeseth 1963; Opalinski & Harland 1981; of the Furuberget Formation. The abundance of out- NORWEGIAN JOURNAL OF GEOLOGY The δ13C chemostratigraphy of the Upper Ordovician Mjøsa Formation 67 crops is illustrated by the fact that Opalinski & Harland The middle portion of the formation was classified into (1981) listed 49 selected localities of the unit and several three, areally more restricted but at least broadly coeval, more are recorded in the literature. Although a thickness members, namely the Eina, Gålås, and Bergevika mem- figure of approximately 100 m has generally been given bers. For a summary of the characteristics of these mem- for the Mjøsa Formation, it should be noted that there is bers, see Owen et al. (1990). Although the Lower Paleo- no section described in which the entire unit and its top zoic sediments in the study area have been subjected to and basal contacts are well exposed. This also applies to considerable folding, faulting, and heating (approxi- its type section at the Eina Quarry in the Toten district, mately 300°C based on Conodont Alteration Index; cf. where most of the unit (85 m) and its basal contact are Bergström 1980), the prevailing opinion is that they are well exposed but where its uppermost portion and the autochthonous, at least in the Nes-Hamar area (Skjeseth top contact are covered (Opalinski & Harland 1981). 1963). For some different interpretations, see Spjeldnæs (1982). Hence, it is very unlikely that the studied succes- As just noted, the Mjøsa Formation rests, probably sion is located on a detached piece of Laurentia. conform ably, on the Furuberget Formation (Fig. 2), the lithology of which is dominated by shales and siltstones Biostratigraphy and with subordinate limestones. The top of the Mjøsa Formation is a prominent karst surface with locally up No graptolites have been recorded from the Mjøsa For- to 3 m deep fissures filled by quartzitic material belong- mation and its shelly macrofossils have not provided ing to the overlying middle Llandovery Helgøya Member any very decisive evidence of the age of the unit. Early of the Sælabonn Formation (Strand & Henningsmoen students (Kiær 1897; Holtedahl 1909) referred the unit 1960; Skjeseth 1963; Worsley et al.

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