Volume 50/1 • Studies in Ordovician geology

A database of fossil occurrences in the Ordovician of Baltoscandia

Hammer, Ø. 2003–04–30: A database of fossil occurrences in the Ordovician of Baltoscandia. Bulletin of the Geological Society of Denmark, Vol. 50, pp. 5–9. Copenhagen. © 2003 by Geological Society of Denmark. ISSN 0011–6297.

A database recording published fossil occurrences in the Ordovician of Baltoscandia has been constructed and made available on the Internet (http://asaphus.uio.no). The database presently contains10340 entries, each recording the first and last appearances of a single species at a single locality, together with other stratigraphical, taxonomic and geographical information. The data are
taken from 141 publications, most of them from 1980 or later but also included are some older key references. The database will be continuously maintained, revised and expanded. In addition to serving as a reference for researchers working on the Ordovician of Baltoscandia, the database hasbeen used to produce biodiversity curves for different fossil groups. These curves reflect the global increase in biodiversity through the Ordovician, but also show interesting local fluctuations.

Key words: Ordovician, Baltoscandia, Baltica, database, diversity.

Øyvind Hammer [ ], Geological Museum, Boks 1172 Blindern, N-0318 Oslo, Norway. 15 June 2001.

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The geochemistry of Lower Palaeozoic sediments deposited on the margins of Baltica

Schovsbo, N.H. 2003–04–30: The geochemistry of Lower Palaeozoic sediments deposited on the margins of Baltica. Bulletin of the Geological Society of Denmark, Vol. 50, pp. 11–27, Copenhagen. © 2003 by Geological Society of Denmark.

A regional survey of the geochemical composition of Lower Palaeozoic shales deposited on Baltica indicates that Llanvirn (Lower/Middle Ordovician) to Lower Silurian shales have higher concentrations of Na, Mg, Cr, Ni and Fe and lower concentrations of K, Rb and Ti compared to Arenig shales. This geochemical signature can be traced from Scania to the Oslo Region, i.e. in areas approximately 500 km apart, but is not present in Middle Ordovician sediments from Avalonia. The geochemical signature matches island arc tholeiites such as those in the Fundsjø Group within the Upper Allochthon of the Norwegian-Swedish Caledonides. Hence, these sediments were probably predominantly derived from island arcs formed during the end phase of closure of the Iapetus Ocean. Simple two component mixing calculations between oceanic and continental sediment sources suggest that the oceanic component diminishes towards the south where modifications related to longer sediment transport distances can be recognised. The introduction of sediment derived from island arcs coincides with increases in subsidence rates in the Oslo Region and may reflect an early stage in foreland basin development. The presence of the geochemical signature in Scania implies that island arcs systems were geographically widespread. The combined evidence indicates that the Arenig/Llanvirn boundary marks an important change in the continuing closure of the Iapetus Ocean. The data suggest that island arcs were obducted onto the outer margins of Baltica presumably during the Arenig. Continued obduction of island arcs in the Mid Ordovician and younger intervals is likely.

Key words: Provenance, Caledonides, Scania, Oslo, Lower Palaeozoic.

Niels H. Schovsbo [ ], Geological Museum, University of Copenhagen, Øster Voldgade 5-7, DK-1350 Copenhagen K, Denmark. 28 August 2001.

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Brachiopod macrofaunal distribution through the upper Volkhov – lower Kunda (Lower Ordovician) rocks, Lynna River, St. Petersburg region

Hansen, J. & Harper, D.A.T. 2003–04–30: Brachiopod macrofaunal distribution through the upper Volkhov – lower Kunda (Lower Ordovician) rocks, Lynna River, St. Petersburg region. Bulletin of the Geological Society of Denmark, vol. 50, pp. 45–53, Copenhagen.

The ranges of 27 brachiopod species, through the upper Volkhov-lower Kunda interval of the Lynna River section, are presented on the basis of over 17,500 specimens. Three depth-related ecological associations are recognized within the succession of alternating limestones and marls forming part of an array of nearshore to mid-shelf communities in an intracratonic setting. Faunal diversity is relatively stable throughout the section with little evidence for abrupt local extinctions or originations. The dominant orthoid and clitambonitoid assemblages, however, form the basis of the distinctive Baltic brachiopod province developed during this time interval.

Key words: Brachiopods, Volkhov, Kunda, Lynna River, biodiversity, brachiopod assemblages.

Jesper Hansen, Geological Museum, Øster Voldgade 5-7, DK-1350 Copenhagen K, Denmark (Present address: Tromsø University Museum, NO-9037 Tromsø, Norway) & David A.T. Harper [ ], Geological Museum, University of Copenhagen, Øster Voldgade 5-7, DK-1350 Copenhagen K, Denmark. 2 January 2002.

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Diversification patterns in the clitambonitoid brachiopods of the Ordovician of Baltoscandia

Vinn, O. & Harper, D.A.T. 2003–04–30: Diversification patterns in the clitambonitoid brachiopods of the Ordovician of Baltoscandia. Bulletin of the Geological Society of Denmark, Vol. 50, pp. 55–61, Copenhagen.

Two intervals of clitambonitoid diversification followed by major extinctions have been recorded in the Ordovician rocks of Baltoscandia, one during the Arenig and a second in the mid Caradoc. The Arenig radiation was characterized by high origination and extinction rates of both genera and species associated with the development of carbonate environments across the region. In addition the migration of numerous stocks of clitambonitoids out of Baltica to a number of other palaeocontinents occurred during this diversity peak during the Arenig, together with the spread of the majority of genera into a range of more local environments. A marked extinction in the Early Llanvirn coincided with shifts in sedimentation patterns whereas the diversity hike in the Mid Caradoc reflects the diversification of clitambonitoid species in both inner and outer shelf environments. The later Caradoc extinction coincided with an extensive eustatic regression event and the major restructuring of the ecosystem and brachiopod fauna. The final extinction of Baltic clitambonitoids in the late Ashgill was gradual and probably reflected an increasing dependence on more restricted niches.

Key words: Brachiopods, Ordovician, Baltoscandia, Clitambonitoidea, biodiversity.

Olev Vinn [ ], Department of Earth Sciences, Laurentian University, Sudbury, Ontario, Canada P3E 2C6 (Present address: Institute of Geology, University of Tartu, Tartu 51014, Estonia). David A.T. Harper [ ], Geological Museum, University of Copenhagen, Øster Voldgade 5-7, DK-1350 Copenhagen K, Denmark. 19 April 2003.

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Conodonts and brachiopods from the Volkhov Stage (Lower Ordovician) microbial mud mound at Putilovo Quarry, north-western Russia

Tolmacheva T., Fedorov P. & Egerquist E. 2003–04–30: Conodonts and brachiopods from the Volkhov Stage (Lower Ordovician) microbial mud mound at Putilovo Quarry, north-western Russia. Bulletin of the Geological Society of Denmark, Vol. 50, pp. 63–74, Copenhagen.

Microbially mediated clay mud mounds are widely developed in the Lower Ordovician succession east of St. Petersburg (Russia) and are associated with a diverse and abundant fauna of brachiopods, ostracodes, echinoderms, bryozoans and conodonts. The lithology of one such mud mound in Putilovo Quarry has previously been studied, but the faunas associated with the mounds have not been investigated to date. Clay lenses in the Putilovo mud mound yield conodont assemblages belonging to the Baltoniodus triangularis and lowermost part of the Paroistodus originalis zones and these stratigraphical intervals are much thicker in the mud mound than in the coeval Lower Ordovician succession lateral to the mound. The compositions of the conodont and brachiopod assemblages are generally the same in the mud mound as in contemporaneous beds. The occurrence of relatively fewer conodont elements in the mud mound than in the surrounding successions probably indicates the higher rate of accumulation of the mud mound clays. Juvenile brachiopods are more numerous in the clays of the mud mound than outside the build-up, supporting the hypothesis that the mounds included ecologically stressed environments.

Key words: Microbial mud mounds, conodonts, brachiopods, Lower Ordovician, Russia.

T. Tolmacheva, All Russian Geological Institute, Sredny 74, 199106 St. Petersburg, Russia., E. Egerquist, Department of Earth Sciences, University of Uppsala, Historical Geology and Palaeontology, Norbyvägen 22, SE-752 36 Uppsala, Sweden. P. Fedorov, Department of Historical Geology, St. Petersburg State University, 7/9 Universitetskaja emb., 199034, St. Petersburg, Russia. 24 August 2001.

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An integrated biostratigraphical analysis of the Volkhov–Kunda (Lower Ordovician) succession at Fågelsång, Scania, Sweden

Stouge, S. & Nielsen, Arne T. 2003–04–30: An integrated biostratigraphical analysis of the Volkhov –Kunda (Lower Ordovician) succession at Fågelsång, Scania, Sweden. Bulletin of the Geological Society of Denmark, Vol. 50, pp. 75–94. Copenhagen.

The Komstad Limestone is composed of dark grey to black micritic limestone deposited in an outer shelf environment on the margin of the East European Platform. It represents an interval of low sea level during the late Volkhov to early Kunda that led to the spread of limestones into the shale-dominated western lithofacies belt.

The Baltoniodus norrlandicus, Lenodus antivariabilis, Lenodus variabilis, Yangtzeplacognathus crassus and Lenodus pseudoplanus conodont zones were identified in the upper Volkhov to lower Kunda interval. Based on the most recent taxonomic and biostratigraphical framework, the Megistaspis limbata (with two subzones), Asaphus expansus and Asaphus raniceps trilobite zones were identified. A shale intercalation in the lower part of the Komstad Limestone contains the graptolites Phyllograptus cor in association with Glyptograptus sp. and is referred to Darriwilian 1 (Upper Arenig). In the Baltoscandian zonation this matches the upper part of the Didymograptus hirundo graptolite Zone. The upper level of the Komstad Limestone and the basal part of the overlying Almelund Shale do not contain graptolites, whereas the succeeding black shales of the Almelund Shale belong to the Holmograptus lentus Zone (Llanvirn). The Arenig-Llanvirn boundary is situated at or very near the top of the Komstad Limestone.

The conodont assemblage in the lower part of the Komstad Limestone at Fågelsång is associated with forms of Gondwanan affinity, which probably reflects the cool water environment of the outer shelf setting. The higher part of the limestone contains the Whiterockian conodont species Dzikodus sp. and Histiodella tableheadensis. These important Laurentian forms occur together with abundant Gothodus sp. 1 and Cyclopyge umbonata in the base of the Asaphus expansus Zone. The arrival of the Laurentian taxa – as well as Cyclopyge with an ‘Gondwana’ affinity – is related to a transient sea level rise at the base of the A. expansus trilobite Zone.

Key words: Komstad Limestone, Almelund Shale, Fågelsång, Scania, Sweden, biostratigraphy, conodonts, trilobites, graptolites, Volkhov and Kunda stages, Arenig–Llanvirn boundary, Lower Ordovician.

Svend Stouge [ ], Geological Survey of Denmark & Greenland, Øster Voldgade 10 DK-1350, Copenhagen K, Denmark. Arne Thorshøj Nielsen, [ ], Geological Museum, Øster Voldgade 5-7, DK-1350 Copenhagen K, Denmark; 15th July 2002.

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