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Tectonically-Induced Lowstand on an Ordovician Ramp – the Björkåsholmen Formation of Scandinavia
Uppsala University, Music and Museums, Museum of Evolution. Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences. (Paleobiologi, Palaeobiology)
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2008 (English)In: 2008 Houston Annual Meeting (5–9 October 2008).: Geological Society of America Abstracts with Programs, Geological Society of America , 2008, 393- p.Conference paper (Other academic)
Abstract [en]

The Tremadoc Björkåsholmen Formation is an approximately one meter thick unit consisting of several carbonate beds with intercalated shales and some glauconite content in its upper part. It is underlain by the Cambrian-Ordovician Alum Shale Formation throughout Scandinavia, and in Norway and westernmost Sweden overlain by the Lower- to Middle Ordovician Tøyen Formation. The Björkåsholmen Formation is remarkably widespread in Scandinavia with relatively little variation in thickness or faunal composition. Internally, the unit consists of thin beds at the base with thicker ones in the center and again thinner beds towards the top. Thin beds are generally mud-rich while thicker ones display packstones. In the Norwegian sections the grain-rich facies have been mostly diagenetically overprinted, whereas eastern Swedish localities are characterized by much better facies preservation regardless of lithology. The overall mud-rich facies of the Björkåsholmen Formation reflects deposition on a low-inclined ramp environment. Coarse-grained sediments represent the proximal facies while increasing mud content shows deeper water middle to outer ramp deposition. Facies trends indicate that the Björkåsholmen Formation consists of at least five regressions and successive transgressions with a maximum lowstand represented by its coarse-grained central portion. As the Björkåsholmen Formation is sandwiched between two prominent outer shelf shales in the Norwegian and westernmost Swedish localities it also represents the most pronounced sea-level lowstand in the Early Ordovician of Scandinavia.This lowstand is within the upper part of the Aorograptus victoriae Biozone and the basal Kiaerograptus supremus Biozone. Deposition of the Björkåsholmen Formation therefore likely represents a fairly short time-span. However, this remarkably strong lowstand in Scandinavia is not paralleled by base-level falls in Bolivia, China or Newfoundland. It is therefore suggested that the Björkåsholmen Formation reflects a tectonic rather than a eustatic event which is restricted exclusively to Baltoscandia.

Place, publisher, year, edition, pages
Geological Society of America , 2008. 393- p.
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Research subject
Earth Science with specialization in Historical Geology and Palaeontology
URN: urn:nbn:se:uu:diva-10122OAI: oai:DiVA.org:uu-10122DiVA: diva2:37890
Geological Association of America, Annual Meeting, Houston,5–9 October 2008
Available from: 2008-10-15 Created: 2008-10-15 Last updated: 2015-01-13

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Ebbestad, Jan OveFrisk, Åsa M.
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Museum of EvolutionDepartment of Earth SciencesPalaeobiology

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