Advanced

Early–Middle Ordovician biotic and sedimentary dynamics in the Baltoscandian paleobasin

Lindskog, Anders LU (2017)
Abstract (Swedish)
Baltoskandien utgör en del av paleokontinenten Baltica, vilken mestadels var täckt av ett grunt epeiriskt hav under den ordoviciska tidsperioden (ca 485,5–444 Ma). Detta uråldriga hav efterlämnade en tunn lagerföljd av sedimentära bergarter. Under tidig–mellanordovicium (ca 485,5–457,5 Ma) befann sig Baltica vid tempererade breddgrader på södra halvklotet och kallvattenkarbonater bildades över stora områden i den Baltoskandiska paleobassängen. Den så kallade ortoceratitkalkstenen är den mest vittspridda bergartstypen från tidig–mellanordovicium i Sverige. Den spred sig successivt geografiskt och stora delar av under–mellanordovicium på Sveriges fastland karaktäriseras av denna litologi. ”Ortoceratitkalkstenens” avsättningsmiljö har länge... (More)
Baltoskandien utgör en del av paleokontinenten Baltica, vilken mestadels var täckt av ett grunt epeiriskt hav under den ordoviciska tidsperioden (ca 485,5–444 Ma). Detta uråldriga hav efterlämnade en tunn lagerföljd av sedimentära bergarter. Under tidig–mellanordovicium (ca 485,5–457,5 Ma) befann sig Baltica vid tempererade breddgrader på södra halvklotet och kallvattenkarbonater bildades över stora områden i den Baltoskandiska paleobassängen. Den så kallade ortoceratitkalkstenen är den mest vittspridda bergartstypen från tidig–mellanordovicium i Sverige. Den spred sig successivt geografiskt och stora delar av under–mellanordovicium på Sveriges fastland karaktäriseras av denna litologi. ”Ortoceratitkalkstenens” avsättningsmiljö har länge varit ofullständigt känd. Detta beror till stor del på en avsaknad av motsvarigheter bland nutida avsättningsmiljöer, men också på luckor i vår kunskap om bergarten i allmänhet. Det råder en generell enighet att ”ortoceratitkalkstenen” representerar en kallvattenkarbonat avsatt i ett epeiriskt hav med mycket begränsat sedimentinflöde, men tolkningar angående rådande vattendjup har skiljt sig åt.
De åtta artiklar som utgör grunden till den här doktorsavhandlingen baseras på olika undersökningar av ”ortoceratitkalkstenen” i Sverige och likåldriga lager i omgivande länder. Detaljerade makro- och mikroskopiska studier av biotiska och sedimentära egenskaper har gett värdefull information om den Baltoskandiska paleobassängen samt den biotiska och miljömässiga utvecklingen under tidig–mellanordovicium. Resultaten visar att ”ortoceratitkalkstenens” avsättningsmiljö varierade avsevärt i både rum och tid; den spände från intertidala områden till miljöer med många tiotals meter djupt vatten. Variationer i övergripande egenskaper och fossilinnehåll i ”ortoceratitkalkstenen” samt likåldriga regionala lager tycks främst avspegla variationer i (relativ) havsnivå.
De sammantagna resultaten tyder på att havsnivån varierade kraftigt genom tidig–mellanordovicium, sannolikt som en följd av klimatvariationer och relaterade förändringar i vattenmassorna globalt. De skönjbara variationerna är samstämmiga mellan flera olika proxies – både abiotiska och biotiska – och cykliska mönster förekommer ofta. Geokemiska data indikerar att klimatet förändrades avsevärt genom tidig–mellanordovicium och att det slutligen gick in i en istidsliknande fas. Inträdet i den senare fasen avspeglas i en tydlig sänkning av havsnivån under mellanordovicium. De tidigare nämnda förändringarna påverkade marina miljöer och bidrog sannolikt till den snabba diversifiering som skedde under den så kallade stora ordoviciska biodiversifieringen (fritt översatt från engelskans Great Ordovician Biodiversification Event). Förfinade absoluta och relativa tidsskalor för mellanordovicium visar att biodiversifieringen var oberoende av ett utdraget meteoritbombardemang under detta tidsavsnitt.
Artiklarna i avhandlingen visar tillsammans att en kombination av infallsvinklar och analysmetoder ger maximal information och tolkningsförmåga. Användning av toppmoderna analys- och avbildningstekniker möjliggör nya upptäckter av tidigare okända bergartsegenskaper och fossil, samt bättre beskrivning av och förståelse för sådana som varit kända sedan tidigare. (Less)
Abstract
The Baltoscandian region forms part of the paleocontinent Baltica, which was largely covered by a shallow epeiric sea throughout much of the Ordovician (c. 485.5–444 Ma). This ancient sea is today recorded by a thin succession of sedimentary rocks. During the Early–Middle Ordovician (c. 485.5–457.5 Ma), Baltica was situated in mid-latitudes on the southern hemisphere and cool-water carbonates formed across large areas of the Baltoscandian paleobasin. The so-called orthoceratite limestone is the most widely distributed rock type from the Early–Middle Ordovician in Sweden. It developed in a time-transgressive manner geographically, but much of the Lower–Middle Ordovician succession in the mainland of Sweden is typically characterized by this... (More)
The Baltoscandian region forms part of the paleocontinent Baltica, which was largely covered by a shallow epeiric sea throughout much of the Ordovician (c. 485.5–444 Ma). This ancient sea is today recorded by a thin succession of sedimentary rocks. During the Early–Middle Ordovician (c. 485.5–457.5 Ma), Baltica was situated in mid-latitudes on the southern hemisphere and cool-water carbonates formed across large areas of the Baltoscandian paleobasin. The so-called orthoceratite limestone is the most widely distributed rock type from the Early–Middle Ordovician in Sweden. It developed in a time-transgressive manner geographically, but much of the Lower–Middle Ordovician succession in the mainland of Sweden is typically characterized by this lithology. The depositional environment of the ‘orthoceratite limestone’ has long remained poorly understood in many respects. This is in large part due to a lack of analogous depositional environments in modern seas, but also due to remaining gaps in our knowledge about the rock type in general. It has generally been agreed upon that the ‘orthoceratite limestone’ is a cool-water deposit formed in a sediment-starved epeiric sea, but interpretations have differed widely with regards to prevailing water depth.
The eight papers appended to this doctoral dissertation are based on various investigations of the ‘orthoceratite limestone’ in Sweden and coeval rocks in surrounding countries. Detailed macroscopic and microscopic studies of the biotic and sedimentary characteristics have added information about the Baltoscandian paleobasin and the biotic and paleoenvironmental development during the Early–Middle Ordovician. It is concluded that the depositional environment of the ‘orthoceratite limestone’ varied considerably through both space and time; it spanned from intertidal areas to settings many tens of meters deep. Variations in
the overall characteristics and fossil content of the ‘orthoceratite limestone’ and coeval regional rocks appear to mainly record variations in (relative) sea level.
The collective results indicate that sea level varied significantly throughout the Early–Middle Ordovician, likely in large part as a response to variations in climate and related changes in global marine water volume. The inferred variations are consistent across multiple different proxies – abiotic and biotic alike – and cyclic patterns occur in the datasets. Geochemical data suggest that the global climate changed considerably during the Early–Middle Ordovician, and that the climate ultimately entered an ‘Icehouse’-like state. The onset of the latter phase is recorded as a distinct drop in sea level during the Middle Ordovician. The aforementioned changes reverberated through the marine realm and likely contributed to the rapid diversification that is seen among fossils during the so-called Great Ordovician Biodiversification Event (GOBE). Based on a refined absolute and relative time scale for the Middle Ordovician, the GOBE can be confidently shown to be unrelated to a prolonged meteorite bombardment that occurred during this time interval.
The papers in the dissertation collectively show that a combination of approaches and analytic techniques leads to maximal information output and confidence in interpretations. The use of state-of-the-art analytic and imaging techniques further allows for the discovery of previously undocumented rock characteristics and fossils, and better description and understanding of such documented before. (Less)
Please use this url to cite or link to this publication:
author
supervisor
opponent
  • Prof. Dr. Munnecke, Axel, Friedrich-Alexander-Universität Erlangen-Nürnberg, Germany
organization
publishing date
type
Thesis
publication status
published
subject
keywords
Ordovician, ‘orthoceratite limestone’, carbonate sedimentology, microfacies, paleoecology, paleoenvironment, paleontology, Baltoscandia
pages
132 pages
publisher
Lund University, Faculty of Science, Department of Geology, Lithosphere and Biosphere Science
defense location
Lecture hall “Pangea”, Geocentre II, Sölvegatan 12, Lund
defense date
2017-03-24 13:15
ISBN
978-91-87847-30-1
978-91-87847-31-8
language
English
LU publication?
yes
id
c902e969-4918-49ef-94f5-4f0492e423ad
date added to LUP
2017-02-24 14:04:39
date last changed
2017-03-10 14:41:11
@phdthesis{c902e969-4918-49ef-94f5-4f0492e423ad,
  abstract     = {The Baltoscandian region forms part of the paleocontinent Baltica, which was largely covered by a shallow epeiric sea throughout much of the Ordovician (c. 485.5–444 Ma). This ancient sea is today recorded by a thin succession of sedimentary rocks. During the Early–Middle Ordovician (c. 485.5–457.5 Ma), Baltica was situated in mid-latitudes on the southern hemisphere and cool-water carbonates formed across large areas of the Baltoscandian paleobasin. The so-called orthoceratite limestone is the most widely distributed rock type from the Early–Middle Ordovician in Sweden. It developed in a time-transgressive manner geographically, but much of the Lower–Middle Ordovician succession in the mainland of Sweden is typically characterized by this lithology. The depositional environment of the ‘orthoceratite limestone’ has long remained poorly understood in many respects. This is in large part due to a lack of analogous depositional environments in modern seas, but also due to remaining gaps in our knowledge about the rock type in general. It has generally been agreed upon that the ‘orthoceratite limestone’ is a cool-water deposit formed in a sediment-starved epeiric sea, but interpretations have differed widely with regards to prevailing water depth.<br/>The eight papers appended to this doctoral dissertation are based on various investigations of the ‘orthoceratite limestone’ in Sweden and coeval rocks in surrounding countries. Detailed macroscopic and microscopic studies of the biotic and sedimentary characteristics have added information about the Baltoscandian paleobasin and the biotic and paleoenvironmental development during the Early–Middle Ordovician. It is concluded that the depositional environment of the ‘orthoceratite limestone’ varied considerably through both space and time; it spanned from intertidal areas to settings many tens of meters deep. Variations in<br/>the overall characteristics and fossil content of the ‘orthoceratite limestone’ and coeval regional rocks appear to mainly record variations in (relative) sea level.<br/>The collective results indicate that sea level varied significantly throughout the Early–Middle Ordovician, likely in large part as a response to variations in climate and related changes in global marine water volume. The inferred variations are consistent across multiple different proxies – abiotic and biotic alike – and cyclic patterns occur in the datasets. Geochemical data suggest that the global climate changed considerably during the Early–Middle Ordovician, and that the climate ultimately entered an ‘Icehouse’-like state. The onset of the latter phase is recorded as a distinct drop in sea level during the Middle Ordovician. The aforementioned changes reverberated through the marine realm and likely contributed to the rapid diversification that is seen among fossils during the so-called Great Ordovician Biodiversification Event (GOBE). Based on a refined absolute and relative time scale for the Middle Ordovician, the GOBE can be confidently shown to be unrelated to a prolonged meteorite bombardment that occurred during this time interval.<br/>The papers in the dissertation collectively show that a combination of approaches and analytic techniques leads to maximal information output and confidence in interpretations. The use of state-of-the-art analytic and imaging techniques further allows for the discovery of previously undocumented rock characteristics and fossils, and better description and understanding of such documented before.},
  author       = {Lindskog, Anders},
  isbn         = {978-91-87847-30-1},
  keyword      = {Ordovician,‘orthoceratite limestone’,carbonate sedimentology,microfacies,paleoecology,paleoenvironment,paleontology,Baltoscandia},
  language     = {eng},
  pages        = {132},
  publisher    = {Lund University, Faculty of Science, Department of Geology, Lithosphere and Biosphere Science},
  school       = {Lund University},
  title        = {Early–Middle Ordovician biotic and sedimentary dynamics in the Baltoscandian paleobasin},
  year         = {2017},
}