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Late Ordovician-Early Silurian facies development and stratigraphy of Jämtland, central Sweden

Dahlqvist, Peter LU (2005) In Litholund theses
Abstract (Swedish)
Popular Abstract in Swedish

Denna doktorsavhandling behandlar de 450-440 miljoner år gamla marina sediment som idag påträffas i form av deformerade och uppveckade sedimentära bergarter vid randen av den svenska fjällkedjan i Jämtland. Målet med studien har varit att kartlägga och tolka hur, och i vilken utsträckning, lokala och globala händelser påverkat de depositionsmiljöer som sedimenten avlagrats i.



Övergången mellan tidsperioderna ordovicium och silur för cirka 450-440 miljoner år sedan var globalt sett en orolig tid. Nedisningar på Sydpolen band periodvis upp stora vattenmassor i form av inlandsisar som växelvis smälte och växte till, med kraftiga variationer i den globala havsnivån som följd. Den... (More)
Popular Abstract in Swedish

Denna doktorsavhandling behandlar de 450-440 miljoner år gamla marina sediment som idag påträffas i form av deformerade och uppveckade sedimentära bergarter vid randen av den svenska fjällkedjan i Jämtland. Målet med studien har varit att kartlägga och tolka hur, och i vilken utsträckning, lokala och globala händelser påverkat de depositionsmiljöer som sedimenten avlagrats i.



Övergången mellan tidsperioderna ordovicium och silur för cirka 450-440 miljoner år sedan var globalt sett en orolig tid. Nedisningar på Sydpolen band periodvis upp stora vattenmassor i form av inlandsisar som växelvis smälte och växte till, med kraftiga variationer i den globala havsnivån som följd. Den Baltiska kontinenten (med bl.a. nuvarande Skandinavien) låg strax söder om ekvatorn vid denna tid, och var på väg att kollidera med den nordamerikanska kontinentplattan. Kollisionen veckade och sköt upp land-, och havsområden som legat mellan/på kontinentkanterna och bildade bergskedjan Kaledoniderna, vars rötter idag utgör den skandinaviska fjällkedjan. På grund av denna händelse skapades mycket höga tryck och temperaturer som omvandlade sedimenten till hårt sedimentärt berg (i vissa fall t.o.m. metamorft berg).



En stor del av avhandlingsarbetet har bestått i fältstudier, d.v.s. dokumentation, av de senordoviciska?tidigsiluriska lagerföljderna i Jämtland. I studierna framkom bl.a. tydliga tecken på upprepade uppgrundningar i form av ler- och sanduppfyllnad av marina miljöer. Men var dessa förändringar enbart lokala företeelser eller kunde de kopplas till vad vi sedan tidigare visste om globala nedisningar och relaterade fluktuationer i havsnivån? De data som framkom under projektet indikerar att uppgrundningarna verkligen kan kopplas till dessa globala händelser. För att nå den slutsatsen var det nödvändigt att mera precist datera när uppgrundningarna ägde rum för att jämföra detta med kända tidpunkter för nedisningar och förändringar i havsnivån. Dateringen gjordes genom att analys av fossil (t.ex. konodonter, graptoliter, och brachiopoder) som förekommer i bergarterna. Man kan med en tidsprecision på +- några hundra tusen år korrelera vissa arters uppträdanden och utdöenden både regionalt och globalt.



Den sista delen av avhandlingsarbetet har bestått i att försöka förstå från vilken berggrund sedimenten ursprungligen framvittrade. En pilotstudie där vi analyserade ett speciellt minerals (zirkon) isotopsignaturer (dessa mineral ingår i bergarterna) och på så sätt fick fram deras bildningsåldrar genomfördes. De data som erhölls pekar mot att materialet huvudsakligen kom från landområden sydväst om Jämtland men även från ett okänt lokalt höjdområde. (Less)
Abstract
This thesis is based on studies of Lower Palaeozoic sedimentary successions within the central Scandinavian Caledonides. These deposits have been transported by considerable nappe displacement (several kilometres), and are today relatively isolated from the nearest coeval outcrops (Dalarna 250 km, Oslo Region 350 km). As a consequence, and due to the previously limited biostratigraphical control the investigated area have received little attention the last decades. Therefore, a detailed investigation, using several untried geological tools was performed.



During the study it became clear that the strata reflected substantial changes in depositional environment and complexity in lateral facies relationship during the Late... (More)
This thesis is based on studies of Lower Palaeozoic sedimentary successions within the central Scandinavian Caledonides. These deposits have been transported by considerable nappe displacement (several kilometres), and are today relatively isolated from the nearest coeval outcrops (Dalarna 250 km, Oslo Region 350 km). As a consequence, and due to the previously limited biostratigraphical control the investigated area have received little attention the last decades. Therefore, a detailed investigation, using several untried geological tools was performed.



During the study it became clear that the strata reflected substantial changes in depositional environment and complexity in lateral facies relationship during the Late Ordovician?Early Silurian. This was a time characterised by global environmental changes such as glacio-eustasy and climate change, in particular during the end-Ordovician (Hirnantian) glaciation. Facies and sequence stratigraphical analysis of two key units (the Ede Quartzite and the Kyrkås Quartzite) and their preceding and succeeding units resulted in reinterpretations and refinement of the stratigraphy. The new data implied that the successions could be linked to known Hirnantian sea-level fluctuations. This correlation was subsequently supported, and our understanding of the timing of changes was improved, by biostratigraphical evidence (conodonts, graptolites, and brachiopods). This revealed a ca 5 myr stratigraphical gap, within the Ede Quartzite, spanning the Ordovician?Silurian boundary. It is concluded that the interaction of allocyclic changes (sea level and climate) were the overriding controls on deposition during the Late Ordovician?Early Silurian in the Jämtland basin.



The contrasting sedimentary architecture of the Ede Quartzite to the west and the partly coeval Kyrkås Quartzite to the east remained enigmatic. Provenance studies (radiometric dating of zircons) tentatively indicate different dominating source areas, but further studies are needed to solve their relationship. (Less)
Please use this url to cite or link to this publication:
author
supervisor
opponent
  • Dr Brenchley, Patrick, Honorary Senior Fellow, Liverpool University
organization
publishing date
type
Thesis
publication status
published
subject
keywords
biostratigraphy, Sedimentology, Stratigrafi, Stratigraphy, Paleontologi, Palaeontology, Jämtland, provenance, brachiopods, conodonts, Hirnantian, Silurian, Ordovician, glaciation, eustasy, Sedimentologi, sedimentology
in
Litholund theses
pages
75 pages
publisher
Department of Geology, Lund University
defense location
Geobiosfärscentrum II Sölvegatan 12 223 62 Lund
defense date
2005-05-12 10:15
ISSN
1651-6648
ISBN
91-86746-60-X
language
English
LU publication?
yes
id
a02bc8e7-11b3-4095-b4d2-54458dce3374 (old id 544734)
date added to LUP
2007-09-04 10:58:04
date last changed
2016-09-19 08:44:58
@phdthesis{a02bc8e7-11b3-4095-b4d2-54458dce3374,
  abstract     = {This thesis is based on studies of Lower Palaeozoic sedimentary successions within the central Scandinavian Caledonides. These deposits have been transported by considerable nappe displacement (several kilometres), and are today relatively isolated from the nearest coeval outcrops (Dalarna 250 km, Oslo Region 350 km). As a consequence, and due to the previously limited biostratigraphical control the investigated area have received little attention the last decades. Therefore, a detailed investigation, using several untried geological tools was performed.<br/><br>
<br/><br>
During the study it became clear that the strata reflected substantial changes in depositional environment and complexity in lateral facies relationship during the Late Ordovician?Early Silurian. This was a time characterised by global environmental changes such as glacio-eustasy and climate change, in particular during the end-Ordovician (Hirnantian) glaciation. Facies and sequence stratigraphical analysis of two key units (the Ede Quartzite and the Kyrkås Quartzite) and their preceding and succeeding units resulted in reinterpretations and refinement of the stratigraphy. The new data implied that the successions could be linked to known Hirnantian sea-level fluctuations. This correlation was subsequently supported, and our understanding of the timing of changes was improved, by biostratigraphical evidence (conodonts, graptolites, and brachiopods). This revealed a ca 5 myr stratigraphical gap, within the Ede Quartzite, spanning the Ordovician?Silurian boundary. It is concluded that the interaction of allocyclic changes (sea level and climate) were the overriding controls on deposition during the Late Ordovician?Early Silurian in the Jämtland basin.<br/><br>
<br/><br>
The contrasting sedimentary architecture of the Ede Quartzite to the west and the partly coeval Kyrkås Quartzite to the east remained enigmatic. Provenance studies (radiometric dating of zircons) tentatively indicate different dominating source areas, but further studies are needed to solve their relationship.},
  author       = {Dahlqvist, Peter},
  isbn         = {91-86746-60-X},
  issn         = {1651-6648},
  keyword      = {biostratigraphy,Sedimentology,Stratigrafi,Stratigraphy,Paleontologi,Palaeontology,Jämtland,provenance,brachiopods,conodonts,Hirnantian,Silurian,Ordovician,glaciation,eustasy,Sedimentologi,sedimentology},
  language     = {eng},
  pages        = {75},
  publisher    = {Department of Geology, Lund University},
  school       = {Lund University},
  series       = {Litholund theses},
  title        = {Late Ordovician-Early Silurian facies development and stratigraphy of Jämtland, central Sweden},
  year         = {2005},
}