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Past dynamics of a marine-terminating glacier in lower Borgarfjörður, west Iceland - Analyses of glaciotectonic sediments and landforms

Sigfusdottir, Thorbjorg LU (2019) 87.
Abstract
Large and complete sections through glacial landforms formed in subaqueous environments are rare, impeding our knowledge of their internal composition and the processes responsible for their formation. Following the last deglaciation of the Icelandic Ice Sheet (IIS), a marine-terminating outlet glacier advanced out of the fjord of Borgarfjörður resulting in large-scale glaciotectonic deformation of glaciomarine sediments. Due to isostatic uplift and erosion, these glaciotectonic formations are now extensively exposed in the region and provide an opportunity to study a glaciotectonised marine sequence on land. The aim of this study is to investigate the inter-relationship between ice-marginal deformation and deposition in a glaciomarine... (More)
Large and complete sections through glacial landforms formed in subaqueous environments are rare, impeding our knowledge of their internal composition and the processes responsible for their formation. Following the last deglaciation of the Icelandic Ice Sheet (IIS), a marine-terminating outlet glacier advanced out of the fjord of Borgarfjörður resulting in large-scale glaciotectonic deformation of glaciomarine sediments. Due to isostatic uplift and erosion, these glaciotectonic formations are now extensively exposed in the region and provide an opportunity to study a glaciotectonised marine sequence on land. The aim of this study is to investigate the inter-relationship between ice-marginal deformation and deposition in a glaciomarine setting, increase the understanding of glaciotectonic processes at the margins of marine-terminating glaciers, and elucidate past glacier dynamics and the regional glacial history.
The main focus of the sedimentological and structural work was on the coastal cliffs of Belgsholt, Melabakkar-Ásbakkar and Skipanes. The sediments and glaciotectonic structures were analysed on a range of scales using sedimentological and structural field methods, high resolution LiDAR scans and micromorphological thin sections. Shells of marine molluscs were sampled for radiocarbon dating and interpreted in the context of the stratigraphy and glaciotectonics to constrain the timeline of the regional glacial history.
The study revealed a series of glaciotectonic moraines in the Melabakkar-Ásbakkar and Belgsholt coastal cliffs. The southernmost moraine is the largest and structurally most complex and is interpreted to indicate the maximum extent of the Borgarfjörður glacier. Other moraines in the series record repeated re-advances of the glacier during its active northward retreat. The moraines were mainly formed by large-scale thrusting and folding of glaciomarine sediments and subsequent deposition of ice-marginal sand and gravel. During the active retreat, glaciomarine sediments accumulated in front of the glacier providing source material for the formation of subsequent moraines.
Detailed analysis of micro- and macroscale structures developed within décollements show that the detachment and transport of unlithified and unfrozen sediment blocks was enabled by overpressurisation of subglacial/ice marginal porewater. This implies that hydrogeology played a key role in the construction of the moraines.
The advances and subsequent active retreat of the Borgarfjörður glacier occurred between c. 13.0 and 11.7 cal. ka BP indicating that it coincided with widespread glacier advances in Iceland and in the North Atlantic region during the Younger Dryas (c. 12.7-11.5 cal. ka BP). In the Early Holocene, after c. 11.3 cal. ka BP, the glacier re-expanded to a position around 5 km inside the Younger Dryas ice limit, indicating more extensive glaciation in the region than previously thought.
The results of this thesis highlight the diversity of sedimentological and glaciotectonic processes involved in the construction of large glaciotectonic moraines at the margins of marine-terminating glaciers, and indicate that such glaciers were more dynamic during the deglaciation than previously thought. (Less)
Abstract (Swedish)
Glaciärer är väldigt effektiva på att forma landskapet och skapa nya sediment och landformer genom erosion, deposition och deformation. Genom att studera dessa landformer och sediment kan värdefull information erhållas om de processer som formade dem, glaciärers historia samt klimat- och miljöförändringar.
Detta projekt fokuserar på glaciala landformer och sediment i den lägre-Borgarfjörður regionen, västra Island. Vid slutet av den senaste istiden avancerade en stor glaciär in i området, vilket resulterade i storskalig deformation (glacitektonisk deformation) av existerande sediment. Området var vid denna tidpunkt beläget under havsnivån och glaciären slutade därför i havet. På grund av landhöjningen efter den slutgiltiga... (More)
Glaciärer är väldigt effektiva på att forma landskapet och skapa nya sediment och landformer genom erosion, deposition och deformation. Genom att studera dessa landformer och sediment kan värdefull information erhållas om de processer som formade dem, glaciärers historia samt klimat- och miljöförändringar.
Detta projekt fokuserar på glaciala landformer och sediment i den lägre-Borgarfjörður regionen, västra Island. Vid slutet av den senaste istiden avancerade en stor glaciär in i området, vilket resulterade i storskalig deformation (glacitektonisk deformation) av existerande sediment. Området var vid denna tidpunkt beläget under havsnivån och glaciären slutade därför i havet. På grund av landhöjningen efter den slutgiltiga deglaciationen är nu glacimarina sediment och glaciala landformer skapade av glaciären belägna på torra land. Ett antal skärningar förekommer genom många av dessa landformer och erbjuder ett sällsynt tillfälle att i detalj studera en serie av glaciala landformer formade i en subakvatisk miljö.
Målet med den här studien är att undersöka deformation vid en isrand och deposition i en glacimarin miljö, öka förståelsen för glacitektoniska processer vid isränder, samt att öka kunskapen om den regionala glaciationshistorien och tidigare isdynamik.
De huvudsakliga studielokalerna var kustklipporna vid Belgsholt, Melabakkar-Ásbakkar och Skipanes, vilka är nästan 6 km långa totalt och upp till 30 m höga. Den exponerade stratigrafin och glacitektoniken vid klipporna analyserades i fält och med högupplöst LiDAR. Prover togs från klipporna för att undersöka mikrostrukturer i mikroskop. Skal av marina mollusker samlades in för kol-14-datering och för att avgränsa tidsramen för de glaciala fluktuationerna.
Resultaten visade att en serie av begravda ändmoräner är exponerade vid Melabakkar-Ásbakkar och Bergsholts kustklippor. Samtliga moräner bildades av sammanpressade sediment vid isränderna och staplande av stora sedimentblock från havsbotten. Detta lösgörande och transport av sedimentblock underlättades genom övertryck av grundvatten som inträffade i samband med glaciärens frammarscher. Smältvatten från glaciären förde också med sig stora mängder sediment ut i havet. De mest grovkorniga sedimenten (sand och grus) avsattes vid isränderna och blev därefter deformerade av isframryckningen. De mer finkorniga sedimenten transporterades längre bort från isränderna och bildade där tjocka sekvenser av stratifierade/laminerade glacimarina sediment.
Den sydligaste belägna moränen i serien är också den störste och indikerar den maximala utbredningen av Borgarfjörður-glaciären. Denna framryckning inträffade mest sannolikt mellan 13 000 och 11 700 år sedan (under Yngre Dryas), vilket indikerar att den var samtida med utbredda glaciärframryckningar annorstädes i Island och i den Nordatlantiska regionen. Andra moräner i serien påvisar temporära återframryckningar, möjligtvis orsakade av återupprepade glaciärsvämningar/hastighetsökningar när glaciären blev successivt mindre på grund av glaciärens minskande massbalans. Den sista påvisade framryckningen skedde under Tidig Holocen (omkring 11 300 år sedan) och visar på att den slutliga deglaciationen av denna region skedde senare än man tidigare har trott.
Allt som allt, den här studien beskriver deformation och deposition i subakvatiska miljöer samt processer involverade i skapandet av stora glacitektoniska moräner. Studien ökar dessutom vår förståelse för Islands glaciationshistoria och belyser det dynamiska beteendet av glaciärer under den senaste deglaciationen.
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Please use this url to cite or link to this publication:
author
supervisor
opponent
  • Dr. Waller, Richard I., Keele University, Staffordshire, UK
organization
publishing date
type
Thesis
publication status
published
subject
keywords
glacier dynamics, marine-terminating glacier, glaciomarine processes, thrust-block moraines, glaciotectonics, micromorphology, Early Holocene, Late Weichselian, Iceland
volume
87
edition
LUNDQUA thesis
pages
123 pages
defense location
Lecture hall "Pangea", Geocentrum II, Sölvegatan 12, Lund
defense date
2019-10-11 13:15:00
ISBN
978-91-87847-47-9
978-91-87847-46-2
language
English
LU publication?
yes
id
59e1e975-2563-4156-8800-9fe4f2b1140c
date added to LUP
2019-09-16 12:54:03
date last changed
2019-10-08 11:10:44
@phdthesis{59e1e975-2563-4156-8800-9fe4f2b1140c,
  abstract     = {Large and complete sections through glacial landforms formed in subaqueous environments are rare, impeding our knowledge of their internal composition and the processes responsible for their formation. Following the last deglaciation of the Icelandic Ice Sheet (IIS), a marine-terminating outlet glacier advanced out of the fjord of Borgarfjörður resulting in large-scale glaciotectonic deformation of glaciomarine sediments. Due to isostatic uplift and erosion, these glaciotectonic formations are now extensively exposed in the region and provide an opportunity to study a glaciotectonised marine sequence on land. The aim of this study is to investigate the inter-relationship between ice-marginal deformation and deposition in a glaciomarine setting, increase the understanding of glaciotectonic processes at the margins of marine-terminating glaciers, and elucidate past glacier dynamics and the regional glacial history.<br/>The main focus of the sedimentological and structural work was on the coastal cliffs of Belgsholt, Melabakkar-Ásbakkar and Skipanes. The sediments and glaciotectonic structures were analysed on a range of scales using sedimentological and structural field methods, high resolution LiDAR scans and micromorphological thin sections. Shells of marine molluscs were sampled for radiocarbon dating and interpreted in the context of the stratigraphy and glaciotectonics to constrain the timeline of the regional glacial history.<br/>The study revealed a series of glaciotectonic moraines in the Melabakkar-Ásbakkar and Belgsholt coastal cliffs. The southernmost moraine is the largest and structurally most complex and is interpreted to indicate the maximum extent of the Borgarfjörður glacier. Other moraines in the series record repeated re-advances of the glacier during its active northward retreat. The moraines were mainly formed by large-scale thrusting and folding of glaciomarine sediments and subsequent deposition of ice-marginal sand and gravel. During the active retreat, glaciomarine sediments accumulated in front of the glacier providing source material for the formation of subsequent moraines.<br/>Detailed analysis of micro- and macroscale structures developed within décollements show that the detachment and transport of unlithified and unfrozen sediment blocks was enabled by overpressurisation of subglacial/ice marginal porewater. This implies that hydrogeology played a key role in the construction of the moraines.<br/>The advances and subsequent active retreat of the Borgarfjörður glacier occurred between c. 13.0 and 11.7 cal. ka BP indicating that it coincided with widespread glacier advances in Iceland and in the North Atlantic region during the Younger Dryas (c. 12.7-11.5 cal. ka BP). In the Early Holocene, after c. 11.3 cal. ka BP, the glacier re-expanded to a position around 5 km inside the Younger Dryas ice limit, indicating more extensive glaciation in the region than previously thought.<br/>The results of this thesis highlight the diversity of sedimentological and glaciotectonic processes involved in the construction of large glaciotectonic moraines at the margins of marine-terminating glaciers, and indicate that such glaciers were more dynamic during the deglaciation than previously thought.},
  author       = {Sigfusdottir, Thorbjorg},
  isbn         = {978-91-87847-47-9},
  language     = {eng},
  school       = {Lund University},
  title        = {Past dynamics of a marine-terminating glacier in lower Borgarfjörður, west Iceland - Analyses of glaciotectonic sediments and landforms},
  url          = {https://lup.lub.lu.se/search/ws/files/69544946/Thorbj_rg_Sigfusdottir_web.pdf},
  volume       = {87},
  year         = {2019},
}