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Process- och miljötolkning av sedimentationen i en subglacial läsideskavitet, Järnavik, S. Blekinge

Nilson, Tomas (1996) In Examensarbeten i geologi vid Lunds universitet
Department of Geology
Abstract
This study deals with the development of a subglacial cavity in a province dominated by stoss- and leeside rockridges.
In the area of Järnavik, south-central Blekinge, there is a small excavation on the leeside of a bedrock ridge. The lithofacies combinations in this section contains both diamictons and sorted sediments. The diamict units are both massive and stratified. The sorted sediments are mainly silty in the lower part of the sequence and sandy in the upper.
This survey revealed that:
The cavity had undergone repeated openings and closures indicated by glacial erosion and rapid changes in sedimentology. When the cavity was closed the glacier overrode and sheared previously deposited sediments. In connection with the glacial... (More)
This study deals with the development of a subglacial cavity in a province dominated by stoss- and leeside rockridges.
In the area of Järnavik, south-central Blekinge, there is a small excavation on the leeside of a bedrock ridge. The lithofacies combinations in this section contains both diamictons and sorted sediments. The diamict units are both massive and stratified. The sorted sediments are mainly silty in the lower part of the sequence and sandy in the upper.
This survey revealed that:
The cavity had undergone repeated openings and closures indicated by glacial erosion and rapid changes in sedimentology. When the cavity was closed the glacier overrode and sheared previously deposited sediments. In connection with the glacial contacts a number of thrusts, normal faults and folds were formed. The differences in cavity size were presumably caused by seasonai variations in meltwater production and ice-movement velocity.
The deposits during cavity closures were mainly deformation till even though pure lodgement till also occur especially in the upper units. At times when the cavity was open debris-flows entered the basin and they turned distally into fine-grained turbidites.
Glaciofluvial sediments were deposited when the meltwater production was so great that a linked cavity system could form. The lithofacies associations (sandy bars, dunes, ripples and planar beds) without any coarser fractions show that the current velocities were rather weak.
Local ice-movement directions, as indicated by particle orientation and the direction of thrust planes, differed considerably from the regional ones. The results show that the ice movement had an easterly direction instead of the more common northerly in the area. The reason for this is most likely that the ice-direction was controlled by the bedrock ridges and that these ridges were able to deflect the direction of the ice flow.
The whole area was situated below the highest shoreline as indicated by the beach deposits on top of the sequence.
Cavities usually form on the leeside of large obstacle where the glacier have difficulties reaching the surface. Thin ice, high ice-velocities, steep obstacles and large pore-water pressures further reduce the ability of the glacier to flow around the obstacle.
All these factors indicate that the deposit at Järnavik has formed during the deglaciation period. When the glacier is retreating from an area it usually gets thinner which could affect the ice movement direction so that it gets controlled by the topography. A thin ice in the area can therefor explain the odd ice direction. The ice velocity, was probably increased due to a calving ice margin in the Baltic Ice Lake.
Since the ice movement direction changes in the upper part of the sequenee from an easterly to a northerly now, this could be the reason for the ultimate cavity closure. (Less)
Please use this url to cite or link to this publication:
author
Nilson, Tomas
supervisor
organization
year
type
H1 - Master's Degree (One Year)
subject
keywords
geografi, geologi, processtolkning, miljötolkning, sedimentation, subglacial läsideskavitet, Järnavik, Blekinge
publication/series
Examensarbeten i geologi vid Lunds universitet
report number
66
language
Swedish
id
2369920
date added to LUP
2012-10-23 10:08:09
date last changed
2012-10-23 10:08:09
@misc{2369920,
  abstract     = {This study deals with the development of a subglacial cavity in a province dominated by stoss- and leeside rockridges.
In the area of Järnavik, south-central Blekinge, there is a small excavation on the leeside of a bedrock ridge. The lithofacies combinations in this section contains both diamictons and sorted sediments. The diamict units are both massive and stratified. The sorted sediments are mainly silty in the lower part of the sequence and sandy in the upper.
This survey revealed that:
The cavity had undergone repeated openings and closures indicated by glacial erosion and rapid changes in sedimentology. When the cavity was closed the glacier overrode and sheared previously deposited sediments. In connection with the glacial contacts a number of thrusts, normal faults and folds were formed. The differences in cavity size were presumably caused by seasonai variations in meltwater production and ice-movement velocity.
The deposits during cavity closures were mainly deformation till even though pure lodgement till also occur especially in the upper units. At times when the cavity was open debris-flows entered the basin and they turned distally into fine-grained turbidites.
Glaciofluvial sediments were deposited when the meltwater production was so great that a linked cavity system could form. The lithofacies associations (sandy bars, dunes, ripples and planar beds) without any coarser fractions show that the current velocities were rather weak.
Local ice-movement directions, as indicated by particle orientation and the direction of thrust planes, differed considerably from the regional ones. The results show that the ice movement had an easterly direction instead of the more common northerly in the area. The reason for this is most likely that the ice-direction was controlled by the bedrock ridges and that these ridges were able to deflect the direction of the ice flow.
The whole area was situated below the highest shoreline as indicated by the beach deposits on top of the sequence.
Cavities usually form on the leeside of large obstacle where the glacier have difficulties reaching the surface. Thin ice, high ice-velocities, steep obstacles and large pore-water pressures further reduce the ability of the glacier to flow around the obstacle.
All these factors indicate that the deposit at Järnavik has formed during the deglaciation period. When the glacier is retreating from an area it usually gets thinner which could affect the ice movement direction so that it gets controlled by the topography. A thin ice in the area can therefor explain the odd ice direction. The ice velocity, was probably increased due to a calving ice margin in the Baltic Ice Lake.
Since the ice movement direction changes in the upper part of the sequenee from an easterly to a northerly now, this could be the reason for the ultimate cavity closure.},
  author       = {Nilson, Tomas},
  keyword      = {geografi,geologi,processtolkning,miljötolkning,sedimentation,subglacial läsideskavitet,Järnavik,Blekinge},
  language     = {swe},
  note         = {Student Paper},
  series       = {Examensarbeten i geologi vid Lunds universitet},
  title        = {Process- och miljötolkning av sedimentationen i en subglacial läsideskavitet, Järnavik, S. Blekinge},
  year         = {1996},
}