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Grounding zone wedges in the Nyköping area, southeastern Sweden

Jónsdóttir, Svava Kristin LU (2025) In Dissertations in Geology at Lund University GEOR02 20251
Department of Geology
Abstract
The Nyköping area, located within the Middle Swedish End Moraine Zone (MSEMZ), contains numerous scattered ridges. While other parts of the MSEMZ show evidence of an ice readvance during the Younger Dryas, such features are absent in this region. Instead, some of the ridges in the Nyköping area exhibit morphologies similar to grounding zone wedges (GZW), which typically form subaqueously at temporary stillstands of the ice margin. However, sedimentological investigations in the area are lacking, and terrestrial GZWs remain poorly studied due to their frequent submergence. This study aims to assess whether the ridges in the Nyköping area represent GZWs, determine the glacial conditions under which they may have formed, and compare them to... (More)
The Nyköping area, located within the Middle Swedish End Moraine Zone (MSEMZ), contains numerous scattered ridges. While other parts of the MSEMZ show evidence of an ice readvance during the Younger Dryas, such features are absent in this region. Instead, some of the ridges in the Nyköping area exhibit morphologies similar to grounding zone wedges (GZW), which typically form subaqueously at temporary stillstands of the ice margin. However, sedimentological investigations in the area are lacking, and terrestrial GZWs remain poorly studied due to their frequent submergence. This study aims to assess whether the ridges in the Nyköping area represent GZWs, determine the glacial conditions under which they may have formed, and compare them to other ridges in the MSEMZ. Three ridges were examined for detailed sedimentological logging. The results show diamicts with underlying sorted sediments suggesting subaqueous deposition at a relatively stable ice margin, without evidence of a major readvance. One ridge appears more likely to represent a GZW than the other two. These findings contribute to a better understanding of glacial retreat dynamics during the Younger Dryas and may support future investigations into the formation and distribution of potential GZWs on land. (Less)
Abstract (Swedish)
Nyköpingsområdet, som ligger i den Mellansvenska ändmoränzonen (MSEMZ), innehåller många spridda moränryggar. Medan andra delar av MSEMZ uppvisar spår av en isframstöt under yngre dryas, saknas sådana tecken i detta område. Istället uppvisar vissa av ryggarna i Nyköpingsområdet morfologier som liknar så kallade grounding zone wedges (GZW), vilka vanligtvis bildas subakvatiskt när iskanten tillfälligt stod stilla. Dock saknas sedimentologiska undersökningar i området, och landbaserade GZW är generellt dåligt studerade eftersom de flesta förekommer på havsbottnen och är täckta av vatten. Denna studie syftar till att bedöma om ryggarna i Nyköpingsområdet kan tolkas som GZW, undersöka under vilka glaciala förhållanden de kan ha bildats samt... (More)
Nyköpingsområdet, som ligger i den Mellansvenska ändmoränzonen (MSEMZ), innehåller många spridda moränryggar. Medan andra delar av MSEMZ uppvisar spår av en isframstöt under yngre dryas, saknas sådana tecken i detta område. Istället uppvisar vissa av ryggarna i Nyköpingsområdet morfologier som liknar så kallade grounding zone wedges (GZW), vilka vanligtvis bildas subakvatiskt när iskanten tillfälligt stod stilla. Dock saknas sedimentologiska undersökningar i området, och landbaserade GZW är generellt dåligt studerade eftersom de flesta förekommer på havsbottnen och är täckta av vatten. Denna studie syftar till att bedöma om ryggarna i Nyköpingsområdet kan tolkas som GZW, undersöka under vilka glaciala förhållanden de kan ha bildats samt jämföra dem med andra ryggar inom MSEMZ. Tre ryggar har undersökts genom detaljerad sedimentologisk loggning. Resultaten visar diamikter med underliggande sorterade sediment, vilket tyder på subakvatisk avsättning vid en relativt stabil isrand utan spår av en större återframryckning. En av ryggarna bedöms mer sannolikt vara en GZW än de andra två. Dessa resultat bidrar till en bättre förståelse av isavsmältningsdynamiken under yngre dryas och kan vara ett underlag inför framtida undersökningar av bildning och utbredning av möjliga GZW på land. (Less)
Popular Abstract
In the Nyköping area of southeastern Sweden, a few interesting ridges may offer insights into past glacial conditions. These ridges lie within the Middle Swedish End Moraine Zone (MSEMZ), an area shaped by ice during the last deglaciation. They might represent a rare example of grounding zone wedges (GZWs) found on land, but they form in water so usually remain submerged. The study examines sediments in these ridges to explore whether they are indeed GZWs, to reconstruct the glacial conditions at the time of formation and how they connect to the MSEMZ.

GZWs typically form where the grounded part of a glacier meets floating ice, in water terminating environments. They are usually studied through geophysical techniques, since they often... (More)
In the Nyköping area of southeastern Sweden, a few interesting ridges may offer insights into past glacial conditions. These ridges lie within the Middle Swedish End Moraine Zone (MSEMZ), an area shaped by ice during the last deglaciation. They might represent a rare example of grounding zone wedges (GZWs) found on land, but they form in water so usually remain submerged. The study examines sediments in these ridges to explore whether they are indeed GZWs, to reconstruct the glacial conditions at the time of formation and how they connect to the MSEMZ.

GZWs typically form where the grounded part of a glacier meets floating ice, in water terminating environments. They are usually studied through geophysical techniques, since they often remain underwater and are not easily accessible. In this project, fieldwork was conducted at three sites, where detailed sedimentological sketches were made (an example is shown in the figure to the right). Five sections were documented in total, and all contain interesting features indicative of glacial influence. The most important observation is that these ridges were indeed formed under water, which supports the interpretation that they could be GZWs.

The morphology of the ridges was also examined. They are notably smaller than most known GZWs, though still within the size range found in one of the few previously studied terrestrial examples. This smaller size might reflect a shorter period of ice-margin stability. Two out of the three ridges studied show an asymmetrical profile, being steeper on the seaward side and gentler on the ice-proximal side, another trait consistent with GZWs.

Theses ridges share some key characteristics with GZWs, such as, formation in subaqueous environment and asymmetrical morphology. However, they also deviate from the typical pattern in several ways. For example, their orientation is parallel to ice flow in two cases, which is unusual, and they are not found in a fjord setting, which is common for many GZWs. One ridge appears to be the most likely candidate for a true GZW due to its internal structure and morphology.

Based on the sedimentological data, it is likely that the ice retreated slowly in this area, with brief standstills or slight readvances. The uppermost sediment layers were difficult to interpret, but the conclusion was that they were deposited as the ice re-advanced over the area. The ridges in the Nyköping area differ a lot from the other parts of the MSEMZ. In the western part of this zone prominent, continuous end morains mark significant ice readvances. In contrast, the Nyköping ridges are irregular, scattered and difficult to trace over long distances. This study suggests that the glacial history of southeastern Sweden was complex and variable. Further research, including larger and more strategically oriented sections, is needed to confirm the origins of these ridges and to refine the reconstructions of the ice margin dynamics in the region (Less)
Please use this url to cite or link to this publication:
author
Jónsdóttir, Svava Kristin LU
supervisor
organization
course
GEOR02 20251
year
type
H2 - Master's Degree (Two Years)
subject
keywords
Grounding zone wedges (GZWs), middle Swedish end moraine zone (MSEMZ), Younger Dryas, glacial retreat, sedimentology, glacial morphology.
publication/series
Dissertations in Geology at Lund University
report number
713
language
English
id
9199690
date added to LUP
2025-06-17 09:27:31
date last changed
2025-06-17 09:27:31
@misc{9199690,
  abstract     = {{The Nyköping area, located within the Middle Swedish End Moraine Zone (MSEMZ), contains numerous scattered ridges. While other parts of the MSEMZ show evidence of an ice readvance during the Younger Dryas, such features are absent in this region. Instead, some of the ridges in the Nyköping area exhibit morphologies similar to grounding zone wedges (GZW), which typically form subaqueously at temporary stillstands of the ice margin. However, sedimentological investigations in the area are lacking, and terrestrial GZWs remain poorly studied due to their frequent submergence. This study aims to assess whether the ridges in the Nyköping area represent GZWs, determine the glacial conditions under which they may have formed, and compare them to other ridges in the MSEMZ. Three ridges were examined for detailed sedimentological logging. The results show diamicts with underlying sorted sediments suggesting subaqueous deposition at a relatively stable ice margin, without evidence of a major readvance. One ridge appears more likely to represent a GZW than the other two. These findings contribute to a better understanding of glacial retreat dynamics during the Younger Dryas and may support future investigations into the formation and distribution of potential GZWs on land.}},
  author       = {{Jónsdóttir, Svava Kristin}},
  language     = {{eng}},
  note         = {{Student Paper}},
  series       = {{Dissertations in Geology at Lund University}},
  title        = {{Grounding zone wedges in the Nyköping area, southeastern Sweden}},
  year         = {{2025}},
}