The Norwegian Channel and our coastal landscapes

Hull-torghatten
07.11.2016

The third geological mystery in this article series relates to the strandflat and fjords. These Norwegian words are used internationally to describe landscape features in coastal areas that have been exposed to repeated glaciations.

The Norwegian coast is a type area for these landscapes. In order to get a full picture of the coastal landscape, we must look at this in correlation with the channels and banks on the continental shelf.

Ever since the late 1800s, Norwegian geologists have been focused on how these features were formed.

 

By Fridtjof Riis
This article has previously been published in Norwegian on www.geoforskning.no

 

The Norwegian Channel

The Norwegian Channel is approximately 100 kilometres wide and reaches 900 kilometres from the outer Oslofjord to the deep sea offshore Stad. Geologically speaking, the bottom of the channel is an erosion surface that is located at a depth of approximately 600 metres, but the channel is partially re-filled with Quaternary sediments. The water is deepest in Skagerrak and is otherwise about 300-400 metres deep along most of the coastline.

For a period of time in the mid-1970s, the Norwegian Channel suddenly became a hot topic that most Norwegians had an opinion about. The people demanded that the significant oil discoveries in the Norwegian sector of the North Sea be developed with landing in Norway to create a foundation for a whole new industry.

People were aware that the water depth was more than 300 metres, and that the technology for laying pipelines in deep waters such as this was under development. I was a geology student at the time and thought it might be appropriate to ask my lecturers how the Norwegian Channel was formed. But no clear answer was forthcoming.

In 1885, Amund Helland launched the hypothesis of the Skagerrak glacier, and suggested that the Norwegian Channel was eroded out from such a glacier. The following discussion lasted 100 years.

Opponents of the hypothesis claimed that the geological discoveries on land did not substantiate such a vast glacier out in Skagerrak. They believed that other hypotheses were needed to explain the Norwegian Channel.

In 1904, Fridtjof Nansen proposed fluvial erosion modified by glacial processes, and Hans Holtedahl interpreted it as a structure which was controlled by tectonic movements.

 

The strandflat and the coastline

 

Topography of the Nordic countries. White arrows: Channels, paleo-ice streams. White line: Outline of the strandflat, from H.Holtedahl. Red line: Outline of high altitude strandflat surface in Rogaland. B: Bjørnøya Fan, N: North Sea fan, J: Jæren

Topography of the Nordic countries.
White arrows: Channels, paleo-ice streams. White line: Outline of the strandflat, from H.Holtedahl. Red line: Outline of high altitude strandflat surface in Rogaland. B: Bjørnøya Fan, N: North Sea fan, J: Jæren

 

A geologist examining a map of Norway with fresh eyes should notice that the coastal areas appear to be divided into different morphologic provinces. Jæren and the Varanger Peninsula mark borders between coastal landscapes.

The Boknafjord is located north of Jæren. There is a clear erosion/abrasion level at an altitude of approx. 30 metres here. It defines the top of many smaller islands and headlands, and gnaws into landscape that extends higher up. The low-lying coastal landscape below it with islands and skerries constitutes the strandflat, The rivers draining into the Boknafjord basin flow in large glacial valleys which continue into spectacular fjords.

Going south from Jæren and further around the southern coast, there is no marked strandflat, and no substantial fjords connected to the major rivers. Further north along the coast, the strandflat continues to Troms. The fjord landscapes are dominant all the way to Varanger and Murmansk, but not at Kola.

The Oslofjord is unlike the fjords of western and northern Norway in that there is no major river ending in the inner fjord. This fjord is not closely related to valleys formed by glaciers, rather it was formed by selective erosion of less resistant sedimentary rocks in fault basins belonging to the Oslo Graben.

 

The Norwegian strandflat

On the coast of western Norway and outside Nordland County, where the strandflat is well-developed, one can often see that the landscape is characterized by several local erosion surfaces at many different altitudes. The top level is commonly located above the highest post-glacial marine limit and the deepest erosion surface can be below sea level.

Large caves and holes formed by marine abrasion can be found at different altitudes, but many of them are located in the upper part, such as the hole in Torghatten.

 

The hole in the Torghatten mountain is a cave formed by marine abration in the uppermost level of the strandflat. From the inside of the hole it is easy to imagine that at such high sea levels the big waves from the open sea will hit the mountain without being damped.

The hole in the Torghatten mountain is a cave formed by marine abration in the uppermost level of the strandflat. From the inside of the hole it is easy to imagine that at such high sea levels the big waves from the open sea will hit the mountain without being damped.

 

The strandflat was defined by Hans Reusch in the 1890s and was examined further by Nansen. Nansen, with his experience from Arctic regions, proved that the strandflat was typical for Arctic areas and believed it was formed during periods where the ocean may have been frozen and with glaciation along the coast.

This theory is generally accepted today, though different scientists may place varying emphasis on the diverse mechanisms that cause erosion of solid bedrock and mass transport when the strandflat is formed.

One noticeable feature of the strandflat off the Nordland coastline is isolated mountain peaks such as the Lofoten islands, Træna, Lovunden and Hestmannen. These rise high above the low strandflat, even far out at sea.

Based on the knowledge of Norwegian mainland geology, one could imagine that these mountains are remnants of a deeply eroded even basement surface. But seismic data, scattered discoveries of Mesozoic glacially transported blocks and remnants from extensive weathering show that several of the deep channels in the strandflat in this area are eroded out in sedimentary rocks of Triassic, Jurassic and probably Cretaceous age.

 

The strandflat in Nordland seen from Åmøy towards the south, with the erosional remnants Rødøyløva, Hestmannen, Lovund and Træna.

The strandflat in Nordland seen from Åmøy towards the south, with the erosional remnants Rødøyløva, Hestmannen, Lovund and Træna.

Sketch showing the flat Middle Jurassic landscape with sediment cover on a weathered basement surface was block faulted and infilled with sediments in the late Jurassic and Cretaceous. The present day landscape was formed by deep erosion. The sedimentary cover has been removed while remnants of the crystalline rocks are preserved. Blue line indicates presen

Sketch showing the flat Middle Jurassic landscape with sediment cover on a weathered basement surface was block faulted and infilled with sediments in the late Jurassic and Cretaceous. The present day landscape was formed by deep erosion. The sedimentary cover has been removed while remnants of the crystalline rocks are preserved. Blue line indicates presen.

 

These sediments were deposited in a low relief landscape that was exposed to rifting in the Late Jurassic and Early Cretaceous. Today, eroded remnants from the resistant core of Caledonian rocks in the fault blocks still stand as dramatic mountains among the low islands. The sketch shows the steps in this geological process.

A cover of loose, easily erodible sediments and weathering material before the ice ages could be an important reason for why the landscape in this area is eroded at such a deep level. This history, which is also important for understanding the geology on the continental shelf, is currently being studied in detail in several projects at the Geological Survey of Norway (NGU).

 

Onshore and offshore – a new perspective

In the early 1980s, I worked with Svein Eggen in the Norwegian Petroleum Directorate to develop a program for basin modelling. In these days, the subsidence in the graben areas in the North Sea was explained and modelled using McKenzies’ formula for extension and thermal cooling.

But when we took a closer look at the subsidence curves and the age of the sediments, there were a number of observations that did not quite add up.

The most important finding was that much of the subsidence and sediment load on the Norwegian Shelf is so young that we believed that it could not be related to rifting in the Jurassic or Paleogene.

This young sediment load was important in order to understand formation and migration of oil and gas on the entire Norwegian Shelf.

The need to proceed with this led to a cooperation within the NPD, where regional seismic mapping was linked with Tor Eidvin’s biostratigraphy studies. We have kept this cooperation going since the 1980s.

Following much laborious work, Tor was able to document in 1989 that the up to 3000 metre thick Bjørnøya fan on the western margin of the Barents Sea was formed under glacial conditions in the Late Pliocene and Pleistocene, the time span that is currently designated as Quaternary.

The same was the case with the fan deposits outside the deep channels in the central Norwegian Shelf – and the North Sea Fan outside the Norwegian Channel.

The vast volumes of data from the Shelf that were collected in the 1970s and 1980s, as well as the need to understand more of the young deposits, provided the basis for new research efforts.

The oil industry became increasingly interested in the youngest parts of the stratigraphic sequence, and a cooperation relating to Quaternary geology grew from the end of the 1980s and 1990s between the research environments and the industry.

Seismic data and drilling operations show the extent of sediments that were built out from the North Sea Channel into the North Sea fan in the Quaternary, a significant portion of it in the late Quaternary. These sediments had to originate from ice erosion in Scandinavia and transport out along the Norwegian Channel.

The current understanding of the processes that formed the Norwegian Channel comes from the work of Eiliv Larsen, Hans Petter Sejrup and their colleagues in the mid-1990s.

They believed that the ice occupying Skagerrak under glacial maximum was an ice stream, not an independent glacier. An ice stream is part of a larger ice cap, but is distinguished from this by the fact that an ice cap has low mobility on its bed of mountains and sediments, while the entire ice stream moves more quickly.

Through field studies, shallow boreholes and datings, Sejrup and Larsen proved that on Jæren the ice stream had left traces on the Norwegian mainland.

In somewhat simplified terms, we can say that the long, low angle slope that separates Low Jæren from High Jæren is the edge of the Norwegian Channel where it comes onto land.

 

The upper strandflat

Ever more people take the ferry from Stavanger to Tau to visit the Pulpit Rock and other sights in Ryfylke. Along the way they will pass a geological sight that is less famous, but plays an important role in Norwegian geology.

At Heia in Jørpeland, farms and cultivated land stretch across an erosional base level or surface at an altitude of approx. 200 metres that is easily visible from the ferry. Once you have noticed this surface, you will find traces of it at many locations throughout the Boknafjord basin. The surface does not have any relation to the bedrock geology.

As a geologist that just moved to Stavanger in the 1980s, I wondered what kind of surface this could be. I eventually met with Edith Fugelli who completed her thesis on the marine clays in High Jæren. These clays are about 40 000 years old, are covered by the glacial deposits from the last part of the Weichselian – and are located at an altitude of up to 200 metres.

When we reviewed the literature, we found descriptions of marine clays on high ground of the same type from Egersund in the south to Hjelmeland in the north. The Sandnes clay is part of this formation.

We were most impressed by Hans Reusch, who described the high altitude plains in Ryfylke in 1913 and interpreted them as an elevated strandflat that was older than the low, “real” strandflat in the Boknafjord.

But what mechanisms could explain a relative sea level that, approximately 40 000 years ago, was more than 150 metres higher than the highest coastline after the last ice age? The strandflat in the rest of Norway generally does not have the same height differences as in Rogaland.

 

Jørpeland and Heia seen from the south. The arrows show the two levels of strandflat formation.

Jørpeland and Heia seen from the south. The arrows show the two levels of strandflat formation.

 

The big picture

Hans Petter Sejrup and Eiliv Larsen came up with an interesting hypothesis after compiling their results from the field work at Jæren.

Their explanation was that the high altitude clays were deposited approximately 40 000 years ago in a bay that occurred between the inland ice in the east and an ice stream in the Norwegian Channel.

During the last glacial maximum, the inland ice in south-western Norway extended so far to the west that the older clays were covered by the glacial deposits, and there was then a consecutive ice cap between the ice stream and the inland ice.

When the ice cap started melting and the ice stream withdrew, the rising caused by isostasy was already well under way.

The relative sea level never became high enough to reach the upper strandflat.

The hypothesis is thus that the upper strandflat at Jæren was formed under special isostatic conditions, with an active ice stream in the Norwegian Channel occurring together with an ice cap over south-western Norway which was smaller than during the last glacial maximum.

This way of thinking provides the key to seeing more correlations. The Norwegian Channel separates from the coast north of Jæren. North of Stad, the channels generally run perpendicular to the coast.

New data presented at the 2015 winter meeting of the Norwegian Geological Society indicates that, north of Jæren, the ice stream broke up and disappeared as early as 20 000 years ago. In Skagerrak, the ice stream stayed longer and was connected to the inland ice for a longer time.

One could assume that the shaping of the strandflat and fjords depended on the interaction between glaciers on land and an open (but often frozen) sea. The time period for such a configuration will be greatest out towards the deep sea and not where the coast was blocked for long periods by large ice streams.

Landscape development should not only be examined from the perspective of the ice. Bedrock and topography also play important roles. Hypotheses must be tested, for example with more field work in Nordland County and modelling of isostasy over south-western Norway.

But I am sure that even Hans Reusch would agree that major advances have been made in understanding the processes that have created the landscapes of our country.


Topics: Geology