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Fjord Formation in Norway Holtedahl, 1967
Lauren Thompson
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What is a fjord? Deep glaciated valleys that have been eroded adjacent to coastlines and are now occupied by marine waters following deglaciation and eustatic sea level rise
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Previous Fjord Formation Theory
JW Gregory (1913): Fjords are of tectonic origin, influence of glaciers negligible Pointed to connection between Tertiary dislocations and development of fjords Pattern of pre-glacial valleys is coincident with tectonic lines (axes of folds, joints, and contacts of geological formations) Shows fluvial erosion and glacial erosion tectonically controlled -He pointed to the causal connection between the Tertiary dislocations and the development of fjords, and as late as in 1927 described Hebridean fjords as being due to the "rending of the crust during and after the Alpine earth movements and to the subsequent gaping of the fissures during the Pliocene uplift of the British area” -The presence of well developed, typical fjords along the westcoast of Norway is a natural consequence of the oblique Tertiary uplift of the Norwegian landmass. The rejuvenation caused increased fluvial erosion, especially active on the slopes facing the ocean, this resulting in more or less deeply cut preglacial valleys.
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Tectonics in Norway Seismic investigations off west coast and south coast of Norway conducted to look at young Cenozoic dislocations on continental shelf Sediment with primary wave velocity of 2 km/s and thickness of 800 m overlying sediment with velocity of 3.25 km/s and thickness of m Strong indication of fault line or fault zone running offshore and parallel to coast Does not prove that fjords form primarily from tectonics
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Glacial Influence on Fjord Formation
Now accepted that glaciers have major significance on formation of fjords Fluvial erosion has also been important Focus one two major Norwegian fjords
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Hardangerfjord and Sognefjord
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Similarities Extend far into the country Both deep and narrow
Surrounded by high mountains inland and low land near the coast Extensive tributary fjords found in inner parts Extend km Sognefjord has a max depth of 1308 m and Hardangerfjord about 900 m Average width of 5 km
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Differences Hardangerfjord
Direction of fjord coincides with main structures that run northeast and southwest Oriented along boundary between Precambrian complex to southeast and Cambro-Silurian schistose rocks to northwest Close connection with fractures The direction of the Hardangerfjord coincides to a great extent with the main Caledonian structures which run north-east and south-west, and the fjord has a situation very much along the boundary between the Precambrian complex to the south-east and the Cambro-Silurian schistose rocks to the north-west. No doubt its situation on the eastern flank of the downfolded Precambrian basement is of importance.
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Differences Sognefjord Cuts through parts of northwestern gneiss area
Crosses over fold trough to the east Also a relationship between fracture systems and trend of main fjord Fracture systems have guided erosion The Sognefjord cuts through parts of the north-western gneiss area with south-west and north-east structures, mainly of Caledonian origin, and then penetrates the Caledonian fold trough in the east. There is no obvious relationship between the east-west trend of the Sognefjord and the fold pattern, with the exception of some of the branches in the inner parts of the main fjord. There is, however, as in the Hardangerfjord, a relationship between fracture systems and the trend of the main fjord and its branches, and these fracture systems have played an important role in guiding erosion. A very marked north-south fracture pattern is present at the mouth of the fjord, being especially well developed in the Devonian rock complex, and a similar northsouth trend is also characteristic in the outer part of the Hardangerfjord.
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Longitudinal Profile: Sognefjord
Increases evenly in depth to the west Passes gradually into a central basin with depths of m Floor rises rapidly Sinks gently with a threshold depth of 150 m
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Longitudinal Profile: Hardangerfjord
Several basins; innermost is deepest Basins further out have floors situated at decreasing depths Basins separated by thresholds Irregularity indicates rocky nature
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Thresholds Thresholds at mouths of fjords: one of strongest indicators of glacial origin Location of thresholds related to sounds, valleys, and lowland Locations where ice able to spread out, meaning it had a less erosive effect Explain differences between longitudinal profiles Sognefjord: ice constricted to about 30 km from coast Hardangerfjord: connection between location of thresholds and location of sounds Sognefjord: Ice constricted to narrow channel in homogenous gneissic rocks to an area 30 km from the coast where spreading was suddenly possible through sounds and valleys Hardangerfjord: more varied geological conditions which cause variation in the width of the fjord There is a connection between the location of thresholds and location of sounds which have acted as “escape routes” for main-fjord glaciers. Threshold at a very narrow part further inland can be attributed to the widening of the fjord basin
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Glacial Erosion Conclusion
Present depth conditions of fjords a result of glacial erosion Influenced by thickness of glaciers Greatest in inner parts where confluence took place Least in outer parts where diffluence occurred
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Subglacial Fluvial Erosion
Subglacially flowing water has significantly contributed to erosion Often neglected Evidence: glaciofluival material found in form of silts and clays, or coarser materials as eskers and outwash accumulations Subglacial streams or possibly water-sheet drainage under hydrostatic pressure
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Fossli Area of Hardangerfjord
From head of fjord, valley continues south as lake basin Mabodal valley branches off towards east as v-shaped valley with steep walls Terminates in a vertical valley head River dashes down in single fall Top of cliff, river continues in broad, open valley From the head of the Hardangerfjord at Eidfjord, the valley continues southwards as a lake basin. South of the lake, the Mabodal valley branches off towards the east as a winding, open v-shaped valley, whose inner part narrows to a gorge with very steep walls, which terminate in a vertical valley head 235 m high. From the southern wall the river Bjoreia dashes down in a single fall of 147 m. At the top of the cliff the river continues in a broad, open and shallow valley with gentle slopes
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Contribution of Glacial Meltwater
Subglacial fluvial erosion very important in formation of Mabodal valley “Dry” canyon evidence for this Canyon incised m into rocky surface Giant potholes m high Subglacial fluvial erosion most likely explanation Current river gorge incised in a surface with a dip perpendicular to trend superimposed Superimposed: having a course not adjusted to the structure of the rocks presently undergoing erosion but determined by a prior erosion cycle -The “dry” canyon is a tributary to the present river gorge and joins it at a point where the Bjoreia river cuts through the sloping rocky hillside just above main waterfall -Incised m into the rocky surface, with vertical walls, continues in the south eastern direction where it ends in a few steps with steep valley ends -As a result of weathering, canyon walls rough and bottom covered in talus At some points, the wall is fresh and polished and giant potholes are seen m high Clearly eroded by running water because the position of the canyon and potholes on the divide cannot be explained by subaerial streams Another piece of evidence is the fact that the present river gorge between the junction of the dry canyon and the main waterfall is incised in a sloping surface, with a dip perpendicular to its trend
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Erosion of Glacial Meltwater vs Current River
Subglacial meltwater was extremely powerful agent of erosion Eroding effect of present river since ice left has been very small The subglacially formed tributary canyon and present river meet at almost the same level River would have lowered bed by actively eroding away material, leaving dry tributary canyon hanging So we know that the current river has not contributed much to eroding the canyon
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Outwash Subglacial meltwater streams are apparent due to the large accumulations of glaciofluvial material that has accumulated as an outwash delta at the head of the fjord Present as raised terraces with maximum thicknesses of 100 m
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Flamsdal Area Flam valley: tributary to Sognefjord
Sharply cuts into a mountainous area with heights of m Ends abruptly in steep wall Cirque-like valley head Step-like longitudinal profile U-formed shape Canyons with potholes common The Flam valley is a fjord-valley that sharply cuts into a mountainous area with heights of m and ends abruptly in a steep wall It has a cirque like valley head, step-like longitudinal profile and U-formed shape, which all point to glacial erosion Canyons are common: usually narrow and fringed with potholes
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Flam Valley Major canyon forming present river channel
Present river runs in very narrow gorge Position not in deepest part of valley, but on sloping side --> superimposed There is a system of canyons, with a major canyon now forming the present river channel, and tributary canyons, which on the whole do not carry streams at the present time. At Berekvam the present river runs in a very narrow gorge, 30 m deep and less than 2 m broad at its narrowest part, and has a position, not in the deepest part of the valley, but on the sloping side, suggesting that it is superimposed
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Longitudinal Profile Step-like profile
Stoss and lee topography ice movement downstream Steps connected to vertical fractures Semicylindrical giant potholes Glacial meltwater moved between vertical walls and ice, forming channel The valley floor has a step-like longitudinal profile Stoss and lee topography indicates ice movement downstream in a northerly direction The steps are closely connected to the occurrence of vertical fractures and have steep and sometimes overhanging walls and show surfaces with fluvial erosional forms, like well-developed giant potholes Glacial meltwater must have moved in channels between vertical rocky walls and ice These channels have formed a system, some with direction towards the central part of the valley, others along the steep side walls
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Furuberget Series of fluvial erosional features present
Furuberget: a riegel with a step on its northern side leading down to broad and flat lower part of the Flam valley Current river runs in narrow gorge through riegel Riegel: a low transverse rock ridge on the floor of a glaciated valley commonly situated at the down- valley end of the flat Potholes common inn these canyons, and most show glacial striae Glacial meltwater had a considerable erosional effect on the valley floor The glacial fluvial erosion in the Flam valley is believed to be caused by a system of subglacial streams probably under hydrostatic pressure
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Evidence for Glacial Fluvial Erosion
Remains of canyons, only one wall exists Giant potholes showing striations Giant potholes at fjord-head close to present sea level Narrow channels stream concentrated and ice-pressure present against rocky wall, indicating thick glacier Glacial fluvial erosion in the Flam valley is believed to be caused by a system of subglacial streams, most likely under hydrostatic pressure There are remains of canyons of which only one wall exists, the other having been formed by ice Giant potholes show striations and are at fjord head close to present sea level Narrow channels mean the stream must have been concentrated and the ice-pressure must have been present against rocky wall, indicating a thick glacier
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Conclusion Depth of fjords depends on thickness of the glacier
Deepest in inner parts of fjord where confluence takes place Subglacial fluvial erosion by meltwater under high hydrostatic pressure was a large contributor to formation of fjords and fjord valleys Still unknown to what degree
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