Marian Harasimiuk, Tadeusz Król Institute of Earth Sciences. Maria Curie-Skłodowska University Lublin, Poland

XX Polar Symposium Lublin, 1993

DEVELOPMENT OF FLUVIAL SYSTEM OF THE DUNDER BASIN (WESTERN SPITSBERGEN)

INTRODUCTION

The Dunder basin situated in NW part of Wedel Jarlsberg Land is the largest practically unglaciated basin in this part of Spitsbergen (a few small glaciers occupy less than 10% of the basin area).

The rectangular shaped basin со veres 142 km2. The lenght of the longer axis of NW-SE direction is 19.2 km with the average width 7.3 km and maximum 8.3 km.

The whole area is situated in the western wing of a vast synclinal structure built of Upper Proterozoic, poorly metamorphosed soft phyllites and more resistant Younger (also Upper Proterozoic) complex series of diamictites (tillites) filling up the axial part of syncline (Dallmann et al. 1990). Besides the Upper Proterozoic rocks in the basin there can be found only thin patches of Pleistocene glacial and fluvioglacial deposits as well as Holocene marine deposits of small thickness (loams as well as sands and gravels).

NE part of the basin is of the mountain range character from 500 m a.s.l. (Frontfjellet) in SE to 775 m a.s.l. (Dalkletten) in NW. This range built of hard sandstones and tillites, is poorly splitted and rises over the ondulated plain up to 50 m a.s.l. as a rectilinear wall with a similar course to a general dislocation zone within the syncline.

SW border of the basin is formed by NW narrowing belt consisting of some distinctly isolated blocks about 600 m a.s.l. in SW up to 340 m a.s.l. in NW. Individual blocks are separated from each other by deep depressions of a dead valley type similar to the dislocation course of NNE-SSW or N-S directions.

SE closure of the basin are some small, partly glaciated mountain ranges of the height 500-800 m a.s.l.

In the central part of the Dunder basin the massif Slettfjellet rises 420-520 m a.s.l. joining the highest part of SW boundary of the basin through the highly situated glaciated pass.

Within the mountain massifs small glaciers of about 1 km2 to about 5 km2

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were preserved. The largest glaciers are found in SE part of the basin and are partially connected with a vast glacial plateau of Western Torell situated about 500 m a.s.l. The largest glacier is Dunder of about 6 km2.

A low-lying part of the basin (situated below 50 m a.s.l.) occupies over 40% area and consists of three distinct parts. The lower part is a vast coastal plain 8 km wide and of the height reaching 25 m a.s.l. at the slope feet. A relative height within this plain does not exceed 12-15 m. There can be found several flat hills of the axis parallel to the main axis of the basin resembling the course of Upper Proterozoic phyllite bassets arranged steeply. Different size depressions some of which were not merged in the surface outflow are found between hills. In SE direction, the lowland becomes narrower to 34 km in the Dunder Valley and about 2 km in the Slett Valley separated from the Dunder Valley by the above mentioned ridge Slett. In this part of lowland the elevations built of almost vertically standing phyllites occupy large areas. They reach about 20-25 m a.s.l. The underslope areas are situated a bit lower. Along the ridge slopes bordering the lowland fragments of marine terraces about 25 m high were preserved in a form of narrow shelves. The upper part of the lowland Dunder has a character of a vast valley about 2 km wide in the bottom level where torrents from glaciers flow down concentrically pouring fans of different sizes. In this zone the rocky background is not exposed.

The morphological axis of the lowland Dunder is river bed about 18 km long with a disproportionately narrow (in relation to the width of whole lowland) zone formed by the contemporary fluvial processes (alluvial plain).

FLUVIAL SYSTEM OF THE DUNDER BASIN

The fluvial system of Dunder is spatially differentiated. In the basin right part there can be found a density system of short (3-5 km) streams perpendicular to the valley axis draining the mountain range. Only the streams Dolter and Gryt flowing out of small glaciers formed in the lowland Dunder vast, flat and at present partially dead outwash fans with a distributive system of beds. In the lower part between a fan of the torrent Gryt and the coast, short torrents flow into shallow lakes or flat basins in the places of lakes. In this part lowland the drainage system is not fully developed yet.

In the left part of basin, density of torrent system is much smaller. The trace of channel system (Slett-Floy) at present separated into three basins parallel to the Dunder river is well readable. Bifurcation flows occur at high spring water levels in the zones where a uniform bed system was broken.

In the upper part of basin a density system of torrents flowing out of glaciers is of similar character to a concentric one. Within vast cones at slope feet there are observed complex distributive bed systems of different ages.

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The analysis of relief of the Dunder Valley bottom (within the lowland Dunder) and the way of its bed development permitted to distinguish 7 charac-teristic segments.

The upper segment (A) is characterized by a complex system. It is formed by strongly eroding torrents draining glaciers of SE basin boundary. At the foot of steepy mountain slopes the torrents build large fans (sandr type) with a comp-licated distributive system of beds. Five fans can be clearly seen (Fig. 1). The two-phases of their development are well readable—into vast older fans with dry or only periodically functioning beds younger fans are put which most frequently use one of main beds of older fans. The surface slope of older cones are 0.012-0.030%. They are built of gravels of thicker fractions than the younger cones put in, the inclination of which is 0.008-0.017%. In the areas of cone sides there are formed multicurrent brook beds about 100 m wide and of 0.0035--0.0045% drop. In the segment about 500 m long a large alluvial braided plain 600 m wide is formed due to merging of braided channels. The plain made of numerous banks and channels is built of 5 cm diameter gravels covered with a several centimeter thick layer of muddy sands and muds. In the abandoned channel thickness of sandy muds increases to over 25 cm. The average bed depth does not exceed 30 cm. The surface of interchannel sandbanks rises over the bed bottom about 60 cm. This is a measure of water level fluctuations in the bed of Dunder River in this segment. The tundra vegetation appears on some sandbanks rising even a few centimeters which is the evidence of their being beyond the reach of water high levels. Such a character of alluvial plain is found on the 3 km long segment down the valley.

Below on 2.5 km long segment the alluvial plane becomes narrower to about 80-150 m and bed development changes from typically of braided character into anastomosis braided or even into anastomosis (Fig. 1, section В and Fig. 2). There takes place a quick transformation from a multicurrent braided system into a multichannel anastomosis system with numerous stable bed islands medially situated and interbed areas developed on the scale of alluvial plain forms (the ratio of interbed area length to bed width — L/w — exceeds the value 7, Teysseyre 1992). A dominishing drop — to about 0.002% is observed along this section. Along the beds the alluvial plain formed at the highest water levels stretches. The depth of beds in this part increases to about 1 m.

Another segment of the Dunder River bed (C) about 4 km long is of ravined character. The gently winding bed, a few meters indented into the background schists is 30-40 m wide, rarely 50 m and width of the whole valley does not exceed 100 m. The gravel banks 5-8 m wide and 30 m long ate situated along the bed. Occasionally there can be found small interbed sandbanks 3 x 15 m. With the maximum water levels the bed is about 1 m deep. According to the estimates of flood wave reach (based on mrophological effects, alluvial sedimentation reach and vegetation cover reach) made on the turn of July and August 1992, the maximum flood water levels in the brook segment axceed by about 30 cm the

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level on the measurement day. In the segment of anastomosis the difference is 40 cm and in the ravine about 50 cm. In the lower part of the ravined segment where the width of side sandbanks is slightly larger the initial natural levee appear.

Another part of the Dunder Valley (2.8 km long — D in Fig. 1) is the alluvial plain over 500 m passing into tributary cones of the Dolter and Slett. The surface of cones is found beyond the reach of maximum flood water levels in the Dunder bed. However, the alluvial plain consists of two elements: the terrace about 0.5 m high above the average water level in the bed covered with the tundra vegetation with thermokarst lakelets and flood water beds as well as a system of interbed sandbanks 10 and 20 cm high above the average water level. The banks are built of gravels (gravel material delivery by the Dolter and Slett tirbutaries) covered by a few centimeter loamy layer but on the terrace thickness of muds and loams lying on gravels reaches about 40-50 cm. As a result, a braided channel system is formed in the average and low water level phases passing into the anastomosing braided in the water high level phase.

Down the valley, a vast alluvial plain becomes narrower rapidly to about 150 m and a complex system of braided channels on about 200 m long part is transformed into a compact, almost rectilinear bed with a few, small interbed sandbanks (Fig. 1; section E, Fig. 3). Along the bed the terrace of about 1.5 m relative height and 3.5-4.0 m a.s.l. stretches. It is built of marine sands lying on compact, hardly erodable marine loams.

The last part of the Dunder Valley has a character of estuary and its length is precisely connected with backwater reach due to marine tides. Tides in the region of Dunder Bay reach the level of 1.7 m. In the phase of maximum tides there is disclosed a vast alluvial plain built of fine gravels with the predominace of slaking gravels of Upper Proterozoic shists covered with ą few or in some places several centimeter layer of muds and loams. A system of quite stable anastomozing beds is developed on the alluvial plain.

THE FACTORS CONTROLLING DEVELOPMENT OF THE DUNDER FLUVIAL SYSTEM

The Dunder fluvial system was developed on a vast abrasive platform which emerged in Holocene due to glacioisostatic movements. Glaciers also took part in forming the platform relief. Their range is determined by the preserved patches of boulder clay found only in the upper and middle parts of the Dunder plain. Emergence of the coastal plain took place in two or three distinct phases which is related to the two cumulative marine terraces preserved in the under slope zones. The lowest terrace (3.5 m a.s.l.) is found within the coastal plain — in the region of the estuary of the rivers Slett and Gryt into Dunder. Slight erosive flattenings stetching along the Dunder bed in the lower part of the ravined segment refer to this terrace.

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Thus the fluvial system of the Dunder was formed gradually with the sea recession (glacioisostatic lifting of the area). Within the coastal plain, the regions formed by the fluvial processes occupy a relatively small area (beyond the areas of outwash cones).

In formation of the Dunder fluvial system the hydrological conditions connected with the periglacial climate of the oceanic character and an enormous delivery of rocky material to the valley bottom in its upper part play an importand role. As a result, a system of unstable distributive beds on the cones connected with a classical multicurrent braided river is formed (segment A).

In the other part a system of main river beds is found in a transitional phase from a braided river to an anastomosis — braided one. The causes of anastomozation are different in different parts of the valley. These are irregular lifting movements of the block cutting the valley transversaly (segment B), supporting the main river bed by the lateral valley cones (segment D) or water damming up in the marine tide conditions of the amplitude up to 1.7 m. Thus in the conditions of the river being close to the threshold value for a given river bed system in the hydrological respect even relatively small deformations of bed drop causes by different local factors can contribute to the tedency of river bed system transformation.

REFERENCES

Dallmann W. K., Hjelle A., Ohta Г., Salvigsen O., BjornerudM. G., Hauser E. C., Maher H. D„ Craddock C„ 1990: Geological Map of Svalbard 1:100 000, Sheet BI 1G Van Keulenfjorden, Norsk Polarinstitutt, Oslo.

Teisseyre A. K., 1992: Rzeki anastomozujące — procesy i modele sedymentacji. Przegl. geologiczny, 4, Wyd. Geol. Warszawa

Addresses of the authors: prof, dr hab. Marian Harasimiuk, mgr Tadeusz Król, Department of Geology, Institute of Earth Sciences, Maria Curie-Skłodowska University, Akademicka 19, 20-033 Lublin, Poland

ROZWÓJ SYSTEMU FLUWIALNEGO DORZECZA DUNDER (ZACHODNI SPITSBERGEN)

Streszczenie

Podczas geograficznej ekspedycji Bellsund 1992 zorganizowanej przez Instytut Nauk o Ziemi Uniwersytetu Marii Curie-Skłodowskiej z Lublina przeprowadzone były badania i obserwacje nad systemem fiuwialnym dorzecza Dunder.

Dorzecze rzeki Dunder położone w NW części Ziemi Wedela Jarlsberga, jest największą i praktycznie nie zlodzoną zlewnią tej części Spitsbergenu, gdyż lodowce zajmują poniżej 10% powierzchni zlewni. Zamknięcie dorzecza stanowi szereg masywów górskich częściowo zlodzonych o wysokości bezwzględnej 500-800 m. Ponad 40% powierzchni dorzecza zajmuje część równinna

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składająca się z trzech wyraźnie wyodrębniających się części. Dolna — to rozległa nizina nadmorska, środkowa—to zwężenie doliny do 2 km powodowane istnieniem tutaj grzebietu Slett oddzielającego SW dopływ rzeki Dunder. Górna część doliny Dunder ma charakter rozległej kotliny, do której koncentrycznie spływają potoki z lodowców sypiąc różnej wielkości stożki.

Osią morfologiczną równiny Dunder jest koryto rzeki o długości około 18 km z wąską strefą przykorytową kształtowaną przez współczesne procesy fluwialne. Sposób rozwinięcia koryta rzeki pozwolił na wyróżnienie 7 charakterystycznych odcinków.

Odcinek górny (Fig. 1 — A) budują rozległe stożki potoków lodowcowych tworzących skomplikowany dystrybutywny system koryt. Niższy odcinek (B) stanowi równina aluwialno--roztokowa, gdzie system koryt dystrybutywnych przechodzi w roztokowe, a następnie w roz-tokowo-anastomozujące i anastomozujące. Następny odcinek (С) o długości około 4 km ma charakter przełomowy, o szerokości koryta 40 m wciętego do kilku metrów w łupki podłoża. Kolejny odcinek (D) stanowi równina aluwialna z systemem koryt roztokowych i anastomozująco--roztokowych, które szybko przechodzą w zwarte prostolinijne koryto (odcinek E) z niewielkimi śródkorytowymi łachami. Ostatni, ujściowy odcinek doliny ma charakter estuariowy, związany z zasięgiem cofki związanej z pływami morskimi, które osiągają tutaj poziom do 1.7 m. Na utworzonej równinie aluwialnej powstał system dość stabilnych koryt anastomozujących.

Cały system fluwialny Dunderdalen rozwinął się na rozległej platformie abrazyjnej wynurzonej w holocenie w wyniku ruchów glacjoizostatycznych. Rozwój następował sukcesywnie w miarę regresji morza i glacjoizostatycznego dźwigania obszaru. Ogromną rolę w ukształtowaniu się systemu fiu wialnego odegrały warunki hydrologiczne związane z klimatem peryglacjalnym o cechach oceanicznych, a także duża dostawa materiału skalnego i zawiesinowego zwłaszcza w górnej części doliny.

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Fig. 1. Situational sketch of the Dunder Basin: 1 — mountain ridges and slopes, 2 — glaciers, 3 — moraine ridges, 4 — watershed, 5 — basin plain area, 6 — outwash cones, 7—pingo, 8 — river system, 9 — characteristic segments channels of Dunder river, 10 — lakelets

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Fig. 2. Channel pattern in the Dunder Valley (I — segment A-B, II — semgent F after Fig. 1): 1 —low ridges and slopes, 2 — active cliff, 3 — escarpments, 4 — sea terrace, 5 — channel patterns on the alluvial plain, 6 — channel patterns on the tide plain, 7 — fluvioglacial fan sandr, 8 — delta fan

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