Quatemary sediments and bedrock geology in the outer Oslofjord and northemmost Skagerrak
ANDERS SOLHEIM & GISLE GRØNLIE
Solheim, A. & Grønlie, G.: Quaternary sediments and bedrock geology in the outer Oslofjord and northernmost Skagerrak. Norsk Geologisk Tidsskrift, Vol. 63, pp. 55-72. ISSN 0029-196X.
Shallow seismic (sparker), magnetic and bathymetric profiling were carried out on two cruises in the outer Oslofjord in 1979 and 1980. The survey area is characterized by deep silled basins defined by the main structural trends of the surrounding land area. The Quaternary scdiments, largcly restricted to these major basins. can be dividcd into three main units of supposed pre-Weichselian to Holocene age. Most of the sediments were probably deposited during relatively short time intervals in the Late Weichselian under ice-proximal conditions, and in the early Holocene. Magnetic total field and scismic data wcrc uscd to map the submarine outlines of the Permo Carboniferous Vestfold lavas, the Perm i an sedimentary rocks and the Larvik i te body. The presencc of other intrusives in the eastern and southern part of the survey area is discussed.
A. Solheim & G. Grønlie, Institutt for geologi, Postboks 1047, Blindern, Oslo 3, Norway. Present address of A. So/heim: Norsk Polarinstitutt, Rolfstangvn. 12, N-1330, Oslo Lufthavn. Present address of G. Grønlie: Det Norske Oljeselskap, Postboks 9556, Egertorget, N-Os/o l.
The survey area is situated in the southern part 3. to map the submarine bedrock geology and of the Permo-Carboniferous Oslo Graben, a structural pattern. downfaulted block of Paleozoic rocks, about 50 In this paper we present data and results ob km wide and more than 200 km long in a north tained during both cruises. northeasterly direction (Ramberg 1976). The subsidence is of the order of 1000-3000 m rela tive to the surrounding Precambrian gneisses. The graben is thought to be linked south-west Data acquisition ward to the North Sea graben system, in the The survey area extends from the island of Skagerrak. Both the bedrock and Quaternary Bastøy and as far south as the island of Jomfru geology on land around the Oslofjord have been land (Fig. 1). The 1979 cruise (Fig. 2) was per investigated in detail (Holtedahl 1960, Feyling formed with the RIV "H. U. Sverdrup". Precise Hanssen 1964, Ramberg 1976, Sørensen 1979), navigation was obtained with a Motorola mini but the submarine part has received relatively ranger system (accuracy approximately 10 m), little attention. During the Norwegian 1963-64, supplemented with Decca �ain chain for the Geotechnical Institute (NGI) carried out a geo outermost lines. The geophysical instrumenta physical survey under the Oslofjord Project tion consisted of an Elsec proton magnetometer, (Richards but the sediment thickness ob 1976), a hull-mounted 12 kHz Simrad echo sounder and tained in some of th� deeper basins was underes a shallow seismic E G & G Sparker system with timated, as shown later in this pa per. an energy output of l kJ. The data were band To augment the data base on the submarine pass filtered (50-400 Hz) and recorded on an parts of the Oslofjord area, the Department of analogue recorder. Geology, University of Oslo, carried out two The weather conditions were generally good in marine geophysical cruises in the outer Oslofjord the inner part of the survey area, while winds up in 1979 and 1980 with the main objectives: to gale strength reduced data quality on the outer l. to map the distribution of Quaternary sedi lines. The ship's speed was approximately 5 ments knots. 2. to obtain information on the stratigraphy and The 1980 cruise (Fig. 2) was performed with discuss the depositional history and the sedi the RIV 'Bjørn Føyn', to supplement the mag mentary processes involved netic measurements of 1979. The navigation was 56 A. So/heim & G. Grønlie NORSK GEOLOGISK TIDSSKRIIT 1 (1983)
ræt Larvikite \ \ \ - Volcanic rocks \ \ \ � Rhomb porphyry conglomerate \ � Cambro silurian sediments \ \ G:9 Precambrian gneiss \ lddefjord granite \ ' F aults and fractures \
. . . . • o • • • .. . • . . . . Skagerrak . .. . 6 • • t ••• • ••••• •• • •• •• • ••• • • • •• • • •• • • • • ••••• •• • • ••••• •
o 10 20 30 km
Fig. l. Geological map of the coast surrounding the survey area (outlined by dotted line). Also to be used for location of names used in the text. (Simplified after Ramberg & Larsen 1978, lddefjord granite after Grønlie et al. 198 based on radar ranging in the fjord, and Decca Geological setting of the surve y area main chain outside the island of Ferder. The Bedrock instrumentation consisted of a hull-mounted 38 kHz echo sounder and an Elsec proton magneto Fig. l shows a simplified map of the land geology meter. surrounding the survey area. The main litholo Weather conditions during this cruise were gies are: good and the ship's speed was held at approxi mately 8 knots. - Precambrian gneisses on the east side of the All analogue records were digitized and stored major fault line, the Oslofjord fault (Ramberg as time series. For the seismic records, the sea 1976). floor and the bedrock surface were digitized to - Precambrian lddefjord granite in the very calculate total thickness of Quaternary sedi southeastern part of the area. This granite ments. body is linked to the Swedish Bohus granite NORSK GEOLOGISK TIDSSKRIFT l (1983) Sediments and bedrock in Oslofjord and Skagerrak 57 NAVIGATION OUTER OSLOFJORD 1980 1979 5 km '-"--'--'--'-' VESTFOLD JOMFRULAND i ! ' ���----�----�--���--L---�---- �----�----�----�----�--�-----L----� 58°48 9"36' 11° Fig. 2. Cruise track 1979 and 1980. Numbers 6a-6d on heavy, dotted lines refer to shallow seismic profiles presented in Fig. 6. (Hageskov 1981), and is slightly younger than via! fans of mainly volcanic material from the the surrounding gneisses (Floden 1973). elevated graben shoulder to the east. - Permo-Carboniferous Javas (Olaussen 1981), - Cambro-Silurian rocks of the Skien-Lange mainly situated in the northwestern part of the sund area, consisting of a more or less continu survey area, but also found in some smaller ous lower Paleozoic section of layered lime areas in the Skien-Langesund area. The Javas stones and shales. consist of a number of rhomb-porphyritic, ba saltic and trachytic flows, mostly of more or Structural lineations are mainly confined to the less local origin (Oftedahl & Petersen 1978). sectors north-northwest to north and north - Permian intrusive body of larvikite in the northeast to northeast, although several trends southwest. This monzonitic rock is one of the occur. These main trends were probably defined major plutonic rock types in the Oslo graben, by zones of weakness before the different stages and defines a distinct southern plutonic region of the Permo-Carboniferous evolution of the (Ramberg 1976). Oslo graben took place (Ramberg & Larsen - Permian rhomb-porphyry conglomerates 1978). In Fig. l only main faults/fractures inside which form some of the islands on the east side the coastline are drawn, except for the major of the Oslofjord. According to Larsen et al. Oslofjord fault. (1978), these sediments were deposited as allu- 5 - Geologisk Tidsskr. 1/83 58 A. Solheim & G. Grønlie NORSK GEOLOGISK TIDSSKRIFT l (1983) OUTER OSLOFJORD BATHYMETRY 5 km Mercator projecUon Contour lnterval : 25m & SOm •134 spot value .. e '" J æ L..-..",-----....______._ ...... __.L...-"'-'------l...... J...... __---'>.....---<...._ _L...I...-L...... L. __.__....__._....L-L...... J58° 54' 10° 24' 11° Fig. 3. Bathymetry of the outer Oslofjord. See text for data sources. NORSK GEOLOGISK TIDSSKR!Ff l (1983) Sediments and bedrock in Oslofjord and Skagerrak 59 Quaternary deep basins, Bastøydjupet, Rauøyrenna, and Hvalerdjupet, separated by two thresholds of The Quaternary history of the Oslofjord-Skager which the southernmost, the Ferder sill, is the rak area is characterized by oscillating ice sheets, shallower and more distinct. Here the seafloor is extending out from central Fennoscandia. At the also highly irregular (with a relief of 10-30 m). end of the Late Weichselian, the ice front re The major basins are characterized by steep treated rapidly northwards. The survey area, slopes and relatively flat bottoms (due to Quater from Hvaler islands and northwards (Fig. 1), was nary deposition) with waterdepths of more than deglaciated from 12,300 years BP to 10,700 300 m. To the sides of the basins, the water years BP (RA sub-stage) (Sørensen 1979). The depths are usually less than 100 m. The slopes retreat took place in a stepwise manner with aften show a stepwise pattern (Fig. 6c). In the stops and minor readvances, producing marginal two northernmost basins the eastern slopes are deposits, end moraines and ice-front deltas. Con the steepest, while the opposite is true for Hva siderable amounts of glaciomarine sediments lerdjupet. The west slope of Hvalerdjupet has in (mostly days) were deposited more distally to some places a gradient of 40 degrees. While the retreating ice front. Bastøyrenna and Rauøyrenna are markedly During and after the retreat of the ice sheet, silled, Hvalerdjupet shallows more gradually to isostatic rebound and eustasy caused a relative wards the southwest, towards the deeper waters vertical shoreline displacement varying from 150 of the Skagerrak. m (near Larvik) to 220 m (100 km north of Apart from these major features, there are a Oslo). Due to this regression, marine sediments number of smaller basins and shoals that com were exposed to current and wave activity, erod monly have northwesterly, northeasterly and ed, transported and redeposited at greater water more northerly trends, which also are the main depths (Roaldset 1979). structural directions of the Oslofjord area. A considerable part of southern Norway is East of the main basins is an elongated trough, drained via outer Oslofjord into Skagerrak. The running between the rhomb-porphyry conglom Oslofjord near-shore sediments consist mainly of erate islands and the mainland. This trough is material derived from glacial deposits and usually interpreted as an expression of the Oslo brought out by rivers, as defined by a high illite fjord fault (Larsen et al. 1978). and chlorite content in the day fraction (Roald set 1979). Nearby in the Skagerrak, however, a relatively high content of smectite and kaolinite shows the influence of the counter-dockwise Jut Quaternary sediments land current system, because these day minerals Distribution originate from Mesozoic and Tertiary sediments in Denmark, southern Sweden and the North An isopach map of the total thickness of Quater Sea (Rønningsland et al. , in prep.). nary sediments in the survey area has been com piled (Fig. 4), based on the sparker data ob tained during the 1979 cruise. No sediment ve locity measurements were made, but an estimat Bathymetry ed mean velocity of 1700 m/s were used to con A bathymetric map of the outer Oslofjord has vert travel time to metres. been compiled (Fig. 3). The northern half of the In general, the outer Oslofjord has a sparse map is based on soundings made by the Norwe and unevenly distributed sediment cover. The gian Hydrographical Survey during the period major thicknesses are restricted to narrow zones from 1961 to 1978. Due to dose line spacing, this following the bathymetric lows. This distribution part has a contour interval of 25 m. The southern is better seen on Fig. 5, where both the bathy half is based on soundings from the cruises in metry and the sediment layer have been plotted 1979 and 1980. As the data density is more sparse perpendicularly to the ship's tracks. In the shal in this area, a contour interval of 50 m is used. lower areas, there is generally sediment present Areas not covered by the two cruises (i.e. doser in pockets, surrounded by more or less barren to the shore) have not been contoured. rock faces (Fig. 6a), but the sparker resolution The Oslofjord consists of a number of silled (10-15 m) is a limitation for mapping thin sedi basins. Predominant in the outer fjord are three ment cover. It should be noted that it may some- 60 A. Solheim & G. Grønlie NORSK GEOLOGISK TIDSSKRIFT l (1983) VESTFOLD ØSTFOLD SEDIMENTTHICKNESS OUTER OSLOFJORD THICKNESS IN METERS l V=1.7 Km/s l ---25m contour - 50,100,150, 200m contour Spot values .,. 5Km ...... _..._ �...... ��.---�------�------�-----W�----�------� 59° 10° 18' 110 Fig. 4. Thickness of Quaternary sediments, based on the sparker profiles of 1979. NORSK GEOLOGISK TIDSSKRIFT l (1983) Sediments and bedrock in Oslofjord and Skagerrak 61 �-----,-----,,------,--�--,------.---,��--�-,59°24' VESTFOLD ØSTFOLD SEDIMENTTHICKNESS OUTER OSLOFJORD [l]]]] WATER • SEDIMENT [1oooms S Km ...... _��....i Fig. 5. Bathymetry and sediment thickness, plotted perpendicularly to the 1979 profiles. 62 A. Solheim & G. Grønlie NORSK GEOLOGISK TIDSSKRIFT l (1983) Two-way travel time (ms) times be difficult to distinguish between bedrock and high! y compacted moraine from shallow seis mic reflection records alone. However, the sharp relief and lack of internal structures suggest that there are no significant morainic deposits in the shallower areas surrounding the main basins. The !argest sediment accumulations are found in the three major basins, with thicknesses ex ceeding 200 m. The shallow Ferder sill has the !east sediments. Further to the south where the water generally gets deeper and the bottom to pography more even, the sediment cover is more continuous, but still with the thickest accumula tion in the Hvalerdjupet basin. Previous work south of Larvik shows that at greater than 150 m water depth, the crystalline basement is covered by a continuous sediment layer (Rønningsland 1976). Three main sedimentary units can be distin guished (Fig. 6b-d): l. The uppermost unit is an acoustically trans parent sediment layer with no internal reflec tors, varying in thickness from a few millisec onds (two-way travel time) in shallow sedi ment pockets, to approximately 130 ms in Hvalerdjupet. In the other deep basins, the top layer is about 40 ms thick. According to Richards (1976), this material consists of "a greyish, sil ty clay with high plasticity", a de scription consistent with the acoustically transparent character of the sediments. 2. An intermediate, acoustically more layered sequence, with thicknesses up to 100 ms (Hvalerdjupet). This unit is only found in the deeper basins and troughs. The individual lay ers are relatively smooth and can be followed across the basins. Both this unit and the top one reflect the underlying topography. 3. A lower unit, up to 8G-90 ms thick, starting with a marked reflector, is seen at ca. 0.5 sin Fig. 6b and ca. 0.6 sin Fig. 6c. This is the !east transparent of the three units. Internal dif fractions, which may indicate larger stones and boulders, can be seen in this layer in Rauøyrenna (Fig. 6c). In Bastøydjupet (Fig. 6b) it may be divided into an upper unit of the same character as in Rauøyrenna, and a low er, layered unit. Fig. 6. Shallow seismic (sparker) profilesacross. a) the Ferder sil!. b) Bastøydjupet (heavy signal in lower right is noise). c) Rauøyrenna. d) Hvalerdjupet. For location, see Fig. 2. Fig. 6a NORSK GEOLOGISK TIDSSKRIFT l (1983) Sediments and bedrock in Oslofjord and Skagerrak 63 o -1 � o 200 l � Øl '< - """" Øl < CD - 3 CD 400 - 3 C/) ...... " b) o 2 4 (km) Fig. 6b The lower sequence cannot be distinguished in and ca. 40 m in Rauøyrenna (Richards 1973, Hvalerdjupet (Fig. 6d), and the whole accumula 1976). The 1979 profiles presented here suggest tion seems to consist of the two upper units. that the total sediment thickness in these troughs Earlier profilingin the outer Oslofjord, using a is considerably larger. The echo sounder in the low frequency echo sounder, have given sedi previous study has obviously not been able to ment thickness of less than 60 min Bastøydjupet penetrate through the intermediate unit. 64 A. Solheim & G. Grønlie NORSK GEOLOGISK TIDSSKRIFT l (1983) o -1 :E o l 200 :E Dl '< - ""'l Dl < CD 400 - 3 CD - 3 (/) - 600 c) o 2 4 6 B(km) Fig. 6c The seismic stratigraphy matches fairly well ter velocity (1500 m/s) gives about 30 m thickness with seismic refraction measurements made by in the two northern basins, and up to 100 m in Øfsthus (1966) just north of the survey area. Hvalerdjupet. It is probable that a relatively Layering can also be seen in small er, local great part of these sediments were deposited dur basins outside the major troughs, but in these ing the first two-three thousand years of the areas the sediments can only be divided in two Holocene, when relative lowering of the sea leve! units based on the sparker records. Recent veloc was most rapid. During this period extensive ity measurements in some of these areas (J. I. submarine areas were exposed to wave action, Faleide, pers. comm.) have given velocities of leading to erosion and redeposition of marine approximately 1500, 1700 and 1900 m/s. This is sediments further out in the fjord. Another im considered to indicate that the same sedimentary portant effect of a sea leve! fall is lowering of the units as seen in the main basins are also found erosional base and thus an increase of the erod outside of these. However, the different units are ing and transporting capacity of rivers. This thinner and the whole sequence is only locally again results in a larger sediment input to the developed. fjord. Arguments for the greater thickness of unit l in Hvalerdjupet are twofold. Firstly, this area Deposition probably received more sediments from the Nor The upper, transparent unit is considered to rep wegian mainland than the other two basins. This resent the Holocene (although a late glacial age is mainly because of the major Glomma river, for part of it cannot be excluded), assuming wa- terminating north of the Hvaler islands. Second- NORSK GEOLOGISK TIDSSKRIFf l (1983) Sediments and bedrock in Oslofjord and Skagerrak 65 o 200 -1 :E o l 400 :E P.> '< - .., P.> < Cl) 600 - 3 Cl) - 3 C/) - d) 800 Fig. 6d ly, this area probably also receives sediments tern could indicate a main transport direction from the south, brought in by the counter clock southwards through the basin. Due to the homo wise Jutland current, which is the main current geneous character of the unit l sediments, the system of the Skagerrak (Svansson 1972). Along latter is, however, considered unlikely. the Swedish coast, recent sedimentation rates of Further north in the fjord, there is less well 30 mm/year have been measured, and this is defined asymmetry in the sediment surface of the most! y material brought in by the Jutland current basins, and it may vary from east to west. This (Fiilt 1982). A southerly sediment source may distribution is somewhat dependent on the un also explain the more continuous sediment cover derlying bedrock topography but may also indi in the southern part of the survey area. cate local variations in the current pattern. Seismic profiles across Hvalerdjupet (Fig. 6d) Assuming a velocity of 1700 m/s for unit 2 show an asymmetrical distribution of unit l implies a thickness up to 45 m in Bastøydjupet sediments. Apart from the less steep basement and Rauøyrenna, and 85 m in Hvalerdjupet. We slope on the east side of the basin, the higher propose that this unit was deposited quite rapidly sediment surface on this side most likely reflects under ice-proximal conditions, probably begin a main sediment supply from east and northeast. ning when the grounded glacier ice started to However, sediment transport from the south, float over the deeper basins. Diffractions, which under the influence of the Coriolis force, may may be interpreted as larger, ice-dropped stones, also be a factor. It should be noted though, that may be seen especially in the lower part of this with current velocities sufficiently high to cause unit. The lower part of the unit has also less well non-deposition or erosion, the asymmetry pat- defined layering than the upper part. A maxi- 66 A. Solheim & G. Grønlie NORSK GEOLOGISK TIDSSKRIFT l (1983) mum of 3000 years may tentatively be given for be due to tida! circulation after the rise in sea the duration of the depositional regime proposed leve!. Tida! activity may also have played an for this unit. active role in the formation of the layering, espe In Kongsfjorden, Spitsbergen, sedimentation cially in the upper part of the second unit in the rates of 10 cm/year have been measured from Oslofjord area. However, in this near-shore, and varves in basins outside a currently active glacier thus ice-proximal environment, differences in (Elverhøi et al. 1980). Although the sedimentary output of coarser material from the glacier are bedrock of Spitsbergen is easier to erode than considered a more important factor. lee rafting is the bedrock of the Oslofjord area, both areas important for transporting material away from have or have had large amounts of loose, glacial the glacier, but density overflow may also carry ly derived material, which is easily eroded by relatively coarse material some distance (Gilbert waves and rivers. The high sedimentation rates 1982, Elverhøi et al. 1980). needed to deposit unit 2 during a time interval of approximately 3000 years is not improbable in this environment. Even higher sedimentation Magnetic measurements rates are reported from other present-day glaci ated areas (Molnia & Sangrey 1979, Powell in Magnetic total intensity has been plotted along press). the profiles for both cruises (Figs. 7 & 8). The The deepest unit may consist of a wide range northern half of the survey area has a quiet of sediment types from a rather wide time span. magnetic field which is of limited use in identify The velocity of 1900 m/s in a smaller, local basin ing geological boundaries. These northern pro may indicate that morainic material is present. files (Fig. 8) should, however, cross three differ The diffractions and lack of reflectors in Rauøy ent types of bedrock: Precambrian gneisses, Per renna support this, but it might also be due to mian lavas, and Permian sediments. The quiet rapid deposition, for instance near the grounding field is also confirmed by the aeromagnetic map line of a floating glacier. Older, pre-Weichselian of the area (Nor. geo!. unders. 1973). Åm & sediments cannot be excluded either, especially Oftedahl (1977) interpret the eastern Vestfold in the bottom of the deep basins. The deepest, Permo-Carboniferous lavas to be underlain by layered sequence in Bastøydjupet (Fig. 6b) might almost unmagnetic Precambrian rocks, and not be such older sediments. The layering could Larvikite. It is, however, reasonable to expect a however also reflect climatic fluctuations during sequence of Cambro-Silurian sedimentary rocks the Late Weichselian glacial retreat, resulting in between the Javas and the Precambrian base glacier oscillations and/or differences in the fall ment. This should not change the character of out from the glacier. the magnetic field. The lowest unit in Rauøyrenna is suggested to Between Rauøy and Hvaler islands there is an consist of morainic material, deposited during area of strong magnetic anomalies. The Permian the later glacial phases, when the glacier was rhomb-porphyry conglomerates found on the relatively thin in this area. Under the effect of small islands along the east side of the fjord are buoyancy, the compaction of the sediments from highly oxidized and thus have low magnetization ice-loading would be rather low. This may ex (K. Storetvedt pers. comm.). The fact that the plain why the sparker so easily penetrates down anomalies seem to cross the Oslofjord fault line to the basement. could indicate that the magnetic sources Iie in the In Bastøydjupet, it is probable that the lower, Precambrian basement, and that the cover of layered unit represents older sediments, com sedimentary rocks is quite thin in this area. A pacted by the Weichselian ice, with a cover of the possible magnetic source body might then be the same type of morainic material as is suggested Precambrian Iddefjord granite. Investigations of for Rauøyrenna. the Swedish Bohus granite body, which is con Work done by Van Weering et al. (1973) and nected to the lddefjord granite (Hageskov 1981), Van Weering (1975, 1982) has shown four depo shows that the granite has higher magnetic sus sitional units further out in the Skagerrak. Our ceptibility than the surrounding gneisses (G. intermediate unit may resemble unit 2 of these Lind pers. comm.). However, since the down surveys, which is also interpreted to be of Late faulting along the Oslofjorden fault is interpreted Weichselian/early Holocene age. The layering in to be !argest in its southern part (Ramberg 1976), Van Weering et al. 's (1973) unit 2 is thought to it is reasonable to expect a thick sequence of NORSK GEOLOGISK TIDSSKRIFT l (1983) Sediments and bedrock in Oslofjord and Skagerrak 67 - -- - 5 9° 2 4' .---� ---,-----,-ITiiTiiiiTiiTTJr-r .---.------o--rr,...,-::-..--;------.---,�==MAGNETIC:::-:-: MEASUREMENTS::-:-::=::-==:o 1979 OUTER OSLOFJORD Total fiektnrvs constant 5 km ØSTFOLD + + ��----�------�------�------�------�------�------_J sgo 110 Fig. 7. Profilesof total magnetic intensity of the 1979 cruise, plotted perpendicular to the ship's track after subtraction of a constant value of 48000nT. According to Fabiano & Peddie (1969), 50000 nT is the approximately correct magnilude of the regional field of the area, and the constant offset is due to an instrumental error. See text for further explanation. Legend of land geology in Fig. l. sedimentary rocks, both Cambro-Silurian and or an intrusive body. In the two cases both sides Permian. Seen from the aeromagnetic map (Nor. of the fault must be affected. If not, the fault geo!. unders. 1973), the granite body may cause must be situated farther to the east (Fig. 1), anomalies, but probably not strong enough to which is unlikely from the bathymetry of the area give a distinct character through a thick sedimen (Fig. 3). tary sequence. A more probable explanation is The magnetic field intensity is also slightly therefore intrusions, rising to some depth below high south of the Hvaler islands, indicating a the seafloor, either in the form of a dyke swarm, continuation of the lddefjord granite in this di- NORSK GEOLOGISK TIDSSKRifT 1 (1983) 68 A. Solheim & G. Grønlie VESTFOLD ØSTFOLD ���----�--�----��--�--�----�----�--�----�--�--�------�58°48' 1 o• 11• Fig. 8. Profiles of total magnetic intensity of the 1980 cruise, plotted perpendicular to the ship's track after subtraction of a constant value of 50000nT. Dotted line in the southwest indicates boundary between magnetic and non-magnetic rocks. See text for further explanation. Legend of land geology in Fig. l. rection but not as far as Torbjømskjær (Fig. 1), About 10 km southeast of Ferder there is a where no granite is exposed (B. Larsen pers. rather sharp positive, doublepeaked anomaly, comm.). recognized on several profiles and paralleling the From the islands of Bolærne southwestwards Hvaler trough to the west. A simple model which to southeast of Langesund there is an area of roughly fits the anomaly is that of two bodies irregular magnetic field and large anomalies. with a susceptibility contrast of 0.006, extending This is interpreted to define the. outline of the downwards at !east 600 m. Two subparallel dykes Vestfold Permian Larvikite intrusive. The are suggested to be the cause of the anomaly. boundary to the less magnetic rocks is clearly seen as a transition into a smoother magnetic field.The irregular appearance of the field in the Larvikite area could be due to susceptibility vari Bedrock geology and structures ations (i.e. variations in the magnetite content). A simple structural map (Fig. 9), showing main The large anomalies east of Tjøme may thus be geological boundaries and different faults and caused by large scale layering of magnetite as fractures, has been compiled from the seismic suggested by Åm & Oftedahl (1977). The ap and magnetic data. pearance of the anomalies, however, also seems As mentioned, the outline of the Larvikite is to indicate a certain dip of the layers. fairly well defined from magnetic measurements. NORSK GEOLOGISK TIDSSKRIFr l (1983) Sediments and bedrock in Oslofjord and Skagerrak 69 l ' l ' ' l l l l VEST�OLD 1 l l l l l l l l l l ØSTFOLp'l l l1 /+ l 1 l l ,, l " l ' l l Il l l/ l il l l il l l jl l l l l l l l l\ FERDER '• '• l l l ,, l l •• l l • Pl l l l l ····· ' ··· ················ .. l . i l i l l BEDROCK STRUCTURE :: l l OUTER OSLOFJORD : l km j l 5 --- Major fault --- Mlnor fault or fracture •••·•• Lithologlcal boundary - Dyke PE: Precambrian Pl Permian lans P l Permian intruaiwes RPC Ahomb porfyry eonglomerate Fig. 9. Simple map of outer Oslofjord bedrock structure, based on data obtained during the 1979- and 1980.cruises. Legend of land geology in Fig. l. 70 A. Solheim & G. Grønlie NORSK GEOLOGISK TIDSSKRIFr 1 (1983) The western boundary, towards what may be structural interpretation of the area. Therefore Cambro-Silurian sedimentary rocks, is shown in no direction of movement is indicated along the Fig. 8. However, since Permian lavas are ex faults. However, it is pro bable that the major posed also west and southwest of the intrusives, troughs owe their formation to downfaulting, it cannot be excluded that the indicated bounda forming a small scale basin and range pattern, ry might be between Permian intrusives and vol later modified by glacial erosion. It should be canics. mentioned, though, that the northern part of The rhomb-porphyry islands on the east side Hvalerdjupet has a somewhat peculiar bathymet of the fjord are all lying on a ridge, especially ric expression (Fig. 3). Sharp, northwesterly well defined in the southem half (Fig. 3). This trending bends in the slopes on both sides are ridge is interpreted to consist of the same materi fairly well aligned with the trough running south al as the islands. Both the northward and the eastwards to the south of the Hvaler islands. This southward continuation of the sediments are un pattern could be indicative of a strike-slip motion certain, but they are not found on the island of (although not shown in Fig. 9) which might be Jeløya (Larsen et al. 1978). partly responsible for the formation of Hvaler The Permo-Carboniferous lavas of Vestfold djupet as a small pull-apart basin. are suggested to continue over most of the fjord The structural pattern elsewhere in the fjord in the northemmost area, underlain by Precam follows the main structural directions on land in brian gneiss and Cambro-Silurian sediments. the area: north-northwest, north-northeast, and Nothing definite can be said about the zone more northerly direction as expected. The more between the rhomb-porphyry conglomerates and northerly direction seems to be most common in the Larvikite, but a probable interpretation may the area interpreted as occupied by Larvikite. be a continuation of the lavas in this area. It seems further reasonable to suggest that these lavas might be interbedded with sediments of a more distal facies, in relation to the graben Summary shoulder to the east, than what is found on the islands. The outer Oslofjord has three major bathymetric In the southeastern part of the survey area, the basins. The two northernmost, Bastøydjupet and seismic profiles show a somewhat different base Rauøyrenna, are markedly silled, while the ment character on either side of Hvalerdjupet third, Hvalerdjupet gradually apens into the Ska (Fig. 6d). The west side has more large-scaled gerrak. The basins have steep, stepwise slopes basement topography, and has also a greater and are probably formed by graben-faulting and water depth than the east side, indicating a dif later modified by glacial erosion. Strike-slip mo ferent type of bedrock. On the east side, a Pre tion is suggested in the southeastern part. cambrian basement seems reasonable. This may The major accumulations of Quaternary sedi either be gneiss or, most probably a continuation ments are restricted to the basins, where they of the Iddefjord granite. Dredging the slopes of exceed 200 m. Three main units can be distin Hvalerdjupet could probably give a more defi guished; an upper acoustically transparent, sup nite answer. On the west side, we assume Per posed Holocene unit, an intermediate, layered mian lava is the most probable bedrock. supposed Late Weichselian unit and a lower unit, On the assumption that most elongated topo probably consisting partly of moraine and partly graphic features in the ara are caused by faults of older, pre-Weichselian layered sediments. In and fractures, basement topography has been Hvalerdjupet, only the two upper units are rec used for mapping of these structures in the fjord ognized. Here, the distribution and larger thick area. Because no displacement can be recognized ness of the Holocene unit may indicate that ma on the seismic records, the interpretation is ten terial is also brought in from south with the tative, and the division between major and minor Jutland current. faults or fractures (Fig. 9) is done only from the From magnetic and seismic profiling the fol size of the topographic features. The outline of lowing inferences can be made about the subma the main eastern fault, the Oslofjorden fault, is rine bedrock geology: taken from earlier published maps (Larsen 1975) wherever there is lack of profile coverage. - The Permo-Carboniferous Vestfold lavas cov The data base is too limited for a detailed er most of the northern part of the survey area ( ) NORSK GEOLOGISK TIDSSKRIFT l 1983 Sediments and bedrock in Oslofjord and Skagerrak 71 and are underlain by rather nonmagnetic Grønlie, G., Johansen, T. E., Karlstad, B. & Heier, K. S. rocks. 1980: Prospecting for geothermal energy in the Iddefjord Large anomalies along the edge of and inside granite, Østfold, Norway. Norsk Geo/. Tidsskr. 60, 263-267. Hageskov, B. 1980: The sveconorwegian structures of the Nor the Permian Larvikite make it possible to map wegian part of the Kongsberg-Bamble-Østfold segment. the outline of this intrusive body. Geo/. For. Stockh. For. 102, 15G-155. Permian rhomb-porphyry conglomerates are Holtedahl, O. (ed.) 1960: Geology of Norway. Nor. geo/. probably restricted to a ridge along the main unders. 208, 540 pp. Larsen, B. T. 1975: Oslo Region tectonic map, l: 250,000. Oslofjorden fault, but Permian sediments, Norwegian Geotechnical Institute, project no. 74628. possibly interbedded with Javas, may also be Larsen, B. T., Ramberg, L B. & Schou Jensen, E. 1978: found west of the ridge. Excursion 3, central part of the Oslofjord. in Dons, J. A. & The Iddefjord granite probably extends farther Larsen, B. T. (eds.), The Oslo paleorift, a review and guide to excursions. Nor. geo/. unders. 237, 105-124. to the southwest from the Hvaler islands. Larsen, B. T., Midtkandal, P. A., Steel, R. & Steinlein, O. lntrusives are suggested to be present north 1981: Guide to excursion B-2: Geology and tectonics of the west of Hvaler, at some depth ( either as a dyke central Oslofjord, Oslo rift. Oslo region research group 7, swarm or as a body), and as two subparallel 51-80. Nor. geo!. unders. 1973: Oslo, l : 250,000 Magnetisk totalfelt. dykes west of Hvalerdjupet. Trondheim, Norway. Molnia, B. F. & Sangrey, D. A. 1979: Glacially derived sedi The submarine structural pattern shows the same ments in the northern Gulf of Alaska - Geology and engi directions as mapped subaerially. neering characteristics. Proc. 1979 Offshore Technology Conference, OTC-3433, 2, 647-655. Øfsthus, A. 1966: Seismiske mdlinger i Oslofjorden. Unpubl. Acknowledgements. - This project was supported by the Nor cand. real. thesis, Univ. of Bergen, 122 pp. wegian Council for Science and the Humanities (NAVF), grant Oftedahl, Chr. & Petersen, J. S. 1978: Excursion 6, southern no. D. 40.31-45. part of the Oslo rift. In Dons, J. A. & Larsen, B. T. (eds.), We thank the Geological Survey of Norway (NGU) and the The Oslo paleorift, a review and guide to excursions. Nor. University of Bergen for providing magnetometer and sparker. geo/. unders. 237, 163-182. Special thanks go to Ivan Rosenqvist, Odd Møller, Åge Midt· Olaussen, S. 1981: Marine incursion in upper Paleozoic rocks hassel, Knut Pederstad, Yngve Kristoffersen, Jo Bergan, Maja of the Oslo Region, southern Norway. Geo!. Mag. 118, 281- Boserup, and the crew of 'Bjørn Føyen' and 'H. U. Sverdrup' 288. for exceUent work during the cruises. Jørn Thiede and Bjørn Powell, R. D. in press: Glaciomarine sedimentation processes Larsen are acknowledged for their critical comments on the in Glacier Bay, Alaska. In Molnia, B. F. (ed.), Glacial manuscript. Marine Sedimentation. Plenum Press, New York. Ramberg, l. B. 1976: Gravity interpretation of the Oslo graben Manuscript received January 1983 and associated igneous rocks. Nor. geo/. unders. 325, 194 pp. Ramberg, l. B. & Larsen, B. T. 1978: Tectonomagmatic evolu tion. In Dons, J. A. & Larsen, B. T. (eds.), The Oslo paleorift, a review and guide to excursions. Nor. geo/. References unders. 237, 55-73. Richards, A. F. 1973: Geotechnical properties of submarine Årn, K. & Oftedahl, Chr. 1977: Brief comments on some soils, Oslofjorden and vicinity, Norway. Nor. Geotech. Inst., aeromagnetic anomalies in the Oslo region. In Heier, K. S. tech. rep. 13, 50 pp. (ed.), The Norwegian geotraverse project, a Norwegian con Richards, A. F. 1976: Marine geotechnics of the Oslofjorden tribution to the international upper mantle project and the region. In Janbu, N., Jørstad, F. & Kjærnsli, B. (eds.), international geodynamics project, 209-222. Laurits Bjerrum memorial volume. Contributions to soil Elverhøi, A., Liestøl, O. & Nagy, J. 1980: Glacial erosion, mechanics. Nor. Geotech. inst., 41-63. sedimentation and microfauna in the inner part of Kongs Roaldset, E. 1979: Pleistocene and Holocene sediments in the fjorden, Spitsbergen. Nor. Polarinst. Skr. 172, 33-62. Oslofjorden-Skagerrak region. Inst. for geo/., Univ. of Oslo, Fabiano, E. B. & Peddie, N. W. 1969: Grid values of total lnt. skriftserie 22, 43 pp. magnetic intensity, IGRF-1965. ESSA Tech. Report C & GS Rønningsland, T. M. 1976: Mineralogi og geokjemi av resente 38, 55 pp. /eirsediment i Skagerrak, Kattegat og tilgrensende fjordom Falt, L. M. 1982: Late Quaternary sea-floor deposits off the rdder. Unpubl. cand. real. thesis, Univ. of Oslo. Swedish west coast. Chalmers University of Technology and Rønningsland, T. M., Roaldset, E. & Rosenqvist, l. Th., in University of Gothenburg, Publ. A37, 259 pp. prep.: Origin and composition of recent clays in the Katte Feyling-Hanssen, R. W. 1964: Foraminifera in late Quaternary gat, Skagerrak, and adjacent Scandinavian coastal waters. deposits from the Oslofjord area. Nor. geo/. unders. 225, 383 Svansson, A. 1972: Physical and chemical oceanography of the Skagerrak and the Kattegat. Fish. Brd. Swe., Inst. Mar. Res. PP· Floden, T. 1973: Notes on the bedrock of the eastern Skager Rpt. No. l, 88 pp. rak with remarks on the Pleistocene deposits. Stockholm Sørensen, R. 1979: Late Weichselian deglaciation in the Oslo contr. geo/. XXIV, 79-102. fjord area, south Norway. Boreas 8, 241-246. Gilbert, R. 1982: Contemporary sedimentary environments on Van Weering, T., Jansen, J. H. & Eisma, D. 1973: Acoustic Baffin Island, N.W.T., Canada: Glaciomarine processes in reflection profiles of the Norwegian Channel between Oslo fjords of Eastern Cumberland Peninsula. Arctic and Alpine and Bergen. Nether/ands Jour. Sea. Res. 6, 241-263. Research 14, 1-12. Van Weering, T. 1975: Late Quaternary history of the Skager- 72 A. Solheim & G. Grønlie NORSK GEOLOGISK TIDSSKRIFT l (1983) rak; an interpretation of acoustical profiles. Geologie en Mijnbouw 54, 130-145. Van Weering, T. 1982: Shallow seismic and acoustic reflection profiles from the Skagerrak; implications for recent sedimen tation. Proc. K. Nederlandse akademie van wetenschappen, Amsterdam, B 85 (2), 129-154.