Bedrock geology and Quaternary sediments in the Lista basin, S. Norway
HANS HOLTEDAHL
Holtedahl, H.: Bedrock geology and Quaternary sediments in the Lista basin, S. Norway. Norsk Geologisk Tidsskrift, Vol. 68, pp. 1-20. Oslo 1988. ISSN 0029-196X.
Sedimentary rocks of U. Triassic and L. Jurassic age are suggested to occur in the submarine area between Lista and the Siregrunnen bank. This conclusion is based on shallow seismic profiling data, as well as on data from dredge- and core-samples. The boundary between the sedimentary rocks and the Precambrian area to the northeast is marked by a major fault outside the island of Hidra, which probably continues northwestwards outside the mainland, and southeastwards along the Eidsfjord. Other faults with similar direction are registered both inside and outside the threshold at the mouth of the Lista basin, which therefore seems to be structurally controlled. While the Quaternary sediment cover over the threshold area is thin and even absent, the basin inside contains sediments with a maximum thickness of about 100 m. These sediments are probably marine and glaciomarine deposits of Late Weichselian and Holocene age. The retreat of the main ice in the basin must have taken place before 12,500yrs B.P., possibly 13,000yrs B.P., which gives a minimum age for the Lista substage. The age of an ice-marginal deposit at the mouth of Fedafjord, possibly representing the Spangereid substage, must be about 12,000yrs B.P. The assumed Mesozoic rocks occurring in the Lista basin are thought to be the source for erratics found in glacial deposits in coastal areas to the northwest, e.g. coal in the 'Skagerrak moraine' at Jæren. Mesozoic sediments have probably covered the Precambrian land areas to the north of the Lista basin, and the present surface is thought to be very much the result of exhumation of a subtriassic weathered surface.
H. Holtedahl, Universitetet i Bergen, Geologisk Institutt, Avd. B, A/Legt. 41, N-5000 Bergen, Norway.
During a cruise in 1984 with the University of addition, bottom sampling was to be carried out, Bergen research ship 'Håkon Mosby', submarine of Quaternary sediments and, if possible, of in moraines and ice-marginal features offthe south situbedrocks. The present paper gives an account coast of Norway were studied (Holtedahl 1986). of the results obtained. In the area west of Lista, where a very marked end moraine, the 'Lista moraine', is thought to occur, continuous seismic profiling showed little evidence of the moraine, but instead the occur rence of sedimentary rocks, in part below Quat Data acquisition ernary sediments, and partly as outcrops. As there Figs. l and 2 show the area of investigation, which was reason to believe that the sedimentary rocks covers about 190 km2 and is bounded to the east might be of Mesozoic age, previously unknown by the Lista mainland, to the west by the Sire in the area, and possibly connected with Mesozoic grunnen bank, and to the north by the Hidra rocks known to exist furtherout in the Skagerrak island and the mainland coast. basin, another cruise was carried out in 1985. The geophysical instrumentati on consisted of a During this cruise, surveying was concentrated in 12 KHz Simrad Scientific Sounder E.K., a hull the area between Lista and the shallow Sire mounted O.R.E. penetration echosounder oper grunnen bank to the west. The main purpose of ated at 3. 7 KHz, and a shallow seismic EG&G the field investigation was to obtain echo-sound Sparker system with an energy output of l KJ. ing profiles and shallow-seismic profiles which The data were bandpass-filtered(60-600 Hz) and would give information on (a) the bathymetry recorded on an analogue recorder. As sparker (which was inadequately known), (b) the bedrock profiles in 1985 were only obtained in profiles geology, and (c) the Quatemary deposits. In transverse to the coast (due to technical failure), 2 H. Holtedahl NORSK GEOLOGISK TIDSSKRIFT 68 (1988)
o 50 100km
o� SOUTH NORWAY
Stavanger
Fig. l. The NorwegianChannel with adjacent coast of S. Norway. The investigated area (Fig. 2) is marked.
some additional sparker- and air-gun profilespar Geological and geomorphological alle! to the coast were run by l. Aarseth (Uni versity of Bergen) in 1986 in connection with setting another program. The positions of the profiles Precambrian rocks occur on land in the area are shown in Fig. 2. Lista-Åna-Sira (Fig. 4). A complex of anor The sediment samples were collected by gravity thositic-charnockitic igneous rocks is common corer and dredge. The dredge was an ordinary northwest of the Listafjord. Gneisses occur on seraper dredge with a triangular steel frame(sides the southern part of Lista as well as north of the of frame about 40 cm) and a meshed bag of the anorthosites. Analyses of the fracture pattem of type used in biological work. The gravity corer this part of Norway (Barth 1939; Ramberg et al. had a 3 m long barrel. As one of the principal 197 7) show a dominance of N-S, and NE-SW objectivesof the sampling was to obtain bedrock, lineations. These are also visible in the landscape, localities for dredging were to some extent selec where valleys and fjords to a great extent follow ted in are as with a minimum of Quatemary cover, a N-S or NE-SW trend. as indicated by the acoustic registrations. The The indentation of the coast at Lista is very sites for sediment sampling are shown in Fig. 2. conspicuous, but with the shallow Siregrunnen The quality of positioning of profilesand sample bank to the west, a lowering of the sea level of stations is assumed to be fairly satisfactory, using 100 m would result in a straight coastline towards the normal navigational means including satellite NW, cut only by the Lista basin. The southern navigation. Strong currents and disturbances from part of Lista is a low-lying plain covered largely land caused certain difficulties, however, espe with Quaternary deposits. A major part of these cially in the western part. are tills, sometimes occurring in drumlinoid ridges NORSK GEOLOGISK TIDSSKRIFT 68 (1988) Bedrock and Quaternary geo/ogy, Lista basin 3
7
5km
• Moraine/till Silt and clay Sa/ l M Thtn lill cover l l ;::_�_-::::: Acoustic profiles
• c.. -
- Oredgo sampleo
Fig. 2. Area of investigation, with tracks of acoustic and seismic profiles, sample positions and distribution ofsurface sediment. Depth contours in m. Profiles described in text shown by coarse dotted line. Bathymetry based on data from the Norwegian Hydrographic Office and own investigatioes.
with a SSW-NNE direction; the main direction of ice movement is shown by glacial striae in Bathymetry the area. Close to the coast morainal hills run The sea area between Lista and the Siregrunnen approximately parallel to the coast and transverse bank is a moderately deep basin with a minimum to the main ice-movement direction. These are depth of about 260 m in its outer part. The thought to be end moraines of the 'Lista substage' threshold has a NW-SE direction and can be formed about 13,5 00 yrs B.P., according to traced between the southwestern part of Lista and Andersen (1979). The submarine extension of the southwestern part of the Siregrunnen bank. this end moraine belt, expressed by submarine The slope leading from this threshold outwards ridges, is thought to occur paraBel with the coast into the Skagerrak ends in a slight longitudinal to the northwest as well as southeast of Lista. depression with a maximum depth of 389m, a Glaciofluvial, tluvialand marine deposits are also feature which continues off the coast eastwards described from the southern part of Lista. The as the northern part of the Norwegian Channel highest marine limit at Lista is thought to be 7- (0. Holtedahl 1940 ). 8 m above sea leve!. The evenness of the basin bottom is broken 4 H. Holtedahl NORSK GEOLOGISK TIDSSKRIFT 68 (1988)
, lkm . !,._ -,-.-�-��--:--:::=="""�"-'-.,----"---'+""' ' Profile37 t WSW ENE �"::.3��------��
TWT Sec. Profile 2
TWT Profile 1 Sec. . 2 �------
.3
.5
.6
Fig. 3a, b. Continuous shallow seismic profiles( sparker profiles) in the Lista basin. For positions see Fig. 2. Numbers within circles mark approximate location of sediment samples. TWT: Two-way travel time. NORSK GEOLOGISK TIDSSKRIFr 68 (1988) Bedrock and Quaternary geology, Lista basin 5
TWT Profile 39 Sec.
TWT Sec.
•3 sw Profile 4 ·· ------�
1km
,..------ 18' + + + 16' + + + + + + + + + + + + + + + + + + + + + + + + + T + + + + + + + + + + + + + + + + + + +Lf�------� . + + + + + + + + + + + + �}+��=+�+�+��+�+�+ + 14' 12' ••••• • Rock boundary V77lSediinentary rocks l.LL....J ILower Jurass1c and upper Triassic?l 1+-=tlPrecambrian � crystallin• rocks -. Stnke and dip Contours in metres Skm ' 30' 40 Fig. 4. Map indicating main geological features in the Lista basin and adjacent land. this map, especially with regard to the boundary talline rocks, with a dip towards the boundary relationships between the Precambrian and sedi (profiles 3 and 4). The fault off Hidra probably mentary rocks. This boundary is unfortunately continues eastwards along Eidsfjord and the val often unclear on the seismic records. What does ley further east (Figs. 4 and 6). The chamockitic seem de ar, however, is the existence of a series rocks which are exposed on the southem valley of faults the most important of which forms the side are strongly fractured and weathered, with boundary between the Precambrian crystalline clay frequently developed in the fractures. A rocks and the sedimentary rocks south of Hidra western continuation of the fault may be found island and off the mainland further to the in the submarine depression between Ånafjord northwest. and the bank outside. In some of the sparker profilesthe sedimentary Other important faults, or fault systems, are rocks are seen to abut abruptly against the crys- present on the inside of the threshold of the Lista 8 H. Holtedahl NORSK GEOLOGISK TIDSSKRIFT 68 (1988) basin, as well as on its outer slope. The latter sen 1966), this may explain why the rocks have faults seem to continue southeastwards outside been eroded to a deeper leve!. Van Weering the coast, with a location roughly along the north (1982) suggests the presence further north of em part of the longitudinal depression which is Upper Jurassic rocks (profile 1), and in profiles part of the Norwegian Channel. further east (profiles2 and 3) Lower Jurassic and While the boundary between the Precambrian possibly Triassic rocks dose to the Precambrian crystalline rocks and the sedimentary rocks out border. side Hidra and the mainland further west is Van Weering's profile l is located somewhat marked by a major fault, the contact with the east of the Lista basin. If the assumption of a Siregrunnen bank to the northwest, and Lista to Jurassic age for the sedimentary rocks close to the southeast, is apparently unrelated to faulting. the Precambrian boundary is correct, a Lower All of the faults mentioned have an orientation Jurassic-Triassic age for the rocks of the Lista more or less following the main outline of the basin would be expected. A similar age is sug coast, which is NW-SE. There are, however, gested by extrapolating data which have been indications of faults with a more northerly direc made available through the Norwegian Petroleum tion. The sedimentary rocks seem to have a south Directorate (P. Blystad pers. comm. 1986) from westerly dip in the threshold area and outside, an area south of the Lista basin. and a northeasterly dip towards the Precambrian boundary outside Hidra. Relationships in this area are undear, and may be due to a complex Description of sediment samples fault pattern. Fig. 2 shows the location of sample stations, and Tab le l summarizes sampling depths and types of sampler used. Age of the sedimentary rocks In the northern part of Skagerrak the boundary Dredge samples between Precambrian rocks and sedimentary An examination of the dredged material was car rocks of assumed Mesozoic age is shown in a ried out. Only material coarser than 4 mm was seismic profile taken south of Kristiansand (Sel analysed. It was decided to restrict the exam levoll & Aalstad 1971). The sedimentary rocks ination to a rough roundness dass analysis in wedge out in the deepest part of the Norwegian order to obtain information on the transport his Channel, a�d increase in thickness southwards tory of the material, and a lithological analysis, towards Jutland. Reflectors with a southerly dip, for provenance study. The roundness of dasts was presumably representing sedimentary layers, are dassified following Olsen (1983), whereby four overlain unconformably, supposedly by Quat degrees of roundness were categorized. In addi ernary sediments. Strong reflectors, which are tion, one dass for fractured, rounded material thought to mark the base of the Upper Creta ceous, sub-crop partway up the southern slope Table l. Sediment samples, type of sampler used, depth of of the channel, indicating the presence of Up sampling, and position of samples related to seismic profilesin per Cretaceous rocks on the upper part of the the Lista bas in. slope (as well as in northern Jutland) and Lower Sample Type of sampler Depth in m Pro file Cretaceous rocks on the lower part of the slope. no. In a continuous seismic profile crossing the l Gravity corer 270 3 Norwegian Channel west of the Lindesnes pen 2 Gravity corer 302 3 insula (east of Lista), van Weering (1982) suggests 3 Gravity corer 309 3 a similar stratigraphy. Assuming the very strong 4 Dredge 330--360 3 refiectors along the southern fiank of the Ska 5 Dredge 298-330 4 6 Dredge 300--360 5 gerrak to be indicative of Upper Cretaceous 7 Gravity corer 198 5 rocks, the adjoining strata to the north are 8 Gravity corer 230 6 thought to be of Lower Cretaceous age. As the 9 Gravity corer 273 6 Lower Cretaceous sequence in the Danish embay 10 Gravity corer 305 6 ment consists primarily of day-rich sediments, 11 Dredge 265-290 38 12 Dredge 120--150 39 which are probably less resistant to erosion (Lar- NORSK GEOLOGISK TIDSSKRIFT 68 (1988) Bedrock and Quaternary geo/ogy, Lista basin 9 Table 2. Roundness classifieation of rock material from the Lista basin. WR: weU rounded, R: rounded, SR: subrounded, A: angular, WR-R/A-SR: clasts with two different degreesof roundness, el: clasts. Sam p le Size WR R SR A WR-R/A-SR no. mm % % % % % Total 4 >16 (7.3) (7.3) (58.5) (14.6) (12.2) 42 5 >32 (16.7) (69.0) (14.2) 42 5 16-32 7.3 57.9 18.2 16.7 166 5 8-16 4.6 8.3 50.5 30.9 6.4 109 6 >32 l el. 7el. 2cl. 15el. 25 6 16-32 1.5 23.1 38.5 36.9 65 6 8-16 0.9 4.7 48.6 21.5 25.2 107 11 >32 9el. 4cl. 6el. 19 11 16-32 9.4 52.8 18.9 18.9 100 11 8-16 8.0 55.0 17.0 20.0 100 12 >8 2.1 6.2 53.6 20.6 17.5 97 was distinguished. The results of the analysis are including chipped rounded and well-rounded peb given in Tables 2, 3 and 4. bles are added, the total percentage of this The data in Table 2 show a great spread in material is remarkable. It occurs especially fre roundness values, with a maximum in the SR quently in sample 6, but also in samples 11 and (subrounded) class. These clasts show definite 12. These data suggest a glacial advance over effectsof wear, often with a striated surface. They previously rounded material, either beach show great similarity to clasts from till material, material or glaciofluvial material. and are no doubt transported and deposited by Based on roundness class studies alone, the glaciers. dredged material from the Lista basin area is Table 3 gives the roundness class distribution mainly of glacial origin, deposited directly from based on the four main classes, excluding the glaciers and possibly also icebergs. WR-R/A- SR class; the rniddle roundness num Lithological analyses were concentrated upon ber is calculated in addition (Olsen 1983). A value three categories of rock types: (1) rocks from the found close to SR (2) for most of the samples is mainland, (2) rocks from the Oslo graben area, indicative of till genesis. The content of pebbles and (3) rocks not known from the mainland and with a high degree of roundness (WR, R) is not possibly of Mesozoic age. As the Lista basin area very high. has undoubtedly been overrun by glaciers many In Tab le 4, however, where all rounded clasts, times during the Quaternary, morainal material with rocks from the mainland must be expected. Erratics from the Oslo area, transported by float ing ice, must also be expected. Mesozoic rocks Table 3. Roundness dass distribution of rock material from the may occur as in situ outcrops in the Lista basin, Lista basin (4 classes), and middle roundness numbers (MR). as glacially transported local material, glacially Well rounded: 4, angular: l. transported material fromother parts of the Ska gerrak, possibly by a 'Skagerrak glacier', or as Sample Size WR(4) R(3) SR(2) A( l) no. mm % % % % MR dropped material from icebergs and floating ice. The major proportion of the dredged samples 4 >16 8.3 8.3 66.7 16.2 2.1 consist of crystalline rocks known fromthe inland. 5 >32 16.7 69.0 14.2 2.0 No attempt has been made to classifythese rocks 5 16-32 8.6 69.4 21.8 1.9 5 8-16 4.9 8.9 53.9 32.3 1.9 and determine their source areas. Anorthosites 6 16-32 2.4 36.6 61.0 1.4 are common, likewise a great variety of gneisses, 6 8-16 1.2 6.2 64.2 28.4 1.8 granites and amphibolitic rocks. These rocks fall 11 16-32 11.6 65.1 23.3 1.9 within the roundness classes mentioned above, 11 8-16 10.0 68.8 21.2 1.9 and with textures typical of glacial till, suggest 12 >8 2.5 7.5 65.0 25.0 1.9 deposition from glaciers coming from inland. 10 H. Holtedahl NORSK GEOLOGISK TIDSSKRIFf 68 (1988) Table 4. Roundness class distribution of rock material from the limestones, sometimes rich in carbonaceous Lista basin (3 classes), showing percentages of angular (A), material, are frequentlypresent, some with plant subrounded (SR), and of total rounded material. remains. In dredge sample 11, siltstones and sand Sample Size Sum: WR, R, SR A stones with carbonaceous matter were especially no. mm WR-R/A-SR % % % frequent in all size classes. A sample of a light grey siltstone with plant remains was investigated 4 >16 26.9 58.5 14.6 by Professor B. Lundblad, Stockholm. Her pre 5 >32 16.8 69.0 14.2 liminary conclusion was that two fragments of 5 16-32 24.0 57.8 18.2 5 s--16 19.3 50.4 30.3 leaves could be recognized as Podozamites sp., a 6 16-32 38.4 23.1 38.5 genus which occurs in the Upper Triassic-M. 6 s--16 30.8 48.0 21.2 Jurassic. Lithologically the siltstone is very similar 11 16-32 28.3 52.8 18.9 to rocks from the lower Li as succession (the 11 s--16 28.0 55.0 17.0 12 >8 25.8 53.6 20.6 Boserup layers) of Skåne, S. Sweden. Thegreat similarity between the Lista specimen· of Podozamites sp. and Podozamites lanceolatus (Lindlay and Hutton) described fromthe Boserup layers is pointed out, and it is therefore deduced Erratics dropped fromicebergs and other types that the Lista siltstone sample is of Lower Lias of floating ice are represented in the samples. age. A study of fossil pollen and spores from The presence of clasts of rhombporphyry and another siltstone sample was carried out by J. larvikites indicates a transport fromthe Oslo gra Vigran, IKU, Trondheim. The similarity to ben area {Table 5), as does a commonly occurring material described from Skåne {T. Nilsson 1958; dark {alum) shale in sample 6. This latter shale is D. Guy 1986) is again suggested, and a Lower strongly striated, with striae in many directions Jurassic age is believed to be most likely. indicating transport by icebergs. Clasts from the Except for coal fragments, which are very Oslo area occur in all samples {Table 5). fragile, most clasts of supposed Mesozoic age Pieces of flint are present in all samples except sedimentary rocks show some degree of wear. A sample 3, and chalk is found in samples 5 and 6. roundness analysis of 124 clasts between 8 mm The source of these rocks may be in the Skager and 80 mm gave the following result: R: 23.4%, rak, where Cretaceous rocks occur, or in ben SR: 53.2%, A: 8.8% and WR-R/A-SR: 14. 6%. mark and southem Sweden. Flint and chalk (as This variation in roundness properties agrees with well as Oslo rocks), supposedly deposited from the values for the whole sample. The percentage floating icebergs, are very common along the of well-rounded and rounded, including chipped entire Norwegian coast. clasts, is 38%, which corresponds to the high Rocks which are thought to occur in the Lista value found in sample 6. The conclusion to be basin are also shown in Table 5. Pieces of coal drawn from this is that most of the assumed are commonly found in the samples and in all size Mesozoic rocks found in the samples have had classes analysed. Sandstones, with or without a some degree of wear, which to some extent must calcareous matrix, are common. Siltstones and have been caused by glacial transport. Table 5. Occurrence of rocks from the Oslo area (rhombporphyry, larvikite etc.); flint and chalk; coal, carbonaceous silt- and sandstones, limestones etc., in dredge samples from the Lista basin. Skagerrak Lista basin (?) Sam p le no. Oslo rocks Flint Chalk Coal Sd. st. Siltst. L. st. Carb. rocks 3 X 4 X X X 5 X X X X X X X 6 X X X X X X 11 X X X X X X X 12 X X X NORSK GEOLOGISK TIDSSKRIFT 68 (1988) Bedrock and Quaternary geology, Lista basin 11 Core samples Of the shorter cores, core 2 is different from The cores were X-rayed in the laboratory, split the longer ones in its greater content of assumed into halves and described. Subsamples were Mesozoic rocks. This is the case particularly in taken, and grain-size analyses as well as a rough the lower part below ca. 40 cm. Grains of coal are lithological analysis of grains larger than 0.125 present throughout the core, with the exception mm carried out. As with the coarse material ob of subsamples 3b and 4. Siltstone, with car tained from dredge samples, it was of special bonaceous layers similar to the Lower Lias Podo interest to observe the degree of grain roundness, zamites siltstone from dredge sample 11, occurs as well as the presence of special rock types. As as a 2 cm clast in subsample 8, and also as finer crystalline rock-grains and clasts are dominant in fragments in the underlying samples. Grains of all cores, only the presence of the following rock coarse sandstones (arkoses), similar to rocks types were noted: flint and chalk, coal and sedi occurring in the dredge samples, were found in mentary rocks rich in carbonaceousmatter, other various subsamples. sedimentary rocks, especially sandstones, silt The clast and grain content of core 9 is similar stones and limestones believed to be of Mesozoic to that of the longer cores. A clast of flint is age. Fig. Sa, b shows the lithostratigraphy, the present in the upper part of the core (subsample grain-size distribution and the content of clasts 2), and a clast of coal in subsample 3. and grains assumed to be of Mesozoic age. The The majority of clasts and grains in the core positions of the sampling sites on the sampling sediments belong to the subrounded dass. Quite profiles, (which are not exactly the same as the a number of pebbles are clearly worn and some sparker profiles), is shown. of them striated. There are also a number of The variation in length of the sediment cores pebbles and finergrains which have a high degree provides a rough indication of the sediment of roundness. A quantificationof this has not been strength. Easily penetrable sediments are present attempted, but cores 8 and 9 have a remarkable at the positions of cores l, 7, 8 and 10, while less amount of rounded material, including non-crys penetrable sediments occor at the positions of talline rocks. cores 2, 3 and 9. The longest cores have sediments In conclusion, the study seems to suggest that consisting of o live grey, or grey, clay and silt, with the core sediments to a great extent have a gla varied amounts of sand and gravel. The sand ciomarine origin, and that deposition from ice may appear as isolated layers with very little fine bergs and other types of floating ice has been material, or it may be poorly sorted, within clay important. and silt units. The grave! fraction may be associ ated with the sand units, or as individual clasts Discussion of dredge and core throughout the core. A num ber of clasts are drop stones, on which glacial striae and polish often material occur. The sediments show considerable textural From the seismi c investigation, the age of the vertical variation, with units of bioturbated silty sedimentary rocks underlying the Lista basin was clays alternating with units of more coarse assumed to be of U. Triassic-L. Jurassic age. material. Direct confirmationof this, i.e. broken slabs from Of the shorter sediment cores, core 9 is similar in situ rocks, was not obtained from the dredge to the longer ones, except for its content of grave! samples. It appears that the dredged material bad which is present throughout its length. Core 2, different source areas and bad been subjected to however, is different, being coarser, with zones varying length and type of transport. As discussed rich in grave! and sand, and showing erosional above, a major part of the coarse material was boundaries. Core 3 contains only a very short deposited by glaciers coming from inland areas. sample consisting of gravelly and sandy mud. By far the majority of rocks are crystalline, to With regard to the content of clasts and grains a great extent gneisses, granites, amphibolites, of Mesozoic(?) rocks, cores l, 7, 8 and 10 do not anorthosites, etc., probably from the Precam differ greatly. Grains of coal occur commonly, brian area adjacent to the Lista basin. A relative!y flint, chalk, carbonaceous rocks and other Meso small amount of the dredged coarse material con zoic(?) rocks less commonly. A clast of flint sists of non-crystalline sedimentary rocks of prob (>2 mm) is found in the upper part of core 8; able Mesozoic age. As the inland-ice advanced other clasts consist mainly of crystalline rocks. over, and surely also eroded the Lista basin, this 12 H. Holtedahl NORSK GEOLOGISK TIDSSKRIFT 68 (1988) CORE 2 Depth 302m CORE 1 Deplh270m CORE 7 Depth 200m eoo..la.!Ne gran Sil:e Cumulative grain size ""' 1 2 3 4 5 1 2 3 4 5 o '*"'! 1 2 3 20 "" 3b 40 5 .. .. 1 "' ,, ...... 00 ::)C X X CORE 3 Deplh 309m 1 23 .. 5 ffilTxl 1.Fifnt ��- 2. Chalk .... Ciay Silt Sand Grave! Qrayef , Coa1 Stones 4. Carbonacews sedi'n. rocks Burrows 5. Other Mesozoic i!MI1/clayMolklscshels (7) rocks 1.Fiinl ..... 2.Chall< Grave! 3.Coal Stor.• 4.Csrbonleeaus sedimroeks Øurro•• 5. OtherMesozoic: i�"'Molusc'"' shels (?)rocks CORE10 Depth 300m COAE9 Depth 270m CORE8 Oepth 230m Cumulatlve grafl aize Cumulative graln slze Cumuletlve gran size t 2 3 4 5 1 2 3 4 5 X X X X X X X XXX 100 X KX :.._-1-:_ l.flil'll =-g= ...... =- -.._ Grave! .,_ -4. CarbO<'Iao;;eoul ..- .. _,_ Burrows ,.. 150 5. OttoerMesozoir;: =- k �acshels l""""' (?)rocts 200 12 :...: ..,. : �· Fig. 5a, b. Lithostratigraphy of sediment cores, content of assumed Mesozoic clasts and grains, and position of samples on bathymetric profiies. NORSK GEOLOGISK TIDSSKRIFT 68 (1988) Bedrock and Quaternary geology, Lista basin 13 small component of sedimentary rocks underlying nection between this glacier and an advancing the basin might seem surprising. A comparison Baltic ice, which would explain the occurrence of with till material dredged from the continental Danish and Baltic erratics. Later, the hypothesis shelf off Møre-Trøndelag (H . Holtedahl 1955), of a Skagerrak glacier covering the area of Jæren however, shows a similar domination of crys up to about 200 m above sea leve! has been criti talline rocks over large areas where the local cized (Andersen 1964; Feyling-Hanssen 1964). bedrock consists of Mesozoic and Tertiary sedi The 'till' is thought to be a glaciomarine sediment ments. The difference in fragility of these rocks deposited at the end of the Saalian glaciation or is, no doubt, an important factor in explaining during the Middle Weichselian (Andersen 1964 ; this discrepancy. The occurrence of coal, car Andersen et al. 1981). bonaceous silt- and sandstones, etc., in the Even if long distance transport by icebergs and dredged samples may therefore well indicate that other types of floating ice is important, the rela these rocks have their source in the Lista basin, tively great number of coal fragments and other only slightly displaced before deposition to their assumed Mesozoic material which was present in present situation by glaciers. the samples from the Lista basin, can scarcely On the other hand, the presence of rocks from have its origin in Skåne in S. Sweden or in the Oslo area in all dredge samples is a good Bornholm in the Baltic. It seems more likely that indication of debris transport by icebergs and it had a local origin and a short transport by both other types of floating ice. Flint and chalk clasts glaciers and floating ice. found in the same samples corroborate this. Thus The core samples collected are all typical of one cannot completely rule out the possibility that glaciomarine sedimentation (see also Van Weer the component of assumed Mesozoic rocks is ing 1982 and Holtedahl1986), and the coarseness derived from glaciers which have assimilated it may suggest a deposition fairly dose to an ice from other areas, and subsequently transported margin. Besides grains and clasts of crystalline it to the Lista basin. rocks, all contain a certain amount of coal frag The presence of rhombporphyry erratics and ments,as well as fragments of silt- and sandstones other characteristic rocks from the Oslo area with carbonaceous matter,limestones, etc., which below the marine limit along the Norwegian coast are similar to the larger clasts in the dredged and continental shelf, show a transport mech material, and assumed to be of Mesozoic age. , anism by icebergs and drifting ice to be very This strongly supports the supposition that the important (Rekstad 1926,O. Holtedahl1953 ,H. Lista basin consists of Mesozoic rocks, probably Holtedahl 1955). The transport and deposition of U. Triassic-L. Jurassic age. These rocks have took place in Late Weichselian time, when the been eroded by ice coming from the inland, and coastal areas were glacio-isostatically submerged redeposited directly from glaciers as till, and/or and in places icefree, while calving of glaciers glaciomarine sediments from floating ice after took place in the Oslo-Inner Skagerrak area. undergoing very limited transport. Flint and chalk are also found frequently as The presence of coal fragments in the 'Ska erratics along the Norwegian coast, even within gerrak moraine' described from Jæren (Horn & the fjords. Their origin must be sought in the Isachsen 1944; Horn 1931),previously thought to Skåne-Sjelland-Kattegatt area. have come from S. Sweden or Bornholm, is now, The 'clayey till' (Skagerrak moraine) (Ander according to the present data, assumed to have sen 1965), occurring abundantly at Jæren (Fig. its origin in the Lista basin. This is further sup 1), contains, besides shell remains, a number of ported by the similar type of coal found in the foreign erratics such as chalk, flint and Oslo rocks, Lista basin samples. It is classified as sub as well as rocks from Swedish and Baltic prov bituminous coal (between brown coal and bitu enance (0. Holtedahl 1953). Of special interest minous coal), and is shown to be coalified from is the tind of coal of brown coal rank, formed wood, probably driftwood (G. Khorasani pers. from wood (H orn & Isachsen 1944). The type of comm. 1987). coal is similar to Lias coals in England and in Bornholm in the Baltic. The 'clayey till' at Jæren was originally thought to be deposited by a pre Quaternary history Weichselian Skagerrak glacier coming from the End moraines of four distinct glacial substages East. Milthers (1911, 1913) suggested a con- are observed in the Lista and adjacent inland 14 H. Holtedahl NORSK GEOLOGISK TIDSSKRIFT 68 (1988) l ' " " " "· " . " " " " .,. ' �'.t.t "._z "';� ...,...... __ Glaclal atrlae ""-. sll6", �� �!Y, "� lee marginal " ·� 58'00N -:--;-••• �� deposits ., ...... -·- Faults . , .. ". . Fig. 6. Assumed position of the ice margin during the Lista- and Spangereid substages (mainly after Andersen 1960). Main pre Quatemary faults shown by broken lines and dots. areas (Andersen 1960 , 1979) . They have been Berglund (1979) the Lista moraine can be cor named, in chronological order,the Lista-, Spang related with the Halland Coastal moraine (Low ereid-,Kristiansand- and Ra substages. The Lista Baltic re-advance) in Sweden. moraine is observed along the south coast of The relative sea levei during the Lista substage Lista (Fig. 6) and, according to Andersen (1960), at Lista is uncertain. It cannot be higherthan 6- continues as a submarine ridge parallel to the 9 m above present sea leve!, which is believed to coast across the Lista basin to the Siregrunnen mark the marine limit in Holocene times; it is bank, and further northwards to the outer coast probably lower (Andersen 1960; Hafsten 1979) . of Jæren. Its continuation across the Boknfjord While the Lista end moraines are observed on further north is suggested by Klemsdal (1969) . land only in a small area on the outer coast of Southeast of Lista, marked submarine ridges, Lista, the Spangereid end moraines are present which parallel the coast and can be followed for in a larger area between the town of Mandal and about 40 km, are thought to be the continuation the Fedafjord (Fig. 6). Sea leve! at the time of of the moraine (Fig. 6) . formation of these moraines was supposedto have The age of the Lista substage is unknown,but been 5-11 m higher than today. The age of the as deposits from the base of a bog at Brøndmyr, Spangereid substage is not known exactly, but, Jæren, 10 km east of the Lista moraine were according to Andersen (1979) ,must be older than radiocarbon dated to about 13 ,150 yrs B.P. 12,550 yrs B.P., probably about 13 ,000 yrs B.P. (Chanda 1965), a tentative age of 13 ,500 yrs B.P. It can possibly be correlated with the Gøteborg was suggested by Andersen (1979). Accordingto moraine in Sweden (Berglund 1979). NORSK GEOLOGISK TIDSSKRIFT 68 (1988) Bedrock and Quaternary geology, Lista basin 15 A number of end mt>raTnes and ice-marginal to a great extent built on the existence of assumed terraces further inland are referred to the Kris morainal ridges outside the coast, these should tiansand substage. The dating of this event is be further investigated. uncertain, but is believed to be of Older Dryas, The retreat of the main ice in the Lista basin is possibly Bølling age. TheRa deposits of Younger supposed to have taken place at !east 12,5 00 yrs, Dryas age (10,000-10,800 yrs B.P.) are very possibly 13,000 yrs B.P. ago. This is shown by distinct features further north, about 35-40km datings of mollusc shells from core l, subsample from the Lista moraines. 12, and core 2, subsample 6 (Fig. 5a). As the As shown above, the assumption of a sub mollusc material was too little for an ordinary 14C marine continuation of the Lista moraine across dating, it was dated by the Tandem Accellerator the Lista basin does not agree with the present technique (G. Possnert, The Tandem Accel data. Possibly the moraine material which covers lerator Laboratory, Uppsala, Sweden). Sample the slope of the submarine promontory northwest 1-12, consisting of a shell of Yoldiella lenticula, of Lista was deposited by a glacier fillingthe Lista gave an age of 11,59 0 ± 200 yrs B.P. (corrected basin during the Lista substage. The main part of for apparent age 410 years (Mangerud 1972)). the promontory, however, is believed to consist of Sample 2.....(i, consisting of a shell of Nuculana pre-Quaternary, mainly Mesozoic, sedimentary pernula, gave an age of 12, 340± 16 0 yrs B.P. rocks. The wide threshold which forms the con ( corrected). tinuation of the promontory towards the north The core samples collected are all of glacio west is definitely a rocky feature, only slightly marine origin, and their partly coarse character covered by morainic and glaciomarine deposits. and content of coal fragments, as well as other As faults have been observed on both sides of the material thought to come from the bottom of the threshold, it may well be structurally determined. Lista basin, suggests a proximity to a retreating Further to the northwest it has not been possible ice frontlying within the basin. Thehomogeneous to verify the previous assumption of a submarine clayey parts of core l, 7 and 10, with traces of moraine covering the eastern slope and the outer burrowing organisms and Jack of dropstones, may part of the Siregrunnen bank. The bank is prob indicate a retreat with subsequent advance of the ably rocky, with a fairly flat surface, and covered ice margin. by glacial reworked material due to its shallow According to Andersen (196 0) end moraines position and exposure to waves and currents. and other ice-marginal deposits belonging to the Seismic data are heavily masked by multiple Spangereid substage are present in an area to the reftectorsand the thickness of the glacial deposits east of Fedafjord (Fig. 6). The continuation of cannot therefore be determined. these features westwards is unclear. A sparker A topographical feature of special note in the profile taken across a sill near the mouth of northwestern part of the Lista basin is the wide Fedafjord shows an ice-marginal deposit which NE-SW oriented ridge. As shown by the seismic probably marks the position of the ice front during data, it consists of glacial material, probably fair! y the Spangereid substage (Fig. 7). The distal lay well consolidated, with a cover of glaciomarine ered sediments are reckoned to have a thickness sediments. As the deposit below the glaciomarine of about 100 m. The deposition of this ice mar sediment could not be sampled, its origin is uncer ginal deposit must have taken place some time tain, but it may well be a till, whose present form after the deposition of the coarse glaciomarine may represent a remnant form of a previously sediment in core 2, dated to 12, 340± 16 0 yrs B.P. larger accumulation, possibly a drumlinoid form. A maximum date of 12,000 yrs B.P. is suggested, The age of the deposit is uncertain, but is in which is less than assumed by Andersen (1979 ). any case greater than the overlying glaciomarine A roundness analysis which was carried out on sediments. the coarse dredge material, as well as on the The Jack of distinct ice-marginal deposits coarse material in the cores, showed a high per between the Lista mainland and the Siregrunnen centage of well-rounded and rounded fragments, bank does not necesarily dispute the idea of a often chipped and showing signs of glacial wear. Lista substage. The ice front has most probably This suggests an advance of the glacier over pre had a pause in its retreat, or even an advance, viously rounded material, either beach material, when it lay at the outer parts of the mainland of or glaciofluvial products (H. Holtedahl 1955; Lista. But as the suggestion of a Lista substage is Bergersen & Garnes 1971; Bergersen 197 3; 16 H. Holtedahl NORSK GEOLOGISK TIDSSKRIFr 68 (1988) TWT Sec . . 3�------=� 0.5nm Fig. 7. Sparker profile across the ice marginal deposit at the mouth of Fedafjord (Spangereid substage ?). Olsen 1983). It is hard to imagine an ice advance registered above the pre-Quaternary surface out over beach material in the Lista basin, as the side the threshold (profile 35), cannot be basin is too deep for deposits to be worked by observed here. Fig. 9 shows the calculated thick waves, even at the maximum eustatic lowering of ness of Holocene and Late Weichselian sedi sea level. Rounding of material by waves can only ments, and Fig. 10 the assumed depth to the have happened in the shallow Siregrunnen area, acoustic basement (Mesozoic rocks mainly and but the bank is situated to the northwest of the till). A sub-Quaternary depression or channel is Lista basin, and can scarcely be the source. The present outside Hidra island and northwestwards other possibility is deposition of material which towards the mainland. The channel is no doubt a has been rounded in meltwater streams within or glacial erosional feature, structurally controlled. below the ice, dose to the ice margin, and later retransported by an advancing ice sheet. As discussed earlier, the sediment infill of the Geomorphology and tectonics Lista basin is thought to represent Late Weich A marginal fault line along the steep coastal slope selian and Holocene glaciomarine and marine from the Langesund Channel to Lista was sug sediments. Theyare easily penetrable acoustically gested by O. Holtedahl (194 0, 1964). From Lil and have distinct internal reflectors visible lesand and westwards this corresponds with the especially on the 3.5 KHz echosounder (ORE) northern slope of the Norwegian Channel. The registrations (Fig. 8). The three acoustic units, indentation of the coastline at Lista was thought NORSK GEOLOGISK TIDSSKRIFT 68 (1988) Bedrock and Quaternary geology, Lista basin 17 TWT sw NE .3+-���--�� Fig. 8. ORE (3.5 KHz echo·sounder) registration of profile 3 (cf. Figs. 2 and 3a). 58°16' N Sl RE GRUNNEN 14' ··· ...... o 10' ·· ·''",;;::·•············ 5km Fig. 9. Isopach map showing thickness in m of Holocene-Late Weichselian marine sediments in the Lista basin. 18 H. Holtedahl NORSK GEOLOGISK TIDSSKRIFT 68 (1988) s• 2o' E 40' 58°16'N 8' 5km 6' Fig. JO. lsopach map showing depth to acoustic basement (pre-Quaternary sediments and till). to be related to a break in the direction of the fault The fault along the southwestern slope of Hidra line, with a transverse fracture which continues island seems to continue northwestwards between inland along the linear Fedafjord. West of Lista the mainland and the Siregrunnen bank. Towards the marginal fault line is supposed to continue the southeast it probably continues along along the western slope of the Siregrunnen bank Eidsfjord and the Listeid isthmus (Fig. 6). The and northwestwards, while another longitudinal rocks on the southern valley side are strongly line of weakness is thought to run along the sound fractured and brecciated. The further continu separating the island of Hi dra from the mainland. ation of the fault across the Framvaren fjord Here a fault line is suggested by Barth (1939) towards Sande is hypothetical. (0. Holtedahl 1940). O. Holtedahl thought the It was previously concluded that the age of the marginal fault lines to be of Cenozoic age and Mesozoic sedimentary rocks in the Lista basin was related to the uplift of the Norwegian landmass. U. Trias-L. Jurassic. These fault lines therefore The present data from the Lista basin strongly developed after the L. Jurassic so it seems natural support the previous suggestion of marginal fault to consider the fault tectonism in the Lista basin lines at the foot of the coastal slope. The fault at in relation to other evidence of volcano-tectonic the outer lower slope of the threshold west of activity in the Skagerrak-Oslo region (Suleng Lista is situated on the northeastern slope of a 1919; Noe-Nygaard 1967; Storetvedt 1966, 1968; depression which continues southeastwards and Halvorsen 1972; Aam 1973; B. T. Larsen 1975; eastwards on the northern flankof the Norwegian Ziegler 1975; Færseth et al. 1976; Ramberg 1976; Channel (Fig. 4). Dons 1977; Ramberg & Spjeldnæs 1978). NORSK GEOLOGISK TIDSSKRIFf 68 (1988) Bedrock and Quaternary geology, Lista basin 19 The uplift of the western part of the Fenno mentary rocks. These are outcropping, or nearly scandian Shield has been associated with the outcropping, on the outer threshold of the basin, opening of the Norwegian Sea. The extent of and on the slope leading down into the Norwegian uplift can, to a certain degree, be estimated by Channel. the altitude of a suggested Tertiary peneplain, Inside the threshold,the sedimentary rocks are which is largely believed to coincide with a Sub covered with an infill of marine and glaciomarine Cambrian peneplain (0. Holtedahl195 3). Ander sediments with a maximum thickness of about sen (196 0) has attempted to reconstruct the sur 100m. A Triassic-Jurassic age is suggested for face of this peneplain in an area adjacent to the the sedimentary rocks. south coast of Norway between Åna-Sira and Dredge- and core-samples contain material Arendal, including the Lista basin area. In con which has been transported by glaciers, and trast to the coast along W. Norway, the surface deposited either directly by ice, or by icebergs of the southern coast shows a moderate rise in and floating ice. The relatively large content of altitude from present sea level and inland. The non-crystalline erratics, inter alia, coal, car presence of Triassic-Jurassic sedimentary rocks bonaceous silt- and sandstones, with plant fossils overlying a Precambrian surface in the Lista of Upper Triassic-Lower Jurassic age, supports basin, presumably with a fault boundary towards the assumption of in situ sedimentary rocks of this the northeast, but assumably not by fault bound age in the basin and outside. aries towards the NW and SE, indicates depo The boundary between the sedimentary rocks sition on a sub-Triassic/Jurassic surface of Pre and the Precambrian area to the northeast is cambrian rocks. No doubt these sediments once marked by a major fault outside the island of covered a fairly extensive area on present land, Hidra, and other faults are present inside and but have since been removed by denudation. outside the threshold more or less parallel to the Considering the sub-Triassic/Jurassic Precam coast. Faulting has not been observed towards brian surface as one which was strongly effected the Precambrian areas to the northwest and by chemical (and physical) weathering, remnants southeast. of this weathered surface might be looked for The submarine extension of the Lista moraine in neighbouring areas. It is of interest in this towards the Siregrunnen bank does not exist as a connection to mention the kaolinite occurrence topographic feature. The sediment cores contain at Dydland between the Jøssingfjord and the marine and glaciomarine deposits of Holocene Ånafjord (Reusch 1901,19 03), and that Barth and Late Weichselian age. The glaciomarine sedi (1 939) assigned the typical morphological features ments have been deposited not far from the ice of the Vest-Agder fjordland, including the Lista margin, but oscillations of the ice front may have area, to chemical weathering under a tropical occurred. The relatively high percentage of climate. This view was supported by the finds of rounded clasts, many of which are chipped and beidelitic clays in altered rocks. Barth believed show glacial scouring, may support this view. that the weathering had taken place in Tertiary The retreat of the main ice in the basin must time on a slightly elevated peneplain, and that have taken place before 12,500 yrs B.P., possibly later ice and water washed away the major part 13,000 yrs B.P. This gives a minimum age of the of the weathering profile. From the present data Lista substage. The age of an ice-marginal deposit · it is now suggested that the chemical weathering at the mouth of Fedafjord, possibly representing took place prior to the sedimentation of the Tri the Spangereid substage,may be about 12,000yrs assic-Jurassic deposits, and that the present land B.P. forms to a certain extent owe their character to the exhumation of this weathered surface. Acknowledgements. - This project was carried out with the use of the University of Bergen's research vessel 'Håkon Mosby'. I thank the officers and crew for their willing assistance in the various operations. I am most grateful to T. Elholm for technical help in connection with the acoustic instruments and sampling; Conclusion E. King for his preliminary core descriptions, work with the grain size analyses, and for correcting the English language, and E. Irgens for preparing the illustrations. I also thank I. Continuous shallow seismic and acoustical reflec Aarseth for supplying me with additional air-gun and sparker tion profilesfrom the basin between Lista and the records. Fossil determinations were kindly carried out by Pro Siregrunnen bank reveal the existence of sedi- fessor B. Lundblad, Stockholm, and J. O. Vigran, I.K.U. 20 H. Holtedahl NORSK GEOLOGISK TIDSSKRIFT 68 (1988) Trondheim. Professor M. 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