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The jæren area, a border zone of the norwegian channel ice stream
Quaternary Science Reviews, Vol. 17, pp. 801—812, 1998 ( 1998 Elsevier Science Ltd. All rights reserved. Printed in Great Britain PII: S0277–3791(98)00019-5 0277—3791/98/$—See front matter
THE JF FREN AREA, A BORDER ZONE OF THE NORWEGIAN CHANNEL ICE STREAM H.P. SEJRUP*, J.Y. LANDVIK-, E. LARSEN‡, J. JANOCKO°, J. EIRIKSSONDD AND E. KING* *Department of Geology, Univ. of Bergen, Allegt. 41, N-5007 Bergen, Norway (E-mail: [email protected]) -The Univeristy Courses on Svalbard (UNIS), P.O. Box 156, N-9170 Longyearbyen, Norway ‡Geological Survey of Norway, P.O. Box 3006, Lade, N-7002, Trondheim, Norway °Department of Geology, University of Troms, N-9037, Troms, Norway DDScience Institute, University of Iceland, Jardfrædahus Haskolans, IS-101 Reykjavik, Iceland Abstract—The coastal lowlands of Jæren, southwestern Norway, have an extensive cover of Quaternary sediments. Lithological, biostratigraphical and geochronological investigations of a series of boreholes across Jæren have allowed us to combine earlier and new information on these deposits into a temporal model of Quaternary sedimentation. The cores show evidence of at least four ice free episodes with marine deposition, during mainly arctic conditions. The oldest of these is considered to be ca. 200 ka old and the youngest one is the Sandnes interstadial which is 14C AMS dated to 32 ka BP. Interbedded between these units are 10—40 m thick units of till with components of marine material. These findings, together with a reevaluation of distinct morphological features on Jæren, as well as earlier provenance studies, strongly suggest that Jæren has been a boundary zone between an ice stream following the Norwegian Channel draining a substantial part of the southern region of the Fennoscandian ice sheet, and local ice from south-western Norway. Through multiple periods of glaciation starting ca. 1.1 myr (million years) ago, the Norwegian Channel ice stream has been the main conduit of erosion products from southern Scandinavia to the deep Norwegian Sea. Comparison with ice stream profiles from Antarctica suggests that the deposition of relatively young marine sediments as high as ca. 200 m a.s.l. on Jæren is a result of glacial isostasy, rather than regional tectonic movements. ( 1998 Elsevier Science Ltd. All rights reserved.
Lotti, 1995; Dansgaard et al., 1993; Johnsen et al., 1992). It is therefore important to study former ice streams bordering the North Atlantic, both in terms of scale and chronology, to obtain concrete data on how their development can be linked with the climate of the ice ages. The existence of large ice streams along the Norwegian Channel has repercussions in terms of our interpretation of a wide variety of data concerning the Quaternary environmental history of the region. Several studies during the last decade have emphasized interpretation of the glacial history of Scandinavia from the record of ice rafted detritus in the Norwegian Sea (Baumann et al., 1995; Jansen and Sj+holm, 1991). It is evident that both the lithology and also volume of IRD will be greatly influenced by the existence of fast flowing ice streams. It is also reasonable to assume that major parts of Norway could have been glaciated with ice reaching the continental shelf, without having an ice stream in the Channel. Several reconstructions of glaciation history have been presented from western Norway during the last two decades (Larsen and Sejrup, 1990; Larsen et al., 1992; Mangerud, 1991). Most of this information is based on on/off records from either cave deposits or raised marine sequences in coastal areas.
INTRODUCTION During the last few years there have been considerable research efforts directed towards glacial fed trough mouth fans bordering the Norwegian Sea (Eidvin et al., 1993; King et al., 1996; Laberg and Vorren 1995, 1996; Sejrup et al., 1996; Vogt et al., 1993). There seems to be a general agreement that these features are a result of fast moving ice streams depositing material as debris flows on the continental slope. One of the larger of these features is the North Sea Fan (Fig. 1) which is inferred to have been deposited primarily by an ice stream following the Norwegian Channel (King et al., 1996). It has been suggested, primarily from studies of cores in the North Atlantic that concentrations of IRD (the so called Heinrich layers) are results of fast moving ice streams/disintegration of ice streams along the coast of North America (Alley and MacAyeal, 1994; Andrews et al., 1993, 1994, 1995; Bond et al., 1993; Bond and Lotti, 1995). Both from the deep sea cores and from the Greenland ice cores it is evident that the Heinrich layers are associated with dramatic changes in the thermohaline circulation and atmospheric temperatures (Alley et al., 1995; Bond et al., 1993; Bond and 801
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FIG. 1. Map of Western Norway, the Norwegian Channel and Jæren (inset with darker shading 200 m a.s.l.). Location of map in Fig. 2 and profiles in Figs 7 and 8 are shown.
Ice streams probably did not develop in all the periods when western Norway was ice covered, so these curves do not neccesarily show ice stream history. In modelling the Fennoscandian ice sheet it is important to take into account the effect of draining by the Norwegian Channel Ice Stream. In previous reconstructions this has not been sufficiently emphasized (Nesje and Sejrup 1988; Vorren and Roaldset, 1977). The effect of extensive iceberg production, probably resulting from decay of the Norwegian Channel Ice Stream, has been traced by the occurrence of Cretaceous nannofossils from the North Sea basin in cores from the North Atlantic (Rahman, 1995). Thus the history of ice streams such as the Norwegian Channel Ice Stream is also of importance for understanding abrupt changes in thermohaline circulation possibly triggered by catastrophic transport of fresh water/icebergs from the continents. BACKGROUND The Jæren area is an anomalous part of southwestern Norway with an extensive cover of Quaternary
sediments and a relatively flat undulating landscape, stretching from Stavanger in the north to Brusand in the south. The boundary with the inland areas, which are almost void of surficial deposits, is very pronounced and extends from sea-level east of Stavanger to more than 250 m a.s.l. in the Storamos area and back to sea-level in the Brusand area (Fig. 1). Based on observations of a clayey till containing marine fossils and erratics from the Oslo area, southern Sweden, and Denmark, Helland (1885) proposed that Jæren had been partly overridden by a glacier following the Norwegian Channel around the southern part of Norway, the so-called ‘Skagerrakbreen’ (breen — Norwegian for glacier). The Norwegian Channel is a ca. 800 km long depression following the coast of Norway from the Oslofjord area to the shelf break off Stad (Fig. 1). The deepest part is found in Skagerrak with depths close to 700 m. From a threshold at ca. 270 m off Jæren, the Norwegian Channel slopes gently northwards to ca. 400 m on the margin to the Norwegian Sea. Previous studies of the Quaternary sediments on Jæren, have indicated that in addition to the till units,
H.P. Sejrup et al.: The Jæren Area, a Border Zone of the Norwegian Channel Ice Stream
numerous successions of sub-till marine beds are present (Andersen, 1964; Andersen et al., 1981, 1987; Bj+rlykke, 1908; Feyling-Hanssen, 1971, 1974; Grimnes, 1910). The chronology of these marine units is not well constrained, but age estimates ranging from Late Weichselian to the last interglacial have been suggested (Andersen et al., 1987). Sediments with marine fossils as high as 200 m a.s.l. have been noted since the turn of the century and have commonly been assumed to be tills deposited by the Skagerrak glacier (Bj+rlykke, 1908; Helland, 1885). More recently, a marine origin for some of these units has been advocated (Andersen et al., 1987; Isachsen, 1943). However, deposition of sediments at such an elevation requires substantial glacio-isostatic depression. Such a model has been questioned since the postglacial marine limit on Jæren is generally only between 20 and 10 m a.s.l. (Fægri, 1940). An alternative explanation for these high level marine deposits has been recent movement along a possible N-S fault line along Jæren. The conclusion of early workers in terms of glacial history was that during an earlier glaciation (Saalian) Jæren was inundated by a Skagerrak glacier depositing tills with far-transported clasts and marine fossils. During the last glaciation, only a relatively thin ice sheet from central south Norway, with its margin close to the present shoreline was thought to have covered the Jæren area (Isachsen, 1943). Recent work carried out in the Norwegian Channel off western Norway and on the North Sea Fan (Fig. 1) has produced strong evidence that the Channel has been occupied on several occasions during the last 1.1 myr (million years) by a fast flowing ice stream (King et al., 1996; Sejrup et al., 1995, 1996). Numerous dates from the Norwegian Channel and the North Sea Fan (Lehman et al., 1991; Sejrup et al., 1994) show that the last deglaciation of the Channel occurred very rapidly close to 15 ka. Basal dates from the small Island of Utsira (Fig. 1) suggest deglaciation prior to 14 ka (Paus, 1990). The oldest radiocarbon dates of the deglaciation from the Bergen area north of Jæren (Fig. 1) are close to 12.7 ka (Mangerud, 1977). Few basal dates are available from the Jæren region, however, deglaciation close to 13 ka for the coastal areas has been suggested (Andersen, 1960). This paper reviews findings from a coring programme carried out on Jæren between 1993 and 1995. In addition, geomorphological and orientation/provenance indicators on Jæren are described and discussed in the light of the possible influence of an ice stream. FIELD EVIDENCE Morphological features The Jæren area is generally divided into two main areas; the La> gjæren (Low Jæren) and H+gjæren (High Jæren). La> gjæren is the lowland along the coast with elevations from the present coastline to ca. 70 m a.s.l., wheras H+gjæren is an inland/upland area with rela-
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tively flat landscapes between 180 and 250 m a.ª s.l. It has been suggested by various authors (FeylingHanssen, 1966) that the boundary between these two areas follows a north south trending fault line from Gannsfjorden to Brusand (Fig. 1). The present study focusses on the southern part of Jæren between Brusand and Nærb+ (Fig. 2) where these major geomorphological elements are particularly well expressed. New information on the Quaternary successions of this area have been obtained from the coring program. The distribution of surficial sediments, together with glacial erosional and depositional features of this area, have been published on three maps on the Quaternary geology of the region (Andersen et al., 1987; Grimnes, 1910; Wangen and Lien, 1990). Based on information from these maps, and our own fieldwork in the area, we have compiled maps which summarize our interpretation of some of the glacial features of the area (Figs. 2 and 3). The southern part of Jæren can be divided into three major morphological provinces (Fig. 2); 1. ¸owland areas with N¼—SE trending ridges. In the low-lying region between Brusand and Nærb+ there are several NNW trending ridges measuring from a few metres to '10 m high, '100 m to several hundred meters wide and 1—3 km long (Fig. 3). The ridges are locally dissected by glacial (E—W) erosion and also by more recent fluvial incision. The set of ridges closest to the coast have been interpreted as marginal moraines (Andersen et al., 1987; Klemsdal 1969), corresponding to the so-called Lista Stage, deposited by ice moving from the east. Based on their overall morphology we favour the conclusion reached by Hansen (1913) that these forms represent drumlins formed by ice flowing along the Norwegian Channel. Superimposed on these is a weaker imprint of later east—west flowing ice. 2. ¸owland areas with strong east—west features. North of area 1, close to Nærb+ (Fig. 3) there is a lowering in the landscape and remnants of the NW trending ridges are only found near the coast. This area has a more hummocky appearance with several shallow lakes and large areas mantled with glacifluvial deposits. This area bears the strong imprint of a young ice lobe from the east depositing several sets of discontinuous recessional moraines during the deglaciation phase. 3. ºpland areas. The upland areas are characterized by a very flat landscape in the west, becoming more hummocky with coarse-grained glacifluvial deposits and dead-ice topography in the east. Bordering the western margin of this landscape are thick sub-till glacifluvial deposits in contact with the most easterly NNW-trending ridges. Large parts of the upland are covered by a till with fine-grained matrix (Andersen et al., 1987) overlying in situ marine sediments (see below). The boundary between the upland and the lowland areas containing N—S ridges forms an escarpment between 100 and 150 m a.s.l. A set of very open wide depressions (valleys) occurs on the western margin of the plateau (Fig. 3) facing the escarpment. They are a few km long, several hundred meters wide
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FIG. 2. Map of the southern part of Jæren (Brusand Nærb+) with the location of the geomorhological provinces; area 1 to 3. Drill sites and location of the seismic profile in Fig. 5 are shown.
and ca. 50 m deep. These valleys have characteristics typical of glacially eroded valleys with very broad Ushaped profiles. The base level of the valleys lies between 140 and 160 m and they end abrubtly, with no signs of accumulation of erosion products, close to the eastern limit of the ridges. We interpret these depressions as hanging glacial valleys, eroded by glaciers feeding into the Norwegian Channel Ice Stream. Striations From the Jæren area there is a relatively large set of striation data indicating a southwesterly ice movement
in the north and more westerly movement in the south (Andersen et al., 1987; Holtedahl, 1953). Due to the paucity of bedrock exposures there is only a single observation of striation (east-west) in area 3 (Fig. 2). From the two lowland areas Andersen et al. (1987). reported two observations of striations from each area with westerly to southwesterly directions. Just south of Brusand (in the region with almost no Quaternary sediment cover; Fig. 1), we have confirmed earlier observations of very strong southwesterly striae. However, we have also found evidence of NNW ice flow in the form of a set of older striations in this area. In addition, a number of sites with NNW striations
H.P. Sejrup et al.: The Jæren Area, a Border Zone of the Norwegian Channel Ice Stream
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FIG. 3. Some characteristic morphological features on southern Jæren visualized from topographic map data: (a) Drumlin morphology on Low Jæren. The drumlins trend is sub-parallell to the coastline. Some have been dissected by fluvial erosion. View from NW. (b) Glacial morphology of the Nærb+ area (Fig. 2). The Lake S+ylandsvatnet lies to the SE of a curvilinear hill interpreted as a recessional moraine formed by a W-trending ice lobe. Strings of mounds parallel to the S+ylandsvatnet hills are seen both outside and inside reflecting steps in the recession history. (c) Two marked incisions into the High Jæren plateau are seen near Elgane (Fig. 2). These forms are interpreted as EW-trending hanging valleys cutting down to a level of 140—160 m a.s.l., thought to be controlled by a NW running ice stream to the west. View from the west.
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rhombporphyrs, Larvikite and sandstones (from the Oslofjord area), chalk and chert (from Denmark), and As land quartzporphyrs, Dalaporphyrs and "stersj+porphyrs (Balthic derivation) from Sweden and As land, provided evidence to suggest that at some stage Jæren had been covered by a large glacier following the Norwegian Channel (Bj+rlykke, 1908; Helland, 1885; Milthers, 1913). However, it was believed that this situation prevailed during a former glaciation (socalled second glaciation) and that the east—west signatures were a result of the last (third glaciation) (Bj+rlykke, 1908). A similar view has also been advocated more recently by Andersen et al. (1987) who kept the possibility open that during the Saalian Jæren could have been overridden by a Skagerrak glacier and that the east-west striations could be the result of the Late Weichselian glaciation. Figure 4 shows some of the provenance data presented by Andersen et al. (1987). Despite doubts regarding the chronology and genesis of some of the investigated units, these data demonstrate two main directions of ice movement. Boreholes
FIG. 4. Content anorthosite and phylite in surficial Quaternary deposits on Jæren. For anothosite, just the the precence in quaternary deposits outside the bedrock province of anorthosite, is indicated. For phylite, percent countour line is give. Based on data from Andersen et al., 1987.
have been reported from the small Island of Utsira (Fig. 1) (Holtedahl, 1953; Unda> s, 1948). Thus to the south and north of Jæren there are glacial striae supporting an ice flow parallel with the coast in the Norwegian Channel. It is also clear that all the youngest striations reflect east—west ice flowing from central Norway across the coastline on Jæren. Provenance There is a long history of clast lithology studies of the surficial deposits on Jæren. Observations of coal fragments in the Quaternary deposits triggered an extensive coring program on Jæren between 1870 and 75 (Bj+rlykke, 1908). Sparse information on the lithology of the drilled units exists today, and no in situ coal has been found below the unconsolidated deposits. Frequent occurrence of far-travelled clasts such as
Four boreholes were drilled along a transect extending from the coastline to about 200 m a..s.l. in the Varhaug area between 1993 and 1995 (Fig. 2). Due to the coarse character of some of the units the recovery was generally low. Detailed accounts of the depositional environments and the geochronology of the cores are presented elsewhere (Janocko et al., 1997). In Fig. 5 the main lithological units and AMS-dates, TL-dates and some amino acid data are presented. The chronological framework for the marine units is illustrated by the degree of epimerization of isoleucine in the benthic foraminifer Elphidium excavatum in Fig 6. The following marine units have been recorded in the corings: The Gr+deland Sand is a 40 m thick marine unit mostly deposited under arctic conditions but it includes an interval with more ameliorated conditions. This part represents an interglacial preceeding the Eemian, most likely isotope stage 7. The Auestad Interstadial represents a marine arctic depositional event within isotope stage 6 and the Sunde Interstadial is a similar event possibly deposited during early parts of the Weichselian. The term ‘interstadial’ is here used simply to refer to ice-free episodes within a cold stage. In the coring at Elgane sediments representing the Sandnes Interstadial, radiocarbon dated to 32 ka, have been encountered 200 m a.s.l. Sediments from this depositional event have been recorded earlier in Jæren (Andersen et al., 1987; Feyling-Hanssen, 1971, 1976). The marine units are found beneath and interbedded with 10—40 m thick, often boulderly, diamictons commonly with a matrix of relatively fine-grained character sometimes containing marine fossils. Coal fragments in the sand fraction were found in many of the units recorded in the drillings. The provenance of this coal could be the Lista basin to the south
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FIG. 5. Simplified stratigraphies of the 93—94 corings (Janocko et al., 1997) carried out in the Varhaug area (Fig. 2). The cores are projected on the seismic profile of Andersen et al., 1987. Radiocarbon dates, TL-dates and aIle/Ile ratios in Elphidium excavatum are also shown.
FIG. 6. Chronology of the marine units recorded in the drillings on Jæren. The degree of epimerization of isoleucine (given as aIle/Ile ratios) in the benthic foraminiferal species Elphidium excavatum is shown with error bars. As the ratios fall within the range for which the epimerization reaction is following linear kinetics (Miller and Brigham-Grette, 1989; Sejrup and Haugen, 1992), we have chosen to use a linear extrapolation approach based on calibration points of known age. The late glacial and Sandnes Interstadial sediments are radiocarbondated and Eemian ratios are from the B+ site and other last interglacial sites recognized in the region (Miller et al., 1983; Sejrup et al., 1995). These provide calibration for linear extrapolation which suggest a stage 7 age (200 kyr) of the Gr+deland Interglacial and a stage 6 age of the Auestad Interstadial. The chronology of these units will be discussed more in detail elsewhere.
(Holtedahl 1988), although local sources cannot be excluded. The sequences recorded in the boreholes bear similarities with the ice stream deposits encountered along
the Norwegian Channel from Jæren to the North Sea Fan. The Quaternary sediments of the Norwegian Channel comprise sequences (Sejrup et al., 1996) which (when they are fully developed) are composed of:
FIG. 7. Cross sections (seismic interpretations) of the Norwegian Channel (for location see Fig. 1). Numerous generations of ‘U’-shaped or half-‘U’-shaped erosion scarps from Channel-parallel ice stream flow are indicated (GES). Marine/glaciomarine deposits constitute a small portion of the deposits (parallel stratification). The thick Quaternary erosional remnants in profile A (redrawn from Haugwitz and Wong, 1993) are a larger scale analogue to Jæren deposits. Jæren (land) profile same as in Fig. 5.
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H.P. Sejrup et al.: The Jæren Area, a Border Zone of the Norwegian Channel Ice Stream
(1) a basal erosional unconformity with extensive glacial sheet or channelized erosion elements termed glacial erosion surfaces (GES), followed by (2) one or more, thick (commonly several to 40 m) till units separated by GESs, and (3) glaciomarine/marine sediments conformably overlying the tills. The till units are clayrich diamictons commonly with more than 20% fine sand and coarser sediments, including Skagerrak-derived chalk, a mixture of reworked fossils, and medium to high shear strength (Sejrup et al., 1995). The marine units have a reduced coarse component, higher water content and in situ fossil assemblages. The clast content, the general lithology and thickness of the tills, and their inter-layering with marine sediments, are similar to the major units on Jæren. This supports the hypothesis that the thick till units on Jæren are a result of erosion and deposition by a Norwegian Channel Ice Stream.
DISCUSSION The Norwegian Channel Ice Stream and the Jæ ren area During the last decade there has been increased interest in the nature and role of large ice streams draining continental ice sheets. Bentley (1987) recommended the definition of an ice stream presented by Swithinbank (1954) as ‘‘. . . part of an inland ice sheet in which the ice flows more rapidly than, and not necessarily in the same direction as, the surrounding ice’’. A large number of studies have been performed on ice streams which are presently only found in the Antarctic (Alley et al., 1989; Bentley, 1987). The role of ice streams in former glaciated
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regions in terms of their location, history, dynamics and effects on continent-ocean sediment flux, is not yet well understood. As mentioned above, an ice stream following the coast around southern Norway was suggested more than 100 years ago (Helland, 1885) based on evidence from the Jæren area. From work recently carried out in the Norwegian Channel and on the glacially fed North Sea Fan at the channel mouth (King et al., 1996; Sejrup et al., 1995, 1996) the existence of repeated occurrences of an ice stream in the channel has been demonstrated. Elements of erosion and deposition from these ice streams, including their relationship to the Jæren deposits, are illustrated in cross sections of the Norwegian Channel and Skagerrak (Fig. 7). Numerous depositional and erosional phases are present, only a small portion of which relates to ice-free (marine) periods. Jæren deposits are thin and perched on the steeper crystalline basement. A possible analogue in terms of lateral ice stream setting and eroded margin is the thick pre—late Weichsel sediment package in Profile A. Based on the north—south trending drumlins, hanging valleys, striations, deformation structures, provenance data, the stratigraphies described above, together with the proximity and similarity with the Norwegian Channel setting, we suggest that Jæren has been repeatedly subjected to erosion and deposition by a high velocity Norwegian Channel Ice stream. The escarpment bordering H+gjæren marks the easternmost boundary of the ice stream, east of which only east—west trending orientation features occur. Based mostly on the data from the Norwegian Channel we assume that the ice stream has been active during
FIG. 8. Profile along the Norwegian Channel ice stream showing the thickness of the Quaternary sediments and our conception of the minimum ice stream profile. This low profile is sufficient for a glacial isostatic depression of ca. 250 m in the Jæren area (see text). For comparison a profile along ice stream B in Antarctica (Bentley, 1987) is shown (note scale differences). Quaternary thicknesses in Skagerrak are from Haugwitz and Wong, 1993 and Sellevold and Sundvor, 1974.
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several major glaciations including the last one. The overprinting of east—west orientation features such as till fabric and striations within the area dominated by the north-south ice stream, show that during the last deglaciation the volume of the Norwegian Channel ice stream decreased, or it disappeared entirely by 15 ka, allowing the boundary between the east—west ice to shift westward beyond the present shoreline. Thus the last glacial imprint is that of the east—west ice, which is reflected in some areas (area 2) more than in others (area 1).
Ice stream and sea-level on Jæ ren Figure 8 shows a longitudinal profile along the Norwegian Channel as far as the Skagerrak, including the thickness of Quaternary sediments. For comparison, profiles along an Antarctic ice stream is also indicated. It should be mentioned that the flux through the Norwegian Channel ice stream must have increased considerably northwards along the coast of western Norway due to contributions from confluent tributary glaciers in south-western Norway.
FIG. 9. Maximum glaciation scenario with an ice stream out of the Norwegian Channel. Relative length of arrows depicts ice velocity. Because of the lack of glacial fed debris flows on the continental slope west of the North Sea fan, a relatively slow moving ice is envisaged for this area. We propose a low saddle between Stavanger and Scotland with mainly slow northwards draining ice north of the saddle and southwards moving ice south of the saddle. During peak glaciation the mountains in Norway act as an obstacle and the major draining of the ice from central Fennoscandia is forced around southern Norway. Partly based on Boulton et al., 1985.
H.P. Sejrup et al.: The Jæren Area, a Border Zone of the Norwegian Channel Ice Stream
The new boreholes on Jæren confirm the existence of in situ marine interstadial sediment (Sandnes Interstadial) at more than 200 m a.s.l. The dating of these sediments to ca. 32 ka reduces the possibility that their present elevation results from regional tectonic movements, as sites of last interglacial age in western Norway suggest a sea level less than 40 m above present (Mangerud et al., 1981; Sejrup, 1987). The existence of an ice stream following the Norwegian Channel would imply an ice load of at least 700 m, as a conservative estimate, in the region. Removal of this ice load, deposition of the interstadial sediments and subsequent glacial isostatic rebound of 200 m in the area can explain their setting. This scenario contrasts with the last deglaciation where sea-level was much lower, so it is important to explain why the deglaciation preceeding the Sandnes interstadial should show this effect. The Late Weichselian was, after all, most probably a period of great Weichselian NW European ice sheet build-up and consequent crustal depression. The reasons for this radically different ice behaviour might include different deglaciation mechanisms between interstadial, with arctic climate and relatively low eustatic sea level, on the one hand, and interglacial, such as the Holocene, with boreal climate and higher eustatic rise in sea level, on the other. For the last deglaciation there is evidence from the North Sea Fan, Norwegian Channel and Denmark that the last ice stream broke up fairly rapidly close to 15 ka (King et al., 1996; Lagerlund and Houmark-Nielsen, 1993; Sejrup et al., 1995). Prior to this the flux of the ice stream possibly decreased and allowed relatively thin westerly-directed ice flow from central Norway to move beyond the coast of Jæren. The oldest basal dates for the deglaciation on Jæren are close to 13 ka (Andersen et al., 1987). This suggests that the crust during the last deglaciation had at least two kyr, probably more, to rebound following removal of the greater ice stream load, before the ocean made an imprint on Jæren. This was apparently enough to allow sufficient isostatic rebound for a marine limit of only 10—20 m. During the deglaciation prior to the Sandnes interstadial, differences in oceanic and atmospheric conditions might have created a contrasting scenario in terms of the relative magnitude of the flux of glacial ice to the Norwegian Channel from southern Norway and from central parts of the Fennoscandian ice sheet. Furthermore, the portion of the ice stream north of Jæren would be more sensitive during deglaciation to factors such as the direction and gradient of slope at its base and to sea level, in addition to atmospherically driven differences. This would indicate some independence between the ice stream and the easterly directed ice across Jæren. Relative timing of deglacial events, together with a lower eustatic sea-level under interstadial conditions could therefore have created a situation whereby the ocean inundated Jæren more rapidly after deglaciation than experienced during the latest retreat.
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CONCLUSIONS From our compilation of new and earlier data on the Quaternary of Jæren we conclude that 1. The Jæren area was inundated by a Norwegian Channel ice stream on several occasions during the last 1.1 m yr. 2. The deposits on Jæren contribute a stratigraphic record of these events going back more than 200 ka. 3. The boundary between the ice stream and westward draining ice was along the escarpment defining the boundary between H+gjæren and La> gjæren. 4. The imprint of the last ice stream phase is expressed in the morphology as N—S trending drumlins which have survived the later east—west flowing ice. 5. Marine interstadial sediments found over 200 m a.s.l. are a result of glacial isostatic depression following an ice stream situation before 32 ka. 6. The Jæren area is one of the few areas where both morphological and stratigraphical records of ice stream deposition and erosion can be studied on land.
REFERENCES Alley, R.B., Blankenship, D.D., Rooney, S.T. and Bentley, C.R. (1989) Sedimentation beneath ice shelves - the view from ice stream B. Marine Geology, 85, 101—120. Alley, R.B., Gow, A.J., Johnsen, S.J., Kipfstuhl, J., Meese, D.A. and Thorsteinsson, T. (1995) Comparison of deep ice cores. Nature, 373, 393—394. Andersen, B.G. (1960) S+rlandet i Sen- og Postglasial tid. Norges Geologiske ºnders~kelse, 210, 144pp. Andersen, B.G. (1964) Har Jæren vært dekket av en Skagerakbre? Er Skagerakmorenen marin leire? Norges Geologiske ºnders~kelse, 228, 5—11. Andersen, B.G., Nydal, R., Wangen, O.P. and "stmo, S.R. (1981) Weichselian before 15,000 years BP at Jæren—Karm+y in south western Norway. Boreas, 10, 297—314. Andersen, B.G., Wangen, O.P. and "stmo, S.R. (1987) Quaternary geology of Jæren and adjacent areas, southwestern Norway. Norges Geologiske ºnders~kelse, Bulletin, 411, 535pp. Andrews, J.T., Erlenkeuser, H., Tedesco, K., Aksu, A.E. and Jull, A.J.T. (1994) Late Quaternary (Stage 2 and 3) Meltwater and Heinrich Events, Northwest Labrador Sea. Quaternary Research, 41, 26—34. Andrews, J.T., Jennings, A.E., Kerwin, M., Kirby, M., Manley, W., Miller, G.H., Bond, G. and MacLean, B. (1995) A Heinrich-like event, H-0 (DC-0): Source(s) for detrital carbonate in the North Atlantic during the Younger Dryas chronozone. Paleoceanography, 10(5), 943—952. Andrews, J.T., Tedesco, K. and Jennings, A.E. (1993) Heinrich events: Chronology and processes, east-central Laurentide ice sheet and New Labrador Sea. NA¹O ASI Series, Ice in the Climate System, I 12, 167—186. Baumann, K.-H., Lackschewitz, K.S., Mangerud, J., Spielhagen, R.F., Wolf-Welling, T.C.W., Henrich, R. and Heidemarie, K. (1995) Reflection of Scandinavian ice sheet fluctuations in Norwegian Sea Sediments during the last 150,000 years. Quaternary Research, 43, 185—197. Bentley, C.R. (1987) Antarctic ice streams: a review. Journal of Geophysical Research, 92(B9), 8843—8858. Bj+rlykke, K.O. (1908) Jæderens Geologi. Norges Geologiske ºnders~kelse, 65, 269. Bond, G., Broecker, W., Johnsen, S., McManus, J., Labeyrie, L., Jouzel, J. and Bonani, G. (1993) Correlations between climate records from North Atlantic sediments and Greenland ice. Nature, 365, 143—147.
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Bond, G.C. and Lotti, R. (1995) Iceberg discharges into the North Atlantic on Millennial Time scales during the last glaciation. Science, 267, 1005—1010. Boulton, G.S., Smith, G.D., Jones, A.S. and Newsome, J. (1985) Glacial Geology and glaciology of the last mid-latitude ice sheet. Journal of the Geological Society, ¸ondon, 142, 447—474. Dansgaard, W., Johnsen, S.J., Clausen, H.B., Dahl-Jensen, D., Gundestrup, N.S. and Hammer, C.U. (1993) Evidence for general instability of past climate from a 250-kyr ice-core record. Nature, 364, 218—220. Eidvin, T., Jansen, E. and Riis, F. (1993) Chronology of Tertiary fan deposits off the western Barents Sea: implications for the uplift and erosion history of the Barents Shelf. Marine Geology, 112, 109—131. Feyling-Hanssen, R.W. (1966) Geologiske observasjoner i Sandnesomra> det. Norges geologiske ºnders~kelser, 242, 26—43. Feyling-Hanssen, R.W. (1971) Weichselian interstadial foraminifera from the Sandnes—Jæren area. Bulletin of the Geological Society of Denmark, 21, 72—116. Feyling-Hanssen, R.W. (1974) The Weichselian section of Foss-Eigeland, south-western Norway. Geologiske Fo( reningens i Stockholm Forhandlingar, 96, 341—353. Feyling-Hanssen, R.W. (1976) The stratigraphy of the Quaternary Clyde Foreland Formation, Baffin Island, illustrated by the distribution of benthic foraminifera. Boreas, 5, 77—94. Fægri, K. (1940) Quate¨rgeologische Untersuchungen im westlichen Norwegen II Zur spe¨tquarte¨ren Geschichte Je¨rens. Bergens Museums As rbok 1939—40, Naturvit. rekke 7, 1—201. Grimnes, A. (1910) Jæderens jordbund. Norges Geologiske ºnders~kelse, 52, 104. Hansen, A.M. (1913) Fra istiderne S+rlandet. Skrifter, »idenskapelig selskap i Kristiania, I. Matematisk-Naturvidenskapelig, Kl. 1, 155. Haugwitz, W.R.v. and Wong, H.K. (1993) Multiple Pleistocene ice advances into the Skagerrak: a detailed seismic stratigraphy from high resolution seismic profiles. Marine Geology, 111, 189—207. Helland, A. (1885) Om Jæderens l+se afleiringer. Meddelelser for Naturhistorisk Forening i Kristiania, 1885. Holtedahl, H. (1988) Bedrock geology and Quaternary sediments in the Lista basin, S Norway. Norsk Geologisk ¹idsskrift, 68, 1—20. Holtedahl, O. (1953) Norges Geologi II. Norges Geologiske ºnders~kelse, 208, 587—1118. Isachsen, F. (1943) Skagerrak-morenens sammensetning og utbredelse. Norsk geologisk ¹idsskrift, 22, 18—46. Janocko, J., Landvik, J., Larsen, E. and Sejrup, H.P. (1997) Stratigraphy and Sedimentology of Middle to Upper Pleistocene Sediments in the new Gr+deland borehole at Javen, SW Norway. Norsk Geologisk Tiddskrift, 77, 87—100. Jansen, E. and Sj+holm, J. (1991) Reconstruction of glaciation over the past 6 Myr from ice-borne deposits in the Norwegian Sea. Nature, 349, 600—603. Johnsen, S.J., Clausen, H.B., Dansgaard, W., Fuhrer, K., Gundestrup, N., Hammer, C.U., Iversen, p., Jouzel, J., Stauffer, B. and Steffensen, J.P. (1992) Irregular glacial interstadials recorded in a new Greenland ice core. Nature, 359, 311—314. King, E.L., Sejrup, H.P., Haflidason, H., Elverh+i, A. and Aarseth, I. (1996) Quaternary seismic stratigraphy of the North Sea Fan: Glacially fed gravity flow aprons, hemipelagic sediments, and large submarine slides. Marine Geology, 130, 293—315. Klemsdal, T. (1969) A Lista-Stage Moraine on Jæren. Norsk Geografisk ¹idsskrift, 23, 193—199. Laberg, J.S. and Vorren, T.O. (1995) Late Weichselian submarine debris flow deposits on the Bear Island Trough Mouth Fan. Marine Geology, 127, 45—72. Laberg, J.S. and Vorren, T.O. (1996) The Middle and Late Pleistocene evolution of the Bear Island Trough Mouth Fan. Global and Planetary Change, 12, 309—330.
Lagerlund, E. and Houmark-Nielsen, M. (1993) Timing and pattern of the last deglaciation in the Kattegat region, southwest Scandinavia. Boreas, 22, 337—347. Larsen, E. and Sejrup, H. P. (1990) Weichselian land-sea interaction; western Norway—Norwegian Sea. Quaternary Science Review, 9, 85—97. Larsen, E., Sejrup, H.P., Olsen, L. and Miller, G.H. (1992) Late Quaternary land-sea interaction: Fennoscandia and Svalbard—The Nordic Seas. Quaternary International, 10–12, 151—159. Lehman, S.J., Jones, G.A., Keigwin, L.D., Andersen, E.S., Butenko, G. and "stmo, S.-R. (1991) Initation of Fennoscandian ice-sheet retreat during the last deglaciation. Nature, 349, 513—516. Mangerud, J. (1977) Late Weichselian marine sediments containing shells, foraminifera and pollen at As gotnes, western Norway. Norsk Geologisk ¹idsskrift, 57, 23—54. Mangerud, J. (1991) The last interglacial/glacial cycle in Northern Europe. In Shane, L.C.K. and Cushing, E.J. (eds), Quaternary landscapes, 229pp. Mangerud, J., S+nstegaard, E., Sejrup, H.-P. and Haldorsen, S. (1981) A continious Eemian — Early Weichselian sequence containing pollen and marine fossils at Fj+sanger, western Norway. Boreas, 10, 137—208. Miller, G.H., Sejrup, H.P., Mangerud, J. and Andersen, B.G. (1983) Amino acid ratios in Quaternary molluscs and foraminifera from western Norway: aminostratigraphy and paleotemperature estimates. Boreas, 12, 107—124. Miller, G.H. and Brigham-Grette, J. (1989) Amino acid geochronology: Resolution and precision in carbonate fossils. Quaternary International, 1, 111—128. Milthers, V. (1913) Ledeblokke i de skandinaviske Nedisningers sydvestlige Grenseegne. Meddelelser Dansk Geologisk Forening, 4, 115—182. Nesje, A. and Sejrup, H.P. (1988) Late Weichselian/Devensian ice sheets in the North Sea and adjacent land areas. Boreas, 17, 371—384. Rahman, A. (1995) Reworked nannofossils in the Northern Atlantic Ocean and subpolar basins: implications for Heinrich events and ocean circulation. Geology, 23, 487—490. Sejrup, H.P. (1987). Molluscan and Foraminiferal biostratigraphy of an Eemian-Early Weichselian Section on Karm+y, southwestern Norway. Boreas, 16, 27—42. Sejrup, H.P. and Haugen, J.-E. (1992) Foraminiferal amino acid stratigraphy of the Nordic Seas: geological data and pyrolysis experiments. Deep Sea Research, 39, 603—623. Sejrup, H.P., Haflidason, H., Aarseth, I., Forsberg, C. F., King, E., Long, D. and Rokoengen, K. (1994) Late Weichselian glaciation history of the northern North Sea. Boreas, 23, 1—13. Sejrup, H.P., King, E., Aarseth, I., Haflidason, H. and Elverh+i, A. (1996) Quaternary erosion and depositional processes: western Norwegian fjords, Norwegian Channel and North Sea Fan. Geological Society of ¸ondon, 117, 187—202. Sejrup, H.P., Aarseth, I., Haflidason, H., L+vlie, R., Bratten, A_ ., Tj+stheim, G., Forsberg, C. F. and Ellingsen, K.I. (1995) Quaternary of the Norwegian Channel: glaciation history and palaeoceanography. Norsk Geologisk ¹idsskrift, 75, 65—87. Sellevold, M.A. and Sundvor, E. (1974) The origin of the Norwegian Channel — a discussion based on seismic measurements. Canadian Journal of Earth Sciences, 11, 224—231. Swithinbank, C.W.M. (1954) Ice streams. Polar Records, 7, 185—186. Unda> s, J. (1948) Trekk fra Utsiras natur og den siste Skagerrakbre. Stavanger Museum As rbok 1948, 59—71. Vogt, P.R., Crane, K. and Sundvor, E. (1993) Glacigenic Mudflows on the Bear Island Submarine Fan. EOS, 74, 449—452. Vorren, T.O. and Roaldset, E. (1977) Stratigraphy and lithology of Late Pleistocene sediments at M+svatn, Hardangervidda, South Norway. Boreas, 6, 53—69. Wangen, O.P. and Lien, R. (1990) Nærb+ Quaternary Map 1 : 50.000. Norges Geologiske ºnders~kelse, Trondheim.
THE JF FREN AREA, A BORDER ZONE OF THE NORWEGIAN CHANNEL ICE STREAM H.P. SEJRUP*, J.Y. LANDVIK-, E. LARSEN‡, J. JANOCKO°, J. EIRIKSSONDD AND E. KING* *Department of Geology, Univ. of Bergen, Allegt. 41, N-5007 Bergen, Norway (E-mail: [email protected]) -The Univeristy Courses on Svalbard (UNIS), P.O. Box 156, N-9170 Longyearbyen, Norway ‡Geological Survey of Norway, P.O. Box 3006, Lade, N-7002, Trondheim, Norway °Department of Geology, University of Troms, N-9037, Troms, Norway DDScience Institute, University of Iceland, Jardfrædahus Haskolans, IS-101 Reykjavik, Iceland Abstract—The coastal lowlands of Jæren, southwestern Norway, have an extensive cover of Quaternary sediments. Lithological, biostratigraphical and geochronological investigations of a series of boreholes across Jæren have allowed us to combine earlier and new information on these deposits into a temporal model of Quaternary sedimentation. The cores show evidence of at least four ice free episodes with marine deposition, during mainly arctic conditions. The oldest of these is considered to be ca. 200 ka old and the youngest one is the Sandnes interstadial which is 14C AMS dated to 32 ka BP. Interbedded between these units are 10—40 m thick units of till with components of marine material. These findings, together with a reevaluation of distinct morphological features on Jæren, as well as earlier provenance studies, strongly suggest that Jæren has been a boundary zone between an ice stream following the Norwegian Channel draining a substantial part of the southern region of the Fennoscandian ice sheet, and local ice from south-western Norway. Through multiple periods of glaciation starting ca. 1.1 myr (million years) ago, the Norwegian Channel ice stream has been the main conduit of erosion products from southern Scandinavia to the deep Norwegian Sea. Comparison with ice stream profiles from Antarctica suggests that the deposition of relatively young marine sediments as high as ca. 200 m a.s.l. on Jæren is a result of glacial isostasy, rather than regional tectonic movements. ( 1998 Elsevier Science Ltd. All rights reserved.
Lotti, 1995; Dansgaard et al., 1993; Johnsen et al., 1992). It is therefore important to study former ice streams bordering the North Atlantic, both in terms of scale and chronology, to obtain concrete data on how their development can be linked with the climate of the ice ages. The existence of large ice streams along the Norwegian Channel has repercussions in terms of our interpretation of a wide variety of data concerning the Quaternary environmental history of the region. Several studies during the last decade have emphasized interpretation of the glacial history of Scandinavia from the record of ice rafted detritus in the Norwegian Sea (Baumann et al., 1995; Jansen and Sj+holm, 1991). It is evident that both the lithology and also volume of IRD will be greatly influenced by the existence of fast flowing ice streams. It is also reasonable to assume that major parts of Norway could have been glaciated with ice reaching the continental shelf, without having an ice stream in the Channel. Several reconstructions of glaciation history have been presented from western Norway during the last two decades (Larsen and Sejrup, 1990; Larsen et al., 1992; Mangerud, 1991). Most of this information is based on on/off records from either cave deposits or raised marine sequences in coastal areas.
INTRODUCTION During the last few years there have been considerable research efforts directed towards glacial fed trough mouth fans bordering the Norwegian Sea (Eidvin et al., 1993; King et al., 1996; Laberg and Vorren 1995, 1996; Sejrup et al., 1996; Vogt et al., 1993). There seems to be a general agreement that these features are a result of fast moving ice streams depositing material as debris flows on the continental slope. One of the larger of these features is the North Sea Fan (Fig. 1) which is inferred to have been deposited primarily by an ice stream following the Norwegian Channel (King et al., 1996). It has been suggested, primarily from studies of cores in the North Atlantic that concentrations of IRD (the so called Heinrich layers) are results of fast moving ice streams/disintegration of ice streams along the coast of North America (Alley and MacAyeal, 1994; Andrews et al., 1993, 1994, 1995; Bond et al., 1993; Bond and Lotti, 1995). Both from the deep sea cores and from the Greenland ice cores it is evident that the Heinrich layers are associated with dramatic changes in the thermohaline circulation and atmospheric temperatures (Alley et al., 1995; Bond et al., 1993; Bond and 801
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FIG. 1. Map of Western Norway, the Norwegian Channel and Jæren (inset with darker shading 200 m a.s.l.). Location of map in Fig. 2 and profiles in Figs 7 and 8 are shown.
Ice streams probably did not develop in all the periods when western Norway was ice covered, so these curves do not neccesarily show ice stream history. In modelling the Fennoscandian ice sheet it is important to take into account the effect of draining by the Norwegian Channel Ice Stream. In previous reconstructions this has not been sufficiently emphasized (Nesje and Sejrup 1988; Vorren and Roaldset, 1977). The effect of extensive iceberg production, probably resulting from decay of the Norwegian Channel Ice Stream, has been traced by the occurrence of Cretaceous nannofossils from the North Sea basin in cores from the North Atlantic (Rahman, 1995). Thus the history of ice streams such as the Norwegian Channel Ice Stream is also of importance for understanding abrupt changes in thermohaline circulation possibly triggered by catastrophic transport of fresh water/icebergs from the continents. BACKGROUND The Jæren area is an anomalous part of southwestern Norway with an extensive cover of Quaternary
sediments and a relatively flat undulating landscape, stretching from Stavanger in the north to Brusand in the south. The boundary with the inland areas, which are almost void of surficial deposits, is very pronounced and extends from sea-level east of Stavanger to more than 250 m a.s.l. in the Storamos area and back to sea-level in the Brusand area (Fig. 1). Based on observations of a clayey till containing marine fossils and erratics from the Oslo area, southern Sweden, and Denmark, Helland (1885) proposed that Jæren had been partly overridden by a glacier following the Norwegian Channel around the southern part of Norway, the so-called ‘Skagerrakbreen’ (breen — Norwegian for glacier). The Norwegian Channel is a ca. 800 km long depression following the coast of Norway from the Oslofjord area to the shelf break off Stad (Fig. 1). The deepest part is found in Skagerrak with depths close to 700 m. From a threshold at ca. 270 m off Jæren, the Norwegian Channel slopes gently northwards to ca. 400 m on the margin to the Norwegian Sea. Previous studies of the Quaternary sediments on Jæren, have indicated that in addition to the till units,
H.P. Sejrup et al.: The Jæren Area, a Border Zone of the Norwegian Channel Ice Stream
numerous successions of sub-till marine beds are present (Andersen, 1964; Andersen et al., 1981, 1987; Bj+rlykke, 1908; Feyling-Hanssen, 1971, 1974; Grimnes, 1910). The chronology of these marine units is not well constrained, but age estimates ranging from Late Weichselian to the last interglacial have been suggested (Andersen et al., 1987). Sediments with marine fossils as high as 200 m a.s.l. have been noted since the turn of the century and have commonly been assumed to be tills deposited by the Skagerrak glacier (Bj+rlykke, 1908; Helland, 1885). More recently, a marine origin for some of these units has been advocated (Andersen et al., 1987; Isachsen, 1943). However, deposition of sediments at such an elevation requires substantial glacio-isostatic depression. Such a model has been questioned since the postglacial marine limit on Jæren is generally only between 20 and 10 m a.s.l. (Fægri, 1940). An alternative explanation for these high level marine deposits has been recent movement along a possible N-S fault line along Jæren. The conclusion of early workers in terms of glacial history was that during an earlier glaciation (Saalian) Jæren was inundated by a Skagerrak glacier depositing tills with far-transported clasts and marine fossils. During the last glaciation, only a relatively thin ice sheet from central south Norway, with its margin close to the present shoreline was thought to have covered the Jæren area (Isachsen, 1943). Recent work carried out in the Norwegian Channel off western Norway and on the North Sea Fan (Fig. 1) has produced strong evidence that the Channel has been occupied on several occasions during the last 1.1 myr (million years) by a fast flowing ice stream (King et al., 1996; Sejrup et al., 1995, 1996). Numerous dates from the Norwegian Channel and the North Sea Fan (Lehman et al., 1991; Sejrup et al., 1994) show that the last deglaciation of the Channel occurred very rapidly close to 15 ka. Basal dates from the small Island of Utsira (Fig. 1) suggest deglaciation prior to 14 ka (Paus, 1990). The oldest radiocarbon dates of the deglaciation from the Bergen area north of Jæren (Fig. 1) are close to 12.7 ka (Mangerud, 1977). Few basal dates are available from the Jæren region, however, deglaciation close to 13 ka for the coastal areas has been suggested (Andersen, 1960). This paper reviews findings from a coring programme carried out on Jæren between 1993 and 1995. In addition, geomorphological and orientation/provenance indicators on Jæren are described and discussed in the light of the possible influence of an ice stream. FIELD EVIDENCE Morphological features The Jæren area is generally divided into two main areas; the La> gjæren (Low Jæren) and H+gjæren (High Jæren). La> gjæren is the lowland along the coast with elevations from the present coastline to ca. 70 m a.s.l., wheras H+gjæren is an inland/upland area with rela-
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tively flat landscapes between 180 and 250 m a.ª s.l. It has been suggested by various authors (FeylingHanssen, 1966) that the boundary between these two areas follows a north south trending fault line from Gannsfjorden to Brusand (Fig. 1). The present study focusses on the southern part of Jæren between Brusand and Nærb+ (Fig. 2) where these major geomorphological elements are particularly well expressed. New information on the Quaternary successions of this area have been obtained from the coring program. The distribution of surficial sediments, together with glacial erosional and depositional features of this area, have been published on three maps on the Quaternary geology of the region (Andersen et al., 1987; Grimnes, 1910; Wangen and Lien, 1990). Based on information from these maps, and our own fieldwork in the area, we have compiled maps which summarize our interpretation of some of the glacial features of the area (Figs. 2 and 3). The southern part of Jæren can be divided into three major morphological provinces (Fig. 2); 1. ¸owland areas with N¼—SE trending ridges. In the low-lying region between Brusand and Nærb+ there are several NNW trending ridges measuring from a few metres to '10 m high, '100 m to several hundred meters wide and 1—3 km long (Fig. 3). The ridges are locally dissected by glacial (E—W) erosion and also by more recent fluvial incision. The set of ridges closest to the coast have been interpreted as marginal moraines (Andersen et al., 1987; Klemsdal 1969), corresponding to the so-called Lista Stage, deposited by ice moving from the east. Based on their overall morphology we favour the conclusion reached by Hansen (1913) that these forms represent drumlins formed by ice flowing along the Norwegian Channel. Superimposed on these is a weaker imprint of later east—west flowing ice. 2. ¸owland areas with strong east—west features. North of area 1, close to Nærb+ (Fig. 3) there is a lowering in the landscape and remnants of the NW trending ridges are only found near the coast. This area has a more hummocky appearance with several shallow lakes and large areas mantled with glacifluvial deposits. This area bears the strong imprint of a young ice lobe from the east depositing several sets of discontinuous recessional moraines during the deglaciation phase. 3. ºpland areas. The upland areas are characterized by a very flat landscape in the west, becoming more hummocky with coarse-grained glacifluvial deposits and dead-ice topography in the east. Bordering the western margin of this landscape are thick sub-till glacifluvial deposits in contact with the most easterly NNW-trending ridges. Large parts of the upland are covered by a till with fine-grained matrix (Andersen et al., 1987) overlying in situ marine sediments (see below). The boundary between the upland and the lowland areas containing N—S ridges forms an escarpment between 100 and 150 m a.s.l. A set of very open wide depressions (valleys) occurs on the western margin of the plateau (Fig. 3) facing the escarpment. They are a few km long, several hundred meters wide
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FIG. 2. Map of the southern part of Jæren (Brusand Nærb+) with the location of the geomorhological provinces; area 1 to 3. Drill sites and location of the seismic profile in Fig. 5 are shown.
and ca. 50 m deep. These valleys have characteristics typical of glacially eroded valleys with very broad Ushaped profiles. The base level of the valleys lies between 140 and 160 m and they end abrubtly, with no signs of accumulation of erosion products, close to the eastern limit of the ridges. We interpret these depressions as hanging glacial valleys, eroded by glaciers feeding into the Norwegian Channel Ice Stream. Striations From the Jæren area there is a relatively large set of striation data indicating a southwesterly ice movement
in the north and more westerly movement in the south (Andersen et al., 1987; Holtedahl, 1953). Due to the paucity of bedrock exposures there is only a single observation of striation (east-west) in area 3 (Fig. 2). From the two lowland areas Andersen et al. (1987). reported two observations of striations from each area with westerly to southwesterly directions. Just south of Brusand (in the region with almost no Quaternary sediment cover; Fig. 1), we have confirmed earlier observations of very strong southwesterly striae. However, we have also found evidence of NNW ice flow in the form of a set of older striations in this area. In addition, a number of sites with NNW striations
H.P. Sejrup et al.: The Jæren Area, a Border Zone of the Norwegian Channel Ice Stream
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FIG. 3. Some characteristic morphological features on southern Jæren visualized from topographic map data: (a) Drumlin morphology on Low Jæren. The drumlins trend is sub-parallell to the coastline. Some have been dissected by fluvial erosion. View from NW. (b) Glacial morphology of the Nærb+ area (Fig. 2). The Lake S+ylandsvatnet lies to the SE of a curvilinear hill interpreted as a recessional moraine formed by a W-trending ice lobe. Strings of mounds parallel to the S+ylandsvatnet hills are seen both outside and inside reflecting steps in the recession history. (c) Two marked incisions into the High Jæren plateau are seen near Elgane (Fig. 2). These forms are interpreted as EW-trending hanging valleys cutting down to a level of 140—160 m a.s.l., thought to be controlled by a NW running ice stream to the west. View from the west.
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rhombporphyrs, Larvikite and sandstones (from the Oslofjord area), chalk and chert (from Denmark), and As land quartzporphyrs, Dalaporphyrs and "stersj+porphyrs (Balthic derivation) from Sweden and As land, provided evidence to suggest that at some stage Jæren had been covered by a large glacier following the Norwegian Channel (Bj+rlykke, 1908; Helland, 1885; Milthers, 1913). However, it was believed that this situation prevailed during a former glaciation (socalled second glaciation) and that the east—west signatures were a result of the last (third glaciation) (Bj+rlykke, 1908). A similar view has also been advocated more recently by Andersen et al. (1987) who kept the possibility open that during the Saalian Jæren could have been overridden by a Skagerrak glacier and that the east-west striations could be the result of the Late Weichselian glaciation. Figure 4 shows some of the provenance data presented by Andersen et al. (1987). Despite doubts regarding the chronology and genesis of some of the investigated units, these data demonstrate two main directions of ice movement. Boreholes
FIG. 4. Content anorthosite and phylite in surficial Quaternary deposits on Jæren. For anothosite, just the the precence in quaternary deposits outside the bedrock province of anorthosite, is indicated. For phylite, percent countour line is give. Based on data from Andersen et al., 1987.
have been reported from the small Island of Utsira (Fig. 1) (Holtedahl, 1953; Unda> s, 1948). Thus to the south and north of Jæren there are glacial striae supporting an ice flow parallel with the coast in the Norwegian Channel. It is also clear that all the youngest striations reflect east—west ice flowing from central Norway across the coastline on Jæren. Provenance There is a long history of clast lithology studies of the surficial deposits on Jæren. Observations of coal fragments in the Quaternary deposits triggered an extensive coring program on Jæren between 1870 and 75 (Bj+rlykke, 1908). Sparse information on the lithology of the drilled units exists today, and no in situ coal has been found below the unconsolidated deposits. Frequent occurrence of far-travelled clasts such as
Four boreholes were drilled along a transect extending from the coastline to about 200 m a..s.l. in the Varhaug area between 1993 and 1995 (Fig. 2). Due to the coarse character of some of the units the recovery was generally low. Detailed accounts of the depositional environments and the geochronology of the cores are presented elsewhere (Janocko et al., 1997). In Fig. 5 the main lithological units and AMS-dates, TL-dates and some amino acid data are presented. The chronological framework for the marine units is illustrated by the degree of epimerization of isoleucine in the benthic foraminifer Elphidium excavatum in Fig 6. The following marine units have been recorded in the corings: The Gr+deland Sand is a 40 m thick marine unit mostly deposited under arctic conditions but it includes an interval with more ameliorated conditions. This part represents an interglacial preceeding the Eemian, most likely isotope stage 7. The Auestad Interstadial represents a marine arctic depositional event within isotope stage 6 and the Sunde Interstadial is a similar event possibly deposited during early parts of the Weichselian. The term ‘interstadial’ is here used simply to refer to ice-free episodes within a cold stage. In the coring at Elgane sediments representing the Sandnes Interstadial, radiocarbon dated to 32 ka, have been encountered 200 m a.s.l. Sediments from this depositional event have been recorded earlier in Jæren (Andersen et al., 1987; Feyling-Hanssen, 1971, 1976). The marine units are found beneath and interbedded with 10—40 m thick, often boulderly, diamictons commonly with a matrix of relatively fine-grained character sometimes containing marine fossils. Coal fragments in the sand fraction were found in many of the units recorded in the drillings. The provenance of this coal could be the Lista basin to the south
H.P. Sejrup et al.: The Jæren Area, a Border Zone of the Norwegian Channel Ice Stream
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FIG. 5. Simplified stratigraphies of the 93—94 corings (Janocko et al., 1997) carried out in the Varhaug area (Fig. 2). The cores are projected on the seismic profile of Andersen et al., 1987. Radiocarbon dates, TL-dates and aIle/Ile ratios in Elphidium excavatum are also shown.
FIG. 6. Chronology of the marine units recorded in the drillings on Jæren. The degree of epimerization of isoleucine (given as aIle/Ile ratios) in the benthic foraminiferal species Elphidium excavatum is shown with error bars. As the ratios fall within the range for which the epimerization reaction is following linear kinetics (Miller and Brigham-Grette, 1989; Sejrup and Haugen, 1992), we have chosen to use a linear extrapolation approach based on calibration points of known age. The late glacial and Sandnes Interstadial sediments are radiocarbondated and Eemian ratios are from the B+ site and other last interglacial sites recognized in the region (Miller et al., 1983; Sejrup et al., 1995). These provide calibration for linear extrapolation which suggest a stage 7 age (200 kyr) of the Gr+deland Interglacial and a stage 6 age of the Auestad Interstadial. The chronology of these units will be discussed more in detail elsewhere.
(Holtedahl 1988), although local sources cannot be excluded. The sequences recorded in the boreholes bear similarities with the ice stream deposits encountered along
the Norwegian Channel from Jæren to the North Sea Fan. The Quaternary sediments of the Norwegian Channel comprise sequences (Sejrup et al., 1996) which (when they are fully developed) are composed of:
FIG. 7. Cross sections (seismic interpretations) of the Norwegian Channel (for location see Fig. 1). Numerous generations of ‘U’-shaped or half-‘U’-shaped erosion scarps from Channel-parallel ice stream flow are indicated (GES). Marine/glaciomarine deposits constitute a small portion of the deposits (parallel stratification). The thick Quaternary erosional remnants in profile A (redrawn from Haugwitz and Wong, 1993) are a larger scale analogue to Jæren deposits. Jæren (land) profile same as in Fig. 5.
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(1) a basal erosional unconformity with extensive glacial sheet or channelized erosion elements termed glacial erosion surfaces (GES), followed by (2) one or more, thick (commonly several to 40 m) till units separated by GESs, and (3) glaciomarine/marine sediments conformably overlying the tills. The till units are clayrich diamictons commonly with more than 20% fine sand and coarser sediments, including Skagerrak-derived chalk, a mixture of reworked fossils, and medium to high shear strength (Sejrup et al., 1995). The marine units have a reduced coarse component, higher water content and in situ fossil assemblages. The clast content, the general lithology and thickness of the tills, and their inter-layering with marine sediments, are similar to the major units on Jæren. This supports the hypothesis that the thick till units on Jæren are a result of erosion and deposition by a Norwegian Channel Ice Stream.
DISCUSSION The Norwegian Channel Ice Stream and the Jæ ren area During the last decade there has been increased interest in the nature and role of large ice streams draining continental ice sheets. Bentley (1987) recommended the definition of an ice stream presented by Swithinbank (1954) as ‘‘. . . part of an inland ice sheet in which the ice flows more rapidly than, and not necessarily in the same direction as, the surrounding ice’’. A large number of studies have been performed on ice streams which are presently only found in the Antarctic (Alley et al., 1989; Bentley, 1987). The role of ice streams in former glaciated
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regions in terms of their location, history, dynamics and effects on continent-ocean sediment flux, is not yet well understood. As mentioned above, an ice stream following the coast around southern Norway was suggested more than 100 years ago (Helland, 1885) based on evidence from the Jæren area. From work recently carried out in the Norwegian Channel and on the glacially fed North Sea Fan at the channel mouth (King et al., 1996; Sejrup et al., 1995, 1996) the existence of repeated occurrences of an ice stream in the channel has been demonstrated. Elements of erosion and deposition from these ice streams, including their relationship to the Jæren deposits, are illustrated in cross sections of the Norwegian Channel and Skagerrak (Fig. 7). Numerous depositional and erosional phases are present, only a small portion of which relates to ice-free (marine) periods. Jæren deposits are thin and perched on the steeper crystalline basement. A possible analogue in terms of lateral ice stream setting and eroded margin is the thick pre—late Weichsel sediment package in Profile A. Based on the north—south trending drumlins, hanging valleys, striations, deformation structures, provenance data, the stratigraphies described above, together with the proximity and similarity with the Norwegian Channel setting, we suggest that Jæren has been repeatedly subjected to erosion and deposition by a high velocity Norwegian Channel Ice stream. The escarpment bordering H+gjæren marks the easternmost boundary of the ice stream, east of which only east—west trending orientation features occur. Based mostly on the data from the Norwegian Channel we assume that the ice stream has been active during
FIG. 8. Profile along the Norwegian Channel ice stream showing the thickness of the Quaternary sediments and our conception of the minimum ice stream profile. This low profile is sufficient for a glacial isostatic depression of ca. 250 m in the Jæren area (see text). For comparison a profile along ice stream B in Antarctica (Bentley, 1987) is shown (note scale differences). Quaternary thicknesses in Skagerrak are from Haugwitz and Wong, 1993 and Sellevold and Sundvor, 1974.
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several major glaciations including the last one. The overprinting of east—west orientation features such as till fabric and striations within the area dominated by the north-south ice stream, show that during the last deglaciation the volume of the Norwegian Channel ice stream decreased, or it disappeared entirely by 15 ka, allowing the boundary between the east—west ice to shift westward beyond the present shoreline. Thus the last glacial imprint is that of the east—west ice, which is reflected in some areas (area 2) more than in others (area 1).
Ice stream and sea-level on Jæ ren Figure 8 shows a longitudinal profile along the Norwegian Channel as far as the Skagerrak, including the thickness of Quaternary sediments. For comparison, profiles along an Antarctic ice stream is also indicated. It should be mentioned that the flux through the Norwegian Channel ice stream must have increased considerably northwards along the coast of western Norway due to contributions from confluent tributary glaciers in south-western Norway.
FIG. 9. Maximum glaciation scenario with an ice stream out of the Norwegian Channel. Relative length of arrows depicts ice velocity. Because of the lack of glacial fed debris flows on the continental slope west of the North Sea fan, a relatively slow moving ice is envisaged for this area. We propose a low saddle between Stavanger and Scotland with mainly slow northwards draining ice north of the saddle and southwards moving ice south of the saddle. During peak glaciation the mountains in Norway act as an obstacle and the major draining of the ice from central Fennoscandia is forced around southern Norway. Partly based on Boulton et al., 1985.
H.P. Sejrup et al.: The Jæren Area, a Border Zone of the Norwegian Channel Ice Stream
The new boreholes on Jæren confirm the existence of in situ marine interstadial sediment (Sandnes Interstadial) at more than 200 m a.s.l. The dating of these sediments to ca. 32 ka reduces the possibility that their present elevation results from regional tectonic movements, as sites of last interglacial age in western Norway suggest a sea level less than 40 m above present (Mangerud et al., 1981; Sejrup, 1987). The existence of an ice stream following the Norwegian Channel would imply an ice load of at least 700 m, as a conservative estimate, in the region. Removal of this ice load, deposition of the interstadial sediments and subsequent glacial isostatic rebound of 200 m in the area can explain their setting. This scenario contrasts with the last deglaciation where sea-level was much lower, so it is important to explain why the deglaciation preceeding the Sandnes interstadial should show this effect. The Late Weichselian was, after all, most probably a period of great Weichselian NW European ice sheet build-up and consequent crustal depression. The reasons for this radically different ice behaviour might include different deglaciation mechanisms between interstadial, with arctic climate and relatively low eustatic sea level, on the one hand, and interglacial, such as the Holocene, with boreal climate and higher eustatic rise in sea level, on the other. For the last deglaciation there is evidence from the North Sea Fan, Norwegian Channel and Denmark that the last ice stream broke up fairly rapidly close to 15 ka (King et al., 1996; Lagerlund and Houmark-Nielsen, 1993; Sejrup et al., 1995). Prior to this the flux of the ice stream possibly decreased and allowed relatively thin westerly-directed ice flow from central Norway to move beyond the coast of Jæren. The oldest basal dates for the deglaciation on Jæren are close to 13 ka (Andersen et al., 1987). This suggests that the crust during the last deglaciation had at least two kyr, probably more, to rebound following removal of the greater ice stream load, before the ocean made an imprint on Jæren. This was apparently enough to allow sufficient isostatic rebound for a marine limit of only 10—20 m. During the deglaciation prior to the Sandnes interstadial, differences in oceanic and atmospheric conditions might have created a contrasting scenario in terms of the relative magnitude of the flux of glacial ice to the Norwegian Channel from southern Norway and from central parts of the Fennoscandian ice sheet. Furthermore, the portion of the ice stream north of Jæren would be more sensitive during deglaciation to factors such as the direction and gradient of slope at its base and to sea level, in addition to atmospherically driven differences. This would indicate some independence between the ice stream and the easterly directed ice across Jæren. Relative timing of deglacial events, together with a lower eustatic sea-level under interstadial conditions could therefore have created a situation whereby the ocean inundated Jæren more rapidly after deglaciation than experienced during the latest retreat.
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CONCLUSIONS From our compilation of new and earlier data on the Quaternary of Jæren we conclude that 1. The Jæren area was inundated by a Norwegian Channel ice stream on several occasions during the last 1.1 m yr. 2. The deposits on Jæren contribute a stratigraphic record of these events going back more than 200 ka. 3. The boundary between the ice stream and westward draining ice was along the escarpment defining the boundary between H+gjæren and La> gjæren. 4. The imprint of the last ice stream phase is expressed in the morphology as N—S trending drumlins which have survived the later east—west flowing ice. 5. Marine interstadial sediments found over 200 m a.s.l. are a result of glacial isostatic depression following an ice stream situation before 32 ka. 6. The Jæren area is one of the few areas where both morphological and stratigraphical records of ice stream deposition and erosion can be studied on land.
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