The Storegga tsunami along the Norwegian coast, its age and runup
S T E I N B O N D E V I K . J O H N I N G E S V E N D S E N . G E I R J O T I N S E N . J A N M A N G E R U D A N D P E T E R E M I L K A L A N D
BOREAS
B o n d e v i k . S . Svendsen. J . I . . . J o h n s e n . G . . M a n g e r u d . J . & K a l a n t l . P. F.. lggi (March): The Storcgga tsunami a l o . - q t h e N o r u c g i a ' c o a s t . its age and runup. Bor.c's. Vol.26. pp. 29 53. oito. tssN 0300_94g3.T h e s t a t i g r a p h l i n 2 5 coastal lakcs shoqs that most of thc Norr.vegian c o a s t l i n e w a s i m p a c t c d by a large t s u n a r n i a b o u t 7 2 0 0 r 1 c BP. The methoclologl has becn t., .o..-a staircasc o1' lakc ' b a s i n s
j o v e t i i e c o n t c m p o r a r y s c a le r " e l i n s e v e r a l a r e a s it n d t o n - t a p t h e t s u n a m i d c p o s i t to its maximum ele1ation. The tsunami w a s r d e n t i f i e d i n t h e s e d i m e n t a r v r e c o r d a s a n c r o s i o n a l unconfornritl' overlain b1, graded or merssive s a n d w i t h s h c l l f r a g m e n t s . l b l l o * e d bl rcdeposited organic detritus. The greatest ,.-r,t1o runup along the co.st ( 1 0 l l nr above high tide) is found in areas most proximal to tie Storegga slide scar on thc Norwegian c o n t i r r e n t a l s l o p c ( S u n n m o r e ) . T o t h c n o r t h a n t l south. runup is less. about 7i z m at Bjugn (250 km norti of S u n n m o r c ) a n d a b o u t 3 5 m in Austrheint (200 km to the south of Sunnmorc). Thrs runup parrern supports the s u g g e s t i o n t h a t t h e t s u n a m i w a - s g e n e r a t e d b1' thc Second Storegga Slide. The rccorded runup heights arc c o n s i s t e n t r i i t h i n a n d b e t $ ' e c n thc invcstigatcd areas. iurd impll rhat the tsunami \4irve \\,lrs not srgnificantll, r n f l u e n c c d b y t h e l o c a l t o p o g r a p h y . su-ugesling a v e 1 1 , l o r . r g u a r , : c l e n g t h . T h e m a p p e c r runup csrrmiltes a r e l n g o o d a g r e e m e n t l " ' i t h a n u m e r i c a l model of thc tsuniimr generated b1, thc Second Storegga slicle. and indicatc t h a t t h e s l i d e * a s a single najor cvent rathcr rhan a sct of smallcr ilides.
Stein &nderik und Jun Mungarutl, Dapurlnu'ttt of Gcolog.t, Ltnittr.sit.t ol Bcrgert. Altcguten ll..\-5007 Bergcn, . \ i o r l l . t l . l . : J t l h n I t t g c S r . e n d ' s t ' n . C e t t t r e f o r S t u d i c s t l l E t l t . i r o n n v r t t t u t t | R c s t l u r t ' e s '
R t ' r g t ' t t ' ' \ o r t u t ' : ( j e i r J t l l m ' ' ' L t t u t t , l P t ' | t ' l ' E t t t i | I ' ' u l u n J . B o | t t n i c u l I t t ' r t i r u t e . n i t e r s i l . l ' , \ ' - 5 0 0 7 fu r g c n . . v o l r ' a . r ' : te(eit(,(l l.sr otttthL'r l9()6. ut.epred lltlt .\oternher 1996.
The hypothesis that a tsunami, generated by the Sec_
ond Storegga Slide. hit the western coast of Norwav w a s f i r s t p r o p o s e d b y Svendsen ( 1 9 8 5 ) a n d Svendsen
& M a n g e r u d ( 1 9 9 0 ) . R e c c n t l y . sedimentological e v i _ dence of such an impact has been documented in s h a l l o w m a r i n e b a s i n s and coastal lakes along the N o r w e g i a n c o a s t ( B o n d e v i k 1996). Thc tsunami de_
posits havc been dated to about l200Bp. which is about the same age as the inferrecl tsunami deposits along the eastcrn coast of Scotland (Dawson .,1 ,r/.
1 9 9 3 ) . In S c o t l a n d it i s recognized a s a n e x t e n s i v e sand layer within raised estuarine sediments passins into peat in a landward direction (Long ct ni. i|9Ai Dawson er ul. 1993). Dawson et ul. (1 9gg) suggested t h a t t h i s t s u n a m i w a s g e n e r a t e d b y a s u b m a r i n e s l i d e on the continental slope off western Norwav. the so-called Second Storegga Slide (Fig. I ) (Bugge .,r a/.
1987; Jansen ct ul. 1987). This assumption has been further supportcd by r.rumerical modelling showing that the eastern coasf ol- Iceland and the weitcrn coast of Norway were most exposed to the tsunami (Harbitz
1992; Pcdersen ct ul. 1995\.
The aim of this article is. based on the geological r e c o r d . t o r e c o n s t r u c t t h c r u n u p f r o m t h i s t s u n a m i along the Norwegian coast. i.e. the maximum distance and elevation the wave(s) surged onto land relative to the contemporary sea level. The coast of Norway is characterized by numerous lakes and boqs in rock b a s i n s f o r m e d b y g l a c i a l e r t r s i o n ( F i g . 2 t . i h e s e small depressions in the bedrock are efficient sediment traos.
and are considered well suited to iclentify and stucly t s u n a n i d e p o s i t s ( B o n d e v i k 1996). O u r a p p r o a c h w a J . first to idcntify the sea level at the timi of tsunami rmpact (7000 1200 BP) at each locality, and then to core lake basins at diffcrent altitudes above this shore_
line. The tsunami had to overflow the oulet threshold rn order to erode and deposit sediments in a lake.
Therefore the elevation of the threshold relative to the contemporary sea level (hereafter called sea level at tsunami timc. abbreviated to SLT) eives a minimum v a l u e o f t h e r u n u p . Our runup cstimate is bracketed between the highcst basin with tsunami denosits and the lowermost basin without tsunami deposits t F i g . 3 r .
A sedimentological model for recognizine tsunami f acies in basins has been established (Bondevik t,r ul. in press). These facies are now used to identifv tsunaml deposits in other basins. Field data have been gathered from a number of near-shore lake basins along the coast, at Blugn. Frosta. Sunnmsre and Austrheim (Fig. I ) covering more than 400 km of the Norwesran west coast lacing the Norwegian an<J North Seasl
Traces of tsunami deposits have also been recog_
nized southwards to Bomlo and possible traces of t[e same tsunami have been reportcd as far north as T r o m s ( C o r n e r & H a u g a n e 1993) in northern Nor_
way. The greatest runup is recorded at Sunnmsre.
d i m i n i s h i n g to t h e n o r r h and sourh (Fig. l). This pattern of runup supports the suggestion that the tsunami was triggered by the Second Storcgga Slide.
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F r e . / . B a t h y m e t r i c m a p o l ' t h e N o n i e g i a n S c a a n d N o r t h S e a s h o u i n g t h c a r c a l c x t c n t o 1 ' t h c S t o r e g g a s u b m a r i n e s l i d e s . T h e h a t c h c d a r c a i n d i c a t e s b o t h t h e e r o s i o n a | a n d c 1 c p c l s i t i o n a l e r t e n t o f t h e s l i d c s ' D o t s a | o n g t h e c o a s t o f N o r u ' a \ ' s h o u ' i n v e s t i g a t e d a r e a s . r L - p r c s e n t l o c a l i t i c s rv i t h t s u n a m i d c p o s i t s . E s t i n r a t e s o f r u n u p a r c i n d i c a t c c l .
Methods
A Russian peat samplcr was used to map the litho- stratigraphic units across each basin in the field. Sam- p l e s l b r l a b o r a t o r y in v e s t i g a t i o n s w e r e n o r m a l l y c o l - lectcd with a piston corer with a diameter ol' I l0 mm"
which provided up to 2 m long PVC core sections.
Thc altitudes of thc basin thresholds were determined b y l c v e l l i n g ( d e n o t c d w i t h " beside the altitude) or read ofT fiom maps in the scale l:5000 (denoted as h b e s i d c th e a l t i t u d e ) . A c c o r d i n g t o N o r w e g i a n M a p - p i n g A u t h o r i t y t h e u n c e r t a i n t y w i t h e l c v a t i o n s o n t h e m a p s s h o u l d n o t e x c e e d 1 . 5 m . b u t e r r o r s o n l a k e levcls are normally less than 0.5 m. After levelling the b a s i n s i n B j u g n . K j e m p e r u d (1 9 8 2 ) c o n c l u d e d t h a t t h e
altitudes indicated on the l:5000 maps are very reli- able within thesc limits.
Laboratory analyses of thc scdiments were done at the Department of' Gcology, University of Bergen.
The cores were first split lengthwise. photographed, described in detail and then subsampled for further analysis. Grain-sizc data were derived by wet sieving.
F o r l o s s o n i g n i t i o n t h e s a m p l e s w e r e d r i e d a t 1 0 5 ' C for 24 h and ignited at 550'C for l h. Weight loss was calculated as a percentage of the dried sample weight.
Empirically, total organic carbon (TOC) is 50'2, of loss or.r ignition.
Convcntional radiocarbon dates of lacustrine gyttja and samples of wood have been carried out at the
BOREAS 26 (1997) The Storegga tsunami: age and runup 3 l
Fig. 2. Photograph of Kulturmyra and Skolemyra viewed towards the southeast. Kulturmyra is a sub-basin of Myklebustvatnet (2I5 m a.s.l.) ufio,rl tOO m long and 70 m wide. Skolemyra, about 70 m x 60 m, is located 200-300 m northwest of Kulturmyra and has a bedrock threshold aL l2.4mi.s.l. In recent years the areal extent of both basins has been reduced by artificial infilling and construction work, but the deepest parts of the basins are undisturbed. In both basins, tsunami deposits have been found. The main coring in Kulturmyra is mariced by a cross, and the line on Skolemyra shows the location of the cross-section presented in Fig. 17. SLT shows the approximate elevation of the shoreline at tsunami time.
Radiological Dating Laboratory in Trondheim, Nor- way (T numbers). To avoid contamination from pene- trating roots from submerged plants, only the NaOH-soluble fraction of the organic-rich sediments was used (A after lab. no.). Small samples of plant remains and shell fragments were dated by accelerator mass spectrometry (AMS) analysis at the T. Svedberg Laboratory, Uppsala University, Sweden (TUa num- bers). The samples for AMS-dating were prepared and targets produced at the Radiological Dating Labora- tory in Trondheim. All ages cited in this article are in radiocarbon years before the present (yrs BP).
Geological setting
Sea-leuel changes
To recognize and reconstruct paleo-tsunamis through time requires that the age and elevation of former
shorelines are known. In western Norway a number of well-documeted sea-level curves have been developed during recent decades, and the shoreline geometry is considered to be relatively well known (Kaland 1984;
Anundsen 1985; Kjemperud 1986; Svendsen &
Mangerud 1987) . The Fennoscandian uplift was pri- marily caused by glacial unloading. For this reason all raised shorelines in western Norway dip towards the ocean, with a decreasing gradient through time (Fig.
4). The greatest emergence in the study area occurred around the inner part of Trondheimsfiorden (Frosta) where marine shorelines are found up to about 180 m a.s.l. (Sveian & Olsen 1984), whereas the marine limit is less than 25 m at localities on the outermost coast of Sunnmore (Svendsen & Mangerud
r987).
In Bjugn and Frosta an uninterrupted emergence has taken place since deglaciation, and consequently all shorelines become younger with lower elevations (Kjemperud 1981, 1986). However, in the western
- t l StL'itr Botrdt'rik L't ul. B O R E A S 1 6 ( 1 9 9 7 )
L a k e b a s i n A
b m a . s . l .
+
runuP Sea level at tsunami time (SLT)
No tsunami deoosil Present sea level
Tsunami deposit
a - S L T < r u n u p < b - S L T
l i , q . - 1 . A s k c t c h s h o u ' i n g h o u v e r t i c a l r u n u p i s c s t i m a t e c l i n t h i s s t u d ) . T s u n a m i d e p o s i t s h a r c b c c n i d e n t i f i e d in b a s i n A b u t n o t i n b a s i n B . R u n u p i s t h u s s o m e u h e r e b e t n ' e e n t h e t h r e s h o l d o f b a s i r r A ( a m a . s . l . ) a n d t h r e s h o l d o 1 ' b a s i n B ( b n r a . s . l . ) . T h c s m a l l e r th e d i f l c r e n c e i n c l c v a t i o n b e t u , e e n b a s i n s A a n d B . t h e m o r c p r e c i s e n t e a s u r c o f r u n u p i s o b t a i n e d .
+ +
+ +
ice-distal areas, such as Sunnmorc and Austrheim, the eustiitic rise of the global sea level was for a period fastcr than the isostatic uplift of the land. and the 9000-year shorcline is crossed by younger shorelines.
This rise of relative sea level is called tl-re Tapes transgressior-r and it culminated at about 6000 BP ( F i g . a ) ( K a l a n d 1 9 8 4 ; S v e n d s e r . r & M a n g e r u d 1 9 8 7 ) . B u.s i tt .s t r u I i g r u p I t.t' u n r I t.t u tt u tr r i J u t' i e,s
T h e s e d i m e n t a r y s c q u c n c e i n t h e b a s i n s re f l c c t s r e l a - t i v e s e a - l e v e l c h a n g e s . A l l i n v e s t i g a t e d b a s i n s . s i t u a t c d b e l o w t h e h i - e h e s t s h o l e l i n e s h o w m a r i n c s e d i m e n t s i n thc lowcr part ol thc scquence. Typically. these colrsist of grey. to brownish grey silt to fine sand with varying a m o u n t s o f ' o r g a n i c m a t t e r ( f a c i e s l . F i g . 5 ) . W h e n t h e basin emerged from the sca. a brackish en'u'ironnrcnt p r e v a i l e d . a n d u s u a l l y a f i n e l y l a m i n a t e d . v e r y d a r k brown to -erccnish-black gyttja was deposited (facies 2 ) . F u r t h e r u p l i l i t o o k t h e b a s i n b e y o n d th e r e a c h o f thc high tidc. and a brownish. lacustrinc gyttja (facies 3) startcd to accumulate. The sea-level curves rcfcrred to above are based on radiocarbon dating of the lacustrine gyttja irnrnediately above (regression) or bclow (transgrcssion) the brackish r,vater gyttja. The a l t i t u d e o f t h e b a s i n th r e s h o l d r e v e a l s t h e h i g h t i d c s e a level at the corresponding time. and tl.rus the sea-level c u r v e s s h o w t h c l c v c l o l t h e s e a a t h i g h t i d e . C o n s e - quently. the reconstructed runup is the elevation a b o v e c o n t e m p o r a n c o u s h i g h t i d e s e a l e v c l . P r e - Holocqrc sediments have not been dcscribed here.
a l t h o u g h th e V e d d e A s h B e d ( 1 0 3 0 0 B P ) ( M a n g c r u d et ul. 1984. Birks rl ul. 1996) is uscd as a stratigraphic n r u r k e r i r r t h e S u n n n r o r c r c c i ( ) n .
The inlerred tsunami deposits form a distinct group of lacics (facies 4 9. Fig. 5) that are dillerent from the e n c l o s i r r g s e d i m e n t s ( B o n d e v i k e t u l . i n p r e s s ) . T h e tsunami deposits rest on an erosional unconformity that typically shows more erosion at the seaward end of the basins. A graded (facies 4) or massive (facies 5) sand bcd with marirre fossils overlics the erosive sur'- face. The sand thins and dccreases in grain size in the landward direction. Abovc. follows an organic cou- glomerate with rip-up clasts of pcat. gyttja and marine s i l t l f a c i c s 6 ) a n d o r g a n i c d e t r i t u s ( f a c i c s 7 ) . I n t h c l o w e r b a s i n s ( < 3 r l a b o v e S L T ) t h e r e m a y be several sanci becls (facies zl or 5) interbeddcd with facies 6 and 7 . w h e r c a s i n t h e h i g h c r b a s i n s (5 l 0 m a b o v e S L T ) only one sand bcd is found. The tsunami lacies are d e s c r i b c d i n m o r e d e t a i l in B o n d e v i k c / a / . i n p r c s s .
The main ar-sunlents thal these facies were de- p o s i t e d b y a t s u n a m i a r e :
l . T h e g e o m c t r y o f t h e b c d ( s e a w a r d e r o s i o n a n d landward fining) and the contcnt of marinc lossils clearly indicate a marine relatcd process fbr its f b r m a t i o n .
2 . R a d i o c a r b o n d a t e s s h o w t h e s a m e a g e in a l l a r e a s . independent ol thc clevation above sea levcl.
3. Earlier. thcsc facies havc sometimcs been inter- prctcd to be the result of thc sea reaching the b a s i n s d u r i n g t h e T a p e s t r a n s g r e s s i o n d a t e d t o b e t w e e n 6 0 0 0 a n d 7 0 0 0 B P ( A k s d a l 1 9 8 6 : S v q r d s e n
& M a n g e r u d 1 9 8 7 ) . W e f i n d t h i s i n t e r p r e t a t i o n u n l i k e l y . b e c a u s e t h e s e d i n . r e n t o l o g y o f t h c d e p o s i t is totally' different front ntarine sediments normally cleposited in basins invadcd by the sea during the t r a n s g r e s s i o n . T r a n s q r c s s i v e s e q u e n c e s . d e p o s i t e d
o B a s i n s w i t h o u l tsunami deposits . B a s i n s w i t h
tsunami deposits
72oo r9l
3 r
3_3
xy , 6
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durir-rg the Tapcs transgression. that have not been disturbcd by the tsunami. arc characterized b1 a graclual trar.rsition from a bror'r't.t gyttja to a brorvn- ish grey silt gyttja (e.g. see Klingrevatr.rct. this article). Furthermorc. these facies are found rvell a b o v e t h e m a x i m u m l e v e l o f t h e t r a n s g r c s s i o u a s w e l l a s i n a r c a s w h e r e t h i s t r a n s g r c s s i o n d i d n o t o c c u f .
, 1 . Many of the charactcristics o f t h e s e d e p o s i t s a r e also reported fiom known modern tsunami dc- p o s i t s . c x t c n s i v e e r o s i o n . r i p - u p c l a s t s " d e c r e a s e in thickness and grair.r size lanchvarcls. alternatior.r betrvccn finer- and coarscr-graitrcd beds and as a whole. the deposit generally fines upwards.
B o n d e v i k t ' t u l . ( i n p r e s s ) c o n c l u d e d t h a t t h e o n l y p r o c e s s t h a t c o u l d h a v c d e p o s i t e d t h e s e ll e i c s i s l t s u n a m l .
Rccortslrtrct ion of runup
T h c p r e s e n c e o f t s u n a m i d e p o s i t s i n a l a k e m e a n s t h a t the tsunar.r.ri overflou'ecl thc outlet threshold. The ele- v a t i o n o f ' t h i s t h r e s h o l d re l a t i v c to S L T -eives a m i n i - m u n t v a l u c o f r u u u p ( F i g . 3 ) . H o w e v e r . r u r t u p i s defined as thc maximum levcl the tsunami reached abovc tl.rc contemporitneous sea level. We thereforc lraced tl.re tsunami deposits successivcly to lakes at higher clcvations. The lirst lake basin we encc'runtcrcd
Tlrt Storaggtt tsununti: ugc urtd ruuult 3 3
1 7 0 1 6 0 1 5 0 1 4 0 1 3 0 120 1 1 0 1 0 0 90 8 0 7 0 6 0 50 4 0 30 2 0 1 0 0
l v i t h n o t r a c e s o f ' a n y i n u n d a t i o n b y t h e t s u n a m r w a s t a k c n b y u s a s a n i n d i c a t i o n th a t t h e t s u n a m i d i d n o t reach this level. The runup estimate is thus bracketed between the highcst basin with tsunami deposits and t h c l o w e r m o s t b a s i n w i t h o u t t s u n a m i d e p o s i t s ( F i g . 3 ) . The smaller the difference in altitr.rde between these t w o l a k e b a s i n s . t h e n r r r l e p l c e i s c r u r ) u p c s l i n l r l c is o b t a i n e d .
As dcscribed above" the identifled SLT refers to the h i g h t i d c l c v c l . C o n s e q u e n t l y . e s t i m a t e s o f r u n u p a r e the elevation abo."'e cc'rntcmporancous high tidc lcl,cl.
If the tsunami happened at low tide. runr.rp would a c t u a l l y b c 2 3 m h i g h e r t h a n o b s e r v e d .
Results
Rttnup itt Bjugn
T h c s t u d i e d b a s i n s i n B j u g n ( F i g . l ) a r e A u d a l s v a t n e t ( 3 1 . 5 m a . s . 1 . ) . K v e n n a v a t n e t ( 1 7 rn a . s . l . ) . G o r r t j o n n a I & I I ( 4 2 m a . s . l . ) a n d J o v a t n e t ( 4 4 m a . s . l . ) ( F i g . 6 ) localcd in a small valley that is oriented nortl.t south t o w a r d s t h e c o a s t ( F i g . 7 ) . A d e t a i l e d d e s c r i p t i o n o f the tsunar.r-ri deposits in these lakes is given in Bclnde- v1k t't ul. (in press).
According to a local sea-level curvc. thc 7200 BP s h o r e l i n e in B j u g n i s a b o u t 3 5 r n a . s . l . ( K . l e m p e r u d
1 9 8 6 ) . H o w e v e r . t h i s c u r v e l a c k e c l d a t a b c t w c c n
1 7 0 1 6 0 1 5 0 1 4 0 1 3 0 120 1 t 0 1 0 0 90 8 0 7 0 6 0 5 0 4 0 3 0 2 0 1 0
6 ( X X ) U P . u h e r e a s in t h c c a s t c r n a r e a s a c o n t i r . r u o u s r e g r e s s i o n h a s t a k e n p l a c e a l i e r t h c d c , s l a c i a t i o n .
D E S C R I P T I O N
INTERPRETATION
Massive gyttja
A few laminae in gyttja silr
Organic detritus i Organic conglomerate 5 Massive sand 4 Graded sand
Erosional unconformity Massive, brown gyttja Gyttja, ofien laminated (dark colour)
Massive sandy silt with org. matter
9 8
3 2
fs ms cs
4-9 Tsunami lacies
9 Lacustrine sediments,bioturbation prevented by saline bottom water 8 Thin bed or laminae of silt
fallen out from suspension 7 Finer organic matter settles slower and thus after facies 6 6 Clasts of peat, gyttja and silt
ripped up lrom floor and edges ol basin
4-5 Rapid deposition of sand settling out from tsunami wave
^.ion bY tsunami-
V"-7-
Lacuslrine sediments Brackish-water sediments Shallow marine sediments 3
2
S t t , i r t B o t t t l t ' t i k e t u l
3 0 m a . s . l . (6 0 0 0 B P ) a n d 4 3 m a . s . l . (8 7 0 0 B P ) a n d i s therelbrc not precise enough to identify the tsunami sea level at 7200 BP.
I n A u d a l s v a t n e t ( 3 3 . 5 m a . s . l . ) t h e t s u n a m i d e p o s i t s are lound in marine sediments 70 90 cm below the marinerlacustrine boundary (Fig. 8), indicating that s e a le v e l w a s w e l l a b o v e 3 3 . 5 m a . s . l . w h e n th e t s u n a m i o c c u r r e d . I n K v e n n a v a t n e t ( 3 7 m a . s . l . ) d i a t o m s s h o w a change from a marine to a brackish environment bclow the tsunami deposit (Fig. 9), whereas above the tsunami deposit there are only lieshwater diatoms (spectres 1 & 2. F-ig. 9). We therefore conclude that Kvennavatnet was a freshwater lake when the tsunami occurred. A moss stem from the brackish zone just below the tsunami deposit was dated to 7350 + 80 ( T U a - 1 2 7 0 , T a b l e l , F i g . 9 ) . i n d i c a t i n g t h a t t h e b a s i n emerged from thc sea only slightly before the tsunami impact. Because of a slow emergence rate at this time ( K j e m p e r u d 1 9 8 6 ) , 3 6 m a . s . l . i s a r e a s o n a b l e e s t i m a t e for the tsunami sea level in the area.
A cross-section from Gorrtjonna I (42m a.s.l.) to- wards Gorrtjonna II (Fig. l0) clearly demonstrates the landward fining of the bed. We tried, but were not able to follow the deposit into Gorrtjonna II, even though the two small lakes were probably part of one bigger lake basin 7200 years ago (they were later separated by peat). A detailed cross-section just inside the outlet of Jsvatnet (44 m a.s.l.) did not reveal any
B O R E A S ] 6 ( I 9 9 7 )
Frg. -i. Dcscription of the lacrcs u s c d i n t h e a r t i c l e . g i v c n a s a n i d e a l i z e d . c o m p l c t e f a c i e s
\ e q L l c n e c t f u c i e ' 4 ( ) ) o l ' l s l l n r m i d e p o s i t s in n e a r - s h o r c l a k e s . T h e e n c l o s i n g s e d i m e n t s a r e a l s o s h o u n . N o t c t h a t i l ' f a c i e s .1 a n d 5 o c c u r i n t h e s a m e s e q u c n c e . t h c y a r e n o r m a l l y s c p a r a t e d b 1 o r g a n i c f a c i e s (f l c i c s 6 o r 7 ) .
candidate for a possible tsunami deposit (Fig. 8). In conclusion, the tsunami clearly inundated Gorrtjonna I (and possibly Gorrtjonna II), but not Jovatnet. This gives a vertical runup of more than 6 m but less than 8 m. Since there is no distinct tsunami deposit in Gorrtjonna II, we assume that this basin was close to the maximum level reached by the tsunami. Therefore we estimate the runup in Bjugn to be 6 7 m. The horizontal runup in the valley is about 500 m, the distance from the 36 m contour line below Gorrtjonna I to the innermost point with certain tsunami deposit.
between Gorrtjonna I & II.
Frosta
The Frosta area is located in the middle part of Tronsheimsfjorden (Fig. l). The geological setting dif- fers from the coastal areas because thick deposits of marine clay and silt cover the bedrock (Reite | 985) and most basins were filled in prior to the tsunami impact.
For this reason very few lake basins close to the 7200 year shoreline exist in the area. The two pre- sented basins, Nesvatnet (60 m a.s.l.) and Nesjovatnet (68 m a.s.l.) are located parallel to the isobase direc- tion. Almost 180 m of emergence has taken place since deglaciation (Kjemperud 1981 ; Sveian & Olsen 1984) of the area between I 0 600 and I 0 I 00 BP. Because of the rapid emergence it is dilficult exactly to determine the
B O R E . \ S t 6 ( 1 9 9 7 )
sca level at 7200 BP. According to the shoreline dia- g r a m ( F i g . 4 ) . t h e 7 2 0 0 y e a r s h o r c l i n e s h o u l d b e c l o s e t o 6 0 m a . s . l . p r o b a b l y b e t w e e n 5 8 a n d 6 0 m a . s . l .
Nesttrlrtcl 60 nt u.s.1.". r'losc to tlte scu lct:cl ut Tsuttuttti t i t n e ( S L T ) . N e s v a t n e t ( F i g . l l ) i s m u c h l a r g c r th a n t h e o t h e r i n v e s t i g a t e d l a k e b a s i n s , 1 8 0 0 m l o n g a n d 750 m wide. The area around thc lake is characterized by thick deposits of marinc silt and clay. Several cores wcrc obtained in the middle of the basin showing a similar stratigraphy: between the marine clayey silt and the lacustrine clayey gyttja there is a distinct.
l 0 c m - t h i c k b e d o f m a s s i v e . s i l t y c l a y ( F i g . l l ) . T h c bed has a sharp lower and upper boundary and fines upwards. Bclow and above the bed there is I 2 cm of finely laminated gyttja (facies 2) (Fig. I l). The lamina- tions probably reflect brackish conditions in the basin.
but the sediments are almost barren of diatoms. so the cnvironmcntal conditions cannot be precisely deter- rnined.
The massive silty clay bcd enclosed in the laminated gyttja is the only possiblc candidate for the tsunami deposit. However, this bed dillers sedimentologically from the known tsunami facies (Fig. 5) and another process for its forn-ration cannot be excluded.
l,{esiarutnet 68 nt u..s.l.''. I l0 trt ubot:e SLT. Lake Nesjovatn is 1.5 km long ancl between 50 and 100 m wide, oriented parallel to thc fjords (Fig. I I ). It should be well exposed to record a tsunami because of the orientation and the narrow Dassase from the sea
F r g . 6 . L o c a t i o n o f s t u d i c d b a s i n s in B j u g n . N u m b c r s i n p a r e n t h e - s e s a r e a l t i t u d e s o f t h e l a k e s .
T l t t ' S l o r t ' g g u L s u r t u r t t i : u g c u n d r w t u p 3 5
side of the basin. We studied the scdirncnts in thc lakc closc to the outlet (southwestern part). where tl.re tsllnami should have entered the lake. Thc sedimqrts showed a normal regression sequence from marine ttr lacustrine sediments. The lower part of the lacustrinc scdiments was thoroughly examined but there were no visible traces of any tsunami in this basin.
Rtrttup in Fro,slu. The possible tsunami in Nesvatnet is interbedded in brackish sediments. This is in agree- ment with the assumed sea level at tsunami time. If the stratigraphy in thc two basins is correctly inter- prctcd, runup is less than 8 l0 m in the area.
Sulu
As part of a sea-level study or.r Sula (Fig. l2), many basins were investigated in the late 1970s and early 1 9 8 0 s ( S v e n d s e n & M a n g e r u d 1 9 8 7 ; M a n g e r u d &
Kristiansen, unpublished). We have now re-examined cores and field dcscriptions obtained l6 18 years ago fron.r Klingrcvatnet, Endrevatnet. Igletjorn and Ratvikvatnet. Based on this examinatiorr we revisited Endrcvatnet and lgletjorn, and the new localities RaudAvatnet. Djupvikvatnet and Rotevatnet (Fig. 13).
The gyttja immediately above the tsunami deposits ir.r Er.rdrevatnct ( 12.5 m a.s.l.) contains only fresh- water diatoms. indicating that sea level was below
1 2 . 5 m a . s . l . w h e n th e t s u n a m i o c c u r r e d . K l i n g r e v a t n e t ( 5 . 5 m a . s . l . ) w a s t r a n s g r e s s e d j u s t a f t e r 8 2 0 0 B P ( F i g .
l4) and thus the sea level was somewhcrc between 5.5 a n d 1 2 . 5 m a . s . l . B y e x t r a p o l a t i n g t h e T a p e s s h o r e l i n c , dated to between 6000 and 7000 BP" sea level is esti- m a t e d to b e l 0 1 l m a . s . l . a t S u l a d u r i n g t h i s p e r i o d . We assume that the 7200 BP shoreline is close to or slightly lower than this altitude (Reite 1968; Greve
1984). and for the estimate of runup we conservatively u s e t h e a l t i t u d c l 0 l l m a . s . l .
Klingret:utrtct 5.5na.s.l.t', 4 5m belov' SLT.
Klingrevatnet is about 300 m long and 200 m wide with a till threshold (boulder) at the northern end.
The basin is partly filled in with gyttja and peat at the southwestern part (Fig. l3) where Kristiansen &
Mangerud (unpublished) described the lithostratigra- phy in several cores, partly published in Svendsen &
M a n g e r u d ( 1 9 8 7 ) .
In the lower part of the basin there is a marine silt (facies I ) with an even transition to a brown lacustrine gyttja (facies 3). This isolation contact is radiocarbon d a t e d t o 9 7 1 0 + 1 8 0 ( T - 3 9 8 3 , A . T a b l e l . F i g . 1 4 ) . Upwards the sediments change gradually to a brown grey, marine silty gyttja (facies 1). This transition.
which shows the onset of the Tapes transgressiotr, is dated to 8220+ 90 (T-39844. Table l. Fig. l4).
Within this marine formation there is a graded sand bed (lacies 4) that lincs upwards from a coarse sand to fine gravel to medium to fine sand with shcll frag-
A u d a l s v a l n e t ( 3 3 . 5 )
Kvennavatner (37) | Go(rtiannar&tr(42)
Afr-Y"/ Jovarner (44)
J O Stein Bondeuik et al BOREAS 26 (t997)
Fig. 7. Aerial photograph of Audalsvatnet, Kvennavatnet, Gorrtjonna I & II and Javarnet. The 7200 Bp shoreline (SLT) is shown as a thick, white line. Tsunami deposits have been found in Audalsvainet (below the sea level of that time), Kvennavatnei, Gorrtjonna I, but not in Gorrtjonna II and Jsvatnet. The basins are located in a small valley oriented north-south towaids the coast. Numbers lre altitudes above present sea level.
l ] O R F \ S 1 6 ( 1 9 9 1 ) T l t t ' . \ t , t r t g . ! t t L s t t t t t t t t t i : t t . g t ' t t t t t l r u t t t t l ; l 7 Run up tsunami + ( 7 - 8 ) m J @ v a l n e t
+ ( 5 - 6 ) m G o r r t j o n n a S e a l e v e l a t
t s u n a m i ti m e
t I
+ ( 0 - 1 ) m K v e n n a v a t n e l
( 2 - 3 ) m A u d a l s v a t n e t
L o s s o n i g n i t i o n 10 20 30"/.
0 5
1
\I {)
Loss on ignition
r 0 20 300.
g p c
l s m s c s
g
c
3 r o .:
tro)
1 5
n" "Fn&*P ,w
F a c i e s 3 ( L a c u s t r i n e ) Brown gyilja F a c i e s 2 ( B r a c k i s h ) Lamrnated gyttja F a c i e s 1 ( M a r i n e ) S i l t / f i n e s a n d
l s m s c s
F a c i e s 4 I ( T s u n a m i ) : j.':";": Sand / f ine gravel
- , C l a s l s A > Plant lragm.
_ _ _ G r a d u a l o o u n o a r y _ S h a r p
oounoary - _ E r o s i o n a l
u n c o n t o r m i t y L o s s o n i g n i t i o n
38 Stcin &tnrlcrik ct ul 'fuhlc
L Rlrdiocarbon dates
B O R E A S 1 6 ( t 9 9 7 )
B a s i n C orc D e p t h ( c m ) M a t e r i a l L r b . n t r A-rre , i r t C
l,ower part of gpttja above the tsunami deposit A l m c s t a d n r y r a I 9 9 1 - l f t . 1 l A u r e t j o r n A r . r r . I
A u r c t j o r n A u r . I ' + E 8 - 4 9 2 5 h n d r L - v a t n e t 5 0 1 - - 1 4 - 0 1 8 i 9 - 8 4 1 E n d r e v a t n c t - t 0 2 - 3 4 - 0 1 8 4 2 - 8 4 5 L n d r c v a t n e t 5 0 2 - - 1 4 - 0 | 8 . + 5 - 8 4 7 . 5
Fro) stadmyra 502-.+0-0 1 17 5-119
F r o v s t a d m r . r a 5 0 2 - 1 0 - 0 1 4 E l - . + 8 5 I - o r l a n t l s r u t n e t F o r l . | 6 6 2 . - 5 - 6 6 8 ( i o r r t . j o n n a l 1 9 9 - l - l - 5 2 2 - 5 2 7 . 5 K \ c r l n a v a l r l c t _ 5 0 4 - 0 1 - 0 1 l l 4 7 - l l 5 l K v e n u l v a t n c t 5 0 4 - 0 1 - 0 1 l l 5 l - 1 1 5 3 R a t v i k r a t n e t 5 0 2 - 3 1 - 0 1 E l 0 - 8 i 3 R a n i k v a t r . r c t 5 0 1 - - l l , 0 l 8 . + 3 - 8 1 7
Skoleml,ra -s02-.+-l-01 161-411
S k c r l c m l ' r a 5 0 1 - . 1 1 - 0 t 1 t t - 1 7 4
Skolcnrl,ra -504-0.1-01 191-194
S k o l c n r y r a 5 0 . + - 0 1 - 0 | 5 0 | - 5 0 5 S k o l c n r l ' r a - s 0 4 - 0 , + - 0 1 5 l l - 5 1 4 S k o l c n r l ' r a 5 0 4 - 0 4 - 0 | 5 l | - 5 2 6 S k o l e m l , r a 5 0 4 - 0 4 - 0 1 5 2 6 A s c t . j o r n A s c t j . I 7 5 1 - j 5 9 A s e t . j o r n A s r - r . j . I j 6 g - 7 j 1 A s c t . j o r n A s e t j . I 7 8 . + - 7 8 9 A s c t j o r n A s c t j . I 7 9 3 . - 5 - 7 9 5 . 5 A s c t . j o r n A s e t . j . I 7 9 9 - 8 0 5 Within the tsunami deposit
A u c l a l s v a t n e r - 5 0 . 1 - 2 5 - 0 2 1 2 5 6 A u r e t j o r n A u r . | 4 9 9 - 5 0 . + F r o l s t a d r n l r a 1 9 9 2 - l 3 6 8 F r o v s t a t l m y r a 5 0 4 - 0 7 - 0 I 2 8 0 - 2 E . +
l:rovstadmy nr 50:l-.+0-0 ] .+91 -493
I " o r l a n d s r , a t n c t F o r l . I 6 7 0 - 6 7 5 ( i o r r t j o n n u I l 9 9 l - l 5 2 7 - 5 1 0 ( i o r r t j o n n a I 1 9 9 3 , 1 5 1 9 - 5 4 2 ( i o r r t j o n n u I 5 0 4 - l l - 0 1 - 5 5 0 - 5 5 3 . 5 K h n g r c v a t n r - t 5 0 2 - l - 5 - 0 1 5 3 2 K u l t u r n r y n r 5 0 . + - 0 5 - 0 1 1 t ) 7
7080 + E0** 26. i * 7 0 2 0 + 9 0 - 2 9 6 7Jl0 + 1.10** 29.8 6 8 6 5 + 1 0 5 - 3 0 . 7 7 1 0 5 + l 3 - 5 3 0 . 6 7 4 9 0 + 9 0 * * - . 1 2 . I
7500 + E0 25.2
7 6 1 5 + 1 5 0 * * 1 6 . 0 7 . + 7 5 + l l 0 * * 2 9 . 9 7 6 0 5 + 1 0 5 * * 2 9 . E 7 1 0 0 + I 2 5 2 9 . 8 7 5 7 0 + 9 0 * * 3 0 . 0
6550 + 100 29.9
7 4 3 0 + 9 5 * * * 2 9 . 5 7 1 3 0 + 9 5 3 0 6 7105 + 90+* -10.2 6 5 7 5 + l l 0 - . 1 0 . 4
6ft90 + 65 30.7
704-5 + 70 10. I 7 6 1 0 l l ( ) ( ) * * - 2 9 . 4 7 8 5 0 + g 5 * * _ 2 7 . 7 * 5 6 9 5 + 1 0 0 3 0 . 6
640-5 + 85 30.2
6 9 9 5 + l t 0 - 1 0 . 7 7 1 8 0 + 9 5 1 0 . 8 7 2 - 3 0 + 1 0 5 * * _ l l . l T u i g
G l t t j a C i l t t i a G1'ttja C1'ttja G1-ttja C1'ttja G v t t j a (i1'ttja G l , t t j a G1'ttja G)'ttja G l t t j a G1'ttja G l t t i a G l t t j a C 1 ' t t l a G1-ttja C1'ttja G y t t j a G l t t j a G1'ttja (;\,tt.ja G1,'tt ja G y t t j a G l t t j a
M o s s s t e n t O r g . d e t r i t u s Tu,ig O r - c . d c t r i t u s O r g . d e t n t u s O r g . c l e t r i t u s Trii-rr T w i g -l-n'ig T.,r'ig
Pf. lragm. ('ullunu'l
TUa--522 T- 1 I 707A T- | I 7064 T- l 0599A T - 1 0 5 9 8 A T - 4 t 6 2 T- I 0s9lA T- 1059-rA T- l l 708A T- I I l.+94 T - t t 2 4 4 A T - l l 0 l 3 A T- I 0595A T- I 0591A T- l 0590A T - l 0 5 9 t A T - t 1 2 7 8 A T - t 1 2 1 7 A T - l 1 2 7 6 A T-il1.+5A T[-ra-86]A T- I I 28lA T - l Il S l A T- | r l80A T- I 2260A T - |l 2 0 2 A
T U a - I 1 5 0 T-t I 705A T U a - 5 2 3 T- l 12.164 T-1961 A T - i l 7 r 0 A TUa-81,{
T U a - 8 3 5 TUa- 1 269 T U a - 1 1 2 2 TUa-8.13
7 3 1 5 + 7 0 8 0 9 0 + 1 2 0 7 6 5 5 + 8 5 7 9 8 5 + I t 5 8480 + I 60 8040 + 1 60 6970 + 175 7930 + 65 711.+5 + 6-5 1 1 7 5 + 1 5 u 2 8 5 + t 8 5
22.9 - 1 8 . 5
2 6 . 1 * 21.8 2 7 . 8 30. s 2 6 . 1 * - 2 6 . 1 *
2 9 5 - 3 0 . 7
ments. plant liagments and gyttja. The bed is about l 0 c m t h i c k i n c o r c 5 0 2 - 3 5 - 0 2 ( F i g . l 4 ) . with a sharp lowcr boundary and a more graudal, but distinCt upper toundary. A plant fiagment from the uppcr p a r r o f r h e b e d i s d a t e d ro 7175 + 75 Bp (TUa_il22.
T a b l c l . F i - e . l4 ) . K r i s t i a n s e n ( u n p u b l i s l r e d ) a l s o d e - scribed A corc with two sand beds interbedded with organic debris (fircies 6 or 7) at the same stratigraphi- c a l p o s i t i o n . T h e b a s i n e m e r g e d a n d t u r n e d in t o a l a k e a t 1 2 4 0 + 1 2 0 B P ( T - 3 9 8 2 A , T a b l e l).
Etrtlrct'u/nt't 12.5 trt u..s.l.t', I 2 nr uhotc SLT.
E n d r e v a t n e t is a p p r o r i m a t e l y 250 x J5m (Fi_e. l3).
The lower parl of tl.re basin contains marine scciiments ( l a c i e s l) ( F i g . l4) with an crosional upper bounclary.
(-uttinuad on lutirtg pttgc
On this unconformity there is a well-defined bed of a poorly sorted coarse sand,fine gravel with shell fiag- ments (facies 5) that in placcs fincs upwards (facies 4).
Twigs and clasts of peat (facies 6) were found on too of facies 4i5 and a fcw clasts also within the sand.
Above thcies 6 there is a thin silty horizon (facies g) followed by a thick sequence of undisturbed lacustrine gyttja (facies 3). Only freshwater diatoms occur across thc boundary from facies 8 to ftrcies 3.
The unconformity below the tsunami depclsits s h o w s th a t e x t e n s i v e e r o s i o n o f l a c u s t r i n c a n d m l t r i n e scdimcnts has taken place. By using pollen analysis and the sea-level curvc from Sula (Svendser.r &
M a n g e r u d 1 9 8 7 ) . w e e s t i m a t e t h a t t h e t s u n a m i e r o d e d sediments that had been deposited during a pericld of
B O R E A S 1 6 ( 1 9 9 7 )
T u h l t : l . ( c o n t d . )
The Storaggu t.yununi: age and runup 39
B a s r n C o r e D e p t h ( c m ) M a t e r i a l L a b . n o . A g e r ! r r C
K v c n n a v a t n c t - 5 0 4 - 0 2 - 0 | K v e n n n v a t n e t 5 0 4 - 0 2 - 0 1 K v e n n a v a t n c t 5 0 4 - 0 2 - 0 1 K v e n n a v a t n e t 5 0 4 - 0 2 - 0 1 R a t v i k v a t n e t 5 0 2 - 3 3 - 0 1 R a t v i k v a t n c t 5 0 2 - 3 1 - 0 1 S k o l e m y r a 5 0 4 - 0 4 - 0 1 S k o l c m i r r a 5 0 4 - . 1 1 - 0 1 S k o l e m y r a 5 0 4 - 0 4 - 0 1 S k o l e n r y r a 5 0 4 - 0 . + - 0 1 S m i c t l l r l c n r ) t r . 5
Asetjorn Asetj. I
A s c t j o r n A s c t l . I A s e t j o r n A s e t j . I Directly below the tsunami deposit
A u r e t j o r n A u r e t j . I F r o y s t a d m v r a 5 0 . + - 0 7 - 0 I F o r l a n d s v a t n c t F o r l . I G o r r t j o n n a I 1 9 9 3 - l K u l t u r n i y r a 5 0 4 - 2 . 1 - 0 1 K v e n n a v a t n c t 5 0 . + - 0 1 - 0 1 A s e t j o r n A s e t 1 . I Isolation/ingression contacts
A s c t j o r n A s c d . I A r i i s v a t n e t A r i i s v . I F r o y s t a d m v r a I 5 0 4 - 0 7 - 0 1 G o r r t j o n n a I 1 9 9 3 - l K l i n g r e v a t n c t 5 0 2 - 3 5 - 0 1 K l i n g r c v a t n e t 5 0 2 - 3 5 - 0 2 K h n g r e v a t n e t 5 0 2 - 3 5 - 0 1 L , t n g . t , t ' , a , C r r l g 1 1 9 . r L o n g e v a t n c t ( l o r c no. 5
T w i g T w i g Shell fragm.
S h e l l f r a g m . Twi_s S t i c k
Bark liagm. Pfurrs Twig
Bark liagm. Belrla O r g . d e t r i t u s Twig Or,c. detrius Twig T w i g
Gyttja Gyttia Gyttja G y t t ja Gyttja M o s s s t e m Gyttja
Gyttja G y t t j a Gyttja G y t t j a Gyttja G y t t j a Gyttja Gyttja Gyttja i l 6 1
l l 7 3 I l6-3 1 1 7 9 8 8 0 901 540-513 495 569-57 I 526-529 507 8 0 5 - 8 1 0 8 7 6 865
290-2.94 694.5-700 5 6 r . 5 - 5 6 7 5 1 1 . 5 - 5 1 7 I 1 8 2 . s 878-884
946.5-9.19.5 t21E-1221 308-l l 2 575-5 E0 3 79-382 57 1.5-57 6 5 9 5 - 5 9 7 . 5
T U a - 8 3 1 TUa-98,{
TUa-832 T U a - E 5 9 T- l 0597 T- I 0596 TUa-861 TUa-524 TUa-860 T - l 1 2 7 5 4 T U a - 8 5 8 T- I 1 279A TUa-864 TUa-861
T- I I 7044 T - 1 t 2 1 7 A T-l I 709A T - l t 2 5 0 4 T - I l 8 3 7 A T U a - 1 2 7 0 T - l 1 2 0 1 A
T - r 1 2 0 3 4 T- l l 204,4 T - t 1 2 4 8 A T - 1 l 2 5 l A T-3982A T-3984A T - 3 9 8 3 A T-22738 T-22748
7240 + 10 7200 + 80 8405 + 70 1 0 7 8 0 + 9 5 7 2 3 0 + 1 0 5 1610 + 7l) 7 2 5 0 + 1 5 7365 + 90 7435 + 75 8 1 1 5 + l l 0 7765 + 80 7 9 1 5 + 7 0 8045 + 75 8 3 5 0 + 8 0
7120 + 80 9 0 2 0 + 1 5 5 7 9 8 5 + 1 5 0 7680 + 70 E.140 + I t5 7-350 + 8t) 7 8 0 5 + I t 5
9590 + 90 9 7 6 5 + I 0 5 9 7 9 5 + 1 6 0 8 1 8 0 + 1 5 0 4210 + 120 8220 + 90 9 7 1 0 + 1 8 0 7 1 9 0 + 8 0 5950 + 70
2 6 . 1 * 1 . 0 * 1 . 0 * - 2 6 . 1 *
2 6 . 1 + 2 6 . 1 * - 2 6 . 1 * - 2 6 . 1 + 24.9 - 2 6 . 1 * - 3 0 . 4 - 2 6 . 1 + - 2 6 . 1 *
- 2 9 . 9 2 6 . I 29.5 29.7 2 2 3 29.6
2 6 . 3 2 6 . 9 2 l .0 3 1 . 0 - 2 8 . 9
2 8 . 9 - 2 6 . 3
l?'l 3 l .2 x Assunred values (not measurcd) used for corrcction to -25%,, JlrC.
* * Ages of samples fronr dircctly abovc the boundary between tsunami deposits and undisturbed lacustrinc g1,ttja.
A rcsen'oir a-qe .,1'4.+0 vears has bccn subtracted fiom the agc of thc shell fragments.
S a m p l e s w i t h l a b . n o . T a r e c o n v e n t i o n a l dates perfornred a t t h e l a b o r a t o r y in T r o n c l h e i m . S a m p l c s w i t h l a b . n o . T L J a a r c a c c e l c r a t o r m a s s s p e c t r o m e t e r v ( A M S ) dates.
about 2000 years. A radiocarbon date of the basal lacustrine gyttja yielded the age 1490 + 90 (T-4162 A.
T a b l e I , F i g . l 4 ) . O r i g i n a l l y , th e erosional unconfor- mity and thc tsunami facies were inferred to be the result of the Tapes transgression (Svendsen &
M a n g e r u d 1 9 8 7 ) . W e c o n c l u d e n o w that this trans- gression did not reach Endrevatnet at all.
Iglet jarn 15.5 m u..\.1.t'. 5 6 m ubote SLT. This small l a k e b a s i n ( 5 0 x l 5 0 m ) i s p a r t l y filled in with sedi- ments and only the middle of the basin is still a small pond. The drainage area of the basin is not much larger than the basin itself and no stream enters the b a s i n .
The tsunami deposit is exceptior.rally thick, up to l 5 0 c m ( F i g . l 4 ) . I t r e s t s o n an erosional unconfor- mity. mainly on marine silt, showing that all lacustrine
sediments have been removed. The tsur.rami deposits start with massive gravel or sand. often with shell fragments (facies 5) and this is followed by organic conglomerate (facies 6) and organic detritus (facies 7).
Facies 6 contains more pieces of larger wood frag- ments (covering the width of the l0 cm wide Russian corer) than the tsunami deposits in other basins. At the transition to the lacustrine gyttja a few laminae are pesent (facies 9). Near the threshold we were unable to penetrate the tsunami deposit with our hand-driven corer; it stopped in poorly sorted gravel.
This shows the coarsening of the bed towards the overflow threshold. Above it, facies 7 and 9 followed.
Ruttikuutnet l6 nt u.s.l.t', 5 6 m ubot)e SLT.
Ratvikvatnet (Fig. 12) is abour 200 x 300 m. Accord- lng to the sea-level curve (Svendsen & Mangerud
c o R E s 0 4 - 0 2 - 0 1 ,10 Stcirt Bottdctik t,t ul
F r ' q . 9 . L o s s o n i g r r i t i o n . radiocurbon uges and diatom anall.sis o1.
c o r r . 5 0 : 1 - 0 2 - 0 1 l l - o n r K v c n n a r a t n c t . A ntoss stem just beJoti.thc t s u n a m i d c p o s i t is t l u t c d to r l - i { ) + 8 { ) (T L r . t - 1 1 7 0 ) r i n d s h o u s t h u t t h c r e is r e r t l i t t l e u n c o n l b r r n i t l . a t t h i s c o r c s i t c . D i a t o n t s s h o u . t h e c r r c r q c n c c ( n t a r i n c - b r a c k i s h - l a c u s t r i n e t r a n s i t i o t r ) o f t h c b a s i n . T h c g l t t J a l r b o v e the tsunanti deposits is donrinated bv fl.eshwater t l i a t o n t s . s u g g e s l i n - s t l ' L t t K \ c n n J \ i l t n e t h r d lust..meruccl a b o r . e s c a l r ' \ c l \ \ lt c l l l \ u t ) i r n l i ( , e ( L l r r c J .
1987) the basin emerged fl-orn the sea around 9500 Bp.
I n s o n t e c o r e s th e t s u n a m i d e p o s i t s r c s t o n m a r i n e s i l t ( f a c i e s l) . whereas in other cores the erosion is Iess and up to 70 80 cm lacustrine gyttja is found between t h e m a r i r r c s e d i m e n t s a n d t h e t s u n a m i d e p o s i t s ( F i g .
l 4 ) . R c s t i n g o n t h e crosional unconformity. thl tsunanti deposits consist of facies 5 followccl by facies 6 and 7. Two woocl fragments within the deposit w,cre d a r e d r o 1 6 t 0 + 7 0 ( T - 1 0 5 9 6 ) a n d 7 2 3 0 i t O S l f _
1 0 5 9 7 ) ( T a b l e l . Fig. l4). and from rhe gyttja above t h e d e p o s i t . a d a r e o f 7 4 3 0 + 95 (T-10594,4. T a b l e l . F i g . l 4 t \ r r s o b t a i r ) c d .
Rutttlcit'ulnct. 20 nt u..t.l.t'. 9 l0 n uhot.c SLT.
R a u d i v a t n e t . a r a t h e r l a r g e lakc (400 m long). con- s i s t s o f t w o s u b - b a s i n s ( F i g s . l3 & l 4 ) . A cross-sectron o f 9 c o r e s in t h e i n n e r s u b - b a s i n ( F i e . l 3 ) shows facies 5 lbllowed by facies 7 in the lower pitrt of the lacus- t n n e s e q u e n c e ( F i g . l 4 ) . I n thc othcr sub-basln n e a r t h e o u t l e t . c o n s i d e r a b l e c r o s i o n i s e v i d e n t . a n c l a coarse sand to fine gravel bed with shell frugnrents rcsts on thc unconformity. The following organic dc_
t r i t u s ( f i r c i e s 7 ) i s s i m i l a r to t h c o n e lb u n d in the inner s u b - b a s i n . F a c i e s 9 i s p r e s e n t in the outer basin as a light brown lamirra followed by a dark greenish- b l a c k l a m i n a , b e d o f e l a s t i c g y t t j a . Severai meters
R E A S 1 6 ( 1 9 9 7 )
of lacustrine gyltja are prescnt above the tsunami o e p o s l t s .
Dlupt'ikruttrct 20 nt u.s.l.t'. 9 l0 n ubor.t, SLT.
D j u p v i k v a t n e r ( 1 5 0 m l o n g ) ( F i g . l3) reveals m u c h t h e same stratrgraphy and lircies association of thc t s u n a r r i d e p o s i t s a s i n R a u d i i v a t n e t . A p r o f i l e o f l 0 c o r e s w A S s a m p l c d fi . o r n the outlet towirids the distal ( w e s t e r n ) p a r t o 1 ' t h e b a s i n . A l o n g t h i s p r o f r l e ( F i g . l 3 ) t h e t s u n a n t i la c i e s r c s t on marine silt. and all the lacustrine gyttja have bcen erorcled. In onc of the c o r e s d i s t a l t o t h e o u t l e t . as much as I m lacustrine gytt;a was prescnt below facies 5. The tsunami deoosit i s t h i c k c s t in t h e w c s t e r n ( d i s t a l ) p a r t of the basirr.
where as much as 2 nr ol- x coi-lrse detritus gyttja with silt and line sand (facies 7) is presenl. Facies- 9 was found in the deeper part of the basin as a light brown lamina followcd by a dark greenish-black laminaibecl of an elastic gyttja On top of the tsunami deposits a p p r o x i m a t e l y 3 m o f lacustrine sediments can be found.
R o t e r u t n a r . 2 1 . 5 n r u . . E . l . t ' , l 0 I I m u b o r e S L T . T s u n a m i d e p o s i t s c o n s i s t i n g o f a l a m i n a o f f a c i e s 5 followed by facies 7 were lbr-rncl at thc basin outlet at t h e n o r t h w e s t e r n e n d of the basin (Fig. l4). The thickness and grain size of' facies 5 diminishes awav from the threshold. In the ir.rner part of the basiri.
between Rotevatnet ancl Otrevatnct (Fig. l3), no traces of tsunami lacies were recognized. Again. this d i s t r i b u t i o n a l p a t t e r n o f t h e l s u n a m i deposits. w i t h a steep sedimentological _eradicnt lionr tl-re outlet area t o w a r d s th c i n n e r p a r t of the basin is seen.
lluntrp on Sulu. If runup was much higher than Rote."'atnet. we would expect extensive erosion in the narrow sound bctwecn Rotevatnet and Otrevatnct.
Because no traces of the tsunami were founcl in this part, we assume that Rotevatnet was situated close to the uppcr limit of runup. The estimate lor SLT is l 0 I I m a . s . l . ( s e c a b o v e ) . Thus. according to these o b s e r v a l i o n s . r . r e e o n c l u d e ( h a l r u n u p o n S u l a is a b o u t
l 0 l l m a b o v e h i g h r i d e . Southern Sunnnrort'
Six basins have been studied on the islands of' Gurskoy, Lcinoy and Bergsoy (Fig. 12). These sites were all previously investigated as part of a sea_level s t u d y b y S v c n d s e n & M a n g c r u d (1 9 g 7 , 1 9 9 0 ) . I n 1 9 g 5 . S v e n d s e n ( 1 9 8 5 ) p u t l b r w a r d t h e hypothesis t h a t s e d i _ ments in Almestadmyra. Gurskoy. could have been deposited by a tsunami generated by the Storegga slidc. In the other basins Svendsen & Mangeiud ( 1 9 8 7 . 1 9 9 0 ) generally misinterpretcd the tsunami d e p o s i t s a s s e d i m e n t s d e p o s i t e j during the Tapes transgression. Thus, the 7000 Bp shoreline was ser roo h i g h .
GORRTJONNA I
1'r.q. /{/. C'ross- t h ic k n e ' s s
B O R I : . A S 1 6 ( 1 9 9 j )
N
T h e s t r a t i g r a p h y i n F r o y s t a d m y r a I. L e i n o y . i n d i - c a t e s th a t t h e t s u n a m i in u n d a t e d th e b a s i n w h c n s e a level was below its threshold. This implies that sea level was bclow 6.4 m. probably by onc to two metres at the tin.rc of the tsunami impact. Thc sea level at t s u n a m i ti m c o n L e i n o y is t h e r e f b r e s e t to 4 5 m a . s . l . Because the shorelines dip westwards. the sea level at tsunami time is estimatcd to be abor.rt 3 zl m on Bergsoy. about 6 7 m at Almcstadmyra on Gurskoy a r r d a b o u t 7 8 r n a t L i t l e v a t n e t ( G u r s k o y ) ( F i g . 4 ) . Kultttrntvru^ 2.5 nt u..s.1.". 0 I nt bclov' SLT. Bergsor.
Kulturmyra (Figs. 2. l2) was first isolated fron.r the s e a a r o u n d l 0 2 0 0 B P ( S v e n d s e n & M a n g e r u d 1 9 9 0 ) . The lacustrine gyttja above the isolation contact has a n c r o s i v e u p p e r b o u n d a r y ( F i g . l 5 ) . T h e g y t t j a r a r i e s in thickness fiom 3 cm to 104 cm. and in several cores it is in fact missing. The tsunamii lacies rest on this unconlbrmity and consist of alternating sand and o r g a n i c f a c i e s ( F i g . l 5 ) . T h e t s u n a m i u n i t i s u p t o 50 cm thick in thc deepest part of the basin where it contains three sand beds separated by organic lhcies.
ln general. the tsunami dcposits start with graded sand lacies on an erosional unconlbrmity and cnd w i t h o r g a n i c c l e t r i t u s f a c i e s ( F i g . l 5 ) .
T h e s a n d l a c i e s c o n t a i n s h e l l fr a g m e n t s a n d a b u n - dant fbraminifera domirratcd bv Cibicidcs lobutulus.
Tlu' Strtrcggu t.suttutrti: u.qt und runup ,11
a s h a l l o w m a r i n c , b r a c k i s h w a t c r s p e c i e s ( A u s t i n &
Sejrup 1994). A plant lragment fiom the organic cor.r_{lomcrate facies. possibly l.reather (Cullunu sp.) w a s d a t e d t o 8 2 8 5 + 1 8 5 B P ( T U a - 8 3 3 . T a b l c l ) . Abovc the tsur.rami facies. a thick unit of a marinc.
g r e y i s h b r o w n . o r g a n i c s i l t ( f a c i e s l) i s p r e s e n t (F i g . 1 5 ) s h o w i n g t h a t t h e b a s i n w a s s i t u a t e d b e l o w s e a level when the tsunami occurrcd. However" in all cores the tsunami fhcies rest on eithcr lacustrine gyttja ( l a c i e s 3 ) o r m a r i n e s e d i m e n t s o f Y o u n g e r D r y a s a g e ( 1 1 0 0 0 l 0 0 0 0 B P ) . A r a d i o c a r b o r . r d a t e j u s t b e l o w the tsunan.ri facics where the lacLrstrine sedimcnts are t h i c k e s t , y i e l d c d 8 3 4 0 + I l 5 B P ( T - l 1 8 3 7 4 . T a b l c l ) . Fra.t'stutlnryru I 6.1 trru.s.l.". I 2m ubote SLT, Lcinay. This basin is presently a bog situated at the h e a d o f a b a y o n L e i n o y ( F i g . l 2 ) . I t i s a b o u t 2 5 0 m long and 100 m wide with a bedrock threshold at thc western end.
The tsunami deposit rests on an erosional unconfbr- mity. In most cores the tsunami facies. previously inferred to be a result of the Tapes transgression ( S v e n d s e n & M a n g e r u d 1 9 9 0 ) . a r e l b u n d o n m a r i n e o r b r a c k i s h s i l t o f Y o u n g e r D r y a s a g e ( l I 0 0 0 1 0 0 0 0 B P ) . T h u s . a l l m a r i n c a n d l a c u s t r i n e s e d i m e n t s deposited at 10000 to 1200 BP have been removcd.
O n l y w i t h i n a s m a l l p a r t o f t h c b a s i n . d i s t a l t o t h e
4 l S t t i r t B r t r t r l t r i A t t , t l
d mil a f f i E I
ffiEl
ooofl._d l::l r I -
E--r=elt - - t l t - . " t t
t - t o l
l - l C l
J L : I i I -ru'-r l_:l I
/ ,t - *Eilil,
( r',
'222{/4
A Nesjsvatnet
0 5 1 0 k m
F i g . / / . I n r , c s t i g a t c c l b a s i n s in F r o s t a . N c s j o v a t n c t (8 l0m above S L T ) d i d n o t c o n t a i n a n 1 , p o s s i b l e tsunami cieposit. In Nesvatnet ( c l o s e to SLT.) a ntassive s i l t y c l a l . b e d c n c l o s e d i n l a r n i n a t e d g y t t j a m a r k c c l T . i s t h c o n l y p o s s i b l c candidate lbr a tsunarni deoosit.
E x p l a n a t i o n t o t h c l o g s i s -siven in Fig. 5. B: brackish.
outlct, a thin bcd of lacustrine gyttja was found betwecn thc lower marinc sediments and the erosional unconlbrmity (Fig. l5). A radiocarbon date from the basc of the lacustrine sediments yielded an age of 9 1 9 5 + 1 6 0 B P ( T - 1 1 2 4 8 , 4 . T a b l e l . F i g . t 5 ) . i n a g r e e - m e n t w i t h S v e n d s e n & M a n g e r u d (1 9 9 0 ) . T h e t s u n a m i deposit is characterizcd by several sand bedsilaminae (lacics 5) separated by organic detritus (facies 6 a n d 7 ) . l n t h c c e n t r a l p a r t o f t h e b a s i n the tsunamr fhcies are thickest (30 cm) and mainly composed of facies 6.
Thrce radiocarbon datcs have been obtained from w i t h i n t h e t s u n a m i d e p o s i t s . A twig was dated to 7 6 5 5 * 8 5 ( T U a - 5 5 3 . T a b l e l. F i g . l 5 ) a n d b u l k d a t c s o f t h e o r g a n i c d e t r i t u s fa c i e s y i e l d e d 7985 -F Il5 and 8 4 8 0 + 1 6 0 y e a r s B P ( T - l 1 2 4 6 , 4 a n d T - 4 9 4 6 7 4 . Table l ) . D i a t o m a n a l y s i s o f c o r e 5 0 4 - 0 7 - 0 1 ( F i g . l 6 ) shows t h a t t h c f i r s t l 0 c m a b o v e t h e t s u n a m i deposit (be- twecn 280 ttnd 210 cn.r) consists ol- lacustrine sedi- ments. Then follows an interval (between 270 and 255 crn) dominated by marine diatoms that probably rcflect that relative sea levcl had risen very close to.
B O R E A S 1 6 ( 1 9 9 7 )
and probably slightly above rhc thrcshold <luring high tidc. This high sea-level stand is interpreted to be the Tapes transgression which culminated after the tsunami impact. dated to about 6300 Bp bv Bierk (1982). However. it is puzzling that we were unubl.
visually or by loss on ignition data to identify the transgression sediments.
Alntestrutlntyru 13.7 nt tt..t.1.". 6 7 m ubot:e SLf.
Gurskoy. The bog is situated in a valley that extends from the head of a small fiord. Gursken, on Gurskoy ( F i g . l 2 ) . T h e outler is across a till threshold at the western end. The basin contains a marine and brack- ish sediment sequence, followed by Holocene lacus- trine gyttja. Howevcr, an crosional unconformitv can be traced throughout the basin. ancl in most places the e a r l y H o l o c e n e s e d i m e n t s a r e r e m o v e d (F i g . l 5 ) . T h e tsunami deposits are recognized as an 0.5-cm-thick sand,fine gravel lan-rinae (lacies 5) resting on this unconformity. The sand is followed bv orsanic con- glomerate (facies 6) and organic detritus lfacies 7).
The latter facies bccomes a silty gyttja in the upper part and grades into a brown lacustrine gyttja. The t h i c k n e s s o f t h e t s u n a m i d e p o s i t s is l0 30cm. A radiocarbon (AMS) date of a small twig from the base of the gyttja above thc rsunami lacieslielded an age o f 7 0 8 0 + 8 0 ( T U a - 5 2 2 . T a b l e I . F i e . 15).
The threshold of the basiu is 4 niabove the Tapes transgression maximum level in this valley and rules o u t t h e p o s s i b i l i t y t h a t t h e s a n d w a s d e p b s i t e d b y a m a r i n e t r a n s g r e s s i o n .
Skolenr.tru 12.4 m u.s.1.". I 9 m ubore SLT, Berssot.
- The pre-Holocene sediments in Skolemyra (Figs. 2
& 12) have previously been studied by Svendsen &
Mangerud (1990). They dated the marine lacustrine boundary to l2 300 BP. A cross-section of the basin ( F i g . 1 7 ) r e v c a l s a chaotic mixture of tsunami de- posits, mainly of facies 6 and 7. resting on an ero- s i o n l r l u n c o n l o r m i t y . l n n r a n y p l a c e s the erosion sropped on rop of the veddc Ash Bed, 10 300 Bp (Fie.
l 7 ) ( M a n g e r u d a t a l . 1 9 8 4 ) . T h e c l a s t s c o n s i s t o f e r &
silt, Vedde Ash. peat (seldom > I cm) and big lumpi of'gyttja (light brown and brown) that filled the l0 cm d i a m e t e r o f t h e c o r e r a n d c o u l d b e l 0 30 cm thick. In addition. thin laminae of medium to fine sand (facies 5) are present. The tsunami deposits are typically 50 70 cm thick in the central parts of the basin and end with two 2 3-mm-thick laminae (facies 9). The lower lamina is yellowish brown. whereas the upper is d i s t i n c t l y b l a c k .
In the thin sand laminae (facies 5) the agglutinated loraminifera Eggerelloidcs .st,uhrunt is quite common.
No other foraminifera species r,',ere lound. According to Austin & Sejrup (1994) this species dominates in s h a l l o w ( 1 . 5 4 m d e p t h s ) s e a water with salinites b e - t w e e n 2 5 % o a n d 3 0 % u in w e s t e r n Norwav. ln Kul- t u r m y r a a n d K v e n n a v a t n e t t o o . ( d e s c r i b e d a b o v e ) ,6 .
B O I T E A S 1 6 ( 1 9 9 7 )
F i g . 1 2 . I n v c s t i - s a t e d b a s i n s o n S u l a a n d s o u l h e r n S u n n m o r c . S o u t h c r n Sunnmore i r . r c l u d e s t h e i s l a n d s o 1 ' B e r g s o y . L e i n o y a n d G u r s k o r ' .
s('ubrum was found in f-acies 5. It is likely that cal- careous foraminifera were also originally deposite<l, but because of the acid organic sediments they later on have been dissolved. Several radiocarbon ages from within and above the tsunami deposits havc been o b t a i n e d ( F i g . 1 5 . T a b l c l ) . T h e y w i l l be discussed b e l o w .
Runttp in soutltcrn Sunnnrore. To determine the runup in the area we cored a few basins situated over l 0 m a b o v e S L T . I n F r o y s t a d m y r a I I ( 1 8 m a . s . l . ) a n d L i t l e v a t n e t ( 1 9 . 2 m a . s . l . ) ( F i g . 1 2 ) , b o t h t 2 l 3 m above SLT, no sign of tsunami facies was found. The large volume ol sedin.rents eroded by the tsunarni in Skolemyra indicates a rapid flow into the basin, and s o t h e t s u n a m i w a s w e l l a b o v e t h c 8 9-m threshold.
The cstimatecl runup must therelbre be somewhere w i t h i n t h e i n t e r v a l 9 1 3 m a . s . l . T h e observed hori- z o n t a l r u n u p i s l a r g e s t in G u r s k e n ( F i g . l2). The distance from the innermost core in Almestadmyra a n d t h e S L T a t 6 I m a . s . l . s h o w s th a t the horizontal r u n u p i s m o r c t h a n 7 5 0 m .
Au.;trheinr
The Austrheim area is situated 50 km north of Bergcn ( F i g . l ) . B a s e d on the stratigraphy i n l a k e b a s i n s . a Holocene sea-level curvc was constructed by Kaland ( 1 9 8 4 ) . A s p a r t o f t h e t s u n a m i p r o j e c t , s i x lake basins
!ou9 been investigated: Longevatnet, Arisvatnet.
Forlansvatnet. Auretjorn. Aseetjorn and Mjosvatnet
Tha Stttreggu tsurtunti: age und runup 43
(Fig. l8). All basins are siruated above the 7200 Bp s e a le v e l . w h i c h i s e s t i m a t e d t o b e l 0 l l m a . s . l . (K a - l a n d 1 9 8 4 ) .
Longerutnet, 10.9 nt u.s.1.". 0 I m ubote SLT.
Kaland (1984) described a silt,.sand horizon and other signs of erosion and resedimentation just above the Preboreal isolation in Longevatnet (layer no. 6, pp.
2 1 4 i n K a l a n d ( 1 9 8 4 ) ) . T h e f o l l o w i n g gytrja, with some plant fragments, had a very high content of minerogenic particles. During a re-examination of the stratigraphy we identilied these sediments as tsunami facies 5 and 7 (Fig. l9). Above the rsunami deposits a l0-cm bed of facies 3 is present where the loss of ignition increases abruptly. The lacustrine gyrrja is overlain by a laminated black-green brackish water gytt1a. A radiocarbon date of the lacustrine gyttja just below the brackish warer gyrrja yielded 7190 + g0 ( T - 2 2 1 3 8 , Table l. Fig. 19) which is a minin'rum ase of the tsunami. The brackish sediments show that tiie high tide was closc to threshold of the basin rieht after t h e t s u n a m i .
Farlund:;t:utttat. l2 nr u..s.1.", I 2 rn uhot:c SLT. The s e d i m e n t s in F o r l a n d s v a t n e t ( 8 1 0 m x 80 m) were cored near the inlet to the basin. Similar tsunami facies as those in Asetjorn were idcntified (Fig.
1 9 ) ( s e e b e l o w ) . G y t t j a ju s t a b o v e th e tsunami deposit w a s d a t e d t o 1 4 7 5 + 1 1 0 B P ( T - 1 1 7 0 8 4 , T a b l e l . F i g . l 9 ) .
R o t e v a t n e t l - Ratvikvatnet
L i t l e v a t n e t
