glaciomarine varves and the character of deglaciation, säveån valley, southwestern sweden

Glaciomarine varves and the character of deglaciation, Saveiin valley, southwestern Sweden RODNEY STEVENS

Stevens, Rodney 1986 12 01: Glaciomarine varves and the character of deglaciation, Savein valley, southwestern Sweden. Boreas, VOI. 15, pp. 289-299. OSIO. ISSN 0300-9483.

GIaciomarine varves, in contrast to glaciolacustrine varves, are primarily dependent upon sedimentation from meltwater overflow. They are usually developed in proximal positions and are a more reliable reflection of deglaciation character within a specific area than ‘classical’ glaciolacustrine varves, which are generally more distal and greater influenced by bottom topography. The close relationship with ice-front processes in the glaciomarine environment is discussed and utilized to suggest correlations between the varve stratigraphy, ice-front positions and climate shifts during the deglaciation of the Saveen valley, where two varve localities have been documented. A varve sequence outside this valley shows similar general trends in varve-thickness variation, and comparison between localities may help in extending the lines connecting positions of concurrent ice-marginal deposition. The study of glaciomarine varves provides a more continuous record of changes in the ice-front character than can be obtained from intermittent moraine positions.

Rodney Stevens, Department of Geology, University of GoteborglChalmers University of Technology, S- 412 96 Goteborg, Sweden; 10th June, 1985 (revised 22nd March, 1986).

BOREAS

Glaciolacustrine varves have played a major role in the dating of deglaciation. Because ‘classical’ varves are distal (Ashley 1975) they are respon- sive to major drainage events and trends, but not the deglaciation character near a particular locality. De Geer (1940) concluded that proximal glaciolacustrine varves were too irregular to be used in geochronologic work. This is especially true when considering the coarser summer intervals deposited from turbidity currents and underflows which are influenced by bottom topography. On the other hand, glaciomarine varves are, when well developed, usually proximal and sedi- mented along meltwater drainageways, and they would expectedly have a closer association with ice-front conditions than distal varves (Stevens 1985). Glaciomarine varves would, furthermore, be expected to be less disturbed by bottom topography than proximal glaciolacustrine varves since both summer and winter portions are often the result of mainly overflow sedimentation. Combined with the possibilities of varve dating and correlation, studies of glaciomarine sediments may be an effective way to complement the discontinuous and generalized record of deglaciation derived from coarser deposits. The thickness variations of glaciomarine varves are, as interpreted and utilized below, tangible re- sponses to the deglaciation conditions. But even non-textural changes, such as color and sediment chemistry, may provide valuable detail reflecting

sedimentation both within the individual varve- years and over longer periods of environment evolution.

The character of the Weichselian deglaciation in southwestern Sweden has, as elsewhere, been largely based upon the mapping and interpretation of coarser moraine deposits. The textural variations within the glaciomarine clay sequences have usually been considered too subtle and too dependent upon marine processes to be reliable as a reflection of the deglaciation character. These reservations are justified if environmental factors cannot be further specified within a depositional model. Recent investigations of modern glaciomarine environments (bibliography in Andrews & Matsch 1983) have greatly increased the possibilities for detailed sedimentologic inter- pretations (e.g. Domack 1984; Mackiewicz et al. 1984). If textural changes in the glaciomarine sediments can be related to ice-front conditions and events, as suggested below by an extention of the depositional model, a more continuous and sensitive record of deglaciation is made available.

The general Weichselian deglaciation along the Savegn valley near Goteborg (Fig. 1) is known from the regional mapping and stratigraphic work of Magnusson (1978), Hillden (1979), Hill- efors (1969, 1979) and others. Recession eastward from the Gothenburg Moraine to the Berghem Moraine is suggested by Berglund (1979) and Lagerlund et al. (1983) to have taken

290 Rodney Stevens BOREAS 15 (1986)

approximately 200 years, whereas Hillden (1979) interpreted the initial deglaciation to have taken only 100 years, followed by a glacial readvance that lasted 100 years before the ice front receded again to Berghem's moraine. These hypotheses, as well as the more detailed character of deglaciation within the Savein valley, are reconsidered after interpretation of the clay sequences at two localities which include varved sediments. A comparison with the varve stratigraphy at a site outside of the valley further illustrates the use of glaciomarine varves to help extend and correlate moraine lines.

Stratigraphy A description of the cored stratigraphy at the locality Kviberg (core L-1, 23 rn) is given in Fig. 2 . The boring stopped at the coarser underlying, friction material. The clay is generally heavy and plastic, but the silt content and the occurrence of silt and sand lamina decrease upwards. The uppermost 2-3 m are siltier again. Isolated pebbles and shells occur mainly above 9 in.

Fig. 1. The localities are shown on 3 paleogeographic map with the distribution of land and sea at the beginning of Berghem Mor- aine deposition, c. 12,400 B.P. The position of the Gothenburg Moraine (c. 12,80&12,600 B.P.) is also indicated. Datings from Berglund (1979).

The lower half of the stratigraphy is further distinguished by rhythmites which are defined by cyclic changes in color, texture, carbonate content and organic content (Stevens 1985). Olive gray (8 Y 611, Munsell color scale) bands, usually 1-10 crn thick, are consistently coarser grained and have a greater frequency of coarser lamina than in adjacent brownish gray (2 Y 60) intervals. The gray bands also have relatively lower contents of carbonate and oxidizable organic material. The average thickness of the rhythmite couplets is 5.5 cm. They are thickest near the bottom. although with notable variation (Fig. 3). Within the banded interval 230 rhythmites were distinguished. The rhythmic character becomes very indistinct at 10 m and may have been in part destroyed by bioturbation which was observed at higher levels.

The stratigraphy at Lerum, 500 m east of Le- rum's church (Figs. 1 and 6 ) , has been studied in cooperation with J. Janovskis (Univ. of Gote- borg) and is summarized in Fig. 4. The generally fining upward trend is similar to that observed at Kviberg, but differing at 12 m where a coarser silt and sand interval occurs and in the uppermost

BOREAS 15 (1986)

lrll c p! . . . . . . .

blri lC

...... ......

...... ......

b r i C

. . . . . . . . . . . . Fig. 2. The stratigraphy of core L-1 at Kviberg (27.5 m above sea level). The facies C-H correspond to those in Fig. 5 .

n 0

5

10

15

20

I

Varves and deglaciation 291

S l i g h t l y s i l t y clay w i t h some p l a n t fragments. Uneven s i l t d i s t r i b u t i o n . bu t l e s s s i l t y towards the bottom o f i n t e r v a l . S i l t laminae a t 1.07 and 1 .36 m. L i g h t g reen ish gray ( 5 GY 7/1) i n general and l i g h t brownish gray (5 YR 6/11 i n c e r t a i n s i l t i e r p o r t i o n s . A few roo ts . Some g rave l p a r t i c l e s .

S l i g h t l y s i l t y c l a y w i t h p l a n t fragments. Greenish gray ( 5 GV 6 / l ) . S h e l l . s h e l l fragments and pebbles (5-20 mm) occur i s o l a t e d s p o r a d i c a l l y w i t h i n the c l a y . Several roo t holes. Genera l l y mare homogeneous than the ove r - l y i n g c l a y i n t e r v a l .

Clay w i t h s w e p l a n t fragments. Greenish gray ( 5 GY 6 / l ) , Some sma l i , b iack - c o l o r e d pockets of probable i r o n s u l f i d e which fade w i t h a i r exposure. Less green ish downwards. The clay i s o f t e n blue-gray (8 B 4/1) when f i r s t exposed. There i s a s i i g h t suggest ion o f banding such a s t h a t desc r ibed below. The c l a y i s ve ry p l a s t i c (heavy) between 5.5 and 5.9 m , w h i l e o the rw ise i t i s very s l i g h t l y s i l t y . Th i s i n t e r v a l a l s o lacks pebbles and s h e l l fragments which occur elsewhere m s t abundant above 5 rn.

Banded c l a y . The banding i s co lo red a5 below but the t e x t u r a l d i f f e r e n c e s

o f t e n somewhat t h i c k e r and i n some cases are n o t d i s t i n c t . The banding i t s e l f becomes mre i n d i s t i n c t towards the top. The gray bands a r e nast s l i g h t l y s i l t i e r than the brownish bands. Black pockets (usually l e s s than I m) o f FeS(7) m i n e r a l i z a t i o n and p o s s i b l e gas ho les were observed w i t h i n the gray bands. Only one s h e l l (9.25 m) and one pebble (8.75) were observed.

Banded, s l i g h t l y s i l t y clay. The banding i s a s desc r ibed below. B lack - co lo red pockets o f m i n e r a l i z a t i o n s (FeS?) w i t h i n some o f t h e gray bands. Somewhat h ighe r s i l t con ten t i n the gray bands. The o v e r a l l s i l t con ten t decreases upwards. No s h e l l s or pebbles.

Banded, s l i g h t l y s i l t y c l a y w i t h t h i n s i l t l ayers . The banding i s a5 desc r ibed below. The s i l t layers (5-20 mm t h i c k ) occur w i t h i n the gray bands and a re graded i n some cases near the bottom. Thin, dark bands and b lack pockets o f FeS(?) m i n e r a l i z a t i o n a l s o occur w i t h i n some gray bands. The brownish bands a r e f i n e r - g r a i n e d and - re homogeneous. No shells or pebbles.

Banded, s i l t y c l a y w i t h t h i n s i l t and sand layers. The banding i s composed o f o l i ve -g ray ( 8 V 6/1) and brownish, o l i v e - g r a y (2 Y 6/1) i n t e r v a l s . The gray bands a re usually S i l t i e r , t h i c k e r and more f r e q u e n t l y c o n t a i n s i l t and sand laye rs . Several o f t he coa rse r i n t e r v a l s a r e graded. The s i l t and sand content i n bo th the brown and gray bands i s h ighes t near t he bottom of the core. No s h e l l s and o n l y two pebbles (19.8 and 23.0 m) were observed.

portion which is also coarser. Rhythmites similar to those at Kviberg are developed in the lower 6 m. The thickness of the couplets, represented by a moving-average curve (Fig. 4), varies considerably. The greatest thicknesses are at 13.4 and 17 m. The banded interval contains approximately 120 rhythmites.

Depositional conditions The evolution of geographic environments in southwestern Sweden in response to the isostatic land uplift has been characteristic of the particular paleogeographic conditions of each area (Ste- vens et al. 1984). Fig. 5 shows a generalized stratigraphic sequence which might be expected within a typical valley due to the changing environmental conditions. The stratigraphic facies

(A-H) are texturally defined and intended to be nongenetic. These facies can often be recognized in described sections and may help to organize observations. A major factor not incorporated in the generalized paleogeographic model, but considered below, is the ultimate control of climate upon meltwater and sediment supply from the glacier.

Although considerably different in detail, the stratigraphic trends observed at both Kviberg and Lerum show a similarity to those in the the- oretical model derived from paleogeographic considerations. The facies transitions C-D-E in connection with the fining-upward trend are interpreted to relate to ice recession and differen- tial settling of sand, silt and clay over increasingly longer transport distances. Nearshore reworking of older deposits was a secondary sediment source that became increasingly important with

292 Rodney Stevens

E

D/ E

D

c

BOREAS 15 (1986)

(ri) C . . . (vxl . . . . -- - -

( 5 i ) C V X

_--_- c I v x )

C

c ( v x l

----

b C

_--- b ( s i ) C

---_ . . . . . .

. . . . . . ......

. . . . . . . . . . . . b l s i l C 1

. . . . . . . --- ...... ...... b s i C . . . &is . . . . ...... . . . . . ...... ...... ...... . . . . . . . ...... . . . . . . , ......

H 0

5

10

15

20

6

10 2

\

\ \

\ \

\ /

/ Ice- I ont positions

+ I

0 10 RHYTHMITE THICKNESS, c m

E e %

4- c 0 U

al w

a

k >

228 220

195

114

160

142

121

102

18

65

52

38

25 Fig. 3. A smoorhcd varvc-thick- 15 ness curve using a 50-em moving average with 25-em steps for the

4 Kviberg core. Suggested ice-front

1 positions are numbered with respect to their correlations in Fig. 6 .

time and land uplift as the shorelines became lower and closer to valley localities. Due to the relative proximity of the shore source, coarser material could be reintroduced to a site at the same time as the meltwater-supplied sediment diminished and became finer grained. The most

obvious expression of this is the coarse conclu- sion of each sequence (facies H). Facies F and G in Fig. 5 are mainly characteristic for Holocene deposition, which is lacking at Kviberg and Le- rum due to the land uplift above sea level.

The rhythmites at Kviberg (the banded clay

BOREAS 1.5 (1986)

M

Vurves and degluciution 293

Fig. 4. The stratigraphy at Lerum and a smoothed varve-thickness curve. The levels 4 and 5 refer to ice-front positions in Fig. 6.

18.15

Clayey, silty sand with plant fragments. Sediments disturbed during sampling. Considerable variation in grain size and sorting. Several thin, organic rich layers. Silt and clay balls observed at 2- 2.5 m. Lenses of clay near bottom.

Slightly silty clay. Shell and shell fragments and isolated pebbles occur below 7 m, most abundant at 9 and 10 m. Dark gray ( N 4 - N 3 ) .

Silty sand and clayey silt. Some pebbles. Only partially recovered in coring.

Banded, silty clay. Bands of dark, yellowish brown fine clay (10 YR 2/21 and thin, olive gray bands (5 Y 4 / l ) . The clay is sandy and has silt lamina in the lowermost 1 . 5 m. Seauence underlain by

deposits) have been discussed earlier (Stevens 1985) and are interpreted to be varves, that is, yearly accumulations. A depositional model for varve development in glaciomarine environments such as in the Savein valley has been presented (Stevens 1985) and will not be dealt with in depth here. Meltwater influence is considered an es- sential prerequisite for varve development. Den- sity stratification, which is best developed during summer meltwater outflow, allows for better sep- aration of silt and clay fractions by limiting the initial effects of coagulation. Furthermore, water stratification inhibits vertical mixing and pre- serves the transporting competence of a meltwater sediment plume. The surface supply of dis- solved oxygen to deeper waters is also limited fa- voring a more reducing geochemical environment during the summer, whereas better mixing and slower sedimentation favor oxidized, brown,

Varve count from bottom 109

101

I1 51 45 32 8

cm RHMHMITE THICKNESS

finer-grained winter sediments. The seasonal fluctuation of sediment supply and meltwater-influenced conditions result in the cyclic deposition interpreted as yearly varves.

A transition to more distal conditions is best reflected upward in the Kviberg stratigraphy where the varves become poorly developed above 10 m. Varves are lacking at both the Kvi- berg and Lerum localities in the upper portions. Distally within the glaciomarine environment or stratigraphically upward during ice recession, water stratification would be expected to be less well developed and distinct varve deposition less probable. Bottom-feeding organisms, perhaps limited by extreme sedimentation rates in proximal environments, should increase in more distal positions and represent a further deterrent for varve preservation.

BOREAS 15 (1986) 294 Rodney Stevens

STRATIGRAPHY

T e x t u r e . . . Facies , , dB ,.:..

...... -

w H

9 E L,

..I W

L-1 a E 0 e: a

I I

i I

w z K

z

U L-1 W

4

€- vi

CI

8

H n

I I

I I I

w z 2 e z oi

I

I I I

- p r e d o m i n a n t s e d i m e n t types

--- o t h e r possible s e d i m e n t s

0 w w a a

m a Y n w

w a o e 2.2 m m

D

€- V w r 0

a

I I I

I i I

> w L-1

> n 0

I

I

z n UI a: m

w Fl

4 0 0 H

n

I I I n 0 4 .. w C

m

5-100 m Fig. 5. A generalized stratigraphy and possible associations with environments of deposition for a typical locality below the marine limit in southwestern Sweden. The facies A-H are described in Stevens er al. (1984).

Sedimentation rates The thicknesses of the varves at Kviberg, which show an overall thinning trend upwards (Fig. 3), agree with the distally decreasing depositional rates observed in modern glaciomarine environments (Elverhpri et ~1.1980; Syvitski & Murray 1981). Climate is often assumed to be the major control of glacial discharge, and climatic variation is, as argued below, likely a major cause of deviation from the upward thinning trend. How- ever, a combination of factors may have been important. Surface overflow may itself be directed by: (1) fallwinds, driving flow in a preferred direction, (2) opposing winds (westerly in this case) and tidal forcing, preventing effective overflow transport, (3) the Coriolis effect, turning flow preferentially to the right (north) along the valley side, and (4) valley topography, restricting outflow direction and spreading. Proglacial bottom topography is not as important to overflow transport as it is to the underflows more characteristic of glaciolacustrine conditions (Elverhoi 1984), although underflows may develop even within gla-

ciomarine settings with very high suspension con- centrations (Powell 1984).

Proglacial sedimentation may also be affected by the character of the ice front and its retreat. Increased sedimentation occurs, for instance, during and after a glacial surge due to several factors (Elverhd 1984): (1) increased meltwater discharge due to frictional heating during a surge, (2) radical change of subglacial meltwater chan- nels, and (3) erosion of glaciomarine sediments reworked by the surging glacier. Surges are a common mode of advance for some glaciers (e.g. on Svalbard, Liestd 1969) and have been related to changes in the balance of shear-stress conditions near the base of the glacier. Ice-tempera- ture changes, generally related to climatic changes, can be responsible, but even topographic irregularities may induce ice damming and subsequent release as a surge (Robin 1969). During periods of general retreat and low glacial gradients surges may not have been common, despite the otherwise favorable topographic conditions along the Saveln valley and the interpreted climatic fluctuations. A stationary ice front

BOREAS 15 (1986) Varves and deglaciation 295

Fig. 6. Ice-front positions along the Savein valley and their possible continuation to the north, east of the Rished locality. Glacioflu- vial sediments, mainlv from mareinal deoosition are shown in black (map data from Magnusson 1978 and Adrielsson & Freden

I

1985).

would, however, also tend to accumulate a thicker bed of glacial debris that would suhse- quently be exposed to erosion with renewed meltwater intensity and perhaps new subglacial drainage patterns; and this would, as in the case of glacial surges, tend to increase proglacial sedimentation.

A step-wise deglaciation along the Savein valley is evidenced by the spacing of thicker till and glaciofluvial deposits indicated as ice-front positions in Fig. 6. Climate is suggested to have been dominantly important in governing when the ice melting would be greatest or least, while topography was an important control upon where the ice front would position itself during these periods. The combined influence of climatic change and

topographic irregularity along a valley reinforce the periodic character of each other such that the rapid retreat over deep and open areas, especially during warm periods, is slowed or stopped during colder phases when the most stable ice- front position is in shallow and narrow areas with more restricted calving. The occurrence of topographic irregularities along the Savein valley and the associated deposits appears to be somewhat regular and may be related to the spacing of zones of conjugate-shear fracturing in connection with pre-Quaternary, east-west compression. A spacing of 2-5 km between major zones seems apparent and may be structurally related to the optimal size of crustal blocks in this area (B. Ronge pers. comm.).


As suggested above, the climatic influence is believed to have been the most dominant control upon meltwater outflow and, thereby, the transport and eventual deposition of glaciomarine sediments. This influence is obviously implied on a yearly scale by the occurrence of varves. Cyclic climatic influence is, furthermore, suggested over even longer periods by the regular spacing of sedimentation highs and lows in Fig. 3, also listed in Table 1. The highest multiple which can be fit- ted to the interval-data is 11.4 years, which is very near the frequency of the 11-year sun-spot cycle. This factor has been often interpreted to be reflected in the thicknesses of glaciolacustrine varves (Schove 1978). There has also been considerable skepticism, especially in Sweden. This is. perhaps, partially a reaction to the extreme influence attributed to the sun-spot cycle by De- Geer (1940), the founder of the Swedish varve chronology. The standard deviation from the 11.4-year value is only 1.66 years for the 11 observed intervals. Although not conclusive in this single example, there is a significant data trend which could be explained by regular variations in solar radiation, presumably further modified by the sedimentation factors discussed above. The rhythmites at Lerum have thickness maxima at 32,57 and 109 varve-years counted from the bottom (Fig. 4). The resolution in this shorter sequence is poor and possible small variations are obscured by factors related to the locality's close proximity to the ice front throughout the time of varve deposition.

Table 1. Relative dating of the sedimentation-rate (varve-thickness) tops from Fig. 3 and their spacing.

Varve count Time interval from bottom (years) Factored;

13 20 1')

13 10 X ?

9.5 x2 9 9

12 12 27 13.5 x 2 24 12 x 2 14 I4 23 11.5 x 2 18 9 x2 38 12.7 x 3

3 / 66

105 129 143 166

222

N = 11; X = 11.47 years; S = 1.66 years. * Largest multiple. but less than twice the smallest observed in-

78

I n4

terval.

Deglaciation along the Savefin valley (1) The first, well-developed ice-marginal deposits east of the Gothenburg Moraine are at Utby (Fig. 6). When the ice front had receded to this position the locality Kviberg had become ice free. The initial sedimentation rates as indicated by the varve thicknesses in Fig. 3 were extremely rapid (10-20 cdyear ) , but decreased quickly to a minimum which is believed to correspond to the lower meltwater outflow during a colder period. The ice front during this break in its retreat would have been most stable at Utby due to the narrower and shallower conditions. The deposits here are interpreted to correspond to this cold phase and the period of warming prior to renewed retreat.

(2) Following retreat from Utby, the sedimentation rate increased again to nearly 15 cdyea r for a short period, leading to a rapid retreat over the open and formerly deep area east of Utby. A new cold period which could correspond to an approximate ice position at Partille is indicated at about varve number 32 (counting upwards) in the Kviberg varve stratigraphy. Thick sequences of glaciofluvial deposits within a wide zone are known from borings at Partille (Magnusson 1978), which may, along with the thinner varves at Kviberg, suggest a colder period over approximately 10 years.

(3) The next documented area of significant coarse glacial material is along the narrow valley section near Jonsered, which is also an extended zone rather than a single defined position. There is considerable variation in the varve thicknesses so that several phases may be represented, mak- ing an exact correlation with the ice-front positions rather uncertain here. The deposits here and at Partille (above) illustrate the tendency for ice-front positioning in the shallow and narrow portions of the valley during standstills or periods of slower retreat. (4) Help in correlating the ice-front deposits

near Stenkullen with the varve stratigraphy at Kviberg is gained by comparison with the Lerum core (Fig. 4). where 120 varves are represented. By assuming that the 100-varve difference between the cores is roughly equivalent to the time before Lerum became ice free, the 120 varves at Lerum would correspond to the sedimentation above 16 m at Kviberg (Fig. 3). Correlation between proximal glaciomarine sequences is com- plicated by variable processes so that the timing

BOREAS 15 (1986) Vurves and degluciution 297

5 10

G

- F

D/ E -

. C

Fig. 7. The stratigraphic column and a smoothed varve-thickness diagram for the deposits at Rished. The ice-front positions refer to Fig. 6 and are discussed in the text.

I ce-fron t p o s i t i o n s

6

of large variations is of greater importance than specific varve thicknesses. A climatic ameliora- tion and subsequent retreat from a position approximately at Stenkullen, which is just east of the Lerum, would be represented by the top in varve thicknesses (8-10 cdyear) recorded at both localities.

(5) Given the relative correlation between the upper portions of the Kviberg varve sequence and the Lerum varves, a similarity can be seen with respect to the major trends. However, the area between Stenkullen and Skallsjo contains considerable coarse material and the variation in the varve thicknesses suggests several stages of different meltwater-flow conditions. The correlation between the varve sequences and particular ice-front positions is therefore somewhat arbi- trary in this area. The last well-defined top in varve thicknesses at both localities is conspicuous (1@18 cm) and may possibly represent a readvance or surge from the restricted valley area east of Skallsjo. Hillefors (1969) observed 50 varves overlain by glaciofluvial sands in a gravel pit to the west of Skallsjo, a sequence most easily explained by a readvance. Glacial readvance from the Berghem Moraine has also been suggested in areas to the south by HilldCn (1979) and Lager- lund et al. (1983) and at Gribo to the north by Hillefors (1969). The sharp peak in varve thick-

RHYTHMITE THICKNESS, cm

E % n E e 'c

+ c 3 0

al "

5

1 > I

19 66 57 43 25 13 0

I 10

nesses at Kviberg, despite the more distal position, indicates a major change in the suspended sediment supply. The broader peak at Lerum is perhaps due to the combined influence of a closer ice front as well as the increase in transported sediment. The lower absolute sedimentation rate at Lerum could be explained by a tunnel position on the opposite (south) side of the valley where the Skallsjo delta deposits are located (Hillefors 1969).

(6) The varves at Kviberg become less distinct upwards, but a final, cold period with lowered sedimentation may be represented at about 11 m. The ice front was then presumably within the Berghem Moraine zone, an area with considerable till and glaciofluvium but representing uncertain ice-front positions other than those suggested at positions 5 and 6 (cf. Morner 1969 and HilldCn 1979). Following the retreat from this zone the ice soon left the principal drainage area of the Saveln valley. This last retreat with more rapid melting and sediment supply may, however, be suggested in the poorly defined increase in varve thicknesses at 10 m. The sand and silt interval overlying the varved interval at Lerum also reflects a period of increased flow and sediment transport. Subsequent drainage from the ice flowed primarily along the wider Larjeln valley to the north. The sedimentation rates are ex-

20 Boreas 15:4 1986


pected to have greatly decreased (to about 5% of the previous rate) so that the upper 9-12 m of each core represent approximately 3,000 years of sedimentation. The subsiding shorelines passed these localities at about 10,OOO B.P. (Svedhage 1985).

As mentioned earlier, Berglund (1979) has es- timated from I4C dates that the ice retreat from the Gothenburg Moraine to the Berghem Mor- aine took 200 years. The interpretation of varve stratigraphy would favor a slightly shorter period to include an approximate 20 years before the front had passed the Kviberg locality and the 155 varves deposited at Kviberg until the ice reached the beginning of the Berghem Moraine zone at Skallsjo (position 5, Fig. 6). The agreement is, nevertheless, good, but it should be noted that there is also some uncertainty about the continuation of the Berghem Moraine to the south so that Berglund’s datings may not correspond to the Skallsjii-GrHbo position (5) in Fig. 6. A major readvance as early as after 100 years (HilldCn 1979) is not supported by the varve sequences, but may have occurred after 175 years (20+ 155) as suggested above.

Johansson (1956) and Hillefors (1979) have suggested that glacial ice moving down the valley sides was an important process during the final deglaciation. Evidence for this could not be infer- red from the varve character. Although this sediment source would be difficult to distinguish in fine sediments without specific lead components, the chance of large amounts of sediment being supplied is small due to the limited meltwater drainage perpendicular to the valley.

Comparisons with a varve sequence outside the Savein valley The locality Rished (Alafors) is located approximately 19 km north of Kviberg in a side valley extending eastward from the larger Gotaalv valley (Figs. 1 and 6). Similar to the SaveHn valley, the drainage westward past Rished was important until the ice had reached the east end of the valley. But in this case the glacial meltwaters were directed by the topography northward after the ice margin receded beyond Kilanda, which lies at the beginning of the Berghem Moraine zone (position 5, Fig. 6).

The stratigraphy at Rished (Fig. 7) shows the fining upward trend observed at the other localities (facies C-D-E). Rhythmites are devel-

oped within two intervals. The upper rhythmites are iron-sulfide bands believed to be organic varves related to early Holocene drainage from the Baltic area. Cat0 (1982) has discussed similar deposits in this region. The lower rhythmites are interpreted to be proximal glaciomarine varves comparable to those in the Savein valley. The varve-thickness diagram (Fig. 7) is useful in a similar way for relating the meltwater supply of sediment to the character of deglaciation at different ice-front positions.

The 80 varves recorded at Rished developed in connection with substantial meltwater flow and drainage past the locality. Only two marked decreases in varve thickness occur which may be interpreted as cold-induced disruptions in the ice retreat. These would most likely correspond to the deposition of the ice-marginal deposits at Starrkarr and east of Kilanda (positions 4 and 5 , Fig. 6). The first cold period appears to have been short since the deposition rates decreased to less then 3 cmlyear for only a few varves. This low, however, may have been limited in its beginning by the vicinity of the ice front. A following period of rapid sedimentation lasted for nearly 50 years. The overlying sand layers are not possible to interpret with much certainty, but they could possibly be explained by a readvance such as that suggested for the similar stratigraphic position at Kviberg and Lerum.

The 50 varves corresponding to the recession between Starrkarr (4) and the beginning of the Berghem Moraine zone (5) are in reasonable agreement with the 60-65-varve interval interpreted at Kviberg. This would suggest that the uncertain extension of the line marked by the Starrkarr marginal deposits should be expected to trend toward the southeast where it may be a continuation of the Stenkullen position (4) in the Savein valley. A study of the varve chronology along the Larjein valley immediately north of the Savein valley would be a logical step to im- prove our understanding of the regional deglaciation.

Conclusions Varve development in a glaciomarine environment is largely dependent upon meltwater outflow and density stratification. Due to the climatic influence upon both varve thickness and, together with topographic influences, the ice- front position within a valley, a tentative correla-

BOREAS 15 (1986) Varves and deglaciation 299

tion between varve stratigraphy and the sequence of deglaciation can often be suggested. This rela-

interpreted character of deglaciation.

McDonald, B. C. (eds.): Glaciofluvial and glaciolacustrine sedimentation, 304-320. Soc. Econ. Min. Paleont. Spec. Publ. 23.

13,500-10,oOO B.P. Boreas 8, 89-118.

tionship Offers better continuity and detail to the Berglund, B, E , 1979: The &glaciation of southern Sweden

The deglaciation of the Savein valley occurred Cato, I. 1982: Grain-size distribution of the cores - with em-

step-wisewith at least six significant c& reflected in the of the ice front during these phases was to slow or

phasis on the sedimentary patterns around the Pleistocene/ Holocene boundary. In Olausson, E. (ed.): The Pleistocene/ Holocene boundary in south-western Sweden. Sver. Geol. Under$. c 794, 5 4 6 5 .

stratigraphy' The response

stop in narrow and shallow valley areas where its De Geer, G. 1940: Geochronologia Suecia Principles. K . Sv. position was most stable. A glaiial readvance or

from the Gothenburg Moraine in the west. The

vetenskapsak.

54. 589-602.

ser. 3, 18:6. 267 PP. Domack, E. W. 1984: Rhythmically bedded glaciomarine sedi-

ments of Whidbey Island, Washington. J. Sediment. Petrol. surge is interpreted 175 years after the retreat

velopment end& after about 250 years. The gen- era1 trends in varve-thickness variation can be recognized and tentatively correlated with those at

ments in the Wedell sea, fjords of Spitsbergen and the Barents Sea: a review. Marine Geol. 57, 5 H 8 .

Elverh@i, A,, Liestol, o, & Nagy, T. 1980: Glacial erosion, sedimentation and microfauna in the inner part of Kongs-

Rished to the north, helping guide the extension of moraine lines Over

The correlations between the varve sequences and with the respective ice-front positions are not conclusive, but it is obvious that certain trends

fjorden, Spitsbergen. Norsk. Polarinsf. Skr. i72, 33-58. - Hillden, A. 1979: Deglaciationen i trakten av Berghemsmor-

anen oster om Goteborg. Univ. of Lund, Dept. of Quat.

Hillefors, A. 1969: Vastsveriges glaciala historia och morfologi. Naturgeografiska studier. Medd. Frdn Lunds Univ. Geogr.

of poor exposure. Geol, Thesis 6.

can be distinguished. The major varve variations are believed to be controlled by climatic and topographic factors, but modified by the local processes. However, in the case of glaciomarine varves, the variations in the meltwater overflow are closely dependent upon climatic conditions. They are, therefore, more reliable for chrono- logic and correlation studies than those proximal varves which originate largely from underflow, such as is characteristic for glaciolacustrine varves which De Geer (1940) considered unreli- able in proximal areas. The details of the varve stratigraphy will become increasingly useful in complementing the interpretation of deglaciation as more localities are considered.

Acknowledgemenfs. -This work has been funded by the Swed- ish Natural Science Research Council. I thank Tore PHsse, Lars Ronnert and Per Wedel for their thoughtful review of the manuscript. Significant improvements were also suggested by E. Lagerlund and another reviewer for Boreas. The discussions and help offered by numerous other members of the Depart- ment of Geology, University of Gothenburg and Chalmer's Technical University are also gratefully acknowledged.

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glaciomarine varves and the character of deglaciation, säveån valley, southwestern sweden

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