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Eiszeitaltern.

Gegenwart

44 — 52

43

3

Hannover

Fig., 2 T a b .

1993

On the Relation between Subglacial Tills and the Substratum in Western Allgäu (Germany) M A T G . G . D EJONG * )

Subglacial tili, M o l a s s e s u b s t r a t u m . Q u a t e r n a r y d e p o s i t s , g r a n u l o m e t r y , gravel p e t r o g r a p h y , heavy m i n e r a l analysis B a d e n - W ü r t t e m b e r g , A l p i n e foreland

Abstract:

Grain-size

distribution,

gravel

petrography,

berg (Austria) have b e e n subject o f many studies dur­

h e a v y m i n e r a l weight p e r c e n t a g e a n d h e a v y m i n e r a l c o m ­

ing the last d e c a d e s ( a . o . CAMMERAAT & RAPPOL 1 9 8 7 ;

p o s i t i o n o f subglacial tills f r o m t h e Alpine f o r e l a n d o f w e s ­

GERMAN

tern A l l g ä u ( G e r m a n y ) h a v e b e e n a n a l y s e d t o a s s e s s t h e relation b e t w e e n t h e s e d i m e n t s a n d t h e substratum.

Ex­

c e p t f o r a few well-defined l o c a l M o l a s s e - d e r i v e d d e p o s i t s - t h e till p e t r o g r a p h y is d o m i n a t e d b y far-travelled debris.

1 9 7 6 ; GERMAN & MADER & KILGER 1 9 7 9 ; D E

J O N G 1 9 8 3 ; DE J O N G & RAPPOL & RUPKE 1 9 8 2 ; R A P P O L 1 9 8 3 ; RAPPOL & VAN GIJSSEL 1 9 8 8 ; SIMONS 1 9 8 5 ; W E I N HOI.D 1 9 7 3 ) .

T h e study o f sedimentary facies a n d p e ­

that r e w o r k i n g o f o l d e r ' Q u a t e r n a r y d e p o s i t s w a s subordi­

trography h a s n o t only resulted in a better u n d e r ­ standing o f the various glacial s.l. p r o c e s s e s active in the past, b u t also e n a b l e d , through a c o m b i n a t i o n with m o r p h o l o g i c a l studies, a detailed r e c o n s t r u c ­ tion o f t h e history o f deglaciation after the last major

nate.

P l e i s t o c e n e ice a d v a n c e ( D E J O N G 1 9 8 3 ; DE J O N G & D E

The

combination

o f petrographic

evidence,

theoretical

c o n s i d e r a t i o n s and b r o a d - s c a l e g e o m o r p h o l o g i c a l

obser­

v a t i o n s l e a d s to the c o n c l u s i o n that m o s t till-forming debris w a s t r a n s p o r t e d to t h e a r e a d u r i n g t h e last g l a c i a t i o n a n d

GRAAFF & RLIPKF. in p r e p . ) .

[Zum Problem des Z u s a m m e n h a n g e s zwi­ s c h e n Grundmoränen-Ablagerungen u n d d e m Substrat im w e s t l i c h e n Allgäu (Deutschland)]

T h e present paper focusses o n the origin o f t h e s u b ­ glacial tills in western Allgäti (Fig. 1), in particular o n the relation b e t w e e n t h e tills and the local substra­ tum. T h e topic has b e e n discussed b y several a u ­ thors in t h e recent past. B a s e d mainly o n heavy m i n ­ eral

Kurzfassung: analysen

Korngrößen-,

würmzeitlicher

Kies- und Schwermineral­

Grundmoränen-Ablagerungen

d e s R h e i n g l e t s c h e r s im b a d e n - w ü r t t e m b e r g i s c h e n

Alpen­

v o r l a n d z e i g e n e i n e n m e i s t relativ niedrigen E i n f l u ß d e s aus M o l a s s e b e s t e h e n d e n S u b s t r a t s a u f d i e Z u s a m m e n s e t ­ zung dieser Sedimente. gungen

A u f G r a n d theoretischer Überle­

und großmorphologischer

geschlossen, daß auch

Beobachtungen

die Aufnahme

quartärer Ablageningen untergeordnet Petrographie

wird

prä-existierender w a r u n d d a ß die

der Moränen-Ablageningen

hauptsächlich

b e s t i m m t w i r d v o m w ä h r e n d d e r letzten V o r l a n d - V e r g l e t scherung v o m Rheingletscher ins Vorland

transportierten

Schutt.

and

gravel

petrographic

analyses,

DE JONG

( 1 9 8 3 ) argued that the influence o f the Molasse s u b ­ stratum o n the c o m p o s i t i o n o f most subglacial tills in w e s t e r n Allgäu was subordinate. He s u g g e s t e d that most till-forming debris h a d b e e n transported b y gla­ ciers t o t h e foreland, assuming little local r e w o r k i n g from sub-till fluvial a n d / o r glaciofluvial d e p o s i t s . B a s e d m a i n l y o n granulometric analyses, CAMMERAAT &

R A P P O L ( 1 9 8 7 ; s e e a l s o RAPPOL 1 9 8 3 ) c o n c l u d e d

that t h e till-forming debris in western AllgäLi a n d Vorarlberg is for a major part o f local, b e d r o c k , ori­ gin. Sedimentary petrographic w o r k o n glacial d e ­ posits in the Rotach valley, straddling t h e b o u n d a r y b e t w e e n G e r m a n y a n d Austria, b y RAPPOL & VAN GIJSSEL ( 1 9 8 8 ) s h o w e d the influence both o f local b e d ­

1 Introduction T h e Quaternary deposits o f t h e hilly a n d mountain­ ous area o f western Allgäu ( G e r m a n y ) a n d o f the adjacent mountains o f c e n t r a l and northern Vorarl*) A l p i n e G e o m o r p h o l o g y R e s e a r c h G r o u p , University o f Amsterdam.

Address o f t h e author: Dr. M. G . G . D E JONG,

Prinsengracht 38c, 1 0 1 5 D W Amsterdam, T h e Netherlands.

r o c k a n d pre-existing gravels and o f far-travelled d e ­ bris o n t h e composition o f subglacial tills, t h e contri­ bution o f t h e first being considered p r e d o m i n a n t in most c a s e s b y these authors. T h e terms 'local' and 'far-travelled' o r 'remote' in t h e discussions refer to the compositional relationship b e t w e e n t h e tills a n d t h e substratum, n o t to t h e a c ­ tual transport distance. A till is termed 'local' w h e n (the largest part of) its sediment-petrography reflects


O n t h e Relation b e t w e e n Subglacial Tills

Fig. 1: G e o l o g i c a l m a p o f t h e region (after FUCHS 1 9 8 0 , D . RICHTER 1 9 5 6 , M. RICHTER 1 9 6 9 , a n d VOLLMAYR 1 9 5 8 ) . A l s o i n d i c a t e d a r e t h e s a m p l i n g l o c a t i o n s o f t h e Q u a t e r n a r y d e p o s i t s in the study a r e a .

45


46

M A T G . G. D E JONG

the c o m p o s i t i o n o f the substratum, irrespective o f the actual transport distance. Conversely, the terms far-travelled' and 'remote' are used for tills with c o m p o s i t i o n s which d o not reflect the l o c a l substra­ tum. T h e e m p h a s i s o n sediment-petrography b e a r s the risk o f under- or overestimating the actual transport distances and may h a m p e r the discussions. The main glacier flow in Rhine a n d 111 valleys (Fig. 1) was at an angle with the SW-NE trending geological zones of the Alps and foreland. T h e s e z o n e s s h o w quite a variation in width (Fig. 1 ) . It is c o n c e i v a b l e that, for instance, Molasse debris in the Alpine foreland o f G e r m a n y , which is underlain by M o l a s s e b e d r o c k , classifies as 'local' b e c a u s e o f petrographic similari­ ties with the substratum although the actual subgla­ cial transport may h a v e b e e n tens o f kilometers. W h e r e a s glacial debris in the Alps after a short trans­ port a c r o s s o n e or m o r e geological z o n e s w o u l d b e c o i n e d 'far-travelled' b a s e d o n c o m p o s i t i o n a l differ­ e n c e s with the b e d r o c k . T o avoid confusion, the terms in the present p a p e r are used with as m u c h specification as possible.

It is not the s c o p e o f t h e paper to present a c o m p r e ­ h e n s i v e overview o f t h e relation b e t w e e n tills a n d their substratum. E m p h a s i s will b e o n a critical re­ v i e w o f s o m e o f the a s p e c t s which have b e e n in the c e n t r e o f debate - grain-size distribution, gravel p e ­ trography, heavy mineral weight p e r c e n t a g e s a n d h e a v y mineral c o m p o s i t i o n . Inspite o f this limita­ tion, a f e w general c o n c l u s i o n s can b e drawn. Information on the m e t h o d s o f sampling and s a m p l e analysis has b e e n p r e s e n t e d by DE J O N G ( 1 9 8 3 ) . T h e results o f the analyses are tabulated in the s a m e pu­ blication. An overview o f the general glacier n e t w o r k in the area during the last glaciation is p r e s e n t e d a.o. by KRASSER ( 1 9 3 6 ) and

2 Sub-till s u b s t r a t u m g e o l o g y T h e study area is underlain by calcareous s a n d s t o n e s and s h a l e s o f the Tertiary Upper F r e s h w a t e r M o ­ lasse ( O b e r e S ü ß w a s s e r Molasse) in t h e s o - c a l l e d Undisturbed Outer M o l a s s e tectonic unit (FUCHS 1980;

HERRMANN

T a b l e 1: Heavy mineral c o m p o s i t i o n s

garnet

Hochgrat-Schuettung Pfaender-Schuatlung Pleistocene sands subglacial tills Molasse Molasse E Switzerland western Allgaeu (Fuechtbauer 1954a) (Hof mann 1957) (Gasser & Nabholz 1969) 48 46 20 26

zoisite

0

epidote

+

0

2

44

30

amphibole

0

0

35

36

staurolite

56

9

5

8

kyanite

2

+

1

2

zircon

1

26

2

+

rutlle

2

17

1

1

sphere

0

+

1

3

1

2

4

8

tourmaline

4

apatite

34

39

ore rock fragments troubled minerals rest grain size

abundant 11 8

1 0.06-0.4 mm

2 0.06-0.4 mm

0.06-0.4 mm

KELLER & KRAYSS ( 1 9 8 0 ) .

2 0.105-0.15 mm

Garnel is given as percentage of total grains counted. The other values are percentages of total grains minus number of garnet grains. For further details refer to De Jong (1983).

SCHWERD

1983;

SCHMIDT

1976;

SCHREINER 1978). Conglome­ rates are absent. Interbedded c o n g l o m e r a t e s d o o c c u r in the Disturbed Outer M o l a s s e a n d Y o u n g e r Inner M o l a s s e t o t h e S a n d SE, in the Pfänder-Hirsch­ b e r g mountain ridge a n d in the R o t a c h and W e i ß a c h valleys (Fig. 1 ) . Sediment-petrographically, the Molasse r o c k s in the study area are part o f the so-call­ e d B o d e n s e e fan ( ' S c h ü t t u n g ' ) , w h i c h includes the P f ä n d e r fan, a n d the Hochgrat fan ( F C C H T BAUER 1954a, 1954b, 1 9 6 7 ; H O F MANN, 1957). T h e B o d e n s e e fan is characterized b y a garnet-zircon-rutile-ore-staurolite h e a v y mineral association ( T a b l e 1 ) . T h e heavy mineral w e i g h t per­ c e n t a g e is less than 0 . 5 % . T h e s e d i m e n t s are quartz-rich, feld­ spar-poor, c a l c a r e o u s ( 3 0 - 5 0 % ) a n d poorly to n o n - d o l o m i t i c . In m o s t respects similar to the B o d e n s e e fan, the H o c h g r a t fan is characterized b y a garnetstaurolite-apatite h e a v y mineral association.

S o - c a l l e d older Q u a t e r n a r y d e ­ posits, which pre-date t h e sub­ glacial tills, overlie t h e M o l a s s e substratum in a n u m b e r o f pla-


O n the Relation b e t w e e n Subglacial Tills

c e s in the study area (GERMAN 1977; DE J O N G 1 9 8 3 ; DE J O N G & RAPPOL & RUPKE 1 9 8 2 ; 1976;

SCHREINER 1976,

1978;

SCHMID 1 9 5 5 ;

SCHMIDT

WEINHOLD 1 9 7 3 ) .

The

a m o u n t a n d distribution o f t h e s e deposits is difficult to assess due to lack o f e x p o s u r e s . Gravels, often sandy, a p p e a r to b e predominant, with subordinate amounts o f debris-flow deposits, fine-grained waterlain sediments, ablation tills and (older) subglacial tills. B a s e d o n a limited n u m b e r o f samples, DE J O N G ( 1 9 8 3 ) c o n c l u d e d that t h e gravel petrography o f the 'older' deposits does not significantly differ from that o f the subglacial tills ( T a b l e 2 ) .

3 P e t r o g r a p h i c a n a l y s e s o f s u b g l a c i a l tills 3.1 Grain-size distribution T h e grain-size distribution o f the subglacial tills ( n u m b e r o f samples n = 6 4 ) in the area o f investigati­ o n has b e e n described b y DE J O N G ( 1 9 8 3 ) a n d w a s la­ ter discussed b y CAMMERAAT & RAPPOL ( 1 9 8 7 ) . In a

T a b l e 2: Gravel petrography Molasse Ss t. (%) average range overlying upper till n=124 upper till n=79

6.4

0-53.7

Crystalline (%) average range 28.7

47

the p r e s e n t study area a n d samples from t h e Molas­ s e area to the S and S E , a n d c o n c l u d e d that the tillforming debris in b o t h areas is for a major part o f lo­ cal ( b e d r o c k ) origin. As m e n t i o n e d b e f o r e , the Mo­ lasse substratum in t h e latter area is c o m p o s e d not o n l y o f sandstones a n d shales, but also o f c o n g l o ­ merates. T h e m o d e in the gravel fraction o f the s a m p l e s from the a r e a o f investigation c a n n o t b e attributed to the incorporation o f e r o s i o n products from t h e Molasse b e d r o c k . T h e Molasse sandstones a n d s h a l e s in the study area are only moderately lithified a n d quite friable, h e n c e d o not w e a t h e r or e r o d e to p e b b l e a n d boulder-size debris. Large M o l a s s e s a n d s t o n e a n d s h a l e clasts are notably absent in t h e deposits o f t h e study area, and t h e Molasse s a n d s t o n e percen­ tage o f the size grade 5 - 8 m m is generally subordinate ( T a b . 2 ) . No evidence exists for the p r e s e n t or for­ m e r o c c u r r e n c e o f M o l a s s e c o n g l o m e r a t e s in the area. T h e s e factors h a v e b e e n s o m e w h a t underex­ p o s e d in previous discussions on the relation b e ­ t w e e n tills and substratum. T h e y imply that the gra­ vel fraction has either b e e n transported t o the area from southerly s o u r c e s o r incorporated from locally p r e s e n t 'older' Quaternary deposits.

3.2 G r a v e l p e t r o g r a p h y

2.8-97.7

nutshell, the distribution is characterized b y a m o d e in the gravel fraction a n d a double-peak m o d e in the fine sand/silt fraction, with a deficiency in the coarse sand fraction.

T h e gravel petrography o f the fraction 5 - 8 m m has b e e n analysed for 7 9 s a m p l e s from the so-called up­ p e r till (DE JONG 1 9 8 3 ) . T h e p e r c e n t a g e o f Molasse s a n d s t o n e pebbles is c o n s i d e r e d the b e s t gravel-petrographic measure for contribution b y t h e local Mo­ lasse substratum. M o l a s s e shale p e b b l e s are nota­ bly a b s e n t . T h e s a n d s t o n e p e r c e n t a g e is generally l o w ( T a b . 2; Fig. 3 ) , e v e n if o n e takes a bias to l o w p e r c e n t a g e s into a c c o u n t due to the destruction o f friable sandstone p e b b l e s during sampling a n d sam­ ple preparation. T h e c o m p o s i t i o n o f m o s t samples is d o m i n a t e d by n o n - M o l a s s e debris. T h i s is diffe­ rent from the observations o f RAPPOL & VAN GIJSSEL ( 1 9 8 8 ) in the Rotach valley, w h o found in m a n y sam­ p l e s from the E y e n b a c h a n d K e s s e l b a c h stream inci­ sions large percentages o f local M o l a s s e debris c o n g l o m e r a t e s , s a n d s t o n e s and siltstones - a n d c o n ­ c l u d e d to an important contribution b y t h e local Mo­ lasse substratum. T h e w o r k by RAPPOL & VAN GIJSSEL S L i g g e s t s that, w h e n important, a contribution by the local Molasse b e d r o c k will b e reflected in high per­ c e n t a g e s o f Molasse c o m p o n e n t s in t h e gravel pe­ trography. Conversely, t h e l o w p e r c e n t a g e s in most s a m p l e s o f the present study area m a y b e s e e n as an indication for a relatively l o w influence o f the local M o l a s s e substratum.

CAMMERAAT & RAPPOL ( 1 9 8 7 : Fig. 2) s h o w e d t h e simi­ larity in the grain-size distribution for s a m p l e s from

R e w o r k i n g o f glaciofluvial deposits into ( t h e basal part o f ) an overlying till has b e e n c o n c l u d e d b y

underlying upper till n=23 overlying lower till n=2 lower till n=3 underlying lower till n=2 notes:

10.5

0-81.3

24.2

5.1-61.7

5.1

0-10.7

22.6

1.4-44.0

1.7

0.7-2.7

20.6

15.7-25.5

4.8

2.3-9.3

22.7

14.7-31.9

4.3

1.7-8.0

31.3

26.8-33.7

'Crystalline' includes granite, gneiss, ophiolite, amphibolite, epidote-rock, quartsite, etc. For further details refer to De Jong (1983).


4S

M A T G . G. D E JONG

RAPPOL & VAN GIJSSEL ( 1 9 8 8 ) for s o m e deposits in the Rotach valley, b a s e d o n corresponding, relatively high percentages o f erratic material. T h e well-round­ ed nature o f s o m e p e b b l e s in the tills o f the Argen lo­ b e area o f the Rhine g l a c i e r suggests reworking o f fluvial a n d / o r glaciofluvial sediments. A 25 c m thick sheared layer at the b a s e o f a till mantle w a s report­

3-3 H e a v y m i n e r a l w e i g h t p e r c e n t a g e s

e d b y D E J O N G & RAPPOL & RUPKE ( 1 9 8 2 ) for a drum-

Data o f heavy mineral weight p e r c e n t a g e s is avail­ a b l e for 39 so-called u p p e r till and 3 so-called lower till s a m p l e s (Fig. 1 a n d 2 ) . T h e weight percentages o f t h e heavy mineral fraction (size grade 0 . 0 6 0 . 4 m m ) o f the B o d e n s e e fan Molasse s a n d s t o n e s are less than 0 . 5 % (HOFMANN 1957).

Lin in west Allgäu consisting largely o f sorted drift. This e v i d e n c e s h o w s that debris from 'older' Quater­ nary deposits has b e e n incorporated into the tills. B e l o w , it will be argued, h o w e v e r , that the degree o f glacial erosion o f pre-existing deposits w a s limited.

S a m p l e s 1143 and 1 1 4 4 , collected in pits n e a r Stei­ n e g a d e n in the eastern part o f the study area (Fig. 1 ) , s h o w heavy mineral weight p e r c e n t a g e s o f 0 . 3 % (in the decalcified size g r a d e 0 . 1 0 5 - 0 . 1 5 m m ) , w h i c h is in g o o d a g r e e m e n t with the f o r e - m e n t i o n e d percen-

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49

O n the Relation b e t w e e n Subglacial Tills tage from HOFMANN ( 1 9 5 7 ) for the Molasse bedrock. T h e s e samples a l s o s h o w garnet-dominated heavy mineral c o m p o s i t i o n s and high Molasse sandstone percentages in the fraction 5-8mm (Fig. 2 and 3 ) and, h e n c e , are c o n s i d e r e d typical representatives o f l o ­ cal, predominantly Molasse-sourced subglacial tills.

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50

M A T G . G . D E JONG

B I K I 9 6 0 ; S M I T SIBINGA-LOKKER 1 9 6 5 ) .

Such 'extremes'

then a r e likely to have u n d e r g o n e c h a n g e s during transport d u e to admixing o f n e w lithologies. It is, h e n c e , difficult if not i m p o s s i b l e t o characterize t h e heavy mineral weight fraction o f the far-travelled d e ­ bris with o n e figure. Using t h e analog o f recent flu­ vial s e d i m e n t s o f the river R h i n e downstream from the c o n f l u e n c e with the river 111 (HAHN 1 9 6 9 ) , a fig­ ure o r r a n g e o f several p e r c e n t s might b e a reason­ able a p p r o x i m a t i o n . The h e a v y mineral c o n c e n t r a t i o n s o f most subglaci­ al till s a m p l e s in the study area are considerably larg­ er than t h o s e o f the Molasse substratum a n d t h e Molasse-derived tills (Fig. 2 ) a n d are interpreted to result from a large if n o t predominant contribution from n o n - M o l a s s e debris. A predominant influence o f the M o l a s s e substratum w o u l d have b e e n reflect­ ed in substantially l o w e r h e a v y mineral c o n c e n t r a ­ tions. Little to n o information is available for the heavy min­ eral fraction o f the 'older' Quaternary deposits ( s e e also HERRMANN & SCHWERD 1 9 8 3 ; RAPPOL & VAN G I J S S E L

1 9 8 8 ) . B a s e d o n the similarity in gravel petrography with t h e subglacial tills ( T a b . 2 ) a n d o n a n analogy with t h e fluvial sediments o f the Rhine ( s e e a b o v e ) , the w e i g h t fractions may b e estimated at several per­ cents. This, then, means that reworking o f older de­ posits into t h e tills is hard t o distinguish from direct sourcing o n the basis o f weight percentages only. As will b e argued b e l o w , h o w e v e r , the glacial erosion o f t h e pre-existing deposits w a s probably n o t veiy large a n d , consequently, t h e contribution o f s u c h erosive products to the till petrography limited.

3.4 H e a v y m i n e r a l c o m p o s i t i o n The h e a v y mineral c o m p o s i t i o n o f the Molasse b e d ­ rock is d o m i n a t e d b y garnet ( T a b . 1 ) . T h e subgla­ cial tills o n average s h o w a garnet-amphibole-epidote association. T h e cross-plot o f the garnet per­ c e n t a g e versus the heavy mineral weight p e r c e n t a g e for 3 9 s o - c a l l e d upper a n d 3 so-called l o w e r tills in the study area (Fig. 2 ) s h o w s a decrease o f garnet with a n i n c r e a s e in heavy mineral weight p e r c e n t a ­ ge, reflecting t h e waning influence o f the Molasse debris. F o r e x a m p l e , t h e s a m p l e s 1 1 4 3 a n d 1 1 4 4 and, t o a lesser degree, 1 9 a n d 3 2 s h o w relatively high garnet percentages a n d l o w heavy mineral c o n ­ centrations. T h e c o n c e n t r a t i o n s o f Molasse sand­ stone p e b b l e s are very high a n d high respectively (Fig. 3 ) , a s o n e w o u l d e x p e c t for samples derived from t h e local Molasse substratum. As m e n t i o n e d before, little to n o information is avai­ lable for t h e heavy mineral fraction o f t h e 'older' Quaternary deposits. B a s e d o n t h e similarity in gra­

vel petrography with the subglacial tills ( T a b . 2 ) a n d on an analogy with the fluvial sediments o f the Rhine (see a b o v e ) , t h e heavy mineral c o m p o s i t i o n s m a y b e quite similar t o those o f the tills. This, then, m e a n s that local sourcing from the older deposits c a n n o t b e differentiated from sourcing b y far-travelled d e b r i s on the basis o f composition only. S e e also b e l o w .

4 Discussion and conclusions The results o f t h e petrographic analyses s h o w that, with a few n o t a b l e e x c e p t i o n s , the subglacial tills a r e c o m p o s e d largely o f non-Molasse debris. T h e tills consist to a large degree o f debris which w a s either transported directly to the sttidy area from southerly sources o r r e w o r k e d from locally present o l d e r QLUIternary deposits. Or, from a c o m b i n a t i o n o f t h e s e . Theoretically, substantial erosion a n d d e f o r m a t i o n o f ( u n c o n s o l i d a t e d ) porous gravels is likely t o o c c u r only at low effective normal pressures at t h e i c e / b e d interface

(BOLILTON 1 9 7 5 ; SMALLEY & UNWIN

1968).

Conditions o f impeded internal drainage t h r o u g h p o r o L i s 'older' Quaternary deposits resulting in l o w effective normal presstires c o u l d c o n c e i v a b l y h a v e existed in the Allgäu, d L i e to the p r e s e n c e o f i m p e r ­ m e a b l e layers o r topographic obstructions. T h e pre­ s e n c e , h o w e v e r , o f drumlins partly o r c o m p l e t e l y c o m p o s e d o f pre-existing gravels (DE J O N G & RAPPOL & R E P K E 1 9 8 2 ) a n d the preservation o f sheet-like lay­ ers o f gravels b e l o w the tills (DE J O N G 1 9 8 3 ) s u g g e s t that subglacial erosion of this type o f S L i b s t r a t u m w a s probably o f limited extent only. As m e n t i o n e d before, the full extent o f t h e "older' Quaternary deposits in the study area c a n n o t b e d e ­ termined due to lack o f exposures. Full terrain c o v e r ­ ing g e o m o r p h o l o g i c a l m a p p i n g o f large parts o f the west Allgäu at scale 1 : 2 5 , 0 0 0 (DEN BESTEN 1 9 8 4 ; CASTEL 1 9 8 2 ; GILLEBERT 1 9 8 5 ; DE J O N G 1 9 8 3 ; W O L F E R T

1 9 8 4 ) , h o w e v e r , has s h o w n that in wide areas s u b ­ glacial till is n o t underlain b y pre-existing deposits. O n e c o u l d argue that this demonstrates that o l d e r deposits h a v e b e e n completely e r o d e d to form t h e subglacial tills. This scenario, h o w e v e r , is c o n s i d e r ­ e d less likely. T h e interpretation o f a p r e d o m i n a n t direct sourcing from southerly areas is preferred b y the present author. The cirques, t h e wide glacial valleys - which a r e of­ ten o v e r d e e p e n e d , e. g. the Upper Rhine, 111 a n d G r o ß w a l s e r valleys in Vorarlberg - a n d t h e overall glacial erosive character o f the Alpine relief w i t n e s s the i m p o r t a n c e o f glacial erosion in creating t h e pre­ sent m o r p h o l o g y . Glaciers have e r o d e d t h e c i r q u e s and valleys a n d have swept freshly e r o d e d a n d o l d e r glacial drift a s well as t h e interglacial and interstadi­ al fluvial a n d m a s s m o v e m e n t debris t o the foreland. The result has b e e n a net transport o f debris from t h e


51

O n the Relation b e t w e e n Subglacial Tills

Alps to the foreland (and b e y o n d ) . A sizeable part o f

5 Acknowledgements

this material forms the subglacial tills in the v i e w o f the present author. RAPPOL & VAN GIJSSEL ( 1 9 8 8 ) a r g u e that glacial e r o s i o n is also an important g e o m o r p h i c process in t h e pied­ mont area. T h e topography o f highs ('Riedel') a n d lows ( ' S e n k e ' ) in the foreland, with differences in elevation u p t o 100m, p r o b a b l y reflects the relief o f

T h e p a p e r is largely b a s e d o n a Ph.D. thesis submit­ ted at t h e University o f Amsterdam in 1983- T h e auth­ or is i n d e b t e d to all those p e o p l e w h o contributed t o the thesis. Appreciation is especially e x t e n d e d t o the m e m b e r s o f the Alpine G e o m o r p h o l o g y R e ­ search G r o u p at the University o f Amsterdam for their stimulating c o o p e r a t i o n .

the M o l a s s e surface (DE J O N G 1 9 8 3 ; s e e also GERMAN et al., 1 9 6 7 , M A D E R 1 9 7 1 , a n d W E I N H O L D 1 9 7 3 ) .

Alter­

nating fluvial a n d glacial e r o s i o n , as d e s c r i b e d b y SCHREINER ( 1 9 7 9 ) for the B o d e n s e e basin, is h e l d res­ ponsible for t h e formation o f this morphological framework. T h e degree o f t h e (glacial) e r o s i o n , however, is clearly o f a l o w e r order than that in t h e Alps.

6 References B E S T E N , W . DEN ( 1 9 8 4 ) : T h e K a r b a c h - W a l d b u r g r ü c k e n area: a s t u d y o f P l e i s t o c e n e l a n d f o r m s a n d s e d i m e n t s . - L'npubl. M . S c . thesis Lab. P h y s . G e o g r . Soil S c . Univ. Amsterdam; Amsterdam. BIK, M.J.J. ( 1 9 6 0 ) : Z u r G e o m o r p h o l o g i e u n d G l a z i a l g e o l o ­ g i e d e s F r ö h d i s c h b a c h - u n d M ü h l t o b e l t a l s in V o r a r l ­

In summary, t h e petrography o f the subglacial tills in the area, e x c e p t for a few well-defined local tills, is dominated b y far-travelled d e b r i s . T h e c o m b i n a t i o n of petrographic e v i d e n c e , theoretical c o n s i d e r a t i o n s and b r o a d - s c a l e g e o m o r p h o l o g i c a l o b s e r v a t i o n s leads to t h e c o n c l u s i o n s that m o s t till-forming debris was transported to the area during the last glaciation and that reworking o f 'older' Quaternary d e p o s i t s was subordinate.

b e r g ( Ö s t e r r e i c h ) . - Diss. Univ. A m s t e r d a m , 1 7 5 p . , 2 3 Abb., 76 Fig., 4 T a b . , 7 Anh., 4 Beil.: A m s t e r d a m . BocLTOV

G . S . ( 1 9 7 5 ) : P r o c e s s e s a n d patterns

o f glacial

s e d i m e n t a t i o n : a theoretical a p p r o a c h . - In: WRIGHT, A.E. & MOSEI.EY, F . (Eds.), I c e a g e s : A n c i e n t a n d m o ­ d e r n : 7 - 4 2 , 17 Fig.; Seel H o u s e Press, L i v e r p o o l . CAMMERAAT, E. & RAPPOL. M. ( 1 9 8 7 ) : O n t h e r e l a t i o n s h i p o f b e d r o c k l i t h o l o g y a n d grain s i z e distribution o f till in w e s t e r n Allgäu ( W e s t G e r m a n y ) a n d V o r a r l b e r g ( A u s ­ tria). - J b . G e o ! B . - A . 130: 3 8 3 - 3 9 0 , 6 Fig.: W i e n .

These

c o n c l u s i o n s differ

from

those

o f RAPPOL

CASIEL, I . I . Y . ( 1 9 8 2 ) : G e o m o r f o g e n e s e v a n e e n g e b i e d in

( 1 9 8 3 ) , CAMMERAAT & RAPPOL ( 1 9 8 7 ) and RAPPOL & VAN

het b e r e i k v a n d e S c h ü s s e n - e n A r g e n l o b ( Z u i d - D u i t s -

GIJSSEL ( 1 9 8 8 ) , w h o conchidecl o n average t o a pre­ dominant infhience o f the local substratum ( s e e also GASSER & NABHOLZ 1969). It is stressed that t h e pre­ sent c o n c l u s i o n s d o not invalidate their w o r k as far as the area outside the p r e s e n t area o f investigation is c o n c e r n e d . All studies s h o w that the petrography of subglacial tills c a n reflect a n y source from the wide range between remote and local. It is b e y o n d the s c o p e o f this p a p e r to a n a l y s e t h e causes for t h e va­ riations. It is noted, however, that the c o r e o f t h e stu­

l a n d ) . - U n p u b l . M. S c . thesis Lab. P h y s . G e o g r . Soil S c . Univ. A m s t e r d a m ; A m s t e r d a m . FUCHS, W . ( 1 9 8 0 ) : D i e M o l a s s e u n d ihr n i c h t h e l v e t i s c h e r Vorlandanteil am Untergmnd einschließlich d e r Sedi­ m e n t e a u f d e r B ö h m i s c h e n M a s s e . - In: OBERHAUSER, R. (Red.), D e r geologische Aufbau Österreichs: 144-176, 11 Fig.; S p r i n g e r Verlag, W i e n . FÜCHTBAUER, H. ( 1 9 5 4 a ) : T r a n s p o r t u n d S e d i m e n t a t i o n d e r westlichen

Alpenvorlandsmolas.se.

-

Heidelberger

Beitr. Min. u. Petr. 4: 2 6 - 5 3 , 6 Fig., 6 T a b . ; H e i d e l b e r g .

dy areas o f R \ P P O L , CAMMERAAT & RAPPOL and RAPPOL

& VAN GIJSSEL is situated in mountainous relief, w h e r e ­ as the present study area lies completely in t h e hil­ ly Alpine foreland. In addition, the distribution o f the samples throtighout t h e present area o f investi­ gation s h o w s s o m e interesting features. T h e Molasse-derived s a m p l e s ( 1 9 , 3 2 , 1 1 4 3 and 1 1 4 4 ) a r e from the eastern part o f the area - t h e foot o f the Pfänder mountain - w h e r e the g l a c i e r flow p r o b a b l y w a s confined b y t h e b e d r o c k relief and m o r p h o l o g y . The samples from the western part o f the area, w h e r e glacier flow w a s less constrained, d o not s h o w a strong Molasse influence. 'Older' Quaternary d e p o ­ sits o c c u r in t h e latter area, b u t as argued b e f o r e it re­ mains q u e s t i o n a b l e if they s h o u l d b e c o n s i d e r e d an important s o u r c e rock for t h e tills.

-

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( 1 9 6 7 ) : D i e S a n d s t e i n e in d e r M o l a s s e n ö r d l i c h d e r Al­ p e n . - G e o l . Rdsch. 56: 266-300, 12 Fig.; Stuttgart.

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Quaternary Science Journal - On the Relation between Subglacial Tills and the Substratum in Wes...