Page 1

Eiszeitalter

u.

Gegenwart

120 — 1 2 8 6 Figures

47

Hannover

1997

Problems o f landslide chronology in the Mätra mountains in Hungary JOSEF SZAFSO* & ENIKÖ FELEGYHAZI* L a n d s l i d e p r o c e s s e s , p o l l e n d i a g r a m s , r a d i o c a r b o n dating, H o l o c e n e , Hungary

A b s t r a c t : A detailed s u r v e y a n d m a p p i n g o f t h e landslide

a u f g e n o m m e n . Als E r g e b n i s d e r

processes

and

k a n n festgestellt w e r d e n , d a ß d i e s e F o r m e n v o r a l l e m a u f

mountains

o f Hungary

surface

forms formed

in

the

in t h e

volcanic

middle

Middle and

Late

folgenden Gebieten

Geländeuntersuchungcn

vorkommen:

M i o c e n e w a s carried o u t in the past d e c a d e . As a result

a ) in d e r R a n d z o n e d e r v u l k a n i s c h e n G e b i r g e ,

o f investigations it c a n b e stated that thier o c c u r r e n c e is

b ) a u f d e r steilen i n n e r e n S e i t e d e r C a l d e r e n d e r e h e m a l i ­

m a i n l y c o n f i n e d t o t h e a r e a s as follows:

gen

a) m a r g i n a l z o n e s o f v o l c a n i c m o u n t a i n s ,

c ) a u f d e n Hängen der die schichtvulkanischen Strukturen

b ) s t e e p inner s l o p e s o f t h e f o r m e r c a l d e r a s o f eruption,

tief e i n - o d e r

c ) v a l l e y s l o p e s cut into s t r a t o v o l c a n i c Structures.

In d e n

In t h e i r characteristic r e g i o n s o f o c c u r r e n c e l a n d s l i d e forms

c h e n s t e l l e n die R u t s c h u n g s f o r m e n

a r e important, in s o m e p l a c e s e v e n d e t e r m i n a n t

manchen

compo­

Ausbruchszentren, d u r c h s c h n e i d e n d e n Tälern.

erwähnten Stellen

charakteristischen V o r k o m m e n s b e r e i ­ einen

einen wichtigen,

entscheidenden

Faktor

an des

n e n t s o f m o r p h o l o g y . T h e o v e r w h e l m i n g majority o f the

m o r p h o l o g i s c h e n B i l d e s dar. D i e ü b e r w i e g e n d e

l a n d f o r m s are n o m o r e a c t i v e a n d c a n b e c o n s i d e r e d fossi­

d e r F o r m e n ist h e u t e nicht m e h r aktiv, u n d die m e i s t e n v o n

Mehrheit

lised. C o n s e q u e n t l y , l a n d s l i d e p r o c e s s e s u s e d t o play a sig­

i h n e n k a n n als fossil b e t r a c h t e t w e r d e n . D e s h a l b ist e s of­

nificant role in forming s l o p e m o r p h o l o g y in p r e v i o u s stag­

f e n s i c h t l i c h , d a ß die R u t s c h u n g s p r o z e s s e in d e r

früheren

es o f geomorphic evolution. Research performed heretofo­

P e r i o d e ( o d e r in d e n

wichtige

früheren

Perioden) eine

re h a s p r o v i d e d o n l y rather a m b i g u o u s a n s w e r t o t h e q u e ­

Rolle

s t i o n c o n c e r n i n g the t i m e o f e m e r g e n c e o f l a n d s l i d e s o n a

Die b i s h e r i g e n F o r s c h u n g e n g a b e n nur e i n e z i e m l i c h un­

theoretical basis.

s i c h e r e , b l o ß prinzipielle Antwort a u f die F r a g e n a c h d e r

D u e t o t h e a b s e n c e o f y o u n g s e d i m e n t s traditional strati-

Entstehungszeit der Formen.

in d e r

Gestaltung

der

F o r m der

Hänge

spielten.

g r a p h i c m e t h o d s c o u l d not yield satisfactory results. T h e

Da

a u t h o r s therefore a t t e m p t e d to d e t e r m i n e t h e t i m e o f the

w ö h n l i c h e n s t r a t i g r a p h i s c h e n M e t h o d e n kein g u t e s E r g e b ­

upfilling o f lakes o r b o g s l o c a t e d in u n d r a i n e d

nis erzielt w e r d e n , d e s h a l b v e r s u c h t e n die V e r f a s s e r d u r c h

depressions

die jungen Sedimente fehlen, konnte durch die ge­

c l a m m e d b y slides a n d t h e m i n i m u m a g e o f t h e landslides

B o h r u n g e n in d e n g e s c h l o s s e n e n , a b f l u ß l o s e n - m i t e i n e m

t h r o u g h t h e a n a l y s e s o f b o r e h o l e s a m p l e s . In t h e Mätra s e ­

S e e o d e r M o o r ausgefüllten - Eintiefungen d e r

d i m e n t s upfilling o f 7 s u c h d e p r e s s i o n s w e r e c a r r i e d out. In

Rutschungen

die

Aufschüttungsdauer

der

größeren

Eintiefungen,

s e v e r a l c a s e s p o l l e n d i a g r a m s w e r e o b t a i n e d a n d in t w o

u n d d a d u r c h das m i n i m a l e Alter der R u t s c h u n g e n z u b e ­

b o r e h o l e s radiocarbon dating o f samples w e r e performed.

s t i m m e n . Im M ä t r a - G e b i r g e w u r d e das Auffüllungsmaterial

T h e p a p e r s u m m a r i s e s t h e results o f t h e b o r e h o l e a n a l y s e s

d e r R u t s c h u n g s e i n t i c f u n g u n t e r s u c h t . In m a n c h e n F ä l l e n

a n d e v a l u a t e s pollen r e c o r d s s u p p o r t e d

k o n n t e e i n e P o l l e n a n a l y s e durchgeführt

b y C'< dating in

w e r d e n , in z w e i

detail.

B o h r u n g e n k a m es auch zu einer »C-Altersbestimmung.

Analyses performed using various methods indicate a b e ­

In d e r v o r l i e g e n d e n S t u d i e w e r d e n die E r g e b n i s s e d e r B o h ­

g i n n i n g o f the b o g formation in t h e Early H o l o c e n e , c o n s e ­

r u n g e n z u s a m m e n g e f a ß t , u n d ausführlich w e r d e n d i e a u c h

q u e n t l y landslides o c c u r r e d p r e s u m a b l y in t h e w a k e o f the

d u r c h d i e i'<C-Altersbestimmung unterstützten

P r e b o r e a l o r prior t o that in a w a r m spell f o l l o w i n g the

gramme

Late G l a c i a l .

D i e mit d e n v e r s c h i e d e n e n M e t h o d e n d u r c h g e f ü h r t e n A n a ­

A l t h o u g h t h e s e results c a n n o t b e e x t r a p o l a t e d t o all t h e

lysen z e i g e n , d a ß die M o o r b i l d u n g in d e n

l a n d s l i d e s identified in t h e t w o m o u n t a i n s ( t h e i r n u m b e r is

Fällen in d e r ersten P e r i o d e d e s H o l o z ä n s b e g a n n , u n d die

c a . 2 0 ) without m o r e a d o n e v e r t h e l e s s t h e y a r e orientating.

R u t s c h u n g e n erfolgten w a h r s c h e i n l i c h a m A n f a n g d e r prä-

Pollendia­

interpretiert.

untersuchten

b o r e a l e n Zeit, o d e r früher, w ä h r e n d e i n e r K l i m a v e r b e s s e [Rutschungschronologische Fragen i m Matra-Gebirge in U n g a r n ]

rung n a c h i r g e n d e i n e r K a l t p e r i o d e der S p ä t g l a z i a l z e i t . D i e e r h a l t e n e n E r g e b n i s s e k ö n n e n w o h l a u f die a n d e r e n - ins­ g e s a m t e t w a 2 0 - R u t s c h u n g s f o r m e n nicht e x t r a p o l i e r t w e r ­

K u r z f a s s u n g : Im letzten J a h r z e h n t w u r d e n b e i d e r Unter­

d e n , a b e r s i e stellen d o c h e i n e Orientierungshilfe im B e z u g

s u c h u n g d e r R u t s c h u n g s p r o z e s s e u n d - f o r m e n in U n g a r n

auf ihr Alter dar.

in d e n Mittelgebirgen, d i e z u m e i s t in d e r Mitte u n d a m En­ d e d e s M i o z ä n s e n t s t a n d e n sind, viele R u t s c h u n g s f o r m e n *) A d r e s s e s o f the authors: Dr. J . SZABO, DR. E. FELEGYHAZI, D e p a r t m e n t o f P h y s i c a l G e o g r a p h y , Lajos K o s s u t h Univer­ sity, D e b r e c e n , H u n g a r y

Introduction A detailed survey and m a p p i n g o f the landslide sur­ face forms in the volcanic middle mountains o f Hun­ gary w a s started in the past decade. G e o m o r p h o l o -


P r o b l e m s o f l a n d s l i d e c h r o n o l o g y i n t h e Mätra m o u n t a i n s in H u n g a r y

gists studying v o l c a n i c ridges previously either did not p a y particular attention to features o f slumping o r c o n s i d e r e d t h e m insignificant additional morpho­ logical elements o f t h e landscape. In o u r earlier pa­ pers ( S Z A B 6 J . 1 9 9 2 , 1 9 9 3 ) short preliminary reports w e r e presented o n t h e results o f field works eluci­ dating the high i m p o r t a n c e o f the part played b y landslide p r o c e s s e s in the g e o m o r p h i c evolution in several regions o f t h e Tertiary v o l c a n i c mountains ( n o t a b l y the V i s e g r ä d Mountains, t h e Mätra Moun­ tains and the basalt v o l c a n o s north o f Lake Balaton) w h e r e slumps are essential, in s o m e c a s e s even de­ terminant c o m p o n e n t s o f the m o r p h o l o g y . Their o c ­ c u r r e n c e is mainly c o n f i n e d to the areas as follows: a) marginal z o n e s o f v o l c a n i c mountains, b ) s t e e p inner s l o p e s o f the former calderas o f erup­ tion, c ) valley slopes cut into stratovolcanic structures. a) This is the most frequent case. W h e r e the b e d o f l o o s e material underlying the extrusive o r loose vol­ c a n i c matter is still o n the surface as for e x e m p l e in the mountain margins, and is p r o b a b l y situated higher than ist surroundings, it h a s lost stability u n d e r the weight o f the volcanic layers and func­ tions as sliding p l a n e for volcanites o n a sufficiently high and steep s l o p e . T h e b e d underlying acid a n d neutral volcanites f o r m e d in the B a d e n i a n stage o f the M i o c e n e is usually the schlieren o f the Carpathi­ an stage (or o c c a s i o n a l l y o l i g o c e n e s e d i m e n t ) while basalts o f the late M i o c e n e generally overlie sandyc l a y e y Pannonian deposits. T h e probability o f lands­ lides was e n h a n c e d b y a ( s o m e t i m e s asymmetric) uplift o f the v o l c a n i c margin, such increasing the ele­ vation over the surrounding area. Sliding planes c o u l d develop within t h e volcanic s e q u e n c e . It w a s m a d e possible b y t h e stratovolcanic structure for sli­ des c o u l d o c c u r a l o n g the surface o f l o o s e tuff hori­ z o n s especially if t h e latter was increasingly w e a t h e ­ red. O n the northern slopes o f the Mätra crest the g e o m o r p h i c e v o l u t i o n and the t e c t o n i c structure c r e a t e d favourable conditions for landslides so a fre­ q u e n t series o f slumps can be o b s e r v e d on the slo­ p e s . In spite o f t h e spatial discontinuity the major forms o f slumping m a k e up a distinct (maximum 1 k m w i d e ) z o n e o f slides, differing morphologically from the foothills o f the mountains dissected b y valleys. b ) T h e previous c e n t r e s o f eruption h a v e survived in remnants only. ( S o . e.g. the more or less intact parts o f the double caldera in the Visegräd Mountains has s l u m p e d in several p l a c e s along the s t e e p inner side a n d at the b a s e o f the caldera walls a series o f huge displaced b l o c k s c a n b e found.) c ) T h e slumps o c c u r r i n g on the s l o p e s o f valleys dissecting the inner portions o f the v o l c a n i c ridges are as a rule slides a l o n g the tuff layers cut during in­ cision. If the w h o l e v o l c a n i c s e q u e n c e h a d b e e n cut in valley evolution t h e underlying b e d o f volcanites

121

c o u l d serve as sliding plane. T h e majority o f the landslide forms are stable and are in a stage o f deg­ radation. Active parts still in m o t i o n o c c u r sporadi­ cally and are o f insignificant size as c o m p a r e d to the earlier slides. Consequently, these landslide forms o f natural origin h a v e relevance primarily not to the prensent-day surface dynamics but b e i n g remnants o f intensive past p r o c e s s e s they are indispensable for the understanding o f the m o r p h o l o g y in the c o n ­ c e r n e d zones o f v o l c a n i c ridges. T h e majority o f t h e m a p p e d landforms are m u c h m o r e older than to b e dated directly ( o n the basis o f historical records, written d o c u m e n t s or maps, p e r h a p s by the d e f o r m e d growth o f trees). At the s a m e time they are m u c h y o u n g e r than the c o v e r o n t h e m or in their immediate surroundings (in the studied areas n o l o e s s series that might have b e e n relevant to dating w e r e found) s o the traditional stratigraphic m e t h o d s o f age determination c o u l d not b e u s e d . Consequently, dating o f the landslide events h a s b e e n carried o u t o n a general theoretical basis ( e . g . b y founding periods o f climates favourable for t h e d e v e l o p m e n t o f landslides) and w a s highly am­ b i g u o u s . A final a n d general solution o f this problem necessitates further and substantial investigations o n landslides a n d the present p a p e r reports o n the initial efforts w h i c h might contribute to a m o r e exact dating o f o c c u r r e n c e o f particular slumps. Extrapo­ lation o f these results is to b e carried out b y due cau­ tiousness.

Principles, range and m e t h o d s o f investigations T h e determination o f the relative a g e o f landslide forms and an approximation o f their absolute age are m a d e p o s s i b l e b y the analysis o f their degrada­ tion (transformation). Degradation has well recognis­ a b l e distinct p h a s e s . A m o n g t h e m e s p e c i a l l y impor­ tant are the characteristic trends o f d e v e l o p m e n t o f depressions formed through d a m m i n g b y the slump­ e d masses. T h e s e depressions h a v e basically two different ways o f evolution. 1. T h e i r c l o s e d ( u n d r a i n e d ) character might b e alter­ e d b y a c o m p l e t e upfilling. In this c a s e after certain period only r o u n d e d steps remain o n the place o f depressions. 2. Closed d e p r e s s i o n s can b e e l i m i n a t e d through drainage. Following landslide events w a t e r provided b y springs at various levels o f the failure surface fills u p the depression a n d ( d e p e n d i n g o n the degress o f r e c h a r g e ) is overflowing its e d g e gradually cuts it a n d o p e n s the basin (certainly w a t e r o f the springs c a n play an important role in the upfilling o f the de­ pression). C h a n c e s for the discharge are e v e n better w h e n this basin is formed in the u p p e r section o f the


122

J . SZAHÖ & E. FELEGYHAZI

Fig. 1:

G e o l o g i c a l f o r m a t i o n s a n d m o r p h o l o g i c a l s k e t c h a l o n g t h e m a i n crest o f t h e Mätra M o u n t a i n s ( C o m p i l e d b y DÄvid I..) A/ 1= E o c e n e b i o t i t e - a m p h i b o l e a n d e s i t e , 2 = K a r p a t i a n - B a d e n i a n a n d e s i t e s , 3 = B a d e n i a n p y r o x e n e - a n d e s i t e l a v a a g g l o ­ m e r a t e , 4 = K a r p a t i a n - B a d e n i a n p y r o x e n e a n d e s i t e - t u f f a g g l o m e r a t e , 5 = O t t n a n g i a n - K a r p a t i a n „lower rhyolite tuff", 6 = B a ­ d e n i a n dacitic rhyolite tuff, 7 = O l i g o c e n e s a n d s t o n e , clay marl, 8 = l o w e r a n d middle a l e u r i t e ( s c h l i e r e n ) , partly s a n d s t o n e , 9 = m a i n crest o f the Mätra M o u n t a i n s

SzabOj. a n d

B / 1 = m a i n crest o f t h e Mätra Mountains, 2 = c l o s e d d e p r e s s i o n d a m m e d b y slide usually o c c u p i e d by l a k e o r b o g , 3 = un­ e v e n surface o f s l u m p h e a p s o r b l o c k s , 4 = e r o s i o n a l valleys, 5- r o c k f o r m a t i o n s b r a n c h i n g from t h e main crest w i t h step­ like s l o p e s , 6 = s l o p e s s t e e p e r t h a n 2 0 °

Abb. 1: Geologische Bildungen und geomorphologische Skizze entlang dem Hauptgrat des Mätra Gebirges (nach der geologischen Karte von Ungarn - 1:200.000. konstruierten Szabö, J. und DÄvid, L.) A, 1= Eozäne Biotit- und Amphibolandesite, 2= Karpatische und Badenische Andesite (Miozän), 3= Lavaagglomeratum aus Piroxenandesit (Badenisch), 4= Tuff und Agglomeratum (Karpatische, Badenische Piroxenandesit), 5= „Untere Rhyolittuff" (Ottnangisch-Karpatisch), 6= Dazitartige Riolittuff (Badenisch), 7= Sandstein und Tonmergel (Oligozän), 8= Untere- und mittlere Schlier (teilweise Sandstein), 9= Hauptgrat des Gebirges. B., 1= Hauptgrat des Gebirges, 2= abgeschlossene Depressionen (im allgemeinen mit Teich oder Moor), 3= unebene Oberfläche der abgenitschten Massen, 4= Erosionstäler, 5= stufenförmige seitliche Felsengrate des Hauptgrates, 6= Hänge (>20 Grad). s l o p e due to the high erosional energy o f the over­ flowing water. In m o s t c a s e s the activities o f springs create a lake in the depression. O n c e r e c h a r g e o f the lake is slowing d o w n or ist upfilling r e a c h e s an a d v a n c e d stage the vegetation develops rapidly and a b o g appears. This

b o g m a y exist for a relatively long time t h o u g h its na­ tural d e v e l o p m e n t might b e interrupted o r terminat­ e d b y t h e arrival o f a substantial amount o f unorganic sediments (provided b y a s u b s e q u e n t s l u m p or surface e r o s i o n ) or b y desiccation due to t h e stop­ p a g e o f recharge.


P r o b l e m s o f landslide c h r o n o l o g y in t h e Mätra m o u n t a i n s in H u n g a r y

Fekete-to pH(HJ3)

123

K o r i s mocsdr Humus"/.

pH(Hp)

654321 i ,i I—

1

1

• I

j

10 1

I

Humus'/. 30 1

I

50 I

50°/o felett

\

- 7 2 5 1 ± 9 8 y r BP. \

7 7 2 1 ± 1 3 8 y r BR

/ /

r*

Fig. 2: Profile o f t h e b o r e h o l e s M / 3 ( F e k e t e L a k e ) a n d M / 5 (Köris B o g ) in t h e Mätra M o u n t a i n s . F o r t h e b o r e h o l e sites s e e Fig 1 / B 1= s a n d with small gravel, 2 = sand, 3= silt,4= mud, 5 = clay ( b l a n k stripes within t h e c o l u m n s c o r r e s p o n d to p l a c e s w h e r e g r a n u l o m e r e analysis is n o t feasible d u e to t h e p r e s e n c e o f plant r e s i d u e ) Abb. 2: Bohaingsprofile (M/3=Fekete-See, M/5" Köris SumpOLage der Bohrungen in Abb. 1/B. 1= Sand mit feinkörnigen Kies, 2= Sand, 3= Staub, 4= Schlamm, 5= Ton (in den leeren Teilen der Profile die Kornzusammen­ setzung wegen der torfigen Pflanzenreste nicht prüfbar)


124

J . SZABÖ & E. FELEGYHAZI

100

200

300 m I

Fig. 3 : S u r r o u n d i n g s o f t h e F e k e t e Lake slide (Mätra M o u n t a i n s ) 1= failure front, 2 = s l u m p e d d o w n m a s s , 3 = lake, 4 = site of the borehole

Abb. 3: Die Umgebung der Rutschung „Fekete See". 1= Hauptabrißwand, 2= abgerutschte Masse, 3= See, 4= Bohrung (M/3). Hohen in m über dem Meeresspiegel. T o unfold details o f the a b o v e m e n t i o n e d p r o c e s s e s and to determine an approximate age o f slumps b o r e ­ h o l e s w e r e drilled in c l o s e d depressions (filled with lakes, b o g s o r o n e s already upfilled and dry) found in v o l c a n i c ridges. 7 h o l e s in the Mätra w e r e drilled d o w n to m a x . 6.5 m depth. In all but 2 cases they r e a c h e d the surface o f a relatively hard v o l c a n i c r o c k (mostly tuff). Pure but not undisturbed samples w e r e collected through

10-20 c m for the subsequent analyses. G r a n u l o m e t r i c composition, humus and calcium carbonate content a n d pH w e r e determined. Palynological analysis w a s carried out in e a c h samples. Absolute dating o f vegetation remnants o f s o m e layers from t w o b o r e s h o l e s i. e. «C investigations w e r e performed b y Ede I Iertelendi in the R a d i o c a r b o n Laboratory o f the Institute o f Nuclear Physics (Debrecen).


P r o b l e m s o f landslide c h r o n o l o g y in t h e Mätra m o u n t a i n s i n H u n g a r y

125

oaet.z-A-v

S3 i m O V t J H d '

YTAHDOAHB-Q

S1D3 1DALLHL O

sniovud S!U3LD0ISA,0 <

ODVLNVID O 3VU3J^138wn *> 3V3NIHVUO M 3V3OVD3DA0 3V33VI0ODON3H3 « -

<O O

1 ?

Fig. 4: Pollen diagram of the bottom sediments of the Fekete Lake (Mätra Mountains), as a percentage of the total amo­ unt of spores/pollen. For the boreholesite (M/3) see Fig.l/B. 1 = clay, 2= mud, 3= sand, 4= debris, 5= peat, plant residue. Absolute time scale by Andersen-de Vries-Zagwijn ( I 9 6 0 ) Abb. 4: Pollendiagramm der Bohrung „Fekete See" (M/3). Pollenzahl in Prozent der Gesamtpollensumme (Spora/Pollen). Die Lage der Bönning in Abb. 1/B. 1= Ton, 2= Schlamm, 3= Sand, 4= Schutt, 5= Torf. Ptlanzenreste. Absolute Chronologie nach Andersen - de Vries - Zagwijn (1960).


126

J . SZABÖ & E . FELEGYHAZI

100

1

\\\

A

0

200

300m J

2

5

Fig. 5: Landslite of the Köris Bog (Mätra Mountains) and its surroundings (altitudes a.s.l. in metres) 1= approximate posi­ tion of the main failure front, 2= heap of the slumped down mass, 3= closed depression dammed by slide (with the lake), 4 = general sloping direction, 5 = erosional trench with bog, 6 = site of the borehole, 7 = spring Abb. 5: Rutschung „Köris Sumpf" und ihre Umgebung = Höhen in m über dem Meeresspiegel) 1- Annähernde Lage der Hauptabrißwand, 2= abgerutschte Massen, 3= abgeschlossene Depression (mit Teich), 4= allgemeine Hang­ richtungen, 5= Eorosionsgraben, 6= Bohrung (M/5), 7= Quelle


P r o b l e m s o f landslide c h r o n o l o g y in t h e Mätra m o u n t a i n s in H u n g a r y

127

Discussion Landslides in the Mätra Mountains T h e overwhelming majority o f landslides found here are resulted from the stratovolcanic structure o f the ridge, the p r e s e n c e o f primary volcanic forms (calderas, lava fields) the slope conditions brought about by subsequent tectonic m o v e m e n t s and the specific character o f the contact o f the volanic seq u e n c e with the less consolidated underlying b e d

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t h e m have d e v e l o p e d o n the steep northern side o f the mountain crest (Fig. 1). T h e western part o f the crest from Agasvär to the Piszkes-tetö and it eastern portion with the K e k e s group to the Tarna Valley are the margins o f the remnants o f a volcanic klippe with a s t e e p northern slope. Its central part, h o w e v e r (betw e e n the Piszkes-tetö and the K e k e s ) is a fragment o f the tectonically disairbed outer side o f a caldera steeply sloping northward (SZEKELY A. 1 9 8 3 , 1987). During the landslides pyroxene-andesite lava, agglomerate and tuff (partly dacitic rhyolite tuff) w e r e moving o n the unconsolidateci b e d underlying the volcanites, generally on Lower and Middle M i o c e n e aleurolite (in s o m e cases weathered tuff layers) as sliding plane. Already fossilised forms o f landslides along the northern side o f the Mätra crest m a k e up a discontinuous zone with distinct morphological features. In the 7 selected depressions d a m m e d b y slumps two b o r e h o l e s ( M / 3 , M / 5 , Fig.2) provided profiles perspective for dating.

Fekete Lake

A

Fig. 6: P o l l e n diagram o f t h e s e d i m e n t s o f t h e Köris B o g (Mätra M o u n t a i n s ) as a p e r c e n t a g e o f t h e total a m o u n t o f s p o r e s / p o l l e n . F o r t h e b o r e h o l e site ( M / 5 ) s e e Fig. 1 / B .

Abb. 6: Pollendiagramm der Bohrung „Köris Sumpf" (M/5). Pollenzahl in Prozent der Gesamtpollensumme (Spora/Pollen) - Zeichen in Abb. 4. Die Lage der Bohrung in Abb. 1/B.

T h e basin o f the F e k e t e Lake located o n the eastern flank o f the Galya-tetö at an altitude 7 3 1 m a.s.l. was created by a single rhyme o f a large-scale rockslide (Fig. 3 ) . Based o n the studies o f the e x p o s e d failure front and the nearby road cut the sliding plane was heavily weathered dacitic rhyolite tuff. According to the information g a i n e d from the b o r e h o l e ( M / 3 ) drilled in the depression with a diameter o f 100 m the evolution o f the lake can b e summarised as follows (Fig. 4 ) . In the depression d a m m e d by the slide a dystrophic lake (pH=5) rich in h u m u s with l o w carbonate and nutrient content d e v e l o p e d m o r e than 9.000 years a g o under cool and moist climate. At that time in the vicinity a mixed grove forest (linden a n d conifers with hazel in the bush level) existed. With the gradual upfilling o f the lake by the e n d o f the Boreal p h a s e in the coastal z o n e bulrush ( T y p h a ) and reed (Phragmites) appeared, then with the advancement o f moss and fern the lake started to turn into bog. It b e c a m e a typical swelling b o g in the Atlantic phase indicated by an e n r i c h m e n t in spores o f acidophilous m o s s e s , especially S p h a g n u m species, those o f the s e d g e family ( C y p e r a c e a e ) and ferns (Athyrium.Cystopteris, Dryopteris). T h e a b o v e age has b e e n confirm e d by a radiocarbon dating o f 6 , 1 5 9 ± 6 0 yr for the


J. SZABÖ & E . FELEGYHÄZI

128

horizon between 4.2 a n d 4.4 m. In this p h a s e there w a s a relative decrease in the share o f the arboreal pollen for it was a swelling b o g with n o trees. Pre­ sently this type is very characteristic under moist o c e a n i c climate ( o m b r o g e n i c b o g ) . In the following phase the reduced a m o u n t or the temporary disap­ p e a r a n c e o f b e e c h ( F a g u s ) and h o r n b e a m (Carpinus) indicate a change in climate with drier summers and increased continental influence. According to C da­ ting this occured 3.300 years a g o i.e. in the subboreal phase. Aridisation o f summers resulted in the appea­ rance o f pine (Pinus silvestris) and in the e m e r g e n c e of a pine-bog. During this dry phase the pollen con­ taining ability o f the accumulating sediments b e c a m e heavily reduced and only the most resistant spores and pollen survived in l o w quantity not suitable for analysis. T h e advancement o f pine is d o c u m e n t e d in other regions, too: e.g. E. KRIPFEL (1986) carrying out stratigraphic analyses o f the b o g s in Slovakia pointed on a similar p h e n o m e n o n having taken p l a c e in the subboreal phase. Drier a n d moister spells w e r e alter­ nating in the Subatlantic phase. T h e plant c o v e r in the vicinity was represented by h o r n b e a m - b e e c h forests. The b o g was settled b y alder (Alnus) a n d a slightly acidic (pH=5-6) dystrophic b o g developed. Climates in the Subatlantic created favourable conditions for the climax o f the s u b m o n t a n e b e e c h forests. Fekete Lake is still situated in the submontane b e e c h wood­ land z o n e with alder grove forests, oak and elm. B a s e d o n the above records on the b o g development the landslide is assumed to have occurred in the wa­ ke o f the Preboreal p h a s e or even earlier, during the improvement o f the climate subsequent to a cold spell o f the Late Glacial. 14

Köris Bog Its basin is situated at an altitude ( 7 0 5 m a.s.l.) similar to that o f the Fekete Lake, and the fonning landslide by the prensent morphological features (Fig. 5.) o c ­ curred in an main single phase. T h e d e e p sliding pla­ ne o f the slide was probably a weathered pyroxeneandesite tuff but it might have b e e n the unconsolida­ ted underlying b e d o f volcanites. T h e landslide o f the Köris B o g forms an organic part o f the landslide zone flanking the northern crest o f the Mätra Mountains. Its dating was based on the analyses o f the ( M / 5 ) bore­ hole profile (Fig. 6). Pollen record s h o w s a dominant coniferous forest with deciduous species during the deposition o f the lowermost horizon. Pine (Pinus silvestris) a n d spruce ( P i c e a ) were associated with s o m e elm (Illmus) and linden (Tilia). This vegetation w a s characteristic for the late Preboreal. Abundant atmospheric precipi­ tation created favourable conditions for the emergen­ ce o f o m b r o g e n i c bogs. O n the acidic debris a dystro­ phic b o g developed p o o r in carbonates a n d nutrients with acidophilous bryum and fern. It d e v e l o p e d into a swelling bog in the Boreal. In this phase the conifers were retreating from the surroundings a n d a deci­

duous forest emerged with linden, elm, o a k a n d hazel in the shrub level. Radiocarbon analyses o f these h o ­ rizons ( 7 . 2 5 0 and 7.720 yr) s e e m to corroborate re­ sults o f the palynological analysis (Fig. 6 ) . T h e rate o f sedimentation slowed d o w n in the Atlantic (from an earlier 4 0 c m to 20 c m per 1 0 0 0 yr). T h e reason might have b e e n that the peat-bog had turned into a moist b o g g y m e a d o w o f s e d g e vegetation surrounded b y high stalk. With the e m e r g e n c e o f grass ( G r a m i n e a e ) a b o g m e a d o w started to develop. T h e Subboreal w a s dominated by beach, hornbeam, oak a n d linden. In­ creasing continentality in climate can b e d o c u m e n t e d with the drop in the share o f b e e c h pollen a n d b y the disappearance o f h o r n b e a m . In the Subatlantic the high s e d g e m e a d o w survived which (after a recurring aridisation) was characterised by the enrichment o f paludal elements. Also this is the present-day situa­ tion. Pollen o f Larix already indicates coniferous plan­ tations. O n the basis o f the b o g development the occur­ r e n c e o f the landslide (similar to that o f the F e k e t e La­ ke) c a n b e put to the Early Holocene or to a transitio­ nal spell from the Late Glacial to the Preboreal. Conclusions First detailed studies o n the time o f the o c c u r r e n c e o f landslides in the volcanic ridges o f Hungary C datings point out 8.000 to 10.000 yr B.P. as a minimum age o f landslides. Theoretically they might e v e n b e ol­ der but the fact that paludal sediments o f the Köris Bog immediately overlie the rock debris o f the slump s e e m s to limit the a g e o f this p h e n o m e n o n . T h o u g h these results cannot b e extrapolated to all the fossil landslides identified in the mountains (their n u m b e r is ca. 2 0 ) without more a d o nevertheless they are orien­ tating. 14

References ANDERSEN, S. 'F., DE VRIES, H. DE., ZAGWIJN.W. H. (I960): I960 Climatic

changes and radiocarbon dating in the Weichselian Glacial of Denmark and The Netherlands. Geo. en Mijnbow. 39- pp. 38-42. DÄVID L. (1992): A Mätra eszaki lejtöinek csuszamläsos felszinfejlödese. Fol. Hist.-nat.Mus. Matr. 17. pp 9-26. KRIPPEL, E. (1986): Postglacialny vyvoj vegetacie Slovenska. p. 307. VEDA. Bratislava. SZABÖ,. J. (1992): Landslide processes and fonns in the Hungarian Moutains of volcanic origin. In: New perspectives in Hungarian geography - Studies in Geography in Hungary 27. Budapest, pp. 63-75. - (1993): Vergleichende Untersuchung der Rutschungsprozes.se in Ungarn. Berliner Geographische Arbeiten, Heft 79, Berlin, pp. 133-161. SZEKELY, A. (1983): Vergleichende vulkanische Mittelgebirgsforschung in Ungarn. Wissenschaftliche Kolloquien der ungari­ schen Wirtschafts- und Kulturtage in Hamburg. Hrsg. W. Bachofer, H. Fischer, München pp. 207-238. SZEKELY, A. (1987): Vulkäni hegysegeink a legujabb kutatäsok tükreben. (The volcanic mountains of Hungary - in the light of recent research. Földrajzi Közl. 35/111) pp. 134-142. Manuskript eingegangen a m 11.03.96

Quaternary Science Journal - Problems of landslide chronology in the Mátra mountains in Hungary  
Quaternary Science Journal - Problems of landslide chronology in the Mátra mountains in Hungary  

Im letzten Jahrzehnt wurden bei der Untersuchung der Rutschungsprozesse und -formen in Ungarn in den Mittelgebirgen, die zumeist in der Mitt...

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