ON AN ANGULAR UNCONFORMITY WITHIN THE CAMBRO-ORDOVICIAN ROCKS OF THE LYS-CAILLAOUAS REGION, AXIAL ZONE,
FRENCH PYRENEES.
Bas den Brok
I n s t i t u u t v o o r Aardwetenschappen Vakgroep S t r u k t u r e l e G e o l o g i e B u d a p e s t l a a n 4, UTRECHT
ABSTRACT I n t h e Cambro-Ordovician r o c k s o f t h e L y s - C a i l l a o u a s r e g i o n a c o n g l o m e r a t e h o r i z o n i s p r e s e n t o f w h i c h t h e l o w e r boundary can be c o n s i d e r e d as an a n g u l a r u n c o n f o r m i t y . O b s e r v a t i o n s o f t h i s u n c o n f o r m i t y a r e c o n f i r m e d by an a p p a r e n t c a r t o g r a p h i c u n c o n f o r m i t y , changes i n o r i e n t a t i o n o f d e l t a l i n e a t i o n s a c r o s s t h e u n c o n f o r m i t y and a change i n d e f o r m a t i o n i n t e n s i t y a c r o s s t h e u n c o n f o r m i t y . T h i s a n g u l a r u n c o n f o r m i t y w i t h i n t h e Cambro-Ordovician r o c k s suggests p r e V a r i s c a n d e f o r m a t i o n .
INTRODUCTION
I n t h e L y s - C a i l l a o u a s a r e a ( s e e f i g u r e 1) r o c k s a r e exposed o f Cambro-Ordocician age showing t h e i m p r i n t o f s e v e r a l phases o f d e f o r m a t i o n . Main phase d e f o r m a t i o n i s c o n s i d e r e d t o be o f V a r i s c a n a g e ( Z w a r t , 1979). Main phase s t r u c t u r e s i n c l u d e f o l d s w i t h a x i a l p l a n e cleavage on v a r y i n g s c a l e s up t o hundreds o f m e t e r s . These s t r u c t u r e s a r e o v e r p r i n t e d by medium grade metamorphism and
d e f o r m a t i o n a l phases o f minor i m p r i n t .
The r o c k s a r e meta-sediments m a i n l y c o n s i s t i n g o f m i c a s c h i s t s , q u a r t z i t e s and m e t a - l i m e s t o n e s . W i t h i n t h e sequence a conglomerate h o r i z o n i s p r e s e n t w h i c h i s v e r y u s e f u l as a marker h o r i z o n t o
u n r a v e l t h e s t r u c t u r e . D e t a i l e d o b s e r v a t i o n s however y i e l d s t r u c t u r e s above and below t h e conglomerate t h a t do n o t f i t w e l l t o g e t h e r ,
making t h e q u e s t i o n a r i s e i f t h e conglomerate has been d e p o s i t e d d i s c o r d a n t over t h e u n d e r l y i n g r o c k s . T h i s i m p l i e s a p r e - V a r i s c a n d e f o r m a t i o n a l phase.
The p r e s e n t paper d i s c u s s e s d i f f e r e n t arguments t h a t make such d i s c o r d a n t d e p o s i t i o n l i k e l y .
f i g u r e 1. G e o l o g i c a l sketch-map o f t h e L y s - C a i l l a o u a s a r e a showing t h e s t u d i e d a r e a ( a f t e r Z w a r t , 1979).
-3-F i g u r e 2 shows a s i m p l i f i e d g e o l o g i c a l map o f t h e s t u d i e d a r e a . Two l i t h o l o g i c a l u n i t s have been marked o u t w i t h i n t h e Cambro-O r d o v i c i a n on t h e map: t h e r o c k s above and t h e r o c k s b e l o w t h e c o n g l o m e r a t e , t h e u n i t above i n c l u d i n g t h e c o n g l o m e r a t e i t s e l f . The boundary between t h e two u n i t s thus r e p r e s e n t s t h e p o s s i b l e d i s c o r d a n c e i n q u e s t i o n .
F i g u r e 3 shows two c r o s s - s e c t i o n s t h r o u g h t h e a r e a on w h i c h main phase f o l d s and cleavage as w e l l as younger l a r g e s c a l e f o l d s o f t h e main phase c l e a v a g e a r e a p p a r e n t .
A d e t a i l e d s t r u c t u r a l and metamorphic d e s c r i p t i o n o f t h e s t u d i e d a r e a i s i n p r e p a r a t i o n ( D e n Brok, i n p r e p ) .
f i g u r e 2. S i m p l i f i e d g e o l o g i c a l map o f t h e s t u d i e d area a f t e r Den B r o k ( i n p r e p ) .
-4-NATURE OF THE LOWER CONGLOMERATE BOUNDARY
I n t h e s t u d i e d a r e a t h e l o w e r c o n g l o m e r a t e boundary l o c a l l y has been a n a l y s e d i n c o n t i n u o u s s e c t i o n . I n most cases t h e b o u n d a r y appeared t o be i n t a c t , t h a t i s , i n most cases i t was n o t d i s t u r b e d by c o p l a n a r f a u l t i n g , as m i g h t be e x p e c t e d as a r e s u l t o f d i f f e r e n c e
i n m e c h a n i c a l b e h a v i o u r between t h e c o n g l o m e r a t e and t h e u n d e r l y i n g s c h i s t s o r q u a r t z i t e s . A t l o c a l i t i e s were t h e boundary seemed t o be a f a u l t , d e t a i l e d i n v e s t i g a t i o n s l e d t o t h e c o n c l u s i o n t h a t i n s e v e r a l cases i t concerned a c o n t a c t between a c o a r s e r and an u n d e r l y i n g f i n e r p a r t o f t h e c o n g l o m e r a t e , t h e r e a l l o w e r b o u n d a r y b e i n g i n t a c t .
T h i s u n d i s t u r b e d n a t u r e o f t h e c o n t a c t makes an e x p l a n a t i o n o f t h e a n g u l a r u n c o n f o r m i t y as a r e s u l t o f f a u l t i n g d o u b t f u l .
S t r u c t u r e s a t t h e l o w e r boundary a r e d i s p l a y e d as i n l i e r s i n
f i g u r e 4. F i v e l o c a l i t i e s have been c o n s i d e r e d t o show c o n v i n c i n g a n g u l a r u n c o n f o r m i t i e s . F o l d s b e i n g c u t o f f a t t h e c o n t a c t have n o t been f o u n d . A t t h e upper boundary o f t h e c o n g l o m e r a t e b e d d i n g i s f o u n d t o be c o n c o r d a n t ever3where. |\j —I 3000 m 2500 2000 — I 2500 m 2000 1500 f i g u r e 3. C r o s s - s e c t i o n s t h r o u g h t h e s t u d i e d a r e a a f t e r Den B r o k ( i n p r e p ) . For l o c a t i o n see f i g u r e 2.
-5-F i g u r e 4. Map showing s t r u c t u r e s a t t h e l o w e r c o n g l o m e r a t e b o u n d a r y ( i n l i e r s ) and d i f f e r e n t l i t h o l o g i e s d i r e c t l y below t h e c o n g l o m e r a t e as w e l l as zones o f one r o c k t y p e t r e n d i n g a t an a n g l e t o t h e c o n g l o m e r a t e boundary.
-6-CARTOGRAPHIC UNCONFORMITY
Rocks d i r e c t l y below t h e c o n g l o m e r a t e d i f f e r i n c h a r a c t e r . W i t h c h a n g i n g l o c a l i t y w h i t e q u a r t z i t e s , l i g h t - c o l o u r e d m e t a - l i m e s t o n e s or r e d - o r s i l v e r - c o l o u r e d q u a r t z i t i c m i c a s c h i s t s a r e p r e s e n t ( s e e f i g u r e 4 ) . T h i s phenomenon has a l s o been r e p o r t e d i n a d j a c e n t a r e a s , e.g. T r o u i l l e r ( 1 9 7 6 ) and A e r d e n ( i n p r e p ) . I n p r i n c i p l e t h i s does n o t n e c e s s a r i l y i n d i c a t e an a n g u l a r u n c o n f o r m i t y o f b e d d i n g . The d i f f e r e n c e i n l i t h o l o g y c o u l d be i n t e r p r e t e d i n terms o f a l a t e r a l change i n d e p o s i t i o n c o n d i t i o n s , t r u e enough a v e r y r a p i d one. However, mapping o f t h e r o c k s u n d e r l y i n g t h e c o n g l o m e r a t e r e v e a l e d zones o f one r o c k t y p e t r e n d i n g a t an a n g l e t o t h e c o n g l o m e r a t e
b o u n d a r y . T h i s has o n l y been o b s e r v e d i n r o c k s below t h e c o n g l o m e r a t e . Above t h e c o n g l o m e r a t e a t a l l l o c a l i t i e s always one r o c k t y p e o r
sequence i s r e p r e s e n t e d .
DELTA LINEATIONS,
T r o u g h o u t t h e a r e a i n t e r s e c t i o n l i n e a t i o n s o f s e d i m e n t a r y b e d d i n g on m a i n phase c l e a v a g e p l a n e s have been measured. F i g u r e 5 d i s p l a y s such i n t e r s e c t i o n l i n e a t i o n s , f r o m now on r e f e r r e d t o as d e l t a l i n e a t i o n s . O r i e n t a t i o n s t r o u g h o u t t h e s t u d i e d a r e a a r e d i s p l a y e d i n f i g u r e 6 and 7.
I t appears t h a t above t h e c o n g l o m e r a t e t h e o r i e n t a t i o n o f t h e d e l t a l i n e a t i o n s i s c o n s t a n t whereas b e l o w t h e c o n g l o m e r a t e o r i e n t a t i o n changes p e r a r e a . Subareas have been d e f i n e d w i t h r e l a t i v e l y c o n s t a n t o r i e n t a t i o n o f t h e d e l t a l i n e a t i o n s ( s e e f i g u r e 8 and 9 ) .
Across t h e l o w e r c o n g l o m e r a t e b o u n d a r y t h e o r i e n t a t i o n o f t h e d e l t a l i n e a t i o n s changes a b r u p t l y o f d i r e c t i o n a t many l o c a l i t i e s , t h e boundary t h u s m a r k i n g o u t a change i n d e l t a l i n e a t i o n o r i e n t a t i o n . From d i r e c t i o n a l s t a t i s t i c s i t f o l l o w s t h a t i n each subarea p o l e s on main phase c l e a v a g e p l a n e s a r e more o r l e s s c o n c e n t r a t e d i n p o i n t -maxima, as w e l l as a r e t h e d e l t a l i n e a t i o n s above t h e c o n g l o m e r a t e
f i g u r e 5. Rocksample showing d e l t a l i n e a t i o n s . Main phase c l e a v a g e p a r a l l e l s t h e p l a n e o f t h e p i c t u r e . The d e l t a l i n e a t i o n s i n t e r s e c t each o t h e r w h i c h i s i n t e r p r e t e d as s e d i m e n t a r y c r o s s - b e d d i n g . S c a l e : sample i s about 15 cm l o n g . s i g n i f i c a n t s p r e a d i n g . T h i s s p r e a d i n g o f t h e d e l t a l i n e a t i o n s can be e x p l a i n e d by m a i n phase f o l d i n g o f e a r l i e r f o l d e d s e d i m e n t a r y s u r f a c e s . Because s p r e a d i n g i s n o t e v i d e n t above t h e c o n g l o m e r a t e i t i s s u g g e s t e d t h a t p r e main phase f o l d i n g t o o k p l a c e b e f o r e d e p o s i t i o n o f t h e c o n g l o m e r a t e . F o l d l i k e s t r u c t u r e s o f d e l t a l i n e a t i o n s on s t r a i g h t u n f o l d e d m a i n phase c l e a v a g e p l a n e s ( s e e f i g u r e 10) a r e i n t e r p r e t e d as p r e m a i n phase s m a l l s c a l e f o l d s . H a v i n g been o b s e r v e d o n l y below t h e c o n g l o m e r a t e t h e y f i t w e l l t h e s u g g e s t e d u n c o n f o r m i t y ,
n o t e 1) I n subarea I I d e l t a l i n e a t i o n s have t h e same o r i e n t a t i o n as t h e d e l t a l i n e a t i o n s above t h e c o n g l o m e r a t e s u g g e s t i n g c o n c o r d a n t d e p o s i t i o n . However, a t t h e c o n t a c t o f t h i s zone and t h e l o w e r c o n g l o m e r a t e boundary a n g u l a r u n c o n f o r m i t y has been o b s e r v e d i n t h e f i e l d ( s e e f i g u r e 4 ) . T h i s c o i n c i d e n c e o f d e l t a l i n e a t i o n s s u b s e q u e n t l y o n l y means t h a t f o l d axes a r e p a r a l l e l .
i I , f
i !
PIC DE
H O U R G A D E
L Y S - C A I L L A O U A S GRANITE
-9-t i g u r e 8. Map w i -9-t h subareas w i -9-t h r e l a -9-t i v e l y c o n s -9-t a n -9-t o r i e n -9-t a -9-t i o n o f -9-t h e d e l t a l i n e a t i o n s and t h e main phase c l e a v a g e .
-10-DIFFERENCE I N DEFORMATION INTENSITY
Below t h e c o n g l o m e r a t e t i g h t and i s o c l i n a l f o l d s have been o b s e r v e d , whereas above i t t i g h t t o open f o l d s o c c u r . P l o t s on sedimentary-b e d d i n g and main phase- c l e a v a g e p l a n e s ( s e e f i g u r e 9) show sedimentary-b e d d i n g and c l e a v a g e p a r a l l e l below t h e c o n g l o m e r a t e , d e f i n e n i n g a p o i n t maximum. Above t h e c o n g l o m e r a t e b e d d i n g d e f i n e s a g r e a t c i r c l e thus
d e v i a t i n g s i g n i f i c a n t l y f r o m t h e main phase c l e a v a g e w h i c h a g a i n appears as a pointmaximum. T h i s suggests t i g h t e r f o l d i n g below t h a n above t h e c o n g l o m e r a t e .
The d i f f e r e n c e i n d e f o r m a t i o n i n t e n s i t y may be e x p l a i n e d i n terms o f d i f f e r e n t d e f o r m a t i o n h i s t o r i e s r e f l e c t i n g p r e m a i n phase f o l d i n g b e i n g r e s t r i c t e d t o t h e u n d e r l y i n g m o s t l y deformed r o c k s . The i s o c l i n a l
f o l d s below t h e c o n g l o m e r a t e must t h e n be r e g a r d e d as m a i n phase r e a c t i v e d p r e main phase f o l d s .
One c o u l d a l s o argue t h a t t h i s d i f f e r e n c e i n f o l d s t y l e i s d e t e r m i n e d by d i f f e r e n c e i n r e s p o n s e t o d e f o r m a t i o n , i . e . by d i f f e r e n c e i n
l i t h o l o g y . However, t h e more i n t e n s e l y deformed r o c k s b e l o w t h e c o n g l o m e r a t e seem t o be more r e s i s t a n t a g a i n s t d e f o r m a t i o n because t h e i r h i g h e r q u a r t z c o n t e n t .
Though r e f o l d e d f o l d s m i g h t be e x p e c t e d below t h e c o n g l o m e r a t e t h e y have n o t been o b s e r v e d i n t h e p r e s e n t a r e a . They do o c c u r however f u r t h e r t o t h e e a s t , i n t h e V a l e e du Lys a r e a ( S c h u t j e n s , i n p r e p ) and i n t h e w e s t e r n L y s - C a i l l a o u a s near Rioumajou(De B r e s s e r , 1985). Both r e f o l d e d f o l d s o c c u r below t h e c o n g l o m e r a t e .
OTHER AREAS
Pre main phase f o l d s i n Cambro-Ordovician r o c k s have been r e c o r d e d b y many w r i t e r s , e.g. S p e k s n i j d e r ( i n p r e p ) who g i v e s a good r e v i e w . Fewer w r i t e r s make m e n t i o n o f an a n g u l a r u n c o n f o r m i t y w i t h i n t h e s e r o c k s . Santanach P r a t ( 1 9 7 4 ) mentions a c a r t o g r a p h i c a l u n c o n f o r m i t y
w h i c h a l s o was d e t e r m i n e d b y a change i n ' o r i e n t a t i o n o f d e l t a l i n e a t i o n s . i n t h e Cerdanya r e g i o n e a s t o f A n d o r r a a t t h e l o w e r boundary o f a
B E D D I N G SQ
M A I N
C L E A V A G E
DELTA
L I N E A T I O N S
whole
area
• • • ly ,u' •' N = 287 " « 'I" ««« N=376 . . I • . . i . . •••• «{-^ '.''It. • • t • •Jjl*. N = 198 BELOW CONGLOMERATEsubarea
I
ft*« * t • • • • • • • N = 60 • t • N = 105 I • N = 67 BELOW CONGLOMERATEsubarea
II
N = 34 • i N=38 N = 4 4 F o r l o c a t i o n o f s u b a r e a s see f i g u r e 8 . I * f • N=65 . 1. N=76^0
. I . • t • N=38 BELOW CONGLOMERATE ABOVE CONGLOMERATE . ...1 •.|..
N = 90 . I . N = 30 1 I • N=49 N = 66 ABOVE CONGLOMERATE
-12-an a n g u l a r u n c o n f o r m i t y below a c o n g l o m e r a t e i n t h e O r r i dome s o u t h o f A n d o r r a .
I n t h e L y s - C a i l l a o u a s a r e a Donnot e t Guerange(1969) r e c o r d a
c a r t o g r a p h i c a l l y deduced a n g u l a r u n c o n f o r m i t y below a c o n g l o m e r a t e near Hospice de France. B a k k e r ( i n p r e p ) m e n t i o n s an u n c o n f o r m i t y under t h e same c o n g l o m e r a t e based upon a b r e a k i n o r i e n t a t i o n o f d e l t a l i n e a t i o n s .
The c o n g l o m e r a t e s t o w h i c h i s r e f e r r e d i n a l l o f t h e s e cases o c c u r w i t h i n t h e Cambro-Ordovician r o c k s and a r e most' l i k e l y to' be
c o r r e l a t e d w i t h t h e c o n g l o m e r a t e i n t h e p r e s e n t a r e a ( D e n B r o k , i n p r e p ) .
f i g u r e 10. Delta, l i n e a t i o n s on m a i n phase c l e a v a g e p l a n e . The c l e a v a g e i s p a r a l l e l t o t h e p l a n e o f t h e p i c t u r e . The l i n e a t i o n s a r e f o l d e d , t h e c l e a v a g e i s s t r a i g h t , t h u s i m p l y i n g p r e m a i n phase s m a l l s c a l e f o l d i n g .
-13-SUMMARY
To summarize, f e a t u r e s have been o b s e r v e d t h a t s e p a r a t e l y have been i n t e r r p e t e d under t h e assumption o f an a n g u l a r u n c o n f o r m i t y .
These f e a t u r e s i n c l u d e :
1. A n g u l a r u n c o n f o r m i t i e s observed i n t h e f i e l d . These u n c o n f o r m i t i e s have n o t been caused by c o p l a n a r f a u l t i n g .
2. Large s c a l e c a r t o g r a p h i c u n c o n f o r m i t i e s . Rocks d i r e c t l y b e l o w t h e c o n g l o m e r a t e d i f f e r i n c h a r a c t e r .
3. D e l t a l i n e a t i o n s change a b r u p t l y i n o r i e n t a t i o n across t h e l o w e r boundary o f t h e c o n g l o m e r a t e . Above i t t h e o r i e n t a t i o n i s c o n s t a n t , b e l o w i t i t shows c o n s i d e r a b l e s p r e a d i n g .
4. Rocks u n d e r l y i n g t h e c o n g l o m e r a t e seem t o be more i n t e n s e l y deformed as i s r e f l e c t e d by t h e occurence o f t i g h t e r f o l d s . These r o c k s seem t o be more r e s i s t a n t t o d e f o r m a t i o n .
C o m b i n a t i o n o f a l l these f e a t u r e s e x p l a i n e d i n terms o f an a n g u l a r u n c o n f o r m i t y makes i t s e x i s t e n c e l i k e l y .
ACKNOWLEDGEMENTS
The f i e l d w o r k has been c a r r i e d o u t w i t h f i n a n c i a l s u p p o r t f r o m t h e M o l e n g r a a f f - F u n d .
-14-REFERENCES
AERDEN, D.G.A.M., i n p r e p . . D e t a i l e d s t r u c t u r a l and metamorphic a n a l y s i s o f the L y s - C a i l l a o u a s M a s s i f between Lac d'Oo and t h e Lys V a l l e y , d o k t . s k r i p t . , v a k g r . S t r u k t . Geol., IVA, U t r e c h t .
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