A Transect Through the Foreland and Transitional Zone of Western

ro cks that ra nge in age f r o m Late P r o t e r o z o i c to M i d d l e O r d o v i c i a n

The l o w e s t most s e q u e n c e , w h i c h rest w i t h p r o f o u n d u n c o n f o r m i t y on

the M i d d l e P r o t e r o z o i c of the G r e e n M o u n t a i n and L i n c o l n m a s s i f s ,

l a r g e l y c o n s i s t s of m e t a w a c k e s , m a f i c v o l c a n i c rocks and p h y l l i t e s

f o r e l a n d b a s i n s e q u e n c e The b o u n d a r y b e t w e e n the two s e q u e n c e s is

c o n g l o m e r a t e s of the a n c i e n t p l a t f o r m m a r g i n and e a s t e r n basin

T h e s e s e q u e n c e s h a v e b e e n s t u d i e d by a n u m b e r of w o r k e r s in the

•past (Cady, 1945; Cady and others, 1962; H a w l e y , 1957; Erwin, 1957;

W e l b y , 1961; S t o n e and D e n n i s , 1964, for e x a m p l e ) and are r e c e i v i n g

w h e r e the f i e l d c o n f e r e n c e w i l l be hel d A d d i t i o n a l i n f o r m a t i o n can be f o u n d in W e l b y (1961) and D o l l and o t h e r s (1961)

n o r t h - t r e n d i n g fo lds and i m b r i c a t e t h r u s t f a u l t s w h i c h are well

d i s p l a y e d on the G e o l o g i c Map of V e r m o n t (Doll and others, 1961) The rift c l a s t i c and p l a t f o r m s e q u e n c e s h a v e each b e e n d i s p l a c e d

s o u t h e r n Q u e b e c to A l b a n y , New Y o r k and p l a c e s the o l d e r p l a t f o r m

s e q u e n c e o v e r the y o u n g e r f o r e l a n d b a s i n s e q u e n c e E s t i m a t e s of

the two, the H i n e s b u r g thrust, p l a c e s t r a n s i t i o n a l and rift c l a s t i c

b e t w e e n the s y n c l i n o r i a l rocks of w e s t e r n V e r m o n t and the G r e e n

M o u n t a i n a n t i c l i n o r i u m D o r s e y and o t h e r s (1983) h a v e d e m o n s t r a t e d that the H i n e s b u r g t h r u s t d e v e l o p e d a l o n g the o v e r t u r n e d limb of a

t h e r e f o r e is q u i t e d i f f e r e n t f r o m the g e o m e t r y of ■the C h a m p l a i n

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E r w i n (1957) and H a w l e y (1957, 1972)

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t h r u s t fault W e s t w a r d d i s p l a c e m e n t is e s t i m a t e d to be 6 km

R e c e n t s e i s m i c t r a v e r s e s a c r o s s w e s t e r n V e r m o n t d e m o n s t r a t e that the C h a m p l a i n t h r u s t dips e a s t w a r d at a p p r o x i m a t e l y 15 d e g r e e s

b e n e a t h the G r e e n M o u n t a i n a n t i c l i n o r i u m and that the m a j o r folds

of w e s t e r n V e r m o n t are f o r m e d by d u p l e x e s and r e l a t e d s t r u c t u r e on

p l a t f o r m and the f o r e l a n d b a s i n s e q u e n c e s H i g h a n g l e n o r m a l

f a u l t s have b e e n m a p p e d a l o n g the C h a m p l a i n t h r u s t fault and in

cut the e a s t e r n part of the p l a t f o r m s e q u e n c e are M e s o z o i c in age

The s t r u c t u r e of w e s t e r n V e r m o n t in the B u r l i n g t o n r e g i o n is best

i l l u s t r a t e d by the r e c e n t w o r k of D o r s e y and o t h e r s (1983) in the

M i l t o n q u a d r a n g l e (fig 3) and L e o n a r d (1985) in S o u t h Hero I s l a n d The cr oss s e c t i o n s for this r e g i o n s h o w that the C h a m p l a i n t h r u s t

f a u l t is e s s e n t i a l l y p l a n a r w h i c h is c o n s i s t e n t w i t h r e c e n t s e i s m i c

s t u d i e s at this l a t i t i u d e D e t a i l e d s u r f a c e m a p p i n g , h o w e v e r , has

T h i s c o n c l u s i o n is c o n s i s t e n t w i t h the c h a r a c t e r of each of the

f a u l t zones The C h a m p l a i n t h r u s t faul t is m a r k e d by gouge, w e l d e d

c o n s i d e r a t i o n s ( S t a n l e y and R a t c l i f f e , 1985) and the a n a l y s i s of

d e f o r m a t i o n in the f o r m of e x t e n s i o n a l f a u l t s and i g n e o u s a c t i v i t y

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V e r m o n t The trip a c r o s s c e n t r a l V e r m o n t w i l l c o n t i n u e a c r o s s the

p r e - S i l u r i a n h i n t e r l a n d t h r o u g h the n o r t h e r n e x t e n s i o n of the root

A s s e m b l e at the A p p l e S t o r e on the s o u t h side of R o u t e 2 in the

the low le vel of d e f o r m a t i o n that c h a r a c t e r i z e s m u c h of the west

s h o r e of the C h a m p l a i n I s l a n d s and the e a s t e r n s h o r e of New Y o r k in

h o w e v e r , do exist w h e r e b e d d i n g p l a n e f a u l t s and h i g h - a n g l e fau l t s

east and is i n t e r p r e t e d to r e s u l t f r o m s i m p l e s h e a r p a r a l l e l to the

T h o s e l ay e r s that are m a r k e d by p r o m i n e n t s l i c k e n l i n e s are b e d d i n g

s o u t h edge of the q u a r r y c o n t a i n s a d j u s t m e n t f a u l t s at its h i n g e

9

that end a l o n g c l e a v a g e z ones w i t h t h i c k clay s e l v e d g e s

The m a j o r s t r u c t u r e s in the q u a r r y are b e d d i n g - p l a n e t h r u s t faults

T h e s e f a u l t s are m a r k e d by c a l c i t e l a y e r s w i t h w e s t - t r e n d i n g

a n t i c l i n e a l o n g the s o u t h w a l l and edge of the q u a r r y is f o r m e d from

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a s s o c i a t e d b l i n d , s y n f o r m a l t h r u s t f a u l t are t r u n c a t e d by the

m a j o r t h r u s t fau lt that is c o n t i n u o u s a c r o s s the n o r t h wall of the

t h o u g h t to be a s s o c i a t e d w i t h a d u p l e x or ramp b e l o w the level of the q u a r r y floor

S T O P 3 - "THE B E A M " - T H I S IS A S U P E R B O U T C R O P THAT S E R V E S AS A

F I E L D L A B O R A T O R Y FOR R E S E A R C H AND T E A C H I N G OF F O R E L A N D D E F O R M A T I O N

P L E A S E S T U D Y IT USE Y O U R C A M E R A S BUT NOT Y O U R H A M M E R S R E F E R TO

F I G U R E S 7 AND 8

a p p r o x i m a t e l y 5 m i l e s west of the e x p o s e d fron t of the C h a m p l a i n

t h r u s t fault or a p p r o x i m a t e l y 4 6 0 0 feet b e l o w the r e s t o r e d w e s t w a r d

p r o j e c t i o n of the t h r u s t s u r f a c e The m a j o r q u e s t i o n s that will be

to d e t e r m i n e if i m b r i c a t e t h r u s t f a u l t s d e v e l o p t o w a r d the f o r e l a n d

c l e a v a g e d e v e l o p m e n t ?, 4* W h a t p r o c e s s e s are i n v o l v e d in the

f o r m a t i o n of fa ult zones ?, 5* Are the re c r i t e r i a that i n d i c a t e the

r e l a t i v e i m p o r t a n c e and d u r a t i o n of m o t i o n a l o n g a f ault zone ?, 6

f a u l t i n g ?, and f i n a l l y 7 W h a t is the s t r u c t u r a l e v o l u t i o n of the

F i v e i m b r i c a t e t h r u s t f a u l t s and a s s o c i a t e d r amp s are e x p o s e d in

p r o f i l e s e c t i o n in a foot t h i c k bed of m i c r i t e that e x t e n d s 45 feet

f u r t h e r c l a s s i f i e d as a c e n t r a l d u p l e x of three h o r s t s that are

s e p a r a t e d f r o m two s i m p l e ra mps at e i t h e r end of the o u t c r o p by

f a u l t s are p r e s e n t a l o n g the u p p e r and l o w e r s u r f a c e of the m i c r i t e

w h e r e they m e r g e w i t h ramp f a u l t s that cut a c r o s s the m i c r i t e bed

at an a n g l e of a p p r o x i m a t e l y 30 d e g r e e s The l o w e r b e d d i n g p l a n e

f a u l t or f l o o r t h r u s t is r e l a t i v e l y p l a n a r and the fault zone is thick In c o m p a r is o n the u p p e r b e d d i n g p l a n e f a u l t s are f o l d e d in the ramp areas, cut by ramp faults, and the f a u l t zones are thin The u p p e r b e d d i n g p l a n e f a u l t f o r m s the r o o f t h r u s t for the c e n t r a l

d u p l e x A l o n g the i n t e r v e n i n g f l a t s the u p p e r f a u l t s are g e n e r a l l y

The l o w e r b e d d i n g p l a n e f a u l t is the m a j o r d e c o l l e m e n t a c r o s s the

o u t c r o p O l d e r b e d d i n g p l a n e s f a u l t s are also p r e s e n t t h r o u g h o u t the s h a l e and are o f f s e t by the p e n e t r a t i v e S1 c l e a v a g e

All the faul t s u r f a c e s are c o v e r e d by l a y e r s of s p a r r y c a l c i t e that

A - 5

89

f o u n d a l o n g the d e c o l l e m e n t or f l o o r t h r u s t w h e r e a s the t h i n nest

d i s c o n t i n u o u s s e l v e d g e of d a r k g r a y sh a l e Each of the l a y e r s are

e s s e n t i a l l y p a r a l l e l f r o m l a y e r to l a y e r (fig 7b) In s e c t i o n s

s l i c k e n l i n e s the sh ale s e l v e d g e s are m o r e p a r a l l e l to each o t h e r

s e c t i o n of the g r o o v e s In the p a r a l l e l s e c t i o n s , h o w e v e r , some of the s e l v e d g e lay e r s are t r u n c a t e d by m o r e c o n t i n u o u s s u r f a c e s One

of th ese can be t r a c e d for 5 feet or m o r e a l o n g the d e c o l l e m e n t

At a n u m b e r of p l a c e s a l o n g the d i f f e r e n t fault zones s m a l l dik es

of s p a r r y c a l c i t e have i n t r u d e d the l o w e r part of the c a l c i t e - s h a l e

a r e a s and a l o n g the w e s t - f a c i n g limb s or s m a l l f l e x u r e s A few are

p r e s e n t in the flat r e g i o n s of the bed The f r a c t u r e s in m a n y of

y o u n g e r g e n e r a t i o n s of en e c h e l o n f r a c t u r e s In the e a s t e r n ramp the h a n g i n g w a l l and f o o t w a l l are cut by n e a r - v e r t i c a l f r a c t u r e s that are f i l l e d w i t h f r a g m e n t s of the s u r r o u n d i n g m i c r i t e e m b e d d e d

in s p a r r y c a l c i t e so as to f o r m c l a s t i c dikes

Two, w e l l d e v e l o p e d , p r e s s u r e - s o l u t i o n c l e a v a g e s are p r e s e n t

o r i e n t a t i o n is an a v e r a g e b a s e d on m e a s u r e m e n t s t a k e n a c r o s s the

v e r t i c a l face of the o u t c r o p b e c a u s e the i n d i v i d u a l S1 s u r f a c e s are

d i a m o n d - s h a p e p a t t e r n on the b e d d i n g s u r f a c e The a c u t e a n g l e ofthe d i a m o n d p a t t e r n is a p p r o x i m a t e l y 30 d e g r e e s in the sha le and 50

d e g r e e s in the m i c r i t e bed T h i s g e o m e t r y i n d i c a t e s a m o d e r a t e

l e v e l of c l e a v a g e d e v e l o p m e n t S1 s u r f a c e s are c o v e r e d by a black,

c a r b o n - r i c h s e l v e d g e of i l l i t e and k a o l i n i t e w h i c h is less than two

v e r t i c a l l y c o n t i n u o u s t h r o u g h the shale, some of t h e m are sho rt and

d i s c o n t i n u o u s w i t h t a p e r e d ends The t h i c k e s t s e l v e d g e o c c u r s onthe m o s t c o n t i n u o u s s u r f a c e s The s u r f a c e s of the s e l v e d g e are not

d i s p l a c e m e n t is g r e a t e s t w h e r e a s e l v e d g e is the t h i c k e s t and it is

g r a d u a l l y r e d u c e d to zero as a s e l v e d g e thins t o w a r d the t a p e r e d

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91

m i c r o l i t h o n s b e t w e e n the S1 s u r f a c e s is 2.2 i n c h e s thick The S1

a c r o s s most of the roof f a u l t s on top of the m i c r i t e bed

A s e c o n d , w e l l d e v e l o p e d c l e a v a g e , St, is r e s t i c t e d to a f o o t - t h i c k

a l o n g the roof th r u s t of the m i c r i t e bed, but the zone is t h i n n e r

and it is more d i f f i c u l t to r e c o g n i z e b e c a u s e the roof f au l t s h a v e

is d e f i n i t e l y r e l a t e d to m o v e m e n t of the t h r u s t f a u l t s b e c a u s e it

is only f oun d n e a r the f a u l t s and it is a b s e n t away from them

c l e a v a g e is r o t a t e d e a s t w a r d so that the s t e e p e r 60 d e g r e e dip in

i n d i c a t e d by the s l i c k e n l i n e s a l o n g the d e c o l l e m e n t This se nse of

d i s p l a c e m e n t is c o n s i s t e n t w i t h the o r i e n t a t i o n of St s inc e the

the m i c r i t e This c l e a v a g e is not d e v e l o p e d u n i f o r m l y t h r o u g h o u t

a r r a y s in the f o o t w a l l and h a n g w a l l that are t r u n c a t e d by the ramp

A -5

92

A -5

f r a c t u r e s i n d i c a t e s that the f r a c t u r e s o p e n e d r a p i d l y and at a

s t r o n g e n o u g h to s u p p o r t the s h a p e of the c a v i t i e s E a s t - c l i m b i n g ,

en e c h e l o n f r a c t u r e a r r a y s are a l s o p r e s e n t in the m i c r i t e w h e r e

they are a b u n d a n t in the ramp r e g i o n s

F a u l t C h r o n o l o g y

o l d e r than these f a u l t s b e c a u s e they are cut and o f f s e t by the S1

c l e a v a g e A l t h o u g h the b e d d i n g p l a n e f a u l t s that are in d i r e c t

c o n t a c t w i t h the m i c r i t e bed may h a v e f o r m e d o r i g i n a l l y at this

time there is a b u n d a n t e v i d e n c e that they c e r t a i n l y were a c t i v e

y o u n g e r

The e v i d e n c e for the r e l a t i v e age of each of the f a u l t s is f o u n d in

the r e g i o n s w h e r e the ramp f a u l t s m e r g e w i t h the roof and f l o o r

f a u l t (Tn-1) cuts a c r o s s the u p p e r - b e d d i n g p lane fau lt (Tn-2) of

the h a n g i n g w a l l b l o c k but m e r g e s a s y m t o p i c a l l y w i t h the f l o o r f a u l t

slip s u r f a c e s in the c a l c i t e - s h a l e f a u l t zone cut a c r o s s the f aul t

same r e l a t i v e c h r o n l o g y a p p l i e s to all the ramps and d u p l e x e s to

the east

i

S e v e r a l i m p o r t a n t c o n c l u s i o n s r e s u l t f r o m this a n a l y s i s First,

the i m b r i c a t e f a u l t s w h i c h c o m p r i s e the roof, ramp, and f l o o r f ault

s y s t e m b e c o m e y o u n g e r to the w e s t or the f o r e l a n d S e c o n d , the

f l o o r fa ult is c o n t i n u a l l y r e a c t i v a t e d d u r i n g the e v o l u t i o n of the

y o u n g e r to the west F u r t h e r m o r e , it is the m o s t c o n t i n u o u s f a u l t

in the o u t c r o p This s e c o n d c o n c l u s i o n e x p l a i n s w h y the f ault zone

for the f l o o r fault is m u c h t h i c k e r t h a n the fau lt zo nes a l o n g the

c o n t i n u i t y of a fau lt zone t h e r e f o r e are d i r e c t l y r e l a t e d to the