I- lV, Structural Features VI, The Bathtub Syncline WI, Structural Analysis of the Bathtub - North Point Area -11, The Problem of the Joints , IX, References t Xllnstrations 1 9 13

- . . I -

l

Geolouv. of t h e B a t h t u b ' - North Point Area

San Mateo C o u n t y , C a l i f o r n i a

3780 7 . H515 BRAN - - .- - -

Rouer Henneberger

Dent" of Geoloqy, //

S t a n f o r d U n i v e r s i t y

Mav 25, 1977

C o n t e n t s

Paae

I . I,

It

I, I n t r o d u c t i o n

11, S t r u c t u r e a n d Pock U n i t s of t h e Half . Voon Ray Reqion

III, Local Rock U n i t s

IV, Probable Sou. rces of S e d i m e n t s

V, S t r u c t u r a l F e a t u r e s

VI, The B a t h t u b S y n c l i n e

W I , S t r u c t u r a l A n a l y s i s of t h e B a t h t u b - North P o i n t Area

-11, The Problem of t h e J o i n t s ,

I X , R e f e r e n c e s

t

X l l n s t r a t i o n s

1

9

13

14

Base

niaqrammatic Cross S e c t i o n s o f t h e B a t h t u b S y n c l i n e ( F i q s , 1-4) lfl

nown P l u n q e V i e w o f t h e Mar, Area Plate 1

S-Pole Diagram for t h e B a t h t u b S y n c l i n e Plate 2

R e c o n s t r u c t i o n o f Crossbed D i p D i r e c t i o n s P l a t e 3

III. Local Rock U n i t s

The rocks of t h e R a t h t u b - North P o i n t area are of

:am q e n e r a l tvDes: t h e Montara Clruartz Diorite (which is

m m o n l v cal led t h e M o n t a r a G r a n i t e , a n d w i l l be so-called i n

:his DaDer), and a set of : c o n f o r m a b l e , l i t h o l o g i c a l l y

s i v i l a r m a r i n e s e d i m e n t a r y rocks, a m a r e n t l y a l l of t h e

31]r isima F o r m a t i o n , I n a d d i t i o n , O u a t e r n a r v d e b r i s is

3 r e s e n t . T w e l v e u n i t s were d i s t i n s u i s h e d , two of which a re

i o c a l l v d i v i d e d i n t o s u b u n i t s , The u n i t s are B r i e f l y

aescribed here.

rgr - T h e m o n t a r a G r a n i t e : a a u a r t z d i o r i t e c o n t a i n i n q - m a i n l y nlaqioclase, n o t a s s i u m feldspar and b i o t i t e , w i t h a

a v e r a b e q r a i n s i z e of a b o u t 2-4 mm, I t is e x n o s e d i n t h e

N o r t h P o i n t area as a b r o w n , i n t e n s e l y weathered and

i o i n t e d rock. I t is e x n o s e d i n , c o n t a c t o n l y w i t h u n i t s Tsm

and T c l , b u t it is Presumed t o be i n c o n t a c t w i t h a t l ea s t

A:

I

t h e u n i t s o v e r l v i n a Tsm,

+-c is a f i n e , d a r k qray s a n d s t o n e w h i c h is e x t e n s i v e l v - e x n o s e d i n t h e R a t h t u b area. I t c o n t a i n s a t l e a s t one and

Drobablv s e v e r a l t h i n layers o f f o s s i l material , c o n s i s t i n s

mostly of d i s a r t i c u l a t e d m o l l u s k s , The fossils t h u s seem

t o be t r a n s n o r t e d from t h e i r o r i s i n a l e n v i r o n m e n t , a n d t h i s

a l o n q w i t h t h e q r a i n s i z e a n d lack of bedt l inq of t h e s a n d

i n d i c a t e t h a t t h e s e d i m e n t was Drobah ly deposi ted on t h e

v i d - t o - o u t e r c o n t i n e n t a l s h e l f , The base of t h e u n i t is

n o t e x n o s e d , so t h e t h i c k n e s s is i n d e t e r m i n a t e , b u t it is

a t l ea s t s e v e r a l meters and may be c o n s i d e r a b l v qreater,

Y*

Tb1 is a t h i n l ayer of h a r d , l i q h t o r a n q e - q r a v , f i n e - breccia c o n t a i n i n q a n a u l a r feldspar, a u a r t z a n d d a r k

m i n e r a l f r a s m e n t s i n a s a n d y m a t r i x , I t is e x p o s e d i n t h e

B a t h t u b area, The l a r se r f r a s m e n t s amear t o be f i r s t

a e n e r a t i o n a n d so are presumed t o be from t h e Montara

G r a n i t e , T h i s u n i t , a s a l l o v e r l v i n q u n i t s , v a r i e s

somewhat i n t h i c k n e s s o v e r s h o r t d i s t a n c e s , T h i s a l o n q

w i t h t h e c o a r s e n e s s of t h e s e d i m e n t i n d i c a t e s a t u r b u l e n t

o r debris-flow tyDe of d e n o s i t i o n a l m e c h a n i s m ,

'PSI is a f ine - to -med ium q ra ined , r ed -o ranqe crossbedded - s a n d s t o n e , m a i n l v a u a r t z o - f e l d s n a t h i c b u t w i t h a 3-5% dark

m i n e r a i c o n t e n t , As i n t h e case of Tbl, i ts small and

v a r i a b l e t h i c k n e s s a n d lack of f o s s i l material i n d i c a t e a

Drobable mass-flow tvpe d e n o s i t i o n , Its orimary e x p o s u r e

i s i n t h e B a t h t u b area, and it is d i v i d e d i n t o two u n i t s i n

t h e s o u t h e r n pa r t , o n e o € w h i c h ( T b l 2 ) l e n s e s o u t t o t h e

s o u t h ,

is a breccia similar t o Tbl, I t is e x p o s e d i n t h e - 1 / 'f s o u t h e r n g a r t o f t h e b a t h t u b and l e n s e s o u t t o t h e n o r t h , F.

\

-ci is a coarse c o n a l o m e r a t e w h i c h c o n t a i n s r o u n d e d cobbles - of q r a n i t i c rock i n a s a n d y m a t r i x , The cobbles c o u l d n o t

b a v e b e e n t r a n s p o r t e d a w e a t d i s t a n c e a n d so probably are

from t h e Montara Grani te , Flowever , t h e a v e r a q e a r a i n s i z e

i n t h e cobble is la rse (4-7 mm) a n d w e a t h e r i n q i s min ima l ,

so it is l i k e l v t h a t t hev were n o t der ived from t h e q r a n i t e

es is sand and beach debris , w h i c h o b s c u r e s t h e older rock a t - some l o c a t i o n s , n a r t i c u l a r l y i n t h e c e n t r a l R a t h t u b ,

IV. Probable S o u r c e of S e d i m e n t s

The s e d i m e n t s , a s n o t e d , are nr imar i lv a u a r t z a f e l d s p a t h i c

s a n d s , a l o n q w i t h c las t s from t h e M o n t a r a G r a n i t e , T h e

d e o o s i t i o n a l e n v i r o n m e n t is a mid-shelf o r o u t e r - s h e l f

e n v i r o n m e n t for Tsf, and fo r t h e o ther u n i t s it is some s e t t i n q c

caoable o f n r o d u c i n q h i q b v e l o c i t v mass-flow d e n o s i t i o n , The

e x t r e m e s i z e of some clasts and t h e lack of a n y s h a l y layers

Takes sloDe d e n o s i t i o n u n l i k e l y , a l t h o u q h n o t impossible, The r: ' ,

I

- O s t n l a u s i h l e e n v i r o n m e n t i's a shallow i n n e r - s h e l f or mid-shelf i$

s e t t i n q , w i t h ranid n e a r b y t e c t o n i ç u n l i f t c r e a t i n q a s o u r c e of i,

:Darse s e d i m e n t w i t h low o r q a n i c a n d s i l t / c l a v comnonents ,

R e c o n s t r u c t e d crossbed d i n s t r e n d a b o u t 3 5 deqrees east of I ' .

. . "...,... - x t h i n t h i s area (P la te 3 ) , so i f t h e c r o s s b e d d i n q is an

~ c u r a t e i n d i c a t o r of n a l e o c u r r e n t d i r e c t i o n , a s e d i m e n t s o u r c e i.

c 4 _ _ t h e s o u t h w e s t m u s t be n o s t u l a t e d , The re is a modern ana loq "

= - c t h i s i n P i l l a r P o i n t , w h i c h is a n area of h i q h re l ie f t o t h e

---:=c o f t h e low- lv inq areas of Half Moon Bav, created t h r o u q h

- - l i f t hv f a u l t i n q , A similar s i t u a t i o n i s c o n c e i v a b l e for t h e

v - - 2 s Reach a rea , w i t h a n of fshore uDthrown f a u l t block n r o d u c i n q

: -Grse s e d i m e n t s w h i c h were denosi ted i n shallow water t o t h e

- - r z h e a s t , %e t r e n d of t h e f a u l t c a u s i n q t h e u n l i f t , based on

- - C z a i e o c u r r e n t r e c o n s t r u c t i o n , w o u l d h a v e b e e n similar t o t h a t

- = olrisltina f a u l t s of t h e r e q i o n ,

-6- ,

L

V, S t r u c t u r a l F e a t u r e s

, . . l - . ..

The primary s r u c t u r a l f e a t u r e o f t h e area is t h e B a t h t u b

s v n c l i n e , described i n d e t a i l below, A l t h o u s h t h e area t o t h e

s o u t h e a s t was n o t manneil, t h e sharp d i s c o r d a n c e of t h e folded

rocks o f t h e b a t h t u b w i t b t h e l i n e a r reefs t o t h e s o u t h e a s t

i n d i c a t e s t h e p r e s e n c e .of a f a u l t , T h e r e are some small-scale

t h r u s t f a u l t s a t t h e head of t h e s y n c l i n e , w i t h o f f s e t slip i n

v a r i o u s d i r e c t i o n s b u t primarily n o r t h e a s t - s o u t h w e s t , A f a i r l y

d i s t i n c t set of j o i n t s Dermeates t h e area, and t h e j o i n t i n q

o c c u r s i n a l l rock u n i t s a n d t h u s was a c t i v e a t l eas t u n t i l a f t e r

d e n o s i t i o n a n d orobalolv a f t e r f o l d i n s of t h e s y n c l i n e rocks,

Some o f these i o i n t s have tonosranhic e x o r e s s i o n i n t h e shapes of

o n s h o r e rock ledses, Mo e v i d e n c e of f a u l t d i m l a c e m e n t

a s s o c i a t e d w i t h t h e j o i n t i n q was o b s e r v e d ,

M u l t i n l e i o i n t s were o b s e r v e d i n t h e W o n t a r a G r a n i t e a t

':orth P o i n t , a n d - m a y commise two or more sets, There may i n

= a c t be some o f f s e t a l o n q t h e S u r f a c e , b u t t h i s c o u l d n o t be

x o v e n d u e t o t h e homogene i ty of t h e rock,

T h e r e apnears t o he some s e c o n d a r v d e f o r m a t i o n a t N o r t h

?!nt and i n t h e t h e area w i t h i n lflfl meters s o u t h of N o r t h P o i n t ,

:S f o l d i n s c a n be s e e n i n t h e down-Plunse view (Plate 1) , The

- r r a c t qeometry o f tb .e N o r t h P o i n t s t r u c t u r e i s m i s r e o r e s e n t e d i n

--is v i e w , t h o u q h , b e c a u s e t h e r e q i o n a l p l u n q e i n t h i s area has

---L.? chanqed as a r e s u l t of t h e s e c o n d a r v w a r n i n q ,

- -- -

-7-

V1,The B a t h t u b S y n c l i n e

The B a t h t u b is t h e t o D o q r a D h i c e x n r e s s i o n of a n eroded

o ï u n o i n a s y n c l i n e , t h e a x i s of w h i c h P u n u e s 12 deqrees i n a

q i r e c t i o n 3 3 desrees west o f N o r t h (Plate 2) , The fo lded rocks

are u n i t s ? s f t h r o u q h TSU, T h e beds o f t h e s o u t h w e s t e r n limb d i D

tuch more steeply t h a n those of t h e n o r t h e a s t e r n limb ( 4 5 " as

oawsed t o 1 f l - 1 S 6 ) , so i t seems likely t h a t i f t h e f o l d i n q was

d u e t o a loca l e v e n t , - t h e n t h e primarv d e f o r m i n s force m u s t h a v e

*en i n t h e s o u t h w e s t , The down-olunqe view shows t h e f o l d t o be

fairlv r e q u l a r e x c e p t í n ' t h e s e c o n d a r i l v deformed areas n o t e d

above. The a n p e a r a n c e of o v e r t u r n i n s i n t h e n o r t h e a s t is d u e t o

the n r e v i o u s l y m e n t i o n e d d i s t o r t i o n i n t h e diaqram: it is

u n l i k e l v t h a t t he re is a n y t r u e o v e r t u r n i n s i n t h e s y n c l i n e ,

T h e q e n e r a l f o l d u e o m e t r v seems t o be t h e r e s u l t of

Since t h e f o l d a x i s oa ra l l e l s t h e r e q i o n a l f a u l t t r e n d s , t h e

s v n c l i n e is mobably closely related t o t h e f a u l t i n g . a c t i v i t y ,

No di rec t e v i d e n c e for s h e a r b e t w e e n beds was f o u n d i n t h e

Q a t h t u h e x c e n t for t h e t h r u s t f a u l t i n q a t t h e s o u t h e a s t e r n Dart

oF t h e s v n c l i n e h e a d , which Drolnably accomodated some s h o r t e n i n q ,

TheCe is a Dossible i n d i r e c t i n d i c a t i o n of shear bv t h e c h a n q e i n

the a b n a r e n t t h i c k n e s s of t h e beds a t t h e h e a d of t h e s y n c l i n e ,

V U and Tsm become d i s t i n c t l y t h i c k e r i n t h e d o w n - d u n q e view,

m 4 this c o u l d be d u e t o s l i D a l o n q b e d d i n u p l a n e s i n s t e a d of

a c t u a l t h i c k e n i n q , S u c h s l i n (or a c t u a l t h i c k e n i n q ) c a n n o t be 1

x m e n c o n c l u s i v e l v d u e t o t h e o r i q i n a l i n c o n s i s t e n t bed

t h i c k n e s s ,

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îu NE

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a P.,

- \ ' / K I ' '

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I!

l

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l

T h e ea r l i e s t recorded e v e n t is t h e s h e l f d e p o s i t i o n of t h e

f o s s i l i f e r o u s s a n d s t o n e Tsf ( W u , l), A l t h o u q h n o t e c t o n i c

a c t i v i t v is d i r ec t ly i n d i c a t e d , it is Prohable t h a t t e c t o n i c

movement and deDost ion were s i m u l t a n e o u s t h r o u s h most or a l l of

t h e Deriod of t h e f o r m a t i o n of t h e R a t h t u b , A t some p o i n t a

maior ,block of M o n t a r a G r a n i t e was u d i f t e d o f fshore t o t h e

s o u t h w e s t o f t h e coast ( p i u , 2 ) " Prom t h i s block. t h e s a n d s t o n e

- breccia - c o n q l o m e r a t e s e d i m e n t s were d e r i v e d , T h e s e u n i t s mav

be n a r a c o n f o r m a b l e w i t h t h e Tsf u n i t , b u t there is no d i rec t

e v i d e n c e t o i n d i c a t e t h e l e n q t h of t h e d e n o s i t i o n a l h i a t u s , i f

anv,

1

As a r e s u l t of t h e same or s u b s e q u e n t d i D - s l i n f a u l t i n s i n

t h e S o u t h w e s t , t h e P u r i s i m a s e d i m e n t s were fo lded i n t o the i r

n r e s e n t s v n c l i n a l form (Fia, 3 ) , There was c o m m e s s i v e stress

of t h e f a u l t block, n r o d u c i n s s h o r t e n i n s which was t a k e n UD by a

c o m b i n a t i o n o f f o l d i n a , small-scale t h r u s t f a u l t i n s , a n d Dossiblv

sl io between beds, If t h e f a u l t i n s was i n f a c t t h e s o u r c e o f t h e

c o m o r e s s i v e force, it would have been no rma l s l i n , a s in.dicated

i n Ficl, 3 ,

F o l l o w i n s , or s i m u l t a n e o u s w i t h t h i s d e f o r m a t , i o n , t h e

ranit te hodv t o t h e n o r t h e a s t deformed t o o r o d u c e low a m d i t u d e

seconi la rv fo lc l incr in t h e North P o i n t area ( F i s , 4 ) , T h i s is

x a b a b l v related t o t h e i o i n t i n a ( a n d Possible f a u l t i n c r ) o b s e r v e d

a t Worth p o i n t , b u t t h e r e 1 a t i o n s h i D is u n c l e a r , a n d i t s

+ t e r m i n a t i o n w o u l d r e q u i r e a more de ta i led s t u d y of t h e q r a n i t e ,

The Dlunqe of t h e s y n c l i n e is mobab ly d u e t o t h e same

e v e n t s t h a t P roduced t h e f o l d i n s , I t is n o t e d t h a t t h e P i l l a r

= o i n t f a u l t h lock aooears t o a l so n l u n q e n o r t h w e s t , a n d so it is

l

I

-12-

mssihle t h a t t h e r e is a tilt cornnonent t o t h e f a u l t d i s n l a c e m e n t

1 i? t h i s area w h i c h p r o d u c e d t h e l oca l n l u n q e ,

VIII, The Problem of t h e J o i n t s

The r e l a t i o n s h i n t o t h e s v n c l i n e of t h e n o r ' t h e a s t - s o u t h w e s t

+ o i n t set r e m a i n s u n c l e a r , 'It may be a r e s u l t of t h e local

% f o r m a t i o n a l stresses, or it mav be a n e x m e s s i o n of a more

cea iona l a a t t e r n o f i o i n t i n s , A s t u d y of t h e i o i n t i n q i n t h e

rrltire resion would be u s e f u l i n r e s o l v i n q t h i s q u e s t i o n , /

I

-13- I

IX, R e f e r e n c e s

C u r t i s , G , , H , , Evernden, J , F , , , and L i p s o n , J , , 19.58, Aqe

n e t e r m i n a t i o n ’ o f some G r a n i . t i c Rocks i n Ca l i f bv t h e R-Ar

method,, Ca, Div, of Mines Snec, Report 5 4 ”

0

g i n t e r , Dw.A,,r 1 9 7 6 , The Geoloqv of Moss B e a c h , C a l i f o r n i a : An

I n t e r D r e t i v e A n a l y s i s o f t h e S t r u c t u r a l F e a t u r e s , S t u d e n t

r e o o r t f o r Der% of Geoloqv, Stanford IJniv,