OPEN FILE REPORT 129
GEOLOGY OF T H E S A W M I L L C A N Y O N A R E A
WF T H E M A G D A L E N A M O U N T A I N S
SOCORRO COUNTY, NEW MEXICO
by
S u s a n J. Roth
Submitted in Partial Fulfillment
of t h e R e q u i r e m e n t s f o r t h e D e g r e e o f
Master of Science in Geology
New Mexico Institute of Mining and Technology
Socorro, New Mexico
.
August, 1980
ii
TABLE OF CONTENTS
ABSTRACT
......................
1NTRODUCTION
vii
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G e o l o g i cS e t t i n g
Objectives
L o c a t i o na n dA c c e s s i b i l i t y
M e t h o d so fI n v e s t i g a t i o n
P r e v i o u s Work
Acknowledgements
*
1
1
2
2
6
9
. . . . . . . . . . . . 11
H e g i o n a lS t r a t i g r a p h y
. . . . . . . . . . . . . . 11
H e l l s MesaTuff
. . . . . . . . . . . . . . . . . 13
U n i to fH a r d yR i d g e
. . . . . . . . . . . . . . . 16
a n d e s i t e s a n d s e d i m e n t a r yr o c k s . . . . . . . . 1 7
r h y o l i t el a v a s
. . . . . . . . . . . . . . . . 19
A - L P e a k T u f f . . . . . . . . . . . . . . . . . . 20
l o w e r member . . . . . . . . . . . . . . . . . 26
r h y o l i t e l a v a . . . . . . . . . . . . . . . . . . .33
u p p e r member . . . . . . . . . . . . . . . . . 33
U n i t o fS i x m i l eC a n y o n
. . . . . . . . . . . . . . 36
l o w e r member . . . . . . . . . . . . . . . . . 37
r h y o l i t e dome c o m p l e x . . . . . . . . . . . 37
v o l c a n i c l a s t i cr o c k s
a n d t u f f s . . . . . . . 38
ash-flowtuff
.
.
.
.
.. .. .. .. .. .. .. .. .. .. .. 40
s a n i d i n er h y o l i t e
.
.
40
u p p e r member . . . . . . . . . . . . . . . . . 4 1
a n d e s i t el a v a s . . . . . . . . . . . . . . . 4 1
t u f fo fC a r o n i t a
Canyon . . . . . . . . . . 42
l o w e r member . . . . . . . . . . . . . . 4 4
u p p e r member . . . . . . . . . . . . . . 45
T u f fo fL e m i t a rM o u n t a i n s
. . . . . . . . . . . . 46
l o w e r member . . . . . . . . . . . . . . . . . 48
u p p e r member . . . . . . . . . . . . . . . . . 48
T u f fo fS o u t hC a n y o n
. . . . . . . . . . . . . . . 51
U n i t o fS a w m i l lC a n y o n
. .. .. .. .. .. .. ..... .. .. .. .. .. 52
52
p y r o c l a s t i cd e p o s i t s
STRATIGRAPHY AND PETROGRAPHY
e
1
iii
. . . . . . . . . . . . . . . . 54
P o p o t o s a F o r m a t i o n . . . . . . . . . . . . . . . '. 5 5
l o w e r member . . . . . . . . . . . . . . . . . 57
a s h - f l o w t u f f . . . . . . . . . . . . . . . . . 57
u p p e r member . . . . . . . . . . . . . . . . . 58
S a n t aF eG r o u p( u n d i f f e r e n t i a t e d )
. . . . . . . . 59
T e r t i a r yi n t r u s i v e
r o c k s . . . . . . . . . . . . . 59
a n d e s i t ed i k e s
.
. . . . . . . . . . . . . . . 60
L a t i t ed i k e . . . . . . . . . . . . . . . . . . 6 0
p o r p h y r i t i cr h y o l i t ed i k e s
.. .. .. .. .. .. .. .. .. .. 606 1
.
.
.
flow-banded
a n rdhi yk
woehlsii tt ee
dome . . . . . . . . . 62
Quaternary-Tertiarydeposits . . . . . . . . . . . 64
alluvi.al-f.an.,gravel.s. . . . . . . . . . . . . 6 4
Q u a t e r n a r yd e p o s i t s
.. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 66 44
p i e d m o n tg r a v e l s
t e r r a c e g r a v e l s . . . . . . . . . . . . . . . . 64
t a l u s a n d c o l l u v i u m . . . . . . . . . . . . . . 65
slump block . . . . . . . . . . . . . . . . . . 65
a l l u v i u m . . . . . . . . . . . . . . . . . . . 65
v a l l e y a l l u v i u m . . . . . . . . . . . . . . . .6 5
r h y o l i t el a v a s
rhyolites
. . . . . . . . . . . . . . . . . . . . . . 66
R e g i o n a l S t r u c t u r e . . . . . . . . . . . . . . . . 66
L o c a lS t r u c t u r e
. . . . .. .. .. .. .. .. .. .. .. .. .. .. .. 68
N o r t hB a l d yc a u l d r o n
70
S a w m i l l Canyon c a u l d r o n . . . . . . . . . . . . 7 1
b l o c kf a u l t i n g
. . . . . . . . . . . . . . . . 72
A L T E R A T I O N AND M I N E R A L I Z A T I O N . . . . . . . . . . . . 75
Alteration. . . . . . . . . . . . . . . . . . . .
75
M i n e r a l i z a t i o n . . . . . . . . . . . . . . . . . . 78
CONCLUSIONS . . . . . . . . . . . . . . . . . . . . .
80
Stratigraphy. . . . . . . . . . . . . . . . . . . .
80
Structure . . . . . . . . . . . . . . . . . . . .
81
STRUCTURE
iv
. . . . . . . . . . 82
REFERENCES . . . . . . . . . . . . . . . . . . . . . .
83
A l t e r a t i o na n dM i n e r a l i z a t i o n
z
V
PLATES
1
G e o l o g i c Map a n dS e c t i o n s
of thS
e a w m i lC
l anyon
area
o ft h eM a g d a l e n aM o u n t a i n s ,S o c o r r oC o u n t y ,
New Mexico
( i n pocket)
2
S t r u c t u r a lG e o l o g y
pocket)
o f t h eS a w m i l l
Canyon Area ( i n
FIGURES
page
1
of t h e
G e n e r a ll o c a t i o n
map s h o w i n g t h e r e l a t i o n s h i p
studyareato.major~physiogra,ph.ic.featuresandroutes
of a c c e s s i n S o c o r r oC o u n t y .
3
A e r i a lp h o t o g r a p ho ft h es t u d ya r e a .
4
A e r i a lp h o t o g r a p ho ft h es t u d ya r e a .
............
........
........
7
R e l a t i o n s h i p o f t h i ss t u d ya r e at op r e v i o u ss t u d i e s .
5
9
10
5
C o r r e l a t i o nd i a g r a mo fp r e v i o u ss t r a t i g r a p h i cs t u d i e s
i nt h eS o c o r r o - M a g d a l e n aa r e a .
. . . . . . . . . . . 10
S t r a t i g r a p h i cc o l u m no ft h es t u d ya r e a
. . . . . . . 12
Mesa T u f f . . 1 5
P h o t o g r a p h o f m e s o b r e c c i ai nt h eH e l l s
P h o t o g r a p h of s u b s p h e r i c a l d e v i t r i f i c a t i o n t e x t u r e
. . . . . . . . . . . . . . 21
f o u n d i n r h y o l i t el a v a s .
P h o t o g r a p h o f t h ec o n t a c tb e t w e e nt h eu n i to fH a r d y
A-L P e a kT u f f .
. . . . . . . . . . . . 22
R i d g ea n dt h e
A-L
P h o t o g r a p h o f s e d i m e n t a r yr o c k su n d e r l y i n gt h e
T u f f . . . . . . . . . . . . . . . . . . . . . . . . 2 3
Peak
I
i n t h e A-L
11
P h o t o g r a p h o f s h e e tj o i n t s
12
P h o t o g r a p h o f l i n e a t e dp u m i c ei nt h e
13
P h o t o g r a p h of a p r i m a r yf o l d
14
P h o t o g r a p ho fr a m p - l i k es t r u c t u r e s
P e a kT u f f .
A-L
i n t h e A-L
P e a kT u f f
P e a kT u f f .
. . 27
. 28
. 31
. . . . . . . . . . . . . . . . . . . . i.n. t.h e. .A-L. .P.e a kT u f f32
vi
c
15
P h o t o g r a p ho d
f e b r i s - f l o wd e p o s i t s
S i x m i l eC a n y o n
16
P h o t o g r a p ho fa n g u l a r
a n d l e n t i c u l a rf r a g m e n t s
t u f fo fL e m i t a rM o u n t a i n s .
17
18
19
20
i n t h eu n i to f
. . . . . . . . . . . . . . . . . . . 39
. . . . . . . . . . i .n .t h.e 5 0
P h o t o g r a p h o f c l i f f - f o r m i n gr h y o l i t el a v a si nt h eu n i t
oS
f a w m i lC
l anyon.
. . . . . . . . . . . . . . . . . 56
P h o t o g r a p ho f
a w h i t er h y o l i t ed i k ei n t r u s i v e
into the
A-L P e a kT u f f ,r o t a t e dt o
a s u b h o r i z o n t a lp o s i t i o n ,a n d
o v e r l a i n by Q u a t e r n a r yt e r r a c eg r a v e l s
63
.......
R e l a t i o n s h i po fc a u l d r o ns t r u c t u r e si nt h eM a g d a l e n a
. . . . . . . 69
M o u n t a i n st ot h et r a n s v e r s es h e a rz o n e
I n t e n s e l yh e m a t i t e - s t a i n e do u t c r o po tf h e
r h y o l i t e dome c o m p l e x i n t h e u n i t o f
S ~ i x m i . l eC a n y o n
. . . . . . . . . . . . . . . . . . . 77
vi i
ABSTRACT
T h es o u t h e r nM a g d a l e n aM o u n t a i n sa r ec h a r a c t e r i z e d
by a t h i c k p i l e o f T e r t i a r y c a u l d r o n - f a c i e s v o l c a n i c r o c k s
i s c o n t r o l l e db y
a n da s s o c i a t e di n t r u s i v e s .S t r u c t u r a ls t y l e
c a u l d r o ns t r u c t u r e sa n dn o r m a lf a u l t st h a ta r er e l a t e dt ot h e
rift.
R i oG r a n d e
i n t h ea r e a
T h eo l d e s tr o c ku n i te x p o s e d
is the
Mesa
H e l l s M e s aT u f fo fO l i g o c e n ea g e .O v e r l y i n gt h eH e l l s
a r e i t s c a u l d r o n - f i l ld e p o s i t sa n d ,
A-L
P e a kT u f f
i n a s c e n d i n go r d e r ,t h e
a n d i t s c a u l d r o n - f i l ld e p o s i t s ,t h et u f fo f
L e m i t a rM o u n t a i n s ,t h et u f fo fS o u t hC a n y o n ,
and t h er h y o l i t e
of S a w m i l lC a n y o n .
l a v a sa n dp y r o c l a s t i cr o c k so ft h eu n i t
An o v e r l y i n gs e q u e n c eo fm u d f l o wd e p o s i t sa n df a n g l o m e r a t e s
i s r e p r e s e n t e db yt h eP o p o t o s aF o r m a t i o no fe a r l yt om i d d l e
M i o c e n ea g e .
S e v e r a lo v e r l a p p i n gc a u l d r o ns t r u c t u r e sa r el o c a t e d
i nt h eM a g d a l e n aM o u n t a i n sa r e a .
132 m.y.)
T h eN o r t hB a l d yc a u l d r o n
u n d e r l i e st h ee n t i r es t u d ya r e aa n d
b u r i e db yy o u n g e rr o c k s .
c a u l d r o n (32-28
m.y.1
is largely
A p o r t i o no ft h eS a w m i l lC a n y o n
margin i s l o c a t e d i n t h i s t h e s i sa r e a
a n dc o n t r o l st h es t r u c t u r a lt r e n dh e r e .
H e g i o n a le x t e n s i o nb e g a n
i n m i d d l eO l i g o c e n et i m e
c o n c u r r e n tw i t hc a u l d r o na c t i v i t ya n dh a sc o n t i n u e du n t i lt h e
p r e s e n td a y .N o r m a lf a u l t i n g
a s . a r e s u l to ft h i se x t e n s i o n
h a sb e e ns u p e r i m p o s e do nc a u l d r o ns t r u c t u r e si n ' ad o m i n a n t l y
viii
*
north-northwestrend.
b l o c k sa r ed r o p p e d
I n t h es o u t h e r nM a g d a l e n aM o u n t a i n s ,
down t o t h e w e s t a n d s t r a t a
is tilted to
t h ee a s t .R h y o l i t ea n dl a t i t ed i k e sh a v eb e e ni n t r u d e da l o n g
t h e s ef a u l t sa n df o l l o wt h e
T h es t u d ya r e a
same g e n e r a l t r e n d .
i s w i t h i nt h er e g i o n a l
in
p o t a s s i u m - m e t a s o m a t i s ma n o m a l y I. n t e n s eh e m a t i t es t a i n i n g
t h ea r e a
i s t h er e s u l t
o f o x i d a t i o no fp y r i t et h a t
is
l o c a l i z e da l o n gf r a c t u r es u r f a c e si nr i n g - f r a c t u r er h y o l i t e
domesand
A-L
P e a kT u f fa l o n gt h em a r g i n
of t h eS a w m i l l
C a n y o nc a u l d r o n .
M i n o rm a n g a n e s em i n e r a l i z a t i o n
s o u t h e r np a r t
is present i n the
o f t h es t u d ya r e aa n dd o e sn o tc o n s t i t u t ea n
e c o n o m i cd e p o s i t
a t p r e s e n tp r i c e s .
G e o l o g i cS e t t i n g
TheMagdalenaRange
i s l o c a t e dw h e r et h eR i oG r a n d e
r i f t i s o v e r p r i n t e do nt h en o r t h e a s tc o r n e r
D a t i l - M o g o l l o nv o l c a n i cp r o v i n c e T
. h er a n g e
of t h e
is a
n o r t h - t r e n d i n gf a u l tb l o c ku p l i f tw i t h i nt h es o u t h e r np o r t i o n
of t h e r i f t (Chapin, 1971a).
A t r a n s v e r s es h e a rz o n et h a t
t r e n d se a s t - n o r t h e a s ts e p a r a t e st h er a n g ei n t ot w os t r u c t u r a l
zonesT
. h en o r t h e r nM a g d a l e n aM o u n t a i n sa r ec h a r a c t e r i z e db y
n o r m a lf a u l t st h a ta r es t e p p e d
t h a t a r e t i l t e dt ot h ew e s t .
w h e r e t h i ss t u d ya r e a
down t o t h e e a s t a n d
The s o u t h e r np a r t
strata
o f t h er a n g e ,
i s l o c a t e d ,c o n t a i n sd o w n - t o - t h e - w e s t
are t i l t e dt ot h ee a s t .
f a u l t sa n ds t r a t at h a t
No
p r e - T e r t i a r yr o c k sa r ee x p o s e di nt h es o u t h e r np a r t
of t h e
r a n g e ,a l t h o u g hP r e c a m b r i a na n dP a l e o z o i cr o c k sa r ee x p o s e d
i n t h en o r - t h e ~ r n .p o r t i o n .
. A t : l e a s t t w oo v e r l a p p i n gc a u l d r o n
i n t h es o u t h e r nM a g d a l e n aR a n g e T
. he
s t r u c t u r e sa r ep r e s e n t
i s w i t h i nt h eN o r t hB a l d yc a u l d r o n ;t h e
e n t i r es t u d ya r e a
s o u t h e r nm a r g i no ft h eS a w m i l lC a n y o nc a u l d r o n
withinthearea
is located
mapped.
Objectives
T h ep r i m a r yo b j e c t i v e so ft h i ss t u d ya r e :
1) t o
e x t e n dt h er e g i o n a ls t r a t i g r a p h i cf r a m e w o r ki n t ot h es o u t h e r n
M a g d a l e n aM o u n t a i n s ,
2) t o e v a l u a t e t h e e x i s t e n c e a n d n a t u r e
3 ) t oe v a l u a t et h e
of s u g g e s t e dc a u l d r o ns t r u c t u r e s ,a n d
of t h ea r e a .
m i n e r a lp o t e n t i a l
Location a n d A c c e s s i b i l i t y
i s l o c a t e da p p r o x i m a t e l y
The' s t u d ya r e a
km) s o u t ha n d
10 m i (16
1 5 m i ( 2 4 km) w e s t o f S o c o r r o i n t h es o u t h e r n
M a g d a l e n aM o u n t a i n (s F i g .
a b o u t2 0s q u a r e
1).
It encompasses a n a r e a of
m i ( 5 2s q u a r ek m ) ,t h eb o u n d a r i e s
of which
a r ed e f i n e db yt h e
limits o f B o w r i n g ' s
n o r t h ,H a r d yR i d g e
t o t h ew e s t ,a p p r o x i m a t e l yt h ee d g e
t o theeast,
S o u t hB a l d yq u a d r a n g l e
Canyon t ot h es o u t h .
(1980) s t u d y t o t h e
a n d t h e mouth o f S a w m i l l
The n o r t h e r nh a l f
w i t h i nt h eC i b o l aN a t i o n a lF o r e s t .P a r t
i s a d m i n i s t e r e db yt h e
of t h es t u d ya r e a
is
of t h es o u t h e r nh a l f
B u r e a u of LandManagement;
U.S.
r e s t i s p r i v a t e l y owned.
of t h e
the
F i g u r e s 2 a n d 3 a r ea e r i a l
p h o t o g r a p h s of t h e s t u d y a r e a .
A c c e s sf r o mS o c o r r o
s o u t ht ot h eS a n
472 t o t h e
i s byway
of I n t e r s t a t e2 5
Marcia1 e x i t , t h e nn o r t h w e s t
on F o r e s tR o a d
mouth of S a w m i l lC a n y o n .F r o mt h e r e ,
tothejunction
of t h e W e s tF o r ka n d
Canyon i s p a s s a b l e u s i n g
a jeep trail
E a s t F o r k of S a w m i l l
a f o u r - w h e e ld r i v ev e h i c l e .
Methods o f I n v e s t i g a t i o n
D e t a i l e dg e o l o g i cm a p p i n gw a sc a r r i e do u td u r i n g
t h e summer a n d f a l l o f
1978 a n dt h es p r i n g
of 1979.
map c o n s i s t s of p o r t i o n s of t h eS o u t hB a l d y ,M o l i n oP e a k ,
T h eb a s e
.
3
Figure L General location map showing relationship of the
study area to major physiographic features and routes of
.access i n Socorro County.
4
o ft h e
southern
F i g u r e .2. A e r i a l p h o t o g r a p h ,l o o k ' i n gn o r t h . ,
o
f
t
h
e
S
a
n
M
o
u
n
t
a
i
n
s
a
n
d
t
h
e
h.e'ad
M
a
r
c
i
a
1
alluvial
Magdalena
fan.
Y-L Ranch i s shown by' t h ew h . i t es p o t si n
the cen'ter
of t h e p h o t o g r a p hT
. h et h e s ' i sa r e ab e g i n s
j u s t south of
t h e V-L R a n c ha n dc o v ' e r sm o s t
o f ' t h ea r e a 'i nt h e' p h o t o g r a p h
n o r t h o f t h er a n c h .T h eh i g h ,b a r ep e a ki nt h eu p p e r '
i s S o u t h .B a l d yP e a k
t o t h e' n o r t ho ft h i s
r i g h t - h a ' n dc o r n e r
T h el o n gr i d g et ot h e
w e s t i s H a r d yR i d g e .
study area.
Figure 3.
Aeriap
l h o t o g r a p h l, o o k i n gn o r t h ,
o f t h en o r t h e r n
half of the thesis area.
The h i g hp e a k
i s S o u t hB a l d yP e a k
w e s t i s H a r d yR i d g eS. a w m i lC
l anyon
a n dt h er i d g et ot h e
i n t h ec e n t e ro ft h ep h o t o g r a p h .
f o r k s a t S a w m i l lR i d g e
a r e p r o m i n e n ti nt h ec e n t e ro ft h e
W h i t er h y o l i t ed i k e s
p h o t o g r a p h west o fS a w m i l lR i d g e T
. h en o r t h e r nb o u n d a r yo f
t h e t h e s i s a r e a i s j u s t n o r t h o f t h e s ed i k e s .
b
P u e r t e c i t o Gap, a n d C i e n e g aR a n c hq u a d r a n g l e so ft h e
G e o l o g i c aS
l urvey
U.S.
7 1 / 2 - m i n u t es e r i e s( 1 : 2 4 , 0 0 0 ) .
U.S.
4-13-71)
F o r e s tS e r v i c ea e r i a lp h o t o g r a p h s( 4 - 1 0 - 7 1a n d
s c a l eo f1 : 1 2 , 0 0 0w e r eu s e dt o
at a
aid i n structural
i n t e r p r e t a t i o na n dl o c a t i o no fo u t c r o p s .F o r t y - s e v e nt h i n
in
s e c t i o n so fr o c k sf r o mw i t h i nt h es t u d ya r e aw e r ea n a l y z e d
o r d e rt od e s c r i b ea n dc o r r e l a t eu n i t s .
M o s to ft h et h i n
s e c t i o n sw e r es t a i n e dw i t hs o d i u mc o b a l t i n i t r i t eu s i n gt h e
( 1 9 6 6 , p.
p r o c e d u r ed e s c r i b e db yD e e r ,H o w i e ,a n dZ u s s m a n
311) t o h e l p d i s t i n g u i s h p o t a s s i u m f e l d s p a r s f r o m q u a r t z a n d
t oe v a l u a t ep o s s i b l ep o t a s s i u mm e t a s o m a t i s mo fp l a g i o c l a s e
was i n c r e a s e dt o
f e l d s p a ra n dt h eg r o u n d m a s s .E t c h i n gt i m e
30 secondsP
. e t r o g r a p h i cr o c kn a m e sa r ea c c o r d i n gt oL i p m a n
(1975, F i g u r e 3 ) ; c o l o r s a r ef r o mt h eG e o l o g i c a lS o c i e t yo f
AmericaRock
Color C h a r t .
A s a m p l ef o rr a d i o m e t r i cd a t i n g
was c o l l e c t e df r o m
i n t h es e d i m e n t a r yi n t e r v a lo ft h eu n i to f
a t u f fi n t e r b e d d e d
S i x m i l eC a n y o n .
P r e v i o u s Work
T h i ss t u d y
i s t h e f i r s t d e t a i l e dg e o l o g i cm a p p i n g
c a r r i e do u ti nt h es o u t h e r nS a w m i l l
Canyon a r e a , a l t h o u g h
s e v e r a lm a p p i n gp r o j e c t sh a v eb e e nc o m p l e t e d
i n adjacent
a r e a so ft h es o u t h e r nM a g d a l e n aM o u n t a i n s .F i g u r e
t h er e l a t i o n s h i p
s u r r o u n d i n ga r e a s .
of t h e s t u d y a r e a t o p r e v i o u s s t u d i e s
M o s to ft h e s e ,i n c l u d i n gt h ep r e s e n t
4
shows
in
7
a
*
i s being
s t u d y ,a r ep a r to ft h e" M a g d a l e n aP r o j e c t "t h a t
c o n d u c t e db yt h e
New M e x i c oB u r e a uo fM i n e sa n dM i n e r a l
Resources.
i n the
E s t a b l i s h m e n to ft h ev o l c a n i cs t r a t i g r a p h y
M a g d a l e n aM o u n t a i n s
i n s u r r o u n d i n gm o u n t a i nr a n g e s
s t u d i e s ,i n c l u d i n gt h o s e
S o c o r r oC o u n t y .
named t h e D a t i l F o r m a t i o nb yW i n c h e s t e r
The f i r s t d e t a i l e dm a p p i n g
e n do ft h eB e a rM o u n t a i n s .
was c a r r i e d o u t
i n the
i n t h en o r t h e r np a r to f
(1.942) i n t h e i r s t u d y o f
b yL o u g h l i na n dK o s c h m a n n
(1957) mapped t h e
t h eK e l l ym i n i n gd i s t r i c t .T o n k i n g
e
(1920) a f t e r t h e
was l o c a t e d a t t h e n o r t h e r n
U a t i l M o u n t a i n s ;t h et y p es e c t i o n
t h er a n g e
in
was o r i g i n a l l y
The e n t i r eT e r t i a r ys e q u e n c e
M a g d a l e n aM o u n t a i n s
o f many
i s a r e s u l to ft h ei n t e g r a t i o n
P u e r t e c i t oq u a d r a n g l en o r t ho fM a g d a l e n aa n dd i v i d e dt h e
D a t i l F o r m a t i ionntthor m
e ee m b e rfsr ;oom
l d eyt sotu n g e s t
a n d L a J a r a PeakMembers.
t h e s ea r et h eS p e a r s ,H e l l sM e s a ,
Weber (1971) l a t e r s u g g e s t e d r a i s i n g t h e D a t i l F o r m a t i o n t o
g r o u ps t a t u s ;t h eS p e a r s ,H e l l sM e s a ,a n d
L a Jara Peak
Members w e r ee l e v a t e dt of o r m a t i o ns t a t u s
b yC h a p i n( l 9 7 l b ) .
Brown (1972) mapped t h es o u t h e r nB e a rM o u n t a i n sa n d
s u b d i v i d e dt h eH e l l s
Mesa F o r m a t i o ni n t ot w oi n f o r m a l
u n i t s - - t h et u f fo fG o a tS p r i n g sa n dt h et u f fo fB e a rS p r i n g s .
t o the
The H e l l s Mesa F o r m a t i o nh a ss i n c eb e e nr e s t r i c t e d
t u f fo fG o a tS p r i n g so f
Brown ( C h a p i n ,
1 9 7 6 ) ; t h et u f fo fB e a rS p r i n g s
e.
R h y o l i t e b yD e aal n dR h o d e s
agreedtoabandontheterm
1974;
D e a la n dR h o d e s ,
was r e n a m e dt h e
(1976).
A-L
Peak
P r e s e n tw o r k e r sh a v e
Datil Formation.
*
Y u r t h e rs u b d i v i s i o n so ft h e s ef o r m a t i o n sh a v eb e e n
madeby
s u b s e q u e n tw o r k e r s
i n d i v i d u a ls e c t i o n s .
a n d w i l l b ed i s c u s s e di n
A c h r o n o l o g i cd e v e l o p m e n o
t ft h e
s t r a t i g r a p h y i s shown i n F i g u r e 5.
Acknowledgements
T h ea u t h o rg r a t e f u l l ya c k n o w l e d g e st h e
many
i n d i v i d u a l s who c o n t r i b u t e dt ot h ec o m p l e t i o no ft h i ss t u d y .
L a r r y S m i t h al1ow.e.d a c c e s st oh i s - p r i v a t ep r o p e r t y .R o b e r t
G.
O s b u r nc o n t r i b u t e d
many u s e f u ls u g g e s t i o n s
i n thefield
a n do f f i c ea n dc r i t i c a l l yr e a dt h em a n u s c r i p t .J o h n
W.
Hawley a i d e d i n f i e l d c l a s s i f i c a t i o n o f t h e S a n t a
a n d Q u a t e r n a r yd e p o s i t s .
e
L.
Norman s e r v e do n
Drs. A n t o n i u s J.
FeGroup
B u d d i n ga n dD a v i d
my t h e s i s c o m m i t t e e a n d c r i t i c a l l y r e a d
t h em a n u s c r i p t .
I w o u l dl i k et og i v e
C h a r l e s E.
a s p e c i a lt h a n k st o
Dr.
C h a p i n , who s e r v e d a s my t h e s i s a d v i s o r a n d
c o n t r i b u t e d a g r e a td e a lt ot h i sp r o j e c t ,b o t hi nt h ef i e l d
a n di no f f i c ed i s c u s s i o n s .T h e
New M e x i c oB u r e a uo fM i n e s
a n d M i n e r a lR e s o u r c e sp r o v i d e df i n a n c i a ls u p p o r ta n du s eo f
fieldvehicle.
a
c
m
n
10
LL
STRATIGRAPHY AND PETROGRAPHY
R e g i o n a lS t r a t i g r a p h y
D u r i n gt h el a t eE o c e n e ,t h ea r e ao fS o c o r r oC o u n t y
was e r o d e d t o
1975).
a s u r f a c eo fl o wr e l i e f( E p i sa n dC h a p i n ,
R e s t i n gu p o nt h i ss u r f a c ea r er o c k so ft h eO l i g o c e n e
S p e a r sF o r m a t i o n ,c o m p o s e do fl a t i t i c - a n d e s i t i c
v o l c a n i c l a s t i cs e d i m e n t a r yr o c k s
a n d m i n o ra s h - f l o wt u f f s .
The S p e a r sr e p r e s e n t st h ea l l u v i a la p r o nw h i c hs u r r o u n d e dt h e
D a t i l - M o g o l l o nv o l c a n i cf i e l dp r i o rt o
( C h a p i na n do t h e r s ,
1978).
i t s i g n i m b r i t ec l i m a x
O v e r l y i n gt h eS p e a r sa r et h i c k
a n de x t e n s i v ea s h - f l o wt u f fs h e e t sf o r m e db yp y r o c l a s t i c
e r u p t i o n sf r o ms u c c e s s i v e o
, v e r l a p p i n gc a u l d r o n s T
. hick
p i l e so fl a v a s ,t u f f s ,a n ds e d i m e n t sa c c u m u l a t e dw i t h i nt h e s e
c a u l d r o n s .D u r i n gt h el a t es t a g e so ft h i sv o l c a n i s m ,
r i f t - r e l a t e de x t e n s i o nf o r m e db a s i n s
t h a t w e r ef i l l e dw i t h
s e d i m e n t so ft h eS a n t aF eG r o u pa n db a s a l t i ca n dr h y o l i t i c
lavas.
R o c k se x p o s e di nt h es t u d ya r e ac o n s i s to fa s h - f l o w
t u f f s ,l a v a s ,
a n d s e d i m e n t a r yr o c k so fO l i g o c e n ea n de a r l y
M i o c e n ea g eo v e r l a i nb yf a n g l o m e r a t ea n dm u d f l o wd e p o s i t s .
The map u n i t s a r e
shown g r a p h i c a l l y on F i g u r e 6.
\ \
"
"
.
A
_
"
" "
U n i t of S a v mC
i lal n y o n :
( 0 - l o 0 0 f t , 305 m) RHYOLITE WHE COHPII
RHYOLITE LAVA €l.ows. MH-FLOW TUFFS: w h i t e to p i n k d
. ense.
o f t ef nl o v - b a n d e d
rhv.: n o a t b l o c k s rare.
CrvsLal-mor
L - a s h - f l o wt u f f as n d
vent b r e c c i a u
s n d e r l i er h y o l i t e .
__
\ \
"7
\
''
\
T u f fo f
SouCh Canyon:
2 6 m.y.
(0-65 f t , 2 0 m) ASH-FLOW N F F S f .
m u l t i p l e - f l o w ,s i m p l e
cooling unit o fr h y o l i c e
ash-flow
Cuff.
Dk. g r a y .m ~ d e r a t e l y t o d e n s e l yv e l d e d .m d c r a t c l ~
xl-rich t u f f VI c h a t o y a n ts a n i d i n ca n de u h e d r a l
quarr:.
I
\
\
\
Tuff of Leemitar H o u n t a i n s :
2 7 . 9 m.y. (1000 f t , 305 m) M E n O J TIJFFS:
m u l c i p l c - f l o w c, o o p o s i t i o n a l l yz o o c d ,
sinplc
Lo compound c o o l i n s u n i r o fd e n s e l yv e l d c dt u f f .L t . - g r a J
Co p a l e - r e dx, l - p o o r
lover menber; ma.-red. x l - r i c h . qcz"
L a n d b i o c i t e - r i c h , 2 f e l d .u p p e r
member.
\
\
,Ed
"
.
"
'"
7
unit of
\
\
\
\
\
"
"
simne
Canyon:
( 2 5 0 0 f c . 762 m) RWOLITE
MKE m m L q
RKYOLITE LAVA FIOVS. ASH-MW TUFFS, ANDE5ITf U V A S , AND
VOLWrr$XJSTlC SEDIHENThRY RDCKS: r h y o l i t e dome E O ~ I C X -
\
"
.
"
-
p i n k .a p h . ,f l o w - b a n d e dr h y .a n do v e r l y i n gd e b r i s - f l o v
rhy. 1-a
flov; bravn, vesicular a n d e s l u
d e p o s i t s ;s a n i d i n c
l a v a s ; a n d t u f f o f Caronita Canyon:
2 9 . 4 m.y.;
mulciplcn o w . s i m p l e c o o l i n g u n i t of d e n s e l y v e l d c d a s h - F l o v
r u f f ; red. x l - p o o r l o v e r member; v h i c c co s a y . xlrich u p p e r member;
"
"
"
\
\
A-L P e a kT u f f :
3 2 - 2 8 m . y . ( 0 - 3 O O I w f t . 914 m) ME-FUIW TUFFS:
rhy., d e n s e l yw e l d e d ,1 - f c l d .
xl-poor t u f f s ; g r a y t o
r e d bm.; f l o w b a n d e d G
. r a yt op i n k x
. l-poor
rhJ.
l a v a s l n t e r b c d d iend
uppcr p a r r .
-
\-
-
Unic of a r d y R i d g e ;
(1000 ft, 30s m) RKYOLITE AND A N D E S I T E
d k . - b r n .a, p h .
a n d e s i r c ; bm. p o r p h .
to. grsy. r l - p o o r rhy.
Dnc rhy. h a s a
a n d e s i t e ;p i n k
t r a c e of b i o ~ l r ca n d p l a g i o c l a s c phenocrysts.
U V A FLOWS:
.
"".
Figure 6.
Hells Xes. Tuff:
3 2 m.y.
( 1 5 0 W fL, 457 m) M H - M V TUFn:
rhy., m u l C . - f l o v ,
sLmplc c o o l i n s - u n i t o f d c m c l ~
fassivc r u f f a .
Plat
v c l d c d , r l - r i c h q t z . - r i c h .2 - f c l d . .
t or c d
b m . Hcsobrcccia f o u n d in z.ouLh-CcnLral p a n
a f s t u a y are=.
S t r a r i g r a p h i c . c o l u m n o f this s t u d y a r e a .
-
13
H e l l s Mesa T u f f
T h eo l d e s tu n i te x p o s e di nt h es t u d ya r e a
Mesa i s a m u l t i p l e - f l o w ,s i m p l e
H e l l s Mesa T u f f T
. h eH e l l s
o f d e n s e l yw e l d e d ,c r y s t a l - r i c ha s h - f l o wt u f f
c o o l i n gu n i t
1978).
( C h a p i na n do t h e r s ,
MesaMember
T o n k i n g ( 1 9 5 7 ) named t h eH e l l s
D a t i l F o r m a t i o na f t e r
o ft h e
Rhodes ( 1 9 7 6 ) r e s t r i c t e d t h e t e r m H e l l s
c r y s t a l - r i c hi n t e r v a lo ft h eH e l l s
The t u f f i s
s i l l o fL o u g h l i na n d
K o s c h m a n n( 1 9 4 2 )a n dt h et u f fo fG o a tS p r i n g so f
1
(1972).
,
+/-
2 . 8 m.y.
I
e
K-Ar
and32.4
+/- 1.5
m.y.
f t
( B u r k ea n do t h e r s ,
others,
1978).
+/- 1.5 m.y.
1963).
Mesa T u f f
(183 m ) o r l e s s K
. rewedl
t h i c k n e s so f3 8 5 0f t
M o u n t a i n sw h e r e
Mesa T u f fa r e3 0 . 6
1 9 6 3 ) a3n2d. 1
OH
u teh
tflh
liloces w
ko nf e s s
n o r m a l l y6 0 0
Brown
d a t e so b t a i n e df o rt h eH e l l s
( W eaB
bneadrs s e t ,
is
( 1 9 7 4 ) mapped a
( 1 1 7 3 m ) i nt h ec e n t r a lM a g d a l e n a
i t f i l l s t h eN o r t hB a l d yc a u l d r o n( C h a p i na n d
E x p o s u r e s o f t h eu n i ti nt h es t u d ya r e aa r e
a l l w i t h i nt h ep r o p o s e dN o r t hB a l d yc a u l d r o n .
The H e l l s Mesa
o u t c r o p sa r eb o u n d e db yf a u l ta n di n t r u s i v ec o n t a c t s .
l o w e rc o n t a c t
and
Mesa t o t h e l o w e r ,
Mesa
Member.
c o r r e l a t i v ew i t ht h er h y o l i t ep o r p h y r y
I
a p r o m i n e n tl a n d f o r m
( 1 9 7 4 ) a n dD e a l
i n t h ee a s t e r nB e a rM o u n t a i n s .C h a p i n
I
is the
i s n o te x p o s e d ,b u t
The
a minimum t h i c k n e s so f1 5 0 0
f t ( 4 5 7 m ) i s p r e s e n t .I nt h es o u t h w e s t e r np a r to ft h es t u d y
a r e a( p o r t i o n s
of S e c s . 9 , 1 5 , a n d 1 6 , T.5S.,
o u t c r o p so ft h eH e l l s
Mesa c o v e ra b o u to n es q u a r e
R.3W.),
m i (2.6
s q u a r e km) i n a r o u g h l yt r i a n g u l a rp a t t e r n .H e r e ,t h eH e l l s
Mesa i s t y p i c a l l y r e d d i s h b r o w n t o g r a y i s h r e d a n d w e a t h e r s
f o r m b l o c k yo u t c r o p s C
. olumnar
a l o n gj o i n tp l a n e st o
j o i n t i n g i s p r e s e n t i n some o u t c r o p sP
. umice
is
i n c O n s p i c u o u s i n m o s to u t c r o p s ;s t r i k ea n dd i pr e a d i n g sa r e
i n a c c u r a t ed u et op o o r l yd e f i n e df o l i a t i o n .L i t h i cf r a g m e n t s
u s u a l l y make u p l e s s t h a n
a
10%. of t h er o c k ;h o w e v e r ,
1976) i s p r e s e n t i n t h es o u t h e a s t
m e s o b r e c c i a( L i p m a n ,
q u a r t e ro fS e c .
9 (T.5S.,
a r e as l a r g ea s
3 f t ( 0 . 9 m ) i n d i a m e t e ra n dc o m p r i s e
a s 75% of t h er o c k .
R.3W.).
H e r e ,t h el i t h i cf r a g m e n t s
a s much
Common l i t h o l o g i e sa r ep o r p h y r i t i c
a n d e s i t e ,c r y s t a l - r i c ht u f f ,r h y o l i t el a v a ,a n db i o t i t e
7 is a
i s l o c a l l ya b u n d a n t .F i g u r e
s c h i s t ,e a c ho fw h i c h
p h o t o g r a p ht h a ts h o w st h et y p i c a la p p e a r a n c eo fH e l l s
Mesa
mesobreccia.
I nt h es o u t h e a s t e r np a r to ft h es t u d ya r e a
Sec.
1 4 , T.5S.,
R.3W.1,
(S
1/2,
small s c a t t e r e do u t c r o p so fH e l l s
Mesa T u f fa r ef o u n d .H e r e ,t h eH e l l s
Mesa i s c o n f o r m a b l y
o v e r l a i n by a p o r p h y r i t i c b a s a l t i c a n d e s i t e l a v a f l o w o f t h e
u n i to fH a r d yR i d g e( T h a l ) .
p a l er e d
The t u f f i s v e r yl i g h tg r a yt o
a n d w e a t h e r st or o u n d e db o u l d e r s .L o c a l l y ,p a l e
r e d ,s u b a n g u l a rt or o u n d e df r a g m e n t sw i t ht h es a m em i n e r a l o g y
a s t h e m a t r i x a r ef o u n dw i t h i nt h ev e r y - l i g h t - g r a yp o r t i o no f
t h et u f f L
. i t h i cf r a g m e n t sa r es p a r s e .
i n t h i ss t u d ya r e a
The H e l l s Mesa T u f f
i s c o n s i s t e n t l yc r y s t a lr i c h ,c o n t a i n i n g
2 5 t o 45% p h e n o c r y s t so fq u a r t z ,s a n i d i n e ,p l a g i o c l a s e ,
b i o t i t e ,a n dm a g n e t i t e .R e l a t i v e l yl a r g e( u pt o
5 mm),
15
F i g u r e 7. P h o t o g r a p h o f m e s o b r e c c i a i n t h e
H e l l s Mesa Tuff. Lens cap is
4 cm in diameter.
Breccia fragments consist of andesite (dark
clasts) and crystal-rich ash-flow tuff.
(E 1 / 2 , Sec. 9, T.5S., R.3W.).
16
a n h e d r a lt os u b h e d r a l ,o f t e nd e e p l ye m b a y e dq u a r t zc o m p r i s e s
abou4
t 0 %o ft h ep h e n o c r y s t s S
. ubequaa
l m o u n t so fs u b h e d r a l
s a n i d i n ea n dp l a g i o c l a s e
make u pa n o t h e r4 0 %o ft h e
p h e n o c r y s t s ;t h ep l a g i o c l a s ec r y s t a l sa r eo f t e nh i g h l y
a l t e r e d .B i o t i t ea n dm a g n e t i t e
make u pt h er e m a i n d e ro f
10% o fe a c h
p h e n o c r y s t s ;a b o u t
i s p r e s e n t .M a g n e t i t e
is
o f t e nf o u n dr e p l a c i n gb i o t i t e .
i s about 6 i n .
A p a l e - p i n ka s h - f a l lt u f ft h a t
cm) t h i c k i s i n t e r b e d d e d i n t h e H e l l s
q u a r t e ro fS e c .
1 4 (T.5S.,
R.3W.).
Mesa i n t h es o u t h w e s t
A t h i ns a n d s t o n ei n t e r v a l
(1 f t , 0 . 3 m ) i s i n t e r b e d d e d i n t h e H e l l s
l o c a t i o n( c e n t e r ,
NE 1 / 4 , SW
Mesa a t o n e
1 / 4 , S e c . 9 , T.5S.,
R.3W.).
D e t a i l e dp e t r o g r a p h i cw o r kh a sb e e nd o n e
H e l l s Mesa Tuffby
Spradlin
(15
on t h e
Brown ( 1 9 7 2 ) , C h a m b e r l i n ( 1 9 7 4 ) , a n d
(1976); t h e i n t e r e s t e d r e a d e r
is referredtotheir
w o r k sf o rf u r t h e ri n f o r m a t i o n .
U n i t o fH a r d yR i d g e
The u n i to fH a r d yR i d g e
was i n f o r m a l l y namedby
Bowring ( 1 9 8 0 ) f o re x p o s u r e sj u s tn o r t ho ft h i ss t u d ya r e a .
The u n i tr e p r e s e n t sc a u l d r o n - f i l ld e p o s i t so ft h eN o r t hB a l d y
a
c a u l d r o na n dc o n s i s t so fs e v e r a la n d e s i t ef l o w sa n dd i k e s ,
t h i c ks e q u e n c eo fr h y o l i t el a v a s ,
a n d m i n o rv o l c a n i c l a s t i c
s e d i m e n t a r yr o c k sa n di n t e r b e d d e da s h - f l o wt u f f s .
A complete
s e c t i o ni nn o te x p o s e dw i t h i nt h es t u d ya r e a ,b u th e r et h e
u n i t i s n o tl e s st h a n
1000 f t (305 m ) thickB
. owring
(1980)
r e p o r t s a t h i c k n e s s o f 8 0 0 f t ( 2 4 4 m ) onHardyRidge
tothe
n o r t h of t h i ss t u d ya r e a .
The u n i t o f H a r d yR i d g eo ' v e r l i e s% h eH e l l s
a t s c a t t e r e dl o c a t i o n st h r o u g h o u tt h e
T u f fa n dc r o p so u t
s t u d ya r e a .
W h e r ee x p o s e dw i t h i nt h eS a w m i l l
33 ( u n s u r v e y e d ) ,T . k S . ,
( S e c s .3 2a n d
>, T.5S.,
R.3W.)
a n dt ot h es o u t h
m a r g i n o f t h eS a w m i l l
1
of t h eS a w m i l l
R.3W.),
Peak T u f f .
R.3W.,
1 4 and
N
i s unconformably
t h eu n i t
To t h ew e s t
of H a r d yR i d g e
4 and
a n d Secs.
o f t h es o u t h e r nt o p o g r a p h i c
o f t h ew e s t e r nm a r g i n
3 0 ( u n s u r v e y e d ) ,T - 4 S - 9
C a n y o nc a u l d r o n( S e c .
R . ~ w . )t h,eu n i t ,
Canyon c a u l d r o n
Canyon c a u l d r o n ( S 1 / 2 ,S e c .
1 / 2 ,S e c .2 3 ,T . 5 S . ,
o v e r l a i n by A-L
1
Mesa
i s d i r e c t l yo v e r l a i n
by t h e
u n i t o f SawmillCanyon.
a n d e s i t e sa n ds e d i m e n t a r yr o c k s C
. o n f o r m a b l yo v e r l y i n gt h e
H e l l s Mesa T u f f
i s a s e r i e s o f t h i na n d e s i t ef l o w s .
a t s c a t t e r e ds i t e st h r o u g h o u tt h es t u d y
a n d e s i t e sc r o po u t
area.
was mapped s e p a r a t e l y ;t h e
One d i s t i n c t i v ef l o w( T h a 2 )
r e s tw e r e
mapped a sT h a l .T h a l
u n i t of H a r d yR i d g e
a n d e s i t ev a r i e s
i s t h eb a s a l
member of t h e
i nt h i ss t u d ya r e a .T h i c k n e s s
of t h e
from 1 0 f t ( 3 m ) t o more t h a n 1 0 0 f t ( 3 0 m ) .
O u t c r o p sa r er e l a t i v e l yn o n - r e s i s t a n t ,o f t e nm a r k e d
slopes.
The
by g r a s s y
The r o c k i s p a l e brown t ol i g h tb r o w n i s hg r a y
f r e s ha n dv a r i o u ss h a d e s
o f t e nf i l l e d
when
o f g r e e n when a l t e r e d .V e s i c l e s ,
w i t h a d a r k - y e l l o w i s h - o r a n g ec l a ym i n e r a l ,a r e
common l o c a l l y .P h e n o c r y s tm i n e r a l o g y
v a r i e s l o c a l l y .I nt h en o r t h e r np a r t
o f t h ea n d e s i t e s
of t h es t u d ya r e a
(N E
18
e
1/4, SeC.
aphanitic.
R.3W.),
3 0T
, .kS.,
I n t h es o u t h
t h ea n d e s i t e
i s dense a n d
14, T.~s.R
, . ~ w .t )h ,e
( S 1 / 2S
, ec.
a n d e s i t el a v ac o n t a i n sa b o u tl O % ; ' p h e n o c r y s t so fp l a g i o c l a s e ,
c l i n o p y r o x e n e , a n d b i o t i t e .F i v ep e r c e n to ft h ep h e n o c r y s t s
a r ep l a g i o c l a s e ,a b o u t
0 . 5 mm l o n g , someofwhichhave
"honeycomb" t e x t u r e .
Four p e r c e n to ft h ep h e n o c r y s t sa r e
a
0 . 4 mm i nd i a m e t e r .
c l i n o p y r o x e n ee u h e d r at h a ta r ea b o u t
The
r e m a i n i n g 1%;o f t h e p h e n o c r y s t s a r e s u b h e d r a l b i o t i t e
mm
c r y s t a l st h a ta p p e a rt ob ea l t e r e d ' ;t h e s ea r ea b o u t0 . 3
long.
One o r 2 % ; m a g n e t i t e i s p r e s e n t ,
some a s r e p l a c e m e n t
p r o d u c t so ft h ep y r o x e n ea n db i o t i t e .P l a g i o c l a s em i c r o l i t e s
a r ep r e s e n t
i n t h eg r o u n d m a s s .
A c o a r s em o s a i co fc a l c i t e
h a sf i l l e di r r e g u l a rp o r es p a c e s .
A d i s t i n c t i v ep o r p h y r i t i ca n d e s i t ef l o w( T h a 2 )
c o n f o r m a b l yo v e r l i e sT h a li nt h es o u t h e r np a r to ft h es t u d y
, ec.
a r e a (S 1/2S
14, T.5S.,
R.3W.).
T h i sa n d e s i t e
400 f t ( 1 2 2 m ) t h i c k a n d i s r e l a t i v e l yr e s i s t a n t ;
c l i f f yo u t c r o p s
f r a g m e n t sT
. h er o c k
i s about
i t forms
a n d w e a t h e r st oh a c k l y ,a n g u l a rt a l u s
i s g r a y i s hr e dt ov a r i o u ss h a d e so f
green a n d c h a r a c t e r i s t i c a l l yc o n t a i n s
p l a g i o c l a s ep h e n o c r y s t s
15 to20%,large
( a s much as 1 ern i nl o n gd i m e n s i o n ) .
About 5%)p h e n o c r y s t so fc l i n o p y r o x e n e ,b i o t i t e ,a n d
h o r n b l e n d ea r ep r e s e n t .E u h e d r a lc l i n o p y r o x e n ea v e r a g i n g
mm i nd i a m e t e rm a k e su pn e a r l y
b i o t i t ea n dh o r n b l e n d eo c c u r
5% o ft h er o c k .T r a c e so f
a s p h e n o c r y s t s .B i o t i t ea n d
m a g n e t i t e commonly r e p l a c ec l i n o p y r o x e n e .
The
groundmass
1
c o n s i s t so fe l o n g a t ep l a g i o c l a s em i c r o l i t e sa n de u h e d r a l
c l i n o p y r o x e n eB
. o t hp h e n o c r y s t sa n dg r o u n d m a s sm i n e r a l sh a v e
a s t r o n g l yp r e f e r r e do r i e n t a t i o n .
A r ~ h y o l i t el a v ao v e r l i e st h ea n d e s i t e s
r h y o l i t el a v a s .
i n t h es o u t hp a r to ft h ea r e a
s c a t t e r e do u t c r o p s
i n
( S 1 / 2 ,S e c
\
14,
a n d NW 1 / 4 ,
Sec.
24,
T.5S.,
i s d i s t i n g u i s h e df r o mT h r 2
R.3W.).
T h i sr h y o l i t e( T h r l )
by t h e p r e s e n c e o f t r a c e s o f
b i o t i t ea n dp l a g i o c l a s ep h e n o c r y s t s .T h eb i o t i t e -a n d
. p l a g i o c l a s e - b e a r i n gr h y o l i t e
i s r e l a t i v e l yr e s i s t a n ta n d
w e a t h e r st ob l o c k yf r a g m e n t s .T h ep r e s e n c eo fs p h e r u l i t e s
a n da l t e r n a t i n gd a r k - g r a ya n dm o d e r a t e - r e df l o wb a n d sm a k e s
t h er o c kc o l o r f u l l yd i s t i n c t i v e
I n t h i ns e c t i o n ,a b o u t
e
p l a g i o c l a s ea r ep r e s e n t
e l o n g a t ea n da v e r a g e s
i n o u t c r o pa n dh a n ds p e c i m e n .
1% p h e n o c r y s t so fb i o t i t ea n d
i n s u b e q u a la m o u n t s .B i o t i t e
is
1 mm i n l o n gd i m e n s i o n .S u b h e d r a l
p l a g i o c l a s ep h e n o c r y s t sr a n g e
i n l e n g t hu pt o
3 mm.
The
groundmass i s s p h e r u l i t i c a l l y d e v i t r i f i e d t o r a d i a t i n g
a g g r e g a t e so fq u a r t za n df e l d s p a r .
A s e c o n dr h y o l i t el a v a( T h r 2 )u n d e r l i e st h e
A-L
P e a kT u f fi nt h en o r t hh a l fo ft h es t u d ya r e a( S e c s .3 2a n d
33 ( u n s u r v e y e d ) ,T . 4 S . ,
R.3W.).
,olor
T h i sr h y o l i t e
f r o ml i g h tg r a y
R.3W., a n dS e c s .
4 a n d 5 , T.5S.,
i s dense a n d a p h a n i t i c a n d ranges i n
or p a l ep i n kt og r a y i s hb l u e .T h er o c k
i s o f t e nf r a c t u r e da n ds i l i c i f i e d .L o c a l l yt h er h y o l i t e sa r e
f l o wb a n d e d ,a n ds p h e r u l i t e sa r e
w i t ht h ef l o wb a n d i n g .
common w i t h i n a n d a l i g n e d
A d i s t i n c t i v ed e v i t r i f i c a t i o n
20
of s u b s p h e r i c a l ,o v e r l a p p i n gm a s s e s
t e x t u r e ,c o n s i s t i n g
is continuous, i s
r h y o l i t et h r o u g hw h i c ht h ef l o wb a n d i n g
8).
common i n t h er h y o l i t e s( F i g .
of
T h e s em a s s e sa r e
a r i n da b o u tw h i c ht h e yw e a t h e ro u t .F o r
s u r r o u n d e db y
of t h i s t e x t u r e , t h e r e a d e r
more d e t a i l e d d e s c r i p t i o n
(1980).
r e f e r r e dt oB o w r i n g
a r ev e r yc r y s t a lp o o r ;
a
is
I n t h i ns e c t i o n ,t h er h y o l i t e s
some c o n t a i n a t r a c e o f m a g n e t i t e .
A s e d i m e n t a r yi n t e r v a lt h a t
i s a s much as 5 0 f t ( 1 5
9 a n d 10).
m) t h i c ko v e r l i e sT h r 2( F i g s .
It c o n s i s t s of
p a l e - r e d ,f i n e -t oc o a r s e - g r a i n e dt h i n l yl a m i n a t e ds a n d s t o n e s
t h a ta r el o c a l l yc r o s sb e d d e d .
A t h i nl i g h t - g r a ya s h - f l o w
t u f f i s i n t e r b e d d e dw i t ht h es a n d s t o n e s .
It c o n t a i n s 5 t o
l O % > ' p h e n o c r y s t so f q u a r t z ,s a n i d i n e ,a n db i o t i t e .L i t h i c
f r a g m e n t s o f r h y o l i t e a n d a n d e s i t e make u pa b o u t
5%)o f t h e
rock.
A-L
P e a kT u f f
The A-L
b yD e a l
was f i r s t named A-L
P e a kT u f f
P e a kR h y o l i t e
( 1 9 7 3 a ) for e x p o s u r e si nt h eS a nM a t e oM o u n t a i n s ;t h e
name h a s s i n c e b e e n m o d i f i e d t o
o t h e r s , 1978).
The t u f f
A-L P e a kT u f f( C h a p i n - a n d
i s c o r r e l a t i v ew i t ht h eb a n d e d
r h y o l i t e o f L o u g h l i n a n d Koschmann ( 1 9 4 2 ) , t h e u p p e r p a r t
Tonking's
S p r i n g so f
( 1 9 5 7 ) H e l l s MesaMember,
Brown ( 1 9 7 2 ) .
S m i t ha n do t h e r s
The A-L
a n dt h et u f f
P e a kh a sb e e nd a t e db y
(1976) a t 31.8 + / - 1 . 7 m.y.
f i s s i o n - t r a c km e t h o d .
of Bear
b yt h e
of
21
F i g u r e 8. P h o t o g r a p h o f b o t r y o i d a l m a s s e s o f
coalescing spherulites, a devitrification texture
f o u n d in the crystal-poor rhvolite l n v n a i n t h i s
study area.
22
F i g u r e 9. P h o t o g r a p h o f t h e c o n t a c t b e t w e e n
the unit o f Hardy Ridge and the A-L P e a k T u f f
A l l contacts are depositional. (r=rhyolite,
(SW 1 1 4 , Sec. 33
ss=sandstone, t=tuff)
(unsurveyed), T.4S., R.3W.I.
23
F i g u r e 10. Photograph of sedimentary rocks
u n d e r l y i n g t h e A-L P e a k T u f f i n t h e n o r t h central part of the study area (SW 1 / 4 ,
Sec. 33 ,(unsurveyed),
T.4S., R.3W.).
24
The A-L
P e a kT u f fh a st h r e er e c o g n i z e d
members
w i t h i n i t s o u t f l o ws h e e t - - t h eg r a y - m a s s i v e ,f l o w - b a n d e d ,a n d
p i n n a c l e sm e m b e r s .
A l l o ft h e
members a r ec r y s t a l - p o o r ,
d e n s e l yw e l d e d ,o n e - f e l d s p a r ,r h y o l i t ea s h - f l o wt u f f s( C h a p i n
1978).
a n do t h e r s ,
Thm
e i d d l ep, u m i c e - r i c hf,l o w - b a n d e d
member i s u s u a l l y w e l d e d t o t h e l o w e r , p u m i c e - p o o r
g r a y - m a s s i v e member a n dt h e s et w om e m b e r sa r en o r m a l l y
mapped
t o g e t h e r .I nt h eL e m i t a ra n dB e a rM o u n t a i n s ,t h eu p p e r ,
p u m i c e - r i c hp i n n a c l e s
member i s s e p a r a t e d f r o m t h e u n d e r l y i n g
flow-banded
member b yb a s a l t i c - a n d e s i t el a v af l o w s( C h a p i n
a n do t h e r s ,
1978).
The s o u r c e a r e a s
Mount W i t h i n g t o nc a u l d r o n
M o u n t a i n s a s a s o u r c e f o r t h e A-L
c u r r e n t l yb e l i e v e d
t h e A-L
P e a kT u f f
(1973a) o r i g i n a l l y
a t t h i st i m e .D e a l
a r ep o o r l yu n d e r s t o o d
p r o p o s e dt h e
f o r membersof
i n t h eS a nM a t e o
P e a k ;h o w e v e r ,
it i s
t h a t a l l members o f t h e t u f f w e r e e r u p t e d
f r o mt h eM a g d a l e n aa n dS a w m i l lC a n y o nc a u l d r o n s .
T h i c k n e s s o f t h e A-L
a r e au s u a l l yv a r i e sf r o m
P e a k i n t h eS o c o r r o - M a g d a l e n a
0 t o 700 f t ( 2 1 3 m ) .
Thickness
e s t i m a t e si nt h es t u d ya r e aa n dv i c i n i t ya r eg e n e r a l l y
u n r e l i a b l ed u et ol a c ko f
v a r i a t i o n sc a u s e d
faultingwithin
horizons.
b ys e c o n d a r yf o l d i n g ,a n du n r e c o g n i z e d
a lithologicallyuniformunitwithnomarker
The u n i t i s e x t r e m e l tyh i c kh, o w e v e rB. o w r i n g
( 1 9 8 0 )e s t i m a t e s
t h en o r t h
a n e x p o s e db a s e ,s t r i k ea n dd i p
a minimum t h i c k n e s s o f 2 0 0 0 f t ( 6 1 0 m ) t o
o f t h i ss t u d ya r e a ,a n dP e t t y
(1979) r e p o r t s a
25
o f t h e s ea r e a s
s i m i l a r t h i c k n e s s i n Ryan Hill C a n y o n ;b o t h
a r ew i t h i nt h eS a w m i l l
t h e A-L
C a n y o nc a u l d r o n .I nt h i ss t u d ya r e a ,
P e a k i s a t l e a s t 3000 f t
S a w m i l lC a n y o nc a u l d r o n ,b u t
(914 m) t h i c k w i t h i n t h e
it t h i n sa b r u p t l ya n db e c o m e s
l a t e r a l l yd i s c o n t i n u o u so u t s i d et h em a i nr i n gf r a c t u r e .
o ft h em a i nr i n gf r a c t u r e ,t h e
A-L
West
i s a b s e n ta n d
P e a kT u f f
i s u n c o n f o r m a b l yo v e r l a i nb yt h et u f f
t h eu n i to fH a r d yR i d g e
o fL e m i t a rM o u n t a i n s .O u t s i d et h es o u t h e r nt o p o g r a p h i c
m a r g i n of t h eS a w m i l l
R.3W.),
t h e A-L
Canyon c a u l d r o n ( S 1/2, S e c .
P e a k i s a b o u t 200 f t (61 m ) t h i c k .
O u t c r o p so f
km) i n t h e n o r t h p a r t
A-L
(unsurveyed), T.kS.,
2 sq m i ( 5 . 2 sq
P e a kc o v e ra l m o s t
of t h e s t u d y
area, w h e r et h eu n i t
a t t a i n s i t s maximum t h i c k n e s s( S e e s .
e
1 4 , T.5S.,
R.3W.I.
2 8 , 2 9 , 3 2 , a n d 33
O u t s i d et h e
m a i n r i n gf r a c t u r e
tohSf ea w m iCl la n y oc na u l d r o tnht,uecf rf o p
o su t
at
s c a t t e r e dl o c a l i t i e st h r o u g h o u tt h ea r e a ,m a i n l yt ot h ee a s t
i s e x p o s e d ,t h e
o fS a w m i l lC a n y o n W
. h e r et h eb a s a lc o n t a c t
A-L
P e a ku n c o n f o r m a b l yo v e r l i e st h eu n i to fH a r d yR i d g e .T h e
a n irregularcontactofthe
p r e s e n c eo f
u n d e r l y i n gr h y o l i t el a v a( S e c .
a n dS e c .
4,
T.5S.,
R.3W.)
The A-L
P e a kw i t ht h e
33 ( u n s u r v e y e d ) , T . b S . ,
s u g g e s t st h ep r e s e n c e
i r r e g u l a re r o s i o ns u r f a c eu p o nw h i c ht h e
deposited.
A-L
A-L
R.3W.,
of a n
Peakwas
Peak i s o v e r l a i n by c a u l d r o n - f i l l
d e p o s i t so ft h eu n i to fS i x m i l eC a n y o n .
I nt h en o r t h e r np a r to ft h es t u d ya r e a ,t h e
P e a k Tuff i s s e p a r a t e d i n t o t w o i n t e r v a l s
A-L
by a r h y o l i t e l a v a
26
f l o w( T a l r ) .
The p o r t i o n of t h e t u f f t h a t o v e r l i e s t h e l a v a
v a r i e s somewhat i n a p p e a r a n c e ,p h e n o c r y s tc o n t e n t ,a n dd e g r e e
w i l l be
of w e l d i n g f r o m t h e p o r t i o n b e l o w t h e l a v a a n d
d i s c u s s e ds e p a r a t e l y B
. owring
(1980) r e p o r t s a s i m i l a r l y
v a r i a b l eu p p e rp a r t
P e a ki m m e d i a t e l yt ot h en o r t h
o f t h e A-L
o f t h i ss t u d ya r e a .
l o w e r member.
The
l o w e r A-L
i s w e l le x p o s e di nt h e
P e a kT u f f
n o r t h e r np a r to ft h es t u d ya r e aa n df o r m se x t e n s i v eo u t c r o p s
of
a l o n gt h ee a s ts i d eo fS a w m i l lC a n y o n .T h ep r e s e n c e
c l o s e l ys p a c e ds h e e tj o i n t sn o r m a lt ot h ef o l i a t i o n( F i g .
small p l a t e l e t s t h a t f o r m
c a u s e st h et u f ft ow e a t h e rt o
The r o c k i s t y p i c a l l yl i g h tb r o w n i s h
e x t e n s i v et a l u ss l o p e s .
a
11)
g r a yw i t hg r a y i s h - r e dp u m i c e .C o a t i n g s
o f r e d d i s h - b r o w ni r o n
o x i d e a n d b l a c km a n g a n e s eo x i d ea r e
common on j o i n t s u r f a c e s .
Thepumice,
i n a m o u n t su p
t o 20%;; a r eh i g h l yf l a t t e n e da n d
I n some a r e a s ,c o a r s e
o f t e ns t r o n g l yl i n e a t e d( F i g .1 2 ) .
v a p o r - p h a s em i n e r a l sh a v ef i l l e de l o n g a t eg a sc a v i t i e s .
P h e n o c r y s t s , i n a m o u n t s of 2 t o lo%>; c o n s i s t of s a n i d i n ea n d
q u a r t z .S u b h e d r a ls a n i d i n e ,a b o u t
1 . 0 mm l o n g , i s g e n e r a l l y
4 t o 5 t i m e s a s a b u n d a n t as q u a r t z ,w h i c h
i s small ( a b o u t
. nhedram
l agnetite
0 . 2 5 m m ) a n da n h e d r a (l o f t e nr o u n d e d ) A
mm i nd i a m e t e r .P l a g i o c l a s ea n d
p h e n o c r y s t sa r ea b o u t0 . 2 5
m a g n e t i t ea r ep r e s e n t
i n t r a c ea m o u n t s .
a n d e s i t el i t h i cf r a g m e n t s ,o f t e nt o o
*
h a n ds p e c i m e n a, r ep r e s e n t S
. h a r ds t r u c t u r e
p r e s e r v e di n
One t of i v ep e r c e n t
small t o b e o b s e r v e d i n
is well
some t h i n s e c t i o n s ; i n o t h e r s , d e v i t r i f i c a t i o n
h a so b s c u r e dp r i m a r yt e x t u r e s .
27
Figure 11. Well-developed sheet joints in the
A-L P e a k T u f f n e a r t h e n o r t h e r n b o u n d a r y o f t h e
study area (SE 1/4, Sec. 2 9 (unsurveyed), T.4S.,
R.3W.).
Foliation in the tuff is nearly
perpendicular to the joint planes.
'
28
F i g u r e 12. Photograph of lineated pumice
in
t h e A-L Peak Tuff. The thickness-to-length
ratio of the pumice is o f t e n a s m u c h a s 1 : l O O .
(SW 1 / 4 , Sec. 33 (unsurveyed), T.4S., R17W.).
1 0 t o 2 0 f t ( 3 t o 6 m ) o ft h el o w e r
T h eu p p e r
A-L
Peak i s u s u a l l y n o n - l i n e a t e d a n d g r a y i s h r e d i n c o l o r w i t h
Sec.
(NW 114,
A t o n el o c a t i o n
p i n k i s h - g r a yd i s c - s h a p e dp u m i c e .
33 ( u n s u r v e y e d ) ,T . 4 S . ,
R.3W.1,
is
t h et o po ft h et u f f
In
b l a c k i s hr e da n de x t r e m e l yd e n s ed u et os i l i c i f i c a t i o n .
a n o t h e ro u t c r o p
( N W 1 1 4 , S e c . 28 ( u n s u r v e y e d ) ,T . 4 S . ,
t h et o po ft h el o w e r
c o n t a i n i n ga b o u t
A-L
R.3W.),
P e a k i s m o d e r a t e l yc r y s t a l - r i c h ,
lZ%, p h e n o c r y s t so fs a n i d i n e ,q u a r t z ,a n d
m a g n e t i t e .N i n ep e r c e n te u h e d r a lt os u b h e d r a ls a n i d i n e
l a t h s ,a b o u t
l%;'small( a b o u t 0.25 m m ) ,
1 mm l o n g ,a n d
a n h e d r a l ,o f t e nd e e p l ye m b a y e dq u a r t z
p h e n o c r y s t s a t t h i sl o c a l i t y .
s u b h e d r a lm a g n e t i t ea b o u t
e
make u pm o s to ft h e
One t oZ % . ' a n h e d r a lt o
0 . 2 5 mm i n d i a m e t e r
p r e s e n t .A n d e s i t el i t h i cf r a g m e n t sc o m p r i s e
a s much a s 5% o f
small t o be v i s i b l e i n h a n d
t h er o c ka n da r es o m e t i m e st o o
specimenP
. u m i c ec o n s t i t u t e sa b o u t
c o n t a i n ss a n i d i n e
is also
1 5 % of t h er o c ka n d
a n d q u a r t z .I no n et h i ns e c t i o n ,
c o n c e n t r i cr i n g so fs e r i c i t ew i t h i nt h ep u m i c ea r ep r o b a b l y
c a u s e db yp e r l i t i cc r a c k sb e i n gf i l l e dw i t ha l t e r a t i o n
p r o d u c t s .S h a r ds t r u c t u r e
i s w e l lp r e s e r v e d
i n the
groundmass.
P r i m a r y l a m i n a r f l o ws t r u c t u r e sa r e
l o w e r A-L P e a k i n t h i s s t u d y a r e a ; t h e s e i n c l u d e
p u m i c ea n dg a sc a v i t i e s ,
phenocrysts,
common i n t h e
1) l i n e a t e d
2 ) r o t a t e dl i t h i cf r a g m e n t sa n d
3 ) f o l d e da n dl i n e a t e df o l i a t i o np l a n e s ,a n d
d i p st o os t e e pt oh a v eb e e nc a u s e d
b ys e c o n d a r yc o m p a c t i o n .
4)
*
P r i m a r y l a m i n a r f l o w s t r u c t u r e sh a v eb e e nr e c o g n i z e d
1 9 6 7 ; W a l k e ra n dS w a n s o n ,
t u f f s( S c h m i n c k ea n dS w a n s o n ,
1 9 7 6 ; C o a t s , 1976; E k r e na n do t h e r s ,
C h a p i na n dD e a l ,
1979).
C h a p i na n dL o w e l l ,
( 1 9 7 9 , p . 152)
C h a p i na n dL o w e l l
"If t h e
t e m p e r a t u r e sd u r i n gt r a n s p o r ta n dd e p o s i t i o n :
i s w e l la b o v et h e
temperature of t h eg l a s s yp a r t i c l e s
w i l l a g g l u t i n a t e ,c o l l a p s e ,
softeningpoint,theparticles
l a m i n a r b o u n d a r yl a y e rt of o r m
f l u i dw i t ht h er h e o l o g i c a lp r o p e r t i e s
...
if
F o l d sr a n g e
O f t e nt h e
A-L
areas ( P e t t y ,
P e a kt h a ti m m e d i a t e l yo v e r l i e s
13).
w i t h o u ta n yc o o l i n gb r e a k s( F i g .
14) a n dc r e n u l a t i o n s
A-L
11
1979; B o w r i n g ,
a fold is
i s w e l d e dt o g e t h e r
R a m p - l i k es t r u c t u r e s
i n l a m i n a ea d j a c e n tt or o t a t e d
l i t h i cf r a g m e n t sa n dp h e n o c r y s t s
o ft h e
. .
i n s c a l ef r o mm i c r o s c o p i ct om a c r o s c o p i c .
u n d i s t u r b e d ,a l t h o u g ht h ee n t i r et h i c k n e s s
(Fig.
will fail
o f l i n e a t i o n s i s a p p r o x i m a t e l ye a s t - w e s t
i nt h i sa n da d j a c e n ts t u d y
1980).
of a r h y o l i t e l a v a .
w i l l occur as l o o s e ash.
t oa d h e r ea n dd e p o s i t i o n
a
a viscous
. . . b e l o w . . ., t h eg l a s s yp a r t i c l e s
T h et r e n d
1978;
o f t h ep r i m a r yf l o wf e a t u r e st oh i g h
a t t r i b u t et h ep r e s e n c e
a n dw e l di nt h e
r e f l e c t t h ev i s c o u sn a t u r e
P e a ka s hf l o w s .
D i pa n g l e s
1968;
1.972; Deal, 1 9 7 3 a , 1 9 7 3 b ;
1972; L o w e l la n dC h a p i n ,
Parker,
i n other
of t h e A-L
P e a kw i t h i nt h eS a w m i l l
C a n y o nc a u l d r o ng e n e r a l l yr a n g e
from 4 0 t o 60 d e g r e e s
c o m p a r e dw i t hd i p so fl e s st h a n
40 d e g r e e si nt h eo t h e r
u n i t s .T h ec o n s i s t e n t l ys t e e p e rd i p s
o f t h e A-L
Peak
F i g u r e 13. P h o t o g r a p h o f a primary fold in
t h e A-L Peak Tuff. The axial plane of this
f o l d i s perpendicular to the lineation and to
the plane of this photograph.
(SW 1 / 4 , Sec. 33 (unsurveyed), T.4S., R.3W.).
32
Figure 14. Photograph
of the A-L PeakrT,uff
showing pumice that have been deformed and
truncated b y an overlying ash flow. Movement
of the flow was left to right. Lens cap is
4 cm in diameter.
(STJ 1.4, Sec. 33 (unsurveyed), T.4S.,
R.3W.).
33
a n d downwarping
p r o b a b l yr e f l e c tc o n t e m p o r a n e o u sd e p o s i t i o n
a l o n gt h ec a u l d r o nm a r g i nd u r i n gs u b s i d e n c e .I no t h e rw o r d s ,
t h e A-L
P e a k was d e p o s i t e do n
a s u r f a c ed i p p i n g
o f t h eS a w m i l l
d e g r e e sn o r t h e a s t w a r dt o w a r d st h ec e n t e r
a n d Lowell
C a n y o nc a u l d r o nC
. hapin
5 t o 25
(1979) r e p o r ts t e e p
i n a s h - f l o wt u f f sd e p o s i t e da l o n gt h em a r g i n s
p r i m a r yd i p s
p a l e o v a l l e y s i n Colorado.
of
Some o f t h es t e e pd i p si nt h e
A-L
P e a k may h a v eb e e ne n h a n c e db yl o c a lf o l d si n d u c e db yi n w a r d
slumping of t u f f .
r h y o l i t el a v a .
The r h y o l i t el a v ai n t e r v a lw i t h i nt h e
as a 2 . 5 m i -
P e a kc r o p so u t
e a s ts i d e
(4.0
km-)
A-L
l o n gs t r i pa l o n gt h e
It i s 700 f t ( 2 1 3 m ) t h i c k a t
of SawmillCanyon.
i t s c e n t e ra n dp i n c h e so u tl a t e r a l l yt ot h en o r t ha n ds o u t h .
The r h y o l i t e i s t y p i c a l l y a p h a n i t i c , g r a y t o p i n k i n c o l o r ,
a n dw e a t h e r st op l a t yf r a g m e n t s .S t r i k e sa n dd i p sa r e
d i f f i c u l tt oo b t a i n
foliation.
o f d i s t i n c tf l o w
on t h i s l a v a d u e t o l a c k
A f l o wb r e c c i a
i s o f t e np r e s e n t
a t t h et o p ,w h e r e
exposed.
u p p e r member.
The
u p p e r A-L
P e a kc r o p so u td i s c o n t i n u o u s l y
a b o v et h er h y o l i t el a v ai n t e r v a la l o n gt h ee a s ts i d e
SawmillCanyon
maximum t h i c k n e s s i s 500 f t ( 1 5 2 m ) .
f o u n di nt h en o r t h e a s tq u a r t e r
u p p e r A-L
(4.0 km).
f o r a d i s t a n c e of 2.5 m i
of Sec.
of
Its
Good e x p o s u r e sa r e
4
(T.5S.,
R.3W.).
The
Peak i s u s u a l l y d e n s e l y w e l d e d , a l t h o u g h i n p l a c e s
a p o o r l yw e l d e dz o n ea b o u t
1 5 f t (4.6 m) t h i c k i s exposed a t
34
*
i t s b a s e .T h ep o o r l yw e l d e di n t e r v a l
i s l i g h tg r a yw i t h
a n d w e a t h e r se a s i l yt oc l a y .
g r a y i s h - g r e e np u m i c e
1 0 t o 20 f t ( 3 t o 6 m ) o f t h ed e n s e l yw e l d e dz o n e
The b a s a l
is crystal
t h i s i n t e r v a l i s p a l er e da n dw e a t h e r s
P o o ra n dl i t h i cr i c h ;
1 t o 2%; p h e n o c r y s t s o f
t ob l o c k yt a l u s .I nt h i ns e c t i o n ,
s a n i d i n e( l a r g e l yp l u c k e do u td u r i n gp r e p a r a t i o n )a n dq u a r t z
A s much a s 1 5 % , ' a n g u l a rl i t h i cf r a g m e n t so f
a r ep r e s e n t .
in
a n d e s i t ea n df l o w - b a n d e dr h y o l i t ea r ep r e s e n ta n dr a n g e
s i z eu pt o
5 cm.
T h eu p p e rp o r t i o no ft h ed e n s e l yw e l d e d
1 2 t o 15%.
zone i s p a l ep u r p l et op a l er e dp u r p l ea n dc o n t a i n s
c r y s t a l s o f s a n i d i n ea n dq u a r t z
a n d a t r a c e of b i o t i t ea n d
1 mm l o n g i s
m a g n e t i t e .E u h e d r a ls a n i d i n ea b o u t
a b u n d a n t as q u a r t z ,w h i c h
e
t i m e s as
i s small, a n h e d r a l ,a n dd e e p l y
1.5 cm i n l e n g t ha n da r eo f t e n
e m b a y e dP
. u m i c ea v e r a g e
spherulitic.
4
A t r a c eo fa n g u l a rr h y o l i t el i t h i cf r a g m e n t s
is
present.
O u t c r o p so f
A-L P e a kT u f ft h a ta r ep r e s e n tb e y o n d
i n a p p e a r a n c ea n d
t h eo u t c r o pa r e ao ft h er h y o l i t el a v av a r y
i n s e c t i o n s 3 and 1 0
w i l l b ed e s c r i b e ds e p a r a t e l y .O u t c r o p s
(T.5S.,
R.3W.)
a r ep o o rd u et ot h ea b u n d a n c e . o ft a l u sf r o m
t h eo v e r l y i n gu n i t s .
a s f l o a tb l o c k s
Some o f t h e s e
i n t h er h y o l i t e
A-L P e a ko u t c r o p so c c u r
dome complex o f t h e u n i t
S i x m i l eC a n y o n .W h e r eo u t c r o p sa r ep r e s e n t ,t h eb a s e
A-L
P e a kc o n s i s t s
of
t h i n ,( a b o u t
of
of t h e
15 f t ( 4 . 6 m ) t h i c k )
m o d e r a t e - b r o w nv i t r o p h y r et h a tc o n t a i n sd a r k - r e d d i s h - b r o w n
m
( o f t e ns p h e r u l i t i c )p u m i c e .
The u n i tt e n d st oc r u m b l e
when
35
a
i s a d e n s e l yw e l d e dz o n eo f
b r o k e n .O v e r l y i n gt h ev i t r o p h y r e
m o d e r a t e - r e dt u f fc o n t a i n i n gd a r kr e d d i s h - b r o w np u m i c e I. n
( N W 1/4, S e c . 1 0 , T . 5 S . ,
o n eo u t c r o p
R.3W.1,
t h et u f f
c o n t a i n ss u b s p h e r i c a ld e v i t r i f i c a t i o nm a s s e s
similar t o t h o s e
f o u n d i n t h er h y o l i t eo ft h eu n i to fH a r d yR i d g e( p .
19; F i g .
0).
The A-L
i n S e c s . 1 4 , 2 2 , a n d2 3
PeakTuff
is
m o d e r a t e l yt od e n s e l yw e l d e da n dw e a t h e r st oa n g u l a rb l o c k s .
i s p a l er e dp u r p l e
The t u f f h e r e
o r g r a y i s hr e dw i t h
v e r y - l i g h t - g r a yp u m i c et h a ta r et y p i c a l l y
1 0 cm by 0 . 5
s i z ea n dc o n t a i ns p h e r u l i t i ca n da x i o l i t i cs t r u c t u r e s .
cm i n
Two
t o 5% p h e n o c r y s t so fs a n i d i n e ,b i o t i t e ,a n dq u a r t za r e
is 3 t o
p r e s e n tS
. anidine
a
4
t i m e s a s a b u n d a n t as q u a r t z .
t r a c e of b i o t i t e i s p r e s e n t .M a g n e t i t e
A
commonly r e p l a c e s
b i o t i t e .T h e s eo u t c r o p sa r eo u t s i d et h et o p o g r a p h i cm a r g i n
o ft h eS a w m i l lC a n y o nc a u l d r o na n dr e p r e s e n to u t f l o w - f a c i e s
A-L
Peak.
O u t s i d et h em a j o rr i n gf r a c t u r eo ft h eS a w m i l l
Canyon c a u l d r o n ,l i n e a t i o n
o u t c r o p s ,w h i l e
i s s t r o n g l yd e f i n e d
i n o t h e r s i t i s p o o r l yd e f i n e d
When p r e s e n t , t h e g e n e r a l t r e n d
north-south
v e r t i c a lv a r i a t i o n
o r absent.
of t h e l i n e a t i o n s
i n c o n t r a s tt ot h eg e n e r a le a s t e r l y
n o r t h e a s t e r l yt r e n d s
i n some
w i t h i n t h ec a u l d r o n .
is
or
The l a t e r a la n d
i n p r i m a r yf l o ws t r u c t u r e s
l a r g e l yt ot e m p e r a t u r ev a r i a t i o n sd u r i n ge r u p t i o n .
i s p r o b a b l yd u e
36
member o f t h e
A t t h ep r e s e n tt i m e ,t h eg r a y - m a s s i v e
A-L
i s b e l i e v e dt oh a v eb e e ne r u p t e df r o mt h e
P e a kT u f f
M a g d a l e n ac a u l d r o nT
. h ef l o w - b a n d e da n dp i n n a c l e m
s embers
a r ec o n s i d e r e dt oh a v eb e e ne r u p t e df r o mt h eM a g d a l e n aa n d
S a w m i lC
l a n y o nc a u l d r o n sr, e s p e c t i v e l yH
. o w e v e rt,h e
p r e s e n c eo ff l o wb a n d i n gi nt h e
A-L
P e a kw i t h i nb o t ho ft h e s e
c a u l d r o n sm a k e st h i sc o r r e l a t i o nu n c e r t a i n A
. lternatively,
t h eS a w m i l l
C a n y o na n dM a g d a l e n ac a u l d r o n s
i n p a r t , w i t h t h ef l o w - b a n d e d
concurrently,
b e e ne r u p t e d
may h a v es u b s i d e d
A-L
P e a kh a v i n g
f r o m b o t hc a u l d r o n s .
U n i to fS i x m i l eC a n y o n
O v e r l y i n gt h e
P e a kT u f fa r em o a td e p o s i t so f
A-L
Canyon c a u l d r o n , f i r s t named t h eS i x m i l e
t h eS a w m i l l
A n d e s i t e byKrewedl
( 1 9 7 4 ) a f t e re x p o s u r e si nt h ee a s t e r n
(1977) c h a n g e dt h e
M a g d a l e n aM o u n t a i n sO
. sburn
oS
f i x m i l eC a n y o n
Canyon
t oi n d i c a t e
name t o u n i t
i t s h e t e r o l i t h i cn a t u r e .
The
u n i tc o n s i s t so fr h y o l i t el a v af l o w sa n dd o m e s ,a n d e s i t el a v a
f l o w s ,s e d i m e n t a r yr o c k s ,a n dt h i n ,l o c a l
t u f f s and i s capped
by a d i s t i n c t i v e a s h - f l o w t u f f i n t e r v a l i n f o r m a l l y
named t u f f
o fC a r o n i t aC a n y o n .
The u n i t o f S i x m i l eC a n y o nc r o p so u tc o n t i n u o u s l y
a l o n gt h ee a s t e r nb o r d e ro ft h es t u d ya r e aa n di n t r u d e sa n d
o v e r l i e st h e
A-L
P e a kT u f fa n dt h eu n i to fH a r d yR i d g e .T h e
u p p e ri n t e r v a lo ft h eu n i t
i s r e p e a t e d i n t h es o u t h w e s t e r n
p o r t i o no ft h i ss t u d ya r e a
by a l a r g ed o w n - t o - t h e - w e s tn o r m a l
fault.
37
e
i s a t l e a s t 2500 f t ( 7 6 2
T h eu n i to fS i x m i l eC a n y o n
m ) t h i c kf o r
(1979) r e p o r t s a
much of i t s e x t e n t ;P e t t y
s i m i l a r t h i c k n e s s i n Ryan H i l l Canyon.Bowring
(1980) h a s
t w o membersbasedupon
i n f o r m a l l ys u b d i v i d e dt h eu n i ti n t o
t h ed e g r e eo fl a t e r a lv a r i a b i l i t yo fl i t h o l o g i e sa n d
s t u d y .T h el o w e r
i n this
w i l l b eu s e d
t h i c k n e s s e s ;h i sm e t h o do fd i v i s i o n
Canyon i s
member o ft h eu n i to fS i x m i l e
c h a r a c t e r i s t i c a l l yv a r i a b l ei nr o c kt y p ea n dt h i c k n e s s .T h e
u p p e r member i s a l a t e r a l l yc o n t i n u o u ss e q u e n c ec o m p o s e do f
a na n d e s i t el a v af l o w
a n d t h eo v e r l y i n gt u f fo fC a r o n i t a
Canyon.
l o w e r member.
The
l o w e r member o ft h eu n i to fS i x m i l e
Canyon
c o n s i s t so fa ni n t r u s i v er h y o l i t ec o m p l e x ,r h y o l i t el a v a
e
a n d m i n o ra s h - f l o w
f l o w s ,v o l c a n i c l a s t i cs e d i m e n t a r yr o c k s ,
tuffs.
r h y o l i t e dome complex ( T x r l ) .
A r h y o l i t e dome
c o m p l e x ( T x r l ) i s l o c a t e d i n t h ec e n t r a lp a r to ft h es t u d y
area.
It i s i n t r u s i v ei n t ot h eH e l l s
H a r d yR i d g e ,a n dt h e
A-L
P e a kT u f fa n d
c a u l d r o n - f i l ld e p o s i t so ft h eS a w m i l l
f l o a tb l o c k sa r e
s q f t t o 0.08
composedof
p o r p h y r i t i ca n d e s i t e ,H e l l s
e
a n d A-L P e a km e s o b r e c c i a .
of
i s o v e r l a i n by
C a n y o nc a u l d r o n .
W i t h i nt h er h y o l i t ea r ef l o a tb l o c k st h a tr a n g e
a r e af r o ms e v e r a l
Mesa T u f f ,t h eu n i t
i n outcrop
s q m i ( 0 . 2 1 s q km).The
a c r y s t a l - p o o ra s h - f l o wt u f f ,
Mesa m e s o b r e c c i a , A-L
P e a kT u f f ,
a
38
s i s t a n t a n do f t e nf o r m
The r h y o l i t e s a r e v e r y r e
s t e e pc l i f f sa n dp i n n a c l e s .C l o s e l ys p a c e d
h i g h l yc o n t o r t e df l o wb a n d i n ga r e
s h e e tj o i n t sa n d
common f e a t u r e s i n t h e
similar
r h y o l i t e s .S p h e r u l i t e sa n dd e v i t r i f i c a t i o nt e x t u r e s
i n t h er h y o l i t e so ft h eu n i to fH a r d yR i d g e
t ot h o s ef o u n d
(p. 19; Fig.
8 ) a r ea l s op r e s e n t .
The r h y o l i t e sa r ep a l er e d
i n texture.
i nc o l o ra n dd e n s ea n da p h a n i t i ct os u g a r y
a t r a c eo fq u a r t z
P h e n o c r y s t sa r eu s u a l l ya b s e n t ,b u t
is
locallypresent.
v o l c a n i c l a s t i cs e d i m e n t a r yr o c k s
and t u f f s (Txs).
A s e d i m e n t a r yi n t e r v a lc o n s i s t i n go fd e b r i s - f l o wd e p o s i t s ,
i n t e r b e d d e ds a n d s t o n e s ,a n d
a t h i na s h - f l o wt u f fc r o p so u t
at
s c a t t e r e dl o c a t i o n si nt h es o u t h e a s t e r np a r to ft h i ss t u d y
area.
T h es e d i m e n t a r yi n t e r v a lo v e r l i e st h er h y o l i t e
complex ( T x r l ) a n d c o n s i s t s l a r g e l y
dome
o f b l o c k so fr h y o l i t e
t h a tw e r ee r o d e df r o mt h ed o m e s ;t h ec o n t a c tb e t w e e nt h e
r h y o l i t ea n dd e b r i s - f l o wd e p o s i t s
is gradational.
O u t c r o p so ft h ed e b r i s - f l o wd e p o s i t sa r ew e l l
i n d u r a t e da n df o r mc l i f f yo u t c r o p s
a t t h eb a s eo ft h ec a n y o n
n e aS
r a w m i lC
l anyon
W i n d m i l(lS e c .
15, T.5S.,
R.3W.l.
The
c l a s t sa r eg e n e r a l l yg r a i n - s u p p o r t e dw i t hl i t t l e
matrix,
p o o r l ys o r t e d ,a n dr a n g eu pt o
A t t h eb a s e
1 m i n diameter.
of t h e u n i t , t h e c l a s t s c o n s i s t e n t i r e l y o f c r y s t a l - p o o r
r h y o l i t e( F i g .
15).
a b u n d a n c eu p w a r d ,a n d
present.
in
F r a g m e n t so fa n d e s i t ei n c r e a s e
a t r a c eo fc l a s t so f
A-L
is
P e a kT u f f
The m a t r i x i s u s u a l l yd a r kr e d d i s hb r o w nb u t
is
39
F i g u r e 15. P h o t o g r a p h o f d e b r i s - f l o w d e p o s i t s
(Txs) found in the unit o f Sixmile Canyon.
Clasts are crystal-poor rhyolite lava.
(SE 1/4, Sec. 10, T.5S., R.3W.).
40
a u s k yy e l l o wg r e e nw h e r ep r o p y l i t i z e d I. n t e r b e d d e d
20 f t ( 6 m ) t h i c k a n d a r e
s a n d s t o n e sa r eu s u a l l yl e s st h a n
g r a y i s hy e l l o wg r e e ni nc o l o r .
An i n t e r b e d d e dw a t e r - l a i d
( ? ) t u f f ,a b o u t
m ) t h i c k ,c r o p so u ti nt h es o u t h e a s tq u a r t e r
(T.5S.,
R.3W.).
10%. f e l d s p a r s a n d
a t r a c e of q u a r t z
1 mm) l i t h i cf r a g m e n t s
p h e n o c r y s t s .S m a l l( l e s st h a n
a l a r g ep o r t i o n
of S e c . 10
i s w h i t e i n c o l o ra n dc o n t a i n s
T h i st u f f
a b o u t 5%; b i o t i t e ,
10 f t ( 3
of t h eg r o u n d m a s s .
make up
was
A sample of t h et u f f
c o l l e c t e d f o r r a d i o m e t r i cd a t i n g .
One o u t c r o p o f T x t i s p r e s e n t
a s h - f l o wt u f f( T x t ) .
w i t h i nt h i ss t u d ya r e a .
The t u f f
i s p a l ey e l l o wa n dc o n t a i n s
a b o u t2 O % > ' p h e n o c r y s t s of s a n i d i n ea n d
a t r a c e of q u a r t z .
t u f f i s b e l i e v e dt ob ec o r r e l a t i v ew i t ht Q et u f f( T x t )
tothenorth
byBowring
(6.4 km)T
. h i sr h y o l i t e
R.3W.I
A t t h et o p
o f S i x m i l eC a n y o n
i s a sanidine-bearing
4
mi
i s approximately 1 0 0 0 f t (305 m)
i t s s o u t h e r ne x t r e m i t y
a n dt h i n st ol e s st h a n
o f t h el o w e r
f o r a d i s t a n c e of almost
r h y o l i t e( T x r 2 )t h a tc r o p so u t
t h i c ka t
mapped
(1980).
s a n i d i n er h y o l i t e( T x r 2 ) .
member o f t h e u n i t
The
( N 1 / 2 ,S e c .
10, T.5S.,
20 f t ( 6 m ) a t t h en o r t h e r n
b o u n d a r y o f t h i ss t u d ya r e a .O u t c r o p s
o f s a n i d i n er h y o l i t e
f o r ms t e e pc l i f f sa n dw e a t h e rt ol a r g eb o u l d e r st h a tf o r m
i m p r e s s i v et a l u ss l o p e st h a to b s c u r eu n d e r l y i n gu n i t s .
r h y o l i t e i s l i g h tb r o w n i s hg r a yt og r a y i s hr e dp u r p l e
The
when
41
fr=sh.
i s 1 0 t o 20
A m o d e r a t e - r e d d i s h - b r o w nv i t r o p h y r et h a t
f.% ( 3 t o 6 m ) t h i c k i s p r e s e n t a t i t s
k t i o n ( S W 1/4, SE 1/4, S e c .
3.m. ) .
u p p e rc o n t a c t
a t one
28 ( u n s u r v e y e d ) ,T . b S . ,
is
J u s tn o r t ho ft h ev i t r o p h y r i cz o n e ,t h er h y o l i t e
p n p y l i t i z e dt o
a g r a y i s h - g r e e nc o l o r .F i v et o
1570,
i n subequal
p k n o c r y s t so fs a n i d i n ea n dq u a r t za r ep r e s e n t
a s much a s 1 cm
m u n t s .I nt h i ns e c t i o n ,s a n i d i n ee u h e d r a
* l e n g t ha r eh i g h l ya l t e r e dt of i n e - g r a i n e dp h y l l o s i l i c a t e s .
W e d r a lt os u b h e d r a lq u a r t zp h e n o c r y s t st h a tr a n g e
i n size
f m m 0 . 2 mm t o 1 . 0 mm a r eo f t e nd e e p l ye m b a y e d .M a g n e t i t e
-ely
.is
d i s s e m i n a t e dt h r o u g h o u tt h ed e v i t r i f i e dg r o u n d m a s s .
w e r member.Theuppermemberof
t h eu n i to fS i x m i l e
Canyon
c m p so u tc o n t i n u o u s l yf r o mt h en o r t h e r nb o u n d a r yo ft h e
o f t h eS a w m i l l
edyareatothesoutherntopographicmargin
Cvcyon c a u l d r o n
(Sec.
14, T . 5 S . ,
R.3W.).
I t c o n s i s t so f
1)
z-msal a n d e s i t e l a v a i n t e r v a l t h a t c o n t a i n s i n t e r b e d d e d
s a r d s t o n e sa n d
2 ) a na s h - f l o wt u f fi n t e r v a l
known a s t h e t u f f
d C a r o n i t a Canyon.
a n d e s i t el a v a s .T h ea n d e s i t e
q p e r memberof
t h eu n i to fS i x m i l e
i.ke s a n i d i n e r h y o l i t e o f t h e l o w e r
a i t h es t u d ya r e a( S e c .
lava i n t e r v a l i n t h e
Canyon i n t e r f i n g e r s w i t h
member i n t h e n o r t h p a r t
2 8 ( u n s u r v e y e d ) , T.kS.,
R.3W.I.
S e d i m e n t a r yr o c k s( u s u a l l ys a n d s t o n e s )t h a ta r ei n t e r b e d d e d
E t ht h ea n d e s i t el a v a s
a r e r e l a t i v e l ya b u n d a n t
I t e t h e r np a r to ft h es t u d ya r e a ,m a k i n gu pm o r et h a n5 0 % .o f
i n the
42
t h et h i c k n e s s
o f t h eu n i t .
Toward t h es o u t h ,t h e
a n d e s i t e - t o - s a n d s t o n er a t i og r a d u a l l yi n c r e a s e st ow h e r et h e
a n d e s i t e makesup
a t l e a s t 9 0 % , o f t h et h i c k n e s s
of t h e u n i t .
O u t c r o p s o f a n d e s i t ea r ev e r yn o n - r e s i s t a n ta n du s u a l l y
The a n d e s i t e i s d e n s e ,a p h a n i t i c ,
w e a t h e rt og r a s s ys l o p e s .
a n db r o w n i s hg r a yi nc o l o r .V e s i c l e sa r e
common a n d a r e
o r d r u s ya n dm o s a i cq u a r t z ,
u s u a l l yf i l l e dw i t hc h a l c e d o n y
a f i b r o u sz e o l i t e( p r o b a b l y
a l t h o u g hc o a r s ec a l c i t ea n d
n a t r o l i t e )a r ef o u n dl o c a l l y .
The a n d e s i t ew e a t h e r s
p r e f e r e n t i a l l y w i t h r e s p e c tt ot h eq u a r t za m y g d u l e s ,w h i c h
a r e more r e s i s t a n t a n d a r ef o u n d
a s e l l i p s o i d a lb o d i e s .
a t h i n ,g r a y i s h - g r e e n
O f t e nt h e s eb o d i e sa r ec o a t e dw i t h
l a y e rt h a t
i s p r o b a b l yc e l a d o n i t e .P h e n o c r y s t sc o n s i s t
1.5 mm l o n g .
t r a c eo fp l a g i o c l a s et h a ta v e r a g e sa b o u t
P l a g i o c l a s ee u h e d r a ,a v e r a g i n g
of a
0 . 4 mm i n l e n g t h ,c o m p r i s e
a b o u t 25%j o f t h eg r o u n d m a s s ;c l i n o p y r o x e n ea n h e d r al e s st h a n
U.l
mm i n d i a m e t e r make u pa b o u t
5%,.
T h er e m a i n i n g
g r o u n d m a s s i s c r y p t o c r y s t a l l i n ea n di n t e n s e l y
hematite-stained.
T u f f o f C a r o n i t aC a n y o n .O v e r l y i n gt h ea n d e s i t e
interval is thetuff
o f C a r o n i t aC a n y o n ,
s i m p l ec o o l i n gu n i tw h i c h
a multiple-flow,
was i n f o r m a l l y named f o r e x p o s u r e s
(1979).
i n t h es o u t h e a s t e r nM a g d a l e n aM o u n t a i n sb yP e t t y
P e t t y (1979) r e p o r t s a r a n g e i n t h i c k n e s so f
(61 t o 305 m ) .
200 t o 1000 f t
However, h i se s t i m a t e dt h i c k n e s s
( 3 0 5 m ) on I t a l i a n Peak i s somewhatlow,
of 1 0 0 0 f t
a s t h el o w e r
member
*
t h e r e i s a t l e a s t 750
a thintuffinterval
( 1 9 7 8 ) mappe d
; h i c kO. s b u r n
i n S i x m i l e Canyon t h a t i s e q u i v a l e n t t o
mapped a
p a r to ft h et u f fo fC a r o n i t aC a n y o n ;B o w r i n g( 1 9 8 0 )
t u f fu n i t3 5 0
f t (107 m ) t h i c k on T i m b e rR i d g e
tothenorth
a s a m p l e o f t h et u f fo f
o ft h i ss t u d ya r e a .B i o t i t ef r o m
at
C a r o n i t a Canyon c o l l e c t e d o nM o l i n oP e a kh a sb e e nd a t e d
29.4
+/-1.1 m.y.
by t h e K - A r
method ( C . E .
C h a p i n ,u n p u b .
date).
The t u f f o f C a r o n i t a
Canyon c r o p so u tc o n t i n u o u s l y
a distance
a l o n gt h ee a s t e r nb o u n d a r yo ft h es t u d ya r e af o r
of
4 m i (6.4
km) a n dc o n t i n u e ss o u t h e a s t w a r di ns c a t t e r e d
o u t c r o p s f o r 1 . 5 m i ( 2 . 4k m )H
. ere,
1 4 , T.5S.,
o fS e c .
R.3W.,
i n t h es o u t h e r np o r t i o n
t h et u f fo fC a r o n i t a
C a n y o np i n c h e s
o u ta g a i n s to l d e ru n i t sa l o n gt h ee a s t - t r e n d i n gs o u t h e r n
t o p o g r a p h i cm a r g i no ft h eS a w m i l lC a n y o nc a u l d r o n .E x p o s u r e s
a r ea l s of o u n d
(Secs.
i n t h es o u t h w e s t e r np a r t
of t h es t u d ya r e a
1 6 a n d2 1 ,T . 5 S ,R . 3 W . ) ,w h e r et h eu n i t
s e v e r a ld o w n - t o - t h e - w e s tf a u l t st h a t
The t u f f o f C a r o n i t a
o ft h eu n i t
i s r e p e a t e db y
w i l l b ed i s c u s s e dl a t e r .
Canyon i,s
t h ey o u n g e s t
member
o f S i x m i l eC a n y o n .T h et u f fn o r m a l l yo v e r l i e s
a n d e s i t e l a v a s ( T x a 2 )e x c e p tn e a rt h es o u t h e r nt o p o g r a p h i c
m a r g i no ft h eS a w m i l lC a n y o nc a u l d r o n .H e r e ,t h eu n i t
o v e r l i e s a r h y o l i t el a v af l o w( T x r 5 )a n dp i n c h e so u ta g a i n s t
t h eo l d e ru n i t
o f H a r d yR i d g e T
. h et u f f
overlainbythetuff
o f L e m i t a rM o u n t a i n s
i s conformably
or u n c o n f o r m a b l y
o v e r l a i n by t h e L o w e rP o p o t o s aF o r m a t i o nH
. owever,
a thin
44
l e n s of s a n d s t o n e o v e r l i e s t h e t u f f
i n o n el o c a la r e a
(NE
1 / 4 , S e c . 11, T.5S.,R.3W.).
l o w e r member.
The
l o w e r member o f t h e t u f f
in
C a r o n i t a Canyon i s r h y o l i t i c t o q u a r t z l a t i t i c
c o m p o s i t i o nT
. h r e et o2 O % , ' p h e n o c r y s t s
of
of p l a g i o c l a s e ,
b i o t i t e ,a n dm a g n e t i t ea r ep r e s e n t .T h ep h e n o c r y s tc o n t e n t
i n c r e a s e su p w a r d
i n t h es e c t i o nw i t hp l a g i o c l a s ec o m p r i s i n g
a b o u t 75% o f t h ep h e n o c r y s t s .
Some p l a g i o c l a s ep h e n o c r y s t s
15%;
show a m a r k e dz o n a t i o n .S u b h e d r a lb i o t i t em a k e su pa b o u t
of t h ep h e n o c r y s t s .A n h e d r a lt os u b h e d r a lm a g n e t i t e
c o n s t i t u t e sa b o u t
10%;of t h ep h e n o c r y s t sa n d
i s sometimes
f o u n dr e p l a c i n gb i o t i t e S
. a n i d i n eq
, u a r t z c, l i n o p y r o x e n e ,
i n t r a c ea m o u n t s .P u m i c ec o n t a i n
a n dh o r n b l e n d ea r ep r e s e n t
p h e n o c r y s t s of p l a g i o c l a s e , b i o t i t e , a n d m a g n e t i t e a n d a r e
u s u a l l ys p h e r u l i t i c .
P e t t y ( 1 9 7 9 ) h a sd i v i d e dt h el o w e r
member i n t o
t h r e ez o n e sb a s e do nt h ed e g r e eo fw e l d i n g ,p h e n o c r y s t
i n his
c o n t e n t ,a n dc o l o r ;d e t a i l e dd e s c r i p t i o n sc a nb ef o u n d
t h e s i s .T h e s ez o n e s ,
i n a d d i t i o nt o
p r e s e n ti nt h es t u d ya r e aa n d
a v i t r o p h y r i cz o n e ,a r e
w i l l b ed e s c r i b e db r i e f l yh e r e .
The z o n a t i o nb e c o m e sl e s sd i s t i n c tt ot h es o u t h
as t h e u n i t
b e c o m e st h i n n e r .
Where e x p o s e d ,t h eb a s e
g r a y i s h - p i n k ,p o o r l yw e l d e dz o n e
-*
of t h e u n i t c o n s i s t s
of a
1 0 t o 20 f t ( 3 t o 6 m ) t h i c k
o v e r l a i n by a b l a c k v i t r o p h y r e e x h i b i t i n g e u t a x i t i c
structure.
Good o u t c r o p so ft h e s ez o n e sa r ef o u n d
i n the
45
s o u t h - c e n t r a lp a r to ft h es t u d ya r e a
R.3W).
11, T.5S.,
(NW
1 1 4 , SW 1 1 4 ,
O v e r l y i n gt h ev i t r o p h y r e
Sec.
i s a l i g h tt o
m o d e r a t eb r o w n ,l i t h i c - r i c h ,c r y s t a l - p o o r ,d e n s e l yw e l d e d
i n t h i si n t e r v a la r el a r g e l y
i n t e r v a l .L i t h i cf r a g m e n t s
f l o w - b a n d e dr h y o l i t e .
memb’er i s
The u p p e r ,c r y s t a l - r i c hz o n eo ft h el o w e r
a n d d e n s e l yw e l d e d .
t y p i c a l l ym o d e r a t er e dt og r a y i s hr e d
i s r e l a t i v e l yr e s i s t a n t
It
a n d b r e a k sa l o n gj o i n tp l a n e st of o r m
a n g u l a rt a l u sf r a g m e n t s T
. h i sz o n eo f t e nd i s p l a y sp r o m i n e n t
e u t a x i t i cf o l i a t i o n .
u p p e r member.
The
l o w e rm e m b e r so ft h et u f f
c o n t a c tb e t w e e nt h eu p p e ra n d
o f C a r o n i t a Canyon was mapped a t a
s h a r p l yg r a d a t i o n a lc h a n g ef r o ma n d e s i t i ct or h y o l i t i c
mineralogy.
An i n c r e a s ef r o m
2 0 % t o 30 o r 4 0 %p h e n o c r y s t s
a l s oo c c u r sa c r o s st h i sb r e a k .
r o u n d e db o u l d e r s ;
The u p p e r member w e a t h e r st o
i t i s w h i t e t o medium g r a ye x c e p tf o rt h e
b a s a lf e wf e e t ,w h i c ha r es o m e t i m e sp a l er e d .
member i s m o d e r a t e l yt od e n s e i yw e l d e d
The u p p e r
a n d c o n t a i n s 30 t o 4 5 %
p h e n o c r y s t so fs a n i d i n e q
, u a r t z a, n db i o t i t e S
. ubhedral
s a n i d i n ea n dl a r g e ,o f t e nd i p y r a m i d a l ,q u a r t zc r y s t a l sa r e
p r e s e n t i n s u b e q u a la m o u n t s
a n d c o n s t i t u t e 90% o f t h e
p h e n o c r y s t s .S m a l l ,s u b h e d r a lb i o t i t ec r y s t a l s
10% of t h ep h e n o c r y s t s .P l a g i o c l a s e
i s present i n trace
a m o u n t s .P u m i c ec o n t a i no n l yt r a c e so fp h e n o c r y s t sa n da r e
o f t e ns p h e r u l t i c .
make u pa b o u t
46
o f C a r o n i t a Canyon v a r i e s f r o m 0 t o
T h eu p p e rt u f f
a s much a s 1100 f t ( 3 3 5 m ) t h i c k i n t h es t u d ya r e a ,a l t h o u g h
thickersections
may b ee x a g g e r a t e db yu n r e c o g n i z e df a u l t i n g .
Many o u t c r o p ss u g g e s t
a t h i c k n e s s of a t l e a s t 500 f t ( 1 5 2 m ) .
Tuff o f L e m i t a rM o u n t a i n s
i s a multiple-flow,
The t u f f o f L e m i t a rM o u n t a i n s
c o m p o s i t i o n a l l yz o n e d ,s i m p l et o
compound c o o l i n g u n i t
1978).
d e n s e l yw e l d e dt u f f( C h a p i na n do t h e r s ,
of
Chamberlin
(1980) d e s c r i b e d a s e c t i o n i n t h eL e m i t a rM o u n t a i n s ,a f t e r
i s named.Using
w h i c ht h et u f f
s a m p l e s of t h e t u f f
t h e K-Ar
m e t h o do nb i o t i t e ,
o f L e m i t a rM o u n t a i n sf r o mt h eS a nM a t e o
a t 27.0
M o u n t a i n sa n dt h eL e m i t a rM o u n t a i n sh a v eb e e nd a t e d
+ / - 1.1
m.y.
a n d2 6 . 3
+ / - 1.0
m.y.,
r e s p e c t i v e l y (C.E.
Chapinu
, npubd
. ate)T
. h r e ed a t e sf r o mt h es a m eo u t c r o pi n
theJoyita
Hills y i e l d e d a g e s
+/-
+/-1.1 m . y .a, n d2 8 . 1
commun.).
The
26.3
m.y.
o f 28.8
1 . 2 m.y.
+/-0.7
(C.E.
m.y.,27.6
Chapino
, ral
d a t ec a nb ee l i m i n a t e dd u et o
p o t a s s i u mm e t a s o m a t i s m ;t h er e m a i n i n gd a t e sa v e r a g e2 7 . 9
C h a p i na n do t h e r s
was t h e s o u r c e
m.y.
( 1 9 7 8 ) p r o p o s e dt h a tt h eS o c o r r oc a u l d r o n
of t h e t u f f
o f L e m i t a rM o u n t a i n s ,b u tt h i s
a s s i g n m e n t i s now i n q u e s t i o n .
The t u f f o f L e m i t a rM o u n t a i n sc r o p so u ta l o n gt h e
w e s ts i d e
o f S a w m i l lC a n y o nw h e r e
s l o p e s ; i t a l s oc r o p so u t
e a s t o f t h ec a n y o n T
. he
it f o r m se x t e n s i v ed i p
i n a f e ws c a t t e r e dp a t c h e st ot h e
tuff i s d i v i s i b l ei n t ot x o
members
47
*
lower, moderately
o a s e du p o np h e n o c r y s tc o n t e n t - - a
crystal-poor
member a n d a n u p p e r ,c r y s t a l - r i c h
i s g r a d a t i o n a lo v e r
c o n t a c tb e t w e e nt h et w om e m b e r s
f t
member.
The
5 t o 50
( 1 . 5 t o 1 5 m ) a n d was mapped a t t h e a b r u p t i n c r e a s e
crystal
variable
in
o f t h e members i s h i g h l y
C o n t e n t .R e l a t i v et h i c k n e s s
as i s t h et o t a lt h i c k n e s so ft h eu n i t .H o w e v e r ,
t h e t u f f o f L e m i t a rM o u n t a i n s
i s v e r yt h i c k
i n t h i ss t u d y
area--as
much a s 1 0 0 0 f t ( 3 0 5 m ) w h e r e a c o m p l e t es e c t i o n
exposed
Sec.
2 1 , T.5S.,
t h i c k n e s s e su pt o
R.3W.).
Petty
is
(1979) r e p o r t s
1 8 0 0 f t ( 5 4 9 m ) i n Ryan Hill Canyon.The
o c c u r r e n c e of t h e s et h i c ks e c t i o n so fo u t f l o w - f a c i e st u f fo f
L e m i t a rM o u n t a i n s
d e p r e s s i o nl e f t
i s p r o b a b l yd u et op o n d i n go ft h et u f fi n
by c o l l a p s e o f t h eS a w m i l l
a
C a n y o nc a u l d r o n .
Donze (1980) r e p o r t s a t h i c k n e s s o f 2 0 0 0 f t ( 6 0 0 m ) o f
L e m i t a r a n d i n t e r b e d d e dm e s o b r e c c i a sa n dm e g a b r e c c i a st ot h e
w e s t o f t h i ss t u d ya r e a .
He s u g g e s t st h a tt h es o u r c e
c a u l d r o nf o rt h et u f fo fL e m i t a rM o u n t a i n s
may b el o c a t e d
in
t h a ta r e a ,b u tu n e q u i v o c a lp r o o fh a sn o tb e e nd o c u m e n t e d .
W i t h i nt h eS a w m i l lC a n y o nc a u l d r o n ,t h et u f f
M o u n t a i n so v e r l i e st h et u f f
t h el o w e rc o n t a c t
of L e m i t a r
o f C a r o n i t aC a n y o n E
. lsewhere,
i s n o te x p o s e da n dt h et u f f
c o n t a c tw i t ht h ey o u n g e rr h y o l i t e so ft h eu n i t
is i n intrusive
of S a w m i l l
Canyon.
Osburn
(1978) m e a s u r e da n dd e s c r i b e d
s t r a t i g r a p h i cs e c t i o n
L e m i t a rM o u n t a i n s
of t h e l o w e r
a
member o f t h e t u f f o f
i n S i x m i l eC a n y o n ;s e eh i st h e s i sf o r
a
48
*
d e t a i l e dd e s c r i p t i o no fz o n a t i o na n dp e t r o g r a p h i c
characteristics.
Donze (1980) d e s c r i b e d a 1 5 0 0 - f(t4 5 0 - m )
t h i c ki n t e r m e d i a t ez o n et h a th a sn o tb e e nr e c o g n i z e d
in this
s t u d ya r e a .
l o w e r member.
The
l o w e r member of t h et u f fo fL e m i t a r
a hackly
M o u n t a i n s i s d e n s e l yw e l d e da n do f t e nb r e a k sw i t h
fracture.
T h er o c k
i s u s u a l l yp a l er e di nc o l o r ,a l t h o u g h
t h eb a s a lf e wf e e ta r em o d e r a t eo r a n g ep i n kw h e r ee x p o s e d .
T h el o w e r
member i s m o d e r a t e l yc r y s t a lp o o r ,c o n t a i n i n g
1 5 % p h e n o c r y s t s .Q u a r t za n h e d r at h a ta r e
d i a m e t e ra n ds o m e t i m e se m b a y e d
1 t o 2 mm i n
make u pa b o u t2 5 % >o ft h e
75%: a r e s a n i d i n e e u h e d r a t h a t a r e 0 . 5 t o
p h e n o c r y s t s ;a n o t h e r
2 . 5 mm l o n g .
10 t o
A t r a c eo fb i o t i t ea n de u h e d r a ls p h e n ea r e
p r e s e n t as p h e n o c r y s t s L
. i t h i cf r a g m e n t so fv a r i o u s
c o m p o s i t i o n sc o m p r i s e
2 t o 5%jof t h er o c k .S h a r ds t r u c t u r e
i s w e l lp r e s e r v e di nt h eg r o u n d m a s s .
u p p e r member.The
u p p e r member o f t h et u f fo fL e m i t a r
M o u n t a i n s ' i sd e n s e l yw e l d e d
boulders.
a n d w e a t h e r st os l a b s
Near i t s l o w e rc o n t a c t t, h er o c k
o r rounded
is usually
g r a y i s hr e dp u r p l ew i t hw i s p yw h i t es t r e a k so fp u m i c e .
r e s to ft h es e c t i o n
The
i s u s u a l l ym o d e r a t er e dt og r a y i s hr e d .
Nearthetopofthesection,thetuffcontainsangularto
l e n t i c u l a rd a r k - r e d d i s h - b r o w nc o g n a t ef r a g m e n t s
t h o s ef o u n d
i n t h eH e l l s
similar t o
Mesa T u f f ;h o w e v e r ,t h ef r a g m e n t s
t h eu p p e rt u f fo fL e m i t a rM o u n t a i n sa r eq u a r t z - f r e e .F i g u r e
in
a
1 6 i s a p h o t o g r a p ho fa no u t c r o po ft h et u f fo fL e m i t a r
M o u n t a i n st h a tc o n t a i n st h e s ef r a g m e n t s .
The l e n t i c u l a r
in
small a n d u s u a l l y
m a s s e s may b ep u m i c e ;t h e ya r e
a l i g n m e n t .T h ea n g u l a rb l o c k s ,h o w e v e r ,r a n g ei nd i a m e t e ru p
t o 3 f t (1 m ) a n d a r ep r o b a b l yl i t h i cf r a g m e n t s .
Good
e x p o s u r e so fo u t c r o p st h a tc o n t a i nt h e s ef r a g m e n t sc a nb e
f o u n da l o n gt h ew e s ts i d eo fS a w m i l l
\unsurveyed)T
, ."iS.,
R.3W.).
is consistently
The u p p e rt u f fo fL e m i t a rM o u n t a i n s
crystal-rich
a n d c o n t a i n s 20 t o $ % i . ' p h e n o c r y s t s .Q u a r t z
p h e n o c r y s t sa r eu n u s u a l l yl a r g ea n d
may b e a s much a s 5 mm i n
d i a m e t e r ,a l t h o u g ht h ea v e r a g es i z e
i s 1 . 5 mm.
a n h e d r a lt oe u h e d r a l ,o f t e ne m b a y e d ,a n d
e
32
Canyon(Sec.
The q u a r t z i s
30 t o 5 0 %
makesup
o ft h ep h e n o c r y s t s .S a n i d i n ea n dp l a g i o c l a s ee u h e d r at h a t
a r e 1 t o 2 mm l o n gc o m p r i s e
5 0 t o 7 0 % . ' o ft h ep h e n o c r y s t s .
a
A l l o ft h ef e l d s p a rp h e n o c r y s t sh a v eb e e na l t e r e dt o
v a r y i n gd e g r e e .I n
some t h i ns e c t i o n s ,p l a g i o c l a s eh a sb e e n
a l t e r e da l m o s te n t i r e l yt of i n e - g r a i n e dp h y l l o s i l i c a t e s
h a sb e e np l u c k e do u td u r i n gp r e p a r a t i o n .
or
The h i g hd e g r e eo f
f e l d s p a ra l t e r a t i o nw i t h i nt h et u f fo fL e m i t a rM o u n t a i n s
makes t h e p l a g i o c l a s e - t o - s a n i d i n e r a t i o d i f f i c u l t t o
e s t i m a t e h; o w e v e r ,
it i s a p p r o x i m a t e l y 3:l.
c o m p r i s e 2 t o 5%; o f t h e r o c k ;
all arepartial'lyalteredto
m a g n e t i t eT
. r a c e so fs p h e n ea n dc l i n o p y r o x e n e
p r e s e n t i n some s a m p l e s .
B i o t i t ee u h e d r a
(?) are
50
F i g u r e 16. Photograph of angular and lenticular
cognate fragments in the tuff'of Lemitar
Mountains.
(NE 1 / 4 , Sec. 3 2 (unsurveyed), T.4S., ,R.3W.).
TuffofSouthCanyon
The t u f f o f S o u t h
Canyon i s a m u l t i p l e - f l o w ,s i m p l e
c o o l i n gu n i to fr h y o l i t ea s h - f l o wt u f f( C h a p i na n do t h e r s ,
1978).
The t u f f was named a f t e r a m e a s u r e ds t r a t i g r a p h i c
1978).
s e c t i o n a t t h e m o u t ho S
f o u t hC a n y o n( O s b u r n ,
Canyon i s c o r r e l a t i v e w i t h t h e u p p e r P o t a t o
t u f fo fS o u t h
(1976).
C a n y o nT u f fo fS p r a d l i n
Canyon was e r u p t e df r o m
a n d o t h e r s , 1978, Fig.
t h e Hop C a n y o nc a u l d r o n( s e eC h a p i n
2).
The t u f fo fS o u t h
m.y.
u s i n gt h e
C a n y o nh a sb e e nd a t e d
K-Ar methodon
Hills (C.E.
a t 26.2
in
+/-
1.0 m.y.
by t h e K - A r
methodon
H i l l s (Bachman
1978).
O n l yt h r e e
Canyon a r e p r e s e n t
small o u t c r o p s o f t h e t u f f o f S o u t h
i n t h i ss t u d ya r e a ;t h e s eo u t c r o p sa r e
i n Sees.
t h es o u t h e r nb o u n d a r yo ft h ea r e a ,
H e r e ,t h et u f fa t t a i n s
(20 m).
P e t t y ( 1 9 7 9 ) r e p o r t st h i c k n e s s e su pt o7 5 0
m ) i n Ryan H i l l Canyon,andDonze
at
21 a n d2 2( T . 5 S . ,
R.3W.).
tuff
1.0
b i o t i t ef r o mr o c k sc o l l e c t e d
s a n i d i n e i n a v i t r o p h y r e ,a l s of r o mt h eJ o y i t a
a n dM e h n e r t ,
+/-
C h a p i nu
, n p u bd
. a t e )T
. h et u f fh a s
a t 25.8
a l s ob e e nd a t e d
(1978)
C h a p i na n do t h e r s
h a v es u g g e s t e dt h a tt h et u f fo fS o u t h
t h eJ o y i t a
The
65 f t
a maximum t h i c k n e s so f
f t (229
(1980) r e p o r t s t h a t t h e
i s 425 f t ( 1 3 0 m ) t h i c k t o t h e w e s t o f t h i s s t u d y a r e a .
T h ea b s e n c eo f
a thickersectionofthetuffofSouth
inthisstudyarea
i s p r o b a b l yd u et o
a f t e r i t s deposition.
Canyon
a n e r o s i o n a lh i a t u s
I n t h i ss t u d ya r e a ,t h et u f fo fS o u t h
Canyon o v e r l i e s t h e u p p e r t u f f o f L e m i t a r M o u n t a i n s a n d
u n c o n f o r m a b l yo v e r l a i nb yt h eP o p o t o s aF o r m a t i o n .
is
e
Canyon i s m o d e r a t e l yw e l d e da n d
The t u f f o f S o u t h
w e a t h e r st oc r u m b l yb l o c k s .
i s p i n k i s hg r a yw i t h
T h er o c k
l i g h t - b r o w n i s h - g r a yp u m i c et h a tc o n t a i na x i o l i t i ca n d
make u p 3 t o 5%; of t h e r o c k .
s p h e r u l i t i cs t r u c t u r e sa n d
Q u a r t za n ds a n i d i n ee a c hc o m p r i s ea b o u t
lO%;of
Q u a r t z i s a n h e d r a lt oe u h e d r a la n da v e r a g e s
a s p h e n o c r y s t sa n da r ep a r t i a l l yr e p l a c e db y
A t r a c e of h o r n b l e n d e was o b s e r v e di nt h i n
magnetite.
section.
1 mm i n d i a m e t e r .
1 . 5 mm l o n g .T r a c e so fb i o t i t e
S a n i d i n ee u h e d r aa r ea b o u t
, u h e d r ao c c u r
thetuff.
i s e x t e n s i v e l ya l t e r e dt o
The
groundmass
p h y l l o s i l i c a t e sa n dc o n t a i n sf i n e l yd i s s e m i n a t e dm a g n e t i t e .
F o r a d e t a i l e dd e s c r i p t i o n
c h a r a c t e r i s t i c s ,s e eO s b u r n
U n i to fS a w m i l l
of z o n a t i o na n dp e t r o g r a p h i c
(1978).
Canyon
A r h y o l i t ei n t r u s i v ec o m p l e x
and associated
p y r o c l a s t i cd e p o s i t si n t r u d ea n do v e r l i et h et u f f
M o u n t a i n s i n t h en o r t h w e s t e r np a r t
u n i t i s y o u n g e rt h a nt h eL e m i t a r ,b u t
of L e m i t a r
of t h es t u d ya r e a .
The
a minimum a g ec a n n o tb e
d e t e r m i n e df r o mf i e l dr e l a t i o n s h i p s .
p y r o c l a s t i cd e p o s i t s
of t h eu n i to fS a w m i l l
H a r d yR i d g e
(Tst).
The b a s a lp y r o c l a s t i cd e p o s i t s
Canyon d i r e c t l y o v e r l i e t h e u n i t
of
i n t h en o r t h w e s t e r np a r to ft h i ss t u d ya r e a ;
Donze (1980) r e p o r t s t h a t t h e
same r e l a t i o n s h i p e x i s t s
w e s tf l a n ko fH a r d yR i d g e .T h ep y r o c l a s t i cd e p o s i t sc o n s i s t
of a s h - f l o wt u f f s ,a n d e s i t e s ,v o l c a n i c l a s t i cs e d i m e n t a r y
on t h e
53
r o c k s ,a n dv e n tb r e c c i a s .
4 5 0 f t ( 1 3 7 m ) t h i c k i n t h i ss t u d ya r e a .
R.3W.)
I n Sec. 5 (T.5S.,
t h eo u t c r o p sc o n s i s to fp o o r l yt od e n s e l yw e l d e dt u f f s
t u f f i s l e s st h a n
o r p l a t yt a l u s .T h e
t h a tw e a t h e rt ob l o c k y
>O f t
i s a s much a s
The p y r o c l a s t i cu n i t
( 1 5 m ) t h i c ka n da p p e a r st oh a v ef i l l e dt o p o g r a p h i c
of Lemitar
d e p r e s s i o n so nt h es u r f a c eo ft h eu n d e r l y i n gt u f f
i n c o l o rf r o ml i g h tg r a yt o
M o u n t a i n sT
. h er o c k sr a n g e
m o d e r a t er e d
a n d c o n t a i n a s much a s l O % ; ' p u m i c et h a tw e a t h e r
The t u f f i s c o n s i s t e n t l yv e r yc r y s t a lp o o r
e a s i l yt oc l a y .
a n dc o n t a i n s
a t r a c eo f
small q u a r t za n h e d r a .L i t h i c
f r a g m e n t sa r eu s u a l l ya b s e n tb u t
b a s a lz o n e .T h er e m a i n i n go u t c r o p s
c r y s t a l - p o o rt u f fa n d
make u p2 0 % i ' o ft h et u f f ' s
of T s t c o n t a i ns i m i l a r
a v a r i e t yo fo t h e rr o c kt y p e st h a t
will
b ed e s c r i b e dl a t e r .L i t h i cf r a g m e n t so fr h y o l i t ea n d
a n d e s i t ea r ea b u n d a n t
a s much a s
i n some u n i t s a n d c o n s t i t u t e
5 0 % , ' o ft h er o c k ;l i t h i cf r a g m e n t so fc r y s t a l - r i c ha s h - f l o w
t u f f ,a sl a r g e
a s 25cm,
a r e common l o c a l l y .
S e v e r a ld i s t i n c t i v eu n i t sa r ef o u n dw i t h i nt h e
of t h es t u d y
a s h - f l o wt u f fi n t e r v a li nt h en o r t h e r n m o s tp a r t
a r e (aS e c3. 0T, . b S . ,
R.3W.).
A r h y o l i t el a p i l l ib r e c c i a
( M a c d o n a l d ,1 9 7 2 ,p . 1 3 1 )c r o p so u ti nt h en o r t h e a s tq u a r t e r
o f S e c .3 0a n dw e a t h e r st ob l o c k yt a l u s .
The u n i tc o n s i s t s
o f5 0 % , ' v e r y - p a l e - o r a n g ep u m i c et h a ta p p e a rr a g g e di nt h i n
s e c t i o na n da b o u t
5% a n g u l a r l i t h i c f r a g m e n t s
of r h y o l i t e .
A m o d e r a t e l yc r y s t a l - r i c ha s h - f l o wt u f fu n i tc r o p s
o u tj u s ts o u t ho ft h er h y o l i t el a p i l l ib r e c c i a
(SE 1/4, NE
54
1/4,
R.3W.).
3 0 , T.hS.,
Sec.
The r o c k i s g r a y i s hr e da n d
w e a t h e r st ob l o c k yt a l u s .
contains
The t u f f i s d e n s e l yw e l d e da n d
15%;p h e n o c r y s t s .S a n i d i n e ,p a r t i a l l yr e s o r b e da n d
l a r g e l ya l t e r e dt os e r i c i t e ,c o m p r i s e sa b o u t
s m a l l a n h e d r a lq u a r t za n d
p h e n o c r y s t sS
. u b e q u a al m o u n t so f
b i o t i t e make u pa b o u t
1 0 % . o ft h e
5%. o ft h er o c k .
A t r a c eo fa n h e d r a l
m a g n e t i t e i s p r e s e n t .L i t h i cf r a g m e n t s .o fr h y o l i t ea n d
a n d e s i t e make u pa b o u t2 %
of t h e r o c k .
An a n d e s i t el a v ac r o p so u t
I
R.3W.).
( u n s u r v e y e d )T
, .kS.,
a t o n el o c a t i o n( S e e .3 2
I t i s d e n s e a, p h a n i t i c a, n d
medium g r a y i n c o l o r .
A v e n tb r e c c i a
R.3W.I.
oS
f e c3. 2( T . b S . ,
i s e x p o s e d i n t h es o u t h w e s tq u a r t e r
I t c o n t a i n sc l a s t so fa n d e s i t e
as l a r g e as 3 f t
a n d t h et u f fo fL e m i t a rM o u n t a i n st h a ta r e
(1 m ) i nd i a m e t e r .
The c l a s t sa r ee n c l o s e d
i n a fragmental
r h y o l i t e matrix.
r h y o l i t e lavas.
A c r y s t a l - p o o rr h y o l i t ei n t r u d e sa n d
o v e r l i e st h eH e l l s
Mesa T u f f ,t h eu n i to fH a r d yR i d g e ,t h e
t u f fo fL e m i t a rM o u n t a i n s ,a n dt h ep y r o c l a s t i cr o c k so ft h e
Good e x p o s u r e s o f
unio
t fS a w m i l C
l anyon.
c a nb es e e ni nt h en o r t h e a s tq u a r t e ro fS e c .
i n t r u s i v ec o n t a c t s
i n the
30and
s o u t h e a s tq u a r t e ro fS e c .3 2( u n s u r v e y e d ) ,T . 5 S . ,
R.3W.).
T h i c k n e s se s t i m a t e sa r ep r o b a b l yu n r e l i a b l ed u et o
u n r e c o g n i z e df a u l t i n gw i t h i nt h er h y o l i t e s ,b u t
100 f t ( 3 0 5 m ) o fl a v a sa r ep r e s e n t .
a
than 2 sq m i (5.2
s q km)on
a s much a s
The l a v a sc o v e r
t h ee a s tf l a n ko fH a r d yR i d g e .
more
The r h y o l i t e sa r ed e n s ea n da p h a n i t i ca n ds o m e t i m e s
w e a t h e rt op l a t yf r a g m e n t s .T h e ya r ee x t r e m e l yr e s i s t a n ta n d
17).
u s u a l l yf o r ms t e e pc l i f f s( F i g .
flow-banded
The l a v a sa r eo f t e n
a n d s p h e r u l i t i c a l l yd e v i t r i f i e da n d
commonly
is f o u n d i n t h e
c o n t a i nt h ed e v i t r i f i c a t i o nt e x t u r et h a t
19,
c r y s t a l - p o o rr h y o l i t e so ft h eu n i to fH a r d yR i d g e( p .
Fig.
8).
P o p o t o s aF o r m a t i o n
was namedbyDenny
T h eP o p o t o s aF o r m a t i o n
(1940)
for e x p o s u r e si nA r r o y oP o p o t o s as o u t h e a s to ft h eL a d r o n
M o u n t a i n sT
. h eP o p o t o s ac o n s i s t so fm u d f l o wd e p o s i t s ,
a n d m u d s t o n e st h a t
c o n g l o m e r a t e s ,s a n d s t o n e s ,s i l t s t o n e s ,
i n an arid to
r e p r e s e n ta l l u v i a lf a n / p l a y ad e p o s i t i o n
1973).
s e m i - a r i dc l i m a t e( B r u n i n g ,
u n i tc o n s i s t so fb a s a l
I n t h i ss t u d ya r e a ,t h e
m u d f l o wd e p o s i t s( T p l )a n du p p e r
c o n g l o m e r a t e sa n ds a n d s t o n e s( T p ) ;
a l l o ft h ed e p o s i t si n
of B r u n i n g ( 1 9 7 3 ) .
t h i sa r e aa r ei nt h ef a n g l o m e r a t ef a c i e s
A l a t e r a l l yd i s c o n t i n u o u sa s h - f l o wt u f f( T p t )
i n Sec.
12 ( T . 5 S . ,
R.3W.
i s interbedded
1.
The P o p o t o s aF o r m a t i o nc r o p so u ta ts c a t t e r e ds i t e s
i nt h es o u t h e r np a r to ft h i ss t u d ya r e a .
a ne r o s i o n a lu n c o n f o r m i t ya n d
i s i nd e p o s i t i o n a lc o n t a c tw i t h
u n i t so fd i f f e r e n ta g e s ;t h ey o u n g e s to ft h e s e
S o u t hC a n y o na n dt h eo l d e s t
P o p o t o s ab e d sw e r eb e v e l e db y
I t was d e p o s i t e do n
i s t h e A-L
i s t h et u f fo f
PeakTuff.The
a p e d i m e n ts u r f a c ea n da r e
now
56
F i g u r e '17. P h o t o g r a p h of cliff-forming rhyolite
lavas in the unit of Sawmill Cadyon.
(NW 1 / 4 , Sec. 3 2 (unsurveyed), T.4S., R.3W.).
i n c o n t a c tw i t hy o u n g e rQ u a t e r n a r yu n i t s .
i n t h i ss t u d y
t h i c k n e s sf o rt h eP o p o t o s aF o r m a t i o n
400 f t ( 1 2 2 m ) ,
An e s t i m a t e d
area i s
a l t h o u g ht h el a c ko fc o n t i n u o u se x p o s u r e s
makes it d i f f i c u l t t o o b t a i n a n a c c u r a t e t h i c k n e s s .
l o w e r member.
The
l o w e rP o p o t o s aF o r m a t i o ni nt h i ss t u d y
a r e ac o n s i s t so f
m u d f l o wd e p o s i t st h a tg r a d eu p w a r di n t o
c o a r s ec o n g l o m e r a t e s T
. h er o c k
w e a t h e r st on a r r o wl e d g e s .
a l t e r e d t o m o d e r a t er e d
i s w e l li n d u r a t e da n d
T h el o w e rP o p o t o s a
i s pervasively
or r e d d i s hb r o w n .C h a p i na n do t h e r s
(1978) s u g g e s t e dt h a tt h er e dc o l o ra n dd e g r e eo fi n d u r a t i o n
a r ea n o m a l o u sa n da r er e l a t e dt oa na n c i e n tg e o t h e r m a l
system.
T h em u d f l o wd e p o s i t sa r eu n s o r t e d ,
m a t r i x - s u p p o r t e d ,a n du s u a l l yu n s t r a t i f i e d ,a l t h o u g hc r u d e
b e d d i n g i s l o c a l l yp r e s e n t .
C l a s t s t h a tr a n g e
i n s i z eu pt o
s e v e r a lm e t e r sa r em a i n l yf r a g m e n t so ft h et u f fo fL e m i t a r
M o u n t a i n sT
. h er o c kb e c o m e si n c r e a s i n g l yh e t e r o l i t h i cu p w a r d
i n t h es e c t i o n
a s s t r a t i f i c a t i o n becomesmorepronounced.
S e d i m e n t a r ys t r u c t u r e st h a tw e r eo b s e r v e di n c l u d ec h a n n e l s
a n dm i n o rc r o s s - b e d d i n g .
a s h - f l o wt u f f .
i s r h y o l i t i ct o
A l o c a la s h - f l o wt u f ft h a t
q u a r t zl a t i t i ci nc o m p o s i t i o no v e r l i e st h el o w e rP o p o t o s a
12 ( T . 5 S . ,
F o r m a t i o nw i t ha n g u l a ru n c o n f o r m i t yi nS e c .
R.3W.).
H e r e ,t h et u f f
G.R. O s b u r n( o r a l
i s l e s st h a n
commun.)
100 f t
r e p o r t st h a t
(305 m ) t h i c k .
a s i m i l a r t u f fc r o p s
*
o u tj u s to u t s i d et h i ss t u d ya r e a .D e p o s i t i o no ft h i st u f f
o c c u r r e da f t e rt h eg e o t h e r m a le v e n tt h a tp e r v a s i v e l ya l t e r e d
t h el o w e rP o p o t o s aF o r m a t i o n .
The t u f f i s p o o r l y t o m o d e r a t e l y w e l d e d a n d
i s p i n k i s hg r a ya n d
w e a t h e r st or o u n d e db o u l d e r s .T h er o c k
c o n t a i n sg r a y i s h - y e l l o w ,d i s c - s h a p e dp u m i c et h a tw e a t h e ro u t
e a s i l y .L i t h i cf r a g m e n t so fv a r i o u sc o m p o s i t i o n sc o m p r i s eu p
t o 2 0 % ;o ft h er o c k .P h e n o c r y s t so fq u a r t z ,p l a g i o c l a s e ,a n d
s a n i d i n e make u pa b o u t
1 0 % . ' o f t h er o c k .Q u a r t za n h e d r aa b o u t
0 . 3 mm i n d i a m e t e r c o m p r i s e
p l a g i o c l a s ea n h e d r a ,
70%. of t h e p h e n o c r y s t s , a n d
0 . 3 mm l o n g , make u p2 0 % . ' o ft h e
phenocrystsT
. e np e r c e n to ft h ep h e n o c r y s t sa r es a n i d i n e
a n h e d r aa b o u t
a r ef o u n d
0 . 3 mm l o n g .T r a c e s
o f b i o t i t ea n dh o r n b l e n d e
as p h e n o c r y s t s .
u p p e r member.
The
u p p e rP o p o t o s aF o r m a t i o n
a r e a w a s d e p o s i t e do n
i n t h i ss t u d y
a n e r o s i o ns u r f a c et h a tf o r m e da f t e r
i
d e p o s i t i o no ft h el o w e r
member.
The
r o c k i s p o o r l yt o
m o d e r a t e l yi n d u r a t e da n dv e r yn o n - r e s i s t a n t ;e x p o s u r e sa r e
c o n f i n e dt ot h es i d e so fa r r o y o s .T h eu p p e rP o p o t o s a
c o n s i s t so fv e r y - l i g h t - g r a yc o n g l o m e r a t e sa n di n t e r b e d d e d
sandstones.
C l a s t s c o n s i s tl a r g e l yo ff r a g m e n t so ft h et u f f
of L e m i t a rM o u n t a i n sa n dc r y s t a l - p o o rr h y o l i t el a v a s ,
a l t h o u g ho t h e rl i t h o l o g i e sa r ep r e s e n t
a m o u n t sT
. h ep a r t i c l e s
i n subordinate
i n t h ec o n g l o m e r a t e sr a n g ef r o m
c l a y - s i z et ob o u l d e r - s i z e ,w i t hb o u l d e r sl o c a l l y
s e v e r a lm e t e r si nd i a m e t e r .T h em o s t
l e s st h a t
1 cm.
as l a r g e a s
common p a r t i c l e s i z e i s
C h a n n e l sa n dc r o s s - b e d d i n ga r e
common s t r u c t u r e s i n
t h eu p p e rP o p o t o s a .I m b r i c a t e dp e b b l e sa r ea b u n d a n ta n dw e r e
u s e dt oo b t a i nt r a n s p o r td i r e c t i o n s .M e a s u r e m e n t sw e r e
u s u a l l y madeon
s e t s of 2 o r 3 p e b b l e s i n c o n t a c tw i t he a c h
o t h e r .F i v et o
1 0 m e a s u r e m e n t sw e r e
made a t e a c ho u t c r o p
and
w e r ea v e r a g e dt oy i e l do n ev a l u ef o rt h a to u t c r o p .T h e
resultsareplotted
on P l a t e 1 a n d i n d i c a t e
a general
d i r e c t i o n o f t r a n s p o r tt ot h es o u t h w e s t .
S a n t aF eG r o u p( u n d i f f e r e n t i a t e d )
T h el o w ,r o u n d e dn o r t h w e s t - t r e n d i n gh i l l st h a ta r e
f o u n d a t t h e m o u t ho fS a w m i l lC a n y o nc o n t a i nn oo u t c r o p s .
However, t h e i r l o c a t i o n a n d t h e l a c k
e
theyare
o f o u t c r o p ss u g g e s tt h a t
made u p o f p o o r l y i n d u r a t e d T e r t i a r y g r a v e l s ,
p r o b a b l yt h eu p p e rP o p o t o s aa n dl o w e rS i e r r aL a d r o n e s
Formations.
T h eS i e r r aL a d r o n e sF o r m a t i o n
(1.978) f o rt h e
t y p es e c t i o n
was namedbyMachette
low f o o t h i l l s of t h eL a d r o nM o u n t a i n s T
. he
i n age
i s e a r l yP l i o c e n et om i d d l eP l e i s t o c e n e
a n dc o n t a i n sa l l u v i a l - f a n ,p i e d m o n t - s l o p e ,a l l u v i a l - f l a t ,
f l o o d - p l a i n ,a n d
a x i a l s t r e a md e p o s i t s( M a c h e t t e ,
1978).
T e r t i a r yi n t r u s i v er o c k s
D i k e sa n dp l u g so fT e r t i a r ya g e
s c a t t e r e dl o c a t i o n st h r o u g h o u tt h es t u d ya r e a .
theirtrend
are found at
I n general,
i s i n a n o r t h - n o r t h w e s t e r l yd i r e c t i o nw h e r et h e y
60
i n t r u d ee x t e n s i o n a lf a u l t st h a ta r ea s s o c i a t e dw i t ht h eR i o
0
Grande r i f t .
i n t o 5 groups:
The i n t r u s i v e sc a nb ec l a s s i f i e d
a n d e s i t ed i k e s ,
a l a t i t ed i k e ,p o r p h y r i t i cr h y o l i t e
i n t r u s i v e s ,f l o w - b a n d e dr h y o l i t e s ,a n dw h i t er h y o l i t ed i k e s
a n d dome.
a n d e s i t ed i k e s .A n d e s i t ed i k e sa r ei n t r u s i v ei n t ot h eH e l l s
Mesa T u f f i n t h e s o u t h w e s t q u a r t e r
#Yhey c o n t a i na b o u t
of Sec.
9 (T.5S., R.3W. ) .
3% p h e n o c r y s t so fe u h e d r a lp l a g i o c l a s e ,
m o s to fw h i c hh a v eb e e np a r t i a l l ya l t e r e dt op h y l l o s i l i c a t e s .
T h eg r o u n d m a s sc o n t a i n sr a n d o m l yo r i e n t e dp l a g i o c l a s e
m i c r o l i t e s a n d f i n e l yd i s s e m i n a t e dm a g n e t i t e .
l a t i t ed i k e s .
One s m a l l l a t i t ed i k e
n o r t h e r nb o u n d a r yo fs e c t i o n
T h i s r o c kh a s
was f o u n do nt h e
3 2 ( u n s u r v e y e d )( T . 4 S . ,
15
a l i g h t - b l u i s h - g r a yg r o u n d m a s sa n dc o n t a i n s
are as l o n g
t o 20% l a r g ep o t a s s i u m - f e l d s p a rp h e n o c r y s t st h a t
a s 1 . 5 cm.
R.3W.).
A t r a c eo f
small q u a r t zp h e n o c r y s t s
was a l s o
observed.
p o r p h y r i t i cr h y o l i t ed i k e s .S e v e r a lc r y s t a l - r i c hr h y o l i t e
d i k e s ( T r l ) a r ef o u n d
i n t h en o r t h - c e n t r a lp a r to ft h es t u d y
a r e a .T h e s ed i k e sa r el i g h tb r o w n i s hg r a yt ol i g h tr e di n
c o l o r a n d a r ev e r yd e n s e ;t h e yw e a t h e rt ob l o c k yt a l u s .
P h e n o c r y s t s make u p 1 5 t o 2 0 % o f t h er o c ka n d
c h a r a c t e r i s t i c a l l yc o n t a i nl a r g eq u a r t z
( a s much as 5 mm i n
diameter)S
. a n i d i n ea n db i o t i t ea r ea l s op r e s e n t .
q u a r t z - t o - s a n i d i n er a t i o
b i o t i t e was o b s e r v e d .
i s a b o u t 3:l a n do n l y
The
a t r a c eo f
01
A n o t h e rp o r p h y r i t i cr h y o l i t ei n t r u s i v e( T r 2 )c r o p s
o u ta to n el o c a t i o n
( N W 1/4, S e c .
i n t h es o u t h e r nh a l f
R.3W.).
11, T.5S.,
o f t h es t u d ya r e a
T h i sr h y o l i t e
r e s i s t a n ta n df o r m ss t e e pc l i f f s .
i s very
The c l i f f - f o r m i n gr h y o l i t e
1 0 t o 15%.
i s l i g h tb r o w n i s hg r a yt ol i g h tr e da n dc o n t a i n s
phenocrysts of s a n i d i n eq
, u a r t z a, n db i o t i t e S
. ubhedral
s a n i d i n e i s a b o u t 1 mm l o n ga n dd e e p l ye m b a y e dq u a r t z
a v e r a g e sa b o u t
1:l.
1 . 5 mm l o n g ; t h e s a n i d i n e - t o - q u a r t z r a t i o
i s present.
A t r a c e o f s m a l l , a n h e d r a lb i o t i t e
S i l i c i f i c a t i o n i n t h ef o r m
is
o f small q u a r t z v e i n s
was s e e n i n
t h i ns e c t i o n .
f l o w - b a n d e dr h y o l i t e sF. l o w - b a n d e dc, r y s t a l - p o o rr h y o l i t e
i n t r u s i v e sa r ea b u n d a n t
i n t h es o u t h e r nt w o - t h i r d s
of t h e
s t u d ya r e a .T h e s er h y o l i t e sa r ei n t r u s i v ei n t ou n i t s
young a s t h et u f f
of L e m i t a rM o u n t a i n s .
( T r f ) t h a ta r e
Some o f t h e
o f a c r y s t a l - p o o ra s h - f l o w
i n t r u s i v e sc o n t a i nf l o a tb l o c k s
tuff
a s l o n g a s 2000 f t ( 6 1 0 m).
O u t c r o p s o f T r 3 a r ev e r yd e n s ea n d
o u t c r o p sn e a rt h eb a s e
as
form b o l d
o f S a w m i l lC a n y o n .D i s t o r t e d
b a n d i n g i s common i nt h i si n t r u s i v e .
flow
A t r a c e of q u a r t z i s
present.
T r 4 i s i n t r u s i v ei n t o
Tr3.
Outcrops of T r 4 a r e
v e r yr e s i s t a n ta n dw e a t h e rt o ,b l o c k yt a l u s .F l o w - b a n d i n gi n
T r 4 h a s a d i s t i n c t i v ea p p e a r a n c e
c o l o rb a n d s
o f . b r o w n i s hg r a y
a n d c o n t a i n sa l t e r n a t i n g
a n d l i g h tg r a y .E a c hb a n d
a b o u t 5 mm t h i c k .Q u a r t zp h e n o c r y s t s ,
is
i n a m o u n t sl e s st h a n
b2
*
mm
2%,, a r es u b h e d r a l ,o f t e nd e e p l ye m b a y e d ,a n da v e r a g e0 . 5
i n d i a m e t e r .F i n e l yd i s s e m i n a t e dm a g n e t i t e
m o s ts a m p l e s .
1 4 (T.5S.,
0 . 2 mm
A t r a c eo fs u b h e d r a lb i o t i t ea v e r a g i n g
a s a m p l ef r o mt h en o r t h e a s tq u a r t e ro f
l o n g was o b s e r v e d i n
Sec.
i s p r e s e n ti n
R.3W.).
The
groundmass
is spherulitically
d e v i t r i f i e d i n a l i g n m e n t w i t h t h ef l o wb a n d i n g .
A m o s a i co f
a t r a c e of b i o t i t e a n d m a g n e t i t e f i l l
q u a r t za n ds a n i d i n ea n d
i r r e g u l a rp o r es p a c e s .
w h i t er h y o l i t ed i k e sa n d
dome.
o f t h es t u d ya r e a ,m a i n l y
dome a r e l o c a t e d a t t h e n o r t h e n d
a l o n gt h eS a w m i l l
a
W h i t er h y o l i t ed i k e sa n d
in a
Canyon F a u l t .T h ed i k e sa r ea r r a n g e d
r o u g h l yr a d i a lp a t t e r no u t w a r df r o mt h e
dome.
The
white
c o l o ro ft h ed i k e sm a k e st h e mv e r yc o n s p i c u o u s .T h ew h i t e
e
18) a n d ,
r h y o l i t ed i k e sh a v eb e e nt i l t e dt ot h ee a s t( F i g .
t h e r e f o r e ,p r e d a t e
someof
the rift faulting.
O u t c r o p so fw h i t er h y o l i t ea r em o d e r a t e l yr e s i s t a n t
a n d w e a t h e rt op l a t yt a l u s .
h i g h l ys i l i c i f i e d .
s o m e t i m e sf l o w - b a n d e d
The c o u n t r yr o c k sa r eu s u a l l y
I n h a n ds p e c i m e n t, h er h y o l i t e
a n d u s u a l l yc r y s t a lp o o rb u t
c o n t a i n a t r a c eo fs a n i d i n ea n dq u a r t z .
c o n t a i n e d 2%. i r o n - s t a i n e d c a s t s
o x i d i z e dp y r i t ec r y s t a l s .
is
may
One s p e c i m e n
t h a t p r o b a b l yr e p r e s e n t
63
F i g u r e 18. P h o t o g r a p h o f a w h i t e r h y o l i t e
dike that has been rotated to a subhorizontal
A-L
position. The dike was intruded into the
P e a k T u f f , rotated to its present position, and
overlain by Quaternary terrace gravels.
(NW 1 / 4 , Sec. 29 (unsurveyed), T . 4 S . , R.3W.).
and sands
a i i u v i a l - f a n .g r a v e l s .P o o r l yc o n s o l i d a t e dg r a v e l s
-
as a l l u v i a l f a n d e p o s iat n
sa dr e
t h aa ct c u m u l a t e d
a t t h e mouth o f
s t r a n d e dh i g ha b o v et h ep r e s e n ts t r e a ml e v e l s
SawmillCanyonweremapped
(Q-Tg).
.
T h e s ed e p o s i t s
now
as Q u a t e r n a r y - T e r t i a r y g r a v e l s
may b e e q u i v a l e n tt op a r to ft h e
S i e rLr a d r o nFeos r m a t i(oJn. W . ' H a w l e 1y 9, '8o0r ,a l
Referto
p.
commun.).
59 f o r : f u r t h e r i n f o r m a t i o n a b o u t t h e S i e r r a
ri
.r
as i s o l a t e d
L a d r o n e sF o r m a t - i o n .T h e s ed e p o s i t sa r ef o u n d
p a t c h e s where t h e y h a v e b e e n . d i s s e c t e d
by s t r e a m e r o s i o n .
Q u a t e r n a r yd e p o s i t s
p i e d m o n tg r a v e l s .P o o r l yc o n s o l i d a t e dg r a v e l sa n ds a n d st h a t
e
a c c u m u l a t ebcdyo a l e s c e n c e
(,
of a l l u v i af la n s
a t t hbea s e
as p i e d m o n t g r a v e l s .
t h eM a g d a l e n aM o u n t a i n sw e r em a p p e d
and
T h e s eg r a v e l sa r et h em o s te x t e n s i v eQ u a t e r n a r yd e p o s i t s
h a v eb e e nd i s s e c t e d ~ t o
'
of
some d e g r e eb ys u b s e q u e n ts t r e a m
.
erosion.
t e r r a c e ~ g r a v e l s .T e r r a c eg r a v e l s
. o fp o o r l yc o n s o l i d a t e d
i n t h i s s t u d y area c o n s i s t
s a n d and g r a v e ld e p o s i t sl o c a t e do n
b e n c h e st h . a ta r eu s u a l l yp a r a l l e lt op r e s e n ts t r e a m s .T h e s e
d e p o s i t ga r ef o u n d
a t s c a t t e r e ds i t e sa l o n gS a w m i l l
Canyon
a n d c a n be^ s e e n a t e l e v a t i o n s as h i g h as 7600 f t (2316 m ) i n
*
t h e. n o r t h e r np a r to ft h es t u d ya r e a .F i g u r e
p h o t o g r a p ho f
a p a r t i c u l a r l y g o o de x p o s u r e
o nt h en o r t h e r nb o u n d a r y
of t h i s s t u d y a r e a .
18 i s a
o f t e r.r a .c e g r a v e l s
a
t a l u sa n dc o l l u v i u m .T a l u sa n dc o l l u v i u ma r eg r a d a t i o n a la n d
t o g e t h e r a s o n eu n i t .T a l u so c c u r s
were
mapped
on
o v e r s t e e p e n e ds l o p e st h r o u g h o u tt h es t u d ya r e aa n dc o n s i s t s
mass w a s t i n g .
o fl o o s er o c kf r a g m e n t st h a tw e r ed e p o s i t e db y
C o l l u v i u mc o v e r s
more m o d e r a t es l o p e sa n dg e n e r a l l yc o n t a i n s
more s o i l t h a n d o e s t a l u s .
s l u m pb l o c k .
A l a r g es l u m pb l o c ko ft u f fo fL e m i t a r
9 (T.5S.,
M o u n t a i n s i s f o u n d i n t h en o r t h w e s tq u a r t e ro fS e c .
R.3W.).
T h eb l o c k
no contacts
i s s u r r o u n d e db yt a l u sa n d
w e r eo b s e r v e d .T h es l u m pb l o c ka p p e a r st oh a v eb e e nd e r i v e d
f r o mt h es t e e pc l i f f s
of L e m i t a r t h a t a r e f o u n d d i r e c t l y
a b o v et h eb l o c k .
e
a l l u v i u m .A l l u v i u m
i n t h i ss t u d ya r e a
i s H o l o c e n e i n a g ea n d
c o n s i s t sl a r g e l yo fp o o r l ys o r t e ds a n da n dg r a v e l ,a l t h o u g h
i n o n es t r e a mb e dt h ed e p o s i t sa r em a i n l yb o u l d e r - s i z e
clasts.
v a l l e ya l l u v i u m .H o l o c e n ea l l u v i u mt h a t
i s c o n f i n e dt o
v a l l e y s a n d c o n s i s t sl a r g e l yo fd e b r i ss h e df r o mt h ev a l l e y
w a l l s was mapped a s v a l l e ya l l u v i u m .T h e s ed e p o s i t su s u a l l y
a r en o tr e s t r i c t e dt on a r r o ws t r e a mb e d s .V a l l e ya l l u v i a l
d e p o s i t sg r a d eu p s l o p ei n t oc o l l u v i u m .
66
STRUCTURE
at the
T h eM a g d a l e n aM o u n t a i n sa r el o c a t e d
r i f t a n dt h en o r t h e a s t e r n
i n t e r s e c t i o no ft h eR i oG r a n d e
c o r n e ro ft h eD a t i l - M o g o l l o nv o l c a n i cf i e l d .
The
w e s t - t r e n d i n gC a p i t a na n dt h en o r t h e a s t - t r e n d i n gM o r e n c i
a n d a t r a n s v e r s es h e a r
l i n e a m e n t si n t e r s e c tn e a rS o c o r r o ,
z o n eo ft h eM o r e n c il i n e a m e n tp a s s e st h r o u g ht h ec e n t r a l
1 9 7 8 , F i g . 1).
M a g d a l e n aR a n g e( s e eC h a p i na n do t h e r s ,
These
s t r u c t u r a le l e m e n t sa r es u p e r i m p o s e du p o ns e v e r a lo v e r l a p p i n g
a c o m p l e xg e o l o g i cs e t t i n g
c a u l d r o ns t r u c t u r e st op r o d u c e
in
t h eS o c o r r o - M a g d a l e n aa r e a .
R e g i o n a lS t r u c t u r e
S e v e r a le p i s o d e so fd e f o r m a t i o np r e c e d e dv o l c a n i c
a c t i v i t yi nt h eS o c o r r o - M a g d a l e n aa r e a .
The a n c e s t r a l Rocky
M o u n t a i n sw e r ef o r m e dd u r i n gl a t eM i s s i s s i p p i a nt oP e r m i a n
o f P a l e o z o i ca n dM e s o z o i c
t i m e .F o l d i n ga n dr e v e r s ef a u l t i n g
r o c k s i n w e s t - c e n t r a l New M e x i c oo c c u r r e dd u r i n gt h eL a r a m i d e
o r o g e n y( L a t eC r e t a c e o u st oM i d d l eE o c e n et i m e ) .
M a g d a l e n aM o u n t a i n sa r e a
u p l i f t( C h a p i na n do t h e r s ,
a r e a was e r o d e d t o
1975).
The
was on t h e f l a n k o f
1978).
a s u r f a c eo f
a b r o a dL a r a m i d e
I n t h el a t eE o c e n e ,t h e
low r e l i e f( E p i sa n dC h a p i n ,
D e t r i t u s f r o m e r o s i o n of t h eL a r a m i d eu p l i f ti nt h e
M a g d a l e n aM o u n t a i n sa r e a
was s h e dn o r t h w a r di n t ot h eB a c a
b a s i na n di n c o r p o r a t e di n t ot h eE o c e n e
( ? ) B a c aF o r m a t i o n .
No p r e - O l i g o c e n er o c k sa r ee x p o s e di nt h i st h e s i sa r e a .
0
i n t h eS o c o r r o - M a g d a l e n aa r e a
V o l c a n i ca c t i v i t y
b e g a n i n e a r l yO l i g o c e n et i m ew i t hd e p o s i t i o n
o f t h eS p e a r s
F o r m a t i o nu n c o n f o r m a b l yu p o nr o c k sr a n g i n gi na g e
1978).
P r e c a m b r i a nt ol a t eE o c e n e( C h a p i na n do t h e r s ,
p y r o c l a s t i ca c t i v i t yb e g a na b o u t3 2
M o u n t a i n sa r e aw i t ht h ee r u p t i o n
from
m.y.
Major
a g o i n t h eM a g d a l e n a
of t h e H e l l s
Mesa T u f fa n d
o f t h eN o r t hB a l d yc a u l d r o n .S e v e r a l
s u b s e q u e n tc o l l a p s e
o v e r l a p p i n gc a u l d r o ns t r u c t u r e sw e r ef o r m e dd u r i n gt h ep e r i o d
f r o m3 2t o2 6
m.y.
a g o ;t h i ss t u d ya r e a
is locatedwithinthe
N o r t hB a l d ya n dS a w m i l lC a n y o nc a u l d r o n s .
The R i oG r a n d e
r i f t b e g a nd e v e l o p i n ga b o u t3 2
1978).
a g oa l o n gp r e e x i s t i n gc r u s t a lw e a k n e s s e s( C h a p i n ,
(1978) h a sd o c u m e n t e dt h ee x i s t e n c e
Chamberlin
m.y.
o'f
high-angle
i n t h e Lemitar M o u n t a i n st h a td e v e l o p e db e t w e e n
n o r m a lf a u l t s
3 2a n d2 3
m.y.
m.y.
ago.
32 t o2 8
H e s u g g e s t s t h a t t h ep e r i o do f
a g o was a t i m e o f h i g h h e a t f l o w a n d r a p i d c r u s t a l
extension.
The R i oG r a n d e
R i f t i s r e p r e s e n t e db y
a s e r i e s of
n o r t h - t r e n d i n gb a s i n st h a tb e g i ni nc e n t r a lC o l o r a d oa n d
w i d e n i n t h eS o c o r r o - M a g d a l e n aa r e at op r o d u c es e v e r a l
p a r a l l e lb a s i n st h a ta r es e p a r a t e db y
(Chapin, 1971a a n d
intrarift horsts
1978).
T h eC a p i t a n
a n d M o r e n c il i n e a m e n t si n t e r s e c t
S o c o r r o - M a g d a l e n aa r e a ,a n d
a t r a n s v e r s es h e a rz o n e
i n the
of t h e
M o r e n c il i n e a m e n td i v i d e st h eM a g d a l e n aR a n g ei n t ot w o
s t r u c t u r a lf i e l d s .
a l o n gf a u l t s
To t h en o r t h
o f t h es h e a rz o n e ,
i s down t o t h e e a s t a n d t h e s t r a t a d i p t o t h e
movement
68
west.
S o u t h of t h e shear z o n e ,
eastward-dipping
down-to-the-west
s t r a t a predominate.
faults and
F o r m a t i o n of t h e
t r a n s v e r s e shear z o n e was c o n c u r r e n t w i t h t h e development o f
a n e x t e n s i o n a l stress f i e l d i n t h e area.
Chapin (1978)
suggests t h a t t h e shear zone follows a n e a r l i e r zone of
weakness i n t h e P r e c a m b r i a n b a s e m e n t .
By 2 6 m.y.
Socorro-Magdalena
ago, a shallow b a s i n i n t h e
area had begun a c c u m u l a t i n g s e d i m e n t s of
t h e Popotosa Formation.
U p l i f t o f t h e a n c e s t r a l Magdalena
M o u n t a i n s i n t h e e a r l y t o m i d d l e Miocene p r o v i d e d a s o u r c e
f o r some o f t h e s e s e d i m e n t s .
T h e a n c e s t r a l M a g d a l e n a s were
l a r g e l y b u r i e d i n t h e i r own d e b r i s by m i d d l e Miocene t i m e
(Bruning, 1974).
Between 7 a n d 4 m - y .
a g o , t h e P o p o t o s a b a s i n was
b r o k e n i n t o a s e r i e s of n a r r o w , p a r a l l e l b a s i n s a n d r a n g e s
t h a t r e f l e c t t h e present topography i n t h e southern Rio
Grande r i f t (Chapin a n d o t h e r s , 1978).
Sediments of t h e
u p p e r S a n t a F e Group were d e p o s i t e d w i t h a n g u l a r u n c o n f o r m i t y
upon t h e m o d e r a t e l y t o s t e e p l y t i l t e d P o p o t o s a F o r m a t i o n .
Local S t r u c t u r e
This study area i s l o c a t e d south of t h e transverse
s h e a r z o n e o f t h e M o r e n c i l i n e a m e n t a n d i s c h a r a c t e r i z e d by
down-to-the-west
rocks.
normal f a u l t s and eastward-dipping T e r t i a r y
The a r e a mapped i s e n t i r e l y w i t h i n t h e N o r t h B a l d y
cauldron (Fig.
19).
The s o u t h w e s t e r n m a r g i n o f t h e S a w m i l l
69
70
Canyon c a u l d r o n i s l o c a t e d w i t h i n t h e s t u d y a r e a a n d g r e a t l y
i n f l u e n c e st h el o c a ls t r u c t u r e .
N o r t hB a l d yc a u l d r o n .
M a g d a l e n aM o u n t a i n s
n o r t h e r nm a r g i n
The o l d e s td o c u m e n t e dc a u l d r o ni nt h e
i s t h eN o r t hB a l d yc a u l d r o n .
The
i s l o c a t e dj u s ts o u t h
o f t h i sc a u l d r o n
of
N o r t hB a l d yP e a ka n dt h es o u t h e r nm a r g i nc r o s s e st h eD e v i l s
Backbone t ot h es o u t ho ft h i ss t u d ya r e a .
m a r g i n i s unmappedand
The s o u t h e r n
i t s e x a c tl o c a t i o n
is uncertain.
The
e a s t e r n a n d w e s t e r nm a r g i n sa r eb u r i e du n d e ry o u n g e rr o c k s .
E r u p t i o n of t h e H e l l s
Mesa T u f f i n i t i a t e d t h e
c o l l a p s e o f t h eN o r t hB a l d yc a u l d r o n .
The H e l l s Mesa i s a s
much as 3850 f t (1173 m ) t h i c kw i t h i nt h ec a u l d r o n( K r e w e d l ,
1.974).
i s w i t h i nt h eN o r t h
E v i d e n c et h a tt h i ss t u d ya r e a
B a l d yc a u l d r o na r e :
1)
The H e l l s Mesa T u f f
i s u n u s u a l l yt h i c kh e r e
( 1 5 0 0 f t , 457 m ) a s c o m p a r e dw i t ho u t f l o w - f a c i e sH e l l sM e s a ,
which i s u s u a l l y 600 f t (1 8 3 m ) t h i c k ;
2 ) C a u l d r o n - f i l ld e p o s i t sa r er e c o g n i z e d
i n the
known a s t h eu n i to fH a r d yR i d g e .S c a t t e r e d
a r e aa n da r e
o u t c r o p so ft h eu n i to fH a r d yR i d g eo c c u rt h r o u g h o u tt h i s
s t u d ya r e ab u ta r el a r g e l yb u r i e db e n e a t hy o u n g e rr o c k s ;a n d
3 )
A m e s o b r e c c i a( L i p m a n ,
1976) i s f o u n d i n t h e
H e l l s Mesa Tuff i n t h e s o u t h w e s t e r n p a r t o f t h e s t u d y a r e a
(SE 1 / 4 , S e c .
9, T . 5 S . ,
R.3W. ) .
of the
S a w m i lC
l a n y o nc a u l d r o nT
. h es o u t h w e s t e r nm a r g i n
S a w m i l lC a n y o nc a u l d r o n
i s l o c a t e dw i t h i nt h i ss t u d ya r e aa n d
c o n t i n u e sn o r t hi n t ot h ea r e a
n o r t h e r nm a r g i n
T i m b e rP e a kb yP e t t y
(1980).
mappedbyBowring
mapped n o r t h o f
o f t h ec a u l d r o nh a sb e e n
(1979).
The
is largely
The e a s t e r nm a r g i n
b u r i e db e n e a t hy o u n g e rr o c k s .
of the
I nt h i ss t u d ya r e a ,t h er i n g - f r a c t u r ez o n e
is
S a w m i l lC a n y o nc a u l d r o nt r e n d sn o r t h - n o r t h w e s ta n d
of t h e
l o c a t e d , f o r much o f i t s l e n g t h ,a l o n ga n dj u s tw e s t
19).
l a n y o n( F i g .
b o t t o m of S a w m i l C
I nt h es o u t h e r nh a l f
t h ea r e a ,t h et r e n db e c o m e se a s t - w e s t .
The r i n g - f r a c t u r e
i s c h a r a c t e r i z e db yr e c u r r e n t
z o n e i s n a r r o wi nt h en o r t ha n d
n o r m a lf a u l t i n ga n dl a t e ri n t r u s i v ea c t i v i t y .
m a r g i n o f t h ec a u l d r o nw i d e n s
s t u d y area.
T h i sw i d ez o n e
of
The s t r u c t u r a l
i n t h ec e n t r a lp a r t
was i n t r u d e db y
complex ( T x r l ) d u r i n gt h ee a r l ys t a g e s
of t h e
a r h y o l i t e dome
of c a u l d r o n - f i l l
a c t i v i t y i n t h eS a w m i l C
l a n y o nc a u l d r o n T
. h er i n g - f r a c t u r e
z o n e was a t l e a s t 1 m i ( 1 . 6 km) i n w i d t ha l o n gt h e
s o u t h w e s t e r ne d g e
o f t h eS a w m i l l
of t h e dome c o m p l e xa r e
Canyon c a u l d r o n , a s o u t c r o p s
of t h i sm a g n i t u d e .
A s t h er h y o l i t e
is
i n t r u s i v e sd i eo u tt ot h es o u t h ,t h er i n gf r a c t u r ez o n e
markedby
a c o m p l e xf a u l tz o n ea n dt h ep r e s e n c e
r h y o l i t ei n t r u s i v e s( s o u t h e a s t e r nq u a r t e r
of l a t e r
o f t h i ss t u d y
area).
. a p s )h a sd o c u m e n t e dt h e
Osburn ( 1 9 7 9 , u n p u b m
a
r e m n a n t s o f t h es o u t h e r nt o p o g r a p h i c
rim of t h eS a w m i l l
'1 2
o f t h i ss t u d y
Canyon c a u l d r o n j u s t e a s t o f ' t h e s o u t h e r n p a r t
s i m i l a r t ot h o s e
a r e a .R e l a t i o n s h i p s
f o u n d i n t h es o u t h e r nh a l f
mappedbyOsburn
are
o f t h i ss t u d ya r e a .H e r e ,
h o w e v e r ,c o n t a c t sa r en o te x p o s e dw e l le n o u g ht o
make a
d e f i n i t ec o r r e l a t i o n .
of t h ep i n n a c l e sa n df l o w - b a n d e d
T h ee r u p t i o n
members o f t h e A-L
P e a kT u f fc a u s e dt h es i m u l t a n e o u sc o l l a p s e
a
o f t h eS a w m i l lC a n y o na n dM a g d a l e n ac a u l d r o n st of o r m
1979).
dumbbell-shapedepression
(Allen,
was f i l l e d w i t h
moat d e p o s i t s o f t h e u n i t
T h id
s epression
o f S i x m i l e Canyon
t h a tc r o po u te x t e n s i v e l yt h r o u g h o u tt h i ss t u d ya r e a .A l l e n
(1979) r e p o r t st h a t
s i m i l a r d e p o s i t so v e r l i et h e
T u f f i n t h eM a g d a l e n ac a u l d r o na n d
e
w i t ht h eS i x m i l e
A-L
Peak
may h a v e i n t e r f i n g e r e d
Canyon d e p o s i t s .
B l o c kf a u l t i n g .N o r m a lf a u l t i n gd u r i n gt h em i d d l et ol a t e
C e n o z o i ch a sd o m i n a t e dt h es t r u c t u r a ls t y l ei nt h es o u t h e r n
i s s o u t h o f t h et r a n s v e r s e
M a g d a l e n aR a n g e T
. h i ss t u d ya r e a
s h e a rz o n ea n d
i n t h ef i e l dd o m i n a t e db yd o w n - t o - t h e - w e s t
strata.
f a u l t sa n de a s t w a r d - d i p p i n g
t h e s ef a u l t s
I n g e n e r a l ,t h et r e n d
i s north-northwestF
. aulting
of
i n t h i ss t u d y
a r e a ,h o w e v e r ,h a sb e e ng r e a t l yi n f l u e n c e db yp r e e x i s t i n g
s t r u c t u r e sp r o d u c e db yc o l l a p s e
cauldron.
o f t h eS a w m i l l
Canyon
F o r e x a m p l e t, h eS a w m i l lC a n y o nf a u l tm a r k st h e
a p p r o x i m a t eb o u n d a r y
o f t h er i n g - f r a c t u r ez o n e
C a n y o nc a u l d r o ni nt h en o r t h e r np a r t
c o l l a p s e o f t h ec a u l d r o n
o f t h eS a w m i l l
o f t h ec a n y o n .T h e
was downward t o t h e e a s t a l o n g t h i s
73
f a u l t .R e c u r r e n t
movement a l o n gt h i sr i n gf r a c t u r ed u et o
r e g i o n a le x t e n s i o nw a s
d i s p l a c e m e n t i n e x c e s s of 6000 f t (1829 m ) .
S a w m i l lC a n y o nf a u l t
i s north-northwest
of t h et h e s i sa r e aa n d
(Sec.
1 6 , T.5S.,
of
down t o t h e w e s t w i t h t h e a m o u n t
of t h e
T h et r e n d
i n t h en o r t h e r np a r t
makes a r a t h e r s h a r p b e n d
i n t h es o u t h
of t h e
R.3W.j t o f o l l o w t h e t r e n d
r i n g - f r a c t u r ez o n e .O t h e rf a u l t sw i t hl e s sd i s p l a c e m e n t h a n
t h eS a w m i l lC a n y o nf a u l ta l s o
show t h i s t r e n d ,
t h ef a u l tt h a to f f s e t st h ea n d e s i t e
Canyon
(Txa2)
R.3W.).
( S 1/2, S e c .
a l o n gt h i sf a u l th a sb e e n
This
of t h eS a w m i l l
f a u l ta p p e a r st of o l l o wt h et o p o g r a p h i cm a r g i n
Canyon
cauldron
of S i x m i l e
of t h eu n i t
1 4 and 1 5 (T.5S.,
i ns e c t i o n s
f o r example,
1 4 , T.5 R.3W.j.
Displacement
down t ot h es o u t h .H o w e v e r ,
of theunit
c a u l d r o n - f i l ld e p o s i t s
o f S i x m i l eC a n y o nw e r e
d e p o s i t e da g a i n s to l d e rr o c k sa l o n gt h et o p o g r a p h i cc a u l d r o n
margin.
I n map v i e w( P l a t e
l), t h ed o w n - t o - t h e - s o u t h
movement a l o n g t h i s u n c o n f o r m i t y g i v e s t h e f a l s e a p p e a r a n c e
of d o w n - t o - t h e . - n o r t hd i s p l a c e m e n t .
A f a u l ti ns e c t i o n
33 ( u n s u r v e y e d )( T . h S . ,
o f d i s p l a c e m e n ts t r i k e s
w i t hs e v e r a lh u n d r e df e e t
R.3W.j
nearly
p e r p e n d i c u l a rt ot h er e g i o n a ls t r u c t u r a lt r e n da n dp r o b a b l y
r e p r e s e n t sr e c u r r e , n t
movement a l o n g p r e e x i s t i n g r i n g
f r a c t u r e s o f t h eS a w m i l l
C a n y o nc a u l d r o n C
. o m p l e xf a u l t i n g
h a so c c u r r e dw i t h i nt h er i n gf r a c t u r ez o n e
i n t h es o u t h e r n
h a l f of t h es t u d ya r e a .W i t h i nt h er i n gf r a c t u r ez o n e ,
f a u l t s o f t e n make r i g h t - a n g l e t u r n s a n d p r o d u c e i s o l a t e d
74
h o r s t s a n d g r a b e n st h a ta r ea t y p i c a lo ft h eb l o c k - f a u l t i n g
i n t h eM a g d a l e n aR a n g e E
. x a m p l e so ft h i s
u s u a l l yf o u n d
11 a n d 1 2
c o m p l e xf a u l t i n gc a nb ef o u n di nS e c s .
(T.5S.,
X. 3W. ) .
One s m a l lb l o c ko fw e s t w a r d - d i p p i n gt u f fo fL e m i t a r
M o u n t a i n s was f o u n d i n S e c s .
b l o c kp r o b a b l ys l u m p e d
1 5 a n d 22 (T.5S.,
o f f t h el a r g eo u t c r o p s
L e m i t a rM o u n t a i n st ot h ew e s ta n d
R.3W.I.
The
o f t u f fo f
was l a t e r b u r i e d b y
s e d i m e n t so ft h eu p p e rP o p o t o s aF o r m a t i o n .E v i d e n c ef o rt h i s
o r i g i ni n c l u d e
1)
c h a o t i ca t t i t u d e sw i t h i nt h i sb l o c ka n d
2)
u n d i s t u r b e do v e r l y i n gP o p o t o s aF o r m a t i o n .
D i p s o f m o s to ft h ef a u l tp l a n e s
a r eu s u a l l ya b o u t
60 d e g r e e s t o t h e w e s t b u t
a s 7 0 d e g r e e s i n t h ew e s t
e a s t .S h a l l o wd i p so f
Chamberlin
o r a ss h a l l o w
1 0 t o 3 0d e g r e e s
i n t h i ss t u d ya r e a
may be a s s t e e p
as 45 d e g r e e s i n t h e
a s r e p o r t e db y
(1978) i nt h eL e m i t a rM o u n t a i n sh a v en o tb e e n
documented i n t h i ss t u d ya r e a .
.
75
ALTERATION AND M I N E R A L I Z A T I O N
Alteration
R e g i o n a lp o t a s s i u mm e t a s o m a t i s m
r o c k si nt h eS a w m i l l
Canyon a r e a a n d
o f t h em e t a s o m a t i s mw e r e
T h ee f f e c t s
i n s u r r o u n d i n ga r e a s .
a netincreaseinthe
a c o r r e s p o n d i n gd e c r e a s e
a m o u n to fp o t a s s i u mi nt h er o c k sa n d
i nt h e
has a f f e c t e d t h e
1978).
a m o u n to fs o d i u m( C h a p i na n do t h e r s ,
Rocks
a l t e r e d i n t h i s m a n n e ru s u a l l yh a v ep l a g i o c l a s e ,a n d
s o m e t i m e ss a n i d i n e ,p h e n o c r y s t sa l t e r e dt of i n e - g r a i n e d
p h y l l o s i l i c a t e s .T r e a t m e n tw i t hs o d i u mc o b a l t i n i t r i t e( r e f e r
t o p.
6 ) o ft h i ns e c t i o n so fp o t a s s i u m - m e t a s o m a t i z e dr o c k s
r e v e a l s a s t r o n gy e l l o w
e
stain.
The i n t e n s ey e l l o w
i n d i c a t e st h ep r e s e n c eo fe x c e s sp o t a s s i u m
stain
i n t h eg r o u n d m a s s
o f t h er o c k .
T h i se v e n t
1978).
a n c i e n tg e o t h e r m a ls y s t e m( C h a p i na n do t h e r s ,
e x t e n to ft h ep o t a s s i u ma n o m a l y
p r e s e n tf r o m
an
i s b e l i e v e dt oh a v eb e e nc a u s e db y
The
i s n o t known, b u t it i s
a t l e a s tt h es o u t h e r nM a g d a l e n aR a n g et ot h e
L a d r o nM o u n t a i n st ot h en o r t h .
I n t h i sa n do t h e rs t u d y
a r e a s ,t h et u f fo fL e m i t a rM o u n t a i n ss e e m st oh a v eb e e n
more
susceptibletopotassiummetasomatismthantheotherunits.
E x t e n s i v eh e m a t i t es t a i n i n g
i s p r e s e n ta l o n gt h e
r i n g - f r a c t u r ez o n eo ft h eS a w m i l C
l a n y o nc a u l d r o n T
. his
h e m a t i t es t a i n i n gc a u s e s
air (Fig. 3).
a c o n s p i c u o u sc o l o ra n o m a l yf r o mt h e
The h e m a t i t e i s a r e s u l to ft h eo x i d a t i o no f
”
a
p y r i t et h a t
i s d i s s e m i n a t e da l o n gf r a c t u r es u r f a c e s
r h y o l i t e dome c o m p l e xa n dt h e
v e r y small ( l e s s t h a n
A s much a s
P e a kT u f f .
A-L
i n the
5%,
1 mm) o x i d i z e dp y r i t ec u b e sc a nb e
The p y r i t e was,
f o u n da l o n gt h e s ef r a c t u r es u r f a c e s .
of t h er h y o l i t e
t h e r e f o r e ,d e p o s i t e da f t e ri n t r u s i o n
dome
complex.
The a r e a s o f m o s ta b u n d a n tp y r i t ea n dm o s ti n t e n s e
h e m a t i t e s t a i n a r e 1) i n t h e A-L P e a kT u f f
at its
of Hardy
d e p o s i t i o n a lc o n t a c tw i t ht h eu n d e r l y i n gr h y o l i t e
R i d g e( T h r 2 )a n d
2 ) i nt h er h y o l i t e
dome complex i n t h e u n i t
o f S i x m i l e Canyon ( T x r l ) , e s p e c i a l l yn e a r
c o n t a c t s( F i g .
20).
its intrusive
The p l a t y ,s h e e t - j o i n t e d
A-L P e a k T u f f
of t h er h y o l i t e
a n d t h eb r e c c i a t e di n t r u s i v ec o n t a c t s
was f a v o r a b l e
c o m p l e xw e r ev e r yp e r m e a b l e .T h i sp e r m e a b i l i t y
f o r l a t e rp a s s a g e
of h y d r o t h e r m a ls o l u t i o n s .
However, t h ea g ea n ds o u r c e
o f t h e s es o l u t i o n s
of t h e
p y r i t e i s l i m i t e dt ot h ev i c i n i t y
unknown.
The
dome
o f t h eS a w m i l l
r i n g - f r a c t u r ez o n e
C a n y o nc a u l d r o n P
. ossible
o r i g i n s o f t h eh y d r o t h e r m a ls o l u t i o n st h a td e p o s i t e dt h e
pyriteare
1) l a t e - s t a g ee m a n a t i o n sf r o m
a s s o c i a t e dw i t ht h eS a w m i l l
s o l u t i o n s ,f r o m
a c o o l i n g magma
C a n y o nc a u l d r o n
o r 2 )h e a t e d
a l a t e rt h e r m a le v e n t ,t h a tc i r c u l a t e da l o n g
c o n d u i t sp r o v i d e d
r i n g - f r a c t u r ez o n e .
by t h eS a w m i l l
C a n y o nc a u l d r o n
is
F i g u r e 20. Intensely hematite-stained outcrop
of t h e r h y o l i t e d o m e c o m p l e x in the unit of
Sixmile Canyon.
(SE 1 / 4 , N E 1 / 4 , Sec. 9 , T.5S., R.3W.).
Mineralization
M i n e r a l i z a t i o n i n t h eS a w m i l l
M a g d a l e n aM o u n t a i n s
Canyon a r e a o f t h e
i s s p o r a d i ca n dc o n s i s t se n t i r e l yo f
l o w - g r a d em a n g a n e s ed e p o s i t s .M o s to ft h ep r o s p e c t si nt h e
s t u d ya r e aa r eb u l l d o z e dc u t s .
Some o ft h e s ep r o s p e c t sw e r e
c u t i n a n d e s i t e st h a tc o n t a i nc e l a d o n i t e - s t a i n e dc h a l c e d o n y
i n s e a r c hf o rc o p p e r
a m y g d u l e sa n dj o i n tf a c e s ,p r o b a b l y
deposits.
I n o t h e r s , n om i n e r a l i z a t i o n
was e v i d e n t i n
in
o u t c r o pH
. o w e v e ro
, r eh a sb e e nr e m o v e df r o mt w ol o c a t i o n s
t h i s s t u d ya r e a .
The g r e a t e s ta m o u n to fo r eh a sb e e nr e m o v e df r o m
t h ep r o s p e c t
R.3W.).
1 7 , T.5S.,
i n t h eH a u n t e dS p r i n ga r e a( S e c .
H e r e , a t t h ei n t e r s e c t i o no ft w om a j o rf a u l t s ,t h e
r o c k sa r eb r e c c i a t e da n ds h e a r e da n dt h ef r a g m e n t sa r e
c e m e n t e dw i t hb o t r y o i d a lp s i l o m e l a n e .
i s a b u n d a n t on j o i n t f a c e s
( T x a 2 ) i s a l t e r e da n dc e l a d o n i t e
a n d c a l c i t ea m y g d u l e s .
The a d j a c e n ta n d e s i t e
A v i s u a le s t i m a t es h o w st h ep i t ' s
x 50 x 100 f t .
d i m e n s i o n st ob ea p p r o x i m a t e l y2 5
i s a l s op r e s e n t
E v i d e n c eo fm i n i n ga c t i v i t y
s e c t i o n s 11 a n d 1 2 , T.5S.,
R.3W.
in
H e r em
, a n g a n e s eo x i d e sh a v e
f o r m e d a c e m e n tf o rf a u l t - b r e c c i af r a g m e n t so ft h el o w e r
P o p o t o s aF o r m a t i o n .A c c o r d i n gt oF a r n h a m
t h eB u r r i su n p a t e n t e dc l a i m s ,
S o c o r r oF. a r n h a mg o e s
ownedby
(1961), t h e s e a r e
B.O.
B u r r i so f
on t o s t a t e :
T h ec l a i m sw e r el o c a t e d
i n t h ee a r l y
1950's by B.O. B u r r i s , a n d s h o r t l y
t h e r e a f t e rc o n s i d e r a b l ee x p l o r a t o r y
work was c o m p l e t e dT
. h eo n l y
known
p r o d u c t i o n was a f e w t e n s o f t o n s o f
h a n d - s o r t e do r et h a t
was s o l d t o
S o c o r r oM a n g a n e s e
Co. by l e s s e e M.V;
Dempsey.
Thework
c o n s i s t e do f
1 2 or more
and
scatteredbulldozertrenches
open-cuts.
T hoep e n i n g s
r a n g e df r o m
2 5 t o 200 f e e t i n
l e n g t h , a n d from a f e wf e e tt o
8
e rp
eroduced
f e e t i n d e p t hT. h o
r e p o r t e d l yc o n t a i n e da b o u t
20 p e r c e n t
m a n g a s e s e ,a n d
was h a n ds o r t e df r o m
t h e dumps o f some o ft h ee x c a v a t i o n s .
..
T h em a n g a n e s ed e p o s i t s
a n dl o wg r a d e
p r e s e n tp r i c e s .
i n t h i ss t u d ya r e aa r e
a n d do n o tc o n s t i t u t ea ne c o n o m i cd e p o s i t
small
at
60
CONCLUSIONS
S e v e r a li m p o r t a n tc o n c l u s i o n sc a nb e
made f r o m t h i s
s t u d yo ft h es o u t h e r nM a g d a l e n aR a n g e .T h e s ea r ed i v i d e d
and alteration
i n t ot h r e es e c t i o n s - - s t r a t i g r a p h y ,s t r u c t u r e ,
andmineralization.
Stratigraphy
Rocks i n t h i s t h e s i s a r e a a r e
a l l Tertiary i n age
i n o r i g i n ,a l t h o u g hm i n o rs e d i m e n t a r y
a n da r em a i n l yv o l c a n i c
u n i t sa r ea l s op r e s e n t .
I n g e n e r a l ,t h er o c k sw i t h i nt h e
s o u t h e r nM a g d a l e n aM o u n t a i n sc o r r e l a t ew e l lw i t ht h er e g i o n a l
s t r a t i g r a p h yd e f i n e d
byChapin
and others
(1978).
Important
c o n t r i b u t i o n st ot h er e g i o n a ls t r a t i g r a p h yt h a th a v eb e e n
madeby
t h i s s t u d ya r e :
1) O u t c r o p s o f
c a u l d r o n - f a c i e sH e l l s
Mesa T u f fa r e
p r e s e n t a t s e v e r a ll o c a l i t i e sw t i h i nt h es t u d ya r e aa n da r e
a s much a s 1 5 0 0 f t ( 4 5 7 m ) t h i c k .
by
Lipman
(1976), i s
A mesobreccia,
p r e s e n t i n t h eH e l l s
as d e f i n e d
Mesa T u f f .
d e p o s i t so ft h eN o r t hB a l d yc a u l d r o no v e r l i et h eH e l l s
Moat
Mesa
T u f fi nt h i st h e s i sa r e a .
2 ) The A-L
t h i c ki nS a w m i l l
P e a kT u f f
Canyonand
i s a t l e a s t 3000 f t
i s u s u a l l yf l o w - b a n d e d .T h e
o fS i x m i l eC a n y o n ,c a u l d r o n - f i l lu n i to ft h eS a w m i l l
cauldron,
i s a t l e a s t 2500 f t ( 7 6 2 m ) t h i c k .
(914 m )
unit
Canyon
A r h y o l i t e dome
complex ( T x r l ) i s i n t r u s i v ea l o n gt h er i n gf r a c t u r e
of t h e
81
S a w m i l lC a n y o nc a u l d r o n ,a n dc o m p l e xf a u l tp a t t e r n sa r e
p r e s e n ta l o n gt h er i n gf r a c t u r ez o n e .
3 ) The t u f fo fL e m i t a rM o u n t a i n s
inthisareaand
may h a v e f i l l e d t h e d e p r e s s i o n l e f t a f t e r
c o l l a p s eo ft h eS a w m i l l
4)
i s u n u s u a l l yt h i c k
C a n y o nc a u l d r o n .
A l a r g er h y o l i t e
dome c o m p l e xp o s t - d a t i n gt h e
more t h a n 2 sq m i ( 5 . 2
t u f fo fL e m i t a rM o u n t a i n sc o v e r s
sq
km) a l o n gH a r d yR i d g ei nt h en o r t h w e s t e r np a r to ft h es t u d y
a r e a .T h i sr h y o l i t ea n da s s o c i a t e dp y r o c l a s t i cd e p o s i t sa r e
h e r e i nd e s i g n a t e d
a s t h eu n i to fS a w m i l lC a n y o n ;t h e ya r e
b e l i e v e dt oh a v eb e e ne x t r u d e da l o n gc o n d u i t sp r o v i d e db yt h e
r i n gf r a c t u r eo ft h eS a w m i l l
C a n y o nc a u l d r o n .
5 ) T h el o w e rP o p o t o s aF o r m a t i o n
was d e p o s i t e du p o n
a n e x t e n s i v ee r o s i o ns u r f a c ei nt h es o u t h e r nM a g d a l e n a
Mountains.
n) V o l c a n i ca c t i v i t yc o n t i n u e da f t e ri n i t i a t i o n
P o p o t o s ad e p o s i t i o n
a s i n d i c a t e db yt h ep r e s e n c eo f
of
an
i n t e r b e d d e da s h - f l o wt u f f .
7 ) F l o wd i r e c t i o n si nt h eu p p e rP o p o t o s aF o r m a t i o n
indicatethat
a h i g h l a n ds o u r c e
was p r e v i o u s l y l o c a t e d t o t h e
n o r t h e a s to ft h em o u t ho fS a w m i l lC a n y o n .
Structure
M a j o rs t r u c t u r a lf e a t u r e so ft h eS a w m i l l
Canyon
a r e aa r en o r t h - n o r t h w e s tt r e n d i n gn o r m a lf a u l t st h a ta r e
*
relatedtothe
X i 0 G r a n d e r i f t a n ds t r o n g l yc o n t r o l l e db yt h e
82
z o n eo ft h eS a w m i l lC a n y o nc a u l d r o n .
ring-fracture
S t r u c t u r a le l e m e n t s
i n t h i st h e s i sa r e aa r e :
i s e n t i r e l yw i t h i nt h eN o r t h
1) T h es t u d ya r e a
B a l d yc a u l d r o na n dc o n t a i n st h eH e l l s
Mesa T u f fa n d
a s s o c i a t e dc a u l d r o n - f i l ld e p o s i t s .
2 ) T h es o u t h w e s t e r nm a r g i no ft h eS a w m i l lC a n y o n
c a u l d r o n i s l o c a t e dw i t h i nt h i ss t u d ya r e a .
The r i n g
f r a c t u r ez o n ed o m i n a t e st h es t r u c t u r a ls t y l ei nt h eS a w m i l l
Canyon a r e a a n d l a r g e l y c o n t r o l s l a t e r e x t e n s i o n a l f a u l t
patterns.
3 ) E x t e n s i o n a lf a u l t sg e n e r a l l yt r e n d
n o r t h - n o r t h w e s te x c e p tw h e r ed e f l e c t e d
by t h e r i n g - f r a c t u r e
z o n eo ft h eS a w m i l lC a n y o nc a u l d r o n .F a u l tb l o c k sa r e
usually down t o t h e w e s t
and strata i s t i l t e d t o t h e e a s t .
A l t e r a t i o na n dM i n e r a l i z a t i o n
C o n c l u s i o n sr e g a r d i n ga l t e r a t i o na n dm i n e r a l i z a t i o n
i nS a w m i l l
Canyon a r e :
1) T h i ss t u d ya r e a
i s l o c a t e dw i t h i nt h er e g i o n a l
potassium anomaly,
2)
I n t e n s eh e m a t i t es t a i n i n g
i s t h er e s u l t
of t h e
o x i d a t i o n of p y r i t e t h a t
i s d i s s e m i n a t e da l o n gf r a c t u r e
s u r f a c e s i n r h y o l i t ea n d
A-L
of t h i s p y r i t e
P e a kT u f f T
. h ea g ea n ds o u r c e
i s unknown.
3 ) M a n g a n e s em i n e r a l i z a t i o n
c o n s t i t u t e a n e c o n o m i cd e p o s i t
i s s p a r s ea n dd o e s
a t p r e s e n tp r i c e s .
not
83
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Survey Bulletin 716-~.
a
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