OPEN FILE REPORT 120
-
T h eG e o l o g y
o f t h eW e s t - C e n t r a lM 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
by
S a m u e l A.
Bowring
Submitted i n P a r t i a lF u l f i l l m e n t
o ft h eR e q u i r e m e n t sf o rt h eD e g r e eo f
i n Geology
M a s t e r o fS c i e n c e
New Mexico I n s t i t u t e o f M i n i n ga n dT e c h n o l o g y
S o c o r r o , New Mexico
May, 1 9 8 0
ii
TABLE OF CONTENTS
.........................
v
ACKNOWLEDGMENTS . . . . . . . . . . . . . . . . . . . .
vii
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . .
1
G e n e r aGl e o l o g iSce t t i n g
. . . . . . . . . . . . . . . 13
Areaofstudy. . . . . . . . . . . . . . . . . . .
Objectives . . . . . . . . . . . . . . . . . . . .
3
P r e v i o u s Work . . . . . . . . . . . . . . . . . . .
5
M e t h o doIsfn v e s t i g a t i o n
............. 7
STRATIGRAPHY AND PETROGRAPHY . . . . . . . . . . . . . . 1 0
G e n e r a lT e r t i a r yS t r a t i g r a p h y
C e n t r aSl o c o r r oC o u n t y
. . .o f. . . . . . . . . . 1 0
H e l l s MesaTuff
. . . . . . . . . . . . . . . . . . 12
U n i t o fH a r d yR i d g e
. . . . . . . . . . . . . . . . 14
a n d e s i t e s and sedimentary rocks . . . . . . . . 15
ash-flowtuffs
.. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 1186
r h y o l i t el a v a s
A-L P e a kT u f f
. . . . . . . . . . . . . . . . . . . 21
l o w e r A-L Peak Tuff . . . . . . . . . . . . . . 24
m e g a b r e c c i a . . . . . . . . . . . . . . . . . . 32
u p p e r A-L P e a k Tuff . . . . . . . . . . . . . . 39
U n i t o f S i x m i l e Canyon . . . . . . . . . . . . . . 4 4
l o w e r member . . . . . . . . . . . . . . . . . 4 5
a n d e s i t e .l a v a s . . . . . . . . . . .. . . . . . . . . . . . . . . .46
rhyolitelavas
. . . . . . .. .. .. .. .. .. .. .. .. .. 50
52
t u f f a n ds e d i m e n ti n t e r v a l
.
.
.
.
.
.
.
.
.
52
sedimentary rocks . . . . . .
.
.
.
.
.
.
.
.
.
53
u p p e ru n i t
o f S i x m i l eC a n y o n
.
.
.
.
.
.
.
.
.
55
t u f f of C a r o n i t a Canyon . . .
l o w e r member . . . . . . . . . . . . . . . . 56
u p p e r member . . . . . . . . . . . . . . . . 56
a n d e s i t e i n t e r v a l . . . . . . . . . . . . . . . 57
T u f fo f
L e m i t a r M o u n t a i n s . . . . . . . . . . . . . . 60
l o w e r member . . . . . . . . . . . . . . . . . 6 1
u p p e r member . . . . . . . . . . . . . . . . . 64
T u fofSf o u t h
Canyon
. . . . . . . . . . . . . . . 75
I n t e r m e d i a tL
eavas
. . . . . . . . . . . . . . . . 78
andesite
. .. .. .. .. .. .. .. .. .. .. .. .. .. .. ..l a v81
79
as
porphyritic
ABSTRACT
. .
lavas
iii
ash-flowtuffsandvolcaniclastic
s e d i m e n t a r yr o c k s
.............
P o p o t o sFa o r m a t i o n
................
l a h a r i cb r e c c i a s
. . . . . . .. .. .. .. .. .. .. .. ..
c o n g l o m e r a t e as n d
sandstones
84
85
87
88
. . . . . . . . . . . 89
L a v a so fM a g d a l e n aP e a k
. . . . . . . . . . . . . . 90
t u f fi n t e r v a l . . . . . . . . . . . . . . . . . 9 2
. . . . . . . . . . . . . . . . 93
r h y o l i t fel o w s
Tertiary
IntrusivR
e ocks
.
.
.
.
.
.
.
.
.
.
.
.
.
95
d a c i t e so fD e e rP l o tT a n k
. .. .. .. .. .. .. .. .. .. .. 96
w h i treh y o l i tien t r u s i v e s
m a fdi ci k e s
. . . . . . . . . . . . . . . . . 97
98
.Te.rtiary - Q u a t e r . n a r yd e p o s i t s
. . . . . . . . . . . 99
p i - e d m o ngtr a v e l s
. . . . . . . . . . . . . . . . 99
Quaternary Deposits . . . . . . . . . . . . . . . . 100
alluvium . . . . . . . . . . . . . . . . . . . 100
. . . . . . . . . . . . . . . 100
talus/colluvium
t e r r a c eg r a v e l so f
unknownage
00
l a n d s l i d ed e p o s i t s
. . . . . . .. .. .. .. .. .. .. .. 1101
STRUCTURE . . . . . . . . . . . . . . . . . . . . . . .
102
R e g i o n a lS t r u c t u r e
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
102
L a r a m i d eu p l i f ta n d
erosion
.
.
.
.
.
.
.
.
.
. . . . . . . . . . . . . 11 00 22
O l i g o c e n ev o l c a n i s m
. . . . . . . . . . . . . . 104
r e g i o n a le x t e n s i o n
L o c a lS t r u c t u r e . . . . . . . . . . . . . . . . . . 1 0 5
. .. .. .. .. .. .. .. .. .. .. 11 00 56
a s h L f l o wt u f f
cauldrons
N o r t hB a l d yc a u l d r o n
.
.
S 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 s
. . . 106
e x t e n s i o n a lf a u l t i n g
. . . . . . . . . . . . . 112
ECONOMIC GEOLOGY . . . . . . . . . . . . . . . . . . . .
116
CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . .
118
S t r a t i g r a p h y . . . . . . . . . . . . . . . . . . . 118
S t r u c t u r e. . . . . . . . . . . . . . . . . . . . .
121
REFERENCES C I T E D . . . . . . . . . . . . . . . . . . . .
123
s a n d s t o n e sa n ds i l t s t o n e s
iv
LIST OF ILLUSTRATIONS
1
'
2
3
G e n e r a l l 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
o ft h es t u d ya r e at om a j o rm o u n t a i nr a n g e sa n d
t h ed i s t r i b u t i o no O
f l i g o c e n ea n d . M i o c e n er o c k s
..
2
Relationshipofthe.studyareatostratigraphic
a n dp h y s i o g r a p h i cf e a t u r e s ,s u r r o u n d i n gs t u d i e s ,
a n rdo u t eosafc c e s s .
...............
4
C o r r e l a t i o nd i a g r a m
of t h e c h r o n o l o g i c
m e n to ft h es t r a t i g r a p h i cn o m e n c l a t u r e
S o c o r r o - M a g d a l e n a area.
6
developi n the
..............
4
5
.6
I
I
7
/ e
a
I
9
L o c a t i o n o f m a j o rc a u l d r o n sa n dt r a n s v e r s es h e a r
z o n e i n t hS
e o c o r r o - M a g d a l e naar e a .
S t r a t i g r a p h i cc o l u m n .
.........
..... . . . . . . . . . .
in
. . . . . . . . . . .' . . . .
A-L
.........
:
S p h e r u l i t e - l i k eb o d i e s
of H a r d yR i d g e
t h eu n i t
P h o t o m i c r o g r a p ho f
S p h e r u l i t e - l i k eb o d i e s
11
t h er h y o l i t el a v a so f
P e a kT u f f .
i n t h e A-L
P e a kT u f f
A-L
P e a kT u f f
S p h e r u l i t e - l i k eb o d i e si nt h e
....
....
........
Gas c a v i t i e s i n b a s e o f u p p e r t u f f
of L e m i t a r
Mountains . . . . . . . . . . . . . . . . . . . . . . .
Ramp s t r u c t u r ei nu p p e rt u f fo f
Lemitar M o u n t a i n s .
Boxwork f a b r i c i n t h e A-L
P e a kT u f f
S t e e p l yd i p p i n gf o l i a t i o n
i n t u f fo fL e m i t a r
M o u n t a i n s . . . . . . . . . . . . . . . . . . . . .
14
8
.20
31
36
37
38
69
70
72
Map o fS 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 s
a n dr e l a t i o n s h i pt oS o c o r r ot r a n s v e r s es h e a r
z o n ea n ds t ' u d y
area
. . . . . . . . . . . . . . . ..log
PLATES
o f t h ew e s t - c e n t r a l
1
G e o l o g i c map a n dc r o s s - s e c t i o n s
M a g d a l e n aM o u n t a i n s( i np o c k e t ) .
2
S t r u c t u r a lf e a t u r e s
of t h ew e s t - c e n t r a lM a g d a l e n a
MountainsS
, o c o ' r r oC o u n t y ,
N e w Mexico ( i n p o c k e t ) .
..
Y
ABSTRACT
T h eO l i g o c e n e
t o M i o c e n eg e o l o g i ch i s t o r yo ft h e
w e s t - c e n t r a lM a g d a l e n aM o u n t a i n sh a sb e e nd o m i n a t e db yt h e
e r u p t i o no fv o l u m i n o u sa s h - f l o wt u f f s
a n d b ye x t e n s i o n a l
of t h e Rio
f a u l t i n ga n du p l i f tr e l a t e dt ot h ed e v e l o p m e n t
Grande r i f t .
are
P o r t i o n s o f t h r e eo v e r l a p p i n gc a u l d r o n s
p r e s e r v e d i n t h i ss t u d ya r e a .
The o l d e s tr o c ku n i te x p o s e d
i s t h e H e l l s Mesa T u f f ( 3 2 m.y.
a g o ) .w h i c h
t h eN o r t hB a l d yc a u l d r o n ;r h y o l i t e s
was e r u p t e df r o m
a n d a s h - f l o wt u f f so ft h e
u n i to fH a r d yR i d g ef i l l e dt h ec a u l d r o nf o l l o w i n g
c o l l a p s e .C o l l a p s eo ft h eS a w m i l l
e
its
Canyon a n d M a g d a l e n a
c a u l d r o nfso l l o w ew
d i tthheer u p t i oont fh e
A-L
P e aTku f f .
A t l e a s t 2 0 0 0 f e e t o fd e n s e l yw e l d e d ,c a u l d r o n - f a c i e s ,
P e a kT u f f
A-L
i s e x p o s e dw 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 e
i n a narrow
S 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 sa r ej o i n e d
z o n e n e a r t h e s u m m i to fS o u t hB a l d ya n df o r m
a large,
d u m b b e l l - s h a p e ds t r u c t u r e T
. h eu n i ot S
f i x m i l eC a n y o n
( a n d e s i t i c l a v a s , r h y o l i t e l a v a s , a s h - f l o wt u f f s ,l a h a r i c
b r e c c i a 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 )p a r t i a l l yf i l l e d
t h ec a u l d r o n s .A n d e s i t i c
l a v a s d o m i n a t ew i t h i nt h eu n i to f
S i x m i l eC a n y o ni nt h en o r t h e r np a r to ft h eS a w m i l lC a n y o n
c a u l d r o n ,r h y o l i t e
l a v a s i nt h es o u t h .
D e p o s i t i o no ft h eo v e r l y i n gt u f fo f
e
Lemitar
M o u n t a i n s was i n f l u e n c e db yt h eS a w m i l lC a n y o n - M a g d a l e n a
vi
c a u l d r o n s ,t h eu p p e ra n dl o w e rm e m b e r sa r ep r e s e n tw i t h i nt h e
c a u l d r o n sw h i l eo n l yt h eu p p e r
A n d e s i t i ct od a c i t i c
member i s p r e s e n t o u t s i d e .
l a v a s a n dt h et u f fo fS o u t h
Canyonwere
I n e a r l yM i o c e n e
d e p o s i t e do nt h et u f fo fL e m i t a rM o u n t a i n s .
t i m e ,f a n g l o m e r a t e s ,l a h a r i cb r e c c i a sa n ds a n d s t o n e so ft h e
P o p o t o s aF o r m a t i o nw e r ed e p o s i t e do v e rm o s to ft h es t u d y
area.
I n t h ew e s t e r np a r to ft h es t u d ya r e a ,r h y o l i t e
( l a v a s o fM a g d a l e n aP e a k )o v e r l i e
lavas
a n d a r ei n t e r b e d d e dw i t h
t h eP o p o t o s aF o r m a t i o n .
The s t r u c t u r a l d e v e l o p n e n t o f t h e w e s t - c e n t r a l
M a g d a l e n aM o u n t a i n sh a sb e e nd o m i n a t e db yt h ei n t e r a c t i o no f
c a u l d r o ns t r u c t u r e sw i t hn o r t h - t r e n d i n gn o r m a lf a u l t sa n d
m a j o rs t r u c t u r a lb r e a k( t h eS o c o r r ot r a n s v e r s es h e a rz o n e ) .
0
The s h e a rz o n et r e n d st h r o u g ht h en o r t h e r np a r to ft h es t u d y
area.
I n g e n e r a l ,s o u t ho ft h es h e a rz o n e ,f a u l tp l a n e sh a v e
b e e nr o t a t e dt ot h e
stepfaulting
is true.
w e s t , s t r a t a d i pt o w a r d st h ee a s t ,a n d
i s down-to-the-west;
t ot h es o u t ht h eo p p o s i t e
a
vii
ACKNOWLEDGMENTS
I a m indebtedtothe
New M e x i c oB u r e a uo fM i n e sa n d
M i n e r a lR e s o u r c e sf o rp r o v i d i n gf i n a n c i a la n dl o g i s t i c a l
s u p p o r to ft h i st h e s i s .
Dr.
C h a r l e s E.
I w o u l d' l i k et ot h a n k
my a d v i s o r ,
C h a p i n ,f o rs u g g e s t i n gt h i ss t u d ya n df o rh i s
e n t h u s i a s t i cs u p p o r t ,c o m m e n t s ,a n da d v i c ed u r i n g
its
completion.
G. R o b e r tO s b u r np r o v i d e d
i n t h ef i e l d
R.
many h e l p f u l s u g g e s t i o n s
a n d c r i t i c a l l yr e a dt h e . m a n u s c r i p t .
S a n f o r da n dJ o h n
R.
Drs.
Allen
M a c M i l l a na l s oc r i t i c a l l yr e a dt h e
manuscript.
G.
R o b e r tO s b u r nd r a f t e dt h e
map a n df i g u r e s .S u e
K e n tt y p e dt w od r a f t so ft h et h e s i sa n dt h ef i n a lm a n u s c r i p t .
INTRODUCTION
G e n e r a lG e o l o g i cS e t t i n g
T h eR i oG r a n d e
rift t r a n s e c t st h en o r t h e a s t e r n
f l a n ko ft h em i d d l eT e r t i a r yD a t i l - M o g o l l o nv o l c a n i cp l a t e a u .
I nt h e
Socorro- M a g d a l e n aa r e a ,t h i si n t e r s e c t i o nh a sb r o k e n
a s e r i e so fn o r t h - t r e n d i n g ,
t h ev o l c a n i cp l a t e a ui n t o
a n d b a s i n sT
. h eM a g d a l e n aM o u n t a i n s ,
f a u l t - b l o c ku p l i f t s
-"-'?Jencia basin,^
-and.-*be
La
Socorra-Lemiter Mountains .are examples
o ft h i sc o m p l e xb a s i n - a n d - r a n g es t r u c t u r e( f i g u r e
1).
F u r t h e rc o m p l i c a t i n gt h es t r u c t u r ei nt h e
Socorro-Magdalena
area i s t h e i n t e r s e c t i o n o f t h e
r i f t a n dt h en o r t h e a s t - t r e n d i n g
s o u t h - t r e n d i n gR i oG r a n d e
t r a n s v e r s es h e a rz o n eo ft h eM o r e n c il i n e a m e n t( C h a p i na n d
others,
1978).
T h et r a n s v e r s es h e a rz o n e
i s a c t i n g as a n
i n c i p i e n tt r a n s f o r mf a u l t ,c o n n e c t i n ge ne c h e l o ns e g m e n t so f
t h eR i oG r a n d e
r i f t , a n ds e p a r a t i n gf i e l d so ~ ft i l t e db l o c k s
u n d e r g o i n gr o t a t i o na n ds t e pf a u l t i n g
( C h a p i na n do t h e r s ,
i n o p p o s i t ed i r e c t i o n s
1978).
T h eM a g d a l e n aM o u n t a i n s ,
a p p r o x i m a t e l y 20 m i l e s l o n gb y
a c o m p l e xu p l i f t e db l o c k
7 m i l e sw i d e ,t r e n d
n o r t h - s o u t ha l o n gt h ew e s ts i d eo ft h eR i oG r a n d ev a l l e yi n
c e n t r a lS o c o r r oC o u n t y ,
New M e x i c oT
. h en o r t h e r nM a g d a l e n a
M o u n t a i n s i s a w e s t - t i l t e df a u l tb l o c k ;t h ee x t r e m es o u t h e r n
M a g d a l e n a sa ne a s t - t i l t e df a u l tb l o c k ;b o t he x h i b i th o g b a c k
t o p o g r a p h y .T h es t r u c t u r ei nt h ec e n t r a lM a g d a l e n aM o u n t a i n s
2
EXPLANATION
Oppro.imolc limit OF bedrock
( u r u o l l y c m p c boundacy
Oligocene fo~corl*
1
.uioccne
"DIC0"iC
." rockc OF D a t i l - M o g o l l o n t i c 1 4
I
1ete M i 0 S c " C . i I i C i C
"
.
Figure 1. General location map showing t h e r e l a t i o n s h i p of the
study area to majormountainrangesand
t h e d i s t r i b u t i o n of
Oligocene t o Miocene rocks.
IO"O,
3
a
i s g r e a t lcyo m p l i c a t ebdtyhper e s e n cofefo uorv e r l a p p i n g
1 9 7 8 ) a n dt h e i ri n t e r a c t i o n
c a u l d r o n s( C h a p i na n do t h e r s ,
w i t h r i f t f a u l t i n ga n dt h et r a n s v e r s es h e a rz o n e .H e r e ,n o
c o n s i s t e n tt o p o g r a p h i ce x p r e s s i o no ft h eg e o l o g y
i s evident.
A r e ao fS t u d y
T h es t u d ya r e a
i s a r e g i o no fa b o u t
1 7 s q u a r em i l e s
i nt h ew e s t - c e n t r a lM a g d a l e n aM o u n t a i n s ,t h ec e n t e ro fw h i c h
. . i s ' y l o c a t e d .a p p r o x i m - a t e l y . . & e nm i l e - s .s o u t h .- o fM a g d a l e n a .T h e
r e l a t i o n s h i po ft h es t u d ya r e at og e o g r a p h i ca n d
p h y s i o g r a p h i cf e a t u r e s ,s u r r o u n d i n gs t u d ya r e a s ,a n dm a j o r
r o u t e so fa c c e s sa r e
shown i n f i g u r e
2.
M o s to ft h es t u d y
i s by
a r e a i s w i t h i nt h eC i b o l aN a t i o n a lF o r e s ta n da c c e s s
U.S.
trails.
F o r e s tS e r v i c er o a d sa n d
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 kt ot h e
w e s t - c e n t r a lM a g d a l e n aM o u n t a i n s ;
t h i c k n e s s e so fa s h - f l o wt u f f
S a w m i l lC a n y o n' a r e a ;a n d
n a t u r eo fc a u l d r o ns t r u c t u r e s
2 ) t oe x p l a i nt h eg r e a t
known f r o mr e c o n n a i s s a n c e
3 ) t oe v a l u a t e. t h ee x i s t e n c ea n d
i n t h es t u d ya r e at h a th a v e
b e e ns u g g e s t e do nt h eb a s i so fr e c o n n a i s s a n c e .
\
i n the
7.2s
/
-J
/
T3
i;
T.4
sac
T.
Figure 2. Relationship of the study area to stratrgrapnlc
features, surrounding studies, and routes of access.
ana p~ly~luylaPl~~u
5
P r e v i o u s Work
T h eC e n o z o i cv o l c a n i cs t r a t i g r a p h ya n ds t r u c t u r ei n
t h eS o c o r r o - M a g d a l e n aa r e ah a sb e e nd e v e l o p e do v e rt h ep a s t
e i g h ty e a r st h r o u g h
a s e r i e s o f d e t a i l e dm a p p i n gp r o j e c t s
1973;
( B r o w n1
, 9 7 2C
; h a p i nu
, n p u b l i s h e dm a p s S
; imon,
Chamberlin,
1 9 7 4 ; W i l k i n s o n , 1 9 7 6 ; O s b u r n , 1978; a n d
Chamberlin, 1978).
T h e s ei n v e s t i g a t i o n s
known as t h e
o n g o i n gs t u d i e s ,i n c l u d i n gt h i so n e ,a r e
M a g . d a l e n aP r o j e c ta n d
,...are supported.;.by..Lhe
a n d M i n e r a lR e s o u r c e s .
oM
f ines
o ft h es t r a t i g r a p h y
as w e l l as s e v e r a l
New M e x i c oB u r e a u
The c h r o n o l o g i cd e v e l o p m e n t
i n t h eS o c o r r o - M a g d a l e n aa r e a
i s shown i n
f i g u r e 3.
O t h e rs t u d i e s( S t a c y ,
1 9 7 3 ; a n dK r e w e d l ,
t h ea r e a .
1968; D e a l , 1973; B r u n i n g ,
1 9 7 4 ) h a v ea d d e dt ot h eu n d e r s t a n d i n g
Deal ( 1 9 7 3 ) mapped a p p r o x i m a t e l y 3 0 0 s q u a r e m i l e s
i nt h en o r t h e r nS a nM a t e oM o u n t a i n so n
%asis.
of
a reconnaissance
He p r o p o s e d a l a r g er e s u r g e n tc a u l d r o n ,c e n t . e r e d
a b o u t Mount W i t h i n g t o n ,a n ds u g g e s t e dt h a t
i t s e a s t e r nm a r g i n
c o u l db ee x t e n d e di n t ot h eM a g d a l e n aM o u n t a i n s K
. rewedl
( 1 9 7 4 ) mapped t h e n o r t h e r n p o r t i o n
o n a r e c o n n a i s s a n c eb a s i sa n dS t a c y
of t h e p r e s e n t s t u d y a r e a
( 1 9 6 8 ) mapped a b o u to n e
s q u a r e m i l e i n t h eL a n g m u i rL a b o r a t o r y - S o u t hB a l d ya r e a .
B r u n i n g( 1 9 7 3 )d e s c r i b e dt h eo u t c r o p so fP o p o t o s aF o r m a t i o n
n e a rS o u t hB a l d y .
e
M a p p i n g i nt h eS o c o r r o - L e m i t a rM o u n t a i n s
( C h a m b e r l i1n9,7a8en)ads t eM
r na g d a l eM
n ao u n t a i(nOs s b u r n ,
6
7
8
1 9 7 8 ) h aess t a b l i s h e d
a d e t a i l esdt r a t i g r a p htyh a t
i s being
e x t e n d e d t o t h ec e n t r a la n ds o u t h - c e n t r a lM a g d a l e n a
D. P e t t y , P. A l l e n , S . R o t h , i n p r o g r e s s ) T
. his
( t h i ss t u d y ,
a n d o t h e r s ( 1 9 7 8 ) t o show t h e
m a p p i n ge n a b l e dC h a p i n
i n t h eM a g d a l e n a
a p p r o x i m a t el o c a t i o no fp r o p o s e dc a u l d r o n s
M o u n t a i n sa n dt od o c u m e n t
a m a j o rt r a n s v e r s es t r u c t u r ew h i c h
of t h i s s t u d y a r e a
p r o j e c t st h r o u g ht h en o r t h e r np o r t i o n
(figure
Mountains
4).
i.P.
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 g
was d o n e a t a s c a l e o f
1 : 2 4 , 0 0 0o nt h eS o u t hB a l d yt o p o g r a p h i cq u a d r a n g l eo ft h e
U n i t e dS t a t e sG e o l o g i c a lS u r v e y
7 1 / 2 - m i n u t es e r i e s U
. nited
S t a t e sF 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 1 )
at a scale
o f 1:12,000 w e r eu s e dt ol o c a t eo u t c r o p s .M a p p i n g
was
c o n d u c t e dd u r i n gt h e
andsummer
summer a n d f a l l
of.1977 a n dt h es p r i n g
o f 1978.
i n the
E i g h t yt h i ns e c t i o n so fr o c k sc o l l e c t e d
s t u d y a r e a w e r eu s e dt od e s c r i b ea n dc o r r e l a t er o c ku n i t s .
Z e i s sb i n o c u l a rp e t r o g r a p h i cm i c r o s c o p e
s e c t i o ns t u d y .
was u s e d f o r t h i n
A l l t h i ns e c t i o n sw e r es t a i n e df o rp o t a s s i u m
(Deer,
w i t hs o d i u mc o b a l t i n i t r a t eu s i n gs t a n d a r dp r o c e d u r e s
H o w i ea, n d
Zussman,
*
A
1 9 6 6 , p . 311).
E t c h i n g times were
i n c r e a s e df r o m1 5 - 2 0s e c o n d st o6 0 - 9 0s e c o n d s P
. etrographic
r o c kn a m e s
a r e f r o mL i p m a n ' s
c l a s s i f i c a t i ofnovro l c a n irco c kasn d
( 1 9 7 6 , f i g . 3 , p.
5)
a r e u s e d i n ainn f o r m a l
-
'
e
.
Figure 4. Location of major cauldron structures and the
transverse shear zone in the Socorro-Magdalena area as
proposed by Chapin and others, 1978. Approximate location
of this study area shown by white block.
9
f a s h i o n a s c h e m i c a la n a l y s e sf o rm o s t
s t u d y a r e a a r en o ta v a i l a b l e .
o f t h er o c k s
in the
Rock c o l o r sr e p o r t e da r ef r o m
t h e GSA r o c k c o l o r c h a r t .
A s a m p l e f o r r 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
t h e . l a v a s i n t e r b e d d e d in t h e P o p o t o s a F o r m a t i o n i n t h e
M u l e s h o eR a n c ha r e a .
, i:
10
a
STRATIGRAPHY AND PETROGRAPHY
G e n e r a lT e r t i a r yS t r a t i g r a p h yo fC e n t r a lS o c o r r oC o u n t y
T h eT e r t i a r ys e q u e n c ei nc e n t r a lS o c o r r oC o u n t y
c o n s i s t so fb o t hv o l c a n i ca n ds e d i m e n t a r yu n i t s .
T e r t i a r yu n i t
The b a s a l
i s t h e B a c aF o r m a t i o n( E o c e n e )w h i c hc o n s i s t s
o fa r k o s i cs a n d s t o n e sa n dc o n g l o m e r a t e s .O v e r l y i n gt h e
F o r m a t i o n i s t h eS p e a r sF o r m a t i o n ,
Baca
a s e r i e so fl a t i t i c -
:conglomerates,. laharic ~breccias, volcaniclastic
s a n d s t o n e s ,a n d e s i t i c
l a v a s , a n d t h r e eo r
more i n t e r b e d d e d
a s h - f l o wt u f f s T
. h i sl a r g e l ys e d i m e n t a r ya c c u m u l a t i o n
r e p r e s e n t st h ea l l u v i a la p r o nt h a ts u r r o u n d e dt h e
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
i g n i m b r i t e c l i m a x ( C h a p i na n do t h e r s ,
i t s Oligocene
1978).
a t h i c ks e q u e n c e
e r u p t i o n sa r er e p r e s e n t e db y
The p y r o c l a s t i c
o f ash-flow
t u f f so v e r l a i na n di n t e r b e d d e dw i t hb a s a l t i c - a n d e s i t ea n d
r h y o l i t e l a v a s (Oligocene- early M i o c e n e )R
. e g i o n ael x t e n s i o n
l a t e s t a g e so ft h i sv o l c a n i s m
c o n t e m p o r a n e o u sw i t ht h e
c r e a t e db a s i n s
i n w h i c ht h ec l a s t i cs e d i m e n t so ft h eS a n t aF e
G r o u pa n db a s a l t i ca n dr h y o l i t i cl a v a sh a v ea c c u m u l a t e d .
T h er o c k se x p o s e d
i n t h es t u d ya r e ac o n s i s to f
a s h - f l o w t u f f s a n dl a v a so fO l i g o c e n et oe a r l yM i o c e n ea g e
a n d a l a t e r s e q u e n c eo ff 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
o v e r l a i nb yr h y o l i t i c
lavas.
g r a p h i c a l l yi nf i g u r e
5.
The
map
u n i t sa r e
shown
11
w
,
PaPotola Formation(0-800 ft 0-244 m ) -IC
BRECCUS, CONGL"
ERliTfS. and SANDSTONES: dense, red, vell-indurated, heterolithic
laharic breccias at base(500 ft ) overlain by and interbedded with
Conglmnrerares and sandstones. Uppelrust portion finely laminated
sandstones and siltstones.
..
,.
Intermediate iaVas(0-400 ft , 0-122 m ) ANDESITES,WODAC~TES:
lavas vlth large
basaltie andesites overlain by rhyodacite
Phenac?rScs Of d h l i feldspar and quartz. locally interbedded
d t h lithic-rich ash-flow tuffs and liharis breccias.
Tuff of South Canyon26.m.y.(0-100
fr , 0-30 m ) ASB-FLOU T
W
F
S
:
multiple-flow. simple cooling voif of rhyolite ash-flow
tuff;
-.phk t w p a y , .moderately to.denselpvelded, crys.-zich,.dirtinctive
streaked pumice.
lava Flows: (OL200 ft
0-60 m ) AMESITE ULVAS: dark red to gray.
dense, basaltic andesites vlth distinctive oxidized
fer-g.
phewcqsts.
,
Tuff of Lemitar Hourains: 28 m.y. (600-1000 ft , 180-305 m 1 ASKFWd TUFFS: rhyolite. multiple-flow. densely welded, ash-flow tuff.
It. gray to palered, crys.-poor, lower member(0-4W ft.) overlain
by med. red. crgs.-rieh, qtr. and hiacite rich, 2 felds. upper
.
member
e
Upper kif of Simile Canyon: 29 m.y.(500-750
ft ,152-230m ) ASBnaU TUFFS,SANDSTONES,ANDESlTE5: Lwer and uppez members of tuff
of Caronita Canyonoverlain by sandstone and andesite.
Lower member
is brick-red.crys.- poor, multiple-flow. and poorlyto mderately
welded. Upper member is gray LO white, crystal-rich. vlth distinetii)e
hipyramidal gyartz. Dverlain by qtr.-rich, bedded. sandstone and
dense,apha*itic,andesites.
Lower Unit of Simile Canyon:(800-2000 ft ) ANDESITE to BASALTIC
ANDESlTE ULVAS, RHYOLITE ULVAS, VOLCANOCLASTICS. AS0 FUW TUFES:
dark. aphanitic. .andesites
in northern porelon of studyarea
pinch out to the south againnst~rhyclite
flows and minor inzerbedded ssh-flov-.fuff and:-sandstone;
A-L Peak Tuff-flow-banded member(7): 32-30 m.y. (2000-2500 ft ,
600-760 m ) ASH F L O W TUFF: Rhyolite. multiple-flow, crys.-poar,
densely welded. tuff. Light grayfo grayish-red purple:
propfee often lineated; llegahreccia member consists of large blocks
of older volcanic rockswhich fell inro the A-L Peak Tllff during
cavldran collapse. Hod. crye.-rich member locally preserved
nasr stratigraphic top. Erupted from the Sawmill CnayOn csuldrou.
U n i t of Bardy Ridge:(500 ft , 152 m ) RHYOLITE ULVAS. ASH-FLOW
TIFFS, ANDESIIE: Massive, aphanitic, &ire to pink flow-banded
rhyolite lavae characterized by ovoid spherulite-likebodies
(6 in.to 1 ft in d i m . ) . Lavas intrude and overlie-poorly uelded,
lithic-rich mffs. locally underlainby aphanizie, green, sndesiresl
Rells Mesa Tuff: 32 m y . (900 ft , 275 m ) ASH-FLUJ TUFFS: rhyolite,
multiple-flow. simple cooling unit
of densely velded. cry*.-rich.
qtr.-rich. 2 felds.. massive tuffs. Erupted from North Baldy cauldron.
*
. Figure 5: Stratigraphicsectionofthethesisarea.
Graphic
section and description modified a f t e r Chapin and others (1978).
Thicknesses are exposed thicknesses and may n o t r e f l e c t t r u e
thicknesses of poorly exposed units.
12
e-
H e l l s Mesa T u f f
i n t h es t u d ya r e a
The o l d e s t T e r t i a r y u n i t e x p o s e d
i s t h e H e l l s Mesa T u f f T
. h eH e l l s
Mesa i s c o r r e l a t i v ew i t h
t h el o w e r ,c r y s t a l - r i c h ,q u a r t z - r i c h ,p o r t i o no fT o n k i n g ' s
( 1 9 5 7 ) H e l l s Mesa
member
Datil Formation.
o ft h e
( 1 9 7 2 )c a l l e dt h i su n i tt h et u f fo fG o a tS p r i n g s
t h eH e l l s
Mesa T u f f :
1) 3 0 . 6 + / - 2.8
1963); 2 )3 2 . 1
t h eB e a rM o u n t a i n s( W e b e ra n dB a s s e t ,
.
m.y.
.
.
.,9.
G a l l i n a s M o u n t a i n s ;a n d
f r o mt h e
f r o mt h eJ o y i t a
Member o f
K-Ar d a t e sh a v eb e e n
M e s aF o r m a t i o nT
. hree
o b t a i n e df o rt h eH e l l s
Brown
..
..
m.y.
from
+/- 5
.
..
,
3 ) 3 2 . 4 + / - 1 . 5 m.y.
H i l l s (Burke a n d o t h e r s , 1963).
The H e l l s
Mesa T u f f w a s e r u p t e df r o mt h eN o r t hB a l d yc a u l d r o n( C h a p i n
1 9 7 8 ) , a remnant o f which i s p r e s e r v e d i n t h e
a n do t h e r s ,
( 1 9 7 4 ) e s t i m a t e dt h e
c e n t r aM
l a g d a l e n aM o u n t a i n sK
. rewedl
a s much a s 3850 f e e tt h i c k .
c a u l d r o nf a c i e st ob e
o u t f l o wf a c i e s ,h o w e v e r ,
. .
The
i s g e n e r a l l y 400 t o 6 0 0 f e e t t h i c k .
a r e a are w i t h i n
. A l l o f - t h ee x p o s u r e si nt h i s .s t u d y
t h ep r o p o s e dN o r t hB a l d yc a u l d r o n( C h a p i na n do t h e r s ,
1978).
I n t h en o r t h e a s tp o r t i o no ft h es t u d ya r e a ,t h eH e l l s
Mesa
T u f fc r o p so u ti n
R.3W.I
a n e a s t - t r e n d i n gb a n d( s e c s .
w i t h n oe x p o s e db a s e .
31, 32, T.3S.,
I n t h i sa r e a ,t h el a c ko f
a
c o n s i s t e n td i pm a k e st h i c k n e s s . e s t i m a t e su n r e l i a b l e .T h i s
*
o u t c r o pb e l t
i s t h es o u t h e r ne n do f
f a c i e sH e l l s
Mesa Tuff t h a t i s c o n t i n u o u s l y e x p o s e d t o t h e
a r e m n a n to fc a u l d r o n
c a u l d r o nm a r g i no nN o r t hB a l d y( K r e w e d l ,
1978).
I tnh iasr e at ,hH
eells
1974; B l a k e s t a d ,
Mesa T u f f i s t y p i c a l l y
13
a l i g h t - g r a yt ob u f fc o l o r ,e x h i b i t sc o l u m n a r
b l e a c h e dt o
j o i n t i n g ,w e a t h e r st ob l o c k yr u b b l e ,a n df o r m ss t e e ps l o p e s .
I ns e v e r a lp l a c e s ,t h i n ,s a n d yl a y e r s ,d e r i v e df r o mt h eH e l l s
i n t h et u f f .T h e s e
Mesa T u f f ,a r ei n t e r b e d d e d
i n lateralextentand
arelimited
i nt h i c k n e s s .
sand l a y e r s
do n o te x c e e dt h r e e. . i n c h e s
i s i n c o n t a c tt ot h es o u t h
The H e l l s Mesa T u f f
an
w i t ht h ey o u n g e rU n i to fS i x m i l eC a n y o na l o n g
e a s t - t r e n d i n gs t r u c t u r et h a t
i s i n t e r p r e t e dt ob et h em 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 n .
.On t h e .e a s t .f l a n ko f
...
T u f fc r o p so u t
T.hS.,
R.3W.I
. . H a r d yR i d g et,h eH e l 1 . s
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( s e c .
a n d i s a minimum
of
Mesa
19,
600 f e e tt h i c k .H e r e ,
it
i s l i g h tg r a yt op a l er e dp u r p l e ,f o r m ss t e e ps l o p e s ,a n d
w e a t h e r s t o b l o c k yt a l u s
0
t h a t f o r m se x t e n s i v et a l u ss l o p e s .
N e a rt h es o u t h e r ne n do ft h i so u t c r o pa r e a ,i n t e n s e
a l t e r a t i o na n db l e a c h i n go ft h e
..
.
. .
H e l l s Mesa T u f fh a so b s c u r e d
a l 1 , p h e n o c r y s t s i n . h a ns dp e c i m e en x, c e q
pu
t a rg
t zr a i na sn d
c o p p e r yb i o t i t e .T h i s . a l t e r t i t i o n
-may b e r e l a t e d t o
b o d yo fr h y o l i t et h a ti n t r u d e st h eH e l l s
a large
Mesa T u f fa b o u t
1000
f e e ts o u t ho ft h es o u t h e r nb o u n d a r yo ft h es t u d ya r e a .
50 t o 1 0 0 f e e t o f
On t h ew e s ts i d eo fH a r d yR i d g e ,
u p p e r m o s t . X e l l s Mesa T u f f i s e x p o s e da l o n g
a major
n o r t h - t r e n d i n gr a n g ef a u l t ;r e c o n n a i s s a n c ei n d i c a t e st h a t
t h i c k e rs e c t i o n so ft h eH e l l s
i m m e d i a t e l yt ot h es o u t h .T h e s e
e
M e s aT u f f
Mesa T u f fa r ee x p o s e d
small e x p o s u r e so fH e l l s
are v e r y similar t o t h o s e
o nt h ee a s t .
Brown ( 1 9 7 2 1 C
, hamberlin
( 1 9 7 4 1 , a n dS p r a d l i n
( 1 9 7 4 ) h a v ed e s c r i b e dt h ep e t r o g r a p h i cc h a r a c t e r i s t i c so ft h e
H e l l s Mesa T u f f i n a r e a sw h e r e
it h a sn o tb e e np e r v a s i v e l y
a l t e r e d ;t h er e a d e rs e e k i n gd e t a i l e dp e t r o g r a p h i ci n f o r m a t i o n
i s r e f e r r e d - t ot h e i r
work.
The H e l l s Mesa T u f fi nt h e. s t u d y
45 t o 55
a r e a i s c o n s i s t e n t l yc r y s t a l - r i c h ,a n dc o n t a i n s
p e r c e n tp 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 ,a n d
biotite.
2 t o 3 mm,
The a v e r a g ep h e n o c r y s ts i z er a n g e sf r o m
a l t h o u g hq u a r t zg r a i n s
commonly r e a c h
6 mm i n d i a m e t e r T
. he
p l k g f o c l a s e p h e n o c - r r y s t s :.ar.e h i g h l y ’ ~ t i l t . e r e d a n d i n h a n d
s p e c i m e na p p e a re i t h e rc h a l k y
o r a s s k e l e t a lr e m n a n t s .
Pumice i s i n c o n s p i c u o u si nm o s to u t c r o p s .
I n t h i ns e c t i o n ,t h es a n i d i n e
t o s e r i c i t ea n d / o rr e p l a c e d
u s u a l l yc o m p l e t e l ya l t e r e d
is partiallyaltered
by c a l c i t e .P l a g i o c l a s e
is
a n d p l u c k e df r o mt h es l i d ed u r i n g
t h i ns e c t i o np r e p a r a t i o n ;r e m n a n t so fs e r i c i t ea n dc l a y
as l a r g e ,
m i n e r a l so c c a s i o n a l l yr e m a i n .- Q u a r t zo c c u r s
r o u n d e dg r a i n st h a ta r eo f t e n
l a t h - s h a p e dc r y s t a l st h a t
S m a l lm a g n e t i t eg r a i n s( 0 . 2
embayed.
B i o t i t eo c c u r s
as
are p a r t i a l l y a l t e r e d t o h e m a t i t e .
mm) a r ef o u n dt h r o u g h o u tt h e
r o c k ,b u tc o m p r i s el e s st h a no n ep e r c e n t .
U n i t o fH a r d yR i d g e
I
A t h i c ks e q u e n c eo fr h y o l i t e
l a v a s , p o o r l yw e l d e d
a s h - f l o wt u f f s ,a n da n d e s i t e sw i t ha s s o c i a t e dv 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 so v e r l i et h eH e l l s
MesaTuff
in the
e
is
s o u t h w e s t e r np o r t i o no ft h es t u d ya r e a .T h i ss e q u e n c e
i n f o r m a l l y named t h eu n i to fH a r d yR i d g ef o rg o o de x p o s u r e s
o nt h eh i g hr i d g ew e s to fS a w m i l lC a n y o n .
The u n i tc r o p so u t
west, b u tn o
e x t e n s i v e l ya l o n gH a r d yR i d g ea n dt ot h e
or east.
o u t c r o p sa r ef o u n dt ot h en o r t h
The c o n t a c t
r e l a t i o n s h i p sb e t w e e ns u b u n i t sw i t h i nt h eu n i to fH a r d yR i d g e
a r eu s u a l l yo b s c u r e db yt a l u sa n d
no m a r k e rh o r i z o n sw e r e
f o u n d .T h e s ep r o b l e m sh i n d e rt h er e c o g n i t i o no ff a u l t sa n d
d e c r e a s et h ea c c u r a c yo ft h i c k n e s se s t i m a t e s .R h y o l i t e
lavas, ~ash-.flow tuffs .-with. minor. ass.ociat.ed. .lavas ,..and
a n d e s i t e sw i t hv 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 sw e r em a p p e d
separately.
A minimum t h i c k n e s s o f 800 f e e t was e s t i m a t e d
f r o mc r o s ss e c t i o n
(A-A').
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
The b a s a lp o r t i o no ft h e
i s c o m p o s e do fa n d e s i t ef l o w sa n d
u n i to fH a r d yR i d g e
a s s o c i a t e dv 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 .
o v e r l i et h eH e l l s
a n o r t h - t r e n d i n gb a n d
Mesa a n d c r o p o u t i n
i n two s m a l l a r e a s o n t h e
o nt h ew e s ts i d eo fH a r d yR i d g ea n d
e a s ts i d e( s e c .
19, T.hS.,
R.3W.I.
M o s to ft h e s ea n d e s i t e s
c o n t a i n 5 t o 20 p e r c e n to x i d i z e dp y r o x e n e
s m a l l e ra m o u n t s
They d i r e c t l y
( ? ) phenocrysts;
o f n o n p o r p h y r i t i ca n d e s i t ea r ea l s op r e s e n t .
S a n d s t o n e sw i t hm i n o rc o n g l o m e r a t ei n t e r v a l so c c u rl o c a l l yo n
topoftheandesite.
D e n s e ,l i g h t - g r a yt om e d i u m - g r a ya n d e s i t e sw i t ha n
a
a p h a n i t i cg r o u n d m a s sc o n t a i n i n g
5 t o 2 0 p e r c e n to x i d i z e d
.
16
p y r o x e n ep h e n o c r y s t s ,
lavas.
2 t o 3 mm l o n g , a r e t h e
m o s ta b u n d a n t
Where v e s i c u l a r a, m y g d a l o i d so fl i g h t - g r e e n . c l a y
m l n e r a l sa n dc a l c i t ea r e
common.
D e n s e ,a p h a n i t i c ,d a r k - g r a yt og r a y i s h - g r e e n
a n d e s i t e s ,w i t hn ov i s i b l ep h e n o c r y s t s ,a r ef o u n dd i r e c t l y
a b o v et h eH e l l s
Mesaon
t h ew e s ts i d eo fH a r d yR i d g e ,i nt h e
e x t r e m es o u t h e r np o r t i o no ft h e
map a r e a .
T h e yo c c u r
I n hand
s p o r a d i c a l l ya l o n gt h ea n d e s i t eo u t c r o pb e l t .
s m a l l , 0.5
s p e c i m e n , some o ft h es a m p l e sc o n t a i n
green -blotches.
In.. . . t h i n . s e c t i o n,.,mo.st
:
v e s i c l ef i l l i n g s .
microlites,
0.2
ovoid,
0.f.
these are. seen to
(?I;
b ec h l o r i t ea n de p i d o t ea f t e ro l i v i n e
mm,
o t h e r s may b e
T h eg r o u n d m a s sc o n s i s t so p
f lagioclase
mm l o n g , p a r t i a l l y o b s c u r e d
by a c l o u d y ,
b r o w na l t e r a t i o np r o d u c t .T h i sm a t e r i a lh a sm o d e r a t e
b i r e f r i n g e n c ea n d
i s p r o b a b l y a c l a ym i n e r a l .E u h e d r a lt o
r o u n d e dm a g n e t i t eg r a i n s ,
0.2
mm i n d i a m e t e r ,
make u p 10 t o
1 5 p e r c e n to ft h er o c k .
a n d s e d i m e n t so nb o t h
a s h - f l o wt u f f sO v e r l y i n gt h ea n d e s i t e s
s i d e so fH a r d yR i d g ea r e
a s e r i e s o fp o o r l yw e l d e dt u f f sw i t h
lavas.
m i n o ra s s o c i a t e dp y r o c l a s t i cb r e c c i a sa n dr h y o l i t e
r e c t a n g u l a ro u t c r o po ft u f f s ,l a v a s ,a n db r e c c i a
t h es o u t h e a s t e r nc o r n e ro fs e c t i o n
i s f o u n di n
1 9 , T.hS.,
R.3W.
The
c o n t a c t s a t t h i s e x p o s u r es u g g e s tt h a tt h e s er o c k sw e r e
d e p o s i t e di n
a s c a l l o p , or c h a n n e l ,d e v e l o p e d
u n d e r l y i n gH e l l s
Mesa.
A c r o s ss e c t i o n
(X-X’)
on t h e
indicates a
A
I
minimum t h i c k n e s so f
4 0 0 f e e t .S i m i l a rr e l a t i o n s h i p sb e t w e e n
thistuffintervalandtheHells
t h es o u t h
Mesa a r e s e e n e x t e n s i v e l y t o
(S. R o t h , 1 9 7 8 , o r a l commun.).
More
e x p o s u r e so fc o r r e l a t i v et u f f sa r ef o u n d
f l a n k so fH a r d yR i d g e .
T.hS.,
R.3W.1,
on t h e c r e s t a n d
A t o n e. l o c a l i t y
thetuffs
extensive
(SW
114,
'set.
19,
a r e o v e r l a i nb yy o u n g e rr h y o l i t e
l a v a s o ft h eH a r d yR i d g eG r o u p .
T h ea s h - f l o wt u f fu n i tw e a t h e r st ob l o c k yt a l u s
t h a tf o r m se x t e n s i v et a l u ss l o p e s .
The t u f f sv a r yi nc o l o r
from:Tinkish .gray to. very pale
blue ,.-.a.r:e~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 da r e
commonly
pumice
and
l i t h i cr i c h .
One
p r o m i n e n tt u f ft h a tc r o p so u to nt h ee a s tf l a n ko fH a r d y
R i d g e ,a n di nt h es o u t h e a s tc o r n e r
m o r et h a n
of s e c t i o n 19, c o n t a i n s
50 p e r c e n tl i g h t - c o l o r e d d, i s c - s h a p e dp u m i c e .
In
m o s to u t c r o p s ,l i t h i cf r a g m e n t sa r ep r e d o m i n a n t l ya n g u l a r
f r a g m e n t s of a p h a n i t i c a n d e s i t e t h a t a v e r a g e
1 t o 2 em i n
d i a m e t e r .O c c a s i o n a l l y ,c l a s t so ft h eu n d e r l y i n gH e l l s
T u f f ,r a n g i n g
tuffs.
Mesa
i n s i z e -f r o m 3 cm t o 30 cm c a nb ef o u n d
i n the
19, a
I n t h ee x t r e m es o u t h w e s t e r nc o r n e ro fs e c t i o n
p y r o c l a s t i cb r e c c i a( F i s h e r ,1 9 6 0 )c r o p so u tw h i c hc o n t a i n s
m o r et h a n
50 p e r c e n t l i t h i c f r a g m e n t s t h a t e x h i b i t
p r e f e r r e da l i g n m e n t .
The l i t h i cf r a g m e n t sa r ef l o wb a n d e d
r h y o l i t ea n da n d e s i t et h a ta v e r a g e
may b e a s l a r g e a s 1 5 cm.
2 i o 3 cm i n d i a m e t e r , b u t
A small a m o u n to fc r y s t a lp o o r
lava i s a s s o c i a t e dw i t ht h et u f f si ns e c t i o n
e
a
t o have a v e r yl i m i t e de x t e n t .
1 9 b u ta p p e a r s
18
-
10 t o 15
The t u f f s u s u a l l y c o n t a i n l e s s t h a n
p e r c e n tp h e n o c r y s t sw h i c hc o n s i s t
o f a p p r o x i m a t e l yt w ot i m e s
a s much q u a r t z a s s a n i d i n ea n d
a small a m o u n to fp l a g i o c l a s e .
T h ef e l d s p a r sa r ep a r t i a l l yt oc o m p l e t e l ya l t e r e dt oc l a y s
i s o f t e nr e p l a c e db yc a l c i t e .Q u a r t . 2g r a i n s
a n dt h ep u m i c e
mm i nd i a m e t e ra n da r eu s u a l l yr o u n d e da n d
a v e r a g ea b o u t0 . 7 5
embayedT
. r a c ea m o u n t so fm a g n e t i t e p
, a r t i a l l yo x i d i z e dt o
h e m a t i t e , are p r e s e n t .
rhyoL&te--,lavas
o v e r l i et h e
in
t u f f s a n da n d e s i t ec r o p so u te x t e n s i v e l y
s e c t i o n 2 3 a n d 24 (T.kS.,
t ot h es o u t h
lavas t h a t l o c a l l y
A s e q u - e n c - e . ~o fr h y o l i t i c
(S.
R.4W.I
a n do n
H a r d y R i d g eM
. apping
1978, o r a l commun.) i n d i c a t e s t h a t
Roth,
s i m i l a r l a v a s b o t hi n t r u d ea n do v e r l i et h eH e l l s
Mesa T u f f
a n dt h el o w e rs u b u n i t so ft h e
u n i t ofHardyRidge.The
c o n t a c t sb e t w e e nt h et u f fa n d
l a v a u n i t s onHardyRidge
a n d ~a c r u d e r a d i a l
a l . s ob ep a r t l y~ i n t s u s i v e .S t e e pd i p s
pattern of dipsintheoutcroplocate~d
i n d i c a t ea ni n t r u s i v e
minimum a g e o f t h e s e
dome.
The
may
in
s e c t i o n 24 may
o n l yc o n s t r a i n to nt h e
lavas i s t h ef a c tt h a tt h et u f fo f
L e m i t a rM o u n t a i n so v e r l i e st h el a v a so nH a r d yR i d g ea n dt o
a t a p p r o x i m a t e l y 2.7 m.y.
t h ew e s ta n dh a sb e e nd a t e d
(R.C.
C h a m b e r l i n ,u n p u b l i s h e dd a t a ) .
I ng e n e r a l ,t h e
lavas a r el i g h tc o l o r e d ,c r y s t a l
p o o r ,f l o wb a n d e d ,a n ds h o wv a r y i n gd e g r e e s
d e v i t r i f i c a t i o na n dh y d r o t h e r m a la l t e r a t i o n .
of
A poorly
19
u n d e r s t o o db u td i s t i n c t i v ea n dw i d e s p r e a dt e x t u r a lf e a t u r eo f
t h e s e l a v a s i s t h eo c c u r r e n c eo fo v e r l a p p i n g ,s p h e r i c a lt o
are l o c a l i z e da l o n gf o l i a t i o n
s e m i - s p h e r i c a lb o d i e sw h i c h
similar t or o u n d e dc o b b l e s
p l a n e s .I no u t c r o p ,t h e ya p p e a r
f r a m 1 cm t o 0.6 m ; t h e a v e r a g e s i z e
a n dr a n g ei ns i z e
t o 20 cm ( f i g u r e 6 ) .
I n some a r e a s ,i n d i v i d u a lf l o wp l a n e s
l a v a t h r o u g ht h eb o d i e sw i t hn o
c a nb et r a c e df r o mt h e
o b v i o u sd e f l e c t i o n .
is 10
When s e p a r a t e df r o mt h ee n c l o s i n g
r h y o l i t e ,t h e yo f t e na p p e a r
a s c i r c u l a rl a y e r s ,l e s st h a no n e
on-%.up o fo n ea n o t . h e r. w i t h
m i l l y m e t e ri nt . h i - c k n e % s ,s t a c k e d
s u c c e s s i v e l y smaller b a d i i .
No l i t h o l o g i c o r t e x t u r a l
d i f f e r e n c eh a sb e e nf o u n db e t w e e nt h eb o d i e sa n dt h e
s u r r o u n d i n g lavas.
t h i c k )s u r r o u n d s
H o w e v e r , a t h i nr i n d
some b o d i e s O
. c c a s i o n a l l y t, h e s eb o d i e s
show i n t e r n a l c o n c e n t r i c
q u i t ed i s t i n c t
arealso
( l e s s t h a n 1 mm
or r a d i a l s t r u c t u r e s , b u t t h e y a r e
i n a p p e a r a n c ef r o ms p h e r u l i t e s .
The l a t t e r
common i n t h e l a v a b u t u s u a l l y d o n o t e x c e e d
1 cm i n
diameter.
I nt h i ns e c t i o n ,t h el a v a sa r ev e r yc r y s t a lp o o r .
U s u a l l y ,t h eo n l yp h e n o c r y s t sa r et r a c ea m o u n t so fi r r e g u l a r
m a g n e t i t eg r a i n s
r o u n d e dg r a i n
3 to
4
a
mm i nd i a m e t e r .I no n es a m p l e ,
( 1 m m ) o fl e u c o x e n e
was n o t e d T
. h eg r o u n d m a s s
of t h e lavas 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 fa l k a l if e l d s p a ra n ds i l i c am i n e r a l s .I n
samples, 3 t o
4
some
mm c a v i t i e s a r e f i l l e d w i t h i n w a r d g r o w i n g
q u a r t zc r y s t a l s .F l o wp l a n e s
a r e d e f i n e db yz o n e s
t h i c k f i l l e d w i t hq u a r t zc r y s t a l s .
1 t o 2 mm
Figure
6.
S p h e r u l i t e - l i k eb o d i e sa s s o c i a t e dw i t ht h e
r h y o l i t e lavas o f t h eu n i t
o f H a r d yR i d g e .
21
T h eu n i to fH a r d yR i d g e
is a heterolithic
a c c u m u l a t i o no fa n d e s i t e s ,s e d i m e n t s ,a s h - f l o wt u f f s ,a n d
r h y o l i t e l a v a s w h i c hp r o b a b l yf i l l e dt h et o p o g r a p h i c
d e p r e s s i o nc a u s e db yc o l l a p s eo ft h eN o r t hB a l d yc a u l d r o n
~.
1978).
( C h a p i na n do t h e r s ,
'
V o l u m i n o u si n t r u s i v er h y o l i t e s ,
l i t h i c - r i c ht u f f s ,a n dp y r o c l a s t i cb r e c c i a sa r es u g g e s t i v eo f
l o c a lv e n ta r e a s ,p o s s i b l ya l o n gr i n gf r a c t u r e s .F u r t h e r
mapping t o t h e s o u t h a n d w e s t
.:.
A-L
P e a kT u f f
The A-L
( C h a p i na n d
e
w i l l testthishypothesis.
P e a kT u f f ,
a c o m p o s i t ea s h - f l o wt u f fs h e e t
Deal, 1 9 7 6 1 , c r o p so u te x t e n s i v e l y
( 1 9 7 3 ) a n d Deal
e a s t - c e n t r a lp o r t i o no ft h es t u d ya r e a .D e a l
and
Rhodes
( 1 9 7 4 ) o r i g i n a l l y named t hteu f ftsh e
t h e name h a ss i n c eb e e nm o d i f i e dt o
1978).
The A-L
P e a kT u f f
" b a n d e dr h y o l i t e "o fL o u g h l i na n d
m i d d l e 6 5 f e e to fT o n k i n g ' s
t u f fo f
A-L
Peak
P e a kT u f f
A-L
P e a kT u f f( C h a p i na n d
i s c o r r e l a t l v ew i t ht h e
Koschmann ( 1 9 4 2 ) ,t h e
( 1 9 5 7 ) H e l l s Mesa Member, a n dt h e
Bear S p r i n g s( B r o w n 1, 9 7 2 ) .
(1974) d a t e dt h e
A-L
i n t h en o r t h e r nS a nM a t e oM o u n t a i n s ;
R h y o l i t ef o re x p o s u r e s
others,
i n the
E.I. S m i t ha n do t h e r s
a t 31.8
+ / - 1 . 7 m.y.
b yt h e
f i s s i o n t r a c k method.
T h e r e a r e t h r e eg e n e r a l l yr e c o g n i z e dm e m b e r so ft h e
A-L
P e a kT u f f - - t h eb a s a lg r a y - m a s s i v e ,t h ef l o w - b a n d e d ,a n d
t h eu p p e rp i n n a c l e sm e m b e r s .
A l l t h r e em e m b e r sa r e
m o d e r a t e l yt od e n s e l yw e l d e d ,c r y s t a l - p o o r ,a s h - f l o wt u f f s
22
w i t h 4 t o1 0p e r c e n tp h e n o c r y s t s .S a n i d i n e ,
1 t o 2 p e r c e n t ,a r et h em o s t
a n d s m a l l r o u n d e dq u a r t z ,
1978).
a l s of o u n d( C h a p i na n do t h e r s ,
T h el o w e rl i g h t - g r a y ,
member g r a d e su p w a r di n t ot h eg r a y
or r e d d i s h - b r o w nf l o w - b a n d e d
l i n e a t e dp u m i c e .
common
o f p l a g i o c l a s ea n db i o t i t ea r e
p h e n o c r y s t s ;m i n o ra m o u n t s
p u m i c e - p o o r . ,g r a y - m a s s i v e
3 t o 8 percent,
member w h i c hh a sa b u n d a n t ,
I n t h eL e m i t a ra n dB e a rM o u n t a i n s
of b a s a l t i c - a n d e s i t e
a tongue
l a v a s s e p a r a t e st h eu p p e rp i n n a c l e s
member f r o mt h ef l o w - b a n d e d
1978).
member ( C h a p i na n do t h e r s ,
.'The* 3 P n n ; a c l e s m e m b e r , t y p i c a l l y h a s . ' . a b x n d a n t n o n - l i n e a f e d
p u m i c ea n d
i s named f o r i t s c h a r a c t e r i s t i c o u t c r o p p a t t e r n .
The t h i c k n e s s o f t h e A-L
P e a kT u f f
S o c o r r o - M a g d a l e n aa r e ag e n e r a l l yv a r i e sf r o m
1978).
( C h a p i na n do t h e r s ,
f r o m a c r o s ss e c t i o n
(B-B')
i n the
0 t o 700 f e e t
I n t h i ss t u d ya r e a ,
an estimate
i n d i c a t e s a minimum t h i c k n e s s of
2000 t o 2 5 0 0 f e e t .T h i st h i c k n e s s
may b ei n a c c u r a t ed u et o :
1) n o n - u n i f o r m . . o r i e n t a t i o n o f f o l i a t i o n i n t h e A-L
l a r g e l yd u et os e c o n d a r yf o l d i n g ;
Peak,
2 ) l a c k of a n e x p o s e db a s e ;
and 3 ) d i f f i c u l t y i n r e c o g n i z i n g f a u l t s w i t h i n
a thickunit
t h a ts h o w sn ow e l l - d e f i n e dv e r t i c a lz o n a t i o na n dh a sn o
m a r k e rh o r i z o n s .
T h es o u r c ea r e a s
u n d e r s t o o d a t t h i st i m e .D e a l
f o r t h e A-L
P e a kT u f fa r ep o o r l y
( 1 9 7 3 ) i n t e r p r e t e d a total^
t h i c k n e s s o f a b o u t 2000 f e e t for a l l t h r e e memberson
Peak t o r e f l e c t
A-L
Peak i n t h e
a s o u r c ec a u l d r o nf o r
A-L
a t l e a s t some o f t h e
S a nM a t e oM o u n t a i n sH
. owever,
all three
23
0
members a r e r e c o g n i z e dw i t h i nt h i ss e c t i o n( C h a p i na n dD e a l ,
19761, a n dt h e r e
in
i s a g r e a td e a lo fv e r t i c a lv a r i a b i l i t y
d e g r e eo fw e l d i n g ,d e v e l o p m e n to fl i n e a t i o n ,a n dp u m i c e
content.
1 7 0 0 f e e t was s e e n
A v e r y s i m i l a r s e c t i o no fa b o u t
i nr e c o n n a i s s a n c eo f
D e v i l s B a c k b o n e ,s o u t ho ft h eM a g d a l e n a
i s c l e a r l yn oc a u l d r o np r e s e n t .
M o u n t a i n s ,w h e r et h e r e
1 9 7 8 , o r a l commun.) t h a tn o n e
C u r r e n tt h o u g h t sa r e( C h a p i n ,
o ft h et h r e em e m b e r sw e r ee r u p t e df r o mt h e
Mount W i t h i n g t o n
c a u l d r o na n dt h a tt h eg r a y - m a s s i v ea n df l o w - b a n d e dm e m b e r s
.
T h e' e x t e n s i v ee x p o s u r e so ft h e
t h es t u d y
are:
t h e A-L
o f t h e knownmembersof
t h eo u t f l o wf a c i e s .
P e a kT u f f
i n the
It c a n n o tb ed e f i n i t e l yc o r r e l a t e d
S a w m i lC
l a n y o nc a u l d r o n .
w i t ha n y
A-L
a c a u l d r o nf a c i e so ft h e
s t u d ya r e aa r eb e l i e v e dt or e p r e s e n t
e
4).
. - - - ' . : . w e r ee r u p t e d . : . f r o m. t . h eM a g . d a l e n a ~c a u l d r o n( f i g .
P e a kr e c o g n i z e d
The c h a r a c t e r i s t i c so ft h e
a r e a t h a t a r e s u g g e s t i v eo f
1) a n o m a l o u st h i c k n e s s ;
c r y s t a lc o n t e n t ,p u m i c e ,
A-L P e a ki n
a c a u l d r o nf a c i e st u f f
2 ) u n i f o r mn a t u r ei nt e r m so f
a n d d e g r e eo fw e l d i n g ;
p r e s e n c eo fm e g a b r e c c i ab l o c k s
in
and 3 )
i n thevicinityofthe
p r o p o s e dc a u l d r o nm a r g i n .
I nt h es t u d ya r e a ,t h e
e x p o s e da l o n gt h e
A-L
P e a kT u f f
is well
E a s t a n d West F o r k so fS a w m i l lC a n y o n .
Good e x p o s u r e s c o n t i n u e t o t h e s o u t h w h e r e t h e
A-L
a p p e a r st op i n c ho u ta g a i n s tr h y o l i t i cl a v a sw h i c h
Peak
may b e
1978, o r a l
e q u i v a l e n tt ot h eu n i to fH a r d yR i d g e
(S.
Roth,
commun.).
A-L
P e a k i s o v e r l a i nb y
To t h en o r t ha n de a s t ,t h e
24
a t h i c ks e c t i o no ft h eu n i to fS i x m i l e
t h o u g h tt ob ec a u l d r o n
f i l l o ft h eS a w m i l l
1978).
( C h a p i na n do t h e r s ,
is
Canyonwhich
Canyon c a u l d r o n
E x t e n s i v ee x p o s u r e so fb o t h
A-L
P e a ka n dt h eu n i to fS i x m i l eC a n y o na r ef o u n dt ot h ee a s ti n
(D. P e t t y , 1 9 7 8 , o r a l commun.).
t h e Ryan H i l l c a n y o na r e a
t h ew e s t ,t h e
A-L
To
P e a k i s i n a p p a r e n tf a u l tc o n t a c tw i t ht h e
H e l l s M e s aT u f fa n dt h eu n i to fH a r d yR i d g e .
This apparent
f a u l t may b et h ec a u l d r o 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 na n d
w i l l b ed i s c u s s e dl a t e r .
The&-&.Peak
a-%.ea:i s d e n s e l y - a e P d e d
in^ t h e s t u d y
It i s c h a r a c t e r i z e db y
w i t hn oo b v i o u sc o o l i n gb r e a k s .
a paucity
p r o m i n e n tp u m i c el e n s e s ,l o wp h e n o c r y s tc o n t e n ta n d
o fl i t h i cf r a g m e n t s .
Much of t h e A-L
p u m i c ef r a g m e n t s .L o c a l l yt h eu p p e r m o s t
P e a ks h o w s
a g r e a td e a l
a strong
and oftenlineated
f o l i a t i o nd e f i n e db yh i g h l yf l a t t e n e d
A-L
P e a kp o s s e s s e s
0 t o 300 f e e to ft h e
of v e r t i c a l v a r i a b i l i t y i n
p h e n o c r y s tc o n t e n t ,d e g r e eo fw e l d i n g ,a n dl i t h i cc o n t e n t ;
t h i su p p e ri n t e r v a l
s t u d ya r e a ,t h eu p p e r
f r o mt h el o w e r
P e t t y , S.
A-L
Roth,
In the
w i l l b ed i s c u s s e ds e p a r a t e l y .
A-L
P e a kT u f f
Peak.Mapping
was n o t mapped s e p a r a t e l y
t ot h es o u t ha n de a s t
1978, o r a l commun.)
similar i na p p e a r a n c et ot h e
t h el o w e r
A-L
l o w e r A-L
P e a kT u f fT h el o w e r
A-L
(D.
i n d i c a t e st h a tt u f f s
P e a k may b es e p a r a t e df r o m
P e a kb yt h i n ,r h y o l i t e
A-L
lava flows.
P e a k i s w e l le x p o s e d
t h r o u g h o u tt h es o u t h - c e n t r a lp o r t i o no ft h es t u d y
area,
25
*
e s p e c i a l l ya l o n gt h e
W e s tF o r ko fS a w m i l lC a n y o nw h e r e
it
f o r m ss t e e ps l o p e sa n dc l i f f s .O u t c r o p sa r eu s u a l l yc u tb y
o n e or more c l o s e l y s p a c e d j o i n t s e t s , o f t e n n o r m a l t o t h e
f o l i a t i o n ;j o i n ts u r f a c e sa n df r a c t u r e sa r eu b i q u i t o u s l y
i r o n o x i d ea n db l a c km a n g a n e s e
s t a i n e dw i t hr e d d i s h - b r o w n
s m a l l p l a t ys l a b s .
o x i d e s .O u t c r o p st y p i c a l l yw e a t h e rt o
The l o w e r A-L
color.
Peak i s u s u a l l y l i g h t g r a y t o g r a y i s h r e d i n
i s g r a y i s hr e d - p u r p l e
I nm o s st p e c i m e n s t, h ep u m i c e
a n dc o n t r a s t sw i t h
m a t r i x a n dp u m i c ea r es h a d e s
p l a c e sb o t h
i n some
a l i g h t - g r a ym a t r i x ;h o w e v e r ,
o f r e d - p u r p l ea n d
i s n o t i m m e d i a t e l yo b v i o u s .
t h ep u m i c e
Much of t h e l o w e r
A-L
Peak T u f f p o s s e s s e s a s t r o n g
f o l i a t i o nd e f i n e db yh i g h l yf l a t t e n e da n do f t e nl i n e a t e d
p u m i c ef r a g m e n t s T
. h ep u m i c ec o n t e n tv a r i e sf r o ml e s st h a n
percenttogreaterthan
20 p e r c e n ta n da v e r a g e s
p e r c e n t .T y p i c a l l yt h ep u m i c ea r e
thick.
2
10 t o
15
2 t o 6 cm l o n g a n d 0 . 5
mm
Where t h ep u m i c ea r ep a r t i c u l a r l yl a r g e ,i n d i v i d u a l
p u m i c ef r a g m e n t s
a r e 1 t o 2 meters l o n ga n d
The l e n g t h t o t h i c k n e s s r a t i o o f t h e p u m i c e
t h a n1 0 0 .D i f f e r e n t i a lc o m p a c t i o n
1 t o 2 cm t h i c k .
i s o f t e ng r e a t e r
i s o f t e ns e e na r o u n d
p h e n o c r y s t sa n dl i t h i cf r a g m e n t s .I np l a n e sp a r a l l e lt ot h e
f o l i a t i o n ,t h ep u m i c ea r et y p i c a l l yc i g a r - s h a p e da n df i l l e d
w i t hg r a i n yv a p o r - p h a s ec r y s t a l s .I nc o n t r a s t ,
A-L
P e a ka n d
i n n o n - l i n e a t e dp o r t i o n s
i n t h eu p p e r
o f t h el o w e r
A-L
Peak,
26
I n t h i ns e c t i o n ,t h el o w e r
A-L P e a kh a s
a n average
o f 3 p e r c e n tp h e n o c r y s t s ;l o c a l l y ,t h ep h e n o c r y s tc o n t e n t
b eo n ep e r c e n t
or l e s s .
may
S a n i d i n e i s f r o mf o u rt of i v et i m e s
a s a b u n d a n t a s q u a r t za n do c c u r s
a s e u h e d r a lt os u b h e d r a l
0 . 5 t o 2.0 mm l o n g .Q u a r t z
c r y s t a l sa p p r o x i m a t e l y
1.0 mm i n
p h e n o c r y s t sa r er o u n d e da n da r eu s u a l l yl e s st h a n
in
d i a m e t e r .E u h e d r a lt or o u n d e dm a g n e t i t eg r a i n st h a tr a n g e
s i z ef r o m
l e s s t h a n 0.05
a n d as p h e n o c r y s t s .
mm t o 0 . 5 mm o c c u r i n t h eg r o u n d m a s s
One t ot w op e r c e n t
small a n d e s i t i c
: - - l - i t h i c f r a g m e n t s , . u s u a l l y l e s s t h a n 1 ..
~ c. m
. . i n
diameter, are
common i n t h el o w e r
A-L
.
P e a k .T h e s el i t h i cf r a g m e n t s ,
h o w e v e r , a r e n o te a s i l ys e e ni nh a n ds p e c i m e n .T r a c ea m o u n t s
o fp l a g i o c l a s e
a r e f o u n d i n some s a m p l e s .
vitroclastictexture
o f t h e A-L
recrystallizationofthe
The o r i g i n a l
P e a kh a sb e e nd e s t r o y e db y
m a t r i x t o a f i n e - g r a i n e di n t e r g r o w t h
i n t h i ns e c t i o nt h e
o fa l k a l if e l d s p a ra n ds i l i c am i n e r a l s ;
t u f f r e s e m b l e s a f l o wb a n d e d , l a v a .
E l o n g a t ep u m i c ef r a g m e n t sa n de l l i p s o i d a lc a v i t i e s - ~
f i l l e dw i t hq u a r t zd e f i n et h ef o l i a t i o n .
c a v i t i e sa v e r a g ea p p r o x i m a t e l y
0.05
The e l l i p s o i d a l
mm x 1 . 0 mm a n d a r e made
u po fc o a r s e l yc r y s t a l l i n ev a p o r - p h a s eq u a r t z .I n
some
s p e c i m e n s ,t h e s eq u a r t zl e n s e s
make u p a s much a s 1 5 p e r c e n t
o ft h er o c k .P u m i c ef r a g m e n t s
a r e c h a r a c t e r i z e db yv a p o r
p h a s em i n e r a l sa n dh e m a t i t es t a i n i n g .
The rims o ft h ep u m i c e
f r a g m e n t s a r e c o m p o s e do fi n t e r l o c k i n g ,i n w a r dg r o w i n g ,
q u a r t za n db l a d e da l k a l if e l d s E a rc r y s t a l s .T h e i rc e n t e r s
a
o f t e nc o n t a i na g g r e g a t e so fe u h e d r a l
c r y s t a l ss u r r o u n d e d
(0.4
mm) a l k a l if e l d s p a r
by l a r g e r a n h e d r a l q u a r t z c r y s t a l s ; t h e
alkalifeldsparcrystals
are o f t e na l t e r e dt oc l a ym i n e r a l s
a n ds t a i n e dw i t hh e m a t i t e .
P r i m a r yl a m i n a rf l o ws t r u c t u r e sa r ed e v e l o p e d
t h el o w e r
A-L
o fp r i m a r y
P e a kT u f f
i n t h es t u d ya r e a .
T h ed e v e l o p m e n t
flow s t r u c t u r e s varies l a t e r a l l ya n dv e r t i c a l l y
w i t h i nt h eu n i ta n da r eb e s to b s e r v e d
i n t h es o u t h - c e n t r a l
l a m i n a r f l o ws t r u c t u r e s
p o r t i o no ft h es t u d ya r e a .P r i m a r y
.
in
,.
~ h a i r e . ' b e e nr e c o g n i z e ' di n
many a s h - f l o w : - - t u f f s( S c h m i n k e ,a n ~ d
Swansoii, 1 9 6 7 ; W a l k e ra n dS w a n s o n ,
1 9 7 2 ; D e a l , 1973; C h a p i na n dL o w e l l ,
1968; L o w e l l a n d Chapin',
1979).
C h a p i na n d
L o w e l l ( 1 9 7 9 ) d i s c u s s ,i nd e t a i l ,t h em e c h a n i s m si n v o l v e d
a
d u r i n gd e p o s i t i o no fa s h - f l o wt u f f ;a n ds u g g e s tt h a t
t e m p e r a t u r eo fe m p l a c e m e n t
a s h - f l o wt u f f
w i l l d e t e r m i n ew h e t h e ro rn o t
an
w i l l e x h i b i tp r i m a r yf l o ws t r u c t u r e s :
If t h e t e m p e r a t u r e o f t h e g l a s s y p a r t i c l e s
i s well
a b o v et h es o f t e n i n gp o i n t ,t h ep 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 ,a n dw e l di nt h el a m i n a r
b o u n d a r yl a y e rt of o r m
a viscousfluidwiththe
r h e o l o g i c a lp r o p e r t i e so f
a r h y o l i t el a v a .
The
r e s u l t a n t t u f f w i l l h a v e a p r i m a r yf o l i a t i o na n d
l i n e a t i o na n d
may c o n t a i n l e n t i c u l e s r e p r e s e n t i n g
p o c k e t s o f g a s e st r a p p e dd u r i n gw e l d i n g .
If t h e t e m p e r a t u r e
i s b e l o wt h es o f t e n i n gp o i n t ,t h e
g l a s s y p a r t i c l e s w i l l f a i l t oa d h e r ea n dd e p o s i t i o n
w i l l o c c u r as l o o s ea s hi n
a l a m i n a r f l o wr e g i m e .I n
t h a tc a s e ,c o l l a p s ea n dw e l d i n g
may o c c u r a f t e r
d e p o s i t i o n , a s i n c r e a s i n gl o a dp r e s s u r ed e p r e s s e st h e
m i n i m u mw e l d i n gt e m p e r a t u r et o ,
or b e l o w , t h e
t e m p e r a t u r eo ft h eg l a s s yp a r t i c l e s .
The t i m e f a c t o r
i s a l s oi m p o r t a n t
i n t h a tp a r t i c l e s a t , or s l i g h t l y
a b o v e ,t h es o f t e n i n gp o i n t
may b e t o o r i g i d t o a d h e r e
i nt h el a m i n a rb o u n d a r yl a y e rb u t
may s l o w l y c o l l a p s e
a n dw e l da f t e rd e p o s i t i o n .
The r e s u l t i n gt u f f
is not
.
28
l i n e a t e da n df o l i a t i o n
i s b ys e c o n d a r yc o m p a c t i o n .
T h et u f fl a c k sc o n s p i c u o u sg a sc a v i t i e sb e c a u s em o s t
o ft h ee n t r a i n e dg a s e se s c a p e dd u r i n gw e l d i n g
( e x c e p t i o n s may e x i s t i n t h i c k , r a p i d l y d e p o s i t e d
tuffs). .
'0
F e a t u r e so ft h e
w h i c ha r ec o n s i s t e n tw i t h
flow are:
P e a kT u f f
A-L
i n t h i ss t u d ya r e a
o r s u g g e s t i v eo fp r i m a r yl a m i n a r
1) f o l i a t i o nd e f i n e d
by h i g h l yf l a t t e n e d ,l i n e a t e d
p u m i c ef r a g m e n t sa n de l o n g a t ec a v i t i e s ;
2 ) f o l d i n go ft h e
3 ) f o l i a t i o nd i p st o os t e e pt oh a v eb e e n
l i n e a t e df o l i a t i o n ;
r o r m e db ys e c o n d a r yc o m p a c t i o n .
. .
-(1979) p a i n t - a u t t h a t t h e
C h a p i n 'a n dL o w e l l
may b e a c o m p o s i t eo f
e u t a x i t i cf a b r i co fa na s h - f l o wt u f f
f o l i a t i o n p l a n e s , g a sc a v i t i e sa n dp u m i c e V
. a p o rp h a s e
c r y s t a l l i z a t i o nh a sd e s t r o y e dt h eo r i g i n a lt e x t u r e so fp u m i c e
e
i n t hl eo w e r
A-L
PeaT
k .h e r e f o r e ,
i t i s d i f f i c u ltto
tell
w h e t h e rt h el o n gr e dp u r p l eb a n d st h a td e f i n et h ee u t a x i t i c
~
f a b r i c o f t h el o w e r
A-L
P e a kr e p r e s e n tl a r g ep u m i c e
r r a g m e n t sP
. u m i c ef r a g m e n t s
may l o c a l i z ee s c a p i n gv o l a t i l e s
d u r i n gw e l d i n ga n dg i v er i s et oa c c u m u l a t i o n so fv a p o r - p h a s e
m i n e r a l st h a te x c e e dt h ed i m e n s i o n so ff l a t t e n e dp u m i c e
f r a g m e n t s .R e g a r d l e s so ft h e i ro r i g i n ,t h er e d d i s hs t r e a k s
a r ep e r v a s i v e l yl i n e a t e di nt h es t u d ya r e a .
was f o u n d t o c o n s i s t e n t l y
strike east-west,
t h e same d i r e c t i o n a s t h a tr e p o r t e db yD e a l
( 1 9 7 8 , o r a l commun.) a n dR o t h
p h e n o m e n aa n dm u s t
i n approximately
(19731, P e t t y
( 1 9 7 8 , o r a l commun.).
Such
w i d e s p r e a dd e v e l o p m e n to fl i n e a t i o nc a n n o tb e
e
The l i n e a t i o n
a local
be a p r i m a r yf e a t u r eo ft h et u f f .T h e
seen i n t h i n s e c t i o n p r o b a b l y
small q u a r t z - f i l l e d l e n s e s
r e p r e s e n tg a sc a v i t i e st h a tw e r ef o r m e d
by e s c a p i n g v o l a t i l e s
a delicate
d u r i n gw e l d i n go ft h et u f f .P r e s e r v a t i o no fs u c h
f e a t u r ea r g u e sf o rp r i m a r yw e l d i n ga n da g a i n s t
much
p o s t - e m p l a c e m e n tc o m p a c t i o n .
i n t h el o w e r
T h ef o l i a t i o n
Peak i s o f t e nf o l d e d
A-L
i s n o td e f l e c t e df r o mp a r a l l e l i s mw i t ht h e
butthelineation
o v e r a l ll i n e a t i o nd i r e c t i o n .
The f o l d sh a v ea m p l i t u d e st h a t
a f e w m i l l i m e t e r st os e v e r a lm e t e r s .S t y l e so f
r a n g ef r o m
foldinginclude-broad,opensynclinesandanticlines,.ti.ght,
i s o c l i n a lt or e c u m b e n tf o l d s ,
The l a t t e r a r e v e r y
( 1 9 7 2 )a n dC h a m b e r l i n
A-L
a n d s h a r pv - s h a p e da n t i f o r m s .
similar t of o l d sd e s c r i b e db y
(1974) i n
Brown
t h ef l o w - b a n d e d
member o ft h e
P e a k .E x c e l l e n te x p o s u r e so ft h e s ef o l d sc a nb es e e no n
t h en o r t hs i d eo fB l a c kB e a rC a n y o n
e l e v a t i o n .I n
a t a b o u tt h e8 1 0 0f o o t
many p l a c e s ,t h es t r i k ea n dd i p
of t h e A-L
P e a k v a r i e s c o n s i d e r a b l yb e t w e e no u t c r o p sl e s st h a n
a p a r t .T h i sv a r i a t i o n
a m p l i t u d e so f
3 meters
i s s u g g e s t i v eo fl a r g ef o l d sw i t h
3 t o 1 5 meters.
D e a l ( 1 9 7 3 ) r e p o r t e dl a r g ef o l d sw i t ha m p l i t u d e s
l a r g e a s 30 m e t e r s i n t h e
M o u n t a i n sa n dP e t t y
f o l d si nt h e
A-L
A-L
P e a kT u f f
as
i n t h eS a nM a t e o
( 1 9 7 8 , o r a l commun.) r e p o r t s similar
P e a k i n t h e Ryan H i l l C a n y o na r e a .A s h - f l o w
t u f f s may d e v e l o pp r i m a r y
f o l d s d u r i n gd e p o s i t i o na n d
s e c o n d a r yf o l d sd u r i n gp o s t - d e p o s i t i o n a sl l u m p i n g C
. hapin
a n dL o w e l l( 1 9 7 9 )n o t e dt h a tt h e
a x i s o fs e c o n d a r yf o l d s
are
31
Figure 7.
P h o t o m i c r o g r a p h o f A-L Peak Tuff. Intricate
folding of f o l i a t i o n a r o u n d p h e n o c r y s t i s a t t r i b u t e d t o
1.5 m m in longest dimension.
laminar flowage. Phenocryst is
30
p a r a l l e lt ot h el i n e a t i o n .U s i n gt h i sc r i t e r i a ,t h el a r g e
f o l d sd e s c r i b e d
i n t h e A-L P e a ka r es e c o n d a r y .
and l i t h i c
E v i d e n c e o f r o t a t e dp h e n o c r y s t s
i n b o t hh a n ds p e c i m e n
f r a g m e n t sc a nb es e e n
D i s t u r b e dz o n e s
a n d t h i ns e c t i o n .
-at t h e e n d s o f i n d i v i d u a l p h e n o c r y s t s
laminae f o r ma s y m m e t r i c
l i t h i cf r a g m e n t s ,w h e r ei n d i v i d u a l
f o l d sa n dc r e n u l a t i o n s ,a r e
or
a m a j o re f f e c t
of t h i s r o t a t i o n .
I n some s p e c i m e n s ,i n d i v i d u a li a m i n a ec a nb et r a c e da r o u n d
is attributedto
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 ;t h i sa l s o
' r o t . a f i o n d u r i n gl a m i n i m
'.flow' ( f i g u r e 7);
. T h e s ef o l d sa n d
c r e n u l a t i o n sh a v ea x e st h a ta r en o r m a lt ot h el i n e a t i o na n d
a r eb e l i e v e dt ob ep r i m a r yf o l d sf o r m e dd u r i n gd e p o s i t i o n
t h et u f f .
of
The f o l d i n g of p r i m a r yl i n e a t i o na n dt h e
p r e s e r v a t i o no f
small c a v i t i e s a l o n g f l o w p l a n e s s u g g e s t s
thatthetuffhadweldedpriortofolding.
t h a n 5 0 d e g r e e sa r e
D i p sc o n s i s t e n t l yg r e a t e r
common o v e rl a r g ea r e a si nt h e
A-L
Peak.
The
a p p r o x i m a t e l y 45 t o 50 d e g r e e s t o t h e
d i p sa r ep r o b a b l yd u et of o l d i n g
r e g i o n a ld i p
east; t h u st h es t e e p e r
or w e r e o r i g i n a l l y s t e e p d u e
t op r i m a r yw e l d i n go np r e - e x i s t i n gs l o p e s .
The e v i d e n c e p r e s e n t e d a b o v e s u g g e s t s t h a t t h e
l o w e r A-L
P e a k w a s d e p o s i t e d . a n dw e l d e d
l a y e r sa l o n gt h eb o t t o m s
d e v e l o p m e n t of t h e p r i m a r y
o f a s h flows.
i n laminar boundary
The f a c t t h a t
features v a r i e s l a t e r a l l y a n d
v e r t i c a l l yp r o b a b l yi n d i c a t e st h a tt h et e m p e r a t u r e s
e r u p t e da s h - f l o w s
varied.
T h i sv a r i a t i o nc o m p l i c a t e s
of t h e
is
32
A-L
c o r r e l a t i o no ft h e
Peak i n t h es t u d ya r e aw i t ht h e
c o r r e s p o n d i n go u t f l o wu n i tw h i c h
may n o t e x h i b i t p r i m a r y
w e l d i n g .F u r t h e rs t u d yo ft h ep r i m a r ya n ds e c o n d a r yw e l d i n g
featuresinthe
A-L
P e a kT u f f
w i l l b eo fv a l u eb o t h
in
c o r r e l a t i n gc a u l d r o n - .a n do u t f l o w - f a c i e st u f f sa n di nm o r e
fullyunderstandingdepositionofash-flowtuffs.
megabreccia
On t h e wes,t s i d eo fT i m b e rR i d g e( n e a rt h e
s o u t h e r nb o u n d a r y
t h eE a s ta n d
o f t h es t u d ya r e a )a n do nt h er i d g eb e t w e e n
W e s tF o r k s
.of S a w m i l. .lC a n y o n ,l a r g eo u t c r o p so f
v o l c a n i cr o c k sb e l i e v e dt ob ec o r r e l a t i v ew i t ht h eu n i to f
H a r d yR i d g ea r ec o m p l e t e l ys u r r o u n d e db yt h el o w e r
A-L
Peak
T u f f .L a r g eb l o c k so ft h e s er o c k so nt h er i d g eb e t w e e nt h e
e
f o r k so fS a w m i l lC a n y o na r ec o m p o s e do fp o o r l yt om o d e r a t e l y
l a v a s , a n dv o l c a n i c l a s t i cs e d i m e n t s .
w e l d e da s h - f l o wt u f f s ,
L a r g eb l o c k so nT i m b e rR i d g ea r ec o m p o s e do fs a n d s t o n e
o v e r l a i nb yp o o r l yw e l d e dt u f f s .
The a t t i t u d e s o f t h e t u f f s ,
l a v a s , a n ds e d i m e n t sw i t h i ni n d i v i d u a lb l o c k s
u s u a l l yn o tc o n f o r m a b l ew i t ht h es u r r o u n d i n g
b l o c k sa r e
i s v a r i a b l ea n d
A-L
P e a kT
. he
commonly s i l i c i f i e d ,f r a c t u r e d ,a n dh y d r o t h e r m a l l y
altered.
T h em o s t
common t u f f w i t h i n t h e b l o c k s
p a l er e dt og r a y i s hp i n ka n d
i s p o o r l yt om o d e r a t e l yw e l d e d .
E x c e l l e n te x p o s u r e so ft h i st u f f
inthe
is generally
West F o r k of S a w m i l lC a n y o n
c a n b ef o u n da l o n gt h e
a t a b o u tt h e
e l e v a t i o n .I nt h i ns e c t i o n ,t h et u f f sc o n t a i n
trail
8100 f o o t
10 t o 25
33
p e r c e n tp h e n o c r y s t s
and
i s a p p r o x i m a t e l yt w i c e
f r a g m e n t s .Q u a r t z
s a n i d i n ea n dp l a g i o c l a s e ;
d i a m e t e ro f
5 t o 1 0 p e r c e n t small, a n g u l a rl i t h i c
0 . k mm.
as abundant as
it i s r o u n d e da n dh a s
a n average
S a n i d i n ep h e n o c r y s t sa r ep a r t i a l l y
a l t e r e dt oc l a ym i n e r a l sa n ds e l d o me x c e e d
1 . 0 mm i n l e n g t h .
P l a g i o c l a s e i s p a r t i a l l ya l t e r e dt op o t a s s i u mf e l d s p a r .
P u m i c ea r eu s u a l l yf i l l e dw i t hc l a ym i n e r a l s .
The t u f f sa r e
v e r y s i m i l a r i n a p p e a r a n c ea n dm i n e r a l o g yt ot u f f s
i n the
u n i t of H a r d yR i d g e .
..
.
A-..distinctive.-porphyri+i.c ,and-e'site o r r h y o d a c i t e
l a v a i s common i n t h el a r g eb l o c k so n ' S a w m i l 1R i d g e .I nt h e
l a r g e ,e m b a y e db l o c ko nt h ew e s ts i d eo fS a w m i l lR i d g e ,t h e
t u f fa n d
l a v a a r ei n t e r b e d d e d .
The l a v a i s t y p i c a l l yg r a y i s h
1
e
b l u ew i t hw h i t et op a l e - p i n kf e l d s p a rc r y s t a l st h a tr a n g e
from
4
mm x 1 mm t o 11 mm x 6 mm.
seenintheunit
of H a r d yR i d g e
s o u t h (S. R o t h , 1978,
No s i m i l a r l a v a s h a v eb e e n
inthisstudyareaortothe
o r a l commun.).
I nt h i ns e c t i o n ,l a r g ep l a g i o c l a s ep h e n o c r y s t s ,
i n an
s o m e t i m e se x h i b i t i n gg l o m e r o p o r p h y r i t i ct e x t u r e ,o c c u r
a p h a n i t i cg r o u n d m a s sc o m p o s e dl a r g e l y
m i c r o l i t e s .T h e
alteredto
of a l t e r e d p l a g i o c l a s e
rims o ft h ep l a g i o c l a s ep h e n o c r y s t s
are
a c l a ym i n e r a lh a v i n gl o wb i r e f r i n g e n c e ;c e n t e r s
o fp l a g i o c l a s ep h e n o c r y s t sa r ep a r t i a l l ya l t e r e dt os e r i c i t e .
L a t h - s h a p e dp h e n o c r y s t so fb i o t i t ea b o u t
0 . 5 mm l o n g a r e
v a r y i n g s t a g e s of a l t e r a t i o n t o m a g n e t i t e a n d h e m a t i t e .
a
E u h e d r a lt os u b h e d r a lh o r n b l e n d e
( ? ) p h e n o c r y s t st h a tr a n g e
in
34
e
i n s i z ef r o m
0.3 mm t o 4 . 0
mm a r ec o m p l e t e l yr e p l a c e db y
A s m a l l a m o u n to fs e c o n d a r yq u a r t z
magnetite.
w a s s e e ni n
o n et h i ns e c t i o n .
o f T i m b e rR i d g ea n ds o u t h e r ns i d e
On t h e w e s t s i d e
of p o o r l y t o
o fB l a c kB e a rC a n y o n ,o n el a r g eb l o c kc o n s i s t s
m o d e r a t e l yw e l d e dt u f f so v e r l y i n g
a medium- t o w e l l - s o r t e d
i s h i g h l yf r a c t u r e da n df a u l t e d ;
s a n d s t o n eT
. h es a n d s t o n e
b r e c c i a t e df a u l tp l a n e sc a nb es e e nd i p p i n gi n
directions.
many
I t i s s t a i n e db r i g h to r a n g ea n dc e m e n t e dw i t h
:,."..:secondar.y s i l i c a n e a r t h e . . c . o n t a c tw i t h
b l o c ko nt h en o r t hs i d eo fB l a c kB e a r
.the.A.-LPeak.The
Canyon c o n s i s t s l a r g e l y
.
o fs a n d s t o n e .
A t t h eb a s eo ft h eb l o c k ,t h es a n d s t o n e
coarse-grained
(1 t o 2 mm g r a i n s ) a n d e x h i b i t s p l a n a r
is
b e d d i n g ; g r a i ns i z ed e c r e a s e st o w a r d st h et o po ft h eo u t c r o p .
A m i n o r a m o u n to fp o o r l yw e l d e dt u f f sa n dt u f f a c e o u s
s e d i m e nt a r y a r e f o u n d n e a r t h e t o p o f t h e o u t c r o p .
T h ec o n t a c tr e l a t i o n s h i p sb e t w e e nm e g a b r e c c i a
b l o c k sa n dt h ee n c l o s i n g
A-L
P e a ka r ew e l le x p o s e da r o u n dt h e
b l o c ko nt h es o u t hf l a n ko fB l a c kB e a rC a n y o na n da l o n gt h e
b a s eo ft h el a r g ee m b a y e db l o c ka l o n gt h e
S a w m i lC
l a n y o n( e x t r e m e
SE
A-L
bothlocalities,the
WestFork
of
1 / 4 , s e c . 18, T a b s . , R.3W.I.
P e a ks u r r o u n d i n gt h eb l o c k s
is
e x t r e m e l yp h e n o c r y s tp o o r ,i np l a c e sl e s st h a no n ep e r c e n t
phenocrysts.
The A-L
locallyexhibits
a
s t r i a t i o n sh a v i n g
P e a k i s o f t e nt i g h t l yf o l d e da n d
a m e g a s c o p i cl i n e a t i o nw i t hr i d g ea n dg r o o v e
a s much a s 2 mm of r e l i e f .R o u g h l y
At
35
s p h e r i c a lm a s s e s ,
much l i k e t h o s e d e s c r i b e d i n t h e l a v a o f
t h eu n i to fH a r d yR i d g e ,a r ef o u n dw i t h i nf i v ef e e to ft h e
8 and 9 ) .
c o n t a c tw i t ht h eb l o c k s( f i g u r e s
a t h i n ,l i g h t - c o l o r e dr i n d
t h e s es p h e r i c a lm a s s e sc o n s i s to f
A-L
s u r r o u n d i n gd e n s e l y - w e l d e d
s u r r o u n d i n g A-L
I nc r o s ss e c t i o n ,
foliation i n the
Peak.
The
P e a ku s u a l l yc o n t i n u e st h r o u g ht h e
masses a n d
t h e yc o n t a i nb o t hl 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 .
P r e l i m i n a r ys t u d ys u g g e s t s
t h a t t h eb o d i e s
c o n c e n t r a t i o n so fs i l i c aa n di r o no x i d e .
.
.
The b e s te x p o s u r e s
of : t h e s e s p b e r u 3 . i t e - l i k a b o d i e s a r e f o u n d . a r o u n d t h e b l o c k . .
t h es o u t hf l a n ko fB l a c kB e a rC a n y o n .
e
may r e p r e s e n t
W.A.
S e a g e ra n do t h e r s
( 1 9 7 6 ) n o t e d" s p h e r u l i t i cb o d i e s
4 t o 6 inches i n diameter"
n e a r" i n t r u s i v ec o n t a c t s "o ft h e
Dona Ana T u f fn e a r
C r u c e s , New Mexico.The
A-L
Las
Dona A n a T u f f i s v e r y similar t o t h e
P e a k i nd e g r e eo fw e l d i n g ,p h e n o c r y s tc o n t e n t ,a n d
d e v e l o p m e n to fl a m i n a rf l o ws t r u c t u r e s .
A n o t h e rc o n t a c tf e a t u r e ,a s s o c i a t e dw i t ht h e
i s what
e m b a y e db l o c ki nt h e ~ W e s tF o r ko ' f . S a w m i l 1C a n y o n ,
appearstobehighlyaltered
b yt h i nv e i n so f
more A-L
a b o x w o r kf a b r i c( f i g u r e
a
P e a kt h a th a sb e e ni n t r u d e d
A-L
Peak.The
r e s u l t i n gs t r u c t u r eh a s
T h e r e a r e a l s o many
10).
a n o m a l o u s l ys t e e pd i p si nt h e
block.
A-L
Peak i n t h ev i c i n i t yo ft h e
I t a p p e a r st h a tl a r g eb l o c k so fp r e - e x i s t i n gr o c k s
fell intothe
A-L
p r o p e r t i e so f
a r h y o l i t el a v a .
P e a kT u f fw h i l e
it h a d t h e r h e o l o g i c a l
The p r i m a r yf o l i a t i o na n d
l i n e a t i o n was d i s t u r b e d a n d d i k e l e t s o f
injected.
on
A-L
P e a kw e r e
36
..
F i g u r e 8.
S p h e r u l i t e - l i k eb o d i e si nt h e
A-L
c o n t a c tw i t hm e g a b r e c c i ab l o c ko nt h es o u t hf l a n k
Bear Canyon.
Peak T u f fn e a r
o f Black
Figure 9.
T i g h t l y f o l d e d A-L P e a k T u f f and spherulite-like
b o d i e s f r o m s a m e o u t c r o p as figure 8.
Figure 10. Boxwork fabric i n A-L Peak Tuff near contact with
menabreccia block, W e s t Fork of Sawmill Canyon.
39
e
T h ec o n t a c tr e l a t i o n s h i p s
o f t h e s eb l o c k s
l a v a s ,a n ds e d i m e n t a r yr o c k sw i t ht h e
A-L
of t u f f s ,
P e a k ,t h e i r
l i t h o l o g i cs i m i l a r i t yt ot h eu n i to fH a r d yR i d g ea n dt h e i r
p r o x i m i t yt ot h eS a w m i l lC a n y o nc a u l d r o nm a r g i n( C h a p i na n d
o t h e r s , 1978, a n df i g .
4 ) s u g g e s t st h a tt h e ya r eb l o c k s
p r e - c a l d e r ar o c k st h a t
f e l l i n t ot h ec a u l d r o nd u r i n ge r u p t i o n
of t h e A-L
of
( 1 9 7 6 ) h a sd o c u m e n t e de x t e n s i v e
Peak.
Lipman
l a n d s l i d ed e p o s i t so fp r e - c a l d e r ar o c k s ,d e r i v e df r o m
walls, t h a t f e l l i n t o c a u l d r o n s i n t h e
o v e r s t e e p e n e dc a l d e r a
SanJuan.v.01cani.w
field during .eruption
and^ c o l l a p s e .
-The
an
g r o u p of b l o c k si nt h i ss t h d ya r e aa r ei n t e r p r e t e dt ob e
example of Lipman's
u p p e r A-L
topofthe
(1976) megabreccia.
P e a kT u f fL o c a l l yp r e s e r v e d
A-L
a t t h es t r a t i g r a p h i c
a great
P e a kT u f fa r ea s h - f l o wt u f f st h a ts h o w
d e a l o f v a r i a b i l i t yi nc r y s t a lc o n t e n t ,p u m i c es i z e ,
and
l i t h i cc o n t e n t .T h i sv a r i a b i l i t y' c o n t r a s t ss t r o n g l yw i t ht h e
u n i f o r m l yd e n s e l yw e l d e d ,c r y s t a l - p o o r ,f l o w - b a n d e d ,l o w e r
A-L
P e a kf o u n dt h r o u g h o u t h es t u d ya r e a .
o ft h eu p p e r
A-L
P e a ka r e :b e n e a t ht h eu n i t
o nt h en o r t hf l a n k
b o r d e ro ft h es t u d y
of S i x m i l e
o f L a n g m u i rL a b o r a t o r y ;
C a n y o ni m m e d i a t e l ys o u t h
F o r ko fS a w m i l lC a n y o n
The b e s te x p o s u r e s
i n the East
a t a b o u tt h e8 6 0 0f o o te l e v a t i o n ;a n d
o f B l a c kB e a rC a n y o nn e a rt h es o u t h e a s t
area.
I nt h e s ea r e a s ,t h et u f f sa r e
c h a r a c t e r i z e db yv a r i a b l ec o l o r ,l a c ko ff l o wb a n d i n g ,
s i z eo fp u m i c ef r a g m e n t sa n d ,l o c a l l y ,i n c r e a s e dp h e n o c r y s t
content.
small
40
T h eu p p e r
A-L
Peakshows
a g r e a td e a lo f
lateral
variation.
A t t h en o r t h e r ne n d so fH a r d yR i d g ea n dt h er i d g e
b e t w e e nt h e
E a s t a n dW e s tF o r k so fS a w m i l lC a n y o na n di nt h e
I
of
E a s t F o r ko fS a w m i l lC a n y o na r ee x c e l l e n te x p o s u r e s
m o d e r a t e l yc r y s t a l - r i c h
(15 t o 30 p e r c e n tp h e n o c r y s t s ) ,
m o d e r a t e l yt od e n s e l yw e l d e du p p e r
ofthis
A-L
t h i c ki nt h e
A-L
P e a k .T h et h i c k n e s s
it i s a t l e a s t 200 f e e t
P e a k i s v a r i a b l eb u t
E a s t F o r ko fS a w m i l C
l a n y o nB
. e t w e e nH a r d y
R i d g ea n dS a w m i l lR i d g e ,t h ec r y s t a l - r i c ht u f fd o e sn o t
A-L
o u t c r o p .H e r e ,t h eu p p e r
f l o wb a n d e d ,c o n t a i n s
5
20
P e a k i s d e n s e l - yw e l d e db u tn o t
1 0p e r c e n tp h e n o c r y s t s ,a n dh a s
v e r yf e wl i t h i cf r a g m e n t s .
T h ec r y s t a l - r i c hp o r t i o no ft h eu p p e r
A-L
Peak
varies i nc o l o rf r o mg r a y i s hr e dp u r p l et ol i g h tg r e e n i s h
g r a yC
. o m m o n l y l, i g h t - p i n ks a n i d i n ep h e n o c r y s t st h a at v e r a g e
1 t o 2 mm i nl e n g t ha r ev i s i b l ei nh a n ds p e c i m e n .Q u a r t z
e a s i l yr e c o g n i z e d
i n h a n ds p e c i m e na n d
is
i s u s u a l l ya b o u t
1 mm
i nd i a m e t e r L
. i t h i cf r a g m e n t s p
, r e d o m i n a n t l ya n d e s i t e ,
a c c o u n tf o r
5 t o1 0p e r c e n to ft h er o c k .P u m i c ef r a g m e n t s
a r en o tl i n e a t e da n d ,i np l a n e sp a r a l l e lt ot h ef o l i a t i o n ,
t h ep u m i c ea r er o u g h l ye q u i d i m e n s i o n a l ;i np l a n e s
p e r p e n d i c u l a r t o t h ~.
ef o l i a t i o n ,
t h e p u m i c ea r el e n s o i d a lw i t h
d e l i c a t e l yf r a y e de n d s .
I n t h i ns e c t i o n ,s a n i d i n e
i s t h ed o m i n a n t
p h e n o c r y s t a n d o c c u r s a s b r o k e ne u h e d r a lc r y s t a l s ,
w h i c h a r e h i g h l y embayed.
Much o ft h es a n i d i n e
many o f
i s a l t e r e dt o
41
clay minerals.
Quartz makes up 1 to 2 percent of the rock
and occurs as rounded, often embayed grains, 0.3 to 0.7 mm in
. diameter.
The sanidine content varies greatly in this tuff
while the quartz remains relatively constant.
Pumice
fragments are typically 2 cm in diameter and 3 to 5 mm thick.
They are usually completely replaced by interlocking,
anhedral quartz and occasionally alkali feldspar crystals;
although in some samples, the pumiceis filled with clay
minerals.
The matrix of the tuff hasrecrystallized to
alkali feldspar..and...silic.a
minerals and is
stained with hematite.
pervasively
Glass shards are sometimes preserved
and exhibit axiolitic texture.
Minor amounts of highly
altered plagioclase feldspar were foundin some specimens.
On the west side ofTimber Ridge, the upper A-L
Peak occurs in scattered outcrops.
On the north side of
Black Bear Canyon at about the 8250-foot elevation are
exposures of a crystal-poor, pumiceous, and very lithic-rich
portion of the A-L Peak.
feet.
Its thickness is a minimum of 200
Individual pumice are small (2 cm x 1 cm x 0 . 5
exhibit a fine, tubular structure.
cm) and
Lithic fragments, 1 to 2
mm in diameter, make up 5 to 20 percent of the rock.
The discontinuous and variable natureof
A-L
the upper
Peak is suggestive ofit having been deposited in
channels or topographic lows developed on the lower A-L Peak.
Mineralogically, it is similar to the lower A-L Peak and may
-e
represent waning stages o f A-L Peak volcanism.
Further work
42
i s n e e d e d t od e t e r m i n et h ee x t e n to f
t ot h ee a s ta n ds o u t h
t h eu p p e r
A-L
P e a ka n d
its relationshiptothelower
A-L
Peak.
B l a k e s t a d ( 1 9 7 8 ) i n t e r p r e t e d a l a r g eb r e c c i a t e d
z o n ea l o n gt h ew e s t e r nm a r g i no ft h eM a g d a l e n aM o u n t a i n s ,
justsouth
of t h e t o w no fM a g d a l e n a ,
a s t h em a r g i no f
member o f t h e A-L
c a u l d r o nt h a te r u p t e dt h ef l o w - b a n d e d
It i s , l i k e l y t h a t t h e g r a y - m a s s i v e
f r o mt h i sc a u l d r o n
(C.E.
a
Peak.
member was a l s o e r u p t e d
C h a p i n , 1 9 7 8 , o r a l commun.).
The
MagdaZeSa" c a u l d r o n . -is - n o t w e l l : : e . x p o s - e d & u e ; . t o b a s i n - a n d - r a n g e
f a u l t i n gH
. owever,
I
a n o t e w o r t h yf e a t u r eo ft h ee x p o s e d
margin i s t h a tt h ep i n n a c l e s
member i s o f t e nc o n s p i c u o u s l y
member i s o v e r l a i nb y
a b s e n ta n dt h ef l o w - b a n d e d
p o s t - c o l l a p s ea n d e s i t e s .T h e s er e l a t i o n s h i p s
a r e n o tf u l l y
understood a t t h i st i m e .
If t h ef l o w - b a n d e d
a n d g r a y - m a s s i v em e m b e r sw e r e
e r u p t e df r o mt h eM a g d a l e n ac a u l d r o n ,
it i s l i k e l y t h a t t h e
p i n n a c l e s member was e r u p t e df r o mt h eS a w m i l lC a n y o n
c a u l d r o nT
. h ec o n t i n u o u ss t r a t i g r a p h i cs u c c e s s i o no f
A-L
P e a kt h r o u g ht h et u f fo fL e m i t a rM o u n t a i n s ,e x p o s e da l o n gt h e
west f l a n k o f T i m b e rR i d g e ,s u p p o r t st h i sh y p o t h e s i s .
A-L
Peak i n t h i s s t u d y a r e a
a b s e n c eo ft h ep i n n a c l e s
If t h e
i s t h ef l o w - b a n d e dm e m b e r ,t h e
member m u s tb ee x p l a i n e d .
A l t e r n a t i v e l y ,t h ea p p a r e n ta b s e n c eo ft h ep i n n a c l e s
i nt h eM a g d a l e n ac a u l d r o nm u s ta l s ob ee x p l a i n e d .
member
43
is that
An o b v i o u sp r o b l e mw i t ha n yi n t e r p r e t a t i o n
t h eo u t f l o wf a c i e s
o f t h ep i n n a c l e s
c h a r a c t e rf r o mt h e
A-L
member i s d i f f e r e n t i n
P e a k i n t h es t u d ya r e a .
The o u t f l o w
member i s u s u a l l yn o tf l o wb a n d e da n d
f a c i e sp i n n a c l e s
is
- c h a r a c t e r i z e db yo v o i do rl e n t i c u l a rp u m i c ef r a g m e n t s .
a n d o u t f l o wf a c i e s
D i f f e r e n c e sb e t w e e nc a u l d r o nf a c i e s
a s h - f l o wt u f f s
byLipman
(C.E.
i n t h eS a nJ u a nv o l c a n i cf i e l dh a v eb e e nn o t e d
1978, o r a l commun.) a n dp r o b a b l y
Chapin,
. r e f l e c td i f f e r e n te m p l a c e m e n tt e m p e r a t u r e sa n d / o rd i f f e r e n t
p o r t i o n s of t h e e r u p t i o n .
F o u rv i a b l e ’ h y p o t h e s i sf o rt h e
1) t h e o u t f l o w f a c i e s p i n n a c l e s
A-L
P e a kT u f fa r e :
member r e p r e s e n t s a n e a r l y
s e r i e s of e r u p t i o n st h a ti n i t i a t e dc a u l d r o nc o l l a p s ew h i l e
e
t h ef l o w - b a n d e dt u f f
i n t h es t u d ya r e ar e p r e s e n t s
. h o t t e re r u p t i v es e q u e n c et h a tp u d d l e d
c o n t e m p o r a n e o u sw i t hc o l l a p s e ;
notrelatedtothepinnacles
a later
i n t h ec a u l d r o n ,
2 ) t h ec a u l d r o nf a c i e st u f f
is
member a n d i s a n i n t r a c a l d e r a
i t s c a u l d r o n ; 3 ) t h eo u t f l o w
L u f fn o tr e p r e s e n t e do u t s i d eo f
f a c i e sa n dc a u l d r o nf a c i e sa r ee q u i v a l e n t ,b u tt h ec a u l d r o n
f a c i e s i s f l o wb a n d e db e c a u s ee m p l a c e m e n tt e m p e r a t u r e sn e a r
t h ev e n t sw e r eh i g he n o u g ht oa l l o wp r i m a r yw e l d i n ga n d
4 ) t h ec a u l d r o n
d e v e l o p m e n t of l a m i n a r f l o ws t r u c t u r e s ;a n d
faciestuffinthestudyarea
t h e A-L
i s t h ef l o w - b a n d e d
P e a kT u f f .F u r t h e rs t u d yo ft h e
A-L
member o f
P e a kT u f f
is
r e q u i r e d t o s o l v e some o f t h e p r o b l e m s r a i s e d i n t h i s r e p o r t .
a
Of particularinterest
Peak Tuff
-
i s t h es i g n i f i c a n c eo ft h eu p p e r
A-L
44
U n i to fS i x m i l eC a n y o n
(secs. 5,
i n S a w m i l lC a n y o n
P e a kT u f f
O v e r l y i n gt h e
A-L
8 , 1 7 , T.hS.,
R.3W.,
unsurveyed) i s a t h i c k
l a v a s , r h y o l i t el a v a s ,a s h - f l o w
s e q u e n c eo fi n t e r m e d i a t e
t u f f 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 ,a n dl a h a r i c
b r e c c i a s .T h i ss e q u e n c e
was i n f o r m a l l y named t h eu n i to f
S i x m i l eC a n y o nb yO s b u r n
(1978).
u n e q u i v o c a l l yd e t e r m i n e
t h e A-L
t h ee a s t
i t s s t r a t i g r a p h i cr e l a t i o n s h i pw i t h
( 1 9 7 4 ) b e l i e v e d i t was
P e a kT u f f ;K r e w e d l
s t r a t i g r a p h i c a l l yb e l o w
He was n o ta b l et o
A-L
P e a k .I nt h i ss t u d ya r e aa n dt o
(D. P e t t y , 1 9 7 8 , o r a l commun.) t h eu n i to fS i x m i l e
A-L
Canyon l i e s d i r e c t l y a b o v e t h e
P e a kT u f fa n db e n e a t ht h e
(1978)
t u f f o f Lemitar M o u n t a i n s C
. h a p i na n do t h e r s
i n t e r p r e t e dt h eu n i to fS i x m i l e
post-collapse
Canyon t o r e p r e s e n t
f i l l o ft h eS a w m i l C
l a n y o nc a u l d r o n T
. he
1973; B l a k e s t a d ,
a n d e s i t eo fL a n d a v a s oR e s e r v o i r( S i m o n ,
19789 o c c u r a t t h e &me s t r a t i g r a p h i c p o s i t i o n
as . t h e u n i t o f
S i x m i l eC a n y o na n da r ep r o b a b l ye q u i v a l e n t .
I n t h eL e m i t a ra n d
Bear M o u n t a i n s a s much a s 5 0 0
f e e to fb a s a l t i ca n d e s i t eo c c u r sb e t w e e no u t f l o wf a c i e s
P e a ka n dt h et u f fo fL e m i t a rM o u n t a i n s( C h a p i n
1978).
A-L
and others,
T h i ss u c c e s s i o nc o n t r a s t sw i t ht h et h i c kc a u l d r o n
f i l l o fi n t e r m e d i a t e
l a v a s , r h y o l i t el a v a s ,a s h - f l o wt u f f s
a n dv o l c a n i c l a s t i cs e d i m e n t st h a tc r o po u ti nt h es t u d ya r e a .
I n t h i sr e p o r t ,t h eu n i to fS i x m i l e
d i v i d e di n t ou p p e ra n dl o w e rm e m b e r s .T h el o w e r
Canyon i s i n f o r m a l l y
member i s
45
c h a r a c t e r i z e db y
l a t e r a l v a r i a b i l i t y i n r o c kt y p e
t h i c k n e s sw h i l et h eu p p e r
and
member i s a r e l a t i v e l y c o n s i s t e n t
i n t e r v a l of
s e q u e n c eo ft w od i s t i n c t i v ea s h - f l o wt u f f sa n da n
b a s a l t i ca n d e s i t e .
l o w e r member
The
west f l a n ko fT i m b e rR i d g e ;h e r e ,
b e l to nt h e
A-L
a
l o w e ru n i to fS i x m i l eC a n y o nc r o p so u ti n
it o v e r l i e s t h e
P e a kT u f fa n du n d e r l i e st h eu p p e ru n i to fS i x m i l eC a n y o n .
A t t h en o r t h e r ne n do ft h i so u t c r o pb e l t ,i nt h en o r t h e a s t
.. c o r n e r s of t h e s t u d y . a r e a ( s e c s .
R. 3W. )
,
the
i s i n d e p o s i t i o n a lc o n t a c tw i t h
l o w e ru n i to fS i x m i l eC a n y o n
t h eH e l l s
5 ..and 6 , T. 4 S . ,
M e s aT u f fa l o n gt h ee a s t - t r e n d i n gS a w m i l lC a n y o n
E a s t F o r ko fS a w m i l l
c a u l d r o nm a r g i n .N e a rt h eh e a do ft h e
Canyon t h e 1owe.rmember
c o v e r s a minimum t o p o g r a p h i c i n t e r v a l
i t may b e a s
o f1 2 0 0f e e ta n dc r o s ss e c t i o n si n d i c a t et h a t
t h i c k as 2000 f e e t .
The l o w e ru n i to fS i x m i l eC a n y o na l s o
c r o p so u to n . t h ew e s tf l a n ko fS o u t hB a l d yi nt h e -v i c i n i t yo f
..
H a r d yS p r i n g( s e c s 1. 2 ,
by f a u l t s o n
13, T.hS.,
t h e w e s t a n ds o u t ha n d
by t h eu p p e ru n i to fS i x m i l e
R.4W.)
Here it i s bounded
i s c o n f o r m a b l yo v e r l a i n
C a n y o n .S m a l le x p o s u r e so ft h e
l o w e ru n i to fS i x m i l eC a n y o na l s oo c c u ri m m e d i a t e l ys o u t ha n d
w e s to fL a n g m u i rL a b o r a t o r y( s e c s .
7 and 18, T-hS.,
a n do nH a r d yR i d g e( s o u t h - c e n t r a lp o r t i o no fs e c .
R. 3W.
R.3W.I
1 8 , T.hS.,
1.
I n t 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
u n i to fS i x m i l eC a n y o n
i s c o m p o s e dl a r g e l yo fa n d e s i t i c
lower
46
l a v a s ; t o t h es o u t h ,r h y o l i t el a v a sa r ed o m i n a n t .T h i s
s e e n on t h ew e s tf l a n ko fT i m b e r
r e l a t i o n s h i pc a nb ec l e a r l y
l a v a s p i n c ho u tt ot h en o r t ha n da r e
R i d g ew h e r et h er h y o l i t e
i n t e r b e d d e dw i t ht h ea n d e s i t i c
m a f i c l a v a s d on o to c c u r
l a v a s ; t h i c ks e q u e n c e so f
i n t h el o w e ru n i to fS i x m i l eC a n y o n
o f t h es t u d ya r e a .T h i sp a t t e r n
i nt h es o u t h e r np a r t
1 9 7 8 , ; P e t t y , 1 9 7 8 , o r a l commun.)
s e e nt ot h ee a s t( O s b u r n ,
1 9 7 8 , o r a l commun.).
a n dt ot h es o u t h( R o t h ,
Canyon a r e r e s t r i c t e d t o
a n o u t c r o pb a n d
is roughlyparalleltothe
a p p r o x i m a t e l yo n em i l ew i d ew h i c h
n o r t h e r nm a r g i n
I nt h ec e n t r a l
mafic l a v a s i n t h e l o w e r
M a g d a l e n aM o u n t a i n s ,t h em a j o r i t yo f
u n i to fS i x m i l e
is also
o f t h eS a w m i l lC a n y o nc a u l d r o n .
l a v a s a n da ni n t e r b e d d e dl a h a r i c
T h ea n d e s i t i c
b r e c c i a ,t w or h y o l i t e
l a v a s , a n d a d i s t i n c t i v ea s h - f l o wt u f f
were mapped s e p a r a t e l y .I na d d i t i o n ,t w oo u t c r o p so f
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 sa l o n gt h en o r t h e r nS a w m i l l
canyon^ c a u l d r o nm a r g i n ,a n dt w o
s m a l l o u t c r o p so fa s h - f l o w
t u f f j u s ts o u t ho fL a n g m u i rL a b o r a t o r yw e r ea l s o
a n d e s i t i c lavas
I nt h en o r t h e r ns t u d y
area, e x t e n s i v e
exposures of a n d e s i t i c t o d a c i t i c
l a v a s o c c u rb u t
T o r t h e s el a v a sw e r ei d e n t i f i e d .
T h em a j o ro u t c r o p s
a n d e s i t i c l a v a s o c c u r :i m m e d i a t e l ys o u t h
L a b o r a t o r y ( N W c o r n e r ,s e c .
f l a n ko fS o u t hB a l d y
1 2a n d
13, T-hS.,
mapped.
no v e n t s
of t h e
o f Langmuir
18, T.bS.,R.3W.);on
t h e west
i n t h ev i c i n i t yo fH a r d yS p r i n g s( s e c s .
R.kW.);
a n da l o n gt h ew e tf l a n k
o f Timber
. 47
R i d g ea n d
(secs.
a t t h eh e a do ft h e
E a s t F o r k o f S a w m i l lC a n y o n
5 a n d 8 , T a b s . , R.3W.I.
S o u t ho fL a n g m u i rL a b o r a t o r y ,i nt h en o r t h w e s t
1 8 , T.4S.,
c o r n e ro fs e c t i o n
d o m i n a n t l ya n d e s i t i c
a minimum t o p o g r a p h i c i n t e r v a l
l a v a s a r e e x p o s e do v e r
f e e ti n
R.3W.,
o f A-L
a t h i nb a n db e t w e e ne x p o s u r e s
o f 1400
P e a kT u f f T
. he
a n d e s i t e s a r e c o n f o r m a b l yo v e r l a i nb yt h eu p p e ru n i to f
S i x m i l eC a n y o nT
. he
common l a v a t h a t c r o p so u ti nt h i sa r e a
i s a d a r k - g r a yt og r a y i s h - g r e e na p h a n i t i ca n d e s i t e .
u s u a l l ye x t r e m e l yu - i i i f o r m
i n a p p e a r a n c e -a n d
It i s
i n places
o f t h es e c t i o n ,
e x h i b i t sc o l u m n a rj o i n t i n g .T o w a r d st h et o p
t h ea n d e s i t eb e c o m e sp o r p h y r i t i ca n dv e s i c u l a r ;m i n o r
p o r p h y r i t i cz o n e s
e
a l s o o c c u rt h r o u g h o u t h es e c t i o n W
. here
p o r p h y r i t i c ,p h , e n o c r y s t so fp l a g i o c l a s ea b o u t1 . 0
l e n g t h makeup
mm i n
a s much a s 20 p e r c e n t o f t h e r o c k ; a l s o
p r e s e n t a r e a b o u to n ep e r c e n te u h e d r a lt or o u n d e dm a g n e t i t e
i s made u po fp l a g i o c l a s e
..
m i c r o l i t e s ,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 ,a n dm i n o r
-phenocrysts.
T h eg r o u n d m a s s
made u p 1 5 t o 20
p y r o x e n e .' I n ' o n es p e c i m e n ,m a g n e t i t e
p e r c e n t o f t h eg r o u n d m a s s .
The p l a g i o c l a s ep h e n o c r y s t sa n d
g r o u n d m a s sa r el a r g e l yr e p l a c e db yc l a ym i n e r a l s
A s i m i l a r l a v a c r o p so u t
S p r i n g ,a l t h o u g hh e r e
p h e n o c r y s t st h a ta v e r a g e
i n t h ev i c i n i t yo fH a r d y
i t c o n t a i n sa b o u t
0.7
and calcite.
2 p e r c e n tq u a r t z
t o 1 . 0 mm i n d i a m e t e r .
s e c t i o n ,t h eq u a r t zp h e n o c r y s t sa r es u r r o u n d e db yr e a c t i o n
0
rims o f p y r o x e n e ( ? ) t h a th a v eb e e nc o m p l e t e l yr e p l a c e db y
In thin
48
clay minerals and calcite.
Phenocrysts of pyroxene
(?I,
also
replaced by clay and calcite, make up approximately 5 percent
o f the rock.
Magnetite occurs as subhedral phenocrysts that
range in size from 0 . 0 5 mm to 0.3 mm.
Also present are small
patches. o f secondary quart.2 probably
unoriented
unrelated
to
emplacement o f the lava.
In the East Fork o f Sawmill Canyon are a variety o f
aphanitic to porphyritic andesites that range
in color from
gray to grayish blue.
Autobrecciated flow tops and bottoms
with minor amountsof:?volcan.iclas.ticsediments are common.
The phenocryst content ranges from0 to 30 percent and
consists o f :
euhedral to subhedral plagioclase (0.5 to 1.0
mm); euhedral to subhedral pyroxene, probably augite; and
a
euhedral to rounded magnetite grains (0.05 to 0.25 mm).
The
matrix is composed o f plagioclase microlites, finely
disseminated magnetite and trace amounts o f augite.
A dist.inctive lava that crops out on the east side
o f Timber Ridge at about the 8800 foot elevation (secs. 8 and
17, T.hS., R.3W.)
is a light-bluish-gray andesite that has 5
to 10 percent euhedral to subhedral augite phenocrysts that
range in size from 0.1 mm to 0.3 mm;
exhibits glomeroporphyritic texture.
much of the augite
.
The groundmass consists
o f altered plagioclase microlites and minor amounts
of
augite.
Above this andesite is a medium- to light-gray
c
dacite which crops out semi-continuously alongTimber Ridge.
49
It c o n t a i n sa b o u t
35 p e r c e n tp h e n o c r y s t s :
p l a g i o c l a s ea b o u t
3 mm l o n g ; 5 t o 1 0 p e r c e n t a l k a l i f e l d s p a r ;
20 p e r c e n t
5 p e r c e n te u h e d r a lt os u b h e d r a lp y r o x e n e ;a n da p p r o x i m a t e l y
p e r c e n tb i o t i t e .
1
T h eg r o u n d m a s sa n dp h e n o c r y s t sa r ea l m o s t
c o m p l e t e l ya l t e r e dt oc l a ym i n e r a l s .
T h el o w e ru n i to fS i x m i l eC a n y o na l s oc r o p so u ti n
a ne a s t - t r e n d i n gb a n d ,s o u t h w e s to fL a n g m u i rL a b o r a t o r y
( s o u t h sec.
7 , T-hS.,
R.3W.I.
T h i ss e q u e n c e , i so fp a r t i c u l a r
Canyon i s
i n t e r e s tb e c a u s et h el o w e ru n i to fS i x m i l e
-i
a n o m a l o . u s l y t b i n -he're+-dn.d. t h eu p p e r : . u n i t : . o fS i x m i l e . C a . n y o n
a p p a r e n t l ya b s e n t .
T h ee x p o s u r e s
is
a r e v e r yp o o ri nt h i sa r e a ,
h o w e v e r it a p p e a r st h a tt h el o w e ru n i to fS i x m i l eC a n y o n
o v e r l i e st h e
e
A-L
i t i s composed l a r g e l yo f
P e a kH
. ere,
a n d e s i t ef l o w s , . w i t hi n t e r b e d d e da n d e s i t i cl a h a r i cb r e c c i a s ,
t u f f a c e o u ss e d i m e n t a r yr o c k sa n dm i n o ra s h - f l o wt u f f .
A n d e s i t i cl a h a r i cb r e c c i ac r o p so u ta l o n gt h eH a r d yC a n y o n
T r a i l a n dc o n s i s t so fs m a l l ,a n g u l a r . t or o u n d e da n d e s i t e
f r a g m e n t s ,u s u a l l yl e s st h a n
dark-green
1 cm i n d i a m e t e r , i n
t o b l a c kf i n e - g r a i n e d
matrix.
a
An e x c e l l e n t
Trail
e x p o s u r eo fa s h - f l o wt u f fo c c u r sa l o n gt h eH a r d yC a n y o n
a t a b o u tt h e9 4 0 0f o o te l e v a t i o n( s o u t h e a s ts e c .
R.3W.I.
< T h eH a r d yC a n y o n
T r a i l i s m i s l o c a t e do nt h eS o u t h
B a l d y 7 1 1 2 - m i n u t eq u a d r a n g l em a p . >T h et u f f
w e l lb e d d e da n df o r m s
d e g r e e st ot h es o u t h w e s t .
7 , T.4S.,
i s p a l eg r e e n ,
a c l i f f yo u t c r o pt h a td i p s
26 t o 45
It c o n t a i n sa p p r o x i m a t e l y
p e r c e n tp h e n o c r y s t sw h i c hc o n s i s to f
20
1 0 p e r c e n tp l a g i o c l a s e
( l a r g e l yr e p l a c e db yp o t a s s i u m - r i c hc l a y s
p e r c e n tq u a r t za n d
approximately
and c a l c i t e ) , 5
5 p e r c e n st a n i d i n e P. u m i c em a k e s . u p
1 5 p e r c e n to ft h er o c k .T h e r ea p p e a r
distinctblocks,althoughthey
t o b et w o
a fault.
may b e s e p a r a t e d b y
T h et u f f . m a y. b ee q u i v a l e n tt ot h et u f fa n ds e d i m e n t a r yr o c k s
i n t e r v a lw i t h i nt h el o w e ru n i to fS i x m i l e
C a n y o nd i s c u s s e d
below.
r h y o l i t el a v a s
:.<. .
T h ei n t e r m e d i a t e
l a v a s t h i na P r u p t l yt ot h e
s o u t h where t h e 1 - o u e r . ~u n i t o f. S i x m i l e ,C a n y o n
i s d o m i n a t e db y
r h y o l i t dl a v a s .E x t e n s i v et a l u sa n ds l i d eb l o c k sf r o mt h e .
o v e r l y i n gu p p e ru n i to fS i x m i l eC a n y o na n dt u f fo fL e m i t a r
M o u n t a i n so b s c u r e
much o ft h er h y o l i t el a v a s .T h e r e
are two
d i s t i n c tr h y o l i t ef l o w su s u a l l ys e p a r a t e db ya n d e s i t ea n d
a
t u f fa n ds e d i r n e n 4 a t - yr o c k si n t e r v a l .
i s p a l er e dp u r p l et o
T h el o w e rr h y o l i t el a v a
g r a y i s hp i n ka n d
i s t y p i c a l l yf l o wb a n d e da n dc r y s t a lp o o r .
I nt h es o u t h e a s tp a r to ft h ea r e a ,t h el o w e rr h y o l i t eh a s
a
a p r o m i n e n tr i d g eo nt h e
thickautobrecciatedtopthatforms
Its thickness is
s o u t hs i d eo fB l a c kB e a rC a n y o n .
a p p r o x i m a t e l y 300 f e e t .I no u t c r o pt h e
l a v a s w e a t h e rt o
small b l o c k y t a l u s . a n d . f o r m s t e e p s l o p e s w i t h o c c a s i o n a l
small c l i f f s .
The p h e n o c r y s tc o n t e n t
i s v a r i a b l eb u t
is generally
b e t w e e n 5 a n d 1 5 p e r c e n t .S a n i d i n ep h e n o c r y s t sr a n g ei n
maximum d i m e n s i o nf r o m
0.3 t o 2 . 0
mm a n d a r e o f t e n a l t e r e d t o
*
clay minerals; some appear perthitic.
Quartz occurs as
embayed crystals about 1.0 mm in diameter; sanidine.is two
to three times as abundant as quartz.
Euhedral to subhedral
magnetite grains about 0.1 mm in diameter make up one half
to
one percent
of
thelava.
The upper rhyolite is likely correlative with the
rhyolite lava flow described by Osburn (1978) in the upper
reaches of Sixmile Canyon (secs. 10, 11,
R.3W. ).
14, and 15, T.hS.,
In this study area, the lavas are pale
pink to
grayish pink and"often have a mott~lea;appearance.
They
weath'er to small cliffs and steep, rubble-covered slopes.
Lithic fragments of brown and gray aphanitic andesite are
abundant locally and may make up
e
rock.
as much as 5 percent of the
Good exposures of a lithic-rich portion of the lava
can be seen in the
vicinity of Hardy Spring (north central
sec. 13, T.bS., R.3W.I.
In
thin section, the lava contains 2 0 t o
phenocrysts of sanidine, quartz and biotite.
40 percent
The groundmass
is spherulitically devitrified and stained with hematite;
this gives the lava itsmottled appearance.
Sanidine occurs
as euhedral to subhedral crystals that average
1.0 mm in
largest diameter, although some are as large as
6.0
m
m
.
Sanidine is usually altered to clay minerals but when
unaltered, the larger crystals exhibit light-blue chatoyancy.
A few
percent o f rounded and embayed quartz phenocrysts that
average 0.5 mm in diameter and 1 t o 2 percent biotite make up
the remainder of
thephenocrysts.
t u f fa n ds e d i m e n t a r yr o c k si n t e r v a l
rhyolite,
Above t h el o w e r
o f a s h - f l o wt u f fa n ds e d i m e n t a r y
a thininterval
r o c k sc r o po u ta l o n gt h ew e s tf l a n ko fT i m b e rR i d g ea n di n
area.
t h eH a r d yS p r i n g
I nb o t hl o c a l i t i e st h et u f fa n d
s e d i m e n t a r yr o c k sp i n c ho u t
t o t h en o r t h .T h i si n t e r v a l
i n the
p r o v i d e s a n i m p o r t a n ts t r a t i g r a p h i cm a r k e rh o r i z o n
l o w e ru n i to fS i x m i l eC a n y o n .
-
The t h i c k n e s so ft h et u f fa n d
60 f e e t .
s e d i m e n t a r yr o c k s
i s v a r i a b l eb u ts e l d o me x c e e d s
The t u f f e x h i b i t s
a p r o m i n e n t ,t h i nb e d d i n gw h i c hw e a t h e r st o
a n d p l a t yt a l u s ' . .
cliffyoutcrops
g r a y i s hy e l l o wg r e e n ;l o c a l l y
,
it^ i s c h a r a c t e r i s t i c a l l y
is
o nT i m b e rR i d g e ,t h ep u m i c e
a d i s t i n c t i v eg r a y i s hg r e e n S
. a n d s t o n e sa n dc o n g l o m e r a t e s
a r eo f t e np r e s e n ta b o v et h e
tuff.
E x c e l l e n te x p o s u r e so f
s a n d s t o n e , 1 0 t o 20 f e e t t h i c k , t h a t a r e d o m i n a t e d b y
a n d e s i t i cd e b r i so c c u ri m m e d i a t e l ys o u t ho fH a r d yS p r i n g .
I nt h i ns e c t i o n ,t h et u f fh a s
10 t o 1 5 p e r c e n tp u m i c e .S a n i d i n e
p h e n o c r y s t sa n d
d o m i n a n tp h e n o c r y s tw i t ha b o u t
plagioclase.
20 t o 2 5p e r c e n t
is the
1 p e r c e n tq u a r t za n d
T h eg r o u n d m a s sh a sb e e nr e p l a c e db yc l a y
m i n e r a l s a n dc h l o r i t ew h i c ho f t e nc o n c e n t r a t ei nt h ep u m i c e .
s e d i m e n t a r yr o c k s
I n t h en o r t h e a s t
co.r.ner of t h es t u d y
area,
are
t w oe x p o s u r e so fc o a r s ev 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
i na p p a r e n td e p o s i t i o n a lc o n t a c tw i t ht h eH e l l s
a l o n gt h en o r t h e r nS a w m i l lC a n y o nc a u l d r o nm a r g i n T
. he
s t r i k ea n dd i po ft h e s ea l l u v i a ld e p o s i t sa r ev a r i a b l ea n d
Mesa T u f f
53
t h i c k n e s se s t i m a t e sa r ed i f f i c u l tt oo b t a i n .H o w e v e r ,j u s t
n o r t h o f t h er o a dt oL a n g m u i rL a b o r a t o r y ,t h ev o l c a n . i c l a s t i c
r o c k s a r e e x p o s e do v e r
feet.
a minimum t o p o g r a p h i c i n t e r v a l o f
400
T h e yw e a t h e rt os t e e ps l o p e sa n dt h eb l o c k so fl a v a
No o u t c r o p s o f t h i sl a v ah a v e
c o u l db em i s t a k e nf o ro u t c r o p .
b e e nf o u n di nt h eu n i to fS i x m i l eC a n y o n ;h o w e v e rc o b b l e sa n d
b o u l d e r s o f i t a r ef o u n di nt h ef l o a ta l o n gt h ec a u l d r o n
east.
m a r g i nt ot h e
O s b u r n ( 1 9 7 8 , o r a l commun.)
r e p o r t st h a t
i n c o a r s eh e t e r o l i t h i c
c l a s t s o f a s i m i l a r l a v ao c c u r
b r e c c i a si nt h eu n i t ' o fS i x m i l e X a n y o nt h a ta r ef o u n dd i p p i n g
A s i m i l a r l a v a i s r e p o r t e db e n e a t ht h e
i n t ot h ec a u l d r o n .
flow-banded
member o f t h e A-L
Baldy
(Krewedl,
1978).
P e a k i nt h ea r e aw e s to fN o r t h
T h i sp o r p h y r i t i c
lava i s
lava f o u n dw i t h i nt h e
lithologically-identicaltothe
It i s c h a r a c t e r i z e d
m e g a b r e c c i ab l o c k sd e s c r i b e dp r e v i o u s l y .
b yg l o m e r o p o r p h y r i t i cp l a g i o c l a s ep h e n o c r y s t s ,b i o t i t el a t h s ,
a n de u h e d r a lp y r o x e n e
( ? ) t h a th a sb e e nr e p l a c e d
by
magnetite.
It i s l i k e l y t h a t t h i s d i s t i n c t i v e l a v a
t h a nt h eu n i to fS i x m i l eC a n y o n ,p e r h a p s
u n i to fH a r d yR i d g e ,a n dt h a t
u p p e ru n i t
a p o r t i o n of t h e
i t was i n c o r p o r a t e d i n b r e c c i a
a n ds e d i m e n t a r yr o c k sd e p o s i t e dn e a rt h e
C a n y o nc a u l d r o nf o l l o w i n g
is older
w a l l s o ft h eS a w m i l l
i t s collapse.
of S i x m i l eC a n y o nA b o v et h el o w e ru n i t
Canyon i s a d i s t i n c t i v e s e q u e n c e
of Sixmile
o f a n a s h - f l o wt u f fc a p p e d
54
w i t h a s a n d s t o n ea n da ni n t e r v a lo fa n d e s i t e .L o c a l l y ,m i n o r
t u f f sa n ds e d i m e n t a r yr o c k so c c u r
a t t h eb a s e .
t u f f . i sc o r r e l a t i v ew i t hO s b u r n ' s
( 1 9 7 8 ) t u f fa n ds a n d s t o n e
i n t e r v a l si nt h eu n i t
tuffaceous unit.
o f S i x m i l eC a n y o n
The a s h - f l o w
a n d S t a c y ' s (1968)
O u t c r o p so ft h eu p p e ru n i to fS i x m i l e
Canyon a r e r e s t r i c t e d t o t h e i n t e r i o r o f t h e S a w m i l l
1978).
c a u l d r o n( C h a p i na n do t h e r s ,
S i x m i l eC a n y o n
..
.
T h eu p p e ru n i o
tf
~ - A - L Peak : a n d - t h e w p p e r . u n i t : o f.Sixm-iL.e:.Canyon.
d i p so ft h e
A-L
P e a ka r e
A-L
the q e g i o n a l ~
4 5 t o 55 d e g r e e s t o t h e e a s t w h i l e
a n g u l a rr e l a t i o n s h i pb e t w e e nt h el o w e ru n i t
a n dt h e
-
Canyon i s 20 t o ~ 3 5d e g r e e s .T h e
t h a to ft h e. u p p e rS i x m i l e
0
of L e m i t a r
i s c o n f o r m a b l yo v e r l a i nb yt h eT u f f
An a p p a r e n ta n g u l a ru n c o n f o r m i t ye x i s t sb e t w e e n
Mountains.
.. .
Canyon
of S i x m i l eC a n y o n
P e a kT u f fc a n n o tb ea c c u r a t e l yd e t e r m i n e d
a l t h o u g ht h e r ea p p e a r st ob e
a g r a d u a lf l a t t e n i n go fd i p
t h r o u g h o u t h el o w e rC a n y o n .T h i sa n g u l a rr e l a t i o n s h i p
pqrsists
commun.
to^ t h e s o u t ha n de a s t
(D. P e t t y , S.
1978, o r a l
Roth,
).
T h eu p p e ru n i to fS i x m i l eC a n y o nc r o p so u ta l o n g
T i m b e rR i d g e ,n e a rH a r d yS p r i n g ,a n di m m e d i a t e l ys o u t ho f
L a n g m u i rL a b o r a t o r y T
. h eu p p e ru n i o
t fS i x m i l eC a n y o nh a s
a v e r a g et h i c k n e s so f
500 t o 600 f e e t i n t h e s t u d y
t h i c k e n sc o n s i d e r a b l yt ot h es o u t h
o r a l colhmun.).
i n t ot h r e e
I.
an
a r e a ; it
(D. P e t t y , S. R o t h , 1978,
I nt h i ss t u d yt h eu p p e ru n i th a sb e e nd i v i d e d
map u n i t s :u p p e ra n dl o w e rt u f fo fC a r o n i t a
Canyon a n d a n o v e r l y i n g a n d e s i t e i n t e r v a l .
55
tuff
0
of S i x m i l eC a n y o n
of C a r o n i t aC a n y o nT h eu p p e ru n i t
d o m i n a t e db y
a s t r o n g l yz o n e d ,m o d e r a t e l y
a s h - f l o wt u f f ,i n f o r m a l l y
is
t p densely.welded
named t h e t u f f o f C a r o n i t a C a n y o n
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 s
f o re x p o s u r e s
( C h a p i n , 1979, o r a l commun..)..
The t u f fo fC a r o n i t aC a n y o n
talus
r o r m ss t e e pc l i f f yo u t c r o p sa n dw e a t h e r st oe x t e n s i v e
s l o p e s ; i t f o r m st h ep r o m i n e n tc l i f fa l o n gt h ew e s tf l a n ko f
a t a b o u tt h e
T i m b eR
r idge
C a r o n i t aC a n y o nc o n s i s t so f
.
..
9600 f o o te l e v a t i o n .
The t u f fo f
a l o w e ra n du p p e r
w e l d e dt o g e t h e r - .b o c a l l yp r e s e r v e d
member t h a t a r e
a t . i t s b a s ea r e
thin
v o l c a n i c l a s t i cr o c k sa n dp o o r l yw e l d e da s h - f l o wt u f f st h a t
w e r en o tm a p p e ds e p a r a t e l y .
Good e x p o s u r e so ft h es e d i m e n t a r yr o c k sc a nb es e e n
j u s t n o r t ho fT i m b e rP e a k
o nT i m b e rR i d g e ,
a t a b o u tt h e
1 0 , 1 5 0f o o et l e v a t i o n H
. e r em
, e d i u m - g r a i n e ds a n d s t o n e sa r e
i n t e r b e d d e dw i t ht u f f a c e o u s
s t r a t a a n dh a v e
a combined
1 5 0 f e e t .J u s ts o u t ho fL a n g m u i rL a b o r a t o r y ,i n
t h i c k n e s so f
t h ee x t r e m en o r t h e a s tc o r n e ro fs e c .
e x c e l l e n te x p o s u r e s
18, T.hS.,
R.3W.,
are
of v o l c a n i c l a s t i c r o c k s a n d p o o r l y
w e l d e d ,l i t h i c - r i c ha s h - f l o wt u f f s .
T h e yp i n c ho u tt ot h e
n o r t h w e s ta l o n gt h ec o n t a c tw i t ha n d e s i t eo ft h el o w e ru n i t
o fS i x m i l eC a n y o n .
One p o o r l yw e l d e d , . l i t h i c - r i c ht u f ff o r m s
a d i s t i n c t i v ec l i f f yo u t c r o pa n d
i s i n t e r b e d d e dw i t h
v o l c a n i c l a s t i cr o c k s .I np l a c e s ,l i t h i cf r a g m e n t s
m o r et h a n
e
make u p
5 0 p e r c e n to ft h et u f fa n dr a n g ei ns i z ef r o m
m i c r o s c o p i ct o
3 t o 4 cm.
A n o t h e re x p o s u r eo f
a similar,
56
p o o r l yw e l d e d
t u f f c r o p s o u t a t t h ee x t r e m es o u t h e r ne d g eo f
t h es t u d ya r e ao nT i m b e rR i d g e
(9100 f e e t ) a n d
was mapped
separately.
l o w e r member
The
l o w e r member o ft h et u f fo fC a r o n i t a
Canyon v a r i e s i n t h i c k n e s s f r o m a b o u t
8 0 f e e t a t t h en o r t h e r n
1 5 0 f e e t a t t h es o u t h e r ne n d .T h e
e n do fT i m b e rR i d g et o
l o w e r member i s t y p i c a l l y g r a y i s h r e d p u r p l e , m o d e r a t e l y
w e l d e d a, n dh a s
l e s s t h a n 5 p e r c e n tl i t h i cf r a g m e n t s .T h e
i s l i g h t g r a y .to , p a l e b l u e , . & o a r l y : . t o m o d e r a t e l y w e l d e d
.:..~.b~ase
a n d c o n t a i n s 1 0 t o 15 p e r c e n t l i t h i c f r a g m e n t s ’ o f a p h a n i t i c
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 e .
Much o ft h el o w e r
member
e x h i b i t s a p r o m i n e n te u t a x i t i cf o l i a t i o n .
I nt h i ns e c t i o n ,t h el o w e rt u f fo fC a r o n i t aC a n y o n
c o n t a i n s 5 t o 1 0 p e r c e n tp l a g i o c l a s ep h e n o c r y s t s ,
p e r c e n t s m a l l r o u n d e dq u a r t zg r a i n s ,a n d
biotite.
1to 2
1 t o 2 percent
The p l a g i o c l a s eh a sb e e nr e p l a c e db yc l a ym i n e r a l s
t h a to f t e nh a v eb e e np l u c k e df r o mt h i ns e c t i o n sd u r i n g
p r e p a r a t i o nP. u m i c ef r a g m e n t s
a r e c h a r a c t e r i z e db y
s p h e r u l i t i cd e v i t r i f i c a t i o n .
u p p e r memberWelded
C a r o n i t aC a n y o n
tuff.
t ot h el o w e r
of
i s a c r y s t a l - r i c h ,d e n s e l yw e l d e da s h - f l o w
It h a s a t h i c k n e s so f ’ 8 O
o fT i m b e rR i d g ea n dn e a rH a r d yS p r i n g ;
o ft h es t u d ya r e a
member o f t h e t u f f
t o 1 0 0 f e e t a t t h en o r t he n d
a t t h es o u t h e r ne d g e
i t i s a b o u t 2 0 0 f e e tt h i c k .I np l a c e s ,t h e
,
57
u p p e r member e x h i b i t s t h i c k b e d d i n g w i t h b e d d i n g p l a n e s . 6
i n c h e st o
2 f e e ta p a r t .
Hand s p e c i m e n sa r eg r a y i s hp i n kt o
v e r yl i g h tg r a ya n da r ec h a r a c t e r i z e db yc o p p e r yb i o t i t ea n d
l a r g e( u pt o
4
mm), o f t e nb i p y r a m i d a l ,q u a r t zp h e n o c r y s t s .
I nt h i n- s e c t i o n ,t h ep h e n o c r y s tc o n t e n tv a r i e sf r o m
25 p e r c e n t a t t h e b a s e t o
4 0 p e r c e n tn e a rt h et o p .S a n i d i n e
i s a p p r o x i m a t e l y 1 . 5 t i m e s a s a b u n d a n t a s q u a r t z ;t h e
latter
o c c u r s a s l a r g e ,s l i g h t l yr o u n d e dc r y s t a l st h a ta v e r a g e
2 mm
a s s m a l l r o u n d e df r a g m e n t st h a t
i nl o n g e s td i m e n s i o no r
a v e r a g e 0 . 3 mm i n d i a m e t e r .
r o u n d e dm a g n e t i t eg r a i n s
One t o t w o p e r c e n te u h e d r a l
make u p t h e r e m a i n d e r o f t h e
phenocrysts.
O v e r l y i n gt h eu p p e r
a
member o f t h e t u f f o f C a r o n i t a
Canyon i s a n i n t e r v a l o f . p i n k i s h - g r a y t o p a l e - r e d f i n e l y
l a m i n a t e ds a n d s t o n e ,1 0t o
mapped s e p a r a t e l y .
50 f e e tt h i c k ,w h i c h
was n o t
The s a n d s t o n e i s s e e nt h r o u g h o u t h e
s t u d ya r e aa n dt ot h es o u t ha n d - e a s t( O s b u r n ,
1 9 7 8 , o r a l c o m m u n . )T. h es a n d s t o n ee x h i b i t s
1978; P e t t y ,
f i n e laminations
(1 t o 3 mm) t h a t a r e t h e r e s u l t o f a l t e r n a t i n g c o a r s e a n d
f i n el a y e r s ;t h eb e d d i n g
i s t h i c k( 0 . 1t o
1 m e t e r ) .I nt h i n
s e c t i o n ,t h es a n d s t o n ec o n s i s t so fp o o r l ys o r t e dq u a r t za n d
feldspargrainswithlocalconcentrations
of magnetite
grains.
a n d e s i t ei n t e r v a lT h r o u g h o u tm o s to ft h es t u d y
area, a
s e q u e n c eo fa n d e s i t ef l o w so v e r l i e st h es a n d s t o n eo ft h et u f f
58
o fC a r o n i t aC a n y o na n du n d e r l i e st h et u f fo fL e m i t a r
t o the
M o u n t a i n s .A l o n gT i m b e rR i d g e ,t h ea n d e s i t e st h i n
a t t h es o u t h e r ne n do ft ' h e
a r e n ol o n g e rp r e s e n t
s o u t ha n d
s t u d ya r e a .O s b u r n
( 1 9 7 8 ) n o t e d s i m i l a r l a v a sb e l o wt h et u f f
o fL e m i t a rM o u n t a i n si nS i x m i l eC a n y o n( s e c .
R.3W.;
1 2 , T. 4S.,
u n s u r v e y e d ) .T y p i c a le x p o s u r e so ft h e s ea n d e s i t e s
c o n s i s to fc o n s p i c u o u s ,c o l l u v i u m - c o v e r e d ,g r a s s yb e n c h e s
b e t w e e nt h eo v e r l y i n gt u f fo f
Lemitar M o u n t a i n sa n dt h e
u n d e r l y i n gt u f fo fC a r o n i t aC a n y o n E
. x c e l l e n te x p o s u r e sc a n
bef.ound
j u s tn o r t ho fT i m b e rP e a kw h e r e. t h ea n d e s i t e sa r e
a p p r o x i m a t e l y1 6 0f e e tt h i c k .
I nh a n ds p e c i m e n ,t h ea n d e s i t e sv a r y
i n c o l o rf r o m
b r o w n i s hg r a yt og r e e n i s hb l a c k .M o s to ft h ef l o w sa r e
e
p o r p h y r i t i cw i t h
a s much a s 1 0 p e r c e n t s m a l l , o x i d i z e d ,
p y r o x e n ep h e n o c r y s t si n
a n a p h a n i t i cg r o u n d m a s s .
A u t o b r e c c i a t e df l o wt o p sa n db o t t o m sa n ds c o r i a c e o u sz o n e s
a r e common.
'<,
i s composedof
I nt h i ns e c t i o n ,t h eg r o u n d m a s s
p l a g i o c l a s e ,c l i n o p y r o x e n ea n dm a g n e t i t em i c r o l i t e s .
P h e n o c r y s t so fc l i n o p y r o x e n e ,o l i v i n e
a c c o u n tf o r
1 5 t o 2 0p e r c e n to ft h er o c k .E u h e d r a lo l i v i n e
( ? ) p h e n o c r y s t s .make.up
a b o u0
t .5
( ? ) a n dm a g n e t i t e
3 p e r c e n to ft h er o c ka n da v e r a g e
t o 1 . 0 mm i n d i a m e t e r .
The o l i v i n e
a r ep a r t l yt oc o m p l e t e l ya l t e r e dt os e r p e n t i n e
'(?I
phenocrysts
( ? ) and
i d d i n g s i t e ;i np l a c e st h e ya p p e a rt ob ep a r t i a l l yr e p l a c e db y
a.
p y r o x e n eT
. h ec l i n o p y r o x e n ep h e n o c r y s t sr a n g e
i n diameter
59
f r o m 0.3
t o 1 . 0 mm a n d make u pa p p r o x i m a t e l y
1 0 p e r c e n to f
t h er o c k ;t h e ya r eo f t e nz o n e da n do c c a s i o n a l l y
show
t w i n n i n g .T h eo p t i c a lp r o p e r t i e so ft h ec l i n o p y r o x e n ea r e
c o n s i s t e n tw i t ht h o s eo fa u g i t e .E u h e d r a lt or o u n d e d
0 . 1 t o 0.3
m a g n e t i t ep h e n o c r y s t s ,a p p r o x i m a t e l y
d i a m e t e r ,a c c o u n tf o r
mm i n
I n most
1 t o 2 p e r c e n to ft h er o c k .
s p e c i m e n s ,t h eg r o u n d m a s s
i s a l t e r e dt oc l a ym i n e r a l s ,
c a l c i t e ,a n dh e m a t i t e ;i n d i v i d u a lp h e n o c r y s t sc a n n o tb e
discerned.
. .
L o c a l l y 5 ; r l tlheinnsoefs~. . . s a = n d s t o n e
f i l l channe1.s
d e v e l o p e d on t o p o f t h e a n d e s i t e s . T h e y a r e t y p i c a l l y
g r a y i s h - r e d ,f i n e - g r a i n e ds a n d s t o n e sc o m p r i s e dm a i n l yo f
a n g u l a rq u a r t za n df e l d s p a rg r a i n s .
The u n i to fS i x m i l eC a n y o n
i s a n a c c u m u l a t i o no f
l a v a s , s e d i m e n t s ,a n da s h - f l o wt u f f st h a tw e r ed e p o s i t e do n
t o po ft h e
A-L
cauldron.
The f a c i e sr e l a t i o n s h i p sw i t h i nt h eu n i to f
P e a kf o l l o w i n gc o l l a p s eo ft h eS a w m i l lC a n y o n
S i x m i l eC a n y o ns u g g e s tt h a tt h el o w e r
member b u r i e d t h e
t o p o g r a p h i cr e l i e fd e v e l o p e dd u r i n gc o l l a p s eo ft h ec a u l d r o n .
T h el o w e r
member c o n s i s t s o f m a f i c l a v a s t h a t f l o w e d i n t o t h e
c a u l d r o nf r o mt h en o r t h ,r h y o l i t e
l a v a s t h a tf l o w e di nf r o m
t h es o u t h ,a n dc o a r s es e d i m e n t sa n db r e c c i a st h a tw e r e
d e p o s i t e da d j a c e n tt ot h ec a u l d r o n
S i x m i l eC a n y o n
wall.
The u p p e ru n i to f
as
w a s d e p o s i t e do ns u b d u e dt o p o g r a p h y
indicatedbytherelativelyuniformthicknessofthetuffof
C a r o n i t a C a n y o na n dt h ew i d e s p r e a do c c u r r e n c eo f
s a n d s t o n e a t t h et o po ft h et u f fo fC a r o n i t a
a uniform
Canyon.
60
0
How much of a depression remained following
-
deposition of
the unit of Sixmile Canyonis not known;
however, as will bediscussed later, the Sawmill Canyon
cauldron has influenced the thickness and distribution
o f the
overlying tuff of Lemitar Mountains
.i.nthe study area.
Tuff o f Lemitar Mountains
Following filling of the Sawmill
Canyon cauldron
with the unit of Sixmile Canyon, the tuff
o f Lemitar
Mountains was deposited over most of.the Socorro-Magdalena
I
area.
The tuff is a distinctive, compositionally zoned,
multiple-flow, simple cooling unit, rhyolite ash-flow tuff
which has been dated at 27.0 + / - 1.1, 26.3 + / - 1.0, and 2 8 - 8
+/-
0.7
m.y.
by the K-Ar technique on biotite (Chapin,
unpublished data).
The tuff of Lemitar Mountains, formerly
tuff of Allen Well, has been
described in detail by several
authors:
Simon (1974)
described samples from the Crouch
..
drill hole in the Silver Hill area
(the unit was
misidentified as tuff of La JenciaCreek due to its presence
in a paleovalley);
Osburn ( 1 9 7 8 ) described a measured
section of the lowerpart of the tuffof Lemitar Mountains in
the eastern Magdalena Mountains;
and R.M.
Chamberlin (1978,
oral commun.) has described a measured section in the Lemitar
Mountains.
The reader seeking detailed petrographic
information is referred to these studies.
61
~
i s d i v i d e di n t ot w o
T h eT u f fo fL e m i t a rM o u n t a i n s
members:
1) a m o d e r a t e l yc r y s t a l - p o o rl o w e r
c r y s t a l - r i c hu p p e r
member.
member a n d 2 ) a
B o t hm e m b e r sa r eb e l i e v e dt oh a v e
and others,
b e e ne r u p t e df r o mt h eS o c o r r oc a u l d r o n( C h a p i n
1 9 7 8 , f i g . 4 ) , t h ew e s t e r nm a r g i n
of w h i c h i s n o t c l e a r l y
d e f i n e d .T h e r ea p p e a r st ob en os h a r pb r e a kb e t w e e nc a u l d r o n
(1978)
f a c i e st u f fo fL e m i t a rM o u n t a i n sd e s c r i b e db yO s b u r n
a n dC h a m b e r l i n
( i n p r o g r e s s )a n dt h eo u t f l o w
(D. P e t t y , 1 9 7 8 , o r a l
i nt 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
-~ commun. )..
I.
( ? ) equivalent
This relationship is-suggestive of "trap-door"
s u b s i d e n c e ,h i n g e d
down t o t h e e a s t ,
o f t h eS o c o r r oc a u l d r o n .
T h ei n f l u e n c eo ft h eS a w m i l lC a n y o nc a u l d r o no n
d e p o s i t i 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 sc a nb ec l e a r l y
of t u f f
s e e ni nt h i ss t u d ya r e aw h e r et h e r ea r et w od o m a i n s
of L e m i t a rM o u n t a i n s :
C a n y o nc a u l d r o nt h e r e
1) w i t h i nt h e
l i m i t s o ft h eS a w m i l l
i s a n o r m a ls u c c e s s i o no ft h el o w e ra n d
u p p e r . m e m b e r so v e r l y i n gt h eu p p e ru n i t - o fS i x m i l eC a n y o n ;a n d
2 ) o u t s i d e o f t h ec a u l d r o nt h e r e
is l i t t l e o r
n ol o w e rt u f f
o f L e m i t a rM o u n t a i n sp r e s e n ta n dt h eu p p e r
member r e s t s
u n c o n f o r m a b l yo nt h eu n i t
I n t h el a t t e r
c a s e ,t h eb a s e
o f H a r d yR i d g e .
o f t h eu p p e r
l a m i n a r f l o ws t r u c t u r e sa n d
member o f t e n e x h i b i t s p r i m a r y
is locallylithicrich.
l o w e r m e m b e rT h el o w e rt u f fo fL e m i t a rM o u n t a i n sc r o p so u t
a l m o s te x c l u s i v e l yw i t h i nt h e
a
c a u l d r o n ;h o w e v e r ,o n e
l i m i t s o f t h eS a w m i l lC a n y o n
small e x p o s u r eo c c u r so u t s i d e
\
of t h e
62
(t
c a u l d r o n i n t h es o u t h - c e n t r a lp o r t i o n
R.4W.
T h el o w e r
o f s e c .2 4 ,T . h S . ,
member o v e r l i e sa n d e s i t e s
of t h eu p p e ru n i t
o f S i x m i l eC a n y o ni nt h en o r t h e r np a r to ft h es t u d y
area and
t h eu p p e rt u f fo fC a r o n i t aC a n y o nt ot h es o u t h .
The b e s t
e x p o s u r e s a r e a l o n gT i m b e rR i d g ew h e r et h el o w e r
member h a s
a n a p p r o x i m a t e minimum t h i c k n e s s of 4 0 0 f e e t .I nt h e
v i c i n i t y of S o u t hB a l d yt h i c k n e s se s t i m a t e s
a r e h i n d e r e db y
p o o ro u t c r o p sw i t he x t e n s i v et a l u s ,l o c a l l yi n t e n s e
s i l i c i f i c a t i o n ,a n dc l o s e l ys p a c e df a u l t s .H e r e ,
minimum
.thi.ckn.ess-:.:estimates.-r.angef r o m 250 ,.to..&.OO-..feet.
( 1 9 7 8 ) f o u n dt h et h i c k n e s so ft h el o w e r
v a r i a b l e ,f r o m
Canyon.
2 0 0 f e e to n
BuckPeak
Osburn
.
member t o b e
t o 820 f e e t i n S i x m i l e
P e t t y ( 1 9 7 9 ) r e p o r t s similar v a r i a t i o ni nt h i c k n e s s
of t h e l o w e r
member t o t h e e a s t a n d s o u t h o f t h i s s t u d y a r e a .
The l o w e r t u f f o f
Lemitar M o u n t a i n s t y p i c a l l y w e a t h e r s t o
s t e e pc l i f f sa n de x t e n s i v et a l u ss l o p e s .
O s b u r n ( 1 9 7 8 ) . d e s c r i b e d a m e a s u r e ds e c t i o no ft h e
l o w e r t u f f o fL e m i t a rM o u n t a i n s
d i s t i n c tz o n e so nt h eb a s i s
i n w h i c hh ed e f i n e d
four
o f o u t c r o pp a t t e r n ,d e g r e e
of
w e l d i n g ,t o t a lp h e n o c r y s tc o n t e n t ,p u m i c ea n dl i t h i cf r a g m e n t
was o b s e r v e d
c o n t e n t ,a n dc o l o r .I ng e n e r a l ,t h i sz o n a t i o n
i nt h i ss t u d ya r e a ,a l t h o u g hl e s s e rt h i c k n e s s e sa n dp o o r
e x p o s u r e sh i n d e r e da c c u r a t ec o r r e l a t i o n s .
L e m i t a rM o u n t a i n s
The l o w e r t u f f o f
i s m o d e r a t e l yp h e n o c r y s t - p o o r
p e r c e n tr a n g e )a n dt y p i c a l l yc o n t a i n sa p p r o x i m a t e l y1 2
p e r c e n ts a n i d i n e ,q u a r t z ,p l a g i o c l a s e ,a n db i o t i t e .
( 8 t o 25
..
63
i s a n average
mineral,
S a n i d i n e ,t h em o s ta b u n d a n t
o f 1 . 0 mm i n l o n g e s t d i m e n s i o n a n d e x h i b i t s a l t e r a t i o n
t e x t u r e sw h i c h
S m a l ll e n s e s
a t t h i st i m e .
a r e n o tc o m p l e t e l yu n d e r s t o o d
o f p o t a s s i cc l a ym i n e r a l si n
a perthitic-like
relationshipwiththesanidineare-pr~esentand
may r e p r e s e n t
I n p l a c e s ,c l a y
a l t e r a t i o no fe x s o l v e da l b i t i cf e l d s p a r .
m i n e r a l sh a v ea l m o s tc o m p l e t e l yr e p l a c e dt h es a n i d i n e
phenocrystsQ
. u a r t zo c c u r s
0.6
a s a n h e d r a lg r a i n sa p p r o x i m a t e l y
mm i nd i a m e t e r .S m a l la m o u n t so fp l a g i o c l a s ep h e n o c r y s t s
~ ~a
l r. S - p r k s e n - t ' i n s o m e ' t h i n s e c t i o n s
bu.C'%-hes.e a r e u s u a l l y
r e p l a c e db yc l a ym i n e r a l s T
. r a c ea m o u n t so fr e d d i s h - b r o w n
b i o t i t ea n dm a g n e t i t ea r ea l s op r e s e n t .
A d i s t i n c t i v es l o p e - f o r m i n gz o n e
e
1978) i s not
l o w e rt u f fo fL e m i t a rM o u n t a i n s( O s b u r n ,
e v e r y w h e r ep r e s e n t
i n t h i ss t u d y
a t t h eb a s eo ft h e
area.
T h i sz o n e
is light
c o l o r e d ,p o o r l yt om o d e r a t e l yw e l d e d ,p u m i c e o u s ,a n dl o c a l l y
c o n t a i n s a s much as 1 5 p e r c e n t a n d e s i t i c a n d r h y o l i t i c r o c k
f r a g m e n t s .W h e r ep r e s e n t ,t h eb a s a lz o n ev a r i e si nt h i c k n e s s
f r o m 1 0 t o 4 0 f e e t .E x c e l l e n te x p o s u r e sc a nb ef o u n d
n o r t h e r ne n do fT i m b e rP e a k
a t a b o u tt h e
at the
10,400 foot
e l e v a t i o n .I nt h i ns e c t i o n ,t h eb a s a lz o n eo ft h el o w e rt u f f
o fL e m i t a rM o u n t a i n sc o n t a i n sa p p r o x i m a t e l y
s a n i d i n e , 4 p e r c e n tq u a r t z ,a n dl e s st h a n
6 percent
1 p e r c e n te a c ho f
biotiteandplagioclase.
O v e r l y i n gt h el o w e rz o n e
most o f t h eo u t c r o pi nt h i ss t u d ya r e a .
i s a z o n et h a tm a k e su p
It i s t y p i c a l l y
64
g r a y i s hr e da n dw e a t h e r st oa n g u l a r ,c l i f f yo u t c r o p s .T h i s
z o n e i s c h a r a c t e r i s t i c a l l yp u m i c e - p o o ra n dc o n t a i n s
a p p r o x i m a t e l y1 0t o
,
1 2 p e r c e n tp h e n o c r y s t so fs a n i d i n ea n d
quartz.
Above t h i s m a i n z o n e , t h e l o w e r t u f f o f L e m i t a r
M o u n t a i n sg r a d u a l l yi n c r e a s e s
i n b o t hp u m i c ea n dp h e n o c r y s t
t h eu p p e rp o r t i o n so ft h el o w e r
member, t h e c o n t r a s t o f
l i g h t - g r a yp u m i c ei n
a pale-red
matrix givesthetuff
streaked appearance.
-"The. c o n t a c t -.wi;t-h t z h e . c o n s ' i s t e n t l y
member was mappedwhen
p h e n o c r y s t - r i c hu p p e r
c o n t e n tr e a c h e da p p r o x i m a t e l y
in
40 f e e t .
c o n t e n to v e ra ni n t e r v a lo fa p p r o x i m a t e l y1 0t o
a
t h ep h e n o c r y s t
25 p e r c e n t ,u s u a l l ya c c o m p a n i e d
b yt h ea p p e a r a n c eo fp i n kb l o t c h e sa n dc r y s t a l - r i c hp u m i c e .
e
15 t o
The u p p e r m o s tl o w e rt u f fo fL e m i t a rM o u n t a i n sc o n t a i n s
20 p e r c e n t s a n i d i n e ,
4 t o 6 p e r c e n tp l a g i o c l a s ea n da b o u t
3
p e r c e n tq u a r t z .
u p p e r member
a n dt h i ss t u d y
area
throughout m o s to ft h eS o c o r r o - M a g d a l e n a
a r e a , t h eu p p e rt u f fo fL e m i t a rM o u n t a i n s
w e l d e d t ot h el o w e r
member.
is
E x c e l l e n te x p o s u r e so ft h eu p p e r
member o c c u ri nr o a dc u t sj u s tw e s to fL a n g m u i rL a b o r a t o r y
at
a ne l e v a t i o no fa b o u t1 0 , 5 0 0f e e t .I nt h es o u t h w e s t e r n
p o r t i o no ft h es t u d ya r e a ,t h eu p p e rL e m i t a r
rests
area, t h e
u n c o n f o r m a b l yo nt h eu n i to fH a r d yR i d g e .I nt h i s
u p p e r L e m i t a r e x h i b i t sf e a t u r e ss u c h
*
f o l d i n g ,a n dg a sc a v i t i e s
w e l d i n gi n
as f l o w - b a n d i n g ,
a l l o fw h i c hs u g g e s tp r i m a r y
a l a m i n a r b o u n d a r yl a y e r( C h a p i na n dL o w e l l ,
flow
65
1979).
Maximum d e p o s i t e dt h i c k n e s so ft h eu p p e rt u f fo f
L e m i t a rM o u n t a i n s
i s u n c e r t a i ns i n c e
it i s u n c o n f o r m a b l y
o r b yi n t e r m e d i a t el a v a s ;
o e r l a i nb yt h eP o p o t o s aF o r m a t i o n
h o w e v e r , a s much a s 600 f e e t o f u p p e r
L e m i t a r a r ep r e s e n t
t h r o u g h o u tt h es t u d ya r e a .
is
T h eu p p e rt u f fo fL e m i t a rM o u n t a i n s
p h e n o c r y s t - r i c ha n dv a r i e s
red.
i n c o l o rf r o mm o d e r a t et ol i g h t
It t y p i c a l l yw e a t h e r st os t e e pt a l u s - c o v e r e ds l o p e sa n d
s m a l l c l i f f yo u t c r o p s .
. t o -35 p e r c e n t .I n - t h es o u t h e a s tc w r n e r
R.hW.,
i s 30
The a v e r a g ep h e n o c r y s tc o n t e n t
o f s e c2. 4T
, .bS.,
5 0 t o 1 0 0 f e e t OY t h e u p p e r t u f f o f L e m i t a r
t h eu p p e r
Mountains i s p i n k i s hg r a yt o
buff i n c o l o r
and appears
h y d r o t h e r m a l l yb l e a c h e d ;t h i sz o n ew e a t h e r st od i s t i n c t i v e
e
o f t h i sz o n e
b e n c h e s .T h ep h e n o c r y s tc o n t e n t
as 50 p e r c e n ta n d
( u pt o
i s c h a r a c t e r i z e db y1 0t o
may b e a s much
1 5 p e r c e n tl a r g e
4 m m ) q u a r t zc r y s t a l sa n dc o p p e r yb i o t i t e .O s b u r n
( 1 9 7 8 ) n o t e d a s i m i l a r z o n ew h i c h
w a s l o c a l l yp r e s e r v e d
at
t h et o po ft h eu p p e rt u f fo fL e m i t a rM o u n t a i n s .
T h eu p p e ro n e - h a l ft oo n e - t h i r do ft h eu p p e r
L e m i t a rc a n n o tb ec o n s i s t e n t l yd i s t i n g u i s h e d
orthinsection
from theHells
c o n t r o l i s missing.
1 3 , 24T
, .hS.,
R.4W.)
Mesa T u f f when s t r a t i g r a p h i c
(secs.
On t h e west s i d eo fH a r d yR i d g e
w h e r et h eH e l l s
MesaTuff
a p p a r e n tf a u l tc o n t a c tw i t ht h eu p p e rL e m i t a r ,
*
i n h a n ds p e c i m e n
v i r t u a l l yi m p o s s i b l et ot e l lt h et w ou n i t sa p a r t .
u n a m b i g u o u sm e a n so fd i s t i n g u i s h i n gt h et w ou n i t s
is i n
it i s
If
is
some
66
d i s c o v e r e d ,t h ec o n t a c t s
T h i sp r o b l e m
i n t h i sa r e as h o u l db er e - e x a m i n e d .
may b e f u r t h e r c o m p l i c a t e d
by t h e d i s c o v e r y t o
1979, o r a l commun.) o fu p p e rt u f fo f
t h es o u t h( O s b u r n ,
L e m i t a rM o u n t a i n si na p p a r e n td e p o s i t i o n a lc o n t a c tw i t hH e l l s
Mesa T u f f .
.~
P u m i c e i n t h eu p p e rt u f fo fL e m i t a rM o u n t a i n s
is
u s u a l l yn o tp r o m i n e n t .E x c e p t i o n sa r ej u s ta b o v et h el o w e r
c o n t a c tw h e r e1 , i g h t - c o l o r e dc r y s t a l - r i c hp u m i c ea r e
common
a n dn e a rt h et o po ft h eu n i tw h e r es t r i n g e r so fl i g h t - g r a yt o
b r o w n ,p h e n o c r y s t - r i c &
pu-mice a r e ' common.
The p h e n o c r y s t
a s s e m b l a g e i n t h eu p p e rt u f fo f ' L e m i t a rM o u n t a i n sc o n s i s t so f
a n d b i o t i t ew i t ht r a c ea m o u n t s
s a n i d i n e ,p l a g i o c l a s e ,q u a r t z ,
o ff a y o l i t e
( ? ) a n dm a g n e t i t e S
. a n i d i n e t, h em o s ta b u n d a n t
p h e n o c r y s t ,v a r i e sf r o m
u s u a l l yo c c u r s
1 5 t o 20 p e r c e n to ft h er o c k .
It
a s b r o k e n ,e u h e d r a lt oe m b a y e dc r y s t a l st h a t
a v e r a g e 1 . 0 t o 1 . 2 mm i n l o n g e s td i m e n s i o n .P e r t h i t i c
e x s o l u t i o nl a m e l l e a
m i n e r a l s ;t h en a t u r e
are p r e f e r e n t i a l l y a l t e r e d t o c l a y
o f t h ee x s o l u t i o np h e n o m e n a
i s extremely
v a r i a b l ee v e no nt h es c a l eo fo n et h i ns e c t i o n .
P l a g i o c l a s ev a r i e sf r o ml e s st h a no n ep e r c e n t
t h r e ep e r c e n t
a t t h et o po ft h el o w e rt u f fo f
M o u n t a i n s t o a maximum o fa b o u t
q u a r t e r so ft h e
Lemitar
10 t o 12 percentthree
way u p i n t h eu p p e rL e m i t a r .
q u a r t e ro ft h eu p p e rt u f fo fL e m i t a rM o u n t a i n s
d r o p i n t h ep l a g i o c l a s ec o n t e n tt o
p l a g i o c l a s e i s o f t e nh i g h l y
to
The u p p e ro n e
i s m a r k e db y
l e s s t h a n 5 p e r c e n t .T h e
embayedand
i s t y p i c a l l ya l m o s t
a
67
c o m p l e t e l ya l t e r e dt os e r i c i t ea n dv a r i o u sc l a ym i n e r a l s .
T h i sa l t e r a t i o nh a sd e s t r o y e da l b i t et w i n n i n g
s p e c i m e n s ,a l t h o u g h
a l b i t et w i n n i n g
i n most
i n o n es a m p l e ,p l a g i o c l a s ew i t hp r e s e r v e d
3 . 5 mm
was f o u n d i n a g l o m e r o p o r p h y r i t i cc l o t
i n diameter.
Q u a r t zp h e n o c r y s t s
a r e e u h e d r a lt oa n h e d r a la n d
1t o 3
f r e q u e n t l yr o u n d e da n de m b a y e d Q
. u a r t zc o m p r i s e s
o f t h eu p p e r
p e r c e n to ft h er o c ki nt h el o w e rt w o - t h i r d s
member; i n t h e u p p e r o n e - t h i r d , q u a r t z
p e r c e n to ft h e. r o c k .
makesup
15
a s much a s
O n e . . t ot w op e r c e n tr e d d i s h - b r o w nt o
l i g h t - g r e e nb i o t i t eo c c u r si nv a r y i n gs t a g e so fa l t e r a t i o nt o
m a g n e t i t ea n dh e m a t i t e T
. r a c ea m o u n t so fm a g n e t i t e
p h e n o c r y s t sa p p r o x i m a t e l y
g r o u n d m a s so ft h eu p p e rt u f fo f
0 . 5 mm i n d i a m e t e ra l s oo c c u r .
The
Lemitar M o u n t a i n s i s u s u a l l y
d e v i t r i f i e dt of i n e - g r a i n e da g g r e g a t e so fq u a r t za n da l k a l i
f e l d s p a r ;i r r e g u l a rp a t c h e so fv a p o r - p h a s eq u a r t za r ea l s o
common.
i n s o m es p e c i m e n s ,e s p e c i a l l yt o w a r d s . t h eb a s e ,t h e
originalvitroclastictextureofthetuff
i s w e l lp r e s e r v e d .
T h r o u g h o u tt h es t u d ya r e a ,t h eu p p e rt u f fo f
L e m i t a rM o u n t a i n s
i s p e r v a s i v e l ya l t e r e d ;t h en a t u r ea n d
d e g r e eo fa l t e r a t i o nv a r i e sc o n s i d e r a b l y .W i t h i nt h e
a p p r o x i m a t e l i m i t s o ft h eS a w m i l lC a n y o nc a u l d r o n ,
p l a g i o c l a s eh a sb e e nc o m p l e t e l yr e p a c e d
by c l a y m i n e r a l s a n d
a l k a l if e l d s p a r ;t h ec l a y sa r eu s u a l l yp l u c k e dd u r i n g
s e c t i o np r e p a r a t i o n .T h i st y p eo fa l t e r a t i o nh a sb e e n
d i s c u s s e di nd e t a i lb yO s b u r n
(1978) a n dC h a p i na n do t h e r s
thin
68
(1978) and i s b e l i e v e dt ob et h er e s u l to fa l k a l i
i n the
limits o ft h ec a u l d r o n ,
metasomatismO
. u t s i d et h e
s o u t h w e s tp o r t i o no ft h es t u d ya r e a ,p l a g i o c l a s eh a sb e e n
i s p r e s e r v e dV
. ery
a l t e r e dt os e r i c i t eb u ta l b i t et w i n n i n g
n e a rt h em a r g i n ,b u to u t s i d e ,
someof
t h ep l a g i o c l a s e
p h e n o c r y s t sh a v eb e e np a r t i a l l ya l t e r e dt os e r i c i t e
a n d some
is a l t e r e d t o both
t op o t a s s i u m - r i c hc l a y s .S a n i d i n e
s e r i c i t ea n dp o t a s s i u m - r i c hc l a y ,b u t
i s i n general less
alteredthantheplagioclase.
In t h e " . s o u ' t h w e s t e r n p o r t i o n - . o f ' ' . t h e s t u d y a c e a , - t h e
u p p e rt u f fo fL e m i t a rM o u n t a d n sl o c a l l ye x h i b i t sp r i m a r y
w e l d i n gc h a r a c t e r i s t i c s .
L e m i t a rM o u n t a i n s
Where t h eb a s eo ft h eu p p e rt u f fo f
r e s t s o nt h eu n i to fH a r d yR i d g e
a
c o n s p i c u o u sz o n eo fl i t h i c - r i c ht u f fw i t ha b u n d a n tg a s
c a v i t i e so c c u r s( f i g u r e
11).
t ob ep o o r l yw e l d e db u t
i n t h i ns e c t i o nh a sh i g h l yf l a t t e n e d
The t u f fm e g a s c o p i c a l l ya p p e a r s
a n dd e f o r m e dg l a s s . s h a r d si n d i c a t i n gm o d e r a t et od e n s e
w e l d i n g .T h i sz o n ev a r i e s
i n t h i c k n e s sf r o m
10 t o
50 f e e t .
E l s e w h e r ei nt h i sa r e a ,t h eu p p e rt u f fo fL e m i t a rM o u n t a i n s
e x h i b i t s l o c a l z o n e so ff l o wb a n d i n g ,f l o wf o l d i n g ,a n dr a m p
s t r u c t u r e s( f i g u r e
12).
is i n
One o ft h eb e s te x p o s u r e so ft h e s ef e a t u r e s
1 4 a n dt h e
t h ee x t r e m es o u t h e a s t e r nc o r n e ro fs e c t i o n
n o r t h e a s tc o r n e ro fs e c t i o n
tuff
2 3 , T.bS.,
R.4W.
H e r e t, h eu p p e r
of L e m i t a r M o u n t a i n sa p p e a r st oh a v eb e e nd e p o s i t e d
p a l e o v a l l e y .R e c o g n i t i o n
a n d i n t e r p r e t a t i o no f
in a
many o ft h e
69
F i g u r e 11. L i t h i c - r i c hb a s e
o f t h eu p p e r
tuff of Lemitar
M o u n t a i n sw i t ha b u n d a n tg a sc a v i t i e s .C r o s ss e c t i o n s
of
t h eg a sc a v i t i e sa r es h o r t ,f l a t t e n e d ,e l l i p s e s .
F i g u r e 12.
O u t c r o po fd e n s e l yw e l d e du p p e r
t u f f o fL e m i t a r
M o u n t a i n ss h o w i n gr a m ps t r u c t u r e .F o l i a t i o nd i p st ot h e
left.
featuresinthispaleovalley
was g r e a t l ye n h a n c e db y
w r i t t e n b yC h a p i na n dL o w e l l
(1979) t h a t p r o v i d e s d e t a i l e d
a paper
an
d e s c r i p t i o n sa n di n t e r p r e t a t i o n so fs t r u c t u r e sp r e s e n ti n
a s h - f l o wt u f ft h a t
a paleovalley i n c e n t r a l
was depositedin
Colorado.
The small o u t c r o p o f 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 i ns e c t i o n s
e a s t - t r e n d i n gp a l e o v a l l e y .
d e g r e e st ot h ee a s t
i n an
14 a n d2 4a r eb e l i e v e dt oo c c u r
30
If t h er e g i o n a ld i po f2 5t o
i s r e m o v e df r o mt h em e a s u r e da t t i t u d e s
a l o n gt h es - o u t h e r nm a r g i no ft h eo u t c r o po fu p p e rt u f fo f
Lemitar M o u n t a i n s ,t h et u Y f
i s f o u n dt oh a v ep r i m a r y
f o l i a t i o nd i p st ot h en o r t h .T h u s ,t h es o u t h e r no u t c r o pe d g e
i s believedtorepresentthevalley
0
a r et o w a r dt h ep r e s u m e dv a l l e y
w a l l a n dt h ep r i m a r yd i p s
axis.
C h a p i na n dL o w e l l
(1979) a r g u ec o n v i n c i n g l yt h a tp r i m a r yf o l i a t i o nd i p ss u c h
as
t h o s eo b s e r v e da l o n gt h es o u t h e r nm a r g i no ft h ep a l e o v a l l e y
a r ep r o b a b l yn o p
t r o d u c e db ys e c o n d a r yc o m p a c t i o n T
. hey
c a l c u l a t et h a ts e c o n d a r yc o m p a c t i o nc a n n o tp r o d u c ef o l i a t i o n s
s t e e p e rt h a na b o u t
2 2d e g r e e s .A f t e rc o r r e c t i n gf o r
p o s t - d e p o s i t i o n a lr e g i o n a lt i l t i n g ,d i p s
d e g r e e sa r e
a s s t e e p a s 40 t o 4 5
c o m m o n l yo b s e r v e da l o n gt h ep a l e o v a l l e y
T h e s ed i p sa r et h er e s u l to fp r i m a r yw e l d i n g
b o u n d a r ya l o n gt h e
wall.
i n a laminar
w a l l ofthevalley.
The p r e s e r v a t i o n o f l a r g e r o d - s h a p e d g a s c a v i t i e s
inthevicinityofthevalley
w a l l ( f i g u r e 13) a l s oa r g u e
a g a i n s t much s e c o n d a r yc o m p a c t i o n .T h e s eg a sc a v i t i e sa r e
73
lineatedparallel
t o t h e a x i s ofthevalleylikethose
d e s c r i b e db yC h a p i n
and Lowell
(1979).
T h e s ee l o n g a t e
c a v i t i e s a r e p r e s e n to n l ya l o n gt h es o u t h e r nm a r g i no ft h e
axis, the
v a l l e y ; as o n e t r a v e r s e s n o r t h t o w a r d s t h e v a l l e y
c a v i t i e sd i s a p p e a ra n dt h et u f fh a s
a p r o m i n e n tf l o w
l i n e a t i o nd e f i n e db yt h i n ,l i g h t - c o l o r e ds t r e a k sa n db y
h i g h l yc o m p r e s s e dp u m i c e .T h i ss t r e a k ye u t a x i t i cf o l i a t i o n
i s d e f i n e db y
a c o m b i n a t i o no ff o l i a t i o np l a n e s ,p u m i c e ,a n d
c o l l a p s e dg a sc a v i t i e s .
. t r a c e df o r
Sormedwhen
Some i n d i v i d u a lf l o wp l a n e sc a nb e
as much.-as 8 f e e t .
t h ev o l u m e
The g a s . c a v i t i e s
may h a v e
of e s c a p i n gv o l a t i l e sb e c a m et o og r e a t
f o rs t o r a g eo nf l o wp l a n e sa n dp o c k e t so fg a sf o r m e d( C h a p i n
a n dL o w e l l ,
1979).
Also p r e s e n t i n t h e u p p e r t u f f o f
Lemitar Mountains
i n t h i sp a l e o v a l l e ya r ei n t r i c a t er e c u m b e n tf o l d sw i t ha x e s
p a r a l l e lt ot h ev a l l e y
axis.
T h e ya r eb e s te x p o s e da b o u t
m e t e r sf r o mt h es o u t h e r nm a r g i no ft h eo u t c r o p .T h e s e
i n t e r p r e t e d a s h a v i n gf o r m e d
when t h e t u f f , w h i c h
100
are
was
d e p o s i t e da l o n gt h eo v e r s t e e p e n e ds i d e so ft h ev a l l e y ,
s l u m p e dt o w a r d st h ev a l l e yb o t t o m .T h r o u g h o u t h ez o n eo f
s o l d i n g ,t h es t r o n ge a s t - w e s tl i n e a t i o no ft h et u f fr e m a i n s
c o n s t a n t I. np l a c e s z, o n e so f - ; r e c u m b e n tf o l d i n ga r eo v e r l a i n
b yu n d i s t u r b e dp l a n a rf o l i a t i o na n dt h ew h o l eu n i t
t o g e t h e rC
. h a p i na n dL o w e l l
in the
i s welded
(19i9) d e s c r i b e similar f e a t u r e s
Wall M o u n t a i nT u f fo fc e n t r a lC o l o r a d o .
74
a
T h ed i s t r i b u t i o no ft h e
t u f f o fL e m i t a rM o u n t a i n s
.
a n dt h ef e a t u r e so b s e r v e di nt h ep a l e o v a l l e ya r eo f
p a r t i c u l a ri n t e r e s tb e c a u s ep r i m a r yw e l d i n gf e a t u r e sh a v en o t
b e e no b s e r v e d
i n t h e t u f f o fL e m i t a rM o u n t a i n si no t h e r
localities-andthearealdistributionof.thesefeatures
b eo fi m p o r t a n c ei nr e c o n s t r u c t i n g
deposition.
may
i t s e n v i r o n m e n to f
I t s h o u l db en o t e dt h a tt h em i n e r a l o g i c a l
as d e s c r i b e d
z o n a t i o no ft h eu p p e rt u f fo fL e m i t a rM o u n t a i n s ,
a b o v e , i s p r e s e n tt h r o u g h o u tt h es t u d ya r e a .T h i si n d i c a t e s
.
..-
. th.at the tuff. wit'bthe. anomalous .features
is equivalent to
I
t h e m o r e" n o r m a l "u b p e rt u f fo fL e m i t a rM o u n t a i n s
and t h a t
d i f f e r e n c e si nt h ee n v i r o n m e n t so fd e p o s i t i o nc a u s e dt h e
t e x t u r a lv a r i a t i o n s .
T h em o s it m p o r t a n vt a r i a b l e s
r e s p o n s i b l ef o rt h eo b s e r v e dp r i m a r yw e l d i n gf e a t u r e sw e r e
p r o b a b l ye m p l a c e m e n tt e m p e r a t u r ea n dt h ea m o u n to fg a s
1979)-
e n t r a i n e di nt h ea s hf l o w s( C h a p i na n dL o w e l l ,
T h es o u t h w e s t e r np o r t i o no ft h es t u d ya r e aa p p e a r s
t oh a v er e m a i n e ds t r u c t u r a l l yh i g hd u r i n gc o l l a p s ea n d
f i l l i n go ft h eS a w m i l l
C a n y o nc a u l d r o nb e c a u s et h e
A-L
Peak
T u f f ,t h eu n i to fS i x m i l eC a n y o n ,a n dm o s to ft h el o w e rt u f f
o fL e m i t a rM o u n t a i n sw e r en o td e p o s i t e dt h e r e .T h es u r f a c e
of this structurallyhighblock
c o n s i d e r a b l et o p o g r a p h i cr e l i e f .
may h a v ep o s s e s s e d
The l o w e rt u f fo fL e m i t a r
M o u n t a i n sf i l l e dm o s to ft h er e m a i n i n gd e p r e s s i o no ft h e
S a w m i l lC a n y o nc a u l d r o na n do n l y
0
d e p o s i t e do u t s i d eo ft h ec a u l d r o n .
s m a l l a m o u n t so f
T h eu p p e r
i t were
member w a s t h e n
75
0
e r u p t e do n t ot h er e l a t i v e l ys m o o t hu p p e rs u r f a c eo ft h el o w e r
member w i t h i n t h e c a u l d r o n a n d f l o w e d o v e r t h e w e s t e r n
SawmilC
l a n y o nc a u l d r o nm a r g i n .
o v e rt h e
I
A s t h eu p p e r
member f l o w e d
m a r g i n o n t o a d i s s e c t e db l o c ko ft h eu n i t
of Hardy
was p r o b a b l y c h a n n e l e d i n t o e x i s t i n g v a l l e y s .
R i d g e , . t h et u f f
T h i sc o n c e n t r a t e df l o w
may h a v ec o n s e r v e dh e a ta n da l l o w e d
h o t t e re m p l a c e m e n t e m p e r a t u r e s .
The r o u g ht o p o g r a p h y
i n t h ea s h
a i r t o b e c o m ei n c o r p o r a t e d
a l s oh a v ec a u s e de n o u g h
may
f l o w s o t h a tg a s e se x p r e s s e dd u r i n gp r i m a r yw e l d i n ge x c e e d e d
t h ev o l u m eo f
.gas t i h a t c o u l d b e - ~ i . n c o r p o r a t e d a l o n g f l o w
p l a n e s ,t h u sf o r m i n gc a v i t i e s .
The o b s e r v a t i o n st h a tm o s to f
t h e s ec a v i t i e so c c u rn e a rt h eb a s e
o f t h eu p p e r
member a n d
a l o n gt h em a r g i n so fp a l e o v a l l e y ss u p p o r tt h i s .
T u f fo fS o u t hC a n y o n
Canyon was t h e l a s t m a j o r
The t u f f o f S o u t h
It i s a
a s h - f l o w t u f f e r u p t e d i n t h eS o c o r r o - M a g d a l e n aa r e a .
m u l t i p l e - f l o w ,s i m p l ec o o l i n gu n i t ,r h y o l i t ea s h - f l o wt u f f
1978).
( C h a p i na n do t h e r s ,
t u f fo fS o u t h
Canyon u s u a l l y o v e r l i e s b a s a l t i c a n d e s i t e
O s b u r n ( 1 9 7 8 ) d e s c r i b e d a m e a s u r e ds e c t i o n
(SW 1 / 4 ,
620 f e e t t h i c k i n t h e e a s t e r n M a g d a l e n a M o u n t a i n s
*
sec.
3 0 , T.3S.,
lavas
Lemitar M o u n t a i n s( C h a p i na n d
t h a to v e r l i et h et u f . fo f
o t h e r s , 1978).
I n t h eM a g d a l e n aM o u n t a i n s ,t h e
R.3W.)
for w h i c ht h et u f f
o fS o u t hC a n y o nh a sb e e nd a t e d
a t 26.2 +/-
i s named.
The
1 . 0 m.y.
tuff
(Chapin,
a t 25.8
u n p u b l i s h e dd a t a )a n d
Mehnert,
1978)
b yt h e
areaforthetuff
a n do t h e r s
K-Ar
+/-
1 . 0 m.y.
(Bochman a n d
m e t h o d on b i o t i t e .
a t t h i st i m e ;C h a p i n
i s p o o r l yu n d e r s t o o d
(1978) s u g g e s t e dt h a t
The s o u r c e
i t was e r u p t e d f r o m t h e Hop
west o f t h i s s t u d y a r e a .
C a n y o nc a u l d r o nl o c a t e d
The t u f fo fS o u t hC a n y o nc r o p so u td i s c o n t i n u o u s l y
t h r o u g h o u tt h ee a s t e r na n ds o u t h e r nM a g d a l e n aM o u n t a i n sa n d
(1979)
i n ~ t h eLemitar M o u n t a i n s .P e t t y
reports a maximum o f
a b o u t 400 f e e ti nt 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 n dA l l e n
..
97 8 ,
o r a l commun.). r e p o r t s a maximum o f 3 0 0 f e e t i n . t h e
a r e a s o u t h w e s t ' o fN o r t hB a l d y .
o fS o u t hC a n y o n
The o n l yo u t c r o po ft h et u f f
i n t h i ss t u d ya r e ao c c u r sj u s te a s to ft h e
s u m m i to fS o u t hB a l d yw h e r ea p p r o x i m a t e l y1 5 0f e e t
e
e x p o s e do n
of t u f f i s
a f a u l tb l o c k .H e r e ,t h eb a s eo ft h et u f f
p o o r l ye x p o s e d ;h o w e v e r ,
a t a b o u tt h e
1 0 , 0 4 0 f o o te l e v a t i o n
a small o u t c r o po f
a l o n gt h er o a dt oL a n g m u i rL a b o r a t o r y ,
i s f o u n db e n e a t ht h et u f f .
r e d d i s h - b r o w ns a n d s t o n e
t h e tuffof
o u t c r o pa r e aa b o v e
is
The
L e m i t a r M o u n t a i n sa n db e l o w
i s o b s c u r e db yc o l l u v i u m ;t h eg e n t l e
t h et u f fo fS o u t hC a n y o n
s l o p e i s s u g g e s t i v e o f a n a n d e s i t ei n t e r v a l .O s b u r n
(1978)
r e p o r t sb a s a l t i ca n d e s i t ei nt h i ss t r a t i g r a p h i cp o s i t i o ni n
t h ee a s t e r nM a g d a l e n aM o u n t a i n s .
(1978) d e s c r i b e dt h et u f fo fS o u t hC a n y o ni n
Osburn
I
d e t a i la n ds u b d i v i d e d
i t i n t ot h r e ez o n e s :
p o o r l y - w e l d e d ,c r y s t a l - p o o rz o n e ;
a lower,
a l i t h o p h y s a lz o n e ;a n da n
r
uppep
r u m i c e o u s" s t r e a k e d "z o n e T
. h er e a d e sr e e k i n gd e t a i l e d
77
*
petrographic information is referred to his study.
The tuff
of South Canyon in this study area is likely equivalentto
Osburn’s upper streaked zone.
Here the tuff of South Canyon
is grayish red to pale red and weathers to angular blocks.
Light-gray to pink pumice, 2 to 5 cm.long, make up from 10 to
20 percent of the rockand give it a pronounced eutaxitic
foliation.
The tuff contains approximately 20 percent
phenocrysts of sanidine and quartz in roughly equal
proportions.
Quartz occurs as euhedr.al t . o subhedral,
occasionally embayed, phenocrysts that average 1.0 mm in
diameter.
Sanidine occurs as subhedral to euhedral tabular
crystals approximately 1.0 mm in length.
e
Exsolution lamellae
within the sanidine are commonly altered to potassium-rich
clays.
Also noted were less than 1 percent relict
phenocrysts that havebeen completely replaced by clay
minerals.
These phenocrysts..closely resemble altered
plagioclase phenocrysts in the tuff
of Lemitar Mountains.
One to three percent lithic fragments of aphanitic andesite
also occur.
The obvious lackof tuff of South Canyon in this
study area is 1ikely.theresult of a combination of erosion,
faulting, and lack of deposition.
The top of the unit
elsewhere in the Socorro-Magdalena area is usually erodedand
greater thicknesses
may have been present.
Large-displacement, down-to-the-west faults along the western
p a r t o ft h es t u d ya r e ah a v en o te x p o s e dt h es t r a t i g r a p h i c
i n t e r v a lw h i c hc o n t a i n st h et u f fo fS o u t h
Canyon.Thus
in
t u f f may b e p r e s e n t b e n e a t h t h e i n t e r m e d i a t e l a v a s
s e c t i o n s 1 0 a n d 12 ( T . & S . ,
R.4W.I.
the
On S o u t hB a l d y ,w h e r et h e
i n t e r m e d i a t el a v a so v e r l i et h et u f fo fL e m i t a rM o u n t a i n s . ,t h e
was p r o b a b l yn o td e p o s i t e d .F u r t h e r
t u f f o fS o u t hC a n y o n
mapping t o t h e
west i s n e e d e d t o d e t e r m i n e t h e a r e a l e x t e n t
o f S o u t hC a n y o ni nt h ew e s t e r n
a n dt h i c k n e s so ft h et u f f
M a g d a l e n aM o u n t a i n s .
I n t e r m e d i a t e Lavas
Above t h e - t u f f
i s a sequence
o f L e m i t a rM o u n t a i n s
o f i n t e r m e d i a t ec o m p o s i t i o nl a v a st h a tr a n g em i n e r a l o g i c a l l y
latite.
f r o ma n d e s i t et oq u a r t z
T h e s e l a v a s c r o po u ti n
a
s c a t t e r e dl o c a l i t i e sn e a rt h es u m m i to fS o u t hB a l d ya n di n
west s i d e o f t h er a n g e( s e c s .
b a n da l o n gt h e
T.hS.,
R.4W.I.
1, 12, 13,
14,
L o c a l l y( s e c 2. 4 )p o o r l yw e l d e da s h - f l o w
t u f f 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 ,a n dl a h a r i cb r e c c i a s
o v e r l i et h e
lavas.
s e q u e n c eo f
l a v a s a n dt u f f sw i t hr e s p e c tt ot h et u f fo fS o u t h
The s t r a t i g r a p h i cp o s i t i o no ft h i s
C a n y o nc a n n o tb eu n a m b i g u o u s l yd e t e r m i n e d
H o w e v e r ,t h ef o l l o w i n gl i n e s
a n dt h es u r r o u n d i n g
i n thisstudyarea.
of e v i d e n c e f r o m t h i s s t u d y a r e a
areas s u g g e s tt h a tt h el a v a s
y o u n g e rt h a nt h et u f fo fS o u t hC a n y o n :
f l a n k of t h e r a n g e t h e
may b e
1) a l o n gt h ew e s t e r n
l a v a s a r ec o n f o r m a b l yo v e r l a i nb y
l a h a r i cb r e c c i a so ft h el o w e rP o p o t o s aF o r m a t i o nw h i c h
79
*
contain abundant clasts of the tuff of South Canyon;
2)
Osburn (1979, oral commun.) reports very similar intermediate
lavas above the
tuff
ofSouth Canyon to the southwest of this
study area; 3) on Water Canyon Mesa, Osburn (1978) reports
scattered outcrops of similar intermediate lavas above the
tuff of South Canyon; and
4) Simon (1973) described
100 feet
of variable porphyritic lavas, now
believed to be above the
tuff of South Canyon, in the Silver Hill
area.
In
this study area thickness estimates for the
lavas range from 100 feet on South Baldy to 300 feet along
the western flank of therange.
Two types of lavas were
mapped within the intermediate lavas.
These are:
1)
aphanitic andesites that are similar to basaltic andesites
found above the tuff ofLemitar Mountains elsewhere in the
Socorro-Magdalena area; and 2) a sequence of variable
porphyritic lavas which overlie and are interbedded with the
aphanitic andesites. Local
ash-flow tuffs and volcaniclastic
sedimentary rocks werealso mapped in this interval.
andesitic lavas
The summit of South Baldy is composed of
approximately 400 feet of andesitic lava which is in apparent
depositional contact with the.tuff of Lemitar
Mountains.
Multiple flows of grayish-red to brownish-gray aphanitic
andesite are marked by autobrecciated flow tops and
bottoms.
Excellent exposures of the andesites can
be seen along the
0
road to Langmuir Laboratory at
an elevation of about 10,500
80
a r e common a n d v e s i c l e sa r eu s u a l l y
f e e t .S c o r i a c e o u sz o n e s
f i l l e dw i t hc l a ym i n e r a l sa n dc a l c i t e .
The a n d e s i t e , w e a t h e r s
t os t e e pr u b b l ys l o p e s .
i s c o m p o s e dm a i n l yo f
I nt h i ns e c t i o n ,t h ea n d e s i t e
p l a g i o c l a s em i c r o l i t e s( 0 . . 2
mm i n l o n g e s td i m e n s i o n )w h i c h
h a v eb e e np a r t i a l l yr e p l a c e db yc l a ym i n e r a l s .A b o u to n e
p e r c e n ta l t e r e dp l a g i o c l a s ep h e n o c r y s t sa l s oo c c u r .R e l i c t
0 . 6 mm i n
p h e n o c r y s t so fp y r o x e n ea n d / o ra m p h i b o l e( 0 . 2t o
d i a m e t e r ) make u p 5 t o
7 p e r c e n to ft h er o c k .A b o u t1 0t o
...p ercent f.inely .d.isseminate.d magnetite,
15
now l a r g e l y . o x i d i z e d ,
m a k e s ' u pt h er e m a i n d e ro ft h er o c k .
D a r k - g r a yt og r a y i s h - r e d - p u r p l ea p h a n i t i ca n d e s i t e s
a r e i n d e p o s i t i o n a lc o n t a c tw i t ht h eu p p e rt u f fo fL e m i t a r
e
M o u n t a i n sa l o n g
S i m i l a rl a v a sc r o po u t
w h e r et h e y
i n t h en o r t h w e s tc o r n e ro fs e c t i o n
are o v e r l a i nb yt h ep o r p h y r i t i cl a v a s ;
betweent,h.e.two.
R.4W.
t h e w e s t e r ne d g eo fs e c t i o n2 4 ,T . h S . ,
is i n t e r b e d d e dt h r o u g h
m e t e r s .A p h a n i t i ca n d e s i t e sa l s oc r o po u t
s e c t i o n s 1 a n d1 2( T . h S . ,
R.4W.).
i an
the contact
intervalofabout
i n a band i n
H e r et h e ya r eo v e r l a i nb y
l a h a r i cb r e c c i a sa n dp e b b l ec o n g l o m e r a t e so ft h eP o p o t o s a
F o r m a t i o n .L o c a l l y ,c h a n n e l sf i l l e dw i t ha n d e s i t i cs a n d s t o n e
a r e f o u n di nt h ea n d e s i t e .
Two l a r g eo u t c r o p so ft h et u f fo f
L e m i t a rM o u n t a i n sa r ef o u n dw i t h i nt h i so u t c r o po fa n d e s i t e .
T h e s eo u t c r o p sp r o b a b l yr e p r e s e n t
o ft h et u f f
of L e m i t a r M o u n t a i n s .
24
exhumed t o p o g r a p h i ch i g h s
6
81
I n t h i ns e c t i o n ,t h e s el a v a sa r e
similar t o t h e
a n d e s i t eo nt h es u m m i to fS o u t hB a l d yb u ta r ei n t e n s e l y
altered.
(?)
A f e wr e l i c p
t yroxene
p h e n o c r y s t sa l t e r e d
to
i n g e n e r a l , a l l t h a tc a nb es e e n
m a g n e t i t ea r ev i s i b l eb u t ,
a r e. r e m n a n t so fp l a g i o c l a s em i c r o l i t e s ;m o s to ft h e
p l a g i o c l a s eh a sb e e nr e p l a c e db yc l a ym i n e r a l s
p o r p h y r i t i c lavas
and sericite.
Southweso
t ft h es u m m i o
t fS o u t hB a l d y
a n o r t h - t r e n d i n gr e c t a n g u l a ro u t c r o po fp o r p h y r i t i cl a v a s
.-'
is
in
f a u l t c o n t a c to nt w o .s i d e sw i t ht h e. u p . p e rt u f fo fL e m i t a r
Moun'tainsT
. h i sr e l a t i o n s h i ps u g g e s t st h a tp r e - l a v af a u l t s
o f t h e l a v a s a s was
may h a v e i n f l u e n c e d t h e d e p o s i t i o n
r e p o r t e db yO s b u r n
(1978).
S t a c y ( 1 9 6 8 ) mapped t h i so u t c r o p
a s t h e Mohawk P e a kr h y o l i t e .
p u r p l et o
I t v a r i e sf r o mg r a y i s hr e d
m e d i u md a r kg r a yi nc o l o r .
The l a v a i s o f t e n
v e s i c u l a ra n dl o c a l l yb e c o m e ss c o r i a c e o u sn e a rt h ee r o s i o n a l
t o po ft h eo u t c r o p .
I n t h i ns e c t i o n ,t h e
c o n t e n tf r o m
lava v a r i e s i n phenocryst
2 t o 5 p e r c e n tw i t ha l k a l if e l d s p a r s ,q u a r t z ,
p l a g i o c l a s e r, e l i c pt y r o x e n e
(?)
and
m a g n e t i tp
eresent.
makes u p 1 t o 5
m o r ep h e n o c r y s tr i c hs p e c i m e n s ,q u a r t z
p e r c e n to ft h er o c k .
e m b a y e d ,g r a i n st h a ta r e
It o c c u r s ' t i sr o u n d e d ,o f t e nd e e p l y
a s much a s 4 . 0
. A l k a l if e l d s p a ro c c u r s
mm i nd i a m e t e r .
a s e u h e d r a lt or o u n d e da n d
e m b a y e dp h e n o c r y s t st h a ta v e r a g ea b o u t
Many o f t h e a l k a l i f e l d s p a r p h e n o c r y s t s
2.0
mm i n l e n g t h .
a r e s u r r o u n d e db y
In
a2
reaction r i m s ,
now r e p l a c e db yc l a ym i n e r a l s .
f e l d s p a rp h e n o c r y s t sr a n g e
The a l k a l i
i n d e g r e eo fa l t e r a t i o nf r o mf r e s h
Many o f t h e a l k a l i
t oc o m p l e t er e p l a c e m e n tb yc l a ym i n e r a l s .
f e l d s p a r se x h i b i tp e r t h i t e - l i k et e x t u r e s ;t h i st e x t u r e
z h er e s u l to fa l t e r a t i o n .M i n o ra m o u n t s
phenocrysts are a l s o seen.
may b e
of p l a g i o c l a s e
Some of t h ep l a g i o c l a s ea p p e a r s
t oh a v eb e e nr e p l a c e db ya l k a l if e l d s p a rb u ti n t e n s e
a l t e r a t i o no b s c u r e st h er e l a t i o n s h i p .
R e l i c tp y r o x e n e
( ? ) p h e n o c r y s t s make u p 5 t o 8
pal percent o f t h - e l a v a ; . t h e y h a v e
.a s h o r h , s t u b b y h a b i t w i t h
The rims of t h e s ep h e n o c r y s t sa r e
h e x a g o n a lc r o s ss e c t i o n s .
a brown,
c o m p l e t e l yr e p l a c e db ym a g n e t i t ea n dt h ec o r e sb y
m o d e r a t e l yb i r e f r i n g e n t ,f i b r o u sm i n e r a lw h i c h
a s much a s 3 mm i n
u r a l i t e .G l o m e r o p o r p h y r i t i cc l o t s
d i a m e t e ro fa l k a l i
occasionally.
may b e
( ? ) f e l d s p a ra n dr e l i c tp y r o x e n ea r e
seen
T h eg r o u n d m a s sc o n s i s t so sf i m p l eo r
C a r l s b a d - t w i n n e dp l a g i o c l a s em i c r o l i t e s ,w h i c hh a v eb e e n
a l t e r e dt oc l a ym i n e r a l sa n ds e r i c i t e ,
a n d a b o u t 1 0 t o 15
p e r c e n tf i n e l yd i s s e m i n a t e dm a g n e t i t e .
A d i s t i n c t i v ep o r p h y r i t i cl a v ac r o p so u ti nt h e
2 3 , T.4S.,
n o r t h e a s tc o r n e ro fs e c t i o n
R-bW.
p e r c e n tl i g h t - c o l o r e df e l d s p a rp h e n o c r y s t s ,
i nl e n g t h ,c o n t r a s ts t r o n g l yw i t h
g r o u n d m a s sT
. h el a v a
A s much a s 30
a s much a s 1 . 0
a p a l e - r e dt ol i g h t - g r a y
i s i nf a u l tc o n t a c tw i t ht h et u f fo f
L e m i t a rM o u n t a i n sa l o n gt h es o u t h e r nm a r g i no ft h e
p a l e o v a l l e yd e s c r i b e da b o v e .
The c o n t a c tr e l a t i o n s h i p sw i t h
cm
83
t h eu n i to fH a r d yR i d g ea r eu n c e r t a i nb u t
i n t r u s i v e .F l o wf o l i a t i o n s
may b e p a r t l y
i n t h el a v aa r eq u i t ev a r i a b l e .
The s i g n i f i c a n t d i f f e r e n c e i n e l e v a t i o nb e t w e e nt h et w o
i n s e c t i o n2 3
o u t c r o p so fu p p e rt u f fo fL e m i t a rM o u n t a i n s
s u g g e s t s ~ t h a tc o n s i d e r a b l ed o w n - t o - t h e - n o r t hr e l i e fe x i s t e d
d u r i n gd e p o s i t i o no fb o t ht h et u f fo fL e m i t a rM o u n t a i n sa n d
l a v a s may h a v eb e e nl o c a l l y
t h ep o r p h y r i t i cl a v a s .T h u s ,t h e
p r e s e r v e da g a i n s t
i n theunit
a s t e e pe s c a r p m e n ta n ds c a l l o p
of H a r d yR i d g e L
. o c a l l y t, h el a v ac o n t a i n sa b u n d a n t
a s much 'as 20 cm i n d i . a ? l l e t e r ,. o fa p h a n i t i c ~ .
fragments,
l a v a f o r m ss t e e pc l i f f sa n dl e d g yo u t c r o p s .
' a n d e s i t eT
. he
S i m i l a rl a v a sw i t hs m a l l e ra n df e w e rp h e n o c r y s t s
c r o po u t
i n t h en o r t h w e s tc o r n e ro fs e c t i o n2 4 ,T . h S . ,
R.4W.
H e r et h ep o r p h y r i t i cl a v a so v e r l i ea n da r ei n t e r b e d d e dw i t h
I n g e n e r a l ,t h e
t h ea p h a n i t i ca n d e s i t e sd e s c r i b e da b o v e .
l a v a s h a v es m a l l e ra n df e w e rp h e n o c r y s t sn o r t h w a r df r o m
s e c t i o n 23.
I nt h i ns e c t i o n ,t h ep o r p h y s i t i cl a v ac o n t a i n s
(?I,
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 ,r e l i c ta m p h i b o l e
biotite,
4
a n d a l k a l if e l d s p a r .Q u a r t zo c c u r s
as l a r g e
(Up
t o
mm) e m b a y e dp h e n o c r y s t s .O f t e n ,t h eq u a r t zp h e n o c r y s t sa r e
f r a c t u r e da n dp a r t l yd i s a g g r e g a t e d
may b es u r r o u n d e db y
s o t h a t a l a r g ep h e n o c r y s t
many s m a l l e rf r a g m e n t s .Q u a r t zm a k e su p
3 t o 8 p e r c e n to ft h er o c k .
P l a g i o c l a s ef e l d s p a r
p h e n o c r y s ta n dm a k e su pa b o u t
i s t h em o s ta b u n d a n t
1 0 p e r c e n to ft h el a v a .' T h e
84
p l a g i o c l a s e i s u s u a l l ya l t e r e dt os e r i c i t ea n dc l a y
a p o c k m a r k e d or s i e v et e x t u r e .
a n do f t e nh a s
minerals
I n many
i s s o c o m p l e t et h a tn or e l i c tt w i n n i n g
s a m p l e s ,a l t e r a t i o n
is
as
v i s i b l eT
. h ep l a g i o c l a s eo c c a s i o n a l l yo c c u r s
a n d c o r r o d e de d g e s .
g l o m e r o p o r p h y r i t i cc l o t sw i t hr o u n d e d
A l k a l if e l d s p a rp h e n o c r y s t s
m a k eu pf r o mo n e
to
fivepercentofthelavaandareonlyslightlyalteredto
s e r i c i t ea n dc l a ym i n e r a l s .
i n l o n g e s td i m e n s i o n .
:
c o n c e n t r i cz o n i n g
One p h e n o c r y s tm e a s u r e d
1.0
cm
Some o ft h ep h e n o c r y s t se x h i b i t
( ? ) a n d a n i n d i s t i n c t wavy e x t i n c t i o n ;
(1978) i n t e r p r e t e d
o t h e r sh a v eu n m i x i n gt e x t u r e s O
. sburn
s i m i l a r f e a t u r e s i n t h eW a t e rC a n y o n
Mesa l a v a s t o b e
i n d i c a t i v eo fs e c o n d a r yp o t a s s i u mf e l d s p a rt h a th a dr e p l a c e d
e
plagioclase.
It i s p o s s i b l et h a tt h e" a l k a l if e l d s p a r
p h e n o c r y s t s "i nt h i sl a v aa r et h er e s u l to fs e c o n d a r y
r e p l a c e m e n to fp l a g i o c l a s e .
Two t o t h r e e p e r c e n t e u h e d r a l t o s u b h e d r a l
amphibole
( ?now
I
r e p l a c e db ym a g n e t i t ea l s oo c c u r .B i o t i t e
invaryingstagesof,alterationtomagnetite
o n ep e r c e n to ft h el a v a .
T h eg r o u n d m a s s
makes u pa b o u t
i s c o m p o s e do f
plagioclasemicrolitesthatarelargelyreplacedbyclay
m i n e r a l sa n ds e r i c i t e .
a s h - f l o w t u f f a n dv 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 sI nt h e
*
n o r t h w e s tc o r n e ra n da l o n gt h ew e s t e r nm a r g i no fs e c t i o n
T..4S.,
R-hW.
p o o r l yw e l d e d ,l i t h i c - r i c ha s h - f l o wt u f f s ,
24,
laharic breccias, and volcaniclastic sedimentary rocks crop
out.
Lithic fragments of aphanitic andesite, phenocryst-poor
rhyolite lavas, and a distinctive, phenocryst-rich ash-flow
tuff are common in this interval.
The ash-flow tuff
fragments are very similar to the upper tuff
o f Lemitar
The thickness o f the tuffs and volcaniclastic
Mountains.
rocks is a maximum o f about 200 feet in the northeast corner
o f section
24.
They weather to rubble-covered slopes and
ledgy outcrops.
In
section,the. tuffs are made up o f 10 t o 15
thin
percent phenocrysts o f quartz, plagioclase, and alkali
feldspar.
Quartz occurs as embayed phenocrysts and varies
from 3 to 5 percent o f the tuff.
The original vitroclastic
texture o f the groundmass has been completely destroyed by
devitrification.
Popotosa Formation
In the Socorro-Magdalena area, the basal
unit o f
the Santa Fe Group consists
of laharic breccias,
conglomerates, sandstones, and playa mudstones.
This
assemblage was named the Popotosa Formation
by Denny (1940)
for exposures along Arroyo Popotosa, southwest
of the Ladron
Mountains.
The Popotosa Formation was deposited as alluvial
fans in a broad sedimentary basin which
spanned the Rio
Grande rift in the Socorro region during the interval from
about 26 may. to
7 m.y. ago (Chapin and Seager, 1975)-
86
S o u r c er e g i o n sf o r
much o ft h eP o p o t o s aF o r m a t i o nw e r et h e
C o l o r a d oP l a t e a ua n dt h eL a d r o nM o u n t a i n st ot h en o r t ha n d
t h eG a l l i n a s ,M a g d a l e n a ,a n dS a nM a t e oM o u n t a i n st ot h ew e s t
a n ds o u t h .
K-Ar
on l a v a f l o w s- w h i c ho v e r l i ea n d
d a t e so b t a i n e d
a r ei n t e r b e d d e dw i t ht h eP o p o t o s aF o r m a t i o ni n d i c a t et h a t
26 m.y.
d e p o s i t i o ns p a n n e dt h ep e r i o df r o ma b o u t
7 m.y.
a g o( C h a p i n ,
1 9 7 8 , o r a l commun.).
.'
P o p o t o s aF o r m a t i o n
20.0
+ / - 0.8
w e s t e r n p a r to ft h i ss t u d ya r e a( s e c .
i nt h eP o p o t o s aF o r m a t i o n .
a t 18.0
114,
sec.
I nt h e
R.4W. ) ,
and are i n t e r b e d d e d
A s a m p l e o f t h ei n t e r b e d d e dl a v a
+/- 0.8 m.y.
( C h a p i n ,u n p u b l i s h e d
of D e e rP l o tT a n ki n t r u d et h eP o p o t o s a
F o r m a t i o ni nt h ee x t r e m es o u t h w e s t e r np o r t i o n
a r e a (SW
m.y.
11, T . 4 S . ,
r h y o l i t e l a v a s o f M a g d a l e n aP e a ko v e r l i e
d a t a ) .T h ed a c i t e s
I n t h ee a s t - c e n t r a l
r e s t s u n c o n f o r m a b J yo nW a t e rC a n y o n . M e s a
l a v a sw h i c hh a v eb e e nd a t e da t
h a sb e e nd a t e d
ago t o about
(1978) r e p o r t st h a tt h el o w e r
M a g d a l e n aM o u n t a i n s O
, sburn
'-
2 3 , T.hS.,
R.4W.);
of t h es t u d y
t h e s e l a v a s h a v en o t
b e e nd a t e d .
B r u n i n g ( 1 9 7 3 ) d i v i d e dt h eP o p o t o s aF o r m a t i o ni n t o
f a n g l o m e r a t ea n dp l a y af a c i e s C
. hapin
a n d o t h e r s (1978)
s u b s e q u e n t l yd i v i d e dt h eP o p o t o s aF o r m a t i o ni n t ol o w e ra n d
u p p em
r e m b e r sT
. h el o w e r
member i s c h a r a c t e r i z e db y
i n t e r t o n g u i n gm u d f l o wd e p o s i t s ,c o n g l o m e r a t e sa n dm i n o r
*
its
l a c u s t r i n ed e p o s i t s ;t h eu p p e r
member, i n t h eS o c o r r oa r e a ,
i s d o m i n a t e db ym u d s t o n e s ,s i l t s t o n e s ,a n ds a n d s t o n e s .
was
I nt h i ss t u d y ,t h eP o p o t o s aF o r m a t i o n
i n f o r m a l l yd i v i d e di n t ot h r e e
map u n i t s :
1) l o w e rl a h a r i c
3)
b r e c c i a s ; 2 ) i n t e r b e d d e dc o n g l o m e r a t e sa n ds a n d s t o n e s ;a n d
f i n e - g r a i n e d l, a m i n a t e d s, a n d s t o n e sa n ds i l t s t o n e s .
The
a n d c o n g l o m e r a t e sa r ep r o b a b l yc o r r e l a t i v e
l a h a r i cb r e c c i a s
(1978) l o w e r member a n dt h el a m i n a t e d
w i t hC h a p i na n do t h e r s '
s i l t s t o n e sa n ds a n d t o n e s
may b e c o r r e l a t i v e w i t h t h e i r u p p e r
member.
l a h a r i cb r e c c i a s .L . a h a r i cb r e c c ' i a sc r o po u t
i n two
near t h es u m m i to fS o u t h
f a u l t - b o u n d e d ,n o r t h - t r e n d i n gs t r i p s
B a l d y .H e r e ,t h e yw e r ed e p o s i t e do nt h eu p p e rt u f fo f
lavas".
L e m i t a rM o u n t a i n sa n dt h e" i n t e r m e d i a t e
b r e c c i a so c c u r
The l a h a r i c
a s r u b b l e - c o v e r e ds u r f a c e st h a th a v eb e e n
h i g h l yd i s t u r b e db yf r o s ta c t i o n ;t h e ya r e
a maximum o f a b o u t
350 f e e t t h i c k .
On t h e
c r o po u t
west^ s i d e o f t h e r a n g e , l a h a r i c b r e c c i a s
i n a. n o r t h - t r e n d i n g b a n d ( s e c s .
1, 12, T.
4S.,
a
a n df o r mv e r ys t e e p ,r u b b l e - c o v e r e ds l o p e s .H e r et h e ya r e
maximum o fo f
500 f e e t i n t h i c k n e s s .
The l a h a r i cb r e c c i a s
w e r ed e p o s i t e do na n d e s i t e sa n d" i n t e r m e d i a t e
B e a rC a n y o n( s e c .
P o p o t o s aF o r m a t i o n
1, T . h S . ; R . h W . ) ,
lavas".
t h el o w e rc o n t a c t
i s o b s e r v e dt ob eg r a d a t i o n a lo v e ro n e
m e t e r ,f r o ma n d e s i t i cr u b b l et om a t r i x - s u p p o r t e dc l a s t so f
andesite.
)
R.4W.
In
of t h e
aa
e
I n o u t c r o p ,t h el a h a r i cb r e c c i a sv a r y
r e d d i s hb r o w nt og r a y i s hr e da n da r e
from pale
so w e l l i n d u r a t e d t h a t
j o i n t sa n d . f r a c t u r e sb r e a kt h r o u g ht h ec l a s t s .
c o n s i s to f
andtuffsin
The b r e c c i a s
a n a s s o r t m e n to fc l a s t sf r o mt h eu n d e r l y i n gl a v a s
a m a t r i x o fc l a yt os a n d - s i z e dp a r t i c l e s .
A v e r a g ec l a s ts i z ev a r i e sf r o mc o b b l et ob o u l d e ra n d
b l o c k so ft h et u f f
d i a m e t e rT
. h em o s t
o f S o u t hC a n y o na r e
some
a s much a s 2 m e t e r s i n
common c l a s tl i t h o l o g i e sa r et u f f
of
S o u t hC a n y o n ,a p h a n i t i ca n d e s i t e s ,a n dp o r p h y r i t i c
i n t e r m e d i a t e lavas.
I n p l a c e s . ,t h e .l a h a r i cb r e c c i a se x h i b i t
5 cm i n
c r u d eb e d d i n gw i t hi n d i v i d u a lb e d sa p p r o x i m a t e l y
t h i c k n e s s ;- h o w e v e r ,i n t e r n a ls t r a t i f i c a t i o n
i s u s u a l l yn o t
present.
c o n g l o m e r a t e sa n ds a n d s t o n e sT h em a j o r i t yo ft h eP o p o t o s a
Formation i n t h i ss t u d ya r e ac o n s i s t s
of i n t e r b e d d e d
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
. h e yc r o po u te x t e n s i v e l y
in
a maximum
t h ew e s t - c e n t r a lp o r t i o no ft h es t u d ya r e aa n dh a v e
t h i c k n e s s of a b o u t 800 f e e t .
The l a v a so fM a g d a l e n aP e a k
and
a s s o c i a t e da s h - f l o wt u f f so v e r l i et h ec o n g l o m e r a t e sa n d
s a n d s t o n e s i n t h en o r t h e r np a r to ft h eo u t c r o p
t h ee a s t e r ne d g eo fs e c t , i o n
11 ( T . b S . ,
R.LW.),
area.
l a v a so f
M a g d a l e n aP e a ka r ei n t e r b e d d e di nt h eP o p o t o s aF o r m a t i o n .
E x c e l l e n te x p o s u r e so ft h ec o n g l o m e r a t e sa n ds a n d s t o n e sc a n
b es e e na l o n gB e a rC a n y o n
( s e c s . 1, 11, T . b S . ,
R.4W.).
Along
89
T h ec o n g l o m e r a t e sa n ds a n d s t o n e sa r ew e l li n d u r a t e d
a n dt y p i c a l l yw e a t h e rt ol e d g yo u t c r o p sa n dc o l l u v i u mc o v e r e d
s l o p e s .T h e ya r eb o t hu s u a l l yd a r kr e d d i s hb r o w nt om o d e r a t e
r e d i n c o l o r .I ng e n e r a l ,c l a s ts i z ed e c r e a s e sw i t h
s t r a t i g r a p h i ch e i g h t ;c l a s t ss i z e sr a n g ef r o mc o b b l e st o
Clast l i t h o l o g i e sa r ed o m i n a t e db ya n d e s i t i c
c o a r s es a n d .
l a v a sa n da s h - f l o wt u f f s ,i n c l u d i n gt h et u f f
o f S o u t hC a n y o n
i n the
a n dt u f fo fL e m i t a rM o u n t a i n s .P e b b l ei m b r i c a t i o n s
c o n g l o m e r a t e si n d i c a t et h a t
of t h e s t u d y
much o f i t w a s d e r i v e ds o u t h w e s t
area.
s a n d s t o n e sa n ds i l t s t o n e s .I nt h es o u t h w e s t e r np o r t i o no ft h e
R.bW.),
s t u d ya r e a( s e c .2 4 ,T . 4 S . ,
f i n e - g r a i n e ds a n d s t o n e s
a n ds i l t s t o n e so v e r l i et h et u f fo fL e m i t a rM o u n t a i n s .
l a h a r i cb r e c c i a s
a r e aS
. m a l al m o u n t so f
The
a n d c o n g l o m e r a t e sa r en o tf o u n d
in this
s i m i l a r s a n d s t o n e sw e r eo b s e r v e dn e a r
t h et o po ft h ec o n g l o m e r a t es e c t i o ni nt h en o r t h e r np a r to f
t h es t u d ya r e as u g g e s t i n g . t h a tt h e ya r et h ey o u n g e s t
of t h e
p r e s e r v e dP o p o t o s aF o r m a t i o ni nt h es t u d ya r e a .
J u s t s o u t ho fD e e rP l o t
Tank ( s e c .
231, a s e q u e n c e
o fw e l l - s o r t e d ,f i n e - g r a i n e d ,l a m i n a t e ds a n d s t o n e sa n d
s i l t s t o n e s i s p r e s e r v e d i n a n a r r o wg r a b e n ;t h i ss e q u e n c e
a minimum o f 8 0 0 f e e tt h i c k .
The s a n d s t o n e sa n ds i l t s t o n e s
v a r y i n c o l o rf r o md a r kr e d d i s hb r o w nt og r a y i s hp i n k ;t h e y
a r eu s u a l l yw e l li n d u r a t e da l t h o u g h
friable.
some h o r i z o n s a r e v e r y
is
*
The d i s t r i b u t i o n o f l i t h o l o g i e s w i t h i n t h e P o p o t o s a
F o r m a t i o ni nt h i ss t u d ya r e ai n d i c a t e ss t r u c t u r a lc o n t r o l
of
t h e i re n v i r o n m e n t so fd e p o s i t i o n S
. o u t ho fH a r d yC a n y o n o
, n
t h es t r u c t u r a lb l o c kw h i c h
i s l a r g e l y madeup
of t h e u n i t o f
H a r d yR i d g eo v e r l a i n- b yt h eu p p e rt u f fo fL e m i t a rM o u n t a i n s
(secs.
23, 2 4 , T.4S.,
R.4W.1,
t h el a h a r i cb r e c c i a sa n d
c o n g l o m e r a t e so ft h eP o p o t o s aF o r m a t i o na r en o tf o u n d T
. his
was h i g h
r e l a t i o n s h i ps u g g e s t st h a tt h i ss t r u c t u r a lb l o c k
d u r i n gd e p o s i t i o n
of t h e s e s t r a t a .
The p r e s e n c eo f
8 0 0f e e to ff i n e l y - l a m h a t e ds a n d s t o n e s
i n d i c a t i v eo f
at l e a s t
and siltstones,
a s t a b l e ,l o w - r e l i e fe n v i r o n m e n to fd e p o s i t i o n ,
s u g g e s t st h eb u r i a lo ft o p o g r a p h yb yt h i sl a t e s tP o p o t o s a .
Thus i t s e e m s l i k e l y t h a t
a s t r u c t u r a lz o n e ,r o u g h l y
c o i n c i d e n tw i t hH a r d yC a n y o ns e p a r a t e d
f r o m a h i g h l a n dt ot h es o u t h .
a basin tothenorth
The b a s i n was f i l l e d w i t h
was t h e n
c o n g l o m e r a t e sa n ds a n d s t o n e sa n dt h ew h o l ea r e a
b l a n k e t e dw i t hf i n e - g r a i n e db a s i n - f l o o rd e p o s i t s .
s o u t h e r nm a r g i no ft h i sb a s i n
i s c o i n c i d e n tw i t ht h ep r o p o s e d
s o u t h e r nm a r g i no ft h eM a g d a l e n ac a u l d r o n
relatedto
The
and i s probably
it.
Lavas o f M a g d a l e n aP e a k
I n t h en o r t h w e s tp o r t i o no ft h es t u d ya r e a ,
s e q u e n c eo fr h y o l i t i c
*
l a v a s a n dm i n o ra s h - f l o wt u f f so v e r l i e s
c o n g l o m e r a t e sa n ds a n d s t o n e s
s o u t ho ft h et o w n
a
o f t h eP o p o t o s aF o r m a t i o n .J u s t
o f M a g d a l e n aa r ee x c e l l e n te x p o s u r e so f
i n thisareaare
similar l a v a sw i t hw h i c ht h el a v a s
t e n t a t i v e l yc o r r e l a t e d .
The r h y o l i t el a v a s
i n t h i ss t u d y
a l a r g ef i e l do fr h y o l i t e sw h i c hc r o po u t
areaarepartof
o f t h eM a g d a l e n aM o u n t a i n s ,f r o m
a l o n gt h ew e s tf l a n k
Mag-dalenaP.eak.on
thenorthto
A l a m e d aS p r i n go nt h es o u t h .
Weber ( 1 9 5 7 ) p r o v i d e sd e t a i l e dp e t r o g r a p h i cd e s c r i p t i o n sa n d
c h e m i c a la n a l y s e sf o rr h y o l i t e sf r o mt h eS t e n d e lP e r l i t e
1 4 , 2 3 , T.3W.,
o f t h es t u d ya r e a( s e c s .
d e p o s i tn o r t h
R.4S. ) .
S a m p l e s o f t h el a v a so fM a g d a l e n aP e a kf r o mt h e
S t e n d e lP e r l i t ed e p o s i ta n dt h et y p el o c a l i t yh a v eb e e nd a t e d
a t 14.0 +/-
0 . 5 m.y.
0.7
m.y.
1 9 6 3 ) a n d 13.1 + / -
( W e b e ra n dB a s s e t t i
1978) r e s p e c t i v e l y .
( C h a p i n J, a h n s a, n do t h e r s ,
A
s a m p l e o f a r h y o l i t el a v af r o mt h i ss t u d ya r e aw h i c h
is
i n t e r b e d d e d i n t h eP o p o t o s aF o r m a t i o nh a sb e e nd a t e d
a t 18.0
+/-
0 . 8 m.y.
( C h a p i n ,u n p u b l i s h e dd a t a ) .T h i sl a v ah a sb e e n
tentativelycorrelatedwiththelavas
a l t h o u g h i t s a g es u g g e s t st h a t
s e q u e n c eo f
o f M a g d a l e n aP e a k
i t may r e p r e s e n t a n o l d e r
l a v a s w h i c hd on o tc r o po u te l s e w h e r ei n
this
s t u d ya r e a .
A c o m m o n l yo b s e r v e do u t c r o ps e q u e n c e
is:
oM
f a g d a l e n aP e a k
i n t h e lavas
1) w e l l - i n d u r a t e dc o n g l o m e r a t e sa n d
s a n d s t o n e so ft h eP o p o t o s aF o r m a t i o no v e r l a i nb y
2) thin,
d i s c o n t i n u o u sl e n s e so fp o o r l yw e l d e d ,l o c a l l yr e w o r k e d ,
l i t h i c - r i c ha s h - f l o w
" f r o t h "r h y o l i t e
t u f f o v e r l a i nb y3 )h i g h l yv e s i c u l a r
l a v a w h i c hg r a d e su p w a r di n t o
l a y e r s o f p o r p h y r i t i cl a v aa n dv i t r o p h y r e .E x c e l l e n t
4) alternating
e x p o s u r e s o f t h i ss e q u e n c ec a nb es e e n
e l e v a t i o no f
i n BearCanyon
at an
8000 f e e t .
t u f fi n t e r v a lT h i nd i s c o n t i n u o u sl e n s e so fp o o r l yw e l d e d
a s h - f l o w . . t u f fa r e
lavas.
commonlyfound
a t t h eb a s eo ft h er h y o l i t e
S i m i l a rt u f f s ,w h i c ha r el o c a l l yw a t e r l a i d ,a r ef o u n d
1 a n d 2 , ThS.,
i ns e v e r a ll a r g eo u t c r o pb l o c k si ns e c t i o n s
R.4W.
H e r e ,t h ec o n t a c t sb e t w e e nt h el a v a sa n dt u f f sa r en o t
w e l el x p o s e d H
. o w e v e rw
, h e r ee x p o s e d t, h e
verticalfoliationandthecontactsare
l a v a h a sn e a r l y
marke.dbyabundant
s i l i c i f i c a t i o na n di n t r u s i v ej a s p e r o i dv e i n l e t s .T h e s e
r e l a t i o n s h i p ss u g g e s tt h a tt h e
l a v a s may b e l o c a l l y
intrusive.
The t u f f i s w h i t et og r a yb r o w n
i n c o l o ra n d
in
c o n t a i n sd a r kr e d d i s h - b r o w nl i t h i cf r a g m e n t st h a tr a n g e
d i a m e t e rf r o m
0.05
t o 1 . 5 cm.
5 t o 20 p e r c e n to ft h er o c k .
e q u i d i m e n s i o n a l( u pt o
f r a g m e n t s .T h et u f f
L i t h i cf r a g m e n t s
make u pf r o m
The t u f fo f t e n .c o n t a i n sl a r g e ,
5.0 cm) y e l l o w i s h - g r a yp u m i c e
i n p l a c e sw h e r e
i s c r u d e l yb e d d e da n d ,
i t h a sb e e nr e w o r k e d ,e x h i b i t sg r a d e db e d d i n g .
I nt h i ns e c t i o n ,t h et u f fc o n t a i n sp h e n o c r y s t so f
q u a r t z , a l k a l i f e l d s p a r ,p l a g i o c l a s ea n db i o t i t e ;t h e
r e l a t i v ea m o u n t so fp h e n o c r y s t sa r eh i g h l yv a r i a b l e .Q u a r t z
i s u s u a l l yt h em o s ta b u n d a n ta n do c c u r s
e m b a y e dp h e n o c r y s t s
m a k e su p
I
2 to
as b o t hr o u n d e da n d
a s w e l l a s a n g u l a rf r a g m e n t s ;q u a r t z
5 p e r c e n to ft h er o c k .A l k a l if e l d s p a r
93
c o m p r i s e s 1 t o 5 p e r c e n to ft h et u f f ;
it o c c u r s a s e u h e d r a l
0 . 2 t o 1 . 5 ,mm i n
t os u b h e d r a lp h e n o c r y s t sw h i c hr a n g ef r o m
in trace
longesd
t i m e n s i o nP
. l a g i o c l a s ef e l d s p a ro c c u r s
amounts a n do c c u r s
r h y o l i t ef l o w s
as t a b u l a rc r y s t a l s
1 t o 2 mm i n l e n g t h .
The l a v a s o M
f a g d a l e n aP e a k
p o r p h y r i t i cf l ~ o w s M
. i n e r a l o g i c a l l y ,t h e
are massive,
l a v a s v a r yf r o m
q u a r t zl a t i t et or h y o l i t e ;c h e m i c a l l y ,h o w e v e r ,t h e ya r e
r h y o l i t e s( W e b e r ,
1957).
The r h y o l i t e sa r eo f t e np a r t i a l l y
v i t ~.
c i c and- dnterlayered vitrophyxes. are characteris-tic. The
c o n t a c tb e t w e e nd e v i t r i f i e dl a v a l a n dv i t r o p h y r e
is
g r a d a t i o n a l .T h eb a s a lp o r t i o n so fi n d i v i d u a lf l o w s ,
i n c l u d i n gt h ev i t r o p h y r e s ,a r eo f t e na u t o b r e c c i a t e d .
t o t a lt h i c k n e s s
The
o f t h e lavas i s d i f f i c u l tt oe s t i m a t eb e c a u s e
it i s a
o ft h ep o s s i b i l i t yo fu n r e c o g n i z e df a u l t i n g ,b u t
minimum o f 5 0 0 f e e t .
Ramp s t r u c t u r e sa n dc o n v o l u t e df l o w
b a n d i n ga r ef o u n d ~ l o c a l l yb u t ,
in g e n e r a l , t h e l a v a s d i p t o
t h e west a t 1 0 t o 2 0 d e g r e e s .
The l a v a s w e a t h e rt o
r l a t - t o p p e dh i l l sw i t hv e r ys t e e ps l o p e s .O u t c r o p sa r e
c h a r a c t e r i z e db ya b u n d a n to p a l i n es i l i c aw h i c h
fills
f r a c t u r e sa n dc a v i t i e s .
I nh a n ds p e c i m e n ,t h e
l a v a s v a r y i n c o l o rf r o m
lightgreenishgraytograyishorangepink
or l i g h t b r o w n .
P h e n o c r y s t s o f f e l d s p a r ,h o r n b l e n d e ,b i o t i t e ,a n dq u a r t z
v i s i b l e .V i t r o p h y r el a y e r sr a n g ei nc o l o rf r o mb l a c kt od a r k
b l u i s hg r a ya n dv a r i o u sh u e so fr e d .
are
.
94
In thin section, the rhyolitescommonly have a
devitr,ified groundmassand spherulitic textures are.sometimes
preserved.
The vitrophyres have both perlitic and
spherulitic textures.
Plagioclase i s the most abundant
phenocryst and makes u p ~ 7to 10 percent of the rock;it
exhibits albite, Carlsbad, and pericline twinsand is often
strongly zoned.
The plagioclase occurs as rounded
phenocrysts which average 1.0 to 1.5 mm in lengthand as
glomeroporphyritic clots as much as5 mm in diameter.
-
..Quartz comprises-titrace to 5 percent o f the rock;
i
1
it occurs as r'ounded and embayed phenocrysts which average 1
to 2 mm in diameter.
In some samples, coarse aggregates of
secondary quartz fill fractures and cavities.
a
Alkali
feldspar is not present in some samples and, where present,
seldom exceeds 2 percent.
The alkali feldspar occurs as
anhedral, corroded phenocrysts which average1 to 1.5 mm in
Hornblende comprises 1 to 2 percent of
longest dimension.
the rhyolite and occurs as euhedral to.subhedra1 phenocrysts
which average 1.5 mm in length; it has often been partially
replaced by magnetite.
Biotite occurs a s lath-shaped
phenocrysts about 1.0 mm in length and makes up 0.5 to 1
percent o f the rock.
The lavas of Magdalena Peak
are part of the bimodal
I
suite of rhyolitic to rhyodacitic and basaltic lavas that
were erupted in the Socorro-Magdalena area over the interval
8
r
from about 20 m.y.
to 7 m.y.
ago.
These lavas are much
95
i n thearea.
y o u n g e rt h a nt h em a j o ra s h - f l o wt u f fc a u l d r o n s
Much o f t h i s s u i t e o f r h y o l i t e a n d b a s a l t
is i n close
4)
p r o x i m i t yt ot h et r a n s v e r s es h e a rz o n e( f i g u r e
which
s u g g e s t st h a tt h i ss t r u c t u r eh a sc o n t r o l l e dt h e i re m p l a c e m e n t
1978).
( C h a p i na n do t h e r s ,
The l a v a so fM a g d a l e n aP e a ka r e
of t h e
a l s ol o c a t e dv e r yc l o s et ot h ew e s t e r nm a r g i n
M a g d a l e n ac a u l d r o na n dt h e i re r u p t i o n
was a l s o l i k e l y
c o n t r o l l e db yt h i ss t r u c t u r e .
T e r t i a r yI k t r u s i v eR o c k s
T e r t i a r yi n t r u s i v er o c k sa r ef o u n dt h r o u g h o u tm o s t
a r e a ; t h e ya r ec o n c e n t r a t e d
o ft h es t u d y
i n t h ec e n t r a l
a r e a a n da r ea b s e n tf r o mt h en o r t h w e s tq u a r t e r
p o r t i o no ft h e
o ft h ea r e a .T h em o s tp r o m i n e n to ft h e s ea r et h ed a c i t e s
of
i n the
D e e rP l o tT a n kw h i c hi n t r u d et h eP o p o t o s aF o r m a t i o n
s o u t h w e s t e r nc o r n e r
T.kS.,
o f t h es t u d ya r e a
(SW 1/4, s e c .
23,
R.4W. ). A l s o p r e s e n ta r e" w h i t er h y o l i t e "a n d
a n d e s i t i cd i k e sw h i c hi n t r u d em o s to ft h eu n i t so l d e rt h a n
t h eP o p o t o s aF o r m a t i o n .
I n t h e SW 1/4, NW 1/4, s e c .
23 ( T . b S . ,
R.bW.1,
two
o u t c r o p so fc o l l u v i a lb r e c c i a ,w i t h i nt h eP o p o t o s aF o r m a t i o n ,
c o n s i s t o f f r a g m e n t so fw h i t er h y o l i t ei n t r u s i v er o c k
muddy m a t r i x .
in a
T h e s eo u t c r o p s ,d o w n f a u l t e dt ot h ee a s t ,
s u g g e s tt h a tt h eP o p o t o s a
t h e s ei n t r u s i v e s .
was a t l e a s t l o c a l l y d e p o s i t e d o n
96
d a c i t e so fD e e rP l o tT a n k
A p o r p h y r i t i cd a c i t el a v ac r o p s
o u t i n t h es o u t h w e s t e r nc o r n e ro fs e c t i o n
More e x t e n s i v eo u t c r o p so ft h e
23 (T.4S.,,
l a v a a r ef o u n ds o u t h w e s to f
l a v a a r ef o u n d
t h es t u d ya r e a .E x c e l l e n te x p o s u r e so ft h e
w h e r e a s t r e a mh a sc u t
c e n t e ro ft h eo u t c r o p .
R-4W.)-
a s t e e p - w a l l e dc a n y o nt h r o u g ht h e
A t t h es o u t h e r ne n do ft h i sc a n y o n ,
a b u n d a n tv e i n l e t s
of j a s p e r o i dc u tt h el a v a .C o n t a c t sw i t h
I
t h eP o p o t o s aF o r m a t i o na r es t e e pa n dh i g h l ys i l i c i f i e dw h i c h
s u g g e s tt h a tt h e
The l a v a s o fD e e rP l o t
l a v a s a r ei n t r u s i v e .
Tank weather to-.-steep,- talus-covered .slopes and cliffs.
' , I nh a n ds p e c i m e n ,t h e
w h i t ea n d
i s o f t e nf l o w - b a n d e d .
-
l a v a i s p a l er e dp u r p l et o
I t c o n t a i n s1 0t o
p h e n o c r y s t so f e l d s p a r ,q u a r t z ,a n db i o t i t e .
20 p e r c e n t
The f e l d s p a r s
i n some c a s e s ,o n l y
may a p p e a rc h a l k yi nh a n ds p e c i m e no r ,
s k e l e t a lr e m a i n sa r ep r e s e n t .S e v e r a lp e r c e n td a r kt o
c o p p e r yb i o t i t ec a na l s ob es e e ni nh a n ds p e c i m e n .
I n , t h i ns e c t i o n ,t h el a v ac o n t a i n s
1 0 t o 20 p e r c e n t
( ? ) , and
p h e n o c r y s t so fq u a r t z ,a l t e r e dp l a g i o c l a s e ,s a n i d i n e
b i o t i t e .Q u a r t zo c c u r s
t h a ta v e r a g ea b o u t
a s e u h e d r a lt o
embayed p h e n o c r y s t s
0.5 mm i n d i a m e t e ra n d
make up 1 0 t o 20
p e r c e n to ft h er o c k .
P l a g i o c l a s ef e l d s p a rp h e n o c r y s t s ,
l o n g e s td i m e n s i o n ,m a k e su pa b o u t
0 . 5 t o 1 . 0 mm i n
5 p e r c e n t of t h e l a v a s ;
m o s th a v eb e e nh i g h l ya l t e r e da l t h o u g ha l b i t et w i n n i n g
o b s e r v e d i n a f e w p h e n o c r y s t s .C l a ym i n e r a l sa n d
was
an
\
o p t i c a l l yc o n t i n u o u sm i n e r a l ,w h i c h
may b e a l k a l i f e l d s p a r ,
97
0
have replaced the plagioclase in most samples.
These
secondary minerals have often been plucked
out during thin
section preparation.
In some specimens, small,
inward-growing quartz crystals line cavities where
plagioclase phenocrysts once existed.
Biotite makes up 3 to
5 percent
o f the rock and
occurs as lath-shaped phenocrysts which are aligned
subparallel to the flow structure.
Some of the biotite,
which occurs within highly altered phenocrysts, appears tobe
secondary.
The groundmass of the.-Lava has been replacedby
clay minerals and silica indicating intense hydrothermal
alteration.
white rhyolite intrusives White rhyolite dikes are very
common in the northernMagdalena Mountains (Blakestad, 1978;
Allen, 1979; Sumner, in progress).
These dikes are thought
t o be related to thenumerous Tertiary granitic plutons in
the northern Magdalenas.
In this study area and to thesouth
and east, the dikesare much less common and often appear to
be related to small domal intrusions (Osburn, 1978; Petty,
1979; Allen, 1979; Roth’, in progress).
The majority of thedikes in this area, and
elsewhere in the Magdalenas, intrude north-trending faults.
The dikes are typically less than 1 5 meters in thickness;
*
however, in
the southwestern part of the area, the outcrops
are suggestive of a larger intrusive body.
The dikes are
o f t e nh i g h l yf r a c t u r e da n dj o i n t e da n dw e a t h e rt op l a t y
i s common w i t h i ns e v e r a lf e e t , o f
t a l u s .S i l i c i f i c a t i o n
i n t r u s i v ec o n t a c t s .
to
I nh a n ds p e c i m e n ,t h ew h i t er h y o l i t e sa r ew h i t e
l i g h tg r a yi nc o l o r .
Most a r ep o r p h y r i t i ca l t h o u g h
s t r o n g l yf l o w - b a n d e da n da p h a n i t i c .
some a r e
I n t h i ns e c t i o n ,r o u n d e d
p h e n o c r y s t so fq u a r t za n ds a n i d i n e ,w h i c ha v e r a g ea b o u t1 . 0
mm i nd i a m e t e r ,
make u p 1 5 t o 20 p e r c e n t o f t h er o c k .I n
a
f e ws p e c i m e n s ,t r a c ea m o u n t so fb i o t i t ea n dp l a g i o c l a s ea r e
found.
in
The s o u r c e f o r m o s to ft h ew h i t er h y o l i t ed i k e s
i s probably a n
t h ec e n t r a lp o r t i o no ft h es t u d ya r e a
i n t r u s i v ec e n t e rm a r k e db y
a
a w h i t er h y o l i t e
o ft h es t u d ya r e ai nS a w m i l lC a n y o n .
dome j u s t s o u t h
A c r u d er a d i a lp a t t e r n
o f d i k e sh a sb e e no b s e r v e da b o u tt h i si n t r u s i v e( R o t h ,o r a l
commun.,
1978).
I n t h es o u t h w e s t e r np a r t
1 / 4 , sec., 23, T.hS.,
o f t h ea r e a
(NW
a ne l l i p t i c a lo u t c r o p
of w h i t e
r h y o l i t es u r r o u n d e db yp e d i m e n tg r a v e l sa n da no u t c r o po f
a
R.bW.),
l i t h i c - r i c hv a r i e t yo fw h i t er h y o l i t ea r es u g g e s t i v eo f
a
l a r g e rb o d y .T h e s ew h i t er h y o l i t ed i k e sa r ei n t r u d e da l o n g
t h eh y p o t h e s i z e ds o u t h e r nm a r g i no ft h eM a g d a l e n ac a u l d r o n .
m a f i cd i k e s
t h ec e n t r a l
A f e wt h i n ,f i n e - g r a i n e d ,m a f i cd i k e so c c u r
p a r t of t h es t u d ya r e a .
The d i k e sa r et y p i c a l l y
a d i s t i n c t i v eg r e e nc o l o ra n da r eu s u a l l yd e e p l yw e a t h e r e d .
T h e ya r eu s u a l l yf o u n dp a r a l l e lt ow h i t er h y o l i t ed i k e sa l o n g
a common s t r u c t u r e .
in
99
In thin section, the dikesconsist o f plagioclase
microlites with about 5 to 10 percent plagioclase phenocrysts
and less than 5 percent relict pyroxene
1s
(?I;
the Plagioclase
largely altered to sericite and the pyroxene
( ? ) to
-chlorite. Trace amounts o f magnetite are also present.
Tertiary-Quaternary Deposits
piedmont gravels
Extensive westward-sloping piedmont
deposits o f unconsolidated gravels are found along the
western flank of the Magdalena Mountains.
The deposits
consist o f coalescing alluvial fans derived largely from the
western flank o f th'e Magdalena Mountains.
They have been
partially dissected and thicknesses in excess o f 5 meters
have been observed in arroyos.
Immediately to the west o f
the study area (sec. 22, T.hS.,
R.bW.),
the gravels rest
unconformably on conglomerates o f the Popotosa Formation.
The gravels consistof cobble to sand-sized clasts
o f ash-flow tuffs and andesitic lava;
the tuffof Lemitar
Mountains is the most common clast lithology. Large blocks
of the
tuff of Lemitar Mountains are often found protruding
from these surfacesand are believed to be large talus blocks
which have come to reston the alluvial fans.
100
Q u a t e r n a r yD e p o s i t s
i n a c t i v ed r a i n a g e sw e r em a p p e d
a l l u v i u mS t r e a ms e d i m e n t s
a l l u v i u m .T h el a r g e s ta r e a sa r e
as
i n s t r e a mb e d sw h i c hd i s s e c t
t h er h y o l i t ea n dP o p o t o s aF o r m a t i o nt e r r a n ei nt h en o r t h w e s t
a perennial
c o r n e ro ft h ea r e aa n da l o n gS a w m i l lC a n y o nw h e r e
s t r e a mf l o w s .
t a l u s / c o l l u v i u mA c t i v et a l u ss l o p e s ,r o c kg l a c i e r s ,a n d
s t a b i l i z e dc o l l u v i u mc o v e rl a r g ep o r t i o n so ft h es t u d ya r e a .
west s i d e o f t h e r a n g e a n d
T h ep r o m i n e n tt a l u ss l o p e so nt h e
a l o n gt h eE a s tF o r ko fS a w m i l l
Canyon a r ec o m p o s e dc h i e f l yo f
t h et u f fo fL e m i t a r . M o u n t a i n s- w h i c hf o r m ss t e e pc l i f f sa b o v e
e
t h et a l u s .C o l l u v i u mc o v e r sg e n t l e rs l o p e sa n dg r a d e si n t o
was mapped
p i e d m ognr ta v egIlensn. e r taal l, u s / c o l l u v i u m
o n l yw h e r ei m p o r t a n tg e o l o g i cc o n t a c t s
or f a u l t s w e r e
obscured.
t e r r a c eg r a v e l so f
u n k n o w na g eA l o n gt h ew e s tf l a n ko ft h e
West F o r ko fS a w m i l lC a n y o na r es e v e r a lo u t c r o p so f
u n c o n s o l i d a t e dg r a v e l s .I nt h es o u t h e r n m o s to u t c r o p ,t h e
g r a v e l s a r e i n e x c e s so f
1 0 0 f e e tt h i c k( f i g u r e
a r et y p i c a l l yp r e s e r v e do nr e m n a n t so fo l dt e r r a c e s
a s 250 f e e ta b o v et h e
They
15).
as much
p r e s e n t c a n y o nf l o o r .S i m i l a rt e r r a c e s
w i t hm i n o ra m o u n t so fg r a v e l sp r e s e r v e da r ea l s os e e na l o n g
*
t h e East Fork o fS a w m i l lC a n y o n .B e l o wt h e s et e r r a c e s ,t h e
c a n y o n w a l l s a r e much s t e e p earn rde f l e c t
a p e r i o odrfa p i d
101
d o w n - c u t t i n g .T h i sp e r i o d
may b e a t l e a s t i n p a r td u et ot h e
c a p t u r eo ft h ea n c e s t r a lR i oG r a n d e
i n m i d d l eP l e i s t o c e n e
time.
The g r a v e l sc o n s i s to fr o u n d e dt os u b r o u n d e d
in a
c o b b l e sa n db o u l d e r so fl o c a l l yd e r i v e dv o l c a n i cr o c k s
m a t r i x o fc o a r s et of i n es a n d .
The c l a s tl i t h o l o g i e s
d o m i n a t e db yt h et u f fo fL e m i t a rM o u n t a i n s ,H e l l s
are
Mesa T u f f ,
a n d e s i t e ,a n dt h eP o p o t o s aF o r m a t i o n .
l a n d s l i d ed e p o s i t sA l o n gt h ee a s t e r ns i d eo fS a w m i l lC a n y o n
a n dj u s ts o u t ho fL a n g m u i rL a b o r a t o r ys e v e r a ll a r g el a n d s l i d e
b l o c k sh a v eb e e nm a p p e d .T h eb l o c k sa r es l i c e so ft h eu n i t
o fS i x m i l e
C a n y o nw h i c hh a v em o v e d
down s l o p ea l o n gl o wa n g l e
d e t a c h m e n tf a u l t s .I n t e r n a ls t r a t i g r a p h y
i s p r e s e r v e dw i t h i n
t h ef a u l ts l i c e s ;h o w e v e r ,t h e ya r ec h a r a c t e r i z e db yh i g h l y
v a r i a b l es t r i k ea n dd i p sa n dr u b b l yo u t c r o p s .
The f a u l t s l i c e s b e c a m e d e t a c h e d a n d
moveddown
s l o p eu n d e rt h ei n f l u e n c eo fg r a v i t yd u et ot h el o ws t r e n g t h
o ft h ep o o r l yw e l d e dt u f f sa n ds e d i m e n t a r yr o c k sw h i c ho c c u r
a t t h e i rs t r a t i g r a p h i cb a s e .
k n o w n ;h o w e v e r ,t h eb l o c k so nt h e
C a n y o na p p e a rt o- r e s to nQ u a t e r n a r y
The a g eo ft h i sf a u l t i n g
e a s t f l a n ko fS a w m i l l
talus deposits.
is not
102
STRUCTURAL GEOLOGY
Regional Structure
The structural development of the Magdalena
Mountains reflects three distinct periods of regional
tectonic activity:
1) Laramide uplift and erosion; 2)
Oligocene calc-alkaline volcanism; and 3 ) regional extension
and block-faulting related to the Rio Grande rift.
Laramide uplift and erosion During
Late Cretaceous to middle
Eocene a regional compressional stress field gave rise to
large north-trending uplifts in the Socorro-Magdalena area.
The area of the present day Magdalena Mountains lay on the
flank of a major north-trending uplift (Loughlin and
Koschman.n, 1942; Chapin and others, 1978).
time (approximately 4 5 m.y.
In middle Eocene
ago), relaxation of compressional
stresses brought an end to uplift; erosion'leveled topography
and formed a regional surface of low relief (Epis
and Chapin,
1975).
In the northern Magdalena Mountains area, erosion of
the Laramide uplift exposed LatePaleozoic rocks and
sediments which were shed northward into the west-trending
Baca Basin to form the Baca Formation
of Eocene age (Chapin,
1978; and oral commun., 1979).
Oligocene volcanism About
37 m.y. ago calc-alkaline volcanic
rocks were erupted onto the lateEocene regional erosion
103
surface.
I n C o l o r a d oa n d , N e wM e x i c ot h e s ee r u p t i o n sb e g a n
theconstruction
of t h e w e l l known S a nJ u a na n d
I n t h eS o c o r r o - M a g d a l e n a
D a t i l - M o g o l l o nv o l c a n i cf i e l d s .
a r e a , t h eS p e a r sF o r m a t i o nm a r k st h eb a s e
D a t i l - M o g o l l o nf i e l d .
andesiticto
The S p e a r sF o r m a t i o nc o n s i s t so f
l a t i t i c b r e c c i ac o m p l e x e sa n dq u a r t z
1978).
a s h - f l o wt u f f s( C h a p i na n do t h e r s ,
m.y.
of t h e
latite
Betw.een 32 a n d 2 6
a g o , a t l e a s t f i v eo v e r l a p p i n gc a u l d r o nc o m p l e x e sf o r m e d
i n t h eS o c o r r o - M a g d a l e n aa r e aa n db l a n k e t e dt h eS p e a r s
F o r m a t i o nw i t h t h o u s a n d so ff e e t
o f a s h - f l o wt u f f sa n d
i n t e r c a l a t e da h d e s i t i cl a v a s .
r i f t i n New
A m a j o rf e a t u r eo ft h eR i oG r a n d e
a s e r i e so fn o r t h e a s t - t r e n d i n g
M e x i c oa n dC o l o r a d oa r e
b a s e m e n tl i n e a m e n t s( C h a p i n
h a v eh a d
a n d o t h e r s , 1 9 7 8 ) ; t h el i n e a m e n t s
a m a j o ri n f l u e n c eo nt h et e c t o n i ca n dm a g m a t i c
h i s t o r y of t h e r i f t .
A s t h eR i oG r a n d e
e ne c h e l o na c r o s st h e s el i n e a m e n t s
r i f t opened, it b r o k e
a n d b a s i n so ne i t h e rs i d e
1978).
d e v e l o p e do p p o s i n gs y m m e t r i e s( C h a p i na n do t h e r s ,
l i n e a m e n t sa r ea c t i n g
a s i n c i p i e n tt r a n s f o r mf a u l t sw h i c h
c o n n e c t e n e c h e l o ns e g m e n t s
of t h e r i f t .
The p o r t i o n so ft h e
l i n e a m e n t sc o n n e c t i n gt h ee n d so fb a s i n sw e r es u b j e c t e dt o
a
s c i s s o r s - l i k et o r q u ei nt h eb r i t t l en e a rs u r f a c er o c k sa n dt o
t r a n s v e r s es h e a r
a t d e p t h( C h a p i n ,
1978).
Some o f t h e s e
t r a n s v e r s es t r u c t u r e sh a v eb e e nt h es i t eo fe x t e n s i v e
v o l c a n i ca c t i v i t y .
C h a p i na n do t h e r s
The
T h eS o c o r r ot r a n s v e r s es h e a rz o n e
( 1 9 7 8 ) i s o n eo ft h e s el i n e a m e n t sa n dt h e
of
104
five overlapping cauldron complexes recognized
in the
Socorro-Magdalena area lie onor near this shear zone, as do
Miocene and Pliocene rhyolite domes and flows.
Regional extension
'Between 32 and 27 m.y.
ago, a regional,
extensional, stress field reactivated the southern Rocky
Mountains which is a north-trending zone
of weakness that
developed during late Paleozoic and early Tertiary orogenies.
This extension marks the beginning
of the Rio Grande rift
i .
(Chapsn and^ Seager, 1975).
Chapin (1978) divided the rift
into three segments, each with its own structural style and
history.
The Socorro-Magdalena area is within the southern
segment of the rift which has undergone the
most extension
a
and iswiderthanthetwonorthern
segments.
The rift
bifurcates near Socorro and a weaker arm (San Augustin rift)
extends southwestward into Arizona (Chapin,
1971).
The
Socorro-Lemitar Mountains and the Magdalena Mountains lie in
a triangular
rift.
area
between
the
San
Augustin
arm and the
main
Much of this triangular area is characterized by
closely spaced, subparallel normal faults which
have
undergone progressive rotation of as much as
6 0 degrees
(Chamberlin, 1978; Osburn, 1978).
Chamberlin (1978) termed
this kind of faulting"domino-style'' faulting and related it
1.0
shallow levels of ductile horizontal extension within the
crust during periods of high heat flow.
e
Domino-style
faulting was common inthe Lemitar Mountains and the
e
M a g d a l e n aM o u n t a i n sf r o ma b o u t2 9t o2 0
f r o m 1 2 t o 7 m.y.
a g oa n d
m.y.
ago a n d a g a i n
was a c c o m p a n i e d b y l o c a l s i l i c i c
rates
v o l c a n i s m .D u r i n gp e r i o d so fl o w e rh e a tf l o wa n dl o w e r
o fe x t e n s i o n
( 2 0 t o 1 2 m.y.
7 m.y.
a g o ,a n d
ago t o p r e s e n t ) ,
f a u l t i n go c c u r r e da l o n gw i d e l ys p a c e dn o r m a lf a u l t s ' w i t h
littleor
1978).
n or o t a t i o no ff a u l tb l o c k s( C h a m b e r l i n ,
B r o a de p i o r o g e n i cu p l i f td i s r u p t e - dt h eb a s i n so f
..
t h eR i oG r a n d e
r i f t betwen 7 and
4
m.y.
o ft h ep r e s e n td a yt o p o g r a p h yw h i c h
a g aon fdo r m e m
d uch
i s c h a r a c t e r i z e db ys t e e p
19781.~
m o u n t a i nr a n g e sa n dn a r r o wb a s i - n s( C h a p i n ,
E p i o r o g e n i cu p l i f th a sc o n t i n u e ds i n c e
much r e d u c e dr a t e .
4
m.y.
The m a j o r i t yo fs t r u c t u r e
a g ob u t
at a
i n t h i ss t u d y
area i s t h er e s u l to fi n t e r a c t i o nb e t w e e nt h eS o c o r r o
e
t r a n s v e rssheezaor ncea,u l d r sotnr u c t u r easen,xd t e n s i o n a l
f a u l t i n g .( F i g u r e
1 4 shows t h el o c a t i o no fc a u l d r o n
s t r u c t u r e s , f a u l t s , a n d t h eS o c o r r os h e a rz o n ei nt h i ss t u d y
area. )
L o c a lS t r u c t u r e
32 t o
a s h - f l o wt u f fc a u l d r o n sD u r i n gt h ep e r i o df r o ma b o u t
25 m.y.
a g o , a t l e a s t f i v em a j o ra s h - f l o wt u f fc a u l d r o n sw e r e
f o r m e d i n t h eM a g d a l e n aa n dS a nM a t e oM o u n t a i n s
a n do t h e r s ,1 9 7 8 ) .P o r t i o n so ft h r e e
area (Chapin
o f t h e s ec a u l d r o n sa r e
e x p o s e di nt h i ss t u d ya r e a ,t h eN o r t hB a l d y ,S a w m i l lC a n y o n ,
a n dM a g d a l e n ac a u l d r o n s .
106
was f o r m e d
N o r t hB a l d yc a u l d r o nT h eN o r t hB a l d yc a u l d r o n
a b o u t 32 m.y.
Mesa T u f f .
a g od u r i n gt h ee r u p t i o no ft h eH e l l s
was c e n t e r e d i n t h eM a g d a l e n aM o u n t a i n sa l t h o u g h
T h ec a u l d r o n
i t s f u l l a r e a l e x t e n t i s n o t w e l l known.
Two r e m n a n t so ft h e
N o r t hB a l d yc a u l d r o na r ee x p o s e di nt h i ss t u d ya r e a ;b o t h
o c c u ra l o n gt h em a r g i no ft h eS a w m i l l
Canyon c a u l d r o n .
I nt h en o r t h e a s tc o r n e r
of t h e a r e a ,
a minimum o f
1 0 0 0f e e to fg e n t l yd i p p i n gc a u l d r o n - f a c i e sH e l l s
Mesa T u f f
i s p a r to fa no u t c r o pb e l tt h a te x t e n d s
i s e x p o s e d ;t h i s
n o r t h w a r d t o N o r t hB a l d y .I nt h es o u t h - c e n t r a la n d . . s o u t h w e s t
p a r t s o ft h es t u d ya r e a( s e c .
19, T . h S . ,
R.3W.1,
H e l l sM e s a -
T u f fa n dt h eu n i to fH a r d yR i d g ea r ee x p o s e d ;t h eu n i t
H a r d yR i d g er e p r e s e n t sc a u l d r o n
of
fill ( l a r g e l yr h y o l i t i c )o f
theNorthBaldy.cauldron.
SawmilC
l a n y o na n dM a g d a l e n ac a u l d r o n sT h em a j o r i t yo f
t h i ss t u d y
a r e a i s l o , c a t e dw i t h i nt h eS a w m i l lC a n y o na n d
M a g d a l e n ac a u l d r o n sw h i c hw e r ef o r m e db e t w e e n3 2a n d2 8m - y .
a o w i t ht h ee r u p t i o no ft h e
A-L
P e a kT u f f .T h i ss t u d y
s u g g e s t st h a tt 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 sa r e
a c o m p o s i t es t r u c t u r ea n da r ej o i n e dn e a rt h es u m m i to fS o u t h
B a l d yC
. hapin
member o f t h e
a n d o t h e r s ( 1 9 7 8 ) b e l i e v et h a t h ef l o w - b a n d e d
A-L
P e a k was e r u p t e df r o mt h eM a g d a l e n a
c a u l d r o na n dt h ep i n n a c l e s
c a u l d r o n .T h es i m i l a r i t yo ft h e
member f r o mt h eS a w m i l lC a n y o n
A-L
Peak i n t h i s a r e a t o t h e
r l o w - b a n d e dm e m b e cr o m p l i c a t e st h i si n t e r p r e t a t i o n H
. owever,
107
t h ep r e s e n c eo ft h e
o f t h eu n i to fS i x m i l e
samemembers
i n the
Canyon i n b o t h c a u l d r o n s s u g g e s t s t h a t c o l l a p s e
M 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
was a p p r o x i m a t e l y
contemporaneous.
T h en o r 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
i n t h en o r t h e a s tc o r n e ro ft h es t u d ya r e a
i s w e l le x p o s e d
w h e r e it c a n b et r a c e de a s t w a r dt o w a r d s
1978;
Petty,
r o a d (sec.
1978).
B u c kP e a k( O s b u r n ,
Due n o r t ho tf h eL a n g m u i L
r aboratory
5 , T.bS.,
R.3W.),
i s a near
t h ee x p o s e dm a r g i n
Mesa T u f f on
v e r t i c a lc o n t a c tb ~ e t w e e nc a u l d r o nf a c i e sH e l l s
t h en o r t ha n da n d e s i t ea n ds e d i m e n t a r yr o c k so ft h eu n i to f
S i x m i l eC a n y o no nt h es o u t h ;f u r t h e rt ot h ew e s t ,
d o w n - t o - t h e - w e s tn o r m a lf a u l t sj u x t a p o s et h eu p p e rt u f fo f
L e m i t a rM o u n t a i n sa g a i n s tH e l l s
Mesa T u f f .
T h ec o n t a c tb e t w e e nt h eH e l l s
Mesa T u f fa n dt h e
u n i to fS i x m i l e
Canyon i s d e p o s i t i o n a la n dt h u sr e p r e s e n t s
t h et o p o g r a p h i c
w a l l o ft h eS a w m i l C
l a n y o nc a u l d r o n T
. he
l a r g eo u t c r o p so fv 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 sa l o n gt h e
c o n t a c ta r ei n t e r p r e t e d
an
a s l a r g eb l o c k sw h i c hf e l lf r o m
u n s t a b l ep o r t i o no ft h ec a u l d r o nw a l li n t ot h ea n d e s i t e so f
t h eu n i to fS i x m i l e
Canyon.
To t h e e a s t o f t h i s s t u d y a r e a ,
(1978).
s i m i l a r r e l a t i o n s h i p sa r er e p o r t e db yO s b u r n
w e s t e r nM a g d a l e n aM o u n t a i n s ,t h eH e l l s
o fS i x m i l eC a n y o na r e
w e s t e r nm a r g i n
In the
Mesa T u f f a n d t h e u n i t
i n d e p o s i t i o n a lc o n t a c ta l o n gt h e
o f t h eM a g d a l e n ac a u l d r o n( A l l e n ,
1979).
108
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 nc a nb et r a c e da l o n gt h ew e s ts i d eo ft h eW e s tF o r k
o fS a w m i l lC a n y o na n dn o r t h w e s t w a r do v e rt h ec r e s to fH a r d y
R i d g eT
. h em a r g i n
i s r e p r e s e n t e d by a n o r t h w e s t - t r e n d i n g
- d o w n - t o - t h e - e a s tf a u l tw i t ht h e 'u n i to fH a r d yR i d g ea n dt h e
f a c i e s A-L
H e l l s Mesa T u f fo nt h ew e s ta n dc a u l d r o n
o nt h e
east.
P e a kT u f f
no
i s p o o r l ye x p o s e d ,h a sl i t t l eo r
T h i sf a u l t
t o p o g r a p h i ce x p r e s s i o n ,a n dd i p ss t e e p l yt ot h es o u t h w e s t .
To t h e s o u t h o f
t h i s s t u d y area, a l a r g er h y o l i t e
dome h a s
b e e ni n t r u d e da l o n gt h ef a u l ta n dh a sc a u s e dl o c a l l yi n t e n s e
a l t e r a t i o no ft h e
A-L
P e a kT u f f .J u s tw e s to ft h ec r e s to f
H a r d yR i d g e ,t h ef a u l tt r e n d sn e a r l yd u ew e s t ;y o u n g e r
d o w n - t o - t h e - w e s tf a u l t sh a v ej u x t a p o s e dt h eu p p e rt u f fo f
L e m i t a rM o u n t a i n sa g a i n s t h eu n i to fS i x m i l eC a n y o n .H e r e ,
t h eS a w m i l lC a n y o nc a u l d r o nm a r g i nj o i n st h e
two
s o u t h w e s t - t r e n d i n gM a g d a l e n ac a u l d r o nm a r g i n T
. h u s t, h e
c a u l d r o n sa r ej o i n e di n
d u m b b e l ls h a p e( f i g u r e
a n a r r o wz o n ea n d . f o r m
14).
P a r a l l e lt oH a r d y
z o n ew h i c h
a crude
I believetobe
Canyon i s a g e o l o g i c a l l yc o m p l e x
a n e x p r e s s i o no ft h es o u t h e r n
m a r g i no ft h eM a g d a l e n ac a u l d r o n .T h i ss o u t h w e s t - t r e n d i n g
z o n eh a si n f l u e n c e dt h ed e p o s i t i o no ft h em a j o rv o l c a n i ca n d
s e d i m e n t a r yu n i t sy o u n g e rt h a nH e l l s
Mesa T u f f R
. ocks
n o r t h of t h i s z o n e a r e d o m i n a t e d b y t h e M i o c e n e
M a g d a l e n aP e a k ,t h eP o p o t o s aF o r m a t i o na n dt h eu n i to f
S i x m i l eC a n y o n .
To t h es o u t h ,t h eu p p e rt u f fo fL e m i t a r
tothe
lavas of
110
M o u n t a i n sa n dt h ei n t e r m e d i a t el a v a sr e s tu n c o n f o r m a b l yo n
it a p p e a r st h a tt h es o u t h e r n
o f H a r d yR i d g e T
. hus,
t h eu n i t
b l o c k w a s s t r u c t u r a l l yh i g hf o l l o w i n ge r u p t i o no ft h e
P e a kT u f f .
A-L
The a b s e n c eo fl o w e rP o p o t o s aF o r m a t i o n
on t h i s
i t r e m a i n e dh i g hd u r i n gd e p o s i t i o no ft h e
b l o c ks u g g e s t st h a t
P o p o t o s aF o r m a t i o n .A n o t h e rp r o m i n e n tf e a t u r eo ft h i sz o n e
i s i t s i r r e g u l a r ,s c a l l o p e da p p e a r a n c ew i t hi n t e r m e d i a t e
lavas and tuffrestingagainst
a b u t t r e s so ft h eu p p e rt u f f
o f L e m i t a rM o u n t a i n sa n dt h eu n i to fH a r d yR i d g e( s e c .2 4 ,
'
T.4S.,
R.4W. ) .- Many o f t h e - r o c k s
i n the ,vicinity of this
' z o n ea r eh i g h l ys i l i c i f i e d ;n u m e r o u sa n dc l o s e l ys p a c e ds h e a r
z o n e s g i v e t h er o c k s
a b r e c c i a t e da p p e a r a n c e .T h i sz o n eh a s
as
a l s ol o c a l i z e ds e v e r a lp r o m i n e n tw h i t er h y o l i t ei n t r u s i v e s
e
l a v a s o fD e e rP l o T
t a n kR
. e c e nw
t ork
w e l l a s t h ei n t r u s i v e
t ot h es o u t h w e s t( D o n z e ,
r h y o l i t i c domeson
i n p r o g r e s s )h a sr e v e a l e d
more
t h e same t r e n d a s t h e s t r u c t u r e
in this
area.
T 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 sa r e
2 . 5 m i l e sl o n g
j o i n e d i n a n a r r o wz o n ea b o u t
by 1 m i l ew i d e
a l o n gt h er i d g ec r e s tb e t w e e nS o u t hB a l d ya n dL a n g m u i r
i s c h a r a c t e r i z e db y
LaboratoryT
. h i sz o n e
.-
t h i n n i n g of t h eu n i to fS i x m i l e
a dramatic
C a n y o na n db yc l o s e l ys p a c e d
n o r m a lf a u l t s ;t h i n n i n go ft h eu n i to fS i x m i l e
indicatesthatthisblock
eruptionofthe
cauldron.
A-L
Canyon
was s t r u c t u r a l l y h i g h f o l l o w i n g
P e a kT u f ff r o mt h eS a w m i l lC a n y o n
To t h ee a s ta n dw e s to ft h i sz o n e ,t h eu n i to f
111
S i x m i l eC a n y o nh a s
a minimum t h i c k n e s so f
2000 a n d 900 f e e t ,
respectively.
A t l e a s tt h r e ee x p l a n a t i o n sa r ep o s s i b l et oe x p l a i n
1 ) it
t h i sz o n eb e t w e e nt h eS a w m i l la n dM a g d a l e n ac a u l d r o n s :
i s a small r e s u r g e n tb l o c kb e t w e e nt h et w oc a u l d r o n sw h i c h
2 ) it i s
r o s ed u r i n ge r u p t i o no ft h eu n i to fS i x m i l e ' C a n y o n ;
a r e m n a n to f
a s e p t u mb e t w e e nt h ec a u l d r o n sw h i c hf o r m e d
t h e i rt o p o g r a p h i c
d i v i d eb e t w e e nt h e
blockthat
w a l l s m e r g e dl o c a l l yt of o r m
t w o d e p r e s s i o n s ;a n d
was " c a u g h t " i n
a n a r r o wz o n e
when
a n a r r o w ,l o w
3 ) it r e p r e s e n t s a
and. d i d - - n o ts u b s i d e
a s much a s t h e a d j a c e n t c a u l d r o n s .
A l t h o u g ht h i sb l o c k
may h a v e b e e n r e s u r g e n t l y
u p l i f t e df o l l o w i n gc o l l a p s eo ft h ec a l d e r a s ,t h e
i t s l o c a t i o nw i t h i nt h ec a u l d r o nc o m p l e x
o ft h eb l o c ka n d
u n l i k et h e
small s i z e
is
many w e l l - d o c u m e n t e dr e s u r g e n tc a l d e r a so ft h eS a n
i n C o l o r a d o( S t e v e na n dL i p m a n ,
J a nv o l c a n i cf i e l d
t h e S a nJ u a nf i e l d ,r e s u r g e n c e
1976).
In
i s g e n e r a l l y a v e r yb r o a d
s c a l ef e a t u r ec e n t r a l l yl o c a t e dw i t h i nt h ec a l d e r a s .T h e
s e c o n dh y p o t h e s i s
may n o t b e f u l l y e v a l u a t e d a s n o r m a l
f a u l t i n gh a sd i s r u p t e dt h eo r i g i n a lg e o m e t r yo ft h ec a u l d r o n s
a n dt h ec o n t a c tb e t w e e nt h es t r u c t u r a l l yh i g hb l o c ka n dt h e
t h i c kc a u l d r o n
f i l l t ot h ew e s t
t h e r e i s n oe v i d e n c et os u p p o r t
A-L
a n e x p e c t e dt h i n n i n go ft h e
P e a kT u f fa g a i n s t h es e p t u m .T h et h i r de x p l a n a t i o n
a p p e a r st ob et h es i m p l e s to n ea n d
e
i s n o et x p o s e dH
. owever,
W i t hi n w a r d - d i p p i n gr i n gf r a c t u r e s ,
i s preferred a t t h i st i m e .
it i s p o s s i b l e t h a t
112
t o p r o c e e d a s r a p i d l yo r
s u b s i d e n c ew o u l dn o tb ea b l e
c o m p l e t e l y i n t h en a r r o wz o n eb e t w e e nt h ec a u l d r o n s . .T h u s ,
a
b l o c kc o u l db e" c a u g h t "b e t w e e nt h em a j o rd e p r e s s i o n sa n d
of t h e A-L
c r e a t e a t o p o g r a p h i ch i g hf o l l o w i n ge r u p t i o n
Peak
Tuff.
e x t e n s i o n a lf a u l t i n g
I n t h i ss t u d y
a r e a , most of t h e f a u l t s
a r en o r t h - t r e n d i n gd, o w n - t o - t h e - w e snt o r m afl a u l t s .
domino-style
of f a u l t i n g c h a r a c t e r i z e d
w h i c h - h a sb e e nd o c u m e n t e d
(Chamberlin,
The
b yb l o c kr o t a t i o n ,
i n t h eL e m i t a rM o u n t a i n s
1978) a n dt h ee a s t e r nM a g d a l e n aM o u n t a i n s
( O s b u r n , 1978;
Petty,
1978) c a n n o bt ew e l dl o c u m e n t e di n
t h i ss t u d ya r e a .T y p i c a l l y ,f a u l tz o n e sa r eo b s c u r e db y
t a l u sa n da r ep o o r l ye x p o s e d .H o w e v e r ,f i e l dr e l a t i o n s h i p s
s u g g e s tt h a t
many o f t h e f a u l t s d i p t o w a r d t h e w e s t ; s o m e a r e
i n a n e a r l yo r t h o g a n o lr e l a t i o n s h i pw i t ht h ev o l c a n i c
strata.
The a g e of f a u l t i n g i n t h i s s t u d y a r e a c a n n o t b e
w e l l c o n s t r a i n e d .E x t e n s i o n a lf a u l t i n gr e l a t e dt ot h eR i o
G r a n d e r i f t b e g a nb e t w e e na b o u t
32 a n d 28 m.y.
c o n t i n u e dt ot h ep r e s e n t( C h a p i n ,
M o u n t a i n s( C h a m b e r l i n ,
(Osburn,
1978).
ago a n d h a s
I n t h eL e m i t a r
1978) a n de a s t e r nM a g d a l e n aM o u n t a i n s
1978) n o r t h - t r e n d i n gf a u l t sh a v eb e e nd o c u m e n t e d
priortotheeruption
of t h e t u f f
of L e m i t a rM o u n t a i n s .
T h e s ef a u l tp l a n e sh a v es u b s e q u e n t l yb e e nr o t a t e d
d e g r e e (sO s b u r n ,
1978; C h a m b e r l i n , 1978).
h a sa l s on o t e dt h a t ,i ng e n e r a l ,t h ed i p
25 t o 60
C h a m b e r l i n (1978)
of f a u l t p l a n e s
113
w i t h i n c r e a s e i n a g e .D u r i n gt h el a t e
b e c o m e ss h a l l o w e r
( 7 t o 4 m.y.
M i o c e n ea n dP l i o c e n e
a g o ) ,t h ep r e s e n t - d a y
M a g d a l e n aM o u n t a i n s ' w e r eu p l i f t e da l o n gm a j o rn o r t h - t r e n d i n g ,
h i g h - a n g l e ,n o r m a lf a u l t s ;t h e s ef a u l t sh a v eb e e n
s u p e r i m p o s e do no l d e rf a u l t s T
. h ec l o s e l ys p a c e d ,
n e a r - v e r t i c a lf a u l t sw h i c hc h a r a c t e r i z e d 't h ec r e s to ft h e
r a n g ep r o b a b l yb e l o n gt ot h i sy o u n g e rs e to ff a u l t sa n d
i n t e r s e c to l d e r ,r o t a t e df a u l t s
i n t h i ss t u d ya r e aa r et h er e s u l to f
w h i c hw e r em a p p e d
...
a t d e p t h .T h u s ,t h ef a u l t s
s u c c e s s i v eg e n e r a t i o n so f ~ n o r m a lf a u l t sw h i c hh a v eu n d e r g o n e
v a r i a b l ea m o u n t s
o f r o t a t i o na n db e e nf u r t h e r -m o d i f i e db y
u p l i f ta n de r o s i o n .I n t e r p r e t a t i o no fd i s p l a c e m e n to nf a u l t s
i s f u r t h e rc o m p l i c a t e db yp a l e o t o p o g r a p h yc r e a t e db yc a u l d r o n
c o l l a p s ew h i c hh a si n f l u e n c e dt h ed e p o s i t i o no fy o u n g e r
units.
F a u l t i n g i n t h i ss t u d y
a r e a h a sb e e ns t r o n g l y
influenced.~by.the.Soc~or.rotransverseshearzoneandby
c a u l d r o nr e l a t e ds t r u c t u r e s .N e a r - s u r f a c em a n i f e s t a t i o no f
o f a c o m p l e xi n t e r m e s h i n g
t h eS o c o r r os h e a rz o n ec o n s i s t s
z o n eo fn o r m a lf a u l t s
o f o p p o s i n gs e n s e ,r a t h e rt h a n
s t r i k e - s l i pf a u l t i n g( C h a p i n ,
1978).
I nt h e
S o c o r r o - M a g d a l e n aa r e a ,t h es h e a rz o n es e p a r a t e sf i e l d so f
t i l t e db l o c k sw h i c hh a v eu n d e r g o n er o t a t i o na n ds t e pf a u l t i n g
i n o p p o s i t ed i r e c t i o n s .
I n g e n e r a l ,n o r t h
f a u l tp l a n e sh a v eb e e nr o t a t e dt o w a r dt h ee a s t ,
t o w a r d st h e
w e s t , a n ds t e pf a u l t i n g
t h es o u t h ,t h er e v e r s e
i s true.
o f t h es h e a rz o n e
strata d i p
i s down-to-the-west;
t o
The S o c o r r os h e a rz o n ep a s s e st h r o u g ht h en o r t h e r n
rocks i s observed i n sec.
6, t h e t u f f
i n d i po ft h e
a n d a d r a m a t i cc h a n g e
p a r to ft h i ss t u d ya r e a
6 , T.kS.,
I n most o f s e c t i o n
R.4W.
o f L e m i t a rM o u n t a i n sd i p st o w a r d st h ew e s tw h i l e
t ot h es o u t ht h ed i p
i s t o w a r d st h ee a s t .I na d d i t i o n ,
a
a d o w n - t o - t h e - e a s t ,e a s t - d i p p i n gf a u l t
s h o r ts e g m e n to f
o f s e c t i o n 6.
mapped i n t h ew e s t e r np o r t i o n
was
of
To t h en o r t h
t h i ss t u d ya r e a ,t h eb e d sc o n s i s t e n t l yd i pt o w a r d st h ew e s t
a n dt h ef a u l t sh a v ed o w n - t o - t h e - w e s td i s p l a c e m e n ta l o n gt h e
1979).
m a r g i n o f t h eM a g d a l e n ac a u l d r o n( A l l e n ,
A n o t h e rf e a t u r ew h i c h
may b e r e l a t e d t o t h e s h e a r
it.
z o n ea r et w om a j o rs p r i n g sw h i c ho c c u ra l o n g
S p r i n g (SW c o r n e r ,s e c .
a
s p r i n gw i t h
water.
5 , T.4S.,
R.3W.)
Baldy
is a perennial
a s i g n i f i c a n ta n dr e l a t i v e l yc o n s t a n to u t p u to f
The s p r i n g i s u n u s u a lb e c a u s e
e l e v a t i o no fn e a r l y
it occurs a t a n
1 0 , 0 0 0 f e e ta n dt h u sh a sv e r y
of Baldy
r e c h a r g ea r e a .A n o t h e rs p r i n go c c u r sd u ew e s t
S p r i n g a t a n e l e v a t i o n o f 8800 f e e t a n d p r o d u c e s
s u p p l yo fw a t e rf o rm o s t
little
o f t h eM u l e s h o eR a n c h .
a year-round
Two t h e r m a l
s p r i n g sn e a rS o c o r r oa l s oo c c u ra l o n gt h es h e a rz o n e .T h e
c o i n c i d e n c eo ft h e s el a r g es p r i n g s
t h a tt h er o c k s
a n d t h es h e a rz o n es u g g e s t
may b e u n u s u a l l y f r a c t u r e d a l o n g t h e s h e a r
zone.
C a u l d r o ns t r u c t u r e sh a v e
a l s o h a d a m a j o ri n f l u e n c e
o nt h en a t u r eo ff a u l t i n gi nt h es t u d ya r e a .F o re x a m p l e ,
e
a l o n gt h ew e s tf l a n ko ft h e
E a s t F o r k o f S a w m i l lC a n y o n
a
115
rotated, down-to-the-west fault with at least 3000 feet of
displacement turns sharply to the west and joins the northern
margin of the Sawmill Canyon cauldron.
A possible
explanation is that the great thickness of cauldron facies
Hells Mesa Tuff,'which occupies thecrest of the range
between North and South Baldy,behaved as a coherent block
and resisted faulting.
Alternatively, the fault may have
followed the structural margin of the SawmillCanyon cauldron
for some distance before continuingon to the north.
The
effect of- the
-Sawmill
Canyon-Magdalena
margin on other faults can be seen in sec'.
where the two cauldrons are joined.
13, T . b S . ,
cauldron
R.hW.,
Here, the displacement
on several large faults is offset and/or redistributed along
the cauldron margin.
A down-to-the-west fault from the
north, which juxtaposes the lower tuff Lemitar
of
Mountains
and A-L Peak Tuff, joins the
west-trending cauldron margin
fault and bends nearly 9 0 degrees.
Original displacement on
both faults is difficult to estimate; however,it is apparent
that the displacement on one of the faults diesout.
The area immediately to the northand south of the
southern Magdalena cauldron marginis characterized by
east-west transverse faults which are subparallel to the
cauldron margin.
This type of faulting is not common in the
rest of this study area or elsewhere in the Magdalena
Mountains and is thought to
be related to the cauldron
margin.
116
E C O N O M I C GEOLOGY
i s u n u s u a l i n t h a t it l a c k s t h e
T h i ss t u d ya r e a
a b u n d a n tp r o s p e c t sa n dm i n e sw h i c hc h a r a c t e r i z e
S o c o r r o - M a g d a l e n aa r e a M
. ost
much o f t h e
o f t h ep r o s p e c t si nt h i sa r e a
a r e ' l o c a t e dn e a rt h en o r t h e a s t - t r e n d i n gS a w m i l l
Canyon
cauldron margin.
T h eo n l ym i n el o c a t e di nt h i ss t u d ya r e a
(P.
C o p p e rL o d ep a t e n t e dc l a i m
A l l e n ,o r a l
l o c a t e da l o n g
6 , T. 4S.,
margin.
R.4W. ).
Themine
is
o f t h eS a w m i l lC a n y o nc a u l d r o n
A s h a f te s t i m a t e dt ob e
a minimum o f 1 0 0 f e e td e e p
was s u n ka l o n gt h ed i k e ,h o w e v e rt h e r e
i s no s i g no fe c o n o m i c
m i n e r a l i z a t i o n i n t h em i n ed u m p s ;t h eH e l l s
v i c i n i t y o f t h ed i k e
p r o s p e c tp i t s
1978)
a n o r t h - t r e n d i n gw h i t er h y o l i t ed i k ew h i c h
intrudestheringfracture
a
commun.,
a t t h ee x t r e m en o r t h e r ne n d
w h i c h i s l o c a t e do nB a l d yR i d g e
o f t h es t u d ya r e a( s e c .
is the
Mesa T u f f i n t h e
i s highly,ailicified.
and s h o r ta d i t sa r e
Numeroushallow
common i n t h e H e l f s
T u f fa l o n gt h ec a u l d r o nm a r g i n ,h o w e v e r
Mesa
no s i g n i f i c a n t
mineralization i s visible.
S m a l la m o u n t s
o f c o p p e rm i n e r a l i z a t i o n( m a l a c h i t e ? )
are f o u n di ns e v e r a ll o c a l i t i e sw i t h i nt h ea n d e s i t et h a tc a p s
S o u t hB a l d y .S e v e r a ls h a l l o wp i t sa r ep r e s e n to nt h es o u t h
f l a n k o f S o u t hB a l d ya n d
a 3 0 - f o o ta d i t
n o r t h - t r e n d i n gf a u l tz o n ei nt h es a d d l ed u e
e
B a l d yS
. m a l al m o u n t so fm a l a c h i t e
i s l o c a t e da l o n g
east of South
( ? ) w h i c ho c c u r s
as
a
fracturefillings
i n t h e dumpsof
may b ef o u n d
these
prospects.
H y d r o t h e r m a lv e i n sf o r m e dl a r g e l yo fq u a r t za n d
c a r b o n a t eh a v eb e e ni n t r u d e da l o n gn o r t h - t r e n d i n gf a u l t si n
t h ev i c i n i t yo fS o u t hB a l d ya n di m m e d i a t e l yt ot h ew e ti n
s e c t i o n s 1 a n d 1 2 (T.hS.,
R.4W.).
T y p i c a l l y ,t h ev e i n s
c o n s i s to fb a n d e dc a l c i t ea n dq u a r t zw i t hv u g sw h i c ha r e
f i l l e dw i t hd r u s ym i n e r a l s .
The v e i n i n s e c t i o n 1 a n d 1 2 h a s
i t , h o w e v e rt h e r e
n u m e r o u s s m a l l p r o s p e c tp i t sd u go n
. ....
.
v i s i b l ee c o n o m i cm i n e r a k i z a t i o n .
.
.
.
.
X.”..
i s no
‘.
.
I
118
CONCLUSIONS
T h ec o n t r i b u t i o n so ft h i ss t u d yt ot h eg e o l o g yo f
t h eS o c o r r o - M a g d a l e n aa r e ac a nb ed i v i d e di n t os t r a t i g r a p h i c
and s t r u c t u r a ls e c t i o n s
a n d a r ed e t a i l e db e l o w .
Stratigraphy
T h e r o c k s i n t h i ss t u d ya r e aa r ep r e d o m i n a n t l y
lavas.
T e r t i a r ya s h - f l o wt u f f s ,a n d e s i t i cl a v a s ,a n dr h y o l i t e
Ingeneral,
t ' h e' s t r a e - i g r a p h i cs e q u e n e ei n- . t h i . ss t u d yf o l l o w s
t h es t r a t i g r a p h i cf r a m e w o r ke s t a d l i s h e df o rt h e
S o c o r r o - M a g d a l e n aa r e a( C h a p i n ,
1978).
H o w e v e r t, h e r e
1974; C h a p i na n do t h e r s ,
a r e a s p e c t so ft h es t r a t i g r a p h yw h i c h
as follows:
a r eu n i q u et ot h i ss t u d ya r e aa n da r e
1)
A s e q u e n c eo fd i s t i n c t i v e ,p h e n o c r y s t - p o o r ,
f l o w - b a n d e d ,r h y o l i t e
l a v a s , a s h - f l o wt u f f s ,a n dm i n o r
a n d e s i t eo v e r l i ~ e sc a u l d r o nf a c i . e sH e l l s
Mesa T u f f i n t h e
is here
c e n t r a lp o r t i o no ft h es t u d ya r e a .T h i ss e q u e n c e
i n f o r m a l l y named t h eu n i to fH a r d yR i d g ea n d
b ec a u l d r o n
i s consideredto
fill o ft h eN o r t hB a l d yc a u l d r o n .
2)
( a t l e a s t 2500 f e e t )
A t h i c ku
, n i f o r m s, e q u e n c e
o fd e n s e l yw e l d e d ,f l o w - b a n d e d
c e n t r a lp o r t i o no ft h i ss t u d ya r e a .
A-L
P e a kT u f fc r o p so u ti n
the
The t u f f i s
c h a r a c t e r i z e db yp r o m i n e n t ,l i n e a t e dp u m i c ea n ds e c o n d a r y
f l o wf o l d s .
The A-L
P e a ki nt h i ss t u d ya r e a
h a v eb e e ne r u p t e df r o mt h eS a w m i l l
i s t h o u g h tt o
Canyon c a u l d r o na n d
may b e
c
c o r r e l a t i v ew i t he i t h e rt h ep i n n a c l e so rf l o w - b a n d e dm e m b e r s
o ft h e
A-L
P e a kT u f f .L a r g eb l o c k so fo l d e rv o l c a n i cr o c k s
a r ef o u n dw i t h i nt h e
w a l l o ft h eS a w m i l l
l a n d s l i d ed e p o s i t sw h i c hf e l lf r o mt h e
i t s collapse.
C a n y o nc a u l d r o nd u r i n g
3)
as
P e a kT u f fa n da r ei n t e r p r e t e d
A-L
is a thick,
The u n i to fS i x m i l eC a n y o n
h e t e r o g e n e o u sa s s e m b l a g eo fa n d e s i t i ca n dr h y o l i t el a v a s ,
a s h - f l o wt u f f s ,a n dv 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 st h a t
f i l l e dt 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 nf o l l o w i n gt h ee r u p t i o no f
i n a narrow
P e a kt u f f .A n d e s i t i cl a v a sp r e d o m i n a t e
b a n dp a r a l l e lt ot h en o r t h e r nS a w m i l l
C a n y o nc a u l d r o nm a r g i n
a n dt h i nd r a m a t i c a l l yt ot h es o u t h ;r h y o l i t el a v a sa r e
e l s e w h e r e i n t h ec a u l d r o n .
0
common
A d i s t i n c t i v es e q u e n c eo f
r h y o l fi lt o
e aw
innsdt e r b e d dveodl c a n i c l a s st iecd i m e n t a r y
r o c k s i s f o u n dw i t h i nb o t ht h eS a w m i l lC a n y o na n dM a g d a l e n a
c a u l d r o nw h i c hs u g g e s t sa p p r o x i m a t e l yc o n t e m p o r a n e o u s
. c o l l a p s eo fb o t hc a u l d r o n s .
4)
T h eS a w m i l C
l a n y o nc a u l d r o ni n f l u e n c e dt h e
i n thisstudy
d e p o s i t i o no ft h et u f fo fL e m i t a rM o u n t a i n s
area.
Two d o m a i n so ft h et u f f
are present:
S 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 s
l o w e ra n du p p e rm e m b e r s
S i x m i lC
e anyon.
2)
1) w i t h i nt h e
a n o r m a ls u c c e s s i o no f
i s f o u n do v e r l y i n g - t h eu n i to f
O u t s i d eo ft h ec a u l d r o n st h e r e
is l i t t l e
o r no l o w e r t u f f o f L e m i t a r M o u n t a i n s p r e s e n t a n d t h e u p p e r
member r e s t su n c o n f o r m a b l yo nt h eu n i to fH a r d yR i d g e .T h e
u p p e rt u f fo fL e m i t a rM o u n t a i n so u t s i d eo ft h ec a u l d r o n s
was
120
d e p o s i t e do n
a s t r u c t u r a l l yh i g hb l o c k .H e r e ,t h et u f fh a s
-.
n o t been
w e l l - d e v e l o p e dp r i m a r yw e l d i n gf e a t u r e sw h i c hh a v e
r e c o g n i z e di nt h eu p p e rt u f fo fL e m i t a rM o u n t a i n se l s e w h e r e
i n t h eS o c o r r o - M a g d a l e n aa r e a .
5)
. I n t e r m e d i a t el a v a sa n dm i n o ra s h - f l o wt u f f s
t h a ta r ep r o b a b l yy o u n g e r
t h a n t h et u f fo fS o u t hC a n y o no c c u r
i n t h ew e s t e r np a r to ft h es t u d ya r e a .
The l a v a sv a r yf r o m
a p h a n i t i ca n . d e s i t e st op o r p h y r i t i cq u a r t zl a t i t e s .L o c a l l y ,
t h el a v a sa r eo v e r l a i nb yp o o r l yw e l d e dl i t h i c - r i c ha s h - f l o w
t u f f sa n dv o l c a n i c l a s x i cs e d i m e n t a r yr o c k s .T h i ss e q u e n c e
is
i
o v e r l a i nb yl o w e rP o p o t o s aF o r m a t i o nl a h a r i cb r e c c i a s .
6)
The d i s t r i b u t i o no fl i t h o l o g i e sw i t h i nt h e
P o p o t o s aF o r m a t i o ni n d i c a t es t r u c t u r a lc o n t r o lo ft h e i r
e n v i r o n m e n t so fd e p o s i t i o n .
c o n g l o m e r a t e sw e r ed e p o s i t e di n
I n g e n e r a l ,l a h a r i cb r e c c i a sa n d
a d e p r e s s i o nw h i c hr o u g h l y
c o i n c i d e sw i t ht h eM a g d a l e n ac a u l d r o n .S o u t h
o f t h ec a u l d r o n
m a r g i n , a s much a s 8 0 0f e e t
o f f i n e l yl a m i n a t e ds a n d s t o n e s
a n ds i l t s t o n e sw e r ed e p o s i t e d
on t h e u p p e r t u f f o f L e m i t a r
Mountains.
7)
R h y o l i t i cl a v a s ,t e n t a t i v e l yc o r r e l a t e dw i t h
t h el a v a so fM a g d a l e n aP e a k ,o v e r l i ea n da r ei n t e r b e d d e dw i t h
t h eP o p o t o s aF o r m a t i o ni nt h i ss t u d ya r e a .
A s a m p l eo f
i n t e r b e d d e dr h y o l i t el a v af r o mt h i sa r e ah a sb e e nd a t e d
1 8 . 0 +/-
0.8
s a m p l e so ft h e
m.y.
T h el a v a
i s s i g n i f i c a n t l yo l d e rt h a n
l a v a s o fM a g d a l e n aP e a kf r o mt h et y p el o c a l i t y
a n dt h eS t e n d e lP e r l i t ed e p o s i t .
an
at
121
Structure
The major structural features of this study area
are the Magdalena and Sawmill Canyon cauldrons, the Socorro
transverse shear zone, and north-trending extensional faults
associated with the Rio Grande
rift.
1)
r
The northern topographic margin of the Sawmill
Canyon cauldron
is well exposed just north
of the Langmuir
Laboratory access road in the northeast portion o f the study
area.
The topographic margin is marked by the unit of
Sixmile Canyon in depositional contact with the Hells Mesa
Tuff and by large blocksof older volcaniclastic sedimentary
rocks which fell into the cauldron. The western margin of
0
the Sawmill Canyon cauldron is represented
by a steeply
dipping fault between cauldron faciesA-L Peak Tuff and Hells
Mesa Tuff.
Megabreccia deposits are found in the A-L Peak
Tuff adjacent to themargin.
2)
The Sawmill Canyon cauldron and the Magdalena
cauldron are joined in a narrow zone near the summit of South
Baldy and form a crude dumbbell shape.
This zone probably
did not subside as much asthe adjacent cauldron and is
characterized by a dramatic thinning of:the unit of Sixmile
Canyon.
3)
The southern margin of the Magdalena cauldron
is represented by a geologically complex zone, parallel to
-e
Hardy Canyon.
This southwest-trending zone has influenced
thedepositionofthemajorvolcanic
and sedimentary units
122
e
younger than theHells Mesa Tuff and has localized daciteto
rhyolite intrusive rocks.
4) In the Socorro-Magdalena area, the Socorro
transverse shear zone separates fields of tilted blocks which
have undergone rotationand step faulting in opposite
directions.
The transverse shear zone passes through this
study area in the vicinity of SouthBaldy.
South of the
shear zone, east-dipping strata, anddown-to-the-west normal
faults predominate; tothe north, west-dipping strata and
down'-tb-the-east- normal-
faults
I
are
found.
123
REFERENCES C I T E D
A l l e n ,P . ,
1 9 7 9 , G e o l o g yo ft h ew e s tf l a n ko ft h eM a g d a l e n a
M o u n t a i n ss o u t h
o f t h eK e l l e yM i n i n gD i s t r i c t ,S o c o r r o
C o u n t y , New M e x i c o :
New M e x i c oI n s t i t u t eo fM i n i n ga n d
T e c h n o l o g y ,u n p u b l i s h e d
M.S. t h e s i s , 1 5 3 p.
Bachman, G . O . ,
a n dM e h n e r t ,
H.H.,
1 9 7 8 , New K-Ar d a t e s a n d
t h el a t eP l i o c e n et oH o l o c e n eg e o m o r p h i ch i s t o yo ft h e
c e n t r a lR i oG r a n d er e g i o n ,
New M e x i c o G
: eological
v. 89, p. 283-292.
S o c i e t yo fA m e r i c aB u l l e t i n ,
1 9 7 8 , G e o l o g yo ft h eK e l l yM i n i n gD i s t r i c t ,
Blakestad, R.B.,
S o c o r r oC o u n t y ,
U n i v e r s i t yo fC o l o r a d o ,
New M e x i c o :
u n p u b l i s h e d M.S. t h e s i s ,1 2 7
p.
Brown, D . M . ,
1 9 7 2 ,G e o l o g yo ft h es o u t h e r nB e a rM o u n t a i n s ,
3 o c o r r o .C o u n t y , New- M e x i c o :
New M e x i c o .I n s t i t u t e o f
M i n i n ga n dT e c h n o l o g y ,u n p u b l i s h e d
M.S. t h e s i s ,1 1 0p .
Bruning, J.E.,
1 9 7 3 , O r i g i n o ft h eP o p o t o s aF o r m a t i o n ,
n o r t h - c e n t r a lS o c o r r oC o u n t y ,
New Mexico:
New Mexico
I n s t i t u t e o f M i n i n ga n dT e c h n o l o g y ,u n p u b l i s h e d
Ph.D.
d i s s e r t a t i o n , 131 p.
B u r k e , W.H.,
K e n n yC
, . S .O
, tto,
J.B.,
a n d Walker, R.D.,
1963,
P o t a s s i u m -a r g o nd a t e s ,S o c o r r oa n dS i e r r aC o u n t i e s ,
New
M e x i c o , i n G u i d e b o o ko ft h eS o c o r r or e g i o n :
New Mexico
G e o l o g i c a lS o c i e t yG u i d e b o o k ,1 4 t hF i e l dC o n f e r e n c e ,p .
224.
. C h a m b e r l i n , R.M.,
1 9 7 4 , G e o l o g yo ft h e . C o u n c i 1R o c kD i s t r i c t ,
S o c o r r o County, N e w Mexico:
New Mexico I n s t i t u t e o f
a
n
d
T
e
c
h
n
o
l
o
g
y
,
u
n
p
u
b
l
i
s
h
e
d
M.S. t h e s i s ,1 3 0
Mining
,
1978, S t r u c t u r a ld e v e l o p m e n t
p.
of the
L e m i t a rM o u n t a i n s ,a ni n t r a r i f tt i l t e df a u l t - b l o c k
N e w M e x i c o( a b s ) ,
i n 1978 I n t e r n a t i o n a l
u p l i f t ,c e n t r a l
S y m p o s i u mo ft h eR i oG r a n d e
R i f t : LosAlamos
Scientific
L a b o r a t o r y ,U n i v e r s i t yo fC a l i f o r n i a ,
p.22-24.
,
i np r e p a r a t i o n ,G e o l o g i cf r a m e w o r k
of
t h eS o c o r r oP e a kg e o t h e r m a la r e a ,S o c o r r oC o u n t y ,
New
M e x i c oC
: o l o r a d oS c h o ool M
f i n e su, n p u b l i s h e d
Ph.D.
dissertation.
Chapin, C.E.,
1 9 7 1 , T h eR i oG r a n d e
r i f t , p a r t 1.:
M o d i f i c a t i o n sa n da d d i t i o n s ,
i n G u i d e b o o ko ft h eS a n
New M e x i c oG e o l o g i c a S
l ociety
Luis B a s i n , Colorado:
G u i d e b o o k ,2 2 n dF i e l dC o n f e r e n c e ,p .1 9 1 - 2 1 1 .
124
, 1 9 7 8 , E v o l u t i o no ft h eR i oG r a n d e
rift:
c o m p a r i s o n sb e t w e e ns e g m e n t sa n dt h er o l eo ft r a n s v e r s e
1 9 7 8 I n t e r n a t i o n a l Symposiumon
s t r u c t u r e s( a b s ) ,i n
Rift:
Los Alamos S c i e n t i f i cL a b o r a t o r y ,
R i oG r a n d e
U n i v e r s i t y o f C a l i f o r n i a , p. 24-27.
the
Chamberlin, R.M.,
Osburn, G.R.,
a n d White,
Chapin, C.E.,
D.W.,
1 9 7 8 , E x p l o r a t i o nf r a m e w o r ko ft h eS o c o r r o
C.E..,
and
g e o t h e r m a la r e a ,
New M e x i c o , i nC h a p i n ,
E l s t o n , W.E.,
e d s . ,F i e l dg u i d et os e l e c t e dc a u l d r o n s
a n da n dm i n i n gd i s t r i c t so ft h eD a t i l - M o g o l l o nv o l c a n i c
Y i e l d , New Mexico:
New M e x i c oG e o l o g i c a S
l ociety
S p e c i a lP u b l i c a t i o n
No. 7 , p.114-129.
Chapin, C.E.,
a n dD e a l ,
E.G.,
1 9 7 6 , The A-L P e a kT u f f ,
A c o m p o s i t ae s h - f l o w
s h e e tG
: e o l o g i c aSl o c i e t y
Mexico:
o fA m e r i c aA b s t r a c t sw i t hP r o g r a m s ,
v. 8 , p . 575.
C h a p i n , C.E.,
J a h n s , R:H.
; ' C h a m b e r l i n , R.M.,
a n dO s b u r n ,
G.R.,
1 9 7 8 , F i r s td a yr o a dl o gf r o mS o c o r r ot oT r u t ho r
C o n s e q u e n c e sv i a ! M a g d a l e n aa n dW i n s t o n ,i nC h a p i n ,
a n dE l s t o n ,
W.E.,
e d s . ,F i e l dg u i d et os e l e c t e d
c a u l d r o n sa n dm i n i n gd i s t r i c t so ft h eD a t i l - M o g o l l o n
v o l c a n i cf i e l d ,
New M e x i c o :
New M e x i c oG e o l o g i c a l
S o c i e t yS p e c i a lP u b l i c a t i o n
No. 7 , p. 1-31.
e
New
C.E.,
Chapin, C.E.,
a n dL o w e l l ,
G.R.,
1979, Primary a n d secondary
f l o ws t r u c t u r e si na s h - f l o wt u f f so ft h eG r i b b l e s
Run
p a l e o v a l l e y ,c e n t r a lC o l o r a d o ,i nC h a p i n ,
C.E. a n d
E l s t o n , W.E.,
e d sA
. s h - f l o wt u f f s G
: eologicaS
l ociety
o fA m e r i c aS p e c i a lP a p e r1 8 0 .
1 9 7 5 , E v o l u t i o no ft h eR i oG r a n d e
C h a p i n , C.E.,
a n dS e a g e r ,
r i f t i nt h eS o c o r r oa n d
Las C r u c e sa r e a ,
i n Las C r u c e s
Country:
New M e x i c oG e o l o g i c a S
l o c i e t yG u i d e b o o k 2
, 6th
F i e l dC o n f e r e n c e ,
p. 297-321.
Deal, E.G., 1 9 7 3 , G e o l o g yo ft h en o r t h e r np a r to ft h eS a n
C o u n t y , New M e x i c o : A s t u d y o f
M a t e oM o u n t a i n s ,S o c o r r o
a r h y o l i t ea s h - f l o wt u f f
c a u l d r o na n dt h er o l eo f
: n i v e r s i t yo f
New
l a m i n a r flow i n ash-flow t u f f sU
M e x i c o ,u n p u b l i s e d
Ph.D. d i s s e r t a t i o n ,1 0 6
p.
a n dR o d e s ,
R.C.,
1976, V o l c a n o - t e c t o n i c
D e a l , E.G.,
s t r u c t u r e s i n t h eS a nM a t e oM o u n t a i n s ,S o c o r r oC o u n t y ,
New M e x i c o , i n E l s t o n , W.E.,
a n d N o r t h r o p ,S . A . ,e d s . ,
i n s o u t w e s t e r n New M e x i c o :
New
C e n o z o i cv o l c a n i s m
M e x i c oG e o l o g i c a lS o c i e t yS p e c i a lP u b l i c a t i o n
No. 5 , p .
51-56.
0
D e e r , W.A.,
Howie, R . A . ,
a n dZ u s s m a n ,
J., 1 9 6 6 , An
i n t r o d u c t i o nt or o c kf o r m i n gm i n e r a l s :L o n d o n ,
GroupL
, td.5
, 2 8p .
Longman
D e n n y ,C . S . ,1 9 4 0 ,T e r t i a r yg e o l o g yo ft h eS a nA c a c i aa r e a ,
J o u r n ao
l fG e o l o g y v, .
4 9 , p.
225-260.
New Mexico:
i n p r o g r e s s , T h eg e o l o g yo ft h es o u t h w e s tf l a n k
Donze, M.A.,
o ft h eM a g d a l e n aM o u n t a i n s :
New Mexico I n s t i t u t e o f
M.S. t h e s i s .
M i n i n ga n dT e c h n o l o g y ,u n p u b l i s e d
E p i s , R.C.,
a n dC h a p i n ,
C.E.,
1 9 7 5 , Geomorphic a n d t e c t o n i c
i m p l i c a t i o n so ft h ep o s t - L a r a m i d e ,l a t e - E o c e n ee r o s i o n
s u r f a c ei nt h es o u t h e r n
R o c k yM o u n t a i n s :
i n Curtis,
B.F.,
e d . ,C e n o z o i ch i s t o r y
o f t h es o u t h e r n Rocky
M o u n t a i n sG
: e o l o g i c aS
l o c i e t yo fA m e r i c aM e m o i r . 1 4 4 p
,.
45-74.
F i s h e r , R.V.,
1 9 6 0 ,C l a s s i f i c a t i o no fv o l c a n i cb r e c c i a s :
G e o l o g i c a lS o c i e t yo fA m e r i c aB u l l e t i n ,
v. 71, p.72-82.
Krewedl, D.A.,
1 9 7 4 , G e o l o g yo ft h ec e n t r a lM a g d a l e n a
M o u n t a i n s ,S o c o r r oC o u n t y ,
New M e x i c oU
: niversity
of
A r i z o n a ,u n p u b l i s h e d
Ph.D. d i s s e r t a t i o n ,1 2 8p .
j_
Lipman, P.W.,
1 9 7 6 , C a l d e r a - c o l l a p s eb r e c c i a s
i n t h ew e s t e r n
S a nJ u a nM o u n t a i n s C
, o l o r a d oG
: e o l o g i c aS
l o c i e t yo f
8 7 , p . 1397-1410.
A m e r i c aB u l l e t i n v
, .
Lipman, P.W.,
1 9 7 5 , E v o l u t i o no ft h eP l a t o r oc a l d e r ac o m p l e x
a n dr e l a t e dv o l c a n i cr o c k s ,
s o u t h e a s t e r nS a nJ u a n
U.S.
M o u n t a i nC
s ,o l o r a d o :
G e o l o g i c a lS u r v e y
P r o f e s s i o n a lP a p e r
853, 1 2 8 p.
L o u g h l i nG
, . F . a, n dK o s c h m a n n ,
A . H . , 1 9 4 2 ,G e o l o g ya n do r e
d e p o s i t so ft h e
M a g d a l e n aM i n i n gD i s t r i c t ,
New M e x i c o :
U.S.
G e o l o g i c a l S o c i e t yP r o f e s s i o n a lP a p e r
2 0 0 , 168 p.
Lowell, G.R.,
a n dC h a p i n ,
C.E.,
1 9 7 2 ,P r i m a r yc o m p a c t i o na n d
f l o wf o l i a t i o ni na s h - f l o w
t u f f s o ft h eG r i b b l e s
Run
p a l e o v a l l e y c, e n t r a C
l oloradoG
: eologicaS
l ociety
of
A m e r i c aA b s t r a c t s 1
, 972p
,7
. 25-726.
O s b u r n , G. R., 1 9 7 8 , G e o l o g yo ft h ee a s t e r nM a g d a l e n a
M o u n t a i n s , Water Canyon t o P o u n dR a n c h ,S o c o r r oC o u n t y ,
N e w Mexico:
New M e x i c oI n s t i t u t eo fM i n i n ga n d
T e c h n o l o g y ,u n p u b l i s h e d
M.S. t h e s i s ,1 5 0p .
1 9 7 9 , G e o l o g yo ft h es o u t h e a s t e r nM a g d a l e n a
P e t t y , D.M.,
M o u n t a i n sS
, o c o r r oC o u n t y ,
New Mexico:
New Mexico
I n s t i t u t e o f M i n i n ga n dT e c h n o l o g y ,u n p u b l i s h e d
M.S.
t h e s i s , 1 5 7 p.
a
R o t h , S.J., i n p r o g r e s s , T h eg e o l o g y
of t h eS a w m i l lC a n y o n
New
a r e a o f t 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 ,
Mexico:
New Mexico I n s t i t u t eo fM i n i n ga n dT e c h n o l o g y ,
u n p u b l i s h e d M.S. t h e s i s .
126
0
Schminke, H.U.,
a n dS w a n s o n ,
D.A.,
1967, L a m i n a r v i s c o u s
i
n
a
s
h
f
l
o
w
t
uffs
s
t
r
u
c
t
u
r
e
s
from Gran C a n a r i a ,
flowage
I
s
l
a
n
d
s
:
J
o
u
r
n
a
l
of
G
e
o
l
o
g
y
,
Canary
v. 75, p. 641-664.
Simon, D.B.,
1 9 7 3 , G e o l o g y o f t h e S i l v e r H i l l a r e a ,S o c o r r o
C o u n t y , New Mexico:
New M e x i c oI n s t i t u t e
o f M i n i n ga n d
t h e s i s , 1 0 1 p.
T e c h n o l o g y , u n p u b l i s h e d M.S.
D e a l , E.G.,
a n dR h o d e s ,
R.C.,
o fT e r t i a r yv o l c a n i ca n d
P l a t e a u ,s o u t h w e s t e r n
New
p l u t o n i cr o c k s ,M o g o l l o n
M e x i c o , i n E l s t o n , W.E.,
a n dN o r t h r o p ,S . A . ,e d s . ,
i n s o u t h w e s t e r n New Mexico:
New
C e n o z o i cv o l c a n i s m
M e x i c oG e o l o g i c a lS o c i e t yS p e c i a lP u b l i c a t i o n
No. 5 , p .
Smith, E.L.,
Aldrich,
J.M.,
1 9 7 6 , F i s s i o n - t r a c ka g e s
117-118.
S m i t h , R.L.,
a n dB a i l e y ,
G e o l o g i c a lS o c i e t y
R.A.,
1968, R e s u r g e n tc a u l d r o n s :
o f AmericaMemoir
1 1 6 , p -6 1 3 - 6 6 2 .
Spradlin, E.J.,
1 9 7 6 , S t r a t i g r a p h y o f T e r t i a r yv o l c a n i c
r o c k s ,J o y i t a' H i l l sa r e a ,
S o c o r r oC o u n t y ,
New Mexico:
U n i v e r s i t y o f New M e x i c o , u n p u b l i s h e d M.S. t h e s i s , 7 3 p.
a
1 9 6 8 , G e o l o g y o f t h ea r e aa r o u n dt h eL a n g m u i r
S t a c y , A.L.,
L a b o r a t o r y ,M 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:
New Mexico I n s t i t u t e o f M i n i n ga n dT e c h n o l o g y ,
u n p u b l i s h e d M.S. t h e s i s , 69 p.
i np r o g r e s s ,G e o l o g y
o f t h eW a t e rC a n y o n - J o r d a n
Sumner, W . ,
Canyon a r e a ,S o c o r r oC o u n t y ,
New M e x i c o :
New Mexico
I n s t i t u t e o f M i n i n ga n dT e c h n o l o g y ,u n p u b l i s h e d
M.S.
thesis.
1 9 5 7 , G k o l o g y o f P u e r t e c i t oq u a d r a n g l e ,
Tonking, W.H.,
S o c o r r oC o u n t y ,
New M e x i c o :
New M e x i c oB u r e a uo fM i n e s
41, 67 p .
a n dM i n e r a lR e s o u r c e sB u l l e t i n
a n d S w a n s o n , D.A.,
1968, L a m i n a rf l o w g e
in a
W a l k e r , G.W.,
P l e i s t o c e n es o d ar h y o l i t ea s h - f l o wt u f f ,L a k ea n dH a r n e y
C o u n t i e sO, r e g o n :
U.S.
G e o l o g i c aSl u r v e P
yr o f e s s i o n a l
P a p e r ~ O O - B , p. B37-B67.
Weber, R.H.,
1 9 5 7 , G e o l o g ya n dp e t r o g r a p h yo ft h eS t e n d e l
P e r l i t e D e p o s i t , S o c o r r oC o u n t y ,
New Mexico:
New M e x i c o
B u r e a u of Mines a n d M i n e r a lR e s o u r c e s
C i r c u l a r 4 4 , 24 p a n d B a s s e t t , W.A.,
1 9 6 3 , K-Ar a g e s o f T e r t i a r y
Weber, R.E.,
v o l c a n i ca n di n t r u s i v er o c k si nS o c o r r o ,C a t r o n ,a n d
G r a n tC o u n t i e s ,
N e w M e x i c o ,i nG u i d e b o o k
o f t h eS o c o r r o
region:
New M e x i c oG e o l o g i c a lS o c i e t yG u i d e b o o k 1
, 4th
F i e l dC o n f e r e n c e ,
p . 220-223.
, 1971,
K-Ar a g e s of T e r t i a r y
i n c e n t r a l a n d w e s t e r n New M e x i c o :
i g n e o u sr o c k s
1, p. 33-45.
I s o c h r o n / W e s t n, o .
"
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"
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"
"
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W i l b o l t , R.H.,
M a c A l p i n , A. J., B a t e s , R.L.,
a n do t h e r s ,
1 9 4 6 , G e o l o g i c map a n d s t r a t i g r a p h i cs e c t i o n so f
o f t h eJ o y i t a
H i l l s , Los P i n o s
P a l e o z o i cr o c k s
M o u n t a i n s ,a n dn a r t h e r nC h u p a d e r aM e s a ,V a l e n c i a ,
T o r r e n c e a, n dS o c o r r oC o u n t i e s ,
New Mexico:
U.S.
G e o l o g i c a lS u r v e y
O i l a n d Gas I n v e s t i g a t i o n sP r e l i m i n a r y
Map 61.
W i l k i n s o n , W.H.,
1 9 7 6 , G e o l o g y o f t h eT r e sM o n t o s a s - C a t
M o u n t a i na r e a S
, o c o r r oC o u n t y ,
New Mexico:
New M e x i c o
M.S.
I n s t i t u t e o f M i n i n ga n dT e c h n o l o g y ,u n p u b l i s h e d
t h e s i s , 1 5 8 p.
W i l l a r d , M.E.,
1 9 5 9 , T e r t i a r ys t r a t i g r a p h y
o f n o r t h e r nC a t r o n
C o u n t y , New M e x i c o , i n G u i d e b o o k o f w e s t - c e n t r a l New
Mexico:
New M e x i c oG e o l o g i c a S
l o c i e t yG u i d e b o o k 1
, 0th
F i e l dC o n f e r e n c e ;
p . 92-99.
1 9 2 0 ,G e o l o g yo fA l a m o s aC r e e kV a l l e y ,
Winchester, D.E.,
S o c o r r oC o u n t y ,
New M e x i c o , w i t h s p e c i a l r e f e r e n c e t o
t h eo c c u r r e n c e
o f o i la n dg a s :
U.S.
G e o l o g i c aS
l urvey
B u l l e t i n 7168.
This dissertation is accepted on behalf of the faculty of the
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