GEOLOGY OF THE SOUTHERN BEAR MOUNTAINS,
SOCORRO COUNTY, N E W MEXICO
A - Thesis
'
Presented t o
t h e F a c u l t y of t h e Department of Geology
New Mexico I n s t i t u t e of MiningandTechnology
,
..
r-.
I n P a r t i a l 'Fulfillment
of the Requirements for the
Degree
Master of Science
.
..
David Mc Kendree Brown
April, ,1972
.t
,
.
. . .. .
.- .
This t h e s i s is accepted on behalf of the faculty of the
I n s t i t u t e by the following committee:
I
.
. .
. .
- .
..
,
..-_
"
,
PAGE
1
1
1
2
'
...............
..................
PreviousInvestigations
4
Acknowledgments
7
STRATIGRAPHY AND PETROLOGY
..............
8
.................
T e r t i a r y Volcanic Rocks . . . . . . . . . . . . . .
.
SpearsFormation . . . . . . . . . . . . . . . . .
Lower Member . . . . . .
................
Upper Member . . . . . . . . . . . . . . . . . .
H e l l s Mesa Formation . . . . . . . . . . . . . . .
TuEf of Goat Spring . . . . . . . . . . . . . . .
Tu€f of BearSprings
..............
Lower cooling U n i t . . . . : . . . . . . . . .
Upper cooling Unit . . . . . . . . . . . . . .
Pre-Tertiary Rocks
.....
..
.
AndesitesInterbedded
T u f f 02 Allen Well
La' 3 a r a Peak Formation
i n t h e ' H e l l s - Mesa
8
9
9
10
13
18
19
31
31
.
....
...............
...............
42
'46
49
50
PAGE
..............
MaficDikes . . . . . . . . . . . . . _ . . . . .
L a r g e rM a f i cI n t r u s i v e s .
.............
T e r t i a r yI n t r u s i v e
Monzonite
55
Rocks
55
59.
....................
61
64
65
PostvolcanicDeposits
. Tertiary
Deposits
Fanglomerate of
...........
......... ....- ..
Dry Lake Canyon . . . . . . . .
.. . . .
66
66
66
68
68
69
Aeolian Sand
'i
.................
7 0'
'
P e t r o g e n e s i so ft h e
A s h Flows
...........
'
70
70
71
Directional Significance of
Laminar-Flow S t r u c t u r e s
76
77
RotatedInclusions
. . . . . . . .' . . . . . .. .
77
78
81
RegionalStructure
........- ...... ..
81
82
.
.
.iv .
PAGE
......... .......
Basin and Range F a u l t s . . . . . .
.......
N o r m a l Faults . . . . . . . . .
.......
O b l i q u e - s l i p Faults
...... .......
Folding . . . . . . . . . . . . .
........
ECONOMIC GEOLOGY . . . . . . . . . . .
.......
O l i g o c e n e Faults
...........
GEOLOGIC HISTORY
Prevolcanic H i s t o r y
REFERENCES
85
86
90
94
98
........ .......
98
. .......
98
............. .......
.............. .......
......
~
84
.......
V o l c a n i c and Postvolcanic H i s t o r y
CONCLUSIONS
83
-.
.. ....
104
107
- v LIST OF ILLUSTRATIONS
PAGE
PLATE
Geologic map and s e c t i o n s o f t h e s o u t h e r n
I.
New Mexico
Mountains,SocorroCounty,
..
Bear
..
. i np o c k e t
PAGE
FIGURE
Index map of New Mexico, showing l o c a t i o n
1.
o f t h e s i sa r e a
.......
:
.........
Generalized composite section
2.
volcanicrocks
Formation
o f the Tertiary
................
Latitic mudflow'breccia in
3.
3
11
the upper Spears
..................
4. Poorly-welded . . l a t i t i c a s h - f l o w t u f f
16
in t h e
.............
upper Spears Formation
16
S t r a t i f i e d o u t c r o p s of t h e Hells Mesa
5.
......
20
............
23
Format-ion .near. Bear Springs draw
6.
T u f f of Goat Spring
7.
Measured s e c t i o n o f
tkie t u f f o f
showing m i n e r a l o g i c a l v a r i a t i o n s
Modal d a t a f o r t h e t u f f o f
8.
Goat Spring
.
.......
27
Goat S p r i n g p l o t t e d
on a three-componentdiagram
. . .. .. . . . .
29
9- Measured s e c t i o n of t h e lower cooling unit in
~
the t u f f of Bear Springs
variations
showing mineralogical
.................
L.. ..
35
- vi PIGURU3
10.
PAGE
Modal d a t a f o r t h e t u f f
ofBearSprings
.....
ofBearSprings
..
p l o t t e d on a three-componentdiagram
tuff
11.
Contortedunit,lower
12.
Measured s e c t i o n o f t h e u p p e r t u f f
39
of Bear
. S p r i n g s showing mineralogical'variations
i
36
13.
Typicaloutcropof
La Jara Peak Andesite
14.
Composite sectionoftheash-flowcooling
...
...
45
52
units. sh0wing.thej.r average mineralogical
composition and r a n g e i n abundanceof
c r y s t aflr a g m e n t s
15.
.....
t u f f of BearSprings
16.
..............
."
'
area
Magdalena
......................
.
. .
79
Location o f t h e s i sa r e aw i t hr e s p e c tt ot h e
major s t r u c t u r a l e l e m e n t s i n t h e
_
.
"
72
Wavelike f o l d si nt h e. c. .o n t o r t e du n i t ,l o w e r
,
.
..............
-
89
- vii LIST OF.TABLES
PAGE
TABLE
I.
Modes i n volume percent from t h e t u f f o f
Goat Spring
13:.
Modes i n volume percent from t h e lower
t u f fo f
111.
.................
BearSprings
'
...........
34
Modes i n 'volume percent from t h e upper
t u f f of Bear s p r i n g s
-..
,
".
.,"
26
- .-. _.
".
..
.............
44
-
*
viii
-
ABSTRACT
Rocks exposed i n the southern BearMountainswere
duringmiddle
and l a t e Cenozoic t i m e .
g r a p h i co r d e r
the majorrock
Formation (37 m.y.),
formed
In a s c e n d i n g s t r a t i -
u n i t sa r e :
(1) t h e S p e a r s .
a thick pile of latitic to andesitic
coiglomerates and sandstones .with l a h a r i c b r e c c i a s , ' a s h - f l o w
'
t u f f s and l a v a f l o w s i n
i t s upper t h i r d ,
(2) the H e l l s Mesa
Formation (31-32 m.y.1,
an a l t e r n a t i n g sequence of q u a r t z
l a t i t i c and r h y o l i t i c ash-flow t u f f s s e p a r a t e d by t h i n ande-
s i t e flows,(3)the
e
La Sara Peak Formation ( 2 4 m.y.),
pile of b a s a l t i c - a n d e s i t e f l o w s w i t h
sedimentaryrocks,and"
a thick
minor v o l c a n i c l a s t i c
(4) the fanglomerates
of Dry Lake
Canyon, a t h i c k wedge of epiclastic sedimentary rocks derived
-
,-
by erosion of t h e La Sara Peak Andesite during
block faulting
l a t e Cenozoic
.
The H e l l s Mesa Formation c o n s i s t s o f e i g h t
t h e southernBearMountains;these
I
i n gs t r a t i g r a p h i co r d e r
are:
Site flows 1, the lower tuff
tuff
*
Of
members i n
members, l i s t e d i n ascend-
t h et u f fo f
Goat Spring, ande-
of B e a r Springs w i t h interbedded
La Jencia Creek, andesite
flows 2, t h e u p p e r t u f f
Bear S p r i n g s , . a n d e s i t e f l o w s 3 , ' a n d t h e t u f f o f A l l e n
Two d i s t i n c t l y d i f f e r e n t t y p e s
Wart2 latitic variety
of ash flows,
of
Well.
a crystal-rich
and a c r y s t a l - p o o r r h y o l i t i c v a r i e t y ,
'"__
,
..
1)
-
ix
o c c urre p e a t e d l iytnh see c t i o n .
-
The two v a r i e t i e s e i t h e r
o r i g i n a t e d from d i f f e r e n t levels of a f r a c t i o n a l l y c r y s t a l l i z i n g magma chamber o r . f r o m two d i f f e r e n t p y r o c l a s t i c
flow s t r u c -
sources a c t i v e a t t h e same time.Primarylaminar
t u r e s i n the t u f f o f
Bear Springs indicate that
t h e source
/t
area l a y somewbere south o r southwest of
Magdalena.
A t l e a s t t w o major periods of faulting have affected
the a r e a s i n c e
middleTertiary
cene,anintense
time.
zone of north-trending
During theLateOligonormal f a u l t s c u t
t h e Speaxs and Hells Mesa Formations and l o c a l i z e d t h e e m - . '
placement of h y p a b y s s a li n t r u s i v e so fa n d e s i t i c ,m o n z o n i t i c ,
a
and g r a n i t i cc o m p o s i t i_..
on
.. .
dated a t 28 m. y .
Two ofthesestoclcshave
.
bee=,
I n Miocene and Pliocene time,, the Bear
Mountains vere u p l i f t e d and t i l t e d westward by b l o c k f a u l t i n g
r e l a t e d t o t h e R i o Grande rift.
trending oblique-slip faults
A t the same
= echelon w i t h
time northeastthe San Augustin
t o form a trough
graben dropped the southern Bear Mountains
between t h e Magdalena Mountains and t h e h i g h p a r t
of the Bear
Mountains.
is r e s t r i c t e d
Surface m i n e r a l i z a t i o n w i t h i n t h e s t u d y a r e a
epithermal vein deposits marginal to the
*
stock.Replacement-typedeposits
La Jencia monzonite
may be found i n t h e u n d e r l y i n g
Paleozoiclimestonesbutsuchdeposits
would beburiedbeneath
a T e r t i a r y v o l c a n i c c o v e r , from 2000 t o 2500 f e e t t h i c k n e a r
"..
..
~~~~
~~
.
e'
.
t h e La J e n c i a s t o c k . S u r f a c e m i n e r a l i z a t i o n i n t h i s a r e a
'
i s very weak and d o e s n o t o f f e r
exploration.
much encouragement f o r d e e p
A p o s s i b l ee x p l o r a t i o nt a r g e t
w e s t o f the map a r e a . U p l i f t
may e x i s t j u s t
and t i l t i n g of t h e Hells Mesa
Formation and i n t e n s e h y d r o t h e r m a l a l t e r a t i o n a l o n g C o u n c i l
Rock Arroyo s u g g e s t t h e p o s s i b i l i t y
this area.
-.
__
of s t o c k i n t r u s i o n i n
INTRODUCTION
Statement of t h e Problem
t ?
..
The purpose of t h i s i n v e s t i g a t i o n i s t o determine the
s t r a t i g r a p h i c and s t r u c t u r a l r e l a t i o n s h i p s
volcanic rocks
of t h e T e r t i a r y
and h y p a b y s s a l i n t r u s i v e r o c k s i n
t h e south-
The study was undertaken as p a r t of an
e r n Bear Mountains.
e x t e n s i v e mapping p r o j e c t of t h e Magdalena a r e a p r e s e n t l y
being conducted by t h e New Mexico Bureau of Mines and Mine r a l Resources.
.
.
A d e t a i l e d g e o l o g i c knowledge of the southern %ear
Mountains is important for
1.
two reasons:
The s t r a t i g r a p h i cu n i t sa r es t r a t e g i c a l l yl o c a t e d
for c o r r e l a t i o n . s o u t h w a r d i n t o t h e
Magdalena min-
__.
~
i n g d i s t r i c t , eastward t o the RioGrande
rift
zone, and westward i n t o t h e h e a r t of t h e Mogollon
..
-- -
Plateau volcanic province.
-
2.
F u t u r ee x p l o r a t i o n
of t h e Magdalena mining d i s t r i c t
i s dependent upon a n a d e q u a t e s t r a t i g r a p h i c
s t r u c t u r a l frameworkwhich
and
heretofore has not been
available.
Location and Accessibi.lity
'
The area o f i n v e s t i g a t i o n
New Mexico.
'
i s located north
of Magdalena,
P h y s i o g r a p h i c a l l y ,t h ea r e ai n c l u d e st h es o u t h
-. .
.!
- 3 -
'
,
-
2L?--s
SO
75Milcr
FIGURE I
e
SCALE
- 4 -
Magdalena 1.5-minute quadrangle t o a 7.5-minute
j o i n i n g it t o p a r t o f
s c a l e and
the S i l v e r H i l l 7.5-minute quadrangle.
U. S . Geological Survey a e r i a l p h o t o g r a p h s o f t h e
GS-VMA
(1956) and GS-VARJ.(1963) series were used a s guides t o t h e
l o c a t i o n and configuration of outcrops.
Eighty t h i n s e c t i o n s were made from samples taken
along a s t r a t i g r a p h i c s e c t i o n
measured j o i n t l y w i t h C. E.
Chapin of the New Mexico Bureau of Mines.Twenty-five
.
a d d i t i o n a l t h i n s e c t i o n s were made of rocks from elsewhere
i n the area.
Petrographic analyses
were made u s i n g a Zeiss
microscope equipped w i t h a Swift automatic point counter.
Modal a n a l y s e s of samples from t h e measured s e c t i o n were
... ..
performed by Douglas
Cowan and 3ames Bruning and checked
were countedon
by t h e a u t h o r . M i n e r a l g r a i n s
gri.d 1/3
nun. x
1.0
mm,.
g i v i n g 2000 t o 2500 p o i n t s p e r t h i n
s e c t i o n f o r t h e upper and lower t u f f o f
1200 t o 1400 p o i n t s f o r t h e t u f f
".
Inv
Per e
s tviiqoaut si o n s
a rectangular
Bear Springs
o f Goat Spring.
.
.
,
r'
The e a r l i e s t work done i n t h e Bear Mountains
Of
generalreconnaissance.In
and
was t h a t
1900, C. L. Herrickconducted
a reconnaissance survey of western Socorro and Valencia
counties.
H e b r i e f l yn o t e dt h a tt h e
Bear,Gallinas,
and
b t i l Mountains are composed of t r a c h y t e and r h y o l i t e i n tnsives.In
1 9 2 0 , D. E. W i n c h e s t e rp u b l i s h e dt h er e s u l t s
..
.
,
- 5 -
e
'
of aninvestigationofthegeologyalong
Alamosa Creek,
now known a s t h e Rio Salado.Winchester
named t h e e n t i r e
T e r t i a r y s e q u e n c e of t h e D a t i l Formation a f t e r t h e D a t i l
Mountains and d e s c r i b e d t h e t y p e s e c t i o n a t
of the BearMountains.
his regionai
'
the n o r t h end
I n 1928, N. H. Darton published
"Red Bed,s" study, which contained a summary
of his .reconnaissance i n t h e a r e a , a s
previous work.
well as Winchester's
.
Loughlin and Koschmann (1942) s t u d i e d t h e Magdalena
mining d i s t r i c t , and n o t e d t h a t
some of t h e T e r t i a r y v o l -
canic rocks extend t o t h e n o r t h . and n o r t h w e s t o u t s i d e t h e
district.
W. N. Tonk.ing (1957) .made t h ef i r s dt e t s . i l e d
s t u d y of the D a t i l v o l c a n i c r o c k s n o r t h o f t h e
H e subdividedthe
area.
.
Magdalena
D a t i l Formation i n t o three mem-
bers; which are i n ascendingorder:
(1) The Spears Member,
..
a thick sequence of l a t i t e t u f f s and volcanic sediments,
..
(2)
The H e l l s Mesa Member, composed of welded r h y o l i t e ash-flow
t u f f s , and ( 3 ) The La J a r a Peak Member, a t h i c k p i l e of
b a s a l t and b a s a l t i c 'andesiteflows.Willard
tatively correlated Tonking's
La Jara Peak Member w i t h t h e
Mangas b a s a l t , a post-Datil sequence.
. suggestion, Weber (1963) excluded the
e
from t h eD a t i l
Formation.
(1959) t e n -
FollowingWillard's
La J a r a Peak Member
Weber (1971) also
suggested
r a i s i n g t h e D a t i l Formation t o group s t a t u s i n o r d e r t o
..
.
- 6 and members.
accommodate subdivisionintoformations
'
Johnson (1955) attempted t o c o r r e l a t e t h e v o l c a n i c
rocks of
the Magdalena mining district w i t h t h o s e of
Tonking'sPuerteciko'Quadrangle.
of t h e a r e a
He produced a crude map
from Tonking's southern boundary
boundary of Loughlin and Koschmann.
s t u d i e d the petrology of the.
which i s l o c a t e d. i n
'
t o t h e northern
D. E . Park (1971)
Anchor Canyon monzonite stock,
t h e Kellymining
district.
An i m p o r t a n t c o n t r i b u t i o n t o the regional geology
t h e Datil-Mogollon volcanic province has
E l s t o n and o t h e r s (1968,1970)
of
been made by
who a r ep r e s e n t l ys t u d y i n g
the volcano-tectonic .framework of t h e Mogollon Plateauarea.
A nu&er
of K-Ar
. post-Datil.rocks
a g e s a r e now a v a i l a b l e f o r D a t i l
i nt h ea r e a .
dated a welded t u f f n e a r
and
Weber and B a s s e t t (1963)
the base of the Hells
Mesa Member,
a s weli '.as t h e N i t t and Anchor Canyon s t o c k s i n t h e Magdalena
mining d i s t r i c t .
Burke and o t h e r s (1963) datedtheSpears
Member and t w o more welded t u f f s from t h e Hells Mesa Member.
Kottlowski, Weber,. and Willard (1969) published a summary of
more t h a n 60 K-Ar d a t e s of T e r t i a r y rock u n i t s i n New Mexico
including many from the Mogollon P l a t e a u and surrounding
a r e a s . . More r e c e n t l y , Weber (1971) haspublished
e
'
K-Ar
d a t e s f r o T e r t i a r yr o c k si nc e n t r a l
Mexico and Chapin (1971-a) has reported
...
i:
a l i s t of
and western New
a d a t e o n t h e La
.
-
,
- 7 -
Jara Peak Member.
Acknowledqements
t h e f i e l d worXwas
Financial support for
provided by
a grant-in-aid from the Bear Creek division of
KenneCQtt
Copper Corporationduringthe
The New
summer of 1969.
Mexico Bureau of Mines and Mineral Resources provided
i l a r support during the
summer of1971.
Appreciation i s a l s o e x t e n d e d t o t h e i n d i v i d u a l s
who.
ideas. and information about various
provided the author with
aspects o f t h e p r o j e c t , i n c l u d i n g
and A. J. Budding.
sim-
R. H. Weber, Kent Condie,
S p e c i a lt h a n k sa r ed u et h et h e s i s
_.. ..
advisor, C. E. Chapin, f o r Yne o r i g i n a l s u g g e s t i o n
thesis a r e a and f o r v a l u a b l e a d v i c e i n
of the
t h e f i e l d and o f f i c e .
.-.
Mr.
and M r s .
DonaldHudgins
and F e l i x and JodySanchez
tended hospitality during the course
of the f i e l d work and
Margie Mora a s s i s t e d i n t h e p r e p a r a t i o n o f t h e
manuscript.
!..
ex-
map and
-_..
,
:.
..
- 8 -
'
STRiiTIGRAPHY AND PETROLOGY
Pre-Tertiary Rocks
.
-.
No r o c k s o l d e r t h a n t h e T e r t i a r y a r e
t h e s i sa r e a .
To thesouth,Loughlin
exposed w i t h i n t h e
and Koschmann's map
(1942) shows t h a t on t h ee a s ts i d eo fG r a n i t e
bas'e of
the Spears Formation
Mountain t h e
rests unconformablyon
Madera Limestone of Pennsylvanian age.
To the north,, Tonking's
map (1957) shows 'chat the b a s e o f t h e S p e a r s
-
a b l y on t h e Baca Formation(Eocene)
disconformably overlies the Crevasse
the
rests conform-
The, Baca, i n t u r n ,
Canyon Formation of
Cretaceousage.
..The Magdalena
area i s s i t u a t e d o n t h e f l a n k s o f
.,
a
Iiaramide u p l i f t which was subjected t o e r o s i o n d u r i n g dep o s i t i o n of t h e
.Ea&
Formation i n a d j a c e n t b a s i n s . C e n t r a l
New Mexico was beveled by an erosion surface of moderately
_"
..
10w.relief p r i o r t o ,the beginning of Oligocene volcanism
. .
(Chapin,1971-b)
so t h a t t h e b a s e
on p r o g r e s s i v e l y o l d e r r o c k s a s t h e
of t h e v o l c a n i c p i l e r e s t s
Magdalena a r e a i s
from
approached and t h e nature of the contact changes
conformable i n t h e ' b a s i n s where t h e Baca accumulated t o an
It i s d i f f i c u l t
angularunconformityonthe.beveled
uplift.
. .
to p r e d i c t where t h i s t r a n s i t i o n o c c u r s w i t h i n t h e s t u d y
.
area since no exposures a r e a v a i l a b l e .
x-....
r
l
.
.
.
,
- 9 Textiary Volcanic Rocks
i n the study area are predominantly
The rocks exposed
volcanic rocks
and interbedded volcani-clastic sedimentary
rocksofTertiaryage.
These rockscanbedividedinto
main u n i t s s e p a r a t e d
byanangularunconformity:
two
(1) the
D a t i l Group.comprised of the Spears and H e l l s MesaForm-
a t i o n s , and (2) t h e La Sara Peak Formationcomprisedof
p o s t - D a t i lb a s a l t i ca n d e s i t ef l o w s .
The combined t h i c k n e s s
may be s i x thousand €eet or more
of t h e s e v o l c a n i c r o c k s
(Figure 2).
,a
Spears Formation
.
i n ' t h e Magdalena a r e a ' i n early Oligocene
Volcanismbegan
time w i t h the accumulation of
a thick sequence of epiclastic
__...
volcanic sediments
and interbeddedash-flow
flows named the Spears
Tonking(1957,
p.
quentlybeenraised
t u f f s and l a v a
member of t h e Datil Formation b y
36-41)
a
The DatilFormationhassubse-
t o Group s t a t u s (Weber, 1971) and the
Spears t o f o r m a t i o n a ls t a t u s
(Chapin,1971-a).
tuff-breccia from the Soyita
Hills, approximately 30 miles
e a s t of t h e Bear Mountains, has been correlated
Spears Foxmation
(I,
A latite
with t h e
and dated by the K-Ar method as 37.1 m.y.
(Weber, 1963, p. 135; 1971, p. ' 4 2 ) .
Marked f a c i e sc h a n g e so c c u rl o c a l l y
i n the Spears.
At
-
10
-
.
H e l l s Mesa, it c o n s i s t s dominantly of f i n e - g r a i n e df l u v i a l
sediments; i n t h e a r e a s t u d i e d h e r e ,
coarse volcanic conglomerates
i n the upper part;
it consists mostly
b u t w i t h some volcanic rocks
on Granite Mountain, it consists of
thick section of lava flows
s o u t h end o f the Kelly d i s t r i c t , it changesback
the
t o a khick
section of volcani-clastic sedimentary rocks with
The b a s e o f ' t h e S p e a r s
a
and ash-flow t u f f s above a reL-
a t i v e l y t h i n lower member of volcanic conglomerates; at
v o l c a n i cr o c k sn e a rt h et o p
of
minor
(Chapin, 1971, o r a l communication).
anywhere i n t h e thesis
is' notexposed
a r e a but at t h e t y p e l o c a l i t y n e a r
H e l l s Mesa, p u r p l i s h
l a t i t i c s a n d s t o n e s n e a r ... ..the base of
t h e Spears are
inter-
bedded w i t h reddish nonvolcanic mudstones and sandstones i n
the upper part of
.the Baca Formation (Tonking, 1957; Potter,
1970).
"
"..
. .
Lower Member.
The lowerportionof
exposed on NippleMountain
area.
t h e Spears i s best
i n the southeast corner
. .
of the
A minimum t h i c k n e s s of 1500 feet has been estimated
from the map, b u t this f i g u r e i s very likely exaggerated
c o n c e a l e df a u l t i n g .
I n t h e Granite Mountain
by
- Nipple
Mountain area, where the' Spears 'is p r o p y l i t i c a l l y a l t e r e d ,
the c o l o r i s generallygreenish-gray.
This c o l o r i s a t y p i c a l
o f t h e r e d d i s ht op u r p l i s h - b r o w nc o l o ro ft h e
a t B e l l s Mesaand
fresh Spears
a t t h e south end of the Kelly
district
x..
..-
':
/.
t u f f of Gozt S ~ r ~ ~ g m ~ i . t ~ . ~ ~ -simple
e - f 1 ocooling
~r
u n i t of p i n k c r y s t a l - r i c h q u a r t z l a t i t a t o r h y o l i t e
ash-flow t u f f s
hemtite-sta.ined
conglomerate
-,
-\
3
.
.
*
Scale (in feat)
...,
The ap2roximate position of.the tuff
o f La Jencia, Creek.
a series of c r y s t a l - r i c h q u a r t z l a t i t e ash-flow tuffs;
is shown by t h e wedge.
I
Figure 2.
a.
.
Generalized connosite section of t h e T e r t i w y v o l c a n i c r o c k s
and volcaniclasticsedimentaryrocks.
i n thesouthern Bear
Xountains.
....
-
12
-
where Eoughlin and Koschmann c a l l e d it t h e p u r p l e a n d e s i t e .
Outcrops of t h e Sower member c o n s i s t of a monotonous se'quence of p r o p y l i t i z e d l a t i t i c t o andesitic conglomerates
and sandstones which range i n g r a i n s i z e from boulder conglomerates t o ' f i n e g r a i n e ds a n d s t o n e s .S t r a t i f i c a t i o n
generally crude except
is
f o r the fine grained sandstone beds,
which are u s u a l l y w e l l - s t r a t i f i e d
and l o c a l l y cross-bedded.
I n hand specimen', t h e lowerSpears
i s well-indurated
and c o n t a i n s rounded l a t i t i c t o a n d e s i t i c c l a s t s r a n g i n g
in
color from grayish-green t o purplish-grayorpink.Varying
percentages o f pale green feldspar, greenish-black horn-
e
blende, b i o t i t e , and magnetiteare
..
v i s i b l e megascopically.
Petrographically, the clasts are similar
l a t i t e s described by Tonking, except that they
l y a l t e r e d - i n t-he Nipple Mountain area.
t o the Spears
are pervasive-
The o r i g i n a l tex-
t u r e s and minerals have been largely obscured
by p r o p y l i t i c
a l g e r a t i o n which uniformly penetrates the entire rock render-.
i n g t h e b o u n d a r i e s between c l a s t s and m a t r i x i n d i s t i n c t .
Most of t h e c l a s t s are p o r p h y r i t i c w i t h
icaLSy aligned feldspar
abundant t r a c h y t -
and hornblende phenocrysts in
a
. .
. .
c r y p t o c r y s t a l l i n e groundmass which s u g g e s t s t h a t t h e y a r e
.
~
a
fragmentsoflava'
flows.
The l a t i t e s a r e . m i n e r a l o g i c a l l y uniform and c o n s i s t
-mainlyofplagioclase,sanidine,hornblende,
".-..
%.
.;
and b i o t i t e .
-
13
-
Individualclastsvaryconsiderably,
however, i n t h e
a b s o l u t e and r e l a t i v e p e r c e n t a g e s of these major cons t i t u e n t s .A c c e s s o r ym i n e r a l si n c l u d ea p a t i t e
Epidote, c h l o r i t e , c a l c i t e ,m a g n e t i t e ,
u s u a la l t e r a t i o np r o d u c t s .
and h e m a t i t ea r e
The g r e e n i s hc o l o r
Spears is caused by the presence
and q u a r t z .
of the lower
of c h l o r i t e and epidote,
while the p i n k i s h c o l o r of some c l a s t s i s probably due
finely divided hematite in
to
the groundmass.
P l a g i o c l a s e is t h e most abundant phenocryst
from 1 0 p e r c e n t t o 20 percent’.
grains from
the
and v a r i e s
It occursassubhedral
0 . 1 t o 1 . 5 mm long and i s h i g h l y a l t e r e d t o
i s approximatelyequal
c l a y s and e p i d o t e . . Sanidine
.. . ..
in
amount t o p l a g i o c l a s e and occurs as subhedral grains from
0.3 to 1.5 mm long.
The s a n i d i n e is g e n e r a l l y l e s s a l t e r e d
t h a n the p l a g i o c l a s e b u t most g r a i n s show varying degrees of
a r g i l l i ca l t e r a t i o n .
Hornblende i s p r e s e n ta se u h e d r a lt o
s ~ h e d r a pseudomorphs
l
ranging i n leng,th up t o 1 . 5
-.
&.
The
original hornblende has been replaced by epidote, chlorite,
andmagnetite.
B i o t i t e i s u s u a l l yr e p l a c e db yc h l o r i t e
and
magnetite.
Upper Member.
A t the top
of NippleMountainabout
feet o€ volcanic rocks overlie the sedimentary rocks
lower Spears.
80
of t h e
The lower 30 f e e t c o n s i s t s of a blue-gray,
*.
-
14
-
amygdaloidal,"turkey-track"andesiteflowwith
zoned.plagioclasephenocrysts.
Nipp'leMountain
Above t h i s ,t h ev e r yt o po f
i s capped by about
in t h i s r e p o r t .
five percent sakidine phenocrysts
a n d e s i t i cl i t h i c s .
f i f t y f e e t of pink,
t u f f , designated the t u f f of
welded, crystal-poor ash-flow
NippleMountain
1 5 percent
The ashflowcontainsabout
and a few percent dark
These volcanicrocks
mark a t r a n s i t i o n
i l i . t h e S p e a r s from t h e lower member of v o l c a n i - c l a s t i c
sedimentary rocks
t o an upper member o f a n d e s i t i c l a v a f l o w s
and l a t i t i c a s h flovis interbedded with laharic breccias
f l u v i a ls e d i m e n t s .
The bestexposuresoccuiin
Box southwest of ,Nipple Mountain,where
more l a h a r i c b r e c c i a s
doesfarthernorth.
and
La Jencia
the section contains
and interbedded andesite
flows t h a n it
I n La Jencia Box theupperSpears
becomes
predominantly mudflow b r e c c i a s which grade upward impercepti\
b l y i n t o welded l i t h i c - r i c h a s h f l o w s : t h e s e c r y s t a l - r i c h
ash flows greatly resemble the basal
20 t o 40 f e e t of t h e
~
t u € f of Goat Spring and c o r r e l a t e w i t h t h e u p p e r l a t i t e
of
Laughlinand, Koschmann (. 1942, p. 2 5 ) .
North of La Jencia Creek the upper
member forms t h e .
lower s l o p e s of t h e t i l t e d f a u l t - b l o c k r i d g e s a l o n g t h e
e a s t e r n margin o f ' t h e a r e a where it crops out poorly because
e.
of mantling by talus from the overlying tuff of
I n this area, outcrops
Goat Spring.
occur only where gulleys have trenched
.. "_
~
.
8
i n t o t h et a l u s .
15
-
Most of t h ef l a t - f l o o r e d
along the eastern margin
ablyunderlainby
pediment s u r f a c e s
of t h e Snake Ranch P l a t s a r e prob-
t h e upper part of the Spears.
A section
of the upper meniber of t h e S p e a r s was measured about
mile northwest of
Goat Spring along
of a prominentpeak.
.3/4-
the s t e e p e a s t e r n s l o p e
The measured s e c t i o n( F i g u r e
2) begins
i n a blyish-gray,prophyritic"turkey-track"andesiteflow,
similar t o the one on NippleMountain.
flows are about
Above t h e a n d e s i t e
220 f e e t ofreddish-brown
conglomerates,sandstones,
and mudstones.
c o n t a i n rounded c l a s t s r a n g i n g i n s i z e
0
t o reddish-purple
The conglomerates
fromoneinch
i n c h e si nd i a m e t e rw i t h , , o c c a s i o n a lb o u l d e r s
to six
up t o one f o o t .
Ike i m b r i c a t i o n of t h e c l a s t s ' i n d i c a t e s a n o r t h e r l y d i r e c t i o n
of t r a n s p o r t f o r.
.
the sediments.
The c l a s t s a r e predominantly
r e d d i s h - b r o w np o r p h y r i t i cl a i i t e
and "turkey-track"andesites
w i t h t h e con-
similar t o the underlyingflows.Interbedded
up t o s i x inches thick.
qlomerates are thin sandstone layers
..
"
Above these sediments,approximately
ash-flow t u f f s were measured.
gray,poorly
200 f e e t o f . l a t i t i c
The t u f f s a r e l i g h t p u r p l i s h -
welded, and o c c a s i o n a l l y show a north-south
lineationdefinedbyelongated
pumice.Chalky
whiteplagio-
c l a s e , .bronzy b i o t i t e , and dark-brown p o r p h y r i t i c a n d e s i t i c
a
lithic f r a g m e n t sc h a r a c t e r i z et h e
the compaction foliation
hand specimens.
is defined by gray streaky
x.
Often
pumice.
..... ,
,
'>
~
~~~~~~
-
16
......
. . . .
e
.......................
....
"
0'
.
~.
F i g u r e 3 . L a t i t i c mudflow b r e c c i a i n t h e upperSpears
Formation showing angular
t o subrounded a n d e s i t e and l a t i t e c l a s t s" f l o a t i n g " in acrystal-richmatrix.
me
white specks in the matrix are altered plagioclase
cryst a l s . Note the l a c ko fs o r t i n g
and s t r a t i f i c a t i o n .
.....
,
........
...
. . . . . .
.-
.
..............
..
J
"
F i g u r e 4. Poorly welded , i a t i t i c ash-flow t u f f i n t h e
upperSpearsFormation.Close-upofsurface
shows
white poorly welded p u m i c e , d a r k a n d e s i t e l i t h i c f r a g ments,andabundantchalky-whitefeldsparcrystals.
I
..
.
In places
-
17
it is d i f f i c u l t t o d i s t i n g u i s h t h e S p e a r s a s h
flows from t h e overlying quartz-poor base of the tuff
of
Goat Spring.
..
The t o p o f t h e S p e a r s
is marked by a d i s t i n c t i v e
The conglomerate i s zero
hematite-stainedconglomerate.
to forty feet thick
clastscoated
and contains hydrothermally altered
with reddish-brownhematite.
Many o f t h e
c l a s t s a r e so a l t e r e d and s i l i c i f i e d a s ' t o resemble chert,
b u t close inspection usually reveals
r e l i c t phenocrysts.
Several small prospect pits'have been
a l o n gt h ee a s t
edgeof
t h es t u d ya r e a .
dug a t t h i s horizon
The hematite-
s t a i n e d c l a s t s a r e e a s i .l.y.. recognized in the f l o a t ; t h u s
t h i s conglomerate provides
of
the
Spears.
.
a u s e f u l markerbed
a t the top
.
Because o f t h e i r - s o f t , p o r o u s n a t u r e , t h e a s h - f l o w
t u f f s i n t h e Spears a t the measured s e c t i o n a r e h i g h l y
. .
a l t e r e d and l i t t l e can be learned about
-.
Theirtexture
them p e t r o g r a p h i c a l l y .
i s p o r p h y r i t i c and s e r i a t e , with abundant f e l d -
from l a t h s 1.0 mm long down
spar phenocrysts grading in size
t o g r a i n sb a r e l yd i s c e r n i b l e
from t h e groundmass.
dividual feldspar grains are often broken
.alignment p a r a l l e l t o t h e
and show a crude
compaction f o l i a t i o n .
eutaxitic structure is visible because of the
welding and t h e high degree
-.
of a l t e r a t i o n .
-
\.\
The in-
Only a f a i n t
low degree of
.- 18 -
e
Mineralogically, t h e ashflows
seem t o resemble t h e
l a t i t e c l a s t s i n t h e lower Spears, except that hornblende
i s absent.
The t h i ns e c t i o n sc o n t a i n . f r o m
40 p e r c e n t f e l d s p a r . p h e n o c r y s t s ; p l a g i o c l a s e
are b o t h p r e s e n t , b u t a l t e r a t i o n
20 p e r c e n t t o
and s a n i d i n e
makes it d i f f i c u l t t o
e s t i m a t e t h e i r relativeproportions.'Plagioclaseappears
t o be more abundant, and occursassubhedral,oftenbroken,
g r a i n s from 0.1 nun t o 1 . 5 mm long.
h i g h l ya l t e r e dt oc l a y s
The phenocrystsare
and c a l c i t e .S a n i d i n er a n g e s
from
0 . 1 mm t o . 1.0 mm i n length, and shows varying degrees
argi11i.calteration,
from i n c i p i e n t t o completereplacement.
A f e w p e r c e n t b i o t i t e occurs
as small
_.. ..
by ironoxide.Traces
andmuscoviteare
of
opaque g r a i n s r e p l a c e d
of clinopyroxene,quartz,apatite,
a l s o present.
and has been replaced
The groundmass i s a p h a n i t i c
by a fine-grained aggregate of clay
minerals and c a l c i t e .
Bells Mesa Formation
The Hells Mesa Formation i s a t h i c k series of welded
quartz. l a t i t e t o r h y o l i t e ash-flow t u f f s and interbedded
andesiteflows.
Tonking (1957, p. 24-30,
56) named t h e
u n i t f o r Hells Mesa a t t h e e a s t e r n edge of the Bear
e
Moun-
t a i n s and measured a t y p e s e c t i o n a t t h i s l o c a l i t y ( S e c ,
31, T. 2N. R. 4 W.; PuertecitoQuadrangle).
He gave the
-
0
19
u n i t member s t a t u s i n t h e D a t i i
-
Formation,butthe
Datil
h a s s i n c e been raised t o group s t a t u s and t h e Hells Mesa
is considered here a s a formation.
Tonkingmeasured
289 feet o f : t h e Hells Mesa a t h i s t y p e s e c t i o n
representsonlythethin
edgeof
southern Bear Mountains the
of about
only
which
t h e formation.
'In t h e
H e l l s Mesa a t t a i n s a t h i c k n e s s
1700 f e e t and h a s b e e n s u b d i v i d e d i n t h i s r e p o r t
i n t o s e v e n members, as d e s c r i b e d i n t h e f o l l o w i n g p a r a g r a p h s .
Tuff of Goat Spring.
The t u f f o f
Goat Spring i s t h e
informal name proposed here f o r t h e b a s a l member o f the
Hells Mesa Formation.
@
Bear,Gallinas,
It i s a d i s t i n c t i v e u n i t i n
San XateoiLemitar,
the
and Magdalena Moun-
t a i n s and w i l l undoubtedly be considered a separate.form-..
,
a t i o n when more r e g i o n a l work i s done.
m i s t a. k. e n l y i d e n t i f i e d a s
This u n i t was
a rhyolite porphyry
sill, f i r s t '
by Loughlin and Koschmann (1942) and l a t e r byJohnson
(1955).
It is. a c t u a l l y a multiple-flow,simplecooling
u n i t o f welded quartz l a t i t e to r h y o l i t e ash-flow t u f f s .
A sample from t h e base of t h e u n i t was dated by
method as 30.6
*
2.8 m. y.
the K-Ar
(Weber and B.assett,1963).
The t u f f of Goat Spring covers about five square
miles of . t h e s t c d y a r e a .
I)
Its main area of outcrop extends
i n a n o r t h e r l y manner from Granite Mountainon
-
".. ...
/
the south
c
F i g u r e 5 , S t r a t i f i e d o u t c r o p s of t h e Hells,Mesa Formation
'near B e a r Springs draw. Double ledge a t t h e t o p of t h e
hill. i s formed by t h e c o n t o r t e d andpumiceousash-flow
u n i t s which cap the lower cooling u n i t i n t h e t u f f of Bear
Springs. Middle slope-forming p a r t i s u n d e r l a i n by t h e
gray massiveunit of t h & t u f f o f Bear S p r i n g s . Dark
ledge-forming outcrops at t h e base and in t h e foreground
a r e t h e tuff of Goat Spring. V i e w is looking southwest;
Tres Montosas are t h e h i g h peak j u s t visible t o t h e right
of c e n t e r .
.
.
along the eastern margin of
north.
-
21
the a r e a t o Hells Mesa on the
A t H e L l s Mesa it forms t h e b a s a l 1 7 5 f e e t of
Its maximum t h i c k n e s sh a s
Tonking'smeasuredsection.
beenmeasured
Sec. 26, T.
a s about 640 f e e t n e a r
Goat s p r i n g (iW
a,
1 S., R. 4 W.) and it .thins rapidlynorthward
i n t o tke Puertec~ito Quadrangle.
The t u f f .of Goat Spring outcrops as prominent
andhogback
ridges.
cliffs
It w e a t h e r s d i s t i n c t i v e l y i n t o l a r g e
rounded blocks bounded by j o i n t s .
Many outcropshave
e l o n g a t e d c a v i t i e s up to four inches long
where pumice has
been removed by weathering.
Fresh hand specimens range
t o purplishgray
i n c o l o r from pinkish-gray
and weat;ier t o b u f f or gray.
Most speci-
mens are densely welded and c r y s t a l - r i c h w i t h phenoclasts
accouiltingfor
40 t o 50 p e r c e n t of the rock.Phenoclasts
i n c l u d es a n i d i n e ,
smoky q u a r t z , and coppery b i o t i t e .
LocaLly, aphanitic purplish-brown lithic fragments
and
g r a y f l a t t e n e d pumice a r e abundant.
In thin section,
samples a r e d i s t i n c t i v e l y p o r p h y r i t i c
and c o n t a i n from 40 t o 50 percent broken
sorbed crystal fragments.
and p a r t i a l l y re-
Comminution of t h e c r y s t a l s
during emplacement has produced' a s e r i a t e t e x t u r e c h a r a c t e r -
1
Tonking'ssection,p.
56, i s upside down.
-
22
-
ized by a complete gradztion from l a r g e c r y s t a l s 3 mm i n
diameter down t o . l r a g m e n t s 0.01 mm o r less i n diameter.
Sanidine is the most abundant phenoclast
from 1 0 t o 30 percent of the rock.
I
t o euhedralgrains
from0.1
averagelengthofabout
1.5
mnl
and v a r i e s
It occursassubhedral
t o 4 mm i n l e n g t h w i t h an
mm.
Most g r a i n s a r e p a r t i a l l y
resorbed and show i n c i p i e n t a l t e r a t i o n t o c l a y s and h e m a t i t e
along cleavages
which g i v e s t h e s a n i d i n e
i n hand
specimen.
'
Plagioclase varies
and averagesabout
e
its pinkish cast
from L O to 20 percent of the rock
1 2 percent.
The g r a i n sa r es u b h e d r a l
t o e u h e d r a l and usually have corroded
,
,..
andembayed
borders.
..
The p l a g i o c l a s e i s g e n e r a l l y s m a l l e r t h a n t h e s a n i d i n e
rangesfrom
and
0 . 1 mm t o 1.5 mm i n length.Variousdegrees
of a l t e r a t i o n t o c l a y s and c a l c i t e a r e p r e s e n t , r a n g i n g
from i n c i p i e n t a l t e r a t i o n a l o n g c l e a v a g e s t o c o m p l e t e
placement.
The compositions of t h e plagioclasephenoclasts
i n a number of samples were
method.
re-
determined u s i n g t h e FouquL
"he averagecomposition
was determinedassodic
andesine and there appears t o be a s y s t e m a t i c v a r i a t i o n
i n the plagioclase composition
from about
An31 near the
b a s e of the u n i t t o about An 36 near t h e t o p .
,*
Quartz occurs as large
3
&
i n diameter.
rounded g r a i n s from 1 mm t o
It i s a b s e n t o r v e r y s c a r c e
i n t h e lower
""
.,
.
Figure 6 . Tuff of Goat Spring. Low magnification (4X)
shows t h e abundanceand range i n s i z e of c r y s t a l f r a g ments. A l t e r e d f e l d s p a r s a r e l i g h t gray, quartz i s
white, b i o t i t e and opaques a r e b l a c k .
.-
24
-
1 0 t o 20 f e e t of the u n i t , t h e n i n c r e a s e s a b r u p t l y t o a b o u t
5 percent of the rock
1.5 p e r c e n t n e a r
and c o n t i n u e s t o i n c r e a s e t o a b o u t
the t o p .
Most grainshave
embayed b o r d e r s
and h o l e s f i l l e d w i t h groundmass g l a s s .
From 1 t o 4 percent biotite occurs as yellowish-brown
pleochroic grains partially to completely altered to iron
oxide.
Minoramounts
suggested by pale
ofpyroxene
may have been present as
brown pseudomorphs w i t h e u h e d r a l o u t l i n e s .
The pseudomorphs a r e composed of c a l c i t e and have magnetite
borders.
The originalcleavagetraceshavebeenpreserved
i n some g r a i n s .
by magnetite replacement
The groundmassof
i s p a l e brownand
these
crystal-rich ash-flow
..
..
tuffs
is u s u 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
aggregatesoffeldspar
and c r i s t o b a l i t e .
n i f i c a t i o n ,s m a l lp a t c h e s
Under high mag-
of glass,brokenphenocryst.
fragments, and f i n e l y d i v i d e d h e m a t i t e c a n
be seen.
i
L i t h i c fragments are
common i n t h e t u f f
of Goat Spring,
.-
e s p e c i a l l y i n the upper and lower p a r t s o f t h e
theyconstitute
i
u n i t where
as much as 1 5 percent of the rock.
I
Most.
'
of the fragments
i n diameter.
are angular and rangefrom
2 mm t o 8 cm
The 'most common type' of l i t h i c i s reddish-
brow=, p o r p h y r i k i c a n d e s i t e w i t h a r g i l l i z e d p l a g i o c l a s e
pl1enocrysts and magnetite pseudomorphs a f t e r b i o t i t e i n
a n opaque,hematized
groundmass.Near
I
.
.
thebaseofthe
'1
i
I
-
-
25
u n i t , a f e w p o r p h y r i t i c" S p e a r s - t y p e "l a t i t ef r a g m e n t s
'are p r e s e n t
of 640 f e e t was measured
A stratigraphic thickness
northwest of Goat Spri.ng (Sec. 26, T . 1 S.,
R.. 4 W . )
.
The
modal v a r i a t i o n s ancl sample l o c a t i o n s are shown diagram,.
a t i c a l l y i n F i g u r e 7.
Phenocryst modes w e r e pointcounted
from t h i n s e c t i o n s b y ' D o u g l a s Cowan; modal d a t a f o r pumice
and l i t h i c s w e r e e s t i m a t e d v i s u a l l y i n
hand specimen by the. author.
t h e f i e l d and i n
The t u f f of Goat Spring i s
r e l a t i v e l y homogeneous throughout i t s thickness, both
mineralogy and degreeofwelding.
T h e t o t a l amount of c r y s t a l s
30 f e e t t o a n a v e r a g e . c f
increases abruptly in the basal
... ..
a more c r y s t a l - r i c h zone i s
about 47 p e r c e n t f o r t h e u n i t ;
evidentbetween
in
150 ,and 250 f e e t above the base.Quartz
i n c r e a s e s upward i n a somewhat e r r a. t i c
manner t o achieve a
.
maximum of 1 5 p e r c e n t i n the upper 3-30 f e e t of the u n i t ; it
i s scarce.toabsentin
the b a s a l 30 f e e t .
s a n i d i n e r a t i o averagesabout
The p l a g i o c l a s e /
0.5 and shows little v a r i a t i o n
vertically within the unit except near the base,
where pla-
g i o c l a s e and s a n i d i n e o c c u r i n n e a r l y e q u a l p r o p o r t i o n s ,
S a n i d i n e ,p l a g i o c l a s e ,
.
and m a f i c m i n e r a l s a r e a l l n e a r l y
.
c o n s t a n t above the b a s a l 30 f e e t .
The t u f f of Goat Spring displays
t y p i c a l of zoned ash-flow cooling
no o b v i o u s v a r i a t i o n s
u n i t s d e s c r i b e d 'in t h e
-._.
c
._
.
.
Modes i n volume percent from t h e ' t u f f of Goat S p r i n g
Table I.
Total
phenocrysts
Sample
Number
,
P r o p o r,tPi h
o en ns o c r y s t
Biotite
Quartz
PlagioSani-
'
.
dine
and '
opaques
clase
Total
points
counted
,
Top of u n i t
1 . "24-37
46.1
22.5:
7.3
2
M21.2'
2446.636
.
10.0
3
"24-34
11.1
22.9
50.9
4
"24-33
24.2 49.9
15.0 : 9.1
M- 24- 3 1
49.0
24.5
' 12.8
5
6
"24-30
28.2
49.8
7
"24-29
7.8
23.4
43.3
8
"24-28
48.7
24.8
7.8 12.5
11.9 27.1 47.9
9
"24-27
10
PI-24-26 27.2 55.7
17.6
11 "24-25
59.1 1385 27
4.9
- 4 10.9 15.9
1 213.7"24-24 29.2
55.3
4.7 28.4
10.4
13
"24-231292
49.6
1 4 4.8 "24-22
13.0 26.8 '49.0
47.1
1 513.9"24-21 23.6
10.0
36.7
20.4
1 6 5.7"24-20
19.5
15.0
40.3
17
"24-19
1 8 . "24-18
28.4
1205 ,14
3.2
*1
0.9 10.2
Base of u n i t
'
10.7
,'
'
~
5.1
7.3
5.8
3.8
1444
..
..._
.
.
"
"
. ~ .
. .
.
.
...,
.
.
. ..
.. . .
."
14.4 1. 3 8 9 1 . 9
13.6
1.8
15
.6
,1323
1.3
1.6
2.8
8.9
4.8
6.1
9.8
2.3
1339
3.6
5.1
3.8
5.6.
5.3
,
1384
1277
1389
1334
1318
1441
1303
1389
6.1
1318
4.4
0.6
0.4
5.4
'
- .
.
"
.
1378
..
27
-
e
’
.. .
Figure 3.
Total
.
Phenocrysts
. .
’
Mafics
Quartz
Plagioclase
Feldspar
Sanidine
Measured s e c t i o n of t h e tuff of Goat Spring showing
mineralogi.ca1 vciriatlons. Dashed l i n e n s a r t h e base
i n d i c a t e s t h e upper limit of quartz-poor l a t i t i c ash flows.
(modal d a t a a r e i n vo%um percent, taken from Table I).
-.
..
4
-
a
28
-
l i t e r a t u r e ' (Ratte! and Steven,1964;Smith
Lipman and others,1966;
and Bailey, 1966;
1967).
Giles,
sheet t o t a l c r y s t a l s , p l a g i o c l a s e ,
and maficminerals
i n c r e a s e upward i n t h e s e c t i o n .
all
The youngestash-flow
u s u a l l y c o n t a i n more c a l c i c p l a g i o c l a s e
The p l a g i o c l a s e d o e s
I n a t y p i c a l zoned
tuffs
and less q u a r t z .
become s l i g h t l y more a n o r t h i t i c upward
i n the t u f f of Goat Spring b u t o t h e r s y s t e m a t i c v a r i a t i o n s
appear t o b e a b s e n t .
Some o f t h e l a r g e r
and more c o n s i s t -
e n t b r e a k s i n the p r o f i l e s of Figure 7 may r e p r e s e n t s l i g h t
d i f f e r e n c e s i n modal composition between individual ash
*
flows (Ratte' and Steven, 1967, p. 3 0 - 3 3 ) .
,
The most notice-
a b l e b r e a k i s the abrupt
, . .. change i n modal composition near
'the b a s e o f t h e s e c t i o n .
s i t i o n a l zonebetween
This change may r e p r e s e n t a t r a n -
two different types of ash flows:
(1) q u a r t z - p o o r l a t i t i c a s h
flows a t t h e b a s e , p o s s i b l y
de-
r i v e d from t h e same magma chamber a s t h e c r y s t a l - r i c h a s h
flows i n the upperSpears'Formation,
rhyolitic to quartz latitic
and ( 2 ) q u a r t z - r i c h
ash flows from another magma.
I n Figure 8 the modal data have been plotted on
a
sanidine-plagioclase-quartz diagram which shows t h a t t h e
..
samples vaxy from l a t i t e t o q u a r t z l a t i t e t o r h y o l i t e
mineralogical composition with
tion parallel to
in
a marked l i n e a r d i s t r i b u -
the r h y o l i t e - q u a r t z l a t i t e
-
boundary. I n
g e n e r a l , the s.amples p l o t p r o .g-.r e s s i v e l y closer t o t h e
.
Quartz
Figure 8.
Xodal data f o r t h e t u f f o f Goat Spring p l o t t e d on a
t h r e e component diagram(diagramrrodified
from
.
Bateman and others, 1963, p. 13).
q u a r t zc o r n e r
'
30
-
m
of t h e diagram as t h e samplesapproach
the t o p o f t h e ' s e c t i o n .
T h i s t r e n ds u g g e s t s
magma was u n d e r g o i n g d i f f e r e n t i a t i o n a l o n g
l i z a t i o n p a t h whichformed
a c o n s t a n tp r o p o r t i o n
that t h e
a crystal-
more q u a r t z b u t e s s e n t i a l l y
of f e l d s p a r sw i t h
time.
Another
e x p l a n a t i o n €or t h e t r e n d i s t h a t t h e magma became
,increasingly' contaminated with quartz
proceeded.
as t h e e r u p t i o n s
The anomalously l a r g eq u a r t zc r y s t a l s
may
a c t u a l l y be xenocrysts accidentally incorporated
d u r i n ge r u p t i o n of t h e t u f f s .
Kowever, the l a r g e
volume of q u a r t z required and i t s s y s t e m a t i c i n c r e a s e
a
a r g daeg a i n s t
t h i s 0rigi.n.
3 3 addition to the
changes i n primary mineralogy
there a r e some s u b t l ~ echanges i n t h e t e x t u r e
t e r a t i o n w i t h i n the t u f f of Goat Spring.
more oxidized toward
and a l -
-
B i o t i t e is
t h e t o p of the c o o l i n g u n i t
where
d e u t e r i c a l t e r a t i o n was l o c a l i z e d b y t h e upward escape
of v o l a t i l e sd u r i n gw e l d i n g .S a n i d i n e
i s more a l t e r e d
and secondary minerals more abundant i n t h e less dense-
l y weldedzones,
which were more porous andunderwent
more i n t e n s e vapor-phase c r y s t a l l i z a t i o n .
Although
r e s o r p t i o n occurs,i n a l l p a r t s of t h e c o o l i n g u n i t ,
0
it
i s best developed i n t h e upper, less densely welded
part.
...
,
.~~
-
~
~~
-
e
Tuff of Bear
Sprincrs.
a
31,-
The t u foff
Bear
Springs
is
t h e informal name proposed f o r the sequen2.e of ash flows,
lava flows.
upperpaxt
and v o l c a n i c l a s t i c s e d i m e n t a r y r o c k s i n
of t h e Hells Mesa Formation.
it has been subdivided into
the
In t h i s r e p o r t ,
two mappable u n i t s s e p a r a t e d
by a c o o l i n g b r e a k and interbedded andesite
flows.
Lower Cooling Unit
The ' l o w e r t u f f of Bear springs
i s a multiple-flow,
compound c o o l i n g u n i t of welded r h y o l i t i c ash-fiow t u f f s
a n a r e a of about f i v es q u a r e
whichoutcropover
..
e
the t h e s i s a r e a .
miles i n
The base o f t h e u n i t i s exposed along
... ..
an approximate north-south line extending
end of t h e La Jencia monzonite stock
from t h e north
t o t h e e a s t side of
~...
Bear Peak.
To t h es o u t h ,
it includesLoughlin
mann's Banded Rhyolite and t o t h e n o r t h
w i t h Tonking's middle
and Kosch-
it c o r r e l a t e s
65 f e e t of t h e Hells Mesa.
The lower tuff of Bear Springs includes several ash
f l o w s , and v a r i e s c o n s i d e r a b l y
istics.
i n i t s outcrop character-
Occasionally it forms rounded h i l l s with a series
of a l t e r n a t i n g l e d g e s and s l o p e s b u t it t y p i c a l l y o u t c r o p s
as weathered, platy slabs
e
which form the d i p slopes of
hogback ridges.
I n handspecimen,
the color of the
..
'L.
,_
.
".
groundmass v a r i e s
w i t ht h ed e g r e e
of weldingfrom
g r a y or reddish-brovm.
stainedbuffto
32
-
Li
l i g h tg r a y
t o pinkish-
Weathered s u r f a c e s a r e u s u a l l y
reddish-brown.
The ash flows are f i n e
1 0 t o 1 5 percent phenocrysts
g r a i n e d and contain only
of s a n i d i n e ,c o p p e r yb i o t i t e ,
The
and smoky quartz.
s a n i d i n e i s euhedral and v a r i e s from c l e a r , w i t h a s i l k y -
white chatoyancy, t o pink and chalky where a l t e r e d .
Samplesfrom
the u p p e r p a r t a r e c h a r a c t e r i z e d b y a s
as 25 percent gray
s i l i c i f i e d pumicewhich
shows vai-ying
Two t y p e s of l i t h i c fragments
degreesofcompaction.
a r e common; one consists of dark
0
much
brown, a p h a n i t i c ande-
s i t e and t h e other i s a p o r p h y r i t i cb i o t i t e - b e a r i n g
rhyolite
_"
...
..
~
P e t r o g r a p h i c a l l y , the ash f l o w s a r e v e r y s i m i l a r
and c o n s i s t o f - b r o k e n , p a r t l y r e s o r b e d p h e n o c r y s t s o f
s a n i d i n e ,q u a r t z ,b i o t i t e ,
and p l a g i o c l a s e i n a n
a p h a n i t i c ,p a r t l yd e v i t r i f i e d
groundmass.
The welded
portions have a eutaxitic structure defined by
and d i s t o r t e d g l a s s s h a r d s
and pumice.
compressed
The groundmass
.
I
i s p a r t l y d e v i t r i f i e d t o fine-grained
(0.01 nun t o 0.1 mm)
cristobalite and f e l d s p a r sometimes i n s p h e r u l i t e s o r
axiolites.
a
I n the less welded and less a l t e r e d samples
the groundmass c o n s i s t s of brown, cuspate and Y-shaped
glassshardsseparatedbyextremelyfinedust.Magnetite
-..- _ _ _
,'
..
..
-
33
-
grains are finely dispersed throughout
t h e groundmass
i n most samples.
Sanidine i s t h e o n l y major c o n s t i t u e n t and v a r i e s
from s i x t o e l e v e n p e r c e n t
by volume.
It o c c u r s a s
unusually well-formed partially resorbed grains
0.2 t o 2.0 mm i n length.Carlsbadtwins
and s t r a i n e d ,
broken crystals w i t h undulatory extinction are
The s a n i d i n e shows varying degrees of
from
common.
unmixing t o per-
t h i t e which is o f t e n i n t e r g r o w n w i t h c l e a r s a n i d i n e i n
irregular .patches controlled by the ‘sanidine cleavages.
The o r i g i n a ls a n i d i n ea p p e a r s
t o havesufferedan
un-
u s u a l d e g r e e o f - p e r t ..h. i t j c e x s o l u t i o n d u r i n g d u e t e r i c
a l t e r a t i o n of the cooling ash flows;
.
~
the p e r t h i t i c .
g r a i n s were counted as s a n i d i n e i n the modes.
Quartz i s a minor c o n s t i t u e n t , and occursassmall,
anhedral to subhedral, resorbed grains usually
less t h a n
1 . 0 mm indiameter.Occasional
shreds of b i o t i t e ,
partialiy replaced by hematite
and o t h e r i r o n o x i d e s ,
.-
make up t h e remainderofthephenocrysts.Usually,only
a f e w small p l a g i o c l a s e g r a i n s c a n b e f o u n d i n
a thin
section.
A s t r a t i g r a p h i c khickness of 507 f e e t was measured
approximatelyone
R. 4 W.,
Sec, 2 2 ) .
mile southeast of Bear Springs
(T. 1 S.,
Figure 9 i s a ‘diagrammatic Column
>.
-._
.
.
Table If. Modes i n volume percent from t h e lower t u f f o f
Sample
Number
clase
dine
Top
- of unit
1. "24-69
2 M-24-68*
3 "24-66
4 "24-65
5 "24-63
6 M-24-62
7 "24-61
8 "24-60
M-24-59
9
i
L O "24-58
;
11 "24-57
12 "24-56
13 "24-55.
14 "24-54
15 "24-53
5.9
16 "24-52
17 "24-51
18 M- 24-50
5.9
0.319 "24-48
5.0
20 "24-47
21 "24-46
.
22 "24-45
23 "24-43
Base o u
f nit
.
Total
phenocrysts
.
2.0
1.9
1.1
5.0
*
Proportions
Phenocryst
PlagioSaniQuartz
.2.4.
3.6
3.8
3 -6
4.0
5.3
3.3
7.0
5.2
4.4
7.1
6.6
5.0
6.3
6.5
6.1
7.0
5.8
5.6
0.3
6.8
7.8
B e a r Springs
'
,
,'
2.3
3.2
3.0.
3.1
4.5
2.7.
6.2
4.8
4.0
6.1
6.3
4.5
\
0.0
0.L
0.1
0.1
0.0:
0.0,
0.0
0.0
tr.
0.0
0.0
0.0
0.0
0.0
0.0
5.8
tr.
5.0
0.0
0.1
4.5
5.3
0.0
6.8
0.0
4.3
tr.
Badly a l t e r e dw
, ith
0.1.
0.3
0.2
0.2
0.2
0.1
0.1
0.1.
0.4
0.1
0.2
0.6
Maf ics
and
opaques
Pumice
Total
points
counted
0.3
0.4
11.4
14.0
1.0
8.6
7.1
27.1
17.8
8.8
16.0
13.3
15.5
6.4
1.2
5.8
0.2
0.0
.2.1
0.0
0.1.
1793
1792
1990
2027
1917
2076
2123
1796
2395
2151
1783
1923
2120
2139
1751
2097
2400
1958
2131
2038
2223
1436
2329
0.3
0.4
0.8
0.7
0.5
0.4
0.3
0.2
0.4
0.2
0.2
0.1
0.1
0.1
0.I
0.3
0.3
0.6
1.0
0.7
0.1
0.5
0.8
0.6
0.5
.
0.3
I
0.4
0.6
0.4
0.4
0.2
0.2
2.4
0.0
0.0
0.5
'
I
W
A
I
numerous phenocryst
holes
. .
-
..
1
35
-
i
20
0
&nice
Of5
Quartz
Feldspar
3
.
-
36
-
-
-
37
showing t h e i n d i v i d u a l s u b u n i t s w i t h i n t h e s e c t i o n , e a c h
one l a b e l l e d a c c o r d i n g t o
its color or characteristic lithol-
a thiclr densely
ogy. The presence of
i n the upper
i s a compound cooling u n i t .
part indicates that the section
Modai d a t a a n d , s a m p l e l o c a t i o n s a r e
distinct break in the
weldedzone
shown t o t h e s i d e . A
modal p r o f i l e s i s evident ne.ar t h e
middle of t h e c o o l i n g u n i t ; t h i s b r e a k c o r r e s p o n d s t o t h e
contact between massive, featureless tuffs
andpumiceous
i n t h e lower half
tuffs with well-developed compaction textures
i n the upper half.
The pumiceous t u f f s c o n t a i n s l i g h t l y
fewer totalcrystals,considerably
more pumice, and s l i g h t -
l y less quartz r e l a t i v e t o s a n i d i n e t h a n t h e i r
more massive
counterparts' i n the. lower half.
I'
.
The graymassiveunit
(G)
i s approximately 300 f e e t t h i c k .
The basal.unwelded zone i s pink i n c o l o r and c o n t a i n s numer-
ous brown a n d e s i t e l i t h i c s and yellowish-white pumice l e n t i l s .
The i n t e r i o r of the ashflow
is l i g h t p u r p l i s h - g r a y ,
moder-
.-
ately to poorly
welded, and very fine-grained.
generally absent except
white, poorly-welded top
Pumice is
i n t h e upper and lowerfew
f e e t . The
of t h e u n i t becomes i n c r e a s i n g l y
reddened and welded upward t h r o u g h . t h e c o n t a c t w i t h t h e o v e r l y i n g pumiceous u n i t .
The pumiceous ashflows
(P) a r e a b o u t f o r t y f e e t t h i c k
and a r e welded t o t h e g r a y massive u n i t , w i t h
-
.
"
A,
a gradational
- 38 -
.e
c o n t a c t over a v e r t i c a lt h i c k n e s s
colorvaries
frompurplish-gray
i s derived froma
of about. t e n f e e t .
t o pinkish-gray.
Its
The name
striking eutaxitic structure defined by
numerous f l a t t e n e d , e l o n g a t e d
pumice, r o t a t e d l i t h i c f r a g -
,.merits, and subparallelimbricatedsanidinephenocrysts.
.
P e t r o g r a p h i c a l l y , the groundmass i s densely welded and t h e
mineralogy i s almost identical with the underlying massive
Pumice ranges i n s i z e from small,almostindis-
ashflow.
tinguishable "ghosts" in the
fraginents 3 cm i n length..
groundmass t o l a r g e e l l i p s o i d a l
The pumice rims havebeendevit-
r i f i e d t o quartz and fine-grained spherulitic aggregates
0
and
the centers o f t e n , c o n t a i n coxscon0~aggregates 04 inward-
pointing euhedral alkali feldspar crystals surrounded
l a r g e ra n h e d r a lG ' a r t eg r a i n s .
.
by
T h e pumice commonly has
.
frayed ends
and contains phenocrysts of quartz and s a n i d i n e .
The c o n t o r t e d u n i t
(C) c o n t a i n s
and/or laminar flow structure
a streakycompaction
which resembles flow banding;
this l e d Laughlin and Koschmann t o g i v e t h e
name "Banded
R h y o l i t e " t o the e q u i v a l e n t of t h i s u n i t i n the Kellymining
district.
The u n i t i s about 1 2 0 feet . t h i c k ,and has a sharp
butgradationalconiiact
*
w i t h t h e pumiceous ashflow.
It h a s
a dark reddish-broh
color on fresh s u r f a c e s and i s c h a r a c t e r -
i z e d by g r a y , h i g h l y
compressedpumicewhich
averages only
2 mm. t h i c k and ranges up t o s e v e r a l i n c h e s l o n g .
highly compressed bands have
ratio of
a maximum length-to-thickness
300:l which i n d i c a t e s t h a t t h e
stretched.
The most
,Some of t h e l a r g e r s t r e a k s
pumice tias r a d i a l l y
may r e p r e s e n t t h e
accumulation of volatiles along shear planes during laminar
flowage and weldingoftheash
contorted ash
flows.
Outcrops o f . t h e
flows usually have a p l a t y f r a c t u r e p a r a l l e l
t o t h e f o l i a t i o n , and the banding i s o f t e n c o n t o r t e d i n t o
t i g h c asynunetric folds.
Microscopically, the ash flows are densely
weldedand
composed’mainly o f h i g h l y f l a t t e n e d pumice i n a glassy,
p a r t l yd e v i t r i f i e d
groundmass.
Pumice i s oftenqolded
around the broke’n and r o t a t e d c r y s t a l f r a g m e n t s .
Above the c o n t o r t e d u n i t
i s aninconspicuouspoorly-
welded a s h flow (PW), about 35 f e e tt h i c k .
It i s s o f t , p i n k i s h -
gray, and c o n t a i n s abundant p a r t i a l l y , c o l l a p s e d c e l l u l a r
pumice stained greenish-black
Thistuff,
which hasonlyabout
by a manganese-bearing mineral.
two p e r c e n t c r y s t a l s ,
r e p r e s e n t the l a s t weak p u l s e of ash
.
may
t o be erupted.
J u s t south of La Jencia Creek t h e lower t u f f ofBear
Springs contains a thin crystal-rich tuff
present i n the.vicinityofthe
which i s not
measured s e c t i o n . T h i s
has been informally designated the tuff of
unit
La J e n c i a Creek
(Chapin, o r a l communication, 1971) ; it i s v e r y s i m i l a r t o t h e
“-__
..
- 41 '
tuffof
h
i t i s welded withinthegraymassive
Goat Springbut
unit.Typicalspecimensaredensely
welded and s l i g h t l y
pumiceous w i t h a l i g h tl a v e n d e r - g r a y
color.
Phenocrysts of
smoky quartz,euhedralsanidine,plagioclase,
and coppery
b i o t i t e make up 30 t o 40 percent of the rock by
t u f f of Goat Spring contains
volume.
The
1 0 t o 20 p e r c e n t more c r y s t a l s
ana i t s m a t r i x i s crowded with s m a l l c r y s t a l f r a g m e n t s .
Judging'from t h e i r low content of phenocrysts and h i g h l y
glassy matrix,
the
ash flows in
probably quenchedfrom
a very hot eruption
liquidus temperature of
0
the tuff of
which was n e a r t h e
t h e o r i g i n a l melt, perhaps as
,900"to 95OoC. based on..the
c a l c u l a t i o n so f
..
This high temperature, together
were
Bear Springs
much a s
Boyd (1961, p. 414).
w i t h their high volatile
c o n t e n t , m a d e ' t h e upper pumiceous ash flows p a r t i c u l a r l y
fluidal.
A s the ash
f l o w s began t o l o s e momentum, exsolved
g a s e s may h a v e a c t e d a s f l u i d i z i n g a g e n t s c a u s i n g
them t o
of
develop laminar flow structures during the final stage
t h e i r emplacement(Noble,
Pumice became h i g h l y
1968).
s t r e t c h e d j.n the d i r e c t i o n o f f l o w a g e
and wave-like flow
f o l d s and some ramp t h r u s t s developed a t r i g h t a n g l e s . t o
pumice l i n e a t i o n . I n t h e
tion trends about
'
the pumice l i n e a -
S 10°W and the a x i a l p l a n e s o f
and t h e ramp t h r u s t sd i p
.
BearMountains,
t o t h es o u t h .
a norkhiqard t r a n s p o r td i r e c t i o n
f l o w folds
These d a t ai n d i c a t e
f o r t h e ashflows,
.
the
which i s
i
-
0
42
-
the recentdiscovery
con.sistentwith
(1971, oral.communication)of
Withingtonarea.
by Deal and Rhodes
a majorcauldron
A g r e a tt h i c k n e s so f
similar stratigraphic position
i n the Mount
ash-flow tuffs of
and l i t h o l o g y were deposited
(2.
G.).
within the cauldron
Upper Cooling U n i t
The u p p e r t u f f
of Bear Springs
ash-flow tuff sequence
i s a welded r h y o l i t e
which c r o p s o u t over about two square
rniles'in the t h e s i s a r e a .
Most oftheexposures
occur north.a
w e s t of Bear Springs and along a ridge extending two miles
. .
southfrom Bear Springs. The upper and lower . u n i t s o f t h e
0
t u f f o f Bear Springs are s e p a r a t e d b y f i v e t o s e v e n t y f e e t
.
.
of andesite flovjs.
__ ..
Except €or t h e p r e s e n c e of t h i s c o o l i n g
break, it would be d i f f i c u l t t o d i s t i n g u i s h t h e
they are very
two s i n c e
similar l i t h o l o g i c a l l y .
The upper u n i t crops out 'as c l i f f s and roundedpinna...
. d e s which weather t o reddish-brown p l a t y s u r f a c e s , o f t e n
h i g h l y , f r a c t u r e d and hackly.
The base i s characterizedby
a zone with egg-shaped s p h e r u l i t e s up t o s e v e r a l i n c h e s l o n g .
.Large slump blocks a r e common o v e r t h e u n d e r l y i n g , r e l a t i v e l y
"
'
. .
s o f t andesites,.
Fresh hand specimens are pinkish-gray
e
t o reddish-brown.
The groundmass i s a p h a n i t i c and h a s a
".-
1
-
._
.._.
.
and weather gray
!
-
#
compaction f o l i a t i o no u t l i n e d
pumice.
Pink,silicified
43"
bysmall,white,compressed
t o 5 c m.
and contorted pumiceup
l o n g form up t o three percent of some specimens.Phenocrysts
are n o t i c e a b l y less abundant than
clude white euhedral sanidine
in t h e lower u n i t and in-
and occasional smoky q u a r t z .
Small a n g u l a r l i t h i c f r a g m e n t s c o m p r i s e l e s s t h a n
one per-
c e n t of most. sainples and a r e s i m i l a r t o the l i t h i c s i n the
lower t u f f of Bear s p r i n g s .
Microscopically, the upper unit contains
from 0 . 1 t o
about 5 percent subhedral, broken sanidine phenocrysts
e
nun.
from 0.2 mm. t o 1.5
whichrange
s a n i d i n e i s only i n c i p i e n t l ya l t e r e d
... .-
i nl e n g t h .U s u a l l y ,t h e
along cracks and cleav-
ages but some g r a i n s a r e c o m p l e t e l y a l b i t i z e d o r a l t e r e d t o
'Anhedral,brokenquartzgrains
clay.
.
and opaque b i o t i t e
pseudomorphs amount t o less t h a n one percent of
"
...,
t h e samples.
..
P l a g i o c l a s e .was not found i n any of t h e t h i n s e c t i o n s .
The
a p h a n i t i c groundmass i s ' l a r g e l y d e v i t r i f i e d and c o n s i s t s of
,
a m i x t u r eo fg l a s s
and s m a l l c r y s t a l l i t e s o f q u a r t z
and
a l k a l i feldspar.
A stratigraphic thickness of
the u p p e r t u f f
of Bear Springs approximately
e a s t 'of BearSprings
e
(T. l
s.,
R. 4 W.,
two miles south-
Sec. 2 2 and 2 7 ) .
shown diagrammatically i n F i g u r e 1 2 , t h e u n i t
".
,
280 f e e t was measured f o r
As
seems t o c o n s i s t
'..
a
.-
'
Table 111. Modes i n volume percent from the u p p e r t u f f
Sample
Nuniber
Total
phenod i n ec r y s t s
Phenocryst Proportions
PlagioQuartz
Mafics
clase
and
S ani-
/
Top of. u n i t
1 "24-88"
2 M-24-87*3.7
3 X-24-86"
3.7
4 M-24"4*
5
M-24-83*
6' "24-82
7
"24-81"
8 "24-806
9 ly-24-79*
4.3
10 "24-78
11 N-24-77
12 "24-76
13 "24-75
14 N-24-73
15 "24-72
Base of u n i t
*
7.0
5.1
5.4
4.3
3.9
3.0
2.6
2.7
5.2
0.2
0.8
0.1
0.7
2.4
0.5
,
5.5
0.0
0.6
0.8
.>
.
'0.7
I
.
.
I
:.
.
2.9
3.0
2.1
2.0'
1.8
0.3
'
0.1
0.4
0.1
0.3
1.4
0.2
Excluding crystal-cich clots
0.5
0.8
0.6
0.6
.0.3 .
0.4
0.6
0.0
0.2
0.0
0.1
0.8
0.2
Pumice
Total
points
counted
12.7
12.1
6.6
18.3
11.6
16.1
13.9
19.0
4.3
9.0
6.1
13.0
5.9
1862
2015
1718
2452
2147
2500.
1920
2480
2170
1806
2062
1971
2225
2500
1922
opaques
'
'
ofBearSprings
0.9
0.6
'1.0
0.6
0.3
0.3
0.3
0.1
0.2
tZ.
0.3
0.2.
0.1
7.8
7.0
I
&
P
I
-
*~
of two ashflows
-
46
welded t o g e t h e r .
The t w o ashflowshave
and i n modal
subtle differences in outcrop characteristics
p e r c e n t a g e s which a r e p l o t.t e d. t o t h e s i d e .
The lower t u f f
'
is"more fnassive'and contains fewer crystal fragments than
t h e upper t u f f , which i s p l a t y and c o n t a i n s d i s t i n c t l y more
pumice ( a s much a s 20 p e r c e n t ) and more c r y s t a l s .
. F o r the c o o l i n g u n i t a s
content increases steadily
awhole,
the t o t a l c r y s t a l
from less t h a n o n e p e r c e n t a t
b a s e t o aboutsevenpercentnearthetop.
spar ratio increases sharply
The quartz/feld-
i n the basal thirty feet
t h e nr e m a i n sn e a r l yc o n s t a n tt o
the top.Figure
t h a t the t w o .c o .o l i n g u n.i. t..s i n t h e t u f f
q u i t e similar p e t r o g r a p h i c a l l y .
and
1 0 shows,
ofSearSpricgsare
T h e most n o t i c e a b l ed i f f e r -
ence i s a s l i g h t l F h i g h e r q u a r t z / f e l d s . p a r r a t i o f o r
upper cooling
the
the
unit.
.. .
AndesitesInterbedded
i n t h e H e l l s Mesa.
d i f f e r e n t t y p e s of andesite flows are
the Hells Mesa ashflows:
andesites containing
found interbedded w i t h
(1) p o r p h y r i t i c" t u r k e y - t r a c k "
abundant l a r g e p l a g i o c l a s e p h e n o c r y s t s ,
s i m i l a r . t o some o f t h e a n d e s i t e s i n
Spears Formation,
Two d i s t i n c t l y
the upper member of &he
and (2) f i n e - g r a i n e d , r e l a t i v e l y
porphyritic andesites lacking plagioclase phenocrysts.
non-
-
e.
The f i r s t type forms
-
47
a d i s t i n c t cooling break between
t h e t u f f of Goat Spring and t h e t u f f
main outcrops form
ofBearSprings.
The
a narrow band extending from h i l l "7090",
w h e r e i t s thickness was measured as 90 f e e t , n o r t h t o Bear
Peakwhere
it appears t o pinchout.
not been observed
These andesiteshave
in the western half
Megascopically, the andesite
of t h e a r e a .
i s p o r p h y r i t i c and b l u i s h -
gray on fresh s u r f a c e s with abundant flow-oriented plagioc l a s ep h e n o c r y s t s
'
and reddish-brownpyroxene
pseudomorphs.
Locally, it c o n t a i n s numerous amygdules f i l l e d w i t h c a l c i t e
.
.
and q u a r t z .
Microscopically,thetexture
,
is h o l o c r y s t a l l i n e and
... ..
porphyritic-aphanitic.Plagioclasephenocrysts
about 20 percent of
the rock and o c c u r a s e u h e d r a l l a t h s
averaging 2.0 mm i n l e n g t h . A l t e r a t i o n
,
make up
is r e s t r i c t e d t o
..
mino;
c l a y s and epidote.
An averagecompositionoffour
plagioclase' phenocrysts determined
by t h e Fouque' method i s
Ans5.
Three g r a i n se x h i b i t i n g
measuredby
the Fouqukmethod;
frQm.An53 a t t h e c e n t e r t o
An
..
percent reddish-brown pyroxene
.hedral grains'up to
e
,
. been replaced
bya
normalzoning
were a l s o
on the average, they range
49
a t the r i m .
About t e n
pseudomorphs occur as
1 . 5 mm i n diameter.
fine-grained aggregate
sub-
The pyroxenehas
of c a l c i t e ,
e p i d o t e , and s i l i c a i n t h e center and by hematite around
.'. .
"
~
':
. .
-
a
the edgesof
the g r a i n s .
-
48
The groundmass i s made o ff e l t y
0.2 nun long and small magnetite
plagioclase microlites about
T h e approximate
g r a i n s which may representalteredpyroxene.
composition of groundmass plagioclase, determined
teen grains by the
Michel-Levymethod,
from f i f -
i s An57.
The second variety of andesite occupies
a cooling break
t u f f of
between t h e upper and l o w e r c o o l i n g u n i t s o f t h e
Bear S p r i n g s ; t h e y
are p r e s e n t a t
this s t r a t i g r a p h i c p o s i t i o n
on Hells Mesa b u t are not shown on Tonking's measured sect u f f ofBear
tion.Andesiteflowsalsooccurbetweenthe
'
' '
Springs and t h e t u f f of A l l e n Well.
Outcropsusually
s l o p e s o r swales between hog-
weather as dark talus-covered
...
,
backs of welded tuff. Lineated
chal.cedony,and
amygdules f i l l e d w i t h q u a r t z ,
calcite are
common and are sometimes s t a i n e d
by greenish-bl.ueceladonite.Thinvolcaniclasticsandstone
layersarelocallyintercalated
c o l o r of hand specimens varies
._
with t h e f l o w s .
from gray
The fresh
t o reddish-brown
while weathered surfaces are usually greenish-brown.
Usually, the flows are
dense and aphanitic with only
a few
phenocrysts of oxidized pyroxene b u t some flopis are vesicu l a ro ra u t o b r e c c i a t e d .
The t h i c k n e s s i s quite v a r i a b l e ,
possibly because. of some minor erosional topography developed
i n the underlyingashflows.Totalthicknessvariesfrom
less t h a n t e n
f e e t t o more t h a n s e v e n t y f e e t .
.
~.
..-.
~
- 49 ten
Microscopically,theseandesitescontainabout
percent reddish-brown
opaque phenocrysts which appear to
be hematizedpyroxene.
The groundmass c o n t a i n sa l t e r e d ,
trachytic plagioclase microlites
g r a i n so fi r o no x i d e s
and i n t e r s t i t i a l opaque
and hematizedpyroxene.
The plagio-
c l a s e i s commonly a r g i l l i z e d and t h e groundmass i s o f t e n
. .
replaced by aggregates of calcite
T u f f of Allen Well.
the s t u d y a r e a
and quartz.
The youngestashflowpresentin
. .
was observed i n three small outcrops along
Dry Lake Canyon
i n t<e s o u t h w e s t c o r n e r o f , t h e s t u d y a r e a .
locatedinsection
A l l e n Well.
.
t u f f which
is a q u a r t z l a t i t i c c r y s t a l - r i c h
3 6 , T.':.l.
S.,
..
R.
The o u t c r o p s a r e
5 W.,
downstreamfrom
the
The t u f f of Allen Well i s separated from t h e
u p p e r c o o l i n g u n i t ' ..o.f t h e
. h f f of Bear S p r i n g s ' b y a series
of andesite flows and. thin sandstones.
.. .. -_
n
I 'hand specimen the
..
t u f f of A l l e n W e l l i s pinkish-.,
gray and c o n t a i n s a s much a s 30 percent phenocrysts of
s a n i d i n e ,p l a g i o c l a s e ,c o p p e r yb i o t i t e ,
v i t r o c l a s t i cm a t r i x .
Goat Spring, but
It stronglyresembles
upon close inspection can
by t h e more abundant matrix,
Well i s a l s o s i m i l a r
.
.
,
the t u f f o f
be d i s t i n g u i s h e d
which is not crowded with small
c r y s t a l . f r a g m e n t s 'as i n t h e t u f f o f
of Allen
and q u a r t z i n a
Goat Spring.
The t u f f
t o t h e t u f f of La J e n c i a
Creek but
50
-
the former' contains fewer t o t a l c r y s t a l s and
p r o p o r t i o n a t e l y less s a n i d i n e .
Petrographically, the
t u f f of Allen Well c o n s i s t s of
phenocrystsofsanidine,plagioclase,quartz,
and b i o t i t e
lithics in a
combined w i t h f l a t t e n e d pumice and afew
reddish-brown d e v i t r i f i e d groundmass.
A modal counton
one t h i n s e c t i o n y i e l d e d 1 4 p e r c e n t s a n i d i n e ,
p l a g i o c l a s e , 4 percentquartz,
7 percent
4 percentbiotite,
1 3 per-
c e n t pumice, and 58 p e r c e n t groundmass.
Trace amounts of
a p a t i t e and pyroxene are also p r e s e n t .
The c r y s t a l s r a n g e
i n s i z e from 0.1 mn t o 3.0 mm, w i t h an average size of
0.5
nun.
..
La J a r a Peak Formation
The La J a r a PeaJc Formation forms
.
the higher elevations
.
i n the BearMountains and crops out over about
miles i n the n o r t h w e s t p a r t
was d e s c r i b e d i n d e t a i l
whonamed
of t h e s t u d y a r e a .
s i x square
The u n i t
by Tonking (1957, p . 44-46; p. 57)
it a f t e r La Jarti Peak,a
prominent volcanic neck
i n the Puertecito Quadrangle,. and designated it a s t h e
upper member of the D a t i l Formation.
The Lower p a r t of t h e La J a r a Peak Formation
series of r e l a t i v e l y s o f t a u t o b r e c c i a t e d f l o w s
volcanicl;7stic sandstones
i s a
and t h i n
and conglomerates whichform
l o w hummocky topography between Bear Springs
the
and t h e e a s t e r n
- 51 escarpment of t h e BearMountains.
well-stratified,
is a
The upperpart
r e s i s t a n t sequenceof
thin basaltic-
a n d e s i t e flows which f o r k t h e main escarpment of
the
The completethickness
of t h e La J a r a
Peak can only be approximated because of
a majox f a u l t
BearMountains.
which has dropped the Bear Mountains over
1 0 0 0 Eeet and
has p l a c e d La J a r a Peak a n d e s i t e s i n j u x t a p o s i t i o n a c r o s s
the f a u l t .
A minimum thicknessofabout
1600 f e e t h a s
been estimated .from s t r u c t u r e c r o s s - s e c t i o n A
-
A'
( P l a t e I ) , . b u t the maximum t h i c k n e s s may b e as much as
*
2900 Eeet.
I n the PuertecitoQuadrangle,
p. 31) e s t i m a t e d the maximum t h i c k n e s s a s
Tonking (1957,
2500 f e e t and
,
measured 1 2 0 5 f e e t a t t h e t y p e s e c t i o n .
Di.ps w e s t o f .t h e H e l l s Mesa Fault range
to f i f t e e n d e g r e e s
to t h ew e s t .
from seven
The basal.part,exposed
the f a u l t , is relatively
o-n ' t h e upthrown e a s t s i d e o f
f l a t - l y i n g and rests ,unconformably on t h e t u f f of Bear
._
Springs.
The a r e a w e s t and southwest of BearSprings
.
.
a p p e ab
ptro
e
aa
osoren
lftd
r o s i o nsaul r f a o
ce
n
Hells Mesa ash flows
the
which was l a t e r f l o o d e d b y
Peak a n d e s i t e s . I n t h e v i c i n i t y
Springs, the contact
a
and Bear
between t h e a n d e s i t e s and the welded
t u f f i i s p a r t l y d e p o s i t i o n a l and p a r t l y a f a u l t c o n t a c t .
.
..
of DeerSpring
La J a r a
."
!
Figure.13. Typical outcrop of La Jara Peak Andesite.
The massive core of
a basaltic andesite flow a t the top
of t h e p i c t u r e g r a d e s a b r u p t l y
downward i n t o an autob r e c c i a t e db a s e .
A t h i n l e d g e of fine-grainedsandstone
.,issandwiched between t h e base of t h i s f l o w u n i t and t h e
upper rubble zone of another f l o w unit.
Talus,
s o i l , and slump b l o c k s p a r t i a l l y o b s c u r e t h e
a distinctive speckled appearance
small hematized pyroxene grains
amygdules f i l l e d w i t h c a l c i t e
have autobrecciated tops
o x i d i z e dt o
from the presence
of
and c o n t a i n s almond-shaped
and q u a r t z .
The lowerflows
and bottoms which a r e o f t e n
a reddish-brown c o l o r .S h e e t i n gp a r a l l e l
to
t h e f o l i a t i o n is common i n the vesicularflows.Lnter-
conglomerates which c o n t a i n reworkedfragments
often c o n t a i n fresh pyroxene along with
pseudomorphs.
of the
the hematite.
Red g l a s s y flows and bombs a r e p r e s e n t
plagioclase phenocrysts.
Many of the massive flows have elongated, lineated
v e s i c l e s which have been plotted on 'the
N 50°E i n t h e western and n o r t h e r n p a r t s
map as i n d i c a t o r s
of t h e outcrop
I n t h e lower p a r t of the formation there a r e a
number of north-northeast-trending
dikes
which are
iden-
t i c a l to t h e La J a r a Peak flows.
These dikescould
occupy l o c a l feeder v e n t s f o r t h e
flows.
The b a s a l t i c -
andcsite flows may have been formed i n c o a l e s c i n g erupt i o n s from w i d e l y s c a t t e r e d f i s s u r e s
w i t h i n the s t u d y a r e a , b u t l a r g e l y
which l a y p a r t l y
t o the west
and south-
west.
In thin section, the andesites
have an aphanitic
groundmass composed of t r a c h y t i c t o f e l t y p l a g i o c l a s e
m i c r o l i t e s w i t h an average length of
I n a f e w samples the p l a g i o c l a s e l a t h s r a n g e
'
'@
long and d i s p l a yc a r l s b a d:,a l b i t e ,
nun.
less t h a n 0 . 0 5
up t o 0.5 nun
and c a r l s b a d - a l b i t e
twinning; the range in ct?rposition of these grains
was
measured as An40 t o Ans5 w i t h a i a v e r a g e c o m p o s i t i o n o f
Ani;
'
( average of f i f t e e n d e t e r m i n a t i o n s by t h e Michel-
Lkvy method).
The groundmass
a l s oc o n t a i n s
.and hematite.
pyroxene,magnetite,
Phenocrysts include
.
microlites of
1 0 fio 20 percentclinopyroxene
andoccasionalxenocrys%sofplagioclase
Clinopyroxeneoccurs
and q u a r t z .
as colorlessto,palegreen,anhedral
to e u h e d r a l g r a i n s from 0 . 2 t o 1 . 5 nun i n length.Basal
sections often display polysynthctic twinning
.
'
e
I
I
. --
..,
g r a i n s a r e zoned.
and a f e w
As much a s h a l f o f the pyroxene is
._
...
a l t e r e d t o hema.tite.
e
55
-
Tonking (1957, p. 4.6) measured the
. .
r e f r a c t i v e i n d i c e s of t h e pyroxene and i d e n t i f i e d it as
i n t e r m e d i a t e between diopside and hedenbergite.
Chemical analyses by Tonking(1957,
(1971-a,
p. 44) i n d i c a t e t h a t
p. 50) and Chapin
the La J a r a Peak flows are
quite rich i n p o t a s s i u m which s u g g e s t s t h a t t h e y
a l k a l i c i n composition.Experimentalstudies
and Ringwood (1967,p.
35 km, t h e l i q u i d u s p h a s e
e
a depth of 1 5 t o
of an a l k a l i b a s a l t magma i s
o l i v i n e i n the e a r l y s t a g e s
.
by Green
142-148) on t h e f r a c t i o n a l c r y s t a l -
l i z a t i o n of b a s a l t i c magmas show t h a t a t
' .
may be
of f r a c t i o n a t i o n f o l l o w e d
c l o s e l y by clinopyroxene.,Plagioclaseappearsonlyat
.
temperatures very near
the solidus.
If t h e La J a r a Peak
magma d i f f e r e n t i a t e d i n t o b a s a l t i c a n d e s i t e a l o n g
the absence of plagioclase phenocrysts
alkali trend, then
i
s
'
an
e x p l a i n e d by the i a t e c r y s t a l l i z a t i o n of plagioclase
the melt.
..
Tertiary Intrusive
from
...,
Rocks
Mafic Dikes
A l a r g e number of dark, aphanitic mafic dikes are
foundthroughout
.*
the a r e a .
They a r ee q u i v a l e n tt o
north-trending lamprophyre dikes in
-
~...
. '
the Kelly district
(Laughlin and Koschmann, 1942, p. 43) and t o the b a s a l t i c
-..
P
the
dikesfound
p. 3 2 ) .
56
-
i n thePuertecitoQuadrangle
Most o f t h e d i l < e s
(Tonking, 1957,
occupy a major f a u l t zone whieh
trends northward through the Kelly district
and a l o n g t h e
e a s t e r n e d g e of t h e BearMountains,varyingfrom
1.5 t o
2 .miles i n width.Although
tocut
t h e monzonite stock, the
none havebeenfound
lower p a r t o f t h e
'
La J a r a Peak
Formation i s intruded by these m a f i c d i k e s i n p l a c e s .
They were probably intruded over
a considerable span of
t i m e , since different 'types are
commonly emplaced s i d e by
sideintothe
Most of t h e dikes were
same f a u l t zone.
probably ernplaced d u r i n g t h e f o r m a t i o n o f t h i s
a
'
major Zault
zone i n t h e Oligocene.
Individual dikes generally vary
from a few f e e t t o a
thousand f e e t i n l e n g t h and one d i k e i s over a mile long.
They a r e g e n e r a l l y narrow but range
from 2 t o 50 f e e t i n
6 feet.
viidth and probablyaverageabout
Most of the
d i k e s t r e n d e i t h e r north-south o r about N 1O0E, b u t a few
trendalmosteast-west.Dipsarepredominantlyeastward
a t angles from 70 degrees t o v e r t i c a l . I n o u t c r o p
the
dikes weather t o shallow d e p r e s s i o n s d e f i c i e n t i n v e g e t a -
t i o n and t h u s , t e n d t o
form v i s i b l e l i n e a m e n t s
on a e r i a l
photographs.
,*
I n hand specimen,. a number of d i f f e r e n t v a r i e t i e s
can be d i s t i n g u i s h e d , b u t
most a r e v e r y a p h a n i t i c , b l a c k
"
.
~
~.
~,
..
__
- 57 t o greenish-gray on fresh surfaces,
e
brown b y l i m o n i t e
and s t a i n e d r u s t y
on weatheredfaces.Phenocrysts
pyroxene,hornblende,biotite,
of
and p l a g i o c l a s e a r e p r e s e n t
i n some varieties and xenocrysts of q u a r t z and p l a g i o c l a s e
a r e common.
A p o r p h y r i t i cv a r i e t yc o n t a i n sp l a g i o c l a s e
phenocrysts up to 5 mm long.
.Most of t h e v a r i e t i e s a r e
too fine-grained and a l t e r e d t o be i d e n t i f i e d p e t r o g r a p h i cally and t h e y a r e b e s t d e s c r i b e d
as lamprophyres i n the
s e n s e t h a t they a r e d a r k , f i n e - g r a i n e d r o c k s i n
which only
fesromagnesianmineralsusuallyoccurasphenocrysts.
lamprophyres have an aphanitic
a
felty plagioclase microlites,
groundmass com2osed of
pyroxene, i r o n o x i d e s , . and
,
accessory amounts o f b i o t i t e and s l e n d e r a p a t i t e p r i s m s .
P l a g i o c l a s e , which formsapproximately
.
The
70 percent of
.
. groundmass, occurs as s u b h e d r a l l a t h s
long.
The compositionofthe
t o 0 . 2 mm
from0.05
groundmass p l a g i o c l a s e i n
w a s measured a s An55 (average
one unusually fresh sample
of 1 5 d e t e r m i n a t i o n s by t h e Michel-Levy method).
samples the groundmass i s h i g h l y a l t e r e d w i t h
clase a l t e r i n g t o c l a y s
calcite.
I n most
the plagio-
and the pyroxenes t o c h l o r i t e and
Pale g r e e n a u g i t e
occurs both
i s the most common mineral and
as groundmass m i c r o l i t e s and as subhedral t o
euhedralphenocrysts
extinction angle
from 0 . 1 t o 2.0 nun long.
( Z ' A c) ' i n t h e p h e n o c r y s t s
x..... .
,
I
the
The maximum
i s about 45:
9
*
58
-
Mos% g r a i n s show s l i g h at l t e r a t i o nt oc a l c i t e c, h l o r i t e ,
and .epidotealongcracks
and cleavages.
Some a u g i t e i s
intergrown with plagioclase as glomeroporphyritic aggregates.
,
The boundaries between the plagioclase
grains are o f t e n r e s o r b e d
and pyroxene
and ,the p l a g i o c l a s e i s sometimes
penetrated py the pyroxene;
'
however, some p l a g i o c l a s e
contains p o i k i l i t i c i n c l u s i o n s
ofpyroxene.Thus,
the
two minerals p r o b a b l y c r y s t a l l i z e d c o n c u r r e n t l y , a t l e a s t
i n part.
There seem t o be two v a r i e t i , e s - o f p l a g i o c l a s e pheno.
.
crystsr
0
(1) f r e s h ,e u h e d r a ll a t h sw i t hs h a r pa l b i t e
twinning, and ( 2 ) composite, o f t e n zoned c r y s t a l sw i t h
i r r e g u l a r ,p a r t i a l l yr e s o r b e db o u n d a r i e s .
'.
The zoned g r a i n s
may be x e n o c r y s t s ' w h i c h c r y s t a l l i z e d a t g r e a t d e p t h s i n c e
they.are usually broken
~. .
and have an undulatory extinction.
.. .
The average composition of the
withslightly
zoned p l a g i o c l a s e i s An5S
more c a l c i c c o r e s .
The compositionofthe
uncorroded,plagioclase averages An
Tonking r e p o r t e d t h e p r e s e n c e
42'
o f hornblende
i n some
of the d i k e s of t h e P u e r t e c i t o Q u a d r a n g l e b u t none has been
found i n t h i s a r e a .
A few d i k e s c o n t a i n
up t o t e n p e r c e n t
b i o t i t e as subhedral phenocrysts partly replaced by magnetite
and c h l o r i t e X
. enocrysts
'
resorbed grains surrounded
..
.
of quartzoccuras
by r e a c t i o n rims..
'._.
rounded
and
"
"
"
~~
-
59
-
The n e a r l y u b i q u i t o u s a l t e r a t i o n o f
t h e dikes,to-
g e t h e r w i t h the presence o€ a l t e r a t i o n z o n e s i n
jacent wall rocks, suggests that
t h e ad-
t h e dikes were e i t h e r
quite w e t and r e a c t i v e d u r i n g emplacement o r t h a t t h e
f r a c t u r e s o c c u p i e d by t h e d i k e s were u t i l i z e d by l a t e r
hydrothermalsolutions.
The composition of t h e d i k e s
the f i r s t p o s s i b i l i t y : t h e i n t r u s i o n o f
arguesagainst
monzonite stocks following dike emplacement and t h e
presence of mineralization along
some of the dikes supports
alteration.
t h e edges and w i t h i n
a hydrothermal origin
Inunedidtelyadjacent
t o t h e dikes t h e w a l l
r o c k s a r e . b a k e d and s i l i c i f i e d b u t t h e wall rocks
.. .
r
of t h e
are o f t e n
bleached a s much a s f i f t y f e e t from thedikes.Sanidine
phenocrysts in
t h e ash flows are
commonly replaced by
l i m o n i t e and epidote.
Larger Mafic Intrusives
Two e l o n g a t e a n d e s i t i c s t o c k s o c c u r
e a s t e r n p a r t of the area along the margins
o f t h e monzonitestock.
n o r t h e r n end of
and t o t h e n o r t h
These i n t r u s i v e s r e p r e s e n t t h e
a l i n e of a u g i t e a n d e s i t e p l u g s
G r a n i t e Mountain.
The s t o c k s a r e
and s t o c k s
crest and w e s t side of
which continues southward along the
andone
i n the south-
300 t o 1 2 0 0 f e e t wide
t o t w o miles long.and were i n t r u d e d i n t o t h e major
..L"..
.
e
,
~~
north-southfault
zone(page
60
-
56) p r i o r t o emplacement of
t h e monzonite stock and w h i t e r h y o l i t e d i k e s .
The a n d e s i t e which forms t h e s t o c k s i s very s i m i l a r
t o the dark mafic dikes in the area
gray and very fine-grained
and t h e s t o c k s a p p e a r
w i t h abundant small plagioclase
phenocrysts and about 10 percent greenish-black pyroxene
phenocrysts.
I n t h i n section, t h e t e x t u r e v a r i e s
from p o r p h y r i t i c -
. s u b h e d r a l ' p l a g i o c l a s e . and '10 t o 1 5 percent subhedral
anhedral clinopyroxene
set i n a n a p h a n i t i c
t r a c h y t i cp l a g i o c l a s em i c r o l i t e s
partlyaltered
groundmass of
and magnetitegrains.
t o c l a y s and epidote.
to
The
The clinopyroxenehas
-
61
-
Monzonite
e,
a monzonite
I n t h e s o u t h e a s t e rpna o
rtth
faer e a ,
i n t r u s i v ec r o p so u ti ns e c t i o n s
4 W.
34 and 35, T. 1 S., R.
and s e c t i o n s 2 and 3 , ' T . 2 S.,
R.
4
W
.
Because of
i t s proximity t o La J e n c i a Creek, it i s h e r e i n r e f e r r e d
t o as the La Jenciastock.
and apophyseswith
%-square mile'.
a total outcrop area
Much o f t h e
a few d i k e s
of approximately
monzonite has been buried by
and i t s t o t a l a r e a l e x t e n t n o r t h
windblownsarid
JenciaCreek
The stockincludes
is probably more t h a n onesquare
La Sencia stock
i s roughlyoval-shapedana
of La
mile.
The
i s actually
..
the northernmost end'of an elongate monzonite
@
'
t r u d e d . i n t o a majqrzoneofweaknessalong
Goat Spring and the lower t u f f o f
.
the viest s i d e
The monzonite i n t r u d e s the t u f f of
of G r a n i t e Mountain.
has been silicified
body in-
Bear Springs:
and warped up i n t o a prominent ridge
a l o n gt h ew e s t e r n
margin of- t h es t o c k .
ash-flowoutcrops
which occurnearthemargins
stock appear to
the l a t t e r
be remnants of
Small, isolated
of t h e
f a u l t b l o c k s formed by
i n t r u s i o n of t h e monzonite.
The monzonite is the y o u n g e s t i n t r u s i v e i n
the area
. w i t h t h e e x c e p t i o n of t h e white r h y o l i t e dikes, a few
latite dikes,
e
and some of theyoungestmaficdikes.
i l a r monzonite bodies i n ,t...h e k e l l y d i s t r i c t , t h e
-- ...
..
.I
...
.
.
..
SimN i t t and
-
.
*
62
-
Anchor Canyon stocks, have been dated
as 28.0
-
3-
by t h e X-Ar method
1 . 4 m. y. and 28.3 f
- 1.4 m. 'y.,, r e s p e c t i v e l y
(Weber and B a s s e t t , 1363, p. 2 2 0 ) .
Monzonite d i k e s re-
l a t e d t o t h e La J e n c i a s t o c k c u t t h e s u r r o u n d i n g a n d e s i t e
i n t r u s i v e s and s i n c e t h e monzonite i s not cut by mafic
d i k e s i n the study area,
and lam-
most o f t h e a n d e s i t i c
prophyric intrusives are probably older than
2 8 m. y .
Those mafic dikes which c u t t h e La Jara Peak Andesite are,
of course, younger.
T h e stcck o u t c r o p s a s a n g u l a r , j o i n t e d
cliff's or
s p h e r o i d a l b o u l d e r s , w i t h a f i n e to medium-grained tex-
*
t u r e . whereas the monzonite d i k e s a r e g e n e r a l l y..f i n e
,,
grained.
Hand specimens a r el i g h to l i v eg r a yo n
fresh
..I
,
s u r f a c e s and weather
t o a yellowish-brown w h i l e more
mafic v a r i e t i e s a r e f i n e r g r a i n e d
and darkergreen.
t e x t u r e v a r i e s from equigrangular
to porphyritic-phaneritic.
The
G r a i n s i z e v a r i e s from 1 t o 4 nun and the p o r p h y r i t i c
up . t o
varieties have chalky-white plagioclase phenocrysts
5 mm l o n g . P h e n o c r y s t s v i s i b l e i n
handspecimeninclude
largeplagioclasegrains,smallerpinkorthoclase,olive
green pyroxene, biotite,
and a f e w rounded q u a r t z g r a i n s .
The monzonite i s cut by a few s m a l l p i n k a p l i t e d i k e s
and near t h e w e s t s i d e of the s t o c k t h e monzonite has in-
'e
c o r p o r a t e d some blocksof
a d a r ks i l i c e o u sr o c k ,
X.".
which may
-
63
-
be xenoliths of Precalnbrian felsite (Loughlin
1 9 4 2 , p. 9 ) .
Along.thewesternmargin
t y p i c a l monzonite grades over
and Koschmann,
of t h e s t o c k , t h e
a d i s t a n c e of s e v e r a l feet
i n t o a dark greenish-black border facies
d i o r i t i c i n m i n e r a l o g i c a l c o m p o s i t i o. n. .
which i s more
. The d i o r i t e c o n t a i n s
more pyroxene and b i o t i t e t h a n f e l d s p a r s
as much p l a g i o c l a s e a s o r t h o c l a s e .
twice
and has about
S m a l l steeply-dipping
s e n o l i t h s o f a dark mafic r o c k s i m i l a r t o
a r e found i n t h e i n t e r i o r o€ t h e s t o c k ,
the border facies
which i n d i c a t e s
t h a t the d i o r i t e may be an e a r l i e r i n t r u s i v e t h a t
was l a t e r
d i s p l a c e d and a s s i m i l a t e d b y t h e monzonite.
. In thin section
t h e monzonite h a s a hyhypav.ton.orphic-
g r a n u l a r t e x t u r e o f i n t e r l o c k i n g s u b h e d,~
ral minerals with
.an averagegrain
s i z e of about 0.75 mm.
The component
80 p e r c e n t f e l d s p a r s w i t h p l a g i o c l a s e
minerals are about
... - . ..
. .
i n excess of o r t h o c l a s e and accompanied by 1 0 t o 15 percentbiotite,from'0
t o 5 percentaugike,about
q u a r t z , . a b o u t 3 percent magnetite,
2 percent
and minor amounts of
a p a t i t e and z i r c o n .P l a g i o c l a s eo c c u r sa ss u b h e d r a lg r a i n s
a s much a s 5
A n . . t o An48.
27
nun.
in length with
a compositionalrangefrom
Most g r a i n s show a r g i l l i c a l t e r a t i o n .
Ortho-
clase ,occurs w i t h q u a r t z a s i r r e g u l a r g r a i n s i n t e r s t i t i a l
to the plagioclase
and pyroxene; some o r t h o c l a s e i s graph-
i c a l l yi n t e r g r o w nw i t hq u a r t z .
The a u g i t e i s palegreen,
-.. ..
-
a
64
anhedraltosubhedralinshape,
graphically intergrown
-
and i s sometimesmicro-
w i t h magnetite.
The monzonite intrudes rocks of,very similar chemical
composition,thuscontact
margins of
e f f e c t s areminimal.
the stock,.'the lower
t u f f of Bear Springs
baked t o a dark greenish-gray hornfels.
is caused by
Near t h e
is
The d a r k c o l o r
the presence of fine-grained epidote replacing
p a r t of ' t h e groundmass.
The w a l l r o c k s a r e s i l i c i f i e d a s
much a s s e v e r a l hundred feet from the c o n t a c t and a r e c u t
by numerous s m a l l q u a r t z v e i n l e t s :
a nunlber of quartz-
c a r b o n a t ev e i n sa r ep r e s e n tn e a rt h ec o n t a c t s .I na d d i t i o n
a
t o s i l i c i f i c a t i o no
, t h e ra.. l. t e r a t i o ne f f e c t si n c l u d eb l e a c h of f e l d s p a r s i n the t u f f s , and hematite
ing, argillization
staining adjacent to fractures.
Porphyritic Latite Dikes
A f e w small latite dikes are present
i n the southeastern
. .
corner of the area.
A l l o f t h e dikes occur within
a two-
m i l e radius of
the La J e n c i a s t o c k which some of them
i n t r u d e ; the l a t i t e i s very similar t o t h e monzoniteexcept
for g r a i n s i z e .
The d i k e s a r e s i m i l a r t o t h e
latite d i k e s mapped byLaughlin
e
The l o n g e s td i k ec r o p so u ti n
three small
and Koschmann (1942, p. 3 3 ) .
an
echelonpattern
f o r about 0 . 6 mile on t h e west s l o p e of N i p p l e Mountain.
'. __
.
.
..
..
The dike trends approximately
6
7 0 d e g r e e se a s t .
monzonite stock
-
65
N 20 E and dips from 60 t o
The o t h e r l a t i t e d i k e s i n t r u d e t h e
and the 'tuff of
Goat S p r i n g i n Goat Spring
The l a t i t e i s grayish
draw and along La J e n c i a Creek.
green on f r e s h s u r f a c e s and i s u s u a l l y h i g h l y a l t e r e d
disintegrated.
Hand specimens arefine-grained
and
and por-
p h y r i t i c w i t h scattered, chalky plagioclase phenocrysts.
Microscopically, the l a t i t e ' c o n t a i n s 10 t o 20 percent
plngioclase phenocrysts
and a r e l a r g e l y r e p l a c e d
and epidote.
e
which average about
3 mm i n l e n g t h
by a g g r e g a t e s o f c l a y s , c a l c i t e ,
About LO percentclinopyroxenephenocrysts
occur as anhedral grains almost entirely replaced by epid o t e ,c h l o r i t e ,
.
.
..
.
The groundmass i s a f i n e -
and' magnetite.
grained intergrohih of about
80 p e r c e n t a r g i l l i z e d p l a g i o -
clase m i c r o l i t e s andabout
1 5 p e r c e n t o r t h o c l a s e which i s
clearlyinterstitialtotheplagioclase.
The remainderof
t h e groundmass i s epidotized pyroxene grains.
White Rhyolite Dikes
The y o u n g e s t i n t r u s i v e r o c k s i n t h e s t u d y a r e a a r e
are restricted t o the south-
w h i t er h y o l i t ed i k e s .O u t c r o p s
e a s t e r n p a r t of the area where d i k e s and small plugs have
~
'
*
been intruded along the 'eastern half
t r e n d i n gf a u l t
of t h e major north-
zone.Individualdikesvaryfrom
..
. .
i
.
.<'
\.. .__.
..
1 0 t o 200
'
66
*
-
feet in width and from s e v e r a l hundred f e e t t o over a thou-
sand f e e t ilne n g t h .
$!Multiple d i k e s a r e common w i t h i n which the white rhyol i t e occursadjacent
to an oldermaficdike.Outcrops
are
o f t e n flow-banded and many c o n t a i n up to 1 0 percent limo-
n i t e pseudomorphs.a€terpyrite.
pinkish-gray on fresh surfaces
The r h y o l i t e i s l i g h t
and weathers yellowish-gray
Phenocrysts of q u a r t z makeup
t o brown.
about. 1 0 p e r c e n t
of the rock and.smal1 altered orthoclase crystals are
sometimes v i s i b l e .
In thin section,
the r h y o l i t e h a s
a dense,micro-
g r a n u l a r groundmass made o f i r r e g u l a r q u a r t z
g r a i n sl a r g e l yr e p l a c e d
by s e r i c i t e .
'
and f e l d s p a r
Quartzphenocrysts
"
v a r y from 0.5 t o 1 . 0 mm i n diameter and occur as.rounded,
r e s o r b e dg r a i n s
w i t h r e a c t i o n rims.
Orthoclasephenocrysts
and sericite.
a r e hZghly a l t e r e d t o c a l c i t e
rare and occurs as subhedral grains replaced
.. . .
Biotite is
by muscovite
and' l i m o n i t e .
.
.
Postvolcanic Deposits
Deposits
"
,
*
.
Fanqlomerateof
..
Tertiary
D r y Lake Canyon.
A series of s o f t ,
.
poorly-sorted volcaniclastic sedimentary rocks occurs interbedded with andoverlyingthe
_.
La Sara PeakFormation.These
'< .
-
67
-
rocks a r e e x t e n s i v e l y exposed i n t h e s o u t h e r n p a r t o f t h e
P u e r t e c i t oQ u a d r a n g l e where Tonking 11957, p. 34) design a t e d them a s t h e e q u i v a l e n t
o r t h e PopotosaFormation.
of either t h e S a n t a Fe Group
Creek area, 1 5
I n t h eS i l v e r
miles t o t h e e a s t , b a s a l t i c a n d e s i t e s i n t e r b e d d e d w i t h t h e
a t 15.8 m. y.
PopotosaFormationhavebeendated
(Weber,
23.8 m. y.
1971) which i s considerably younger than the
age -oft h e La J a r a Peak Formation(Chapin,1971-a).
PopotosaFormation
,
(Denny, 1940), a b a s a l u n i t
The
of t h e
S a n t a Fe Group, was deposited in middle
Miocene t o P l i o -
cene t i m e i nb l o c k - f a u l t e db a s i n sa l o n g
t h e Rio Grande
rift incentral
New Mexico.
It c o n s i s t s o€ fanglomerates,
c
p l a y ad e p o s i t s ,
andinterbeddedvolcanicrocks;
represent an inverted section
the c l a s t s
of t h e D a t i l Group w i t h o l d e r
rocks becoming more.abundant upward a s erosion cut deeper
i n t o the bordering u p l i f t s .
The fanglomerate of Dry Lake
Canyon c o n t a i n s m o s t l y d e t r i t u s
from t h e La J a r a Peak
I
Formation but i s s i m i l a r i n mode of d e p o s i t i o n 'and perhaps
age to the Popotosa.
The formation outcrops along
the study area in the. vicinity of
t h e western margin
Dry Lake Canyon and i n
t h e l o w a r e a w e s t of t h e main BearMountainsdivide..
lower part
of
The
i s interbedded w i t n f l o w s i n the upper La J a r a
PeakFormation.
The formationdipsabout
.. ... .
6 degrees t o t h e
,.--_._,
..
- 110 Smith., R. L . , and Bailey, R. A . , J.966, The BandelierTuff:
A
,tudy
of
ash-flow
eruption
cycles
from
zoned
magma
chanibers:
S
. B u l l . Volcanologique, V. 29, p. 83-103.'
Snyder, D. O., 1971, S t r a t i g r a p h i c a n a l y s i s of the Baca Format i o n ,w e s t - c e n t r a l
New Mexico: unpublished Ph.D. d i s s e r t . ,
Univ. New Mexico.
T i t l e y , S . R., 1959,Geological summary of t h e Magdalena miningdistrict,Socorro
County, New Mexico, i n Guidebook of
west-central New Mexico: N. Mex. Geol. SOC., Guidebook,
1 0 t h F i e l d Conf., p. 144-148.
Geology of PuertecitoQuadrangle,Socorro
Tonking, W. H.;1957,
County, New Mexico: ' N . Mex. S t a t e B u r . Mines Mineral Resources, B u l l . 41.
. Weber, R. E . , 1963,CenozoicvolcanicrocksofSocorro
County,
'
. i n Guidebook o f theSocorroregion:
N. Mex. Geol. Soc.,
Guidebook, 1 4 t hF i e l dC o n f . ,
p. 132-143.
, 1971, K-Ar agesofTertiaryigneousrocks
c e n t r a l and western. New Mexico: Isochron/West,
p. 33-45.
in
n.1,
, and B a s s e t t , W. A., 1963, K-Ar agesofTertiary
volcanic and i n t r u s i v e r o c k s i n Socorro, Catron, and Grant
Counties, New Mexico, i n Guidebook of the Socorro region:
X. Mex. Geol. SOC., Guidebook, 1 4 t hF i e l d Conf., p. 220-223.
Willard,' M. E., 1959, T e r t i a r y s t r a t i g r a p h y of northernCatron
County, New Mexico, i n Guidebook of west-central New Mexico:
'N. Mex. Geol. Soc'., Guidebook, 10thFieldConf.,
p. 92-99.
Winchester, D. E . , 1920, Geology, of Alanosa Creek Valley,
Socorro County, New Mexico, w i t h s p e c i a l r e f e r e n c e - t o t h e
occurrence of o i l and gas: U.S. Geol.SurveyBull.
716-A.
west and has been eroded into
heterolithic clasts
6% -
s t r i k e ridges covered with
of the underlying volcanic rocks.
The d e p o s i t s c o n s i s t o f s i l t s t o n e s , s a n d s t o n e s ,
conglomerates, and mudflow breccias most l i k e l y d e p o s i t e d
a s westward-sloping alluvial fans along
BearMountains
uplift.
Outcropsrange
the marginof
i n c o l o r from pink
t o l i g h t brown and a r e g e n e r a l l y p o o r l y . s o r t e d
cementcld.by c a l c i t e i n
a s i l t ym a t r i x .
containsubrounded.pebbles,cobbles,
a s o n ef o o ti nd i a m e t e r .
the
and loosely
The conglomerates
and b o u l d e r s a s
much
Most of t h e c l a s t s a r e d e r i v e d
from t h e La J a r a Peak Formation b u t fragments from a l l
p a r t s of the D a t i l Group a r e p r e s e n t
i n minoramounts.
..
Cross-bedding and channeli.ng a r c common s t r u c t u r e s .
roadcut just east
In a
of Dry Lake Canyon, a white a i r f a l l t u f f
and some s t r a t i f i e d t u f f a c e o u s s a n d s t o n e s a r e i n t e r b e d d e d
with t h e other d e p o s i t s .
The a i r f a l l t u f f c o n t a i n s
pumice l a p i l l i up t o one inch long
-
cementedby
white
reddish-
brown s i l t y m a t e r i a l .
'
Quaternary
Deposits
Pediment Gravels.
The o l d e s t s u r f i c i a l d e p o s i t s i n
the a r e a are t h i n v e n e e r s of poorly-consolidated pediment
gravels.
The most extensive pediment surfacesoccuralong
the e a s t e r n margin of t h e a r e a a s g e n t l e e a s t w a r d - d i p p i n g
..
"><
x.
.
..
surfaces at the
Flats).
edgeof
the La Sencia basin
The pedimentscontainvolcanicpebbles
(Snake Ranch
and cobbles
d e r i v e d . from the u n d e r l y i n g D a t i l Group, and i n , t h e e x t r e m e
of Paleozoic and Precambrian
southeastern corner, clasts
r o c k s . . A prominent caliche zone caps the pediment gravels
and extends onto
t h e t a l u s conesalong
the e a s t s i d e of t h e
southern Bear Mountains.
.. ..
Talus,Landslides,
For the purposes 'of
and Colluvium.
t h i s s t u d y ,t a l u s ,l a n d s l i d e s ,
and colluvium were mapped
c o l l e c t i v e l y t o d e l i m i t the aprons and fans derived
wasting.
The t a l u s d e p o s i t s
s o i l and g r a s s andmost
'
from mass-
are s t a b i l i z e d byamantleof
are b e i n g d i s s e c t e d by shallow gulleys.
They a rree l a t i v e l o
y l d e p o s i tospf r o b a b l y
have a well-developedcalichezone
P l e i s t o c e n e age,
.
.
on t h e i r lower slopes, and
merge downslope i n t o the pediment surfaces.
Boulders as
downhill from
much a s t e n f e e t i n d i a m e t e r h a v e
the cliff-forming ash-flow
partiallyburiedbycolluvium.
slumped
t u f f s and have been
The upper t u f f ofBearSprings
i s p a r t i c u l a r l y s u s c e p t i b l e t o slumping over the relatively
incompetentandesites.
T h e r o t a t e d slump b l o c k sr a n g ei n
s i z e to a s much a s s e v e r a l h u n d r e d
feet in length
o€ten l e a v e c i r q u e - l i k e s c a r s a t t h e i r h e a d s .
a.
\.
..
..
and t h e y
1
!
- IO
1
Aeolian Sand.
T h e pediment s u r f a c e si nt h es o u t h e r n
h a l f of t h e a r e a h a v e b e e n l a r g e l y
windblown sand.
of
w i t h a definite northeast-
The thickest accumulations 0ccu.r along La
SenciaCreek,where
pediment gravels,
buried by a veneer
The sand shows up c l e a r l y on the a e r i a l
photos as light-colored areas
t r e n d i n gg r a i n .
-
up to t w e n t y f e e t
of the sand overlies
and i n dunes which have developed on the
windward s i d e of some r i d g e s .
The windblown sandprovides
a shallowground-waterreservoirbytrappingmeteoricwater
above the r e l a t i v e l y impermeableadobe
t h e pediment surfaces
and .usually supports' a t h i c k growth of
I n this report,alluvium
recent stream gravels
drainages.
s o i l s and c a l i c h e of
,.. .
j u n i p e r and sage.
Alluvium.
'
is used t o i n d i c a t e
which have been d e p o s i t e d i n t h e m a j o r
It a l s oi n c l u d e sd e p o s i t s
of deeplyweathered
v a l l e y f i l l which.have obscured. the bedrock.
The o l d e r a l -
luvium interfingers'with and.is approximately the
same age
as the windblown sand.
-.P e t r o q e n e s i s of the Ash Flows
The purpose of this s e c t i o n i s t o summarize and in-.
t e r p r e t some o f t h e p e t r o g r a p h i c
and f i e l d d a t a p e r t a i n i n g
i n order to reconstruct
to the a s h f l o w s . i n t h e s t u d y a r e a
p a r t of t h e a r e a ' s v o l c a n i c h i s t o r y
.-.
.... The
data is r e s t r i c t e d
~
- 71 -
*
to t h e Hells Mesa ash flows which have widespread outcrops
i n thearea
'
mapping.
andhavebeensampled
i n some d e t a i l d u r i n g
A more comprehensive p e t r o l o g i cs t u d y
the
of t h e
e n t i r e s u i t e of voLcanic rocks n e c e s s i t a t e s s y s t e m a t i c chemi c a l . a n a l y s e s b u t some preliminary conclusions can be
drawn
i n the p r e s e n t i n v e s t i g a t i o n .
Petrographic Data
Figure 14 is a composite section whichcompares
the
mineralogy and abundance of c r y s t a l s i n each of the ash-
f l o w coolingunits.The'diagram
shows t h a t t h e r e
t i n c t gap i n the abundance of phenocrysts between
e
poorash
flows and t h e c r y s t a l - r i c h a s h
...
have an upper
flows.
is a disthe c r y s t a l -
Tne former
..
l i m i t . of 8.0 p e r c e n t w h i l e t h e l a t t e r h a v e
a
,.. .
lower limit of about 30 percent.
The r e l a t i v ep e r c e n t a g e s
of t h e component minerals are designated by the histograms
which a l s o d i s p l a y . . a marked d i f f e r e n c e b e t w e e n t h e c r y s t a l -
,khe t u f f o f Bear Springs
poorsamplesfrom
rich samples.
'
The two c o o l i n g u n i t s i n
Springsarealmostidentical
and t h e c r y s t a l -
t h e t u f f of Bear
to each other but contain prac-
t i c a l l y no p l a g i o c l a s e and r e l a t i v e l y more. s a n i d i n e t h a n the
crystal-rich units
..
.*
which a r e a l s o q u i & s i m i l a r t o e a c h o t h e r .
Peterson'and:-Roberts
(1963)~s t u d i e d a number of ash-
f l o w sheets throughout the Basin
and Range province of the
1
I
i
i
$
i
i
ii
:
- 73 0
western U n i t e dS t a t e s
and found t h a t t h e r e
between the chemical composition
the t u f f s .
is a r e l a t i o n s h i p
and t h e c r y s t a l c o n t e n t o f
The c r y s t a l - p o o r welded t u f f s c o n t a i n
25 p e r c e n t c r y s t a l s
less t h a n
and c h e m i c a l l ya r er h y o l i t e s .
The c r y s t a l -
r i c h welded t u f f s have more than 2 5 percent phenocrysts
chemicallyrange
from q u a r t z l a t i t e t o d a c i t e .
and
The r h y o l i t i c
ash flows a r e a l s o composed of s m a l l e r s i z e d v i t r i c f r a g m e n t s
l a t i t i c ashflows.Although
t h a nt h eq u a r t z
in
theashflows
the study area have not yet been chemically analyzed, the
mineralogicalcompositionsindicatethatthecrystal-poorash
flows i n the Hells Mesa F o r m a t i o n a r e r h y o l i t i c ( F i g u r e
w h i l et h ec r y s t a l - r i c ha s h
q u a r t zl a t i &
..
10)
flows p l o tc l o s et ot h er h y o l i t e .
boundary(Figure
8).
Furthermore,petrographic
examination shows t h a t t h e c r y s t a l and v i t r i c fragments are
generally smaller in the crystal-poor tuffs.
.
.. .. .
Peterson and Roberts
@. u.)
suggested
two p o s s i b l e
hypotheses t oe x p l a i nt h e s er e l a t i o n s h i p s :
(1) d i f f e r e n t i a t i o n
by'cxystalsettling
v o l a t i l e s , and ( 2 )
p x t i a l melting.
theformation
02
and/orupward-migrating
The end r e s u i t of e i t h e r p r o c e s s
a crystal-poor rhyolite, high
would be
i n s i l i c a s and
a l k a l i s , and a c r y s t a l - r i c h q u a r t z l a t i t e o r d a c i t e , p o o r
s i l i c a and a l k a l i e s .
0
in
Although it is d i f f i c u l t t o d e m o n s t r a t e
w h e t h e rf r a c t i o n a lc r y s t a l l i z a t i o n
or p a r t i a l m e l t i n g h a s
formed a p a r t i c u l a r s u i t e of rocks, the euhedral faces on
..
many
-
74
-
of the c r y s t a l s and t h e presence of p o s s i b l e d i f f e r e n t i a t i o n
t r e n d s i n the modal compositions suggest that the ash flows
i n t h e H e l l s Mesa were formedby
d i f f e r e n t i a t i o n i n a shallow
magma chaniber a l t h o u g h t h e o r i g i n a l
magmamay
have been de-
r i v e d by anatectic melting in the upper mantle or lower crust.
The following sequence
of events i s p o s t u l a t e d by the w r i t e r
as a model of the c r y s t a l l i z a t i o n h i s t o r y :
1.
A m e l t ofintermediatecomposition
was emplaced
i n t o a r e l a t i v e l y shallow holding chaniber and
began c r y s t a l l i z i n g .
Early-formedmafic
and c a l -
cic minerals s e t t l e d toward the bottom o f t h e
magma chamber wllile v o l a t i l e s and a l k a l i e s as...
.-
bendEd toward t h e top.Gradually
the magma seg-
r e g a t e d i t s e l f i n t o an u p p e r c r y s t a l - p o o r r h y o l i t i c
.
.
p a r t and a lower c r y s t a l - r i c h l e v e l
of q u a r t z la-
t i t i c composition.
2.
Fissurestapped
t h e lower l e v e l and c r y s t a l - r i c h
ash f l o w s ( t h e t u f f of Goat Spring) were erupted.
These ash €lows were l a t i t i c t o q u a r t z l a t i t i c
in
composition and c o n t a i n e d r e l a t i v e l y h i g h p e r c e n t ages of mafic minerals
3.
Later,.another
and p l a g i o c l a s e .
s e t of f i s s u r e st a p p e dt h eu p p e r
l e v e l of t h e magma chaniber and erupted a s e r i e s
of crystal-poor ash
flows ( t h e t u f f of Bear Springs).
-
a
75
-
were r h y o l i t i c i n composition and
Theseash€lows
w e r e d e f i c i e n t i n p l a g i o c l a s e and mafic minerals.
.
were r i c h i n v o l a t i l e s
I na d d i t i o n ,- t h e y
and hence
more pumiceous and more f l u i d a l a s e v i d e n c e d
t h e i r l i n e a t e d pumiceand
by
other laminar flow
structures.
4.
From time t o time t h e f i r s t s e t of f i s s u r e s w e r e
reopened and more crystal-rich ash flows
(the
t u f f s of La J e n c i a Creek and Allen Well) were erupted almost simultaneously
poor ash
e
with t h e c r y s t a l -
flows.
Although t h i s model is undoubtedly oversimplified,
~.. ..
it
.
does provide a reasonable conception of .the mechanisms which
may have produced
tionships.
the observed petrographic
The compositesectioninFigure
h y p o t h e s i s byshowing
that crystal-poor
flows alternate w i t h each other in
d u r i n gt h e
and c r y s t a l - r i c h a s h
t h e top of
the
tuff of Bear Springs, the
ash flows appear to have
the t u f f
the two t y p e s of magma
waning s t a g e s of t h a t e r u p t i o n .
upper p a r t .of
0
1 4 supports the
a somewhat c y c l i c a l manner.
The clots of crystal-rich material near
of Bear Springs suggests mixing of
and f i e l d rela-
Except f o r t h e
two v a r i e t i e s of
been e r u p t e d i n i n t e r m i t t e n t p u l s e s
from two d i f f e r e n t sets of feeder vents.
sufficient to determine whether
The data. i s i n -
these v e n t s were p a r t of a
._
I
.- 76
e
plumbingsystem
..
which t a p p e d d i f f e r e n t l e v e l s
magma chamber or e n t i r e l y d i f f e r e n t
general location
of a s i n g l e
magma bodies.
of t h e ash-flowsource
or sources
The
is d i s -
cussed i n more d e t a i l i n t h e f o l l o w i n g s e c t i o n .
DirectionalSignificance
of Laminar Plow S t r u c t u r e s
Ash-flow t u f f s a r e t h o u g h t
to be emplaced i n a h i g h l y
turbulent gas-charged eruption but descriptions of laminarflow structures in ash flows are
'
literature.
tions, folds,
e
..
becoming common i n t h e
Most o ft h e s es t r u c t u r e s ,
which i n c l u d el i n e a -
and flow banding have been attributed
secondary flowage caused
by remobilization of t h e ash
,..
after d e p o s i t i o n .
to
..
flOV7S
However, some primaryflowstructures
are
the ash flows came
produced by laminar. flowage just before
to rest and t h e s e s t r u c t u r e s may be used t o determine the
..'
'
.. .,
d i r e c t i o n of movement (Schmincke and Swanson, 1967; Noble,
1968:Lowell,
1969: Elston and Smith,1970).Theseprimary
I
I
.laminar flow structures
were f i r s t described by Schmincke
and Swanson (1967) and include:
(1) s t r e t c h e d pumice f r a g -
ments; ( 2 ) r o t a t e d and broken pumice fragments: ( 3 ) t e n s i o n
c r a c k s i n t h e matrix:
(4) hollows around rotated inclusions:
(5) f o l d s ;( 6 )i h h r i c a t e dc r y s t a l s ,l i t h i c ,
merits; and (7) ramp s t r u c t u r e s .
...
.
and pumice f r a g -
The ashflows
area c o n t a i n some o f t h e s e f e a t u r e s ,
"_
i nt h es t u d y
which a r e d e s c r i b e d
and
.
e
77
-
.
i n t e r p r e t ei d
ndividually
below.
..
&ineated Pumice.
Compressed pumice e l l i p s o i d s and
c a v i t i e s l e f t b y t h e weathering of
each of t h e c o o l i n g u n i t s .
c e n t e r and t a p e r a t
pumice a r e common i n
These e l l i p s o i d s b u l g e i n
e i t h e r end.
the
The l o n g a x e s a r e p a r a l l e l
and define a l i n e a t i o n which t r e n d s c o n s i s t e n t l y n o r t h - s o u t h
in the tuff of
Goat Spring and' from north-south to'
the tuffofBearSprings.
were producedduring
N 30°E i n
Assuming t h a t these l i n e a t i o n s
emplacement of t h e a s h
f l o w s , t h e sense
of movement appears t o have been almost north-south
e
s l i g h t l y more n o r t h e a s t e r l y s e n s e f o r t h e . t u f f
with a
of Bear Springs.
. . ..
Rotated
Inclusions.
schmincke
and Swanson (1967, p .
653) described f l o w s t r u c k u r e s produced by d i f f e r e n t i a l f l o w ..-
age i n the matrix around r o t a t e d i n c l u s i o n s .
a r e typically spindle-shaped
ondiagonallyopposite
The s t r u c t u r e s
and a r e d e f i n e d b y
sides of t h e i n c l u s i o n .
of t h e s e c a v i t i e s h a s d i r e c t i o n a l s i g n i f i c a n c e
two cavities
The asymmetry
since the
hollow on t h e downslope s i d e of t h e i n c l u s i o n i s c o n s i s t e n t l y
h i g h e r t h a n the one on
the upslope side.
The 'ash flows of t h e thesis a r e a ' l o c a l l y c o n t a i n abundant lithic fragments
up t o s e v e r a l c e n t i m e t e r s i n d i a m e t e r .
a'
I n p l a c e s , pumice h a s molded around these l i t h i c s .or
aroundphenocrysts
,
"
,
~~
t o 'form a crudely spindle
/;"lf&d
structure
- 78 -
'e
s i m i l a rt ot h e s ed e s c r i b e d
ly imbricated relative
deformedpumice
The i n c l u s i o n s are slight-
above.
to t h e ,compaction f o l i a t i o n , and t h e
is o f t e n h i g h l y c r e n u l a t e d on t h e downslope
s i d e of the i m b r i c a t i o n where the f o l i a t i o n wasjammed
a g a i n s t t h e i n c l u s i o n a t the p o i n t of g r e a t e s t p r e s s u r e .
When viewed i n a v e r t i c a l p l a n e p a r a l l e l t o t h e l i n e a t i o n ,
the Long axes of these spindle-shaped structures dip souththe source.
ward,'presumablytoward
F o l d s and Ramp S t r u c t u r e s .
I n p l a c e s , t h e compaction
been warped i n t o
foliation in the contorted ash flow has
a n t i c l i n e s and synclines'with amplitudes ranging from several
,
inches t o s,e.veraZ f e e t .... ..Theshape o f , t h e s e f o l d s v a r i e s
broad,wavelikeundulations
axes of
from
to t i g h t , V-shaped f l e x u r e s .
The
a l l t h e folds observed are oriented west-northwest,
normal t o the pumice l i n e a t i o n . A x i a l p l a n e s o f .
f o l d su s u a l l yd i ps t e e p l ys o u t h w a r d .
the asyrmnetric
The broad warps have
wavelengths up t o s e v e r a l f e e t and may be ramp s t r u c t u r e s , so
named bySchminckeand
ramps,found
Swanson'(1967;p.
656).
n e a r 3 e a r .Springs, i s about s i x f e e t l o n g
highly asymmetric
of w e s t .
The f o l d s a r e p r o b a b l y p r i m a r y f e a t u r e s
'
and
w i t h t h e s t e e p side southward and with an
a x i s which trends about twenty degrees north
.
One of these
compressionalbucklingofthecontorted
produced by t h e
ash flow a f t e r the
...
. .
'
pumicehad
eo
-
l a r g e l y c o l l a p s e d and the ash flow
a laminar manner j u s t b e f o r e
dip of
coming t o r e s t .
The southward
of the
the a x i a l p l a n e s i n d i c a t e s t h a t t h e a s h f l o w s
t u f f of Bear Springs advancedfrom
'
was moving i n
south t o n o r t h and t h e i r
f o l i a t i o n began t o wxinlcle l i k e a r u g a s
momentum.
S i m i l a rf l o ws t r u c t u r e s
t h e flow f r o n t l o s t
which i n d i c a t e t h e same
s e n s e of movement have been mapped by Richard Chamberlin
( o r a l communication, 1971) i n t h eG a l l i n a s
possible source cauldron
Mountains.
A
f o r the t u f f of Bear Springs has
recently been discovered by
Ed Deal and Rodney Rhodes
( o r a l communication, 1971) i n t h e Mount Withington area of
t h e San Mateo Range which i s located about
..
south-southwestof
30 miles t o the
..
t h e study area.
A sourceinthatgeneral
area i s c o n s i s t e n t - w i t h the l a m i n a r f l o w s t r u c t u r e s
in this report.
.
"
described
-
81
-
STRUCTURE
ReqionalStructure
.The r e g i o n a l s t r u c t u r a l
framework of west-central
Mexico h a s n o t b e e n s t u d i e d i n d e t a i l .
New
According t o Eardley
(1962), the a r e a i s p a r t of t h e Sonoran-Chihuahua system, a
s u b d i v i s i o n of the Basin and Range province characterized
bylate-Cenozoicblockfaulting
and volcanism.
The Bear
and Magdalena Mountains represent a north-northwest-trending,
westward-tilted uplift bordered
on t h e east by a doum€aulted
b a s i n known a s khe Snake Ranch F l a t s o r La Jencia basin.
The l a t e Cenozoic b l o c k - f a u l t i n g r e l a t e d t o t h e R i o Grande
r i f t zone has been superimposed on Laramide
"..
b a s i n s which were beveled by
of r e g i o n a l e x t e n t . W i t h i n
u p l i f t s and
a pre-volcanic erosion surface
t h e framework of t h e Mogollon
the type-Datil rocks of t h e Bear
Plateau volcanic province
.
.
Mountains are thought by Elston
(1968) t o occupy t h e o u t e r
r i m of a l a r g e v o l c a n o - t e c t o n i c d e p r e s s i o n c e n t e r e d i n t h e
Nogollon Plateau.
Laramide forces warped the pre-volcanic rocks of the
area into
a series of plunging homoclinal folds
and t h r u s t
b e l t s (Darton,1928;Kelley
and Wood, 1946;Tonking,.1957;
KelleyandClinton,
Laughlin and Koschmann (1942)
1960) ,
'._.
-
82
-
f o u n de v i d e n c et h a tt h eP a l e o z o i cr o c k si n
t h e Magdalena
D i s t r i c t were arched into an elongated anticline
which
the Tertiary
was beveled. by erosion prior to deposition of
E a r l i e r w r i t e r s a t t r i b u t e d these f o l d e d -
volcanicrocks.
s t r u c t u r e s t o east-west compressive forces but Eardley
a r i s i n g column of magma d u r i n g t h e Laramideorogeny.
surrounding Basin and Range province have been subjected
t o c r u s t a ld i s t e n s i o n
accompaniedby
volcanism.
The cause
of t e n s i o n a l f o r c e s may be a northwestward d r i f t of t h e
e
ColoradoPlateaublock,..
which h a s openedup
t h e R i o Grande
rift '(Eardley, 1962; Chapin, 1971-b) .
f.
LocalStructure
The area of i n v e s t i g a t i o n h a s hada
h i s t o r y of r e c u r r e n t f a u l t i n g frommiddle
until the present.
explainseve.ra1
An i n t e r p r e t a t i o n o f
long and complex
Tertiary time
t h e s t r u c t u r e must
anomalous f e a t u r e s of t h e area.
F i g u r e 1 6 shows t h a t the study area occupies a
between t h e Magdalena Mountainson
Mountains on t h en o r t h .
First,
low saddle
the' south and t h e Bear
Second, t h e r e i s a marked dis-
cordance between t h e general west-southwestward
tilt o f t h e
-
1)
83
-
rocks and the north-south alignment
b l o c kr i d g e s .T h i r d ,r e v e r s a l s
of t h e i n d i v i d u a l f a u l t -
of t h er e g i o n a l
westward d i p
are present in places.
OliqoceneFaults.
The oldestdeformation
t h e s u r f a c e w i t h i n the study area
spaced normal
exposed a t
i s a s e r i e s of c l o s e l y
f a u l t s which have broken t h e e a s t e r n p a r t of
the a r e a i n t o a numker of t i l t e d blocks p r o g r e s s i v e l y
stepped down t o the e a s t .
These f a u l t s were developeddur-
i n g t h e Oligocene b e c a u s e t h e y c u t a s h - f l o w t u f f s d a t e d a t
about 30 m. y. and are intruded by monzonite stocks dated
e
a t 28 m. y.
The i n d i v i d u a l f a u l t s t r e n d
south t o N l S W.
a ta n g l e sf r o m
from aboutnorth-
The f'ault p l a n e s d i p s t e e p l y
60 degrees t o n e a r l y v e r t i c a l .
t o the east
The out-
c_..
crop pattern of the
map s u g g e s t s t h a t some b l o c k s a r e bound-
ed a t e i t h e r end b y t r a n s v e r s e f a u l t s
which trend approxi-
mately e a s t - w e s t .
Some of t h e . f a u l t zones i n t h i s system have lpcalized
t h e emplacement of the a n d e s i t e and monzonite s t o c k s a s
well a s numerous s m a l l e r d i k e s .
aboutnorth-south
and d i p s t e e p l y e a s t , b u t
criss-crosspattern,trending
e
Most o f t h e d i k e s t r e n d
many form a
from N 30 E t o N 1 0 W.
This
p a t t e r n may be r e f a t e d t o a conjugate s e t of f r a c t u r e s
developedcontemporaneouslywiththefaults.
The d i k e
pattern, together
84
-
w i t h the predominant north
to north-
northwest t r e n d o f t h e f a u l t s , i n d i c a t e s t h a t
t h e f r a c t u r i n g were o r i e n t e d e a s t -
stresses which produced
n o r t h e a s t t o east-west.
p a r t of a major zone
This system of f a u l t s and d i k e s is
of Oligocene f a u l t i n g and i n t r u s i o n
which h a s a known length of
over 2 5 miles from the c e n t r a l
p o r t i o n of the MagdalenaMountains
d i s t r i c t and northward along
ainstonorth
the tensional
through the Kelly mining
t h e e a s t edge of t h e Bear Mount-
ofRiley(Chapin,oral
communication, 1971).
The zone is about 1% miles wide i n t h e Kelly d i s t r i c t , 3 miles
wide i n t h e thesis a r e a , and about 5 miles wide i n t h e Riley
@
area.
It i s characteriz,ed by closelyspaced
intruded by mafic dikes
and bymonzonite
related l a t i t i c t o ” ’ r h y o 1 i t i c dikes.
significance of
normal f a u l t s
s t o c k s and t h e i r
The r e g i o n a le x t e n t
and
this s t r u c t u r a l zone is o n l y p a r t l y known
b u t 3.t was a m a j o r f a c t o r i n
t h e l o c a l i z a t i o n of monzonite
s t o c k s .and o r e d e p o s i t s i n t h e
Magdalena a r e a .
Basin and Ranqe Faults. During the period of Basin
Range f a u l t i n g , the Magdalena Range was u p l i f t e d i n t o
and
a
..
.
~
*
westward-tilted horst
bounded on t h e e a s t s i d e by t h e La
Jenciabasin.During
the e a r l y s t a g e s o f u p l i f t
a s e t of
n o r t h e a s t - t r e n d i n g ’ f a u l t s dropped t h e s t u d y a r e a i n t o
orientedobliquelytothe
Magdalena-BearMountains
.‘ .
a graben
horst.
The
-
-
85
main period of u p l i f t p r o b a b l y t o o k p l a c e d u r i n g
Late Miocene
t o Pliocene time b e c a u s e t h e f a u l t i n g p o s t d a t e s t h e
PeakFormation
(24
La S a r a
m. y . ) and both accompanied and followed
d e p o s i t i o n of the PopotosaFormation
( t h e S i l v e r Creek Andes-
i t e interbedded i n the lower p a r t of the Popotosa has been
d a t e d a t 16 m. y .
(Weber, 1971).
Formation have'been found
Outcropsof
on both sides and on t h e crest of
the Magdalena Range and p l a y a d e p o s i t s o f
been uplifted along
ern Bear Mountains
e
t h e Popotosa have
the Socorro-LemitarMountains(Chapin,
The majorexpression
1971-b) .
t h e Popotosa
of this f a u l t i n g i n t h e s o u t h -
i s a prominent north-northwest trending
escarpment along the western margin of
t h e La J e n c i a , b a s i n .
This escarpment probably represents
a zone of closely spaced
normal f a u l t s p r o g r e s s i v e l y s t e p p e d
down t o t h e e a s t .
presenceof
a recent fault scarp just outside the
The
mapped a r e a
i n d i c a t e s t h a t movement along t h i s f a u l t zone has continued
up to the p r e s e n t .
...
. .
Normal F a u l t s
F a u l t s of this system generally trend
N 20°W.
from N 1O0E t o
Some of the n o r t h w e s t e r l y - t r e n d i n gf a u l t s
may rep-
r e s e n t renewed .movement along t h e Oligocene zones of weakness.
The f a u l t z o n e s a r e o f t e n
a
b r e c c i a ,c a l c i t e ,
free of d i k e s , but c o n t a i n f a u l t
and s p r i n gd e p o s i t s .
...
Evidence
from
slicken-
86
-
s i d e s u r f a c e s and s t r a t i g r a p h i c d i s p l a c e m e n t i n d i c a t e
t h e f a u l t plan'es of the northeast-trending
that
set .dip an average
and a r e downthrown to the w e s t .
of 75 d e g r e e s t o . n e a r - v e r t i c a l ,
The Hells Mesa f a u l t (Tonking, 1957, p. 38) extends f o x
f i f t e e n miles i n a north-northwesterly direction through
the
P u e r t e c i t o Quadrangle and i t s s o u t h e r n c o n t i n u a t i o n has been
six miles i n t h e p r e s e n t a r e a
traced fox an additional
it trends approximately north-south along the
f a u l t h a s dropped t h e La 3 a r a Peak Formation
onethousand
east s i d e o f
The west s i d e o f t h i s
the h i g h Bear Mountains a x i s .
dom
feet. ' I n t h e P u e r t e c i t o Q u a d r a n g l e ,
estimated the throw as
...500
normal.
more t h a n
Tonking
t o 600 f e e t and t h e h o r i z o n t a l
displacement as g r e a t e r t h a n one mile.
appears t o b e p a r t ' o f
where
The Hells Mesa f a u l t
a system of subparallel faults.which
are progressively stepped
down toward the vest i n t o a graben
which lies between t h e Bear-Magdalena and Gallinas-San Mateo
uplifts.
-. .
Oblique-Slip Faults
In the northeast corner
controlled ridge along
of t h e a r e a ,
a prominent,fault-
the south side' of B e a r S p r i n g s d r a w
j u t s o u t i n t o +he La J e n c i a b a s i n t r a n s e c t i n g t h e r e g i o n a l
northwesterly.structura1 g r a i n .
Bear Springs
T h e whole areasouth
of
draw i s p h y s i o g r a p h i c a l l y and s t r u c t u r a l l y low
is...
.
-
*
and appears
t o have been
-
87
moveddown
and t o t h e e a s t with
r e s p e c t t o the main BearMountains
the Bear Springs area
to the northwest..
i s approachedfrom
the s t r i k e of t h e o u t c r o p s swingsfrom
t r e n d and becomes almosteast-west.
the north
and south
the regional N 1 5 W
The r i d g e s wrap around
and are t r u n c a t e d a t t h e i r n o r t h e r n e n d s .
are warpedup
As
Many outcrops
next t o t h e f a u l t s and t i l t e d
or down by drag
eastward on t h e downthrown blocks.
.The c a u s e o f t h e s e p h y s i o g r a p h i c f e a t u r e s
i s a zone of
northeast-trending faults in the vicinity of Bear Springs.
The'zone i s aboutone
e
mile wideand
the a g g r e g a t e v e r t i c a l
throw i s approximatelyonethousandfeet.Individualfaults
I
t r e n d from t h i r t y t o f o r t y - f i v e d e g r e e s e a s t o f n o r t h
d i s p l a y a left-lateralsenseof
i:3
and
movement i n p l a n view.
It
not possible t o prove strike-slip displacement in the study
area 'because no d i k e s o r o t h e r v e r t i c a l s t r u c t u r e s a r e o f f s e t
by t h e f a u l t s .
However, l e f t - l a t e r a l movement can b e demon-
strated along the
southhermost o b l i q u e - s l i p f a u l t
Highway 60, where a zone of Oligocenedikes
b e e no f f s e ta b o u t
0.8 mile.
o f f s e t may be caused
rocks.
P a r to f
zone near
and f a u l t s h a s .
t h e apparent strike-slip
by t h e r e g i o n a l s o u t h w e s t e r l y d i p
of t h e
The n o r t h e a s t - t r e n d i n g f a u l t s i n t h e s t u d y . a r e a a r e
p a r t o f a major oblique-slip:fault system
t h. ..
e Tertiary volcanic rocks into
which has dropped
a low saddle bounded o n t h e
,
~
. .
-
a
88
-
south by the Magdalena Mountains and on t h e north by t h e
main BearMountains(Figure
1 6 ; Chapin, 1971-b, p - 1 9 9 ) .
The o r i g i n a l f a u l t % r a c e s of t h e o b l i q u e - s l i p s y s t e m
have been largely obscured
north-trending Basin
f a c ee x p r e s s i o n
the
by more recent u p l i f t a l o n g
and Range f a u l t s and t h e i r major sur-
is a s t r i k e - s l i p disp1,acement of about1.5
miles acrossBearSprings
.
Draw.
.
Complex, mutuallycross-
.
c u t t i n g r e l a t i o n s h i p s between t h e two f a u l t systems are
suggested by the geologic map, which i m p l i e s t h a t t h e o b l i q u e -
slip f a u l t s are g e n e t i c a l l y r e l a t e d t o
' '
0
t h e t e n s i o n a l forces
whichhave
u p l i f t e d and t i l t e d t h e Magdalena and Bear Mount-
a i n ss i n c e
Miocene time.
!rhe o b l i q u e - s l i pf a u l t sp r o b a b l y
became dormant some t i m e ago while the north-trending
normal
faults have continued to uplift the study area. relative to
the La Sencia graben.
.. .
The t w o major f a u l t s i n the o b l i q u e - s l i p s y s t e m a r e
Deer S p r i n g f a u l t
and Be'ar S p r i n g s f a u l t .
The blockbetween
these tvio s t r u c t u r e s i s a northeast-trending graben
which h a s
d i s p l a c e d a s e c t i o n o f t h e 'La J a r a Peak Formation down
and t o the east.
Deer S p r i n g f a u l t t r e n d s n o r t h e a s t e r l y
a c r o s s Beer Spring Canyon and o f f s e t s r o c k s o f t h e D a t i l
I
I
*
Group about 1 0 0 0 feet down along i t s southeast 'side.. South
of Deer Spring the f a u l t f o l l o w s
a topographic lineament
. t h r o u g h t h e La Sara Peak Formation i n a north-northeasterly
-..
.
-
. .
c
.
89,-
,
Figure 16,Location of t h e s i s a r e a w i t h r e s p e c t t o t h e
major s t r u c t u r a l
elements i n t h e i-iagdalena area. Dark l i n e s r e 2 r e s e n t m a j o r .
high-angle faults. Kountain ranges are
outlined bjr the
7500-foot topographiccontour,
-
*
direction, subparallel
90
-
.to t h e Hells Mesa f a u l t . . Bear Springs
fault trends northeasterly in
Bear Springs Canyon.
a topographic lineament along
South of B e a r Springsranch,
this f a u l t
trends south-southwesterly for about one
mile and then bends
southwesterlyagain.
of t h e f a u l t i s up-
thrown and southward
The s o u t h e a s ts i d e
r o t a t i o n of the upthrown block has
f l a t t e n e do rr e v e r s e dd i p sa l o n gt h ee n t i r el e n g t h .
p o s s i b l e t h a t Bear Springs fault
on the southeast side
It i s
was o r i g i n a l l y downthrown
and the present sense
r e s u l t e d from r e a c t i v a t i o n o f t h e f a u l t
of movement h a s
zone during more
recent uplift.
e
.
Folding
The o n l y m a j o r f o l d v i s i b l e a t
t h e s u r f a c e i s a broad,
<-
flat-bottomedsyncline.
The a x i s of t h i s f l e x u r e t r e n d s i n
a north-northwestexly direction
and bisects t h e s o u t h e r n h a 1 f
limb which d i p s t o the southwest
of t h e a r e a i n t o a n e a s t e r n
and a western lipib which d i p s t o thesouthwest.
the ash-flow t u f f s ' a r e n e a x l y h o r i z o n t a l .
'
explanationsfor
o c c u rt ot h ea u t h o r :
A t l e a s t four
this r e v e r s a l of t h e r e g i o n a l westward d i p
(1) east-westcompressional
( 2 ) compaction dips along the. sides
0
N e a r theaxis
of a paleovaliey,
i n t r u s i v e doming, and (4) f a u l t - b l o c kt i l t i n g .
explanation,favoredbyJohnson
.;:
(1955),
.
".-
stresses,
(3)
The f i r s t
seems l e a s t l i k e l y
-
0
91 -
i n view of the t e n s i o n a l stresses presentsincemiddle
T e r t i a r y time
.
The o r i g i n a l o u t c r o p p a t t e r n
siderably modified
of t h e f o l d h a s b e e n
by f a u l t i n g , which makes it d i f f i c u l t t o
e v a l u a t e the r e m a i n i n g p o s s i b i l i t i e s . I f
ern BearMountains
t h e e n t i r e south-
a r e on the southwestern limb'of
a major
1957, p. 37), t h e n it i s
southward-plungingarch(Tonking,
possible that the
Oligoceneashflows
northwest-trendingsynclinalvalley.
were emplaced along a
A s h flowsarehighly
mobile and a r e known t o f i l l i n low a r e a s l i k e w a t e r .
*
support of
con-
t h i s p o s s i b i l i t y , t h e a x i s of t h e f o l d
i s almost
on t r e n d w i t h the a x i s ..o f a s y n c l i n a lp a l e o v a l l e yi n
PuertecitoQuadrangle(Tonking,
1957, p. 24)
.
In,
the
'If t h e ash
f l o w s of the D a t i l Group were emplaced i n t o t h i s p a l e o v a l l e y ,
compaction dips along the
t h e d i p of the ash flows
sides would e x p l a i n t h e r e v e r s a l
and the t r e n d of t h e v a l l e y a x i s
i n the
would e x p l a i n t h e c o n s i s t e n t n o r t h - s o u t h l i n e a t i o n s
' a s h flows.
However, it seems l i k e l y t h a t
the prevolcanic
the low r e l i e f on
s u r f a c e would have been l a r g e l y f i l l e d and
l e v e l l e d by S p e a r s s e d i m e n t a t i o n p r i o r t o e r u p t i o n
flows u n l e s s e r o s i o n
'.
in
of t h e ash
exhumed p a r t of the topography.
An a l t e r n a t i v e e x p . l a n a t i o n for "ihe f o l d i s t h a t the w e s t
X i n i b of t h e s y n c l i n e was domed up by a concealed intrusive
s i m i l a r t o t h e La J e n c i a monzonite s t o c k which warped up p a r t
'.
I
*
o ft h e
east limb.
92
-
The a t t i t u d e s of t h ev o l c a n i cr o c k s. i n
the s o u t h w e s t e r n p a r t of the study area
form a crudely
a r c u a t e p a t t e r n around the h i g h e s t p o i n t
t r e n d i n gr i d g e
which forms p a r t of t h e west limb.
If such
i t s form was l a r g e l y o b s c u r e d
a dome o r i g i n a l l y e x i s t e d ,
by normal
on t h e n o r t h -
f a u l t i n g which collapsed the c e n t e r o f ' t h e in-
t r u s i o n i n t o a graben bounded . o n t h e e a s t s i d e
Mesa f a u l t .
by t h e Hells
Although no surfaceexposuresof
any l a r g e i n -
w e s t of t h e La J e n c i a s t o c k , t h e ash-
trusives have been found
f l o w t u f f s i n the v i c i n i t y of Allen Well are hydrothermally
altered along fracture
zones and most of t h e t u f f s a r e
i f i e d t o some degreeup .. . t o one mile northof
silic-
La J e n c i a Creek.
. T h i s a l t e r a t i o n w a s probably produced by the l o c a l i z a t i o n of
hydrothermal fluids along
numerous f a u l t s , b u t e v i d e n c e
lacking-to determine whether the fluids ascended
is
from an under-
lying intrusive.
Another explanation for the fold
block f a u l t i n g .
i s t i l t i n g due t o
The r e g i o n a ls o u t h w e s t e r l yd i pr e v e r s e s
becomes s o u t h e a s t e r l y a l o n g
and
a line passing diagonally across
t h e s t u d y area from Bear Springs draw
on t h e n o r t h e a s t a c r o s s
t h e n o r t h e r n end of the s y n c l i n a l s t r u c t u r e and southwestward
I
*
beneath the alluvium
towards
Allen
thisline,
whichcoincides
faults, the outcrops
I
Well.
i i t h a zoneof
Immediately
south
northeast-trending
either flatten or dip southeastward
..-
of
as
. . ...
.
i f t i l t i n g and/or drag has
a
p r e v i o u sa t t i t u d e s .
93
"
-
warped t h e s t r a t a o u t
of t h e i r
The w e s t l
i
m
b of the s y n c l i n a ls t r u c t u r e
i s l o c a t e d a t t h e i n t e r s e c t i o n of north-trending Basin
and
Range f a u l t s w i t h n o r t h e a s t - t o - e a s t t r e n d i n g t r a n s v e r s e f a u l t s
w h i c h have dropped
t h e major portion of
the study area into
a
northeast-orientedgraben.Unfortunately,sandhascovered
t h e c r i t i c a l zone of i n t e r s e c t i o n i n the southwestern corner
of the a r e a but t h e topography and t h e r e p e t i t i o n of u n i t s
s u g g e s t s t h a t t h e w e s t limb of t h e s y n c l i n e i s a h o r s t bounded
on t h e west and n o r t h bymajor
downdropped blocks.
The r o t a -
t i o n a l s e n s e of the h o r s t would be away from t h e s e two grabens, whichmightproduce
0
a l o n gt h er i d g eh e l d
the southeastward
up by t h e h o r s t .
,
.. .
tilt observed
,However, t h e r i d g e i s
o v e r 3 miles i n l e n g t h and t h e s o u t h e a s t e r l y d i p s p e r s i s t t o
t h e southboundaryofthe
map a r e a .
T h i s seems t o o g r e a t
a
d i s t a n c e t o e x p l a i n by t i l t i n g along t h e northeast-trending
faults
.
A curious f e a t u r e of t h e western l
imb of t h e s y n c l i n e
i s t h a t the s t r i k e o f t h e b e d s
i s generally transverse to
b o t h t h e s y n c l i n a l a x i s and t o t h e n o r t h - t r e n d i n g f a u l t s
have.truncated
it on t h e w e s t .
which
This makes the s t r u c t u r e d i f -
f i c u l t t o e x p l a i n by either compressional folding
or b l o c k
f a u l t i n g and l e n d s some credence t o an o r i g i n by i n t r u s i v e
doming o r compaction over buried topography.
to the south
.
F u r t h e r mapping
may shed some l i g h t on t h i s problem.
.. . .
..
-
94.
-
ECONOMIC GEOLOGY
The s t u d y a r e a
is situated just north of
t h e Kelly
miningdistrict,animportantproduceroflead,zinc,
s i l v e r ores u n t i l about 1949 ( T i t l e y ,1 9 5 9 ) .
minecalization occurs along the
and
Most of t h e
west s l o p e of the Magdalena
Mountains i n a major f a u l t zonewhich
t r e n d s N 1 O 0 W and i s
o v e r a mile i n width. Past production has been mainly from
v e i n and replacement-type ore d e p o s i t s e x t e n d i n g t o
the south
o f the N i t t Stock f o r a b o u t , t h r e e miles a l o n g t h e f a u l t z o n e .
The € a u l t zoneand
the monzonite stocks extend northward
along t h e w e s t s i d e of Granite
c o r n e r of t h e s t u d y a r e a
Mountain i n t o t h e s o u t h e a s t e r n
where a number of weakly-mineralized
..
.
epithermal veins occur in
t h e v i c i n i t y of t h e La J e n c i a mon-
__-.
zonite stock.
Most of t h e v e i n s i n
the study area are located along
the westernandnorthernmargins
veins vary in
of t h e La Jencia stock.
width f r o m a f r a c t i o n of a n i n c h t o
much as f i v e feet wide.
the western contact
east.
e
-.
,
zones a s
One set o€ v e i n s r o u g h l y p a r a l l e l s
of the s t o c k ; t h e s e t r e n d
north t o N 2O0E and d i p 70Oto 85'east.
from about
The
N 4S0E t o almost east-west
fromabout
due
A second set t r e n d s
and d i p s 55Oto 7S0south-
A t h i r d s e t p a r a l l e l s a s t r o n g N 30°W j o i n t ' set w i t h i n
the stock.
~
Many o f t h e v e i n s a r e l o c a l i z e d n e a r c o n t a c t s o f
. the monzonitewith
t h e t u f f ofBearSprings
x.-.
or w i t h t h e andes-
- 95 Gangue m i n e r a l si n c l u d eq u a r t z ,c a l c i t e ,s i d e r i t e ,
barite.
The q u a r t z was d e p o s i t e d e a r l y i n t h e
v e i n l e t s andmassive
silica.
and
form ofsmall
The caebonateminerals
occupy
v e i n l e t s and c a v i t i e s i n b r e c c i a t e d c h e r t y q u a r t z . A p p a r e n t l y
the e a r l y q u a r t z - f i l l e d v e i n s
carbonateminerals
were reopened and sealed with
and b a r i t e . S u l f i d e m i n e r a l s
are found
p r i m a r i l y i n v e i n s which c u t t h e t u f f of Bear Springs
it i s warpedupand
contact.
*
silicified along
where
a ridge parallel to the
The s u l f i d e si n c l u d es m a l lg r a i n so fp y r i t e ,g a l e n a ,
c h a l c o p y r i t e , and s p h a l e r i t e d i s s e m i n a t e d i n t h e m a s s i v e
quartz.
A t r a c e of malachiFe is a l s op r e s e n ti n
some areas.,
Some of t h e c a l c i t e h a s a b l a c k c o l o r from t h e presence of
.
internally
disseminated
cryptomelane,
a potassium-bearing
form of psilomelane(Charles
Walker, o r a l communication, 1 9 7 1 )
The s u l f i d e - b e a r i n g v e i n s i n
t h e t u f f of Bear Springs have
. b e e n mined t o a shallow depth with shafts connected
The absence of s i g n i f i c a n t amounts o f o r e m i n e r a l s i n
dumps i n d i c a t e s t h a t t h e v e i n s
by d r i f t s .
the
may have been worked f o r t h e i r
gold or silver value.
The La J e n c i a monzonite s t o c k and i t s a s s o c i a t e d v e i n s
appear t o r e p r e s e n t the roof zone of a l a r g e i n t r u s i v e body
vihich was emplaced w i t h i n 2000 f e e t of the Oligocene surface
(C.' E. Chapin, o r a l cormnunication, 1 9 7 1 ) .
I
.~
Evidencefound
in
-
96
-
the study area which s u p p o r t s t h i s c o n c l u s i o n i n c l u d e s t h e
presence of roof pendanks of the tuff of
Goat Spring within
the monzonite, the h i g h s t r a t i g r a p h i c p o s i t i o n o f
J e n c i a stock, and the s t r u c t u r a l l y
area relative to the
lo\Ai
t h e La
p o s i t i o n of t h e s t u d y
Magdalena Mountains.Although
the s i l i c i c
ash-flow t u f f s now exposed a t t h e s u r f a c e were n o t v e r y r e a c t i v e
t o hydrothermal fluids,
the Paleozoic limestones buried
depth might c o n t a i n r e p l a c e m e n t d e p o s i t s s i m i l a r
f a r t h e r south, i n the K e l l y d i s t r i c t .
dicates t h a t such deposits
volcanic cover about
at
t o those
The p r e s e n t stv.dy in-
hiould be buried beneath
a Tertiary
2000 t o 2530 f e e t t h i c k i n t h e v i c i n i t y
of La J e n c i a stock.
..
The L a J a m Peak Andesite i s a l s o mineralized, but not
the S i l v e r H i l l a r e a t o ' t h e s o u t h -
within the study area. In
west 'and a t 'the Baca prospect near
are c u t b y v e i n s
H e l l s Mesa, the a n d e s i t e s
which €0110~
brecciatedfaultzones.
The
veins contain quartz
and c a l c i t e a s gangue minerals and
-
chxysocolla together
w i t h minor amounts of copper darbonates
a n ac h a l c o c i t ea s
ore minerals(Lasky,1932,p.38-41).
deposits are localized along
s u g g e s t s a second period
positionof
the Hells Flesa f a u l t zone, which
o,f mineralization subsequent
t h e , L a J a r a Peak Andesite(24
and the beginning of Basin
Another area
The
t o de-
m. y . , Chapin,1971-a)
and Range f a u l t i n g ;
of m i n e r a l i z a t i o n o c c u r s i n t h e
.-.
._
La J a r a
-
-
97
La J e n c i a
Peak A n d e s i t e s o u t h of the study area, between
e.
Creek and
Magdalena
(Lasky,
1932, p. 41-42;
Koschmann, 1942, p. 163).
Loughlin and
Mapping i n t h i s a r e a
by C. E.
Chapin ( o r a l communication, 1 9 7 1 ) hasrevealed
of west-northw,est,northwest,
the existence
and northeast-trendingveins
c o n t a i n i n gq u a r t z ,b a r i t e ,c a r b o n a t e s ,
and minor amounts of
galena.
The exposed portion of the L a Jencia monzonite stock
..
'
q u i t e fresh and b a r r e n and o f f e r s l i t t l e encouragement for
f i n d i n gd i s s e m i n a t e d . s u l f i d em i n e r a l i z a t i o n .L o u g h l i n
.
.
.
.
Koschmann (1942, p. 1 1 1 - 1 1 2 )
a
is
concluded t h a tm i n e r a l i z a t i o n
i n t h e Kelly d i s t r i c t is n o t r e l a t e d t o t h e
but rather
t o major faults
a great depth.
may e x i s t j u s t west
A more f a v o r a b l e e x p l o r a t i o n t a r g e t
of t h e s t u d y a r e a ,
where two f a c t o r s s u g g e s t
a concealedintrusive:
structure(p-
exposed s t o c k s
which served a s conduits along
which t h e o r e f l u i d s ascendedfrom
. -. -
and
t h e presence o f
(1) t h e w e s t limb o f t h e s y n c l i n a l
90) may be r e l a t e d t o magmatic doming, and
.
.
( 2 ) a small. area of i n t e n s e h y d r o t h e r m a l a l t e r a t i o n o c c u r s
alongCouncil
Rock Arroyo,
of t h e mapped a r e a .
i n t h e extremesouthwestcorner
The p o s t u l a t e d i n t r u s i v e would be
l o c a t e d somewhere between t h e Bear and Gallinas Hountains.
-
*,
98
-
GEOLOGIC I-IISTORY
Prevolcanic I-Iistory
I n l a t e Cretaceous and e a r l y T e r t i a r y t i m e , L a r a n i d e
orogenic forces uplifted the area to the north
of t h e Bear
Mountains t o form the north-trending Lucero arch and Comanche
'thrust b e l t .
Anothermajor
u p l i f t formed t o t h e s o u t h
and
i t s western flank trends north-
southeast of the study area:
eastward through Magdalena and tho southern Bear Mountains.
re-
Erosion of t h e s e h i g h l a n d s d u r i n g e a r l y T e r t i a r y t i m e
s u l t e d i n the d e p o s i t i o n of a s much a s 1 0 0 0 f e e t of a r k o s i c
sediments t o form t h e Baca Formation of Eocene age (Kelley
e
Wood, i946;. Snyder, -1971)
.
.
and
.
By the end of t h e Eocene; erosion had beveled the
up-
lifts and f i l l e d t h e b a s i n a l ' a r e a s t o form a l o w , undulating
s u r f a c ew i t hn o r t h w e s t - t r e n d i n gs y n c l i n a lv a l l e y s .
l a r g e s t of t h e s e v a l l e y s p r o b a b l y
headed west of t h e Lucero
u p l i f t and drained southeastujard through the
s t u a ya r e a
(Tonking,1957,
p . 24)
One o f t h e
center of t h e
.
Volcanic and Postvolcanic ITistory
I n early Oligocene.time,, a v o l c a n i c l a s t i c a l l u v i a l a p r o n
spread. across the Magdalena a r e a a s
a r e s u l t of erosion of
@ a n d e s i t e and l a t i t i c volcanic rocks being erupted
of t h e Mogollon Plateau volcanic province
'.. ..
i n the heart
t o the south
and
.
>
-
m'
southwest.
-
A f t e r d e p o s i t i o n of more than 1 0 0 0 f e e t o fa l l u -
vial detritus, the eruptions
and mudflows, ashflows,
s p r e a do v e r
99
encroached on t h e Magdalena area
and lava flows from local centers
flows
the a l l u v i a la p r o n .P o r p h y r i t i ca n d e s i t e
and t h e t u f f of Nipple Mountain spread over
the alluvial apron to
form a ' v e r y u s e f u l
a wide area of
marker horizon above
t h e t h i c k and monotonous sequence of Eluvial conglomerates.
C o n t i n u e d , t r a n s p o r t of v o l c a n i c d e t r i t u s away from the c e n t e r
of
.
.t h e f i e l d b u r i e d the t u f f of Nipple Mountain.
of t h e e r u p t i o n s ,
The n a t u r e
however, became i n c r e a s i n g l y v i o l e n t
so
t h a t f l u v i a l d e p o s i t i o n gave way t o mudflows and t h e n t o c u l -
a
m i n a t i n ge r u p t i o n s
of l a t i t i c ash f l o w s .
..
.
~
hydrothermal alteration
An ensuingperiod
of
and e r o s i o n is recorded i n t h e h e m a t i t e -
stained conglomerate whichmarks
t h e t o p of the Spears Formation
i n the southern Bear Mountains.
._ -. ~.. .
The ash flows
i n t h e upper Spears signaled
t h e beginning
of a period of c a t a s t r o p h i c p y r o c l a s t i c e r u p t i o n s
t h e Magdalena area beneathnearly
f i r s t of these g r e a t e r u p t i o n s
(30.6 4- 2,8
and 32.4
+-
2000 f e e t of t u f f s .
The
formed. t h e t u f f of Goat Spring
m. y., Weber and Bassett, 1963;
1 . 5 m. y . ,
which b u r i e d
32.1
Weber, 1971, p . 40-41).
t h e lower p a r t . o f a f r a c t i o n a l l y c r y s t a l l i z i n g
to form these verycrystal-richashflows.
+-
1.5 m. y .
F i s s u r e st a p p e d
magmacham33er
The e x a c t , l o c a t i o n .
of t h e source i s
not known, b u t f l o w s t r u c t u r e s . s u g g e s t t h a t
...
-..._
....
~
.
.
-
e
it may havebeen
to the south
100
-
of Magdalena.
I n t h e study area,
the a s h flows may have flowed northward along
a broad paleo-
valley largely filled with Spears sediments.
Welding and cooling of t h e t u f f of Goat Spring was f o l lowedby
t h e e r u p t i o n of p o r p h y r i t i c a n d e s i t e f l o w s . P a r t i a l
c o l l a p s e of t h e magma chamber roof may have sealed off the
A periodofrela-tivequiessence
f i s s u r ev e n t st e m p o r a r i l y .
was marked byminor
erosional reworking of
the s u r f a c e and
the d e p o s i t i o n of a n d e s i t e flows.
Renewal of pyroclastic volcanism began with the eruption
of t h e t'uff of BearSprings
e
which has not yet been dated.
s o u r c e of t h i s ash flow sequence appears
cauldron centered in the
t o have been
The
a major
Mount W i t h i n g t o n a r e a a t t h e n o r t h
end of the San Mated Mountains (Ed Deal, o r a l communication,
1971).
A very hot, c r y s t a l - p o o rs e r i e s
the study area
from the southwest
of ash flows entered
andwelded
t o g e t h e r to form
were
t h e lowercoolingunit.The,upperashflowsofthisunit
pumiceous and v e r y f l u i d a l : t h e y
flowed i n a laminar manner
j u s t b e f o r e coming t o rest as evidence by stretched
and abundant small
pumice
f l o w f o l d s whose axes are perpendicular
to t h e pumice l i n e a t i o n s .
The t u f f o f
La J e n c i a Creek was
erupted contemporaneously w i t h t h e non-pumiceous t u f f s i n t h e
lower tuffofBearSprings,but
advanced only
these c r y s t a l - r i c h a s h f l o w s
as f a r a s t h e s o u t h e r n
..
margin of
the study area.
--
_...
,
.._.
... . .. .
...... ,,-,,, ..
//
.i_I
_,,
=.: L"._I.
.
e
A short period
-
101
. . ., . . . .ij
.
i."
,
and minor e r o s i o n a l
A s e r i e s of
the lowercoolingunit.
thin andesite flows then entered the area along the
low cen-
t r a l a x i s of t h e paleovalley.Continuingeruptions
from t h e
of
Mount Withington cauldron deposited the upper cooling unit
the t u f f of Bear Springs
... .
-
of quiescence followed
r e l i e f was developedon
,_..
""
I
in the central part
of t h e a r e a .
Foliowing t h e d e p o s i t i o n of more a n d e s i t e flows and t h i n
volcaniclastic sandstones,
These crystal-rich ash
t h e t u f f of Allen Well was'erupted.
flows r e p r e s e n t t h e y o u n g e s t D a t i l
Other ash
Group volcanicrocksfoundwithinthestudyarea.
a
flows may have been emplaced d u r i n g t h e waning stages of volcanism and s t r i p p e d b y e r o s i o n
.end of t h i s period of volcanism,
much a s f o u r m i l l i o n y e a r s ,
from the sttldyarea.
By t h e ,
which may havespanned
the prevolcanic drainages
as
were
l a r g e l y f i l l e d and t h e a r e a w a s p a r t of an extensive constructional plain.
Tn l a t e Oligocene t i m e , the volcanic rocks
were broken
and t i l t e d by block f a u l t i n g . a l o n g a wide n o r t h - t r e n d i n g f a u l t
zone. Faulting
may h a v e b e e n . i n i t i a t e d b y
of magma d u r i n g t h e l a s t s t a g e s
of e f f u s i v e a c t i v i t y .
ber of d i k e s and s t o c k s w e r e intruded along
.e
the upward movement
A num-
t h i s f a u l t zone,
i n c l u d i n g the N i t t and Anchor Canyon monzonite s t o c k s (28.0
1.4 m. y.and
I
28.3
2
-
i-
1 . 4 m. y . , . r e s p e c t i v e l y ; Weber and
B a s s e t t , 1963, p. 2 2 0 ) . The La J e n c i a monzonitestock,
vihich,
._.
"
- 102 h a sn o t
been d a t e d , was emplaced c l o s e t o t h e s u r f a c e a t
this time.
Eydrothermalfluidsascendingalongthemargins
of t h e s e stocks deposited quartz veins
and minor lovi-
temperature sulfides in fractures.
A f t e r t h i s . p e r i o d .of s t r u c t u r a l m o d i f i c a t i o n , e r o s i o n
dissected the volcanic rocks into
Volcanism began again
sion of the
a low undulating topography.
i n e a r l y Miocene time 4 t h t h e e x t r u -
La Sara PeakFormation
(23.8
-1-
-
1 . 2 m. y.
, Chapin,
1971-a).Thinbasalticandesiteflowsfloodedthetopography;
t h e s e s p o r a d i c - e r u p t i o n s were separated by local surface
'*
working.and
re-
sedimentation i n lava-dammed streams.
After a p e r i o d of time. s u f f i c i e ntto
d e p o s iat s
a s two thousand f e e t of lava flows
much
~.
and sediments, the volcanic
a c t i v i t y b e g a n t o subside and f a u l t i n g r e l a t e d t o t h e o p e n i n g
of t h e R i o Grande r i f t began.
t i l t e d t o t h e w e s t by normal
of the
The a r e a was u p l i f t e d and
f a u l t s a l o n g ' t h e west boundary
La Sencia graben. While the area
was b e i n g u p l i f t e d ,
a set of o b l i q u e - s l i p f a u l t s dropped the southern Bear
t a i n s i n t o a northeast-trending trough
Moun-
bounded by t h e h i g h
Magdalena Range on t h e s o u t h .
Concurrently with uplift,
PeakAndesite
'
a great thickness of
was stripped by erosion
north-trending graben
from t h e h i g h a r e a s .
bounded o n t h e e a s t s i d e b y t h e
Mesa f a u l t p r e s e r v e d a t h i c k s e c t i o n o f t h e
'...
..
La Sara
Hells
La J a r a Peak
A
-
103
-
Formationand
became filled w i t h the fanglomarateof
Lake Canyon.
A i r f a l l t u f f s interbedded w i t h t h e fanglom-
exates indicate that volcanism
Dry
may have accompanied f a u l t i n g
i n the nearby area.
During t h e l a s t s t a g e
of deformation
of D r y Lake Canyon was u p l i f t e d , t i l t e d ,
removed by e r o s i o n .I nQ u a t e r n a r y
and p a r t i a l l y
time, an extendedperiod
of e r o s i o n d i s s e c t e d t h e t i l t e d f a u l t b l o c k s
extensive pediment surfaces
the fanglomerate
and developed
which were l a t e r b u r i e d by sand
blown i n t o t h e a r e a from ihe southwest, possibly
San Augustin Plains
and La Jencia Creek.
-.
.. ..
I,
__
.-
'. ._
from t h e
-
104
-
CONCLUSIONS
"he southern Bear Mountains have
l e a s t two major periods
been t h e s i t e of a t
of volcanism separated
of s e v e r a lm i l l i o ny e a r s :
by a span
(1) an Oligoceneperiod
l a t i t i c and a n d e s i t i c e r u p t i o n s
of e a r l y
and l a t e r r h y o l i t i c pyro-
c l a s t i c e r u p t i o n s which produced t h e rocks known a s t h e
D a t i l Group, and ( 2 ) a Miocene period of b a s a l t i c a n d e s i t e
volcanism which producedthe
La J a r a PeakFormation.
conibined t h i c k n e s s o f t h e s e T e r t i a r y v o l c a n i c r o c k s
c l a s t i c d e r i v a t i v e s may be s i x t h o u s a n d f e e t o r
The H e l l s Mesa Formation, whichforms
The
and t h e i r
more.
the upper part
of t h e D a t i l Grwcp is t h i c k e r and mare 'varied than previous-
l y d e s c r i b e d byTonking
~
f l o w units are occupied
by t h i n andesite flows
r o c k s .C r y s t a l - p o o rr h y o l i t e s
. ..
Cooling breaks betweenash
(1957).
. ..
and c l a s t i c
~
i
and c r y s t a l - r i c hq u a r t zl a -
. .
t i t e s form t w o d i s t i n c t l y d i f f e r e n t t y p e s
These c o n t r a s t i n g t u f f s a p p e a r t o
andRoberts
of ash-flow
tuffs.
f i t t h e model of P e t e r s o n
(1963), by which the pumiceous r h y o l i t e s a r e
thought t o be erupted from v e n t s which t a p t h e v o l a t i l e - r i c h
upper level
of f r a c t i o n a t i n g magma chamber, w h i l e t h e c r y s t a l -
rich q u a r t z l a t i t e s a r e t h o u g h t t o
be d e r i v e d f r 0 m . a c r y s t a l
mush accumulating i n t h e lower p a r t of t h e magma chamber.
two v a r i e t i e s of ash flows
-""
were erupted by overlapping and
..
The
simu1,taneous a c t i v i t y .
Laminar f l o w s t r u c t u r e si n -
dicate that the source of the tuff of Bear Springs lay
some-
where t o t h e s o u t h o r 'southwest.
Following, or p o s s i b l y accompanying, t h e l a s t s t a g e s o f
Oligocenevolcanism,north-trending
t h e rocks westward
sal intrusives.
area, the
normal f a u l t s t i l t e d
and l o c a l i z e d t h e emplacement of hypabys-
The l a r g e s t of t h e s e i n t r u s i v e s i n
La J e n c i a monzonitestock,
the study
was f o r c e f u l l y i n j e c t e d
close t o . t h e s u r f a c e and t i l t e d the volcanic rocks along
.
westernmargin.Hydrothermal
its
.
f l u i d sl a t e rd e p o s i t e de p i -
thermal veins in fractures
and f a u l t zones near the stock.
.The vein&.are generally... . b a r r e n of ore, and t h e p r e s e n t s t u d y
indicates that
any lead-zinc replacement deposits in the
Paleozoic limestones
would be buried beneath
a Tertiary vol-
..
caniccoverabout
2000- t o 3500-feet-thick
in the southeastern
p a r t of t h e a r e a and from 4000- t o 6000-feet-thick
western corner of
i n the south-
t h e a r e a where a t h i c k s e c t i o n o f t h e
La
...
J a r a Peak Formation has been preserved
Eruption of
by down-faulting.
the La J a r a Peak Andesite began i n e a r l y
Miocene time a f t e r a n e r o s i o n a l h i a t u s
of a b o u t f o u r m i l l i o n
..
years.
*
The flows were eruptedontoanerosionalsurfaceof
l o w r e l i e f fromfissure-typevents.Intermittentvolcanism
and minor sedimentationover
an unknown t i m e spandeposited
as 'mu-ch as 2500 t o 3000 f e e t . o f m a t e r i a l i n
t h e study area.
.
.
.....................
. . . . . . . . . . . .....,..".
i
.
.
~ .. .~............
.,. . . . . . . . . .. .. ..-.
.. .. .. . . . . .. . . . . . . .
"
- 106
.. -~.....
"
"
.-:-.,;...-.. :--..::;:.+
..........
-
I n l a t e Miocene time t h e a r e a was transected by two
major fault zones related to the opening of
rift,
The volcanicrocks
north-trending blocks
w e s t shoulderof
t h e RioGran6e
were u p l i f t e d and b r o k e n i n t o
by Basin andRange
faulting along the
t h e ri€t., A t approximatelythe
n0rtheas.t-trending faults at the northeast
same t i m e ,
end o f t h e San
Augustin arm of the rift dropped p a r t of t h e s t u d y a r e a a t
l e a s t one thousand
f e e t r e l a t i v e t o the Bkar and Magdalena
uplifts.
...
,
C"
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. . ...