OPEN FILE REPORT 106
.GEOLOGY OF THESOUTHEASTERN
MAGDALENI1
MOUWTAINS, SOCOR3OCOUNTY,
-Y
NEW I4EX:ICO
l?
David M. P e t t y
Submitted i n P a r t i a l F u l f i l l m e n t
of t h e Requirements for theDegree
Master of ScienceinGeology
of
N e w Mexico I n s t i t u t e of Mining and Technology
Socorro, N e w Mexico
May, 1 9 7 9
This thesis is accepted on behzt l f oE the faculty of
I n s t i t u t e by the follming comnittee:
CONTENTS
KY
..
. .
Page
...............................................
.INTRODUCTION ............................................
ABSTRACT
....................
Location and Accessibility.
......................
Physiographic Setting ...........................
Previous Work ...................................
1
Purpose of the Investigation
1
4
6
.........................
Acknowledgments .................................
STRATIGPAPKY AND PETROGRAPHY
..........................
Prc-Tertiary Rocks...............................
a
Method of'Investigation
9
1.0
10
.........
Tertiary Volcanic and Sedimentary Rocks
...
.
.
...........................
Track" Andesite
...............
. .
. .
Spears Formation
"Turkey
1
..............
Tuff ofGranite~Mountain
11
12
1.5
17
. .
............................
Hells Mesa Tuff
.............................
Unit of Hardy Ridge
..........................
Rhyolite Lava5........................
Sedimentary Rocks and Tuffs
......;....
Upper
Unit
20
. .
...............................
A-L
PeakTuff.
22
26
27
28
29
Interbedded Rhyolit.?:Lavas, Sandstones,
..............................
Sixmile Canyon
.....................
and Tuffs
Unit
I
of
43
45
Lower Intervalof Tuffs, Sandstones and
Lavas
..................................
.
i
47
'
Page
........................
RhyoliteLavas................
Andesite Lavas
49
Sanidine
53
Andesite Lavas
.........................
.
58
.............
Canyon
...............
.sandstones"*and Siltstones
58
Tuff of Caronita
59
~
.....................
Upper Member......................
Sandstones ............................
Andesite Lavas ........................
..
Tuff of Lemitar.Mountains ....................
Lower Member................... .......
Upper Nember ...........................
Tuffs 'and Sandstones ........ ..............
Rhyolite Lavas .............................
Basalt and Basaltic Andesite Lavas
.........
Rhyolite Dome ..............................
Tuff.of South Canyon .......................
Basalt and Basaltic Andesite Lavas
..........
Lower Popotosa Formation...................
Pre-Lava Ash-Flow Tuffs....................
Lower P4ember
. .
.w
.
61
'
.
64
66
67
~
i
67
70
72
75
76
71
....
.79
80
85
88
-93
....$................ 93
95
Upper Pound Ranch Lavas
....................
98
Tertiary Intrusive Rocks.........................
-98
'White Rhyolite Dikes.......................
Rhyolite Intrusions .........................
100
Dikes
Monzonite
.............................
102
Lower Pound-RanchLavas
~.
'G
. .
ii
. . .
:
....
-
#
ILLUSTRATIONS
PLATE
PAGE
I.
Geologic map and sections .of the southeastern
.. County, New-MexMagdalena Mountains, Socorro
ico ..........................................in
pocket
11.
Structural features, intrusions, mines and
prospects in the southeastern Magdalena Mounin.pocket
tains, Socorro County, New Mexico
.........
FIGURE
1.
.I
Location map of the study area
.................
2
.2. Location map' showing physiographic features of
the study area, surrounding studies and access
routes
3
3.
.........................................
Panoramic view of the study area
......... ;..
I
2 .
5
4. Oligocene stratigraphic section f$r the southeastern Magdalena Mountains
13
Photomicrograph of the "tcrkey track," andesite.
18
....................
.
5.
6.
"
7a.
7b.
-. 8.
9.
._
........
Lineated pumice in the A-L Peak Tuff
..........
I
Close-up of the outcrop shown in Figure ....
7a
Photomicrograph of the A-L Peak Tuff..........
View of the eastern rim of Ryan Hill Canyon
...
Primary flow fold in the ALL Peak!Tuff
35
37
37
42
-
55
-10. Example of flow banding in the sanidine rhy~.
olitelava
56
....................................
Red pumice tn the upper member
of;thetuff of
Lemi'iarMountains
74
12. Basal, poorly welded, moderately lithic-rich
zone of the tuff of South Canyon 1.
83
11.
..............................
!.............
13. Miocene to Holocene stratigraphic section for
rocks in the eastern Magdalena Mountains and
west-central Chupadera Mountains
d.............
86
5;
14. Outcrop
....
of Lower Popotosa mudflow!deposits.
I
V
I
92
-.. .
Page
Mafic Dikes
................................
103
Tertiary-Quaternary Rocks'........................
Sierre
Ladrones
Sediments
.,................
. 104
...
Formation
.............................
!
Alluvial
Alluvium
.................................
Fan Deposits......................
..................................
I
110
110
.......................................
, ,'.
STRUCTURE ............................................
."
.
1.13
Structure
.
Local Structure
North
113
,
..................................
Baldy
117
,I
........ :.............
118
...................
119
Cauldron
Sawmill Canyon Cauldron
!
................
I ."
Faulting............................
I
CauldronSocorro
Block
...
.Gold Mine
Area
i . . . . . . . . .
126
129
......... !.............
137
I
Southern
Marginof the
SQwmill
'Canyon
I
.............................
13.8
ECONOMIC GEOLOGY....................................
14o
. ,
Gold Mine Area
................... !............. 142
Manganese Mineralization........................
147
CCNCLUSIONS ...........................................
- 152
Cauldron
I
I
~
I
. ,
....................................
........................
Stratigraphy
Structure
152
i..............
iii
,
I
,
.
c
111
...............................
Regional
106
110
Talus
,
,104
104
Easaltic Lavas ........................
Quaternary Units
,
.
155
Page
Econom:kc Geology
.................................
157
159
..
. .
. .
..
.
-.
~
!
..
:
Page
Figure
of t h e ,lower Popotosa
15.
Close-up of an outcrop
16.
Whiterhyolitedike
17.
Outcropof Sierra LadronesFormaeion
stream bed
I.
............................
19.
20.
21.
22.
23.
25.
105
108
112
I
SocIorro-Magdalena
,
...........................................
V i e w from t h e g o l d mine r o a d s h o i i n g t h e
n o r t h e r n m a r g i n - o f t h e . . Sawmill Ca,nyon c a u l d r o n .
Geometricmodelpresented
by Lipman' ( 1 9 7 6 ) t o
ill.ustratetheformationofcauld'roncollapse
1.
breccias
........................................
,
115
122
125
V i e w o fh o g b a c k sf o r m e db yf a u l t e d ,e a s t -
d i p p i n gs t r a t a
24.
map o f t h e
99
.
I
Regionalstructure
area
92
in.a
......;. .....
V i e w l o o k i n g east from.Molino Pedk ............
Active talus deposit
i n Ryan hill^ Canyo:1 .......
......................
1.
18.
..
.................................
130
Model developedbyMorton
'and d c k (1975) t o
show p r o g r e s s i v e b l o c k t i l t i n g
132
Boulder of b a n d e d , b l a c k a n d w h i t e c a l c i t e a n d
minor manganese minerals
148
.................
.......................
..
-,':,,
ABSTRACT
.t
The-southeasternMagdalena
Mountains
have
a
complex
middle to late Tertiary geologic history. This study
area is located entirely within the Norkh Baldy cauldron,
'
I
from which the Hells Mesa Tuff erupted
about 3 2 to 33 m.y. .
!
ago.
The Hells.MesaTuff is at least2000 feet thick in
\
the study.area. The unit
of Hardy Ridge (rhyolite lavas
.
I
and domes, and
volcaniclastic sedimentary rocks) filled
n
-1
~
the
North
Baldy
cauldron
after
the
Hells
MesawasTuff
I
erupted.
!
!
The A-L Peak Tuff (pinnaclesmembq
3)
erupted from
l
the Sawmill Canyon cauldron about
3 2 m.y. ago.
is at least 3 0 0 0 feet
thick
in
the
This tuff
soutkeastern
Magdalena
2500
Mountains. The unit of.SixmileCanyon (as much as
feet thick) is a heterogeneous assemblage ofrhyolite.and
I
:.
..
.
andesite lavas, tuffs, and volcaniclagtic sedimentary rocks
that
filled
the
Sawmill
Canyon
cauldron
A-L the
Peak
after
Tuff was erupted.
The
part
of
the
Sawmill
delineated so far-isroughly
Canyon
elongate
cauldron
that
has
been
in
an east-northeast
direction. The east-northeast trending Morenci lineament
may have influenced the shape of the Saiimill Canyon cauldron.
The
northern
cauldron
margin
transects
!
h
e
northwestern
portion of this study area; part of thiL margin is the ring
'
fracture
The
and
southern
part
ring
of
it is
the
fracture
topographic
of
the
rim
of the
Sawmill
vii
I
1
cauldron.
Canyon
cauldron
t
i s exposed i n Ryan H i l l Canyon; e l s e w h e r e , it i s covered
/..,
..
rim hasbeen
mapped s o u t h o f t h i s s t u d y , a r e a .
b e t w e e nt h es o u t h e r nr i n gf r a c t u r e
.
sosliern topographic
A p o r t i o no ft h e
w i t hy o u n g e rr o c k s .
graphic r i m indicates 'that
.A widezone
and t h e .s o u t h e r n. t o p o -
consider able^ s l u m p i n g a n d c a v i n g '
I
,
o ft h ec a u l d r o nv j a l lt o o kp l a c e .A p p a r k n t l y ,
l i t t l e o'rno
I
'
in
slumping occurred along the .northern c+ildron margin
.
this study area.
The'
-
tuff
I
,-
.
!
of Lemitar Mountains erupted from the Socorro
1
c a u l d r o na b o u t
:
27 m.y.
ago.
.
.
The wester+ marginof
.
this
1
i t d o e s , it
c a u l d r o n may p a s st h r o u g ht h i ss t u d ya r e a .I f
. .. .
l
i s a hingezonean
a t r a p - d o o rc a u l d r o n t
The t u f fo fL e m i t a r
Mountainsthickensslightlyacrossthiscauldronmargin,
b u t it i s u n u s u a l l y. .,-t h. .i c ko u t s i d et h ec a u l d r o n .T h i s
.:*
be because the tuff
may
I
o f Lemitar Mountains "partially filled
a d e p r e s s i o nr e m a i n i n g
l
in^ t h e 'S a w m i l l
.
.
Canyon c a u l d r o n .
!
.
'
Sedim.entary 'rocks a n d i n t e r b e d d e d i a v a f l o w s i n t h e
miles i n t h e eastern
S a n t a Fe Group c o v e r s e v e r a l s q u a r e
portion of the study area.
:
- A number o f , r e g i o n a l and l o c a l
u n c o n f o r m i t i e se x i s tb e t w e e nt h ey o u n g e ' s tr o c k so ft h eS a n t a
FeGroupand
theOligocenerocks.
..
i
Numerous f a u l t s . a s s o c i a t e d w i t h e x t e n s i o n o f t h e R i o
Grande r i f t o c c u r i n the s t u d ya r e ? .
The transverse s h e a r
I
zoxe,whichtransectsthenorthwestern$ortionofthisstudy
I
area, s e p a r a t e s f i e l d s o f o p p o s i t e l y t i l t e d . f a u l t b l o c k s .
Most o f t h i s s t u d y a r e a
&>
transverse
is south of the
shear
east And most of t h ef a u l t s
z o n ew
, h e r et h es t r a t ad i pt ot h e
I
I
viii
I
' I
. ,
.
..
. . ,..
. .,.
I
I
dip
to
western
.
the
wes+&
.However,'someof,the strata
portion
of
in
I
the
the
north.I
study
areat$e(on
transverse shear
,
zone) dip to the west. The structural heformation that
I
!
produces
the
tilted
strata
'has
been
called
"domino-style''
!
faulting. This style involves progressive tilting0.f
'
,
both the strata and the'faults. New
fahts form when the
I
older
faults
become
too
1
shallow
€or
,
movement
cauldron
was
the
toonoccur
.them.
The
Sawmill
Canyon
major
structural
..
control
for
most
of
the
mineralization
I .
in
this
study
area.
Gold and silver mineralization occurs aiong the northern
cauldron margin. This mineralizction is closely associated
..
with
white
faults
and
rhyolite
the
dikes
that
east-trending
intruded~
ring
fractkre
.
north-trending
of
the
Sawmill:
Canyon cauidron. Manganese 'rnineralizat$.on occurs a in
wide
zone
between
the
southern
ring
fracture
and
.
topographic rim of the Sawmill Canyon cauldron.It is
especially
of
the
intense
Sawmill
at
Canyon
the.
intersection
I
.cauldron
with
,
.
of
the
ring
has
Socorro cauldron. A regional potassium; metasomatism
. .
the . southern
.. .
.,
. ..,
fracture
hinge
o f the zone
the'
affected some of the rocks
in the study area..
.
~
. .
. .
.
..
..
r
.
.
c
..
..
- -
.
....
!
,
~
. .
I
..
INTRODUCTION
'
. .
I
,
.
,
Purpose 'of the
Investigation
I
1
,
,. .
The'objectives of.th.is study ara:i1) -to define the
I
..
volcanic,
.and
sedimentary
Magdalena
Mountains
and
stratigraphy
the
o
f
southeastern
'
known
fitintolthe
itI ,
to
strati-
I
,
.
graphic section in the Socorro-Magdalena region,
2 ) to
I
map
and
interpret
located
within
the
the
..
thesis
extensional
structures
related
to
the
mineral
potential
Location
and
The
geology
of overlapping
area,
map
.+nterpret
and
of
this
the
'I'
area.
area
covers
.
I
i
about
35 sqvare
miles
in
the
southeasternMagdalenaMountains,Socorro'.County,New
is^, bounded
on the
..
Mexico. The area shown in' Figure
1
northwest
by
Timber
may
I
Accessibility
study
t,o
I
thesis 'area that
!
Grande
rift,-md 4) it0 evaluate the
Rio
within
3)
cauldrons
Peak,
on the
southhest.
I
by
.
.'
I'talian
Peak, and on the east by the Chupaderal Mountains.
Figure
I
2 shows
the general physiographic feathres
of the area.,
.
. .
About.one-half~
of
surrounding studies, and access routes1
the
study
study
area in
is the
area
can
be
Cibola
Forest.
National
The'
reached U.S.
from Highway 60 by
following
I
to the Water Canyon Campground.An unForest Route 235
paved
portionof Forest
campground
b
This road
to
the
passes
Route
Langmuir
235
continues
Laboratory
on South
through
the northwestern
.
,
,,
.
.
beyond
Baldy.
portionof the
..
the
.
-.
be
NEW.hIEXIC0
..
I
,
-
.
.
_I
..
.
.
..
.
.
Figure ,I.
Location
:
map o f study are2
.-
.
.
_.
F i g u r e 2. L o c a t i o n map s h o w i n gp h y s i o g r a p h i cf e a t u r e so ft h e . s t u d y
area,surroundingstudies
andacc.es,sroutes.
f
4
4
,
study area (secs. 3 ,
',.. 9
and 5, T. 4 S., R. 3 - W . , unsurveyed,
and sec. .34, T. 3 S . , .R. 3 W.)
.can
be
reached
road
passes.
until
can
to
60
it
the
the
be
Pound
which
Marcia1
by
leaves
. _
Ranch
and
following
exit,
then
This
conti.nues
Forest 4Route
3 ~ 2 . . The
with.
reached
San
road
..
-about
6 . 3 miles -we&] of Socorro.
connects
also
The eastern portion
following.
an unpaved
.by
~.
U. S. Highway
.
south-southwest
eastern
Interstate
following.
portion
Highway
Forest
25
Route
472 northwest.
Physiographic Setting
.
A high,
+..
north-trending ridge
connecting'Timber.Peak
(10,510.feet) .and Italian
Peak ( 9 , 0 5 2 feet) forms.the
western boundary of the study area. South danyon and Sixmile
Canyon
$rain
northeastward
from
the,
north
.
.end
.,
:.. .
of
, '
..
, .
..
ridge; Ryan Hill Canyon, the largest canyon in the study
:, ' .:,.
.I .
area, drains southeastward along the east side of this
ridge. About 2500 feet of topographic relief exists
between Timber Peak and .the floor of Sixmile Canyon,
1.1
'.
miles to the east. The lowest' elevations .'(5600-6200 feet)
are
found
in
the
north-trending
graben
that
separates
the
3 is a panoramic
Magdalena and Chupadera Mountains. Figure.
view
which
Most
of
shows
the
some
of
the
southeastern
dominated by east-tilted
fault
features
mentioned
above.
Magdalena
Mountains
are
blocks,
kome of which
form
prominent hogbacks. A transverse shear zone (Chapin and
&
others, 1978) transectsthenorthwesternportion
of the
't
I
F i g u r e 3. Panoramic v.iew o € t h et h e s i s area f r o mt h es o u t h e a s t .
The n o r t h t r e n d i n gr i d g ec o n n e c t i 7 . gT i m b e rP e a k
( 1 0 , 5 1 0 f e e t ) and I t a l i a nP e a k( 9 , 0 5 2
f e e t )f o r m st h ew e s t e r nb o u n d a r yo ft h et h e s i s
area. MolinoPeak(8,902
f e e t ) i s t h e h i g h e s t summit e a s t of Ryan H i l l Canyon.The
eastern boundary
of t h et h e s j . s area i s l o c a t e di nt h ew e s t e r nC h u p a d e r aM o u n t a i n s .
A better
view o ft h eC h u p a d e r aE o u n t a i ' n sc a n
be .seen i n F i g u r e 1 8 . The d i s t a n c e
between I t a l l a n P e a k a n d t h e e a s t e r n ' b o u n d a r y
i s a b o u t 8 . 5 miles.
6
*
,f"..
-
area. Morth of this transverse shear zone,.
the'strata
*;.
dip
to
found
the'west. Five
overlapping
~
in
the
Magdalena
..
.
cauldrons
Mountains;'the
.
have
study
been
area
includes
-
portions of threeof these cauldrons. In the easternmost
portion of the study area, nearly flat-lying Pliocene
(?)
basalts
cap
Previous
older
sediment6
of
the Fe
Santa
Group.,
Work
Winchester (1920) examirsd the volcanic rocks along
-
the
northern
and-
named
edge
of the
them.
Mountains.
Datil-Mogollon
the
Datil
volcanic
Formation
after
field
the
Datil
His type section, however, is at the north
. .
end o f .the .BearMountains. Laughlin and"Koschmann (1942)
publishedadetailed
z
study.ofthegeologyandore
deposits of.the Kelly mining district..Kalish (.1953)
studied
the
geology
of, the
Water
Canyon
area
northeast
of this study area. Miesch (1956) mapped the Luis Lopez
manganese
district
and
the
northern
-
Chupadera
, 1.
Mountains.
Tonking (1957) published the first .detailed of
study
the
volcanic
rocks
He mapped
the
Fdrmation
into
in
Puertecito
. .
three
the
Datil
Formationofnorth
Magdalena.
quadrangle
members;
and
divided
the
Datil
oldest
to youngest - .
from
these are the Spears, Hells Mesa, and La Jara Peak 'Members.
Givens (1957) mapped the Dog Spring Quadrangle and divided
the Hells Mesa Member into seven mappable units. Weber
(1963) excluded the La Jara Peak Member from the Datil
I
.
.
.
i
1
..
,
7
I
I
.
..
.
.
.
.
.*..
Formation. Later, Weber (1971) elevated.%he Datil Formation to group status..' T.he Spears, Hells Mesa, and La
Jara
Peak
Members
were
raised
to
formational
status
by Chapin (1971a). .Brown (1972) mapped &he southern
Bear Mountains and subdivided the Hells Mesa Formation
into two informal units: the tuff of Goat Springs and the.
tuff of Bear Springs. The Hells Nesa Formation has since
been restrictedto.the quartz-rich, crystal-rich ash-flow
tuffs
that
are
the
basal
unit
of
Tonking's
Hells
Mesa
Memljer and the tuff of Goat Springs ,of Brown (Chapin,
1974;
Deal and Rhodes, 1976). The tuff of Bear Springs was
renamed
theA-L Peak
Rhyolite
by
Deal
and
Rhodes
(1976).
-Elston (1976) recommended that the term Datil Group be
abandoned.
Recently, a number of theses and dissertations
have.
been
completedin
further
defined
... Figure 2 shows
preparation
the
the
the
in
Socorro-Magdalena
volcanic
location
the
and
of
Magdalena
area .have
which
sedimentary
published
and
Mountains
in
order.
to
evaluate
mapsmaps
andin
Chupadera
Krewedl (1974) studied the geology
of'the.
the
stratigraphy.
Mountains.
central
Magdalena
economic
potential
of
that area. A portion of the southern part of Krewedl's
area was remapped for this study. Blakestad (1978) mapped
hnd
in
described
the
studied
the
geology the.Kelly~min.ing
of
district
northwestern
the
Magdalena
of the
geology
Mountains
southeastern
while
Magdalena
Osburn
(1978)
Mountains
just north of this thesis area. Bowring (in preparation)
.
.
. 8
mappeir the upper Sawmill
Canyon a r e a c o n c u r r e n t l y w i t h
t
ofTimberPeak.Roth
( i np r o g r e s s )
p o r t i o no fS a w m i l l
crest
common b o u n d a r y a l o n g t h e
thisstud,andshared
is mapping t h e ' l o w e r
t h i s study
Canyon, west-sout.bwestof
a r e a .O t h e rr e c e n ts t u d i e si nt h eS o c o r r o - M a g d a l e n aa r e a
i n c l u d et h o s e
by Park ( 1 9 7 1 ) , , B r u n i n g( 1 9 7 3 ) ,D e a l( 1 9 7 3 ) ,
Simon (1973) ,, Woodward (1973) , Chamberlin ( 1 9 7 4 ) , Siemers
'
( 1 9 7 4 , 1 9 7 8 ) ,D e a l
(1976), Spradlin (1976),
andRhodes'
N i l k i n s o n ( 1 9 7 6 ) , and I o v e n i t t i ( 1 9 7 7 ) .
The c o n t r i b u t i o n s
A.
will b e d i s c u s s e d i t i m o r e
oftheseandotherauthors
detail in the sections
Thisstudyarea
structure.
on s t r a t i g r a p h y a n d
is l o c a t e d i n t h e R i o G r a n d e r i f t ;
summary
..
p a p e r s on t h i s c o n t i n e n t a l ' r i f t h a v e b e e n p u b l i s h e d
by
Chapin(1971b),ChapinandSeager(1975),andCordell
(1978).
.Methodsof
.
.
. -. . .
.
.
..
Investigation.
Detailed geologic mapping.was done
1 : 2 4 , 0 0 0 ' ona
a t a scale of
b a s e map c o n s i s t i n g of p a r t s of t h e Molino
PeakandSouthBaldy
7 1/2-minute .quadrangles.
photographs""'of t h e GS-VAVL series (8-6-64;
and1:8,276)andthe
GS-V"A
~.
series (3-7-56;
were u s e d a s a n a i d i n l o c a t i n g o u t c r o p s a n d
Aerial
scale, 1:34',682
s c a l e ,1 : 9 , 3 1 7 )
making
s t r u c t u r a li n t e r p r e t a t i o n s .A p p r o x i m a t e l yn i n et ot e n
months of f i e l d work were donebetween
J u n e , 1 9 7 7 and
November, 1978.
Ninety-three thin sections
were made fromsamples
I
-
i
collected throughout the study area. Most of the thin *w
sectjlons were
stained
for
potassivT. sodium
. with cobal*
tinitrite. (after Deer, Howie and Zussman;
1966, p. 311).
Petrographic analyses'were
done
on
a
Zeiss
binocular
microscope. All samples were described usin9 the petrographic classification of Travis'(1955).. Visual estimates
were
used
to
determine
the
min_eral
.
m"
abundances.
.
.
Acknowledgements
.
L
-:
The
who
author
made
would
suggestipns
like
and
to
thank
provided
the
many
individuals
'information
for
this
project. Graduate students conducting geologic studies
in
G.R.
the
Socorro-Magdalena
Osbiurn who
provided
area
were
valuable
very
aid
helpful,
in
'the
especially
field
and
office discussions. He also took the photomicrographs
.
.
presented in this thesis. Dr. A.J. Budding and Dr. K.C..
Condie
the
servedon
my
thesis
committee
and
critically
. .
read
manuscript.
The
author
would
like
to
extend
special
thanks
to
Dr. Charles E. Chapin, who originally suggested this
problem and then served as thesis advisor for this study.
Financial
by
theNew
support
and'field
Mexico
B'ureau'of
.
to
whom
the
author
is
transportation
Mines
.
grateful.
and
were
Mineral
provided
Resources,
. .
in
10
.b
PETROGRAPHY
STRATIGRAPHY
AND
6 '!
h
*
Pre-Tertiary
. .
Rocks
Pre-Tertiary rocks are not exposed in the study
area;
"
but they do drop out about one-half mile north of the study.,.'
. .'
~.
area. . The
'pre-Tertiary
.rocks
in. the
central
Magdalena.
-. .
. .
Mountains have been described recently by Krewedl'(l974);
.
-.
Siemers (1974, 1978)
, and Osburn (1978).
..
'at,
The
closest
Precambrian
rocks
crop
out
along
Water
.Canyon about 1.5 miles northeast
of the study area. These
rocks
are
metasedimentary
rocks'mapped
as
argillite
by
Krewedl (,1974). Further north, Krewedl also mapped
,
Precambrian granite intruding the argillite.' Sumner
(in preparation) has mapped the Precambrian rocks in the
,
north-central
Magdalena
.
Mountains.
. .
The' Paleozoic rocks in the Magdalena Mountains '*.:
have
'
been
discussed
by
Laughlin
and
Koschmann
(1942),
Arnstrong
(1958, 1963), Kottlowski (1960, 1963), Krewedl (1974)
,'
Siemers
(1974,1978), .Blakestad
'
(1978)
, and
Osburn
(1978)
Krewedl (1974) divided the Paleozoic rocks in the central
Magdalena Mountains,intofive
mapunits:,
these
Kelly
is
the
Mississippian
The
oldestq f '
Limestone, i.s.a
which
light-bluish-gray, medium- to coarse-grained, crinoidal
biosparrite
andis )approximately
80-100
.feet
thick
in the
Water Canyon area. Above this is the Sandia Formation
of
k . .
Pennsylvanian age, which Krewedl (1974) divided ainto
I
lower quartzite .member and
a n upper shale member. The
.
. .
.
.
.
.
11
.
-.
1
Sandia
5'50 feet thick in the
. . Formation is approximately
Water Canyon area. The thickest Paleozoic unit in the
Water Canyon area-is %he
which
.
.
isa s much
Mountains
a
Pennsylvanian
as1800 feet
thick
(Siemers;1 9 7 8 ) . .. The
Madera
in
Madera
the
'Limestone,
central
Limestone
Magdalena
is
mostly
thick,homogeneou,? sequenceof gray to black micritic.
-
limestone, although' the lower part contains some interbedded
'quartzose
sandstones..
'The contact
-.,
betyeen
the
Madera
Limeskone and the underlying Sandia sandstones and shales
-.
is gradational; Krewedl placed the base
of the Madera
Limestone'at the
over
sandstone
horizon
and
above
which
limestone
shale.
,.~Zn
the
Water
Canyon
.
predominates
area.,
.
Krewedl ( 1 9 7 4 ) reports that the Madera Limestone is
overlain
'
north
in
by
the
the
separates
Spears
Formation
Nagdalena
theMad& and
of
?4ountains,
Spears
"
.Oligocene
age; further
the
Permian
-Formations
Abo
Formation
(Laughlin
and.
. ...',
Koschmann, 1 9 4 2 ) .
-
i
Tertiary
Volcanic
During
".
the
and
late
Sedimentary
Eocene',
Rocks.
central
New
Mexico
to a surface of low relief (Epis and Chapin,1 9 7 5 ) .
.
of
this
able
to
The basal
erosion
surface,
spread
over
Oligocene
OIigocene
large
rocks
areas
in
the
volcanic
in
were
New
area
Mexico.
are
andesitic and latitic conglomerates, laharic breccias,
'.
and sandstones ofthe Spears Formation. Lavas and ash-flow
a
Y
tuffs of similar composition are abundant in the upper
, . I .
I
I
.
eroded
Because
rocks
central
Magdalena
was
. .
12
'
.
I
x.
Spears
Formation.
.
The S p e a r s i s t y p i c aotlfhaen d e s i t i c -
l a t i t i c a:.rons of volca-1ic .and volcaniclastic rocks found
a t t h e b a s e of . .t h e f o r e l a n d v o l c a n i c f i e l d s
. .
U n i t e d S t a t e s (Lipmanand
of.about
others, 1970).
of t h e western
A f t e rd e p o s i t i o n
2 0 0 0 f e e t o ft h eS p e a r sF. o. r m a t i o n ,e x t e n s i v ea s h - f l o w
. .
tuffsheets
were formedby
pyroclasticeruptionsfrom
N e w Mexico.
c a u l d r o n si ns o u t h - c e n t r a la n ds o u t h w e s t e r n
. I n t h e , Magdalena' Mountains, several of
o v e r l a po n ea n o t h e r( F i g . '
'
.
these cauldrons
thickpiles
20);
.
of l a v a s , t u f f s ,
and volcaniclastic sediments usually fill each cauldron.
Beginniilg i n l a t e Oligocene-e"a1yMiocene
formedal@cgthe
time, basins
Rio G r a n d e ' r i f t and were f i l l e d w i t h
c l a s t i c s e d i m e n t so ft h eS a n t a
Fe Group..
B a s a l t f c' a n d
r h y o l i t i c lavas a r e i n t e r b e d d e d w i t h t h e s e s e d i m e n t s , i n ,
t h es t u d y
area.
A l l of t h e r o c k se x p o s e di nt h e
mapped a r e Cenozoic i na g e .S t r a t i g r a p h i c
the Oligocene units in Figure
area
columns d e p i c t
'.:,
4 a n dM i o c e n e - P l i o c e n eu n i t s
i n F i g u r e 13.
..
SpearsFormation
i s a group of i n t e r b e d d e d v o l c a n i -
The SpearsFormation
Xava f l o w s , a n d a s h - f l o w t u f f s
c l a s t i c s e d i m e n t a r yr o c k s ,
which represent the beginning of Oligocene
t h e Socorro-Magdalena area.
thick section
of t h e s e r o c k s
-
Tonking(1957)measured
a
on t h e Guy S p e a r s r a n c h
in the
Puertecito Quadrangle and named'
o ft h e
volcanism i n
them t h e S p e a r s Member
D a t i l F o r m a t i o n .C h a p i n( 1 9 7 1 a )r a i s e dt h eS p e a r s
..
' ,
-
R
...
gnit .
,r sixnilc Carwont '~1000~2000
f t . ) ANDESITE t o BASALTIC AWESITE LAVAS.
H5iYULZE ??.VAS. AS%OLi
TUFFS, LAVARIC BHECClAS and S A N W I S W S : c a u l d r o n
fill Of Sawmill Cnnyon t a u l d r o n : l o s e r i n t e r v a l o f
xl.-pooF, Pk. t o l t - - W Y . *
r h y o l i t e l a v a s , tufIs a n d l i z h i c - r i c h 5s. o v e r l a i n by dense, cry. t o &rY.p r p l . , a n d e s i t el a v a si n t e r b e d d e dw i t hp a l e - r d .t op k . - g r y .r h y o l i t e
lavas
a n dn i n o rd k . - e r y .t o
prpl.. h e t e r o l i t h i c ,1 a h a r i . cb r e c c i a s .T h i n
S S . and
siltst. o v e r l i e t h e l a v a s i n
places. TUFF
OF
CAROliiTA CAiqYON ( i n unper p a r t
o f u n i t ) is a m u l t i n l e - f l o w . s i m o l e c o o l i n e . u n i t o f a & - f l a w t u f f c o n s i s t i n g
of a l o i e r ~ m e m b e r0: brick-&d. t b cry., x1;-poor,
andesiteash-flowtuffand
a n "mer member of gry, x l - r i c h , q t z . - r i c h . r h y o l i t e a s h - f l o w t u f r ' i q t Z . - a n d
feld;'-rich ES. o c c u r s n e a r t o p o f u n i t .
'"La Jara Peak liite" b a s a l t i c
a n d e s i a e s o v e r l i e these a n d u n d e r l i e t h e t u f f
O f Lemitar Mountains.
-
A-L PeakTuff
o i n n a c l c s nembber(?): 32 m.y.'(3?00+ f t . ) ASH-FLO'N TUPFSI
t u f f s : g r y .t or d . - b m .
r h g o l i t e ,d e n s e l y welded.l-celd.,XI.-poor,pum~cecu~.
withgry.-rd.
t o brn. pumice. l e a t h e r s co small, p l a t y t a l u s < pu:nice l i n e a z c d .
Unit believed erupted fro2 Sarrmill
Canyon c a u l d r o n : p r o b a b l y c o r r e l a t i v e w i t h
p i n n a c l e s member. Base n o t knovm. b u t u n i t is minimally several thousand f t .
t h i c k . Gry. t o p n k . r n y c l i t e l a v a 5
With 10-15S phen.of
s a n i d i n ea n dq t z . .
l i t h i c - r i c h SS. a n d t h i n . p o o r l y
welded t u f f s i n t e r b e d d e d i n u p p e r p a r t o f
unit,
t i n i t of HardV a i d q e t (400-900(?)f t . ) RHYOLITE
LAVAS
and DC.XES. ASH-FLOW
TUFFS and SANDSTONESr cauldron fill of NorthBaldycauldron.fiostly
pk. t o
em.. X ~ . - D O O ~r h v o l i t e l a v a s and doms.Thin.
DoorlY welded.ash-flowtuffs
;nil i e d . - c i s . - g n d ; ,l i t h i c - r i c h
SS. o v e r l i e t h e t h y o l i t e l a v a .
HellS.%esaTuff8
32-33 m. y. (200Ot f t . ) ASH-PLOLJ
TUFFS,
r h y o l i t e ,m u l t i p l e ' flow, s i x p l e c o o l i n g u n i t . d e n s e l y w e l d e d . x 1 . - r i c h , q t z . - r i c h , 2 - f e l d , .
'ma'ss.
tuffs. Pk. t o rd.-brn. when-fresh.gry.
when p r o p y l .a l t e r e d .W e a t h e r st o
b l o c k yb l d r s .
nasal t u f f s similiar t o t u f f f G r a n i t e Kcn.: a b r u p ti n c r e a s e
in
10-25 f t . abovebase.
Source is NorthBaldyCauldron.
st;.
-
Soears Parmation
Umer laws and b r e c c i a s ,
( 5 2 0 I t . ) ANDESITE to IATITE
WYhS andminor MCD-FLOW DEPOSITS: mostlydense.
latitetoandesitelavas.
p r p l .t o dk.-gry.
One type is a p h a n i t i cl a t i t e ;o t h e rt y p e
is p o r p h y r i t i c
a n d e s i t e .C o n t a i n s
minor i n t e r b e d d e d a n d e s i t i c t o
l a r i t i c mud-flow b r e c c i a s .
-
soearsFormation
t u f f of G r a n i t e !:ountaint
(400-500 f t . ) ASH-FLOW TUF.S~
l a t i t e . m u l t i p l e - f l o w ,s i m p l ec o o l i n gu n i t .d c n s e l yw e l d e d ,X I . - r i c h .l i r h i c rich,qt2.-poor.
mass. tuffs1rd.-brn
when f r e s h . dk.-grn.-gry.
when p r o p y l .
altered.
-
SoearsFormation
'"turkeytrack.'andesite,
(500 f t . ) ANDESITE
LAYAS,
gry.r d .t op r p l .p o r p h y r i t i ca n d e s i t ew i t hc r u d e l yf l o w - a l i m e dp l a g .p h e n .
U n d e r l i e s t u f f o r Nipple Iltn. in northern Kagdalena
Nits.4 t u f f n o t recagnized here.
L
0
Figure 4. Oligocene stratigraphic section for focks in the
eastern Magdalena Mountains'. Graphic sections and descriptions
modified after,Chapinand others, 1978, and Osburn, 1978.
~-
,
toformationalstatus;
however, t h e terms D a t i l F o r m a t i o n
1 9 7 1 ) .havebeenabandoned(Elston,
and D a t i l Group'(Weber,
1 9 7 6 , p. 1 3 4 ) .B u r k ea n do t h e r s( 1 9 6 3 )r e p o r t e d
date of 37.1 m.y.
a X-Ar
from b i o t i t e i n a b m l d e r from t h e u p p e r
part of the Spears.
The SpearsFormation
i s f,ound o n l y i n t h e n o r t h w e s t e r n
'(secs. 3 and 4 , T. 4 S . ,
p o r t i o no ft h es t u d ya r e a
secs. 33 and34,
unsurveyed,and
t h er o a d
T.
t o Langmu'ir Laboratory.
" q u a r t e r s of
a
._
.
s q u a r e mile.
3 S.,
R.' 3 W . )
R.
3 W.,
along
It c o v e r sa b o u tt h r e e
According t o Krewedl ( 1 9 7 4 ) ,
theSpearsFormation~unconformablyoverliesthe
Madera
Limest.one i n m o s t p l a c e s ; , however - a t t h e h e a d o f P a t t e r s o r ;
Canyon, it o v e r l i e st h e
S p e a r s? o r m a t i o n
Abo F o r m a t i o n T
. he'base
i s notexposed
of t h e
.
i n thi.sstudyarea.
The S p e a r s F o r m a t i o n h a s s e v e r a l d i f f e r e n t r o c k t y p e s
which may undergomarked
another.
facies changes from one area
to
Brown ( 1 9 7 2 ) , Chamberlin ( 1 9 7 4 ) , S p r a d l i n ( 1 9 7 6 )
andNilkinson
upperandlower
( 1 9 7 6 ) , d i v i d e d t h e S p e a r sF o r m a t i o ni n t o
members.
I n ~ g e n e r a l ,t h el o w e r
member
c o n s i s t s of a n d e s i t i c - l a t i t i c v o l c a n i c l a s t i c c o n g l o m e r a t e s
andsandstoneswithminorlavaflows;
w h i l e theupper
member c o n s i s t s of l a t i t i c a s h - € l o w t u f f s , a n d e s i t i c t o
l a t i t i c lc7va f l o w s , l a h a r i c b r e c c i a s
r o c k s .U s u a l l yt h eb o u n d a r yb e t w e e nt h e
and o t h e r s e d i m e n t a r y
two member:, i s
p l a c e d a t t h e base o f a " t u r k e y t r a c k " a n d e s i t e t h a t
underlies the tuff of Nipple Mcuntain.
15
. .
..
The
A
Spears
Formation
has
recently
been
Mountainsby ICrewedl (197.4)
,,Blakestad
Magdalena
mapped
in
the
(1978)
and Osburn (1978). In the central
Magdalena~Mountains,
'
,
. .
.
the tuff of NipFle Mountain
is not present. Here, .Krewedl
(1974) divided the Spears Formation into three mappable
witha "turkey track" andesite '(middle member)
units,
separating
to
an
upper
Krewedl,the."turkey
equivalent ,to the
.. '. a lo,wer
member
from'
track"
.
"turkey'
member.
,According
andesite.
track"
-
he
mapped
is
andesite
.mapped
by
Brown.
. .
(1972). If this correlation is' correct, then Krewedl's
. .
middle
and
upper
members
are
equivalent
to Brown's
upper
member. . Krewedl's lower memberis not exposed.in this
thesis area, but his middle and upper member's are. For
. .
this
study,
mappable
the
units,
Spears
Formatio:.
which
from
was d,ivided into three
oldest
to youngest
"turkey track" andesite, the tuff
of Granite
and an upper
unit
of
andesitic-latitic
.,
are
the
Mountain,
lavas
and-laharic
,breccias. The tuff of Granite Mountain and the upper
unit
:
of
andesitic-latitic
lavas
and
laharic
breccias
were
mapped together by Krewedl
as his upper member.
"Turkey Track". Andesite. A distinctive series of
porphyritic lava 'flows, which exhibit a "turkey track"
texture
imparted
phenocrysts,
by
covers
a
subparallel
about
one
alignment
of
plagioclase
quarter
of a square mile
in
the northwestern portion of the study area. They crop
.
i:.,
out
alongthe Langmuir
.
Laboratory
access
road
and
continue
16
. .
n o r t h w e s t w a r d . i n t o the area mapbed )by Krewedl.
.
.
.~
"turkey track" a'ndesite
. .
.
..
'
.
. c .
The
A s probably about. 550 . f e e t . t h i c k ,
..
~
b u t f a u l t i n g makes i-t ' d i f f i c u l t ' t o , g e t . a n a c c u r a t e estimate.
.
. ....
.
.
-
.The .base of .'$he ' ' t u r k e y t r a c k ' ? a n d e s i t e
~.
.
.
. .
,
. . ..
.~
i 7
..
.
.
.
the
northern^ bou'ndary i n sec.
.
R.
3
O.'l mile
3 4 , T. 3 S.,
.."
-
:w
. .
.*
n o t exposed i n !
. .
. t h i s t h e s i s ' a m a , ..bur 'it.
. ,~ i s e x p o s e d less t h a n
.
.. . ,. ~.
northof
.
.
O u t c r o p s . o f . t h e " . t u r k e y t r a c k. ". . a n d e s i t e a r e p o o r l y
. .. .
.
.
,
it weathers very
exposed .and 'scattered because
and i s ..o f t e n c o v e r e d b y c o l i u v i u m .
easily
ex-
Some o f t h e b e s t
..
p o s u r e s are foundalong
tlie r o a d t o Langmuir L a b o r a t o r y ;
elsewhere, t h e "turkey -track" and.3site. usually forms
i s o l a t e d ,r o u n d e do u t c r o p ss e p a r a t e db yt a l u s
t o highiy
The j o i n t s v a r y f r o m b e i n g r o u g h l y p l a n a r
.
and s o i l .
.
irregular;consequen'tly,the.talusandcolluviumblocks
t o somewhat rounded.
v a r yf r o ma n g u l a r
.%
The " t u r k e y t r a c k " a n d e s i t e i s p u r p l e t o g r a y
fresh,but
when
it 'is f r e q u e n t l y a. .-l t e.r e d t o a g r e e n i s h - g r a y
color i n t h es t u d ya r e a .1 t . u s u a l l yc o n t a i n s
20 t o 40
percent plagioclase' phenocrysts in an aphanitic matrix.
Theseplagioclasephenocrysts
1 nun t o a b o u t 1 2
are tabular and vary,from
i n length, 'but they
,mm
i n s i z e w i t h i na n y ' o n es p e c i m e n . .
'are f a i r l y u n i f o r m
The t A i c k n e s s o f t h e
u n i t and t h e v a r i a t i o n s i n L D l a g i o c l a s e s i z e a n d a b u n d a n c e
suggeststhatthere
are severalflows;
not p o s s i b l e i n t h e f i e l d t o
Amygdules f i l l e d w i t h
Gowever, it was
map i n d i v i d u a l f l o w s .
c a l c i t e , c e l a d o n i t e ,c h a l c e d o n y ,
.
.
li
.
and
.
.. .
quar'tz
'are common
. .
.
.,,
in
.
,
the
.. .
.
.
.
.
."tugkey
track"
.
andesite.
..
.
In thin section, the "turkey
track'' andesite consists.
.
of
:
-
about30 percent
.
phenocrysts
.
of
'euhedral
plagioclase
.~
(An3*; Plichel-Levy method,8 grains) distributed ina
.~
.
subparallel' alignment (see
Fig.
5 ) . The.plagioclase
. .
.
.
. .
.
varies from about0.5 to 5 mm in length and averages
about
2.0 m
m.
. .
These plagioclase phenocrysts are always
partially altered to clays., fine-grained quartz, and
.
r
calcite(?).' The rock also contains about
10 percent
xenocrysts ( ? ) that have been a,ltered to a porous honeycomblike structureof,fine-grained quartz' and clays.
The
'
~.
xenocrysts ( ? ) often
that
range
The rock
exhibit
from
0.4 to 4 mm
also
eight-sided
in
diameter
euhedral
and
contains
about.2 to 3 percent
outlines
average
magnetite
1.8
mm.
and
about 1 percent pyroxene' (probably hypersthene), which is
..
about 0.5 mm in diameter. The matrix consists mostly of
..
0.1 mm).
small crystalsof plagioclase (average length about
About 10 percent of the
which
occurs
as
thin
tiny
section
blades
and
consists
of hematite,
irregular
masses
replacing
the groundmass.
Tuff of Granite Mountain.The tuff of Granite Mountain
,
.
is a latitic, crystal-rich, quartz-poor, ash-flow
tuff.which
I
overlies the "turkey track" andesite in the area mapped.
It is a
densely
welded,
multiple-flow,
simple
~
cooling
unit
(Chapin and others, 1978). The tuffof Granite Mountain
is an informal name proposed by Chapin (1974).
The type
18
.
..
. .
.
.
.
.
.
..
,..
.
'
' '
'
'
Figure 5. Photomicrograph of the,"turkeytrack" andesite.
The blocky, euhedral phenocrysts are altered plagioclase.
The subparallel alignmentof these phenocrysts gives
the
rock its distinctive "turkey track" texture in hand specimen. The large grainon the right side
of the photomicrograph is partof .a xenocryst ( ? ) . These xenocrysts consist
entirely of a fine-grained aggregateclay.minerals
of
and
calcite and are too altered to identify. All
of the rocks
in the northwestern portion
of the study area .have been
intensely a.ltered. The plagioclase phenocrysts in the
"turkey track" andesite are always partially altered
to
clays, fine-grained quartz and calcite.The alteration
products seen in this photomicrograph (gray, cloudy
material) consists mostly Jf clay minerals. Most of the
opaque minerals in the groundmass are secondary hematite.
Crossed nichols,31.25~.
, .. . . . ..
.
..~.
.. :
.~
.,.'
.
,
.
,
19
of Magdalena.
l o c a l i t y is o n G r a n i t e M o u n t a i n n o r t h e a s t
The t u f f a f G r a n i t e M o u n t a i n
was i n c l u d e d . b y Krewedl
(1974) i nh i su p p e r
T h i st u f f, h a sa l s o
member.
d e s c r i b e d by Brawn ( 1 9 7 2 ) ,
( 1 9 7 6 ) , Blakestad (19781,
'
been
Chamberlin ( 1 9 7 4 ) , Wilkinson
anq
Osburn
(1978).
It covers
a b o u t one. q u a r t 3 . r ' o f a s q u a r e mile, o r ' l e s s , . i n t h e
.. .
northwesternportion
of! t h i s s t u d y area where i t i s a b o u t
~.
Tres Montosas area, . t h e t u f f of
500 feet t h i c k .I nt h e
GraniteMountain
is immediatelyoverlainbythe
Mesa Pormakion;however,
Hells
area, t h e two t u f f
inthisstudy
x*:
u n i t s are a l w a y s s e p a r a t e d . s t r a t i g r a p h i c a l 1 y by a series
of l a v a f l o w s and6 b r e c c i a s .
I n h a n ds p e c i m e n ,t h et u f fo fG r a n i t eM o u n t a i n
is a
m o d e r a t e l y t o d e n s e l yw e l d e d ,c r y s t a l - r i c h ,q u a r t z - p o o r ,
lithic-rich,
l a t i t i c a s h - f l o wt u f f .
Plagioclase, s a n i d i n e ,
and b i o t i t e a r e t h ep r e d o m i n a t ep h e n o c r y s t s .
lithicfragments
.;. l i t h i c f r a g m e n t s
are u s u a l l y p r e s e n t ;
-.
A few p e r c e n t '
the most common
are red-to-gray," finelgrained andesitic
f r a g m e n t s t h a t are. u s u a l l y , a b o u t 0 . 5 t o 3 crn i n . d i a m e t e r .
Pumice i s p r e s e n t i n
percent.
amountsrangingfromabout
The r o c k i s u s u a l l yp u r p l e
i s f r e q u e n t l ya l t e r e d
when f r e s h , b u t
t o a g r e e nc o l o r .
of t h i s t u f f . are on t h e r o a d t o t h e
L l s e w h e r e ,s c a t t e r e do u t c r o p s
2 t o .lO
it
The best e x p o s u r e s
LangmuirLaboratory.
are u s u a l l y low,rounded
exposures p a r t i a l l y c o v e r e d with c o l l u v i u m .
In thin section, the tuff
of G r a n i t e M o u n t a i n
contains
1
5 0 t o 6 0 percent p h e n o c r y s t s , 5 t o 1 0 p e r c e n t l i t h i c
. .
20
,.
fragments, and about
. . 5 percent pumice. All the feldspars
. .
are
highly
.altered,
so^ $6 is difficult
to
estimate
abun-
dances. About 80 .to..'90 percent of the.phenocrysts are
feldspars, a?d it. appears tl-at plagioclase (andesine)
is
four to five timesas abundant.as potash feldspar. However,
some of Lhe feldspars
are too altered
to identify. .Previous
workers have'reported that the plaGioclase is only slightly
.
.
more abundant than the potash feldspar.
have
an
average
g r a m size
of
The feldspars
about
1.1 to 1.2 'm, but
some phenocrystsare'as much as 2.6 mm long.
In thin
section, the tuff also contains about
to 54 percent
2 percent
to the
The
biotite,
magnetite,1 percent Quartz and a trace of pyroxene.
The quartz averages
embayed.
"
-
about2 7 mm in
diameter
and
is
,deeply
The pumice and matrix are devitrified and altered
point
that
pumiceis now
little
mostly
vitroclastic'texture
represented
by
remains.
streaks
of
vapor-
phase quartz and hematite stain.
Upper Unit. In some partsof the Magdalena area, the
tuff
of
Granite
Mountain
is
immediately
overlain
by
the
Hells Mesa Tuff (Chamberlin,
1974).,. In this study area,
however, the two tuffs are separated
by a group of
andesitic to latitic
lavas
and
mud-flow,deposits
.
were
mappedas
These
rocks
the
upper
unit
cover
about
one
of
.
which
,
the
Spears
of. a, square
quarter
Formation.
mire,or
less, and are about
500 feet thick.
4
At least two different.'lava flows are present.
One
\
..
.-.
,.
21
.
.
..?
is a porphyritic andesite
with.10 to 20 percent feldspar
.
I
8 s 1 cm in length. The
phenocrysts, that are' as'much
feldspars
color.
are
usually
altered
to
a
white
or
light-green
The~otherlava flow is a very-fine-grained latite
or quartz latite. Both lavas are very dense. and gray to
grayish-purple when fresh. They usually break along joints
to form sharp,.angular.talus fragments. Occasionally,
mudflow
conglomerates
and
breccias
are
exposed
in
the
r_,
.I
upper unit. This rock type consists of unsorted, unstratified, subrounded to angular, andesitic and latitic
fragments in a muddymatzrix. .Thc.porphyritic lava appears
to
be
the
most
abundantthe-three
of
rock
types
in
.
the
..
upper unit; the crystal-poor lava is in
next
abundance.
The
mudflow
deposits
are
exposed
in
only
a
few
places.
In thin section, the porphyritic lava contains about
10 percent
phenocrysts
of
feldspar,
some
with
a
glomero. ....
'
x..
'
..
porphyritic texture. Xost of the phenocrysts are subrounded
plagioclase (andesine, to labradorite); although about
2 to
3 percent of .the rockis sanidine phenocrysts. The
feldspar
phenocrysts
average
about
3 to 4 mm in
diameter,
but may be as much
a s 6 mm in diameter:
The feldspars.
are
and
Five
all
partially
toC percent
altered
of
the
to
rock
clays
consists
of
'
fine-graised
opaque
quartz.
minerals.
About one-half of this
(2 to 3 percent of the rock; is
probably primary magnetite. The remainder consists ,of
hematite
and
magnetite
which.have replaced
(?)
biotite
or other mafic minerals.
'The groundmass appears to have
.
.
~~~
~
I
22
,...
been holocrystalline,'although it
has
been
altered
and
partially replaced by other minerals.It consists mainly
of feldspar microlites (about 0.05
mn~long), but also
contains larger fragments of feldspar, crystals,
a few of
which range in size up t.hat
to of the phenocrysts. Most
of
the
small
are highly .altered, but
crystals
feldspar
they are probably plagioclase.'i'hFre.is a crude subparallel
alignment
of
the
crystals
in
the.groundmass
of
..
this rock. About 5' percent
of the rock in thin section
is secondary quartz; traces of pyroxene are also present.
The fine-grained lava discussed earlier aislatite
or quartz latite. The average size of'phenocrysts in thin
section is about 0.1 mm; the
largest
was about 0 . 3 m - i ndiameter.
holocrystalline,
but'
it
altered in some places.
phenocryst
.observed
This rock appears to be
has
been
partially
to completely
..... .
In thin section, it contains
approximately equal amounts
of plagioclase' (An52) and
..sanidine. About 5 to 10 percent quartz'and3 to 5'percent
magnetite
perqent
phenocrysts
hematite
are
occurs
as
present
The Hells
.. Mesa
the
rock.',
a,replacement other
of
Hells
.
in
Mesa
About
minerals.
Tuff
Tuffis a"multip1e-flow,
simple
cooling
_
u.nit of densely welded; crystal-rich, quartz-rich ash-flow
tuff (Chapin and others,1978):.
Detailed petrographic
work by &own (1972), Chamberlin (1974), and Spradlin
t
(1976)
.'
.
has
-*.
I .
shown
that
the
Hells
5
Mesais Tuff
a quartz
. .
23
latite to rhyolite in mineralogical composition. Tonkinq
-
(1957) orjqinally named the Hells llesa
Member of the Datil
Formation
after
Hells
Mesa
in
the
eastern
Bear
Mountains.
His type locality was in the Puertecito auadrangle (sec.
31, T. 2 N., R. 4 W.). The Hel-1s i4esawas'elevated to
. .
formational status by Chapin (1971a). Brown (1972) conducted
a
detailed
study
of the
Hells
Mesa
Formation
and
. divided it into the tuff
of.Goat Spring and the tuff
of'
Bear Springs. Chapin (1974) .andDeal and Rhodes (1976)
restricted the Hells Mesa Tuff
to the
lower,
crystal-rich,
quartz-rich unit, formerly called the tuff of Goat Spring
by Brown (1972). Brown's tuff
of Bear Springs wa's renamed
'
the A-2 Peak Rhyolite by Deal and Rhodes (1976). X-Ar
dates obk-ained from the Hells Mesa are 30.6
2 2.8 m.y.
32.1 5 1.5
(Weber and Bassett, 1963; Weber, 1971), and
and 32.4
The
'
'.
. . ...
2 1.5 m.y. (Burke andothers, 1963).
Hells
mininq-district
Mesa
of
Tuff
was
the
first
Kagdalena
studied
'.,'..
in
Mounta'ins
the
by
Kelly
Laughlin
ana Koschmann (1942), although they mistakenly interpreted
it tobe a. rhyolite porphyry sill. Blakestad (1978) has
remapped
the
Kelly
mining
district
and
Xrewedl
(1974)
and Osburn (1978) have mapped some
of the Hells Mesa Tuff
in the south-central and southeastern parts of the
Magdalena Mountains, respe-tively. Based on their mapping,
the
source
be
theNorth
of
the
Baldy
Hells
cauldron
Mesa
(see
Tuff
has
Fig.
20, and
~
been
to
determined
Chapin
and
others, 1978).. The Hells MesaTuff mapped for this thesis
.-
..-
.
.
.
i s e n t i r e l yw i t h i nt h eN o r t hB a l d yc a u l d r o n .E x p o s u r e s .
of Hells Mesa a r e c u t o f f t o t h e s o u t h
. .
.
.
1
s
by t h e y o u n g e r
Sawmill Canyon c a u l d r o n (see P l a t e s 1 and 2),.
The Hells Mesa.Tuff i s f o u n d o n l y . i n t h e n o r t h w e s t e r n
.area(secs.
p o r t i o no ft h i ss t u d y
R.
W.,
3
3 , 4 , and5;
.secs. 3 2 and 3 3 , T. 3
unsurveyed,and
where it c o v e r sa na r e ao fa b o u t1 . 5s q u a r e
,
T. 4 S.,
R.
S.,
miles.
e ,
3'.W.)
Krewodl
( 1 9 7 4 ) r e p o r t sa na n g u l a ru n c o n f o r m i t yb e t w e e nt h e
'
Hells
,-.
i n t h e c e n t r a l Magdalena
Mesa T u f f a n d t h e S p e a r s F o r m a t i o n .
were i n a d e q u a t e t o ' v e r i f y it i n
M o u n t a i n s ,b u te x p o s u r e s
thisthesisarea.
The s t r a t i g r a p h - i c t o p ,of t h e Hells Mesa
Tuff i s n o t e x p o s e dd u et oe r o s i o n .o t h e rs t u d i e so u t s i d e
ofthe
r e p o r t a t h i c k n e s s of a b o u t 6 0 0
MagdalenaMountains
is w i t h i n ' t h e
t o 800 Ceet, b u t s i n c e t h i s s t u d y a r e a
Hells~MesaT u f f may
i n f e r r eN
d o r tB
h a l dcya u l d r o nt h
,e
b e much t h i c k e r .
n e s so f
Xrewedl ( 1 9 7 4 ) r e p o r t e d a maximum t h i c k -
3850 f e e t i n t h e c e n t r a l
MagdalenaMountains,but
t h i s t h i c k n e s s may b e e x c e s s i v e b e c a u s e o f u n r e c o g n i z e d
f a u l t i n g .F a u l t s
are v e r y d i f f i c u l t
c a u l d r o n - f a c i e s Hells Mesa T u f f .
Hells Mesa p r o b a b l ye x c e e d s
The b e s t e x p o s u r e s
t o recognize i n thick,
In t h i s s t u d y a r e a , ~ t h e
20.00 f e e t i n " t h i c k n e s s .
of t h e H e l l s Mesa o c c u r a l o n g t h e
..
road . t o t k 3 Langmuir L a b o r a t o r y a n d t h e
Along t h e s e r o a d s , t h e
'
gold mine road.
Hells Mesa i s u s u a l l y , h i g h l y j o i n t e d ;
t h e s e j o i n t s may be s h e e t - l i k e i n
h i g h l y erratic i n o t h e r a r e a s .
some areas o r c u r v e d a n d
The % e i l s Mesa u s u a l l y
1
;c
._
I
..
.
.
.~
.
.
'
.
't
~
25
b r e a k sa l o n gj o i n t st o
.-.
form s h a r p ,b l o c k yt a l u s .
of t h e a r e a i s covered"by
Much
thistalus,butoccasionally
t h e Hells Mesa f o r m s s t e e p l e d g e s
and c l i f f s .
., .
I n hand spccimen, tfie Hells Mesa T,uff i s u s u a l l y
.
. ..
w h i t et og r a yb e c a u s eo fs t r o n gb l e a c h i n ga n dp r o p y l i t i c
alteration.
Hells Mesa is r e d d i s h -
Mhere u n a 1 t e r e d ; t h e
(Brown, 1 9 7 2 ; S p r a d l i n , ' 1 9 7 G ) .
brown i nc o l o r
.
It usually
..
c o n t a i n s 4.0 t o 50 p e r c e n t p h e n o c r y s t s o f . s a n i d i n e , p l a g i o c l a s e ,q u a r t za n db i o t i t e .Q u a r t zo f t e no c c u r sa sd i s t i n c t i v e
much a s 4 mm i n d i a m e t e r .t h e
" e y e s "m e a s u r i n ga s
^
-.
Hells
I
Mesa i s u s u a l l y v e r y p u m i c e p o o r , a l t h o u g h t h e a l t e r a t i o n
may make t h e pumice d i f f i c u l t t o see i n some a r e a s .
a few p l a c e s , t h e
Hells Mesa i s m o d e r a t e l y p u m i c e r i c h .
Detailedpetrographic
b e e nd o n e . b y
In'
workon
t h e Hells Mesa h a s
Brown ( 1 9 7 2 ) , Chamberlin ( 1 9 7 4 ) a n dS p r a d l i n
.
.
(1976).
Brown (1972, p. 21-24)
Mesa Tuff ( h i s t u f f o f
r e p o r t e dt h a tt h e
Hells
G o a tS p r i n g )u s u a l l yc o n t a i n s
'
40 .
, t o 50 percentphenocrystsofbrokenandpartiallyresorbed
c r y s t a lf r a g m e n t s .S a n i d i n e
.'
varies from~1.0 t o 30 p e r c e n t
o € t h er o c k ,w h i l ep l a g i o c l a s ev a r i e sf r o m ' l 0t o
20
Y.
p e r c e n ta n da v e r a g e sa b o u t
scarce in the lower
12 p e r c e n t .Q u a r t z
is very
10 t o 20 f e e t of Brown's Hells Mesa,
b u t it i n c r e a s e s t o 5 to, 15 perce:lt o f l t h e r o c k h i g h e r
Ln t h e
section.^
of t h e r o c k .
..
f r o mt h es t u d ya r e a .
.
I'.
One t h i n s e c t i o n o f t h e
"8\r ,'
1 to 4 percent
B i o t i t ec o m p r i s e sa b o u t
Hells'Mesa Tuff was examined
I t c o n t a i n s. a b o u t
4 S p e r c e n tp h e n o c r y s t s .
.. ..
26
47
Quartzcomprisesabout
'10 t o 1 2 p e r c e n t o f t h e r o c k
q.
andoccur?
as s u b h e d r a l .b u td e e p l y
embayed p h e n o c r y s t s
ranging. i n s i z e from 0.3 t o 4 . 0 mm a n d a v e r a g i n g a b o u t
mm i nd i a m e t e r . .F e l d s p a r - c o m p r i s e s
t h er o c k .
0 . 7 5 mm in d i a m e t e r .
averagesabout
toclays
. .
..
-
s i z e from 0 . 4 t o 4 . 0 mm i nd i a m e t e ra n d
w h i c hr a n g e si n
'. f e l d s p a r
abou',: 3 0 p e r c e n to f
'the feldspar is sanidine,
About 65 p e r c e n to f
.
1.8
.
The rest 0.f t h e
i s p l a g i o c l a s e .which h a s b e e n p a r t i a l l y ' a l t e r e d
and c a l c i t e .
About 2 t o 3 p e r c e n t b i o t i t e . o c c u r s
assmall,,ruhedralgrains,
mm i n ' d i a m e t e r .
i n t h er o c k .
w h i c hr a n g e ' f r o m
0.25 t o 1.1
A t r a c ea m o u n t ' b fm a g n e t i t ea l s oo c c u r s
-. .
The m a t r i xh a s
iieeil c o m p l e t e l y d e v i t r i f i e d
and altered.
Unit ofHardyRidge'
The u n i t o f
I!ardy R i d g e c o n s i s t s o f p h e n o c r y s t - p o o r
..
rhyolite lavas; poorly welded, lithic-rich,, ,-ash-flow
. t u f f s ;a n dv o l c a n i c l a s t i cs e d i m e n t a r yr o c k s .T h i su n i t
overlies the
Hells Mesa Tuffand
is b e l i e v e d t o - b e p a r t
o ft h ec a u l d r o nf i l lo f 't h eN o r t hB a l d yc a u l d r o n .
was named byBowring
It
( i n preparation)^ f o r e x p o s u r e s o n
Hardy Ridge. . i n. t h e s o u t h e r n Magdalena Ftountains. . So f a r ,
this.unithasonlybeenfoundon
Hardy Ridge,andin'Sawmil1,
Ryan H i l l a n d C a r o n i t a c a n y o n s ( a l l i n t h e s o u t h e r n
d a l e n aM o u n t a i n s ) .T h e s eo u t c r o p sa r ep r o b a b l y
Mag-
i n the
southern portion of the North'Baldy cauldron.
ti,
In this study area, the unit of
HardyRidgedccurs
in
..
.
-
27
..
s o u t h e r n Ryan H i l l Canyon (secs:25,
K\
R.
3 W.,
unsurveyed;and
sec. 36', T . 4 S., R. 3 W.).
It
a p h e n e c r y s t - p o o rr h y o l i t e
lava
was mapped a s two u n i t s :
( T h r ) ,' and a g r o u p o f 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
posed i n tlie s t u d y area.
However, one small o u t c r o po f
Hells Mesa Tuff i s exposed i n S / 2 ,
i s o v e r l a i n by t h e u n i t o f
.
sec. 3 6 .
HardyRidge.South
..
a b u n d a n ta ~ st h et o p o g r a p h i cm a r g i no ft h eS a w m i l l
c a u l d r o n i s approached(Osburn,
-.
of t h i s
HardyRidge
.
Canyon
1 9 7 9 , o r a lc o m m u n i c a t i o n ) .
inthisstudy
35 2nd 3 6 ) a p p a r e n t l y l i e s between t h e
-
T h i s outcrop
Hells' Mesa Tuff become more
e x p o s u r e ,o u t c r o p so ft h e
The u n i t o f
rocks and
i s seldomex-
The b a s eo ft h i su n i t
welded t u f f s ( T h r s ) .
4 S.,
2G a n d . 3 5 , T.
area (sec.
25, 2 6 ,
topographic r i m and
I
t h e mhin r i n g f r a c ' t u r e of t h e Sawmill .Canyon c a u l d r o n .(see
P l a t e s 1 and 2 ) . . The main r i n g f r a c t u r e d o v ~ n f a u l t e d t h e
u n i t ofHardyRidge
1 0 0 0 f e e t or. more, so it i s no l o n g e r
e x p o s e dn o r t ho ft h a ts t r u c t u r e .
T b e . u n i to f
i s o v e r l a i n by t h e A-L Peak Tuff; which
HardyRidge
becomes t h i n n e r
a s it a p p r o a c h e s t h e t o p o g r a p h i c r i m of t h e S a k i 1 1 Canyon
cauldron.
R h y o l i t eL a v a s( T h r ) .
Most o f . t h e u n i t o f
lavas.
c o n s i s t so fp h e n o c r y s t - p o o rr h y o l i t e
exposures of the 'rhyolite
H i l l Canyon.
are along the floor
Along t h es i d e s
The b e s t
o f Ryan
of t h ec a n y o n ,t h eo u t c r o p s
are p o o r .T h e r e ,t h e yu s u a l l yo c c u r
.c>
HardyRidge
as i s o l a t e de x p o s u r . e s
a f e w fee.E i n s i z e , which a r e s e p a r a t e d f r o m . e a c h o t h e r
.
by
.
:..,
.
28
9
t a l u s f r Q m o v e r l y i nugn i t s ; ,
minimum t h i c k n e s s o f a b o u t
8 0 0 f e e t i n places.
The r h y o l i t e l a v a h a s a
250 f e e t , b u t
it may exceed
0.f t h e r h y o l i t e .is
S i n c et h eb a s e
a maximum t h i c k n e s s c . a n n o t b e
notcontinu3usly'exposed,
:
. .
determined.
It i s o v e r l a i n by a sequence of s e d i m e n t a r y
. .
t uw
i fes l d ae nd d r. o c k s
i s a pink t o g r a y
. .
I t is u s u a l l ym a s s i v e ,a l t h o u g ho c c a s i o n a l l y
I n handspecimen,
color.
-
(.Thrsj.
therhyolitelava
.
it may be flow-banded or. b r . e c c i a t e d . T h e r h y o l i t e l a v a
-.
is p h e n o c r y s tp o o r ;o n l y
a few small c r y s t a l s ( S e c o n d a r y
q u a r t z 7 ) can b e seen i n hand.specimens.
I n thin section; the rhyolite
l a v a c o n t a i n s a b o u t 25
percent s p h e r u l i t i c c h a l c e d o n y a n d
LO t o 2 0 percent q u a r t z
i \
c r y s t a l s which a r e a b o u t 0 . 1 t o 1 mm i n d i a m e t e r .
'
These
in
q u a r t z c r y s t a l s are a n h e d r a l a n d o c c a s i o n a l l y o c c u r
c l u s t e r s ;p a r t
The r e s t - o f t h e r o c k c o n s i s t s
quartz.
'":
o r all of them a r e probab-lysecondary
.
.
( 0 . 0 2 t o 0 . 1 mm) q u a r t z , c h a l c e d o n y a n d s a n i d i n e
Sedimentary Rocks a n d
T u f f 's( T h r s )
(?)
.
.'
. A s e q u e n co
ef
tuffs
volcaniclasticsedimentaryrocksandpoorlywelded
_...
o v e r l i e s t h er h y o l i t el a v ai n
crops o f t h i s u n i t
. outcrops occur
Ryan H i l l Canyon.The
on t h e east side of Ryan H i l l Canyon (SW/4,
T h e r e ,t h eb e t t e ro u t c r o p so c c u r
as b l o c k y ' e x p o s u r.e. s i n
ravines where erosion has
.
t.1 .
t hoe v e r l y i n g
.
. .
.
out-
a r e poor i n most places; t h e b e s t
'sec. 2 5 and sec. 36).
talus.
This. u n iht a s
.
of f i n e - g r a i n e d
- .
.
,. .
removed
an a v e r a gteh i c k n e s s
..
-.
29
-
.*
67
of a b o u t - 5 0 f e e t , a l t h o u g h t h e . t h i c k n e s s may v a r y from
-
0 t o 100 feet.
t u f f s are c h a r a c t e r i s t i c a l l y
The s e d i m e n t a r y r o c k s a n d
verythin
and. d i s c o n t i n u o u s l a t e r a l l y ,
d e s c r i p t i o no ft h e
rocks i n t h i s u n i t
so o n l y a g e n w a l
i s given.
'The s e d i "
m e n t a r yr o c k sc o n s i s tm o s t l y
of even,para.lle1-bedded,
p o o r l ys o r t e d ,l i t h i c - r i c h ,s a n d s t o n e s .O c c a s i o n a l l y ,
. c o n g l o m e r a t e sa n db r e c c i , a so c c u ri n ,t h i su n i t .
The t u f f s
are u s u a l x y g r a y 'to g r e e n , p o o r l y t o m o d e r a t e l y w e l d e d ,
and l i t h i c r i c h . .
They u s u a l l y c o n t a i n a b o u t 1 0 . t o
p e r c e n tl i t h i cf r a g m e n t so fa n d e s i t ea n dr h y o l i t e ,
*.
20
5 to 10
p e r c e n tq u a r t zp h e n o c r y s t s ,
5 tcj 1 0 percent f e l d s p a r ( m o s t l y
s a n i d i n e ' a n dp e r t h i t e ) ,a n d
5 t o 20 p e r c e n t pumice.
A-L .Peak Tuff
.
.
The A-L Peak T u f f i s a c o m p o s i t e sheet of d e n s e l y
welded crystal-poor, one-feldspar, rhyolitic
,.
( C h a p i na n do t h e r s ,1 9 7 8 ) .
t h e A-L P e a k R h y o l i t e f o r
. a.s h - f l o w
Deal andRhodes(1976)
.
.
tuff.
named
a 2000-foot-thick section in the
n o r t h e r n San Mateo Mountains. Most w o r k e r si nt h eS o c o r r o . ,. .
Magdalena 'area now refer t o t h i s ' f o r m a t i o n a s t h e .A-L Peak
_"
Tuff
(Chapinandothers,1978).The
equivalenttothe
Koschmann ( 1 9 4 2 ) .
" b a n d e dr h y o l i t e "
,
A-L -PeakTuff
of Laughlinand
I t i s a l s oe q u i v a l e n t
t u f f of Bear S p r i n g s , w h i c h
is
to Brown's
(1972)
is t h e u p p e r p a r t ofTonking's
(1957) Hells-Mesa Formation. . Brown (19721, Simon ( 1 9 7 3 ) ,
a I:
andSpradlin
(1976). have presented detailed petrographic
30
I
,
d e s c r i p t i o notsfh e
A-L P e aTku fC
f .h a p iannodt h e r s
(1978)div'idedthe
which
.
.. .
.
.. .
.
..
.
..
.
. ..
..
from o l d e s t t o y o u n g e s t a r e t h e g r a y - m a s s i v e
. flow-bandea.. m @ m D-e r , a npdi n n a c l e s
..
(1976) obtained
. . .
',
A-L' Peal: Tuff i n t o t h r e e members,
'
. t h e A-L
'
PeakTuff
-a
member.
member,
S m i tahnodt h e r s
,
f i s . s i o n - t r a c k d a t e of 3 1 . 8 k-1.7 m.y.
for
a t the t y p e l o c a l i t y .
The A-L P e a k T u f f g e n e r a l l y c o n t a i n
p h e n o c r y s t so fs a n i d i n . ea n d. q u a r t z .
2 t o 1 0 percen,t
The s a n i d i n e pheno-
c r y s t s are u s u a l l y several times m o r e a b u n d a n t t h a n t h e
as euhedral to subhedral
quartzphenocrystsandarefound
g r a i n sa b o u t
1 t o 2 mm i n l e n g t h .
a r e u s u a l l y sm?.il (less t h a n ' 0 . 5
The q u a r t zp h e n o c r y s t s
mm) and ' a n h e d r a l .T h e
lower gray-massive member of t h e A-L Peak, Tuff .i s. g e n e r a l l y
lightgray
a'nd pumice poor.
It i s o v e r l a i nb yt h ef l o w ,
.
banded member, which i s g r a y t o r e d d i s h ' b r o w n
andcharac-
t e r i z e d byabundant,higlilywelded,lineatedpumice.Flow:'
f o l d s a r e o c c a s i o n a M yf o u n d
i n t h e flow-banded member.
, - . T h F p i n n a c l e s member u s u a l l y c o n t a i n s a b u n d a n t , n o n - l i n e a t e d
pumice; however, it may h a v eh i g h l yw e l d e d ,l i n e a t e dp u m i c e
-
i n t h e Sawmill Canyon c a u l d r o n .
Each of t h e members of t h e A-L Peak T u f f h a s b e e n
c o r r e l a t e d w i t h a major ash-flow t u f f e r u p t i o n a s s o c i a t e a
w i t hc a u l d r o nc o l l a p s e( C h a p i na n ao t h e r s ,
1978).
The
gray-massive member was p r e v i o u s l y t h o u g h t t o h a v e c a u s e d
t h e i n i t i a l c o l l a p s e of t h e Mt. W i t h i n g t o n c a u l d r o n
b
(Deal, 1973; D e a l andRhodes,
i978).
1 9 7 6 ; C h a p i na n do t h e r s ,
More r e c e n t work,however,
i n d i c a t e st h a tt h e
31
1
*?
4
gray-massive member probably'came from the Magdalena
d
cauldron(Chapin,
banded member was a l s o e r u p t e d
from t h e Magdalena cauldron
and i s welded' t o t h e gsay-massive member.
member i s b e l i e v e d t o
cauldron.
The p i n n a c l e s
have e r u p t e d f r o m t h e S a w m i l l
The Sawmill Canyon c a u l d r o nc o n t a i n s
of A-L PeakTuffwhich
tc
ohafeu l d r o n
.
.
Canyon
a section
may exceed 3 0 0 0 f e e t i n t h i c k n e s s ,
b u t s i n c e t h e bottomof
.. .
T?e flow-
1 9 7 9 , oralcommv?ication).
this tuff
i s notexposed
i n most
abnedc a uposofes s i bcloen c e a lfeadu l t s ,
t h i s i s o n l ya ne s t i m a t e .
The p i n n a c l e s member i s n o t
flow banded i n i t s outflow facie;, but the A-L.Peak Tuff
i n t h e Sawmill'Canyon cauldron contains flow-banded,
l i n e a t e d pumice.
b'
"
I
I ft h ep i n n a c l e s
member d i de r u p tf r o m
it was h o t enough . t o
t h e Sawmill Canyon c a u l d r o nt h, e n
causeextremeweldingandproduceflowbandinginthe
c a u l d r o n ,b u t , n o ti n ' t h eo u t f 1 o w . f a c i e . s .
I.
andflow-banded
The p i n n a c l e s
members a r e v e r y s i m i l a r p e t r o g r a p h i c a l l y ,
so it i s d i f f i c u l t t o d e t e r m i n e which m e m b e r . f i l l e d t h e
i n the
Sawmill Canyon c a u l d r o n b y o n l y e x a m i n i n g t h e r o c k s
cauldron.
In this study area, the
headofSixmile
'A-L PeakTuff
Canyon (sec. 1 0 , T. 4 S . , R.
I
veyed) alld i n Ryan H i l l Canyon.
cauldron, the
Mesa Tuff.'
i s found a t t h e
Tuff occurs entirely inside the
and t h e b a s e o f t h e
unsur-
. O u t s i d e ' t h eS a w m i l l
A-L Peak T u f f u s u a l l y
I n t h i ss t u d ya r e a ,
3 W.,
Canyon
overlies t h e B l l s
!
however, t h e A-L Peak
Sawmill Canyon c a u l d r o n ,
A-L P e a k ' T u f f is n o t e x p o s e d e x c e p t i n
-32
K\\
.f
-
,the southern part 'of Ryan Hill Canyon (secs.
25, 26 and 3 6 ) .
Here,
This
theA-L Peak
area
is
Tuff
between
overlies
the
the
unit
topographic
of
wall
Hardy
and
Ridge.
the
main
ring fractureo: the Sawmill Canyon cauldron (see Plates
1 and 2 ) .
."
The A-L Peak Tuff thins as it approaches the
topographic
margin
of
the
cauldron
to 300 feet thick in section
36.
and
is
only 200
about
It apparently overlies
an unconformity formed inside the topographic wall.
Osburn (1979, oral commun.) reports that the unit of
Sixm'ile Canyon (part of the fill of the Sawmill Canyon
cauldron) also thins
to the south against the topographic
wall
of
the
cauldron.
I
...
..
.
North of the
southern
ring
fracture
of
the
Sawmill
Canyon
cauldron (Plate 2 ) , the A-L Peak Tuff thickens dramatically.
The base of the tuff is not exposed, but the tuff is at
least 2000 feet
thick
less
than
1 mile
north
of
the
r.ing
fracture. South of the ringfracture,,itis usually about
,500 feet thick. Further northin Ryan'HillCanyon, the
A-L
.
Peak
may
exceed
3000 feet
in
thickness.
The A-L Peak Tuffis faulted againdt younger units
on
the
it
is
Canyon.
west
side
overlain
of
Ryan
Hill
stratigraphically
Canyon;
by
the
on
the
unit
of
east
side,
Sixmile
The unit of Sixmile Canyen
fihed the Sawmill
I
A-L Peak Tuff erupted. ProCanyon cauldron after the
G
nounced
lateral
facies
changes
occur
in the
unit
of
Sixmile
Canyon.
In the northern part of the kanyon, andesite lavas
33
overlie theA-L Peak; whereas further south, rhyolite
lavas,
82:rlstones, tor
'lin ash-flow
At the
the
.
unit
head
of
of
Sixmile
Sixmile
tuffs
may
overlie
Canyon, A-L
thePeak
Canyon
are
it.
. ..
faulted
Tuffand.
together
corn-in
a
.
plex manner. This area' is on the transverse shear zone
2, this studv)
. It 'is
(Chapin and others, 1978; Plate
'
also cl'ose to the
northernSawmill'Canyon-cauldron
margin,
so complex structures are likely. Slide blocks emplaced
in the Sawmill
Canyon
cauldron
could
also
..account
for
some
of the outcrop complexities. The
most'likelypossibility
though, is that these complexities result from 'complicated
normal faulting like that described in the section on
structural geology (p. 137). In sixmile Canyon, the A-L
Peak
Tuff
is
overlain
bysequence.of:thin
a
welded
,
tuffs
and volcaniclastic sedimentary rocks, which are of
part
the unitof Sixmile Canyon. These' rocks are very similar
,to the
Peak
tuffs
Tuff
sedimentary
rocks
that
overlie
A-L the
in
Ryan-HillCanyon.
The A-L
closely
and
Peak
spaced
Tuffis
characterized
by
abundant,,
sheetjoints;
it is almost
collect a handspecimen'free of
joints.
impossible
to
In the less
densely welded zones, the joints are
more irregular.
In
I
the
most
breaks
densely
both
along
welded
zones,
the
foliation
the
A-LiPeak
and
the
Tuff
sheet
joints
I
form small "platelets". In some places, outcrops are
Q
I
4L
largely masked by a covering
of these "platelets". The
A-LPeakTuff
is exposedmoderatelywell
commonly
in SixmileCanyon,.
to
34
".
I -
'
. . z-
Ryan H i l l Canyon,, it i s o f t e nf o u n d
b u tf u r t h e rs o u t hi n
i n isolated, cliffy exposures surrounded .by talus.
I n handspecimen,
t h e A-L PeakTuff
c o l o ri nn o r t h w e s t e r nS . i x m i l e
t or e d d i s h
A-L
Canyon, b u t it becomes g r a y
brown f u r t h e r s o u t h i n
Ryan H i l l Canyon.
and q u a r t z ; however, t h e p h e n o c r y s t
c o n t e n tr a n g e sf r o m
T u f fc o n t a i n s
The
5 t q 6 percent.pheno-
P e a kT u f fu s u a l l yc o n t a i n sa b o u t
c r y s t so fs a n i d i n e
is usually a gray
2 t o 15 p e r c e n t .
Most o f t h e
A-L Peak
r.
less than 5 p e r c e n t l i t h i c f r a g m e n t s , b u t - t h e
may c o n t a i n a s much a s 1 0 t o 1 5
upper part of the tuff
p e r c e n tl i t h i cf r a g m e n t s .T h e s ec o n s i s tm o s t l yo ff r a g m e n t s
o fa n d e s i t ea n dr h y o l i t e .
TheA-L.Peak. .
. .
Tuff i n S i x m i l e
Canyon i s moderately pumice p o o r , b u t i n t h e c e n t r a l t o
Ryan H i l l Canysn, it may c o n t a i n a s much
southern part of
a s 2 5 p e r c e n t pumice.
.,
..
. .
I n some placesT'the A-L Peak Tuff i s so highlywelded
t h a t it may resemble a flow-banded r h y o l i t e lava.
pumice i s a l s o h i g h l y l i n e a t e d
these areas, the
. .
'.
. .
In
and
o c c a s i o n a l l y some p r i m a r y f l o w f o l d s,. c a, .n b e s e e n i n t h e
t u f f .F i g u r e s
6 and 7
show some o ft h e s ef e a t u r e s .
These structures are thought to be primary in origin;
t h a t i s , t o haveformedbeforethe'forwardmotionofthe
t u f f h a dc e a s e d .S t r u c t u r e so f
tl:is t y p ei ni g n i m b r i t e s
i n c e n t r a l Colorado have .been described by Chapin and
L o w e l l( i np r e s s ) .
f l o w sb e g i nt o
t h a td e p o s i t i o n
They s u g g e s tt h a tu n u s u a l l yh o ta s h
weld b e f o r ef o r w a r dm o t i o n
ceases and
occurs i n a l a m i n a rb o u n d a r yl a y e r .T h i s
'
'
36
,
I
. .
F i g u r e l a . L i n e a t e d pumice i n t h e A-L PeakTuff
(SW/4,
sec. 1 0 , T. 4 S . , h. 3 W . , u n s u r v e y e d ) T
. h el i n e a t e d
pumice i s p r o b a b l y a p r i m a r y s t r u c t u r e , t h a t
i s , it
forms i n c. t u f f t h a t b e g i n s
t o weldbeforethetuff
h a s come t o rest. During t h i ss t a g e ,t h et u f fd e v e l o p s
a high viscosity and thpumice
is stretched in the
d i r e c t i o n of flow.
F i g u r e 7b.
Close-up of l i n e a t e d purnice'in
Figure l a
35
:
.Figure 6. Primary flow fold in'the A-L :Peak Tuff (NW/4,
'sec. 15, T. 4 S., R. 3 W., unsurveyed). ' Such folds are
occasionally seen in the cauldron-facies A-I, Peak Tuff
(pinnacles member ? ) within the study area (Sixmile
Canyon and northern Ryan Xi11 Canyon); however, they
are restricted to the.flow-banded member, in the outflow
facies.
..
..
..
1
. . > .
-
.
37
38
Q
causes
tuff
50 develop
the
a
high'viscosity
and
to
behave
as a rhyo?itelava. Dvxing deposition, the pumice is
stretched in the direction of movement of the tuff. Continued
movement
of
the
tuff
may
produce
shear
planes
and
folds, similar'to some structures in rhyolite lavas. The
primary
folds
to
direction
the
Nine
are
aligned
of
with
their
axes
perpendicular
pumice
are
shown
movement.
measurementson lineated
on
Plate 1. Most o f the symbols on the map represent several
measurements taken at the same location. Although there is
some
variation
in
the
measurements,
they
average
about
N 8 0 E, if one erratic value is omitted. This lineation
is
1'
_ ,
almost
perpendicular
to
the
other
measurements
recorded
nearby.
Bowring
(oral
communication)
reports
that
in
Sawmill Canyon, just west
of this study area, the lineations
were
oriented
The A-L Peak
approximately
Tuff
has
both
.,
east-west.
vertical
and
lateral
varia-
tions in the study area. In Sixmile Canyon,
it is usually
gray, pumice-poor, densely welded,
and'containsonly 1
to ,2 percent
Primary
they
phenocrysts
folds
are
not
In Ryan
are
and
1 to 2 percent
occasionally
found
in
lithic
this
fragments.
area,
although
common.
Hill
Canyon,
The A-L Peak
welded than in SixmileCarzyon.
Tuff
is
less
densely.
The Pumiceis still lineated,
but notas highly as in the Sixmile Canyon area.
A few
primary
6;;
but
folds
few,
if
were
any,
seen
in
the
northernof part
the canyon,
occur
in
the
central
to
o$
southern
'part
39
f?
t h ec a n y o n .I n
Ryan H i l l Canyon, t h e A-L Peakmatrix
usually gray to reddish
g r a y i s h - r e dt o
is usually a
brown and>hepumice
dark-brown c o l o r .
is
in
The A-L PeakTuff
Ryan H i . 1 1 Canyon i s s l i g h t l y more c r y s t a l - and l i t h i c - r i c h
t h a n it i s i n S i x m i l e Canyon.
3 to 8 .
I t u s u a l l yc o n t a i n s
p e r c e n tp h e n o c r y s t so fs a n i d i n ea n dq u a r t z ,a l t h o u g h
it
may c o n t a i n a s much as 15 p e r c e n t p h e n o c r y s t s i n t h e
Two t o 1 5 p e r c e n t1 i t h i c . f r a g m e n t s
u p p e rp a r to ft h et u f f .
of andesite
and r h y o l i t e o c c u r i n t h e
A-L
PeakTuff
in
Ryan H i l l Canyon.
On t h e e a s t e r n s i d e
of Ryan H i l l Canyon, t h e s t r a t a
( ? I t ou p p e r
d i pt ot h ee a s t ,t h e r e b ye x p o s i n gt h em i d d l e
portion of the
A-L
u n i to fS i x m i l e
PeakTuffand
Canyon.
the lower part of the
U n f o r t u n a t e l y , much of t h i s ,
especially i n thenorthernpartofthecanyon,
is u n s u i t -
c o v e r e dw i t ht a l u s .A l t h o u g hm o s to ft h i sa r e a
able for taking
is largely
a measured section, g e n e r a l i z a t i o n s c a n
made a b o u t t h e u p p e r
A-L
Peak based
gvseeenrqteuirceaanllc e s .
on!mappingand
be
the
I
A t thetopofthesection,
severa~lt h i n r h y o l i t e l a v a s ,
s a n d s t o n e s and t u f f s are i n t e r b e d d e d i;”t h e A-L P e a k T u f f .
Theserocks
are d e s c r i b e d i n t h e f o l l o w i n g s e c t i o n ( p .
Below t h e l a v a s , t h e
A-L P e a k T u f f c o n t a i n s
p h e n o c r y s t so fs a n i d i n ea n dq u a r t z ,a n d
l i t h i cf r a g m e n t s .
43).
4 t o 6 percent
1 0 t o 15 p e r c e n t
The p e r c e n t a g eo fl ’ i t h i cf r a g m e n t s
increases g r a d u a l l y f r o m t h e b o t t o m o f t h e c a n y o n
to the
b a s e of t h er h y o l i t el a v a - s a n d s t o n ei n t e r v a l .A l t h o u g ht h e
40
c
F\
*/
tuff
of
is
highly
weldin?
The tuff
decreases
below
generally
welded
the
cvntains
throughout
slightly
to
section,
the
degree
upwards
in thesection.
rhyolite
three
the
lava-sandstone
interval
five
much
times
a3
sanidine
as quartz.
The A-L Peak
"
Tuff
is similarto the
above
the
lava-sandstone
underlyingA-L Peak,
except
interval
that
it
con-
. tains 8 to.10 percent phenocrysts of quartz and sanidine,
and the quartz is about as abundant
as the sanidine. The
-
tuff
-
in
interval
is only
this
In
thin
section,
the
moderately
Peak
Tuff
A-L
to
d,ensely
welded.
in Sixmile
Canyon
is very crystal-poor, containing only about
1 to 2 percent
phenocrysts of sanidine and quartz. Sanidine is more
i'
,
abundant
than
the
quartz
and
usually
occurs
as subhedral
' 5
to euhedral grainsas much as 1.0 mm in length. The
sanidine
I
from
in
which
Some of
this
thin
the
area,
sections
sanidines
birefringent
as as
well
all
clay,
were
were
giving
the
made,
partially
them
a
other
was
partially
replaced
cloudy
areas
by
altered.
a
appearance,
low
while
.
others were completely replaced by sericite and other
.
clay minerals.
The quartz occursas small (0.2 mm or
less), anhedral grains. Hematite is fairly ubiquitous
throughout the thin section, usually occurring as small,
.
bladed or acicular crysta;s.
Pumice occurs in amounts
of about 5 percent
in most'
the highly
This pumice
100 to 1; in
has
or
less
to thickness
length
ratios
welded
greater
zones.
than
thin section the pumice rarely exceed
0.3 mm in
41
thickness. Crystals of vapor-phase quartz with a diameter
of 0.1 m
m, or less, occur in the pnmice and along shear
-
“a
8).
planes in the matrix (see Fig.
The A-L Peak
Tuff
in
Ryan
Hill
2anyon
(except
that
above the rhyolite 1ava)usually contains
3 to 5 percent
phenocrysts of sanidine and quartz. The sanidine has
an
average length of about
0 . 1 5 nun and is euhedral to submore. abundant
hedral. It is between three and five times
than the quartz. The sanidine is always altered to some
extent. The quartz has an average diameter
of about 0.2
mm and is anhedral. Trace amoucts of
biotite.occur in
thin
section,
and
usually
a
few
percent
hematite
is
present
A-L Peak usually
as bladed and acicular crystals. This
about2 to 10 percent lithic fragments, which
contains
are probably andesite and, rhyolite fragments, and 10 to
15 percent pumice. The amounts of pumice and lithic
fragments generally increase upward in, the tuff.
The
pumice
and
matrix
are
both
devitrified’
to
a
fine-grained
assemblage of quartz and feldspar; occasional spherulitic
and axiolitic structures were seen
in ;the matrix. Outlines
of
the
dust
original
shards
are
preserved
by
hematite
inthe matrix. Most of the thin sections also contain
extensivl
quartz
has
glass
vapor-phase
much 0.3
as nun in
as
been
minerals
completely
consisting
of feldspar
I
diameter. Most of the
,
altered anto opaque
mixture
and
feldspar
of
clay
minerals. Much of the pumice has been replaced by small
feldspar
by
crystals
anhedral
which
quartz.
point
inward
and
are
surrounded
42
Figure 8 . Photomicrograph of flow-banded
A-L Peak Tuff.
This sample came from Sixmile Canyon where the tuff is
very highly welded and appears flow banded in hand specimen
and outcrop. The phenocryst in the photograph is sanidine.
The shear planes in the tuff are defined by light-colored
vapor-phase minerals. Crossed nichols,,31.25 x.
. .
43
The uppermost A-L P e a k T u f f ( t h a t t u f f s e p a r a t e d f r o m
f
....~
'i
t h e rest by t h e r h y o l i t e l a v a - s a n d s t o n e i n t e r v a l ) c o n t a i n s
8 t o 1 0 p e r c e n tp h e n o c r y s t so fq u a r t za n ds a n i d i n e ,a n d
1 0 t o1 5p e r c e n k
l i t h i c fragments.
between t h i s t u f f
and t h e t u f f
i s t h a t it c o n t a i n s a b o u t a s
below t h e r h y o l i t e l a v a s
much q u a r t z a s f e l d s p a r .
and 3 t o 4 p e r c e n ts a n i d i n e .
0.7 nun d i a m e t e r .
a b o u t 0.4'mm.
The q u a r t zh a sa na v e r a g e
nun; t h e l a r g e s t q u a r t z p h e n o c r y s t
.
is
.
The s a n i d i n eh a sa na v e r a g e ,l e n g t ho f
The l i t h i cf r a g m e n t s
are m o s t l ya n d e s i t e
Pumice c o m p r i s e s . 1 5t o 20 p e r c e n t
and r h y o l i t ef r a g m e n t s .
of t h er o c k .
The
5 t o 6 percentquartz
thinsectionstudiedcontainsabout
diameter ofabout0.3
The m a i nd i f f e r e n c e
The A-L PeakTuff
in this unit
is only
e'
moderately to densely welded.
i.
I n t e r b e d d e dR h y o l i t e
t h ec e n t r a lp o r t i o n
2 6 , T. 4 S., R.
-.
'
'
3
Lavas, S a n d s t o n e sa, n dT u f f s .
In
of R y a n ' H i l l Canyon (secs. 22,23,and
VI.,
unsurveyed), a t h i n seyence of
, .
r h y o l i t e lavas ( T a l r ) , and s a n d s t o n e s and t u f f s ( T a l s )
i s i n t e r b e d d e di nt h eu p p e r
A-L Peak T u f f .
t h i c k n e s s of t h i s u n i t d o e s
n o t exceed! 200 t o 3 0 0 feet:
The t o t a l
it i s p r o b a b l y less t h a n t h i s i n m o s t p l a c e s .
The r o c k s of t h i s u n i t a r e u s u a l l y v e r y p o o r l y e x p o s e d ;
o u t c r o p s are u s u a l l y small and separated b y c o n s i d e r a b l e
talus.
Because of t h ep o o re x p o s u r e s , s i t
to trace i n d i v i d u a l r o c k u n i t s l a t e r a l l y
G
few t e n s of feet.
However, t h e g e n e r a l
is impossible
f o r more t h a n a
vertical sequence
'..
44
a
t,
c abnees t a b l i s h eidn
a few p l a c e s .
A s t h e map shows
lava i s u n d e r l a i n e n d o v e r l a i n
( P l a t e l)., t h e r h y o l i t c .
by a s a n d s t o n e a n d t u f f i n t e r v a l i n m o s t p l a c e s .
is gr?.y t o p i n k
The r h y o l i t e l a v a ( T a l r )
c o n t a i n s 1 0 t o 1 5 p e r c e n tp h e n o c r y s t s .
a r em o s t l yq u a r t za n df e l d s p a r ;i n
matrix is aphanitic.
'
and u s u a l l y
The phenocry.sts
handspecimen,the
One t h i ns e c t i o n
a b o u t 1 5 p e r c e n tp h e n o c r y s t s .A b o u t
e x a m i n e dc o n t a i n s
7 p e r c e n to ft h er o c k
is subhedral,partiallycorrodedquartzwithanaverage
d i a m e t e ro fa b o u t
a b o u t 1 . 5 mm i n d i a m e t e r .T h i sr o c kc o n t a i n s
feldspar.
-.
1
is
0.5 mm; t h e l a r g e s t q u a r t z p h e n o c r y s t
About 3 p e r c e n to ft h er o c k
two t y p e so f
i s p e r t h i t e ,w i t h
0 . 6 t o 0.7'mm. , A l s o , a b o u t
a na v e r a g ed i a m e t e ro f
p e r c e not h
tfr eo ccko n s i s o
t sf
2
a p h e n o c r y tsht ah ta s
been r e p l a c e d by c l a y m i n e r a l s a n d
is, t o o a l t e r e d t o i d e n t i f y .
About 2 p e r c e n tb i o t i t e( l a r g e l yr e p l a c e d
p r e s e n ti nt h er o c k .
is
byhematite1
The m a t r i xc o n s i s t sm o s t l yo ff . i n e -
m, o r less) c h e r t y a n d s p h e r u l i t i c q u a r t z ,
.grained ( 0 . 1 m
f e l d s p a r( s a n i d i n e
?),
a n dc h a l c e d o n y .A l s oh e m a t i t ed u s t
matrix.
is s c a t t e r e d t h r o u g h t h e
The sandstoneand
tuff unit(Tals)consistsof
a
heterogeneous mixture o f p o o r l y w e l d e d , l i t h i c - r i c h t u f f s ,
a n di n t e r b e d d e ds a n d s t o n e s .
The s a n d s t o n e s are m o s t l y
grayishgreen,
medium g r a i n e d , l i t h i c r i c h , f e l d s p a r . r i c h
andmoderately
to p o o r l ys o r t e d .
brown t o p i n k a n d ' c o n t a i n
f e l d s p a r andqua.rtz,and
The t u f f s a r e g e n e r a l l y
5 t o 1 0 percentphenocrysts
of
1 0 t o 25 p e r c e n t l i t h i c f r a g m e n t s
"
45
I
9.
..
(mostly andesite). The tuffsare moderately to poorly
welded
and
frequently
pumice
Unit
The
unit
of
of
rich.
Sixmile
Sixmile
Canyon
Canyon
is
a
thick
sequence
of
andesite and rhyolite lavas, ast:-flow tuffs, voland
caniclastic sedimentary rocks. The rocksin this unit
undergo rapid lateral facies changes; in some places,
thick
andesite
lavas
interfinger
with
thick
rhyolite
lavas. Krewedl (1974) named the Sixmile Canyon andesite
after
some
outcrops
in
the
heads
of
Sixmile
and
South
Canyons. This name was changed to the unit of Sixmile
Canyon by Osburn (1978). Krewedl (1974). also mapped a
unit in
the
head
of
Sixmile
Canyon
which
he
called
the
Sawmill Canyon Formation. Krewedl's Sawmill Canyon Formation
consists
mostly
of
A-L
Peak
Tuff,
and
to
a
lesser
of Sixmile Canyon (as mapped for this
extent, of the unit
thesis). Krewedl's South Baldy Peak andesite is also
included in the unit
c.f Sixmile Canyon. The unit of
Sixmile
Canyon
has
not
yet
been
dated,
but
the
underlying
and overlying units have been dated at m.y.
31.8 (A-L
Peak Tuff) and26.3 m.y. (tuff of Lemitar Mountains).
The
and is
unitof Sixmile
part
of
the
Canyon
cauldron
overlies
of the
fill
the
A-L
Sawmill
Peak
Tuff
Canyon
cauldron.
So far, it has not been found outside of the
cauldron,
but.rocks
position
are
present
oral communication).
of
in
similar
the
appearance
Magdalena
and
cauldron
stratigraphic
(Chapin,
46
..
f.
...
The n o r t h e r n r i n g f r a c t u r e o f t h e S a w m i l l
Canyon
'
cauldron transects the northwestern pcxtion of this
s t u d ya r e a( P l a t e
I t i s a n e a r - v e r t i c a lc o n t a c t
2).
between t h e Xells Mesa Tuffand
the unit of
S i x m i l e Canyon.
Part of the northern cauldron margin represents
r i m and p a r t o f
tionalcontactalor3thetopographic
t h er i n gf r a c t u r e( P l a t e
2).
the cauldron margin in
a depo-si-
it i s
L a t e rf a u l t i n gh a sr e a c t i v a t e d
some p l a c e s ; p a r t o f t h e , c a u l d r o n
margin has also been uplifted in
a horst block.
The s o u t h e r n r i n g f r a c t u r e
of t h e c a u l d r o n ( P l a t e
2)
a p p a r e n t l y was n o t r e a c t i v a t e d b y - f a u l t i n g a f t e r t h e
depositionofthe
u n i t o fS i x m i l e
Canyon, s i n c e t h a t u n i t
c o n t i n u e su n f a u l t e do v e rt h er i n gf r a c t u r e .
Mapping by
Osburn ( i n p r o g r e s s ) i n d i c a t e s t h a t t h e u n i t o f S i x m i l e
Canyon p i n c h e s o u t a g a i n s t t h e t o p o g r a p h i c c a u l d r o n m a r g i n
on t h es o u t hs i d eo ft h ec a u l d r o n .
The u n i to fS i x m i l e
Canyon h a s a maximum t h i c k n e s s of a b o u t 2500 - f e e t .
I n some p l a c e s , t h e
.
T u f fi n
s t r i k e s and d i p s i n t h e
A-L Peak
Ryan H i l l Canyon d i f f e r from t h o s ei nt h eo v e r -
l y i n g u n i t o fS i x m i l e
Canyon.
T h i sc o u l di n d i c a t et h a t
t h e r e i s anangularunconformitybetweenthe
two u n i t s :
however, f a u l t i n g i s p r o b a b l y r e s p o n s i b l e f o r m o s t o f t h e
d i f f e r e n c ei nt h es t r i k e s
a n dd i p s .
Canyon i s o v e r l a i n by t h e t u f f o f
The u n i t of Sixmile
Lemitar Mountains.There
i s no..apparentangularunconformitybetweenthetuffof
C a r o n i t a Canyon ( u p p e r p a r t o f t h e
and t h e o v e r l y i n g t u f f
u n i t of Sixmile Canyon)
of L e m i t a r M o u n t a i n s , a l t h o u g h t h e r e
is a n e r o s i o n s u r f a c e b e t w e e n k h e ' t w o .
47
Lower I n t e ro
v fa l
6"
4.
T u f f s , Sandstones
Lavas.
and
A group
"
oftuffs,sandstones
ar.? l a v a s o v e r l i e s
A t t h e headofSixmiJeCanyon,
i n m o s to ft h es t u d ya r e a .
and s a n d s t o n e s ,a n d
this g r o u p c o n s i s t s e n t i r e l y o f t u f f s
was mapped a s T x t .
I n. s o u t h e r n
Ryan H i l l Canyon, t h i s group
c o n s i s t so fr h y o l i t el a v a s ,t u f f s
r h y o l i t el a v a( T x r l )o v e r l i e s ,
sandstonesequence(Txtl
i nT x t ,T x t l ,a n d
20-Et)
t h e A-L Peak Tuff
and s a n d s t q n e s .
and i s o v e r l a i nb y ,
and T x t 2 ) .
a tuff-
The t u f f s a n ds a n d s t o n e s
Txt2 typicallyconsist
of t h i n (5- t o
i n t e r v a l so fi n t e r b e d d e d ,p o o r l yw e l d e dt u f f sa n d
sandstones.
There axe s e v e r a l d i f f e r e n t t u f f s i n
a few p l a c e s .
these
were o n l y
units,butmostofthesearelocaltuffsthat
s e e ni n
The
The T x t 2u n i ta l s oc o n t a i n s
a thin
andesitelava.
The complex o u t c r o p p a t t e r n s i n t h e h e a d
of S i x m i l e
Canyon are i n t e r p r e t e d t o be c a u s e d m o s t l y b y f a u l t i n g ;
Canyon (Txt) c o u l d
however, some o f t h e u n i t o f S i x m i l e
havebeenemplaced
as s l i d e b l o c k s i n
( T x a l ) .T h i sa r e a
is d i s c u s s e df u r t h e ri nt h es e c t i o n
the andesite lava
on
structural geology.
The Txtl,
Txrl,andTxt2unitsalloccurinsouthern
Ryan H i l l Canyon (secs. 2 5 , 2 6 , and36,,
unsurveyed).
f r e q u e n t l yc o v e r e dw i t ht a l u s .
6;
3 W.,
4 S., R.
The T x t l u n i t o n l y c r o p s o u t o n t h e e a s t e r n
s l o p e of Ryan Hill Canyon.
l4
T.
i n r a v i n e s where e r o s i o n h a s
A rhyolitelava
It is a t h i n u n i t t h a t
is
The b e s te x p o s u r e so c c u r
removed t h e o v e r l y i n g t a l u s .
(Txrl) o v e r l i e s t h e t u f f s a n d s a n d s t o n e s :
48
of the Txtl unit. This lava is in turn overlain by another
r
tuff and sandstone unit (Txt2). mhe rhyolite
l m a and Txt2
units
Hill
are
fairly
Canyon
and
well
in
exposed
the
upper
on,.the
reaches
eastern
of
rim
of
Caronita
Ryan
Canyon.
The rhyolite lava occurs
as low, blocky to cliffy outcrops,
while the Txt2'unitoccurs as low, .rounded outcrops.
The tuffs in Txt, Txtl and Txt2 exhibit a wide variety
of characteristics in hand specimens. In general, they
are
.
lithic-rich,
moderately
crystal-rich
and
poorly
welded.
In thin section, most of these tuffs contain
5 to 15
percent quartz and5 to 1 0 percent feldspar (mostly sanidine
and some perthite).'They also contain5 to 25 percent lithic
-.
.
. .
fragments,
.
which
.
consist
mostly
of
andesite
and
lesser
amounts of rhyolite. Also, a few lithic fragments are
fine-grained
silicic
fragments
which
consist
mostly
of
anhedral quartz grains. The tuffs are usually moderately
10 to 40 percent of
pumice rich; the pumice may comprise
the rock.
The
sandstones
usually
consist
of even,
parallel
beds, although occasionally, cross-bedding
was observed.
( 0 . 2 5 mm) sandThey are mostly medium- to coarse-grained
".e
stones, consistingof angular grainsof quartz, feldspar
and lithlc fragments. The proportion of these grains
may
vary,
all the
but
sandstones
are
lithic
rich
contain 10 to 50 percent lithic fragments..One thin
section of the sandstone contains40 to 50 percent .
lithic fragments,25 to 30 percent quartz,10 percent
and
.
"'
49
sanidine, 5 percent
plagioclase
and
5 percent
opaque
minerals
(probably Fagnetitel. Tke sandstone is poorly sorted and
the average grain diameter is about
0.9
m
m
.
A number of
different types of lithic fragments were observed. The most
.
common types are fine-grained Silicic fragments
ivhich are
.
.
.
probably rhyolites. Lesser amounts of andesite or latite
fragments
occur
in
the
sandstone.
. .
The rhyolite lava (Txrl) in southern Ryan Hill Canyon
is pink to white and phenocrysts poor. Occasionally, the
rhyolite is flow banded or brecciated, but usually it is
massive a d aphanitic.
A few crystals can be seen in' hand
specimens]
are
but
these
Andesite Lavas (Txal)-
probably
A
secondary
quartz.
thick sequence of andesite
flows crops out in the upper portions
of Sixmile, South and .
Sawmill Canyons. Krewedl (1974) included the andesites in
bothhisSixmileCanyonandesiteandSouthBaldyPeak
andesite,
although
there
is
no
..
difference
between
the
two
units. Osburn (1978) included' the andesites
in the unitof
Sixmile
Canyon.
..
These andesite lavas('lxal) prob~blyinterfinger with
the sanidine rhyolite lava (TxrZ) tli
at
e head of Ryan Hill
Canyon (sec. 15, T. 4
the
unit
of
S.
Sixmile
,
R. 3 W. I unsurveyed)
Canyon
has
been
.
Most of
removed
by
erosion
this area. However, Osburn (1978) and ,Bowring (in preparation) report that the andesite lavas probably interfinger
with the rhyolite lavas
in their study :areas. There' is, a
in
50
thin
layer
lava
on
of
the
andesite
lava
northeastern
beneath
side
of Ryan
the
Hill
sanidine
Canyon;
a
rhyolite
thin
andesite lava flow{.Txa2) overlies the rhyolite lavas in.
..
Chavez Canyon.
In a fewplaces, the andesite lavas (Txal) may overlie
the A-L Peak
Tuff;
however,
in
most
places
overlie tuffs and sandstones mapped
as Txt
for
the
andesite
lavas
this.thesis.
The andesite lavas are probably about
2000 feet thick in
the
Timber
Peak
area;
sandstones which'are
here,
in
the
they
upper
are'overlain
part
of
the
by
unit
tuffs
of
and
Six-
mile Canl7on.
The
best
outcrops
of
the
andesite
lavas onoccur
the
ridge north of Timber Peak. Here, the andesites usually
break along closely spaced joints to form sharp, blocky
C'
"
I
outcrops. On the sides of this ridge, the andesites are
frequently covered with talus. They are usually well exposed on
.Timber
the
ridges
at
the
head
of
Sixmile
.
Peak)
Canyon (east
. .
Osburn (1978) divided the andesite lavas into three
types:
1) dense, aphanitic; 2
)
pyroxene-porphyritic (La
Jara Peak-like); and 3) plagioclase-porphyritic. Rock typ'es
similar to these were recognized
in this study area. The
pyroxene-porphyritic
lavas
were
maL-ped
as Txa3
for
this
stJdy. The andesites in Txal consist mostly of dense,
aphanitic and plagioclase-porphyritic lavas. Also, afew
pieces of a very coarse-grained
(2 cm or more), porphyritic
'
lava were seen in.talus, although no definite outcrops of
of
51
this unit were observed. Because the andesites in this
study
area
occix
on .?tip
the slope
of
Timber
Peak,'
and
because of faulting, it is difficult to determine the
vertical
distribution'of
the
different
andesite
types.
The most abundant rock type in the study area. is a
gray-to-purple, aphanitic andesite. This rockis often
.I
dense and massive, but occasionally, it is brecciated or
.contains moderately abundant to abundant vesicles. The
vesicles are frequently filled with calcite, quartz or
"
celadonite. Occasionally, this rock type may contain
1
to 3 percent small (1 to 2 mm) phenocrysts of feldspar,
or 1 to 4 percent small (about1 mm) phenocrysts of
magnetite.
$?
.
Inthinsection,thisrockconsistsmostlyoffine-
< ,
.grained
plagioclase
with
an average
length
of
0.1
about
mm. Three to4 percent phenocrysts of magnetite were
. .
also observed in one thin section. These phenocrysts
are euhedral and have an average diameter
of 1
mm. Ten
to 20 percent o f the
of
thin
minerals (magnetite ? ) .
about 0.02 to 0.03
section
consists
small
opaque
They have an average length
of
mm. This rock generally has a trachytic
texture.
Second
in
abundance
are
the
plagioclase-porphyritic
andesites. These lavas contain 20 to 50 percent plagioclase. Most of the plagioclase phenocrysts are about
2
G
to 3
m
m
'
in
length
and
are
frequently
altered
to white,
chalky clays. Differences in mineralogy indicate that
52
thereprobablyismorethan
5
f?
one type of plagioclase-
porphyritic andesite. One thin seelion. examined contains
about 40 percent
phenocrystso f plagiocla'se
mm. The matrix consists
average length of about
1.9
mostly of plagioclase
0.1 mm.
with
an
with
an
average
length
of about
About 3 0 percent of the matrix consists of
opaque minerals. Some of these minerals consist of
hematized
mafic
anh
are
some
minerals
probably
(probably'
magnetite.
The
pyroxene
sizeof the
minerals varies from about
0.01 to 0.25
section
examined
contains
.,
about
2 3 percent
biotite)
-
opaque
Another thin
phenocrysts
plagioclase with an average diameter
of'about 2.0
m
m
.
also
with
contains
about
15 percent
c ,
%.
nm.
and
average length of about
consist
mostly
of
1.0
opaque
minerals
of
It
an
mm. -Theseminerals probably
pyroxene
which
has
been
partially
altered
to hematite., Some of the opaque minerals may also be
magnetite.
!
The very-coarse-grained, porphyritic lava was found
only at one locality(NW/4,
unsurveyed).
SW/4,
sec. 4 , T. 4 S.,
R. 3 W.,
It consists of about 20 percent plagioclase(?)
phenocrysts
which
The plagioclase
range
from
phenocrysts
about
1 to 4 cm
are
a1tered:toa
inlength.
yellowish-
white, chhlky clay. The matrix is purpie and codtains 5
to 10 percent pyroxene (?) which
In
i
G
a
few
The breccias
are
places,
coarse
comprised
a foot indiametersetin
of
is2 to 3 mm in length.
andesitic
clasts
breccias
crop
averaging
6 inches
a finer-grained(silttosandsize),
to
out.
53
f2
p u r p l e - t o - g r a ym a t r i x .T h e s er o c k sa r ep r o b a b l yl a h a r i c
brecciasdepositedalongthe
walls 7 f t h e
Sawmil? Canyon
cauldron.
SanidineRhyoliteLavas(Txri)-
Brown t op i n k i s h - g r a y ,
porphyritic,rhyolitelavaswithsanidinephenocrysts
l a r g e a s 1 cm, c r o p o u t o v e r
a r e a .T h e s el a v a s
as
a largepartofthestudy
were mapped a sT x r 2 .
They were f i r s t
d e s c r i b e d by Osburn ( 1 9 7 8 ) w;:o mapped a p a r t o f t h i s u n i t
Sixmile
in
Canyon.
... .
3 s q u a r e 'miles
The s a n i d i n e r h y o l i t e l a v a s c o v e r a b o u t
in the study area
and c a p t h e n o r t h e a s t e r n
H i l l Canyon ( F i g . 9 ) .
i'
'.
r i m of Ryan
The r h y o l i t el a v a sa r ea l s oe x p o s e d
i nt h eu p p e rp o r t i o n so fC h a v e z ,C a r o n i t a ,X o l i n o ,a n d
I
MaderaCanyons.These
r h y o l i t el a v a sh a v eb e e n
Bowring ( i np r e p a r a t i o n )a n d
Canyon.
Roth ( i np r o g r e s s )i nS a w m i l l
Osburn ( i n p r o g r e s s )
S o u t ho ft h i ss t u d ya r e a ,
to pinch out against
reports that the rhplite lavas appear
thesoutherntopographjc
r i m o ft h eS a w m i l l
The r h y o l i t e l a v a s a r e p r o b a b l y a b o u t
feetthick
mappedby
I
Canyon c a u l d r o n .
1100 t o 1200
on t h e eastern r i m of Ryan H i l l Canyon.
south,inCaronita
Canyon, t h e r h y o l i t e l a v a s
2000 f e e t i n t h i c k n e s s .
Further
may exceed
However, u n r e c o g n i z e df a u l t i n g
may b e p a r t i a l l y r e s p o n s i b l e f o r t h i s a p p a r e n t t h i c k n e s s .
Where t h e b a s a l c o n t a c t
i s exposed,thesanidine
rhyolitelavasoverliethetuffs,sandstonesandrhyolite
Q;;
lavas t h a t c o m p r i s e t h e u n i t s
Txtl,
T x t Z , andTxrl.
On t h e
54
4
"
'
northeastern
may
also
slope
overlie
of
the
Ryan
A-L
Hill
Peak
Canyon,
Tuff,
or
the'sanidine
andesite
rhyolite
lavas
from
the Txal unit; however, the contact.is always covered with
talus. The sanidine rhyolite lava
2 s usually overlain by
either
sedimentaryr o c k of
the
Txsl
unit
or
andesite
lavas
,of the Txa2 unit in Chavez and Caronita Canyons. At the
head of Molino and Madera Canyons, the sanidine rhyolite
may
be
overlain
by
either
the
upper
or
lower
members
of
the tuff of Caronita Canyon. The sanidine rhyolite lava
probably
interfingers
to
the
north
with
andesite of
lavas
"..
the Txal unit.
The
ridges,
d\
~
sanidine
but
they
rhyolite
lava's may form bold, prominent
are
frequently
covered
with
small
blocky
talus, as on the northeastern rim of Ryan Hill Canyon (Pig.
.I
9).
Occasionally, the rhyolite lavas display excellent
flow banding (Fig.10).
This flow bandingis not displayed
everywhere and, in general, beeomes more prevalent towards
the south. Secondary quartz and chalcedony which formed
.a. r
.
along shear planes in the rhyolite lava enhance
flow
the
'
banding.
In hand specimen, the rhyolite usually contains
15 to
40 percent phenocrystsof sanidine,
quartz and biotite.
is sanidine, whichis
About 15 to 30 percent of the rock
usually about twiceas abundant as quargz. Most of the
sanidine
G
which
,consists
of
euhedral
to-
subhedral
Phenocrysts
2 to 3 mm, although
havean average length of about
some of themare
aslong as 1 cm.
The sani'dine is usually
55
F i g u r e 9. V i e w l o o k i n gn o r t h e a s t a t t h e e a s t e , r n r i m of
Ryan H i l l Canyon. The r i m is' capped by a s a n i d i n e
r h y o l i t e lava ( T x r 2 ) t h a t d i p s t o t h e east 'and forms a
p r o m i n e n th o g b a c k .T a l u s ,d e r i v e dm o s t l yf r o mt h e
r h y o l i t e l a v a , covers xuch of t h e e a s t e r n slope of Ryan
H i l l Canyon. T h i st a l u s . c o m p l e t e l yc o v e r st h er h y o l i t e
a n du n d e r l y i n gu n i t s
on t h eu p p esr l o p e , otfh ec a n y o n .
F i g u r e ' 1 9 p r e s e n t s a b e t t e r v i e w of t h e l a r g e , a c t i v e
t a l u s d e p o s i t on t h i s s l o p e .
Molino Peak i s o n t h e f a r
s
i
d
e
o
f
right
thephotograph.
3
I
;t
56
F i g u r e 1 0 . Example of flow banding i nt h es a n i d i r i e
r h y o l i t e lava ( T x r 2 )i n Chavez Canyon ( N N / 4 , sec. 2 5 ,
T. 4 S., R. 3 N., u n s u r v e y e d ) . The area where t h i s
photograph was t a k e n c o n t a i n s t h e m o s t s p e c t a c s l a r
flow banding i nt h es a n i d i n er h y o l i t el a v a .N o r t ho f
thisarea,theflowbanding
i s p o o r l yd e v e l o p e d o r
a b s e n t .S o u t ho ft h i s
a r e a , t h e flow banding i s o n l y
Flow f o l d s and ' e r r a t i c s t r i k e s
s p o r a d i c a l l yd e v e l o p e d .
and d i p s a r e common.
57
altered
to
a
pink
or
white
color,
although
occasionally,
they are.clear and display chatoyaray. The quartz usually
occurs
as
small,
anhedral
diameter of about0.5
nun.
phenocrysts
with
an
average
The rhyolite also contains1
to 3 percent biotite. Two to 3 percent lithic fragments
.*
of brown, fine-grained andesite occur in 'bf
most
the
rhyolite
lavas. The
matrix
,
is
usually
aphanitic
and
dense
in hand specimen.
There may have been two or more flows
lava in
this
15 to
unit. The rocks in the lower part xsually contain
2 5 percent
phenocrysts, while the rocks in the upper part
contain 2 5 to 35 percent phenocrysts. The sanidine/quartz
ratio i,s always about
2 to 1, but the rock contains about
1 percent
the
biotite
at
the
base 4and
percent
at
the
top
of
unit.
I
Thin
sectionsof the
between 2 0 and 35 percent
sanidine
.
rhyolite
phenocrysts
.
lava
contain
consist
of
which
sanidine, quartz and biotite. Sanidine, which comprises
about 10 to 2 0 percent
of
the
rock,
is
usually
about
as abundant as quartz. The sanidineis^ euhedral and has
an average
length
served
thin
in
of
about
1.1 mm; the
section
is,about 3 . 3
maximum
mm.
~
The
length
ob-
quartz
is
anhedral 20 subhedral and has an average diameter
of about
0.5
nun; the
maximum diameter is about
1!0
m
m
.
Some of the
quartz is slightly embayed. .Biotite, with
an average
diameter of 0.3 mrn to 0.4 mrn, comprises 1 to 4 percent
the rock. Most of the biotite is a deep red color.
A
of
twice
58
I
4-.,
trace to 1 percent opaque minerals (probably magnetite)
also
occurs
in
The matrix
the
rhyolite.
consists
mostly
of
chalcedony,
and
fine-
Some rocks contain
grained quartz and sanidine( ? ) .
spherulitic structuresin.the matrix. Layers of quartz
and
chalcedony
These
minerals
define
the
probably
flow
formed
banding
along
in
shear
these
rocks.
planes
in
the
rhyolite.
Andesite Lavas (TxaZ1, A thin sequence of andesite
lavas overlies the sanidine rhyolite lavas
in Chavez and
Caronita Canyons. In a few places, these andesite lavas.
are overlain by sedimentary rocks (Txsl); elsewhere, they
” <
are overlain by the tuff of Caronita Canyon.
A complete
section of these lavas is not exposed; however, the total
thickness
feet.
with
the
lavas
probably
does
not 30
exceed
to 40
The lavas are always partially to completely covered
talus
.
of
derived
from
the
overlying
tuff
of
Caronita
I
Canyon.
In hand specimen, the
andesite
lavas
are
brownish
gray, dense to vesicular and fine grained.
The vesicles
are usually filled with quartz, opal
or celadonite.
The
andesites usually contain
1 to 3 percent small (less than
1 mm) phenocrysts of feldspar (probably plagioclase). The
matrix
G
has
an
aphanitic
texture.
Sandstones and Siltstones(Txsl)’
sandstones
and
siltstones
crops
out in
A thin sequence of
Caronita
Canyon
59
(sec. 31, T. 4 S., R. 2 W.) and on Timber Peak. In Caronita
Canyon, this unit overlies an andesfte (Txa;)
lava and is
,
overlain by the tuff of Caronita Canyon. This unit50 is
feet thick or less in Caronita Canyon and is exposed in
only a few elongate outcrops. White to
red, very thinly
bedded (1 cm or less) siltstones comprise most of the
unit in Caronita Canyon. Pieces of
red, medium- to coarsegrained
this
also observed in talus in areas where
were
sandstone
unit
This
was
unit
covered.
also
occurs
in
a
few
outcrops
on Timber
Peak, where it has a maximum thickness of about 150 feet.
The
best
outcrops
are
on
the
north
side
of
Timber
Peak.
Here, this unit consists mostly of greenish-gray, poorly
bedded siltstones and sandstones. A few coarser grained
rocks (mostly .sedimentary breccias) also occur on Timber
Peak.
The clasts in this breccia consist mostly of
. .
andesite fragments.
Tuff of CaronitaCmyon.
is
a'multiple-flow,
It consists
of
a
simple
lower
The tuff of Caronita Canyon
cooling
unit;of ash-flow tuff.
member.
of 'to
poorly
densely
welded,
crystal-poor, moderately lithic-rich, andesitic (in
mineralogy)
tuff
and
an
upper
member
of
imoderately
densely welded, crystal-rich, rhyolite ash-flow tuff.
Good
exposures
of
these
tuffs
can
be in
found
Caronita
T. 4 S., R. 3 W., unsurveyed;
and Chavez Canyons (sec. 25,
and secs. 30 and.31, T. 4 S., R. 2 W.).
The tuff of
to
.
'
C a r o n i t a Canyon is e q u i v a l e n t t o p a r t o f t h e
6%
*@-.,
mapped byOsburn(1978)
T6t t u f f s
i nS i x m i l ea n dS o u t hc a n y o n s .
Qsburn's Tst tuffs also included
The t u f f o f C a r o i l i t a
some o t h e r t u f f u n i t s .
Canyon h a s a l s o b e e n
mappedby
( i n p r e p a r a t i o n )a n dR o t h( i np r o g r e s s )i nS a w m i l l
Bowring
Canyon.
Osburn ( o r a l c o m m u n i c a t i o n )r e p o r t st h a tt h i su n i tp i n c h e s
out south of this study area against
g r a p h i c r i m oftheSawmill
The t u f f o f C a r o n i t a
t h e southerntopo-
Canyon c a u l d r o n .
Canyon i s w i d e l y d i s t r i b u t e d
western h a l fo ft h es t u d ya r e a .
t h r o u g h o u tt h e
u n i t occursonTimber
e x p o s u r eo ft h i s
largerexposuresoccur
Peak.
on I t a l i a n Peakandon
s i d e of Ryan H i l l Canyon.Most
F u r t h e rs o u t h ,
the western
o ft h ee a s t e r ns l o p eo f
I t a l i a n Peak c o n s i s t s o f t h e u p p e r
C a r o n i t a Canyon.
A small
member o f t h e t u f f o f
1 s q u a r e mile
T h i st u f fc o v e r sa b o u t
i n ChavezandCaronitaCanyons.
The t h i c k n e s s o f t h e
t u f f of C a r o n i t a Canyon i s
v a r i a b l et h r o u g h o u tt h es t u d ya r e a .
It i s about 200 f e e t
~.
t h i c k onTimberPeak,but
'
on . I t a l i a n P e a k .T h e s ed i f f e r e n c e s
p r o b a b l yd u e ,
i n t h i c k n e s sa r e
i n part,. to differences in elevation of
t h e s u r f a c e onwhich
d e p o s i t e d .T h e r e
the upper reaches
the tuff of Caronita
Canyon was
are a l s o d i f f e r e x e s i n t h e
t h i c k n e s s e so ft h eu p p e r
t&
1000 feet t h i c k
it may be almost
andlower
members.
relative
I n p a r t s of
of Madera Canyon, t h e l o w e r member is
missing;whileinparts
of ChavezCanyon,
may b e 500 t o 600 f e e t t h i c k .
thelower
member
61
On TimberPeak,
thetuffofCaronita
and t s f f s i n Txsl.
s e d i m e n t a r -r o c k s
Canyon o v e r l i e s '
F u r t h e rs o u t h ,t h e
in
t u f f of C a r o n i t a Canyon may o v e r l i e s e d i m e n t a r y r o c k s
a n d e s i t e so fT x a 2 ,o rt h er h y o ? . i t el a v a so fT x r
Txsl,
The t u f f o f C a r o n i t a
2'
Canyon i s o v e r l a i n by s a n d s t o n e s
(Txs ) i n some p l a c e s ; these s a n d s t o n e s c o n s i s t m o s t l y o f
2
m a t e r i a le r o d e df r o mt h eu p p e r
member.
Where t h es a n d s t o n e s
Canyon i s o v e r l a i n by
'areabsent,thetuffofCaronita
eitherthetuffofLemitarMountainsorandesitelavas
(Txa3).
The a n d e s i t el a v a so c c u rm o s t l y .
and t h e r i d g e
betwee.nTimberPeakand
Lower Member.
Thelower
onTimberPeak
I t a l i a n Peak.
member o ft h et u f fo fC a r o n i t a
Canyon i s a na n d e s i t ei nm i n e r a l o g i c a lc o m p o s i t i o n .
It
contains 3 t o 20 percentphenocrystsofplagioclase,biotite
and m a g n e t i t e .
The d e g r e eo fw e l d i n gi n c r e a s e sf r o m
poorlyweldedbase
t o a d e n s e l yw e l d e dt o p .
The m i n e r a l
proportions are.rather constant throughout the lower
butthephenocrystcontent
a
member,
increases upward i n t h e t u f f
I
t h ed e g r e eo fw e l d i n gi n c r e a s e s .T h i s
dividedintothr.eezonesbased
member c a nb e
on t h e d e g r e e o f w e l d i n g ,
p h e n o cco
roya
nlsn
toe
tdrn. t
, .
The b a s a l z o n e o f t h e l o w e r . m e m b e r ! i s
poorly welded and usually contains
4 t o ' 5 p e r c e n t pheno-
c r y s t s , 5 t o 1 0 p e r c e n tl i t h i cf r a g m e n t s ,a n d
,
brown t o p i n k ,
1 0 t o 20
.
p e r c e n t p u m i c e .T h i sb a s a lz o n ec r o p so u tp o o r l y .
b e s te x p o s u r e s
are i n s t r e a mb e d s ;e l s e w h e r e ,t h i sz o n e
The
as
62
n
is usually covered with: talus from the overlying, .more
. .
d e n s e l yw e l d e dz o n e s .T h i sz o n e
I
i s u s t l a l l ya b o u t
50 f e e t
t h i c k , or l e s s , a l t h o u g h it may be as much as 2 0 0 f e e t
t h i c k e a s t o f MoiinoPeakand
in parts of Caronita
Canyon.
,ei
make it d i f f i c u l t t o
The p o o r e x p o s u r e s o f t h i s z o n e
o b t a i na na c c u r a t et h i c k n e s s .T h i sb a s a lz o n e , i sf r e q u e n t l y m i s s i n g from t h e lower member.
lower member i s g r a y t o brown,
The middlezoneofthe
5 t o 1 0 percentphenocrysts,
m o d e r a t e l yw e l d e da n dc o n t a i n s
5 t o 20 percentlithicfragments
and 5 t o 1 0 p e r c e n tp u m i c e .
low l e d g e s .
a n g u l a ro u t c r o p sw h i c ho c c a s i o n a l l yf o r m
It
to. f o r ma n g u l a rt a l u sf r a g m e n t s .T h i s
b r e a k sa l o n gj o i n t s
zone i s u s u a l l y 50 to 2 0 0 f e e t t n i c k
whereexposed.
i s b r i c k - r e d ,d e n s e l yw e l d e d ,a n d
The upperzone
c o n t a i n s 1 0 t o 20 p e r c e n t ' p h e n o c r y s t s , a b o u t
l i t h i cf r a g m e n t s ,a n d1 5
1 0 percent
t o 25 percentpumice.Thepumice
is u s u a l l y a d a r k e rr e d
,
low,
well, andforms
T h i sz o n eu s u a l l yc r o p so u tm o d e r a t e l y
color t h a n t h e m a t r i x .
The u p p e r '
zone i s u s u a l l y t h e b e s t e x p o s e d o f t h e t h r e e z o n e s a n d
' f r e q u e n t l yf o r m sl e d g y
a l o n gj o i n t s
or c l i f f yo u t c r o p s .
t o form s h a r p ,a n g u l a r
u p p e r zone i s a l w a y s p r e s e n t
n o te x p o s e d ) .
talus f r a g m e n t s .
The upperzone
feet thick, although occasionally,
are n o t a l w a y s p r e s e n t
i s u s u a l l y 50 t o 1 0 0
it may b e a s much a s
I
250 i e e t t h i c k .
In thin section, the
TI-&
when t h e lower member i s ex-
p o s e d ;w h e r e a s ,t h eu n d e r l y i n gz o n e s
(02
It breaks
lower member o f t h e t u f f o f
63
Caronita
Canyon
contains
2 to 2 0 percent
plagioclas-, biotiteam? magnetite.
phenocrysts
of
It also contains
traces of sanidine, quartz and clinopyroxene. Usually,
about 70 percent of the phenocrysts are plagioclase (about
An48; Michel-Levy method', 3 grains) .
The plagioclase is
euhedral to subhedral and has an average lenath
of
0.9 mm.
Biotite
is
second
in
abundance
and
usually
makes
15
up
.to.25 percent of the phenocrysts. Biotite is usually
euhedral and has an average length 0.75
of
mm. Magnetite
may comprise as much 10
as percent of the phenocrysts (2
..
percent
df
the
rocl:)
.
Quartz,is'present'
in
trace
amounts
in the lower half of the lower member, but it may comprise
-.
~.
5 to 10 percent
of the phenocrysts in the uppermost
10 to
2 0 feet.of the lower member. Trace amounts of, sanidine
and
clinopyroxene
The lower
also
member
occur
also
in
the
rock.
2 to 20. percent
.
contains'
lithic
fragments. These consist mostly of andesite fragments and
fine-grained silicic rock fragments. In hand specimen,
brown, fine-grained lithic fragments with closely spaced
(a few mm or less) parallel bands were seen. These may be
equivalent
'to
of the
some
fine-grainedsilicic fragments
observed in thin section.
The
pumice
has
been
extensively
replaced
by
cherty
quartz and spherulitic chaicedony. In most of
the'thin
sections
G
examined,
the
matrix
is
devitrified
and
occasionally
contains spherulites. However, one thin section from a
vitrophyre
collected
by
G.R. Osburn at Torreon
Springs
64
f:
( s e c t i o n 8 , T. 5 s . , R. 2
w.)
fresh
c o n s i s t sm o s t l yo f
glass.
Upper Member.
The upper member o ft h et u f , fo fC a r o n i t a
Canyon i s a w h i t e t o g r a y , d e n s e l y w e l d e d , , c r y s t a l - r i c h ,
r h y o l i t ea s h - f l o wt u f f .
Where o k e r v e d , t h e . b a s e
upper member i s welded t o t h e t o p o f t h e l o w e r
c o n t a c tb e t w e e nt h eu p p e ra n dl o w e r
member.
becomesmore
1 0 t o 20 f e e t ,
p h e n o c r y s t rich
and t h e c o l o r c h a n g e s f r o m r e d i n t h e l o w e r
o rw h i t e
i n t h eu p p e r
member.
The
members i s g r a d a t i o n a l
through a s t r a t i g r a p h i c i n t e r . r a l . o f a b o u t
i n which t h e t u f f g r a d u a l l y
of t h e
member t o g r a y
T h el o w e rp o r t i o no ft h e
upper member.niay b,e l i g h t brown i n p l a c e s , b u t a b o v e t h e
b a s a lp o r t i o n ,t h eu p p e r
a ,
"
I
member b e c o m e s ' g r a yt ow h i t e .
thebaseofthetransitioninterval,thelower
member
c o n t a i n s a few p e r c e n tq u a r t za n d . s a n i d i n e .
in this interval
The q u a r t z
is frequentlyverylarge(as
i n d i a m e t e r )a n dc o n ~ p i c u o u s ,e v e n
At
much a s 4 mm
i n handspecimens.
Upward i n t h e t r a n s i t i o n i n t e r v a l , q u a r t z a n d s a n i d i n e
becomesmore
abundant, while the plagioclase content
decreases.
The upper member u s u a l l y c r o p s o u t v e r y p o o r l y
H i l l Canyon.
T h e r e ,t h eu p p e r
i n Ryan
member w e a t h e r s t o l a r g e
b o u l d e r s (1 meter o r more i n d i a m e t e r ) t h a t t e n d t o c o v e r
t h eo u t c r o p s .
The o u t c r o p st h a td oo c c u r
and somewhat rounded.
G
are u s u a l l y low
I n Chavez andCaronitaCanyons,
andaroundMolinoPeak,theupper
member u s u a l l y f o r m s
'
crests of r i d g e s a n d o n t h e h i l l t o p s .
good o u t c r o p s a l o n g t h e
#-
.. .
There,theupper
member o c c u r s a s r o u n d e d t o b l o c k y o u t c r o p s .
member i s
On t h e s i d e s o f t h e s e r i d g e s a n d h i l . l s , ' t h e u p p e r
or , i s c o v e r e d w i t h t a l u s .
only partially exposed
The upper member o f t h e t u f f ' o f C a r o n i t a
t a i n s 4 0 t o 55 p e r c e n t p h e n o c r y s t s
biotite.
Canyon con-'
of s a n i d i n e , q u a r t z a n d
35 t o
The t o t a lp h e n o c r y s tc o n t e n ti n c r e a s e sf r o m
4 0 p e r c e n t a t t h e base t o a b o u t 55 p e r c e n t a t t h e t o p o f
t h eu n i t .
However, t h em i n e r a lp r o p o r t i o n s
c o n s t a n tt h r o t i g h o u tt h eu p p e r
The b a s e o f t h e u p p e r
Sanidine is t h e
member.
mostabundantphenocrystandquartz
are f a i r l y
i s second i n abundance.'
member c o n t a i n s a b o u t
20 p e r c e n t
sanidineand15percentquartz,whilethetopcontainsabout
c
,
*_
.
30 percent s a n i d i n e and 20 t o 25; percent q u a r t z .
The
s a n i d i n eo c c a s i o n a l l yd i s p l a y sc h a t o y a n c yi nh a n ds p e c i m e n s .
The q u a r t z i s d i s t i n c t i v e . b e c a u s e of i t s l a r g e s i z e
a s 6 mm d i a m e t e r )a n db e c a u s e
(as much
it f r e q u e n t l y o c c u r s a s
e u h e d r a l ,d i p y r a m i d a lc r y s t a l s .
c o n t a i n s 2 t o 4 p e r c e n tb i o t i t e .
The u p p e r member a l s o
Most hand'specimens
also
' c o n t a i n 1 t o 2 p e r c e n t .small l i t h i c f r a g m e n t s , c o n s i s t i n g
m o s t l yo f
red a n d e s i t i c ( ? ) material.
Theupper
member
c o n t a i n s 5 t o 1 0 p e r c e n t pumice a t t h e base a n d 1 0 t o 1 5
p e r c e n tp u m i c e
a t t h e top.
T h i sp a m i c e
i s white t o gray
axdcrystalpoor.
I n t h i n section, t h e s a n i d i n e i s s u b h e d r a l a n d r a n g e s
45
from 0.15 t o 3.5 mm i n l e n g t h .
s u b h e d r a la n do c c a s i o n a l l y
The q u a r t z i s e u h e d r a l t o
embayed.
I t h a sa na v e r a g e
'
66
/'4
8,
of1.6 nun, but
diameter
thin secticn is 4.0
the
nun.
maximum
diameter
observed
in
Two to 4 percent biotite and1
to 2 percent magnetite occur in the rock.
The biotite
0.4 mm.' The matrix
has an average length of about
devitrified in all
of
the
thin
sections
was
examined.
Sandstones ( T x s 2 k Overlying the tuffof Caronita
Canyon is a20- to 80-foot interval
of fixely laminated to
cros..-bedded sandstc.xe. This sandstone
was mapped by
Osburn (1978) in Sixmile Canyon.It also occurs on Timber
Peak
and
was
found
in
talus
on the
western
side.of'
Ryan
Hill Canyon, although no outcrops were found. East of
Ryan Hill Canyon, it occurs in scattered outcrops in Caronita
/.
and Chavez Canyons, and in the upper reaches
,
of Xolino and
. .
Madera Canyons. The sandstone consists mostly of detritus
derived
from
Caronita
large,
the
underlying
Canyon;
euhedral
in
many
quartz
upper
member
theof
tuff
places,
it consists
grains
similar
of
mostlyof,
to
the
quartz-
phenocryst
in the underlying tuff. The sandstone unit is usually
20
to 50 feet
80
thick,
but
in.a few
pdacesit may
be
as
much
as
feet thick.
The sandstone often crops out in ledgy slopes. Usually,
'(0.1 to 1.0 m.) and
the sandstones have even, thick bedding
fine
to
very
(0.1 to a few
fine
mm.), planar, parallel
laminations. A few outcrops containing ,coarse-grained
sandstone
G
display
planar
cross-bedding.
'
In hand
to gray.
the sandstone is usually brownish gray
specimen,
It consists
67
mostly
of
poorly
to
moderately
sorted,
fine-
to
coarse-
grained quartz and feldspar in a torc?graymat:-ix. . The
sandstone
usually
contains
feldspar,
although
quartz
subequal
usually
amounts
quarkz-and
of
predominates
over
feldspar.
It usually containsa few percent magnetite( ? ) grains and
rock fragments.
.Andesite Lavas(Txa3)'
overlies
the
tuff
A sequence of andesite lavas
of
CaroniL.
on TimGsr
Canyon
Peak
and
on
the ridge between Timber Peak and Italian Peak. The andesite
lavas apparently pinch out to the south; they do not occur
27 and 34', and they were not
east of Italian Peak in secs.
seen in Caronitz and Chavez Canyons. However, a thin
sequence
of
andesite
lavas
overlies
the:
tuff
of
Caronita
feet
Canyon north of Molino Peak. The lavas: are 150
about
of Timber Peak, they
thick on Timber Peak. South and east
thin to about5 0 feet in thickness. Occasionally, the
andesites
form
ledgy
outcrops,
but
usuakly
they
form
talug-
covered slopes. In many places south
of Timber Peak, these
1
,
andesites are only seen in talus and coiluvium.
The.rocks
in
this
unit
consist
of
greenish-gray
to
reddish-gray
andesites and basaltic andesites. The .andesites are
dense
and
dense
to
fine
grained,.
vesicular
and
while
the
contain
as much
basaltic
andesites
are
as10 percent
hematized pyroxene ( ? ) phenocrys'ts inan aphanitic matrix.
Tuff of Lemitar
The
tuff
of
Lemitar
Mountains
Mountains
is
a
multiple-flow,
68
compagitionally zoned, simple to compound cooling ofunit
e
densely welded rhyolite tuff (Chapin and others, 1978).
ThE
outflow
sheet
can
be
divided ainto
crystal-rich
upper
member and a more crystal-poor lower'member.
The Socorro
cauldron is beiieved'to be the source for the tuff of
Lemitar
Mountains.
(Chapin
and
others,
Detailed petrographic workon the
-
1978)
tuff
of
Lemitar
Mountains has been done by Simon (1973), Osburn (1978)
and Chamberlin (in preparation). Simon (1973) studied
samples
(he
from
the
Crouch
misidentified
the
"drill
unit
as
hole
tuff
in
of
theIlill
Silver
area
La
Jencia
Creek
due
to its presence in a paleovalley): Osburn (1978) described
the tuff of Lemitar Ilountains in the southeastern Magdalena
Mountains, just north of this study area.
He measured and
described a stratigraphic section
of the lower, moderately
crystal-poor member in Sixmile Canyon (sec.
7, T. 4
R. 2 W.) .
in
the
K-Ar
S.,
Chamberlin (in preparation) described a section
Lemitar
dates
of
26.3
Mountains,
2
after
the tuff
which
is
named.
1.0 m.y., 27.0, 2 1.11 m.y., and 28.5
2 0.7 m.y. have been obtained from the tuff
of Lemitar
Mountains (Chapin, unpub. data). All are biotite dates;
the averageis 27.4 m.y.
The tuff
of
Lemitar
Mountains
is widely
distributed
throughout the study area.It caps Timber Peak and occurs
in scattered
outcropson the
ridge
trending
south
from
Timber Peak. It also occurs in relatively continuous
outcrops
throughout
the
central
portion
of
the
study
,
area.
69
The
tuff
of
Lemitar
Mountains
is about500 feet
thickon
of Italian Peak (sec.
Timber Peak (erosional t.op); east
3 5 , T. 4 S., R. 3 W.,
unsurveyed), it is about
1400 to
1500 feet thick. Throughout the central part of the study
area,
thetuff of Lemitar Nountains is about
1 5 0 0 to 1 8 0 0
feet thick. Chamberlin (oral communication) reports that
it may be
as much
as 2900 feet thick in the northern
. Chupadera Nountains. However, Osburn
the
tuff
NW/4,
is
only
650 feet thick .at the Tower (ND7/4,
Mine
sec. 7, T. 4 S., R. 1 W.)
The
( 1 9 7 8 ) reports that
relative
.
thicknesses
of
the
upper
and
lower
members
is also variable over the study area. East of Italian
Peak, the lower member varies
from'about 2 0 0 to 4 0 0 feet
i
,-
in thickness, while the upper member is'about 1100 .to.1230
feet thick. Further east, in Chavez Canyon, the lower
member variesfrom.about 300 to.800 feet thick and the
upper member is about
These
ways.
.
1100 to 1200 feetthick.
thickness
variations
can
be
explained
in
two
First, the tuff of Lemitar'Mountains could have
partially
filledan irregular
depression
remaining
after
the Sawmill Canyon cauldron formed. The tuff of Lemitar
Mountains is usually
about400 feet
thick
in
the
outfrow
1979, oral
facies in the Lemitar Mountains (Chamberlin,
communication), comparedtu an average
thicknessof about
1500 feet in the study area. Second, the slight increase
in
G
thickness
from
west
to east
could
represent
a
increase across a hinge zone along the western margin
thickness
of
70
the Socorro cauldron if it istlieoftrap-door type (Plate
2)
-
*.
The
tuff
of
Lemitar
Mountains
usually
overlies
the
tuff of Caronita Canyon; however, a-few
in
places, it overlie5
andesite lavas of the Txa
unit (on.Timber
Peal; and on the
3
ridge southof Timber Peak) or the sanidine rhyolite lava
(north of Molino Peak). In the central part t
of
lle study
area, the tuffof Lemitar
Mountains
is
overlain
by
either
of South Canyon. The
basaltic andesites (Tbal) or the tuff
basaltic andesites apparently pinch out to the south (in
sec. 33, T. 4
R. 2 W.).
S.,
and 36, T. 4 . S . ,
...
I.
i
I
Furtiler west, in secs. 35
it is overlain at different
R. 3 W.,
locations by oneof the following: tuffs and sandstones
(Ttl), rhyolite lavas
(Trl), the tufE of South Canyon, or
mudflow deposits of the Popotosa Formation(Tpl).
..
.
This
. .
. .
variation
in
topographic
Mountains.
overlying
units
depressionson top
was
of
probably
the
tuff
caused
of
by
Lemitar
The.Popotosa Formation alsoIoverlies'a regional
unconformity
formedon the-older
Oiigocene
rocks.
Lower Member. Osburn (1978).divided the lower member
of
the
basis
tuff
of
of
outcrop
Lemitar
Mountains
pattern,
degree
into
of
three
zones
on
welding,
total
pheno-
cryst content, pumice and lithic content, and color.
In
general,
.G
the
lower
member
13 percent
avarages
of sanidine, quartrand biotite.
and
sizes
are
fairly
(Osburn 1978, Fig. 9).
constant
phenocrysts
The phenocryst.morpho1ogies
throughout1ower.member
the
the
71
.
Thebasalzoneofthelowermemberislightgrayto
4\
'i ..
brown
in
color,
poorly
to
moderately
welded,
moderately
lithic rich, and pumiceous. East' of Italian Peak (secs.
21 and 34, T. 4 S.,
is
R. 3 W., unsurveyed), the basal zone
veryp o o r l y welded
and
thus
forms
rounded
to
very
irregularly shaped olxtcrops. On Timber and Molino Peaks,
is absent or scarce, and the basal
the poorly welded tuff
zone is usually, gray to white and moderately we'lded. There,
it forms
moderately
steep
slopes
and
weathers
to
angular
talus fragments. The basal zone usually contains
10 to 15
percent phenocrystsof sanidine and quartz, and
5'to 10
percent lithic fragments. However, one thin section of
,/
C'
the
very
poorly
welded
tuff
only
5 to 8 percent
contains
phenocrysts.Osburnreportedtraceamountsofplagioclase
in the lower zone; however, none was observed in thin
.
. .
.
...,
sections from the lower zone
in this study area. This
1.
50 t o 100 feetthick,althoughit
basalzoneisusually
may be absent or very thin in Chavez Canyon.
It may be
I
as much
as
100
to
150
feet
thick
onPeak.
Molino
Above .this basal zone, the lower member is grayish red
to reddish brown. Outcrops are often steep and cliffy and
they may appear brecciated; the tuff breaks to form sharp,
angular fragments. This second zone has a gradational
contact w i t h the basal zone. It usually contains8 t o 12
percent
biotite.
and
phenocrystsof quartz
and
sanidine,
and
minor
This zone usually contains a few lithic fragments,
pumice
is
n o t prominent.
72
1
c
o.f t h e lower member o f t e n
The t h i r d andupperzone
increase i n t h e
h a s a s t r e a k e da p p e a r a n c ep r o d u c e db ya n
s i z e andabundance
of light-gray,hi'ghlycompressedpumice
See &burn
a n db yd a r k e n i n go ft h em a t r i x .
4 1 ) f o rp h o t o g r a p h s
t h i s zone.
( 1 9 7 8 , p. 38-
and a d e s c r i p t i o n o f t h e m i n e r a l o g y o f
The p h e n o c r y s tc o n t e n t . of t h i s z o n ei n c r e a s e s
from 8 t o 1 5 p e r c e n t upward i n t h e s e c t i o n .
an a v e r a g e q u a r t z c o n t e n t
of 2.5percen.;.
is t h e m o s t a b u n d a n t p h e n o c r y s t
Inthinsection,sanidine
(8 t o 1 0 p e r c e n t ) , h a s a n
The r o c kh a s
mm,
average l e n g t h o f a b o u t 0 . 9
and may show a p e r t h i t i ct e x t u r e( O s b u r n ,1 9 7 8 ,p .3 5 ) .
Q u a r t zo c c u r s
i n amountsbetween
a v e r a g ed i a m e t e ro f
5'
k,
0.7
1 and 6 p e r c e n t a n d h a s
m
m
: Trace
amountsof
m a g n e t i c e a l s o occur i n t h e lower member.
fin
b i o t i t e and
O s b u mr e p o r t e d
a t r a c e amount of p l a g i o c l a s e ; b u t . it w a s l a r g e l y a l t e r e d
....
to clay minerals.
Upper Member. upper
?he
i
member L
o feot'm
u
tfhfieft a r
Mountains i s u s u a l l y . r e d t o g r a y i s h r e d ;
andusuallycontains
it i s c r y s t a l r i c h
30 t o 3 5 p e r c e n t p h e n o c r y s t s o f s a n i -
d i n e ,p l a g i o c l a s e ,q u a r t z ,
and b i o t i t e .
~
I t u s u a l l yw e a t h e r s
t o form rounded outcrops which may f o r m l r o u n d e d , t a l u s
c o v e r e ds l o p e s ,w i t ho c c a s i o n a l
c l i f f s and l e d g e s .I n
thesouth-centralportionofthestudy
area, t h e u p p e r
member o c c a s i o n a l l y w e a t h e r s t o form " s l a b b y " outcrops.
These "slabs" a r e u s u a l l y a f e w i n c h e s t h i c k a n d o n e
G
t o several f e e t i n w i d t h a n d l e n g t h .
foot
73
. ,
f3'
The c o n t a c t w i t h t h e u n d e r l y i n g
lower member i s
g r a d a t i o n s ' . over a n i n t e r v a l o f 5 t o 50, f e e t . T h i s
i s markedby
!
contact
a g r a d u a l increase i n t h e ' t o t a l p h e n o c r y s t
c o n t e n t t o aljout 35 p e r c e n t ,a n d
a parallelincreasein
p l a g i o c l a s ec o n t e n t .O s b u r n- ( 1 9 7 3 )r e f e r r e dt ot h i s - . z o n e
member. was
. .
o r where
a s t h e " t r a n s i t i o nz o n e . "T h e . b a s eo f' t h eu p p e r
mapped a t t h e i n c r e a s e
i n p h e n o c r y s tc o n t e n t ,
. t h e p r o m i n e n t pumice s t r e a k i n g i n t h e lower member e n d s .
T h e upperfew
of t h e t u f f o f
tens of feet
Lemitar
Mountains may b eg r a yo rp i n k i s hg r a y .O s b u r nb e l i e v e d
t h i s t o be a z o n a t i o n i n t h e t u f f r a t h k r t h a n a l t e r a t i o n ,
were f r e s h .
s i n c em o s to ft h ep h e n o c r y s t s
crystscontentrangesupto
p"
.
I
p l a g i o c l a sceo n t e n t
The t o t a l pheno-
almost 50 p e r c e n t , b u t t h e
i s o n l ya.b o u t
2 p e r c e nitn
t h i ps a r t
of t h e t u f f .
The upper member c o n t a i n s b o t h g r a y a n d r e d p u m i c e .
i s scarce i n t h e n o r t h e r n
Osburnreportsthattheredpumice
part of his
to the south.',
area, b u t becomesmoreabundant
Both t h e g r a y and r e d pumiceoccur
in this study area, but
I
t h er e d
pumicepredominates.
The r e d pumice i s ,quartz-and
it may
s a n i d i n e - p o o r ,b u tp l a g i o c l a s e -a n db i o t i t e - r i c h ;
occur as l a r g e s t r e a k s s e v e r a l
feet i n :length(see.Fig.
11).
I n t h i n section, s a n i d i n e is t h e most abundantphenoc r y s t i n t h eu p p e r
member and may comprise a s much a s 25
p e r c e n t of t h e lower p o r t i o no ft h eu p p e r
member.
(1978)reportsthatplagioclasecompri'sesabout
Osburn
.7 p e r c e n t
.
F i g u r e 11. Xed pumice i n t h e u p p e r member o f t h e t u f f
of LemitarMountains
(SW/4, sec. 1 9 , T . 4 S., X. 2 W.) .
The t u f f a l s o c o n t a i n s g r a y p u m i c e , b u t
it i s less
abundant than the .red pumice
i n t h e area where t h i s
photograph was t a k e n . The red p u m i c ec o n t a i n sa b u n d a n t
p l a g i o c l a s e and b i o t i t e , b u t l i t t l e q u a r t z and s a n i d i n e .
Some of t h i s pumice may b e several f e e t i n l e n g t h .
75
a n dq u a r t za b o u t
i n thislowerportion.
1 percentofthe'rock
I
Thinsectionsfromtheupperportio-oftheupper
member
c o n t a i n a s much a s 50 p e r c e n t t o t a l p h e n o c r y s t s :s a n i d i n e
( a v e r a g el e n g t h ,
1 . 4 m m ) comprisesabout
r o c k ,q u a r t z( a v e r a g el e n g t h ,
p e r c e n to ft h er o c k
25 p e r c e n t o f t h e
1 . 6 mm) comprisesabout
and b i o t i t e c o m p r i s e s
3 t o 5 percent
i s t o oa l t e r e d
oftherock.Anotherphenocryst,which
i d e n t i , f y ,c o m p r i s e sa b o u t
15
to
2 p e r c e n to ft h er o c k .T h i s
m i n e r a l may be p l a g i o c l a s e .
TuffsandSandstones
A s e q u e n c eo ft h i n ,w e l d e dt u f f sa n ds a n d s t o n e so v e r -
l i e s t h et u f fo fL e m i t a rM o u n t a i n si n
-.
R.
3 iv.,
sec. 35, T. 4
in a
unsurveyeT
d .hui snpi tr o b a b loyc c u r s
paleovalley which formed
s.,
on a n e r o s i o n s u r f a c e
on t o p of
the tuff of Lemitar Mountains.
These tuffs
graphic interval
and s a n d s t o n e s o c c u r
a t t h e same s t r a t i -
as ".he u n i t ofLuisLopez,
mappedby
C h a m b e r l i n( i np r e p a r a t i o n )i nt h e. n o r t h e r nC h u p a d e r a
Mountains.
The u n i t o fL u i s
Socorrocauldronand
ChupaderaMountains.
Lopez i s t h e m o a t f i l l o f t h e
i s widelyexposed
i n thenorthern
However, b e c a u s eo ft h el i m i t e d
e x p o s u r e s , it is n o t p o s s i b l e t o d e t e r m i n e i f t h e
andsandstones
tuffs
i n the study area correlate with the unit
o f L u i s Lopez.
The t h i c k n e s s o f t h i s s e q u e n c e o f t u f f s a n d s a n d -
G
s t o n e sv a r i e s
from 0 t o 350 f e e t .
The sequence i s composed
76
..
4'.<
3-
of at least three tuEf units and one sandstone unit. Each
of these units is
100 feet, or less, in thickness.
The
tuffs
consist
mostly
of
pink-to-brown,
crystal-
poor (1 to 5 percent phenocrysts), moderately lithic-rich
(10 to 20 percent), poorly to moderately welded, rhyolitic
(?)
ash-flow tuffs. They weather readily and form very poor
outcrops. The sandstone occurs towards the top of the
sequence. This sandstone consists of very coarse, angular
grains of quartz, feldspar and lithic fragments.
In a
few places, it may
consistof angular
breccia
clasts.
Rhyolite Lava (Trl)
A gray, crystal-poor, spherulitic, rhyolite lava
- .~
.,
overlies the tuffs and sandstones of the Ttl unit in NW/4
sec. 35, T. 4 S . ,
R. 3 W., unsurveyed.
It also unconform-
ably overlies the tuff .of Lemitar Mountains. This rhyolite
lava, like the underlying tuffs' and sandstones, probably
..
occurs
in
a
paleovalley
cut
in
the
underlyiny
tuff
of
-.
Lemitar Mountains. The rhyolite lava
i_s overlain by the
tuff of South Canyon. Like the underlying tuffs and sandstones,
the
unit of Luis
to
determine
The
thick.
rhyolite
Lopez,
lava
but
could
there
be
are
correlative
insufficient
with
the
exposures
this.
rhyolite
lava
is probably
about200 to.250 feet
It crops out in a few blocky outcrops; elsewhere,
it is largely covered with colluvium. This rhyolite is
I '
seen i n hand
sp.ecimen.
of this rhl3lite
Thesespherulites
The most c h a r a c t e r i s t ifce a t u r e
is t h e :.bundant
s p h e r u l i t e s it c o n t a i n s .
are u s u a l i y 1 t o 2 cm indiameterand
t e n dt oo c c u ri nc l u s t e r so f
1 0 t o 20
o r more.
These
c l u s t e r s a r e bound t o g e t h e r by f l o w - b a n d e d r h y o l i t e .
I n d i v i d u a l s p h e r u l i t e s may beeggshaped
shaped,andhave
to fqotball
a l e n g t h of 5 t o . I 5 c m .
Basalt ana B a s a l t i c AndesiteLavas(Tbal)
A s e q u e n c eo fp o r p h y r i t i c ,p y r o x e n e - b e a r i n g ,b a s a l ta n d
basaltic andesite
lava f l o w s o v e r l i e s t h e t u f f o f
Mountains throughout
area.
-.
most of t h e c e n t r a l p a r t
of t h e s t u d y
are n o t
These l a v a s p i n c ho u tt ot h es o u t ha n d
p r e s e nitnhseo u t h - c e n t r aplo r t i oontfhset u d y
. ,
(sec. 3 3 , T. 4 S., R.
2 W.)
.
Lemitar
area
Osburn(1978)
mapped t h e s e
area.
b a s a l t i ca n d e s i t e sn o r t ho ft h i ss t u d y
Chamberlin
(oral communication) reports that' basaltic andesite
lavas
a r e p r e s e n t a t t h i s s t r a t i g r a p h i c l e v e l ' i n t h e Lemitar
Mountains.
B a s a l t i c a n d e s i t e l a v a s a r e ' a l s o p r e s e n tb e t w e e n
t h e t u f f o f ..
Lemitar Mountainsand
the tuff
of S o u t h Canyon
i n t h e ' J o y i t a Hills ( S p r a d l i n , 1 9 7 6 ) a n d e l s e w h e r e i n t h e
Magdalena area.
The l a v a s a r e u s u a l l y a b o u t
50 f e e t t h i c k , o r
less.
However, i n W/2, sec. 21,. T; 4' S., R. 2 ,W., t h e y may b e
a s much as 350 f e e t t h i c k .
Osburn(1978)
r e p o r t st h a t
t h e s e lavas may be as much a s 600 f e e t t h i c k a l o n g S o u t h
7%
appear to cut the tuff
of L e m i t a r M o u n t a i n s , b u t n o t t h e
J\
t u f f of South Canyon: these f a u l t s may c o n t r o l t h e t h i c k -
n e s s o ft h eb a s a l t i ca n d e s i t e s .F a u l t sd on o ta p p e a r
to
controltheirthicknessinmost'placesinthisstudy
However, a few p l a c e s d o c o n t a i n
normalaccumulations
area.
somewhat t h i c k e r t h a n
of t h e l a v a s a n d t h e s e c o u l d h a v e
a c c u m u l a t e dn e x tt of a u l t s .
as
T h e l a v a su s u a l l yo c c u r
' l o w , rounded h i l l s c o v e r e d w i t h c o l l u v i u m
.
of small, a n g u l a r
f r a g m e n t s ;o c c a s i o n a l l y ,t h e yc r o po u ta l o n gt h es i d e so f
stream b e d s .
I n handspecimen,
lavas are usually gray
brown.They
-.
the basalt
and b a s a l t i c a n d e s i t e
t o b l a c k and o c c a s i o n a l l y g r a y i s h
a r e p o r p h y r i t i c and may c o n t a i n a few p e r c e n t
phenocrystsof'pyroxene,plagioclaseandolivine
a p h a n i t i c matrix.
set i n a n
S m a l l o l i v i n e andpyroxenephenocrysts
a b o u t 1 . 0 mm i n d i a m e t e r a r e common: t h e o l i v i n e s a r e
frequently altered to iddingsite and/or hematite.
One t h i n s e c t i o n o f t h e b a s a l t i c a n d e s i t e s c o n t a i n s
4 0 t o 4 5 p e r c e n tp h e n o c r y s t so fp l a g i o c l a s e
'
(Ans0; Michela seriate
Levymethod,
1 0 g r a i n s ) .T h i sp l a g i o c 1 a s . eh a s
texture with
s i z e s r a n g i n g from a b o u t 0 . 5 mm t o 1.3
The p l a g i o c l a s e i s m o s t l ye u h e d r a l .
crysts of plagioclase
m
m
.
The smaller pheno-
are a r r a n g e d i n a v e r y c r u d e p a r a l l e l
4 to
a l i g n m e n tC
. l i n o p y r o x e n e( p r o b a b l ya u g i t e )c o m p r i s e s
5 percentoftherockandoccurs
G
phenocrystswith
a s subhedral t o euhedral
an average d i a m e t e r o f a b o u t
Some o f t h e p y r o x e n e o c c u r s
0.35
nun.
i n glomeroporphyriticclumps
79
which may be a s much a s 1 . 8 mm i n d i a m e t e r .
Q
Two t o 3
p e r c e n t p h e n o c r y s t s o f. . w h e d r a l t o s u b h e d r a l o l i v i n e ,
0 . 5 mm.
w i t h a n a v e r a g ed i a m e t e ro f
o u tt h er o c k .
o c c u rs c a t t e r e dt h r o u g h -
The o l i v i n e h a s b e e n p a r t i a l l y r e s o r b e d .
The o u t e r rims ofmostof
the olivine are altered to
i d d i n g s i t e ( ? ) o rh e m a t i t e ;
a l m o s tc o m p l e t e l yr e p l a c e d
some o l i v i n e c r y s t a l s a r e
by t h e s em i n e r a l s .
The m a t r i x
.of the rock consists.mostly of glass and microlites of
I t a l s oc o n t a i n s
plagioclase.
olivine.
a few percentpyroxeneand
The t h i ns e c t i o nc o n t a i n sa b o u t
v e s i c l e s ; a f e w ofwhich
1 0 percent
are filled with calcite.
R h y o l i t e Dome ( T r 2 )
f'
-
7
A p i n k - t o - g r a y ,p h e n o c r y s t - p o o r ,s p h e r u l i t i c ,r h y o l i t e
.
dome c r o p so u ti n
sec. 1 9 ,
'2. 4
i s o v e r l a i n by
o v e r l i e st h eb a s a l t i ca n d e s i t e s( T b a l )a n d
t h et u f f
of South Canyon.The
T h i sr h y o l i t e
S., R.'2,.W.
r h y o l i t e i s d e s c r i b e da s
dome because oC i t s r e s t r i c t e d l a t e r a l e x t e n t .
it c o u l d a l s o b e
distribution.
a rhyolite lava with
Mountains).
However,
a very restricted
The r h y o l i t e o c c u r s a t t h e
i n t e r v a l as t h e u n i t o f L u i s
same s t r a t i g r a p h i c
Lopez ( i n &henorthernChupadera
It h a sa . m a x i m u m . t h i c k n e s s ~ o fa b o u t
600 f e e t .
The r h y o l i t e o c c u r s i n r e l a t i v e l y c o n t i n u o u s , b l o c k y o u t cropsalongthe
crest of a h i l l ; a l o n g t h e s i d e s o f - t h i s
hill, the .rhyolite crops out
i n more ledgy exposures
s u r r o u n d e d ' b y colluvium.
G
I n handspecimen,
a
the rhyolite usually has
a pink
80
matrix,which
i s f i l l e d w i t hg r a y
and chal-edony.
massesofsecondar'yquartz
are u s u a l l y a b o u t
t o w h i t e s p h e . r u l i t e s a'nd
,
Thesk s p h e r u l i t e s
1 t o . 2 cm i n d i a m e t e r a n d t h e y v a r y f r o m
0 t o a b o u t 2 0 p e r c e n ti na b u n d a n c e .U s u a l l y ,
1 0 t o 50 p e r c e n to ft h er o c k .
q u a r t z andchalcedonyforms
I t o c c u r sm o s t l y
t h e secondary
as small, rounded t o i r r e g u l a r m a s s e s ,
a
few mm t o 2 c m , o r more, i n diameter, and scattered througho u tt h er o c k .I nt h i ns e c t i o n ,
t h e r h y o l i t ec o n s i s t s
m o s t l y of s p h e r u l i t i c a g g r e g a t e s o f
a l k a l i f e l d s p a r and
The t h i n s e c t i o n . a l s o
s i l i c a m i n e r a l s ,a n dc h e r t yq u a r t z .
c o n t a i n s a few c r y s t a l s of s e c o n d a r y q u a r t z
0 . 3 mm i n diameter.
.
a s l a r g e as
.
TuffofSouth
Canyon
Canyon i s a m u l t i p l e - f l o w ,s i m p l e
The t u f fo fS o u t h
c o o l i n g u n i t of r h y o l i t e a s h - f l o w t u f f ( C h a p i n
1978).Osburn(1978)
named t h i s t u f f a f t e r
.,
and o t h e r s ,
a measured
s t r a t i g r a p h i c sectio:I a t t h e mouth of S o u t h Canyon (SW/4,
sec. 3 0 , T. 3 S . , R. 3
W.). All
o ft h e r t u f fu n i t sa b o v e
t u f f of L e m i t a r Mountainshavebeen
by some r e c e n t workers.
called t h e u p p e r t u f f s
Simon ( 1 9 7 3 ) mapped u p p e rt u f f s
i n t h e S i l v e r H i l l area, b u t t h e r e l a t i o n s h i p
u p p e rt u f f sa n dt h et u f fo f. S o u t h
between h i s
Canyon i s unknown.
The
tu.ff of S o u t h Canyon i s e q u i v a l e n t t o t h e u p p e r " P o t a t o
Canyon T u f f "o fS p r a d l i n( 1 9 7 6 )
i n t h e J o y i t a Hills.
The
t u f f of S o u t h Canyon h a s r e c e n t l y b e e n d e s c r i b e d
by A l l e n
( i np r e p a r a t i o n )a n d
the
Bowring ( i n p r e p a r a t i o n ) i n
. . 5.
81
Magdalena Mountains, and Chamberlin (in preparation) in
..
._
the Lemitar and Chupadera Mountains. Chapin and others
(1978) have correlated the eruption tof
kie tuff of South
Canyon with the collapse of the Hop Canyon cauldron. The
tuff
of
using
South
Canyon
theK/Ar method
has
on
dated
26.2at?I 1.0 m.y.
been
biotite
from
rocks
Canyon
occurs
in
collected
in
the Joyita Hills.
The
tuff
of
South
outcrops
which
trend
north-northwest in the central portion of the study area.
There, it overlies the tuff of Lemitar Mountains, the
(Tr2).
basaltic andesite lavas (Tbal) or a rhyolite dome
It is
overlain
in
most
of
this
area
by
basalt
and
andesite lavas (Tba2), although in secsl
17 and 18, T. 4
basaltic
S.,
-,
R. 2 W., it is overlain by tuffs of the Twt, unit. The tuff
. *
of South .Canyon is usua.lly between 250
7 5 0 and
feet
Most
of
the
variation
in.thicknessis
due
to
thick.
erosion
. ..._
.
on
top of the tuff of Lemitar Mountains. The tuff of South
Canyon also crops out in sec.
36, T. 4 S . , R. 3 W., on the
,
western side of Ryan Hill Canyon. There, it overlies
.'
either the tuffof Lemitar Mountainsor rhyolite lavas of
the
A
Trl
small
unit
and
is overlain
outcropof the
by
the
tuffof South
lower
Canyon
Popotosa
also
Formation.
occurs
on the western side of Ryan
Hill Chnyon in sec. 21, T. 4
R. 3
W., unsurveyed.
See Osburn (1978) for a detailed
0;
S.,
description of the
mineralogy and zonation of the tuff
of South Canyon. In
general,
the
tuff
of
South
Canyon
contains
subequal
amounts
82
f"?
of
quartz
and
sanidine
where san'idine may
be
except
nearly
the
of the
top unit,
near
.?s abundant ac quartz.
?,ice
The sanidine often'displays chatoyancy. The total pheno3 percent at the base
cryst content increases from about
of
the
tuff
middle'of the
to
a
tuff
maximum
and
then
of 21
about
percent
above
the
percent
'to 12
about
decreases
at the top of the tuff (see Osburn,
1978, Fig. 14).
of
the
tuff
contains
a
trace
of
several
Nost
percent
lithic
fragments; the basal portion of the tuff may contain as
much as 8 percent lithic fragments. The'tuff contains 10
I
to 35 percent pumice.
Tho. jpff of South Canyon
can be
divided
into
three
of the pumice, .
zones based on degree of welding, character
?.
.
I
and presence of lithophysal cavities.
poorly
welded,
.
The basal.zone is
,
crystal
poor.(3 to.9 percent phenocrysts),
'
'
and may contain3 to 8 percent lithic fragments. This . ..
basal interval is best exposed
in s'ec.
where
is,
T. 4 S.,
it.
is about 50 to 100 feet thick.! South
X. 2 W.,
of
this
area,
the basal zone is usually
10 to 5 0 feet thick. This zone
usually
weathers
topographic
very
highs
of
easily
and>may form
the
overlying
saddles
material
between
(see
Fig.
In sec. , 3 3 , T. 4 S., R. 2 W., a portion of this zone
12).
I
was unwelued.
Above the basal zone
is a
highly
G
welded
and
may
lithophysal
contain
zone
which
is
lithophysjl
,cavities
as
much
as 6 inches in length. This zone occurs sporadically in
sec. 18, T.. 4 S., R. 2 W.; elsewhere, it isusually
exposed.
not
a3
Figure 12. Basal, poorly welded,' moderately lithic-rich
zone of the tuff of South Canyon
(SE/4, sec. 18, T. 4 S.,
.R. 2 X.). This could bean air-fall tuff or a series of
thin, ash-flow tuffsat the base of
the,unit. Osburn
of
(oral communication) reports that the basal portion
the tuffof South Canyon occasionally contains bedded
tuffs in Sixmile and South Canyons.
84
The
f"
upper
abundant
zone
has
light-gray
a
streaked
pumice
in
a
appearance
darker,
imparted
pale-red
or
by
grayish-
red matrix. The upper zone is the
most common of the three
zones;
most
If
tne
tuff
of
South
Canyon
in
this
study
belongs to this zone. The upper zone is moderately to
densely
covers
welded
most
and
of
weathers
the
to
angular,
blocky
talus
which
exposures.
In thin section, the tuff of South Canyon consists
mostly of sanidine and quartz, which are usually present
in subequal amounts. Osburn (1978) reported traces of
badly
altered
plagioclase,
but
no'plagioclase
was
observed
in thin sections from this study area. Trace amounts. of
magnetite
i '
__
(?)
and small, 'euhedral biotite phenocrysts occur
in the tuff of South Canyon.
As much as 8 percent lithic
fragments occur. in the lower, crystal-poor part of. the
'
tuff,
but
they
decrease
in
abundance
upward
in
the
tuff.
The lithic fragments consist of gray-to-red, aphanitic,
silicic rock fragments (some are flaw-banded), andesite,
.
and crys'tal-poorto crystal-rich tuff( ? ) fragments.
''
Fragments from both the lower and upper tuff of Lemitar
Mountains
also
occur
in
the
tuff
of
South
Canyon.
The sanidine is subhedral to euhedral and has an
average length of
1.0
mm. The lary,est phenocryst in the
section hasa length of3.0
mm.
Carlsbad .twinning is
common in sanidine, and much of the 'sanidine shows ex-
G
solution of albite in thin section (see Osburn,
1978,
Fig. 16).
Quartz occurs as subhedral to euhedral crystals,
area
~~
~
~~
~-
85
d ..~,
which are occasionally embayed and have 'an average diameter
' _
of 1.0
mm. They often s:.ow internal conchoidal fracturing.
Basalt and Basalti.2 Andesite iavas
)
2
a
h
!
'
(
A
lies
sequence of basalt and basaltic andesite lavas overthe
tuff
of South
Canyon
in
the
central
portion
of the
study area. These rocks are similar to the basalt and
'
basaltic andesite lavas (Tbal) that underiie the tuff of
Souti, Canyon. They asually contain10 to 25 percant pheno"
crysts of pyroxene, olivine and plagioclase. These lavas
have
not
been
dated,
so they
could
be
anywhere
from
12
m.y. old (age of overlying Pound Ranch lavas)26.2
to m.y.
old (age of the underlying tuff of South Canyon).
Thethickness'oftheselavasvariesfrom
e.
0 to 600 (3)
feet in this study area. Osburn (1978) 'reports that they
post-date erosion of the tuff of South Canyon and are
cut
by a later period'
.
.
. #
._
of erosion in the area he mapped. This
appears to be the case in this study area as well, and it
probably
explains
much.
of thickness'variation
the
through-
out the area.' The lavas 'pinch out to tde of
north
this
''
study area in secs.
17 and 18, T. 4 S., 13. 2 W., but they
are
exposed
again
further
north
in
the
area
mapped
by
Osburn (1978).
I
In secs. 16, 17 and 21, T.-4
R. 2 W., the basaltic
S.,
.
andesite
lavas
are
overlain
by either
,
tuffsof the
Twt
unit or the Pound Ranch lavas. Elsewhere in the study area,
4LY
the upper contact of these lavas is not ;exposed.
..
..
.
.
86
o r n r u i y area.
OLIGOCENE
.
..
.
Iiolocene
stra"ii&phic
section
for
F i g u r e 13: Miocene
r o c k s i n t h e e a s t e r n . Wagdalena Mountains and.ivest-centra1
Chupadera. Blountains. D e s c r i p t i d n s ~m o d i f i e da f , t e rO s b u r n ,
. .
1978 *
-
.
..
.
..
.
~.
.
-.
.
.
..
'.
87
These
lavas
usually
form
gentle,
rounded
slopes
covered
with colluvium. In hand specimen,+.he lavas are usually
gray
to
black
when
fresh,
and
brownish
gray
when
altered.
They contain10 to 25 percent phenocrysts of pyroxene,
olivine and plagioclase. Frequently, both the pyroxene
andolivinearereplacedbyhematite,
I
to estimate
the
relative
so itisdifficult
abundance
of these
minerals
in
hand specimen.
-
Osburn ( 1 9 7 8 ) reported
that
some
of these
rocks
in
his study area contain olivine clots
as much
-
as
2 . 0 cm.in
diameter. These rocks are coarsely porphyritic and contain
as much as 25 percent phenocrysts of plagioclase, olivine
c '.
-
I
and
pyroxene.
The
percent
(Osburn,
Si02
content
of these
rocks
is 49.5
1 9 7 8 , appendix B).
A thin sectionof a sample from
SF7/4, section
T. 4
S.,
R.
2 W., contains about15 percent phenocrysts
of olivine, pyroxene and plagioclase
in'a matrix
consists mostlyof plagioclase.
an
average
21,
that
0livine.phenocrysts with
diameter
of 0.4 mm comprise
about5 percent
of
have.been partially
the rock. Most of them are subhedral and
,(?I
resorbed. They are usually partially aitered, to iddingsite
and hematite. Subhedral to anhedral pyroxene phenocrysts'
(probably augite).with an
comprise
diameker
of 0.75 mm
about5 percent of the rock.
Plagioclase
1) '.-
average
comprises
percent of the phenocrysts.
most
of
the
katrix
and
I.t has an average diameter of
0.2 mm.but has a seriate texture. Osburn ( 1 9 7 8 ) reported
a
few
I
88
I
,
c
6 -?
.
that the plagioclase compositions .inthe rocks he studied
(Rittmann zone method,6 grains).
vary betwe.-n An49 and
Ar.
70
The plagioclase in the thin section studied for this thesis.
'
has a composition of An (Michel-Levy method,
68
8 grains).
The matrix is compo'sed mostly of plagioclase with a
..
pilotaxitic texture. Thirty to 40 percentof the matrix
also consists of small pyroxene, oliv.ine and magnetite
. grains:
Lower
Popotosa
Formation
The Popotosa Formation was named by Denny (1940) for
exposures of fanglomerates and playa deposits of the lower
Santa Fe Group
X'
.
along
Arroyo
Popotosa
of the
southeast
Ladron
Mountains. The Popotosa consists of a ,variety of rock types,
I
including mudstones, siltstones, sandstones, conglomerates
,
and mudflow deposits. Only the mudflow; deposits
theof
I
-lower
Popotosa
Formation
are
exposed
in'
this
study
area.
Most of t.hese belong
to the "fanglomerate facies" described
by Bruning (1973).
The .lower
.
Popotosa
Formation
crops
out
tions. One outcrop occurs in sec. 13,:T. 4
inthree
S.,
R. 2
loca-
W.
The base of this outcrop is not exposed! the
and top is .
eroded; north of this study area., in tee areas mapped by
Osburn (1978) and Chamberlin (in preparation), the lower
Popotosa is overlain Ijy playa
mudstones
that
probably
belong to the upper Popotosa. Several 'smaller outcrops
G
of the lower Popotosa occur in
sec. 18, T. 4 S., R. 2 W.
89
Osburn (1978) called these pre-Pound Ranch lava sediments
. (Ts),
but
their
age
and
lithologic
-haracteristi,s
correlate
with those of the lower Popotosa. These sedimentary rocks
overlie
the
upper
member
of
the
tuff
of
Lemitar
I4ountains
and the tuff of South Canyon. They are overlain by the
upper Pound Ranch lavas. They consist mostly
of reddishbrown,
well-cemented
mudflow
breccias
and
conglomerates.
In sec. 3 6 , T. 4 S., R. 3 W., tile Popotosa overlies both
the
tuff
of
South
Canyon
and
the
upper
member
of
the
tuff
of Lemitar Mountains. There, the Popotosais overlain by
a poorly welded.tuff( T t Z ) of uncertain age. This tuff
may
be
equivalent
to
the
Twt
tuffs
that
underlie.the
I
upper Pound Ranch lavas further north.
1
.i
'
The age of these
sedimentaryrocks must
be
between
that of the underlying tuff of South Canyon
( 2 6 . 2 miy.1
and the overlying Pound Ranch lavas
(10.4 m.y.).
They
.
overlie
the
tuff
of.
South
Canyon
with
angular
-
.
unconformity
in sec. 18, so they are probably a few million years younger
than that unit. In sec. 3 6 , the Popotosa dips steeply(35
degrees), but not as steeply as the underlying units.
The
Popotosa
appears
to
be
cut
by
an
erosion
surface
in
18; Osburn (1978) reported that the lower Popotosa appears
to be
relief
cutby an
erosional
of
considerable
southeastof the Pound Ranch.
The Magdalena
g>
uncpnformity
Mountains
were
probably
uplifted
eroded during Miocene time, thus acting
as a source for
the Popotosa Formation (Bruning,1973).
The Popotosa
and
sec.
90
.
Formation
probably
producing
thickness
Subsequent
accumulated
in
variations
at
erosion
may
have
.
topographic
the
time
produced
of
depressions,
deposition.
even
greater.variations
in thickness. 11, sec. 18, The Popotosa is about200 feet
thick and in sec.36, it may beas thick as 500 feet.
Elsewhere, an accurate thickness cannot be determined.
The
lower
Popotosa
mudflow
deposits
usually
form
well-
exposed outcrops. They.frequently crop outas cliffsand
ledges. Usually, the outcrops are fairly massive and
display few sedimentary structures: however, in a few
places, the mudflow deposits are crudely bedded (see Fig.
14). The Popotosa mudflow deposits'areiwell indurated,
but they frequently weather to individual. boulders
(Fig. 15).
The
rocks
are
usually
reddish
brown
in.color
and
consist of angular to rounded, boulder and cobble clas.ts
in a sandy to silty matrix. The clasts' are very poorly
sorted and the larger clasts are often supported by the
finer-grained matrix. This is characteristic of clasts
transported by mudflows. Fragments of the upper member
of
the
tuff
o f Lemitar
Mountains
.the
are,
most
abundant
type of clast; they usually comprise
50:to7 5 percent of
the cobble and boulder clasts. Thc other clasts consist
of fragments of the
tuff of South Canyon, andesite
or
basaltic andesite, the lower memberthe
oftuff
Lemitar
Mountains
and
the
tuff
of Caronita
of
Canyon.
91
4
F i g u r e 14. Outcropof lower P o p o t o s am u d f l o vd e p o s i t s
(SPJ/4, sec. 3 6 , T . 4 S. , R. 3
, unsurveyed) These
rocks dip 35 t o 4 0 d e g r e e s t o t h e e a s t .
.
W
.
’.
F i g u r e 1 5 . O u t c r o po f lower Powotosa mudflow d e p o s i t s
from same l o c a t i o n a s i n F i g u r e
14. T h k s e d e p o s i t s
consistofangulartorounded,cobbletoboulder
clasts.
Fragments of t h e upper member o f t h e t u f f o f
Lemitar
Mountains (I,)compriseabout 7 5 p e r c e n t of t h e c l a s t s
i n t h i s area. The o t h e r c l a s t s c o n s i s t , m o s t l y . o f
a n d e s i t e ( A ) ; the lower member o f t h e t u f f . o f
Lemitar
M o u n t a i n s ,a n dt h et u f f
o f C a r o n i t a Canyon (lower member,
Cl; upper member, C u ) .
I
:
93
Pre-Lava Ash-Flow T u f f s (Twt and T t 2 ( ? ) )
ard pumice-
A thinsequenceofpoorly.welded,lithic-
r i c h ,l i g h t - c o l o r e da s h - f l o wt u f f s
(Twt) und,erlies the
Pound Ranch l a v a s i n t h e PoundRanch
area (Osburn,1978).
i n thenorth-
Only a few small e x p o s u r e s o f t h i s u n i t o c c u r
of t h i s s t u d y
c e n t r a lp o r t i o n
R.
2 V?.)
.
A small o u t c r o p of . t u f f s
( T t 2 ) o c c u r si n
SE/4,
Ryan H i l l Canyon.
andpumice
area (secs'. 1 7 and 18, T. 4 S . ,
SW/4,
of S i m i l a ra p p e a r a n c z
sec. 3 6 , T. 4 S.,
R.
3
W.
in
a r e p o o r l yw e l d e d ,l i t h i c
T h e s et u f f s
r i c h and l i g h t brown; t h e y may b e e q u i v a l e n t
t o t h e Twt t u f f s , b u t l i m i t e d e x p 3 s u r e s
imake it i m p o s s i b l e
todemonstratethis.
1
Ryan H i l l C a n y o n ~ a n dsecs. 1 7 .and
The t u f f s i n b o t h
c_
I
The T w t t u f f s are
1o8v e r l aieenr o s i o n u
a ln c o n f o r m i t y .
o v e r l a i n by t h e u p p e r
lavas, w h i l e t h e
PoundRanch
t u f f sh a v ea ne r o s i o n a lt o p .
Tt2
The T w t t u f f s a r e 0 t o 2 0 0
,.
f e e t t h i c k i n secs. 1 7a n d1 8 ,a n dt h e
. '
T t 2 t u f f s are 0 t o
a b o u t 60 f e e t t h i c k .
O s b u r n( 1 9 7 8 )r e p o r t e dt h a tt h e
Twt tuffs contain
as
much a s 20 p e r c e n t p h e n o c r y s t s o f p l a g i o c l a s ' e , s a n i d i n e ,
q u a r t z ,b i o t i t ea n dh o r n b l e n d e .T h i sm i n e r a l o g y
to that of the
'lower PoundRanch
i s similar
lavas a n d t h e two u n i t s
may b e r e l a t e d .
Lower PoundRanch
C r y s t a l - r i c h ,q u a r t z - r i c h
G
T. 4 S . , R. 2 W . ,
Lavas
lavas crop o u t i n sec. 15,
a l o n gt h en o r t h e r nb o r d e r
of t h e s t u d y
. .
94
area.
Osburn(1978)
-1avas.
named t h e s et h el o w e r
a t 1 1 . 8 ? 0 . 5 m.y.
T h e s el a v a sh a v eb e e nd a t e d
u s i n gt h e
K / A r methodon
PoundRanch
b i o t i t e (Osburn, 1 9 7 8 ) .
m i n e r a l o g y ,t h el a v a sa r e
In
'
latites orquartzlatites;but
a r e r h y o l i t e s ( 7 3 . 0 p e r c e n t S i 0 2 ; Osburn,
c h e m i c a l l y ,t h e y
1 9 7 8 ; appendix C ) .
C h a m b e r l i n( i np r e p a r a t i o n )h a s
mapped
lavasinterbeddedinthePopotosaFoimationintheSocorro
Peak area t h a t a r e s i m i l a r i n l i t h o l o g y a n d a g e t o t h e
Pound
Ranch
lavas.
Thelower
.
Pound Ranch l a v a s c r o p o u t i n a much smaller
a r e at h a nt h eu p p e r
Pound Ranch lavas.
'Thelower
a s q u a r e mile
Ranch l a v a s c o v e r a b o u t o n e q u a r t e r o f
inthisstudy
area.
T h e s el a v a s
Pound
a r e n o te x p o s e dm o r et h a n
a b o u t a mile s o u t h o f t h e n o r t h e r n b o r d e r o f t h i s t h e s i s
I
area.
It i s p o s s i b l et h a tt h e
lava's neverflowed
much
further south than their present exposures.
..
Osburn(1978)reported
f e e t f o r t h el o w e r
a minimum t h i c k n e s s of 4 0 0
PoundRanch
probablythisthickin
lavas.
sec. 1 5 , b u t t h e t o p of t h e s e lavas
i s e r o d e d .T h e s el a v a su s u a l l yw e a t h e r
boulders that cover
The l a v a s a r e
t o form rounded
much of t h e o u t c r o p , a l t h o u g h o c c a s i o n a l
c l i f f s a n dl e d g e sd oo c c u r .
I n hand specimen, t h e lower P c m d Ranch l a v a s are
usdally light
brown or p i n k a n d c o n t a i n ~ a b u n d a n t , coarse
p h e n o c r y s t s of p l a g i o c l a s e , s a n i d i n e , q u a r t z a n d
Crude flow banding can o c c a s i o n a l l y b e s e e n
i n t h e lavas;
t h e flow banding i s d e f i n e d by g r a y s t r e a k s i n t h e
matrix.
..
biotite.
brOVM
95
e
In thinsection,therockcontains
825 t o 30 p e r c e n t
p h e n o c r y s t so fq u a r t z ,p l a g i o c l a s e ,s a n i d i n ea n db i o t i t e .
About 1 0 t o 12 p e r c e n t q u a r t z o c c u r s a s a n h e d r a l t o
s u b h e d r a l ,f r - e q u e n t l y
embayed g r a i n s a s mdch a s 4 . 0 mm
1 . 0 mm.
i nd i a m e t e r ,b u ta v e r a g i n ga b o u t
c l a s e , which v a r i e s i n l e n g t h
mm,
Zoned pl'agio-
from 0 . 5 t o 4 . 0 rcm-and h a s
comprises 9 t o 1 0 p e r c e n t
a na v e r a g el e n g t ho f
1.5
.of therock.Osburn
( 1 9 7 8 ) r e p o r t e dc o m p o s i t i o n sf r o m
Anz4 t o And3 (Rittmannzonemethod,
10 g r a i n s ) .
Some o f t h e p l a g i o c l a s e o c c u r s i n g l o m e r o p o r p h y r i t i c
a g g r e g a t e s ,w h i c h
may p o i k i l i t i c l y e n c l o s e c r y s t a l s o f
5 p e r c e n ts u b h s d r a ls a n i d i n eo c c u r si n
b i o t i t e .T h r e et o
t h er o c k .
f
'
;
I t h a s a n a v e r a g el e n g t h
o npe h e n o c r y sot b s e r v e d
i n length.
of a b o u t 1 . 6
i n t h i ns e c t i o nm e a s u r e s
Some e u h e d r a ls a n i d i n ep h e n o c r y s t s
2 . 0 c m o c c u ri n
handspecimens.
mm,
but
1 . 0 cm
as l o n ga s
The s a n i d i n e i n t h i n
section often. poikiliticly encloses crystals of biotite
a n dp l a g i o c l a s e .
Two to 3 p e r c e n t b i o t i t e ,
I
1 percent
m a g n e t i t e and a t r a c e amount of hosnblende also occur
i nt h e
rock,.
The groundmass c o n s i s t s of s p h e r u l i t e s
( p r o b a b l y i n t e r g r o w t h s of a l k a l i f e l d s p a r s a n d s i l i c a
m i n e r a l s ) , and f i n e - g r a i n e d q u a r t z a n d p o t a s s i u m f e l d s p a r
UpperPoundRanch
The upper PoundRanch.
Lavas
lavas are porphyritic lavas
c o n t a i n i n g 1 0 t o 25 p e r c e n t p h e n o c r y s t s o f p l a g i o c l a s e ,
G
b i o t i t e andhornblende.Theyoccur
i n e x t e n s i v eo u t c r o p s
(?I.
96
5'
in the area south
of the Pound Ranch. Osburn (1978)
.~
mapped
most
about
one
north
of
of
these
mile
outcrops
southwest
this
study
of
area)..
and
lncatdd
Pound
Ranch
they
never
flowed
much
probpbie
(about
one
vent
mile
These crop
lavas outin secs.
15, 16, 17, 18, 21 and 22, T. 4 -S., R.' 2 VI.
that
a
farther
It L
i iikely
south
than
their
present exposures. These lavas have been dated at
2
10.5
0.4 m.y. using the K/Ar method,
on biotite
1978).
(Osburn,
In mineralogy, these lavas are andesites, but
(72.3
chemically, they are rhyolites or quartz latites
percent SiOz; Osburn,1978; Appen-iix C).
The upper
Pound
Ranch
lavas
are
probably
600
feet
thick, or more, in sec. 16; they apparently pinch out
- ,
to the south. These lavas were deposited
surface
and
they
may
on an erosion
unconformably
overlie
all units
above
the tuff of Lemitar Ilountains. In this study area,. the
~
lavas
are
not
overlain
by
any
units
except
-
Quaternary
alluvium. The upper Pound Ranch lavas usually weather
to
rounded
boulders
however, theyalso crop
that
cover more^
thegentle
outas cliffs
and
slopes;
ledges
on
the
I
steeper slopes.
The
upper
Pound
Ranch
lavas
are
zoned
texturally,
but not mineralogically (Osburn, 1978). Close to the
G
vent,
this
by
finely
a
unit
consists
of a
flow-foliated
basal
zone
vitrophyre,
ovc:lain
which
gradually
becomes
more massive upward. The finely flow-foliated zone is
not presentin this study area; Osburn reports that it
97
isrestrictedtowithinaboutamile
of the vent.
The
I
basal vitrophyre is a few feet 1 to
0 0 feet thick. It
22; elsewhere, the
crops out fairly regularly in sec.
outcrops are mor? sporadic. Above the vitrophyre, the
lavas are usually grayish red to light
r'ed. 'They frequently
are brown to brownish red
on the weathered surfaces. A
crudely
some
defined
outcrops
flow
appear
banding
is
usually'
present,
although
almost
massive:'
Osburn (1978) reported an upward increase in the
phenocryst content from10 to 25 percent. Thin sections
1 5 to 20 percent phenoexamined for this thesis contain
crysts o€ plagioclase, biotite, and hornblende. Plagioclase
has
3.3
comprises70 to 80 percent of the phenocrysts and
an
average
length
m, and
of m
1.5
a
maximum
length
of
nun. It occurs as euhedral to subhedral phenocrysts,
and occasionally in glomeroporphyritic aggregates. Much
of the plagioclaseis strongly zoned; Osburn(1978)
reported
that
the
composition vary.from
may
An32
(Rittmann zone method,
20 grains). Some
'.
to
An55
of this plagioclase
poikiliticly encloses crystals of biotite and possibly
hornblende.
Biotite
and
hornblende
each
2 to 3 percent
comprise
of
the rock. They are both pleochroic and display greenishbrawn to red colors. The euhedral to subhedral biotite
phenocrysts have an average length of about
0.7
hornblende
is
also
euhderal
1.0
cross-sectional diameter of
to
subhedral
mm. The
and
has
mm. The matrix of the
an
average
98
c?
..
vitrophyre
undergone
consists
yartial
mostly
glass, of
some
which
of
spherulitic
has
devitrification.
Tertiary Intrusive Rocks
White
The
white
rhyolite
Rhyolite
dikes
Dikes
are
light
color,
in
massive
to flow banded, and contain
0 to 30 percent phenocrysts
.of.quartz,sanidine and biotife.' A few of the more crystalrich dikes also contain plagioclase; dikes that have been
mineralized may contain sulfides. Mostof the dikes occur
.
in the ncrthkestern portion
of the study area (secs.3 , 4,
5 and 10, T. 4
R. 3 W., unsurveyed), although one also
S.,
26, T. 4 S . , R. 3 W.,
occurs in Ryan Hill Canyon (sec.
i'
.?
unsurveyed).Mostofthedikeswereemplacedalongnorth
or north-northwest trending faults; some dikes also trend
east-west,
parallel
with
the
northern
Sawmill
Canyon
ring
fracture. Most of the dikes are near vertical, although
locally the dip may vary. The dikes usually range from
a
few
the
inches
more
to
about
150 feet
resistant
dikes
in
thickness..
stand
up as "walls"
Some
above
of
the
surrounding surface (Fig.16).
In hand specimen, the white rhyolite dikes are usually
white
or
gray
and
may
contain
30 percent
asas much
phenocrysts. Quartz is usually the dominant phenocryst
and may comprise
G
Sanidine
is
as
much 20
as percent of
second
in abundance
but
is
therock.
usually
not
recognized.inhand specimens. Traces to a few percent
easily
99
F i g u r e 1 6 . White r h y o l i t e d i k e . i n NE/4, sec. 4 , T . 4 S . ,
R. 3 W., unsurveyed.Thesedikes
are a f e w inches t o
a b o u t1 5 0f e e t
i n w i d t n . Some o ft h el a r g e s td i k e sf o r m
p r o m i n e n to u t c r o p s , as shown h e r e , b u t m o s t o u t c r o p s
are
much more subdued.
100
I
e
biotite may also be present. Many
of the white rhyolite
dikes have been silicified and minsr-alized.
Pyr'te
(frequently altered to limonite)
and.minor chalcopyrite
SW/4, sec. 3 , T. 4 S..
occur in'a few dikes. The gold mine in
R. 3 W., unsurveyed, is located along a white rhyolite dike
(see section on economic geology).
In thin section, the quartz is usually euhedral to
of 0.4
subhedral and has an average diameter
mm. Sanidine
is subhedral and has an average length
of 0.9.m;
it has
usually been partially replaced by clay,minerals. Biotite
usually has an average length 1.0
of
mm.'
phenocrysts are bleached and
colorless.'
f'
altered
plagioclase
occur
in
a
few
.
Some biotite(?)
Traces
of badly
rocks.
. I
Rhyolite Intrusions
Two types
area:
of rhyolite intrusions occur in the study
one contains sanidine and minor quartz and the
other is very crystEl poor. The sanidine rhyolite intrusions
occur
in
the
!?.pperreaches
of
Rincon-Madera
Canyon (sec. 13, T. 4 S., R. 3 W., and sec. 18, T. 4 S.,
!
R. 2 W.).
This rock type occurs two'intrusions
in
in the
study area; Osburn(1978) also mapped one of these intrusions
and
reported
that
at
least
two
others
occur
the area he studied. These intrusions may have been
emplaced.
along
alternatively,
emplacement.
I
.fault
they
may
zones
have
(see iPlates
and 2);
been
faulted
since
their
in
.
101
The crystal-poor rhyolite intrusions occur in Italian
Canyon (secs.. 21, 27 and 28, T. 4 S., R! 3 W., unsurveyed)
3 W.).
and Ryan Hill Canyon (sec.3 6 , T. 4 S . , ' R .
intrusion
in
Itaiian
Canyon
appears
to
The
have
partially
domed
the overlying strata: some of the units
west of the intrusion
..
dip
to
the
west. but the
regional
dip
is
to
the
east.
In outcrop, the crystal-poor rhyolite frequently
stands
These
out
as
blocky
rhyolites
are
cliffs
partially
flow
banded-tomassive
covered
and
with
are
talus.
usually
pink to gray in color. Occasionally,
theycontain abundant
spherulites. Breccia occurs along the marginof the
intrusion
The
I
F
' ,
%
in
Italian
sanidine
Canyon.
rhyolite
intrusions
low,subduedoutcrops,exceptwhere
usually
weather
to
the,rocks aresilicified.
These rocks closely resemble the Txr2 rhyolite lavas.
They..
are
pale
red
to
pinkish
gray
and
contain
10 to 2 0 percent
.
phenocrysts. Sanidine, the predominent phenocryst,
I
frequently occurs as large, tabular crystals, which may
I
.
be almost 1.0 cm in length. The quartz'is small and not
conspicuous in hand specimen.A few andesite lithic fragments occur in some areas.
One
thin
section
of
the
sanidine
rhyolite
about 6 to 8 percent sanidine and
2 to 4 percent
contains
quartz
phenocrysts. The sanidine has a maximum length 4.5
of
mm, but averages about2.0
mm.
The quartz is subhedral
and has an average diameterof 0.4
biotite
and
a
trace
of
mm. About 1 percent
magnetite
occurs
in the
rock.
.
102
c
In
thin
section,
the
crystal-poor
rhyolites
contain
few, if any, phenocryst?. A few small crystals of quartz
(about 0.1 mm diameter) occur in the thin section, but
most of these are probably secondary quartz. The matrix
(0.01 mm or
consists of fine-grained'
less
diameter)
quartz
and potassium feldspar ( ? I .
Monzonite
Two
monzonite
Cikes
(NV/4,
of the study area
Dikes
occur
in
the
sec. 3 and S E / 4 ,
northwestern
sec. 4, T. 4
S.
portion
,
X. 3 W., unsurveyedj. These rocks are usually a mottled
green color and contain very coarse
(5 cm
or
more
in
length) feldspar phenocrysts. The dikes in this study
area are 30 to 50 feet thick'and can be followed for onlv
g ~>
. I
.
a short distanceon the surface. Some o'f these dikes in,
the MagdalenaMountains stand up as walls; however, in this
study area, the dikes usually form.-low, rounded outcrops.
Hand specimens ofthe.monzonitedikes
are
grayish
green and coarsely porphyritic. Phenocrysts of feldspar,
quartz
and
mafic
minerals
may
comprise
25 to 30 percent
of the rock. The feldspars probably consist of both
sanidine
and
plagioclase,
but
they
are
altered
to
a
white,
clayey material. Many of the feldspar phenocrystsare 1
cm or more in length,' andfew
a
The
quartz
diameter,
.G
Most
of
usually
occurs as
and
the
may
mafic
are
small
as
much
as 5 cm
grains,
1 to 2 nm in
comprise
as much as 5 percent
minerals
are
long.
probably
of
the
biotite,
some pyroxene and magnetite may also be present.
.
rock.
although
103
area have
Both of t h e m o n z o n i t e d i k e s i n t h i s ' s t u d y
b e e ni n t e n s e l ya l t e r e d .
The s u r r o u n d i n gc o u n t r yr o c kh a s
occasionally been altered
b eb l e a c h e d .
and t h e i n t r u s i v e c o n t a c t s
Krewedl(1974).reported
maficminerals
hadbeenreplaced
p r o d u c e dt h eg r e e nc o l o r
may
t h a t many o ft h e
by c h l o r i t e ; t h i s p r o b a b l y
common i n t h e r o c k s . S i l i c i f i c a -
t i o no ft h ed i k ea n dc o u n t r yr o c k s
are a l s o common.
'
K a f i cD i k e s
a r e common i n secs.
Small, fine-grainedmaficdikes
3 and 4 , T. 4 S.,
R.
p o r t i o no ft h es t u d y
3
W., unsurveyed, i n t h e n o r t h w e s t e r n
A few d i k e s a l s o o c c u r i n
area.
9 and 10, T. 4 S., X. 3 N., un'surveyed.
<\
secs.
Most of t h ed i k e s
p r o b a b il ny t r u d eadl o nf ag u l tasl,t h o u gdhi s p l a c e m e na el o n g
"
t h e s ef a u l t s
may b e d i f f i c u l t t o ' p r o v e . I n
same f a u l t s a s t h e w h i t e r h y o l i t e
m a f i cd i k e si n t r u d et h e
d i k e sU
. s u a l l yt, h e
many c a s e s , t h e
mafic d i k e as p p e a r
'
..
t o be
younger
The mafic dikc.s a r e a f e w
t h a nt h ew h i t er h y o l i t ed i k e s .
,
i n c h e s t o a few f e e t i n w i d t h a n d u s u a l l y c a n n o t b e f o l l o w e d
f o r more t h a n a few t e n s of f e e t a l o n g s t r i k e .
Themaficdikerocks
f i n e g r a i n e d . ,O c c a s i o n a l l y ,
are generally green
i n color and
a f e w small p h e n o c r y s t s of
p l a g i o c l a s e ( ? ) c a nb es e e ni nh a n ds p e c i m e n .
Alteration
t e n d s t o o b s c u r et h et e x t u r e s .T h i ns e c t i o n se x a m i n e d
for
t h i s s t u d y c o n s i s t m o s t l y of a f i n e - g r a i n e d assemblage of
c a l c i t e , q u a r t za n dc l a ym i n e r a l s .
andpyroxene
Relics of plagioclase ( ? )
( ? ) p h e n o c r y s t sc a nb es e e n .
Traces of h e m a t i t e
..
... .,. .
. .~.
. .
104
are
f?
present
as
replacement
Tertiary-Quarternary
of
other
minerals.
Rocks
Sierra
Sediments.
products
Ladrones
Formation
The'Sierra Ladrones Formation consists of
gently dipping, poorly consolidated sands and gravels,
boulder
alluvium
and
interbedded
basalts,
which
comprise
the upper part of the Santa Fe Group. The Sierra Ladrones
Formation was named by Machette
(1978) for the Sierra
Ladrones,
In
the
Pliocene
which
type
to
are
low foothills
area,
the
middle
Sierra
Pleistocene
of
the
Ladron
Ladrvnes
in
age
Hountains.
Formation
and
the
is
early
sediments
consist of alluvial-fan, piedmont-slope, alluvial-flat,
<'
.
flood-plainandaxial-streamdeposits(Xachette,
I
1978).
In this study area, the Sierra Ladrones Formation
crops
out
mainly
in
a
group
secs. 1 3 , 14 and 24, T. 4 S.,
capped mesa in secs.
25, 26,35
A few
small
outcrops
1 4 , 1 5 , 2 3 , and 24;
.
also
R.
of
2 W.,
low,
rounded
in
S.,
stream
R;
2 W.
beds
in
Figure 17 shows a .typical outcrop
in
a stream bed.
In the rounded hills (secs.13, 14, and 2 4 ) , the
outcrops
are
almost
always
mantled
by
a
lag of
gravel
rounded cobbles and boulders. Outcrops
,can only
be seen
in a few places in sec.
13, where stream erosion has
F
Q!Y
_,
in
and on a basalt-
and 36, T. 4
occur
hills
removedthelaggravel.Theseoutcropsconsistmostly
of sandy gravels, although occasionally, outcrops
of
secs.
,.,-
105
e
Figure 17. Outcrop of Sierra Ladrones Formationin a.
stream bed (SN74, sec. 14, T. 4 S., R. 2 N . )
The
sands and gravels of this unit are usually partly con-'
solidated, but they crop out poorly. The sediments
in the outcrop shown-here dip about
10 degrees to the
east; quaternary alluviumoverlies the Sierra Ladrones
Formation.
.
106
boulder alluvium occur. The boulders and cobbles.in the
coarser-grained
deposits
probably
remain
behind
after
the
outcrops are eroded and the finer-grained sediments
removed. This e;rentually results in a layer
of boulders
and
cobbles
which
cover
the
. These
deposits.
older
boulders
and cobbles are inostly rounded and consist
of fragments
of
tuff of Lemitar Nountains, tuff of South Canyon, basaltic
andesites and Pound Ranch lavas.A few scattered outcrops
of basaltic lavas (Tsbl) crop out
at the base of the
low hills in secs.13 and 14.
These basalts are probably
interbedded in the sediments.A minimum thickness for the
Sediments of about
3 5 0 feet is indicated by the topographic
relief.
Outcrops of the sediments in the Sierra Ladrones
Formation on the basalt-capped mesa (secs.
2 5 , 26, 3 5 ,
and 3 6 ) consist mostly of sandy gravels.The sediments
and
overlying
basalt
flows
are
nearly
Locally, the dips may 3beto 5 degrees.
flat
lying
The topographic
relief indicates that the sedimentary unit
is at
300 feet thick on the basalt-capped mesa.
This
erodes
easily
and
is
therefore
poorly
least
unit
exposed
mostin
places: the best outcropsare in NW/4, sec. 25.
imbrications at this'location indi,ate
although,
transport
Pebble
directions
1). TransfrJm the southeastto the northwest (see Plate
14 also indicate
port directions from outcrops in sec.
transport
from
the
southeast the
to northwest.
Basaltic Lavas. A series of basaltic lava flows
are
107
interbedded
in
the
Sierra
Ladrones
Formation
portion of the study ares. Theselavas'
in
eastern'
the
all
have similar
petrographic characteristics; they consist of a fine-grained
mosaic of plagioclase, pyroxene and oliviae. These rocks
were mapped as two lava flows (Tsbl and. Tsb2), .based on
their general stratigraphic and field relationships.
The Tsbl lavas in Secs. 13,
14 and NW/4, sec. 24,
.T. 4
R.
S.,
2 W. are probably interbedded in sands and
gravels ofthe Sierra Ladrones Formation,.
.These larvas may
dip 5 degrees or .more to the northeast.
The Tsb2 lavas
overlie the sands and gravels, andnot
areoverlain by
younger units. They are essentially flat lying, although
they may-dip as much as 3 degrees to the northeast. Figure
&.
_
.
.
I
I
18 shows the outcrop expression of some of these basalts
in the western Chupadera Mountains. Orientations can
occasionally
in
most
on vesicles in
obtained
be
places,
The basalt
it
flows
is
difficult
have
the
to
lavas,
obtain
but
strike-dip
not
yet been dated, but
data.
they
are interbedded in the Sierra Ladrones Formation, which is
probably
of.
Pliocene
to middle
of about 200 feet;
eroded.
obtain
G
age
(Machette,
The Tsb2 lavas probably have
a maximum thickness
1978).
less
Pleistocene
however, the top
of these lavas is
The Tsbl lavas are not well enough exposed
to
an
than
These
accurate
100
rocks
thickness
estimate
but
they
are
feet
thick.
usually
weather
to gentle
slopes
covered
with blocky colluvium. Many of these blocks are several
I
probably
108
F i g u r e1 8 .
V i e w lookingeastfromMolinoPeak.
The
ChupaderaMountains a r e i n t h e c e n t e r of t h e p h o t o g r a p h .
j u s t abovethetop
of t h e
The RioGrandecanbeseen
center
ChupaderaMountains.
The d a r ko u t c r o p si nt h e
of t h e p h o t o g r a p h a r e b a s a l t s c a p p i n g
alowmesa;
the
w e s t e r n p o r t i o n ' of t h i s mesa i s i n t h e t h e s i s a r e a .
The b a s a l t s o v e r l i e s a n d s a n d g r a v e l s o f t h e S i e r r a
A p o r t i o n of a manganesemine
LadronesFormation.
i n NW/4, sec. 3 0 , T. 4 S . , R. 2.V7. c a nb es e e n
in the
lower right corner
of t h e p h o t o g r a p h .
~
~~~~~
109
feet
in
whether
diameter.and
they
are
it
is
frequently
difficult
to
determine
outcrops 1-ose
or blocks.
true
In hand specimen, the basaltic lavas are usually
dense, gray to black, and fine grained. They consist mostly
of plagioclase, pyroxene
and.olivine phenocrysts with a
diameter of about 1
mm.
Occasionally, a few larger
phenocrysts (2 to 4mm diameter) can be seen. Someof the
basalts conta'inas much
occasionally
filled
as15 percent
with
vesicles,
which
are
calcite.
In thin section, the basalts consist5 to
of 10 percent
corroded
olivine
grains
surrounded
by
an
intergranular
,mixture of euhedral to subhedral labradorite and interflows (Tsbl and
stitial grains of augite. The two lava
Tsb2) both contain similar percentages of constituents,
axthough
few
the
percent
amounts
between
of
individual
minerals
may
'vary
by
a
flows.
. .
10 grains)
Plagioclase (Ani4; Michel-Levy method,
comprises about60 to 70 percent of the rock.It has a
maximum 'lengthof 1.9 mm and
averages0.7 mm in length.
10 to 25
Anhedral pyroxene (probably augite) comprises
percent
of
The pyroxene
the
has
rock
an
and
is
average
with
length1.0ofmm, but
average
phenocrysts areas long as 4.0
intergrown
the
plagioclase.
some
mm. The olivine has an
lengthof about 0.75 mm; the largest phenocl-yst
is
1.85 mm in length. Some of the
olivine has been partially
altered to iddingsite. Oneto 3 percent magnetite and
ilmenite ( ? ) also occur in the basalts.
110
f'.
Quaternary Units
. .
_,
.
.
Alluvial. Fan Deposits
Poorlyconsolidatedgravelsandsandsthataccumulated
as a l l u v i a l - f a n d e p o s i t s i n t h e e a s t e r n p o r t i o n o f t h e
s t u d y area were mapped a s Q u a t e r n a r y g r a v e l s . ( Q g ) .
alluvial fan deposit;: occur in this area.
f a n s are ccrrentlybeingdissectedby
t h en o r t h e r nf a n
Two
. Bothofthese
a c t i v e str'eams, b u t
(secs. 2 1 , 2 6 , and 2 7 , T. 4 S . , R.
2 W. )
i s more d i s s e c t e dt h a nt h es o u t h e r nf a n .T h e s ef a n s
were
apparently deposited on an erosion surface that
hadbeen
S a n t a Fe Group.
fdrmed a f t e r t h e d e p o s i t i o n o f t h e
Outcrops of t h e Q u a t e r n a r y 'gravels a r e u s u a l l y
2'
..
poorlyexposed..
Where t h e y d o , o c c u r , t h e y c o n s i s t o f
poorly sorted deposits which contain
s i z e clasts.
diameter.
thecoarser
are 1 f o o t or more i n
Some o ft h eb o u l d e r s
Most o ft h e
sand t o b o u l d e r
c l a s t s . a r e subrounded t o well rounded;
c l a s t s ( b o u l d e r sa n dc o b b l e s )
are better
f i n e r clasts.
They c o n s i s t m o s t l y of
- . t u f f of Lemitar M o u n t a i n s , w i t h
lesser amounts of t u f f
r o u n d e dt h a nt h e
o fC a r o n i t a
Canyon, r h y o l i t e l a v a a n d a n d e s i t e .
Alluvium
Quaternary alluvium includes xostly Holocene deposits
of p o o r l ys o r t e ds a n d
and g r a v e l .
I n most p l a c e s ,t h e s e
sediments were d e p o s i t e d i n a c t i v e stream c h a n n e l s .
G
However, i n t h e e a s t e r n p o r t i o n
of t h e s t u d y
area, some
of t h e s e s e d i m e n t s may h a v e b e e n d e p o s i t e d o n r e l a t i v e l y
undissectedalluvialfans.
111
Talus
Material
mappedas talus
includes
mostly
clasts
of
cobble to boulder size. Some house-size blocks may have
slid down the stzeper slopes, especially in Sixmile
and.
. This
unit includes both active and
Ryan Hill Canyons.
stabilized talus'deposits. Some rock glaciers, as much
as a
mile
19).
They frequently exhibit wave-like ridges that were
probably
in
length,
formed
during
occur
in €5.11
RyanCanyon'( s e e Fig.
movement
of
the
talus
pile.
. ..
112
F i g u r e 1 9 . A c t i v et a l u sd e p o s i ti n
Ryan H i l l Canyon
(NE/4, sec. 2 2 , and SE/4, sec. 1 5 , T. 4 S., 1
5. 3 W.,
u n s u r v e y e d ) .T h i sd e p o s i t
i s a b o u t 0 . 5 miles l o n g
t ide.
Some o f t h e l a r g e s t t a l u s d e p o s i t s
and 7 5 0 f e ew
w
a
v
e
l
i
k
e
r
i
d
g
e s on t h e i r s u r f a c e s .
display
113
STRUCTURE
R e g i o n a lS t r u c t u r e
The Socorro-Magdalenaarea
is loqated at the junction
o fs e v e r a li m p o r t a n tr e g i o n a ls t r u c t u r e s .I nt h i sa r e a ,
t h e X i 0 Grande r i f t t r a n s e c t s t h e n o r t h e a s t e r n p o r t i o n
o ft h eD a t i l - M o g o l l o nv o l c a n i cf i e l d .N o r t ho fS o c o r r o ,
t h e r i f t i s c o n f i n e dm o s t l yt o
a series o f l a r g e , s i n g l e
b a s i n s .I nt h eS o c o r r o - M a g d a l e n aa r e a ,t h er i f tw i d e n s
toincludeseveralnorth-trendingpararlelbasinsseparated
by i n t r a r i f th o r s t s( C h a p i n ,
S o c o r r o , t h e Co.loradoPlateauforms
that has resisted
-
1
mation.
Northwestof
1971, 1 9 7 8 ) .
a h i g h ,r i g i db l o c k
much o f t h e T e r t i a r y s t r u c t u r a l d e f o r -
The n o r t h e a s t - t r e n d i M
n go r e nlci ni e a m eantnthde
w e s t - n o r t h w e s t - t r e n d i n gC a p i t a nl i n e a m e n ti n t e r s e c ti nt h e
. .
S o c o r r oa r e a
(see Chapinand
regional structural
.
.
o t h e r s , 1978, Fig. 1 ) . These
elements combine t o p r o d u c e a complex
setting for the local geology.
Pre-existing structures have undoubtedly influenced
l a t e rs t r u c t u r a ld e f o r m a t i o n .C e n t r a l
deformedinlateMississippiantoPermian
Rocky M o u n t a i n s ) a n d a g a i n d u r i n g t h e
middleEoceneLaramideorogeny.
'apparently broke along
N e w Mexico w a s
time ( a n c e s t r a l
Late C r e t a c e o u s t o
The Rio Grande r i f t
c r u s t a l weaknesses formed d u r i n g
t h e s e two s t r u c t u r a le v e n t s( C h a p i na n dS e a g e r ,
1975).
I n w e s t - c e n t r a l New Mexico, t h e P a l e o z o i c a n d M e s o z o i c
. .
r o c k s were f o l d e d and t h r u s t f a u l t e d d u r i n g t h e L a r a m i d e
114
+f\
$..~/
orogeny (Kelly and Wood, 194.6; Tonking, 1957;.and Kelly
and Clinton, 1960). These structures are still poorly
understood,
but
they
probably
influenced
the
geometry
of
the later Tertiaxy structures. Since no .pre-Oligocene
rocks
at the
areexposed in this study area, is
it not possible
present
time
to
evaluate
the
effects
of
Laramide
structures on subsequent structures.
After cessationo f the Laramide orogeny (middle
Eocene),
the
existing
uplifts
were
worn
down
to
produce
an erosion surface of
low relief (Epis and Chapin, 1975).
Detritus
eroded
from
some
of
the
Laramide
uplifts
was
shed
into the Baca basin to produce the Eocene
( ? ) Baca Formation (Snyder, 1971).. The subsequent mid-Oligocene
-,
(I
. I
volcanicandvolcaniclasticsediinentaryrocksweredeposited
.on a relatively flat surface and were able to spread over
.. .. .
wide areas. Between about 32 and 26 m.y. ago, several
overlapping
.(Fig. 20).
and
cauldrons
formed
in
the
Socorro-Magdalena
area
Portions of the North Baldy, Sawmill Canyon
Socorro
cauldrons
The Morenci
and
are
located
Capitan
within
lineaments
this
intersect
study
the
area.
Rio
Grande rift in the Socorro-Magdalena area (Chapin and others,
1978). These lineaments are major crustal weaknesses and
therefore
may
have
had
a
major
influence
on the
development
of. subsequent structures. The geometry of the Sawmill
Canyon
G
cauldron
may
have
Xiorenci lineament (p. 120).
been
influenced
by
the
pre-existing
The Rio Grande rift began
to
form between 3 2 and 27 m.y. ago (Chapin, 1978), concurrent
LEGEND
Transvere Shear
"
c
.
Zone
-c
'\\.
Major Faults
(Basin and Range!
;
,,
4 Cauldron Margin
...
Outline of Mountain
=..Ranges
..
6
F i g u r e 20. R e g i o n a ls t r u c t u r e map o f theSocorro-Magdalcna
area. From o l d e s tt oy o u n g e s t ,
t h e c a u l d r o n sa n dt h e i ra s h are: 1) NorthBaldy--HellsNesaTuff,
2 ) Mt.
. f l o wt u f fs h e e t s
Withington--A-LPeak
Tuff,gray-massive member ( ? ) , 3 ) Kagdalmember, 4 ) Sawmill Canyon-ena--A-L Peak'Tuff,flow-banded
A-L Peak T u f f ,p i n n a c l e s member, 5 ) Kt. klithington--Potato
Hop
Canyon T u f f , 6 ) S o c o r r o - - t u f f o f L e m i t a r K o u n t a i n s , 7 )
Blakestad
Canyon--tuff of S o u t h Canyon. F i g u r em o d i f i e da f t e r
(1978), Chapinandothers(1978).,and,Osburn(1978).
. .
116
with
cauldron
development t.he
in Socorro-Magdalena
The structl'ra.1 geologyof the
Socorro
cauldron
area.
may
be
complicated by contemporaneous rift faulting.It has
recently
been
recognized
that
the
Morenci
lineament
separates fields of oppositely tilted fault blocks (Chapin
North of this "transverse shear
zone,"
and others, 1978).
the strata dip to the west; while of
south
it, the strata
.dip to the east. Chapin and others(1978, p. 115) state
that this shear zone "is acting
as,an incipient transform
fault connecting en echelon s.egments of the Rio Grande
rift. "
This "transform fault" apparently connects the
Albuquerque
graben
to
the
northeast,
and
the
Mulligan
Gulch and Winston grabens to the southwest. Since this
transverse
magmas
shear
have
zone
penetrates
tended rise
to
along
deeply
this
zone
into
the
more
crust,
readily
than in other areas. Modern magma bodies also appear
to accumulate along this zone (Chapin and others,
1978).
The transverse shear zone is about
1.5 km or more in
width. Within this zone, the 'structures may be quite
complex
and
produce
blocks
with
erratic
orientations.
During the early stages
of rifting ( 3 2 to 26 m.y.
ago) , basaltic
andesite
lavas
were
interbedded
with
high-
silica ash-flow sheets. Although the silicic volcanism
died
out,
abundant
basaltic
andesite
until about 2 0 m.y. ago. Chamberlin
volcanism
continued
(1978) correlated
this period ( 3 2 to 20 m.y. ago) with high heat flow,
rapid spreading, and "domino-style'' structural deformation
117
(p. 133) in the Lemitar Mountains.. This Same
structural
g.
z
style occurre6 in the Flagdalena
MOW
tainc,probatly
during
the same time interval. In the early Miocene (about 24 ( ? )
m.y. ago), a broad basin (the Popotosa basin) began to
develop in the Socorro-Magdalena area.The ancestral
Magdalena
Yountains
were
uplifted
to
provide
detritus
for
this basin in the early to middle Miocene
( 2 3 ( ? ) to 15
m.y. ago).
uplift
was
By the middle Miocene (15 to 11 m.y. ago), this
largely
worn
down
and
covered
by
Popotosa
sedimen
(Bruning, 1974).
Silicic lavas, domes and intzusions were emplaced
along the transverse shear zone in the Socorro and eastern
-.
Magdalena Mountains. After this period (during the interval
. .
12 to 7 m.y. ago) large intrarift horsts formed, creating
the modern topography (Chapin,
1978) and disrupting the
'
Popotosa basin. Upper Santa Fe sediments (early Pliocene
to middle Pleistocene) were deposited with angubar unconformity
on
moderately
to
steeply
tilted
strata
of
the
Popotosa Formation. These upper Santa
Fe sediments are
cut by some high-angle faults, but the sediments usually
have
gentle
dips.
Local Structure
The
transverse
shear
zone
divides
the
Magdalena
Moun-
tains intotwo structural provinces. North of the transverse
shear
6;
zone,the Magdalena
Mountains
consist
of
west-tilted
fault blocks that expose Precambrian, Paleozoic and Tertiary
118
rocks. South of the transverse shear zone, east-tilted
fault b1ocl:s expose mostly Tertiary volcacic r0cl.s.The
transverse
shear
zone
transects
of this study area (see Plate
2).
.the
northwestern
corner
Most of this study area
is southof the shear zone and the strata dip to the east.
However, in secs. 3 and. 4, T. 4
S.,
R.
3
W., unsurveyed
(on.the transverse shear zone)some of the dips are to
the west. North of this study area, the western dips
become
more
consistent.
The study area includes parts
of th.ree overlapping.
cauldrons: the North Baldy, Sawmill Canyon and Socorro
cauldrons. The marginsof the Sawmill Canyon and Socorro
cauldrons p a s s through this study area.Plate 2 shows
these structures.
North
Saldy
Cauldron
. :
The North Baldy cauldron (Fig.
20) is the oldest
cauldron
that
has
Little is known
be$n
about
identified
the
.in
shape
of this
the
Magdalena'Nountains.
cauldron
it
since
is largely buried beneath younger volcanic rocks. The
northern
cauldron
margin
passes
just
south
of North
Baldy
of the cauldron
Peak and' trends east-west. This part
margin was described by Blakestad (1978).
The southern
cauldron
Backbone
margin
traverses
the
Devils
(the
southern
tip of the Magdalena Mountains) with an east-west
( ? ) trend.
This part
of
the
cauldron
has yet
notbeen
mapped,so the
details remain to be worked out. The eastern western.
and
119
1
p"i
ror.'ts
parts of the North Baldy cauldron have been buried beneat$.
.
younger
-
The Hells Mesa T u f f e r u p t e d f r o m t h e N o r t h B a l d y
cauldronandcaused
i t s collapse ( C h a p i na n do t h e r s ,1 9 7 8 ) .
O u t s i d e <he c a u l d r o n , t h i s t u f f
is usuallyabout
o r less, i n t h i c k n e s s ; i n s i d e t h e c a u l d r , o n ,
600 f e e t ,
it may be a s
1
.
much a s 3850 f e e t t h i c k
'Mesa T u f fe r u p t e d ,
(Xrewedl, 1 9 7 4 ) .
A f t e rt h e
a s e q u e n c eo fr h y o l i t el a v a s ,
and v o l c a n i c l a s t i cs e d i m e n t sf i l l e dt h ec a u l d r o n .
Hells
domes ( ? )
Bowring
u n i t of
( i np r e p a r a t i o n )i n f o . r m a l l y
named t h e s e r o c k s t h e
HardyRiCge.
so f a r been mapped i n Sawmill
T h i su n i th a s
and Ryan 9ill canyons,and
onHardyRidge.
The d e t a i l s
of the cauldron fill of the North Baldy cauldron are
g?
a_,
p o o rul n
yderstod
ou
de
t o limited
xposures.
In this study area, the
hasbeen
f i l l of t h e N o r t h B a l d y c a u l d r o n
mapped i n two a r e a s t: h en o r t h w e s t e r nc o r n e ro f
(secs. 3 , 4 , and 5, T. 4 S., R. 3 W., un-
t h et h e s i sa r e a
The o u t c r o p s i n t h e
surveyed) and sec. 36, T. 4 S., R. 3 W.
northvestern corner of the study
t h i c ks e q u e n c eo f
o n eo u t c r o po f
..
area c o n s i s t e n t i r e l y o f
Hells Mesa T u f f . I n
Ryan H i l l Canyon,
Hells Mesa T u f fo c c u r si n
t u f f i s more l i t h i c - r i c h t h a n t h e
sec. 36.
This
Hells Mesa. t o t h e n o r t h ,
b u t it i s v e r y similar i n mineralogy.The
Hells Mesa i n
t h i s area i s o v e r l a i n by r h y o l i t e lavas a n d v o l c a n i c l a s t i c
sedimentary rocks
of t h e u n i t of Hardy Ridge.
Sawmill Canyon Cauldron
The S a w m i l l C a n y o n c a u l d r o n e n c o m p a s s e s p a r t o f t h e
a
. .
120
c
s o u t h e a s t e r n Magdalena
Mountains
(Fig.
of the
nor':hern
on P l a t e 2.
Portions
20).
a n ds o u t ' l e r nc a u l d r o nm a r g i n sa r e
shown
The a p p r o x i m a t ep o s i t i o n 'o ft h ew e s t e r n
i s shown i n F i g u r e 2 0 ; h o w e v e r , . s i n c e t h e
cauldronmargin
" .
e x a c tl o c a t i o no f
this margin i s n o t y e t
shown on P l a t e 2 . .
known, it i s n o t
S. Roth i s c u r r e n t l y mappin4 i n s o u t h e r n
Sawmill Canyon and a more p r e c i s e l o c a t i o n o f t h e w e s t e r n
.margin may be known s h o r t l y .
The e a s t e r n limit of t h e
time since t h a t a r e a
c a u l d r o n i s n o t known a t t h e p r e s e n t
i s covered with younger rocks.
The Sawmill Canyon c a u l d r o n d o e s n o t h a v e t h e c i r c u l a r
s h a p ea t t r i b u t e dt om o s tc a u l d r o n s
of t h i st y p e .T h e r e
a r es e v e r a lp o s s i b l ee x p l a n a t i o n sf o rt h i s .
The Sawmill
Canyon c a u l d r o n is l o c a t e d on theMorencilineament(Chapin
a n do t h e r s ,
1978).
T h i sl i n e a m e n t
i s a d e e ps t r u c t u r e
that probably controlled the ascent of the
t h a t producedtheSawmill
Canyon c a u l d r o n .
magma body
The c a u l d r o n
is roughlyelongateparallelwiththislineament(Plate
The magma body t h a t p r o d u c e d t h e c a u l d r o n
2).
may have been
elongateparalleltothelineament,andthegeometryofthe
c a u l d r o n may r e f l e c t t h i s e l o n g a t i o n .
is t h a t t h e p o r t i o n o f
Anotherpossibleexplanation
thecauldron
shown on P l a t e 2 i s o n l y a s m a l l p a r t o f
l a r g e rc a u l d r o n .
e a s to ft h ea r e a
6;;
The Sawmill Canyon c a u l d r o n may widen
shown on P l a t e 2.
Sincethispart
c a u l d r o n i s b u r i e db e n e a t hy o u n g e rr o c k s ,
a t the present
cauldron.
a
it is d i f f i c u l t
time t o d e t e r m i n e t h e o v e r a l l s h a p e
*
of t h e
of t h e
121
The e r u p t i o n of t h e p i n n a c l e s member o f t h e
Tuffhasbeencorrelatedwiththec,llapseofthc
Peak
Sawmill
1 9 7 8 ; a l s o see p. 3 1 ,
Canyon c a u l d r o n( C h a p i na n do t h e r s ,
t h i st h e s i s ) .T h i st u f f
A-L
may b e as much as 3 0 0 0 f e e t t h i c k
i n s i d et h ec a u l d r o n .I ft h ep i n n a c l e s
from t h i s c a u l d r o n , t h e n
member d i d e r u p t
it was hot enough t o d e v e l o p
f l o wb a n d i n ga n dl i n e a t e dp u m i c ei n s i d et h ec a u l d r o n ,b u t
n o to u t s i d eo f
it.
.
Insidethecauldron,the
A-L PeakTuff
by a n d e s i t e and r h y o l i t e l a v a s , t u f f s
t h e u n i t of S i d l e Canyon.
is overlain
and s a n d s t o n e s i n
T h i su n i t
2500 f e e t t h i c k i n s i d e t h e c a u l d r o n , b u t
may be a s much as
it i s n o t p r e s e n t
outsidethecauldron,exceptpossiblyintheolder
Magdalena
cauldron.
The n o r t h e r nc a u l d r o nm a r g i n( F i g .
21) t r e n d sa p p r o x i -
miles n o r t h -
matelyeast-westfromSouthgaldytoabout0.5
west of puck Peak: t h e n it t r e n d sn o r t h e a s tu n t i l
t h ey o u n g e rS o c o r r oc a u l d r o n
t h e s i s ; andBowring,
(see Osburn, 1 9 7 8 ; F i g . 2 0 , t h i s
i np r e p a r a t i o n ) .T h i sc a u l d r o nm a r g i n
Mesa T u f f o n t h e n o r t h a n d t h e
to be the ring fracture
Hells
u n i t of S i x m i l e Canyonon
2 shows t h a t p a r t o f t h i s m a r g i n
is interpreted
l i t t l e o r noslumping
occurredalongthenortherncauldronmargin,andthetopo4”
graphic r i m ofthecauldron
fracture.
the
and p a r t of it t o b e t h e t o p o g r a p h i c
r i m o ft h ec a u l d r o n .A p p a r e n t l y ,
is l o c a t e d c l o s e t o t h e r i n g
The n o r t h e r nc a u l d r o nn i a r g i n
~,
it i n t e r s e c t s
is a s t e e p l y d i p p i n g t o v e r t i c a l c o n t a c t b e t w e e n t h e
s o u t h .P l a t e
.’
i s exposed t o
’
Figure 21. View from the gold mine road showing the northern Sawmill Canyon
cauldron margin (shown
by dashed lines on photograph). The go13 nine (crossed
pattern) is locatedon this cauldron margin. Timber Peak is capped by the
of
lower member of the tuff of Lemitar Mountains. The treeless ridge north
Timber Peak (right skyline) consists of andesite (Txal) that filled the
northern margin of the Sawmill Canyon cauldron. The area shown here is
also on the transverse shear zone.
123
considerable
excess
that
of
has
depth
1000
because
feet
uplifted
a
and
of
a
topogsaphic
relief
in
north-trending
mile-long
segment
horst
of
the
block
cauldron
margin in excess of
1000' feet. The level of exposure .of
the
cauldron
2000 feet
margin
in
the
deeper than
at the
gold
north
mine
end
area
of
is
more
Timber
than
ridge,
about 1.5 miles to the west. Therefore, the northern
cauldron
margin
is
mapped
as the
ring
fracture
in
the
horst block (gold mine area), but
as the topographic rim
outside the horst block (Plate
2).
Osburn (1979, oral communication) has recently
identified
a
remnant
of
the
topographic
rim
of
cauldron margin southof this study area (Plate
2).
mapping
has
shown
that
the
margin
has
an
the
southern
His
east-northeast
trend. This margin probably intersects this study area
along the southern edge of secs.
3 3 or
34;'unfortunately,
this
location
with
This
topographic
between
the
is
entirely
margin
unit
of
covered
is
expressed
Sixmile
Canyon
younger
as
an
and
rocks.
unconformity
either
the
under-
lying Hells Mesa Tuff or unit of Hardy Ridge. The southern
topographic
rim
has
also
been
identified
in
Sawmill
Canyon
by S. Roth.
The
caaldron
southern
has
been
ring
identified
canyons (see Plate2 ) .
the A-L Peak
Tuff
fracture
in
of
the
Ryan
Sawmill
Hill
and
Canyon
Sawmill
This ring -fracture is placed where
suddenly
becomes
thicker
of the unitof Hardy Ridge end (sec.
26, T. 4
S.,
and
outcrops
R. 3 W.,
124
4-".
unsurveyed in Ryan Hill Canyon).The outcrops of the unit
of
Hardy
ring
RGdge
in this
fracture
cauldron.
Hardy
and
the
Tile A-L
Ridge
Ftudy
area
are
between
topographic of
rimthe
the
main
Sawmill
Canyon
Peak Tuff that overlics the unit of
appears
to
thin
to
the
southeast
against
the
topographic rimof the cauldron. The wide zone between
the
.of
southern'
the
ring
cauldron
caving of this
fracture
indicates
cauldron
and
that
wall
southern
topographic
considerable
rim
slumping
and
occurred
cluring cauldron
collapse (Flate2 ) .
A fcw small outcrops of tuffs ana rhyolites in Ryan
Hill Canyon (5/2,
sec. 2 6 ) may be megabrecciadeposited~
just inside the ring fracture. Lipman(1976) discussed
e.
the origin and occurrence
of breccias intermixed in
intracaldera ash-flow tuffs.. He described "megabreccia"
as deposits
fragmental
"in
which
nature
of
many
the
clasts soare
large
deposit
is
obscure
individual outcrops" (Lipman, 1976, p. 1397).
is a
model
that
Lipman
used
to illustrate
that
the
the
in
many
Figure 2 2
way
in
which most megabreccia forms; Most megabreccia accumulates
of the cauldron. Lipman described
in the deeper portions
the
deep
consist
portions
mostly
interbedded tuff^.
contain
G
only
a
of
of
some
cauldron
megabreccia
with
fill
deposits
only
minor
that
amounts
The uppcr portionsof most.cauldrons
small
amount
of
megabreccia
1976,
(Lipman,
p. 1 4 0 8 ) .
Bowring (in preparation) has mapped what he interprets
of
F i g u r e 22.Geometric model p r e s e n t e d b y Lipman ( 1 9 7 6 ) t o
illustratetheformationofcauldroncollapsebreccias.
o f thecaldera
AccordingtoLipman,majorslumpingandcaving
walls w i l l t a k e p l a c e d u r i n g t h e e a r l y s t a g e s
of cauldron
c o l l a p s e . Most of t h i s b r e c c i a will b ei n t e r b e d d e dw i t ht h e
in the deeper portions o f the
intracaldera ash-flow tuff
f r o m Lipman, 1976, F i g u r e 12.
cauldron.Reproduced
126
to be megabreccia in Sawmill Canyon. However, megabreccia
is
generally
>f the
in:exposures
uncommon
.fill
Oi the
Sawmill Canyon cauldron.This may indicate that only the
upper
portions
Resurgent
of
the
cauldron
domingof the
are
Sawmill
exposed.
Canyon
cauldron
cannot be demonstrated in this study area. Block faulting
has
to
obscured
prove
many
of
resurgence
the
if
relationships
it
Socorro
The
Socorro
cauldron
has
that
would
be
needed
occurred.
Cauldron
is
one
of
the
youngest
cauldrons
in the Magdalena Nountains and is believed to have collapsed
during eruption of the tuff
of Lemitar Mountains (Chapin
and others, 1978).
Figure 20 shows the inferred geometry
of the Socorro cauldron. Much of the cauldron has been
down-faulted and buried beneath younger
rocks, so the
exact
boundaries
are
not
known
in
.
most
places.
.
Chamberlin (in preparation) has delineated the northern
cauldron margin on Soccrro Peak. The tu.ff
of Lemitar
Mountains
is
as
much 3000
as feet
Chupadera
Mountains
but
only
thickin
the
about
400 feet
thick
Lemitar Mountains (Chapin and others,
1978).
has
also
mapped
cauldron
collapse
northern
the
Chamberlin
breccia
of Lipman, 1976) and moat
in the terminology
in
(mesobreccia
deposits
(unit of Luis Lopez) in the Chupadera Mountains. The
central
been
and
mapped,
southern
but
Chupadera
reconnaissance
Nountains
indicates
have
that
not
the
yet
127
1
'.
southern
{>
-
cauldron
margin
probably
transects
this
mountain
range.
The
western
margin
of
the
Socorro
cauldron
is
as not
well defined as ';he northern margin. If the location of
the
cauldron
shown
on Plate 2 and Figure 20. is
margik
correct, then this mergin is probably the hinge zone of
a trap-door cauldron. This conclusionwas reached because
the tuffof Lemitar
Mountains
does
not
exhibit
a
marked
thickness
increase
across
the
cauldron
margin.
Insec.
I
3 3 , T. 4 S:, R. 2 W.
(inside the cauldron), the tuff of
1800 feet
Lemitar Mountains is about
.
thick;
3 . 5 miles west, in sec.
35, T. 4 S.,
-.
.
while
about
.
R:
3
W., unsurveyed
(outside the cauldron), the tuff of Lemitar Mountains is
stillabout 1400 to 1500 feetthick.
..
:
Part of the cause for this thick section, both inside
... ..
and.outside
Mountains
the
cauldron,
filled
a
may
depression
be
that
remaining
the
o f tuff
Lemitar
after
the
Sawmill
Canyon cauldron had collapsed. The slight increase in
.
thickness
'.
across
the
hinge
zone
would
then
represent
thickening across the cauldron margin.
About 650 feet
measured
at
the
of
the
Tower
Mountains (Osburn, 1 9 7 8 ) .
tiie tuff
of
Lemitar
tuff
Mine
in the
of
Lemitar
western
Mountains
was
Chupadera
About 1 km to the southeast,
Mountains
is a s much as 2 8 0 0 feet
thick. Chapin and others ( 1 9 7 8 , p. 120) presented several
G
possible explanations for these thickness changes.One
possibility
is
that
the
buried
Sawmill
Canyon
cauldron
128
e
a c t e d is a r i g i d b l o c k w h i c h d i d n o t s u b s i d e a s
much a s
t h ee a s t e r nh a l fo ft h eS o c o r r oc a u l d r o n .A n o t h e rp o s s i b l e
e x p l a n a t i o n i s t h a t a m a j o r ,n o r t h - t r e n d i n g ,d o w n - t o - t h e - e a s t
rift fault located just east
of t h e Tower minewas
active
d u r i n gt h ec o l l a p s eo ft h eS o c o r r oc a u l d r o n .T h i sc o u l d
havecausedtheeasternhalfoftheSocorrocauldronto
s u b s i d e more t h a , nt h ew e s t e r nh a l f .
The e a s t e r n h a l f
.theSocorrocauldronalsocontainsas
m o a td e p o s i t s( u n i to f
of
much a s 2 0 0 0 f e e t of
L u i s L o p e z )b e t w e e nt h er e s u r g e n t
dome i n t h e n o r t h e r n C h u p a d e r a M o u n t a i n s a n d t h e n o r t h e r n
cauldron margin.
is believedtoberoughly
AlthoughtheSocorrocauldro.1
.
c i r c u l a r (Chapinandothers,
1973, Fig.
21,
it i s . p o s s i b l e
.
I
.
I
t h at thm
e a j ofra u lste p a r a t i ntgheea s t e rannw
d estern
portions is actually part of the ring fracture
c a u l d r o n .T h i s
e l o n g a t ei n
zone o f t h e
would imply t h a tt h eS o c o r r oc a u l d r o n
a n o r t h - s o u t hd i r e c t i o n .
is
The S o c o r r oc a u l d r o n
erupted during the early stages of .rifting along the Rio
Grande r i f t , 'so t h e c a u l d r o n may b e e l o n g a t e p a r a l l e l w i t h
the rift.
Osburn(1978)delineated
a zone t h a t may r e p r e s e n t
thecauldronmargininhisstudyarea(northofthisstudy
area).
H e c i t e d a change i n s t r u c t u r a l s t y l e , t h e p r e s e n c e
ofintrusionsand
t h ec a u l d r o nm a r g i n .
6
more inte:lse a l t e r a t i o n a s e v i d e n c e f o r
The i n f e r r e d l o c a t i o n
m a r g i ni nt h i ss t u d ya r e a( P l a t e
m a r g i nd e l i n e a t e d
by Osburn.
of t h e c a u l d r o n
2 ) i s c o n t i n u o u sw i t ht h e
Two r h y o l i t e i n t r u s i o n s a n d
129
4. "
a rhyolite dome occur along this cauldron margin. Extensive
-,
manganese
mincralization
also
occur.'
along
this
r.srgin
in
Chavez Canyon. The zone of intersection between.the Sawmill
Canyon and Socorro cauldrons contains the most intense
mineralization. The Socorro cauldron margin shown on
Plate 2 delineates a boundary between continuous outcrops
of the tuff of Lemitar Mountains inside the cauldron, and
more scattered outcrops outside the cauldron (see Plate
1).
The general regional structural trends in the eastern
Magdalena
Mountains
seem
to
parallel
the
curvature
of
the
Socorro cauldron. However, these trends seem to continue
outside
-..
.,
the
and
t h e y may
cauldron
be
unrelated
to
the
I
cauldron development. The evidence presented above does
not conclusively delineate the western margin of the
Socorro cauldron; however, if the cauldron margin is
. .
a
hinge zone, then it would not display all the features
normally
attributed
to
cauldron
Block
margins.
Faulting
Middle to,late Cenozoic block faulting has extensively
deformed
the
strata
in
the
southeastern
Magdalena
Ellountains.
The transverse shear zone (Plate
2 and Fig. 20), which
transects
the
northwestern
corner
of this
study
area,
separates fields of oppositely tilted blocks. Most of this
study area lies to the south of the transverse shear zone,
l
e;
where
the
strata
dip
to
the
west.
Thestructuralstylefrequentlyproduceshogbacks
23.
such as those shown in Figure
east
and
the
faults
dip
to
the
13 0
Figure 23. View looking northwest at hogbacks formed by .
faulted, east-dipping strata. This areais about 4 miles
south of the transverse shear zone,
so the strata dip
uniformly to theeas.;. The left arrow on the photograph
points to a hogback formed by the tuff of Lemitar Mountains
about 30 to 3 5 degrees to the east.
(27 m.y. old), which diZs
The rightarrow points to
a hogbackformed by the
upper
Pound Ranch lavas (10.5 m.y. old). Dips in the Pound Ranch
10 dip
to
lavas are very erratic, but the lavas probably
2 0 degrees to the east. These lavas unconformably overlie
older Oligocene rocks that were faulted, tilted and eroded
before the lavas were deposited. Later, both the Pound
Ranch lavas and the older Oligocene rocks were cut by
younger faults.
'
131
f'
.
.
.
The
dipof fault
planes
varies
within
the
study
area.
In some places (for example, on Molino Peak), faults with
dips
as
steep 70
as degrees
intersect
faults
with
dips
as
shallow as 30 degrees. Chamberlin (1978) has mapped normal
faults in the Lemitar Mountains wh,ichas dip
shallow
as10
degrees. The Leraitar Mountains are north of the transverse
the and the
shear zone,s o the strata there dip to west
faults dip to the east;, otherwise, the structuralisstyle
the sameas that south of the transverse'shear zone.
Morton and Black(1975) presented a model for the
origin
of
tilted
fault
blocks
in
the
Afar
region
of
Africa.
This area has undergone rifting and extension along a
'
spreading center; Their model involves progressive tilting
. _
.
ofboththestrataandthefaultplanes.Earlyfaults
form with a dip Gof
O to 70 degrees; butas extension
proceeded, these.fault planes are tilted so that the dips
~.
become more shallow. When the strata are tilted
20 to 30
degrees,
the
fault
planes
dip
about
4 0 degrees in the
opposite direction. Morton and Black felt that once this
point
was
reached,
continued
extension
would
produce
new
faults with .dips
of 60 to 70 degrees. Continued crustal
extension
of the
tilting
and
blocks
may
eventually
produce
a third set of faults. Figure
2 4 Lllustrates the concept
of progressive block tilting. The second and third
generations
&
of
faults
are more
closely
spaced
according
to this model.
Regional
mapping
in
the
Socorro-Magdalena
area
has
6'
133
c ?.
f
defined a structural style that appears to be very similar
to that described. by Morton Band
la-k.
Chamherl'n
(1978)
referred to this as "domino-style" faulting.
In areas with
this structural style, the oldest rocks (Oligocene and early
Miocene rocks) are tilted the steepest, are cut by lowangle normal faults, and are the most structurally complex.
Most
Pliocene
degrees
or
Morton
and
less
and
Quaternary
and
are
Black
strata
only
cut
envisioned
have
by
the
20
dips
high
upper
of
angle
crust
faults.
undergoing
progressive block tilting over lower crust that behaved in
a ductile manner., They estimated that faulting would
extend about 10 to 30 km into the crust. Although precise
depths cannot be given, Chamberlin
( 1 9 7 8 ) considers
I'
. ,
progressive block tilting to 'have occurred in the Lemitar
Mountains
when
the
rigid
crust
. M ~ S
thinner
than
'
when
horst-and-graben-style structures formed.
"Domino-style'' structural deformation apparently
occurred
in
the
southeastern
Although
no
horizontal
normal
Magdalena
faults
Mountains
were
as
mapped,
well.
some
normal faults have dips
as shallow as 30 degrees. These
faults occur only in Oligocene rocks. Chamberlin
(1978)
defined two periods of "domino-style'' normal faulting in
the Lemithr Mountains: 29 to 20 m.y. ago, and 12 to 7 m.y.
ago. Since most of the rocks in this study area are
Oligocene
G
in
age,
it
is
not
possible
to
date
all
of
faulting. However, based on the timing of the faulting
that
can
be
dated,
the
"domino-style"
faulting
probably
the
134
!
occurred
during
the
two
time
intervals
outlined
by
Chamberlin.
Osburn (19-9) reported tYat some faults in Sixmile Canyon
(about 1 mile
tuff
of
north
Lemitar
of
this
study
Mountains,
but
area)
appear
tu cut
nottui'f
thnof
the
South
Canyon.
If this is correct, then'regional extension started at
least 26 m.y. ago in this area.
The
.of the
is
A-L'
Peak
Tuff
containsofcome
the steepest
dips
rocksin the southeastern Magdalena Mountains;
it
also
among
the
oldest
rocks
in
this(32area
m.y.).
However, this A-L Peak contains laminar flow structures
and some of these steep dips could
be.primary dips. The
remaining
.,
Oligocene
rocks
have
moderate
to steep
dips
(about 30 to 65 degrees) and are occasionally unconformably
overlain by younger rocks. In southern Ryan Hill Canyon
(sec. 3 6 ) , Popotosa mud-flow breccias (early to middle.
Pliocene age) with dips of about 3 5 degrees unconformably
overlie
The
by
Oligocene
Popotosa
younger
with
the
rocks
Formation
with
is,
tuffs
of uncertain
Twt
tuffs
and
dips
in
between
42 and 6 5 degrees.
turn,
age'
. .
unconformably
(possibly
overlain
associated
therefore 10
about
to 12 m.y. old)
which have a dip
of 10 degrees or less. The Popotosa
Formation in sec.3 6 overlies a low angle fault
( 3 5 degrees)
with
more
than
2000 feet
of
displacement
but
it
iscutnot
by this fault. Apparently, the fault was inactive by the
time
the
Popotosa
sediments
were
deposited.
The Pound Ranch lavas and associated tuffs
(10 to 12
f)
m.y.
old) in the north-central part
of the study area
.
%
.
135
unconformably
%'
~-
Osburn
was
overlie
able
to
older
feet
several edhundr
demonstrate
displacement on faults
(Fig. 23).
rocks
Oligocene
whichcut
theOligocene
of
rocks
but
1978, Fig. 27 and
not the Pound Ranch lavas (see Osburn,
Plate 1, section A-A').
Osburn
they
described
Although faults suchas those
could
undouhtedly
not
be
mapped
exist'beneath
the
in
Pound
The youngest documented faults in
the study
Pliocene
this
study
Ranch
area
area?
lavas.
cut
basalts in the eastern portion of the study
(?)
area.
In well-exposed outcrops, the faults in this study
area are expressed
as breccia
zones
or
as' slickensided
surfaces, or a combination of the two. Outcrops that
* \
.
display
these
characteristics
are
the
most
abundant
the relief is moderately ,low (about
500 feet or less).
where
In
places with higher relief (for example, Timber Peak, Italian
:
Peak and Ryan Hill Canyon), the faults
are rarely exposed
and
mapping
displacement.
of
faults
must
be
based
on
stratigraphic
As Plates1 and 2 show, the faults sometimes
bifurcate and rejoin again. Also, high-angle faults may
intersect
*low-angle
faults,
or east-dipping
faults
intersect
west-dipping faults. When this happens, the older fault
is displacod
There
by
are
the
younger
several
fault.
places
in
thestudy
areawhe:.e thick,
massive units conceal faulting. For example, south
of
&
Molino Peak inSE/4,
R. 3 W.,
sec. 23 and SW 4 , sec. 2 4 , T. 4
S.,
unsurveyed, several faults can be mapped which cut
13 6
the
tuff
of
Lemitar
Mountains
,and.
the
tuff
of
Caronita
'
Canyon. However, after these faults enter the sanidine
rhyolite lava (Txr2), they can no longer be followed since
they do not dispiace any mappable units. Primary structures
within the rhyolite lava (erratic foliation, shear planes
.and breccia) make it difficult
fo follow the faults. In
this case, the faults probably turn to the southwest and
follow the trend of the rhyolite lava. Several other
thick units occasionally conceal faulting. These include
the
Hells
Nesa
Tuff,
A-L
Peak
Tuff
and
the
tuff
of
Lemitar
Mountains.
Mor-ton and Black (1975) hypothesized that the faults
in the Afar
region
originally
formed
with
a
dip
of 70
60 to
>.
..
degrees.However,inthesoutheasternMagdalenaMountains,
,
the
fau1.t~
are
thought
to
have
formed
with
a
steeper
dip
(75 to90 degrees) because wheSe the faults are well exposed,
they
intersect
the
bedding
with
an
angle
of almost.90 degrees.
In Rincon-Madera Canyon (sec.
20, T. 4 S., R. 2 W.); a major
fault (about1500 feet displacement) that dips about 57
'
degrees to the west intersects bedding that dips 30
about
degrees to the east. Osburn (1978) observed the same
relationships
The
in
fault
the
area
surfaces
he
studied.
appear
to,
Le
essentially
planar
ovzr the distance through which they can be mapped. The
faults
-ib;
on
the
cross
section
1) were
(Plate
drawn
as
straight lines, although it is uncertain whether is
there
by Morton and
any. curvature at depth. The model developed
137
6-l
i
Black ( 1 9 7 5 ) c o u l d employ e i t h esrt r a i g h t
or c u r v efda u l t
(197?.) developed mode'l
a
. s u r f a c e s .9 n d e r s o n
f o re x t e n s i o n a l
structures that requires curved fau1.t planes to converge at
a few k i l o m e t e r s d e p t h i n t o
Mortonand'Black
a s u b h o r i z o n t d lz o n eo fd e % o l l e m e n t .
( 1 9 7 5 ) f e l t t h a t m o v e m e ~ ~ t a l o n g s u c h a sub-
horizontalfaultinanareaundergoingextensionaltectonics
would b e u n l i k e l y . . I n e i t h e r c a s e , o v e r t h e d e p t h t h r o u g h
.which t h e f a u l t s a r e
shown i n t h e c r o s s s e c t i o n s , . t h e
faults are thought to be nearly straight.
Gold Mine A r e a .A l t h o u g ht h eg e n e r a ls t r u c t u r a l
g e o l o g yo ft h i ss t u d ya r e ah a sb e e nd e s c r i b e d ,t h e r ea r e
two s r n a l l e ra r e a st h a tn e e df u r t h e rd i s c u s s i o n .
- .
w e s t e rcno r n e r
of t h i s t u d y
The n o r t h -
area i s a complex
region
where
thetransverseshearzoneintersectsthemarginofthe
.
m i l l Canyon c a u l d r o n .
t h i sa r e ah a v ep r o d u c e d
Saw.
Two m a j o rn o r t h - t r e n d i n gf a u l t s
a h o r s tb l o c k .
.
in
The w e s t e r n m o s t
f a u l t is downthrown t o t h e west and d i s p l a c e s t h e t u f f o f
LemitarMountainsagainstthe
Sixmile Canyon).
the east
mine.
Txt u n i t ( i n t h e u n i t o f
The e a s t e r n m o s tf a u l t
and i s l o c a t e d a b o u t
i s downthrown'to
0 . 5 miles e a s t o f t h e g o l d '
T h i sh o r s tb l o c ka p p a r e n t l ye x p o s e sr o c k sf r o m
deeper in the crust than those rocks on either side of
thehorst.
I t c o n t a i n s a h i g hc o n c e n t r a t i o n
of w h i t e .
r h y o l i t e d i k e s and a complex series o f o u t c r o p s .
S o u t h ' o ft h eg o l d
&
u n s u r v e y e d )t ,h e
mine ( i n sec. 1 0 , T. 4 S . , R. 2 W.,
A-L Peak Tuff
and
u n i t of S i x m i l e Canyon
.
I
138
I
I
crop out in confusing outcrops. There are several possible
explanations for these outcrop patterns: Some ofthe strata
in
this
area
as80 degrees
dip
as steeply
to
the
east.
However, all of the mappable faults ill.that area are nearvertical. The most likely interpretation for this is that
the
area
has
undergone
two
or
more
periods
of faulting.
The early faults were tilted along with the strata. These
earl'y faults were then cut by younger, high-angle faults..
The
result
is
a complex juxtaposition of blocks such as
that shown in Figure24, stages 3 , 4 or 5.
After an area
such as this is eroded, the result.ing outcrops may appear
confusing.
Some
as
slida
ofthe tuff
units.(Twk)
blocks
the
in
could
andesite
have
lava
been
while
emplaced
the
Sawmill
Canyon cauldron was being filled. A combination of slide
block
emplacement
occurred,
but
and
most
subsequent
of
the
faulting
complexities
could
are
also
.'.
have
probably
due to
faulting.
2
Southern Marginof the Sawmill Canyon Cauldron. Plate
shows
a
southern
This
area
wide
zone
between
topographic
apparently
rim
of
the
the
contains
southern
Sawmill
some
ring
fracture
Canyon
anomalous
cauldron.
structural
features. Much of the manganese mineralization in Chavez
Canyon
occursin brecciated rock, but no
mapped.' Also, some majorfaults appear
4;
bifurcate as they approach the ring fracture.
faults
to
die
could
out
and
be
or
For example,
139
..f'.~-
several
major
faults
sec. 30, T. 4 S., R. 2
see.
the
cauldron
north
of the
manganese
W. appear to
dic out
mines NW/4,
in
as'theyapproacl.
margin.
G. R. Czburn is currently mapping south
of this
area.
study
He reports (1979, oral communication) that many of
the faul'ts turn shari>ly inside this zone. This appears to
be
the
case
the western
feet
in
this
study as
area
well.
sideof Ryan
displacement
turns
Hill
A major
faulton
Canyon
with'more'than2 0 0 0
sharply
as it
margin (see Plates1 and 2 ) .
.
crosses
the
cauldron'
Also, some of the faults in
Rincon-Madera Canyon (sec.28, T. 4 S.
,
X. 2
W.) apparently
bifurcate as they approach the ring fracture zone..
:x
.,
:
-
I
.
14 0
ECONOMIC GEOLOGY
Two
types
of
mineralization
occur
in this
study
area:
gold and silver in the northwestern cornei
the.study
of
area, and manganese in the south-central portion
of the
study area. Although the mineralogy of these two types
of deposits is different, all
of the ore deposits in this
.study
area
are
localized
along
the
Sawmill Canyon cauldron (see Plate
2).
cauldron
margins
of
the
Local geologic
features (see following sections) also influenced the
distribution
of
the
mineralization.
Some of the best documented cauldron structures in the
world are located in the San Juan volcanic field. Lipman
C'
.
I
and others
(1976) reported that much
of the mineralization
in that area was localized by the cauldron structures
but
that mineralization occurred
2 to 15 m.y. after
cauldron
1
collapse. The mineralization is, therefore, genetically
unrelated to the magmas that produced the calderas. Lipman
and others (1976) state that this situation is typical
of
many
cauldrons
in
the
San
Juan
Mountains.
Apparently, cauldrons serve primarily
as a
structural
control for the emplacement of later mineralization.
The
cauldron
margins
are
deep-seated
structures
that
are
especially susceptible to later intrusion, alteration and
mineralization.
This appears to be the
case in the Sawmill
.
.
Canyon cauldron. The gold-silver mineralizationon the
northern
margin
is
concentrated
in
a
narrow
zone
along
the
141
ring fracture. However, the manganese mineralization
on the
southern
margin
occurs
in
a
wide
zone
between
the
ring
The central
2).
fracture and the topographic rim (Plate
so far,
portion of the cauldron, as it has becn defined
contains
no
significant
Although
the
mineralization.
regions
described
above
contain
several
types of.local alteration, there also appears to be a
regional
potassium
alteration
that
was
associated anwith
ancient geothermal system. This alteration has been
previously discussed by Chapin and others (19781, and
Osburn (1978), so it will only be mentioned briefly here.
Julie D'Andrea (Florida State University) is currently
conducting
-.
a
more
detailed
geochemical
study
of this
potassi3m metasomatism. In general, it is characterized
by an
additionof potassium
into
the
rocks
and
a
depletion
of sodium in the same rocks. In the
more K20-rich rocks,
the
plagioclase
feldspars
have
been
completely
replaced
by a "clay-like" aggregate.'
The
exact
limits
of
this
alteration
zone
are
yet not
known. Chapin and others (1978) reported that the alteration
extends
from
near
south
Baldy
in
the
Magdalena
Range
northward to the Ladron Mountains. Some of the rocks
in
this stud:? area have also been affected.It is apparently
most intensive in certain stratigraphic units, such as the
.
f
;
i
tuff of Lemitar Mountains and the tuff of South Canyon.
Other
units,
such
as the
Hells
Mesa
Tuff
and A-L
thePeak
Tuff, have undergone sericitic alteration (in the feldspars),
but not potassium metasomatism.
.
1 42
Gold Mine Area
A gold-silvermine(TimberPeakmine)'andseveral
preciousmetalprospectsoccurinthenorthwesterncorner
(secs. 3 and 1 0 , T.. 4 S . ,
o ft h es t u d ya r e a
i s l o c a t e da t
s u r v e y e d ) .T h i sa r e a
t h e Water Canyon d i s t r i c t ( J o n e s ,
1 9 0 4 ; Lasky, 1932).
inthesouthernpartofthe
district, but gold
major ores.
W., un-
the. s o u t h e r n end of
Jones ( 1 9 0 4 ) r e p o r t e d t h a t g o l d , s i l v e r
hadbeenfound
E(. 3
a n dc o p p e ro r e s
Water Canyon
and s i l v e r a r e l i k e l y t o b e t h e o n l y
Although l i t t l e i s known a b o u tt h eh i s t o r y
of t h e TimberPeakmine,Jones
( 1 9 0 4 ) p r e s e n t e dt h em o s t
detailed account of the mining 'operation.
.*
.
The TimberPeakMining
Company i n t h e
earlyseasonof
1 9 0 0 completed a 1 5 0 t o n
r o l l - c r u s h i n gc o n c e n t r a t i n gp l a n te n c l o s e d
i n a s t e e l b u i l d i n g . The p l a n t was o p e r a t e d
o n l y a s h o r t time a n d t h e n c l o s e d
down i n definitely.
The p l a n t was d i s m a n t l e d t h e f o l l o w i n g
season and a g r e a t e r p o r t i o n of the. machinery
was shipped t o K e x i c o ; t h e b u i l d i n g
was removed a c r o s s t h e r a n g e t o C a t M o u n t a i n .
S e v e r a l causes a r e a s s i g n e d t o t h i s f a i l u r e :
b u t it seems c e r t a i n t h a t t h e blame s h o u l d n o t
belaidtothe
mine. TheTimberPeak
o r eb o d i e s
a r e t h e l a r g e s t i n t h e d i s t r i c t , b u t mustbe
c l a s s e d a s low g r a d e ; b e n e a t h t h e s u p e r f i c i a l
o x i d i z e dz o n eh e a v ys u l f i d e sa r ee n c o u n t e r e d .
The o r e i s a b o u t e q u a l l y d i v i d e d b e t w e e n
g o l d and s i l v e r .
(Jones, 1904~, p. 1 2 7 )
Thelowerworkingsof
t h i s mine a r e now i n a c c e s s i b l e ;
one short adit remains open, but. only minor mineralization
was s e e ni nt h a tt u n n e l .A p p a r e n t l y ,t h e" h e a v ys u l f i d e s "
t h a t Jones r e f e r r e d t o are n o l o n g e r a c c e s s i b l e
The g o l d - s i l v e r m i n e r a l i z a t i o n
i n themine.
is located i n a
143
'
4
structurally complex area. The transverse shear zone
~~
traverses
+his
is probably
and
area
responsible
for
most
of the complex faulting. The mineralization is also
located on the northern ring fracture of khe Sawmill Canyon
cauldron (Plate 2).
Also, a north-trending horst block,
transverse to the cauldron margin, has uplifted the
mineralized are'a by1000 to 2,000 feet (p. 137).
The mineralization in this area is also associated
with white rhyolite dikes. These dikes intruded several
north-trending faults and the east-trending ring fracture
of the Szwmill Canyon cauldron. The gold mine occurs at
a
place
where
several
thick
north-trending
white
rhyolite
.dikes curve to intersect the ring fracture.
I
\
Although
there
are
several
varieties
of
white
rhyolite
dikes, the most common varieties are a fine-grained,
phenocryst-poor rhyolite and a porphyritic rhyolite that
. .
contains 10 to 20 percent quartz phenocrysts. The mineralization
some
occurs
dikes
The
mostly
contain
topographic
along
the
margins
disseminated
expression
of
of
the
mineralization
the
dikes
dikes,
in
although
places.
indicates
1 and
that they intruded near-vertical faults (Plate
Fig. 16).
Some of the strata in this area dip as steeply
as 80 degrees. If the model presentedin the structural
geology section is correct, then the early faults would
have been tilted along with the strata. Therefore, any
near-vertical, north-trending faults are probably young.
v
It'would be
risky
to
date
these
faults
(and
the
dikes
that
144
intruded them) basedon their dip, but it is likely that
/\
..I
.-
.
they are much ,younger (at least several miy.) than the
.
Sawmill
Canyon
The
cauldron.
relationships
described
above
are
similar
to those
described by Lipman and others (1976) in the San Juan volcanic
field.
He reported that t'ne major post-caldera mineraliza-
tion is closely associated with distinctive, quartz-bearing
igneous rocks that were emplaced several
m.y. after the
calderas had formed. These quartz-bearing igneous rocks
were
apparently
not
associated
with
the
igneous
activity
that produced the'calderas. Instead, the calderas act
as a major structural control for later igneous activity.
Rrewedl (1974, Fig. 4 , Section F-F') speculated that
.
the golil mine
area
was
located
over
a
large
intrusion
that
produced the white rhyolite dikes.
The main evidence for
this is the existence of the dikes themselves. There also
appears
block
to
a
previously
The
tion.
be
gold
higher
concentration
dikes.in
of
the
horst
discussed.
mine
area
contains
several
types
of
altera-
The andesites in the Spears Formation and of
some
the andesites
in
the
unit
of
Sixmile
Canyon
have
been
propylitically altered. The most intense propylitic
alteratior. occurs in andesites (from the unit of Sixmile
Canyon) found in small dumps around the gold mine.The
'
G
.HellsMesa
altered
area.
Tuffis intensely
bleached
throughouttke northwestern
and
portion.
propylitically
of
the
study
The feldspars inthe Hells Mesa have been partially
145
replaced by calcite, sericite, and clay minerals
(?).
All of
(see
the
dikes
in
the
area
have
been
intens'ely
altered
sectionson white rhyolite, mafic and latite dikes).
Si1icificatic.n commonly occurs along faults and in association with the whLte rhyo.lite dikes. The intrusive contacts
of some of the white rhyolite dikes are bleached. This
is
especially
Most
near
t h e gold
true
of
the
nine.
mineralization
is
confined
to
the
area
immediately around the gold mine.A s mentioned previously,.
the
lower
the
mine
that
The
most
intensively
around
workings
of the
the
The
exposed
entrance
host
andesites
is
rocks
in
the
mine
are
not
contains
mineralized
to
the
for
unit
only
rocks
minor
were
the ofpart.
mineralization.
found
scattered
adits.
this
of
exposed;
mineralization
Sixmile
Canyon,
consist
white
of
rhyolite'
dikes and Hells Mesa Tuff.No mineralization was seen in
the andesites that crop out at the surface; however, the
most
iatense
mineralization
occurs
in
andesite
. .
fragments
found near the mine entrance. These andesites are part
of
the
near
unit
the
of
ring
Sixmile
fracture
Canyon;
and
they
adjacent
probably.occurred
to.the
white
rhyolite
dikes before they were mined.
The white rhyolite dikes
also
contain
whether
the
mineralization,
rhyolites
or .tke
mineralization
Only
minor
Away
since
the
gold
is
is
andesites
of the
most
mineralization
was
from
but
mine
seenin
mine,
contain
is
the
only
difficult
now
the
to
determine
most
inaccessible.
Hells
Mesa
Tuff.
traces
of mineralization
146
' -0
were seen. This mineralizationwas a l y a y s confined to
the
white
rhyolite
dikestoor
the 'adjacent
intruded
rocks. ICrewedl (1974) believed 'that the1Ttineralization
(he described only silica veinlets an3 goethite)
is
confined to the HellsMesa.Tuff. However, it appears
that
the
mineralization
is
associated
mostly
with
the
whitCrhyolite dikes, so trace amounts of mineralization
could he found in &atever rocks the dikes have intruded.
A detail.ed
mineralogical and chemical study
of the
ore deposits was not conducted for this study. Samples
were
examined
with'
a hand
lense
'and
a
binocular
microscope.
Based on this study, the mineralization consists of barite,
pyrite, chalcopyrite, galena and sphalerite. The sulfide
X'
.,
minerals are usually disseminated throughout the host rock.
However, the barite usually occurs
as veins along joints
and white rhyolite dikes; it probably filled open spaces.
The
only
mineralization
Elsewhere,
samples
are
only
that
those
trace
contain
.
significant
sulfide
...
collected
at the gold mine.
amounts
of
pyrite
and
chalcopyrite.
(?I
were seen. The gold mine samples always contain
less than
20 percent disseminated sulfide minerals.At least 9 0
percent of this mineralization consists
of pyrite. Only
a few percent of the
other
sulfide
minerals
were
seen
and
these occur in only
a few samples. These minerals fchalcopyrite, galena and sphalerite) usually occur
as small (less
G
than 1 nun diameter) crystals disseminated in the host rock
and in small quartz veinlets. Limonite pseudomorphs (after
147
pyrite)
are
common
in
the
rocks
and
limonite
stain
1
frequently
coats
the
joint
surfaces.
( 1 9 0 4 ) probably
The gold and silver reported by Jones
occurs as impurikies or inclusions in other minerals or in
microscopic crystals. No gold or silver minerals could be
identified in hand specimens
or under
the
binocular
micro-
scope.
Manganese
-
Nost
Mineralization
of
the
manganese
mineralizationthis
in study,
area is concentrated in a wide zone between the southern
-
ring fracture and the southern topographic rim of the
Sawmill Canyon cauldron. Minor manganese mineralization
-.
also occurs north of this zone; it consists mostly of
. ,
manganese
minerals
coating
joint
surfaces,
but
no
mines
or prospects were found.A small manganese prospect occurs
in
the
Pliocene.(?)
basalts
near
the eastern
margin
of
the
area. study
i
Numerous
1
'.
prospects
zone (Plate 2).
and
T. 4 S.,
R.
3
occur
within
the
mineralized
The-mostintense manganese mineralization
is in MW/r?, sec. 30, T. 4 S.
.
mines
, 3.
W., unsurveyed.
2 W. ; and NE/4,
sec
.
25,
This area is locatedat the
!
intersection of the
Canyon
cauldron
The boulder
shown
southern
with
in
the
ring
fracture
of the
hinge of
zone
the Socorro
Figure
2 5 came from a
large
Sawmill
cauldron.
calcite
vein in this area. Most of the ore' in the mineralized
I
c
zone was probably low grade. All the
oremined occurred
148
F i g u r e 25.
Boulderofbandedblackandwhite
c a l c i t e and'
minor manganese minerals from
a manganesemine i n XE/4,
sec. 2 5 , T. 4 S., R. 3 W;, unsurveyed.ThisLoulder
cane
from a 15- t o 2 0 - f o o t - w i d e v e i n w h i c h h a d b e e n s t r i p
mined.
I
149
within
about,
50 feet
stripped
from
Although
of
shallow
the
the
surface;
pits,
cauldron
it
was
since
no adits
margin
apparently
were
is
the
seen.
major
regional
control for che mineralization, local
strxtures control
the precise location of
.the ore. The rocks in the zone
between
the
ring
fracture
and
the
topographic
rim
have
n
been brecciated in many pl.aces where faults could not be
.mapped. The exact cause of this brecciation is clear,
not
but it could be related to the existence of
cauldron'
the
margin. Usually the brecciation produced large spaces
between khe breccia fragments. These fragments were
usually not rotated extensively or sheared. Much the
of
breccia
has
been
cemented
with
manganese
minerals
and
L'
. *
calcite.
Nost
of
the
north-northwest-trending
faults
shown on Plates.1 and 2 are also mineralized. This
mineralization
as an
occurs
open-space
The
as
a
filling
mineralization
coating
on joint: surfaces and
where
occurs
of Sixmile Canyon and
as young
larger
in
as
voids
were
rocksoldasas the
formed.
unit
( ? ) basalts
Pliocene
the
(Tsb2). However, only one prospect.was found in the young
basalts
and
the
Oligocene
rocks
appear
to
be
more
highly
mineralized than the hasalts.It appears that the'main
control
for
mineralization
in
the
Oligocene was
rocks
structural. Therefore, m w h of the mineralization probably
occurred
6
-
afterall of the
Oligocene
rocks
were
deposited
but before the Pliocene
( ? ) basalts erupted.
The
host
rock
in
the
immediate
vicinity
of the ore
150
#\
?
deposits
is
frequently
bleached
and
the
feldspars
be may
altered to clays. However, this alteration appears
to be
very
irregularly
distributed
around 'ore
the deposits.
Occasionally, fault zones are bleache3 in places where
there is little or no mineralization. Silicification
occasionally occurs around faults and adjacent to mineralization, but it is much less abundant than in the gold mine
area. Calcite is commonly found along faults and in
association
with
the
mineralization.
Several large calcite veins occur in the study area.,
sec. 2 5 , T. 4 S., R. 3 W.,
The largest vein occurs XE/4,
in
unsurveyed, and is at least
10 feet wide. The boulder
-.
. ,
shown in Figure25 is from this vein. Osburn( 1 9 7 8 )
reported
thatone calcite
vein
in
his
study
area
at is
least 100 feet wide. The large veins in this study area
consist
mostly
of
banded,
calcite,
which
is
interlayered
black
calcite
and
A detailed
manganese
coarsely
manganese
in
this
lesser
white
amounts
of
oxides.
mineralogical
minerals
with
crystalline,
and
study
geochemical
area
study
of the
has
yetnot
been
conducted. The major minerals reported in
otherparts of
Socorro County are psilomelane, hollandite, cryptomelane
and coronildite (Hewett, 1964; Willard, unpublished report)'.
Miesch (1956, p. 2 5 ) concluded that psilomelane was
the
most
The
prominent
Luis
Lopez
ore
mineral the
in Luis
district
is
a
Lopez
district.
msjor
hanganese
district
located about 5 to 10 miles east of the manganese mineralization in this study area.
151
All of the manganese oxiaes occur as open-space filling.
Most of
the
minerals'occur.in
massive., botryoidal and banded.
aggregates. Accessory
minerals include calcite, quartz,
barite and fiuorite (only found in one
ar.?a).
152
COSCLUSIONS
This
study
has
made
several
contri.butions
to
the
geology of the Southeastern Magdalena Mountains. These
contributions are divided into three sections: stratigraphy,
structural geology and economic geology.
Stratigraphy
All
of the rocks exposed in this study area are of
Tertiary
age;
volcanic
-stratigraphicunits.
units
comprise
the majority
of
the
A smaller number of sedimentary units
are interbedded in the Tertiary volcanic rocks. In general,
these
-.
. .
volcanic
and
sedimentary
units
fit well
withthe
regional stratigraphy defined by Chapin and others
(1978).
However, there are several features
of these units in this
..
study
area
that
are
unique
to
the
.
southeastern
.
Magdalena
Mountains, or that are important in understanding the
regional geology of the Socorro-Magdalena area. These
features
are
listed
below
from
oldest
to
youngest.
1) A thick accumulation (at.least2000 feet) of Kells
Mesa
The
Tuff
in
the
southeastern
HellsEIesa Tuff is unusually
because
Baldy
exists
the
area
cauldron,
is
located
which
was
Tuff (Chapin and others,
1978).
thick
in
entirely
the
Magdalena
source
this
within
for
Mountains.
study
the
the
area
North
Hells
Mesa
A heterogeneous assemblage
of rhyolite lavas and domes
( ? ) , tuffs, and volcaniclastic
G
sediments (the unit of Hardy Ridge) filled the cauldron
after
the
Hells
Mesa
Tuff
was
erupted.
153
2)
A-L
( a t l e a s t 3000 f e e t ) of
A t h i c ka c c u m u l a t i o n
Peak Tuff crops out
i n Ryan H i l l and Sixmile canyons.
Thisunusualthicknessoccurswithinthe
c a u l d r o n ( P l a t e s 1 and 2 , andFig.
Sawmill Canyon
201,
which i s t h o u g h t
t o have collapsed during eruption of the pinnacles
o ft h e
member
A-L P e a kT u f f .I n s i d et h ec a u l d r o n ,t h e
Tuff i s v e r y d e n s e l y w e l d e d ;
A-L
Peak
it i s flowbandedandcontains
l i n e a t e d pumice i n many p l a c e s .
The A-L P e a kT u f ft h i c k e n s
of t h e c a u l d r o n :
dramaticallyinsidetheringfracture
it
a p p e a r s t o t h i n a s it a p p o a c h e s t h e s o u t h e r n t o p o g r a p h i c
rim o f t h e c a u l d r o n .
3)
.-
.
The u n i t of S i m i l e Canyon i s a t h i c k ( a t l e a s t
2500 f e e t ) , h e t e r o g e n e o u sa s s e m b l a g eo fr h y o l i t ea n d
a n d e s i t el a v a s ,t u f f s ,a n dv o l c s n i c l a s t i cs e d i m e n t a r yr o c k s
that filled the
Sawmill Canyon c a u l d r o n a f t e r t h e
Tuff was e r u p t e d . T h i s u n i t
A-L
Peak
i s c o n f i n e d t o t h e Sawmill
Canyon c a u l d r o n .
4)
The t u f f of Lemitar K o u n t a i n s i s u n u s u a l l yt h i c k
i n t h es o u t h e a s t e r n
e r u p t e df r o mt h e
a slight increase
t h e“ h i n g ez o n e ”
M a g d a l e n aM o u n t a i n s .T h i st u f fp r o b a b l y
Socorro cauldron(Chapin
i n t h i c k n e s s occurs i n t h e t u f f
of t h eS o c o r r oc a u l d r o n .
Lemitar Xountains may h a v e p a r t i a l l y
remaininginthe
d
5)
across
The t u f f of
f i l l e d a depression
Sawmill Canyon c a u l d r o n : t h i s c o u l d
accountfortheunusualthickness
Socorro cauldron.
and o t h e r s , 1 9 7 8 ) ;
of t h e t u f f o u t s i d e t h e
.
.
D e p o s i t s similar i n a p p e a r a n c ea n ds t r a t i g r a p h i c
154
c
position t o the unit of
L u i s Lopez (moat d e p o s i t s of t h e
are p r e s e n t i n t h e s o u t h e a s t e r n M a g d a l e n a
S o c o r r oc a u l d r o n )
M o u n t a i n s( w e s t e r np a r t
are
of t h e ' S o c o r r o c a u l d r o n ) , b u t
u s u a l l y less t h a n 50 f e e t t h i c k .T h i sp r o b a b l yi n d i c a t e s
that the eastern part of.the Socorro cauldron subsided
f u r t h e rt h a nt h ew e s t e r np a r t .T h e s er o c k si n c l u d eb a s a l t i c
a n d e s i t el a v a s
( % a l ) , t u f f sa n ds a n d s t o n e s ,r h y o l i t el a v a s
.and a r h y o l i t e dome ( ? ) .
Some of these r o c k s were d e p o s i t e d
i n a l a r g ep a l e o v a l l e yi ns o u t h e r n
Ryan H i l l Canyon.
The
b a s a l t i ca n d e s i t el a v a s( T b a l )p i n c ho u tf r o mn o r t ht o
south.
6)
The lower P o p o t o s aF o r m a t i o n( m o s t l y
d e p o s i t s ) was d e p o s i t e d on a n e x t e n s i v e e r o s i o n
f'
i
I
t hseo u t h e a s t e r n
Magdalena
Mountains.
thelowerPopotosa
mudflow
surface i n
I n t h isst u d y
was p r o b a b l y c o n f i n e d m o s t l y
arec.,
t o paleo-
. v a l l e y s anddepressions.
7)
A t h i c ks e q u e n c e
l a v a s (PoundRanch
of 1 1 . 8 t o 1 0 . 5 m-y. o l d , r h y o l i t e
lavas) e r u p t e df r o m
1 mile n o r t ho ft h i ss t u d y
a v e n tl o c a t e da b o u t
area. (Osburn,1978).These
lavas are c o n f i n e d t o t h e n o r t h - c e n t r a l p o r t i o n
s t u d y area.
Theyunconformahly
i n c l u d i n gb a s a l t i ca n d e s i t e
.
of t h e
overlie several u n i t s ,
lavas (Tba2), t h e t u f f of S o u t h
Canyon a n dt h el o w e rP o p o t o s a . F o r m a t i o n .
Lavas o fe q u i v a l e n t
age and similar c o m p o s i t i o r . o v e r l i e t h e P o p o t o s a F o r m a t i o n
i n theSocorroPeak
8)
area.'
The Sierra L a d r o n e sF o r m a t i o n ,p r o b a b l y
Pliocene t o middlePleistocene
of e a r l y
age, c r o p s o u t i n t h e
155
eastern portion o f the study area. This formation consists
.-
of
poorly
consolidated
sands
and
gravels
with
interbedded
basaltic lavas. Paleocurrent directions, taken from
pebble imbrications in the sedimentary units, are from the
southeast to the northwest (Plate
1).
Structure
Parts of several overlapping cauldron structures
occur in the southeastern
PlaFrlalena Mountains. These
cauldrons
have
had
a
major
influence
on the
geologic
evolution of this area. Basedon this study, several
conclusions
.
. *
be
made
about
these
cauldron
structures:
This study area occurs entirely inside the North
1)
i.
can
A thick sequence of Hells Mesa Tuff, which
Baldy cauldron.
erupted from the cauldron and partially filled
it, crops
of the study area. T h e
out in the northwestern corner
unit
of
occurs
Hardy
in
also
southern
part
of the
part
of
cauldron fill,
. .
Ryan Hill Canyon.
Parts of the northern and southern cauldron margins
2)
of the
the
Ridge,
Sawmill
(Plate 2).
Canyon
cauldron
traverse
this
study
area
A segment of the ring fracture and part of the
rimof 'the
topographic
cauldron
have
been
delineated
along
the northern cauldron margin. Apparently, little or
no
slumping
margin
occurred
and
the
along
ring
this
fracture
part
and
of
the
northern
topographic
cauldron
margin
nearly
coincide. Considerable slumping, however, occurred along
+",
the
southern
margin
and
produced
a
wide
zone
between
the
156
ring
0
.-
fracture
and
the
southern
topographic
rim
of
the
cauldron.
The part of the Sawmill Canyon cauldron that has
3)
been defined so far
appears
to
northeast direction (Plate2 ) .
be
elongateanin
east-
The Morenci lineament,
of similar trend, transects the northwestern portion of
this study area and may have influenced the shape of the
Sawmill Canyon cauldron..
4)
not
The western margin of the Socorro cauldron does
display
all
of
the
features
usually
attributed
to
cauldron margins. However, the tuff of Lemitar Mountains
does
appear
whicn
.
to
appears
thicken
to
be
from
a
west
hinge
east
zone
on
a trap-door
Two intrusions and a rhyolite
do:ne
cauldron
to
(?)
across
this
margin,
cauldron.
occur near the
margin.
This study area is located within the Rio Grande rift.
Extensive
block
faulting,
caused extension'in
by
the rift,
has broken the study area. Several conclusions can be
reached about the structures related to extension
the.
in
:
rift:
1)
The transverse shear zone (Chapin and others,
1978) transects
the northwestern portion of this study area.
This structure separates fields of oppositely tilted fault
blexks. Most of this study area is located south of the
transverse
t:.;
shear
zone,
where
the
strata
dip to
the
east
and most of the faults dip to the west. However, a small
part of the study area (the northwestern portion) contains,
strata
that
dip
to
the
west.
157
g-.
2)
The fault-blocl-; tilting probably occurred ain
manner
similarto that
envisioned
by
Morton
and
Black
(1978)
(1975) for the Afar rift in Africa. Chamberlin
described a similar structural style in the Lemitar
Mountains which he called "domino-style" faulting. This
structural style involves the progressive tilting of fault
blocks and the formation rof
iew generations
of
faults
when
.the old fau1.t~ become too shallow for movement to occur
#
along them (Fig. 2 4 ) .
The southern cauldron margin of the Sawmill Canyon
3)
cauldron has probably influenced the later faulting. Some
of
the
extensional
faults
appear
to die
out
or
bifurcate
as they approach the cauldron margin. Also, some the
of
-,
',
. -
faults
bendas they
crossthe cauldron
margin.
Economic Geology
Several
conclusions
can
be
reached
about
the
mineraliza..
tion in this study area:
1) All of the significant mineralization is localized
along
The
the
main
structure
cauldron
margin
of the
importance
of
which
act
a's a
may
this
Sawmill
cauldron
control
for
Canyon
is
cauldron.
that
it
is
subsequent
mineralization.
2)
a
Gold-silver mineralization is concentrated in
narrow
zone
along
the
northern
margin
of the Sawmill
Canyon cauldron. The mineralization is also closely
t;
associated with the white rhyolite dikes that intruded
many of the north-trending faults and the east-trending
a
deep
158
4'-
r i n g f r a c t u r e of t h e Sawmill Canyon c a u l d r o n .
3)
The manganese m i n e r a l i z a t i o n i s c o n c e n t r a t e di n
a widezonebetweentheringfractureandthesouthern
t o p o g r a p h i c r i m of t h e S a w m i l l Canyon c a u l d r o n .T h i s .
mineralization occurs as a n o p e n - s p a c e f i l l i n q i n
a n da l o n gf a u l t s .
breccia
The m i n e r a l i z a t i o no c c u r s . i qr o c k s
o l d a s t h e u n i t of S i x m i l e Canyonand
P l i o c e n e ( ? ) h a s a l t s( T s h Z ) .
as
a s young as t h e
However, t h eO l i g o c e n er o c k s
c o n t a i n most o f t h e m i n e r a l i z a t i o n .
159
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