Geology of the Florida Mountains, Luna County, New Mexico

GUIDEBOOK
Geology of the Florida Mountains
Luna County, New Mexico
EDITOR
LeRoy L . Corbitt
Field Trip Leaders
LeRoy L. Corbitt
Fred L. Nials
Ronnie J . Varnell
t
Eighth Annual Field Trip, May 4 and 5 , 1 9 7 4 .
CONTENTS
Page
President ' s Message
ii
El Paso Geological Society Officers
iii
iv
Introduction
Index map '
v
Stratigraphic Nomenclature chart
ROAD LOGS
Deming to Capital Dome
Capital Dome to Mahoney Park
ARTICLES
The Capital Dome Section, Forida Mountains
Christina Lockman-Bal k
Structure and Stratigraphy, Florida Mountains
LeRoy L. Corbit
Mineral Deposits, Florida Mountains
George B. Griswold
The Lower Ordivician Florida Mountains Formation
Stratatype, Luna County, New Mexico
Dave V . LeMone
Preliminary Radiometric Age Determination from
the Florida Mountains, New Mexico
Douglas G. Brookins and Roger E. Denison
vi
The El Paso Geological Society
DEPARTMENT OF GEOLOGICAL SCIENCES
UNIVERSITY
OF TEXAS
AT EL PASO
El Paso, Texas 79968
-
Telephone 747-5501
THE PRESIDENT' S MESSAGE
I t i s my pleasure t o welcome you t o t h e e i g h t h
annual f i e l d t r i p of t h e E l Paso Geological Society.
This y e a r ' s excursion w i l l view t h e s t r u c t u r e and
s t r a t i g r a p h y o f t h e F l o r i d a Mountains.
Special thanks a r e extended t o D r . Leroy C o r b i t t
and h i s colleagues and students o f t h e Department o f
Geology, Eastern New Mexico U n i v e r s i t y who organized
t h e t r i p and prepared t h e guidebook.
The El Paso Geological Society
DEPARTMENT OF GEOLOGICAL SCIENCES
UNIVERSITY
OF TEXAS
AT EL PASO
E l Paso, Texas 79968
-
Telephone 747-5501
EL PAS0 GEOLOGICAL SOCIETY
Officers For 1974'
Jerry M. Hoffer
William C. Cornell
John H. Earl
John G. Lay
Marion-E. Spitler
C. Tom Hollenshead
President
Vice President
Secretary-Treasurer
Counci 1 1 or
Councillor
Counci 11 or
Past Presidents
Earl M.P. Lovejoy
Robert D. Habbit
William N. McAnulty
Charles J. Crowley
John M. Hills
Dale E. Lockett
C. Tom Hollenshead
iii
INTRODUCTION
-
LeRoy L . C o r b i t t
I
s?
The eighth annual E l Paso Geological Society f i e l d t r i p i s through
the n o r t h e r n and w e s t e r n portions of the F l o r i d a Mountains where s t r u c t u r e s
s i m i l a r t o those present i n the S i e r r a De Juarez can be observed. The
F l o r i d a Mountains a r e located i n the C o r d i l l e r a n foldbelt of southwestert?
New M e x i c o near the contact w i t h the c r a t o n i c foreland. B l o c k faulting
d u r i n g the T e r t i a r y created the present mountains by u p l i f t along rangem a r g i n a l faults; however the i n t e r n a l s t r u c t u r e s w l t h i n the mountains a r e
p r i n c i p a l l y t h r u s t faults of L a r a m i d e age.
A t stop one (Capitol Dome) a l o w e r Paleozoic section o v e r l i e s P r e cambrian m e t a m o r p h i c and igenous r o c k s and sedimentary d i a m i c t i t e of
possible g l a c i a l o r i g i n . T h e postorogenic Lobo F o r m a t i o n and overlying
andesite agglomerate w i l l a l s o be seen.
A t stop two and three (Mahoney P a r k ) a l o w e r Paleozoic section w i t h
complex i m b r i c a t e t h r u s t s l i c e s i s seen. A l s o contacts that have been
i n t e r p r e t e d t o be both f a u l t and i n t r u s i v e can be observed.
1
I would l i k e t o express m y gratitude to a l l who have contributed so
generously of t h e i r t i m e and knowledge. Your president D r . ' J e r r y M
H o f f e r f i r s t suggested the t r i p . Ronnie V a r n e l l and F r e d N i a l s aidedg r e a t l y i n preparing the r o a d log. The authors of a l l a r t i c l e s gave f r e e l y
of t h e i r t i m e and knowledge. M r s . Pete A n n Braught, s e c r e t a r y f o r the
G e o l o j y Department a t E a s t e r n New M e x i c o U n i v e r s i t y typed the manus c r i p t . M a r l e n e Bachicha drafted the cover design.
.
We hope you enjoy the t r i p and contribute some good healthy arguments a t the outcrops f o r the benefit of a l l interested i n t h i s a r e a .
I
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ROAD LOG FROM DEMING T O C A P I T O L DOME AND MAHONEY
PARK IN T H E FLORIDA MOUNTAINS
LeRoy L . C o r b i t t
A S S E M B L Y POINT: R a m a d a Inn, Deming
.
0.0 Log b e g i n s at R a m a d a Inn. F a c e w e s t .
0.6
0.6 T u r n l e f t (south) f r o m U .S. 70-80 on t o C o l u m b u s R o a d , N . M . 11
0.7
1 .3 J o u n N.M. 11 t o C o l u m b u s .
3.1
4 . 3 9:30 L i t t l e F l o r i d a M o u n t a i n s , T e r t i a r y v o l c a n i c s .
10:OO-11:OO F l o r i d a M o u n t a i n s (7448'), P r e c a m b r i a n , P a l e o z o i c , M e s o z o i c ,
and T e r t i a r y .
12:OO T r e s H e r m a n a s M o u n t a i n s (5802 '), T e r t i a r y q u a r t z m o n z o n i t e i n t r u s ~ v e .
1:00 Sierra d e l a s P a l o m a s , M e x i c o (7552') P a l e o z o i c and C r e t a c e o u s . 2:00 C e d a r k \ o u n t a i n s (621 5') T e r t i a r y v o l c a n ~ c sand P a l e o z o i c s t r a t a .
2:30 S n a k e H ~ l l s ,low o u t c r o p s cf t e c t o n i c a l l y m i x e d l o w e r P a l e o z o i c s
3: 0 0 Red Mountain (5422 '), T e r t i a r y r h y o l i t e d o m e .
2 .o
6 . 3 T u r n l e f t ( e a s t ) on t o f a r m t o m a r k e t r o a d . S i g n s a y s Rockhound State P a r k .
1.2
7 . 5 G r a v e l p i t on r i g h t .
2.8
. I 0 . 3 T u r n r l g h t (south) o n t o V e n t u r a B o u l e v a r d of Deming R a n c h e t t e s .
2.0
. *
1 2 . 3 M a k e jog t o r i g h t t h e n l e f t on r o a d t o t h e F l o r i d a M o u n t a i n s .
1 .8
14.1 T a k e right f o r k a t Y .
0.1
14.2 Arroyo
0.6
1 4 . 8 S T O P # 1 - See g u i d e b o z k a r t i c l e by ' q c k m a n B a l k f o r d e t a i l d e s c r i p t i o n of
Capitol Dome S e c t i o n .
C l i m b t o t o p of P r e c a m b r ~ a ng r a n i t e hill a t 10:OO t o c o n t a c t with t h e
Bliss Formation.
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.
.
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P a n o r a m i c View ( f r o m t o p of hill):
12:OO (north) C o o k s Peak, 8404'; 2 8 , m i l e s a w s y . T e r t i a r y g r a n o d i o r i t e
1:00 L i t t l e F l o r i d a ~ o u n t a i n sc o m p o s e d of T e r t i a r y r h y o l i t i c v o l c a n i c s .
7 : 0 0 T r e s H e r m a n a s M o u n t a i n s , T e r t i a r y q u a r t z m o n z o n ~ t ei n t r u s ~ v e
8: 0 0 Big H a t c h e t P e a k , 8441 : 56 m i l e s a w a y . P e r m i a n H o r q u ~ l l aL i m e s t o n e s .
9 : 0 0 S n a k e H i l l s a n d V i c t o r i a M o u n t a m s In b a c k g r o u n d .
10:OO Red M o u n t a m , a r h y o l i t e d o m e , 5422,; ' B u r r o P e a k in d stance, 7965;
56 m ~ l e a
s way. *
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1 1 : 0 0 B l a c k M o u n t a i n . T e r t i a r y r h y o l ~ t i cr a c k s capped by b a s a l t .
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A
v'
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-- - *
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Stop # 1. Capitol Dome. Precambrian metamorphics (Pcm) , Precambrian
granite ( P ~ g n,) Precambrian diamictite (Pesed) , Bliss (Gbl) , E l Paso, (Oep) ,
Montoya (Om)., Mesozoic ( ? ) Syenite (msn), Mesozoic gabbro (mglj), Lobo
Formation (tl) , Andesite Agglomerate (Tag).
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" ,
I)
C
..
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D I S C U S S I O N S T O P #I
,I
!
I
T h e P r e c a m b r i a n r o c k s beneath C a ~ i t o D
l o m e are g r a n i t e a n d s y e n i t e t h a t
a r e i n t r u s i v e i n t o o l d e r m e t a m o r p h i c r o c k s . T h e m e t a m o r p h i c r o c k s c o n s i s t of
interbedded g r a n i t e g n e i s s , p o r p h y r o b l a s t i c g r a n i t e , biotite s c h i s t , hornblende
g n e i s s a n d a m p h i b o l i t e a n d r e g i o n a l e v i d e n c e s u g g e s t t h e r o c 6 5 a r e 1 . 4 t o 1'.8
b . y . o l d . T h e g r a n i t e b e n e a t h t h e B l i s s at C a p i t o l D o m e is d a t e d at . 4 5 t o .6 b . y
o l d . M o s t cf t h e i g e n o u s body b e t w e e n C a p i t o l D o m e a n d M a h o n e y P a r k t o t h e s o u t h
( S T O P S # 2 & # 3 ) is s y e n i t e y i e l d i n g s i m i l a r d a t e s .
.
",
1
I
1
A p p r o x i m a t e l y 1/2 m i l e t o the n o r t h Upper P r e c a m b r i a n p ) d i a m i c t i t e i s pres e r v e d b e n e a t h t h e B l i s s F o r m a t i o n . D i a m i c t i t e s of s i m i l a r s t r a t i g r a p h i c p o s i t i o n
a n d l ~ t h o l o g yo c c u r r i n g f r o m A l a s k a t o C a l i f o r n i a h a v e b e e n i n t e r p r e t e d t o b e
e s s e n t i a l l y s y n c h r o n c u s u n i t s on t h e b a s i s of t h e i r u n i q u e l i t h o l o g y a n d p o s s i b l y
r e p r e s e n t a t i m e of g l a c i a t i s n .
I
1
H e r e t h e s a n d y B l i s s f o r m a t i o n r a n g e s f r o m 5-1 1 0 f e e t t h i c k . T h e o v e r l y i n g
E l P a s o G r o u p is a b o u t 1 1 50 f e e t t h i c k a n d i s d i v i d e d i n t o a l o w e r d a r k - g r a y t o
black dolomite, a middle white t o blue limestone and a u p p e r c h e r t y l i m e s t o n e .
T h e E l P a s o i s o v e r l a i n b y t h e M o n t o y a G r o u p a n d p o r t i o n s of t h e d a r k U p h a m
" Dolomite and c h e r t y A l e m a n F o r m a t i o n are p r e s e r v e d a t Capitol D o m e .
.
T e r t i a r y (?) L-obo r e d b e d s a n d c o n g l o m e r - a t e s u n c o n f o r m a b l y o v e r l i e t h e Prec a m b r i a n , the P a l e o z o i c s t r a t a and the syenitic' r o c k s . Interbedded with and overl y i n g t h e L o b o F o r m a t i o n a r e a b o u t 1 6 0 0 f e e t t o L o w e r T e r t i a r y (Eocene-Oligocene?)
a n d e s i t i c r o c k s which m a k e u p Capitol D o m e .
.
T u r n a r o u n d a n d h e a d b a c k t o N . M . 11
4.5
1 9 . 3 T u r n left f r o m Ventura Boulevard t o County r o a d .
4.0
2 3 . 3 T u r n l e f t ( s o u t h ) o n N . M . 11
3.0
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2 6 . 3 D i p i n h i g h w a y , 12:OO T r e s H e r m a n a s M o u n t a i n s .
1 .o
2 7 . 3 S u n s h i n e s c h o o l on r ~ g h t .
2 .o
2 9 . 3 T u r n l e f t ( e a s t ) on c o u n t y r o a d , c r o s s c a t t l e g u a r d . S i g n on left--Koeningls
Hereford Ranch.
1 .I
3 0 . 4 Cattleguard. F a r m buildings on r i g h t .
1 .o
31 . 4 C a t t l e g u a r d . R o a d i n t e r s e c t i o n . Continue. s t r a i g h t a h e a d .
10:OO ' T e r t i a r y a n d e s i t e a g g l o m e r a t e .
I 1 :00 C a p t i o l D o m e w i t h r e d d i s h P r e c a m b r i a n a t b a s e of m o u n t a i n s .
11 :00-12:30 M e s o z o i c p ) s y e n i t e - - d a r k e r w e a t h e r i n g , c u t by w h i t e ' r h y o l i t e d i k e s .
1 2 : 3 0 B a l d y P e a k c o m p o s e d of E l P a s o l i m e s t o n e intrud.ed (?) by s y e n i t e .
1:00
hill foreground shows repeated F usselman-Montoya sections.
1 :00-2: 00 P r e c a m b r i a n (?) g r a n i t e a n d s y e n i t e
3: 00 T r e s H e r m a n a s M o u n t a i n s .
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LO^
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1.5
32.9 Road on left leads to White H i l l s composed of T e r t i a r y felsite. Continue ahead.
0.6
33.5 Road bears to right, fence on left.
1 .6
35.1 Gate. 2:OO-3:30, h i l l shows repeated Fusselmarr-Montoya sections.
0.3
35.4 3: 00 H i l l on immediate right--faulted Fusselman Montoya
2:00 H i l l east of gap has E l Paso a t base overlain by the Montoya Group and
Fusselman Dolomite. Northeast slde of h i l l has syenitic rocks i n fault
o r intrusive contact with Paleozoics.
12:OO Baldy Peak with syenite intrusive (?) into E l Paso Group.
9:00-12:OO Syenite cut by white rhyolite dikes.
0.2
35.6 3:00 H i l l to south through gap has repeated Fusselman-Montoya section.
0.2
35.8 STOP # 2
Jeep t r a i l on r i g h t leads through gap. Follow jeep t r a i l to south
on foot. The following roadlog pertains to the walk along the jeep t r a i l .
0.1
0.1 Bottom of arroyo.
0.1
0.2 E l Paso L-imestone on l e f t overlain by Montoya Group and Fusselman ~ o l o m i t e .
0 .I
0.3 Outcrops on r i g h t a r e Montoya and F usselman. E l Paso, Montoya, and
Fusselman are exposed on h i l l to left.
0.3
0.6 Deep arroyo. End of t r a i l , Repeated Fusselman and Montoya on h i l l ahead
(south).
0.2
36.0 Abandoned ranch house on l e f t .
0.1
36.1 C o r r a l and windmill--road bears to r i g h t . I n 0.05 m i l e a jeep t r a i l leads to
left, continue straight ahead.
0.3
36.4 Abandoned buildings on right. Mesozoic p) syenite exposed on bcth sides of road.
0.1
36.5 Sharp bend i n road to right,
0.1
36.6 Top of h i l l composed OF Mesozoic (?) syenite.
12:00 Low h i l l s ahead are Fusselman Dolomite.
10:00-1:00 Rugged h i l l s i n background a r e Precambrian (?) granite and syenite.
Continue ahead down h i l l , road bears sharply to left.
0.2
36.8 Bottom of arroyo. H i l l s on r i g h t a r e Fusselman Dolomite.
0.2
37.0 Top of h i l l .
9:00 Low h i l l capped by white E l Paso limestone shows intrusive nature of syenite.
1:00 Black outcrop of Devonian Percha Shale.
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The environment of deposition of the upper 9 feet of conglomeratic arkose
appears a l s o t o have been a surface of moderate relief. At present it cannot
be determined whether the conglomerate accumulated a t the Lase of a low
s e a cliff or a s valley fill in a fault valley. The preservation of the conglomerate suggests that the region had begun t o downwarp and the ai-kose may
well represent a coarse beach deposit. It is highly probable that this conglomeratic arkose belongs to the same cycle of sedimentation a s the arkosic,
coarse-grained, hematitic lower sandstone of the Bliss Formation in section B
In this northern exposure Woodward (i970, p 29) reported that the Bliss
Formation unconformably overlies the conglomeratic arkose Clearly, the
Bliss Formation referred t o by Woodward (1970) and Corbitt and Woodward
(1973) is the whiie t o greenish white quartzose sandstone (lithofacies 1 and 3)
which occurs widely a t the b a s e of the Paleozoic section i r i the Florida Mountains The report of a n unconformable (i.e , disconformable) relation between
the conglomeratic arkose and the widespread white quartzose sands tone which
belongs t o the early Ordovician cycle of sedimentation favors the
assignment of a Late Cambrian age t o the conglomeratic arkose and the valleyf i l l hematitic sandstones of section B It is probable that the marine transgressions into this region occurred a t the same times during the Franconian, a s they
did in the regions t o the north where the lower portion of the Bliss Formation
is dated by the presence of marine faunules
.
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Three lithic units may be recognized within the E l Paso Group. At the
base is 155 t o 260 feet of dark-gray, thin-bedded, medium-grained crystalline
arbitrarily in seciion A-A a t tne
dolomite. The b a s e of the unit is drawn
lowest beds of this lithology. The change from the Bliss unit is fairly sharp,
but one t o two feet of buff sandy limestone can be found above these lowest
dolomites In section A-A no fossils were found, but many of the beds contain small (1/4 inch) concretionary nodules, probably of algal origin, which
are characteristic of tl-eunit. At this position in the section and in this type
of matrix, brachiopods and trilobites correlative with Ross' Zone C of the
Gasconade have been obtained a t other localitites. The unit is e a s i l y
recognized in exposures (text figures 2 , 3) by its darker color and a series
of low cliffs In section A-A there is a long dip slope a t the top of the unit.
In contrast to the transgressive marine conditions of deposition of the underlying sandy limes tone and limy sandstones of the early Ordovician Bliss
Formation, this dolomite unit records the onset and persistence of regress ive
conditions. Downwarping was extremely slow and the algal build-up was able
t o maintain the surface of the carbonate banks constantly in a n intertidal t o
supratidal position. The resultant diagenetic doiomitization destroyed the
sedimentary structures of the carbonate bank lithotopes a s well a s most of
the organic remains
.
.
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The middle unit records, the return of transgressive conditions over the
carbonate banks, when increased downwarping of the area resulted in the
carbonate sedimentation accumulating predominately in the infralittoral t o
intertidal zones The middle unit consists of 740 t o probably 775 feet of
thin-, medium- and thick-bedded, white-gray t o blue-gray limestones s hvwing
a variety of sedimentary structures. Five or- s i x lithic subunits , representing
live or s i x different lithotopes , are rapeatad over and over again in the sections,
and indicate the long m t i n u e d shifting of the different lithotopes ,of the
carbonate bank back and forth across the region as the bank built up.
.
At the base of the middle unit there is a narrow transition zone in which
the dark crystalline dolomites are interbedded with limestone pebble conglomerate lenses and light-gray, fine-grained limestones with crinkled s i l t
laminae (algal mats). Blue-gray and white-gray limestone with crinkled s i l t y
laminae or patches is a very characteristic subunit of the middle lithic unit,
and a t some horizons tne s i l t y component may comprise nearly 60% of a bed.
Traces of fos s ils appear almost immediately in the section.
At 65 feet above the base of the middle unit, twig-like algal structures
may be s e e n on the surface of the b e d s . This algal type is considered characteristic of sublittoral bottoms, 50 feet or s o in depth, and these unbedded
masses of blue-gray limestone are believed t o be small sublittoral reef structures, In the overlying 40 feet they are interbedded with pink and buff pellet
beds of dolomitic limes tone which are moderately f os s iliferous (the s edimentation of the interreef basins and pools). At 100 feet lenses of calcirudite
occur, indicating portions of the bank had been in the intertidal zone At
180 feet specimens of G i r v a n a are found,.and the overlying 20 feet form a
conspicuous cliff of the dense silty laminated limestone interbedded with
numerous thin pellet and pebble beds, the second lithic subtype. The clasts
average around 5 mm in long dimens ion, the smaller ones becoming more
spherical while the larger specimens are roller or blade shape, and they lie a t angles
to the bedding. Each long thin lense or bed had a n uneven lower and upper
surface, and appears t o have originated a s a coating of fine c l a s t i c debris
over the surface of deposition. These pellet beds are a conspicuous lithic
subtype a t intervals throughout the El Pas o Group.
.
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A good exposure of a stromatolite and sponge reef occurs between 220
and 240 feet above the b a s e of the middle unit. Specimens of sponges,
gastropods and long thin cephalopod siphuncles are noticeable. The stromatolites form short pillars (8 inches by 3 t o 4 inches) of fine, controted lime
laminae. At this exposure the irregulx top of one dt the reefs may be s e e n ,
and appears a s if it had actually been exposed subaerially before being covered
by the overlying coarsely crystalline limestone. In the ensuing 200 foot
interval this fossiliferous reef subtype intercalated with numerous thin pellet
beds is well exposed.
At 450 feet above the base of the middle unit, a fourth lithic subtype is
noticeably developed: -pink and buff pebbly dolomitic limestone beds occurring
in bands 1 t o 1 1/2 feet in thickness. This subtype appears sporadically in
the interval 75 t o 100 feet above the base, but is more conspicuously developed
a t this higher position. It alternates with bands of blue-gray, fine-grained
limestone in which sponges and gastropods are common. In the highest beds
placed in the middle unit, the fine-grained s i l t y laminated limestones, the
unbedded blue-gray reef limestones, and the numerous pellet beds alternate
up t o the fault.
Above the fault a n upper unit of 115 feet was measured t o the base of the
Upham Dolomite of the Montoya Group. This urlit is composed of the same
three alternating lithic subtypes which occur in the beds immediately below
the fault, but the upper unit is readily distinguished from the beds of the
middle unit by a n abundance of fresh chert which may make up a s much a s 20
t o 30 percent of a bed. The unit forms a reddish-brown-weathering cliff or
s t e e p slope below the Upham.
In section A-A very little of the Montoya Group has escaped erosion.
The Upham Dolomite forms a low cliff no more noticeable than the upper unit
of the El Paso Group, but farther t o the south the entire Upham Dolomite and
some of the Aleman Formation are present. The Cable Canyon sandy dolomite
may be distinguished a t the base of the Upham, overlain by dark-brown crystalline dolomite, succeeded a t the top by light-gray brecciated dolomite
or limestone conglomerates. In section A-A later erosion has removed a l l but
30 feet of the Upham. To the south the level of this eroded surface rises
stratigraphically s o that immediately below the nose of Capitol Dome the
Upham totals 50 t o 60 feet in thickness and is meslain by 70 t o 80 feet of
thin-bedded, buff-gray , very cherty limestones of the Aleman Formation.
The Lobo Formation overlies this Paleozoic section and forms long slopes
up t o the base of the Tertiary agglomerate cliff. The age of this formation is
not known. Fossils have not been found. The formation is known only in
the northern and southeastern parts of the Florida Mountains where the exposures start on the northwest side of the range, then cross the range in a
southeasterly direction south of &co d e l Diablo ( ~ l o r i d aPeak) t o Lobo
Draw, the type locality, The Lobo Formation overlaps the bounding faults of
the Paleozoic fault block and r e s t s upon both Ordovician and the Precambrian
rocks with noticeable angular unconformity Its areal distribution in the
range is the same a s the overlying Tertiary volcanics At the base of the
formation in section A-A there occurs a coarse conglomerate of variable thickness (3 t o over 10 feet) composed of Paleozoic limestone and chert boulders
in a buff s i l t y limestone matrix. At his locality a 40-foot lense of striking
gray-purple, dense nodular limestone intimately mixed with a maroon-red
silt matrix is well exposed. The lens is overlain by 2- t o $-foot coarse
buff conglomerate. The basal unit is succeeded rather abruptly by mediumt o thin-bedded buff calcarous s ilts tones and thin-bedded gray s hales Higher
in the section, units of maroon-red shales and calcarous siltstones appear
and alternate with the buff and gray beds. The Lobo Formation ranges in
thickness from 250 t o 350 f e e t . Throughout the section lenses of coarse sand
and granules of anglular t o subangular chert appear sporadically in the siltstones. About 40 feet from the top, a coarse conglomerate occurs. The matrix
is a buff calcareous siltstone like the overlying and underlying beds; the boulders are predominately s ubangular t o s ubrounded limestones and cherts , but
pink granite boulders appear for the first time This conglomerate is overlain by red and buff calcareous s iltstones typical of the Lobo At Captiol
Dome where red siltstone is the top unit, the contact with the basal conglomerate assigned t o the Tertiary sequence is dis conformable But a t several
localities t o the north where the buff siltstones and conglomerates are the
top beds, the contact appears gradational.
.
.
.
.
.
.
At Capitol Dome, the basal conglomerate associated with the Tertiary
volcanics is composed predominately of well-rounded boulders of limestone,
cherty limestone, and pink granite, with rare angular pieces of andes ite,
in a gray-green tuffaceous matrix, Within this conglbmerate for a s high a s
8 feet above the base t k e occur lenses and thin beds of buff and maroon-red
calcareous siltstones apfiarently identical with the Lobo lithofacies By analogy
with similar andesites in the region which k v e been dated, these volcanics
are probably of Eocene-Oligocene age.
.
Acknowledgements
Dr. Frank Kottlowski, New Mexico Bureau of Mines and Mineral Resources,
very kindly reviewed t h i s revised version of my 1958 article and up-dated
the regional tectonic and volcanic data.
References
.
Corbitt, L .L. and Woodward Lee A., 1973 Upper Cambrian (?) Diamictite
of Florida Mountains, Southwestern New Mexico: Bull. Geol
Soc. America V. 84, pp. 171-174.
..
Flower, R H , 1958, Cambrian-Miss iss ippian beds of southern New Mexico:
pp. 60-78.
Roswell Geological Society, Guidebook 11th Field Conf
.,
Lockman-Balk, Christina , 19 72, Cambrian of the Rocky Mountain Region:
Geologic Atlas of the Rocky Mountain Region, Rocky Mountain
Geologic Association, p
.
Woodward, Lee A., 19 70, Precambrian rocks of southwestern New Mexico:
New Mexico Geological Socie.ty, Guidebook 2 1st Field Conf ,
pp. 25, 29.
.
STRUCTURE AND STRATIGRAPHY
OF THE FLORIDA MOUNTAINS
BY
LEROY L. CORBITT
INTRODUCTION
The Florida Mountains a r e in the Basin and Range Province in south central
Luna County, New Mexico (Fig. 1) about 8 miles southeast of Deming. Block
faulting during the Tertiary created the present mountains by uplift along rangemarginal faults; however, the internal structures within the mountains, a r e
principally thrust faults of Laramide a g e . A t l e a s t 3 , 0 0 0 f e e t of Cambrian
through Permian strata a r e preserved above Precambrian r o c k s . Higher peaks
in the range exceed 7 , 0 0 0 f e e t in elevation, rising about 2 , 8 0 0 f e e t above the
adjacent bolsons
.
Some of the major structures and stratigraphic relations within the range
were recognized by Darton (1917) during reconnaissance mapping for the Deming
Folio. Kelley and Bogart (1952) recognized that the Gym Limestone a s described
by Darton w a s mostly Fusselman Dolomite and Bogart (1953) concluded the term
Gym should be abandoned in favor of the name Hueco. Complex thrusts within
the Montoya-Fus selman stratigraphic interval were noted by Kottlows ki (195 7)
Lockman-Balk (1958) mapped and described the a r e a near Capitol Dome in the
northern part of the Florida Mountains. The paleotectonic setting of the region
during Pennsylvanian time w a s d i s c u s s e d by Kottlowski (195 8) . Griswold (196 1)
described the mineral deposits of the Florida Mountains.
This report i s b a s e d mainly on a Ph. D. dissertation a t the University of
New Mexico by Corbitt under the supervision of Woodward. Financial support
and a i r photographs were provided by Forest Oil Corporation. Mobile Research
and Development Corporation provided nine R ~ / S ~ - K / dAa~t e s through the courtesy
of R . E . Denison. Eastern New Mexico University Institutional Research Funds
helped defray e x p e n s e s for radiometric d a t e s provided by Douglas G. Brookins.
STRATIGRAPHY
The igneous rocks in the Florida Mountains (Fig. 2) c o n s i s t of a t l e a s t three
m a s s e s of granite and syenite t h a t are intrusive into older metamorphic rocks.
Regional evidence suggest the metamorphic rocks a r e probably 1 . 4 t o 1 . 8 b . y .
old (Woodward, 1970). The rocks c o n s i s t of interlayered granite g n e i s s , porphyroblastic granite, biotite g n e i s s , biotite s c h i s t , hornblende g n e i s s and
amphibolite .
The northernmost intrusive mass i s the granite beneath the Bliss Formation
near Capitol Dome. This granite i s dated a t .45 t o .60 b y . (Murphy e t .a l . ,
1970).
.
.
F i g 1 Tectonic m a p of southwestern
New M e 3 i c o and p a r t of adjacent M e x i c o
showing s t r u c t u r e s o f C o r d i l l e r a n foldb e l t (modified f r o m C o r b i t t and W o ~ d w a r d ,
1973). Numbers r e f e r to l o c a l i t i e s mentioned i n 1 973 text.
Syncline.
~x1.s of
Antlcllne
.
on upper plate.
Northern erosional
llmlt of
Laramlde thrusts of Cordilleran
Foldbelt .
Generalized
trends of olaaely
compressed folds rhowlng direction
of overturning.
Overturned
of
Axls
Barbs
of Anticline.
.
Axls
Fault
Preorogenlc and Synorogenic R o c k s
Thrust
S ISTEM
PRECAMBRIAN
ORDOVICIAN
GROUP
FWdlO%Yh
Lalu MIS
mito
unndnwd
FORMaTION
1
I
Bliss
w
LOWU
Upp%r
DEVONIAN
ILova I
AGE
MISSISSIFPIAN
PERMIAN
CRETACEOUS
TERTIARY
QUATERNARY
\
'
1
#
'
-%
7
,
1
'cs
;:
,'qp --
,
1;,
/
ogpknnerate and tuff,
Alluwum
50-75
:
'
O
;
.
:
,
;
',
'
:'.(:.
:-
,
of liahi md
dmk may Qlom~te
Moniw W tan YQth.nng dolamito,
Sikc~fied cnrrt
conqla-reraie
I n i u W red and buff colcaecus
h u s and l i m r s t w congbnwrate
Andmits- latite
and
DESCRIPTION
Growls
--
I600
'
- Q, -?
\
, -
'22- LlTHOUm
i:90
0- 7
unm
chart pebble
grovels
t w r (7) alhrnatiq
siltstones,
vith inkrb.dd.d
I
i
I
A central body lying between Capitol Dome and the large thrust f a u l t south
of Mahoney Park c o n s i s t s predominantly of syenite. Its age of formation/emplacement is uncertain a s f a u l t , sedimentary and apparent intrusive contacts
with Paleozoic sediments are noted (Brookins and Corbitt 1974, in press)
Age of formation/emplacement based on geologic inference range from Precambrian t o Mesozoic. ~ b / S rd a t e s range from 0.40 t o 0.75 b . y . even when
weathering contamination i s n o t s u s p e c t e d . A s no evidence for Paleozoic
intrusive activity i s known for this area we propose that t h i s body represents
contaminated Precambrian material possibly emplaced in the Mesozoic. This
pr-oblem is critical due t o the mineralization a s s o c i a t e d with Mesozoic intrus i v e s elsewhere in the Southwest.
.
Most of the southern part of the range appears to be a mass of quartz syenite
t o granite thrust northward over the other rocks. This igneous body yields Rb/Sr
whole rock isochron of 1.13 f 0.20 b . y . with initial Sr (87/86) = 0.704 c o n s i s t a n t
with a Precambrian origin (Brookins and Corbitt , 1974 in press) However, one
Cretaceous date is reported t o have been obtained from a sample collected near
Copper Canyon (Kottlowski , personal communication, 1972).
.
Approximately 1/2 mile north of Capitol Dome a n Upper Precambrian ( ? )
diamictite overlies Precambrian mafic g n e i s s (Corbitt and Woodward 1973).
The diamictite sequence i s 12 m thick and c o n s i s t s of c l a s t s 3 . 0 cm t o 1 . 2 m
in diameter in a s h a l e matrix. The diamictite r e s t s unconformably on mafic g n e i s s
and is unconformably overlain by the Bliss Formation.
Of 12 7 c l a s t s t h a t were examined, 45 percent were composed of lithic sandstone with thin cherty limestone interbeds; 17 percent were s i l t y hematite breccia;
17 percent were siliceous ironstone; 12 percent were diabase or basalt; and 10
percent were granite. The lithic sandstone is composed of grains of quartzite
with s i l i c a and c a l c i t e cement; limestone intercalated with t h e sandstone occurs
in beds a s thick a s 15 cm. The silty hematite breccia c o n s i s t s of angular fragments
of hematite in a siltstone matrix. Rarely, pebbles and cobbles are striated and
some have poorly faceted f a c e s . In addition t o t h e c l a s t s described a b o v e , pellets
of c l a y reaching a maximum of 2 cm in diameter a r e present enclosed by shale
matrix.
The b a s a l 1 . 8 m of the diamictite c o n s i s t s of reddish, fissile s h a l e with c l a s t s
of a l l the lithic types noted above. The next 7.5 m is composed of greenish gray,
f i s s i l e s h a l e containing a l l the varieties of c l a s t s . Some of the boulders have
deformed the bedding within the underlying s h a l e a s though the boulders were
dropped into unconsolidated mud. The upper 3 m c o n s i s t s of granitic boulders
reaching a maximum of 30 c m in diameter in a n arkose matrix.
At l e a s t 80 percent of the c l a s t are not similar t o any known Precambrian rocks
in New Mexico o r Texas. We propose that the lower part of t h e diamictite was
formed by ice rafting of c l a s t s , and t h e upper part by deposition of locally derived
granite debris t h a t filled a downfaulted depression.
Approximately 3 ,000 f e e t of Paleozoic strata are preserved above the Precambrian rocks (Fig. 2 ) . Because of complex thrusting in the middle and upper
Paleozoic part of the section precise stratigraphic thicknesses cannot be determined. The stratigraphic relationships in the Florida Mountains are summarized
in Fig. 2 . The Paleozoic strata are mostly shelf carbonates; the Bliss Formation
of Late Cambrian and Early Ordovician age and the Upper Devonian Percha Shale
are the only Paleozoic c l a s t i c units present.
The stratigraphic section a t Capitol Dome is described in detail by LockmanBalk (195 8 , s e e guidebook article)
.
The Bliss Formation in the Florida Mountains is from 0-200 f e e t thick and con-
sist of arkosic t o quartzose sandstone with conglomerate and limestone. Where
the Bliss i s a b s e n t , the E l Paso contacts with the underlying igneous rocks appear
to be either intrusive or fault in origin.
*
The Lower Ordovician E l Paso Group a t Capitol Dome i s approximately 1 , 0 5 0
feet thick and is d i v i ' b d into a lower, dark-gray t o black dolomite, a middle,
white t o blue limestone and a upper, cherty limestone. The lower, dark dolomite
is not recognized elsewhere in the Florida Mountains. The Upper Ordovician
Montoya Group is approximately 450 feet thick and c o n s i s t s of a lower dark Upham
Dolomite with a b a s a l sandy Cable Canyon unit, a middle cherty Aleman Formation
and the upper Cutter Dolomite.
The Cutter Dolomite is overlain unconformably by the Middle Silurian Fusselman
Dolomite which is approximately 1000 ( ? ) feet thick. The Fusselman appears t o
c o n s i s t of four alternating units of light and dark-grey dolomite. The l e a s t deformed
Montoya-Fusselman section is on the first hill south of Mahoney Park a t s t o p two.
The Fusselman Dolomite is unconformably overlain by the olive-green Upper
Devonian Percha Shale which appears t o be approximately 250 f e e t thick. The
Lower Mississippian Lake Valley Formation conformably overlies the Percha Shale
and i s about 2 00 feet thick. Approximately 5 00 feet of massive Lower Permian
Hueco Limestone overlies the cherty Lake Valley limestones. The Hueco-Lake
Valley contact appears t o be both a thrust f a u l t and a sedimentary unconformable
contact. Pennsylvanian rocks are a b s e n t in the Florida Mountains.
The syenite body between Capitol Come and the large thrust fault which c r o s s e s
the range south of Mahoney Park appears t o intrude rocks stratigraphically a s high
a s the Fusselman Dolomite and may represent Precambrian material emplaced in
the Mesozoic.
A small outcrop consisting of 50 t o 75 f e e t of highly deformed silicified conglome r a t e , overlying E l Paso limestone in the southeastern portion of the range i s interpreted t o be Early Cretaceous in a g e .
I
The post-orogenic Lobo Formation lies unconformably on the Precambrian, the
Paleozoic, the syenitic intrusive rocks and t h e t h r u s t s , (see guidebook article by
Lockman Balk for description of the Lobo Formation a t Capitol Dome). The upper
portion of the Lobo Formation is interbedded with and overlain by 1600 feet of
andesitic deposits.
A t the b a s e of t h e mountains and extending up the valleys a r e deposits of gravel
and alluvium. The flanks of the range a r e composed of thick alluvial deposits which
a t depth, probably grades into the Tertiary Santa Fe-Gila Formation. Well data
indicates the alluvium and interbedded volcanics surrounding t h e mountains a r e a t
l e a s t 4 , 0 0 0 feet thick.
PALEOTECTO NIC SETTING
From Cambrian through M i s s i s s i p p i a n time t h e Florida Mountains area w a s part
of a s t a b l e shelf t o the south of the Transcontinental arch. At l e a s t three e p i s o d e s
of epeirogenic uplift during the Middle Ordovician, Early Silurian, and Late
Silurian-Middle Devonian a r e recorded by stratigraphic breaks In Pennsylvanian
time t h i s shelf w a s deformed e peirogenically and the Pedregosa basin developed
in the far southwestern corner of New Mexico (Kottlowski , 195 8 ) . A mildly positive
a r e a , the "Florida Islands " , bordered the basin on the northeast, near the present
F lorida Mountains (Kottlowski , 195 8)
.
.
Pennsylvanian strata a r e a b s e n t in t h e Florida Mountains. However, in a l l
surrounding ranges Pennsylvanian shelf carbonates with minor c l a s t i c units overlie M i s s i s s i p p i a n carbonates. The Permian Hueco Formation overlies the M i s s issippian Lake Valley Formation in the Florida Mountains indicating shelf conditions prevailed again during Wolfcamp time. Immediately t o the north i n the Cooks
Range, red b e d s and conglomerates, u p t o 5 0 f e e t t h i c k , containing angular
fragments of igneous and metamorphic rock a r e considered t o be Permian Abo
(Greenwood e t . a t . , 1970) and indicate nearby uplift and erosion t o the Precambrian.
These conglomerates overlie Pennsylvanian limestone and underlie the Lower
Cretaceous Sarten Sandstone.
Kottlowski (195 8) suggested t h a t t h e "Florida Islands " were emergent in Late
Pennsylvanian time. Sam Thompson I11 (1972, personal communication) concluded
t h a t the Florida Mountain area w a s not emergent until possibly Wolfcamp time,
during which Pennsylvanian rocks were eroded. Corbitt (1970, 1971) indicated
t h a t Pennsylvanian strata were a b s e n t in the Florida Mountains b e c a u s e of tectonic
elimination along Laramide thrust f a u l t s rather than pre-Hueco erosion.
The only outcrop of Hueco in the Florida Mountains i s in the canyon south of
Gym Peak. Here a major reverse fault h a s brought Precambrian ( ? ) granitic rocks
over a thrust s l i c e of Permian Hueco and Devonian Percha Shale which overlies
the Mississippian Lake Valley Formation. The Hueco-Lake Valley contact appears
t o be both a thrust fault and a sedimentary unconformable contact. The western
I
.
portion of t h e contact is a fault breccia zone, silicified in most exposures Thinbedded limestones and cherts beneath t h e breccia zone are drag-folded and overturned t o the north. To the e a s t the contact appears t o b e sedimentary with angular
t o subrounded fragments of Lake Valley limestone surrounded by rock of Hueco
lithology and fauna. The b a s a l Hueco limestone in the Florida Mountains is not
indicative of nearby uplift and erosion t o the Precambrian a s the "Lobo-Abo ( ? ) "
conglomerates a r e immediately t o t h e north in Cooks Range.
The location of the Wolfcampian Abo-Hueco "Florida Islands" t h a t provided t h e
angular c l a s t s of Precambrian rock in the Cooks Range i s not apparent. It is doubtful that it w a s in the Florida Mountain a r e a . Also the Wolfcampidn isopach and
f a c i e s map prepared by Kottlowski (1963, p. 5 2) indicates a distant northern source
area rather then the "Florida Islands" t o the south in central and northwestern Luna
County. The map gives no indication of a n uplift in Luna County during Wolfcampian
time. Corbitt (1971) suggested that both Lower Cretaceous and Permian Abo had been
mapped a s Lobo Formation in the Cooks Range and t h a t the conglomerates which d i s conformably ( ? ) underlie the Cretaceous Sarten Sandstone and contain c l a s t s of Precambrian rock were Lower Cretaceous in a g e . This problem needs more study.
a
During the Mesozoic the Florida Mountains area was a southeastern extension of
the Burro uplift described by Elston (1958). These positive elements comprised
part of the more extensive Deming a x i s (Turner, 1962) Elston (195 8) suggested that
the Burro uplift w a s formed during the Early Cretaceous; Turner (1962), however,
indicated t h a t t h e Deming a x i s w a s probably positive throughout much of Mesozoic
time. The syenite between Capitol Dome and the large fault crossing the range
south of Mahoney Park appears t o be contaminated Precambrian material emplaced
in the Mesozoic.
.
A Cretaceous trough t h a t w a s present in southwestern New Mexico (Kottlowski,
1963) probably was bordered on the north by the Deming a x i s . Northward-yielding
thrust f a u l t s and overturned folds developed during Laramide time within and along
the northern margin of t h i s trough creating the internal structure of the Florida
Mountains. Block faulting during the Tertiary created the present mountains by
uplift along range-marginal f a u l t s .
LARAMIDE STRUCTURES
In the southern Florida Mountains thrust and steeply dipping reverse faults cut
or deform rocks a s young a s Early Cretaceous ( ? ) and syenitic rocks probably
emplaced in the Mesozoic and are in turn unconformably overlain by the Tertiary
Lobo Formation. Thus t h e s e faults appear t o have formed during the Laramide
orogeny .
The most conspicuous fault is a northwe st-trending , steeply dipping reverse
fault t h a t h a s brought Precambrian ( ? ) granitic rocks over the Paleozoic in the
southern part of the mountains south of Mahoney Park. This fault i s s t e e p a t
deep structural l e v e l s , but flattens upward. There i s a t l e a s t 2,000 feet of
stratigraphic separation on t h i s fault.
In t h e Paleozoic strata beneath t h i s f a u l t , on the northeast s i d e , complex
imbricate thrust s l i c e s a r e particularly w e l l exposed in the second ridge south
of Mahoney Park in S e c s . 34 and 3 5 , T 25 S , R 8 W. A t t h i s location undulating
t o gently dipping thrusts have repeated parts of the Ordovician-Silurian section
three times. There i s tectonic elimination of s t r a t a , a s well a s repetition along
some of the f a u l t s . The thrusts and the strata dip s o u t h e a s t a t a n average of
about 15'.
The fault surfaces commonly are marked by brecciated zones up t o
60 f e e t thick that locally are silicified. A precise calculation of s l i p on t h e s e
faults-cannot be made, but there must be a t l e a s t 4 , 0 0 0 f e e t of horizontal yielding i n a northerly direction. Presumably t h e s e imbricate s l i c e s are related
genetically to the s t e e p e r fault described a b o v e , where granite h a s been brought
over the Paleozoic rocks; possibly the imbricate thrusts are slightly older than the
s t e e p f a u l t and have been offset by the latter. However, the a b s e n c e of Paleozoic
rocks on the south s i d e of the s t e e p fault precludes proving t h i s possibility. A t
any r a t e , a l l the thrusts and s t e e p reverse faults probably formed during the same
general orogenic e p i s o d e .
Complex thrusting of the Paleozoic rocks beneath the major s t e e p reverse fault
a l s o i s s e e n a t Gym Peak. Here the thrusts mainly involve tectonic elimination
of strata although locally there i s repetition. In the canyon south of Gym Peak,
below the major reverse f a u l t , a thrust plate composed of Permian Hueco Limestone
and Devonian Percha Shale overlies the Mississippian Lake Valley Formation.
These faults have been tilted by Basin-Range deformation and their present attitudes
do not indicate their original orientations. Mostly, the faults are subparallel with
bedding. Exact amounts of movement cannot be determined, but the movement probably
w a s several thousand f e e t or more.
On the hill north of Victorio Canyon on t h e East side of the Florida Mountains,
the Ordovician El Paso Formation h a s been thrust onto strata ranging from t h e El
Paso t o the Silurian Fusselman Dolomite, with a maximum stratigraphic separation
of 1 , 3 00 f e e t . The horizontal component of movement i s a t l e a s t a few thousand
f e e t . Some of the stratigraphic units have been tectonically thinned a t this locality.
BASIN
- RANGE STRUCTURE
North-South block faulting during the Tertiary created the present uplift along
range-marginal f a u l t s . This uplift is commonly assumed t o be Miocene in a g e .
Elston(et a l . , 1973) dated the beginning of Basin and Range faulting in the
Mogollon-Datil Province a t 20 m. y. B. P. However, w e s t of the Franklin Mountains , a n d e s i t e s dated a t 4 7 . 1 -I- 2 . 3 m y are younger t h a n , and guided by some
north striking Basin and Range faults (Lovejoy, 1972) ihdicating uplift probably
began in mid-Eocene .
. .
4
If the data reported in the Angelus # 2 drilled in the NE $ SE of sec. 8 , T . 2 6
S . , R . 8 W . (Kottlowski and o t h e r s , 1969) i s correct, more than 3 , 0 0 0 f e e t of d i s placement occurred on the w e s t s i d e of the range. This t e s t drilled about one mile
from the lowest outcrop a t the foot of the mountain, immediately w e s t of South Peak,
reported Quaternary and Tertiary sediments t o a total depth of 3 , 3 6 5 f e e t . At l e a s t
Figure 3. Tectcnc mop showing interpretation of Basin and
Range faults of Florida Mountains.
t h i s much valley f i l l is present five miles e a s t of the range. Two t e s t wells drilled
in s e c . 4 and 8 , T. 25 S. , R . 6 W . , reported Tertiary volcanics in the lowest
samples a t 3,815 f e e t (Kottlowski and o t h e r s , 1969). Thus the block forming the
Florida Mountains appears t o have been uplifted a t l e a s t 4 , 0 0 0 f e e t along rangemarginal faults (Fig. 3 ) .
DISCUSSION
North of the Florida Mountains (Fig. 1) a few small structurally isolated thrusts
have been observed (Corbitt and Woodward, 1973). These thrusts are local and
have no consistant direction of yielding; their geometry indicates t h a t they are either
upthrusts t h a t become nearly vertical a t depth or gravity-slide m a s s e s derived from
uplifted blocks. Thus , t h e s e structures are very similar t o many other Rocky
Mountain foreland features and are dominated by vertical movements whereas the
foldbelt structures indicate a strong component of horizontal movement.
The major Laramide thrust faults exposed in southwestern New Mexico (Fig. 1)
have yielded northeastward toward the foreland.
Uncertainty concerning the extent of basement involvement impedes a n a l y s i s of
the mechanics of thrusting. If the basement is involved in thrusting, a s i t appears
to be in the Florida Mountains, gravitational gliding probably w a s not the driving
mechanism. Also, no evidence h a s been presented t o indicate the presence of
extensional structures within the thrust plates. Possibly, however, a gravitational
model similar t o that proposed by Price and Mountjoy (1970) may have been operative,
with upward and outward pushing from a n uplifted central core in the foldbelt.
Southwestern New Mexico w a s marginal t o any hypothetical central foldbelt uplift,
and the thrusts in t h i s region appear t o have moved upslope.
The amount of displacement on the thrusts cannot be determined accurately,
but movement probably w a s a t most s e v e r a l miles and in many a r e a s w a s no more
than a few thousand f e e t .
By analogy with a r e a s in the foldbelt in Nevada and Utah (Mackin, 1960),
anomalous rriinor structures may be superimposed on the foldbelt. These structures
include doming by intrusions and gravity-slide plates derived from intrusive doming
or from block uplift during Basin-Range deformation. Obviously, however, the thrust
faults and compressional folds are not related t o Basin-Range deformation b e c a u s e
the range-marginal faults of the Basin-Range uplifts truncate the thrusts and folds.
Thrust faults along tectonic strike of the New Mexico thrust b e l t are reported in
the Chiricahua and Dos C a b e z a s Mountains of e a s t e r n Arizona (Sabins, 1357).
Northward-yielding thrust faults a l s o are present in central Cochise County, Arizona
(G.illuly, 195 6) Drewes (1968) dated northeast-directed thrust faults in southeastern
Arizo-na a s beginning about 90 rn,y . a g o and ending about 5 2 mey . a g o , with a time
of quiescence between 62 and 5 7 m . y . ago. Hayes (1970) dated initial phases of
the Laramide orogeny in southwestern New Mexico a s late Turonian.
.
Lovejoy (1974) indicated Laramide thrusting and folding in the Juarez Mountains
had ended prior t o the emplacement of a n d e s i t e s believed t o b e 50-45 m.y. old.
The postorogenic a n d e s i t e s interbedded with and overlying the Lobo Formation in
the Florida Mountains may be of similar a g e . However, a n d e s i t e s of the Upper
Cretaceous -Lower Tertiary ( ? ) Hidalgo Formation are involved in Laramide thrusting
in the Little Hatchet Mountains and Brockman Hills (Fig. 1).
The zone of frontal breakthrough marked by overthrusts along the northern
margin of the New Mexico segment of the foldbelt appears t o continue southeast
into southwestern Texas (King, 1969) and into t h e eastern Chihuahua tectonic b e l t
(Gries and Haenggi , 1960) . Thus the northern margin of the Laramide thrust zone
coincides with the Texas lineament. Consequently, in southwestern New Mexico,
the Texas lineament (Albritton and Smith, 195 7) appears t o b e a zone separating t h e
foldbelt from the foreland on t h e north.
REFERENCES CITED
. .
.
Albritton , C C , Jr. , and J . F Smith, Jr. , 195 7 , The Texas lineament, i n Tomo
2 of Relaciones entre l a tectonica y la sedirnentacion: 20th Internat. Geol.
C o n g . , Mexico, D.F. , 1956, s e c . 5 , p. 501-518.
.
Bogart, J. E , 1953 , The Hueco (Gym) Limestone, Luna County, New Mexico :
M S t h e s i s , University of New Mexico, 91 p.
. .
~ r
of igneous rocks of
Brookins, D. G. , and Corbitt , L. L. , Preliminary ~ b / study
the Florida Mountains, N M : American Geophysical Union, April 1974.
. .
Corbitt, L.L., and Woodward, L.A. , 1970, Thrust faults of the Florida Mountains
New Mexico and their regional tectonic significance: New Mex, Geol. Soc.
Guidebook, Tyrone-Big Hatchet Mountains-Florida Mountains Area, pp. 69-74.
.
Corbitt , L. L , 19 7 1 , Structure and Straigraphy of t h e Florida Mountains , New
Mexico: Ph. D. d i s s e r t . , University of New Mexico, 115 p.
Corbitt, L. L . , and Woodward, L.A. , 1973 a , Upper Precambrian ( ? ) diamictite of
F lorida Mountains, Southwestern New Mexico: Geol . Soc Am. v . 8 4 , p . 171- 173.
.
Corbitt , L. L. , and Woodward, L. A. , 1973 b , Tectonic framework of Cordillsran
foldbelt in southwestern New Mexico: Am. Assoc Petroleum Geol. Bull. , v , 5 7 ,
p. 2207-2216.
.
Darton, N. H. , 1917 , Deming Folio, New Mexico: U. S. Geol. Survey Geological
Atlas, Folio 207.
.
Drewes , H. , 1968, The Laramide orogeny of Arizona s o u t h e a s t of Tucson (abs ):
Geol. Soc America Cordilleran Sec Mtg , Tucson, p. 5 2 -5 3; 1969, Geol .
Soc. America Spec. Paper 1 2 1 , p. 5 01 -5 02.
.
.
Elston, W. E . , 195 8 , Burro uplift, northeastern limit of sedimentary basin of southwestern New Mexico and southeastern Arizona: A m . A s s o c . Petroleum Geologists
Bull., v . 4 2 , p. 2513-2517.
Elston, W. E . , Damon, P.E. , C o n e y , P. J.,Rrodes , R . C . , Smith, E . I. , and Bikerman,
M . , 1 973. Tertiary Volcanic Rocks, Mogollon-Data1 Province, New Mexico and
Surrounding Region: K-Ar D a t e s , Patterns of Eruption, and Periods of Mineralization.
Gilluly, J . , 1956, General geology of central Cocnise County, Arizona. U . S, Geol.
Prof. Paper 281, 169 p.
.
.
Greenwood, E , F. E Kottlowski , a n d A. K. Armstrong, 1 9 7 0 , Upper Paleozoic and
C r e t a c e o u s stratigraphy of t h e Hidalgo County a r e a , New M e x i c o , in Guidebook
of t h e Tyrone-Big Hatchet Mountains-Florida Mountains region: New Mexico
Geol. Soc. , p. 33-43.
Gries , J . C . , a n d W . T. Haenggi , 1 9 7 0 , Structural evolution of t h e e a s t e r n Chihuahua
t e c t o n i c b e l t , in K. Seewald and D. Sundeen, e d s , The geologic framework of
t h e Chihuahua t e c t o n i c belt: W e s t Texas Geol. Soc. -Texas Univ. , Austin, p.
45-48.
.
Griswold , G . B. , 1961 , Mineral d e p o s i t s of Luna County, New Mexico: New Mexico
Bur. Mines a n d Mineral Resources Bull. 72-157 p.
H a y e s , P. T. , 19 7 0 , C r e t a c e o u s paleogeography of s o u t h e a s t e r n Arizona and a d j a c e n t
a r e a s : U.S. Geol. Survey Prof. Paper 658-B, 42 p.
Kelley, V . C . , a n d Bogart, L . E . , 1952 Gym Limestone, New Mexico: Am. A s s o c .
Petroleum Geol. Bull. , v . 3 6 , pp. 1644-1648.
King, P. B. , 1969, The t e c t o n i c s of North America - A d i s c u s s i o n t o accompany t h e
Tectonic Map of North America, scale 1:5 ,000,000: U. S. Geol. Survey Prof.
Paper 6 2 8 , 95 p.
.
Kottlowski , F E . , 195 7 , High-purity dolomite d e p o s i t s of south-central New Mexico:
New Mexico Bur. Mines and Mineral Resources C i r c 47 , 43 p
.
.
195 8 , Pennsylvanian a n d Permian rocks n e a r t h e l a t e Paleozoic Florida
i s l a n d s , in The H a t c h e t Mountains and t h e Cook Range-Florida M o u n t a i n s , G r a n t ,
Hidalgo, a n d Luna Counties , southwestern New M e x i c o , 1l t h Field Conf . Guidebook: Roswell Geol. S o c . p. 79-87.
1 9 6 3 , Paleozoic and M e s o z o i c s t r a t a of southwestern a n d south-central
New Mexico: New Mexico Bur. M i n e s a n d Mineral Resources Bull, 79-1 00 p.
R. W . F o s t e r , and S .A. Wengerd, 1 9 6 9 , Key o i l t e s t s a n d stratigraphic
s e c t i o n s in s o u t h w e s t New M e x i c o , in Guidebook of t h e border region: New
Mexico Geol. S o c . , p. 186-196.
Lockman-Balk, C . , 195 8 , The Capitol Dome s e c t i o n , Florida Mountains; Roswell
Geol . Soc . Guidebook, Hatchet Mountains a n d Cooks Range-F lorida Mountain
a r e a , pp. 47-52.
Love joy, Earl M . P. , 1972 , Basin-Range faulting r a t e s , Franklin Mountains, Texas*,
Geol S o c . America Abstracts v . 3 , p. 3 87-3 88.
.
Love joy, Earl M . P. , 1974, Supplement t o the E l Paso Geological Soc. sixth annual
field trip guidebook, The Stratigraphy and Structure of the Sierra DeJuarez
Chihuahua, Mexico, p. 5 .
Mackin, J . H. , 1960, Structural significance of Tertiary volcanic rocks in southwestern Utah: Am. Jour. S c i . , v . 258, p. 81-131.
Murphy, R . E . , Corbitt, L. L. , and Kenney, E . E . , 1970, Road log from Deming t o
Capitol Dome and Mahoney Park in the Florida Mountains! New Mexico Geol.
Soc. Guidebook, Tyrone-Big Hatchet Mountains-F lorida Mountains Area, pp.
2-3-26.
Price, R.A. , and E . W. Mountjoy, 1970, Geologic structure of the Canadian Rocky
Mountains between Bow and Athabasca Rivers-a progress report in Structure of the
Southern Canadian Cordillera: Geol. Assoc. Canada Spec. Paper 6 , p. 7-25.
Sabins , F. F. , Jr. , 195'7, Geology of the Cochise Head and western part of the Vanar
quadrangles, Arizona: Geol. Soc. America Bull. , v. 6 8 , p. 1315-1341.
Turner, G. L. 1962, The Deming a x i s , southeastern Arizona, New Mexico, and TransPecos Texas, in Guidebook of the Mogollon Rim region: New Mexico Geol. Soc. ,
p. 59-71.
"
Woodward, L , A . , 1970, Precambrian rocks of southwestern New Mexico, in Guidebook of the Tyrone-Big Hatchet Mountains-Florida Mountains region: New Mexico
Geol. S o c . , p. 27-31.
MINERAL DEPOSITS FLORIDA MOUNTAINS
by
George B . Griswold
This condensation i s from the New Mexico Bureau of Mines and Mineral
Resources Bulletin 72 , Mineral Deposits of Luna County, New Mexico by
George B. ~ r i s w o l d ,1961.
Lead, z i n c , copper, s i l v e r , and gold have been mined from the district
in the p a s t , mostly i n the period 1880 - 1920. Manganese deposits were
exploited on the southeast s l o p e s during the 1 9 5 0 1 s , when the manganese purchasing program of the U. S. Government w a s in effect. Mining activity i s now
a t a standstill.
Description of Mines and Prospects:
Silver Cave Mine
a
This deposit is located on the south slope of Gym Peak in the SW sec.
7 , T - 25-S , R 7 W . The deposit i s s a i d t o have been worked in the period
1881 - 85 . During t h i s time 1 ,800 tons of oxidized lead-silver o r e , valued a t
$ 6 0 , 0 0 0 , was shipped. There i s no known production s i n c e 1885
.
.
The mine i s located on the north slope of a n arroyo that drains t o the e a s t .
Near the mine, the arroyo follows a large northwest-trending fault t h a t h a s d i s placed Paleozoic sediments (on the northeast) a g a i n s t Precambrian granite (on
the southwest). Farther e a s t , the f a u l t trace climbs the south slope of the
canyon and then abruptly swings t o the south.
The block of Paleozoic rocks northeast of the fault i s a homocline dipping
t o the e a s t and forms the bulk of Gym Peak. The beds range from Ordovician t o
Permian in a g e . Darton (1917) placed the Silver Cave mine in a n a r e a underlain
by what he called Gym (Permian) limestone. Actually, the mine i s within a
m a s s i v e , gray dolomite that the writer believes t o be Fusselman dolomite. Complicated structure and intense silicification make accurate age definition difficult.
In the vicinity of the mine, the Fusselman ( ? ) dolomite strikes N . 45O E . an&
dips 40° SE. A b a s i c dike about 5 feet wide, trending N. 5' E . and dipping 7Q0
W. , h a s cut the dolomite. A s t e e p incline w a s driven along the hanging wall of
this dike. Little mineralization occurs a t t h i s location, but the incline apparently
served a s a n extraction opening for ore stoped farther up into the hillside.
A shallow shaft w a s sunk about 150 f e e t northeast of the portal of the incline,
and some stoping w a s done on a N . 80° W . fracture zone containing replacement
pods of oxidized lead-zinc ore. Little ore i s left. The ore mineral w a s c e r u s s i t e ,
accompanied by smithsonite (?), limonite, c a l c i t e , and quartz. The stope openings
are not a c c e s s i b l e without the aid of l a d d e r s , but they a r e believed t o extend
downward t o the level of the incline. The mine dumps do not indicate a large
amount of underground development.
Lucky John (Mahoney) Mine
A lead-zinc-copper deposit i s located on a high ridge about one mile north
of Gym Peak in the central part of s e c . 1 , T. 2 6 S . , R . 8 W. The date of discovery
is not known, but Darton indicated t h a t the mine w a s worked in 1 9 1 4 . Several ore
shipments are believed t o have been made in 1915-1 7 and 1 9 2 6 .
The deposit i s located in a block of Paleozoic sediments almost completely
surrounded by Precambrian granite. The block dips gently t o the e a s t for the most
part, but locally the attitude varies greatly.
The lead-zinc-copper mineralization is limited t o a s e r i e s of east-trending
' v e r t i c a l veins in a gray dolomite believed t o be Fusselman. In the vicinity of the
mine, the beds strike almost due north and dip l o 0 - 50' E . Five distinct e a s t trending v e i n s , a s well a s s e v e r a l other weakly mineralized z o n e s , crop out on
the c r e s t of the ridge. Development c o n s i s t s of trenching on the v e i n s , several
vertical s h a f t s , and one long a d i t driven into the ridge from the e a s t s i d e about
200 f e e t below the c r e s t .
Several s t o p e s have broken through t o the surface; in t h e s e openings the vein
widths range from 0 . 5 foot t o 4 f e e t . The vein material is much oxidized a t the
s u r f a c e , containing smithsonite , c e r u s s i t e , malachite, and azurite a s the ore
minerals, in a gangue of limonite, quartz, barite, and c a l c i t e . A few specimens
of galena were noted; apparently some sulfide ore w a s mined a t depth. The
strength of the vein outcrops tempts one t o recommend this property for further
development, but its remoteness would make the c o s t of such a program extremely
high.
San Antonio Mine
A lead-zinc deposit is located about a quarter of a mile w e s t of Capitol Dome
near t h e b a s e of the mountains in the SE sec. 1 0 , T . 25 S. , R . 8 W.
The local geologic setting is a block of east-dipping Ordovician sediments
bounded on the north and south by east-trending f a u l t s , on the w e s t by Precambrian
granite underlying the sediments, and on the e a s t by the Tertiary Lobo Formation
and andesitic agglomerate that overlie the Ordovician sequence.
The lead-zinc deposits are restricted t o the lower dolomite member of the
El Paso group. The principal veins occur in and near a west-draining arroyo,
where the sediments strike N . l o 0 - 25O E . and dip 30° - 40' E . On the north
s i d e of the arroyo, two shallow shafts have been sunk on two east-striking
veins that dip steeply t o the north. The v e i n s a r e very narrow a t the s u r f a c e ,
the maximum width being about 2 f e e t . Several hundred f e e t south of the arroyo,
two other narrow veins have been prospected, one striking N . 30' W. , the other
N . 85O E . The exposed parts of the veins contain minor amounts of c e r u s s i t e and
smithsonite ( ? ) in a gangue of limonite, c a l c i t e , and quartz.
"
Still farther s o u t h , a short a d i t w a s driven on a vertical vein striking due
e a s t just above the b a s e of t h e El Paso group. The dip of the sediments i n c r e a s e s
t o 60° E . h e r e , but otherwise the mineralization is similar t o t h a t described above.
The amount of dump material present on the property indicates only a minor
amount of underground .vork; l i k e w i s e , the amount of ore shipped must have been
small.
Stenson Mine
A small copper mine, called the Stenson, is located about 1 mile south of the
San Antonio mine, near the center of s e c . 1 4 , T. 25 S . , R . 8 W .
.
The deposit is described by Lindgren et a1 (191 0) a s having had about 1 ,000
f e e t of underground development by 1910. Since that time, another a d i t h a s been
driven a n additional 650 f e e t . An examination of the workings indicated t h a t little
stoping had been done; therefore, the amount of ore shipped must have been small.
The deposit is located within the large exposure of Precambrian rocks on the
western slope of the range. The Precambrian conSists of red and gray, coarsegrained granite, gabbro , and diokite,
.
.
A small vein-fault zone trending N 70' E , with dips ranging from 65' t o 85'
S. , traverses the Sunny Slope and Georgia claims. Development c o n s i s t s of three
? ? i t s and one s h a f t . The upper adit i s caved a t the portal, but i s reported t o be
50 f e e t long (Lindgren e t a l . , 1910 ) . A shaft from the surface extends through
k c i s l e v e l about 70 f e e t in from the portal. The middle a d i t i s a c c e s s i b l e for 390
f e z t , a t which point it is badly c a v e d . A r a i s e , located 320 f e e t inside the portal,
~c seported t o connect with a sublevel 60 f e e t above; the s h a f t from the surface a l s o
connects with the sublevel. The lower adit contains a total of 65 0 f e e t of workings,
but of t h i s distance only 500 f e e t w a s driven along the vein structure.
The exposures in the middle adit show a weak vein containing chalcopyrite,
pyrite, magnetite, and quartz. The vein i s partially oxidized, yielding malachite
and limonite. The vein width varies widely, but the mean width i s 3 f e e t . Much
postore f a u l t movement is evident along the vein.
In the lower a d i t , a definite vein is not v i s i b l e . Instead, a zone 40 f e e t
or more wide contains numerous randomly oriented veinlets and disseminations
of pvrite and chalcopyrite. On the s u r f a c e , in the vicinity of the portal, the
diorite(?) is altered by bleaching and contains much limonite. The average grade
of the zone explored by the lower a d i t i s low, probably only a fraction of 1 perc e n t copper.
Bradley Mine
A lead-zinc-silver prospect is located in s e c . 18, T. 25 S. , R . 7 W.
This mine is believed t o be one of the o l d e s t in the d i s t r i c t , having been
worked in the early 1900's or earlier. The writer w a s not able t o obtain a complete record of production, but one shipment w a s recorded in 1 9 2 7 .
The deposit is located in the Tertiary agglomerate sequence. In t h e vicinity
of the mine, the agglomerate is composed of light brownish-gray tuffs and conglome r a t e beds that strike northwest and dip 1O0 - 30' NE An east-striking vein k i t h
vertical dip c u t s the agglomerate. The vein h a s been developed by trenching and
stoping for about 200 feet along the strike. The material on the dump indicates
t h a t the vein contains g a l e n a , sphalerite ( ? ) , and minor chalcopyrite, in a gangue
of pyrite, limonite, c a l c i t e , and quartz. The vein width i s variable, but the
average is estimated t o b e 2 f e e t . Two large c u t s were made in a tuff bed a few
hundred f e e t w e s t of the vein outcrop. These c u t s do not show a distinct vein
structure; copper s t a i n s , however, a r e present a s streaks in the tuff.
.
The Park (Hilltop) Mine
An old zinc mine is located in what i s known a s The Park, a large amphitheater
on the w e s t side of the Florida Mountains. The precise location is the center of the
S$sec.35,T. 25S.,R. 8W.
The d a t e of discovery and p a s t ore shipments are not known.
The deposit i s located near a northwest-trending fault that h a s displaced Precambrian granite on the northeast a g a i n s t Lower Paleozoic sediments on the southw e s t . The general attitude of the sediments is one of northeast strike and southe a s t dip. In the immediate a r e a of the mine, however, the beds are contorted,
presumably c a u s e d by movement on the aforementioned f a u l t , which l i e s immediately
t o t h e northeast. The sediments a t the mine a r e gray, crystalline, c a l c i c dolomite
b e d s , which weather t o tan and grayish black. The beds are probably members of
either the Montoya or Fusselman dolomite.
Several t r e n c h e s , shafts'and pits expose indistinct veins that trend northwes't and dip steeply t o the southwest. The most promising vein strikes N . 550
W . and dips 65' S . The vein material is thoroughly oxidized, and the ore mineral
.
is "dry bone " smithsonite accompanied by a minor amount of c e r u s s i t e Limonite
is abundant. A grab sample from a small pile of handsorted ore near one of the
pits a s s a y e d 26.4 percent z i n c and a trace of l e a d . The sample is not t o b e
considered a s representative of the ore in place.
An a d i t w a s noted a quarter of a mile south of The Park mine, but it w a s not
examined. It i s in Precambrian rock.
There are numerous manganese deposits in the Florida Mountains. Prominent
among t h e s e are:
Birchfield mines, secs. 5 and 6 , T . 26 S. , R . 7 W. , and s e c s 31 and 3 2 ,
T. 25 S . , R . 7 W .
White King mine, SE sec. 3 1 , T. 26 S . , R . 7 W .
Wet King mine, NW sec. 1 3 , T . 26 S . , R . 8 W .
These mines a r e reported t o have shipped small tonnages during World
War 11, and again in the middle of 1950 ' s .
.
4
4
The Florida mining district h a s remained dormant, except for manganese mini n g , for a number of y e a r s . During the field investigation of the d i s t r i c t , t h e writer
did not have the advantages of a n experienced guide t o show him where the various
mines and prospects a r e located. H e n c e , i t is certain that a number of deposits
were completely missed. After the termination of field work, the writer learned of
the existence of several additional p r o s p e c t s , but time would not allow a reexamination of the a r e a . These deposits are:
Birchfield zinc prospect, a -zinc prospect located in t h e SW sec. 3 2 , T .
25 S. , R . 7 W. A carload of ore w a s reported t o have been shipped from the
deposit in 1949.
Shaw prospect, a copper prospect located in sec. 3 5 , T. 25. 25 S . , R . 8 W.
This prospect may be t h e a d i t noted south of The Park mine
Waddel prospect a g a l e n a , b a r i t e , and fluorspar deposit located in the S $
sec. 24, T. 25 S. , R . 8 W. This deposit may have been renamed the Atir
mine, in which c a s e i t w a s briefly explored by the Consolidated Minerals
Corp. in 195 9.
Edna Belle prospect, a lead prospect located in the W Q sec. 18, T. 25 S . , R
7 W.. This deposit may be the same a s the Bradley mine described above.
Window Mountain mine. Darton (1917) mentions a prospect of t h i s name but
f a i l s t o describe it.
Uranium occurrence. Purple fluorite containing a radioactive mineral is reported from the d i s t r i c t , but the location of the occurrence i s now known.
4
REFERENCES CITED
..
D a r t e n , N. H. , 1 9 17 , Deming F o l i o , New Mexico: U S G e o l o g i c a l Survey
G e o l o g i c a l A t l a s , Folio 207.
Griswold, G . B. , 1961 , Mineral d e p o s i t s of Luna C o u n t y , New Mexico:
N. M e x . Bureau of M i n e s and Mineral Research Bull. 7 2 , pp. 117 - 12 7.
I
.
. .
Lindgren , Waldemar , Graton , L. C , a n d Gordon, C H , 1 9 10 , The ore d e p o s i t s
of New Mexico: U S Geology Survey Prof. Paper 6 8 , pp . 2 85 - 2 95
. .
.
T H E LOWER ORDOVICIAN FLORIDA MOUNTAINS FORMATION STRATOTYPE
LUNA COUNTY, NEW MEXICO
by
David V. LeMone
Department of Geological Sciences
The University of Texas a t E l Paso
The derivation of the name F l o r i d a Mountains i s f r o m that mountain range of
the same name i n Luna County, New Mexico. The word i s Spanish i n o r i g i n and
means l'flowery" and r e f e r s to the many flowers that grow on the slopes of the
range particularly after the summer (July-August) rainy season (see Pearce,
1965, p. 57). The local pronunciation i s Spanish, not English. The general type
locality i s i n the east-central part of the range.
The formation was defined by Flower (1 964, p. 149) who indicated the general
type locality. Flower originally designated the formation as F l o r i d a which had
been pre-empted (G V. Cohee, personal communication, 1968). LeMone (1 969,
p. 22) subsequently designated the formation as F l o r i d a Mountains and located the
precise type locality.
.
The F l o r i d a Mountains Formation represents the youngest Canadian (Lower
Ordovician) rocks exposed i n the Texas-New Mexico area of the southwestern United
States. The F l o r i d a Mountains Formation i s the uppermost of the ten formations
comprising the Canadian Series E l Paso Group. It i s equivalent to U n i t C which
i s an informal designation made by Cloud and Barnes (1 948) i n West Texas. Ordovician age rocks have been recognized i n boreholes i n the United States and Mexico.
The F l o r i d a Mountains Formation, i f present, i n these petroleum tests i s not
separable a t this time. The Canadian E l Paso Group f o r m s a regional angular unconf o r m i t y that dips to the south and confines the F l o r i d a Mountains to a narrow belt
that i s recognized only f r o m the F l o r i d a Mountains, New Mexico i n the west to the
Van Horn area (Beach Mountain) i n the east. The El Paso Group i s recognized as a
time-transgressive unit f r o m west to east originating i n the Cordilleran Geosyncline.
The nearest surface outcrops of the Lower Ordovician t o the south i s the Placer de
Guadalupe area south and east of Chihuahua City, Chihuahua, Mexico some 250
m i l e s south-southeast of the F l o r i d a Mountains stratotype area. The Placer de
Guadalupe section lacks the precision to indicate correlation relationships a t this
time.
The F l o r i d a Mountains Formation stratotype i s located i n the N E / ~ , sec. 6, T .
26s., R.7W., Luna County, New Mexico (see Figure 1). Examination of the Gym
Peak 7.5 minute quadrangle w i l l indicate that the holostratotype can be reached
by means of an unimproved d i r t road that parallels the east slope of the F l o r i d a
UPHAM FM
MONTOYA GROUP
JPPER
IRDOVlClAN
"
14--
TF 10
45
I
covered
Stratotype Florida
Mount ians Formation
I
s
I3
Limestone
L irnestme
with interbedded Iihs+one
Chert
A
Statigraphic Section Location
I
covered
11;
Covered
2
TF
0
meters
1
I
Covered
1
feet TF
0
SCENIC DRIVE
Figure 1
FM
from
GYM PEAK
05
N. M E X .
Z5
Quadrangle
Mountains. Local conditions should be checked at time of entry. A secondary
reference hypostratotype of the easily accessible, intensively studied Southern
F r a n k l i n Mountain Scenic D r i v e section w i l l be subsequently published elsewhere.
The thickness of the stratotype i s 14.295 meters (46.9 feet). The u n i t i s
typically a slope-former below the overlying, massive Upper Ordovician Upham
Formation of the Montoya Group. The stratotype i s a medium t o poorly exposed
u n i t which i s p a r t i a l l y covered (see Figure 1). I t consists of v e r y fossiliferous,
calcareous carbonate with interbedded siltstones and s i l t y limestone with several
zones of light brown to pale yellow brown chert. A lithologic description of the
stratotype i s included i n Appendix I
.
.
Carbonate petrography (see Appendix 11) of the stratotype indicates considerable
recrystallization throughout the section. Styolitization and pressure solution of
allochemical grains a r e also noted. P y r i t e i s common. Carbonate g r a i n sorting
i s typically poor and usually angular. Slides which were stained with A l i z a r i n RedS over 50% of the surface revealed some m i n o r dolomitization.
The F l o r i d a Mountains Formation i s v e r y fossiliferous as described i n Appenaix Iand 11. The flora contains megascopically and microscopically recognizable
stromatolithic units. Thin-section analysis reveals algal corrosion of grains which
can probably be assignable to Girvanella. Nuia siberica (Maslov) i s recognized
a t the stratotype as w e l l as the hypostratotype a t E l Paso.
-
The fauna i s varied and rich. Abundant pelmatozoan ossicles are recognized
i n thin-section and outcrop. Ostracods a r e f a i r l y abundant but have been recognized only i n thin-section.
Conodonts a r e undoubtedly present but have not been
processed a t the stratotype. T r i l o b i t e s are abundant and represented by species
of the genera of Pseudocybele , Pseudomaria, G oniotelus , and Isoteloides (7').
Nautiloids a r e represented by Buttsoceras Cyptendoce r a s Protocycloceras , and
Brachiopods include species of Diaparalasma,
and unidentified tarphyceroid fragment
Riberoid bivalves and monoplacophorans (high
Tritoechia, and Syntrophopsis
and low-conical forms) have also been recovered. P o r i f e r a has been recognized
i n thin-section and as questionable fragments i n the outcrop., Gastropods are common. Calaurops has been recognized by Flower. Flower i s i n the process o f preparing a detailed paleontological study of the formation.
.
.
,
,
The fauna i s typical of the foreland (West Texas-New Mexico) Buttsoceras
Chronozone. It i s correlatable to the Hintz (1 951 , 1952) and Ross (1 951) composite
Garden City-Ibex, Utah miogeosynclinal western North A m e r i c a standard series (see
Figure 2). I t would include Chronozone J-Pseudocybele nasuta and Chronozone KHespernomiella minor. It i s Upper Cassinian Stage o r West Granville Substage. I t
i s probable chronostratigraphic equivalent of the following formations: Wahwah
(Utah); Upper West Spring Creek (Oklahoma); S m ithville-Black Rock (Ozarks); Odenv i l l e (Southern Appalachians); Pinesburg Station (Northern Appalachians): Upper
Copake p) (Southern Hudson Valley); P r o v i d e n c e Island ( F o r t Ann, New York);
Bridport-Beldens (Vermont); C o r e y and Basswood C r e e k (Phillipsburg , Quebec);
and the U p p e r m o s t St. G e o r g e G r o u p (Western Newfoundland). T h e r e is not
a p p a r e n t equivalents t o t h i s f o r m a t i o n in e i t h e r the S p i t z b e r g e n o r t h e B a l t i c
s e q u e n c e s . T h e F l o r i d a Mountains F o r m a t i o n is U p p e r m o s t Canadian Series
or the equivalent t o a portion of the Upper Arenig Series of G r e a t B r i t a i n .
T h e F l o r i d a Mountains F o r m a t i o n c a n b e recognized f r o m the underlying
L o w e r C a s s i n i a n S t a g e S c e n i c D r i v e F o r m a t i o n by a n increasing s i l t content,
yellowish-brown weathering c h a r a c t e r , thin b e d s , and c h e r t development. T h e
l o w e r contact i s covered. W h e r e the s u r f a c e i s well exposed e l s e w h e r e , it d o e s
not f o r m a p a r t i c u l a r l y d i s t i n c t disconformity and m a y p r e s e n t s o m e identification p r o b l e m s t o the uninitiated.
T h e U p p e r O r d o w c i a n Cable Canyon M e m b e r of the Upham F o r m a t i o n of the
Montoya G r o u p overizci- ".e F l o r i d a Mountains F o r m a t i o n at the s t r a t o t y p e T h e
mottled (bioturbated), cLrrT-forming m a s s i v e c a r b o n a t e i s e v e r y w h e r e d i s t i n c t
and unmistakable. It i s Eden-Gobourg S t a g e of the Cincinnatian S e r i e s . I t is
equivalent t o the C a r a d o c i n t h e G r e a t B r i t a i n Series. T h e s p o r a d i c a l l y o c c u r r i n g
Champlainian (Middle Ordovician) R i o M e m b r e s S a n d s t o n e (LeMone, 1969, p 22-23)
"is not recognized at t h e s t r a t o t y p e .
.
.
T h e F l o r i d a Mountains F o r m a t i o n , w h e r e p r e s e n t , i s regignally o v e r l a i n by
the profound unconf o r m i t y that s e p a r a t e d the u p p e r m o s t S a u k f r o m the Lowerofs Sloss. T h e overlying chronostratim o s t Tippecanoe S e q u e n c e s or S ~ ~ p e r s y s t e m
g r a p h i c unit (Chronozone L- Orthoambonites subalatus) is r e p r e s e n t e d only i n
t h e d e e p e r b a s i n s o r miogeosynclinal areas s u c h as t h e G a r d e n City-Ibex region of
Utah. T h e basal Champlainian S e r i e s - W h i t e r o c k S t a g e J u a b S u ~ s t a g ei s recognized
in t h e duab L i m e s t o n e . T h e J u a b S u b s t a g e i s t h e p a r t i a l equivalent of the lower
p a r t of the S p i t z b e r g e n Valhallfonna F o r m a t i o n and i s equivalent t o t h e B a l t i c
\ b l k h o v F o r m a t i o n . The J u a b S u b s t a g e is equivalent t o the u p p e r m o s t A r e n i g and
l m v e ~ r n o s tL l a v i r n of the G r e a t B r i t a i n Series.
T h e F l o r i d a F o r m a t i o n is i n t e r p r e t e d t o be a f o r e l a n d , tidal t o subtidal probably
i n ; :ial r e g r e s s i v e f a c i e s of the Candian in W e s t T e x a s and New Mexico.
I
I
REFERENCES
I
Clcud, Preston E., Jr., and Barnes, V i r g i l E., (I
948), The Ellenburger Group
of Central Texas, Univ. Texas, Bur. Econ. Geol. Pub. 40, 473 p.
Flower, Rousseau H., (1 964), The nautiloid order Ellesmeroceratida (Cephalopoda),
New Mex. Inst. Min. and Tech., State Bur. Mines and Mineral Res., Mem
12, p. 149.
.
4
Hintze, Lehi, (1 951), Lower Ordovician detailed stratigraphic section f o r western
Utah, Utah Geol. and Min. Surv. Bull. 39.
------ , (1952),
Lower Ordovician trilobites f o r western Utah and eastern Nevada,
Utah Geol. and Min. Surv. Bull. 48, 249 p.
LeMone, David V., (1 969), Cambrian-Ordivician i n E l Paso Border Region i n
Border Startigraphy Symposium, New Mex. Inst. Min. and Tech., State
Bur. Mines and Mineral Res , Circ. 104, p. 22-23
.
.,
(1 965), New Mexico Place Names, The Univ. of New Mexico Press,
Pearce, T .M
Albuquerque, New Mexico, p. 57.
1
Ross, Ruben, (1951), Stratigraphy of the Garden City Formation i n north-eastern
Utah and i t s trilobite faunas, Yale Univ. Peabody Mus. Nat. Hist. Hull.
6, 161 p.
TYPE FLORIDA FORClATION
AREA:
Southeastern Flank o f the F l o r i d a Mountains, Lena County, New Mexico.
T o t a l Thickness = 14.295 meters o r 46.9 f e e t .
llPPER ORDOVICIAN
MONTOYA GROUP
llPHAM FORMATION
Sample No.
TF-1OCC
Description
Thickness
Calcareous sand; c o l o r weathered-not observed;
c o l o r f r e s h - p a l e red, 10R 6/2: sedimentary features : Cable Canyon o f t h e Upham Formation o f
the Mon toya Group. Specimen taken l a t e r a l l y
along the outcrop. D i s t i n c t c o l o r change f i n e
t o medium sandstone: fauna and f l o r a : n o t observed: sandy mudstone: sandy m i c r i t e .
LOWER ORDOVI CIA4
EL PAS0 GROllP
FLORI DA MOUNTAINS FORMATION
TF- 9
Sedimentary features : covered. No p o s i t i v e outcrop. Probable t o p o f t h e F l o r i d a Mountains sect i o n estimated a t 46.9.
Sample o f Cable Canyon
taken l a t e r a l l y a t t h i s p o i n t . I n t h e l i n e o f
s e c t i o n no outcrops can be found i n place.
1.829
Limestone-coarsely c r y s t a l l i n e ; c o l o r (weathered)medium grey (N5); c o l o r (fresh)-medium l i g h t grey
(N6) ; sedimentary features : s l i g h t l y s o f t e r a t 2-3
feet coquina l i k e , Very f r i a b l e ; choppable m a t e r i a l
r e p l e t e w i t h good fauna o f tri 1o b i tes , gastropods ,
brachiopods , pelmatozoan fragments ; packstonecoarse grad ned 1imestone; b i o s p a r r u d i t e
1.524
.
Covered.
TF- 8
.45 7
Limes tone : c o l o r (weathered) -some 1i h t brown (5Y R
6/4) c h e r t and s i l t , medium grey (N5 : c o l o r ( f r e s h ) medi urn grey (N5) : sedimentary features : c a l careni t i c ,
some c h e r t (nodular) i n t r a c l a s t i c , s o f t sediment
s t r u c t u r e , flame s t r u c t u r e ; excel l e n t fauna, pelmatozoan fragments, questionable algae, brachiopods ; packs tone : i n t r a m i c r i t e and b i omi c r u d i t e .
1.067
3
Covered, no c e r t a i n outcrops.
1.494
Thi ckness
Oes c r i p ti on
Sample No.
TF- 7
Limestone and minor. s i l tstone ; color (weathered)s i l t l i g h t brown as scattered lenses, mediurn grey
(N5) : color (fresh)-medi um l i g h t grey (N6) ; sedimentary features : i n t r a c l a s t i c (fine orange s i l t )
a1 ternating in part w i t h f o s s i l i ferous calcarenites;
fauna and f l o r a : repl aced forms, pelmatozoan fragments : wackestone t o packstone; intramicri t e and
intramicrudi t e .
TF- 6
Limestone and s i l t ; color (weathered)-sil tstone,
l i g h t brown (5YR 5 / 6 ) , limestone, l i g h t rey (N7 ;
color (fresh)-sil tstone , moderate brown 5YR 4/4 ,
limestone., niedium l i g h t grey (N6): sedimentary
features : like TF-5. Very tough t o sample. Contains reddish zones which take on the aspect of a
paper shale. A1 ternates w i t h coquina-ti ke dark
zones. Coarse grained alternating w i t h fine micrit i c laminated beds; fauna and flora: t r i l o b i t e s ,
brachiopods , pelmatozoans : wackestone w i t h s i l t stone; biomicrudi t e w i t h s i l t s t o n e .
?
1
1.067
Limestone and s i l t s (minor l i g h t brown chert) ; color
(weathered)-medium grey (N5); color (fresh)-medium
l i q h t grey (N6); sedimentary features: This i s the
lowest unit with extensive limonite. S i l t y interbeds.
Forms a massive slope forming u n i t . Coarse grained.
Surface s i l i c i f i c a t i o n . Styoli ti c. Mineralized i n
p a r t ; fauna and flora: contains choppable t r i l o b i t e s ,
algal structure?, brachi opods , pel matozoans ; packs tone
.GI0
t o grainstone ; biomi crudi t e t o b i osparrudi t e .
Cove red
.
.914
TF- 4
Limestone and chert; color (weathered)-chert grey
orange pink (5YR 7/2), medium l i g h t grey (N6) ; color
(fresh)-chert l i g h t brown (5YR 5/6), medi um grey (N5) ;
sedimentary features : s i l ty , d i fferenti a1 compaction,
chert and limestone intermixed. Chert more bedded;
fauna and flora-none observed. Questionable a1 gal
1.554
material. Possible bioturbation ; mudstone; micrite.
TF- 3
Limestone; color (weathered)-moderate brown (chert and
s i l i c i f i e d fauna) (5YR 4/4), medium grey (N5) : color
(fresh)-medium dark grey (M4); fauna and flora: Ind i s t i n c t , repl aced pelmatozoans (moderate brown),
questionable gastropods , brachiopods ; wackestone t o
packs tone ; b i omi crudi t e
.
Covered.
,213
1.554
Sample No.
TF-2
Description
Th4 c h e s s
Limestone: color (weathered)-medi um grey (N5) ; color
(fresh)-medi urn grey (N5) ; sedimentary features : a1 temating minor l u t i ti c carbonates and cal careni tes
fossilifersus material. Som orangjsh s i l i c i f i e d
material ; fauna and flora: bivalves, t r i l o b i tes ,
brachiopods pelmatozoans ( s l i g h t l y s j l i cl fled) ,
trochoi d gastropods ; packstone t o waekestone; biomf cri t e t o biomf crudi t e
.I22
Cove re 8.
.975
.
TF- 1
Limestone and chert; color (weathered)-pale red brown
(102 5 / 4 ) ( c h e r t ) , ale ye1 low brown (10YR 6/21, colow
(fresh)-wdium grey (N5); sedimentary features: well
developed chert; fauna and flora: possibly some small
debris ; muds tone ; m i cri t e
.305
Covered.
.I22
.
TOTAL THICKNESS
14.295
LOWER ORPOVI CI AN
EL PAS0 GROUP
SCENIC DRIVE FORMATION
TF-RASE
Limestone and chert; color (weathered)-moderate orange
p i n k (5YR R/4) ( c h e r t ) , medium grey (N5) ; color (fresh)moderate brown ( 5 Y R 4/4) ( c h e r t ) , medium P l g h t grey (N6) ;
sedimentary features : Scenic Dri ve Formation, Base of
section i s an orange chert u n i t . Sample taken 1 foot
be1 ow l i tho1 ogi c change. Grey muddy cal careni tes and
cal c i l u t i t e s . Soft sediment deformation (compaction).
Laminated i n part w i t h orangish s i l t y laminae; fauna
arid flora: Tubula~.material vertically situated i n
section (3.6 inches h i g h w i t h a cross-sectbon diameter
- 3 7 -inches). Material interpreted t o be dSg9tate
al gae . Tt-f 1 obl t e s , gastropods, erni deegtf ffable f m g ments ; n~udstoneand wackestone ; sparse $tom3 cri t e e
-nc
TF-4
6
6
A
A
TF-1flCC
TF-9
-
TF-8
TF- 7
TF-6
TF-5
P
TF-3
TF-2
-
TF- 1
-
No.
'TF-Case
Chert and recrystallized limestone
Recrystallized
b i ope1 m i crudi t e
Intraclasti c (re- '
xtal ) b i osparrudi t e
A1 gal bi omi cri t e
Chert laminated
rextal lized
Pelmicri t e
Rextal lized intraclas ti
' b i ope lmi crudi t e
Chert, lavinated s i l t stone and pelrcicrite
Intraclasti c biomi crudi te and i n t r a c l a s t i c
bi omi cri t e
Pextal lized intraclasts
1 b i o ~ e l m i c rtie
1 Pextal lized
b i osparrudi t e
Si 1i ci fiec!
carbonate
,
FOLK'S CLASS
1 Siltstone and recrysta'
11 blized
spicular ( ? )
i on11crud1 t e
TF-Series (Type Section - Fl ori da Mountains , Luna County New Me xi co)
Florida Mountains Formation
No.
PT-tlase
G = Good,
F
=
F a ' l " ~ ,P = Poor; A , SA: SR, R.
DIINIiAF!'S CLASS
( Si 1t s t o n e and wacke-
I
P-F $A-
L
TF-SerSes (Type Section - FSori da Mountains , Luna County , New Mexico)
FSori da Mountains Formati on
P r e l i m i n a r y Radiometric Age Determinations f r o m the F l o r i d a Mountains,
New Mexico
.
Douglas G Brookins
University of New Mexico
Roger E. Denison
M o b i l Research and Development Corporation
Introduction
The bulk of the igneous rocks of the F l o r i d a Mountains can be divided into
three groups: Old (-1.35
b.y. ?) g r a n i t i c rocks i n the northernmost p a r t of the
area, granite to (quartz) syenite of uncertain age which occurs south of the
m a j o r high angle reverse fault which roughly divides the F l o r i d a s into northern
and southern parts (Corbitt, 1971; C o r b i t t and Woodward, 1973), and enigmatic
syenitic rocks, also of uncertain age, which occur north of the m a j o r high angle
reverse fault. T h i s last group of rocks i s of extreme importance i n that C o r b i t t
0971) has proposed a Mesozoic age of emplacement f o r them although Griswold
(1961) has e laced them i n the Precambrian; m i n e r a l i z a t on, albeit somewhat minor,
i s m o r e o r less confined to occurrences within o r near the northern body.
It i s the purpose of this r e p o r t to summarize pre i m i n a r y gecchronologic
data f o r t!?e m a j o r occurrences of igneous r o c k s f r o m the F l o r i d a Mountains
(i
.e. those w i t h syenitic o r quartz syenitic to g r a n i t i c affinities) and to attempt
t o i n t e r p r e t t h e i r h i s t c r y . K-A.r and Rb-Sr m i n e r a l and Rb-Sr whole r o c k age
data have been obtained; further, geochemical and normative data a r e included
f o r assistance i n interpretation.
Previous w o r k
The most comprehensive f i e l d study of the F l o r i d a Mountains has ueen that
of C o r b i t t (1971). He mapped o r re-mapped the entire range and was able to precisely tie down the high angle reverse fault separating the northern and southern
parts of the Floridas; the tectonic map resulting f r o m h i s study has been r e ~ o r t e d
by C o r b i t t and Woodward 0971; 1973). The most d i f f i c u l t part of Corbitt's work
to evaluate, however, i s the age of the igneous rocks w i t h syecitic affinities which
occur north of the reverse fault. I n b r i e f , C o r b i t t 0971) argues f o r intrusion of
the syenite (Note: C o r b i t t f s terminology w i l l be used f o r the remainder of this
section except where noted) into the Paleozoic sedimentary seqcence i n the Mesozoic; he notes probable intrusive contacts of syenite-sedimentary rocks i n several
Further, as the syenite i s overlain by the sedimentary rocks
places (p. 61-67).
of the Lobo Formation (Cretaceous-?-Tertiary-?)
he places the age of intrusion
as Mesozoic. Because of s i m i l a r i t i e s between the m a j o r igneous rocks to the
north and south of the reverse fault those south of the thrust fault are also mapped
as Mesozoic syenites (or quartz syenites).
T h e r e a r e three basic problems involved: 0 ) The age(s) o f the s y e n i t i c
r o c k s . (2) The nature of the contacts of the s y e n i t i c r o c k s w i t h the Paleozoic
sedimentary r o c k s . (3) The p r o p e r r o c k c l a s s i f i c a t i o n c f the m a j o r igneous
r o c k s n o r t h and south of the t h r u s t f a u l t (hereafter s i m p l y r e f e r r e d t o as the
n o r t h e r n and southern bodies). A s stated i n the Introduction, the r o c k s of the
southern body a r e i n f a u l t contact w i t h the Paleozoic and older r o c k s and a r e
o f u n c e r t a i n age; hence comments concerning age based on geologic evidence
p e r t a i n t o the n o r t h e r n body only.
The contact of the n o r t h e r n body w i t h the Paleozoic r o c k s i s not clear;
Coebitt 0971) cites the following evidence f o r low grade, contact alteration:
(a) r e c r y s t a l l i z a t i o n and/or d o l o m i t i z a t i o n of carbonates a t the contact, (b)
replacement of arenaceous contact m a t e r i a l b y chlorite, clinbzoisite, c o r d i e r i t e
and epidote, (c) fine-crystalline syenite exhibiting f l o w s t r u c t u r e w i t h i n the
m a r g i n a l p a r t s of the n o r t h e r n body, (d) zones o f m i n e r a l i z a t i o n possibly r e l a t e d
t o syenite emplacement, (e) zones of induration a n d / r e c r y s t a l l i z a t i o n near the
syenite contact, (f) baking and a l t e r a t i o n i n contact zones.
T h e contact relationships of the n c r t h e r n body a r e b y no means c l e a r .
F. E. K o t t l o w s k i ( w r i t t e n c o m m u n i c a t i m t c R. E Denison, 1972) mentions fault,
"edimentary,
and possible cold i n t r u s i v e - t y ~ econtacts of the syenite w i t h the
Paleozoic r o c k s . I t i s impossible t o comment f u r t h e r on the nature of these
contacts without f u r t h e r f i e l d work; i t i s s u f f i c i e n t t o s i m p l y state h e r e that the
problem i s not unequivocally resolved and the basic problem of age of the syenite
r e m a i n s uncertain. P r e v i o u s w o r k e r s (See Griswold, 1962; f o r example) have
placed a l l of the m a j o r igneaus r s c k s of the F l o r i d a Mountains i n the Precambrian;
l i t t l e d e t a i l has been paid t o the "intrusive'' types of contacts noted, however.
.
Of f u r t h e r importance, i f f o r no other reason than f o r comparative petrographic classification, i s the c h e m i s t r y of the r o c k s involved. C o r b i t t 0971) r e f e r s
t o both the n o r t h e r n and southern bodies as syenites t o quartz syenites; others
have called the r o c k s granites, a l k a l i granites, granites w i t h s y e n i t i c a f f i n i t i e s ,
etc. T h i s w i l l be commented on l a t e r i n t h i s r e p o r t .
T h e sample locations a r e shown i n F i g u r e One; only the l a s t two d i g i t s of
each number have been plotted.
Results f o r The Southern Bodv
a.
Rb-Sr whole r o c k age determinations
T h e r e s u l t s of Rb-Sr age determinations f o r eleven whole r o c k s a r e shown
i n Table One; these include f o u r samples wh ich, based on t h i n section examination,
a r e unsuitable f o r age w o r k (i.e. presence of carbonates indicating open system
0.2 b y
conditions). T h e remaining seven samples y i e l d a mean age of 1.13
assuming i n i t i a l S r (87/86) = 0.705. T h e f o u r a l t e r e d samples y i e l d apparent
ages rangirig f r o m 0.4 t o 0.6 b.y. t o which no significance can be d r a w n a t t h i s
time.
f
..
GEOLOGIC
-
M A P O F T H E FLORIDA
MOUNTAINS
L U N A COUNTY. N E W M E X I C O
,
-
r
Tu
A
SOUTH
PEAK
SCALE
,?
,*, a2.d
----
LEGEND
W U S T FAULT
Table 1
SampleNo.
K/Rb
K/( W S r )
SiO,( % )
Apparent
Rb-Sr Age
Carbonate
Alteration
Rock
I. Samples from North of High Angle Reverse Fault
1183
1189
*
543
2.5
69.9
41
Quartz Syenite
532
1.45
66.7
064
Quartz Syenite
64.3
.51
Quartz Syenite
78.1
.70
Granite
1191
870
19.3
1197
241
1200
605
11.6
63.7
-
Quartz Syenit e
1186
600
5.1
63.5
.57
Quartz Syenite
.58
11. Samples from South of High Angle Reverse Fault
I l a . South and west of South Pealc
+
Granite
+
Granite
-
Granite
-
Granite
+
Granite
-
Granite
+
Granite
-
Granite
IIb. South and east of South Peak
-
Granite
Granite
Granite
Notes t o Table 1.
1. Age calculated from t = (sre7/sr
86 )measured
(h )
(
w
-
87 86
( s r /sr )initial
~
Jmeasured
~ /
~
~
~
~
Table 2
Mineral Ages
Sample No.
Rock Type
1650-1F
Quartz Syenite
Quartz Syenit e
Quartz Syenite
Quartz Syenite
Quartz Syenite
Quartz Syeni t e
Hornblende gabbro
Notes : (1) F = Feldspar, H = hornblende.
(2) Rb-Sr ages calculated using
h=
1.39 x
b.
Whole r o c k chemistry
The chemistry of rocks f r o m the southern and northern bodies w i l l be
reported on i n detail elsewhere; roughly they v a r y i n S i 0 2 content f r o m 71
percent to 80 percent. The compositions c f rocks f r o m the southern body a r e
plotted on a normative quartz-albite-K-feldspar diagram i n F-igure 2. A l l
samples plot w i t h i n l i m i t s of e r r o r f o r normal a l k a l i granites.
c.
Discussion
The age of the rocks of the scuthern body i s Precambrian (1.13
0.2
b.y.) but i t i s not clear as to whether i t belongs ta the 1.4 b.y. old province
o r 1.0 b .y. old province as discussed by Denison and Hetherington (1969) and
Wasserburg and others (1965); as m o r e samples a r e contemplated f o r analysis
f u r t h e r statemefits concerning age of formation would be premature here.
I
l
f
The rocks southeast of South Peak a r e granites and not syenites; although
those south and west of South Peak a r e alkalic. They contain abundant modal
and normative quartz (in quantities such that even the use of the t e r m "quartz
syenite'' i s to be discouraged).
I
.
I
I
I
-
The s a m ~ l e sf r o m the southern body have been f u r t h e r subdivided into a
k o u t h and western group and a southeastern grour, f r o m South Peak f o r convenience; the f o r m e r (Group IIa) yield apparent ages ranging f r o m 0.5 to 0.87 b .y.
f o r three unaltered samples and low ages f o r altered samples. K/Rb ratios f o r
unaltered samples range f r o m 198 to 257 f o r the three unaltered samples w i t h K/
( R b / ~ r )ranging f r o m 0.50 t o 0.73. F o r altered samples, K/Rb ranges f r o m 282
to 611, a range m o r e characteristic of syenitic rocks (Erlank, 1968) and K/(Rb/
S r ) f r o m 1.45 to 2.7.
F o r samples f r o m Group I I b (all unaltered) K / R ~v a r i e s f r o m 197 to 400 and
K / ( R ~ / s ~ f)r o m 0.38 t o 0.95. The f o r m e r r a t i o s a r e characteristic of acidic,
non-syenitic rocks (Erlank, 1968) and the low K / ( R ~ / s ~ )r a t i o s coupled w i t h the
high (i.
e
1 b .y.) ages support t h e i r classification as Precambrian granites.
.-
I
I
Of interest, though, are the altered samples of Group IIa; the high (relative to unaltered samples) K/(Rb/sr) r a t i o s as w e l l as low ages a r e readily
explained by addition of S r i n the f o r m of S r i n carbonates added during alteration. Not readily explained a r e the higher K/Rb r a t i o s f o r three of the four
altered samples. P r e f e r e n t i a l addition of K relative t o Rb (or, alternately,
removal of Rb preferential to K) i s implied but the mechanism t o account f o r
this unknown.
Results f o r The Northern Body
a.
i
I
K-Ar and Rb-Sr m i n e r a l and Rb-Sr whole r o c k age determinations
F r o m Rb-Sr age determinations f o r feldspars f r o m the quartz syenite range
f r o m 456
56 m .y. to 51 5 26 m.y. while two hornblendes f r o m quartz syenite and
three hornblendes f r o m penecontomporaneous (See Corbitt, 1971) hornblende gabbro
range f r o m 418 - 8 m.y. to 555
11 m .y. F o u r whole r o c k Rb-Sr age determinations range f r o m 500 m .y. to 750 m . y . f o r quartz syenites which show no signs
2
f
+
2