NEW MEXICO BUREAU OF GEOLOGY AND MINERAL RESOURCES
27
Tt1
763
Tt2
36
84
75
74
83
63
68
Kvx
69
70
81
73
80
TKm
62
3638
59
Qy
87
24
!
18
39
46
Qy
TKm
73
Qy
TKgc
TKm
TKgc
!
Ks
33
Qy
Qy
TKgc
15
Kvx
24
Qy
!
!
75
Kvx
87
Qy
Qm
Kvx
10
TKm
TKm
TKm
Qy
20
20
Kvx
Qy
85
15
14
TKm
9
3
4
TKm
Ks
TKm
6 5
Kvx
49
3
Qy
!
!
15
Kvx
TKm
TKm
Ks
Ks
D
Ks
47
Ks
23
TKm
D
D
o
Ks
TKm
Kvx
30
D
Kvx
D
Kvx
D
D
D
Kvx
Ks
D
Ks
10
7
Ks
TKm
Ks
TKm
5
Trp
63
Ks
Ks
54
30
54
27
Trp
3630
Ks
Kvx
42
Kvx
25
Ks
50
Qy
Kvx
30
Qy
!
6
3
40
83
Tco
Tco
! !
! !
Qy
Kvx
Tco
TKm
Kvx
Qy
Ks
TKm
10
Qy
Trp
Tco
Tco
Qy
Kvx
14
Ks
Kvx
TKm
Qo
B
TKm
3
Ks
Tco
2
14
Tb
Tco
12'30"
Kvx
Ttnx
TKm
Tco
Qo
Ttnx
5
764
0.5
1000
0
1
SILVER CITY
TYRONE
FORT
BAYARD
HURLEY WEST
ALLIE CANYON
Tcy
Tb
Ttnx
Ttnx
Ttnx
Qy
10'
0
1000
2000
3000
5000
6000
0.5
0
55
7
Qm
Tt2
Ttnx
3628000 m N
Qm
Ttn
Ts
Ttnx
Tcy
766
2
4
Qo
767
Tts 3
13
Qm
Qy
Ttn
10
Qy
Tts
Tco
Tts
769000 m E
B'
A
A'
Trace of axial plane of anticline, dashed where approximately
known, dotted where concealed.
Cross sections are constructed based upon the interpretations of the author made from geologic
mapping, and available geophysical, and subsurface (drillhole) data. Cross-sections should be used as
an aid to understanding the general geologic framework of the map area, and not be the sole source
of information for use in locating or designing wells, buildings, roads, or other man-made structures.
The map has not been reviewed according to New Mexico Bureau of Geology and Mineral Resources
standards. The contents of the report and map should not be considered final and complete until
reviewed and published by the New Mexico Bureau of Geology and Mineral Resources. The views and
conclusions contained in this document are those of the authors and should not be interpreted as
necessarily representing the official policies, either expressed or implied, of the State of New Mexico, or
the U.S. Government.
Steve J. Skotnicki1 and Charles A. Ferguson2
281 West Amoroso Drive, Gilbert, AZ 85233
2
114 North Fork Road, Centennial, WY 82055
1
New Mexico Bureau of Geology and Mineral Resources
Open-file Geologic Map 152
Mapping of this quadrangle was funded by a matching-funds grant from the STATEMAP program
of the National Cooperative Geologic Mapping Act, administered by the U. S. Geological Survey,
and by the New Mexico Bureau of Geology and Mineral Resources (Dr. Peter A. Scholle, Director
and State Geologist, Dr. J. Michael Timmons, Geologic Mapping Program Director).
TKm
Crystal-rich welded tuff (Tertiary) — This welded tuff contains about 5-15% subhedral sanidine and quartz from 1-2 mm across and
1% subhedral biotite flakes 1-2 mm across, all in a light gray aphanitic matrix. Abundant flattened pumice fragments up to 10 cm long
form a moderate to strong eutaxitic foliation which dips about 35° at the southern margin (the base of the unit) and becomes less steep
upward and northward. Layering is mostly visible from a distance. One thin, 1-2 meter, thinly bedded air-fall tuff interval is exposed in
the wagon road, but otherwise the tuff is rather structureless and massive.
Bisbee Group equivalent?
UNCONFORMITY
Pa
&s
Mo
Pebbly conglomerate (Late Cretaceous?) — Interbedded medium- to coarse-grained sandstone and pebble conglomerate. The sand
grains are composed mostly of moderately to poorly sorted subangular quartz and minor feldspar moderately cemented by silica. Pebbles
are well rounded and composed of subrounded to well rounded of light to dark gray chert, light gray quartzite, and vein quartz commonly
between 1-4 cm, but locally up to 10 cm across. Petrified wood is locally abundant and has been replaced by dark gray, fine-grained
chert. Some pieces of petrified wood are well rounded, indicating they may have been eroded from an older rock unit. This unit forms a
thin layer a few meters thick resting on top of the Colorado Formation (Ks). Locally, pebbly conglomerate is interbedded with the Ks less
than about 10 meters from the top of the unit. If equivalent, exposures in the northwest corner of the map are close to 100 feet thick.
Colorado Formation (Late Cretaceous?) — Interbedded shale and sandstone. The lower part of this unit is mostly medium to dark
gray, very fine-grained sandstone and siltstone, with subordinate shale. Sandstone beds are locally coarse-grained and contain well
sorted subangular to subrounded quartz and subordinate feldspar grains. Coarse-grained beds in the lower part of the unit pinch and
swell between 10 and 50 cm thick over a distance of about 10 meters, and typically grade downward to well rounded chert-pebble
conglomerate with clasts up to 3 cm across. Some siltsone beds contain septarian concretions. Though rare on outcrop some weatheredout concretions are up to 40 cm across. Thinly laminated dark siltstones locally exhibit low-angle cross-beds in sets up to 2 cm thick.
Sandstone beds dominate in the upper part of the unit, where they are also fine- to medium-grained subangular to subrounded quartz and
subordinate feldspar, in tan-colored thin to medium beds. They locally exhibit planar to trough cross-beds in sets up to 70 cm thick. Thin,
dark brown sandy limestone beds up to 20 cm thick form conspicuous, but mostly covered, outcrops in the upper part of the unit. Where
siltstone and shale dominate the unit forms smooth, rounded hills and slopes with poor outcrop. Sandstone beds commonly form steps
or small ledges a few feet high. May be equivalent to similar deposits beneath the Springerville volcanic field in east-central Arizona, or
to the Bisbee Group in southern Arizona.
Beartooth Quartzite (Cretaceous) — This clean, light gray sandstone contains well sorted, fine-grained quartz grains that are partially
to completely fused together to form protoquartzite and quartzite. Bedding is mostly medium to thick and commonly wavy. Ripple marks
are locally visible and planar cross-beds are abundant in sets up to about 1 meter thick, but more commonly in sets 20-40 cm thick.
TKg
Basalt (Tertiary) — This light bluish gray rock contains 2-5% subhedral phenocrysts all altered to red opaque minerals in a microcrystalline
matrix. Large ovoid cavities up to 1 cm across superficially resemble flattened pumice but are mostly vacant and are likely vesicles. This
rock is exposed in only one place at the base of the Tertiary sequence less than one mile west of Century Hill.
Pyroxene and hornblende porphyritc quartz diorite to syenite (Cretaceous or Tertiary?).
Hornblende quartz diorite (Cretaceous or Tertiary?) — This medium to light gray medium-grained granitic rock contains anhedral to
subhedral light gray feldspar 1-3 mm across and about 15-30% euhedral biotite 1-3 mm across and subhedral hornblende 1-6 mm long,
in a fine-grained granular matrix. Quartz and feldspar are difficult to distinguish from one other in hand-samples. On some weathered
surfaces K-feldspar is abundant and is visible as nearly square, medium to light gray phenocrysts up to 8 mm across. The unit forms a
sill-like body north of Bayard (the Fort Bayard stock), where it intrudes sandstone and shale beds of the Colorado Formation (Ks). Fresh
road cut exposures, particularly along Highway 180, are light to medium gray in color, and reveal sparse rounded xenoliths up to about
6 cm across that have the same mineralogy but contain more hornblende and, thus, appear darker. This intrusive body exhibits several
well-defined joint directions, locally mineralized. Outcrops are typically highly weathered and are rusty orange and crumble into sand,
but rounded to subangular boulders are common on the surface.
Tcy
Ttnx
Younger conglomerate (Tertiary) — This unit everywhere overlies the welded tuff breccia (Ttnx) but is nowhere well-exposed. The
unit contains abundant subangular to subrounded clasts of greenish-gray sandstone and clean quartzite derived from the Colorado
Formation (Ks) and Beartooth Quartzite (Kb). In the east these clasts dominate. In the west the unit contains more abundant pyroxenerich hypabyssal rocks derived from dikes or from the volcanic breccia (Kvx). In all areas, more so in the east, dark, dense iron clasts from
1-40 cm across are abundant and diagnostic. Also in the east the unit contains abundant subangular clasts derived from the hornblende
quartz diorite (TKqd), which are commonly fresher than nearby exposures of the intrusive unit. Rounded, lighter colored clasts of
crystal-rich welded tuff (resembling the Kneeling Nun Tuff) are locally common, particularly in the east, where they occur as cobbles to
large boulders over 1 meter across. Forms smooth rounded hills in the southern part of the map area.
Kvx
Welded tuff breccia (Tertiary) — This unit contains very poorly sorted clasts of crystal-rich welded tuff (Ttn) from sand-size to large
boulders >10 meters across. Eutaxitic foliation, visible in the larger clasts, is rotated mostly randomly with respect to foliation in the
neighboring clasts. In some of the exposures north of Bayard a consistent eutaxitic foliation in some outcrops dips northwest between
30° and 50°. Where visible, the matrix also appears to be composed of the same crystal-rich welded tuff, or sand derived from it. This
unit caps the hill north of Bayard and superficially resembles and intact flow. Elsewhere it forms a thin layer a few meters thick that is
Quarry-cut showing Beartooth Quartzite.
Pa
&s
Cretaceous and Paleozoic Sedimentary and Volcanic Rocks
Location of geologic cross section.
Trace of axial plane of syncline, dashed where approximately
known, dotted where concealed.
by
Ks
UNCONFORMITY
Outcrop of the Pinos Altos granite.
Tertiary Volcanic and Sedimentary Rocks in the South
Normal fault, ball-and-bar on downthrown side. Solid where
exposed, dashed where approximately known, dotted where
concealed.
7
Ks
Medium- to coarse-grained granite (Cretaceous or Tertiary?) — Central core of Piños Altos stock. Contains about 15%
biotite+hornblende. Biotite is euhedral and up to 3 mm across. Hornblende is tabular and up to 6 mm long. Feldspar and quartz form
a granular texture, and on weathered surfaces it is difficult to distinguish the two minerals. Locally, K-feldspar is very abundant and
rectangular, up to 8 mm. Forms rounded to subangular bouldersand light rusty orange sandy soil.
Crystal-poor welded tuff (Tertiary) — This welded tuff contains <1% sanidine and 2-5% subhedral biotite flakes 1-3 mm wide. The
biotite is characteristically a slight copper-red color. Where exposed near the north end of Cameron Creek, the basal 3 meters of the
tuff is bedded. Most outcrops are massive and non-bedded and contain partially flattened pumice fragments several centimeters across.
Some places are more densely welded than others. In the northeast part of the quadrangle this tuff is separated from the stratigraphically
lower, crystal-rich tuff by conglomerate (Tc), but farther west near Cameron Creek the conglomerate pinches out and the two tuffs are
in contact.
32° 45' 00" N
108° 07' 30" W
Geologic contact. Solid where exposed or known, dashed where
approximately known, dotted where concealed or inferred.
1 KILOMETER
Geologic Map of the Fort Bayard Quadrangle,
Grant County, New Mexico.
Tb
TtTs 7
EXPLANATION OF MAP SYMBOLS
SANTA RITA MINE
A geologic map displays information on the distribution, nature, orientation, and age relationships
of rock and deposits and the occurrence of structural features. Geologic and fault contacts are
irregular surfaces that form boundaries between different types or ages of units. Data depicted
on this geologic quadrangle map may be based on any of the following: reconnaissance field
geologic mapping, compilation of published and unpublished work, and photogeologic interpretation.
Locations of contacts are not surveyed, but are plotted by interpretation of the position of a given
contact onto a topographic base map; therefore, the accuracy of contact locations depends on the
scale of mapping and the interpretation of the geologist(s). Any enlargement of this map could cause
misunderstanding in the detail of mapping and may result in erroneous interpretations. Site-specific
conditions should be verified by detailed surface mapping or subsurface exploration. Topographic
and cultural changes associated with recent development may not be shown.
Tt2
10
Qls
16
4
768
NATIONAL GEODETIC VERTICAL DATUM OF 1929
COMMENTS TO MAP USERS
Glance Conglomerate equivalent?
Exposure of andesite breccia showing smaller clasts in volcaniclastic matrix.
3629
TKg
85
TKg
37
Tts
7000 FEET
May 2007
Ksc
2
45
Tcy
1 MILE
4000
UNCONFORMITY
Tco
Tco
Tcy
765
Kvx
TKg
Tco
Tco
Rhyolite porphyry lithic breccia (probably Tertiary) — Coarse-grained feldspar porphyritic, commonly flow-foliated lithic breccia
containing 5-80% pebble to several meter-diameter xenoliths of volcanic breccia (Kvx) and sandstone-shale of probable Colorado
Formation (Ks). The unit resembles the megabreccia of some ash-flow tuffs, which it may grade into in some areas.
83
33
Ttnx
Qy
Tco Tb
Ksc
Kb
Tcy
CONTOUR INTERVAL 20 FEET
Magnetic Declination
March, 2006
10º 8' East
At Map Center
HURLEY EAST
Ks
14
Tcy
41
Tco
NEW MEXICO
TWIN SISTERS
14
Qy
1:24,000
1
READING MOUNTAIN
Qm
Tcy
4
2
Tcy
763
762
Base map from U.S. Geological Survey 1992, from photographs taken 1987, field checked in 1989, map edited in 1992.
1927 North American datum, UTM projection -- zone 12N
1000-meter Universal Transverse Mercator grid, zone 12, shown in red
QUADRANGLE LOCATION
25
12TKg
13
2
Qy
Tcy
FOLDING
Abundant petrified wood weathering out of the quartz-pebble conglomerate.
9
8
6
5
Qo
Rhyolite porphyry dikes and plugs (probably Tertiary) — Coarse-grained (up to 20 mm), euhedral to subhedral feldspar (potassium
feldspar and plagioclase), typically strongly flow-foliated porphyry dikes locally with abundant 1-4 mm euhedral biotite. An isolated
plug occurs in the southwest corner associated with a north-striking swarm of dikes that extend to north, nearly the entire length of the
map area. The dikes and parts of the plug are typically crumbly, and weather into light-colored fragments, but locally, a dark grayish
green vitric matrix is preserved. Ubiquitous biotite is typically very fresh, and the dikes are characteristally non-magnetic in contrast
with most of the older dikes in the area. The unit also occurs along Twin Sisters Creek east of Fort Bayard as a north-striking chain
of plugs. A minor swarm of west-northwest striking dikes occur to the northwest of the north end of this chain. The plugs in this area
contain abundant xenoliths. The unit is locally differentiated into a flow-banded hypabyssal rock (Trp) and lithic breccia (Trpx).
3630
5
l
761
760
3
Tco
Qo
Tco
759
72
21
Qy
Ttnx
6
TKgc
6
Tco
758
Tco
3
Tco
3627
Tcy
32
5
Ttnx
Intrusive
Rocks
derived from the Colorado Formation (Ks), and possibly older quartzite. Only one boulder-size clast of non-foliated coarse-grained
granite was seen. This xenolith is similar to xenoliths found within the dike complex and along intrusive contacts within the fine-grained
monzonitic unit of the Pinos Altos complex. In many areas the matrix and the clasts have a nearly identical composition, making
distinction of the clasts from the matrix difficult.
11
75
53
33
TKg
Tco
Tts
4
Ks
TKg
26
Tb
!
!
Kvx
33
Ks 32
Kvx
Ks
84
82
Tco
Tb
Fort Bayard
Stock
TKm
85
Ttnx
Tb
25
3628
85
27
4
Tco
4
13
Qo
10
Qy
!
!
Tco
Tcy
86
6
!
!
!
Ks
Trpx
TKg
81
9
Tco
10
!
!
Kvx
!
!
Tco
Trp
6
TKg
Cameron Creek dike swarm
Older conglomerate (Tertiary) — Conglomerate and sandstone. Nearly flat-lying to gently dipping (<10°) thin to medium bedded
sandstone and conglomerate containing mostly clasts of pyroxene porphyry derived either from dikes of map unit TKh or from the
volcanic breccia (Kvx). Subordinate clasts include fine- to medium-grained pyroxene-bearing hypabyssal rocks and minor sandstone
derived from the Colorado Formation (Ks). North of Bayard these deposits also contain minor boulders of crystal-rich welded tuff. Most
clasts are angular to subrounded. Bedding in this unit is relatively planar, but paleochannels up to 1 meter deep are common. Except for
the exposures immediately north of Bayard the largest clasts are between 40 and 60 cm across, but more typically between 20 and 30
cm. The sand fraction is composed of lithic fragments of the same rock types listed above. Quartz is a very minor constituent of the sand
fraction. The unit is almost everywhere moderately cemented with calcite.
Spheroidal weathering is common in dikes.
81
15
!
Kvx
Tco
7
6
8
Conglomerate and sandstone (Tertiary) — Conglomerate and sandstone. Nearly flat-lying to gently dipping (<10°) thin to medium
bedded sandstone and conglomerate containing mostly clasts of pyroxene porphyry derived either from dikes of map unit TKh or from
the volcanic breccia (Kvx). Most clasts are angular to subrounded. Locally, the unit is dominated by subrounded to rounded large cobbles
and small boulders up to 60 cm across composed of crystal-rich welded tuff that resembles the Kneeling Nun Tuff (Ttn). These tuff
clasts tend to weather out of eroded slopes and give the impression that the slope is an exposure of welded tuff. Bedding in this unit
is relatively planar, but paleochannels up to 1 meter deep are common. The largest clasts are between 40 and 60 cm across, but more
typically between 20 and 30 cm. The sand fraction is composed of lithic fragments of the same rock types listed above. Quartz is a very
minor constituent of the sand fraction. The unit is almost everywhere moderately cemented with calcite, and is interbedded with Tertiary
volcanic rocks along the northeast side of the quadrangle. The best exposure is on the west side of Avalanche Peak, where a cliff face
exposes a 20-meter thick section. Overall this unit closely resembles the older conglomerate (Tco) in the southern part of the quadrangle
and may be partially equivalent.
79
Ks
Tco
TKm
Basaltic andesite (Tertiary) — This volcanic rock contains 30-40% tabular/rectangular euhedral plagioclase phenocrysts up to 6 mm
long, and 2-5% green glassy pyroxene also up to 6 mm long, and small crystals <1 mm across altered to red opaques that may be olivine
altered to iddingsite, all in a dark gray aphanitic matrix. Commonly vesicular. This unit forms a prominent dark cliff and bench where
the overlying conglomerates have eroded back. These lava flows pinch out eastward north of Avalanche Peak.
85 88
82
81
25
Ks
Kvx
82
77
12
86 83
7
Ks
Â
81
86
Tco
Ks
18
85
83
Ks
Tco
Tco
Qy
84
Ks
8
Ks
32° 45' 00" N
108° 15' 00" W
80
Ks
Tco
Tco
7
40
16
3631
83
Qo
10
Qm
20
10
88
47' 30"
40
Ks
25
Tc
15
26
Qo
Ks
16
12
11
85
&s
9
11
13
3632
TKg
Kb
10
85
10
6
11
TKg
68
12
15
23
82
Kvx
4
15
19
22
10
18
86 18
Trp
Ks
Ks
Qo
20
21
78
TKg
5
TKg
Ks
Tba
3
Qy
10
15
87
Tco
Qy
26 20
37
Ks
TKg
15
18
20
Ks
Trpx
10
TKg
&s
Ks
Ks
A
20
Ks
78
10
Tb
Outcrop of aphyric lava, NE corner of quad.
37
61
65
Tco
3633
11
Qy
Kb
64
27
Qy
Qy
7
82
21
Kvx
Ks
24
83
Ks
Tts
17
TKg
20
62
Ks
9
A'
9
8
87
Ks
19
24
Ks
40
Tb
85
31
Trp
Trp
20
54
&s
TKg
6
8
57
Ks
57
$
Trp
78
58
Trp
Trp
Ks
31
51
Ks
13
Tt
Possible
Ttn clasts
8
23
TKg
Trpx
9
12
21
Trp
Trp
14
Qy
Trp
Pinos Altos
Stock
Dikes and other Intrusive Bodies
4
18
9
82
Trp
Qy
25
TKm
Trp
37
Kb
50
Trpx
13
7
Tc
Trp
Aphyric lava (Tertiary) — These light purple-tan lava flows are nearly aphyric except for minute phenocrysts <1 mm altered to
red opaque minerals. Outcrops are typically very platy and resistant. Individual flows are visible because of vegetation bands on the
mountain northeast of the northeast corner of the map. North of the quadrangle they appear to form a thick sequence several hundred
feet that is overlain by a light tan unit that resembles a tuff from a distance.
Trp
11
32
Kb
48
Trpx
16
Qy
D
3631
TKm
Trp
Trp
D
47'30"
28
20
Ks
Qy
24
Kb
Qy
54
Tt2
UNCONFORMITY
Typical exposure of conglomerate (Tco).
Qy
30
Qy
11
Ts
Kb
20
5
Ks
21
Ks
Tba
Tb
Basalt (Tertiary) — This dark gray rock contains 1-2% phenocrysts of altered red opaque minerals (iddingsite) up to 2 mm across, in
a medium to dark bluish gray aphanitic matrix. The rock weathers into thin robust platy outcrops that are very difficult to break with a
hammer.
Mo
25
Ks
Ksc
30
Ks
Trp
9
3629
6
Ttn
Tt1
Sugarlump Tuff (Tertiary) — This welded tuff contains about 1% subhedral biotite and sanidine phocrysts up to 1 mm in a light bluish
gray aphanitic to partially glassy matrix. Contains abundant flattened pumice fragments. The base is very sharp and planar and overlies
a light pink clay-rich layer 1-2 meters thick that may be altered nonwelded tuff. This welded tuff unit appears to be one flow unit as no
distinct flow breaks are visible and the unit forms one prominent cliff up to 5 meters high.
24
Qm
TKg
47
85
9
34
35
Tva
5
29
15
Ttnx
UNCONFORMITY
Older Quaternary alluvium (early Pleistocene) — This unit forms a thin, sheet-like deposit capping the highest hills in the southern
part of the quadrangle. In most areas it contains volcanic clasts almost exclusively, which are composed mostly of basaltic andesite (from
Tba), aphyric lava (from Tva), vesicular basalt, and occasional clasts of welded tuff. The unit contains a very well-developed argillic
horizon (abundant red-brown clay) and appears dark brown in color. Most surface clasts are moderately varnished and, where rarely
exposed, carbonate is abundant in the matrix below the argillic horizon.
3634
TKg
28
Kvx
Kb
48
42
47
Kvx
Trp
Ks
20
16
D
D
TKm
D
27
Ks
Kvx
Exposures in the North
Bedded lithic tuff (Tertiary) — These thinly bedded deposits form two distinct tuff units in the far southeast corner of the map area.
The lowermost unit is thin, a few meters thick, and is interbedded with conglomerate rich containing clasts of pyroxene porphyry and/
or andesite (Tc). The uppermost tuff is considerably thicker (10+ meters), though exposures are poor, and is overlain by float of green
sandstone (Ts). Beds are thin, platy, and light yellowish gray. Both tuff layers contain abundant subangular sand to small pebbles of
maroon to dark red quartzite.
10
Qy
16
18
20
16
Ks
26
Kvx
30
13
Ks
Ks
TKm
Trp
10
25
22 27
42
Qy
Kvx
Qy
Qy
Trp
Tco
80
20
Ks
TKm
3632
6
Ks
Trp
30
Ksc
21
34
80
!
!
Ks
Ks
15
Kvx
33
3633
88
Exposures in the South
Qo
&s
9
Ks
Kvx
Ks
TKm
18
20
31
20
82
Trp
3635
Qm
Tcy
28
Kvx
Qy
30
Tva
11
20
56
Kvx
Green sandstone (Tertiary) — Thick sequence (100+ feet) of moderately well sorted sandstone, gravelly sandstone, and thinly bedded
light tan-colored pumaceous (?) gravelly sandstone. The overall forest green color of this unit is quite striking. In all beds the larger
grains are dominated by fine-grained, light gray to maroon quartzite and other siliceous grains, some of which resemble flow-banded
rhyolite. The sequence is thin to medium bedded. The light yellowish tan layers are sandy and commonly more gravelly than the greener
beds, and contain abundant siliceous, aphanitic gravel-size clasts that resemble pumice. Small channels less than 1 meter across and 20
cm deep contain subrounded pebbles and cobbles, but the exposures were inaccessible so their composition is uncertain.
Tertiary Volcanic and Sedimentary Rocks in the North
14
Qls
UNCONFORMITY
12
5
Qy
86
38
Kvx
Qy
Kb
Qm
18
Ks
Landslide deposits (Pleistocene) — This deposit was mapped in the far southeast corner of the quadrangle where it is composed of
small to large, poorly sorted blocks of Kneeling Nun Tuff (Ttnx). It is not clear if this unit is related to the breccia deposits also containing
poorly sorted clasts of Kneeling Nun Tuff, but it appears to post-date the north-northeast-striking fault in the same area. Because it is
lower in the landscape than map unit Qo it may be younger than Qo.
Pa
6
16
35
72
12
85
10
3636
!
30
D
!
Kvx
Kvx
3634
20
80
Ksc
15
32
38
84
24
21
Ks
Qy
TKm
TKm
12
Ks
23
20
50'
Qo
&s
Pa
23
Ks
30
TKm
Qm
Qy
Ks
Ksc
14
Ks
17
32
Ks
Ksc
Kvx
43
43
3635
Trp
Qm
18
Kvx
25
27
Ks
Qm
Qm
Qy
Qm
Qy
10
10
16
Tco
Middle Quaternary alluvial deposits (middle to late Pleistocene) — Interbedded silt, sand, and gravel. These deposits are typically
coarser than the younger Holocene deposits (map unit Qy) and contain abundant subrounded to rounded cobbles and locally small
boulders up to 40 cm across. Many of the clasts are Tertiary volcanic rocks. These deposits form elevated terraces from 5 to 10 meters
above the modern stream channels, and commonly have moderately developed argillic horizons.
Kneeling Nun welded tuff (Tertiary?) — This crystal-rich welded tuff contains about 30-50% anhedral to subhedral quartz and
sanidine, and 1-2% subhedral biotite flakes, all 1-3 mm across in a light tan to brown aphanitic matrix. The rock may contain rare
hornblende. Pumice clasts are partially flattened but eutaxitic foliation is difficult to discern, even on weathered surface. The base of
the unit is locally nonwelded and is light gray and nonbedded, but the same phenocryst assemblage is visible. Contains sparse angular
xenoliths 1-8 cm across composed of fine-grained maroon quartzite.
Ks
9
19
Ks
Qm
Th
20
13
Ks
Ksc
!
Qls
23
4
Qy
Kvx
3637
Qm
Ksc
Qy
30
Qy
15
11
23
Ksc
67
30
Kvx
Qy
TKgc
9
7
33
Qy
Qm
!
40
23
Ks
Qm
45
Qm
Qm
Ks
Ks
!
Kvx
18
Ksc
Ks
20
10
Qy
85
88
24
17
Qm
21
Stream-cut showing Holocene sedimentary deposits. Note low-angle cross-bedding and soil horizons.
87
Ks
TKm
Tb
27
Ksc
21
20
4
15
Kvx
3638
Ks
Kvx
Ksc
Qy
Ksc
50'
Tva
Tt1
35
82
Trp
3637
5
4
10
5
Tc
TKgc
74
Tt
Tts
70
76
Tt2
3639
81
Qy
Kvx
85
76
Tc
Tt1
Tb
TKm
67
2
5
Qm
Qy
TKgc
78
3
4
86
Kvx
Tt2
12
76
55
73
5
87
Ts
Tva
2
Tc
Ttn
Tt1
Kvx
84
66
77
72
68
Younger Quaternary alluvium (Holocene) — These deposits are comprised of mostly unconsolidated, locally laminated, silt and sand,
with subordinate gravel and clay. Gravel commonly forms relatively thin beds between about 10 and 30 cm thick. In good stream-cut
exposures, paleosol horizons are visible. These are darker, non-bedded, clay-rich horizons exhibiting vertical cutan formation. Also in
stream-cut exposures, 10 to 50 cm thick shallow paleochannels are common and are filled with gravel or sand. The unit forms very flat
deposits along the modern stream valleys that have almost everywhere been dissected and entrenched by the modern streams. In the
southern part of the quadrangle dissection is between 2-3 m, while in the north it is up to 8-10 m.
Qy
Qm
8
79
68
Tc
8
Tt1
TKm
66
75
Tc
83
76
TKm
72
45
Tt1
70
86
73
2
Tt2
80
Kvx
Qm
56
3640
Tba
Qy
Tc
Kvx
TKgc
87
Tc
Qy
TKm
73
Tt2
71
77
72
84
TKgc
6
Tc
80
78
82
Tc
Qy
l
70
45
85
50
84
81
Tva
Tt2
CORRELATION OF MAP UNITS
quite pervasive stratigraphically. Although Jones and others (1970) originally interpreted this unit as having formed from removal of the
softer crystal-poor welded tuff below and collapse of the Kneeling Nun Tuff, intact exposures of the crystal-poor tuff below outcrops of
the welded tuff breccia suggest a different genesis. Pervasive jointing in the Kneeling Nun Tuff, and the proximity of breccia deposits
to steep outcrops of Kneeling Nun Tuff in the extreme southeast corner of the map, suggest that the breccia deposits may have been
deposited as rock avalanches or debris flows shed from intact cliffs of welded tuff.
Quaternary Surficial Deposits
l
73
Mo
Kvx
Kvx
79
DESCRIPTION OF MAP UNITS
Tc
3
Tba
3
5
Tba
Tc
Tc
Tc
Tc
Tba
73
78
&s
Qy
Tt2
Tc
Kvx
70
7
3639
77
86
Tt2
Tt1
108° 07' 30" W
32° 52' 30" N
768
767
Tva
Tva
Tc
Qy
TKm
Kb
87
766
o
83
10
13
Mo
Tba
87
80
TKgc
37
Tt2 Tba
Tc
Tba
Tt1
68
Th
765 10'
764
QUATERNARY
58
TKm
Ks
9
l
12
&s
6
762
83
5
3640000 m N
12' 30"
761
Tt1
TERTIARY
760
759
60
CRETACEOUS
758000 m E
MISSISSPENNIPPIAN SYLVANIAN PERMIAN
108° 15' 00" W
32° 52' 30" N
NMBGMR Open-file Geologic Map 152
Last Modified 27 October 2008
A DIVISION OF NEW MEXICO INSTITUTE OF MINING AND TECHNOLOGY
Volcanic breccia (Cretaceous?) — Composed of poorly sorted angular to subrounded clasts of pyroxene-bearing volcanic and
hypabyssal rocks. The most extensive unit in the map area is a mixed andesitic lava, heterolithic andesitic breccia, and volcaniclastic
unit (the ‘andesite breccia’ of Jones et al., 1970). The unit consists of a wide variety of andesitic lavas that are invariably intruded
by intermediate to mafic dikes, the density ranging from less than 5% to greater than 80%. Three main varieties of lavas, based on
phenocryst assemblages, abundances, sizes, and size ranges can be easily recognized from place to place, but due to poor exposure
and the prevalence of cross-cutting dikes with similar phenocryst assemblages it would be impossible to regionally differentiate these.
Most of the andesite is magnetite bearing. Three principle varieties are, in decreasing order of abundance: 1) fine-grained (<2 mm)
plagioclase phenocryst-rich with subordinate mafics of hornblende and/or pyroxene, ± olivine, 2) medium- to coarse-grained (2-6 mm,
but up to 10 mm) hornblende-porphyritic and/or pyroxene-porphyritic, and 3) elongate, medium-grained (2-4 mm), sparsely plagioclaseporphyrititc lava with very sparse mafics. Individual lava flows, consisting of massive coherent facies lava mixed to varying degrees
with autobreccia, are complexly interbedded with a wide variety of heterolithic volcanic and epiclastic breccias, and lesser amounts of
mafic volcaniclastic rocks. Clasts in the breccias and volcaniclastic rocks are dominated by andesite, but locally include sparse material
Abo Formation (Early Permian) — Interbedded fine-grained red sandstone, and red to purple sandstone and shale. Poorly exposed
near the east side of the map area where it only locally crops out beneath the Beartooth Quartzite (Kb). Forms red-colored clay-rich
slopes mostly mantled by clasts shed from the overlying quartzite. Small outcrops exposed by quarrying operations show thin bedding
in the fine-grained red sandstones.
Syrena Formation (Pennsylvanian) — Chert-rich limestone. This light blue-gray limestone is micritic to finely crystalline. Irregularly
shaped, light yellowish tan chert stringers locally occupy 50% or more of the rock and weather out to cover the surface in a tan lag
that superficially resembles float of fine-grained sandstone. The chert has incompletely replaced the host carbonate such that even
the chert fizzes lightly with acid. Chert stringers are commonly roughly parallel to bedding, but also cut across bedding in irregular
patterns. Bedding in this unit is thin to thick, and planar. Although mostly micritic, the unit is locally coarsely crystalline, and contains
abundant crinoid stems, brachipods, and bryozoa, in some places silicified but in most areas are limestone. Fossils are most visible on
weathered surfaces. Sequences of limestone are interbedded with purple shales that form poorly exposed slopes. The best place to see
this relationship is in the road cut along the east side of Highway 152.
Strike and dip of bedding, horizontal bedding.
26
GEOLOGIC CROSS SECTIONS
Strike and dip of joints, vertical joints.
A
Mineralized joints.
75
27
Flow foliation of volcanic rocks.
Eutaxitic foliation of volcanic rocks.
Eolian cross-bed flow direction.
A΄
Southwest
6,000’
Fort Bayard
State Medical Center
Rio de
Arenas
7,000
feet ASL
Qy
Qy
Qy
Qy
Northeast
Cameron
Creek
7,000
feet ASL
TKg
Qy
Qy
&s
Mo
6,000’
Outcrop of Pennsylvanian limestone on east side of quad.
Kvx
EXPLANATION OF DIKE SYMBOLS
5,000’
Ks
Pyroxene porphyritic rocks.
4,000’
Rhyolite porphyritic rocks.
Ks
TKm
5,000’
TKg
Trp
Ks
? ? Pre-Mississippian Rocks ? ?
&s
Mo
TKg
3,000’
3,000’
Plagioclase porphyritic rocks.
REFERENCES
Hernon, R. M., Jones, W. R., and Moore, S. L., 1964, Geology of the Santa Rita quadrangle, New Mexico: U.S. Geological Survey
Geologic Quadrangle Map GQ-306, scale 1:24,000.
Feldspar megacrystic rocks.
INDEX TO GEOLOGIC MAPPING
B
B΄
West
Rio de
Arenas
7,000
feet ASL
Both
6,000’
Fort Bayard
5,000’
Qo
Qy
Qo
4,000’
Steve Skotnicki
3,000’
Twin Sisters
Creek
Qo
Qy
Tco
Tcy
Qo
Tco
Qy
Ks
TKm
Ttnx
Qo
Tcy
Cameron
Creek
Tcy
Qy
Tb
Kvx
Ks
Charles Ferguson
Oswaldo Formation (Mississippian) — Medium to dark gray fossiliferous limestone. This unit contains almost no chert and possesses
thick beds that are typically massive. Bedding is best visible from a distance. The best exposures are just outside the map area to the east
in a large open-pit quarry operation. In these pits the apparent upper surface of the unit is deep red and appears karsted.
4,000’
Ks
TKg
Granodiorite porphyritic rocks.
Kb
Mo
Tco
Bend in
Section
Ttn
Ttnx
Ts
Tts
Ks
Tco
Jones, W. R., Moore, S. L., and Pratt, W. P., 1970, Geologic map of the Fort Bayard quadrangle, Grant County, New Mexico: U.S.
Geolgical Survey Geologic Quadrangle Map GQ-865, scale 1:24,000.
6,000’
Seager, William, 2004, Laramide (late Crtetaceous-Eocene) tectonics of southwestern New Mexico: in: Mack, G. H., and Giles, K. A.,
eds., The Geology of New Mexico, a geologic history: New Mexico Geological Society Special publication 11, p. 183-202.
5,000’
Spencer, A. C., and Paige, S., 1935, Geology of the Santa Rita mining area, New Mexico: U.S. Geological Survey Bulletin 859, 78 p.
4,000’
Thoman, M. W., North, R. M., and Worthington, W. T., 2006, Santa Rita porphyry copper deposit, Grant County, New Mexico; a
summary: Arizona Geological Society Guidebook, Fall Field Trip, November 4-5, 2006, 55 p.
Tts
Tco
TKg
7,000
feet ASL
Ttn
Qy
Tco
Ks
Ks
East
Whitewater
Creek
Bayard
Jones, W. R., Hernon, R. M., and Moore, S. L., 1967, General geology of the Santa Rita quadrangle, Grant County, New Mexico: U.S.
Geological Survey Professional Paper 555, 144 p.
TKg
3,000’