?

?

?

?

?

?

?

?

?

?

? ?

?

?

10

3

4

7

6

4

10

Integrated

Age (Ma)±error (2σ) MSWD Age (Ma)±error (2σ) MSWD n/(total) Age (Ma)±error (2σ) Age (Ma)±error (2σ) MSWD

RGR–421† 415631 4069545 Tbx Xenocrystic basalt of Hill 8489 2.12±0.02 2.12±0.02 1.32 2.12±0.02 3.0 19/(19) 303±3 2.13±0.03 whole rock

RGR–009 411835 4084940 Tlx Xenocrystic trachyandesite 2.78±0.05 2.78±0.05 1.65 2.87±0.09 4.1 12/(13) 291±10 2.82±0.06 whole rock

RGR–017† 412362 4066591 Trx Basaltic trachyandesite of Red Hill 2.81±0.04 2.81±0.04 0.77 2.84±0.05 1.7 15/(15) 294±15 2.83±0.04 whole rock

RGR–012 406854 4084244 Tsr Rio San Antonio member of Servilleta Basalt 2.94±0.12 2.94±0.12 1.49 2.9±0.5 2.0 12/(13) 300±30 2.95±0.15 whole rock

RGR–010 (no. 1) 408888 4083000 Tau Upper dacite of San Antonio Mountain 2.9±0.3 3.0±0.2 1.56 2.9±0.3 1.9 11/(13) 299.6±1.7 3.0±0.2 whole rock

RGR–010 (no. 2) 408888 4083000 Tau Upper dacite of San Antonio Mountain 2.9±0.2 6.9 14/(16) 298±3 2.84±0.16 whole rock

RGR–285 410033 4078787 Tau Upper dacite of San Antonio Mountain 3.14±0.08 94.0 13/(18) 286±8 3.06±0.11 whole rock

RGR–035 407415 4077090 Tau Upper dacite of San Antonio Mountain 3.13±0.09 26.0 14/(15) 280±30 3.08±0.07 whole rock

RGR–231 410159 4082952 Tan Lower trachyandesite of north San Antonio Mountain 2.91±0.07 3.45±0.15 7.3 13/(18) 272±7 2.91±0.07 whole rock

RGR–133 410590 4073355 Tas Lower trachyandesite of south San Antonio Mountain 3.0±0.04 3.02±0.06 34.0 15/(15) 290±20 3.0±0.04 whole rock

RGR–235 412885 4073069 Tsw Wissmath Craters member of Servilleta Basalt 3.20±0.05 3.20±0.05 1.34 3.37±0.12 9.7 18/(18) 295±4 3.29±0.11 whole rock

RGR–013 404240 4079557 Tsm Pinabetal Mesa member of Servilleta Basalt 3.33±0.14 3.33±0.14 1.19 3.5±0.3 1.3 10/(13) 288±18 3.33±0.14 whole rock

RGR–374 399646 4090465 Trl Basaltic andesite of Rio de los Piños 3.39±0.04 3.39±0.04 2.0 3.49±0.10 7.9 18/(18) 293±3 3.32±0.09 whole rock

RGR–149 (no. 1) 399235 4091508 Trlc Basaltic andesite of Rio de los Piños 3.47±0.09 2.91 3.3±0.3 5.7 9/(16) 307±19 3.39±0.09 whole rock

RGR–149 (no. 2) 399235 4091508 Trlc Basaltic andesite of Rio de los Piños 3.4±0.2 11.0 9/(16) 303±9 3.52±0.10 whole rock

RGR–131† 399410 4080612 Tca Basaltic andesite of Cañada los Ranchos 3.29±0.05 1.02 3.16±0.12 3.5 6/(15) 304±4 3.28±0.09 whole rock

RGR–221 405826 4086085 Tca Basaltic andesite of Cañada los Ranchos 3.32±0.06 3.32±0.06 1.67 3.48±0.11 4.4 15/(17) 296±3 3.39±0.10 whole rock

RGR–015 404862 4075955 Tcx Basaltic trachyandesite of Los Cerritos de la Cruz 3.36±0.05 3.36±0.05 2.95 3.41±0.06 6.5 13/(15) 280±30 3.38±0.05 whole rock

Tna Rhyolite of No Agua Peaks* 3.88±0.06 3.88±0.06 glass

Tna Rhyolite of No Agua Peaks* 4.1±0.03 4.1±0.03 glass/feldspar

RGR–226 400749 4089240 Tcb Basalt of Chino Peak 4.24±0.10 2.7 4.3±0.7 41.0 18/(18) 295±8 4.0±0.3 whole rock

RGR–014 400691 4087852 Tcbc Basalt of Chino Peak 4.31±0.07 4.31±0.07 1.28 4.1±0.4 2.9 14/(15) 304±18 4.27±0.09 whole rock

RGR–146 403003 4093022 Tb Basalt and basaltic andesite of Valdez Tank (basaltic andesite) 4.37±0.05 1.48 4.33±0.11 2.8 15/(15) 301±7 4.36±0.06 whole rock

RGR–371 398841 4091038 Tbc Basalt and basaltic andesite of Valdez Tank (basaltic andesite) 4.37±0.03 4.32±0.03 1.01 4.37±0.03 1.3 15/(17) 278±3 4.23±0.07 whole rock

RGR–357† 395381 4095867 Tb Basalt and basaltic andesite of Valdez Tank (silicic basalt) 4.25±0.02 1.02 4.30±0.03 0.9 16/(17) 274±6 4.24±0.05 whole rock

RGR–362 397801 4093198 Tb Basalt and basaltic andesite of Valdez Tank (silicic basalt) 4.36±0.04 4.24±0.03 1.45 4.36±0.04 2.3 15/(17) 200±20 4.22±0.07 whole rock

RGR–364 398025 4091882 Tb Basalt and basaltic andesite of Valdez Tank (basalt) 4.36±0.05 4.36±0.07 4.3 17/(18) 298±5 4.36±0.05 whole rock

RGR–036 403603 4072691 Tlb Basalt of Lucero Lakes 4.55±0.07 4.55±0.07 1.21 4.60±0.11 2.9 15/(15) 295±4 4.54±0.08 whole rock

RGR–378† 392235 4108018 Thl Hinsdale Formation, alkaline basalt to basaltic trachyandesite 20.61±0.07 20.61±0.07 1.58 20.94±0.24 whole rock

RGR–148 402621 4092353 Tht Hinsdale Formation, tholeiitic basalt 25.54±0.24 2.09 25.11±0.35 whole rock

RGR–456† 388663 4076306 Tht Hinsdale Formation, tholeiitic basalt 25.67±0.20 25.67±0.20 0.7 25.51±0.49 whole rock

RGR–033† 395099 4073140 Thb Hinsdale Formation, basaltic andesite 26.05±0.21 26.05±0.21 1.61 25.47±0.35 whole rock

RGR–494 399855 4077961 Thp Hinsdale Formation, porphyritic basalt 26.17±0.07 26.17±0.07 1.34 26.17±0.07 whole rock

RGR–466 396803 4091475 Ttc Chiquito Peak Tuff 28.94±0.31 28.94±0.31 0.66 sanidine

* ages reported by Dickens (2007) and represent weighted mean of multiple ages determined on grains of glass and (or) feldspar. Reported ages calculated relative to Fish Canyon Tuff neutron flux monitor age of 28.02 Ma. Ages shown here are adjusted by a factor of 1.006 (determined by 28.201/28.02 = 1.006) to account for different flux monitor age.

† indicates sample location outside map area.

Material 

analyzed

Plateau Isochron Single grain total fusion Sample ID Easting Northing Map Unit

Symbol Map Unit Name Preferred unit age 

(Ma)±error (2σ)

Table 2. Summary of 40Ar/39Ar geochronologic ages. 

[Table shows ages determined using plateau, inverse isochron, and integrated age calculation methods for samples analyzed by step-heating methods, and total fusion ages for samples measured by single crystal analysis. Ages interpreted and preferred by the authors are shown in bold for each sample. Preferred unit age indicates preferred age for unit when multiple samples were analyzed.

Northing and Easting values in meters using North American Datum 1983 (NAD83), Universal Transverse Mercator (UTM) zone 13. MSWD, mean square of weight deviates; n, number of heating steps used in calculation of isochron age; Ma, million years; σ, sigma; λ, lamda; Ar, argon; K, potassium. Ages determined using Fish Canyon Tuff as a neutron flux monitor with

an age of 28.201±0.023 Ma (1σ) (Kuiper and others, 2008), 40K decay constants of λ = (5.463±0.214) × 10-10yr-1 and λ(e-) = (0.580±0.007) ×10-10yr-1 (Min and others, 2000), and a 40Ar/36Ar ratio of trapped argon equal to atmospheric value of 298.56 (Lee and others, 2006). Analytical data for all samples can be accessed at https://doi.org/10.5066/F72N51M5 (Turner and others, 2018)]

40Ar/36Ar

no plateau

no plateau

no plateau

no plateau

no plateau

no plateau

Insufficient dispersion of step-heating analyses to calculate

Insufficient dispersion of step-heating analyses to calculate

Insufficient dispersion of step-heating analyses to calculate

Insufficient dispersion of step-heating analyses to calculate

Insufficient dispersion of step-heating analyses to calculate

Map Unit Tbx Tlx Trx Tsr Tau Tan Tas Tse Tsw Tsw Tls Tsm Trlc Tcx Tcx Tca Tca Tna Tcb Tcb Tbc Tb Tb Tlb Thl Tht Thb Tha Thp

Sample ID RGR–421† RGR–009 RGR–017† RGR–012 RGR–285 RGR–231 RGR–133 RGR–134 RGR–235 RGR–438 RGR–135 RGR–492 RGR–149 RGR–229 89T019 RGR–496 RGR–495 RGR–130 RGR–226 RGR–534 RGR–371 RGR–254 RGR–362 RGR–036 RGR–248 RGR–250 RGR–230 RGR–228 RGR–494

Easting 415631 411835 412362 406854 410033 410159 410590 416671 412885 410781 414439 403211 399235 402389 404520 405697 405760 414889 400749 399706 398841 396445 397801 403603 395064 398648 402143 402320 399855

Northing 4069545 4084940 4066591 4084244 4078787 4082952 4073355 4078917 4073069 4073371 4075125 4083198 4091508 4077055 4074982 4087623 4084426 4066595 4089240 4089481 4091038 4093574 4093198 4072691 4094003 4094143 4076634 4077317 4077961

XRF Lab 1 1 1 1 1 2 1 1 2 1 1 1 1 2 1 1 1 1 2 2 2 1 2 3 1 1 2 2 1

SiO2 50.83 54.62 51.41 51.88 63.46 61.20 56.57 50.54 48.40 51.38 53.16 50.11 51.50 51.40 57.07 49.27 53.77 73.87 48.30 49.60 52.40 48.52 50.50 50.17 50.26 49.55 52.60 56.00 48.34

TiO2 1.35 1.64 1.79 1.19 0.72 0.78 1.26 1.22 1.30 1.20 1.06 1.23 1.27 1.42 1.13 1.29 1.14 0.07 1.45 1.71 1.29 1.40 1.41 1.38 1.65 1.47 1.69 1.84 1.67

Al2O3 15.91 15.91 16.44 16.45 15.86 15.80 15.89 15.80 16.20 15.95 15.53 16.29 15.83 16.20 15.80 15.97 15.57 13.40 15.60 16.50 16.80 16.72 16.70 16.10 15.14 14.73 14.20 15.40 13.92

Fe2O3 11.38 9.16 10.53 9.95 5.38 6.03 8.06 11.41 12.80 11.53 10.42 11.34 11.43 11.10 8.56 12.47 9.98 0.70 12.10 12.00 10.30 11.61 11.30 11.61 12.41 13.35 11.60 10.60 10.67

MgO 6.05 3.17 4.36 5.75 1.95 2.82 3.36 6.81 8.03 7.30 6.73 7.12 6.58 6.84 4.35 7.09 5.76 0.06 9.54 6.66 6.24 7.32 7.59 8.03 6.55 7.34 6.32 2.67 8.06

CaO 8.29 5.93 7.17 8.26 4.41 4.98 5.92 8.83 9.18 8.99 7.12 9.19 8.52 8.07 5.90 8.81 7.45 0.77 9.01 8.76 8.37 9.80 9.04 8.91 8.15 9.25 8.17 5.85 8.58

Na2O 3.39 4.03 4.01 3.48 3.97 3.85 4.03 3.09 3.07 3.07 3.12 3.18 3.28 3.47 3.66 3.27 3.09 4.01 3.09 3.58 3.52 3.27 3.41 3.28 3.53 2.94 2.91 3.58 3.06

K2O 1.26 2.65 1.91 1.27 3.14 2.94 2.60 0.87 0.49 0.58 1.42 0.78 0.77 1.12 2.59 0.82 1.44 4.71 0.44 1.01 1.39 0.55 0.91 0.56 1.18 0.56 1.23 2.44 1.80

P2O5 0.28 0.97 0.83 0.46 0.29 0.40 0.68 0.34 0.20 0.19 0.19 0.26 0.19 0.29 0.38 0.26 0.20 0.01 0.21 0.34 0.24 0.20 0.23 0.20 0.33 0.20 0.31 0.53 0.59

MnO 0.17 0.13 0.14 0.14 0.09 0.09 0.12 0.16 0.18 0.15 0.15 0.16 0.15 0.15 0.13 0.17 0.14 0.08 0.17 0.17 0.15 0.16 0.15 0.17 0.16 0.18 0.16 0.14 0.15

Cr2O3 n.m. n.m. n.m. n.m. n.m. n.d. n.m. n.m. 0.02 n.m. n.m. n.m. n.m. 0.02 n.m. n.m. n.m. n.m. 0.04 0.02 0.02 n.m. 0.03 n.m. n.m. n.m. 0.02 n.d. n.m.

Total 98.90 98.19 98.58 98.83 99.25 98.89 98.48 99.068 99.87 100.36 98.88 99.65 99.51 100.08 99.57 99.43 98.53 97.68 99.95 100.35 100.72 99.55 101.27 100.41 99.34 99.55 99.21 99.05 96.83

LOI 0.60 0.99 0.81 0.77 0.47 1.65 1.11 0.76 0.24 n.d. 1.27 0.00 0.51 0.22 0.96 0.06 0.38 2.74 n.d. 0.13 0.29 0.32 n.d. n.d. 0.35 0.40 0.81 0.39 2.20

Miocene and Oligocene

Table 1. Representative major element geochemical analyses for select samples from the map area

[These are representative samples only and are not placed on the map. Northing and Easting values in UTM meters using North American Datum 1983 (NAD 83), Universal Transverse Mercator (UTM) zone 13. X–ray fluorescence (XRF) Laboratories: (1) U.S. Geological Survey, Denver, Colo.; (2) SGS, Ontario, Canada; (3) Washington State University, Pullman, Wash. Oxide abundance reported as weight percent oxide; abbreviations include: n.d., not detected; n.m., not measured; LOI, loss on ignition.

All major element and trace element compositions for the complete set of samples can be found at https://doi.org/10.5066/F72N51M5 (Turner and others, 2018)] 

Pliocene

† Indicates sample location outside map area.

Figure 2. Landsat 7 satellite image (30-m meter [m] band 7–4–2 merged with 15-m band 8) acquired on

Oct. 14, 1999, of the area surrounding San Antonio Mountain (image clip from Sawyer and others, 2004). Image 

shows selected geographic names referred to in unit descriptions. Yellow line indicates the map boundary. Red 

asterisks indicate known or inferred (with query) vent locations associated with the Taos Plateau volcanic field. 

At least two additional vent locations are probably concealed beneath dacite lavas at San Antonio Mountain.

Figure 1. Shaded-relief index map of the southern and central parts of the San Luis Basin and adjacent areas 

showing the outline of the San Antonio Mountain area map boundary and the outline of Rio Grande del Norte 

National Monument. Abbreviations for labeled physiographic and volcanic features include: CA, Cerro del Aire; 

CO, Cerro de la Olla; CC, Cerro Chiflo; CM, Cerro Montoso; CT, Cerro de los Taoses, GM, Guadalupe Mountain; 

LM, Los Mogotes; NA, No Agua Peaks; SM, San Antonio Mountain; TO, Tres Orejas; and UM, Ute Mountain.

Figure 3. Total alkali-silica classification diagram of Le Bas and others (1986). Major element compositions 

are recalculated to volatile-free with ferric/ferrous iron ratios as proposed by Middlemost (1989). Grayed out 

field names did not have any rocks that fell within that respective category.

Shaded-relief base generated from U.S. Geological Survey 10-m Digital Elevation Model (DEM) data from the National Elevation Dataset, accessed June 2016

at http://ned.usgs.gov/.

Rio Grande del Norte National Monument boundary simplified from U.S. Bureau of Land Management at

http://www.blm.gov/nm/st/en/prog/blm_special_areas/national_monuments/rio_grande_del_norte.html on Aug. 12, 2013.

EXPLANATION

Xenocrystic basalt of Hill 8489 (Tbx)

Xenocrystic trachyandesite (Tlxc/Tlx)

Basaltic trachyandesite of Red Hill (Trxc/Trx)

Rio San Antonio member of Servilleta Basalt (Tsr)

Upper dacite of San Antonio Mountain (Tauc/Tau)

Lower trachycandesite of north San Antonio Mountain (Tan)

Lower trachyandesite of south San Antonio Mountain (Tas)

Pinabetal Mesa member of Servilleta Basalt (Tsm)

Basaltic andesite of Rio de los Piños (Trlc/Trl)

Basaltic andesite of Cañada los Ranchos (Tca)

Section 35 member of Servilleta Basalt (Tse)

Wissmath Craters member of Servilleta Basalt (Tsw)

Xenocrystic basaltic andesite of La Segita Peaks (Tls)

Basaltic trachyandesite of Los Cerritos de la Cruz (Tcxc/Tcx)

Rhyolite of No Agua Peaks (Tna)

Basalt of Chino Peak (Tcbc/Tcb)

Basalt and basaltic andesite of Valdez Tank (Tbc/Tb)

Basalt of Lucero Lakes (Tlb)

Alkaline basalt to trachybasalt, Hinsdale Formation (Thl)

Tholeiitic basalt, Hinsdale Formation (Tht)

Trachyandesite, Hinsdale Formation (Tha)

Basaltic andesite, Hinsdale Formation (Thb)

Porphyritic basalt, Hinsdale Formation (Thp)

14

12

10

8

6

4

2

0

35 40 45 50 55 60 65 70 75 80

Na O + K 2 O (weight percent)

2

SiO2 (weight percent) 

Basalt

Picrobasalt

Basaltic

andesite

Andesite

Dacite

Rhyolite

Trachydacite

Trachyte

Trachyandesite

Tephrite/Basanite

Phonotephrite

Tephriphonolite

Phonolite

Trachybasalt

Basaltic

trachyandesite

0 3 6 MILES

0 3 6 KILOMETERS

Rio San

Antonio

Rio

Piños

San Antonio

Mountain

Wissmath

Craters

La Segita

Peaks

No Agua 

Peaks

Broke Off

Mountain

Laguna

Larga Pinabetal Mesa

Lucero

LakesLos Cerritos de la Cruz

Chino

Peak

Dike with

cinders

Red Hill Red HillRed Hill

Bighorn

Peak

Valdez

Tank

COLORADO

NEW MEXICO

106°07'30" 106°

37°

36°52'30"

36°45'

285

285

Ortiz

de los

EXPLANATION

Vent location—Identity or existance

inferred where queried

Base from Landsat 7 satellite image, October 14, 1999

30 meter band 7–4–2 merged with 15 meter band 8

37°00'

36°30'

106°00' 105°30'

RIO GRANDE

DEL NORTE

NATIONAL MONUMENT

River

Grande

Red

Rio

San

Antonio

Rio de los Pinos

Rio San Antonio

Rio

Rio

Grande

64

64

64

285

285

522

522

159

142

518

17

Taos

Tres Piedras

Questa

San Luis

Antonito

UM

CO CC

CM

CT

TO

GM

SAN

LUIS

HILLS

TUSAS MOUNTAINS

SOUTHEAST 

SAN JUAN MOUNTAINS

SANGRE

SAN LUIS

BASIN

Costilla

Plain COLORADO

NEW MEXICO

San

Pedro

Mesa

SM

LM

CA

NA

TAOS

PLATEAU

DE CRISTO

MOUNTAINS

15 KILOMETERS

0 5 10 15 MILES

0 5 10

Map

area

Red Hill. View to west of the quarry at Red Hill where cinder deposits of basaltic trachyandesite of Red Hill (unit Trxc) are being quarried. Photograph by K. 

Turner, May 3, 2016.

Dike. Looking west across the 

canyon of the Rio San Antonio 

where a dike cross-cuts flows of 

basaltic andesite of Cañada los 

Ranchos (unit Tca). Above the dike 

are flows of the Pinabetal Mesa 

member of the Servilleta Basalt 

(unit Tsm) that are not cut by the 

dike. The white line indicates 

approximate contact between the 

dike and surrounding flows. The 

dike is petrographically and 

compositionally similar to the 

basaltic andesite of Cañada los 

Ranchos and is surrounded by 

loose cinder deposits near the rim 

of the canyon. Photograph by

K. Turner, September 19, 2012.

Esquibel member. Cobble-rich facies of the Esquibel 

Member of the Los Piños Formation (unit Tle) taken north 

of the Rio de los Piños. These beds are locally dipping 

eastward up to 10 degrees. In upper right is the valley 

incised by the Rio de los Piños with the northern Tusas 

Mountains in the background. The back- pack in the 

center of the photograph is about 0.75 meter in length. 

Photograph by K. Turner, June 12, 2013.

Rio de los Piños. View toward the east looking down the Rio de los Piños. The 

lowest distinctive ledge above the trees is held up by tholeiitic basalt flows of the 

Hinsdale Formation (unit Tht). The flat, upper surface north of the river (right side of 

the photograph) is held up by basaltic andesite flows of Rio de los Piños (unit Trl) and 

Cañada los Ranchos (unit Tca). The slope forming unit intercalated with the lava 

flows is the Esquibel Member of the Los Piños Formation (unit Tle). Photograph by K. 

Turner, June 11, 2011.

Lava tube. View into the 

collapsed ceiling of a lava tube 

in flows of the Pinebetal Mesa 

member of the Servilleta Basalt 

(Unit Tsm). Sediments and 

fossils are preserved in the lava 

tube that are older than 0.85 

Million year (Rogers and others, 

2000). Fossils were excavated 

from sediments preserved in the 

lava as early as the 1950s 

(Rogers and others, 2000). 

Photograph by K. Turner, July 21, 

2014. 

GQ–1750

(Thompson and 

Lipman, 1994b)

GQ–1749

(Thompson and 

Lipman, 1994a)

MF–1451

(Manley, 1982a)

SIM 3417

(Turner and

others, 2018)

106°15' 106°7'30" 106° 105°52'30" 105°45'

37°

37°07’30"

36°45'

36°37'30"

36°52'30"

COLORADO

NEW MEXICO PINABETOSO

PEAKS LA SEGITA

PEAKS NE

CERRO

DE LA OLLA

TRES

PIEDRAS NE

BURNED MULE CANYON TRES PIEDRAS

MOUNTAIN

FOX CREEK ANTONITO LOBATOS KIOWA HILL

LA SEGITA

PEAKS

BROKE OFF

MOUNTAIN

LOS PINOS

SAN ANTONIO

MOUNTAIN

BIGHORN

PEAK

Index map showing the map area in yellow with 

dashed red outline and adjacent 7.5' U.S. Geological 

Survey (USGS) quadrangles. For USGS publications, 

the publication number, author, and year published 

are shown. Gray area indicates geologic mapping by 

Eppler (1976). 

REFERENCES

American Geological Institute, 1982, Grain-size scales used by American geologists, 

modified Wentworth scale, data sheets (2d ed.): Falls Church, Va., American 

Geological Institute, sheet 17.1.

Appelt, R.M., 1998, 40Ar/39Ar geochronology and volcanic evolution of the Taos Plateau 

volcanic field, northern New Mexico and southern Colorado: Socorro, New Mexico 

Tech, M.S. thesis, 58 p.

Benson, L.V., Madole, R.F., Landis, G.P., and Gosse, J.C., 2005, New data for late 

Pleistocene Pinedale alpine glaciation from southwestern Colorado: Quaternary 

Science Reviews, v. 24, p. 49–65.

Benson, L.V., Madole, R.F., Phillips, W.M., Landis, G.P., Thomas, T., and Kubic, P., 

2004, The probable importance of snow and sediment shielding on cosmogenic ages 

of north-central Colorado Pinedale and pre-Pinedale moraines: Quaternary Science 

Reviews, v. 23, p. 193−206.

Bingler, E.C., 1968, Geology and mineral resources of Rio Arriba County, New Mexico: 

New Mexico Bureau of Mines and Mineral Resources, Bulletin 91, 158 p., 7 pl.

Breese, R., 1984, Rhyolite domes and flows at No Agua Peaks, New Mexico, in

Baldridge, W.S., Dickerson, P.W., Riecker, R.E., and Zidek, J., eds., Rio Grande Rift—

Northern New Mexico: New Mexico Geological Society, Guidebook 35, p. 373–374. 

Butler, A.P., Jr., 1971, Tertiary volcanic stratigraphy of the eastern Tusas Mountains, 

southwest of the San Luis Valley, Colorado-New Mexico, New Mexico Geological 

Society Fall Field Conference Guidebook: New Mexico Geological Society, v. 22,

p. 289–300.

Chamberlin, R.M., and Barker, J.M., 1996, Genetic aspects of commercial perlite 

deposits in New Mexico, in Austin, G.S., Hoffman, G.K., Barker, J.M., Zidek, J., 

and Gilson, N., eds., Proceedings of the 31st forum on the geology of industrial 

minerals—The Borderland Forum: New Mexico Bureau of Mines and Mineral 

Resources Bulletin 154, p. 171–186.

Cole, J.C., Mahan, S.A., Stone, B.D., and Shroba, R.R., 2007, Ages of Quaternary Rio 

Grande terrace-fill deposits, Albuquerque area, New Mexico: New Mexico Bureau 

of Geology and Mineral Resources, New Mexico Geology, v. 29, no. 4, p 122−132.

Colucci, M.T., Dungan, M.A., Ferguson, K.M., Lipman, P.W., and Moorbath, S., 1991, 

Precaldera lavas of the southeast San Juan volcanic field—Parent magmas and 

crustal interactions: Journal of Geophysical Research, v. 96, p. 13413–13434.

Cruden, D.M., and Varnes, D.J., 1996, Slope movement types and process, in Schuster, 

R.L., and Krizek, R.J., eds., Landslide investigation and mitigation: Washington,

D.C., National Academy Press, Transportation Research Board Special Report 247,

p. 36–75.

Dickens, A.K., 2007, Obsidian hydration and its consequences for the Ar-Ar dating 

method: Socorro, New Mexico Institute of Mining and Technology, M.S. thesis, 182 p.

Drenth, B.J., Turner, K.J., Thompson, R.A., Grauch, V.J.S., Cosca, M.A., and Lee, J.P., 

2011, Geophysical expression of elements of the Rio Grande rift in the northeast 

Tusas Mountains—Preliminary interpretations, in Koning, D.J., Karlstrom, K.E., 

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INTRODUCTION

This map summarizes the geology for the San Antonio Mountain area of northern 

New Mexico and approximately 10 square kilometers (km2) of southern Colorado. This 

geologic investigation was carried out with support from the U.S. Geological Survey 

(USGS) National Cooperative Geologic Mapping Program between 2012 and 2016. The 

map area is physiographically on the western edge of the Taos Plateau within the 

southern San Luis Valley. The San Luis Valley is the geomorphic expression of the San 

Luis Basin, an extensional basin of the northern Rio Grande rift. Approximately half of 

the map area is within the Rio Grande del Norte National Monument (fig. 1). The map 

area encompasses two full 7.5' quadrangles (see quad index on sheet); the San Antonio 

Mountain and Los Pinos, and three partial 7.5' quadrangles: Bighorn Peak, Pinabetoso 

Peaks, and La Segita Peaks. The partial quadrangles (Pinabetoso Peaks and La Segita) are 

included to the east to fully encompass the San Antonio Mountain volcano, and in the 

northwest area (Bighorn Peak) to include Pliocene volcanic rocks related to the Taos 

Plateau volcanic field. Previous work in the area included small-scale maps by Bingler 

(1968), Butler (1971), and Manley and others (1987). More detailed mapping included 

the Bighorn Peak 7.5' quadrangle (Manley, 1982a), San Antonio Mountain area (Eppler, 

1976); San Antonio Mountain 7.5' quadrangle (Thompson and Lipman, 1994b), and Los 

Pinos 7.5' quadrangle (Thompson and Lipman, 1994a).

Deposits within the map area record volcanic, sedimentary, and tectonic processes 

over the last ~33 million years (m.y.). Oldest exposed deposits include Oligocene 

volcanic rocks associated with the southeast San Juan Mountains locus of volcanism 

within the Southern Rocky Mountains volcanic field. The Southern Rocky Mountains 

volcanic field is a composite mid-Tertiary volcanic field that covered parts of southern 

Colorado and northern New Mexico (Lipman, 2007), of which the San Juan Mountains 

are one locus of volcanism. Overlying deposits of the Southern Rocky Mountains 

volcanic field are volcaniclastic sedimentary rocks interbedded with predominantly 

basaltic lava flows of Oligocene to Miocene age. Basalt to rhyolite volcanic rocks of the 

Pliocene to Pleistocene Taos Plateau volcanic field unconformably overlie Oligocene to 

Miocene volcanic and sedimentary deposits. Superposed on the Tertiary deposits are 

Pleistocene to Holocene alluvial and colluvial deposits.

North- to northwest-trending faults displace rocks within the map area. Faults are 

assumed to have normal, chiefly dip-slip, displacement but are identified with varying 

degrees of certainty as their recognition within the map area is complicated by generally 

small displacements in young, brittle volcanic deposits that often lack an obvious fault 

scarp or in the poorly indurated Los Pinos Formation deposits that rarely preserve a 

prominent fault scarp and may not be exposed sufficiently to document offset. Normal 

faults attributed as ‘accurate’ or ‘approximately located’ were observed displacing map 

units where ‘approximately located’ indicates a lower degree of spatial certainty. Normal 

faults attributed as ‘inferred’ are associated with distinct topographic lineaments and 

cutoff ledges but lack direct observation of displaced lava flows.

Magnitude of deformation is broadly correlative with age of the deposits inasmuch 

as Oligocene to Miocene rocks display a greater degree of fault displacement and east 

tilting than Pliocene volcanic rocks. Bedding attitudes recorded during field investigation 

are sparse as a result of (1) poorly indurated and poorly exposed Los Pinos Formation; 

(2) lava flows that are sufficiently magnetic to deflect a compass arrow; and (3) lava

flows that rarely preserve a measurable surface representative of the attitude of the flow.

Within the map area, faults displace Oligocene to Miocene deposits 10–30 meters (m)

with generally down-to-west offset, and the units dip eastward 3–7 degrees. However,

west of the map area fault displacement is as much as 200 m down-to-west along the west

side of Broke Off Mountain (fig. 2) (Manley, 1982b). Pliocene volcanic rocks exhibit

shallower eastward dips inferred primarily from the slope of upper lava flow surfaces that

dip eastward 1–3 degrees. However, proximal to vent areas, dips increase and dip in all

directions away from the vent. Fault offset of Pliocene volcanic rocks is likely

underrepresented on the map due to cover by unconsolidated surficial deposits, but

exposed faults displace Pliocene lavas less than 5 m. The magnitude of post-emplacement

tilting of Pliocene lava flows is difficult to constrain because the lavas are interpreted to

have flowed predominantly down an east-dipping paleoslope. Flow directions shown on

the map are inferred from vent locations and topographic expression, and display

predominantly easterly flow directions.

Geology was mapped using a combination of remotely sensed data and field 

investigation. Natural-color, 1-m resolution imagery gathered by the U.S. Department of 

Agriculture National Agricultural Imagery Program (NAIP) from 2009 and 2014 for Rio 

Arriba and Taos Counties, New Mexico, and 2009 for Conejos County, Colorado, was 

used as an interpretive layer for new mapping and to evaluate previous mapping (later 

acquisition years of NAIP available as quarter quadrangle downloads from 

https://nationalmap.gov). New mapping utilized argon-argon (40Ar/39Ar) geochronology, 

geochemical data, and thin-section petrography to identify, correlate, and constrain the 

stratigraphic position and ages of volcanic units. Volcanic rock names are based on the 

standard International Union of Geological Sciences (IUGS) classification scheme of Le 

Bas and others (1986) using major element compositions recalculated as volatile-free 

with ferric/ferrous iron ratios as proposed by Middlemost (1989); a result of this 

calculation can cause a slight shift to higher concentrations in the major element values 

plotted in figure 3 relative to the original data reported in table 1. Geochemical analyses 

of select representative samples are presented in table 1 and 40Ar/39Ar ages are reported in 

table 2. The complete datasets that support these tables are available through ScienceBase 

(https://www.sciencebase.gov) as part of the data release that accompanies this report at 

https://doi.org/10.5066/F72N51M5 (Turner and others, 2018). 

Qa

Tbx

Tlxc

Tlx

Qsw

Qsa

Qsp

Qf

Qc

Tcxc

Tcx

Trlc

Trl

Tca

Trxc

Trx

Tsr

Tauc

Tau

Tan

Tas

Tse

Tna

Tsw

Tls

Tsm

Tcbc

Tcb

Tbc

Tb

Tlb

Tlc

Tle

Thl

Tht

Thb

Thp

Tt

Ttc

Tcf

Tha

DESCRIPTION OF MAP UNITS

[Due to the addition of a shaded relief base, colors in the Correlation of Map Units and 

the Description of Map Units may not exactly match unit colors on the map]

SURFICIAL DEPOSITS

The surficial units on this map are informal allostratigraphic units of the North 

American Stratigraphic Code (North American Commission on Stratigraphic 

Nomenclature, 2005). Mapped surficial deposits are known or estimated to be at least 1 

meter (m) thick. Ages of time boundaries are those of the U.S. Geological Survey 

Geologic Names Committee (2018) except those for the middle-early Pleistocene and 

late-middle Pleistocene boundaries, which are those of Gibbard and others (2010). Age 

assignments for surficial deposits are based chiefly on (1) the relative heights above 

modern streams or channels of ephemeral streams, (2) topographic relationships with 

other surficial deposits, and, to a lesser extent, (3) relative degree of erosional 

modification of original (depositional) surface morphology. Stages of secondary calcium 

carbonate morphology (referred to as stages I and II that are formed on the bottom of 

clasts) are from Gile and others (1966) and Machette (1985).

In this report, the terms “alluvium” and “alluvial” refer to sediment transported by 

running water confined to channels (stream alluvium), whereas those deposited by 

running water not confined to channels are referred to as sheetwash. The terms 

“colluvium” and “colluvial” refer to sediment transported downslope chiefly by 

mass-movement (gravity-driven) processes—such as debris flow, rock fall, and 

near-surface creep—aided by running water not confined to channels (Hilgard, 1892; 

Merrill, 1897). Surficial map units that include debris-flow deposits probably also include 

hyperconcentrated flow deposits. These latter deposits are intermediate in character 

between stream-flow and debris-flow deposits.

Eolian deposits are present in the map area but are isolated deposits that are too small 

to show at the scale of the map. Much of the silty sheetwash alluvium (Qsw) frequently 

observed on Pliocene lava flows and much of silty fine sand in the matrix of colluvium 

(Qc) probably is derived chiefly from silty eolian sand. The latter may have been derived 

in part from silty fine sand eroded from the Los Pinos Formation and was deposited on 

sparsely vegetated, actively aggrading floodplains of the Rio de los Piños and other major 

streams within and near the map area. The sparsely vegetated, silty fine sand on these 

floodplains probably was later deflated and transported by strong winds and deposited as 

eolian sand.

Grain or particle sizes of surficial deposits are based on field estimates, using the 

modified Wentworth scale (American Geological Institute, 1982). In the descriptions of 

surficial map units, the term “clasts” refers to particles larger than 2 millimeters (mm) in 

diameter, whereas all finer material is called “matrix.” Most of the clasts in fan deposits 

(Qf) within the map area are very angular, whereas most of those in alluvial deposits are 

very angular to subrounded.

ALLUVIAL DEPOSITS

Stream alluvium (Holocene and late Pleistocene)—Mostly sand and gravel 

in stream channels as well as sand, silty sand, and gravel underlying 

adjacent floodplains and minor low stream terraces. Stream-channel 

deposits along the Rio de los Piños are composed of slightly bouldery, 

cobbly pebble gravel. Top of unit Qa is about 1–2 m above modern 

stream level. Unit Qa locally includes small fan deposits (Qf) and 

narrow aprons of sheetwash alluvium (Qsw) along valley margins. 

Low-lying deposits are prone to periodic stream flooding. Estimated 

thickness 1–5 m along the Rio de los Piños, 1–3 m along other streams

Sheetwash alluvium (Holocene and late Pleistocene)—Chiefly slightly 

pebbly to pebbly, slightly silty to silty sand in aprons that overlie gentle 

slopes on Pliocene lava flows near northeastern corner of the map area. 

Unit Qsw locally includes deposits of stream alluvium (Qa) that are too 

small to show at map scale. Much of the silt- to fine-sand-size fraction in 

these deposits is likely to be of eolian origin. Low-lying areas of unit 

Qsw are susceptible to sheet flooding due to unconfined overland flow. 

Estimated thickness is 1–5 m

Sheetwash alluvium and stream alluvium, undivided (Holocene and late 

Pleistocene)—Chiefly silty and sandy sheetwash deposits (Qsw) on 

gentle slopes and undifferentiated sandy and pebbly stream alluvium 

(Qa) along and near ephemeral streams mainly on Pliocene lava flows 

and Los Pinos Formation in the eastern part of the map area. Low-lying 

areas of unit Qsa in and adjacent to stream channels may be subject to 

periodic stream flooding; adjacent slopes may be subject to periodic 

sheet flooding. Estimated thickness is 1–5 m

ALLUVIAL AND POND DEPOSITS

Sheetwash alluvium and pond deposits, undivided (Holocene and late 

Pleistocene)—Composed chiefly of silty fine sand that accumulated in 

circular depressions commonly on Pliocene lava flows in the central part 

of the map area. Much of the unit consists of eolian sediment that was 

redeposited by unconfined overland flow as sheetwash alluvium (Qsw) 

or as lacustrine sediment in small ephemeral water bodies. Unit locally 

may contain marsh deposits. Low-lying areas of unit may be subject to 

periodic inundation by sheet flooding and ponded water. Estimated 

thickness is 1–5 m, possibly locally as much as 10 m

ALLUVIAL AND COLLUVIAL DEPOSIT

Fan deposits (Holocene to middle? Pleistocene)—Unit forms an extensive 

sedimentary apron that was deposited by debris flows and sedimentcharged, ephemeral streams along the lower flanks of San Antonio 

Mountain. Unit commonly consists of clast- and matrix-supported, 

locally bouldery, pebbly and cobbly gravel with a silty sand matrix. Near 

the lower part of the fan apron, deposits locally consist of pebbly and 

cobbly, slightly silty sand that contains gravel lenses. Rock fragments in 

deposits of unit Qf are composed of dacite (Tau) eroded from San 

Antonio Mountain. Eolian sediment may have played an important role 

in the genesis of debris-flow deposits (Shroba and others, 2007). Stage I 

and weak stage II carbonate morphology on the bottom of clasts in 

deposits of unit Qf suggests that these deposits accumulated during the 

Pinedale glaciation (see table 2 in Machette, 1985), about 12–30 ka 

(Nelson and others, 1979; Benson and others, 2004, 2005, and references 

cited therein). Some of the deposits could be as old as 40–47 ka (Cole 

and others, 2007), and may have been deposited during an early advance 

of Pinedale ice (Sturchio and others, 1994). Unit locally may include 

small fan deposits of middle Pleistocene age. Near the lower part of the 

fan apron, unmapped deposits of sheetwash alluvium (Qsw) and stream 

alluvium (Qa), too small to show at map scale, locally overlie or are 

inset into deposits of unit Qf. Low-lying areas adjacent to stream 

channels are prone to periodic stream flooding and debris-flow deposition. Estimated thickness is 1–10 m

COLLUVIAL DEPOSIT

Colluvium, undivided (Holocene to middle? Pleistocene)—Deposits of 

non-sorted and non-stratified, mostly matrix-supported, sandy sediment 

and rock debris on and near steep slopes. Deposits range in size from 

pebbly silty sand to cobbly and bouldery rubble with a sandy matrix. 

Unit Qc consists chiefly of debris-flow, rock-fall, and creep deposits, as 

defined by Cruden and Varnes (1996). Unit locally includes small talus 

deposits on the upper flanks of San Antonio Mountain, and mass-movement (periglacial?) deposits above an altitude of 9,957 feet (ft) near the 

head of a small valley on the southeast side of San Antonio Mountain 

near the summit. Maximum thickness possibly about 15 m

DEPOSITS OF THE TAOS PLATEAU VOLCANIC FIELD

The Taos Plateau volcanic field (Lipman and Mehnert, 1979) is a basaltic to 

rhyolitic, Pliocene to Pleistocene volcanic field underlying the Taos Plateau. The volcanic 

field is predominantly basaltic with lesser intermediate and rhyolitic compositions that 

erupted from no fewer than 55 distinct vents. The Taos plateau occupies the southern San 

Luis Basin in northern New Mexico and southern Colorado and is bordered on the east by 

the Sangre de Cristo Mountains and on the west by the Tusas Mountains and southeastern 

San Juan Mountains (fig. 1).

Within the map area, rock compositions include basalt to rhyolite (fig. 3; table 1) 

and range in age from 4.55–2.12 Ma (table 2). At least 15 exposed vent areas are known, 

or inferred, that vary from low-relief shields to the steep-sided volcano at San Antonio 

Mountain, which is composed of glassy dacite lava flows and a summit cinder cone (fig. 

2; Eppler, 1976; Thompson and Lipman, 1994a, b). Volcanic edifices composed of 

basaltic trachyandesite with vent areas concealed by overlying upper dacite of San 

Antonio Mountain (Tau) may represent cogenetic precursors to the dacite lavas; however,

a more thorough investigation is needed to confirm this relationship.

Olivine tholeiitic basalt of the Servilleta Basalt is the most voluminous volcanic rock 

in the Taos Plateau volcanic field. Ages throughout the Taos Plateau range from 5.3 Ma to 

about 1 Ma (Lipman and Mehnert, 1979; Appelt, 1998; Thompson and others, 2015). The 

low viscosity olivine tholeiitic basalts were emplaced as floods from low-relief shield 

volcanoes throughout the Taos Plateau and share general characteristics: flow thickness 

generally less than 10–12 m; olivine phyric; rarely xenocrystic; vesicular segregrations in 

the form of vertical pipes and (or) horizontal planes; and groundmass that is usually 

medium grained with moderate to well-developed diktitaxitic texture. Composition is 

dominantly tholeiitic basalt, but more complete sampling and compositional 

characterization of flows throughout the volcanic field reveal basaltic andesite to basaltic 

trachyandesite compositions that have been included in the Servilleta Basalt (Lipman and 

Mehnert, 1975, 1979; Dungan and others, 1986; Thompson and Lipman, 1994a). Within 

the map area the designation of Servilleta Basalt is restricted to lavas with characteristics 

described above and are classified as basalt or basaltic andesite on the IUGS total-alkalisilica classification diagram (LeBas and others, 1986). The Servilleta Basalt is subdivided 

into informal members based on age, known or inferred source vent, composition, and 

textural characteristics that, in some cases, are atypical of Servilleta Basalt elsewhere in 

the Taos Plateau. The Servilleta Basalt members included here are the Rio San Antonio 

(Tsr), Section 35 (Tse), Wissmath Craters (Tsw), and Pinebetal Mesa (Tsm) members.

Xenocrystic lavas, less common in other parts of the Taos Plateau volcanic field, are 

prolific within the map area. Whole-rock compositions vary from basaltic to basaltic 

andesite but also include mildly alkaline compositions ranging from trachybasalt to 

basaltic trachyandesite (fig. 3). Xenocrysts are dominantly quartz and plagioclase but rare 

xenocrystic pyroxene glomerocrysts are also observed. Grains identified as xenocrysts 

generally demonstrate highly rounded or extensively embayed grain morphologies, and 

plagioclase characteristically displays moderate- to well-developed, sieve-textured 

interiors. Grains interpreted as primary phenocrysts generally display euhedral 

morphologies with sharp grain boundaries and solid interiors, but mildly embayed grain 

boundaries, particularly in olivine, are considered phenocrysts. In rocks with a continuous 

range in grain morphologies, rounded grains with similar characteristics to euhedral 

grains are taken into account in modal estimates of phenocrysts. This classification 

ignores complexities in the magmatic system where phenocrysts formed at higher 

temperature and pressure conditions can be resorbed as the magma ascends and cools, 

which may explain the continuous range in grain morphologies from euhedral to highly 

embayed as observed in some rocks. Nevertheless, without extensive investigations of 

mineral chemistry, this approach to classify xenocrysts is reasonable for our descriptive 

purposes.

Glomerocrysts are frequently present in lava flows of the Taos Plateau volcanic 

field. Unit descriptions do not categorize glomerocrysts separately relative to their modal 

percentage. Rather, modal phenocryst percentages include contributions from 

glomerocrysts. Although glomerocrysts may represent remobilization of either crystals 

settled in a magma chamber or crystal accumulations along magma chamber walls, in 

general, phenocrysts that compose the glomerocrysts are petrographically 

indistinguishable from isolated grains. In cases where grains within glomerocrysts are 

distinct from isolated grains, the glomerocrysts are identified as xenocrystic.

Xenocrystic basalt of Hill 8489 (early Pleistocene)—Black to dark gray, 

moderately vesicular and porphyritic basalt (51 weight [wt.] percent 

silicon dioxide [SiO2

]; table 1) lava flows and near-vent pyroclastic 

deposits (not on map). Phenocrysts include 4–6 percent elongate to 

tabular plagioclase, 0.3– 2 mm in length; 4–6 percent euhedral to 

anhedral olivine with minor iddingsite replacement, up to 1.8 mm; and 

less than 1 percent anhedral orthopyroxene. Glomerocrysts of 

plagioclase, olivine, and orthopyroxene are common. Up to 2 percent 

xenocrysts that include clear quartz grains mostly 1–5 mm, although 

some grains are larger, and plagioclase 1–10 mm. Biotite is present but is 

likely a secondary mineral as grains are only in some small vesicles and 

voids in glomerocrysts and plagioclase xenocrysts. Groundmass is 

aphanitic to glassy and composed of glass, plagioclase, opaque and other 

mafic minerals. Flows are preserved in the southeast corner of the map 

and near-vent deposits are preserved east of the map boundary near the 

northeast flank of No Agua Peaks. Outflow was mostly east-northeast of 

vent area where it flowed over 13 kilometers (km) from the vent. 

Whole-rock 40Ar/39Ar age is 2.12±0.02 Ma (RGR–421; table 2).

Xenocrystic trachyandesite (Pliocene)—Dark gray, sparsely porphyritic 

trachyandesite (55 wt. percent SiO2) lava flows and near-vent pyroclastic 

deposits. Up to 1 percent phenocrysts that include a combination of 

orthopyroxene that is weakly pleochroic from light green to light brown, 

0.4–1.2 mm, and rare clinopyroxene and opaque mineral pseodomorphs 

after olivine. Opaque mineral pseudomorphs of euhedral and embayed 

olivine are up to 0.5 mm and associated with orthopyroxene grains in 

glomerocrysts. Xenocrysts are about 1 percent and include plagioclase, 

0.4–8 mm, and quartz up to 2.5 mm. Fine-grained groundmass with 

trachytic and intersertal textures is composed of plagioclase, pyroxene, 

glass, olivine, and opaque minerals. Vent area is located on northeast 

flank of San Antonio Mountain. Lipman and Mehnart (1979) report a 

potassium-argon (K-Ar) age of 2.24±0.15 Ma. Whole-rock 40Ar/39Ar age 

from lava near the vent is 2.78±0.05 Ma (RGR–009; table 2)

Near-vent pyroclastic deposits—Cinder and spatter agglutinate with 

minor interbedded lava flow material

Lava flows—Massive lava flows 5- to 10-m-thick. A distinct flow lobe 

extends to northeast nearly 7 km from the vent area and may be a 

compound single flow. A second flow is more proximal to the vent area

Basaltic trachyandesite of Red Hill (Pliocene)—Dark gray, vesicular to 

non-vesicular, porphyritic basaltic trachyandesite (51 wt. percent SiO2) 

lava flows and near-vent pyroclastic deposits. Phenocrysts include 5–6 

percent tabular to highly elongate plagioclase up to 1 mm that is 

occasionally intergrown with olivine; 1 percent euhedral to anhedral 

olivine, 0.1–0.4 mm, with iddingsitized cores in euhedral grains and 

often highly skeletal grains with little or no iddingsite alteration in 

anhedral grains; less than 1 percent orthopyroxene, 0.2–0.4 mm, with 

slight pleochroism from tan to light pink, and reaction rims at the grain 

boundary composed entirely of iddingsitized grains that may have 

originally been olivine or clinopyroxene; and less than 1 percent tabular 

clinopyroxene, up to 0.6 mm, with only slightly rounded grain boundaries. Fine-grained to aphanitic groundmass with intergranular and weakly 

developed trachytic textures composed of plagioclase, opaque minerals, 

and pyroxene. Vent is marked by a large cinder cone at Red Hill about 5 

km south of San Antonio Mountain. Whole-rock 40Ar/39Ar age for sample 

along U.S. 285 is 2.81±0.04 Ma (RGR–017; table 2)

Near-vent pyroclastic deposits—Cinder and spatter agglutinate with 

minor interbedded lava flow material. Cinder cone is being quarried

Lava flows—Massive lava flows 3- to 5-m-thick. At least two flows extend 

over 4 km from the vent area to south-southeast. At least three more 

proximal flows extend to the west and north

Rio San Antonio member of Servilleta Basalt (Pliocene)—Black, moderately vesicular to non-vesicular, sparsely phyric basaltic andesite (51–52 

wt. percent SiO2

). Phenocrysts include 5–10 percent microphenocrysts of 

anhedral to euhedral olivine, 0.1–0.8 mm. Plagioclase is likely a 

phenocryst phase as well but a distinction cannot be made from groundmass based on inspection of thin section alone. Groundmass is medium 

grained with intersertal, intergranular, and occasionally weakly trachytic 

textures and composed of elongate plagioclase up to 2.5 mm in length, 

but equant to tabular grains not uncommon; mostly devitrified glass; and 

subophitic clinopyroxene, up to 1.5 mm. Tsr lavas may have erupted 

from a single vent or multiple vents approximately coeval as all lavas 

mapped as Tsr occupy the same stratigraphic position. One possible vent 

may be a circular depression northwest of San Antonio Mountain; 

however, flows are higher in elevation south of the depression, suggesting 

Tsr flows may have erupted from multiple vents or were tilted eastward 

after emplacement. Flow thickness 3–8 m. Whole-rock 40Ar/39Ar age for 

sample from northwest of San Antonio Mountain indicates an age of 

2.94±0.12 Ma (RGR–012; table 2)

Upper dacite of San Antonio Mountain (Pliocene)—Black to gray, non-vesicular to moderately vesicular, sparsely porphyritic dacite to trachydacite

(63–65 wt. percent SiO2) lava flows and near-vent pyroclastic deposits. 

Phenocrysts include 2 percent euhedral to subhedral orthopyroxene, 

slightly pleochroic from light green to light brown and often with darker 

brown cores, 0.2–0.4 mm; and 1–2 percent elongate to tabular plagioclase

up to 0.5 mm in long dimension. Orthopyroxene often occurs in glomerocrysts. Fine-grained to glassy groundmass with weakly trachytic to 

felty texture and composed of glass, plagioclase, pyroxene, and opaque 

minerals. Lipman and Mehnart (1979) report a whole-rock K-Ar age of 

3.12±0.17 Ma. The preferred whole-rock 40Ar/39Ar age sampled from a 

flow on the north flank of San Antonio Mountain is 2.9±0.3 Ma 

(RGR–010 [no. 1]; table 2)

Near-vent pyroclastic deposits—Cinders and spatter with interlayered 

black glass to highly oxidized red flow material. Marks late-stage vent 

area along the southern peak of San Antonio Mountain

Lava flows—Lava flows up to 30-m-thick that radiate outward from a 

central vent area. Typical flow structure is black and glassy at base of 

flow; light gray and fine grained in the middle; and coarse breccia top 

with red, highly oxidized and black, glassy blocks. Eppler (1976) 

provides a more detailed discussion of the internal flow structure

Lower trachyandesite of north San Antonio Mountain (Pliocene)—Black, 

fine-grained, and highly to moderately vesicular, sparsely porphyritic 

trachyandesite (63–64 wt. percent SiO2) lava flows. Phenocrysts include 

1–3 percent subhedral orthopyroxene often with slightly rounded grain 

boundaries, slightly pleochroic from light green to light brown and often 

with a darker brown core, 0.4–0.8 mm; and less than 1 percent tabular 

plagioclase, mostly 0.5 mm. Orthopyroxene often occurs in glomerocrysts. Xenocrysts of quartz are rare but can be greater than 1 mm. 

Fine-grained to glassy groundmass with weakly trachytic to felty and 

intersertal textures common and composed of, in decreasing order of 

abundance, glass, plagioclase, pyroxene, and opaque minerals. Exposures 

limited to the north flank of San Antonio Mountain. Lavas may represent 

an earlier eruptive episode preceding the main dacitic volcano building 

event based on similar composition and nearly identical phenocryst 

content and character. The preferred whole-rock 40Ar/39Ar age is 

2.91±0.07 Ma (RGR–231; table 2)

Lower trachyandesite of south San Antonio Mountain (Pliocene)—Black, 

sparsely porphyritic trachyandesite (55–56 wt. percent SiO2

) lava flows. 

Phenocrysts include 1–2 percent elongate to tabular and rarely equant 

plagioclase, 0.4–1.5 mm; 1 percent anhedral and embayed olivine, 

0.1–0.7 mm; and rare tabular to irregular grains of clinopyroxene (much 

less than 1 percent), up to 0.5 mm. Occasionally, plagioclase phenocrysts 

are intergrown with olivine and clinopyroxene. Olivine preserved within 

plagioclase also exhibits embayed grain boundaries. Fine-grained to 

glassy groundmass where intersertal texture is dominant but weekly 

trachytic to felty textures present and composed of plagioclase, glass, 

pyroxene, and opaque minerals. Unit is exposed only on south and 

southeast flanks of San Antonio Mountain. Whole-rock 40Ar/39Ar 

geochronology indicates age of 3.00±0.04 Ma (RGR–133; table 2)

Section 35 member of Servilleta Basalt (Pliocene)—Black, sparsely phyric 

basalt (50–51 wt. percent SiO2

) lava flows. Basalt is equigranular and 

medium grained and as a result phenocryst determination other than 

olivine is unclear. Olivine phenocrysts 5–7 percent as euhedral to 

anhedral grains, 0.2–1 mm. Groundmass exhibits well-developed 

diktytaxitic, intergranular, and intersertal textures and is composed of 

elongate plagioclase, 0.2–2.5 mm; equant clinopyroxene, 0.1–0.5 mm; 

opaque minerals; and brown to completely devitrified glass. Source of 

lava flows is undetermined but is inferred to be buried beneath the 

eastern flank of San Antonio Mountain or the alluvial fans that extend 

east from San Antonio Mountain based on lava distribution. The lava 

flows overlie flows of the Wissmath Craters member of the Servilleta 

Basalt (Tsw) east of the map boundary. Lava flow thickness about 10 m

Wissmath Craters member of Servilleta Basalt (Pliocene)—Black, 

sparsely vesicular, porphyritic basalt to basaltic andesite lava flows 

(48–52 wt. percent SiO2

). Phenocrysts include 1–5 percent euhedral to 

irregular olivine, less than 2.8 mm, irregular grains with sometimes 

highly embayed boundaries suggesting some grains may be xenocrystic; 

2–7 percent plagioclase with sharp grain boundaries and solid interiors. 

Glomerocrysts of plagioclase and olivine present in some samples. Less 

than 1 percent xenocrysts that include plagioclase, up to 6 mm, mostly 

with highly developed sieve-textured interiors and a thin mantle, but also 

present as large solid-cored grains with highly embayed grain boundaries;

and orthopyroxene with irregular grain boundaries rimmed by clinopyroxene (Dungan, 1987). Aphanitic to medium-grained groundmass

with intergranular texture and patchy subophitic pyroxene that forms a 

mottled appearance in thin section. Groundmass composed of plagioclase,

pyroxene, olivine, opaque minerals, and minor glass. Lavas are compositionally similar despite the range in SiO2

 and are interpreted to have 

erupted from a central vent marked by the depression at Wissmath Craters 

located immediately east of the eastern map boundary (fig. 2). However, 

multiple source vents could explain the petrographic variability and 

range in SiO2

 content. Additionally, exposed in a road cut along highway 

U.S. 285 northwest of No Agua Peaks (fig. 1), a thin sedimentary 

interbed separates compositionally similar lavas indicating a hiatus in 

eruption, which could indicate multiple sources. Lava flows are observed 

stratigraphically above xenocrystic basaltic andesite of La Segita Peaks 

(Tls) in the southern part of Wissmath Craters. Flows associated with 

Wissmath Craters shield within the map area are over 6 km southwest of 

the crater, but east of the map area, flows extend 10 km east of Wissmath 

Craters. Whole-rock 40Ar/39Ar age from sample in the southwest area of 

the map near U.S. 285 is 3.20±0.05 Ma (RGR–235; table 2)

Xenocrystic basaltic andesite of La Segita Peaks (Pliocene)—Black, 

moderately vesicular, sparsely porphyritic basaltic andesite (53–55 wt. 

percent SiO2

) lava flows and red, scoriaceous near-vent pyroclastic 

deposits (east of map area). Phenocrysts include 1–2 percent euhedral to 

subhedral olivine, 0.2–1.3 mm. Up to 2 percent xenocrysts that include 

plagioclase with solid cores surrounded by fine sieve texture and thin 

mantle at outer grain boundary; and quartz grains up to 6 mm. Groundmass

exhibits intersertal to intergranular textures and is composed of 

plagioclase, pyroxene, olivine, glass, and opaque minerals. Vent area is 

inferred from thick accumulation of near-vent scoriaceous deposits and 

agglutinated spatter that underlie most of the high points at La Segita 

Peaks located immediately east of the eastern map boundary. Unit 

extends to the southwest but is mostly concealed below Wissmath 

Craters member of the Servilleta Basalt (Tsw). 40Ar/39Ar geochronologic 

analysis on a whole-rock sample resulted in discordant data and therefore 

is not reported in table 2. However, analytical results are reported in the 

data release that supports this report (sample RGR–135, Turner and 

others, 2018)

Pinebetal Mesa member of Servilleta Basalt (Pliocene)—Gray, moderately 

vesicular, sparsely phyric to porphyritic basalt lava flows (49 wt. percent 

SiO2

). Rock is generally coarse grained and equigranular rendering 

phenocryst determination in most samples difficult with the exception of 

a sample from the depression rim at the main shield, which is porphyritic 

with a fine-grained to aphanitic groundmass. Phenocryst percentages 

based on porphyritic sample include 10–15 percent elongate plagioclase, 

0.2–5 mm, that are occasionally intergrown with olivine and other 

plagioclase; and 7–10 percent subhedral to anhedral olivine, 0.1–2 mm, 

occasionally with moderately embayed grain boundaries. Samples with 

coarse-grained groundmass exhibit diktytaxitic, intergranular, and 

intersertal textures and are composed of platy plagioclase, up to 8 mm; 

subophitic pyroxene; glass, usually devitrified; and opaque minerals. 

Vent area for some lava flows is a shield with a central depression located

northwest of San Antonio Mountain on Pinabetal Mesa. Flows may also 

have erupted from north-aligned depressions west of the main shield. 

Some lavas flowed south from the main vent where they ponded in 

paleotopographic lows and formed flows 20–30 m thick where exposed

along the Rio San Antonio. Most lavas flowed north and northeast of the 

vent area for at least 10 km, at which point they are concealed by overlying

Rio San Antonio member of Servilleta Basalt (Tsr). One lava tube is 

known to exist on Pinabetal Mesa, accessed through a collapsed ceiling 

at Cisneros Mine (Rogers and others, 2000). Flows in the northeastern 

corner of the map are also distinctly coarse grained and occupy the same 

stratigraphic position so are included in the Pinabetal Mesa member of 

the Servilleta Basalt; although, it is unknown if these lavas erupted from 

the same vent. With exception of ponded lava flows, flow thickness 

generally 5–10 m. Whole-rock 40Ar/39Ar age is 3.33±0.14 Ma (RGR–013; 

table 2) on a sample from the rim of Rio San Antonio west of San 

Antonio Mountain

Basaltic trachyandesite of Los Cerritos de la Cruz (Pliocene)—Gray to 

black, vesicular to nonvesicular, porphyritic basalt to trachyandesite 

(51–57 wt. percent SiO2

) lava flows and near-vent pyroclastic deposits. 

Phenocrysts are difficult to distinguish because of high xenocryst 

content, but grains interpreted to be phenocrysts are cumulatively 2–3 

percent and include tabular to elongate plagioclase, 0.5–1.5 mm, with 

either sharp or only slightly rounded grain boundaries; euhedral to 

anhedral grains and grain fragments of olivine, 0.2–1.5 mm, with moderately to highly embayed grain boundaries and resorbed interiors suggesting 

some olivine may be xenocrystic; and rare euhedral clinopyroxene, up to 

1.5 mm. Xenocrysts are up to 7 percent of rock and include plagioclase 

and alkali feldspar 5 mm and larger; rounded quartz, 3 mm and larger 

(Thompson and Lipman, 1994b); orthopyroxene, 0.5–1 mm, with grain 

boundaries rimmed by clinopyroxene; and glomerocrysts usually consisting of 20 or more grains of 0.5 mm clinopyroxene. Very fine grained 

groundmass with intersertal to intergranular textures composed of 

plagioclase, pyroxene, olivine, and sparse glass. North and south cinder 

cones mark vent areas. Mapped as a single unit because stratigraphic 

order between outflow from north and south vents could not be established

in the field and eruptive activity is inferred to be similar in age; however, 

aeromagnetic data indicate normal polarity for the south cone but 

reversed polarity for the north cone indicating some amount of elapsed 

time between eruptions of the north and south cones (Drenth and others, 

2011). Whole-rock 40Ar/39Ar age on lava flow located between north and 

south cones is 3.36±0.05 Ma (RGR–015; table 2)

Near-vent pyroclastic deposits—Cinder and spatter agglutinate and minor 

lava flow material preserved at two cones

Lava flows—Multiple lava flows 3–8 m thick originating from both vent areas

Basaltic andesite of Rio de los Piños (Pliocene)—Gray, sparsely vesicular, 

porphyritic basalt to basaltic andesite (50–53 wt. percent SiO2

) lava 

flows and near-vent pyroclastic deposits. Phenocrysts include 2–4 

percent tabular to elongate plagioclase up to 2 mm with occasionally 

rounded grain boundaries; and 1–3 percent anhedral to subhedral olivine, 

0.1–1.2 mm, some grains exhibit skeletal morphologies or are slightly 

embayed, and iddingsite replacement varies from moderate to none. 

Glomerocrysts of plagioclase and olivine are ubiquitous. Less than 1 

percent xenocrysts of plagioclase (2.5 mm) and quartz (3 mm). 

Fine-grained groundmass with intergranular, moderate- to well-developed trachytic texture, and patchy subophitic pyroxene results in a 

mottled appearance. Groundmass composed of plagioclase, subophitic 

pyroxene, olivine, opaque minerals, and sparse glass. Flows originated 

from a single vent area at an unnamed hill north of the Rio de los Piños. 

Lavas that flowed to the northeast extend over 10 km from the main 

vent, drape older Valdez Tank lava flows (Tb), and appear to have filled 

a paleochannel. Lavas that flowed east rest on basaltic andesite of 

Cañada los Ranchos (Tca) south of the Rio de los Piños. Lavas are 

difficult to differentiate from Valdez Tank lavas in hand sample but 

basalt of Rio de los Piños has fewer plagioclase xenocrysts, occasionally 

has quartz xenocrysts, and lacks pyroxene phenocrysts. Preferred 

whole-rock 40Ar/39Ar age is 3.39±0.04 Ma (RGR–374; table 2)

Near-vent pyroclastic deposits—Cinder and spatter agglutinate with 

minor lava flow material

Lava flows—Multiple lava flows from 5- to 10–m-thick 

Basaltic andesite of Cañada los Ranchos (Pliocene)—Black to dark gray, 

moderately vesicular to highly vesicular, porphyritic basaltic andesite 

(52–53 wt. percent SiO2

) lava flows. Phenocrysts include 2–3 percent 

euhedral to anhedral olivine, 0.2–1 mm, skeletal grains are common and 

some grains exhibit embayed grain boundaries; 1–2 percent elongate 

plagioclase, 0.5–1 mm. Xenocrysts (less than 1 percent) include plagioclase up to 3 mm, quartz up to 0.6 mm and rare (only observed in one 

thin section) elongate orthopyroxene, 0.2–0.6 mm, with rounded grain 

boundaries and rimmed by clinopyroxene. Fine-grained to glassy 

groundmass with trachytic, intersertal and intergranular textures common 

and patchy subophitic pyroxenes result in a mottled appearance in thin 

section. Groundmass composed of plagioclase, pyroxene, glass, olivine, 

and opaque minerals. Main vent is located near western map boundary, 

north of Rio San Antonio, where a circular depression is at the summit of 

a low-angle shield. At least seven moderately oxidized, highly vesicular, 

1- to 2-m-thick flows are exposed in the wall of the depression. Northeast of the main vent area along the Rio San Antonio gorge is a dike

associated with minor cinder deposits interlayered with lava flows

indicating a possible fissure eruption linked with the main vent. Flows

extend at least 16 km to the northeast. Preferred whole-rock 40Ar/39Ar age

is 3.32±0.06 Ma (RGR–221; table 2). Although the mean age is younger

than overlying basaltic andesite of Rio de los Piños (Trl), the ages are

indistinguishable within analytical error suggesting the units were

erupted at about the same time

Rhyolite of No Agua Peaks (Pliocene)—Gray to pale brown perlitized 

rhyolite (73 wt. percent SiO2

) flows, tuffs, and breccias. Lava flows are 

commonly flow banded, include obsidian to microcrystalline groundmass,

and have onionskin and perlitic fracture textures. Degree of vesiculation 

varies from 2–20 percent of rock, where less vesicular varieties are 

generally less perlitized (Neart and others, 1980). Phenocrysts 

(xenocrysts?) include alkali feldspar as euhedral grains and irregular 

fragments. Previous investigations suggest No Agua rhyolites erupted 

from 1–4 vents (Neart and others, 1980; Lipman and Mehnert, 1979; 

Whitson, 1982; Breese, 1984; Chamberlin and Barker, 1996). No Agua 

rhyolite is present in the southeastern corner of the map area with all 

inferred vents located southeast of the map boundary. Dickens (2007) 

determined 40Ar/39Ar ages based on laser fusion of glass and feldspar 

separates indicating temporally distinct eruptive episodes at 3.88±0.06 

and 4.1±0.03 Ma. Some single-grain laser-fusion ages determined on 

alkali feldspar yielded ages of 25.35±0.13 Ma, which Dickens (2007) 

suggests indicates incorporation of feldspar xenocrysts from an Amalia 

Tuff-related source

Basalt of Chino Peak (Pliocene)—Gray to black, moderately vesicular and 

porphyritic basalt (49–50 wt. percent SiO2) lava flows and near-vent 

pyroclastic deposits. Although range in SiO2 content is narrow, lavas 

included in map unit have substantial variability in wt. percent magnesium oxide (MgO), potassium oxide (K2

O), and numerous trace elements 

including rubidium (Rb), niobium (Nb), and lanthanum (La). Petrographically, rocks are broadly similar. Phenocrysts include 1–4 percent 

euhedral to anhedral, and commonly skeletal, olivine, 0.1 to 2.5 mm; 1 

percent tabular to elongate plagioclase, 0.1 to 1 mm. Sparse xenocrysts 

include plagioclase and orthopyroxene where grain boundary is irregular 

and mantled by completely iddingsitized olivine(?) grains. Mediumgrained to aphanitic groundmass with intergranular to rarely intersertal 

texture composed of plagioclase, pyroxene, olivine, and opaque minerals. Micro enclaves of quenched melt inclusions that consist dominantly 

of brown glass, opaque minerals, elongate plagioclase and pyroxene are 

present in samples with greater than 8.0 wt. percent MgO (8.04–9.55) 

and wt. percent K2O less than 0.5 (0.43–0.49). Samples with wt. percent 

MgO less than 7.0 (5.95–6.77) and wt. percent K2O greater than 0.9 

(0.9–1.02) do not contain enclaves of melt inclusions. Two compositionally transitional samples with 7.40–7.95 wt. percent MgO and 0.75–0.77 

wt. percent K2O also contain enclaves of melt inclusions. These compositional and petrographic characteristics may indicate a magma mixing 

relationship. A single vent area is identified by a cinder cone at Chino 

Peak; however, highly oxidized cinders and agglutinate are located 

northwest of the cinder cone just east of a small fault. The preferred 

whole-rock 40Ar/39Ar age is 4.31±0.07 Ma (RGR–014; table 2) from a 

sample of lava with MgO less than 7.0 wt. percent that is interbedded 

within the cinder cone

Near-vent pyroclastic deposits—Cinder and spatter agglutinate and minor 

lava flow material

Lava flows—Multiple flows originating from vent area up to 18-m-thick 

where exposed along the Rio de los Piños

Basalt and basaltic andesite of Valdez Tank (Pliocene)—Black to gray, 

weakly to moderately vesicular, porphyritic basalt to basaltic andesite 

lavas and near-vent pyroclastic deposits. Unit includes three compositional groups that form a broadly continuous progression in composition 

and possess similar petrographic characteristics; lava types include basalt 

(48–51 wt. percent SiO2), silicic basalt (50–51 wt. percent SiO2), and 

basaltic andesite-basaltic trachyandesite (51–55 wt. percent SiO2). Finegrained to aphanitic groundmass composed of plagioclase, clinopyroxene,

opaque minerals, olivine, and sparse minor glass. Seriate texture 

common in low-silica basalt and shows continuous grain size from less 

than 0.05 mm up to 1.5 mm. Porphyritic, intergranular, and trachytic 

texture common in all types with groundmass up to 0.5 mm and 

phenocrysts up to 1.5 mm. Low-silica basalt phenocrysts include 1–2 

percent plagioclase and 1–2 percent olivine; silicic basalt and basaltic 

andesite-basaltic trachyandesite groups include phenocrysts of 1–2 

percent plagioclase, 1 percent clinopyroxene, 1 percent orthopyroxene, 

and up to 1 percent olivine. Glomerocrysts are common and consist of 

plagioclase only, plagioclase and both pyroxenes, and plagioclase and 

olivine with and without pyroxenes. All three compositional groups 

contain up to 1 percent xenocrysts of plagioclase (up to 3.5 mm). 

Multiple vent areas are identified: low-silica basalt correlates to a 

1.5-m-wide dike approximately 2.5 km north of Bighorn Peak, north of 

the map boundary; basaltic andesite-basaltic trachyandesite correlates to 

a low-profile cinder cone along the north rim of Rio de los Piños and 

cinder deposits and lavas approximately 2 km to the north-northwest; 

and silicic basalt correlates to cinder deposits and lavas about 2 km north 

of Bighorn Peak just north of the map boundary. Lava types are grouped 

as a single map unit because differentiation in the field was not possible 

and the continuous compositional progression and similar eruption ages 

suggest they may be cogenetic. Preferred whole-rock 40Ar/39Ar ages for 

the distinct compositional types include ages of 4.36±0.05 Ma 

(RGR–364; table 2) for basalt; 4.36±0.04 Ma (RGR–362; table 2) for 

silicic basalt; and 4.37±0.03 Ma (RGR–371; table 2) for basaltic andesitebasaltic trachyandesite. Stratigraphically, basaltic andesite-basaltic 

trachyandesite lavas overlie both basalt types but no relative order was 

determined between basalt and silicic basalt types. If eruptions were 

coeval stratigraphic order may not be consistent

Near-vent pyroclastic deposits—Cinder and spatter agglutinate and minor 

lava flow material

Lava flows—Lava flows originating from multiple vent areas north of Rio de 

los Piños. Individual lava flows are 3–12 m thick, but, where tops are 

eroded erosional remnants can be less than 1 m

Basalt of Lucero Lakes (Pliocene)—Light gray to gray, fine-grained, sparsely

phyric basalt (49.5–50.1 wt. percent SiO2) lava flows. Microphenocrysts 

are difficult to differentiate from groundmass because of seriate texture 

but include 1–2 percent olivine, 0.1–0.25 mm with little or no iddingsite 

replacement, and 1–2 percent tabular to elongate pyroxene, 0.1–0.6 mm, 

dark green with moderate to extensive iddingsite replacement and some 

with well-developed skeletal texture. Sparse xenocrysts of quartz, 

plagioclase and pyroxene only observed in one sample. Fine-grained to 

aphanitic groundmass with intergranular and seriate textures and patchy 

subophitic pyroxene forms a mottled appearance in thin section. Groundmass composed of plagioclase, pyroxene, olivine, opaque minerals, and 

minor glass. Flows originated from Lucero Lakes area (fig. 1) and were 

channelized by paleovalleys with east- to southeast-directed flow. Lava 

flow thickness from 5–10 m. A northwest-trending, approximately 

0.5-m-wide, dike that is sparsely phyric with rare green olivine phenocrysts

protrudes above the surface in the southeast flank of Lamy Peak. 

Compositional data for the dike are not available; however, the physical 

characteristics of the dike are consistent with the basalt of Lucero Lakes 

rather than the Hinsdale Formation basaltic andesite (Thb) that caps Lamy 

Peak, and, therefore, the dike is considered part of the basalt of Lucero 

Lakes. Whole-rock 40Ar/39Ar age is 4.55±0.07 Ma (RGR–036; table 2)

OLDER REGIONAL VOLCANIC AND VOLCANICLASTIC ROCKS

Volcanic and volcaniclastic deposits that predate deposits of the Taos Plateau 

volcanic field record volcanism of the Southern Rocky Mountains volcanic field 

(SRMVF) and filling of early extensional basins. The SRMVF is a composite mid-Tertiary 

(38–23 Ma) volcanic field in northern New Mexico and Colorado (Steven, 1975; Lipman, 

2007). Loci of volcanism of the SRMVF surrounding the map area include the San Juan 

Mountains, San Luis Hills, and the Latir volcanic locus near the town of Questa, New 

Mexico. Volcanic deposits associated with the southeastern San Juan Mountains are 

present in the northwestern part of the map area. Conejos Formation (Tcf) deposits 

represent the early magmatic system in the San Juan Mountains and are composed of 

mostly andesite to dacite lavas flows and breccias and derivative volcaniclastic rocks 

associated with central volcanoes. Eruption of intermediate-composition lavas was 

superseded by multi-cyclic caldera formation at the Platoro caldera complex associated 

with eruption of at least seven major ignimbrites (individual ignimbrite volumes range 

between 75–1,000 km3) between ~29–30 Ma that collectively form the Treasure Mountain Group (Lipman and others, 1996). The Chiquito Peak Tuff (Ttc) is the only distinct 

ignimbrite present in the map area, exposed along the Rio de los Piños in the northwest 

part of the map. Along the western margin of the San Luis Valley, the Chiquito Peak Tuff 

has previously been mapped as Masonic Park Tuff (Lipman,1975a, b; Manley, 1982a) 

and was inferred to have erupted from a buried source in the central San Juan Mountains 

(Lipman, 1975a). However, based on compositional, mineralogical, geochronologic, and 

stratigraphic evidence, Lipman and others (1996) determined previous usage of the 

Masonic Park Tuff combined two distinct ignimbrites and they renamed the ignimbrite 

that erupted from the Platoro caldera as the Chiquito Peak Tuff. The Chiquito Peak Tuff is 

the youngest widely distributed ignimbrite originating from the Platoro caldera complex 

and erupted at 28.94 Ma (RGR–466; table 2). Undivided Treasure Mountain Group 

deposits (Tt) below the Chiquito Peak Tuff are largely covered by rock-fall and colluvial 

deposits.

The Los Pinos Formation is composed of redeposited volcanic material sourced 

from surrounding volcanic highlands (Manley, 1981). In the northern part of the map 

area, clasts within Los Pinos deposits are dominantly intermediate-composition volcanic 

rocks from the southeast San Juan Mountains and the San Luis Hills, which is diagnostic 

of the Esquibel Member (Tle). The base of the Esquibel Member is placed at the top of 

the Chiquito Peak Tuff (28.94 Ma). In the southern part of the map area, intermediatecomposition clasts become smaller and volumetrically minor compared to abundant 

rhyolitic clasts of the Amalia Tuff that characterize the Cordito Member (Tlc). Manley 

(1981) placed the base of the Cordito Member at the top of the Amalia Tuff (25.4 Ma; 

Zimmerer and McIntosh, 2012) where the units are in stratigraphic position south of the 

map area. The contact between the two members is time-transgressive as deposition of 

the Esquibel Member continued in the northern part of the map area coeval with deposition of Cordito Member.

Interbedded within the Los Pinos Formation is the Hinsdale Formation, which 

includes basalt to rhyolite lava flows throughout its regional extent including the San 

Juan Mountains, San Luis Hills, and Sangre de Cristo Mountains adjacent to the southern 

San Luis Valley. Lipman and Mehnert (1975) suggested all basaltic lava flows interbedded

within the Los Pinos Formation should be included in the Hinsdale Formation including 

some Pliocene lava flows correlative with flows within the map area. However, the 

Hinsdale Formation is here restricted to Oligocene and Miocene volcanic rocks due to the 

substantial age gap between the Miocene and Pliocene rocks and because the Pliocene 

volcanic rocks are compositionally and temporally related to the Taos Plateau volcanic 

field. Within the map area, deposits of the Hinsdale Formation consist of basalt to 

trachyandesite lava flows and are predominantly Oligocene in age. A single remnant of 

Miocene lava flow (Thl) is preserved capping Bighorn Peak in the northwest part of the 

map area. This lava flow is compositionally distinct from Pliocene and Oligocene lavas in 

the map area but is similar (fig. 3) to Miocene lavas more widely distributed to the north 

of the map area in southern Colorado (Lipman, 1975b).

Los Pinos Formation (Miocene and Oligocene)—Beds and lenses of poorly 

sorted to moderately well sorted sandy pebble to slightly bouldery cobble 

conglomerate and friable, well stratified fine-grained to pebbly, 

coarse-grained, tuffaceous(?) sandstone. Much of the moderately well 

sorted conglomerate appears to be of fluvial origin, whereas the poorly 

sorted, sparsely bouldery cobble conglomerate may reflect debris flow 

deposits. Unit locally includes thin siltstone lenses and sandy-over-gravelly

sheetflood(?) deposits. Unit locally may include lenses and thin beds of 

claystone derived from the alteration of volcanic ash. Sheetflood deposits 

consist of rhythmically bedded couplets consisting of alternating thin 

(5–40 centimeters [cm]), well-stratified layers composed of small-pebble, coarse-grained sandstone that commonly overlie slightly cobbly 

pebble conglomerate. Conglomerate clasts commonly are subrounded to 

well rounded and range in size from 1–2 cm up to 1.5 m in diameter. 

Clasts are dominantly volcanic and intrusive in origin shed from volcanic 

highs in the southeast San Juan Mountains, San Luis Hills, and the Latir 

volcanic locus. Locally, deposits include clasts of Proterozoic granitic 

gneiss and quartzite. Formation consists of the upper Cordito Member 

and the lower Esquibel Member (Manley, 1981) separated by a time 

transgressive boundary. Estimated thickness, as much as 400 m

Cordito Member—Commonly contains clasts of Amalia Tuff and other 

volcanic and intrusive rocks from the Latir volcanic locus of the Southern 

Rocky Mountains volcanic field. Typically, the matrix of Cordito is 

lighter colored than that of the underlying Esquibel Member due chiefly 

to a more tuffaceous matrix and more abundant felsic clasts transported 

from the Latir volcanic locus along west- to northwest-directed 

paleodrainages that flowed into, and through, the Tusas Mountains

Esquibel Member—Commonly contains abundant clasts of andesite, 

dacite, and basalt volcanic and intrusive rocks eroded from the southeast 

San Juan Mountains and the San Luis Hills. Clasts of welded ignimbrite 

erupted from the Platoro caldera complex in the southeast San Juan 

Mountains are also observed. North of the map area, in southern Colorado, 

early Miocene basaltic lava flows are interbedded with deposits of the 

Esquibel Member (Lipman and Mehnert, 1975). Miocene Hinsdale 

Formation lava flow that caps Bighorn Peak (Thl) suggests the Esquibel 

Member is as young as Miocene in the map area. Distinction from the 

Cordito Member is based chiefly on the absence of Amalia Tuff clasts. 

Clast provenance from the San Luis Hills and southeast San Juan 

Mountains indicates transport along paleodrainages that flowed south to 

southwest into and through the northern Tusas Mountains

Hinsdale Formation (Miocene and Oligocene)—Includes Oligocene-aged, 

porphyritic basalt (Thp), trachyandesite (Tha) , basaltic andesite (Thb), 

and tholeiitic basalt (Tht), as well as a single isolated outcrop of Miocene

basaltic lava flow (Thl) that caps Bighorn Peak. Tholeiitic basalt (Tht) 

and basaltic andesite (Thb) are most abundant Hinsdale Formation types 

within the map area whereas trachyandesite and porphyritic basalt types 

are more aerially restricted. Miocene basaltic lava is compositionally 

distinct from all other Hinsdale in the map area but is correlated to 

similar rocks to the north, which are about 20 Ma (table 2) 

Alkaline basalt to trachybasalt (Miocene)—Black to gray, mediumgrained porphyritic basalt lava flows (50 wt. percent SiO2

). Phenocrysts 

include 5–7 percent euhedral to anhedral olivine, 0.2–0.5 mm, where 

some grain boundaries are embayed and have minor to extensive 

iddingsite replacement. Xenocrysts of plagioclase are rare. Fine- to 

medium-grained groundmass with intergranular and rarely intersertal 

textures composed of plagioclase, subophitic pyroxene, olivine, opaque 

minerals, and minor glass. Only one lava flow is preserved capping 

Bighorn Peak. Unit is more extensive north of map area (Lipman, 

1975b). Whole-rock 40Ar/39Ar age from sample north of the map area is 

20.61±0.07 Ma (RGR–378; table 2)

Tholeiitic basalt (Oligocene)—Gray to dark-gray, moderately to nonvesicular, basalt lava flows (49–50 wt. percent SiO2

). Phenocrysts include 

olivine and plagioclase, however, due to the medium- to coarse-grained 

groundmass, modal plagioclase is difficult to distinguish from groundmass grains. Euhedral to subhedral olivine is 7–15 percent of rock, 0.1–1 

mm, and slightly to completely iddingsitized; larger grains often have 

slightly rounded to embayed grain boundaries. Groundmass is intergranular, intersertal, and weakly diktitaxitic and composed of plagioclase, 

0.1–2.4 mm, some grains as large as 4 mm; clinopyroxene, 0.05 to 0.2 

mm, individual equant grains and subophitic grains up to 1 mm; 3–5 

percent opaque minerals; and 0–7 percent glass, partially devitrified with 

intergrown opaque minerals. Rare orthopyroxene grains intergrown with 

olivine have thick clinopyroxene overgrowths and are up to 2 mm. 

Plagioclase grains with sieve-textured interiors are sparse. Outcrop 

trends southwesterly from northeast part of map area along the Rio de los 

Piños, where 7–10 flows are present. Number of lava flows decreases to 

the southwest suggesting lavas erupted from a source to the northeast of 

the map area. Flow thickness 5–8 m. Preferred whole-rock 40Ar/39Ar age 

is 25.67±0.20 Ma (RGR–456; table 2) from a sample west of the map 

area, which is significantly older than a previously reported K-Ar age of 

15.0±0.8 Ma (Lipman and Mehnert, 1975). Sample RGR–148 

(25.54±0.24 Ma) was collected near the sample of Lipman and Mehert 

(1975), therefore the K-Ar age is considered incorrect

Basaltic andesite (Oligocene)—Dark-gray to gray, sparsely phyric basaltic 

andesite lava flows (52–54 wt. percent SiO2). Phenocrysts include 

olivine and most likely plagioclase; however, plagioclase grains larger 

than the groundmass are not observed. Subhedral to anhedral olivine 5–7 

percent of rock, 0.1–0.5 mm, with completely iddingsitized cores but 

rims of grains are generally unaltered. Medium- to fine-grained groundmass has intergranular to intersertal, and weakly developed diktytaxitic 

textures and is composed of plagioclase, 0.2–1.0 mm; glass, dark brown 

to black mostly devitrified; clinopyroxene, 0.1 to 0.5 mm, equant to 

tabular grains; and opaque minerals. One grain with orthopyroxene core 

and thick clinopyroxene rim was observed. A single lava flow observed 

in most localities but possibly up to three lava flows present on Broke 

Off Mountain. Source unknown but may be east of the map area and 

buried by Pliocene volcanic rocks. Whole-rock 40Ar/39Ar age is 

26.05±0.21 Ma (RGR–33; table 2) on a sample west of the map area on 

Broke Off Mountain.

Trachyandesite (Oligocene)—Black, sparsely phyric, trachyandesite lava 

flow (56–57 wt. percent SiO2). Microphenocrysts likely include olivine 

and plagioclase but there is no distinction in size from groundmass. Fine 

grained to glassy with intersertal texture and composed of 50–60 percent 

tabular plagioclase, up to 0.4 mm; 30–35 percent glass that varies from 

fresh and light-brown with little to no devitrification to dark brown and 

highly devitrified; 3–5 percent equant and elongate clinopyroxene, up to 

1 mm; 3–5 percent opaque minerals that often fill areas between 

plagioclase grains or are equant cubic grains; and trace equant olivine, 

0.1 mm, with minor to no iddingsite replacement, or, rarely, as 

phenocrysts up to 1 mm that are highly iddingsitized. Trachyandesite 

occurs as a single lava flow up to 8-m thick with highly stretched 

vesicles. Lava flow is overlain by tholeiitic basalt (Tht, RGR–456, 

25.67±0.20 Ma; table 2), northwest of Laguna Larga, and basaltic 

andesite lavas (unit Thb, RGR–033, 26.05±0.21 Ma; table 2) in the area 

of Los Cerritos de la Cruz. Two vent areas are inferred. The first is 

northwest of Laguna Larga and west of western map boundary where 

sparsely phyric and highly oxidized scoriaceous deposits, inferred to 

represent near-vent pyroclastic deposits, are eroding from the slope 

below tholeiitic basalts. A second vent is inferred west of the northern 

cone of Los Cerritos de la Cruz where two intersecting dikes are sparsely 

phyric and petrographically similar to outflow lavas

Porphyritic basalt to trachybasalt (Oligocene)—Black to dark gray, 

porphyritic alkaline basalt to trachybasalt (48 wt. percent SiO2) lava 

flow. Phenocrysts include 3–5 percent clinopyroxene, up to 1.5 mm; 3–5 

percent euhedral to subhedral olivine, up to 2 mm; and 1 percent 

plagioclase, where phenocrysts are differentiated from groundmass 

grains based on more equant form and presence of mafic inclusions. 

Pyroxene is often in glomerocrysts with as many as 10 grains giving 

appearance in hand sample of larger phenocrysts. Groundmass is 

medium grained with intergranular texture and includes plagioclase, 

clinopyroxene, olivine, and opaque minerals. Groundmass plagioclase up 

to 2 mm in long dimension and groundmass clinopyroxene and olivine 

grains up to 0.5 mm. Lava occurs as a single flow up to 5-m thick. 

Source vent is unknown. Whole-rock 40Ar/39Ar age is 26.17±0.07 Ma 

(RGR–494; table 2) from a sample south of Rio San Antonio near the 

western map boundary

Treasure Mountain Group, undivided (Oligocene)—Ignimbrite, air fall 

tuff, and volcaniclastic deposits associated with recurrent eruption and 

collapse of the Platoro caldera complex in the southeast San Juan 

Mountains. Within the map area, Treasure Mountain Group includes a 

distinct ledge of Chiquito Peak Tuff (Ttc) and a non-distinct, mostly 

covered slope stratigraphically between the Chiquito Peak Tuff and the 

underlying Conejos Formation. Undivided Treasure Mountain Group (Tt) 

includes predominantly volcaniclastic tuffaceous sedimentary deposits. 

Thickness of slope-forming unit below Chiquito Peak Tuff up to 60 m

Chiquito Peak Tuff—Pink to white, moderately welded, low-silica 

rhyolite to trachydacite ignimbrite. In areas proximal to the Platoro 

caldera, phenocryst content is 40–50 percent, but within the map area, 

along the Rio de los Piños, phenocryst content varies from 10–20 

percent. Phenocrysts include plagioclase, biotite, opaque minerals, and 

sanidine, ± augite (Lipman, 1975a; Lipman and others, 1996). Feldspar 

phenocrysts are equant to tablular and up to 1.5 mm. Biotite phenocrysts 

are up to 2 mm and are variably altered; grains with unaltered cores 

show pleochroism from light brown to greenish brown. Ignimbrite is 

only exposed along the Rio de los Piños in the northwestern part of map 

where it forms a single cooling unit up to 30 m thick. Single-grain, 

total-fusion age on sanidine from a sample collected along the Rio de los 

Piños is 28.94±0.31 Ma (RGR–466; table 2)

Conejos Formation (Oligocene)—Chiefly composed of gray matrix-supported

volcaniclastic debris flows. Clasts are predominantly dark-brown, 

porphyritic pyroxene-bearing andesitic and light-gray dacitic lavas. 

Clasts are angular to subangular and range from 1–75 cm. Unit exposed 

along the Rio de los Piños, but exposures are discontinuous in part 

because of colluvial debris from overlying poorly resistant Los Pinos 

Formation deposits. Thickness of unit in map area as much as 50 m but 

base is not exposed. Colucci and others (1991) report hornblende and 

biotite 40Ar/39Ar ages from Conejos Formation in the southeast San Juan 

Mountains between 33.5 and 29.5 Ma

Qa Qsw Qsa Qsp

Qf Qc

Tbx

Tauc Tau

? ?

Tlxc Tlx Trxc Trx

Tsr

Tle

Tht

Thb

Tsw

Tha

Thp

Tls

Tlc Thl

Tcbc Tcb Tbc Tb

Tan

Tas

Tsm

Tlb

Tcxc Tcx

Tca

Trlc

Tse

Tna

SURFICIAL DEPOSITS

OLDER REGIONAL

VOLCANIC AND 

VOLCANICLASTIC ROCKS

DEPOSITS OF THE

TAOS PLATEAU

VOLCANIC FIELD

Tcf

late

middle

early

11.7 ka

126 ka

781 ka

2.58 Ma

5.33 Ma

23.03 Ma

34.09 Ma

ALLUVIAL

DEPOSITS

ALLUVIAL

AND

POND

DEPOSIT

ALLUVIAL

AND

COLLUVIAL

DEPOSIT

COLLUVIAL

DEPOSIT

CORRELATION OF MAP UNITS

Holocene

Pleistocene

QUATERNARY

TERTIARY

Pliocene

Oligocene

Miocene

Trl

Tt

Ttc

SCALE 1: 50 000

1 1/ 2 0 1 2 3 4 MILES

3000 0 3000 6000 9000 12000 15000 18000 21000 FEET

1 .5 0 1 2 3 4 5 KILOMETERS

EXPLANATION OF MAP SYMBOLS

Contact—Solid where location is accurate; dashed where location is 

approximate

Contact separating individual lava flows within same map unit—Identity 

and existence certain, location accurate

Rim of volcanic crater—Solid where location is accurate, dashed where 

location approximate; queried where identity or existence is questionable.

Hachures point into crater

Lava flowline—Arrow indicates direction of lava flow, as inferred from vent 

locations and topography 

Normal fault—Solid where location is accurate; long dashed where location is 

approximate, short dashed where location is inferred, and dotted where 

location is concealed. Queried where identity or existence questionable. 

Ball and bar on downthrown block

PLANAR POINT FEATURES

Strike and dip of bedding

Inclined

Inclined, approximate

40Ar/39Ar geochronologic age sample location—With sample ID, preferred 

sample age shown in millions of years (Ma), and plus or minus (±) error 

(2 sigma [2σ]). See table 2

AREAL FEATURE

Quarry

RGR–012

2.92±0.12 Ma

6

10

RGR–362

4.36±0.04 Ma

RGR–364

4.36±0.05 Ma

RGR–466

28.94±0.31 Ma

RGR–146

4.37±0.05 Ma

RGR–148

25.54±0.24 Ma

RGR–149 (no.1)

3.39±0.09 Ma

RGR–371

4.37±0.03 Ma

RGR–226

4.24±0.10 Ma

RGR–014

4.31±0.07 Ma

RGR–374

3.39±0.04 Ma

RGR–221

3.32±0.06 Ma

RGR–012

2.94±0.12 Ma

RGR–010 (no. 1)

2.9±0.3 Ma

RGR–285

3.06±0.11 Ma

RGR–035

3.08±0.07 Ma

RGR–133

3.0±0.04 Ma RGR–235

3.2±0.05 Ma

RGR–15

3.36±0.05 Ma

RGR–036

4.55±0.07 Ma

RGR–494

26.17±0.07 Ma

RGR–231

2.91±0.07 Ma

RGR–013

3.33±0.14 Ma

RGR–09

2.78±0.05 Ma

Quarry

Tca

Tca

(dike)

Tca

Tca

Tca

Tsm

Tsr

Tsr

Tsr

Tsr

Tsm

Tsm

Tsm

Tsr

Tsm

Qa

Qa

Tcf Tcf Ttc?

Dike

(unit unidentified)

Ttc?

Ttc

Ttc

Ttc

Qsw

Tb

Tb

Tb

Tb

Tht

Tht

Tht

Tht

Tcb

Tcbc

Tcb

Tb

Tb

Thl

Tle

Tle

Trl

Trl

Trl

Trl

Trl

Trlc

Tle

Tle

Tle

Tle

Tle

Tle

Tle

Tle

Tt

Tt

Tt

Tle

Tle Tle

Tle

Tbc

Tbc

Tbc

Qsw

Qsw

Qsw

Qsw

Qsw

Tlx

Tlxc

Qsw Qsw

Qsw

Qsa

Qsa

Qsa

Qsa

Qsp

Qsp

Qsp

Tca

Tca

Tca

Tle

Tle

Tle

Tle

Tle

Tle

Tle

Tle

Qsp

Qsp

Tha

Tle

Qsp

Qsp

Qsa

Qsa

Qa

Qa

Qf

Tan

Tse

Tan

Tan

Qf

Qf

Qf

Qsa

Qsa

Qsa

Qsa

Qsa

Qf

Qf

Qa

Qc

Qc

Qsp

Qsw

Qsp

Qsp

Qc

Qc

Tbx

Tls

Tls

Qc

Qa

Qc

Tau

Tsw

Tsw

Tsw

Tau

Tau

Tauc

Tau

Tau

Qc

Qc

Qc

Tas

Tcx

Tcx

Tcxc

Tcxc

Tcx

Trx

Trx

Trxc

Tas

Qc

Qa

Qa

Qa

Qa

Tha

Thp

Thp

Thp

Thp

Thp

Thb

Thb

Thb Tlb

Tlb Tlb Tlb

Tlb

Thb

Thb

Tle

Tlc

Thb Tlc

Tlc

Tls

Tlc

Tna

Tlc

Tlc

Tlc

Tlc

Tlc

Tle

Tlc

Thb

Tlb?

(dikes)

Thb

Tha

Tha

(dike)

?

K.J. Turner and R.A. Thompson carried out mapping of 

bedrock units and structural features; M.A. Cosca and 

L.E. Morgan carried out 40Ar/39Ar geochronologic analysis; 

R.R. Shroba carried out mapping of unconsolidated 

surficial deposits and characterized Tertiary sedimentary 

units, and C.F. Chan assisted in field work and preparation 

for analytical work.

GIS database and digital cartography by K.J. Turner

Publishing support provided by the Science Publishing 

Network, Denver Publishing Service Center

Edit and digital layout by L. Binder

Manuscript approved for publication September 12, 2018

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Map 3417

NEW MEXICO

MAP LOCATION

COLORADO

Geologic Map of the San Antonio Mountain Area, Northern New Mexico and Southern Colorado

By

Kenzie J. Turner, Ren A. Thompson, Michael A. Cosca, Ralph R. Shroba, Christine F. Chan, and Leah E. Morgan

2018

Base modified from U.S. Geological Survey US Topo 7.5-minute quadrangles, 2017: San Antonio 

Mountain, N. Mex.; Los Pinos, N. Mex.-Colo.; Bighorn Peak, N. Mex.-Colo.; Pinabetoso Peaks,

N. Mex.-Colo; and La Segita Peaks, N. Mex., from The National Map, accessed August 24, 2017, at 

https://viewer.nationalmap.gov/launch/

North American Datum of 1983 (NAD 83)

Projection and 1,000-meter grid: Universal Transverse Mercator, zone 13N

Boundary of the Río Grande del Norte National Monument accessed on May 30, 2018 at

https://navigator.blm.gov/data?keyword=rio%20grande%20del%20norte%20national%20monument

CONTOUR INTERVAL 100 FEET

NATIONAL GEODETIC VERTICAL DATUM OF 1988

APPROXIMATE MEAN

DECLINATION, 2018 TRUE NORTH

MAGNETIC NORTH

81⁄2°

Divisions of Quaternary, Neogene, and Paleogene Time

Used in this Report1

Period or

subperiod Epoch Age

Quaternary

Holocene 0–11.7 ka

late 11.7–126 ka

Pleistocene middle 126–781 ka

early

Neogene Pliocene

Miocene

Oligocene

Paleogene Eocene

Paleocene

1

Ages of time boundaries are those of the U.S. Geological Survey Geologic Names 

Committee (2018) except those for the late-middle Pleistocene boundary and 

middle-early Pleistocene boundary, which are those of Gibbard and others (2010). 

Ages are expressed in ka for kilo-annum (thousand years) and Ma for mega-annum 

(million years).

781 ka–2.58 Ma

2.59–5.33 Ma

5.33–23.03 Ma

23.03–34.09 Ma

34.09–55.8 Ma

55.8–65.5 Ma

To learn about the USGS and its information products visit

http://www.usgs.gov/ 

1-888-ASK-USGS

For product and ordering information visit:

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ISSN 2329-132X (online)

https://doi.org/10.3133/sim3417

Any use of trade, product, or firm names in this publication is for descriptive 

purposes only and does not imply endorsement by the U.S. Government

This map or plate is offered as an online-only, digital publication. Users should 

be aware that, because of differences in rendering processes and pixel 

resolution, some slight distortion of scale may occur when viewing it on a 

computer screen or when printing it on an electronic plotter, even when it is 

viewed or printed at its intended publication scale

Conversion Factors

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch 

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

Area

square kilometer (km2

) 0.3861 square mile (mi2)

Volume

cubic kilometer (km3

) 0.2399 cubic mile (mi2)

SI to Inch/Pound

?

?

RIO GRANDE DEL NORTE

NATIONAL MONUMENT

Suggested citation: Turner, K.J., Thompson, R.A., Cosca, M.A., Shroba, R.R., 

Chan, C.F., and Morgan, L.E., 2018, Geologic map of the San Antonio Mountain 

area, northern New Mexico and southern Colorado: U.S. Geological Survey 

Scientific Investigations Map 3417, scale 1:50,000,

https://doi.org/10.3133/sim3417.

For more information concerning the research in this report, contact the 

Center Director, USGS Geosciences and Environmental Change Science Center

Box 25046, Mail Stop 980

Denver, CO 80225

(303) 236-5344

Or visit the Geosciences and Environmental Change Science Center website at

https://www.usgs.gov/centers/gecsc

Digital files available at https://doi.org/10.3133/sim3417 

ScienceBase data release GIS and other files that support this report are

available at https://doi.org/10.5066/F72N51M5

Altitude, as used in this report, refers to distance above the

 vertical datum.

COLORADO

NEW

MEXICO

Map

area

COLORADO

NEW

MEXICO

Map

area