Pamphlet to accompany

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Map 3179

Prepared in cooperation with the National Park Service 

Geologic Map of the Montoso Peak Quadrangle, Santa Fe 

and Sandoval Counties, New Mexico
Cover: View to the northeast of eroded cinder cone of Twin Hills, from 

northeast rim of Cerro Rito. Photograph by Ren A. Thompson, 1996. Insert: 

U.S. Forest Service twelve hundred water well.
Geologic Map of the Montoso Peak 

Quadrangle, Santa Fe and Sandoval 

Counties, New Mexico

By Ren A. Thompson, Mark R. Hudson, Ralph R. Shroba, Scott A. Minor, and 

David A. Sawyer

Prepared in cooperation with the National Park Service

Pamphlet to accompany Scientific Investigations Map 3179

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2011

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Suggested citation:

Thompson, R.A., Hudson, M.R., Shroba, R.R., Minor, S.A., and Sawyer, D.A., 2011, Geologic map of the Montoso Peak 

quadrangle, Santa Fe and Sandoval Counties, New Mexico: U.S. Geological Survey Scientific Investigations Map 

3179, 20 p. pamphlet, 1 sheet, scale 1:24,000.
iii

Contents

Introduction.....................................................................................................................................................1

Description of Map Units..............................................................................................................................1

Surficial Deposits..................................................................................................................................1

Artificial-Fill Deposits..................................................................................................................6

Alluvial Deposits...........................................................................................................................6

Alluvial and Colluvial Deposits ..................................................................................................7

Colluvial Deposits.........................................................................................................................7

Lava Flows and Related Deposits of the Cerros Del Rio Volcanic Field......................................8

Acknowledgments.......................................................................................................................................18

References Cited..........................................................................................................................................18

Figures

1. Location map and general geology of the Española Basin, adjacent basin, 

and volcanic fields of the Rio Grande rift .................................................................................2

2. Landsat 7 satellite image of Cerros del Rio, New Mexico area showing 

geologic features and select geographic names....................................................................3

3. Interpreted lithologic column from U.S. Forest Service Twelve Hundred 

well driller’s log .............................................................................................................................4

4. Landsat 7 image of Cerros del Rio volcanic field showing approximate 

eruptive extent, mapped vent areas, and presumed buried vents based on 

aeromagnetic data .......................................................................................................................5

5. Total alkali vs. silica classification diagram for volcanic rocks exposed in the 

map area and adjacent areas.....................................................................................................8

Tables

1. Map units, eruption ages, and volcanic rock names for the Cerros del Rio 

volcanic field. Map sheet....................................................................................see map sheet

2. Whole rock major oxide and trace element analysis of representative 

Cerros del Rio volcanic field samples ...............................................................see map sheet

3. Location coordinates for Cerros del Rio volcanic field samples..........................................9

Map Sheet

1. Geologic map of the Montoso Peak Quadrangle, Santa Fe and Sandoval

Counties, New Mexico.............................................................................................................link
iv

Conversion Factors

SI to Inch/Pound

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

Divisions of Quaternary, Neogene, and Paleogene time 

used in this report1

Period or 

Subperiod Epoch Age

Holocene 0—11.7 ka

Quaternary

late 11.7—132 ka

Pleistocene middle 132—788 ka

early 788 ka—2.59 Ma

Neogene Pliocene 2.59—5.33 Ma

Miocene 5.33—23.0 Ma

1

Ages of time boundaries are those of the U.S. Geological Survey Geologic Names Committee (2010) except those 

for the late-middle Pleistocene and middle-early Pleistocene boundaries, which are those of Richmond and Fullerton 

(1986). Ages are expressed in ka for kilo-annum (thousand years) and Ma for mega-annum (million years).
Introduction

The Montoso Peak quadrangle is underlain by volcanic 

rocks and associated sediments of the Cerros del Rio volcanic 

field in the southern part of the Española Basin (figs. 1–3) 

that record volcanic, faulting, alluvial, colluvial, and eolian 

processes over the past three million years. The geology was 

mapped from 1997 to 1999 and modified in 2004 to 2008. 

The geologic mapping was carried out in support of the 

U.S. Geological Survey (USGS) Rio Grande Basins Project, 

funded by the USGS National Cooperative Geologic Mapping Program. The primary mapping responsibilities were as 

follows:Thompson and Hudson mapped the volcanic deposits 

and faults and prepared the digital compilation of the geologic 

map, Shroba mapped the surficial deposits, Mark R. Hudson 

and Scott A. Minor mapped surface exposures of faults, 

Hudson conducted paleomagnetic studies for stratigraphic correlations, and David A. Sawyer assisted with unit correlations 

with exposures on the adjoining Tetilla Peak quadrangle to the 

south (Sawyer and others, 2002).

The mapped distribution of units is based primarily on 

interpretation of 1:16,000-scale, color aerial photographs 

taken in 1992, and 1:40,000-scale, black-and-white, aerial 

photographs taken in 1996. Most of the contacts on the 

map were transferred from the aerial photographs using a 

photogrammetric stereoplotter and subsequently field checked 

for accuracy and revised based on field determination of 

allostratigraphic and lithostratigraphic units. Determination 

of lithostratigraphic units in volcanic deposits was aided 

by geochemical data, 40Ar/39Ar geochronology, aeromagnetic and paleomagnetic data (Thompson and others, 2006). 

Supplemental revision of mapped contacts was based on 

interpretation of USGS 1-meter orthoimagery. This version of 

the Montoso Peak quadrangle geologic map uses a traditional 

USGS topographic base overlain on a screened shaded relief 

base generated from 10-m digital elevation model (DEM) data 

from the USGS National Elevation Dataset (available at 

http://ned.usgs.gov/); sun illumination is from the northwest 

at 45° above the horizon.

Faults are identified with varying confidence levels in the 

map area (fig. 4). Recognizing and mapping faults developed 

near the surface in young, brittle volcanic rocks is difficult 

because (1) they tend to form fractured zones tens of meters 

wide rather than discrete fault planes, (2) the youth of the 

deposits has allowed only modest displacements to accumulate 

for most faults, and (3) many may have significant strike-slip 

components that do not result in large vertical offsets that are 

readily apparent in offset of sub-horizontal contacts. Those 

faults characterized as “certain” either have distinct offset of 

map units or had slip planes that were directly observed in the 

field. Faults classed as “inferred” were traced based on linear 

alignments of geologic, topographic and aerial photo features 

such as vents, lava flow edges, and drainages inferred to preferentially develop on fractured rock. Lineaments defined from 

magnetic anomalies form an additional constraint on potential 

fault locations.

Description of Map Units

[Color of units from Munsell Color, 1973]

Surficial Deposits

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

(North American Commission on Stratigraphic Nomenclature, 

1983), whereas the other map units are informal lithostratigraphic units. For this reason, subdivisions of stratigraphic 

units use time terms “late” and “early” where applied to surficial units, but use position terms “upper” and “lower” where 

applied to lithostratigraphic units. The mapped surficial deposits (Qba and younger) are known or estimated to be at least 

1 m thick. Most of these deposits are poorly exposed. Thin 

(< 50 cm), discontinuous sheetwash deposits (Qsw) locally 

mantle gently sloping map units. We did not map deposits 

that are (1) of limited extent (less than about 25 m wide), (2) 

exposed in steep cuts, and (3) fill material (unit af) that was 

not observed on aerial photographs or orthoimagery.

Age assignments for surficial deposits are based chiefly 

on the (1) relative degree of modification of their original 

surface morphology, (2) relative heights above modern stream 

channels, and (3) degree of soil development. Soil-horizon 

Geologic Map of the Montoso Peak Quadrangle, Santa Fe 

and Sandoval Counties, New Mexico

By Ren A. Thompson, Mark R. Hudson, Ralph R. Shroba, Scott A. Minor, and David A. Sawyer
2 Geologic Map of the Montoso Peak Quadrangle, Santa Fe and Sandoval Counties, New Mexico

25

40 25

Abiquiu

embayment

Chama

Rio

JEMEZ

MTS Basin

Española

Grande

Basin

Albuquerque

Sangre de Cristo

Mountains

Sandia Mts

Nacimiento Mts

Española

Valles

caldera

Rio

LB

JZ

Rio

Grande

LB

PJ

EB

EB

Rio

Jemez

CdRvf

TPvf

SF

Toledo

embayment

Taos

SANTA FE

SAMvf

Albuquerque

San

Luis

Basin

0 10 20 30 MILES

0 10 20 30 KILOMETERS

35°00'

35°30'

36°00'

107°00' 106°30' 106°00' 105°30'

36°30'

Montoso

Peak Agua

Fria

Cochiti

Dam

White Rock

Tetilla

Peak

Rio Grande

Rift

NEW

MEXICO

Area of 

fig. 1

EXPLANATION

25 Interstate highway

Reverse Fault—Dotted where concealed

Fault—Bar and ball on downthrown side

Contact

Precambrian rocks

Lower Oligocene–Paleozoic rocks

Pleistocene-Miocene volcanic rocks

Pleistocene-Pliocene basaltic volcanic rocks

Holocene-Oligocene sediments

Figure 1. Location map and general geology of the Española Basin, adjacent basin, and volcanic fields 

of the Rio Grande rift. Geology modified from Smith and others (2001, fig. 3). Approximate location of the 

Toledo caldera (Toledo embayment) is modified from Gardner and Goff (1996). Volcanic fields: CdRvf, 

Cerros del Rio volcanic field; SAMvf, Santa Ana Mesa volcanic field; TPvf, Taos Plateau volcanic field. 

Faults: EB, Embudo fault, JZ, Jemez fault; LB, La Bajada fault; PJ, Pajarito fault; SF, San Francisco fault.
Description of Map Units 3

Figure 2. Landsat 7 satellite image (30-m band 7-4-2 merged with 15-m band 80 acquired on October 14, 1999, 

of Cerros del Rio, New Mexico area (image clip from Sawyer and others, 2004), showing geologic features 

and select geographic names. Volcanic rocks and sediments of the Cerros del Rio volcanic field are visible 

throughout the image, dark-blue river in the northwest part of the image is the Rio Grande, light-orange area 

and linear feature in the southeast part of the image are a cinder quarry and haulage road, light-green area in 

the southeast corner of the image is a recreation area. Yellow rectangle is the boundary of the Montoso Peak 

7.5-minute quadrangle.

106°15’00” 106°07’30”

35°37’30”

35°45’00”

Cerrita

Portillo

Cerrito

Pelado

Montoso

Peak

Ortiz

Mountain

Twin

Hills Cerro

Micho

Colorado

Peak

Thirty-one Draw

Cerrita

Portillo

Cerrito

Pelado

Montoso

Peak

Ortiz

Mountain

Twin

Hills Cerro

Micho

Colorado

Peak

Thirty-one Draw

ROCK

CAN YON

SANTA FE

Cochiti NATIONAL FOREST

volcano

Rio

Grande

Boundaries modified from U.S. Geological Survey, Montoso Peak, NM, 1:24,000-scale map, 2002.

COCHITI INDIAN

RESERVATION

0 1 2 3 4 5 MILES

0 1 2 3 4 5 KILOMETERS
4 Geologic Map of the Montoso Peak Quadrangle, Santa Fe and Sandoval Counties, New Mexico

Twelve Hundred Well (water)

clay

basalt

clay & cinders

red

alluvium +

basalt

cinders

gray, hard

basalt

red, 

yellow

sandy silt

+ black cinders

basalt

basalt

basalt

basalt

basalt

clay

26

50

76

86 10

79

155

151

306

107

413

27

440

157

447

604

26

630

130

762

ash 800

840

880

988

38

28

sandy silt + gravel

prod. zone 102

40

108

1,054

1,090

25

48

sandy silt - 

1,204 prod. base 1,207

15

23

184

192

clay

125

256

268

301

307

354

368

93

47

332

340

812

elevation = 5,718 ft

Lithology

clay + gravel

sand + gravel

sandy silt, interbedded 

cobble gravel

sand

41

50

1,115

1,163

Dept

 (m)

Depth

(ft)

Thickness

(ft)

SWL = -1,007

main

producing

zone

Figure 3. Interpreted lithologic column from U.S. Forest Service Twelve Hundred well 

driller’s log (modified from Sawyer and Minor, 2006). Eight horizons identified as basalt 

represent lava flows, flow packages and associated pyroclastic deposits. Interbedded 

clastic rocks and fine-grain deposits are relatively minor except for the drilling interval 

93–125 m. Interpreted penetration of Santa Fe Group sediments occurred at 256 m 

depth, static water level (SWL) occurs at a depth of 307 m. 
Description of Map Units 5

Figure 4. Landsat 7 image (30-m 

band 7-4-2 merged with 15-m band 80 

acquired on October 14, 1999; image 

clip from Sawyer and others, 2004) of 

Cerros del Rio volcanic field showing 

approximate eruptive extent as purple 

dashed line, mapped vent areas as 

red stars, and presumed buried vents 

based on aeromagnetic data as yellow 

stars. Geographic abbreviations: LA, Los 

Alamos; WR, White Rock; CL, Cochiti 

Lake; BM, Buckman Mesa; OM, Ortiz 

Mountain; MP, Montoso Peak; CM, Cerro 

Micho; TH, Twin Hills; CP, Cerro Portillo; 

CC, Cerro Colorado; CV, Cochiti Volcano; 

TP, Tetilla Peak; MJ, Mesita de Juana. 

Major mapped faults and lineaments 

are shown in red and aeromagnetic 

lineaments related to buried faults are 

shown in purple. Fault abbreviations: PJ, 

Pajarito fault; SF, San Francisco fault; LB, 

La Bajada fault; TT, Tetilla fault; CM, Cerro 

Micho fault; CC, Cerro Colorado fault; 

CO, Cochiti fault; TH; Twin Hills fault; CP, 

Cerro Portillo fault; LT, Las Tetillitas fault; 

AC, Arroyo Calabasas fault. Quadrangle 

outline in yellow.

106°15’ 106°07’30”

35°45’

35°37’30”

PJ

SF

LB

CO

CC

CM

CP

TH

AC

LB

TT

LT

0

0

5 KILOMETERS

5 MILES

LA

Pajarito

Plateau WR Rio

Grande

BM

Cañada

Ancha

Cañada

Ancha

TM

MP

CM

CP CV

CC

TH

Santa Fe

Airport

TP

CL

Santa Fe

River

MJ
6 Geologic Map of the Montoso Peak Quadrangle, Santa Fe and Sandoval Counties, New Mexico

designations are based on those of Guthrie and Witty (1982), 

Birkeland (1999) and the Soil Survey Staff (1999). Some of 

the surficial deposits contain secondary calcium carbonate of 

pedogenic (soil) origin. Stages of secondary calcium carbonate morphology (referred to as stages I through III in Bk, 

Bkb, Btkb, K, and Kb soil horizons) are from Gile and others 

(1966) and Machette (1985). Thin (less than 1 m) deposits of 

volcanic rubble that locally overlie basaltic and andesitic lava 

flows commonly are cemented by calcium carbonate having 

stage IV morphology.

In this report, the terms “alluvium” and “alluvial” refer 

to material 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 material transported on slopes chiefly by mass-wasting (gravity-driven) 

processes—such as creep, debris flow, and rock fall—aided by 

running water not confined to channels (sheetwash) (Hilgard, 

1892; Merrill, 1897). Surficial map units that include debrisflow deposits probably also include hyperconcentrated flow 

deposits. These latter deposits are intermediate in character 

Artificial-Fill Deposits

af	 Artificial fill (latest Holocene)—Uncompacted mine tailings and storage piles composed of dark red 

(10R about 3/6), cinder, spatter, and agglutinate excavated from basaltic pyroclastic deposits at 

Cerrito Pelado, near the southeastern corner of the map area. Locally may include in-place 

pyroclastic depositis. Estimated thickness less than 15 m

Alluvial Deposits

Qg Gravelly alluvium (Holocene? and late Pleistocene)—Small, scattered deposits of gravelly stream alluvium that form small low hills along and near Arroyo Tetilla in the southern part of the map area. 

Unit contains deposits of pebbly and cobbly sandy gravel and pebbly and cobbly silty sand. The 

top of the unit is capped by a gravelly lag that is about 5 m or less above the adjacent channels of 

intermittent streams. Boulders as long as 1 m are locally present in the gravelly lag. Some deposits 

of Qg may be gravelly facies of Qac. Estimated thickness 5–10 m

Qsw Sheetwash deposits (Holocene to middle? Pleistocene)—Slightly pebbly to pebbly and cobbly, 

slightly silty to silty, very fine to medium sand. The matrix is slightly to moderately plastic. Some 

of the silt- to fine sand-size fraction in these deposits may be of eolian origin and have been transported by strong southwesterly winds that deflated sediment from former sparsely vegetated or actively aggrading alluvial deposits. Some of the silt- and clay-size fraction in Qsw may have been 

derived from more distant sources (Shroba and Thompson, 1998). Sediments in Qsw were deposited chiefly by unchannelized surface flow and, possibly in part, by gravity-driven slope processes. 

Unit Qsw locally contains rare fragments of reworked pumice from the 1.61 Ma Guaje(?) Pumice 

Bed of the Otowi Member of the Bandelier Tuff (Bailey and others, 1969; Izett and Obradovich, 

1994) and possibly pumice (Qec) from younger eruptions in the nearby Jemez Mountains, about 

25 km northwest of the map area (fig. 1). The unit also locally includes small deposits of undivided 

alluvium and colluvium (Qac) and basaltic alluvium (Qba). Although loess locally mantles lava 

flows about 50–60 ka in the Valles caldera (Wolff and others, 1996) (fig. 1) and rock-glacier debetween stream-flow and debris-flow deposits. The term 

“plastic” in the descriptions of sheetwash unit (Qsw) refer to 

the extent to which moist sediment changes shape continuously under the influence of an applied stress and that retain 

the impressed shape on the removal of the stress (Soil Survey 

Staff, 1951).

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 mm in 

diameter, whereas the term “matrix” refers to particles smaller 

than 2 mm in diameter. Most of the clasts in surficial deposits 

within the map area are angular and subangular fragments 

derived chiefly from basaltic and andesitic lava flows and 

pyroclastic deposits. 

Colors of the surficial deposits were determined by comparison of dry matrix with Munsell Soil Color Charts (Munsell 

Color, 1973). These colors generally are similar to those of 

the sediment and volcanic rock from which they were derived. 

The colors of the surficial deposits commonly range from 

white (2.5Y 8/1) to light reddish brown (7.5YR 6/4).
posits about 10–14 ka near Lake Peak in the Sangre de Christo Range (Shroba, 1977, 1987), loess 

deposits were not observed within or on Qsw. Surface soils formed in Qsw in the adjacent Agua 

Fria and Tetilla Peak quadrangles (Shroba and others, 2006; Sawyer and others, 2002) locally 

have Bw, Bt/Bk, or Btk/K horizon morphology depending in part on the age of the deposit. Bt 

and Btk horizons have thin clay films on ped faces. Bt horizons are 20–70 cm thick; Btk horizons 

are 10–45 cm thick and have stage I–II carbonate morphology. Bk horizons are as much as 80 cm 

thick and have stage I to II carbonate morphology. K horizons are as much as 75 cm thick and 

have stage III carbonate morphology. These properties suggest that some of the more strongly 

developed soils are formed in deposits that may be about 120–160 ka (Shroba and Birkeland, 

1983; Nelson and Shroba, 1998; Birkeland and others, 2003). Although undifferentiated, younger 

deposits locally overlie older deposits of Qsw. Older sheetwash deposits are locally greater than 

2 m thick and have upper surfaces marked by buried Bkb, Btkb, or Kb horizons greater than 30 

cm thick. Presence of surface and buried soils formed on and in Qsw indicate periodic deposition 

followed by surface stability and soil development. The last major episode of deposition may have 

been associated with enhanced summer rainfall during the early Holocene (Reneau and others, 

1996b). Low-lying areas of Qsw are susceptible to sheet flooding due to unconfined overland 

flow, and locally to stream flooding and gullying. Disturbed surface of Qsw may be susceptible to 

minor wind erosion. Exposed thickness 1.5–4.5 m; possibly as much as 10 m 

Alluvial and Colluvial Deposits

Qac Alluvium and colluvium, undivided (Holocene and late Pleistocene?)—Chiefly undifferentiated 

flood plain and stream channel deposits, sheetwash (Qsw), and minor fan alluvium and debrisflow deposits, undivided (Qfd). Unit ranges from slightly pebbly, silty sand to poorly sorted, clastand matrix-supported, locally bouldery, cobbly and pebbly gravel with a sandy matrix. Low-lying 

areas of the map unit are prone to periodic flooding and locally to debris-flow deposition. Deposits composed of moderately well sorted silty sand are prone to gullying. Thickness possibly about 

1–10 m

Qfd Fan alluvium and debris-flow deposits, undivided (Holocene to middle? Pleistocene)—Clast-andmatrix supported, locally bouldery, pebbly and cobbly gravel with a sandy matrix, and locally 

pebbly and cobbly, slightly silty sand that contains gravel lenses. The unit commonly forms 

fan-shaped and lobate masses of sediment deposited at and below the mouths of narrow valleys. 

Sediments were deposited by sediment-charged, intermittent streams and debris flows. Levees 

that contain cobbles and boulders are locally present on debris flows. Small deposits of unmapped 

sheetwash (Qsw) locally overlie unit Qfd. Soils formed in younger deposits have Bk horizons; 

those formed in older deposits have thin (about 30 cm) K horizons with strong stage III carbonate 

morphology. Low-lying areas of the map unit adjacent to stream channels are prone to periodic 

flooding and debris-flow deposition. Maximum thickness possibly about 20 m

Colluvial Deposits

Qc Colluvium, undivided (Holocene to middle? Pleistocene)—Deposits of unsorted 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 soil-creep, debris-flow, and rock-fall deposits, as defined by Varnes (1978) and 

Cruden and Varnes (1996). Unit locally includes deposits of sheetwash (Qsw) and alluvium and 

colluvium, undivided (Qac) that are too small to map separately. Unit Qc may be susceptible to 

continued movement or reactivation due to natural and human-induced processes. Some of the 

rock fragments in unit Qc are as much as 3 m in diameter; some of the sand in the unit probably 

was deposited as sheetwash and debris flows. Maximum thickness possibly about 20 m

Description of Map Units 7
8 Geologic Map of the Montoso Peak Quadrangle, Santa Fe and Sandoval Counties, New Mexico

Lava Flows and Related Deposits of the Cerros 

Del Rio Volcanic Field

The Cerros del Rio volcanic field is a predominantly 

basaltic to andesitic volcanic plateau along the southeast flank 

of the Jemez Mountains (fig. 1) of northern New Mexico. 

Lavas and related pyroclastic deposits of this field are locally 

exposed over 700 km2

 and reflect eruption of at least 120 km3

of rift-related mafic magma, mainly between 2.7 and 1.1 Ma 

(table 1, see map sheet). Most of the lava flows are of Pliocene 

and early Pleistocene age and predate large-volume silicic 

caldera eruptions in the nearby Jemez volcanic field; however, 

late-stage eruptions from the Cerros del Rio volcanic field 

post-date eruption of the Tshirege Member (1.2 Ma; Valles caldera) of the Bandelier Tuff in the Jemez volcanic field. Most 

of the eruptive centers in the Cerros del Rio volcanic field are 

central-vent volcanoes ranging from low-relief shield centers 

to remnants of steep-sided, breached cinder cones. These lavas 

have from 49 to 64 weight percent SiO2

 and exhibit a strong 

correlation between landform and whole-rock chemistry 

(fig. 5; table 2, see map sheet; and table 3). The low-silica, 

subalkaline basaltic lavas erupted from broad shield volcanoes 

and formed thin (< 3–4 m) low-viscosity flows that traveled 

far; conversely transitional to mildly alkaline basalts and 

basaltic andesites formed thick (as much as 30 m) discontinuous lavas flows that erupted from high-relief, steep-sided, 

dissected vents. Dacitic lavas are related to late-stage dome 

growth and eruption of thick (as much as 50 m), even more 

viscous blocky lava flows that issued from one well-defined 

vent area at Tetilla Peak about 2 km south of the map area 

(Sawyer and others, 2002) and locally in tributary canyons of 

the Rio Grande, west of the map area (Dethier, 1997).

EXPLANATION

Montoso Peak

quadrangle lavas

Other CDRVF lavas

Weight percent Na2O+K2

O

Weight percent SiO2

0

2

4

6

8

10

12

40 45 50 55 60 65 70

trachy- trachydacite

andesite

basaltic

trachyandesite

dacite

andesite

basaltic

basalt andesite

trachybasalt

Figure 5. Total alkali vs. silica 

classification diagram (Le Bas and 

others, 1986) for volcanic rocks exposed 

in the map area (filled diamonds) and 

adjacent areas (open squares). CDRVF, 

Cerros del Rio volcanic field.

Volcanic deposits of the Cerros del Rio volcanic field 

are divided into a three-fold classification representing early, 

middle, and late phases of eruption that persisted from about 

2.7 Ma to 1.1 Ma. Volcanic units in the map area are believed 

to represent products of monogenetic eruptive centers of the 

early (>2.7–2.59 Ma), middle (2.59–2.1 Ma) and late (1.5–1.1 

Ma) volcanic phases of Thompson and others (2006). These 

subdivisions are based on 1:24,000-scale geologic mapping, 

stratigraphic studies, 40Ar/39Ar geochronology, and paleomagnetic and aeromagnetic data. Integration of these datasets results in approximate eruption ages reflected in table 1. 

Some geochemical data were used to identify rock lithologies 

within the stratigraphic subdivisions for the entire field and 

were incorporated in unit descriptions, where available, for 

volcanic deposits in the map area. Volcanic rock names are 

based on the standard IUGS classification scheme (Le Bas 

and Streckeisen, 1991); however, only root names are used 

in map unit description. Some volcanic units contain rocks 

of variable composition, spanning classification boundaries. 

In these cases, the dominant rock type was used for the unit 

name and ranges of compositions are presented in table 1. 

Preliminary results of new 40Ar/39Ar age determinations are 

presented in unit descriptions. In places, map units represent 

the consolidation of deposits of limited extent and are based 

on similar lithologic character, stratigraphic position, inferred 

age, aeromagnetic and paleomagnetic signatures, and aerial 

extent of similar or related deposits. Map-unit nomenclature 

for the volcanic deposits in part reflects that proposed by 

Aubele (1979). Colors of the lava flows and cinder deposits 

used in unit descriptions were determined by comparison with 

a Geological Society of America Rock-Color Chart (RockColor Chart Committee, 1995).
Description of Map Units 9

Table 3. Location coordinates for Cerros del Rio volcanic field samples.

[Geographic Coordinate System NAD 27 Datum]

Sample No. Quadrangle Map Unit Latitude Longitude

96TTP01 Tetilla Peak Tat 35°36'29" 106°12'33" 

04TMP23 Montoso Peak Tcm 35°44'22" 106°14'47" 

6RTCD10 Montoso Peak Tcm 35°42'52" 106°14'50" 

3MRG–1 Montoso Peak Tbcc 35°42'56" 106°11'16" 

96TMP18 Montoso Peak Tbc 35°42'50" 106°11'01" 

96DTP04 Tetilla Peak Tbs 35°37'11" 106°08'15"

96TTH02A Turqoise Hill Tbs 35°37'01" 106°07'29" 

02TMP02 Montoso Peak Tayc 35°39'22" 106°10'15" 

2MRG–11 Montoso Peak Tay 35°39'02" 106°10'24" 

96TMP38 Montoso Peak Tcac 35°39'49" 106°13'10" 

96TMP20 Montoso Peak Tca 35°40'33" 106°12'47"

96TMP24 Montoso Peak Tca 35°38'24" 106°13'49" 

MP3260601 Montoso Peak Tcp 35°41'38" 106°09'48" 

96TMP04 Montoso Peak Tcp 35°41'56" 106°09'11" 

96TMP36 Montoso Peak Tcp 35°40'01" 106°09'56" 

96TMP08 Montoso Peak Tm 35°44'10" 106°12'54" 

96TMP07 Montoso Peak Tm 35°43'50" 106°13'38" 

96TMP35 Montoso Peak Tbcl 35°39'07" 106°09'16"

04TMP21 Montoso Peak Tbcl 35°39'03" 106°09'17" 

04TMP22 Montoso Peak Tbcl 35°39'11" 106°09'14" 

96TMP27 Montoso Peak Ttx 35°43'52" 106°09'33" 

2MRG–2 Montoso Peak Qbu 35°44'12" 106°08'22" 

96TMP15 Montoso Peak Qbu 35°44'11" 106°08'21" 

4MRG–17 Montoso Peak Qbx 35°37'46" 106°11'08"

96TMP09 Montoso Peak Qbx 35°38'45" 106°10'10" 

MP3270601 Montoso Peak Qae 35°42'06" 106°14'24" 

05TMP01 Montoso Peak Qae 35°42'30" 106°12'50" 

96TMP22a Montoso Peak Qbt 35°38'00" 106°13'46" 

96TMP22b Montoso Peak Qbt 35°38'00" 106°13'46" 

2MRG–5 Montoso Peak Qcb 35°38'34" 106°13'36" 

3MRG–2 Montoso Peak Qcb 35°39'52" 106°13'45" 

MP3260602 Montoso Peak Qto 35°42'22" 106°09'52"

96TMP03 Montoso Peak Qto 35°41'53" 106°09'09"

96TMP17 Montoso Peak Qo 35°44'56" 106°10'30" 

96TMP14 Montoso Peak Qo 35°44'17" 106°08'25"

3MRG–8 Montoso Peak Qtd 35°43'56" 106°11'14" 

96TMP19 Montoso Peak Qtd 35°42'22" 106°11'20" 

04TMP24 Montoso Peak Qtdc 35°42'45" 106°12'03" 

96DCD01 Cochiti Dam Qb 35°37'56" 106°16'04" 

96TCD01 Cochiti Dam Qb 35°38'03" 106°15'36"

6RTCD4 Cochiti Dam Qcl 35°39'35" 106°18'00" 

DN–96–28b Cochiti Dam Qcl 35°39'52" 106°17'55" 

97TMP50 Montoso Peak Qcu 35°40'32" 106°14'37"

3MRG–4 Cochiti Dam Qcu 35°41'20" 106°16'58" 
Qec El Cajete Member of the Valles Rhyolite (late Pleistocene)—Small scattered deposits of white, 

coarse sand- to small pebble-size (0.5–12 mm) pumice fragments that locally mantle hill slopes in 

the northern part of the map area. The pumice is about 50–60 ka and was erupted from the nearby 

Valles caldera (Bailey and others, 1969; Reneau and others, 1996a), about 20 km northwest of the 

map area (fig. 1). Some or much of Qec was redeposited by alluvial and possibly by mass-movement or eolian processes. Maximum thickness possibly about 5 m

Qba Basaltic alluvium (early? to middle? Pleistocene)—Several poorly exposed deposits of locally 

bouldery, cobbly and pebbly gravel with a sandy matrix and pebbly sand are exposed near the 

eastern boundary of the map area. Clasts are commonly subangular and consist of basalt, basaltic 

andesite, and andesite. The unit locally contains carbonate-cemented lenses composed of pumice 

fragments about 2 mm to 10 cm in diameter. This pumice was probably derived from the nearby 

Jemez Mountains. Likely sources of the pumice are the 1.22 Ma Tsankawi Pumice Bed of the 

Tshirege Member of the Bandelier Tuff, and the 1.61 Ma Guaje Pumice Bed of the Otowi Member of the Bandelier Tuff (Bailey and others, 1969; Izett and Obradovich, 1994). Unit Qba is 

mostly stream alluvium, but also probably includes fan alluvium and debris-flow deposits as well 

as sheetwash deposits. Unit Qba is equivalent to the alluvium on basalt of Spiegel and Baldwin 

(1963). Thickness possibly about 3–10 m 

Andesite of Cochiti Volcano (early Pleistocene)—Reddish brown to medium gray lava flows and oxidized 

cinder and spatter erupted from Cochiti Volcano, which is on the Cochiti Dam quadrangle, near 

the southwestern boundary of the map area. Cochiti Volcano deposits post-date down-to-west offset along the Cochiti fault, whose footwall scarp formed a topographic barrier to eastward deposition of lava flows. Unit consists of an upper and lower sequence of preserved lava flows and near 

vent pyroclastic material associated with the upper unit. Vent area for upper unit is coincident with 

eroded, conical summit of Cochiti Volcano. Maximum exposed thickness is 40 m, but deposits 

thicken dramatically to the west in the adjacent quadrangle

Qcuc Upper unit, cinder deposits—Near-vent pyroclastic deposits, predominantly scoria and spatter agglutinate and minor flow material. Deposits are only present along the western edge of the map area

Qcu Upper unit, lava flows—Medium to dark gray andesite lava flows (59–62 weight percent SiO2); typically thin (3–5 m), platy and discontinuous and are everywhere overlain by near-vent deposits, 

predominantly cinder and lesser amounts of spatter. Sparse phenocrysts include olivine, pyroxene 

and plagioclase in decreasing order of abundance. An 40Ar/39Ar isotopic age of 1.51±0.05 Ma was 

obtained from a lava flow near the base of the section on the northwest side of Cochiti Cone in the 

adjacent Cochiti Dam quadrangle. These flows have reverse aeromagnetic and paleomagnectic 

signature

Qcl Lower unit—Medium to dark gray andesite lava flows (53–59 weight percent SiO2

), typically thin, 

less than 3 m in map area but locally thicken in adjacent Cochiti Dam quadrangle. Sparse phenocrysts include olivine, pyroxene and plagioclase in decreasing order of abundance; slightly higher 

proportions of olivine than in Qcu. Flows are interpreted to have reversed magnetic polarity based 

on aeromagnetic signature. Poorly exposed, maximum thickness 40 m, base not exposed 

Basalt of Hill 7033 (early Pleistocene)—Lava flows and associated near-vent deposits erupted from small, 

eroded cinder and spatter cones in southeast part of map area. Part of a chain of northeast-trending 

vents that coincides with mapped lineament inferred to be one of several north to northeast trending faults in the map area. Exposed thickness typically 2–25 m, thicker near vent areas

Qbxc Cinder deposits—Dissected cinder cones composed of near vent pyroclastic deposits, predominantly 

scoria, spatter agglutinate and minor flow material

10 Geologic Map of the Montoso Peak Quadrangle, Santa Fe and Sandoval Counties, New Mexico
Qbx Lava flows—Thin (<3 m), medium gray basalt lava flows (47–49 weight percent SiO2

) exposed in 

upper reaches of Arroyo Calabasas, on the adjacent Turquoise Hill quadrangle, in the southeast 

part of the map area. Flows are laterally discontinuous with massive interiors and vesiculated 

flow tops, largely covered with sheet wash alluvium. Olivine phenocrysts (1–2 mm) are common 

in a fine-grained microcrystalline groundmass. An 40Ar/39Ar isotopic age of 2.03±0.35 Ma was 

obtained from a lava flow south of the map area on the adjacent Tetilla Peak quadrangle (Sawyer 

and others, 2002). Reversed paleomagnetic and aeromagnetic signature

Andesite of Ortiz Mountain (early Pleistocene)—Massive andesite lava flows composed of a lower aerially 

extensive, basal crystal-rich lava flow and discontinuous near-vent deposits on the upper flanks 

of Ortiz Mountain. Ortiz Mountain is the southernmost of a series of andesite eruptive centers of 

largely unknown compositional range outside of the map area in the adjacent White Rock Canyon 

quadrangle (Dethier, 1997) 

Qoc Cinder deposits—Near-vent pyroclastic deposits, predominantly scoria, spatter agglutinate and minor 

flow material

Qo Lava flows—Medium-gray to black-glassy, blocky andesite lava flows (59–63 weight percent SiO2

) 

containing abundant phenocrysts of plagioclase and pyroxene, and locally minor olivine, hornblende and opaque minerals. An 40Ar/39Ar isotopic age of 2.32±0.06 Ma (sample 96TMP14) was 

obtained from near the southeastern extent of the basal lava flow. Reverse aeromagnetic and 

paleomagnetic signature. Maximum exposed thickness is 225 m

Basaltic andesite of Twin Hills (early Pleistocene)—Comprises three adjacent, dissected vent areas and 

associated lava flows in the Twin Hills and Cerro Rito areas. Flank-lava flows from the Twin Hills 

vent complex and the unnamed eruptive center 1 km to the south largely flowed northeast. Exposures of paleovalley-filling lavas erupted along Thirty-One Draw may include petrologically similar flows from both the Twin Hills and Cerro Rito eruptive centers. The Twin Hills and unnamed 

centers to the south are cut by northeast trending dikes. The Cerro Rito center is characterized by 

a prominent breached summit crater cut by discontinuous northwest trending dikes.

Qtoc Cinder deposits—Near vent pyroclastic deposits, predominantly scoria, spatter agglutinate, locally 

interbedded with medium gray to red oxidized lava flows

Qto Lava flows—Thin (<3 m) medium-gray lava flows (50–53 weight percent SiO2) forming cliff exposures along Thirty-One Draw. Lava flows thin to the north and were likely sourced near the current head of Thirty-One Draw and were restricted to deposition in a shallow paleovalley bordered 

on the east by Thirty-One Draw. Flows contain olivine and clinopyroxene phenocrysts (2–3 mm) 

and sparse glomerocrysts of olivine + pyroxene. Thin platy fracturing is common, particularly 

near the base of lava flows. An 40Ar/39Ar isotopic age of 2.53±0.04 Ma (sample 2TMP01) was 

obtained from a northeast-trending dike on the western flank of Twin Hills. Maximum exposed 

thickness in the Thirty-One Draw area is 20 m. Near vent deposits are medium-gray to red oxidized vesicular lava flows and blocky erosional flow remnants intercalated with spatter agglutinate 

and scoria in the Twin Hills and Cerro Rito areas. Flows contains ubiquitous olivine phenocrysts 

(< 1 mm) and resorbed plagioclase and quartz xenocrysts (1.3 mm) in a fine-grained holocrystalline groundmass. Reversed aeromagnetic and paleomagnetic signature. Maximum exposed thickness is 175 m

Basalt of Hill 7065 (early Pleistocene)—Eroded and dissected near vent deposits and flank lava flows preserved on the northwest and southeast facing flanks of extensive lavas flows erupted from a small 

center marked by near vent pyroclastic deposits of an eroded cinder cone on the southeast side of 

Cerro Micho. These lava flows filled a west- to northwest-trending paleotopographic valley currently occupied by Arroyo Eighteen. Lavas erupted from two source areas on the northwest flank 

of Cerro Micho and filled a small north-trending paleovalley cut into deposits of Tcmc

Description of Map Units 11
Qtdc Cinder deposits—Near vent pyroclastic deposits, predominantly scoria, spatter agglutinate, locally 

interbedded with medium gray to red oxidized lava flows 

Qtd Lava flows—Medium to dark gray basaltic foliated lava flows (50–52 weight percent SiO2), typically 

2–5 meters thick with aa bases and vesicular tops. Most flows are sparsely phyric containing small 

(less than 1 mm) olivine and pyroxene phenocrysts in a fine-grained intersertal to glassy groundmass. Flows thickened locally where ponded against paleotopographic barriers and thin proximal 

to vent areas. An 40Ar/39Ar isotopic age of 2.55±0.04 Ma (sample 5MRG-4) was obtained from 

a lava flow on the northwest flank of Cerro Micho. Reverse aeromagnetic and paleomagnetic 

signature. Maximum exposed thickness is 30–35 m in Arroyo Eighteen and thicker (up to 100 m) 

where comprised primarily of eroded near vent deposits northwest of Cerro Micho

Basalt of Tetilla Hole (early Pleistocene)—Lava flows and associated near-vent pyroclastic deposits near the 

confluence of Arroyo Tetilla and Arroyo Colorado in the southwest part of the map area. Locally, 

unconformably overlies deposits of Tca and is overlain by deposits of Qcb in the upper reaches 

of Arroyo Colorado

Qbtc Cinder deposits—Dissected cinder cones composed of near-vent pyroclastic deposits, predominantly 

scoria, spatter agglutinate and minor flow material

Qbt Lava flows—Massive, medium-gray, basaltic (50 weight percent SiO2) lava flows that underlie the 

upper reaches of Arroyo Tetilla. Flows are thick, up to 17 m, with aa bases, vesicular tops with 

elongate vesicles, and minor interbedded bombs and scoria. Flow direction is to the north and 

flows grade laterally to the south into preserved remnants of vent area composed of cinder, scoria, 

bombs and agglutinate. Olivine phenocrysts and small pyroxene clots are common, groundmass 

is dense and fine grained. An 40Ar/39Ar isotopic age of 2.49±0.05 Ma (sample 96TMP22a) was 

obtained from the uppermost lava flow at Tetilla Hole. Reverse aeromagnetic and paleomagnetic 

signature. Maximum exposed thickness is 80 m

Basalt of Colorado Peak (early Pleistocene)—Includes erosional remnants of two volcanic centers built 

on the northwest and southeast flanks of Colorado Peak. Deposits include near vent pyroclastic 

deposits and associated flank lava flows. Erupted lavas flowed away from the paleotopographic 

high underlain by Tca/Tcac toward the current drainages occupied by Arroyo Tetilla to the south 

and Arroyo Eighteen to the north. An 40Ar/39Ar isotopic age of 2.53±0.01 Ma (sample 2MRG–5) 

was obtained from a lava flow near the base of the section in Arroyo Colorado. Reverse polarity 

based on paleomagnetic determination and aeromagnetic signature

Qcbc Cinder deposits—Dissected cinder cones comprised of near vent pyroclastic deposits, predominantly 

scoria, spatter agglutinate and minor flow material

Qcb Lava flows—Medium- to dark-gray, massive, basaltic (49–50 weight percent SiO2) lava flows (3–5 m 

thick). Phenocrysts of olivine, typically small, less than 1 mm diameter but occur up to 4 mm in 

diameter, and clinopyroxene with common olivine+clinopyroxene clots up to several millimeters 

in a fine grained holocrystalline groundmass. Xenocrysts of resorbed quartz and plagioclase up to 

several millimeters in diameter are common. Maximum exposed thickness is 120 meters although 

the section may reflect apparent thickening due to offset along north-trending normal faults

Basaltic andesite of Arroyo Eighteen (early Pleistocene)—Lava flows and associated near-vent pyroclastic 

deposits erupted from small cinder cones at the head of Arroyo Eighteen. Erupted lavas flowed 

westward along a paleodrainage coincident with Arroyo Eighteen. An 40Ar/39Ar isotopic age of 

2.56±0.02 Ma (sample 3MRG–6) was obtained from a lava flow in a tributary to Arroyo Montoso 

near the western edge of the map area. Reverse polarity based on paleomagnetic determination 

and aeromagnetic signature

12 Geologic Map of the Montoso Peak Quadrangle, Santa Fe and Sandoval Counties, New Mexico
Qaec Cinder deposits—Dissected cinder cones comprised of near vent pyroclastic deposits, predominantly 

scoria, spatter agglutinate and minor flow material

Qae Lava flows—Medium-gray, massive basaltic andesite (55–57 weight percent SiO2) lava flows (3-5 m 

thick) containing sparse olivine±pyroxene phenocrysts and abundant xenocrysts of plagioclase 

and quartz. Maximum exposed thickness is 65 m but is locally thickened by down-to-west normal 

faults

Basalt of Hill 6929 (early Pleistocene)—Lava flows and near vent pyroclastic deposits from two centers, preserved in the northcentral part of the map area. Locally, near-vent deposits unassociated with any 

lava flows, are preserved on the southeast flank of Montoso Peak. Additional deposits occur west 

of Ortiz Mountain where a prominent northeast-trending dike cuts eroded cinder deposits. Lava 

flows abut deposits of Ttx, Qto and Tm to the southeast, south and west respectively. Reverse 

magnetic polarity based on fluxgate magnetometer determination

Qbzc Cinder deposits—Dissected cinder cones composed of near vent pyroclastic deposits, predominantly 

scoria, spatter agglutinate and minor flow material

Qbz Lava flows—Medium- to dark-gray, basaltic to basaltic andesite lava flows, proximal to associated cinder deposits are thin (2–3 m) and discontinuous. Some lava flows along the eastern and 

western flanks of Montoso Peak are likely derived from vent areas to the north of the map area on 

the White Rock quadrangle. Phenocrysts of olivine, ubiquitously altered to iddingsite, and lesser 

amounts of pyroxene are typical. Quartz and plagioclase xenocrysts are present locally. Maximum 

exposed thickness is 30 m, base not exposed

Basalt of Cañada Ancha (early Pleistocene)—Medium- to dark-gray lava flows exposed along the northeastern margin of the map area and along Cañada Ancha in the adjacent Agua Fria quadrangle (Shroba 

and others, 2006). Comprises three informal map units produced by discrete eruptive events from 

unknown source vents presumed to underlie younger deposits of the map area. Only the upper 

units (Qbu and Qbuc) are exposed in the map area but the nomenclature is retained for consistency with unit designation in the adjacent Agua Fria quadrangle. The basalt of Cañada Ancha is 

overlain by the andesite of Ortiz Mountain at the northern exposed extent

Qbuc Cinder deposits—Dissected cinder cones composed of near vent pyroclastic deposits, predominantly 

scoria, spatter agglutinate and minor flow material. Locally includes thin, isolated deposit of 

cinder, not necessarily reflecting near vent proximity. This deposit may reflect lava flow rafting 

of near-vent pyroclastic deposits away from vent areas now presumed buried beneath Qtoc in the 

Twin Hills area

Qbu Lava flows—Basalt to basaltic andesite lava flows that cap an eastward-sloping surface on the west 

side of Cañada Ancha. Lava flows contain moderately abundant, equant olivine phenocrysts, 

sparse clinopyroxene phenocrysts, and ubiquitous resorbed plagioclase xenocrysts. Lava flows are 

lobate and discontinuous, and vary in thickness from 1.5 to 5 m. Flows have reversed magnetic 

polarity based on paleomagnetic signature and overlie 2.3–2.4 Ma lava flows of unit Tbm in the 

Agua Fria quadrangle (Shroba and others, 2006). Maximum exposed thickness is 8 m

Basalt of Montoso Peak (Pliocene)—Basalt lava flows and near vent pyroclastic deposits forming Montoso 

Peak and adjacent lowlands southwest of Montoso Peak. An 40Ar/39Ar isotopic age of 2.59±0.05 

Ma was obtained from a lava flow forming a prominent bench on the south side of Montoso Peak 

(sample 96TMP08). Normal magnetic polarity from both paleomagnetic and aeromagnetic signature. Maximum exposed thickness is 210 m

Description of Map Units 13
Tmc Cinder deposits—Dissected monogenetic cinder cone composed of near vent pyroclastic deposits, 

predominantly scoria, spatter agglutinate and minor flow material. Volcanic bombs are common near the summit of Montoso Peak

Tm Lava flows—Dark-gray to medium-gray basalt (49–52 weight percent SiO2) lava flows, typically less 

than 5 m thick containing common phenocrysts of olivine, pyroxene and plagioclase, and locally xenocrysts of quartz and hornblende

Andesite of Colorado Peak (Pliocene)—Andesitic lava flows form the lower flanks of Colorado Peak and 

grade transitionally into predominantly near-vent pyroclastic deposits near the summit and dissected southern flank. Most flows appear to have erupted from a central vent area characterized by 

a conspicuous summit crater and associated ring dikes and incipient flow breccias. Summit cone 

area is cut by northeast- and northwest-trending dikes. An 40Ar/39Ar isotopic age of 2.60±0.10 Ma 

was obtained from a lava flow near the summit of Colorado Peak. Normal magnetic polarity based 

on paleomagnetic and aeromagnetic signature. Maximum exposed thickness is 225 m, base not 

exposed

Tcac Cinder deposits—Dissected summit cone composed of near vent pyroclastic deposits, predominantly 

scoria, spatter agglutinate, bombs and abundant flow material. Near-vent deposits are sparsely 

olivine and pyroxene phyric and contain few xenocrysts

Tca Lava flows—Dark-gray basaltic andesite to andesite (56–62 weight percent SiO2) lava flows up to 15 

m thick containing phenocrysts of olivine and pyroxene and abundant xenocrysts of resorbed plagioclase and sparse quartz. Lava flows are lobate in form and directed predominantly to the west 

and northwest but truncated to the west by offset along the down-to-west Cochiti Cone normal 

fault

Basaltic andesite of Thirty-One Draw (Pliocene)—Discontinuous, poorly exposed outcrops of lava flows and 

minor associated pyroclastic deposits forming the north flank of Twin Hills underlying Qto. Interpreted to be remnants of a larger, eroded and buried volcanic center. Normal magnetic polarity is 

inferred from aeromagnetic signature and is consistent with an eruption age greater than 2.59 Ma 

(table 1)

Ttxc Cinder deposits—Pyroclastic deposits, predominantly scoria and spatter agglutinate which may not 

represent immediate proximity to eruptive vent. Deposits are thin and discontinuous 

Ttx Lava flows—Medium-gray to dark-gray basaltic andesite lava flows (52–53 weight percent SiO2

) 

underlying vegetated north slopes of Twin Hills. Lava flows are thin, (less than 3 m), vesicular, 

aphyric to sparsley phyric containing small olivine phenocrysts (less than 1 mm) in a fine-grained, 

holocrystalline groundmass. Preserved lava flow directions are to the north. Maximum exposed 

thickness is 50 m

Andesite of Cerrita Portillo (Pliocene)—Lava flows and associated near-vent pyroclastic deposits of Cerro 

Portillo volcano exposed in the central and eastern margin of the map area. An 40Ar/39Ar isotopic 

age of 2.60±0.10 Ma (sample 96TMP36) was obtained from a lava flow on the south flank of 

an eroded distal flow lobe. Normal magnetic polarity based on paleomagnetic and aeromagnetic 

signature. Maximum exposed thickness is 175 m

Tcpc Cinder deposits—Near-vent pyroclastic deposits, predominantly spatter with lesser amounts of scoria

Tcp Lava flows—Medium-gray massive, blocky andesite lava flows (58–61 weight percent SiO2) erupted 

from vents in the central part of the map area. Lava flows form broad surfaces on lava-flow 

sequences as much as 100 m thick. Lava flows contain ubiquitous phenocrysts of hornblende, 

14 Geologic Map of the Montoso Peak Quadrangle, Santa Fe and Sandoval Counties, New Mexico
plagioclase, clinopyroxene, Fe-Ti oxides, and minor olivine. Resorbed plagioclase xenocrysts are 

commonly associated with glomerocrysts of altered pyroxene. Dominant flow directions are to the 

northeast, east and southeast

Basalt of Caja del Rio (Pliocene)—Lava flows and associated near-vent pyroclastic deposits associated with 

a northeast trending alignment of small volume eruptive centers in the southeast part of the map 

area. Comprised predominantly of near-vent deposits with lesser flow material erupted from lower 

flanks of individual cinder cones. An 40Ar/39Ar isotopic age of 2.59±0.04 Ma (sample 02TMP02) 

was obtained from a lava flow on the southeast flank of the largest cinder and spatter remnant. 

Normal magnetic polarity based on paleomagnetic and aeromagnetic signature. Maximum 

exposed thickness is 135 m, base not exposed

Tayc Cinder deposits—Multiple dissected cinder cones composed of near-vent pyroclastic deposits, predominantly scoria, spatter agglutinate, bombs and abundant flow material

Tay Lava flows—Thin (<4 m), medium-gray basalt to basaltic andesite (49–53 weight percent SiO2) lava 

flows exposed in the uppermost reaches of Arroyo Calabasas. Sparsely phyric with olivine phenocrysts (1–2 mm) in microcrystalline groundmass. Ubiquitous, resorbed plagioclase xenocrysts are 

typically 2–3 mm but can be up to 0.5 cm 

Tb Basalt of La Bajada Rim (Pliocene)—Medium- to dark-gray basaltic (48–50 weight percent SiO2) 

lava flows exposed only in the lower reaches of Arroyo Tetilla in the southeast part of the map 

area. Includes both tholeiitic and transitional olivine basalts and locally basaltic andesites. Phenocrysts include olivine+pyroxene±plagioclase and lesser iron-titanium oxides. Locally overlies 

paleochannel deposits of the Rio Grande east of the map area in the Cochiti Dam quadrangle. 

Likely erupted from buried and (or) eroded volcanic centers overlain by Tca. Maximum exposed 

thickness is 40 m

Basaltic andesite of Arroyo Calabasas (Pliocene)—Medium- to dark-gray lava flows exposed along Arroyo 

Calabasas and three tributary arroyos in the southeastern part of the map area and in the adjacent 

Agua Fria quadrangle (Shroba and others, 2006). Comprises two map units produced by discrete 

eruptive events, the younger from an unknown source vent presumed to underlie younger surficial 

deposits east of the map area. Only the lower unit is exposed in the map area but the nomenclature is retained for consistency with unit designation in the adjacent Agua Fria quadrangle. 

An 40Ar/39Ar isotopic age of 2.53±0.20 Ma (sample 4MRG–5) was obtained from a lava flow in 

Arroyo Calabasas but a normal magnetic polarity based on the measured paleomagnetic signature 

of the same sample suggests the age must be greater than 2.59 Ma and is within the analytical 

error of the argon age determination. Maximum exposed thickness is 90 m at Cerrito Pelado, 

outflow thickness in Arroyo Calabasas is typically 25 m or less

Tbclc Cinder deposits—Near vent pyroclastic deposits, predominantly scoria, spatter agglutinate, bombs 

and abundant flow material preserved at Cerrito Pelado. Active quarry operations have removed 

and (or) recontoured the northeastern third of Cerito Pelado exposing interbedded flow material 

and pyroclastic material and lesser intrusive dikes too small to show at map scale

Tbcl Lava flows—Medium- to dark-gray, basaltic andesite (53–55 weight percent SiO2) lava flows containing sparse olivine phenocrysts in a fine-grained holocrystalline groundmass. Lava flows are 

typically discontinuous and thin, typically less than 1.5–2 m thick. In most places the map unit 

represents flow lobes of a single flow or eruptive event. The source area is inferred to be the vent 

area of Cerrito Pelado, based on geochemical and paleomagnetic correlation with lava flows in 

Arroyo Calabasas

Description of Map Units 15
Basalt of Tsinat Mesa (Pliocene)—Interbedded flows and near-vent pyroclastic deposits erupted from vents at 

the northeast corner of the Tetilla Peak quadrangle (Sawyer and others, 2002). Flows are interbedded with brown to reddish-brown cinders, spatter and other undifferentiated near-vent pyroclastic 

deposits. Locally the cinder cone is composed of oxidized cinders, scoria, blocks and bombs, and 

cut by numerous dikes of fine-grained basaltic andesite. Lava flows that erupted from the cinder 

cone in the southeast corner of the map area flowed predominantly south and southeast and form 

the eastern rim of the Cerros del Rio volcanic field adjoining the village of Cieneguilla in the Turquoise Hill and Tetilla Peak quadrangles. An 40Ar/39Ar isotopic age of 2.68±0.03 Ma was obtained 

from a dike that locally intrudes the cinder cone on the adjacent Tetilla Peak quadrangle to the 

south (Sawyer and others, 2002). Cinder deposits are locally quarried for decorative landscape 

material. Normal magnetic polarity based on paleomagnetic and aeromagnetic signature. Exposed 

thickness is 30–35 m, but deposits thicken to the south 

Tbsc Cinder deposits—Near-vent pyroclastic deposits, predominantly scoria, spatter agglutinate, bombs 

and abundant flow material

Tbs Lava flows—Medium-gray basalt to basaltic andesite (51–53 weight percent SiO2) lava flows containing small olivine phenocrysts and common resorbed plagioclase and quartz xenocrysts. Commonly interbedded with scoria and cinders 

Tbta Basaltic tephra and Ancha Formation, undivided (Pliocene)—Poorly exposed deposits of basaltic 

tephra about 15 cm to 5 m thick interstratified with sand, pebbly sand, and poorly sorted basaltic 

tephra-bearing sandstone of the Ancha Formation. Some of the basaltic tephra-bearing sandstone 

may have been deposited as hyperconcentrated flows. Mapped near the southeastern corner of the 

map area beneath the basaltic andesite of Arroyo Calabasas (Tbcl). Maximum exposed thickness 

is 20 m 

Basaltic andesite of Cerro Micho (Pliocene)—Thin, discontinuous lava flows associated with two small, 

eroded cinder cones on the northeast flank of Cerro Micho. Locally cut by northeast trending 

dikes. An 40Ar/39Ar isotopic age of 2.73±0.03 Ma (sample 3MRG–1) was obtained from a lava 

flow interbedded with scoria on the larger of the two cinder cones. Normal magnetic polarity 

based on paleomagnetic and aeromagnetic signature. Maximum thickness is 115 m

Tbcc Cinder deposits—Near-vent pyroclastic deposits, predominantly scoria, spatter agglutinate, bombs 

and abundant flow material

Tbc Lava flows—Medium- to dark-gray, thin (<3m) basaltic andesite (52–53 weight percent SiO2

) lava 

flows. Contains abundant olivine and lesser pyroxene phenocrysts (< 2 mm) and ubiquitous plagioclase and quartz xenocrysts in a fine grained holocrystalline groundmass

Andesite of Cerro Micho (Pliocene)—Lava flows and associated near-vent deposits erupted from several 

eroded vent areas, the largest of which is preserved at Cerro Micho. Thick lava flows up to 

8–10 m, underlie the eastern rim of White Rock Canyon in the northwest part of the map area. 

Between White Rock Canyon and Arroyo Montoso, the section is exposed in several, down-towest fault blocks. Scattered near-vent deposits, predominantly cinders, overlie associated lava 

flows and are thickest along a NW-SE trending alignment of dissected cinder and spatter cones 

extending from the northwest corner of the map to Cerro Micho on the southeast. An 40Ar/39Ar 

isotopic age of 2.76±0.03 Ma (sample 3MRG–5) was obtained from a lava flow in a tributary to 

Arroyo Montoso. Normal magnetic polarity based on paleomagnetic and aeromagnetic signature. 

Maximum exposed thickness is 300 m although thickness varies considerably and likely reflects 

eruption of lava flows onto an irregular paleotopographic surface

16 Geologic Map of the Montoso Peak Quadrangle, Santa Fe and Sandoval Counties, New Mexico
Tcmc Cinder deposits—Near-vent pyroclastic deposits, predominantly scoria, spatter and lesser agglutinate

Tcm Lava flows—Medium-gray to reddish brown andesite (57–60 weight percent SiO2) lava flows. 

Although variable, most lava flows characteristically contain olivine and clinopyroxene phenocrysts, sparse to abundant plagioclase phenocrysts ± resorbed plagioclase xenocrysts in a finegrained holocrystalline groundmass

Tat Andesite of Tetilla Peak (Pliocene)—Massive light-gray lava flows that underlie the southern rim 

of Arroyo Colorado in the southwest part of map area and most of Tetilla Peak to the south in 

the adjacent Tetilla Peak quadrangle (Sawyer and others, 2002). Basaltic andesite and andesite 

lava flows (56.5–62.7 percent SiO2) contain common plagioclase and clinopyroxene phenocrysts, 

sparse xenocrystic quartz, and hornblende that locally is a prominent phenocryst in upper part of 

lava flows at Tetilla Peak (Zimmerman and Kudo, 1979). An 40Ar/39Ar isotopic age of 3.04±0.21 

Ma from a lava flow in the adjacent Tetilla Peak quadrangle. Normal magnetic polarity based on 

paleomagnetic and aeromagnetic signature. Individual flows range from 50 to 70 m in thickness; 

aggregate thickness as much as 200 m

Description of Map Units 17
18 Geologic Map of the Montoso Peak Quadrangle, Santa Fe and Sandoval Counties, New Mexico

Acknowledgments

The authors thank Michael N. Machette and Kenzie 

Turner of the U.S Geological Survey for their thorough 

reviews that greatly improved the report. V.J.S. Grauch prepared, and assisted with interpretation of, aeromagnetic data 

used to constrain lithostratigraphic units in the volcanic rocks. 

We gratefully acknowledge Daniel P. Miggins of the U.S. 

Geological Survey and William McIntosh of the New Mexico 

Institute of Mining and Technology for 40Ar/39Ar age determinations of volcanic rocks referenced in this report. 

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20 Geologic Map of the Montoso Peak Quadrangle, Santa Fe and Sandoval Counties, New Mexico

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R.A. Thompson and others—Geologic Map of the Montoso Peak Quadrangle, Santa Fe and Sandoval Counties, New Mexico—SIM 3179