RGR–32

RGR–145

RGR–144

RGR–39

RGR–156

RGR–276

Sample no. Easting Northing Map Symbol Map Unit Name

RGR–62* 438706 4065243 Tsb Servilleta Basalt

RGR–78* 436080 4090439 Tsb Servilleta Basalt

RGR–6* 442165 4065515 Tag Trachyandesite of north Guadalupe Mountain 

RGR–276 442869 4068688 Tag

RGR–305* 432527 4065550 Tao Trachyandesite of Cerro de la Olla

RGR–143* 435125 4066920 Tvc Dacite of Cerro Chiflo

RGR–145 434436 4071090 Tmd Basaltic trachyandesite dike

Table 3. 40Ar39Ar incremental heating data for select samples in or near map area

[Sample numbers starting with RGR determined using whole rock chips. Sample numbers starting with RA reported from Appelt (1998) determined from groundmass

concentrate (g.c.); location coordinates for RA sample are approximate]

Northing and Easting values in UTM meters using NAD 27, zone 13. 

*Indicates sample site located outside of map area.

Data Analysis

Age (Ma) error Material Method

3.19 0.10 (2 sigma) whole rock Plateau

3.43 0.12 (2 sigma) whole rock Plateau

5.02 0.05 (2 sigma) whole rock Plateau

5.02 0.03 (2 sigma)

4.61 0.16 (2 sigma) Plateau

10.2 1.0 (2 sigma) Plateau

17.63 0.09 (2 sigma) Plateau

RA–102 434312 4071893 Tb Trachybasalt 4.47 0.06 (1 sigma) g.c. Isochron

Trachyandesite of north Guadalupe Mountain whole rock

whole rock

whole rock

Plateau

biotite

Table 2. Geochemical analyses of select samples from the map area and adjacent regions

[LOI, Loss on ignition; --, no data]

Sample no.

Easting 434408 434436 435125 439711 442165 442790 443687 438245 442869 434352 438599 438675 436360 436404 436404 441084 438227 436080 440104 428457 429548 429190 429283

Northing 4071849 4071090 4066920 4067494 4065515 4068767 4065058 4063692 4068688 4070196 4056656 4056699 4092652 4092649 4092649 4059563 4063711 4090439 4057961 4065107 4066409 4067550 4067702

Map Unit Tb Tmd Tvc Tvc Tag Tag Tag Tag Tag Tsb Tsb Tsb Tsb Tsb Tsb Tsb Tsb Tsb Tsb Tao Tao Tao Tao

SiO2 50.38 51.20 66.32 66.69 61.17 60.89 60.34 59.44 52.80 51.16 49.86 49.44 49.74 49.08 50.93 49.94 48.64 51.14 58.67 54.79 56.69 55.75

TiO2 1.43 1.79 0.51 0.54 0.78 0.82 0.80 0.89 1.19 1.24 1.15 1.26 1.21 1.18 1.28 1.15 1.26 1.28 0.97 1.04 0.94 0.95

Al2O3 16.21 15.04 15.12 15.32 15.87 15.44 15.84 16.04 16.52 16.54 15.90 16.49 16.77 16.92 15.96 15.70 15.80 16.45 16.17 16.67 16.22 16.24

Fe2O3 10.94 9.49 3.99 4.05 5.81 5.39 5.87 6.40 11.26 10.68 12.10 12.29 11.71 11.72 11.26 12.25 12.16 10.62 6.75 8.66 7.74 8.21

MgO 7.58 4.72 1.42 1.34 2.60 2.34 2.99 3.04 6.76 6.22 7.64 6.76 6.85 6.89 6.91 7.77 7.69 5.94 3.39 4.57 4.82 5.34

CaO 8.40 8.37 2.88 3.19 4.70 3.99 4.75 5.25 8.77 8.60 8.71 9.58 9.76 9.72 8.53 8.70 8.79 8.41 5.77 7.58 6.73 7.03

Na2O 3.74 4.20 3.92 3.98 4.26 4.37 4.11 4.39 3.10 3.45 3.17 3.13 3.10 3.01 3.45 3.15 3.45 3.52 4.54 3.78 3.80 3.75

K2O 1.33 2.20 3.81 3.63 2.88 3.51 2.87 2.55 0.71 0.88 0.61 0.39 0.37 0.39 0.85 0.60 0.80 0.88 2.50 1.96 2.23 1.98

P2O5 0.39 0.67 0.19 0.21 0.35 0.67 0.34 0.34 0.22 0.19 0.18 0.20 0.19 0.18 0.19 0.13 0.30 0.19 0.47 0.38 0.37 0.34

MnO 0.16 0.14 0.06 0.08 0.09 0.07 0.09 0.10 0.15 0.15 0.16 0.17 0.16 0.16 0.16 0.17 0.17 0.15 0.11 0.14 0.13 0.13

LOI (%) 0.00 1.24 0.60 0.54 0.89 2.08 3.21 0.69 0.55 0.47 0.42 0.88 0.84 1.85 0.76 0.80 0.89 0.29 0.46 0.74

Total 100.54 99.06 98.82 99.55 99.39 99.59 101.20 98.44 102.17 99.67 99.96 100.11 100.74 100.10 101.36 99.56 99.82 99.37 100.22 99.84 100.12 100.46

RGR–32a RGR–145b RGR–143b* RGR–39a RGR–6b* RGR–156b RGR–157b* RGR–68b* RGR–276b RGR–144b RGR–4b* RGR–5b* RGR–22a* RGR–23a* RGR–24a* RGR–43b* RGR–67b* RGR–78b* RGR–140b* RGR–282b* RGR–283b*RGR–284b* RGR–290b*

Major elements

weight % oxides

Chemical analysis determined by WDXRF (Wavelength Dispersive X-ray Fluorescence). Analysis carried out by: aWashington State University; bThe U.S. Geological Survey.

Northing and Easting values in UTM meters using NAD 27, zone 13.

*Indicates sample site located outside of map area.

--

61.43

0.85

15.63

5.45

2.26

4.17

4.69

3.32

0.70

0.08

98.77

0.20 --

Qa

Qay

Qai

Qao3

Qao2

Qao1

1

3

6

18

30

35

–

–

–

56

63

76

–

–

–

73

82

–

Map unit

Ephemeral stream

channels in

Sunshine Valley North South

Rio Grande

Table 1. Approximate height, in meters, of tops 

of valley-floor and piedmont-slope alluvium 

above ephemeral stream channels in Sunshine 

Valley and the Rio Grande in the Sunshine 7.5' 

quadrangle.

[North, indicates deposits in the northern part of the 

map area; South, indicates deposits in the southern part 

of the map area]

Basalt

Basaltic

andesite

Andesite

Dacite

Rhyolite

Trachyte

Trachyandesite

Basaltic 

trachyandesite

Trachybasalt

Tephriphonolite

Phonotephrite

Basanite/Tephrite

Picrobasalt

Phonolite

0

2

4

6

8

10

12

14

35 40 45 50 55 60 65 70 75 80

Basaltic trachyandesite dike (Tmd)

Trachybasalt (Tb)

Dacite of Cerro Chiflo (Tvc)

Trachyandesite of north 

Guadalupe Mountain (Tag)

Cerro de la Olla (Tao)

Servilleta Basalt (Tsb)

Na O + K 2 O (weight percent)

2

SiO2 (weight percent)

EXPLANATION

Figure 3. Total alkali-silica rock classification diagram of Le Bas and others (1986). Data points on diagram are based on 

recalculation of analyses in table 2 to determine FeO and Fe2O3 based on iron oxidation ratios determined by Middlemost (1989) 

for rock type.

Photograph 1. View east from Cerro Chiflo toward the northern end of Guadalupe Mountain 

in the southern part of the map area. Servilleta basalt capped gorge of the Rio Grande is 

visible in the foreground and Sangre de Cristo Range is visible in the distance. Parking for 

Sheep Crossing trailhead is visible on the east side of the gorge and is one of numerous gorge 

overviews and trail access points along the Rio Grande Wild and Scenic River of the Rio 

Grande del Norte National Monument. Photograph by Ren Thompson, 2013.

105°45'00" 105°37'30"

36°52'30"

36°45'00"

Cerro Chiflo

Guadalupe 

Mountain

Sunshine Valley

Latir Creek Latir Creek Latir Creek

Cerro

de la

Olla

Rio

Grande

Rio

Grande

0 1 2 3 4

3 4

5 MILES

0 1 2 5 KILOMETERS

Figure 2. Landsat 7 satellite image (30-m band 7‒4‒2 merged with 15-m band 8) acquired on Oct. 14, 1999, of 

Sunshine, New Mexico area (image clip from Sawyer and others, 2004), showing select geographic names. 

Volcanic rocks and sediments of the Taos Plateau volcanic field are visible throughout the image. Yellow 

rectangle is the boundary of the Sunshine 7.5' quadrangle. Yellow dot indicates the location of photograph 1.

Cerro

Sunshine Sunshine Sunshine unshine

Location of Location of photograph 1 photograph 1Location of photograph 1Location of 

photograph 1

64

64

64

285

285

522

522

159

142

518

Sunshine

Quadrangle

15 KILOMETERS

0 5 10 15 MILES

0 5 10

37°00’

106°00’ 105°30'

36°30'

Questa

San Luis

Antonito

Tres Piedras

Taos

SAN

LUIS

TUSAS MOUNTAINS

SANGRE DE CRISTO MOUNTAINS

HILLS

Rio

Grande

Red River

COLORADO

NEW MEXICO

SAN

PEDRO

MESA

SM

LM

UM

GM

CDO

CDA

CM

CT

TO

NA

CC

Rio

Grande

17

RIO GRANDE

DEL NORTE

NATIONAL MONUMENT

Taos

Plateau

San Luis

 Basin

Costilla

Plain

Figure 1. Shaded relief index map of the Taos Plateau and adjacent regions of southern Colorado and northern New 

Mexico showing outline of Sunshine geologic map area. Labeled physiographic volcanic features include: LM, Los 

Mogotes; UM, Ute Mountain; SM, San Antonio Mountain; CDO, Cerro de la Olla; NA, No Agua; CC, Cerro Chiflo; 

GM, north and south Guadalupe Mountain; CDA, Cerro del Aire; CM, Cerro Montoso; CT, Cerro de los Taoses; TO; 

Tres Orejas.

Shaded-relief base generated from U.S. Geological Survey 10-m Digital Elevation Model (DEM) data from the National Elevation Dataset, accessed April 2013

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.

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ACKNOWLEDGMENTS

The authors thank Chris Fridrich and Paul Carrara of the U.S Geological Survey for their 

thorough reviews that greatly improved the report. B.J. Drenth prepared aeromagnetic data used to 

constrain faults in the volcanic rocks. 

INTRODUCTION

The Sunshine 7.5' quadrangle is located in the south-central part of the San Luis Basin of 

northern New Mexico, in the Rio Grande del Norte National Monument (fig. 1), and contains 

deposits that record volcanic, tectonic, and associated alluvial and colluvial processes over the past 

four million years. Sunshine Valley, named for the small locale of Sunshine, is incised by a series of 

northeast-trending drainages cut into Tertiary and Quaternary alluvial deposits forming an extensive 

alluvial apron between the east flank of the Sangre de Cristo Mountains and the Rio Grande. These 

deposits predominantly overlie gently eastward-dipping lava flows of Pliocene Servilleta Basalt 

erupted from centers west of the map area. Servilleta Basalt lava flows terminate to the south 

against the elevated topography of three volcanic centers of the Taos Plateau volcanic field. From 

west to east these are Cerro de la Olla, Cerro Chiflo, and Guadalupe Mountain that are exposed in 

the southern part of the map area (fig. 2). Remnants of Miocene volcanic rocks are exposed near 

the southwestern edge of the map area and record evidence of an eroded volcanic terrain underlying 

deposits of the Taos Plateau volcanic field. These deposits are likely fault bounded to the east, 

roughly coincident with north to northwest trending, down-to-east faults in the southwestern 

quarter of the map area. The down-to-east normal faults reflect the basinward migration of the 

western margin of the Sunshine Valley sub-basin of the southern San Luis Basin (Ruleman and 

others, 2013).

Pliocene and younger basin deposition was accommodated along predominantly 

north-trending fault-bounded grabens and is preserved as poorly exposed fault scarps that cut lava 

flows of Ute Mountain volcano, north of the map area. The Servilleta Basalt and younger surficial 

deposits record largely down-to-east basinward displacement. Faults are identified with varying 

confidence levels in the map area. Recognizing and mapping faults developed near the surface in 

relatively 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 relative youth of the deposits has 

resulted in only modest displacements on most faults, and (3) some of the faults 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. Lineaments defined 

from magnetic anomalies form an additional constraint on potential fault locations. 

The mapped distribution of units is based primarily on interpretation of U.S. Geological Survey 

(USGS) 1:40,000-scale, black-and-white, aerial photographs, U.S. Department of Agriculture (USDA) 

color orthoimagery, and 2012 GeoEye (http://www.digitalglobe.com/resources/satellite-information) 

satellite imagery. 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, and aeromagnetic data (Bankey and others, 2005). Supplemental revision of 

mapped contacts was based on interpretation of USGS 1-meter orthoimagery. The geologic 

mapping was carried out in support of the USGS Rio Grande Basins Project, funded by the USGS 

National Cooperative Geologic Mapping Program. 

EXPLANATION

Contact

Normal fault—Dashed where approximately located, dotted where 

concealed; bar and ball on apparent downthrown side

Geochemical analysis sample location—With sample number. See table 2 

for values

RGR–32

Period or

subperiod

Epoch Age

Quaternary Holocene

Pleistocene

late

middle

early

0–11.7 ka

11.7–126 ka

126–781 ka

781–2.59 Ma

Neogene Pliocene

Miocene

2.59–5.33 Ma

5.33–23.0 Ma

DIVISION OF QUATERNARY AND NEOGENE TIME

USED IN THIS REPORT1

1Ages 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 Gibbard and others (2010). Ages are expressed in ka for kilo-annum (thousand years) and 

Ma for mega-annum (million years).

CONVERSION FACTORS

Multiply By To obtain

inches (in.)

inches (in.)

0.3937

0.03937

3.281 feet (ft)

0.6214 mile (mi)

0.3861

0.2399

millimeters

centimeters (cm)

meters (m)

square meters (m2)

kilometers (km)

square kilometer (km2

)

cubic kilometer (km3

)

10.76 square foot (ft2

)

square mile (mi2

)

cubic mile (mi3

)

Index map showing map area in yellow, and adjacent 7.5' 

quadrangles. For U.S. Geological Survey publications the 

publication number, author(s), and year of publication are shown. 

For non-USGS publications the State agency, author, and 

publication year are shown. 

37°00’00”

105°52’30” 105°45’00” 105°37’30” 105°30’00”

36°52’30”

36°45’00”

36°37’30”

LA SEGITA

PEAKS NE

UTE MOUNTAIN COSTILLA

CERRO DE LA

OLLA

SUNSHINE CERRO

TRES PIEDRAS NE GUADALUPE

MOUNTAIN

QUESTA

SIM 3283

(Thompson and

others, 2014)

SIM 3284

(Thompson and

others, 2014)

OF 169

NM Bureau of Geol.

and Min. Resources

(Kelson and

others, 2008)

DESCRIPTION OF MAP UNITS

[The surficial 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 (Qao1 and younger) are known or estimated to be at least 1 m thick. Most of these 

deposits are poorly exposed. 

Surficial map units locally include sheetwash alluvium (Qsw), and alluvium and colluvium, 

undivided (Qac), that are too small to show as separate map units. Many of the deposits of 

valley-floor alluvium (Qa), colluvium, undivided (Qc), and talus (Qt) in the Rio Grande gorge are 

too narrow to show at map scale. Thin (<50 cm), discontinuous sheetwash deposits (Qsw) locally 

mantle gently sloping map units. 

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.

The heights of units Qao1, Qao2, and Qao3 above the Rio Grande and the gradient of the 

southward-flowing Rio Grande increase from north to south within the map area. In the northern 

four-fifths of the map area (upstream of the 7,220-ft contour) the average gradient of the Rio 

Grande is about 2 m/km. However, in the southern one-fifth of map area (south of the 7,220-ft 

contour), the average gradient of the Rio Grande steepens to about 9 m/km. The southward increase 

in heights of units Qao1

, Qao2, and Qao3 is due chiefly to the average gradients of the Rio Grande 

(2 and 9 m/km) being greater than the southward decrease in altitude of the tops of these units 

(about 0.5 m/km). The average depth of the Rio Grande gorge is about 52 m in the northern 

four-fifths of the map area and increases to about 67 m in the southern one-fifth.

Stages of secondary calcium carbonate morphology (referred to as stages I and II observed 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 were not identified in the map area; 

however, some of the silty sheetwash alluvium (Qsw) noted on fan deposits (Qf) may be derived 

from loess. The terms “sticky” and “plastic” in the description of the Santa Fe Group, upper part 

(QTsf) are defined as follows: The former term refers to moist sediment, which after pressure is 

applied, adheres to both thumb and finger and tends to stretch somewhat rather than pulling free 

from either digit. The latter term refers to the extent to which moist sediment changes shape 

continuously under the influence of an applied stress and then retains 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 fan deposits within the map area 

are angular and subangular, whereas most of those in alluvial deposits are angular to subrounded.

Colors of the Santa Fe Group, upper part (QTsf) were determined by comparison of dry 

matrix with Munsell Soil Color Charts (Munsell Color, 1973). Due to the addition of a shaded relief 

base, colors on the List of Map Units and the Correlation of Map Units may not exactly match unit 

colors on the map]

SURFICIAL DEPOSITS

Alluvial deposits

Valley-floor alluvium (Holocene)—Deposits along the Rio Grande are locally 

boulder, cobbly pebble gravel. Deposits along other streams consist of sand and 

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

plains and adjacent low terraces. The top of unit Qa is about 1 m above channels 

of ephemeral streams in Sunshine Valley and about 2 m below the top of unit 

Qay (table 1). Unit Qa locally includes a minor amount of the fan deposits (Qf) 

and narrow sheetwash aprons (Qsw) along valley margins. Low-lying deposits 

are prone to periodic stream flooding. Estimated thickness 1–5 m along the Rio 

Grande, 1–3 m along other streams

Sheetwash alluvium (Holocene and late Pleistocene)—Alluvial aprons composed 

chiefly of slightly pebbly to pebbly, slightly silty to silty sand that accumulates 

on gentle slopes, such as those on Servilleta Basalt (Tsb). Some of the silt- to 

fine sand-size fraction in these deposits may be of eolian origin (Shroba and 

Thompson, 1998). Deposits of unit Qsw along the shores of intermittent ponds 

or small lakes on Servilleta Basalt (Tsb) locally may include or overlie playa 

deposits. Low-lying areas of unit Qsw are susceptible to sheet flooding due to 

unconfined overland flow, and locally to stream flooding and gullying. Recently 

disturbed surface of unit Qsw may be susceptible to minor wind erosion. 

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

Sheetwash and valley-floor alluvium, undivided (Holocene and late Pleistocene)— 

Chiefly silty and sandy sheetwash deposits (Qsw) on gentle slopes and a minor 

amount of undifferentiated sandy and pebbly valley-floor alluvium (Qa) along 

and near ephemeral streams on Servilleta Basalt (Tsb) near the western boundary 

of the map area. Low-lying areas of unit Qsa in and adjacent to stream channels 

may be subject to periodic stream flooding, gullying, and possibly debris-flow 

deposition; adjacent slopes may be subject to periodic sheet flooding. Estimated 

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

Piedmont-slope alluvium

Five map units of alluvial deposits of three age groups form an extensive alluvial apron (or 

bajada) between the east flank of the Sangre de Cristo Mountains and the Rio Grande. The alluvial 

apron is about 13–18 km wide; has average slopes of about 10–50 m/km near the mountain front 

and about 1–6 m/km within the map area; and was deposited by westward and southwestward 

flowing streams sourced in the Sangre de Cristo Mountains and in the piedmont west of the 

mountain front. The westward-sloping landform that developed on the alluvial apron is known as 

the Costilla Plain (Upson, 1939). Piedmont-slope alluvium commonly consists of pebbly sand and 

matrix-supported, pebble gravel with a sandy matrix. Some deposits of pebble gravel contain a 

minor amount of small cobbles. Pebble- and cobble-sized clasts in piedmont-slope alluvium are 

composed chiefly of volcanic rock fragments from the mid-Tertiary Latir volcanic field (Lipman 

and Reed, 1989), including Amalia Tuff, as well as granite, quartz, feldspar, pegmatite, felsic 

gneiss, and a minor amount of pale-green quartzite from Proterozoic sources. These clasts were 

eroded from bedrock in the Sangre de Cristo Mountains. Small, unmapped deposits of sheetwash 

alluvium (Qsw) and alluvium and colluvium, undivided (Qac) locally overlie deposits of 

piedmont-slope alluvium. The heights of piedmont-slope deposits above Costilla Creek are higher 

than those of equivalent deposits along and near channels of unnamed intermittent or ephemeral 

streams near the northeast and southeast corners of the map area. Deposition of units Qao1

, Qao2, 

and Qao3

 predate major downcutting of the Rio Grande gorge (Thompson and Shroba, 2007), 

which began after about 440 ka (Machette and others, 2007, 2013). The deposits of units Qao1

, 

Qao2

, and Qao3 may have accumulated under pluvial climatic conditions associated with pre-Bull 

Lake glaciations in the Sangre de Cristo Mountains prior to about 440 ka (Ruleman and others, 

2013). Deposits of map units Qao1

, Qao2, and Qao3 accumulated prior to canyon cutting by the 

Rio Grande (Shroba and Ruleman, 2007), which began after about 440 ka (Machette and others, 

2007, 2013). Low-lying areas adjacent to stream channels are prone to periodic stream flooding.

Younger piedmont-slope alluvium (late Pleistocene)—Top of unit Qay is about 3 m 

above channels of ephemeral streams in the western part of Sunshine Valley and 

about 3 m below the top of unit Qai (table 1). In the adjacent Ute Mountain 

quadrangle, deposits of unit Qay appear to be graded to deposits of unit Qf. 

Stage I and weak stage II carbonate morphology on clasts in unit Qf suggests that 

unit Qf, as well as unit Qay, accumulated during the Pinedale glaciation (table 2 

in Machette, 1985), about 12–30 ka (Nelson and others, 1979; Benson and 

others, 2004, 2005). Deposits of unit Qay could be as old as 40–47 ka (Cole and 

others, 2007). Estimated thickness is 1–3 m

Intermediate piedmont-slope alluvium (late? and middle Pleistocene)—Top of 

unit Qai is about 6 m above channels of ephemeral streams in the western part of 

Sunshine Valley and about 6 m below the top of unit Qao3

 (table 1). Unit Qai

may have accumulated during the Bull Lake glaciation, about 120–170 ka (Sharp 

and others, 2003; Pierce, 2004). Estimated thickness is 1–3 m

Lower older piedmont-slope alluvium (middle Pleistocene)—Top of unit Qao3

 is 

about 18 m above channels of ephemeral streams in the western part of Sunshine 

Valley, and about 12 m below the top of unit Qao2 (table 1). In the northern part 

of the map area the top of unit Qao3 is about 56 m above the Rio Grande, and 

about 7 m below the top of unit Qao2. In the southern part of the map area the 

top of unit Qao3

 is about 73 m above the Rio Grande, and about 9 m below the 

top of unite Qao2 (table 1). Deposits of unit Qao3 are preserved on both the east 

and west sides of the Rio Grande gorge (Cañon del Rio Grande). Estimated 

thickness is 1–3 m

Higher older piedmont-slope alluvium (middle Pleistocene)—Top of unit Qao2

 is 

about 30 m above channels of ephemeral streams in the western part of Sunshine 

Valley, and about 5 m below the top of unit Qao1 (table 1). In the northern part 

of the map area the top of unit Qao2

 is about 63 m above the Rio Grande, and 

about 13 m below the top of unit Qao1. In the southern part of the map area the 

top of unit Qao2 is about 82 m above the Rio Grande. Estimated thickness is 2–3 m

Highest older piedmont-slope alluvium (middle Pleistocene)—Top of unit Qao1

near the northeast corner of the map area, is about 35 m above channels of 

ephemeral streams in the western part of Sunshine Valley, about 13 m above the 

top of unit Qao2

, and about 76 m above the Rio Grande (table 1). Estimated 

thickness 2–3 m

Alluvial and colluvial deposits

Alluvium and colluvium, undivided (Holocene and late? Pleistocene)—Consists 

chiefly of valley-floor alluvium (Qa), sheetwash alluvium (Qsw), and fan 

deposits (Qf) primarily along minor tributary drainages of the Rio Grande. Unit 

ranges from slightly pebbly, silty sand to poorly sorted, clast- and matrix-supported, locally bouldery, cobbly and pebbly gravel with a sandy matrix. Low-lying areas of the map unit are prone to periodic stream and sheet flooding and 

locally to debris-flow deposition. Deposits composed of silty sand are prone to 

gullying. Estimated thickness is 1–10 m

Fan deposits (Holocene and late? Pleistocene)—Chiefly clast-and-matrix supported, 

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

locally consist of pebbly and cobbly, slightly silty sand that contains gravel 

lenses. Rock fragments in unit Qf, near the southern boundary of the map area 

(from east to west) are composed chiefly of trachyandesite of Guadalupe 

Mountain (Tag), dacite of Cerro Chiflo (Tvc), and trachyandesite of Cerro de la 

Olla (Tao). One small deposit of unit Qf, that overlies alluvium of unit Qay near 

the west end of Sunshine Valley, is composed of sediment eroded from the upper 

part of the Santa Fe Group (QTsf). Deposits of unit Qf commonly form 

fan-shaped and lobate masses of sediment deposited by debris flows and by 

sediment-charged, ephemeral streams. Eolian sediment may have played an 

important role in the genesis of these debris flows (Shroba and others, 2007). 

Levees that contain abundant cobbles and boulders formed at the margins of 

some debris flows. Erosional modification of original surface morphology of fan 

deposits in areas between washes ranges from little, if any, to minor. Washes 

commonly have steep sides that are only slightly modified by erosional 

processes. Small, unmapped deposits of sheetwash alluvium (Qsw) and alluvium 

and colluvium, undivided (Qac) locally overlie deposits of unit Qf, such as on 

the large fan apron along the base of Cerro Chiflo. Deposits of unit Qf may have 

accumulated under pluvial climatic conditions associated with the Pinedale 

glaciation, as well as during major precipitation events, such as intense summer 

thunderstorms during the Holocene. Low-lying areas adjacent to stream channels 

are prone to periodic stream flooding and debris-flow deposition. Maximum 

thickness possibly about 50 m

Colluvial deposits

Landslide deposits (Holocene and late Pleistocene)—Deposits of unsorted to partly 

unstratified debris that commonly display hummocky topography. All of the 

landslides and landslide deposits are within the Rio Grande gorge. They are 

formed from the Servilleta Basalt (Tsb) and sediment between or below some of 

the lava flows. Sliding of the Servilleta Basalt is probably due in part to reduction of lateral support promoted by incision of the floor of the gorge by the Rio 

Grande. The largest landslide deposits are in the southern half of the map area 

where displacement along faults near the Rio Grande gorge may have significantly weakened the Servilleta Basalt, and may have made it susceptible to sliding. 

Much of unit Qls probably includes material displaced chiefly by translational 

sliding and locally by rock creep as defined by Cruden and Varnes (1996). 

Deposits of unit Qls are susceptible to reactivation due chiefly to stream incision 

by the Rio Grande. Unit Qls locally include minor deposits of talus (Qt) and 

colluvium, undivided (Qc). Maximum thickness possibly about 85 m 

Talus deposits (Holocene to middle? Pleistocene)—Composed mostly of angular, 

tabular blocks of dacite of Cerro Chiflo (Tvc) deposited chiefly by rock fall on 

the flanks of Cerro Chiflo along the southern boundary of the map area. Also 

includes small deposits exposed along Rio Grande gorge composed mostly of 

angular blocks of Servilleta basalt (Tsb). Maximum thickness possibly about 15 m

Colluvium, undivided (Holocene to middle Pleistocene)—Chiefly 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 bouldery 

rubble with a sandy matrix. Unit Qc consists mostly of debris-flow, rock-fall and 

creep deposits, as defined by Varnes (1978) and Cruden and Varnes (1996). Unit 

locally includes small deposits of sheetwash alluvium (Qsw), and alluvium and 

colluvium, undivided (Qac). Two large deposits in the Rio Grande gorge include 

talus (Qt) and landslide (Qls) deposits. Unit Qc may be susceptible to continued 

movement or reactivation due to natural and human-induced processes. 

Maximum thickness possibly about 20 m

OLDER SEDIMENTARY DEPOSIT

Santa Fe Group, upper part (middle? Pleistocene to Pliocene)—Exposed deposits 

at scattered outcrops consist of silty to very silty, mostly very fine to medium 

sand that overlies Servilleta Basalt (Tsb). Deposits are weakly consolidated, 

slightly sticky, plastic to very plastic, and are commonly brown, strong brown, 

and reddish yellow (7.5YR 5/4, 5/6, 6/6, and 6/7) in color (Munsell Color, 1973). 

The abundance of medium sand and finer sediment suggests deposition in a low 

energy, closed basin environment, such as ephemeral playa lakes (Summers and 

Hargis, 1984). Sediments in the lower part of the Santa Fe Group are not 

exposed, but drill-hole data indicate that they are preserved in the subsurface 

beneath the Servilleta Basalt (Winograd, 1959). Unit QTsf overlies, and is 

younger than, the about 3.5 Ma-Servilleta Basalt. The minimum age of unit QTsf

is not well constrained in the map area. However, tephra in nearby areas help to 

constrain its minimum age. About 25 km south of the map area near Questa, New 

Mexico, the upper part of the Santa Fe Group contains the Tsankawi Pumice Bed 

of the Tshirege Member of the Bandelier Tuff (Pazzaglia and Wells, 1990) that 

has a K-Ar age of 1.12 ± 0.03 Ma (Izett and others, 1981). About 33 km 

northeast of the map area near San Luis, Colorado, the upper part of the Santa Fe 

Group contains Lava Creek B ash less than 2 m below the eroded(?) top of the 

unit (Machette and others, 2008). The ash has a 40Ar/39Ar age of 639 ± 2 ka 

(Lanphere and others, 2002). Exposed thickness is 1–2 m, possibly as much as 50 

m (Winograd, 1959)

LAVA FLOWS AND RELATED DEPOSITS

OF THE TAOS PLATEAU VOLCANIC FIELD

The Taos Plateau volcanic field is a basaltic to rhyolitic volcanic plateau of the southern San 

Luis Basin (Lipman and Mehnert, 1979) of northern New Mexico and southern Colorado (fig. 1). 

The volcanic plateau is bordered by the high mountains of the Sangre de Cristo Range, up to 4,000 

m to the east, and the east-dipping ramp of the Tusas Mountains to the west (fig. 1). Predominantly 

basaltic to andesitic lava flows underlie the approximately 2,300-m elevation tablelands; 

punctuated by eroded volcanic edifices of more evolved compositions reaching elevations greater 

than 3,000 m. Lavas and related pyroclastic deposits of this field are locally exposed over 500 km2

and reflect eruption of predominantly mafic and intermediate composition magma, mainly between 

5 and 1 Ma. No fewer than 55 distinct eruptive centers of the Taos Plateau volcanic field have been 

identified ranging from small volume (<1 cubic kilometer) cinder cones, to the largest centers such 

as Ute Mountain that record original eruptive volumes likely in excess of 12 km3

. Most of the 

eruptive centers in the Taos Plateau volcanic field are central-vent volcanoes ranging from 

low-relief shield centers and steep-sided breeched cinder cones to remnants of large composite 

volcanoes and lava domes. These lavas have from 48 to 68 weight percent SiO2

 and exhibit a 

strong correlation between landform and whole-rock chemistry. The low-silica, subalkaline basaltic 

lavas erupted from broad shield volcanoes and formed variably thin (<3–5 m) to thick (>15 m) 

low-viscosity flows that traveled far following paleotopography; conversely, transitional to mildly 

alkaline basalts and basaltic andesites formed thick (as much as 20 m) discontinuous lavas flows 

that erupted from high relief, dissected vents. Dacitic lavas form lava domes and large composite 

centers, often with poorly preserved stratigraphy, and rhyolite is restricted to steep-sided lava 

domes with associated small volume proximal pyroclastic deposits. 40Ar/39Ar ages are dominantly 

Pliocene (M.A. Cosca and J.P. Lee, unpub. data, 2013; Appelt, 1998). 

Lithologic units were delineated on the basis of map distribution, stratigraphic position, 

petrography, geochemistry, 40Ar/39Ar geochronology and, in places, rock magnetic properties 

derived from field magnetic fluxgate determinations and aeromagnetic signature (Bankey and 

others, 2005). Volcanic rock names are based on the standard International Union of Geological 

Sciences (IUGS) classification scheme (Le Bas and others, 1986) modified by ferric/ferrous iron 

ratios as proposed by Middlemost (1989). Geochemical analyses of select samples are presented in 

table 2. Some volcanic units contain rocks of variable composition, spanning classification 

boundaries. In these cases, the dominant rock type was used for the unit name. Results of new 

40Ar/39Ar age determinations are presented in unit descriptions. Select 40Ar/39Ar results are 

presented in table 3. Some map units consist of multiple deposits of limited extent that have been 

consolidated based on similar lithologic character, stratigraphic position, inferred age, and 

aeromagnetic signature. 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 (Munsell 

Color, 2009).

Servilleta Basalt (Pliocene)—Black to dark-gray tholeiitic basalt to basaltic andesite 

(49‒53 weight percent SiO2) lava flows in western map area. Highly vesicular, 

having characteristic vesicle pipes and segregation trains; intergranular, seriate, 

weakly defined trachytic and diktytaxitic groundmass textures common with 

plagioclase, clinopyroxene, olivine, and oxides. Plagioclase (An33‒65) composes 

60–80 percent of rock; grains are elongate ranging in size from 0.4 to 2.4 mm in 

length and less than 0.04–0.4 mm in width. Clinopyroxene (En33‒39, Fs16‒23, 

Wo41‒44) composes 15‒20 percent of rock as phenocrysts, groundmass and as late 

stage ophitic to subophitic grains, grain size ranges 0.01–1.0 mm, generally light 

green to colorless, often with iddingsitized rims. Olivine (Fo65‒82) composes 5–15 

percent of rocks as euhedral to anhedral phenocrysts, commonly skeletal, that 

average 0.3–0.4 mm, but as large as 1.5 mm and smaller than 0.2 mm. 40Ar/39Ar 

geochronology on samples (samples RGR–78 and RGR–62; table 3) from the 

Rio Grande gorge rim, west of Ute Mountain and south of Cerro Chiflo, yield 

interpreted ages of 3.43 ± 0.12 Ma and 3.19 ± 0.10 Ma respectively. Ages 

throughout the Taos Plateau volcanic field range from 2.4 to 5.3 Ma (M.A. Cosca 

and J.P. Lee, unpub. data, 2013; Appelt, 1998). Lava flow thickness commonly 

3–5 m in the map area but is consistently thicker (>10 m) to the south, where 

exposed in the Rio Grande gorge near Taos, New Mexico 

Trachybasalt (Pliocene)—Gray, fine-grained trachybasalt (50 weight percent SiO2

) 

lava flows and associated near vent pyroclastic deposits including cinder, spatter 

and spatter agglutinate. Likely a small, eroded vent complex near the western 

edge of map area. Intergranular and weakly trachytic groundmass textures with 

0.1–0.15 mm plagioclase and 0.01–0.02 mm pyroxene. Only phenocrysts of 

olivine present, about 5 percent of rock, as skeletal, euhedral and anhedral grains 

and fragments of euhedral grains ranging in size from 0.15 to 0.9 mm, with no 

alteration or minor iddingsitized rims and fractures. Appelt (1998) reports a 

40Ar/39Ar date of 4.47 ± 0.06 Ma from this center (sample RA–102; table 3). Lava 

flows are thin (<3 m), but poorly exposed

Trachyandesite of Cerro de la Olla (Pliocene)—Gray to black basaltic 

trachyandesite to trachyandesite (55–59 weight percent SiO2

; McMillan and 

Dungan, 1988). Distal lava flows erupted from prominent Cerro del la Olla 

edifice southwest of map area. Groundmass with intersertal texture composed of 

glass, feldspar microlites, and opaque minerals. Phenocrysts include olivine, 

pyroxene, and plagioclase. Light-green olivine composes about 7–10 percent of 

rock as 0.2–1 mm euhedral to anhedral phenocrysts. Some grains show olivine 

cores with pyroxene overgrowths. Pyroxene grains are dark brown to black in 

hand sample and compose 3–5 percent of rock as 0.3–1 mm euhedral to anhedral 

grains. Plagioclase phenocrysts are more equant than highly elongate grains in 

groundmass and range in size from 0.1 to 0.5 mm. Glomerocrysts composed of 

olivine, pyroxene, and plagioclase are abundant and range in size from 0.5 to 4 mm.

Sparse quartz xenocrysts observed as large as 2 mm. A 40Ar/39Ar date of 4.61 ± 

0.16 Ma (sample RGR–305; table 3) obtained from Cerro de la Olla southwest of 

map area.Thin lava flows in map area (<2‒3 m); flows thicken closer to edifice

Trachyandesite of north Guadalupe Mountain (Pliocene)—Gray to black 

trachyandesite to trachyte (59‒63 weight percent SiO2

) lava flows in the 

southeast map area. Groundmass with intersertal texture composed of glass, 

feldspar microlites, and opaque minerals. Phenocrysts include orthopyroxene, 

clinopyroxene, and feldspar in all samples; olivine present only in lowest 

stratigraphic flows sampled along the Rio Grande gorge. Enstatite (En66‒82, 

Fs14‒32, Wo2‒4) composes 3–7 percent of rock and occurs as isolated euhedral 

grains and fragments or within glomerocrysts where it is the sole constituent, or 

is associated with augite grains; occasionally enstatite grains are highly elongate 

up to 1.3 mm in length with 0.01 mm width; grain size is highly variable from 

0.1 to 1 mm and glomerocrysts solely composed of enstatite are up to 3 mm. 

Augite (En43‒48, Fs11–15,Wo39‒45) composes less than 1–5 percent of rock and 

occurs as isolated euhedral grains and fragments or within glomerocrysts where 

it is the sole constituent or is associated with enstatite; augite grains are 

commonly twinned including sector twins; grain size highly variable from 0.1 to 

1 mm and glomerocrysts solely composed of augite up to 1 mm. Feldspar 

composes 1–7 percent of rock as highly elongate grains up to 1 mm length or as 

more tabular grains up to 0.2–0.4 mm width and 0.3–0.6 mm length. Olivine 

(Fo72–76) only present in one sample but where present comprises 3 percent of 

rock as euhedral to anhedral grains commonly skeletal or with embayed 

boundaries; grains vary from no alteration to near complete replacement by 

iddingsite. 40Ar/39Ar geochronology on a sample 1.5 km south of quadrangle 

boundary indicates an age of 5.02 ± 0.05 Ma (sample RGR–6; table 3). An 

additional 40Ar/39Ar date of 5.02 ± 0.03 Ma obtained from lava flow in southwest 

part of map area (RGR–276, table 3). Distinguished from lava flows of south 

Guadalupe Mountain, on adjacent quadrangle to the south, by reversed 

aeromagnetic polarity. Thickness of lava flows typically 5–15 m

Dacite of Cerro Chiflo (Miocene)—Pink to light-gray lava flow remnants and 

associated lava dome deposits of predominantly dacite composition (66‒67 

weight percent SiO2

) along south map border. Glassy groundmass with feldspar 

microlites and elongate amphibole grains up to 0.15 mm length and 0.01 mm 

width. Phenocrysts include amphibole, feldspar, biotite, and opaques. Amphibole 

composes about 15 percent of rock, is pleochroic from light brown to dark 

brown, euhedral to anhedral grains ranging in size from 0.2 to 1 mm, and some 

grains are skeletal or have resorbed edges. Feldspar phenocrysts are about 12–15 

percent of rock; euhedral to subhedral grains range in size from 0.2 to 1 mm, 

rounded edges or embayed edges are ubiquitous. Biotite is minor, 1–3 percent of 

rock, with tabular grains ranging in size from 0.2 to 0.8 mm; strongly pleochroic 

from light brown to dark brown that produces anomalous interference colors. 

Rare, conspicuous orthopyroxene grains observed; tabular grains are pleochroic 

light green to tan. Parts of intrusion contain high proportions of schist and gneiss 

xenoliths. Lipman and Mehnart (1979) report K-Ar age determination for 

intrusion from biotite and plagioclase as 10.15 ± 0.53 Ma and 10.28 ±0.57 Ma, 

respectively. A 40Ar/39Ar date of 10.2 ± 1.0 Ma was obtained from the west flank 

of Cerro Chiflo (RGR–143, table 3). Base not exposed

INTRUSIVE ROCKS

Basaltic trachyandesite dike (Miocene)—Gray, fine-grained basaltic trachyandesite 

(51 weight percent SiO2

) vertical, east-west trending dike approximately 30 m 

wide in southwest part of map area. Mapped unit shows dike and local deposits 

adjacent to dike which likely represent erosional remnants of associated extrusive 

deposits. Intergranular and weakly trachytic groundmass texture with 

plagioclase, pyroxene, and opaques with grain size 0.02–0.2 mm. Phenocryst 

assemblage consists of pyroxene and olivine. Pyroxene, probably clinopyroxene, 

based on petrographic characteristics, composes up to 7 percent of rock as 

glomerocrysts and isolated phenocrysts; glomerocrysts 0.3 mm up to 1.5 mm in 

size with individual euhedral to anhedral grains 0.05–0.2 mm; isolated grains 

generally similar in size to grains in glomerocrysts but some grains up to 0.8 mm. 

Olivine composes 2 percent of rock as euhedral to subhedral grains with bimodal 

size distribution. Grains 0.15–0.3 mm are completely or nearly completely 

replaced by iddingsite, larger grains 1–2 mm have iddingsitized rims or are 

unaltered. A 40Ar/39Ar date of 17.63 ± 0.09 (sample RGR–145; table 3) was 

obtained from the dike interior

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Alluvial deposits Piedmont-slope alluvium Colluvial deposits

OLDER SEDIMENTARY

DEPOSIT

LAVA FLOWS AND RELATED DEPOSITS

OF THE TAOS PLATEAU VOLCANIC FIELD

INTRUSIVE

ROCKS

CORRELATION OF MAP UNITS

SURFICIAL DEPOSITS

Alluvial and 

Colluvial

deposits

Late

Middle

Early

Holocene

Pleistocene

Pliocene

Miocene

TERTIARY

QUATERNARY

11.7 ka

126 ka

781 ka

2.59 Ma

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SCALE 1: 24 000

1 1/ 2 0 1 MILE

1000 1000 0 2000 3000 4000 5000 6000 7000 FEET

1 0 .5 1 KILOMETER

Base from U.S. Geological Survey, Sunshine, New Mex., 1963

Shaded-relief base generated from U.S. Geological Survey 10-m Digital 

Elevation Model (DEM) data from the National Elevation dataset, 

accessed April 2013 at http://ned.usgs.gov/

Polyconic projection. 1927 North American Datum (NAD 27)

10,000-foot grid based on New Mexico coordinate system, central zone

1,000-meter Universal Transverse Mercator grid tics, zone 13 CONTOUR INTERVAL 20 FEET

DOTTED LINES REPRESENT 10-FOOT CONTOURS

NATIONAL GEODETIC VERTICAL DATUM OF 1929

NEW MEXICO

QUADRANGLE LOCATION

The geology was mapped from 2006–2009 and modified in 2011 and 2012. 

R.A. Thompson and K.J. Turner mapped the volcanic deposits and faults 

and prepared the digital compilation of the geologic map.

R.R. Shroba and C.A. Ruleman mapped the surficial deposits, M.A. Cosca 

and J.P. Lee carried out the 40Ar/39Ar sampling, analysis and interpretation, 

and assisted with stratigraphic interpretations.

T.R. Brandt produced the digital topographic base map.

Edit and cartographic layout by L.J. Binder

Denver Publishing Service Center

APPROXIMATE MEAN

DECLINATION, 2012 TRUE NORTH

MAGNETIC NORTH

9°

Any use of trade, firm, or product names is for descriptive purposes only and does not 

imply endorsement by the U.S. Government.

Although this information product, for the most part, is in the public domain, it also 

contains copyrighted materials as noted in the text. Permission to reproduce copyrighted 

items must be secured from the copyright owner.

This database, identified as SIM 3283, has been approved for release and publication by 

the U.S. Geological Survey (USGS). Although this database has been subjected to 

rigorous review and is substantially complete, the USGS reserves the right to revise the 

data pursuant to further analysis and review. Furthermore, it is released on condition 

that neither the USGS nor the U.S. Government may be held liable for any damages 

resulting from its authorized or unauthorized use.

Publishing support provided by: 

Denver Publishing Service Center

Manuscript approved for publication Nov. 19, 2013

For more information concerning this publication, contact:

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 Web site at:

http://gec.cr.usgs.gov/

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This report is available at:

Suggested citation: http://pubs.usgs.gov/sim/3283

Thompson, R.A., Turner, K.J., Shroba, R.R., Cosca, M.A., Ruleman, C.A., Lee, J.P., and Brandt, T.R., 2014, 

Geologic map of the Sunshine 7.5’ quadrangle, Taos County, New Mexico: U.S. Geological Survey 

Scientific Investigations Map 3283, scale 1:24,000, http://dx.doi.org/10.3133/sim3283.

U.S. Department of the Interior

U.S. Geological Survey

Geologic Map of the Sunshine 7.5' Quadrangle, Taos County, New Mexico

By

Ren A. Thompson, Kenzie J. Turner, Ralph R. Shroba, Michael A. Cosca, Chester A. Ruleman, John P. Lee, and Theodore R. Brandt

2014

ISSN 2329-132X (online)

http://dxdoi.org/10.3133/sim3283

Scientific Investigations Map 3283