RSN

RSS

D.D. Vaughn

No. 1 well

Qao

Qay

Qai

Qay

Qai

QTsf

Tsb

QTsf

Qa

Qc

Qai

Qa

Qai

Qay

Tsf

Qac

Qa

Tsf

Qai

Qac

Qai

QTsf

Qao

Qao

Tsf

Qay Qac

Qay

Qao

Qfp

Qay

Qa

Qao

Qaf

Qac

QTsf

Qa

Qfp

Tsf

Qai

Tsb

Qaa

Qa

Qa

Qay QTsf

Tsb

Qao

QTsf Tsf

Qlso

Qay

Qlsy

Qc

Qc

QTsf

Qac

Qac

Qac

Qc

Qlso

Qac

Tsb

Qay

Qlsy

Qes Qes

Qc

Tsb Tsf

Tsb

Qay

Qes

Qlsy Qlso

Qac

Qac Qay

Tsb

Qac

Qlsb

Qla

Qac

Qla

Qc

Tsf

Qlso

Qac

Qla

Qlsb

Qac

Qlsb

Qaa Qay

Qay

Qay

Qa

Qay

Qay

Qac

Qai

Qay

Qfp

Qfp

Qfp

Qaa

Qay

Qay

Qay

Qay

Qay Qac

Qfp

Qa

Qla

Qac Qac

Qac

Qlsb

Qlsb

Qc

Qla

Qay

Qao

Qlso

Qao

Qao

Qao Qao

Qay

Qay

Tsf

Qaf

Qc

Qlsy

Qac

Qac

Qac

Qai

Qao Qai

Qay

Tsf

Qay

Qay

QTsf

QTsf

QTsf

Qay

Qay

Qa

Qfp

Tsf

Tsf

Qlsb

Qac

Qac

Qac

Qay

Qay Qaa

Qaa

QTsf

QTsf

QTsf

Tsf

Qao

Qai

Qay

QTsf

Qa

Qfp

Tsf

Qc

Qlso

Qai

Qai

Qay

Qlsy

Qlsb

Qlsy

Qlsb

Qlsb

Tsb

Qac

Qla

Tsf

Tsf

Qao

Tsf

Qc

Qa

Qa

Qlsy

Qaa

Qla

Qay

Qc

Tsf

Tsf

Qai

Qfp

Qac

Qay

Qa

Any use of trade names is for descriptive purposes only and does not imply 

endorsement by the U.S. Government

For sale by U.S. Geological Survey Information Services

Box 25286, Federal Center, Denver, CO 80225

1-888-ASK-USGS

GIS files and a PDF file for this map are available online at: 

http://pubs.usgs.gov/sim/2963 

Suggested citation:

Machette, M.N., Thompson, R.A., and Drenth, B.J., 2008, Geologic map of the San Luis 

quadrangle, Costilla County, Colorado: U.S. Geological Survey Scientific Investigations 

Map 2963, 1 sheet, scale 1:24,000. 

MAGNETIC NORTH

9.3°

APPROXIMATE MEAN

DECLINATION, 2008 TRUE NORTH

CENTRAL SANGRE DE CRISTO

FAULT ZONE

SAN LUIS FAULT ZONE

CENTRAL SANGRE DE

CRISTO FAULT ZONE

SAN LUIS FAULT ZONE

SAN LUIS FAULT ZONE

SOUTHERN SANGRE DE CRISTO

 FAULT ZONE

5.3 Ma

3.7– 4.8 Ma

Lava Creek B ash

639±2 ka

23.0 Ma

33.9 Ma

11.48 ka

1,800 ka

780 ka

130 ka

1.8 Ma

Qes

Qaf

Qla

Qa

Qaa

Qay

Qai

Qao

Qac

Qc

Qlsy

Qlso

Qfp

Qaa

Qfp

Qaf Qla

Qa

Qc Qlsy

Qlso

Qes

Qay

Qai

Qao

Qac

Tsb

Qlsb

Tsf

QTsf

QTsf

Qlsb

Tsb

Tsf

Eolian Lacustrine Manmade Alluvial

SURFICIAL DEPOSITS

CORRELATION OF MAP UNITS

Alluvial

and 

colluvial

Colluvial Landslide

Holocene

Pleistocene

Pliocene

middle 

lower 

QUATERNARY

TERTIARY

Volcanic

Rocks

Sedimentary Rocks

Miocene

Oligocene

Santa Fe Group

upper

upper

lower 

GEOLOGIC MAP OF THE SAN LUIS QUADRANGLE,

COSTILLA COUNTY, COLORADO

By

Michael N. Machette, Ren A. Thompson, and Benjamin J. Drenth

2008

Geology mapped in 2006 using aerial photographs, aeromagnetic and 

gravity data (Bankey and others, 2006), and field reconnaissance. 

Geology compiled on 1:24,000-scale topographic base map using a 

photogrammetric plotter (Kern PG-2) and orthophotographic maps

Digital data prepared by Ted Brandt

Editing and digital cartography by Lisa Ramirez Rukstales

Manuscript approved for publication March 17, 2008

CONTOUR INTERVAL 10 FEET

DATUM IS MEAN SEA LEVEL 

1 .5 0 1 KILOMETER

SCALE 1:24 000

1 1/ 2 0 1 MILE

U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY 

SCIENTIFIC INVESTIGATIONS MAP 2963

COLORADO

MAP LOCATION

Base from U.S. Geological Survey, San Luis, Colorado, 1967.

Polyconic projection. 1927 North American datum

10,000-foot grid based on Colorado coordinate system, south zone

1,000-meter Universal Transverse Mercator grid ticks, zone 13

Ojito Peak

(Wallace and 

Soulliere, 1996) 

La Valley

(Kirkham and

others, 2004) 

Blanca SE

(Machette and

Thompson,

 unpub. mapping,

2007)

San Luis Hills

(Thompson 

and Machette,

1989)

Fort Garland

SW (Kirkham 

and Heimsoth,

2002)

Taylor Ranch

(Kirkham and

others, 2003) 

San Luis

(this map)

37°

37°15’

105°30’ 105°15’

Sanchez Reservoir

and Garcia (partial),

(Thompson and

others, 2007) 

INDEX MAP SHOWING SOURCES OF GEOLOGIC DATA

COLORADO

MAP LOCATION

ALAMOSA 30’ X 60’ SHEET

MAP AREA

37°00’

37°30’

106° 105°

SAN PEDRO MESA

SAN PEDRO

MESA

CULEBRA GRABEN

UNNAMED MESA

(N. SAN PEDRO)

CULEBRA GRABEN

EXPLANATION

Highly magnetic rock (Tsb)

 >200 m deep or absent

Window through Tsb

(unit eroded)

San

Luis

Chama

Los

Fuertes

San

Pablo

San

Pedro

Figure 1. Simplified map showing relative depth to magnetic rocks on and east of San Pedro mesa as determined 

from high-resolution aeromagnetic data of Bankey and others (2006). Area west of San Luis is relatively deep to 

magnetic rocks owing to erosion of the Pliocene Servilleta Basalt (unit Tsb). These same depths probably extend to 

the north and south beneath the basalt-covered mesas, although Proterozoic rocks that are possibly rooted are 

present on San Pedro mesa, about 4.5 km south of the San Luis quadrangle (see Thompson and others, 2007).

CENTRAL SANGRE DE CRISTO FAULT ZONE

SAN LUIS FAULT ZONE

SAN LUIS 

FAULT ZONE

WESTERN FOOTHILLS

OF CULEBRA RANGE

Quaternary fault and fold, dashed 

 where approximately located,

 dotted where concealed

Pre-Quaternary fault and fold, dashed

 where approximately located,

 dotted where concealed

Major road

105°30’ 105°22’30’’

37°15’

37°07’30’’

CENTRAL SANGRE DE

 CRISTO FAULT ZONE

SOUTHERN SANGRE DE

 CRISTO FAULT ZONE

159

159

152

152

242

242

Highly magnetic rock (Tsb)

 at surface

Highly magnetic rock (Tsb)

 near surface (<100 m deep)

REFERENCES CITED

Bankey, Viki, Grauch, V.J.S., Drenth, B.J., and Geophex Ltd., 2006, Digital data from the 

Questa-San Luis and Santa Fe East helicopter magnetic surveys in Santa Fe and Taos 

Counties, New Mexico, and Costilla County, Colorado: U.S. Geological Survey 

Open-File Report 2006–1170 (available at http://pubs.usgs.gov/of/2006/1170/).

Birkeland, P.W., 1999, Soils and Geomorphology, 3rd edition: New York, Oxford University Press, 430 p.

Crone, A.J., and Machette, M.N., 2005, Paleoseismic activity on the Sangre de Cristo fault 

near San Luis, Colorado [abs.]: Geological Society of America Abstracts with Program, 

v. 37, no. 7, p. 558, Abstract 256–2.

Crone, A.J., Machette, M.N., Bradley, L.-A., and Mahan, S.A., 2006, Data related to late 

Quaternary surface faulting on the Sangre de Cristo fault, Rito Seco site, Costilla 

County, Colorado: U.S. Geological Survey Scientific Investigations Map 2955, 1 sheet.

Kirkham, R.M., Brister, B.S., Grauch, V.J.S., and Budahn, J.R., 2005, Culebra graben—A 

major intrarift structure in the San Luis basin, Rio Grande rift [abs.]: Geological Society 

of America Abstracts with Programs, v. 37, no. 6, p. 14.

Kirkham, R.M., and Heimsoth, C.M., 2002, Geologic map of the Fort Garland SW 

quadrangle, Costilla County, Colorado: Colorado Geological Survey Open-File Map 

02–6, 1 sheet, scale 1:24,000, 24 p. pamphlet.

Kirkham, R.M., Lufkin, J.L., Lindsay, N.R., and Dickens, K.E., 2004, Geologic map of the 

La Valley 7.5 minute quadrangle, Costilla County: Colorado Geological Survey 

Open-File Map 04–08, 1 sheet, scale 1:24,000, pamphlet.

Kirkham, R.M., Shaver, K.C., Lindsay, N.R., and Wallace, A.R., 2003, Geologic map of the 

Taylor Ranch 7.5 minute quadrangle, Costilla County, Colorado: Colorado Geological 

Survey Open-File Map 03–15, 1 sheet, scale 1:24,000, 30 p. pamphlet.

Lanphere, M.A., Champion, D.E., Christiansen, R.L., Izett, G.A., and Obradovich, J.D., 

2002, Revised ages for tuffs of the Yellowstone Plateau volcanic field—Assignment of 

the Huckleberry Ridge Tuff to a new geomagnetic polarity event: Geological Society of 

America Bulletin, v. 114, no. 5, p. 559–568.

Machette, M.N., 1985, Calcic soils of the southwestern United States, in Weide, D.L., ed., 

Soils and Quaternary geology of the southwestern United States: Geological Society of 

America Special Paper 203, p. 1–21. 

Miggins, D.P., 2002, Chronologic, geochemical, and isotopic framework of igneous rocks 

within the Raton basin and adjacent Rio Grande rift, south-central Colorado and 

northern New Mexico: Boulder, University of Colorado, M.S. thesis, 417 p.

Ruleman, C.R., and Machette, M.N., 2007, An overview of the Sangre de Cristo fault 

system and new insights to interactions between Quaternary fault in the northern Rio 

Grande rift, in Machette, M.N., Coates, M.-M., and Johnson, M.L., eds., 2007 Rocky 

Mountain Section Friends of the Pleistocene Field Trip—Quaternary geology of the San 

Luis Basin of Colorado and New Mexico, September 7–9, 2007: U.S. Geological 

Survey Open-File Report 2007–1193, Chapter J, p. 187–197.

Thompson, R.A., and Machette, M.N., 1989, Geologic map of the San Luis Hills area, 

Conejos and Costilla Counties, Colorado: U.S. Geological Survey Miscellaneous 

Investigations Series Map I–1906, 1 sheet, scale 1:50,000.

Thompson, R.A., Machette, M.N., and Drenth, B.J., 2007, Preliminary geologic map of the 

Sanchez Reservoir quadrangle and eastern part of the Garcia quadrangle, Costilla 

County, Colorado: U.S. Geological Survey Open-File Report 2007–1074, scale 

1:24,000, 1 plate.

Wallace, A.R., and Machette, M.N., 2008, Revised geologic map of the Fort Garland 

quadrangle, Costilla County, Colorado: U.S. Geological Survey Scientific Investigations 

Map 2965, scale 1:24,000, 1 plate.

Wallace, A.R., and Soulliere, S.J., 1996, Geologic map of the Ojito Peak quadrangle, 

Costilla County, Colorado: U.S. Geological Survey Miscellaneous Field Studies Map 

MF–2312–B, 1 sheet, scale 1:24,000.

BASIC STRUCTURE AND BRIEF DESCRIPTION OF GEOLOGIC HISTORY

San Luis, the oldest town in Colorado, is located in a broad valley that is the direct 

result of tectonism, volcanism, and alluviation. West and south of town are two mesas (the 

informally named San Luis mesa west of San Luis, and San Pedro mesa south of the Rio 

Culebra) that form a north-trending horst, whereas the valley to the east of the mesas is a 

north-northwest-trending graben (Culebra graben, restricted sense). The foothills to the east 

of town, define the western margin of the uplifted Culebra Range, which is part of the 

Sangre de Cristo Mountains. The faults that bound the horst and graben show evidence of 

Quaternary movement because they offset surficial deposits as well as juxtapose older units, 

such as the Servilleta Basalt (Tsb) against sediment of the Santa Fe Group (QTsf and Tsf).

The mesas are bounded by the San Luis fault zone on the east and the southern Sangre 

de Cristo fault zone on the west. The Culebra graben is bounded on the east by the central 

Sangre de Cristo fault zone, which has Holocene (<10 ka) movement. Such recent movement indicates that the fault zone is active and thus poses a hazard to the town of San Luis 

and surrounding region. The general trace of the central Sangre de Cristo fault zone forms 

a consistent pattern of right steps, from Sanchez Reservoir on the south (Thompson and 

others, 2007) toward Fort Garland on the north (see Kirkham and others, 2002; Wallace 

and Machette, 2008). The central Sangre de Cristo is one of three fault zones (northern, 

central, and southern) that comprise the 250-km-long Sangre de Cristo fault system, which 

extends from Poncha Pass, Colo., on the north to Taos, N. Mex., on the south (see 

Ruleman and Machette, 2007, for a discussion of fault nomenclature in this region). 

However, the individual strands in the central Sangre de Cristo fault zone are not simple 

linear structures; they are complex, curvilinear, overlap, and form en echelon stepovers. At 

Rito Seco Creek in the northeastern corner of the map, Crone and others (2006) found 

evidence that four surface-faulting earthquakes occurred on the central Sangre de Cristo 

fault zone in about the past 48 k.y., and the most recent event occurred about 

9,000±2,000 years ago (Crone and Machette, 2005). No other paleoseismic data exist for 

the central Sangre de Cristo fault zone, the southern Sangre de Cristo fault zone (to the 

west), or the San Luis fault zone. However, our mapping suggests that late Quaternary 

movement (<130 ka) has probably occurred on all these fault zones. In addition to faulting 

hazards, many slopes below the mesa rims are covered with landslide deposits, colluvium, 

and (or) unmapped rockfall debris. These unstable slopes are caused by hard, competent 

basalt (Tsb) lying on soft, incompetent sediment of the Santa Fe Group (chiefly Tsf). 

The oldest map units exposed in the quadrangle are sedimentary rocks of the upper 

Oligocene to lower middle Pleistocene Santa Fe Group (QTsf and Tsf). As mapped, unit Tsf

is upper Oligocene to Pliocene in age and generally underlies the Servilleta Basalt (Tsb), or is 

present on the uplifted, eastern side of the central Sangre de Cristo fault zone (see Kirkham 

and others, 2003). The Servilleta Basalt was erupted from numerous vents on the Taos 

Plateau to the southwest, and perhaps locally from unmapped or buried vents in the 

southern San Luis Valley. The Pliocene Servilleta Basalt typically ranges in age from 

4.8–3.7 Ma in southern Colorado, although 40Ar/39Ar dates on these rocks in the San Luis 

quadrangle range from 4.59 to 4.46 Ma (see description of Tsb). These basalt flows lapped 

against the ancient sedimentary foothills of the Culebra Range and, thus, are only preserved 

east of the central Sangre de Cristo fault zone in a few localities. In the Culebra graben, 

multiple flows of the Servilleta Basalt are reported to be about 143 m thick in the Energy 

Operating Co. Williamson No. 1 drill hole (Kirkham and others, 2005; R.M. Kirkham, 

written commun., 2005), south of Sanchez Reservoir. On San Pedro mesa to the west, the 

Servilleta Basalt is typically 30–40 m thick (Thompson and others, 2007), but thins to the 

northwest to several meters thickness. 

The upper part of the Santa Fe Group (QTsf, Pliocene to lower middle Pleistocene) is 

exposed mainly west of the central Sangre de Cristo fault zone, having been either eroded 

or sparsely deposited on the uplifted, eastern side of the fault. Aeromagnetic data, which is 

sensitive to the depth of strongly magnetic rocks such as basalt (fig. 1), suggests that the 

Servilleta Basalt (Tsb) is buried to depths of 200 m or more in parts of the Culebra graben 

southeast of San Luis (Bankey and others, 2006; Thompson and others, 2007). The 

Servilleta Basalt appears to be rather shallow at several locations in the graben and on San 

Luis mesa. 

The upper part of the Santa Fe Group contains tephra (Lava Creek B ash) from the 

639±2 ka eruption of the Yellowstone caldera in northwestern Wyoming. This tephra is 

interbedded in fine-grained sediment (QTsf) that dips about 3° to the north and is capped by 

3–5 m of coarse gravel (Qao). Thus, in the Culebra graben, the uppermost (and youngest) 

part of the Santa Fe Group is younger than about 640 ka (early middle Pleistocene). The 

older stream alluvium, which forms a complex alluvial fan on the Costilla Plains (southwest 

of the quadrangle, see Thompson and Machette, 1989), is everywhere coarser than and 

unconformable on sediments of the Santa Fe Group. We suspect that the older alluvium 

may be as old as 0.6–0.5 Ma on the basis of the thickness and morphology (advanced stage 

III) of calcic soils developed on the fan surface.

Uplift of the San Pedro mesa horst and subsidence of the adjacent Culebra graben to 

the east was mostly accomplished by movement on the bounding faults, as mentioned 

previously. Although Culebra Creek has probably been superposed across the horst since 

deposition of the Servilleta Basalt, it managed to keep pace with up-to-the-west-uplift on the 

San Luis fault zone. Old stream alluvium (Qao) is preserved along the banks of the creek in 

the horst and west of it; conversely, in the graben, old and intermediate stream alluvium 

(units Qao and Qai, respectively) are faulted and downdropped. Deposition of alluvium 

from the Culebra Range has kept pace with ongoing graben formation such that Culebra 

Creek flows across the sill formed by San Pedro mesa. However, the creek labors to 

maintain westward flow; both east and west of the San Luis fault zone, Culebra Creek has a 

meandering course whereas the course is more linear across the fault zone. Most of the 

topographically low part of the graben, which is covered by upper Pleistocene to Holocene 

alluvium, has a high water table, typically within 5 m of the surface. Outside of the graben, 

the ground-water table is tens to hundreds of meters deep, especially beneath the mesas and 

in the foothills of the Culebra Range.

Contact

Fault—Bar and ball on downdropped side; shown in red where fault has 

Quaternary movement. Solid where certain; dashed where approximately 

located or inferred; dotted where concealed

Landslide escarpment—Topographic headwall or escarpment associated with 

landslides chiefly along margins of San Pedro mesa where Servilleta Basalt 

(Tsb) is underlain by sediment of the lower Santa Fe Group (Tsf). Large 

slide blocks of basalt mapped separately as unit Qlsb. Only prominent or 

continuous escarpments are shown. Hachure on downdropped side

Fold—Solid where accurately located; dashed where approximately located or 

inferred; dotted where concealed. 

Anticline—Surface trace of gentle anticline (Quaternary movement shown in 

red) expressed on terrace surface of old alluvium (Qao) and in sediment of 

lower Santa Fe Group (Tsf) south of Culebra Creek and in Servilleta Basalt 

(Tsb) on the northwest part of San Pedro mesa

Syncline—Surface trace of gentle north-trending syncline in Servilleta Basalt 

(Tsb) on the northwest part of San Pedro mesa

Location and age of dated rock or sediment—Samples of unit Tsb were 

dated using 40Ar/39Ar methods by Miggins (2002). Sample of volcanic ash 

(tephra) from upper part of unit QTsf is chemically correlated to 639±2 ka 

Lava Creek B ash (Lanphere and others, 2002) from the Yellowstone 

caldera, Wyoming (Elmira Wan, USGS-Menlo Park, Calif., written 

commun., 2007)

Approximate location of the D.D. Vaughn No. 1 well (NW1/4 SW1/4 Sec. 

24, T.3N., R.72W.)—This well was drilled in 1962 about 1.6 km east of 

San Luis. An electric log by Welex (Tusas Minerals, Inc.) reported a total 

logged depth of 3,273 ft (about 1,000 m), but no lithologies were noted

Location of trenches across central Sangre de Cristo fault zone—Results 

described by Crone and others (2006). RSN—Rito Seco North, RSS—Rito 

Seco South

DESCRIPTION OF MAP UNITS

Map-unit names and stratigraphic associations are based on mapping of the 

entire Alamosa 30' x 60' sheet, which contains many more units. See 

Birkeland (1999) and Machette (1985) for soil horizon nomenclature and 

carbonate morphology. OIS indicates marine oxygen isotope stage

SURFICIAL DEPOSITS

Man-made Deposits

Artificial fill (historic)—Mainly material in small earth-fill dams for reservoirs 

and sewage treatment plant

Eolian Deposits

Eolian sand (Holocene)—Forms small pronounced dunes as much as 3 m high 

and 5- to 10-m-thick ramps against the western base of San Pedro mesa. 

Some deposits are very young (active dunes); others contain weak buried 

soils (A horizons) indicating multiple episodes of landscape stability and 

redeposition of sand

Lacustrine Deposits

Lake sediment (Holocene)—Comprised of organic-rich, fine-grained sand, silt, 

and clay in small natural depressions on San Pedro mesa 

Alluvial Deposits

Undifferentiated stream alluvium (Holocene)—Fine-grained, organic-rich silt 

and clay to medium-grained sand and gravel that form active stream 

channels and floodplains along most drainage courses, some of which are 

too small to map separately as either unit Qaa or Qfp. Grain size and 

lithology vary depending on source material in stream headwaters. Gravel 

clasts are generally <10 cm in diameter and fine to the west along Culebra 

Creek; slightly coarser (10–15 cm) along streams draining the Culebra 

Range to the east of San Luis

Active stream alluvium (upper Holocene)—Fluvial sand and gravel in modern 

to recently active stream channels that retain fresh geomorphic expression 

(that is, oxbow loops or abrupt terrace escarpments). Gravel clasts are 

generally <10 cm in diameter and fine to the west along Culebra Creek; 

slightly coarser (10–15 cm) along streams draining the Culebra Range to 

the east of San Luis

Floodplain alluvium (lower Holocene)—Fine-grained, organic-rich silt and clay 

to medium-grained sand and pebble gravel that form floodplains adjacent to 

active channels. Floodplains commonly have muted depositional morphology (that is, infilled oxbow loops, subtle terrace margins). Gravel clasts are 

generally <10 cm in diameter along streams draining the Culebra Range to 

the east of San Luis and fine to the west along Culebra Creek

Younger stream alluvium (upper Pleistocene)—Fluvial sand and gravel in 

former floodplains that are now terraces 1–2 m above modern stream level 

or as alluvial fans and piedmont slopes along the foothills of the Culebra 

Range. Commonly has weak soil with Bw and (or) Bk horizons. Qay is 

probably 10–50 ka (see dating of similar units by Crone and others, 2006) 

and associated with OIS 2 or 3 

Intermediate stream alluvium (upper and middle Pleistocene)—Fluvial sand 

and gravel at elevations about 2–5 m above modern stream level in 

terraces, alluvial fans, and piedmont slopes along the foothills of the 

Culebra Range. Forms major intermediate-level (13–15 m high) terraces 

along Culebra Creek, west of San Pedro mesa. The relict surface of the 

intermediate stream alluvium has moderately developed soil (Bt and stage 

II–III Bk horizons). Locally, thin deposits of unit Qay overlie unit Qai or 

Qao. Most of unit is probably associated with Bull Lake age (OIS 4) glacial 

outwash from the Culebra Range, although unit may include younger 

(undivided) materials deposited during OIS 4–5. Thus, most of unit Qai is 

probably 50–150 ka

Older stream alluvium (middle Pleistocene)—Fluvial sand and gravel about 

5–15 m above modern streams as terraces or as alluvial fans and piedmont 

slopes along the foothills of the Culebra Range. Elevations of the alluvium 

surfaces vary and commonly converge downstream because of fault activity. 

Forms major high-level (25–30 m high) terrace and composite alluvial fan 

south of Culebra Creek, west of San Pedro mesa. The relict surface of the 

old stream alluvium commonly has strongly developed calcic soil (thick Bt 

and/or stage III Bk horizons). East of San Luis, old stream alluvium lies 

unconformably on sediment of the Santa Fe Group (QTsf) that contains 

639±2 ka Lava Creek B ash. Most of unit is probably 150–450 ka, and 

associated with Bull Lake age (OIS 6) or pre-Bull Lake age (OIS 8 or 12) 

glacial outwash from the Culebra Range

Alluvial and Colluvial Deposits 

Alluvium and colluvium, undivided (Holocene and upper Pleistocene)— 

Fine- to coarse-grained alluvium and colluvium that forms gentle to moderately sloping surfaces adjacent to and on bedrock-cored hills, such as San 

Pedro mesa. Material is typically pebbly-to-cobbly, silty sand to pebbly 

sand. Most of the silt and sand is probably eolian, but has been reworked by 

alluvial and colluvial processes (slopewash). Contains weak to moderately 

developed zonal soils with A, Bw, and (or) Bk horizons. Surface of deposits 

is typically not dissected

Colluvial Deposits

Colluvium, undivided (Holocene and upper Pleistocene)—Poorly sorted, 

nonstratified, sandy to gravelly deposits; common on moderate to steep 

slopes adjacent to resistant bedrock. Typically includes minor eolian 

deposits and locally derived alluvium. Thickness probably 2–10 m

Landslide Deposits

Young landslides (Holocene and upper Pleistocene)—Remobilized sediment 

of the Santa Fe Group (QTsf) with poorly sorted, nonstratified, cobbly-tobouldery fragments of Servilleta Basalt (Tsb); typically on moderate to steep 

slopes beneath steep cliffs. Young deposits commonly have a hummocky 

surface with sediment-filled closed depressions. They also appear to have 

some characteristics of flowage, perhaps having formed during moist 

climates 

Old landslides (upper and middle Pleistocene)—Remobilized sediment of the 

Santa Fe Group (QTsf) with poorly sorted, nonstratified, cobbly-to-bouldery 

fragments of Servilleta Basalt (Tsb); typically on moderate to steep slopes 

beneath steep cliffs. Old deposits commonly have hilly, but smoothed 

surface with thick accumulations of eolian sand, alluvium, and ponded 

deposits between masses of broken basalt. These deposits have abrupt 

bases, suggesting that they are displaced by the southern Sangre de Cristo 

fault zone. Commonly the fault has not displaced adjacent young deposits 

(Qa, Qay, and Qes) and therefore is rarely visible in these deposits, which 

is evidence of late Pleistocene but not Holocene fault movement in this 

area. Large slabs and blocks of Servilleta Basalt (Tsb) are mapped 

separately in large old landslides

Landslide blocks (upper and middle Pleistocene)—Large slabs of relatively 

coherent Servilleta Basalt (Tsb) within landslide masses (Qlsy and Qlso); 

common on moderate to steep slopes beneath steep cliffs or on basaltcovered mesas. Mapped chiefly in the south-central part of quadrangle 

where the Rio Culebra has trimmed the eastern margin of San Pedro mesa, 

which has been uplifted along the San Luis fault zone. Thickness commonly >20 m to 50 m reflecting thickness of adjacent Servilleta Basalt (Tsb)

Sedimentary Rocks

Santa Fe Group (lower middle Pleistocene to upper Oligocene)—Weakly to 

strongly consolidated, poorly to locally well-exposed siltstone, sandstone, 

and pebble-to-cobble conglomerate; poorly to moderately well sorted and 

bedded. This regionally extensive basin-fill unit contains subrounded clasts 

of early Proterozoic felsic and mafic metamorphic and igneous rocks, 

Paleozoic limestone and sandstone, and Tertiary volcanic rocks, all derived 

from rocks exposed in the Culebra Range to the northeast and east of the 

mapped area. Clast lithology generally reflects inverted bedrock stratigraphy; that is, lower parts of the section are rich in Tertiary volcanic rocks 

and upper parts of the section are rich in Proterozoic rocks. Clast imbrications indicate generally westward to southwestward stream transport. 

Bedforms are sheeted, with locally to widespread, deeply incised channels, 

crossbedding, and graded bedding. Mostly preserved in the subsurface, but 

locally exposed beneath the San Luis and San Pedro mesas, and in the 

foothills east of San Luis. Where these sediments are interbedded with the 

Servilleta Basalt (Tsb), the group is subdivided into upper and lower parts 

above and below the basalt, respectively

Upper part (lower middle Pleistocene to Pliocene)—Slightly oxidized, 

poorly consolidated, friable sandstone, siltstone, and pebble-to-cobble 

conglomerate that lie above Servilleta Basalt (Tsb). Mostly preserved in the 

subsurface, but locally exposed beneath the mesas west and southwest of 

San Luis, in landslide scars around the mesas, and in the foothills east of 

San Luis. In the uppermost part of the section, sediment locally contains 

white, silty, fine-sand-size volcanic ash (Lava Creek B) that was erupted 

from the Yellowstone caldera in northwestern Wyoming at 639±2 ka. 

Exposed thickness of upper part of Santa Fe Group is probably 100–200 m 

but is about 350 m thick in Energy Operating Co. Williamson No. 1 drill 

hole (Kirkham and others, 2005), which is located south of Sanchez 

Reservoir quadrangle to the south (Thompson and others, 2007)

Lower part (Pliocene to upper Oligocene)—Moderately oxidized, moderately consolidated sandstone, siltstone, and pebble-to-cobble conglomerate 

that lie beneath the Servilleta Basalt (Tsb). Also contains clayey mudstones 

deposited in ancient playas. Deposits in most exposures are relatively fine 

grained considering proximity to their source area in the Culebra Range, 

suggesting that the coarse-grained component may have been confined to 

the Culebra graben (the modern valley occupied by the town of San Luis). 

Primarily in subsurface, but locally exposed beneath basalt-covered mesas, 

and east of the central Sangre de Cristo fault zone. Exposed thickness of 

lower part of Santa Fe Group is probably 100–200 m but is about 1,500 m 

thick in Energy Operating Co. Williamson No. 1 drill hole (Kirkham and 

others, 2005), which is located south of Sanchez Reservoir in the 

quadrangle to the south (Thompson and others, 2007). In addition, unit 

Tsf must have been found in the D.D. Vaughn No. 1 well in the Culebra 

graben east of San Luis, but the lithologic log is not available to the authors

Volcanic Rocks 

Servilleta Basalt (Pliocene)—Thin, dark-gray flows of tholeiitic basalt containing small olivine phenocrysts, diktytaxitic texture, and local vesicle pipes and 

segregation veins. As many as six flows are exposed on western rim of San 

Pedro mesa south of the quadrangle (Thompson and others, 2007). Likely 

erupted from vents to the south and west of the map area. 40Ar/39Ar age 

determinations of 4.46±0.10 Ma (DM-02-53c) and 4.59±0.02 Ma 

(DM-99-53) were reported by Miggins (2002) from flows on San Luis mesa 

(informally named mesa west of San Luis, see sample locations on this 

map), whereas dates of 4.8–3.7 Ma have been obtained on other samples 

of Servilleta Basalt in the region. Maximum exposed thickness is about 30 

m; as much as 143 m thick in the Energy Operating Co. Williamson No. 1 

drill hole (Kirkham and others, 2005), which is located south of Sanchez 

Reservoir in the quadrangle to the south (Thompson and others, 2007) 

DM-99-53

RSN

SL-MM06-91

639±2 ka

DM-99-53

4.59±0.02 Ma

DM-02-53c 

4.46±0.10 Ma

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