MAGNETIC NO

RTH 

65

56

NORTHERN BASIN 

FAULT 

SOUTHERN 

BASIN 

FAULT 

CEN

T

RAL S

A

G

RE 

DE

CRISTO 

FAULT 

ZONE 

CENTRAL 

SANGRE 

DE 

CRISTO 

FAULT 

ZONE 

CENTRAL 

SANGRE 

DE 

CRISTO 

FAULT 

ZONE 

FAULT 

UTE CREEK BASIN 

Forbes block 

CULEBRA GRABEN

ZONE

Garland mesas block 

CENTRAL SANGRE

DE

CRISTO 

NORTHERN BASI AULT 

SOUTHERN BASIN FAULT 

San Pedro 

Mesa 

VALLEY 

CULEBRA 

RANGE 

Blanca 

Peak 

Fort Garland

quadrangle 

San Luis 

Fort

Garland 

Blanca 

Smith

Reservoir 

BASALTIC HILLS 

SAN 

LUIS 

CULEBRA 

GRABEN

 Culebra R

Foothills of the

ange 

CULEBRA 

RANGE 

U.S. DEPARTMENT OF THE INTERIOR SCIENTIFIC INVESTIGATIONS MAP 2965 

U.S. GEOLOGICAL SURVEY 

55 11 CORRELATION OF MAP UNITS LATE TERTIARY AND QUATERNARY STRUCTURAL GEOLOGY 58 Qrt Rockfall and talus deposits, undivided (Holocene to upper 37 57 19 SURFICIAL DEPOSITS Tv Tsf Pleistocene)—Poorly sorted, nonstratified, gravelly to bouldery deposits 48 Tba 51 Tsfv All of the faults in the Fort Garland, Colo., quadrangle formed during extension of broken rock; typically on moderate to steep slopes adjacent to related to the development of the Neogene Rio Grande rift (Wallace, 1995, 2004). 55 Qao1 MASS-WASTING DEPOSITS 27 resistant rocks such as Tsb. Only mapped in southeast corner of Qao1 Rifting began in the late Oligocene about 26 Ma, and the Quaternary faults (shown in quadrangle below the southwest side of North Garland mesa. 49 red) reflect ongoing extension in the mapped area. The Neogene faults reflect uplift 34 Alluvial Rockfall 30 Qao 

Tsf 

and and Commonly associated with landslide headscarps and steep cliffs. of two rift-related mountain ranges: the Culebra Range to the east and Blanca Peak 

85 Qa 23 Man-made Eolian Lacustrine Alluvial colluvial Colluvial talus Landslide Thickness at least 2–10 m; base of unit not exposed to the north (fig. 1). Structural evidence in the Russell Colo., quadrangle to the Tba Qay Qai Qa Qfp present 47 22 northeast (see quadrangle index map; Wallace, 1997b), coupled with sedimentological 23 Qaf Qla Qaa upper Qls Landslide deposits (Holocene to middle Pleistocene)—Chaotic to locally Tv Qa 45 evidence from the Santa Fe Group in this quadrangle and along the southern and Qes Qa Holocene coherent deposits composed of poorly sorted, nonstratified, cobbly to western base of Blanca Peak (A.R. Wallace, USGS unpub. data, 1995), indicate that Xg 40 40 Qao Qfp lower bouldery fragments of Servilleta Basalt and remobilized sediment of the 1 Blanca Peak rose slightly later than the Culebra Range (fig. 1). With time, upliftrelated faulting along the Culebra Range and the southern side of Blanca Peak Qay 28 Qac Qc Qrt 11.77 ka Santa Fe Group (QTsf and Tsf); commonly on moderate to steep slopes 37 40 Tsf Qai Qay beneath steep cliffs or mesas. Younger deposits have a hummocky Tsfv 1 Qls migrated west and south through the Ute Creek basin and into the San Luis Basin (fig.

Qay surface with sediment-filled closed depressions, whereas older deposits 1), thus creating an intermediate-level structural block (Ikes Creek block, fig. 2) cored 

by Proterozoic (Xg) and Tertiary volcanic (Tv) rocks. This relation is clearly evident in 

26 Qa upper Qay have hilly, but smoothed surface with thick accumulations of eolian sand, Qa 87 40 FTG-AW04-1 45 Qa 2 Qlsb 50 alluvium, and ponded deposits between masses of broken basalt. Only 14.76 Ma ± 0.45 Ma 33 the Fort Garland quadrangle, where faults with known Quaternary movement are mapped in southeast corner of quadrangle below the west side of the 130 ka outboard from the range fronts. These faults displace Quaternary alluvium (Qai and 

Qao) and, locally, younger alluvium (Qay). Fault scarps that are 1.5–2 m high on unit 

Qa 61 Qai Garland mesas and in the valley of Trinchera Creek where it bisects the QUATERNARY 28

55 mesas. Thicknesses commonly 2–20 m 26 Qay (upper Pleistocene) within the town of Fort Garland attest to recent movement 21 Qao1 44 Qay Pleistocene Qlsb Landslide blocks (upper and middle Pleistocene)—Large slabs of relatively on this part of the fault zone. No Quaternary movement has been found on other Tsf Qao coherent Servilleta Basalt (Tsb) within landslide masses (Qls); common normal faults in the map area. However, Quaternary movement has been Qao 53 32 2 middle Tba Qai on moderate to steep slopes beneath steep cliffs or on basalt-covered documented to the north along the base of Blanca Peak (Colman and others, 1985)

Qay 

80 50 Qa and to the south along the Culebra Range front into northernmost New Mexico 

(Kirkham and Rogers, 1981; Colman and others, 1985; Thompson and others, 2007) mesas. Only mapped in southeast corner of quadrangle on and below the 52 Qay2 SEDIMENTARY ROCKS west side of the Garland mesas and in the valley of Trinchera Creek 45 15 780 ka where it bisects the mesas. Thickness commonly >20 m to 50 m on the central Sangre de Cristo fault zone (see Ruleman and Machette, 2007).

McCalpin (1982) estimated a recurrence interval of 25–30 k.y. for large earthquakes 

along the range-bounding northern Sangre de Cristo fault zone, which appears to 

Qao1 42 reflecting thickness of adjacent Servilleta Basalt (Tsb) lower 52 Qay 30 43 Qai 51 QTsf Qao1 1,800 ka 1.8 Ma SEDIMENTARY ROCKS terminate on the south face of Blanca Peak. From there south to the New Mexico VOLCANIC ROCKS 17 State line (near Amalia, New Mex.), the central Sangre de Cristo fault zone deforms 43 36 Tsb 3.66 Ma Pliocene Santa Fe Group (middle Pleistocene to upper Oligocene)—Weakly to 55 25 late Pleistocene age and older alluvial units. At Rito Seco Creek, northeast of San

Qfp strongly consolidated, poorly to locally well-exposed sandstone and Luis, Colo., Crone and Machette (2005) have documented early Holocene (9±2 k.y.) 

movement and three earlier faulting events over the past 48 k.y. on the central 

Qfp 

5.3 Ma pebble-to-boulder conglomerate; poorly to moderately well sorted and 

Qai 

Qes 

Qfp

N

Tv Qai 34 38 17 bedded. This regionally extensive basin-fill unit contains subrounded Tsf Sangre de Cristo fault zone.

Miocene clasts of Early Proterozoic felsic and mafic metamorphic and igneous In addition to the central Sangre de Cristo fault zone, we mapped two eaststriking faults that are herein named the "northern basin fault" and the "southern basin Tsf Tsfv Tba 14.8 Ma TERTIARY rocks, Paleozoic limestone and sandstone, and Tertiary volcanic rocks, 16 30 14 all derived from rocks exposed as far as 35 km to the northeast of the 23.0 fault"; they bound the Fort Garland basin (fig. 2) portion of the Culebra graben. Ma 5 mapped area. Clast lithology generally reflects inverted bedrock These features are marked by escarpments that trend east-west; they have greater 15 stratigraphy; that is, lower parts of the section are rich in Tertiary Qao1 Local unconformity apparent displacements on their east ends than on their west ends, and the northern 10 38 Qay1 Oligocene volcanic rocks and upper parts of the section are rich in Proterozoic fault appears to have about 2 to 3 times the surface offset of the southern fault. At 36 Qa Qaa Tv 29–30 Ma rocks. Clast imbrications indicate generally westward to southwestward first glance, these escarpments appear to be fluvial because they trend subparallel to 14 stream transport. Clasts derived from rocks with distinctive lithology on Tsf 17 33.9 Ma local drainages. However, both escarpments terminate at the western bounding fault Blanca Peak to the north (fig. 1) are conspicuously absent in most of the of the central Sangre de Cristo fault zone, diminish in displacement west of the fault 

zone, and have greater scarp heights on older deposits (progressive growth with time). 

24 Qa 33

Qai 5 23 Major group, indicating late Tertiary uplift of Blanca Peak (see Wallace, 2004). 29 unconformity The predominant facies is light-brown to reddish silt and sand with a In addition, the surface of unit Qai is tilted northward on the footwall (northern block) 

of the northern basin fault (for example, near the MacMullan Cemetery, northwest of Qay prevalence of reddish Proterozoic felsic gneiss matrix and clasts but less Qac Qa 1.6 Ga EARLY Qay Blanca Trinchera Xg than 30 percent volcanic clasts. Bedforms are sheeted, with broad to Qao the Sierra Grande School). As such, these faults form a trap-door-like half-graben 1 No. 1 well PROTEROZOIC locally deeply incised channels, cross-bedding, and graded bedding. that has its maximum displacement adjacent to the central Sangre de Cristo fault 

zone. 3 Tsf Mostly preserved in the subsurface, but locally exposed beneath the 18 Qa 

Garland mesas, in landslide scars around the mesas, and well exposed in Trinchera Creek, in the southeastern part of the quadrangle, was an antecedent the foothills east of the mesas and north and west of Fort Garland Qay DESCRIPTION OF MAP UNITS stream that incised itself into the Servilleta Basalt and Santa Fe Group strata as the

Qai 

Qes Qay 

Qay 

Qa 

Qai 

Qai 

Qaa Qai Intermediate stream alluvium (upper Pleistocene and upper middle 

QTsf Garland mesa block (fig. 2) began to tilt northeastward sometime after about 3.7 Ma. [This map is revised from Wallace (1997a) and includes new mapping of Quaternary Pleistocene)—Unconsolidated, poorly exposed sand and pebble- to Upper part (lower middle Pleistocene to Pliocene)— Where basin-fill 39 Large blocks of the Servilleta Basalt, possibly triggered by Quaternary seismic activity, units and faults in the Fort Garland 7.5-minute quadrangle. Map-unit names and cobble-size gravel in former floodplains that are now terraces, in alluvial sediment lies on or above the Servilleta Basalt (Tsb), it is mapped as an

Qao1 stratigraphic associations are based on ongoing mapping of the entire Alamosa 30' × 60' fans, or in piedmont slopes that cut across Santa Fe Group sedimentary informal upper member (QTsf) of the Santa Fe Group. These sediments 

Tsf slid along a failure plane into the developing canyon and dammed Trinchera Creek. 

The dam created a lake that extended east along Trinchera Creek, depositing 

Qay2 

Qao1 Qa sheet. See Machette (1985) and Birkeland (1999) for soil horizon nomenclature and rocks. Unit contains moderately weathered, slightly fractured, are generally less deformed, are less oxidized, and contain fewer basin- sediment that forms fertile soil along that valley (Wallace, 1996). This lake may have 

overflowed slightly through a small gap into the lower Ojito Creek drainage to the 5 carbonate morphology. OIS; marine oxygen isotope stage] subrounded clasts derived from Early Proterozoic metamorphic rocks and center (playa-like) deposits than in the lower part of the Santa Fe Group Qao1 21 Paleozoic limestone and sandstone exposed to the north and northeast of (Tsf). More than 60 m of unit is exposed north of Mountain Home south, but Trinchera Creek ultimately overflowed and cut through the dam to resume 

its westerly course. The dam for Mountain Home Reservoir (Qaf) was constructed at 

SURFICIAL DEPOSITS the quadrangle. The relict surface of unit has moderately developed soils Reservoir in the Garland mesas block (fig. 2). Total thickness unknown 20

(Bt and stage II-III Bk horizons) and, in places, is covered by thin deposits due to faulting Qao Qaf Man-made deposits (historic)—Earth-fill embankment for Mountain Home 1 the toe of the landslide and within the central Sangre de Cristo fault zone. Qai (<1 m thick) of eolian sand and silt (loess). Unit has multiple subunits 4 Reservoir in the southeast part of map area and in areas of road fills (not Tsf Lower part (lower Pliocene to upper Oligocene)— Where basin-fill (undivided herein) that have been moderately deformed by strands of the mapped) Qac sediment lies below the Sevilleta Basalt and north or east of the central

Qai central Sangre de Cristo fault zone, and thus are preserved at a range of Sangre de Cristo fault zone (uplifted block), it is mapped as an informal 10 Tsf 12 Qaa Qes Eolian deposits (Holocene)—Fine- to medium-grained quartz-rich sand that elevations relative to modern stream level. Where the unit forms broad 105°30' 105°22'30" lower member (Tsf) that comprises the majority of the Santa Fe Group. 37°30' Qai Qao1 has been transported by the wind. Forms small pronounced dunes as terraces, it is about 27–32 m above the modern channels of Ute and This sediment is generally more deformed, more oxidized, and contains 

more basin-center (playa-like) deposits than in the upper part of the Ikes Creek

Qai Qao1 much as 3 m high in western part of map area; more extensive to west Sangre de Cristo Creeks. However, these elevations vary because of 

Qa of quadrangle. Some deposits are very young (active dunes) especially deformation across the central Sangre de Cristo fault zone. The Santa Fe Group (QTsf). Total thickness unknown due to faulting; about block 

1,100 m of basin-fill sediment (Santa Fe Group, undivided) was 

penetrated in the Blanca Trinchera No. 1 well drilled in 1952 by the Qac 

piedmont facies forms broad surfaces cut across sediment of the Santa 

Fe Group (mainly Tsf) 25–32 m above modern stream level, but is rarely 

Qac Qay 

Qai Qai Qao1 

on downwind side of center-pivot irrigation circles, others contain weak 

buried soils (A horizons) indicating multiple episodes of landscape preserved at the surface west of the central Sangre de Cristo fault zone stability and redeposition of sand in the middle to late Holocene (that is, 

Qa past 6,000 years; see Machette and others, 2007)

Sunny Valley Oil Co. The precise location of this well is unknown, but it 3 because of burial on the footwall (western) side of the fault. However,

Qai Qai Qao1 

is about 5 km NNE of Fort Garland. In the Trinchera Ranch quadrangle away from the fault zone the piedmont slope formed by this unit is 

Qla to the east, Wallace (1996) reported that the lower Santa Fe Group Lacustrine deposits (upper Holocene)—Lake sediment composed of locally preserved as islands 2–5 m above modern stream level, or is might be more than 3,800 m thick Qai organic-rich, fine-grained sand, silt, and clay in Mountain Home buried by younger alluvium along the western margin of the quadrangle, Qai Qay Reservoir, east of the mesas southwest of Fort Garland (informally east of Smith Reservoir (fig. 1). Most of unit is probably associated with Tsfv Volcanic-rich facies (Miocene to upper Oligocene)— This informal 

Qa Qao1 known as the Garland mesas). Only exposed on margins of reservoir Bull Lake age (OIS 4) glacial outwash in the Blanca Peak quadrangle to member of the Santa Fe Group is much like Tsf except that it contains

when water level is low. Thickness about 1–5 m; unit is thickest where the north (A.R. Wallace, USGS, unpub. mapping, 1995), although unit 35 abundant clasts (>60 percent of total clasts) of Tertiary volcanic rock.

streams enter the reservoir and along now-inundated former channel of may include younger (undivided) materials deposited during OIS 4–5. The unit is light gray due to an abundance of volcanic-rich clasts and N F

Qaa Qa Trinchera Creek Thus, most of Qai is probably 50–150 ka. Deposits mapped as Qai on matrix, but it also contains clasts of Paleozoic limestone and arkose and Qai QTsf this map were previously designated as lower deposits of older alluvium subordinate Proterozoic metamorphic rocks. Exposed in northwestern Alluvial deposits (Quaternary)—Unconsolidated, poorly sorted, silt- to Qao2 (mapped as unit Qoal by Wallace, 1997a). Thickness is 2–5 m and 5–15 Fort Garland quarter of quadrangle in the Ikes Creek block (fig. 2). In the Trinchera 

Ranch quadrangle to the east (Wallace, 1996), the volcanic-rich facies Qa boulder-size material along and adjacent to streams. Locally includes m beneath piedmont slopes and terraces (respectively); commonly >10 m

colluvium on slopes adjacent to major streams. Where colluvium is a thick in alluvial fans Qaa typifies the lower part of the Santa Fe Group, but in the Ojito Peak

Qai significant component, the materials are mapped as unit Qac (alluvium quadrangle to the southeast, it is enclosed by the predominantly volcanic- Qao Older alluvium (middle Pleistocene)—Moderately consolidated, poorly Fort Garland 

basin 

Qay QTsf and colluvium). Clasts along smaller drainages are mostly reworked from poor facies (Wallace and Soulliere, 1996). Thickness unknown, but as Tsf exposed sand and pebble- to cobble-size gravel in high terraces, alluvial 

fans, or piedmont slopes that cut across Santa Fe Group sedimentary 

Qa sediment of the Santa Fe Group; clasts along Ute and Sangre de Cristo much as 1,100 m of sediment is exposed in northern part of mapped 15 Tsf Creeks were derived both from bedrock source areas to the north and area rocks. Gravel clasts are subrounded, fractured and (or) weathered, and east, and reworked from Santa Fe Group sediment exposed in adjacent 

slopes. The base of upper Pleistocene and Holocene alluvial deposits are derived from Early Proterozoic metamorphic rocks and Paleozoic VOLCANIC ROCKS Qay limestone and sandstone exposed to the north and northeast of the along Ute and Sangre de Cristo Creeks and basinward of the central quadrangle. The relict surface of unit has strongly developed calcic soils Tsb Servilleta Basalt (Pliocene)—Vesicular to massive, thin flows of dark-gray to Qfp Sangre de Cristo fault zone is buried, so thicknesses are unknown Qai (thick Bt and (or) stage III Bk horizons) and, in places, is covered by thin black tholeiitic basalt characterized by small olivine phenocrysts; flows

Qa 37°22'30" Recent stream alluvium, undivided (Holocene)—Fine-grained, organic- (1–2 m thick) deposits of eolian sand and silt (loess); basal 2–3 m of unit locally have columnar joints. Lower Santa Fe Group sediment (Tsf) 

Qay rich silt and clay to medium-grained sand and gravel in stream channels has ground-water oxidized, reddish-brown matrix. The alluvium has two beneath flows contains a 1-m-thick reddish baked zone at the contact. Figure 2. Major structural elements in the Fort Garland Qa 

and floodplains along modern drainage courses, but which are too small map subunits (Qao1 and Qao2) that are strongly deformed by strands of Basalt is composed of millimeter-size plagioclase and olivine phenocrysts quadrangle, Colo. Faults shown as black lines; dashed 

or discontinuous to map separately as units Qaa or Qfp. Grain size and the central Sangre de Cristo fault zone (fig. 2) and preserved at a wide in a microcrystalline groundmass containing abundant plagioclase laths where approximately located or inferred; dotted where range of elevations relative to modern stream level. Mapped as and triangular void spaces (that is, diktytaxitic texture). Two 40Ar/39Ar Qlsb Qao lithology vary depending on source materials in drainage system. Gravel concealed. Bar and ball on downthrown side. Qa 1 undivided Qao where subunits could not be recognized or reliably whole-rock dates of 3.66 Ma were obtained from (1) basalt west of clasts are generally <15 cm in diameter, but become finer (< 5–10 cm) correlated. Forms broad terraces and piedmont slopes preserved as Mountain Home Reservoir dam (sample locality 93TR-17; Wallace, 

high, isolated remnants of formerly broad surfaces that cut across 1997a) and (2) basalt on west side of the North Garland mesa (sample Qfp to the west along Ute and Sangre de Cristo Creeks in the western part of

the mapped area and along Trinchera Creek downstream of Mountain sediment of the Santa Fe Group (mainly Tsf). Alluvial fans comprised of locality DM-99-55; Miggins, 2002). These flows are the youngest part Home Reservoir. Thickness at least 2–3 m; base of unit not exposed Tsb Qao are not preserved west of the central Sangre de Cristo fault zone of an extensive volcanic sequence of the Taos Plateau that is as old as REFERENCES CITED


Qay Qaa Active stream alluvium (upper Holocene)—Fluvial silt, sand, and gravel owing to burial on the footwall (southwest) side of the fault. Qao is 4.75 Ma (Miggins, 2002). Unit preserved in southeastern quarter of

Qao1 in modern to recently active stream channels that retain fresh probably 150–450 ka. Deposits mapped as older alluvium on this map quadrangle in the Garland mesas block (fig. 2). Likely erupted from now- Birkeland, P.W., 1999, Soils and Geomorphology (3rd ed.): New York, Oxford Qaa geomorphic expression as seasonally flooded channels, oxbow loops, and were also designated as older alluvium (Qoal and Qoau) by Wallace buried vents within the Culebra graben (fig. 1), between the Garland University Press, 430 p. Qac abrupt terrace escarpments. Mapped primarily along major streams, (1997a). Qao and its subunits are 2–10 m thick beneath terraces and mesas and the Basaltic Hills to the southwest. Thickness 20–30 m Colman, S.M., McCalpin, J.P., Ostenaa, D.A., and Kirkham, R.M., 1985, Map 18

such as Ute Creek and lower reach of Sangre de Cristo Creek, and west piedmont slopes, and commonly >10 m thick in alluvial fans Tba showing upper Cenozoic rocks and deposits and Quaternary faults, Rio Grande Basaltic andesite (Miocene)—Dark-brown, black, and locally reddish of the quadrangle. Thickness at least 2 m; base of unit not exposed rift, south-central Colorado: U.S. Geological Survey Miscellaneous Investigations Qao1 Older stream alluvium, subunit 1 (middle Pleistocene)—Lower, vesicular to massive basaltic andesite flow and agglutinate. Well to Series Map I–1594, 2 sheets, scale 1:125,000. 

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

22

Qfp Floodplain alluvium (lower Holocene)—Fine-grained, organic silt and younger subunit of stream alluvium. Adjacent to Ute Creek, two poorly exposed in northwestern quarter of quadrangle in the Ikes Creek 

clay to medium-grained sand and pebble gravel that form slightly elevated remnants of subunit Qao1 QTsf are preserved about 75–87 m above present block (fig. 2); where poorly exposed, it forms a dark-colored rubble Cristo fault near San Luis, Colorado [abs.]: Geological Society of America floodplains adjacent to active channels. Floodplains have muted stream channel. Within the central Sangre de Cristo fault zone (fig. 2), within Santa Fe Group strata. Contains 1–2 mm plagioclase and 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 

Qa 20 4

depositional features, such as infilled oxbow loops or subtle terrace the surface of Qao1 is 46–58 m above larger stream channels. Subunit pyroxene phenocrysts in a microscopic groundmass of plagioclase and Tsf Qao1 is probably associated with Bull Lake age (OIS 6) glacial outwash in pyroxene. A new margins. Primarily mapped adjacent to active stream alluvium (Qaa) in 40Ar/39Ar plateau date of 14.76±0.45 Ma (D.P late Quaternary surface faulting on the Sangre de Cristo fault, Rito Seco site, Qa Qai wide valleys, such as Ute and Sangre de Cristo Creeks and along the Blanca Peak quadrangle to the north (A.R. Wallace, USGS, unpub. Miggins, USGS, written commun., 2007) was obtained from basaltic Costilla County, Colorado: U.S. Geological Survey Scientific Investigations Map mapping, 1995) 2955, 1 sheet, various scales. Qao andesite interbedded in the lower Santa Fe Group (Tsfv) in the northern 2 western margin of the quadrangle, upstream of where these streams and Qa 

Qa 

Tsf part of the mapped area (locality FTG-AW04-1). This flow would Trinchera Creek enter Smith Reservoir (fig. 1). Thickness at least 2–3 Johnson, B.R., and Bruce, R.M., 1991, Reconnaissance geologic map of parts of the Qao2 Older stream alluvium, subunit 2 (middle Pleistocene)—Higher, older m; base of unit not exposed probably lie in the medial part of the Hinsdale Formation of Lipman and Twin Peaks and Blanca Peak quadrangles, Alamosa, Costilla, and Huerfano subunit of stream alluvium Within the central Sangre de Cristo fault zone Mehnert (1975). Thickness 10–20 m Counties, Colorado: U.S. Geological Survey Miscellaneous Field Studies Map Qay (fig. 2), the surfaces of Qao2 Qaa are deformed and lie 112–150 m above Qay Younger stream alluvium (upper Pleistocene)—Unconsolidated fluvial MF–2169, 1 sheet, scale 1:24,000. 

Kirkham, R.M., and Heimsoth, C.M., 2002, Geologic map of the Fort Garland larger stream channels. Subunit Qao2 is probably associated with pre- Tv Older volcanic rocks (Oligocene)—Andesite and dacite flows, poorly 

Bull Lake age (OIS 8 and older) glacial outwash in Blanca Peak welded tuff, laharic breccia, and volcaniclastic rocks exposed in 

quadrangle to the north (A.R. Wallace, USGS, unpub. mapping, 1995) northwestern quarter of quadrangle in the Ikes Creek block (fig. 2). 

FG-CF04-01 sand and gravel in alluvial fans or in former floodplains that are now

Qa Qay Qac 19 No data terraces as a result of stream downcutting and (or) faulting. Gravel clasts Southwest quadrangle, Costilla County, Colorado: Colorado Geological Survey

are subrounded, unweathered, and derived from Early Proterozoic Open-File Map 02–6, 1 sheet, scale 1:24,000. 

Kirkham, R.M., and Rogers, W.P., 1981, Earthquake potential in Colorado: 

Colorado Geological Survey Bulletin 43, 171 p. 

Lipman, P.W., and Mehnert, H.H., 1975, Late Cenozoic basaltic volcanism and 

Flows are dark brown, black, or red; massive, with some agglutinate. 

Andesite contains plagioclase and pyroxene phenocrysts; dacite also 

includes phenocrysts of amphibole, biotite, sanidine, and rare quartz. 

Qay metamorphic rocks and Paleozoic limestone and sandstone that are Qac Alluvial and colluvial deposits, undivided (Holocene to upper 

Pleistocene)—Fine- to coarse-grained alluvium and colluvium that form 

Qfp 

Qlsb exposed to the north and northeast of the quadrangle. The deposits Qa DM-99-55

3.66 commonly have weak soils with Bw and Bk horizons where the local Ma ± 0.12 Ma gentle to moderately sloping surfaces adjacent to steep hillslopes. Unit Qay Qls Tuff is light to dark brown and forms coarse platy talus; phenocrysts water table has remained several meters below the surface. Unit Qay is development of the Rio Grande depression in the southern Rocky Mountains, in 

Curtis, B.F., ed., Cenozoic history of the Southern Rocky Mountains: Geological 

Qa consists of pebbly to cobbly silty sand to pebbly sand. Most of the silt include biotite, hornblende, plagioclase, sanidine, and locally quartz and preserved in terraces 3–10 m above modern streams or is inset against and sand was probably eolian, but it has been reworked by alluvial and NORTH GARLAND MESA pumice. Similar to, and perhaps identical to, tuff in Trinchera Ranch Qrt older alluvial deposits (Qai and Qao) and sediment of the Santa Fe Society of America Memoir 144, p. 119–154. colluvial processes (slopewash). Contains weak to moderately developed quadrangle to east dated at 29.6±0.1 Ma by Qa 40Ar/39Ar geochronology Tsf Group (QTsf or Tsf) within the foothills of the Culebra Range (fig. 1). Machette, M.N., 1985, Calcic soils of the southwestern United States, in Weide, zonal soils with A, Bw, and (or) Bk horizons. Surface of deposits Tsb (Wallace, 1996). Breccia is brown with angular, subrounded boulder-size Unit forms a major composite terrace with two levels (subunits Qay1 and D.L., ed., Soils and Quaternary geology of the southwestern United States: 

Geological Society of America Special Paper 203, p. 1–21. typically is not dissected. Thickness at least 2–3 m; base of unit not 22 clasts and variable amounts of interstitial matrix. Volcaniclastic rocks are Qay2). West of the central Sangre de Cristo fault zone, Qay forms 

alluvial fans and piedmont slopes graded to local base levels in the Qa exposed Qaa Qfp 93TR-17 light brown to gray interbedded with massive to thinly laminated, poorly

3.66 Ma ± 0.03 Ma Machette, M.N., Coates, M-M., and Johnson, M.L., eds, 2007 Rocky Mountain Qla MASS-WASTING DEPOSITS to well-sorted sediment; laterally continuous volcaniclastic unit near western and southwestern part of the quadrangle. Qay is probably 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 

Qfp Qfp 18 Qaf middle of volcanic section contains abundant pumice and lithic clasts and 10–50 ka (see dating of similar units by Crone and others, 2006). [Locally derived deposits of colluvium, landslides, rockfalls, and talus exposed FG-MM05-66 forms massive, subrounded to thinly bedded outcrops. Lower part of Thickness is 2–10 m; locally more common in alluvial fans at base of Survey Open-File Report 2007–1193, 210 p. (available at 

http://pubs.usgs.gov/of/2007/1193) 

21 No data around the margins of the Garland mesas where Servilleta Basalt (Tsb) is volcanic section is juxtaposed against Early Proterozoic granite (Xg) by a steep slopes underlain by the Santa Fe Group (QTsf or Tsf). Typically includes some Qa west-dipping fault; upper contact with Santa Fe Group strata is an eolian sand and locally debris-flow deposits, stream deposits, and ponded McCalpin, J.P., 1982, Quaternary geology and neotectonics of the west flank of the Qay1 Younger stream alluvium, subunit 1 (upper Pleistocene)—Lower, erosional unconformity. Unit may be partly equivalent to 29–30 Ma northern Sangre de Cristo Mountains, south-central Colorado: Colorado School 

of Mines Quarterly, v. 77, no. 3, 97 p. Qay Qac 34 alluvium that cover areas too small to map separately. Thickness variable, but

13 Qlsb younger subunit of stream alluvium along major composite terrace on volcanic rocks of the Conejos Formation in the San Luis Hills, 15–20 km

east side of Ute Creek and at the south edge of quadrangle (see also commonly 5–20 m at base of steep slopes] to southwest (Thompson and Machette, 1989), and to similar volcanic Miggins, D.P., 2002, Chronologic, geochemical, and isotopic framework of igneous Qc Colluvium, undivided (Holocene and upper Pleistocene)—Poorly sorted, mapping by Kirkham and Heimsoth, 2002). Probably associated with rocks in adjacent quadrangles (see quadrangle index map). Thick basal 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. 

Qaa nonstratified, sandy to gravelly deposits; typically on moderate to steep lahar sequence found in the Trinchera Ranch (Wallace, 1996) and Ojito Qaa Pinedale glaciation of OIS 2

Qfp slopes adjacent to resistant rocks such as Tsb. Only mapped in Peak (Wallace and Soulliere, 1996) quadrangles may have been Qay Ruleman, C.A., and Machette, M.N., 2007, Chapter J—An overview of the Sangre Tsf Qay2 Younger stream alluvium, subunit 2 (upper Pleistocene)— Higher, southeast corner of quadrangle where unit continues south into the Fort eliminated by faulting at the base of the section in Fort Garland de Cristo fault system and new insights to interactions between Quaternary faults 

in the Northern Rio Grande Rift, in Machette, M.N., Coates, M-M., and Johnson, older subunit of stream alluvium along major composite terrace on east Garland SW quadrangle (see Kirkham and Heimsoth, 2002). Commonly quadrangle. Thickness 425–600 m where faulted, may be thicker along

Qai side of Ute Creek and at the south edge of quadrangle (see also mapping includes some eolian deposits and locally derived alluvium. Thickness 2–5 north boundary of quadrangle 20 Qac M.J., eds., 2007 Rocky Mountain section friends of the Pleistocene field 17 by Kirkham and Heimsoth, 2002). Probably associated with OIS 2 or 3 m; base of unit not exposed Xg Granite, undivided (Early Proterozoic)—Pinkish-tan to reddish, 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, p. equigranular, medium-grained granite exposed in northwestern quarter of 187–197. (available at http://pubs.usgs.gov/of/2007/1193) 

SOUTH GARLAND MESA 

quadrangle in the Ikes Creek block (fig. 2). Foliated to massive but Qai Tsb 

Qay Thompson, R.A., and Machette, M.N., 1989, Geologic map of the San Luis Hills 2 15 deeply weathered; forms grusy massive to subdued outcrops, and float Qai Tsf area, Conejos and Costilla Counties, Colorado: U.S. Geological Survey Qay consists of orange grus. Granite has phenocrysts of microcline, biotite, Miscellaneous Investigations Series Map I–1906, 1 sheet, scale 1:50,000. Qay Qai quartz, and subordinate plagioclase, but does not resemble any 

Qay Qay1 COLORADO Proterozoic rocks exposed to the north and northeast in the Blanca Peak Thompson, R.A., Machette, M.N., and Drenth, B.J., 2007, Preliminary geologic map 2 Qay2 Qai Qac of the Sanchez Reservoir Quadrangle and eastern part of the Garcia Quadrangle, (fig. 1) and upper Ute Creek areas (A.R. Wallace, USGS, unpub.

Qls 20 105°37'30" 105°15' mapping, 1995). Thickness unknown; base of unit not exposed Costilla County, Colorado: U.S. Geological Survey Open-File Report Qay Qc 1 Qa Alamosa 30 × 60-minute sheet 37°37'30" 2007–1074, 1 sheet, scale 1:24,000.
 

Wallace, A.R., 1995, Cenozoic rift-related sedimentation and faulting, northern 


Culebra Range, southern Colorado, in Bauer, P.W, Kues, B.S., Dunbar, N.W., 37°30' Map area 

37° 9° 106° Karlstrom, K.E., and Harrison, Bruce, eds., Geology of the Santa Fe Region: 105° Base from U.S. Geological Survey, Fort Garland, 1965 (photorevised 1982) SCALE 1:24 000 Geology mapped by A.R. Wallace and M.N. Machette in 2006 MAP LOCATION New Mexico Geological Society 1995 Guidebook, 46th Field Conference, p. 1 0 1 MILE using aerial photographs and field reconnaissance. Geology Polyconic projection. 1927 North American Datum 1/ 2 COLORADO compiled on a 1:24,000-scale topographic base map using a Contact 147–153.

Wallace, A.R., 1996, Geologic map of the Trinchera Ranch Quadrangle, Costilla 

TRUE NORTH

Quaternary fault—Bar and ball on downdropped side; solid where well located; County, Colorado: U.S. Geological Survey Miscellaneous Field Studies Map

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

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

photogrammetic plotter (Kern PG-2) and digital orthophotographic maps 1 .5 0 1 KILOMETER 

MAP LOCATION 105°37'30" 105°30' 105°22'30" 105°15' Digital data by T.R. Brandt CONTOUR INTERVAL 20 FEET 37° 37' 30" dashed where approximately located or inferred; dotted where concealed MF–2312–C, 1 sheet, scale 1:24,000. APPROXIMATE MEAN Wallace, A.R., 1997a, Geologic map of the Fort Garland Quadrangle, Costilla 

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

NATIONAL GEODETIC VERTICAL DATUM OF 1929 TWIN PEAKS BLANCA PEAK RUSSELL Editing and digital cartography by L.J. Binder DECLINATION, 2008 

MF–2169 Fault—Bar and ball on downdropped side; solid where well located; dashed where (Wallace, Denver Publishing Service Center MF–2312–D (Johnson and 

Bruce, 1991) mapping, 1995)

unpublished (Wallace, 1997b) approximately located or inferred; dotted where concealed MF–2312-E, 1 sheet, scale 1:24,000.

Manuscript approved for publication February 22, 2008 Wallace, A.R., 1997b, Geologic map of the Russell Quadrangle, Costilla County, Head scarp—Hachures on downdropped side. Solid where well located, dashed Colorado: U.S. Geological Survey Miscellaneous Field Studies Map MF–2312–D, where approximately located or inferred. Headwall failure due to landsliding 37°30' 1 sheet, scale 1:24,000.

FORT TRINCHERA Strike and dip of horizontal bedding Wallace, A.R., 2004, Evolution of the southeastern San Luis Basin margin and the

GARLAND RANCH Culebra embayment, Rio Grande rift, southern Colorado, in Brister, B.S., Bauer, 30 BALDY BLANCA SIM–2965 Strike and dip of inclined bedding P.W., Read, A.S., and Lueth, V.W., eds., Geology of the Taos Region: New MF–2312–C (revised from Mexico Geological Society 2004 Guidebook, 55th Field Conference, p. 

181–192. 

(Wallace, 1996) Wallace, 1997a) (Machette and 45 Strike and dip of inclined foliation Thompson, 37°22'30" unpublished mapping, 38 Wallace, A.R., and Soulliere, S.J., 1996, Geologic map of the Ojito Peak quadrangle, 2007) FORT Paleocurrent direction—Based upon orientation of imbricate clasts in Santa Fe

GARLAND OJITO PEAK Costilla County, Colorado: U.S. Geological Survey Miscellaneous Field Map MF­ Group sedimentary rocks. Symbol combined with strike and dip of bedding 

symbols SW 2312–B, 1 sheet, scale 1:24,000. MF–2312–B 

(Wallace and LASAUSES BLANCA SE (Kirkham and 

Heimsoth, 2002) Soulliere, 1996) DM-99-55 Sample locality for 40Ar/39Ar age determination 

37°15' Approximate location of the Blanca Trinchera No. 1 well (SW1⁄4 sec. 2, T. 30 Any use of trade, product, or firm names is for descriptive purposes only

and does not imply endorsement by the U.S. Government. Index map showing Fort Garland quadrangle and

REVISED GEOLOGIC MAP OF THE FORT GARLAND QUADRANGLE, COSTILLA COUNTY, COLORADO S., R. 72 W.) 

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 adjacent 7.5-minute quadrangles. For U.S 

Geological Survey publications, number, if available, reproduce copyrighted items for other than personal use must be secured 

By author, and date of publication are shown for from the copyright owner. 

quadrangle; for non-U.S. Geological Survey For sale by U.S. Geological Survey Information Services 37°07'30" publications, author and year of publication are Box 25286, Federal Center, Denver, CO 80225

Alan R. Wallace and Michael N. Machette
 

Printed on recycled paper 2008 

shown. Figure 1. Place names and features mentioned in 1-888-ASK-USGS Suggested Citation: the text. The Fort Garland quadrangle is indicated Wallace, A.R., and Machette, M.N., 2008, Revised geologic map of the Fort Garland quadrangle, A PDF of this report is available at: 

by the yellow box. Costilla County, Colorado: U.S. Geological Survey Scientific Investigations Map 2965, scale 1: 24,000. http://pubs.usgs.gov/sim/2965