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DESCRIPTION OF MAP UNITS

SURFICIAL DEPOSITS

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

Stratigraphic Code (North American Commission on Stratigraphic Nomenclature, 1983). 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 El Cajete tephra, erupted from the Jemez Mountains at ~50 to 60 ka (Reneau and others, 1996) 

provides an important late Pleistocene time-stratigraphic marker in much of the area. Soil development (presence and type of B horizons) and the morphologic stage of pedogenic carbonate 

accumulation, Stage I to Stage IV, provide another means of estimating the approximate age of 

Quaternary deposits in the area (Birkeland, 1999).

Artificial-Fill deposits

Artificial fill (latest Holocene)—Rock and earth fill of Cochiti Dam and associated 

structures

Alluvial deposits

Fan alluvium (Holocene to middle Pleistocene)—Cross-bedded, poorly sorted 

cobble to boulder gravel and poorly sorted, matrix-rich debris-flow deposits. 

Older deposits are preserved as remnants 5–15 m above active alluvial fans on 

slopes from 4° to 10°. Includes eolian deposits and local areas of ~55 ka El 

Cajete tephra along La Bajada fault escarpment in the southeast part of map area. 

Soils within deposit are poorly exposed, but locally thicker than 1 m, with Stage 

III or weak Stage IV carbonate morphology. Thickness probably 4–20 m; most 

contacts obscured by colluvium

Alluvium (Holocene)—Cross- to planar-bedded sand, pebbly sand, and thin beds of 

silty sand exposed along the Rio Grande, along active channels of tributary 

arroyos, beneath adjacent low (< 3 m) terraces and on low-gradient alluvial fans 

at the mouths of Sanchez and Medio Canyons. Beds generally less than 0.5 m 

thick. Includes some late Pleistocene deposits in upland areas northeast of the 

Town of Cochiti Lake. Actual thickness is probably greater than 10 m along the 

Rio Grande. Soils thin (< 0.5 m) ranging from A/C profiles to profiles with Bw 

and (or) Bk horizons with Stage II carbonate morphology on low terraces. 

Exposed thickness 2–4 m, base not exposed

Alluvial deposit 5 (Holocene to late Pleistocene)—Cross-to planar-bedded sand and 

pebbly sand, and thin beds of silty sand beneath narrow terraces 3–10 m above 

the Rio Grande. Beds generally <0.5 m thick. Most extensive deposits located 1 

km south of Alamo Canyon. Soils thin (<0.5 m) ranging from A/C profiles to 

profiles with Bw and (or) Bk horizons with Stage II carbonate morphology. 

Exposed thickness 3–10 m, base not exposed

Alluvial deposit 4 (late Pleistocene)—Well-sorted cobble to boulder gravel, 

cross-bedded sand, and thin-bedded sand of fluvial origin and fluvial and eolian 

silty sand beneath terrace remnants 14–20 m above the Rio Grande. Exposed 

mainly at mouths of tributaries such as Medio Canyon and in the southern part of 

the map area. Overlies Santa Fe Group deposits, landslide deposits, or older 

alluvium. Deposits are extensive near Cochiti Dam, where 5–10 m of axial river 

cobble-gravel overlies basaltic boulders and underlies piedmont alluvium mainly 

derived from the west. Deposits apparently lie beneath the ~55 ka El Cajete 

tephra. Soils <1.0 m thick and poorly exposed with Stage II or weak Stage III 

carbonate morphology. Thickness probably 4–20 m

Alluvial deposit 3 (middle Pleistocene)—Well-sorted cobble to poorly sorted 

boulder gravel, cross-bedded sand, and thin-bedded sand beneath terrace 

remnants 25–45 m above the Rio Grande, preserved at mouths of tributaries and 

in an extensive deposit near Alamo Canyon that contain angular boulders as large 

as 3.5 m. Clasts are predominantly axial river lithologies but are locally rich in 

andesitic and dacitic rocks derived from volcanic terrane to the west. Overlies 

and truncates rocks of the Santa Fe Group, landslide deposits, older alluvium or 

hydromagmatic deposits. Extensive deposits on both sides of Rio Grande near 

the mouth of Alamo Canyon are overlain by El Cajete tephra. Soils 0.5–1.0 m 

thick contain Stage II or Stage III carbonate morphology and local buried Bt 

horizons. Thickness 4–15 m

Alluvial deposit 2 (middle? Pleistocene)—Well-sorted cobble to boulder gravel 

containing layers composed of boulders as large as 3 m, exposed beneath terrace 

remnants approximately 50 to 70 m above the Rio Grande. Contact relations best 

exposed on both sides of Cochiti Lake, south and southwest of the Tetilla Peak 

overlook, where 8–25 m of gravelly axial channel deposits overlie lower 

Pleistocene and Pliocene cobble gravel of the Santa Fe Group; axial facies. These 

deposits contain laterally extensive boulder layers. Sandy to silt rich piedmont 

alluvium 2–8 m thick derived from eastern sources and layers of angular 

boulders of basalt interfinger with and overlie the axial alluvium in the southeastern part of map area. Overlies Santa Fe Group or landslide deposits. Deposits 

contain clasts of upper Bandelier Tuff and a local bed of ~0.55 Ma tephra derived 

from Valles caldera. Amino-acid ratios from gastropods in the upper 5 m of the 

deposits suggest a local age of 250 to 300 ka (see Dethier and McCoy, 1993). 

Deposits are overlain by El Cajete tephra (~55 ka; Reneau and others, 1996) and 

may be in part coeval with unit Qal. Soils are 0.8–1.5 m thick and contain Stage 

IV carbonate in gravel; finer piedmont deposits expose several buried Bt 

horizons locally and carbonate development is Stage III. Thickness generally 

10–30 m

Alluvial deposit 1 (early Pleistocene)—Poorly sorted boulder gravel containing 

clasts of volcanic rock as large as 4 m, well-sorted cobble gravel and beds of 

coarse sand exposed beneath terrace remnants 80–130 m above the Rio Grande 

south of the mouth of Capulin Canyon. Boulder-rich beds contain abundant clasts 

of andesite and welded Bandelier Tuff and minor basalt or axial-river lithologies 

derived from sources northwest of the modern river. Most exposures contain 

boulders of, and are thus younger than, the 1.22 Ma Tshirege Member of the 

Bandelier Tuff (Izett and Obradovich, 1994); basal exposures at several sites 

contain no axial lithologies and fill narrow paleodrainages cut into the Tshirege 

Member of the Bandelier Tuff. Deposits SE of the Town of Cochiti Lake contain 

boulders of the 1.61 Ma Otowi Member of the Bandelier Tuff (Izett and 

Obradovich, 1994). Deposits overlie basaltic or andesitic rocks in most areas. 

Overlain by eolian deposits, El Cajete tephra, and locally derived alluvium. Soils 

are 1.0 to > 2.0 m thick; finer overlying piedmont deposits expose multiple 

buried Bt horizons locally. Carbonate development is Stage III+ or Stage IV; 

carbonate-rich horizons locally brecciated and broken by vertical veins. Deposits 

at several sites between Alamo Canyon and Sanchez Canyon fill narrow 

paleocanyons cut before deposition of lower Bandelier Tuff (Reneau and Dethier, 

1996). Locally overlain by upper Bandelier Tuff. Thickness generally 10–30 m, 

but thicker near the mouth of Medio Canyon

Piedmont deposits

Piedmont alluvial deposit 5 (Holocene to late Pleistocene)—Cross-to planar-bedded 

sand and pebbly sand and poorly sorted gravel. Clasts predominantly andesite 

and silicic tuff derived from the Jemez Mountains to the west; matrix rich in 

lithic fragments and quartz. Deposits 0.5– 4.0 m thick are exposed beneath 

terraces 3–10 m above Rio Chiquito and adjacent tributaries graded to the Rio 

Grande. Beds generally <0.5 m thick. Soils are thin (<0.5 m) ranging from A/C 

profiles to profiles with Bw and (or) Bk horizons with Stage II carbonate. 

Exposed thickness 3–10 m, base not exposed

Piedmont alluvial deposit 4 (late Pleistocene)—Poorly sorted cobble to boulder 

gravel, sand, and local silty sand. West of the Rio Grande, clasts are predominantly andesite and silicic tuff derived from the Jemez Mountains to the west and 

the matrix is rich in lithic fragments and quartz. To the east, granitic and 

metamorphic clasts predominate and matrix is arkosic. Deposits 0.5–8.0 m thick 

are exposed beneath terraces 14–20 m above present channels graded to the Rio 

Grande. Exposures are extensive near Rio Chiquito and south to Bland Canyon. 

Overlies Cochiti Formation or Otowi Member of the Bandelier Tuff. Soils 

generally thin (<1 m) and moderately developed having local Stage III carbonate 

morphology and buried Bt horizons

Piedmont alluvial deposit 3 (middle Pleistocene)—Poorly sorted cobble to boulder 

gravel and sand. West of the Rio Grande, clasts are predominantly andesite and 

silicic tuff derived from the west and the matrix is rich in lithic fragments and 

quartz. East of the Rio Grande granitic and metamorphic clasts predominate and 

matrix is arkosic. Overlies Cochiti Formation, Otowi Member of the Bandelier 

Tuff, or Santa Fe Group deposits. Extensive exposures are preserved north of 

Bland Canyon. Soils are 0.5–1.0 m thick with Stage II or Stage III carbonate 

morphology; strongly developed Bt horizons locally are buried by El Cajete 

tephra. Deposits 0.5–8.0 m thick are exposed beneath terraces 25–40 m above 

present channels graded to the Rio Grande

Piedmont alluvium (middle? Pleistocene)—Poorly sorted cobble to boulder gravel 

and massive sand and local silt beds in the southeast part of the map area beneath 

terrace remnants 45–60 m above present channels. West of the Rio Grande, clasts 

are predominantly andesite and silicic tuff derived from sources in the Jemez 

Mountains west of the map area and the matrix is rich in lithic fragments and 

quartz. East of the Rio Grande, clasts are mainly granitic and metamorphic clasts 

and matrix is arkosic. Overlies Cochiti Formation, lower Bandelier Tuff, or 

deposits of the Santa Fe Group. Extensive exposures west of Santa Cruz Arroyo 

are interfingered with middle(?) Pleistocene alluvial deposits (Qa2) and include 

several carbonate horizons with Stage II or Stage III carbonate morphology and 

locally strongly developed buried Bt horizons 

20

8

4

20

14

8

8

11

Piedmont alluvium (early? Pleistocene)—Poorly sorted cobble to boulder gravel 

forming terrace remnants >60 m above present channels. Clasts rich in andesite 

and silicic tuff derived from sources in the Jemez Mountains. Overlies Cochiti 

Formation or Otowi Member of the Bandelier Tuff. Near the Town of Cochiti 

Lake, deposits overlie or grade laterally into deposits of unit Qa1. Poorly 

exposed

Sheetwash deposits

Sheetwash deposits (Holocene and early Pleistocene)—Sand, fine sand, silt, and 

one or more buried soils capping mesas isolated from major arroyos. Locally 

includes eolian deposits, some reworked. Most deposits include primary and 

reworked El Cajete tephra and admixed basaltic colluvium. Overlies basalt and 

andesite flows or older alluvial deposits. Overlain by eroded late Pleistocene or 

Holocene fluvial and eolian deposits. Soils within unit are 1.0 to > 2.0 m thick 

and contain buried Bt horizons and local Stage IV carbonate morphology and are 

locally brecciated. Thickness 2–8 m

Colluvial deposits

Colluvium (Holocene to middle Pleistocene)—Rockfall, debris-flow, and poorly 

sorted alluvial deposits at the base of cliffs, particularly along White Rock 

Canyon and major tributaries such as Capulin Canyon. Texture and clast 

lithology is variable. Deposits include ~55 ka El Cajete tephra in many areas. 

Overlies Bandelier Tuff in the northwest part of map area and volcanic rocks and 

landslides elsewhere. Northwest of the Pajarito fault zone, Goff and others 

(1990) showed colluvial deposits "...only where extensive or where covering 

critical contacts." Surface soils are thin (<0.5 m) and weakly developed at most 

locations, but lenses of El Cajete tephra, buried soils with strong Bt horizons 

(west of the Rio Grande) or K-horizons (in White Rock Canyon) demonstrate 

that many deposits are locally older than late Pleistocene. Thickness generally 

4–10 m, but locally exceeds 25 m along White Rock Canyon

Landslide deposit (Holocene to early Pleistocene)—Massive gravity slide deposits 

mantled by basaltic colluvium, mainly exposed along the steep walls of White 

Rock Canyon, along the La Bajada fault scarp and in the Jemez Mountains. 

Deposits consist of slump blocks with coherent internal stratigraphy near canyon 

rims and progressively deformed slumps and debris flows closer to the Rio 

Grande. Stratigraphic dips in the slump blocks range from 10° to 30° toward 

head scarps. Failures mainly occurred along steeply dipping planes rooted in the 

Santa Fe Group, and in hydromagmatic deposits and other altered volcanic rock. 

Slide material overlies rocks of the Santa Fe Group at most sites. Deposit 

morphology and inclusion of upper Bandelier Tuff in some deposits suggests that 

slides were active in early Pleistocene time, but that many stabilized in middle to 

late Pleistocene time. Carbonate morphology is Stage IV at some sites, and Stage 

III carbonate morphology and buried Btk horizons are present in most exposures. 

Soils are generally 0.8 to 1.4 m thick. Most slides are inactive, but lake 

drawdown after high levels in the mid-1980s reactivated several major slides 

along the western margin of Cochiti Lake. El Cajete tephra (~55 ka) overlies 

landslide deposits near Cochiti Lake and along the La Bajada escarpment 

SEDIMENTARY ROCKS AND DEPOSITS

Santa Fe Group and deposits

Cochiti Formation (lower Pleistocene to Miocene)—Weakly lithified pebble to 

cobble gravel, massive to planar-bedded sand, thin (<0.3 m) beds of rhyolitic 

tephra and pumiceous alluvium, beds of fine sand and silt, and silt-rich debris 

flows containing volcanic clasts. Gravel fraction rich in andesitic and silicic 

rocks derived from west of the map area. The sand fraction is rich in lithic 

fragments and quartz. Includes older alluvium of Goff and others (1990) and 

Smith and others (1970). On both sides of the Rio Grande, between Sanchez 

Canyon and the Town of Cochiti Lake, upper 2–10 m of the Cochiti Formation 

consists of dark-colored sand containing beds and concentrations of pumice, 

some of it tubular and nearly aphyric, correlated with the ~1.8 Ma San Diego 

Canyon ignimbrite (Spell and others, 1990; Smith and Kuhle, 1998). Interbedded 

with unit Tsfa south of the Town of Cochiti Lake and near the intersection of 

Cochiti and Bland Canyons. Underlies lower Bandelier Tuff or piedmont 

alluvium. Dips gently northeast near the Town of Cochiti Lake, more steeply 

(>10°) near faults. Northwest of the Pajarito fault zone, Cochiti Formation of 

Goff and others (1990) is a black to gray volcaniclastic unit consisting of debris 

flows, local zones of fluvial sediments and andesitic cinder cone debris, and 

flows of andesite too thin to show at map scale, interbedded with volcanic rocks 

of the Keres Group (Gardner, 1985; Gardner and others, 1986; Smith and others, 

2001). Thick deposits tend to be covered by colluvium, estimated maximum 

thickness >30 m, base not exposed

Santa Fe Group, axial channel facies (lower Pleistocene and Pliocene)—Slightly 

lithified pebble to cobble gravel rich in rounded clasts, arkosic sand, and thin 

beds of silty sand. Coarse units are 0.5–3.0 m thick, massive to cross-bedded, and 

locally planar-bedded. Gravel generally >70 percent quartzite, granite and other 

resistant rocks from northern New Mexico; andesitic and dacitic rocks from the 

Jemez Mountains common locally; vesicular basalt uncommon. Matrix quartzose 

or arkosic. Along White Rock Canyon, locally includes areas of pink silty 

sandstone that may correlate with the middle Santa Fe Group. Maximum 

thickness along Cañada de Cochiti; extensive exposures near Medio Canyon and 

in the southern part of the quadrangle on both sides of the Rio Grande. Fills 

channels in and locally interbedded with Cochiti Formation (QTc). Lies beneath 

landslide deposits, Pliocene basalt, or hydromagmatic deposits at most 

exposures. Paleocurrent direction measured on channels, gravelly crossbeds, and 

imbricated cobbles range from 140°–230°, and averages about 180°. Mainly 

undeformed except near faults. Near the mouth of Medio Canyon interlayered 

with a basaltic flow dated at about 2.5 Ma (Bachman and Mehnert, 1978). 

Thickness 5 to >70 m

Undivided Santa Fe Group (Pliocene to upper Miocene)—White, tan and pink, 

moderately lithified, planar-bedded to massive silty sandstone, siltstone, and 

cross-bedded pebbly sand in sparse channels. Sandstone beds up to 4 m thick; 

channels <1 m. Coarse units locally cemented with sparry calcite. Clasts mainly: 

(1) intermediate volcanic rocks and quartzite from southern Colorado and the 

northern Sangre de Cristo Range, and (2) Precambrian granite and associated 

rocks from the southern Sangre de Cristo Range. Matrix arkosic. Locally beneath 

tuffs of Canovas Canyon (Tkct) and unconformably overlies Galisteo Formation 

(Tgs) in the northwestern map area. Dips gently (<8°) to the west and northwest. 

Correlates, in part, with the Tesuque Formation of Galusha and Blick (1971), and 

the Ancha Formation of Spiegel and Baldwin (1963). Thickness >100 m

Pre-Santa Fe Group sedimentary deposits

Galisteo Formation (Eocene)—Tan and pink to brownish red beds of well-indurated 

sandstone, siltstone, arkose, and conglomerate; conglomerate beds locally 

contain limestone, chert, and granitic fragments from pebble to boulder size 

eroded from Paleozoic and Precambrian rocks. Exposed on rotated fault block 

with beds dipping steeply west in northwest part of map area. Unconformably 

underlies Santa Fe Group; base of unit not exposed; maximum observed 

thickness about 200 m

VOLCANIC ROCKS AND DEPOSITS

Tewa Group

The Pajarito Plateau area of the southeastern Jemez Mountains (fig. 1) is underlain by the Otowi 

(Qbo) and Tshirege (Qbt) Members of the Pleistocene Bandelier Tuff, erupted during collapse of 

the Valles caldera. These tuffs were deposited unconformably on underlying basaltic volcanic rocks 

of the Cerros del Rio volcanic field and on precaldera volcanic rocks of the Keres Group. Postcaldera volcanic deposits in the La Bajada constriction area are limited to reworked pumice and ash 

deposits of the approximately 55 ka El Cajete tephra (Reneau and others, 1996; Wolf and others, 

1996). This pumice and ash blanket the northeastern slopes of eroded volcanic highlands east of the 

Rio Grande.

El Cajete tephra (late Pleistocene)—White to gray, poorly sorted rhyolitic pumice 

fall deposits; pumice clasts contain sparse phenocrysts of quartz, sanidine, 

biotite, and clinopyroxene. Most exposures have been reworked and concentrated 

by fluvial or slope processes. Overlies buried soil containing a well-developed Bt 

horizon invaded by carbonate. Derived from the El Cajete vent in the Valles 

Caldera (fig. 1); age ~50 to 60 ka (Reneau and others, 1996). Exposed in local, 

dune-like outcrops as thick as 8 m

Bandelier Tuff

Tshirege Member of Bandelier Tuff (early Pleistocene)—White to pink slightly 

welded pyroclastic flows and a thin (< 0.5 m) pumiceous fall unit, both of 

rhyolitic composition and containing abundant phenocrysts of chatoyant sanidine 

and quartz and trace clinopyroxene, hypersthene, and fayalite. Consists of two to 

five pyroclastic flows separated by pumice concentrations or thin, sorted 

partings, exposed along deep canyons west of the Rio Grande; thinner (<25 m) 

flow sections are preserved locally in shallow canyons east of the Rio Grande. 

Thickness generally <60 m, but locally >90 m along Medio Canyon. Paleoflow 

directions to the southeast. Overlies lower Bandelier Tuff or pumiceous piedmont 

alluvium of the Toledo interval. Derived from the Valles caldera area northwest 

of the map area. 40Ar/39Ar age is 1.22 ± 0.02 Ma (Izett and Obradovich, 1994)

Otowi Member of Bandelier Tuff (early Pleistocene)—White to pink, nonwelded 

rhyolitic ash-flow tuff and a laminated to poorly sorted compound pumiceous fall 

unit as thick as 10 m, both containing abundant phenocrysts of chatoyant 

sanidine, quartz and sparse mafic phenocrysts. The Otowi Member consists of 

one to three thick ash-flow units comprising a compound-cooling unit as thick as 

30 m. Lithic fragments generally are more abundant than in unit Qbt. Best 

exposed south of Capulin Canyon and southwest of the Town of Cochiti Lake. 

Overlies 2 to 5 m of gray pumiceous alluvium (upper Cochiti Formation) in areas 

southwest of Lummis Canyon. Locally overlies axial gravel deposits (Qa1). 

Overlain by upper Bandelier Tuff, and andesitic flows. Derived from the Valles 

caldera northwest of the map area. 40Ar/39Ar age is 1.61±0.01 Ma (Izett and 

Obradovich, 1994)

LAVA FLOWS AND RELATED DEPOSITS

 OF THE CERROS DEL RIO VOLCANIC FIELD

The Cerros del Rio volcanic field is a predominantly basaltic to andesitic volcanic plateau 

along the southeast flank of the Jemez Mountains (fig. 1) of northern New Mexico. Lavas and 

related pyroclastic deposits of this field are locally exposed over 700 km2 and reflect eruption of at 

least 120 km3 of rift-related mafic magma, mainly between 2.7 and 1.1 Ma. Most of the lava flows 

are of Pliocene and early Pleistocene age and predate large-volume silicic caldera eruptions in the 

nearby Jemez Volcanic Field; however, late-stage eruptions from the Cerros del Rio volcanic field 

post-date eruption of the Tshirege Member (1.2 Ma; Valles Caldera) of the Bandelier Tuff in the 

Jemez volcanic field. Most of the eruptive centers in the Cerros del Rio volcanic field are centralvent volcanoes ranging from low-relief shield centers to remnants of steep-sided, breached cinder 

cones. These lavas have from 49 to 64 weight percent SiO2 and exhibit a strong correlation 

between landform and whole-rock chemistry. The low-silica, subalkaline basaltic lavas erupted 

from broad shield volcanoes and formed thin (< 3–4 m) low-viscosity flows that traveled far; 

conversely transitional to mildly alkaline basalts and basaltic andesites formed thick (as much as 30 

m) discontinuous lavas flows that erupted from high-relief, steep-sided, dissected vents. Dacitic 

lavas are related to late-stage dome growth and eruption of thick (as much as 50 m), even more 

viscous blocky lava flows that issued from one well-defined vent area at Tetilla Peak about 2 km 

south of the map area (Sawyer and others, 2002) and locally in tributary canyons of the Rio 

Grande, west of the map area.

Volcanic deposits of the Cerros del Rio volcanic field are divided into a three-fold classification representing early (1), middle (2), and late (3) phases of eruption that persisted from about 2.7 

Ma to 1.1 Ma. Volcanic units in the map area are believed to represent products of monogenetic 

eruptive centers of the early (>2.7–2.59 Ma), middle (2.59–2.1 Ma), and late (1.5–1.1 Ma) volcanic 

phases of Thompson and others (2006). These subdivisions are based on 1:24,000-scale geologic 

mapping, stratigraphic studies, 40Ar/39Ar geochronology, and paleomagnetic and aeromagnetic 

data. Some geochemical data were used to identify rock lithologies within the stratigraphic 

subdivisions for the entire field and were incorporated in unit descriptions, where available, for 

volcanic deposits in the map area. Volcanic rock names are based on the standard IUGS classification scheme (Le Bas and Streckeisen, 1991), however, only root names are used in map unit 

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

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

compositions are presented in table 1. Preliminary results of new 40Ar/39Ar age determinations are 

presented in unit descriptions. Locations for samples referenced in the text are presented in table 2. 

In places, map units represent the consolidation of deposits of limited extent and are based on 

similar lithologic character, stratigraphic position, inferred age, aeromagnetic and paleomagnetic 

signatures, and aerial extent of similar or related deposits. Map-unit nomenclature for the volcanic 

deposits in part reflects that proposed by Aubele (1978, 1979). Colors of the lava flows and cinder 

deposits used in unit descriptions were determined by comparison with a Munsell color chart 

(Munsell Color, 1995).

Andesite of Little Cochiti cone (early Pleistocene) —Reddish brown to medium 

gray andesite (58–60 weight percent SiO2) lava flows (Qlc) and associated 

pyroclastic deposits (Qlcc) erupted from small volcanic cone in the southeastern 

part of the map area herein referred to as Little Cochiti cone. Little Cochiti cone 

deposits post-date down-to-west offset along the La Bajada fault, whose footwall 

formed a topographic escarpment over which westward flowing lava cascaded 

toward the present day course of the Rio Grande. Subsequent down-to-west 

offset along the La Bajada fault may have accentuated the offset of deposits 

preserved on the hanging wall and footwall blocks. Basal lava flows are typically 

thin (3–5 m), platy and discontinuous. Sparse phenocrysts include olivine, 

pyroxene and plagioclase in decreasing order of abundance, xenocrysts of 

plagioclase and lesser quartz are common near the vent area centered about hill 

6598' on the Caja del Rio. Maximum exposed thickness is 70 m

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

agglutinate and minor flow material, present only in proximity to eroded vent 

area

Lava flows—Medium to dark gray andesite lava flows; typically thin (3–5 m), 

platy and discontinuous and are everywhere overlain by near-vent deposits, 

predominantly cinder and lesser amounts of spatter. An 40Ar/39Ar isotopic age of 

1.14±0.13 Ma was obtained from a lava flow near the head of Santa Cruz Arroyo 

(sample 96TCD08). Lava flows have reversed aeromagnetic signature and 

reverse magnetic polarity based on magnetic fluxgate determinations 

Dacite of Arroyo Montoso (early Pleistocene)—Dark gray to black, fine grained to 

glassy, aphyric dacite lava flows and dome remnants (64–65 weight percent 

SiO2) best exposed near the confluence of the Rio Grande and Arroyo Montoso. 

Lava flows are thick (up to 20 m) and massive with brecciated flow bases and 

flow foliated tops. Sparse phenocrysts of olivine ± pyroxene ± plagioclase occur 

locally in a microcrystalline to cryptocrystalline groundmass. Partially resorbed 

xenocrysts of plagioclase ± quartz are sparse but ubiquitous. An 40Ar/39Ar 

isotopic age of 1.31±0.08 Ma was obtained from a lava flow above the Rio 

Grande (sample DN85113). Locally overlies deposits of unit Qbo. Maximum 

exposed thickness is 170 m

Andesite of Cochiti volcano (early Pleistocene)—Reddish brown to medium gray 

lava flows and oxidized cinder and spatter erupted from Cochiti volcano near the 

southwestern boundary of the map area. Cochiti volcano deposits post-date 

down-to-west offset along the Cochiti fault, whose footwall scarp formed a 

topographic barrier to eastward deposition of lava flows. Consists of an upper 

(Qcu) and lower (Qcl) sequence of preserved lava flows and near vent pyroclastic material (Qcuc) associated with the upper unit. Vent area for upper unit is 

coincident with eroded, conical summit of Cochiti volcano in on the eastern 

margin of the map area. Maximum exposed thickness is 350 m

Upper unit, cinder deposits—Near-vent pyroclastic deposits, predominantly 

scoria and spatter agglutinate and minor flow material. Deposits are only present 

along the eastern edge of the map area

Upper unit, lava flows—Medium to dark gray andesite lava flows (59–62 weight 

percent SiO2); typically thin (3–5 m), platy and discontinuous and are everywhere overlain by near-vent deposits, predominantly cinder and lesser amounts 

of spatter. Sparse phenocrysts include olivine, pyroxene and plagioclase in 

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

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

Cochiti volcano (sample 3MRG3). These flows have reverse aeromagnetic and 

paleomagnetic signature

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

SiO2) containing sparse phenocrysts include olivine, pyroxene and plagioclase in 

decreasing order of abundance; slightly higher proportions of olivine than in unit 

Qcu. Flows are interpreted to have reversed magnetic polarity based on 

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

exposed 

Basaltic andesite of Cochiti Lake (early Pleistocene)—Medium- to dark-gray, 

basaltic andesite (54 weight percent SiO2) lava flows, typically 2–4 meters thick 

exposed along the Rio Grande near the southern edge of the map area. Sparsely 

phyric containing phenocrysts of olivine±pyroxene. An 40Ar/39Ar isotopic age of 

2.52±0.04 Ma (sample DN9612) was obtained from a lava flow on the west bank 

of the Rio Grande southeast of the community of Cochiti Lake. Flows are 

interpreted to have reversed magnetic polarity based on aeromagnetic signature. 

Maximum thickness 25 m

Basaltic andesite of Arroyo Eighteen (early Pleistocene)—Medium-gray, massive 

basaltic andesite (55–57 weight percent SiO2) lava flows (3–5m thick) containing 

sparse olivine ± pyroxene phenocrysts and abundant xenocrysts of plagioclase 

and quartz. Lava flows erupted from small cinder cones at the head of Arroyo 

Eighteen on the adjacent Montoso Peak quadrangle to the east. Erupted lavas 

flowed westward along a paleodrainage coincident with Arroyo Eighteen. An 

40Ar/39Ar isotopic age of 2.56±0.02 Ma (sample 3MRG6) was obtained from a 

lava flow in a tributary to Arroyo Montoso near the eastern edge of the map area. 

Reverse polarity based on paleomagnetic determination and aeromagnetic 

signature. Maximum exposed thickness is 65 m but is locally thickened or 

repeated by down-to-west normal faults

Basalt of La Bajada rim (Pliocene)—Medium- to dark-gray basaltic (48–50 weight 

percent SiO2) lava flows exposed principally along topographic rim of the La 

Bajada fault escarpment in the southeast part of the map area. Lava flows occur 

locally to the west of the escarpment in the hanging wall of the La Bajada fault. 

Includes both tholeiitic and transitional olivine basalts and locally basaltic 

andesites. Phenocrysts include olivine + pyroxene ± plagioclase and lesser 

iron-titanium oxides. Locally overlies paleochannel deposits of the Rio Grande. 

Likely erupted from buried and (or) eroded volcanic centers overlain by units 

Qcu. Maximum exposed thickness is 40 m

Basalt of Cochiti Lake (Pliocene)—Dark gray basalt lava flows (48–49 weight 

percent SiO2) preserved as prominent benches west of the Rio Grande, northeast 

of the community of Cochiti Lake. Lava flows are typically 1.5 to 2 m thick, 

having massive interiors and vesicular, rubbly flow bases and tops and contain 

small (<1 mm) phenocrysts of olivine and pyroxene in a fine grained to microcrystalline groundmass. Locally contains xenoliths of rounded Precambrian 

pebbles, likely recycled from underlying Santa Fe Group sediments. Normal 

polarity based on magnetic fluxgate determinations. Maximum exposed 

thickness is 70 m 

Andesite of Colorado Peak (Pliocene)—Andesitic lava flows grade transitionally 

into predominantly near-vent pyroclastic deposits near the summit and dissected 

southern flank of Colorado Peak, east of the map area on the Montoso Peak 

71⁄2-minute quadrangle. Most flows appear to have erupted from a central vent 

area characterized by a conspicuous summit crater and associated ring dikes and 

incipient flow breccias. Summit area is cut by northeast- and northwest-trending 

dikes. Dark-gray basaltic andesite to andesite (56–62 weight percent SiO2) lava 

flows up to 15 m thick containing phenocrysts of olivine and pyroxene and 

abundant xenocrysts of resorbed plagioclase and sparse quartz. Lava flows are 

lobate in form and directed predominantly to the west and northwest but 

truncated and offset to the west by down-to-west Cochiti normal fault located on 

the Montoso Peak quadrangle. An 40Ar/39Ar isotopic age of 2.60±0.10 Ma was 

obtained from a lava flow near the summit of Colorado Peak east of the map area 

(sample 96TMP38). Normal magnetic polarity based on paleomagnetic and 

aeromagnetic signature. Maximum exposed thickness is 75 m, but exceeds 225 m 

to the east on the adjacent Montoso Peak quadrangle

Andesite of Sanchez Canyon (Pliocene)—Medium gray to reddish brown (56 weight 

percent SiO2) andesite lava flows and associated near vent pyroclastic deposits 

exposed along the Rio Grande principally between Bland Canyon on the south 

and Capulin Canyon on the north. Locally includes oxidized andesitic scoria, 

breccia, spatter and spatter agglutinate interbedded with lava flows, particularly 

well preserved in the lower reaches of Bland Canyon where deposits may 

represent deeply dissected near vent deposits. Lava flows are discontinuous and 

variable in thickness from 1.5 to 10s of meters thick depending on local 

underlying paleotopography. Porphyritic lavas contain phenocrysts of olivine + 

pyroxene ± plagioclase in a fine-grained matrix. Locally, partially resorbed 

xenocrysts of plagioclase and quartz are present. Maximum thickness is 100 m

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

erupted from several eroded vent areas, the largest of which is preserved at Cerro 

Micho on the adjacent Montoso Peak quadrangle to the east. Thick lava flows up 

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

of the map area. Between White Rock Canyon and Arroyo Montoso, the section 

is exposed in several, down-to-west fault blocks. Scattered near-vent deposits, 

predominantly cinders, overlie associated lava flows and are thickest along a 

northwest-southeast trending alignment of dissected cinder and spatter cones 

extending from the northwest corner of the map to Cerro Micho. Medium-gray to 

reddish brown andesite (57–60 weight percent SiO2) lava flows. Although 

variable, most lava flows characteristically contain olivine and clinopyroxene 

phenocrysts, sparse to abundant plagioclase phenocrysts ± resorbed plagioclase 

xenocrysts in a fine-grained holocrystalline groundmass. Near-vent deposits at 

scattered localities along White Rock Canyon are reddish-brown pyroclastic 

deposits, predominantly scoria, spatter and lesser agglutinate. An 40Ar/39Ar 

isotopic age of 2.76±0.03 Ma (sample 3MRG5) was obtained from a lava flow in 

a tributary to Arroyo Montoso. Normal magnetic polarity based on paleomagnetic and aeromagnetic signature. Maximum exposed thickness is 300 m 

although thickness varies considerably and likely reflects eruption of lava flows 

onto an irregular paleotopographic surface

Andesite and basaltic andesite, undivided (Pliocene)—Undifferentiated lava flows 

and some interbedded pyroclastic and (or) hydromagmatic deposits preserved in 

White Rock Canyon north of Arroyo Montoso. Deposits locally include andesite 

and basaltic andesite lava flows and associated breccias overlying hydromagmatic deposits of unit Tbm and underlying lava flows of unit Tcm. Maximum 

exposed thickness is 130 m

Hydromagmatic deposits (Pliocene)—Bedded to massive fall, surge, and flowage 

deposits composed of basaltic tuff and cinders, accidental fragments of the Santa 

Fe Group and thin (<10 m), interlayered basaltic flows. Thickest exposures are 

along Arroyo Montoso and along the Rio Grande south of Alamo Canyon. 

Ash-fall beds 0.3–3.0 m thick, composed mainly of ash and lapilli containing 

sparse bombs of accidental and basaltic fragments. Surge beds are planar and 

cross-bedded, locally rippled, coarse silt to pebbly sand, generally 0.1–0.4 m 

thick. Flowage deposits mainly matrix-rich pebble to boulder gravel in discontinuous beds 1 to 4 m thick. Near maars the frequency of accidental fragments 

decreases up-section. Locally sheared, slumped or brecciated. Thickness of 

fragmental flow deposits and interlayered basalt flows ~60 m along Arroyo 

Montoso (Self and others, 1996). Interlayered with Pliocene basalt flows and 

generally lies above Pliocene axial deposits of the Santa Fe Group. Lies beneath 

flows of basalt and andesite. Older than ~2.5 Ma; maximum age not closely 

constrained

VOLCANIC ROCKS AND DEPOSITS OF THE KERES GROUP

Keres Group (Miocene)—All precaldera volcanic and volcaniclasitic deposits of the Pajarito 

Plateau area in the western part of the map area (units Tkpt, Tkbd, Tkhd, Tkha, Tkpa, Tka,

Tkca, Tkoa, and Tkct) belong to the Keres Group as defined by Bailey and others (1969) and 

modified by Gardner and others (1986) and Smith and others (2001). The Keres Group rests 

unconformably on Santa Fe Group sediments and locally on 16-Ma basaltic lava flows (unit Tsfb) 

related to rifting (Goff and others, 1990). The Keres Group consists predominantly of andesite to 

dacite lavas, pyroclastic deposits, volcanic breccias, and sedimentary deposits (Gardner and others, 

1986; Lavine and others, 1996). Age of the volcanic rocks ranges from approximately 12 Ma near 

the base of the Paliza Canyon Formation to approximately 6.5 Ma, the youngest age reported for 

the Bearhead Rhyolite; most volcanism occurred between 10 and 8 Ma (Gardner and others, 1986; 

McIntosh and Quade, 1995; Lavine and others, 1996; Justet, 1999). Unit descriptions are modified 

from Goff and others (1990).

Paliza Canyon Formation

Peralta Tuff Member of Bearhead Rhyolite—White to tan, lithic-rich ash-fall tuff; 

pumice fragments contain phenocrysts of quartz, potassium feldspar, 

biotite+plagioclase; lithic fragments consist of volcanics from nearby sources. 

Tkpt fills in rugged volcanic topography on earlier Keres Group rocks; sources 

for all tuff units unknown; K-Ar age of lowermost tuff bed at Peralta Canyon 

type locality 6.81±0.15 Ma; maximum observed thickness about 170 m

Paliza Canyon Formation

Biotite dacite—Domes, flows, and minor intrusive rocks of gray to pink porphyritic 

dacite containing phenocrysts of complexly zoned plagioclase and smaller 

phenocrysts of plagioclase, biotite, clinopyroxene, opaque minerals, ± 

hornblende, ± hypersthene, ± quartz, ± potassium feldspar; flows massive to 

sheeted; may contain flow breccia; K-Ar age of dome in upper Cochiti Canyon 

8.8±0.5 Ma; K-Ar age of dome in upper Bland Canyon 9.11±0.19 Ma; maximum 

observed thickness about 150 m

Hornblende dacite—Domes, flows, and minor intrusive rocks of gray to pink 

porphyritic dacite containing phenocrysts of complexly zoned plagioclase and 

smaller phenocrysts of plagioclase, hornblende, clinopyroxene, opaque minerals, 

± biotite, ± hypersthene, ± apatite, ± potassium feldspar; flows massive to 

sheeted; may contain flow breccia. K-Ar age of dome at head of Sanchez Canyon 

9.5±2.5 Ma; K-Ar age of dome on southeast slope of Boundary Peak 9.48±0.44 

Ma; maximum observed thickness about 60 m

Hornblende andesite—Black to gray to pink domes and flows of glassy to devitrified 

andesite having small phenocrysts of plagioclase, clinopyroxene, oxidized 

hornblende ± hypersthene ± rare biotite; may display flow banding or contain 

flow breccia; ages of various flows unknown; maximum observed thickness 

about 45 m

Porphyritic andesite—Gray to black domes and flows of coarse porphyritic andesite 

having large phenocrysts of plagioclase and abundant phenocrysts of clinopyroxene and hypersthene; flows may be sheeted or contain flow breccia; K-Ar age of 

flow at summit of St. Peters Dome 8.69±0.38 Ma; K-Ar age of dome west of 

Rancho Canada 9.33±0.19 Ma; maximum observed thickness about 150 m

Undivided andesite—Gray to pink to black flows, domes, and minor intrusives of 

porphyritic andesite containing phenocrysts of plagioclase, clinopyroxene, and 

hypersthene; flows massive to sheeted; may contain flow breccia; displays 

pervasive hydrothermal alteration in canyon bottoms; ages of various flows 

unknown; maximum observed thickness about 50 m

Clotted andesite—Gray to black flows of porphyritic andesite having abundant 

distinctive plagioclase-pyroxene clots 2 mm in diameter and phenocrysts of 

plagioclase, clinopyroxene, and hypersthene; flows massive, rarely sheeted or 

brecciated; ages of flows unknown; maximum observed thickness about 50 m

Olivine andesite—Black to gray domes and flows and minor red cinder deposits of 

plagioclase, clinopyroxene, hypersthene and iddingsitized olivine; flows massive 

with vesicular flow tops; flows rarely sheeted; ages of various flows unknown; 

maximum observed thickness about 210 m

Canovas Canyon Rhyolite—White to pink ash-fall and ash-flow tuffs best preserved 

in laharic deposits of the Cochiti Formation; "pink tuff" apparently underlies all 

other Keres Group volcanic rocks and overlies Santa Fe Group on south and east 

flanks of St. Peters Dome; estimated age range of unit 9.0–13.0 Ma; maximum 

observed thickness about 20 m

Basalt flows in Santa Fe Group

Basalt flows in Santa Fe Group (Miocene)—Thin discontinuous flows, pillow basalt 

and palagonite tuff composed of black alkali basalt and basanite containing small 

phenocrysts of olivine ± clinopyroxene ± plagioclase; vesicles commonly filled 

with calcite; olivine may be altered to iddingsite; basalts occur in upper 

exposures of Santa Fe Group on lower south and east flanks of St. Peters Dome 

(Frijoles 71⁄2-minute quadrangle); one small intrusive plug of alkali basalt in 

Galisteo Formation is correlated with this unit; K-Ar age of thin flow in upper 

Medio Canyon 16.5±1.4 Ma. Maximum observed thickness about 5 m

af

Qaf

Qa

Qa5

Qa4

Qa3

Qa2

Qa1

Qp5

Qp4

Qp3

Qp2

Qp1

Qsw

Qc

Qls

QTc

QTsfa

Tsf

Tgs

Qec

Qbt

Qbo

Qlcc

Qlc

Qd

Qcuc

Qcu

Qcl

Qac

Qae

Tb

Tbc

Tca

Tas

Tcm

Ta

Tbm

Tkpt

Tkbd

Tkhd

Tkha

Tkpa

Tka

Tkca

Tkoa

Tkct

Tsfb

R

R

R

R

R

R

R

N

N

N

INTRODUCTION

The Cochiti Dam quadrangle is located in the southern part of the Española Basin (fig. 1) and 

contains sedimentary and volcanic deposits that record alluvial, colluvial, tectonic and volcanic 

processes over the past seventeen million years. The geology was mapped from 1997 to 1999 and 

modified from 2004 through 2008. The primary mapping responsibilities were as follows: Dethier 

mapped the surficial deposits, basin-fill sedimentary deposits, Miocene to Quaternary volcanic 

deposits of the Jemez volcanic field, and a preliminary version of fault distribution. Thompson and 

Hudson mapped the Pliocene and Quaternary volcanic deposits of the Cerros del Rio volcanic field. 

Thompson, Minor, and Hudson mapped surface exposures of faults and Hudson conducted 

paleomagnetic studies for stratigraphic correlations. Thompson prepared the digital compilation of 

the geologic map.

The mapped distribution of units is based primarily on interpretation of 1:16,000-scale, color 

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

1996 (U.S. Geological Survey National Aerial Photography Program (NAPP) available at 

http://eros.usgs.gov/#/Guides/napp). Most of the contacts on the map were transferred from the 

aerial photographs using a photogrammetric stereoplotter and subsequently field checked for 

accuracy and revised based on field determination of allostratigraphic and lithostratigraphic units. 

Determination of lithostratigraphic units in volcanic deposits was aided by geochemical data, 

40Ar/39Ar geochronology, aeromagnetic and paleomagnetic data (Grauch and Bankey, 2003; 

Grauch and others, 2006; Thompson and others, 2006). Supplemental revision of mapped contacts 

was based on interpretation of U.S. Geological Survey (USGS) 1-meter orthoimagery. This version 

of the Cochiti Dam quadrangle geologic map uses the USGS 1:24,000-scale Cochiti Dam, New 

Mexico quadrangle as the topographic base overlain on a screened shaded relief base generated 

from 10-m digital elevation model (DEM) data from the USGS National Elevation Dataset (NED, 

http://ned.usgs.gov/); sun illumination is from the northwest at 45° above the horizon.

For continuity, geologic units are delineated in areas now inundated by Cochiti Lake and in 

areas where natural topography was altered by excavation of material used in the construction of 

Cochiti Dam. Contacts in these areas are drawn using topography mapped in 1953, guided by 

aerial-photographic interpretation and mapping in adjacent areas. Northwest of the Pajarito fault 

zone in the northwest part of the map area, contacts and map unit descriptions were derived largely 

from Goff and others (1990). Major tributary canyons to the Rio Grande are labeled in figure 2 and 

a simplified fault diagram is shown as figure 3.

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the Santo Domingo basin, New Mexico: New Mexico Geology, v. 20 (1), p. 21–27.

Smith, R.L., Bailey, R.A., and Ross, C.S., 1970, Geologic map of the Jemez Mountains, New 

Mexico: U.S. Geological Survey Miscellaneous Geologic Investigations Map I–571, scale 

1:125,000.

Smith, G.A., McIntosh, William, and Kuhle, A.J., 2001, Sedimentologic and geomorphic evidence 

for seesaw subsidence of the Santo Domingo accommodation-zone basin, Rio Grande rift, 

New Mexico: Geological Society of America Bulletin, v. 113, p. 561–574.

Spell, T.L., Harrison, T.M., and Wolff, J.A., 1990, 40Ar/39Ar dating of the Bandelier Tuff and San 

Diego Canyon ignimbrites, Jemez Mountains, New Mexico—Temporal constraints on 

magmatic evolution: Journal of Volcanology and Geothermal Research, v. 43, pp. 175–193.

Spiegel, Zane, and Baldwin, BBrewster, 1963, Geology and water resources of the Santa Fe area, 

New Mexico: U.S. Geological Survey, Water-Supply Paper 1525, 258 p.

Thompson, R.A., Sawyer, D.A., Hudson, M.R., Grauch, V.J.S., and McIntosh, W.C., 2006, 

Cenozoic volcanism of the La Bajada constriction area, in Minor, S.A., ed., The Cerrillos 

Uplift, the La Bajada Constriction, and hydrogeologic framework of the Santo Domingo basin, 

Rio Grande rift, New Mexico, chap. C: U.S. Geological Survey Professional Paper 1720, p. 

43–60.

U.S. Geological Survey Geologic Names Committee, 2010, Divisions of geologic time—Major 

chronostratigraphic and geochronologic units: U.S. Geological Survey Fact Sheet 2010–3059, 

2 p.

Wolf, J.A., Gardner, J.N., and Reneau, S.L., 1996, Field characteristics of the El Cajete pumice 

deposit and associated southwestern moat rhyolites of the Valles caldera, in Goff, Fraser, Kues, 

B.S., Rogers, M.A., McFadden, L.D., and Gardner, J.N., eds., New Mexico Geological Society 

47th field conference, Jemez Mountains Region, Guidebook 47, p. 311–316.

ACKNOWLEDGMENTS

The authors thank Margaret E. Berry and Florian Maldonado of the U.S. Geological Survey 

for their thorough reviews that greatly improved the report. V.J.S. Grauch prepared, and assisted 

with interpretation of, aeromagnetic data used to constrain lithostratigraphic units in the volcanic 

rocks. We gratefully acknowledge Daniel P. Miggins of the U.S. Geological Survey and William 

McIntosh of New Mexico Institute of Mining and Technology for previously unreported 40Ar/39Ar 

age determinations of volcanic rocks referenced in this report. 

af

Qaf

Qa

Qa5 Qp5

Qa4 Qp4

Qa3 Qp3

Qa2 Qp2

?

?

Qa1 Qp1

Qls

SURFICIAL DEPOSITS

Artificial-fill

deposits

Alluvial

deposits

Colluvial

deposits

Piedmont

deposits

Sheetwash

deposits

QTc

QTsfa

SEDIMENTARY ROCKS

AND DEPOSITS

Qec

TEWA

GROUP

LAVA FLOWS AND RELATED

DEPOSITS OF THE

CERROS DEL RIO VOLCANIC FIELD

VOLCANIC ROCKS AND DEPOSITS

VOLCANIC ROCKS AND DEPOSITS OF THE KERES GROUP

Paliza Canyon Formation

CORRELATION OF MAP UNITS

11.7 ka

126 ka

781 ka

2.59 Ma

late

middle

early

Holocene

Pleistocene

Pliocene

Miocene

Eocene

TERTIARY

QUATERNARY

Qsw

Qc

Tsf

Tgs

Qbt

Qbo

Qlcc

Pre-Santa Fe Group sedimentary deposits

Santa Fe Group deposits

Qlc Qd

Qcuc Qcu

Qcl

Qac

Qae

Tb

Tbc

Tca Tas

Tcm

Ta

Tbm

Tkpt

Tkbd

Tkhd Tkha

Tkpa

Tka

Tkca

Tkoa

Tkct

Tsfb

1.14 Ma 1.31 Ma

2.52 Ma

2.56 Ma

2.76 Ma

2.76? Ma

>2.59 Ma

>2.59 Ma

2.6 Ma >2.59

Ma

1.7–1.5 Ma

Bandelier

Tuff

N

R R

R

R

R

R R

N

N

White and black characters represent aeromagnetic polarity signature and paleomagnetic signature respectively.

N, normal; R, Reverse. Approximate eruption ages from table 1.

Basalt flows in 

Santa Fe Group

2.59 Ma

MATUYAMA

Reverse polarity chron

GAUSS

Normal polarity chron

5.33 Ma

23.0 Ma

View southwest from above the confluence of Arroyo Montoso and Rio Grande in White Rock Canyon. Photograph by Ren Thompson, 20 Vi th t f b th fl f A Mt d Ri G d i Whit R k C Ph t h b R Th 2006.

Yucca baccata. Photograph by

Stan Shebs, 2004. Image available at: 

http://upload.wikipedia.org/wikipedia/co

mmons/9/96/Yucca_baccata_whole.jpg.

Rubbly base of basaltic andesite lava flow typical of flow morphology of mafic lavas exposed along La Bajada Fault. Photograph

by Ren A Thompson, 2006.

View northwest toward mouth of Tetilla Canyon in the southeastern part of the map area. Photograph by Ren Thompson, 2006.

Roadrunner. Image WO3862 by Gary Kramer,

U.S. Fish and Wildlife Service, 2008.

http://digitalmedia.fws.gov/.

Sample no. Quadrangle Map unit *Latitude *Longitude

96TCD08 Cochiti Dam 35o38' 48" 106o16'43" 

DN85113 Cochiti Dam 35o43'05" 106o16'16" 

3MRG3 Cochiti Dam 35o41'13" 106o16'33" 

DN9612 Cochiti Dam 35o38'32" 106o19'11" 

3MRG6 Montoso Peak 35o42'57" 106o14'07" 

96TMP38 Montoso Peak 35o39'49" 106o13'10" 

3MRG5 Montoso Peak 35o43'01" 106o13'57" 

*Geographic Coordinate System NAD 27 Datum 

Qlc

Qd

Qcu

Qac

Qae

Tca

Tcm

Table 2. Locations for samples referenced in the text

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

Eruption Age1

(Ma) Volcanic rock name2

Andesite of Little Cochiti cone Qlc 3 1.14 Trachyandesite

Dacite of Arroyo Montoso Qd 3 1.31 Dacite/trachyte

Upper andesite of Cochiti volcano Qcu 3 1.51 Andesite/trachyandesite

Lower andesite of Cochiti volcano Qcl 3 1.7–1.5 Andesite/basaltic andesite

Basaltic andesite of Cochiti Lake Qac 2 2.52 Basaltic trachyandesite

Basaltic andesite of Arroyo Eighteen Qae 2 2.56 Basaltic trachyandesite

Basalt of La Bajada Rim Tb 1 >2.59 Basalt/basaltic andesite

Basalt of Cochiti Lake Tbc 1 >2.59 Basalt

Andesite of Sanchez Canyon Tas 1 >2.59 Andesite/trachyandesite

Andesite of Colorado Peak Tca 1 2.6 Trachyandesite

Andesite of Cerro Micho Tcm 1 2.76 Trachyandesite

Andesite and basaltic andesite, undivided Ta 1 >2.76? Andesite/trachyandesite

/basaltic andesite

1Eruption ages are based on one or more ages where determined for the map area or estimated based on stratigraphic position

2Modified root names based on IUGS rock classification (LeBas and Streckeisen, 1991)

Map unit name Map unit

symbol

Eruptive

series

106°15’00” 106°07’30”

35°45’00”

35°37’30”

PJ

SF

LB

CO

CC

CM

CP

TH

AC

LB

TT

LT

PJ

SF

LB

CO

CC

CM

CP

TH

AC

LB

TT

LT

0

0

5 KILOMETERS

5 MILES

LA

WR Rio Grande

BM

Cañada Ancha

TM

MP

CM

CP CV

CC

TH

TP

CL

Santa Fe River

MJ

LA

Pajarito

Plateau

Pajarito

Plateau WR Rio Grande

Rio

Grande

BM

Cañada AnchaCañada

Ancha

TM

MP

CM

CP CV

CC

TH

Santa Fe

Airport

TP

CL

Santa Fe RiverSanta Fe

River

MJ

Figure 3. Landsat 7 image (30-m band 7–4–2 merged with 15-m band 8) acquired on October 

14, 1999; image clip from Sawyer and others, 2004) of Cerros del Rio volcanic field showing 

approximate eruptive extent as purple dashed line, mapped vent areas as red stars and 

presumed buried vents based on aeromagnetic data as yellow stars. Geographic abbreviations: 

LA, Los Alamos; WR, White Rock; CL, Cochiti Lake; BM, Buckman Mesa; OM, Ortiz 

Mountain; MP, Montoso Peak; CM, Cerro Micho; TH, Twin Hills; CP, Cerro Portillo; CC, 

Cerro Colorado; CV, Cochiti Volcano; TP, Tetilla Peak; MJ, Mesita de Juana. Major mapped 

faults and lineaments are shown in red and aeromagnetic lineaments related to buried faults are 

shown in purple. Fault abbreviations: PJ, Pajarito fault; SF, San Francisco fault; LB, La 

Bajada fault; TT, Tetilla fault; CM, Cerro Micho fault; CC, Cerro Colorado fault; CO, Cochiti 

fault; TH; Twin Hills fault; CP, Cerro Portillo fault; LT, Las Tetillitas fault; AC, Arroyo 

Calabasas fault. Quadrangle outline in yellow.

106°22’30”

35°45’00”

106°15’00”

35°37’30”

0123 4 5 MILES

0 1 2 3 4 5 KILOMETERS

Cochiti

volcano

Cochiti

volcano

Cochiti

Lake

Cochiti

Lake

Rio

Grande

Bland

Canyon BlandCanyon

Cochiti Cochiti

Canyon

Sanchez Sanchez Sanchez

Canyon

Medio

Canyon

Capulin Capulin

Canyon

PAJARITO PAJARITO

PLATEAU PLATEAU

Alamo

Canyon

Lumis Canyon

Arroyo Montoso Montoso

WHITE

ROCK

CANYON

Cochiti

Lake

Cochiti

Lake

BANDELIER

NATIONAL MONUMENT

BANDELIER WILDERNESS

SANTA FE

NATIONAL FOREST

Boundaries modified from U.S. Geological Survey, Cochiti Dam, NM, 1:24,000-scale map, 2002. DOME WILDERNESS SANTA FE NATIONAL FOREST

Figure 2. Landsat 7 satellite image (30-m band 7–4–2 merged with 15-m band 8) acquired on October 14, 

1999, of Cerros del Rio, New Mexico area (image clip from Sawyer and others, 2004), showing select 

geologic features and geographic names. Yellow rectangle is the boundary of the Cochiti Dam 7.5-minute 

quadrangle.

25

40 25

Abiquiu

embayment

Chama

Rio

JEMEZ

MTS Basin

Española

Grande

Basin

Albuquerque

Sangre de Cristo

Mountains

Sandia Mts

Nacimiento Mts

Española

Valles

caldera

Rio

LB

JZ

Rio

Grande

LB

PJ

EB

EB

Rio

Jemez

CdRvf

TPvf

SF

Toledo

embayment

Taos

Santa Fe

SAMvf

Albuquerque

San

Luis

Basin

0 10 20 30 MILES

0 10 20 30 KILOMETERS

35°00'

35°30'

36°00'

107°00' 106°30' 106°00' 105°30'

36°30'

Montoso

Peak Agua

Cochiti Fria

Dam

White Rock

Tetilla

Peak

Bland Frijoles

Cañada

Santo

Domingo

Pueblo SW

Santo

Domingo

Pueblo

Rio Grande

Rift

NEW

MEXICO

Area of 

fig. 1

Holocene-Oligocene sediments

Pleistocene-Pliocene basaltic volcanic rocks

Pleistocene-Miocene volcanic rocks

Lower Oligocene–Paleozoic rocks

Precambrian rocks

EXPLANATION

25

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

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

embayment) is modified from Gardner and Goff (1996). Volcanic fields: CdRvf, Cerros del Rio volcanic field; SAMvf, 

Santa Ana Mesa volcanic field; TPvf, Taos Plateau volcanic field. Faults: EB, Embudo fault, JZ, Jemez fault; LB, La 

Bajada fault; PJ, Pajarito fault; SF, San Francisco fault.

Contact

Fault—Bar and ball on downthrown side

Reverse Fault—Dotted where concealed

Interstate highway

U.S. Department of the Interior Scientific Investigations Map 3194

U.S. Geological Survey

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

CONTOUR INTERVAL 20 FEET

NATIONAL GEODETIC VERTICAL DATAUM OF 1929 

The geology was mapped from 1997–1999 and modified in 2004–2008. The primary 

mapping responsibilities were as follows: Dethier mapped the surficial deposits, 

basin-fill sedimentary deposits, Miocene to Quaternary volcanic deposits of the Jemez 

volcanic field, and a preliminary version of fault distribution. Thompson and Hudson 

mapped the Pliocene and Quaternary volcanic deposits of the Cerros del Rio volcanic 

field. Thompson, Minor and Hudson mapped surface exposures of faults and Hudson 

conducted paleomagnetic studies for stratigraphic correlations. Thompson prepared the 

digital compilation of the geologic map.

APPROXIMATE MEAN

DECLINATION, 2008 TRUE NORTH

MAGNETIC NORTH

10°

NEW MEXICO

QUADRANGLE LOCATION

Base from U.S. Geological Survey Cochiti Dam, New Mex., 1953, revised 1993

North American Datum of 1927 (NAD 27)

Projection and 10,000-foot ticks: Universal Transverse Mercator

New Mexico coordinate system, central zone

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

Period or

subperiod

Epoch Age

Quaternary Holocene

Pleistocene

late

middle

early

0–11.7 ka

11.7–132 ka

132–788 ka

788–2.59 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 Richmond and Fullerton (1986). 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 miles (mi)

millimeters

centimeters (cm)

meters (m)

kilometers (km)

For more information concerning this publication, contact:

Center Director, USGS Geology and Environmental Change Science Center

Box 25046, Mail Stop 980

(303)236-5344

Or visit the Geology and Environmental Change Science Center Web site at:

http://esp.cr.usgs.gov/ 

Any use of trade, product, or firm 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 for other than personal use

must be secured from the copyright owner. 

Publishing support provided by: 

Denver Publishing Service Center

Edit and digital layout by L.J. Binder

Manuscript approved for publication August 31, 2011

This and other USGS information products are available at

http://store.usgs.gov/ 

U.S. Geological Survey

Box 25286, Denver Federal Center

Denver, CO 80225

To learn about the USGS and its information products visit

http://www.usgs.gov/ 

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This report is available at:

http://pubs.usgs.gov/sim/3194

Suggested citation:

Dethier, D.P., Thompson, R.A., Hudson, M.R., Minor, S.A., and Sawyer, D.A., 2011, Geologic 

map of the Cochiti Dam quadrangle, Sandoval County, New Mexico: U.S. Geological 

Survey Scientific Investigations Map 3194, 1 sheet, scale 1:24,000. 

Prepared in cooperation with the

National Park Service

SANTO DOMINGO

PUEBLO SW

SANTO DOMINGO

PUEBLO

TETILLA PEAK

35°52’30”

Index map showing Cochiti Dam quadrangle, in yellow, 

and adjacent 7.5-minute quadrangles. For U.S. 

Geological Survey publications, number, authorship, 

and date of publication are shown; for the non-U.S. 

Geological Survey publication, number, authorship, 

year of publication, and affiliation are shown.

CAÑADA COCHITI DAM MONTOSO PEAK

FRIJOLES WHITE ROCK

35°37’30”

35°30’00”

106°30’00” 106°22’30” 106°15’00”

35°45’00”

106°22’30”

BLAND

SIM–3179

(Thompson and

others, 2011)

MF–2352

(Sawyer and

others, 2002)

SIM–3194

(Dethier and

others, 2011)

GM–73

(Dethier, 1997)

New Mexico Bureau of

Geology and

Mineral Resources

Geologic Map of the Cochiti Dam Quadrangle, Sandoval County, New Mexico

By

David P. Dethier, Ren A. Thompson, Mark R. Hudson, Scott A. Minor, and David A. Sawyer

2011

EXPLANATION

Contact

Normal fault—Dashed where inferred from aerial photographs, dotted where 

concealed; bar and ball on apparent downthrown side

Lineament—Located using aerial photographs and reduced to pole

aeromagnetic map; coincides with zones of minor fracturing and faulting

Lava flow direction—Inferred lava flow direction for select volcanic rock 

units

Dike

Inclined bedding—Showing strike and dip

3MRG3 Sample location—With sample number

5

AM