GRAND

MESA

SAN LUIS

VALLEY

SAN

SAN JUAN BASIN

 UPLIFT

NACIMIENTO

VALLES CALDERA SANGRE DE CRISTO MOUNTAINS

CHAMA BASIN

BRAZOS

UPLIFT

JUAN

MOUNTAINS

ANTICLINORIUM

UNCOMPAHGRE PLATEAU

ARCHULETA

SAN JUAN BASIN RIM Durango

Arkansas River

Rio Grande

Pagosa Springs

 Quadrangle

Aspen

Gallup

Gypsum

DENVER

Alamosa

Trinidad

Montrose Gunnison

SANTA FE

Farmington

Grand

Junction

Colorado

Springs

Pueblo

Rio Grande

Colorado River

COLORADO

NEW MEXIO

SAN JUAN

DOME

U.S. Department of the Interior

U.S. Geological Survey

Pamphlet to accompany

Scientific Investigations Map 3419

Geologic Map of the Pagosa Springs 7.5' Quadrangle, 

Archuleta County, Colorado
Cover. Photograph showing town of Pagosa Springs, Colo. The low terraces are north of town, and 

Pagosa Peak is on skyline. Taken about 2 miles south of Pagosa Springs, just outside of southern map 

area border, looking north. Photograph by David Moore, 1980.
Geologic Map of the Pagosa Springs 7.5' 

Quadrangle, Archuleta County, Colorado

By David W. Moore and David J. Lidke

Pamphlet to accompany

Scientific Investigations Map 3419

U.S. Department of the Interior

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

RYAN K. ZINKE, Secretary

U.S. Geological Survey

James F. Reilly II, Director

U.S. Geological Survey, Reston, Virginia: 2018

For more information on the USGS—the Federal source for science about the Earth, its natural and living 

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Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials 

as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

Suggested citation:

Moore, D.W., and Lidke, D.J., 2018, Geologic map of the Pagosa Springs 7.5' quadrangle, Archuleta County, Colorado: 

U.S. Geological Survey Scientific Investigations Map 3419, pamphlet 24 p., scale 1:24,000, 

https://doi.org/10.3133/sim3419.

The ScienceBase data release GIS files that support this report can be found at https://doi.org/10.5066/P9WRSUI6

ISSN 2329-132X (online)
iii

Contents

Location and General Geology ....................................................................................................................1

Brief History....................................................................................................................................................1

Precambrian Time and Paleozoic Era ...............................................................................................1

Mesozoic Era.........................................................................................................................................1

Jurassic Period ............................................................................................................................1

Middle and Late Jurassic Epoch (175−145 Ma).............................................................1

Cretaceous Period.......................................................................................................................4

Early Cretaceous Epoch (145−100 Ma)............................................................................4

Late Cretaceous Epoch (100−66 Ma)...............................................................................4

Cenozoic Era..........................................................................................................................................4

Paleogene Period (66−23 Ma)....................................................................................................4

Paleocene, Eocene, and Oligocene Epochs ..................................................................4

Neogene Period (23−2.6 Ma) .....................................................................................................5

Miocene and Pliocene Epochs.........................................................................................5

Quaternary Period (2.6 Ma−Present)........................................................................................5

Pleistocene and Holocene Epochs..................................................................................5

Surficial Deposits...........................................................................................................................................6

Gravel......................................................................................................................................................6

Other Deposits.......................................................................................................................................6

Structure..........................................................................................................................................................6

Cross Sections and Subsurface Information ............................................................................................8

Acknowledgments.......................................................................................................................................10

Description of Map Units............................................................................................................................11

Surficial Deposits................................................................................................................................11

Artificial Fill .................................................................................................................................11

Precipitated Rock ......................................................................................................................11

Alluvial Deposits.........................................................................................................................11

Glacial and Glaciofluvial Deposits ..........................................................................................13

Mass Wasting Deposits ...........................................................................................................14

Bedrock Units......................................................................................................................................14

Sedimentary Rock......................................................................................................................14

Igneous and Metamorphic Rock.............................................................................................18

References Cited..........................................................................................................................................19

Glossary.........................................................................................................................................................23

Sheet

Geologic Map of the Pagosa Springs 7.5' Quadrangle, Archuleta County, Colorado .................. link
iv

Conversion Factors

U.S. Customary Units to International System

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Figures

1. Location and setting of Pagosa Springs quadrangle showing outline of quadrangle......2

2. Photograph showing town of Pagosa Springs, Colo. The low terraces are

north of town, and Pagosa Peak is on skyline .........................................................................3

3. Oldest fluvial terrace gravel (Qtg5) in gravel pit, on Oakbrush Hill in the map area,

section 6, Township 35 N., Range 2 W; thickness 18 ft...........................................................7

4. Pinedale glacial outwash (Qop), 15–25-foot-thick, overlying shale of the Mesaverde

Group (Kmv) exposed at Mesa Ditch in the northeast part of the Pagosa Springs

7.5' quadrangle..............................................................................................................................8

5. Stratigraphic column showing map units of the Pagosa Springs 7.5' quadrangle ...........9
v

Divisions of Geologic Time Used in This Report1

Period Epoch Age2

Quaternary

Holocene3

late 0−4.2 ka

middle 4.2−8.2 ka

early 8.2−11.7 ka

Pleistocene4

late 11.7−126 ka

middle 126−781 ka

early5 781 ka−2.58 Ma

Cretaceous Upper6

Maastrichtian 66.0−72.1 Ma

Campanian 72.1−83.6 Ma

Santonian 83.6−86.3 Ma

Coniacian 86.3−89.8 Ma

Turonian 89.8−93.9 Ma

Cenomanian 93.9−100.5 Ma

1

Divisions of geologic time were selected for emphasis in this study. 

2

Ma, million years; ka, thousand years.

3

Subdivisions of the Holocene are informal divisions advocated by Walker, M.J.C and others (2012).

4

Subdivisions of the Pleistocene are informal divisions from Cohen and others (2013; updated).

5

Early Pleistocene as applied here is the combination of the Calabrian and Gelasian Ages from Cohen and others 

(2013; updated).

6

Subdivisions of the Upper Cretaceous are divisions from Cohen and others (2013; updated). 

Geologic Map of the Pagosa Springs 7.5' Quadrangle, 

Archuleta County, Colorado

By David W. Moore and David J. Lidke

Location and General Geology

The Pagosa Springs 7.5' quadrangle is in southwest 

Colorado between the San Juan Basin to the southwest and 

the San Juan Mountains north and east; it covers an area of 

about 59 sq. miles (mi) (fig. 1). The town of Pagosa Springs, 

population 1,727 (U.S. Census, 2010) is the Archuleta 

County seat. U.S. Highway 160 runs east-west through the 

center of town. The San Juan Basin, centered in northwestern 

New Mexico, is enclosed by outcrops of Late Cretaceous 

sedimentary rocks. Pagosa Peak (12,640 ft) north of the 

quadrangle and other nearby peaks of the San Juan Mountains to the east, are composed of volcanic rocks (figs. 1 and 

2). The San Juans are thick remnants of nearly level layers 

of tuff erupted 30−25 million years ago (Ma) from explosive, 

caldera-forming volcanoes (McIntosh and Chapin, 2004; 

Lipman, 2006). The tuff covers older volcanic rocks that 

were deposited on a late Eocene low-relief erosion surface. 

Later, in the Neogene and Quaternary, streams and glaciers 

carved the volcanic layers into rugged peaks. Tuff may have 

once covered the map area (Atwood and Mather, 1932), but 

the only remaining volcanic rock today is gravel washed in 

by streams.

The rocks that crop out in the quadrangle are shales and 

sandstones deposited in an inland sea mainly in Late Cretaceous time about 100 to 70 Ma. These rocks are as much 

as 1,690±300 feet (ft) thick. After the inland sea withdrew 

from the region, an arch (anticlinorium) lifted and tilted 

the formerly horizontal layers of shale and sandstone and 

the layers were later faulted. Weathering and erosion of the 

land accentuated the hard sandstone layers, preserving them 

as tilted (dipping) ledges evident along U.S. Highway 160. 

Beneath the surface, pre-Cretaceous Mesozoic rock layers lie 

unconformably on Precambrian gneiss and granite (depicted 

in cross sections). In Pagosa Springs, a geothermal test drill 

hole 1,483 ft deep, penetrated Precambrian rocks at a depth 

of 1,380 ft (Galloway, 1980). Similarly, a drill hole in the 

northeast part of the map area and another just outside the 

western map border also penetrate Mesozoic and Precambrian rock.

Brief History

Precambrian Time and Paleozoic Era

Precambrian gneiss and granite formed 1,650−1,450 Ma 

in this region when volcanic island arcs converged on and 

docked onto the south edge of Laurentia (early North America), 

adding new terrane to the continent. About 1,400 Ma, deep 

masses of granitic magma heated the basement rock and reset 

the radiometric ages younger than their original crystallization 

ages (Karlstrom and Bowring, 1988; Condie, 1992; Shaw and 

others, 2005). In Pagosa Springs, biotite in Precambrian rock 

was dated by K/Ar at 1,152±39 Ma on a sample from 1,475 ft 

depth in a drill hole (Galloway, 1980).

For most of the Paleozoic era, from about 540−300 Ma, 

the continental Western Interior straddled the equator alternately as a low plain or shallow sea on a wide, sloping 

continental shelf. Fossiliferous marine sediments accumulated 

on the shelf. In the Pennsylvanian Period about 300 Ma, as 

the North American and African continents collided, two large 

island mountain ranges (the Ancestral Rockies) lifted in what 

is now Colorado. As they lifted, erosion shed thick debris into 

adjacent basins. However, rocks of the time interval apparently are absent from the quadrangle.

The following history of the quadrangle began after the 

Paleozoic Era.

Mesozoic Era

Jurassic Period

Middle and Late Jurassic Epoch (175−145 Ma)

Buried sandstone, gypsiferous limestone, and shale of 

the Wanakah Formation (Todilto Formation in northern New 

Mexico; Anderson and Lucas, 1992) and Entrada Sandstone 

were penetrated by petroleum and geothermal exploration 

drill holes in Pagosa Springs and nearby areas (see cross sections). These formations are made of rock debris eroded from 

mountains in eastern Nevada and possibly from remnants of 

the Ancestral Rockies, carried to and deposited in basins by 

streams and wind.
2 Geologic Map of the Pagosa Springs 7.5' Quadrangle, Archuleta County, Colorado

Figure 1. Location and setting of Pagosa Springs quadrangle showing outline (red rectangle) of 

quadrangle.

GRAND

MESA

SAN LUIS

VALLEY

SAN

SAN JUAN BASIN

 UPLIFT

NACIMIENTO

VALLES CALDERA SANGRE DE CRISTO MOUNTAINS

CHAMA BASIN

BRAZOS

UPLIFT

JUAN

MOUNTAINS

ANTICLINORIUM

UNCOMPAHGRE PLATEAU

ARCHULETA

SAN JUAN BASIN RIM Durango

Arkansas River

Rio Grande

Pagosa Springs

 Quadrangle

Aspen

Gallup

Gypsum

DENVER

Alamosa

Trinidad

Montrose Gunnison

SANTA FE

Farmington

Albuquerque

Grand

Junction

Colorado

Springs

Pueblo

Rio Grande

Colorado River

COLORADO

NEW MEXIO

40°

109° 108° 107°

35°

36°

37°

38°

105°

39°

106°

SAN JUAN

DOME

0 30 60 KILOMETERS

0 30 60 MILES

Base from ESRI ArcGis Online, World Shaded Relief Map, derived from 30 m digital elevation data, accessed August 2017

Universal Transverse Mercator projection, zone 13N

North American Datum 1983
Brief History 3

Figure 2. Photograph showing town of Pagosa Springs, Colo. The low terraces are north 

of town, and Pagosa Peak is on skyline. Taken about 2 miles south of Pagosa Springs, just 

outside of southern map area border, looking north. Photograph by David Moore, 1980.
4 Geologic Map of the Pagosa Springs 7.5' Quadrangle, Archuleta County, Colorado

The oldest exposed rock unit, the Morrison Formation 

(Jm), is made of sediment deposited late in the Jurassic on a 

vast, low plain. The Morrison Formation underlies the Dakota 

Sandstone (Kd) in Martinez Canyon (southwest corner of the 

quadrangle) where talus cover prevented a close study. Drill 

holes nearby (Galloway, 1980) also record Morrison Formation 

beneath the Dakota Sandstone, in normal stratigraphic order. 

The plain on which Morrison sediment accumulated covered 

the Western Interior (Blakey and Ranney, 2008) from Mexico 

to Canada, including what is now the eastern Colorado Plateau, 

Rocky Mountains (not yet formed), and western Great Plains. 

Rivers flowing from Idaho, eastern Nevada, western Utah, and 

the Mogollon Highlands in Arizona (Peterson, 1988) brought 

sediment to the plain. Uniform bedding suggests deposition in 

sluggish, migrating channels, on mud flats, in swamps and in 

large shallow, short-lived alkaline or freshwater lakes (Turner 

and Fishman, 1991) on a semiarid, savannah-like landscape 

(Kowallis and others, 1998; Turner and Peterson, 2004). Ash 

erupted from volcanoes in southwestern Arizona settled onto 

the plain (Dott, 1979). The ash weathered to clay minerals, 

mostly smectite (bentonite) and illite (Skipp and Aubrey, 

1992). The clay and organic matter was oxidized and zeolitized 

in soil zones, which colored tuffaceous claystone to the characteristic pale hues of tan, maroon, and green. The Morrison at 

other localities has yielded entire fossil skeletons of sauropods 

(dinosaurs with “lizard feet”) and pterosaurs (flying reptiles) 

(King and others, 2006).

Cretaceous Period

Early Cretaceous Epoch (145−100 Ma)

While streams eroded the land in southern Colorado 

(Aubrey, 1992), rising sea level began flooding low coastal 

areas around the Gulf of Mexico, depositing calcareous 

marine mud on the shelf in southern Arizona, New Mexico, 

and Texas (Bilodeau and Lindberg, 1983). Simultaneously in 

Colorado, upper parts of the Morrison Formation were being 

eroded, evidenced by the regional unconformity beneath the 

overlying Dakota Sandstone (Cobban and Reeside, 1952; 

Pipiringos and O’Sullivan, 1978). Early Cretaceous southwestern Colorado was slightly above sea level as was the 

Western Interior of the continent.

Not until latest Early Cretaceous (about 105 Ma) did the 

Western Interior Seaway invade southwest Colorado. Globally, 

sea levels were rising (Miller and others, 2005), hypothetically 

caused by large outpourings of lava like the extensive Ontong 

Java and Manihiki Oceanic Plateaus in the southwest Pacific, 

or rapid spreading of the mid-Atlantic ridge (Duncan and 

Richards, 1991; Taylor, 2006); both would displace seawater 

from ocean basins onto low-lying continental areas. Geographically, Cretaceous rocks indicate that flooding advanced 

(transgressed) southward from the Arctic Ocean onto the 

midcontinent, covering areas now occupied by the Rocky 

Mountains and Great Plains, and westward into north-central 

Colorado (Weimer and Land, 1972).

Late Cretaceous Epoch (100−66 Ma)

The sediment composing most local rocks was deposited 

during the Late Cretaceous. By 100−97 Ma (Aubrey, 1992), 

the inland sea covered much of Colorado (Gillette and Thomas, 

1985) and sediment accumulated on coastal plains and shorelines, forming dunes, deltas, offshore sand bars, and lagoons. 

These environments shifted landward and seaward repeatedly 

for the next 30–25 Ma, forming the Dakota Sandstone (Kd) 

and overlying Cretaceous rocks throughout the midcontinent 

(Obradovich, 1993; Molennar and others, 2002). Volcanic ash 

blew in from a tectonically active west edge of the continent. 

Whereas shell-making organisms living in shallow warm seas 

in Texas made calcareous mud, reefs, and shoals (Moore, 

2010), in Colorado mostly terrigenous clay and sand was 

deposited on deltas (the Mesaverde Group) and in the inland 

sea. The clayey sediment would become the Mancos and Lewis 

Shales. Thin layers of calcareous nannoplanktonic ooze settled 

on low-energy, deeper central parts of the seafloor and lithified 

to chalky limestone of the Bridge Creek Limestone and Fort 

Hays Limestone Members of the Mancos Shale. 

As the Western Interior Seaway deepened, older sediments and underlying crust subsided. Marine sediments 

accumulated layer on layer. The depth of the sea varied as 

the crust rose or subsided and global sea levels varied (Haq 

and others, 1987). At its largest, about 600 mi wide, the sea 

divided the continent into eastern and western land masses. An 

equable and warm Cretaceous climate (Barron, 1983) nurtured 

abundant coastal vegetation and marine life in the expansive 

sea. Marine fauna included fishes, sharks, rays, turtles, large 

air-breathing reptiles (mosasaurs and plesiosaurs), calcium 

carbonate shell-forming clams, oysters, single-celled plankton, 

foraminifera, and coccolithophores (microscopic algae), and 

various ammonite (cephalopod) species. Crocodiles lived in 

coastal swamps (Cope, 1885). Depositional conditions were 

favorable for preserving fossils, and museums worldwide 

are able to represent the now extinct Cretaceous fauna with 

diverse and high-quality fossil displays. 

Uppermost Cretaceous Kirtland Shale, Fruitland Formation, and Pictured Cliffs Sandstone include economic coal 

(Carroll and others, 2011) deposits that formed in densely 

vegetated coastal swamps and mires. These units were eroded 

from the map area, but presently form the hogback of the San 

Juan Basin about 20 mi southwest of Pagosa Springs.

Cenozoic Era

Paleogene Period (66−23 Ma)

Paleocene, Eocene, and Oligocene Epochs

Near the close of the Mesozoic Era, the sea retreated from 

Colorado for the last time. Concurrently, the Rocky Mountains began to rise (Laramide orogeny) (Fassett, 1974; Cather, 

2003). Although no mountains rose in the Pagosa Springs 

quadrangle, not far north and east, the Laramide, San Luis 

highland was rising where the Uncompahgre Plateau, eastern 
Brief History 5

San Juans, and San Luis Valley are today (fig. 1; Tweto, 1975). 

The Nacimiento uplift also came up 90 mi south (Baltz, 1967). 

Both highlands sourced sediment to San Juan Basin. Other 

nearby Laramide uplifts are the Needle (San Juan dome) and 

Brazos (Tusas) Mountains (Muehlberger, 1967). The early 

Paleocene Archuleta anticlinorium, which is in part a broad, 

north-trending uplift, extends into the Pagosa Springs quadrangle (fig. 1). This feature is likely related to the Laramide 

uplifts mentioned above, and it extends south of the map area 

where it may connect with the Nacimiento uplift about 90 mi to 

the south.

The Laramide orogeny probably was caused by 

continental-scale, east-west compression driven by thermal 

convection in Earth’s upper mantle (Dickinson and Snyder, 

1979; Livaccari and others, 1981). Dickinson and Snyder 

(1979) and Bird (1988) posited that as the Pacific oceanic 

crust, the Farallon plate, subducted under west-moving North 

America, it remained flat at a shallow depth, “scraping” the 

base of continent and far eastward into it (roughly 600 mi), 

making east-west directed forces of friction and compression. 

According to Liu and others (2010), a large oceanic plateau 

riding on the Farallon plate caused vertical movements that 

may explain the Late Cretaceous marine flooding and Paleogene mountain building. The forces pushed up basement-cored 

blocks on steep faults, making the north-trending modern 

Rocky Mountains. The forces possibly also folded rock layers 

of the Archuleta anticlinorium. As the ranges emerged, basins 

among them, including the San Juan Basin, subsided and 

received Paleogene sediment. These hypotheses also suggest 

a period of tectonic quiescence (“relaxation”) followed the 

compression, causing regional east-west extension, which may 

have formed the Stinking Springs fault and other faults, 2−4 mi 

west of town (see map). 

Later, during the tectonically quiet late Eocene Epoch, 

streams eroded a wide, low-gradient bedrock surface in southern and central Colorado (Epis and Chapin, 1975). 

Astounding events followed this quiescence. Titanic 

volcanic eruptions occurred northeast of Pagosa Springs 

37−24 Ma (mostly in the Oligocene Epoch). Two phases are 

recognized. An early phase (37−30 Ma) erupted andesitic 

and intermediate composition lavas and volcaniclastic debris 

that composes the Conejos Formation (Steven and others, 

1974). The later phase, 30−24 Ma, saw colossal eruptions 

of ash-flow tuffs exploded from stratovolcanoes so huge 

(“supervolcanoes”) that masses of rock collapsed into underlying magma reservoirs, making 15 large calderas (Steven and 

Lipman, 1976; Lipman, 2006; Bachmann and others, 2000). 

These explosions were some of the largest known in Earth 

history. Dense, fiery clouds of ash and particles raced outward, 

burying large parts of southwest Colorado and covering older 

volcanic rocks that overlie the late Eocene erosion surface 

mentioned above (Dunn, 1964); residual heat welded and fused 

the ejecta to hard, thick tuff layers of the San Juan Mountains.

Oligocene intrusive rock units at Jackson Mountain, about 

10 mi northeast of Pagosa Springs, were dated at 25.1 Ma 

based on U-Pb ages obtained from zircons (Gonzales, 2015). 

Neogene Period (23−2.6 Ma)

Miocene and Pliocene Epochs

During the late Neogene, the Western Interior, Colorado 

Plateau, and western Great Plains were lifted (epeirogenic 

uplift) (Eaton, 2008). Precisely when and how much is 

debated; some attribute the uplift to the earlier Laramide 

orogeny (Pederson and others, 2002). Others believe that 

after Laramide uplift of individual mountain ranges, the entire 

region, including basins, was lifted even more (McQuarrie 

and Chase, 2000). Notwithstanding the debate, evidence 

for regional uplift is apparent. Cretaceous marine sediments 

in the quadrangle and surrounding states stand more than 

6,000 ft above sea level. Empowered by the uplift, rivers cut 

deeply into Precambrian basement rock. Examples include 

the Black Canyon of the Gunnison and Royal Gorge of the 

Arkansas Rivers; Piedra River canyon, northwest of the map 

area, is another nearby example. The Colorado River has cut 

5,200 ft below the 10 Ma-old basalt cap on Grand Mesa (U.S. 

Geological Survey, 1966; Scott and others, 2001). McMillan 

and others (2006) believe that intermountain basins first filled 

with sediment during Oligocene slow subsidence, followed 

by uplift in late Miocene that continued from about 7 Ma to 

the present. They posit that regional uplift followed the crustal 

extension cited above, driven by isostatic forces (slow flow of 

upper mantle rock responding to loading of the crust above) 

and cited east-tilted Pliocene deposits on the western Great 

Plains as support (McMillan and others, 2002). Eaton (2008) 

proposed domal uplift over buoyant mantle anomalies caused 

by Neogene intrusion of hot magma under the Rockies. 

Quaternary Period (2.6 Ma−Present)

Pleistocene and Holocene Epochs

North American continental glaciers may have begun 

about 2.5 Ma, evidenced by glacial deposits in oceanic sediment (Shackleton and others, 1984). However, alpine valley 

glaciers formed much later in the San Juan Mountains based 

on end-moraines about 300–140 ka (thousand years) old in 

the Animas River Valley at Durango (Scott and Moore, 2007). 

Steven and others (1974) stated that younger moraines formed 

roughly after 140 ka in the Piedra River headwaters only 7 mi 

north of the quadrangle.

Traditionally throughout the Rockies, field studies by 

geologists have found and named two major glaciations 

(Blackwelder, 1915). The older, Bull Lake, was thought 

to have begun about 200 ka and ended with the Sangamon 

warm interglacial 130–127 ka (Fullerton and others, 2004). 

The younger, Pinedale, was about 30–14 ka, peaking about 

20–18 ka, concurrent with maximum extent of the Wisconsin 

continental ice sheet (Richmond, 1965). More specific ages 

have resulted from cosmogenic nuclide dating; a few examples are cited here. Benson and others (2005) measured 10Be 

surface-exposure ages of boulders on crests of Pinedale end 

moraines in southwest Colorado; they concluded that glacier 
6 Geologic Map of the Pagosa Springs 7.5' Quadrangle, Archuleta County, Colorado

retreat began about 16.8±0.3 ka. Leonard and others (2017) 

obtained 10Be surface-exposure ages from moraine boulders 

and ice-polished outcrops of about 21 ka until 17–16 ka, the 

time when Pinedale valley glaciers were at maximum extent 

in the Sangre de Cristo Mountains. 10Be surface-exposure ages 

of Pinedale terminal moraines in the upper Arkansas River 

Valley reported by Schweinsberg and others (2016) averaged 

21.8±0.7 ka. The ages vary owing to preexisting factors such 

as snow cover, rock erosion, and sample site location. Interested readers are referred to the published work.

We mapped possible glacial deposits mixed with definite 

landslide deposits (diamicton, Qd) in the northeast corner of 

the quadrangle. As climate warmed and the last alpine glaciers 

melted, volcanic-clast-rich stream gravel (outwash) washed 

southward into the quadrangle.

Surficial Deposits

Gravel

Stream gravel is present in narrow deposits in modern 

channels, for example, along Fourmile Creek. These waterworn gravels are derived mostly from volcanic rock in the San 

Juan Mountains and were rounded by abrasion during stream 

transport. Layers of river gravel armor the underlying shale, 

slowing erosion of the river bed, and with time, the landscape 

inverts—parts that were low become high, and vice versa. 

Shale not covered by gravel is eroded deeply. This process 

forms a succession of vertically elevated gravel-capped 

benches (fluvial terraces) along modern stream channels.

The gravel here is mined for road base. The walls of a 

gravel pit (active in 2015) on Oakbrush Hill (left center of map 

area) expose coarse, rounded clasts of volcanic river gravel 

(Qtg5, fig. 3). Similar deposits compose the Qg, Qtg1−4, 

Qop, and Qob units, which were all made by south-flowing 

streams in former channels; their remains now stand at various 

heights. The gravel deposits flanking Fourmile Creek (Qop

and Qob, northeast part of quadrangle) are glacial outwash 

and some of the other gravels may also be outwash. Boulders 

as large as 3 ft in diameter within the deposits suggest glacial 

meltwater discharges were more powerful than the discharges 

of existing and precursor streams of the present interglacial 

(Holocene, approximately during the last 12,000 years). 

Higher still in the landscape is an older outwash gravel deposit 

(Qob) that flanks McCabe and Fourmile Creeks. Besides the 

outwash, some high, hill-capping gravel (Qpg) in the north 

part of the quadrangle is pediment gravel. Others are landslide 

and debris-flow deposits (Qdf).

Erosion acting over thousands of years sculpted the vertical sequence of gravel-veneered fluvial terraces. The Qtg1

deposits are only 10 ft or less above modern streams. In the 

uplifted map area, streams have cut downward to modern levels, abandoning former higher stream levels that are now evidenced by the fluvial terrace remnants (Qtg1−5). Relatively 

high (older) gravels, for example Qtg4, have advanced (and 

thickened) soil development—an earmark of weathering 

for thousands of years—which includes blocky or prismatic 

structure in an oxidized, red-brown clayey B soil horizon that 

typically has whitish, calcium carbonate built up in its lower 

part. Recognition of well-developed soils and the heights of 

old terrace deposits stem from the study of geomorphology 

and soil science (Shroba and Birkeland, 1983). 

The highest terrace gravel Qtg5 is on Oakbrush Hill 

(elevation 8,069 ft; southwest part of map area). It may be 

older than the Bull Lake outwash gravel (Qob), because of 

its greater height (430−440 ft above Martinez Creek). Bull 

Lake-age gravels (Qob, Qtg4?) are about 200–140 ka in age 

(Hall and Michaud, 1988; Schweinsberg and others, 2016); 

Pinedale-age gravels (Qop, Qtg3?) about 30−14 ka; and postglacial gravels (Qtg2, Qtg1, Qal) are <14 ka (fig. 4; Pierce, 

2003). The names derive from alpine glaciations named in 

the Wind River Mountains, Wyoming (Blackwelder, 1915; 

Richmond, 1965).

Other Deposits

Loess (wind-deposited silt) was not mapped separately 

because it is thin and spotty. Loess is commonly mixed with 

sheetwash alluvium and is reddish-brown silt, silty clay, and 

clayey silt that veneers nearly level land, chiefly on terrace 

surfaces. In the northeast part of quadrangle, a poorly exposed 

surficial deposit of uncertain origin is mapped as diamicton 

(Qd); it includes old gravels, which might be glacial or glaciofluvial in origin (late middle Pleistocene?), that are mixed with 

and modified by younger landslide deposits (Holocene to latemiddle Pleistocene?). Other scattered surficial deposits include 

alluvial-fan, debris-flow, pediment gravel, and landslide 

deposits as well as talus, colluvium, sheetwash alluvium, and 

travertine. Soils and surface-capping residuum are widespread, 

but they were not mapped. In general, deposits were mapped 

where they exceeded 4−5 ft in. thickness.

Structure

Sedimentary rock layers dip gently northeast as part 

of the broad northeastern flank of the northwest-trending 

Archuleta anticlinorium (see fig. 1). High-angle faults cut 

and offset the Cretaceous layers locally. Two prominent 

northwest-trending faults cut rock layers in the southwest 

part of the quadrangle. The longer one, the Stinking Springs 

fault (see map), extends 6 mi; the other extends 2.5 mi. Both 

are high-angle normal faults. Bedding is offset approximately 

480 ft along the Stinking Springs fault, measured in SW1⁄4, 

NE1⁄4, sec. 22, T. 35 N., R. 2 W. The Dakota Sandstone (Kd), 

the Graneros Shale Member of the Mancos Shale (Kmgr), 

and underlying units are down-dropped on the southwest side 

of these two faults.
Structure 7

Figure 3. Oldest fluvial terrace gravel (Qtg5) in gravel pit, on Oakbrush Hill in the map area, 

section 6, Township 35 N., Range 2 W.; thickness 18 ft. Pick is 2 ft in length. Photograph by 

David Moore, 1980.
8 Geologic Map of the Pagosa Springs 7.5' Quadrangle, Archuleta County, Colorado

The Archuleta anticlinorium is a broad, north-trending 

arch of crustal rock that includes numerous folds and faults in 

northeast New Mexico and several folds and faults in southern 

Colorado. It is a Laramide structure of early Paleocene age 

(Cather, 2003), perhaps modified later by extensional forces. 

The anticlinorium defines the northeast margin of the San 

Juan Basin and its broad eastern limb extends into the quadrangle and is expressed by the gentle east dip of Cretaceous 

rock units throughout most of the quadrangle. Smaller scale 

anticlines, synclines, and a monocline are mapped mostly in 

the northwestern part of the map area and might be related to 

the anticlinorium. However, most of these mapped folds are 

northeast-trending and a direct relationship to the north-trending anticlinorium is not obvious. According to Baltz (1967), 

the Archuleta anticlinorium formed at the same time as the 

Nacimiento uplift farther to the south.

Cross Sections and Subsurface 

Information

The map sheet includes three cross sections that portray 

structural and stratigraphic relations in the subsurface across 

the map area. The broad arching of strata and faults discussed 

above, and the stratigraphy described below, are depicted 

in these sections. Together with the stratigraphic column of 

the map area (fig. 5), they provide glimpses of subsurface 

relations beneath the Pagosa Springs quadrangle. The cross 

sections are constrained by surface exposures of contacts 

between rock units, by measured bedding orientations of the 

units, and by depths to the rock-unit contacts in the subsurface 

as recorded in drill-hole data.

The five wells shown in the cross sections (one shown in 

two cross sections) are the only wells in and nearby the map 

area that have recorded information regarding the rock units 

penetrated during drilling. Data for the deep geothermal test 

well in the southeast corner of the quadrangle (see C–Cʹ) are 

Figure 4. Pinedale glacial outwash (Qop), 15–25-foot-thick, overlying shale of the Mesaverde Group (Kmv) exposed at Mesa Ditch 

in the northeast part of the Pagosa Springs 7.5' quadrangle. Terrace is about 140 ft above the nearby Fourmile Creek. Boulders on the 

shaded slope have cascaded downslope from outwash above; they mantle gently dipping shale exposed in slope in right-central part 

of photo. Photograph by David Moore, 1980.
Cross Sections and Subsurface Information 9

Figure 5. Stratigraphic column showing map units of the Pagosa Springs 7.5' quadrangle. Modified from the 

drill hole based stratigraphic column shown in Galloway (1980).

Concretions

Fossil clams

EXPLANATION

Unconsolidated

Shale

Crystalline rock

Limestone

Sandstone

Surficial deposits

THICKNESS, IN FEET

(NOT TO SCALE)

250 (+)

200–600

0–260

800–950

14–16

70–150

20–30

130–233

17–25

100–170

150–250

280–750

0–96

0–230

0–30

unknown

Units shown only in subsurface

of cross sections where existence

is based on drill hole data. 

EraCeno- zoic

Quaternary

Period Age Unit Symbol

Morrison Formation

Entrada Sandstone

gneiss and granite

Cretaceous

Upper

Turonian Cenomanian

Jurassic

Precambrian

Mesozoic

Upper Middle

Proterozoic

Fort Hays Limestone Member

Montezuma Valley member

Juana Lopez Member

Carlile shale member

Bridge Creek Limestone Member

Graneros Shale Member

Dakota Sandstone

Wanaka Formation

(Todilto Formation)

Coniacian

Lewis Shale

Mesaverde Group

Mancos Shale,

undifferentiated upper part

Q

Kl

Klmv

Kmu

Kmf

Kmmv

Kmj

Kmc

Kmb

Kmgr

Kd

Jm

Jw

Je

=

Campanian

Epoch

Lewis Shale and Mesaverde

Group, undifferentiated

Kmv
10 Geologic Map of the Pagosa Springs 7.5' Quadrangle, Archuleta County, Colorado

available in Galloway (1980); drill-hole data for the other 

wells shown in the cross sections were acquired from the 

Colorado Oil and Gas Conservation Commission (COGCC) 

website (http://cogcc.state.co.us/maps.html#/gisonline). Some 

of the drill holes shown in the cross sections extend deep 

enough to penetrate older sedimentary and igneous-metamorphic (crystalline) rock units that do not crop out at the surface 

in the quadrangle (discussed below).

Other drill holes shown only on the map (7 wells), those 

that are not also shown in the cross sections, either to our 

knowledge have no record or their records on the COGCC 

website do not specify the rock units penetrated in the drill 

hole. Wells that have no record (4 wells) are labeled “NR” on 

the map. Wells shown on the map whose records lack rockunit information are Cloman 1, Adios-Dinero 1, and Adios 

Dinero 2. 

Drill holes that were used to constrain subsurface relations in the cross sections are shown and labeled on the cross 

sections and on the geologic map. There is one exception; 

the Strat no.1 well is shown projected into the southwest end 

of cross section A–Aʹ. This well is located about 1 mi northwest of the A–Aʹ cross section intersection with the west map 

border and consequently, it is outside of the map area and not 

shown on the map. The Strat no. 1 well, the H.R. Macht no. 1 

well in the northeast part of the map area, and the CGS Test 

Well P-1 in the southeast corner (downtown Pagosa Springs), 

provide very significant subsurface information in that they 

all penetrate beneath the Jurassic and Cretaceous sedimentary 

rock units exposed in the quadrangle and bottom in significantly older underlying Precambrian crystalline rocks (see 

cross sections).

The Jurassic Morrison Formation (Jm) is the oldest rock 

unit exposed in the map area and data from the three deep 

wells mentioned above indicate that the Morrison overlies 

differing older rock units at depth. In the drill hole in the 

northeastern part of the map area (H.R. Macht no. 1), the 

Morrison Formation unconformably overlies Precambrian 

crystalline rocks at depth (see B–Bʹ). However, in the drill 

hole just west of the map area (Strat no. 1) and in the drill 

hole in the southeast corner of the map (CGS Test Well P–1), 

the Morrison Formation overlies older Jurassic sedimentary 

rock units consisting of the Wanakah Formation (Todilto 

Formation in northern New Mexico) and the underlying 

Entrada Sandstone; the Entrada in turn unconformably overlies Precambrian crystalline rocks near the bottom of these 

two drill holes (see A–Aʹ and C–Cʹ). For the map area, the 

existence of the Wanakah Formation, Entrada Sandstone, and 

Precambrian crystalline rocks is known only from the subsurface drill-hole data of these three wells. These subsurface 

units are shown in the cross sections near the deep wells that 

penetrate them, but their possible lateral continuations are 

not inferred elsewhere in the cross sections where subsurface 

information is lacking.

The lines of the cross sections (A–Aʹ, B–Bʹ, and C–Cʹ) 

intentionally intersect the locations of some of the wells 

shown on the map. This allows the subsurface drilling 

information from these intersected wells to be directly represented at those intersected localities in the subsurface of 

those cross sections. Cross sections A–Aʹ and B–Bʹ also show 

wells that are projected into the lines of these cross sections 

from well sites that are as much as 1.75 mi from the line of the 

cross section. These wells are projected along the strike of the 

strata and perpendicular to the line of the cross section. The 

relative positions of the tops and bottoms of projected wells 

are adjusted in the cross sections based on the elevation of 

the topographic surface at the well site relative to the surface 

elevation of the location where the well projection intersects 

the line of the cross section. These projected wells provide 

constraints on the relations and thicknesses of rock units in 

the subsurface at these localities in the cross sections. We 

consider these projections of well information over distances 

of less than 2 mi to be reasonable and perhaps the best method 

available for extrapolating rock-unit relations and thicknesses 

in the subsurface at these cross-section localities. In the cross 

sections, intersected wells are portrayed as solid lines in the 

subsurface and projected wells are shown as dashed lines.

Acknowledgments

During field studies, numerous landowners graciously 

granted access permissions to their land. Jan Tulley (Pagosa 

Springs Town Hall) helped locate data related to the town’s 

geothermal project. We thank the following geologists 

for their thorough and insightful technical reviews that 

significantly improved the map and report: Orin Anderson 

(resident of the Pagosa Springs area and retired from the 

New Mexico Bureau of Mines and Geology), Margaret Berry 

(USGS), and Chris Carroll (formerly with the Colorado and 

Wyoming Geological Surveys). This map was first digitized 

by Chris McGuire using a program created by Van Williams 

(USGS). Darren (Paco) VanSistine (USGS) was responsible 

for updating the initial digital version of the map data and 

topographic base and for the creation of the geodatabase and 

metadata for the digital map data; these are published and 

available on ScienceBase (Moore and others, 2018). Jeremy 

Havens (USGS) assisted with initial drafts of some of the 

figures. Julie Herrick (USGS) provided the geologic names 

review, was responsible for final editing and layout of the 

map sheet and pamphlet, and for many aspects of the final 

figures shown in the pamphlet and on the map sheet. Any 

errors that remain are those of the authors. 
Description of Map Units 11

DESCRIPTION OF MAP UNITS

SURFICIAL DEPOSITS

Bases of deposits are not exposed; thickness estimated.

ARTIFICIAL FILL

af Fill (modern)—Earth and rock fragment in dams; crushed aggregate as fill and base 

material for airport runways; thickness 3–30 ft

PRECIPITATED ROCK

Qtr Travertine (late Holocene)—Limestone, calcium carbonate (CaCO3) precipitated from 

warm spring water; very pale orange, white, weathers light gray and light brown, finely 

crystalline CaCO3, fibrous or reed-like, porous, “honeycomb-like” rock, in places is 

cavernous, thickly laminated, densely layered, or concretionary; has built up a large 

domal structure on the inside of a meander along the San Juan River in Pagosa Springs. 

Forms 6-ft-high mounds of accumulated carbonate layers that have silt-size, dark-brown 

organic inclusions. Total thickness unknown, but at least several ft thick along San Juan 

River where base is covered 

ALLUVIAL DEPOSITS

Qsw Sheetwash alluvium (Holocene)—Widely present on low, nearly flat parts of shale bedrock 

near the Dakota Sandstone-Graneros Shale Member contact. Common in valleys and 

around reservoirs; only the thickest deposits were mapped; an example in NE 1⁄4 sec. 22, 

T. 35 N., R. 2 W. is very dark brown massive clay and dark grayish brown silt; 

30−50 percent coarse sand-sized shale fragments; sticky when wet and hard when dry; 

mixed with sandstone fragments near faults; unit 2−5 ft thick

Qal Alluvium (Holocene)—Gravel and calcareous sand and silt. McCabe Creek alluvium is 

typical: grayish-brown, very dark grayish brown, and black, clayey, lithic sand; local 

lenses of reworked shale or limestone fragments and rounded volcanic porphyritic tuff 

pebbles and cobbles; lithic sand-size grains of shale, siltstone, limestone, volcanic rock, 

and <5 percent quartz. The alluvium is massive, crudely stratified; sticky when wet, 

hard when dry; commonly contains shells of small land snails; locally 2 ft-thick basal 

gravel composed mostly of shale clasts. Dutton Creek alluvium is about 80 percent 

Dakota Sandstone subangular blocks 2–24 inches (in.) and 20 percent rounded volcanic 

tuff pebbles and cobbles. It is covered by 1–2 ft thick, humic dark gray silty clay. Map 

unit includes a few low river terrace deposits about 10 ft or less above streams; unit 

thickness 6–12 ft; San Juan River deposit may be as thick as 24 ft

Qg Gravel deposit (Holocene to late Pleistocene)—Small, isolated gravel remnants that 

cap benches and hills in places at various heights above streams, generally 80–160 ft. 

Likely correlative with a few of the units in the terrace gravel sequence, but some are 

thin, nonbedded lag gravels. Unit was mapped separately because these isolated gravel 

occurrences could not be confidently correlated with specific terrace gravel units. Unit 

includes waterworn, volcanic porphyry pebbles and cobbles and locally includes angular clasts of Dakota Sandstone. Small deposits capping small hills just north of Pagosa 

Springs are 85 percent blocky, Dakota Sandstone cobbles and 15 percent rounded, 

volcanic porphyry pebbles and cobbles. Thickness of unit 1–5 ft

Qtg1 Fluvial terrace gravel, youngest (Holocene)—Unit consists of about 65 percent blocky 

(slightly rounded edges) Dakota Sandstone cobbles, 30 percent well-rounded volcanic 

porphyry pebbles and cobbles, and 5 percent rounded granite, shale, and quartzite clasts. 

Unit has a grayish brown clayey sand matrix; forms a terrace 20 ft above San Juan 

River near downtown Pagosa Springs. Thickness 3–6 ft
12 Geologic Map of the Pagosa Springs 7.5’ Quadrangle, Archuleta County, Colorado

Qtg2 Fluvial terrace gravel, young (late Pleistocene)—Similar to other terrace-gravel units 

in being composed primarily of Dakota Sandstone and volcanic porphyry clasts in 

clay-sand matrix. West and south of Dutton Creek (sec 7 and 8, T. 35 N., R. 2 W.), unit 

consists of about 90 percent slightly rounded cobbles and boulders of Dakota Sandstone 

and 10 percent well-rounded cobbles and boulders of volcanic porphyry in a brown, 

clayey sand (loamy) matrix; many clasts deeply weathered and unit is covered by slope 

wash (1−4 ft thick) that is very dark, grayish brown clayey silt and locally capped by 

soil that includes a dark brown 2-ft-thick B horizon. Forms terrace 50 ft above San Juan 

River at highway bridge (Highway 160) over river in town and a terrace 35–40 ft above 

Dutton Creek. Unit ranges from about 5 to 15 ft thick near Dutton Creek 

Qtg3 Fluvial terrace gravel, intermediate age (late Pleistocene)—Rounded pebbles and cobbles 

of volcanic porphyry containing less than 2 percent blocks (0.3−1.5 ft in diameter) of 

Dakota Sandstone in sandy matrix; many clasts deeply weathered in a yellowish-brown 

sticky sandy clay matrix; upper 3−6 ft is yellowish-brown clayey silt (loess?). Surface 

soil developed in the silt is typically well-developed with a reddish-brown B horizon, 

the lower part of which is carbonate-enriched to as much as stage III carbonate morphology (Gile and others, 1966). Soil development suggests intermediate age. Terrace 

surface is 160–180 ft above San Juan River (east of town) and 160–180 ft above Dutton 

Creek (near center of map area). Unit may be correlative in age with Pinedale outwash 

(Qop) based on their similar height in the landscape and weathering characteristics. Unit 

generally 4−8 ft thick with sharp contact on black shale; 9−11 ft thick near confluence of 

Dutton and Martinez Creeks in southwest part of map area (sec. 18, T. 2 W., R. 35 N.)

Qtg4 Fluvial terrace gravel, old (late middle Pleistocene)—Rounded to subrounded pebbles 

and cobbles composed mostly of volcanic porphyry (rhyolitic?) in sandy silty matrix. 

Forms river terraces 300–330 ft above San Juan River and 330 ft above Martinez 

Creek. Essentially like the Qtg3 unit in composition, clast size, soil development, and 

thickness. Unit may be correlative in age with Bull Lake outwash (Qob) based on their 

similar height in the landscape and weathering characteristics. Thickness not measured, 

but may range from about 5−15 ft

Qtg5 Fluvial terrace gravel, oldest (late middle Pleistocene)—River terrace gravel 420 ft above 

San Juan River (east of town); 430−440 ft higher than Martinez Creek (on Oakbrush 

Hill, west-central part of map). Matrix-supported, rounded, mainly volcanic porphyry 

and minor quartzite cobble gravel; matrix is dark-yellowish-brown, mostly poorly 

sorted, silty medium-grained sand and minor clay; crude bedding; lenticular interbeds of cemented to unconsolidated pebbly sand; unit commonly covered by 2–4 ft 

of silty loess and sheetwash alluvium. At Oakbrush Hill (east half of sec. 6, T. 35 N., 

R. 2 W.), an active gravel pit revealed that clasts in this deposit vary widely in size; 

boulders >2 ft in diameter are10–20 percent of deposit, boulders ≤1 ft in diameter are 

5–15 percent, cobbles are 15–25 percent, pebbles 20–30 percent, and sandy-silty matrix 

is about 20 percent; ≤10 percent of clasts are deeply weathered and crumbly. Soil at top 

of unit on Oakbrush Hill is well developed with dark-brown silty, friable, A horizon 

(9 in. thick); dark-yellowish-brown silty AB horizon (6 in. thick); and strong brown 

silty-clay, plastic B horizon with strong angular blocky soil structure (12−18 in. thick). 

Typical thickness range of unit is about 6 to 22 ft, but about 30 ft thick at south edge of 

Oakbrush Hill where it overlies black shale on a sharp basal contact

Qfy Fan alluvium, young (Holocene)—Generally pale-brown and dark yellow brown pebbly 

sand, silt, and clay. Mapped along and near base of some slopes flanking active 

channels; surface soil is minimally developed, suggesting relatively young age. Bedding 

not exposed. Thickness 1–6 ft

Qfo Fan alluvium, old (late Pleistocene)—Exposed east of San Juan River (NE 1⁄4 sec. 24, 

T. 35 N., R. 2 W.) and near Hidden Valley. Locally overlies Qob and Qtg3. Dark gray 

and grayish brown silty clay that is sticky and plastic; contains abundant, small pieces 

of very dark gray fissile shale and contains 2−5 percent rounded volcanic cobbles and 

pebbles. Unit caps low ridges and surfaces above and isolated from active channels and 

typically has moderately developed soil; elevated position in landscape and soil development indicate unit is older than Qfy. Although old in form, may periodically receive 
Description of Map Units 13

new thin layers of sediment on surface from sheetwash events. Thickness near edge of 

deposit about 4 ft, thickens to about 12 or 18 ft in central part of deposits

Qpg Pediment gravel deposit (late and middle Pleistocene?)—Rounded gravel mostly of volcanic porphyry rock and minor dark, crumbly mafic rock, in a sand matrix; clasts mostly 

12–18 in. diameter, but generally range from <1 in. to about 30 in. diameter; commonly 

thinly veneered by very dark grayish brown silt (loess?). Forms gently south-sloping, 

nearly flat-topped remnants of water-transported gravel on relatively small, high interfluves, on foothills in northern third of map area (about 20 separate locations). Unit 

mapped at various heights above active channels, suggesting deposits are remnants 

of pediments of differing ages. The deposit on Coyote Hill (west-central part of map, 

elevation 8,452 ft, NW 1⁄4 sec. 30, T. 36 N., R. 2 W.) has a weak soil profile and is 700 ft 

higher than adjacent Martinez Creek; it is the highest and oldest pediment gravel and 

it may be the oldest Pleistocene deposit in the map area; however, only moderately 

developed soil profile, thin loess, and poor exposure suggest its surface is actively being 

eroded. Rock types include cobbles and boulders of dark purple porphyrytic, welded, 

ash-flow tuff; few greenish cobbles and pink, coarse-grained porphyry; few dark mafic 

rocks; weathering rinds on clasts 0.019–0.058 in. thick; many rocks pitted or partly disintegrated, indicating a long time of surface exposure. At Coyote Hill, thickness ranges 

from 25 ft to possibly 35 ft at its south end. Average thickness of unit in map area 6–15 ft

GLACIAL AND GLACIOFLUVIAL DEPOSITS

Qop Pinedale outwash (late Pleistocene)—Rounded clasts of volcanic porphyry in a sand 

matrix; contains unusually large rounded boulders. Presumed source was meltwater 

from a former valley glacier in the headwaters of Fourmile Creek where glacial cirques 

are preserved. Forms terrace about 140−150 ft above Fourmile Creek, a height above 

the creek that implies a late Pleistocene age. Thickness 10−25 ft

Qob Bull Lake outwash (late middle Pleistocene)—Rounded small cobbles in a clayey sand 

matrix that is 10−20 percent of deposit. Remnants of this deposit extend relatively 

continuously from the north edge of the map area about 7 mi south to <1 mi north of 

town of Pagosa Springs. Presumed source was meltwater from former valley glacier in 

headwaters of Fourmile Creek north of quadrangle where glacial cirques are preserved. 

Measured and described in a gravel pit in SW 1⁄4, NW 1⁄4, sec. 26, T. 36 N., R. 2 W.; 

rounded volcanic porphyry pebbles, cobbles, and few boulders, sandy gravel and 

gravelly sand, clayey and silty very fine quartz and lithic sand, brownish yellow matrix. 

Predominant diameter of rounded volcanic clasts is 2–12 in.; one large rounded cobble 

of very coarse granite was seen. Distinguished from Qop, principally by its higher position in landscape and by more mature soils that locally cap it. Qob terrace remnant just 

north of Pagosa Springs is 270 ft above McCabe Creek and this remnant is 330 ft above 

the nearby San Juan River to the south. Distinct glacial spillways, which are depicted on 

map, cut Qob between Fourmile Ditch and Fourmile Creek in northeast part of quadrangle (about 0.5 mi southeast of mouth of Quien Sabe Creek); here, Qob is 180−200 ft 

above Fourmile Creek. Unit thickness typically about 10 ft, but locally as much as 20 ft

Qk Kame terrace (?) deposit (late middle Pleistocene)—Crudely stratified sand and gravel 

made of rounded volcanic clasts. This layered deposit is enigmatic because it is higher 

and isolated from other stratified gravel deposits and is unusually thick. Presumed 

source was former valley glacier in headwaters of Fourmile Creek where glacial 

cirques are preserved. If its origin is not glacial, it may be a remnant of an old terrace or 

pediment gravel deposit. Mapped only in one location (NE1⁄4, SW1⁄4, sec. 10, T. 36 N., 

R. 2 W.) at 8,240–8,320 ft elevation. This deposit of presumed glaciofluvial origin is 

about 120 ft above nearby Bull Lake outwash (Qob). Thickness at least 80 ft based on 

geomorphology; base covered

Qd Diamicton (Holocene to late middle Pleistocene?)—Brownish-gray and very dark grayish-brown, moderately compact, unsorted, nonbedded, clay, silt, sand and gravel that 

is significantly modified by slope processes, mostly landsliding and slumping. Poorly 

exposed and mapped only in northeast corner of quadrangle where unit primarily 
14 Geologic Map of the Pagosa Springs 7.5’ Quadrangle, Archuleta County, Colorado

mantles hummocky, moderately steep, southwest-facing valley flank of Fourmile Creek. 

Gravel may be glacial-glaciofluvial in origin (outwash and [or] till), in part based on the 

presence of glacial cirques in the headwaters of Fourmile Creek. If not glacial or glaciofluvial in origin, gravel may be lag derived from former, old pediment gravels. Although 

the gravel’s position in landscape is significantly modified by younger landslides and 

slumps, its overall relatively high position suggests a Bull Lake or older age (late middle Pleistocene?). Hummocky topography characterizes landslides and slumps within 

unit and implies a younger, Holocene to late Pleistocene age for these features. Unit 

mapped as diamicton and assigned a broad age range to reflect that it includes remnants 

of old gravel deposits mixed with and modified by younger landslides, slumps, and 

colluvium. Gravel includes granules, pebbles, cobbles, and boulders; most are rounded, 

but some are subangular. Gravel clasts are mainly composed of volcanic tuff, but some 

sandstone and sparse coarse-grained granite clasts are also present. Thickness unknown, 

but in general relatively thin, perhaps mostly <25 ft thick

MASS WASTING DEPOSITS 

Qt Talus (Holocene)—Cobble- and boulder-size angular blocks of Dakota Sandstone below 

cliffs and fault scarps; mapped only along the Stinking Springs fault; thickness 5–10 ft

Qc Colluvium (Holocene)—Gravelly sand, silt, clay, and dark yellow brown clay chips. 

Includes pebble-, cobble-, and boulder-size rock fragments of Dakota Sandstone (Kd) 

derived from upslope and transported downward mainly by creep. Locally contains 

unreworked deposits of terrace or pediment gravel; commonly overlies yellowish-brown 

clayey residuum on shale; contains slump blocks and thin beds of possible mudflow 

deposits in north part of map area. Widespread on shale in northern part of map area. 

Mostly present on lower hillslopes; colluvium ≤3 ft thick was not mapped. On dip 

slopes of Dakota Sandstone, the unit locally includes jumbled, angular fragments of 

broken sandstone as large as 2 ft across in a strong brown matrix of silt and fine sand. 

Thicknesses 2–6 ft

Qls Landslide deposit (Holocene and late Pleistocene)—Sandstone and shale fragments, 

mixed with sand and clay. Forms chaotic hummocky landform on steep hillslopes 

mainly in north part of quadrangle; water seeps from deposit in wet season, many show 

signs of downhill creep; thickness unknown, possibly 6–15 ft 

Qdf Debris-flow deposit (late to late middle Pleistocene?)—Unsorted volcanic gravel, sand, 

silt, and clay that caps isolated, relatively steeply southward sloping even surfaces that 

are not gullied and aligned more or less north-to-south. Thin loess scattered on surface. 

Poorly exposed and bedding, if present, was not observed. Gravel consists of weathered 

and rounded large cobbles and small boulders (pock marked, largest boulder 30 in.) of 

pale-red, biotite-rich porphyry and dark brown and greenish volcanic porphyry clasts. 

Mapped as several small, scattered, deposits in northwest part of map area; the unusual 

“J” shaped deposit in south half of sec. 28, T. 36 N., R. 2 W., suggests that it is a remnant of a formerly more extensive deposit; thickness 6−10 ft

BEDROCK UNITS

The bedrock units listed below consist of sedimentary rocks that are exposed at the surface in the map area 

(described directly below) and older, subsurface rock units (sedimentary, igneous, and metamorphic rocks) 

that are not exposed at the surface. The subsurface rock units are known from drill holes that penetrate 

these units; they are described at the end of this section, are only shown in the cross sections, and they are 

also briefly discussed in the “cross sections and subsurface information” section of this report.

SEDIMENTARY ROCK

Names of stages and formations and members follow Dane and others (1966) and Leckie and others (1997). 

We did not identify biostratigraphic zones as did other workers for some of these units elsewhere (for 

example, Cobban and others, 2006), who used index fossil ammonites and clams.
Description of Map Units 15

Kl Lewis Shale (Upper Cretaceous, Campanian)—Dark olive gray to dark grayish brown 

fissile shale, firm to soft, interbeds of very fine sandy carbonaceous siltstone and silty, 

very fine sandstone, slightly clayey. Unit poorly exposed. Observed one horizon of 

thick, discoid concretions 2−3 ft diameter cut by veins of calcite spar; very hard calcareous septarian concretions grayish brown, weathers pale yellow; apparently several 

horizons of concretions; poor exposures preclude reliable count; extensive slumping in 

this shale. An unknown thickness of the upper part is eroded. Eroded thickness approximately 250 ft 

Klmv Lewis Shale and Mesaverde Group, undifferentiated (Upper Cretaceous, 

Campanian)—In the map area, the boundary between these two undifferentiated units 

(Lewis Shale and Mesaverde Group) is not recognizable, owing to its gradational and 

interfingering character, thick colluvial and vegetation cover, and apparent low dip 

of bedding. In part, this undifferentiated unit represents a zone of transition between 

marine shale of the Lewis and nonmarine sandy units of the Mesaverde Group. Unit 

underlies an approximately 4-mile long line of northeast-trending hills northeast of 

McCabe Creek. The first distinct beds near the base, observed in road cuts, are thin 

0.8–2.3 in., brownish yellow, very fine grained sandstone beds; sandstone contains 

heavy minerals and lithic grains and displays planar bedding and oscillation ripples. 

Sandstone beds are as much as 0.4–5 in. thick and have interbeds of grayish brown 

shale that are 0.4–0.8 in. thick. Upper part of unit is mostly shale and very few sandstone beds. Sandstone is pale yellow, light gray silty very fine sand. Mapped thickness 

ranges from approximately 200 to about 600 ft 

Kmv Mesaverde Group, undivided (Upper Cretaceous, Campanian)—Members named in the 

type area (Mesa Verde National Park) are indistinguishable in the Pagosa Springs map 

area. Unit is mostly shale that is dusky yellow, grayish brown, or olive gray. On slopes, 

unit mantled by weathered residuum from which thin (1−3 in. thick) sandstone interbeds protrude and break off to form “float” of thin, flaggy sandstone pieces. Abundant 

very thin, coarse-grained siltstone beds, yellowish-brown, slightly sandy and clayey and 

strongly calcareous, sparsely micaceous; common trails, burrows, prod marks on bedding surfaces; planar beds break to chips and very fine plates 0.1−0.2 in. thick and as 

much as 16 in. across; Sandstone interbeds and laminae are pale yellow silty, very fine 

subangular quartz and lithic grains (sublitharenite); thickest sandstone bed seen was 5 ft. 

Some sandstone beds are calcareous and contain sparse subrounded quartz sand grains; 

sandstone planar beds uniformly 0.8−2 in. thick, finely burrowed, sparse fragments of 

Inoceramus fossil clams; matrix typically is 30–40 percent of sandstone beds and is 

hydrous iron oxide and iron oxide-clay minerals. Unit contains minor organic shale, 

traces of heavy minerals, a few calcareous light-yellowish-brown concretions (as much 

as 3 ft across), zones of thinly laminated siltstone, and some low amplitude (0.8−1.5 in.) 

current and oscillation ripples of 4 in. wave length on bedding planes. Base of unit 

interbedded shale and sandstone grading to Mancos Shale below. Thickness in Pagosa 

Springs quadrangle as much as 260 ft. Molenaar and others (2002) report that the unit is 

thicker west of the quadrangle

Mancos Shale (Upper Cretaceous)—Eight members of the Mancos Shale, consisting of 

four formal and four informal members, were identified in the map area. The upper two 

members, the Cortez and Smoky Hill members (informal after Leckie and others, 1997), 

were not mapped separately and instead were combined and mapped together as the 

undifferentiated upper part of the Mancos Shale. The other six members were mapped 

separately. As such, the Mancos Shale comprises seven map units that are shown on the 

geologic map and described below

Kmu Mancos Shale, undifferentiated upper part (Upper Cretaceous, lower Campanian to 

middle Coniacian)—Equivalent to, from top to bottom, Cortez and Smoky Hill members of Mancos Shale (Leckie and others, 1997). Dark gray and grayish brown fissile 

shale weathers white on flat surfaces; silty; probably accumulated in relatively deep 

marine water. Lower part of unit (Smoky Hill member) is chalky shale and clayey chalk 

with probable coccoliths (Neugebauer, 1975) and associated nannofossils. Uppermost 

shale in this unit grades upwards into thin-bedded, olive brown calcareous very fine, 
16 Geologic Map of the Pagosa Springs 7.5’ Quadrangle, Archuleta County, Colorado

somewhat lithic sandstone (sublitharenite). Shale contains innumerable white flecks 

visible with magnifier, and shiny, transparent spheres (0.0025 in.) singly or in clusters 

(foraminifera?); very small pyrite crystals; sparse bluish white, phosphatic bone pieces 

0.04–0.08 in. long and small (0.2–0.8 in.) oyster type shells attached to fragments of 

prismatic clam shells 0.1–0.8 in. thick, grooved rectangular-shaped shell fragments, a 

few 2–3 in. long. Thin beds of oyster coquina are present. Few beds containing small 

fossil fringed oysters (Lopha lugubris), fossil selachian fish scales (cartilaginous fishes 

including sharks and rays) light bluish-white and fine sand size; ptychodus skate teeth 

believed evolved for crushing clam and oyster shells. Ptychodus teeth collected near 

McCabe Creek, 3 mi north of Pagosa Springs. Thickness 800–950 ft 

Kmf Fort Hays Limestone Member (Upper Cretaceous, lower Coniacian)—Interbedded 

chalky limestone and chalky shale. Very finely crystalline and microcrystalline chalky 

limestone, broken surface is medium olive gray, weathers light olive gray, light gray, 

and yellowish-gray; limestone contains foraminifera, common to abundant calcite 

prisms derived from disintegrated shells of inoceramid clams (pelecypods). Unit 

contains thin beds of bentonite (devitrified volcanic ash); whole and fragmentary 

Inoceramus fossils (cf. Mytiloides?), some are auriculate (have an ear-like extension 

at the beak of shell). In outcrop, unit typically has thin limestone ledges, each ranging 

in thickness 8–16 in., interbedded with calcareous gray mudstone and shale. Outcrop 

covered in places with brownish-gray oxidized (weathered) silty, clayey, and sandy 

residual soil of Mancos Shale and weathered, colluvial volcanic gravel clasts. Outcrop 

commonly sloughs angular, flat limestone fragments 0.3−3 in. on a brown shale 

hillslope; in stream cutbanks and road cuts, unit crops out as distinct alternating ledges 

and clayey layers. Locally shown on map as colored line where this distinctive and continuous unit is too thin to otherwise portray at the map scale. Thickness of unit 14–16 ft

Kmmv Montezuma Valley member (informal name after Leckie and others, 1997; Upper 

Cretaceous, upper Turonian)—Medium olive gray silty to slightly sandy, calcareous 

shale. Contains a few bentonite layers and septarian concretions some of which contain 

inoceramid clam shells. Unit is 70−150 ft thick 

Kmj Juana Lopez Member (Rankin, 1944; Upper Cretaceous, upper Turonian)—

Predominantly very dark gray mudstone, shale, and brown-weathering interbeds of 

calcarenite (limestone composed of calcite grains and crystals). Some freshly broken 

calcarenite has petroliferous odor. Outcrops form low cuestas of dark gray (light-gray 

weathering) calcareous clayey shale and mudstone with light gray layers of bentonitic 

clay and gray (unweathered) and dark yellowish-brown, olive, and brown (all weathered), platy calcarenite interbeds that are 0.5−8 in. thick, and rarely as thick as 15 in.; 

limestone concretions, as much as 2.5 ft in diameter (some are septarian). Small, platey 

fragments of calcarenite slough off exposed beds, veneering shale slopes below. The 

unit contains abundant Lopha lugubris (ribbed oyster shells) that are 1 in. or less in 

diameter and commonly mixed with other shell fragments in concretions. The Juana 

Lopez occurs in the San Juan and the Raton Basins, and in several Great Plains states. 

Calcarenite consists of sand-size calcite grains that are rounded or “prism-like,” 

about 0.5 mm long, and commonly tapered at one end. They are abraded calcite 

prisms derived from the ostracum layer of shells of Inoceramid pelecypods (clams); 

the calcarenite contains thin skeletal grainstone beds and lenses containing planktonic 

foraminiferal tests, fish scales, teeth, and vertebrae (fish? bone). Lenses and thin wavy 

beds, wavy parallel laminations, parallel orientation of long axes of prisms, laminated 

crossbedding, and flaser bedding (separate lenses) of calcarenite intercalated with shale 

indicate that bottom currents swept the calcite prisms into ripples on a muddy seafloor.

Locally shown on map as colored line where this distinctive and continuous unit 

is too thin to otherwise portray at the map scale. Thickness of member is 20−30 ft in 

Pagosa Springs quadrangle, but Dane and others (1966) report thicknesses of 90−135 ft 

in the eastern San Juan Basin near Ventana, New Mexico

Kmc Carlile shale member (informal following Dane, 1948; Upper Cretaceous, middle 

Turonian)—Very dark gray, calcarenitic, silty, fissile shale. Covered in south-central 

part of map area by residuum mixed with gray silt, clayey sheetwash, and loess; this 
Description of Map Units 17

surface material typically is about 4−5 ft thick and overlies weathered, plastic shales of 

this member on a sharp contact. Unit veneered in places by angular limestone fragments, 6–24 in. diameter. A layer of concretions (about 3 ft diameter) exists 20−30 ft 

below lowest calcarenite bed of the overlying Juana Lopez Member. Thickness of unit 

130−233 ft

Kmb Bridge Creek Limestone Member (Upper Cretaceous, middle to lower Turonian)—

Gray weathers very pale brown. Rhythmic alternating, planar, thin (1.5–12 in.) beds of 

silty, calcareous gray shale and chalky limestone; includes five to six moderately hard 

limestone beds 12–20 in. thick that protrude from outcrop, separated by calcareous 

nonfissile shale (marl); vertical joints facilitate breaking limestone to chips and pencil-shaped fragments. Limestone contains few carbonaceous impressions of Inoceramus 

clams. An inocermid collected from the upper part of this unit was identified by W.A. 

Cobban (USGS, deceased) as Mytiloides mytiloides. Unit crops out somewhat subtly on 

gentle hillslopes, but distinctly in stream banks and roadcuts. Thickness 17–25 ft 

Kmgr Graneros Shale Member (Upper Cretaceous, upper Cenomanian)—Noncalcareous, 

dark gray, fissile shale, silty in places and contains a thin sandstone bed in places, 

especially in the lower 10–30 ft. Contains few altered volcanic ashfall (bentonitic clay) 

layers; concretions of subspherical shape (some mushroom-like), 3–4 ft across, occur 

in layers at 20–65 ft above base of unit; in places, unit veneered by 2−3 ft of silty clay 

loess, sheetwash, thin (<1 ft thick) sandstone lag gravel, and residuum. Thickness 

100−170 ft.

A Tertiary (Neogene?) small mafic (lamprophyre or diabase?) dike is exposed in 

outcrop (NW 1⁄4 sec 14, T. 35 N., R. 2 W. [1.4 mi; 2.2 km northwest of the Pagosa hot 

springs]). Dike is about 3 ft wide. May be an outlier of the north-south oriented dike 

field associated with the Archuleta anticlinorium, or related to the Jackson Mountain 

intrusive stock northeast of the map area 

Kd Dakota Sandstone (Upper Cretaceous, Cenomanian)—Very fine to fine grained, 

silty quartz sandstone composed mostly of subrounded grains (few well rounded); 

typically yellowish-gray, grayish-orange, light gray and dark gray; weathers dark 

yellowish-orange. Sandstone is mostly silica-cemented and hard (forms cliffs), but 

weathered zones are easily broken and fragmented. Includes 5−10 percent white, 

opaque small, elongated prisms of feldspar, altered to kaolinite(?), trace of muscovite 

and heavy minerals or lithic silt, and sparse, partial casts of ribbed shells (clams?); 

flinty (milky opaque chert cement) texture in places; many bedding planes show 

abundant burrows with lens-shaped cross sections; includes medium-scale (1–2 ft) 

interbeds of cross-bedded, fine-grained sand (about 95 percent quartz) and some herringbone crossbedding; unit chiefly thin to thick bedded and well sorted; locally has 

euhedral quartz overgrowths on sand grains. The Dakota contains few thin beds of silt 

and sandy shale and thin, wavy laminae and beds of lignite or dark gray bioturbated 

carbonaceous clay and fine-grained quartz sandstone that weathers like shale and contains disseminated, fine particles of coalified vegetation. The base of Dakota rests on a 

paraconformity on the Morrison Formation (Aubrey, 1992). Covering the surface of the 

Dakota, about 1.5−2 mi (1.5−3 km) west of town (sec. 22, T. 35 N., R. 2 W.), is angular 

lag sandstone gravel; this is atypical in the map area. On dip slopes of 8 percent grade 

or less near faults (upthrown block), the Dakota is covered by 1−3 ft of dark brown and 

reddish-brown loamy, very silty fine sand residuum and loess, and small blocks of sandstone; on slopes more than about 8 percent grade, it is bare rock. In Martinez Canyon 

area (southwest corner of map) total thickness of unit is about 200−230 ft, and within 

canyon, 48 ft of unit is well exposed in a cliff face; a partial exposure about 55 ft thick 

was observed in Piedra Canyon northwest of map area; total thickness reported in drill 

holes in town of Pagosa Springs was as much as 200 ft (Galloway, 1980), and ranges 

between 150 and 250 ft in other drill holes in and nearby map area

Jm Morrison Formation (Upper Jurassic; Brushy Basin Member)—In Martinez Canyon 

(southwest corner of map) two white quartzose sandstone beds 15−20 ft thick crop 

out in a poorly exposed 159-ft-thick partial section of the Morrison Formation below 

a sharp contact (unconformity) with the overlying Dakota Sandstone. The Morrison 
18 Geologic Map of the Pagosa Springs 7.5’ Quadrangle, Archuleta County, Colorado

sandstones are torrentially cross-bedded in places (20° dipping foreset beds) and are 

separated by presumed claystone (covered). The sandstone is soft, sugary texture in 

places, and contains medium-grained quartz sand that is well rounded, frosted, and 

moderately well sorted. Thin, planar sandstone beds weather to slabs. No fossils were 

observed. In most places in Colorado, the Morrison also displays distinctive beds of 

pale varicolored claystone and minor nodular limestone; the partial and poorly exposed 

Morrison sequence in Martinez Canyon may contain such interbeds, but none were 

observed. In the map area and San Juan Basin, the Morrison is truncated at the top by a 

regional unconformity at the base of the overlying Dakota Sandstone (Aubrey, 1992). 

Mudstones of the Morrison have yielded well preserved entire dinosaur skeletons, for 

example in Garden Park, Colorado, and Dinosaur National Monument, Utah. Galloway 

(1980) reported 681 ft of Morrison Formation (undifferentiated) in a deep geothermal 

well (CGS Test Well, P−1) in Pagosa Springs; other wells in and nearby map area 

indicate thicknesses of as much as 750 ft, and as little as 280 ft. In the northeastern part 

of map area, the Morrison Formation unconformably overlies Precambrian granite and 

gneiss in the subsurface (H.R Macht no.1 well; cross section B−B'); in other wells, it 

overlies the Wanakah Formation in the subsurface (see cross sections A−A' and C−C'). 

Thickness of upper part of Morrison Formation in Martinez Canyon is 110−120 ft, base 

is covered

Jw Wanakah Formation (upper Middle Jurassic)—Present at least locally in subsurface and 

only shown in cross sections. Limestone and anhydrite (CaSO4). Where encountered 

in drill holes, underlies Morrison Formation and overlies Entrada Sandstone. Unit is 

correlative to Todilto Formation in northern New Mexico (Anderson and Lucas, 1992). 

Thickness in drill hole in southeast part of map is 96 ft (CGS Test Well, P1; see C−C'); 

thickness in drill hole just west of map area is 91ft (Strat no. 1 well; see A−Aʹ); absent 

in drill hole in northeast part of map (H.R Macht no.1 well; see B−Bʹ). Apparently, unit 

was either not deposited or was eroded prior to deposition of the Morrison Formation 

(Jm) in the northeastern part of the map area where the H.R Macht no. 1 well is located

Je Entrada Sandstone (upper Middle Jurassic)—Present at least locally in subsurface and 

only shown in cross sections. Sandstone, quartzose and slightly feldspathic, fine- to 

coarse-grained, poorly sorted, some frosted grains. Where encountered in drill holes, 

underlies Wanakah Formation and overlies crystalline, Precambrian rocks. Thickness in 

drill hole in southeast part of map is 159 ft (CGS Test Well, P–1; see C–Cʹ); thickness 

in drill hole just west of map area is 91ft (Strat no.1 well; see A–Aʹ); absent in drill hole 

in northeast part of map area (H.R Macht no. 1 well; see B–Bʹ). Unit is a minimum of 

about 230 ft thick where it is exposed in Piedra Canyon west of the map area. Like the 

Wanakah (Jw), this formation apparently was either not deposited or was eroded prior 

to deposition of the Morrison Formation (Jm) in the northeastern part of the map where 

the H.R Macht no.1 well is located 

IGNEOUS AND METAMORPHIC ROCK

p_ Gneiss and granite (Precambrian, Proterozoic)—Present at least locally in subsurface 

and only shown in cross sections. Contains pegmatite, migmatite, and biotite schist 

(Galloway, 1980). Underlies Morrison Formation in drill hole in northeast part of map 

area (H.R. Macht no. 1 well; see B−Bʹ) and underlies Entrada Sandstone in drill hole in 

southeast part of map area (CGS Test Well, P1; see C–Cʹ) and in drill hole west of map 

area (Strat no. 1 well; see A−Aʹ). These three wells all bottomed in these Precambrian 

rocks; 112 ft of this unit penetrated in CGS Test Well, P–1; 6 ft penetrated in Strat no.1 

well, and only 2 ft penetrated in H.R Macht no. 1 well; true thickness unknown, likely 

many miles thick
References Cited 19

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Glossary 23

Glossary

Anticlinorium Regional, elongated arch 

of sedimentary rock formed by compressive 

forces; includes smaller, second-order anticlines and synclines.

Ash-flow tuff Extrusive volcanic rock 

formed by violent eruptions and spread by a 

dense, ground-hugging cloud of extremely 

hot gas and rock particles, a so-called “fiery 

cloud.” 

B horizon A middle or lower zone of 

a soil, under the A horizon and above a C 

horizon or bedrock; in the map area, usually 

enriched with clay (<0.002 mm diameter) and 

the lower part of older B horizons are also 

enriched with carbonate. Downward-seeping meteoric water carries clay and colloids, 

which accumulate in this layer whose texture, 

color, and coherence are distinctive. Oxidation of iron minerals in the B reddens it; 

centuries in age.

Calcarenite Limestone consisting 

of sand-size grains of calcium carbonate 

(calcite).

Coccolith Microscopic calcium carbonate plate (about 1 micron) of the phytoplankton spherical exoskeleton (one-celled marine 

plants) which is about 25 microns across. 

Seafloor ooze of the plates becomes chalky 

limestone. 

Colluvium Surficial rock debris and soil 

moved downslope by gravity.

Diamicton Debris of uncertain origin; 

unbedded, unsorted (clay to boulders), unconsolidated material possibly deposited by a 

glacier or landsliding.

Epicontinental sea On the surface of 

a continent; a relatively shallow inland sea; 

differs from deep oceans, such as the Atlantic.

Epeirogeny Uplift of a region of continental crust.

Fluvial terrace deposit A tabular body 

of gravel that covers a terrace (bench or 

tread); usually bound by a scarp; an erosional 

remnant of river channel gravel of rounded 

resistant stones.

Foraminifera Single-celled marine 

organism (ameboid protist) that secretes a 

small (<1−4 mm) shell of calcium carbonate 

extracted from seawater.

Ga (Giga annum) billion years.

Grainstone A rock having depositional 

texture that lacks mud and is grain supported.

Index fossil A distinctive, relatively 

short-lived, widespread fossil; used to correlate sedimentary rock units of similar age.

ka kilo-annum (thousand years).

Laramide orogeny Mountain building 

period in western North America, about 70 to 

40 million years ago, when the modern Rocky 

Mountains formed; first recognized in the 

Laramie Mountains, Wyoming.

Ma Mega-annum, million years ago.

Mass wasting Gravity-driven movement of rock and soil down slopes.

Micritic (microscopically crystalline) 

A limestone texture having distinct tiny crystals viewed with microscope.

Outwash Gravel sand and finer detritus 

deposited by glacial meltwater.

Pediment deposit Gently sloping thin 

cover of detritus on bedrock. In northern part 

of quadrangle, isolated, south-sloping pediment-deposit remnants are located on foothills 

of mountains to north.

Pleistocene Epoch Young geologic 

time (<2.6 Ma) informally divided into early, 

middle, and late for purposes of this map; part 

of the Quaternary Period.

Porphyry (adjective: porphyritic) 

Igneous rock made of large mineral crystals 

“floating” in a finely crystalline matrix. 

Residuum In-place mineral particles 

and soil formed by rock weathering; not 

transported. 

Septarian concretion Large, nearly 

oblate spheroid of clayey limestone made of 

irregular blocks separated by calcite-filled 

cracks. Occur in shale and may contain fossil 

mollusks. 

Subduction Sinking of a tectonic 

plate, usually dense oceanic crust, into the 

mantle below another plate as the two collide 

(converge).

Terrace Flat or gently inclined surface,

generally long and narrow, bound by descending steep slope at one edge and ascending 

scarp at the opposite edge, widthwise.
24 Geologic Map of the Pagosa Springs 7.5’ Quadrangle, Archuleta County, Colorado

Tuff A volcanic rock (ignimbrite) 

deposited by a dense, fiery cloud eruption of 

rapidly moving suspension of superhot ash, 

and small particles. Usually very dense (particles welded together by residual heat after 

coming to rest).

Weathering rind Crust of oxidized 

iron mineral enveloping a cobble or boulder; 

formed by prolonged weathering.
For more information concerning this publication, contact

Center Director, USGS Geosciences and Environmental 

Change Science Center

Box 25046, Mail Stop 980

Denver, CO 80225

(303) 236-5344

Or visit the Geosciences and Environmental Change Science 

Center Web site at https://gec.cr.usgs.gov/
GRAND

MESA

SAN LUIS

VALLEY

SAN

SAN JUAN BASIN

 UPLIFT

NACIMIENTO

VALLES CALDERA

SANGRE DE CRISTO MOUNTAINS

CHAMA BASIN

BRAZOS UPLIFT

JUAN

MOUNTAINS

ANTICLINORIUM

UNCOMPAHGRE PLATEAU

ARCHULETA

SAN JUAN

BASI

N

R

I

M Durango

Arkansas River

Rio Grande

Pagosa Springs

 Quadrangle

Aspen

Gallup

Gypsum

DENVER

Alamosa

Trinidad

Montrose Gunnison

SANTA FE

Farmington

Grand

Junction

Colorado

Springs

Pueblo

Rio Grande

Colorado River

COLORADO

NEW MEXIO

SAN JUAN

DOME

Moore and Lidke—Geologic Map of the Pagosa Springs 7.5' Quadrangle, Archuleta County, Colorado—Scientific Investigations Map 3419

ISSN 2329-132X (online)

https://doi.org/10.3133/sim3419