J.S. Department of the Interior 

J.S. Geological Survey

Geologic Framework of the San Juan Structural Basin 

:>f New Mexico, Colorado, Arizona, and Utah with 

Emphasis on Triassic through Tertiary Rocks

Regional Aquifer-System Analysis

professional Paper 1420

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GEOLOGIC FRAMEWORK OF THE SAN JUAN 

STRUCTURAL BASIN OF NEW MEXICO, COLORADO, 

ARIZONA, AND UTAH, WITH EMPHASIS ON 

TRIASSIC THROUGH TERTIARY ROCKS

By STEVEN D. CRAIGG

REGIONAL AQUIFER-SYSTEM ANALYSIS SAN JUAN BASIN, NEW MEXICO 

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1420
U.S. DEPARTMENT OF THE INTERIOR 

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY 

Charles G. Groat, Director

Any use of trade, product, or firm names in this report is for identification purposes only and does not constitute endorsement 

by the U.S. Government.

Reston, Virginia 2001

Library of Congress Cataloging in Publications Data

Craigg, Steven D.

Geologic framework of the San Juan structural basin of New Mexico, Colorado, Arizona, and Utah, with emphasis on Triassic through 

Tertiary rocks / by Steven D. Craigg.

p. cm. (Regional aquifer-system analysis San Juan Basin, New Mexico) (U.S. Geological Survey professional 

paper ; 1420)

Includes bibliographical references (p. -). 

ISBN 0-607-88067-8

1. Geology, Stratigraphic Mesozoic. 2. Geology, Stratigraphic Teriary. 3. Geology San Juan Basin (N.M. and Colo.) 

4. San Juan Basin (N.M. and Colo.) I. Title. II. Series. III. Series : U.S. Geological Survey professional paper ; 1420. 

QE675.C76 2000

557.89 dc20 95-37186

CIP

For sale by U.S. Geological Survey, Branch of Information Services, 

Box 25286, Federal Center, 

Denver, CO 80225
FOREWORD 

THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM

The RASA Program represents a systematic effort to study a number of the Nation's 

most important aquifer systems, which, in aggregate, underlie much of the country and 

which represent an important component of the Nation's total water supply. In general, 

the boundaries of these studies are identified by the hydrologic extent of each system 

and, accordingly, transcend the political subdivisions to which investigations have 

often arbitrarily been limited in the past. The broad objective for each study is to 

assemble geologic, hydrologic, and geochemical information, to analyze and develop 

an understanding of the system, and to develop predictive capabilities that will 

contribute to the effective management of the system. The use of computer simulation 

is an important element of the RASA studies to develop an understanding of the natural, 

undisturbed hydrologic system and the changes brought about in it by human activities 

and to provide a means of predicting the regional effects of future pumping or other 

stresses.

The final interpretive results of the RASA Program are presented in a series of U.S. 

Geological Survey Professional Papers that describe the geology, hydrology, and 

geochemistry of each regional aquifer system. Each study within the RASA Program 

is assigned a single Professional Paper number beginning with Professional Paper 

1400.

Charles G. 

Director

CONTENTS

	Page

Foreword........................................................................................................................................................... iii

Abstract............................................................................................................................................................. I

Introduction....................................................................................................................................................... 1

Purpose and Scope .................................................................................................................................. 2

Location and Geographic Setting............................................................................................................ 2

Previous Investigations........................................................................................................................... 2

Acknowledgments................................................................................................................................... 4

Regional Geologic Setting ....................................................................... ........................................................ 4

Tectonic Evolution.................................................................................................................................. 4

Structural Boundaries and Other Major Structural Elements ................................................................. 6

Structural Boundaries.................................................................................................................... 6

Other Major Structural Elements.................................................................................................. 9

Geologic Units.................................................................................................................................................. 10

Triassic Rocks......................................................................................................................................... 10

Moenkopi and Moenkopi(?) Formations ...................................................................................... 10

Chinle Formation .......................................................................................................................... 15

Dolores Formation ........................................................................................................................ 18

Jurassic Rocks......................................................................................................................................... 18

Wingate Sandstone........................................................................................................................ 18

Entrada Sandstone..................... ................................................................................................... 19

Wanakah Formation...................................................................................................................... 19

Cow Springs Sandstone................................................................................................................. 20

Junction Creek Sandstone............................................................................................................. 21

Morrison Formation...................................................................................................................... 21

Cretaceous Rocks.................................................................................................................................... 23

Burro Canyon Formation .............................................................................................................. 23

Dakota Sandstone.......................................................................................................................... 27

Mancos Shale................................................................................................................................ 30

Gallup Sandstone .......................................................................................................................... 30

Crevasse Canyon Formation......................................................................................................... 32

Point Lookout Sandstone.............................................................................................................. 35

Menefee Formation....................................................................................................................... 35

Cliff House Sandstone................................................................................................................... 38

Lewis Shale......................................................... ......................................................................... 41

Pictured Cliffs Sandstone.............................................................................................................. 44

Fruitland Formation and Kirtland Shale ....................................................................................... 44

Tertiary Rocks...................................................................................................................................... 48

Ojo Alamo Sandstone .......................................... ........................................................................ 48

Animas Formation......................................................................................................................... 49

Nacimiento Formation .................................................................................................................. 53

San Jose Formation....................................................................................................................... 53

Undivided Tertiary Sedimentary Rocks above the Ojo Alamo Sandstone in the Central Basin.. 54

Chuska Sandstone......................................................................................................................... 54

Summary........................................................................................................................................................... 54

Selected References .......................................................................................................................................... 57

Appendix Method of Contouring Maps Using Surface Modeling and Geographic Information

System Software............................................................................................................................................ 65
VI CONTENTS

ILLUSTRATIONS

(Plate is in pockpt)

PLATE 1. Geologic map of the Regional Aquifer-System Analysis study area in the San Juan structural basin. New Mexico, Colorado,

Arizona, and Utah 

FIGURE 1. Map showing location of the Regional Aquifer-System Analysis study area in the San Juan structural basin.................................................... 3

2. Map showing structural features of the San Juan Basin and adjacent areas.......................................................................................................... 5

3. Diagrammatic north-south-trending geologic sections showing principal structural features of the San Juan Basin........................................... 7

4. Diagrammatic east-west-trending geologic sections showing principal structural features of the San Juan Basin ............................................. 8

5. Diagram showing stratigraphic framework and nomenclature of the San Juan Basin in a general north-south direction .................................. 11

6. Diagrammatic northeast-trending stratigraphic section through the San Juan Basin showing the trough-like structure..................................... 12

7. Map showing approximate thickness of undivided Triassic rocks......................................................................................................................... 13

8. Diagrammatic subsurface stratigraphic correlation of units in the Chinle Formation ........................................................................................... 16

9-13. Maps showing:

9. Approximate thickness of the Morrison Formation......................................................................................................................................... 24

10. Approximate depth to the top of the Morrison Formation .............................................................................................................................. 25

11. Approximate altitude and configuration of the top of the Morrison Formation ............................................................................................. 26

12. Approximate depth to the top of the Dakota Sandstone................................................................................................................................. 28

13. Approximate altitude and configuration of the top of the Dakota Sandstone................................................................................................. 29

14. Diagrammatic subsurface stratigraphic correlation of the Gallup Sandstone and adjacent units in New Mexico............................................... 31

15-31. Maps showing:

15. Approximate depth to the top of the Gallup Sandstone................................................................................................................................... 33

16. Approximate altitude and configuration of the top of the Gallup Sandstone.................................................................................................. 34

17. Approximate depth to the top of the Point Lookout Sandstone ...................................................................................................................... 36

18. Approximate altitude and configuration of the top of the Point Lookout Sandstone..................................................................................... 37

19. Approximate depth to the top of the Menefee Formation............................................................................................................................... 39

20. Approximate altitude and configuration of the top of the Menefee Formation .............................................................................................. 40

21. Approximate depth to the top of the Cliff House Sandstone........................................................................................................................... 42

22. Approximate altitude and configuration of the top of the Cliff House Sandstone ......................................................................................... 43

23. Approximate depth to the top of the Pictured Cliffs Sandstone...................................................................................................................... 45

24. Approximate altitude and configuration of the top of the Pictured Cliffs Sandstone..................................................................................... 46

25. Approximate thickness of the combined Fruitland Formation and Kirtland Shale ........................................................................................ 47

26. Approximate depth to the top of the combined Fruitland Formation and Kirtland Shale.............................................................................. 49

27. Approximate altitude and configuration of the top of the combined Fruitland Formation and Kirtland Shale............................................. 50

28. Approximate depth to the top of the Ojo Alamo Sandstone ........................................................................................................................... 51

29. Approximate altitude and configuration of the top of the Ojo Alamo Sandstone .......................................................................................... 52

30. Approximate thickness of the undivided Tertiary sedimentary rocks above the Ojo Alamo Sandstone in the Central Basin

of the San Juan Basin ...........................................................................................................................................................................^ 55

31. Approximate altitude and configuration of the bottom of the undivided Tertiary sedimentary rocks above the Ojo Alamo Sandstone

in the Central Basin of the San Juan Basin..................................................................................................................................................... 56

32. Geographic information system coverage of control points intersected with a fine-mesh grid used to compute approximate altitude

and configuration of the top of the combined Fruitland Formation and Kirtland Shale....................................................................................... 67

33. Undipped and unannotated geographic information system line coverage of contours representing approximate altitude and

configuration of the top of the combined Fruitland Formation and Kirtland Shale.............................................................................................. 68

34. Clipped and annotated geographic information system coverage of the approximate altitude and configuration of the top of the combined

Fruitland Formation and Kirtland Shale ................................................................................................................................................................. 69

TABLES

TABLE 1. Summary of geologic characteristics for Tertiary through Triassic formations in the San Juan Basin Regional Aquifer-System

Analysis study area .......................................................................................................................................................................................... 14
CONTENTS VII

CONVERSION FACTORS AND VERTICAL DATUM

Multiply By To obhiin

inch 25.4 millimeter

loot 0.3048 meter

mile 1.609 kilometer

square mile 2.590 square kilometer

Sea level: In this report, "sea level" refers to the National Geodetic Vertical Datum of 1929 (NGVD of 1929) a geodetic datum derived from a general adjustment 

of the first-order level nets of the United States and Canada, formerly called Sea Level Datum of 1929.

REGIONAL AQUIFER-SYSTEM ANALYSIS SAN JUAN BASIN, NEW MEXICO

GEOLOGIC FRAMEWORK OF THE SAN JUAN

STRUCTURAL BASIN OF NEW MEXICO, COLORADO,

ARIZONA, AND UTAH, WITH EMPHASIS ON

TRIASSIC THROUGH TERTIARY ROCKS

By STEVEN D. CRAIGG

ABSTRACT

The San Juan Basin is located in the Four Corners 

region of northwestern New Mexico and southwestern Colorado, with a smaller part in northeastern Arizona and southeastern Utah an area of about 21,600 square miles. The 

basin is a structural depression formed during Laramide time 

with a maximum structural relief of about 10,000 feet. It contains about 14,400 feet of sedimentary rocks mainly ranging 

in age from Devonian through Tertiary.

The Regional Aquifer-System Analysis study area is 

slightly smaller than the San Juan structural basin, having an 

area of about 19,400 square miles, and is that part of the 

structural basin containing rocks of Triassic age and younger. 

This report describes the regional geologic and stratigraphic 

framework of the study area and includes detailed descriptions of the geologic units along with maps showing the 

thickness, depth to the top, and altitude and configuration of 

the top of the strata. The results presented in this report provide a hydrogeologic framework for the ground-water-flow 

modeling and water-quality studies of the San Juan Basin 

Regional Aquifer-System Analysis.

Triassic sedimentary rocks were generally deposited in 

continental (nonmarine) environments and collectively attain 

a maximum thickness of about 1,600 feet. The Chinle and 

Dolores Formations constitute the most important hydrogeologic units; regionally they could be considered confining 

units, but locally they are water yielding.

Jurassic rocks also mainly represent deposition in continental environments and collectively attain a maximum thickness of about 1,500 feet in the northwestern part of the basin. 

Although all these rocks locally yield some water to wells,

the Westwater Canyon Member of the Morrison Formation is 

regionally the most important aquifer.

Cretaceous sedimentary rocks represent continental, 

marginal marine, and marine environments associated with 

transgressing and regressing seas. During Cretaceous time, at 

least 6,500 feet of strata were deposited. Regionally, the 

Dakota and Gallup Sandstones are important water-yielding 

formations, although other units also yield dependable local 

supplies of water.

Tertiary sedimentary rocks were deposited in continental 

environments and attain a maximum thickness of about 

3,800 feet. The Ojo Alamo Sandstone and San Jose Formation are the most important hydrogeologic units.

Maps showing depth of the top, altitude, configuration 

of the top, and, if possible, thickness of the major formations 

were prepared from a data base of about 24,000 oil- and gastest wells in the basin using a surface-contour-generating 

software package interfaced with a Geographic Information 

System. These maps were used as input to a digital, threedimensional ground-water-flow model of the regional aquifer 

system in the basin.

INTRODUCTION

This report is a result of the U.S. Geological Survey's 

Regional Aquifer-System Analysis (RASA) study of the San 

Juan structural basin, a project that began in October 1984. 

The objectives of the San Juan Basin RASA are to: (1) define 

and evaluate the ground-water system; (2) assess the effects 

of past, present, and potential ground-water use on aquifers

1
GEOLOGIC FRAMEWORK OF THE SAN JUAN STRUCTURAL BASIN

and streams; and (3) determine the availability and quality of 

ground water (Welder, 1986).

PURPOSE AND SCOPE

This report identifies and defines the regional geologic 

and stratigraphic framework, and the extent and geologic 

characteristics of Triassic through Tertiary sedimentary rocks 

in the study area. Data used in this report were collected during the study or were derived from existing records in the 

U.S. Geological Survey's computerized National Water Information System (NWIS) data base and the Petroleum Information Corporation's data base.

LOCATION AND GEOGRAPHIC SETTING

The San Juan structural basin encompasses parts of 

New Mexico, Colorado, Arizona, and Utah and has an area 

of about 21,600 square miles (fig. 1). The structural basin is 

about 140 miles wide and 200 miles long. The study area is 

that part of the structural basin that contains rocks of Triassic 

or younger age and, therefore, is less extensive than the 

structural basin. It is about 140 miles wide (about the same 

as the structural basin), 180 miles long, and has an area of 

about 19,400 square miles. Triassic through Tertiary sedimentary rocks are emphasized in this study because they are 

the principal aquifers and confining units in the basin.

Altitudes in the study area range from about 4,500 feet 

above sea level in southeastern Utah to about 11,000 feet in 

the southeastern part of the basin. Annual precipitation in the 

mountainous areas along the northern and eastern margins of 

the basin is 20-30 inches; annual precipitation in the lower, 

central part of the basin generally is less than 8 inches 

(Craigg and others, 1989). Mean annual precipitation in the 

study area is about 12 inches.

Data obtained from the U.S. Bureau of the Census 

(1980, 1985) were used to estimate the population of the 

study area. The population in 1970 was estimated to be 

about 134,000. The population increased to about 194,000 by 

1980, 212,000 by 1982, 221,000 by 1984, and then 

decreased to about 210,000 in 1985. The economy of the 

basin is supported by exploration and mining of uranium, 

coal, and oil and gas; urban enterprise; farming and ranching; tourism; and recreation. The fluctuation in population 

was related to changes in the economic strength of the minerals and oil and gas industries, and their support services. 

Uranium mining and milling increased rapidly until the late 

1970's, when most uranium-mining activity ended in the 

study area. Likewise, the oil and gas industry prospered until 

about 1983 and then decreased rapidly, also affecting many 

jobs in support industries.

PREVIOUS INVESTIGATIONS

Numerous geologic investigations have been made in 

the San Juan structural basin, mainly because of its wealth of 

energy resources but also because of academic interest the 

area is a classic example of basin structural evolution that is 

somewhat penecontemporaneous with basin sedimentary 

deposition. Numerous hydrologic investigations also have 

been made in this basin; many of them relate to the evaluation of ground-water resources for Indian tribes, public-land 

management agencies, and municipalities, as well as the 

determination of potential effects of various energy-development plans on water resources. It is beyond the scope of this 

report to list all these works. Many of the major reports, 

however, are listed in the Selected References section; some 

of the major references for structure, stratigraphy, and hydrogeology are listed in this section; and other references are 

given throughout the text. For a comprehensive listing, 

Wright (1979) compiled an extensive bibliography of San 

Juan Basin geologic and hydrologic literature.

Dutton (1885) was among the first geologists to visit 

the San Juan Basin, where he made a geologic reconnaissance of the area. His study of Mount Taylor and the Zuni 

Plateau was published in the now-classic Sixth Annual 

Report of the U.S. Geological Survey. The general structure 

of the basin was discussed in an early New Mexico Geological Society field-trip guidebook by Kelley (1951). Hunt and 

Dane (1954), Baltz (1967), and Woodward (1974, 1987) also 

discussed structure and tectonics of the basin.

Geologic maps of the San Juan Basin area are available 

at a scale of 1:500,000 for each of the States in the study 

area: Dane and Bachman (1965) for New Mexico, Wilson 

and others (1969) for Arizona, Tweto (1979) for Colorado, 

and Hintze (1981) for Utah. These State geologic maps were 

used in compiling the geologic map of the study area for this 

report (pi. 1). The geology of the I°x2° (scale 1:250,000) 

quadrangles that cover the area has been determined and the 

maps are listed in the Selected References section of this 

paper. Numerous 7.5-minute quadrangles (scale 1:24,000) of 

the basin also are widely available; only those maps specifically referred to in the text are listed in the Selected References. In addition, Woodward (1987) compiled results of 

comprehensive geologic investigations along the eastern 

boundary of the San Juan Basin and adjacent areas.

Although not discussed specifically in this report, Precambrian and Paleozoic rocks in the basin are detailed in the 

following major works:

Precambrian: Cross andPurington(1899),Crossandothers(1899, 

1905a, b), Wood and Northrop (1946), Kelley (1950), 

Fitzsimmons (1963, 1967), and Woodward (1987). 

Cambrian:Ignacio Quartzite Cross and others (1899), Barnes

(1954), and Rhodes and Fisher (1957). 

Devonian:Aneth Formation Knight and Cooper (1955); 

Elbert Formation Knight and Cooper (1955) and
INTRODUCTION

Boundary 

of the 

San Juan 

structural 

basin

Boundary 

of the 

Colorado 

Plateau

200

I

400 MILES

200 300 400 KILOMETERS

FIGURE 1. Location of the Regional Aquifer-System Analysis study area in the San Juan 

structural basin, New Mexico, Colorado, Arizona, and Utah.

Rhodes and Fisher (1957); Ouray Limestone Cross 

and others (1899), Burbank (1941), and Knight and 

Baars(1957).

Mississippian: Leadville Limestone Emmons and others 

(1894) and Burbank (1941).

Pennsylvanian: Molas Formation Cross and others 

(1905a, b) and Wengerd and Matheny (1958); 

Madera Formation Read and others (1944) and 

Kelley and Wood (1946); Hermosa Formation 

Cross and others (1899), Bass (1944), and Wengerd 

and Matheny (1958); Honaker Trail Formation 

Wengerd and Matheny (1958); Paradox 

Formation Bass (1944) and Wengerd and Matheny 

(1958); Pinkerton Trail Formation Wengerd and 

Matheny (1958).

Permian: Abo Formation Needham and Bates (1943), 

Kelley and Wood (1946), and Wood and Northrop 

(1946); Cutler Formation Cross and others 

(1905 a, b), Baker and Reeside (1929), and Wengerd

and Matheny (1958); De Chelly Sandstone Gregory (1916); Yeso Formation Needham and Bates 

(1943), Kelley and Wood (1946), and Wood and 

Northrop (1946); Glorieta Sandstone Needham 

and Bates (1943); San Andres Limestone Needham 

and Bates (1943) and McKee and others (1959). In 

addition, Baars (1962) provided a comprehensive 

general reference for Permian rocks of the Colorado 

Plateau.

Major references for Triassic strata include Cross and 

others (1899), who named the Dolores Formation; Ward 

(1901), who named the Moenkopi Formation; and Gregory 

(1917), who named the Chinle Formation. Repenning and others (1969) also reported in detail on the stratigraphy of the 

Moenkopi and Chinle Formations, as did Stewart and others 

(1972a, b).

The stratigraphy of various Jurassic rocks in the basin 

was the subject of Emmons and others (1894), in which the 

Morrison Formation was named by G.H. Eldridge; Cross and
GEOLOGIC FRAMEWORK OF THE SAN JUAN STRUCTURAL BASIN

others (1899), who conducted early investigations in southwestern Colorado; Gilluly and Reeside (1928), who named 

the Entrada Sandstone; Burbank (1930), who first named the 

Wanakah Formation; and Baker and others (1936, 1947), 

who made regional correlations of Jurassic units throughout 

the Four Corners area. Craig and others (1955) conducted 

important work on the stratigraphy and uranium deposits of 

the Morrison in the Colorado Plateau. Other works concentrating on Jurassic rocks include Goldman and Spencer 

(1941), Harshbarger and others (1951), Smith (1953), and 

Green and Pierson (1977). Recent changes in nomenclature 

of certain Jurassic rocks have been made by Condon and 

Peterson (1986), Condon and Huffman (1988), and Condon 

(1989a, b).

Major investigations of Cretaceous rocks have been 

made by Holmes (1877), who named the Pictured Cliffs 

Sandstone; Cross and Purington (1899), who named the 

Mancos and Lewis Shales; Bauer (1916), who named the 

Kirtland Shale and Fruitland Formation; Collier (1919), who 

named the three-part Mesaverde Formation (now Mesaverde 

Group); and Sears (1925), who named the Gallup Sandstone. 

Sears and others (1936) defined the stratigraphy and coal 

resources of Cretaceous rocks in the southern part of the 

basin. Sears and others (1941) is a major work that established the concept of sediment balance in a subsiding basin 

as a major factor controlling transgression-regression of a 

sea. More recent work on Cretaceous rocks has included 

publications by Fassett and Hinds (1971) on the Fruitland 

Formation, Owen (1973) and Owen and Siemers (1977) on 

the Dakota Sandstone, and Molenaar (1973, 1974, 1977b) on 

the Gallup Sandstone.

Investigations that have defined rocks of Tertiary age 

include those by Emmons and others (1894), in which the 

Animas Formation was named by Whitman Cross; Keyes 

(1906), who named the Nacimiento Formation; Brown 

(1910), who named the Ojo Alamo Sandstone; and Gregory 

(1916), who named the Chuska Sandstone. Simpson 

(1948a, b) named the San Jose Formation, which later was 

subdivided by Baltz (1967). The Ojo Alamo Sandstone has 

more recently been investigated in detail by O'Sullivan and 

others (1972), Powell (1973), and Fassett (1974).

Some of the hydrologic investigations made in the San 

Juan Basin include those by Gregory (1916), who conducted 

a geographic and hydrographic reconnaissance of the Navajo 

Indian Reservation and adjacent areas, and Renick (1931), 

who conducted a ground-water investigation of the southeastern part of the basin. Irwin (1966) investigated groundwater resources of the Ute Mountain Ute Indian Reservation 

in Colorado and New Mexico; Baltz and West (1967) investigated ground-water resources of the Jicarilla Apache Reservation in New Mexico; and Cooley and others (1969) 

conducted a regional hydrologic investigation of the Navajo 

and Hopi Indian Reservations in Arizona, New Mexico, and 

Utah. Ground-water investigations in the southern part of the

San Juan Basin include those by Gordon (1961), Cooper and 

John (1968), Mercer and Cooper (1970), Shomaker (197la), 

and Risser and Lyford (1983). Stone and others (1983) published the results of a regional hydrogeologic investigation of 

the New Mexico part of the San Juan Basin. Master's theses 

that cover selected 15-minute quadrangles in the basin were 

done by Brown (1976), Anderholm (1979), Brod (1979), and 

Craigg (1980). Craigg (1992) conducted a ground- and surface-water investigation of the Jemez, Zia, and Santa Ana 

Pueblos in the southeastern part of the basin. The San Juan 

Basin RASA staff has prepared U.S. Geological Survey 

Hydrologic Investigations Atlases (HA-720-A through J) for 

10 major aquifers in the basin; included are Craigg and others (1989, 1990); Kernodle and others (1989, 1990); Dam 

and others (1990a, b); Levings and others (1990a, b); and 

Thorn and others (1990a, b).

ACKNOWLEDGMENTS

The author expresses appreciation to the following U.S. 

Geological Survey geoscientists in Denver, Colo., for providing useful discussion, ideas, review comments, timely stratigraphic information, and recent or pending nomenclature 

changes for various geologic units: S.M. Condon, J.E. Fassett, A.C. Huffman, Jr., and C.M. Molenaar; Vickie L. Clark 

deserves special thanks for providing access to computer 

facilities. Also, I am indebted to J.M. Kernodle (U.S. Geological Survey, Albuquerque, N. Mex.) for his expertise and 

assistance with Geographic Information Systems.

REGIONAL GEOLOGIC SETTING

The San Juan structural basin is an approximately circular, asymmetric structural depression located primarily in the 

east-central part of the Colorado Plateau (fig. 1). The basin is 

one of several large structural basins underlying the Colorado Plateau. The San Juan Basin straddles the Four Corners 

area but mainly is located in northwestern New Mexico and 

southwestern Colorado; smaller parts are located in northeastern Arizona and southeastern Utah (figs. 1 and 2). The 

basin is about 140 miles wide, 200 miles long, and has an 

area of about 21,600 square miles. Faulting is common, 

especially in the northeastern, southeastern, and south-central 

parts of the basin, and also along the north-central and eastcentral margins (pi. 1).

TECTONIC EVOLUTION

The tectonic evolution of the San Juan Basin is extensive and complex. The Precambrian geologic history of the 

region is obscured by complex metamorphism, deformation, 

intense erosion, and subsequent burial under Phanerozoic 

rocks; Kelley (1950, p. 101) reported that there is no evi-
REGIONAL GEOLOGIC SETTING

108e

COLORADO __ co 

NEW MEXICO r

FIGURE 2 Structural features of the San Juan Basin and adjacent areas. Modified from Kelly (1951).
GEOLOGIC FRAMEWORK OF THE SAN JUAN STRUCTURAL BASIN

dence to indicate any structural depression during Precambrian time in the present San Juan Basin area.

Tectonism began during late Paleozoic time along the 

areas bordering the present basin (San Juan Uplift in the 

north, Nacimiento Uplift in the east, Zuni Uplift in the south, 

and Defiance Uplift in the west; fig. 2). Recurrent uplift and 

minor deformation continued throughout Mesozoic time 

(Kelley, 1951, p. 129).

As noted by Kelley (1950, 1951, 1957), it was not until 

latest Late Cretaceous and early Tertiary time that the principal structural elements of the present San Juan Basin began 

to form. The San Juan Basin is, therefore, predominantly a 

Laramide feature. After the culmination of the Laramide 

orogeny, minor doming associated with intrusion by laccoliths and other igneous bodies during middle and late 

Tertiary time modified some of the older structures and reactivated faults in the southeastern part of the basin (Woodward and Callender, 1977, p. 209).

STRUCTURAL BOUNDARIES AND OTHER MAJOR 

STRUCTURAL ELEMENTS

Major structural elements of the San Juan Basin were 

first delineated by Kelley (1950), with minor later revisions 

by Kelley (1951, 1957, 1963), Kelley and Clinton (1960), 

Baltz (1967), and Woodward and Callender (1977). Structural boundaries are distinct and well defined in many areas, 

whereas in other areas the basin merges transitionally into 

adjacent structural depressions or uplifts. Numerous small 

structural embayments or sags extend from the San Juan 

Basin proper into adjacent areas (fig. 2) (Kelley, 1950, 

p. 101).

Structural boundaries and elements of the San Juan 

Basin are of three major types: (1) large, elongated, domal 

uplifts; (2) low, marginal platforms; and (3) abrupt monoclines. The following discussion of these features is summarized mainly from the works of Kelley (1950, 1951, 1957) 

and Woodward (1987).

STRUCTURAL BOUNDARIES

Along the northern margin, the San Juan Basin is 

bounded by the San Juan Uplift (figs. 2, 3A, and 3£), with 

its Precambrian core of crystalline rocks rising abruptly from 

the basin rim. The San Juan Uplift is a northwest-trending 

feature about 75 miles long and 35 miles wide. Structural 

relief between the uplift and the San Juan Basin is as much 

as 20,000 feet (Kelley, 1957, p. 48).

The basin is bounded on the northeast by the broad 

Gallina-Archuleta Arch, a northwest-trending, asymmetric 

anticlinorium (fig. 2). This arch separates the much deeper 

San Juan Basin to the west from the shallower Chama Embayment to the east. Maximum structural relief between the arch 

and the San Juan Basin is about 13,000 feet, whereas the 

Chama Embayment is structurally only about 1,500 feet lower 

than the arch (Woodward and Callender, 1977, p. 211).

The eastern margin of the San Juan Basin is bounded by 

a prominent and structurally complex feature, the Nacimiento 

Uplift (fig. 2). In a classic geologic report, Woodward (1987) 

described the complex structural relations and geologic history of this area; the following summary is from that report.

The Nacimiento Uplift is a north-trending mountain 

block about 50 miles long and 6-10 miles wide and represents 

the southwestern limit of the Rocky Mountains. The northern 

part of the uplift terminates in a broad, north-plunging anticline that merges with the Gallina-Archuleta Arch, whereas 

the southern part terminates in south-plunging folds covered 

unconformably by Tertiary sedimentary rocks. In the most 

simplified model, the uplift generally consists of a block of 

Precambrian crystalline rocks that has been overthrust westward (fig. 4B). Structural relief between the highest part of the 

uplift and the San Juan Basin is at least 10,000 feet (Woodward, 1987, p. 49).

This report can only summarize the extremely complex 

structure of the western flank of the Nacimiento Uplift. In 

general, the western flank consists of northern and southern 

segments of two separate, major faults. The Nacimiento Fault 

(pi. 1) is a thrust fault extending northward from San Miguel 

Canyon. At great depths, the dip of this fault is steep to vertical (fig. 4£), but at shallow levels the dip flattens and 

becomes virtually horizontal (Woodward, 1987, p. 51, figs. 

17-19). The maximum stratigraphic displacement along the 

Nacimientio Fault is about 4,000 feet; the maximum westward movement of the hanging wall is about 2,500 feet 

(Woodward, 1987, p. 52).

The Pajarito Fault (pi. 1) is a steep-angle reverse fault 

extending southward from San Miguel Canyon. The fault 

trace ranges in thickness from a knife edge to a 50-foot-wide 

zone of sheared Precambrian crystalline rocks. Maximum 

stratigraphic separation on the Pajarito Fault is about 

3,600 feet (Woodward, 1987, p. 52). Craigg (1992) described 

the hydrogeology of saline springs associated with the discharge of deep ground water from the San Juan Basin along 

the Pajarito Fault.

Numerous other smaller faults (synthetic reverse faults, 

east-trending faults, antithetic reverse faults, and normal 

faults), en echelon folds, and other complex structures exist 

along the western face of the Nacimiento Uplift. A complexly 

faulted, steep, sometimes westward-dipping, sometimes overturned sinuous hogback of Paleozoic, Mesozoic, and Cenozoic rocks extends along the western face of the uplift; this 

hogback is the most complex part of the Hogback Monocline 

shown in figure 2. For detailed discussion and analysis of all 

aspects of the Nacimiento Uplift, the interested reader is 

referred to Woodward (1987).
REGIONAL GEOLOGIC SETTING

SOUTH NORTH

Zuni 

uplift

EXPLANATION

Tertiary strata 

Cretaceous strata 

Jurassic strata

Triassic strata 

Paleozoic strata

Precambrian crystalline 

rocks

Contact Dashed where inferred

FIGURE 3. Diagrammatic north-south-trending geologic sections showing principal structural features 

of the San Juan Basin. A, section from Acoma Sag through Chaco Slope, Central Basin, and Hogback 

Monocline to San Juan Uplift. 5, section from Zuni Uplift through Chaco Slope, Central Basin, Hogback Monocline, and Four Corners Platform to San Juan Uplift.

The San Juan Basin is bounded on the southeast by 

three features: the extensively fractured Rio Grande Rift (a 

post-Laramide structure), the Ignacio Monocline, and the 

Lucero Uplift (fig. 2). The gradual structural rise of the 

southeastern part of the San Juan Basin terminates against 

the complexly faulted western margin of the Rio Grande 

Rift, a late Cenozoic extensional feature that is partly 

bounded on the west by the Ignacio Monocline. The Lucero 

Uplift is a northeast-trending structure about 30 miles long

and 14 miles wide with a structural relief of about 2,500 feet 

(Kelley, 1950, p. 103). The broad, synclinal Acoma Sag is 

located between the Lucero Uplift on the east and the Zuni 

Uplift on the west (fig. 2).

The south-central margin of the San Juan Basin is 

bounded by the northwest-trending Zuni Uplift (figs. 2, 35, 

and 4C). This domal feature consists of a Precambrian 

crystalline-rock core and prominent southwest- and northeast-facing dip slopes of upper Paleozoic sedimentary rocks.
GEOLOGIC FRAMEWORK OF THE SAN JUAN STRUCTURAL BASIN

WEST EAST

EXPLANATION

 R

Pz

p 

^

Triassic strata 

Paleozoic strata

Precambrian crystalline 

rocks

direction of movement

Tertiary strata 

Cretaceous strata 

Jurassic strata 

Contact

FIGURE 4. Diagrammatic east-west-trending geologic sections showing principal structural 

features of the San Juan Basin. A, section from Four Corners Platform through Hogback 

Monocline (west limb) and Central Basin to Hogback Monocline (east limb). B, section 

from Chuska Mountains and Defiance Uplift through Chaco Slope and Central Basin to 

Hogback Monocline (east limb) and Nacimiento Uplift. C, section from Gallup Sag through 

Nutria Monocline, Zuni Uplift, Chaco Slope, and Puerco Fault Zone to Puerco Platform.
REGIONAL GEOLOGIC SETTING

The southwestern limb of the uplift is known as the Nutria 

Monocline. The Zuni Uplift is about 80 miles long and as 

much as 35 miles wide. Structural relief between the highest 

part of the uplift and the San Juan Basin is as much as 5,500 

feet (Kelley, 1950, p. 103).

In the southwest, the San Juan Basin is bounded by the 

northwestern part of the Zuni Uplift and the southern end of 

the north-trending Defiance Uplift (fig. 2). The steep eastern 

limb of the Defiance Uplift is known as the Defiance Monocline. A synclinal structural embayment, the Gallup Sag 

(fig. 2) is formed by the opposite-dipping limbs of the Nutria 

and Defiance Monoclines. The Zuni Embayment is a southern extension of the Gallup Sag.

Along the western margin, the San Juan Basin is 

bounded by the northern end of the north-trending Defiance 

Uplift. This prominent uplift forms the structural divide 

between the San Juan Basin to the east and the Black Mesa 

Basin to the west (fig. 2). The steeply dipping, sinuous Defiance Monocline (figs. 2, 4B) forms the eastern face of the 

uplift. The Defiance Uplift is about 100 miles long and 

30 miles wide, with a maximum structural relief of about 

8,000 feet (Kelley, 1957, p. 44) above the San Juan Basin.

In the northwest, prominent laccolithic masses of the 

Carrizo Mountains, Sleeping Ute Mountain, and La Plata 

Mountains bound the San Juan Basin (fig. 2). These laccoliths rise from the extensive, low structural feature known as 

the Four Corners Platform.

OTHER MAJOR STRUCTURAL ELEMENTS

The Puerco Platform and Puerco Fault Zone 

(figs. 2,4Q mark the southeastern limit of the San Juan 

Basin and Colorado Plateau. Collectively, these northeasttrending features are about 35 miles long and 7-22 miles 

wide (Kelley, 1957, p. 47). To the north, the Puerco Platform 

merges with the Puerco Fault Zone and the Chaco Slope. To 

the east, the platform is bounded by the Ignacio Monocline 

and the Rio Grande Rift. To the south, the platform merges 

with the Acoma Sag, and to the west, it is bounded by 

Mount Taylor. The Puerco Fault Zone is characterized by 

northeast-trending, steep-angle normal faults (pi. 1), most of 

which are downthrown to the west (Kelley, 1957, p. 47; 

Craigg, 1980, p. 27). Most of the faults have maximum displacements ranging from only a few tens of feet to as much 

as 150 feet (Craigg, 1980, p. 27), but Kelley (1957, p. 47) 

reported between 1,000 and 2,000 feet of displacement on a 

few faults; however, he did not specify the location of these 

faults. Craigg (1980, p. 27) reported that the major geologic 

units typically are not completely offset along these faults. 

Also, the dip of the Mesozoic strata becomes steeper and the 

faults rise in a steplike manner toward the western flank of 

the Nacimiento Uplift (Craigg, 1980, p. 27). It is uncertain 

how deep the faulting in the Puerco Fault Zone actually

extends into the subsurface. Also present on the Puerco 

Platform and within the Puerco Fault Zone are a series of 

northeast-trending basaltic dikes, exhumed volcanic necks, 

and Tertiary basalt flows (Craigg, 1980, p. 27-31).

The prominent Chaco Slope (fig. 2) is a somewhat arbitrarily defined structural element of the San Juan Basin 

(Kelley, 1957, p. 46). The slope is a broad, northwest-trending platform that is south of the Central Basin and north of 

the Puerco Platform, Puerco Fault Zone, and Zuni Uplift 

(fig. 2). The Chaco Slope is about 110 miles long and 30- 

40 miles wide. The regional dip of the rocks forming this 

platform is about 1° northward, and structural relief between 

the platform and the Central Basin is about 2,500 feet 

(Kelley, 1957, p. 46). Locally, the dip of the rocks forming 

the Chaco Slope is steeper, as it is along the northern part of 

the Zuni Uplift and also where the platform merges with the 

southern part of the Central Basin (figs. 3, 4C).

The Four Corners Platform (fig. 2) is a broad, prominent, northeast-trending structural element in the northwestern part of the San Juan Basin. The platform is distinctly 

bounded by the San Juan Uplift in the north, the Hogback 

Monocline in the east, and the Defiance Uplift in the southwest (figs. 2, 3B, and 4fi). However, the platform boundary 

is difficult to delineate where it merges gradually with the 

Central Basin (southeast), Chaco Slope (south), and the 

Blanding and Paradox Basins of Utah and Colorado, which 

are to the northwest of the San Juan Basin (fig. 2). The dip 

of the rocks forming the platform steepens where it merges 

with the Central Basin along the Hogback Monocline 

(fig. 4A). The maximum structural relief between the Four 

Corners Platform and the Central Basin is about 4,000 feet 

(Kelley, 1957, p. 44).

The Hogback Monocline is a long, arcuate structural 

feature that extends northward from the Rio Grande Rift in 

the southeastern part of the San Juan Basin to the San Juan 

Uplift in the northern part of the basin and then southwestward for about 100 miles (fig. 2). Along the western flank of 

the Nacimiento Uplift, the rocks forming the Hogback Monocline are complexly faulted and locally overturned (Woodward, 1987, p. 55; Craigg, 1992). The Hogback Monocline 

forms all but the southern and southwestern boundaries of 

the Central Basin (fig. 2). The rocks forming the monocline 

dip steeply into the Central Basin (figs. 3, 4A, and 4B).

The Central Basin is a nearly circular (fig. 2) bowlshaped depression that is deeper in the north and northeast. 

Along its southern margin, the Central Basin has a slightly 

dipping limb that merges with the Chaco Slope (figs. 3, 4B, 

and 4Q. The axis of the San Juan Basin strikes eastward 

through the northern part of the Central Basin. The deepest 

part of the San Juan Basin is in the north-central part of the 

Central Basin, beneath the Navajo Reservoir (pi. 1) in New 

Mexico. The depth to Precambrian crystalline rocks in the 

deepest part of the basin is about 14,400 feet (Fassett and 

Hinds, 1971, p. 4):
10 GEOLOGIC FRAMEWORK OF THE SAN JUAN STRUCTURAL BASIN

GEOLOGIC UNITS

Professional Paper 1420 was approved for publication 

by the Director, U.S. Geological Survey, in 1992. The stratigraphic nomenclature used herein reflects the state of geologic knowledge circa 1990. Subsequent workers may have 

proposed minor changes in the nomenclature of certain geologic units in the interim between Director's approval and 

publication of this report.

No attempt is made herein to incorporate any stratigraphic nomenclature changes made since Director's 

approval, because the other major reports for the San Juan 

Basin RASA (Geochemistry Dam, 1995; Ground-Water 

Modeling Kernodle, 1996; and Summary Levings and 

others, 1996) cite Professional Paper 1420 as in press and 

use many of the same illustrations including the stratigraphic 

diagrams, formation thickness maps, altitude of top of formation maps, and cite the geologic map. The ten Hydrologic 

Investigations Atlases (HA-720-A through J) published during this RASA also use the same stratigraphic nomenclature.

Superposed on the San Juan Basin is a thick sedimentary rock sequence mainly ranging from Devonian through 

Tertiary age, with the largest portion being Pennsylvanian 

through Tertiary (fig. 5). The maximum thickness of this 

sequence above the Precambrian crystalline-rock basement 

of the Central Basin is about 14,400 feet (Fassett and Hinds, 

1971, p. 4). Basin structural history, as well as depositional 

history, is recorded in the unconformities and intertonguing 

of these rocks, in the faults that transect and monoclines that 

tilt the rocks, and in the igneous bodies that intrude the 

rocks. Evidence of the Laramide orogeny during latest Late 

Cretaceous and early Tertiary time is especially evident in 

the unconformities at the bases of lower Tertiary rocks (Condon and Huffman, 1989, p. 17, 18).

The sedimentary rock sequence of the San Juan Basin 

dips inward from the basin margins toward the trough-like 

structural center, or deepest part of the Central Basin (fig. 6; 

pi. 1). Older sedimentary rocks crop out around the basin 

margins and are successively overlain by younger strata 

toward the basin center, much like a set of nested bowls.

The San Juan Basin RASA study area, as defined earlier, is that part of the San Juan Basin that contains rocks of 

Triassic through Tertiary age. These rocks are emphasized 

because they contain the principal aquifers and confining 

units in the basin. The general geologic characteristics of the 

rocks are summarized in table 1. Although not discussed in 

this report, volcanic rocks of Tertiary and Quaternary age 

and various unconsolidated surficial deposits (alluvial, 

eolian, landslide, talus, and terrace deposits) also are present 

in the basin (pi. 1).

TRIASSIC ROCKS

Sedimentary rocks of Triassic age in the San Juan Basin 

consist of the Moenkopi Formation, Moenkopi(?) Formation, 

Chinle Formation, and Dolores Formation. The Moenkopi 

Formation is of Early and Middle Triassic age, and the 

Chinle and Dolores Formations are of Late Triassic age. Triassic rocks crop out along the basin boundaries (pi. 1) and 

are present in the subsurface throughout the basin. Regionally, Triassic rocks disconformably overlie Permian rocks 

throughout most of the San Juan Basin (fig. 5); in the eastern 

and northern parts of the basin, however, Triassic rocks disconformably overlie Pennsylvanian or Precambrian rocks 

(O'Sullivan, 1977, p. 139). In most of the basin, Triassic 

rocks are disconformably overlain by the Entrada Sandstone 

of Middle Jurassic age.

Triassic rocks generally consist of deposits from various 

nonmarine (stream channel, flood plain, eolian, and lacustrine) environments. In the San Juan Basin, the maximum 

total thickness of Triassic strata is about 1,650 feet; the 

thickness increases toward the southwest, from zero near 

Pagosa Springs, Colo., to a maximum of about 1,650 feet 

along the New Mexico-Arizona State line (fig. 7) (O'Sullivan, 1977, p. 140). The general lithologies and maximum 

thicknesses of Triassic rocks are summarized in table 1.

MOENKOPI AND MOENKOPK?) FORMATIONS

The Moenkopi Formation was named by Ward (1901) 

for exposures along the mouth of Moenkopi Wash at the 

junction with the Little Colorado River in Coconino County, 

Ariz. In the San Juan Basin, the Moenkopi is present only 

along an isolated strip west of the New Mexico-Arizona 

State line in the Defiance Uplift. In this area, the Moenkopi 

consists solely of the Holbrook Member (Stewart and others, 

1972b, p. 26), which generally consists of reddish-brown 

mudstone and siltstone with lesser amounts of fine-grained 

sandstone. The maximum thickness is about 50 feet (O'Sullivan, 1977, p. 139).

Although throughout most of its extent the Moenkopi 

Formation was deposited on a broad shelf along the margins 

of a transgressing sea, in the San Juan Basin it mainly was 

deposited in fluvial environments (O'Sullivan, 1977, p. 145). 

The Moenkopi disconformably overlies sedimentary rocks of 

Permian age and is disconformably overlain by the Shinarump Member of the Chinle Formation of Late Triassic 

age.

Along the southern margin of the San Juan Basin, in the 

Zuni Mountains-Fort Wingate-Bluewater Lake area, outcropping strata tentatively have been correlated with the Moenkopi Formation. These deposits have been referred to as the 

Moenkopi(?) Formation by some geologists (Stewart and 

others, 1972b, p. 26; O'Sullivan, 1977, p. 139). A 30-mile
GEOLOGIC UNITS 11

TERTIARY

EOCENE

PALEOCENE

UPPER

LOWER

UPPER

SAN JOSE FORMATION

KIRKLAND SHALE FARMINGTON SANDSTONE MEMBER 

Lower shale member 

MCDERMOT 

MEMBER

_______SATAN TONGUE

JdOSfATONGUE

I GIBSON COAL MEMBER 

CREVASSE

CANYON ^ < 'MULATTOTONGUE 

FORMATION

DALTON SANDSTONE MEMBER

TOCITO 

SANDSTONE LENTIL

MANGOS SHALE

<^EL VADO SANDSTONE MBR^

MANGOS SHALE SEMILLA SANDSTONE MEMBER

BRIDGE CREEK (GREENHORN) LIMESTONE MEMBER"

TWOWELLS TONGUE OF DAKOTA SANDSTONE . GRANEROS MEMBER

PAGUATE TONGUE OF DAKOTA SANDSTONE-^

DAKOJA^SANDSTONEiENCINAJL CANYON_M EMBER, CUB_ERO TONGUE^AND OAK CANYON_MEM_BER) 

JACKPILE SANDSTONE MEMBER BURRO CANYON FORMATION

WESTWATER CANYON MEMBER^

BRUSHY BASIN MEMBER

RECAPTURE MEMBER

MORRISON FORMATION SALT WASH MEMBER

JUNCTION CREEK SANDSTONE

JURASSIC COW SPRINGS SANDSTONE HORSE MESA MEMBER

MIDDLE \N RAFAEL GROUP BECLABITO MEMBER

TODILTO LIMESTONE MEMBER

- WANAKAH Upper part

FORMATION PONY EXPRESS LIMESTONE MEMBE"R

PERMIAN

DE CHELLY SANDSTONE

ABO SANDSTONE

CUTLER FORMATION

PENNSYLVANIAN

MISSISSIPPIAN

DEVONIAN

SILURIAN

ORDOVICIAN

CAMBRIAN

PRECAMBRIAN

MADERA FORMATION HERMOSA FORMATION HERMOSA GROUP HONAKER TRAIL FORMATION 

PARADOX FORMATION

PINKERTON TRAIL FORMATION

MOLAS FORMATION

ELBERT FORMATION

GRANITIC AND HIGH-RANK METAMORPHIC ROCKS

NOT TO SCALE

FIGURE 5. Stratigraphic framework and nomenclature of the San Juan Basin in a general north-south direction. Modified from Molenaar

(1977a, b; 1989).
SOUTHWEST 

TRIASSIC JURASSIC 

ROCKS ROCKS

CRETACEOUS 

ROCKS

TERTIARY 

ROCKS

SAN JOSE, NACIM

AND ANIMAS FORM

PALEOZOIC ROCKS 

UNDIVIDED

PRECAMBRIAN

CRYSTALLINE

ROCKS

NOT TO SCALE

FIGURE 6. Diagrammatic northeast-trending stratigraphic section through the San Juan Basin showing the trough-like structure. Modified fr
GEOLOGIC UNITS 13

106^30'

37 0

0 10 20 30 40 50 60 MILES

I I

36 00

0 10 20 30 40 50 60 KILOMETERS 34 45'

EXPLANATION

Outcrops of Triassic rocks From Dane and 

Bachman (1965), Wilson and others (1969), 

and Tweto (1979)

Boundary of study area

 7650 Structure contour Shows approximate thickness 

of Triassic rocks. Interval 330 feet

FIGURE 7. Approximate thickness of undivided Triassic rocks in the San Juan Basin study area. Modified from 

O'Sullivan (1977, fig. 1). Contour interval is 330 feet.
14 GEOLOGIC FRAMEWORK OF THE SAN JUAN STRUCTURAL BASIN

TABLE 1. Summary of geologic characteristics for Triassic through Tertiary formations in the San Juan Basin Regional Aquifer-System

Analysis study area

[Formations are listed in stratigraphic order with the youngest at the top. Figures 3-6 and 8 show strati graphic relations of these formations]

System Formation

Approximate maximum 

depth to top (feet)

Approximate 

maximum

thickness (feet) General lithologic description

Tertiary......... Chuska Sandstone

San Jose Formation

Nacimiento Formation

Animas Formation 

(includes McDcrmott 

Member)

Ojo Alamo Sandstone

Cretaceous..... Kirtland Shale and

Fruitland Formation 

(combined)

Pictured Cliffs Sandstone

Lewis Shale

Cliff House Sandstone

Menefee Formation

At surface in western 

part of basin

At surface inside the 

Central Basin

2,600

1 2,600

3,500

3,500

4,000 

4,000-4,500

6,000 

6,000-6,500

Point Lookout Sandstone 6,000-6,500

Crevasse Canyon Formation ' 3,200

1,750

2.700

1,300

2,700

400

2,000

400

2,400

Individual tongues 

discussed in text

2,000

415

750

Gallup Sandstone

Mancos Shale

Dakota Sandstone

4,500

7,000

8,500

300

2,300

500

Massive, crossbedded, very fine to coarsegrained sandstone; minor interbeds of 

siltstone and shale; eolian

Interbedded very fine to coarse-grained, 

locally conglomeratic arkosic sandstone 

and siltstone and variegated shale; fluvial

Interbedded gray shale and discontinuous 

lenses of arkosic sandstone; locally 

constant carbonaceous lenses; lacustrine 

and fluvial

Interbedded tuffaceous sandstone,

conglomerate, and shale; McDermott 

Member distinctly purple; fluvial

Overlapping sheetlike sequences of 

arkosic sandstone and conglomerate; 

locally contains interbedded lenses of 

shale; fluvial

Interbedded repetitive sequences of 

lenticular sandstone, siltstone, shale, and 

claystone; carbonaceous shale and coal 

common in Fruitland; fluvial and "coal" 

swamp environments

Upward-coarsening, very fine to medium 

grained sandstone with thin interbeds of 

dark shale in lower part; regressive marine

Dark shale and silty shale with thin 

interbeds of silty limestone, siltstone, and 

fine-grained sandstone in lower part; 

intertongues with Pictured Cliffs and Cliff 

House Sandstones; offshore marine

Several very fine to fine-grained sandstone 

tongues; common interbeds of dark shale; 

transgressive marine

Interbedded sequences of lenticular 

sandstone, siltstone, dark shale, and 

claystone; carbonaceous shale and coal 

common in lower and upper parts; fluvial 

and "coal" swamp environments

Very fine to medium-grained sandstone 

with thin interbeds of dark shale in lower 

part; regressive marine

Interbedded sequences of lenticular 

sandstone, siltstone, shale, and claystone 

with carbonaceous shale and coal 

common in lower and upper parts; fluvial 

and "coal" swamp environments. Also 

contains regressive marine deposits; 

Dalton Sandstone Member is in middle 

part (members discussed in text)

Sandstone with some conglomerate, shale, 

carbonaceous shale, and coal; mainly 

regressive marine but also contains 

deposits of continental environments

Dark shale and silty shale with thin 

interbeds of silty limestone, siltstone, and 

fine-grained sandstone; intertongues with 

Point Lookout, Gallup, and Dakota 

Sandstones; offshore marine

Several members and tongues of fine- to 

coarse-grained sandstone with dark shale, 

siltstone, and minor carbonaceous shale; 

mainly transgressive marine, but lower
GEOLOGIC UNITS 15

TABLE 1. Summary of geologic characteristics for Triassic through Tertiary formations in the San Juan Basin Regional Aquifer-System

Analysis study area Continued.

[Formations are listed in stratigraphic order with the youngest at the top. Figures 3-6 and 8 show stratigraphic relations of these formations]

System Formation

Approximate maximum 

depth to top (feet)

Approximate 

maximum

thickness (feet) General lithologic description

Burro Canyon Formation Not known

Jurassic.......... Morrison Formation

Triassic.

8,500

Junction Creek Sandstone Not known

Cow Springs Sandstone Not known

Wanakah Formation 9,500

Entrada Sandstone J 9,300

Wingate Sandstone Not known

Dolores Formation Not known

Chinle Formation ' 9,700

Moenkopi Formation Not known

Moenkopi(?) Formation Not known

200

1,100

500

300

2200

330

Not known 

1,200(?)

1,650

50

150

part contains fluvial deposits 

Fine- to coarse-grained sandstone and

conglomeratic sandstone interbedded

locally with shale; fluvial 

Fine- to coarse-grained locally

conglomeratic sandstone, sandy siltstone,

shale, and claystone; also contains thin

limestone beds; various continental

environments (members discussed in text) 

Crossbedded fine- to coarse-grained

quartzose sandstone with arkosic

sandstone and shale lenses; mostly eolian 

Crossbedded fine- to medium-grained

quartzose and arkosic sandstone; eolian 

Limestone, gypsum, claystone, very fine to

fine-grained silty sandstone, and coarsegrained sandstone; lacustrine, eolian, and

fluvial (members discussed in text) 

Crossbedded silty sandstone and very fine

to medium-grained quartzose sandstone;

eolian 

Crossbedded very fine to medium-grained

sandstone; eolian 

Siltstone and very fine to fine-grained

sandstone with limestone conglomerate

lenses; fluvial 

Variegated claystone and shale, siltstone,

sandstone, conglomerate, and limestone;

various continental environments

(members discussed in text) 

Variegated mudstone and siltstone with

lesser amounts of fine-grained sandstone;

fluvial 

Variegated mudstone and siltstone,

sandstone, and quartz-pebble

conglomerate; fluvial

'Stone and others, 1983. 

2Molenaar, 1989.

gap exists between outcrops of the Moenkopi(?) and their 

probable equivalents in the Defiance Uplift area to the northwest (Stewart and others, 1972b, p. 28).

The Moenkopi(?) Formation disconformably overlies 

the San Andres Limestone of Permian age and is disconformably overlain by the Shinarump Member of the Chinle 

Formation. The Moenkopi(?) was deposited by streams 

draining into the Triassic sea in Arizona (O'Sullivan, 1977, 

p. 145). Lithologically, the Moenkopi(?) generally consists of 

variegated sequences of slope-forming mudstone and siltstone, ledge-forming sandstone, and quartz-chert pebble conglomerate (Stewart and others, 1972b, p. 26-28).

The thickness of the Moenkopi(?) Formation ranges 

from about 50 feet in the Zuni Mountains to about 150 feet 

in the Bluewater Lake area. Outcrops of the Moenkopi(?) 

might not be present at all Triassic-rock localities in this

area, and the eastward extent of the formation is uncertain 

(Stewart and others, 1972b, p. 27). The geologic characteristics (fine-grained lithologies, isolated lenses, and limited 

areal extent) of the Moenkopi and Moenkopi(?) Formationsmake them unimportant as aquifers in the San Juan Basin.

CHINLE FORMATION

Gregory (1917, p. 42-48) defined the Chinle Formation 

for characteristic exposures in Chinle Valley west of Canyon 

de Chelly, Apache County, Ariz. The Chinle crops out along 

the western flank of the Sierra Nacimiento, along Interstate 

40 between Grants and Gallup, N. Mex., and in the western 

part of the Defiance? Uplift along the New Mexico and Arizona State line (pi. 1). The Chinle is present in the subsur-
16 GEOLOGIC FRAMEWORK OF THE SAN JUAN STRUCTURAL BASIN

SOUTHWEST

Window Rock area 

(Arizona)

Rock Point Member

C^/NXVXNXNX

3k Member

Fort Wingate area 

(New Mexico)

Mount Taylor area 

(New Mexico)

NORTHEAST

Gallina area 

(New Mexico)

Entrada Sandstone (Jurassic)

Petrified Forest Member 

(upper part)

Chinle 

Formation

Petrified Forest Member

(upper shale member of Wood and

Northrop, 1946, and Woodward, 1987)

Poleo Sandstone Lentil

Sonsela Sandstone Bed

Petrified Forest Member (lower part)

Shinarump Member

Shinarump 

Member

Permian

rocks 

(undivided)

Agua Zarca

Sandstone Member

(includes informal

sandstone member

of Stewart and

others, 1972a)

NOT TO SCALE

FIGURE 8. Diagrammatic subsurface stratigraphic correlation of units in the Chinle Formation. Modified from Stewart and others (1972a, fig. 8)

and O'Sullivan (1974, fig. 2).

face throughout the San Juan Basin; its equivalent, the 

Dolores Formation (fig. 5), is present in the subsurface and 

crops out in southwestern Colorado (pi. 1). Regionally, the 

Chinle disconformably overlies rocks of Permian age 

(figs. 5, 8). The Chinle typically forms broad valleys (especially along Interstate 40 between Grants and Gallup, 

beneath the red cliffs of the Entrada Sandstone); locally, erosion-resistant sandstone or conglomerate layers cap small 

buttes or form rough ledges, cliffs, or dip slopes.

In general, the Chinle Formation consists of strata 

deposited in various continental (stream channel, flood plain, 

lake, swamp, and dune) environments. The formation contains variegated claystone, shale, siltstone, sandstone, conglomerate, and limestone beds of varying thickness. The 

Chinle has been subdivided into several members. The following descriptions mainly are taken from Wood and 

Northrop (1946), Repenning and others (1969), Stewart and 

others (1972a), O'Sullivan (1977), Woodward (1987), and 

Dubiel (1989a, b).

In the southern, western, and northwestern parts of the 

San Juan Basin, the Chinle Formation includes (in ascending 

order) the Shinarump, Monitor Butte, Petrified Forest (and 

Sonsela Sandstone Bed), Owl Rock, and Rock Point 

Members (fig. 8). The Shinarump Member mostly is lenticular, crossbedded, coarse-grained, conglomeratic sandstone. 

Color varies from yellowish orange, grayish purple, and yellowish gray to brown; locally the sandstone is mottled. 

Minor pale-red to greenish-gray mudstone lenses also are

present. The contact with underlying Permian rocks is a 

deeply scoured erosion surface. In the San Juan Basin, the 

Shinarump Member is present as isolated lenses of local 

extent with a maximum thickness of about 25 feet near Fort 

Wingate (O'Sullivan, 1977, p. 139).

The Monitor Butte Member mainly consists of interbedded red mudstone, gray to red siltstone, sandstone, and 

conglomerate. It locally intertongues with the underlying 

Shinarump and overlying Petrified Forest Members (Repenning and others, 1969, p. 20). The thickness of the Monitor 

Butte decreases from about 300 feet in the Canyon de Chelly 

area of Arizona until it pinches out near Mount Taylor in 

New Mexico (fig. 8) (O'Sullivan, 1974, fig. 2; and 1977, 

p. 141).

The Petrified Forest Member is the most widespread 

member of the Chinle Formation throughout the Colorado 

Plateau and the San Juan Basin (fig. 8) (Repenning and others, 1969, p. 21; O'Sullivan, 1974, fig. 2, p. 172, 173). It 

mainly consists of variegated gray, brown, red, and purple 

mudstone, shale, and siltstone. In the southern and western 

parts of the basin, it is divided into a lower and upper part 

by the Sonsela Sandstone Bed (fig. 8). The lower part thins 

from about 300 feet in the Canyon de Chelly area to about 

100 feet in the eastern part of the Zuni Uplift (O'Sullivan, 

1977, p. 141). The upper part thickens from about 400 feet 

in the Canyon de Chelly area, to about 850 feet near Fort 

Wingate, to about 1,100 feet near Thoreau, N. Mex. (Repenning and others, 1969, p. 23). Several isolated sandstone
GEOLOGIC UNITS 17

lenses as much as 40 feet thick are present in the upper part 

of the Petrified Forest Member (O'Sullivan, 1977, p. 141). 

The Sonsela Sandstone Bed is a gray to yellowish-gray, 

crossbedded, conglomeratic sandstone unit with interbedded 

siltstone and shale. This bed is widespread in the southwestern part of the basin and averages about 50 feet thick; near 

Grants, it is as much as 300 feet thick (O'Sullivan, 1977, 

p. 141). Cooley (1959, p. 71), Strobell (1964, p. 100), and 

O'Sullivan (1974, fig. 2, p. 172-173) suggested that the Sonsela Sandstone Bed and the Poleo Sandstone Lentil of the 

Chinle Formation in the eastern San Juan Basin might be 

correlative units.

The Owl Rock Member of the Chinle Formation mainly 

consists of mottled pink to red, calcareous silty shale interbedded with erosion-resistant, ledge-forming cherty limestone and calcareous siltstone and sandstone. This member 

forms a conspicuous ledge below the Entrada Sandstone 

cliffs north of Interstate 40 between Gallup and Thoreau. In 

the Canyon de Chelly area, the Owl Rock Member is about 

300 feet thick but thins eastward until it disappears near 

Thoreau because of erosion prior to deposition of the 

Entrada Sandstone (fig. 8) (O'Sullivan, 1974, fig. 2).

Recent revisions in the nomenclature of the Chinle Formation and the Jurassic Wingate Sandstone have resulted in 

removing the Rock Point Member from the Wingate and 

placing it in the upper part of the Chinle. Dubiel (1989b, 

p. 213) has shown that the Rock Point Member is stratigraphically equivalent to the Church Rock Member of the 

Chinle Formation in the Monument Valley area of northeastern Arizona and also the upper member of the Dolores Formation in southwestern Colorado. The Rock Point Member 

is present in the Defiance Uplift-Gallup Sag-Zuni Embayment areas and generally consists of alternating beds of horizontally laminated or crossbedded, variegated siltstone, silty 

sandstone, or very fine grained sandstone (Stewart and others, 1972a, p. 44). This part of the Rock Point Member probably was deposited in alternating lacustrine and eolian 

environments (Stewart and others, 1972a, p. 99; Dubiel, 

1989b, p. 219). In the southern part of the depositional area, 

the member locally contains conglomerate and conglomeratic 

sandstone beds of fluvial origin that have informally been 

termed "the beds at Lupton" by O'Sullivan and Green (1973, 

p. 77). The maximum thickness of the Rock Point Member is 

about 500 feet in the northern part of the Defiance Uplift, 

but it thins in all directions from that area (Stewart and others, 1972a, fig. 24; Cooley and Weist, 1979, p. 60).

In the north-central and eastern parts of the San Juan 

Basin, the Chinle Formation consists of (in ascending order) 

the Agua Zarca Sandstone Member, Salitral Shale Tongue, 

Poleo Sandstone Lentil, and Petrified Forest Member (fig. 8) 

(Stewart and others, 1972a; O'Sullivan, 1974, 1977; Woodward, 1987; and Dubiel, 1989a). The Agua Zarca Sandstone 

Member was named by Wood and Northrop (1946) for exposures southeast of the village of Gallina in Rio Arriba

County, N. Mex. This member is a cliff-forming fluvial unit 

that disconformably overlies Permian rocks and has gradational contacts with the overlying Salitral Shale Tongue and 

locally with the overlying Petrified Forest Member (Woodward, 1987, p. 31). The Agua Zarca Sandstone Member typically consists of white to light-gray and red to purple, very 

fine to coarse-grained, crossbedded, locally conglomeratic 

sandstone. Wood and Northrop (1946) and Woodward (1987, 

p. 31) recognized the Agua Zarca Sandstone Member from 

the San Pedro Mountain area southward to the area around 

San Ysidro, N. Mex. Stewart and others (1972a, p. 21-24) 

and O'Sullivan (1974, p. 171), however, did not recognize 

the Agua Zarca Sandstone Member as far south as San 

Ysidro and instead designated an informal member they 

called the "sandstone member." In this report, the terminology of Wood and Northrop (1946) and Woodward (1987, 

p. 31) is used because the two members intertongue, have the 

same general lithology, and occupy the same stratigraphic 

position. The thickness of the Agua Zarca Sandstone Member is variable but generally thins northward from as much 

as 250 feet near San Ysidro until it disappears north of San 

Pedro Mountain, near Gallina, (Woodward and Ruetschilling, 

1976; Woodward, 1987, p. 1).

The Salitral Shale Tongue also was named by Wood and 

Northrop (1946) for exposures between the villages of Gallina and Coyote in Rio Arriba County, N. Mex. The Salitral 

Shale Tongue conformably overlies the Agua Zarca Sandstone Member and disconformably overlies Permian rocks in 

the Mount Taylor area. It is disconformably overlain by the 

Poleo Sandstone Lentil (fig. 8). The Salitral Shale Tongue 

was deposited in a lacustrine environment and consists 

mainly of red, maroon, brown, and greenish-gray shale with 

ubiquitous calcareous nodules. Lenticular, very fine to 

coarse-grained sandstone beds locally also are present. The 

Salitral Shale Tongue is lithologically similar to the Petrified 

Forest Member and cannot be recognized as a separate unit 

where the overlying Poleo Sandstone Lentil is absent (O'Sullivan, 1974, p. 172; Woodward, 1987, p. 1). The thickness of 

the Salitral Shale Tongue is variable but generally decreases 

toward the north and east from a maximum of about 300 feet 

just northeast of La Ventana, N. Mex., to about 110 feet in 

the northern part of the Sierra Nacimiento, to zero near Gallina (O'Sullivan, 1974, p. 171; Woodward, 1987, p. 31).

The term "Poleo" was first used in a geologic sense by 

Huene (1911), who named exposures just southwest of Coyote the "Poleo top sandstone." Wood and Northrop (1946) 

redefined the unit as a lentil in the Chinle Formation. The 

Poleo Sandstone Lentil disconformably overlies the Salitral 

Shale Tongue, and its upper contact is conformable and gradational with the Petrified Forest Member (fig. 8) (Stewart 

and others, 1972a, p. 35; O'Sullivan, 1974, fig. 2). The Poleo 

Sandstone Lentil is present in the northern part of the Sierra 

Nacimiento and in the San Pedro Mountain area, where it 

attains its maximum thickness of about 130 feet; the lentil
18 GEOLOGIC FRAMEWORK OF THE SAN JUAN STRUCTURAL BASIN

pinches out southward near San Miguel Canyon, northeast of 

La Ventana (Woodward, 1987, p. 31). The Poleo Sandstone 

Lentil is a high-energy fluvial deposit mostly consisting of 

ridge- and cliff-forming, yellowish-gray to greenish, very 

fine to coarse-grained, crossbedded sandstone. The sandstone 

locally is conglomeratic with pebbles of chert, quartz, 

quartzite, and limestone.

The Petrified Forest Member in the north-central and 

eastern San Juan Basin is about 1,100 feet thick in the southern part of the Sierra Nacimiento and thins northward to 

about 800 feet north of San Pedro Mountain near Gallina 

(Stewart and others, 1972a, p. 36, fig. 8; Woodward, 1987, 

p. 34). Wood and Northrop (1946) and Woodward (1987, 

p. 33, fig. 10) used the informal terminology "upper shale 

member" for that part of the Chinle above the Poleo Sandstone Lentil. Stewart and others (1972a, fig. 8), however, 

correlated the same part of the Chinle Formation with the 

Petrified Forest Member of the southern and western parts of 

the basin; their terminology is followed in this report.

In the Chinle Formation, water-yielding units mainly 

consist of local, lenticular sandstone bodies such as the Shinarump and Agua Zarca Sandstone Members (Cooley and 

Weist, 1979, p. 61-62; Stone and others, 1983, p. 41; Craigg, 

1992). The Poleo Sandstone Lentil and Sonsela Sandstone 

Bed also have potential as minor aquifers.

DOLORES FORMATION

The Dolores Formation was defined by Cross and others 

(1899) from exposures in the Dolores River valley in 

southwestern Colorado. The term Dolores Formation, as presently used, generally is applied to rocks that are equivalent to 

parts of the Chinle Formation (fig. 5) (Stewart and others, 

1972a, pi. 2). The Dolores was deposited in fluvial environments 

(O'Sullivan, 1977, p. 145) and disconformably overlies the 

Cutler Formation of Permian age. The Dolores is disconformably overlain by the Entrada Sandstone of Middle Jurassic age 

(fig. 5) (Stewart and others, 1972a, fig. 15).

The Dolores Formation is presently defined only in Colorado (pi. 1), and within the study area has been subdivided 

into informal lower, middle, and upper members by Stewart 

and others (1972a, p. 33) and O'Sullivan (1977, p. 143). These 

authors considered the lower and middle members to be equivalent to the Petrified Forest and Owl Rock Members of the 

Chinle Formation, and the upper member equivalent to what 

they considered the Rock Point Member of the Wingate Sandstone. These stratigraphic relations were discussed and shown 

graphically by Stewart and others (1972a, p. 34, 48^9, pi. 2). 

As was discussed in the preceding section, the Rock Point 

Member was subsequently removed from the Wingate Sandstone and placed in the upper part of the Chinle Formation 

(Dubiel, 1989b, p. 214).

The lower member of the Dolores Formation consists of 

ledge-forming, greenish-gray to yellowish-green, very fine to 

fine-grained, crossbedded sandstone with some limestone conglomerate. The thickness of the lower member ranges from 

about 20 feet along the Piedra River west of Pagosa Springs, 

Colo., to about 160 feet in the subsurface southwest of the 

outcrops near Durango, Colo. (Stewart and others, 1972a, p. 34, 

pi. 2).

The middle member of the Dolores Formation is a slopeforming, variegated red-bed unit consisting of siltstone, silty 

sandstone, and very fine grained sandstone. Thin, lenticular 

limestone conglomerate beds also are present. The thickness 

of the middle member ranges from about 130 feet along the 

Piedra River to about 270 feet along the Animas River north 

of Durango (Stewart and others, 1972a, p. 48).

The upper member of the Dolores Formation forms steep 

slopes. It also is a red-bed unit consisting of light-brown to 

reddish-brown siltstone and sandy siltstone containing isolated 

lenses of limestone conglomerate, but there are fewer such 

lenses in this member than in the middle member. The thickness 

of the upper member ranges from about 260 feet along the 

Animas River north of Durango to about 700 feet in the 

subsurface west of the Animas River. The upper member is 

absent in the Piedra River area (Stewart and others, 1972a, 

p. 48, 49).

JURASSIC ROCKS

Jurassic sedimentary rocks in the San Juan Basin consist 

of the Wingate Sandstone, Entrada Sandstone, Wanakah Formation, Cow Springs Sandstone, Morrison Formation, and 

Junction Creek Sandstone. The Wingate is of Early Jurassic 

age, and the Entrada, Wanakah, and Cow Springs are of 

Middle Jurassic age. The majority of the Junction Creek is of 

Late Jurassic age, but the lower part is Middle Jurassic (Condon and Huffman, 1988, p. 10). The Morrison Formation is 

of Late Jurassic age.

Jurassic rocks consist of strata deposited in various nonmarine environments (stream channel, flood plain, lake, 

dune, sabkha). The strata collectively attain a maximum 

thickness of about 1,500 feet in the west-northwestern part of 

the basin. The erosion surface at the base of the Upper Cretaceous Dakota Sandstone progressively truncates the Upper 

Jurassic strata toward the south until just south of the Zuni 

Uplift, where the entire Jurassic sequence has been removed 

and the Dakota disconformably overlies Upper Triassic rocks 

(Green and Pierson, 1977, p. 147). The general lithologies 

and thicknesses of Jurassic rocks are summarized in table 1; 

outcrops of these rocks are shown on plate 1.

WINGATE SANDSTONE

The Wingate Sandstone (of the Glen Canyon Group) 

originally was named by Dutton (1885) for the red cliffforming unit north of Fort Wingate, N. Mex., but the formation has had a confusing history of nomenclature problems. 

Dutton's (1885) definition included all strata between the
GEOLOGIC UNITS 19

present Owl Rock Member of the Upper Triassic Chinle Formation and the Todilto Limestone Member of the Middle 

Jurassic Wanakah Formation. The cliff-forming part of the 

stratotype eventually was correlated with the Middle Jurassic 

Entrada Sandstone of southeastern Utah (Baker and others, 

1947). West of the stratotype, only the lowermost deposits 

between the Owl Rock Member and the base of the Entrada 

were considered to be Wingate (Harshbarger and others, 

1957, p. 8; Green and Pierson, 1977, p. 149). Nomenclature 

problems continue to develop because recent work by Green 

(1974) and Dubiel (1989b) has left the Wingate without a 

stratotype.

As currently defined, the Wingate Sandstone is recognized only along the extreme northwestern margins of the 

San Juan Basin (Green, 1974, p. 1; Green and Pierson, 1977, 

p. 149; Dubiel, 1989b, p. 213). It is found only west of a 

northeast-trending line that approximately connects Fort 

Defiance, Ariz., Shiprock, N. Mex., and Cortez, Colo. 

(O'Sullivan and Green, 1973, fig. 3-D). Where present, the 

Wingate unconformably overlies the Chinle Formation 

(Cooley and Weist, 1979, p. 60) and is unconformably overlain by the Entrada Sandstone (Green and Pierson, 1977, 

p. 150) (fig. 5).

The Wingate Sandstone formerly consisted of a lower 

unit, the Rock Point Member, and an upper unit, the Lukachukai Member (Harshbarger and others, 1957, p. 8). Green 

(1974), however, correlated rocks of the Lukachukai Member 

at the Fort Wingate section with the overlying Entrada Sandstone and renamed them the lyanbito Member of the Entrada 

Sandstone. In the most recent nomenclature revision, Dubiel 

(1989b) has placed the Rock Point Member in the upper part 

of the Chinle Formation. Although the Wingate still exists as 

a sedimentary rock body, the work of Green (1974) has left 

the Wingate without a stratotype; Dubiel (1989b) formally 

abandoned the name Lukachukai Member because it was the 

only remaining member of the Wingate.

The Wingate Sandstone was deposited in an eolian 

environment and consists of homogeneous, light- to reddishbrown to orange, very fine to medium-grained sandstone. 

High-angle, large-scale crossbedding and massive vertical 

cliffs are characteristic features (Harshbarger and others, 

1957, p. 11; O'Sullivan and Green, 1973, p. 76). The limited 

extent of the Wingate in the San Juan Basin makes it unimportant as a regional water-yielding unit.

ENTRADA SANDSTONE

The Entrada Sandstone was first described by Gilluly 

and Reeside (1928, p. 76) for outcrops at Entrada Point in 

the San Rafael Swell of southeastern Utah. In the San Juan 

Basin, the Entrada is the basal formation of the San Rafael 

Group (Entrada Sandstone, Wanakah Formation, and Cow 

Springs Sandstone). The Entrada crops out around the basin

margins and typically forms steep cliffs above the contact 

with the nonresistant Chinle Formation. The distinctive, 

bright-red cliffs along the northern side of Interstate 40 

between Grants and Gallup are the Entrada Sandstone 

capped by the Todilto Limestone Member of the Wanakah 

Formation.

The Entrada Sandstone is present throughout the San 

Juan Basin and adjacent Colorado Plateau (Green and Pierson, 1977, p. 150, 151). Throughout most of the basin, the 

Entrada unconformably overlies the Chinle Formation (and 

equivalent rocks of the Dolores Formation in Colorado) 

except in the northwest, where it unconformably overlies the 

Wingate Sandstone. Rocks presently identified as Entrada 

along the southern margin of the basin originally were called 

Wingate Sandstone by Dutton (1885). Eventually, Baker and 

others (1947) revised the nomenclature and correlated the 

Entrada Sandstone of the stratotype in southeastern Utah 

with Dutton's (1885) Wingate Sandstone.

The Entrada Sandstone generally consists of reddishorange, mottled red and white to light-brown silty sandstone 

and very fine to medium-grained well-sorted quartz sandstone interbedded with thinner reddish-brown siltstone 

(Smith, 1959, p. 74; O'Sullivan and Craig, 1973, p. 81; 

Green and Pierson, 1977, p. 151). High-angle, large-scale 

crossbedding is a characteristic feature, indicating deposition 

in an eolian environment, whereas the siltstone represents 

deposition in interdune and sabkha environments (Green and 

Pierson, 1977, p. 151).

In the southern and western parts of the San Juan Basin, 

the Entrada Sandstone has been divided into one formal and 

two informal members. In ascending order, these are the 

lyanbito Member defined by Green (1974), the middle siltstone member, and the upper sandstone member (Green, 

1974, fig. 2). The lyanbito Member contains silty sandstone 

and crossbedded sandstone and is recognized only in the 

southern part of the basin, whereas the middle siltstone and 

upper sandstone members also are recognized in the western 

part of the basin. In the eastern and northern parts of the 

basin, the Entrada is undifferentiated (Green and Pierson, 

1977, p. 151).

The thickness of the Entrada Sandstone is variable. Green 

and Pierson (1977, p. 151) reported a basinwide range in 

thickness of 60-330 feet. The maximum thickness is in the 

area north of Interstate 40 between Grants and Gallup (O'Sullivan and Craig, 1973, p. 81). Along the eastern basin margin, 

thickness ranges from about 100 feet near San Ysidro, 

N. Mex., to about 300 feet north of Cuba, N. Mex. (Woodward, 1987, p. 34). In the Four Corners area, along McElmo 

Creek north of Sleeping Ute Mountain in southwestern Colorado, the thickness is only 80-100 feet (Irwin, 1966, p. 18). 

Although not a productive aquifer, the Entrada has the potential of yielding small quantities of water to wells (Cooley and 

Weist, 1979, p. 58,59; Lyford, 1979, p. 6; Stone and others, 

1983, p. 41).
20 GEOLOGIC FRAMEWORK OF THE SAN JUAN STRUCTURAL BASIN

WANAKAH FORMATION

Burbank (1930) defined the Wanakah Member of the 

Morrison Formation for exposures in the Wanakah mine near 

Ouray, Colo., north of Durango. Eckel (1949, p. 28-29) 

raised the Wanakah to formation rank and applied the name 

to strata between the Entrada and Junction Creek Sandstones. 

Strata equivalent to the Wanakah in northwestern New Mexico and northeastern Arizona, however, have had a problematic nomenclature history; in that region, the name Wanakah 

replaces the Summerville Formation and the upper part of 

the Bluff Sandstone (Condon and Huffman, 1988, p. 3; Condon, 1989a, p. 231). The Wanakah has limited potential as an 

aquifer (Cooley and Weist, 1979, p. 57, 58; Stone and others, 

1983, p. 40); Thomas (1989) considered the formation to be 

a confining unit.

In New Mexico, the Wanakah Formation contains (in 

ascending order) the Todilto Limestone, Beclabito, and 

Horse Mesa Members (fig. 5) (Condon and Huffman, 1988, 

p. 3). In southwestern Colorado, the Wanakah consists of the 

basal part of the Pony Express Limestone (equivalent to the 

Todilto Limestone Member in New Mexico) and Bilk Creek 

Sandstone Members; a third member of informal usage, the 

marl member, also is present (Eckel, 1949, p. 28, 29). 

Molenaar (1989) recognized only the Pony Express Limestone Member and an informal upper member in Colorado 

(fig. 5).

The Todilto Limestone Member in New Mexico and 

equivalent Pony Express Limestone Member in Colorado are 

arbitrarily separated by the State line (Condon and Huffman, 

1988, p. 5). These members contain two major lithofacies, a 

lower gray limestone unit and an upper gypsum and anhydrite 

unit (Green and Pierson, 1977, p. 151) and some siltstone and 

sandstone. The limestone facies is present throughout the 

study area except in the Gallup Sag and in the extreme northwest (Condon and Huffman, 1988, fig. A5). It conformably 

overlies the Entrada Sandstone and typically forms an erosion-resistant caprock. The maximum thickness of the limestone facies is about 40 feet (Green and Pierson, 1977, p. 151; 

Condon and Huffman, 1988, p. 5). The massive light-gray to 

white gypsum and anhydrite facies is present only in the eastern one-half of the study area (Condon and Huffman, 1988, 

fig. A5). Its thickness ranges from zero to a maximum of 

about 100 feet (Green and Pierson, 1977, p. 151). The Todilto 

Limestone and Pony Express Limestone Members were 

deposited either in a fresh to saline inland lake or in a 

restricted marine basin (Condon and Huffman, 1988, p. 5).

The Beclabito Member of the Wanakah Formation 

(Summerville Formation of former usage) is present throughout northeastern Arizona and northwestern New Mexico. 

This member was deposited either in a marginal lacustrine or 

marginal marine (sabkha) environment, depending on one's 

interpretation of the underlying Todilto Limestone and Pony 

Express Limestone Members (Green and Pierson, 1977,

p. 151; Condon and Huffman, 1988, p. 7). The Beclabito 

Member conformably and gradationally overlies the Todilto 

Limestone and Pony Express Limestone Members and, in the 

western and southwestern part of the San Juan Basin, intertongues with the Cow Springs Sandstone (Condon and Peterson, 1986, fig. 4a; Condon and Huffman, 1988, fig. A3). The 

Beclabito Member consists of interbedded white to reddishorange and reddish-brown claystone, massive or planar-bedded to crossbedded siltstone, and silty, very fine to finegrained sandstone (Green and Pierson, 1977, p. 151; Condon 

and Huffman, 1988, p. 6). The basal part of the member 

grades into a coarse-grained fluvial conglomerate south and 

southeast of Grants. The Beclabito Member probably correlates with the Bilk Creek Sandstone Member and the marl 

member of the Wanakah Formation of southwestern Colorado (Condon and Huffman, 1988, p. 6-7). The thickness of 

the Beclabito Member ranges from about 125 to 200 feet 

(Condon and Huffman, 1988, p. 6).

The Horse Mesa Member of the Wanakah Formation 

(lower part of Bluff Sandstone of former usage) is present 

throughout northeastern Arizona and northwestern New 

Mexico. It represents deposition in eolian dune, interdune, 

localized fluvial, and sabkha environments (Condon and 

Huffman, 1988, p. 7). The Horse Mesa Member conformably 

overlies the Beclabito Member. Toward the east, it grades 

into the Beclabito Member, and toward the southwest it 

grades into the Cow Springs Sandstone (Condon and Peterson, 1986, fig. 4a; Condon and Huffman, 1988, p. 7, fig. A3; 

Condon, 1989b, fig. 2). The Horse Mesa Member mainly 

consists of cliff-forming, white to orange and red, planarbedded and crossbedded, very fine to coarse-grained quartzose sandstone (Condon and Huffman, 1988, p. 7; Condon, 

1989a, p. 235). The thickness of the Horse Mesa Member is 

about 40 feet at its stratotype. It thins to the north, grades 

laterally into the Cow Springs Sandstone to the south, has 

been removed by Cenozoic erosion to the west, and grades 

into the Baclabito Member in the subsurface to the east 

(Condon and Huffman, 1988, p. 7). Condon and Huffman 

(1988, p. 7) reported that the lower part of the Junction 

Creek Sandstone in southwestern Colorado probably correlates with the Horse Mesa Member.

COW SPRINGS SANDSTONE

The term Cow Springs Sandstone first appeared on a 

geologic section constructed by J.W Harshbarger (in McKee, 

1949, p. 46, fig. 6). The formation was named by Harshbarger and others (1951, p. 97) for exposures on the north 

face of Black Mesa near Cow Springs Trading Post, 

Coconino County, Ariz.

The Cow Springs Sandstone is present in the southwestern part of the San Juan Basin (Condon, 1989b, fig. 2), 

where its outcrops form prominent smooth and rounded 

cliffs. The Cow Springs intertongues with the Beclabito
GEOLOGIC UNITS 21

Member of the Wanakah Formation throughout much of its 

extent and thins somewhat and intertongues with the Horse 

Mesa Member of the Wanakah Formation to the east and 

northeast (Condon and Huffman, 1988, p. 7; Condon, 1989b, 

fig. 2), forming a broad, vertical intergradational zone with 

the Wanakah Formation (Condon and Huffman, 1988, p. 7). 

The Cow Springs is conformably and gradationally overlain 

by the Recapture Member of the Morrison Formation (fig. 5) 

(Condon and Huffman, 1988, p. 7, figs. A2, A3). The upper 

part of the Cow Springs recently has been placed in the 

Recapture Member of the Morrison Formation and is now 

called the eolian facies of the Recapture Member (Condon, 

1989a, p. 236). Cooley and Weist (1979, p. 56) reported that 

the Cow Springs yields some water to wells where it is 

hydraulically connected with the Morrison Formation in the 

southwestern part of the San Juan Basin. The Cow Springs 

is not present southwest of a lobate area approximately connecting the villages of Thoreau and Crystal, N. Mex. (Condon, 1989b, fig. 1).

The Cow Springs Sandstone represents deposition in 

eolian and interdune environments (Green and Pierson, 1977, 

p. 151). It mainly consists of crossbedded to flat-bedded, 

greenish-gray, light-yellowish-gray, and light-brown, wellsorted, fine- to medium-grained quartzose and arkosic sandstone; color banding is a characteristic feature (Harshbarger 

and others, 1951, p. 97; Green and Pierson, 1977, p. 151; 

Stone, 1979).

The thickness of the Cow Springs Sandstone at the 

stratotype is 342 feet (Harshbarger and others, 1951, p. 97). 

In the San Juan Basin, the maximum thickness is about 

300 feet at Lupton, Ariz. The thickness decreases irregularly 

northward to about 200 feet south of Crystal and eastward to 

about 90 feet near Thoreau (Condon, 1989b, fig. 2).

JUNCTION CREEK SANDSTONE

The Junction Creek Sandstone has a history of nomenclature changes. The stratigraphic interval containing this 

formation was first identified by Cross and Purington (1899) 

as the La Plata Sandstone. Goldman and Spencer (1941) 

abandoned the term La Plata Sandstone and renamed its 

informal subdivisions as new lithostratigraphic units. Unit 5 

was renamed the Junction Creek Sandstone Member of the 

Morrison Formation, represented by outcrops a few miles 

north of Durango (Goldman and Spencer, 1941, 

p. 1750, 1751). Eckel (1949, p. 1) abandoned the member 

status of the Junction Creek and interpreted it to be a separate formation. Craig and Cadigan (1958, p. 182) stated that 

the Junction Creek Sandstone of southwestern Colorado 

directly correlates with the Bluff Sandstone of southeastern 

Utah. Irwin (1966, p. 20) also correlated the Junction Creek 

with the Bluff Sandstone and attributed confusion with terminology to problems at the Colorado-Utah State line. O'Sullivan (1980) reassigned the Bluff Sandstone of southeastern 

Utah and northeastern Arizona to member status in the Morrison Formation, and Condon and Huffman (1988, p. 10) 

stated that the Bluff Sandstone Member and Junction Creek 

Sandstone are correlative.

The Junction Creek Sandstone has been mapped only in 

southwestern Colorado, in the northern part of the San Juan 

Basin (Haynes and others, 1972; Steven and others, 1974). 

The formation, however, reportedly continues southward in 

the subsurface for several townships into New Mexico 

(Steven M. Condon, U.S. Geological Survey, oral commun., 

1989). Stone and others (1983, sheet 4) also showed the 

Junction Creek as extending several miles into New Mexico 

in the subsurface. The Junction Creek yields small amounts 

of water to wells, mainly in southwestern Colorado, and is 

hydraulically connected with the Morrison Formation 

(Cooley and Weist, 1979, p. 57).

The Junction Creek Sandstone typically forms prominent smooth and rounded cliffs; locally, it weathers to hoodoo forms (Ekren and Houser, 1958, p. 74). The Junction 

Creek conformably or gradationally overlies the Wanakah 

Formation (fig. 5) (Eckel, 1949, p. 29) and is conformably or 

gradationally overlain by the Morrison Formation.

The Junction Creek Sandstone was deposited in an 

eolian environment (Haynes and others, 1972). The major 

part of the formation consists of crossbedded, white to buff, 

light-brownish-gray, pink, or reddish-orange, poorly sorted, 

fine- to coarse-grained quartzose sandstone (Irwin, 1966, 

p. 20, 21; Haynes and others, 1972). Thinner lenticular, 

horizontally bedded, arkosic sandstone beds and partings of 

gray or red shale also are present (Eckel, 1949, p. 29).

The thickness of the Junction Creek Sandstone is variable, ranging from 200 to 500 feet in southwestern Colorado 

(Goldman and Spencer, 1941, p. 1750; Eckel, 1949, p. 29). 

Irwin (1966, p. 21, 22) reported a range of 230-300 feet on 

the Ute Mountain Ute Indian Reservation and the thinning of 

the unit southward into New Mexico. Haynes and others 

(1972) stated that the Junction Creek in southwestern Colorado thins eastward and that toward the north, it merges with 

the upper part of the underlying Summerville Formation 

(now Wanakah Formation; fig. 5).

MORRISON FORMATION

The Morrison Formation was named by G.H. Eldridge 

(in Emmons and others, 1894) for exposures near the town 

of Morrison, Jefferson County, Colo., about 10 miles southwest of Denver. Major sandstones in the Morrison typically 

form erosion-resistant cliffs and dip slopes, whereas shales 

form topographic saddles and slopes.

The Morrison Formation is present throughout the San 

Juan Basin (pi. 1) (Green and Pierson, 1977, p. 151) and 

crops out around the basin margins. It conformably overlies
22 GEOLOGIC FRAMEWORK OF THE SAN JUAN STRUCTURAL BASIN

the Wanakah Formation and Cow Springs Sandstone (Condon and Peterson, 1986, p. 24). In the northern part of the 

basin, the Morrison conformably overlies and probably intertongues with the Junction Creek Sandstone (fig. 5). The 

Morrison is disconformably overlain by the Dakota Sandstone throughout most of the San Juan Basin except the 

northern part, where it is conformably overlain by and 

locally intertongues with the Burro Canyon Formation 

(fig. 5) (Green and Pierson, 1977, p. 151).

The Morrison Formation generally consists of yellowishtan to pink, fine- to coarse-grained, locally conglomeratic 

sandstone that is interbedded with sandy siltstone and green to 

reddish-brown shale and claystone; minor limestone beds also 

are present (Woodward and Schumacher, 1973, p. 3-5; Green 

and Pierson, 1977, p. 151; Stone and others, 1983, p. 38). 

Morrison strata were deposited in various continental environments including stream channels, flood plains, and lakes 

(Green and Pierson, 1977, p. 151).

In the San Juan Basin, the Morrison Formation consists 

of five members (Gregory, 1938; Craig and others, 1955; 

Cadigan, 1967; Green and Pierson, 1977; Owen, 1984). In 

ascending order, they are the Salt Wash Member, Recapture 

Member, Westwater Canyon Member, Brushy Basin Member, and Jackpile Sandstone Member.

The most prolific regional aquifers in the San Juan 

Basin are in the Morrison Formation, particularly in the 

Westwater Canyon Member. Although coarser grained units 

within each member of the Morrison are aquifers, shaly and 

clayey zones within the members act as confining units. Several authors have reported on the hydrogeologic characteristics of the Morrison (Kelly, 1977; Cooley and Weist, 1979, 

p. 54, 55; Lyford, 1979, p. 8; Stone and others, 1983, 

p. 38, 39; Dam and others, 1990a). Kelly (1977) detailed the 

hydrogeology of the Westwater Canyon Member in the 

southern part of the basin. Dam and others (1990a) discussed 

the hydrogeology of the Morrison throughout the basin. The 

Westwater Canyon Member also is the major uranium-bearing formation in the basin.

The Salt Wash Member of the Morrison Formation is 

present only in the northern and northwestern parts of the 

San Juan Basin (Craig and others, 1955, fig. 21; Condon and 

Peterson, 1986, p. 21). It forms ledgy slopes and mainly consists of white to grayish-yellow and pale-orange, crossbedded, fine- to medium-grained and locally conglomeratic 

fluvial sandstone. The sandstone may be present either as 

individual lenses that are as much as 20 feet thick or as 

stacked beds as much as 80 feet thick. Interbedded with the 

sandstone are reddish-brown, grayish-red, and greenish-gray 

silty claystones that locally contain platy or slabby gray 

limestone beds (Craig and others, 1955, p. 135, 136). The 

Salt Wash Member mainly was deposited in a fluvial environment, but it also contains minor eolian sandstone and 

lacustrine limestone beds (Condon and Peterson, 1986, 

p. 21). The maximum thickness of the Salt Wash Member in

the San Juan Basin is about 200 feet near the Four Corners. 

Southeastward, 30-50 miles into New Mexico, the Salt Wash 

Member intertongues with and pinches out in the overlying 

Recapture Member (Craig and others, 1955, p. 137, fig. 21).

The Recapture Member of the Morrison Formation is 

present throughout much of the San Juan Basin (Craig and 

others, 1955, fig. 22) as the lower part of the Morrison Formation. Recent revisions in nomenclature and in the definition of the Recapture have extended this member southward 

from its original limit (Condon and Peterson, 1986, p. 21, 22; 

Condon, 1989a, p. 236, 237). The Recapture Member 

recently has been subdivided into two major facies: the 

lower eolian facies and the upper fluvial facies (Condon and 

Peterson, 1986, p. 21). The eolian facies forms cliffs or 

steeper slopes than does the fluvial facies, which forms 

varicolored slopes. Generally, the eolian facies is dominant 

in the southern and southwestern parts of the basin and the 

fluvial facies is dominant in the northwest (Condon and 

Peterson, 1986, p. 22). The eolian facies has previously been 

called the Cow Springs Sandstone, Zuni Sandstone, or upper 

part of the Bluff Sandstone (Condon and Peterson, 1986, 

p. 21). It is characterized by large-scale crossbedding consisting of light-brown, yellowish-gray, or pale-red, fine- to 

medium-grained sandstone; locally, claystone and siltstone 

beds are present. The thickness of the eolian facies ranges 

from 75 to 300 feet (Condon and Peterson, 1986, p. 21). 

Condon (1989a, p. 237) considered the eolian facies of the 

Recapture to be time equivalent to the Bluff Sandstone 

Member of the Morrison Formation in southeastern Utah and 

the Junction Creek Sandstone in southwestern Colorado. 

South of the San Juan Basin, the Recapture Member is 

disconformably truncated by the Dakota Sandstone erosion 

surface (Craig and others, 1955, fig. 22; Condon, 1989a, 

p. 237).

The fluvial facies of the Recapture Member constitutes 

the classic description of the member in previous geologic 

literature (prior to the report by Condon and Peterson, 1986). 

It mainly consists of pale-red to grayish-red, silty and sandy 

claystone interbedded with some light-brown to pinkish-gray, 

fine- to medium-grained sandstone (Craig and others, 1955, 

p. 140); gray limestone beds also are present locally (Woodward and Schumacher, 1973, p. 3). The thickness of the fluvial facies in the San Juan Basin varies from 40 to 370 feet 

on the east side, to about 200 feet on the south side, to 

between 200 and 500 feet on the west side; the facies 

pinches out south of Grants (Condon and Peterson, 1986, 

p. 21). In the Cuba area, it cannot be differentiated from the 

Beclabito Member of the Wanakah Formation, which 

prompted Woodward and Schumacher (1973, p. 3) to call it 

the lower member of the Morrison Formation.

The Westwater Canyon Member of the Morrison Formation typically forms cliffs or ledgy slopes. This member 

was deposited by a braided-stream complex and consists of 

yellowish-gray to tan, pink or light-brown, fine- to coarse-
GEOLOGIC UNITS 23

grained locally conglomeratic sandstone interbedded with 

shale or claystone (Craig and others, 1955, p. 153; Woodward and Schumacher, 1973, p. 3). Grain size increases 

toward the west-central part of the basin until the 

member consists wholly of conglomeratic sandstone (Craig 

and others, 1955, p. 154). The thickness of the Westwater 

Canyon Member increases from about 100 feet on the north, 

east, and south sides of the San Juan Basin to about 300 feet 

in the west-central part of the basin (Craig and others, 1955, 

p. 154).

The Brushy Basin Member of the Morrison Formation 

is present throughout the San Juan Basin (Craig and others, 

1955, fig. 29). This member consists of variegated (green, 

reddish-brown, and grayish-purple) calcareous and bentonitic 

claystone and mudstone of lacustrine origin that form slopes 

in the outcrop. Fluvial deposits of light-brown to yellowishgray, fine- to medium-grained sandstone, and locally thin 

beds of limestone deposited in freshwater lakes also are 

present (Craig and others, 1955, p. 156; Condon and Peterson, 1986, p. 22). The thickness of the Brushy Basin Member ranges from about 150 feet along the northern and 

eastern basin margins to about 250 feet in the Central Basin. 

In the Defiance Uplift area, the thickness ranges from zero to 

about 100 feet. Along the southern basin margin, the Brushy 

Basin Member is disconformably overlain by the Dakota 

Sandstone (Craig and others, 1955, p. 156, fig. 29).

The Jackpile Sandstone Member of the Morrison Formation (Owen, 1984, p. 45) is found mostly in the southeastern part of the San Juan Basin and crops out discontinuously 

between the villages of Laguna and Cuba, N. Mex. The 

Jackpile Sandstone Member is present in the subsurface as 

far north as the Chaco Culture National Historical Park area 

(Owen, 1984, fig. 9). It forms cliffs or ledgy slopes and gradationally or disconformably overlies the Brushy Basin 

Member and is in turn disconformably overlain by the 

Dakota Sandstone (fig. 5). The Jackpile Sandstone Member 

is lithologically similar to and occupies the same stratigraphic horizon as the Burro Canyon Formation to the north 

(Aubrey, 1988, p. 62). The Jackpile Sandstone Member was 

deposited by a braided-stream system and mainly consists of 

white to yellowish-tan, crossbedded, medium- to coarsegrained, locally conglomeratic arkosic sandstone; lenses of 

variegated (red to pale-green) bentonitic mudstone also are 

present locally (Owen, 1984, p. 46-51). The average thickness in the vicinity of the stratotype (the Jackpile-Paguate 

uranium mine north of Laguna) is about 100 feet with a 

maximum of about 300 feet. South of the stratotype, the 

thickness decreases to zero because of disconformable truncation by the Dakota Sandstone (Owen, 1984, p. 50).

The thickness of the Morrison Formation in the San 

Juan Basin ranges from about 200 feet near Grants to about 

1,100 feet in the northwestern part of the basin. The greatest 

thickness is in a north-northwest-trending zone from southeast of Crownpoint, N. Mex., to Cortez, Colo. (fig. 9).

The depth to the top of the Morrison Formation ranges 

from zero in areas of outcrop to about 8,500 feet below land 

surface in the northeastern part of the study area (fig. 10). 

The rapid increase in depth in the area northeast of Grants is 

due to the local topography of Mount Taylor.

The altitude and configuration of the top of the Morrison Formation is shown in figure 11. Because the top of the 

Morrison is a key marker horizon throughout the San Juan 

structural basin, figure 11 also illustrates the overall structural configuration of the basin. The top of the Morrison 

decreases from a maximum altitude of about 10,000 feet 

above sea level on the outcrops along the northern basin 

margin near Durango to about 1,500 feet below sea level in 

the subsurface beneath Navajo Reservoir in the northeastern 

part of the study area.

CRETACEOUS ROCKS

Sedimentary rocks of Cretaceous age in the San Juan 

structural basin (in ascending order) consist of the Burro 

Canyon Formation, Dakota Sandstone, Mancos Shale, Gallup 

Sandstone, Crevasse Canyon Formation, Point Lookout 

Sandstone, Menefee Formation, Cliff House Sandstone, 

Lewis Shale, Pictured Cliffs Sandstone, Fruitland Formation, 

and Kirtland Shale. All are of Late Cretaceous age with the 

exception of the Burro Canyon Formation, which is Early 

Cretaceous.

Cretaceous rocks mainly consist of transgressive and 

regressive marine shore-zone sediments that were deposited 

in shallow seas that encroached into the San Juan Basin area 

from the northeast, and also nonmarine clastic deposits from 

source areas southwest of the basin. Several transgressions 

and regressions of the shallow seas occurred during Late 

Cretaceous time, and the area has been referred to by Fassett 

(1974, p. 225) as the SCI-SWO (sea came in-sea went out) 

zone. Molenaar (1977b, p. 159, 160) presented a concise 

summary of the depositional history of the San Juan Basin 

during the Late Cretaceous, an epoch in which at least 

6,500 feet of sediments were deposited. The general lithologies and thicknesses of Cretaceous rocks are summarized in 

table 1; outcrops of these rocks are shown on plate 1.

BURRO CANYON FORMATION

The Burro Canyon Formation was named by Stokes and 

Phoenix (1948) for exposures in Burro Canyon, just east of 

the Dolores River in San Miguel County, Colo. The Burro 

Canyon mainly is present in southwestern Colorado, but 

lenses of the formation also are present in northwestern New 

Mexico, northeastern Arizona, and southeastern Utah (Ekren 

and Houser, 1959, p. 200). The Burro Canyon typically is 

mapped with the Dakota Sandstone; however, locally it can 

be mapped separately (Haynes and others, 1972; Steven and
24 GEOLOGIC FRAMEWORK OF THE SAN JUAN STRUCTURAL BASIN

107 00'

37 OO1 -

36 OO1

35 00'

0 10 20 30 40 50 60 KILOMETERS 34 45'

EXPLANATION

Outcrops of Morrison Formation From Dane and Bachman 

(1965). Wilson and others (1969), Tweto (1979), and Hintze 

(1981)

Outcrops of Zuni Sandstone (of former usage) From Dane 

and Bachman (1965). Now included in Morrison Formation 

(Condon. 1989a, b)

Outcrops of Morrison and Wanakah Formations and Entrada 

Sandstone, undivided From Tweto (1979)

Outcrops of Morrison Formation and San Rafael Group, 

undivided From Wilson and others (1969)

Boundary of study area

106 ; 30'

__ woo __ Line of equal approximate thickness of Morrison Formation 

Interval 100 feet

FIGURE 9. Approximate thickness of the Morrison Formation in the San Juan Basin study area. Modified from

Dam and others (1990a).
GEOLOGIC UNITS 25

37 00

36'OO1

35~00'

0 10 20 30 40 50 60 KILOMETERS 34~45'

EXPLANATION

Outcrops of Morrison Formation From Dane and Bachman 

(1965), Wilson and others (1969). Tweto (1979), and Hintze 

(1981)

Outcrops of Zuni Sandstone (of former usage) From Dane 

and Bachman (1965). Now included in Morrison Formation 

(Condon, 1989a. b)

Outcrops of Morrison and Wanakah Formations and Entrada 

Sandstone, undivided From Tweto (1979)

Outcrops of Morrison Formation and San Rafael Group, 

undivided From Wilson and others (19b9)

Boundary of study area

__ 2QQQ _ Line of equal depth to top of Morrison Formation Interval 

500 feet. Datum is land surface

106~30'

0 10 20 30 40 50 60 MILES

FIGURE lO.-Approximate depth to the top of the Morrison Formation in the San Juan Basin study area. Modified

from Dam and others (1990a).
26 GEOLOGIC FRAMEWORK OF THE SAN JUAN STRUCTURAL BASIN

37 00

0 10 20 30 40 50 60 MILES

I I I I I

36 OO 1

0 10 20 30 40 50 60 KILOMETERS

EXPLANATION

Outcrops of Morrison Formation From Dane and Bachman 

(1965). Wilson and others (1969). Tweto (1979). and Hintze 

(1981)

Outcrops of Zuni Sandstone (of former usage) From Dane 

and Bachman (1965). Now included in Morrison Formation 

(Condon. 1989a. b)

Outcrops of Morrison and Wanakah Formations and Entrada 

Sandstone, undivided From Tweto (1979)

Outcrops of Morrison Formation and San Rafael Group, 

undivided From Wilson and others (1969)

Boundary of study area

__2000 __ Structure contour Shows altitude of top of Morrison Formation. 

Contour interval 500 feet. Datum is sea level

FIGURE 11. Approximate altitude and configuration of the top of the Morrison Formation in the San Juan Basin 

study area. Modified from Dam and others (1990a).
GEOLOGIC UNITS 27

others, 1974). Irwin (1966, p. 26) reported that the Burro 

Canyon either is absent or cannot be recognized south of the 

San Juan River in New Mexico. The Burro Canyon is 

present in the Chama Embayment, which is adjacent to the 

northeastern margin of the San Juan Basin (fig. 2), but 

whether it is present in the northeastern part of the San Juan 

Basin has been the subject of some debate (Saucier, 1974, 

p. 212-215; Owen and Siemers, 1977, p. 179, 180; Anderholm, 1979, p. 44; Woodward, 1987, p. 35; Aubrey, 1988, 

fig. C2). A conglomeratic sandstone does persist at the base 

of the Dakota Sandstone in this area, but whether it belongs 

in the Dakota or the Burro Canyon is still open to discussion.

Throughout its extent, the Burro Canyon Formation 

conformably overlies the Brushy Basin Member of the 

Morrison Formation (fig. 5) and probably represents a 

continuation of deposition from Late Jurassic through Early 

Cretaceous time. Ekren and Houser (1959, p. 192, 193) 

reported that locally in the Four Corners area, Burro Canyon 

sandstone lenses intertongue with mudstone of the Brushy 

Basin Member. The Burro Canyon is disconformably overlain by the Dakota Sandstone (fig. 5).

Strata of the Burro Canyon Formation were deposited in 

various fluvial environments such as streams and flood 

plains (Ekren and Houser, 1965, p. 18). Ridgley (1977, 

p. 157, 158) reported that the lower part of the Burro Canyon 

was deposited by a series of high-energy braided to meandering streams, whereas the upper part of the formation was 

deposited by low-energy, meandering streams.

Lithologically, the Burro Canyon Formation consists of 

lenticular, white, light-gray, and pale-brown to tan, fine- to 

coarse-grained sandstone and conglomeratic channel 

sandstone. These sandstone lenses are quartzose and kaolinitic, with pebbles of varicolored chert. The lenses commonly are crossbedded and vertically stacked, forming rough 

ledges and cliffs. Interbedded with the sandstone are greenish-gray and reddish-brown shale and mudstone. Southward, 

the sandstone becomes thinner and discontinuous and mudstone beds become predominant (Ekren and Houser, 1959, 

p. 194, 195; and 1965, p. 18, 19; Irwin, 1966, p. 26-28).

The thickness of the Burro Canyon Formation is variable. In the Sleeping Ute Mountain area, it ranges from 30 to 

200 feet (Ekren and Houser, 1965, p. 18), averaging about 

150 feet in the general northwestern part of the San Juan 

Basin (Haynes and others, 1972). Southward, toward the San 

Juan River, the Burro Canyon thins to an irregular pinch-out 

edge, after which only a few discontinuous lenses of conglomeratic sandstone are found; the unit also thins toward 

the northeast to a series of discontinuous mudstone and sandstone lenses (Haynes and others, 1972). The Burro Canyon 

yields small quantities of water to wells and springs (Cooley 

and Weist, 1979, p. 53).

DAKOTA SANDSTONE

The Dakota Sandstone generally is thought to be of earliest Late Cretaceous age, although the lowermost part may 

be of latest Early Cretaceous age (Fassett, 1974, p. 225). The 

formation crops out around the basin margins (pi. 1), where 

it typically caps mesas and forms erosion-resistant dip slopes 

and hogbacks.

The Dakota Sandstone in the San Juan Basin and vicinity 

was deposited on a regional erosion surface; the strata represent a transition from continental alluvial-plain deposition in 

the lower part of the formation to marine shore-zone deposition in the upper part. Owen (1973, p. 39-51) presented a 

comprehensive depositional model for the formation in the 

San Juan Basin. The Dakota disconformably overlies the 

Brushy Basin Member of the Morrison Formation throughout 

much of the basin. However, it also disconformably overlies 

the Westwater Canyon Member of the Morrison in the southwest, the Jackpile Sandstone Member of the Morrison in the 

southeast, as well as the Burro Canyon Formation in the north 

(fig. 5). The upper contact of the Dakota Sandstone is conformable with the Mancos Shale, and intertonguing of these 

two formations is common near the contact.

The stratigraphy of the Dakota Sandstone is complex. 

The formation consists of a main sandstone body in the north 

that branches into members and tongues in the rest of the 

San Juan Basin. The Dakota consists of five formal subdivisions (Landis and others, 1973; Owen, 1973; Aubrey, 1988, 

p. 59); in ascending order, these are the Encinal Canyon 

Member, Oak Canyon Member, Cubero Tongue, Paguate 

Tongue, and Twowells Tongue. The two upper members 

intertongue with the Graneros Member of the Mancos Shale.

The Dakota Sandstone contains three principal lithologies: a sequence of buff to brown, crossbedded, poorly 

sorted, coarse-grained conglomeratic sandstone and moderately sorted, medium-grained sandstone in the lower part; 

dark-gray carbonaceous shale with brown siltstone and lenticular sandstone beds in the middle; and yellowish-tan, finegrained sandstone interbedded with gray shale in the upper 

part (Owen, 1973, p. 39-48; Merrick, 1980, p. 45-47).

The thickness of the Dakota Sandstone has an overall 

range of a few tens of feet to about 500 feet; Stone and others (1983, p. 37) reported that 200-300 feet probably is the 

most common range. Data reported by Molenaar (1977b, 

p. 160, 161) and Stone and others (1983, fig. 66), along with 

data obtained from Petroleum Information Corporation, indicate that the thickness of the Dakota generally increases 

from the northern, northwestern, and western margins of the 

basin toward the southern, southeastern, and eastern margins.

The depth to the top of the Dakota Sandstone ranges 

from outcrops around the basin margins to about 8,500 feet 

in the northeastern part of the study area (fig. 12). The rapid 

increase in depth in the area northeast of Grants reflects the 

local topography of Mount Taylor.
28 GEOLOGIC FRAMEWORK OF THE SAN JUAN STRUCTURAL BASIN

106 30'

3600'

35"00'

0 10 20 30 40 50 60 KILOMETERS 34 45'

EXPLANATION

Outcrops of Dakota Sandstone From Dane and 

Bachman (1965) and Wilson and others (1969)

Outcrops of Burro Canyon Formation and Dakota 

Sandstone, undivided From Tweto (1979) and 

Hintze (1981)

Boundary of study area

 2000 Line of equal depth to top of Dakota Sandstone 

Interval 500 feet. Datum is land surface

FIGURE 12.-Approximate depth to the top of the Dakota Sandstone in the San Juan Basin study area Modified from

Craigg and others (1989).
GEOLOGIC UNITS 29

The altitude and configuration of the top of the Dakota 

Sandstone are shown in figure 13. The top of the Dakota 

decreases from a maximum altitude of about 9,500 feet 

above sea level on the outcrops along the northern basin 

margin near Vallecito Reservoir in Colorado, to about 

1,500 feet below sea level in the subsurface beneath Navajo 

Reservoir in the northeastern part of the study area. Because 

the Dakota is a key marker zone in the San Juan Basin, the 

overall structural configuration of the basin also is reflected 

in figure 13.

The Dakota Sandstone forms a dependable aquifer 

throughout much of the San Juan Basin, but well yields generally are small. Locally, in the southern part of the basin, 

the Dakota is hydraulically connected with the underlying 

Morrison Formation (Cooley and Weist, 1979, p. 52, 53; 

Stone and others, 1983, p. 37, 38; Craigg and others, 1989).

MANCOS SHALE

The Mancos Shale was named by Cross and Purington 

(1899) for characteristic exposures in the Mancos River valley near Mesa Verde National Park, Colo. Topography 

formed by the Mancos typically is rolling to flat, rough and 

broken, and has a badlands appearance. Localized, erosionresistant siltstone, sandstone, or limestone caps isolated 

buttes or hillocks, whereas softer shale forms slopes and 

broad valleys or flats. The Mancos commonly forms steep 

slopes below mesas or buttes capped by Upper Cretaceous 

regressive marine sandstones (Gallup Sandstone, Dalton 

Sandstone Member of the Crevasse Canyon Formation, and 

Point Lookout Sandstone; fig. 5).

Basinwide, the Mancos Shale conformably overlies and 

intertongues with the transgressive marine Dakota Sandstone 

(fig. 5) and is conformably overlain by and intertongues with 

the regressive marine Point Lookout Sandstone. In the southwestern part of the basin, the Mancos intertongues with the 

regressive marine Gallup Sandstone. In the extreme southern 

part of the basin, the Mancos locally intertongues with the 

Dalton Sandstone Member of the Crevasse Canyon Formation, a local regressive marine sandstone, and the Hosta 

Tongue of the Point Lookout Sandstone, a local transgressive 

marine sandstone.

The Mancos Shale is present throughout the San Juan 

Basin and comprises most of the Upper Cretaceous marine 

deposits (pi. 1). It was deposited in relatively shallow offshore marine environments, possibly no more than 400 feet 

deep (Molenaar, 1977b, p. 161). In general, the Mancos consists of gray to black shale and claystone with discontinuous 

yellowish-gray, calcareous siltstone and sandstone (O'Sullivan and Beikman, 1963; Haynes and others, 1972; Steven 

and others, 1974; Hackman and Olson, 1977). Thin bentonite 

beds are common in the lower part of the formation (Wyant 

and Olson, 1978). The Mancos also contains zones of calcareous concretions, some thin limestone beds, and offshore 

sandstone bar deposits (Molenaar, 1977b, p. 161). Cooley 

and Weist (1979, p. 50) reported that the Mancos contains 

more sandstone in the southern and western parts of the 

basin, and more limestone and calcareous shale in the northern and northeastern parts of the basin. Regionally, the Mancos is a confining unit, but locally, sandstone bodies within 

the formation yield small quantities of water to wells 

(Craigg, 1980, p. 57, 58).

The Mancos Shale consists of a main body in the north 

that attains a maximum thickness of about 2,300 feet 

(Molenaar, 1977a, 1989) from which several members or 

tongues extend to the southwest. These are the Graneros, 

Bridge Creek (formerly Greenhorn) Limestone, Semilla 

Sandstone, Juana Lopez, and El Vado Sandstone Members; 

the Tocito Sandstone Lentil; the Pescado Tongue (a localized 

tongue in the southwestern part of the basin), and the 

Mulatto and Satan Tongues. Fassett (1974) and Molenaar 

(1977b, 1983) presented concise summaries of the geologic 

characteristics of these rocks.

GALLUP SANDSTONE

The Gallup Sandstone has a smaller areal extent than 

the other major Upper Cretaceous sandstones in the San Juan 

Basin and is present only in New Mexico and a small part of 

Arizona. The Gallup crops out in an arcuate pattern around 

the margins of the southwestern part of the basin (pi. 1) 

where the formation typically forms erosion-resistant cliffs 

and dip slopes.

As originally defined by Sears (1925) and discussed in 

detail by Dane and others (1957) and Molenaar (1973, 1974, 

1983), the Gallup Sandstone actually consists of several rock 

types including conglomerate, sandstone (the predominant 

lithology), shale, carbonaceous shale, and coal. The interval 

delineated by these rocks represents the first major regression of the Late Cretaceous sea in the San Juan Basin area. 

The strata indicate deposition in various marine and nonmarine environments including shore zone, transitional offshore, 

coal swamp, fluvial and distributary channel, and eolian dune 

(Molenaar, 1973, 1974, 1983; Campbell, 1979).

The Gallup Sandstone is restricted to the southwestern 

one-half of the San Juan Basin, partly because of stratigraphic pinch out and partly because of truncation 

(figs. 5 and 14) (Molenaar, 1973, p. 85). From its outcrops, 

the Gallup dips toward the center of the basin and terminates 

in a northwest-trending truncation line that extends from the 

southeastern part of the basin, through the central part, to the 

area northwest of Shiprock (fig. 15). The Gallup is not 

present beyond this truncation line, which is a pre-Niobrara 

erosion surface (Penttila, 1964; Molenaar, 1973, 1974, 1983). 

Molenaar (1973, p. 85) reported that as much as 300 feet of
30 GEOLOGIC FRAMEWORK OF THE SAN JUAN STRUCTURAL BASIN

107 OO1

37 00

36 OO1

106 301

I I I I T 

0 10 20 30 40 50 60 KILOMETERS 34 45

EXPLANATION

I" I Outcrops of Dakota Sandstone From Dane and 

L ___I Bachman (1965) and Wilson and others (1969)

I I Outcrops of Burro Canyon Formation and Dakota 

I____| Sandstone, undivided From Tweto (1979) and 

Hintze (1981)

Boundary of study area

-2000 Structure contour Shows altitude of top of Dakota 

Sandstone. Contour interval 500 feet. Datum is sea 

level

FIGURE 13. Approximate altitude and configuration of the top of the Dakota Sandstone in the San Juan Basin study

area. Modified from Craigg and others (1989).
GEOLOGIC UNITS 31

Upper Cretaceous strata has been removed northeast of this 

truncation line.

Isolated marine sandstone lenses are present in the 

transgressive Mulatto Tongue of the Mancos Shale northeast 

of the truncation line. These lenses have been referred to by 

informal names such as basal Niobrara sandstone, transgressive Gallup sandstone, Tocito sandstone, and Stray sandstone; they are now formally named Tocito Sandstone Lentils 

of the Mancos Shale (Molenaar, 1983, p. 38). Many of these 

lenticular sandstones are found at stratigraphic horizons that 

suggest the presence of the Gallup Sandstone (Molenaar, 

1973, 1974, 1983; Kernodle and others, 1989), and some 

authors have suggested that the Gallup is present throughout 

the San Juan Basin (for example, see Miller and others, 

1990). The Tocito Sandstone Lentils, however, are not connected with or depositionally related to the Gallup Sandstone 

(Molenaar, 1973, 1974).

As noted by several authors (Beaumont, 1957; Budd, 

1957; Dane and others, 1957; Molenaar, 1973, 1974, 1983; 

and Campbell, 1979), the stratigraphy of the Gallup Sandstone is complex. The stratigraphic relations are complicated 

by numerous facies changes within the formation, intertonguing with adjacent rocks, and thinning and erosional truncation toward the basin center. Nonuniform and inconsistent

SOUTHWEST

Zuni uplift area Crownpoint area

geologic names also have complicated the stratigraphic interpretations of the Gallup. In detailed stratigraphic studies, 

Molenaar (1973) identified seven regional sandstone bodies 

in the interval referred to as the Gallup Sandstone. He proposed that the uppermost of these (a fluvial sandstone) be 

called the Torrivio Sandstone Member; the remaining six 

sandstone bodies were informally referred to using letter designations A through F. Mizell and Stone (1979) published 

maps showing the areal extent and thickness of these sandstone bodies. Molenaar (1983), in a classic stratigraphic 

paper, established the principal reference section for the Gallup, revised the stratigraphic nomenclature of this interval, 

and formally named the uppermost nonmarine sandstone 

body the Torrivio Member. The complex stratigraphic relations within the Gallup Sandstone and its relation to adjacent 

rocks are shown diagrammatically in figure 14.

Throughout its extent, the Gallup Sandstone conformably overlies and intertongues with the lower part of the 

Mancos Shale (fig. 14). Along its outcrop belt and throughout 

the southwestern part of the basin, the Gallup is conformably 

overlain and intertongues with the Dilco Coal Member of the 

Crevasse Canyon Formation. Regressive deposits of the Gallup grade seaward (northeast) into offshore deposits of the 

lower part of the Mancos and grade landward (southwest) into

NORTHEAST

Farmington area

Gibson Coal 

Member

Upper part of Mancos Shale

Crevasse

Canyon

Formation

Tocito Sandstone Lentil

Borrego Pass 

Dilco Coal Member Lentil

Northeastern extent 

of Gallup Sandstone

Pre-Niobrara 

unconformity

Lower part of Mancos Shale

FIGURE 14. Diagrammatic subsurface stratigraphic correlation of the Gallup Sandstone and adjacent units in New Mexico. Modified from

Molenaar (1973, 1974, and 1983).
32 GEOLOGIC FRAMEWORK OF THE SAN JUAN STRUCTURAL BASIN

nonmarine deposits of the Crevasse Canyon. Beginning a few 

miles northeast of the town of Gallup, the basal transgressive 

marine Mulatto Tongue of the Mancos Shale unconformably 

overlies both the Gallup Sandstone and the nonmarine deposits of the Dilco Coal Member (figs. 5 and 14) (Molenaar, 

1973, 1974, 1983).

Stone (1981, p. 6) reported that sandstone bodies in the 

Gallup Sandstone generally are pink to light gray, fine to 

medium grained, and are moderately to well sorted. The 

thickness of the Gallup is variable and decreases from about 

300 feet near the outcrops along the southwestern margin of 

the basin until it is cut off to the northeast by the northwesttrending truncation line (Molenaar, 1974, fig. 2). The thickness also generally decreases southeastward from the Nutria 

Monocline to the Arroyo Chico-Rio Puerco area (Craigg, 

1980, p. 59; Stone and others, 1983, fig. 58). The thickness 

of individual sandstone bodies (as defined by Molenaar, 

1973, 1974, and 1983) ranges from a pinch-out edge to a 

maximum of about 150 feet (Stone, 1981, p. 7).

The depth to the top of the Gallup Sandstone ranges 

from outcrops to about 4,500 feet below the land surface in 

the truncated area about 20 miles south of Farmington, 

N. Mex. (fig. 15). The rapid increase in depth in the area 

northeast of Grants reflects the local topography of Mount 

Taylor.

The altitude and configuration of the top of the Gallup 

Sandstone are shown in figure 16. The altitude decreases 

from a maximum of about 7,500 feet above sea level along 

the outcrops in the western and southern parts of the basin, 

to about 1,500 feet above sea level in the subsurface near 

where the formation is truncated southwest of Farmington.

The Gallup Sandstone is a productive aquifer in the 

study area and is the major source of water for the town of 

Gallup (Cooley and Weist, 1979, p. 51; Stone and others, 

1983, p. 36, 37; Kernodle and others, 1989). In the southeastern part of the basin, the U.S. Bureau of Land Management has constructed almost 200 miles of pipeline for a 

livestock-water distribution system that distributes water 

from a single, flowing artesian well completed in the Gallup 

Sandstone (Craigg, 1980, p. 62).

CREVASSE CANYON FORMATION

Alien and Balk (1954, p. 91-93) defined the Crevasse 

Canyon Formation for predominantly nonmarine sedimentary 

rocks between the top of the Gallup Sandstone and the bottom of the Hosta Tongue of the Point Lookout Sandstone. 

The stratotype is along the southern end of the Chuska 

Mountains in McKinley County, N. Mex.

The Crevasse Canyon Formation is present only in New 

Mexico and a small part of Arizona, in the south-southwestern part of the San Juan Basin (pi. 1). From its outcrops, the 

Crevasse Canyon dips northeast toward a northwest-trending

truncation zone (similar to but not as far north as the truncation line of the Gallup Sandstone) that extends from the 

southeastern part of the basin, through the Chaco Culture 

National Historical Park area, to the area near Crystal in the 

Chuska Mountains. The Crevasse Canyon is not present 

northeast of this truncation zone; the upper part of the formation wedges against the Mancos Shale, and the lower part is 

truncated by a pre-Niobrara erosion surface (Molenaar, 1974, 

p. 252, figs. 2 and 3).

The Crevasse Canyon Formation conformably to disconformably overlies the Gallup Sandstone and is disconformably overlain by the Hosta Tongue of the Point Lookout 

Sandstone. In the Gallup Sag area, the Menefee Formation 

gradationally overlies the Crevasse Canyon (Molenaar, 

1977b, p. 163, fig. 1). As defined by Alien and Balk (1954, 

p. 91-93), the Crevasse Canyon consists of (in ascending 

order) the Dilco Coal, Dalton Sandstone, and Gibson Coal 

Members. Beaumont and others (1956, p. 2153) recognized a 

fourth member, the Bartlett Barren Member. This member, 

however, is only of local significance and merges with the 

Gibson Coal (Molenaar, 1977b, p. 163). Correa (1970) 

named a fifth member as the Borrego Pass Sandstone Lentil, 

which is the same unit as the informal Stray sandstone of 

Sears and others (1941).

The Dilco Coal Member (fig. 5) consists of interbedded 

gray shale and claystone, carbonaceous shale, coal, yellowish-gray to pale-orange siltstone, and lenticular channel sandstone (Hackman and Olson, 1977; Stone, 1979). It forms 

ledges and slopes, and outcrop areas tend to have a badlands 

appearance. The Dilco Coal Member was deposited in continental environments (stream channels, flood plains, and 

swamps). The thickness ranges from zero along a wedgeedge in the subsurface between Crystal and the Chaco 

Culture National Historical Park in the southeastern basin to 

a maximum of about 300 feet in the Gallup area (Molenaar, 

1974, figs. 2-4; Hackman and Olson, 1977).

The Dalton Sandstone Member (fig. 5) is a regressive 

marine shore-zone deposit (Molenaar, 1977b, p. 163). It 

mainly consists of cliff-forming, grayish-orange to grayishyellow, very fine to medium-grained quartzose sandstone 

with some coarse-grained or conglomeratic sandstone (Hackman and Olson, 1977; Molenaar, 1977b, p. 163). The maximum thickness of the Dalton Sandstone Member is about 

150 feet. It extends eastward to the Mesa Chivato area, 

where it maintains a uniform thickness of about 100 feet 

(Sears and others, 1941, p. 113). The Dalton Sandstone 

Member merges laterally into nonmarine, undivided Crevasse Canyon Formation deposits along a broad zone from 

about 5 miles northeast of Gallup to about 7 miles northeast 

of Window Rock, Ariz. (Hackman and Olson, 1977). The 

member pinches out into the Mancos Shale and merges with 

the Hosta Tongue of the Point Lookout Sandstone in the subsurface about halfway between Crownpoint and the Chaco
GEOLOGIC UNITS 33

106°30'

37^0

i i i r

0 10 20 30 40 50 60 KILOMETERS 34 45-

EXPLANATION

I I Outcrops of Gallup Sandstone In Arizona, contains Pescado Tongue 

I____I of the Mancos Shale. From Dane and Bachman (1965) and Hackman 

and Olson (1977)

I I Outcrops of Mesaverde group, undivided Contains Gallup Sandstone 

I____I From Dane and Bachman (1965)

 Approximate subsurface extent of Gallup Sandstone From Molenaar (1973)

 Boundary of study area

 2000 Line of equal depth to top of Gallup Sandstone Interval 500 feet. Datum is 

land surface

FIGURE 15. Approximate depth to the top of the Gallup Sandstone in the San Juan Basin study area. Modified from

Kernodle and others (1989).
34 GEOLOGIC FRAMEWORK OF THE SAN JUAN STRUCTURAL BASIN

107 00'

36'DO

35 00'

"i r

0 10 20 30 40 50 60 KILOMETERS 34 45'

EXPLANATION

i Outcrops of Gallup Sandstone In Arizona, contains Pescado Tongue of 

L ___| the Mancos Shale. From Dane and Bachman (1965) and Hackman and 

Olson (1977)

I I Outcrops of Mesaverde group, undivided Contains Gallup Sandstone. 

I____I From Dane and Bachman (1965)

 Approximate subsurface extent of Gallup Sandstone From Molenaar (1973)

 Boundary of study area

 2000 Structure contour Shows altitude of top of Gallup Sandstone. Contour interval 

500 feet. Datum is sea level

106 30'

0 10 20 30 40 50 60 MILES

I I I I I I

FIGURE 16. Approximate altitude and configuration of the top of the Gallup Sandstone in the San Juan Basin study

area. Modified from Kernodle and others (1989).
GEOLOGIC UNITS 35

Culture National Historical Park area (Kirk and others, 

1988).

The Gibson Coal Member is the uppermost member of 

the Crevasse Canyon Formation. South of the pinch out of 

the Hosta Tongue of the Point Lookout Sandstone, the Gibson Coal Member is gradationally overlain by the Cleary 

Coal Member of the Menefee Formation (fig. 5) (Molenaar, 

1977b, p. 163). The Gibson Coal Member consists of the 

same types of continental deposits as those described for the 

Dilco Coal Member. The thickness of the Gibson Coal Member ranges from zero along a wedge-edge in the subsurface 

where the Dalton Sandstone Member and Hosta Tongue 

merge, to about 550 feet (Alien and Balk, 1954, p. 93).

Although the Dalton Sandstone Member probably is the 

most important aquifer in the Crevasse Canyon Formation, 

lenticular channel sandstone in the other members also yields 

water to wells locally. The lower part of the formation is 

hydraulically connected with the underlying Gallup Sandstone (Cooley and Weist, 1979, p. 50; Lyford, 1979, p. 12).

The Borrego Pass Lentil (Correa, 1970; Robertson, 

1990), which replaces the informal Stray sandstone of Sears 

and others (1941), is situated between the Mulatto Tongue of 

the Mancos Shale and the Dilco Coal Member of the Crevasse Canyon Formation (fig. 5). Molenaar (1974, p. 256) 

stated that the Borrego Pass Lentil actually is a transgressive 

onlap deposit (a Tocito Sandstone Lentil of the Gallup Sandstone) associated with the pre-Niobrara unconformity. The 

lentil extends as a tongue-shaped wedge northwestward from 

the Mount Taylor area to the Crownpoint area (Sears and 

others, 1941, pi. 26). The lentil consists of cliff-forming, 

gray, fine- to coarse-grained, locally conglomeratic, crossbedded sandstone (Hackman and Olson, 1977). The thickness increases from zero along a wedge-edge near 

Crownpoint, to about 75 feet north of Thoreau, to about 

100 feet near Grants (Sears and others, 1941, pi. 26). The 

small areal extent of this lentil makes it regionally insignificant as an aquifer, although locally it probably is capable of 

yielding small quantities of water to wells.

POINT LOOKOUT SANDSTONE

The Point Lookout Sandstone crops out beyond the 

margins of the Central Basin (pi. 1). The outcrops typically 

form cliffs, cap mesas and buttes, or form erosion-resistant 

dip slopes and hogbacks (as along the base of the Hogback 

Monocline).

The Point Lookout Sandstone, named by Collier (1919) 

for exposures at Mesa Verde National Park in southwestern 

Colorado, is the lowermost formation of the Mesaverde 

Group, which consists of the Point Lookout Sandstone, 

Menefee Formation, and Cliff House Sandstone in the San 

Juan Basin. The Point Lookout is the most extensive regressive marine beach sandstone in the basin (Molenaar, 1977b,

p. 164). It conformably overlies the Mancos Shale throughout the basin (fig. 5); the contact is characterized by a distinct offshore marine transition zone consisting of 

interbedded thin sandstone, siltstone, and shale (Shetiwy, 

1978; Craigg, 1980; Wright, 1984;. The Menefee Formation 

conformably or disconformably overlies the Point Lookout 

Sandstone, with some local intertonguing at the contact 

(Tabet and Frost, 1979).

In the southern part of the San Juan Basin, the Point 

Lookout Sandstone is separated into two parts by the Satan 

Tongue of the Mancos Shale (fig. 5). The upper unit is the 

main body of the Point Lookout. The lower unit is the Hosta 

Tongue, which crops out between Crownpoint and the 

Arroyo Chico-Rio Puerco area in New Mexico (pi. 1). The 

Hosta Tongue is a transgressive marine beach sandstone. The 

main body of the Point Lookout and the Hosta Tongue 

merge along the southern margin of the basin, where they are 

about 250 feet thick (Sears and others, 1941; Beaumont and 

others, 1956, p. 2154). The Hosta Tongue is of limited areal 

extent, pinching out about 30 miles northeast of its outcrop 

(Beaumont, 1971, p. 22; Craigg, 1980). The tongue attains a 

maximum thickness of about 160 feet (Beaumont and others, 

1956, p. 2155).

The Point Lookout Sandstone generally consists of 

light-gray, thick- to very thick bedded, very fine to mediumgrained, locally crossbedded sandstone (Shetiwy, 1978; 

Craigg, 1980; Wright, 1984). Thin interbeds of dark marine 

shale also are present, especially in the lower part of the formation. The Point Lookout yields small quantities of water 

to wells, especially in the Mesa Chivato area of the southeastern San Juan Basin (Cooley and Weist, 1979, p. 49; 

Craigg, 1980, p. 48-52; Craigg and others, 1990).

The thickness of the Point Lookout Sandstone is variable. Beaumont (1971, p. 22) reported the formation as varying irregularly from about 100 feet in the southern part of 

the basin to about 350 feet near the Colorado-New Mexico 

State line. Molenaar (1977a) reported a maximum thickness 

of 300 feet. Stone and others (1983, p. 34) reported a thickness range from 40 to 415 feet in New Mexico.

The depth to the top of the Point Lookout Sandstone 

ranges from outcrops to between 6,000 and 6,500 feet in the 

northeastern part of the study area (fig. 17). The increase in 

depth in the area northeast of Grants reflects the local topography of Mount Taylor; the increase in depth in the area 

northeast of Window Rock reflects the synclinal structure of 

the Gallup Sag.

The altitude and configuration of the top of the Point 

Lookout Sandstone are shown in figure 18. The top of the 

Point Lookout decreases from a maximum altitude of about 

8,000 feet above sea level on the outcrops along the northcentral and southeastern basin margins, to about 500 feet 

above sea level in the subsurface in the northeastern part of 

the study area.
36 GEOLOGIC FRAMEWORK OF THE SAN JUAN STRUCTURAL BASIN

107=00'

106 301

35 00'

0 10 20 30 40 50 60 KILOMETERS

EXPLANATION

Outcrops of Point Lookout Sandstone From Dane and 

Bachman (1965)

Outcrops of Hosta Tongue of Point Lookout Sandstone 

From Dane and Bachman (1965)

Outcrops of Menefee Formation and Point Lookout 

Sandstone From Tweto (1979)

Outcrops of Mesaverde Group, undivided Contains 

Point Lookout Sandstone. From Dane and Bachman (1965) 

and Tweto (1979)

Boundary of study area

 2000 Line of equal depth to top of Point Lookout Sandstone

Interval 500 feet. Datum is land surface

FIGURE 17. Approximate depth to the top of the Point Lookout Sandstone in the San Juan basin study area. Modified from Craigg and others (1990).
GEOLOGIC UNITS 37

108C 00' 

37 45'

301 107 00'

109'30' 109'00' 

37 "3

-I \'H37 0

3600

I 1 I I I 

0 10 20 30 40 50 60 KILOMETERS 34'45'

106=30'

EXPLANATION

Outcrops of Point Lookout Sandstone From Dane and 

Bachman (1965)

Outcrops of Hosta Tongue of Point Lookout Sandstone 

From Dane and Bachman (1965)

Outcrops of Menefee Formation and Point Lookout 

Sandstone From Tweto (1979)

Outcrops of Mesaverde Group, undivided Contains 

Point Lookout Sandstone. From Dane and Bachman (1965) 

and Tweto (1979)

Boundary of study area

 2000 Structure contour Shows altitude of top of Point Lookout 

Sandstone. Contour interval 500 feet. Datum is sea level

FIGURE 18. Approximate altitude and configuration of the top of the Point Lookout Sandstone in the San Juan 

Basin study area. Modified from Craigg and others (1990).
GEOI .OGIC FRAMEWORK OF THE SAN JUAN STRUCTURAL BASIN

MENEFEE FORMATION

The Menefee Formation (fig. 5) crops out beyond the 

margins of the Central Basin (pi. 1). Erosion-resistant sandstone in the Menefee commonly caps isolated buttes and hillocks, whereas less resistant shale units form slopes and 

broad valleys or flats. The topography on Menefee outcrops 

typically is rolling to rough, broken and steep, and generally 

has a badlands appearance. The upper part of the Menefee 

commonly forms steep slopes below mesas or buttes capped 

by the erosion-resistant Cliff House Sandstone.

The Menefee Formation, named by Collier (1919) for 

exposures near Mesa Verde National Park in southwestern 

Colorado, is the middle formation of the Mesaverde Group 

in the San Juan Basin. The Menefee conformably or disconformably overlies the Point Lookout Sandstone and is conformably or disconformably overlain by the Cliff House 

Sandstone; local intertonguing occurs at both contacts (Tabet 

and Frost, 1979; Craigg, 1980; Stone and others, 1983). 

Some authors have reported the Menefee to be conformably 

overlain by the Lewis Shale in the southeastern part of the 

basin (Dane, 1936; Beaumont and others, 1956). South of the 

pinch out of the Point Lookout in the vicinity of Gallup, the 

Menefee conformably and gradationally overlies the 

Crevasse Canyon Formation.

The Menefee Formation generally consists of interbedded and repetitive sequences of variously thick sandstone, 

siltstone, shale, claystone, carbonaceous shale, and coal beds 

(Collier, 1919; Sears and others, 1936; Mannhard, 1976; 

Tabet and Frost, 1979; Craigg, 1980). Typically, the (1976) 

stone is lenticular, light brown to gray, thick to very thick 

bedded, and fine to medium grained, with a clay matrix and 

various types of cement (calcium carbonate, iron oxide, 

silica). The siltstone commonly is tabular, gray, and thin to 

thick bedded; the shale and claystone typically are lightbrownish gray and thick to very thick bedded (Mannhard, 

1976; Tabet and Frost, 1979; Craigg, 1980). Although the 

Menefee acts as a regional confining unit, the lenticular 

channel sandstone beds yield dependable quantities of water 

to stock wells throughout the formation's extent (Cooley and 

Weist, 1979, p. 48; Lyford, 1979, p. 12; Craigg, 1980, 

p. 39, 40; Stone and others, 1983, p. 34; Levings and others, 

1990a).

The Menefee Formation usually is mapped as an undivided formation; however, in the southern part of the basin, 

two formal members and one informal member can easily be 

differentiated (Tabet and Frost, 1979; Craigg, 1980). In 

ascending order, these are the Cleary Coal Member, consisting of abundant carbonaceous shale and coal beds, gray shale 

and claystone, siltstone, and scattered channel sandstone; the 

Allison Member, mainly a thick sequence of stacked channel 

sandstone with some siltstone, gray shale, and claystone; and 

the upper coal-bearing member, which is similar to the 

Cleary Coal Member and includes the Hogback Mountain 

tongue of Shomaker and Whyte (1977).

The thickness of the Menefee Formation increases from 

north to south. It ranges from zero where the unit pinches 

out between the Point Lookout and Cliff House Sandstones 

in Colorado, to about 2,000 feet in the Chaco Culture p. 164; 

National Historical Park-Torreon areas (Molenaar, 1977b, 

Tabet and Frost, 1979). The depth to the top of the Menefee 

ranges from outcrops to between 6,000 and 6,500 feet in the 

northeastern part of the study area (fig. 19). The altitude and 

configuration of the top of the Menefee are shown in figure 

20. The top surface decreases from a maximum altitude of 

about 8,000 feet above sea level on the outcrops along the 

north-central basin margin near Durango to about 1,000 feet 

above sea level in the subsurface beneath Navajo Reservoir 

in the northeastern part of the study area.

CLIFF HOUSE SANDSTONE

Cliff House Sandstone outcrops form the margins of the 

Central Basin (pi. 1). Typically, the Cliff House caps mesas 

(as in the Chaco Culture National Historical Park area and 

southwest of Cuba) and forms erosion-resistant dip slopes 

and hogbacks (as on the Hogback Monocline). The Cliff 

House Sandstone, named by Collier (1919) for exposures at 

Mesa Verde National Park in southwestern Colorado, is the 

uppermost formation of the Mesaverde Group in the San 

Juan Basin.

The Cliff House Sandstone is conformably overlain by 

and intertongues with the Lewis Shale and in turn conformably and unconformably overlies the Menefee Formation, with 

which it locally intertongues (Molenaar, 1977b, p. 164; 

Craigg, 1980, p. 7). In some areas where Cliff House tongues 

pinch out, the Lewis may directly overlie the Menefee (Stone 

and others, 1983, p. 33). In the western part of the basin near 

the confluence of Coyote Wash and the Chaco River, the Cliff 

House merges with the Pictured Cliffs Sandstone, pinching out 

the Lewis (fig. 5). Cliff House strata consist of several sandstone tongues that represent marine shore-zone deposits of an 

overall transgressing shallow sea. Molenaar (1977b, p. 164) 

noted that these sandstone tongues actually are offlap, or 

regressive, deposits formed during stillstands and minor 

regressions of the shoreline.

The stratigraphy of the Cliff House Sandstone is complex. Continuing nomenclature controversies, differing geologic interpretations, and informal oil-field terminology tend 

to further complicate stratigraphic correlations. The geometry 

of the Cliff House is much different from that of the Point 

Lookout and Pictured Cliffs Sandstones (Fassett, 1977, 

p. 193). The stratigraphic sequence of the Cliff House consists of several sandstone tongues of varying thicknesses and 

areal extents. Some of these sandstone tongues have been 

formally named, whereas informal nomenclature is applied to 

others.

Fassett (1977), who gave an excellent, consise summary 

of the Cliff House Sandstone, reported that the sequence
GEOLOGIC UNITS 39

107 OO 1

37 0

36'00

35r OO'

106'30'

0 10 20 30 40 50 60 MILES

\ \ I I 

0 10 20 30 40 50 60 KILOMETERS 34 45

EXPLANATION

Outcrops of Menefee Formation From Dane and Bachman (1965)

Outcrops of Menefee Formation and Point Lookout Sandstone 

From Tweto (1979)

Outcrops of Mesaverde Group, undivided Contains Menefee 

Formation. From Dane and Bachman (1965) and Tweto (1979)

Boundary of study area

 5000 Line of equal depth to top of Menefee Formation Interval 500 feet. 

Datum is land surface

FIGURE 19. Approximate depth to the top of the Menefee Formation in the San Juan Basin study area. Modified

from Levings and others (1990a).
40 GEOLOGIC FRAMEWORK OF THE SAN JUAN STRUCTURAL BASIN

0 10 20 30 40 50 60 MILES

36°00' -

35-001

0 10 20 30 40 50 60 KILOMETERS 34"> 451

EXPLANATION

Outcrops of Menefee Formation From Dane and Bachman (1965)

Outcrops of Menefee Formation and Point Lookout Sandstone 

From Tweto (1979)

Outcrops of Mesaverde Group, undivided Contains Menefee 

Formation. From Dane and Bachman (1965) and Tweto (1979)

Boundary of study area

__5000 _ Structure contour Shows altitude of top of Menefee Formation. 

Contour interval 500 feet. Datum is sea level

FIGURE 20.-Approximate altitude and configuration of the top of the Menefee Formation in the San Juan Basin 

study area. Modified from Levings and others (1990a).
GEOLOGIC UNITS 41

consists of a few sandstone lenses at the base (the "basal" 

Cliff House Sandstone), the thick La Ventana Tongue in the 

middle, and a major, unnamed tongue at the top. The stratigraphic relations of these tongues with one another and with 

adjacent formations are shown diagrammatically in figure 5.

The basal Cliff House Sandstone tongues are common 

in the deeper part of the Central Basin but are of limited 

thickness (aggregate thickness of about 300 feet); these basal 

tongues also are of limited areal extent and pinch out to the 

northeast (Molenaar, 1977b, p. 164; Stone and others, 1983, 

sheets 2-4). Some of these tongues actually have been 

named, but the names are not commonly used. For example, 

the Barker Dome Tongue was named by Wanek (1954), and 

the Cholla Canyon Tongue and Beechatuda Tongue were 

named by Hayes and Zapp (1955)

South of Cuba, in the southeastern part of the basin, the 

thick sandstone sequence named La Ventana Tongue of the 

Cliff House Sandstone crops out (pi. 1). According to 

authors most familiar with the regional stratigraphy of the 

San Juan Basin (Fassett, 1977; Molenaar, 1977b, p. 164), 

La Ventana Tongue can be traced from its outcrops in the 

southeast, through the subsurface across the basin, to outcrops on the Hogback Monocline a few miles west of Farmington. Other authors (Mannhard, 1976; Fuchs-Parker, 1977) 

have reported that La Ventana Tongue is a more localized 

sandstone sequence that is present only in the southeastern 

part of the basin and represents deposition in a deltaic environment rather than in a marine shore-zone environment. 

Mannhard (1976) and Tabet and Frost (1979) showed the 

tongue pinching out 15-20 miles west of the village of La 

Ventana.

La Ventana Tongue unconformably to conformably 

overlies the Menefee Formation, and intertonguing is common; the Lewis Shale conformably overlies La Ventana 

Tongue, and intertonguing also is common. The maximum 

thickness of La Ventana Tongue according to Molenaar 

(1977b, p. 164) is about 800 feet. Mannhard (1976, p. 39) 

and Fuchs-Parker (1977, p. 199), however, reported a maximum thickness of about 1,000 feet in outcrops along State 

Highway 44 south of Cuba.

The upper, unnamed tongue of the Cliff House Sandstone represents the stratotype at Mesa Verde National Park 

and also forms the prominent outcrop belt that bounds the 

southern part of the Central Basin in the vicinity of the 

Chaco Culture National Historical Park. This tongue formerly was called the Chacra Sandstone Member of the 

Mesaverde Formation (Fassett, 1977, p. 196). To avoid confusion with informal oil-field terminology, Fassett (1977, 

p. 197) has suggested the term Tsaya Canyon Tongue for this 

interval. The thickness of the tongue is about 400 feet at the 

stratotype (Collier, 1919, p. 297). Molenaar (1977b, p. 164) 

reported a range in thickness of 150-300 feet, whereas Stone 

and others (1983, p. 33) reported that the thickness of this 

tongue throughout most of its extent in New Mexico ranges

from zero to about 250 feet. The upper, unnamed tongue 

intertongues with both the underlying Menefee Formation 

and the overlying Lewis Shale.

The Cliff House Sandstone generally consists of tan, 

light-brown, or yellowish-brown, thick to very thick bedded 

and locally crossbedded sandstone with calcite or silica 

cement and clay matrix. Grain size ranges from very fine to 

fine and the sandstone is well to very well sorted (Stone and 

others, 1983, p. 28, 33). Interbeds of gray shale and silty 

shale are common (O'Sullivan and Beikman, 1963; Haynes 

and others, 1972; Craigg, 1980). Cooley and Weist (1979, 

p. 47) reported that the Cliff House generally is a dependable, small-yielding aquifer. Thorn and others (1990a) 

reported on the hydrogeology of the formation.

The depth to the top of the Cliff House Sandstone 

ranges from outcrops to about 6,000 feet in the northeastern 

part of the study area (fig. 21). The altitude and configuration 

of the top of the Cliff House is shown in figure 22. The top 

of the formation decreases from a maximum altitude of 

about 8,500 feet above sea level on the outcrops along the 

northeastern and northern rims of the Central Basin, to about 

1,000 feet above sea level in the subsurface beneath Navajo 

Reservoir, near the structural center of the basin.

LEWIS SHALE

The Lewis Shale crops out inside the margins of the 

Central Basin (pi. 1). The topography formed on the 

formation outcrops is rolling to flat, rough and broken, and 

generally has a badlands appearance. Localized erosion-resistant siltstone and sandstone cap isolated buttes and hillocks, 

whereas less resistant shale forms slopes and broad valleys 

or flats. The upper part of the Lewis commonly forms steep 

slopes below mesas or buttes capped by the Pictured Cliffs 

Sandstone.

The Lewis Shale was named by Cross and others (1899) 

for exposures in the La Plata River valley, about 15 miles 

southwest of Durango. The Lewis conformably overlies and 

intertongues with the Cliff House Sandstone (fig. 5). This 

intertonguing is especially evident in the southeastern part of 

the basin, where several fingers of La Ventana Tongue of the 

Cliff House Sandstone are present in the lower part of the 

Lewis (Mannhard, 1976, p. 76; Molenaar, 1977b, p. 164). 

Locally, the Lewis conformably overlies the Menefee Formation (Shomaker, 197 Ib, p. 94; Stone and others, 1983, p. 33) 

and is in turn conformably overlain by the Pictured Cliffs 

Sandstone; the latter contact is characterized by a distinct 

offshore marine transition zone consisting of interbedded 

thin shale, siltstone, and sandstone (Reeside, 1924, p. 19; 

Fassett and Hinds, 1971, p. 8).

The Lewis Shale was deposited in an offshore marine 

environment. The strata represent the final major transgression of the Cretaceous sea in the San Juan Basin and, thus,
42 GEOLOGIC FRAMEWORK OF THE SAN JUAN STRUCTURAL BASIN

37'OCT

35°00'

36=00

I i I I I

10 20 30 40 50 60 KILOMETERS 34 45'

EXPLANATION

Outcrops of Cliff House Sandstone From Dane and Bachman (1965) and 

Tweto (1979)

Outcrops of La Ventana Tongue of Cliff House Sandstone From Dane 

and Bachman (1965)

Outcrops of Mesaverde Group, undivided Contains Cliff House Sandstone. 

From Dane and Bachman (1965) and Tweto (1979)

Boundary of study area

 5000 Line of equal depth to top of Cliff House Sandstone Interval 500 feet. 

Datum is land surface

FIGURE 21 . Approximate depth to the top of the Cliff House Sandstone in the San Juan Basin study area. Modified

from Thorn and others (1990a).
GEOLOGIC UNITS 43

107=00'

37"0

35°00'

0 10 20 30 40 50 60 MILES

I I I I I I

\ I I I 

0 10 20 30 40 50 60 KILOMETERS

EXPLANATION

Outcrops of Cliff House Sandstone From Dane and Bachman (1965) and 

Tweto (1979)

Outcrops of La Ventana Tongue of Cliff House Sandstone From Dane 

and Bachman (1965)

Outcrops of Mesaverde Group, undivided Contains Cliff House Sandstone 

From Dane and Bachman (1965) and Tweto (1979)

Boundary of study area

 5000 Structure contour Shows altitude of top of Cliff House Sandstone. Contour 

interval 500 feet. Datum is sea level

FIGURE 22. Approximate altitude and configuration of the top of the Cliff House Sandstone in the San Juan Basin

study area. Modified from Thorn and others (1990a).
44 GEOLOGIC FRAMEWORK OF THE SAN JUAN STRUCTURAL BASIN

the Lewis is stratigraphically the highest marine shale in the 

basin (Fassett and Hinds, 1971, p. 6). In general, the Lewis 

consists of light- to dark-gray and black shale and silty shale 

with thinner interbeds of silty to sandy limestone, siltstone, 

and fine-grained sandstone; calcareous concretions also are 

present in the unit (Baltz, 1967, p. 15, 16; Fassett and Hinds, 

1971, p. 6). Baltz (1967, p. 15) reported that the sandstone 

interbeds are most common in the lower part of the formation. The Lewis also contains several bentonite time-marker 

beds. The thickest and most extensive of these, the Huerfanito Bentonite Bed named by Fassett and Hinds (1971, 

p. 6), is in the upper part of the Lewis and is used for 

regional subsurface correlations.

The thickness of the Lewis Shale increases from southwest to northeast (fig. 5). The thickness ranges from zero 

where the formation pinches out between the Cliff House 

and Pictured Cliffs Sandstones near the junction of the 

Chaco River and Coyote Wash in New Mexico (pi. 1), to a 

maximum of about 2,400 feet in the Colorado part of the San 

Juan Basin (Fassett and Hinds, 1971, p. 6). The depth to the 

top of the Lewis ranges from 4,000 to 4,500 feet below land 

surface.

The Lewis Shale generally acts as a confining unit 

between the Cliff House and Pictured Cliffs Sandstones 

(Cooley and Weist, 1979, p. 46). Water wells reportedly 

completed in this formation actually may be completed in 

various sandstone tongues of the underlying and intertonguing Cliff House Sandstone.

PICTURED CLIFFS SANDSTONE

The Pictured Cliffs Sandstone was named by Holmes 

(1877, p. 248) for exposures north of the San Juan River 

between Shiprock and Farmington. The Pictured Cliffs crops 

out inside the margins of the Central Basin (pi. 1), where the 

sandstone caps mesas and buttes or forms erosion-resistant 

dip slopes. The Pictured Cliffs typically is a cliff former, 

although along the southeastern outcrop belt, the unit is very 

thin and forms low, nonresistant slopes. There is some debate 

as to the presence of the Pictured Cliffs Sandstone along the 

eastern margin of the San Juan Basin. Baltz (1967, p. 17) 

considered the formation to be present in that area but stated 

that it is represented only by a thin, silty and shaly offshore 

marine transition zone that merges with the upper part of the 

Lewis Shale. Fassett and Hinds (1971, p. 16), however, 

stated that the Pictured Cliffs is not present along the eastern 

side of the basin and that the Lewis Shale is overlain unconformably by the undivided Fruitland Formation and Kirtland 

Shale.

The Pictured Cliffs Sandstone is a regressive marine 

beach deposit (Molenaar, 1977b, p. 165). The formation conformably overlies the Lewis Shale (fig. 5), with the contact 

characterized by a distinct offshore marine transition zone

consisting of interbedded thin sandstone, siltstone, and shale 

(Reeside, 1924, p. 19; Fassett and Hinds, 1971, p. 8). The 

Upper Cretaceous Fruitland Formation conformably overlies 

the Pictured Cliffs, and intertonguing locally occurs between 

the two formations (Fassett and Hinds, 1971, p. 8).

The Pictured Cliffs Sandstone generally consists of an 

upward-coarsening sequence of light-gray to yellowish-gray, 

thick- to very thick bedded, very fine to medium-grained, 

locally crossbedded and bioturbated sandstone. Thin interbeds of dark marine shale also are present, especially in the 

lower part of the formation (Baltz, 1967, p. 17, 18; Fassett 

and Hinds, 1971, p. 8). The Pictured Cliffs is an aquifer 

throughout the Central Basin, but well yields are small 

(Cooley and Weist, 1979, p. 46; Stone and others, 1983, 

p. 32; Dam and others, 1990b).

The thickness of the Pictured Cliffs Sandstone is variable. Molenaar (1977a) reported a maximum thickness of 

400 feet but also reported that the average thickness is much 

less (Molenaar, 1977b, p. 165). Fassett and Hinds (1971, 

p. 17) stated that thickness ranges from zero on the east side 

of the basin to about 400 feet in the north-central part of the 

basin. Stone and others (1983, p. 33) reported a thickness 

range of 25-280 feet in New Mexico.

The depth to the top of the Pictured Cliffs Sandstone 

ranges from outcrops to about 4,000 feet in the northeastern 

part of the study area (fig. 23). The altitude and configuration 

of the top of the Pictured Cliffs is shown in figure 24. The 

top surface decreases from a maximum altitude of about 

8,000 feet above sea level on the outcrops along the northern 

rim of the Central Basin, to about 3,000 feet above sea level 

in the northeastern part of the study area, beneath Navajo 

Reservoir.

FRUITLAND FORMATION AND KIRTLAND SHALE

The Fruitland Formation and Kirtland Shale crop out 

inside the margins of the Central Basin (pi. 1). The topography formed by their outcrops varies from rolling to rough 

and typically resembles badlands. Erosion-resistant sandstone 

commonly caps isolated buttes and hillocks, whereas less 

resistant shale forms slopes and broad valleys or flats. The 

upper part of the Kirtland Shale generally forms steep slopes 

below mesas or buttes capped by the overlying erosion-resistant Ojo Alamo Sandstone.

The Fruitland Formation and Kirtland Shale were 

named by Bauer (1916) for exposures near the villages of 

Fruitland and Kirtland, N. Mex., along the San Juan River 

west of Farmington. The Fruitland conformably overlies the 

Pictured Cliffs Sandstone, with local intertonguing at the 

contact, and is conformably overlain by the Kirtland Shale. 

The Kirtland is unconformably overlain by the Ojo Alamo 

Sandstone of Tertiary age (fig. 5) (Baltz, 1967; Fassett and 

Hinds, 1971; Molenaar, 1977b). In southwestern Colorado,
GEOLOGIC UNITS 45

107-00'

35 00'

106 30'

0 10 20 30 40 50 60 MILES

I I .1.1. I I I I I \ T I I I

0 10 20 30 40 50 60 KILOMETERS 34 45'

EXPLANATION

Outcrops of Pictured Cliffs Sandstone From Dane and 

Bachman (1965)

Outcrops of Pictured Cliffs Sandstone and Lewis Shale, 

undivided From Tweto (1979)

Boundary of study area

 2000 Line of equal depth to top of Pictured Cliffs Sandstone 

Interval 500 feet. Datum is land surface

FIGURE 23. Approximate depth to the top of the Pictured Cliffs Sandstone in the San Juan Basin study area. Modified from Dam and others (1990b).
46 GEOLOGIC FRAMEWORK OF THE SAN JUAN STRUCTURAL BASIN

107°00'

35=00'

0 10 20 30 40 50 60 MILES

I I I I I I

36°00

I I I I 

0 10 20 30 40 50 60 KILOMETERS 34-451

EXPLANATION

Outcrops of Pictured Cliffs Sandstone From Dane and 

Bachman (1965)

Outcrops of Pictured Cliffs Sandstone and Lewis Shale, 

undivided From Tweto (1979)

Boundary of study area

 4000 Structure contour Shows altitude of top of Pictured Cliffs 

Sandstone. Contour interval 500 feet. Datum is sea level

FIGURE 24. Approximate altitude and configuration of the top of the Pictured Cliffs Sandstone in the San Juan 

Basin study area. Modified from Dam and others (1990b).
GEOLOGIC UNITS 47

the upper part of the Kirtland is overlain by and intertongues 

with the Upper Cretaceous McDermott Member of the Animas Formation (fig. 5).

In general, the Fruitland Formation and Kirtland Shale 

both consist of variable thicknesses of interbedded and repetitive sequences of nonmarine channel sandstone, siltstone, 

shale, and claystone. Coal beds and carbonaceous shales are 

common in the Fruitland. The Kirtland does not contain coal 

and has been divided into the following three members (in 

ascending order): the lower shale member of informal usage, 

the Farmington Sandstone Member, and the upper shale 

member of informal usage (fig. 5) (Bauer, 1916). Regionally, 

the Fruitland and Kirtland together probably act as a confining unit; however, the formations yield water to wells locally 

(Baltz and West, 1967, p. 16, 17; Brown, 1976, p. 53, 54; 

Anderholm, 1979, p. 36, 37; Cooley and Weist, 1979, 

p. 45, 46).

The thickness of the combined Fruitland Formation and 

Kirtland Shale ranges from zero on the eastern side of the 

basin (because of the uncomformity at the base of the Ojo 

Alamo Sandstone) to a maximum of about 2,000 feet in the 

northwestern part of the basin (fig. 25) (Fassett and Hinds, 

1971, p. 22, 26; Molenaar, I977b, p. 165). The Fruitland 

ranges in thickness from zero along the eastern side to about 

500 feet in the northwestern part of the basin (Fassett and 

Hinds, 1971, p. 23), with an average thickness of 300- 

350 feet (Molenaar, 1977b, p. 165). The thickness of the 

Kirtland ranges from zero along the eastern side to about 

1,500 feet in the northwestern part of the basin; the lower 

shale member, Farmington Sandstone Member, and upper 

shale member of the Kirtland Shale each are as much as 

500 feet thick (Fassett and Hinds, 1971, p. 26; Molenaar, 

1977b, p. 165; Stone and others, 1983, p. 31).

The depth to the top of the Kirtland Shale ranges from 

outcrops to about 3,500 feet in the eastern part of the basin 

(fig. 26). The altitude and configuration of the top of the 

Kirtland is shown in figure 27. It decreases from a maximum 

altitude of about 8,000 feet above sea level on the outcrops 

along the northeast to about 3,500 feet above sea level in the 

subsurface in the eastern part of the study area.

TERTIARY ROCKS

Sedimentary rocks of early Tertiary age in the Central 

Basin consist of the Ojo Alamo Sandstone, upper part of the 

Animas Formation (the lower part of the Animas, the 

McDermott Member, is of Late Cretaceous age; however, to 

avoid repetition, both parts are discussed in this section), 

Nacimiento Formation, and San Jose Formation (fig. 5). In 

the western part of the basin, the Chuska Sandstone (not 

shown in fig. 5) caps the Chuska Mountains. Tertiary rocks 

in the Central Basin disconformably overlie Upper 

Cretaceous rocks.

Tertiary rocks were deposited in various nonmarine 

(stream channel, flood plain, eolian, and lacustrine) environments. The maximum combined thickness of these rocks in 

the Central Basin is about 3,800 feet; the thickness of the 

strata in the Central Basin generally increases toward the 

east. The general lithologies and thicknesses of Tertiary 

rocks are summarized in table 1; their outcrops are shown on 

plate 1.

OJO ALAMO SANDSTONE

The Ojo Alamo Sandstone is of Paleocene age (fig. 5). 

It crops out inside the Central Basin (pi. 1) and typically 

forms cliffs and dip slopes or caps low mesas and forms 

rounded hills. The majority of Ojo Alamo rocks are in New 

Mexico. The formation pinches out in the northwest, about 

halfway between Farmington and the Colorado State line, 

west of La Plata River. In the northeast, Ojo Alamo outcrops 

can be traced into Colorado, where the formation pinches out 

a few miles north of the State line, south of Pagosa Springs 

(Fassett, 1974, p. 228). Subsurface studies by Fassett and 

Hinds (1971, fig. 9 and p. 29) indicate that the Ojo Alamo is 

not present north of a line connecting the northernmost outcrops of the formation.

Throughout most of its extent, the Ojo Alamo 

Sandstone disconformably overlies the Kirtland Shale. On 

the east side of the Central Basin, however, the Kirtland was 

completely eroded prior to deposition of the Ojo Alamo, and 

the Ojo Alamo disconformably overlies the Fruitland 

Formation; locally, in places where the Fruitland has been 

completely eroded, the Ojo Alamo disconformably overlies 

the Lewis Shale (Fassett, 1974, p. 228). The contact of the 

Ojo Alamo with underlying rocks has been described by 

O'Sullivan and others (1972, p. 56) as a distinct, wavy 

erosional surface. Fassett and Hinds (1971, p. 28) reported 

large-scale channeling at the base of the Ojo Alamo and 

stated that some of these channels extend 50 feet or more 

into the underlying shale or sandstone of the Fruitland 

Formation or Kirtland Shale. Throughout most of its extent, 

the Ojo Alamo is conformably overlain by the Nacimiento 

Formation, and intertonguing at the contact is common 

(Fassett and Hinds, 1971, p. 29).

Strata of the Ojo Alamo Sandstone primarily represent 

overlapping stream-channel deposits (Baltz, 1967, p. 33), but 

flood-plain deposits also are present (Powell, 1973, p. 116). 

Powell (1973) presented an alluvial-plain depositional model 

for the formation. In general, the Ojo Alamo consists of 

overlapping sheetlike sequences of sandstone and conglomeratic sandstone that locally contain interbedded shale lenses. 

The sandstone is arkosic, light brown to rusty brown or buff 

and tan, and contains abundant silicified wood. The 

sandstone is medium to very coarse grained and commonly 

conglomeratic, containing pebbles of various compositions 

that decrease in size and quantity from west to east across
48 GEOLOGIC FRAMEWORK OF THE SAN JUAN STRUCTURAL BASIN

107°00'

106 30'

36 W

30'

0 10 20 30 40 50 60 KILOMETERS

EXPLANATION

Outcrops of Kirtland Shale and Fruitland Formation, undivided 

From Dane and Bachman (1965) and Tweto (1979)

Boundary of study area

 7000 Line of equal thickness of the Kirtland Shale and Fruitland Formation, 

undivided From Fassett and Hinds (1971). Interval 100 feet

FIGURE 25. Approximate thickness of the combined Fruitland Formation and Kirtland Shale in the San Juan Basin 

study area. Modified from Fassett and Hinds (1971, fig. 11).
GEOLOGIC UNITS 49

107=00'

109°30 

37°3

35 W

0 10 20 30 40 50 60 MILES

I I I I I I

36 00

r r

0 10 20 30 40 50 60 KILOMETERS 34 45'

EXPLANATION

Outcrops of Kirtland Shale and Fruitland Formation, undividedFrom Dane and Bachman (1965) and Tweto (1979)

Boundary of study area

 2000 Line of equal depth to top of Kirtland Shale Interval 500 feet. 

Datum is land surface

FIGURE 26. Approximate depth to the top of the combined Fruitland Formation and Kirtland Shale in the San Juan 

Basin study area. Modified from Kernodle and others (1990).
50 GEOLOGIC FRAMEWORK OF THE SAN JUAN STRUCTURAL BASIN

107 00'

37 0

0 10 20 30 40 50 60 MILES

I I I I I I

i i i i i r

0 10 20 30 40 50 60 KILOMETERS

EXPLANATION

Outcrops of Kirtland Shale and Fruitland Formation, undivided 

From Dane and Bachman (1965) and Tweto (1979)

Boundary of study area

 4000 Structure contour Shows altitude of top of Kirtland Shale. Contour 

interval 500 feet. Datum is sea level

FIGURE 27. Approximate altitude and configuration of the top of the combined Fruitland Formation and Kirtland 

Shale in the San Juan Basin study area. Modified from Kernodle and others (1990).
GEOLOGIC UNITS 51

the Central Basin (Baltz and West, 1967, p. 17; Fassett and 

Hinds, 1971, p. 28). The Ojo Alamo Sandstone is a dependable aquifer and yields water to wells and springs mainly in 

its outcrop area (Brimhall, 1973, p. 199-202; Brown, 1976, 

p. 52; Anderholm, 1979, p. 29-33; Cooley and Weist, 1979, 

p. 45).

The thickness of the Ojo Alamo Sandstone is variable. 

Baltz (1967, p. 32) reported that the thickness ranges from 

70 to 200 feet. O'Sullivan and others (1972, p. 57) also 

reported a maximum thickness of 200 feet, whereas Stone 

and others (1983. p. 31) reported a thickness range of 70- 

300 feet. Fassett and Hinds (1971, p. 28, 29) reported that 

throughout the Central Basin, the thickness of the Ojo Alamo 

ranges from 20 to 400 feet but that a range of 50-150 feet is 

most common. Fassett and Hinds (1971, p. 28) also stated 

that thickness varies according to the number of sandstone 

beds that constitute the formation at any given location.

The depth to the top of the Ojo Alamo Sandstone ranges 

from outcrops to about 3,500 feet in the east-central part of 

the Central Basin (fig. 28). The altitude and configuration of 

the top of the Ojo Alamo is shown in figure 29. The top 

surface decreases from a maximum altitude of about 8,000 

feet above sea level along the narrow outcrop band in the 

northeast to about 4,000 feet above sea level in the subsurface in the east-central part of the study area.

ANIMAS FORMATION

Most of the Animas Formation is of Paleocene age, but 

the lower part is of latest Late Cretaceous age (fig. 5) 

(Barnes and others, 1954). The formation crops out principally inside the northern margin of the Central Basin (pi. 1). 

The Animas is present only in the northern part of the 

Central Basin, mainly in Colorado, where it is found south of 

a zone that connects Dulce, N. Mex., and La Plata River 

valley near the New Mexico-Colorado State line. Along this 

zone, the Animas grades laterally into and occupies the same 

stratigraphic interval as the Nacimiento Formation (fig. 5) 

(Fassett and Hinds, 1971, p. 34; Fassett, 1974, p. 229). The 

Animas conformably overlies the Kirtland Shale in the north; 

farther south (near the New Mexico-Colorado State line), the 

upper part of the formation might unconformably overlie the 

Ojo Alamo Sandstone (Fassett and Hinds, 1971, p. 34).

The Animas Formation consists of the McDermott 

Member of Late Cretaceous age and the unnamed upper 

member of Paleocene age (fig. 5) (Barnes and others, 1954). 

The upper member disconformably overlies the McDermott 

Member, the unconformity representing a hiatus of about 

6 million years (Fassett, 1988b, p. 319). The Animas Formation mainly consists of fluvial and volcaniclastic deposits. 

The McDermott Member is present only in the Animas River 

valley area of Colorado and is laterally equivalent to the 

upper shale member of the Kirtland Shale. The McDermott

Member consists of varicolored (dominantly purple) tuffaceous sandstone and conglomerate with some variegated 

shale (Reeside, 1924, p. 25). The upper member is present in 

New Mexico and Colorado and also consists of varicolored 

and interbedded tuffaceous sandstone, conglomerate, and 

shale (Fassett, 1974, p. 229). The diagnostic characteristic of 

the Animas Formation is the presence of macroscopic volcanic material (Fassett and Hinds, 1971, p. 33).

The thickness of the Animas Formation ranges from 

about 230 feet at the stratotype along the Animas River at 

Durango (Barnes and others, 1954), to about 2,700 feet near 

the La Plata-Archuleta County line in Colorado (Fassett and 

Hinds, 1971, p. 33). The Animas could be expected to yield 

small quantities of water to wells, with water-yielding 

characteristics similar to those of the Nacimiento Formation 

(Cooley and Weist, 1979, p. 44).

NACIMIENTO FORMATION

The Nacimiento Formation is of Paleocene age (Baltz, 

1967, p. 35). It crops out principally in a broad band inside 

the southern and western margins of the Central Basin and 

also in a narrow band along the western face of the 

Nacimiento Uplift (pi. 1). The Nacimiento is a nonresistant 

unit and typically erodes to low, rounded hills or forms 

badlands topography.

The Nacimiento Formation is present in the southern 

two-thirds of the Central Basin where it conformably overlies and intertongues with the Ojo Alamo Sandstone (Baltz, 

1967, p. 41; Fassett, 1974, p. 229). Along a zone connecting 

Dulce and the La Plata River valley near the New MexicoColorado State line, the Nacimiento grades laterally into the 

upper part of the Animas Formation (Fassett and Hinds, 

1971, p. 34; Fassett, 1974, p. 229). Along this zone, the two 

formations occupy the same stratigraphic interval (fig. 5).

Strata of the Nacimiento Formation mainly were deposited in lakebeds, with lesser deposition in stream channels 

(Brimhall, 1973, p. 201; Fassett, 1974, p. 229). In general, 

the strata consist of drab, interbedded black and gray shale 

with discontinuous white, medium- to very coarse grained 

arkosic sandstone (Fassett, 1974, p. 229; Stone and others, 

1983, p. 30). Anderholm (1979, p. 25) reported local carbonaceous shale and lignite in the formation. Baltz (1967, p. 39) 

stated that the sandstone percentage increases northward. 

Stone and others (1983, p. 30) suggested that the formation 

might contain more sandstone than is often reported because 

some geologists might have assumed the slope-forming strata 

in the unit to be shale, whereas in some localities these strata 

actually are poorly consolidated sandstone.

The thickness of the Nacimiento Formation ranges from 

500 to 1,300 feet (Molenaar, 1977a). The formation generally thickens from the Central Basin margins toward the 

center (Baltz, 1967, p. 38; Steven and others, 1974; Stone
52 GEOLOGIC FRAMEWORK OF THE SAN JUAN STRUCTURAL BASIN

36°00

106°30'

0 10 20 30 40 50 60 MILES

I I I I I___ I

i i i \ r

0 10 20 30 40 50 60 KILOMETERS 34"45'

EXPLANATION

I I Outcrops of Ojo Alamo Sandstone From Dane and Bachman (1965) 

I I and Fassett and Hinds (1971)

 Approximate subsurface extent of Ojo Alamo Sandstone From 

Fassett and Hinds (1971)

Boundary of study area

 7000 Line of equal depth to top of Ojo Alamo Sandstone Interval 500 feet 

Datum is land surface

FIGURE 28. Approximate depth to the top of the Ojo Alamo Sandstone in the San Juan Basin study area Modified

from Thorn and others (1990b).
GEOLOGIC UNITS 53

107°00'

106=30'

0 10 20 30 40 50 60 MILES

I I I I I I

I 1I I 

0 10 20 30 40 50 60 KILOMETERS 34 45'

EXPLANATION

Outcrops of Ojo Alamo Sandstone From Dane and Bachman (1965) 

and Fassett and Hinds (1971)

Approximate subsurface extent of Ojo Alamo Sandstone From 

Fassett and Hinds (1971)

Boundary of study area

 4000 Structure contour Shows altitude of top of Ojo Alamo Sandstone. 

Contour interval 500 feet. Datum is sea level

FIGURE 29. Approximate altitude and configuration of the top of the Ojo Alamo Sandstone in the San Juan Basin 

study area. Modified from Thorn and others (1990b).
54 GEOLOGIC FRAMEWORK OF THE SAN JUAN STRUCTURAL BASIN

and others, 1983), but the thickness and extent of sandstonelenses lessen because their depositional environment was in 

the more localized stream channels (Brimhall, 1973, p. 201). 

Discontinuous, fine-grained sandstone bodies in the 

Nacimiento Formation yield variable quantities of water to 

wells; however, the formation is only dependable as a local 

aquifer (Brimhall, 1973, p. 201). Cooley and Weist (1979, 

p. 44) reported that the Nacimiento probably is a better aquifer toward the northeast, where sandstone becomes more 

prevalent. Brown (1976, p. 43, 44) and Anderholm (1979, 

p. 25, 27) also reported on the water-yielding characteristics 

of the Nacimiento.

SAN JOSE FORMATION

The San Jose Formation of Eocene age was defined by 

Simpson (1948a, b). With the exception of the isolated 

Chuska Sandstone near the western edge of the study area, 

the San Jose Formation is the youngest sedimentary rock 

unit in the San Juan Basin, and its outcrops form the land 

surface throughout much of the Central Basin area (pi. 1).

The San Jose Formation is present in New Mexico and 

Colorado. It generally overlies the Nacimiento Formation 

south of the State line and overlies the Animas Formation 

north of the State line (Fassett, 1974, p. 229). The basal contact of the San Jose Formation varies with location in the 

San Juan Basin. Along the basin margins, this contact is a 

disconformity whereas along the Nacimiento Uplift it is an 

angular unconformity; in the Central Basin, the contact is 

conformable (Baltz, 1967, p. 54; Fassett, 1974, p. 229).

The San Jose Formation was deposited in various fluvial 

environments (Baltz, 1967, p. 44-55) and consists of an 

interbedded sequence of sandstone, siltstone, and variegated 

shale. The sandstone is buff to yellow and rust colored, 

crossbedded, very fine to coarse-grained arkose that is 

locally conglomeratic and contains abundant silicified wood 

(Baltz, 1967, p. 46; Fassett, 1974, p. 229; Anderholm, 1979, 

p. 23).

Baltz (1967, p. 45) recognized four members of the San 

Jose Formation in the east-central part of the Central 

Basin. These members and their principal lithologies are (in 

ascending order) the Cuba Mesa Member (sandstone), 

Regina Member (shale), Llaves Member (sandstone), and 

Tapicitos Member (shale). Baltz (1967) also identified but 

did not name a fifth member in the northeastern part of the 

Central Basin, The stratigraphic relations and consequent 

mappability of these members are complicated by extensive 

intertonguing and pinch cuts (Fassett, 1974, p. 229; Anderholm, 1979, p. 23; Stone and others, 1983, p. 25). Whether 

the members are regionally mappable throughout the Central 

Basin has been the subject of some discussion among San 

Juan Basin geologists.

The thickness of the San Jose Formation is variable but 

generally increases from west to east. Fassett (1974, p. 229)

reported a maximum thickness of 2,400 feet in the east-central 

part of the Central Basin, and Stone and others (1983, p. 25) 

reported a range from about 200 feet in the west and south to 

. almost 2,700 feet in the center of the Central Basin.

Brimhall (1973, p. 202) and Stone and others (1983, 

p. 25) reported that the sandstones of the San Jose Formation 

are largely untested as aquifers. Baltz and West (1967, p. 62) 

and Brown (1976, p. 33, 35) stated that the San Jose should be 

considered as a major water source. Anderholm (1979, p. 24) 

reported that the Cuba Mesa Member has good potential for 

yielding water to wells on the eastern side of the San Juan 

Basin.

UNDIVIDED TERTIARY SEDIMENTARY ROCKS ABOVE THE 

OJO ALAMO SANDSTONE IN THE CENTRAL BASIN

The thickness of the undivided Tertiary sedimentary 

rocks above the Ojo Alamo Sandstone in the Central Basin 

(including the McDermott Member of the Animas Formation) is shown in figure 30. The thickness ranges from about 

500 to about 3,500 feet in the east-central part of the Central 

Basin.

The altitude and configuration of the bottom of the 

undivided Tertiary sequence (actually, the undivided 

Nacimiento and Animas Formations) above the Ojo Alamo 

Sandstone in the Central Basin is shown in figure 31. The 

bottom of these rocks decreases from a maximum altitude of 

about 8,000 feet above sea level in the northeast to about 

4,000 feet above sea level in the east-central part of the Central Basin.

CHUSKA SANDSTONE

The Chuska Sandstone was originally described by Gregory (1916, pi. 21) as being of Eocene(?) age. Later investigations, however, determined the unit to be at least as young 

as early Oligocene (Hackman and Olson, 1977). As such, the 

Chuska is the youngest sedimentary rock unit in the San 

Juan Basin but is restricted to the western part of the basin, 

where it caps the Chuska Mountains along the New MexicoArizona State line (pi. I).

The Chuska Sandstone unconformably overlies Mesozoic sedimentary rocks along the Defiance Monocline. The 

unit was deposited principally in an eolian environment but 

also contains deposits of fluvial origin in the lower part. In 

general, the Chuska consists of friable, light-brown, yellowish-gray, or white, massive, crossbedded, very fine to coarsegrained sandstone; some interbeds of reddish-brown siltstone 

and shale also are present (O1 Sullivan and Beikman, 1963; 

Hackman and Olson, 1977). Locally, the lower part of the 

Chuska consists of as much as 250 feet of fluvial sandstone 

and shale and was formerly referred to as the Deza 

Formation (Wright, 1956). Because of sparse distribution of
GEOLOGIC UNITS 55

107"00'

106-30'

35 00

0 10 20 30 40 50 60 KILOMETERS 34 45'

EXPLANATION

Outcrops of San Jose Formation From Dane and Bachman (1965). 

and Tweto (1979)

Outcrops of Nacimiento Formation From Dane and Bachman (1965), 

Fassett (1974). and Tweto (1979)

Outcrops of Animas Formation From Dane and Bachman (1965), 

Fassett (1974), and Tweto (1979)

Boundary of study area

 2500 Line of equal thickness of San Jose, Nacimiento, and Animas 

Formations Interval 500 feet

FIGURE 30. Approximate thickness of the undivided Tertiary sedimentary rocks above the Ojo Alamo Sandstone 

in the Central Basin of the San Juan Basin. Modified from Levings and others (1990b).
56 GEOLOGIC FRAMEWORK OF THE SAN JUAN STRUCTURAL BASIN

107 00'

106"30'

37°0

35°00'

36°00

0 10 20 30 40 50 60 MILES

I 

0 10 20 30 40 50 60 KILOMETERS 34 45'

EXPLANATION

Outcrops of San Jose Formation From Dane and Bachman (1965) 

and Tweto (1979)

Outcrops of Nacimiento Formation From Dane and Bachman (1965), 

Fassett (1974), and Tweto (1979)

Outcrops of Animas Formation From Dane and Bachman (1965), 

Fassett (1974), and Tweto (1979)

Boundary of study area

 4000 Structure contour Shows altitude of bottom of Nacimiento and Animas 

Formations. Contour interval 500 feet. Datum is sea level

FIGURE 31. Approximate altitude and configuration of the bottom of the undivided Tertiary sedimentary rocks 

above the Ojo Alamo Sandstone in the Central Basin of the San Juan Basin. Modified from Levings and others 

(1990b).
SUMMARY 57

outcrops and questionable mappability, the name Deza 

Formation has been abandoned.

The Chuska Sanstone's thickness throughout most of the 

Chuska Mountains ranges from 500 to 1,200 feet, but Wright 

(1956, p. 416) reported a maximum local thickness of about 

1,750 feet. Cooley and Weist (1979, p. 42) stated that water 

wells have not been completed in the Chuska Sandstone, but 

the formation yields water to many springs in the Chuska 

Mountains (Lyford, 1979, p. 16).

SUMMARY

The San Juan Basin, with an area of about 21,600 

square miles, encompasses northwestern New Mexico, southwestern Colorado, northeastern Arizona, and southeastern 

Utah. The basin is a structural depression formed during the 

Laramide orogeny; maximum structural relief is about 

10,000 feet. A Cambrian through Tertiary sedimentary rock 

sequence about 14,400 feet thick fills the basin.

The RASA study area that is the subject of this report 

consists of that part of the basin that contains Triassic 

through Tertiary sedimentary rocks; it is somewhat smaller 

(about 19,400 square miles) than the structural basin. The 

older Triassic rocks crop out around the basin margins and 

are sequentially overlain by Jurassic, Cretaceous, and Tertiary rocks toward the center of the basin. These strata dip 

toward the troughlike center, the structurally deepest part of 

the basin.

Triassic rocks represent deposition in various nonmarine 

environments and consist of the Moenkopi, Moenkopi(?), 

Chinle, and Dolores Formations. Triassic strata attain a maximum combined thickness of about 1,650 feet in the westcentral part of the basin. Because of its limited areal extent, 

the Moenkopi Formation is not an important aquifer in the 

basin. Regionally, the Chinle and Dolores Formations act as 

confining units, but local sandstone bodies are 

water yielding.

Jurassic rocks also were deposited in continental 

environments and attain a maximum combined thickness of 

about 1,500 feet in the northwestern part of the basin. 

Jurassic formations include the Wingate Sandstone, Entrada 

Sandstone, Wanakah Formation, Cow Springs Sandstone, 

Morrison Formation, and Junction Creek Sandstone. The 

Wingate Sandstone is unimportant as an aquifer because of 

its limited areal extent, but parts of the other formations are 

water yielding. The Westwater Canyon Member of the Morrison Formation is regionally the most productive aquifer in 

the Jurassic strata.

Cretaceous rocks were deposited in continental, 

marginal marine, and marine environments associated with 

several advances and retreats of a large, shallow inland 

seaway that bisected the continent. Cretaceous rocks attain

a combined thickness of at least 6,500 feet in the San Juan 

Basin. The strata include the Burro Canyon Formation, 

Dakota Sandstone, Mancos Shale, Gallup Sandstone, 

Crevasse Canyon Formation, Point Lookout Sandstone, 

Menefee Formation, Cliff House Sandstone, Lewis Shale, 

Pictured Cliffs Sandstone, Fruitland Formation, and Kirtland 

Shale. The McDermott Member of the Animas Formation 

also is of Cretaceous age. Although parts of all these formations are water yielding, the Dakota and Gallup Sandstones 

are regionally the most productive aquifers in Cretaceous 

rocks.

Tertiary sedimentary rocks represent deposition in 

various nonmarine environments and in the Central Basin 

include the Ojo Alamo Sandstone, upper part of the Animas 

Formation, Nacimiento Formation, and San Jose Formation. 

Maximum combined thickness of these rocks is about 

3,800 feet. In the west-central part of the San Juan Basin, the 

Chuska Sandstone, the youngest sedimentary unit in the 

basin, caps the Chuska Mountains and attains a maximum 

thickness of about 1,800 feet. The most productive aquifers 

in Tertiary rocks are the Ojo Alamo Sandstone and sandstone 

bodies in the San Jose Formation.

Geologic maps showing the thickness, depth to the top, 

and altitude and configuration of the top of major formations 

were prepared from a data base of about 24,000 oil- and gastest wells in the basin, supplemented with water-well data 

from the U.S. Geological Survey's NWIS data base and data 

from outcrops. These maps were constructed using a surfacecontour-generating package interfaced with a Geographic 

Information System. The maps were used as input to a threedimensional, ground-water flow model of the regional aquifer system in the San Juan Basin.

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 1981, Hydrogeology of the Gallup Sandstone, San Juan Basin, northwest New Mexico: Groundwater, v. 19, no. 1, p. 4 11.

Stone, W.J., Lyford, P.P., Frenzel, P.P., Mizell, N.H., and Padgett, E.T., 

1983, Hydrogeology and water resources of San Juan Basin, New Mexico: Socorro, New Mexico Bureau of Mines and Mineral Resources 

Hydrologic Report 6, 70 p.
SELECTED REFERENCES 63

Strobell, J.D., Jr., 1956, Geology of the Carrizo Mountains area in northeastern Arizona and northwestern New Mexico: U.S. Geological Survey Oil 

and Gas Investigations Map OM-160, scale 1:48,000, 2 sheets.

 1964, Sonsela Sandstone Bed in Chinle Formation, in Geological Survey Research, 1964, chapter A: U.S. Geological Survey Professional Paper 501-A, p. A100.

Tabet, D.E., and Frost, S.J., 1979, Environmental characteristics of Menefee 

coals in the Torreon Wash area, New Mexico: Socorro, New Mexico 

Bureau of Mines and Mineral Resources Open-File Report 102, 134 p.

Thomas, B.E., 1989, Simulation of the ground-water system in Mesozoic 

rocks in the Four Corners area, Utah, Colorado, Arizona, and New 

Mexico: U.S. Geological Survey Water-Resources Investigations 

Report 88-4086, 89 p.

Thorn. C.R., Levings, G.W., Craigg, S.D., Dam, W.L., and Kernodle, J.M., 

1990a, Hydrogeology of the Cliff House Sandstone in the San Juan 

structural basin, New Mexico, Colorado, Arizona, and Utah: U.S. Geological Survey Hydrologic Investigations Atlas HA-720-E, 2 sheets.

 1990b, Hydrogeology of the Ojo Alamo Sandstone in the San Juan 

structural basin, New Mexico, Colorado, Arizona, and Utah: U.S. 

Geological Survey Hydrologic Investigations Atlas HA-720-B, 

2 sheets.

Turner-Peterson, C.E., Santos, E.S., and Fishman, N.S., eds., 1986, A basin 

analysis case study The Morrison Formation, Grants uranium region, 

New Mexico: Association of Petroleum Geologists Studies in Geology 

22, 391 p.

Tweto, Ogden, 1979, Geologic map of Colorado: U.S. Geological Survey, 

scale 1:500,000, 2 sheets.

U.S. Bureau of the Census, 1980, Master area reference file for 1980 Census: 

Washington, D.C., U.S. Bureau of the Census.

 1985, Technical documentation, population, and per capita income estimates: Washington, D.C., U.S. Bureau of the Census, Governmental 

Units, 6 p.

Wanek, A.A., 1954, Geologic map of the Mesa Verde area, Montezuma 

County, Colorado: U.S. Geological Survey Oil and Gas Investigations 

Map OM-152, scale 1:63,360.

Ward, L.F., 1901, Geology of the Little Colorado Valley: American Journal 

of Science, v. 12, no. 72, p. 401^13.

Welder. G.E., 1986, Plan of study for the Regional Aquifer-System Analysis 

of the San Juan structural basin, New Mexico, Colorado, Arizona, and 

Utah: U.S. Geological Survey Water-Resources Investigations Report

85-4294, 23 p.

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v. 42, no. 9, p. 2048-2106.

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sediments: Journal of Geology, v. 30, no. 5, p. 377-392.

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129.

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J.F., 1977, Tectonic framework of the San Juan Basin, in Fassett, J.E., 

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quadrangle, New Mexico: Socorro, New Mexico Bureau of Mines and 

Mineral Resources Geologic Map 37, scale 1:24,000.

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southeastern San Juan Basin, New Mexico: Socorro. New Mexico 

Bureau of Mines and Mineral Resources Circular 129, 7 p.

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Basin, New Mexico, Colorado, Arizona, and Utah: U.S. Geological 

Survey Bulletin 1481, 123 p.

Wright, H.E., Jr., 1956, Origin of the Chuska Sandstone, Arizona-New 

Mexico A structural and petrographic study of a Tertiary eolian 

sediment: Geological Society of America Bulletin, v. 67, p. 413-433.

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Cretaceous Point Lookout Sandstone Implications for shoreline 

progradation and basin tectonic history, San Juan Basin, New Mexico: 

Houston, Texas, Rice University, Ph.D. dissertation, 404 p.

Wyant, D.J., and Olson, Annabelle, 1978, Preliminary geologic map of the 

Albuquerque 1° by 2° quadrangle, northwestern New Mexico: 

U.S.Geological Survey Open-File Report 78-467, scale 1:250,000.

Young, R.G., 1973, Cretaceous stratigraphy of the Four Corners area, in James, 

H.L., ed.. Guidebook of Monument Valley and vicinity, Arizona and 

Utah: New Mexico Geological Society, 24th Field Conference, p. 86-93.

APPENDIX

APPENDIX 67

APPENDIX METHOD OF CONTOURING

MAPS USING SURFACE MODELING AND

GEOGRAPHIC INFORMATION SYSTEM

SOFTWARE

A surface-contour-generating software package, Interactive Surface Modeling (ISM) by Dynamics Graphics, Inc., 

was used to construct geologic contour maps (subsurface 

altitude and configuration of the top, depth to the top and, if 

possible, thickness) of major formations in the San Juan 

Basin. An interface was developed between ISM and Environmental Systems Research Institute's ARC/INFO Geographic Information System. The contour maps were 

converted to ARC/INFO polygon coverages for later input to 

a digital, three-dimensional ground-water flow model of the 

regional aquifer system in the basin; these techniques were 

discussed in detail by Kernodle and Philip (1988).

The general process of preparing geologic contour maps 

is discussed below and illustrated in figures 32-34. The process illustrated here is for the preparation of maps of the 

combined Fruitland Formation and Kirtland Shale, but the 

same process was used for the other formations. Data points 

used to generate the maps were derived from 10 U.S. Geological Survey Hydrologic Investigations Atlases prepared 

during the San Juan Basin RASA (see Craigg and others, 

1989, 1990; Kernodle and others, 1989, 1990; Dam and others, 1990a, b; Levings and others, 1990a, b; and Thorn and 

others, 1990a, b). The Petroleum Information Corporation's 

data base of about 24,000 oil- and gas-test wells in the San 

Juan Basin was used in the computerized data-retrieval process. Only the reported altitudes of the tops of formations 

picked from geophysical logs were selected from this data 

base. The data were evaluated for accuracy by comparing the 

altitude above sea level of each selected formation top on the 

logs with the mean value of the formation tops of neighboring wells. This was accomplished by intersecting an ARC/ 

INFO point coverage of the wells with a polygon coverage 

of a fine-mesh grid (fig. 32 shows this intersection for wells 

having an altitude for the top of the combined Fruitland Formation and Kirtland Shale; grid size is about 0.6 square 

mile). Wells were collected by grid cell, and a mean value of 

the altitude was determined for each cell. Points with formation tops that differed from the local mean by 50 or more 

feet were assumed to be erroneous and were deleted from the 

data file for each formation.

After a satisfactory contour map was produced and converted to ARC/INFO coverage, the ARC/INFO system was 

used to edit contours (if necessary), to annotate contours, and 

to clip (a "cookie-cutter" routine) the contour coverage with 

a polygon coverage of the inner (basinward) part of the 

appropriate geologic outcrop (figs. 33 and 34). The contour 

map was then combined with a base map of other ARC/ 

INFO geographic coverages (hydrography, outcrop, political 

boundaries, study area, structural basin, and roads), produced 

in hard-copy format, and published in U.S. Geological Survey Hydrologic Investigations Atlases HA-720-A through J 

Craigg and others, 1989, 1990; Dam and others, 1990a, b; 

Kernodle and others, 1989, 1990; Levings and others, 

1990a, b; and Thorn and others, 1990a, b).

The techniques described above for preparing regional 

geologic contour maps were useful tools during the San Juan 

Basin RASA for defining the geologic framework of the 

basin's multilayered aquifer system. In turn, this framework 

provided the basis for the ground-water-flow modeling part 

of this RASA investigation as well as a basis for future 

regional and local hydrogeologic studies of this basin. These 

techniques and their modification and refinement also should 

have transfer value to other hydrogeologic studies.

It is important, however, that the investigator be experienced and knowledgeable about the geology of the study 

area and its relation to the ground-water-flow system(s). A 

conceptual framework of the hydrogeology of the study area 

needs to be developed before any digital-mapping techniques 

are used. The hydrogeologist needs to be able to recognize 

reasonable and erroneous results immediately, regardless of 

how impressive and attractive the maps might be. These 

important concepts also were expressed by Berry (1987) in 

an informative discussion of some of the pitfalls to be 

avoided in the production of computer-generated maps.

In summary, computerized analysis of large quantities 

of geologic data using a Geographic Information System 

coupled with surface-modeling software (a geoscience information system) and the resulting contour maps representing 

various aspects of the hydrogeologic framework of an area 

have important application to the science of hydrogeology. 

But it also is important that the resulting maps never be 

"untouched by the human mind."
68 GEOLOGIC FRAMEWORK OF THE SAN JUAN STRUCTURAL BASIN

106°30'

37°0

35°00-

36°00

0 10 20 30 40 50 60 MILES

I I I I I I

I I I I I T 

0 10 20 30 40 50 60 KILOMETERS 34°45'

EXPLANATION

Outcrops of Kirtland Shale and Fruitland Formation, undivided 

From Dane and Bachman (1965) and Tweto (1979)

Geographic Information System polygon coverage of fine-mesh 

grid

Boundary of study area

Control point Used to compute approximate altitude of the top of 

the combined Kirtland Shale and Fruitland Formation

FIGURE 32. Geographic Information System coverage of control points intersected with a fine-mesh grid used to 

compute approximate altitude and configuration of the top of the combined Fruitland Formation and Kirtland 

Shale in the San Juan Basin study area.
INTRODUCTION 69

106=30'

35°00'

0 10 20 30 40 50 60 MILES

I I I I I I

0 10 20 30 40 50 60 KILOMETERS 34045.

EXPLANATION

Outcrops of Kirtland Shale and Fruitland Formation, 

undivided From Dane and Bachman (1965) and 

Tweto (1979)

Boundary of study area

- 2000 Undipped and unannotated structure contour 

Shows approximate altitude relative to sea level. 

Contour interval 500 feet

FIGURE 33. Undipped and unannotated Geographic Information System line coverage of contours representing 

approximate altitude and configuration of the top of the combined Fruitland Formation and Kirtland Shale in 

theSan Juan study area.
70 GEOLOGIC FRAMEWORK OF THE SAN JUAN STRUCTURAL BASIN

107=00'

37 0

36=00

30'

35 OO1

106 30'

IM 

-- - {--- 

0 10 20 30 40 50 60 MILES

0 10 20 30 40 50 60 KILOMETERS

EXPLANATION

Outcrops of Kirtland Shale and Fruitland Formation, 

undivided From Dane and Bachman (1965) and 

Tweto (1979)

Boundary of study area

 5000 Clipped and annotated structure contour 

Shows approximate altitude relative to sea level 

Contour interval 500 feet

FIGURE 34. Clipped and annotated Geographic Information System coverage of the approximate altitude and 

configuration of the top of the combined Fruitland Formation and Kirtland Shale in the San Juan Basin study area. 

Modified from Kernodle and others (1990).
Selected Series of U.S. Geological Survey Publications

Books and Other Publications

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Geophysical Investigations Maps (GP's) are on topographic 

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