Regional Diagenesis of Sandstones in the 

Upper Jurassic Morrison Formation, 

San Juan Basin, New Mexico and Colorado: 

Geologic, Chemical, and Kinetic Constraints

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Chapter H

Regional Diagenesis of Sandstones in the 

Upper Jurassic Morrison Formation, 

San Juan Basin, New Mexico and Colorado: 

Geologic, Chemical, and Kinetic Constraints

By PAULA L HANSLEY

A multidisciplinary approach to research studies of sedimentary 

rocks and their constituents and the evolution of sedimentary 

basins, both ancient and modern

U.S. GEOLOGICAL SURVEY BULLETIN 1808 

EVOLUTION OF SEDIMENTARY BASINS SAN JUAN BASIN
DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR., Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L Peck, Director

Any use of trade, product, or firm names in this publication is for descriptive 

purposes only and does not imply endorsement by the U.S. Government.

UNITED STATES GOVERNMENT PRINTING OFFICE: 1990

For sale by the

Books and Open-File Reports Section

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Library of Congress Cataloging-in-Publication Data

Hansley, Paula L.

Regional diagenesis of sandstones in the Upper Jurassic Morrison 

Formation, San Juan Basin, New Mexico and Colorado : geologic, chemical, 

and kinetic constraints / by Paula L. Hansley

p. cm. (U.S. Geological Survey bulletin ; 1808) 

Includes bibliographical references. 

Supt. of Docs, no.: I 19.3:1808-H

1. Sandstone New Mexico. 2. Sandstone Colorado. 3. Geology, 

Stratigraphic Jurassic. 4. Morrison Formation. I. Title. II. Series. 

QE75.B9 no. 1808 

[QE471.15.S25] 

557.3 s-dc20

[552'.5] 89-600194

CIP
CONTENTS

Abstract HI 

Introduction HI 

Geologic setting H3

Regional tectonic setting and climate H3 

Stratigraphy H3

Depositional environments and general lithology H4 

Methods of study H4 

Petrologic data H5

Heavy minerals H5 

Other framework minerals H5 

Salt Wash Member H5 

Recapture Member H5 

Westwater Canyon Member H7 

Brushy Basin and Jackpile Sandstone Members H9 

Cretaceous sandstones H9 

Diagenesis H9

Heavy minerals H9 

General sandstone diagenesis H12 

Salt Wash Member H12 

Recapture Member H13 

Westwater Canyon Member H13 

Brushy Basin Member H20 

Jackpile Sandstone Member H23 

Cretaceous sandstones H24 

Burial history H24 

Interpretation of diagenesis H25

Influence of Lake T'oo'dichi' H27 

Alterations related to meteoric water H29 

Comparison of diagenetic interpretations H29 

Chemistry H30

Kinetic model for diagenesis H30 

Summary and conclusions H31 

References cited H31

FIGURES

1. Map showing major tectonic features, outcrop areas of rocks of Morrison 

Formation, and location of sampled outcrops and cores, San Juan 

basin H2

2. Ternary diagram showing classification of sandstones in the Salt Wash,

Recapture, Westwater Canyon, and Brushy Basin Members of the Morrison 

Formation H8 

3-5. Photomicrographs showing:

3. Felsic volcanic rock fragment in the Westwater Canyon Member H9

4. Vitric textures preserved in analcime-cemented sandstone of the Brushy 

Basin Member H10

5. Anti-rapakivi textures of detrital plagioclase grains Hll 

6. Scanning electron micrograph showing deeply etched garnet from the 

Morrison Formation H12

Contents III
7. Diagram showing occurrence of garnet textures in the Morrison 

Formation H13

8. Maps showing regional distribution of major authigenic clay minerals in 

members of the Morrison Formation H16

9. Diagram showing paragenesis of major authigenic phases and diagenetic 

events in the Morrison Formation H19

10. X-ray diffractogram of authigenic nonferroan chlorite in sandstone of the

Westwater Canyon Member H19 

11-12. Scanning electron micrographs showing:

11. Clinoptilolite crystals in a sandstone pore of the Westwater Canyon 

Member H19

12. Clinoptilolite intergrown with authigenic idiomorphic quartz in 

sandstone pore of the upper part of the Westwater Canyon 

Member H20 

13-15. Photomicrographs showing:

13. Analcime-cemented sandstones in the Morrison Formation H21

14. "Chessboard albite" grain in sandstone of the upper part of the 

Westwater Canyon Member H22

15. Potassium feldspar overgrowths H22

16. X-ray diffractogram of well-ordered illite/smectite in a sandstone near the 

top of the Westwater Canyon Member H23

17. Scanning electron micrograph showing authigenic chlorite rosette and 

intergrown illitic/smectite filaments from the Westwater Canyon 

Member H23 

18-21. Photomicrographs showing:

18. Several generations of chalcedony cement in sandstone of the Brushy 

Basin Member H23

19. Authigenic pore-filling albite overgrowths in a sandstone of the 

Brushy Basin Member H23

20. Detrital plagioclase grain with albite overgrowth and later chalcedony 

cement H24

21. Pore-filling kaolinite cement in sandstone H24 

22-24. Scanning electron micrographs showing:

22. Illite/smectite in sandstone of the Brushy Basin Member H24

23. Intergrowth of illitic illite/smectite needles and authigenic quartz in 

sandstone of the Brushy Basin Member H25

24. Illitic overgrowths on Clinoptilolite in sandstone pore of the Brushy 

Basin Member 1125

25. Map showing vitrinite reflectance values for coals in the Upper Cretaceous 

Dakota Sandstone and Fruitland Formation, San Juan basin H26

26. Scanning electron micrograph showing illitic illite/smectite on margins of 

analcime cement in sandstone pore of the Westwater Canyon 

Member H27

27. Ion activity diagram showing the relations between some common 

authigenic minerals in the Morrison Formation H28

TABLES

1. Petrographic data derived from point counts of selected thin sections H6

2. Results of X-ray diffraction analysis of the < 2-(im fraction of selected 

sandstones H14

IV Contents
CONVERSION FACTORS FOR SOME SI METRIC AND U.S. UNITS OF 

MEASURE______________________________________

___To convert from__________To_____________Multiply by____

Feet (ft) Meters (m) 0.3048

Miles (mi) Kilometers (km) 1.609

Pounds (Ib) Kilograms (kg) 0.4536

Degrees Fahrenheit (°F) Degrees Celsius (°C) Temp °C = (temp °F-32)/1.8

Contents

EVOLUTION OF SEDIMENTARY BASINS-SAN JUAN BASIN

Regional Diagenesis of Sandstones in the 

Upper Jurassic Morrison Formation, 

San Juan Basin, New Mexico and Colorado: 

Geologic, Chemical, and Kinetic Constraints

By Paula L Hansley

Abstract

Early authigenic mineral assemblages (such as zeolite, 

silica, and feldspar cements) and vitroclastic textures are 

extraordinarily well preserved in upper sandstones of the 

Upper Jurassic Morrison Formation in the San Juan basin of 

New Mexico and Colorado. Early diagenetic reactions were 

driven by rapid dissolution of rhyolitic volcanic ash, which 

was incorporated into Morrison sediments at the time of 

deposition. The distributions of these authigenic minerals 

were controlled, in part, by chemical gradients in a large 

saline, alkaline lake (Lake T'oo'dichi') that existed in late 

Morrison time. Sandstones on lake margins were cemented 

by smectite and silica, whereas sandstones nearer the lake 

center, in which waters were most saline and alkaline, were 

cemented by zeolites. Diagenetic alterations in sandstones 

were promoted by alkaline interstitial waters that emanated 

from adjacent fine-grained, tuffaceous lake beds. Metastable 

phases that precipitated first were replaced relatively quickly 

by more stable, ordered phases in the geochemically favorable environment of the closed basin setting. Sandstones in 

the lower part of the Morrison Formation contained less ash 

at the time of deposition and thus have fewer diagenetic 

alterations. Diagenesis was dominated by precipitation of 

early pore-filling calcite cement and mixed-layer clay 

minerals. In the northern part of the basin, quartz is an 

abundant cement.

Vitrinite reflectance values for Cretaceous coals indicate 

that the Morrison Formation has remained relatively cool at 

basin margins throughout diagenesis. Elevation of temperatures above the geothermal gradient was provided by 

the influx of warm, deep-basin waters that locally modified 

early diagenetic assemblages during burial diagenesis. In 

organic- (and commonly also uranium ore-) bearing sandManuscript approved for publication, June 7, 1989.

stones located primarily in the southern part of the basin, 

complex diagenetic assemblages resulted from water/rock 

reactions involving soluble organic complexes. Uplift of 

recharge areas promoted the downdip migration of meteoric 

waters through permeable sandstones of the Morrison in the 

Early Cretaceous and late Tertiary that caused widespread 

precipitation of kaolinite.

INTRODUCTION

The Upper Jurassic Morrison Formation is the 

major uranium- and vanadium-bearing unit on the Colorado Plateau and, as such, has been the focus of 

numerous sedimentologic, petrologic, and geochemical 

studies. (See Adams and Saucier, 1981, and TurnerPeterson and others, 1986, for bibliographies.) As a 

result, most research has concentrated on uranium orebearing strata; few studies have examined the regional 

petrology of the Morrison Formation. The major goal of 

the research discussed in this report was to extend a 

petrologic study of the Morrison Formation in eight U.S. 

Geological Survey (USGS) cores taken from drill holes 

in the Grants uranium region (fig. 1) in the southwestern 

part of the San Juan basin (Hansley, 1986a, b) to the rest 

of the basin in order to determine if observed diagenetic 

alteration patterns identified in the cores reflect basinwide trends. A second objective was to use the mineralogy and paragenetic relationships of diagenetic 

assemblages to infer chemical processes responsible for 

observed postdepositional modifications of sandstones.

In order to accomplish these goals, sandstone 

samples were collected from 22 outcrops of the Morrison 

Formation and overlying Cretaceous sandstones around 

the margins of the basin (fig. 1). The Galisteo Dam

Regional Diagenesis of Sandstones in the Morrison Formation, New Mexico and Utah H1
section is 30 km northeast of Albuquerque, just east of 

the area shown in figure 1. In addition, sandstones from 

15 cores in the southern part of the basin were examined 

to enlarge the core data set. New petrologic data were 

combined with previously published data to facilitate 

interpretations of basinwide patterns.

Virtually all outcrop localities were barren of 

uranium ore, with the exception of small uraniumvanadium claims in the Salt Wash Member of the Morrison near Oak Springs and in the Westwater Canyon 

Member of the Morrison at the Dennison-Bunn outcrop. 

Some uranium enrichment (discrete coffinite) was noted 

in the cores. A lack of ore-bearing samples was not a 

detriment to this study, however, because primary uranium mineralization in the Morrison Formation was a 

very early diagenetic process (Ludwig and others, 1984), 

and most diagenetic alterations discussed herein 

occurred subsequent to ore formation or formed in parts

SAN JUAN 

VOLCANIC FIELD

Cuba

Senorita Canyon

San Pablo Canyon

Dennison-Bunn

Figure 1 . Major tectonic features and outcrop areas of rocks of Morrison Formation, San Juan basin. 

Location of sampled outcrops (triangles) and cores (solid circles) also shown. Numbers (1-8) indicate 

USGS core localities; letters (A-N) indicate other core localities (S.A. Adams, Colorado School of Mines). 

Tectonic features from Kelley and Clinton (1960); base map modified from Turner-Peterson (1986, 

fig. 1).

H2 Evolution of Sedimentary Basins San Juan Basin
of the basin where the Morrison Formation is not known 

to contain ore. Furthermore, uranium enrichment in 

some of the cores and descriptions of uranium ore zones 

in previous studies provided a basis for comparing the 

petrology of barren sandstones to mineralized 

sandstones.

Acknowledgments. I express appreciation to Isabelle Brownfield and Gary Skipp, USGS, for careful 

laboratory preparation of samples and for running heavymineral separations and clay-mineral diffraction patterns. In addition, I thank Neil Fishman and Christine 

Turner-Peterson, USGS, for furnishing some samples 

and thin sections from the northern and western parts of 

the basin.

GEOLOGIC SETTING

Regional Tectonic Setting and Climate

Although formation of the San Juan basin dates 

back to the Paleozoic, its present configuration reflects 

Laramide deformation (Kelley, 1951). In the Late Jurassic, the San Juan basin was bounded by the Zuni uplift to 

the south and the Defiance uplift to the west, and small 

positive areas also existed in southeastern Utah and 

southwestern Colorado (Santos and Turner-Peterson, 

1986). Although the Defiance and Zuni uplifts may have 

been moderately active, furnishing local detritus to the 

basin during Morrison deposition (Santos and TurnerPeterson, 1986), major source areas for the Morrison 

were a magmatic arc west of the Colorado Plateau in 

present-day southeastern California, southwestern Arizona, and northwestern Sonora, Mexico (Hamilton, 

1978; Dickinson, 1981), and the Mogollon uplift that 

extended from central Arizona into western New Mexico 

(Cooley and Davidson, 1963; Martinez, 1979; Lowy, 

1982). In these source areas, volcanic, intrusive igneous, 

metamorphic, and minor sedimentary strata furnished 

detritus to northeastward- and eastward-flowing streams. 

Abundant rhyolitic to dacitic volcanic ash (Cadigan, 

1967) that was blown eastward from volcanoes in the 

magmatic arc into the San Juan basin comprised a large 

part of the original matrix (now largely smectite) of 

Morrison sediments (Waters and Granger, 1953). Thick 

sequences (>100 m) comprised predominantly of 

altered ash beds as thick as a meter in lacustrine facies of 

the Brushy Basin Member attest to the enormous 

volumes of ash that were deposited (Turner-Peterson, 

1987).

The climate during Morrison time was semiarid as 

evidenced by sedimentary structures, fluvial style (such 

as ephemeral, braided streams), the presence of eolian 

deposits in the lower part of the Morrison, and saline,

alkaline lake deposits in the upper part of the Morrison 

(Bell, 1983, 1986; Turner-Peterson, 1987).

Stratigraphy

The Morrison Formation consists of five formal 

members in the basin (in ascending order): the 

Recapture, Westwater Canyon, Brushy Basin, Jackpile 

Sandstone, and Salt Wash. (The lowest of these, the Salt 

Wash Member, is only in the Four Corners region (Craig 

and others, 1955).) The contacts of members with each 

other are sharp to gradational and occasionally interfingering. The Morrison is overlain unconformably 

throughout most of the basin by the Upper Cretaceous 

Dakota Sandstone or by the Lower Cretaceous Burro 

Canyon Formation in the Chama basin and in the 

northern San Juan basin. The Jackpile is overlain by 

fluvial sandstones at the base of the Dakota Sandstone in 

the southeastern part of the basin (Aubrey, 1986). The 

Morrison is underlain by various formations of the 

Middle Jurassic San Rafael Group throughout the region 

(Turner-Peterson, 1986).

The average thickness of the Morrison Formation 

in the basin is 150 m (Turner-Peterson and Fishman, 

1986). The Morrison thins erosionally and depositionally 

to the south where it is truncated by the Dakota Sandstone; hence, the Dakota rests on progressively older 

beds of the Morrison to the south. In the southwestern 

corner of the basin, the Brushy Basin has been eroded 

completely and the Dakota rests on the Westwater 

Canyon Member (Turner-Peterson, 1986).

The Salt Wash Member is present in the northwestern part of the basin where it reaches a thickness of 

61 m (Condon and Peterson, 1986). It thins to the south 

and interfingers locally with the lower part of the 

Recapture Member. The Recapture Member is present 

throughout the basin, but it is not differentiated from the 

Beclabito Member of the underlying Middle Jurassic 

Wanakah Formation in the Cuba area (Condon and 

Peterson, 1986). The Recapture lies directly under the 

Brushy Basin Member in the Durango area (TurnerPeterson, 1987). The Recapture is as thick as 152 m on 

the west side of the basin. The Westwater Canyon 

Member thins to the south, north, and east; it pinches out 

depositionally south of the San Juan basin and north of 

the New Mexico-Colorado State line and is last recognized as far east as Galisteo Dam (Turner-Peterson, 

1987). The Westwater Canyon Member is thickest 

(about 100 m) in outcrop along the western margin of the 

basin (Turner-Peterson, 1987, fig. 11) and is as thick as 

135 m in the subsurface northeast of Gallup (Kirk and 

Condon, 1986). The Brushy Basin Member is thickest in 

the vicinity of Durango (fig. 1), where it is as thick as 

about 105 m (Turner-Peterson, 1987, fig. 32).

Regional Diagenesis of Sandstones in the Morrison Formation, New Mexico and Utah H3
In the southeastern part of the basin, the Jackpile 

Sandstone Member lies above the Brushy Basin Member 

just under a regional Jurassic-Cretaceous unconformity. 

The type Jackpile is exposed in the Laguna area where it 

locally contains uranium ore and is as thick as 65 m 

(Owen and others, 1984). From Laguna, Owen and 

others extended the Jackpile north to Cuba and east to 

the Galisteo Dam area. The Jackpile was deposited along 

a syndepositional syncline (Hilpert and Moench, 1960), 

the trace of which trends to the northeast into the 

subsurface where a sandstone thought to be the Jackpile 

continues to appear at the top of the Morrison Formation 

(Flesch, 1975). This correlation has been extended to 

sandstones at the top of the Morrison Formation in 

surface exposures along the Nacimiento uplift (Ruetschilling, 1973; Woodward and Schumacher, 1973; Santos, 1975). Flesch (1975) mapped 7-30 m of Jackpile 

near San Ysidro, and Santos (1975) mapped 0-50 m of 

Jackpile in the San Ysidro area. Others, however, have 

questioned this identification because the Jackpile 

occupies the same stratigraphic position as the fluvial 

Burro Canyon Formation in the Chama basin (Aubrey, 

1986). For example, Saucier (1974) extended the Jackpile northeastward from Laguna to the San Ysidro area, 

but, from San Ysidro along the Nacimiento uplift to the 

northeast toward the Chama basin, he replaced the 

Jackpile with the Burro Canyon Formation. Stratigraphic 

correlation is difficult because these two fluvial units are 

similar in composition except for a lack of conglomerate 

in the Jackpile (Flesch, 1975; Aubrey, 1986). The reason 

for this similarity is that the Burro Canyon consists 

mostly of reworked Morrison detritus (Santos, 1975).

Recently, the Encinal Canyon Member of the 

Dakota Sandstone, also a fluvial unit, was named 

formally in the southeastern San Juan basin (Aubrey, 

1986). It locally cuts out the Jackpile in the southeastern 

part of the basin and includes parts of what have been 

called Burro Canyon Formation and Jackpile Sandstone 

Member along the Nacimiento uplift (Aubrey, 1986). In 

its type area, the Encinal Canyon Member includes part 

of the Jackpile as defined by Grant and Owen (1974). 

Where these fluvial units occur together, the sandon-sand relationship is difficult to decipher because of 

the quartzose composition of all three units. The Jackpile 

is less conglomeratic and may contain more feldspar and 

volcanic-rock fragments than the Cretaceous units. The 

Burro Canyon Member is conglomeratic and contains 

abundant kaolinite, tripolitic chert (Santos, 1975), and 

oolitic, fossiliferous, silicified limestone pebbles 

(Ridgely, 1977). Generally, sandstones in the Encinal 

Canyon Member contain distinctive white, chalky chert 

pebbles (Aubrey, 1986). The uppermost Jurassic and 

Cretaceous stratigraphy used in this report follows that of 

Aubrey (1986, fig. 6) in the eastern part of the basin.

Depositional Environments and 

General Lithology

The Salt Wash Member was deposited by meandering and braided streams that originated in source 

areas to the west of the Colorado Plateau (Peterson, 

1984). The Salt Wash consists of light-greenish-gray to 

light-brown, very fine to medium grained, moderately 

well sorted sandstone interbedded with greenish-gray to 

reddish-brown mudstone.

The Recapture Member was deposited in fluvial, 

lacustrine, and eolian environments (Condon and Peterson, 1986). Streams generally came into the basin from 

the southwest. The Recapture consists of very fine to fine 

grained (occasionally medium grained), light-gray to 

pale-reddish-brown sandstone beds interbedded with 

variegated red-brown to purplish silty mudstone and 

claystone beds.

The Westwater Canyon Member was deposited by 

braided streams that flowed northeastward to southeastward across the basin from distant source areas to 

the west and southwest of the Colorado Plateau (TurnerPeterson, 1986). Sandstones of the Westwater Canyon 

Member are poorly to well sorted, fine to medium 

grained, and locally conglomeratic. The Recapture and 

Westwater Canyon Members are reddish in outcrop 

owing to late Tertiary oxidation but are generally a drab 

gray in the subsurface (Turner-Peterson, 1986). Interbedded greenish-gray mudstones and claystones are 

bentonitic.

Sediments of the Brushy Basin Member were 

deposited in a classic saline, alkaline lake, Lake 

T'oo'dichi' (Turner-Peterson, 1987), in which mudflat 

facies around the margins grade into more saline, playalake facies toward the center of the basin (Bell, 1983, 

1986; Turner-Peterson, 1987). Fluvial sandstones are 

light brown to gray and are similar in composition to 

sandstones of the Westwater Canyon Member. Sandstones are interbedded with thick sequences of bentonitic 

to zeolitic mudstone, claystone, altered tuff beds, and 

thin limestone units.

The Jackpile Sandstone Member was deposited by 

braided streams (Moench and Schlee, 1967) that flowed 

in a northeasterly direction at Laguna and in an easterly 

direction along the Nacimiento uplift (Flesch, 1975; 

Owen and others, 1984). Sandstone units are chalky gray, 

white to yellow, fine to medium grained, and locally 

conglomeratic and contain minor interbedded claystone. 

Silicified or coalified plant remains are locally present.

METHODS OF STUDY

Sandstone samples (217) were collected from 22 

outcrops of the Morrison Formation on the margins of

H4 Evolution of Sedimentary Basins San Juan Basin
the San Juan basin. Some localities were sampled in 

detail, and others were sampled sparsely to check 

variations in petrology. Petrographic thin sections of 

outcrop samples were impregnated with blue-dyed epoxy 

and stained with sodium cobaltinitrate for potassium 

feldspar identification and with Alizarin Red-S for calcite 

identification. Point counts (300 points per section) were 

made of representative thin sections for sandstone 

classification. Additional petrographic thin sections (84) 

(donated by S.S. Adams, Colorado School of Mines) 

from 15 cores in the southern part of the basin were 

examined; point counts were not made of these thin 

sections.

Heavy-mineral separations of the <60>200 

micron fraction of selected fine- to medium-grained 

samples were made in Bromoform (S.G. 2.87). The 

magnetic fraction was removed from the heavy fraction 

with a hand magnet, and the remaining nonmagnetic 

fraction (or a split thereof) was mounted with Canada 

balsam on a petrographic slide. Point counts of the 

heavy-mineral mounts were not conducted during this 

study. (See Hansley, 1986a, for point counts of samples 

from the USGS cores.)

Selected samples were examined with a Cambridge 

Stereoscan Mark 2 scanning electron microscope (SEM) 

with an attached Tracor Northern energy-dispersive system so that textural (paragenetic) relationships among 

authigenic minerals in sandstone pores could be 

described and the genetic nature (authigenic versus 

detrital) of clay minerals determined.

The <2-/im fraction of selected samples was 

analyzed by X-ray diffraction for determination of clay 

mineralogy. Three X-ray diffractograms were generated 

from each sample air dried, glycolated, and 550 °C 

runs in order to make clay-mineral identifications. The 

percentage of illite in interstratified illite/smectite (I/S) 

was determined by measuring the 001/002 and 002/003 

peak positions (Reynolds, 1980); the notation of 0.71 

means 70 percent illite in I/S. The presence of perfect 

ordering (ISIS, HIS, and so forth) was evaluated by 

looking for a superlattice peak, noting positions of nonintegral peaks, and comparing the results to published 

(Reynolds, 1980) and unpublished (Maynard Slaughter, 

Colorado School of Mines, 1987) clay-mineral tables.

PETROLOGIC DATA

Heavy Minerals

Tourmaline, zircon, garnet, staurolite, apatite, and 

iron-titanium oxides comprise the bulk of heavy-mineral 

suites. Minor heavy-mineral species include epidote, 

rutile, hornblende, sphene, and monazite. The most

notable change in composition among the members is the 

appearance of abundant staurolite in the Westwater 

Canyon and Brushy Basin Members, as compared to a 

lack of staurolite in the Salt Wash Member and a trace of 

staurolite in the Recapture Member. In general, staurolite is more abundant in the Westwater Canyon and 

Brushy Basin Members (including the Jackpile Sandstone Member) in the eastern part of the basin than in 

the rest of the basin, but it is notably absent at Galisteo 

Dam. Epidote is present in the Recapture and Westwater 

Canyon Members along the Nacimiento uplift and is 

most abundant in rocks at Senorita Canyon. Sphene was 

noted only at Galisteo Dam. Garnet is common 

throughout the Morrison except at the Durango Hospital 

section, where the Recapture and Brushy Basin 

Members contain no garnet. The Middle and Upper 

Jurassic Junction Creek Sandstone, which underlies the 

Recapture Member at Durango, does not contain garnet 

but does contain distinctive green hornblende not present 

in the overlying Morrison Formation.

Only a few Cretaceous samples were analyzed for 

heavy minerals. The marine Oak Canyon Member of the 

Dakota Sandstone is characterized by zircon and tourmaline; no garnet or apatite was observed. The underlying Encinal Canyon Member contains abundant zircon, 

tourmaline, and staurolite, but no garnet was noted in the 

samples analyzed.

Other Framework Minerals

The detrital mineralogy of sandstones in the Morrison Formation is also relatively consistent around the 

basin. Sandstones were classified according to Folk's 

scheme (1974). Classifications generally agree with those 

of previous workers (Cadigan, 1967; Schmitt, 1982; 

Steele, 1984a, b). Point counts of selected thin sections 

are shown in table 1.

Salt Wash Member

Sandstones of the Salt Wash Member are 

moderately well sorted, fine-grained subarkoses (fig. 2/4) 

composed of quartz, microcline, orthoclase, sodic plagioclase, sanidine(?), and various rock fragments. The most 

common rock fragments are chert, limestone (micrite), 

silicified limestone, and fine-grained varieties such as 

siltstone and shale. Igneous (plutonic) and metamorphic 

(quartzite) lithic fragments are rare. Most plagioclase is 

untwinned; some displays albite twinning or zoning.

Recapture Member

Sandstones of the Recapture Member are mainly 

subarkoses and lithic arkoses (fig. 25) that have 

compositions similar to those of the Westwater Canyon

Regional Diagenesis of Sandstones in the Morrison Formation, New Mexico and Utah H5
Table 1 . Petrographic data derived from point counts of selected thin sections

[Data in volume percent; based on 300 points per thin section. Kdec, Encinal Canyon Member of Dakota Sandstone; Kbc, Burro Canyon Formation; 

Jmu, Monison Formation (undifferentiated). Monison Formation: Jmj, Jackpile Sandstone Member; Jmb, Brushy Basin Member; Jmw, Westwater 

Canyon Member, Jmr, Recapture Member, Jmsw, Salt Wash Member. Petrographic components: frag, fragments; Muse, muscovite; min, minerals; 

Anhy, anhydrite; Zeol, zeolites; ovgr, overgrowths; Chal, chalcedony]

Strat 

unit

Sample 

number Qtz Kspar Flag

Rock 

frag BiotiteMusc

Heavy 

min CalciteAnhy Barite Zeol

Qtz Kspar Albite 

ovgr ovgr ovgr Chal ClayHematite Void

Senorita Canyon

Jmb 

Jmb 

Jmw 

Jmw 

Jmw

Sc-1 

Sc-2 

Sc-10 

Sc-11 

Sc-13

144 

121 

119 

134 

132

12 

6 

53 

19 

29

15 

15 

33 

10 

11

25 

56 

37 

29 

13

0 

0 

0 

0 

0

2 

0 

0 

0 

0

000 

000 

000 

0 106 0 

0 113 0

0 

0 

0 

0 

0

50 

21 

0 

0 

0

4 

33 

11 

0 

0

0 

0 

0 

0 

0

0 

0 

0 

0 

0

1 

1

7 

2 

1

35 

37 

1 

0 

1

0 

0 

0 

0 

0

12 

10 

39 

0 

0

Capulin Peak

Kbc 

Kbc 

Jmw 

Jmw 

Jmw 

Jmw 

Jmw

Cp-1 

Cp-2 

Cp-8 

Cp-9 

Cp-10 

Cp-11 

Cp-14

209 

179 

140 

194 

127 

144 

141

3 

24 

51 

25 

28 

43 

42

0 

0 

3 

12 

11 

8 

5

23 

19 

19 

25 

23 

20 

16

0 

0 

0 

0 

0 

0 

0

0 

0 

0 

0 

0 

0 

0

000 

000 

0 50 0 

0 10 0 

0 107 0 

060 

0 89 0

0 

0 

0 

0 

0 

0 

0

0 

0 

0 

0 

0 

0 

0

2 

0 

0 

0 

0 

0 

0

0 

0 

0 

0 

0 

0 

0

0 

0 

0 

0 

0 

0 

0

0 

44 

1 

0 

0 

0 

0

7 

12 

27 

12 

0 

19 

5

0 

0 

0 

0 

0 

0 

0

56 

16 

9 

22 

0 

57 

2

San Pablo Canyon

Jmj 

Jmw

Sp-2 

Sp-5

138 

138

29 

22

16 

21

40 

40

0 

0

0 

0

000 

000

0 

0

0 

4

75 

54

0 

3

0 

0

1 

2

0 

9

0 

0

1 

7

Durango Hospital

Jmr 

Jmr 

Jmr 

Jmr 

Jmr 

Jmb 

Jmb

Dh-2 

Dh-3 

Dh-4 

Dh-5 

Dh-6 

Dh-8 

Dh-11

112 

136 

138 

162 

151 

153 

79

11 

25 

11 

12 

10 

9 

14

7 

7 

14 

12 

13 

18 

16

9 

9 

6 

8 

9 

11 

1

0 

0 

0 

0 

0 

0 

0

0 

0 

0 

0 

0 

0 

0

0 11 25 

0 20 0 

080 

0 70 0 

0 47 0 

030 

030

6 

0 

0 

0 

0 

0 

0

0 

0 

0 

0 

0 

0 

1

117 

66 

84 

29 

55 

25 

0

0 

0 

0 

0 

0 

0 

0

0 

0 

0 

0 

0 

20 

0

0 

0 

3 

0 

3 

0 

169

2 

37 

35 

2 

12 

22 

2

0 

0 

0 

0 

0 

0 

0

0 

0 

1 

0 

0 

39 

0

White Mesa

Jmj 

Jmj 

Jmj 

Jmb 

Jmw 

Jmw 

Jmr

Wm-la 

Wm-1 

Wm-2 

Wm-3 

Wm-11 

Wm-12 

Wm-13

204 

145 

186 

112 

137 

103 

165

17 

30 

18 

25 

22 

25 

30

0 

23 

9 

17 

23 

33 

20

18 

35 

24 

48 

26 

37 

24

0 

0 

0 

0 

0 

0 

0

0 

0 

0 

0 

0 

0 

0

000 

000 

000 

0 76 0 

000 

000 

1 0 0

0 

0 

0 

0 

0 

0 

0

0 

0 

0 

0 

0 

99 

0

1 

10 

0 

0 

0 

0 

4

0 

1 

0 

0 

0 

0 

4

0 

0 

0 

0 

0 

0 

2

8 

56 

0 

0 

90 

0 

0

12 

0 

45 

20 

1 

0 

7

0 

0 

0 

1 

0 

1 

0

40 

0 

18 

1 

0 

0 

43

Laguna

Jmj 

Jmj 

Jmj 

Jmb 

Jmb 

Jmw 

Jmw 

Jmw 

Jmr

Lg^ 

Lg-5 

Lg-6 

Lg-7 

Lg-8 

Lg-9 

Lg-10 

Lg-11 

Lg-13

194 

160 

160 

185 

149 

151 

140 

143 

178

13 

26 

31 

18 

19 

16 

3 

11 

15

1 

5 

2 

11 

14 

6 

4 

1 

5

15 

23 

26 

18 

23 

21 

58 

19 

17

0 

0 

0 

0 

0 

0 

0 

0 

0

0 

0 

0 

0 

0 

0 

0 

0 

0

000 

000 

000 

000 

000 

1 36 0 

0 93 0 

0 126 0 

0 85 0

0 

0 

0 

0 

0 

0 

0 

0 

0

0 

0 

0 

0 

0 

57 

0 

0 

0

20 

0 

20 

17 

34 

6 

0 

0 

0

0 

0 

0 

0 

2 

0 

1 

0 

0

0 

0 

0 

0 

4 

0 

0 

0 

0

0 

0 

0 

0 

0 

0 

0 

0 

0

12 

28 

22 

25 

15 

4 

0 

0 

0

0 

4 

2 

0 

0 

0 

0 

0 

0

45 

54 

37 

26 

40 

2 

2 

0 

0

Encinal Canyon

Kdec 

Kdec 

Kdec 

Kdec

Ec-1 

Ec-2 

Ec-3 

Ec^

172 

177 

177 

207

12 

18 

21 

18

0 

0 

0 

0

21 

38 

17 

19

0 

0 

0 

0

0 

0 

0 

0

0 38 0 

0 1 0 

000 

000

0 

0 

0 

0

10 

0 

0 

0

29 

19 

43 

4

0 

0 

0 

0

0 

0 

0 

0

0 

0

1 

0

0 

26 

25 

49

0 

0 

0 

0

28 

21 

16 

3

Galisteo Dam

Kbc 

Jmb 

Jmu 

Jmu

Gd-1 

Gd-5 

Gd-6a 

Gd-8

207 

98 

134 

153

0 

56 

22 

18

0 

0 

27 

10

30 

19 

19 

15

0 

0 

0 

0

0 

0 

0 

0

000 

050 

0 87 0 

000

0 

0 

0 

0

Q

0 

0 

0

0 

0 

8 

59

0 

0 

0 

0

0 

0 

0 

0

0 

110 

0 

0

43 

12 

3 

11

0 

0 

0

22

3 

0 

0 

12

H6 Evolution of Sedimentary Basins San Juan Basin
Table!. Continued

Strat Sample 

unit number Qtz Kspar Plae

Rock 

frag BiotiteMusc

Heavy 

min CalciteAnhy Barite Zeol

Qtz

ovgr

Kspar Albite 

ovgr ovgr Chal Clay Hematite Void

Ghost Ranch

Kdec Gr-1 207

Kbc Gr^ 158

Jmb Gr-5 150

1

8

28

0

1

1

8

31

18

0

0

1

0

0

0

0

0

0

0

11

0

0

0

0

0

0

0

0

0

0

38

2

0

0

0

0

0

0

0

15

73

0

6

6

90

1

7

0

24

3

10

Dennison-Bunn

Kdoc Db-Kd 211

Jmj Db-1 187

Jmj Db-2 238

Jmb Db-5 174

Jmb Db-6 182

Jmw Db-8 191

Jmw Db-9 190

Jmw Db-10 147

Jmw Db-11 160

Jmr Db-12 127

Jmr Db-14 163

14

19

13

32

20

20

16

36

33

27

38

0

0

0

0

1

4

1

2

9

6

6

6

20

6

26

4

18

16

40

25

35

15

0

0

0

0

0

0

0

0

1

0

1

0

0

0

0

0

0

0

0

1

0

0

0

0

0

0

0

0

0

0

1

3

0

0

0

0

0

0

0

0

0

13

97

1

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

1

0

0

0

0

0

0

0

0

0

0

0

19

48

0

0

1

24

15

15

5

0

0

0

2

0

0

12

11

13

10

5

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

1

0

0

0

27

7

1

24

21

7

28

19

18

5

12

0

0

1

0

1

0

0

0

2

0

1

23

17

41

44

56

25

21

30

25

0

62

Beclabito dome

Jmsw Bd^ 207

Jmsw Bd-5 125

Jmsw Bd-6 181

Jmw Bd-8 167

Jmw Bd-11 206

Jmw Bd-13 187

Jmb Bd-18 113

Jmb Bd-22 158

12

9

10

12

13

17

3

2

3

1

10

15

18

14

11

8

13

14

17

20

15

10

12

23

0

0

0

0

0

0

0

0

0

0

0

0

0

1

0

0

1

0

0

0

0

0

0

0

24

29

17

1

0

6

1

2

2

0

0

0

0

0

0

0

1

0

0

0

0

0

0

0

0

0

0

0

6

6

58

1

16

27

20

47

3

14

8

0

0

0

0

1

0

0

0

0

0

0

0

9

2

0

0

1

0

0

0

0

0

0

88

0

7

3

3

23

11

25

0

3

0

0

0

0

0

0

0

0

13

22

42

5

26

20

0

1

Oak Springs

Jmsw Os-17 173

Jmsw Os-20 185

Jmsw Os-22 176

Jmr Os-25 169

Jmr Os-26 174

Jmr Os-30 137

Jmw Os-32 186

Jmw Os-34 218

Jmw Os-38 207

Jmw Os^2 194

Jmb Os-43 136

Jmb Os-50 152

21

26

25

29

26

29

19

15

23

12

15

13

4

7

14

13

14

15

9

4

4

7

9

12

10

22

22

6

13

23

29

18

23

15

32

29

0

0

0

0

0

0

0

0

0

0

0

1

0

0

0

0

0

1

0

0

0

0

0

0

2

1

0

3

1

1

0

0

0

0

0

0

39

2

6

0

0

74

11

2

2

0

57

0

1

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

1

0

25

0

0

0

0

0

0

0

0

0

3

0

0

1

12

0

1

0

0

12

4

26

29

0

0

0

0

0

0

0

0

0

0

2

2

0

0

0

0

0

0

0

0

0

0

0

0

1

0

0

0

0

0

0

0

0

0

0

0

79

0

19

0

3

6

11

13

4

18

6

11

13

0

0

0

0

0

1

0

0

0

0

4

0

25

37

45

77

56

8

33

27

19

30

0

0

Member. Micritic fragments were noted at the 

Dennison-Bunn and Oak Springs sections, and micritic 

oolites occur at Laguna. Most rock fragments are chert, 

other fine-grained sedimentary rocks such as shale and 

siltstone, volcanic (felsite), and metamorphic (stretched 

polycrystalline quartz, chlorite) types. The most notable 

aspect of framework mineralogy is the (rare) appearance 

of detrital plagioclase grains rimmed with potassium 

feldspar (anti-rapakivi texture) near the top of the 

Recapture Member. (See discussion on Westwater 

Canyon Member in next section.) Some plagioclase is 

untwinned; twinned varieties are dominantly albite and 

minor pericline and carlsbad. Low angles between twin 

planes and cleavage traces and the thinness of twin 

laminae suggest that plagioclase compositions range 

from albite to andesine.

Westwater Canyon Member

Sandstones of the Westwater Canyon Member are 

principally subarkoses and lithic arkoses (fig. 2C) 

characterized by large amounts of volcanic material 

including sanidine and volcanic rock fragments. The 

dominant fragments are fine-grained felsic volcanic rocks 

containing rare quartz or feldspar phenocrysts (fig. 3). 

Other rock fragments are (in decreasing order) polycrystalline quartz (metamorphic and plutonic igneous), 

chert, and chlorite. Rare plutonic fragments are very fine 

grained and have a "granitic" composition. Relict shard 

textures that were preserved by early smectite, calcite, or 

zeolite cements (fig. 4) indicate that volcanic ash 

composed much of the original matrix of these 

sandstones.

Regional Diagenesis of Sandstones in the Morrison Formation, New Mexico and Utah H7
QUARTZ

^OUARTZARENITE

SUBLITHARENITE

B QUARTZ

kQUAHTZARENITE

SUBLITHARENITE

FELDSPAR ROCK 

FRAGMENTS

FELDSPAR ROCK 

FRAGMENTS

QUARTZ

VQUAHTZARENITE

SUBLJTHAHENITE

FELDSPATHIC

LITHARENITE \LITHARENITE

QUARTZ

VQUARTZARENITE

SUBLITHARENITE

FELDSPAR ROCK 

FRAGMENTS

FELDSPAR ROCK 

FRAGMENTS

Figure 2. Classification of sandstones in the Morrison Formation. A, Salt Wash Member; B, Recapture Member; C, Westwater 

Canyon Member; D, Brushy Basin Member. Classification scheme after Folk (1974).

Plagioclase grains having potassium feldspar rims 

(for example, anti-rapakivi texture) occur throughout the 

Westwater Canyon Member (fig. 5/4) but are most 

common in medium-grained sandstones of the middle to 

upper part of the Westwater Canyon Member. The only 

place that these grains do not occur is in the Morrison 

Formation at Durango, where most feldspars have been 

completely albitized. Locally, the plagioclase core has 

been partly to totally dissolved such that only a rim of 

potassium feldspar enclosing a void remains (fig. 5B). 

Although some potassium feldspar-rimmed grains display overgrowths (fig. 5C), most potassium feldspar 

extends inward from the rims of plagioclase-cored grains 

without overgrowth development. Remnants of plagioclase cores are commonly albitized as indicated by high 

birefringence (fig. 5D) and electron microprobe analyses 

showing 99 mole percent albite (Hansley, 1986b). 

Associated mixed-layer I/S may or may not be illitized.

In the Grants uranium region, the presence of 

hollow potassic sanidine rims without plagioclase cores 

led Austin (1963,1980) to conclude that these distinctive 

potassium feldspar rims were skeletal detrital sanidine 

grains. Subsequently, these rims were reinterpreted to be 

metasomatic replacement of plagioclase by potassium 

feldspar and (or) authigenic potassium feldspar overgrowths that had precipitated on detrital plagioclase 

grains; rims became hollow when the plagioclase 

dissolved (Hansley, 1986b). This conclusion was based 

on detailed quantitative electron microprobe analyses, 

observations that the potassium feldspar rim was 

composed of many euhedral, micron-size authigenic 

potassium feldspar crystals having different optical 

orientations, and the presence in the sample suite of the 

total spectrum from a detrital plagioclase having a 

potassium feldspar rim to a potassium feldspar rim 

enclosing a void (Hansley, 1986b). The potassium feldH8 Evolution of Sedimentary Basins San Juan Basin
Figure 3. Photomicrograph showing felsic volcanic rock 

fragment in the Westwater Canyon Member at Oak Springs. 

Plane-polarized light; length of field, 0.2 mm.

spar rims did not exhibit cathodoluminescence and are 

almost pure potassic sanidine containing 16.27 percent 

K2O, 0.15 percent Na2O, and 0.003 percent CaO (Hansley, 19865); thus, they formed at low temperatures 

(Kastner, 1971). What was not considered in either 

Hansley's or Kastner's study was the possibility that the 

potassium feldspar rims are in part detrital.

Petrographic observations made during the present 

study suggest that plagioclase grains having anti-rapakivi 

textures were inherited from the source area: (1) 

development of potassium feldspar rims even at grain 

contacts, (2) potassium feldspar-rimmed plagioclase in 

sandstones cemented by early quartz and zeolite 

cements, and (3) lack of potassium feldspar overgrowth 

development on adjacent detrital potassium feldspars. 

The abrupt appearance of these distinctive grains in the 

upper part of the Recapture Member suggests that 

intermediate volcanic rocks were unroofed in the source 

area or, alternatively, that a new source area began to 

feed sediments into the San Juan basin. The appearance 

of these grains coincides with the appearance of other 

types of volcanic material such as sanidine and euhedral 

zircons. Thick sequences of Mesozoic rhyolitic and 

dacitic volcanic rocks in southern Arizona (Cooley and 

Davidson, 1963) were potential sources for these grains 

as well as for the potassic volcanic rock fragments. An 

orogenic belt to the west of the Colorado Plateau may 

have been another source of andesitic material. Plagioclase grains having anti-rapakivi textures are common in 

dacites and andesites (Williams and others, 1954).

Brushy Basin and Jackpile Sandstone Members

Most fluvial sandstones of the Brushy Basin and 

Jackpile Sandstone Members are subarkoses (fig. 2D) 

having compositions very similar to those of sandstones 

in the Westwater Canyon Member. Moench and Schlee 

(1961) reported 80-95 percent quartz and 1-19 percent 

feldspar in sandstones of the Jackpile, but Adams and 

others (1978) reported only 60-79 percent quartz and 13 

percent feldspar. Sandstones of the Brushy Basin and 

Jackpile Sandstone Members contain fewer rock 

fragments owing to their generally smaller grain size. In 

some Brushy Basin samples, early silica and zeolite 

cements apparently protected feldspars from dissolution 

resulting in a higher feldspar content than in other 

sandstones of the Morrison Formation. Potassium 

feldspar-rimmed plagioclases were noted only in coarser 

sandstones of the lower part of the Brushy Basin 

Member.

Cretaceous sandstones

Most sandstones of the Encinal Canyon Member of 

the Dakota Sandstone are sublitharenites; sandstones 

containing more feldspar are subarkoses. Major constituents are quartz, potassium feldspar, and silicic rock 

fragments such as chert and quartzite. Unlike sandstones 

of the Jackpile, sandstones of the Encinal Canyon contain 

no plagioclase. This absence may be the result of diagenesis, or it may reflect an original lack of plagioclase; too 

few samples were examined to be able to determine the 

reason for this absence.

DIAGENESIS

Heavy Minerals

The results of previous studies (Hansley, 1986a) 

and of the present study indicate that of the common 

heavy-mineral species in the Morrison, staurolite and 

garnet were the most affected by intrastratal solution, 

and zircon, tourmaline, and apatite show virtually no 

effects of diagenetic alteration. In the USGS cores, 

staurolite and garnet were deeply etched and possibly 

totally dissolved locally by diagenetic processes (Hansley, 

1986a). In the outcrop samples of this study, however, 

both are preserved (not etched). Deeply etched garnets 

(fig. 6) texturally similar to those in the USGS cores 

(Hansley, 1986a, 1987) were found only in the Salt Wash 

Member in the northwestern part of the basin (fig. 7) in 

sandstones stratigraphically equivalent to ore-bearing 

sandstones. At Durango Hospital, the lack of garnets 

may be due to intrastratal dissolution. In other outcrop 

sections, many garnets display minor surface etching, but

Regional Diagenesis of Sandstones in the Morrison Formation, New Mexico and Utah H9
Figure 4. Photomicrographs showing vitric textures preserved in analcimecemented sandstone of the Brushy Basin Member from Senorita Canyon (planepolarized light). A, Clinoptilolite-and smectite-rimmed shard (S) infilled with analcime 

(An). Analcime also fills voids. Q, detrital quartz grain. Length of field, 0.6 mm. B, 

Shard (S) infilled by analcime (S's are at each end of the shard); shard is outlined by 

micron-size yellow Clinoptilolite grains. Analcime-filled pores are reddish purple due to 

insertion of gypsum plate. Note grain (G) that has been replaced entirely by analcime 

and preponderance of floating grains. Length of field, 0.8 mm.

most are unetched. The Encinal Canyon Member does distinction between sandstones of the Encinal Canyon

not contain any garnets, probably as a result of attrition 

during transport or during the reworking of older 

sediments. Unfortunately, the sample number (13) is not 

large enough to ascertain whether this is truly a major

H10 Evolution of Sedimentary Basins San Juan Basin

and the Jackpile, which are otherwise very similar in 

composition.

Several varieties of authigenic heavy minerals 

including leucoxene, anatase, and pyrite are common in
Figure 5 (above and following page). Photomicrographs showing anti-rapakivi textures 

of detrital plagioclase grains. Note inward extension of potassium feldspar from detrital 

rims. Plagioclase grains are in sandstone of the Westwater Canyon Member at Beclabito 

dome. A, Partly dissolved plagioclase grain (P) rimmed by potassium feldspar overgrowth 

(stained yellow green by potassium cobaltinitrate). V, void (blue); Q, quartz. Planepolarized light; length of field, 0.4 mm. B, Two potassium feldspar overgrowth rims (yellow) 

that remain after dissolution of plagioclase cores: one rim contains plagioclase remnants 

(P) in the dissolution void (V); the other is filled in with calcite (Ca). Plane-polarized light; 

length of field, 0.7 mm. C, Partly dissolved, albitized detrital plagioclase grain with 

potassium feldspar rim (Ko, stained greenish yellow) near detrital potassium feldspar 

grains (K) that do not have overgrowths. Note quartz overgrowth (Qo) on detrital quartz 

grain (Q). Crossed polars; length of field, 0.8 mm.

Regional Diagenesis of Sandstones in the Morrison Formation, New Mexico and Utah H11
the Morrison Formation. Pyrite is locally abundant as a 

cement and as euhedral grains in the Westwater Canyon 

Member associated with organic matter and uranium 

enrichment in cores; it is rare in outcrop sections. Euhedral titanium dioxide minerals such as anatase are 

present in most heavy-mineral grain mounts and are 

thought to be alteration products of detrital irontitanium oxide minerals. Comprehensive studies of irontitanium oxide minerals and their alteration products in 

the Morrison Formation in the San Juan basin were 

conducted by Adams and others (1974) and Reynolds 

and others (1986).

Salt Wash Member

Lower sandstones of the Salt Wash Member are 

sporadically well cemented with quartz overgrowths and 

calcite. Upper sandstones are characterized by dissolution as evidenced by skeletal plagioclase, corroded 

quartz grains, moderately etched garnets, and remnant 

calcite cement. Small uranium-vanadium claims occur in 

the upper part of the Salt Wash near Oak Springs.

The clay mineralogy of the Salt Wash Member was 

determined only at Beclabito dome where the dominant 

authigenic clay mineral is chlorite. The 002 and 004 

peaks do not have an appreciably greater intensity than

General Sandstone Diagenesis

The distributions of authigenic minerals in all 

members reveal distinct trends across the basin 

highlighted by an increase in the complexity of alterations 

to the north. The regional distributions of major authigenic clay minerals in sandstones of the Recapture, 

Westwater Canyon, and Brushy Basin Members are 

shown in figure 8. Scanning electron microscope observations of (delicate) textures and of trends in expandability of mixed-layer clay minerals in the Morrison 

Formation indicate that these clays are authigenic. (See 

table 2 for point counts of authigenic phases in thin 

section.) Diagenesis has severely altered frameworkmineral assemblages around the basin, and the inferred 

paragenesis of major postdepositional alterations is 

shown in figure 9.

Figure 6. Scanning electron micrograph showing deeply 

etched garnet from the Morrison Formation. Length of field, 

0.30 mm. Sample is from USGS core 7.

H12 Evolution of Sedimentary Basins San Juan Basin
DEEPLY 

ETCHED

MODERATELY 

ETCHED

SLIGHTLY 

ETCHED

UNETCHED

(Ore in upper 

Salt Wash 

near Oak 

Springs)

Westwater Canyon 

Member

Figure 7. Occurrence of garnet textures in the Morrison Formation at Oak Springs. Note correlation 

between deeply etched garnets and uranium ore. One sandstone in the middle part of the Salt Wash 

Member containing unetched garnets is fine grained; the other sandstones are medium grained.

than the 001 and 003 peaks (fig. 10), indicating that the 

chlorite is not ferroan (Brown and Brindley, 1980). Both 

minor ordered I/S having 70-90 percent illite layers 

(0.7-0.91) and kaolinite are present in most samples.

Recapture Member

In general, sandstones in the Recapture Member 

are characterized by abundant (early) calcite cement, 

which commonly fills as much as 30 percent of the 

primary pores; little additional alteration is present 

except for minor anhydrite and barite cement. Where 

dissolution of calcite has taken place, skeletal plagioclase 

is present.

Diagenesis is most advanced in the eastern part of 

the study area at Galisteo Dam where the Recapture 

equivalent is present and at the Durango Hospital 

section. At Galisteo Dam, the lower sandstones in the 

Morrison (Recapture equivalent?) contain abundant 

quartz overgrowths and (or) minor albite overgrowths. 

Hematite cement coats the overgrowths. In the northernmost part of the study area at the Durango Hospital 

section, the Recapture, which lies directly under the 

Brushy Basin, contains abundant quartz and albite overgrowths (for example, even on detrital potassium feldspar) and calcite, and locally anhydrite, barite, and 

hematite cements. Intermittent sericitization of plagioclase grains suggests that sericitization occurred before 

deposition.

Clay-mineral trends in the Recapture parallel the 

trends of other authigenic minerals in that more 

advanced diagenetic phases occur in the northern part of 

the basin. At Beclabito dome, chlorite and ordered I/S 

( > 0.751) are dominant, and kaolinite is a minor phase. 

At the northernmost section of this study, Durango 

Hospital, the Recapture contains major I/S (>0.9I) and 

minor kaolinite. In contrast, along the eastern side of the 

basin from Capulin Peak south to White Mesa, clays are 

dominantly kaolinite and expandable I/S (<0.1I). In the 

USGS cores from the southern part of the basin, authigenic clays in the Recapture are also dominantly smectite 

(Whitney, 1986).

Westwater Canyon Member

Diagenesis in sandstones of the Westwater Canyon 

Member is characterized by a general upward increase in 

amount of diagenetic alteration in outcrops and in cores 

and by an increase in intensity of alteration to the north.

In the northwestern part of the basin at Beclabito 

dome, sandstones of the Westwater Canyon Member 

contain abundant quartz and albite overgrowths and 

minor potassium feldspar overgrowths. Tabular clinoptilolite crystals ( > 50 /*m) partly fill pores (fig. 11) and may 

be intergrown with authigenic quartz (fig. 12) in upper 

sandstones. Analcime occurs locally as a cement or as an 

infilling in skeletal plagioclase grains (fig. 13/4), and

Regional Diagenesis of Sandstones in the Morrison Formation, New Mexico and Utah H13
Table 2. Results of X-ray diffraction analyses of the <2-u,m fraction of selected sandstones 

[+++, 3,000 counts; ++, >2,000 counts; + >500<2,000 counts; Trace, <500 counts; leaders ( ) indicate none 

detected. <0.1I indicates <10 percent illite in illite/smectite]

Stratigraphic unit Sample No. Chlorite Illite/smectite Kaolinite

Beclabito Dome

Burro Canyon Formation

Dakota Sandstone

Brushy Basin*

Do.

Do.

Do.

Westwater Canyon*

Do.

Do.

Do.

Do.

Do.

Recapture*

Do.

Do.

Do.

Do.

Salt Wash1

Do.

Do.

Do.

Do.

Do.

81-Bd-37

81-Bd-36

81-Bd-35

81-Bd-34

81-Bd-33

81-Bd-32

81-Bd-31

81-Bd-30

81-Bd-29

81-Bd-28

81-Bd-26

81-Bd-25

81-Bd-24

81-Bd-23

81-Bd-22

81-Bd-21

81-Bd-20

81-Bd-19

81-Bd-18

81-Bd-16

81-Bd-15

81-Bd-17

81-Bd-14

Trace

Trace

+

+

Trace

Trace

+

Trace

Trace

+

++

+

++

Trace

-H-+

+

-H-+

+

-H-+

++

+++ (<0.3I)

Trace (0.81, >0.9I)

Trace (0.81, 0.651)

Trace

Trace (0.751, 0.51)

Trace (0.51 0.81)

Trace (0.71, <0.6I)

Trace (2 phases)

+ (0.71)

Trace

Trace

Trace

+ (>0.6I)

Trace (>0.6I)

Trace (0.71)

+ (0.71)

Trace

Trace (0.61, 0.71)

+ (>0.9I)

Trace (>0.9I)

+ (>0.8I)

Trace (>0.9I)

+ (>0.8I)

+

++

+

Trace

Trace

+

+

Trace

Trace

+

+

Trace

Trace

Trace

+

Trace

Trace

Trace

+

Sanostee Wash

Brushy Basin1

Do.

Do.

Westwater Canyon1

Do.

Do.

Do.

Do.

85-Sw-13

85-Sw-12

85-Sw-ll

83-Sw-10

83-Sw-7

83-Sw-5

83-Sw-2

83-Sw-l

+

+

+

+ (<0.1I)

++(<0.1I)

++(<0.1I)

+ (<0.1I)

++(<0.1I)

++ (<0.2I)

+ (<0.2I)

+ (<0.2I)

+++

+

Trace

Trace

Trace

+

+

+

Asaayi Lake

Westwater Canyon1

Do.

Do.

Do.

Do.

Do.

83-A1-12

83-A1-10

83-A1-9

83-A1-7

83-A1-3

83-A1-2

Trace

Trace

+

Trace

+

+

Trace

Trace

+ (<0.1I)

+ (<0.1I)

+ (<0.1I)

++

+++

++

++

+

++

Toadlena

Westwater Canyon1

Do.

Do.

Do.

Do.

83-Td-10

83-Td-8

83-Td-7

83-Td-3

83-Td-l

+

+

Trace

++

+

++ (smectite)

Trace (smectite)

Trace(smectite)

(smectite)

Trace (smectite)

++

+++

+++

+++

-H-+

Ghost Ranch f\ 

Encinal Canyon^

Morrison Formation

Do.

86-Gr-l

86-Gr-6

86-Gr-3

+++ (<0.1I)

++ (>0.9I)

-H-+

Trace

-HLaguna

Jackpile Sandstone1

Do.

Do.

86-Lg-6

86-Lg-4

86-Lg-5

Trace Trace (>0.9I)

(<0.1I)

-H-(<0.1I)

+++

-H-+

-H-+

H14 Evolution of Sedimentary Basins San Juan Basin
Table 2. Continued

Stratigraphic unit Sample No. Chlorite Illite/smectite Kaolinite

San Pablo Canyon

Westwater Canyon* 85-Sp-5 +

White Mesa

Jackpile Sandstone* 

Do. 

Do.

85-Wm-13 

85-Wm-2 

85-Wm-l

+ 

+

++(<0.1I) 

++(<0.1I) 

+ (<0.1I)

+

Encinal Canyon

Encinal Canyon^ 

Do. 

Do. 

Do.

86-EC-4 

86-Ec-3 

86-Ec-2 

86-Ec-l

Trace 

Trace 

Trace

Trace 

Trace 

Trace (0.81 ordered) 

++ (0.61 ordered)

+-H- 

+-H- 

+-HTrace

Galisteo Dam

Burro Canyon Formation 

Brushy Basin1 

Morrison Formation 

Do. 

Do. 

Recapture1

86-Gd-l 

86-Gd-2 

86-Gd-6 

86-Gd-4 

86-Gd-3 

86-Gd-7

+++ (>0.9I)

-HH-(>0.9I) 

Trace 

+ (>0.75I) 

-HH- (>0.9I) 

Trace

+ 

++

+

Senorita Canyon

Brushy Basin1 

Westwater Canyon1 

Do.

85-Sc-l 

85-Sc-7 

85-Sc-6

+++ (<0.1I) 

Trace 

++ (<0.1I)

Trace

Capulin Peak

Brushy Basin1 

Do. 

Westwater Canyon1 

Do. 

Do. 

Recapture 1 

Do. 

Do. 

Do.

85-Cp-6 

85-Cp-4 

85-Cp-10 

85-Cp-9 

85-Cp-8 

85-Cp-14 

85-Cp-13 

85-Cp-12 

85-Cp-ll

-HH- (<0.1I)

+

-HH- (<0.1I) 

-HH- (<0.1I) 

-HH-(<0.1I) 

++ (<0.1I) 

-HH- (<0.1I) 

-HH- (<0.1I) 

-HH- (<0.1I)

++ 

++ 

++ 

++ 

++ 

+-H- 

+++ 

+-H- 

+

Durango Hospital

Recapture1 

Do. 

Do.

85-Dh-6 

85-Dh-4 

85-Dh-3

+ (>0.9I)

+ (>0.9I)

+ (>0.9I)

...

Dennis on-Bunn

Burro Canyon Formation 

Do.

Brushy Basin1 

Westwater Canyon1 

Do. 

Do. 

Do. 

Recapture 1 

Do. 

Do.

85-Db-2 

85-Db-l 

85-Db-6 

85-Db-l 1 

85-Db-l 0 

85-Db-9 

85-Db-8

85-Db-l 6 

85-Db-15 

85-Db-14

+ 

Trace

Trace 

Trace

-HH-(<0.1I) 

-HH- (<0.1I)

+ (<0.1I) 

Trace 

++ (<0.1I)

-HH- (<0.1I) 

-HH-(<0.1I) 

-HH-(<0.1I)

+ 

+++ 

++

+

Trace 

Trace

1 Member of Morrison Formation.

y

'Member of Dakota Sandstone.

analcime-cemented sandstones are adjacent to analcimized tuff beds (fig. 135). Potassium feldspar grains are 

albitized, and complete albitization has formed 

"chessboard albite" grains (fig. 14) that look identical to 

those reported by Walker (1984); however, electron 

microprobe analyses were not made of the grains to

check for potassium feldspar remnants such as Walker 

found. Just to the south of Beclabito dome, at Oak 

Springs, sandstones contain abundant potassium feldspar 

overgrowths but fewer albite overgrowths than at Beclabito dome. Farther south, diagenetic alterations are less 

complex, except in the upper part of the Westwater

Regional Diagenesis of Sandstones in the Morrison Formation, New Mexico and Utah H15
37<

35<

109° 107°

SAN JUAN 

VOLCANIC FIELD

0 10 20 30 40 50 KILOMETERS I r~^ h r

OSan 

Ysidro

Albuquerque O

Figure 8 (above, facing, and following page). Regional distribution of major authigenic clay minerals in 

the Morrison Formation (pattern), San Juan basin. Sample localities indicated by triangles. K, kaolinite; 

I/S, illite/smectite, 0.71 means 70 percent illite in I/S; C/S, chlorite/smectite (>0.9C). A, Recapture 

Member; B, Westwater Canyon Member; C, Brushy Basin Member.

Canyon Member where quartz overgrowths, but no detrital plagioclase, are present. The lack of plagioclase is due 

to dissolution because, proceeding downward, skeletal 

plagioclase grains appear and finally near the base of the 

member entire grains are preserved. Kaolinite (as an 

alteration of plagioclase?) is a common pore-filling 

cement. Along the southern margin of the basin, 

sandstone grains commonly are coated with thick

hematite rims, which in turn are overlain by chlorite, 

quartz overgrowths, kaolinite, and calcite cement.

Additional potassium metasomatism of plagioclase 

grains having anti-rapakivi textures and formation of 

potassium feldspar overgrowths were caused by fluids 

that escaped into the sandstones from associated saline, 

alkaline beds. (See, for example, Oak Springs and 

Dennison-Bunn sections.) The precipitation during early

H16 Evolution of Sedimentary Basins San Juan Basin
SAN JUAN 

VOLCANIC FIELD

I/S (>0.9 I) 

K, C/S

I/S (>0.9 1,0.66 I); 

C/S; K

CHAMA 

BASIN

I/S(>0.8I,<0.1 I)

C/S

K

I/S (>0.6 

C/S,K

I/S (<0.1 I) / J'Oownpoint

35°-

diagenesis of potassium feldspar overgrowths is suggested by the occurrence of these overgrowths under 

poikilotopic calcite cement in which framework grains 

float (fig. 154). Potassium feldspar overgrowths are 

coated with early hematite cement in sandstones at 

the Dennison-Bunn section (fig. 155). Potassiummetasomatized plagioclase has also been found in Cenozoic saline, alkaline lake deposits (Hay, 1966).

Not all development of authigenic potassium feldspar was caused by fluids from the Brushy Basin Member 

because these distinctive grains occur throughout the 

basin in all depositional facies, not just in sandstones 

adjacent to or in the saline, alkaline lake facies.

Abundant authigenic potassium feldspar also occurs in 

and adjacent to ore zones. Where the paragenetic 

sequence could be determined in ore zones, a major 

episode of potassium metasomatism and overgrowth 

formation could be seen to have occurred after chlorite 

rim precipitation.

Trends in the distributions of authigenic clay 

minerals exhibit similar patterns. For instance, at Beclabito dome authigenic I/S ranges from 0.51 to 0.91, and 

near the top of the section rectorite (a perfectly ordered 

ISIS mixture) is indicated by the presence of a superlattice peak (fig. 16). Chlorite is the most abundant clay 

mineral at the top of the Westwater Canyon Member; it

Regional Diagenesis of Sandstones in the Morrison Formation, New Mexico and Utah H17
SAN JUAN 

VOLCANIC FIELD

10 20 30 40 50 KILOMETERS

35°-

overgrows expandable I/S and is intergrown with illitic 

I/S (X-ray diffractograms indicate the presence of two I/S 

phases). Similar authigenic clay mineralogy is present in 

the Chaco Canyon cores (fig. 17). Variable amounts of 

kaolinite and chlorite occur in most samples. Directly to 

the south of Beclabito dome, at Oak Springs, I/S 

(0.3-0.61) is dominant and is accompanied by major 

chlorite, minor kaolinite, and illitic I/S (>0.8I). 

Proceeding farther southward along the western side of 

the basin, expandable I/S (< 0.1-0.51) occurs at Sanostee 

Wash and more expandable I/S (<0.1I) at Asaayi Lake 

and Toadlena. Kaolinite is dominant at all three localities

but is more abundant to the south, whereas chlorite 

becomes less abundant southward.

On the eastern side of the basin, authigenic claymineral trends are also evident. At the easternmost 

locality, Galisteo Dam, clay minerals are mainly kaolinite 

and ordered I/S (0.661), whereas along the Nacimiento 

uplift authigenic clays are more expandable(<0.11). 

Major kaolinite occurs with minor expandable I/S 

(<0.1I) at Capulin Peak, and just to the south at San 

Pablo Canyon clays are predominantly kaolinite. In the 

southeastern part of the basin, at Laguna, I/S ( < 0.11) is 

also expandable.

H18 Evolution of Sedimentary Basins San Juan Basin
DIAGENETIC PHASE 

OR PROCESS

Pyrite

Smectite

Zeolites

Fe-Ti oxide alteration

Humate material

Uranium

Illite-smectite

Chlorite

Calcite

Ankerite

Potassium feldspar

Albite

Silica

Sulfate minerals

Plagioclase dissolution

Garnet etching

Kaolinite

Iron oxide minerals

PARAGENESIS

DEPOSITION PRESENT

Dissolution

Figure 9. Paragenesis of major authigenic phases and diagenetic events in the Morrison Formation.

GLYCOL

3.5 A 

(004)

10 15 20 

DEGREES 26

25 30 35 40

Figure 10. X-ray diffractogram of authigenic nonferroan 

chlorite in sandstone of the Westwater Canyon Member at 

Beclabito dome.

In the cores, diagenetic alterations also increase 

both vertically and to the north. In the northernmost and 

deepest core (Chaco Canyon core), analcime has 

replaced some plagioclase grains (also noted in other 

Chaco Canyon cores by Hicks, 1981); however, most

Figure 11. Scanning electron micrograph showing clinoptilolite crystals (Cl) in a sandstone pore of the Westwater 

Canyon Member at Beclabito dome. Length of field, 

0.16 mm.

plagioclase is albitized. Chlorite rosettes occur commonly in pores. Two stages of quartz overgrowths are 

present, and in the lower part of the Westwater Canyon 

Member anhydrite fills pores rimmed by quartz overgrowths. Hollow albite rims that look identical to hollow 

potassium feldspar rims occur in the cores farthest to the 

northeast. Associated plagioclase grains are albitized, 

and rare chlorite rims are under albite overgrowths.

Regional Diagenesis of Sandstones in the Morrison Formation, New Mexico and Utah H19
Figure 12. Scanning electron micrograph showing clinoptilolite (Cl) intergrown with authigenic idiomorphic quartz (Qa) 

in sandstone pore of the upper part of the Westwater Canyon 

Member at Beclabito dome. Length of field, 45 jum.

In the other cores, authigenic chalcedony cement, quartz 

overgrowths, and skeletal plagioclase grains are most 

common in the upper part of the Westwater Canyon 

Member.

Many diagenetic alterations are best developed in 

and adjacent to uranium-bearing sandstones; such is the 

case in the USGS cores (Hansley, 1986b). For example, 

in and adjacent to coffinite-bearing sandstone in core 

H-26-22, grains are cemented with pyrite, large quartz 

overgrowths, and potassium feldspar overgrowths. 

Abundant early authigenic potassium feldspar and 

hollow potassium feldspar rims are notable among 

alterations associated with uranium enrichment. As 

noted previously, the high amount of primary porosity 

(filled with calcite) in these sandstones provides evidence 

that potassium feldspar overgrowths formed early. A 

similar association of uranium enrichment and authigenic silica has been noted in the Morrison Formation in 

other areas of the Colorado Plateau (Goldhaber and 

others, 1987).

Brushy Basin Member

Regional diagenetic alteration patterns are well 

established in sandstones of the Brushy Basin Member. 

As in the other members, diagenetic alterations are more 

advanced from south to north; however, the severity of 

diagenesis varies widely from sandstone to sandstone 

within one outcrop. The Brushy Basin Member is characterized throughout the basin by mixed-layer clays, 

silica, and zeolite cements. As many as four distinct 

generations of chalcedony cement may be present 

(fig. 18), and the minimal diagenetic alteration of 

framework grains in these silica-cemented samples 

indicates that the silica cements were precipitated during 

early diagenesis.

In the northern part of the basin at the Durango 

Hospital and Piedra River outcrops, porosity is close to 

zero because the sandstones are cemented with quartz 

and albite overgrowths (fig. 19) and later formed porefilling chalcedony and calcite (fig. 20). Small potassium 

feldspar overgrowths are on some detrital potassium 

feldspar grains at Durango, but just to the east, at Piedra 

River, most potassium feldspar has been completely 

albitized. Most plagioclase grains are albitized, and those 

at the top of the member commonly are replaced by 

analcime. At both sections, hematite, dolomite, and 

anhydrite cements locally replaced earlier diagenetic 

minerals.

On the other hand, in the southeastern part of the 

basin at Laguna, plagioclase grains are skeletalized and 

albitized and large areas of framework grains are 

cemented by kaolinite (fig. 21). Remnants of calcite 

cement occur in pores, and albite and quartz overgrowths 

are abundant. To the east, at Galisteo Dam, chert and 

chalcedony cement are major diagenetic alterations, and 

micron-size, authigenic, twinned and doubly terminated 

albite crystals occur in pores.

On the eastern side of the basin, along the 

Nacimiento uplift, diagenetic alterations decrease from 

north to south. Sandstone grains are intensely zeolitized 

at Senorita Canyon: analcime has replaced feldspar and 

molds of shards are infilled with analcime. Grainrimming chert or smectite is superceded by pore-filling 

chalcedony or analcime. Plagioclase exhibits minor 

dissolution textures. On the other hand, sections to the 

south contain few zeolites. Sandstone matrix at San Pablo 

Canyon, just to the south of Senorita Canyon, is mostly 

smectitic and contains few zeolitized or albitized plagioclase grains and minor zeolite cement. Farther to the 

south, at White Mesa, sandstones are characterized by 

abundant smectite and by silica cement that has been 

superceded locally by calcite.

In the northwestern corner of the basin, at Beclabito dome and Oak Springs, some sandstone beds have a 

matrix comprised of authigenic smectite; other beds 

contain a wide variety of zeolites, authigenic feldspars, 

and illitic I/S. Many sandstone grains are coated with 

low-birefringent smectite rims surrounding voids filled 

with analcime, calcite, or chalcedony. Shardlike molds 

are filled with microcrystalline silica and rimmed by 

albite(?). Albitization of potassium feldspar has 

produced grains that appear to be "chessboard albite", 

and untwinned plagioclase locally is skeletalized and 

albitized. Albite overgrowths are more abundant at 

Beclabito dome than at Oak Springs, and potassium 

feldspar overgrowths were not noted at either section. At 

both sections, sandstones near the base of the member 

contain oxidized pyrite, barite cement, and small 

potassium feldspar overgrowths. An inverse correlation 

exists between the abundance of analcime and albite:

H20 Evolution of Sedimentary Basins San Juan Basin
Figure 13. Photomicrographs showing 

analcime-cemented sandstones in the 

Morrison Formation. A, Detrital plagioclase (P) replaced partly by analcime 

(An) in sandstone of the Westwater 

Canyon Member at Senorita Canyon. 

Note ghosts of twin lamellae. Rim on 

plagioclase is chert, which precipitated 

before analcime cement. Q, detrital 

quartz grain; V, void. Plane-polarized 

light; length of field, 0.7 mm. B, 

Analcime-cemented sandstone adjacent 

to analcimized tuff in outcrop at Beclabito 

dome.

Regional Diagenesis of Sandstones in the Morrison Formation, New Mexico and Utah H21
Figure 14. Photomicrograph showing "chessboard albite" 

grain (Al) in sandstone of the upper part of the Westwater 

Canyon Member at Beclabito dome. Note albite overgrowths 

(Ao) on grain; V, void. Crossed polars; length of field, 0.5 

mm.

sandstone beds either have abundant analcime and few 

quartz or feldspar overgrowths or have abundant albite 

and quartz overgrowths and little or no analcime. Grains 

in the highest sandstones at Oak Springs contain 

fractures filled with analcime.

Clay minerals exhibit similar regional alteration 

patterns: to the south, mixed-layer clay minerals are 

highly expandable, whereas to the north and northwest 

they become increasingly illitic. In the northwestern part 

of the basin, at Beclabito dome, major I/S ( > 0.81) and 

minor chlorite (fig. 22) occur in the lower half of the 

Brushy Basin Member; dominant chlorite and minor I/S 

(0.6-0.71) and kaolinite occur in the upper half. The I/S 

is intergrown with authigenic quartz (fig. 23) and appears 

to overgrow clinoptilolite (fig. 24). At the top of the 

Brushy Basin Member, well-defined rectorite (0.61) and 

minor kaolinite occur; no chlorite was observed. Just a 

few miles to the south at Oak Springs, sandstones of the 

lower Brushy Basin contain major chlorite and I/S 

(< 0.11) and minor kaolinite. In sandstones of the upper 

Brushy Basin at Oak Springs, I/S (0.3-0.61) is the major 

clay mineral. The clay mineralogy of the northernmost 

section at Durango is similar to that of Beclabito dome in 

that I/S (>0.9I) is dominant.

Along the Nacimiento uplift, major authigenic clay 

minerals are kaolinite and expandable I/S (<0.1I); 

farther to the east, at Galisteo Dam, major kaolinite and 

minor I/S (0.851; ISII stacking) are present. Chlorite is 

not a major phase in these eastern sections.

Figure 15. Photomicrographs showing potassium feldspar 

overgrowth on detrital potassium feldspar, Dennison-Bunn 

section. A, Early potassium feldspar overgrowth (Ko) on 

detrital potassium feldspar (K) in calcite-cemented (Ca) 

sandstone. Note floating grains and point contacts. Planepolarized light; length of field, 0.2 mm. B, Potassium feldspar 

overgrowth (Ko) coated with hematite cement (H) in 

sandstone. K, detrital potassium feldspar. Plane-polarized 

light; length of field, 0.05 mm.

H22 Evolution of Sedimentary Basins San Juan Basin
100

30 28 26 24 22 20 18 16 14 12 10 8 6 

DEGREES 26

Figure 16. X-ray diffractogram of well-ordered I/S (rectorite; ISIS ordering) showing development of a superlattice reflection at 27.3 A. Rectorite peaks are labeled in 

angstroms. Q, detrital quartz; K, kaolinite; C, calcite. 

Sample is from a sandstone near the top of the Westwater 

Canyon Member at Beclabito dome.

Figure 17. Scanning electron micrograph showing authigenic chlorite rosette (Ch) and intergrown illitic I/S filaments. 

Chlorite is resting on expandable I/S that has illitic overgrowths (I). Length of field, 35 ^m. Sample is from the 

Westwater Canyon Member in the Chaco Canyon core 

(CC-12, fig. 1).

Figure 18. Photomicrograph showing several generations 

of chalcedony cement in sandstone of the Brushy Basin 

Member at the Piedra River section. Crossed polars; length 

of field, 0.2 mm.

Figure 19. Photomicrograph showing authigenic porefilling albite overgrowths (Ao) on albitized plagioclase (P) in a 

sandstone of the Brushy Basin Member at the Durange 

Hospital section. V, void. Crossed polars; length of field, 0.1

mm.

Jackplle Sandstone Member

At the Laguna type section, the Jackpile Sandstone 

Member is characterized by abundant kaolinite cement 

especially at the top, few plagioclase grains, and quartz 

grains displaying feathery dissolution edges. Toward the 

base, plagioclase grains commonly are albitized. This

Regional Diagenesis of Sandstones in the Morrison Formation, New Mexico and Utah H23
Figure 20. Photomicrograph showing detrital twinned plagioclase grain (P) with albite overgrowth (Ao) and later 

chalcedony cement (Ch) in pore and calcite (Ca) in fracture. 

Sample is from the Brushy Basin Member at the Durango 

Hospital section. Crossed polars; length of field, 0.15 mm.

distribution of kaolinite at the top and albite at the 

bottom was also noted by Nash (1968) and Adams and 

others (1974). Quartz overgrowths are locally abundant, 

and minor hematite cement is present. In nearby Encinal 

Canyon, the Jackpile contains kaolinite and minor I/S 

( < 0.11) and chlorite. Farther to the northeast, at White 

Mesa, two expandable I/S phases are present (0.31 and 

<0.1I). In contrast to Laguna, plagioclase grains are 

unaltered at White Mesa and sandstones are more silicic 

because thin quartz overgrowths are superseded by 

chalcedony cement. Farther to the north at San Pablo 

Canyon, two generations of (early) chalcedony commonly fill primary pores, and framework grains are fresh 

except for thin (early) potassium feldspar overgrowths on 

potassium feldspar.

Cretaceous sandstones

The overlying fluvial Cretaceous units, the Burro 

Canyon Formation (Lower Cretaceous) and the Encinal 

Canyon Member of the Dakota Sandstone (Upper 

Cretaceous), are more siliceous than most sandstones of 

the Morrison. The Cretaceous sandstones are comprised 

of quartz, chert, quartzite, and little or no plagioclase. 

Kaolinite is the most abundant clay mineral and is 

accompanied by minor detrital(?) I/S (>0.9I). At

Figure 21. Photomicrograph showing pore-filling kaolinite 

cement (K) in sandstone at Laguna section. Crossed polars; 

length of field, 0.25 mm.

Figure 22. Scanning electron micrograph showing I/S 

(>0.8I) in sandstone of the Brushy Basin Member at Beclabito dome. Length of field, 45 fj,m.

Beclabito dome, the overlying fluvial Cretaceous 

sandstones (Burro Canyon? Formation) have an authigenic smectite matrix.

BURIAL HISTORY

A map of vitrinite reflectance values (R0 ) 

measured on coals in the Dakota Sandstone and Fruitland Formation in the San Juan basin (fig. 25) was 

constructed using data from many sources (Russell, 

1979; Dow, 1982; Keal, 1982; Rice, 1983; Jacobson and 

others, 1985; Bayliss and Schwarzer, 1987, 1988; N.H. 

Bostick, USGS, written commun., 1988; V.F. Nuccio, 

USGS, written commun., 1988). Values increase from 

southwest to northeast across the basin, and lower values

H24 Evolution of Sedimentary Basins San Juan Basin
Figure 23. Scanning electron micrograph showing intergrowth of illitic I/S (>0.8 I) needles and euhedral authigenic 

quartz (Q) in sandstone of the Brushy Basin Member at 

Beclabito dome. Length of field, 9 /on.

are generally in the southern part of the basin and along 

basin margins. As Rice (1983) pointed out, lines of equal 

vitrinite reflectance curve in the northwestern part of the 

basin such that they appear to follow the trend of the 

Hogback monocline. Maximum burial of the Morrison 

Formation was approximately 2,300-3,200 m in the 

deepest part of the basin (Weeks and Garrels, 1959, table 

1); however, maximum R0 values do not coincide with 

either maximum burial depths or the basin axis because 

the highest R0 values in the basin (>2.5) (Rice, 1983; 

V.F. Nuccio, written commun., 1988) in the basin are just 

south of Durango.

The Morrison Formation may have been subjected 

to a higher geothermal gradient in the northern San Juan 

basin because of proximity to the San Juan Volcanic 

Field (fig. 1). Bond (1984) constructed a Lopatin 

diagram of a well just south of Durango for which R0 

values are the highest in the basin that led him to 

conclude that maximum burial of the geologic section 

coincided with a "heat flash" caused by Oligocene 

intrusive activity related to the San Juan Volcanic Field. 

If this is the case, however, R0 values should be elevated 

in the Durango area, but R0 values of Cretaceous coals 

are relatively low in the area of the San Juan Volcanic 

Field (Nuccio and Johnson, 1988) and in outcrops (0.8) 

(Rice, 1983) in the Durango area just north of the 

anomalously high values. As an alternate explanation for 

the high R0 values just south of Durango, Clarkson and 

Reiter (1987) suggested that a buried heat source (such 

as a magma) just south of Durango accounts for the 

disparity in R0 values over a distance of less than 24 km. 

Along the Hogback monocline, the Morrison Formation 

was exposed from Late Cretaceous to early Tertiary time 

(Kelley and Clinton, 1960). Thus, shallow burial may 

explain the low levels of maturity along the monocline,

Figure 24. Scanning electron micrograph showing illitic I/S 

(>0.8I) clay (I) on clinoptilolite (Cl) in sandstone pore. 

Sample is from the Brushy Basin Member at Beclabito dome. 

Length of field, 25 /tm.

and the abnormally high maturity of the section in the 

area south of Durango may be explained by maximum 

heating having occurred during deepest burial. 

Coincidence of deepest burial with maximum heating of 

sediments results in rapid maturation and, in many cases, 

overwhelms the effects of time (Waples, 1985).

Regardless of the reason for the wide variation in 

R0 values, even at inferred minimum depths of burial 

(about 2,000 m) (Haun and Weimer, 1960) of the 

Morrison Formation along the Hogback monocline, 

temperatures would have been approximately 74-98 °C 

due to burial alone assuming a geothermal gradient of 

27-34 °C for the Rocky Mountain region (American 

Association of Petroleum Geologists and U.S. Geological Survey, 1975).

INTERPRETATION OF DIAGENESIS

Regional diagenetic alteration patterns in sandstones of the Morrison Formation indicate that controls 

on diagenetic reactions were basinwide and included 

chemical gradients in saline, alkaline Lake T'oo'dichi'; 

updip migration of warm ground water due to 

compaction; differential burial; and influx of meteoric 

water. In addition, diagenetic alterations were influenced 

locally by chemical reactions related to concentrations of 

organic matter (Hansley, 1986b).

The refractory nature of the present-day detrital 

heavy-mineral assemblage reflects the silicic composition 

of source rocks, long-distance transport from source 

areas, and postdepositional diagenetic processes. 

Upward increases in euhedral apatite and zircon abunRegional Diagenesis of Sandstones in the Morrison Formation, New Mexico and Utah H25
109° 107°

37'

35

O San 

Ysidro

AlbuquerqueO

Figure 25. Vitrinite reflectance values for coals in the Upper Cretaceous Dakota Sandstone and 

Fruitland Formation, San Juan basin. Location of outcrops of Morrison Formation (pattern) also shown. 

Sources of data: 1, V.F. Nuccio (USGS, written commun., 1988); 2, Rice (1983); 3, N.H. Bostick (USGS, 

written commun., 1988); 4, Russell (1979); 5, Dow (1982); 6, Keal (1982); 7, Jacobson and others (1985); 

8, Bayliss and Schwarzer (1987, 1988). Triangles indicate vitrinite reflectance value of less than 0.8 

percent; half-filled circles indicates vitrinite reflectance value of 0.8 percent; solid circles indicate vitrinite 

reflectance value of greater than 0.8 percent. Dashed line delineates approximate boundary between 

immature (to the south) and mature (in oil generation window) parts of the Morrison Formation.

dances reflect an increased influx of volcanic material 

towards the end of Morrison deposition (Hansley, 

1986a). The appearance of epidote, abundant staurolite, 

and sphene along the eastern margin of the basin

suggests that a local source area contributed igneous and 

metamorphic detritus to the basin. The abundance of 

potassium feldspar, perthitic feldspars, and igneous rock 

fragments in the area supports this conclusion.

H26 Evolution of Sedimentary Basins San Juan Basin
Influence of Lake T'oo'dichi'

The earliest authigenic phases, such as smectitic 

clay minerals, silica, zeolites, and feldspars, formed 

during alteration of the large amount of rhyolitic volcanic 

ash that was incorporated into the upper part of the 

Morrison Formation at the time of deposition. Relatively 

rapid dissolution of the labile ash created alkaline 

interstitial waters that quickly became saturated with 

respect to SiO2, Na+ , and K+ . Diagenesis in upper 

sandstones of the Morrison commonly paralleled diagenesis in adjacent tuffaceous, fine-grained lacustrine beds, 

as alkaline and saline pore waters from the lake 

sediments escaped into adjacent sands due to salinity 

gradients and compaction. For example, sandstone beds 

adjacent to analcimized tuff beds in the Brushy Basin 

Member at Beclabito dome are cemented with analcime 

(fig. 13); sandstones adjacent to clinoptilolite-rich, 

altered tuff beds in the Brushy Basin Member at Senorita 

Canyon are cemented with clinoptilolite; and sandstones 

cemented with large albite overgrowths are adjacent to 

albitized tuff beds at the Durango Hospital section.

Formation of smectite from ash dominated early 

diagenesis on the southern, western, and eastern margins 

of the basin where pore waters were fresher. To the 

north, where Lake T'oo'dichi' waters were more saline 

and alkaline, early diagenetic products of ash dissolution 

include zeolites and I/S. Near the center of the lake 

(Beclabito dome and Durango Hospital sections), high 

minus cement porosity values (30-34 percent in this 

study) indicate that primary analcime and albite formed 

from ash dissolution products. This conclusion was also 

reached by Fishman and others (1986). Analcime in the 

northernmost cores in the southern part of the basin 

(fig. 1) coincides with hypothetical extension of the 

zeolite facies across the basin between outcrops on the 

western and eastern margins of the basin as mapped by 

Turner-Peterson (1987).

Smectite becomes increasingly illitic in altered tuff 

beds toward the center of ancient Lake T'oo'dichi' 

(Turner-Peterson and others, 1987). Data presented in 

this report show that smectite in associated sandstones 

also becomes illitic towards the center of the basin. 

Turner-Peterson and others (1987) concluded that early 

precipitation of I/S in the Brushy Basin Member was 

promoted by the extreme alkalinity and salinity of pore 

waters; thus, the most illitic clays, which are accompanied 

by authigenic analcime and albite, formed where 

interstitial waters were the most saline. This interpretation is in contrast to the classical "Gulf Coast" 

theory in which illitization is considered to be primarily a 

thermally driven process (Perry and Hower, 1970). In the 

current study of sandstone diagenesis in the Morrison, 

illitization of analcime cement where it is in contact with 

smectitic grain rims (fig. 26) indicates that I/S formed

somewhat later than analcime; however, the parallelism 

between diagenesis in the tuffs and adjacent sandstones 

strongly suggests that I/S formed during early diagenesis.

Ordering of I/S may have occurred slightly later 

during burial diagenesis. Recent studies have shown that 

ordering occurs at about 60 percent illite when no 

smectite layers are bounded by other smectite particles 

(Nadeau and others, 1985) and involves dissolution and 

reprecipitation of new clay phases (Whitney and 

Northrup, 1988). The coexistence of two authigenic 

mixed-layer I/S phases exhibiting different degrees of 

ordering in many sandstones of the Morrison Formation 

is explained by this model.

Preservation of shard textures within analcime 

cement in sandstones of the Westwater Canyon Member 

at Senorita Canyon indicates that analcime precipitated 

during early diagenesis and may have directly replaced 

volcanic glass. Theoretical chemical activity diagrams 

indicate that zeolites may form directly from glass if silica 

activity is high enough (fig. 27). Additional evidence that 

analcime formed early is the presence of analcimecemented rip-up clasts in fluvial sandstone of the Westwater Canyon Member at Beclabito dome. Although 

direct formation of analcime from glass may have 

occurred in some sandstones, the presence of thin smectite or clinoptilolite rims on the shards and clinoptilolite 

inclusions within the shards suggest that analcime 

replaced a precursor zeolite (in this case, clinoptilolite). 

Analcime-replaced shards in the Eocene and Oligocene 

Vieja Group in Texas were also interpreted to indicate 

that the analcime replaced clinoptilolite (Walton, 1975). 

This interpretation corroborates Sheppard's (1971) 

conclusion that analcime in sedimentary rocks does not 

form directly from glass. Analcime-filled stylolites and 

fractures in sandstones at Beclabito dome and at

Figure 26. Scanning electron micrograph showing illitic I/S 

that has formed from the margins of analcime cement (An) in 

a sandstone pore. Length of field, 25 pm. Sample is from the 

Westwater Canyon Member at Beclabito dome.

Regional Diagenesis of Sandstones in the Morrison Formation, New Mexico and Utah H27
Senorita Canyon formed somewhat later than the porefilling analcime because major deformation in the region 

occurred during the Laramide orogeny (Late Cretaceous 

to early Tertiary) when the Morrison was buried under 

several hundred meters of Upper Cretaceous sediments. 

The precipitation of at least some authigenic 

potassium feldspar in sandstones of the upper part of the 

Westwater Canyon Member and in the Brushy Basin 

Member is related to chemical gradients in Lake 

T'oo'dichi' (Hansley, 1984; Turner-Peterson, 1987). 

Where potassium feldspar overgrowths are best developed in outcrop sandstones (such as Oak Springs and 

Dennison-Bunn), sandstones lie in or under the zeolite 

and potassium feldspar altered tuff facies of the Brushy 

Basin Member. Potassic volcanic rock fragments and 

evolved pore fluids provided intrinsic sources of 

potassium for the overgrowths, and potassium may have 

also been supplied by fluids expelled from tuff beds. A 

potential problem with this interpretation is that the 

mixed-layer clay minerals in some sandstones that 

contain potassium feldspar overgrowths, notably those 

along the Nacimiento uplift, are expandable. If 

potassium-rich fluids had been in the sandstones, why 

wasn't smectite illitized? One possibility is that high silica 

activities (>10~2'5 ) in these fluids (confirmed by 

widespread quartz overgrowths and chalcedony cement) 

inhibited the formation of illite (Garrels, 1984; Eslinger 

and Pevear, 1988). Smectite can coexist with potassium

LOG

-6 -5 -4

LOGa Si0 2

Figure 27. Ion activity diagram showing the relations 

between some common authigenic minerals in the Morrison Formation Note that illite and montmorillonite are 

treated as two separate phases. Modified from Garrels 

(1984).

feldspar at high silica activities (fig. 27). Potassium 

feldspar forms early as an alteration of tuff beds in 

Cenozoic saline, alkaline lake deposits; potassium feldspar, which formed indirectly (through intermediate 

phases) from volcanic ash, has been dated as 60,000- 

100,000 years old (Hay, 1966). Because they formed in a 

similar closed basin setting, tuff beds in the Brushy Basin 

Member that have altered mostly to potassium feldspar 

and large potassium feldspar overgrowths in associated 

sandstones are inferred also to have formed during early 

diagenesis. Potassium feldspar overgrowths that formed 

later may have precipitated from K+ -rich magmatic 

waters because intrusions in the northwestern San Juan 

basin are minettes, which are enriched in potassium 

feldspar (Shoemaker, 1956).

Fishman and others (1986) concluded that albitization of tuffs and sandstones in the Brushy Basin 

Member was also a very early diagenetic process that 

took place in the most saline, alkaline facies of Lake 

T'oo'dichi'. Their conclusion is also partly on the 

presence of high minus-cement porosity values wherever 

albite occurs and partly on the cogenetic habit of the 

albite-quartz-chalcedony assemblage. Early albitization 

also took place in upper sandstones of the Westwater 

Canyon Member at Beclabito dome and in upper 

sandstones of the Recapture Member at the Durango 

Hospital section; minus-cement porosity values of from 

30 to 34 percent in sandstones from these sections 

indicate that the albite and quartz overgrowths precipitated before compaction. Elevated Na+ activity and 

low water activity in the central part of the lake facilitated 

early albitization. Analcimic textures of albitized tuff 

beds (Turner-Peterson, 1987) indicate that albite locally 

may have been an early product of analcime dissolution, 

and analcime remnants occur in calcite cement in albitized sandstones at Piedra River. Albite replaces analcime by dehydration and silicification: analcime + silica 

= albite + water (Liou, 1971). Elevated temperatures 

and (or) high salinity cause this reaction to proceed to the 

right, but in the Morrison Formation high salinity was the 

main factor in causing albite to form because temperatures were close to ambient during early diagenesis. 

Likewise in the Eocene Green River Formation, albite is 

inferred to have replaced analcime at the relatively low 

temperatures of 50-75 °C because of the high salinities of 

the pore waters (Hay, 1966).

To estimate the temperature at which authigenic 

albite in the Morrison Formation formed, albite overgrowths in sandstones of the Brushy Basin Member from 

the Durango Hospital section were examined for fluid 

inclusions, but no vapor phases were found in any of the 

tiny (<10 p,m) inclusions. If the albite were formed 

during later diagenesis at elevated temperature, the lack 

of vapor bubbles could be due to the difficulty in nucleation of vapor bubbles in small inclusions (Roedder,

H28 Evolution of Sedimentary Basins San Juan Basin
1981). Burial history reconstruction (Bond, 1984) shows 

that maximum temperatures in the Durango area were 

about 75 °C, high enough to form vapor phases in 

inclusions. However, textures indicate that the albite 

formed early, either as a direct precipitate from saline 

pore waters or as an early replacement of analcime.

The assemblage of authigenic chlorite, potassium 

feldspar, I/S, and quartz (overgrowths and idiomorphic 

crystals) commonly observed in sandstones of the Morrison Formation in this study and in the Morrison in the 

Grants uranium region (Hansley, 1986b) is interpreted 

to have formed during burial diagenesis. Chlorite 

precipitation may be explained by the release of Mg2+ , 

Fe3 + , and silica during dissolution of random I/S and 

reprecipitation of ordered I/S. This process provides an 

explanation for the lack of significant chlorite in samples 

containing expandable I/S on the western, southern, and 

eastern margins of the basin.

Alterations Related to Meteoric Water

Thick (about 5 jim) hematite rims underlying chlorite rims and quartz overgrowths on detrital sand grains 

in the Morrison Formation along the southern margin of 

the San Juan basin indicate that early oxidation occurred 

before the formation of chlorite and quartz overgrowths 

(Hansley, 1986b). Dissolution of plagioclase grains and 

precipitation of kaolinite in the uppermost part of the 

Morrison Formation along the western and southern 

margins of the basin may have been caused by downward 

percolation of acidic, oxidizing meteoric water during 

Early Cretaceous erosion. The occurrence of increasingly 

higher percentages of detrital plagioclase in the Morrison 

Formation proceeding downward from the unconformity 

indicates that the dissolution episode was related to 

exposure and erosion either during the time represented 

by the unconformity or slightly later due to corrosive 

solutions emanating from the Dakota Sandstone. 

Dissolution of plagioclase and precipitation of kaolinite 

may have been caused by acidic solutions derived from 

overlying Dakota coals and carbonaceous shales (Schlee 

and Moench, 1961; Granger, 1962). This idea remains 

speculative, however, because neither Flesch (1975) nor 

Schlee (1957) found an obvious relationship between 

kaolinite in the Jackpile and the presence of coal in the 

overlying Dakota Sandstone. Some later oxidation and 

additional(?) plagioclase dissolution may have occurred 

during the period late Tertiary to the present (Saucier, 

1980), as evidenced by limonite and (or) hematite 

cement on potassium feldspar and oxidized pyrite in 

outcrops and shallow sandstones.

Comparison of Diagenetic Interpretations

Previous diagenetic studies of the Morrison Formation in the southern part of the San Juan basin identified 

early alteration patterns that were attributed to chemical 

gradients in Lake T'oo'dichi' (Hansley, 1984, 1986b). 

Continuation of the same patterns northward into the 

central and northern parts of the basin supports this 

conclusion. In previous studies (Whitney and Northrop, 

1987), updip movement of warm, deep-basin fluids 

through the Westwater Canyon Member was inferred by 

stable isotope data that indicated that cogenetic ordered 

I/S and chlorite precipitated from interstitital fluids at 

temperatures exceeding 100 °C. Deeply etched garnets 

occur in precisely the same zone as the isotopically 

distinct clay minerals (Whitney and others, 1986; Hansley, 1987). In laboratory experiments, warm (>75 °C), 

oxalic acid-bearing solutions produced systematic etch 

features on garnets identical to those on naturally etched 

garnets (Hansley, 1987). Because the Morrison 

Formation contains garnets throughout the Colorado 

Plateau (Cadigan, 1967), the present lack of garnets at 

Durango was apparently caused by complete dissolution 

of detrital garnets. Burial history data indicate that 

temperatures during diagenesis in the Durango area 

were as high as 75 °C; thus, warm pore waters were 

available to facilitate dissolution of the garnets. Whether 

organic acids are necessary for dissolution or were 

involved in dissolution of garnets at Durango is unknown.

A diagenetic mineral assemblage (idiomorphic 

quartz, adularia, chlorite, ordered I/S) similar to that in 

the zone of elevated temperature noted above occurs in 

the upper Salt Wash Member at Beclabito dome and Oak 

Springs. Oligocene intrusive bodies in the Four Corners 

area apparently did not elevate formation temperatures 

because vitrinite reflectance values of Cretaceous rocks 

just downdip from Morrison Formation outcrops on the 

western margin of the basin are relatively low. 

Furthermore, diagenetic maturity varies dramatically in a 

sequence of tuff beds at Beclabito dome: some beds are 

zeolitized, some have been altered to potassium feldspar, 

and others have remained pure smectite (TurnerPeterson, 1987). This diagenetic assemblage may have 

formed as a result of action by organic acids generated 

from local concentrations of organic matter within the 

Salt Wash Member, movement of warm (organic acidbearing?) fluids through the Salt Wash Member, or 

unknown causes.

In earlier studies of core material in the southern 

part of the basin, amorphous, epigenetic interstitial 

organic matter enriched in uranium and other metals was 

found to be locally abundant (Leventhal, 1980; Hansley, 

1986b). During early diagenesis, the organic matter was 

solubilized by alkaline fluids and was then precipitated in

Regional Diagenesis of Sandstones in the Morrison Formation, New Mexico and Utah H29
sandstone pores. In subsequent diagenetic reactions, 

bacterially mediated sulfate reduction resulted in 

precipitation of pyrite and carbonates, as seen in most of 

the cores from the southern part of the basin. Samples in 

the present study, however, contain only trace amounts of 

amorphous organic matter, although evidence of 

reactions involving organic matter (coffinite, pyrite) 

exists in several of the cores that were examined. By 

analogy, detrital organic matter in the Salt Wash 

Member also influenced diagenetic reactions in that 

hydrolysis of volcanic ash created alkaline waters 

favorable for leaching soluble organic matter from 

decaying plant material. In the vicinity of organic matter 

(detrital or epigenetic), therefore, chemical reactions 

were facilitated by the release of soluble organic 

complexes during low-temperature bacterial metabolism. Soluble organic complexes have been shown in 

experiments (Surdam and Crossey, 1985) and in natural 

settings (Bennett and Siegel, 1987) to dramatically 

increase the solubility of chemical species, including 

silica. This process may explain the abundance of 

potassium feldspar and quartz overgrowths in the vicinity 

of ore zones.

The alteration patterns in the Jackpile Sandstone 

Member reflect basinwide depositional and diagenetic 

patterns. Albitization at the base of the Jackpile is a 

result of saline fluids migrating into basal sandstones 

from the playa facies of the Brushy Basin Member, and 

kaolinization of the upper part of the Jackpile was caused 

by fluids migrating downward during exposure in the 

Early Cretaceous Epoch. Adams and others (1978) 

reached the same conclusions during their study of 

diagenesis in the Jackpile Sandstone Member.

CHEMISTRY

During early diagenesis of the Morrison 

Formation, interstitial waters were moderately alkaline 

(pH about 8.5) due to the dissolution of volcanic ash. 

Formation of early authigenic phases, such as smectite 

and zeolites, was controlled by several variables including 

the activities of Na+ , K+ , H4 SiO4 °, and A1(OH)4 -, pH, 

and total ionic strength of pore waters. Of these, silicic 

acid (H4SiO4 °) and aluminate ion (A1OH4~) activities 

are the most important because most early diagenetic 

phases were aluminosilicates. Aluminate ion solubility 

increases dramatically above a pH of 7, whereas silica 

solubility does not increase markedly until a pH of 9.9 at 

25 °C (Drever, 1982; Donahoe and Liou, 1985). The 

solubility of aluminum, therefore, increases more rapidly 

than the solubility of silica between pH 7 and 9.9. Above 

pH 9.9, silica solubility becomes a dominant control of 

the Si/Al ratios in aluminosilicate minerals. Aluminate 

activity thus was probably a major factor in diagenetic 

reactions because pH values were almost certainly below

9.9 except in the most saline, alkaline conditions near the 

center of the lake. In addition, the presence of soluble 

organic matter in early interstitial waters further 

increased the solubility of aluminum because of the 

excellent complexing ability of the organics. Aluminate 

ion concentration is considered to be a constant in many 

published chemical reactions, a practice that seems 

unrealistic considering the normal pH range of most 

sediments.

The solubility of silica in alkaline solutions is 

controlled by the solubility of amorphous silica rather 

than by that of quartz (Garrels and Christ, 1965). It 

follows, therefore, that in Cenozoic saline/alkaline lacustrine deposits zeolites and glass are associated with 

amorphous silica (such as opal) (Hay, 1966). Opal has 

not been found in Morrison sediments because in the 

time elapsed since early diagenesis it would have 

converted to a more stable, crystalline form of silica such 

as quartz or chert. The high silica activity in tuffaceous 

sediments may explain the occurrence of zeolites in some 

samples without significant clay mineral development 

(Boles and Surdam, 1979).

A puzzling aspect of diagenetic assemblages in 

sandstones of the upper Morrison sandstones and 

associated fine-grained beds, in view of the alkaline 

chemistry, is the paucity of early carbonate minerals. 

Cenozoic saline, alkaline lakes and playa-lake facies of 

the Eocene Green River Formation have abundant 

authigenic carbonate and bicarbonate minerals (Milton 

and Eugster, 1959). The lack of preserved authigenic 

carbonate minerals in the Morrison Formation is partly 

due to replacement by later diagenetic phases because in 

many samples remnants of carbonate cement are present

Organic complexes are very soluble in moderately 

alkaline water. Therefore, during early diagenesis, when 

pH values were buffered to moderately basic values by 

the dissolution of volcanic ash, the concentration of 

organics in solution was locally high, particularly where 

detrital organic matter was present. During later diagenesis, locally elevated temperatures (>75 °C) again 

favored the solubility of organic complexes, and the 

resultant acid pore waters (combined with the excellent 

complexing ability of organic acid anions) caused 

dissolution of framework grains and cements.

KINETIC MODEL FOR DIAGENESIS

Dibble and Tiller (1981) stated that "chemical 

equilibrium***is least likely to be attained in sedimentary rocks containing volcanic glass." Thus, diagenetic reactions in the Morrison during early diagenesis 

were mostly kinetically, rather than equilibrium 

controlled (Senderov, 1983) because at the time of 

deposition the Morrison Formation contained an 

abundance of unstable volcanic material. Dissolution of

H30 Evolution of Sedimentary Basins San Juan Basin
volcanic glass was rapid relative to formation of alteration products; therefore, as glass dissolved, interstitial 

solutions quickly became supersaturated with respect to 

many mineral phases. The phases that precipitated under 

conditions of supersaturation were metastable, fastgrowing phases such as smectite, aluminosilicate gels, 

and disordered zeolites and feldspars (Dibble and Tiller, 

1981). The thermodynamic drive of water-rock systems is 

toward the lowest free energy state (Garrels and Christ, 

1965). The total free energy of a system is decreased 

most rapidly by precipitation of fast-growing, metastable 

phases rather than by formation of slow-growing, stable 

phases (Dibble and Tiller, 1981). Ordered feldspars and 

I/S formed somewhat later than earliest diagenesis 

through transformation of earlier phases in conditions of 

lower supersaturation rather than directly from dissolution of glass. These phases formed earlier, however, 

than in normal marine sediments because of the favorable chemistry of interstitial waters in the upper part of 

the Morrison Formation.

SUMMARY AND CONCLUSIONS

Widespread preservation of vitric textures in 

sandstone matrix and of early zeolite cements in primary 

pores for 100 million years or more is remarkable. The 

dissolution of volcanic ash coupled with chemical 

gradients in Lake T'oo'dichi' were the major variables 

that determined the composition of authigenic mineral 

phases in the upper members of the Morrison Formation 

(descending: Jackpile Sandstone, Brushy Basin, and 

upper part of the Westwater Canyon Members) at a 

particular locality in the basin during the first stages of 

diagenesis. Early diagenetic alterations in the upper part 

of the Morrison Formation in the San Juan basin are well 

preserved in distinct, concentric patterns that were 

relatively unaffected by subsequent burial history as 

indicated by the lack of correlation between vitrinite 

reflectance values and diagenetic patterns. Later diagenesis in sandstones was influenced by elevated thermal 

gradients caused by burial diagenesis, by updip migration 

of warm waters from deeper parts of the basin, by local 

influence of organic acids derived from the diagenesis of 

organic matter, and by downdip migration of meteoric 

waters due to uplift of recharge areas. Sandstones in the 

lower members, the Salt Wash and Recapture, contain 

predominantly early calcite and (or) anhydrite cements 

and exhibit little grain alteration except where uranium 

enrichment is found.

In the closed-basin environment of high ionic 

strength and alkalinity that existed near the center of 

Lake T'oo'dichi' in late Morrison time, the interval 

between formation of metastable and stable phases was 

foreshortened. The first authigenic phases to form were 

those that grew the fastest at ambient temperatures and

pressures (such as disordered zeolites, feldspars, and clay 

minerals), but these phases were not the most stable 

thermodynamically. In some sandstones near the center 

of the lake, disordered(?) albite and quartz overgrowths 

apparently precipitated directly from solution. In other 

sandstones, dissolution of metastable phases such as 

analcime and random I/S was followed by precipitation of 

stable phases such as albite and ordered I/S in response 

to favorable chemical conditions well before maximum 

burial and diagenetic temperatures were achieved.

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in Geology 22, p. 315-330.

___ 1988, Experimental investigation of the smectite to illite 

reaction; dual reaction mechanisms and oxygen-iostope 

systematics: American Mineralogist, v. 73, p. 77-90.

Whitney, C.G., and Northrop, H.R, 1987, Diagenesis and fluid 

flow in the San Juan basin, New Mexico; regional 

zonation in the mineralogy and stable isotope composition of clay minerals in sandstone: American Journal 

of Science, v. 287, p. 353-382.

Whitney, C.G., Northrop, H.R., and Hansley, P.L., 1986, The 

use of mineral alteration patterns and stable isotope 

compositions to infer a paleohydrologic regime, in 

Carter, L.M.H., ed., USGS Research on Energy 

Resources 1986, Program and Abstracts: U.S. 

Geological Survey Circular 974, p. 73-74.

Williams, Howell, Turner, F.J., and Gilbert, C.M., 1954, 

Petrography: San Francisco, W.H. Freeman, 406 p.

Woodward, L.A., and Schumacher, O.L., 1973, Morrison 

Formation of the southeastern San Juan basin, New 

Mexico: New Mexico State Bureau of Mines and Mineral 

Resources Circular 129, 7 p.

Regional Diagenesis of Sandstones in the Morrison Formation, New Mexico and Utah H35

L.S. GOVERNMENT PRINTING OFFICE: 1990-774-283



SELECTED SERIES OF U.S. GEOLOGICAL SURVEY PUBLICATIONS

Periodicals

Earthquakes & Volcanoes (issued bimonthly). 

Preliminary Determination of Epicenters (issued monthly).

Technical Books and Reports

Professional Papers are mainly comprehensive scientific reports of 

wide and lasting interest and importance to professional scientists and engineers. Included are reports on the results of resource studies and of 

topographic, hydrologic, and geologic investigations. They also include 

collections of related papers addressing different aspects of a single scientific topic.

Bulletins contain significant data and interpretations that are of lasting scientific interest but are generally more limited in scope or 

geographic coverage than Professional Papers. They include the results 

of resource studies and of geologic and topographic investigations; as well 

as collections of short papers related to a specific topic.

Water-Supply Papers are comprehensive reports that present significant interpretive results of hydrologic investigations of wide interest 

to professional geologists, hydrologists, and engineers. The series covers 

investigations in all phases of hydrology, including hydrogeology, 

availability of water, quality of water, and use of water.

Circulars present administrative information or important scientific 

information of wide popular interest in a format designed for distribution 

at no cost to the public. Information is usually of short-term interest.

Water-Resources Investigations Reports are papers of an interpretive nature made available to the public outside the formal USGS publications series. Copies are reproduced on request unlike formal USGS 

publications, and they are also available for public inspection at 

depositories indicated in USGS catalogs.

Open-File Reports include unpublished manuscript reports, maps, 

and other material that are made available for public consultation at 

depositories. They are a nonpermanent form of publication that may be 

cited in other publications as sources of information.

Maps

Geologic Quadrangle Maps are multicolor geologic maps on 

topographic bases in 71/2- or 15 -minute quadrangle formats (scales mainly 1:24,000 or 1:62,500) showing bedrock, surficial, or engineering geology. Maps generally include brief texts; some maps include structure 

and columnar sections only.

Geophysical Investigations Maps are on topographic or planimetric 

bases at various scales; they show results of surveys using geophysical 

techniques, such as gravity, magnetic, seismic, or radioactivity, which 

reflect subsurface structures that are of economic or geologic significance. 

Many maps include correlations with the geology.

Miscellaneous Investigations Series Maps are on planimetric or 

topographic bases of regular and irregular areas at various scales; they 

present a wide variety of format and subject matter. The series also includes 71/2-minute quadrangle photogeologic maps on planimetric bases 

which show geology as interpreted from aerial photographs. Series also 

includes maps of Mars and the Moon.

Coal Investigations Maps are geologic maps on topographic or 

planimetric bases at various scales showing bedrock or surficial geology, stratigraphy, and structural relations in certain coal-resource areas.

Oil and Gas Investigations Charts show stratigraphic information 

for certain oil and gas fields and other areas having petroleum potential.

Miscellaneous Field Studies Maps are multicolor or black-andwhite maps on topographic or planimetric bases on quadrangle or irregular areas at various scales. Pre-1971 maps show bedrock geology 

in relation to specific mining or mineral-deposit problems; post-1971 

maps are primarily black-and-white maps on various subjects such as 

environmental studies or wilderness mineral investigations.

Hydrologic Investigations Atlases are multicolored or black-andwhite maps on topographic or planimetric bases presenting a wide range 

of geohydrologic data of both regular and irregular areas; principal scale 

is 1:24,000 and regional studies are at 1:250,000 scale or smaller.

Catalogs

Permanent catalogs, as well as some others, giving comprehensive listings of U.S. Geological Survey publications are available under 

the conditions indicated below from the U.S. Geological Survey, Books 

and Open-File Reports Section, Federal Center, Box 25425, Denver, 

CO 80225. (See latest Price and Availability List)

"Publications of the Geological Survey, 1879-1961" may be purchased by mail and over the counter in paperback book form and as a 

set of microfiche.

"Publications of the Geological Survey, 1962-1970" may be purchased by mail and over the counter in paperback book form and as a 

set of microfiche.

"Publications of the U.S. Geological Survey, 1971-1981" may be 

purchased by mail and over the counter in paperback book form (two 

volumes, publications listing and index) and as a set of microfiche.

Supplements for 1982,1983,1984,1985,1986, and for subsequent 

years since the last permanent catalog may be purchased by mail and 

over the counter in paperback book form.

State catalogs, "List of U.S. Geological Survey Geologic and 

Water-Supply Reports and Maps For (State)," may be purchased by mail 

and over the counter in paperback booklet form only

"Price and Availability List of UJS. Geological Survey Publications," issued annually, is available free of charge in paperback booklet form only.

Selected copies of a monthly catalog "New Publications of the U.S. 

Geological Survey" available free of charge by mail or may be obtained 

over the counter in paperback booklet form only. Those wishing a free 

subscription to the monthly catalog "New Publications of the U.S. 

Geological Survey" should write to the U.S. Geological Survey, 582 

National Center, Reston, VA 22092.

Note. Prices of Government publications listed in older catalogs, 

announcements, and publications may be incorrect Therefore, the 

prices charged may differ from the prices in catalogs, announcements, 

and publications.