Zeolitic Diagenesis of Tuffs in the 

Miocene Chalk Hills Formation, 

Western Snake River Plain, Idaho 

U.S. G EO LOG I CAL SURVEY BULLETIN 1963 
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Zeolitic Diagenesis of Tuffs in the 

Miocene Chalk Hills Formation, 

Western Snake River Plain, Idaho 

By RICHARD A. SHEPPARD 

Silicic vitric tuffs south of Oreana are 

completely altered 

to clinoptilolite with or 

without smectite and opai-CT 

U.S. GEOLOGICAL SURVEY BULLETIN 1963 
U.S. DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR., Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

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

UNITED STATES GOVERNMENT PRINTING OFFICE: 1991 

For sale by the 

Books and Open-File Reports Section 

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Library of Congress Cataloging-in-Publication Data 

Sheppard, Richard A. 

Zeolitic diagenesis of tuffs in the Miocene Chalk Hills formation, western 

Snake River Plain, Idaho I by Richard A. Sheppard. 

p. cm.-(U.S. Geological Survey bulletin ; 1963) 

Includes bibliographical references. 

1. Volcanic ash, tuff, etc.-Idaho-Chalk Hills Formation. 

2. Zeolites-Idaho-Chalk Hills Formation. 3. Diagenesis-Idaho-Chalk 

Hills Formation. 4. Geology, Stratigraphic-Miocene. 5. Geology-Idaho. 

6. Chalk Hills Formation (Idaho) I. Title. II. Series. 

QE75.B9 no. 1963 

[QE461] 

557.3 s-dc20 

[552'.23] 90-26075 

CIP 
CONTENTS 

Abstract 1 

Introduction 1 

Previous work 2 

Scope of present investigation 2 

Laboratory methods 3 

Acknowledgments 3 

Geologic setting 5 

Stratigraphy and lithology of the Chalk Hills Formation 5 

Sandstone 6 

Siltstone and mudstone 6 

Diatomite 6 

Tuff 7 

Diagenetic zones 8 

Distribution and field description 8 

Petrology of tuffs in the zeolite zone 8 

Diagenetic minerals 10 

Clinoptilolite 10 

Erionite 12 

Mordeni te 12 

Phillipsite 12 

Smectite 13 

Opal-CT 13 

Quartz 14 

Potassium feldspar 14 

Other authigenic minerals 14 

Paragenesis 15 

Genesis of diagenetic silicate minerals 15 

Formation of zeolites from silicic glass 16 

Chemical changes during zeolitic diagenesis 16 

Age of zeolitic diagenesis 17 

Conclusions 17 

References cited 18 

Appendix 20 

FIGURES 

1. Map showing margins of the western Snake River Plain 2 

2. Photograph of Castle Creek zeolite deposit with Ben-Jel bentonite mine in 

background 3 

3. Generalized geologic map showing sample localities in the Chalk Hills 

Formation 4 

4. Photograph of gently dipping Chalk Hills Formation unconformably overlain by 

the Glenns Ferry Formation 5 

5. Generalized composite columnar section of the Chalk Hills Formation 6 

Contents Ill 
6--8. 

9. 

10. 

11. 

12. 

13-25. 

TABLES 

Photographs: 

6. Exposure of Chalk Hills Formation west of Castle Creek 7 

7. Clinoptilolite-rich marker tuff and underlying tuffaceous mudstone 7 

8. Lower part of unaltered marker tuff east of Hart Creek 7 

Map of the southern part of the western Snake River Plain showing the 

distribution of the zeolite zone in the Chalk Hills Formation 9 

Photograph of lower part of Chalk Hills Formation showing basinward 

gradation of fresh tuff to zeolitic tuff 9 

Diagrammatic cross section showing interfingering of fresh-glass and zeolite 

zones 9 

Photographs of clinoptilolite-rich marker tuff west of Castle Creek 10 

Scanning electron micrographs: 

13. Dissolution features of silicic, vitric shards in marker tuff 10 

14. Shard pseudomorph consisting of smectite, clinoptilolite, and 

mordenite 11 

15. Hollow pseudomorph of shards consisting of smectite and 

clinoptilolite 11 

16. Well-formed platy to blocky clinoptilolite from the marker tuff 11 

17. Blocky clinoptilolite on star-shaped aggregates of opal-CT 11 

18. Threadlike fibers of erionite 12 

19. Threadlike fibers of erionite among corroded diatoms 13 

20. Mordenite fibers draped over blocky to platy clinoptilolite 13 

21. Mordenite fibers interstitial to and draped across clinoptilolite 13 

22. Pseudomorph of a shard consisting of smectite and clinoptilolite 14 

23. Pseudomorph of a shard consisting of smectite and opal-CT 14 

24. Pseudomorph of a shard consisting of mammillary opal-CT and 

clinoptilolite 14 

25. Fibrous erionite on corroded clinoptilolite 15 

1. Chemical analyses of unaltered shards separated from tuffs of the Chalk Hills 

Formation 8 

2. Chemical analyses and unit-cell contents of clinoptilolite separated from the marker 

tuff of the Chalk Hills Formation 12 

3. Chemical analyses and chemical comparison of unaltered and clinoptilolite-rich 

marker tuff from the Chalk Hills Formation 17 

IV Contents 
Zeol itic Diagenesis of Tuffs in the 

Miocene Chalk Hills Formation, 

Western Snake River Plain, Idaho 

By Richard A. Sheppard 

Abstract 

Zeolites are common diagenetic alteration products of 

silicic, vitric tuffs in Cenozoic lacustrine deposits of the 

western United States, especially those that were deposited in 

highly saline and alkaline water. Such deposits have received 

much attention in the last 25 years because they contain 

economic or potentially economic phillipsite, chabazite, erionite, and clinoptilolite. In contrast, study of zeolites in 

relatively freshwater lacustrine deposits has been neglected. 

The Miocene Chalk Hills Formation of southwestern Idaho 

consists of about 100m (meters) of mainly mudstone, siltstone, 

and interbedded vitric tuffs that were deposited in a chiefly 

freshwater lake. Throughout most of the outcrop belt, the tuffs 

are unaltered, but, locally, the tuffs are completely altered to 

clinoptilolite with or without smectite and opal-CT. This study 

was undertaken to delineate the pattern of alteration, to 

characterize the authigenic minerals and their paragenetic 

relationships, and to suggest a possible origin. 

About 300 samples of fresh and diagenetically altered 

tuffs and tuffaceous sediments were collected throughout the 

formation and from an area of about 500 km2 (square 

kilometers). A marker tuff, 1-13 m thick, is the thickest and 

most continuous tuff in the area, and this tuff was most 

extensively sampled and studied. The relative abundance of 

zeolites and other authigenic minerals in all samples was 

determined by X-ray powder diffraction. Selected samples 

were also studied by scanning electron microscopy and thinsection optical microscopy to ascertain the process of genesis 

and the paragenetic relationships of the minerals, and by X-ray 

fluorescence analysis to chemically characterize the zeolites 

and to provide data on the chemical changes during alteration. 

The zeolite zone in the Chalk Hills Formation is south of 

Oreana, Idaho, and extends parallel to the southern margin of 

the Snake River Plain, northwestward from Birch Creek to Hart 

Creek, about 21 km. Fresh, rhyolitic, vitric tuffs that are about 

1 em (centimeter) to 13 m thick grade laterally into zeolitic 

tuffs. Original textures and sedimentary structures are 

Manuscript approved for publication, November 7, 1990. 

generally preserved in the tuffs of the zeolite zone. Fresh 

(unaltered) tuffs are light gray and soft, but altered tuffs are 

white, light yellow, or light brown and relatively hard, commonly forming ledges. Zeolitic tuff generally consists of 

clinoptilolite, smectite, and opal-CT. Although some tuffs are 

locally nearly monomineralic clinoptilolite, most contain 

40-90 percent clinoptilolite. Trace to minor amounts of erionite are associated with the clinoptilolite at a few localities. 

Trace amounts of fibrous to filiform mordenite are commonly 

associated with the clinoptilolite, but the mordenite can be 

recognized only by scanning electron microscopy. Phillipsite 

was recognized at only one locality where it makes up about 

70 percent of the basal centimeter of the clinoptilolite-rich 

marker tuff. Other authigenic minerals locally recognized 

in the zeolitic tuffs are quartz, calcite, and gypsum. The 

common paragenetic sequence (listed early to late) is glasssmectite-cl i nopti lol ite-morden it e. Opai-CT crystal I ized 

after smectite and either before or after clinoptilolite. Erionite, 

quartz, and calcite generally crystallized later than the clinoptilolite, and quartz and calcite locally replaced all of the earlier 

authigenic silicate minerals. 

The zeolites and associated silicate minerals undoubtedly formed during diagenesis by hydrolysis and dissolution of 

rhyolitic glass by pore water that was trapped in the tuffs 

during lacustrine sedimentation. Where tuffs are still unaltered, 

the original lake water probably was fresh. The zeolite zone 

probably coincides with a part of the lake that had a low to 

moderate salinity and a pH of 7-9. 

INTRODUCTION 

The Miocene Chalk Hills Formation is chiefly a 

lacustrine deposit that crops out discontinuously along the 

southwestern margin of the western Snake River Plain from 

Adrian, Oregon, southeastward to Bruneau, Idaho, a distance of about 140 km (fig. 1). Most of this area is in the 

northwestern part of Owyhee County, Idaho. The Snake 

River Plain is the only major expanse of relatively gentle 

topography in Idaho, and this part of the Snake River Plain 

Introduction 
is characterized by badlands at the base of conspicuous 

northeast-trending benches. The Chalk Hills Formation 

crops out mainly in the badlands. 

Zeolites are common alteration products of silicic, 

vitric tuffs in Cenozoic lacustrine deposits of the western 

United States, especially those that were deposited in highly 

saline and alkaline water (Hay, 1966). These deposits have 

received much attention in the last 25 years because they 

contain economic or potentially economic phillipsite, chabazite, erionite, and clinoptilolite. In contrast, study of 

zeolitic tuffs in relatively freshwater lacustrine deposits has 

been neglected. The Chalk Hills Formation contains at least 

20 interbedded vitric tuffs that were deposited in a chiefly 

freshwater lake. Throughout most of the outcrop belt, the 

tuffs are unaltered, but, locally, the tuffs are completely 

altered to mainly clinoptilolite with or without smectite and 

opal-CT. 

Previous Work 

The Miocene Chalk Hills Formation, part of the Idaho 

Group of Miocene to Pleistocene age, was named by Malde 

and Powers (1962), but upper Cenozoic lacustrine deposits 

were recognized in the western Snake River Plain as early 

as the 1870's (Kimmel, 1979). Since 1962, numerous 

studies have been conducted on the stratigraphy, paleontology, and depositional environment of the formation, and 

these have been reviewed by Kimmel (1979, 1982). Swirydczuk and others (1982) used chemical analyses and field 

studies to attempt correlation of tuffs in the Chalk Hills 

Formation throughout the Snake River Plain. Ekren and 

I 

l 

:zl 

0'0 

(5 1 I 

~1;3 o·-

116° 00' 

Figure 1. Map showing the margins (hachured lines) of the 

western Snake River Plain. 

others ( 1981) mapped the geology of the western part of 

Owyhee County, Idaho, which includes the present study 

area. 

Clinopti1olite was first discovered in the Chalk Hills 

Formation by T .H. Eyde in 1962 at the so-called "Castle 

Creek zeolite deposit" during a nationwide exploration 

program for zeolites by the Union Carbide Corporation 

(Mumpton, 1983). The Castle Creek zeolite deposit is about 

2.5 km west of Castle Creek and about 9 km south of 

Oreana, Idaho (fig. 2; loc. 1, fig. 3). Zeolites in the Chalk 

Hills Formation have not received much study, and 

published information concerning them is meager. Regis 

(1978) investigated the mineralogy and properties of bentonite from the Ben-Jel mine, located in the Chalk Hills 

Formation about 1.5 km southeast of the Castle Creek 

zeolite deposit (fig. 2; loc. 2, fig. 3), and found that some of 

the bentonite contains major amounts of clinoptilolite. Also, 

Ekren and others ( 1981) reported that tuffaceous material in 

the formation is commonly altered to zeolites and montmorillonite, but these authors did not identify the zeolites or 

give the localities. Two brief reports by Sheppard and Gude 

(1982, 1983) presented mineralogical, chemical, cationexchange, and gas-adsorption data for clinoptilolite-rich tuff 

from the Castle Creek deposit. 

In 1975, the Double Eagle Petroleum and Mining 

Company (Casper, Wyo.) filed claims on a clinoptiloliterich tuff at the Castle Creek deposit. Since then, this 

company has mined, crushed, and stockpiled clinoptiloliterich tuff but has shipped only a minor tonnage for use 

chiefly in agricultural experiments and tests. Additional 

claims were made in 1987 on a clinoptilolite-rich tuff east of 

Castle Creek by Teague Mineral Products (Adrian, Ore.). 

Scope of Present Investigation 

After a brief reconnaissance of the alteration of the 

Chalk Hills Formation throughout the outcrop belt of the 

formation in 1984, this present investigation concentrated 

on the extensively altered area south of Oreana, Idaho 

(fig. 3). About 300 samples of fresh and diagenetically 

altered tuffs and tuffaceous sediments from that area were 

collected and studied during 1985-88. The tuffaceous rocks 

of the Chalk Hills Formation were chosen for detailed study 

for the following reasons: (1) the formation contains 

numerous tuffs that are both fresh (unaltered) and altered to 

a variety ?f authigenic silicate minerals, (2) the formation 

was subjected to relatively shallow burial and shows only 

slight deformation, and (3) the formation is well exposed, 

and individual tuffs can be traced throughout much of the 

extent of the formation. Objectives of this investigation 

were to delineate the pattern of zeolite alteration, to 

characterize the authigenic minerals and their paragenetic 

relationships, and to suggest a possible origin for the 

zeolites and associated silicate minerals. 

2 Zeolitic Diagenesis of Tuffs, Chalk Hills Formation, Snake River Plain, Idaho 
Figure 2. View eastward of Castle Creek zeolite deposit, Idaho. Pit in foreground is in the clinoptilolite-rich marker tuff that was 

excavated by the Double Eagle Petroleum and Mining Company. Disturbed area in upper right (arrow) is the Ben-Jel bentonite 

mine. Castle Creek is just beyond the Ben-Jel mine. 

laboratory Methods 

The mineralogy of bulk samples was determined by 

X-ray diffractometer analysis. Samples were first ground to 

a powder and then packed in aluminum sample holders, 

exposed to copper radiation, and scanned at 2° 28 per 

minute over the range of 2°-40° 28. Relative abundances of 

the minerals were visually estimated from the diffractometer patterns following the procedure outlined by Sheppard 

and Gude (1968, p. 2-3). Estimates are probably less 

reliable for samples containing glass or opal-CT because 

these materials yield rather poor X-ray records. 

Textures and paragenetic relationships of the diagenetic minerals were examined in thin sections and 

immersion oil mounts by transmitted-light microscopy and 

by scanning electron microscopy of freshly broken fragments. Selected samples to be studied by scanning electron 

microscopy were coated with a film of gold (about 300 A 

(angstrom) thick) just prior to examination. 

The chemical compositions of bulk samples and pure 

separates of clinoptilolite and glass shards were determined 

by standard X-ray fluorescence analyses. The H20 content 

was determined by weight loss on ignition at 900 °C. An 

energy-dispersive X-ray unit, attached to the scanning 

electron microscope, was used to determine the Si:Al ratios 

of selected zeolites. The pure clinoptilolite and glass-shard 

concentrates were prepared by crushing the tuff and then 

disaggregating it in an ultrasonic bath. Clinoptilolite and the 

glass shards were then concentrated by repeated centrifuging in a heavy-liquid mixture of bromoform and acetone. 

Acknowledgments 

Grateful appreciation is expressed to those in the U.S. 

Geological Survey who provided technical assistance 

during this investigation. Arthur J. Gude, 3d, helped during 

the field reconnaissance of the western Snake River Plain in 

1984 and performed the initial X-ray diffractometer 

analyses. Gary L. Skipp assisted in the field in 1985 and 

1986 and performed X-ray diffractometer analyses and 

mineral separations in the laboratory. The guidance and 

patience of James M. Nishi during my use of the scanning 

electron microscope is greatly appreciated. I also thank 

Introduction 3 
/ 

/ QTs 

..____ ----------

QTs 

5 KILOMETERS 

3 MILES 

EXPlANATION 

Glenns Ferry Formation (Pliocene) and younger 

sedimentary rocks (Pleistocene to Holocene) 

Chalk Hills Formation (Miocene) 

Volcanic rocks (Miocene) 

Granitic rocks (Cretaceous) 

-----Contact 

-----Fault-Dashed where approximately located, 

dotted where concealed 

e

3 Sample locality-Showing number 

~ 

\ 

MAP LOCATION 

I 

Figure 3. Generalized geologic map showing sample localities in the Chalk Hills Formation south of Oreana, Owyhee County, 

Idaho; modified from Ekren and others (1981 ). Mineralogic composition of samples, as estimated from X-ray diffractometer 

patterns, is given in the appendix. 

4 Zeolitic Diagenesis of Tuffs, Chalk Hills Formation, Snake River Plain, Idaho 
Figure 4. View west to gently dipping Chalk Hills Formation (Tch), about 9 km south of Oreana, Idaho. The light-colored Chalk 

Hills Formation crops out in the lower part of the conspicuous bench and in badlands at the base of the bench. The Glenns Ferry 

Formation (Tgf) unconformably overlies the Chalk Hills Formation. A veneer of Quaternary gravel caps the bench. 

Barbara A. Lockett for preparing excellent thin sections 

from difficult materials. 

GEOLOGIC SETTING 

The western Snake River Plain (fig. 1) is an intermontane rift basin that received several thousand meters of 

volcanic and sedimentary deposits in Miocene and later 

time. The western Snake River Plain trends northwest, in 

contrast to the northeast-trending eastern Snake River Plain. 

Highlands south of the western Snake River Plain near the 

study area are underlain chiefly by Cretaceous granitic 

rocks and Miocene rhyolitic ash-flow tuffs (Ekren and 

others, 1981 ). The upper Miocene Chalk Hills Formation 

unconformably overlies silicic ash-flow tuffs of Miocene 

age and is unconformably overlain by sedimentary rocks of 

the Glenns Ferry Formation of Pliocene age. Throughout 

most of the study area south of Oreana, the basal part of the 

Glenns Ferry Formation is a brown, conspicuous, ledgeforming, calcareous oolite (Swirydczuk and others, 1979). 

Similar to the Chalk Hills Formation, the Glenns Ferry 

Formation contains numerous silicic, vitric tuffs. Unlike the 

Chalk Hills Formation, however, the tuffs in the Glenns 

Ferry Formation are unaltered or are partly altered to 

smectite throughout the region. 

Most of the Chalk Hills Formation is lacustrine, but 

fluviatile, flood-plain, and beach deposits have also been 

recognized (Kimmel, 1979; Middleton and others, 1985). 

The formation resulted from the interplay of alluvial and 

lacustrine depositional processes in an active volcanotectonic setting. Erosion of the Chalk Hills Formation 

generally has resulted in badlands. South of Oreana, the 

formation crops out in badlands at the base of conspicuous 

northeast-trending benches that are capped by Quaternary 

gravel (fig. 4). The formation commonly dips about 3°-10° 

northeastward (basinward) and is cut by numerous northwest-trending faults. 

STRATIGRAPHY AND LITHOLOGY OF THE 

CHALK HILLS FORMATION 

The Chalk Hills Formation is about 100 m thick and 

consists chiefly of mudstone, siltstone, sandstone, diatomite, and numerous thin volcanic ash beds or tuffs. For 

convenience throughout the remainder of this report, the 

volcanic ashes are referred to as tuffs even though they are 

relatively unconsolidated and unaltered. Along the southwestern margin of the Snake River Plain south of Oreana, 

the thickness of the formation is commonly less than 50 m. 

This reduced thickness is due to nondeposition on topographic highs of Miocene volcanic rocks and to slight 

basinward tilting of the Chalk Hills Formation and erosion, 

particularly at the extreme margin of the basin, prior to 

deposition of the overlying Glenns Ferry Formation. In 

addition to the numerous discrete tuffs, most epiclastic 

rocks in the formation have a tuffaceous component, 

whether the rock is mudstone or sandstone. Fossil fish, 

mammals, mollusks, ostracodes, and plants have been 

collected by others from the Chalk Hills Formation, and 

their significance has been discussed by Malde and Powers 

(1962) and Kimmel (1979). 

A generalized composite columnar section of the 

Chalk Hills Formation (fig. 5) was compiled from several 

partial measured sections from Hart Creek southeast to 

Birch Creek (fig. 3). Tuffaceous mudstone and siltstone are 

the predominant lithologies, but tuffaceous sandstone is 

especially common in the lower part of the section, 

particularly near the basin margin. Locally, such as at the 

head of Castle Creek and its. tributaries, most of the exposed 

formation consists of sandstone. The thickest and most 

continuous tuff in the area studied is in the lower half of the 

Stratigraphy and Lithology of the Chalk Hills formation 5 
formation and is termed the "marker tuff' (figs. 5, 6). The 

marker tuff is 1-13 m thick and averages about 5 m thick. 

This marker tuff was most extensively sampled and studied. 

Sandstone 

Sandstone is generally yellowish gray or light brown 

and is highly variable in grain size, thickness, and induration. Most commonly, sandstone is medium to coarse 

grained and medium to thick bedded. Crossbedding and 

ripple marks are locally common. Dips of foresets near the 

basin margin at Castle Creek and its tributaries indicate a 

northeast (basinward) transport direction. Cementation is 

common, especially where interbedded tuffs are zeolitic. 

Cements are chiefly clinoptilolite and smectite, but opalCT, gypsum, and calcite have also been recognized. 

The framework constituents of sandstone consist of 

varying amounts of mineral grains, rock fragments, and 

vitroclastic particles. Sorting co~monly is poor, and the 

clasts are generally angular to subrounded. The detrital 

mineral grains are quartz and feldspar with lesser amounts 

of biotite, muscovite, hornblende, zircon, and epidote. 

Feldspar and quartz are commonly present in nearly equal 

amounts, and sodic plagioclase is more abundant than alkali 

feldspar. Rock fragments are mostly volcanic and granitic. 

The tuffaceous component commonly makes up 20-60 

percent of the framework and varies from fresh to completely altered. 

Siltstone and Mudstone 

Siltstone and mudstone are the predominant rocks in 

the Chalk Hills Formation. Both are medium to thick 

bedded, yellowish brown to grayish brown, and have an 

earthy luster. These fine-grained rocks break with a 

conchoidal to subconchoidal fracture and, where weathered, 

have a punky "popcorn" coating. Most mudstones contain, 

in addition to clay minerals, detrital sand grains, diatoms, 

vitroclastic particles, and authigenic minerals. Angular to 

subrounded detrital mineral grains commonly make up 

10-40 percent of mudstones and are the same suite that is in 

the sandstones. Locally, diatoms and vitroclastic grains are 

represented by pseudomorphs of clinoptilolite. Authigenic 

minerals include clinopti1olite, opal-CT, potassium feldspar, 

gypsum, and calcite. Near-vertical joints in the fine-grained 

rocks are locally filled with gypsum or starkeyite 

(MgS04 ·4H20) (fig. 7). 

Diatomite 

Diatomite is a light-gray, fine-grained, lightweight, 

porous rock that breaks with a blocky to subconchoidal 

fracture. It is commonly medium to thick bedded and shows 

X X X X 

X X X 

X X X X 

EXPLANATION 

I I Mudstone, siltstone, diatomite 

.· :·./·>/ ~. ·.:·:".:· d Tuffaceous sandstone 

I x >< >< >E >E I Tuff 

x x x Marker tuff X X X X 

X X X 

X X X X 

.. 

·.·.·.· .. ·.··. :_: ,': ·:: :: .'· ... : ·. · .· . ·: 

METERS 

20 

10 

Figure 5. Generalized composite columnar section of the 

Chalk Hills Formation, Idaho. 

6 Zeolitic Diagenesis of Tuffs, Chalk Hills Formation, Snake River Plain, Idaho 
Figure 6. Excellent exposure of most of the Chalk Hills 

Formation (Tch) west of Castle Creek, about 9 km south of 

Oreana, Idaho. Formation consists chiefly of yellowish-brown 

and grayish-brown mudstone and siltstone. White unit in 

lower part of the formation is the zeolitic marker tuff (m) where 

it has been excavated for clinoptilolite. The basal oolite of the 

Glenns Ferry Formation (Tgf) unconformably overlies the 

Chalk Hills Formation. 

Figure 7. Closeup view of excavation in figure 6, showing 

contact (arrow) between the white, clinoptilolite-rich marker 

tuff and the underlying tuffaceous mudstone in the Chalk Hills 

Formation. Near-vertical joints in the mudstone are locally 

filled with gypsum or starkeyite. The cliff-forming basal oolite 

of the Glenns Ferry Formation can be seen in the background. 

no sedimentary structures. In the field, diatomite superficially resembles altered fine-grained tuff. All the 

examined diatomites in the Chalk Hills Formation contain 

as much as 50 percent nondiatom constituents, including 

clay minerals, vitroclastic particles, and angular clastic 

grains of quartz and sodic plagioclase. Because of the 

amorphous character of the diatom frustules and the vitroclastic material, X-ray diffractometer patterns of diatomite 

tend to show less than the amount of these constituents in 

diatomite. 

Tuff 

Silicic, vitric tuffs in the Chalk Hills Formation make 

up about 10-12 percent of the stratigraphic section and are 

the most conspicuous and continuous strata. At least 20 tuffs 

were recognized, and they are about 1 em to 13 m thick, 

although most are less than 0.5 m thick. The thicker tuffs are 

generally more continuous than the thinner ones. Thin tuffs 

are commonly single beds, but most tuffs that are thicker 

than 25 em are multiple bedded (fig. 8). Individual beds of 

either single-bedded or multiple-bedded tuffs are commonly 

graded, being coarser at the base. Some thin tuffs and the 

lower part of thick tuffs are the result of ash falls directly 

into the lake. Other tuffs (particularly the upper part of thick 

tuffs) consist of reworked vitroclastic material mixed with 

epiclastic grains derived from highlands south of the 

depositional basin. 

Two or more thin (less than 25 em thick) basaltic tuffs 

are locally present in the upper part of the Chalk Hills 

Formation. The pattern of alteration of these basaltic tuffs is 

Figure 8. Natural exposure of lower part of the marker tuff 

east of Hart Creek, about 8 km southwest of Oreana, Idaho. 

The tuff is gray, friable, and multiple bedded. Silicic, vitric 

shards are unaltered. The contact (arrow) with underlying 

brown mudstone is sharp. 

Stratigraphy and Lithology of the Chalk Hills Formation 7 
similar to that of the silicic ones, although the detailed 

diagenetic mineralogy differs. The alteration of the mafic 

tuffs will be the subject of another report and is not 

discussed further herein. 

Fresh silicic tuff is light to dark gray and has a 

characteristic vitreous luster. It is very friable and easily 

disaggregated with the fingers. Natural exposures are poor; 

therefore, thin fresh tuffs could easily be overlooked. The 

lower contact of a tuff generally is sharp (fig. 8), whereas 

the upper contact is commonly gradational into the overlying rock, regardless of lithology. Ripple marks, laminations, and contorted bedding are present but rare. 

The silicic tuffs are vitric but contain varying 

amounts of crystal and rock fragments, and, locally, diatoms. Most are fine to medium grained. The vitric particles 

consist of platy bubble-wall shards and pumice shards, but 

the platy bubble-wall type predominates. The index of 

refraction of unaltered shards ranges from 1.496 to 1.503. 

Angular crystal and rock fragments commonly make up 

1-10 percent of the tuffs. Pyrogenic crystals consist of 

quartz, sadie plagioclase, sanidine, biotite, hornblende, 

zircon, and magnetite. The rock fragments are spherulitic 

and hyalopilitic lavas, ash-flow tuff, granite, and chert. 

Mineral grains of epiclastic origin are microcline, muscovite, and epidote. 

Chemical analyses of unaltered shards separated from 

the marker tuff and a tuff stratigraphically above the marker 

tuff indicate a rhyolitic composition (table 1). Although the 

shards do not contain any authigenic minerals, the high 

(greater than 5 percent) H20 content is evidence that even 

the freshest shards are hydrated. All other silicic tuffs are 

presumed to be rhyolitic on the basis of the low index of 

refraction of the glass shards and the coexisting pyrogenic 

minerals as described above. 

DIAGENETIC ZONES 

Distribution and Field Description 

Throughout the extent of the Chalk Hills Formation, 

the tuffaceous rocks are unaltered or partly altered to 

smectite with or without opal-CT and are herein considered 

to be in the fresh-glass zone. South of Oreana, from Hart 

Creek southeastward to Birch Creek (fig. 3), a distance of 

about 21 km, some or all tuffaceous beds are altered to 

clinoptilolite and are considered to be in the zeolite zone, 

which is generally 1-3 km wide and roughly parallel to the 

southwestern margin of the Snake River Plain (fig. 9). The 

zeolite zone is gradational, interfingers into the fresh-glass 

zone, and pinches out to the northwest and southeast. Near 

marginal parts of the basin, such as along an eastern 

tributary to Castle Creek (fig. 1 0), the gradation from 

unaltered tuff to completely altered zeolitic tuff in a 

Table 1. Chemical analyses of unaltered shards separated 

from tuffs of the Chalk Hills Formation, Owyhee County, Idaho 

[Values are in percent. Chemical analyses by X-ray fluorescence, H20 

determined by loss on ignition at 900 °C. Analysts: J.E. Taggart, A.J. 

Bartel, and K.C. Stewart. <, less than] 

Sample ................................... .. 2 

Si02 ....................................... . 72.3 72.1 

Al203'''"''''''''''''''''''''''''''''''''' 11.3 11.5 

Fe20 3 ..................................... . 1.84 1.69 

MgO ...................................... . 0.11 0.21 

CaO ....................................... . 0.55 0.57 

Na20 ..................................... . 1.94 2.42 

K20 ....................................... . 5.54 5.58 

Ti02 •••••••.••••••••.••.•••••••••••••••••••• 0.19 0.20 

P20s ...................................... . <0.05 <0.05 

MnO ...................................... . 0.03 0.04 

H20 ....................................... . 5.93 5.37 

Total .................................. . 99.73 99.68 

SAMPLE DESCRIPTIONS 

1. Marker tuff, Hart Creek; NW114SEY4sec. 4, T. 5 S., R. 1 W.; 

sample No. CC--46D. 

2. Tuff in the upper part of the formation, between Browns and 

Castle Creeks; NW1

14SEY4 sec. 18, T. 5 S., R. 1 E.; sample 

No. CC-32. 

basinward direction is well exposed and has occurred over 

a distance of about 300 m. The zeolite zone is thickest 

between Browns Creek and Castle Creek (fig. 9) where the 

entire Chalk Hills Formation is zeolitic. To the northwest 

and southeast, the zeolite zone thins and is restricted to the 

marker tuff and (or) stratigraphically lower tuffaceous beds 

(fig. 11). 

Tuffs of the fresh-glass zone are typically light gray 

and friable; shards have a distinct vitreous luster. Unlike 

tuffs of the fresh-glass zone, zeolitic tuffs are resistant and 

ledge forming. The zeolitic tuffs are white to light yellow or 

brown, relatively hard, and dull or earthy. Even those 

zeolitic parts of otherwise fresh tuffs stand out in a natural 

exposure. Original textures and sedimentary structures are 

generally preserved in the zeolitic tuffs. Tuffs of the zeolite 

zone break with a platy, hackly, or conchoidal fracture 

(fig. 12). Tuffs of the fresh-glass zone . generally can be 

distinguished from altered tuffs in the field, but the zeolite 

content in altered tuff cannot be predicted without the aid of 

X-ray diffractometry. Altered tuff consisting mainly of 

smectite and opal-CT superficially resembles clinoptiloliterich tuff in the field. 

Petrology of Tuffs in the Zeolite Zone 

The mineralogy of the altered tuffs was determined 

mainly by study of X-ray diffractometer powder patterns of 

bulk samples (see appendix), supplemented by thin-section 

8 Zeolitic Diagenesis of Tuffs, Chalk Hills Formation, Snake River Plain, Idaho 
5 KILOMETERS 

Figure 9. Southern part of the western Snake River Plain 

showing the distribution of the zeolite zone (stippled) in the 

Chalk Hills Formation, south of Oreana, Idaho. The unpatterned area is the fresh-glass zone. The shaded area is 

highlands along the southern margin of the Snake River Plain. 

These highlands are underlain chiefly by Cretaceous granitic 

rocks and Miocene volcanic rocks. 

study and scanning electron microscopy. Thin sections and 

scanning electron microscopy were especially useful for 

determining the age relationships of the authigenic minerals, but these techniques were not used until the mineralogy was known by X-ray methods. 

Silicic, vitric tuffs in the zeolite zone consist mainly 

of variable amounts of authigenic zeolites, smectite, and 

opal-CT. Clinoptilolite is by far the most abundant zeolite in 

the Chalk Hills Formation, but minor or trace amounts of 

erionite, mordenite, and phillipsite have been recognized 

locally. These diagenetic minerals locally coexist with relict 

glass. Near the boundary between the fresh-glass zone and 

the zeolite zone, the relict shards show obvious dissolution 

features, including pitting and corrosion (fig. 13). Even 

where no relict glass remains, the vitroclastic texture is 

generally preserved in the tuffs. 

Typical pseudomorphs after bubble-wall shards 

consist of a thin (2-1 0 11m, micrometer) marginal layer of 

smectite that is succeeded inwardly by crystals of one or 

more zeolites (fig. 14). The pseudomorphs may be solid or 

hollow, and commonly both types can be recognized in the 

Figure 10. View north, along the eastern tributary of Castle 

Creek, of the lower part of the Chalk Hills Formation, Idaho, 

showing basinward gradation of fresh tuff (foreground) to 

zeolitic tuff (left of standing figure in middle ground). The fresh 

tuff in the foreground is gray and friable, but altered tuff left of 

the figure is light yellow and resistant. 

~ 

(I) 

(I) 

u 

V) 

§ ~ 0 d 

NW tXs U SE ~--------------------------------------~ 

Glenns Ferry Formation 

EXPLANATION 

Zeolite 

D Glass 

Figure 11. Diagrammatic northwest-southeast cross section 

showing the interfingering relationship between the fresh-glass 

zone and the zeolite zone in tuffaceous rocks ofthe Chalk Hills 

Formation, Idaho. Between Browns Creek and Castle Creek, 

the entire thickness of the formation is zeolitic, but to the 

northwest and southeast, the boundary between the zones is at 

the marker tuff (m) or stratigraphically lower beds. The Glenns 

Ferry Formation unconformably overlies both the fresh-glass 

zone and the zeolite zone. Tuffaceous rocks in the Glenns 

Ferry Formation are mostly unaltered and lack zeolites. 

same specimen. Hollow ones are more common than solid 

ones and can be recognized even with a hand lens. The 

larger pseudomorphs generally are the hollow ones. The 

most common hollow pseudomorphs in tuffs of the Chalk 

Hills Formation consist of the marginal layer of smectite 

succeeded inwardly by a single layer of clinoptilolite 

Diagenetic Zones 9 
;t, 

,~/.. ,-,. .. 

~.) • _....__. r ' -~1 ...... 

.1 

~·~ ',..., -..:· 

./.(, 

•, 

I • I> ' j 1 

Figure 12. Clinoptilolite-rich marker tuff in exploration pits west of Castle Creek, Idaho. A, hackly to platy fracture in upper part 

of the tuff. 8, conchoidal fracture in lower part of the tuff. 

Figure 13. Scanning electron micrographs showing dissolution features of silicic, vitric shards in the marker tuff, Chalk Hills 

Formation, Idaho. A, corroded surface of platy shard. 8, crenulated authigenic smectite (S) that grew on pitted shard. 

crystals oriented quasi-perpendicular to the shard wall 

(fig. 15). Hollow pseudomorphs are convincing evidence 

that at least some of the clinoptilolite and coexisting zeolites 

crystallized in cavities from which the glass had already 

been dissolved. 

DIAGENETIC MINERALS 

Clinoptilolite 

Clinoptilolite occurs in altered tuffs as well-formed 

platy or blocky crystals that are 1-30 ~m long, but most are 

1-15 ~m long (figs. 16, 17). Although some tuffs contain 

only a trace of clinoptilolite and some tuffs are nearly 

monomineralic, most tuffs of the zeolite zone contain 40-90 

percent clinoptilolite. The tuffs seem consistently rich in 

this zeolite between Browns Creek and Castle Creek (fig. 

3). The marker tuff, in particular, is consistently rich in 

clinoptilolite and thick (generally 5-13 m thick) in this area. 

X-ray fluorescence analyses of three pure separates of 

clinoptilolite from the marker tuff (table 2) show that the 

Si:(Al+Fe3

+) ratio is about 3.8--4.1 and that the 

exchangeable cations are variable. Additional energydispersive X-ray analyses of clinoptilolite from other tuffs 

in the Chalk Hills Formation have an average Si:Al ratio of 

10 Zeolitic Diagenesis of Tuffs, Chalk Hills Formation, Snake River Plain, Idaho 
Figure 14. Scanning electron micrograph of shard pseudomorph consisting of a marginal layer of smectite (S) that is 

succeeded inwardly by platy clinoptilolite and then fibrous 

mordenite, Chalk Hills Formation, Idaho. 

Figure 15. Scanning electron micrograph of hollow pseudomorphs of shards consisting of a thin marginal layer of smectite 

(S) that is succeeded inwardly by a single layer of platy 

clinoptilolite crystals oriented quasi-perpendicular to shard 

wall, Chalk Hills Formation, Idaho. 

about 4.15. The Na:Ca ratio for clinoptilolite from the 

marker tuff seems to show a systematic northwest to 

southeast transition from about 2.4 to 0.2. It is unknown, 

however, whether clinoptilolite in the other tuffs has the 

same apparent transition and whether the present Na:Ca 

ratio represents the ratio at the time of crystallization or 

reflects postcrystallization cation exchange. 

Thermal stability has been used as a simple way to 

distinguish clinoptilolite from heulandite; both are members 

of the heulandite structural group. Mumpton (1960) suggested overnight heating in an oven at 450 °C, followed by 

examination by X-ray diffraction, as a means to differentiate 

the zeolites. Under these conditions, Mumpton found that 

the X-ray diffractometer pattern for clinoptilolite was essentially unaffected but that the pattern for heulandite was 

Figure 16. Scanning electron micrograph of well-formed 

platy to blocky clinoptilolite from the marker tuff, Chalk Hills 

Formation, Idaho. 

Figure 17. Scanning electron micrograph of blocky clinoptilolite that grew on star-shaped aggregates of opai-CT, Chalk 

Hills Formation, Idaho. 

destroyed. Later workers (Alietti, 1972; Boles, 1972) 

showed that the thermal behavior of members of the 

heulandite structural group was more complex and that the 

behavior was affected by the Si:Al ratio and divalent cation 

content of the zeolite. Thus, Alietti ( 1972) proposed that the 

overnight heating test of Mumpton be increased to 550 °C. 

The three analyzed clinoptilolite samples of table 2 were 

heated in an oven at 550 oc for 16 hours. After cooling to 

room temperature, the samples were examined by X-ray 

diffraction. The diffractometer pattern for all samples persisted, but the peak intensities were diminished. For 

example, the (020) reflection showed about a 30-80 percent 

reduction in intensity after heating. The sample showing the 

most reduction is also the most calcic, and this thermal 

behavior is consistent with the findings of Alietti (1972) and 

Boles (1972). 

Diagenetic Minerals 11 
Table 2. Chemical analyses and unit-cell contents of 

clinoptilolite separated from the marker tuff of the Chalk Hills 

Formation, Owyhee County, Idaho 

[Chemical analyses by X-ray fluorescence; H20 determined by loss on 

ignition at 900 °C. Analysts: J.E. Taggart, A.J. Bartel, and K.C. Stewart. 

<,less than. Unit-cell contents based on 72 oxygen atoms (Ti, P, and Mn 

were omitted in calculation of the unit cell)] 

Sample .............. . 

Si02 •••••••••••••••••• 

Al20 3 .••.•.....•••... 

Fe2 0 3 .•.......•..•.•. 

MgO ................. . 

CaO .................. . 

Na2 0 ................ . 

K20 .................. . 

Ti02 •••••••••••••••••• 

P20s·················· 

MnO ................. . 

H20 .................. . 

Total ............. . 

Si ...................... . 

AI ..................... . 

Fe3+ .................. . 

Mg .................... . 

Ca ..................... . 

Na ..................... . 

K ...................... . 

H20 .................. . 

Si:(Al+Fe3+) .... . 

Chemical analyses 

62.6 

13.4 

0.70 

1.04 

1.96 

2.59 

2.35 

0.38 

0.06 

<0.02 

14.0 

99.08 

Unit-cell contents 

28.65 

7.22 

0.24 

0.71 

0.96 

2.30 

1.37 

21.36 

3.84 

SAMPLE DESCRIPTIONS 

2 

63.9 

12.3 

1.37 

1.02 

2.39 

1.32 

2.78 

0.19 

<0.05 

<0.02 

14.5 

99.77 

29.09 

6.60 

0.47 

0.69 

1.16 

1.16 

1.61 

22.01 

4.11 

3 

63.3 

12.8 

0.91 

1.21 

4.08 

0.36 

1.71 

0.17 

<0.05 

<0.02 

15.9 

100.44 

28.87 

6.88 

0.31 

0.82 

1.99 

0.32 

1.00 

24.18 

4.02 

1. Between Hart and Browns Creeks; SEV4SEV4 sec. 3, T. 5 S., 

R. 1 W.; sample No. CC-42C. 

2. Browns Creek; NE1/4NE1/4 sec. 24, T. 5 S., R. 1 W.; sample 

No. CC-10B. 

3. Birch Creek; NW1/4NEV4 sec. 15, T. 6 S., R. 1 E.; sample 

No. CC- 21-5B. 

Erionite 

Trace to minor amounts of fibrous erionite occur with 

clinoptilolite at several localities in tuffs of the Chalk Hills 

Formation (see appendix). In addition to the erionite 

identified by X-ray diffraction, some was recognized by 

scanning electron microscopy in tuffs where the abundance 

is so low (less than 1 percent) that the erionite could not be 

detected by X-ray diffraction of the bulk samples. Most 

erionite in the tuffs of lacustrine deposits occurs as acicular 

or prismatic crystals or compact bundles of such crystals, 

but the erionite from the Chalk Hills Formation occurs as 

threadlike fibers that are 10-70 11m long and about 0.1-

0.2 11m wide. The erionite fibers commonly occur in loose 

clusters (figs. 18, 19). Where associated with clinoptilolite, 

the erionite generally formed later than the clinoptilolite. 

Energy-dispersive X-ray analyses show that the Si:Al ratio 

of the erionite is about 3.2-3.8, which is about the same 

range determined for erionite from a variety of lacustrine 

deposits throughout the world (Gude and Sheppard, 1981 ). 

Mordenite 

Thin fibers or filaments of presumed mordenite are 

commonly associated with clinoptilolite in the tuffs, but the 

abundance is so low that the mordenite cannot be 

recognized in X-ray diffractometer patterns of bulk 

samples. The morphology, size, and mode of occurrence are 

very similar to mordenite from a variety of clinoptilolite 

deposits examined by scanning electron microscopy by 

Mumpton and Ormsby (1976). Individual mordenite fibers 

from the Chalk Hills tuffs are 2-40 11m long and about 

0.1-1.2 11m wide. The mordenite fibers commonly lie across 

clinoptilolite crystals or are interstitial to them (figs. 20, 21 ), 

indicating that the mordenite postdates the clinoptilolite. 

Energy-dispersive X-ray analyses yield Si:Al ratios of about 

4.6-5.2 for the mordenite. This range of Si:Al ratios is well 

within the range of 4.2-5.9 that was determined for mordenites from a variety of localities and rock types by 

Passaglia (1975). The Chalk Hills mordenite cannot be 

distinguished from erionite solely on the basis of 

morphology. The combination of morphology and Si:Al 

ratios, however, gives credence to the identification in the 

absence of X-ray diffraction data. 

Phillipsite 

Phillipsite was recognized only at one locality (10, 

fig. 3), just east of Browns Creek, where it locally makes up 

as much as 70 percent of the basal centimeter of the 

Figure 18. Scanning electron micrograph of a cluster of 

threadlike fibers of erionite, Chalk Hills Formation, Idaho. 

12 Zeolitic Diagenesis of Tuffs, Chalk Hills Formation, Snake River Plain, Idaho 
Figure 19. Scanning electron micrograph of a cluster of 

threadlike fibers of erionite that grew among corroded diatom 

tests in a tuff from the upper part of the Chalk Hills Formation, 

Idaho. 

Figure 20. Scanning electron micrograph of mordenite fibers 

that are draped over blocky to platy crystals of clinoptilolite, 

Chalk Hills Formation, Idaho. 

otherwise clinoptilolite-rich marker tuff. The phillipsite 

occurs as malformed length-slow spherulites that are 

60-300 j.lm in their longest dimension. The mean index of 

refraction is 1.475 and is consistent with a relatively 

siliceous and alkali-rich phillipsite (Galli and Loschi Ghittoni, 1972). This index is only slightly higher than that for 

a suite of phillipsites from silicic tuffs in saline, alkalinelake deposits (Sheppard and Fitzpatrick, 1989). These 

saline, alkaline-lake phillipsites have Si:Al ratios of about 

3.1-3.4; thus, the Si:Al ratio of the Chalk Hills phillipsite is 

probably only slightly lower than this range. Where phillipsite is associated with clinoptilolite and erionite in the 

basal marker tuff, the crystallization of phillipsite seems to 

have preceded that of the other zeolites. 

Figure 21. Scanning electron micrograph of mordenite fibers 

that are interstitial to and draped across platy crystals of 

clinoptilolite, Chalk Hills Formation, Idaho. 

Smectite 

Smectite is a nearly ubiquitous authigenic mineral in 

the tuffaceous rocks of the Chalk Hills Formation. The 

smectite content in completely altered tuffs of the zeolite 

zone commonly ranges from a trace to about 30 percent. 

Even in the fresh-glass zone, smectite is locally common 

and, at some localities, is associated with opal-CT (see 

appendix). Where smectite is the major constituent of a tuff, 

a punky "popcorn" coating is typical of weathered natural 

exposures. Scanning electron micrographs of smectite show 

the characteristic crenulated to flaky morphology (fig. 22). 

The smectite has a first-order basal spacing of about 14.0 A 

for oriented, air-dried samples and of 17.0-17.3 A for 

oriented, glycolated samples. The (060) reflection is 

1.49-1.50 A and is typical for dioctrahedral smectite 

(montmorillonite). Where associated with other authigenic 

silicate minerals in the tuffs, the smectite is the earliest 

mineral to have crystallized. 

Opai-CT 

Opal-CT is difficult to recognize in thin sections of 

altered tuffs because of its isotropic and finely crystalline 

character. Most identifications of opal-CT in the Chalk Hills 

Formation are based on X-ray diffractometer data or 

scanning electron microscopy. The opal-CT has relatively 

broad diffractometer peaks at the following d spacings: 

4.23--4.29 A, 4.07--4.09 A, and 2.49-2.50 A. The peak near 

4 A has, by far, the strongest intensity. Scanning electron 

micrographs show that opal-CT occurs as clusters of minute 

(less than 1 j.lm) crystals. The morphology of the clusters is 

commonly ragged blades (fig. 23), lepispheres, mammillary 

coatings (fig. 24), or star-shaped aggregates (fig. 17). The 

abundance of opal-CT in the tuffs ranges from a trace to 

Diagenetic Minerals 13 
Figure 22. Scanning electron micrograph of pseudomorph of 

a shard that consists of marginal smectite (S) followed inwardly 

by platy clinoptilolite (Cp), Chalk Hills Formation, Idaho. The 

smectite shows the typical crenulated texture. 

nearly 100 percent, but most tuffs contain less than 30 

percent. Where associated with smectite and (or) clinoptilolite, opal-CT crystallized after smectite and either before or 

after clinoptilolite. 

Quartz 

Authigenic quartz is relatively rare in the altered tuffs 

of the Chalk Hills Formation. Most of the quartz identified 

by X-ray diffraction of bulk samples (see appendix) is 

epiclastic or pyrogenic. The authigenic quartz occurs in 

irregular patches of nearly equidimensional, minute (less 

than 2 j.lm in diameter) anhedral grains or of aggregates of 

fibers that commonly are 10-30 j.lm long. Both types of 

Figure 23. Scanning electron micrograph of a pseudomorph 

of a shard showing marginal smectite (S) followed inwardly by 

ragged blades of opai-CT (0), Chalk Hills Formation, Idaho. 

Figure 24. Scanning electron micrograph of a pseudomorph 

of a shard showing marginal mammillary opai-CT (0) followed 

inwardly by blocky clinoptilolite (Cp), Chalk Hills Formation, 

Idaho. 

quartz have wavy extinction. The patches of quartz have 

replaced authigenic smectite and clinoptilolite and commonly have obliterated the vitroclastic texture. 

Potassium Feldspar 

Potassium feldspar of probable authigenic ongm 

occurs in tuff and tuffaceous siltstone and mudstone of the 

Chalk Hills Formation. The feldspar was identified on the 

basis of its distinctive X-ray diffractometer pattern (Sheppard and Gude, 1969) in bulk samples where it makes up as 

much as 20 percent of the rock. The relationship of this 

authigenic potassium feldspar to other diagenetic minerals 

is unknown because the feldspar was not recognized in thin 

sections or in scanning electron micrographs. X-ray diffractometry (see appendix) showed that the feldspar occurs 

from Browns Creek to Birch Creek but is most common 

between Browns Creek and Castle Creek (fig. 3). Studies of 

authigenic potassium feldspar in lacustrine tuffs from other 

deposits generally indicate that the feldspar is finely 

crystalline (less than 10 j.lm in size), monoclinic, and pure or 

nearly pure potassium end member (Sheppard and Gude, 

1969). Such authigenic feldspar crystallized after the 

coexisting authigenic zeolites and commonly from the 

precursor zeolites. 

Other Authigenic Minerals 

Zeoli tic tuffaceous rocks in the Chalk Hills Formation 

locally contain authigenic calcite or gypsum. Thin-section 

and scanning electron microscopy studies show that these 

constituents postdate the zeolites. Luster mottling by calcite 

is locally common in the lower part of the zeolitic marker 

14 Zeolitic Diagenesis of Tuffs, Chalk Hills Formation, Snake River Plain, Idaho 
tuff. The calcite crystals are as much as 8 em long, and they 

have replaced authigenic smectite, clinoptilolite, and opalCT. Gypsum, in trace amounts, was recognized by X-ray 

diffraction in certain tuffs and tuffaceous mudstones. 

Gypsum also occurs locally in zeolitic tuff as small 

spherical concretions (as much as 2 em in diameter) and as 

veins of fibrous satin spar. 

PARAGENESIS 

The relative age relationships of diagenetic minerals 

in the tuffaceous rocks of the Chalk Hills Formation were 

determined by the sequence of filling of voids and oncehollow pseudomorphs of vitric particles and by replacement. The mineral in the interior of a void or pseudomorph 

presumably formed later than the mineral(s) nearer the 

periphery. The following sequences of crystallization were 

compiled from thin-section and scanning electron microscopy examination (the earliest mineral is listed first): 

Smectite-clinoptilolite 

Smectite-clinoptilolite-mordenite 

Smectite-clinoptilolite-opal-CT 

Smectite-clinoptilolite-calcite 

Smectite-clinoptilolite-quartz-calcite 

S mecti te-philli psi te-clinoptilo li te±eri oni te 

S mecti te-erionite 

Smectite-clinoptilolite-erionite 

Smectite-clinoptilolite-quartz 

Opal-CT -clinoptilolite 

Smectite--opal-CT -clinoptilolite 

The first three sequences listed above are the common ones 

observed in tuffs of the zeolite zone. Smectite consistently 

crystallized prior to any zeolite. At the only occurrence of 

phillipsite in the Chalk Hills Formation, the phillipsite 

formed earlier than the coexisting zeolites. Except for this 

phillipsite occurrence, clinoptilolite is the zeolite that generally succeeded smectite. Mordenite and erionite postdated 

clinoptilolite, but the paragenetic relationship between mordenite and erionite is unknown because they have not been 

observed to coexist in the tuffaceous rocks studied. Erionite 

rarely crystallized on clinoptilolite crystals that had been 

affected by partial dissolution (fig. 25). Both quartz and 

calcite are late diagenetic minerals and locally have 

replaced smectite, clinoptilolite, and opal-CT. Calcite seems 

to have been the latest mineral to form because it locally has 

replaced authigenic quartz. 

GENESIS OF DIAGENETIC SILICATE 

MINERALS 

Factors that may control the formation and distribution of zeolites and other diagenetic silicate minerals 

Figure 25. Scanning electron micrograph of fibrous erion.ite 

(E) that grew on corroded clinoptilolite (Cp), Chalk Hills 

Formation, Idaho. Energy-dispersive X-ray analyses indicated 

that the Si :AI ratios of clinoptilolite and erionite are 4.1-4.2 

and 3.2-3.8, respectively. 

in sedimentary rocks were summarized by Hay (1966, 1986) 

as composition, permeability, and age of the host rock, and 

metastable crystallization, temperature, pressure, and chemistry of the pore water. Inasmuch as all the diagenetic 

silicate minerals in the Chalk Hills Formation occur within 

a given tuff, differences in the composition, permeability, 

and age of the host rock cannot explain their distribution 

pattern. The Chalk Hills Formation was subjected to only 

shallow burial; therefore, the temperature and pressure 

during diagenesis were relatively low. Besides, the difference, if any, in the depth of burial between the fresh-glass 

zone and the zeolite zone was seemingly not great enough to 

cause the observed pattern. The present diagenetic silicate 

mineralogy of the tuffs and the pattern of distribution of the 

diagenetic zones probably reflect differences in the chemistry of pore water during diagenesis. 

Experimental work by others and theoretical considerations indicate that the activity ratio of alkali ions to 

hydrogen ions and the activity of silica are the major 

chemical parameters of the pore water that control whether 

clay minerals or zeolites will crystallize under those conditions approximating surface temperatures and pressures 

(Hemley, 1959; Garrels and Christ, 1965; Hess, 1966, 

Barth-Wirsching and Holler, 1989). Zeolites are favored 

over clay minerals by relatively high alkali ion to hydrogen 

ion activity ratios and by relatively high silica activities. 

Such conditions in nature are especially common in the 

depositional environment of a saline, alkaline lake (Surdam 

and Sheppard, 1978). 

The zeolites and associated authigenic silicate minerals in the tuffaceous rocks of the lacustrine Chalk Hills 

Formation undoubtedly formed during diagenesis by 

hydrolysis and dissolution of silicic glass by interstitial 

water. The pattern of alteration and the lateral gradation of 

unaltered (fresh) glass to an assemblage of smectite, 

Genesis of Diagenetic Silicate Minerals 15 
clinoptilolite, and opal-CT strongly suggest that the 

interstitial water causing the alteration was the connate 

water trapped in the tuffaceous rocks during lacustrine 

deposition and that the lake water was laterally chemically 

zoned. 

Although regional studies by Kimmel ( 1979) and 

sedimentologic studies by Middleton and others ( 1985) 

indicate that the Chalk Hills Formation and the overlying 

Glenns Ferry Formation were deposited in large, essentially 

freshwater lakes, the lake water coinciding with the distribution of the zeolite zone in the Chalk Hills Formation 

was probably characterized by low to moderate salinity and 

a pH of 7-9. Zeolites have not been recognized in the 

overlying Glenns Ferry Formation where freshwater conditions remained throughout that depositional interval. 

Neither evaporite minerals or molds of evaporite minerals, 

indicative of high salinities, nor Magadi-type chert, indicative of high salinities and high pH (9-11), have been 

recognized anywhere in the Chalk Hills Formation. 

Diatomites and diatomaceous sediments are common 

in the fresh-glass zone of the Chalk Hills Formation but are 

absent or rare in the zeolite zone. Mudstone, siltstone, and 

tuff from the zeolite zone are either barren of diatoms or 

contain highly corroded tests. Zeolitic tuffaceous rocks 

locally contain diatoms that have been replaced by 

clinoptilolite. Diatom-bearing mudstone was collected from 

the fresh-glass zone near the zeolite zone, and its presence 

there suggests deposition in lake water that was essentially 

fresh but slightly saline and alkaline (J.P. Bradbury, written 

commun., 1986). 

Ostracodes provide information on the chemical 

composition of the ancient lakes where they lived (Forester, 

1986). Clinoptilolite-bearing tuffaceous sandstone from one 

locality (NE1

/4SW1/4 sec. 33, T. 5 S., R. 1 E.) in the zeolite 

zone of the Chalk Hills Formation contains a diverse 

assemblage of ostracode species that suggest a lake-water 

salinity greater than 300 mg/L (milligrams per liter) but less 

than 3 g/L (grams per liter) and a pH of about 8-9 (R.M. 

Forester, written commun., 1988). 

Thus, the indirect evidence from the Chalk Hills 

Formation suggests that the lake water trapped in the 

tuffaceous rocks that now contain clinoptilolite was of low 

to moderate salinity and alkalinity. 

Formation of Zeolites from Silicic Glass 

Solution of silicic glass by the lake water trapped 

during deposition of the tuffaceous rocks provided the 

materials necessary for the crystallization of zeolites. The 

initial alteration of rhyolitic glass in the tuffaceous rocks by 

hydrolysis to form the early smectite probably raised the 

pH, activity of Si02 , and (Na++K+):H+ activity ratio of the 

pore water to levels where clinoptilolite crystallized. The 

increased pH and salinity of the pore water also caused 

continual and rapid dissolution of the v1tnc material 

(Mariner and Surdam, 1970). Assuming a relatively closed 

system during the alteration, the crystallization of smectite 

and clinoptilolite resulted in an excess of Si02 that 

crystallized chiefly as opal-CT. Dissolution of diatoms also 

provided additional Si02 to the interstitial fluid. 

During the later stages of diagenesis, slight changes in 

the pH and cation ratios of the pore water probably were 

responsible for the local minor to trace amounts of mordenite and erionite that crystallized after clinoptilolite. 

Experimental work by Mariner and Surdam (1970) indicated that the Si:Al ratio of the zeolite is controlled by the 

Si:Al ratio of the solution from which the zeolite crystallized. Their experiments showed that the Si:Al ratio of the 

solution decreased as the pH increased. Thus, relatively 

aluminous zeolites, such as erionite and phillipsite, are 

favored over siliceous zeolites, such as clinoptilolite and 

mordenite, as the pH is increased. Relatively high Na:K 

ratios favor mordenite over clinoptilolite (Hawkins, 1981; 

Barth-Wirsching and Holler, 1989). As scanning electron 

micrographs (fig. 25) have shown, clinoptilolite in tuff of 

the Chalk Hills Formation was only rarely corroded prior to 

the crystallization of erionite. Although mordenite also 

postdated the clinoptilolite in the tuffs, there is no evidence 

of dissolution of clinoptilolite prior to crystallization of 

mordenite. 

Chemical Changes During Zeolitic 

Diagenesis 

Chemical analyses of bulk samples of clinoptiloliterich marker tuff from the Chalk Hills Formation are given in 

table 3, along with a chemical analysis of an unaltered 

(fresh) bulk sample from the same tuff in the fresh-glass 

zone. In addition to the obvious and significant increase in 

the H20 content, other approximate changes in chemical 

composition from fresh glass to zeolitic tuff were inferred 

by assuming that the Al20 3 content remained constant 

during diagenetic alteration, although this assumption has 

not been rigorously tested. Gains and losses of chemical 

components for the clinoptilolite-rich tuffs given in table 3 

were determined by (1) recalculation of all the analyses 

to the anhydrous form and normalizing to 100 percent, 

(2) then, recalculation of these normalized analyses of the 

clinoptilolite-rich tuffs, assuming constant Al20 3 content of 

the fresh glass, and (3) finally, comparison of the Al20 3-

constant analyses with the anhydrous analysis of the fresh 

glass. 

The clinoptilolite-rich tuffs differ significantly in 

composition from the fresh glass. Relative to the unaltered 

glass, all the clinoptilolite-rich tuffs show consistent gains 

of MgO and CaO and losses of Si02 and K20. Two of the 

three zeolitic tuffs also show losses of Fe20 3 and Na20. The 

most obvious chemical changes in the clinoptilolite-rich 

16 Zeolitic Diagenesis of Tuffs, Chalk Hills Formation, Snake River Plain, Idaho 
Table 3. Chemical analyses and chemical comparison of unaltered and clinoptilolite-rich marker tuff from the Chalk Hills 

Formation, Owyhee County, Idaho 

[H20 detennined by loss on ignition at 900 oc. A, uncorrected analysis in weight percent; B, analysis recalculated to anhydrous and nonnalized to 100 

percent; C. gains ( +) and losses (-) in weight percent assuming the constant AI 20 3 value of the unaltered tuff (analysis 1 B). Analyses by X-ray fluorescence 

by A.J. Bartel, K.C. Stewart, and J.E. Taggart] 

Sample ....... 1 2 3 4 

A B A B c A B c A B c 

Si02 .......... 71.7 76.90 62.8 73.72 -18.70 62.4 73.29 -14.83 62.0 74.33 -17.64 

Al20 3 ...••..• 11.4 12.23 13.2 15.49 12.3 14.44 12.8 15.34 

Fe20 3 .•.•••.• 1.87 2.00 0.66 0.77 -1.39 2.77 3.25 +0.75 0.84 1.01 -1.20 

MgO .......... 0.12 0.13 1.01 1.18 +0.80 1.37 1.61 +1.23 1.26 1.51 +1.07 

CaO ........... 0.55 0.59 1.60 1.88 +0.89 3.36 3.94 +2.75 4.01 4.81 +3.24 

Na2 0 ......... 1.92 2.06 3.11 3.65 +0.82 1.70 2.00 -0.37 0.61 0.73 -1.48 

K2 0 ........... 5.46 5.86 2.36 2.77 -3.67 0.98 1.15 -4.89 1.70 2.04 -4.23 

Ti02 •.••.•.•.. 0.19 0.20 0.46 0.54 +0.23 0.21 0.25 +0.01 0.19 0.23 -0.02 

MnO .......... 0.03 0.03 <0.02 -0.03 0.06 0.07 +0.03 <0.02 -0.03 

H20 ........... 6.37 14.4 14.9 15.7 

Total.. .... 99.61 100.00 99.60 100.00 100.05 100.00 99.11 100.00 

SAMPLE DESCRIPTIONS 

1. Unaltered tuff, Hart Creek; NW11<1SEY4 sec. 4, T. 5 S., R. 1 W.; sample No. CC-46D. 

2. Clinoptilolite-rich tuff, between Hart and Browns Creeks; SE11<1SEY4 sec. 3, T. 5 S., R. 1 W.; sample No. CC-42C. 

3. Clinopti1o1ite-rich tuff, Castle Creek; SW1

1<1SE1,4 sec. 32, T. 5 S., R. 1 E.; sample No. CC-37 A. 

4. Clinoptilolite-rich tuff, Birch Creek; NWY4NE1,4 sec. 15, T. 6 S., R. 1 E.; sample No. CC-21-5B. 

tuffs are the gains of alkaline earth elements and the losses 

of alkalis and silica. Similar gains and losses have been 

noted for the diagenetic alteration of silicic glass to mainly 

clinoptilolite in the John Day Formation (middle Tertiary) 

of Oregon (Hay, 1963) and the Ricardo Formation (Miocene 

and Pliocene) of California (Sheppard and Gude, 1965). 

Age of Zeolitic Diagenesis 

Zeolites can form relatively rapidly from silicic vitric 

material in a saline, alkaline lake. Their formation from 

rhyolitic glass probably is facilitated by the rapid rate of 

solution and high solubility of the glass at a pH above 9 

(Surdam and Sheppard, 1978). Rhyolitic tuffs near the 

surface of Teels Marsh, a saline, alkaline playa in western 

Nevada, are altered to zeolites. Studies of Teels Marsh by 

Taylor and Surdam (1981) indicated that alteration of the 

uppermost tuff at a depth of about 30 em took 1,000 years 

or less. In the highly saline, alkaline diagenetic environments of Searles Lake, California, silicic ash altered to 

zeolites in less than 9,000 years (Hay and Guldman, 1987). 

The age of the zeolitic alteration of tuffs in the Chalk 

Hills Formation is unknown, but the zeolitization probably 

was not as rapid as that given for the above lacustrine 

deposits because both the salinity and pH of the pore water 

in the zeolite zone of the Chalk Hills Formation were lower 

than that in these examples. Nevertheless, indirect evidence 

suggests that zeolitization of the Chalk Hills tuffs was 

completed prior to deposition of the unconformably overlying Glenns Ferry Formation. Some of the calcareous ooids 

in the basal oolite of the Glenns Ferry Formation contain 

nuclei of subrounded, clinoptilolite-rich tuff. Although the 

source of the zeolitic nuclei is uncertain, a probable source 

is zeolitic tuff in the Chalk Hills Formation. Inasmuch as the 

unconformity separating the two formations represents 

more than a million years (Swirydczuk and others, 1982), 

ample time was available for zeolitization, tilting, and 

erosion of the marginal part of the Chalk Hills Formation 

prior to deposition of the Glenns Ferry Formation. 

CONCLUSIONS 

A significant deposit of clinoptilolite, having economic potential, formed during diagenesis of rhyolitic, 

vitric tuffs in the lacustrine Miocene Chalk Hills Formation 

of southwestern Idaho. The marker tuff, in particular, is 

5-13 m thick between Browns and Castle Creeks and 

generally consists of 70 to nearly 100 percent clinoptilolite. 

Authigenic smectite and opal-CT commonly coexist with 

the clinoptilolite. About 35 million short tons of 

clinoptilolite-rich marker tuff are still preserved between 

Hart and Birch Creeks. 

The salinity and pH of the Chalk Hills lake water 

were probably lower than that of typical Cenozoic saline, 

alkaline-lake deposits of the western United States (Surdam 

and Sheppard, 1978). As a result, abundant clinoptilolite 

formed rather than chabazite, erionite, phillipsite, and (or) 

analcime. Experimental work by Mariner and Surdam 

Conclusions 17 
(1970) and Barth-Wirsching and Holler (1989) showed that 

a relatively low alkalinity (pH=7-9) should favor the 

crystallization of clinoptilolite rather than a zeolite of lower 

Si:Al ratio, such as phillipsite. The assemblage of clinoptilolite and associated minerals in the Chalk Hills tuffs and 

their inferred origin are similar to the alteration recently 

described by Hay and Guldman (1987) for vitric, silicic 

tuffs in the lower part of Searles Lake, California. 

REFERENCES CITED 

Alietti, Andrea, 1972, Polymorphism and crystal-chemistry of 

heulandites and clinoptilolites: American Mineralogist, 

v. 57, p. 1448-1462. 

Barth-Wirsching, Ulrike, and Holler, Helmut, 1989, Experimental 

studies on zeolite formation conditions: European Journal of 

Mineralogy, v. 1, p. 489-506. 

Boles, J.R., 1972, Composition, optical properties, cell dimensions, and thermal stability of some heulandite group 

zeolites: American Mineralogist, v. 57, p. 1463-1493. 

Ekren, E.B., Mcintyre, D.H., Bennett, E.H., and Malde, H.E., 

1981, Geologic map of Owyhee County, Idaho, west of 

longitude 116° W.: U.S. Geological Survey Miscellaneous 

Investigations Map I-1256. 

Forester, R.M., 1986, Determination of the dissolved anion composition of ancient lakes from fossil ostracodes: Geology, 

v. 14, p. 796-798. 

Galli, Ermanno, and Loschi Ghittoni, A.G., 1972, The crystal 

chemistry of phillipsites: American Mineralogist, v. 57, 

p. 1125-1145. 

Garrels, R.M., and Christ, C.L., 1965, Solutions, minerals, and 

equilibria: New York, Harper and Row, 450 p. 

Gude, A.J., 3rd, and Sheppard, R.A., 1981, Woolly erionite from 

the Reese River zeolite deposit, Lander County, Nevada, 

and its relationship to other erionites: Clays and Clay 

Minerals, v. 29, p. 378-384. 

Hawkins, D.B., 1981, Kinetics of glass dissolution and zeolite 

formation under hydrothermal conditions: Clays and Clay 

Minerals, v. 29, p. 331-340. 

Hay, R.L., 1963, Stratigraphy and zeolitic diagenesis of the John 

Day Formation of Oregon: University of California Publications in Geological Sciences, v. 42, no. 5, p. 199-262. 

___ 1966, Zeolites and zeolitic reactions in sedimentary rocks: 

Geological Society of America Special Paper 85, 130 p. 

___ 1986, Geologic occurrence of zeolites and some associated 

minerals, in Murakami, Y., Iijima, A., and Ward, J.W., eds., 

New developments in zeolite science and technology: 

Tokyo, Kodonsha, 7th International Zeolite Conference, 

Proceedings, Tokyo, August 1986, p. 35-40. 

Hay, R.L., and Guldman, S.G., 1987, Diagenetic alteration of 

silicic ash in Searles Lake, California: Clays and Clay 

Minerals, v. 35, p. 449-457. 

Hemley, J.J., 1959, Some mineralogical equilibria in system 

K20-Al20 3-Si02-H20: American Journal of Science, 

v. 257, p. 241-270. 

Hess, P.C., 1966, Phase equilibria of some minerals in the 

K20-Na20-Al20 3-Si02-H20 system at 25°C and 1 atmosphere: American Journal of Science, v. 264, p. 289-309. 

Kimmel, P.G., 1979, Stratigraphy and paleoenvironments of the 

Miocene Chalk Hills Formation and Pliocene Glenns Ferry 

Formation in the western Snake River Plain, Idaho: Ann 

Arbor, University of Michigan, Ph.D. dissertation, 331 p. 

___ 1982, Stratigraphy, age, and tectonic setting of the 

Miocene-Pliocene lacustrine sediments of the western 

Snake River Plain, Oregon and Idaho, in Bonnichsen, Bill, 

and Breckenridge, R.M., eds., Cenozoic geology of Idaho: 

Idaho Bureau of Mines and Geology Bulletin 26, p. 559-

578. 

Malde, H.E., and Powers, H.A., 1962, Upper Cenozoic stratigraphy of western Snake River Plain, Idaho: Geological 

Society of America Bulletin 73, p. 1197-1220. 

Mariner, R.H., and Surdam, R.C., 1970, Alkalinity and formation 

of zeolites in saline alkaline lakes: Science, v. 170, p. 977-

980. 

Middleton, L.T., Porter, M.L., and Kimmel, P.G., 1985, 

Depositional settings of the Chalk Hills and Glenns Ferry 

Formations west of Bruneau, Idaho, in Flores, R.M., and 

Kaplan, S.S., eds., Cenozoic paleogeography of the westcentral United States: Denver, Colo., Rocky Mountain 

Section, Society of Economic Petrologists and Mineralogists, p. 37-53. 

Mumpton, F.A., 1960, Clinoptilolite redefined: American Mineralogist, v. 45, p. 351-369. 

___ 1983, Discovery and commercial interest in zeolite 

deposits examined during Zeo-Trip '83, in Mumpton, F.A., 

ed., Zeo-Trip '83: Brockport, N.Y., International Committee on Natural Zeolites, p. 66-72. 

Mumpton, F.A., and Ormsby, W.C., 1976, Morphology of zeolites 

in sedimentary rocks by scanning electron microscopy: 

Clays and Clay Minerals, v. 24, p. 1-23. 

Passaglia, Elio, 1975, The crystal chemistry of mordenites: 

Contributions to Mineralogy and Petrology, v. 50, p. 65-77. 

Regis, A.J., 1978, Mineralogy, physical, and exchangeable 

chemistry properties of bentonites from the Western United 

States, exclusive of Montana and Wyoming: U.S. Bureau of 

Land Management Technical Note T/N 315, 35 p. 

Sheppard, R.A., and Fitzpatrick, J.J., 1989, Phillipsite from silicic 

tuffs in saline, alkaline-lake deposits: Clays and Clay 

Minerals, v. 37, p. 243-247. 

Sheppard, R.A., and Gude, A.J., 3d, 1965, Zeolitic authigenesis of 

tuffs in the Ricardo Formation, Kern County, southern 

California, in Geological Survey research 1965: U.S. 

Geological Survey Professional Paper 525-D, p. D44-D47. 

___ 1968, Distribution and genesis of authigenic silicate 

minerals in tuffs of Pleistocene Lake Tecopa, In yo County, 

California: U.S. Geological Survey Professional Paper 597, 

38 p. 

___ 1969, Diagenesis of tuffs in the Barstow Formation, Mud 

flills, San Bernardino County, California: U.S. Geological 

Survey Professional Paper 934, 35 p. 

___ 1982, Mineralogy, chemistry, gas adsorption, and NH4 +-

exchange capacity for selected zeolitic tuffs from the 

Western United States: U.S. Geological Survey Open-File 

Report 82-969, 16 p. 

___ 1983, Guide to selected zeolite deposits in eastern Oregon, 

southwestern Idaho, and northwestern Nevada, in Mumpton, F.A., ed., Zeo-Trip '83: Brockport, N.Y., International 

Committee on Natural Zeolites, p. 1-59. 

18 Zeolitic Diagenesis of Tuffs, Chalk Hills Formation, Snake River Plain, Idaho 
Surdam, R.C., and Sheppard, R.A., 1978, Zeolites in saline, 

alkaline-lake deposits, in Sand, L.B., and Mumpton, F.A., 

eds., Natural zeolites--occurrence, properties, use: Elmsford, N.Y., Pergamon Press, p. 145-174. 

Swirydczuk, Krystyna, Larson, G.P., and Smith, G.R., 1982, 

Volcanic ash beds as stratigraphic markers in the Glenns 

Ferry and Chalk Hills Formations from Adrian, Oregon, to 

Bruneau, Idaho, in Bonnichsen, Bill, and Breckenridge, 

R.M., eds., Cenozoic geology of Idaho: Idaho Bureau of 

Mines and Geology Bulletin 26, p. 543-558. 

Swirydczuk, Krystyna, Wilkinson, B.H., and Smith, G.R., 1979, 

The Pliocene Glenns Ferry oolite-Lake-margin carbonate 

deposition in the southwestern Snake River Plain: Journal of 

Sedimentary Petrology, v. 49, p. 995-1004. 

Taylor, M.W., and Surdam, R.C., 1981, Zeolite reactions in the 

tuffaceous sediments at Teels Marsh, Nevada: Clays and 

Clay Minerals, v. 29, p. 341-352. 

References Cited 19 
N APPENDIX 0 

N Mineralogic composition of tuffaceous rocks of the Chalk Hills Formation, Owyhee County, Idaho, as estimated from X-ray diffractometer 

I'D patterns of bulk samples 

~ n· [---,looked for but not found; Tr, trace. Mica includes pyrogenic biotite, detrital muscovite and biotite, and authigenic illite. Quartz includes pyrogenic, detrital, and 

0 authigenic varieties. Other: chiefly hornblende; sample CC-IOA contains 7 parts phillipsite and a trace of erionite; samples CC-10-30, CC-I3A, CC-13B, and E:i. 

C1Q CC-58C contain a trace of erionite; sample CC-33J contains one part erionite; sample CC-60B contains 5 parts apatite. Sample location column shows position I'D 

= of sample in meters above(+) or below(-) base of marker tuff (M); U, above marker tuff but exact stratigraphic position unknown; L, below marker tuff but exact I'D 

Cll stratigraphic position unknown; Uk, stratigraphic position unknown; 0, diatomite; Ms, mudstone; Ss, sandstone; St, siltstone, T, tuff. X-ray analyses in parts of 10] (ii. 

0 --1 Locality c Sample Sample Clinop- Potassium Opal- PI agio-

~ No. No. location Glass Mica Smectite tilolite feldspar Quartz CT clase Calcite Gypsum Other 

~ (fig. 3) :::r 

llJ 

;::: I CC-I A -3.05, Ms --- Tr 3 4 Tr 

~ -IO +O.OI, M --- --- --- 3 --- --- --- --- --- 7 

iii -IE +2.44, M --- --- --- 8 --- --- 2 '"TI -IF +5.26, M 0 --- --- I 9 .... 

3 -IG +8.76, M --- --- --- IO --- --- --- --- --- Tr 

a 

o· -IH +I0.82, M --- --- I 9 ... = 

Vl -IN +I5.36, T --- --- I 9 

= llJ -IP +22.38, T --- Tr I 3 I I 4 ll':" 

I'D -II +26.06, St --- I 2 4 2 I --- --- --- Tr 

:;:· "" -IQ +29.30, T --- I 9 ~ 

-= -IR +30.58, T --- --- I 9 E:i"' .?. -IS +37.70, Ms --- Tr 2 3 2 --- 2 Tr 

c: -IT +46.IO, Ss --- I I 3 --- 2 --- 3 llJ 

:::r -IU +61.22, T 10 --- --- --- --- --- --- Tr 0 -IV +66.26, T --- Tr Tr 3 2 --- 5 

-IW +73.82, T --- Tr 2 3 --- 3 2 Tr 

2 CC-550 M, lower part --- Tr 5 --- --- 5 

-55F M, middle part --- Tr 9 --- --- I 

-55E M. upper part --- --- 4 2 --- --- 4 

3 CC-3A L,T --- Tr I 3 I 5 --- Tr 

-3B L, Ss Tr 3 4 --- 2 --- I 

4 CC--4A L,T --- --- 2 3 --- 5 Tr 

--4B L,T --- --- I 3 --- 6 --- Tr 

-56B L, Ss --- Tr I 4 --- 4 --- 1 

5 CC-5 M, upper part --- Tr Tr 6 --- --- 4 

6 CC-6A L. Ss --- Tr Tr 4 --- 4 2 

-6B L,T --- --- --- 3 --- 6 

7 CC-7A L, Ss I Tr 5 --- 2 2 Tr 

-7B L,T --- --- --- Tr --- 9 
Mineralogic composition of tuffaceous rocks of the Chalk Hills Formation, Owyhee County, Idaho, as estimated from X-ray diffractometer 

patterns of bulk samples-Continued 

Locality Sample Sample Clinop- Potassium Opal- PI agio- No. Glass Mica Smectite Quartz Calcite Gypsum Other 

(fig. 3) No. location tilolite feldspar CT clase 

8 CC-8A M, lower part --- Tr 10 

-8B M, lower part --- --- Tr 2 --- --- --- --- 8 

-8C M, upper part --- Tr 4 --- 5 

9 CC-9A M, lower part --- --- --- 4 2 3 

-9B M, middle part 8 --- 2 

-9C M, middle part 10 

-9D M, upper part 2 --- 2 3 --- 2 

10 CC-10-1 -0.01, Ms Tr 2 5 1 Tr 2 Tr 

-lOA +0.01, M --- --- l 2 --- --- --- --- --- 7 

-10-2B +0.03, M --- Tr 9 

-l0-2C +0.06, M --- --- Tr 10 

-lOB +1.00, M --- --- Tr 10 

-lOC +2.75, M --- Tr 10 --- Tr 

-lOD +6.65, M --- --- 2 6 --- --- 2 

-10-3A +14.95, T --- Tr 1 4 1 --- 4 

-10-3B +21.72, T --- Tr 1 4 1 --- 4 

-10-3C +23.36, T --- --- Tr 4 1 --- 5 Tr 

-10-3D +27.00, T --- --- 7 2 --- --- 1 --- --- Tr 

-10-3E +33.02, T --- --- 5 3 --- --- 2 Tr 

-10-4 +38.07, St --- Tr 1 5 2 1 1 

-10-5 +46.23, Ss --- 1 1 3 --- 2 Tr 3 --- Tr 

-10-6A +58.27, St --- l Tr 6 --- 2 

-10-6B +61.12, T --- --- Tr 10 

11 CC-11A L,Ss --- Tr Tr 6 --- 2 --- 2 

-liB L,T --- --- Tr --- --- 9 

-11C L,T --- --- Tr 9 

12 CC-12A -0.30, St 2 --- 6 

-12B +0.02, M 5 --- 5 

-l2C +5.00, M 7 2 

-12D +9.40, Ss 6 Tr 3 

-12F +16.50, Ss 6 --- 3 --- Tr 

> "'0 -12G +25.00, T "'0 7 --- 2 ~ 

:I -12H +26.00, T 7 --- 2 Q. 

;;(' -121 +26.30, T 7 1 Tr 

-12J +27.00, T 6 1 2 

-l2K +27.00, T 6 --- 3 

!:: -12L +28.00, T 5 --- 2 --- --- 2 
N Mineralogic composition of tuffaceous rocks of the Chalk Hills Formation, Owyhee County, Idaho, as estimated from X-ray diffractometer N 

patterns of bulk samples-Continued 

N 

Locality Potas- I'!) 

Sample Clinop- Opal- PI agio- 0 Sample ..... No . No. location Glass Mica Smectite tilolite sium Quartz CT clase Calcite Gypsum Other 

r;· (fig. 3) feldspar 0 

j)j" 13 CC-13A U, T, lower part --- --- 4 5 --- --- l --- --- Tr ~ 

= -l3B U, T, upper part --- --- 1 7 --- --- --- 1 --- 1 Tr I'!) 

Vl 

v;· 

0 14 CC-I4 

- L,T --- Tr Tr 4 --- I 5 Tr 

-1 15 CC-15 L, Ss --- Tr Tr 6 --- 2 --- 2 --- Tr 1: 

__ a: I6 CC-16 L,T --- Tr Tr 7 --- 2 --- 1 --- Tr 

li 

:T I7 CC-I7-2 U,T 6 --- 2 --- --- Tr e/.. 

~ -I7-3A U,T 7 --- 2 --- --- --- --- Tr 

::t: -I7-3B U,Ms 2 I 4 --- --- I --- 1 --- 1 

Vi -l7-3C U, T, lower part 1 --- 1 --- --- --- --- 2 --- 6 Tr "TI 

0 -I7-3D U, T, middle part 8 --- --- --- --- --- --- 2 Tr --- Tr 

3 

~ s· -17-3E U,T, upper part 8 --- Tr 

.. = -17-3F U,T 10 --- Tr --- --- --- Tr fJ) 

= -17-30 U,Ms --- 1 4 --- --- I 2 2 --- Tr ~ 

I'!) 

~ -17-4A U,Ms --- 1 4 --- --- 2 1 2 

:;110 -17-4B U,T 9 :;:-

I'!) 

... 

'"g 18 CC-18A -0.15,Ms --- Tr 2 3 --- --- 4 I --- Tr 

~ -18B +0.02, M --- --- 1 8 --- --- --- --- 1 .. = 

Q:. -I8C +0.30, M --- Tr Tr 8 --- --- 2 --- --- Tr 

~ -18D +0.70, M --- --- Tr 8 --- --- 2 :T 

0 

I9 CC-19A L,T --- --- --- --- --- 10 --- Tr 

-19B L,T --- --- --- Tr --- --- IO 

20 CC-20D -0.50,Ms --- Tr 1 5 2 1 Tr 

-20A +0.02, M --- Tr 3 7 

-20B +1.00, M --- --- Tr 10 

-20C +1.70, M --- --- Tr 7 --- --- 3 

21 CC-21-1 -1.27, Ss --- Tr Tr 2 --- 4 --- 4 

-21-2 -0.74, T --- --- 1 5 --- 2 --- 2 

-21-3 -0.36,Ms --- Tr 1 6 2 I 

-21-4 -0.10, Ss --- --- --- 2 --- 3 --- 3 2 

-21-5A +0.02, M --- --- --- 6 --- --- --- --- 4 

-21-5B +0.30, M --- --- Tr 10 

-21-5C +1.00, M --- --- Tr IO 

-2l-5D +2.00, M --- --- Tr 9 --- --- 1 

-21-5E +3.53, M --- Tr 1 4 Tr --- 5 

-2I-7 +7.15, T --- Tr 1 4 --- 1 4 Tr 
Mineralogic composition of tuffaceous rocks of the Chalk Hills Formation, Owyhee County, Idaho, as estimated from X-ray diffractometer 

patterns of bulk samples-Continued 

Locality Sample Sample Clinop- Potas- Opal- Plagio- No. No. location Glass Mica Smectite tilolite sium Quartz CT clase Calcite Gypsum Other 

(fig. 3) feldspar 

-21-8 +14.44, Ms --- Tr 5 --- --- 2 1 2 

-21-9 +15.74, T 7 Tr 1 --- --- 2 

-21-lOA +19.90, T 6 Tr 2 

-21-108 +38.20, T 5 --- 3 

-21-lOC +43.10, T 6 Tr 2 --- --- 1 1 Tr 

22 CC-22 U,T --- Tr 1 4 Tr 5 Tr 

23 CC-23-1 -14.37, Ss --- Tr 1 3 --- 2 2 2 

-23-2 -13.85, T --- 1 Tr 4 --- --- 5 

-23-3 -9.57, Ms --- Tr 4 3 --- 1 1 1 --- Tr 

-23-4A -5.I7, T --- Tr 3 2 --- 5 Tr 

-23-4B -4.61, Ss --- Tr --- 3 --- 4 3 

-23-5 -3.37, Ss --- --- Tr 1 --- 5 --- 4 

-23-6A -1.72, T --- --- 1 8 

-23-6B -0.20, Ms --- Tr 1 4 2 1 1 I --- Tr 

25 CC-25A Uk, T --- --- Tr I --- --- 9 

-25B Uk, T --- --- --- 1 --- 1 6 Tr 2 

26 CC-26A -0.04, Ms --- --- 2 2 --- I 4 

-26B +0.02, M --- Tr 5 5 

-26C +0.17, M --- Tr 2 8 

-26D +1.12, M --- Tr 2 4 --- --- 4 --- --- Tr 

-26E +2.50, M --- Tr 1 6 --- --- 3 --- --- Tr 

27 CC-27 Uk,T --- Tr 2 5 

28 CC-28 U,T 6 --- 3 

29 CC-29 Uk, T 6 Tr 2 

30 CC-30 Uk, T 8 Tr I --- --- Tr 

> 31 CC-31A L,T 5 Tr 5 --- --- Tr --- Tr "C 

"C -318 L,T 6 Tr 2 --- --- 1 --- I ~ 

;:, 

c.. 

>(' 32 CC-32 U,T 10 

33 CC-33A U,Ms 4 Tr 4 --- --- 1 --- 1 --- Tr 

-33B U,Ms 4 Tr 5 --- --- Tr N 

w -33C U,T 7 Tr 1 --- --- Tr 
N Mineralogic composition of tuffaceous rocks of the Chalk Hills Formation, Owyhee County, Idaho, as estimated from X-ray diffractometer 4:o 

patterns of bulk samples-Continued 

N 

Locality Potas- I'D 

~ No. Sample Sample Glass Mica Smectite Clinop- sium Quartz Opal- Plagio- Calcite Gypsum Other 

;;· (fig. 3) No. location tilolite feldspar CT clase 

0 

~· -330 U,T 10 --- --- --- --- --- --- Tr ~ 

= -33E U,T 8 --- 1 --- --- Tr --- 1 --- Tr Tr I'D -33F U,T 10 V> 

;:;;· 

0 -330 U,T 8 --- 1 --- --- Tr 

-l -33H U,Ms 2 1 5 --- --- 1 c 

~ -331 U, T, lower part 2 Tr 5 1 2 ("") --- --- ---

::r 

~ -331 U, T, upper part --- --- 7 --- --- 1 --- 1 ~ -33K U, Ss Tr l 2 --- --- 4 --- 3 --- --- Tr 

~ iii 

., 34 CC-60A U,Ms Tr 6 --- --- 3 --- Tr 0 -608 U,Ms --- Tr 5 --- --- --- --- --- 5 3 

el. 

a· 35 CC-35A U,Ms 1 Tr 6 --- --- 2 

,= -358 U,T 9 Tr Tr fJ) 

= 

~ 

;II:" 

I'D 36 CC-36A L,T --- I 9 --- --- --- Tr ;:ll:l -368 L, Ss --- Tr Tr 4 --- 3 --- 3 <" I'D -36C ... L, Ss --- 1 Tr 5 --- 2 --- 2 

""CC 

F. 37 CC-37A +0.20, M --- --- 1 9 --- --- --- Tr ,= 

0: -378 +3.25, M --- Tr 6 --- --- 4 ~ -37C +5.90, M --- --- Tr 6 --- --- 4 ::r 

0 

38 CC-38 M, middle part --- --- 1 9 --- --- --- --- Tr 

40 CC-40A -0.20, Ms --- Tr 2 1 1 1 5 Tr 

-408 +0.15,M --- --- 4 2 --- --- 4 

-40C +0.80, M 5 Tr 5 

-400 +1.68, M 5 5 --- --- --- --- Tr 

-40E +7.50, St --- Tr Tr 4 --- --- 5 I 

-40F +IO.OO, Ms --- Tr l 1 1 7 Tr 

41 CC-41A M, lower part Tr 1 4 --- --- 5 

-418 M, upper part --- --- 4 2 --- --- 4 Tr 

42 CC-42A +0.02, M --- Tr 3 3 --- 2 --- 2 

-428 +0.80, M --- Tr 2 2 --- 3 --- 2 

-42C +1.12, M --- Tr --- 10 --- --- --- Tr --- Tr 

-420 +1.68,M --- Tr Tr 8 --- --- --- 2 

43 CC-43A U,T 9 --- --- --- --- --- --- 1 --- --- Tr 

-438 U,O 3 1 2 --- --- 2 --- 2 

-43C U,Ms 2 1 3 --- --- 2 --- 2 
Mineralogic composition of tuffaceous rocks of the Chalk Hills Formation, Owyhee County, Idaho, as estimated from X-ray 

diffractometer patterns of bulk samples-Continued 

Locality Sample Sample Clinop- Potas- Opal- PI agio- No. No. location Glass Mica Smectite tilolite sium Quartz CT clase Calcite Gypsum Other 

(fig. 3) feldspar 

44 CC-44A --0.20, Ms --- --- 5 2 --- 1 2 Tr 

-44B +0.20, M --- --- 2 3 --- --- 4 1 

-44C +0.90, M --- --- 1 6 --- --- 3 

-440 +4.43, M 5 --- 5 --- --- Tr --- --- --- Tr 

-44E +8.10, M 5 --- 5 --·- Tr --- Tr 

-44F +8.30, M 5 --- 5 --- --- --- --- Tr 

45 CC-45-1 --0.20, Ms --- Tr 3 2 --- --- 3 2 --- Tr 

-45-2 +0.30, M 6 --- 2 --- --- --- 2 Tr 

-45-4 +40.41, 0 3 1 4 --- --- 1 --- 1 --- Tr 

46 CC-46A +0.05, M 10 --- --- --- --- --- --- Tr 

-460 +0.10, M 10 

-46B +0.30, M 5 --- 5 --- --- --- Tr Tr 

-46C +1.99, M 7 Tr 3 --- --- --- Tr 

47 CC-47 M, lower part 8 --- 2 --- --- --- --- Tr 

48 CC-48 Uk,Ss --- 1 1 --- 4 --- 4 

49 CC-49A -3.41, T 5 --- 5 --- --- --- Tr 

-49C -1.68, Ms --- Tr 4 2 --- 2 --- 2 

-490 --0.02, Ms --- 1 6 1 --- 1 1 Tr 

-49E +0.63, M 10 Tr --- --- --- Tr 

-49F +3.36, M 9 Tr 1 --- --- Tr 

50 CC-50A L, Ss --- Tr 3 --- --- 3 --- 4 

-SOB L, Ss --- Tr 2 1 --- 3 --- 4 

-SOC L,T --- --- --- Tr --- Tr 5 Tr 5 

51 CC-51A L, St --- Tr 2 2 --- 2 3 1 

-SIB L, Ss --- Tr Tr 4 --- 2 1 3 

52 CC-52A M, lower part 1 --- Tr 9 

-52B M, middle part 9 --- 1 Tr --- --- --- Tr 

> "t:l -52C M, upper part --- --- 1 1 --- Tr 8 "t:l 

I'D 

~ 

c.. 53 CC-53A --0.50, Ms --- 1 2 3 --- 2 >(' 

-53B +0.05, M --- --- Tr 10 

-530 +1.75, M --- --- 1 9 --- --- --- --- Tr 

-53E +3.50, M --- --- 1 8 --- Tr N 

c.n 
N Mineralogic composition of tuffaceous rocks of the Chalk Hills Formation, Owyhee County, Idaho, as estimated from X-ray a'\ 

diffractometer patterns of bulk samples-Continued 

N Locality Potas- (!) 

S= No. Sample Sample Glass Mica Smectite Clinop- sium Quartz Opal- PI agio- Calcite Gypsum Other 

;;· (fig. 3) No. location tilolite feldspar CT clase 

0 

6j' 54 CC-54 L, Ss --- Tr 1 4 3 --- 2 --- Tr C1Q 

(!) 

:::l 

(!) 

57 CC-57A M, lower part Tr 2 6 2 Cl> --- --- --- iii' 

0 -578 M, upper part --- --- 2 6 --- --- 2 

--1 

c 58 CC-58A U,T 2 ~ 6 --- --- --- 1 1 Tr --- --- Tr 

("') -588 U,Ms --- Tr 7 --- --- 2 --- 1 Tr Tr :r -58C U, Ss --- --- 1 --- --- 9 Tr --- --- --- Tr 1:.1 

;:: 

~ 59 CC-59 U,Ms --- Tr 2 2 --- 1 5 Tr --- Tr 

iii 

'TI 

0 61 CC-61A L,T 9 --- 1 --- --- Tr --- Tr 3 

~ -618 L,T --- --- Tr --- --- --- 10 

s· -61C L,T --- Tr 1 5 --- 2 --- 2 ... :::~ 

Vl 

:::l 63 CC-63A U,T 8 --- 2 1:.1 

~ 

(!) -638 U,Ms --- 1 5 --- --- 2 ~ -63C ::t U,T 5 --- 2 --- --- 2 Tr (!) 

"'t 

'"l:l 64 CC-64A M, lower part 4 --- 2 --- --- 2 1 1 --- Tr [ 

... :::~ -648 M, middle part 6 --- 3 --- --- --- Tr 1 Tr 

c::. 

1:.1 

:r 65 CC-65A M, lower part --- --- 3 4 --- --- 3 0 -658 M, middle part --- --- 2 5 --- --- 3 

-65C M, upper part --- --- 3 5 --- --- 2 

66 CC-66A -0.05, Ms --- 1 6 --- --- 2 

-66B +0.30, M 8 --- 2 

67 CC-67A -0.20, Ms --- --- 5 --- --- 3 Tr 2 

-678 +0.04, M 9 Tr 

-67C +0.20, M 10 

68 CC-68A -0.20, Ms 2 Tr 2 4 

-688 +O.lO,M 7 --- Tr 1 --- Tr 2 Tr 

-68C +0.40, M 9 --- 1 Tr 

-68D +2.33, M 2 --- Tr 7 

-68E +3.10, M 8 --- I 1 --- --- --- Tr 

69 CC-69 L, T --- --- Tr 10 

70 CC-70A M, lower part --- --- --- 8 --- --- 2 

-708 M, upper part --- --- 2 8 
c * 0 

Gl 

0 

< m 

:D 

z 

;5: 

m 

z 

-i 

-c 

:D 

z 

-i 

z 

Gl 

~ 0 

!1:1 

~ u, 

-..j 

~ .,.. 

~ "' 

a> 

~ 

> '"CI 

'"CI 

t'l) 

= c.. 

>(' 

N 

....... 

Mineralogic composition oftuffaceous rocks of the Chalk Hills Formation, Owyhee County, Idaho, as estimated from X-ray diffractometer 

patterns of bulk samples-Continued 

Locality Potas- Sample Sample Clinop- Opal- PI agio- No. Glass Mica Smectite sium Quartz Calcite Gypsum Other No. location tilolite CT clase (fig. 3) feldspar 

71 CC-71-1 -25.56, T --- --- 1 5 --- 2 2 

-71-2 -22.35, T --- 1 2 2 5 --- Tr 

-71-3 -19.00, Ss --- Tr 1 3 --- 4 --- 2 

-71-5A +0.04, M --- --- Tr 10 

-71-5B +3.75, M --- --- 1 9 

-71-5C +6.50, M --- Tr 10 

-71-6 +8.27, St --- --- 2 6 --- 2 

-71-7 +9.22, St --- Tr 2 5 --- 1 --- 2 

72 CC-72A +0.20, M --- --- Tr 10 

-72B +2.00, M --- Tr 10 

-72C +5.60, M Tr 1 8 --- Tr 

73 CC-73A +0.20, M --- --- 1 5 --- --- 4 

-73B +1.00, M --- --- 1 6 --- --- 3 

-73C +2.00, M --- 1 9 

-73D +4.50, M --- --- 2 8 

-73E +6.25, M --- 1 7 --- --- 2 

74 CC-74 M, lower part --- --- 1 7 --- 2 

75 CC-75 L, T 6 --- 4 

76 CC-76A Uk, T, lower part --- --- --- 6 --- --- 4 

-76B Uk, T, middle part --- --- --- 7 --- --- 3 

-76C Uk, T, upper part --- Tr 2 6 2 --- --- --- Tr 

-76D Uk, Ss --- Tr --- 2 4 1 3 

77 CC-77 Uk, T 8 Tr 2 

78 CC-78A M, lower part 9 Tr 

-78B M, upper part 9 Tr 1 --- --- Tr 

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