Contributions to Industrial-Minerals Research

U.S. Department of the Interior

U.S. Geological Survey

Bulletin 2209–I

Geology of a Middle Tertiary Clay Deposit in the 

Patagonia Mountains near Harshaw, Santa Cruz 

County, Southeastern Arizona

Chapter I of
Contributions to Industrial-Minerals Research

James D. Bliss, Phillip R. Moyle, and Keith R. Long, Editors

Bulletin 2209–I

U.S. Department of the Interior

U.S. Geological Survey 

Geology of a Middle Tertiary Clay Deposit in the 

Patagonia Mountains near Harshaw, Santa Cruz 

County, Southeastern Arizona

By Brenda B. Houser

Chapter I of
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

Version 1.0, 2005

This publication is available only online at:

http://pubs.usgs.gov/bul/b2209-i/

Text edited by George A. Havach

Layout by Stephen L. Scott

Manuscript approved for publication, July 24, 2003

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 
CONTENTS

Abstract--------------------------------------------------------------------------------------------------- 1

Introduction----------------------------------------------------------------------------------------------- 1

Geologic Setting----------------------------------------------------------------------------------------- 2

Northeastern Tuff with Clay Facies and Limestone Cap--------------------------------------- 2

 Depositional Environment of the Harshaw Clay------------------------------------------------ 5

Description of the Harshaw Clay----------------------------------------------------------------------- 5

Summary-------------------------------------------------------------------------------------------------- 6

Acknowledgments--------------------------------------------------------------------------------------- 6

References Cited----------------------------------------------------------------------------------------- 7

Figures

1. Map of southeastern Arizona, showing location of the Harshaw clay relative to 

 Tucson and major highways----------------------------------------------------------------- 1

2. Map of Harshaw, Ariz., area in Coronado National Forest, showing areas of

 Tertiary tuff with which the Harshaw clay is associated--------------------------------- 2

3. Part of a geologic map of the Nogales and Lochiel quadrangles, southeastern 

 Arizona, showing outcrop areas of Tertiary tuff with which the Harshaw 

 clay is associated and relation of tuff to Cretaceous andesite and Tertiary 

 and Quaternary alluvium--------------------------------------------------------------------- 3

4. Detailed geologic map of the Tertiary tuff and limestone and the Harshaw clay 

 in the Harshaw quadrangle, southeastern Arizona--------------------------------------- 4

5. Photographs of outcrops of the Harshaw clay near sample locality shown in 

 figure 4------------------------------------------------------------------------------------------ 5

6. Photographs of test tiles of Harshaw clay collected at sample locality shown 

 in figures 4 and 5------------------------------------------------------------------------------ 6

III
1

Abstract

A middle Tertiary rhyolite tuff on the northeast side of 

the Patagonia Mountains in Santa Cruz County, southeastern 

Arizona contains lenses of calcareous low-swelling montmorillonite clay, as much as 10 to 15 m thick. The presence of 

the tuff has been known for years, but the clay has not been 

described previously. The clay lenses, which are virtually siltand sand-free, were probably formed by diagenetic alteration 

of fairly clean ash-fall-tuff beds. In preliminary tests, the clay 

exhibited only about 9 percent shrinkage on drying and about 

1 percent shrinkage on firing. Cracking and distortion were 

minimal in both drying and firing. Further testing needs to be 

done on the clay to determine its suitability as a specialty clay 

or as an additive to other clays.

Introduction

This chapter reports on a previously undescribed montmorillonite clay deposit associated with upper Oligocene 

rhyolite tuff. The deposit, which is in Coronado National 

Forest on the northeast flank of the Patagonia Mountains 

in southeastern Arizona (figs. 1, 2), is informally termed 

the “Harshaw clay” after the nearby mining ghost town of 

Harshaw. Although the deposit may consist of common clay 

and thus may not have a high value, its characteristics have 

not been tested adequately. Additional testing may show it 

to have value as a specialty clay or as an additive to other 

clays.

Sedimentary clay deposits of middle Miocene through 

Holocene age (younger than the Harshaw clay) are fairly 

abundant and widespread in the southern Basin and Range 

Province of Arizona, where they were deposited chiefly in 

lakes and playas of closed basins (Patterson, 1969). Because 

of their position in valleys, the younger deposits generally are 

easily accessible and near population centers. However, most 

clays from these geologic settings are common clays that 

contain an unacceptable amount of mixed-layer smectite, an 

expandable-clay mineral which may cause excessive shrinkage, warping, and cracking during drying. Thus, many clay 

deposits in Arizona are economically undesirable.

The Harshaw clay of the present study is composed of 

smectite but in the form of low-swelling Ca montmorillonite. 

Preliminary tests show that shrinkage of the clay is about 10 

percent upon drying, with no cracking and with minimal additional shrinkage upon firing. The relatively low shrinkage, 

and identification of the dominant clay mineral as Ca montGeology of a Middle Tertiary Clay Deposit in the 

Patagonia Mountains near Harshaw, Santa Cruz 

County, Southeastern Arizona

By Brenda B. Houser

Figure 1. Southeastern Arizona, showing location of the Harshaw 

clay (in red) relative to Tucson and major highways. Base from U.S. 

Geological Survey 1:500,000-scale map (1974).

 Introduction
2 Contributions to Industrial-Minerals Research

morillonite, are encouraging enough to recommend further 

testing of material from this deposit.

Geologic Setting

The Harshaw clay crops out on the northeast side of the 

Patagonia Mountains, where it is associated with Tertiary 

rhyolite tuff (units Tt, Tl, fig. 3; Simons, 1974). The tuff, 

which is exposed on both the northeast and northwest sides 

of the mountains, probably was deposited across the intervening area as well, but it was removed by erosion during 

Basin and Range faulting and uplift of the Patagonia Mountains in late Tertiary time. The source of the tuff is unknown, 

although it is coeval with the Oligocene rhyodacitic to 

rhyolitic Grosvenor Hills Volcanics to the northwest in the 

adjacent southern Santa Rita Mountains (fig. 1; Drewes, 

1968, p. C14–C15; Drewes, 1971; Simons, 1974). Simons 

reported four K-Ar ages for the tuff that average 25.3 Ma. 

A new 40Ar/ 39Ar date of 27.65 ± 0.25 Ma was obtained on 

hornblende in the tuff for this study (M.J. Kunk, written 

commun., 2004). Because of the greater precision of the 

40Ar/ 39Ar method, 27.65 ± 0.25 Ma is the preferred age the 

tuff. 

The tuff is similar on both sides of the range, although 

its massive clay facies apparently is present only on the 

northeast side. The section on the northwest side of the Patagonia Mountains consists of as much as 45 m of pumiceous 

biotite rhyolite ash-fall and ash-flow tuff. Simons (1974) 

reported that the contact is unconformable between the tuff 

and underlying Upper Cretaceous volcanic rocks. The top 

of the tuff is unconformably(?) overlain by middle or upper 

Tertiary conglomerate.

Northeastern Tuff with Clay Facies and

Limestone Cap

The tuff section on the northeast side of the Patagonia 

Mountains also is as much as 45 m thick, but in places 10 to 

15 m of the section consists of massive lenticular clay bodies, 

in addition to ash-fall and ash-flow tuff (figs. 4, 5). Furthermore, in the easternmost 0.6 km of the outcrop belt (east of 

U.S. Forest Service Road 58, fig. 4), the upper 2 to 5 m of the 

section is an ash-fall tuff that has been altered to very hard 

limestone and opaline chalcedony with wavy laminar bedding 

(Tl of Simons, 1974). The presence of clay and the limestone 

cap suggests that the tuff on the northeast side of the Patagonia Mountains had a different diagenetic history from that on 

the northwest side.

The northeastern tuff crops out in a band, about 4 km 

long by 0.6 km wide, between Upper Cretaceous volcanic 

rocks (unit Ka, figs. 3, 4) on the north and middle and upper 

Cenozoic sedimentary rocks (units Tal, QTal) on the south. 

Along the north edge of the exposure (west of U.S. Forest 

Service Road 58, fig. 4), the tuff unconformably overlies 

Upper Cretaceous trachyandesite flows. The detailed geologic 

map (fig. 4) shows that east of the road, a basal conglomerate, 

about 10 m thick, lies between the tuff and trachyandesite. 

The basal conglomerate is also exposed beneath the tuff in 

two outcrops farther to the south (fig. 4). These two exposures may have been topographic highs around which the tuff 

was deposited. The basal conglomerate is dated at probably 

middle Tertiary (unit Tal), on the basis of its position just 

below and conformable with the 27.65-Ma tuff.

Along its south edge, the tuff either underlies or is in 

lateral facies relation with the indurated conglomerate (unit 

Tal, fig. 4) that probably is coeval with the tuff, as mentioned 

above (fig. 4). The trachyandesite, tuff, and middle Tertiary 

conglomerate are all overlain unconformably by poorly indurated gravelly deposits of upper Tertiary basin-fill alluvium 

and a thin layer of piedmont alluvium commonly dated as 

Quaternary (unit Qtal, figs. 3, 4). On the detailed geologic 

map (fig. 4), the middle Tertiary conglomerate (unit Tal) 

is differentiated from the basin-fill alluvium and piedmont 

gravel (unit QTal), whereas Simons (1974) mapped all the 

conglomerate and gravel together as his unit QTal (fig. 3).

The stratigraphy of the tuff and clay is not uniform 

throughout the deposit, possibly because of high-energy 

emplacement of the ash-flow tuff. Thus, in some places the 

clay facies underlies the tuff, in other places at the east end 

of the exposure the clay facies is absent, and at the west end 

the clay appears to lie between upper and lower ash-flow-tuff 

beds. Along the south edge of the western part of the exposure, the clay facies appears to extend to the top of the ridge 

(fig. 4), indicating that the clay may be more than 20 m thick 

here, although this indicated thickness is tenuous because of 

inadequate exposures.

Both the tuff and the middle Tertiary conglomerate are 

mildly deformed and dip generally about 5°–25° S. A zone 

Figure 2. Harshaw, Ariz. area in Coronado National Forest, showing 

areas of Tertiary tuff (Tt, Tl) with which the Harshaw clay is associated. 

White areas, privately owned land. Base from U.S. Bureau of Land 

Management Surface Management Status map of the Nogales 

1:100,000-scale quadrangle (1990).
3

Figure 3. Part of geologic map of the Nogales and Lochiel quadrangles, southeastern Arizona (Simons, 1974), showing 

outcrops of Tertiary tuff, limestone, and clay (units Tt, Tl) with which the Harshaw clay is associated and relation of tuff to 

Cretaceous andesite (unit Ka) on the north and to Tertiary and Quaternary alluvium (unit Qtal) on the south.

 Geologic Setting
4 Contributions to Industrial-Minerals Research

Figure 4. Detailed geologic map of the Tertiary tuff (unit Tt) and limestone (unit Ttl) and the Harshaw clay (unit Ttc) 

(modified from Simons, 1974). Massive clay facies is present in both western and eastern parts of map area, but 

because of poor exposure, the Harshaw clay (unit Ttc) was not differentiated west of U.S. Forest Service Road 58 (FS 

58). Base from U.S. Geological Survey 1:24,000-scale map of the Harshaw quadrangle (1958).
5

of steep (25°–65°) northerly dips in the limestone cap of the 

tuff (fig. 4) may indicate the presence of faulting, although no 

faults were observed. Northeast-trending faults are common 

in the middle Tertiary conglomerate and probably also in the 

tuff, but because of poor outcrop, mapping of the faults or 

determination of their displacement is difficult. The two outcrops of the basal conglomerate (unit Tal) surrounded by tuff 

in the eastern part of the exposure may be fault controlled, or, 

as suggested above, they may have been topographic highs 

over which the tuff was deposited.

Depositional Environment of the Harshaw Clay

The presence of clay lenses, as much as 15 m thick, 

within the tuff section and of interlaminated limestone and 

opaline chalcedony at the top of the tuff (east of U.S. Forest 

Service Road 58, fig. 4) may indicate that the tuff was partly 

deposited in a body of standing water. In fact, Simons (1974) 

described his unit Tl as freshwater limestone. However, 

detailed inspection in the present study revealed no sedimentary structures, such as crossbedding, mud cracks, trace fossils, or ripple marks on bedding planes, nor such pedogenic 

features as paleosols and root casts. No ostracode tests were 

visible in hand specimens of the limestone or in crushed 

samples of the clay in oil.

Given the absence of sedimentary structures, I interpret 

the clay lenses and limestone cap of the tuff as products of 

diagenesis in the presence of Ca-rich alkaline ground water. 

The massive clay beds most probably represent altered ashfall tuff that, because of its distance from the likely eruptive 

source, contained no lithic fragments or pumice and few 

crystals. The interlaminated microcrystalline limestone and 

opaline chalcedony facies at the top of the tuff in the easternmost 0.6 km of the outcrop belt (east of U.S. Forst Service 

Road 58, fig. 4) is traceable downsection into featureless, 

very fine grained light-gray material that also appears to have 

originally been a thin-layered ash-fall tuff.

Description of the Harshaw Clay

The Harshaw clay occurs both as massive, slope-forming 

beds, 10 to 15 m thick, near the base or middle of the Tertiary 

rhyolite tuff and as thin laminated beds interbedded with tuff 

near the top of the section. The massive clay near the base or 

middle of the tuff is pale red (10R 6/2) (Goddard, 1948) or 

yellowish gray (5Y 7/2); the laminated clay interbedded with 

tuff, commonly near the top, is yellowish gray. The massive 

clay was the subject of further testing, as detailed below.

Except for their color, both the pale-red and yellowishgray varieties of the massive clay are nearly identical. Clay 

of both colors contains very little silt and virtually no sandsize detritus. Both varieties are semi-indurated and break into 

blocky fragments, less than 1 cm across (fig. 5A). Although 

the clay does not slake upon contact with water, wetting and 

drying causes it to break up into smaller and smaller fragments so that weathered slopes are covered with a layer of 

loose sand-size clay particles, about 15 cm deep (fig. 5B). 

The induration of the Harshaw clay is probably due to its 

middle Tertiary age. The presence of calcite cement also may 

be a factor, although in places where the clay is not calcareous, it still is well indurated.

X-ray-diffraction analysis of samples of both varieties of 

the massive clay collected at the locality shown in figures 4 

and 5 demonstrated the mineral composition to be montmorillonite, muscovite, calcite, quartz, orthoclase, and dolomite 

(Carol Gent and Steve Sutley, written commun., 2002). The 

clay was not examined in thin section, and so the distribution of mineral phases is unknown, although montmorillonite is predominant. Calcite, composing about 20 weight 

percent by solution in HCl, does not occur as visible crystals 

or nodules in hand specimen; and in outcrop, calcite-filled 

Figure 5. Outcrops of the Harshaw clay near sample locality at the 

west end of figure 4. A, Fresh surface of the Harshaw clay in outcrop, 

showing blocky fracture pattern and absence of obvious bedding. 

B, Weathered clay slope developed on pale-red and yellowish-gray 

massive clay. Larger shrubs are about 1 m high.

 Description of the Harshaw Clay
6 Contributions to Industrial-Minerals Research

joints are relatively uncommon. Muscovite occurs as widely 

separated, very small flakes (sericite). Orthoclase and dolomite were detected in trace amounts. Clear quartz grains and 

hornblende crystals are rare and are the only minerals in the 

massive clay that are clearly of original volcanic rather than 

diagenetic origin. Interestingly, hornblende was observed 

only in the yellowish-gray colored clay.

Samples of the pale-red and yellowish-gray varieties of 

the massive clay were ground in a mortar and pestle, mixed 

with water, and then dried and aged for 2 to 3 days to a working consistency. The clay was rolled out into slabs and cut 

with a template to form tiles 2.5 by 10.2 cm (fig. 6A). Dryingand-firing tests were run on these tiles. Drying shrinkage was 

about 9 percent (slightly greater for the pale-red than for the 

yellowish-gray variety), with no cracking or warping. This 

amount of shrinkage is about the average of the approximately 

30 other noncommercial clays tested. After drying, the tiles 

were fired under mostly oxidizing conditions in a propane kiln 

to 900°C for about 4 hours (Paul Thornburg, written commun., 

2003). An additional shrinkage of 1 percent or less occurred 

upon firing, with no cracking or warping (fig. 6B). After firing, 

the surface of the yellowish-gray clay was rough with white 

speckles (fig. 6B), and its color was light brown (5YR 6/4), 

whereas the surface of the pale-red clay was smooth, and its 

color was pale reddish brown (10R 5/4). The rough texture 

and speckling of the yellowish-gray clay after firing may be 

due to less thorough grinding than for the pale-red clay.

Summary

The Harshaw clay consists of Ca montmorillonite probably derived from diagenetic alteration of Tertiary (27.65 

Ma) rhyolite tuff. The source of the tuff was volcanic eruptions, most likely in the vicinity of the present-day Santa Rita 

Mountains. The tuff was deposited as ash flows and ash falls.

The tuff is exposed in a narrow belt, about 4 km long by 

0.2 to 0.6 km wide. Clay lenses, as much as 15 m or more 

thick, interbedded with relatively unaltered tuff, occur along 

the length of the exposure. Thinner laminated clay beds are 

interbedded with ash-fall tuff near the top of the tuff at the 

east end of the exposure.

Although exposures are poor, the thick clay lenses appear 

to be fairly clean and homogeneous. Virtually no sand or silt 

is present, and the clay seems to be uniformly calcareous, 

although the easternmost clay body (fig. 4) is noncalcareous 

in a few places. The clay is semi-indurated, possibly with 

some calcite cement, and requires grinding before it can be 

mixed with water and formed. Shrinkage upon drying and 

firing is about 9 and 1 percent, respectively with no cracking or warping. The clay is pale red or yellowish gray; after 

firing, it is pale reddish brown and light brown, respectively.

Because the Harshaw clay is composed of montmorillonite and is relatively stable upon forming and firing, additional 

testing is needed to determine whether it is suitable for use 

as a specialty clay or as an additive to other clays. If the Harshaw clay does have useful characteristics, then a drilling program should be undertaken to determine whether the deposit 

is sizable enough to offset its relatively remote location.

Acknowledgments

Norman Hale and his family kindly allowed access 

across their property to the clay deposits. I thank Paul and 

Laurel Thornburg, potters in Sonoita, Ariz., for their interest 

Figure 6. Test tiles of Harshaw clay. Clay samples were collected 

at locality shown in figures 4 and 5. R, pale-red variety of clay; 

Y, yellowish-gray variety of clay. “4 in. wet” notation printed on 

tiles refers to initial length (10.2 cm) when wet. A, Test tiles after 

drying but before firing. Tiles are about 9.2 cm long, demonstrating 

shrinkage of about 9 percent on drying. B, Test tiles after firing. 

Upper tile is 9.1 cm long, and lower tile is 8.9 cm long, demonstrating 

additional shrinkage of less than 1 percent on firing.
7

in this project and for preparing, drying, and firing samples 

of the Harshaw clay. The X-ray-diffraction analysis was performed by Carol Gent and Steve Sutley at the U.S. Geological 

Survey laboratories in Denver.

References Cited

Drewes, Harald, 1968, New and revised stratigraphic names in the 

Santa Rita Mountains of southeastern Arizona, in Contributions to stratigraphy, 1968: U.S. Geological Survey Bulletin 

1274–C, p. C1–C15.

Drewes, Harald, 1971, Geologic map of the Mount Wrightson quadrangle, southeast of Tucson, Santa Cruz and Pima Counties, 

Arizona: U.S. Geological Survey Miscellaneous Geologic 

Investigations Map I–614, scale 1:48,000.

Goddard, E.N., chairman, 1948, Rock-color chart: Washington, 

D.C., National Research Council, n.p. [distributed by the Geological Society of America, Boulder, Colo.].

Patterson, S.H., 1969, Clay, in Mineral and water resources of Arizona: Washington, D.C., report prepared by the U.S. Geological Survey, Arizona Bureau of Mines [now Arizona Geological 

Survey], and U.S. Bureau of Reclamation for the U.S. Senate’s 

Committee on Interior and Insular Affairs, p. 324–336.

Simons, F.S., 1974, Geologic map and sections of the Nogales 

and Lochiel quadrangles, Santa Cruz County, Arizona: U.S. 

Geological Survey Miscellaneous Investigations Series Map 

I–762, scale 1:48.000.

 References Cited