Correlation of Upper 

Triassic and Triassic(P) 

Formations Between 

Southwestern Utah and 

Southern Nevada

By RICHARD F. WILSON and JOHN H. STEWART

CONTRIBUTIONS TO STRATIGRAPHY

GEOLOGICAL SURVEY BULLETIN 1244-D

Based on work done on behalf of the 

U.S. Atomic Energy Commission
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary

GEOLOGICAL SURVEY 

William T. Pecora, Director

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1967

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402 - Price 15 cents
CONTENTS

Page

Abstract_______-_____-__-__-_-_-___-__-____---_---__-__-___---_ Dl

Introduction..-________________________________________________ 1

Stratigraphic terminology_________________________________________ 3

Moenkopi Formation._________________________________________ 7

Upper unit of Moenkopi(?) Formation___________________________ 8

Chinle Formation________________________________________________ 8

Shinarump Member_______________________________-_____-_____ 10

Petrified Forest Member and probable equivalent__________.-_-__ 11

Chinle Formation (restricted)-Glen Canyon Group contact _____________ 13

Glen Canyon Group and probable equivalents_.__________--___--____ 14

Moenave and Kayenta Formations and probable equivalents______ 14

Southwestern Utah________________________________________ 15

Southern Nevada________________________________________ 17

Navajo Sandstone and Aztec Sandstone__________________________ 18

References cited___________________________________________________ 19

ILLUSTRATIONS

Page

FIGURE 1. Map showing location of measured sections and areas of outcrop of the combined Chinle, Moenave, and Kayenta Formations, and equivalent rocks undivided in southwestern 

Utah, northwestern Arizona, and southern Nevada ________ D2

2. Chart showing recent and proposed terminology for Upper

Triassic and Lower Jurassic strata.______________________ 4

3. Correlation chart of Upper Triassic and Upper Triassic (?)

formations_ _ ________________________________________ 9

in

CONTRIBUTIONS TO STRATIGRAPHY

CORRELATION OF UPPER TRIASSIC AND TRIASSIC(F)

FORMATIONS BETWEEN SOUTHWESTERN UTAH

AND SOUTHERN NEVADA

By RICHARD F. WILSON and JOHN H. STEWART

ABSTRACT

Field studies in southwestern Utah and southern Nevada indicate that the 

Chinle Formation (Upper Triassic) as employed by previous workers in southern 

Nevada is the probable correlative of the Chinle Formation (Upper Triassic), 

the Moenave Formation (Triassic?), and the Kayenta Formation (Triassic?) 

of the Glen Canyon Group in southwestern Utah.

Three principal stratigraphie units make up the strata between the Moenkopi 

Formation (Lower and Middle? Triassic) and the Navajo Sandstone (Triassic? 

and Jurassic) in southwestern Utah and between the Moenkopi Formation and 

the equivalent of the Navajo Sandstone, the Aztee Sandstone, in southern Nevada. 

These units are a basal unit of sandstone and conglomerate; a conformably 

overlying middle unit of variegated purple, red, and gray sandstone and bentonitic claystone; and an unconformably overlying upper unit of reddish-brown 

nonbentonitic siltstone and sandstone that grades upward into the overlying 

Navajo Sandstone or Aztec Sandstone. Prior to recent work, the basal unit in 

southwestern Utah generally was termed the Shinarump Conglomerate, and the 

middle and upper units constituted the Chinle Formation. As now recognized in 

southwestern Utah, the lower and middle members are called, respectively, the 

Shinarump and Petrified Forest Members of the Chinle Formation. Probable 

equivalents of these members are recognized in southern Nevada. The upper 

unit in southwestern Utah comprises the Moenave and the Kayenta Formations 

of the Glen Canyon Group. Here these two formations are readily distinguished. 

In southern Nevada, no such division can be made; the strata of the upper unit 

as a whole are believed to be the probable undifferentiated equivalents of the 

Moenave and Kayenta Formations.

A unit that is lithologically similar to the Chinle Formation is present between 

the Shinarump(?) Member of the Chinle Formation and the subjacent unquestioned Moenkopi Formation in southern Nevada. This unit was included in the 

Moenkopi Formation by previous workers in Nevada. Until more is known about 

this unit, the authors tentatively retain it in the Moenkopi Formation and call 

it the upper unit of the Moenkopi (?) Formation.

INTRODUCTION

Correlation of rocks between the Moenkopi Formation (Lower and 

Middle? Triassic) and the Navajo Sandstone (Triassic? and Jurassic)

Dl
D2 CONTRIBUTIONS TO STRATIGRAPHY

from southwestern Utah to southern Nevada has suggested a reassignment of the boundaries of the Chinle Formation (Upper Triassic) in 

southern Nevada and the tentative correlation of the Moenave (Triassic?) and Kayenta (Triassic?) Formations of the Glen Canyon Group 

in southwestern Utah with the upper part of the Chinle Formation, as 

denned by previous authors, in southern Nevada.

The present study was undertaken by the U.S. Geological Survey 

on behalf of the Division of Eaw Materials of the U.S. Atomic Energy 

Commission as part of a larger regional program concerned with the 

stratigraphy of rocks of Triassic age on the Colorado Plateau. The 

study involved the tracing of units in the St. George area in southwestern Utah into southern Nevada by means of stratigraphic sections 

measured at several localities (fig. 1). The distance between some of 

the sections is fairly large, but no well-exposed rocks of Late Triassic 

age are known to crop out between the sections. Outcrops in the 

Virgin Mountains in Arizona proved unsuitable for the measurement

114°

Cedar

10 10 20 MILES

0 100200300 MILES

EXPLANATION

Areas of outcrop 

Location of measured section

SPRING 

MOUNTAINS

37"

FIQUBE 1. Location of measured sections and the areas of outcrop of the combined Chinle, Moenave, and Kayenta Formations and equivalent rocks undivided in southwestern Utah, northwestern Arizona, and southern Nevada 

(compiled largely from Wells, 1952, general pi. 1; Bowyer and others, 1958; 

and Tschanz and Pampeyan, 1961).
UPPER TRIASSIC AND TRIASSIC (? ) FORMATIONS, UTAH-NEVADA D3

of sections. Despite large gaps between some of the sections, most of 

the general stratigraphic units of Late Triassic age in southwestern 

Utah are identifiable in southern Nevada.

The area studied in Nevada is structurally complex; Triassic strata 

are exposed in complicated thrust-faulted and folded fault-block mountains. The major stratigraphic units are easily recognized, but thickness of units in some areas is difficult to determine accurately because of 

concealed faults and the problems in measuring the correct attitude of 

beds. Longwell (1928) measured stratigraphic sections near some of 

those described in this paper; his thicknesses for some units are markedly different from those of the authors. Some of these differences 

are probably the result of different interpretations of local faulting or 

attitude of beds.

STRATIGRAPHIC TERMINOLOGY

Until 1955, the rocks between the Moenkopi Formation and the 

Navajo Sandstone in most of southwestern Utah were divided into the 

Shinarump Conglomerate and the Chinle Formation (fig. 2). This 

division was made in the Zion National Monument, directly north of 

Zion National Park (Gregory and Williams, 1947), in part of the 

Zion Park region (Gregory, 1950b), the Cedar City area (Thomas and 

Taylor, 1946; Gregory, 1950a), the Leeds area (Proctor, 1953), the 

Beaver Dam Mountains (Eeber, 1952), and the St. George area 

(Dobbin, 1939). Gregory (1950b) divided the Chinle Formation into 

four members, in ascending order: (1) the lower sandstones, (2) the 

Petrified Forest Member, (3) the Springdale Sandstone Member, and 

(4) the upper sandstones. Proctor (1953) divided the Chinle in the 

Leeds area into three principal members, in ascending order: (1) the 

lower Chinle member, which corresponds to Gregory's lower sandstones and Petrified Forest Member; (2) the Silver Reef Sandstone 

Member, which is identical with Gregory's Springdale Sandstone 

Member; and (3) the Duffin Sandstone and shale, which corresponds 

to most of Gregory's upper sandstones. Proctor divided his lower 

Chinle member into three local units, in ascending order: the Hartley 

Shales and Sandstones, the Fire Clay Hill Bentonitic Shales, and the 

Trail Hill Sandstone.

Stratigraphic studies by Averitt, Detterman, Harshbarger, Repenning, and Wilson (1955) and Wilson (1959) have shown that the 

upper part of the Chinle Formation as defined by earlier workers in 

northwestern Arizona and southwestern Utah lithologically correlates 

with the Moenave and Kayenta Formations of the Glen Canyon Group 

in northern Arizona. The top of their restricted Chinle Formation 

is placed at an erosion surface from 200 to 250 feet below the base of 

the Springdale Sandstone Member and within the Petrified Forest
D4 CONTRIBUTIONS TO STRATIGRAPHY

SOUTHERN NEVADA

Longwell

(1928)

Jurassic(?)

sandstone

Chinle

Formation

Shinarump

Conglomerate

yv\/\/\/\/\A/w\

Moenkopi

Formation

Clock

(1929)

Crossbedded

sandstone

Chinle

Formation

Shinarump

Conglomerate

<rv/v\/\/\/\/\/\/\/x

Moenkopi

Formation

Hewett

(1931)

Longwell

(1949)

Aztec

Sandstone

Chinle

Formation

Shinarump

Conglomerate

-VA/\/V/\/\/X^\/XX

Moenkopi

Formation

This

report

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Aztec

Sandstone

(Navajo

Sandstone)

Probable

equivalent

of

Moenave

and

Kayenta

Formations

v\/\AyN/\/wx 

Probable

equivalent

of

Petrified

Forest

Member

Shinarump(?)

Member 

i

Upper unit of

Moenkopi(?) Formation

W\/V\/\/N?'\/\/\/VW

Moenkopi

Formation

FIGTTBE 2. Recent and proposed terminology for Upper Triassic and

Member of the Chinle as used by Gregory (1950b). The Moenave 

Formation named and defined by Harshbarger, Eepenning, and 

Irwin (1957) in the western part of the Navajo Indian Reservation in 

northeastern Arizona includes all strata between this erosion surface 

and the top of the Springdale Sandstone Member. The strata between 

the top of the Springdale and the base of the Navajo were assigned by 

Averitt, Detterman, Harshbarger, Eepenning, and Wilson (1955) to 

the Kayenta Formation. Averitt, Detterman, Harshbarger, Repenning, and Wilson (1955) divided the Moenave Formation in south-
UPPER TRIASSIC AND TRIASSIC(?) FORMATIONS, UTAH-NEVADA D5

SOUTHWESTERN UTAH

Gregory and

Williams (1947)

Gregory

(1950a, 1950b)

o

a a 

0 2 

cO

O

o

(0

E

£

a>

!E

0

Navajo 

Sandstone

Upper

sandstones

Springdale

Sandstone

Member

Petrified

Forest

Member

Lower

sandstones

Shinarump

Conglomerate 

/"x/x/x/x/x/x/

Moenkopi

Formation

Proctor

(1953)

Navajo 

Sandstone

c

(0

E

£

"E

.c

O

Duffin

Sandstone

and Shale

Silver Reef

Sandstone

Member

Member

c

O

Lower

Trail 

Hill 

Sandstone

Fire' Clay

Hill Bentonitic Shales

Hartley 

Shales and 

Sandstones

Shinarump

Conglomerate

X/X/X/X/X/XA/^

Moenkopi

Formation

Averitt

and

others

(1955)

a

o

0

c

re 

0

c

W

,/w

c

Chinie Formatio

Navajo 

Sandstone

Kayenta

Formation

r

o

(0

L.

oenave

^

\A

Springdale

Sandstone

Member

Dinosaur

Canyon 

Sandstone 

Member

<x/x/x/x/\/

Petrified

Forest 

Member

Lower

sandstones

Shinarump

Conglomerate 

y\/x/x/x/x/N^v

Moenkopi

Formation

Wilson

(1966)

a

0

(5

c

o

c

O

c

s/XA

Navajo 

Sandstone

Kayenta

Formation

c

(0

F

L.

oenave

2

/xr

Springdale

Sandstone

Member

Whitmore

Point

Member

Dinosaur 

Canyon

Member

v/X/X/X^W

Chinie 

Formation 

(not described)

This

report

a

o 

w

c

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c

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c

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Formatic

JB

0

XV/N

Navajo 

Sandstone

Kayenta

Formation

c Springdale

.2 Sandstone

"TO Member

£ Whitmore

u- Point

S Member

c Dinosaur 

o Canyon

2 Member

y\A/X/\/N/X/N

Petrified 

Forest 

Member

Shinarump

Member

/N^X/XT\A\^\

Moenkopi

Formation

Lower Jurassic strata in southwestern Utah and southern Nevada.

western Utah into two members: a lower Dinosaur Canyon Sandstone 

Member and an upper Springdale Sandstone Member. The Dinosaur 

Canyon Sandstone Member originally defined as a formation by Colbert and Mook (1951) was placed in the Moenave Formation by 

Harshbarger, Eepenning, and Irwiii (1957). The Springdale is thS 

same unit that Gregory (1950b) originally named Springdale Sandstone Member of the Chinie Formation.

In some parts of the Zion Park region, Gregory (1950b) recognized 

the Wingate Sandstone and Kayenta Formation above strata that he

234-113 66 2
D6 CONTRIBUTIONS TO STRATIGRAPHY

designated as Chinle Formation. The Wingate as recognized by 

Gregory was found by Wilson (1959) and by Averitt, Detterman, 

Harshbarger, Kepenning, and Wilson (1955) to be a tongue of the 

Navajo Sandstone within the Kayenta Formation. Averitt, Detterman, Harshbarger, Repenning, and Wilson (1955) and Wilson (1959) 

discussed the terminology of these units.

Wilson (1959,1966) used much of the terminology of Averitt, Detterman, Harshbarger, Kepenning, and Wilson (1955) but dropped the 

word "sandstone" from the term Dinosaur Canyon Sandstone Member. 

The lithologic designation is not appropriate for southwestern Utah, 

where the Dinosaur Canyon Member is composed predominantly of 

siltstone. Wilson (1966) also proposed the name Whitmore Point 

Member of the Moenave Formation for a unit previously included in 

the Dinosaur Canyon Member by Averitt, Detterman, Harshbarger, 

Repenning, and Wilson (1955).

The present authors use the same terminology as Wilson for strata 

of the Glen Canyon Group in southwestern Utah. The term Shinarump Member of the Chinle Formation is substituted for the term 

Shinarump Conglomerate, following Stewart's redefinition (1957) of 

the Chinle Formation to include the Shinarump. The designation 

"lower sandstones of the Chinle Formation" is not used. This unit 

probably represents a transition zone between the Shinarump and 

Petrified Forest Members and is placed in the Petrified Forest Member 

of the Chinle as defined in this report.

In southern Nevada the rocks between the Moenkopi Formation and 

the Aztec Sandstone have been divided into the Shinarump Conglomerate and Chinle Formation; a division made by Glock (1929) 

and Hewett (1931) in the Spring Mountains to the southwest of Las 

Vegas and by Longwell (1928,1949) in the Muddy Mountains area to 

the east of Las Vegas. Their usage corresponds to that of Gregory 

(1950b) for the Zion Park region.

As a result of new correlations, the present authors propose several 

new interpretations of the Upper Triassic and Triassic(?) stratigraphy in southern Nevada. A thin unit is recognized that corresponds 

to the uppermost part of what other geologists have called Moenkopi 

Formation but that is lithologically more similar to the Chinle Formation. For the present, the unit is questionably retained in the Moenkopi Formation as the upper unit of the Moenkopi(?) Formation. 

The name Shinarump (?) Member of the Chinle Formation is used 

by the authors in southern Nevada. The correlation of the Shinarump 

into southern Nevada from southwestern Utah is questionable because 

of large gaps between outcrops and because of some differences in 

lithology of the Shinarump in the two areas. The Shinarump was also 

recognized as a member of the Chinle Formation in southern Nevada 

by Longwell (1952, p. 34).
UPPER TRIASSIC AND TRIASSIC (?) FORMATIONS, UTAH-NEVADA D7

The Chinle Formation as defined by previous workers in southern 

Nevada is divided into two units by the present authors: a lower unit 

that is the probable equivalent of the Petrified Forest Member of the 

Chinle in southwestern Utah and an upper unit that is the probable 

equivalent of the Moenave and Kayenta Formations (fig. 2). In southwestern Utah, the Moenave and Kayenta Formations are distinctly 

separate units. In southern Nevada, corresponding separations cannot 

be made; the upper part of the Chinle Formation as defined by previous workers in Nevada is considered the probable undifferentiated 

equivalent of the Moenave and Kayenta Formations.

As pointed out by Baker, Dane, and Eeeside (1936, p. 20 and table 

1), a unit in southern Nevada termed Aztec Sandstone proposed by 

Hewett (1931) for the Spring Mountains area and later extended by 

Longwell (1949) to the Muddy Mountains area corresponds to the 

Navajo Sandstone in southwestern Utah. The present authors likewise 

believe that the Aztec and Navajo are equivalent and that they represent the same lithostratigraphic unit. Nevertheless, the term Aztec 

Sandstone is retained in this report for this unit in southern Nevada.

Although the correlations and stratigraphic interpretations presented in this paper are believed to be correct, an informal terminology 

is used for units in southern Nevada. More detailed work should 

permit formal terminology proposals.

MOENKOPI FORMATION

In southwestern Utah and southern Nevada, rocks unquestionably 

assigned to the Moenkopi Formation consist of interstratified limestone, gray siltstone and gypsum, and red siltstone and sandstone. 

Both marine and continental deposits are represented; the percentage 

of marine deposits increases to the west. Throughout southwestern 

Utah, the uppermost part of the Moenkopi consists of red beds the 

upper red member (Gregory, 1950b). This member is believed to be 

present in southern Nevada also.

The upper red member is composed predominantly of very thick 

bedded siltstone and lesser amounts of ripple-laminated siltstone. 

Cross-laminated sandstone and minor amounts of gypsum are also 

present. The member forms a slope beneath cliffs formed by the 

Shinarump Member of the Chinle Formation. In sections examined 

in Nevada, the top several feet of the upper red member contains one 

or more beds 1-4 feet thick of sandstone composed of rounded grains 

of limestone or conglomerate composed of rounded grains to pebbles 

of limestone interbedded with red or gray siltstone. These sandstone or conglomerate beds are useful for determining the top of the 

upper red member in southern Nevada.
D8 CONTRIBUTIONS TO STRATIGRAPHY

UPPER UNTT OF THE MOENKOPI(P) FORMATION

In southwestern Utah, the upper red member of the Moenkopi Formation unconf ormably underlies the Shinarump Member of the Chinle 

Formation. In the sections examined in southern Nevada, however, 

a peculiar unit is present between the Shinarump (?) Member of the 

Chinle and the undoubted Moenkopi Formation. This unit is lithologically similar to the Chinle Formation but is questionably retained 

in the Moenkopi Formation as the upper unit of the Moenkopi (?) 

Formation. The base of this unit directly overlies or is within a few 

feet of the uppermost of the calcarenite or conglomerate beds of the 

upper red member of the Moenkopi Formation. Thus, in southern 

Nevada, the position of the contact between the Moenkopi and the 

Chinle Formations and therefore the position of the mid-Triassic 

unconformity is in doubt and may be either at the top or at the base 

of this questionable Moenkopi unit.

The upper unit of the Moenkopi (?) Formation is probably represented in all the sections measured by the authors in southern Nevada. 

The thickness of this unit seems to decrease to the east (fig. 3) at 

Spring Mountains, approximately 55 feet of strata are assigned to the 

unit; at Valley of Fire, 20 feet; and at Horse Spring Valley, 8 feet.

The upper unit of the Moenkopi(?) Formation is poorly exposed 

at many places and is quite variable in lithology. At Horse Spring 

Valley, it is composed of greenish-gray claystone to clayey sandstone; 

at the Spring Mountains the stratigraphic section is of grayish-red 

silty claystone. The claystone at both of these localities is composed 

of clays that expand in water and apparently belong, at least in part, 

to the montmorillonite group. These clays probably represent bentonitic material. At Valley of Fire, the Shinarump (?) Member of the 

Chinle Formation is underlain by a poorly exposed unit of grayishred-purple siltstone that may represent the upper unit of the Moenkopi (?) Formation.

The swelling clay, clayey sandstone, and grayish and purplish colors 

of this unit at various localities are much more characteristic of the 

Chinle Formation than they are of the upper red member of the Moenkopi Formation, which contains nonswelling clay and is red to reddishbrown. Therefore, the transfer of this unit to the Chinle Formation 

would seem justified. In places, however, this unit might represent an 

altered zone, possibly a soil, at the top of the Moenkopi Formation. 

Further study of this unit seems necessary before its relationships to 

the overlying and underlying units are completely understood.

CHINLE FORMATION

The Chinle Formation in southwestern Utah is divided into two 

units: the Shinarump Member below and the Petrified Forest Mem-
UPPER TRIASSIC AND TRIASSICK?) FORMATIONS, UTAH-NEVADA D9

liiiiif ilDi trt ?° TT C3 m

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D10 CONTRIBUTIONS TO STRATIGRAPHY

her above. That part of the section formerly considered the upper 

part of the Chinle is now assigned to the Moenave and Kayenta Formations of the Glen Canyon Group (Averitt and others, 1955). In 

southern Nevada, the Chinle Formation, as restricted in this paper, 

includes the Shinarump(?) Member and the probable equivalent of 

the Petrified Forest Member. The upper part of the Chinle Formation, as defined by previous workers in Nevada, is considered the 

probable equivalent of the Moenave and Kayenta Formations; it is 

discussed in the section on the Glen Canyon Group and its equivalents.

SHINARUMP MEMBER

Strata belonging to the Shinarump Member, or questionably assigned to the member, occur in all outcrops of Late Triassic age examined in southwestern Utah and southern Nevada. The thickness 

of these strata is variable and ranges from 29 feet at Horse Springs 

Valley, Nev., to 162 feet near Leeds, Utah (fig. 3).

Near Leeds, the lithology of the Shinarump Member is identical 

with the lithology of the member in its type area at the Shinarump 

Cliffs, about 50 miles east-southeast of Leeds. The Shinarump is composed predominantly of pale-orange to yellowish-gray fine- to coarsegrained horizontally laminated to cross-laminated sandstone and interstratified lenses of conglomeratic sandstone and conglomerate. 

Minor amounts of ripple-laminated siltstone are present in the member in some localities. The sandstone in the Shinarump contains 

trough sets of small to medium scale, low- to very low angle, crosslaminae 1 and probably represents stream-channel deposition. Pebbles 

in the conglomeratic parts of the member consist of quartz, quartzite, 

and chert. The member typically weathers to form prominent cliffs 

or hogbacks.

In southern Nevada, rocks questionably assigned to the Shinarump 

Member are composed predominantly of medium- to coarse-grained 

cross-laminated sandstone and lesser amounts of conglomeratic sandstone and conglomerate. These rocks differ from those of the Shinarump Member in southwestern Utah in that locally, as in Horse Spring 

Valley, they contain some clayey matrix. In addition, chert pebbles 

seem to be more abundant in the conglomerate beds of the Shinarump (?) Member in Nevada than they are in the type area of the 

Shinarump in southwestern Utah. At the Spring Mountains stratigraphic section (fig. 3), the lower 12 feet of the Shinarump(?) Member is composed entirely of chert-pebble conglomerate.

The contact of the Shinarump Member with the overlying Petrified 

Forest Member, or with strata in Nevada probably equivalent to the

1 Most of the stratification terminology used in this report is after McKee and Weir 

(1953).
UPPER TRIASSIC AND TRIASSIC (?) FORMATIONS, UTAH-NEVADA Dll

Petrified Forest Member, is transitional in most localities; it is arbitrarily placed at the base of the lowest claystone, siltstone, or clayey 

sandstone unit above the ledge-forming sandstone of the Shinarump 

Member. Above this contact at many localities are sandstone units 

similar in lithology to the Shinarump. These sandstone units may 

represent tongues of the Shinarump Member in the Petrified Forest 

Member.

PETRIFIED FOREST MEMBER AND PROBABLE EQUIVALENT

The Petrified Forest Member, or its probable equivalent, forms 

most of the Chinle Formation in southwestern Utah and southern Nevada. A thickness of 350-450 feet for the unit is fairly constant 

throughout the area. A maximum thickness of 752 feet for the unit 

was measured in Horse Spring Valley, but this measured thickness may 

be too great, owing to difficulties in determining accurate dips and to 

the possible repetition of beds by concealed faults along the line of 

section.

In southwestern Utah and southern Nevada, the Petrified Forest 

Member, or its probable equivalent, exhibits the same lithology that 

characterizes the member in its outcrops in the Painted Desert and 

Petrified Forest in Arizona and elsewhere on the Colorado Plateau. 

Variegated hues of red, purple, yellow, and gray make it the most 

distinctive Triassic unit of this region. In general, the member is 

composed of lenses, lenticular beds, and interstratified masses of claystone, silty claystone to clayey siltstone, siltstone, clayey sandstone, 

sandstone, and minor conglomeratic sandstone to conglomerate. Most 

of the claystone and clayey constituents of the unit are composed of 

clay minerals that swell in water and presumably belong in part to the 

montmorillonite group of clays. Alien (1930), Waters and Granger 

(1953), and Schultz (1963, p. C37-C39) have shown that much of 

the clay of the Petrified Forest Member was probably formed by the 

devitrification of volcanic ash, pumice fragments, and tuffaceous material. This clay may be classed as bentonite. The claystone commonly 

weathers to form frothy surfaced slopes. A weathered zone extends 

several feet downward from the surface of the claystone and obscures 

the internal bedding structure. In general, however, the claystone 

seems to form horizontally bedded layers with bedding planes from 

1 foot to many feet apart.

The sandstone in the Petrified Forest Member and its probable 

equivalent occurs in lenses a few inches to 50 feet thick interstratified 

with claystone and siltstone; this sandstone ranges from clayey and 

silty to coarse grained and is poorly to well sorted. Many of the 

clayey sandstone lenses are composed of fine to medium grains of 

quartz, chert, and sand-sized aggregates of clay in a clay matrix. The
D12 CONTRIBUTIONS TO STRATIGRAPHY

sand-sized clay grains were probably formed by the decomposition of 

water-deposited vitric particles of volcanic material that once were 

similar in size to the more stable grains of quartz and chert (Schultz, 

1963, p. C33). The clayey sandstone in general is poorly indurated 

and weathers to form slopes, along with the claystone and siltstone of 

the member.

The better sorted, and less clayey, silty sandstone and sandstone in 

the Petrified Forest Member weathers to form ledges and cliffs and is 

better exposed than the clayey sandstone. This better-sorted sandstone 

is typically composed of thin to thick trough and minor planar sets of 

small- to medium-scale low-angle crosslaminae; the sandstone represents probable stream-channel deposits. Quartz and lesser amounts of 

chert and feldspar are the predominant minerals in the sandstone. 

Some of the sandstone units include lenses of conglomeratic sandstone 

and conglomerate composed of granules and pebbles of quartz, quartzite, chert, limestone, and siltstone.

The percentage of sandstone in the Petrified Forest Member and its 

probable equivalent increases toward the south or southwest. In 

southwestern Utah, the percentage of sandstone ranges from 16 to 19 

percent. It increases to about 35 percent in southern Nevada and 

reaches a maximum of 42 percent in the Valley of Fire; this indicates 

that the source area lay toward the south. Additional evidence that 

the Chinle Formation in southern Nevada was deposited close to its 

source area is furnished by a bed of conglomeratic sandstone 190 feet 

above the base of the Petrified Forest Member equivalent in the Valley 

of Fire. Pebbles in this conglomeratic sandstone are largely composed 

of unstable volcanic rock, probably not transported far from source.

In the Leeds area, the Petrified Forest Member consists of two claystone units separated by a medial sandstone unit (fig. 3). The upper 

claystone unit is composed predominantly of grayish-red-purple claystone that contains several layers of limestone nodules. These nodules 

range from a fraction of an inch to more than 2 inches in diameter. 

The lower claystone unit is composed predominantly of grayish-red 

and gray claystone and lesser amounts of siltstone; limestone nodules 

are absent. At Horse Spring Valley, the medial sandstone unit cannot 

be differentiated, but the upper 100 feet of the Petrified Forest equivalent consists of purple claystone that contains limestone nodules. This 

purple claystone may be in part correlative with the upper claystone 

unit of the Petrified Forest Member at Leeds. Farther west, in the 

Valley of Fire, the upper 65 feet of the Petrified Forest equivalent 

is composed of purple claystone with limestone nodules concentrated 

in the top 30 feet; it may also be partly correlative with the upper claystone unit in the Leeds area. Still farther west, in the Spring Mountains, the purple claystone unit cannot be recognized; no limestone
UPPER TRIASSIC AND TRIASSIC (? ) FORMATIONS, UTAH-NEVADA D13

nodules are present in the Petrified Forest equivalent. The purple 

claystone is the only unit in the Petrified Forest Member that may be 

correlative from southwestern Utah to southern Nevada.

CHINLE FORMATION (RESTRICTED)-GLEN CANYON 

GROUP CONTACT

In southwestern Utah, the contact between the Petrified Forest 

Member of the Chinle Formation and the overlying beds of the Glen 

Canyon Group is a well-defined horizon that can be easily distinguished in the field. This same horizon is clearly recognizable in 

southern Nevada between the probable equivalent of the Petrified 

Forest Member of the Chinle and the probable equivalents of the 

Moenave and Kayenta Formations the upper part of the Chinle as 

used by previous authors. The contact lies along an erosion surface 

that marks a stratigraphic hiatus that in regions to the east is occupied 

by the Owl Rock and Church Rock Members of the Chinle Formation, 

the youngest members of the Chinle (Stewart, 1957), and probably 

at least part of the Wingate Sandstone, which, where present, is the 

basal unit of the Glen Canyon Group. The erosion surface thus marks 

an important stratigraphic break in the Upper Triassic Series.

In southwestern Utah, the basal unit of the Glen Canyon Group, 

directly overlying the Chinle Formation, is the Dinosaur Canyon 

Member of the Moenave Formation. In southern Nevada, the Moenave 

equivalent, if present, could not be differentiated from the overlying 

Kayenta equivalent. Here the Chinle Formation (restricted) is overlain by beds collectively considered by the authors to be the probable 

equivalents of the Moenave and Kayenta Formations, undifferentiated. 

These beds comprise the upper part of the unrestricted Chinle Formation of previous reports.

East of the Leeds area, the basal bed of the Dinosaur Canyon 

Member is commonly a siltstone or very fine grained sandstone that 

contains laminae and scattered grains of coarse sand and fragments 

of reworked claystone and siltstone derived from the Chinle Formation. Lenses and layers of conglomeratic sandstone or conglomerate 

that contain pebbles of chert and minor quartzite are present in some 

localities but are not abundant. In most localities examined west of 

Zion National Park, however, the basal bed of the Glen Canyon Group 

or its probable equivalents consists of conglomeratic sandstone or of 

lenses of conglomeratic sandstone in a sandstone matrix. This bed 

ranges from 1 inch to 18 feet in thickness, and seems to be a persistent 

unit. The conglomeratic material contains granules to cobbles of chert 

and lesser amounts of quartzite and quartz. Siltstone and limestone 

pebbles are common. In the Spring Mountains, a considerable amount
D14 CONTRIBUTIONS TO STRATIGRAPHY

of the conglomeratic material in this bed consists of volcanic rock 

fragments.

Although the pebble assemblage of this bed at various localities is 

very similar to pebble assemblages in the Chinle Formation, there 

is one significant difference. Many of the siliceous pebbles in the basal 

bed of the Glen Canyon Group or its probable equivalent have pitted 

surfaces; some are faceted. These pebbles may be ventifacts. Within 

the Chinle Formation, pitted or faceted surfaces on pebbles in conglomerate units are rare. The pebbles in the basal bed of the Glen 

Canyon Group probably accumulated as lag gravel deposits on the 

erosion surface at the top of the Chinle Formation. No siliceous 

pebbles are in the Moenave or Kayenta Formations, or in their equivalents, above this basal bed; therefore, the pebbles probably were derived by reworking from the underlying Chinle Formation.

Topographic relief along the contact reaches a maximum of about 

10 feet on the Vermilion Cliffs east of Zion National Park but is not 

particularly noticeable in the region west of the park. The contact 

surface is almost flat, and the average relief is not more than a few 

inches.

Several criteria, stated in terms of Utah nomemclature but also 

effective in southern Nevada, are useful for field recognition of the 

contact. First, the contact marks the uppermost occurrence of 

bentonitic clay in the Upper Triassic and Triassic (?) strata. No 

bentonitic claystone or clay is known to be present in the Moenave 

or the Kayenta Formations. Second, there is a definite color change 

at the contact from the grayish-red to grayish-red-purple strata of the 

uppermost part of the Chinle Formation to the pale-reddish-brown, 

grayish-brown, and light-brown strata characteristic of the Moenave 

and Kayenta Formations. Third, there is an increase in grain size 

at the contact from claystone and silty claystone of the uppermost 

"beds of the Chinle Formation to sandstone and conglomeratic sandstone of the basal bed of the Glen Canyon Group. This basal bed 

of the Glen Canyon Group marks the highest occurrence of siliceous 

pebbles in rocks of Late Triassic and Triassic (?) age in this region, a 

fact that serves as an additional criterion for identifying the contact.

GLEN CANYON GROUP AND PROBABLE EQUIVALENTS

MOENAVE AND KAYENTA FORMATIONS AND PROBABLE 

EQUIVALENTS

In southwestern Utah, the Moenave and Kayenta Formations, which 

make up the lower part of the Glen Canyon Group in this area, form 

separate lithologic units that may be easily distinguished. In southern 

Nevada, the Moenave equivalent, if present, could not be separated
UPPER TRIASSIC AND TRIASSIC(?) FORMATIONS, UTAH-NEVADA D15

from the Kayenta equivalent; the strata between the Chinle Formation 

(restricted) and the Aztec Sandstone form a relatively homogeneous 

unit. Owing to differences in the lithology of the Moenave and 

Kayenta Formations and their probable equivalents, the two areas are 

discussed separately.

SOUTHWESTERN UTAH

In most of southwestern Utah, the Moenave Formation is divided 

into three members, in ascending order: the Dinosaur Canyon, the 

Whitmore Point, and the Springdale Sandstone Members.

The Dinosaur Canyon Member in the Leeds area is approximately 

200 feet thick. It is composed predominantly of horizontally thickbedded to laminated pale-reddish-brown siltstone and weathers to form 

a slope. The upper 60 feet of the member forms a ledgy sequence that 

is composed largely of beds of coarse siltstone to sandy siltstone that 

are in part cross laminated on a small to medium scale. Some ripplelaminated beds composed of both parallel and cuspate ripple marks 

(McKee, 1954, p. 57-60) are present in the member.

In the Leeds area, the Whitmore Point Member is a distinctive unit 

approximately 60 feet thick composed of greenish-gray, dark-greenishgray, and grayish-red horizontally stratified siltstone and claystone. 

It overlies the coarser siltstone of the Dinosaur Canyon Member with 

a sharp, even contact. The Whitmore Point Member intertongues 

with the overlying Springdale Sandstone Member; the contact between the two is transitional in some areas. Eastward from Zion 

National Park, the Whitmore Point Member tongues into and pinches 

out in the lower part of the Springdale Sandstone Member (Wilson, 

1966).

The Springdale Sandstone Member is the uppermost and most distinctive member of the Moenave Formation. It typically forms a 

prominent irregular or vertical cliff that lies between slopes formed on 

the overlying Kayenta Formation and the underlying Whitmore Point 

Member. In Zion National Park and southward, the Springdale 

ranges from 150 to 200 feet in thickness. To the west, however, the 

Springdale thins and at Leeds is only 95 feet thick. The Springdale 

consists of a series of large overlapping or partly overlapping lenses 

and lenticular beds, from several feet to several hundred feet in length, 

composed of pale-red to light-brown fine-grained to very fine grained 

quartzose sandstone. The stratification of the sandstone lenses consists of thin to thick trough and planar sets of medium- to large-scale 

low-angle to very low angle cross-laminae and also of some sets of 

horizontal laminae or thin beds. Horizons of cuspate ripple-laminae 

are common in some areas. Most of the sandstone lenses are separated 

from each other by erosion surfaces; thin lenses of intraformational
D16 CONTRIBUTIONS TO STRATIGRAPHY

claystone and siltstone pellet conglomerate that fill or line channels cut 

into underlying sandstone lenses are characteristic. The Springdale 

represents deposition in stream channels in contrast to the predominant 

flood-plain or alluvial-plain environments that seem to have characterized the overlying and underlying units. The member is very similar 

in lithology to that in the type area, or "typical f acies," of the Kayenta 

Formation.

Immediately to the east and the southeast of Leeds, the contact 

between the Springdale Sandstone Member of the Moenave Formation 

and the Kayenta Formation is a rather sharp, even surface. This 

surface marks a change in topography from a cliff below to a slope 

above, a change in color from hues of pale red below to a pale reddish 

brown above, a change in grain size from sand below to silt above, and 

a change in stratification from cross-lamination below to horizontal 

stratificaton above. Locally, a minor amount of intertonguing is present. The Springdale in the Leeds area, however, seems to intertongue to a considerable extent with the Kayenta Formation. At 

Leeds, the Springdale contains a unit of horizontally stratified claystone, siltstone, and sandstone 14 feet thick; this unit is similar in 

appearance to strata of the Kayenta Formation but is separated from 

the Kayenta by 18 feet of sandstone that exhibits typical Springdale 

lithology. This silty unit probably represents a tongue of the Kayenta Formation that extends into the Springdale Sandstone Member 

from the west; the westward thinning of the Springdale from Zion 

National Park to Leeds, therefore, is probably due to intertonguing 

with the Kayenta Formation.

In the Leeds-St. George area, the Kayenta Formation ranges in 

thickness from ,930 to more than 1,000 feet and may be divided into 

two parts. The lower part consists of a slope-forming series, 550-650 

feet thick, of interstratified grayish-red to pale-reddish-brown siltstone, sandy siltstone, and lesser amounts of silty claystone and very 

fine grained silty sandstone. The strata are predominantly horizontally laminated to very thick bedded, but sets of parallel ripplelaminae are common, and some of the sandstone and sandy siltstone 

units are cross laminated on a small to medium scale. Locally, the 

Kayenta contains lenses of pale-red cross-laminated fine-grained sandstone. At Leeds, the Kayenta contains a lens of sandstone 20 feet 

thick about 250 feet above the base of the formation that is similar in 

lithology to the Springdale Sandstone Member. A flood plain or 

alluvial plain was the probable environment of deposition of the lower 

part of the Kayenta, but the cross-stratified sandstone lenses, such as 

that at Leeds, represent stream-channel deposits.

The upper part of the Kayenta Formation, about 300-400 feet thick, 

contains a considerably higher percentage of sand than the lower part
UPPER TRIASSIC AND TRIASSIC(?) FORMATIONS, UTAH-NEVADA D17

and represents a transition into the Navajo Sandstone. It weathers to 

form a series of cliffs and ledgy slopes below the massive Navajo Sandstone. The Kayenta is composed of interstratified pale-reddish-brown 

horizontally to wavy-stratified siltstone to sandy siltstone and lightbrown, moderate-reddish-orange, and white horizontally stratified to 

cross-stratified sandstone. Most of the cross-stratification in the sandstone consists of thin to thick trough and planar sets of low-angle 

small- to medium-scale cross-laminae and is the result of streamchannel deposition. Some of the lighter colored sandstone units, 

however, are composed of thick wedge-planar sets of medium- to largescale high- and low-angle cross-laminae. These units probably represent eolian deposited tongues of the Navajo Sandstone interstratified 

with water-laid deposits of the Kayenta Formation.

As indicated, the contact between the Kayenta Formation and the 

Navajo Sandstone is transitional, the result of intertonguing between 

the two formations. The upper contact of the Kayenta Formation is 

placed arbitrarily at the top of the uppermost unit of well-defined 

horizontally stratified to small-scale cross-stratified red siltstone or 

sandstone below the massive cliff of medium- to large-scale crossstratified sandstone of the Navajo. Owing to intertonguing, the 

contact is not a continuous horizon from place to place. In southwestern Utah, the tongues of Navajo Sandstone in the Kayenta 

Formation merge with the Navajo to the northeast and pinch out into 

the Kayenta to the southwest. This relation indicates that the Kayenta-Navajo contact rises and becomes younger in age to the southwest 

(Wilson, 1959, p. 116).

SOUTHERN NEVADA

In southern Nevada, the strata between the Chinle Formation (restricted) and the Aztec Sandstone (Navajo Sandstone) are 920 feet 

thick at Horse Spring Valley and 820 feet thick in the Spring Mountains. These strata are here considered to be the probable equivalents 

of the Moenave and Kayenta Formations of southwestern Utah. A 

minimum thickness of 630 feet was measured in the Valley of Fire, 

but this measured thickness may be too small, owing to the cutting out 

of beds by possible concealed faults along the line of section.

The probable equivalents of the Moenave and Kayenta Formations 

in Nevada the upper part of the Chinle Formation of previous reports are very similar in lithology to the Kayenta Formation of the 

Leeds area and consist of a lower silty part and an upper sandy part 

that, in places, is transitional with the Aztec Sandstone (Navajo Sandstone) . Units correlative with the various members of the Moenave 

Formation of southwestern Utah were not recognized.

The lower silty part of the probable equivalents of the Moenave and 

Kayenta Formations ranges from 450 to 650 feet in thickness. It is
D18 CONTRIBUTIONS TO STRATIGRAPHY

composed predominantly of pale-reddish-brown to grayish-red horizontally laminated to very thick bedded siltstone to sandy eiltstone. 

Lesser amounts of silty sandstone are interstratified in thin to very 

thick sets with the siltstone. Much of the sandstone is cross laminated 

and ripple marked. Minor amounts of gypsum are also present. At 

Valley of Fire, a prominent ledge of pale-red sandstone, 75 feet thick, 

is present 22 feet above the base of the interval. The lithology of this 

sandstone is similar to that of the Springdale Sandstone Member of 

the Moenave Formation in southwestern Utah, and the two units may 

be correlative. This sandstone unit is absent, however, farther to the 

east at Horse Spring Valley. Thus, the correlation of the Springdale 

Sandstone Member into the Valley of Fire is questionable at best and 

probably should not be attempted.

The upper sandy part of the probable equivalents of the Moenave 

and Kayenta Formations is identical in lithology to the upper part of 

the Kayenta Formation in the Leeds area and also represents a transition into the overlying Navajo equivalent, the Aztec Standstone. The 

upper sandy part ranges in thickness from 230 feet at Horse Spring 

Valley to 360 feet sot the Spring Mountains. Wavy-stratified coarse 

siltstone and horizontally stratified to cross-laminated sandstone are 

characteristic, but in some localities much of the siltstone is also horizontally stratified. In the Spring Mountains, a thick unit of white 

to pinkish-gray sandstone composed of wedge-planar sets of high- and 

low-angle small- to medium-scale cross-laminae is present 237 feet 

below the base of the Aztec Sandstone. This sandstone unit is probably 

of eolian origin and may represent a tongue of the Aztec Sandstone. 

Like the base of the Navajo Sandstone in southwestern Utah, the base 

of the Aztec Sandstone in southern Nevada is arbitrarily placed at 

the top of the highest horizontally stratified to wavy-stratified red 

sandstone or siltstone below the massive cliff of lighter colored Aztec 

Sandstone.

The percentage of sandstone in the lower part of the equivalents of 

the Moenave and Kayenta Formations in southern Nevada and in the 

Moenave Formation and the lower part of the Kayenta Formation in 

southwestern Utah increases northeastward from about 15 to 20 percent in southern Nevada to about 34 percent near Leeds, Utah; this 

increase suggests that the source area of these sediments lay to the east 

or northeast of southern Nevada.

NAVAJO SANDSTONE AND AZTEC SANDSTONE

The Navajo Sandstone and its equivalent in Nevada, the Aztec Sandstone, are believed to be a homogeneous unit throughout the region 

examined. In southwestern Utah, the Navajo has a maximum recorded thickness of 2,280 feet in Zion National Park (Gregory, 1950b,
UPPER TRIASSIC AND TRIASSIC (?) FORMATIONS, UTAH-NEVADA D19

p. 83). The top of the Aztec is missing everywhere in southern Nevada, 

either through faulting, as in the Spring Mountains, where the Aztec 

is overlain by Cambrian rocks in thrust contact (Hewett, 1931), or 

through erosion, as in the Muddy Mountains and Horse Spring Valley, 

where the Aztec is separated from overlying strata by an angular 

unconformity (Longwell, 1928,1949). The formation has a maximum 

thickness of 2,500 feet, however, in the Valley of Fire (Longwell, 1949) 

and 2,100 feet in the Spring Mountains at the northern border of the 

Goodsprings quadrangle (Hewett, 1931).

The Navajo or Aztec consists of moderate-orange-pink, yellowishgray and light-brown fine-grained to very fine grained well-sorted 

sandstone, typically composed of large wedge-planar sets of highangle medium- to large-scale cross-laminae. It weathers to form a 

high cliff, or a hogback, above the ledges and slopes formed on the 

Kayenta Formation in southwestern Utah or on the probable equivalents of the Moenave and Kayenta Formations in southern Nevada.

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D20 CONTRIBUTIONS TO STRATIGRAPHY

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o