Revisions to Stratigraphic Nomenclature 

of Jurassic and Cretaceous Rocks 

of the Colorado Plateau

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Revisions to Stratigraphic Nomenclature 

of Jurassic and Cretaceous Rocks 

of the Colorado Plateau

Revisions in Nomenclature of the Middle Jurassic Wanakah Formation, 

Northwestern New Mexico and Northeastern Arizona

By STEVEN M. CONDON and A. CURTIS HUFFMAN, JR.

Stratigraphy and Nomenclature of Middle and Upper Jurassic Rocks, 

Western Colorado Plateau, Utah and Arizona

By FRED PETERSON

The Encinal Canyon Member, a New Member of the Upper Cretaceous 

Dakota Sandstone in the Southern and Eastern San Juan Basin, New Mexico

By WILLIAM M. AUBREY

U.S. GEOLOGICAL SURVEY BULLETIN 1633 A-C
DEPARTMENT OF THE INTERIOR 

DONALD PAUL MODEL, Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

UNITED STATES GOVERNMENT PRINTING OFFICE: 1988

For sale by the

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Any use of trade names is for descriptive purposes only and does not imply 

endorsement by the U.S. Geological Survey.

Library of Congress Cataloging-in-Publication Data

Revisions to stratigraphic nomenclature of Jurassic and Cretaceous rocks of the

Colorado Plateau.

(U.S. Geological Survey bulletin ; 1633 A-C)

Includes bibliographies.

Supt. of Docs, no.: I 19.3:1633A-C

Contents: Revisions in nomenclature of the Middle Jurassic Wanakah 

Formation, northwestern New Mexico and northeastern Arizona / by Steven M. 

Condon and A. Curtis Huffman Stratigraphy and nomenclature of Middle 

and Upper Jurassic rocks, western Colorado Plateau, Utah and Arizona / by 

Fred Peterson The Encinal Canyon Member, a new member of the Upper 

Cretaceous Dakota Sandstone in the southern and eastern San Juan Basin, New 

Mexico / by William M. Aubrey.

1. Geology, Stratigraphic Jurassic Nomenclature. 2. Geology, 

Stratigraphic Cretaceous Nomenclature. 3. Geology, Stratigraphic 

Nomenclature Colorado Plateau. I. Condon, Steven M. II. Series. 

QE75.B9 no. 1633 A-C 557.3 s 87-600053 

[QE681] [551.7'66'014]
CONTENTS

[Letters indicate the chapters]

(A) Revisions in nomenclature of the Middle Jurassic Wanakah Formation, 

northwestern New Mexico and northeastern Arizona, by Steven M. 

Condon and A. Curtis Huffman, Jr. 1

(B) Stratigraphy and nomenclature of Middle and Upper Jurassic rocks, 

western Colorado Plateau, Utah and Arizona, by Fred Peterson 13

(C) The Encinal Canyon Member, a new member of the Upper Cretaceous 

Dakota Sandstone in the southern and eastern San Juan Basin, New 

Mexico, by William M. Aubrey 57

Contents III

Chapter A

Revisions in Nomenclature of the 

Middle Jurassic Wanakah Formation, 

Northwestern New Mexico and 

Northeastern Arizona

By STEVEN M. CONDON and A. CURTIS HUFFMAN, JR.

"Wanakah Formation" replaces 

"Summerville Formation" in 

New Mexico and Arizona

U.S. GEOLOGICAL SURVEY BULLETIN 1633-A

REVISIONS TO STRATIGRAPHIC NOMENCLATURE OF JURASSIC AND CRETACEOUS ROCKS 

OF THE COLORADO PLATEAU
CONTENTS

Abstract 3

Introduction 3

Stratigraphy and depositional environments

Upper and lower contacts 7

Reference section 10

References cited 10

FIGURES

Al. Location map of northwestern New Mexico, northeastern Arizona, and adjacent areas 4

A2. Chart showing nomenclature applied by various authors to Jurassic rocks 

in the Four Corners region 5

A3. Cross section of the Wanakah Formation from Toh Atin Mesa, Ariz., to 

Navajo, N. Mex. 6

A4. Photographs of Wanakah Formation and adjacent formations at measured 

section localities 8

A5. Map showing Todilto-Pony Express basin of deposition and the approximate distribution of the gypsum-anhydrite unit 10

A6. General paleogeography of the Four Corners region in Todilto-early 

Beclabito-early Cow Springs time 10

A7. Reference section of the Wanakah Formation and its Todilto Limestone 

Member and type section of its Beclabito and Horse Mesa Members 11

Contents
REVISIONS TO STRATIGRAPHIC NOMENCLATURE OF JURASSIC AND CRETACEOUS ROCKS 

OF THE COLORADO PLATEAU

Revisions in Nomenclature of the Middle Jurassic 

Wanakah Formation, Northwestern New Mexico 

and Northeastern Arizona

By Steven M. Condon and A. Curtis Huffman, Jr.

Abstract

Use of the name Summerville Formation in northwestern 

New Mexico and northeastern Arizona is here discontinued, 

and this name is replaced by the name Wanakah Formation 

in that area. In New Mexico and Arizona the Wanakah consists of the basal Todilto Limestone Member and two new 

members, the Beclabito and Horse Mesa Members. The 

Wanakah overlies the Entrada Sandstone, underlies the Morrison Formation, and intertongues laterally with the Cow Springs 

Sandstone. On the basis of previously established stratigraphic 

relations, the Wanakah is considered to be Middle Jurassic in 

age.

INTRODUCTION

The Middle Jurassic Wanakah Formation of the 

San Rafael Group is here reinstated throughout northwestern New Mexico and northeastern Arizona and 

replaces the name Summerville Formation in those areas. 

This change in stratigraphic nomenclature is necessary 

because recent stratigraphic reports (O'Sullivan, 1980a, 

1981; O'Sullivan and Pierce, 1983) have documented the 

reasons for limiting use of the name Summerville Formation to the San Rafael Swell-Monument upwarp area 

of Utah (fig. Al). These reports have applied the name 

Wanakah Formation to red beds in southeastern Utah 

and southwestern Colorado where the Summerville was 

previously recognized.

The Wanakah was named by Burbank in 1930 for 

exposures in the Wanakah mine, near Ouray, Colo., and 

was originally assigned as the basal member of the Morrison Formation (fig. A2). The Wanakah near Ouray included all strata between the Upper(?) Jurassic sandstone 

(now named the Entrada Sandstone) and the lower part 

of the sandstone member of the Morrison (now assigned 

to the Tidwell Member of the Morrison; Peterson, this 

volume) (Burbank, 1930, p. 172). Goldman and Spencer

(1941) also considered the Wanakah to be a member of 

the Morrison; however, they restricted the name to sandy, 

red, argillaceous strata that are now called the upper or 

marl member of the Wanakah. They named an underlying red sandstone the Bilk Creek Sandstone Member of 

the Morrison, and assigned Burbank's "Pony Express 

beds" to the base of the Morrison as the Pony Express 

Limestone Member. Eckel (1949, p. 28) elevated the 

Wanakah to formation rank and extended the name to 

the La Plata Mountains area (fig. Al), where it consists 

of the Pony Express Limestone Member at the base, the 

Bilk Creek Sandstone Member, and the marl member. 

In this report, our use of the name Wanakah Formation 

in northwestern New Mexico and northeastern Arizona 

follows that of Baker and others (1947, p. 1668) and 

Imlay (1952, p. 960), who included the Todilto Limestone, an equivalent of the Pony Express, as the basal 

member of the Wanakah Formation (fig. A2).

In this report we are establishing a reference section for the Todilto Limestone Member and the Wanakah 

Formation, and a type section for the Beclabito (bek-'labito) and Horse Mesa Members at Horse Mesa, Arizona 

and New Mexico, in sec. 17, T. 38 N., R. 31 E. (Gila/Salt 

River grid) and sees. 26 and 27, T. 29 N., R. 21 W. (New 

Mexico grid). At Horse Mesa the Wanakah consists of 

the Todilto Limestone Member at the base, the Beclabito 

Member, and the Horse Mesa Member. The Beclabito 

is named in this report for exposures at Beclabito dome, 

7 mi (11 km) north-northeast of Horse Mesa. The name 

Horse Mesa Member is applied here to rocks between the 

Beclabito Member of the Wanakah and the Salt Wash 

Member of the Morrison Formation on Horse Mesa. A 

comparison of the nomenclature of this report with that 

used previously and in other areas is shown as figure 

A2. The measured section at Horse Mesa is shown as 

figure A7.

The Todilto Limestone, named by Gregory (1917) 

for exposures at Todilto Park, N. Mex. (fig. Al), is here

Jurassic Wanakah Formation, New Mexico and Arizona
38°

36° 

110°

SAN RAFAEL 

SWELL

108° 106°

/ I I

UTAH

Area of 

this figure

COLORADO

Horse Mesa (Reference section of 

Todilto and Wanakah. Type 

section of Horse Mesa and 

Beclabito Members

I \ Sanostee Wash 

Toadlena

SAN JUAN BASIN

SIERRA 

NACIMIENTO

Line of section' | | Navajo 

shown on figure 3

ARIZONA NEW MEXICO

0 100 MILES

_I

I I 

0 50 100 KILOMETERS

Figure A1. Location map of northwestern New Mexico, northeastern Arizona, and adjacent areas.

lowered in rank and made the basal member of the 

Wanakah Formation, because its equivalent in Colorado, 

the Pony Express Limestone, is the basal member of the 

Wanakah in that area (fig. A2). This reassignment makes 

the base of the Wanakah consistent from Colorado to 

Arizona and New Mexico. The name Beclabito Member 

is here introduced to describe the red beds previously 

named Summerville Formation in Arizona and New

Mexico. Above these red beds the name Bluff Sandstone 

was applied incorrectly to both the Bluff Sandstone 

Member of the Morrison Formation and an underlying 

sandstone unit in the Carrizo Mountains area (Strobell, 

1956); use of the name Bluff Sandstone in the Carrizo 

Mountains area is here restricted, and the lower sandstone unit or "A unit" of the Bluff is renamed the Horse 

Mesa Member of the Wanakah. The Horse Mesa is the

Jurassic and Cretaceous of Colorado Plateau
Burbank (1930) 

Ouray, Colorado

Goldman and 

Spencer (1941) 

Southwest 

Colorado

Eckel (1949) 

La Plata Mountains

Imlay (1952) 

Northwest New Mexico 

Northeast Arizona

Strobell (1956) 

Carrizo Mountains

O'Sullivan (1980b) 

Southeast Utah

This report 

Northwest New Mexico 

Northeast Arizona

Cretaceous rocks

_o

I

o

o

 =

o

5

Shale 

member

Sandstone 

member

(Unit not present)

Wanakah Member

Shale 

beds

Sandstone 

bed

"Pony Express 

beds"

Upper(?) Jurassic 

sandstone

_o

I

o 

o 

o

5

Post-Junction 

Creek

Junction Creek 

Sandstone

Wanakah Marl 

Member

Bill Creek 

Sandstone 

Member

Pony Express 

Limestone 

Member

Entrada 

Sandstone

Morrison 

Formation

Junction Creek 

Sandstone

o

*

Marl 

member

Bill Creek 

Sandstone 

Member

Pony Express 

Limestone 

Member

Entrada 

Sandstone

Morrison 

Formation

0

e

o

i

Sandstone 

and shale 

member

Todilto 

Limestone 

Member

Entrada 

Sandstone

Morrison 

Formation

Bluff 

Sandstone

Summerville 

Formation

Todilto 

Limestone

Entrada 

Sandstone

i

o

o

 =.

o

5

Salt Wash 

and younger 

members

Bluff 

Sandstone 

Member

Wanakah 

Formation

(Unit not present)

Entrada 

Sandstone

1 

1

Middle Jurassic

o

E 

o

1

O

o 

o

3

Recapture 

and younger 

members

Salt Wash 

Member

Bluff 

Sandstone 

Member

Horse Mesa 

Member

Beclabito 

Member

Todilto 

Limestone 

Member

\

<

< 

<

Springs Cow Sandstone

Entrada 

Sandstone

Figure A2. Nomenclature applied by various authors to Jurassic rocks in the Four Corners region. This report establishes new 

nomenclature for northeastern Arizona and northwestern New Mexico; the nomenclature of Eckel (1949) is still applied in 

southwestern Colorado, and that of O'Sullivan (1980b) is used in southeastern Utah.

uppermost member of the Wanakah in the Carrizo 

Mountains area.

Figure A3 is a cross section from Toh Atin Mesa 

in northeastern Arizona to Navajo in northwestern New 

Mexico that shows the thickness of the Wanakah in the 

area of this report. See Condon (1985a) for detailed 

lithologic descriptions of the stratigraphic sections used 

for the cross section. Figure A4 comprises photographs 

of most of the outcrops that are control points for the 

cross section. Also see Condon (1985b) for lithologic 

descriptions of the Wanakah Formation and adjacent 

units along the south side of the San Juan Basin.

STRATIGRAPHY AND 

DEPOSITIONAL ENVIRONMENTS

The Todilto Limestone is the basal member of the 

Wanakah Formation in northeastern Arizona and northwestern New Mexico and is the same lithostratigraphic 

unit as the Pony Express Limestone Member of southwestern Colorado (Wright, 1959, p. 62). The nomenclature change from Todilto to Pony Express is placed 

arbitrarily at the New Mexico-Colorado State line. The 

Todilto Limestone Member consists of limestone, 

gypsum, anhydrite, and minor amounts of calcareous

sandstone and siltstone (fig. A4Z>). The limestone forms 

a unit that ranges in thickness from 0 to 40 ft (12 m); 

it is light to dark gray, is laminated to structureless, and 

contains algal structures, ostracodes, and a few fish but 

is otherwise generally unfossiliferous. In part of the 

Todilto-Pony Express basin of deposition, the limestone 

is overlain by a white gypsum-anhydrite unit 0 to 125 ft 

(38 m) thick (fig. A5). This unit is predominantly gypsum at or near the surface and predominantly anhydrite 

in the subsurface; it includes bedded and nodular types 

of both minerals. Near the margins of the basin of deposition of the Todilto-Pony Express (fig. A5), the limestone 

grades laterally into calcareous sandstone and siltstone 

of the Beclabito Member or equivalents. Green and Jackson (1976) and Condon (1985b) report that granules and 

pebbles of chert occur at the stratigraphic position of the 

Todilto in the Gallup, N. Mex., area, where the limestone 

pinches out. The Todilto is recognized throughout most 

of the New Mexico part of the San Juan Basin, as well 

as in the Chama basin, the Hagan basin, and other areas 

of northwestern New Mexico (figs. Al, A5).

The Todilto has been interpreted alternatively as 

deposits of (1) a fresh to saline inland lake (Anderson 

andKirkland, 1960; Tanner, 1965, 1970; Rawson, 1980) 

or (2) a restricted marine basin (Harshbarger and others, 

1957; Ridgley and Goldhaber, 1983; Ridgley, 1984).

Jurassic Wanakah Formation, New Mexico and Arizona
NORTHWEST

Toh Atin 

Mesa

Sec 20 

T40N 

R27E*

Right angle 

bend in section

SOUTH

Tsitah 

Wash 

Sec 7 

T40N 

R29E*

Morrison 

Formation

Wanakah 

Formation

- 11 mi 

(17.7 km)

Bluff Sandstone Member

16 mi (25.7 km)-

Sanostee 

Wash 

Sec 5 

T25N 

R20W*

18 mi (29 km)

Salt Wash Member

Toadlena 

Sec 33 

T23N 

R19W*

18 mi (29 km) *u 15 mi (24.1 km) 

Recapture Member

Todilto

Park 

Sec 13 

T20N 

R20W*

Navajo 

Sec 31 

T20N 

R20W*

Figure A3.

figure A1.

*Unsurveyed; grid location projected 

from adjacent surveyed area

Cross section of the Wanakah Formation from Toh Atin Mesa, Ariz., to Navajo, N. Mex. See line of section on

Five main reasons have been given for interpreting 

the Todilto as a continental deposit: (1) A nonmarine 

ostracode species has been found in the Todilto (Swain, 

1946). (2) No fossils that are clearly marine have been 

found (Anderson and Kirkland, 1960, p. 45). (3) The 

Todilto doesn't interfinger with or extend into marine 

deposits (Tanner, 1970, p. 286). (4) The paleogeographic 

position of the Todilto basin of deposition has been interpreted by some to be an inland, shallow depression that 

was bordered by low hills on three sides and open to an 

alluvial plain on the east (Tanner, 1965, p. 568). (5) No 

dolomite is associated with gypsum in the Todilto 

(Rawson, 1980, p. 305).

Reasons given for interpreting the Todilto as a 

marine deposit are these: (1) Limestone is associated with 

gypsum in the member, and chloride salts are absent 

(Harshbarger and others, 1957, p. 46). (2) Whole-rock carbon, oxygen, and sulfur isotope studies point to a marine 

rather than a lacustrine origin for limestone of the Todilto 

(Ridgley and Goldhaber, 1983). (3) Other paleogeographical reconstructions indicate deposition in a craton-margin 

basin (Ridgley, 1984). (4) Recent re-evaluation of Jurassic 

fish from the Western Interior suggests that two fish taxa 

in the Todilto entered the Todilto basin from marine waters 

(Schaeffer and Patterson, 1984, p. 12).

The data presented are thus somewhat contradictory, and this issue remains unresolved. It seems to the 

present writers that the isotope studies of Ridgley and 

Goldhaber (1983) are perhaps the best quantitative data 

for supporting a marine origin over a lacustrine origin. 

Figure A6 is a general paleogeographical reconstruction 

of the Four Corners region in Todilto-early Beclabitoearly Cow Springs time.

The Beclabito Member of the Wanakah (the Summerville Formation of previous reports) conformably 

overlies the Todilto and consists of 125-200 ft (38-61 m) 

of interbedded reddish-orange to reddish-brown, finegrained, well-sorted sandstone, siltstone, sandy siltstone, 

and mudstone. Sedimentary structures consist of ripple 

cross-stratification, small-scale trough crossbeds, and 

wavy, subparallel, horizontal laminations. In most of the 

outcrops examined in Arizona and New Mexico, the 

Beclabito includes a thick-bedded to massive, dominantly 

structureless sandstone unit at its base, immediately 

overlying the Todilto Limestone Member. This sandstone, which is about 35 ft (10.6 m) thick at the type section (figs. A4E", A7), occupies the same stratigraphic 

position as the Bilk Creek Sandstone Member in Colorado, although exact equivalence of the units has not 

been established. The carnelian sandstone marker bed,

Jurassic and Cretaceous of Colorado Plateau
which is at the top of the Bilk Creek Sandstone Member 

in many areas of southwestern Colorado, was not found 

in the Carrizo Mountains area. The Beclabito Member 

is recognized throughout northwestern New Mexico and 

northeastern Arizona where the Summerville Formation 

was previously recognized.

At the type section, 108 ft (33 m) of interbedded 

sandstone, siltstone, sandy siltstone, and mudstone 

overlie the basal sandstone of the Beclabito Member (fig. 

A7). Within this interval is the bed at Butler Wash 

(O'Sullivan, 1980b), which has been traced into northeastern Arizona and northwestern New Mexico from 

southeastern Utah but has not been identified in outcrops 

south of Horse Mesa (fig. A3). In many areas of southeastern Utah and northeastern Arizona, the part of the 

Wanakah below the bed at Butler Wash is greatly deformed (fig. A44). In some localities, as at Tsitah Wash, 

Ariz. (fig. A4B), the entire Wanakah is contorted, but 

in other places there is little or no significant deformation of the formation (Beclabito Dome and Sanostee 

Wash, figs. A4C and A4F).

The Beclabito Member was deposited in environments marginal to the Todilto-Pony Express basin of 

deposition and is present in many areas of northwestern 

New Mexico, northeastern Arizona, southeastern Utah, 

and southwestern Colorado. It has been traced throughout the San Juan Basin in the subsurface (Summerville 

Formation of Lupe, 1983). The Beclabito intertongues 

with the Cow Springs Sandstone along outcrops south 

of Todilto Park (fig. A3) and in the subsurface west of 

Gallup, N. Mex. We stress that the contact between the 

Beclabito Member and the Cow Springs Sandstone is a 

broad zone of vertical and horizontal intertonguing (fig. 

A4G), and therefore the two units overlap somewhat. The 

unit that we are calling Cow Springs Sandstone in this 

report is the same as the Cow Springs Member of the 

Entrada Sandstone, as used by Peterson in the Black 

Mesa, Ariz., area. (See Peterson, this volume.)

The interpretation of the environment of deposition of the Beclabito depends in part on the interpretation of the underlying Todilto Limestone Member; that 

is, the Beclabito is either marginal lacustrine or marginal 

marine. In this case also, the rock unit has no truly 

diagnostic features such as algal mats, desiccation 

features, or macrofossils that allow for an unequivocal 

interpretation. Deposition at or near base level and 

periodic inundation by water are suggested by some 

features, such as the thin beds of limestone at the base 

of the member, the thin, intercalated mudstone and 

claystone beds within the generally silty or sandy sequence, the lateral continuity of beds, and the lack of 

lenticular, scour-based fluvial sandstone beds. The lateral 

north-to-south gradation of the Beclabito into the eolian 

Cow Springs Sandstone and its position above the 

marine(?) Todilto Limestone Member also indicate that 

the Beclabito has a coastal sabkha-marginal marine

origin. Additionally, the Beclabito grades southward into 

coarse fluvial conglomerates in an area south and 

southeast of Grants, N. Mex. (figs. Al, A6). This area 

is shown on figure A6 as a positive area, although the 

extent and magnitude of this upland area is unclear.

The Horse Mesa Member is the uppermost member 

of the Wanakah. At the type section the Horse Mesa consists of 36.5 ft (11.1 m) of red to orange, fine- to coarsegrained, crossbedded and flat-bedded sandstone. In some 

places its contact with the underlying Beclabito Member 

is sharp; in other places the contact is somewhat arbitrary 

and is placed at the top of the highest siltstone bed of 

the Beclabito Member. The Horse Mesa was deposited 

in eolian dune, interdune, and sabkha environments that 

prograded northward following the withdrawal of the 

body of water in which the Todilto-Pony Express was 

deposited.

The Horse Mesa was previously correlated with the 

Bluff Sandstone (Strobell, 1956; Huffman and Lupe, 

1977), mainly on the basis of lithology and stratigraphic 

position. However, recent stratigraphic studies indicate 

that the Horse Mesa Member underlies the type Bluff 

Sandstone Member at Bluff, Utah, and is separated from 

it by an unconformity; hence the Bluff has now been 

reassigned as the basal member of the Morrison Formation (O'Sullivan, 1980b). A key outcrop on the west side 

of the Carrizo Mountains at Tsitah Wash, Ariz., clearly 

shows that the Horse Mesa Member was contorted along 

with the rest of the Wanakah prior to deposition of the 

overlying Bluff Sandstone Member (fig. MB). We believe 

that the Bluff Sandstone Member is not present above 

the Horse Mesa Member in outcrops in northwestern New 

Mexico.

The Horse Mesa Member is recognized in northeastern Arizona and northwestern New Mexico. It thins 

northward from Arizona, and at Bluff, Utah, is a sandy 

unit at the top of the "middle member and younger parts 

of the Wanakah Formation" as used by O'Sullivan 

(1980b, sections 44, 46, 47, 50, and 52). Southward, the 

Horse Mesa Member grades laterally into the Cow 

Springs Sandstone. Westward, it has been removed by 

Cenozoic erosion, and eastward, in the subsurface, it 

grades into the Beclabito Member (Summerville Formation of Huffman and Lupe, 1977, p. 278). Outcrops in 

southwesternmost Colorado were not evaluated for the 

presence of the Horse Mesa Member in this study. 

However, the lower unit of the Junction Creek Sandstone 

(Ekren and Houser, 1965, p. 13) probably correlates with 

the Horse Mesa.

UPPER AND LOWER CONTACTS

The Wanakah Formation conformably overlies the 

eolian Middle Jurassic Entrada Sandstone in northeastern 

Arizona and northwestern New Mexico. O'Sullivan

Jurassic Wanakah Formation, New Mexico and Arizona 7
«&U?J - , .: * - SHfct - M -IHe ' *^^ "WS^WS

^^ J to^ *' *- *^TJ^^-| ,1 ijiigiitlih yw£ m"^t ^^ C^ ijtojg®^^ ** *^ ^ Hft^*»*^, ^ ^*-"^^i*i^v>ni _^_^ft^ « %fe H^ * . , "s. T* . ^-a-^aiP^^ r-"'i ^

,4, Toh Atin Mesa, Ariz. (sec. 28, T. 40 N., B, Tsitah Wash, Ariz. (sec. 7, T. 40 N., R. 29 E. (projected)). Note that Horse Mesa 

R. 27 E. (projected)). Member was folded and later truncated by the Bluff Sandstone Member.

C, Beclabito Dome, N. Mex. (sec. 24, T. 30 N., R. 21 W.). D, Reference section for Todilto Limestone Member of

Wanakah Formation at Horse Mesa, Ariz. (sec. 17, T. 38 N., 

R. 31 E.). Photograph by J. L. Ridgley.

Figure A4 (above and facing page). Wanakah Formation and adjacent formations at measured section localities in northeastern 

Arizona and northwestern New Mexico. Je, Entrada Sandstone; Jwb, Beclabito Member of the Wanakah; Jwbb, bed at Butler 

Wash; Jwh, Horse Mesa Member of the Wanakah; Jmbs, Bluff Sandstone Member of the Morrison Formation; Jms, Salt Wash 

Member of the Morrison. Note that large-scale deformation features below the bed at Butler Wash are present at Toh Atin Mesa 

(A) but not at Horse Mesa (E). Also note that the percentage of sandstone in the Wanakah increases southward from C to E to F.

8 Jurassic and Cretaceous of Colorado Plateau
f, Type section for Beclabito and Horse Mesa Members of Wanakah Formation at Horse Mesa, Ariz. (sec. 17, T. 38 N., R. 31 

E.). Massive, parallel-bedded sandstone behind tree is near the base of the Beclabito Member.

F, Sanostee Wash, N. Mex. (sec. 5, T. 25 N., R. 20 W. G, Near Navajo, N. Mex. (sec. 31, T. 20 N., R. 20 W.

(projected)). (projected)). Jcs, Cow Springs Sandstone; "T," calcareous zone

that corresponds to the Todilto Limestone Member. The Todilto 

pinches out about 1.5 mi (2.5 km) east of this outcrop (R. E. 

Thaden, oral commun., 1984). The Cow Springs forms tongues

Figure A4. Continued.

of light-colored, crossbedded sandstone that thin northward 

into the Wanakah.

Jurassic Wanakah Formation, New Mexico and Arizona 9
Coastal sabkhamarginat marine

35° 

i r

50 100 KILOMETERS

50 100 KILOMETERS 

EXPLANATION

Figure A5. Outline of the Todilto-Pony Express basin of 

deposition and the approximate distribution of the gypsumanhydrite unit. Modified from Wright (1959).

Eolian dunes Interdunes

Figure A6. General paleogeography of the Four Corners 

region in Todilto-early Beclabito-early Cow Springs time.

(1980b) shows that the lower part of the Wanakah intertongues with and is replaced by the Entrada Sandstone 

in southeastern Utah.

In parts of southwestern Colorado, southeastern 

Utah, and northeastern Arizona the Wanakah is overlain 

by the B unit of the Bluff Sandstone Member of the Morrison Formation or the correlative B unit of the Junction Creek Sandstone (Craig and Cadigan, 1958, p. 182). 

The A units of the Bluff and the Junction Creek are now 

known to be pre-Morrison in age. As now used, the Junction Creek Sandstone is therefore of Middle and Late 

Jurassic age and is not considered part of the San Rafael 

Group in this report. In the northwestern part of the San 

Juan Basin either the Salt Wash Member or the Recapture Member of the Morrison Formation overlies the 

Wanakah at a sharp, erosional contact. Although 

O'Sullivan (1980b) recognizes an unconformity between 

the Morrison and the Wanakah in southeastern Utah, the 

presence of an unconformity at this position in Arizona 

and New Mexico has not been determined.

REFERENCE SECTION

The reference section for the Wanakah Formation 

in northeastern Arizona and northwestern New Mexico 

is at Horse Mesa (Horse Mesa 7.5-minute quadrangle), 

approximately 6 mi (9.6 km) north of the Red Rock 

Trading Post, Ariz. Figure A7 shows the lithology and 

thickness of the Wanakah at the reference section.

REFERENCES CITED

Anderson, R. Y., and Kirkland, D. W., 1960, Origin, varves, 

and cycles of Jurassic Todilto Formation, New Mexico: 

American Association of Petroleum Geologists Bulletin, 

v. 44, no. 1, p. 37-52.

Baker, A. A., Dane, C. H., and Reeside, J. B., Jr., 1947, Revised correlation of Jurassic formations of parts of Utah, 

Arizona, New Mexico, and Colorado: American Association of Petroleum Geologists Bulletin, v. 31, no. 9, 

p. 1664-1668.

10 Jurassic and Cretaceous of Colorado Plateau
EXPLANATION

Sandstone

Siltstone and

mudstone 

Limestone

Trough crossbeds

Tabular planar 

crossbeds

Wavy

laminations

Morrison Formation

c 

o

ro

E

o

LJ_

Wanakah

Entrada Sandstone

Salt WshMember

CD

<D

CO 

COd>

d>

CO

o

I

Member ito 

CO

o CD 

GO

I

Upper sandy member

[-Coarse sand 

i Medium sand 

I Fine sand

 Very fine sand

 Silt 

I Mud 

I Clay

50

r-180

_-170

-160

-150 r

-140

40-

-130

-120

-110

30-

20-

10

0-L0

-1001

-90

-80

-60

-50

-40

-30

-20

Not described.

Sandstone, moderate-reddish-brown (10R 4/6), fine- to medium-grained (some coarse 

grains), well to moderately well sorted, subangular to subrounded; wavy laminated with 

minor tabular planar and trough crossbedding; contains black accessory minerals and 

abundant white chert; 36.5 ft (11.1 m) thick.

Sandstone, siltstone, and mudstone: sandstone, moderate-reddish-brown (10R 4/6) and 

light-brown (5YR 6/4), very fine to medium-grained, well to moderately well sorted, 

subangular to subrounded; wavy laminated with minor trough crossbedding; contains 

black accessory minerals, sparse white chert, and bleached clasts of algal(?) material. 

Fine-grained interbeds of siltstone and mudstone, grayish-red (10R 4/2); wavy 

laminated. 33.0 ft (10.0 m) thick.

Sandstone, grayish-orange-pink (5YR 7/2) to light-brown (5YR 6/4), medium-grained, well 

sorted, subrounded; moderate angle trough crossbedding; 5 ft (1.5 m) thick

Sandstone, moderate-reddish-brown (10R 4/6) to moderate-reddish-orange (10R 6/6), 

fine- to very fine grained, well sorted, subangular to subrounded; wavy laminated with 

minor low-angle trough crossbedding; contains black accessory minerals; minor 

interbedded siltstone and mudstone; 37.0 ft (11.3 m) thick.

Sandstone, siltstone, and mudstone: sandstone, moderate-reddish-brown (10R 4/6) to 

moderate-reddish-orange (10R 6/6), very fine grained, well sorted to silty, subrounded, 

massive to wavy laminated; load casts at base of beds; contains black accessory 

minerals and bleached clasts of algal(?) material. Fine-grained interbeds of siltstone and 

mudstone, dark-reddish-brown (10R 3/4), wavy laminated. 33 ft (10.0 m) thick.

Sandstone, light-brown (5YR 5/6) to moderate-reddish-brown (1 OR 4/6), very fine grained, 

well sorted, subrounded; massive to wavy laminated; banded with white (N9); contains 

black accessory minerals; massive, rounded weathering; 34.5 ft (10.5 m) thick.

Limestone, sandstone, and siltstone: limestone, light-brownish-gray (5YR 6/1) to mediumlight-gray (N6), wavy laminated; weathers to slabby ledges. Sandstone, pinkish-gray 

(5YR 8/1), fine-grained, very well sorted, subrounded to rounded, wavy laminated, very 

calcareous. Siltstone at base, pale red (10R 6/2), wavy laminated, calcareous; 5.5 ft 

(1.7 m) thick.

Not described.

Figure A7. Reference section of the Wanakah Formation and its Todilto Limestone Member (Jwt), and type section of its Beclabito 

and Horse Mesa Members at Horse Mesa, Ariz. and N. Mex. Color terms are those of Goddard and others (1948).

Jurassic Wanakah Formation, New Mexico and Arizona 11
Burbank, W. S., 1930, Revision of geologic structure and 

stratigraphy in the Ouray district of Colorado, and its bearing on ore deposition: Colorado Scientific Society Proceedings, v. 12, no. 6, p. 153-232.

Condon, S. M., 1985a [1986], Lithologic descriptions of selected 

intervals of Jurassic rocks in southeastern Utah, northeastern Arizona, and northwestern New Mexico: U.S. Geological Survey Open-File Report 85-223, 100 p.

____1985b [1986], Lithologic descriptions of selected Middle and Upper Jurassic rocks from Gallup to Laguna in 

northwest New Mexico: U.S. Geological Survey Open-File 

Report 85-126, 68 p.

Craig, L. C., and Cadigan, R. A., 1958, The Morrison and adjacent formations in the Four Corners area, in Sanborn, 

A. F., ed., Guidebook to the geology of the Paradox basin: 

Intermountain Association of Petroleum Geologists Ninth 

Annual Field Conference, 1958, p. 182-192.

Eckel, E. B., 1949, Geology and ore deposits of the La Plata 

district, Colorado: U.S. Geological Survey Professional 

Paper 219, 179 p.

Ekren, E. B., and Houser, F. N., 1965, Geology and petrology 

of the Ute Mountains area, Colorado: U.S. Geological 

Survey Professional Paper 481, 74 p.

Goddard, E. N. (chm.) and others, 1948, Rock-color chart: 

Washington, D.C., National Research Council; reprinted 

by the Geological Society of America, Boulder, Colo., 

1975.

Goldman, M. I., and Spencer, A. C., 1941, Correlation of 

Cross' La Plata sandstone, southwestern Colorado: 

American Association of Petroleum Geologists Bulletin, 

v. 25, no. 9, p. 1745-1767.

Green, M. W., and Jackson, T. J., 1976, Geologic and structure contour maps of the Gallup East quadrangle, 

McKinley County, New Mexico: U.S. Geological Survey 

Open-File Report 76-453, scale 1:24,000.

Gregory, H. E., 1917, Geology of the Navajo country A 

reconnaissance of parts of Arizona, New Mexico, and 

Utah: U.S. Geological Survey Professional Paper 93, 161 p.

Harshbarger, J. W., Repenning, C. A., and Irwin, J. H., 1957, 

Stratigraphy of the uppermost Triassic and the Jurassic 

rocks of the Navajo country: U.S. Geological Survey Professional Paper 291, 74 p.

Huffman, A. C., Jr., and Lupe, R. D., 1977, Influences of 

structure on Jurassic depositional patterns and uranium 

occurrences, northwestern New Mexico, in Fassett, J. E., 

ed., San Juan Basin III, Guidebook of northwestern New 

Mexico: New Mexico Geological Society, 28th Field Conference, 1977, p. 277-283.

Imlay, R. W., 1952, Correlation of Jurassic formations of North 

America, exclusive of Canada: Geological Society of 

America Bulletin, v. 63, p. 953-992.

Lupe, Robert, 1983, Stratigraphic sections of subsurface 

Jurassic rocks in the San Juan Basin, New Mexico,

Colorado, Utah, and Arizona: U.S. Geological Survey Oil

and Gas Investigations Chart OC-118. 

O'Sullivan, R. B., 1980a, Stratigraphic sections of Middle

Jurassic San Rafael Group and related rocks from the

Green River to the Moab area in east-central Utah: U.S.

Geological Survey Miscellaneous Field Studies Map

MF-1247. 

____1980b, Stratigraphic sections of Middle Jurassic San

Rafael Group from Wilson Arch to Bluff in southeastern

Utah: U.S. Geological Survey Oil and Gas Investigations

Chart OC-102. 

____1981, Stratigraphic sections of some Jurassic rocks from

near Moab, Utah to Slick Rock, Colorado: U.S. Geological 

Survey Oil and Gas Investigations Chart OC-107.

O'Sullivan, R. B., and Pierce, F. W., 1983, Stratigraphic 

diagram of Middle Jurassic San Rafael Group and 

associated formations from the San Rafael Swell to Bluff 

in southeastern Utah: U.S. Geological Survey Oil and Gas 

Investigations Chart OC-119.

Rawson, R. R., 1980, Uranium in Todilto Limestone (Jurassic) 

of New Mexico Example of a sabkha-like deposit, in 

Rautman, C. A., compiler, Geology and mineral 

technology of the Grants uranium region, 1979: New Mexico Bureau of Mines and Mineral Resources Memoir 38, 

p. 304-312.

Ridgley, J. L., 1984, Paleogeography and facies distribution 

of the Todilto Limestone and Pony Express Limestone 

Member of the Wanakah Formation, Colorado and New 

Mexico: Geological Society of America, Abstracts with 

Programs, Rocky Mountain Section, v. 16, no. 4, p. 252.

Ridgley, J. L., and Goldhaber, Martin, 1983, Isotopic evidence 

for a marine origin of the Todilto I imestone, north-central 

New Mexico: Geological Society of America, Abstracts 

with Programs, v. 15, no. 4, p. 414.

Schaeffer, Bobb, and Patterson, Colin, 1984, Jurassic fishes 

from the western United States, with comments on Jurassic 

fish distribution: American Museum Novitates, no. 2796, 

p. 1-86.

Strobell, J. D., Jr., 1956, Geology of the Carrizo Mountains 

area in northeastern Arizona and northwestern New Mexico: U.S. Geological Survey Oil and Gas Investigations Map 

OM-160.

Swain, F. M., 1946, Middle Mesozoic nonmarine Ostracoda 

from Brazil and New Mexico: Journal of Paleontology, 

v. 20, p. 543-555.

Tanner, W. F., 1965, Upper Jurassic paleogeography of the 

Four Corners region: Journal of Sedimentary Petrology, 

v. 35, p. 564-574.

____1970, Triassic-Jurassic lakes in New Mexico: The Mountain Geologist, v. 7, p. 281-289.

Wright, J. C., 1959, San Rafael (Entrada) studies: U.S. Geological Survey Trace Elements Investigations Report 

TEI-751, p. 61-66.

12 Jurassic and Cretaceous of Colorado Plateau
Chapter B

Stratigraphy and Nomenclature of 

Middle and Upper Jurassic Rocks, 

Western Colorado Plateau, 

Utah and Arizona

By FRED PETERSON

A revision of units in the 

San Rafael Group and the 

Morrison Formation

U.S. GEOLOGICAL SURVEY BULLETIN 1633-B

REVISIONS TO STRATIGRAPHIC NOMENCLATURE OF JURASSIC AND CRETACEOUS ROCKS 

OF THE COLORADO PLATEAU
CONTENTS

Abstract 17

Introduction 17

Acknowledgments 20

Methods 20

Middle part of the San Rafael Group 20

Entrada Sandstone 20

Informal members 21 

Cow Springs Member 27 

Upper part of the San Rafael Group 29

Curtis Formation 29

Summerville Formation 30 

Chocolate member 30 

Brick-red member 30

Romana Sandstone 32 

Morrison Formation 33

Tidwell Member 35

Salt Wash Member 42

Fiftymile Member 42

Brushy Basin Member 45 

Name and location of measured sections 46 

Measured sections 48

Section 1, principal reference section, Entrada Sandstone 48

Section 2, type section, Romana Sandstone 52

Section 3, type section, Tidwell Member 52

Section 4, type section, Fiftymile Member 53 

References cited 54

14 Contents
FIGURES

B 1. Index maps showing locations of measured sections, lines of stratigraphic 

sections, and other features in southeastern Utah and northeastern 

Arizona 18 

B 2. Chart showing stratigraphic nomenclature of Jurassic and Cretaceous

rocks on west side of Colorado Plateau 19 

B 3. Columnar section for reference section of the Entrada Sandstone at Pine

Creek 21

B 4. Photograph of Pine Creek reference section of the Entrada Sandstone 22 

B 5. Stratigraphic section A-A' from Mount Carmel Junction, Utah, to

Kaiparowits basin 23

B 6. Photograph showing bleaching at top of middle member of Entrada Sandstone beneath J-3 unconformity 24

B 7. Stratigraphic section B-B' along south side of Kaiparowits basin 25 

B 8. Stratigraphic section C-C' from Cow Springs, Ariz., to the Straight

Cliffs 26 

B 9. Photograph of middle and upper members of Entrada Sandstone near

Henrieville in western Kaiparowits basin 26 

BIO. Stratigraphic section D-D' along north, east, and south sides of Black

Mesa, Ariz. 28

fill. Stratigraphic section E-E' along Waterpocket Fold 30 

B12. Photographs showing truncation of beds beneath the J-3 and J-5 unconformities 31

B13. Stratigraphic section F-F' from Henry basin to Kaiparowits basin 32 

B14. Photographs showing outcrops of Romana Sandstone 34 

B15. Columnar section of the type section of the Romana Sandstone 35 

B16. Photograph of coarse and very coarse grains at base of Romana

Sandstone 36

B17. Schematic section showing facies relationships in San Rafael Group 36 

B18. Diagram showing relationships of members in the Morrison Formation 37

B19. Photograph of type section of the Tidwell Member of the Morrison 37 

B20. Columnar section of type section of the Tidwell Member 38 

B21. Stratigraphic section G-G' from Escalante to Duma Point, Utah 39 

B22. Photograph of Tidwell Member in South Desert 40 

B23. Photograph of basal bed of Tidwell Member along Waterpocket Fold 41 

B24. Photograph of Morrison Formation in southeastern Kaiparowits basin,

showing Fiftymile Member 43 

B25. Columnar section of type section of Fiftymile Member of the

Morrison 44 

B26. Photograph of type section of Fiftymile Member 45

Middle and Upper Jurassic Rocks, Utah and Arizona 15

REVISIONS TO STRATIGRAPHIC NOMENCLATURE OF JURASSIC AND CRETACEOUS ROCKS 

OF THE COLORADO PLATEAU

Stratigraphy and Nomenclature of Middle and 

Upper Jurassic Rocks, Western 

Colorado Plateau, Utah and Arizona

By Fred Peterson

Abstract

Recent studies on the west side of the Colorado Plateau 

revealed certain key problems in the existing stratigraphic concepts and terminology for the upper part of the San Rafael 

Group and the Morrison Formation:

1. An eolian sandstone unit at Black Mesa, Ariz., that 

was formerly known as the Cow Springs Sandstone 

does not interfinger with the Summerville and Morrison Formations, as had been supposed.

2. Marginal marine and eolian strata in the Kaiparowits 

basin, previously included in the Entrada Sandstone 

or the Morrison Formation, are separated from both 

of these formations by unconformities, are a sandstone facies of the Summerville Formation, and are 

sufficiently distinct to merit formation status.

3. A predominantly mudstone unit at the base of the 

Morrison, variously assigned to either the Salt Wash 

Member or the underlying Summerville Formation, 

is separated from the Summerville by an unconformity, interfingers with the Salt Wash, and is lithologically distinct from the Salt Wash.

4. A thick sequence of interbedded mudstone and sandstone in the Kaiparowits basin of southern Utah, included by some workers in the upper part of the Salt 

Wash Member, correlates largely with the Brushy 

Basin Member but is readily distinguished from both 

the Brushy Basin and the Salt Wash. 

To rectify these problems, the following revisions in 

Jurassic stratigraphic nomenclature on the west side of the Colorado Plateau are here made:

1. The unit formerly known as the Cow Springs Sandstone is reduced in rank to the Cow Springs Member 

of the Entrada Sandstone of the San Rafael Group.

2. The southern facies of the Summerville Formation in 

the Kaiparowits basin is named the Romana Sandstone of the San Rafael Group.

3. The predominantly mudstone unit at the base of the 

Morrison Formation is named the Tidwell Member 

of the Morrison Formation.

4. The southern facies of the Brushy Basin Member in

the southeastern Kaiparowits basin is named the Fiftymile Member of the Morrison Formation. 

These revisions clarify stratigraphic relationships in the 

Jurassic System of the Colorado Plateau and facilitate regional 

sedimentologic and paleotectonic syntheses.

INTRODUCTION

Although considerable effort has been devoted to 

unraveling the stratigraphic and sedimentologic framework of Middle and Upper Jurassic strata on the Colorado Plateau, the rocks remain, as Oriel and Craig (1960, 

p. 43) pointed out more than 25 years ago, "the stormy 

center of prolonged geologic controversy." Many of the 

older problems concerning the stratigraphic relationships 

of these rocks have been studied and, to varying degrees, 

resolved, but a review of the accomplishments of the past 

40 years shows that almost every resolution of a specific 

stratigraphic problem has led to another controversy.

This report revises the stratigraphic framework for 

these rocks based on an understanding of their relationships that has evolved during the course of field work 

during the past 20 years. The studies clearly demonstrate 

that the previous nomenclature is not only inadequate but 

confusing and, hence, the need for the revisions. This 

study chiefly considers two stratigraphic problems in the 

region: (1) the lateral relationships of the various lithologic units and (2) the nature of certain surfaces that are 

thought to be unconformities.

The stratigraphic nomenclature varies from region 

to region in the area of study on the west side of the Colorado Plateau (fig. Bl). The new nomenclature and the 

approximate time-stratigraphic relationships of the 

various Middle and Upper Jurassic rocks are depicted 

schematically in figure B2.

The San Rafael Group is considered Middle 

Jurassic in age and the Morrison Formation is considered

Middle and Upper Jurassic Rocks, Utah and Arizona 17
113

EMERY Tidwe" 

cmtnr Bottoms

BEAVER \ 

/

Area shown 

in figure B1B

GARFIELD 15.6

Westwater D n Recapture

Creek Creek

22\26 Canyon Canyon

KAIPAROWITS

36° 

50 100

100 MILES

150 KILOMETERS

Figure B1 (above and facing page). Locations of measured sections, lines of stratigraphic sections, and other features mentioned in this report. Triangles indicate type or principal reference sections for units defined or revised herein. Names and 

locations of measured sections 1-93 are listed at end of text; stratigraphic sections A-A' through F-F' are presented in figures 

B5, B7, B8, B10, B11, B13, and B21. A (above), Regional map of southeastern Utah and northeastern Arizona. E, S, and C 

indicate Entrada, Summerville, and Curtis Points, respectively.

18 Jurassic and Cretaceous of Colorado Plateau
ARIZONA ,

37° _J ______uimi_

0 50 100 KILOMETERS 

Figure B1 Continued. B, Enlarged map of south-central Utah.

System Series Black Mesa basin, Arizona Kaiparowits basin, Utah

Unconformity

CRETACEOUS

Upper 

(part)

Lower

Upper

JURASSIC

Middle

Dakota Sandstone

m

Dakota Formation

Westwater Canyon Member

Recapture Member

Salt Wash 

Member

Entrada 

Sandstone

*Tidwell Member

*Cow Springs Member

Lower sandy member

Salt Wash Member

Middle member

Lower member

Carmel Formation Carmel Formation 

Page Sandstone

^rnrrrrrrrrrrmrrrrrrrrrrrfrnrr^

K-2

K-1

J-5

J-3

J-2

Figure B2. Stratigraphic nomenclature of Jurassic and Cretaceous rocks on the west side of the Colorado Plateau that are 

discussed in this report. Asterisks indicate new or reassigned units. Unconformity designations at right are those of Pipiringos and O'Sullivan (1978).

Middle and Upper Jurassic Rocks, Utah and Arizona 19
Late Jurassic in age, according to a fairly recent evaluation by Imlay (1980). Throughout the region of study, 

the San Rafael is underlain by the Glen Canyon Group 

of Late Triassic to Early Jurassic age. The Morrison is 

unconformably overlain by either the Lower Cretaceous 

Cedar Mountain Formation or, where the Cedar Mountain is missing, the lowermost Upper Cretaceous Dakota 

Sandstone or Formation (Peterson and Kirk, 1977). Prior 

to deposition of the Dakota, the southwestern part of the 

Colorado Plateau was elevated, allowing the Dakota to 

truncate progressively lower formations southwestward.

This report only discusses strata in the middle and 

upper parts of the San Rafael Group and the Morrison 

Formation in the western part of the Colorado Plateau 

(fig. Bl). Stratigraphic relationships of older strata in the 

San Rafael Group, including the Temple Cap, Page, and 

Carmel Formations, were described in an earlier report 

(Peterson and Pipiringos, 1979).

Middle and Upper Jurassic strata on the Colorado 

Plateau contain several marker surfaces of broad regional 

extent that help to separate the sedimentary sequence into smaller subdivisions of related formations or members. 

Conflicting opinions concerning the conformable or unconformable nature of these surfaces may be the result 

of differences in the surfaces at different localities. Recent 

studies by Condon and Kirk (1983), Kirk and Condon 

(1986), Peterson (1980b, 1984, 1986), and Santos and 

Turner-Peter son (1986) indicate that many of the basins 

and uplifts on the Colorado Plateau were technically active during the Jurassic, allowing the possibility that more 

or less continuous deposition in the deeper parts of some 

of the basins may have coincided with nondeposition or 

even erosion on flanking uplifts. Consequently, a surface 

that is unconformable in one area may be conformable 

in another area. Furthermore, an unconformity may be 

planar and separate beds that are parallel to that surface 

(a paraconformity in the terminology of Dunbar and 

Rodgers, 1957), making it difficult to determine from 

physical relationships whether any particular surface is 

indeed an unconformity. Locally, angular relationships 

are so slight that the beds appear to be parallel and conformable; in many such cases, angular beveling is only 

apparent when the broad regional relationships are considered. The lack of age-diagnostic fossils and conflicting results from isotopic dating also preclude accurate 

dating of these rocks, which would aid immeasurably in 

identifying any given surface as an unconformity.

ACKNOWLEDGMENTS

Discussions with S. M. Condon, L. C. Craig, 

M. W. Green, M. E. MacLachlan, R. B. O'Sullivan, 

C. E. Turner-Peterson, and H. D. Zeller were very 

helpful in the evolution of my thoughts concerning the 

relationships of Middle and Upper Jurassic strata of the

Colorado Plateau. Capable assistance during the course 

of field work was lent by Craig Buth, J. D. Craig, M. A. 

Cutler, Brad Esslinger, M. J. Larson, D. C. Logan, Paul 

Milde, R. A. Moore, M. C. Pope, and J. V. Roberts. 

The report was significantly improved by the helpful comments and thoughtful reviews of M. E. MacLachlan, 

P. L. Martin, and R. B. O'Sullivan.

METHODS

About 150 or more sections were measured during 

this investigation. Sections were measured with Brunton 

compass and tape or with a 5-foot Jacob's staff where 

the beds dip more than approximately 5 degrees. All of 

the Stratigraphic units proposed in this report, whether 

formations or members, are considered mappable entities, 

and some have already been mapped as informal stratigraphic units in the southeastern Kaiparowits region 

(Peterson, 1975, 1980a).

Crossbedding was measured by the methods of 

Reiche (1938) and Curray (1956), and vector resultants 

and consistency factors (vector mean strength) were 

calculated using a small programmable pocket calculator 

with programs prepared by M. B. Sawyer (U.S. Bureau 

of Mines) or the author. Where possible, 25 or more 

measurements were made at each locality. However, in 

some places, lack of outcrops or difficult access made 

it impossible to obtain 25 measurements and a smaller 

number (not less than 15) had to suffice. The vector 

resultants obtained from these studies aid in distinguishing some of the crossbedded eolian sandstone units. The 

consistency factor (Reiche, 1938; multiplied by 100 to 

eliminate decimals and read as percent) is a measure of 

vector strength and aids in evaluating the amount of 

variation between the individual crossbedding measurements that make up the resultant. Consistency factors 

near 100 percent (the maximum) indicate a tendency 

toward unidirectional-dipping cross-strata; low values 

reflect a tendency for the individual measurements to dip 

in many directions. In most cases, consistency factors less 

than about 20 percent are considered random and therefore meaningless.

MIDDLE PART OF THE SAN RAFAEL GROUP

The middle part of the San Rafael Group includes 

the Entrada Sandstone and correlative rocks formerly 

known as the Cow Springs and Henrieville Sandstones. 

Regional studies by Imlay (1980) indicate that the Entrada 

is Middle Jurassic (early to middle Callovian) in age.

Entrada Sandstone

The Entrade Sandstone was named by Gilluly 

and Reeside (1928) for strata at Entrada Point in the

20 Jurassic and Cretaceous of Colorado Plateau
northeastern part of the San Rafael Swell (fig. Bl). There, 

the formation consists largely of red, very fine- to finegrained, flat-bedded silty sandstone and lesser quantities 

of interbedded red mudstone that tend to form slopes or 

a series of ledges and slopes. The term "earthy" facies 

has been applied to this sequence and similar rocks in and 

near the type section. South and east from Entrada Point, 

beds approximately correlative with the type Entrada 

grade into a cliff-forming red or white crossbedded sandstone unit referred to by some as the "slick rock" facies. 

Many authors consider the slick rock facies more representative of the formation as a whole than the earthy 

facies, not only because it is the dominant facies 

throughout much of the central and eastern parts of the 

Colorado Plateau, but also because it is present at many 

of the well-visited scenic and recreation areas in the 

region. West of the Monument up warp the Entrada is 

separated from overlying beds in the San Rafael Group 

by the J-3 unconformity of Pipiringos and O'Sullivan 

(1978). O'Sullivan (1980b) showed that the J-3 unconformity fades out eastward along the crest of the Monument uplift and that the undivided Entrada east of the 

uplift is approximately equivalent to the Entrada and 

some of the younger beds of the San Rafael Group farther west.

Informal members

In most of the Colorado Plateau, the Entrada can 

be readily divided into several members based on contrasting lithologies. A representative section of the Entrada of the Colorado Plateau, consisting of three clearly 

defined members, is well exposed at Pine Creek, about 

5 km north of Escalante, Utah, in the northern part of 

the Kaiparowits basin (fig. Bl). Because this section is 

representative of the Entrada in much of the western Colorado Plateau, has frequently been cited (for example, 

Baker, Dane, and Reeside, 1936), and because it is readily 

accessible, the Pine Creek section is here proposed as the 

principal reference section for the Entrada Sandstone 

(figs. B3, B4; measured section 1 at end of report).

At Pine Creek, the three units of the Entrada are 

informally designated the lower, middle, and upper 

members. The lower member consists of a reddishorange, flat-bedded, slope-forming silty sandstone of the 

earthy facies, similar to the sequence at the type section 

of the Entrada. (See map of nearby area by Zeller, 1973.) 

About 25 km southeast of Pine Creek, the lower member 

grades into a reddish-orange, largely crossbedded, cliffforming slick rock facies that is present throughout the 

eastern and southern parts of the Kaiparowits basin. (See 

maps by Peterson, 1975, 1980a.) The lower member at 

Pine Creek apparently correlates with the entire section 

of the type Entrada in the northeastern part of the San 

Rafael Swell.

Formation

and 

Member

Description

Tidwell Mbr of 

Morrison Fm (part)

Upper 

member

Middle 

member

Interbedded red mudstone and white 

sandstone

White sandstone, crossbedded; 

contains scattered coarse grains

Upper part: interbedded light-green 

mudstone and light-brown to white 

sandstone

Middle part: interbedded red mudstone 

and silty sandstone; includes some 

white sandstone

Lower part: red mudstone and 

sandstone interbedded with white 

sandstone

Lower 

member

Reddish-orange silty sandstone; 

includes scattered thin red 

mudstone beds

Carmel 

Formation (part)

Red silty sandstone and mudstone 

interbedded with white gypsum

EXPLANATION

Color code

Gray, light green, light

brown 

Red, reddish orange,

purple

Thin bentonite bed

Sandstone 

Mudstone

Sandstone and

mudstone 

Gypsum

Figure B3. Columnar section for the principal reference section of the Entrada Sandstone at Pine Creek near Escalante, 

Utah. This is section 1 on figure B1.

Middle and Upper Jurassic Rocks, Utah and Arizona 21
Figure B4. Photograph of the Pine Creek principal reference section for the Entrada Sandstone. The strata dip about 45 degrees 

to the upper left. For scale, the Entrada Sandstone is 314.1 m thick. The principal reference section at the end of the report 

was measured where the units are labeled. J~Rn, Navajo Sandstone; Jphw, Harris Wash Tongue of Page Sandstone; Jcjh, Judd 

Hollow Tongue of Carmel Formation; Jptp, Thousand Pockets Tongue of Page Sandstone; Jcau, upper member of Carmel Formation; Entrada Sandstone: Jel, lower member, Jem, middle member, Jeu, upper member; Morrison Formation: Jmt, Tidwell Member, 

Jms, Salt Wash Member; Kcm, Cedar Mountain Formation; Kd, Dakota Formation; Kt, Tropic Shale.

The middle member at Pine Creek consists of red 

to light-gray or light-greenish-gray, generally flat-bedded 

sandstone and silty sandstone that tends to weather to 

slopes in the northern part of the Kaiparowits basin. To 

the southeast, the content of resistant crossbedded sandstone increases and the member weathers to steep slopes 

or cliffs, especially where protected by resistant overlying strata. The middle member is similar to the earthy 

facies of the lower member that is present in the northern 

and western parts of the Kaiparowits basin. The two 

members may be distinguished by the bedding, which 

tends to be thinner in the middle member (generally less 

than a meter thick) and thicker in the lower member 

(generally more than a meter thick). Also, the lower

member is mostly reddish orange, whereas the middle 

member is reddish brown or reddish orange and contains 

several thin but conspicuous, very light-gray sandstone 

beds (fig. B3).

The upper member at the Pine Creek principal 

reference section consists of light-gray, largely crossbedded, cliff-forming sandstone. In a few localities, the 

upper member is reddish orange. Uppermost beds of the 

middle member at Pine Creek have been bleached to very 

light gray or light greenish gray (figs. B3, B4, B5) and 

therefore resemble the upper member in color. The lightcolored parts of the two members are readily distinguished, however, by bedding differences: the middle 

member is predominantly flat-bedded and the upper

22 Jurassic and Cretaceous of Colorado Plateau
A

WEST

Upper part of 

Carmel Formation

Figure B5. Stratigraphic section A-A' from Mount Carmel Junction, Utah, to the northeast side of the Kaiparowits basin showing bleached zones beneath unconformities. Section 13 at Henrieville is the type section of the Henrieville Sandstone (H) of 

Thompson and Stokes (1970); section 21 at Point 5802 is at or near the type section of the White Point Member (W) of the 

Summerville Formation of Thompson and Stokes (1970). Neither name is retained in this report. Kcm, Cedar Mountain Formation; J-3, J-5, unconformities of Pipiringos and O'Sullivan (1978). For location see figure B1.

member is predominantly crossbedded. The upper 

member is truncated by the J-3 unconformity in several 

places (figs. B6, B12). Locally along the Straight Cliffs 

(fig. Bl), in an area of poor exposures, the lowermost 

beds of the upper member appear to grade southeastward 

into the uppermost part of the middle member (fig. B5). 

The three members are also readily identified in the 

the Henry basin, where their lithologies are essentially the 

same as in the Kaiparowits basin. The lower member consists of flat-bedded silty sandstone of the earthy facies 

in the northern part of the Henry basin, and crossbedded 

sandstone of the slick rock facies in the central and 

southern parts of the basin. Along the Waterpocket Fold 

on the west side of the Henry basin, the middle member 

contains a cliff-forming crossbedded sandstone unit about 

20 m thick that is lithologically similar to the slick rock 

facies of the lower member. Although the middle member 

is largely concealed by colluvium along the Waterpocket 

Fold, studies there suggest that crossbedded sandstone 

becomes progressively more dominant in the member 

toward the southern end of the fold. The upper member 

apparently is cut out in the northwestern half of the 

Henry basin by the J-3 unconformity. Part of the upper 

member is present in the southeastern half of the basin, 

but its relationships are not clear owing to poor access,

the lack of Stratigraphic marker beds or surfaces, and a 

southeastward change of its color to moderate reddish 

orange, similar to the colors in underlying members. 

Observations from a light plane suggest that the upper 

part of the upper member is cut out there but that the 

lower part may be present. Thus, the lower and middle 

members and part of the upper member appear to grade 

southeastward into moderately reddish-orange, cliffforming crossbedded sandstone of the undivided Entrada 

in the southeastern Henry basin.

The lower contact of the Entrada is a surface generally marking a sharp vertical change in lithologies, but 

it is not an unconformity. Detailed work in the southeastern Kaiparowits basin along the Colorado River (now 

inundated by Lake Powell) showed some interfingering 

involving about 2 m of beds in the Entrada and Carmel 

Formations. The presence of large, highly irregular, 

slumped masses of Entrada that moved downward into 

the upper part of the Carmel in the Kaiparowits and 

Henry basins (Peterson, 1975, 1980a) supports the contention that no significant period of erosion, nondeposition, or compaction separates the Carmel and the 

Entrada.

Some of the colors in the Entrada and in other 

Jurassic formations were caused by bleaching, which has

Middle and Upper Jurassic Rocks, Utah and Arizona 23
Figure B6. Bleaching at the top of the middle member of the Entrada Sandstone (Jem) beneath the J-3 unconformity. Southwestern 

Kaiparowits basin at the type section of the Romana Sandstone (Jr) (fig. B1, section 2). The red band marker bed is the dark 

zone at the base of the Romana and is about 2.6 m thick.

caused considerable confusion in deciphering the stratigraphic relationships within the Entrada and between the 

Entrada and other formations. When viewed from a 

distance of half a kilometer or more, a color boundary 

between bleached and unbleached rocks appears as a 

planar surface that might easily be mistaken for a contact between stratigraphic units because it is so obvious. 

However, from a distance of several meters or tens of 

meters, the color boundary can be seen to cut across the 

bedding; it clearly is the result of postdepositional alteration processes (fig. B6). Regional stratigraphic studies also 

confirm this (figs. B5, B7, B8).

Bleached sandstone beds generally are white, very 

light gray, very light yellowish gray, very light greenish 

gray, or light grayish brown, and contrast markedly with 

the reddish-brown to reddish-orange unbleached sandstone beds. Bleached mudstone beds tend to be medium 

to light greenish gray, which also contrasts with unbleached dark-reddish-brown mudstones, but some 

unbleached mudstone beds deposited in marine and 

lacustrine waters are also medium to light greenish gray

and therefore are not readily distinguished from bleached 

mudstones. For the most part, the thickness of rock affected by bleaching is considerably greater for porous, 

permeable sandstone strata than for less permeable 

mudstone strata. In the southwestern part of the 

Kaiparowits basin just northwest of Page, Ariz., the entire 200-m thickness of the Entrada, consisting largely of 

porous and permeable eolian sandstone, was bleached to 

white or very light gray (fig. B7). In this same area the 

bleaching fluids altered only the uppermost 3 m of mudstone in the underlying Carmel Formation.

Bleaching is common throughout the region 

beneath the J-3 unconformity (underlying the Curds, 

Summerville, or Romana Formations) and the J-5 unconformity (base of the Morrison Formation), and it is 

especially extensive beneath the unconformity at the base 

of the Upper Cretaceous Dakota Formation. The 

bleaching is particularly evident where the Dakota contains coaly and carbonaceous mudstone beds and where 

underlying rocks consist of porous, permeable sandstone. 

This relationship strongly suggests that the bleaching was

24 Jurassic and Cretaceous of Colorado Plateau
B

WEST

Cummings 

Mesa N.

B'

EAST

Navajo 

Point

Upper member 

of Carmel 

Formation

Bleached zone

20 KILOMETERS

Figure B7. Stratigraphic section B-B' along the south side of the Kaiparowits basin showing bleached zones beneath unconformities. For location see figure B1.

caused by organic-rich fluids that seeped into underlying strata and altered some of the minerals, especially 

feldspars and the more readily destroyed iron-bearing 

minerals. Mackin and Schmidt (1956) and Lynd (1960) 

showed that organic-rich fluids can easily destroy irontitanium oxide minerals. Organic matter is not abundant 

in the Curtis, Summerville, Romana, and Morrison Formations, but the Romana and Morrison do contain petrified logs, and the Morrison also contains carbonized plant 

material, which is locally abundant in lacustrine mudstone 

and some fluvial sandstone beds (Peterson, 1980b). Thus, 

it appears likely that bleached zones beneath unconformities are the product of alteration by organic-rich fluids 

that seeped into underlying rocks while the overlying formation was being deposited. Alternatively, organic-rich 

fluids produced by soil-forming processes might have 

seeped into underlying rocks during an erosion interval 

and caused the bleaching.

Westward from the Kaiparowits basin to Mount 

Carmel Junction (fig. B5), a bleached zone is at the top 

of Jurassic rocks just below the Dakota Formation. The

bleached zone cuts across the various formation and 

member boundaries and clearly bears no relationship to 

the stratigraphy. Elsewhere in the study area, bleached 

rocks are present beneath the J-3 and J-5 unconformities, 

indicating that the bleaching is not a phenomenon 

restricted solely to strata underlying the Dakota.

The Henrieville Sandstone of Thompson and 

Stokes (1970) was named for light-gray sandstone and 

mudstone strata that lie beneath the Dakota Formation 

near Henrieville in the western part of the Kaiparowits 

basin (fig. B9). Regional relationships show that these 

rocks are within the bleached zone beneath the Dakota 

Formation (fig. B5) and actually comprise the upper part 

of the middle member and all of the upper member of 

the Entrada of this report. Because the bleached zone has 

no significance as far as Stratigraphic relationships are 

concerned, and because the Henrieville includes part or 

all of two Stratigraphic units that do have Stratigraphic 

importance and are recognized elsewhere in the Kaiparowits basin, the Henrieville Sandstone is not recognized 

here as a valid formation.

Middle and Upper Jurassic Rocks, Utah and Arizona 25
c

SOUTH

BLACK MESA 

BASIN

C

NORTH

Big Hollow 

KAIPAROWITS BASIN Spring

27

Upper member 

of Carmel 

Formation

Bleached zone

Figure B8. Stratigraphic section C-C' from Cow Springs, Ariz., to the Straight Cliffs escarpment on the northeast side of 

the Kaiparowits basin. Section 6 at Cow Springs is the type section of the Cow Springs Member of the Entrada Sandstone; 

section 2 near Romana Mesa is the type section of the Romana Sandstone. For location see figure B1.

Figure B9. Middle (Jem) and upper (Jeu) members of the Entrada Sandstone exposed on an unnamed butte about 3 km west 

of Henrieville in the western Kaiparowits basin, NW14 sec. 28, T. 37 S., R. 2 W., Garfield County, Utah. The Henrieville Sandstone of Thompson and Stokes (1970) is the bleached zone beneath the Dakota Formation (Kd) that extends down to red beds 

(dark gray in the photo) near the base of the slope. The upper member of the Entrada is about 25 m thick.

26 Jurassic and Cretaceous of Colorado Plateau
Cow Springs Member

Bleaching beneath unconformities is also present 

in the region south and southeast of the Kaiparowits basin 

(figs. B7, B8). Southeastward from the Kaiparowits 

region the Dakota truncates successively older strata, and 

at Cow Springs, Ariz., it lies on strata here considered 

to be part of the Entrada Sandstone.

The Entrada consists of two stratigraphic units at 

Cow Springs. A lower unit contains mostly reddishorange crossbedded to flat-bedded sandstone that was 

called the lower sandy member of the Entrada by Harshbarger and others (1957). An overlying unit consisting 

largely of light-colored crossbedded sandstone was 

originally named the Cow Springs Sandstone by Harshbarger and others (1951). Like similar strata farther 

north, in southern Utah, the porous and permeable, 

crossbedded sandstone beds of the Cow Springs have been 

bleached to very light gray, very light greenish gray, or 

light grayish brown.

Exposures of the basal contact of the Cow Springs 

are poor at the type locality. A color boundary that cuts 

across bedding is identifiable in some of the small nearby canyons but is clearly a postdepositional phenomenon. 

To the east, at exposures just southwest of Kayenta, Ariz., 

and in the eastern and southeastern parts of Black Mesa, 

the large-scale crossbedded, cliff-forming sandstone of the 

Cow Springs can be distinguished reasonably well from 

the underlying Entrada strata, which consist of flatbedded silty sandstone in that area. However, northwest 

of Cow Springs, around Square Butte and White Mesa 

(sections 44 and 45 on fig. Bl), the contact between the 

Cow Springs and underlying beds of the Entrada is within 

predominately crossbedded sandstone strata and is difficult to define, even though the exposures are good. In this 

area, as at the type locality, the color boundary cuts across 

the bedding and is therefore clearly postdepositional.

Thus, the Cow Springs is closely related to underlying strata of the Entrada Sandstone but is also distinct 

from these underlying beds in much of its range. The Cow 

Springs serves as a stratigraphic marker that is useful in 

evaluating sedimentologic and paleotectonic conditions 

at the close of deposition of the Entrada Sandstone. For 

these reasons, the crossbedded, cliff-forming sandstone 

unit at the top of the Entrada at Black Mesa is here reduced in rank to the Cow Springs Member of the Entrada Sandstone. The Cow Springs was not included in 

the San Rafael Group by the original workers (Harshbarger and others, 1951, 1957), but its reassignment here 

to the Entrada places it within the San Rafael Group.

The Cow Springs Member may be roughly equivalent in age to the upper member of the Entrada in 

southern Utah or, as the correlations in figure B8 suggest, 

the Cow Springs may be slightly older than the upper 

member. Correlations along this line of section indicate

that both members lie beneath the J-3 unconformity. 

Because the upper member is at the top of the Entrada 

but beneath the J-3 unconformity, and because it is 

similar lithologically to the Cow Springs, the two 

members once may have been deposited as a single large 

sandstone body. If so, the sandstone body was later divided into two parts by upwarping and erosion, during 

the J-3 erosion interval, in the area between Black Mesa 

and the Kaiparowits basin. The name Cow Springs is not 

applied to similar strata of the upper member of the Entrada in the Kaiparowits and Henry basins because the 

regional relationships of the two members are not yet 

clear. The Cow Springs could not be distinguished from 

underlying beds of the Entrada in the area north and 

northwest of White Mesa and Square Butte (sections 44 

and 45, fig. B8).

The Cow Springs, as here defined, is recognized on 

the northwest side, part of the east side, and the southeast 

side of Black Mesa basin (fig. BIO), where it contrasts 

markedly with underlying red sandstone and silty sandstone beds in the Entrada Sandstone. It is not present on 

the southwest side of the basin owing to southwestward 

regional truncation beneath the Dakota Sandstone, and 

it is difficult to distinguish from crossbedded sandstones 

of the Entrada on the west side of the basin. Farther east, 

in northwestern New Mexico, the Cow Springs is recognized on the southwest side of the San Juan Basin, and 

formation status (within the San Rafael Group) is retained 

there pending further studies (Condon and Peterson, 

1986).

The relationships of the lower sandy, medial silty, 

and upper sandy members of the Entrada (Harshbarger 

and others, 1957) in Black Mesa basin to the members 

of the Entrada recognized farther northwest, in the 

Kaiparowits basin, are not clear. Tentative correlations 

suggest that the three informal members in the Black 

Mesa region may correlate with the lower member and 

possibly the middle member of the Entrada in the 

Kaiparowits basin.

Assignment of the Cow Springs Member to the Entrada means that its relationship to other Middle and Upper Jurassic rocks on the Colorado Plateau must be 

reconsidered. The Cow Springs was originally thought 

to be a southern eolian sandstone facies of parts of the 

Summerville and Morrison Formations (Harshbarger and 

others, 1957, p. 48). As shown in figure B8, however, 

the type Cow Springs is entirely older than the Morrison 

Formation and the Romana Sandstone, which is a correlative of the Summerville in the Kaiparowits basin 

discussed later in this report. Thus, the Cow Springs, at 

least as represented by strata at the type locality, cannot 

be considered equivalent in age to, and a facies of, the 

Summerville and Morrison Formations.

Large-scale crossbedded eolian sandstone, which 

typifies the Cow Springs, also is present in some beds of

Middle and Upper Jurassic Rocks, Utah and Arizona 27
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the Romana Sandstone and the Morrison Formation, and 

therefore the name Cow Springs has been applied to some 

of these eolian beds by previous workers. Studies of crossbedding dip vectors suggest one method of distinguishing eolian beds of the Cow Springs from those of the 

Romana and Morrison Formations. Crossbedding in the 

Cow Springs at Black Mesa tends to dip toward the 

southeast, south, southwest, and west (fig. BIO). Some 

thin units within the Cow Springs have crossbedding that 

dips to the northeast but these anomalous dips are insignificant in dip-vector studies of an appreciable thickness 

of strata or of the entire member. Crossbedding in eolian 

strata of the Recapture Member of the Morrison Formation at Black Mesa dips consistently toward the northeast 

or east-northeast (fig. BIO), about 70 degrees or more 

away from crossbedding dip vectors in the Cow Springs 

Member. A thick, largely crossbedded, predominantly 

eolian sandstone unit near the town of Bluff, Utah (fig. 

Bl), is recognized as the Bluff Sandstone Member of the 

Morrison Formation (O'Sullivan, 1980a). Crossbedding 

dip vectors in the Bluff measured about 5 km north of 

Bluff, Utah, dip north 81 degrees east with a consistency 

factor of 70 percent, based on 40 measurements. These 

data suggest that the direction of crossbedding dip in 

eolian strata is a helpful method of distinguishing the Cow 

Springs from eolian beds in the Morrison Formation. Furthermore, eolian crossbedding in the Romana Sandstone 

of the southwestern Kaiparowits basin dips north 87 degrees east with a consistency factor of 81 percent, based 

on 55 measurements. Hence, the same method can be 

used to distinguish the Romana from the Cow Springs.

Studies northeast of Black Mesa suggest that the 

Cow Springs of the Black Mesa region grades into some 

of the flat-bedded silty sandstone beds of the Wanakah 

Formation in northeasternmost Arizona, southeastern 

Utah, and along the west side of the San Juan Basin in 

northwestern New Mexico, (O'Sullivan, 1978; Condon 

and Peterson, 1986). The unit called the Cow Springs 

Sandstone on the southwest side of the San Juan Basin 

(as restricted by Condon and Peterson, 1986) appears to 

correlate with the Cow Springs Member of Black Mesa.

In the south-central part of the San Juan Basin, 

eolian strata in the Recapture Member of the Morrison 

Formation can be distinguished from eolian strata in the 

Cow Springs by differences in lithologies (Peterson and 

Condon, 1984). Recent observations show that some of 

the eolian crossbedding in the Recapture Member dips 

southeast or east-southeast in the southern San Juan 

Basin, suggesting that crossbedding dip vector differences 

may not be as helpful in distinguishing Recapture and 

Cow Springs eolian strata in the San Juan Basin as they 

are farther west in Black Mesa basin.

The Entrada is interpreted as largely eolian and 

sabkha in origin (Kocurek and Dott, 1983). For the Entrada in general, eolian strata dominate over sabkha strata

throughout most of the Colorado Plateau. Sabkha and 

possibly tidal-flat or shallow-water hypersaline marine 

deposits dominate in the westernmost part of the plateau 

and farther west in the easternmost part of the Basin and 

Range province of west-central Utah.

UPPER PART OF THE SAN RAFAEL GROUP

The upper part of the San Rafael Group includes 

the Curtis and Summerville Formations and the Romana 

Sandstone. The Romana is newly named in this report 

for uppermost San Rafael Group strata in the Kaiparowits basin that correlate with the Summerville Formation 

and possibly the Curtis Formation farther north. Regional 

studies by Imlay (1980) and Pipiringos and Imlay (1979) 

indicate that these beds are late Middle Jurassic (middle 

and late Callovian) in age.

Curtis Formation

The Curtis Formation was named by Gilluly and 

Reeside (1928) for sandstone strata lying above the Entrada Sandstone at Curtis Point in the northeastern part 

of the San Rafael Swell (fig. Bl). The formation generally 

forms greenish-gray cliffs or ledges, which contrast 

markedly with red beds in underlying and overlying 

formations.

The Curtis is mainly composed of grayish-green, 

glauconitic, flat-bedded and crossbedded sandstone. Thin 

beds of grayish-green mudstone also are present, thin beds 

of dense gray limestone have been found near the depositional edge of the formation in the Henry basin, and a 

thin bed of gypsum was found at the top of the formation at Penitentiary Point about 10 km south of Hanksville, Utah. Stratigraphic relationships of the Curtis to 

other rocks in part of the region are shown in figure Bl 1.

On the west side of the Colorado Plateau, the basal 

surface of the Curtis, or of the Summerville where the 

Curtis is missing, is the J-3 unconformity (Pipiringos and 

O'Sullivan, 1978). Intraformational folds or primary inclined bedding within the Entrada are truncated by this 

surface in several places in the Henry basin and the San 

Rafael Swell (fig. B12; Gilluly and Reeside, 1928; Hunt 

and others, 1953). Farther south, in the Kaiparowits 

basin, the Romana Sandstone, a southern facies of the 

Summerville, also truncates intraformational folds in the 

middle and upper members of the Entrada (figs. B5, B13). 

The time represented by the erosion surface probably was 

brief (Imlay, 1980, p. 77); the unconformity appears to 

die out eastward and is not recognized east of the Monument uplift on the east side of the Colorado Plateau 

(O'Sullivan, 1980b).

The Curtis is mostly a marine formation, as indicated by marine fossils found in the formation in the 

San Rafael Swell (Gilluly and Reeside, 1928). The

Middle and Upper Jurassic Rocks, Utah and Arizona 29
£

NORTHWEST

E'

SOUTHEAST

Depositional 

Sequences

Jsb 

Jsc 

Jss 

Jcu

Brick-red member 1 SummerviUe 

Chocolate member >- Formation 

Sandstone unit -J 

Curtis Formation

Figure B11. Stratigraphic section £-£' along the Waterpocket Fold showing stratal relationships in the upper part of the San 

Rafael Group and the Morrison Formation. J-3J-5, unconformities of Pipiringos and O'Sullivan (1978). For location see figure B1.

presence of fairly abundant glauconite also suggests a 

marine origin. Thin lenses of gypsum indicate local 

evaporative conditions, probably during brief periods of 

shallowing of the Curtis sea, or perhaps in small and 

isolated embayments.

SummerviUe Formation

The SummerviUe Formation was named by Gilluly 

and Reeside (1928) for strata at SummerviUe Point in the 

northeastern part of the San Rafael Swell (fig. Bl), where 

it consists largely of slope-forming reddish-brown 

mudstone. The formation is here divided into two 

members informally named the chocolate member and 

the brick-red member (fig. Bll) based on slight differences in the color of the mudstones and differences 

in some of the lithologies.

Chocolate member

The chocolate member of the SummerviUe is as 

much as 96 m thick and is the part of the SummerviUe 

that is present at the type locality. The member consists 

largely of laminated to ripple cross-laminated, reddishbrown mudstone and siltstone, although it also contains 

minor thin beds of light-greenish-gray mudstone. The 

member tends to weather to fairly steep slopes or, where 

capped by a resistant sandstone bed near the base of the 

Morrison Formation, it may form steep slopes or cliffs 

(fig. B12). The chocolate member contains scattered thin 

beds of very fine-grained, laminated, reddish-brown

sandstone. In the northern part of the Henry basin and 

in much of the San Rafael Swell, the member contains 

scattered thin beds as much as 3 m thick of white or 

very light-red gypsum. In some places, several sandstone 

beds are present in a poorly defined zone in the middle 

of the member, and these are helpful for determining 

Stratigraphic relationships to the overlying Morrison Formation (fig. Bll).

Brick-red member

The brick-red member, at the top of the SummerviUe, is only present in the western part of the San Rafael 

Swell, where it ranges in thickness from 0 to 21 m. These 

beds were recognized as slightly different from typical 

SummerviUe strata by Smith and others (1963, p. 30), but 

thus far they have received little study. The member consists largely of laminated to ripple cross-laminated, bright 

red mudstone. It also includes thin beds of very fine- to 

fine-grained, laminated, red or white sandstone and 

scarce but conspicuous cliff-forming beds (3-6 m thick) 

of pebbly, crossbedded, light-brown sandstone. Thin 

white gypsum beds, generally less than 0.3 m thick, are 

scarce but locally present. Thus far, no fossils have been 

found in, or reported from, the member.

The brick-red member ultimately may prove to be 

more significant than is now apparent. It is the youngest 

unit in the San Rafael Group and could be earliest Late 

Jurassic (early to middle Oxfordian) in age if it proves 

to correlate with the Redwater Shale Member of the Sundance Formation. The Redwater is found in much of 

Wyoming and extends southward into northeastern Utah

30 Jurassic and Cretaceous of Colorado Plateau
-

:

Figure B12. Truncation of beds beneath the J-3 and J-5 unconformities in southeastern Utah. A, East side of San Rafael Swell, 

several hundred meters south of Interstate Highway 70 in the SE!/4 sec. 7, T. 22 S., R. 14 E. B, Folds in the Entrada Sandstone 

truncated beneath the J-3 unconformity along the Waterpocket Fold. Je, Entrada Sandstone: Jem, middle member, Jeu, upper 

member; Jcu, Curtis Formation; Js, Summerville Formation; Morrison Formation: Jmt, Tidwell Member, Jms, Salt Wash Member. 

For scale, in A, the cliff-forming gypsum bed at the base of the Tidwell is about 6 m thick, and, in B, the entire Tidwell Member 

is about 20 m thick.

Middle and Upper Jurassic Rocks, Utah and Arizona 31
KAIPAROWITS BASIN

23 24

15 KILOMETERS

CIRCLE CLIFFS UPLIFT

Salt Wash Member

of

Morrison Formation 

(part)

Ti dwell 

Member

of

Morrison 

Formation

Tidwell Member of Morrison Formation 

J-5

Summerville 

Formation

Romana 

Upper Sandstone

member

of

Entrada

Sandstone

(part)

Middle 

member

of

Entrada

Sandstone

(part)

Upper member

of

Entrada Sandstone 

(part)

EXPLANATION

Conglomerate 

Pebbly sandstone 

Sandstone

Mudstone

Crossbedded sandstone 

Authigenic chert

Figure B13. Stratigraphic section F-F' from the Henry basin to the Kaiparowits basin showing correlation of stratigraphic units 

and truncation of beds beneath the J-3 and J-5 unconformities. Jrr, red band marker bed at the base of the Romana Sandstone. 

For location see figure B1.

and northwestern Colorado, where it is truncated by the 

J-5 unconformity at the base the Morrison Formation 

(Pipiringos and O'Sullivan, 1978). The J-4 unconformity is at the base of the Red water, but thus far an unconformity has not been identified at the base of the brick-red 

member. For this reason the brick-red member is here 

considered a part of the Summerville and of late Middle 

Jurassic age.

Romana Sandstone

Regional correlations indicate that rocks in the 

Kaiparowits basin, here named the Romana Sandstone,

most likely correlate with the Summerville Formation (fig. 

B13), although correlation with the uppermost part of 

the Curtis cannot be ruled out. Correlations into the 

Kaiparowits region are based on the stratigraphic position of the Romana between the J-3 and J-5 unconformities at the top of the Entrada Sandstone and the base 

of the Morrison Formation, respectively (fig. B13). The 

Romana was previously referred to informally as a gray 

sandstone unit (Peterson, 1974) and as the sandstone at 

Romana Mesa (Peterson, 1975, 1980a).

The Romana Sandstone is here named for a sandstone unit in the Kaiparowits region as much as 46 m thick 

that tends to form a sheer cliff between the Entrada Sandstone and the Morrison Formation (fig. B14). The

32 Jurassic and Cretaceous of Colorado Plateau
Romana takes its name from Romana Mesa, an imposing promontory on the north side of Lake Powell about 

12 km southeast of the type section (fig. Bl, locality 2). 

The type section (fig. B14) is in Crosby Canyon, a small 

and easily accessible tributary canyon to Warm Creek 

canyon in southernmost Utah about 18 km north of Page, 

Ariz. Romana Mesa received its name from a nearby 

campsite (Camp Santa Francisca Romana) that was used 

by members of the Dominguez-Escalante expedition on 

November 5, 1776 (Bolton, 1950).

The formation is 44.2 m thick at the type section, 

where it consists largely of very fine- to fine-grained, 

moderately sorted to well-sorted, flat-bedded and 

crossbedded gray sandstone (fig. B15; measured section 

2 at the end of this report). It contains scattered coarse 

and very coarse grains at or near the base in many places 

(fig. B16), especially where a basal red marker bed is absent. The coarse and very coarse grains are of great value 

in distinguishing the Romana from underlying sandstone 

beds of the Entrada Sandstone, which generally lack such 

coarse constituents. In the southwestern part of the 

Kaiparowits basin, the Romana locally contains scarce 

small pebbles of chert concentrated along some of the 

bedding planes.

In much of the structurally deeper part of the 

Kaiparowits basin, the basal part of the Romana contains 

a thin unit of flat-bedded red mudstone or silty sandstone, 

informally named the red band, which makes a convenient marker bed (fig. B15). The red band pinches out 

southeastward by lateral gradation into the gray sandstone that is typical of the rest of the formation. At 

localities where the red band is missing, the scattered 

coarse and very coarse grains are especially helpful in 

identifying the basal contact (fig. B16).

The Romana has a limited distribution; it has been 

mapped in the Kaiparowits basin in Kane County, Utah, 

and it extends southward into northernmost Coconino 

County, Ariz. Correlations in the southern part of the 

Kaiparowits basin (fig. B7) and south from there toward 

Black Mesa (fig. B8) indicate that the Romana is beveled to some degree by the unconformity at the base of 

the Morrison Formation and, farther south, it is cut out 

by the unconformity at the base of the Dakota Formation. The Romana Sandstone is not present at Cow 

Springs, Ariz., and neither it nor correlatives of the Summerville and Curtis Formations appear to be present 

anywhere else in the Black Mesa area.

The Summerville, Curtis, and Romana are interpreted as having been deposited in or along the margin 

of a narrow seaway that extended as far south as the 

Kaiparowits basin. Paleontologic evidence indicates that 

the Curtis was largely deposited in normal marine waters 

(Gilluly and Reeside, 1928; Imlay, 1980) during the initial 

stages of advance of the seaway into south-central Utah. 

The chocolate member of the Summerville probably is

a shallow-water hypersaline marine unit deposited mostly 

during a regressive phase when the seaway became 

restricted farther north, permitting gypsum to precipitate 

locally and intermittently. The brick-red member was 

deposited largely in overbank floodplain and mudflat environments and locally in hypersaline environments; most 

of the crossbedded pebbly sandstone beds were deposited 

by streams that flowed eastward or northeastward from 

a highland source area farther west.

The lower part of the Curtis thins by onlap onto 

the J-3 unconformity, but some of the upper beds of the 

Curtis grade southward and southeastward into basal 

Summerville red mudstone strata (fig. Bl 1), as Hunt and 

others (1953, p. 69) showed in the Henry basin and 

McKnight (1940, p. 90) and O'Sullivan (1980) showed farther north, suggesting that hypersaline conditions may 

have existed in the shallow waters near the edge of the 

Curtis sea. The Romana Sandstone is interpreted as a 

southern facies of the Summerville Formation (fig. B17) 

and quite likely of the chocolate member of the Summerville. Crossbedding in eolian strata of the Romana dips 

mostly due east, suggesting that the shallow Summerville 

seaway in the Kaiparowits region was gradually filled in 

by windblown sand that prograded eastward across the 

southwestern margin of the seaway. Eolian sandstone 

beds are lacking in the Summerville of the San Rafael 

Swell, indicating that the dune field did not extend much 

farther north than the Kaiparowits region.

MORRISON FORMATION

The Morrison Formation was named by Eldridge 

(1896) for rocks exposed near the town of Morrison, in 

east-central Colorado. At the type locality, the formation largely consists of greenish-gray mudstone interbedded with thin beds of gray limestone and light-brown 

sandstone (Waldschmidt and LeRoy, 1944). Lithologically, the type Morrison is not typical of most of the formation on the Colorado Plateau, although it most closely 

resembles the Tidwell Member of this report. The Morrison is divided into eight members on the Colorado 

Plateau, including the two new members proposed in this 

report (fig. B18).

The Morrison can be divided into two large parts, 

each of which contains several members. The lower part 

includes the Salt Wash (Lupton, 1914), Bluff Sandstone 

(Gregory, 1938), and Tidwell (of this report) Members; 

the upper part includes the Westwater Canyon (Gregory, 

1938), Fifty mile (of this report), Brushy Basin (Gregory, 

1938), and Jackpile Sandstone (Owen and others, 1984) 

Members. Additionally, beds of the Recapture Member 

(Gregory, 1938) occur in both the lower and upper parts 

of the formation (fig. B18). Although there is some 

interfingering between strata in the two parts of the

Middle and Upper Jurassic Rocks, Utah and Arizona 33
34 Jurassic and Cretaceous of Colorado Plateau
formation on the west side of the Colorado Plateau, the 

interfingering generally does not involve as great a thickness of beds as the interfingering that exists between the 

various members within each of the parts.

The surface at the base of the Morrison Formation 

is considered an unconformity because it truncates 

underlying strata of the San Rafael Group (figs. B5, B7, 

B8, BIO, Bll, B12, B13). Slightly angular relationships 

can be seen locally as well as regionally along this unconformity (figs. Bll, B12, B13; Gilluly and Reeside, 

1928; Hunt and others, 1953). A stratigraphic section on 

the west side of the Henry basin (fig. Bl 1) shows truncation of the brick-red member of the Summerville Formation in the northwestern part of the Waterpocket Fold. 

Farther southeast, the unconformity bevels out strata 

from the top of the chocolate member of the Summerville down through a sandstone unit in the middle of that 

member. Similarly, the unconformity bevels out the 

Romana Sandstone in the Kaiparowits basin (figs. B5, 

B13). Thinning of the Romana is not due entirely to 

depositional onlap, for a stratigraphic section on the 

northeast side of the Kaiparowits basin (fig. B15) shows 

that the red band at the base of the Romana does not 

thin toward the wedge edge of the formation but, instead, 

is truncated by the J-5 unconformity.

Tidwell Member

The Tidwell Member is here named for grayishgreen mudstone strata at the base of the Morrison Formation exposed about 26 km southwest of the town of 

Green River, Utah (fig. Bl, locality 3). The member takes 

its name from Tidwell Bottoms, a marshy area along the 

San Rafael River about 5-10 km north of the type section. The type section is shown in figures B19 and B20, 

and the measured type section is described at the end of 

the report. The name Tidwell was first applied to these 

beds in unpublished maps in the files of the U.S. Department of Energy (formerly the U.S. Atomic Energy Commission), which were made by R. G. Young in the 1950's 

(R. G. Young, oral communication, 1986), though no 

authorship was credited. I have decided to resurrect this 

name and formally apply it to these beds.

Figure B14 (on facing page). Outcrops of the Romana Sandstone (Jr) in south-central Utah. A, Northwest side of Cummings 

Mesa, above Lake Powell in Glen Canyon. B, Type section in 

Crosby Canyon, southwest side of the Kaiparowits basin in SE% 

sec. 8, T. 43 S., R. 4 E., Kane County, Utah. The type section 

was measured along the left side of the photographed area. 

The red band marker bed (Jrr) shown at the base in B, is not 

present at the outcrop in A; there, the basal contact of the 

Romana was identified by differences in grain size. (See fig. 

B16.) Entrada Sandstone: Jel, lower member; Jem, middle 

member. Jms, Salt Wash Member of the Morrison Formation. 

The Romana is 25.6 m thick in A and 44.2 m thick in B.

Salt Wash Member

of Morrison 

Formation (part)

Sandstone, laminated and 

burrowed

Sandstone; contains scattered 

coarse grains, small pebbles 

and mudchips

Romana 

Sandstone

Sandstone; channel-fill containing 

scattered mudchips and very coarse 

grains

Sandstone, crossbedded

Sandstone; thin purple bentonite 

bed near top

Red band 

(marker bed)

Sandstone; scattered very coarse 

grains at base

Middle member of

Entrada Sandstone

(part)

Color code 

Gray to greenish gray

Red

Lithology 

Sandstone

Mudchips 

Chert pebbles 

Bentonite

L 0

Figure B15. Columnar section of the type section of the 

Romana Sandstone in Crosby Canyon, southwestern 

Kaiparowits basin. For location, this is section 2 on figure B1.

Middle and Upper Jurassic Rocks, Utah and Arizona 35
Recognition of the Tidwell as a separate member 

of the Morrison helps resolve longstanding problems concerning the nature of the basal contact of the formation

..** 

Jr

Figure B16. Scattered coarse and very coarse grains at the base 

of the Romana Sandstone (Jr) marking the contact with the middle member of the Entrada Sandstone (Jem). The middle 

member is very fine to fine grained and lacks the large grains 

found in the Romana. Dangling Rope Canyon on the southeast 

side of the Kaiparowits basin, sec. 17, T. 42 S., R. 8 E., Kane 

County, Utah. The pick head is scaled in inches.

SOUTH

KAIPAROWITS 

BASIN

CIRCLE CLIFFS UPLIFT 

Eroded

and the relationships of the Summerville Formation to 

the Morrison. Strata here assigned to the Tidwell apparently were not included in the original definition of the 

Salt Wash Member by Lupton (1914). (At the time of 

Lupton's study, strata from the base of the Entrada Sandstone up through the Lower Cretaceous Cedar Mountain 

Formation, including the Salt Wash Member, were considered part of the "McElmo Formation," which has 

since been abandoned as a formation name.) Lupton 

(1914) originally named the Salt Wash for exposures near 

Duma Point, about 34 km east of the type section of the 

Tidwell (fig. B21), although he did not recognize an unconformity at or near the base of the Salt Wash. Gilluly 

and Reeside (1928) and Gilluly (1929), in their study of 

the San Rafael Swell, recognized the unconformity at the 

base of strata here included in the Tidwell Member but 

they included these beds in the Salt Wash Member of the 

Morrison Formation. Hunt and others (1953) placed gray 

mudstone strata (Tidwell of this report) on the east side 

of the Henry basin in the Salt Wash and recognized an 

unconformity at the base of that member. However, on 

the west side of the basin the Tidwell Member is red (fig. 

B22), and they apparently included it with other red beds 

in the Summerville Formation. Later, but also along the 

west side of the Henry "basin, Davidson (1967) placed the 

base of the Salt Wash at about the middle of the Tidwell 

at the base of the lowest limestone bed present. Near 

Green River, Utah, Trimble and Doelling (1978) placed 

the lower contact of the Salt Wash at the top of the thick 

gypsum bed that is here included in the lower part of the 

Tidwell. They considered the irregular surface at the top 

of this gypsum bed to mark an unconformity, but this 

surface is here interpreted to be a result of local dissolution of the gypsum rather than an erosion surface.

NORTH

HENRY BASIN

Nearshore marine 

Hypersaline marine 

Normal marine

Figure B17. Schematic north-south section showing interpreted facies relationships among formations in the upper part of the 

San Rafael Croup. (Not to scale.)

36 Jurassic and Cretaceous of Colorado Plateau
The Tidwell is at the base of the Morrison through- directly on the J-5 unconformity (figs. B7, B8, B14, B21).

out most of the western part of the Colorado Plateau, (2) On the southwest side of the San Rafael Swell in the

with three known exceptions: (1) In much of the south- outcrop belt between the NEVi sec. 28, T. 25 S., R. 5

eastern part of the Kaiparowits basin the Salt Wash lies E. and the NEVi sec. 13, T. 24 S., R. 6 E., Sevier and

NOT TO SCALE

Figure B18. Relationships of members in the Morrison Formation on the Colorado Plateau. J-5, unconformity of Pipiringos 

and O'Sullivan (1978); this unconformity has not been clearly identified in the San Juan Basin.

** " X ^L -*'-*^v " ^(N -' *tef %, j/^ *-6i|j88*»» JR ^ V

 *rV: 5 "-*." > * ' ^t'fu --. rv

to, " »t? ": :, ' * "' Mir*fi " " - , *«^

 * <i-"' ~V >

\ ,:^- v:_v,

 T ->\ii j£ '*\

-^« *.

 <**s- **&I

Figure B19. Type section of the Tidwell Member of the Morrison Formation (Jmt) about 26 km southwest of Green River, Utah. 

Js, Summerville Formation; Jms, Salt Wash Member of the Morrison Formation. The Tidwell Member is 29.3 m thick here. This 

is section 3 on figure B1.

Middle and Upper Jurassic Rocks, Utah and Arizona 37
Emery Counties, Utah, the entire lower part of the Morrison is locally missing and the Brushy Basin Member 

rests directly on the brick-red member of the Summerville Formation. In this area the Tidwell and Salt Wash

Stratigraphic 

unit Description

Salt Wash 

Member (part)

Tidwell 

Member

Pebbly sandstone

Interbedded mudstone and sandstone

Mudstone and some sandstone

Interbedded sandstone and mudstone

Mudstone; scarce thin sandstone beds 

near base

Chocolate member 

of Summerville 

Formation (part)

5 METERS

EXPLANATION

Color code

Gray, greenish gray 

Red, purple 

Mixed red and gray 

Lithology 

Sandstone 

Mudstone 

Gypsum

Sandstone lens 

Limestone lens 

++ Authigenic chert

Figure B20. Columnar section of the type section of the 

Tidwell Member of the Morrison Formation, 26 km southwest 

of Green River, Utah. This is section 3 on figure B1.

thin and pinch out by onlap onto a paleotopographic high 

that is attributed to growth on the Emery uplift just 

before or during deposition of these members (Peterson, 

1986). (3) At Black Mesa the Tidwell pinches out by onlap 

onto another paleotopographic and paleostructural high 

(fig. BIO; Peterson, 1986).

Recognition of the Tidwell helps resolve some of 

the differences of opinion concerning the nature of the 

basal contact of the Morrison. The contact of the 

Morrison with the Summerville was thought to be conformable and interfingering by some workers and an unconformity by others. Either interpretation may seem 

justified, depending on where the contact is placed. If 

the lower contact of the Morrison is placed at the base 

of strata here included in the Tidwell Member, then an 

unconformity is apparent (for example, fig. Bl 1). Some 

geologists considered erosional irregularities at the base 

of the lowest fluvial sandstone bed in the Salt Wash to 

represent the basal unconformity of the formation. However, this interpretation does not seem entirely consistent 

because erosional irregularities are present at the base of 

most fluvial sandstone beds.

On the other hand, interfingering between the Morrison and the Summerville can easily be interpreted if the 

contact is placed somewhere within beds here assigned 

to the Tidwell. The lowest limestone bed, selected by 

several geologists as the base of the Morrison, lies above 

the J-5 unconformity. Furthermore, limestone beds in 

the Tidwell are highly lenticular, and a contact based on 

this criterion would be placed at different Stratigraphic 

positions in different areas. This variability would suggest that the boundary between the Morrison and Summerville is interfingering. Most if not all of these problems 

dealing with interfingering can be resolved by recognizing 

that the Tidwell Member interfingers with lowermost Salt 

Wash strata and that the proper base of the formation 

is the base of Tidwell strata.

The Tidwell ranges in thickness from 0 to 89 m, 

though it is generally about 8-21 m thick in the Henry 

basin and 0-30 m thick in the San Rafael Swell. The 

member consists largely of grayish-green calcareous 

mudstone at the type section (figs. B19, B20; section 3 

at the end of this report), around the San Rafael Swell, 

and along the east side of the Henry basin. In contrast, 

red mudstone is the dominant lithology along the west 

side of the Henry basin (fig. B22) and in the Kaiparowits 

basin. Beds of white gypsum as much as 14 m thick are 

at the base of the member in the northern part of the 

Henry basin, around most of the San Rafael Swell, and 

at the type section (fig. B19). Thin beds of gray, dense 

limestone, some of which have a fetid odor, are present 

at most localities where grayish-green mudstone is the 

principal rock type. Dark-gray to dark-greenish-gray 

mudstone containing minute flecks of carbonized plant

38 Jurassic and Cretaceous of Colorado Plateau
G'

6)

D.

C o

m KAIPAROWITS BASIN 

15 1 79 80 81 21 22

CIRCLE CLIFFS UPLIFT 

26 27 82 83 (?) 78

HENRY BASIN 

84 85 86 87

Duma 

Point

88

Brushy ^ Basin *=1 := 

Member

Depositional sequences in Tidwell 

and Salt Wash Members

Figure B21. Stratigraphic section C-C' from Escalante to Duma Point, Utah, showing relationships of members and sequences in the Morrison Formation on the west 

side of the Colorado Plateau. Circled section numbers are type localities of the Fiftymile (4), Salt Wash (5), and Tidwell (3) Members. For location see figure B1.
Figure B22. Tidwell Member of the Morrison Formation (Jmt), about 15 m thick, well exposed in South Desert near the confluence of Deep Creek with the Fremont River, western Henry basin, sec. 24, T. 29 S., R. 7 E., Wayne County, Utah. Beds in 

the Tidwell are irregular and slightly contorted, whereas Summerville beds Us) tend to be planar. A thin conglomerate bed about 

15 cm thick is locally present at the base of the Tidwell. Jms, Salt Wash Member of the Morrison Formation.

fragments is a relatively rare lithology found at the type 

section and several other places. Some of these beds in 

the eastern part of the Henry basin yielded an excellent 

suite of palynomorphs, including the lacustrine alga 

Botryococcus sp. identified by R. H. Tschudy (written 

commun., 1978-80).

A stratigraphic marker in the Tidwell that is useful 

in some places consists of authigenic red and (or) yellow 

chert in small blebs 1-2 mm in diameter or in nodules 

or concretions as much as a meter long. The chert replaces 

limestone, gypsum, mudstone, and flat-bedded sandstone, but it has only rarely been found in any of the 

scarce crossbedded fluvial sandstone beds in the member. 

In most places the chert is about 7-11 m above the base 

of the member, although locally it is as much as 38 m 

above the base in the area about 8-30 km southeast of 

Escalante, Utah. The greater thickness of Tidwell beneath 

the chert zone in this area probably reflects deposition 

in ancient valleys that did not exist elsewhere in the 

region. Because the chert is found at several stratigraphic 

levels at the type section (fig. B20) and several other 

localities, it is not an ideal stratigraphic marker. However, 

the chert does seem to be useful where it is present only 

at one stratigraphic horizon, as is the case in most of the 

Henry and Kaiparowits basins. Where several zones are 

present, the most persistent zone of this chert generally 

is the most useful for correlation purposes. The red and 

yellow chert has been named Sunset Agate by rock collectors, who use it for making polished rock pendants, 

belt buckles, and bolo ties.

Thin beds of very fine- to fine-grained, laminated 

to massive, light-brown sandstone are fairly common, 

although not dominant, in the member, and beds of finegrained, crossbedded, light-brown pebbly sandstone are 

scarce. In much of the central and eastern parts of the 

Colorado Plateau, the base of the member is marked by 

a flat-bedded, fine-grained, light-brown sandstone bed 

containing scattered coarse and very coarse grains or 

small pebbles, often of colorful varieties of chert (fig. 

B23). This bed is especially helpful in identifying the basal 

contact where it separates overlying red mudstone of the 

Tidwell from red beds of the Summerville Formation. The 

basal bed is informally called bed A by O'Sullivan (1980b, 

1984) and is thought to correlate with the Windy Hill 

Member of the Sundance Formation of Wyoming, 

northern Colorado, and northeastern Utah (Pipiringos, 

1968, Pipiringos and O'Sullivan, 1976, 1978).

Locally, the Tidwell Member contains very fine- to 

fine-grained, very light-brown sandstone beds or lenses 

several meters thick that are laminated, crossbedded, or 

massive. Where these beds are at the base of the Tidwell 

their lower surfaces are flat, but the basal surfaces of 

similar beds farther up in the Tidwell tend to be concave 

upward. These beds and lenses are interpreted as 

lacustrine bars, and the configuration of their lower surface apparently was determined by the degree of lithification of the underlying strata. Where the lacustrine bars 

were deposited directly on the Summerville Formation, 

the basal surface is flat because the Summerville was at 

least partly lithified prior to deposition of the Tidwell and

40 Jurassic and Cretaceous of Colorado Plateau
Figure B23. Basal pebbly sandstone bed of the Tidwell Member of the Morrison Formation (Jmt) along Waterpocket Fold in 

the western Henry basin. The bed contains scattered colorful chert pebbles and contrasts markedly in texture and color with 

underlying mudstone beds of the Summerville Formation (Js). Near section 70 (figs. B1, B11), sec. 7, T. 32 S., R. 8 E., Carfield 

County, Utah.

therefore resisted deformation due to loading. Where the 

lacustrine bars occur higher in the member and are enclosed entirely within Tidwell mudstone, the basal surface is concave upward because the enclosing mudstone 

beds were not lithified during deposition of the bars, 

allowing them to slump downward into the relatively soft, 

unconsolidated muds.

Channelized fluvial beds composed of fine-grained, 

crossbedded, light-brown, pebbly sandstone are scarce in 

the Tidwell but have been found in several areas. One 

of these beds was found at the base of the Tidwell in the 

Kaiparowits basin about 38.6 km southeast of Escalante, 

Utah.

The Tidwell also contains rare beds of fine-grained, 

well-sorted, light-gray to white sandstone that contains 

thick sets of trough and tabular crossbedding. These are 

interpreted as eolian strata deposited in small dune fields. 

Only two of these beds have been found in the region; 

one is in the Kaiparowits basin 8 to 24 km southeast of

Escalante, Utah, and the other is in the Henry basin, extending about 2 km along the outcrop near Trachyte 

Ranch (section 87, figs. Bl, B21). Crossbedding dip vector resultants obtained from the eolian sandstone beds are 

oriented due east to northeast, essentially the same as resultants obtained from eolian strata in the Recapture 

Member of the Morrison at Black Mesa, Ariz. (fig. BIO). 

The part of the Morrison Formation that includes 

the Tidwell and Salt Wash Members has been divided into 

three sequences that are useful in describing and understanding the sedimentology of these rocks in south-central 

Utah and north-central Arizona (Peterson, 1980b). The 

distinction between the sequences is based on differences 

in bedding ratios and crossbedding dip vector resultants 

(Peterson, 1980b) and not on obvious lithologic differences. For this reason, the three sequences do not constitute members. Nevertheless, recognition of the three 

sequences is helpful in understanding the stratigraphic 

relationships of the Salt Wash and Tidwell Members

Middle and Upper Jurassic Rocks, Utah and Arizona 41
(fig. B21) and for reconstructing the sedimentologic and 

paleotectonic evolution of the region during Late Jurassic 

time (Peterson, 1984; 1986).

The Tidwell interfingers with the Salt Wash 

Member in many places in the study area (figs. BIO, Bl 1, 

B13, B21). Not only does the Tidwell replace lowersequence fluvial sandstone beds that are included in the 

Salt Wash, but it replaces the entire Salt Wash at a few 

localities along the west side of the San Rafael Swell. In 

that area, the Tidwell can be distinguished readily from 

the overlying Brushy Basin Member where the Salt Wash 

is missing; the Tidwell contains thick gypsum beds 

(generally 3-20 m thick) and mudstone composed largely of nonswelling clays whereas the Brushy Basin lacks 

these lithologies and consists largely of mudstone rich in 

swelling clays. Thompson and Stokes (1970) considered 

part of the Tidwell Member a southern extension of the 

Summerville Formation and called these beds the White 

Point Member of the Summerville Formation in the 

southeastern part of the Kaiparowits basin. However, 

regional stratigraphic relationships indicate that these 

beds are part of the Morrison Formation (fig. B5), and 

therefore the White Point Member of the Summerville 

Formation of Thompson and Stokes (1970) is not recognized in this report.

The Tidwell Member was deposited in a variety of 

environments, including lacustrine, evaporative, mudflat, 

and minor eolian and fluvial environments. These beds 

were distal or lateral equivalents of Salt Wash alluvial 

complexes deposited by streams that flowed northeastward to southeastward across much of the Colorado 

Plateau from highland source regions in southeastern 

Nevada and southwestern Utah (Peterson, 1984).

Salt Wash Member

The Salt Wash Member of the Morrison Formation was named by Lupton (1914) for exposures of cliffforming sandstone beds interbedded with minor quantities of red or grayish-green mudstone near Duma Point, 

Utah (fig. B21; O'Sullivan, 1984, fig. 4). The sandstone 

beds are resistant to erosion and, in the southeastern 

Kaiparowits basin, the member tends to form steep darkbrown cliffs (figs. B14, B24), which add much to the 

scenic beauty of Glen Canyon and Lake Powell. Sandstone beds in the Salt Wash are generally light grayish 

brown to light gray in fresh exposures, fine to medium 

grained, and highly crossbedded. Although beds composed solely of flat-laminated sandstone are found in the 

member, they are not nearly as common as in the overlying Fiftymile Member. Mudstone beds in the Salt Wash 

are reddish brown to light grayish green and laminated 

to very thin bedded. Thin beds of gray to grayish-green, 

laminated to very thin-bedded, carbon-bearing mudstone

containing comminuted and carbonized plant fragments 

are scarce but have been found in the member on the east 

side of the Henry basin, where they are associated with 

tabular sandstone-type uranium-vanadium deposits 

(Peterson, 1980b). Mudstone strata in the Salt Wash tend 

to be thin and inconspicuous, forming narrow benches, 

short steep slopes, or thin notches between the thick sandstone beds.

Fiftymile Member

A series of interbedded sandstone and mudstone 

beds as much as 107 m thick lies above the Salt Wash 

Member and below the Dakota Formation in the southeastern part of the Kaiparowits basin. Previously, these 

beds were included in the Salt Wash because they contain a significant amount of sandstone. However, regional 

relationships indicate that these beds are a facies of the 

Brushy Basin Member and therefore should be assigned 

to the upper part of the Morrison. Moreover, important 

lithologic differences distinguish these beds from the Salt 

Wash. For these reasons, this thick sandstone-mudstone 

sequence is here named the Fiftymile Member of the Morrison Formation. The type section is about 2 km northwest of Fiftymile Point, a bold promontory at the foot 

of the Straight Cliffs on the northeastern side of the 

Kaiparowits basin approximately 80 km southeast of 

Escalante, Utah (figs B25, B26).

The Fiftymile Member tends to form a series of 

ledges and slopes above the steeper cliffs of the Salt Wash 

Member (figs. B24, B26). Sandstone beds in the Fiftymile 

are light brown to light gray, are fine grained, contain 

pebbles, and are crossbedded, flat-laminated, or massive. 

Sandstone beds in the lower part of the member tend to 

be laminated to very thin bedded, as contrasted with 

highly crossbedded sandstone beds in the underlying Salt 

Wash Member. In addition, conglomerate beds are scarce 

but, where present, as at the type section (fig. B25; section 4 at the end of this report), they contain pebbles of 

red, orange, tan, green, black, gray, white, brown, and 

banded light- and dark-gray chert.

The suite of brightly colored pebbles has been 

known by field workers as the "Christmas tree" conglomerate or pebble suite, in allusion to the colorful ornaments on a Christmas tree. The "Christmas tree" suite 

of pebbles is especially common in the Brushy Basin 

Member, and Craig and others (1955) used this suite of 

pebbles to distinguish the Salt Wash and Brushy Basin 

Members. Similar colorful chert pebbles have been found 

in the Salt Wash and Tidwell Members, in the Romana 

Sandstone, and in westernmost exposures of the Entrada 

Sandstone, but in general they are not nearly as common 

in those units as they are in the Brushy Basin and Fiftymile Members. Exceptions are the uppermost bed of the

42 Jurassic and Cretaceous of Colorado Plateau
!""H>- 

^- 5&T* iy%^^» 5* >;?£ ; . ,*vr-

*.« ,,. .*f -*" ?'««'^^.* "i!: '-i»*/; 'J '^ * 

'it*' ;i ^» ' 6 . g-:jgr- ' > : »-. JT - * . ' *^-

« .'*X*»'' » ^iJ^^1*' "»" " « ** ? I ' 'I * - " "*" 

i.T - ,«* % .» 1 - " . . *V , S"i!» 'X. . .»> "- %t ». *»» -^

Figure B24. Morrison Formation, about 158.3 m thick, in the southeastern Kaiparowits basin on the east side of Rock Creek 

canyon, sec. 18, T. 43 S., R. 7 E., Kane County, Utah. The Fiftymile Member of the Morrison Formation (Jmf) contains more 

mudstone and less sandstone than the underlying Salt Wash Member Cms); it tends to form ledges and slopes whereas the Salt 

Wash tends to form irregular cliffs and thin slopes. The uppermost sandstone beds in the Fiftymile are nearly white from alteration by fluids that seeped down from the Dakota Formation (Kd). Jem, middle member of the Entrada Sandstone; Jr, Romana 

Sandstone.

Salt Wash Member, which contains this colorful suite of 

pebbles at many localities in the Henry basin and the San 

Rafael Swell, and most Salt Wash sandstone beds in the 

southwestern Kaiparowits region. Mudstone strata in the 

Fiftymile are reddish brown to light grayish green, are 

laminated to very thin bedded, and weather to form 

slopes that are covered or partly covered with rubble from 

overlying sandstone beds. Commonly, the uppermost 

beds of the Fiftymile have been bleached from fluids that 

seeped into the member from the overlying Dakota Formation. The uppermost sandstone bed of the Fiftymile 

generally is white (figs. B5, El, B8, B24), and the intense 

alteration has obliterated all traces of bedding in many 

places. For the most part, mudstone beds at or near the 

top of the Fiftymile have been altered to grayish green. 

The lower contact is placed where the lowest thick 

mudstone unit in the Fiftymile lies on the uppermost thick 

sandstone bed of the Salt Wash. Some interfingering

involving about 5-10 m of beds has been observed in 

mapping this contact, but, in most localities, it is a fairly 

planar surface; replacement of appreciable thicknesses of 

one member by the other has not been observed.

The upper contact of the Fiftymile Member is the 

regional unconformity at the base of the Dakota Formation, which cuts out the entire Morrison Formation, as 

well as the Romana and Entrada Sandstones, in southwestern Utah (figs. B5, B7, B8, BIO, B21). The basal 

stratum of the Dakota generally is a thin, light-gray sandstone bed that may be difficult to distinguish from sandstone beds in the Fiftymile. The basal sandstone bed of 

the Dakota generally contains minute carbonaceous plant 

fragments, which are lacking in the Fiftymile. Locally, 

the basal stratum of the Dakota is a conglomerate bed 

that is easily recognized by the lack of the colorful red, 

orange, or green chert pebbles that are characteristic of 

conglomerates in the Fiftymile. Basal Dakota beds,

Middle and Upper Jurassic Rocks, Utah and Arizona 43
Stratigraphic 

unit Description

Fiftymile 

Member

Salt Wash 

Member (part)

I

%

i 

I

Conglomerate and conglomeratic sandstone; colorful suite of chert pebbles

Mudstone

Sandstone; contains scattered pebbles 

of white, light-gray or gray chert

Mudstone and some sandstone

Conglomeratic sandstone

iitB

Mudstone and some sandstone

:'-'A-."-'.;.V::.: :-:-'.

Sandstone; contains scattered mudchips

Mudstone and some sandstone

Conglomeratic sandstone

Color code

H

Gray to light brown 

Mixed red and gray 

Black to dark gray

f- 10 METERS

u 0

Lithology

Sandstone

Mudstone

Conglomerate

Mudchips

pebbles

Figure B25. Columnar section of the type section of the Fiftymile Member of the Morrison Formation in the southeastern 

Kaiparowits basin near Fiftymile Point. This is section 4 on 

figures B1 and B21.

whether sandstone or conglomerate, may contain or interfinger with carbonaceous mudstone beds that are also 

diagnostic of the Dakota.

It is not possible to physically trace the Fiftymile 

Member into the Brushy Basin Member on the outcrop; 

therefore, the stratal relationships between these two 

members were determined from regional relationships. 

About 10-50 km southeast of Escalante, Utah, the unconformity at the base of the Dakota truncates the Fiftymile and Salt Wash Members, allowing the Dakota to 

locally rest on uppermost beds of the Tidwell (fig. B21). 

The Fiftymile Member cannot be traced northward across 

the Circle Cliffs uplift into Brushy Basin strata of the 

Henry basin because Tertiary erosion has removed these 

rocks from the uplift.

Correlation of the Fiftymile with the Brushy Basin 

is based on its Stratigraphic position between the Salt 

Wash and Cretaceous strata and on lithologic and 

sedimentologic differences, previously discussed, between 

the Fiftymile and the Salt Wash. Thus, the regional 

Stratigraphic framework indicates that the Fiftymile 

Member is a southern facies of the lower part of the 

Brushy Basin. (As shown in figure B18, the uppermost 

beds of the Brushy Basin may be younger than the Fiftymile.) This interpretation considerably revises earlier 

thoughts about the Salt Wash Member. Heretofore, the 

Salt Wash was considered to be a large alluvial fan deposit 

whose apex was in the southern Kaiparowits region, 

because the greatest thickness of Salt Wash was thought 

to be there and because pebbly sandstones of the Salt 

Wash in that region were thought to represent the most 

proximal facies of the member. However, the revised correlations show the greatest thickness of the Salt Wash is 

in the southern part of the Henry basin, where it is related 

more to subsidence of that basin during deposition than 

proximity to the source region (Peterson, 1980b). In addition, a more proximal facies of the Salt Wash is present in the northwestern part of the Henry basin and 

southwestern San Rafael Swell, where thick conglomerate 

beds contain chert and quartzite pebbles, cobbles, and 

rare boulders as much as a meter long.

Correlations in north-central Arizona suggest that 

the lower part of the Fiftymile Member correlates with 

the upper part of the Recapture Member of the Morrison 

Formation in the Black Mesa, Ariz., area (figs. BIO, B18). 

It also seems likely that some of the upper beds of the 

Fiftymile correlate with strata currently known as the 

Westwater Canyon Member of the Morrison at Black 

Mesa. Reconnaissance studies between Black Mesa and 

the San Juan Basin of northwestern New Mexico by C. E. 

Turner-Peterson (oral commun., 1985) suggest that some 

of the unit formerly called the Westwater Canyon Member 

of the Morrison at Black Mesa could constitute an alluvial 

complex within the Recapture Member. However, further 

work is needed to adequately test this hypothesis.

44 Jurassic and Cretaceous of Colorado Plateau
Jmf Jms 

/

Figure B26. View northwest from Fiftymile Point, southeastern Kaiparowits Plateau, toward the type section (4) of the Fiftymile 

Member of the Morrison Formation (Jmf). The Fiftymile is 107.1 m thick and tends to form a series of slopes and ledges above 

irregular cliffs of the Salt Wash Member (Jms). Jemr, middle member of Entrada Sandstone and Romana Sandstone, undivided; 

the Romana is only about 5.1 m thick here. Kd, Dakota Formation.

Brushy Basin Member

The Brushy Basin Member (Gregory, 1938) is at the 

top of the Morrison Formation in most of the Colorado 

Plateau. However, the Fiftymile Member replaces the 

Brushy Basin in the southeastern part of the Kaiparowits 

basin, and neither of these members is present in 

southwestern Utah, owing to pre-Dakota erosion. A 

reported occurrence of the Brushy Basin on the east side 

of Black Mesa (Harshbarger and others, 1957, pi. 3) is 

provisionally interpreted as a mudstone bed that was 

originally deposited within the alluvial complex that includes the Recapture Member and possibly part of the 

Westwater Canyon Member of the Morrison. By this interpretation, sandstone beds of the Westwater Canyon 

that normally overlie this mudstone bed were removed 

by pre-Dakota erosion at Black Mesa. Generally, the 

Brushy Basin forms a red and light-gray slope at lower 

elevations where there is little vegetation cover. At higher 

elevations with greater precipitation, the Brushy Basin 

forms a tree-covered, hummocky slope affected by many 

landslides.

The Brushy Basin is composed predominantly of 

reddish-brown and light-greenish-gray, laminated to thinbedded mudstone. The mudstone contains appreciable 

quant ties of swelling clays in many areas and weathers 

to produce a frothy surface resembling red or gray popcorn. Light-brown ledges composed of fine-grained, 

laminated to very thin-bedded or crossbedded pebbly 

sandstone and dark-brown crossbedded conglomerate 

also form a small but conspicuous part of the member. 

Other minor lithologies are thin beds of red or very light 

brown bentonite, thin beds or lenses of dense gray

limestone, and thin zeolite beds (J. E. Johnson, University of California at Berkeley, written commun., 1980).

The lower contact of the Brushy Basin Member is 

placed at various stratigraphic levels by different 

geologists. Opinions differ mainly over the stratigraphic 

placement of a pebbly sandstone or conglomerate marker 

bed at the boundary. This marker bed contains the 

"Christmas tree" suite of colorful chert pebbles found 

throughout a large part of southeastern Utah and western 

Colorado. Craig and others (1955, p. 156) put the contact at the base of this bed and included the marker bed 

in the Brushy Basin Member. In so doing, they place a 

mudstone bed at the top of the Salt Wash Member, which 

differs from Lupton's (1914) original definition of the 

Salt Wash and also differs from the generally held concept that the Salt Wash is the cliff-forming, predominantly sandstone unit beneath the slope-forming, 

predominantly mudstone unit of the Brushy Basin 

(Gregory, 1938). Where the colorful chert pebbles are absent, Craig and others (1955) placed the contact at the 

top of the highest sandstone bed in the Salt Wash. It is 

noteworthy that some of the coauthors of the Craig and 

others (1955) study placed the contact at the top of the 

bed containing the colorful chert pebble suite. (Compare 

various sections in Craig and others, 1959.)

In southeastern Utah and adjacent parts of 

Arizona, New Mexico, and Colorado, the Recapture 

Member lies above the Salt Wash. The contact in these 

areas is placed at the top of the highest sandstone bed 

in the Salt Wash Member and at the base of a relatively 

thick series of slope-forming mudstone and sandstone 

beds typically included in the Recapture (Craig and 

others, 1955). In a somewhat similar manner in this

Middle and Upper Jurassic Rocks, Utah and Arizona 45
report, the contact of the Salt Wash with the Fiftymile 

Member is placed at the top of the highest typical sandstone bed in the Salt Wash and at the base of the lowest 

thick mudstone unit in the Fiftymile. Likewise in this 

report, the contact between the Salt Wash and Brushy 

Basin Members is placed at the top of the thick series of 

sandstone beds, or conglomerate if present, of the Salt 

Wash. This closely follows the original and most commonly held definition of the Salt Wash as a predominately 

cliff-forming sandstone member (Lupton, 1914), whereas 

the Brushy Basin is a predominately slope-forming 

mudstone member (Gregory, 1938).

In some areas on the west side of the Colorado 

Plateau, the Brushy Basin Member is overlain by the 

Lower Cretaceous Cedar Mountain Formation. The 

Cedar Mountain is difficult to distinguish from the 

Brushy Basin where basal conglomerate or sandstone 

units of the Cedar Mountain are absent. The Cedar 

Mountain above the basal coarse elastics consists of lightgreenish-gray bentonitic mudstone that is, for all intents 

and purposes, identical to the mudstone at the top of the 

Brushy Basin. Because of the similarity of these lithologies, the light-greenish-gray mudstones are variously 

assigned either to the top of the Brushy Basin or to the 

base of the Cedar Mountain Formation where the basal 

coarse elastics of the Cedar Mountain are absent an inconsistent and unsatisfactory stratigraphic arrangement.

Craig (1981, p. 195) reports that Cedar Mountain 

mudstones can be distinguished from Brushy Basin 

mudstones by the lack of brilliant colors and color banding and by the presence of abundant limestone nodules 

in the Cedar Mountain. However, mudstone beds at the 

top of the Brushy Basin also tend to lack brilliant colors 

or color banding, and limestone nodules are locally common in undoubted Brushy Basin mudstones but are absent from the Cedar Mountain at many localities. Thus, 

no satisfactory sedimentologic method is currently known 

that will help in distinguishing Cedar Mountain and 

Brushy Basin mudstone beds where the basal coarse 

clastic beds of the Cedar Mountain are missing. Isotopic 

dating of minerals in altered ash beds of the Brushy Basin 

and Cedar Mountain (for example, see Kowallis and 

Heaton, 1984) might help, but this requires considerable 

laboratory work and is not a satisfactory solution to what 

is basically a field problem. The problem obviously is in 

need of further study.

Dark-gray carbonaceous mudstone beds or sandstone and conglomerate beds that contain or interfinger 

with carbonaceous mudstone within the Dakota Formation readily distinguish it from the Brushy Basin Member 

of the Morrison.

The Brushy Basin is poorly understood because few 

detailed studies have been done on it. The member apparently was deposited in mudflat and lacustrine environments, but a small part was also deposited in fluvial and 

overbank floodplain environments. Strata containing

authigenic zeolites, analcime, and feldspars, indicating 

deposition in saline-alkaline lakes, have been identified 

in the Brushy Basin in northwestern New Mexico (TurnerPeterson, 1985; Bell, 1986), and the same authigenic 

minerals have also been found in southeastern Utah and 

southwestern Colorado (Keller, 1962; J. E. Johnson, 

University of California at Berkeley, written commun., 

1980; C. E. Turner-Peterson, oral communication, 1985; 

F. Peterson, unpublished data). The Fiftymile Member 

is interpreted as a proximal facies of the Brushy Basin 

and probably was deposited in mudflat, nearshore 

lacustrine, and fluvial environments. Bentonite beds in 

the Brushy Basin represent alteration products of volcanic 

ash that may have been reworked slightly by fluvial and 

lacustrine processes. The ash probably was blown into 

the region from volcanic areas far to the west. One of 

the geologic enigmas on the Colorado Plateau is the 

nature of conditions that allowed deposition of a few 

high-energy fluvial conglomerate beds within the abundant low-energy mudstone deposits that compose the bulk 

of the Brushy Basin Member. The Brushy Basin generally is considered Late Jurassic in age, but a recent discovery of Early Cretaceous zircons in ash beds of the member 

(Kowallis and Heaton, 1984; Kowallis and others, 1986) 

suggests that this interpretation may be in error and that 

further study is needed.

NAME AND LOCATION OF 

MEASURED SECTIONS

[Quotation marks denote local names not shown on U.S. Geological 

Survey maps]

Section 

No. Name

1. Pine Creek

2. Crosby Canyon

3. Moonshine Tanks

4. Fiftymile Point

5. Duma Point

6. Cow Springs

7. Mount Carmel 

	Junction

8. Brown Canyon

Location

SWV4 sec. 29, SEV4 sec. 30, T. 

34 S., R. 3 E., Garfield County, Utah

SEV* sec. 8, SWV* sec. 9, T. 43 

S., R. 4 E., Kane County, 

Utah

SEV* SEV* sec. 24, T. 22 S., R. 

13 E., Emery County, Utah

C-S'/z sec. 14, T. 41 S., R. 8 E., 

Kane County, Utah

NEV4 sec. 24, T. 23 S., R. 17 E.; 

NW sec. 19, T. 23 S., R. 18 

E., Grand County, Utah

36°24'15"N., 110°47'45"W., 

Coconino County, Ariz.

Sec. 12, 25, NEV4 sec. 36, T. 41 

S., R. 8 W., Kane County, 

Utah

SE!/4 sec. 7, NEV4 sec. 19, T. 41 

S., R. 5 W.; NWV4 sec. 32, T. 

40 S., R. 5 W., Kane County, Utah

46 Jurassic and Cretaceous of Colorado Plateau
Section

No. Name

9. Carly Knoll

10. Lick Wash

11. Little Bull Valley

12. Kodachrome Flat

13. Henrieville

14. "Upper Valley #1 

	unit"

15. Black Hills

16. Point 7037

17. Point 6077

18. Tenmile Spring

19. Slickrock Water

20. Collet Canyon

21. Point 5802

22. Point 5740

23. Point 5549

24. Point 5603

25. Point 5523

26. Point 5437

27. Big Hollow Spring

28. "Sand Valley"

29. Blue Pool Wash

30. "Twin Red Buttes'

31. Wahweap Creek

32. Castle Rock

33. Romana Mesa

Section 

Location No.

W»/2 sec. 30, SW K4 sec 31, T. 40 34.

S., R. 4W.;SE1/4 sec 21, T.

40S.,R.41/2 W.,KaneCoun- 35.

ty, Utah 

SE!4 sec. 36, T. 39S..R. 4W., 36.

Kane County, Utah 

Sec. 19, T. 38S.,R. 3W.,Kane 37.

County, Utah 

E!/2 sec. 3, T. 38 S., R. 2 W., 38.

Kane County, Utah 

NW!/4 sec. 23, T. 37 S., R. 2 39.

W., Garfield County, Utah 

NW/4 sec. 12, T. 36 S., R. 1 E., 40.

Garfield County, Utah 

SB 1/* sec. 13, T. 34S..R. 2E., 41.

Garfield County, Utah 

SE'/4 sec. 19, T. 35 S., R. 3E.,

Garfield County, Utah 42. 

NW/4 sec. 27, T. 36 S., R. 3 E.,

Garfield, County, Utah 43. 

SEY4 sec. 17, W!/2 sec. 20, T. 36

S., R. 4 E., Garfield County, 44.

Utah 

SWV4 sec. 14, T. 37S.,R. 4E., 45.

Garfield County, Utah 

EVi sec. 35, T. 37 S., R. 4 E., 46.

Garfield County, Utah 

WVi sec. 5, SE1/4 sec. 6, T. 38 47.

S., R. 5 E., Kane County,

Utah 48. 

SW!/4 sec. 10, T. 38S..R. 5E.,

Kane County, Utah 49. 

SE!4 sec. 23, SW!4 sec. 24, T.

38 S., R. 5 E., Kane County, 50.

Utah 

SE>/4 sec. 24, T. 38S.,R. 5E., 51.

Kane County, Utah 

NW/4 sec. 30, T. 38 S., R. 6 E., 52.

Kane County, Utah 

SW 1/* sec. 29, T. 38 S., R. 6 E., 53.

Kane County, Utah 

SEY4 sec. 11, sec. 12, T. 39 S., 54.

R. 6 E., Kane County, Utah 

SE/4 sec. 33, SVi sec. 34, T. 42 55.

N., R. 7 E., Coconino County, Ariz. 

C-S!/2sec. 30, T. 43S.,R. 3E., 56.

Kane County, Utah 

NE/4 sec. 22, T. 43S..R. 3E., 57.

Kane County, Utah 

W/2 sec. 29, E/2 sec. 32, T; 43 58.

S.,R. 4E.;E/2 sec. 5, T. 44

S., R. 4 E., Kane County, 59.

Utah 

NE/4 sec. 3, T. 44 S., R. 4 E., 60.

Kane County, Utah 

NE^sec. 21, T. 43 S., R. 5 E., 61.

Kane County, Utah

Name

Cummings Mesa W 

Cummings Mesa N 

Cummings Mesa E 

Navajo Point 

Rock Creek N 

Rock Creek E

Little Valley

Canyon 

Kane Wash

Cummings Mesa

SW 

Tsai Skizzie Rock

White Mesa NE 

White Mesa E 

Klethla Valley 361 

Klethla Valley 362 

Klethla Valley 364 

Black Mesa Road 

Long House Valley 

Tsegi 

Kayenta 

Rough Rock 

Lohali Point

Balakai (Salahkai) 

Mesa

Steamboat East 

Star Mountain 

Jones Bench 

Hartnet Draw NW 

Hartnet Draw 

The Notch

Location

SW/4 sec. 8, T. 43 S., R. 7 E.,

San Juan County, Utah 

37 003'30"N., 111°07'00"W.,

San Juan County, Utah 

37 005'30"N., 111°01'30"W.,

San Juan County, Utah 

Sees. 24, 25, T. 42 S., R. 81/2 E.,

Kane County, Utah 

SW/4 sec. 5, T. 41 S., R. 7 E.,

Kane County, Utah 

37°13'30"N., 111°09'00"W.,

Kane County, Utah 

37°14'00"N., 111°16'30"W.,

Kane County, Utah 

SEY4 sec. 1, T. 43 S., R. 5 E.;

NW/4 sec. 7, T. 43 S., R. 6

E., Kane County, Utah 

36 0 59'30"N., 111°05'30"W.,

Coconino County, Ariz. 

36 048'30 ff N.,lll°06'10"W.,

Coconino County, Ariz. 

36°35'00"N., 110°56'00"W.,

Coconino County, Ariz. 

36°33'00"N., 110°56'30"W.,

Coconino County, Ariz. 

36°26'40"N., 110°40'40"W.,

Navajo County, Ariz. 

36°27'22"N., 110°40'10" W.,

Navajo County, Ariz. 

36°27'50"N., 110°39'15"W.,

Navajo County, Ariz. 

36°33'20"N., 110°30'00"W.,

Navajo County, Ariz. 

36°34'40"N., 110°28'17"W.,

Navajo County, Ariz. 

36°37'30"N., 110°27'07"W.,

Navajo County, Ariz. 

36°39'37"N., 110°14'45" W.,

Navajo County, Ariz. 

36°26'45"N., 109°52'05" W.,

Apache County, Ariz. 

36°08'40"N., 109°48'35"W.,

Apache County, Ariz. 

36°56'10 ff N., 109°51'15"W.,

Navajo County, Ariz.

35°46'05 ff N., 109°46'20"W.,

Navajo County, Ariz. 

SW/4 sec. 27, T. 26 N., R. 19

E., Navajo County, Ariz. 

NW/4 sec. 21, T. 26S.,R. 5E.,

Sevier County, Utah 

NE/4 sec 14, T. 27 S., R. 5 E.,

Wayne County, Utah 

C-E/2 sec. 26, T. 27 S., R. 6 E.,

Wayne County, Utah 

C-W/2 sec 28, T. 28 S., R. 7 E.,

Wayne County, Utah

Middle and Upper Jurassic Rocks, Utah and Arizona 47
Section

No.

62.

63.

64.

65.

66.

67. 

68.

69.

70.

71.

72.

73.

74.

75.

76.

77.

78.

79.

80.

81.

82.

83.

84.

85. 

86.

87.

88.

89.

90.

Name

Fremont River

Tergeson Flats

Sandy Creek 1

Sandy Creek 2

Dogwater Creek

Sandy Creek 

Benches 

Spring Canyon

Bitter Creek Divide

The Post

Muley Twist

Deer Point

"Red Slide"

"Wingate Bridge"

Hall Divide

Long Canyon

"Pine"

Baker Ranch

Escalante

Escalante Rim

Point 5948

Point 6069

Sooner Slide

Bullfrog Creek

"Shootaring 

Canyon" 

Star Spring Road

Woodruff Hole

Trachyte Ranch

Little Egypt

Goatwater Spring

Location

SE!/4 sec. 14, T. 29S.,R. 7E.,

Wayne County, Utah 

SEi/4 sec. 27, T. 30 S., R. 8 E.,

Wayne County, Utah 

NW1/4 sec. 5, T. 31 S.,R. 8E.,

Garfield County, Utah 

NEV4 sec. 17, T. 31 S., R. 8 E.,

Garfield County, Utah 

NE!/4sec. 29, T. 31S..R. 8E.,

Garfield County, Utah 

NW!/4 sec. 4, T. 32S.,R. 8E.,

Garfield County, Utah 

NEV4 sec. 8, T. 32 S., R. 8 E.,

Garfield County, Utah 

SW^sec. 17,T. 33S..R. 8E.,

Garfield County, Utah 

NE'/isec. 25, T. 34S..R. 8E.,

Garfield County, Utah 

NW/4 sec. 18, T. 35 S., R. 9E.,

Garfield County, Utah 

NW/4 sec. 29, T. 35 S., R. 9E.,

Garfield County, Utah 

C-W/2 sec. 33, T. 35 S., R. 9

E., Garfield County, Utah 

NWVS sec. 4, T. 36S.,R. 9E.,

Garfield County, Utah 

C-sec. 27, T. 36 S., R. 9 E.,

Garfield County, Utah 

NW>4 sec. 30, T. 36 S., R. 10

E., Garfield County, Utah 

NE»/4sec. 3,T. 37S..R. 10 E.,

Garfield County, Utah 

SEV* sec. 5, T. 38S..R. 10 E.,

Kane County, Utah 

SWV* sec. 7, T. 35 S., R. 3 E.,

Garfield County, Utah 

SWY* sec. 4, T. 36 S., R. 3 E.,

Garfield County, Utah 

NE!4 sec. 9, T. 37 S., R. 4 E.,

Garfield County, Utah 

SW!/4 sec. 24, T. 40 S., R. 7 E.,

Kane County, Utah 

Sec. 32, T. 40 S., R. 8 E., Kane

County, Utah 

SB 1/* sec. 34, T. 36 S., R. 10 E.,

Garfield County, Utah 

SE !/4 sec. 16, T. 35S.,R. HE.,

Garfield County, Utah 

NW!4 sec. 31, T. 34 S., R. 12

E., Garfield County, Utah 

NE>/4sec. 8, T. 34S..R. 12E.,

Garfield County, Utah 

NE'/4 sec. 6, T. 33S.,R. 12 E.,

Garfield County, Utah 

NW/4 sec. 30, T. 31 S., R. 12

E., Garfield County, Utah 

SW 1/* sec. 36, T. 30S..R. HE.,

Wayne County, Utah

Section

No. Name Location

91. Little Flat Top

92. Hatt Ranch

93. Little Grand fault

SW»/4 sec. 15, T. 26 S., R. 13 E., 

Emery County, Utah

NW'/4 sec. 26, SE!4 sec. 27, T. 

22 S., R. 14 E., Emery County, Utah

C-sec. 29, T. 21 S., R. 16 E., 

Emery County, Utah

MEASURED SECTIONS

[The locations of the measured sections are shown on figure B1]

Section 1. Principal reference section of the 

Entrada Sandstone

[Measured about 5 km north of Escalante, Utah, just west of Pine Creek, 

in the SWV4 SW/4 sec. 29, SEV4 sec. 30, T. 34 S., R. 3 E., Garfield 

County, Utah. Shown as section 1 on figs. Bl and B5]

Thickness

(meters)

Morrison Formation (part): 

Tidwell Member (part):

75. Sandstone, dark-reddish-brown, very finegrained; bedding indistinct; forms slope . 0.3

74. Sandstone, yellow to light gray, fine-grained, 

bedding indistinct but appears to be very 

thin-bedded to low-angle crossbedded; slight 

ledge ................................ 0.9

Total measured Tidwell Member of Morrison 1.2

J-5 unconformity. 

Entrada Sandstone: 

Upper member:

73. Sandstone, light-gray, light-reddish-brownweathering, very fine- to fine-grained; basal 

2.1 m is irregularly very thin bedded, remainder is crossbedded with some sets as 

much as 6.1 m thick; medium and coarse 

grains locally concentrated along some of the 

bedding planes; about 8.2 m above base is 

0.3-m-thick reddish-brown band, and about 

12.2 m above base is another reddish-brown 

band about 1.5 m thick; forms cliffs and 

ledges ............................... 69.5

Total upper member ................... 69.5

48 Jurassic and Cretaceous of Colorado Plateau
Thickness

(meters)

Entrada Sandstone Continued 

Middle member:

72. Sandstone, light-reddish-brown to pinkishbrown, fine- to medium-grained, irregularly very thin-bedded; forms slight ledge ..

70. Sandstone, light-gray, very fine- to finegrained; some low-angle crossbedding apparent; forms slight ledge .............

68. Sandstone, pink, light-grayish-greenweathering, very fine- to fine-grained, 

laminated; forms slight ridge ...........

64. Sandstone, yellowish-brown, very fine- to 

fine-grained, laminated; forms slight ridge

2.4

71. Sandstone, light-grayish-green, very fine- to 

fine-grained, very thin- to thin-bedded; alternates with light-green, laminated to very 

thin-bedded mudstone; top 2.5 cm is pinkishbrown sandstone; forms notch ......... 0.6

0.9

69. Sandstone, light-gray to light-grayish-green, 

very fine- to fine-grained, very thin- to thinbedded; basal 0.2 m is sandstone alternating 

with light-green mudstone; forms notch . 1.8

0.6

67. Sandstone, light-grayish-green, very fine- to 

fine-grained, very thin- to thin-bedded; alternates with light-green, laminated to very 

thin-bedded mudstone; forms notch .... 0.3

66. Sandstone, yellowish-brown, very fine- to 

fine-grained, laminated; 0.9 m above base is 

2.5 cm of light-green mudstone; 3.0 m above 

base is 2.5 cm of light-green mudstone; 

forms slight ridge ..................... 5.5

65. Sandstone, light grayish-green, very fine- to 

fine-grained, very thin- to thin-bedded; includes some light-green mudstone; forms 

slope ................................ 0.5

6.9

63. Sandstone, light-grayish-green, very fine- to 

fine-grained, very thin- to thin-bedded; in 

thin beds alternates with light-green, 

laminated to very thin-bedded mudstone; 

forms notch ......................... 1.5

62. Sandstone, light-grayish-green, very fine- to 

fine-grained, laminated to very low-angle 

crossbedded; forms slight ridge ......... 0.6

61. Sandstone, light-grayish-green, very fine- to 

fine-grained, very thin- to thin-bedded; alternates with light-green, laminated to very 

thin-bedded mudstone; forms slight notch 0.9

Thickness 

(meters)

60. Sandstone, light-grayish-green, very fine- to 

fine-grained; some low-angle crossbedding 

apparent; 0.3 m above base is 5-cm zone of 

light-green mudstone; contains yellowishorange-brown nodules of pyrite(?) surrounded by limonitic material in a zone 

about 0.6-0.9 m above base of unit; 3.4 m 

above base is 5-cm-thick light-green bentonitic mudstone bed; 6.1 m above base is 

1.0-m-thick yellowish-brown zone; 7.0 m 

above base is zone 0.3 m thick of light-green 

mudstone alternating with light-grayishgreen sandstone; top of unit holds up slight 

ridge ................................ 10.1

59. Sandstone, silty, reddish-brown (mottled 

very light gray), very fine- to fine-grained, 

irregularly very thin-bedded; includes some 

low-angle crossbedding; contains about 

20-40 percent dark reddish-brown, laminated mudstone; forms slope .......... 0.2

58. Sandstone, light-gray, very fine- to finegrained; indistinct bedding; 2.1 m above base 

a reddish-brown sandstone bed makes a 

0.5-m ledge; forms slope .............. 3.7

57. Sandstone, silty; same as unit 59 

56. Sandstone; same as unit 58 .....

55. Sandstone, silty; same as unit 59

0.3 

0.3 

0.6

54. Sandstone, light-gray (mottled with some 

reddish brown), very fine- to fine-grained, 

irregularly laminated to very thin-bedded; 

1.5 m above base is 1.5-m-thick zone that is 

light reddish pink to reddish brown at the 

top; 4.9 m above base is 0.3 m of light-green 

mudstone, overlain by 0.5 m of darkreddish-brown mudstone, 0.6 m of pink 

sandstone, and 1.2 m of light-gray sandstone; forms ridge .................... 7.5

53. Sandstone, silty; same as unit 59 ....... 0.5

52. Sandstone; same as unit 58 ............ 0.6

51. Sandstone, silty; same as unit 59 ....... 0.5

50. Sandstone; same as unit 58 ............ 1.8

49. Sandstone, silty; same as unit 59 ....... 1.2

48. Sandstone; same as unit 58 ............ 1.2

47. Sandstone, silty; bedding indistinct; 1.5 m 

above base is 0.6 m of white sandstone; 

otherwise same as unit 59 ............. 6.1

Middle and Upper Jurassic Rocks, Utah and Arizona 49
Thickness 

(meters) 

Entrada Sandstone Continued

Middle member Continued 

46. Sandstone; same as unit 58 ............ 1.8

45. Sandstone, silty; same as unit 59 ....... 2.7

44. Sandstone; same as unit 58 ............ 1.5

43. Sandstone, silty, reddish-brown and some 

light-gray, very fine- to fine-grained, irregularly very thin-bedded and some low-angle 

crossbedded; contains some dark-reddishbrown mudstone; 7.5 cm of dark-reddishbrown and light-green mudstone at the base; 

forms slope .......................... 3.0

42. Sandstone; same as unit 58; 1.2 m above base 

is 0.3 m of reddish-brown sandstone and 

dark-reddish-brown to light-green mudstone; 

forms ridge .......................... 3.4

41. Sandstone, silty; same as unit 59 

40. Sandstone; same as unit 58 .....

38. Sandstone; same as unit 58 ..... 

37. Sandstone, silty; same as unit 59

0.9 

1.8

39. Sandstone, silty; bedding indistinct; otherwise same as unit 59 .................. 2.1

1.5 

1.5

36. Sandstone; bedding indistinct; otherwise

same as unit 59 ...................... 1.5

35. Sandstone, silty; 0.6 m above base is 7.5-cmthick bed of dark-reddish-brown mudstone; 

1.2m above base is 0.3-m-thick bed of darkreddish-brown to purple mudstone; otherwise same as unit 59 .................. 4.3

34. Sandstone; same as basal part of unit 58 0.6

33. Sandstone, silty; 4.6 m above base is 0.3 m 

of dark-reddish-brown mudstone; 8.5 m 

above base is 0.6 m of white, irregularly thinbedded sandstone; top 0.6 m is interbedded 

grayish-green mudstone and sandstone; 

otherwise same as unit 59 ............. 13.7

32. Sandstone; same as unit 58 but tends to form 

ledge; unit is highest of the ledge-forming 

light-gray sandstone beds in the middle 

member ............................. 1.2

Thickness 

(meters) 

31. Sandstone, silty; basal 0.9 m is a partly

covered slope; same as unit 59 ......... 5.8

30. Sandstone, light-gray; top 1.2 m is partly

covered slope; forms ridge ............. 3.4

29. Sandstone, silty; top 0.4 m consists of 0.2 

m of light-gray sandstone overlain by 0.2 m 

of grayish-green mudstone; otherwise same 

as unit 59 ........................... 1.1

28. Covered slope; probably same as unit 59 1.1

27. Sandstone; same as unit 58 but forms ridge 1.4

26. Sandstone, silty; same as unit 59 ....... 1.4

25. Sandstone; top 0.6 m contains low-angle 

crossbedding; otherwise same as unit 58; 

forms ridge .......................... 2.1

24. Sandstone, silty, reddish-brown, very fineto fine-grained, very thin- to thin-bedded; 

basal meter includes three beds of white 

sandstone each 0.2 m thick; forms slope 4.7

23. Sandstone; 0.3 m below top is a 0.2-m-thick 

dark-reddish-brown mudstone bed; otherwise same as unit 58 .................. 1.5

22. Sandstone, silty, reddish-brown (with minor 

white mottles), very fine- to fine-grained, irregularly very thin bedded; some low-angle 

crossbedding; contains about 20-40 percent 

dark-reddish-brown mudstone; forms slope 5.2

21. Sandstone, light gray (mottled with some 

reddish-brown), very fine- to fine-grained, 

irregularly laminated to very thin-bedded; 

some low-angle crossbedding; top has closely 

spaced ripples with crests about 2 cm apart; 

forms ridge .......................... 5.8

Total middle member .................. 132.1

Lower member:

20. Sandstone, orange-brown, very fine- to finegrained, laminated to very low-angle 

crossbedded; 1.2 m above base are ripple 

cross-laminae; 10.7 m above base is 1-cmthick dark-reddish-brown mudstone bed; 

15.5 m above base is some dark-reddishbrown mudstone interbedded with approximately equal proportions of reddish-brown 

sandstone; forms ridges and swales ..... 23.8

50 Jurassic and Cretaceous of Colorado Plateau
Thickness

(meters)

Entrada Sandstone Continued 

Lower member Continued

19. Sandstone, reddish-brown to orange-brown, 

very fine-grained, includes several darkreddish-brown sandstone beds about 0.3 m 

thick; bedding obscure; forms slope .... 2.3

18. Sandstone, reddish-brown, very finegrained, and dark reddish-brown mudstone; 

both very thin-bedded; 2.5-cm-thick pink 

limey siltstone or silty limestone in middle; 

forms slope .......................... 0.6

17. Sandstone, orange-brown; same as unit 19; 

0.6 m above base is 0.3-m-thick bed of darkreddish-brown, very fine-grained sandstone; 

contains some thin sets of crossbeds; forms 

slope ................................ 7.8

16. Sandstone, dark-reddish-brown, very finegrained, and mudstone, dark-reddish-brown; 

very thin-bedded; includes about 5 percent 

white sandstone; forms slope .......... 1.5

15. Sandstone, reddish-brown to orange-brown, 

very fine-grained; locally altered to very light 

gray; 1.5m above base is 0.2 m of very finegrained sandstone; forms slope ......... 9.8

14. Sandstone, dark-reddish-brown, very finegrained, and dark-reddish-brown mudstone; 

both very thin-bedded; top has 0.3-m-thick 

interval that is largely pink platey limestone 

or possibly highly calcareous siltstone; forms 

slope ................................ 1.1

13. Sandstone; same as unit 19 but laminated to

very thin bedded in part; forms slope ... 6.1

12. Sandstone, dark-reddish-brown, very finegrained, and dark-reddish-brown mudstone; 

both very thin-bedded; about 5 percent is 

mottled white; forms slope ............ 0.6

11. Sandstone; 2.7 m above base is 7.5-cm-thick 

bed of dark-reddish-brown mudstone; otherwise same as unit 19; forms slope ...... 6.4

10. Sandstone, dark-reddish-brown, very finegrained; interbedded with about 20 percent 

dark-reddish-brown mudstone; very thin 

bedded; appears as a conspicuous dark band; 

forms slope .......................... 0.5

Thickness 

(meters)

9. Sandstone, reddish-brown to orange-brown, 

very fine-grained; bedding difficult to identify but appears very thin to thin and includes 

some thin sets of crossbeds; forms slope 12.8

8. Mudstone, dark-reddish-brown to purple, 

very thin-bedded; grades upward into silty 

sandstone containing biotite grains, sandstone locally altered to very light gray; forms 

slope ................................ 0.3

7. Sandstone, reddish-brown to orange-brown, 

very fine-grained; contains several darkreddish-brown sandstone beds about 0.3 m 

thick; largely covered with red soil; 3.4 m 

below top is 5.0-cm-thick bed of darkreddish-brown, silty mudstone overlain by 

about 25-cm-thick dark-reddish-brown sandstone bed; basal contact essentially a planar 

surface; best exposed in stream cuts of Pine 

Creek; forms slope ................... 39.0

Total lower member .................... 112.6

Total Entrada Sandstone ............... 314.2

Carmel Formation (part): 

Upper member (part):

6. Mudstone, reddish-brown, purple weathering, laminated to very thin-bedded; forms 

slope. Used as the contact marker bed in the 

vicinity of Escalante, Utah, where the lower 

member of the Entrada is reddish-brown silty 

sandstone similar to that in the underlying 

upper part of the Carmel ..............

5. Sandstone, reddish-brown, very finegrained; bedding indistinct; forms slight 

ledge ................................

4. Mudstone, reddish-brown; forms notch .

3. Sandstone, reddish-brown, very finegrained, irregularly very thin-bedded and ripple cross-laminated; forms slight ledge ..

2. Sandstone, silty, reddish-brown, very finegrained; bedding indistinct; forms notch

1. Gypsum, gray, wavy-bedded; stratigraphically highest gypsum bed in the Carmel; forms 

ledge ................................

Total measured upper member of Carmel 

Formation ............................

0.3

1.8 

0.5

0.3 

0.6

0.3

3.8

Middle and Upper Jurassic Rocks, Utah and Arizona 51
Section 2. Type section of the Romana Sandstone

[Measured on northeast side of Crosby Canyon, a side canyon to Warm 

Creek canyon. Section measured in the SE14 NEV4 SE14 sec. 8., T. 

43, S., R. 4 E., Kane County, Utah. Shown as section 2 on figs. Bl 

and B8.]

Thickness

(meters)

Thickness

(meters)

Morrison Formation (part): 

Salt Wash Member (part):

14. Sandstone, grayish-brown; weathers darkyellowish-brown; fine grained but contains 

scattered conglomerate lenses and stringers; 

contact locally has about 0.3-0.6 m of relief. 

Thickness estimated ...................

9. Sandstone, light-greenish-gray, fine-grained, 

moderately sorted, crossbedded to very thinbedded; scattered coarse and very coarse 

grains in middle; top includes 2.5-cm-thick 

grayish-green mudstone bed containing 

sandstone-filled desiccation cracks; forms 

cliff .................................

8. Sandstone, light-grayish-green, fine-grained, 

moderately sorted; scattered coarse to very 

coarse grains; some crossbeds; contains scattered mud chips; unit fills a channel, 

thickness measured in deepest part of channel; forms notch .....................

7. Sandstone, light-grayish-green, fine-grained, 

well-sorted, crossbedded; scattered coarse 

grains; forms cliff ....................

4.6

Total measured Salt Wash Member (incomplete) 4.6

J-5 unconformity. 

Romana Sandstone:

13. Sandstone, light-greenish-gray, fine-grained, 

moderately sorted, crossbedded; contains 

scattered coarse to very coarse grains; forms 

cliff .................................

12. Mudstone, dark-reddish-brown and minor 

grayish-yellow-green; sandstone-filled desiccation cracks near top; forms notch ....

11. Sandstone, light-greenish-gray, fine-grained, 

moderately sorted, crossbedded; contains 

scattered coarse and very coarse grains; 

forms cliff ...........................

10. Sandstone, light-greenish-gray, fine-grained, 

moderately sorted to well-sorted, crossbedded; includes scattered mud chips near 

base; forms cliff ......................

0.6

0.3

15.2

4.9

6. Sandstone, light-grayish-green to light-olivegreen, very fine-grained, moderately sorted, 

very thin-bedded; forms cliff ...........

5. Sandstone, silty, reddish-brown to lightgrayish-green, very fine-grained, moderately sorted, very thin-bedded; basal 2.5 cm is 

a purple bentonite bed; forms ledge ....

4. Sandstone, very light-gray, very fine-grained, 

moderately sorted, very thin- to thin-bedded; 

forms cliff ...........................

3. Sandstone, reddish-brown (locally mottled to 

light-greenish-gray), very fine-grained, 

moderately to poorly sorted, very thin- to 

thin-bedded; includes some dark-reddishbrown silty mudstone; forms notch. Unit is 

the red band or marker bed that is lithologically related to the Summerville Formation

2. Sandstone, light-greenish-gray, white, and 

right-yellowish-green (locally mottled to reddish brown near top), fine-grained, 

moderately-sorted, very thin-bedded; scattered very coarse grains; lower part forms 

slightly overhanging ledge; basal contact is 

a planar surface ......................

1.5

0.3

4.9

2.6

0.2

Total thickness Romana Sandstone ...... 44.2

J-3 unconformity. 

Entrada Sandstone (part): 

Middle member (part):

1. Sandstone, white to very light-gray, very 

fine-grained, crossbedded; forms cliff. 

Thickness incomplete ................. 3.0

Total measured Entrada Sandstone 3.0

4.6

Section 3. Type section of the Tidwell Member 

of Morrison Formation

[Measured on the east side of a small southward-draining canyon in 

the NE'/4 SE14 SE»4 sec. 24, T. 22 S., R. 13 E., Emery County, Utah. 

Shown as section 3 on figs. Bl and B21.]

1.8

7.3

Morrison Formation (part): 

Salt Wash Member (part):

8. Sandstone, light-gray, grayish-brownweathering, fine-grained, moderately sorted, 

crossbedded; forms cliff ...............

Thickness 

(meters)

2.4

52 Jurassic and Cretaceous of Colorado Plateau
Thickness 

(meters)

Thickness

(meters)

Morrison Formation (part) Continued 

Salt Wash Member (part) Continued

Total measured Salt Wash Member 

(incomplete) .................... 2.4

Tidwell Member:

7. Mudstone, grayish-green and dark-reddishbrown, laminated to very thin-bedded; contains several thin beds of light-gray, very 

fine- to fine-grained, moderately sorted, 

locally burrowed sandstone; scattered small 

chert pebbles in a bed in middle of unit; 

forms slope ..........................

6. Mudstone, grayish-green to grayish-yellowgreen (with a few thin grayish-purple to darkreddish-brown beds), laminated to very thinbedded; 4.9 m above base is a 1-m-thick lens 

of sandstone that is very fine grained and ripple cross-laminated; 7.3 m above base is 

0.3-m-thick bed of gray mudstone containing black carbonaceous blebs; forms slope

5. Mudstone, grayish-green (with a few grayishpurple beds), laminated to very thin-bedded; 

contains several thin sandstone beds that are 

light gray, very fine to fine grained, and 

laminated to very thin bedded or small-scale 

crossbedded; one sandstone bed 1.4m above 

the base contains scattered small chert pebbles; forms slope .....................

4. Mudstone, grayish-green, grayish-yellowgreen, and light-grayish-green; a few beds are 

dark reddish brown; small blebs of red 

authigenic chert at several horizons; thin bed 

of gray, dense limestone found 2.7m above 

base; scarce scattered small chert pebbles in 

some beds; forms slope ...............

3. Gypsum, white and minor very light-grayishgreen and light-grayish-pink, nodular; locally 

contains small blebs and nodules of red or 

yellow authigenic chert; includes about 20 

percent grayish-green mudstone; one bed of 

light-grayish-green mudstone 0.3 m thick and 

0.3 m below top forms a notch; otherwise 

the unit tends to form a cliff ..........

2. Mudstone, light-grayish-green and minor 

dark-reddish-brown, laminated to very thinbedded; contains gypsum as veins, nodules, 

and thin beds; thin lenses of gray, dense 

limestone about 0.6 m above base; forms 

slight ledge ..........................

J-5 unconformity. 

Summerville Formation (part): 

Chocolate member (part):

1. Mudstone, dark-reddish-brown, very thin- to 

thin-bedded; contains several thin beds 0-15 

cm thick of white gypsum; also includes 

many thin gypsum veins; upper 0-15 cm 

bleached to light grayish green; forms cliff; 

thickness incomplete .................. 4.6

6.4

Total measured Summerville Formation .. 4.6

Section 4. Type section of the Fiftymile Member 

of the Morrison Formation

[Measured on the southeast side of a northeast-trending ridge in the 

SWVi SEVi sec. 11, NW/4 NE 1/* sec. 14, T. 41 S., R. 8 E., Kane County, Utah. Shown as section 4 on figs. Bl and B2L]

9.0 Thickness 

(meters)

Dakota Formation (part):

3.4

11. Mudstone, black to dark-gray, carbonaceous, laminated to very thin-bedded; includes some light-gray sandstone; forms 

slope. Basal contact is planar; regional relationships indicate it is an unconformity. 

Thickness incomplete .................

Total measured Dakota Formation .......

3.0

3.0

5.3

3.5

1.7

Total thickness Tidwell Member ......... 29.3

Morrison Formation (part): 

Fiftymile Member:

10. Sandstone, light-gray, medium-grained, 

moderately to poorly sorted, thin-bedded to 

crossbedded, conglomeratic; forms cliff .

9. Conglomerate, grayish-brown, very thin- to 

thin-bedded and crossbedded; pebbles include white, very light-gray, gray, black, 

light-brown, orange, light-green, and darkred chert and some dark-red quartzite; 

average pebble size about 4 mm; contains 

scattered cobbles and small blocks of 

sandstone as much as 20 cm long; also 

contains several thin sandstone beds; forms 

cliff .................................

8. Mudstone, dark-purple and light-green, 

laminated to very thin bedded, and some red 

siltstone; forms slope .................

7.6

7.8

1.2

Middle and Upper Jurassic Rocks, Utah and Arizona 53
Thickness 

(meters) 

Morrison Formation (part) Continued

Fiftymile Member Continued

7. Sandstone, light-gray, fine-grained, moderately sorted, laminated and crossbedded; 

contains scattered pebbles of white, lightgray, light-brown, and gray chert; forms 

cliff ................................. 36.9

6. Mudstone, dark-reddish-brown, and several 

thin beds of light-gray sandstone; partly 

covered; forms slope .................. 5.6

5. Sandstone, light-gray, fine-grained, moderately sorted, crossbedded, conglomeratic; 

forms cliff ........................... 5.8

4. Mudstone, dark-reddish-brown; includes 

several thin beds of light-gray sandstone; 

partly covered slope .................. 15.4

3. Sandstone, light-gray, fine-grained, moderately sorted, laminated; contains some crossbeds; forms cliff ..................... 13.4

2. Mudstone, dark-reddish-brown; includes 

several thin beds of light-gray sandstone; 

partly covered slope .................. 13.4

Total thickness of Fiftymile Member ..... 107.1

Salt Wash Member (part):

1. Sandstone, light-gray, fine-grained, moderately to poorly sorted, crossbedded; contains 

scattered pebbles and conglomerate lenses; 

basal 4.6 m includes several thin darkreddish-brown mudstone lenses to the north 

of the line of section; about 15.2 m above 

base is a 1-m-thick lens of dark-reddishbrown mudstone; 17.7 m above base is 

another dark-reddish-brown mudstone bed 

that wedges out to the north; about 24.4 m 

above base is a 2- to 3-m-thick conglomerate 

lens; forms large cliff ................. 40.5

Total measured Salt Wash Member

(incomplete) ........................... 40.5

REFERENCES CITED

Baker, A. A., Dane, C. H., and Reeside, J. B., Jr., 1936, Correlation of the Jurassic formations of parts of Utah, 

Arizona, New Mexico, and Colorado: U.S. Geological 

Survey Professional Paper 183, 66 p.

Bell, T. E., 1986, Deposition and diagenesis of the Brushy Basin 

Member and upper part of the Westwater Canyon Member 

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Fishman, N. S., eds., A basin analysis case study The 

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in Geology no. 22, p. 7-26.

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the Morrison and related formations, Colorado Plateau 

region, a preliminary report: U.S. Geological Survey 

Bulletin 1009-E, p. 125-168.

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and others, 1959, Measured sections of the Morrison and 

adjacent formations: U.S. Geological Survey open-file 

report, 173 measured sections.

Curray, J. R., 1956, The analysis of two-dimensional orientation data: Journal of Geology, v. 64, no. 2, p. 117-131.

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Dunbar, C. O., and Rodgers, John, 1957, Principles of 

stratigraphy: New York, John Wiley and Sons, Inc., 356 p.

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Gilluly, James, 1929, Geology and oil and gas prospects of part 

of the San Rafael Swell, Utah: U.S. Geological Survey 

Bulletin 806-C, p. 69-130.

Gilluly, James, and Reeside, J. B., Jr., 1928, Sedimentary rocks 

of the San Rafael Swell and some adjacent areas in eastern 

Utah: U.S. Geological Survey Professional Paper 150-D, 

p. 61-110.

Gregory, H. E., 1938, The San Juan country, a geographic and 

geologic reconnaissance of southeastern Utah: U.S. Geological Survey Professional Paper 188, 123 p.

Harshbarger, J. W., Repenning, C. A., and Irwin, J. H., 1957, 

Stratigraphy of the uppermost Triassic and the Jurassic 

rocks of the Navajo country: U.S. Geological Survey Professional Paper 291, 74 p.

54 Jurassic and Cretaceous of Colorado Plateau
Harshbarger, J. W., Repenning, C. A., and Jackson, R. L., 

1951, Jurassic stratigraphy of the Navajo country: New 

Mexico Geological Society Guidebook, 2d Field Conference, p. 95-99.

Hunt, C. B., Averitt, Paul, and Miller, R. L, 1953, Geology 

and geography of the Henry Mountains region, Utah: U.S. 

Geological Survey Professional Paper 228, 234 p.

Imlay, R. W., 1980, Jurassic paleobiogeography of the conterminous United States in its continental setting: U.S. Geological Survey Professional Paper 1062, 134 p.

Keller, W. D., 1962, Clay minerals in the Morrison Formation 

of the Colorado Plateau: U.S. Geological Survey Bulletin 

1150, 90 p.

Kirk, A. R., and Condon, S. M., 1986, Structural control of 

sedimentation patterns and the distribution of uranium 

deposits in the Westwater Canyon Member of the Morrison Formation, northwestern New Mexico A subsurface study, in Turner-Peterson, C. E., Santos, E. S., and 

Fishman, N. S., eds., A basin analysis case study The 

Morrison Formation, Grants uranium region, New Mexico: American Association of Petroleum Geologists, Studies 

in Geology no. 22, p. 105-143.

Kocurek, Gary, and Dott, R. H., Jr., 1983, Jurassic 

paleogeography and paleoclimate of the central and 

southern Rocky Mountain region, in Reynolds, M. W., and 

Dolly, E. D., eds., Mesozoic paleogeography of the westcentral United States: Society of Economic Paleontologists 

and Mineralogists, Rocky Mountain Section, Rocky Mountain Paleogeography Symposium 2, p. 101-116.

Kowallis, B. J., and Heaton, Julie, 1984, Fission track 

stratigraphy of Upper Jurassic and Lower Cretaceous bentonites from the Morrison and Cedar Mountain Formations of south-central Utah: Geological Society of America 

Abstracts with Programs, v. 16, no. 6, p. 565.

Kowallis, B. J., Heaton, J. S., and Bringhurst, Kelly, 1986, 

Fission-track dating of volcanically derived sedimentary 

rocks: Geology, v. 14, no. 1, p. 19-22.

Lupton, C. T., 1914, Oil and gas near Green River, Grand 

County, Utah: U.S. Geological Survey Bulletin 541, 

p. 115-134.

Lynd, L. E., 1960, Study of the mechanism and rate of ilmenite 

weathering: American Institute of Mining and Metallurgical 

Engineers Transactions, v. 217, p. 311-318.

Mackin, J. H., and Schmidt, D. L., 1956, Uranium- and 

thorium-bearing minerals in placer deposits in Idaho, in 

Page, L. R., Stocking, H. E., and Smith, H. B., compilers, 

Contributions to the geology of uranium and thorium by 

the United States Geological Survey and Atomic Energy 

Commission for the United Nations International Conference on Peaceful Uses of Atomic Energy, Geneva, 

Switzerland, 1955: U.S. Geological Survey Professional 

Paper 300, p. 375-380.

McKnight, E. T., 1940, Geology of area between Green and 

Colorado Rivers, Grand and San Juan Counties, Utah: 

U.S. Geological Survey Bulletin 908, 147 p.

Oriel, S. S., and Craig, L. C., 1960, Lower Mesozoic rocks in 

Colorado, in Weimer, R. J., and Haun, J. D., eds., Guide 

to the geology of Colorado: Denver, Colo., Geological 

Society of America, Rocky Mountain Association of 

Geologists, and Colorado Scientific Society, p. 43-58.

O'Sullivan, R. B., 1978, Stratigraphic sections of Middle 

Jurassic San Rafael Group from Lohali Point, Arizona, 

to Bluff, Utah: U.S. Geological Survey Oil and Gas Investigations Chart OC-77.

____1980a, Stratigraphic sections of Middle Jurassic San 

Rafael Group from Wilson Arch to Bluff in southeastern 

Utah: U.S. Geological Survey Oil and Gas Investigations 

Chart OC-102.

____1980b, Stratigraphic sections of Middle Jurassic San 

Rafael Group and related rocks from the Green River to 

the Moab area in east-central Utah: U.S. Geological Survey 

Miscellaneous Field Studies Map MF-1247.

____1984, The base of the Upper Jurassic Morrison Formation in east-central Utah: U.S. Geological Survey Bulletin 

1561, 17 p.

Owen, D. E., Walters, L. J., Jr., and Beck, R. G., 1984,'The 

Jackpile Sandstone Member of the Morrison Formation 

in west-central New Mexico A formal definition: New 

Mexico Geology, v. 6, no. 3, p. 45-52.

Peterson, Fred, 1974, Correlation of the Curtis, Summerville, 

and related units (Upper Jurassic) in south-central Utah 

and north-central Arizona: Geological Society of America 

Abstracts with Programs, v. 6, no. 5., p. 466-467.

____1975, Geologic map of the Sooner Bench quadrangle, 

Kane County, Utah: U.S. Geological Survey Miscellaneous 

Investigations Series Map 1-874.

____1980a, Geologic map and coal deposits of the Big Hollow 

Wash quadrangle, Kane County, Utah: U.S. Geological 

Survey Coal Investigations Map C-84.

____1980b, Sedimentology of the uranium-bearing Salt Wash 

Member and Tidwell unit of the Morrison Formation in 

the Henry and Kaiparowits basins, Utah, in Picard, M. D., 

ed., Henry Mountains Symposium: Salt Lake City, Utah, 

Utah Geological Association, p. 306-322.

____1984, Fluvial sedimentation on a quivering craton 

Influence of slight crustal movements on fluvial processes, 

Upper Jurassic Morrison Formation, western Colorado 

Plateau: Sedimentary Geology, v. 38, no. 1/4, p. 21-49.

____1986, Jurassic paleotectonics in the west-central part of

the Colorado Plateau, Utah and Arizona, in Peterson, 

J. A., ed., Paleotectonics and sedimentation in the Rocky 

Mountain region, United States: American Association of 

Petroleum Geologists Memoir 41, p. 563-596.

Peterson, Fred, and Condon, S. M., 1984, Stratigraphy of 

Middle and Upper Jurassic eolian rocks, Black Mesa, 

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4, p. 251.

Peterson, Fred, and Kirk, A. R., 1977, Correlation of the 

Cretaceous rocks in the San Juan, Black Mesa, 

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Plateau, in Fassett, J. E., ed., San Juan Basin HI, 

Guidebook of northwestern New Mexico: New Mexico 

Geological Society, 28th Field Conference, 1977, 

p. 167-178.

Peterson, Fred, and Pipiringos, G. N., 1979, Stratigraphic relations of the Navajo Sandstone to Middle Jurassic formations, southern Utah and northern Arizona: U.S. 

Geological Survey Professional Paper 1035-B, p. B1-B43.

Pipiringos, G. N., and Imlay, R. W., 1979, Lithology and subdivisions of the Jurassic Stump Formation in southeastern

Middle and Upper Jurassic Rocks, Utah and Arizona 55
Idaho and adjoining areas: U.S. Geological Survey Professional Paper 1035-C, p. C1-C25.

Pipiringos, G. N., and O'Sullivan, R. B., 1978, Principal unconformities in Triassic and Jurassic rocks, Western Interior United States A preliminary survey: U.S. 

Geological Survey Professional Paper 1035-A, p. A1-A29.

Reiche, Parry, 1938, An analysis of cross-lamination The 

Coconino Sandstone: Journal of Geology, v. 46, no. 7, 

p. 905-932.

Santos, E. S., and Turner-Peterson, C. E., 1986, Tectonic setting of the San Juan Basin in the Jurassic, in TurnerPeterson, C. E., Santos, E. S., and Fishman, N. S., eds., 

A basin analysis case study The Morrison Formation, 

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p. 27-33.

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R. G., 1963, Geology of the Capitol Reef area, Wayne and 

Gar field Counties, Utah: U.S. Geological Survey Professional Paper 363, 102 p.

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the San Rafael Group, southwest and south central Utah: 

Utah Geological and Mineralogical Survey Bulletin 87 

(replacement copy, May 1971), 53 p.

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uranium-vanadium deposits of the San Rafael River mining area, Emery County, Utah: Utah Geological and 

Mineralogical Survey Bulletin 113, 122 p.

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primary uranium ore deposits, Grants uranium region, New 

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Bulletin, v. 69, no. 11, p. 1999-2020.

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Bulletin, v. 55, no. 4, p. 1097-1114.

Zeller, H. D., 1973, Geologic map and coal resources of the 

Dave Canyon quadrangle, Garfield County, Utah: U.S. 

Geological Survey Coal Investigations Map C-59.

56 Jurassic and Cretaceous of Colorado Plateau
Chapter C

The Encinal Canyon Member, a 

New Member of the Upper Cretaceous 

Dakota Sandstone in the Southern and Eastern 

San Juan Basin, New Mexico

By WILLIAM M. AUBREY

Studies of a newly recognized 

fluvial sandstone unit clarify 

the nature of a major regional 

unconformity

U.S. GEOLOGICAL SURVEY BULLETIN 1633-C

REVISIONS TO STRATIGRAPHIC NOMENCLATURE OF JURASSIC AND CRETACEOUS ROCKS 

OF THE COLORADO PLATEAU
CONTENTS

Abstract 59

Introduction 59

Pre-Dakota rocks 59

Dakota Sandstone 62

Encinal Canyon Member 62 

Oak Canyon Member 63

Previous correlations 65

Age 65

Depositional history 65

Conclusions 66

Measured section 66

References cited 68

FIGURES

Cl. Map of southeastern San Juan Basin showing location of sections 60 

C2. Cross section of uppermost Jurassic and lowest Cretaceous rocks, southern

part of the San Juan Basin 61 

C3. Photograph of the type section of Encinal Canyon Member, near Laguna,

N. Mex. 63 

C4. Columnar section of the Encinal Canyon Member at the type section 64

58 Contents
REVISIONS TO STRATIGRAPHIC NOMENCLATURE OF JURASSIC AND CRETACEOUS ROCKS 

OF THE COLORADO PLATEAU

The Encinal Canyon Member, a New Member of the 

Upper Cretaceous Dakota Sandstone in the Southern 

and Eastern San Juan Basin, New Mexico

By William M. Aubrey

Abstract

A thin, discontinuous, fluvial conglomeratic sandstone 

unit that occurs at the base of the Upper Cretaceous Dakota 

Sandstone throughout most of the southern and eastern San 

Juan Basin and contiguous areas in north-central New Mexico 

is herein named the Encinal Canyon Member of the Dakota 

Sandstone. The unit is characterized by conglomeratic beds 

that contain abundant white, angular, chalky chert clasts, by 

dark-gray shale and mudstone beds, and by locally abundant 

carbonaceous debris. In the past, part or all of the sandstone 

beds that are placed here in the Encinal Canyon have been 

included in the Jackpile Sandstone Member of the Morrison 

Formation, in the Lower Cretaceous Burro Canyon Formation, 

or in the Upper Cretaceous Dakota Sandstone.

Recognition of the Encinal Canyon Member helps clarify 

the nature of the sub-Dakota unconformity throughout the 

region. In the past, this erosion surface was thought by many 

workers to be at the base of the marine and paralic sedimentary rocks of the Oak Canyon Member of the Dakota Sandstone, which overlies the Encinal Canyon Member. The 

unconformity is shown here to be at the base of the fluvial rocks 

of the Encinal Canyon Member.

Local relief of as much as 100 feet (30 m) at the base of 

the Encinal Canyon indicates that the sub-Dakota erosion 

surface formed during a time of regional degradation. Easterly 

flowing streams carved this paleo-land surface, eroding the 

underlying units and locally cutting completely through the 

Burro Canyon Formation and the Jackpile Sandstone Member. 

The Encinal Canyon was deposited in valleys as the base-level 

rose, in response to the initial transgression of the Dakota sea. 

As the sea inundated the area, waves and currents reworked 

some of the Encinal Canyon and truncated the tops of high areas 

of Lower Cretaceous or older bedrock intervening between the 

valleys. As the transgression continued paralic and marine sediments of the overlying Oak Canyon Member were deposited.

INTRODUCTION

Most previous workers have recognized fluvial rocks 

at the base of the Dakota Sandstone in the northern and

western parts of the San Juan Basin, but not in the 

southeastern part of the basin (fig. Cl). In this area, it 

was believed that basal Dakota strata consisted almost 

entirely of marginal marine or marine rocks of the Oak 

Canyon Member (Landis and others, 1973; Molenaar, 

1977; Owen, 1973) and that the sub-Dakota regional unconformity lay at the base of these rocks. Studies now 

show, however, that fluvial strata genetically and temporally related to the Dakota Sandstone underlie the Oak 

Canyon Member and that the regional unconformity is 

at the base of these fluvial rocks. A newly recognized unit 

consisting of these thin, discontinuous, locally conglomeratic fluvial sandstone strata is here named the Encinal 

Canyon Member of the Dakota Sandstone. Recognition 

of the Encinal Canyon Member is important because it 

marks the correct stratigraphic position of the sub-Dakota 

regional unconformity and because it changes concepts 

concerning sedimentologic processes during the early 

stages of the deposition of the Dakota.

PRE-DAKOTA ROCKS

The Dakota Sandstone truncates successively older 

stratigraphic units from northeast to southwest along the 

southeastern side of the San Juan Basin (fig. C2). In 

ascending order, the units that lie directly beneath the 

Dakota in one place or another in the study area are the 

Zuni Sandstone, the Brushy Basin and Jackpile Sandstone 

Members of the Morrison Formation, and the Burro Canyon Formation. These units are briefly described below.

The Zuni Sandstone consists largely of fine- to 

medium-grained, generally yellowish-gray or grayishyellow-green eolian sandstone (Maxwell, 1976). It is 

Jurassic (probably Middle Jurassic) in age and lies below 

the Encinal Canyon at El Morro National Monument and 

in the Acoma area. The Zuni lies below the Morrison Formation on the east side of the basin.

The Brushy Basin Member of the Morrison Formation consists of thick montmorillonitic beds of

Encinal Canyon Member of Dakota Sandstone, New Mexico 59
109

36° 

35° 

25 50 MILES

25 50 KILOMETERS

Figure C1. Locations of stratigraphic cross section A-A', measured sections 1-17 (both shown in fig. C2), and other features 

of interest in and near the San Juan Basin, New Mexico. Patterned areas are outcrops of Dakota Sandstone.

60 Jurassic and Cretaceous of Colorado Plateau
METERS 

0

30.5 --100

FEET 

200

-0

1

El Morro

National

Monument

Sec 6, T9N, R14W

LACUNA AREA 

2

Acoma Pueblo 3 4 

(Modified from Encinal Canyon Spring Number 

Maxwell, 1976) (Type Section) Fifteen Draw 

Sec 34, T8N, R7W Sec 26, T1 ON, R6W Sec 21, T1 ON, R5W

43 mi 

(69 km)

14 mi 

(22.5 km) (5km

24% mi 

(39.5 km)

Sec 

SAN YSIDRO AREA LA VENTANA-CUBA AREA 

9 10 

Zia Holy 11 12 13 14 

Indian Ghost Arroyo Elk Arroyo de Campo El 1 5 

1 Resv. Spring Semilla Spring los Pinos Deseo Cuba 

Bernalillito 8 Sec 26 Sec 1 1 Sec 25 Sec 36 Sec 1 1 Sec 24 Sec 1 

Mesa San Ysidro T1 7N T17N T1 8N T1 9N T1 9N T20N /T20N 

Sec27,T15N, R1W Sec28,T15N, R1W R1W R1W R1W R1W R1W R1W /R1W Sec13

Jmj

-z - 

Jmb

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5% mi

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 * z * - ^^ '

12 mi

(9 km) (19.5 km)

2% mi 

(4km)

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/

3% mi

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5% mi

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4 mi

 ^

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\ 

t

/

2 mi

(5.5 km) (9 km) (6.5 km) (7 km) (3 km|

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16'/2 mi

(26.5 km) 

EXPLANATION

Encinal Canyon Member of Dakota Sandstone 

Burro Canyon Formation

Jackpile Sandstone Member of Morrison 

Formation

Brushy Basin Member of Morrison Formation 

Zuni Sandstone

Figure C2. Schematic cross section A-A' showing uppermost Jurassic and lowest Cretaceous rocks in southeastern part of San 

where uncertain. Datum is the base of the Oak Canyon Member of the Dakota Sandstone. See figure C1 for location.
light-grayish-green mudstone, clay stone, and siltstone 

which alternate with thinner lenses of sandstone, conglomeratic sandstone or conglomerate. The mudstone 

unit also contains scattered thin limestone beds and thin, 

well-indurated, orange and grayish-green zeolitic beds. 

This unit is considered to be Late Jurassic in age in the 

southeastern San Juan Basin (Santos, 1970).

The Jackpile Sandstone Member of the Morrison 

Formation locally overlies the Brushy Basin Member in 

the southeastern part of the basin. The Jackpile is a white, 

kaolinitic, fine- to medium-grained, crossbedded fluvial 

sandstone with thin interbeds of pale-green mudstone and 

siltstone. It interfingers with the underlying Brushy Basin 

mudstones in the Laguna area and is generally thought 

to be of Late Jurassic age, although no age-diagnostic 

fossils have been found in the unit.

Sandstone similar to the Jackpile rests on Brushy 

Basin mudstones in the northern part of the region in the 

Chama basin. This sandstone is considered probable 

Burro Canyon Formation by most recent authors 

(McPeek, 1965; Saucier, 1974; Ridgley, 1977, 1979). The 

Burro Canyon Formation in the Chama basin is a tan, 

kaolinitic, fine- to medium-grained, fluvial, crossbedded, 

locally conglomeratic sandstone with interbeds of green 

and red clay stone and siltstone. It generally has a sharp 

basal contact and may be disconformable with the underlying Brushy Basin (Ridgley, 1979). No age-diagnostic 

fossils have been found in the unit in the Chama basin. 

However, it may be Early Cretaceous in age if it correlates 

with the Burro Canyon Formation at its type locality in 

southwestern Colorado (Stokes, 1952; Craig, 1981), about 

150 miles (240 km) to the northwest.

The Jackpile and the Burro Canyon are lithologically similar fluvial sandstone units that occupy the same 

stratigraphic position between the Brushy Basin mudstone 

unit and the Dakota Sandstone; the relationship between 

these two units is uncertain (Owen and Siemers, 1977). 

Probably the most obvious difference between the Burro 

Canyon in the Chama basin and the Jackpile Sandstone 

Member in the Laguna area is that the Burro Canyon is 

conglomeratic. Conglomeratic beds occur in the JackpileBurro Canyon interval as far south as the La Ventana 

area (Saucier, 1974) and consist of varicolored chert and 

quartzite and abundant tan chert pebbles. The tan chert 

is generally nonporous and has a somewhat earthy appearance. It is commonly rounded; however, it may be 

fragmented. Broken surfaces are generally even or have 

conchoidal fractures.

DAKOTA SANDSTONE 

Encinal Canyon Member

The Encinal Canyon Member tends to form lightgray or light-brown cliffs beneath ledge-forming sandstone or slope-forming shale units in the overlying parts

of the Dakota. It is generally well exposed throughout 

most of the study area. However, because of faulting, exposures are scattered and scarce north of Cuba, N. Mex., 

along the western flank of the Nacimiento uplift. The type 

section is in Encinal Canyon, approximately 4 miles northwest of Laguna, N. Mex. (fig. Cl), and is shown in figure 

C3. The type section is illustrated in figure C4 and the 

measured section is given at the end of the report.

The Encinal Canyon Member is a very light-gray 

or light-brown, quartzose, fine- to medium-grained, 

cross-stratified sandstone unit that is commonly conglomeratic, especially at its base. Typically it has a clayrich or calcareous cement or matrix but locally it is 

cemented with silica, especially near its top. The Encinal 

Canyon commonly contains black disseminated carbonaceous debris and thin, discontinuous lenses of gray 

mudstone. Thin beds of coal are also locally present. 

Typically the Encinal Canyon is trough crossbedded but 

it also has tabular planar crossbeds and, more rarely, 

horizontal or nearly horizontal laminations at some 

localities. Massive and horizontally stratified conglomeratic beds are interbedded with poorly defined crossbedded sandstone beds locally on the eastern side of the 

basin north of Cuba along the Nacimiento uplift.

Clasts in the conglomeratic beds of the Encinal 

Canyon consist of varicolored chert and quartzite and 

abundant and distinctive white chalky chert. This chalky 

chert is generally finely porous and pulverulent in appearance. Although the white chalky chert pebbles may 

be rounded, they are more commonly fragmented and 

have highly irregular shapes because of uneven, rough 

fracture surfaces. Their brilliant white color and angular 

shapes give the chalky chert pebbles a distinctive appearance, especially on a broken surface. The chert clasts 

generally range in size from very coarse sand to large pebbles, but rare chert boulders as large as 1.5 feet (0.5 m) 

are seen locally. The chalky chert is one of the most 

distinctive characteristics of the Encinal Canyon Member, 

and its recognition aids in the correlation of this important unit throughout the region.

The Encinal Canyon Member ranges from 0 to 75 

feet (23 m) in thickness in most of the region. However, 

it may be as thick as 100 ft (30 m) along the Nacimiento 

uplift north of Cuba on the eastern flank of the San Juan 

Basin (fig. C2). The Encinal Canyon Member occurs at 

least as far south and west as El Morro National Monument, and it extends at least as far northeast as Mesa 

Laguna in the Chama basin (fig. C2). In Poison Canyon, 

north of Grants, N. Mex., a thin, lithologically similar 

sandstone unit containing thin coaly interbeds may be 

equivalent to the the Encinal Canyon Member. West and 

northwest of Grants the Encinal Canyon along with the 

Oak Canyon and other members that make up the lower 

part of the Dakota Sandstone in the southeastern San 

Juan Basin is undifferentiated and is included in the 

main body of the Dakota Sandstone (Landis and others,

62 Jurassic and Cretaceous of Colorado Plateau
' *^ f-m- , * . s, 1 w 

f4^^^^^l^"? ' ^-*

Figure C3. Type section of the Encinal Canyon Member of the Dakota Sandstone (sec. 26, T. 10 N., R. 6 W.) on the east side 

of the road in Encinal Canyon northwest of Laguna, N. Mex. Jmj, Jackpile Sandstone Member of the Morrison Formation; Kdec, 

Encinal Canyon Member of the Dakota Sandstone; Kdoc, Oak Canyon Member of the Dakota Sandstone.

1973). Encinal Canyon equivalents probably occur at the 

base of the main body of the Dakota in the western and 

northern San Juan Basin.

The base of the Encinal Canyon is a scour surface 

with tens of feet of local relief, and its lowest beds locally contain rip-up clasts of sandstone or mudstone 

derived from the underlying units. Locally this surface 

cuts completely through underlying units; for example, 

it cuts through the Jackpile Sandstone Member in the subsurface in the L-Bar mine area northeast of Laguna 

(H. F. Bartlett, Sohio Petroleum Co., oral commun., 

1983) and has probably scoured through the Jackpile and 

the Burro Canyon Formation in outcrops along the Nacimiento uplift north of Cuba (fig. C2). The scour surface 

at the base of the Encinal Canyon truncates progressively 

older units from north to south (fig. C2) and is considered 

to be a regional unconformity. In the southern part of 

the area the unconformity is slightly angular but has less 

than 1 degree of angular discordance.

The Jackpile Sandstone Member and the Burro 

Canyon Formation, which underlie the Dakota in most 

of the study area, are both fluvial sandstone units similar

to the Encinal Canyon Member. They differ from the Encinal Canyon, however, in that they are devoid of black 

carbonaceous debris and they contain green, greenishgray, or red mudstone rather than gray mudstone or shale 

interbeds. Conglomeratic beds in the Jackpile-Burro Canyon interval in the northern part of the region are also 

different from those found in the Encinal Canyon. Conglomerate in this interval is characterized by abundant 

earthy chert clasts, whereas conglomerate in the Encinal 

Canyon is characterized by abundant chalky chert clasts. 

Jackpile-Burro Canyon clasts are generally tan and well 

rounded, and they tend to break with conchoidal fracture surfaces. Encinal Canyon clasts, however, are commonly white, are highly irregular in shape, and break with 

rough and uneven fracture surfaces. Chert clasts larger 

than 2 inches (5 cm) appear to be restricted to the Encinal Canyon Member.

Oak Canyon Member

At its type section in the Laguna area, the Oak Canyon is divided into two parts, a lower part consisting

Encinal Canyon Member of Dakota Sandstone, New Mexico 63
Series

Lower 

Cretaceous

Upper 

Jurassic

Formation

Dakota 

(part)

Morrison 

(part)

Member

Oak 

Canyon 

(part)

Encinal 

Canyon

Jackpile 

Sandstone

/ Brushy \ 

Basin

Lithology

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 ^ r f^.

^s^

4^m-:\

!

Thickness

>13.8ft 

(4.2 m)

30.5ft 

(9.3 m)

>29.0ft 

(8.8 m)

Description

Shale, dark gray

Sandstone, brown, fine- to medium-grained, 

crossbedded

Sandstone, light-gray to brown, fine- to 

medium-grained; locally contains 

conglomeratic beds, thin gray 

mudstone beds and carbonaceous 

debris; crossbedded

Sandstone, white, fine- to mediumgrained; contains thin green 

mudstone beds; crossbedded

Mudstone, green

EXPLANATION

Sandstone

Siltstone

Mudstone

Conglomeratic sandstone or conglomerate

Trough crossbedding

Herringbone crossbedding

Woody plant impressions

Figure C4. Columnar section of the Encinal Canyon Member of the Dakota Sandstone at the 

type section in Encinal Canyon, southeastern San Juan Basin, New Mexico.

largely of paralic and marginal marine sandstone and 

shale with some possible fluvial sandstone beds, and an 

upper part consisting of beds of marine shale (Landis and 

others, 1973). Sandstone is only locally present at the base 

of the Oak Canyon. Northeast of Laguna, in the San 

Ysidro and Holy Ghost Spring areas, the paralic sandstone and shale appear to be absent, and marine shales 

lie directly on the pre-Dakota rocks.

The contact between the Oak Canyon Member and 

the underlying fluvial sandstone of the Encinal Canyon 

Member of the Dakota Sandstone is a sharp, planar surface. It is placed where the light-gray to light-brown, 

mostly clay-rich or calcite-cemented, cliff-forming sandstone units of the Encinal Canyon Member are in contact with the overlying, generally grayish-orange, 

silica-cemented sandstone and black shale of the Oak

64 Jurassic and Cretaceous of Colorado Plateau
Canyon Member. Where present, sandstone beds at the 

base of the Oak Canyon form a resistant, slightly overhanging ledge above the Encinal Canyon. In the past the 

Oak Canyon was thought to be the basal unit of the 

Dakota. Recognition of Encinal Canyon has resulted in 

a redefinition of the Dakota contact in the southeastern 

San Juan Basin. The base of the Encinal Canyon is a 

regional unconformity, and the base of the Oak Canyon, 

previously thought to be a major unconformity, is now 

considered to be only a discontinuity resulting from minor 

reworking of previously deposited Dakota strata as the 

sea transgressed across the area.

PREVIOUS CORRELATIONS

In the past, the sandstone beds that compose the 

Encinal Canyon were included in the Dakota Sandstone, 

in the Jackpile Sandstone and Brushy Basin Members, 

and in the Burro Canyon Formation. The probable reason 

for miscorrelation of Encinal Canyon beds with the Jackpile and the Burro Canyon is that all of these fluvial units 

closely resemble one another physically and lithologically. 

In addition, the irregular surface at the base of the Encinal Canyon was probably misidentified as a local scour 

surface. Irregular scour surfaces of this type are fairly 

common in the Jackpile and the Burro Canyon. Some 

workers have recognized local but discrete fluvial sandstone units at the base of the Dakota Sandstone at various 

places throughout the study area, and these are included 

in the Encinal Canyon. Maxwell (1976) mapped a discontinuous "lower fluviatile conglomerate and sandstone" 

unit at the base of the Dakota in the Acoma area. Nash 

and Kerr (1966) reported a thin unit of "reworked" 

Jackpile that truncates sandstone-hosted uranium ore in 

the Jackpile at the Jackpile mine near Laguna. This unit 

probably is part of the Encinal Canyon Member. 

Reutschilling (1973, p. 46) informally called Encinal 

Canyon equivalents the "unnamed basal sandstone" in 

the San Ysidro area. Owen and Siemers (1977) recognized 

fluvial sandstone units at the base of the Dakota on the 

east side of the basin in the San Ysidro (Ojito Springs) 

area, but they included these units in the Oak Canyon 

Member of the Dakota Sandstone. The lowest unit of the 

Dakota Sandstone in the Chama basin is a conglomeratic 

fluvial sandstone 30 to 60 ft (9 to 18 m) thick (Grant and 

Owen, 1974; Ridgley, 1977, 1979). Based on stratigraphic 

position and similarity of lithology, particularly the 

presence of white chalky chert clasts, the Encinal Canyon 

and lower fluvial unit of the Dakota Sandstone in the 

Chama basin probably are correlative. Correlation of units 

between the Chama basin and the region farther south 

along the eastern flank of the San Juan Basin is difficult 

because of faulting and scarcity of outcrops along the west 

side of the Nacimiento uplift north of Cuba.

AGE

Palynomorphs from the Encinal Canyon Member 

collected by R. H. Tschudy (U.S. Geological Survey, oral 

commun., 1983) in the Jackpile mine near Laguna indicate that it is earliest Late Cretaceous (early Cenomanian) in age. A very similar age (Cenomanian) has been 

assigned to the overlying Oak Canyon Member of the 

Dakota Sandstone based on fossil mollusks (Cobban, 

1977) and palynomorphs (R. H. Tschudy cited in Cobban, 1977). The presence of carbonaceous material in the 

Encinal Canyon also suggests its age is close to that of 

overlying Dakota strata. Carbonaceous debris is common 

in the Dakota Sandstone, whereas it is rare or absent from 

Lower Cretaceous and Upper Jurassic rocks in the region. 

Moreover, regional stratigraphic relationships indicate 

that the Encinal Canyon is part of the Dakota. Its basal 

contact is an unconformity that truncates Middle and Upper Jurassic and Lower Cretaceous rock units, whereas 

its upper contact, which separates it from the overlying 

Oak Canyon Member, is only a discontinuity that formed 

as the sea transgressed across the region.

DEPOSITIONAL HISTORY

The large amount of relief (several tens of feet) on 

the base of the Encinal Canyon indicates that the subDakota erosion surface in the southeastern San Juan 

Basin formed as a surface of degradation. This surface 

probably was the result of a lowering of base level that 

postdated deposition of the Burro Canyon Formation. 

Easterly flowing streams, which carved the paleo-land 

surface preserved at the unconformity, scoured all the 

underlying units and in some places cut completely 

through the Jackpile Sandstone Member or the Burro 

Canyon Formation. A main trunk of this stream system 

probably passed through the area north of Cuba on the 

east flank of the San Juan Basin, where the Encinal 

Canyon Member is thickest (fig. C2). Encinal Canyon 

streams began to aggrade and the member was deposited 

as backfill, first in the lower part of the river valleys and 

later in topographically higher areas, in response to a rise 

in base level that was probably associated with the encroachment of the Dakota sea into the region. Most of 

the Encinal Canyon is interpreted to be fluvial in origin. 

However, locally, near its top, the Encinal Canyon may 

include some paralic beds deposited during minor stillstands as the sea intermittently transgressed across the 

area. The top of the Encinal Canyon is placed just below 

the flat surface that formed as the Late Cretaceous sea 

advanced over the area. During this marine transgression 

waves and currents reworked some of the previously 

deposited Encinal Canyon Member and truncated the 

tops of high areas of bedrock intervening between the

Encinal Canyon Member of Dakota Sandstone, New Mexico 65
valleys (composed of Lower Cretaceous or older rocks). 

After the shoreline advanced across the region, the Oak 

Canyon was deposited in paralic, marginal marine, and 

offshore marine environments.

CONCLUSIONS

The Dakota Sandstone, in the southeastern San 

Juan Basin, was thought to consist of marginal marine 

and marine sedimentary facies, and its base was thought 

to be an erosional surface formed during a marine transgression. The newly recognized Encinal Canyon Member 

is important because it changes our understanding of the 

stratigraphy of this interval and, consequently, of the 

depositional history in the region during earliest Late 

Cretaceous time. The member is a mappable fluvial unit 

lying beneath strata previously thought to be at the base 

of the Dakota. Significant relief on the base of the Encinal Canyon indicates that the sub-Dakota erosion surface is a result of regional degradation by streams flowing 

over the area. This surface is a major unconformity that 

truncates Lower Cretaceous and Upper Jurassic formations. However, the contact between the Encinal Canyon 

and the overlying Oak Canyon is only a discontinuity 

formed as the sea transgressed across the region.

MEASURED SECTION

Type section of the Encinal Canyon Member of the 

Dakota Sandstone

[Measured on east side of road in Encinal Canyon approximately 4 miles 

(6.4 km) northwest of Laguna, N. Mex. (fig. Cl), in the W!/2 NE 1/* 

sec. 26, T. 10 N., R. 6 W., Laguna V'/i-minute quadrangle, Valencia 

County, N. Mex.]

Thickness 

Feet Meters

Top of measured section. 

Dakota Sandstone (part): 

Oak Canyon Member (part):

28. Sandstone, light-brown (5YR5/6) to 

dusky-brown (5YR2/2), fine- to coarsegrained, moderately well sorted, subrounded; has silica cement (quartz overgrowths); quartzose with less than 1 

percent red accessory grains; tabular herringbone crossbeds in sets 1-2 ft (0.3- 

0.6 m) thick contain graded foresets Yi 

to 1 in. (1.3 to 2.5 cm) thick that exhibit 

opposing dip directions ............ 9.0 2.7

Thickness 

Feet Meters

27. Sandstone, light-brown (5YR5/6) to 

dusky-brown (5YR2/2), fine- to 

medium-grained, moderately well sorted, 

subrounded; has silica cement (quartz 

overgrowths); quartzose with less than 

1 percent black accessory grains; wedgeshaped crossbeds in sets 6-10 in. 

(15.2-25.4 cm) thick ............... 2.8 0.9

26. Sandstone, light-brown (5YR5/6) to 

dusky-brown (5YR2/2), fine- to 

medium-grained, moderately well sorted, 

subrounded; has silica cement (quartz 

overgrowths); quartzose with less than 

1 percent pink and black accessory 

grains; wedge-shaped crossbed sets approximately 2 ft (0.6 m) thick contain 

graded foresets 1-3 in. (2.5-7.6 cm) 

thick; forms an overhanging resistant 

ledge above the Encinal Canyon .... 2.0 0.6

Total thickness of the Oak Canyon

Member (part) .....................13.8 4.2

Encinal Canyon Member:

25. Sandstone, very light gray (N8) and 

moderate-reddish-orange (10R6/6) to 

moderate-reddish-brown (10R4/6), fineto medium-grained, moderately well 

sorted, subrounded; has silica (quartz 

overgrowths), calcite (minor), and clay 

(minor) cement; quartzose with less than 

1 percent black accessory grains; trough 

crossbeds in sets as thick as 8 in. (20.3 

cm) contain graded foresets 1-2 in. 

(2.5-5.1 cm) thick ................. 1.5 0.5

24. Sandstone, moderate-reddish-orange 

(10R6/6) to moderate-reddish-brown 

(10R4/6), fine-grained, well sorted, 

subrounded; has silica (quartz overgrowths), calcite (minor), and clay 

(minor) cement; quartzose with less than 

1 percent black accessory grains; massive 2.0 0.6

23. Shale, light-brownish-gray (5 YR6/7) and

grayish-black (N2) ................ 2.0 0.6

22. Covered interval .................. 5.0 1.5

21. Sandstone, very light gray (N8) to lightgray (N7) and moderate-yellowishbrown (10YR5/4), very fine- to mediumgrained, moderately well sorted, 

subangular; has silica (quartz overgrowths) and calcite (minor) cement;

66 Jurassic and Cretaceous of Colorado Plateau
Thickness 

Feet Meters

Thickness 

Feet Meters

Dakota Sandstone (part) Continued 

Encinal Canyon Member Continued

quartzose; woody plant fragment impressions along the basal surfaces of 

some troughs; broad trough crossbeds in 

sets approximately 1 ft (0.3 m) thick 5.0

15.

1.5

20. Shale, medium-gray (N5) to mediumdark-gray (N4) .................... 0.5 0.2

19. Sandstone, light-gray (N7), darkyellowish-brown (10YR4/2), and duskyyellowish-brown (10YR2/2), fine- to 

medium-grained, well-sorted, subrounded; has silica (quartz overgrowths) 

and calcite (minor) cement; quartzose 

with less than 1 percent black accessory 

grains; limonite and hematite staining 

and woody plant fragment impressions 

concentrated along basal surface; faintly crossbedded .................... 1.5 0.5

18. Sandstone, pebbly, very light gray (N8), 

dark-yellowish-orange (10YR6/6), and 

dark-reddish-brown (10R3/4), mediumto coarse-grained (with some pebbles as 

long as 1A in.; 0.6 cm), poorly sorted, 

subangular, clay-cemented, quartzose; 

pebbles include white angular chalky 

chert; locally contains woody plant fragment impressions; trough crossbeds approximately 1 ft (0.3 m) thick contain 

graded foresets 4-6 in. (10.2-15.2 cm) 

thick ............................ 2.0 0.6

17. Sandstone, very light gray (N8), darkyellowish-orange (10YR6/6), and 

moderate-reddish-brown (10R4/6), fineto medium-grained, moderately to 

moderately well sorted, subrounded, 

clay- and calcite- (minor) cemented, 

quartzose; trough crossbeds in sets 1-2 

ft (0.3-0.6 m) thick contain graded 

foresets as thick as '/z in. (1.3 cm); 

limonite stain and woody plant fragment 

impressions concentrated along basal 

scour surfaces of trough cross sets .. 3.5 1.1

16. Sandstone, very light gray (N8), darkyellowish-orange (10YR6/6), and 

moderate-reddish-brown (10R4/6), fineto coarse-grained, moderately sorted, 

subrounded, calcite- and clay-cemented; 

quartzose with less than 1 percent green 

and black accessory grains; also contains 

thin carbonaceous laminations and

14.

13.

abundant disseminated carbonaceous 

plant debris; poorly defined trough 

crossbeds(?) in sets approximately 1 ft 

(0.3 m) thick contain graded foresets 

/2-1 in. (1.3-2.5 cm) thick ......... 2.5 0.8

Sandstone, pebbly, dark-yellowishorange (10YR6/6), and very light gray 

(N8), fine- to coarse-grained (with pebbles as long as 1A in.; 1.3 cm), poorly 

sorted, subangular, clay-cemented; pebbles include clasts of white, angular to 

subangular, chalky chert and subrounded black and gray smooth chert as 

long as 1A in. (0.6 cm), and very light 

gray siltstone clasts as long as l/2 in. (1.3 

cm); contains thin carbonaceous laminations; massive .................... 0.5 0.2

Sandstone, very light gray (N8), darkyellowish-orange (10YR6/6), and 

grayish-orange-pink (5YR7/2), fine- to 

coarse-grained, moderately well sorted, 

subrounded, clay- and silica- (minor) 

cemented, quartzose; contains occasional thin laminations of carbonaceous 

material; faint trough crossbeds in sets 

as thick as 8 in. (20.3 cm) contain graded 

foresets 1-4 in. (2.5-10.2 cm) thick; 

scoured basal surface .............. 3.5 1.1

Pebble conglomerate, sandy, darkyellowish-orange (10YR6/6) and very 

light gray (N8), poorly sorted, subangular, matrix-supported, clay-cemented, 

quartzose; matrix is fine to coarse 

grained; pebbles are as long as 3A in. (1.9 

cm) and include abundant white, angular 

chalky chert clasts and black, gray, and 

white subrounded smooth chert clasts; 

locally contains woody plant fragment 

impressions and limonite staining near 

or along the basal scour surface; poorly 

defined crossbedding .............. 1.0 0.3

Total thickness of the Encinal Canyon

Member ........................... 30.5 9.3

Morrison Formation (part): 

Jackpile Sandstone Member:

12. Sandstone, white (N9; locally stained 

dark yellowish orange, 10YR6/6), 

fine-grained, moderately well sorted to 

well sorted, subrounded, claycemented, quartzose, massive ..... 5.0 1.5

Encinal Canyon Member of Dakota Sandstone, New Mexico 67
Thickness 

Feet Meters

Morrison Formation (part) Continued 

Jackpile Sandstone Member Continued

11. Silty mudstone, light-brownish-gray 

(5YR6/1) and grayish-yellow-green 

(5GY7/2); locally fissile .. ......... 0.5 0.2

10. Sandstone, white (N9; locally stained 

dark yellowish orange, 10YR6/6), finegrained, moderately well sorted to well 

sorted, subrounded, clay-cemented, 

massive .......................... 7.0 2.1

9. Sandstone, grayish-orange (10YR7/4) 

and very pale orange (10YR8/2), fineto medium-grained, moderately well 

sorted, subrounded, clay-cemented; 

quartzose with less than 1 percent black 

accessory grains; massive ........... 5.0 1.5

8. Sandstone, silty, white (N9) and 

moderate-reddish-orange (10R6/6); silt 

to medium-grained sand; poorly sorted 

to moderately sorted; trace of calcite cement; contains sandy siltstone rip-up 

clasts as long as Vi in. (1.3 cm) ..... 0.5 0.2

7. Siltstone, sandy, light-brownish-gray 

(5YR6/1) and light-greenish-gray 

(5G8/1); silt to very fine grained sand; 

moderately well sorted; trace of calcite 

cement; quartzose ................. 1.5 0.5

6. Sandstone, moderate-orange-pink 

(5YR8/4), medium- to coarse-grained 

(with a few scattered pebbles as long as 

l/4 in.; 0.6 cm), poorly to moderately 

sorted, subrounded, clay-cemented; 

quartzose with less than 1 percent black 

accessory grains; granules and pebbles 

include quartz clasts and gray chert 

clasts; faint trough crossbeds contain 

graded foresets 2-3 in. (5.1-7.6 cm) thick 4.0 1.2

5. Sandstone, pebbly, moderate-orangepink (5YR8/4), medium- to coarsegrained (with some pebbles as long as !/2 

in.; 1.3 cm), poorly sorted, claycemented, quartzose; granules and 

pebbles include clasts of quartz, gray 

chert, and white rhyolite; trough crossbedded; scoured basal surface ...... 1.0 0.3

4. Sandstone, moderate-orange-pink 

(5YR8/4), medium- to coarse-grained, 

moderately well sorted, subrounded, 

clay-cemented, quartzose; faint

Thickness 

Feet Meters

crossbeds contain graded foresets 1-2 in.

(2.5-5.1 cm) thick ................. 2.5 0.8

3. Sandstone, very pale orange (10YR8/2), 

medium-grained, moderately well sorted, 

subrounded, clay-cemented, quartzose, 

massive .......................... 1.0 0.3

2. Sandstone, pebbly, very pale orange 

(10YR8/2), medium- to coarse-grained 

(with some pebbles as long as 3/4 in.; 1.9 

cm), poorly sorted, subrounded, calciteand clay- (minor) cemented; quartzose 

with less than 1 percent pink and black 

accessory grains; pebbles include clasts 

of tan, light-gray, and black chert, white 

rhyolite, and green mudstone; massive 1.0 0.3

Total thickness of the Jackpile

Sandstone Member ............... 29.0 8.9

Brushy Basin Member (part):

1. Mudstone, grayish-yellow-green

(5GY7/2); forms slope .......... Not measured.

REFERENCES CITED

Cobban, W. A., 1977, Fossil mollusks of the Dakota Sandstone 

and intertongued Mancos Shale of west-central New Mexico, in Fassett, J. E., ed., Guidebook of San Juan Basin 

III, northwestern New Mexico: New Mexico Geological 

Society, 28th Annual Field Conference, p. 213-220.

Craig, L. C., 1981, Lower Cretaceous rocks, southwestern Colorado and southeastern Utah, in Wiegand, D. L., ed., 

Geology of the Paradox Basin: Rocky Mountain Association of Geologists Field Conference, 1981, p. 195-200.

Grant, Keith., and Owen, D. E., 1974, The Dakota Sandstone 

(Cretaceous) of the southern part of the Chama basin, New 

Mexico A preliminary report on its stratigraphy, paleontology, and sedimentology, in Siemers, C. T., ed., Silver 

Anniversary Guidebook Ghost Ranch, central-northern 

New Mexico: New Mexico Geological Society, 25th Annual Field Conference, p. 239-249.

Landis, E. R., Dane, C. H., and Cobban, W. A., 1973, 

Stratigraphic terminology of the Dakota Sandstone and 

Mancos Shale, west-central New Mexico: U.S. Geological 

Survey Bulletin 1372-J, 44 p.

Maxwell, C. H., 1976, Geologic map of the Acoma Pueblo 

quadrangle, New Mexico: U.S. Geological Survey Geologic 

Quadrangle Map GQ-1298.

McPeek, L. A., 1965, Dakota-Niobrara (Cretaceous) stratigraphy and regional relationships, El Vado area, Rio Arriba 

County, New Mexico: Mountain Geologist, v. 2, no. 1, 

p. 23-34.

68 Jurassic and Cretaceous of Colorado Plateau
Molenaar, C. M., 1977, Stratigraphy and depositional history of Upper Cretaceous rocks of the San Juan Basin 

area, New Mexico and Colorado, with a note on economic resources in Fassett, J. E., ed., Guidebook of San 

Juan Basin III, northwestern New Mexico: New Mexico 

Geological Society, 28th Annual Field Conference, 

p. 159-166.

Nash, J. T., and Kerr, P. F., 1966, Geologic limitations of the 

age of uranium deposits in the Jackpile sandstone, New 

Mexico: Economic Geology, v. 61, p. 1283-1290.

Owen, D. E., 1973, Depositional history of the Dakota Sandstone, San Juan Basin area, New Mexico, in Fassett, J. E., 

ed., Cretaceous and Tertiary rocks of the southern Colorado Plateau: Four Corners Geological Society Memoir, 

p. 37-51.

Owen, D. E. and Siemers, C. T., 1977, Lithologic correlation 

of the Dakota Sandstone and adjacent units along the 

eastern flank of the San Juan basin, New Mexico, in 

Fassett, J. E., ed., Guidebook of San Juan Basin III, northwestern New Mexico: New Mexico Geological Society, 28th 

Annual Field Conference, p. 179-183.

Reutschilling, R. L., 1973, Structure and stratigraphy of the

San Ysidro quadrangle, Sandoval County, New Mexico: 

Albuquerque, N. Mex., University of New Mexico M.S. 

thesis, 73 p.

Ridgley, J. L., 1977, Stratigraphy and depositional environments of Jurassic-Cretaceous sedimentary rocks in the 

southwest part of the Chama basin, New Mexico, in 

Fassett, J. E., ed., Guidebook of San Juan Basin III, northwestern New Mexico: New Mexico Geological Society, 28th 

Annual Field Conference, p. 153-158.

_____1979, Preliminary geologic map of the Arroyo Del Agua 

quadrangle, Rio Arriba County, New Mexico: U.S. Geological Survey Open-File Report 79-657.

Santos, E. S., 1970, Stratigraphy of the Morrison Formation 

and structure of the Ambrosia Lake district, New Mexico: 

U.S. Geological Survey Bulletin 1272-E, 30 p.

Saucier, A. E., 1974, Stratigraphy and uranium potential of 

the Burro Canyon Formation in the southern Chama Basin, 

New Mexico: New Mexico Geological Society 25th Annual 

Field Conference, p. 211-219.

Stokes, W. L., 1952, Lower Cretaceous in the Colorado Plateau: 

American Association of Petroleum Geologists Bulletin, 

v. 36, no. 9, p. 1766-1776.

Encinal Canyon Member of Dakota Sandstone, New Mexico 69

* U.S. GPO: 1988/673-047/86,015



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