The Base of the Upper 

Jurassic Morrison 

Formation in 

East-Central Utah 

U.S. GEOLOGICAL SURVEY BULLETIN 1561 

The Base of the Upper 

Jurassic Morrison 

Formation in 

East-Central Utah 

By ROBERT B. O'SULLIVAN 

U.S. GEOLOGICAL SURVEY BULLETIN 1561 

A review of the first description of 

the base of the Morris on Formation 

in east-central Utah and some later 

modifications 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1984 
UNITED STATES DEPARTMENT OF THE INTERIOR 

WILLIAM P. CLARK, Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

Library of Congress Cataloging in Publication Data 

O'Sullivan, Robert Brett, 1923-

The base of the Upper Jurassic Morrison Formation in east-central Utah. 

(Geological Survey bulletin ; 1561) 

Bibliography: 17 p. 

Supt. of Docs. no.: I 19.3:1561 

1. Geology, Stratigraphic-Jurassic. 2. Geology-Utah. I. Title. II. Series. 

QE75.B9 no. 1561 [QE684] 557.3 s [551.7'66] 84-600219 

For sale by the Branch of Distribution 

U.S. Geological Survey 

604 South Pickett Street 

Alexandria, VA 22304 
Abstract .. 

Introduction 

Stratigraphy 

Base of Morrison Formation 

Previous usage 

Present usage 

References cited . . 

CONTENTS 

ILLUSTRATIONS 

Page 

1 

1 

3 

7 

7 

11 

16 

Page 

FIGURE 1. Index map of east-central Utah and adjacent parts of Colorado . 2 

2. Some Jurassic nomenclature used in east-central Utah 4 

3--5. Photographs showing: 

3. Jurassic rocks at Summerville Point . . . . . . . . 8 

4. Jurassic rocks along White Wash at type locality of Salt Wash 

Member of Morrison Formation . . . . . 9 

5. Jurassic rocks at section near Duma Point . . . . . . . . 12 

III 

THE BASE OF THE UPPER JURASSIC 

MORRISON FORMATION IN 

EAST -CENTRAL UTAH 

By ROBERT B. O'SULLIVAN 

ABSTRACT 

The contact between the Middle Jurassic San Rafael Group and the overlying Upper 

Jurassic Morrison Formation in the San Rafael Swell of Utah was originally described 

as an unconformity that is both angular and erosional. This surface is now recognized 

as the J-5 unconformity, a widespread break in Jurassic rocks of the Western Interior 

of the United States. In east-central Utah, a sequence of channel sandstones-the Salt 

Wash Member of the Morrison Formation-lies 5-30 m above the J-5 unconformity. 

The basal contact of the Morrison has also been placed at the base of the Salt Wash 

Member, or somewhere between the Salt Wash and the J-5 unconformity. In east-central Utah, the base of the Morrison Formation is here placed at the widespread and 

consistent J-5 unconformity. 

INTRODUCTION 

The Morrison Formation of Late Jurassic age is present over large 

areas of east-central Utah. Throughout the region, the Morrison lies 

upon rocks of the San Rafael Group of Middle Jurassic age. Investigations by the author in eastern Utah indicate that geologists have 

selected different horizons as the base of the Morrison Formation. 

In introducing the term Morrison into the San Rafael Swell (fig. 1), 

Gilluly (1929, p. 111) described the lower contact as an unconformity 

that is both angular and erosional. In some areas the lower contact 

of the Morrison has been placed by many geologists at the base of 

a sequence of channel sandstones-the Salt Wash Member of the Morrison-which lies as much as 30 m above the unconformity observed 

by Gilluly. The base of the Morrison has also been placed by many 

geologists somewhere between the Salt Wash Member and the unconformity noted by Gilluly. This report reviews Gilluly's description of 

the base of the Morrison in east-central Utah and some later modifications. In addition, the basal contact of the Morrison is here placed 

at the unconformity described by Gilluly. The lithology and lateral 

stratigraphic relations of the various rock units above and below the 

unconformity are only briefly discussed in this report; more complete 

details may be found in publications cited throughout the text. 
2 JURASSIC MORRISON FORMATION, EAST-CENTRAL UTAH 

iiJ Woodside 

s 0 

X Cl) XSu_mmerville 

Cedar POint 

Mountain 

39° jjf 

Ll 

: 

Monticello 0 

ARIZONA 

PHOENIX 

0 

1 go 

Grand 

Junction 

v"Vco ~,() 

~~ 0 

Gateway >$>~ 

50 KILOMETERS 

Glenwood 

Springs 

0 

DENVER 

0 

COLORADO 

SAN JUAN 

BASIN SANTA FE 0 

NEW MEXICO 

0 200 KILOMETERS L-J____j 

~ ~~ ~&Montrose 

FIGURE 1. -Index map of east-central Utah and adjacent parts of Colorado. 

The unconformity at the base of the Morrison Formation is one 

of several found in Jurassic rocks of east-central Utah. This surface 

that separates the Morrison Formation from the San Rafael Group 

is termed the J-5 unconformity and is recognized over much of the 

Western Interior of the United States. The unconformities found in 
STRATIGRAPHY 3 

Jurassic rocks and their regional stratigraphic significance are discussed in a report by Pipiringos and O'Sullivan (1978). 

George N. Pipiringos first introduced me to the Jurassic rocks of 

the San Rafael Swell in 1964. Subsequently, he shared with me his 

extensive knowledge of the Jurassic in the Western Interior, and contributed materially to my understanding of the contact relations described herein. 

STRATIGRAPHY 

The San Rafael Group, its subdivisions, and the Morrison Formation 

were established in east-central Utah in a press memorandum and 

four reports released over a period of 6 years. The San Rafael Group 

was defined in a report by Gilluly and Reeside (1928, p. 73) to include 

in ascending order the Carmel Formation, Entrada Sandstone, Curtis 

Formation, and Summerville Formation. The names and lithologies 

of the divisions of the San Rafael Group and the Morrison Formation 

were described in a U.S. Department of the Interior Memorandum 

for the Press, number 6064, dated March 30, 1926. A description of 

the San Rafael Group, its divisions, and the Morrison Formation and 

a diagram showing stratigraphic relations were included in a report 

by Baker and others (1927). The type sections of the Entrada Sandstone and the Curtis and Summerville Formations were given by Gilluly (1929, p. 104-110). The Carmel Formation was later defined and 

described by Gregory and Moore (1931, p. 72-76). 

Geologic investigations in the San Rafael Swell produced the reports 

mentioned above. Field work in the San Rafael Swell was begun in 

July 1924 under the direction of E. M. Spieker. In August 1924, the 

investigations were assumed by James Gilluly, who then completed 

the field work in September 1925. In December 1925, oil was discovered in a drill hole on the Cane (Kane) Creek anticline about 15 km 

southwest of Moab. This discovery caused considerable interest in the 

petroleum potential of east-central Utah. Consequently, some of the 

results of the field work in the San Rafael Swell were incorporated 

in the Memorandum for the Press in 1926, the summary report by 

Baker and others (1927), and the regional study by Gilluly and Reeside 

(1928). The detailed description of the geology of the San Rafael Swell 

by Gilluly (1929) was undoubtedly delayed because the report contained a multi-colored geologic map that took some time to prepare. 

This report by Gilluly (1929) is treated herein as the principal reference because it includes the type sections of the Summerville and 

related formations. 

The four formations (fig. 2) included in the San Rafael Group by 

Gilluly (1929, p. 99-110) are well exposed in the San Rafael Swell. 
4 JURASSIC MORRISON FORMATION, EAST-CENTRAL UTAH 

San Rafael Swell Area southeast of 

Gilluly (1929) Green River, Utah, This report 

Lupton (1914) 

Cretaceous rocks Cretaceous rocks Cretaceous rocks 

cc Brushy Basin Member 

Morrison Salt Wash o.o 

(/) .... 

Formation c sandstone member ·::E Salt Wash Member 

0 o ..... "_;:j :2:0 ro u.. Tidwell Member E ~~Unconformity ~ 0 ~ ~J5 Unconformity~ u.. 0. Summerville Formation Summerville Formation () ::l 0 "iii (5 0 0. Curtis J Lower Summerville (/) 

Curtis Formation E ::l ro e Formation of McKnight (1940) :s Qi w 

' ~ () ~ 

ro Entrada Sandstone :2: 2 Entrada Sandstone a: ro c a: ro 

(/) Carmel Formation c Carmel Formation ro 

(/) 

Page Sandstone 

Older rocks Older rocks 

Older rocks 

FIGURE 2.-Some Jurassic nomenclature used in east-central Utah. 

On Cedar Mountain, the thickness of the group totals about 325 m 

(Wright and others, 1979, p. 26-42). The San Rafael Group shows 

a variety of lithologies including red and green siltstone and shale, 

red earthy sandstone, gray clean sandstone, glauconitic sandstone, 

and minor amounts of limestone and gypsum. The Carmel and Curtis 

Formations are of marine origin as indicated by marine fossils. The 

Entrada Sandstone and Summerville Formation are most likely marginal-marine deposits. The uppermost division of the San Rafael Group, 

the Summerville Formation, at Summerville Point (the type locality) 

is 49.6 m thick (Gilluly, 1929, p. 109). The Summerville consists of 

red siltstone and lesser amounts of sandstone and shale, all in very 

thin beds. Gypsum is present particularly in the upper part of the 

Summerville as thin beds, nodules, and seams that cut across bedding. 

The assignment of some units of the San Rafael Group has recently 

been somewhat changed. The Page Sandstone, named by Peterson 

and Pipiringos (1979, p. B20--B30), is a gray crossbedded sandstone 

about 30m thick near Cedar Mountain, which was formerly assigned 

to older Jurassic rocks. The Page is now assigned as the basal formation of the San Rafael Group. East of the Green River, equivalents 

of the Curtis Formation extending towards Moab beneath the Summerville are recognized as the lower Summerville of McKnight (1940), 

a unit now known to be entirely older than the Summerville Formation 

above the Curtis in the San Rafael Swell. (See O'Sullivan, 1980a.) 

The Morrison Formation rests unconformably on the San Rafael 
STRATIGRAPHY 5 

Group. In east-central Utah, the formation is 180-245 m thick (Craig 

and others, 1955, fig. 30). The Morrison consists of three parts. The 

Tidwell Member at the base is successively overlain by the Salt Wash 

and Brushy Basin Members. Strata forming the Tidwell have previously been assigned to the lower beds of the Salt Wash Member 

(O'Sullivan, 1981c) or included as an undivided part of the Salt Wash 

Member (L. C. Craig, written commun., 1980) in regional studies of 

the Morrison Formation (Craig and others, 1955). 

A terrestrial origin for most of the Morrison is generally accepted. 

The fossil flora and fauna together with sedimentary bedding features 

support this environmental interpretation. Locally, however, some of 

the basal part of the Morrison may be marine or quasi marine. Imlay 

(1980, p. 95), for instance, stated, "* * * the presence of massive 

gypsum at the base of the Morrison Formation in the San Rafael 

Swell * * * suggests deposition in lagoons at the margin of a shallow 

sea * * *." Similarly Holt (1940) noted that near Grand Junction, 

Colo., adjacent to east-central Utah, "* * *basal beds of the Morrison 

Formation * * * were deposited in lagoons on the borders of the 

Upper Jurassic * * * sea." The marine or marginal-marine beds, just 

described, in the basal part of the Morrison Formation above the J-5 

unconformity correlate with the quasi-marine Windy Hill Sandstone 

Member of the Sundance Formation overlying the J-5 unconformity 

in north-central Colorado. 

The Tidwell Member is at the base of the Morrison Formation 

throughout east-central Utah. The Tidwell forms a siltstone slope that 

extends from the J-5 unconformity up to the base of the Salt Wash 

Member. The thickness of the Tidwell at 24 measured sections in eastcentral Utah averages 17.2 m and ranges from as little as 4.8 m to 

as much as 28.6 m. There is a pronounced color shift in the Tidwell 

from gray in the San Rafael Swell to red in the Moab area. As will 

be explained, this color shift probably accounts for some of the different placements of the basal contact of the Morrison. 

The lithology of the Tidwell Member is somewhat varied. Siltstone 

is the dominant rock type. Chert beds as much as 1.5 m thick, rounded 

limestone nodules, and gray limestone beds are conspicuous lithologic 

features of the Tidwell Member. Gypsum is also present in some abundance from the San Rafael Swell to just east of the Green River. 

Gray ledge-forming sandstones as much as 2m thick, in which bedding 

is absent or not apparent, crop out at many localities; light-gray crossbedded channel sandstone beds typical of the overlying Salt Wash 

Member are absent at most places, but where present form a minor 

lithology in the slope-forming Tidwell Member. At places, the Tidwell 

contains persistent thin ledge-forming sandstone beds, generally less 

than 1 m thick, which are blocky, ripple marked, and commonly carry 
6 JURASSIC MORRISON FORMATION, EAST-CENTRAL UTAH 

coarse grains of chert. A widespread sandstone, termed for convenience bed A, marks the base of the Tidwell Member at most places. 

Throughout large areas of east-central Utah, bed A is generally less 

than 0.5 m thick but locally is as much as 2.5 m thick. Here and 

there the bed contains coarse grains, is ripple marked, and tends to 

form a resistant ledge that overhangs the J-5 unconformity and underlying rocks. 

The Tidwell is overlain in ascending order by the Salt Wash and 

Brushy Basin Members. In east-central U tab, the Salt Wash ranges 

in thickness from 60 to 110m, and the Brushy Basin ranges in thickness from 90 to 125 m (Craig and others, 1955, figs. 21 and 29). The 

Salt Wash is made up of a sequence of light-colored lenticular sandstone ledges interspersed with red and gray siltstone and sandstone. 

The sandstone beds, of fluviatile origin, are generally crossbedded 

and locally are coarse grained and conglomeratic. The Salt Wash forms 

high impressive cliffs, ledgy slopes, and broad benches. The lower 

boundary of the Salt Wash is at the base of the lowest channel sandstone that is a part of a continuous zone of channel sandstones (fig. 

4). The stratigraphic position of the contact varies considerably from 

place to place because of intertonguing with the underlying Tidwell 

Member. Above the Salt Wash, the slope-forming Brushy Basin Member consists mostly of gray, green, and red siltstone, shale and minor 

limestone, and conglomeratic sandstone. 

All the strata now assigned to the San Rafael Group and Morrison 

Formation were once included in the McElmo Formation near Green 

River (fig. 2). In an area southeast of the town of Green River and 

extending through Duma Point, Lupton (1914, p. 127) noted, "A gray 

coarse-grained sandstone, conglomeratic in places* * *about 350 feet 

below the top of the McElmo formation * * *" for which he proposed 

the name Salt Wash Sandstone Member of the McElmo Formation. 

In a general section of rocks in the area southeast of Green River, 

Lupton (1914, p. 124) recognized a Salt Wash Sandstone Member 46-

53 m thick; the thicknesses of the underlying and overlying unnamed 

parts of the McElmo were about 213 m and 99-107 m, respectively. 

The Salt Wash Sandstone Member was positioned somewhere above 

the middle of the McElmo Formation (figs. 2 and 4). 

In the later work of Gilluly and Reeside (1928) and Gilluly (1929), 

the newly defined San Rafael Group replaced the lower part of the 

McElmo Formation and the Morrison Formation replaced the upper 

part. In discussing the Morrison Formation, Gilluly (1929, p. 118) 

noted, "The rocks here correlated with the Morrison formation have 

* * * been hitherto referred to the "McElmo" formation. * * * There 

has been so wide diversity in the usage of the term "McElmo" 

* * * that it has seemed advisable to use the term Morrison for these 
BASE OF MORRISON FORMATION 7 

beds rather than to attempt to restrict the usage of McElmo. There 

seems ample lithologic and fossil evidence for the extension of the 

term Morrison formation * * * [into the San Rafael Swell]." 

BASE OF MORRISON FORMATION 

PREVIOUS USAGE 

The contact between the San Rafael Group and the Morrison Formation was carefully described in the San Rafael Swell by Gilluly (1929, 

p. 111): "There is an unconformity, both angular and erosional, at 

the top of the Summerville formation. * * * Above this unconformity 

lie the clay, sandstone, and conglomerate of the Morrison formation." 

This unconformity that is both angular and erosionall is now recognized 

as the J-5 unconformity. 

In the San Rafael Swell, the Salt Wash has heretofore been recognized as the basal member of the Morrison. In describing the Morrison, Gilluly (1929, p. 111) noted, "The basal portion of this formation 

on the east flank of the Swell, is a series of conglomeratic sandstone 

lenses of the channel type, interbedded with far subordinate amounts 

of clay and some gypsum and limestone. * * * From the descriptions 

given by Lupton * * * this series is very probably his Salt Wash 

sandstone member * * *." It is clear from the description and measured sections that Gilluly (1929, p. 111-118) believed the Salt Wash 

extended down to the J-5 unconformity in the San Rafael Swell. Summerville Point makes a convenient locality to show the J-5 surface 

and its relationships to associated strata inasmuch as it includes the 

type section of the Summerville Formation-the youngest unit of the 

San Rafael Group beneath the J-5 unconformity. The Summerville 

Formation is overlain by the Morrison Formation that was extended 

somewhat casually into east-central Utah, conseqwmtly the Morrison 

lacks a nearby reference section. At Summerville Point, where the 

Morrison is well displayed, the lowest gray channel sandstone in the 

Morrison lies 9-16 m above the J-5 unconformity (fig. 3). These channel sandstones are lithologically the same as and are equivalent to 

the gray channel sandstones assigned by Lupton (1914) to the Salt 

Wash along White Wash (fig. 4) in the area east of the Green River 

where the Salt Wash was named. The sequence between the lowest 

channel sandstone and the J-5 unconformity is now recognized as the 

Tidwell Member. The Tidwell in the Swell is mostly gray siltstone 

with some thin gray limestone ledges. At Summervillle Point, the gray 

bed A at the base of the Tidwell is 0.15 m thick, makes a slight 

ledge, and contains sparse coarse grains as much as 1. 5 mm across. 

About 1.2 m above bed A is a gray persistent ripple--marked sandstone 
8 JURASSIC MORRISON FORMATION, EAST-CENTRAL UTAH 

FIGURE 3.-View northwest of Jurassic rocks at Summerville Point. Upper part of 

Curtis Formation (J c) makes the flat at base of steep slope of the Summerville Formation (Js). Tidwell Member about 15 m thick (Jmt); Salt Wash Member (Jms). The 

J-5 unconformity (J-5) separates the San Rafael Group and Morrison Formation. 

Summerville Point is in SE1

/4 sec. 9 (unsurveyed) T. 19 S., R. 13 E. 

about 1 m thick that can be traced for several kilometers to the north 

and south of Summerville Point. The sandstone just described is typical of the Tidwell Member but is unlike the lenticular sandstone beds 

in the Salt Wash. 

The J-5 unconformity was miscorrelated from the San Rafael Swell 

eastward towards the Green River. The detailed stratigraphy of the 

San Rafael Swell was extended eastward in the report by Gilluly and 

Reeside (1928). Measured sections in the San Rafael Swell were correlated with a section measured by Lupton (1914, p. 126) along White 

Wash (Gilluly and Reeside, 1928, plate 15). The unconformity at the 

base of the Morrison in the Swell was correlated with the base of 

the Salt Wash along White Wash rather than with the J-5 unconformity about 18 m below the Salt Wash along White Wash. The miscorrelation from the Swell eastward occurs partly because the Tidwell 

Member between the J-5 unconformity and the Salt Wash was overlooked at Summerville Point and along White Wash. Consequently, 

what was regarded as Salt Wash in the Swell, which included the 
BASE OF MORRISON FORMATION 9 

FIGURE 4.-View eastward of Jurassic rocks along White Wash at type locality of 

Salt Wash Member of Morrison Formation. Tidwell Member (Jmt) is about 18 m 

thick. Summerville Formation (J s) is only about 5 m thick here because most of 

it has been beveled out by the J-5 unconformity (J-5) at the base of the Morrison 

Formation. Entrada Sandstone (Je); the lower Summerville of McKnight (1940), a 

Curtis Formation equivalent, (Jsl); Salt Wash Member (Jms); and Brushy Basin Member (Jmb). Topographic high points in picture are in NW1

/4 sec. 19, T. 23, S., R. 

18 E. 

Tidwell Member, was correlated with what was defined by Lupton 

as Salt Wash along White Wash. In addition, the Tidwell Member 

underlying the Salt Wash is gray in the Swell and mostly red along 

White Wash. The red Tidwell along White Wash might be confused 

at first glance with the red Summerville Formation beneath the J-5 

unconformity. 

The Summerville Formation has been recognized east of White 

Wash and the Duma Point area. Gilluly and Reeside (1928, plate 15) 

traced the Summerville eastward to Dewey Bridge. However, recent 

stratigraphic studies (O'Sullivan, 1980a) show that all of the Summerville that overlies the Curtis Formation in the San Rafael Swell is 

truncated beneath the J-5 unconformity near Dubinky well not far 

east of White Wash. The strata previously treated as Summerville 

east of Dubinky well are now assigned to the Tidwell Member. In 
10 JURASSIC MORRISON FORMATION, EAST-CENTRAL UTAH 

selected parts of east-central Utah, the base of what was termed Morrison Formation has been variously described by Baker (1933) in the 

Moab area, by Dane (1935) in the Salt Valley and adjacent areas, 

and by Stokes (1952) in the Thompson area. Each author recognized 

a Salt Wash Sandstone Member underlain by what was termed Summerville Formation. Each seemed somewhat perplexed by the gradational nature of their selected base of the Morrison inasmuch as the 

base of the Morrison in the San Rafael Swell was described as an 

unconformity that is both angular and erosional. 

In describing the Moab area, Baker (1933, p. 51) wrote: "The top 

of the Summerville is placed at the base of the first ledge of white 

sandstone in the Morrison formation, which was deposited on an uneven surface that may represent an erosional unconformity of more 

than local extent. The red sandstones and mudstones interbedded with 

the white sandstone ledges of the lower Morrison are similar to the 

material forming the body of the Summerville formation, and this apparent unconformity may be of no more significance than several other 

irregular contacts in the irregularly bedded overlying material. Elsewhere in southeastern Utah * * * this contact is marked by a pronounced unconformity, and it is inferred, therefore, that the unconformity at this horizon in the Moab district is more significant than is 

indicated by local evidence." 

In his discussion of the Salt Valley and adjacent areas, Dane (1935, 

p. 112) stated: "The contact between the Morrison formation and the 

underlying Summerville is locally difficult to locate with precision, 

because of some resemblances in lithology between the formations. 

In most places it can be ascertained precisely and is a sharp, slightly 

irregular contact regardless of whether the basal bed of the Morrison 

is gray limestone, shale, or crossbedded sandstone. Within the area 

mapped no angular unconformity was observed, and the continuity 

and slight variation in thickness of the underlying Summerville formation suggest that there was not a significant erosional interval between 

the periods of deposition of the two formations. Angular unconformity 

has been reported at this contact in the San Rafael Swell * * *." 

In the Thompson area, Stokes (1952, p. 11) found that: "The Morrison-Summerville contact * * * is somewhat gradational and difficult 

to determine. In general, the sediments involved in the transition 

zone reflect a gradual change from deposition in shallow, still, or 

slowly moving water to deposition under changing fluvial conditions 

which resulted in better sorting and greater lateral variation in sediments. The contact has been drawn where red, shaly, thin-bedded 

sediments give way upward to predominantly gray or brownish, 

sandy, lenticular types with distinct cross-bedding and less limestone 

and chert. Locally, however, sandstones of Morrison lithology cut into 
BASE OF MORRISON FORMATION 11 

the Summerville and conversely, red Summerville-type rocks occur 

in the lower part of the Morrison." Stokes (1952, p. 14) further noted, 

in describing the uppermost part of the San Rafael Group, "* * * 

slight angular unconformities are found in the San Rafael area (Gilluly 

and Reeside, 1928, p. 81) * * *." 

The puzzling absence of an unconformity near the base of the Salt 

Wash is due to a misunderstanding of the position of the J-5 unconformity with respect to the Salt Wash Member. The J-5 unconformity 

in the Moab area, in the Salt Valley and adjacent areas, and in the 

Thompson area is well below the Salt Wash and is at the base of 

what was described as Summerville Formation but which is instead 

the Tidwell Member. The Tidwell is red and, superficially at least, 

resembles the red Summerville Formation in the San Rafael Swell. 

The Tidwell is completely gradational with the overlying Salt Wash; 

no sharp contact separates the two stratigraphic units. 

The base of the Morrison Formation has also been placed at a horizon that indicates a change in environment of deposition. Craig and 

others (1955, p. 134) stated: "The base of the Morrison formation is 

defined * * * as the base of the terrestrial, fluvial Jurassic deposits 

overlying beds of the marine or marginal marine San Rafael group." 

In detail, according to L. C. Craig (written commun., 1980), the basal 

contact of the Morrison is selected at the lowest indication of terrestrial deposition, a contact that lies at the base of the lowest channel 

sandstone or lowest limestone or lowest lenticular bed of sandstone, 

mudstone, or siltstone. In east-central Utah, this contact is beneath 

the Salt Wash Member and lies 1 m or more above the J -5 unconformity. 

PRESENT USAGE 

The selection of any horizon but the J-5 unconformity as the base 

of the Morrison Formation in east-central Utah constitutes a departure from the horizon specified by Gilluly (1929, p. 111-118) when 

he extended the Morrison into east-central Utah. The base was clearly 

described in the San Rafael Swell as an unconformity that is both 

angular and erosional between the Summerville and Morrison Formations. 

The base of the Morrison over wide areas has obviously been put 

by many geologists above the horizon selected by Gilluly (1929). Accordingly, the base of the Morrison is here placed at the unconformity 

described by Gilluly-now recognized as the J-5---throughout eastcentral Utah and adjacent parts of western Colorado. Moreover, this 

returns the base of the Morrison to the unconformity defined by Gilluly (1929, p. 111) as the top of the San Rafael Group. The Salt Wash 
12 JURASSIC MORRISON FORMATION, EAST-CENTRAL UTAH 

FIGURE 5.-View northward of Jurassic rocks at the section near Duma Point. Top 

of Slick Rock Member (SR) of Entrada Sandstone; lower Summerville of McKnight 

(1940) includes lower beds (LB), bed at Black Steer Knoll (K), and upper beds (UB); 

the basal red ledge (RL) of Summerville Formation. Morrison Formation here includes Tidwell Member (Jmt) and Salt Wash Member (Jms). Tidwell Member is 10.8 

m thick. Bed A (A) at base of Tidwell overlies J-5 unconformity. Section is exposed 

mostly in NW% sec. 30, T. 23 S., R. 18 E. 

Member as described and defined by Lupton (1914, p. 126) lies above 

the J-5 unconformity throughout east-central Utah. Strata between 

the Salt Wash and the base of the Morrison in east-central Utah now 

make up the Tidwell Member. 

A section (fig. 5) near Duma Point is presented in the following 

to show the lithology and stratigraphic arrangement of units associated with the J -5 unconformity. The section is easily accessible 

by dirt road and lies about 2.3 km south of the locality on White 

Wash where Lupton (1914, p. 126) measured what is commonly regarded as the type section of the Salt Wash Member. Strata at the 

Duma Point section include in ascending order the uppermost part 

of the Entrada Sandstone, the lower Summerville of McKnight (1940), 

the Summerville Formation, and the Morrison Formation. The correlation of the various stratigraphic units to the east and west of the 

Duma Point section is shown in a report by O'Sullivan (1980a). 
BASE OF MORRISON FORMATION 

Duma Point Section 

[Measured in NEV.NEV.SWV. and SEV.SEV.NWV. sec. 30, T. 23 S., R. 18 E.] 

Morrison Formation: 

Salt Wash Member: 

Sandstone, gray, crossbedded; forms a cliff; only basal part of Salt 

Wash measured . . . . . . . . . . . . . . . . . 

Incomplete thickness of Salt Wash Member . . . . . . . . . . 

Tidwell Member: 

Siltstone and shale; reddish-brown in lower 3 m, upper part is bluish 

gray; 0.3-m white sandstone makes a ledge 3.6 m above base of 

unit; limestone nodules and small bits of chert and gray limestone 

litter surface; forms a slope . . . . . . . . . . . . . . . . . . . . 

Sandstone, brown, flat to slightly wavy and thinly bedded; coarse 

grains as much as 2.4 mm across; minor interbedded reddish-brown 

siltstone; forms a conspicuous ledge. This sandstone is bed A . 

Total thickness of Tidwell Member . . . . . . . . . . . . . . . 

J--5 unconformity. 

Summerville Formation: 

Siltstone, reddish-brown; poorly exposed in a slope . . . . . . . . 

Red ledge: 

Sandstone and interbedded siltstone, reddish-brown; makes a conspicuous ledge-forming marker bed in White Wash-Dubinky well area. 

Total thickness of Summerville Formation .......... . 

Lower Summerville of McKnight (1940): 

Upper beds: 

Siltstone, reddish-brown; 0.4-m reddish-tan sandstone 3 m above base 

of unit makes a ledge and co11tains rounded medium grains less than 

0.4 mm across; upper 8 m of unit contains at least three thin reddishtan sandstone beds that form ledges; unit mostly forms a poorly 

exposed slope . . . . . . . . . . 

Total thickness of upper beds . . . . . . . . . . . 

Bed at Black Steer Knoll: 

Sandstone, reddish-brown, flatbedded; forms a ledge 

Siltstone, reddish-brown; forms a re-entrant . . . . . 

Sandstone, reddish-tan, flatbedded; forms a conspicuous ledge 

Total thickness of bed at Black Steer Knoll . . . . . . . . 

Lower beds: 

Sandstone, silty, reddish-brown mottled gray, evenly bedded; in eight 

or nine beds; four very thin reddish-brown clay beds at 2.6, 3.6, 

13 

Thickness 

in meters 

4.6+ 

4.6+ 

8.8 

2.0 

10.8 

4.6 

3.0 

7.6 

12.5 

12.5 

.8 

.3 

.7 

1.8 

4.0, and 5.2 m above base of unit; forms a slope . . . . . . . . 5.5 

Siltstone, reddish-brown mottled gray; 0.1-m light-greenish-gray sandstone 1.1 m above base of unit; thin purple clay bed 3.2 m above 

base of unit, another thin purple clay bed at top of unit; on vertical 

cliffs unit weathers to rounded spheroidal forms; elsewhere unit 

forms slope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 

Sandstone and siltstone, reddish-brown; interbedded in thin beds less 

than 0.15 m thick; four thin purple clay beds at base and at 0.24, 

1.5, and 1. 7 m above base of unit; forms a slope . . . . . . . . 2.4 
14 JURASSIC MORRISON FORMATION, EAST-CENTRAL UTAH 

Lower Summerville of McKnight (1940)-Continued 

Lower beds-Continued 

Sandstone, reddish-tan mottled gray; contains dark reddish-brown clay 

galls; forms ledge . . . . . . . . . . . . . . . . . . . . . . . . . . 

Siltstone, reddish-brown; six thin reddish-brown or purple clay beds 

at 0.06, 0.3, 0.4, 0.5, 0.9, and 1.1 m above base of unit, thin reddishbrown clay at top of unit; forms slope . . . . . . . . . . . . . . 

Sandstone, reddish-brown, irregularly to flatbedded; thin reddishbrown siltstone bed in middle of unit contains a very thin reddishbrown clay bed; forms a slight ledge . . . . . . . . . . . . . . . 

Siltstone, reddish-bro\\"'11; thin reddish-brown clay 0.2 m above base 

of unit; forms a slope . . . . . . . . . . . . . . . . . . . . . . . . 

Sandstone, reddish-brown, streaked white, flatbedded; forms ledge . 

Siltstone, reddish-brown; contains a few thin reddish-brown locally 

mottled gray sandstone beds; thin reddish-brown clay at base, three 

other thin reddish-brown or purple clay beds at 0.15, 1.9, and 2.3 

m above base of unit; forms slope . . . . . . . . . . . . . . . . 

Sandstone, brown, flatbedded; slightly overhangs underlying Entrada 

Sandstone, weathers unevenly; forms thin reworked zone at base 

of lower beds . . . . . . . . . . . . . . . . . . . . . . . . . , . 

Total thickness of lower beds . . . . . . . . . . . . . . . . 

Total thickness of lower Summerville of McKnight (1940) 

Entrada Sandstone: 

Slick Rock Member: 

Thickness 

in meters 

.2 

1.3 

.3 

.6 

.2 

2.5 

.2 

16.6 

30.9 

Sandstone, reddish-brown, flatbedded 2.6 

Sandstone, reddish-tan, crossbedded 3.5 

Sandstone, reddish-tan, flatbedded 1.6 

Sandstone, gray, crossbedded . . . . 1.0 

Sandstone, reddish-tan, flatbedded . 2.1 

Sandstone, reddish-tan, crossbedded 1.1 

Sandstone, reddish-tan, flatbedded . .9 

Sandstone, reddish-tan, crossbedded .8 

Sandstone, reddish-tan, flatbedded . . 7 

Sandstone, reddish-brown, flatbedded 3.0 + 

Base of measured section. Base of Entrada Sandstone not exposed in area 

where section was measured. 

Incomplete thickness of Slick Rock Member of Entrada Sandstone . 17.3 + --

In the Duma Point area, strata that form the Tidwell Member have 

been recognized previously as beds that are younger than the San 

Rafael Group. The area was investigated in the summer of 1940 by 

Baker and others (1952) not long after the work by Gilluly and Reeside 

(1928). The report was not published until1952 because of World War 

II. In describing the base of the Morrison in the Duma Point area, 

they (Baker and others, 1952, p. 81) noted, "* * * typical red beds 

of the Summerville are overlain by gray and lavender shale and 

mudstone, thin beds of sandstone, and some blue-gray limestone, and 

these in turn are overlain by the Salt Wash sandstone member 

* * *." The description of the beds between the Summerville and 

the Salt Wash aptly applies to the Tidwell Member. Later, however, 
BASE OF MORRISON FORMATION 15 

Baker and others (1952, p. 81) described these strata as follows, 

"* * * the basal unit of the Salt Wash sandstone member of the Morrison consists of 35 feet of gray to variegated shale and mudstone containing many thin beds of sandstone and some nodular limestone. 

These are overlain by highly lenticular sandstone beds 30 feet or more 

thick, interbedded with shale and mudstone." The inclusion of these 

beds as a "basal unit" of the Salt Wash in the Duma Point area constitutes a change of the Salt Wash as designated by Lupton (1914) in 

the same area. A comparison of a measured section by Baker and 

others (1952, pl. 3) with sections measured by O'Sullivan (1980a) in 

the same Duma Point area show that the basal contact of the Morrison 

Formation as used by Baker and others coincides with the J-5 unconformity or nearly so. 

The Tidwell Member is found over wide areas of Utah and Colorado. 

It has been traced from the San Rafael Swell eastward to Bridgeport 

and Uravan in western Colorado. South and southwest of the Swell, 

the Tidwell is recognized in the Kaiparowits Plateau and Henry Mountains area (Peterson, 1980, fig. 4). In southeastern Utah between Monticello and Bluff, the Tidwell is replaced laterally by the Bluff Sandstone Member of the Morrison. Near Glenwood Springs in western 

Colorado, the overlying Salt Wash Member wedges out and the Tidwell merges eastward with an undivided Morrison Formation. 

The J-5 unconformity forms a consistent and widespread base of 

the Morrison Formation. At some places the J-5 is angular on underlying rocks. Generally, the beds above and below the unconformity are 

parallel or nearly so and the J-5 unconformity shows only slight relief. 

Regionally, however, the unconformity bevels progressively across 

older rocks from the San Rafael Swell eastward into Colorado. Along 

the Gunnison River east of Black Canyon, the Morrison Formation 

rests on Precambrian rocks (Hedlund and Olson, 1973); Jurassic strata 

beneath the surface of unconformity are absent. The specific stratigraphic position of the J-5 at many localities in eastern Utah and 

western Colorado is shown in reports cited in following text. 

The J-5 unconformity is recognized over large areas of Utah and 

adjacent States. It has been traced from the San Rafael Swell through 

the Moab area to Bluff (O'Sullivan and Pierce, 1983). It is recognized 

from the Moab area southeastward to Slick Rock, Colo. (O'Sullivan, 

1981a). The J-5 unconformity has been traced from the Moab area 

to Dewey Bridge (O'Sullivan, 1981b). From Dewey Bridge, the J-5 

has been followed around the Uncompahgre Plateau through the 

Grand Junction area to Bridgeport, Colo. (O'Sullivan and Pipiringos, 

1983), and from Dewey Bridge to Uravan (O'Sullivan, 1984). In northwestern New Mexico, a regionally extensive disconformity has been 

identified by R. D. Lupe (U.S. Geological Survey, 1980, p. 48) at 
16 JtJRASSIC MORRISON FORMATION, EAST-CENTRAL UTAH 

the base of the Morrison throughout the subsurface of the San Juan 

Basin. This surface, which "* * *likely corresponds to the J-5 unconformity * * *" (Lupe, 1983), forms an important datum and has been 

traced westward in the subsurface to near Bluff, Utah. At Bluff, the 

datum used by R. D. Lupe does indeed coincide with the J-5 unconformity traced into that area from the north (O'Sullivan, 1980b). Consequently, the unconformity recognized by Lupe in New Mexico enlarges the areal extent of the surface beyond Utah and Colorado. 

REFERENCES CITED 

Baker, A. A., 1933, Geology and oil possibilities of the Moab district, Grand and San 

Juan Counties, Utah: U.S. Geological Survey Bulletin 841, 95 p. 

Baker, A. A., Dobbin, C. E., McKnight, E. T., and Reeside, J. B., Jr., 1927, Notes 

on the stratigraphy of the Moab region, Utah: American Association of Petroleum 

Geologists Bulletin, v. 11, p. 785--808. 

Baker, A. A., Duncan, D. C., and Hunt, C. B., 1952, Manganese deposits of southeastern Utah: U.S. Geological Survey Bulletin 979-B, p. 63-155. 

Craig, L. C., Holmes, C. N., Cadigan, R. A., Freeman, V. L., Mullens, T. E., and 

Weir, G. W., 1955, Stratigraphy of the Morrison and related formations, Colorado 

Plateau region, a preliminary report: U.S. Geological Survey Bulletin 1009-E, p. 

125-168. 

Dane, C. H., 1935, Geology of the Salt Valley anticline and adjacent areas, Grand 

County, Utah: U.S. Geological Survey Bulletin 863, 184 p. 

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., and Moore, R. C., 1931, The Kaiparowits region, a geographic and 

geologic reconnaissance of parts of Utah and Arizona: U.S. Geological Survey Professional Paper 164, 161 p. 

Hedlund, D. C., and Olson, J. C., 1973, Geologic map of the Carpenter Ridge Quadrangle, Gunnison County, Colorado: U.S. Geological Survey Geologic Quadrangle 

Map GQ 1070, Scale 1:24,000. 

Holt, E. L., 1940, The Morrison and Summerville Formations of the Grand River Valley 

and their vertebrate and invertebrate fauna [abs.]: Colorado University Studies, 

v. 26, no. 3, p. 55. 

Imlay, R. W., 1980, Jurassic paleobiogeography of the conterminous United States 

in its continental setting: U.S. Geological Survey Professional Paper 1062, 134 p. 

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. 

Lupton, C. T., 1914, Oil and gas near Green River, Grand County, Utah: U.S. Geological Survey Bulletin 541, p. 115-133. 

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. 

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. 
REFERENCES CITED 17 

--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. 

--1981a, 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. 

--1981b, Stratigraphic sections of Middle Jurassic Entrada Sandstone and related 

rocks from Salt Valley to Dewey Bridge in east-central Utah: U.S. Geological Survey Oil and Gas Investigations Chart OC-113. 

--1981c, The Middle Jurassic San Rafael Group and related rocks in east-central 

Utah: New Mexico Geological Society Field Conference, 32nd, Western Slope Colorado, 1981 Guidebook, p. 89-95. 

--1984, Stratigraphic sections of Middle Jurassic San Rafael Group and related 

rocks from Dewey Bridge, Utah, to Uravan, Colorado: U.S. Geological Survey 

Oil and Gas Investigations Chart OC-124. 

O'Sullivan, R. B., and Pierce, Frances Wahl, 1983, Stratigraphic diagram of Middle 

Jurassic San Rafael Group and associated formations from the San Rafael Swell 

to Bluff in Utah: U.S. Geological Survey Oil and Gas Investigations Chart OC-119. 

O'Sullivan, R. B., and Pipiringos, G. N., 1983, Stratigraphic sections of Middle Jurassic 

Entrada Sandstone and related rocks from Dewey Bridge, Utah, to Bridgeport, 

Colorado: U.S. Geological Survey Oil and Gas Investigations Chart OC-122. 

Peterson, Fred, 1980, Sedimentology as a strategy for uranium exploration-Concepts 

gained from analysis of a uranium-bearing depositional sequence in the Morrison 

Formation of south-central Utah, in Turner-Peterson, C. E., (ed.)., Uranium in 

sedimentary rocks-Application of the facies concept to exploration: Rocky Mountain Section, Society of Economic Paleontologists and Mineralogists, Short Course 

Notes, p. 65-126. 

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. Bl-B43. 

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. Al-A29. 

Stokes, W. L., 1952, Uranium-vanadium deposits of the Thompsons area, Grand 

County, Utah-with emphasis on the origin of carnotite ores: Utah Geological and 

Mineralogical Survey Bulletin 46, 51 p. 

U.S. Geological Survey, 1980, Geological Survey research 1980: U.S. Geological Survey 

Professional Paper 1175, p. 48. 

Wright, J. C., Dickey, D. D., Snyder, R. P., Craig, L. C., and Cadigan, R. A., 1979, 

Measured stratigraphic sections of Jurassic San Rafael Group and adjacent rocks 

in Emery and Sevier Counties, Utah: U.S. Geological Survey Open-File Report 

79-1317, 157 p. 

·,'?U.S. GOVERNMENT PRINTING OFFICE: 1985-576-049/20,008 REGION NO.8 




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