Marine Trace Fossils 

in the Upper Jurassic 

Bluff Sandstone, 

Southeastern Utah

c; K o L o c; i c: A L s v R v i< Y B r L L K r IN i 3 y 5 -1

Marine Trace Fossils 

in the Upper Jurassic 

Bluff Sandstone, 

Southeastern Utah

By ROBERT B. O'SULLIVAN and JOHN O. MABERRY

CONTRIBUTIONS TO STRATIGRAPHY

GEOLOGICAL SURVEY BULLETIN 1395-1

A description of newly discovered 

trace fossils and their 

significance

UNITED STATES GOVERNiMENT PRINTING OFFICE, WASHINGTON: 1975
UNITED STATES DEPARTMENT OF THE INTERIOR 

ROGERS C. B. MORTON, Secretary

GEOLOGICAL SURVEY 

V. E. McKelvey, Director

Library of Congress Cataloging in Publication Data

O'Sullivan, Robert Brett, 1923-

Marine trace fossils in the Upper Jurassic Bluff Sandstone, southeastern Utah.

(Contributions to stratigraphy) (Geological Survey Bulletin 1395-1)

Bibliography: p.

Supt.ofDocs.no.: 119.3:1395-1

1. Trace fossils Navaho Indian Reservation. 2. Paleontology Jurassic.

I. Maberry, John O., joint author. II. Title. III. Series. IV. Series: United States

Geological Survey Bulletin 1395-1. 

QE75.B9 no. 1395-1 [QE720.5] 557.3'08s [560'.176] 75-619006

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

Washington, D. C. 20402

Stock Number 024-001-2641^
CONTENTS

Page

Abstract........................................................................................................................... II

Introduction.................................................................................................................... 1

San Rafael Group........................................................................................................... 1

Bluff Sandstone........................................................................................................ 3

Trace fossils............................................................................................................. 4

Significance..................................................................................................................... 9

References cited............................................................................................................... 11

ILLUSTRATIONS

Page

FIGURE 1. Map showing generalized distribution of Bluff Sandstone and the

trace fossil locality...................................................................................... 12

2. Diagram showing arrangement and distinguishing characteristics

of the members of the Bluff Sandstone...................................................... 5

3. Photograph showing trace fossil burrows on North Red Mesa ................... 6

4. Oblique Photograph of hand specimen of trace fossils showing characteristics of burrow structure.................................................................... 7

5. Vertical photograph of hand specimen of trace fossil showing morphology and burrow density...................................................................... 8

6. Photograph showing member D of Bluff Sandstone.................................... 10

III

CONTRIBUTIONS TO STRATIGRAPHY

MARINE TRACE FOSSILS IN THE UPPER

JURASSIC BLUFF SANDSTONE,

SOUTHEASTERN UTAH

BY ROBERT B. O'SULLIVAN and JOHN O. MABERRY

ABSTRACT

The Bluff Sandstone, as much as 340 feet (104 metres) thick, is the uppermost 

formation of the San Rafael Group in southeastern Utah and extreme northeastern 

Arizona. Trace fossils, 4 to 9 millimetres across and of indeterminate length, are 

found as burrows in the lower part of the Bluff Sandstone. The depositional environment represented is the upper foreshore of a marine beach. The middle part of the 

Bluff is eolian but the upper part may be marine. The upper contact of the Bluff is 

almost certainly an unconformity.

INTRODUCTION

The Bluff Sandstone, of rather limited geographical extent (fig. 

1) in southeastern Utah and northeastern Arizona, is the youngest 

formation in the San Rafael Group. Fossils have not heretofore 

been found in the Bluff Sandstone or, for that matter, in any other 

part of the San Rafael Group in southeastern Utah. Discussions of 

the age and depositional history of the San Rafael Group in this 

area have been based largely on regional relationships. Trace fossils described herein, although not diagnostic as to age, are helpful 

in interpreting the geologic history and depositional environment 

of the Bluff Sandstone and associated underlying parts of the San 

Rafael Group. This report briefly describes the Bluff Sandstone, 

describes the trace fossils, and discusses their significance.

This report is part of a continuing study of the geology of the 

Navajo Reservation and was done on behalf of the U.S. Bureau of 

Indian Affairs in cooperation with the Navajo Tribe.

SAN RAFAEL GROUP

The name San Rafael Group was applied (Baker and others, 

1927, p. 787) to a sequence of rocks in the San Rafael Swell of central Utah where the group consists of, in ascending order, Carmel 

Formation, Entrada Sandstone, and the Curtis and Summerville

Ii
12 CONTRIBUTIONS TO STRATIGRAPHY

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FIGURE 1. Map showing generalized distribution of Bluff Sandstone (stippled) and the 

trace fossil locality. Quaternary alluvium and eolian sand (lined) shown in Red MesaBoundary Butte area, where they conceal the Bluff Sandstone. Compiled in part from 

Haynes, Vogel, and Wyant (1972).
MARINE TRACE FOSSILS, BLUFF SANDSTONE, UTAH 13

Formations. The Curtis Formation grades southward from the 

Swell into the Summerville Formation and is not recognized in 

southeastern Utah. Instead, in that area, the Bluff Sandstone is at 

the top of the San Rafael Group.

Parts of the San Rafael Group are fossiliferous, particularly the 

Carmel Formation in southwestern and central Utah and the Curtis Formation in the San Rafael Swell. The fossils indicate that the 

lower part of the Carmel Formation is of Middle Jurassic age and 

the remainder of the group is of Late Jurassic age.

The San Rafael Group, in southeast Utah, is overlain by the 

Morrison Formation of Late Jurassic age and is underlain by the 

Navajo Sandstone of the Glen Canyon Group. The Glen Canyon 

Group is mostly of Late Triassic age, but its uppermost part is 

thought to include rocks of Jurassic age.

BLUFF SANDSTONE

The Bluff Sandstone was named formally by Gregory (1938, p. 

58) for the excellent exposures about the settlement of Bluff, where 

the sandstone forms bold cliffs along the San Juan River. Gregory 

treated the Bluff Sandstone as a member of the Morrison Formation, as had been done previously by Baker, Dane, and Reeside 

(1936, p. 15). Later, the Bluff was recognized as a tongue (Craig and 

others, 1955, p. 133) of the Cow Springs Sandstone, a formation of 

eolian origin whose type locality lies about 50 miles (80 km) southwest of Bluff on Black Mesa in Arizona. At present the Bluff Sandstone is considered to be a separate formation at the top of the San 

Rafael Group (Sears, 1956, p. 202; Harshbarger and others, 1957, p. 

42).

The Bluff Sandstone near the San Juan River forms prominent 

cliffs. North and south of the river the topographic expression of 

the Bluff Sandstone is more subdued, and over large areas the formation forms a rolling hummocky surface largely veneered by 

windblown sand. The Bluff is well displayed near the ArizonaUtah State line near the tops of the three mesas that are collectively referred to in this report as Red Mesa and are distinguished 

individually as North, West, and East Red Mesa (fig. 1). For a distance of 4 to 5 miles (6.4 to 8 km) to the north and south of Red Mesa, 

the Bluff Sandstone is almost completely obscured by windblown 

sand and alluvium of Quaternary age.

The Bluff Sandstone is thickest near the settlement of Bluff 

where it is 338 feet (103 m) thick in the cliffs just south of the San 

Juan River. The Bluff thins gradually northward and pinches out 

in the Summerville Formation about 24 miles (39 km) north of the 

San Juan River. Similarly, the Bluff thins gradually southward 

and is only 75 feet (22.8 m) thick at the north end of North Red
14 CONTRIBUTIONS TO STRATIGRAPHY

Mesa. The Bluff is 25 feet (7.6 m) thick near the north end of West 

Red Mesa and is only 7 feet (2.1 m) thick at the west end of East Red 

Mesa.

Near Bluff, the massive Bluff Sandstone is yellowish gray to very 

light gray and weathers tan or buff. Although cliff-forming, the 

Bluff Sandstone is friable. It is composed of well-sorted very fine to 

fine subrounded to rounded grains of quartz and has minor 

amounts of feldspar, varicolored chert, and dark accessory minerals. Well-rounded medium to coarse grains are either disseminated 

throughout the formation or concentrated along laminae.

The Bluff Sandstone is divided into four informal members, 

referred to as members A, B, C, and D, in ascending order(Cadigan, 

1952; Craig and Cadigan, 1958). The distinguishing characteristics of each of these members are shown in figure 2. Members A and 

D have been interpreted as subaqueous deposits, member B as an 

eolian deposit, and member C as an eolian deposit that has been, in 

part, reworked by water.

The four members of the Bluff are all found together near the San 

Juan River; elsewhere one or more of the members is absent (Craig 

and Cadigan, 1958, p. 185). Only member A is recognized throughout the distribution area of the Bluff Sandstone. From the town of 

Bluff, members B and C both lose definition northward by internal 

thinning, and near the northern limits of the Bluff Sandstone, 

member D lies directly on member A. South of the San Juan River 

the overlying Morrison Formation progressively truncates member D and the thinning remnants of members B and C in the area 

near the south end of White Mesa. At Red Mesa the Morrison Formation lies on a southward-thinning wedge of member A.

TRACE FOSSILS

Trace fossils (fig. 3) are present in about the upper half of the 

Bluff Sandstone on the eastern side of North Red Mesa in the 

NE^SE^NEVi sec. 25, T. 43 S., R. 23 E., and near the eastern 

boundary of the same township. At that locality only member A, 

about 68 feet (21 m) thick, is present. The trace fossils occur in two 

beds, each about 2.5 feet (75 cm) thick and separated from each 

other by a bed of gently crossbedded sandstone about 5.5 feet (1.7 

m) thick. That part of member A below the lower trace fossil bed 

consists of 38 feet (11.6 m) of flat-bedded sandstone. The upper trace 

fossil bed is overlain by 20 feet (6.1 m) of sandstone that is mostly 

gently crossbedded but with some flat-bedded units. The lower 

trace fossil bed extends for a distance of about 100 feet (30 m) along 

the outcrop and is more persistent than the upper bed.
MARINE TRACE FOSSILS, BLUFF SANDSTONE, UTAH 15

Formation

and 

member

Morrison 

Formation

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Thickness

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100

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160

Metres

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Description

Light-gray and yellow sandstone 

and reddish-brown and green 

siltstone and shale

Mostly massive and structureless

ering units and horizontally 

bedded units; medium grains

sparse or absent

Mostly small-scale low-angle crossbedded sandstone; numerous

horizontal truncation planes;

medium grains in moderate

abundance

Large-scale high-angle crossbedded

sandstone; coarse and medium

grains abundant particularly in

lower part

Contains flat-bedded and small-scale

crossbedded sandstone units;

numerous horizontal bedding

planes; reddish-brown siltstone

partings. Medium and coarse

grains abundant

Dark-reddish-brown siltstone and 

shale with interbedded thin 

white and tan sandstone

FIGURE 2. Diagram showing arrangement and distinguishing characteristics of the members of the Bluff Sandstone. In part from Craig and Cadigan (1958).
16 CONTRIBUTIONS TO STRATIGRAPHY

1

FIGURE 3. Trace fossil burrows on North Red Mesa. Burrows at the top of the lower 

trace fossil bed have been etched out by weathering and truncated by overlying beds.

The most conspicuous trace fossils found to date in the Bluff 

Sandstone are burrows; there are no other extraneous tracks or 

trails. The sandstone surrounding the burrows contains abundant 

fecal material and aggregates of sand grains excavated from the 

burrows. The burrows are in a very fine to fine-grained sandstone consisting of well-rounded, clear smooth-surfaced quartz 

grains. All mud and other fine-grained particles have been removed, undoubtedly by the winnowing action of water. Bioturbation due to burrowing destroyed all bedding structures within the 

trace fossil beds prior to consolidation. The sandstone matrix, burrow filling, and burrow lining are all slightly calcareous and effervesce slowly in dilute hydrochloric acid.

The burrows range in diameter from 4 to 9 mm based on 63 

separate measurements; most fall in the 7 to 8 mm range. Original 

lengths of the burrows are indeterminate because the structures 

have been weathered away in their upper parts or have been 

truncated by overlying beds. The burrows commonly terminate 

downward in a bulbous structure with a rounded bottom. The bulb 

ranges in size from 25 to 50 mm in length and 8 to 14 mm in maximum diameter. The bulb is filled with ovoid and spindle-shaped 

fecal pellets in random orientation, mixed with fine-grained sandstone.

The burrows at places are abundant. In two hand specimens (fig.
MARINE TRACE FOSSILS, BLUFF SANDSTONE, UTAH 17

FIGURE 4. Oblique photograph of hand specimen of trace fossils showing characteristics

of burrow structure.

4) measuring 13 by 16 by 6 cm and 10 by 20 by 7.5 cm, there are 85 

and 74 recognizable burrow structures, respectively. At one locality 

on the outcrop there is a small bench, approximately parallel to the 

bedding, near the top of the lower trace fossil bed. Numerous white 

circles packed no more than 12 mm apart are exposed on the bench. 

Each white circle is a cross section of a burrow. Due to variations in 

animal behavior relative to burrow habitation and construction, 

however, it is impossible to estimate animal density during occupation of the substrate by organisms.

The burrows are straight, nontapering, unbranched, nonintersecting shafts lined with quartz grains firmly bound by calcite cement; phosphate cement has not been detected (G.A. Desborough, 

oral commun., 1974). The burrows are filled with homogeneous 

sand that is similar to the matrix rock. There is no evidence of spreiten, which are linear sediment traces indicative of sequential excavation and backfilling in response to some external stimulus 

such as sedimentation rates.

Most of the burrows have an internal canal near the circumference but separate from the burrow wall (fig. 5). The canal is 1 to 2 

mm across, and commonly is poorly preserved. In weathered specimens where the burrow wall is partially worn away, the canal is 

preserved as a U -shaped structure, convex toward the center of the 

burrow.
18 CONTRIBUTIONS TO STRATIGRAPHY

FIGURE 5. Vertical photograph of hand specimen of trace fossil showing morphology

and burrow density.

The burrows are very conspicuous on the weathered outcrop because the burrow tubes and fillings are bleached white in a matrix 

of brownish-gray sandstone. The burrows are much less conspicuous lower down in fresh white sandstone. Preservation is much 

better in the weathered zone than in fresh rock.

The full-relief trace fossils are oriented vertically to steeply inclined to the surface. Although the inclined burrows appear to have 

a random orientation, many of them have an inclination to the 

south or southwest. Modern burrows generally are inclined away 

from the dominant wave action. The inclination to the south and 

southwest of many of the trace fossil burrows suggests an ancient 

beach in that direction.

The burrows were undoubtedly domichnia (Seilacher, 1964) of decapod crustaceans and are similar to burrows of living Crustacea 

such as shrimp and crabs. Such decapod crustaceans are suspension feeders, which feed on particulate organic matter suspended in 

the water and do not burrow within the substrate for nourishment.

The environment of the burrows is in the upper foreshore of a 

marine beach. Lined small burrows are accepted by ichnologists as 

indicating a zone of relatively high water-energy. Burrows in quiet 

water rarely have linings, and either have random orientation relative to the sediment-water interface or are mostly horizontal or in-
MARINE TRACE FOSSILS, BLUFF SANDSTONE, UTAH 19

clined at low angles. Fauna who feed within the sediment usually 

have branching burrows, which have linings of organic-rich mud 

to delineate them from their matrix. Also, quiet waters commonly 

have a more diverse fauna. If the environment had been subaerial, 

the burrows probably would not be lined; if lined, the lining would 

not be likely to be as thick as these. Also, in a subaerial environment, population density would not be as high as in these rocks. If 

the environment had been a lake, there would be more mud, a more 

diverse fauna, and structures with thin walls or uncemented walls. 

Marine microenvironments other than upper foreshore have different fauna, different types of structures, and different lithologies 

than do the burrows described here.

SIGNIFICANCE

The trace fossils described in this report are helpful in interpreting the environment of deposition of the Bluff Sandstone. Member 

A, characterized by both flat-bedding and crossbedding and containing marine trace fossils, was probably deposited near the shore 

of an oscillating sea that reworked dunes which formed at the end 

of Summerville time. Member B contains high-angle crossbeds and 

is undoubtedly of eolian origin; it is perhaps akin to similar sandstone bodies in the Entrada Sandstone, which probably represent 

deposition under subaerial conditions on a beach near a sea. Member C, with low-angle crossbeds and numerous truncation planes, 

records the return of water which reworked dune sand. Bedding 

structures and the decrease in grain size and increase in carbonate 

cement compared to the underlying members show that member D 

was a subaqueous deposit (Craig and Cadigan, 1958, p. 185).

The waters in which member D was deposited and which reworked the dune sands of member C may indicate a return of the sea in 

which member A was deposited. Certain bedding features support 

this conception of deposition in marine waters although it is unsupported by fossil data. Member D locally weathers to rounded 

spheroidal weathering features termed hoodoos. Similar hoodoos 

in the underlying Entrada Sandstone are found in parts of the formation that are of marine origin (Harshbarger and others, 1957, p. 

44). In addition, the bedding features and general aspect of member D, at places (fig. 6), are strikingly similar to the Summerville, a 

formation also regarded as marine or marginal marine in origin.

The entire San Rafael Group in southeastern Utah is characterized by marine, marginal marine, and associated eolian deposits, 

which contrast sharply with the lithofacies of the underlying and 

overlying formations. The overlying Morrison Formation is dominantly a fluviatile and flood-plain deposit. The underlying formations of the Glen Canyon Group are dominantly eolian deposits
no CONTRIBUTIONS TO STRATIGRAPHY

P

FIGURE 6. Member D of Bluff Sandstone southeast of the town of Bluff. Exposures are 

in the NEV, sec. 8, T. 41 S., R. 23 E., located 8.5 miles (13.7 km) southeast of Bluff. 

A basal channeling sandstone of the Morrison Formation caps the cliff.

having minor fluviatile deposits. The Glen Canyon Group represents a long period of continental deposition and is everywhere 

separated from the marine deposits of the San Rafael Group by a 

great regional unconformity.

The contact between the San Rafael Group and the Morrison Formation also marks an important and widespread break in deposition. In the Red Mesa area of southeastern Utah, the contact separates the terrestrial fluviatile Morrison Formation from the marine 

Bluff Sandstone and the contact is an unconformity. Elsewhere on 

the Colorado Plateau, the contact represents a depositional hiatus. 

According to Cadigan (1967, p. 109), a widespread uplift and consequent withdrawal of marine waters brought to an end the tectonically quiescent environment of San Rafael time. After the uplift, 

erosion of the uppermost part of the San Rafael Group provided the
MARINE TRACE FOSSILS, BLUFF SANDSTONE, UTAH 111

initial fluviatile deposits of the Morrison Formation. R. A. Cadigan (written commun., 1968) further stated that "* * * the sequence 

of tectonic events which occurred after the deposition of the Bluff 

Sandstone and before deposition of the Morrison and which produced major correlated changes in lithology and sedimentary 

structures throughout the * * * Colorado Plateau regions very definitely did represent a considerable amount of geologic time."

The relationships described in this report suggest that the Bluff 

Sandstone did not extend very far into Arizona. The Bluff Sandstone, as previously discussed, thins markedly southward to a 

thickness of only 7 feet (2.1 m) at the west end of East Red Mesa. 

Furthermore, the inclination of the trace fossil burrows indicates a 

nearby ancient beach in Arizona beyond which the marine part of 

the Bluff Sandstone could not have extended. The Bluff, however, 

has been widely recognized in Arizona in the Carrizo Mountains 

area (Strobell, 1956) and in Black Mesa (Harshbarger and others, 

1957; Craig, 1959). It has also been recognized in New Mexico in the 

southern part of the San Juan Basin (Freeman and Hilpert, 1956; 

Green .and Pierson, 1971; Moench and Schlee, 1967; Thaden and 

Ostling, 1967). The relationship of the type Bluff Sandstone in 

southeastern Utah to these other sandstone bodies in Arizona and 

New Mexico called Bluff is as yet incompletely understood.

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. Geol. 

Survey Prof. Paper 183, 66 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: Am. Assoc. Petroleum Geologists Bull., v. 11, no. 8, p. 785-808.

Cadigan, R. A., 1952, The correlation of the Jurassic, Bluff, and Junction Creek 

Sandstones in southeastern Utah and southwestern Colorado: Pennsylvania 

State College M.S. thesis, 163 p.

___1967, Petrology of the Morrison Formation in the Colorado Plateau region: 

U.S. Geol. Survey Prof. Paper 556, 113 p.

Craig, L. C., 1959, Measured sections of the Morrison and adjacent formations: 

U.S. Geol. Survey open-file rept.

Craig, L. C., and Cadigan, R. A., 1958, The Morrison and adjacent formations 

in the Four Corners area, in Intermountain Assoc. Petroleum Geologists 

Guidebook 9th Ann. Field Conf., Paradox basin, 1958: p. 182-192.

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. Geol. Survey Bull. 1009-E, 

p. 125-168.

Freeman, V. L., and Hilpert, L. S., 1956, Stratigraphy of the Morrison formation 

in part of northwestern New Mexico: U.S. Geol. Survey Bull. 1030-J, p. 309- 

334.
112 CONTRIBUTIONS TO STRATIGRAPHY

Green, M. W., and Pierson, C. T., 1971, Geologic map of the Thoreau NE quadrangle, McKinley County, New Mexico: U.S. Geol. Survey Geol. Quad. Map 

GQ-954.

Gregory, H. E., 1938, The San Juan country, a geographic and geologic reconnaissance of southeastern Utah, with contributions by Malcolm Rutherford 

Thorpe and Hugh Dinsmore Miser: U.S. Geol. Survey Prof. 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 [Colorado 

Plateau]: U.S. Geol. Survey Prof. Paper 291, 74 p.

Haynes, D. D., Vogel, J. D., and Wyant, D. G., 1972, Geology, structure, and 

uranium deposits of the Cortez quadrangle, Colorado and Utah: U.S. Geol. 

Survey Misc. Geol. Inv. Map 1-629.

Moench, R. H., and Schlee, J. S., 1967, Geology and uranium deposits of the Laguna district, New Mexico: U.S. Geol. Survey Prof. Paper 519, 117 p.

Sears, J. D., 1956, Geology of Comb Ridge and vicinity north of San Juan River, 

San Juan County, Utah: U.S. Geol. Survey Bull. 1021-E, p. 167-207.

Seilacher, Adolf, 1964, Biogenic sedimentary structures, in Imbrie, John, and 

Newell, N. D., eds., Approaches to paleoecology: New York, John Wiley and 

Sons, Inc., p. 296-316.

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

Arizona and northwestern New Mexico: U.S. Geol. Survey Oil and Gas Inv. 

Map OM-160.

Thaden, R. E., and Ostling, E. J., 1967, Geologic map of the Bluewater quadrangle, Valencia and McKinley Counties, New Mexico: U.S. Geol. Survey Geol. 

Quad. Map GQ-679.

&U.S. GOVERNMENT PRINTING OFFICE: 1975-677-310/47