Dolomite and Siliciclastic Dikes and Sills in 

Marginal-Marine Cretaceous Coals of 

Central Utah

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Dolomite and Siliciclastic Dikes and Sills in 

Marginal-Marine Cretaceous Coals of 

Central Utah

By John K. Hardie

CLASTIC DIKES IN CRETACEOUS COALS IN UTAH AND COLORADO

U.S. GEOLOGICAL SURVEY BULLETIN 2087-A

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1994
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, Director

Published in the Central Region, Denver, Colorado 

Manuscript approved for publication February 9, 1994 

Edited by Judith Stoeser

Graphics by John Hardie and Mari L. Kauffmann 

Photocomposition by Mari L. Kauffmann

For Sale by U.S. Geological Survey, Map Distribution

Box 25286, MS 306, Federal Center

Denver, CO 80225

Any use of trade, product, or firm names in this publication is for descriptive purposes only and 

does not imply endorsement by the U.S. Government

Library of Congress Cataloging-in-Publication Data

Hardie, John K.

Dolomite and siliciclastic dikes and sills in marginal-marine 

Cretaceous coals of central Utah / by John K. Hardie.

p. cm. (U.S. Geological Survey bulletin : 2087-A)

Includes index.

Supt. of Docs, no.: 119.3:2087A

1. Dikes (Geology) Utah. 2. Sills (Geology) Utah. 3. Geology, 

Stratigraphic Cretaceous. 4. Coal Geology Utah. I. Title. 

II. Series.

QE75.B9 no. 2087A 

[QE611.5.U6]

557.3 s dc20 94-6890 

[551.8'8] CIP
CONTENTS

Abstract................................................................................................................................. Al

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

Depositional History and Stratigraphy................................................................................. 2

Previous Studies................................................................................................................... 3

Classification of Dikes.......................................................................................................... 3

Load-Compaction Dikes....................................................................................................... 3

Distribution................................................................................................................. 3

Description.................................................................................................................. 4

Pyramidal-Shaped Dikes...................................................................................... 9

Sigmoidal-Shaped Dikes...................................................................................... 9

C-Shaped Dikes.................................................................................................... 10

Sills....................................................................................................................... 10

Mineralogy.................................................................................................................. 10

Interpretation.............................................................................................................. 12

Discussion................................................................................................................... 13

Channel-Margin Dikes......................................................................................................... 15

Description.................................................................................................................. 15

Interpretation.............................................................................................................. 16

Fracture-Fill Dikes................................................................................................................ 17

Description.................................................................................................................. 17

Interpretation.............................................................................................................. 17

Significance of Dikes............................................................................................................ 18

References Cited................................................................................................................... 18

FIGURES

1. Index map showing location of study area in central Utah.................................................................................... A2

2. Schematic section showing emplacement of clastic dikes and sedimentary architecture...................................... 4

3. Drawing showing distribution and orientation of load-compaction dikes and dike swarms in Trail Mountain

mine..............................................................................._ 5

4. Negative prints of sigmoidal dike.......................................................................................................................... 6

5. Photograph and schematic drawing showing sandstone floor rock cut and heaved by low-angle dike................. 7

6-10. Photographs showing:

6. Pyramidal-shaped dikes with different forms of secondary injection features attached to their termini..... 8

7. Sigmoidal-shaped dike with sill attached to outside radius of sharp fold.................................................... 10

8. C-shaped dike in Hiawatha coal seam.......................................................................................................... 11

9. Multiple clastic sills near base of coal seam................................................................................................. 11

10. Core containing small-scale dikes at top of marl lens below base of Hiawatha coal seam in Star Point

Sandstone...................................................................................................................................................... 12

11. Diagram showing trends of structural elements and related features in vicinity of Trail Mountain mine ............ 15

12. Photograph showing channel-margin dike attached to sandstone channel body in roof of coal seam.................. 16

13, 14. Sketches showing:

13. Large-scale channel-margin dike.................................................................................................................. 16

14. Typical fracture-fill dike............................................................................................................................... 17

m

Dolomite and Siliciclastic Dikes and Sills in Marginal-Marine

Cretaceous Coals of Central Utah

By John K. Hardie

ABSTRACT

Clastic dikes intrude 13 of 15 commercial coal seams 

that span more than 250 m of section along the eastern flank 

of the Wasatch Plateau and the western Book Cliffs of central Utah. Dikes are grouped into three principle populations 

that reflect different modes of origin, distribution, and geometry. Channel-margin dikes are scattered siliciclastic bodies 

that rarely exceed 5 cm in thickness and seldom penetrate 

deeper than the upper meter of coal seams. Commonly, they 

are physically attached to channel-sandstone bodies that 

make up the roof of coal seams. These dikes form by infilling 

tears that develop in the peat along channel-body margins. 

Fracture-fill dikes are rare, high-angle siliciclastic bodies 

that rarely exceed 1 m in thickness and can cut entire coal 

seams. Their fault-bound margins, which cut entire coal 

seams, indicate that they formed by injection along fractures. 

Load-compaction dikes are by far the most abundant type of 

dike in the central part of the Wasatch Plateau. They can 

exceed 2 m in thickness and extend more than 450 m laterally. Load-compaction dikes are further divided into three 

subgroups: broad, pyramidal-shaped dikes that penetrate the 

lower part of coal seams; sigmoidal-shaped dikes, which are 

ribbonlike features that commonly cut entire coal seams at 

high angles; and rare C-shaped dikes, which are broad tabular injections that appear to cut entire coal seams. Load-compaction dikes are composed principally of dolomite and form 

from the sediment loading of water-saturated lenses of dolomitic marl interbedded in sandstone directly beneath peat. 

Evidence for this type of origin includes exposures showing 

sandstone floor rocks that have been heaved upward by dolomitic dikes that penetrate into the base of coal seams, the 

presence of dikes in swarms, and the petrology of the dike 

material, which is indistinguishable from that of the underlying marl lenses. Paleotopography, irregularities along the 

depositional surface of the peat, overpressuring of shoreface 

sandstone, and the physical characteristics of the peat may 

have had significant influence on dike formation. Clastic 

sills are common features that can exceed 10 cm in thickness 

and extend laterally more than 100 m from their origins at 

sigmoidal and pyramidal dikes. The sills are composed of 

microcrystalline dolomite that was squeezed from dikes

during peat compaction. Evidence for this type of origin 

includes the physical attachment of the sills to pyramidaland sigmoidal-shaped dikes, the abundant transposition 

structures in the sills, the fibrous character of the peat at the 

time of sill formation, and the similar petrology of sills and 

dikes.

Clastic dikes are common in coal seams that 

accumulated in marginal-marine settings and have caused a 

wide variety of serious mine problems. They were the 

principle factor in closing operations at the Cameo and 

Cameo No. 1 mines in western Colorado. Their presence in 

the Appalachian Basin has rendered some coal seams 

unminable.

INTRODUCTION

Clastic dikes (also called horsebacks, clay slips, rock 

faults, mud slips, rock spars, dumb faults, stone dikes, clay 

veins, clay seams, mud seams, nips, and rock-jumbles), as 

discussed herein, are tabular bodies that cut across coal 

bedding. They are composed of sedimentary material that 

was either deposited or injected from a source(s) outside the 

host coal seam. Clastic sills are tabular concordant bodies 

composed of sedimentary material that was extruded from a 

source(s) either inside (such as clastic dikes) or outside the 

host coal seam.

Dikes penetrate 13 of 15 minable coal seams that span 

more than 250 m of section along the Wasatch Plateau and 

western Book Cliffs of central Utah (fig. 1) (Hardie, 1991). 

They range from small features a few centimeters thick to 

massive intrusions more than 2 m thick that extend laterally 

as much as 480 m. In mines where dikes are abundant and 

(or) their thicknesses exceed approximately 10 cm and cut 

deep into coal seams, they cause a wide variety of problems 

(Hardie and Fleck, 1991; Hardie, 1992; Shea-Albin, in press) 

including roof instability (Savage, 1910; Kerns, 1970; 

Dunrud, 1976; Hill and Bauer, 1984; Chase and Ulery, 

1987), impaired coal production (Kerns, 1970; K. Fleck, 

Deer Creek mine, oral commun., 1990; L. Ruschke, 

Roadside mine, oral commun., 1993), roof vibration when 

cut (Ellenberger, 1979; Moebs and Stateham, 1986),

Al
A2 CLASTIC DIKES IN CRETACEOUS COALS, UTAH AND COLORADO

increased ash (Doelling, 1979; Johnson, 1982), channeling 

of water and increased methane emissions (Shea-Albin, in 

press), methane ignition from sparks generated by coal-cutting equipment (L. Adair, oral commun., 1991), damage and 

increased wear of coal-cutting equipment (Ellenberger,1979; Moebs and Stateham, 1986), and increased 

disposal handling of mine waste (Ellenberger, 1979), and 

they are a hazard to mine personnel. Dikes that are either too 

thick or too hard to be cut by conventional mining equipment 

must be drilled and blasted repeatedly, sometimes delaying 

production for days (K. Fleck, oral commun., 1991). In both 

the eastern and western United States, clastic dikes have 

been a significant factor in mine closures (Boreck, 1986). In 

western Colorado, near the town of Grand Junction, dikes 

were a principal factor in closing operations at the Cameo 

(Dunrud, 1976) and Cameo No. 1 mines (L. Rushke, 

Roadside mine, oral commun., 1993). At the turn of the 

century, Gresley (1897, p. 39) wrote that the presence of 

dikes in the Appalachian Basin has rendered some coal 

seams worthless. He went on to say that "portions of the 

Pittsburgh coal seam in West Virginia have been found so 

thickly intersected thereby that mining operations had to be 

abandoned in such areas."

This investigation was conducted to determine the 

nature, distribution, geometry, and predictability of clastic

nri5' 11 TOO1 1KT451

0 20 40 KILOMETERS

1,1 I

Figure 1. Index map showing location of study area in central 

Utah.

dikes and sills and to determine their relation to processes 

active during and after coal accumulation. The results of this 

study add to the understanding of coal geology, dolomite 

formation, coal development and production, basin development, and shoreline processes. They also provide mining 

strategies for operators producing coal from seams cut by 

clastic dikes.

Data used in this study were collected primarily from 

direct examination of outcrops and mine workings, oral and 

written reports from local mine operators, surface and inmine drill cores, and well logs. The large number of 

underground mines in the study area using room-and-pillar 

mining methods has resulted in widely scattered threedimensional exposures along entries and crosscuts. Where 

mine workings were not accessible to the author because of 

unsafe mine conditions or because they had been sealed off 

following coal extraction, mine operators frequently 

provided photographs, samples, sketches, and detailed mine 

maps showing the geometry and location of dikes and sills. 

Continuous exposures along canyon walls provided 

excellent conditions for examining dikes, sills, and adjacent 

strata. At one location, excavation equipment was used to 

expose dikes for detailed examination. Drill core was used 

primarily to study the mineralogy and sedimentology of the 

floor and roof rocks; however, dikes were present in some 

core. Clastic material was analyzed using X-ray diffraction, 

cathodoluminesccnce, and binocular and petrographic 

examination. Quantitative results were determined by 

optical methods and by point counting 300 mineral grains 

per thin section.

Acknowledgments. I am grateful for the cooperation 

and information provided by numerous central Utah coal 

companies, particularly their geology and engineering staffs. 

Kenneth Fleck helped to photograph and collect mine 

samples and provided mine expertise in the Trail Mountain 

mine. Mine geologists Mark Bunnell of the Skyline mine, 

Tom Lloyd of the Cottonwood mine, John Mercier of the 

Plateau mine, and Laine Adair and Mike Glasson of the Centennial mine also provided valuable information, particularly 

where mine workings were no longer accessible. Rodger Fry 

of PacifiCorp provided drill core and valuable mine data. 

James Cathcart and William Betterton of the U.S. Geological 

Survey assisted in sample analysis.

DEPOSITIONAL HISTORY AND 

STRATIGRAPHY

Commercial coal seams along the Wasatch Plateau and 

western Books Cliffs were assigned to the Upper Cretaceous 

Blackhawk Formation by Young (1965). These coal seams 

accumulated along an eastward-prograding shoreline of the 

Cretaceous epeiric seaway during Campanian time. Major 

uplifts along the Sevier orogenic belt, which extended along
DOLOMITE AND SILICICLASTIC DIKES AND SILLS A3

the western margin of Cretaceous foreland basin, caused 

pulses of terrigenous clastic material to be shed across 

nearby lower coastal plain and marginal-marine coastal 

environments (Armstrong, 1968; Balsley, 1980). Progradation, sea-level fluctuations, and lateral shifting and 

abandonment of coastal complexes resulted in repeated 

transgressive-regressive sedimentation that is recorded as a 

series of vertically stacked, forward-stepping, coarseningupward sedimentary packages, each of which is tens of 

meters thick. Typically, these packages consist of deepwater shale overlain by thick shoreface deposits and capped 

by coal-bearing terrestrial deposits (fig. 2). Many coal seams 

accumulated directly on upper shoreface deposits, as 

evidenced by root horizons across the upper bounding 

surfaces of shoreface sandstones. These rooted bounding 

surfaces indicate emergence of shoreface deposits prior to 

the development of peat swamps. Examination of core cut 

from shoreface deposits reveals multiple lenses of marl that 

range from several centimeters to more than a meter in thickness and are composed principally of detrital dolomite. During Cretaceous time, Paleozoic carbonate-rich sediments 

exposed along the orogenic belt were transported to depositional sites along the seaway margin (Balsley, 1980). Longshore processes redeposited these carbonate sediments along 

a northwest-trending paleoshoreline (Flores and others, 

1982, 1984) as elongate marl lenses. Previous studies suggest that thick and continuous coal seams along the Wasatch 

Plateau accumulated along protected sites behind barrier 

islands (Young, 1965; Howard, 1969; Flores and others, 

1982, 1984). Thick coal seams slightly higher in the section, 

exposed along the western Book Cliffs, accumulated in 

wave-dominated delta-plain settings (Balsley, 1980).

PREVIOUS STUDIES

Since dikes were described in central Europe by Agricola in 1556 (Hoover and Hoover, 1950), their presence in 

coal fields and in non-coal-bearing sequences around the 

world has been well documented. In a classic study, 

Newsom (1903) stated that coal beds are particularly favorable for the formation of clay and sandstone dikes. Although 

dikes are common and widespread in North American coal 

fields (Crane, 1898), most studies of clastic dikes and sills in 

coal measures are dated or of limited scope. A summary of 

work on clastic dikes in North American is given by 

Damberger(1973).

In central Utah, clastic dikes and sills were first studied 

by Doelling (1979) in a small and heavily mined area in the 

central Wasatch Plateau. Doelling proposed that dikes in the 

Hiawatha coal seam formed from infilled desiccation cracks 

in the peat. Johnson (1982), in a mine study conducted in the 

same study area as Doelling, concluded that surface 

lineaments did not appear to be directly related to the

orientation or presence of clastic dikes, but he proposed no 

specific mode of origin. In a later study, several local mine 

geologists working along the northern part of the Wasatch 

Plateau concluded that the origin of dikes was controlled by 

tectonic or structural fabric (Lindberg and others, 1983). In a 

petrographic study of rocks collected from the Cottonwood/ 

Wilberg mine, a few kilometers east of Doelling's and 

Johnson's area, Jepsen (1987) proposed that dikes formed 

when desiccation cracks formed in the peat and were infilled 

by windborne detritus. He pointed out that desiccation 

cracks more than 0.6 m deep are present in peat beds in the 

Big Cypress Swamp in the southeastern United States.

Bunnell and Taylor (1987) studied clastic dikes at two 

widely separated mine sites in the northern part of the 

Wasatch Plateau. They proposed that some of the dikes in the 

mine workings near the town of Price (fig. 1) originated from 

fluvial sand bodies immediately overlying the coal but 

suggested no specific mode of origin. At the other mine site, 

about 15 km to the west and near the town of Scofield, they 

reported that dike trends are similar to regional fault trends 

and vertical joint planes in the coal and overlying rocks. 

They proposed that dike formation may be related to local 

tectonic activity including earthquake shocks.

CLASSIFICATION OF DIKES

Clastic dikes have been divided into three principal 

groups: load-compaction dikes, channel-margin dikes, and 

fracture-fill dikes (Hardie, 1991). Dikes within each group 

have a similar mode of origin and physical and compositional characteristics. On the basis of their geometry and 

attitude, load-compaction dikes can be further divided into 

pyramidal, sigmoidal, and C-shaped dikes. Clastic sills are 

included with load-compaction dikes because they are 

secondary features that formed directly from loadcompaction dikes.

LOAD-COMPACTION DIKES

DISTRIBUTION

Load-compaction dikes, with the exception of rare 

C-shaped dikes, are particularly abundant in an area more 

than 5 km wide in the central Wasatch Plateau that includes 

the Trail Mountain mine (figs. 1,3) and several nearby abandoned mine properties that produced coal from the Campanian Hiawatha coal seam. Dikes are also present, but are 

much less common, in mine properties about 27 km to the 

north, near the town of Wattis, and about 48 km to the south, 

north of the community of Emery (fig. 1). Dikes are present 

in several coal seams that span more than 60 m of section, 

although rare and poorly documented C-shaped dikes have
A4 CLASTIC DIKES IN CRETACEOUS COALS, UTAH AND COLORADO

Maximum 

sediment loading

Hypothetical 

paleotopography

Marine shale 

interbedded with 

sandstone beds 

(Masuk Member 

of the Mancos 

Shale)

Terrestrial deposits 

including fluvial channel 

bodies

Coal cut by three types 

of clastic dikes:

A. Load-compaction dike 

B. Channel-margin dike 

C. Fracture-fill dike

Shoreface sandstone

containing scattered

lenses of dolomitic marl

(Star Point Sandstone)

EXPLANATION

Migration of dike material

Fault Arrows show direction 

of movement

Swaley bedding

Low-angle crossbeds

Marine bivalve

Burrow

Trough crossbeds

(NOT DRAWN TO SCALE) 

Figure 2. Schematic section showing emplacement of clastic dikes and sedimentary architecture.

been reported only in the Hiawatha seam at the Trail Mountain mine. There are no known load-compaction dikes along 

the western Book Cliffs. Clastic sills are common in the 

vicinity of the Trail Mountain mine, rare throughout most of 

the remaining Wasatch Plateau, and absent along the western 

Book Cliffs.

DESCRIPTION

Load-compaction dikes range from a few centimeters to 

more than 2 m in thickness and have been traced laterally for 

more than 450 m along mine entries and crosscuts. They 

penetrate all or portions of coals seams at low to high angles
DOLOMITE AND SILICICLASTIC DIKES AND SILLS A5

ALL DIKES 

«=242

ALL SOLITARY

DIKES

n=28

Figure 3. Drawing showing distribution and orientation of load-compaction dikes and dike swarms in the Trail Mountain 

mine. Rose diagrams include dikes longer than 10m and are weighted by a factor of one for each hundred meters of dike 

length. Solid lines indicate clastic dikes; shaded areas indicate excavated mine workings.
A6 CLASTIC DIKES IN CRETACEOUS COALS, UTAH AND COLORADO

Figure 4. Negative prints of a sigmoidal dike. The photograph on the right (field, 6 cm by 4 cm) is an enlarged view of the framed area in 

the photograph on the left (field, 15 cm by 9 cm). It shows millimeter-scale faults and near-horizontal vitrain bands adjacent to dike contacts 

and delicate fibrous plant fragments (marked by arrows) that interfinger with dike margins.

and have a dominant central trend of N. 32° E. and a subordinate trend of N. 50° W. (fig. 3A). Dikes are present both as 

solitary intrusions and, more commonly, in swarms. Solitary 

dikes have a dominant trend of N. 40° W. and a subordinate 

trend of N. 10° E. (fig. 3B). Swarms are as closely spaced as 

200 m apart and cover areas that range from less than 150 m 

to more than 1,000 m in width (fig. 3). Individual swarms 

range from less than 10 to more than 150 dikes and include 

both pyramidal- and sigmoidal-shaped dikes and commonly 

clastic sills. The central trend of dikes within individual 

swarms generally varies from one swarm to another as 

shown in figures 3C-G. Coal cleat, oriented roughly 

N. 65° E. and N. 15° W., can be traced cutting across the 

entire thickness of some dikes. Dikes, particularly those that 

are massive, are commonly cut by several medium- to highangle faults that have offsets that measure from less than a 

centimeter to as much as 20 cm.

Dike contacts range from smooth to finely serrated to 

coarsely jagged. Where contacts are smooth, dikes are bound 

by numerous sets of slickensides that are generally oriented 

parallel to subparallel to the direction of dike shortening. 

Jagged contacts are composed of a series of crude wedgeshaped projections of interfingering coal and dike material. 

Detailed impressions of sticks and other coarse vegetable 

matter, some showing the texture of their bark, are fairly 

common along the subhorizontal surfaces of individual projections. The coal along the margins of dikes is commonly 

cut by hundreds of millimeter- to centimeter-scale faults (fig. 

4). In some exposures, bright vitrain bands are bent strongly 

upward near dike margins; however, in others, banding is 

almost horizontal. Examination of the polished block of dike 

material and coal shown in figure 4 confirmed the presence 

of horizontal to low-angle coal layering adjacent to the 

margins of some dikes. Figure 4 also shows that the delicate
DOLOMITE AND SILICICLASTIC DIKES AND SILLS A7

* *

Figure 5. Photograph and schematic drawing showing sandstone floor rock of the Star Point Sandstone cut and heaved 

by a low-angle dike.
A8 CLASTIC DIKES IN CRETACEOUS COALS, UTAH AND COLORADO
DOLOMITE AND SILICICLASTIC DIKES AND SILLS A9

Figure 6 (above and facing page). Pyramidal-shaped dikes with 

different forms of secondary injection features attached to their 

termini. Scale cards (circled) in A and B are 15 cm high and the 

pick in C is about a meter high.

coalified fibrous plant material that penetrates the margins of 

dikes is roughly horizontal to low angled and approximately 

perpendicular to the direction of dike shortening. 

Examination of some thin sections of dike contacts shows a 

similar orientation of fibrous plant material.

Dike material is composed of light- to medium-gray, 

well-indurated, sandy dolostone. In some dikes, irregularshaped clasts or plugs of clay stone and shale more than 10 

cm across appear to be lodged in narrow portions of dikes. 

Fresh exposures of dike material commonly reveal two types 

of flow structures, circular swirls and less common laminar 

structures, both composed of alternating light and dark 

material produced by differences in grain size. Concentric 

swirls, approximately a centimeter or more across, 

commonly are present in massive dikes, where they produce 

a mottled appearance. Laminar flow structures rarely are 

more than 5 mm thick and are composed of very thin laminae 

that alternate between light- and dark-gray rock. Laminae

are generally oriented parallel to the lower contact of lowangle dikes and sills and follow smoothly and evenly around 

irregularities along dike contacts.

In rare mine exposures, coal-cutting equipment has 

exposed sandstone floor rocks that are offset as much as 2 cm 

on either side of dikes that cut through the floor rock and into 

the base of the coal. Figure 5 shows one such exposure 

where the floor rock directly above the inclined dike is offset 

more than 15 cm above the adjacent floor rock on the 

opposite side of the dike. Examination of thin sections cut 

from the dike material and from the floor rock on either side 

of the dike shows contrasting mineralogy. The floor rock is 

composed of typical quartzose Star Point Sandstone, 

whereas dike material is composed of approximately 65 

percent dolomite and 35 percent quartz, chert, and feldspar. 

In the sandstone floor rock along the dike-floor contacts are 

scattered pockets of microcrystalline dolomite that measure 

from several millimeters to roughly a centimeter in width 

and display well-defined laminar flow structures accentuated 

by streaks of organic-rich material.

PYRAMIDAL-SHAPED DIKES

Pyramidal dikes are common intrusions that measure 

from about 30 cm to more than 2 m thick at their base and 

rapidly taper to a terminus about 1-2 m above the floor. 

Attached to their termini are various forms of secondary 

injection features that include sills and discordant contorted 

bodies of clastic material such as those shown in figure 6. 

Dike contacts range from almost smooth to jagged. Where 

contacts are jagged, fishtail-shaped projections of coal as 

thick as several centimeters commonly penetrate the margins 

of dikes.

SIGMOIDAL-SHAPED DIKES

Sigmoidal dikes are common features that are present 

as crenulated ribbonlike bodies that cut through most or all 

of coal seams at medium to high angles. High-angle dikes, 

those greater than about 70°, display complex geometries 

that are intensely crenulated and highly faulted and swollen 

(fig. 4). In rare exposures, sills extend laterally for tens of 

meters from the outside radius of sharp folds in high-angle 

dikes (fig. 7). Dikes are generally less than 20 cm thick and 

taper toward the roof; however, in one exposure where heavy 

equipment was used to excavate the lower part of an 

average-size dike, the base of the dike abruptly flared to 

more than 2 m across within about 15 cm of the sandstone 

floor rock. Such thickening suggests that sigmoidal dikes 

may grade into pyramidal dikes. Where dikes are present in 

swarms, they commonly cut adjacent dikes at various angles 

to form complex interconnected networks. Dikes have been 

traced laterally along the roofs of mine entries, where they 

abruptly split into two or more diverging branches. Some 

sigmoidal dikes cut into the roof rock where they generally
A10 CLASTIC DIKES IN CRETACEOUS COALS, UTAH AND COLORADO

Figure 7. Sigmoidal-shaped dike with sill attached to the outside 

radius of a sharp fold. Scale card (circled) is 15 cm high.

taper and pinch out within a few centimeters of the coal-roof 

contact. Here, just beneath the roof line, they abruptly swell 

to as much as four times their normal thickness, then 

abruptly thin at the point where they enter the roof rock. In 

other exposures, particularly where the roof rock is 

composed of well-indurated organic-rich claystone, dikes 

penetrate only to the base of the roof rock, where they 

become highly swollen and create convex-upward 

depressions along the coal-roof contact that measure many 

centimeters across.

C-SHAPED DIKES

When viewed in cross section, C-shaped dikes are 

tabular bodies that are shaped like a broad and crudely 

formed C (fig. 8). They range in thickness from about 15 cm 

to 40 cm, but their thickness varies greatly when traced from 

floor to roof. Dikes appear to cut entire coal seams; 

however, poor exposures conceal their affinity with floor 

and roof rocks. Dikes are sharply bound and commonly 

display bulbous irregularities along their contacts that are 

caused by uneven distribution of dike material.

SILLS

Sills are fairly common in areas where pyramidal dikes 

are present and are less common where sigmoidal dikes are 

found. They are typically attached to the terminus of 

pyramidal dikes (fig. 6A) and to the outside radii of tight 

folds in sigmoidal dikes (fig. 7). Sills are as thick as 10 cm 

and can extend laterally for more than 100 m. They are 

generally present in multiples of from 2 to 10 closely spaced 

sills in the lower half of coal seams (fig. 9). They have sharp 

contacts and where traced laterally display fairly uniform 

thicknesses, with the exception of an occasional bulbous 

area that is swollen with sill material. Millimeter-scale, lowto medium-angle dikes commonly connect a thick sill with 

one that is considerably thinner. It is likely that thicker sills 

provided clastic material for the development of thinner 

ones. Well-defined millimeter-scale laminar flow structures 

are common along the lower contact of sills.

MINERALOGY

Forty-two stained and unstained thin sections 

representing dike and sill material, coal-seam floor and roof 

rock, and marl were analyzed petrographically. Because of 

the heterogeneity of dike material observed in the thin 

sections, large samples from which the thin sections were cut 

were analyzed by X-ray diffraction to determine bulk 

mineralogy. Seven samples were examined using cathodoluminescence.

Dike material consists of heterogenous dolostone 

containing scattered millimeter- to centimeter-scale pockets 

and stringers of quartz and (or) dolomite grains that are 

generally of a different grain size than that of the host rock. 

Clastic material can be either matrix- or grain-supported and 

is composed of 46-85 percent dolomite, 9-44 percent quartz,

2-6 percent feldspar, and trace amounts of chert and lithic 

fragments of sedimentary origin. Although generally absent, 

where present calcite rarely exceeds 2-3 percent. Dolomite 

is present in at least four forms that include 2-6 percent of 

total dolomite content as partially replaced feldspar grains,

3-9 percent as cement, 1-12 percent as aggregate detrital 

grains, and 42-78 percent as monocrystalline detrital grains. 

Feldspar grains are abraded and partially altered to dolomite. 

Dolomite cement is iron-rich anhedral pore fill that has 

eliminated any visible porosity. Subrounded to wellrounded aggregate dolomite grains are poorly to well sorted 

polycrystalline and range from less than 50 to more than 

300 |i in diameter. Individual component grains of dolomite 

are poorly to well sorted, rounded to well-rounded grains 

that range from less than 10 to 100 |Ll in diameter. Monocrystalline dolomite grains rarely exceed 250 |U in diameter and 

are generally poorly sorted and rounded to well rounded. 

Grains commonly display optically continuous ferroan rims; 

however, minor euhedral to subhedral grains ranging from
DOLOMITE AND SILICICLASTIC DIKES AND SILLS All

Figure 8. C-shaped dike in the Hiawatha coal seam. Rubble is in area beneath hammer (circled).

Figure 9. Multiple clastic sills near base of a coal seam. Hammer (circled) is shown for scale.
A12 CLASTIC DIKES IN CRETACEOUS COALS, UTAH AND COLORADO

Jf*.-.

parent 

material 1"

G*-a;(* 

214$

 * « 

Figure 10. Core containing small-scale dikes at the top of a marl 

lens 1.5 m below the base of the Hiawatha coal seam in the Star 

Point Sandstone.

sharp-edged to slightly abraded rhombs also are present and 

may be incorrectly grouped with detrital material. This 

variation in grain angularity suggests variable reworking of 

dolomite grains. Under cathodoluminescence, some rounded 

grains contain well-defined cores; however, the fact that 

slightly abraded to sharp-edged dolomite rhombs do not contain such cores suggests that euhedral grains did not form 

from secondary overgrowth of detrital cores but that entire 

grains precipitated at sites nearby. Highly fractured dolomite grains that are iron stained along their fractured edges 

are common and probably resulted from postdepositional 

compaction followed by contact with iron-bearing magnesium-rich fluids. Individual pockets and stringers of clastic 

material, commonly seen in thin section, display a narrow 

range of grain sizes; however, within a single dike, different 

stringers and pockets display grain diameters that range from 

microcrystalline to more than 300 |i.

Petrographic examination of marl lenses, cored from 

depths ranging from a few centimeters to more than 40 m 

below the base of the coal seams, shows that the marl is

similar in composition to dike material. Marl is composed of 

31 -75 percent dolomite, 18-58 percent quartz, 4-9 percent 

feldspar, and trace amounts of chert and lithic fragments. 

The four forms of dolomite in the dike material are present 

in similar quantities in marl lenses. The major difference 

between the two types of rock is that the marl is homogenous, lacking the stringers and pockets of sedimentary material common in dikes. It is interesting that small centimeterscale dikes were found at the top of a cored marl lens, 

approximately 1.5 m beneath the top of the Upper Cretaceous Star Point Sandstone (fig. 10).

About 300 m east of the main portal at the Trail 

Mountain mine, a narrow westerly trending lens-shaped 

sandstone body, interpreted by local mine geologists as a 

possible washover fan, was scoured into the top of the Star 

Point Sandstone and is overlain by the Hiawatha coal seam. 

Petrographic examination of the fine-grained rock along the 

margins of the lens shows that the sedimentary material has 

a composition very similar to that of the dike material.

Sill material is composed of more than 90 percent ironrich microcrystalline dolomite and minor amounts of fine- to 

medium-grained detrital quartz and dolomite grains.

INTERPRETATION

Intense folding, swelling, and faulting of high-angle 

sigmoidal dikes indicates that they were injected early, prior 

to significant compaction and decomposition of the peat. A 

compaction ratio of four to one was determined for a sigmoidal dike; its height was restored by outlining its margins with 

a string and then measuring the length of string. This ratio 

does not, however, include the amount of height that was lost 

through faulting, dike swelling, and lateral migration of clastic material. The fibrous character of plant material along 

dike margins and the well-preserved impressions of bark texture along dike contacts show that time was insufficient for 

significant decomposition of plant material to occur. In a 

study on coal metamorphism, Teichmuller and Teichmuller 

(1968) stated that the detailed features of initial plant material are often recognized in peat but are not recognizable in 

rank stages from soft brown coal and beyond. The presence 

of cleated dikes and the absence of coal cleats filled with 

dike material indicate that dikes formed prior to cleat development. Close (1991) estimated that cleat development in 

Cretaceous coals occurred within 4 m.y. of peat accumulation in the northern San Juan Basin and within 1.5-5.8 m.y. 

in the Raton Basin. Although it is widely accepted that cleat 

development is interdependent on many factors (Close, 

1992), if one accepts Close's figures as an approximate reference frame for coal cleat development in central Utah 

coals, then dikes formed within roughly 6 m.y. of peat accumulation.

Several lines of evidence demonstrate that dike material 

was injected from below. Mine exposures showing floor
DOLOMITE AND SILICICLASTIC DIKES AND SILLS A13

rocks that have been cut by dikes and that are offset on either 

side of the dikes leave little doubt that injection was from 

below. Here, pressure beneath the floor caused by the 

upward migration of dike material probably caused the floor 

to heave directly above the dike. The offset fracture created 

along the floor rock provided an avenue for the dike material 

to penetrate into the base of the peat. Further evidence for 

injection from below includes convex-upward depressions, 

interpreted as pressure points, along coal-roof rock contacts 

just above the terminus of some dikes and dikes that are 

swollen just below the roof rock contact. Here, the upward 

migration of dike material was probably impeded at the base 

of resistant roof sediment, resulting in dike swelling and the 

formation of pressure points along the roof line. The 

abundance of dikes that taper upward and that pinch out 

before reaching the roof of coal seams is further evidence 

that injection was from below.

Dike material was derived from multiple sources within 

the Star Point Sandstone. The composition of typical Star 

Point Sandstone and of the Masuk Member of the Upper 

Cretaceous Mancos Shale (Davis and Doelling, 1977; 

Jepsen, 1987), as compared to that of dike material, 

precludes either unit as a possible source. Petrographic 

analysis of cored marl lenses from within the Star Point 

Sandstone shows, however, that dike material and marl are 

almost identical both mineralogically and petrologically. 

Mine exposures where the sandstone floor rocks are offset 

along dikes that cut into the base of the coal demonstrate that 

the source material is at least 15 cm below the top of the Star 

Point Sandstone. In addition, centimeter-scale dikes at the 

top of cored marl lenses possibly are small-scale clastic 

dikes that failed to develop into larger forms. The presence 

of dikes in swarms suggests that multiple marl lenses within 

the Star Point Sandstone provided clastic material for dikes. 

Individual or several closely spaced dike swarms within the 

same area are believed to correspond to an isolated body of 

marl.

Injection of dike material was probably a gradual or 

intermittent process that did not occur everywhere at the 

same time as evidenced by the crosscutting relations of some 

dikes. In addition, the heterogeneity of dike material, 

including the presence of parallel laminar flow structure 

recognized in thin section, suggests a less than violent 

injection process. Hesse and Reading (1978), in a study on 

clastic dikes in non-coal-bearing strata, recognized similar 

flow structure and suggested that the extrusion of sediments 

took place in a laminar fashion but may have been more 

violent in the case of homogenized dike material. They 

pointed out, however, that if injection were sudden and 

violent the rate of flow of clastic material probably would 

have waned some time after the initial triggering event and 

produced features indicative of gradual migration of clastic 

material.

The attitude of vitrain bands and the jagged 

interfingering projections of coal along some dike contacts

suggest that dike material migrated both laterally and 

vertically through the peat. Where jagged projections are 

turned up and vitrain bands are sharply bent upward, dike 

material was probably injected vertically. The drag between 

migrating clastic material and the peat resulted in 

deformation of both banding and interlayered projections. 

Where coal banding and interfingering coal are relatively 

horizontal and undisturbed, dike material probably migrated 

laterally. Fine and delicately interlayered primary plant 

components that interfinger with dike material (fig. 4) 

suggest that these were sites of lateral migration. It is 

reasonable that the path of migration was both vertical and 

horizontal during the injection process. Another possibility 

is that dike material was a low-viscosity slurry of mostly 

water and minor suspended detrital grains that imposed little 

drag on plant material.

Sills are secondary features that formed from dikes. 

The common occurrence of sills that are physically attached 

to the termini of pyramidal-shaped dikes and to tight folds of 

high-angle sigmoidal dikes demonstrates that sills formed 

from dikes. Their sharply bound concordant contacts along 

coal bedding planes and their well-preserved laminar flow 

structure indicate that dike material advanced along 

horizontal zones of weakness within the peat. Elliott's 

(1965) classification of subaqueous sedimentary structures 

designates laminar flow structures as horizontal 

transposition structures whose geometry suggests that the 

sediments were subject to lateral extension. Furthermore, 

sill material is almost identical in composition to dike 

material.

DISCUSSION

Although the origin of load-compaction dikes is not 

completely understood, the principal driving force 

responsible for mobilization of carbonate-rich sediments is 

gravity acting on the sediments overlying lenses of marl. 

Other factors, including irregularities along the surface of 

accumulation, overpressuring of the Star Point Sandstone, 

and the channeling of water from the underlying marine 

shale, probably played a role in dike development. Kerns 

(1970) proposed that dikes in the Pittsburgh coal seam in 

southern Pennsylvania were mainly controlled by vertical 

stress and differential compaction. Vertical stress caused by 

the weight of overlying sediments, acting uniformly across a 

lens of water-saturated marl, would pre-load the lens (Athy, 

1930), but the sediments would remain in place. Uneven 

load stress caused by paleotopography could cause a portion 

of the lens to collapse where load stress was high, squeezing 

out the dike material as in pinching a tube of toothpaste (fig. 

2). The Star Point Sandstone that surrounds the marl lenses 

probably acted as a resistant barrier that directed the 

migration of dike material upward, toward low-pressure sites 

along the base of peat beds. Sites where dike material entered 

the base of peat beds may have been controlled by
A14 CLASTIC DIKES IN CRETACEOUS COALS, UTAH AND COLORADO

irregularities along the depositional surface of peat 

accumulation (Wilson, 1916). Local irregularities along the 

base of the Hiawatha coal seam at the top of the Star Point 

Sandstone may have formed from the development of 

runnels and surge channels (Johnson, 1979). These linear 

irregularities probably formed laterally extensive pressure 

points along the base of the peat where tears or zones of 

weakness formed that later became low-pressure sites for the 

migration of dike material. If the Star Point Sandstone was 

overpressured and if the plant matter and hydrostatic 

pressure within the peat swamp were adequate to provide a 

seal, release of overpressure may have aided in dike formation. It is reasonable that either increasing pressure within 

the sandstone or decreasing hydrostatic pressure in the peat 

reached a critical point at which the overpressure was vented 

upward into the base of the peat. Dewatering of the marine 

shale beneath the Star Point Sandstone probably aided 

injection by providing a prolonged source of water to 

maintain the mobility of the marl slurry. In addition, the 

upward migration of water may have established preferred 

avenues through the peat, similar to that of springs or boils 

in peat bogs (Bennett and others, 1991), that later became 

avenues for dike material.

Dike location and possibly orientation were controlled 

by longshore processes. Carbonate-rich sediments were 

transported by streams to depositional sites along the 

Cretaceous seaway (Balsley, 1980). Longshore drift currents 

probably redeposited these sediments (Flores and others, 

1982) in elongate bodies oriented parallel to subparallel with 

the paleoshoreline. The presence of marl lenses within the 

shoreface sandstone shows that dolomite was redeposited 

under shallow-marine conditions. Although the 

N. 10°-20° W. paleoshoreline trend proposed by Flores and 

others (1982) only partly agrees with principal dike trends 

shown in figure 3, the lack of more convincing alignment 

may reflect local irregularities in the paleoshoreline. Probably the most widely favored modes of origin for clastic dikes 

in coal beds are infilling of desiccation cracks (Cady, 1942; 

Doelling, 1979; Jepsen 1987), dike formation controlled by 

structural trends (Linderg and others, 1983; Hardie, 1992; 

Hardie and Bostick, 1993), and intrusion resulting from seismic shocks from earthquakes and (or) volcanic activity 

(Gresley, 1897; Crane, 1898; Kendall, 1919; Raistrick and 

Marshall, 1939; Shirley, 1955; Williamson, 1967; Damberger, 1973; Doelling, 1979). Dike formation attributed to 

desiccation cracks requires sufficient dehydration of peat to 

develop large wedge-shaped cracks that are later infilled 

from above by subaerial processes (Doelling, 1979; Jepsen, 

1987). Although large dissection cracks more than 60 cm 

deep have been reported in peat beds in the Everglades 

National Park (Jepsen, 1987), the load-compaction dikes in 

the study area were clearly injected from below. Pyramidshaped dikes have broad bases that pinch out in an upward

direction some distance below the roof rock. This upward 

pinchout is inconsistent with infilled desiccation cracks that 

are widest at the surface and narrow downward. In addition, 

the formation of desiccation cracks requires considerable 

drying of the peat, which would have produced oxidized 

zones not observed in Utah coal seams. Dike formation 

related to local or regional structural trends is unlikely 

because of the wide range of dike trends shown in figures 

3B-G. Results of a similar study conducted in the Piceance 

Basin of western Colorado (Hardie, 1992; Hardie and 

Bostick, 1993) show that dikes in Cretaceous coals are structurally controlled and cited as one of the principal lines of 

evidence, the narrow range of dike trends corresponding to 

joint sets in the coal floor rock. In the case of the Trail Mountain mine, however, a comparison of the trends of all loadcompaction dikes (fig. 3A) with structural trend data (fig. 11) 

shows that the axis of the Straight Canyon syncline, located 

less than 2 km northwest of the mine property, is roughly 

parallel with the dominant trend of dikes. It is noteworthy 

that the coal cleat trends shown in figure 11, widely accepted 

as a strong indicator of tectonic stresses present during early 

coalification, do not appear to be related to the dominant 

N. 32° E. trend of the dikes. Dike formation resulting from 

seismic shock is not likely, although seismic activity was 

probably present, given the proximity to the nearby 

cordillera. Genesis by seismic shock implies sudden and 

violent injection facilitated by thixotropic behavior of watersaturated sediments. Load-compaction dikes form less 

suddenly. Seismic shock commonly produces crustal 

deformation features over wide areas, particularly if the area 

affected is near a coastline. For example, Foster and 

Karlstrom (1967) reported that during the 1964 Alaskan 

earthquake crustal deformation, including widespread outpourings of mud and sand, pressure ridges, mounds, fissures, 

collapsed pits, mud and sand surface flows, and slump 

features, occurred over an area more than 100 km across but 

was most common along coastal settings, such as mud flats. 

Similar features covering more than 10,500 km2 formed 

during the New Madrid earthquake series (Saucier, 1977), 

and earthquake deformation occurred in an area of roughly 

18,304 km2 during an 1869 earthquake in eastern India (Oldhan and Mallet, 1872). In the vicinity of the Trail Mountain 

mine, sedimentologic features that could be interpreted as 

earthquake deformation features, other than dikes and 

scattered exposures exhibiting convolute bedding, are rare to 

absent. In addition, the areal distribution of load-compaction 

dikes is not characteristic of widespread deformation 

induced by seismic shock. Dikes are limited to three small, 

widely separated areas along the Wasatch Plateau. In the 

vicinity of the Trail Mountain mine, dikes are absent less 

than 2 km to the south in the abandoned Black Diamond 

mine, less than 2 km to the east in the Cottonwood mine, 

about 5 km to the north in the Deer Creek mine, and in
DOLOMITE AND SILICICLASTIC DIKES AND SILLS A15

-JOINTS IN COMROOF R°CK

Figure 11. Trends of structural elements and related features in 

the vicinity of the Trail Mountain mine. Range of trends indicated 

by width of vector segments. Data from Davis and Doelling (1977), 

Johnson (1982), and local mine operators.

recently opened mine workings on the western margin of the 

Trail Mountain mine. In addition, the near absence of dikes 

that pinch out farther than about 30 cm into the roof rock is 

inconsistent with earthquake-related eruptions of sand and 

water, which commonly breach the surface in the form of violent sandblows or outpourings of mud that form large pools 

along the surface. Furthermore, it is hard to imagine that 

broad C-shaped dikes in coal and parallel laminar flow structures in dike material that wrap smoothly around sharp irregularities along dike-coal contacts would form from a sudden 

seismic event. It is possible, however, that a small-magnitude 

earthquake (or series of earthquakes) could have released sufficient energy to trigger the formation of dikes that failed to 

breach the surface.

Attempts have been made by the author and by other 

investigators to determine peat-compaction ratios based on 

restoration of crenulated dikes to their original heights. 

Caution should be exercised when applying these data 

because the ratios do not measure peat compaction that was 

assimilated through dike swelling, faulting, and lateral migration of dike material from the original site of injection. Furthermore, ratios may be an extremely poor measure of peat 

compaction if they were determined from dike crenulation 

produced during the lateral migration of dike material along 

irregular planes of weakness some distance from the original 

point of vertical injection.

CHANNEL-MARGIN DIKES 

DESCRIPTION

Channel-margin dikes are present in small numbers in 

almost all coal mines throughout the Wasatch Plateau and 

western Book Cliffs and appear to be randomly oriented. 

These dikes are probably more abundant than previously 

reported because they are generally small-scale features that 

have little impact on coal production and are often ignored 

by mine personnel. Dikes are present both in swarms and as 

solitary intrusions in at least 10 major coal seams that span 

more than 250 m of section that includes the Hiawatha seam. 

They are commonly attached to sandstone channel bodies in 

the roof (fig. 12) and in rare cases are attached to sandstone 

rolls in the floor. Channel-margin dikes rarely exceed 5 cm 

in thickness, can extend more than 100 m laterally, and 

generally taper downward to a terminus within the upper 

meter of coal seams. High-angle dikes, greater than about 

50°, display a poorly developed crenulated character and 

have slightly jagged margins. Low-angle dikes have more 

uniform thicknesses and display almost smooth contacts. 

Dikes attached to channel bodies commonly can be traced 

downward for 10-20 cm, at which point they change to a 

horizontal direction, parallel with the base of the channel 

body. The margins of channel bodies that are attached to 

dikes are commonly outlined by vitrain bands that conform 

with the general shape of the channel. Results of 

petrographic analysis and X-ray diffraction show that 

individual dikes and their companion channel sandstone 

body are indistinguishable in composition. They are 

composed of grain-supported quartzose sandstone with 

dolomite and (or) calcite cement.

Dikes that emanate from floor rolls share the same 

characteristics as those associated with roof channels except 

that they are smaller and have shallower penetration. They 

rarely exceed a few centimeters in thickness, penetrate less 

than 0.5 m into the base of coal seams, and extend laterally 

less than 100 m. Mine operators along the western Book 

Cliffs have reported a small number of channel-margin dikes 

that are large scale and that display a variety of shapes far 

different from those previously described in this report. Rare 

and poorly documented large-scale dikes have been reported 

that are shaped like crude teardrops that cut entire coal seams 

(fig. 13). These dikes narrow near their attachment point 

near the base of roof channels and thicken dramatically 

toward the floor. The narrow portion of these dikes has 

jagged and well-defined contacts. Sills and (or) dikes less 

than a few centimeters thick can emanate from this narrow 

part. Near the floor, the dikes appear extremely swollen and 

blotted and display smooth and fairly even contacts.
A16 CLASTIC DIKES IN CRETACEOUS COALS, UTAH AND COLORADO

Figure 12. Channel-margin dike attached to a sandstone channel body in the roof of a coal seam. Centimeter scale card 

shown in lower left.

Figure 13. Sketch of large-scale channel-margin dike.

INTERPRETATION

Channel-margin dikes probably developed later than 

load-compaction dikes when sediment loading forced channel bodies into the top of peat beds forming tears that were 

infilled by channel material (fig. 2). The absence of welldeveloped crenulated character in high-angle dikes also 

suggests that channel-margin dikes formed later than loadcompaction dikes, after significant compaction of the peat. 

The widespread association of dikes and roof channels, 

particularly those that are physically attached to the margins 

of channels, and the similar composition of dikes and roof 

channel material demonstrate that channel bodies are the 

source of dike material. Vitrain bands that conform to the 

shape of channel bodies indicate that channel bodies were 

pressed into the upper surface of peat or lignite beds by the 

weight of the overlying sediments. Channel bodies, 

composed of grain-supported material, experienced little 

flattening out and compaction as they were imbedded into 

the surface of peat beds. The shallow and nonlinear 

penetration of dikes into the tops of coal seams and the 

attachment of dikes to channel bodies suggest that dikes 

formed from infilled tears in the peat. Once the tears were 

filled with channel material, continued sediment loading and 

compaction of the peat or lignite probably squeezed clastic 

material farther into the peat. The rare dikes that emanate 

from floor rolls probably formed in a similar manner. Their 

smaller size may be related to the effects of gravity. Kerns 

(1970) proposed a similar control for dikes associated with 

floor and roof rolls in the Pittsburgh coal seam of
DOLOMITE AND SILICICLASTIC DIKES AND SILLS A17

southwestern Pennsylvania. He suggested that tension 

cracks developed in areas adjacent to sand bodies where the 

peat was stretched and filled with sediments. Tectonism is 

not believed to have played a significant role in the formation of channel-margin dikes because of the apparent randomness of dike trends, the lack of consistent penetration by 

the dikes into the roof rock, and the relatively short lateral 

extent of the dikes.

The formation of large-scale dikes is unclear but probably is related to different physical states of the peat. The 

presence of jagged margins and small-scale sills and dikes 

emanating near the top of the dike may have resulted from 

injection of clastic material along zones of weakness in the 

younger and more fibrous peat after the infilling of tears. The 

smooth and bloated appearance of the dike in older peat near 

the bases of the beds may be the result of injection in peat 

that was more decomposed and gel-like. Had the entire peat 

bed been composed of fibrous vegetable matter, smaller and 

more typical channel-margin dikes probably would have 

formed. Another possibility is that large-scale dikes are actually load structures. Howard and Lohrengel (1969) described 

12 pillow-shaped structures that range from 6 to 22 m in 

length in the northern part of the Wasatch Plateau and in 

approximately the same stratigraphic interval as large-scale 

channel dikes. They proposed that these features were large 

ball-and-pillow load structures detached from overlying 

strata and formed in a thick siltstone bed. If large-scale dikes 

formed in a similar manner, it is possible that the small-scale 

sills and dikes that emanate from the top of large-scale dikes 

resulted from differential lateral slippage along horizontal 

planes in the upper part of the peat bed.

FRACTURE-FILL DIKES

DESCRIPTION

Information on fracture-fill dikes is sketchy because of 

their small number and the limited opportunity the author 

has had to investigate them directly. Fracture-fill dikes have 

been reported in the northern and central parts of the 

Wasatch Plateau in at least three coal seams that span about 

50 m of section that includes the Hiawatha seam. They are 

present as solitary features that measure from about 20 cm to 

more than a meter across near the floor and generally thin 

upward to a terminus in the upper to middle portions of coal 

seams, although a few dikes penetrate entire coal seams (fig. 

14). Dikes are mostly composed of moderate to poorly indurated sandstone. In rare exposures, the terminus of these 

sandstone dikes is composed of siltstone or claystone. 

Although their lateral extent is unknown, the dikes have been 

reported to cut across multiple mine entries more than a hundred meters wide and disappear into mine walls. Dikes cut

Figure 14. Sketch showing typical fracture-fill dike.

coal seams at high angles and are sharply bound along their 

uppermost contact by high-angle normal faults, although 

rare dikes have been reported that display no visible offset. 

Commonly, faults cut entire coal seams and display as much 

as 25 cm of offset that dies out near the roof line. The lower 

contact of dikes has a nonlinear and undulate character and 

displays no visible signs of offset (fig. 4).

INTERPRETATION

Fracture-fill dikes probably formed relatively late, after 

considerable coalification, as indicated by fault-bound 

contacts and the near absence of compaction-related 

features, including crenulation, microfaults, dike swelling, 

and coal fishtail structures. The upward thinning of fracturefill dikes and their tendency to pinch out near the top of coal 

seams show that dike material entered the base of coal 

seams. The presence of a single, sharp, fault-bound contact 

opposite an undulate contact suggests that offset occurred 

either after or during injection. If clastic material was 

injected into a fracture that was already faulted, both dike 

contacts would likely display a similar nonlinear character. 

At the time of injection, both contacts probably had a similar 

undulate character, but subsequent faulting probably healed 

those irregularities along the fault-bound contact. An 

apparent contradiction seems to exist, however, concerning 

the physical state of the peat at the time of faulting and 

injection. Characteristically, peat is a tough flexible material 

that would respond to the shear movement associated with 

faulting by stretching and tearing. The presence of smooth 

fault-bound dikes indicates that the peat was fairly well 

lithified when faulted; however, undulate dike contacts 

opposite faulted contacts suggest that the peat was flexible 

and easily deformed at the time of injection. One possible 

explanation involves the heterogeneity of vegetable matter 

within the peat. Peat beds are typically heterogenous bodies 

composed of layers of different types of vegetable matter 

having different physical characteristics including hardness.
A18 CLASTIC DIKES IN CRETACEOUS COALS, UTAH AND COLORADO

Injection into peat beds may have caused undulate contacts 

to develop as a result of different levels of yield associated 

with different layers within the peat. Fault-plane contacts 

may have experienced a healing of irregularities as faulting 

proceeded following injection.

SIGNIFICANCE OF DIKES

Clastic dikes in central Utah are believed to have at 

least three modes of origin, and each of the three major types 

of dikes probably formed at different times. Loadcompaction dikes probably formed very early, channelmargin dikes formed somewhat later but while organic 

material was still in the peat or lignite stage, and fracture-fill 

dikes formed relatively late after considerable coalification. 

Subsequent investigators of clastic dikes may find 

conflicting evidence of dike formation within a study area, 

only to find that dike formation involved more than one 

mechanism and that dike formation occurred during more 

than one time period.

Clastic dikes have created a range of extraordinary 

problems in underground coal mines that have resulted in 

increased production costs, mine closures, and injury of 

mine workers. Where dikes are present in numbers, 

recommended mining strategies include mining the upper 

part of coal seams where possible to avoid the thicker 

portions of dikes near the base of coal seams. Roof failure 

related to the cantilevering of roof rocks along zones of 

weakness at dike contacts can be reduced by driving mine 

entries at high angles to the central trend of dikes to reduce 

dike alignment with main entries.

Load-compaction dikes in central Utah are particularly 

interesting because of the many forms of dolomite present in 

the dike material, particularly the authigenic fraction. The 

presence of dolomite-rich dikes in peats that accumulated 

along a shoreline that was repeatedly transgressed suggests 

that seawater provided the magnesium necessary for precipitation of dolomite. Land (1985) proposed that seawater is 

the only common magnesium-rich fluid in the Earth's crust 

and must be the primary agent for the formation of dolomite. 

He went on to say that dolomite must form relatively early in 

the depositional history of sediments, when seawater or seawater-derived fluids can be actively pumped through 

sediments. Sibley (1991) wrote that, if one accepts Land's 

proposition, then the optimal situation for dolomitization is 

prolonged sea-level highstands or slow subsidence. The 

study area is an ideal laboratory for investigating dolomite 

formation because of the depositional setting, good 

exposures, and abundant mine access. Furthermore, the 

acidic nature of peat swamps and the early injection of 

dolomitic material are an interesting apparent contradiction 

that is worthy of investigation.

Clastic dikes are common in many North American 

basins where coal beds are associated with marine sequences

and are uncommon to absent where coal accumulated in 

continental settings. Alluvial- plain coals, such as those of 

the Powder River Basin, Williston Basin, and Denver Basin, 

contain few, if any, dikes, according to the literature and the 

investigations conducted by the author. Dikes are common 

in many marginal-marine coals, including those in the Appalachian Basin, Illinois Basin, and Unita and Piceance Basins, 

and are particularly common in coals that accumulated 

directly on shoreface deposits. This association may be 

related to one or more factors including sustained water 

production through dewatering of marine shale, 

overpressuring of sand bodies beneath coal beds, and 

differential compaction of marine and marginal-marine 

sediments. Future investigations that compare and contrast 

coal-forming basins, both with and without clastic dikes and 

that also include non-coal-bearing marine sequences 

intruded by dikes may lead to a more comprehensive model 

of dike formation.

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