Fracture History of the Divide Creek and 

Wolf Creek Anticlines and Its Relation to 

Laramide Basin-Margin Tectonism, 

Southern Piceance Basin, Northwestern 

Colorado

U.S. GEOLOGICAL SURVEY BULLETIN 1 787-Z
AVAILABILITY OF BOOKS AND MAPS OF THE U.S. GEOLOGICAL SURVEY

Instructions on ordering publications of the U.S. Geological Survey, along with the last offerings, are given in the current-year issues of 

the monthly catalog "New Publications of the U.S. Geological Survey." Prices of available U.S. Geological Survey publications released prior 

to the current year are listed in the most recent annual "Price and Availability List." Publications that are listed in various U.S. Geological 

Survey catalogs (see back inside cover) but not listed in the most recent annual "Price and Availability List" are no longer available.

Prices of reports released to the open files are given in the listing "U.S. Geological Survey Open-File Reports," updated monthly, which 

is for sale in microfiche from the U.S. Geological Survey Books and Open-File Reports Sales, Box 25425, Denver, CO 80225.

Order U.S. Geological Survey publications by mail or over the counter from the offices given below.

BY MAIL

Books

Professional Papers, Bulletins, Water-Supply Papers, Techniques of Water-Resources Investigations, Circulars, publications 

of general interest (such as leaflets, pamphlets, booklets), single 

copies of periodicals (Earthquakes & Volcanoes, Preliminary Determination of Epicenters), and some miscellaneous reports, including some of the foregoing series that have gone out of print at the 

Superintendent of Documents, are obtainable by mail from

U.S. Geological Survey, Books and Open-File Report Sales

Box 25425 

Denver, CO 80225

Subscriptions to periodicals (Earthquakes & Volcanoes and 

Preliminary Determination of Epicenters) can be obtained ONLY 

from

Superintendent of Documents

U.S. Government Printing Office

Washington, DC 20402

(Check or money order must be payable to Superintendent of 

Documents.)

Maps

For maps, address mail order to

U.S. Geological Survey, Map Sales

Box 25286 

Denver, CO 80225

Residents of Alaska may order maps from

U.S. Geological Survey, Map Sales

101 Twelfth Ave., Box 12

Fairbanks, AK 99701

OVER THE COUNTER

Books

Books of the U.S. Geological Survey are available over the 

counter at the following U.S. Geological Survey offices, all of 

which are authorized agents of the Superintendent of Documents.

 ANCHORAGE, Alaska^l230 University Dr., Rm. 101

 ANCHORAGE, Alaska-£05 West 4th Ave., Rm G-84

 DENVER, Colorado-Federal Bldg., Rm. 169,1961 Stout St.

 LAKEWOOD, Colorado-Federal Center, Bldg. 810

 MENLO PARK, California-Bldg. 3, Rm. 3128,345

Middlefield Rd.

 RESTON, Virginia-National Center, Rm. 1C402,12201 

Sunrise Valley Dr.

 SALT LAKE CITY, Utah-Federal Bldg., Rm. 8105,125 South 

State St.

 SAN FRANCISCO, California-Customhouse, Rm. 504, 555 

Battery St.

 SPOKANE, Washington-U.S. Courthouse, Rm. 678, West 

920 Riverside Ave.

 WASHINGTON, D.C.-U.S. Department of the Interior Bldg., 

Rm.2650, 1849CSt.,NW.

Maps

Maps may be purchased over the counter at the U.S. Geological Survey offices where books are sold (all addresses in above list) 

and at the following Geological Survey offices:

 ROLLA, Missouri-1400 Independence Rd.

 FAIRBANKS, Alaska-New Federal Building, 101 Twelfth 

Ave.
Chapter Z

Fracture History of the Divide Creek

and Wolf Creek Anticlines and Its Relation to

Laramide Basin-Margin Tectonism,

Southern Piceance Basin, Northwestern

Colorado

By MARILYN A. GROUT and EARL R. VERBEEK

A multidisciplinary approach to research studies of sedimentary 

rocks and their constituents and the evolution of sedimentary basins, 

both ancient and modern

U.S. GEOLOGICAL SURVEY BULLETIN 1787

EVOLUTION OF SEDIMENTARY BASINS UINTA AND PICEANCE BASINS
U.S. DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR., Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

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

UNITED STATES GOVERNMENT PRINTING OFFICE: 1992

For sale by the

Books and Open-File Reports Section

U.S. Geological Survey

Federal Center

Box 25425

Denver, CO 80225

Library of Congress Cataloging-in-Publication Data

Grout, Marilyn A.

Fracture history of the Divide Creek and Wolf Creek anticlines and its relation 

to Laramide basin-margin tectonism, southern Piceance Basin, northwestern 

Colorado/ by Marilyn A. Grout and Earl R. Verbeek.

p. cm. (U.S. Geological Survey bulletin ; 1787) (Evolution of 

sedimentary basins Uinta and Piceance basins; ch. Z)

"A multidisciplinary approach to research studies of sedimentary rocks and their 

constituents and the evolution of sedimentary basins, both ancient and modern. "

Includes bibliographical references.

Supt. of Docs, no.: I 19.3:

1. Joints (Geology) Colorado Piceance Creek Basin. 2. Geology, 

Structural Colorado Piceance Creek Basin. I. Verbeek, Earl R. II. Title. 

III. Series. IV. Series: Evolution of sedimentary basins Uinta and Piceance 

basins ; ch. Z. 

QE75.B9 no. 1787-Z 

[QE605]

557.3 s dc20 91-26628 

[551.8'4'09788] CIP
CONTENTS

Abstract Zl

Introduction Zl

General geology Z2

Lithology Z5

Structural setting Z6

Economic significance of fractures Z6

Previous studies of joints in the Piceance basin Z7

Procedure and techniques Z7

Joint sets of the Piceance basin and Grand Hogback monocline Z8

Piceance system Z8

Hogback system Z10

Joint sets of the Divide Creek and Wolf Creek anticlines Z10 

Discussion Z14

Distribution of MV2 set Z15

Presence and prominence of local fold-parallel fractures Z16

Distribution of fractures of the Piceance system Z18

Fracture history and fluid movement Z20 

Timing of fracture formation Z20 

Fluid movement through upper Mesaverde and lower Wasatch

strata Z22

Comments on coal Z22 

Concluding remarks Z22 

References cited Z23 

Appendix Summary of characteristics of fracture sets Z28

FIGURES

1-2. Maps showing:

1. Location of study area, late Paleozoic uplifts, and Tertiary basins and

uplifts, northeastern part of Colorado Plateau and adjacent Rocky Mountain 

foreland Z3

2. Geology of area of Divide Creek and Wolf Creek anticlines and part of

Grand Hogback monocline Z4 

3. Vertical section through study area showing combined interpretation of seismic,

gravity, and drill-hole data Z5 

4 6. Maps showing:

4. Regional distribution and orientation of joints of Fj F4 sets in Piceance 

basin Z9

5. Average orientation of joints of MV2 fracture set in study area, southern 

Piceance basin Z12

6. Average orientation of joints of fold-parallel fracture sets in study area, 

southern Piceance basin Z13

7. Photographs showing fold-parallel fractures in sandstone bed of Wasatch 

Formation on Divide Creek anticline Z14

8. Map showing average orientation of joints of F: fracture set in study area, 

southern Piceance basin Z15

Contents III
9. Photograph showing joints of F2 and F4 fracture sets in interbedded 

sandstones of Wasatch Formation on Divide Creek anticline Z16

10. Map showing average orientation of joints of F2 fracture set in study area, 

southern Piceance basin Z17

11. Photograph showing especially prominent joints of F2 fracture set in sandstone of Wasatch Formation on Wolf Creek anticline Z17 

12-14. Maps of study area in southern Piceance basin showing average orientation 

of:

12. Joints of F3 fracture set Z18

13. Joints of F4 fracture set Z19

14. Joints of F5 fracture set Z20

TABLES

1. Average axial trends of Divide Creek and Wolf Creek anticlines and strike range 

of joints of each set measured in the study area Z10

2. Summary of fracture characteristics common to sets in the Divide Creek-Wolf 

Creek area Zll

3. Fracture chronology for the Divide Creek-Wolf Creek area Z21

IV Contents
EVOLUTION OF SEDIMENTARY BASINS UINTA AND PICEANCE BASINS

Fracture History of the Divide Creek and 

Wolf Creek Anticlines and Its Relation to 

Laramide Basin-Margin Tectonism, 

Southern Piceance Basin, Northwestern Colorado

By Marilyn A. Grout and Earl R. Verbeek

Abstract

The Divide Creek and Wolf Creek anticlines are two 

gentle, north-northwest-trending, gas-producing intrabasin 

folds near the eastern margin of the Piceance basin of 

northwestern Colorado. Both folds developed above a decollement on the basinward side of a large, basement-involved 

thrust wedge that resulted from southwest- to west-southwestdirected compression late during the Laramide orogeny. The 

nearby Grand Hogback monocline, which defines the eastern 

margin of the basin, is the surface expression of this wedge. 

Natural gas in the Divide Creek-Wolf Creek area is produced 

principally from fractured sandstone reservoirs and coals of the 

Upper Cretaceous Mesaverde Group, the uppermost part of 

which is exposed sparingly on both folds.

Upper Cretaceous through Eocene strata in the Divide 

Creek-Wolf Creek area contain eight sets of extension joints, 

three of them related to the same period of Laramide 

compression that produced the folds and the remaining five of 

post-Laramide age. Joints of the oldest set known from the area 

are rare but form one of the major sets along the Grand 

Hogback, where they are interpreted to have formed during 

the early (Paleocene-early Eocene) stages of thrusting as the 

beds were gently warped upward over the advancing thrust 

wedge. The abundance of these early joints wanes 

dramatically from the monocline into the basin, reflecting the 

transition from the gently warped strata above the wedge to the 

relatively undeformed rocks farther west. Bed dips at this time 

were everywhere still low.

As shortening proceeded during the main (Eocene) phase 

of late Laramide compression, two sets of inclined joints 

formed in modest abundance in the study area, principally on 

the Divide Creek fold. Both sets strike parallel to the fold axis

Manuscript approved for publication June 4, 1991.

but dip 60°-70° in opposite directions, one toward and the 

other away from the fold crest. Differences in joint size and dip 

angle between the two sets suggest that they formed as the 

beds were tilting and that they are an expression of stratal 

extension above the neutral surface of a growing fold. Joints of 

this period of deformation are restricted to thick beds of 

sandstone and to coals; the rest of the strata remained little 

fractured.

The five youngest joint sets are of post-Laramide age and 

collectively dominate the fracture network of the Divide 

Creek-Wolf Creek area. All five sets are of regional extent and 

have been traced in stratigraphic and areal continuity from the 

Upper Cretaceous and Paleocene rocks of the study area 

through the youngest (~43 Ma, middle Eocene) rocks 

preserved in the basin interior. All five of these sets are present 

also in the neighboring Uinta basin to the west, suggesting that 

post-Laramide joint-set formation is due more to regional 

stresses affecting much of the northeastern Colorado Plateau 

than to events associated with the evolution of a single basin or 

basin margin.

The relatively young (<43 Ma) age of almost all the joints 

in the Upper Cretaceous (75-66 Ma) reservoir rocks in the 

Divide Creek-Wolf Creek area and areas to the west implies 

that formation fluids moved through these rocks primarily by 

matrix-dominated flow for the first 20-30 m.y. after deposition. 

More rapid fluid flow through a vertically continuous network 

of interconnected fractures became possible only after middle 

Eocene time.

INTRODUCTION

The Piceance basin of northwestern Colorado lies 

along the northeastern edge of the Colorado Plateau (fig. 1) 

and is one of several Laramide structural and depositional 

basins of the Colorado Plateau-Rocky Mountain foreland

Fracture History, Southern Piceance Basin, Colorado Z1
region that is economically important as a source for natural 

gas (Spencer, 1987; Johnson and Rice, 1990), crude oil 

(Dunn, 1974; Donaldson and MacMillan, 1980), and shale 

oil (Dyni, 1987). Sedimentation in the basin occurred 

primarily during the Paleocene and Eocene epochs and was 

in part contemporaneous with late Laramide deformation of 

the basin's eastern margin, the early stages of which are 

reflected in depositional thinning of Paleocene strata toward 

the deforming area (R.C. Johnson, oral commun., 1991). 

The later, main (Eocene) phase of deformation culminated 

in formation of the large Grand Hogback monocline (fig. 1), 

which formed as a basement-involved thrust wedge 

advanced southwestward into the basin (Perry and others, 

1988) and which now defines both the eastern margin of the 

basin and part of the tectonic boundary between the mildly 

deformed Colorado Plateau and more highly deformed 

orogenic regions of the Rocky Mountains (Kelley and 

Clinton, 1960). Interpretation of the fracture history of part 

of the southern Piceance basin as related to these tectonic 

movements forms one of the principal themes of this paper.

The southern part of the Piceance basin was chosen 

for study in part because it contains obvious intrabasin 

folds, the Divide Creek and Wolf Creek anticlines, of 

previously unknown origin adjacent to the tectonically 

thrusted and folded eastern basin margin (fig. 2). The 

anticlines are broad folds, trend north-northwest to 

northwest, and are bordered on the north and east by the 

S. 70° E.- and S. 10° E.-trending portions, respectively, of 

the Grand Hogback monocline. Both anticlines are of 

similar geometry in the shallow subsurface, approximately 

25-35 km long and 10-15 km wide as measured on top of 

the Rollins Sandstone Member of the Upper Cretaceous 

Mesaverde Group (Johnson, 1983). Structure-contour maps 

of deeper horizons depict the anticlines as continuous down 

to the crystalline Precambrian basement (Murray and Haun, 

1974), implying basement folding as a cause of anticline 

formation. New seismic and gravity data (Abrams and 

Grout, 1987, 1990; Grout and others, 1991) show, however, 

that the anticlines, like the Grand Hogback, are products of 

late Laramide thrusting (fig. 3). The distribution of several 

fracture sets discussed in this report is related to this 

deformation.

Acknowledgments. This work was supported by the 

U.S. Geological Survey Evolution of Sedimentary Basins 

Program with partial support from the U.S. Department of 

Energy Western Tight Gas Sands Program. D.B. Yager, 

C.W. Johannes, R.E. M'Gonigle, and G.S. Hess assisted in 

the field, the.latter two as volunteers. D.B. Yager, R.E. 

Sabala, and M.F. Duncan, and also R.V. Grout and Christy 

Jones as volunteers, provided drafting support. R.C. 

Johnson generously shared with us his unpublished 

stratigraphic data pertaining to early movements along the 

Grand Hogback. Manuscript report versions of this 

manuscript were reviewed by M.C. Reheis, C.C. Barton, 

R.L. Wheeler, W.C. Swadley, and S.A. Minor. R.L.

Wheeler, in particular, provided encouragement to retain 

some of the more controversial ideas in this paper; to him 

our thanks.

GENERAL GEOLOGY

The Piceance basin encompasses approximately 

10,500 km2 and is divided roughly in half by the Colorado 

River (fig. 1). Elevations within the basin range from 

1,370 m to 3,580 m and along the Grand Hogback from 

1,980 m to 3,230 m. Present structural relief between the 

Piceance basin and the White River uplift east of the Grand 

Hogback is about 9,100 m.

Generally flat-lying Cambrian through Cretaceous 

sedimentary rocks having an aggregate thickness of about 

7,700 m (MacLachlan and Welder, 1987) were deposited on 

crystalline Precambrian basement rocks in the area of the 

Piceance basin prior to basin development. To these were 

added another 3,300 m of Paleocene to Eocene strata 

deposited in the basin proper (MacLachlan, 1987). The 

pre-basin rocks are most extensively exposed along the 

Grand Hogback monocline (Tweto and others, 1978; 

MacLachlan, 1987; MacLachlan and Welder, 1987), where 

the stratal succession can be followed eastward through 

increasingly older rocks and into the Precambrian metamorphic core of the uplifted White River block. Exposures 

are abundant also along the Douglas Creek arch and the 

Book Cliffs along the western and southwestern basin 

margins, respectively (Williams, 1964; Cashion, 1973; 

Tweto and others, 1976). Strata near the top of the pre-basin 

sequence, most notably those of the Upper Cretaceous 

Mesaverde Group, are exposed sparingly on the Divide 

Creek, Wolf Creek (fig. 2), and Coal Basin anticlines 

(Tweto and others, 1978) but not at all within the rest of the 

basin interior. The deepest drill holes in the study area, the 

California Company-Hurd-Government Divide Creek 

No. 1, the W.R. Grace & Company Wolf Creek No. 35-1, 

and, most recently, the Texaco Thunderhawk No. 1, have 

penetrated this sequence only into the Lower Permian and 

Middle Pennsylvanian (Grout and others, 1991, fig. 7).

The Paleocene and Eocene Wasatch Formation and 

the younger Eocene Green River Formation are exposed 

throughout the Piceance basin except on mesa tops, on 

forested slopes, and in areas of post-Eocene plutonic and 

volcanic activity. The overlying Uinta Formation, the 

youngest (middle Eocene) sedimentary unit preserved in the 

basin, is exposed chiefly north of the Colorado River. 

Remnants of Pliocene and Miocene alkali basalt flows are 

preserved locally, chiefly on Grand Mesa in the 

southwestern part of the basin where they total 224 m thick; 

dikes and plugs of similar composition may have been 

feeders to the flows (Tweto and others, 1978). Middle 

Tertiary, generally quartz monzonitic rocks are present as 

stocks, dikes, sills, laccoliths, and irregular bodies locally in 

the southeastern part of the basin (Tweto and others, 1976,

Z2 Evolution of Sedimentary Basins Uinta and Piceance Basins
50 MILES

Figure 1. Northeastern part of Colorado Plateau and adjacent Rocky Mountain foreland. Locations of study area (diagonalpatterned box), late Paleozoic uplifts (shaded), and Tertiary basins and uplifts are shown. Dotted line indicates Colorado Plateau 

boundary; dashed line delineates inferred extent of penesaline deposits within Pennsylvanian Eagle paleobasin (from Dodge and 

Bartleson, 1986); barbed lines delineate topographic and structural basins; solid circle indicates location of MWX site. Modified 

from Kelley and Clinton (1960), Howard and others (1972), Tweto (1980), and Cries and Dyer (1985).

Fracture History, Southern Piceance Basin, Colorado Z3
107°45' 107°30' 107°15'

39°22'30"

39°15' 

EXPLANATION

Pliocene and Miocene basalt 

Middle Tertiary intrusive rocks 

Tertiary Green River Formation

Tertiary Wasatch Formation Includes uppermost 

Cretaceous Ohio Creek Member of Hunter Canyon 

Formation of Mesaverde Group

Upper Cretaceous Mesaverde Group

Upper part of Upper Cretaceous Mancos Shale

Lower part of Upper Cretaceous Mancos Shale 

through Upper Triassic Chinle Formation, 

undifferentiated

Lower Permian and Pennsylvanian Maroon Formation

Middle Pennsylvanian Eagle Valley Formation

Contact

Normal fault Bar and ball on downthrown side

 A _ A _ Inferred zone of blind splays from low-dip 

reverse fault Sawteeth on upper plate

Anticline 

Syncline 

Monocline

Figure 2. Generalized geology of Divide Creek and Wolf Creek anticlines and part of southern leg of Grand Hogback monocline, 

southern Piceance basin. Locations of fracture stations (solid circles) and approximate locations of gravity (line G) and seismic (line 

S) surveys (Grout and others, 1991) discussed in text are shown. Geology from Tweto and others (1978), Berry (1959), Tichy and 

Rettger (1962), and Johnson (1983). Gas fields (shaded) from Petroleum Information Corp. and Barlow and Haun, Inc. (1986).

1978). A small stock, Haystack Mountain, crops out 

immediately south of the Divide Creek anticline (fig. 2), and 

minor sills beneath the same fold have been penetrated by 

drill holes (J.R. Dyni, oral commun., 1988). Most of the 

volcanic and plutonic rocks, however, are confined to the 

mountainous areas south and east of the basin.

The Grand Hogback monocline bordering the study 

area on the east (fig. 1) is part of the boundary between two 

major physiographic provinces, the Colorado Plateau and 

the Rocky Mountain foreland. Along its sinuous

135-km-extent, the northern leg trends approximately S. 5° 

E. for one-third of the distance, bends abruptly to form the 

50-km long central segment of S. 70° E. trend, and then 

bends abruptly once more to trend S. 10° E. for an 

additional 40 km. The monocline dies out near the western 

edge of the Elk Mountains (Tweto and others, 1978) and 

may be draped over the northern continuation of the 

southwest-directed Elk Range overthrust (Poole, 1954). 

Strata along the basinward, west- to southwest-facing, steep 

limb of the monocline typically dip from 45° to slightly

Z4 Evolution of Sedimentary Basins Uinta and Piceance Basins
WEST EAST SOUTHWEST NORTHEAST

DIVIDE CREEK 

ANTICLINE WOLF CREEK 

ANTICLINE

GRAND HOGBACK 

MONOCLINE

Complexly thrusted 

and folded shale

 1 KILOMETER

Figure 3. Vertical section through study area showing combined interpretation of seismic, gravity, and drill-hole data. 

Modified from Grout and others (1991). Arrows indicate movement direction of upper plates of faults; faults dashed where 

inferred. Geologic units: Tw, Tertiary Wasatch Formation; Kmv, Upper Cretaceous Mesaverde Group; Km, Upper Cretaceous 

Mancos Shale; ~6-Kd, Triassic strata through Upper Cretaceous Dakota Sandstone, undivided; Pwm, Lower Permian to Middle 

Pennsylvanian Weber Sandstone and Maroon Formation; Pmbe, Middle Pennsylvanian Minturn Formation and Middle 

Pennsylvanian Belden Formation, including Middle Pennsylvanian Eagle Valley evaporite sequence; M-Cr, Mississippian 

through Cambrian rocks, undivided; p , Precambrian metamorphic basement rocks.

overturned (Murray, 1966). The basin west of the monocline is only mildly deformed relative to most parts of the 

adjacent Rocky Mountain foreland. A series of westnorthwest-trending, broad, gentle anticlines and synclines is 

present in the northern part of the basin, and northwesttrending folds characterize the southern part (Dunn, 1974; 

Murray and Haun, 1974; Johnson, 1983). Limb dips on 

most of these folds at the surface are less than 5°. Apart 

from the ubiquitous joints, the basin strata are cut only by 

scattered, minor thrust faults and small kink bands, products 

of late Laramide compression, and by later west-northwesttrending normal faults of small to moderate throw.

LITHOLOGY

Rock units studied in the Divide Creek-Wolf Creek 

area include the Upper Cretaceous Mesaverde Group 

(principally the Ohio Creek Member of the Hunter Canyon 

Formation) and the Paleocene and Eocene Wasatch 

Formation. These are described briefly below, from oldest 

to youngest. Simplified stratigraphic sections are shown in 

MacLachlan (1987) and Johnson (1989).

The Upper Cretaceous Mesaverde Group is a 

regressive marginal-marine sequence that includes minor 

transgressive marine and transitional facies that interfinger 

toward the base of the unit (Dunn, 1974). Marginal-marine 

blanket shoreline sandstone members at the base of the 

Mesaverde grade upward from the underlying Upper 

Cretaceous Mancos Shale, which contains thin sandstone 

layers in its upper part, into a paludal interval consisting 

predominantly of carbonaceous shale, coal, and lenticular

sandstone (Johnson and Nuccio, 1984). These rocks are 

overlain by a sandier, fluvial sequence of overlapping 

lenticular sandstone bodies interbedded with shale and 

mudstone, the whole truncated at the top by a regional 

unconformity. Maximum thickness of the Mesaverde Group 

is 1,860 m north of the Colorado River and 1,250 m south 

of the river (Tweto and others, 1978).

The Upper Cretaceous Ohio Creek Member, which 

caps the Mesaverde Group, is 122 m thick near the southern 

border of the Piceance basin and gradually thins to about 15 

m in the northern part. Lorenz and Rutledge (1985) 

described the Ohio Creek Member as "a generally coarsegrained to locally conglomeratic sandstone*** which 

ranges from a fluvial facies in the southwestern and 

southern parts of the basin to a paralic facies in the 

east-central part of the basin." The upper part of the unit is 

weathered and kaolinitic and is thought to mark a paleoweathering profile (Johnson and May, 1980). Joint stations 

in this unit are shown in figure 2 as being in the Tertiary 

Wasatch Formation because the Ohio Creek Member 

previously was mapped as Tertiary in age (Tweto and 

others, 1978) and the area has not been remapped in 

conformance with the new stratigraphic data. The Ohio 

Creek Member is separated from the Paleocene and Eocene 

Wasatch Formation by a regional unconformity.

The Wasatch Formation is about 1,770 m thick 

(Tweto and others, 1978), is composed mainly of thick, 

variegated mudstone and subordinate channel sandstone 

lenses (Donnell, 1969), and contains carbonaceous shale 

and lignite near its base (Tweto and others, 1978). Lenses of 

pebble conglomerate of different age and clast composition

Fracture History, Southern Piceance Basin, Colorado 25
from those of the underlying Ohio Creek Member (Johnson 

and May, 1980) are present locally near the base of the 

Wasatch Formation.

STRUCTURAL SETTING

Most of the Tertiary basins of the Colorado Plateau 

are bounded along at least one margin by monoclines 

(Kelley, 1955) that formed during northeast-southwest 

oceanic and cratonic convergence during the Laramide 

orogeny (Davis, 1978). By analogy with the well-exposed 

Grand Canyon examples, monocline formation by 

reactivation of pre-existing, steeply dipping fault zones in 

Precambrian basement commonly was presumed; the Grand 

Hogback has been depicted repeatedly as one such fold 

(Tweto, 1975, 1980; Stearns, 1978). From recent seismic 

data, however, it has become increasingly apparent that 

some of the monoclines, especially in the Rocky Mountain 

foreland near the boundary with the Colorado Plateau, 

overlie a west-, southwest-, or south-directed thrust system 

(for example, Berg, 1962; Sales, 1968; Gries, 1983; Stone, 

1986; Brown, 1988).

The effects of two major deformational episodes, 

widely separated in time, are evident in the study area, from 

seismic, gravity, drill-hole, and outcrop data from the 

eastern Piceance basin and Grand Hogback monocline 

(Perry and others, 1988; Grout and others, 1991). The first 

episode was during the Pennsylvanian Period, when Upper 

Cambrian through Middle Pennsylvanian strata were block 

faulted and subsequently buried beneath younger sediments; 

these faults now lie at depths of 4 5 km below the Divide 

Creek and Wolf Creek anticlines (fig. 3). Several such block 

faults in correlative strata have been recognized in the 

central Piceance and adjacent Eagle basins (Waechter and 

Johnson, 1985; 1986; De Voto and others, 1986; Johnson 

and others, 1988). The second episode occurred during the 

latter part of the Laramide orogeny when the Piceance basin 

was formed, filled, and then deformed by thrusting along its 

eastern margin. The Grand Hogback monocline formed 

above the tip of a blind, basement-involved thrust wedge 

that moved southwest and west-southwest into the Piceance 

basin, culminating within the Upper and Lower Cretaceous 

Mancos Shale (Perry and others, 1988). The Divide Creek 

and Wolf Creek anticlines, whose crests lie respectively 19 

km and 10 km west of the anticlinal axial trace of the Grand 

Hogback monocline, are related to this same thrust system. 

Both anticlines overlie a decollement (fig. 3) that developed 

within the mechanically weak Middle Pennsylvanian evaporites beneath the tip of the thrust wedge and that died out 

basinward as a series of imbricated splay faults in the 

Mancos Shale (Grout and others, 1991). The Divide Creek 

anticline formed above the tip lines of these splay faults as 

thrusting locally overthickened the shale and repeated the 

sandstone units between the shale and the evaporites. The 

Wolf Creek anticline, however, may overlie a pre-existing

graben within which the Middle Pennsylvanian sequence is 

both thicker and richer in evaporites than to either side; 

syntectonic thickening of these mobile rocks along the 

decollement resulted in formation of an evaporite pillow 

(readily visible on the seismic line) and doming of the 

overlying strata. Gravity data (Abrams and Grout, 1987, 

1990) confirm that excess material of relatively low density 

lies beneath the Wolf Creek structure, whereas material of 

relatively higher density (overthickened shale) lies beneath 

the Divide Creek anticline.

ECONOMIC SIGNIFICANCE OF 

FRACTURES

Upper Cretaceous and lower Tertiary sandstone, coal, 

shale, and mudstone in the southern Piceance basin contain 

large undeveloped reserves of natural gas (ICF Resources 

Inc., 1989; Johnson and Rice, 1990). Matrix permeabilities 

of the reservoir rocks are only 0.01-3.0 microdarcies 

(Branagan and others, 1984; Seccombe and Decker, 1986; 

Lorenz and others, 1989), but formation (in situ) 

permeability is orders of magnitude greater at 20-100 

microdarcies (Branagan and others, 1985; Lorenz and 

others, 1989). The large difference between laboratory- and 

field-tested permeabilities has been attributed to a partially 

open, three-dimensional system of interconnected fractures 

in the reservoir rocks. Economic production of natural gas is 

so dependent on fractures in these and other gas fields of the 

western United States that systematic efforts are underway 

to determine the extent to which the geometry of subsurface 

fracture networks can be predicted from surface studies.

The area of the Divide Creek and Wolf Creek 

anticlines (fig. 2) contains the principal natural-gasproducing fields in the southern Piceance basin; production 

is almost exclusively from sandstone strata and interbedded 

coal of the lower part of the Mesaverde Group (Dunn, 1974; 

Donaldson and MacMillan, 1980; Petroleum Information 

Corp. and Barlow and Haun, Inc., 1986). Anticlinal 

structure, however, apparently has had only a limited effect 

on accumulation of hydrocarbons within the producing 

horizons because commercial gas is found also in areas that 

have no obvious structural traps (Gunter, 1962). One of 

these is the U.S. Department of Energy's Multiwell 

Experiment (MWX) site (fig. 1), where recent slant-hole 

drilling in a direction perpendicular to the dominant fracture 

set reaffirmed, through large gas kicks, the importance of 

natural fractures to gas production (Lorenz and Hill, 1991; 

Paul Branagan, oral commun., 1991).

Coal beds of the lower Mesaverde Group in the 

Piceance basin are being studied for direct production of 

methane. These beds are estimated to contain 20-430 

trillion cubic feet of methane in a drillable area of 10,570 

km2 and have the greatest potential for near-term production 

of coalbed methane of any basin in the western United

Z6 Evolution of Sedimentary Basins Uinta and Piceance Basins
States (Choate and others, 1984; McFall and others, 1986; 

ICF Resources Inc., 1990). The southeastern quarter of the 

basin, which includes the study area, is considered to have 

the highest potential. Technological methods for 

maximizing methane extraction are being perfected. A 

favored method is drilling angled and horizontal holes 

perpendicular to the direction of greatest permeability in the 

coal beds; generally this direction corresponds to the strike 

of the face cleats in the coal (see Gas Research Institute, 

1983-present). Success of the method is predicated on 

knowing the orientations of the face and butt, or end, cleats 

in each seam under consideration. Coalbed-methane study 

sites in the Piceance basin include the Red Mountain area 

south of Plateau Creek in the south-central part and a second 

area at the northwest end of the Divide Creek anticline.

PREVIOUS STUDIES OF JOINTS IN THE 

PICEANCE BASIN

Uinta Formations. Reports on the fracture history of the 

basin are in Verbeek and Grout (1983, 1984a, b, 1986, 

1987), Grout and Verbeek (1983, 1985, 1987, 1989), Grout 

(1988, 1991), and Grout and others (1991). These reports 

discuss regional and local fracture histories, the effects of 

lithology and bed thickness on joint properties, relations 

between episodes of fracture and tectonic events, and the 

prediction of joint-network properties at depth from surface 

studies. In the present paper, the sequence of formation and 

prominence of joint sets in the Upper Cretaceous Mesaverde 

Group and the Paleocene and Eocene Wasatch Formation at 

53 stations in the Divide Creek-Wolf Creek area (fig. 2) are 

interpreted within the context of the regional tectonic 

setting. The tectonic evolution of the Divide Creek and 

Wolf Creek anticlines was discussed recently by Grout and 

others (1991).

PROCEDURE AND TECHNIQUES

Various aspects of the surface pattern of joints in the 

Grand Hogback monocline were mapped or studied by 

Harper (1964), Murray (1966, 1967), Dula (1981, 1984), 

and Garrett and Lorenz (1989). In the Piceance basin, Smith 

and Whitney (1979) mapped orientations of fractures both 

in outcrop and from aerial photographs, Kelley and Clinton 

(1960) and Welder (1971) mapped fracture patterns from 

aerial photographs, and Clark (1983) mapped fractures in 

outcrop and in core in an attempt to interpret aspects of the 

late Cenozoic stress history of the southeastern part of the 

basin. Mineral fillings, orientations, and abundance of 

fractures in oriented cores from Mesaverde Group strata at 

the MWX site were discussed by Lorenz (1985), Pitman and 

Sprunt (1986), Lorenz and Finley (1987), Finley and Lorenz 

(1989), and Lorenz and Hill (1991). Orientations of cleats in 

coal beds of the Mesaverde Group along the southern rim of 

the Piceance basin were reported by Geological Services of 

Tulsa, Inc. (1980) and Boreck and Strever (1980). Cleat 

orientations in cores from correlative strata in the westcentral part of the basin, at the Gas Research Institute's Red 

Mountain site, were reported by Seccombe and Decker 

(1986) and Horner (1986). In addition, several unpublished 

industry reports contain data on the orientations of surface 

fractures. With the exception of the studies by Lorenz and 

his colleagues, Pitman and Sprunt, and Verbeek and Grout 

(mentioned below), generally only one aspect of fractures 

was measured their orientation.

The Piceance basin fracture study by the U.S. 

Geological Survey, initiated in 1979, has involved the 

collection of fracture data from more than 900 outcrops and 

manmade cuts in the Piceance basin and surrounding 

uplifts, in strata that range in age from Precambrian through 

Eocene and Quaternary. Of these, about 750 stations are in 

the Upper Cretaceous through Eocene rocks of the Mesaverde Group and overlying Wasatch, Green River, and

The methods used here to characterize joint sets in 

sedimentary rocks and to interpret their fracture history 

were developed during field investigations in the Piceance 

basin beginning in 1979 (Grout and Verbeek, 1983; Grout, 

1988, 1991) and subsequently refined as our research 

expanded to other areas of the Colorado Plateau and Rocky 

Mountain foreland. These methods utilize joint styles the 

brittle analogue of fold styles to define and correlate joint 

sets on the basis of multiple criteria in addition to their 

orientation. The concept of joint style was initially 

developed by Russell L. Wheeler, now of the U.S. 

Geological Survey, and his colleagues (Holland, 1976; 

Wheeler and Stubbs, 1976; Wheeler and Dixon, 1980; 

Wheeler and Holland, 1981) and includes such attributes of 

joints as their dimensions, overall configuration, surface 

structures, nature and sequence of mineral fillings, spacings 

between adjacent members, and the manner in which the 

joints terminate laterally and vertically. Collectively, these 

attributes uniquely define each joint set as the product of an 

episode of brittle failure whose age relative to other 

episodes is known. Several aspects of the use of joint styles 

in the field study of complex joint networks are summarized 

briefly here and described more fully in Grout and Verbeek 

(1983) and Grout (1988).

Two of the most important elements of joint style are 

surface structures and abutting relations among coexisting 

joints of different age. The types of structures observed on 

fracture surfaces are the most direct and reliable indicators 

of the mode of failure during each fracture event. Moreover, 

the origin point for each fracture, its direction of 

propagation at any place on its surface, and some estimate 

of the relative velocity of fracture propagation in different 

directions commonly can be determined by observing the 

directional components of the surface structures. Important 

directional indicators for joints formed in extension include

Fracture History, Southern Piceance Basin, Colorado Z7
plumose structures, arrest lines, and twist-hackle fringes, all 

common among the joint sets of the Piceance basin. The 

study of such structures fractography has long been used 

in the glass, ceramics, and materials science industries. 

Much practical information on fracture-surface structures is 

summarized in Kulander and others (1979, 1990), who also 

related the study of fractography to structures on rockfracture surfaces in outcrop and in core.

The manner in which fractures of coexisting sets 

terminate against or cut across older fractures is the single 

most important criterion for determining their relative ages. 

Most fractures in the Piceance basin formed in extension as 

open and initially cohesionless subvertical planes. 

Extension joints, unlike faults, cannot cross pre-existing 

open fractures, and thus younger joints commonly terminate 

against older joints. Where joints of an older set have been 

cemented by precipitated minerals, however, younger joints 

can either cut across or abut them, depending on the 

mechanical properties of the joint filling relative to those of 

the adjacent rock (see, for example, Engelder, 1982; Barton, 

1983; Grout and Verbeek, 1983; Verbeek and Grout, 1983). 

Crosscutting fractures containing cements of different age 

can be treated much like intersecting dikes for determination of the sequence of fracture. Both abutting and 

crosscutting relations are common among the fractures of 

the Piceance basin, and from them a consistent picture of the 

sequence of set formation has emerged.

The characteristics of the joints of each set and the 

sequence of set formation were recorded at each exposure to 

facilitate unambiguous correlation of sets across the region. 

The data for each set were then entered into a computer 

database program developed for the Piceance basin study 

(Grout, 1988) and which allows sorting of the data from any 

desired combination of characteristics. Fracture data for the 

Divide Creek-Wolf Creek area are presented in a 

combination of tabular, narrative, and stereonet form in 

Grout (1988) and are summarized in the appendix of this 

paper.

JOINT SETS OF THE PICEANCE BASIN AND 

GRAND HOGBACK MONOCLINE

The Upper Cretaceous through middle Eocene rocks 

of the Piceance basin and adjacent Grand Hogback monocline are cut by two systems of fractures, one mostly older 

and the other younger than the basin itself. The older of the 

two is termed the Hogback system (Verbeek and Grout, 

1984a, b) and is composed chiefly of two sets of joints, the 

MV, and the MV2 sets, which dominate the fracture 

network of the Upper Cretaceous and lower Paleocene rocks 

along the entire length of the Grand Hogback but do not 

extend into the younger basin rocks to the west. In addition, 

two older sets of the Hogback system recently have been 

identified along the southern Grand Hogback, but only in

Upper Cretaceous and older rocks. The younger Piceance 

system of fractures cuts Paleocene through middle Eocene 

rocks of the basin proper and is exposed over a large area 

west of the Grand Hogback, extending to the Colorado-Utah 

border and beyond into correlative rocks of the eastern 

Uinta basin. It is composed of five regional sets of joints 

designated Fl (oldest) through F5 (youngest) (Verbeek and 

Grout, 1983, 1984a, b). Along the eastern margin of the 

Piceance basin, in the transition zone between the basin and 

pre-basin rocks, strata already cut by joints of the Hogback 

system locally were fractured anew as the later Piceance 

sets developed; the base of the Piceance system and the top 

of the Hogback system thus overlap stratigraphically. All 

five sets of the Piceance system, as well as the youngest 

(MV2) set of the older Hogback system, are found in the 

Divide Creek-Wolf Creek area.

Piceance System

The joints of each set of the Piceance system are 

steeply dipping extension fractures of regional distribution 

but differing relative abundance from place to place (fig. 4). 

The F! joints strike generally N. 10°-30° W. and record a 

minor fracture event; in most places they are absent or form 

only an insignificant part of the total fracture network. 

Locally, however, they are more abundant, particularly in 

oil shale of the Green River Formation and in lenticular 

channel sandstone bodies of the Wasatch Formation. The 

later F2 joints strike N. 60°-75° W. and in the northern part 

of the basin constitute by far the dominant set; their 

importance, however, wanes to the south, where instead the 

F3 joints, of N. 60°-80° E. strike, are the most prominent 

elements of the fracture system. Both sets are present in 

almost all rock types. Field relations indicate that the F2 and 

F3 sets formed during a period when the stress field 

progressively rotated counterclockwise with respect to the 

basin, forming first the F2 set, then the F3 set, and still later 

resulting in left-lateral shear on both (Verbeek and Grout, 

1984a, 1986). The relative ages of the first three sets of the 

Piceance system have been established from consistent 

abutting relations, but their absolute ages remain only 

loosely constrained from about 43 to 10 Ma.

The remaining sets of the Piceance system formed 

under conditions of declining lithostatic load and probably 

began to form upon renewed regional uplift 8-10 Ma 

(Whitney and Andrews, 1983). As erosion progressively 

unloaded the rocks, fractures parallel to bedding the 

sedimentary equivalent of "sheeting" joints in crystalline 

rocks formed throughout the region, commonly along the 

contacts between beds of dissimilar lithology and within 

beds possessing a well-developed planar fabric, such as the 

laminated oil shales of the Green River Formation. The 

presence of the bed-parallel fractures is responsible for the 

small size of the F4 and F5 joints relative to those of the 

earlier pre-uplift sets. Continued unloading led to the

Z8 Evolution of Sedimentary Basins Uinta and Piceance Basins
108°30' 108°00' 107°30'

40°00'

39°30' i

38°45' 38°45'

108°30' 108°00' 107°30' 107°00' 108°30' 108°00 107°30'

40°00'

39°30'

38°45'

30 KILOMETERS

40°00'

39°30'

39°00' -

38°45'

Figure 4. Regional distribution and orientations of joints of F-,-F4 sets in Piceance basin. Data from Grout (1988), Grout and Verbeek 

(1985), Verbeekand Grout (1983,1984a, 1987), and unpublished data of both authors. Short-dashed line is base of Tertiary outcrop. 

A, F, set. B, F2 set. C, F 3 set. D, F4 set.

development of the F4 set, a prominent regional set of 

stress-relief "cross" fractures that consistently strike 

perpendicular to whichever of the earlier sets is dominant. 

The F4 joints thus strike N. 15°-30° E. over much of the 

northern part of the basin and N. 10°-30° W. over much of 

the southern part. Their tendency to form perpendicular to 

earlier joints extends even to the outcrop level, as noted

repeatedly in exposures where F2 and F3 joints coexist in 

differing relative abundance from bed to bed. The F4 joints 

are almost ubiquitous throughout the basin and form one of 

its most prominent sets. The later F5 set, in contrast, is the 

most weakly expressed set of the Piceance system and 

nowhere forms more than a minor part of the overall 

fracture network. Its joints, of N. 60°-75° W. strike, are

Fracture History, Southern Piceance Basin, Colorado Z9
almost parallel to the present-day in-situ direction of 

maximum horizontal compressive stress (Bredehoeft and 

others, 1976). The F5 and earlier F2 joints are of almost 

identical orientation but much different style and relative 

age.

Hogback System

The older joints of the Hogback system are exposed at 

the surface only where Laramide folding and later erosion 

has exhumed the Paleocene and older rocks that contain 

them, as along the Grand Hogback and in the Divide 

Creek-Wolf Creek area. The subsurface continuation of this 

system beneath the basin rocks is poorly documented, but 

oriented cores from the MWX site, about 25 km northwest 

of the crest of the Divide Creek anticline, contain abundant 

joints of the MVj set and far fewer of the MV2 set (Lorenz 

and Finley, 1987; Finley and Lorenz, 1989; Lorenz and Hill, 

1991). The core was extracted from the lower and middle 

parts of the Mesaverde Group at depths of 1,675-2,375 m. 

The somewhat younger upper Mesaverde and Wasatch 

rocks of the Divide Creek-Wolf Creek area contain only 

sparse joints of the MV2 set. Both sets are strongly 

expressed along the Grand Hogback, where during Laramide folding they were tilted with the beds to new attitudes. 

Restoration of their original orientations by stereographically "unfolding" the beds reveals that average 

strikes of the MVl joints were N. 70°-85° W. and of the 

MV2 joints N. 10°-30° E., in agreement with core data from 

the MWX site and outcrop data from the Divide Creek 

Wolf Creek area. Joints of both sets in steeply tilted beds 

along the Grand Hogback commonly are slickensided, the 

result of minor slip during folding.

JOINT SETS OF THE DIVIDE CREEK AND 

WOLF CREEK ANTICLINES

The Upper Cretaceous and Tertiary strata on the 

Divide Creek and Wolf Creek anticlines (fig. 2) show 

evidence of a more complex stress and fracture history than 

that of equivalent strata elsewhere in the Piceance basin. In 

all, eight sets of joints have been identified (table 1). Six of 

these can be correlated with regional sets in surrounding 

rocks, but the remaining two are local and probably formed 

during folding. These fold-related sets, late Laramide in age, 

postdate the MVj and MV2 sets of the Hogback system and 

predate all five sets of the Piceance system.

Characteristics of each joint set of the Divide CreekWolf Creek area are described briefly below, from oldest to 

youngest, and summarized in table 2. A more detailed 

discussion, based on data and field observations reported 

previously (Grout, 1988), is given in the appendix.

MV2 set. Joints of the MV2 set are abundant only 

within and adjacent to the Grand Hogback monocline,

Table 1 . Average axial trends of Divide Creek and Wolf 

Creek anticlines and strike range of joints of each set 

measured in the study area 

[Relative ages of joint sets are confirmed by their abutting relations]

Divide Creek anticlir 

Wolf Creek anticline

Joint set (from 

oldest to youngest)

MV2 ............................

Fold-parallel sets ......

pTJ ................................

2 

F3 ................................ J

4 ................................ 

F.................................

ic ...... .................... N. 20°

............................. N. 20°

Range of average 

strike of joints

...... N. 10°-25°E.

...... N.28°-55°W.

...... N.6°-36°W.

N 46°-74° W

...... N.54°-88°E.

......N.5°W.-N.44°E.

...... N.50°-80°W.

-45° W. 

-25° W.

Percent of stations 

where joints of 

set occur

4 

12 

27 

76 

49 

73 

8

particularly in strata of the Mesaverde Group and in the 

transition zone within the lower Wasatch Formation where 

fractures of the Piceance and Hogback systems overlap. In 

correlative strata on the Divide Creek and Wolf Creek 

anticlines, however, MV2 joints are sparse (fig. 5) and 

formed only within thick beds of sandstone (appendix). The 

MV2 joints strike north-northeast (table 2), dip steeply, and 

die out laterally as hairline cracks within the rock. Their 

characteristically large size (about 3 m high and 8-10 m 

long) and lack of termination against other fractures reflect 

their unrestrained development in unfractured rock. Surface 

structures indicate they are extension joints.

Fold-parallel sets. Two local sets of fractures are 

present on the Divide Creek anticline; they strike parallel to 

the axial trace of the fold (fig. 6) and dip moderately steeply 

to intersect bedding at typical angles of 60°-70°. These 

fractures at each outcrop studied form two sets that dip in 

opposite directions (fig. 7), one toward and the other away 

from the axial trace of the anticline. Many are large and 

span the full thickness of the beds in which they formed, but 

others either terminate vertically against each other or die 

out as hairline cracks in the rock. The inclined fold-parallel 

joints strike northwest to north-northwest (table 2) and are 

restricted to thick beds of sandstone (appendix). Plumose 

structure is preserved locally on some of these fractures and 

shows them to be extension joints, despite the superficial 

resemblance of their outcrop pattern to conjugate shear 

fractures. Similar fractures were found at only one locality 

on the Wolf Creek anticline. Fractures of similar strike but 

forming only one set perpendicular to bedding have been 

found in coals in the Divide Creek area (Grout, 1991) and 

probably belong to the same period of deformation.

Z10 Evolution of Sedimentary Basins Uinta and Piceance Basins
Table 2. Summary of fracture characteristics common to sets in the Divide Creek-Wolf Creek area

 n 

3

n

I

I

5>*

0

O

c

3-

n

3

 ^

8W

3

a?

J5°

n

o

o

i.

o

MV2 Fold-parallel

Wolf Creek (local)- Divide Creek1

Areal and Grand Hogback; (common)- Wolf

stratigraphic Mesaverde Group Creek (rare);

distribution and lower part Mesaverde Group

Wasatch Formation and Wasatch

Formation

Strike2 Common range N. 10°-25° E. N. 28°-55°W.

Median N. 16° E. N. 47° W.

Common Height 1-3 m 15-6 m

dimensions Length 15->25m 1-7 m

Spacings Variable; 0.75->4 m 0.25-5 m; commonly

05-3 m

Surface Plumes, arrest lines; Plumes, arrest lines;

structures; planar to subplanar planar to subplanar

shape

Mineraliza- No mineral fillings Local fibrous calcite

tion found patches

Terminating Against no other Against each other

relations joints

*Most joints are on northern end of Divide Creek anticline where trend of axis is N. 45° W.

F,

Divide Creek-Grand

Hogback (local);

Mesaverde Group

and Wasatch

Formation

N. 6°-36° W.

N. 31° W.

0.1-5 m

0.75-7 m

0.01-5 m; commonly

05-4 m

Plumes, arrest lines,

rare twist hackle;

planar to subplanar

No mineral fillings

found

Vertically against

fold-parallel

fractures. Laterally

none found4

F2

Divide Creek-Wolf

Creek; Mesaverde

Group and Wasatch

Formation

N. 46°-74° W.

N. 55° W.

0.3-4 m

0.3-5 m

0.08-4 m; commonly

0.1-15 m

Plumes, arrest lines,

twist hackle; planar

to subplanar

Calcite, barite,

quartz; in different

areas

Vertically against

fold-parallel

fractures. Laterally

none found5

F3

Divide Creek-Wolf

Creek; Mesaverde

Group and Wasatch

Formation

N. 54°-88° E.

N. 71° E.

0.25-7 m

0.3-7 m

0.25-8 m; commonly

0.3-4 m

Plumes, arrest lines

or twist hackle,

hooks3; planar to

subplanar

Generally no mineral

fillings; massive

calcite at one locality

Laterally against

joints of earlier

formed sets

^Dips of joints generally are within 10° of vertical; the fold-parallel sets in sandstones, however, dip S7°-7S° , either southwest or northeast.

^Although not surface structures, hooks indicate the shape of the surface.

^n other areas, Fj joints terminate against MV2 joints.

^In other areas, F, joints terminate against MV2 and Fj joints.

N
107°45' 107°30' 107°15'

39°22'30" -

39°15' -

EXPLANATION

Average strike and dip of joints in 

the MV2 fracture set

 Fracture station where the MN/2 set 

is not present

Figure 5. Average orientation of joints of MV2 fracture set in study area, southern Piceance basin.

Ft set. Joints of the Ft set are present only in widely 

scattered areas of the basin (fig. 4) but in some places are 

prominent elements of the local fracture network. Within 

the study area (fig. 8) the Fj joints are large, conspicuous 

fractures that strike north-northwest (table 2), more 

northerly on average than the fold-parallel fractures, and 

that also dip more steeply, typically within 5° of vertical. 

Common plumose structure and local arrest lines and twist 

hackle are evidence of extensile failure. Many Fl joints 

terminate against no other fractures and die out laterally as 

hairline cracks in the rock, except on the Divide Creek 

anticline, where in a few places they abut earlier fractures of 

the fold-parallel sets. Though Fj joints were nowhere 

observed in association with MV2 joints in the study area, 

abutting relations elsewhere in the basin (Verbeek and 

Grout, 1984a) consistently establish Fj as the younger set.

F2 set. Joints of the F2 set are large (fig. 9) and 

prominent throughout the study area (fig. 10); at many 

stations they constitute the dominant set. They also 

dominate the fracture network in the northern two-thirds of 

the basin (fig. 4) but, with the exception of the Divide 

Creek-Wolf Creek area, are more weakly expressed in the 

southern part. The F2 joints strike west-northwest (table 2),

dip steeply, and terminate laterally against MV2 and F: 

joints or vertically against the fold-parallel sets, if present. 

Surface structures typical of extension fractures, including 

plumose structure, arrest lines, and twist hackle, are 

common on F2 joints.

A notable property of the F2 and older sets is that they 

are mutually exclusive to a considerable degree (fig. 11); 

only rarely are two of these sets well developed in the same 

place. This is a common property of F2 joints basinwide and 

probably arises because the F2, F1? and fold-parallel sets are 

sufficiently close in average strike (table 2) that strain 

during the F2 fracture episode could be accommodated by 

reopening of the older fractures without forming new ones. 

Development of the F2 set thus was suppressed wherever 

the F! or fold-parallel sets had already formed. Terminating 

relations thus are uncommon but nevertheless consistent 

and sufficiently numerous to establish firmly both the 

existence and relative ages of the three sets.

F3 set. Joints of the intermediate-age F3 set strike 

east-northeast to northeast (table 2), dip steeply, and 

terminate laterally against the MV2 , fold-parallel, F,, or F2 

joints, if present. The F3 joints are large, planar fractures in 

those few outcrops where they were the first set to form;

Z12 Evolution of Sedimentary Basins Uinta and Piceance Basins
107°3D' 107°15'

39°15' -

EXPLANATION

=f= Average strike and dip of joints in 

64 fold-parallel sets

 Fracture station where the fold-parallel 

fracture sets are not present

Figure 6. Average orientation of joints of fold-parallel fracture sets in study area, southern Piceance basin. Paired strike-and-dip 

symbols are on either side of station location.

elsewhere they are smaller on average than the older joints 

and more irregular in shape, both natural consequences of 

jointing of a repeatedly fractured and thus increasingly 

anisotropic rock. Common plumose structure, arrest lines, 

and twist hackle furnish abundant evidence of extensile 

failure. Joints of this set are present throughout the study 

area (fig. 12) but are not as numerous as joints of the F2 set 

and were found in less than half the outcrops studied.

Joints of the F3 set are sparse in the northern part of 

the basin but greatly increase in abundance southward; west 

of the study area they dominate the fracture network. The 

general north-to-south increase in abundance of the F3 joints 

across the basin is the opposite of that for the F2 set (fig. 4). 

Wherever F! joints are found, however, the F3 set 

commonly is present also. Again these effects are a probable 

result of one joint set influencing development of another: 

the F! joints strike at high angles (about 80°) to the F3 joints 

and thus were unfavorably oriented to accommodate 

extensional strains during F3 time, whereas oblique 

reopening of F2 joints during the same deformation was 

possible. Reactivation of F2 joints in a sense compatible 

with stress orientations during F3 time has been documented

at several localities near the study area (Verbeek and Grout, 

1984a). The F3 joints thus formed preferentially where Ft 

joints were present and F2 joints least abundant.

F4 set. Joints of the F4 set are present throughout the 

study area (fig. 13) and are prevalent as well throughout the 

rest of the Piceance basin. In most places they formed at 

high angles to whichever of the F2 or F3 sets was dominant; 

the F4 joints thus strike north-northwest in beds cut by 

abundant F3 joints but strike north-northeast wherever the 

F2 set is most strongly expressed. This effect is seen not 

only on a regional scale (figs. 4, 13) but also in individual 

outcrops and accounts for the large dispersion in orientation 

(strike range of 50°, table 2) of the F4 set.

The F4 joints formed during and after regional uplift 

of the area, as evidenced by their common terminations 

vertically against bed-parallel partings (unloading joints) 

(Verbeek and Grout, 1983). Laterally they generally 

terminate against whatever earlier joints were present unless 

these were firmly cemented, in which case the F4 joints cut 

across some of the older joints and are larger than normal. 

The variable but generally small size (0.08-3 m long, 

0.1-2 m high) and irregular, sinuous, and commonly

Fracture History, Southern Piceance Basin, Colorado Z13
Figure 7. Especially prominent and sharply formed foldparallel fractures in 10-m-thick sandstone bed of Wasatch 

Formation at station 644 (fig. 2) on Divide Creek anticline. A, 

View of jointed bed looking northwest. Note that the two 

inclined sets are not equally developed. Although fractures of 

one set appear to crosscut fractures of another, on closer 

inspection it can be shown that they do not. B, Closeup view. 

Note that dips of fractures are unequal. Delicate surface 

structures, barely visible on the left-dipping fracture, indicate 

strike-parallel propagation. Fractures facing observer are 

members of the F, set.

branching surfaces (appendix) are properties so 

characteristic of joints of this set that in many places they 

can be recognized on sight. Arrest lines indicate they are 

extension fractures.

F5 set. The youngest joints, the F5 set, as in other 

parts of the Piceance basin are sparse in the study area (fig. 

14). They are parallel to the F2 set in orientation but are 

unlike them in all other style characteristics (appendix) and 

cannot be mistaken for them. F5 joints terminate vertically 

against bed-parallel parting joints and laterally against the 

F4 joints and thus are uniformly small. Their surfaces are 

irregular, even more so than those of the F4 set. Local arrest 

lines and twist hackle show that they are extension joints.

Cleats in coal. Knowledge of cleat orientations in 

coal is important for the economic recovery of coalbed 

methane through directional drilling. The most productive 

coals, those of the lower part of the Mesaverde Group, are 

not exposed in the Divide Creek-Wolf Creek area, but 

scattered exposures of coaly stringers in the lower part of 

the Wasatch Formation provide data on cleat directions that 

may be applicable to the underlying coals. The face cleats 

(major fractures) in the study area are small, subvertical, 

sharply planar fractures whose surfaces abundantly show 

such extension-fracture propagation structures as plumes, 

arrest lines, and twist hackle (Grout, 1988). The face cleats 

generally strike N. 35°-50° W., similar to the two early 

fold-parallel sets discussed above (tables 1, 2). In the brittle 

coals, however, only one set of fold-parallel cleats 

perpendicular to bedding is present, in contrast to the two 

sets oblique to bedding in the thick sandstones nearby. The 

end cleats have formed almost perpendicular to the 

prevailing face-cleat direction and thus generally strike 

east-northeast to northeast, about parallel to the F3 set. The 

end cleats everywhere are subordinate to the face cleats in 

size and commonly also in abundance; typically they are 

short fractures that terminate at both ends against the earlier 

formed face cleats (Grout, in press). Neither the face cleats 

nor the end cleats are mineralized.

That cleat sets in coal can be matched to joint sets in 

associated clastic rocks is true of the rest of the Piceance 

basin also (Grout, 1991), though different sets comprise the 

face and end cleats in different areas. The F3 joint set, for 

example, forms the face cleats of most coal seams to the 

west of the study area but corresponds to the end cleats on 

the Divide Creek fold, where instead an older fracture set 

not present farther west forms the face cleats. The timing of 

cleat formation is discussed at greater length later in this 

report.

DISCUSSION

Extension joints in areas of mildly deformed 

sedimentary rocks are common and sensitive indicators of 

stress history. In the Piceance basin, for example, the Upper

Z14 Evolution of Sedimentary Basins Uinta and Piceance Basins
107°45' 107°30' 107°15'

39°22'30" -

39°15' -

Average strike and dip of joints in 

the FI fracture set

EXPLANATION

Strike of vertical joints in the FI Fracture station where the FI set 

fracture set is not present

Figure 8. Average orientation of joints of F, fracture set in study area, southern Piceance basin.

Cretaceous through middle Eocene rocks were subjected to 

only two periods of deformation in the conventional sense 

one gave rise to the Grand Hogback and related folds and 

the second to scattered normal faults yet these same rocks 

are cut by nine regional and two local sets of joints. Eight of 

these are represented in the Divide Creek-Wolf Creek area. 

Opportunity to relate fracture and stress histories to 

tectonic evolution of the Piceance basin only recently has 

become possible. Fracture data from outcrops in the Grand 

Hogback monocline (Verbeek and Grout, 1983; 1984a, b; 

Grout and Verbeek, 1987) and from oriented cores from the 

MWX site in the Colorado River valley (Lorenz and Finley, 

1987; Finley and Lorenz, 1989; Lorenz and Hill, 1991) have 

gone far in revealing the connection between the younger 

Piceance and older Hogback systems of fractures. Improved 

understanding of subsurface structure along the 

southeastern margin of the basin has come from recently 

acquired seismic and gravity data (Abrams and Grout, 1987, 

1990; Perry and others, 1988; Grout and others, 1991), and 

recent stratigraphic and thermal maturity studies (Bostick 

and Freeman, 1984; Johnson and Nuccio, 1984, 1986; 

Nuccio and Johnson, 1984, 1989; Johnson, 1989) have 

provided new insights into the depositional, thermal, and

burial and uplift histories of the basin rocks. From these 

studies we now know that some of the fracture sets in the 

basin are related to the Grand Hogback and associated folds 

and resulted from basin-margin tectonism, whereas others 

provide clues to events in the regional stress history that left 

no other known signature upon the rocks. Some of these 

topics are discussed below. Also discussed is the timing of 

fracture development with respect to the change in the 

hydrocarbon reservoir rocks from pore-dominated to 

fracture-dominated fluid flow.

Distribution of MV2 Set

North-northeast-striking joints of the MV2 set are 

abundant along the entire 135-km-long outcrop belt of the 

Grand Hogback and cut more than 2.000 m of section, from 

the lower Wasatch Formation through the underlying 

Mesaverde Group and into the Mancos Shale beneath. They 

formed about perpendicular to earlier joints of the equally 

widespread and even more prominent MV1 set, but the MV2 

joints persist to higher stratigraphic levels and in parts of the 

lower Wasatch Formation are the oldest joints.

Fracture History, Southern Piceance Basin, Colorado Z15
Figure 9. Prominent, sharply formed joints of F2 fracture set 

in interbedded sandstones of Wasatch Formation, as viewed 

from below and along strike in a vertical face on west side of 

Divide Creek anticline. Joints of F4 fracture set (sunlit, dipping 

toward observer) are bounded laterally by F 2 fractures and are 

approximately perpendicular to them. Station 754 (fig. 2); view 

to west-north west.

The MV2 joints formed while bed dips were still quite low 

and subsequently were rotated with those beds to new 

attitudes during monocline development (Verbeek and 

Grout, 1984a, b; Grout and Verbeek, 1987). Along parts of 

the northern leg of the Grand Hogback, for example, they 

are now subhorizontal in beds tilted to vertical. Their wide 

geographic extent and pre-monocline age are properties 

suggestive of a regional set of joints unrelated to basinmargin tectonism. If so, however, their near absence at both 

the MWX site (Lorenz and Finley, 1987; Finley and Lorenz, 

1989) and in the Divide Creek-Wolf Creek area (fig. 5) 

(Grout, 1988) is puzzling; both areas are only 10-18 km 

from the Grand Hogback. That a prominent joint set 

traceable for more than 135 km along the length of the 

monocline should die out so dramatically within 20 km 

away from it was the first strong clue that the distribution of 

this set was related to events along the basin margin.

The probable nature of the relation between the MV2 

joints and basin-margin tectonism became apparent only 

recently, as knowledge accumulated on the underlying 

structure of the monocline and associated intrabasin folds 

(Perry and others, 1988; Grout and others, 1991) and as 

stratigraphic relations near the monocline were investigated 

in greater detail. Though the bulk of the thrusting to form 

the Grand Hogback occurred during Eocene time, marked 

thinning of the Paleocene (lower Wasatch) section toward 

the monocline (R.C. Johnson, written commun., 1991) is 

suggestive of earlier stages of movement. So too is the 

uniformly low rank of coals along the Grand Hogback 

(Nuccio and Johnson, 1989), which indicates that Mesaverde strata along that fold never were deeply buried. In 

Johnson's view, the Mesaverde and overlying lower 

Wasatch strata during the Paleocene epoch were gently 

warped over the leading edge of the White River block the 

future site of the Grand Hogback as it began to thrust 

southwestward into the Piceance basin. That the MV2 joints 

are found only in the lower Wasatch and older strata 

suggests that they too formed about this time, as a set of 

uplift-induced joints perpendicular to joints of the earlier 

(and more widespread) MVa set. Strata west of the 

deforming edge of the White River block were minimally 

affected by these early movements and developed few or no 

MV2 joints. Bed dips during these initial movements 

remained low, as reflected in the consistent nearperpendicularity of the MV2 joints with respect to bedding, 

but later movements during the main phase of thrusting 

sharply tilted the beds and their contained joints to form the 

Grand Hogback monocline. In this view, then, genesis of the 

MV2 set is related to early basin-margin tectonism, and the 

marked decrease in prominence of the set toward the basin 

is a natural consequence of differential tectonic movement.

Presence and Prominence of 

Local Fold-Parallel Fractures

The thickest and coarsest grained sandstone beds on 

the Divide Creek anticline are hosts for two local sets of 

relatively abundant, large fractures that dip at angles 

of 55°-70° to bedding and that commonly strike 

N. 28°-55° W. (fig. 6), parallel to the axial trace of the 

Divide Creek anticline. Although the strikes of these 

fractures overlap those of the regional Fl and F2 joints, their 

dips are less steep (table 2) and abutting relations indicate 

that they are older, the Fj and F2 fractures are shorter and 

consistently terminate vertically against them. The early age 

of these local joints, their parallelism to the Divide Creek 

anticline, and especially their abundance on the flanks and 

crestal regions of that fold (fig. 6) contrasted to their 

absence elsewhere collectively suggest that the joints and 

the fold may share a common genesis.

The fold-parallel fractures on the Divide Creek anticline generally form two sets, each parallel to the axial

Z16 Evolution of Sedimentary Basins Uinta and Piceance Basins
107°45' 107°30' 107°15'

39°22'30"

39°15'

Average strike and dip of joints in 

the F£ fracture set

EXPLANATION

Strike of vertical joints in the F2 

fracture set

 Fracture station where the ?2 set 

is not present

Figure 10. Average orientation of joints of F 2 fracture set in study area, southern Piceance basin.

Figure 11. Especially prominent and closely spaced joints of 

F 2 and F4 fracture sets in sandstone of Wasatch Formation at 

station 552. View south-southeast from crest of Wolf Creek 

anticline along F2. A few joints of the F3 regional set, at an 

angle of about 35° to F2 joints, are near the center right of 

photograph.

trace of the fold but inclined in opposite directions, westsouthwest for one set and east-northeast for the other. 

Although this geometry is suggestive of conjugate shear

fractures, with O2 parallel to fracture strike and G1 

approximately perpendicular to the beds, surface structures 

show that they are extension fractures. Delicate plumes and 

arrest lines are common, and twist hackle was noted in a few 

places (fig. IB). The overall direction of propagation 

indicated by the surface structures is parallel to fracture 

strike. Small patches of calcite growth fibers attached to the 

asperities of three fracture surfaces indicate local, minor 

normal-slip movement postdating fracture formation. In 

some places a fracture of one set may appear to cut across 

another of opposite dip (fig. 1A), but on closer inspection 

(fig. IE) the shorter is seen to terminate against the longer. 

Fractures of the two fold-parallel sets are unequal in 

size (appendix); in most places the largest fractures dip 

toward the axial trace of the anticline and the smallest dip 

away, regardless of their location on the fold. In addition, 

the set dipping toward the fold axis consistently has the 

shallower dip, by amounts of 2°-l 1°. Together these two 

facts are revealing: the first suggests that the Divide Creek 

anticline at the time of fracture already had some structural 

expression so that position on the fold determined which set 

would grow to larger size; the second suggests that folding 

continued after joint formation so that joints dipping toward

Fracture History, Southern Piceance Basin, Colorado Z1 7
107°45' 107°30' 107°15

39°15'

82

Average strike and dip of joints in 

the F3 fracture set

EXPLANATION

Strike of vertical joints in the FS 

fracture set

 Fracture station where the FS set 

is not present

Figure 12. Average orientation of joints of F3 fracture set in study area, southern Piceance basin.

the fold axis were rotated to shallower dips and their 

counterparts to steeper ones. Joint development during 

folding thus best explains the observed geometry.

Fracture sets bearing consistent geometric relations to 

folds, and by some workers attributed to the same general 

deformation responsible for the folding, have been reported 

from many areas (Price, 1966; Hancock, 1985). A common 

and simple pattern consists of a single set of vertical 

extension fractures perpendicular to the axial trace of the 

fold and thus parallel to the compression direction during 

folding; such patterns have been reported, for example, by 

Stearns and Friedman (1972) and Engelder and Geiser 

(1980). Also common in some areas are conjugate sets of 

fractures parallel to the fold axis and dipping at low angles 

to bedding, such as those discussed by Reches in Israel 

(1976). Other patterns and the possible significance to be 

attached to each have been discussed by Stearns and 

Friedman (1972) and Hancock (1985), among others. We 

have not yet found, however, a published analogue to the 

fractures described here. Minor normal faults along the 

crestal regions of anticlines (Suppe, 1985 and references 

cited therein) offer the closest geometric analogy, with 

fractures striking parallel or subparallel to the fold axis

and steeply inclined to bedding, but we emphasize again 

that surface structures on the Divide Creek fractures 

indicate that they are extension, not shear, fractures. 

Although minor normal offsets have occurred along some of 

them, those movements were secondary. Nevertheless, as in 

the case of crestal faulting, we infer that these fractures 

developed in response to stratal lengthening during 

formation of the Divide Creek anticline and that they 

constitute another means by which extensional strain is 

accommodated above the neutral surface of a growing fold 

(see Hobbs and others, 1976; Suppe, 1985). In this context 

it is perhaps worthy of note that the inclined fold-parallel 

fractures formed only in thick beds of sandstone, where 

strain accommodation by interstratal slip would have been 

least effective.

Distribution of Fractures of the 

Piceance System

Reference has been made earlier in this paper to areal 

variations in joint abundance among the sets of the Piceance 

system. The Fj joints, for example, are moderately abundant

Z18 Evolution of Sedimentary Basins Uinta and Piceance Basins
107°45'

\ Hagpole

\ Mountain,'

X

39°15' -

Average strike and dip of joints in 

the F4 fracture set

EXPLANATION

Strike of vertical joints in the F4 

fracture set

 Fracture station where the F4 set 

is not present

Figure 13. Average orientation of joints of F4 fracture set in study area, southern Piceance basin.

on the Divide Creek anticline but scarce on the Wolf Creek 

fold, and on both folds the F2 joints are more abundant than 

in bordering areas to the west, where instead the F3 set 

generally is dominant. It is tempting, therefore, to suggest 

that formation of the Divide Creek and Wolf Creek folds in 

some manner influenced the development of the F! and F2 

sets. We discount that hypothesis, however, on three 

grounds: (1) the post-Laramide age of all five sets of the 

Piceance system, (2) their regional extent beyond the 

confines of the Piceance basin, and (3) the common 

existence of comparable variations in relative abundance of 

the same sets elsewhere, where intrabasin folds are absent.

A post-Laramide age for all five sets of the Piceance 

system is suggested from the following lines of evidence:

1. Fj and F2 joints on the Divide Creek anticline 

maintain near-vertical attitudes regardless of bed dip. The 

four steepest beds measured, at stations 753, 754, 759, and 

625 (Grout, 1988) are 9°, 10.5°, 11°, and 20°, respectively. 

The Fj set is at three of these locations and the F2 set at all 

four, but average dips of the joints in all seven cases are 87° 

or more. Both sets are oriented such that post-joint folding 

should have resulted in readily detectable tilt of the joints, 

but no such tilts were observed.

2. Similarly, joints of the Piceance system in steeply 

tilted rocks of the Wasatch Formation along the Grand 

Hogback and in subhorizontal basin rocks farther west have 

comparable orientations, suggesting that the joints were 

superimposed on already folded strata. Slickenside 

striations that resulted from reactivation of old joints during 

monocline formation are common on joints of the Hogback 

system but lacking on the later joints of the Piceance 

system.

3. Minor, calcite-cemented thrust faults within thin 

dolomite layers of the lower part of the Green River 

Formation near the Grand Hogback are cut through by F2 

joints. Similar relations were observed at scattered localities 

elsewhere in the basin.

4. Kink bands related to Laramide compression 

were observed within the Green River Formation at more 

than a dozen localities and at each of them predate all joints 

sets present.

From these observations we infer that joints of the 

Piceance system mostly postdate structural movements 

along the eastern margin of the basin and that areal 

variations in the relative abundance of sets cannot be 

explained in terms of basin-margin tectonism. The broad

Fracture History, Southern Piceance Basin, Colorado Z19
107°45' 107°30' 107°15'

39°22'30" -

39°15' -

EXPLANATION

85 Average strike and dip of joints in 

the F§ fracture set

 Fracture station where the F5 set 

is not present

Figure 14. Average orientation of joints of F5 fracture set in study area, southern Piceance basin.

geographic extent of this system, which affected an area far 

larger than the Piceance basin and whose outer boundaries 

remain undefined, suggests instead that its formation is 

related more to regional stresses affecting much of the 

northeastern Colorado Plateau than to events associated 

with the evolution of a single basin or basin margin. The 

distributions of the F2 and F3 joint sets are a case in point. 

During the F2 fracture episode, most strata in the northern 

part of the basin were stressed to the point of failure and 

those farther southwest progressively less so, whereas later, 

during counterclockwise rotation of the stress field, 

effective stresses weakened in the north, strengthened in the 

south, and culminated in formation of the F3 set. We find 

nothing in the thermal and burial history of the rocks to 

explain such an effect and instead ascribe it to regional 

variations in intraplate tectonic stress. Except for the joints, 

little record exists of these events.

Additional factors controlling differences in joint-set 

expression on a more local scale potentially are many; rock 

type, fluid pressure, and lithostatic load are among the more 

obvious. Stress amplification due to rock-body geometry, as 

iin lenticular masses of sandstone embedded in mudstone, 

may also play some role (J.C. Lorenz and N.R. Warpinski,

written commun., 1990). The effect of previous fracture 

history in discouraging the formation of new joint sets at 

low to moderate angles to older ones has already been 

mentioned. These factors, singly or in combination and 

including others not mentioned here, probably are 

responsible for much of the local variations in joint-set 

prominence in the Piceance basin.

Fracture History and Fluid Movement

Timing of Fracture Formation

The timing of fracture formation in Upper Cretaceous 

reservoir rocks in the Divide Creek-Wolf Creek and 

adjacent areas is of importance to studies on gas and 

petroleum generation, migration, and production in the 

basin. Much of the gas presently contained within the 

Mesaverde Group originated within the interbedded coal 

and carbonaceous shale of the lower part of the group, 

together with an uncertain but possibly substantial 

contribution from the thick underlying Mancos Shale 

(Johnson, 1989, p. 35-36). Johnson suggested that

Z20 Evolution of Sedimentary Basins Uinta and Piceance Basins
Table 3. Fracture chronology for the Divide Creek-Wolf Creek area

Fracture set Probable age Abundance in area

MV .Paleocene-early Eocene

Fj ............................Late middle Eocene or younger (<43 Ma).

F2 ............................Late middle Eocene or younger (<43 Ma).

F3 ............................Early to middle Miocene (?).........................

F4 ............................ Late Miocene or younger ( < 10 Ma)............

F«; ............................Late Miocene or younger (<10 Ma)...........,

.Rare.

.Moderate (Divide Creek); rare (Wolf Creek).

.Moderate.

.Abundant.

.Abundant.

.Abundant.

.Rare.

generation of thermal gas from the lower part of the Mancos 

Shale could have begun in the deepest part of the basin near 

the close of the Cretaceous Period, during the CretaceousTertiary depositional hiatus, but that temperatures in the 

overlying Mesaverde beds at that time probably were too 

low to generate significant amounts of thermal gas. 

Subsequent burial of the Mesaverde Group during 

deposition of the overlying Wasatch and Green River 

sediments raised formation temperatures so that, by early 

Eocene time, some gas probably was being generated within 

the Mesaverde source rocks in the deepest parts of the basin. 

By the close of the Eocene epoch, however, "gas was being 

generated by at least the lower part of the Mesaverde 

throughout much of the basin" (Johnson, 1989, p. 1). Gas 

generation continued until about 10 Ma, when regional 

uplift and erosional downcutting lowered formation 

temperatures. When during this long period of gas generation lasting roughly 55 m.y. did fractures form in the 

reservoir and source rocks?

The oldest fractures known from the Divide CreekWolf Creek area are members of the MV2 set. The MV2 

joints are particularly abundant along the Grand Hogback 

and tentatively are regarded as late Paleocene to early 

Eocene on the basis of stratigraphic evidence: nowhere have 

we found them in beds younger than those of the lower 

Wasatch Formation. As noted previously, however, the 

prominence of the MV2 set wanes markedly from the 

monocline westward to the Divide Creek area, where MV2 

joints are present at only 2 of the 53 sites studied. Moreover, 

MV2 joints in the Divide Creek area penetrate little of the 

stratigraphic section but instead are confined to thick beds 

of sandstone, and they did not become interconnected with 

fractures of other sets until much later. As conduits for early 

hydrocarbon migration in the Divide Creek-Wolf Creek 

area, then, their influence probably was negligible.

The fold-parallel joints of the Divide Creek anticline 

probably date from the main phase of basin-margin 

compression, but precise dating of structural movements 

along and near the Grand Hogback is difficult due to erosion 

of post-Wasatch strata from the monocline. Some 

movement during deposition of the Green River Formation 

is indicated from stratigraphic studies in the basin (R.C.

Johnson, oral commun., 1991), and tilting of the same beds 

near the monocline shows that movement continued through 

late middle Eocene time or later. Post-Green River 

Formation movement also is indicated by the presence in 

that unit of local thrust faults and kink bands, all compatible 

with southwest- to west-southwest-directed tectonic 

compression, but thrusting probably had ceased by the end 

of Eocene (Johnson, 1989) or early Oligocene time (Tweto, 

1978). The existing evidence thus permits the probable age 

of the fold-parallel sets to be given only as Eocene. Similar 

to the MV? joints, fold-parallel joints in the Divide Creek 

area are restricted to certain rock types, principally thick 

beds of sandstone and also coal, but are present only in 

modest abundance and are missing from other strata. The 

fracture network, though increased in complexity by the 

formation of the fold-parallel sets, still was poorly 

interconnected and both stratigraphically and 

geographically limited.

All other fracture sets in the Upper Cretaceous and 

Paleocene rocks of the Divide Creek-Wolf Creek area are 

of demonstrated post-Green River age. The earliest (Fj) set 

of the Piceance system, for example, formed within upper 

Green River strata at scattered but numerous places 

throughout the basin, and joints probably corresponding to 

this set have been found sparingly in the overlying Uinta 

Formation also. Joints of the F2 and younger sets are 

abundant in both units and have been documented at 

hundreds of localities. The late middle Eocene (about 43 

Ma) age of the Uinta Formation, the youngest unit preserved 

in the basin, thus provides an approximate upper bound on 

the age of the oldest joints of the Piceance system. Field 

evidence (M.A. Grout, unpub. data, 1984-86) suggests that 

the F2 joints are older and the F3 joints younger than dikes 

provisionally mapped as Miocene (Tweto and others, 1978) 

in the southernmost Piceance basin, but the exact relation 

between the dikes and joints is not yet clear and the dikes 

have not been dated. Finally, the maximum likely age for 

the F4 set probably is fixed by the time of renewed regional 

uplift 8-10 Ma (Whitney and Andrews, 1983, as discussed

Fracture History, Southern Piceance Basin, Colorado Z21
in Verbeek and Grout, 1983) to which the set is related. 

Taken together, the limited evidence so far available 

suggests a fracture chronology similar to that given in 

table 3.

Fluid Movement Through Upper Mesaverde and 

Lower Wasatch Strata

The upper Mesaverde and lower Wasatch strata of the 

Divide Creek-Wolf Creek area are the upper part of a 

gas-bearing sequence of low-permeability rocks that, 

slightly lower in the section, includes the principal reservoir 

rocks of the basin. The fracture network as it exists today in 

these rocks is the product of at least seven episodes of brittle 

failure spanning a time interval of more than 50 m.y. 

Though the absolute ages of some fracture sets are only 

loosely constrained, it is clear from field relations that (1) 

members of the earliest joint set in exposed strata of the 

Divide Creek-Wolf Creek area are of inconsequential 

abundance; (2) the syntectonic fold-parallel fractures are 

only locally prominent and restricted to certain rock types; 

and (3) all of the other joint sets those of the Piceance 

system, which collectively dominate the fracture network are of late middle Eocene age or younger. Thus, 

with the exception of coals (discussed below) and some 

thick sandstone beds, the change from matrix-dominated to 

fracture-dominated fluid flow in these rocks did not occur 

early in their history. Most of the upper Mesaverde Group in 

this area, for example, remained unfractured for more than 

20 m.y., the elapsed time between deposition of the strata 

(75-66 Ma for the group as a whole) and the earliest time at 

which fractures of the Piceance system could have started to 

form (43 Ma, the approximate age of the Uinta Formation). 

During this long period of time, vertical leakage of 

formation fluids water, plus any early formed gas, if 

present through most of the section could have occurred 

only through intergranular pore space. More rapid crossformation fluid flow through a vertically continuous 

network of interconnected fractures became possible only 

after middle or late Eocene time.

Comments on Coal

Data on cleat directions in buried coal seams beneath 

the Piceance basin are few, but nonetheless consistent with 

the notion that face cleats near the Grand Hogback are older 

than those farther west. Other than the Red Mountain site, 

the most complete data are from the MWX site, where face 

cleats of west-northwest strike (J.C. Lorenz, oral commun., 

1990) probably correspond to the MV\ fracture set, the 

dominant set of the Mesaverde Group along the entire 

length of the monocline. From outcrop data in the Divide 

Creek area, where coaly stringers of the lower Wasatch 

Formation are cut by the syntectonic fold-parallel joints, it 

seems reasonable to assume that the deeper coals of the

Mesaverde Group are cut by this or an older set. Finally, 

face cleats in an exposed coal seam on the Coal Basin 

anticline, immediately south of the Divide Creek-Wolf 

Creek area, correspond to the MV} set in part of the coal 

seam and to the fold-parallel sets in another part (Grout, in 

press). In all these areas the face cleats are of pre-Piceance 

system age. Fracture-dominated fluid flow through these 

coal beds may have been achieved as early as early 

Paleocene time for some beds and sometime during the 

Eocene epoch for others.

The relatively early age of the face cleats in the 

Divide Creek-Wolf Creek area is anomalous for the 

southern Piceance basin and, like the anticlines themselves, 

is a result of basin-margin tectonism. Farther west, in the 

rest of the southern Piceance basin where strata were little 

affected by these movements, the majority of face cleats 

both in outcrop (Boreck and Strever, 1980; Geological 

Services of Tulsa, Inc., 1980; Grout and Verbeek, 1985) and 

in core (Horner, 1986; Seccombe and Decker, 1986) strike 

east-northeast and are members of the widespread and 

regionally prominent F3 set (Grout and Verbeek, 1985). 

Locally, as in the Red Mountain area in the south-central 

part of the basin, some face cleats in core strike westnorthwest (Horner, 1986), parallel to the F2 joints (Grout, 

1991) also present in this area. The coals were deposited 

75-73 Ma and the cleats within them are of post-Uinta 

Formation age (<43 Ma), pointing to at least a 30-m.y. 

hiatus between deposition of the coal beds and generation of 

the first major fracture set within them. This long interval 

may seem surprising in view of the common brittleness and 

low tensile strength of coal at shallow to intermediate 

depths of burial and the demonstrated or inferred early time 

of coal fracturing in other regions (see, for example, 

Nickelsen and Hough, 1967; McCulloch and others, 1974; 

Perry and Colton, 1981). Nevertheless, our data on cleat 

orientations are in accord with those of other workers 

(Boreck and Strever, 1980; Geological Services of Tulsa, 

Inc., 1980; Seccombe and Decker, 1986), and the young, 

post-Uinta age of the F2 and F3 joint sets is well established.

CONCLUDING REMARKS

Gas fields producing from the Mesaverde Group and 

part of the Wasatch Formation are widely scattered across 

the Piceance basin, but those near the eastern margin of the 

basin tend to be the best producers (Pitman and Sprunt, 

1986; Johnson, 1989). The most productive fields, Divide 

Creek in the southern part of the basin and Piceance Creek 

in the northern part, are both located on northwest- to 

west-northwest-trending anticlinal folds whose crests are 

within 20 km of the Grand Hogback. The Piceance Creek 

fold has a gentle northeast flank but a much steeper 

southwest flank and, similar to the Divide Creek fold, may 

be underlain by a blind thrust fault at depth (Pitman and 

Sprunt, 1986). Production from both fields is enhanced by

Z22 Evolution of Sedimentary Basins Uinta and Piceance Basins
abundant natural fractures in the reservoir rocks. Core 

analysis and comprehensive review of well records from 

throughout the Piceance basin (Pitman and Sprunt, 1986) 

indicate that highly fractured reservoir rocks are 

characteristic of gas fields near the eastern margin of the 

basin but that fractures are considerably less well developed 

within the Mesaverde and Wasatch reservoir rocks farther 

west. Pitman and Sprunt concluded that abundant natural 

fractures, along with structural closure on doubly plunging 

anticlines, have enhanced the productivity of gas fields in 

the eastern part of the basin. Both the anticlinal folds (Grout 

and others, 1991) and enhanced fracture permeability (this 

paper), it now appears, are related effects of basin-margin 

tectonism.

The oldest fracture set possibly related to thrusting 

along the eastern edge of the Piceance basin is the MVl set. 

Thus far we have said little about this set because it is not 

present in the study area. The west- to west-northweststriking joints of this set, however, dominate the fracture 

network throughout the Mesaverde Group and lowermost 

Wasatch Formation along the entire length of the Grand 

Hogback (Verbeek and Grout, 1984a, unpub. data, 

1986-1988) and also are abundant within the cored intervals 

from the lower and middle Mesaverde Group at the MWX 

site about 20 km distant from the monocline (Lorenz and 

Finley, 1987; Lorenz and others, 1989). Their absence from 

the younger, uppermost Mesaverde and lower Wasatch 

rocks in the Divide Creek-Wolf Creek area contrasted with 

their abundance in these same rocks along the monocline 

suggests that the stratigraphic distribution of this set wanes 

westward. Moreover, no trace of the MVj fracture set has 

been found in the extensive exposures of the Mesaverde 

Group ( and underlying Mancos Shale) along the southern 

and southwestern margins of the basin, where instead these 

rocks are cut only by the relatively young joints of the 

Piceance system (Verbeek and Grout, 1984a, b; Grout and 

Verbeek, 1985). The available information thus suggests 

that the MVl fracture set is most strongly expressed along 

the eastern margin of the basin, wanes in strength and 

stratigraphic distribution westward, and is missing entirely 

from the western part of the basin. The presence along the 

Grand Hogback of the MVj set within strata of the lower 

Wasatch Formation, but nowhere in younger beds, suggests 

that the set formed during the earliest stages of the same 

compression that later gave rise to the Grand Hogback and 

associated intrabasin folds during basin-margin thrusting. If 

so, early compression was directed almost east-west and 

later rotated to more west-southwesterly and southwesterly 

directions. The late MV2 joints, as discussed earlier, then 

formed as a set of "cross joints" with respect to the MVl set 

within uplifted beds that gently arched over the advancing 

wedge. This set, too, decreases markedly in strength with 

increasing distance from the monocline. Still later, during 

the main phase of thrusting, two local fracture sets formed 

parallel to the growing Divide Creek fold as thrusting and

splay faulting proceeded at depth. Progressive thrusting 

along the eastern margin of the basin thus gave rise, in this 

view, to the sequential development of three and possibly 

four fracture sets, each of limited geographic distribution 

above the advancing thrust wedge and within the upper 

plate of the associated decollement. The presence of these 

fractures in Upper Cretaceous and Paleocene reservoir rocks 

probably is responsible, at least in part, for the enhanced 

fracture permeability and higher gas yields for fields near 

the eastern margin of the basin relative to those farther west. 

The later fractures of the Piceance system are of much wider 

geographic distribution, show no consistent tendency to 

either increase or decrease in abundance with proximity to 

the Grand Hogback, and probably postdate the late 

Laramide (Paleocene-Eocene) structural movements 

responsible for the Grand Hogback monocline and related 

intrabasin folds. Their presence in both the Piceance basin 

of western Colorado and the neighboring Uinta basin of 

eastern Utah suggests to us that their formation is related 

more to stress fields affecting much of the northeastern 

margin of the Colorado Plateau than to the development of 

a single basin.

REFERENCES CITED

Abrams, G.A., and Grout, M.A., 1987, Complete Bouguer profiles 

and principal facts for gravity stations in the Divide Creek 

anticline area, Piceance basin, Colorado: U.S. Geological 

Survey Open-File Report 87-647, 9 p.

____ 1990, Principal facts for gravity stations in the Wolf Creek 

anticline area, Piceance basin, Colorado: U.S. Geological 

Survey Open-File Report 90-261, 7 p.

Barton, C.C., 1983, Systematic jointing in the Cardium Sandstone 

along the Bow River, Alberta, Canada: New Haven, 

Connecticut, Yale University, Ph.D. dissertation, 302 p.

Berg, R.R., 1962, Mountain flank thrusting in Rocky Mountain 

foreland, Wyoming and Colorado: American Association of 

Petroleum Geologists Bulletin, v. 46, no. 11, p. 2019-2032.

Berry, G.W., 1959, Divide Creek field, Garfield and Mesa 

Counties, Colorado, in Haun, J.D., and Weimer, R.J., eds., 

Symposium on Cretaceous rocks of Colorado and adjacent 

areas: Rocky Mountain Association of Geologists, p. 89-91.

Boreck, D.L., and Strever, M.T., 1980, Conservation of methane 

from Colorado's mined/minable coal beds A feasibility 

study: Colorado Geological Survey Open-File Report 80-5, 

95 p.

Bostick, N.H., and Freeman, V.L., 1984, Tests of vitrinite 

reflectance and paleotemperature models at the Multiwell 

Experiment site, Piceance Creek basin, Colorado, in 

Spencer, C.W., and Keighin, C.W., eds., Geologic studies in 

support of the U.S. Department of Energy Multiwell 

Experiment, Garfield County, Colorado: U.S. Geological 

Survey Open-File Report 84-757, p. 110-120.

Branagan, Paul, Cipolla, C, Lee, S.J., and Wilmer, R., 1985, 

Comprehensive well testing and modeling of pre- and 

post-frac performance of the MWX lenticular tight gas 

sands: Society of Petroleum Engineers Report 13867, 

p. 19-22.

Fracture History, Southern Piceance Basin, Colorado Z23
Branagan, Paul, Cotner, Gene, and Lee, S.J., 1984, Interference 

testing of naturally fractured Cozzette sandstone A case 

study at the DOE MWX site: SPE/GRI/DOE 

Unconventional Gas Recovery Symposium, Society of 

Petroleum Engineers, Dallas, Texas, Proceedings, 

p. 359-366.

Bredehoeft, J.D., Wolff, R.G., Keys, W.S., and Shuter, Eugene, 

1976, Hydraulic fracturing to determine the regional in situ 

stress field, Piceance basin, Colorado: Geological Society of 

America Bulletin, v. 87, no. 2, p. 250-258.

Brown, W.G., 1988, Deformational style of Laramide uplifts in the 

Wyoming foreland, in Schmidt, C.J., and Perry, W.J., Jr., 

eds., Interaction of the Rocky Mountain foreland and the 

Cordilleran thrust belt: Geological Society of America 

Memoir 171, p. 1-25.

Cashion, W.B., 1973, Geologic and structure map of the Grand 

Junction quadrangle, Colorado and Utah: U.S. Geological 

Survey Miscellaneous Geologic Investigations Map 1-736, 

scale 1:250,000.

Choate, R., Jurich, D., and Saulnier, G.J., Jr., 1984, Geologic 

overview, coal deposits, and potential for methane recovery 

from coalbeds, Piceance basin, Colorado, in Rightmire, 

C.T., Eddy, G.E., and Kirr, J.N., eds., Coalbed methane 

resources of the United States: American Association of 

Petroleum Geologists Studies in Geology 17, p. 223-251.

Clark, J.A., 1983, The prediction of hydraulic fracture azimuth 

through geological, core, and analytical studies: Society of 

Petroleum Engineers/Department of Energy Symposium on 

Low Permeability, March 14-16, Denver, SPE/DOE Paper 

11611, p. 107-114.

Davis, G.H., 1978, Monocline fold pattern of the Colorado 

Plateau, in Matthews, Vincent, III, ed., Laramide folding 

associated with basement block faulting in the western 

United States: Geological Society of America Memoir 151, 

p. 215-233.

De Voto, R.H., Bartleson, B.L., Schenk, C.J., and Waechter, N.B., 

1986, Late Paleozoic stratigraphy and syndepositional tectonism, northwestern Colorado, in Stone, D.S., ed., New 

interpretations of northwest Colorado geology: Rocky 

Mountain Association of Geologists, p. 37-49.

Dodge, C.N., and Bartleson, B.L., 1986, The Eagle basin A new 

exploration frontier, in Stone, D.S., ed., New interpretations 

of northwest Colorado geology: Rocky Mountain 

Association of Geologists, p. 113-122.

Donaldson, J.C., and MacMillan, Logan, 1980, Oil and gas 

History of development and principal fields in Colorado, in 

Murray, O.K., ed., Colorado geology: Rocky Mountain 

Association of Geologists, p. 175-189.

Donnell, J.R., 1969, Paleocene and lower Eocene units in the 

southern part of the Piceance Creek basin, Colorado: U.S. 

Geological Survey Bulletin 1274-M, p. M1-M18.

Dula, W.F., Jr., 1981, Correlation between deformation lamellae, 

microfractures, and in situ stress measurements, White 

River uplift, Colorado: Geological Society of America 

Bulletin, v. 92, part I, p. 37-46.

____ 1984, Laramide compression and crustal segmentation, 

White River uplift, Colorado: Geological Society of 

America Abstracts with Programs, v. 16, no. 6, p. 495.

Dunn, H.L., 1974, Geology of petroleum in the Piceance Creek 

basin, northwestern Colorado, in Murray, D.K., ed., 

Guidebook to the energy resources of the Piceance Creek 

basin, Colorado: Rocky Mountain Association of 

Geologists, p. 217-223.

Dyni, J.R., 1987, The origin of oil shale and associated minerals, 

in Taylor, O.J., ed., Oil shale, water resources, and valuable 

minerals of the Piceance basin, Colorado The challenge 

and choices of development: U.S. Geological Survey 

Professional Paper 1310, p. 17-20.

Engelder, Terry, 1982, Reply to Comment on "Is there a genetic 

relationship between selected regional joints and 

contemporary stress within the lithosphere of North America?": Tectonics, v. 1, p. 465-470.

Engelder, Terry, and Geiser, P.A., 1980, On the use of regional 

joint sets as trajectories of paleostress fields during the 

development of the Appalachian Plateau, New York: 

Journal of Geophysical Research, v. 85, p. 6319-6341.

Finley, S.J., and Lorenz, J.C., 1989, Characterization and 

significance of natural fractures in Mesaverde reservoirs at 

the Multiwell Experiment site: Society of Petroleum 

Engineers Joint Rocky Mountain Regional/LowPermeability Reservoir Symposium and Exhibition, Denver, 

SPE Paper 19007, p. 721-728.

Garrett, C.H., and Lorenz, J.C., 1989, Fracturing along the Grand 

Hogback, Garfield County, Colorado: Sandia National 

Laboratories Report SAND88-3298, 29 p.

Gas Research Institute, 1983-present: Methane from Coal Seams 

Technology Quarterly Review, Chicago.

Geological Services of Tulsa, 1980, Geologic framework and 

potential structural control of methane in coal beds of 

southeastern Piceance Creek basin, Colorado: U.S. 

Department of Energy, Morgantown Energy Technology 

Center; prepared for TRW Energy Systems Group (under 

TRW Subcontract J44432JJDOE and U.S. Department of 

Energy Contract DE-AC-21-78MCO8089), 34 p.

Cries, R.R., 1983, North-south compression of Rocky Mountain 

foreland structures, in Lowell, J.D., and Gries, R.R., eds., 

Rocky Mountain foreland basins and uplifts: Rocky 

Mountain Association of Geologists, p. 9-32.

Gries, R.R., and Dyer, R.C., eds., 1985, Seismic exploration in the 

Rocky Mountain region: Rocky Mountain Association of 

Geologists and Denver Geophysical Society, 300 p.

Grout, M.A., 1988, Fracture data for the Divide Creek and Wolf 

Creek anticlines area, southern Piceance basin, 

northwestern Colorado: U.S. Geological Survey Open-File 

Report 88-204, 66 p.

____ 1991, Cleats in coal beds of the southern Piceance basin, 

Colorado Correlation with regional and local fracture sets 

in associated clastic rocks, in Schwochow, S.D., Murray, 

O.K., and Fahy, M.F., ed., Coalbed methane of western 

North America: Rocky Mountain Association of Geologists, 

p. 35^7.

Grout, M.A., Abrams, G.A., Tang, R.L., Hainsworth, T.J., and 

Verbeek, E.R., 1991, Late Laramide thrust-related and 

evaporite-domed anticlines in the southern Piceance basin, 

northeastern Colorado Plateau: American Association of 

Petroleum Geologists Bulletin, v. 75, no. 2, p. 205-218.

Z24 Evolution of Sedimentary Basins Uinta and Piceance Basins
Grout, M.A., and Verbeek, E.R., 1983, Field studies of jointsInsufficiencies and solutions, with examples from the 

Piceance Creek basin, Colorado, in Gary, J.H., ed., 16th Oil 

Shale Symposium, Proceedings: Golden, Colorado School 

of Mines Press, p. 68-80.

____ 1985, Fracture history of the Plateau Creek and adjacent 

Colorado River valleys, southern Piceance basin Implications for predicting joint patterns at depth: U.S. Geological 

Survey Open-File Report 85-744, 17 p.

____ 1987, Regional joint sets predate folding of the Grand

Hogback monocline: Geological Society of America 

Abstracts with Programs, v. 19, no. 5, p. 279.

____ 1989, Prediction of fracture networks at depth in lowpermeability reservoir rocks, Piceance and Washakie 

basins, western U.S.: American Association of Petroleum 

Geologists Bulletin, v. 73, no. 9, p. 1158.

Gunter, C.E., 1962, Oil and gas potential of Upper Cretaceous 

sediments, southern Piceance basin, in Mott, M.R., ed., 

Exploration for oil and gas in northwestern Colorado: 

Rocky Mountain Association of Geologists, p. 114-118.

Hancock, P.L., 1985, Brittle microtectonics Principles and 

practice: Journal of Structural Geology, v. 7, nos. 3-4, 

p. 437-457.

Harper, M.L., 1964, Mechanics of basement deformation in 

Glenwood Canyon, Colorado: Boulder, University of 

Colorado, Ph.D. dissertation, 102 p.

Hobbs, B.E., Means, W.D., and Williams, P.E., 1976, An outline 

of structural geology: New York, John Wiley and Sons, 

571 p.

Holland, S.M., 1976, Style elements of some systematic joints, 

south-central Preston County, West Virginia: Morgantown, 

West Virginia University, unpublished M.S. problem report, 

117 p.

Homer, D.M., 1986, Deep coal seam project: Methane from Coal 

Seams Technology Quarterly Review, v. 4, no. 2, p. 19-27.

Howard, A.D., Williams, J.W., and Raisz, Erwin, 1972, 

Physiography, in Mallory, W.W., ed., Geologic atlas of the 

Rocky Mountain region: Rocky Mountain Association of 

Geologists, p. 29-31.

ICF Resources, Inc., 1990, The United States coalbed methane 

resource: Methane from Coal Seams Technology Quarterly 

Review, v. 7, no. 3, p. 10-28.

Johnson, R.C., 1983, Structure contour map on the top of the 

Rollins Sandstone Member of the Mesaverde Formation and 

Trout Creek Sandstone Member of the lies Formation, 

Piceance Creek basin, Colorado: U.S. Geological Survey 

Miscellaneous Field Studies Map MF-1667, scale 

1:250,000.

____ 1989, Geologic history and hydrocarbon potential of Late 

Cretaceous-age, low-permeability reservoirs, Piceance 

basin, western Colorado: U.S. Geological Survey Bulletin 

1787-E, 51 p.

Johnson, R.C., and May, Fred, 1980, A study of the CretaceousTertiary unconformity in the Piceance Creek basin, Colorado The underlying Ohio Creek Formation (Upper 

Cretaceous) redefined as a member of the Hunter Canyon or 

Mesaverde Formation: U.S. Geological Survey Bulletin 

1482-B, 27 p.

Johnson, R.C., and Nuccio, V.F., 1984, Late Cretaceous through 

early Tertiary stratigraphy and structural geology of the 

Piceance Creek basin, Colorado, in Spencer, C.W., and 

Keighin, C.W., eds., Geologic studies in support of the U.S. 

Department of Energy Multiwell Experiment, Garfield 

County, Colorado: U.S. Geological Survey Open-File 

Report 84-757, p. 14-20.

____ 1986, Structural and thermal maturity of the Piceance 

Creek basin, western Colorado, in relation to hydrocarbon 

occurrence in the Mesaverde Group, in Spencer, C.W., and 

Mast, R.F., eds., Geology of tight gas reservoirs: American 

Association of Petroleum Geologists Studies in Geology 24, 

p. 165-206.

Johnson, R.C., and Rice, D.D., 1990, Occurrence and geochemistry of natural gases, Piceance basin, northwest 

Colorado: American Association of Petroleum Geologists 

Bulletin, v. 74, no. 6, p. 805-829.

Johnson, S.Y., Schenk, C.J., and Karachewski, J.A., 1988, 

Pennsylvanian and Permian depositional systems and cycles 

in the Eagle basin, northwest Colorado, in Holden, G.S., ed., 

Geological Society of America Centennial Meeting, 

Denver, 1988, Field Trip Guidebook: Colorado School of 

Mines Professional Contributions 12, p. 156-174.

Kelley, V.C., 1955, Monoclines of the Colorado Plateau: 

Geological Society of America Bulletin, v. 66, p. 789-804.

Kelley, V.C., and Clinton, N.J., 1960, Fracture systems and 

tectonic elements of the Colorado Plateau: University of 

New Mexico Publications in Geology 6, 104 p.

Kulander, B.R., Barton, C.C., and Dean, S.L., 1979, The 

application of fractography to core and outcrop fracture 

investigations: U.S. Department of Energy Report 

METC/SP-79/3, 174 p.

Kulander, B.R., Dean, S.L., and Ward, B.J., Jr., 1990, Fractured 

core analysis Interpretation, logging, and use of natural 

and induced fractures in core: American Association of 

Petroleum Geologists Methods in Exploration 8, 88 p.

Lorenz, J.C., 1985, Tectonic and stress histories of the Piceance 

Creek basin and the MWX site, from 75 million years ago 

to the present Preliminary report: Sandia National 

Laboratories Report SAND84-2603, 50 p.

Lorenz, J.C., and Finley, S.J., 1987, Differences in fracture 

characteristics and related production of natural gas in 

different zones of the Mesaverde Formation, northwestern 

Colorado: Society of Petroleum Engineers Annual 

Technical Conference and Exhibition, 62nd, Dallas, SPE 

Paper 16809, p. 589-593.

Lorenz, J.C., and Hill, R.E., 1991, Subsurface fracture spacing- 

 Comparison of inferences from slant/horizontal core and 

vertical core in Mesaverde reservoirs: Society of Petroleum 

Engineers Joint Rocky Mountain Regional/Low 

Permeability Reservoir Symposium, April 15-17, Denver, 

SPE Paper 21877, p. 705-716.

Lorenz, J.C., and Rutledge, A.K., 1985, Facies relationships and 

reservoir potential of the Ohio Creek interval across the 

Piceance Creek basin, northwestern Colorado: Sandia 

National Laboratories Report SAND84-2610, 52 p.

Fracture History, Southern Piceance Basin, Colorado Z25
Lorenz, J.C., Warpinski, N.R., Branagan, Paul, and Sattler, A.R., 

1989, Fracture characteristics and reservoir behavior of 

stress-sensitive fracture systems in flat-lying lenticular 

formations: Journal of Petroleum Technology, v. 4, no. 1, 

p. 11-16.

MacLachlan, M.E., 1987, General geology of the Piceance basin, 

in Taylor, O.J., ed., Oil shale, water resources, and valuable 

minerals of the Piceance basin, Colorado The challenge 

and choices of development: U.S. Geological Survey 

Professional Paper 1310, p. 7-15.

MacLachlan, M.E., and Welder, F.E., 1987, Paleozoic and Mesozoic Formations and their potential as ground-water 

reservoirs, in Taylor, O.J., ed., Oil shale, water resources, 

and valuable minerals of the Piceance basin, Colorado 

The challenge and choices of development: U.S. 

Geological Survey Professional Paper 1310, p. 95-106.

McCulloch, C.M., Deul, Maurice, and Jeran, P.W., 1974, Cleat in 

bituminous coalbeds: U.S. Bureau of Mines Report of 

Investigations 7910, 25 p.

McFall, K.S., Wicks, D.E., Kuuskraa, V.A., and Sedwick, K.B., 

1986, A geologic assessment of natural gas from coal seams 

in the Piceance basin, Colorado: Gas Research Institute 

Topical Report 87/0060, 76 p.

Murray, D.K., and Haun, J.D., 1974, Introduction to the geology of 

the Piceance Creek basin and vicinity, northwestern 

Colorado: Rocky Mountain Association of Geologists, 

p. 29-39.

Murray, F.N., 1966, Stratigraphy and structural geology of the 

Grand Hogback monocline, Colorado: Boulder, University 

of Colorado, Ph.D. dissertation, 219 p.

____ 1967, Jointing in sedimentary rocks along the Grand 

Hogback monocline, Colorado: Journal of Geology, v. 75, 

no. 1, p. 340-350.

Nickelsen, R.P., and Hough, V.N.D., 1967, Jointing in the 

Appalachian Plateau of Pennsylvania: Geological Society of 

America Bulletin, v. 78, no. 5, p. 609-630.

Nuccio, V.F., and Johnson, R.C., 1984, Thermal maturation and 

burial history of the Upper Cretaceous Mesaverde Group, 

including the Multiwell Experiment (MWX), Piceance 

Creek basin, Colorado, in Spencer, C.W., and Keighin, 

C.W., eds., Geologic studies in support of the U.S. 

Department of Energy Multiwell Experiment, Garfield 

County, Colorado: U.S. Geological Survey Open-File 

Report 84-757, p. 102-109.

____ 1989, Variations in vitrinite reflectance values for the 

Upper Cretaceous Mesaverde Formation, southeastern 

Piceance basin, northwestern Colorado Implications for 

burial history and potential hydrocarbon generation: U.S. 

Geological Survey Bulletin 1787-H, 10 p.

Perry, W.J., Jr., and Colton, G.W., 1981, A summary of the 

methodology and results of regional-joint studies in the 

central and northern Appalachian Basin conducted by the 

U.S. Geological Survey as a result of Department of Energy 

Interagency Agreement EX-76-C-01-2287: U.S. 

Geological Survey Open-File Report 81-1341, 16 p.

Perry, W.J., Jr., Grout, M.A., Tang, R.L., and Hainsworth, T.J., 

1988, Wedge model for late Laramide basement-involved 

thrusting, Grand Hogback monocline and White River 

uplift, western Colorado: Geological Society of America 

Abstracts with Programs, v. 20, no. 7, p. 384-385.

Petroleum Information Corporation and Barlow and Haun, Inc., 

1986, Northwest Colorado regional structure contour map 

on Dakota Formation, showing oil and gas fields overlayed 

on Landsat satellite image, in Stone, D.S., ed., New 

interpretations of northwest Colorado geology: Rocky 

Mountain Association of Geologists, scale 1:500,000.

Pitman, J.K., and Sprunt, E.S., 1986, Origin and distribution of 

fractures in lower Tertiary and Upper Cretaceous rocks, 

Piceance basin, Colorado, and their relation to the 

occurrence of hydrocarbons, in Spencer, C.W., and Mast, 

R.F., eds., Geology of tight gas reservoirs: American 

Association of Petroleum Geologists Studies in Geology 24, 

p. 221-233.

Poole, F.G., 1954, Geology of the southern Grand Hogback area, 

Garfield and Pitkin Counties, Colorado: Boulder, University 

of Colorado, M.S. thesis, 128 p.

Price, N.J., 1966, Fault and joint development in brittle and 

semi-brittle rock: New York, Pergamon Press, 176 p.

Prucha, J.J., Graham, J.A., and Nickelsen, R.P., 1965, Basementcontrolled deformation in Wyoming province of Rocky 

Mountain foreland: American Association of Petroleum 

Geologists Bulletin, v. 49, no. 7, p. 966-992.

Reches, Ze'ev, 1976, Analysis of joints in two monoclines in 

Israel: Geological Society of America Bulletin, v. 87, 

p. 1654-1662.

Sales, J.K, 1968, Crustal mechanics of Cordilleran foreland 

deformation A regional and scale-model approach: 

American Association of Petroleum Geologists Bulletin, 

v. 52, p. 2016-2044.

Seccombe, J.C., and Decker, A.D., 1986, Geologic and reservoir 

characteristics of the Red Mountain coalbed methane test 

site in the Piceance basin: Gas Research Institute Topical 

Report GRI-86/0109, 89 p.

Smith, R.S., and Whitney, J.W., 1979, Map of joint sets and 

airphoto lineaments of the Piceance Creek basin, 

northwestern Colorado: U.S. Geological Survey 

Miscellaneous Field Studies Map MF-1128, scale 

1:100,000.

Spencer, C.W., 1987, Hydrocarbon generation as a mechanism for 

overpressuring in the Rocky Mountain region: American 

Association of Petroleum Geologists Bulletin, v. 71, no. 4, 

p. 368-388.

Steams, D.W., 1978, Faulting and forced folding in the Rocky 

Mountain foreland, in Matthews, Vincent, III, ed., Laramide 

folding associated with basement block faulting in the 

western United States: Geological Society of America 

Memoir 151, p. 1-38.

Stearns, D.W., and Friedman, Melvin, 1972, Reservoirs in 

fractured rock: American Association of Petroleum 

Geologists Memoir 16, p. 82-106.

Stone, D.S., 1969, Wrench faulting and central Rocky Mountain 

tectonics: The Mountain Geologist, v. 6, p. 67-79.

____ 1986, Geology of the Wilson Creek field, Rio Blanco 

County, Colorado, in Stone, D.S., ed., New interpretations 

of northwest Colorado geology: Rocky Mountain 

Association of Geologists, p. 229-246.

Suppe, John, 1985, Principles of structural geology: New Jersey, 

Prentice-Hall, 537 p.

Z26 Evolution of Sedimentary Basins Uinta and Piceance Basins
Tichy, J.R., and Rettger, R.W., 1962, Divide Creek field, in Parker, 

J.M., ed., Colorado-Nebraska oil and gas fields 1961: 

Rocky Mountain Association of Geologists, p. 110-111.

Tweto, Ogden, 1975, Laramide (Late Cretaceous-early Tertiary) 

orogeny in the Southern Rocky Mountains, in Curtis, B.F., 

ed., Cenozoic history of the Southern Rocky Mountains: 

Geological Society of America Memoir 144, p. 1-44.

____1980, Tectonic history of Colorado, in Kent, H.C., and 

Porter, K.W., eds., Colorado geology: Rocky Mountain 

Association of Geologists, p. 5-9.

Tweto, Ogden, Moench, R.H., and Reed, J.C., Jr., 1978, Geologic 

map of the Leadville I°x2° sheet, Colorado: U.S. 

Geological Survey Miscellaneous Investigations Series Map 

1-999, scale 1:250,000.

Tweto, Ogden, Steven, T.A., Hail, W.J., Jr., and Moench, R.H., 

1976, Preliminary geologic map of the Montrose 1°X2° 

quadrangle, southwestern Colorado: U.S. Geological 

Survey Miscellaneous Field Studies Map MF-761, scale 

1:250,000.

Verbeek, E.R., and Grout, M.A., 1983, Fracture history of the 

northern Piceance Creek basin, northwestern Colorado, in 

Gary, J.H., ed., 16th Oil Shale Symposium, Proceedings: 

Golden, Colorado School of Mines Press, p. 26-44.

____ 1984a, Fracture studies in Cretaceous and Paleocene strata 

in and around the Piceance basin, Colorado Preliminary 

results and their bearing on a fracture-controlled natural-gas 

reservoir at the MWX site: U.S. Geological Survey OpenFile Report 84-156, 30 p.

____ 1984b, Prediction of subsurface fracture patterns from 

surface studies of joints An example from the Piceance 

Creek basin, Colorado, in Spencer, C.W., and Keighin, 

C.W., eds., Geologic studies in support of the U.S. 

Department of Energy Multiwell Experiment, Garfield 

County, Colorado: U.S. Geological Survey Open-File 

Report 84-757, p. 75-86.

____ 1986, Cenozoic stress rotation, northeastern Colorado 

Plateau, in Stone, D.S., ed., New interpretations of 

northwest Colorado geology: Rocky Mountain Association 

of Geologists, p. 97.

____1987, Systematic joints within oil shales and associated 

rocks of the Green River Formation, in Taylor, O.J., ed., Oil 

shale, water resources, and valuable minerals of the 

Piceance basin, Colorado The challenge and choices of 

development: U.S. Geological Survey Professional Paper 

1310, p. 45-55.

Verbeek, E.R., Grout, M.A., and Van Gosen, B.S., 1988, Structural 

evolution of a Grand Canyon breccia pipe The Ridenour 

copper-vanadium-uranium mine, Hualapai Indian 

Reservation, Coconino County, Arizona: U.S. Geological 

Survey Open-File Report 88-006, 75 p.

Waechter, N.B., and Johnson, W.E., 1985, Seismic interpretation 

in the Piceance basin, northwest Colorado, in Cries, R.R., 

and Dyer, R.C., eds., Seismic exploration in the Rocky 

Mountain region: Rocky Mountain Association of 

Geologists and Denver Geophysical Society, p. 247-258.

____ 1986, Pennsylvanian-Permian paleostructure and stratigraphy as interpreted from seismic data in the Piceance basin, 

northwestern Colorado, in Stone, D.S., ed., New 

interpretations of northwest Colorado geology: Rocky 

Mountain Association of Geologists, p. 51-64.

Welder, F.A., 1971, Map showing joint pattern inferred from aerial 

photographs: Report on Economics of Environmental 

Protection for a Federal Oil-Shale Leasing Program, Water 

Resources in Colorado Oil Shale, Advisory Committee 

Study, Denver, scale 1:380,000.

Wheeler, R.L., and Dixon, J.M., 1980, Intensity of systematic 

joints Methods and application: Geology, v. 8, 

p. 230-233.

Wheeler, R.L., and Holland, S.M., 1981, Style elements of 

systematic joints An analytic procedure with a field 

example, in O'Leary, D.W., and Earle, J.L., eds., 

Proceedings, Third International Conference on Basement 

Tectonics: Basement Tectonics Committee Publication 3, 

p. 93-104.

Wheeler, R.L., and Stubbs, J.L., Jr., 1976, Style elements of 

. systematic joints Statistical analysis of size, spacing, and 

other characteristics, in Podwysocki, M.H., and Earle, J.L., 

eds., Proceedings, Second International Conference on 

Basement Tectonics: Basement Tectonics Committee 

Publication 2, p. 491-^99.

Whitney, J.W., and Andrews, E.D., 1983, Past and present 

geomorphic activity in the Piceance Creek drainage basin, 

northwestern Colorado, in Gary, J.H., ed., 16th Oil Shale 

Symposium, Proceedings: Golden, Colorado School of 

Mines Press, p. 566-577.

Williams, P.L., 1964, Geology, structure, and uranium deposits of 

the Moab quadrangle, Colorado and Utah: U.S. Geological 

Survey Miscellaneous Investigations Series Map 1-360, 

scale 1:250,000.

Fracture History, Southern Piceance Basin, Colorado 227
APPENDIX SUMMARY OF 

CHARACTERISTICS OF 

FRACTURE SETS

The relative ages, areal and stratigraphic distribution, 

styles, modes of failure, mineralization, and orientations of 

the five regional joint sets of the Piceance system and the 

youngest set of the Hogback system in the Divide CreekWolf Creek area are described below, from oldest to 

youngest. In addition, two local sets of fold-parallel 

fractures, apparently related to layer-parallel stretching on 

the Divide Creek anticline, are described and placed within 

the time frame of regional joint-set formation. All field data 

for the fracture sets at each of the 53 stations studied have 

been compiled, entered into a computer database, and 

published (Grout, 1988). General characteristics of the 

joints of each set, as described below and summarized in 

table 2, are drawn from this database.

HOGBACK SYSTEM OF FRACTURES

MV2 Set

Areal and stratigraphic distribution. Joints 

comprising the youngest set of the Hogback system, the 

MV2 set, were found at only two outcrops in the Divide 

Creek area but are much more abundant farther east, near 

and within the Grand Hogback monocline (fig. 5). Joints of 

this set in the study area formed only in thick (2-5 m) 

sandstone units that are coarse medium grained and poorly 

sorted. Broadly planar, low-angle crossbeds typify the 

sequence, but steep-angled crossbeds are present locally. 

These primary bedding structures seem not to have 

influenced either joint style or orientation. The sandstone 

layers are porous, noncalcareous to only slightly calcareous, 

locally quartz cemented, weakly to firmly indurated, and 

have rip-up clasts locally near the base. Although these 

layers in the two outcrops are of different appearance 

(medium gray and light tan on fresh surface) and age (Late 

Cretaceous and Paleocene), they nonetheless are lithologically similar.

Orientation. Joints of the MV2 set commonly strike 

N. 10°-25° E. and dip steeply to subvertically, 

approximately perpendicular to bedding.

Dimensions. Joints of the MV2 set are large, 

averaging 3 m high and 8-10 m long. Their large size is in 

part attributable to the lack of pre-existing joints in the rock, 

which otherwise would have impeded their lateral growth. 

Large size is common among early formed joints 

throughout the Piceance basin.

Spacings. Spacings of the MV2 joints are variable, 

from 0.75 m to greater than 4 m. Too few beds containing 

the MV2 set are exposed in the study area for the variation 

in spacing to be ascribed to any specific cause, but lithology 

(degree of induration) and layer thickness are the dominant 

factors elsewhere.

Surface structures and shape. Surface features on 

MV2 joint planes include plumose structures, arrest lines, 

and twist hackle, all of which are indicative of extensile 

fracture and whose pattern on the fracture surfaces indicates 

that the vertical joints propagated laterally, parallel to 

bedding. Surfaces of the joints are planar in the thinner beds 

but broadly sinuous and only locally planar in the more 

massive units.

Mineralization. Mineral coatings or fillings were 

not observed at either outcrop on MV2 joints, despite their 

early age. This is not unexpected, however, because joint 

surfaces are easily stripped of mineral coatings during 

weathering. Along the Grand Hogback, also, mineral 

coatings have been removed from MV2 joints in most places 

but remnant patches of calcite nevertheless observed at 

dozens of other places where the joints have been most 

protected from weathering.

Terminating relations. The MV2 joints die out 

laterally as hairline cracks within the body of the rock. They 

were the first set of joints to form in the Divide Creek area 

and do not terminate against any other fractures.

FOLD-PARALLEL SETS

Areal and stratigraphic distribution. Sets of 

moderately dipping, fold-parallel fractures were observed in 

ten outcrops on the Divide Creek anticline but were 

abundant at only six (fig. 6) and were found at one locality 

on the Wolf Creek anticline. They were not found west of 

the study area. The fold-parallel sets formed in the coarser 

grained sandstone of both the Wasatch Formation and the 

Mesaverde Group, and only in the thicker (1.5-10 m) layers 

of each unit (for example, fig. 7). The sandstone beds are 

planar-crossbedded to massive, porous, calcareous to noncalcareous, and weakly to moderately indurated. Rip-up 

clasts of mudstone and coaly material are common near the 

bases of two of the layers; contorted bedding is apparent in 

another.

Although the fold-parallel and MV2 fracture sets were 

found in similar rock types, they were not found in the same 

outcrops, undoubtedly because of the rarity of the MV2 set. 

Joints of other sets associated with the fold-parallel 

fractures generally are smaller, abut the fold-parallel 

fractures, and are not abundant.

Orientation. The relatively wide range in strike of 

the fold-parallel fractures N. 10°-58° W., but in most 

places N. 28°-55° W. reflects the curving 

(N. 20°-46° W.) axial trace of the Divide Creek anticline. 

Those joints that dip toward the axis of the anticline 

generally have lower dip angles than those that dip in the 

opposite direction. The fractures range in dip from 57° to 

75°, and, among pairs of oppositely dipping fractures in the 

same outcrop, the difference in dip magnitude is from 2° 

to 11°.

Z28 Evolution of Sedimentary Basins Uinta and Piceance Basins
Dimensions. Generally, the fold-parallel fractures 

that dip toward the axis of the Divide Creek anticline are 

larger and more abundant than their oppositely dipping 

counterparts, and they dominate the sandstone layers in 

which they are found. Some of these fractures cut the entire 

thickness of the layer, as much as 7 m. Upon close 

inspection, however, such large fold-parallel fractures 

generally are observed not to be single surfaces but instead 

are zones 5-10 cm wide of very closely spaced, subparallel, 

shorter fractures whose traces overlap but generally do not 

touch. These zones are very similar to those of the subvertical Fj set described in the next section.

Spacings. Spacings of the fold-parallel fractures are 

0.25-5 m and most commonly 0.5-3 m. Spacings 

commonly are closer in the thinner beds than in the thicker 

ones.

Surface structures and shape. Surfaces of the foldparallel fractures appear planar from a distance but actually 

are slightly undulatory downdip, especially where they 

intersect internal bedding contacts within the sandstone. 

Plumose structures and arrest lines (fig. IB) on several of 

the fractures indicate not only that fracture propagation was 

lateral, parallel to bedding, but also that the fold-parallel 

fractures are extension joints, despite their geometric 

resemblance to conjugate shear sets.

Mineralization. Mineral coatings on surfaces of the 

fold-parallel sets are sparse. Local patches of fibrous calcite 

attached to asperities on three fracture surfaces indicate 

minor downdip slip postdating fracture formation.

Terminating relations. Age relations between the 

MV2 and fold-parallel sets are not directly confirmable 

because the sets are not present in the same outcrops. 

However, the fold-parallel fractures are assumed to be 

younger because the MV2 joints formed during the earliest 

stages of thrusting and the fold-parallel fractures during the 

main stage (see text). The largest fold-parallel fractures of 

each set also are the oldest; they terminate against no other 

fractures but instead commonly cut through the full 

thickness of the bed. Shorter fractures of both sets, however, 

commonly terminate against larger ones of opposite dip; the 

two sets, therefore, probably formed coevally as products of 

the same period of deformation.

PICEANCE SYSTEM OF FRACTURES

F, Set

Areal and stratigraphic distribution. Joints of the Pl 

set, the oldest set of the Piceance system, were found in 

more than one-fourth of the outcrops studied (fig. 8). Joints 

of this set are more numerous here than elsewhere in the 

southern Piceance basin (Grout and Verbeek, 1985), where 

in most places they are either absent or form only a minor 

part of the composite fracture network.

Fj joints most commonly formed in sandstone 

lithologically similar to that in which fractures of the MV2

set and the fold-parallel sets are present. Despite the 

similarity in host lithology, however, the Pl joints are not 

present in the same layers as fractures of the MV2 set and 

only rarely are present with the fold-parallel sets (compare 

figs. 5, 6, and 8); these early sets are mutually exclusive to 

a considerable degree. Most sandstone layers containing Pl 

joints are 2-8 m thick and are crossbedded, though locally 

massive; the crossbeds range in appearance from low-angle 

planar to trough and do not affect joint strike. The sandstone 

generally is porous and moderately indurated but varies 

widely in its content of calcite cement.

Some fractures of the Fj set, unlike those of the MV2 

and fold-parallel sets, were observed (at three localities) in 

thin- to medium-bedded sequences of sandstone, siltstone, 

and mudstone.

Orientation. The Fj joints commonly strike 

N. 6°-36° W. and are subvertical. Their surfaces either 

curve slightly but broadly in strike or are sinuous in both 

vertical and lateral profile. The undulatory nature of the 

subvertical Fj planes can be attributed in part to slight 

changes in dip where they cross lithologic discontinuities 

within compound sandstone units.

Dimensions. Heights of the Fl joints generally are 

equal to the thickness of the sandstone layers in which they 

formed. In the thickest (>3 m) beds, however, individual Pl 

joints shorter than bed thickness in many places form 

subvertical zones <0.25 m wide that span the entire 

thickness of the bed and locally extend above and below 

into transitional, more silty units. Individual fractures within 

such zones overlap vertically, are spaced a few millimeters 

to centimeters apart, and commonly are 2 m high or less; the 

zones themselves, however, are as much as 7 m in vertical 

extent. Exposed lengths of the zones, where observed, are 

3-7 m, but single fractures within each zone generally are 

much shorter, and the zones themselves probably are 

considerably longer than the portions presently exposed.

Where Fj joints and the inclined fold-parallel joints 

are present in the same outcrop, the Fl joints terminate 

either upward or downward against the inclined fractures 

and thus are less high than normal.

Spacings. Spacings of F, joints most commonly are 

0.5 4 m; observed extremes are 0.01-5 m. The largest 

values in thick beds correspond generally to spacings 

between Fj joint zones rather than between individual 

joints. For any given lithology, spacings of the F} joints or 

joint zones generally increase with increasing layer 

thickness, as noted previously in many other parts of the 

Piceance basin (Verbeek and Grout, 1984a).

Surface structures and shape. Large plumose 

structures are common on Fl joints, arrest lines are less 

common, and twist hackle is fairly rare. The arrangement of 

these structures on the fracture surfaces indicates that the Fj 

joints, similar to those of the MV2 set, are vertical extension 

fractures that propagated laterally, parallel to bedding.

Fracture History, Southern Piceance Basin, Colorado Z29
Mineralization. Mineral coatings on joints of the Fj 

set were not observed.

Terminating relations. Terminating relations 

between MV2 and Fj joints could not be studied in the 

Divide Creek area because the two sets were nowhere found 

in the same outcrop. In other parts of the basin, however, the 

two sets locally coexist, and there the Fj joints do terminate 

laterally against MV2 joints, confirming the younger age of 

the F] set. Vertical terminations of F, joints against the 

inclined fold-parallel sets at several localities on the Divide 

Creek anticline further establish the chronology given here: 

MV2 > fold-parallel sets > F^

F2 Set

Areal and stratigraphic distribution. More than 

three-fouths of the outcrops studied in the Divide CreekWolf Creek area contain well-formed, prominent joints 

that strike northwest to west-northwest (fig. 10) and that are 

correlative with the regional F2 set. The abundance of the F2 

joints in the study area is unusual; elsewhere in the southern 

part of the Piceance basin the F2 joints are present in only 

about half of the outcrops studied and the later F3 joints 

instead constitute the dominant set (Grout and Verbeek, 

1985).

In most of the Piceance basin the presence of F2 joints 

in the same beds as members of the Fj set is rare (Verbeek 

and Grout, 1983; Grout and Verbeek, 1985), in part due to 

the common absence of the F! set and in part because the 

development of younger F2 joints at strike directions only 

25°-30° different from those F^ joints already present was 

unlikely except where the Fj joints were few or had already 

been thoroughly "healed" by mineralization. A similar 

effect was noted in the Divide Creek-Wolf Creek area: 

wherever the F2 and Fj sets were found together, the F2 

joints are most abundant in those parts of the outcrop where 

the Fj joints are least abundant. Joints of both sets, however, 

are sufficiently abundant that they coexist in almost a 

quarter of the outcrops studied (compare figs. 8, 10).

Orientation. The F2 joints commonly range in strike 

from N. 46°-74° W. and are subvertical.

Dimensions. Many F2 joints are as large as the Fj 

joints and commonly are 0.3-4 m high, depending on layer 

thickness. Where fractures of the F^ set are prominent and 

relatively closely spaced, however, the F2 joints, if present, 

commonly are diminished both in abundance and size and 

are somewhat more irregular in shape than usual.

F2 joints that appear from a distance to cut several 

meters of outcrop (for example, fig. 11) actually are not 

single fractures but instead are narrow zones of very closely 

spaced, shorter individual joints that overlap vertically, 

similar to the zones described for the Fj set. Heights of 

individual joints in such zones are 2 m or less. The zones are 

as long as 15 m, but the individual joints within them 

commonly are only 0.3-5 m long and rarely as long as 7 m.

Spacings. Spacings of the F2 joints are 0.1-1.5 m. 

As is usual for this set, large F2 joints in thick beds generally 

are spaced farther apart than smaller joints in thinner strata.

Surface structures and shape. The usual 

complement of surface structures indicative of extensile 

failure plumose structure, arrest lines, and twist 

hackle is common on many of the joints of this set.

Mineralization. Mineral fillings are more common 

in joints of the F2 set than in any other, due both to the sheer 

numbers of F2 joints and their relatively large original 

apertures, commonly 0.5-1.5 mm on a basinwide scale. 

Most of the mineral-filled F2 joints observed are on the 

Wolf Creek anticline and near the Grand Hogback monocline; fewer are in the Divide Creek area. The three types of 

mineral fillings identified, in order of decreasing 

abundance, are calcite, barite, and quartz, but in their 

distribution these minerals may be mutually exclusive; only 

one type was found at each outcrop. Barite is of particularly 

restricted occurrence and was found in F2 joints only on the 

south end of the Wolf Creek anticline.

Terminating relations. The F2 joints terminate 

laterally against joints of the Fj and MV2 sets and both 

laterally and vertically against the moderately dipping 

fold-parallel fractures. The F2 joints therefore postdate these 

other sets.

F3 Set

Areal and stratigraphic distribution. Joints of the F3 

set are present in half of the outcrops studied (fig. 12) but 

are not as abundant as in other parts of the southern 

Piceance basin (Grout and Verbeek, 1985). The reduced 

prominence of the F3 set probably is linked to the increased 

prominence of the F2 set (figs. 10, 12) in the same way as 

previously discussed for other sets: the more a given rock 

layer is fractured, the less likely a new fracture set will form 

at low or moderate angles to an existing set unless the rock 

has already regained cohesion through extensive 

cementation of the earlier fractures. On a basinwide scale, 

the F3 joints are abundant only where the earlier F2 set is 

weakly expressed and are much less common, or absent, in 

those areas where the F2 set is well developed.

Orientation. F3 joints are subvertical and commonly 

strike from N. 54°-88° E.

Dimensions. In the four outcrops in the study area 

where F3 joints are the first-formed set, and therefore 

achieve their greatest expression, their surfaces are large, 

fairly planar, and regularly spaced, similar to those of the F2 

set. In the numerous other outcrops where older joint sets 

are present, however, the F3 joints are smaller and more 

irregular in shape. Heights and lengths of F3 joints thus vary 

considerably and are 0.25-7 m and 0.3-10 m, respectively.

Spacings. Spacings of F3 joints were measured in 

more than half of the outcrops that contain them and

Z30 Evolution of Sedimentary Basins Uinta and Piceance Basins
commonly are 0.3^ m; however, where the joints are 

small less than 1 m2 in surface area the spacings locally 

are too variable for meaningful measurement.

Surface structures and shape. F3 joints are planar 

only in those few outcrops not cut by older sets. Elsewhere, 

the F3 joints generally are broadly curved or sinuous in 

strike and dip and thus are more variable in shape and 

orientation. Plumose structure, twist hackle, and arrest lines 

are common on the surfaces of many F3 joints, though not 

many possess all three in combination.

Mineralization. Joints of the F3 set commonly are 

unmineralized in this part of the Piceance basin, but thin 

seams of massive brown calcite were observed in one place 

on the south end of the Divide Creek anticline.

Terminating relations. The F3 joints terminate 

laterally against joints of all previously formed sets. In 

addition, the extremities of many F3 joints curve so as to 

terminate against an adjacent, earlier formed F3 joint almost 

at right angles, a phenomenon called hooking (Kulander and 

others, 1979) and a common form of crack interaction 

among propagating extension fractures. Hooking among 

joints of the F3 and other sets is quite common throughout 

the Piceance basin (Grout and Verbeek, 1983).

F4 Set

Area! and stratigraphic distribution. Joints of the F4 

set were found in nearly three-fourths of the outcrops 

studied in the Divide Creek-Wolf Creek area (fig. 13) and 

are not restricted to any particular stratigraphic interval or 

rock type.

Orientation. F4 joints are subvertical and strike 

from N. 5° W. to N. 44° E. Such variable strikes are 

common for joints of this set because the joints are 

stress-relief fractures that, during regional uplift and 

erosion, tended to form perpendicular to whichever of the 

older sets dominated the local fracture network. Thus, F4 

joints strike north-northeast to northeast where F2 joints are 

strongly expressed (fig. 9), as in much of the study area, but 

strike more northerly to north-northwest in those places 

where instead F3 joints are dominant. In addition, because 

F4 joints formed within polygonal blocks of rock bounded 

by the joints of previous sets and variably cemented 

together, stress trajectories during fracture likely were 

heterogeneous, especially so from one block to another; this 

too probably contributed to the orientational variability (and 

irregular shape; see below) of the F4 joints.

Dimensions. F4 joints generally are much smaller 

than joints of the older sets; they commonly are 0.1-2 m 

high and only 0.08-3 m long. Their small size is due to two 

factors: not only did the F4 joints form in previously 

fractured rock, where generally there was not enough space 

to grow to great length, but their heights commonly were 

constrained by bed-parallel unloading joints (fig. 9) that 

formed during or after regional uplift as overburden

pressures were lessened by erosion. Small size and vertical 

termination against bed-parallel unloading joints are 

characteristic of the F4 set basinwide (Verbeek and Grout, 

1983, 1984a; Grout and Verbeek, 1985).

Spacings. Measured spacings of the F4 joints, 

similar to those of the F3 set, are quite variable, from a few 

centimeters to 8 m. At some outcrops the spacings are too 

variable for meaningful measurement, but the widest 

spacings generally are between the largest joints.

Surface structures and shape. Few F4 joints 

approximate planar surfaces; most are irregular, subplanar 

to nonplanar fractures, as befits a late-formed set in 

repeatedly fractured rock. Sinuosity of F4 joints along 

strike, hooking of their extremities into previously formed 

joints, and moderate to strong curvature in dip are all 

common. Surface structures are few; the most common are 

arrest lines.

Many F4 joints during growth split into two segments 

of lesser height, the upper one terminating downward and 

the lower one upward against some bed-parallel surface, 

either a lithologic discontinuity or an unloading joint. 

Lateral growth of the two segments then continued along 

similar but noncoplanar directions to produce a compound, 

discontinuous joint surface, which, despite its complex 

geometry, is the product of a single fracture event. Such 

split joints are common among the F4 joints but are almost 

unknown from earlier sets.

Mineralization. Thin seams of brown microcrystalline calcite were observed on some F4 joints at one 

locality on the south end of the Divide Creek anticline. 

Elsewhere there is no evidence of mineralization, but the 

characteristically thin films of calcite that coat F4 surfaces 

elsewhere in the basin are readily stripped by weathering.

Terminating relations. The F4 joints terminate 

laterally against joints of all previously formed sets and 

vertically against both bed-parallel unloading joints (fig. 9) 

and lithologic discontinuities, such as mudstone partings 

within sandstone beds or contacts between beds of 

dissimilar lithology.

F5 Set

Areal and stratigraphic distribution. The F5 set is 

the most weakly developed of the Piceance system of 

fractures in the study area; it was found at only four 

outcrops and is absent from the Divide Creek anticline (fig. 

14).

Orientation. The F5 joints are subvertical and 

commonly strike N. 50°-80° W., parallel to the F2 joints. 

Joints of the two sets can be distinguished easily, however, 

by their different styles and especially by their different 

terminating relations (Verbeek and Grout, 1983, 1984a; 

Grout and Verbeek, 1985).

Dimensions. The F5 joints are smaller than the F4 

joints and are confined vertically between even more

Fracture History, Southern Piceance Basin, Colorado Z31
closely spaced bed-parallel unloading joints. Only the 

largest F5 joints were measured. These are 0.06-2.5 m high, 

but most are less than 1.5 m high. Their lengths, commonly 

0.25-2.5 m, were constrained by the distance an F5 joint 

could propagate before encountering another fracture of a 

previous set, especially those of the F4 set.

Spacings. Spacings of the F5 joints are extremely 

variable because this set was the last to form in repeatedly 

fractured rock. Particularly large and abundant F5 joints at 

one locality on the Wolf Creek anticline are spaced 0.75-3 

m apart.

Surface structures and shape Most F5 joints are 

irregular, subplanar to nonplanar fractures similar in

appearance to those of the F4 set but even more crudely 

shaped. Surface structures are few, but the largest F5 joints 

at one outcrop on the Wolf Creek anticline display arrest 

lines and twist hackle. Hooking of some F5 joints into 

nearby members of the same set was observed at the same 

outcrop and provides additional evidence of extensile 

failure.

Mineralization. No evidence of mineralization was 

observed on any F5 joint in the study area nor in the rest of 

the Piceance basin.

Terminating relations. Joints of the F5 set terminate 

laterally against joints of all older sets except for the F2 

joints, to which they are parallel.

Z32 Evolution of Sedimentary Basins Uinta and Piceance Basins

*U.S. GOVERNMENT PRINTING OFFICE: 1992-673-049/46025
SELECTED SERIES OF U.S. GEOLOGICAL SURVEY PUBLICATIONS

Periodicals

Earthquakes & Volcanoes (issued bimonthly). 

Preliminary Determination of Epicenters (issued monthly).

Technical Books and Reports

Professional Papers are mainly comprehensive scientific reports 

of wide and lasting interest and importance to professional scientists and 

engineers. Included are reports on the results of resource studies and of 

topographic, hydrologic, and geologic investigations. They also include 

collections of related papers addressing different aspects of a single scientific topic.

Bulletins contain significant data and interpretations that are of 

lasting scientific interest but are generally more limited in scope or geographic coverage than Professional Papers. They include the results of 

resource studies and of geologic and topographic investigations; as well 

as collections of short papers related to a specific topic.

Water-Supply Papers are comprehensive reports that present significant interpretive results of hydrologic investigations of wide interest 

to professional geologists, hydrologists, and engineers. The series covers 

investigations in all phases of hydrology, including hydrology, availability of water, quality of water, and use of water.

Circulars present administrative information or important scientific information of wide popular interest in a format designed for distribution at no cost to the public. Information is usually of short-term interest.

Water-Resources Investigations Reports are papers of an interpretive nature made available to the public outside the formal USGS publications series. Copies are reproduced on request unlike formal USGS 

publications, and they are also available for public inspection at depositories indicated in USGS catalogs.

Open-File Reports include unpublished manuscript reports, maps, 

and other material that are made available for public consultation at depositories. They are a nonpermanent form of publication that maybe cited in other publications as sources of information.

Maps

Geologic Quadrangle Maps are multicolor geologic maps on topographic bases in 7 1/2- or 15-minute quadrangle formats (scales mainly 1:24,000 or 1:62,500) showing bedrock, surficial, or engineering 

geology. Maps generally include brief texts; some maps include structure 

and columnar sections only.

Geophysical Investigations Maps are on topographic or planimetric bases at various scales, they show results of surveys using geophysical techniques, such as gravity, magnetic, seismic, or radioactivity, which 

reflect subsurface structures that are of economic or geologic significance. Many maps include correlations with the geology.

Miscellaneous Investigations Series Maps are on planimetric or 

topographic bases of regular and irregular areas at various scales; they 

present a wide variety of format and subject matter. The series also includes 7 1/2-minute quadrangle photogeologic maps on planimetric 

bases which show geology as interpreted from aerial photographs. The 

series also includes maps of Mars and the Moon.

Coal Investigations Maps are geologic maps on topographic or 

planimetric bases at various scales showing bedrock or surficial geology, 

stratigraphy, and structural relations in certain coal-resource areas.

Oil and Gas Investigations Charts show stratigraphic information for certain oil and gas fields and other areas having petroleum potential.

Miscellaneous Field Studies Maps are multicolor or black-andwhite maps on topographic or planimetric bases on quadrangle or irregular areas at various scales. Pre-1971 maps show bedrock geology in relation to specific mining or mineral-deposit problems; post-1971 maps 

are primarily black-and-white maps on various subjects such as environmental studies or wilderness mineral investigations.

Hydrologic Investigations Atlases are multicolored or black-andwhite maps on topographic or planimetric bases presenting a wide range 

of geohydrologic data of both regular and irregular areas; the principal 

scale is 1:24,000, and regional studies are at 1:250,000 scale or smaller.

Catalogs

Permanent catalogs, as well as some others, giving comprehensive 

listings of U.S. Geological Survey publications are available under the 

conditions indicated below from the U.S. Geological Survey, Books and 

Open-File Reports Sales, Box 25425, Denver, CO 80225. (See latest 

Price and Availability List.)

"Publications of the Geological Survey, 1879-1961" may be purchased by mail and over the counter in paperback book form and as a set 

microfiche.

"Publications of the Geological Survey, 1962-1970" may be purchased by mail and over the counter in paperback book form and as a set 

of microfiche.

"Publications of the U.S. Geological Survey, 1971-1981" may be 

purchased by mail and over the counter in paperback book form (two 

volumes, publications listing and index) and as a set of microfiche.

Supplements for 1982, 1983, 1984, 1985, 1986, and for subsequent years since the last permanent catalog may be purchased by mail 

and over the counter in paperback book form.

State catalogs, "List of U.S. Geological Survey Geologic and Water-Supply Reports and Maps For (State)," may be purchased by mail and 

over the counter in paperback booklet form only.

"Price and Availability List of U.S. Geological Survey Publications," issued annually, is available free of charge in paperback booklet 

form only.

Selected copies of a monthly catalog "New Publications of the 

U.S. Geological Survey" is available free of charge by mail or may be 

obtained over the counter in paperback booklet form only. Those wishing 

a free subscription to the monthly catalog "New Publications of the U.S. 

Geological Survey" should write to the U.S. Geological Survey, 582 National Center, Reston, VA 22092.

Note.-Prices of Government publications listed in older catalogs, 

announcements, and publications may be incorrect Therefore, the prices 

charged may differ from the prices in catalogs, announcements, and publications.