Thermal Effects of Thin igneous Intrusions on 

Diagenetic Reactions in a Tertiary Basin of 

Southwestern Washington

U.S. GEOLOGICAL SURVEY BULLETIN 2085-C

JK'
Cover. Steeply dipping beds (fluvial channel deposits) of the Eocene Puget Group in the 

upper part of the Green River Gorge near Kanaskat, southeastern King County, 

Washington. Photograph by Samuel Y. Johnson, July 1992.
Thermal Effects of Thin Igneous Intrusions on 

Diagenetic Reactions in a Tertiary Basin of 

Southwestern Washington

By Kenneth J. Esposito and Gene Whitney

EVOLUTION OF SEDIMENTARY BASINS CENOZOIC SEDIMENTARY BASINS 

IN SOUTHWEST WASHINGTON AND NORTHWEST OREGON

Samuel Y. Johnson, Project Coordinator

U.S. GEOLOGICAL SURVEY BULLETIN 2085-C

A multidisciplinary approach to research studies of 

sedimentary rocks and their constituents and the 

evolution of sedimentary basins, both ancient and modern

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1995
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, Director

For sale by U.S. Geological Survey, Information Services

Box 25286, Federal Center

Denver, CO 80225

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

does not imply endorsement by the U.S. Government

Library of Congress Cataloging-in-Publication Data

Esposito, Kenneth J.

Thermal effects of thin igneous intrusions on diagenetic reactions in a Tertiary basin of 

southwestern Washington/by Kenneth J. Esposito and Gene Whitney.

p. cm. (Evolution of sedimentary basins Cenozoic sedimentary basins in 

southwest Washington and northwest Oregon; C) 

(U.S. Geological Survey bulletin; 2085)

Includes bibliographical references.

Supt. of Docs, no.: 119.3:2085C

1. Intrusions (Geology) Washington (State). 2. Diagenesis Washington (State). 

3. Geology, Stratigraphic Tertiary. 4. Geology Washington (State). I. Whitney, 

Gene. II. Title. III. Series. IV. Series: U.S. Geological Survey bulletin; 2085. 

QE75.B9 no. 2085-C 

[QE611.5.U6] 

557.3 s dc20

[552'.5] 94-23449

CIP
CONTENTS

Abstract........................................................................................................................... Cl

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

Tectonic History..................................................................................................... 2

Thermal History...................................................................................................... 2

Stratigraphy............................................................................................................ 2

Previous Work on the Illitization Reaction............................................................ 4

Mineralogy of Illite-Smectite ......................................................................... 5

Illitization in Contact Metamorphic Environments........................................ 5

Materials and Methods ................................................................................................... 7

Sample Acquisition and Preparation ...................................................................... 7

Clay-Mineral Analysis............................................................................................ 7

X-Ray Fluorescence Spectrometry......................................................................... 8

Scanning Electron Microscopy............................................................................... 8

Petrographic Thin-Section Examination ............................................................... 8

Vitrinite Reflectance............................................................................................... 8

Results............................................................................................................................ 8

Petrography of Igneous Rocks........................................................................................ 8

Diagenesis of Sedimentary Rocks.................................................................................. 9

Clay-Mineral Composition..................................................................................... 9

Variation in Illite-Smectite Expandability.............................................................. 9

Variation in Other Clay Minerals........................................................................... 10

Variation in Vitrinite Reflectance........................................................................... 11

Heat-Flow Calculations.................................................................................................. 11

Discussion....................................................................................................................... 22

Reaction Kinetics.................................................................................................... 22

Factors Affecting Illitization Reaction Rates ......................................................... 25

Calculations of the Extent of the Illitization Reaction ........................................... 25

Conduction, Diffusion, and Advection Near Intrusions......................................... 29

Paragenesis of Other Minerals................................................................................ 29

Summary and Conclusions............................................................................................. 34

References Cited............................................................................................................. 34

Appendix........................................................................................................................ 37

	FIGURES

1. Map of western Washington showing coal rank of Eocene macerals and location of cores examined

	for study................................................................................................................................................................. C3

2. Chart showing simplified Tertiary stratigraphy in the Puget Lowland.................................................................. 4

3. Diagram showing sample locations and distribution of rock types in cores ......................................................... 6

4. X-ray diffraction pattern showing peak positions used for determining illite-smectite expandability ................. 7

5. Diagram showing igneous rock identification in terms of total alkalis and silica for samples in study................ 10

6. Schematic sections of cores WC-83-5 and WC-83-6 showing position of igneous rocks................................. 11

7. Diagram showing point-count data and percentage of illite in illite-smectite, core WC-83-6 ............................ 12

8. Graph showing expandability of illite-smectite, core WC-83-6 .......................................................................... 20

9. Scanning electron micrograph showing typical morphology of illite-smectite..................................................... 21

10. Graph showing expandability of illite-smectite, core WC-83-5 .......................................................................... 21

11. Graph showing percentages of kaolinite and chlorite, core WC-83-6................................................................. 21

III
IV CONTENTS

12. Scanning electron micrograph of kaolinite............................................................................................................ 23

13. Scanning electron micrograph showing chlorite replacing biotite...................................................... 23

14-17. Graphs showing calculated temperature at various times after emplacement:

14. Between intrusions 6 and 3 m thick.................................................................................................... 24

15. Of intrusions in core WC-83-6 .......................................................................................................... 24

16. Of 26-m-thick intrusion in core WC-83-5 ......................................................................................... 24

17. Of 61-m-thick intrusion in core WC-83-5 ........................................................................................ 24

18. Graph showing calculated illitization reaction extent using only the heat provided by intrusions

matching observed illitization near the intrusions or at the midpint between the intrusions.............. 27

19-30. Graph showing comparison of calculated and observed percent illite in illite-smectite for core:

19. WC-83-6 using 900°C intrusions for calculations............................................................................. 27

20. WC-83-6 using 1,000°C intrusions for calculations.......................................................................... 27

21. WC-83-6 using 1,100°C intrusions for calculations.......................................................................... 28

22. WC-83-6 using 1,200°C intrusions for calculations.......................................................................... 28

23. WC-83-5 using 26-m-thick, 900°C intrusion for calculations ......................................................... 30

24. WC-83-5 using 26-m-thick, 1,000°C intrusion for calculations ....................................................... 30

25. WC-83-5 using 26-m-thick, 1,100°C intrusion for calculations........................................................ 31

26. WC-83-5 using 26-m-thick, 1,200°C intrusion for calculations........................................................ 31

27. WC-83-5 using 61-m-thick, 900°C intrusion for calculations........................................................... 32

28. WC-83-5 using 61-m-thick, 1,000°C intrusion for calculations........................................................ 32

29. WC-83-5 using 61-m-thick, 1,100°C intrusion for calculations........................................................ 33

30. WC-83-5 using 61-m-thick, 1,200°C intrusion for calculations........................................................ 33

TABLES

1. Lithological and clay mineralogical data and distance to adjacent igneous intrusions for samples from 

core WC-83-6 .......................................................................................................................^^^

2. Lithological and clay mineralogical data and distance from pluton for samples from core WC-83-5................. 16

3. Lithological and clay mineralogical data for samples from core WC-83-17 ....................................................... 18

4. Mean vitrinite reflectance values for samples from cores WC-83-5, WC-83-6, and WC-83-17....................... 20

5. Mathematical relationships for heat-flow calculations........................................................................................... 20

6. Values for variables used in heat-flow calculations................................................................................................ 22

7. Published kinetic values used in the Arrhenius equation for activation energy and pre-exponential function

during illitization.........................................................._ 26

8. Kinetic values used for activation energy and pre-exponential function to calculate illitization .......................... 29
Thermal Effects of Thin Igneous Intrusions on

Diagenetic Reactions in a Tertiary Basin of

Southwestern Washington

By Kenneth J. Esposito and Gene Whitney

ABSTRACT

Tertiary sedimentary rocks in the Puget Lowland of 

southwestern Washington are intruded by thin igneous sills 

that increase in both thickness and frequency toward the 

Cascade Mountains. Shale, mudstone, and sandstone were 

sampled from three drill cores: the core nearest the Cascades 

contains four basaltic sills in a 244-m-thick (800 ft) section, 

the second contains a single basaltic sill in a 457-m-thick 

(1,500 ft) section, and the third contains no igneous sills. 

Detailed clay-mineral analyses were performed to assess the 

thermal influence of these igneous intrusions on illitization 

and other clay-mineral reactions in the sedimentary rocks.

The extent of illitization is a function of temperature, 

which depends on maximum burial depth and proximity to 

intruded sills. In the two cores that contain intrusions, temperatures resulting from burial to a depth of about 4 km and 

a geothermal gradient of 35°C/km have produced inter stratified illite-smectite with about 60-70 percent illite layers. 

The abundance of illite in illite-smectite increases to about 

75-95 percent immediately adjacent to sills. Furthermore, 

the extent of the reaction (or percentage of illite layers in 

illite-smectite) is proportional to the thickness of the sill. The 

third core, which is farthest from the Cascade Range, contains no sills and has never been deeply buried, and only 

highly smectitic illite-smectite is present.

Modeling of the illitization reaction using conductive 

heat-flow calculations and reaction kinetics confirms that the 

extent of the illitization reaction as a result of heating by the 

sills diminishes rapidly away from the sill contact. The intrusions affect the surrounding rocks to a distance roughly 

equivalent to the thickness of the sill.

INTRODUCTION

Smectite converts to illite via mixed-layer illite-smectite in both natural and experimental systems under the influence of increased temperature, and the proportion of 

smectite layers decreases as the reaction progresses (Perry

and Hower, 1970; Eberl and Hower, 1976; Hower and 

others, 1976; Boles and Franks, 1979; Pytte, 1982; Inoue and 

Utada, 1983; Ramseyer and Boles, 1986; Buhmann, 1992; 

Inoue and others, 1992; Velde and Vasseur, 1992; Whitney, 

1992). In natural systems, the heat that drives the smectite to 

illite reaction may come from burial, from circulating hydrothermal fluids, or from igneous bodies. For example, shale in 

sedimentary basins exhibits a systematic decrease in the percentage of smectite layers in the mixed-layer illite-smectite 

(illite-smectite) with increasing temperature during burial 

(see, for example, Perry and Hower, 1970; Hower and others, 1976; Boles and Franks, 1979; Velde and Vasseur, 

1992). Similar clay-mineral reactions result from contact 

metamorphism, manifested as a decrease in the expandability of illite-smectite near igneous intrusions (Pytte, 1982; 

Buhmann, 1992). It has been suggested that the illitization 

reaction can be used as a low-temperature geothermometer 

(Hoffman and Hower, 1979; Pollastro, 1993).

We investigated the effects of heat from intrusive rocks 

on the illitization reaction by examining clay-mineral reactions in sedimentary rocks near intrusions in a Tertiary basin 

in southwestern Washington. Samples of intrusive and host 

sedimentary rocks were obtained from three drill cores in 

Lewis, Pierce, and King Counties, southwestern Washington, between the Cascade Range and the Puget Lowland. The 

intrusive rocks are hosted by the fluvial-deltaic Puget Group, 

which consists dominantly of sandstone and siltstone and 

includes several thin beds of carbonaceous shale and coal. 

Several intrusive sills provide the basis for determining the 

extent to which the thermal maturity (increasing percentage 

of illite in illite-smectite) of the clays is dependent on the 

intercalated intrusions. Mineralogical and chemical characterizations of the clay minerals were used to establish 

illite-smectite composition as a function of distance from the 

intrusions. Conductive heat-flow modeling of the intrusions 

provided a heating profile in the country rock that was then 

combined with published kinetic expressions for the illitization reaction to determine the extent to which observed thermal maturity patterns resulted from heating effects of the 

thin intrusions.

Cl
C2 CENOZOIC SEDIMENTARY BASINS IN SOUTHWEST WASHINGTON AND NORTHWEST OREGON

Acknowledgments. This research was funded by the 

U.S. Geological Survey Evolution of Sedimentary Basins 

Program. We thank Weyerhauser Corporation, and 

particularly Peter Hales, for access to cores. Sam Johnson 

provided guidance in sampling and geologic background for 

the study. Vito Nuccio provided the vitrinite reflectance 

data. We thank Sam Johnson and Rich Pollastro for constructive reviews.

TECTONIC HISTORY

The geology of western Oregon and Washington exhibits the complex structural, lithological, and sedimentological 

relationships characteristic of colliding plate boundaries. 

During the Paleocene, low-angle subduction of the Kula and 

Farallon oceanic plates under the westward-moving North 

American plate (Ewing, 1980) created the Challis volcanic 

arc system, which extended from Idaho to the Dakotas 

(Nilsen and McKee, 1979). The configuration of this subduction changed in the late Paleocene when basaltic rocks of 

the Crescent Formation and the Siletz River Volcanics were 

erupted as seamounts along the continental margin. The 

Crescent Formation and the Siletz River Volcanics form the 

basement of the Oregon and Washington Coast Range. Published models for the origins of these rocks suggest that (1) 

they formed in an intracontinental rift setting somewhat 

analogous to the volcanism in the Andaman Sea and the Gulf 

of California (Wells and others, 1984; Snavely, 1988; Babcock and others, 1992) or (2) they formed on a spreading 

center between the Kula and Farallon plates and were subsequently accreted to the continental margin by plugging of a 

subduction zone that resulted in the initiation of a new subduction zone to the west (Wells and others, 1984; Duncan 

and Kulm, 1989). The intracontinental rift model predicts 

that the eastern boundary of the rift province is a dextral 

strike-slip fault, consistent with the observations of Johnson 

(1984, 1985) and Johnson and Yount (1992) concerning the 

structural and subsidence histories of sedimentary basins in 

western Washington.

Pre-Tertiary metasedimentary rocks form the basement 

of the Cascade block east of the Coast Range in southern 

Washington. The structural boundary between the Cascade 

and Coast Range provinces extends northward through the 

southeastern Puget Lowland. This boundary is covered by 

Oligocene and younger deposits but on the basis of geophysical data (gravity, magnetics, magnetotellurics) (S.Y. 

Johnson, oral commun., 1993) is inferred to be about 10-20 

km (6.2-12.4 mi) west of the borehole sites examined in the 

study. Eocene sedimentary basins east of this boundary were 

characterized by rapid subsidence associated with transtension and sedimentary fill as thick as 6,000 m (20,000 ft) 

(Johnson, 1985; Johnson and Yount, 1992). The rocks of the 

Puget Group examined for this study make up a part of this 

thick sedimentary fill.

In the latest Eocene or early Oligocene, following a significant tectonic transition along the continental margin, the 

Cascade volcanic arc was initiated. Since that time, the 

Cascade foothills and Coast Range province have occupied 

part of the Cascade forearc basin. Since the Eocene, the 

region has also experienced significant heterogeneous 

(decreasing to the east) clockwise rotation (as much as 75°). 

Several causes of rotation have been suggested (Wells and 

Heller, 1988), including oblique subduction of the oceanic 

plates against North America, block rotations between parallel and subparallel transcurrent faults, and basin and range 

extension.

THERMAL HISTORY

The thermal history of western Washington has been 

inferred primarily from the distribution of indicators of thermal maturity in the sedimentary rocks in the region. Mean 

vitrinite reflectance of coal macerals is one of the primary 

methods for estimating thermal maturity of sedimentary 

rocks that contain organic matter (Waples, 1985). Coals in 

the Puget Lowland of Washington are mostly lignite and 

subbituminous, but coal rank increases eastward toward the 

Cascades, where coals are low-volatile bituminous or anthracite (Walsh and Lingley, 1991) (fig. 1). Walsh and Lingley 

(1991) related the regional increase in coal rank toward the 

Cascades to the elevated heat flow associated with late Oligocene to middle Miocene intrusive activity in the Cascade 

arc. They suggested that this elevated heat flow affected coal 

rank for a distance of several kilometers. In addition to heating from burial and nearby intrusions, other areas in the 

Pacific Northwest also show local evidence for convective 

heating by hydrothermal fluids (Summer and Verosub, 

1989). Unfortunately, coal beds are not distributed with sufficient regularity or frequency to map heat flow in any 

greater detail than at a regional scale.

STRATIGRAPHY

Our research focused on the thermal effects of igneous 

intrusions on the sedimentary rocks of the Puget Group. 

Because of the complex geology, formation names change in 

different geographic areas, and a simplified stratigraphic section of local Tertiary formations is represented in figure 2.

The geology in the study area, was first described in 

detail by Fisher (1957, 1961). The sedimentary rocks in the 

area are lithologically similar to the Puget Group to the north 

in the Black Diamond area where isotopic and paleontologic 

dates and stratigraphic relationships indicate a middle to late 

Eocene age (Vine, 1962; Turner and others, 1983; Frizzell 

and others, 1984). The Puget Group consists of predominantly feldspathic sandstone and siltstone and lesser
THERMAL EFFECTS OF INTRUSIONS ON DIAGENETIC REACTIONS C3

121 C

32 Mount 

4 * Rainer

0 20 

I i i i I

40 60 

I

80 KILOMETERS 

J

Figure 1. Map of western Washington showing coal rank of Eocene macerals and location of the three cores examined for this study. Core 

WC-83-17 is in King County, sec. 13, T. 22 N., R. 6 E.; core WC-83-6 is in Lewis County, SWV4 sec. 29, T. 14 N., R. 5 E.; and core 

WC-83-5 is in Pierce County, sec. 28, T. 15 N., R. 6 E. Coal rank of Eocene macerals (solid circles; from Walsh and Phillips, 1983) is indicated by number: 1, anthracite; 2, low volatile bituminous; 3, medium volatile bituminous; 4, high volatile bituminous; 5, subbituminous; 

6, lignite.
C4 CENOZOIC SEDIMENTARY BASINS IN SOUTHWEST WASHINGTON AND NORTHWEST OREGON

Glacial deposits

Wilkes Formation

Columbia River 

Basalt Group

£

1

f8 |

Q. <O

TO O

-E LL

O

Goble an<f~-- 

Hatchet Mountain

vVolcanics \- \_

Undifferentiated Puget Group 

Spiketon Formation

Carbonado Formation 

Undifferentiated

Raging <^^ 

River Formatjorj? Puget Group

Metasedimentary 

basement rocks

Figure 2. Simplified Tertiary stratigraphy in 

the Puget Lowland in the Seattle and Carbon 

River areas. Modified from Armentrout and 

others (1983).

amounts of carbonaceous shale and coal. It was deposited in 

a variety of fluvial-deltaic environments including prodelta, 

delta front, interdistributary bay, swamp, fluvial distributary 

channel, natural levee, and braided stream (Buckovic, 1979; 

Evans, 1989). Igneous dikes, sills, and flows of varying compositions and thicknesses are present in the sedimentary

sequence (Fisher, 1961). The Puget Group includes several 

formations, including the Carbonado and Spiketon Formations (Armentrout and others, 1983). Rocks from the two 

southern cores of this study (fig. 1) are probably part of the 

Carbonado Formation, whereas the northern core contains 

Undifferentiated upper Puget Group rocks correlative with 

the Spiketon and Skookumchuck Formations (S.Y. Johnson, 

oral commun., 1993).

PREVIOUS WORK ON THE ILLITIZATION 

REACTION

The illitization reaction, in which smectite is converted 

to illite via an intermediate mixed-layer illite-smectite, has 

been observed and studied in burial diagenetic settings 

(Perry and Hower, 1970; Hower and others, 1976; Boles and 

Franks, 1979; Velde and Vasseur, 1992), in contact metamorphic environments (Pytte, 1982; Ramseyer and Boles, 

1986; Buhmann, 1992), and during hydrothermal fluid 

migration (Inoue and Utada, 1983; Inoue and others, 1992). 

It has also been studied experimentally (Eberl and Hower, 

1976; Whitney and Northrop, 1988; Huang, 1992; Whitney, 

1992; Whitney and Velde, 1993) to provide a better understanding of the reaction mechanism and processes in natural 

systems.

The basic reaction of smectite to illite can be written: 

Smectite+Al3++K>=Illite+Si4+.

The exact mechanism of the reaction from smectite to 

illite is poorly understood and has been the subject of considerable research. Several mechanisms have been proposed 

including a solid-state reaction (Hower and others, 1976), 

neoformation and crystal growth (Nadeau and others, 1985; 

Yau and others, 1987; Inoue and others, 1988; Eberl and others, 1990), interlayer dissolution (Altaner, 1989), and smectite cannibalization (Boles and Franks, 1979).

In the solid-state reaction model, phyllosilicate layers 

remain intact (do not completely dissolve) as ionic substitutions within the low-charge smectite layers produce 

high-charge illite layers (Weaver and Beck, 1971; Hower 

and others, 1976). The layer charge is built by ionic substitution of aluminum for silicon in the tetrahedral sheet and (or) 

magnesium for aluminum in the octahedral sheet. As the 

negative charge on the layers increases, electrical balance is 

maintained by the addition of interlayer potassium ions. 

When the layer charge increases to about 0.7 equivalents per 

Oio(OH)2, water is driven off and the interlayer potassium 

becomes fixed, transforming a smectite layer to an illite 

layer. In the solid-state model, ions are supplied by the dissolution of potassium feldspar or other accessory phases. 

This model has been suggested for the early formation of
THERMAL EFFECTS OF INTRUSIONS ON DIAGENETIC REACTIONS C5

disordered illite-smectite during illitization (Inoue and others, 1987; Whitney and Northrop, 1988).

Two models rely on the complete dissolution of smectite layers as illitization proceeds. The neoformation and 

crystal growth model proposes that decreased expandability 

of illite-smectite results from the dissolution of thin unstable 

smectite crystallites and the subsequent crystallization of 

thin illite crystallites, which themselves dissolve and grow 

into larger illite crystals. An increase in particle thickness 

(more 2:1 layers in each crystallite; Eberl and Srodon, 1988) 

and lateral dimensions (each 2:1 layer growing in the a and 

b crystallographic directions; Inoue and others, 1988) of 

illite-smectite suggests that illite recrystallization occurs by 

an Ostwald ripening process in which small illite particles 

dissolve and nourish the growth of larger illite particles. 

Nadeau and others (1984) showed that large illite-smectite 

particles grow larger at the expense of dissolving smaller 

particles. Similarly, the smectite cannibalization model proposes that dissolution of the smectite layers provides all of 

the aluminum needed for the reaction to proceed; no external 

source of aluminum is needed because aluminum is conserved (Boles and Franks, 1979; Pollastro, 1985).

In the interlayer dissolution model, parts of adjacent 

smectite 2:1 layers dissolve and recrystallize around the 

interlayer space as an illite layer, and the remaining parts of 

the smectite 2:1 layers are unaffected (Altaner, 1989; 

Altaner and Ylagan, 1993). Transmission electron microscopy (TEM) studies (Ahn and Peacor, 1986) suggest that 

illite-smectite can be thought of as a stack of high-charge 

(illite) and low-charge (smectite) interlayers such that an 

individual 2:1 layer can have a high-charge tetrahedral sheet 

on one side and a low-charge tetrahedral sheet on the other. 

This model, in essence, proposes the dissolution of the 

low-charge half of the 2:1 layer and reprecipitation of a 

high-charge tetrahedral sheet. Research by Drits (1987) 

demonstrates that the reaction of smectite to illite is accompanied by redistribution of octahedral cations, as well as by 

an increase in tetrahedral charge. Thus, if the interlayer dissolution model operates, the octahedral sheet must also be 

affected.

Although the reaction is written as if illite and smectite 

are in chemical equilibrium, the reaction is, for all practical 

purposes, irreversible. Because the reaction is believed to be 

controlled by reaction kinetics, both the maximum temperature and the duration of the heating event play crucial roles 

in extent of illitization (Ramseyer and Boles, 1986). Velde 

and Vasseur (1992) contended that the duration of heating is 

more important than maximum temperature in determining 

the reaction extent for burial lasting millions to tens of millions of years; however, for short time spans, the effects of 

maximum temperature on the illitization reaction is uncertain. Velde and Lanson (1993) believed that in the Salton Sea 

area temperature was the most important variable controlling

extent of illitization over time spans of 104-105 years, 

although they concluded that in the Paris Basin short-lived 

heating due to an intrusion had little effect on the progress of 

the illitization reaction.

MINERALOGY OF ILLITE-SMECTITE

Illite-smectite is composed of illite layers and smectite 

layers stacked together either randomly or regularly along 

the c-axis (Reynolds, 1980). As the illitization reaction proceeds, the expandability (percentage of smectite layers) in 

the illite-smectite decreases, and the arrangement of the layers exhibits increasing regularity in stacking order. In our 

study we used the Reichweite (or "reach back") terminology 

consistent with the method of expandability determinations 

(Srodon, 1980). During the early stages of illitization the few 

illite layers (I) formed are distributed randomly within a 

stack of the smectite layers (S). This disordered stacking is 

referred to as R=0 and is seen in samples containing less than 

60 percent illite layers. As illitization proceeds to about 60 

percent illite layers, the random arrangement between the 

illite and smectite layers is lost, and each smectite layer is 

separated by one illite layer. The perfect alternation between 

illite layers and smectite layers (ISISIS) is expressed as R=l 

ordering. The mineral rectorite, in which each smectite layer 

is separated by an illite layer, is an example of R=l ordering. 

R=l ordering can persist through continued illitization until 

about 80-85 percent illite (15-20 percent expandable). As 

illitization continues beyond about 80 percent illite layers, 

each smectite layer is separated by three illite layers (ISIIISIIISII), which is expressed as R=3 ordering. In summary, as 

illitization proceeds the percentage of illite layers increases 

and ordering increases from random stacking (R=0), to 

short-range ordered stacking (R=l), to long-range ordered 

stacking (R=3). In the present study, the term "percent illite" 

refers only to the percent illite in mixed-layer illite-smectite 

(100 percent-percent smectite), not to the percentage of discrete illite in the clay-size fraction, and the term "percent 

expandable" refers to the percentage of smectite layers in 

illite-smectite.

ILLITIZATION IN CONTACT METAMORPHIC 

ENVIRONMENTS

The illitization reaction has been previously documented in contact metamorphic environments. Pytte 

(1982) detailed the smectite to illite reaction in contact-metamorphosed rocks of the Pierre Shale in 

south-central Colorado. A pattern of decreasing expandability with increasing temperature was observed in rocks 

that were heated to temperatures ranging from 70°C to 

250°C for durations of approximately 10 years to 300 m.y.
C6 CENOZOIC SEDIMENTARY BASINS IN SOUTHWEST WASHINGTON AND NORTHWEST OREGON

WC-83-17 

210ft_____64m

1,735ft

WC-83-5

517ft,_____,158m

EXPLANATION

Sedimentary rock

Igneous rock

 Sample location

« Number of samples

collected at location

WC-83-6 

1,221 ft,_____ 372 m

FEET METERS 

0-rO

20-

30-

528.8 m 1,995 ft I 1608m 1,999ft 309.3 m

Figure 3. Sample locations and distribution of rock types in cores WC-83-5, WC-83-6, and WC-83-17.

Pytte suggested that the reaction is controlled by kinetic 

rather than equilibrium factors. He used measured 

expandabilities in lithologically homogenous sedimentary 

rocks to assign an activation energy to a sixth-order 

kinetic expression for the overall illitization reaction, 

first-order with respect to potassium availability and 

fifth-order with respect to starting smectite concentration. 

Pytte and Reynolds (1989) documented a zone of 

decreased expandability extending into the country rock a 

distance of approximately twice the intrusion thickness. 

They used the sixth-order model to explain the illitization 

process in the field and concluded that the correct kinetic 

rate law is probably a chain of low-order reactions, each 

having different kinetic constraints.

Buhmann (1992) identified obvious temperature-dependent trends in illite-smectite composition in the

vicinity of intrusive sills in the Karoo Basin in the Republic 

of South Africa. In contrast to the almost homogeneous 

Pierre Shale, Karoo Basin strata contain interbedded 

sandstone, siltstone, and carbonaceous mudstone. A systematic decrease in the expandability in illite-smectite from 70 

to 5 percent was reported in the vicinity of the sills. Near the 

intrusion, where illitization proceeded to ordered phases 

(R=l) of illite-smectite, correlation of illitization with 

vitrinite reflectance data was relatively good. Closer to the 

intrusion, higher ordering in illite-smectite (R>1) corresponded to greater vitrinite reflectance values; however, farther from the intrusion, correlation of illitization with 

vitrinite reflectance values was poor because vitrinite reflectance values increased again, and highly expandable disordered illite-smectite (R=0) predominated. Buhmann was 

unable to explain this discrepancy between vitrinite maturity
THERMAL EFFECTS OF INTRUSIONS ON DIAGENETIC REACTIONS C7

(5.25°) 

16.82

80 percent expandable R=0 illite-smectite

-A26-

(10.21°) 

8.66

(15.89°)

5- 58 (20.75°) 

4.28

(31.80°) 

2.81

10 15 20 

DEGREES 29

25 30 35

Figure 4. X-ray diffraction pattern of a mixed-layer illite-smectite containing 20 percent randomly interstratified illite layers. Peak 

positions are labeled by d-spacing and 26 (in degrees)*Cu^ radiation). Arrows indicate direction of migration of peak positions as 

the percentage of illite layers increases. The difference between 

these two peaks (A29) is used for determining illite-smectite expandability.

and illite-smectite immaturity. In general, the effect of igneous intrusions on sedimentary rocks is primarily dependent 

on the proximity and thickness of the intrusion, although the 

distance over which changes can occur in illite-smectite 

composition may vary considerably with vitrinite 

reflectance values (Buhmann, 1992).

MATERIALS AND METHODS 

SAMPLE ACQUISITION AND PREPARATION

This investigation was based on 117 sedimentary and 

12 igneous rock samples collected from three cores, 

WC-83-5, WC-83-6, and WC-83-17 (fig. 1), which span 

49 km (30 mi) of the Puget trough near the Cascade Range. 

They were collected at the Weyerhauser core storage facility 

in Dupont, Washington. Samples were taken at varying 

intervals from all three cores, with particular attention to the 

sandstone and shale adjacent to the intrusive rocks (fig. 3). 

The core closest to the Cascades, WC-83-6, contains eight 

igneous bodies (four intrusions and four flows) that have an 

average thickness of 4 m (13 ft) in 235 m (771 ft) of core. 

Core WC-83-6 was sampled between 372 and 609 m 

(1,221-1,999 ft) depth. The second core, WC-83-5, contains three flows, each about 3 m (10 ft) thick, and an igneous 

intrusion at least 26 m (85 ft) thick at the bottom of the core. 

This core was sampled between 158 and 608 m (517-1,995 

ft) depth. In core WC-83-17, the farthest from the Cascades, 

there are no intrusions between the sampled depths of 64 and 

529 m (210-1,735 ft).

Whole-rock samples were ground in a corundum 

mortar and pestle to a size of 325 mesh and loaded into an

aluminum sample holder for randomly oriented powder 

X-ray diffraction (XRD) analysis (Moore and Reynolds, 

1989). Separation of the <l-(om fraction was obtained by 

coarsely grinding samples in a jaw crusher (approximately 2-mm chips), followed by ultrasonic disaggregation 

and centrifugation. The <l-|om size fraction was separated 

at 1,000 rpm for 8.2 minutes (Jackson, 1969). Sodium 

hexametaphosphate (0.1 molal solution) was used to disperse flocculated samples. Oriented samples were prepared 

by Millipore filtration and transferred to glass slides (Pollastro, 1982). The oriented clay samples were X-rayed 

both air-dried and ethylene glycol-saturated. Ethylene glycol solvation of the sample was achieved by exposure to 

ethylene glycol vapor for at least 8 hours at 60°C.

A total of 129 samples were analyzed for clay-mineral 

composition by XRD. Analyses were performed on an automated Siemens D500 diffractometer using CuKa radiation at 

30 milliamps and 40 kV. A curved graphite crystal monochrometer was used on the diffracted side of the beam path. 

Routine scans were run from 2° to 50° 20, at 2 seconds per 

step and 50 steps per degree. Selected samples were analyzed using much longer count times for detection and resolution of minor peaks.

CLAY-MINERAL ANALYSIS

In an ethylene glycol-saturated, oriented sample, pure 

(100 percent expandable) smectite is identified by a basal 

(001) spacing of 17A and a regular sequence of OO/ reflections in the XRD pattern. Irregular basal OO/ reflections are 

characteristic of interstratified clay minerals. Srodon (1980) 

developed XRD methods for precise identification of 

illite-smectite interstratification. We used the method of differential 2-theta (A20) to estimate expandability of the 

illite-smectite. In this method the precise measurement 

between the illite-smectite 001/002 peak (near 9°-10° 20) 

and 002/003 peak (near 17° 20) is used to estimate the 

expandability of a given illite-smectite phase. (By convention the illite-smectite 001/002 peak refers to the 

mixed-layer composite peak; for example, the 002 reflection 

of smectite migrates toward the 001 peak of illite as illitization proceeds.) Figure 4 shows the peak positions and A20 

measurements on an XRD pattern. Estimating the expandability using the A20 method has many advantages over single-peak position estimations. Because it is a differential 

measurement, it is relatively insensitive to goniometer alignment and sample placement problems (Moore and Reynolds, 

1989). Expandability estimations are also enhanced because 

these two peaks shift in different directions as illite composition changes.

The measured expandability of illite-smectite includes 

analytical error from domain size effects (Srodon, 1980), 

generally on the order of 5 percent but sometimes as high 

as 30 percent, for the method used for this study. In addition to analytical error, precise determination of the peak
C8 CENOZOIC SEDIMENTARY BASINS IN SOUTHWEST WASHINGTON AND NORTHWEST OREGON

positions needed to measure expandability (with 20 peaks 

near 10° and 17°) is affected by large amounts of discrete 

illite with peaks at 8.9° and 17.8°. When a large amount 

of discrete illite is present, the illite-smectite peaks appear 

as shoulders on the larger illite peaks.

One-dimensional XRD patterns were calculated for 

several expandabilities as an independent verification of 

expandabilities estimated with the A20 method. Several 

calculated patterns were generated for specified expandabilities and then compared to actual data. All calculated 

XRD patterns were modeled using the commercially available program NEWMOD (Reynolds, 1985).

Relative abundances of kaolinite and chlorite as a 

function of depth were calculated as ratios from the intensity of their respective peaks. Percentages were calculated 

by dividing the intensity of the kaolinite or chlorite XRD 

peak against the combined intensity for both minerals. The 

002 peak of kaolinite and the 004 peak of chlorite, both 

of which are near 25° 20, were measured on oriented 

mounts. Although this provides only a gross semiquantitative measure of abundance, it does provides a means of 

detecting changes in relative abundance of the two minerals with depth. The rank of clay-mineral abundances 

given in tables 1, 2, and 3 was based on gross peak intensities in oriented mounts of the <l-jom size fraction.

pie holders with epoxy and allowed to dry thoroughly. 

Samples were then sputter coated with gold for 2-4 minutes. Qualitative spot chemical analyses were obtained 

using a Tracer Northern 5500 energy-dispersive X-ray 

spectrometer.

PETROGRAPHIC THIN-SECTION 

EXAMINATION

Conventional thin sections of the sedimentary rocks 

were prepared using blue-epoxy impregnation for porosity 

determination and potassium feldspar stain. Point-count 

analysis (350 points minimum) of sedimentary rocks was 

used to identify and quantify any postdepositional authigenic phase that replaced detrital grains or pore space. 

Thin sections from all of the igneous bodies (prepared 

without feldspar stain) were inspected to determine (1) if 

they are intrusive or extrusive and (2) a reasonable intrusion temperature. Due to the very fine grained nature of 

the groundmass (and extensive alteration), classification 

of igneous rocks based on mineral abundances in thin sections could not be determined. Classification was based 

on major-element chemical analysis as proposed by 

LeBas and others (1986) for volcanic rocks.

X-RAY FLUORESCENCE SPECTROMETRY

Major-element chemical analysis was performed on 

all igneous rocks for purposes of classification and on 

selected sedimentary rocks in an attempt to detect metasomatism. Chemical analyses were carried out on an ARL 

Model 8420+ wavelength-dispersive X-ray fluorescence 

spectrometer. Samples were prepared as fused glass 

beads, using a method modified from Tertian and Claisse 

(1982). Finely ground samples were mixed with lithium 

tetraborate in a flux to sample ratio of 10:1. The mixture 

was fused over a gas flame at 1,200°C for 30 minutes, 

then poured into a 50-mm mold and slowly cooled for 

several hours. Samples were analyzed for silicon, aluminum, phosphorus, potassium, calcium, magnesium, 

sodium, titanium, manganese, and iron (total iron as 

F^Os) using conventional crystal and detector combinations. Each analysis consisted of averaging five repetitive 

counts of 10 seconds for each element. Loss on ignition 

was determined by heating samples to 900°C prior to glass 

bead preparation and measuring weight loss.

SCANNING ELECTRON MICROSCOPY

Clay textures and the paragenetic relationships among 

the clay minerals were examined using a Cambridge 

Stereoscan electron microscope at 20 kV accelerating 

voltage, with a LaBg crystal, operating at 3.1 nanoamps. 

Small chips (5-10 mm) were mounted on aluminum samVITRINITE REFLECTANCE

Vitrinite, a maceral derived from woody plant material, is common in coal and carbonaceous shale. Vitrinite 

reflectance is a measurement of the proportion of light 

reflected from a polished vitrinite grain. It is related to 

the degree of metamorphism of the vitrinite grain and can 

be directly converted to coal rank and petroleum generation.

One coal sample from core WC-83-6, three coal 

samples from core WC-83-17, and four coal samples 

from core WC-83-5 were prepared by crushing, mounting in epoxy on a microscope slide, planing off when 

hardened, and polishing. The mean vitrinite reflectance 

from randomly oriented indigenous vitrinite grains was 

determined using plane-polarized incident white light and 

a 546-nm monochromatic filter, in immersion oil, on a 

reflected light microscope with a nonrotating stage. Several measurements were made for each sample. The data 

were plotted on histograms for interpretation (but not 

included in this study).

RESULTS

PETROGRAPHY OF IGNEOUS ROCKS

The igneous rocks in the cores are composed predominantly of plagioclase feldspar that ranges in
THERMAL EFFECTS OF INTRUSIONS ON DIAGENETIC REACTIONS C9

composition from 45 to 65 percent anorthite (based on 

extinction angles and XRD). The larger phenocrysts of 

feldspar usually exhibit compositional zoning. 

Clinopyroxene and biotite are present in most samples in 

relatively low abundances.

Detailed mineralogical analysis of the igneous rocks 

in thin section was complicated by the presence of 

fine-grained matrix and abundant alteration. Two basic 

textures were used to separate rocks on the basis of intrusive or extrusive origin. Intrusive textures include fine- to 

medium-grained euhedral interlocking crystals. Extrusive 

textures include very few, fine-grained phenocrysts in a 

glassy devitrified matrix.

Alteration of the igneous rocks is pervasive in all but 

the largest intrusions. Carbonate and opaque oxide minerals replace phenocrysts and matrix. Chlorite replaces 

biotite in most samples, and chlorite replaces clinopyroxene in the most highly altered samples.

Precise petrographic identification of mineralogy 

was impossible because of the fine-grained nature of the 

intrusive rocks and the pervasive alteration. Classification, therefore, was based on chemical analyses (fig. 5). 

Although the pervasive alteration must have also had 

some affect on the chemical composition of these rocks 

(especially loss of the alkali metals), the chemical analyses provided a much more consistent classification than 

did petrographic identification. Based on total alkalis and 

silica content, a basaltic composition is indicated. Basaltic rocks can have intrusion temperatures as high as 

1,200°C (Best, 1982). Figure 6 and appendix table 2 indicate that the igneous rocks are either intrusive (sills) or 

extrusive (flows).

DIAGENESIS OF SEDIMENTARY ROCKS

Figure 7 shows the relationship between the percentages of replacement and authigenic minerals and proximity to the intrusive body for samples 60 through 65 from 

core WC-83-6. These samples were taken from a single 

continous sandstone bed adjacent to an intrusion to minimize differences in facies, detrital mineralogy, porosity 

and fluid chemistry. Point-count analysis of thin sections 

was used to identify and quantify any postdepositional 

authigenic phase that replaced detrital grains or pore 

space. Carbonate replacement of framework grains and 

authigenic growth in pore spaces has occurred near the 

intrusion. Authigenic carbonate increases near the intrusion (fig. 7), and carbonate abundance decreases sharply in 

contact with the intrusion. XRD analysis failed to document this trend because of the small percentage of carbonate minerals in each sample; however, wherever 

carbonate was detected in the core by XRD, the

composition of the carbonate varied considerably and 

included calcite, dolomite, and siderite.

CLAY-MINERAL COMPOSITION

Sample depths, hand-specimen descriptions, and mineral compositions of the <l-jom size fraction are presented 

in tables 1, 2, and 3 for cores WC-83-6, -5, and -17, 

respectively. In the cores containing igneous intrusions, 

the <l-|om fraction is dominated by illite-smectite that has 

variable expandability and ordering and also includes 

smaller amounts of kaolinite, chlorite, and mica. (In this 

study, any 10A phase is called mica; muscovite, biotite, 

and sericite are present in most thin sections.) The core 

containing no intrusions (WC-83-17) contains similar 

clay-mineral assemblages, although the expandability of 

the illite-smectite is consistently high and in most samples pure smectite is present with no interstratified illite. 

Although all three cores contain a wide range of lithologies, there is no apparent correlation between relative 

clay-mineral abundance and lithological variables such as 

rock type or grain size.

VARIATION IN ILLITE-SMECTITE 

EXPANDABILITY

Of primary importance in this study is the decrease in 

expandability of illite-smectite near intrusions. Figures 8 

and 9 show illite-smectite expandability as a function of 

depth and proximity to intrusions. Near the midpoints 

between the intrusions in core WC-83-6 (fig. 8), the 

illite-smectite expandability levels out at a minimum of 

about 35 percent (65 percent illite in illite-smectite). 

Immediately adjacent to the intrusions, the percentage of 

illite in illite-smectite increases by at least 5 percent and in 

some samples by as much as 25 percent. The texture of 

the illite-smectite ranges from thin flakes to feathery 

blades, indicative of authigenic formation (fig. 9). The 

illite-smectite in core WC-83-5 is about 30 percent 

expandable (70 percent illite in illite-smectite) between the 

intrusions and exhibits a pronounced decrease in 

expandability near the large intrusion at the base of the 

sampled interval (fig. 10). Adjacent to the intrusion, only a 

few smectite layers remain (98 percent illite in 

illite-smectite).

In addition to increasing the percentage of illite in 

illite-smectite near the intrusions, ordering of the 

illite-smectite also increases in the vicinity of the intrusions. 

In the few instances where large intrusions (>8 m thick) are 

present, expandability is 10 percent or less, and ordering 

increases to R=3. Core WC-83-17, which contains no
CIO CENOZOIC SEDIMENTARY BASINS IN SOUTHWEST WASHINGTON AND NORTHWEST OREGON

14 

45 50

ULTRABASIC, 

ULTRAMAFIC

BASIC, 

MAFIC

65 70 75 

ACID, FELSIC

55 60 

INTERMEDIATE

Si02, IN WEIGHT PERCENT

Figure 5. Igneous rock identification in terms of total alkalis and silica for samples in this study. Intrusive (x) and extrusive (F) determinations were made on the basis of petrographic observations. Sample information and analyses are given in appendix table 2. Classification 

modified from LeBas and others (1986).

intrusions, contains only very expandable illite-smectite: 

pure smectite is present throughout most of the core, 

although expandabilities decrease by 5-10 percent locally.

VARIATION IN OTHER CLAY MINERALS

Although the clay mineralogy appears to be independent of lithology, the abundances of kaolinite and chlorite 

vary with vertical distance from intrusions within each core. 

Although there is a significant amount of scatter in the data, 

kaolinite decreases in abundance immediately near the 

intrusions (fig. 11), and chlorite increases in abundance. 

Kaolinite is present primarily as replacement of feldspar, 

and large authigenic kaolinite books were observed in thin 

section and using scanning electron microscope (fig. 12).

Based on texture and distribution, kaolinite probably is an 

alteration product that formed before the intrusive event. 

Emplacement of the intrusions may have led to dissolution 

of kaolinite near the intrusions.

In contrast to kaolinite, the proportion of chlorite 

shows an opposite trend: chlorite increases in the vicinity 

of the intrusions and decreases at increasing distances. 

Both in thin section and using scanning electron microscope, chlorite replaces mafic grains such as biotite. Scanning electron microscopy of the chlorite shows an 

authigenic texture that resembles delicate corn flakes when 

replacing a biotite grain (fig. 13). The relationship between 

the intrusions and chlorite and kaolinite content is not consistent throughout all cores, and the distribution of chlorite 

is not as well defined as that of kaolinite. Detrital mica 

(muscovite and biotite) probably has a distribution similar 

to that of kaolinite most abundant away from the
THERMAL EFFECTS OF INTRUSIONS ON DIAGENETIC REACTIONS Cll

200

250

300

c/) 350

CC. 

LU1-

LU

Z

JE 400

D_ 

LU

D

450

500

550

WC-83-5

-

- ~

-

-

Flow I.

i. iw K

Flew _

_

-

~ -

Intrusion 1 "~

suu

600

700

800

900

1,000

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ILJJ

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X

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1,400

1,500

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600

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WC-83-6

FlowH

FlowG

Intrusion F

Flow E

Intrusion D

Intrusion C

Intrusion B

Flow A

1,200

1,300

1,400

1,500

1,600 -

1,700

1,800

1,900

2,000

Figure 6. Position of igneous rocks in cores WC-83-5 and WC-83-6. Intrusive and extrusive determinations 

were made based on petrographic observations. Labels as to type correspond to those in tables 1 and 2 and 

appendix tables 1 and 2.

intrusions and least abundant near the intrusions. Based on 

dissolution features and the manner in which they are 

draped over other grains, the mica flakes probably are predominantly detrital.

VARIATION IN VITRINITE REFLECTANCE

Mean vitrinite reflectance values and observed 

illite-smectite expandability from cores WC-83-6 and 

WC-83-5 are shown in figures 8 and 10. Table 4 lists mean

vitrinite reflectance values (V. Nuccio, written commun., 

1993) and core positions.

HEAT-FLOW CALCULATIONS

In order to evaluate whether the thin intrusions provide a reasonable heat source for the observed illitization, 

quantification of host-rock heating due to the intrusions is 

necessary. Modeling heat flow adjacent to a cooling
C12 CENOZOIC SEDIMENTARY BASINS IN SOUTHWEST WASHINGTON AND NORTHWEST OREGON

439 1

442 

445 

448 ' r-jrr :rr:_:::_60

TOTAL

AUTHIGENIC

MINERALS

CARBONATE 

REPLACEMENT

0 30

AUTHIGENIC

CLAY

| i i i i | 

64 69

ILLITE IN 

ILLITE-SMECTITE

Figure 7. Point-count data (in percent) showing amounts of replacement and authigenic phases in rocks above a sill in core WC-83-6. 

Percentages of illite layers in illite-smectite are also shown. Numbers refer to sample numbers in table 1.

intrusion yields a time-temperature profile (Jaeger, 1964) 

that can be used to predict, if combined with an appropriate reaction rate expression, the extent of the illitization 

reaction as a function of distance from the intrusion. Heating was calculated for only those igneous bodies interpreted to be intrusions (fig. 6). The equations of Jaeger 

(1964, 1968) were used to calculate temperature profiles 

adjacent to igneous intrusions during the cooling process. 

The equations for the most common intrusive geometries 

are provided by Jaeger; thus, it is possible to calculate the 

temperature profile between two parallel sills (see also 

Furlong and others, 1991). These basic conductive 

heat-flow calculations provide a time-dependent temperature profile around the intrusions.

In order to manage the mathematics of heat-flow calculations, several simplifying assumptions were made. (1) The 

thermal conductivities and diffusivities of the country rock 

and the intrusion are approximately the same. (2) Intrusion is 

assumed to take place instantaneously, with no preheating of 

country rock. (3) The heat released by crystallization of the 

magma cannot be ignored, but it can be handled easily and 

accurately by increasing the initial temperature of the

magma (see following). (4) All thermal properties of both 

sedimentary and igneous rocks are assumed to be independent of temperature. This latter assumption may introduce 

the most error, but the effect of increasing temperature on the 

thermal diffusivity and conductivity of rocks is poorly 

understood. The importance of several complicating factors 

was assessed by Jaeger (1968), who concluded that errors 

associated with these assumptions have the greatest affect on 

the temperature profile inside the intrusion. Outside the 

intrusion, the general equations hold tolerably well. All 

dimensions (intrusion thickness, time after emplacement, 

and distance from contact) and thermal properties (thermal 

diffusivity and conductivity) are expressed in the two parameters T and e (Lovering, 1935). Table 5 summarizes Jaeger's 

(1964, 1968) mathematical relationships and defines the 

terms used in calculating the heating effect of intrusions. The 

temperature at any given point away from an intrusive sheet 

under deep cover can be expressed as a function of the 

parameters e,T. Because time, thermal diffusivity, and position relative to the intrusion are combined in this function, 

the approximate temperature at any point is the initial temperature multiplied by this function.
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Continued.

Sample 

number

161

160

159

158

157

156

Intrusion I

Thickness of Percent illite 

Depth intrusion Lithology in Ordering Mineralogy of <l-um 

(feet) (feet) illite-smectite

1,903.5

1,906.5

1,910

1,910.7

1,912.2

1,913.8

1,955.2 >84

Siltstone and mudstone, gray-black, parallel-laminated

Siltstone and mudstone, gray-black, parallel-laminated

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Mudstone, dark-gray

Mudstone, dark-gray

Mudstone, dark-gray

Medium- to coarse-grained igneous rock, olive-black

90

95

95

98

98

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R3+(i;

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R3

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) Illite-smectite, kaolinite, chlorite

pyrite

Kaolinite, illite-smectite, chlorite

Kaolinite, siderite, quartz, chlori

Illite-smectite, kaolinite, chlorite

Illite-smectite, kaolinite, chlorite

Table 3. Lithological and clay mineralogical data for samples from core WC-83-17, sec. 13, T. 22 N., R. 6 E., King County, Washington.

[Location of core hole shown in figure 1. Ordering of mixed-layer illite-smectite is given in Reichweite notation; parentheses denote trace amount of phase indicate

given in order of abundance; illite-smectite is mixed-layer illite-smectite and mica is muscovite or illite]

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Mineralo

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122 635.5 Fine- to medium-grained sandstone, gray-white; some organic lamination 0

121 637 Fine- to medium-grained sandstone, gray-white; some organic lamination 0

120 730.7 Fine-grained sandstone and siltstone, parallel-laminated 7 RO

119 807 Very fine grained to fine-grained sandstone and siltstone, gray-white, massive 6 RO

118 840.4 Fine-grained sandstone and siltstone, parallel-laminated 0

117 879.6 Very fine grained to fine-grained sandstone and siltstone, brown, parallel-laminated, 0

	bioturbated

116 900.5 Carbonaceous mudstone 0

115 902 Medium-grained sandstone, gray-white, massive 0

114 905 Medium-grained sandstone, gray-white, massive 0

113 934.7 Fine-grained sandstone and siltstone, gray, parallel-laminated, bioturbated 0

112 998.2 Very fine grained to fine-grained sandstone and siltstone, gray, parallel-laminated <5 RO

111 1,033.7 Mudstone (ash?) 0

110 1,053.8 Mudstone (ash?) 0

109 1,092.6 Very fine grained to fine-grained sandstone and siltstone, gray, parallel-laminated, 18 RO

	bioturbated

108 1,112.4 Fine-grained sandstone and siltstone, gray-brown, massive, bioturbated 0

107 1,136.2 Fine-grained sandstone, gray, massive; organic fragments 0

106 1,146 Medium-to fine-grained sandstone, white, massive; near coal 0

105 1,205.8 Medium-grained sandstone and siltstone, white; organic lamination 0

104 1,234.2 Medium-grained sandstone and siltstone, white; organic lamination 0

Illite-smectite, kaol

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Smectite, kaolinite, 

Smectite, kaolinite

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o o

& &

ooovoo oo

 aa

a" a^ a" | | "o a" a' a"

' ' '

 aa

E 

on

ON oo ON ON ONONONON ONONONONOO
C20 CENOZOIC SEDIMENTARY BASINS IN SOUTHWEST WASHINGTON AND NORTHWEST OREGON

400

450

500

550

600

\ \

_R0= 0.86

1,200

1,300

1,400

1,500

1,600

1,700

1,800

1,900

2,000

10 20 30 40 50 60 70 80 90 100 

PERCENT ILLITE IN ILLITE-SMECTITE

Table 5. Mathematical relationships for heat-flow calculations.

T = Time-dependent temperature at any given point. 

T0 = Initial temperature of magma.

k= Thermal diffusivity of country rock. Values for

sedimentary rocks range from 0.004 to 0.012 cm2/sec and 

for igneous rocks from 0.004 to 0.012 cm2/sec; this study 

assumes 0.01 cm2/sec.

d = Half-thickness of the intrusion.

t= Time since intrusion.

x = Distance from midplane of intrusion.

T = Dimensionless time, where

-7

£ = Proportional distance from the intrusion, where

-3

I = Distance between the midplanes of the two intrusions.

Figure 8. Observed expandability of illite-smectite (diamonds) in 

relation to intrusive rocks (shaded areas), core WC-83-6. 

Illite-smectite expandabilities were calculated using the A29 peak 

positions described in figure 4. Vitrinite reflectance value (Rj,, in 

percent) is shown for carbonaceous material analyzed.

Table 4. Mean vitrinite reflectance values for samples from 

cores WC-83-5, WC-83-6, and WC-83-17, Washington. 

[Location of core holes shown in figure 1]

Core

WC-83-5

WC-83-5

WC-83-5

WC-83-5

WC-83-6

WC-83-17

WC-83-17

WC-83-17

Mean vitrinite reflectance 

Depth (percent)

548.64m (1,800 ft)

534.26m (1,720 ft)

440.74m (1,446 ft)

170.69 m (560 ft)

593.75m (1,948 ft)

509m (1,670 ft)

438.9 1m (1,440 ft)

273.41 m (897 ft)

1.69

1.34

1.09

0.96

0.86

0.35

0.38

0.25

Jaeger (1964) reduced the triple integral of heating by 

diffusion to the more manageable one-dimensional form,

T = r0 [<|>(e,T)]

where

(1)

(2)

Equations 1 and 2 can be combined to calculate the temperature at a specified distance from an intrusive sheet under 

deep cover,

,e +

(3)

The temperature at a point x between two intrusive bodies is simply the combined effect of the two heat sources,

L

Tn

^ I,) (4)

where (£I,TI) are dimensions and conductivities of the first 

sheet, (£2>T2) a*6 those of the second, and (d^) is the thickness 

of the second, defined as follows:

= (l-x)/d2

Kt_

d

2 '

(5)

(6)

Using the estimations or measurements for the variables listed in table 6, the time-dependent temperature (T) 

can be calculated for a specified distance or range of distances from the intrusion.

Uncertainty in the original temperature of the intrusion 

is potentially a source of error. The crystallization of minerals from a magma is an exothermic reaction releasing significant quantities of heat (Jaeger, 1964). This latent heat of 

crystallization increases the heat flow from the intrusions 

above that expected for an intrusion undergoing simple 

cooling without crystallization. The thermal effects of the
THERMAL EFFECTS OF INTRUSIONS ON DIAGENETIC REACTIONS C21

Figure 9. Scanning electron micrograph showing typical morphology of illite-smectite. Core 

WC-83-6, sample 19, depth 1,953 ft (595 m).

PERCENTAGE OF CHLORITE RELATIVE TO KAOLINITE 

100 90 80 70 60 50 40 30 20 10 0

200

250

300

CO

£ 350

LU

? 400

t

g 450

500

550

600

I 

- R0= 0.96

_

_

JS0= 1 .09

-R0=1.34

=H0= 1.69

i r ^

*

^

» t

%

| 

l-rtrt

 

~

 

* -

 

 X -|

^

 ^

<

***t>

^^^^^^^^fc , . . "=3==

* __

*

 

3UU

600 ^ * * ** 

400^ » 

700 ' t

* **

800

* * -

900 ^

1,200 yj ^

1,300 ^ H

LU ^

1,200 ^ z

' * *** *

'.*

Z ~ 500 I 

1,300 x- fE | ,,__* _, .,* *

I LU mtUtf1 """"" ""' "~ . . ^ 

1,400 gj Q f

D *

1,500

550 1 enn

-j 1.700

 1 1,800

' t/^1900 6°°

g^=55=^= ^T S!|!:=jln= '

l» ** *

*

* t *

I ' **

I . - *

I,^UU

1,300

1,400

1,500

LLJ 

LLJ 

U_

z

1,600 ~

X

t

LU

Q

1,700

1,800

1,900

?.noo

^BBMHmBKL- » 2 ooo ° 10 2° 30 40 50 60 70 " 80 9° 10°

0 10 20 30 40 50 60 70 80 90 100 

PERCENT ILLITE IN ILLITE-SMECTITE

Figure 10. Observed expandability of illite-smectite (diamonds) 

in relation to intrusive rocks (shaded areas), core WC-83-5. Illite-smectite expandabilities were calculated using the A29 peak 

positions described in figure 4. Vitrinite reflectance (Rg, in percent) 

values are shown for carbonaceous material analyzed.

PERCENTAGE OF KAOLINITE RELATIVE TO CHLORITE

Figure 11. Percentages of kaolinite and chlorite relative to each 

other in relation to intrusion position in core WC-83-6. Percentages 

were calculated by measuring peak intensities of both kaolinite and 

chlorite and calculating the ratio of individual peak intensity to the 

combined peak intensity.
C22 CENOZOIC SEDIMENTARY BASINS IN SOUTHWEST WASHINGTON AND NORTHWEST OREGON 

Table 6. Values for variables used in heat-flow calculations.

Independent variable

Thermal diffusivity (k)

Initial temperature (T0)

Time since intrusion (f)

Intrusion thickness (d)

Distance from intrusion (x)

Distance between intrusions (/)

Value

0.01 cm2/sec

900°C-1,200°C

0.01-100 years

2.6-9.75 m 

(8.5-32 ft)

0.03-57.3 m 

(0.1-188 ft)

32.3-63.4 m 

(106-208 ft)

Jaeger (1968).

Source

Estimates from igneous petrology.

Estimates based on initial calculations from intrusion thicknesses.

Measured in core.

Measured in core.

Measured in core.

latent heat of crystallization can be simulated by artificially 

raising the initial temperature of the intrusions. In addition, 

the presence of water in the melting process reduces the 

magma temperature. Depression of the dry melting temperature of basalt by water was probably offset by the temperature increase attributed to the latent heat of crystallization. 

In these samples it was impossible to determine a more precise estimation of water content using the petrographic 

descriptions, and thus it was difficult to estimate intrusion 

temperatures. A range of probable intrusion temperatures of 

900°C-1,200°C was used for the heat-flow calculations.

The calculations were performed on a Macintosh 

computer using spreadsheets developed in Microsoft 

Excel. Individual spreadsheets were used to calculate the 

temperature at various times and distances between intrusions of the core. Temperature distributions were calculated in log years (0.01, 0.1, 1, 10, and 100 years) 

because heat flow is time dependent. Heating beyond 100 

years was not calculated because cooling from intrusions 

of this thickness is essentially complete within 100 years. 

Temperature distributions are expressed as the ratio of the 

initial intrusion temperature to the temperature after each 

time increment. For graphing purposes this temperature 

ratio was converted to actual temperature based on 

assumed intrusion temperatures.

The temperature of the host rock near the intrusions 

was calculated for each sample and plotted in relation to 

depth (figs. 14-17). Figure 14 illustrates the calculated 

temperature distribution between two intrusions in core 

WC-83-6. The temperature distribution between all the 

parallel intrusions of core WC-83-6 is shown in figure 15. 

Calculating the temperature distribution related to the

intrusion in core WC-83-5 is somewhat less complex 

because there is only one intrusion and thus no additive 

heating from other intrusions. Although the calculations 

for the single intrusion of core WC-83-5 are less complex, the calculated temperature distribution has much 

more uncertainty because of the unknown thickness of the 

intrusion at the bottom of the core. Two temperature distributions were calculated. In one distribution (fig. 16), 

we assumed that the intrusion ends just slightly beyond 

the end of the core, making the intrusion 26 m (85 ft) 

thick; in the second (fig. 17), we assumed that the intrusion is 61 m (200 ft) thick.

DISCUSSION 

REACTION KINETICS

Prior to reaching equilibrium, the rate at which a chemical reaction proceeds can be expressed in terms of temperature, time, and concentration of reactants. Based on the 

hard-shell theory of reaction kinetics, a fundamental chemical reaction requires the collision of two molecules, and the 

reaction rate must be proportional to the frequency of collisions (Laidler and Meiser, 1982). The frequency factor is a 

measure of the number of times per second that atoms are 

close enough to react. For a reaction to occur between stable 

molecules, a certain amount of energy, termed "activation 

energy," must be absorbed to weaken the bonds holding the 

reactant molecules together. Complex mineral reactions 

such as the illitization reaction consist of numerous fundamental chemical reactions proceeding along parallel and (or) 

sequential paths. Nevertheless, the overall reaction can be
THERMAL EFFECTS OF INTRUSIONS ON DIAGENETIC REACTIONS C23

Figure 12. Scanning electron micrograph of kaolinite. Core WC-83-6, sample 79, depth 

1,283 ft (391 m).

Figure 13. Scanning electron micrograph of chlorite replacing biotite. Core WC-83-6, 

sample 73, depth 1,318.8 ft (402 m).
C24 CENOZOIC SEDIMENTARY BASINS IN SOUTHWEST WASHINGTON AND NORTHWEST OREGON

o

o 

o 

cc

If) 

O

I

cc

13

< 

CC

500

450

400

cc 

o

10 15 20 25 

DISTANCE, IN METERS

30 35

Figure 14. Calculated temperature between two intrusions (shaded) 6 m and 3 m (19.5 and 10 ft) thick at various times after emplacement. Intrusion temperature is assumed to be 1,000°C. 

Midpoints of the two intrusions are 32.3 m (106 ft) apart.

400

450

.- 500 cr

550

600

1,200

 1,300

1,400

1,500

1,600

= 1,700

1,800

1,900

2,000

0 100 200 300 400 500 

INCREASE IN TEMPERATURE OF HOST ROCK, IN °C

Figure 15. Calculated temperature between intrusions (shaded) 

in core WC-83-6 at various times after the simultaneous emplacement of the intrusions. Intrusion temperature is assumed to be 

1,000°C.

200 -

100 200 300 400 

INCREASE IN TEMPERATURE OF HOST ROCK, °C

2,000

500

Figure 16. Calculated temperature at various times after emplacement of a 26-m-thick (85 ft) intrusion (shaded) in core 

WC-83-5. Intrusion temperature is assumed to be 1,000°C. A 

26-m-thick intrusion assumes that the intrusion terminates just beyond the bottom of the retrieved drill core.

200

250 

300 -

350 -

400

450 -

500 -

550

600

0 100 200 300 400

INCREASE IN TEMPERATURE OF HOST ROCK, IN °C

Figure 17. Calculated temperature at incremental times after 

emplacement of a 61-m-tbick (200 ft) intrusion (shaded) in core 

WC-83-5. Intrusion temperature is assumed to be 1,000°C. A 

61-m-thick intrusion assumes a much thicker intrusion than that 

shown in figure 16, one that does not terminate just beyond the bottom of the retrieved drill core but has a similar sheetlike geometry.
THERMAL EFFECTS OF INTRUSIONS ON DIAGENETIC REACTIONS C25

expressed in terms of a single simplified kinetic expression 

using a single activation energy and a single pre-exponential 

function.

For example, the reaction of smectite to illite, expressed 

as a first-order reaction, would obey the equation,

dS r-= kS, (7)

where

S0 = Initial percentage of smectite layers

k - First-order rate constant (reciprocal units of 

time)

t = Time

dS = Incremental change in smectite content

dt = Incremental change in time.

The effect of temperature on the rate constant (k) is 

expressed in the Arrhenius equation,

k= Ae

u_ ~RT

(8)

where

A = Pre-exponential function (reciprocal units of 

time), also called the frequency factor

U = Activation energy (kJ/mole)

R = Gas constant = 1.987 cal/K mol or 8.314 J/K mol

T = Temperature (Kelvin).

The Arrhenius equation may be substituted into the 

first-order rate law,

dS

u_ ~RT

(9)

Equation 9 relates the progress of a reaction, with 

respect to time, at a given temperature. In this case dS/dt is 

the change in the number of smectite layers in illite-smectite 

for a given change in time.

This rather simplistic treatment of illitization reaction 

kinetics is complicated by the fact that a number of researchers have proposed widely divergent values for reaction 

order, activation energy, and pre-exponential function (table 

7). These differences arise from the fact that the reaction 

involves a chemically diverse and imperfectly characterized 

group of minerals reacting under a variety of temperature-time-chemistry conditions.

FACTORS AFFECTING ILLITIZATION 

REACTION RATES

In general, fluid-rock interactions affect chemical and 

mineralogical reactions in country rock near intrusions (Kerrick, 1991). Within such an alteration zone, the availability 

of potassium and chemistry of the fluids can profoundly 

affect the rate and extent of illitization (Eberl and Hower, 

1977; Roberson and Lahann, 1981; Whitney, 1992). Huang 

and others (1993) suggested that, although magnesium

significantly retards the illitization reaction rate, calcium 

may not retard the reaction rate as much as previously 

thought, and sodium retards the reaction rate only in high 

concentrations. Additionally, the chemical composition of 

the starting smectite affects illitization reaction rates because 

in hydrothermal environments trioctahedral (magnesium-rich) smectite and iron-rich smectite react to illite 

much more slowly then does dioctahedral aluminous smectite (Eberl and others, 1978; Whitney, 1983; Chang and others, 1986; Couto Anjos, 1986). The activity of water in the 

system is also an important factor in illitization (Whitney, 

1990). In fluid-deficient systems (where pore spaces are not 

saturated), the rate and extent of illitization may be significantly inhibited. Even differences in texture may affect the 

illitization reaction (Whitney and Velde, 1993). In comparison of massive and laminated shales, Howard (1987) documented increased illitization in laminated shale that he 

interpreted to be a result of the laminations acting as fluid 

conduits.

Thus, the lithology, chemical composition, and texture 

of the rocks into which igneous sills are intruded can influence solution chemistry, water availability, and starting mineralogy, and these variables in turn affect illitization rates 

and contribute to scatter in the observed data. The 

lithological inhomogeneity of the rocks in this study results 

from varying sedimentary facies associated with the 

fluvial-deltaic depositional environment. In this setting, 

small-scale variations in bulk chemistry, potassium availability, organic matter, and fluid flow are expected. Each of 

these small-scale variations can locally affect the illitization 

reaction rate and contribute to the observed scatter in the 

data. Despite these uncertainties, it is possible to model the 

extent of the illitization reaction in response to heating 

caused by an igneous intrusion in order to test whether the 

heat from the intrusions accounts for the patterns of illitization observed in the core.

CALCULATIONS OF THE EXTENT OF THE 

ILLITIZATION REACTION

As mentioned above, different researchers have 

reached different conclusions regarding the reaction kinetics 

of the illitization reaction. Table 7 shows the range of values 

for the kinetic expressions for the illitization reaction proposed by different authors. Some workers believe that the 

overall reaction of smectite to illite obeys a first-order rate 

law with respect to the expandability (percent smectite layers) of the illite-smectite (Eberl and Hower, 1976; McCubbin and Patton, 1981), whereas other researchers have 

suggested a higher order rate law for the reaction (Pytte, 

1982; Whitney and Northrop, 1988; Huang, 1992). The 

kinetic formulation for illitization used in the present study 

is based on reaction rates calculated from burial in natural 

sedimentary basins by Velde and Vasseur (1992). Velde and 

Vasseur' s model uses sequential first-order kinetic formulations that divide the reaction into two stages, the first stage
C26 CENOZOIC SEDIMENTARY BASINS IN SOUTHWEST WASHINGTON AND NORTHWEST OREGON

Table 7. Published kinetic values used in the Arrhenius equation for activation energy and pre-exponential function during illitization. 

[Arrhenius equation is given in text]

Researcher

Velde and Vasseur (1992)

Eberl and Hower (1976)

Howard and Roy (1985)

Pytte (1982)

Huang and others (1991, 1993)

Roberson and Lahann (1981)

Activation energy U (kJ/mol)

R0=69.7

Rl=37.4

82

12.5-14.5

138

117.2

125.6

Setting

Burial

Laboratory

Laboratory

Contact metamorphism

Laboratory

Laboratory

Reaction order

1

1

1

1

6

3

1

Pre-exponential function)

RO=e2L5/Ma

Rl=e7-2/Ma

en- 8/day

en- 8/day

e49/second

en-3/second

e22-5/day

involving the formation of disordered R=0 illite-smectite 

and the second stage the formation of ordered R=l 

illite-smectite. The two-stage reaction is based on an empirical analysis of illitization in several basins having different 

ages, rates of subsidence, and geothermal gradients. This 

two-stage kinetic model gives a relatively good estimation of 

illite-smectite expandability for a wide variety of sedimentary conditions. Separate rate constants are provided for each 

stage of the sequential formation of the R=0 and R=l phases, 

which combine to produce the total reaction extent. Using 

this kinetic approach, the final expandability can be calculated based on specific time (age) and temperature (maximum depth of burial and the geothermal gradient) 

conditions.

We attempted to simulate the observed illitization reaction profiles in the intruded sedimentary rocks by combining 

the thermal profiles calculated using the equations of Jaeger 

(1964) and the kinetic expressions developed by Velde and 

Vasseur (1992). As a first attempt, the calculations were 

made using only heat from the intrusions and no other heat 

sources. Assuming pure smectite as a starting material, 

kinetic variables (activation energy and pre-exponential 

function) can be selected such that the calculated illitization 

matches the measured maximum illitization adjacent to the 

intrusions but does not accurately simulate the expandability 

of illite-smectite farther away from the intrusions; even at 

small distances from the intrusions the reaction would not 

proceed at all (no illite in illite-smectite) (fig. 18A). Conversely, if kinetic variables are selected that match the 

observed illitization midway between the intrusions, the 

reaction would proceed to completion in huge haloes extending far from the intrusions (100 percent illite in illite-smectite) (fig. 185). The only way to make the calculated reaction 

extent profiles match the observed data exactly, using only 

heat from the intrusions, is to choose unique activation energies and pre-exponential functions for each sample, which is 

chemically unreasonable.

A geologically more reasonable approach to matching 

the observed expandability profile is to assume a starting 

mineralogy consisting of illite-smectite containing about 

50-70 percent illite layers. There are two possible ways to

get this starting material: (1) detrital illite-smectite may have 

been initially 50-70 percent illite in illite-smectite, or (2) the 

starting material was initially 100 percent expandable, and 

another source of heat (burial diagenesis?) drove the illitization reaction to about 50-70 percent illite prior to emplacement of the intrusions. Scanning electron microscopy of 

samples far away from the intrusions shows delicate 

illite-smectite textures that indicate authigenic formation, 

not detrital deposition (fig. 9). This textural evidence suggests that the sediments were affected by heating due to 

burial in addition to heating by the intercalation of the intrusions. Thus, to model realistically the combined extent of 

illitization in the basin, an estimate of illitization caused by 

burial must be included.

The extent of illitization produced by burial can be calculated using estimated depths of burial and geothermal gradients. Stratigraphic reconstruction of measured field 

sections suggests a minimum depth of burial of 3 km (Fisher, 

1957). A slightly elevated geothermal gradient of about 

35°C/km is based on organic thermal maturity profiles 

(Walsh and Lingley, 1991). The extent of the illitization 

reaction produced by heat derived from burial constitutes a 

baseline for calculating the extent of the reaction resulting 

from heating by the intrusions. The straight lines in figures 

19-22 reflect the calculated extent of illitization in core 

WC-83-6, based on 3.3 km of burial and a geothermal gradient of 35°C/km. The straight lines in figures 23-30 show 

the calculated extent of illitization in core WC-83-5, based 

on a similar geothermal gradient and slightly deeper (4.1 km) 

burial. All estimates of burial diagenesis assume a 

depositional age of 40 million years for these Eocene sedimentary rocks. Calculations used to estimate the expected 

extent of illitization resulting from burial are based on published values of rate parameters for burial environments 

(Velde and Vasseur, 1992). The first stage of the reaction 

(R=0) is modeled using an activation energy of 69.7 kJ/mol 

and a pre-exponential factor of e21-5/Ma. The second stage of 

the reaction (R=l) is modeled using an activation energy of 

37.4 kJ/mol and a pre-exponential factor of e7 - 2/Ma.

The increase in illitization caused by heat from intrusion was estimated using the temperature-time distributions
THERMAL EFFECTS OF INTRUSIONS ON DIAGENETIC REACTIONS

calculated in the previous section. The kinetic model used for 

burial was also used to calculate the increase in illitization due 

solely to intrusions. Activation energies employed to calculate reaction extent were identical to those used for burial calculations, R0=69.7 kJ/mol and Rl=37.4 kJ/mol (Velde and 

Vasseur, 1992). Figures 19-30 show the calculated total reaction extent due to the combined effect of heat provided by 

burial and by intrusion for a range of intrusion temperatures 

from 900°C to 1,200°C. Because the true thickness of the 

intrusion in core WC-83-5 is unknown, figures 23-30 show 

illitization for a temperature range of 900°C-1,200°C for two

C/3

rr

LLJ 

LLJ

t

LJU 

Q

5

10

15

20

25

30

35

A

DC

LJU 

Q

5

10

15

20

25

30

35

B

60 I

Q_

80 g

100

0

20

h40 [Jj

LL

60 ^

X 

80 t

LLJ 

Q

100

0 10 20 30 40 50 60 70 80 90 100 

PERCENT ILLITE IN ILLITE-SMECTITE

Figure 18. Calculated illitization reaction extent using only heat 

provided by the intrusions (shaded). Using only the heat provided 

by the intrusions in a single reaction model, the illitization reaction 

can be modeled either (A) to fit the observed illitization near the intrusions or (B) to fit the observed illitization at the midpoint between the intrusions, but not both.

Figure 20 (facing column). Comparison of calculated and observed percent illite in illite-smectite for core WC-83-6 using 

1,000°C intrusions (shaded) for calculations. Illitization from burial 

only assumes burial depth of 3 km, geothermal gradient of 

35°C/km, and depositional age of 40 million years.

400 

450

500 

550

600

Observed percent illite 

in illite-smectite

1,900

2,000

0 10 20 30 40 50 60 70 80 90 100 

PERCENT ILLITE IN ILLITE-SMECTITE

Figure 19. Comparison of calculated and observed percent illite 

in illite-smectite for core WC-83-6, using 900°C intrusions (shaded) for calculations. Illitization from burial only assumes burial of 

3 km, geothermal gradient of 35°C/km, and depositional age of 40 

million years.

EXPLANATION

Illitization from burial *

and intrusions 

Illitization from burial o 

Observed percent illite

1,200

1,300

1,400

1,500

1,600

= 1,700

1,800

1,900

600

2,000

0 10 20 30 40 50 60 70 80 90 100 

PERCENT ILLITE IN ILLITE-SMECTITE
C28 CENOZOIC SEDIMENTARY BASINS IN SOUTHWEST WASHINGTON AND NORTHWEST OREGON

400

450

500

550

600 -

Utilization from burial 

and intrusions 

Utilization from burial only

1,200

 1,300

- 1,400

1,500

1,600

1,700

1,800

1,900

2,000

0 10 20 30 40 50 60 70 80 90 100 

PERCENT ILLITE IN ILLITE-SMECTITE

Figure 21. Comparison of calculated and observed percent illite in illite-smectite for core WC-83-6 using 1,100°C intrusions 

(shaded) for calculations. Illitization from burial of 3 km, geothermal gradient of 35°C/km, and depositional age of 40 million years.

400 V

EXPLANATION

Illitization from burial 

and intrusions 

Illitization from burial only 

500 » Observed percent illite

550

600

1,200

1,300

1,400

1,500

1,900

2,000

0 10 20 30 40 50 60 70 80 90 100 

PERCENT ILLITE IN ILLITE-SMECTITE

Figure 22. Comparison of calculated and observed percent illite 

in illite-smectite for core WC-83-6 using 1,200°C intrusions 

(shaded) for calculations. Illitization from burial only assumes burial depth of 3 km, geothermal gradient of 35°C/km, and depositional 

age of 40 million years.

thicknesses. Superimposed on the calculated curves are the 

observed values o f illitization (from tables 1 and 2) of core 

samples.

Although the same activation energy values were used 

in both burial and intrusive heating scenarios, satisfactory 

curves could not be calculated unless the pre-exponential 

function was changed. According to these calculations, the 

illitization reaction would not proceed adjacent to the intrusions if pre-exponential functions used for the burial setting 

were applied to the intrusive heating situation. The 

pre-exponential value used for burial diagenesis is based on 

low temperatures and long times. A higher value is appropriate for contact metamorphism during which temperatures 

are much higher for shorter periods (Lasaga, 1981). Therefore, pre-exponential factors were adjusted on a trial and 

error basis until observed illite-smectite expandabilities 

were matched by the calculated curves. The pre-exponential 

factor for the second (R=l) stage of the kinetic calculation 

was adjusted upward from 4.24x10~n/sec to 2.12xlO~6/sec 

for the multiple intrusions of core WC-83-6 and to 

2.12xl(T7 for core WC-83-5. These pre-exponential factors are about halfway between the values used in the burial 

model above and experimentally determined estimates for 

short-duration heating events of about 1.5 per second (Eberl 

and Hower, 1976). Because the pre-exponential factor is a

temperature-dependent function based on molecular collision frequency, the higher values of pre-exponential factor 

chosen for these calculations are justified (Lasaga, 1981). 

The first stage (R=0) of the reaction was not adjusted similarly because that part of the reaction had mostly gone to 

completion as a result of burial alone. Table 8 summarizes 

the kinetic values used to calculate expected illitization 

reaction extent.

The calculated models of reaction extent for the two 

intruded cores, including both burial and intrusive heat 

(figs. 19-30), fit the observed data reasonably well. In the 

case of four parallel intrusions (core WC-83-6), the calculated expandabilities compare well with the observed data 

especially at higher intrusive temperatures (figs. 21, 22, 

1,100°C-1,200°C) and below a depth of 457.2 m (1,500 

ft). The simulations match the data much better for the 

section of the core between the intrusions at a depth of 

457.2-579.1 m (1,500-1,900 ft) than for the shallower 

part of the well, for which the observed illitization is significantly less than the predictions. The reason for these 

unexpectedly high expandabilities at about 396.2 m 

(1,300 ft) is not clear. It may be possible to explain this 

anomaly using something as simple as a change in sediment source. Alternately, very thin extrusive bodies 

(labeled as flows in fig. 6) may have been releasing
THERMAL EFFECTS OF INTRUSIONS ON DIAGENETIC REACTIONS 

Table 8. Kinetic values used for activation energy and pre-exponential function to calculate illitization.

C29

Setting

Burial (Velde and Vasseur, 1992)

Intrusions (core WC-83-5)

Intrusions (core WC-83-6)

Activation energy U (kJ/mol) Reaction order

R0=69.7 1 

Rl=37.4 1

R0=69.7 1 

Rl=37.4 1

R0=69.7 1 

Rl=37.4 1

Pre-exponential function A

R0=e215/Ma 

Rl=e7- 2/Ma

RO=e21 -5/Ma 

Rl=e18/Ma

RO=e21 -5/Ma 

Rl=e15-7/Ma

magnesium, iron, and calcium from unstable mafic grains 

and locally saturating the surrounding sediments and 

inhibiting the burial illitization reaction. The only other 

very highly expandable sample (sample 57, 458.4 m 

(1,504 ft)) in this core also is within I m of another flow.

The large intrusion at the bottom of core WC-83-5 is 

modeled best by a 61-m-thick (200 ft), 1,000°C intrusion 

(fig. 28). The anomalously expandable samples at about 

579.1 m (1,900 ft) in this core are more difficult to explain 

because they are closer to the thick intrusion and there are no 

extrusive rocks nearby, but they may also reflect a change in 

sediment source. Alternatively, if they represent a continuous sediment source, their anomalous expandability may be 

an indicator of the timing of the intrusion and the burial. If 

burial preceded intrusion, the observed illitization would be 

relatively smooth and continuous (as are the dark lines in 

figs. 19-30) and would serve as a starting point for illitization due to intrusions (the thin lines in figs. 19-30). If, however, intrusion preceded burial (or occurred after very little 

burial), illitization would proceed rapidly and almost completely in a narrow zone near the intrusion. Highly expandable illite-smectite (or pure smectite) would remain stable at 

some distance from the intrusion (similar to fig. ISA). If fluids rich in sodium, calcium, magnesium, and iron were 

released from the intrusion as it cooled, the fluids would 

inhibit any subsequent illitization. Although this hypothesis 

cannot be proven by this study, such a scenario would produce an amount of illitization less than that used as a baseline 

in an area adjacent to an intrusion where the model otherwise 

predicts illitization well above the baseline.

CONDUCTION, DIFFUSION, AND 

ADVECTION NEAR INTRUSIONS

Near intrusions, heat may be transferred by conduction, or it may accompany the advection of fluids (Ferry 

and Dipple, 1991). In the case of heat transfer, the model 

used in this study (Jaeger, 1968) considers host-rock heating resulting purely from conduction. Convective fluid flow 

near igneous intrusions may redistribute the heat in the 

country rock but does little to affect the rate of heat transfer

from the intrusion (Furlong and others, 1991). Thus, if fluid 

movement carried heat away from the intrusion, the temperature gradients might not be as steep as indicated in figures 14-17, although the duration of heating would be 

similar. Stable-isotope techniques have been used to study 

conduction-dominated contact metamorphic systems 

around large plutons (Sheih and Taylor, 1969), and unless 

there is strong evidence to suggest otherwise it is best to 

assume that conductive heat transfer dominates (Furlong 

and others, 1991). In the present study, the narrow thermal 

alteration zone adjacent to the intrusions and the lack of 

textural evidence for fluid movement in the sediments near 

the intrusion suggest that advective effects were minimal.

PARAGENESIS OF OTHER MINERALS

The formation of authigenic carbonate minerals (calcite, dolomite, and siderite) in rocks near the intrusions 

implies that calcium, magnesium, and iron were mobile, at 

least over short distances. Whether these ions came from fluids released by the cooling intrusions or from dissolution of 

framework grains is unclear, but their presence may have 

locally inhibited the illitization reaction as described previously. Additionally, the presence of authigenic albite in the 

<l-(om size fraction indicates that sodium is abundant 

throughout most of the core. In contact metamorphic environments containing abundant sodium, the conversion of 

mafic minerals to chlorite is very common (Barton and others, 1992). Petrographic data show that chlorite formed predominantly as replacement of mafic minerals, not as 

pervasive pore filling or grain coatings. Formation of chlorite may have predated the formation of illite-smectite, or 

chlorite may have precipitated contemporaneously with the 

illite-smectite during intrusive heating. The general increase 

in chlorite abundance near the intrusions suggests that it is a 

byproduct of the heating from the intrusions. Likewise, the 

depletion of kaolinite near the intrusions suggests that 

kaolinite predated the intrusions and was destroyed during 

the contact metamorphic processes when the intrusions were 

emplaced.
C30 CENOZOIC SEDIMENTARY BASINS IN SOUTHWEST WASHINGTON AND NORTHWEST OREGON

200

250

300

350

400

t

450

500

550

600

Illitization from burial 

and intrusion

Illitization from burial only

Observed percent illite 

in illite-smectite

500

600

700

800

900

1,000

1,100

1,200

LL

1,300

1,400

1,500

1,600

1,700

2,000

60 70 80 90 100 

PERCENT ILLITE IN ILLITE-SMECTITE

EXPLANATION

200 -

250 -

300 -

350 -

" 400

450 -

500 -

550 -

500

- 600

- 700

600

50 60 70 80 90

PERCENT ILLITE IN ILLITE-SMECTITE

EXPLANATION

2,000

100

Illitization from burial 

and intrusion

Illitization from burial only

Observed percent illite 

in illite-smectite

Figure 23. Comparison of calculated and observed percent illite 

in illite-smectite for core WC-83-5 using a 26-m-thick (85 ft), 

900°C intrusion (shaded) for calculations. Illitization from burial 

only assumes burial depth of 4.1 km, geothermal gradient of 

35°C/km, and depositional age of 40 million years.

Figure 24. Comparison of calculated and observed percent illite 

in illite-smectite for core WC-83-5 using a 26-m-thick (85 ft), 

1000°C intrusion (shaded) for calculations. Illitization from burial 

only assumes burial depth of 4.1 km, geothermal gradient of 

35°C/km, and depositional age of 40 million years.
THERMAL EFFECTS OF INTRUSIONS ON DIAGENETIC REACTIONS 

500

C31

200 

- 600

- 700

 1,800

1,900

2,000

100

PERCENT ILLITE IN ILLITE-SMECTITE 

EXPLANATION

Utilization from burial 

and intrusion

Utilization from burial only

Observed percent illite 

in illite-smectite

Figure 25. Comparison of calculated and observed percent illite 

in illite-smectite for core WC-83-5 using a 26-m-thick (85 ft), 

1,100°C intrusion (shaded) for calculations. Illitization from burial 

only assumes burial depth of 4.1 km, geothermal gradient of 

35°C/km, and depositional age of 40 million years.

200 -

250 -

300

350

x~ 400

t

450

500

550

600

Utt

50 60 70 80 90

PERCENT ILLITE IN ILLITE-SMECTITE

EXPLANATION

500

600

700

800

900

1,000

1,100

1,200

LL

1,300

1,400

1,500

1,600

1,700

1,800

1,900

2,000

100

Utilization from burial 

and intrusion

Utilization from burial only

Observed percent illite 

in illite-smectite

Figure 26. Comparison of calculated and observed percent illite 

in illite-smectite for core WC-83-5 using a 26-m-thick (85 ft), 

1,200°C intrusion (shaded) for calculations. Illitization from burial 

only assumes burial depth of 4.1 km, geothermal gradient of 

35°C/km, and depositional age of 40 million years.
C32 CENOZOIC SEDIMENTARY BASINS IN SOUTHWEST WASHINGTON AND NORTHWEST OREGON

200 -

250 -

300 -

350 -

400 -

450 -

500 -

550 -

600

500

- 600

PERCENT ILLITE IN ILLITE-SMECTITE 

EXPLANATION

Illitization from burial 

and intrusion

Illitization from burial only

Observed percent illite 

in illite-smectite

200 -

250 -

300 -

350 -

400 -

450 -

500 -

550 -

600

500

- 600

2,000

_______ _ 2,100 

50 60 70 80 90 100 

PERCENT ILLITE IN ILLITE-SMECTITE

EXPLANATION

 Illitization from burial 

and intrusion

 ^^ Illitization from burial only

^ Observed percent illite 

in illite-smectite

Figure 27. Comparison of calculated and observed percent illite 

in illite-smectite for core WC-83-5 using a 61-m-thick (200 ft), 

900°C intrusion (shaded) for calculations. Illitization from burial 

only assumes burial depth of 4.1 km, geothermal gradient of 

35°C/km, and depositional age of 40 million years.

Figure 28. Comparison of calculated and observed percent illite 

in illite-smectite for core WC-83-5 using a 61-m-thick (200 ft), 

1,000°C intrusion (shaded) for calculations. Illitization from burial 

only assumes burial depth of 4.1 km, geothermal gradient of 

35°C/km, and depositional age of 40 million years.
THERMAL EFFECTS OF INTRUSIONS ON DIAGENETIC REACTIONS C33

500

- 600

200 -

250 -

300 -

350 -

400 -

450 -

500 -

550 -

600

500

50 60 70 100

PERCENT ILLITE IN ILLITE-SMECTITE

EXPLANATION

 Utilization from burial 

and intrusion

^~~~~~ Utilization from burial only

^ Observed percent iltite 

in illite-smectite

Figure 29. Comparison of calculated and observed percent illite 

in illite-smectite for core WC-83-5 using a 61-m-thick (200 ft), 

1,100°C intrusion (shaded) for calculations. Illitization from burial 

only assumes burial depth of 4.1 km, geothermal gradient of 

35°C/km, and depositional age of 40 million years.

200 -

250 -

300 -

350 -

400 -

450

500 -

550 -

600

2,100

50 60 70 80 90 100 

PERCENT ILLITE IN ILLITE-SMECTITE

EXPLANATION

 Utilization from burial 

and intrusion

 Illitization from burial only

^ Observed percent illite 

in illite-smectite

Figure 30. Comparison of calculated and observed percent illite 

in illite-smectite for core WC-83-5 using a 61-m-thick (200 ft), 

1,200°C intrusion (shaded) for calculations. Illitization from burial 

only assumes burial depth of 4.1 km, geothermal gradient of 

35°C/km, and depositional age of 40 million years.
C34 CENOZOIC SEDIMENTARY BASINS IN SOUTHWEST WASHINGTON AND NORTHWEST OREGON

SUMMARY AND CONCLUSIONS

Thin igneous intrusions locally affected the thermal 

maturity of sedimentary rocks within a lithologically heterogeneous sedimentary basin in southwestern Washington. 

The expandability of illite-smectite decreases sharply in a 

narrow zone adjacent to each intrusion, whereas the abundance of kaolinite decreases as the abundance of chlorite 

increases and the amount of carbonate replacement increases 

near the intrusions. Alteration zones associated with the thin 

sills are very narrow and resulted from short-lived thermal 

pulses from the intrusions. Illite-smectite from two cores that 

contain intrusive bodies exhibits a greater extent of reaction 

throughout the core interval sampled (mean value for both 

cores is about 60 percent illite in illite-smectite, R=l) compared to illite-smectite from the core containing no intrusions (almost fully expandable; -0-10 percent illite in 

illite-smectite, R=0).

Conductive modeling of the heat flow produced by the 

intrusions predicts short-lived and locally very intense heating. Sediments cooled to pre-intrusion temperatures only a 

few hundred years after the injection of these thin intrusions. 

Temperatures remained high enough to drive the kinetically 

controlled illitization reaction to a significant degree for only 

about 100 years. In the case of thin intrusions, the size of the 

illitization aureole was only slightly affected by variations in 

the intrusion temperature. Kinetic modeling suggests that 

substantial illitization (=10-25 percent reduction in expandability) progressed in the host rock for a distance approximately equal to the thickness of the intrusion, matching the 

observed data tolerably well.

Finally, the regional pattern of thermal maturity 

reflected in coal grade throughout western Washington has 

probably been little affected by igneous intrusions. Even 

though a coal bed intruded by a sill would certainly exhibit 

strong thermal effects, the thermal maturity trends observed 

in coal on a regional scale probably resulted from thermal 

processes more widespread than the local heating produced 

by the thin igneous intrusions described herein.

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natural gas potential of western and central Washington: Washington Division of Geology and Earth Resources Open-File 

Report 91-2, 26 p.

Walsh, T.J., and Phillips, W.M., 1983, Rank of Eocene coals in 

western Washington State A reflection of Cascade Plutonism?: Washington Division of Geology and Earth Resources Open-File Report 83-16, 21 p.

Waples, D.W., 1985, Geochemistry in petroleum exploration: Boston, International Human Resources Development Corporation, p. 217.

Weaver, C.E., and Beck, K.C., 1971, Clay water diagenesis during 

burial How mud becomes gneiss: Geological Society of 

America Special Paper 134, p. 96.

Wells, R.E., Engebretson, D.C., Snavely, P.O., Jr., and Coe, R.S., 

1984, Cenozoic plate motions and the volcano-tectonic evolution of western Oregon and Washington: Tectonics, v. 3, p. 

275-294.

Wells, R.E., and Heller P.L., 1988, The relative contribution of 

accretion, shear, and extension to Cenozoic tectonic rotation in 

the Pacific Northwest: Geological Society of America Bulletin, 

v. 100, p. 325-338.

Whitney, G., 1983, Hydrothermal reactivity of saponite: Clays and 

Clay Minerals, v. 31, p. 1-8.

 1990, Role of water in the smectite to illite reaction: Clays 

and Clay Minerals, v. 38, p. 343-350.

 1992, Dioctahedral smectite reactions at elevated temperatures Effects of K-availability, Na/K ratio and ionic strength:

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Whitney, G., and Northrop, H.R., 1988, Experimental investigation

of the smectite to illite reaction Dual reaction mechanisms

and oxygen-isotope systematics: American Mineralogist, v. 73,

p. 77-90. 

Whitney, G., and Velde, B., 1993, Changes in particle morphology

during illitization An experimental study: Clays and Clay

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Yau, Y., Peacor, D.R., andMcDowell, S.D., 1987, Smectite-to-illite

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Published in the Central Region, Denver, Colorado 

Manuscript approved for publication August 10, 1994 

Edited by Judith Stoeser

Graphics by Norma J. Macs and Kenneth Esposito 

Photocomposition by Norma J. Macs
APPENDIX
Appendix table 1. Major-element chemical analyses for samples from core WC-83-6. 

[Location of core hole shown in text figure 1 . Samples shown in italics are igneous rocks]

Sample 

number

85

84

83

82

81

FlowH

FlowG

80

79

78

77

76

74

73

72

75

71

70

69

68

67

66

65

64

63

62

61

60

Intrusion F

59

58

57

FlowE

56

55

54

53

52

51

Depth 

(feet)

1,263

1,270.2

1,271.6

1,273.2

1,273.8

1,278

1,281.4

1,282.5

1,283

1,283.5

1,305.2

1,313.6

1,317.4

1,318.8

1,336

1,341

1,353

1,353.5

1,362.5

1,378.5

1,400

1,402.8

1,446.8

1,463.6

1,466.5

1,469.2

1,470

1,470.4

1,494

1,501.9

1,503.3

1,504.3

1,501

1,505.5

1,506.2

1,507.2

1,546.5

1,554.8

1,558.5

SiO2

(percent)

63.16

66.62

63.31

63.97

63.07

56.52

53.47

65.74

66.82

67.62

57.86

53.10

59.50

71.05

66.06

63.51

66.17

59.88

66.56

67.09

64.00

61.54

77.64

76.87

74.42

78.03

76.58

76.27

59.79

71.19

77.14

67.78

44.28

67.25

64.25

56.55

58.28

63.17

64.79

A12O3 

(percent)

19.32

18.17

17.47

21.18

20.33

17.87

18.97

18.52

16.71

17.45

16.28

24.24

20.61

14.87

17.88

18.82

16.34

22.49

12.04

17.68

17.93

18.89

12.54

13.26

13.47

12.15

12.81

13.28

15.88

11.17

10.48

15.29

16.49

14.76

18.02

15.12

24.37

17.75

19.00

FeTO3 

(percent)

4.11

4.98

7.71

4.46

4.55

7.45

8.23

4.47

6.18

5.37

14.74

9.31

8.78

3.37

7.42

7.50

5.57

5.97

3.58

6.06

10.68

8.99

2.29

2.85

2.60

1.68

2.08

3.04

7.59

4.06

2.34

6.20

15.83

6.70

6.60

16.59

3.50

5.08

4.97

MnO 

(percent)

0.05

0.06

0.11

0.03

0.03

0.17

0.24

0.03

0.07

0.06

0.34

0.03

0.05

0.06

0.10

0.08

0.05

0.07

0.15

0.07

0.30

0.18

0.04

0.05

0.05

0.06

0.05

0.05

0.11

0.26

0.12

0.10

0.63

0.10

0.07

0.16

0.04

0.05

0.04

MgO 

(percent)

1.71

1.91

2.32

2.09

2.11

2.33

2.62

2.13

2.12

1.97

2.57

1.19

2.22

1 .47

2.05

1.67

1.50

1.62

1.12

1.79

2.03

2.35

0.76

0.96

1.10

0.71

0.92

1.20

4.94

1.61

1.12

2.14

5.62

2.70

2.27

2.57

2.87

2.19

1.97

CaO

(percent)

1.78

0.83

0.77

0.64

0.58

9.97

11.73

0.60

1.26

1.05

1.84

4.23

0.42

0.90

0.72

0.81

1.03

1.52

9.87

0.73

1.12

1.36

0.93

1.36

1.25

2.49

1.85

1.04

6.10

8.64

4.48

3.04

14.48

2.46

1.34

1.13

1.55

0.77

0.65

Na2O 

(percent)

2.32

2.12

1.85

1.90

1.82

3.27

3.27

1.74

1.59

1.58

1.80

3.21

1.57

2.39

2.00

1.36

1.77

2.25

2.00

2.14

1.74

1.96

2.17

3.15

2.31

2.36

2.16

2.80

4.01

1.84

2.78

2.20

3.06

2.85

2.56

1.96

1.60

2.46

1.69

K2O

(percent)

2.04

2.37

2.25

2.95

2.80

0.89

0.49

2.76

2.40

2.42

2.23

0.91

2.88

1.95

2.52

3.24

2.38

2.43

1.63

2.79

2.67

2.68

2.19

2.33

2.17

2.13

2.12

2.26

/.35

1.41

1.53

1.65

0.44

1.77

2.74

2.26

3.63

2.53

2.98

Ti02 

(percent)

0.89

0.83

0.83

0.86

0.84

0. 78

0.87

0.73

0.75

0.77

0.87

1.64

0.81

0.63

0.88

0.96

0.74

1.21

0.47

0.82

1.10

0.92

0.34

0.48

0.40

0.31

0.33

0.47

0.93

0.41

0.20

0.63

1.05

0.71

0.89

0.81

0.38

0.82

0.95

(percent)

0.30

0.24

0.30

0.15

0.16

0.14

0.12

0.23

0.29

0.27

0.38

0.38

0.20

0.24

0.28

0.31

0.35

0.39

0.20

0.25

0.30

0.26

0.16

0.17

0.18

0.16

0.17

0.17

0.14

0.16

0.14

0.22

0.09

0.27

0.26

0.22

0.14

0.22

0.25

T

(pe

9

9

9

9

9

9

9

9

9

9

9

9

9

9

9

9

9

9

9

9

10

9

9

10

9

10

9

10

10

10

10

9

70

9

9

9

9

9

9
Appendix table 1. Major-element chemical analyses for samples from core WC-83-6 Continued.

Sample 

number

50

49

48

47

46

45

44

Intrusion D

43

42

41

40

39

38

37

36

Intrusion C

35

34

33

32

31

30

28

27

26

25

Intrusion B

23

22

21

Flow A

19

18

17

16

15

Depth 

(feet)

1,578.7

1,601

1,611

1,613.3

1,651

1,654.5

1,655.5

1,672

1,700

1,722.6

1,743.5

1,760

1,766

1,768.8

1,769.8

1,778

1,801

1,804.5

1,805.3

1,806

1,807.3

1,808.8

1,810.8

1,814.7

1,816.7

1,828.4

1,854.8

7,903.3

1,928.5

1,940.7

1,944

7,945

1,953

1,962.8

1,965.4

1,991

1,999

SiO2 

(percent)

73.27

68.06

76.53

67.10

65.14

70.16

65.57

57.23

66.39

68.66

65.37

70.89

61.45

63.72

68.21

69.53

51.43

61.90

65.89

64.65

68.55

65.83

63.74

69.24

63.45

49.94

60.69

57.27

67.79

66.46

54.60

57.47

70.19

72.05

67.94

66.27

56.96

A1203 

(percent)

13.60

15.06

10.25

17.86

13.42

16.95

20.51

18.59

16.87

14.40

18.14

14.08

12.40

17.56

17.03

17.06

76.57

18.18

18.18

17.18

16.39

16.56

16.26

15.59

19.55

12.80

16.80

76.96

18.38

13.30

17.78

76.75

17.01

14.54

11.32

19.91

18.05

FeTO3 

(percent)

3.80

4.24

1.84

5.07

4.84

5.30

4.57

10.48

4.71

4.36

5.92

3.90

3.85

6.04

5.00

5.83

70.76

6.30

4.77

6.34

5.21

5.30

4.49

5.71

6.05

7.14

8.59

9.74

4.50

6.37

9.13

9.99

6.37

4.34

7.83

5.07

7.83'

MnO 

(percent)

0.05

0.05

0.20

0.02

0.15

0.05

0.03

0.17

0.06

0.09

0.09

0.06

0.22

0.07

0.04

0.05

0.76

0.07

0.04

0.04

0.05

0.06

0.14

0.07

0.06

0.31

0.15

0.22

0.04

0.12

0.19

0.24

0.09

0.07

0.09

0.06

0.09

MgO 

(percent)

1.18

1.40

0.58

1.47

2.55

1.42

1.65

6.57

1.79

1.24

1.62

1.24

1.31

1.97

1.79

1.76

4.07

2.65

2.07

2.58

1.83

1.88

1.77

1.82

2.10

3.41

2.68

3.66

1.75

2.61

3.58

4.10

1.38

1.22

3.81

1.59

4.21

CaO

(percent)

1.09

1.18

5.55

0.63

5.31

0.54

0.38

5.84

0.70

3.87

0.99

1.94

14.89

0.78

0.62

0.76

70.39

2.15

1.05

0.76

1.12

1.37

5.96

1.85

0.63

23.86

5.28

73.00

0.54

5.48

9.03

77.52

0.97

3.23

2.82

0.60

6.68

Na2O 

(percent)

2.20

2.24

2.27

2.08

2.11

1.90

1.47

3.42

2.05

1.80

1.49

2.40

1.73

2.16

1.83

1.50

3.06

1.82

1.82

1.90

1.70

1.77

1.61

2.15

1.57

1.03

1.35

2.37

1.71

2.11

2.30

3.00

1.70

2.36

2.98

1.88

3.47

K2O

(percent)

2.28

2.35

1.74

3.48

1.69

2.73

3.28

0.84

2.62

2.14

2.72

2.28

1.57

3.03

2.98

2.74

0.56

2.04

2.89

2.31

2.29

2.32

2.03

2.24

2.97

0.94

1.92

0.43

2.64

1.50

0.71

0.50

2.51

1.49

0.91

3.13

0.74

TiO2 

(percent)

0.59

0.61

0.21

0.82

0.51

0.87

0.95

0.97

0.71

0.58

0.87

0.46

0.63

0.78

0.74

0.86

7.3

0.94

0.85

0.81

0.78

0.79

0.75

0.73

0.90

0.66

0.88

1.28

0.84

0.47

1.34

1.25

0.93

0.58

0.43

0.96

1.18

(percent)

0.21

0.20

0.15

0.26

0.21

0.28

0.20

0.05

0.26

0.23

0.29

0.23

0.23

0.28

0.24

0.25

0.18

0.33

0.33

0.31

0.32

0.28

0.27

0.33

0.32

0.25

0.41

0.37

0.28

0.24

0.34

0.20

0.18

0.22

0.16

0.29

0.54

(

1

1

1

1

1
Appendix table 2. Major-element chemical analyses for samples from core WC-83-5 and WC-83-6.

[Location of core holes shown in text figure 1]

Core

WC-83-5

WC-83-5

WC-83-5

WC-83-5

WC-83-6

WC-83-6

WC-83-6

WC-83-6

WC-83-6

WC-83-6

WC-83-6

WC-83-6

Sample

FlowL

FlowK

Flow!

Intrusion I

FlowH

FlowG

Intrusion F

FlowE

Intrusion D

Intrusion C

Intrusion B

Flow A

Sample Midpoint of Intrusion

depth igneous sheet thickness

(feet) (feet) (feet)

1,115.3 1,120.5 10.0

1,264.5 1,264.5 10.8

1,276 1,274.9 5.2

1,973 1,955.2 >84

1,278 1,276.5 8.4

1,281.4 1,281.4 2.2

1,494.8 1,486.2 31.3

1,505.4 1,505.2 1.7

1,672 1,674.5 28.0

1,801 1,794.8 19.5

1,903.3 1,892 10.0

1,945 1,945.6 3.7

Si02

51.68

50.27

49.02

51.31

56.52

53.47

59.79

44.28

51.23

51.43

51.21

51.47

A12O3

16.82

16.45

15.74

18.82

17.87

18.97

15.88

16.49

18.59

16.57

16.96

16.75

FeTO3

9.42

10.47

11.33

9.17

7.45

8.23

7.59

15.83

10.48

10.16

9.74

9.99

MnO MgO CaO Na2O

0.12 5.63 11.35 2.45

0.14 7.07 10.16 2.52

0.17 7.94 11.20 2.35

0.14 6.19 5.62 5.75

0.17 2.33 9.91 3.27

0.24 2.62 11.73 3.21

0.11 4.94 6.10 4.01

0.63 5.62 14.48 3.06

0.17 6.51 5.84 3.42

0.16 4.07 10.39 3.06

0.22 3.66 13.00 2.31

0.24 4.10 11.52 3.00

K2O

0.51

0.50

0.41

0.98

0.89

0.49

1.35

0.44

0.84

0.56

0.43

0.50

Ti

1

1

1

1

0

0

0

1

0

1

1

1