Evidence of Contemporary and Ancient

Excess Fluid Pressure in the New Madrid

Seismic Zone of the Reelfoot Rift,

Central United States

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1538-N

SSOURI

wm* "sb^ •

ARKANSAS
Cover. Gray, shaded-relief map of magnetic anomaly data. Map area includes parts of 

Missouri, Illinois, Indiana, Kentucky, Tennessee, and Arkansas. Illumination is from the 

west. Figure is from Geophysical setting of the Reelfoot rift and relations between rift 

structures and the New Madrid seismic zone, by Thomas G. Hildenbrand and John D. 

Hendricks (chapter E in this series).
Evidence of Contemporary and Ancient 

Excess Fluid Pressure in the New Madrid 

Seismic Zone of the Reelfoot Rift, 

Central United States

By F.A. McKeown and S.F. Diehl

INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE 

Edited by Kaye M. Shedlock and Arch C. Johnston

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1538-N

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

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, Director

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

Box 25286, MS 306, Federal Center

Denver, CO 80225

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

does not imply endorsement by the U.S. Government

Library of Congress Cataloging-in-Publication Data

McKeown, F.A. (Francis Alexander), 1920-

Evidence of contemporary and ancient excess fluid pressure in the New Madrid 

seismic zone of the Reelfoot rift, Central United States 

/ by F. A. McKeown and S.F. Diehl.

p. cm. — (Investigations of the New Madrid seismic zone; N) 

(U.S. Geology Survey professional paper; 1538-N) 

Includes bibliographical references. 

Supt.ofDocs.no.: 119.16:1538-N

1. Seismology—Missouri—New Madrid Region. 2. Geology, Structural— 

Missouri —New Madrid Region. 3. Aquifers—Missouri—New Madrid Region. 

I. Diehl, S.F. II. Title. III. Series. IV. Series: U.S. Geological Survey professional 

paper; 1538-N. 

QE535.2.U6I59 1994 vol. N 

551.2'2'09788985 s—dc20

[551.2'2'09788985] 93-41468

CIP
CONTENTS

Abstract........................................................................................................................... Nl

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

Summary of Geologic History........................................................................................ 2

Evidence of Contemporary Excess Fluid Pressure......................................................... 4

Drill-Hole Data..................................................................................................... 4

McNairy-Nacatoch Aquifer.................................................................................. 10

Evidence of Ancient Excess Fluid Pressure................................................................... 11

Stylolites............................................................................................................... 11

Mineral-Filled Fractures....................................................................................... 14

Shale Fabric.......................................................................................................... 14

Causes and Estimates of Excess Fluid Pressures............................................................ 15

Cause of Contemporary Excess Fluid Pressure.................................................... 16

Cause of Ancient Excess Fluid Pressure .............................................................. 16

Implications of Excess Fluid Pressure on Earthquakes in

the New Madrid Seismic Zone.................................................................................. 17

Conclusions..................................................................................................................... 20

References Cited............................................................................................................. 22

FIGURES

1. Index map of the New Madrid region showing epicenters, plutons, rift boundaries, selected drill holes,

Blytheville-Pascola arch complex, and seismic reflection profile lines ..................................................................... N3

2. Schematic diagrams showing temporal and structural evolution of the Reelfoot rift, the Blytheville-Pascola

arch complex, and the Mississippi Embayment syncline ........................................................................................... 5

3. Coherency-filtered, unmigrated seismic reflection record section of line PI and line drawing extending from 

southeast boundary of Reelfoot rift northwestward across southwest end of the Blytheville-Pascola arch 

complex....................................................................................................................................................................... 6

4. Coherency-filtered, unmigrated north-south seismic reflection record section of line P2 and line drawing

processed to intersect Blytheville-Pascola arch complex at about 90°....................................................................... 7

5. Columnar section of hydrogeologic units in the upper Mississippi Embayment........................................................ 8

6. Map showing locations of wells that penetrate Paleozoic rocks................................................................................. 9

7. Map showing locations of wells that penetrate the Upper Cretaceous aquifer........................................................... 10

8. Map showing zones of anomalously warm water and contours of P-wave velocities in upper 14 km of the crust

in relation to the Reelfoot rift and the Blytheville-Pascola arch complex.................................................................. 12

9. Thin section photomicrographs of Stylolites in the Wilson drill hole and deformed stylolites in the Garrigan

drill hole...................................................................................................................................................................... 13

10. Thin section photomicrograph showing network of vertical and horizontal fractures filled with epigenetic

carbonate minerals...................................................................................................................................................... 14

11. Scanning electron photomicrographs of well-laminated and randomly oriented clay platelets in shale .................... 15

12. Generalized map derived from figures 1,6, and 7 showing relationship of the Blytheville-Pascola arch complex

and excess-fluid-pressure zone to seismicity.............................................................................................................. 21

TABLE

1. Estimates of excess pressures at bottom of drill pipes................................................................................................ N19

in
IV CONVERSION FACTORS

METRIC CONVERSION FACTORS

Multiply inch-pound unit By To obtain metric (SI) unit

inches 0.02540 meters (m)

feet (ft) 0.3048 meters (m)

miles (mi) 1.609 kilometers (km)

gallons per minute (gal/min) 6.309xlO~2 liters per second (L/s)

pound-force per inch2 (psi) 6.895xlO~3 megapascals (MPa)

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius as follows:

°C = 5/9 (°F-32) 

Pressure in bars may be converted to megapascals by multiplying by 0.10
EVIDENCE OF CONTEMPORARY AND ANCIENT

EXCESS FLUID PRESSURE IN THE NEW MADRID

SEISMIC ZONE OF THE REELFOOT RIFT,

CENTRAL UNITED STATES

By F.A. McKeown and S.F. Diehl

ABSTRACT

Geophysical and seismological data indicate that most 

earthquakes in the New Madrid seismic zone are associated 

with a structural high composed of intensely disrupted rocks 

along the axis of the Reelfoot rift; the high is inferred to be 

structurally weak and, therefore, is more likely to fail than 

adjacent rocks. The structural high is within a part of the 

upper Mississippi Embayment where hydrologic data show 

that excess fluid pressure is pervasive in aquifers below the 

Paleocene Midway Group confining unit where the group is 

more than 200 ft thick. In addition, low seismic velocities 

and relatively low resistivities to depths of more than 5 km 

have been interpreted to be caused by excess fluid pressure. 

The principal evidence of excess fluid pressure is artesian 

flow from aquifers in Paleozoic rocks at depths greater than 

1 km and from wells in the Upper Cretaceous McNairy-Nacatoch aquifer. Hydraulic continuity of the Paleozoic and 

Cretaceous aquifers and circulation of water from great 

depths are indicated by chemical and thermal data of water 

samples. Vertical continuity through heterogeneously distributed fracture zones in the Blytheville-Pascola arch complex is interpreted from seismic reflection profiles. Because 

most seismicity in the New Madrid zone is spatially coincident with areas where excess fluid pressure is present in the 

disrupted arch complex, a causal relationship is suggested. 

Special conditions for gravity flow and confining layers that 

may explain contemporary and ancient excess fluid pressures were present during the late Paleozoic and probably 

early Mesozoic as well as since the Eocene. Excess ancient 

fluid pressure is clearly evident in petrographic studies of 

samples of lower Paleozoic rocks from drill holes. Dilation 

and mineral filling of stylolites and fractures and the fabric 

of shales are the principal evidence of ancient excess fluid 

pressure. The paragenetic sequence of minerals determined 

from these studies and a wealth of data on the source of ore 

fluids for lead-zinc deposits in the Ozarks show that several 

episodes of excess fluid pressure occurred during a period of

several million years. The special conditions for 

contemporary excess fluid pressure probably existed since 

the Eocene; however, the style of faulting differed from that 

of today. The northeasterly strike of normal faults in Tertiary 

and Quaternary rocks indicate that the direction of maximum 

principal stress was rotated about 35° counterclockwise from 

the direction of the present stress field. Seismicity trends and 

focal mechanisms, therefore, would have been unlike those 

of today in the embayment. The lack of northeast-striking 

normal faults since about the Pleistocene suggests that the 

stress field changed to its present direction and that current 

seismicity associated with strike-slip movement on northeasterly striking faults may have started at about that time. 

Current seismicity in the Blytheville-Pascola arch complex, 

therefore, may be a long-lived phenomenon and not an aftershock sequence of the 1811-12 earthquakes. Estimates of 

the excess fluid pressures in the New Madrid seismic zone 

are less than the pressure changes known to trigger earthquakes. Because of much uncertainty about the state of stress 

at hypocentral depths and evidence of changes in seismicity 

related to very small changes in fluid pressure, consideration 

is warranted of a causal relationship of the excess fluid pressure to seismicity in the New Madrid region. If the relationship is causal, many ramifications should be considered. 

These include establishing a physical basis for delimiting 

seismic zones, estimating when background seismicity 

started, and possibly estimating recurrence rates of earthquakes in a region as a function of source-zone size and the 

time required to pressurize a source zone by fluid flow.

INTRODUCTION

A difficult problem in evaluating earthquake hazards is 

to establish a physical basis for delimiting earthquake source 

zones. Although the location, stress directions, and mechanisms of earthquakes can be obtained from seismicity, information about the physical properties of rocks in an

Nl
N2 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

earthquake source zone must be obtained from geologic and 

geophysical data. The term physical properties as used in this 

report refers to the gross or volume-averaged properties of 

rock, which may be quite unlike physical properties measured on centimeter-sized samples. If the physical properties 

known to contribute to failure of rocks in active seismic 

zones are known and the regional state of stress is fairly uniform, as indicated by the stress map of Zoback and Zoback 

(1981), the probable extent of the zone can be mapped independently of the current seismicity. Furthermore, identification, or inference, of similar properties in other areas may be 

used in the preparation of earthquake hazard maps—this 

information from other areas may also enhance the credibility of the maps. Resolving this problem requires: (1) identification of the physical properties of a seismically active 

source zone, (2) identification of the geologic factors that 

produced those properties, and (3) an explanation of why 

such properties cause or are conducive to the generation of 

earthquakes.

Geophysical and seismological data show that most 

earthquakes in the New Madrid seismic zone occur within a 

structural high in lower Paleozoic rocks along an approximately 10-km-wide axial zone of the Reelfoot rift (Crone and 

others, 1985; Hamilton and McKeown, 1988; McKeown and 

others, 1990). The Blytheville (axial) structural high merges 

with the Pascola arch, which is about 40 km wide. Because 

the Blytheville and Pascola are coextensive and apparently 

cogenetic, we refer to them herein as the Blytheville-Pascola 

arch complex (fig. 1). Seismic reflection profiles indicate 

that the rocks in the high are intensely disrupted, which 

implies that the zone is structurally weak and which explains, 

at least in part, the occurrence of earthquakes (Crone and others, 1985; Hamilton and McKeown, 1988; McKeown and 

others, 1990). In addition, low seismic velocities and relatively low electrical resistivities of these rocks suggest that 

they contain pore fluids that have excessively high fluid pressure compared to adjacent rocks (Al-Shukri and Mitchell, 

1988; Hamilton and Mooney, 1990; Stanley and Rodriquez, 

1992). Sibson (1990) has suggested that, in the northern part 

of the New Madrid seismic zone, reverse faults unfavorably 

oriented to the current stress field are seismically active 

because of fluid-overpressured cracks.

We use "excess fluid pressure" to describe any pressure 

that is greater than the hydrostatic pressure and the term 

"pore pressure" primarily to refer to fluid pressure in fractures or faults and interstitial pore space. Undoubtedly, some 

of the rock is porous and has fluid pressure in the pores, but 

the extent and abundance of porous rock is not known. 

Lithostatic pressure is analogous to overburden pressure and 

is defined as equal to the pressure caused by the weight of a 

column of overlying rock.

The principal objectives of this paper are: (1) to present 

direct and circumstantial evidence of excess fluid pressure in 

the New Madrid seismic zone, (2) to present geologic reasons for contemporary excess fluid pressure, (3) to present

evidence that excess fluid pressure existed in late Paleozoic 

and probably early Mesozoic time, and (4) to discuss the 

implications of excess fluid pressure to seismicity in the 

New Madrid seismic zone.

Perhaps the most significant observation reported in 

this paper is that most seismicity in the New Madrid seismic 

zone occurs primarily where excess fluid pressure is inferred 

in the arch complex. Seismicity decreases abruptly to the 

north of the arch complex where fluid pressure is not excessive (but where structural disruption very likely exists) 

because this area coincides with the intersection of major 

northeast- and northwest-striking fault zones (Hildenbrand 

and others, 1992). Seismicity also decreases abruptly to the 

south where the arch complex terminates but where fluid 

pressures are inferred to be excessively high. We suggest 

that earthquake activity in the New Madrid seismic zone 

requires both structurally disrupted rocks and excess fluid 

pressure.

The primary emphasis of most earthquake studies is to 

identify the seismic source zones. The ultimate reason for 

this paper, however, is an attempt to explain why certain 

geologic structures in the New Madrid seismic zone are seismically active whereas others are not, despite an abundance 

of major faults in the region that are suitably oriented for slip 

in the contemporary stress field.

Acknowledgments.—We wish to thank several U.S. 

Geological Survey colleagues for helpful criticisms and discussion. John Bredehoeft, Anthony Crone, Evelyn Roeloff, 

and Martin Goldhaber provided thoughtful and helpful 

reviews. William Ellis and Henri Swolfs kept us from making some very erroneous statements about pressure and 

stress. Harold Olsen explained how to calculate frictional 

loss of pressure in pipes. Rus Wheeler raised some pertinent 

questions on our early draft, and Walter Mooney encouraged 

us to write the paper. Discussions with John Van Brahana 

were invaluable, and he provided much of the hydrologic 

data that we discuss in the paper.

SUMMARY OF GEOLOGIC HISTORY

Major advances in knowledge of the subsurface structure in the embayment have resulted from studies of a variety 

of geophysical data and a few deep drill holes (Kane and others, 1981; Mooney and others, 1983; Howe and Thompson, 

1984; Hildenbrand, 1985; McKeown and others, 1990; Diehl 

and others, 1992; Swolfs, 1992). These studies have emphasized the tectonic history of the upper Mississippi Embayment region. However, the data on the distribution and cause 

of excess fluid pressures require a broader perspective of the 

geologic history than just tectonics. Important aspects of the 

geologic history are the location of upland areas at various 

periods of geologic time and the ages, lithology, and erosion 

of sedimentary rocks. Of particular importance is the geologic record and history of fluid flow in the region, a topic
EVIDENCE OF EXCESS FLUID PRESSURE IN THE NEW MADRID SEISMIC ZONE N3

/MALDEN I + 

PLUTON/

EXPLANATION

— — — Rift boundary

+ Eathquake epicenter

Figure 1. Index map of the New Madrid region showing earthquake epicenters (Andrews and others, 1985), plutons, rift boundaries, 

Blytheville-Pascola arch complex, and selected drill holes. Solid line boundary of arch complex is from seismic reflection profiles and 

drill-hole data; long dashed line boundary is from drill-hole data. PI and P2 show locations of east-west and north-south seismic reflection 

profiles, respectively.
N4 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

that has been studied extensively for many years in efforts to 

explain the distribution and origin of Mississippi 

Valley-type lead-zinc ore bodies (Leach and Rowan, 1986). 

In addition, recent hydrologic studies of flow directions and 

the chemistry of ground water in the upper Mississippi 

Embayment region bear directly on postulating the existence 

and distribution of excess fluid pressure in the shallow and 

deep subsurface (Brahana and Mesko, 1988).

The Reelfoot rift is inferred to have formed in Late Proterozoic or Early Cambrian time (Ervin and McGinnis, 

1975). No definitive evidence of the rift's Late Proterozoic 

age is known, but rifting occurred during the Cambrian as 

indicated by the thick section of Cambrian rocks along the 

middle of the rift (Howe and Thompson, 1984; McKeown 

and others, 1990). During the Late Cambrian, valleys were 

incised into Precambrian igneous rocks in the Ozark dome 

(Schwalb, 1982) and were filled with sandstone. The Ozark 

and Nashville domes were structural and topographic highs 

for part of the time after the Early Ordovician (Wanless, 

1975; Buschbach and Atherton, 1979), but several major 

transgressions of shallow seas, which deposited carbonate 

rocks over broad areas, were interspersed with minor uplifts 

in the Ordovician, Silurian, Devonian, and Mississippian. 

Sandstone and siltstone were commonly deposited adjacent 

to the uplifts. By Pennsylvanian time, the Ozark and Nashville domes were low landmasses and were covered by shallow lakes, swamps, lagoons, and shallow seas. Later in the 

Pennsylvanian, uplift exposed mainly carbonate rocks in 

which a complex of sinkholes and caves formed. Some of the 

caves and sinks are filled with Pennsylvanian rocks more 

than 90 m thick (Wanless, 1975).

Because of the close association of earthquakes and the 

possibility of structural traps for hydrocarbons, the origin 

and structural characteristics of the Blytheville-Pascola arch 

complex have been of considerable importance since the 

complex was first discovered (Howe and Thompson, 1984; 

Crone and others, 1985). McKeown and others (1990) proposed that the Blytheville-Pascola arch complex formed by 

diapirism starting during the late Paleozoic. Diapiric movement in the thickest section of clastic rocks (composed 

largely of shale, siltstone, and fine-grained sandstone) along 

the axial part of the Reelfoot rift may have been triggered by 

unloading when a thick section of Mississippian and Pennsylvanian rock was eroded, or it may have been triggered by 

overpressure conditions in the clastic rocks. As we discuss 

later in this report, excess fluid pressure probably existed in 

the Cambrian and Ordovician rocks at this time and could 

have been a major factor in making the fine-grained clastic 

rocks that fill the axial part of the rift more ductile and less 

dense than the laterally contiguous rocks. After formation of 

the Blytheville-Pascola arch complex, a long period of uplift 

and erosion began, which lasted until the Late Cretaceous, 

when downwarping began to form the Mississippi Embayment syncline (Stearns and Marcher, 1962). Cretaceous, 

Paleocene, and lower Eocene rocks, amounting to about 1

km in thickness, were deposited in the upper part of the 

embayment during a sequence of five marine transgressions 

and regressions (Stearns, 1957). The maximum transgression, during the Paleocene, deposited the Porters Creek Clay. 

A generalized evolution of the Reelfoot rift and the 

Blytheville-Pascola arch complex, as described above, is 

schematically shown in figure 2. The principal evidence for 

the stages of rifting has been described by McKeown and 

others (1990) and is shown in the seismic reflection profile 

of figure 3. Identification of the principal reflectors, which is 

critical to the structural events interpreted in the profile, was 

based on a synthetic seismic reflection profile made from 

sonic, density, and lithologic logs of the Dow Chemical No. 

1 Wilson hole (fig. 1) (E. Luzietti, written commun., 1989). 

The arching of strata by the Blytheville-Pascola arch complex, shown in figures 3 and 4, is well documented in numerous other seismic reflection profiles.

It is particularly important to note in figure 2 that stratigraphic units known, or inferred to contain, many thick confining layers, which could act as caps to underlying aquifers, 

were present during only two periods: one was when Mississippian and Pennsylvanian rocks with many confining layers 

covered much of the region (fig. 2) (Wanless, 1975); the second was when Tertiary clays were deposited in the 

embayment.

EVIDENCE OF CONTEMPORARY 

EXCESS FLUID PRESSURE

Well-documented data on water levels and water chemistry provide abundant evidence that the current fluid pressure is excessively high in several aquifers in the upper 

Mississippi Embayment. Interpretations of hydrologic data 

are commonly made by combining stratigraphic units that 

are hydrologically distinct into hydrostratigraphic or hydrogeologic units (Maxey, 1964; Seaber, 1988). The hydrogeologic units used in this study (fig. 5) are from Brahana 

(written commun., 1992) and Imes and Smith (1990).

DRILL-HOLE DATA

The most spectacular evidence in recent years of contemporary excess fluid pressure in Paleozoic rocks was the 

artesian flow of water from the New Madrid test well 

(fig. 6) (Crone and Russ, 1979; Crone, 1981). Dolomite 

was first penetrated in the well at a depth of 2,023 ft. The 

rocks in the well above the Paleozoic and Upper Cretaceous are predominantly fine grained, of which the most 

impermeable are the Paleocene Porters Creek Clay (314 

ft) and the Eocene Cook Mountain Formation (178 ft). 

The very high clay content was determined from core and 

is indicated by the low resistivity on geophysical logs of 

the New Madrid test well (Crone and Russ, 1979, fig. 3
EVIDENCE OF EXCESS FLUID PRESSURE IN THE NEW MADRID SEISMIC ZONE N5

Middle(?) and Late Cambrian Figure 2. Schematic diagrams showing temporal 

^————— and structural evolution of the Reelfoot rift, the 

Blytheville-Pascola arch complex, and the Mississippi 

Embayment syncline. Bold arrows with tails indicate 

rst stage of rifting rfft movement Arrows without tails indicate flow

Crystalline rocks directions.

Late Cambrian

Crystalline rocks

id stage of rifting

Late Cambrian-Ordovician

Knox and Arbuckle Group equivalents

Final rift boundary

Crystalline rocks 

Post Ordovician-pre Upper Cretaceous, Post OrdOVician-pre-Late CretaCCOUS

Knox and Arbuckle Group equivalents

Crystalline rocks

Post Ordovician-pre-Upper Cretaceous Pennsylvanian and Permian

Crystalline rocks

BLYTHEVILLE ARCH X

Late Paleozoic-Cretaceous

Crystalline rocks
SOUTHEAST

Figure 3. Coherency-filtered, unmigrated seismic reflection record section of line PI and line drawing that shows area extending from southeast boundary of Reelfoot rift northwestward 

across southwest end of the Blytheville-Pascola arch complex. T/K, Tertiary-Upper Cretaceous contact; K/Pz, Upper Cretaceous-Paleozoic contact; R, a prominent reflector in the undifferentiated Knox and Arbuckle Groups; BC, top of basal clastic rock unit; B, Paleozoic-crystalline rock contact. Location of line PI shown on figure 1.

O

iC/5

I

O

ns
SOUTHEAST NORTHWEST

HORIZONTAL SCALE

-WAYTR/

u

u - 

1 - 2 - 

3 - 4 - c; - , , T/K • —————————————— • - — ———— ——————— — ——————————————————— T75-T/K ——————————— ————— ———————— — ————— K/ft — -^__ ivrt R ———— — R —— —— ~-Rr? "-- - • R BC ——— - - — BC ————————————— ——————— — BC -"' •"->>' "^ — -— ^— """" •• ^" • ~-^_lj *• - - — • """" "^ *^' » ~" O"5 ,^^^_ ^^ __, — . ———— ' m - Q f • ——— — — ___ _ ~ —— "^~^ ~~ ^=~ ^ — — ~ ~~~ ^, — ' ^- ——— Figure 4. Coherency-filtered, unmigrated north-south seismic reflection record section of line P2 and line drawing, which has been processed to intersect Blytheville-Pascola arch 

complex at about 90° (W. Agena, written commun., 1989). T/K, Tertiary-Upper Cretaceous contact; K/Pz, Upper Cretaceous-Paleozoic contact; R, a prominent reflector in the undifferentiated Knox and Arbuckle Groups; BC, top of basal clastic rock unit; B, Paleozoic-crystalline rock contact. Location of line P2 shown on figure 1.

I

W

gm

ICO
N8 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

Period/Epoch

Quaternary

Tertiary

Late Cretaceous

Pre-Late Cretaceous 

and 

Middle Ordovician

Late Cambrian

Late Cambrian

Precambrian

Principal Stratigraphic Units

Alluvium

Undifferentiated 

Claiborne Formation

Wilcox Group

(lower part)

Midway Group

McNairy Sand and 

Nacatoch Sand

Undifferentiated 

units

Knox and Arbuckle

Group 

equivalents

Elvins Group

Bonnetterre Formation 

and 

Lamotte Sandstone

Crystalline rocks

Hydrogeologic Units

Alluvial aquifer

Undifferentiated 

aquifers and 

confining units

^> Lower <-\^ 

^^ Wilcox aquifer J/>

< /

Midway 

confining unit

McNairy-Nacatoch 

aquifer

Ozark 

confining unit

Ozark ^^

aquifer

St. Francois 

confining unit

St. Francois 

aquifer

Basement confining unit

Figure 5. Columnar section of hydrogeologic units in the upper Mississippi Embayment (after J.V. Brahana, written commun. 

1992).

and appendix 2). Crone and Russ (1979, p. 5) reported 

that, almost immediately below casing set at 2,060 ft, 

"artesian water from Paleozoic rocks began to flow at the 

rate of 4 to 6 gal/min at the surface. The flow gradually 

increased to about 30 gal/min as the well was deepened. 

At 2,303 ft, the well intersected an 8-ft-thick fracture 

zone and the artesian flow increased to more than 500 

gal/min." After failing to cement the fractured zone, the 

well was completed at a depth of 2,316 ft. Water flowed 

from a string of drill pipe, the top of which was 27 m 

above ground level (Crone and Russ, 1979). This flow 

indicated that the excess fluid pressure at the surface was 

about 0.28 MPa.

The U.S. Bureau of Mines, R.B. Oliver, h. No. 1 well 

(drilled in 1945, about 35 km southwest of New Madrid, 

Mo., in the northwestern part of the rift (fig. 6)), is more significant than the New Madrid test well because of the depth 

from which the artesian flow originated. The artesian flow of 

water apparently came from fractured zones in the Boneterre 

Formation (Upper Cambrian) and the Elvins Group (Upper 

Cambrian). Grohskopf (1955, p. 79) reported that:

Between depths 2,920 and 3,000 ft, water was encountered which flowed 

50 ft above ground level at an estimated 700 gal/min.* **Another flow of 

water was encountered at 3,500 ft, and circulation was lost at several points 

below that depth. Several attempts were made to cement off this water but

all failed, making it necessary to set a string of 4-inch standard pipe at 

3,828 ft.

Another well with artesian flow of water that penetrated 

Paleozoic rocks was the Strake Petroleum Incorporated, 

T. P. Russell No. 1 well, located in Pascola, Mo. (fig. 6). 

Grohskopf (1955) reported that, upon completion of this 

well, water with a temperature of 92°F flowed at 150 gal/min 

at the surface, but no information is given about the head of 

flow above the ground surface. The flow, as interpreted from 

the log of the well in Grohskopf (1955), apparently came 

from Paleozoic rocks at a depth of 2,208 ft. Grohskopf 

(1955) also reported that water with a temperature of 110°F 

flowed at 70 gal/min from a depth of 4,405 ft in the Benedum-Trees Oil Co., C.W. Mack No. 1 well, which is located 

about 11 km east of Blytheville, Ark. (fig. 6). These observations of artesian flow of water from deeply buried lower 

Paleozoic rocks are important to some arguments about the 

heterogeneity and magnitude of excess fluid pressure made 

later in this report.

Additional measurements of water levels in Paleozoic 

rock from wells within the embayment are few. Boswell and 

others (1965) reported that, although a number of flowing 

wells occur in the region, their discharge could not be estimated. In addition to the Oliver, Strake, and Benedum-Trees 

wells, Brahana and Mesko (1988, fig. 9) report
EVIDENCE OF EXCESS FLUID PRESSURE IN THE NEW MADRID SEISMIC ZONE

90°30 90°00' 89°30' 89°00'

N9

U.S. Bureau of Mines 

R.B. Oliver, Jr., No. 1

ARKANSAS . , , .

/ / -* \

Strake Petroleum Inc. / / 27 ^ew Madrid 

T.P. Russell No. 1 ' / - -

. 1 , 

Spence Trust & I

— --

Benedum-Trees Oil Co.

W.

X

Blyiheville^ C.W.MackNo.1

36°00'

35°30' —

35°00'

EXPLANATION

—————— Rift boundary Pluton

Figure 6. Map showing locations of wells that penetrate Paleozoic rocks. Number next to well symbol is altitude, in meters, of water above 

ground level. Wells with "F" next to symbol were flowing, but level of water above ground level is unknown. Level of water in wells 

without number or letter is unknown.

measurements in eight wells drilled into Paleozoic rocks 

within the western part of the embayment in the "bootheel" 

of Missouri (fig. 6). Most of the wells are very close to the 

northwestern edge of the embayment (fig. 6). The altitude of 

the water in these wells is about the same as the altitude of 

the land surface, which indicates normal hydrostatic fluid 

pressure in the Paleozoic rocks. The lack of excess fluid 

pressure in these wells is probably because of the lack of a

confining layer. The large flow of water from the Oliver well 

may be anomalous. This well is above the Maiden pluton, 

which is estimated to be Mesozoic in age (Hildenbrand, 

1985) (fig. 1). The fracture zones in the Bonneterre Formation, an aquifer, and the Elvins Group, generally a confining 

unit (fig. 5), may have been caused by intrusion of the pluton, which may have caused fracturing and alteration of the 

Paleozoic rocks into which it intruded.
N10 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

90°30' 90°00' 89°30'

36°00' —

35°30

35°00'

EXPLANATION

—————— Rift boundary Pluton

Figure 7. Map showing locations of wells that penetrate the Upper Cretaceous aquifer. Number next to well symbol is altitude, in meters, 

of water above ground level. Wells with "F" next to symbol were flowing, but level of water above ground level is unknown. Level of 

water in wells without number is below ground level. Contours indicate thickness, in feet, of Porters Creek Clay.

MCNAIRY-NACATOCH AQUIFER

Artesian heads of water from wells in the Upper Cretaceous McNairy-Nacatoch aquifer are important for 

municipal water supplies (Boswell and others, 1965; 

Luckey, 1985). Measurements of water levels in 58 wells 

that are north of lat 35°N. in the embayment and that penetrate this aquifer are shown in figure 7. Twenty-eight of

these measurements are from figure 10 of Brahana and 

Mesko (1988), and 30 are from Brahana (written commun., 1992). Water levels in many of these wells are a 

few meters to tens of meters above the ground surface. 

For example, the static water level in a well into the 

McNairy-Nacatoch aquifer near Memphis, Tenn., is about 

33 m above the ground elevation (Wells, 1931). Near the 

eastern and western edges of the embayment, water
EVIDENCE OF EXCESS FLUID PRESSURE IN THE NEW MADRID SEISMIC ZONE Nil

levels in this aquifer are commonly below the surface altitude (fig. 7). The water levels plotted in figure 7 were 

measured at different times over a period of 10 years or 

more, and the highest levels were the earliest measurements, indicating that pumping over a long time period 

has reduced the levels. The water levels plotted in figure 

7 are the highest recorded. A correlation of water levels 

with the principal zone of seismicity is somewhat speculative because of a lack of data. Where measured, water 

levels correlate well with the thickness of the Porters 

Creek Clay confining unit (fig. 7). If unmeasured pressures are also high where the Porters Creek Clay is thick, 

then the area of high water levels would contain the earthquake epicenters. The absence of potentiometric data for 

the McNairy-Nacatoch aquifer within the rift and south 

of the wells shown in figure 7 is because adequate water 

supplies are obtained less expensively from shallower 

aquifers (Boswell and others, 1965; Brahana and Mesko, 

1988).

A comparison of the water levels and the thickness contours of the Porters Creek Clay confining unit shows an 

approximate positive correlation (fig. 7); that is, the water 

levels are highest where the thickness of the Porters Creek 

Clay is greatest. Water levels are below the ground surface 

where the confining unit is less than 200 ft thick. In addition, 

Brahana and Mesko (1988) interpret the higher water levels 

in the Upper Cretaceous McNairy-Nacatoch aquifer near the 

center of the embayment to be caused by thinning of the 

Paleozoic-Cretaceous confining unit (fig. 5) (Brahana and 

Mesko, 1988) toward the center of the embayment. This 

thinning results in a hydraulic connection between the Paleozoic and Cretaceous aquifers and allows the pressure in the 

aquifers to equalize. The water levels in wells in the overlying Tertiary rocks do not indicate significantly high fluid 

pressure. Aquifers in these rocks, however, supply very large 

amounts of water for irrigation as well as for public and 

industrial purposes, which causes a considerable drawdown 

of water levels. Prior to extensive pumping, artesian flow 

from Tertiary rocks was probably widespread (J.V. Brahana, 

written commun., 1992) and may have been a factor in the 

extensive liquefaction of alluvium that occurred during the 

1811-12 New Madrid earthquakes (Fuller, 1912).

Brahana and Mesko (1988, table 2) interpret that the 

McNairy-Nacatoch aquifer is recharged with fluid from 

Paleozoic rocks in zones above the northeastern part of the 

northwest boundary of the Reelfoot rift and northwestward 

to about the Bloomfield pluton (figs. 1 and 2). This is 

because high chloride content, high dissolved-solids concentrations, and trace-element constituents that are characteristic of water in the Paleozoic aquifer occur in water in 

the Upper Cretaceous aquifer. Also, they report Tom 

Kraemer's (written commun., 1986) interpretation of 

radium-isotope-ratio data in water from the Upper Cretaceous aquifer to indicate that the water was derived from 

deep Paleozoic rocks. Therefore, the chemical composition

and isotope data suggest that fluid pressures at depth are 

high enough to cause water to flow from Paleozoic rocks 

into the Upper Cretaceous McNairy-Nacatoch aquifer.

Zones of hydrothermal or anomalously warm water 

(fig. 8) in the McNairy-Nacatoch aquifer are also inferred by 

Brahana and Mesko (1988) to indicate recharge from underlying Paleozoic rock. Brahana and Mesko (1988) believe 

that a linear zone of hydrothermal water close to the western 

boundary of the rift indicates upward leakage of water from 

Paleozoic rocks to the Upper Cretaceous aquifer through 

faults along the rift boundary. Luckey (1985) speculates that 

the high temperatures and the artesian pressure in the Upper 

Cretaceous aquifer may make it a possible heat source. The 

highest water temperatures above the normal geothermal 

gradients are generally between the northwestern boundary 

of the rift and the center of the Blytheville-Pascola arch complex. Swanberg and others (1982, p. 188) also reported a 

"well-defined thermal anomaly associated with the New 

Madrid seismic zone." They considered several explanations 

of the anomaly and ultimately favored convective heat flow 

from deep ground-water circulation along fractures in the 

upper crust as the best explanation.

EVIDENCE OF ANCIENT EXCESS 

FLUID PRESSURE

Detailed petrographic and scanning-electron-microscope studies of core samples of Cambrian rocks from the 

Reelfoot rift show rock fabrics that indicate former 

exces-fluid-pressure conditions. Structural fabrics that suggest fluid migration and excess fluid pressure include: (1) 

dilated horizontal and, subsequently, mineralized stylolites; 

(2) vertical and horizontal mineral-filled fractures; and (3) 

abundant pore space created by randomly oriented clay 

platelets in silty shale.

STYLOLITES

Stylolites are pressure-solution surfaces that occur as 

well-defined column-and-socket or sutured structures and as 

irregular seams lined with insoluble residues from the host 

rock (Park and Schot, 1968). Stylolites are commonly parallel to bedding, but they may also form at any orientation normal to the maximum principal stress direction. Stylolites are 

present both in arenaceous and shaly rocks in the Reelfoot 

rift as low-amplitude, wavy, clay-filled seams that form 

planes of weakness. Petrographic data show that the Stylolites expanded into open fractures in the past when the 

hydrostatic pressure exceeded lithostatic pressure. The Stylolites commonly contain layers of quartz, organic and clay 

residue, carbonate minerals, and minor amounts of sulfide 

minerals, and they are interpreted to once have been flow
N12 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

90°

EXPLANATION

Blytheville-Pascola arch complex 

P-wave velocity contours, contour 

interval 0.1 km/s

Areas of anomalously warm water, 

showing temperature (in °Q above 

geothermal gradient

2-5

>5

Figure 8. Map showing zones of anomalously warm water (from Brahana and Mesko, 1988) and contours of P-wave velocities (in 

km/s) in upper 14 km of the crust (from Al-Shukri and Mitchell, 1988) in relation to the Reelfoot rift and the Blytheville-Pascola arch 

complex. Solid line boundary of arch complex is from seismic reflection profiles and drill-hole data; long dashed line boundary is from 

drill-hole data.
EVIDENCE OF EXCESS FLUID PRESSURE IN THE NEW MADRID SEISMIC ZONE N13

Figure 9. A, Thin-section photomicrograph showing stylolites in 

the Wilson drill hole (12,782 ft) filled with residual clay minerals, 

organic residue, and minor epigenetic minerals. Horizontal field of 

view is approximately 2.5 mm. B, Thin-section photomicrograph 

showing that stylolites in the Garrigan drill hole (7,999.9 ft) have 

commonly undergone deformation and are filled with a succession 

of carbonate cements. Horizontal field of view is approximately 5 

mm.

paths for solutions (Braithwaite, 1989; Bathurst, 1990; Diehl 

and others, 1992).

We compared deformation features in stylolites in samples of Cambrian rocks from two deep drill holes in the Reelfoot rift—the Dow Chemical No. 1 Garrigan, which is in the 

intensely deformed Blytheville-Pascola arch complex, and 

the Dow Chemical No. 1 Wilson, which is southeast of the 

complex (fig. 1). Stylolites in rocks from the Wilson drill 

hole are parallel to subparallel to bedding and are characteristically lined with insoluble organic matter and residual clay 

(fig. 9A); they also contain rare epigenetic monazite and 

pyrite that must have precipitated from hydrothermal fluids 

(Diehl and others, 1992). The stylolites also have slickensided clay, which is indicative of differential slip subparallel 

to the plane of the stylolite.

In contrast, the bedding-parallel stylolites in samples 

from the Garrigan drill hole commonly show evidence for 

multiple fluid-flow and precipitation events (fig. 9B). The 

residual clayey material in these samples is commonly 

sheared, and several generations of carbonate cement fill the 

dilated stylolites—siderite, dolomite, and minor ferroan 

dolomite. The carbonate minerals have cone-in-cone structure, which is typical of calcareous layers in overpressured 

shales (Stoneley, 1983) (fig. 95). Epigenetic pyrite, sphalerite, and minor rare-earth minerals apparently accompanied 

carbonate mineralization in the sheared stylolites (Diehl and 

others, 1992) (fig. 9#). For horizontal bedding-parallel

stylolites to open and allow fluid flow, pore-fluid pressure 

must have exceeded the lithostatic pressure of the 

overburden.

The stylolites in samples from the Wilson drill hole are 

largely in siliciclastic rocks, whereas stylolites in the Garrigan drill hole samples are in both siliciclastic rocks and 

shale. Swarms of microstylolites in shale from the Garrigan 

drill hole are both parallel and normal to bedding, which 

indicates a change in the maximum stress direction. The 

greater degree of fracturing and stylolite formation in the 

shale may be due to the location of the Garrigan drill hole in 

the severely disrupted Blytheville-Pascola arch complex or 

to the different physical properties and diagenetic changes 

between siliciclastic rocks and shale.

Carozzi and Von Bergen (1987) related enhanced 

porosity in connection with sutured stylolites in deeply buried carbonate rocks and the precipitation of secondary carbonate cements. But they also described low-amplitude, 

non-sutured stylolites in shale that had no influence on 

porosity development. In contrast, in our studies of rocks 

from the Reelfoot rift, the non-sutured stylolites in samples 

from deeply buried shale have been important fluid conduits 

in the past and have been episodically mineralized by carbonate cements (Braithwaite, 1989). The stylolites in the rift 

have reacted to maximum stress directions other than those 

that result from compaction. In responding to tensile 

stresses, the opened stylolites acted as lateral porous
N14 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

Figure 10. Thin-section photomicrograph showing a network of 

vertical and horizontal fractures filled with epigenetic carbonate 

minerals (Strake Petroleum No. 1 Russell drill hole, 4,118 ft). Vertical field of view is approximately 2.5 mm.

pathways through the rock. Diagenetic cements filling the 

stylolites are precipitated from fluids that may have migrated 

in response to seismic events, not from local sources due to 

dissolution of minerals along the stylolite (Sibson and others, 1975).

MINERAL-FILLED FRACTURES

A network of mineral-filled vertical and horizontal 

fractures with planar surfaces is present at mesoscopic and 

microscopic scales in core and cuttings from the Garrigan, 

Wilson, and Strake drill holes in the Reelfoot rift (fig. 6) 

(Swolfs, 1992). This network of fractures suggests vertical 

and horizontal continuity of permeability at the time of their 

formation. S wolfs (1992) interprets the vertical fractures in 

brittle sandstone to have formed as a result of extension that 

was caused by excess fluid pressure. Micro-thrust faults and 

veinlets with incremental fillings of quartz and carbonate

minerals indicate multiple episodes of dilation, possibly due 

to excess fluid pressures (fig. 9fi). Mineral fillings in the 

horizontal fractures, like the mineral fillings in stylolites, 

almost certainly indicate excess fluid pressure. In the Garrigan drill hole, a set of vertical veinlets, normal to bedding 

and limited to the siliciclastic rocks, commonly terminates 

against horizontal, clay-lined stylolites where strain energy 

was relieved along the ductile surface. Another, older, set of 

vertical fractures extends over a vertical distance of 3,000 ft 

through both the siliciclastic rocks and shale (Swolfs, 1992). 

These fractures may have provided flow paths for fluids 

(Diehl and others, 1992). The Strake, Garrigan, and Wilson 

drill holes are approximately 80 km apart and all have fractures in Paleozoic rocks filled with the same sequence offerroan dolomite followed by ferroan calcite cements (fig. 10). 

Assuming these carbonate minerals precipitated from a common source of fluids during late Paleozoic time, this same 

paragenetic sequence suggests regional hydraulic continuity 

and widespread excess fluid pressures. To summarize the 

above, opening of the fractures is required to allow deposition of minerals, and this opening suggests that fluid pressure 

in the rocks had to be greater than the lithostatic load.

Although the vertical fractures in the Wilson and Garrigan drill holes have the same sequence of mineralization (siderite, dolomite, ferroan dolomite, and ferroan calcite), these 

minerals are not present in stylolites in the Wilson drill hole. 

Only stylolites in the Garrigan drill hole and the horizontal 

fractures in the Strake drill hole, both of which are in the 

Blytheville-Pascola arch complex, have a similar sequence of 

authigenic carbonate mineralization. Samples from the Wilson drill hole, which is outside of the Blytheville-Pascola 

arch complex, contain minor dolomite-filled, horizontal fractures and do not show evidence of repeated opening of these 

fractures or of the stylolites. This suggests that rocks southeast of the arch complex had lower fluid pressure than the 

rocks within the arch complex during the period of fracturing.

SHALE FABRIC

Scanning-electron-microscope studies of samples from 

the thick sequence of Cambrian-age shale in the Garrigan 

drill hole show fabrics that indicate excess fluid pressure. 

Layers of randomly oriented illitic clay platelets, in which 

porosity is high, are interlayered with well-oriented clay 

platelets, in which porosity is low (figs. 1 \A and 1 IB). These 

clay-mineral fabrics are characteristic of overpressured and 

normal shale, respectively, in the Gulf Coast region (O'Brien 

and Slatt, 1990) where high-porosity shale layers have 

greater fluid pressure than low-porosity layers.

The high silt content of the shale in the rift is also a factor that contributes to excess-fluid-pressure conditions. The 

silt grains mixed with clay in the shale suggest that the rate 

of deposition and burial of the shale was rapid. During 

deposition, clay platelets tend to settle in random orientation
EVIDENCE OF EXCESS FLUID PRESSURE IN THE NEW MADRID SEISMIC ZONE N15

ORIENTATED 

CLAY PLATELETS

Figure 11. A, Scanning-electron photomicrograph showing well-laminated clay platelets parallel to bedding of shale (Dow Chemical No. 

1 Garrigan drill hole, 8,002 ft). B, Scanning-electron photomicrograph showing randomly oriented clay platelets in shale (Dow Chemical 

No. 1 Garrigan drill hole, 11,420 ft).

around a quartz grain, creating pore space that traps fluids. 

Energy-dispersive X-ray analyses indicate that the clay in 

the shale is mostly illite with some chlorite. Throughout the 

Paleozoic and with deep burial, diagenetic reactions may 

have added interstitial water to the shale by converting smectite to illite (Hower and others, 1976). Adding interstitial 

water to pore spaces by this process would elevate the fluid 

pressure, especially if the water could not migrate rapidly 

through overlying confining layers.

All of these factors—unoriented clay fabric, porous 

silty-shale layers, and the possible release of water through 

diagenetic conversion of clay—may have been factors that 

could contribute to the development of excess fluid pressure.

CAUSES AND ESTIMATES OF EXCESS 

FLUID PRESSURES

The evidence cited above leave little doubt that some 

stratigraphic units and fault zones in the upper 

Mississippi Embayment are currently under excess fluid 

pressure and that parts of the stratigraphic section during 

late Paleozoic and possibly early Mesozoic time had 

excess fluid pressure. The causes of excess fluid pressure 

have been studied extensively because of its importance 

to drilling for oil and gas and because of its importance in 

understanding the process of hydrocarbon migration and 

accumulation (Dutta and Levin, 1987). Some of the most 

commonly cited reasons for excess fluid pressure include: 

(1) mechanical-compaction disequilibrium, (2) fractures 

formed by tectonic extension or compression (i.e., extension would increase the volume of a mass of rock and 

decrease the pressure; compression would decrease the 

volume of rock and increase the pressure), (3) diagenetic 

phase changes of minerals, (4) tectonism, (5) aquathermal 

pressuring, (6) organic metamorphism, and (7) gravity

fluid flow (Dutta and Levin, 1987, chapters 1 and 2). 

Modeling studies show that compaction-driven fluid flow 

is too slow to maintain the temperatures of the ore-bearing fluids at the point at which ore was deposited in the 

discharge area of the Mississippi Valley-type deposits in 

the Ozarks (Cathles and Smith, 1983; Bethke, 1986). 

Fluid flow caused by changes in rock volume is analogous to compaction, and, presumably, the flow would also 

be slow. No evidence of large-scale diagenetic phase 

changes, aquathermal pressuring, or organic metamorphism is currently known. Of the several causes of 

excess fluid pressure listed above, gravity flow appears 

to be the most likely cause of current excess fluid pressure in the upper Mississippi Embayment. Our concept of 

current gravity flow is shown in the post-Late Cretaceous 

panel of figure 2. The one requirement in common for 

excess fluid pressure to exist over geologic time is that 

fluid flow must be retarded or restricted. No rock, however, is totally impervious, and some fluid will always 

flow away from an excessively pressured zone.

We infer that paleo-topography is the primary cause of 

ancient excess fluid pressures, although compaction or mineral diagenesis may also have been contributing factors at 

times. Both mechanisms require that relatively impermeable 

rock units be present to restrict flow. Compression may be a 

factor for excess pressure today, particularly with the presence of confining layers.

Our concept of the cause and direction of ancient fluid 

flow is shown in the Pennsylvanian and Permian panel of 

figure 2. In addition to favorable topographic conditions for 

gravity fluid flow, the structural setting of the Reelfoot rift 

and the Mississippi Embayment syncline, both of which are 

structural lows, enhance excess-fluid-pressure conditions 

because aquifers and confining units increase in depth 

toward the lows, thereby increasing the potential hydraulic 

head.
N16 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

CAUSE OF CONTEMPORARY EXCESS FLUID 

PRESSURE

As shown in figure 7, the highest fluid pressures in the 

upper Mississippi Embayment correlate closely with the 

thickness of the Midway Group confining unit (Porters 

Creek Clay), which overlies the McNairy-Nacatoch aquifer. 

Also, as stated above, this aquifer is in hydraulic continuity 

with aquifers in the underlying Paleozoic rocks. The flow 

paths of fluids from recharge areas for these aquifers cannot 

be explicitly defined, but recharge into the highly fractured 

rocks and through sinkholes in the Ozarks is well documented (Harvey, 1980).

The flow paths are controlled by fracture systems. Fractures occur within the hydrogeologic units and contribute to 

their permeability. Fractures in fault zones, however, 

transect the hydrogeologic units and provide permeable 

zones through which fluids can flow vertically or horizontally from one hydrogeologic unit to another. Also note that 

no rock is a perfect confining unit and some leakage of fluid 

occurs through all rocks.

Locally, much of the recharged water resurfaces at 

large springs, but a large volume of the water must be driven 

by gravity toward the topographic and structural low in the 

embayment. This flow is self-evident from the artesian flow 

of water from wells into Paleozoic and Cretaceous rocks 

within the embayment and from the chemical composition 

and temperature of the water in the wells. Permeability in the 

Paleozoic rocks is largely a function of the extent and abundance of fractures, both of which may be highly variable. 

Thus, the permeability is probably very heterogeneous. The 

evidence of this heterogeneous permeability of the Paleozoic 

rocks in the Ozarks is shown by the distribution of many 

very large springs and by tracing the flow paths of water that 

feeds the springs (Harvey and others, 1983; Harvey, 1980).

The Elvins Group, which is generally a confining unit, 

is overlain by the Potosi Dolomite (Upper Cambrian), which 

is one of the best aquifers in the Cambrian section (Harvey, 

1980). If the axial part of the rift was not disrupted by the 

Blytheville-Pascola arch complex, a permissible interpretation is that excess fluid pressure due to topographic relief of 

the Potosi Dolomite would not extend below the Elvins 

Group. However, as indicated in figures 3 and 4, the entire 

section of Paleozoic rocks is warped up against the 

Blytheville-Pascola arch complex. Formation of this complex resulted in intense fracturing and faulting as shown by 

seismic reflection and refraction profiles (Hamilton and 

McKeown, 1988; Hamilton and Mooney, 1990). Fluids in 

Paleozoic rocks, such as the Potosi Dolomite, are probably in 

hydraulic continuity with rocks in the arch. The Paleozoic 

aquifers are permeable because of fractures or sedimentary 

fabric, and these rocks are warped up against the complex. 

Fluids in the Paleozoic rocks probably enter fault and fracture zones in the arch complex, and the fluid pressure would 

be transmitted vertically through confining units in rocks

adjacent to the complex. Several wells provide evidence that 

this does occur. The fluid from the New Madrid test well 

flowed from a fault zone in part of the Elvins Group (Crone, 

1981), and water flowed from several zones in the Bonneterre Formation and Elvins Group in the Oliver and Benedum Trees wells. This strongly indicates fluid flow in the 

lower part of the Paleozoic section of rocks. Heterogeneous 

permeability is also evident in petrographic studies of samples of the Elvins Group in the Garrigan drill hole (Diehl and 

others, 1992; Swolfs, 1992). As described in the section Evidence of Ancient Excess Fluid Pressure, some parts of the 

drill core and cuttings show pores formed by randomly oriented clay platelets and other parts show clay platelets compressed with little evidence of porosity. Although 

heterogeneous permeability to depths of several kilometers 

is indicated by several kinds of data, magnetotelluric data 

indicate that rocks to depths of greater than 3 km in the arch 

complex have much lower resistivity than adjacent rock 

(Stanley and Rodriquez, 1992). This implies that, although 

the magnetotelluric method does not resolve zones of lower 

resistivity in detail, the rocks in the arch as a whole contain 

more saline fluid and are under higher fluid pressure than the 

rocks adjacent to the arch complex.

CAUSE OF ANCIENT EXCESS FLUID 

PRESSURE

The geologic history of the New Madrid region shows 

that topographic and structural conditions similar to those of 

today probably existed at the end of the Paleozoic Era. As 

stated in the previous section, the requirements for long-lasting excess fluid pressure are gravity flow of fluid into aquifers sealed by confining strata. Although fractured carbonate 

rocks and minor interbeds of sandstone would have been 

aquifers throughout most of Paleozoic time, widespread confining units were not deposited until the Mississippian and 

Pennsylvanian (fig. 2). The Ozark and Nashville domes, 

which have been structurally high since Cambrian time, 

were exposed during most of the Pennsylvanian (McKee and 

Crosby, 1975, plates 15A and 15B). Many of the sinkholes 

and caves, which are exposed today in the Ozarks, started to 

form before Pennsylvanian time (Wanless, 1975). Prior to or 

concomitant with the formation of the Blytheville-Pascola 

arch complex, a hydrologic regime with fluids driven by 

gravity flow very probably existed during the Pennsylvanian. The petrographic studies we described earlier show 

that excess fluid pressure existed not only in the most deeply 

buried rocks in the middle of the rift but also in rocks on the 

shelf adjacent to the rift. Post-Pennsylvanian uplift of the 

region caused erosion of the confining strata and destroyed 

the stratigraphic conditions needed to maintain excess fluid 

pressure.

A reversal of fluid-flow directions apparently occurred 

during Late Pennsylvanian or Early Permian time. This
EVIDENCE OF EXCESS FLUID PRESSURE IN THE NEW MADRID SEISMIC ZONE N17

reversal is inferred from multidisciplinary studies of the 

lead-zinc ore deposits in the Ozarks. Most geologists who 

have studied these ores concluded that they were deposited 

from hot brine solutions migrating through deep basins 

(Leach and Rowan, 1986). Identification of the basin(s) from 

which the brines originated and the fluid driving force, however, is controversial (Ohle, 1980). Leach and Rowan (1986) 

proposed that the source of the fluids was the Arkoma Basin 

and that gravity flow from the topographically high Ouachita 

Mountains south of the basin forced fluids out of the basin 

and up to the Ozark dome. Diehl and others (1992) and Goldhaber (oral commun., 1991) propose that the Reelfoot rift 

was possibly a flow path and that the topographically high 

Appalachian Mountains east of the rift forced gravity flow of 

fluids northwestward out of deeply buried sedimentary rocks 

in the rift and into the Ozark dome. Gregg and Shelton 

(1989) interpret the distribution of trace elements as evidence of multiple-basin flow paths for lead-zinc mineralization. They inferred that ore fluid came from basins both 

north and south of the lead-zinc deposits in the Ozarks. We 

agree with the suggestion of multiple basins and flow paths 

and also with the suggestion of Bethke (1986) and Click 

(written commun., 1988) that the Pascola arch area was high 

enough to cause gravity flow away from the arch. Bethke 

(1986) proposed flow northward, toward the Illinois Basin, 

and Glick (written commun., 1988) proposed that flow was 

westward, toward the Ozarks, during latest Paleozoic time 

and possibly throughout the Jurassic. Our arguments are 

similar to Click's, but we base our broader interpretations on 

additional data. Seismic reflection profiles (figs. 3 and 4) and 

drill-hole data show that nearly all Paleozoic rocks above the 

arch complex have been eroded. A pre-Upper Cretaceous 

unconformity truncates the arch of faulted Paleozoic strata. 

Stearns and Marcher (1962) estimated 8,000 ft to 15,000 ft 

of rock were eroded off the crest of the arch. Because the 

Ozark dome has always remained a structural and topographic high, it was never covered by as thick a section of 

rocks as those deposited in the Reelfoot rift. The thickness of 

Cambrian rocks on the dome compared to the thickness of 

Cambrian rocks in drill holes in the rift provides insight into 

the amount of rock eroded from the arch complex. Cambrian 

rocks on the north flank of the Ozark dome are about 1,200 

ft thick (Howe and Koenig, 1961). The thickness of Cambrian rocks in the Wilson drill hole is about 5,600 ft (E.E. 

Glick, written commun., 1989), and the same stratigraphic 

interval in the Garrigan drill hole is about 7,400 ft thick (Collins and others, 1992). These data show that this section of 

Cambrian rocks alone is at least 6,200 ft thicker in the middle 

of the rift than on the Ozark dome. Adding the remainder of 

the Paleozoic section to this thickness would yield a total 

thickness of rocks along the axial zone of the Reelfoot rift 

prior to formation of the arch complex quite compatible with 

the estimate of 8,000 ft to 15,000 ft made by Stearns and 

Marcher (1962). The removal of such a great thickness of 

rocks in the arch compared to the thickness of equivalent

rocks on the dome during the same time suggests that uplift 

of the arch complex must have been much greater than the 

uplift of the Ozark dome during the same time.

Based on stratigraphic data, the time of uplift is inferred 

to be between Late Pennsylvanian and Late Cretaceous (E.E. 

Glick, written commun., 1988). Several sets of independent 

data consistently indicate this approximate time of uplift. 

Paleomagnetic data show that the ore deposits in the Ozarks 

formed during Late Pennsylvanian and Early Permian (Wu 

and Beales, 1981; Wisniowiecki and others, 1983). Petrographic and fluid-inclusion studies of samples from the Bonneterre Formation in the Amoco Spence Trust No. 1 drill 

hole (fig. 6) show three episodes of burial and uplift in the 

Reelfoot rift (Tobin, 1991). Tobin (1991) concludes that the 

last uplift after deepest burial of sediments occurred in Late 

Mississippian to Early Pennsylvanian time and that, after 

subsidence during the Cretaceous and Tertiary, uplift has 

been occurring and continuing to the present. If the ores in 

the Ozarks were deposited from brines, some of which may 

have originated or passed though the arch complex, then 

reversal of fluid flow (from the flow direction that existed 

prior to formation of the arch complex) must have occurred 

during the same time interval. This uplift and erosional history strongly indicates that the arch complex started forming 

during the Pennsylvanian when the rift was filled with the 

greatest thickness of sedimentary rocks and when the constrained fluid was under the greatest pressure.

The above geologic history of the Ozark and upper Mississippi Embayment region provides an explanation of why 

and where excess fluid pressure conditions existed at the end 

of the Paleozoic Era, and it provides an explanation of the 

forced opening of stylolites, which were filled with mineralizing fluids. The regional structural, topographic, and lithologic conditions at the end of the Paleozoic Era were similar 

to those of today, except that the location of topographically 

high areas were shifted, and, therefore, the direction of fluid 

flow was reversed.

IMPLICATIONS OF EXCESS FLUID

PRESSURE ON EARTHQUAKES 

IN THE NEW MADRID SEISMIC ZONE

The apparent spatial correlation of earthquakes in the 

New Madrid seismic zone with excess fluid pressure 

observed in artesian wells (fig. 12) and inferred from low 

P-wave seismic velocities and magnetotelluric data seems 

more than fortuitous and implies a causal relation between 

the earthquakes and the excess fluid pressure as well as with 

the structural high. Al-Shukri and Mitchell (1988) noted a 

general correlation of lower P-wave velocities with 

seismicity, which is apparent by comparing the seismicity of 

figure 1 with the P-wave velocity contours on figure 8. 

Al-Shukri and Mitchell (1988) attribute the lower velocities 

to high pore pressure. They supported their arguments for the
N18 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

lower velocities due to high pore pressure with the laboratory data of Nur and Simmons (1969), who showed that 

P-wave velocities are 15-20 percent less in water-saturated 

samples that have pore pressure equal to external pressure. 

Al-Shukri and Mitchell (1988) also noted that the greatest 

reduction in velocity is in the upper 5 km of sedimentary 

rocks, where water is probably of meteoric origin. The data 

and arguments in this report conform to their interpretations.

Quantitative arguments supporting a causal relationship 

cannot be made because the failure criterion and state of 

stress of rocks in the hypocentral zone of the earthquakes are 

not known. However, estimates of excess fluid pressure at 

the surface and at depths from which artesian water flowed 

from Paleozoic rocks can be made and may provide insight 

into whether earthquakes could be expected to occur as the 

result of excess fluid pressure.

Assuming the pressure gradient for the commonly briny 

water from wells in Paleozoic rocks is 0.01 MPa/m, the 

excess hydrostatic pressure from the 27-m head of water 

estimated at the New Madrid test well is about 0.27 MPa and 

is a minimum value because water was flowing from the drill 

pipe. This low excess pressure is unlikely to trigger earthquakes based on the results of a detailed study of the amount 

of pore pressure required to initiate earthquakes (Raleigh and 

others, 1972). By determining the state of stress, the coefficient of friction, and by monitoring changes in pore pressure 

in the Rangely oil field, Raleigh and others (1972) found that 

earthquakes could be initiated or stopped by pore pressure 

changes of 3.5 MPa. However, in a study of a northeastern 

Ohio earthquake that occurred in 1986, 12 km from wells 

into which hazardous waste was being injected under high 

pressure, Nicholson and others (1988) estimated that the 

fluid injection into the wells did not raise the pore pressure 

in the hypocentral zone of the earthquake more than 0.4 

MPa, which does not agree with the Rangely data. They recognized, however, that some earthquakes have been caused 

by changes in pore pressure of only a few bars and concluded 

that their data were inadequate to determine the minimum 

amount of pore pressure increase necessary to cause the 

1986 earthquake.

Several factors common to the Rangely and Ohio areas 

are: (1) the pore pressure at the depths of the earthquakes 

were on the order of 20-25 MPa, (2) focal mechanisms of 

earthquakes in both areas showed strike-slip movements, 

and (3) both areas are in parts of the continental interior considered to be tectonically stable. These attributes are similar 

to those in the New Madrid seismic zone, and, thus, the 

results of these studies might be applicable to the New 

Madrid zone.

Based on the above studies, an obvious conclusion is 

that the small excess fluid pressure of 0.27 MPa estimated at 

the surface of the New Madrid test well would not be likely 

to trigger earthquakes. The multiple correlations of direct 

and circumstantial evidence of excess fluid pressure with the 

earthquakes in the New Madrid seismic zone, however, seem

to be more than a fortuitous relationship and require some 

explanation. Several possibilities are obvious. One is that 

only a small change (a few bars) in the pore pressure of rocks 

that are close to failure may be needed to trigger earthquakes. Historic and instrumental records of abundant and 

ongoing seismicity (Nuttli, 1973; Stauder and others, 1976) 

in the New Madrid seismic zone seems to indicate that rocks 

in the seismic zone are and have been near failure for at least 

several decades. A second possibility is that the pore pressure at hypocentral depths is much greater than the pressures 

indicated by hydrostatic head measured at the surface in artesian wells. Both of these possibilities are supported by circumstantial and documented evidence. A third possibility is 

that the failure criterion and state of stress are quite different 

at hypocentral depths than assumed from conventional methods of estimating them. This possibility cannot be evaluated 

within the scope of this report.

The evidence that increases in pore pressure as small as 

0.01 MPa can trigger earthquakes is documented by studies 

of reservoir-induced seismicity (Simpson, 1986) and by the 

correlation of seismicity with changes in river stages. Some 

investigators suggest that increases in pore pressure as small 

as 0.03 MPa, which are related to changes in water levels of 

rivers, can trigger earthquakes in regions where rocks are 

close to failure (Costain and others, 1987). The explanation 

of the process by which small changes of water levels near 

the surface can propagate to the hypocentral depths of earthquakes is controversial, but the relation of increases and 

decreases in seismicity with changes in water levels appears 

to be sound.

Several arguments can be made that pore pressure may 

exceed hydrostatic pressure in some lithologic or fracture 

zones at hypocentral depths in the New Madrid seismic zone. 

We cite the opening of stylolites as evidence of ancient 

excess fluid pressure, and we argue that the fluid movement 

was primarily gravity driven, but compaction or diagenetic 

phase changes may have contributed to hydraulic gradients. 

Also, we cite differences in the porosity of Upper Cambrian 

shale and the distribution of fractures as evidence of heterogeneous permeability. These conditions apparently existed 

during the late Paleozoic and possibly the early Mesozoic. 

The opening of stylolites, described in earlier sections, 

required that the hydrostatic fluid pressure be at least equal 

to the lithostatic pressure. Because structural, lithologic, and 

topographic conditions today are similar to those during the 

late Paleozoic, the inference may be made that zones of 

higher porosity and permeability with fluid overpressure 

exist today in the lower part of the Paleozoic section, as in 

the past. Evidence of this is from the excess pressure of water 

from the lower parts of the Paleozoic section penetrated in 

the New Madrid test, the Strake, the Oliver, and the Benedum-Trees wells. Although excess-fluid-pressured zones 

cannot be identified explicitly within the New Madrid seismic zone, the abundant evidence of excess fluid pressure 

from hydrologic, stratigraphic, petrographic, geophysical,
EVIDENCE OF EXCESS FLUID PRESSURE IN THE NEW MADRID SEISMIC ZONE N19

Table 1. Estimates of excess fluid pressures at bottom of drill 

pipes.

[See fig. 6 for complete names and locations of drill boles. Altitude 1, altitude (in m) 

of recharge area; altitude 2, altitude (in m) of top of drill pipe; estimated excess pressure 1, estimated excess pressure (in MPa) from difference in altitude and pressure gradient of 0.01 MPa/m; excess pressure 2, excess pressure (in MPa) equivalent to 

frictional loss in drill pipe]

Well 

name

New Madrid

test well......

Oliver............

Stroke............

Benedum

Trees...........

Flow 

(L/min)

. 1,892.5

. 2,649.5

.... 567.8

....265.0

Altitude 1

305

305

305

305

Altitude 2

114

98

82

82

Estimated 

excess 

pressure 1

1.9

2.1

2.2

2.2

Excess 

pressure 2

1.1

3.1

0.1

0.4

and seismological data suggests that excess-fluid-pressure 

zones may be very common, if not abundant, particularly in 

parts of the Reelfoot rift that are highly faulted.

Excess hydrostatic pressure may be considerably 

higher at the depth at which water enters a drill pipe than 

the pressure estimated at the top of the pipe. It is obvious 

that for flow to occur up the pipe, the pressure at the bottom of the pipe must be greater than at the top. Two methods of estimating the pressure at the bottom of a drill pipe 

are: (1) assume static conditions for water level in the pipe 

(that is, no flow): the pressure at the bottom of the pipe is 

the difference in altitude between the recharge area and 

the altitude at the top of the pipe multiplied by the pressure gradient of the water (0.01 MPa/m), (2) calculate frictional loss of pressure in the pipe from the depth at which 

water enters the pipe to the top of the pipe where water is 

discharging. Some of the available data needed to apply 

these methods have considerable uncertainties. For example, estimating the altitude of the recharge areas is a subjective judgment that is based on the concept that the 

pressure at any depth cannot exceed the highest altitude 

that water is entering a hydrologic system, even though 

much of the water may be discharged through springs or 

leakage through confining layers in transit to the greatest 

depth at which the water is tapped. On the basis of structure contours of the tops of the Ozark and St. Francois 

aquifers (Imes, 1990a, 1990b), the altitude of the highest 

recharge areas for both aquifers is about 305 m. Computation of frictional losses in a pipe is based mainly upon 

empirical data and a standard method of calculating frictional losses given by Rouse (1950, p. 112). This method 

involves computing the Reynolds number from which a 

frictional coefficient is estimated and used in the Weisbach equation that relates frictional loss of pressure in a 

pipe as a function of velocity of flow, the diameter of the 

pipe, and the length of the pipe. Probably the most important parameter in these calculations is the uncertainties in 

estimates or measurements of flow out of the top of a

drill pipe because loss of pressure through a pipe is a 

function of the velocity of flow squared. Given these caveats, table 1 shows the estimated excess fluid pressures 

using each method for the depths at which water entered 

drill pipes of four wells that penetrated Paleozoic rocks 

and lists the parameters used to make the estimates.

Pressures at the bottom of a well, calculated by the two 

methods given above, should be equal values. The frictional 

loss of pressure for all wells, except the Oliver well, is less 

than the pressure estimate from the difference in altitude of 

the recharge area to the bottom of the well. This difference 

may be the result of loss of pressure through the aquifers. 

The excessive pressure computed from frictional loss in the 

drill pipe of the Oliver well is 1 MPa more than the pressure 

estimated from differences in altitude. We cannot explain 

this difference except as noted above that the rate of flow is 

a critical parameter in the frictional-loss equation. Flow from 

the New Madrid test well was measured through a weir (A.J. 

Crone, oral commun., 1992) and is probably a good estimate. 

We suspect that rates of flow for the other three wells were 

only estimated and could have large errors.

More important than the pressure estimates at the bottom of the drill holes in Paleozoic rocks is the pressure at the 

source of the water entering the drill holes. Analogous to the 

reasoning for computing frictional loss of pressure in the 

drill pipe, the pressure at the source must be greater than that 

at the bottom of the pipe in order for water to rise to the bottom of the pipe. Frictional loss of pressure would occur for 

water flowing up through rocks from the source to the bottom of a drill pipe. Data are inadequate, however, to estimate this loss in pressure. The minimum pressure that might 

be expected is the pressure estimated at the bottom of the 

drill pipe plus the calculated hydrostatic pressure to the 

source.

If a causal relationship between contemporary earthquakes and changes in fluid pressure exists as we suggest, 

contemporary earthquakes may be representative of 

long-lived background seismicity and are not aftershocks of 

the 1811-12 sequence of earthquakes because the current 

fluid-pressure conditions have probably existed in the 

Blytheville-Pascola arch complex since at least Eocene time 

(after deposition of the Porters Creek Clay). The evidence 

that excess fluid pressure existed in the lower part of the Porters Creek Clay is the presence of sandstone dikes in the Porters Creek at several places in western Kentucky (Glenn, 

1906; Olive, 1980). The suggestion that the current background of seismicity has existed since the Eocene is contradicted, however, by the fact that, during the Tertiary, the 

state of stress was probably very different than it is today. 

Northeast-striking normal faults are common in Tertiary 

rocks, and some displace the Lafayette Gravel (Grohskopf, 

1955; Olive, 1980; McKeown, 1982), whose age is considered to be Pliocene or Pleistocene. McKeown and others 

(1988) summarize the evidence and arguments for these 

ages. Northeast-trending normal faults indicate that the max-
N20 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

imum principal horizontal stress would have been oriented 

northeast-southwest, which is about 35° from its present orientation. If, therefore, seismicity started when the state of 

stress changed to its present orientation, the current rate of 

seismicity may have been occurring since the Pliocene (5 

Ma) or Pleistocene (1.6 Ma). This inference is somewhat 

surprising because of the lack of evidence of more than three 

(Russ, 1979) (or possibly only one (Wesnousky and Leffler, 

1992)) large-magnitude earthquakes in the New Madrid seismic zone during the Quaternary. If the recurrence rate of 

large earthquakes is about 600 years, as estimated by Russ 

(1979), then more than 10,000 large earthquakes would be 

expected to occur during about the last million years. However, there is no evidence for this large number of earthquakes. A major assumption in postulating the large number 

of earthquakes is that the level of stress today, as well as the 

orientation, has been the same during the past million years 

or so.

It is possible that the recurrence of earthquakes within 

excess-fluid-pressure zones may be a function of the rate of 

flow of fluids from one zone to another or of the time 

required to recharge a fault zone to excess fluid pressure 

after failure of the zone. Recharge of a fault zone large 

enough to cause large earthquakes may take thousands of 

years, depending upon the rate of fluid flow and the rate of 

leakage from a zone. The changes in porosity and permeability of fault zones caused by fracturing from a large earthquake must also be considered; past pressures could have 

been higher or lower than contemporary pressures. Fluctuations in excess fluid pressure over hundreds or thousands of 

years certainly may be expected as the result of changes in 

rainfall or changes in the access of water to aquifers as the 

result of erosion and incision of streams into carbonate rocks 

in the Ozarks. In addition, even though leakage through confining layers may occur, subsurface flow of water under a 

high hydrostatic head will occur. Dissolution and deposition 

of minerals will be a continuous, although slow, process that 

will ultimately change the porosity and permeability of the 

rocks through which water flows. Because artesian flow of 

water from deeply buried Paleozoic rocks is occurring today, 

dissolution and deposition of minerals must be considered 

active contemporary processes. Qualitative estimates of 

these processes and their rates could be made based upon 

mineralogic and geochemical studies related to ore deposits, 

but those estimates are beyond the scope of this report.

If our conclusion is correct that earthquakes may occur 

where faults are under excess fluid pressure in a major zone 

of structural deformation such as the Blytheville-Pascola 

arch complex, similar pressure and structural conditions 

elsewhere should be considered as potential earthquake 

source zones. Hamilton and Mooney (1990) suggested that 

the 8-km-wide fault zone that is the southeastern boundary 

of the Reelfoot rift has low seismic-velocity characteristics, 

similar to those of the arch complex. These authors also 

noted that minor earthquake activity occurs in this zone.

Excess fluid pressure in the Cretaceous aquifer near Memphis, Tenn. (Wells, 1931), and presumably in underlying 

Paleozoic rocks, implies that the zone may have the same 

hydrologic characteristics as the arch complex. The concordance among these data suggests that the fault zone bounding the southeastern margin of the Reelfoot rift may be a 

source zone with a potential for earthquakes nearly as great 

as that of the arch complex. Available seismic reflection 

data, however, do not show unambiguously that faults in the 

southeastern rift boundary zone extend to depths as great as 

those that bound the Blytheville-Pascola arch complex. 

Earthquakes on faults in the southeast boundary zone presumably would have smaller source dimensions than faults 

in the arch complex. Rupture of faults in the southeast 

boundary fault zone of the Reelfoot rift, therefore, may not 

produce earthquakes as large as those that may occur in or 

along the boundaries of the Blytheville-Pascola arch complex. Much more data are required to resolve this question.

CONCLUSIONS

The principal conclusions made from the data and arguments presented in this report are summarized as follows:

1. Hydrologic data provide direct evidence of excess 

fluid pressure at depths of about 1.3 km, and this 

pressure is the result of gravity-driven flow. These 

data also show that pressure is maintained by confining units, which severely retard vertical flow 

from underlying aquifers. The principal confining 

unit is the Porters Creek Clay in the Midway 

Group of Paleocene age.

2. The geologic history of the upper Mississippi 

Embayment and adjacent highlands and the petrographic evidence of ancient excess fluid pressure 

indicates that the regional geologic and hydrologic 

regime that is causing excess pressure today may 

be special. The late Paleozoic and possibly early 

Mesozoic appear to be the only other times when 

a similar regime existed. Because of the similarity 

of regimes and evidence of contemporary deep 

subsurface flow of fluids, it is reasonable to 

assume that high fluid pressure, and dissolution 

and precipitation of minerals at hypocentral 

depths, may be occurring today. These processes, 

of course, would continually change porosity and 

permeability of the rocks through which fluids 

flowed. The rate of change of these parameters, 

however, is beyond the scope of this report.

3. Drill-hole data provide direct evidence of fluid 

flow from fault zones. Interpretation of seismic 

reflection profiles indicate that the Blytheville-Pascola arch complex has many faults that extend 

through the Paleozoic section; these faults are 

inferred to provide zones of vertical continuity for
EVIDENCE OF EXCESS FLUID PRESSURE IN THE NEW MADRID SEISMIC ZONE N21

v '- 

Blytheville /

36°00'

35°30'

35°00'

EXPLANATION

—————— Rift boundary + Earthquake epicenter Pluton

Figure 12. Generalized map derived from figures 1,6, and 7 showing relationship of the Blytheville-Pascola arch complex and excessfluid-pressure zone to seismicity. Excess-fluid-pressure zone is within the heavy line, dashed where uncertain.

the flow of fluid and thus allow pressure to be distributed heterogeneously. The chemical characteristics and thermal data of water from wells in 

Paleozoic rocks and the contiguous Cretaceous 

aquifer indicate upward flow of water from depths 

of several kilometers.

4. The spatial correlation of earthquakes with the 

Blytheville-Pascola arch complex and the distribution of excess fluid pressure within the Reelfoot

rift (fig. 12) appears to be more than fortuitous. 

Estimates of excess fluid pressures at shallow 

hypocentral depths, however, are less than those 

required to trigger earthquakes according to conventional ideas on the state of stress in the Earth's 

crust. Yet, the correlation between changes in seismicity that result from very small changes in pore 

pressure related to filling of reservoirs and river 

stages suggest that current ideas on the state of
N22 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

stress may need to be modified. 

5. If earthquakes in the New Madrid seismic zone 

are causally related to zones of excess fluid pressure in the Blytheville-Pascola arch complex, then 

many important ramifications should be considered. These include providing a physical basis for 

delimiting seismic zones, determining the duration 

of background seismicity, and possibly estimating 

recurrence rates of earthquakes in source zones as 

a function of size of the zone and time required to 

pressurize it by fluid flow.

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Published in the Central Region, Denver, Colorado 

Manuscript approved for publication September 28,1993 

Edited by Richard W. Scott, Jr. 

Graphics prepared by Wayne Hawkins 

Photocomposition by Norma J. Macs

* U.S. QOWHaWHOT PRINTING OFFICE: 1994-673-046/86C54