Magnetic and Gravity Study of the

Paducah I°x2° CUSMAP Quadrangle,

Illinois, Indiana, Kentucky, and Missouri

U.S. GEOLOGICAL SURVEY BULLETIN 2150-C

Paducah I°x2°

Quadrangle
Cover. Index map showing the approximate location of the Illinois Basin (shaded 

area) and the location of the Paducah I°x2° CUSMAP quadrangle.
Magnetic and Gravity Study of the 

Paducah I°x2° CUSMAP Quadrangle, 

Illinois, Indiana, Kentucky, and Missouri

By Thomas G. Hildenbrand, Robert P. Kucks, and Paul C. Heigold

THE PADUCAH CUSMAP QUADRANGLE: RESOURCE AND TOPICAL INVESTIGATIONS 

Martin B. Goldhaber, Project Coordinator

U.S. GEOLOGICAL SURVEY BULLETIN 2150-C

A joint study conducted in collaboration with the Illinois State 

Geological Survey, the Indiana Geological Survey, the 

Kentucky Geological Survey, and the Missouri 

Division of Geology and Land Survey

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1996
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

Hildenbrand, T. G.

Magnetic and gravity study of the Paducah I°x2° CUSMAP Quadrangle, Illinois, 

Indiana, Kentucky, and Missouri / by Thomas G. Hildenbrand, Robert P. Kucks, and 

Paul C. Heigold.

p. cm. (The Paducah CUSMAP Quadrangle, resource and topical 

investigations ; C) (U.S. Geological Survey bulletin ; 2150)

Includes bibliographical references.

Supt.ofDocs.no.: I 19.3:2150-C

1. Geology, Structural Middle West. 2. Magnetic anomalies Middle West. 

3. Gravity anomalies Middle West. [. Kucks, R. P. II. Heigold, Paul C. 

III. Title. IV. Series. V. Series: U.S. Geological Survey bulletin ; 2150. 

QE75.B9 no. 2150-C 

[QE627.5.M55] 

557.3 s dc20

[526'.7'0977] 95-42708

CIP
CONTENTS

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

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

Geologic History................................................................................................................... 4

Aeromagnetic and Gravity Data........................................................................................... 5

Depth to Magnetic Basement................................................................................................ 5

Magnetic and Gravity Features............................................................................................. 6

Paducah Gravity Lineament (PGL)............................................................................ 7

South-Central Magnetic Lineament (SGML)............................................................. 8

Fault Zones................................................................................................................. 9

Profile Modeling................................................................................................................... 9

Profile W-W............................................................................................................... 10

Profiles X-X' and Y-Y'............................................................................................... 10

Discussion............................................................................................................................. 14

Rifts............................................................................................................................. 15

Paducah Gravity Lineament....................................................................................... 15

South-Central Magnetic Lineament............................................................................ 17

Shallow Dikes............................................................................................................. 18

Summary and Conclusions ................................................................................................... 18

Acknowledgments................................................................................................................ 20

References Cited................................................................................................................... 20

FIGURES

1. Reference map of the northern Mississippi Embayment region showing the study area (the Paducah I°x2°

quadrangle)................................................................................................................................................................... C2

2. Reduced-to-pole magnetic anomaly map..................................................................................................................... 3

3. Residual gravity anomaly map produced by removing a second-order orthogonal polynomial (shown in fig. 4)

from the gravity field ................................................................................................................................................... 4

4. Regional gravity anomaly map generated by fitting the gravity field with a second-order orthogonal

polynomial surface....................................................................................................................................................... 6

5. Maximum depth to magnetic basement....................................................................................................................... 7

6. Boundaries of the Paducah gravity lineament (PGL) and the south-central magnetic lineament (SGML)................. 8

7. Theoretical models and potential-field anomalies along W-W, X-X', and Y-Y'........................................................ 11

8. Interpreted large intrusions .......................................................................................................................................... 14

9. Gravity anomaly map of the northern Mississippi Embayment region ....................................................................... 16

10. High-pass magnetic anomaly map ............................................................................................................................... 19

III

MAGNETIC AND GRAVITY STUDY OF THE

PADUCAH I°x2° CUSMAP QUADRANGLE,

ILLINOIS, INDIANA, KENTUCKY, AND MISSOURI

By Thomas G. Hildenbrand, 1 Robert P. Kucks,2 and Paul C. Heigold3

ABSTRACT

Magnetic and gravity data in the Paducah 1 °x2° quadrangle (the study area) provide a geologic picture of the subsurface that indicates a long and complex tectonic and 

magmatic history. A prominent, continental-scale magnetic 

lineament (called the south-central magnetic lineament) possibly delineates a Precambrian northwest-trending shear 

zone intruded by large, intermediate igneous bodies. Models 

crossing these intrusions suggest that the sources are composed of highly magnetic, intermediate rock, such as quartz 

diorite. Two Early Cambrian rifts (the Reelfoot and Rough 

Creek) trend into the study area. Interpreted structures and 

magnetic basement depths suggest that the Reelfoot graben 

bends eastward to join with the Rough Creek graben. Paralleling the south-central magnetic lineament to the southwest 

is a pronounced gravity lineament that may represent dense 

mafic intrusions emplaced along northwest-trending faults 

and axial faults of the Reelfoot graben. The age of these 

intrusions is interpreted to be Cambrian (syn-rift) or 

younger. Strike-slip motion along the northwest-trending 

faults is suggested by deflections of magnetic anomalies 

along the margins of the Reelfoot graben.

Post-rifting geology is characterized by thick sequences 

of sedimentary rocks, as evidenced by the deepening of magnetic basement (>6 km) in the Rough Creek graben in the 

eastern part of the Paducah quadrangle. Shallow, large magnetic intrusions of post-rift age may be present in or near the 

Wabash Valley fault system and in the region of tripoli 

(microcrystalline silica) deposits in southwestern Illinois. A 

high-pass magnetic anomaly map delineates as many as 14 

shallow (<1 km), plug-like or laccolithic intrusions in the 

eastern part of the study area. Three of these shallow

'U.S. Geological Survey, 345 Middlefield Road, Menlo Park, 

California 94025.

2 U.S. Geological Survey, Box 25046, Mail Stop 964, Denver Federal 

Center, Denver, Colorado 80225.

Illinois State Geological Survey, 615 East Peabody Drive, 

Champaign, Illinois 61820.

intrusions coincide with known or indicated shallow intrusive centers at Hicks dome, Omaha dome, and Coefield; 

other interpreted shallow intrusions may also have mineral 

potential similar to that at Hicks dome.

INTRODUCTION

The study area (Paducah I°x2° quadrangle, fig. 1) is 

surrounded by major uplifts and basins. To the west, in 

southeastern Missouri, Precambrian basement rises toward 

the crest of the Ozark uplift and crops out in the St. Francois 

Mountains. Basement also rises to the east along the northsouth-trending Cincinnati arch in central Kentucky. To the 

north and southwest, basement descends beneath the Illinois 

Basin and Mississippi Embayment, respectively. The study 

area is structurally complex and includes at least seven major 

fault systems. Two rifts intersect or join within the quadrangle. Moreover, a continental-scale crustal discontinuity 

crosses the study area. Numerous igneous intrusions have 

been emplaced along this discontinuity. The study area is 

structurally one of the most complex regions in the Midcontinent. Regional potential-field studies aid in our understanding of the structures of the Paducah quadrangle and can 

provide a meaningful geologic picture of the subsurface.

Past potential-field studies have contributed significantly to the understanding of structures and lithologies 

within the study area. Ervin and McGinnis (1975) combined 

gravity, seismic, stratigraphic, and petrologic data to suggest 

that the Mississippi Embayment is the site of a northeasttrending, late Precambrian rift, which they called the Reelfoot rift. Subsequent gravity and magnetic studies (Kane and 

others, 1981; Hildenbrand and others, 1982; Hildenbrand, 

1985b; Hildenbrand and Hendricks, 1995)'have further 

defined the buried rift structures, some of which extend into 

the study area. Cordell (1977) suggested that the Reelfoot rift 

extends northward into southern Illinois and western Kentucky, where broad gravity highs are interpreted as evidence 

for a fossil rift cushion at the crust-mantle boundary. An 

east-west-trending rift graben (Rough Creek graben),

Cl
C2

92

THE PADUCAH CUSMAP QUADRANGLE: RESOUCE AND TOPICAL INVESTIGATIONS 

91° 90° 89° 88°

38

Bloomfield \^i /^ \^/ '// - -'' ;"

pluton :/ ' >$> X^ /' \

\ . /V o,^_ \

^ Maiden pluton

____ A

Paragould

0 10 20 30 40 50km

EXPLANATION

Northern limit of coastal-plain material 

of the Mississippi Embayment

Mafic or ultramafic intrusions within the 

Mississippi Embayment identified in 

drill-hole cuttings. Intrusions labeled 

PO and LO are also locations of the 

Pure Oil McGregor No. 1 and the Lion 

Oil Company No. 1 wells, respectively.

Mafic intrusion within the Mississippi

Embayment interpreted from the magnetic 

field. Approximate boundaries of 

intrusions determined from zero contour 

of associated anomaly on a second vertical 

derivative map.

Long dashed lines show principal magnetic 

lineaments reflecting faulting and lithologic 

contrasts in magnetic basement, a and b 

are lineaments discussed in text.

Fault- Dashed where inferred

Possible or hypothetical fault. Locations based 

on subsurface data or exceptionally strong 

lineaments from aerial photographs.

Approximate margins of the Reelfoot graben.

Locations of the Big Chief (BC) and Dow 

Chemical 1 Wilson (DW) drill holes.

City or town Earthquake epicenter
MAGNETIC AND GRAVITY STUDY OF THE PADUCAH I°x2° CUSMAP QUADRANGLE C3

Figure 1 (facing page). Reference map of the northern Mississippi Embayment region showing the study area (the Paducah I°x2° 

quadrangle; heavy black line). After Hildenbrand (1985b).

defined with gravity, magnetic, and drill-hole data (Soderberg and Keller, 1981), meets the northeast-trending Reelfoot rift within the study area. Braile and others (1982, 1986) 

used gravity, magnetic, and seismic data to interpret a quadruple rift junction, which they called the New Madrid rift 

complex. In their model, the Reelfoot and Rough Creek rifts 

form two failed rift arms; potential-field anomalies extending northeast and northwest from the juncture of the Reelfoot 

and Rough Creek rifts may represent the remaining two 

arms.

Prominent northwest-trending magnetic and gravity 

anomalies cross the study area and are aligned along mapped 

and inferred faults (Lidiak and Zietz, 1976; Hildenbrand and

others, 1982,1983). Of particular interest is the south-central 

magnetic lineament (Hildenbrand and others, 1983; Hildenbrand, 1985a; Ravat, 1984), which reflects a continentalscale geologic feature that extends from eastern Tennessee to 

Missouri.

This paper reviews these earlier potential-field studies 

and considers the tectonic and magmatic development of the 

study area and their interrelationships. For this purpose, aeromagnetic and gravity anomaly maps (figs. 2 and 3) were 

prepared from available digital data. The nature of digital 

data allowed application of several analytical techniques to 

enhance and invert the anomalies and provide new interpretive information. Some geophysical anomalies correlate well 

with major known Precambrian and Paleozoic tectonic features and aid in delineating their lateral extent and associated 

structure. Although the sources of some anomalies are not 

completely understood, observed patterns and correlations 

yield new insights into the tectonic and magmatic evolution 

of the region.

38°

37°

-300 -200 -100 0

10

100 200

20 30 40

300 400

50 KILOMETERS

500 NANOTESLAS

Figure 2. Reduced-to-pole magnetic anomaly map. Models along profiles X-X', Y-Y', and W-W are shown in figure 7. Anomalies A 

through F are discussed in the text. Hachure patterns depict occurrence of silica deposits in southwestern Illinois (Lamar, 1953).
C4

38°

THE PADUCAH CUSMAP QUADRANGLE: RESOUCE AND TOPICAL INVESTIGATIONS

89° 88°

37°

-4 8

10

12

20 30

16

40

20 24

50 KILOMETERS

28

J

MILLIGALS

Figure 3. Residual gravity anomaly map produced by removing a second-order orthogonal polynomial (shown in fig. 4) from the gravity 

field. Models along profiles X-X', Y-Y', and W-W are shown in figure 7. Anomalies A through F are discussed in the text.

GEOLOGIC HISTORY

Precambrian crystalline basement contains the major 

gravity and magnetic sources within the study area. Precambrian rocks crop out only in the St. Francois Mountains to the 

west near the crest of the Ozark Uplift, where unmetamorphosed volcanic and related epizonal intrusive rocks (~ 1.5 

Ga) (Bickford and others, 1981; Kisvarsanyi, 1981) are 

exposed. This granite-rhyolite terrane is extensive in the 

southern and central interior of the craton (VanSchmus and 

others, 1993). Magnetite-trachytes and diabase dikes are also 

present in the St. Francois terrane, but they are not areally 

extensive. In Missouri, the granite-rhyolite terrane overlies a 

gneissic pre-volcanic basement (about 1.6 Ga), generally 

having a northwesterly structural grain (Kisvarsanyi, 1984). 

To the south within the Reelfoot graben, the Lee Wilson well 

(DW, fig. 1) encountered granitic gneiss, once buried as deep 

as 15 km (Denison, 1984). To the north in the Illinois Basin,

drill holes reaching Precambrian basement have encountered 

granite-rhyolite rocks (Kolata and Nelson, 1991), except in 

Lawrence County, Indiana (lat 38°48' N. long 86°24/ W.) 

where ophitic basalt was recovered (Lidiak and others, 

1985). In eastern Kentucky and Tennessee, the Grenville 

Province rocks (1.1 Ga) developed in and adjacent to the 

granite-rhyolite St. Francois terrane. This province consists 

of a variety of rock types: among these are mafic and felsic 

volcanics, amphibolite, and mafic granulite (Lidiak and others, 1985).

North America was part of a larger supercontinent in 

Late Proteiozoic time. During the Late Proterozoic to Early 

Cambrian, continental breakup resulted in the formation of a 

passive continental margin along the present southern and 

eastern edge of the proto-North American craton (Hendricks, 

1988; Kolata and Nelson, 1991). Several aulacogens (or 

failed rifts) and strike-slip faults developed in conjunction 

with the formation of this new continental margin; the
MAGNETIC AND GRAVITY STUDY OF THE PADUCAH Fx2° CUSMAP QUADRANGLE C5

Reelfoot rift (Ervin and McGinnis, 1975; Hildenbrand, 

1985b; Hendricks, 1988) and the Rough Creek graben 

(Soderberg and Keller, 1981) also formed during this breakup, probably in Cambrian time (Thomas, 1991). The Reelfoot rift trends northeast from east-central Arkansas to the 

study area and is reflected as a 70-km-wide graben with a 

structural relief of about 2 km (Hildenbrand, 1985b). Large 

plutonic bodies (fig. 1) were emplaced along the margins of 

the graben probably during Cretaceous time or later. The 

Rough Creek graben, with a maximum structural relief of 

greater than 6 km, trends east-west in western Kentucky and 

southern Illinois (Soderberg and Keller, 1981). Kolata and 

Nelson (1991) suggested that the Rough Creek graben is a 

simple extension of the Reelfoot graben, and, thus, the quadruple junction of Braile and others (1986) does not exist.

Due to the confusion that sometimes involves the term 

"rift," we define "rift" here as a fundamental flaw in continental crust along which the entire lithosphere has ruptured 

under extension. A clear distinction is made between rift and 

graben, in that the graben is one near-surface manifestation 

of the rift.

A post-rifting phase in early Paleozoic time included 

thick accumulation of sediments in the Reelfoot basin of 

western Kentucky and southeastern Missouri (Schwalb, 

1978). From early Paleozoic until Middle Pennsylvanian, 

regional deposition and subsidence continued intermittently 

throughout the region (Click, 1975, 1982); late Paleozoic 

and Mesozoic uplift resulted in the formation of the Pascola 

arch (fig. 1). The rise of the arch created the southern margin 

of the Illinois Basin. Subsidence and accompanying deposition of predominantly marine clastic sediments, attaining a 

thickness of 1 km west of Memphis, Tenn., began in the Late 

Cretaceous and are preserved in, and directly adjacent to, the 

present Mississippi Embayment (Stearns and Marcher, 

1962). The Reelfoot graben, lying beneath the Mississippi 

Embayment, noticeably contains the area of principal seismicity in the Midcontinent (fig. 1) and, in particular, the epicentral line of the devastating 1811-12 New Madrid 

earthquake series (Nuttli, 1982).

Dikes, diatremes, and plugs as young as Late Cretaceous intrude Precambrian basement and younger formations within the study area and surrounding regions. For 

instance, the emplacement of intrusions in the Devonian 

(Zartman and others, 1967) may have been accommodated 

by the Ste. Genevieve fault (fig. 1). In the Illinois-Kentucky 

fluorspar district near lat 37°30' N. and long 88°15' W., Permian mica peridotite dikes and diatremes are restricted to 

northwest-trending fractures (Watson, 1967). Permian lamprophyric dikes have been encountered in drill holes (fig. 1) 

in rocks of Cambrian and Ordovician age on the Pascola arch 

(Zartman, 1977). Late Cretaceous lamprophyre, syenite, and 

nepheline syenite (Moody, 1949; Kidwell, 1951) have been 

encountered in wells in southwest Tennessee. The Magnet 

Cove complex of ring dikes in east-central Arkansas (Erickson and Blade, 1963) and the syenite intrusions near Little

Rock (Gordon and others, 1958) are composed of Late Cretaceous alkalic rocks (Zartman, 1977).

AEROMAGNETIC AND 

GRAVITY DATA

The digital set of aeromagnetic data for the Paducah 

quadrangle was compiled by Kucks (1990) from several 

published surveys. Flight-line spacing of these surveys was 

1.6 km (1 mi), except for a small survey in the southwest corner of the quadrangle where the spacing was 0.8 km (0.5 mi). 

A 0.5-km grid of magnetic values was prepared for each survey using minimum curvature (Briggs, 1974) and a computer algorithm developed by Webring (1982). The grids 

were analytically continued to a consistent 305 m (1,000 ft) 

above mean terrain before compositing to create a compatible data set. Data were reduced to the- north magnetic pole, a 

procedure that shifts the anomalies to positions above their 

sources. The resulting map is shown in figure 2.

The gravity data were earlier compiled by Hildenbrand 

and others (1979) at a scale of 1:250,000. All data were tied 

to the IGSN-71 gravity datum (Morelli and others, 1974) and 

reduced to simple Bouguer anomaly values using a reduction 

density of 2.67 g/cm3 and the 1967 formula for theoretical 

gravity (International Association of Geodesy, 1967). 

Approximately 4,800 gravity stations at an average spacing 

of 4 km were used to construct a 1-km grid of values.

By the polynomial fitting method, a regional field was 

removed from the gravity data in an attempt to eliminate 

effects of deep sources and shallow, broad sources and therefore enhance the gravity signature of shallow, local features. 

A second-order orthogonal polynomial (SURFIT Branch 

of Geophysics, 1989) was derived from a gravity field over 

a region extending 2° beyond the boundaries of the Paducah 

quadrangle. This regional field over the Paducah area is 

shown in figure 4 and was subtracted from the gravity field 

(Hildenbrand and others, 1979) to produce the residual field 

shown in figure 3.

DEPTH TO MAGNETIC BASEMENT

Magnetic basement is defined here as Precambrian 

crystalline rocks or igneous rocks of younger age. Phanerozoic sedimentary rocks are assumed to produce little or no 

effect on the magnetic field.

An interpretational method developed by Vacquier and 

others (1951) was employed to roughly estimate the maximum depth to magnetic basement (fig. 5). In this method, we 

compared observed anomalies with theoretical anomalies 

produced by vertical prisms of various dimensions and different orientations of the Earth's ambient field. It is assumed 

that all magnetic sources are uniformly polarized in the same
C6 THE PADUCAH CUSMAP QUADRANGLE: RESOUCE AND TOPICAL INVESTIGATIONS

38°

37°

-18 -16 -14 -12 MILLIGALS

0 10 20 30 40 50 KILOMETERS

Figure 4. Regional gravity anomaly map generated by fitting the gravity field with a second-order orthogonal polynomial surface.

direction as the Earth's present field. When the areal shape 

of a theoretical anomaly closely matches that of the observed 

anomaly, maximum depth to the magnetic source is determined by comparing lengths of horizontal gradients associated with the two anomalies. The method yields maximum 

depths to magnetic sources because the theoretical models 

used in the curve-matching process are assumed to have vertical sides. In real geologic situations, the causative body 

generally has sloping sides; therefore, its actual depth of 

burial is shallower than the computed depth.

In the Paducah quadrangle, 90 estimates of maximum 

depth to magnetic basement below sea level were determined. The depths reported here must be regarded as rough 

estimates. It should be noted, however, that depths of magnetic basement compare reasonably well with depths of Precambrian basement, determined by seismic and drill-hole 

data (fig. 5). Explanations for discrepancies in depths 

between magnetic and Precambrian basements (e.g., southwestern Illinois and the northeast part of the study area) are

offered later. Magnetic basement shallows to roughly 1 km 

in the west on the flanks of the Ozark uplift and descends to 

depths greater than 6 km in the east in the Rough Creek 

graben.

MAGNETIC AND GRAVITY FEATURES

The amplitudes and wavelengths of anomalies exhibited on the compiled magnetic and gravity anomaly maps 

vary considerably within the study area. For example, the 

gravity low in the eastern part of the study area reflects the 

deepening of Precambrian basement to depths greater than 6 

km (fig. 5). Magnetic and gravity anomalies with steeper 

gradients in the west depict the shallowing of basement 

westward across the quadrangle. Lithologic variation 

accounts for some of the wide variety of anomaly patterns. 

This is evident at the western edge of the study area, where 

two corresponding oval magnetic and gravity lows (A, figs.
MAGNETIC AND GRAVITY STUDY OF THE PADUCAH I°x2° CUSMAP QUADRANGLE C7

90

38°

37°

-6 -5 -4 

10 20 30 40

-2

50 KILOMETERS

-1 DEPTH TO MAGNETIC 

BASEMENT, IN 

KILOMETERS

Figure 5. Maximum depth to magnetic basement. Dots denote locations of depth estimates. Heavy line (dashed where inferred) is the interpreted northwest margin (NWM) of the northeast-trending Reelfoot graben as it merges with the east-trending Rough Creek graben. Contours (1-km interval) are depths to Precambrian basement provided by Michael Sargent (Illinois State Geological Survey, written commun., 

1991) from drill-hole and seismic data.

2 and 3) probably reflect tin-granite plutons. The magnetic 

lows are bounded by a distinct ring of magnetic highs. These 

characteristic geophysical patterns are observed over several 

exposed tin-granite plutons to the west in the St. Francois 

Mountains (Cordell and Knepper, 1987). The prominent 

gravity and magnetic highs to the south of these ring anomalies (in the southwest corner of the study area) express the 

northern edge of the Bloomfield pluton (fig. 1).

On a more regional scale, Ervin and McGinnis (1975) 

and Cordell (1977) proposed that a broad northeast-plunging positive gravity anomaly associated with the Mississippi Embayment is caused by an anomalously high density 

mass at the base of the crust. They interpreted this regional 

gravity high (which includes the prominent positive anomalies in the Paducah quadrangle) as the fossil Reelfoot rift 

cushion underlying the Mississippi Embayment. It will be 

shown later that, although some positive anomalies in the 

study area may be related to deep crustal sources, many of

the high-amplitude gravity anomalies are caused by dense 

igneous intrusions at depths shallower than 10 km.

PADUCAH GRAVITY LINEAMENT (PGL)

Many of the individual gravity peaks within the broad 

northwest-trending gravity high zone in the southwest part of 

the quadrangle coincide with isolated magnetic highs. 

Herein, this northwest-trending gravity high zone will be 

referred to as the Paducah gravity lineament (PGL). The 

presence of both magnetic and very dense sources suggest 

that the area of the PGL is heavily intruded by mafic rock. 

Within this broad zone of igneous rocks, large plutons are 

interpretable and are sketched in figure 8. The locations of 

three of these interpreted plutons appear to be reflected in 

drainage patterns of rivers.
C8 THE PADUCAH CUSMAP QUADRANGLE: RESOUCE AND TOPICAL INVESTIGATIONS

38°

37°

Fault, dashed 

where inferred

10 20 30 40 50 KILOMETERS

Figure 6. Boundaries of the Paducah gravity lineament (PGL) and the south-central magnetic lineament (SGML). Heavy line depicts the 

interpreted northwest margin (NWM) of the Reelfoot graben as it merges with the east-west-trending Rough Creek graben. Major fault zones 

include the Ste. Genevieve (SG), Cottage Grove (CG), Rough Creek (RC), Lusk Creek (LC), and Wabash Valley (WV) fault zones. Fourteen 

small enclosed black areas represent shallow, dike-like intrusions reflected in the high-pass magnetic anomaly map (fig. 10). Three known 

or indicated shallow intrusive centers (Hicks dome (HD), Omaha dome (OD), and Coefield (CF)) coincide with highs in the high-pass data 

shown in figure 10.

As the PGL crosses the Illinois-Missouri State boundary, the steep gradient on its northeastern edge coincides 

with the Ste. Genevieve fault zone (SG, fig. 6). Isopach data 

on sedimentary formations suggest that the oldest movement 

along this fault zone is Middle Ordovician (Timothy Hayes, 

oral commun., 1991). Kolata and Nelson (1991) suggested 

that deformation along the Ste. Genevieve fault zone 

resulted in the emplacement of Devonian diatremes of mica 

peridotite southwest of the fault near Avon, southeastern 

Missouri (fig. 1, near lat 37°45' N. and long 90° 15' W.). 

About 80 plugs and dikes (Kidwell, 1947) occur here at shallow depths in a small area. Although the mappable, southeast-trending Ste. Genevieve fault terminates in Union 

County, Illinois, the northeastern margin of the PGL continues to the southeast. We propose that the northeastern edge 

of the PGL reflects a Precambrian fault zone; thus the Ste. 

Genevieve fault zone may represent a Paleozoic reactivation 

of this interpreted Precambrian fault zone. The continuation

of the PGL beyond the terminus of the Ste. Genevieve fault 

zone suggests that the ancestral Precambrian fault zone continues southeastward in the subsurface into western Kentucky (Lidiak and Zietz, 1976; Hildenbrand and others, 

1982).

The gravity high near the center of the quadrangle (B, 

fig. 3) disrupts the gradient related to the Ste. Genevieve 

fault zone. Because of the presence of a corresponding magnetic high (B, fig. 2), we interpret the source to be a large 

mafic intrusion.

SOUTH-CENTRAL MAGNETIC 

LINEAMENT (SCML)

A prominent northwest-trending zone of magnetic 

highs is present in the northeastern part of the quadrangle

and

is is present in the northeastern part or the quadrangle 

parallels the PGL. This magnetic anomaly is part of the
MAGNETIC AND GRAVITY STUDY OF THE PADUCAH I°x2° CUSMAP QUADRANGLE C9

south-central magnetic lineament (SGML) named by 

Hildenbrand and others (1983). Hildenbrand (1985a) suggested that the SGML extends northwestward from eastern 

Tennessee to possibly as far as southeastern Nebraska. 

Although some north-trending anomalies associated with 

Keweenawan structures in eastern Tennessee cross the 

SGML, other anomalies end abruptly at the SGML. This 

anomaly pattern prompted Hildenbrand (1985a) to propose 

that the sources of the SGML developed during or prior to 

Keweenawan time. The SGML is clearly expressed as a 40- 

km-wide band of magnetic highs from the Tennessee-Kentucky boundary at long 86°30' W. (Hildenbrand and Keller, 

1983) to eastern Missouri.

Near the Kentucky-Illinois border, lamprophyre and 

mica peridotite dikes and sills, together with intrusive breccias (Clegg and Bradbury, 1956; Koenig, 1956; Heyl and 

others, 1965), have been mapped and encountered in drill 

holes in the vicinity of the magnetic highs along the SGML; 

mica and amphibole from these rocks have Rb-Sr and K-Ar 

ages of approximately 270 Ma (Zartman and others, 1967, 

reconverted to the decay constants of Steiger and Jager, 

1978). Hicks dome, a structural feature of the Kentucky-Illinois fluorspar mineral district (HD, fig. 6), is on the southwestern flank of this northwest-trending magnetic feature. 

The dome is underlain by a body of mineralized breccia 

(Brown and others, 1954) that is interpreted as a diatreme 

formed in alkalic to carbonatitic magmatism (Bradbury and 

Baxter, 1992). Small lamprophyre and phlogopite peridotite 

dikes and plugs and intrusive breccias occur on the dome's 

flanks (Heyl and others, 1965).

Magnetic highs generally coincide with gravity highs 

along the SGML in the study area. These gravity highs are, 

however, lower in amplitude than those along the PGL. In 

contrast, the magnetic highs along the PGL have lower magnetic intensities than those along the SGML. The implication 

of this reverse correlation in magnetic and gravity anomaly 

intensities is discussed later.

FAULT ZONES

Hildenbrand and Hendricks (1995) suggest that some 

fault zones correlate with geophysical features. For example, 

the Ste. Genevieve fault zone may be expressed as steep 

gravity gradients related to the northeast edge of the PGL. 

Also, the Cottage Grove fault system may have provided 

channelways for magma forming intrusions related to the 

SGML. On the other hand, the numerous northeast-trending 

faults in the eastern part of the study area have no apparent 

expression on the gravity and magnetic anomaly maps. 

Hildenbrand and Keller (1983) suggested that fault displacements here are too small to be detected by regional potentialfield methods.

PROFILE MODELING

The principal goal of potential-field studies is to detect 

and quantify changes in magnetic and mass properties at 

depth. To translate observed magnetic and gravity anomalies 

into a meaningful geologic picture of the subsurface requires 

inversion or modeling programs. We used a 2 1 /9-dimensional 

modeling program, SAKI (Webring, 1985), based on generalized inverse theory to derive upper crustal models (fig. 7) 

along three individual profiles shown in figures 2 and 3. The 

program requires an initial estimate of model parameters 

(depth, shape, density, and magnetization of sources) and 

then varies selected parameters in an attempt to reduce the 

weighted root-mean-square error between the observed and 

calculated gravity and magnetic fields. Due to the lack of 

information on remanent magnetization, total magnetization 

was assumed to be in the direction of the Earth's present-day 

magnetic field (inclination = 66° N. and declination = 2° E.).

Knowledge of several parameters facilitated the selection of the initial estimate for the models. Several hundred 

measurements of physical properties of rocks from drill 

holes and outcrops in the St. Francois Mountains (west of the 

Paducah quadrangle) (Eva Kisvarsanyi, Missouri Department of Natural Resources, written commun., 1990) were 

used to assign a value for density and susceptibility of Precambrian basement in the western part of the study area. Statistical measurements on these laboratory results (Joseph 

Rosenbaum, written commun., 1991) indicate that unweathered St. Francois granite-rhyolite terrane has an apparent 

average density of 2.67 g/cm3 and susceptibility of 0.009 SI

(47T CgS = SI).

Geophysical logs from drill holes located west, north, 

and east of the quadrangle suggest a wide range of densities 

for Paleozoic units. For example, average densities range 

from 2.4 g/cm3 for the Cambrian Lamotte Sandstone to 2.73 

g/cm3 for the Cambrian Eminence Dolomite. A value of 2.65 

g/cm3 was obtained for the average density of the Paleozoic 

section (based on visual inspection of density logs) and 

agrees with the estimated density of similar rocks at depth in 

Oklahoma (based on density-depth data from drill samples Athy, 1930; Cordell, 1977). This value was used in our 

models. A more detailed model for the Paleozoic is considered inappropriate due to the lack of information on depths 

and densities of the Paleozoic units within the broad study 

area. The estimated depths to magnetic basement (fig. 5) were 

used to approximate the base of Paleozoic sedimentary rocks.

The velocity-density relationship of Birch (1961) and 

velocities cited by Mooney and others (1983) were used to 

select a representative density for middle crust (2.75 g/cm3 ). 

Middle crust susceptibilities were assigned a value of 0.006 

SI, a reasonable estimate for metamorphic rocks (Carmichael, 1982).

Before inverting the data on long profiles, individual 

anomalies related to inferred intrusions were modeled to 

provide reasonable estimates of their thicknesses. Two
CIO THE PADUCAH CUSMAP QUADRANGLE: RESOUCE AND TOPICAL INVESTIGATIONS

gravity anomalies (C and D, fig. 3) were modeled along the 

PGL using a three-dimensional inversion algorithm (GI3, 

Cordell, 1968). Assuming the intrusions have mafic compositions (e.g., gabbro) and densities of about 2.9 g/cm3 , the 

modeling results suggest that the depth to the bottom of an 

intrusion along the PGL is about 10 km below the surface. A 

similar modeling exercise was carried out for two magnetic 

anomalies (using the algorithm SAKI) along the SGML (E 

and F, fig. 2). If the depth to the bottom of intrusion is placed 

at 10 km, some calculated susceptibilities were unreasonably 

high (>0.15 SI); therefore, the base of an intrusion along the 

SGML is assumed to lie at 15 km.

The interpretation of potential-field data yields nonunique solutions because an infinite number of geometrical 

models will have an associated field that closely matches the 

measured field. Available drill-hole information, simultaneous inversion of gravity and magnetic data, and geological 

reasoning have aided in deriving a suitable geophysical 

model to represent the geologic situation in the study area. It 

should be noted, however, that increasing the density and 

decreasing the thickness of an intrusion will generally not 

produce an appreciable change in the computed fields.

Profile W-W (figs. 2 and 3) was selected with a northwest trend that avoided the major positive anomalies related 

to the PGL and SGML. The magnetic low along W-W parallels the SGML southeastward into Tennessee. In the Midcontinent, other linear magnetic lows of similar intensity and 

length exist but are uncommon. The SGML may be flanked 

on the southwest by a trough filled with low-density, 

reversely magnetized rocks (e.g., an old rift filled with volcanic rocks). Ravat (1984) modeled a reversely magnetized 

body in the midcrust to explain the presence of this pronounced magnetic low. The existence of reversely magnetized rocks is plausible but not necessary to explain the steep 

gradient between the geophysical low and the SGML. We 

assume here that the crystalline rocks beneath the sedimentary sequence along W-W' have negligible magnetic properties (i.e., zero susceptibilities). Associated densities of the 

crystalline rocks at the northwest end of profile W-W' are 

assumed to be similar to those in the nearby St. Francois 

Mountains (2.67 g/cm3 ).

The data along profile W-W' were inverted first, based 

on the assumed physical properties at its northwestern end 

(density of 2.67 g/cm3 and zero susceptibility). Knowledge 

of the resulting change in these physical properties eastward 

along profile W-W' then helped to constrain Precambrian 

properties at the junctures of this profile with the remaining 

profiles. Profiles X-X' and Y-Y' trend northeast and are normal to the PGL and SGML.

PROFILE W-W

A distinct change in the gravity field occurs along profile W-W' (fig. 7A). In the northwestern part of the profile,

crystalline basement beneath the sedimentary rocks is 

assumed to have a density of 2.67 g/cm3 . To the southeast, 

the increase in the gravity field reflects a more dense (2.74 g/ 

cm3 ) Precambrian basement within the region of the Rough 

Creek graben. This change in basement density occurs 

across an interpreted igneous intrusion (B, fig. 3) near the 

center of the profile. We suggest that this change in Precambrian basement composition represents the northern extension of the faulted northwestern margin of the Reelfoot 

graben as it joins with the northern margin of the Rough 

Creek graben. An alternative explanation for the increase in 

gravity southeastward along profile W-W' is that the 

regional field (fig. 4) does not adequately reflect deep 

sources. The level change in gravity along the profile may 

therefore be related to lateral density variations associated 

with a dense rift cushion at depth. We prefer the modeling 

results because a density increase in Precambrian basement 

is anticipated over areas where the low-density St. Francois 

terrane is absent or thin. Early Paleozoic uplift related to rifting may have resulted in substantial erosion of the St. Francois terrane, as similarly proposed by Denison (1984) for the 

region to the south where granitic gneiss (once buried as 

deep as 15 km) was encountered in a drill hole (DW, fig. 1).

PROFILES X-X' AND Y-Y'

Profile X-X' (fig. IB) and profile Y-Y' (fig. 1C) are situated normal to the northwest-trending Paducah gravity lineament and the south-central magnetic lineament. The 

proposed igneous bodies producing the PGL and the SGML 

appear to be dense and magnetic. Computed densities and 

susceptibilities are reasonable for a variety of igneous rocks 

such as syenite, quartz diorite, gabbro, and peridotite (Daly 

and others, 1966; Carmichael, 1982;Coleman, 1971). Calculated densities along the PGL range from 2.78 to 2.91 g/cm3 , 

whereas, along the SGML, their range is 2.72 to 2.78 g/cm3 . 

On the other hand, calculated susceptibilities are generally 

greater along the SGML (0.021 to 0.103 SI) than those along 

the PGL (0.009 to 0.090 SI). This reverse correlation in both 

physical properties and anomaly intensities (figs. IB, 1C) 

suggests that the igneous bodies along the PGL have different compositions and origin than those along the SGML. An 

alternative explanation is that the igneous intrusions have 

similar origins but ultramafic rocks along the SGML have 

been chemically altered during serpentinization, whereby 

Fe-rich silicates such as olivine and pyroxene were oxidized 

to form ferrimagnetic material. During serpentinization, 

magnetizations of the ultramafic rocks significantly 

increase, but their densities decrease (Coleman, 1971; Saad, 

1969; Toft and others, 1990). We do not favor this process 

here, however, because the linear zone of intrusions associated with the SGML is considerably larger than the size of 

known serpentinized ultramafic rock bodies.
MAGNETIC AND GRAVITY STUDY OF THE PADUCAH I°x2° CUSMAP QUADRANGLE Cll

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Figure 7 (above and following pages). Theoretical models and potential-field anomalies along profiles A, W-W; 5, X-X'; and C, Y-Y'. Profile 

locations are shown in figures 2 and 3. Triangles and crosses in the potential-field plots are the observed data (figs. 2 and 3); solid lines are calculated fields based on the model shown at the bottom of each part of the figure. Numbers in model represent calculated densities in g/cm3 and 

susceptibilities in SI units (in parentheses). Hachured bodies in model represent igneous intrusions. Labels PGL and SGML in B and C show 

lateral extent of the Paducah gravity lineament and south-central magnetic lineament along the profile. Vertical exaggeration in the models is 

1.65x.
C12 THE PADUCAH CUSMAP QUADRANGLE: RESOUCE AND TOPICAL INVESTIGATIONS

-400 "-

-500

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30 40 50 60 70 

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MAGNETIC AND GRAVITY STUDY OF THE PADUCAH I°x2° CUSMAP QUADRANGLE

Y

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C14 THE PADUCAH CUSMAP QUADRANGLE: RESOUCE AND TOPICAL INVESTIGATIONS

90 38°

37°

10 20 30 40 50 KILOMETERS

Figure 8. Interpreted large intrusions. Solid enclosures denote boundaries of magnetic sources. Dashed enclosures depict gravity sources. 

Magnetic intrusions with hachure pattern lie at depths considerably shallower (>1 km shallower) than the depth of Precambrian basement 

(see fig. 5). Heavy lines are interpreted faults (some with strike-slip offsets), also shown in figure 9. Intrusions A are interpreted Precambrian 

tin-granite plutons, similar to those to the west in the St. Francois Mountains. Intrusion B lies along the interpreted northwest margin of the 

Reelfoot graben.

The steep gravity gradients bounding the PGL require 

near-vertical intrusions lying at shallow depths. On the other 

hand, the intrusions along the SGML may dip about 60°. It is 

worth noting that the intrusions appear to be laterally differentiated (i.e., their cores may be generally more magnetic 

and dense).

DISCUSSION

Four major geophysical features cross the study area: 

anomalies associated with the Reelfoot and Rough Creek 

grabens, the south-central magnetic lineament (SGML), and 

the Paducah gravity lineament (PGL). The physical and geologic relations among these four features and their associated 

structures (e.g., the many fault zones) are poorly understood 

and raise the following questions. Does the Reelfoot rift simply merge with the Rough Creek rift or are there more

complicated rift structures involving several failed arms? 

What are the origins of the PGL and SGML? How do the 

fault zones relate to these four features? The following discussions address these and other questions.

To facilitate the discussions of igneous bodies, a map of 

interpreted large intrusions was prepared and is shown as 

figure 8. Intrusive boundaries were defined using two techniques: horizontal gradient maxima of the gravity and 

pseudogravity fields (Cordell and Grauch, 1982) and body 

edges indicated on first- and second-vertical-derivative 

anomaly maps.

Also shown in figure 8 (hachure pattern) are interpreted 

intrusions that appear to lie at depths substantially above 

Precambrian basement. In comparing depths of magnetic 

basement and Precambrian basement (fig. 5), four interpreted intrusions appear to lie at depths shallower (>1 km) 

than the depth of Precambrian basement. Although highly 

speculative, we propose the two shallow intrusions in the
MAGNETIC AND GRAVITY STUDY OF THE PADUCAH I°x2° CUSMAP QUADRANGLE C15

northeast part of the study were emplaced during late 

Paleozoic in or near the Wabash Valley fault zone. The late 

Paleozoic to early Mesozoic was a period when the craton 

was experiencing both compression and extension when 

Pangea was formed and then broke up. The other two shallow intrusions in the western part of the study area lie along 

the Paducah gravity lineament and are discussed later.

RIFTS

Hildenbrand and others (1982) and Braile and others 

(1982) suggested that the graben related to the Reelfoot rift 

continues northeastward in the Mississippi Embayment 

where it merges with or intersects the Rough Creek graben 

(Soderberg and Keller, 1981) in southern Illinois and western Kentucky. Hildenbrand and others (1982) and Hildenbrand and Keller (1983) described the southeast margin of 

the Reelfoot graben as bending to the east along the Pennyrile fault zone east of the study area, where it becomes the 

southern margin of the Rough Creek graben (fig. 1); they 

therefore suggested that the Reelfoot graben simply terminates in the Paducah quadrangle and merges with but does 

not cross the Rough Creek graben.

A more complex rift structure was proposed by Braile 

and others (1982). Their interpretation includes a quadruple 

junction in the study area with the Reelfoot and Rough Creek 

grabens representing two failed rift arms. A third rift arm 

would extend northeastward along the Wabash Valley fault 

system, whereas the fourth arm is bounded by the SGML and 

PGL and extends northwest to about St. Louis, Mo. They 

named this entire system and its quadruple junction the New 

Madrid rift complex. Nelson (1990), however, suggested 

that there are geologic and proprietary seismic data to dispute the existence of a quadruple junction. The following 

discussions expand on earlier potential-field interpretations 

(Hildenbrand and others, 1982) of the juncture of the Reelfoot rift and Rough Creek graben in the study area.

South of the study area, magnetic anomaly maps clearly 

define the Reelfoot graben (fig. 1), which contains about 2 

km of low-density fill and is flanked by dense and highly 

magnetic plutons. A second-vertical-derivative magnetic 

anomaly map (Hildenbrand, 1985b) clearly shows the southeast margin of this graben trending northeast into Kentucky, 

where the southeast margin gently bends eastward (lat 37° N. 

and long 87°45/ W.) along the southern margin of the Rough 

Creek graben. The sharp change in the magnetic field 

reflects crossing the edge of the deep graben floor to the shallowing terrane of the dipping and possibly wide graben margin. It should be noted that we cannot determine the outer 

edge of the graben margins due the complex magnetic signatures related to the magnetic intrusions emplaced along the 

margins.

On the other hand, the nature of the northern extension 

of the northwest margin of the Reelfoot graben beyond the

Bloomfield pluton (fig. 1) is equivocal. Here northwesttrending geophysical features (mainly the SCML and PGL) 

may mask the more subdued gradients of the graben's northwest margin. Indirect evidence for its northward continuation into the study area is seen in the geophysical model of 

profile W-W' (fig. 7A). A change in basement composition 

may occur at about lat 37°30/ N. and long 89° W., where an 

intrusion may also have been emplaced along the curved 

margin in this region (B, figs. 2, 3, and 8). Extending the graben's northwest margin to this area does not imply a widening of the graben northward from the Bloomfield pluton; 

rather our definition of the line representing the margin is 

changed. Because the expression of the edge of the graben 

floor is not obvious here, the line representing the northwest 

margin is based on subtle anomalies of northeast-trending 

features lying within the broad region of the graben margin. 

Close inspection of regional magnetic and gravity anomaly 

maps suggests that this northwest margin does not continue 

northeastward beyond the central area of the Paducah quadrangle (i.e., beyond the SCML). This margin may therefore 

bend to the east and merge with the northern margin of the 

Rough Creek graben, roughly paralleling its southern margin. Such an interpretation is easily visualized in the depth to 

magnetic basement map (fig. 5). A sketch of the juncture of 

the two grabens in the study area is shown in figures 1, 5, and 

6. The offset in the margin (lat 37° 10' N. and long 89° 15' 

W.) is based on magnetic basement depths (fig. 5) and interpreted strike-slip faults related to the PGL (discussed later). 

It is worth noting that the zone of northeast-trending faults in 

southern Illinois and western Kentucky is largely bounded 

between the interpreted margins of the Reelfoot graben (figs. 

1 and 6).

Hildenbrand and Hendricks (1995) suggest that the 

abrupt change in the trend of the Reelfoot rift (northeast) to 

the trend of the Rough Creek graben (east) is due to either:

1. A preferred strain direction Rifting may have 

propagated northeastward and then followed existing east-west structures in western Kentucky that 

represented a path of less resistance; or

2. An obstacle The competent, homogeneous 

batholithic rocks deep beneath the eastern flank of 

the Ozark uplift may have represented a crustal 

obstacle, diverting rift propagation to the east. 

Alternatively, they propose that rifting may have originated 

in the east and propagated westward along the trend of the 

Rough Creek graben before being diverted to the southwest 

along the trend of the Reelfoot graben.

PADUCAH GRAVITY LINEAMENT

Figure 9 shows the gravity field for a region larger than 

the study area. Four prominent gravity gradients (A-D, fig. 

9) trend northwest into the study area. These gradients probably represent strike-slip faults. Evidence for strike-slip
C16

38°

THE PADUCAH CUSMAP QUADRANGLE: RESOUCE AND TOPICAL INVESTIGATIONS

89° 88°

37°

36°

MILLIGALS

12

0

-4

-8

-12

-16

-20

20 40 60 80 100 KILOMETERS

Figure 9. Gravity anomaly map of the northern Mississippi Embayment region. Dashed lines A-D are interpreted faults located at the 

junction of the Reelfoot graben and Rough Creek graben. Positive gravity anomalies E and F are discussed in the text. The intrusion related 

to anomaly E lies along the axis of the Reelfoot graben (see fig. 1). A southeast displacement of the block between faults C and D is inferred 

from a bend in the southeast margin of the Reelfoot graben (Hildenbrand, 1985b, and fig. 1).

motion is based largely on a noticeable southeastward 

deflection in the southeast margin of the Reelfoot graben 

(between magnetic lineaments a and b, fig. 1). The region 

between gravity lineaments C and D (fig. 9) also appears to

be offset southeastward by about 12 km (Hildenbrand and 

others, 1982). The inferred Precambrian fault, along which 

the Ste. Genevieve fault developed, may extend into Kentucky either along lineament C (Hildenbrand and others,
MAGNETIC AND GRAVITY STUDY OF THE PADUCAH I°x2° CUSMAP QUADRANGLE C17

1982) or lineament D (Lidiak and Zietz, 1976). Analysis of 

magnetic data indicates that gravity lineaments C and D continue southeast into Tennessee (a and b, fig. 1). Hildenbrand 

and others (1982) proposed that this faulted region represented a transition from a rift structure to a block-faulted 

region.

Of particular interest is that anomalies E and F (fig. 9) 

have sharp corners on their southeastern edges. Hildenbrand 

and others (1982) suggested that the source of anomaly E is 

an intrusion emplaced along axial faults of the Reelfoot graben (see intrusion located on the Kentucky-Tennessee border, fig. 1). We propose here that the intrusions associated 

with anomaly E and those along the PGL were emplaced 

along both the Reelfoot axial faults and the proposed northwest-trending faults shown in figure 9. The sharp corners of 

anomalies E and F indicate that the associated edges of the 

intrusions are structurally controlled by the intersecting 

northwest-trending Precambrian faults and the early Paleozoic axial faults of the Reelfoot graben. The age of the intrusions should therefore be early Paleozoic or younger.

The coincidence of magnetic highs (fig. 2) and 

microcrystalline silica (tripoli) argues for a post-rift igneous 

event along the PGL. Berg and Masters (1994) recognized 

this correlation and proposed that intrusions provided the 

heat source to drive hydrothermal waters that deposited 

silica in southwestern Illinois. The age of these deposits is 

unknown, although they are hosted by Early Devonian 

sedimentary rocks. The magnetic intrusions in the region of 

the silica deposits (figs. 2 and 8) lie at depths substantially 

above (>1 km) Precambrian basement. Nearby interpreted 

magnetic intrusions (fig. 8) may have also been heat sources 

with associated silica deposits. The strong correlation 

between silica deposits and shallow magnetic intrusions 

infers that some of the igneous bodies along the PGL are of 

Early Devonian or younger age. To the northwest, Early to 

Middle Devonian igneous rocks lie along the trend of the 

PGL near Avon, Mo. (fig. 1). Without additional data, it is 

difficult to speculate on the age(s) of the intrusions that 

produce the PGL. Intrusions of different ages may be 

present. Probable ages based on the age of nearby igneous 

rocks and structures include Cambrian (syn-rift), Devonian, 

late Paleozoic, and (or) Cretaceous.

Does the PGL represent a third rift arm? The lateral 

extent of PGL (about 80 km beyond the northwest margin of 

the Reelfoot graben) is considerably smaller than the length 

of either the Reelfoot rift or Rough Creek graben. If a third 

rift arm is expressed by the PGL, it has a lateral extent 

uncharacteristic of other rifts. There are also no data to support the existence of an associated graben or rift-related 

sedimentary rocks along the PGL (Kolata and Nelson, 1991). 

Hildenbrand and Hendricks (1995) provide an alternative 

interpretation for the structures related to the PGL: a strikeslip fault zone that accommodated strain at the juncture of 

the Reelfoot rift and Rough Creek graben.

The calculated densities of the intrusions along the PGL 

suggest a mafic source rock, such as gabbro or diorite. Many 

of the steep gradients and small widths of large-amplitude 

gravity anomalies along the PGL (fig. 7) clearly require that 

these mafic intrusions lie at relatively shallow depths (<10 

km). Some of the anomalous mass producing the prominent 

gravity highs in the study area, therefore, resides in the upper 

crust. An anomalously high density mass at the base of the 

crust may also be present, as previously proposed by Cordell 

(1977). The regional gravity field in figure 4 may reflect, in 

part, anomalous crust immediately above the mantle.

SOUTH-CENTRAL MAGNETIC LINEAMENT

In the study area, the SGML reflects an igneous event 

that emplaced large volumes of high-susceptibility rocks 

along a 40-km-wide northwest-trending zone (figs. 6 and 8). 

Occurrence of Permian alkalic, ultramafic dikes along the 

SGML (Baxter and others, 1989) suggests that the SGML 

developed during Early Permian as the supercontinent Pangea was assembled. Kolata and Nelson (1991) proposed that 

this major tectonic event resulted in Permian deformation 

and associated igneous activity in the southern part of the 

Illinois Basin. There are, however, reasons to propose that 

the source of the SGML developed earlier than Permian 

time:

 In east-central Tennessee, the lineament appears to 

reflect contrasts in magnetic properties in 

Keweenawan rocks (Hildenbrand, 1985a).

 Seismic lines crossing the region of magnetic anomalies E and F (fig. 2) indicate horizontal layering in 

the Phanerozoic sedimentary rocks, with no indication of massive invasion of magma, except possibly 

for dipping horizons in the Precambrian (Heigold 

and Kolata, 1993).

Thus, the SGML possibly delineates a Precambrian 

basement feature that developed along a north west-trending 

structure of the old (1.6 Ga) metamorphic basement, similar 

to that underlying Missouri. The exceptional linearity of the 

SGML suggests that the source is a shear zone. Because 

Keweenawan (1.1 Ga) features may be structurally related to 

this proposed shear zone, its possible origin spans a period of 

time from about 1.6 to 1.1 Ga. The age of the igneous intrusions along the SGML may be 1.6 Ga or younger. Moreover, 

because the SGML appears linear (Hildenbrand, 1985a) in 

the region where the Reelfoot graben and Rough Creek graben merge, the source of the SGML would have been minimally affected by deformation during rifting or rift 

reactivation.

The calculated physical properties (figs. IB, 1C} indicate an unusual rock type for the SGML intrusions. Densities 

(2.72 to 2.78 g/cm3 ) suggest an intermediate rock such as 

granodiorite or quartz diorite. The high susceptibilities 

(0.021 to 0.103 SI), however, are typical of more mafic and
CIS THE PADUCAH CUSMAP QUADRANGLE: RESOUCE AND TOPICAL INVESTIGATIONS

denser rock types such as diorite, gabbro, or peridotite. As 

previously discussed, the intrusive thickness has a major 

effect on the calculated values of the physical properties. For 

example, if their thickness is reduced from 12 km to 6 km, 

density increases but susceptibility also increases to about 

0.15 SI. This high susceptibility value is uncommon in the 

Precambrian rocks in the Midcontinent (except for ironformations). On the other hand, rocks can acquire additional 

magnetic properties during their formation while cooling. 

The resulting natural remanent magnetization can be much 

greater than the induced magnetization (susceptibility x geomagnetic field intensity). Because there is no evidence to 

support large remanent magnetizations, we prefer an intermediate composition for the intrusions along the SGML

Paralleling the SGML to the southwest is a magnetic 

and gravity low. We assume that the geophysical low depicts 

a zone lying between zones heavily intruded by dense and 

magnetic rocks along the PGL and SGML. In a model (WW, fig. 1A) along the axis of this geophysical low, it was 

assumed that crystalline basement underlying the sedimentary rocks has negligible magnetic properties to a 10-km 

depth. This assumption is based on the intensity of the magnetic low, which is uncharacteristic in the magnetic field of 

the Midcontinent. Additional investigations are needed to 

establish whether the magnetic low is associated with a thick 

basin of nonmagnetic sediments or reversely magnetized 

rocks.

SHALLOW DIKES

Permian ultramafic alkalic dikes and sills crop out or 

are encountered in drill holes at very shallow depths (<1 km) 

within the study area (Koenig, 1956; Clegg and Bradbury, 

1956). These occurrences are much shallower than the computed depths to magnetic basement (2-6 km, figs. 5 and 7). 

The dimensions of these very shallow dikes (generally less 

than 7 m wide; Clegg and Bradbury, 1956) are too small to 

produce anomalies of sufficient amplitude to be observed on 

figures 2 and 3.

To enhance short-wavelength anomalies and delineate 

slightly larger igneous masses of economic interest such as 

Omaha dome, a high-pass filter (removing wavelengths 

greater than 10 km) was applied to the magnetic data east of 

long 89° W. (fig. 10). The data west of long 89° W. were not 

included because shallow Precambrian magnetic basement 

here can produce high-passed anomalies that are similar to 

those produced by shallow Permian igneous masses. Interpreted shallow intrusions are shown in figure 6. It should be 

noted that, because high-frequency anomalies can be 

generated by the deeper intrusions along the PGL and the 

SGML, only small intrusions offset from these lineaments 

are shown in figure 6. Gravity anomalies could not be used 

to delineate shallow intrusions due to the coarse station spacing (average spacing of about 4 km).

Of particular importance is that shallow intrusions (fig. 

6) correlate well geographically with the known or indicated 

shallow intrusive centers near Omaha dome, Coefield, and 

Hicks dome. Other short-wavelength magnetic highs may 

delineate shallow Permian intrusions. Two intriguing possibilities would be the interpreted shallow magnetic bodies 

lying west-northwest of Hicks dome (intrusive-related mineralization occurs at Hicks dome) (Bradbury and Baxter, 

1992).

SUMMARY AND CONCLUSIONS

The potential-field data have provided a geologic picture of the subsurface that indicates that the study area experienced a long and complex tectonic and magmatic history. 

In Middle Proterozoic time (about 1.6 Ga), basement consisted of gneissic rocks with a pervasive northwesterly structural grain (Kisvarsanyi, 1984). Some of these structures 

may have been zones of weakness along which later structures developed, such as the proposed shear zone expressed 

as the SGML. The SGML is a prominent, linear magnetic 

feature that trends northwest from eastern Tennessee into 

Missouri. In the study area, the SGML is clearly expressed 

as a 40-km-wide band of magnetic highs that, on the basis of 

modeling, may reflect large plutons emplaced along the 

shear zone. Calculated susceptibilities and densities indicate 

a highly magnetic intermediate source rock, such as quartz 

diorite. Because Keweenawan (1.1 Ga) features to the east of 

the Paducah quadrangle may be structurally related to this 

shear zone, emplacement of the intrusions may span an interval from 1.6 to 1.1 Ga. The age of the intrusions along the 

SGML, however, has yet to be determined because they may 

have been emplaced after the development of the shear zone. 

Although highly speculative, we base our interpretation of 

an intruded Precambrian shear zone on the exceptional linearity of the SGML. The relation of the shear zone to the tectonic evolution of interior craton has yet to be determined.

To the west of the Paducah quadrangle, volcanic activity formed the St. Francois Mountains at about 1.5 Ga and 

laid down a granite-rhyolite terrane over, at least, the western parts of the study area (Bickford and others, 1981). Two 

oval magnetic and gravity lows on the western edge of the 

Paducah quadrangle probably represent subcropping tingranite plutons that were emplaced during this igneous 

event.

In early Paleozoic time (about 570 Ma), the breakup of 

the North American supercontinent resulted in the formation 

of several aulacogens (Thomas, 1991), which included the 

Reelfoot and Rough Creek rifts. Our preferred interpretation 

of the relation between these two rifts is that the Reelfoot 

graben simply bends eastward to merge with the Rough 

Creek graben. This interpretation is based on the lack of geophysical features trending northeast across the SGML. 

Instead, interpreted lineaments and depressions in magnetic
MAGNETIC AND GRAVITY STUDY OF THE PADUCAH I°x2° CUSMAP QUADRANGLE

89° 88°30' 88°

C19

38°

37°30'

37°

NANOTESLAS

16

14

12

10

8

6

4

2

0

04812 16 20 KILOMETERS

Figure 10. High-pass magnetic anomaly map. Wavelengths greater than approximately 10 km have been removed from the magnetic data in 

figure 2. Three known or indicated shallow intrusive centers (Hicks dome (HD), Omaha dome (OD), and Coefield (CF)) coincide with positive 

magnetic anomalies.

basement trend northwest or bend eastward at the juncture of 

the rifts. This interpreted bend in the rift may be related to a 

preferred strain direction or to an obstacle such as competent, homogeneous batholithic rocks of the St. Francois 

Mountains.

At the juncture of the rifts, northwest- and northeasttrending faults along the axis of the Reelfoot rift may have 

provided channelways for magma. Prominent gravity highs 

parallel these faults and form the PGL. Of particular 

interest is that these anomalies have sharp edges at the
C20 THE PADUCAH CUSMAP QUADRANGLE: RESOUCE AND TOPICAL INVESTIGATIONS

intersections of the faults. This geophysical pattern suggests that large intrusions were emplaced in a block-faulted 

region. Strike-slip motion along the northwest-trending 

faults is indicated by deflections in the southeast margin of 

the Reelfoot rift. Because the axial faults of the Reelfoot 

graben also appear to have been channelways for the 

magma, the igneous event(s) probably occurred during rifting (Cambrian) or later. Because Devonian-hosted silica 

deposits correlate with some of the magnetic highs along 

the PGL, the age of the associated intrusions appear to be 

Devonian or younger. One could argue, however, that 

nearby igneous bodies and structures indicate a late Paleozoic or Cretaceous age for these intrusions. In summary, 

the age or ages of the mafic intrusions along the PGL have 

yet to be determined, but likely possibilities include Cambrian, Devonian, late Paleozoic, and (or) Cretaceous.

These mafic intrusions form a 100-km-wide zone near 

the axis of the Reelfoot graben and trend northwest for 180 

km into eastern Missouri where the zone narrows to 30 km. 

This nose extends northwest about 80 km beyond the interpreted bend of the northwestern margin of the Reelfoot graben. The PGL probably does not represent a failed rift arm 

due to its short lateral extent and absence of rift-related early 

Paleozoic sedimentary rocks (Kolata and Nelson, 1991). The 

heavily intruded zone is interpreted here to be the result of 

reactivation of old faults during rifting (Cambrian) or subsequent tectonism.

The post-rifting phase included thick accumulations of 

sediments, as evidenced by the deepening of magnetic basement (>6 km) in the eastern part of the study area. Because 

magnetic basement is considerably shallower than Precambrian basement in two areas, post-rifting igneous activity 

may have emplaced large intrusions along northwest-trending faults in southwestern Illinois and along or near the 

north-northeast-trending Wabash Valley fault system in the 

northeastern part of the study area (fig. 8).

Other evidence for post-rifting intrusions are the subtle, 

local magnetic highs, enhanced by a high-pass filter. These 

magnetic highs probably delineate Permian ultramafic alkalic plugs or laccoliths at shallow depths (<1 km). Of particular importance is that positive anomalies coincide with 

known or indicated shallow intrusive centers near Omaha 

dome, Coefield, and Hicks dome. Perhaps some of the other 

11 positive anomalies delineate shallow intrusions with mineral potential, similar to that at Hicks dome.

ACKNOWLEDGMENTS

The study has benefited from discussions with Lindrith 

Cordell, Andrew Griscom, Bill Hinze, Richard Reynolds, 

and Joe Rosenbaum, although the conclusions contained in 

this paper are the responsibility of the authors. We appreciate 

the assistance by Bob Sikora in the preparation of figures. 

The manuscript improved greatly from the excellent reviews 

by Timothy Hayes, John Hendricks, and John Nelson.

REFERENCES CITED

Athy, L.F., 1930, Density, porosity, and compaction of sedimentary 

rocks: American Association of Petroleum Geologists Bulletin, 

v. 14, p. 1-24.

Baxter, J.W., Bradbury, J.C., Kisvarsanyi, E.B., Gerdemann, P.E., 

Gregg, J.M., and Hagni, R.D., 1989, Precambrian and 

Paleozoic geology and ore deposits in the Midcontinent region: 

American Geophysical Union, International Geologic Conference Field Trip T147, 36 p.

Berg, R.B., and Masters, J.M., 1994, Geology of microcrystalline 

silica (tripoli) deposits, southernmost Illinois: Illinois State 

Geological Survey Circular 555, 89 p.

Bickford, M.E., Harrower, K.L., Hoppe, W.J., Nelson, B.K., Nussbaum, R.K., and Thomas, J.J., 1981, Rb-Sr and U-Pb geochronology and distribution of rock types in the Precambrian 

basement of Missouri and Kansas: Geological Society of 

America Bulletin, pt. 1, 92, p. 323-341.

Birch, F., 1961, The velocity of compressional waves in rocks to 10 

kilobars: Journal of Geophysical Research, v. 66, 

p. 2199-2224.

Bradbury, J.C., and Baxter, J.W., 1992, Intrusive breccias at Hicks 

dome, Hardin County, Illinois: Illinois State Geological Survey 

Circular 550, 23 p.

Braile, L.W., Keller, G.R., Hinze, W.J., and Lidiak, E.G., 1982, An 

ancient rift complex and its relation to contemporary seismicity 

in the New Madrid seismic zone: Tectonics, v. 1, p. 225-237.

Braile, L.W., Hinze, W.J., Keller, G.R., Lidiak, E.G., and Sexton, 

J.L., 1986, Tectonic development of the New Madrid rift complex, Mississippi Embayment, North America: Tectonophysics. v. 131, p. 1-21.

Branch of Geophysics, 1989, Potential-field geophysical programs 

for VAX 7xx computers: U.S. Geological Survey Open-File 

Report 89-115, A-D.

Briggs, I.C., 1974, Machine contouring using minimum curvature: 

Geophysics, v. 39, p. 39-48.

Brown, J.D., Emery, J.A., and Meyer, P.A., Jr., 1954, Explosion 

pipe in test well on Hicks dome, Hardin County, Illinois: Economic Geology, v. 49, p. 891-902.

Carmichael, R.S., 1982, Magnetic properties of minerals and rocks, 

in Carmichael, R.S., ed., Handbook of Physical Properties of 

Rocks: Boca Raton, Florida, CRC Press, Inc.

Clegg, K.E., and Bradbury, J.C., 1956, Igneous intrusive rocks in Illinois and their economic significance: Illinois State Geological Survey Report of Investigations 197, 19 p.

Coleman. R.G., 1971, Petrologic and geophysical nature of serpentinites: Geological Society of America Bulletin, v. 82, 

p. 897-918.

Cordell, Lindrith, 1968, Iterative three-dimensional solution of 

gravity anomaly data using a digital computer: Geophysics, v. 

33, p. 596-601.

 1977, Regional positive gravity anomaly over the Mississippi Embayment: Geophysical Research Letters, v. 4, p. 285-287.

Cordell, Lindrith, and Grauch, V.J.S., 1982, Mapping basement 

magnetization zones from aeromagnetic data in the San Juan 

Basin, New Mexico [abs.]: Society of Exploration Geophysicists, Abstracts with Programs, 1982, Annual Meeting, Dallas, 

Texas, p. 246-247.
MAGNETIC AND GRAVITY STUDY OF THE PADUCAH I°x2° CUSMAP QUADRANGLE C21

Cordell, Lindrith, and Knepper, D.H., 1987, Aeromagnetic 

images Fresh insight to the buried basement, Rolla quadrangle, southeast Missouri: Geophysics, v. 52, p. 218-231.

Daly, R.A., Manger, Edward, and Clark, S.P., Jr., 1966, Density of 

rocks, in Clark, S.P., Jr., ed.. Handbook of Physical Constants: 

Geological Society of America Memoir 97, Section 4, 

p. 20-26.

Denison, R.E., 1984, Basement rocks in northern Arkansas,//! McFarland, J.D., and Bush, W.V., eds., Contributions to the Geology of Arkansas Volume II: Arkansas Geologic Commission 

Miscellaneous Publication 18-B, p. 33^49.

Erickson, R.L., and Blade, L.V., 1963, Geochemistry and petrology 

of the alkalic igneous complex at Magnet Cove, Arkansas: U.S. 

Geological Survey Professional Paper 425, 95 p.

Ervin, C.P., and McGinnis, L.D., 1975, Reelfoot rift Reactivated 

precursor to the Mississippi Embayment: Geological Society of 

America Bulletin, v. 86, p. 1287-1295.

Click, E.E., 1975, Arkansas and northern Louisiana, paleotectonic 

investigations of the Pennsylvanian System in the United 

States, Part I, Introduction and regional analyses of the Pennsylvanian System: U.S. Geological Survey Professional Paper 

853-1, p. 157-175.

 1982, Stratigraphy and structure of sediments above the 

Newport pluton of northeastern Arkansas, in McKeown, F.A., 

and Pakiser, L.C., eds.. Investigations of the New Madrid, Missouri, Earthquake Region: U.S. Geological Survey Professional Paper 1236, p. 151-174.

Gordon, M., Jr., Tracey, J.I., Jr., and Ellis, M.W., 1958, Geology of 

the Arkansas bauxite region: U.S. Geological Survey Professional Paper 229,268 p.

Heigold, P.C., and Kolata, D.R., 1993, Proterozoic crustal boundary 

in the southern part of the Illinois Basin: Tectonophysics, 

v. 217, p. 307-319.

Hendricks, J.D., 1988, Bouguer gravity of Arkansas: U.S. Geological Survey Professional Paper 1474, 31 p.

Heyl, A.V., Jr., Brock, M.R., Jolly, J.L., and Wells, C.E., 1965, Regional structure of the southeast Missouri and Illinois-Kentucky mineral districts: U.S. Geological Survey Bulletin 

1202-B,p. B1-B20.

Hildenbrand, T.G., 1985a, Magnetic terranes in central USA from 

the interpretation of digital data, in Hinze, W.F., ed., The Utility of Regional Gravity and Magnetic Anomaly Maps: Tulsa, 

Okla., Society of Exploration Geophysicists, p. 248-266.

 1985b, Rift structures of the northern Mississippi Embayment from the analysis of gravity and magnetic data: Journal of 

Geophysical Research, v. 90, no. B14, p. 12607-12622.

Hildenbrand, T.G., and Hendricks, J.D., 1995, Geophysical setting 

of the Reelfoot rift and relations between rift structures and the 

New Madrid seismic zone, chap. E of Shedlock, Kaye, and 

Johnston, Arch, Investigations of the New Madrid Seismic 

Zone: U.S. Geological Survey Professional Paper 1538-E, 

p. 1-30.

Hildenbrand, T.G., Kane, M.F., and Hendricks, J.D., 1982, Magnetic basement in the upper Mississippi Embayment region A 

preliminary report, in Pakiser, L., and McKeown, F.A., eds.. 

Investigations of the New Madrid, Missouri, Earthquake Region: U.S. Geological Survey Professional Paper 1236-E, 

p. 39-53.

Hildenbrand, T.G., Kane, M.F., and Stauder, William, 1977, Magnetic and gravity anomalies in the northern Mississippi

Embayment and their spatial relation to seismicity: U.S. Geological Survey Miscellaneous Field Studies Map MF-914, 

scale 1:500.000.

Hildenbrand, T.G., and Keller, G.R., 1983, Magnetic and gravity 

features of western Kentucky Their geologic significance: 

U.S. Geological Survey Open-File Report 83-164, 9 p.

Hildenbrand, T.G., Kucks, R.P., and Kane, M.F., 1979, Simple 

Bouguer gravity map of the Paducah I°x2° quadrangle, Illinois, Kentucky, and Missouri: U.S. Geological Survey OpenFile Report 79-1179, scale 1:250,000.

Hildenbrand, T.G., Kucks, R.P., and Sweeney, R.E., 1983, Digital 

magnetic-anomaly map of Central U.S. Description of major 

features: U.S. Geological Survey Geophysical Investigations 

Map GP-955, scale 1:1,000,000.

International Association of Geodesy, 1967, Systeme geodesique 

de reference 1967 (English translation): International Association of Geodesy Publication Special, no. 3, 115 p.

Kane, M.F., Hildenbrand, T.G., and Hendricks, J.P., 1981, A model 

for the tectonic evolution of the Mississippi Embayment and its 

contemporary seismicity: Geology,- v. 9, p. 563-567.

Kidwell, A.K., 1947, Post-Devonian igneous activity in southeastern Missouri: Missouri Geological Survey and Water Resources, Report of Investigations 4, 85 p.

 1951, Mesozoic igneous activity in the northern Gulf Coast 

Plain: Transactions of the Gulf Coast Association of Geologists 

Society, v. l,p. 182-199.

Kisvarsanyi, E.B., 1974, Operation basement Buried Precambrian rocks of Missouri: Their petrography and structure: 

American Association of Petroleum Geologists Bulletin, v. 58, 

p. 674-684.

 1981, Geology of the Precambrian St. Francois terrane, 

southeastern Missouri: Report of Investigation 64, Missouri 

Department of Natural Resources, Division of Geology and 

Land Surveys, 58 p.

 1984, The Precambrian tectonic framework of Missouri as

interpreted from the magnetic anomaly map: Missouri Department of Natural Resources, Contribution to Precambrian Geology No. 14, Part B, 19 p.

Koenig, J.B., 1956, The petrography of certain igneous dikes of 

Kentucky: Kentucky Geological Survey, Series 9, Bulletin 21.

Kolata, D.R., and Nelson, W.J., 1991, Tectonic history of the Illinois Basin, in Leighton, M.W., Kolata, D.R., Oltz, D.F., and 

Eidel, J.J., eds., Interior Cratonic Basins: Tulsa, Okla., American Association of Petroleum Geologists Memoir 51 (World 

Petroleum Basins), p. 263-285.

Kucks, R.P., 1990, Aeromagnetic anomaly map of the Paducah 

I°x2° quadrangle, Missouri, Illinois, and Kentucky: U.S. Geological Survey Miscellaneous Field Studies Map MF-2131, 

scale 1:250,000.

Lamar, J.E., 1953, Siliceous materials of extreme southern Illinois: 

Illinois State Geological Survey, Report of Investigations 166, 

39 p.

Lidiak, E.G., Hinze, W.J., Keller, G.R., Reed, J.E., Braille, L.W., 

and Johnson, R.W., 1985, Geologic significance of regional 

gravity and magnetic anomalies in the east-central Midcontinent, in Hinze, W.F., ed., The Utility of Regional Gravity and 

Magnetic Anomaly Maps: Tulsa, Okla., Society of Exploration 

Geophysicists, p. 287-307.

Lidiak, E.G., and Zietz, Isidore, 1976, Interpretation of aeromagnetic anomalies between latitudes 37° N. and 38° N. in Eastern and
C22 THE PADUCAH CUSMAP QUADRANGLE: RESOUCE AND TOPICAL INVESTIGATIONS

Central United States: Geological Society of America Special 

Paper 167, 37 p.

Moody, C.L., 1949, Mesozoic igneous rocks of the northern Gulf 

Coastal Plain: American Association of Petroleum Geologists 

Bulletin, v. 33, p. 1410-1428.

Mooney, W.D., Andrews, M.C., Ginzburg, A., Peters, D.A., and 

Hamilton, R.M., 1983, Crustal structure of the northern Mississippi Embayment and a comparison with other continental 

rifts: Tectonophysics, v. 94, p. 327-348.

Morelli, Carlo, Gantar, C., Honkasala, Tauno, McConnel, R.K., 

Tanner, J.G., Szabo, Bela, Uotila, U.A., and Whalen, G.T., 

1974, The International Gravity Standardization Net 1971 

(I.G.S.N. 71): Paris Bureau Central de 1'Association Internationale de Geodesic Special Publication 4, 193 p.

Nelson, J.W., 1990, Comment and reply on "Major Proterozoic 

basement features of the eastern Midcontinent of North America revealed by COCORP profiling" by Pratt and others (1989): 

Geology, April, p. 378-379.

Nuttli,O. W., 1982, Damaging earthquakes of the central Mississippi valley, in McKeown, F.A., and Pakiser, L.C., eds., Investigations of the New Madrid, Missouri, Earthquake Region: U.S. 

Geological Survey Professional Paper 1236-B, p 15-20.

Ravat, D.N., 1984, Magnetic investigations in the St. Louis arm of 

the New Madrid rift complex: Purdue University, M.S. thesis, 

102 p.

Saad, A.H., 1969, Magnetic properties of ultramafic rocks from Red 

Mountain, California: Geophysics, v. 34, no. 6, p. 974-987.

Schwalb, H.R., 1978, Paleozoic geology of the New Madrid area 

Annual progress report Fiscal year 1978, contract NRC-76- 

321, New Madrid seismotectonic study Activities during fiscal year 1978, [edited by T.C. Buschbach, prepared for the Division of Reactor Safety Research, Office of Nuclear 

Regulatory Research, U.S. Nuclear Regulatory Commission, 

contract NRC 04-76-204]: Springfield, 111., Illinois State Geological Survey.

Soderberg, R.K., and Keller, G.R., 1981, Geophysical evidence for 

a deep basin in western Kentucky: American Association of 

Petroleum Geologists Bulletin, v. 65, p. 226-234.

Stearns, R.G., and Marcher, M.V., 1962, Late Cretaceous and subsequent structural development of the northern Mississippi

Embayment area: Geological Society of America Bulletin, v. 

73, p. 1387-1394.

Steiger, R.H., and Jager, E., 1978, Subcommission on Geochronology: Convention on use of decay constants in geochronology 

and cosmochronology, in Cohee, G.V., Glaessner, M.F., and 

Hedberg, H.G., eds., Contributions to the Geologic Time Scale: 

Student Geology 6, American Association of Petroleum Geologists, p. 67-71.

Thomas, W.A., 1991, The Appalachian-Ouachita rifted margin of 

southeastern North America: Geological Society of America 

Bulletin, v. 103, p. 415-431.

Toft, P.B., Arkani-Hamad, Jafar, and Haggerty, S.E., 1990, The effects of serpentinization on density and magnetic susceptibility A petrophysical model: Physics of the Earth and Planetary 

Interiors, v. 65, p. 137-157.

Vacquier, Victor, Steenland, N.C., Henderson, R.G., andZietz, Isidore, 1951, Interpretation of aeromagnetic maps: Geological 

Society of America Memoir 47, 151 p.

VanSchmus W.R., Bickford, M.E., Sims, P.K., Anderson, R.R., 

Shearer, C.K., and Treves, S.B., 1993, Proterozoic geology of 

the Western Midcontinent basement, in Precambrian Conterminous U.S., Geological Society of America, C-2, p. 239-259.

Watson, K. D., 1967, Kimberlites of eastern North America, in 

Wyllie, P.J., ed., Ultramafic and Related Rocks: New York, 

JohnWiley, p. 312-323.

Webring, Michael, 1982, MINC A gridding program based on 

minimum curvature: U.S. Geological Survey Open-File Report 

81-1224, 43 p.

 1985, SAKI -FORTRAN program for generalized linear 

inversion of gravity and magnetic profiles: U.S. Geological 

Survey Open-File Report 85-122, 29 p.

Zartman, R.E., 1977, Geochronology of some alkalic rock provinces in Eastern and Central United States: Earth and Planetary 

Science Letters Annual Review, v. 5, p. 257-286.

Zartman, R.E., Brock, M.R., Heyl, A.B., and Thomas, H.H., 1967, 

K-Ar and Rb-Sr ages of some alkalic intrusive rocks from the 

Central and Eastern United States: American Journal of Science, v. 165, p. 848-870.

Published in the Central Region, Denver, Colorado 

Manuscript approved for publication August 1, 1995 

Edited by Richard W. Scott, Jr. 

Photocomposition and cover design by Shelly A. Fields

it U.S. GOVERNMENT PRINTING OFFICE: 1996 - 774-045 / 20034 REGION NO. 8