GEOLOGIC INVERPRETATION 0F SELECTED

ANOMAUES WHEN THE MONROWA QUADRANGLE, .

UBERIA. USING AN SMERGRATION 0F
GEOPHYSEAL METHODS

Thesis for the Degree of M. S.
MlCHiGAN STATE UNIVERSH'Y
NATHANIEL REGINA]. RiCHARDSON, Jr.
1973

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SUPPLEMENTAP"
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ABSTRACT

GEOLOGIC INTERPRETATION OF SELECTED ANOMALIES
WITHIN THE MONROVIA QUADRANGLE, LIBERIA,
USING AN INTERGRATION OF
GEOPHYSICAL METHODS

BY

Nathaniel Reginal Richardson, Jr.

In the tropics, because of the high degree of
weathering, information from geophysical surveys are used
in the making of geologic maps. Faults, geologic contacts,
and structural trends, are often hard or impossible to
detect solely on the grounds of surface geologic mapping.
This is the case in Liberia.

In 1968 a joint magnetic and radiometric survey was
flown over the entire country of Liberia by Lockwood,
Kessler and Bartell (Behrendt and Wotorson, 1971a). A
detailed gravity station network has been started in the
Monrovia Quadrangle and will eventually cover the entire
country. Rock samples were collected in the summer of
1972 and their densities and magnetic susceptibilities
were determined and compared with the grativy and magnetic

pattern in this study. In general these physical properties

Nathaniel Reginal Richardson, Jr.

show very little correlation with the gravity and magnetic
patterns of the area.

The aeromagnetic data show that the Monrovia
Quadrangle has primarily a north-east structural trend.
Superimposed on this trend are north-west trending diabase
dikes which occur in two zones: A coastal zone, and a
zone beginning about 90 kilometers inland.

The Bouguer and Free-air anomaly gravity maps show
a positive trend which runs parallel to the coast. From
the bouguer anomaly map, a gradient of 3-4 milligal/km is
associated with this feature. Behrendt and Wotorson (1972),
felt that uplift of granulite and crustal thickening were
both responsible for this feature, since the gradient is
too steep to be explained by crustal thickening alone. The
mean free air anomaly for the entire country is + 22
milligals which would mean that essentially, the area of
Liberia is isostatically under compenstated. For the
region of Liberia, a crustal thickness of about 32 km. was
calculated using Woollard's curve (Woollard, 1957).

The total-count gamma radiation map shows that
there is a positive correlation of felsic and mafic rocks
with radioactivity. The felsic rocks have a greater
radioactivity than the mafic rocks, which is due to their
higher potassium content. The radiometric pattern of the
quadrangle also reflects the Northeast and Northwest

trends that are found on the aeromagnetic map.

Nathanial Reginal Richardson, Jr.

In all, six anomalies within the Monrovia Quad-
rangle are interpreted in this discussion. In general, it
would appear that the anomalies that occur in the coastal
area are caused mainly by faulting, accompanied at times
by the intrusion of diabase dikes. Where basins have
resulted from the faulting, sediments have accumulated to
substantial thicknesses. Those anomalies that occur
inland are mainly caused by density contrasts of large

rock bodies.

GEOLOGIC INTERPRETATION OF SELECTED ANOMALIES
WITHIN THE MONROVIA QUADRANGLE, LIBERIA,
USING AN INTERGRATION OF

GEOPHYS ICAL METHODS

BY

Nathaniel Reginal Richardson, Jr.

A THESIS

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

MASTER OF SCIENCE

Department of Geology

1973

ti} ACKNOWLEDGMENTS

The author wishes to express his thanks and
appreciation to Dr. H. F. Bennett whose assistance
throughout this study was invaluable. The many useful
suggestions by both Dr. R. S. Carmichael and Dr. T. A.
Vogel are acknowledged. Special thanks goes to Mr. Cletus
Wotorson of the Liberian Geological Survey (L.G.S.) for
his assistance to me while collecting the samples. Lastly,
my sincere thanks and appreciation goes to J. C. Behrendt,
United States Geological Survey (U.S.G.S.) and C. S.
Wotorson (L.G.S.) for the unrestricted use of all their

geophysical data.

ii

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TABLE OF CONTENTS

LIST OF TABLES . . . . . . . . . . .

LIST OF FIGURES. . . . . . . . . . .

Chapter

I.

II.

III.

INTRODUCTION . . . . . . . . .

Purpose of the Study . . . . . .
Location of Study Area . . . . .
Previous Investigation . . . . .
Geology of Study Area. . . . . .

Metamorphic Rocks . . . . . .
Igneous Rocks. . . . . . . .
Sedimentary Rocks . . . . . .

PROPERTIES OF THE ROCK SAMPLES . . .

Introduction. . . . . . . . .
Magnetic Susceptibility of the Rocks.
Density of the Samples . . . . .
Opaques . . . . . . . . . .

GEOPHYSICAL DATA . . . . . . . .

Previous Geophysical Investigation
Magnetic Survey of the Study Area. .

Residual Magnetic Map of the Area .

Gamma Radiation Survey of the Area .
Gravity Survey of the Study Area . .

Bouguer Anomaly Map. . . . . .
Free-Air Anomaly Map . . .

Residual Gravity Map of the Study Area

iii

Page

vi

H

bPQFJH

\JON-b

11
16
17

24

24
24

27

29
30

3O
33
34

Chapter

Comparison of the Physical Properties
Determined From the Rock Samples With
the Aeromagnetic and Gravity Maps . .

Relationship of Opaques to Magnetic
Susceptibility Values, and Magnetic
Susceptibility Values to the Aero-
magnetic Map . . . . . . . .

Relationship of the Opaques to Density
and Densities to the Gravity Maps .

IV. INTERPRETATION. . . . . . . . . .

Introduction. . . . . . . . . .
Anomaly l. . . . . . . . . . .

Anomaly 2. . . . . . . . . . .
Anomaly 3. . . . . . . . . . .
Anomaly 4. . . . . . . . . . .
Anomaly 5. . . . . . . . . . .
Anomaly 6. . . . . . . . . . .

V. RECOMMENDATIONS, SUMMARY AND CONCLUSION .

Recommendations. . . . . . . . .
Summary and Conclusions . . . . . .

REFERENCES CITED . . . . . . . . . . .

APPENDIX 0 O O O O O O O O O O O O 0

iv

Page

42

43
45
47
47
47
50
51
52
53
55
57

57
58

60

62

L I ST OF TABLES

Table Page
1. Magnetic Susceptibility Values According to

Rock Type . . . . . . . . . . . . 15

2. Density of Rocks in the Monrovia Quadrangle . l8

3. Percent Opaque According to Rock Type . . . 22

Figure

l.

10.

11.

12.

13.
14.

15.

LIST OF FIGURES

Map Of Liberia 0 O O O O O O O O O O

Monrovia Quadrangle Showing Sample Location for
Susceptibility Determination. . . . . .

Monrovia Quadrangle Showing Sample Locations .

Monrovia Quadrangle Showing Specific Gravity
Sample Location . . . . . . . . . .

Monrovia Quadrangle Showing Sample Location
for Opaques . . . . . . . . . . .

Aeromagnetic Map of the Monrovia Quadrangle
(See Folder at Back)

Residual Total Magnetic Intensity Map . . .

Gamma Radiation Map of the Monrovia Quadrangle
(See Folder at Back)

Bouguer Anomaly Map of the Monrovia Quadrangle
(See Folder at Back)

Residual Bouguer Anomaly Map of the Monrovia
Quadrangle (See Folder at Back)

Free-Air Anomaly Map of Liberia . . . . .
Monrovia Quadrangle Showing Profile Location
Used In Construction of the Regional
Gravity Map . . . . . . . . . . .
Profiles AA' and CC' . . . . . . . . .
Profiles BB', GG' and NN' . . . . . . .

Profiles DD' and FF' . . . . . . . . .

vi

Page

10

19

23

28

31

36
38
39

4O

Figure Page
16. Profiles II' and HH' . . . . . . . . . 41

17. Monrovia Quadrangle Showing Anomalies
Location O I O O O O O O O O O O 48

18. Aeromagnetic Map of the Gbanka Quadrangle
(See Folder at Back)

19. Magnetic Profiles AA' and BB' and Gravity
Profile CC' Over Cape Mount Anomaly . . . 54

vii

CHAPTER I

INTRODUCTION

Purpose of the Study

 

The purpose of this study is to integrate magnetic,
gravity and radiometric data with recent geologic infor-
mation, in order to interpret certain geophysical anomalies
in the Monrovia Quandrangle, Southwestern Liberia. These
anomalies are readily apparent on some or all of the
geophysical maps of the quadrangle, but they are not all
easily correlatable to features on the geologic map of the
area.

It is hoped that an integration of the geophysical
methods with the geologic information will place re-
strictions on the possible solutions, thus viable and
geologically sound models for these anomalies might be
suggested. Towards this end, samples were collected with
the hope that they would add further restrictions to the

models.

Location of Study Area

 

Liberia lies on the south-west coast of west

Africa, between 4 degrees and 9 degrees N. latitude and

between 7 degrees and 12 degrees W. longitude. To the
northwest is Sierra Leone. Guinea lies to the north and
the Ivory Coast to the east. To the south and south-west
is the Altantic Ocean. This thesis covers the Monrovia
Quadrangle, which is one of several quadrangles that makes
up the entire map of Liberia. The Monrovia Quadrangle is
shown by the shaded area in Figure 1. The region has low
relief near the coast, with a hilly interior. The highest
point in the quadrangle can be found at Cape Mount. The
region, like the rest of the country, lies in the tropical
rain forest belt, and is therefore heavily forested. Due
to the intense weathering, there are few outcrops from
which to collect samples. In such a region, the outcrops
that offered the best samples were those that were found
in river beds, and those that were left exposed in road or

railroad cuts.

Previous Investigation

 

Previous investigations in this area consist of
geological and geophysical studies. The geological studies
were conducted by A. W. Leo and R. w. White (1969), whose
contributions are mainly the Reconnaissance geology of
western Liberia, mapping of both sedimentary and weathered
crystalline rocks, and summarizing the age provinces of
Liberia. L. V. Blade (1970) studied the geology of the
Bushrod Island-New Georgia clay deposit near Monrovia.

P. M. Hurley et a1. (1971) were instrumental in dating and

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classifying the various age provinces of Liberia. All
previous geophysical work in the area which includes
grativy, magnetics and radiometrics, have been done by
J. C. Behrendt (U.S.G.S.) and C. S. Wotorson (L.G.S.)

between 1968 and 1972.

Geology of Study Area

 

The Quadrangle, like most of Liberia lies within
the Guinea shield and is underlain by precambrian cristal-
line rocks mainly of granite composition. Rocks in the
Quadrangle fall into two age provinces. The Pan-African
and the Liberian Age provinces, have been dated as 500 M.Y.
and 2700 M.Y. respectively (Hurley, et a1., 1967). In the
Liberian age province both the foliation of the gneiss and
metasedimentary structural features trend northeastward.
This is not the case in the Pan-Africa age province where
the structural grain generally trend northwestward (White
and Leo, 1969). Figure 2 shows the age provinces of the

Monrovia Quadrangle.

Metamorphic Rocks

 

The metamorphic rocks range in grade from the
amphibolite to granulite facies, and there is considerable
exposure of amphibolite, quartzite, schist, and metasedi—
mentary iron formation found in the area. The amphibolite
facies occurs mainly east-northeast of Monrovia, whereas,
the granulite facies is found mainly from Monrovia towards

the northwest and onward to the Sierra Leone border. The

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amphibolite and other mafic rocks in Liberia are generally
not as deeply weathered as the granitic rocks and schist,
therefore they have a tendency to form ridges and divides
(R. W. White, 1970).

The quartzite and schist are in many places
associated with iron formations, as is the case in the
Goe-Fantro Ranges. The schist in this region contains
kyanite, quartz and muscovite. The metasedimentary iron
formations, locally known as itabirite, are finely lami-
nated rocks with grain size increasing with metamorphism
(C. H. Thorman, 1972). Major deposits of this itabirite
are found in the area of Bomi Hills and the Bong Range.
The magnetite-itabirite is the most common although
hematite-itabirite is also found (R. W. White, 1969-70).

The granulite facies crops out in a 15 mile wide
belt which runs parallel to the coast from Monrovia toward
Sierra Leone. Granulite of igneous and sedimentary origin

occurs in this belt (R. W. White and G. W. Leo, 1969).

Igneous Rocks

 

The igneous rocks are mainly intrusive in nature
and include diabase, norite and pegmatite. Some granite
is also found in the Quadrangle. The diabase appears
mainly as vertical dikes having surface expressions that
can be traced for tens of miles. The longest continuous
dike, which occurs within an inland northwest trending

dike zone, is the Gibi dike and can easily be seen on the

aeromagnetic map of the quadrangle (Figure 6A in back
folder). Another diabase dike zone strikes along the
coast, and here, flat lying sill-like diabase bodies have
been mapped (R. W. White and A. W. Leo, 1969). The norite
which forms a prominent hill near Robertsport is believed
to be a plug—shaped intrusive body (Brehrendt and Wotorson,
1971c). Its relationship with the granulite facies rocks
in the area is not known since this hill is surrounded by
unconsolidated sand on one side and the Atlantic Ocean on
the other. Pegmatites appear sparsely in the Quadrangle
and have been placed into two categories. These categories
are (l) granite, quartz—muscovite-microcline-plagioclase
rocks with minor hornblend or biotite, and (2) massive bull
quartz, with minor amounts of feldspar and mafic minerals

(C. H. Thorman, 1972).

Sedimentarprocks

 

In the Quadrangle, sedimentary rocks are found
mostly between Monrovia and Buchanan. The sandstones lay
unconformably on the metamorphics and are of Paleozoic,
lower Cretaceous and Tertiary ages (J. C. Behrendt and
C. S. Wotorson, 1969). They are mainly white sandstone
which are heavily stained with iron in places, arkose,
graywacke, siltstone, mudstones and conglomerates. The true
thickness of this section is not known since from geo—
physical data (Behrendt and Wotorson, 1969) it has been

shown that there is at least one major basis underlying a

part of the area. Sedimentary rocks also crop out northwest
of Lake Piso and at Gibi mountain (C. H. Thorman, 1972).
Just north of Monrovia and in the area of Lake Piso there
are large areas of thin unconsolidated sandy sediments. In

these places they are slightly thicker than 15 meters

(R. W. White, 1969-70).

CHAPTER II

PROPERTIES OF THE ROCK SAMPLES

Introduction

 

More than sixty rock samples were collected during
the summer of 1972 to determine general physical properties
of these materials. The exact location of these samples
are shown in Figure 3 and numbers on the map refer to the
sample number in the Appendix. The samples were collected
for several reasons which will be mentioned here and
discussed more fully below. They were collected to identify
the rock types, to determine their magnetic susceptibili-
ties and densities and to identify and estimate the per-
centages of opaques.

The sampling scheme used in this study was based
primarily on the accessibility of the outcrops and on the
degree to which weathering had proceeded. With this in
mind, those outcrops that were heavily weathered were
passed over, and only those with relatively fresh rock
were sampled. Because of the intense weathering problem,
most of the rocks collected were mafic rocks rather than

felsic rocks. In Liberia the former seem to be less

10

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effected by the weathering than the latter. Several of
the samples were collected by Mr. C. Thorman of the
U.S.G.S. from outcrops in the Lofa and St. Paul River

channels.

Magnetic Susceptibility of the Rocks

The reason for measuring the magnetic susceptibi-
lity was to have base data that would give the range and
magnitude of magnetic susceptibilities. It was felt that
this information could probably be utilized in the interpre-
tation of the magnetic maps.

The magnetic property of any material, including
rocks, is due to both permanent and, or induced magneti-
zation. Permanent or remanent magnetization refers to that
portion of magnetism which remains fixed in the material or
rock. If the rock is moved about or exposed to an external
magnetic field, the permanent magnetization will be un-
affected. However for a weakly, permanently magnetized
rock, the permanent magnetization can change with time if
the rock is placed in a different magnetic field. An
ordinary bar magnet is a good example of permanent magneti-
zation. A study of the remanent magnetization of these
samples was not conducted therefore only the susceptibility
is discussed in this study.

Induced magnetization refers to that portion of
magnetism that is created when a material is placed in an

external magnetic field, such as the magnetic field of the

12

earth. When this external field is present, the material
will behave like an ordinary bar magnet which is oriented
with the external field. If the strength and direction of
this external field is removed, the induced magnetization
will disappear. An electromagnet is a good example of
induced magnetization. The induced magnetization is
proportional to the field through the constant k, where k
is the magnetic susceptibility of the material. The mag-
netic susceptibility of a material therefore is a measure
of the ability of that material to take on induced mag-
netization when it is placed in an external field.

The susceptibility of a rock is dependent upon

several characteristics listed below.

1. Dependence on composition.--The variation in
composition, of the magnetic mineral suite within rocks,
can lead to variations in susceptibility. Since weathering
produces oxides of iron from the primary magnetite, the
relatively nonmagnetic oxides reduce the susceptibility.
The formation of hematite from pre-existing magnetite, due
to hydrothermal solutions, will cause a reduction in the
susceptibility, whereas the formation of magnetite from the
reduction of hematite, in highly metamorphosed sediments,

will increase the susceptibility (Grant and West, 1966).

2. Dependence on field strength.--According to
Strangway (1967), the susceptibility of a material will

increase rapidly as the field strength approaches 150

13

oersteds, above this, the susceptibility starts to decrease.
However, this factor is not important in the earth's

relatively low magnetic field.

3. Dependence on state of magnetization.--The state
of magnetization is very important to the susceptibility
of a sample. If a sample has remanent magnetization its
susceptibility will be different than if it had no perma-
nent magnetization. According to Strangway (1967), it is
found in general that susceptibility decreases with an

increased remanence.

4. Dependence on_grain size.--Experiments carried out
on powdered samples of magnetite show that a decrease in
the particle size greatly reduced the effective suscepti-
bility (J. J. Jakosky, 1957). This means that plutonic
rocks have generally higher magnetic susceptibilities than

their volcanic equivalents.

5. Dependence on amount of magnetic minerals present.
--Not only does magnetization depend on the type of mag-
netic mineral present, but also on the quantity present.
Jakosky (1957), found that rocks which have widely dis-
persed magnetite show the lowest susceptibility. However,
with increased grain size one might reasonably expect an

increase in susceptibility.

6. Dependence on orientation.--Certain metamorphic

rocks especially those that exhibit a platy, schistose or

14

gneissic texture will have different susceptibility values
in different directions. As a general rule, the bulk
susceptibility will be smaller in the direction perpen-
dicular to the schistosity or gneissosity then in the
direction parallel (Grant and West, 1965).

Other variations in the susceptibility values
could have come from the following sources. Some of the
samples were slightly below minimum size, and although a
conversion factor was used, error might have been intro-
duced in this conversion. Variations in five repeated
measurements on the same sample were found to be less than
1 percent, which is the estimated error in Table 1.

Since some of the samples are metamorphic in
origin, this might be a source of error in these samples.
The samples measured might not be representative of the
general rock type. For example, D. Sanderson (Ph.D.
Thesis, 1972) showed that magnetic susceptibility is apt
to change quite markedly from one location to another
within the same rock body. Table l and the susceptibility
values will be discussed in Chapter III.

Briefly, the magnetic susceptibility of a rock is
measured by placing coils, referred to as the primary coil
and the secondary coil, around the rock sample. When a
current is passed through the primary coil, it causes a
galvanometer, which is connected to the secondary coil, to

be deflected (Dorbrin, 1960). This deflection is a measure

15

TABLE l.--Magnetic Susceptibility Values According to Rock Type.
Errors in the Repeatibility of These Readings is Less Than
1 Percent.

 

 

Number of Susceptibility Medium Arithmetic

Rock Type Samples Range x 10"6 Value Mean
Granite Gneiss 8 117.0-1838 783 733
Granite 1 20.0 . . . .
Gneiss 1 146.0 . . . .
Metabasic 1a 2 2870-4210 . . . .
Diabase 6 2188-5793 3990 3889
Gabbro 2 5099-5546 . . . .
Chlorite Schist 1 158.0 . . . .
Norite 1 1728 . . . .
Weathered-
Quartzite 1 756.0 . . . .
Metabasic 2b 6 126-1678 192 376

 

aMetabasic l--locally called amphibolite, is a dark gray,
medium grained massive rock containing approximately equal amounts of
plagioclase and hornblende.

bMetabasic 2--locally called granulite, is a reddish gray,

medium to course grained massive rock containing pyroxene, plagioclase
and often garnet.

Note: These values represent selected samples, and should not
be taken as the susceptibility values, for the representative rock
type in the Monrovia Quadrangle.

16

of the magnetic susceptibility of the rock. The magnetic
susceptibility values determined in this study are given

in Table 1 and the location of the samples used are shown
in Figure 2. The susceptibility bridge used in this study

was the model MS-3 made by Geophysical Specialities Company.

Density of the Samples

 

The sample densities were determined to provide
base information on the range and magnitude of the rock
densities in the area. It was felt that this information
would aid in the interpretation of the gravity survey data.

The densities of the samples used in this study
were determined by dividing the weight in air by the loss
of weight in water. For igneous and metamorphic rocks,
this method presents no serious problems since the porosity
of these rocks is very low. For sedimentary rocks, the
density depends mainly on the amount of pore spaces and on
the degree to which these pore spaces are filled with
water.

Errors in the determination of densities of the
samples could have come from one or both of the following

sources.

1. Air bubbles were clinging to several of the rock
samples when they were submerged for the measurements.
The error due to this was minimized by allowing the sample
to stay submerged for a period of time, during which time

most of the bubbles broke loose from the sample and floated

17

to the surface. The bubbles that remained were removed

with a small brush.

2. A few of the samples had surfaces that had developed
a weathered crust. This problem was reduced mostly by
chipping off the weathered surface with a rock hammer.
Readings that might be in error due to this factor will
appear with an asterisk to their upper right hand corner
in the Appendix.

The estimated relative error based on five repeated

measurements of the same sample, was found to be less than
1 percent. Densities obtained by the method described
above are shown in Table 2 and for the location of samples

used in the density determination refer to Figure 4.

Opagues

The primary purposes for studying the opaques in
the rocks were to look at the size of the magnetite grains,
the degree to which these grains had altered, and the
secondary minerals that had been produced by the alteration
of the magnetite. It was hoped that these properties could
be related to the magnitudes of the magnetic susceptibili—
ties, and to the density ranges of the rocks studied.

The method chosen for this study was the point
count method. The point count method, using transmitted
light, allowed the estimation of the total opaques in the

rock sample. The identification and degree of alteration

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n msoH>mum OHHwEEHHua EsHpmz wuHmcmo mo umnEdz

 

.ucmonmm H Gene mmmH mH mucmEmHSmmmz mHnmummmmm co vmmmm
uounm m>HumHmm UmumEHumm one .mHmcmupmso mH>ouc02 may :H mxoom mo muHmcmall.m mqmde

l9

 

 

 

 

 

2322.583 3.2.:— e. you... .3233 23...; 5:323 22.2.3.5 2.2.3: V .mml
o: :0:
so .unensoa-
‘Q
m. . 2 Vuvo
h. 2.3.3! o
u.vm
.0 o \k
a a Q?
= 2 NR
‘. v.
a
n.
3 2.
. 2. ~— NM. 3
x 3
n. «ma... N... 3:... 8
.N 2 a. . L 2. 33......

m“. mu on. 3

H 3.

2. :3? . t 2

nu - on.
ow.
an. fl“.
.. e ..
.—

Jono— —

20

of the opaques were then carried out using an ore micro-
scope.

In the point count method both standard thin
sections and thin sections made from rock cores were used
in the estimation of the opaque percentages. Although the
total average count for the core thin sections were about
750 as compared to 1000 for the standard thin sections,
both results were placed on the same table.

After the counts were made, the thin sections were
polished in order that the opaques be identified. The most
abundant opaque identified was magnetite although.in places
it was almost completely altered to ilmenite. Ilmenite was
the next in abundance followed by limonite or hematite. On
one or two thin sections leucoxene was identified.

The quality of the standard thin sections, those
made with Canadian Balsalm cement, deteriorated with
polishing and therefore this made identification of the
opaques harder. The polished thin sections made with
epoxy cement were far superior for opaque identification.

This study shows that in general, the larger the
grain size of the magnetite and the higher the total
opaque percentage, the higher the magnetic susceptibility
and density are likely to be. This does not appear to be
a linear relationship since the ranges of densities and
susceptibility differ widely with rock types. Specifically,

the physical properties of individual rock samples may not

21

individually correlate with gravity and magnetic fields,
on the other hand, the ranges in the measured physical
properties of these samples may be useful in the interpre-
tation.

The percent opaques based on rock type can be
found in Table 3 and Figure 5 shows the location of the
samples used in this study. For the percentage of opaques

of the individual rocks refer to the Appendix.

22

TABLE 3.--Percent Opaque According to Rock Type. Tr. is
Less Than 1 Percent.

 

 

 

Range of
Number of Opaque Medium Arithmetic

Rock Type Samples Percentages Value Mean
Granite Gneiss 7 Tr.-4.4 .5 1.48
Granite l Tr. . . . .
Gneiss 5 Tr.-3.0 .2 .78
Metabasic la 1 4.06 . . . .
Diabase lO 1.9—9.4 3.82 3.88
Gabbro 2 8.68-8.80 . . . .
Chlorite Schist 2 Tr. . . . .
Hornblendite l .2 . . . .
Norite l 2.5 . . . .
Weathered-
Quartzite l 6.58 . . . .
Metabasic 2b 7 Tr.-4.4 .66 1.77

 

aMetabasic l--locally called amphibolite, is a dark
gray, medium grained massive rock containing approximately
equal amounts of plagioclase and hornblende.

bMetabasic 2--locally called granulite, is a

reddish gray, medium to course grained massive rock con-
taining pyroxene, plagioclase and often garnet.

23

3

 

«oases:

.3

 

.223...— o_.:..nm :32.» 25.2.2...

2.3:: m .umu

on 5

 

 

e v
w o
O
0.... 23:38 o \K
cm
... *Vv
A v
=
n.
a. =
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.N a a 2 2. gm... .5358.
2...
m... 8
on. . .
. n.
«N 3.
an.
son on
on 3.03:

CHAPTER III

GEOPHYSICAL DATA

Previous Geophysical Investigation

The geophysical data used in this study have been
compiled and briefly discussed by J. C. Behrendt and C. S.
Wotorson in several reports put out from 1969 to 1972.
The geophysical work covered in these reports include
magnetics, gravity and radiometrics. All of the geo-
physical maps with the exception of the residual bouguer
anomaly map, which appear at the end of this study, have
been taken from their reports. For the methods used in
the reduction of the geophysical data and the compilation
of the maps, refer to Behrendt and Wotorson (1969—1972).
Some off-shore geophysical data was collected by the
U.S.C.G.S. Discoverer in 1968 but will not be discussed in
this study (Behrendt and Wotorson, 1972). For reference

to the geophysical maps, see the back folder.

Magnetic Survey of the Study Area
The aeromagnetic map of the Monrovia Quadrangle

(Figure 6, back folder), can be divided into four provinces,

24

25

based on the amplitudes of the magnetic anomalies in the
areas. Two provinces have high magnetic amplitudes, one
in the southeastern section of the quadrangle, and the
other in the northern section. The third province is an
area of rather low amplitude magnetic anomalies, and is
found in the central portion of the quadrangle. The
fourth province is off-shore and is characterized by
reasonable high amplitude, very broad anomalies.

In the northern province the high amplitude,
rather narrow magnetic anomalies lie mainly over the Bong
Mines and the Bomi Hills mining districts. The trend of
the iron formations is mainly east-west with a slight change
to a north-eastern trend in the eastern part of the
quadrangle. Most of the high amplitude anomalies are
caused by the magnetite itabirite and not the ore itself
(Behrendt and Wotorson, 1972). Due to these high east-west
trending anomalies, the anomalies caused by the lower
amplitudes northwest trending diabase dikes are not easily
seen, and in many instances are completely lost in the
anomalies caused by the itabirites. Within this province
there are two areas of low amplitude anomalies, one just
north of Bong mines, and the other north-west of the Bomi
Hills.

The central province is characterized by rather
low amplitude anomalies and here the north-west trending
dikes can easily be observed since their amplitudes are

higher than the background noise. It is in the eastern

26

portion of this province that the Gibi dike can best be
seen. This dike, passing through the Gibi Mountain, is
the longest continuous dike in the country.

The most noticeable features in the southeastern
province are the Goe-Fantro Ranges. Thorman (1972),
described these rocks as being hematite and magnetite
itabirites that are associated with quartzite, schist and
amphibolite. Two low amplitude anomaly areas can be found
in this province, one just south of the Fantro Range and
the other just south of Roberts Field. On the western side
of this province is an anomaly, called Anomaly A (Behrendt
and Wotorson, 1971a). This anomaly is the longest north-
east trending linear feature in this province. Northeast
of the Goe Range there is a low amplitude northeast
trending feature, which might be the continuation of
Anomaly A on up through the central province and into the
Gibi mountains. A large section of the Fantro Range has
the same northeast trend and along with Anomaly A, these
features might be controlled by a northeast trending
fracture pattern in this area. In Chapter IV, portions
of this area are more fully discussed.

The seaward province has reasonably high amplitude,
rather broad anomalies. It is interesting to note, that
there seem to be an equal distribution of high and low
anomalies, having no preferred trends. Because of the
broadness of these anomalies, their sources are probably

rather deep. From the general pattern of these anomalies,

27

and the fact that they have no preferred orientation, one
might conclude that the basement is rather complexly
faulted. There are no radiometric or gravity data avail-
able for this province. This province, will not be
discussed hereafter.

Residual Magnetic Map
of the Area

 

 

The residual map (Figure 7) shows the effects of
broad features and because of this, it tends to show only
the effect due to the influence of deeper bodies. It is
not surprising then, since the aeromagnetic map shows
mainly shallow features, that these two maps are somewhat
dissimilar.

Two rather small northwest trending features
appear on the residual map just northeast of Monrovia. On
the aeromagnetic map these are the two low amplitude
linear features that appear in this area. Behrendt and
Wotorson (1971a), attributed them to shallow diabase dikes.
These features however are the only two diabase dikes that
have a correlatable linear feature on the residual map.
This might indicate that these dikes are either much
deeper than previously believed, or that they are the
single surface exposure of a dike swam that exist at depth.
The low residual value in the area of Roberts Field corre-
sponds to one of the low amplitude areas described above

on the aeromagnetic map.

28

 

 

 

 

00‘

I
O

11

Magnetic Intensity

Total

From

Fig. 7 Residual

1971a

and Wotorson,

Behrendt

In Gamma:

ts

29

In general, there are slight indications that the
Northeast and Northwest trends that exist on the aero-
magnetic map also exist on the residual map. This would
indicate that these trends extend reasonably deep in the
crust and are probably the reflection of gross tectonic

patterns in the crust.

Gamma Radiation Survey of the Area

 

The radiometric method is based on the detection
of naturally occurring isotopes of K, U and Th. In general,
the higher radioactive counts, those above 250 c.p.s., are
in the area of the granite gneiss. This area is located
toward the eastern portion of the quadrangle (Figure 8
back folder).

In areas where the rocks are mainly sedimentary,
iron formations, granulites or other mafic rocks, the
general count is less than 250 c.p.s. In the areas of low
background counts, the low readings caused by the iron
formations and dikes, are masked much like the high mag-
netic values of the dikes were masked by the even higher
magnetic values of the itabirite.

The highest radiometric readings in the quadrangle,
are those in the north central portion and those north of
the Fantro Range. The high radiometric values that are
north of the Fantro Range are in the vicinity of the lower
portion of the broad gravity low that is found on the

eastern side of the Bouguer Anamaly map (Figure 9 back

30

folder). The rocks in this area would probably be high in
feldspars and low in mafic minerals, since a highly
feldspathic granite gneiss would tend to be less dense
than granite gneiss containing amphibolites, thus ex-
plaining the low anomaly area on the Bouguer anomaly map.

The high north central values have been attributed
to monazite and zircon by Behrendt and Wotorson (1971b).
The high values north of the Fantro Range correspond to an
area of low magnetic readings which might indicate that
this area has highly felsic rocks.

The narrow anomalies that are seen along the St.
Paul River and the local highs along the beach, have been
attributed to clay deposits and black sands respectively
(Blade, 1970; Behrendt and Wotorson, 1971b). The black
sands contain zircon and monazite (S. P. Srivastawa,

personal communication, 1971).

Gravity Survey of the Study_Area

 

The Bouguer anomaly map and the Residual Bouguer
anomaly map can both be found in the back folder and are
numbered Figure 9 and Figure 10 respectively. The Free air

anomaly map Figure 11 is found below.

Bouguer Anomaly Map

In the compilation of the Bouguer anomaly map,
terrain corrections were not made because of the lack of
topographic maps, but should be less than 1 mgal (Behrendt

and Wotorson, 1972).

31

2.

 

 

 

 

I
I’

 

L_ ’2’! “Henri
'. \ /.,\ 2;

o [I l
at“ \V \\/

E53. \ O

\ .

'r
004
J.»
O
3.

9 10 Km 4.
v 5.

Fig.11 Free Air Anomaly Map 0' liberia

From Behrendt and WotorsonJS‘lZ Values In Milligals

f i ,
u Monrovua Quadrangle

32

There are four easily noticeable features on this
map. The first is the 40 to 50 milligal feature which runs
parallel to the coast all the way from Sierra Leone down
through the southeastern section of this quadrangle and
into the Ivory Coast. Behrendt and Wotorson (1971) wrote,
"this feature has a gradient of 3 to 4 milligals / km, in
the area between Sierra Leone and Buchanan, which is too
steep to be more than partially the result of crustal
thickening at the continental margins." This anomaly they
feel is the result of uplift of granulite from deep within
the crust.

A second area of notice is the broad low gravity
area found on the eastern side of the quadrangle. The
gravity data in this area of the quadrangle, are too sparse
for one to come up with any definite conclusions as to the
origin of this feature. However, it is probably the
result of a granite gneiss which has a lower density than
the surrounding rocks. The radiometric values in this
area show that the rocks are probably highly felsic, and
if these rocks are also low in density, they could be
responsible for the lower portion of the broad, low
Bouguer gravity anomaly. On the free-air anomaly map
(Figure ll), this second area is coincidental with an area
of relatively low contour values. The area is probably the
only area in Liberia that is in isostatic equilibrium and

this might also be due to the lower density granite gneiss.

33

The third feature that comes to ones' attention is
the well defined gravity low that is known as the Roberts
Basin. A model study of this structure (Behrendt and
Wotorson, 1972) show that it is a down faulted basin
filled, to a considerable depth, with cretaceous sediments.

The anomaly at Cape Mount is the fourth major
anomaly found on the Bouguer anomaly map. This feature
will be discussed in Chapter IV. There are several areas
on the Bouguer anomaly map that show the contour widening
or closing and a few of these areas will also be discussed
later. Even though the station density is greater along
the coast than inland, in general, it appears that the
anomalies inland are broader than those along the coast.
From this one might conclude that the sources producing

the anomalies are shallower near the coast.

Free-Air Anomalngap

Free—air gravity anomalies are best suited for
studying tectonic adjustments since they do not take into
account the gravitional attraction due to the mass included
between sea level and the elevation of the observation
site. Therefore they become first approximations to the
isostatic map. The Free-air anomaly map of Liberia
(Figure 11) (Behrendt and Wotorson, 1972) shows that the
coastal feature present on the Bouguer anomaly map is also
present on the Free-air anomaly map. Behrendt and Wotorson

(1972), found that the calculated mean free-air anomaly for

34

the entire country is + 26 milligals, but because the
coastal gradient might have biased the value, the area of
steep gradient was left out and they came up with an
average mean of + 22 milligals. This value of + 22

milligals would mean that although locally there might be

isostatic equilibrium, for example the broad low free-air

gravity area, the general area of Liberia is essentially

isostatically under compensated. The reason for this
might be that the uplifted basement material has not been
compensated for at depth. Assuming that coastal adjust-
ments to a free-air anomaly of 0 milligal requires a
downward movement of approximately 500 feet, and that this
calculation assumes a mantle density of 3.3 gm/cc, the
resulting elevation after isostatic adjustment would be
approximately 100 feet. This value corresponds to a
coastal thickness of about 32 km based upon Woollard's
curve (Woollard, 1959). The above calculation uses an
estimated mean elevation for Liberia of 600 feet (Behrendt

and Wotorson, 1972).

0

Residual Gravity Map of
ofithe Study Area

Many anomalies caused by near surface geologic
bodies are often found superimposed on anomalies (or
gradients) that are caused by large deep seated structural
features. The magnitude and direction of the regional
gradient depends entirely on depth, distance and con-

figuration of the large structural features involved.

35

There are several sources of regional gravity variations.
"In addition to large-scale geologic structures, there are
density effects caused by intrabasement lithologic changes
as well as isostatic variations which may often be in-
determinate" (Dobrin, 1960).

In order to interpret near surface geology, the
gravity anomalies caused by these features must somehow
be removed from the regional background (Grant and West,
1965). Methods, such as the direct computation of the
residual using a grid system, and graphical methods have
been proposed for taking out the regional. The method
chosen here, is a graphical method known as cross-
profiling. For best results, one set of profiles should
be drawn perpendicular to the direction of steepest
gradient and the second set parallel to the gradient. In
this study, the profiles were drawn along lines that
represented reliable control points and not necessarily
along lines that were perpendicular to each other.

The process involves smoothing out the profiles
and being certain that where profiles cross the smoothed
values are the same. According to Grant and West (1965),
"This method has the merit of being highly flexible, since
it allows the interpreter to incorporate into the process
his personal judgment, or sense of 'rightness' about the
forms of the residual anomalies."

More than 16 profiles were made, but only a few

of them will be presented here. Figure 12 shows the

36

:8 3.3.9 3......- o: uni-a 3 1::

«2:2; 3:3...” 3:23.: 2.22.: NF.U_l on:

 

 

 

 

 

.3 . .
571:5... / .
u a
./ o
.. 2 VW
oo
“.>e-zo!
.. . \k»?
x..— ’ vv
«v
. _ \
. u
a. 4 5p
aeludgo
.... I F .
a n
.a ..
2.. c. a 3.8....

37

location of the profiles presented in this study. In
using results from the cross-profile method, one must
always keep in mind that the removal of the regional is
purely an interpretive process, and there is no unique
solution. Because of this fact, the interpretation based
on a residual obtained by this method, could be somewhat
different than one obtained by another method. Admittedly
this method might introduce error in the interpretation,
therefore this must be kept in mind. Figure 13 through
Figure 16 are profiles obtained by the cross-profiling
method. The difference between the observed curve and the
smoothed out curve was plotted and contoured to give the
residual gravity anomaly map (Figure 10 see back folder).
The residual Bouguer gravity anomaly map has
brought out several anomalies which are not readily
apparent on the Bouguer anomaly map. An example is
anomaly 3, Figure 10. This anomaly although slightly
visible on the Bouguer anomaly map has been more definitely
defined by the cross-profiling method. On Figure 10, the
anomalies labeled 1 to 5 are interpreted in Chapter IV but
those labeled A to E will be briefly discussed here.
Between the Roberts Basin and Robertsport, the
steep gradient of the gravity contour on the Bouguer
anomaly map, appears to have caused the anomalies that
appear on the residual Bouguer anomaly map, to be shifted
slightly northward from where they appeared on the Bouguer

anomaly map. A good example is anomaly 3 which on the

38

 

 

 

 

 

3:023, .233... III .3 a... .= ”3:3... 0-. .0:—
a a . . s
A. ll m. ow pm a? ..~ a" pm a m 8..
Eli ll .30
a
.2
.3
.2.
.0 0:
m u . _ s
m. m. a. o.” w... a. t k. m. w J3-

 

m 3an=<

  

 

.0

 

39

 

 

5‘

3,32: ||.|.. 1.3.3: In .I II

a ..~ ...~

sg¢3llllw

C_mmm @.u.—Qnom

.5. 2......2. 2...... V9.03.

  

a . . .2
.m p. N. . ..

 

m 23:22 a

 

 

 

Ir

2'8 32

23

1'6
MILES

1'2

 

 

 

-Z.

40

   

, ANOMALY 2

 

 

Ll.__‘_¥ .
v 2 9

s [I II II' n
' 0

1'2 1'3 2‘0 2'4 2':
n l l r s

V

 

Observed

 

no' and n'

Pmfiles

Fig. 15

ii 21 28 32

1':

M l l E S

 

 

 

~20

4()

    

 

 

‘u.
-=
’:5
(\l
3..
_, a
‘1
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‘li
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55 S: "

-2.

I l l E 8

Observed

 

DD' and FF,

Pfofiles

Fig. 15

 

2'3 32

23

1':
MILES

i2

 

 

4()

    

(\l .:=
>-
_‘ %
<
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I l l E S

— - Itetional
Observed

 

on' and rr'

Profiles

Fig. 15

 

18 20 2} ii 32 5.
n l l e 5

f3

 

 

41

 

3
q-
s

 

 

16 is 23 z:

13
m I l r s

 

Observed

Proliles II. and HH'

l=ig;.'1(5

42

Bouguer anomaly map is centered about 25 miles north of
Monrovia whereas on the residual Bouguer anomaly map its
center is almost five miles further north.

Anomaly A and B are two anomalies that are not
visible on the Bouguer anomaly map. They are probably
the result of an area within the granulite that have a
slightly lower density. Anomaly C has been interpreted by
Behrendt and Wotorson (1971c), as being caused by a down
faulted block in which there has accumulated a large
thickness of sediment. Anomaly D also like C, is probably
caused by a down faulted block in which sediments have
accumulated, but not to the thickness of those found at
anomaly C. On the residual total magnetic intensity map
of the area anomaly D is vaguely delineated by a small
saddle in the contour lines. Anomaly E is found a few
miles north of Monrovia and is probably another one of
these down faulted sediment filled basins.

Comparison of the Physical Properties

Determined;From the Rock Samples With
the Aeromagnetic and Gravity Maps

 

 

In general the magnetic susceptibilities, densities
and percent opaques for individual samples, did not corre-
late with the magnetic or gravity patterns in the study
area. In the next two sections, these discrepancies and

possible reasons for them are discussed.

43

Relationship of Opaques to
Magnetic Susceptibility Values,
and Magnetic Susceptibility
Values to the Aeromagnetic Map

 

 

 

 

The values of magnetic susceptibilities are found
in Table l and show the following relationships. The
magnetic susceptibility values less than 1900 x 10.6 c.g.s.
seem to be restricted to the granite gneiss and granulite
zones. The granite gneiss zone on the aeromagnetic map is
in the low amplitude central province. The lowest value
of 20 x 10-6 c.g.s. is the measured susceptibility of a
granite which is one of the host rocks at the Bong Mine
iron mine. The granulites have susceptibilities that
range from (125 to 1678) x 10"6 c.g.s. These rocks are the
most uniform in their susceptibility values. This might
explain why on the aeromagnetic map, the magnetic values
are so smooth over the area that has been mapped as
granulite (White and Leo, 1969). The magnetite in most of
the granulites is highly altered, mostly to ilmenite. The
grain size of the opaques are also relatively small
compared with those of most of the diabases or gabbros.
These two reasons probably explain the low susceptibili—
ties of these rocks.

The amphibolites have an average susceptibility
that is considerably higher than the granulite. The range
is from (1668-4239) x 10’6 c.g.s. and it would seem that
any model that would explain the origin of these rocks must

also explain this difference. The grain size of some

44

opaques in these samples are also larger than those in the
granulite. The norite body at Cape Mount has a susceptibi—
lity of about 1728 x 10.6 c.g.s.

The highest values obtained in the susceptibility
measurements were those from the highly mafic diabases and
gabbros. The average values here were 4402 x 10-6 c.g.s.
and 5685 x 10.6 c.g.s. respectively. It was among these
rocks that the highest opaque percentages and largest
magnetite grains were found. The scatter for the diabase
was much greater than that for the gabbro. This is
probably because the opaques in the diabase show a greater
degree of weathering.

There are some correlations between the aero-
magnetic map and the high and low susceptibility values.
The best examples are found over the diabase dikes where
aeromagnetic highs correlate with high susceptibilities.
In places however, where there are low amplitude aero-
magnetic anomalies, there are some high susceptibility
values. Probably this is because the sample represents a
very narrow diabase dike which, although large enough to
provide an outcrop, is too small to give an aeromagnetic
anomaly. Behrendt and Wotorson (1970), wrote, "Some
exposed dikes have no magnetic anomaly due to their limited
extent and many of the 'dikes' are shown where there are

no known exposures. This statement might also explain
why there are low susceptibility values where there are

aeromagnetic highs. In general it was found that there

45

was not as good a correlation of sample susceptibility and
opaques, with the aeromagnetic map as was expected at the
beginning of the study. However, the above observation by
Behrendt and Wotorson may help to explain this discrepancy.
Relationship of the Opaques to

gensityand Densities to the
Gravity_Maps

 

The density values obtained in this study (Table 2)
show a good correlation with a similar study performed by
Samuel Rosenblum of the U.S.G.S., however there does seem
to be a difference in the densities determined for the
amphibolites. His value of 3.08 gm/cc is probably more
correct since he used many more samples than were used in
this study. In general the samples with greater percent-
ages of opaques (Table 3) had higher densities but since
the degree of weathering of the opaques varies with rock
type and even within the same rock type, the relationship
of high opaque percentage with high density samples was
found to be unreliable at times.

It was also found that in general sample densities
show no correlation with high and low anomalies on the
Bouguer and residual Bouguer anomaly maps. Some places
where there are high density values there are low gravity
anomalies and vice versa. Unlike the aeromagnetic
anomalies, the anomalies on the gravity maps are broader
and this is probably why the density values show poor

correlation. For example, one sample might not represent

46

the overall density of the rocks in an area. Another
explanation for this inconsistency is that the surface

rock type or density might differ from those at depth.

For example, if a surface rock such as quartzite with a
density of 2.68 gm/cc is underlain by a granulite with a
density of 3.1, the gravity anomaly in this area would not
be representative of what is expected of the quartzite. A
third explanation for the inconsistency of the gravity maps
with the rock densities is that the surface samples have

had their densities decreased due to weathering.

CHAPTER IV

INTERPRETATION

Introduction

 

Several anomalies within the Monrovia Quadrangle
have been briefly discussed by Behrendt and Wotorson
(1969 through 1972). These authors have discussed the
Roberts Basin anomaly in detail but have hardly mentioned
the others that are in the quadrangle. In this study, six
anomalies labeled 1 through 6 in Figure 17, excluding the
Roberts Basin anomaly have been studied and given a
geological interpretation. All of the geophysical maps

used in this study are found in the back folder.

Anomaly 1

Anomaly 1 (Figure 17) is located on the Bouguer
anomaly map (Figure 9, back folder) at latitude 6 degrees
30 minutes north and longitude 10 degrees 53 minutes west.
On the Bouguer anomaly map anomaly l is recognized by a
slight negative gravity nose facing a southerly direction.
The nose is recognized by the spreading of the contour
lines that form it. On the residual Bouguer anomaly map

(Figure 10), anomaly 1 is more clearly defined by a

47

48

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a no :22; «1.2.3.2

:32; ._..:;:a

«23...! NP .mul .30:

 

. 0%.:-

 

a...

mo

0

.-

 

 

2

3.09.2

48

 

 

 

 

can: _..:_£2.- a .323... 32-523 .233... «3.3325 23:3: NP .U_l boom—.-

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49

similar nose also facing in a southerly direction. Anomaly
l is not represented either on the radiometric or residual
aeromagnetic maps of the quadrangle.

On the aeromagnetic map, Figure 6A, anomaly 1 is
represented by an area of relatively low magnetic relief,
nowhere going much below 4600 or much higher than 4800
gammas. The 200 gamma difference is small compared with
differences of 500 just outside the area. Off shore from
anomaly l the magnetic contours widen considerably giving
a rather smooth area, and then within six miles from the
shoreline these contours take on a rather complex high and
low appearance. Although this rather smooth magnetic area
is slightly seaward of the location of anomaly l, the
structure causing this gravity anomaly is probably the
same structure responsible for the smooth magnetic ex-
pression described above. According to Behrendt and
Wotorson (1972), "Black faults are inferred in the pattern
indicated along the coastal area and continental shelf."
Anomaly l is therefore interpreted as being the result of
a sediment filled basin similar to the Roberts Basin east
of Monrovia (see Figure 9 and Behrendt and Wotorson,
1969). The interpreted basin, causing anomaly l, was
probably caused by the same tectonic forces that caused
the formation of the Roberts Basin; their ages therefore

might be similar.

50

Anomaly 2

 

Anomaly 2 with its location shown on Figure 17 is
seen on the Bouguer anomaly map (Figure 9), on the residual
Bouguer anomaly map (Figure 10), but not on the radio-
metric map (Figure 8A). On both the Bouguer and residual
Bouguer anomaly maps, anomaly 2 appears as a positive
feature. On the residual Bouguer anomaly map the location
of anomaly 2 seems to be shifted slightly northwest of its
location on the Bouguer anomaly map. This slight difference
in position is probably due to the effect of the steep
gravity gradient that parallels the coast in the Monrovia
quadrangle.

The aeromagnetic map (Figure 6A) shows two low
amplitude northwest trending negative features in the
approximate location of anomaly 2. The area adjacent to
and between these two features is magnetically smooth just
like the background in the adjacent areas.

The residual total magnetic intensity map (Figure
7) shows two northwest trending troughs in the location
of anomaly 2. The southern most trough is in the exact
location of anomaly 2 and is probably related to this
gravity anomaly. From the aeromagnetic and gravity data,
it would appear that the sources causing this anomaly on
the respective maps might be different. On the aero-
magnetic map, the source for the low amplitude magnetic

anomalies, location 2, are interpreted as being shallow

51

and probably the result of shallow diabase intrusions. The
above mentioned troughs on the residual magnetic map are
probably the results of the total effect of these diabase
bodies.

On the gravity maps anomaly 2 is interpreted as
being caused by a much deeper feature in the crust. The
feature could be an upfaulted block of a denser rock type

than those rocks adjacent to it.

Anomalyy3

 

Anomaly 3 (Figure 17) appears on the Bouguer
anomaly map (Figure 9, back folder) as a broading of the
contour lines. Anomaly 3 is also visible on profile AA'
(Figure 13) and on the residual Bouguer anomaly map
(Figure 9), where it appears as a rather well defined 5
milligal closure. On the aeromagnetic map (Figure 6A)
there is a cluster of narrow magnetic highs and lows in
the general area of the anomaly. The narrowness of these
features might indicate that their sources are near the
surface.

On the southwest edge of anomaly 3, the radio-
metric map shows a pattern of moderate width and amplitude
radiometric anomalies. From a background radiometric count
of 125 c.p.s., just outside the area, this portion of
anomaly 3 attains radiometric readings of 250 and 375
c.p.s. Geologic information from the area of anomaly 3

shows that the rock type is leucocratic gneiss containing

52

rather large areas of amphibolites. This is not the case
immediately outside the area covered by anomaly 3 where
the rock type is generally granite gneiss.

Anomaly 3 is interpreted as being the result of a
density increase of the rocks in the area caused by the
increased density of amphibolities in the lencocratic
gneisses. This model would give the observable gravity
pattern and because of a possible increase in the ferro-
magnesium minerals, due to the amphibolites, it could also
explain the aeromagnetic pattern that is observed. The
relative increase in potassium which might be associated
with the leucocratic gneisses could explain the radio-

metric pattern over anomaly 3.

Anomaly 4

 

Anomaly 4 (Figure 17) is most prominently displayed
on the aeromagnetic map (Figure 6A) of the Monrovia
Quadrangle. On the aeromagnetic map it is marked "Anomaly
A" (Behrendt and Wotorson, 1971a), and occurs as a north—
east trending series of reasonably narrow, low amplitude,
trough like features. The pattern for anomaly 4 is
stronger near the coast than farther inland. The magnetic
pattern disappears at the Goe Range but reappears, at a
much lower amplitude further north near the Gibi Mountain
where it continues on into the Bong Range (Figure 18).

On the Bouguer and residual Bouguer anomaly maps

(Figure 9 and Figure 10 respectively), there are

53

distortions in the contour lines along the lower portion
of anomaly 4. The distortions on the residual Bouguer
anomaly map tend to curve toward the seaward extension of
the anomaly. Along anomaly 4 diabase sills were mapped
but these sills are not different from other diabase
intrusions in the area (White, 1969). Behrendt and
Wotorson (1971a), felt that because of the seizable mag—
netic amplitude along this anomaly, these diabase bodies
are not simple sills but may have considerable vertical
extent.

Anomaly 4 is interpreted in agreement with Behrendt
and Wotorson (1971a) as being a fault in the crust that has
been accompanied by the emplacement of diabasic material.
There appears to be two possible extensions of the trend
of anomaly 4. The first is that anomaly 4 continues at a
lower amplitude, past the Goe Range and up into the Bong
Range (Figure 18). The second possible extension is that
anomaly 4 is the continuation of the St. John's fault,
along the St. John's river (Figure 18) and has been dis-

placed to its present position along the Goe-Fantro Ranges.

Anomaly 5

 

Anomaly 5 (Figure 17) has a 35 milligal gravity
anomaly associated with it and is observable both on the
Bouguer and residual Bouguer anomaly maps of the Monrovia
Quadrangle. Gravity profile CC' (Figure 19) shows a rapid

increase in gravity over the noritic body forming Cape

54

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55

Mount. There are no corresponding radiometric anomalies
associated with anomaly 5, although on the radiometric
map (Figure 8B) there are high radiometric counts in the
vicinity of Cape Mount. This small radiometric anomaly is
attributed to the black sands which contain monazite and
zircon. These sands are not related to the norite body.
Anomaly 5 is interpreted in agreement with Behrendt
and Wotorson (1971c) as being caused by a norite plug.
These authors calculated that for a density contrast of
0.3 to 0.1 gm/cc, the thickness of the body would range
from 1.6 km to 4.8 km respectively. Although the closest
exposed bedrock to the norite body is found to be granulite
it is difficult to explain the positive gravity anomaly
since the granulites are higher in density than the norite.
More probably, the country rocks surrounding the norite
body is granite gneiss, which is of lower density than
norite and thus the positive gravity anomaly would be
consistent with the model. However, since granite gneiss
has a lower susceptibility than norite it is difficult to
explain the low magnetic anomaly except by assuming that

the norite body has reversed polarization.

Anomaly 6

 

Anomaly 6 (Figure 17) can best be seen on the
radiometric map (Figure 8A,B) of the Monrovia Quadrangle.
There are no gravity data available for this area therefore

anomaly 6 does not appear on either the Bouguer or Residual

56

Bouguer anomaly maps. Off the coast of Liberia, and on
line with the east—west section of anomaly 6, is a fracture
zone which has only been observed off shore in the basement
(Behrendt, personal communication, 1973). Anomaly 6 might
be the landward continuation of this fracture zone. The
increase in radiometric values along this zone could well
be due to mineralization along the landward extension of
the fracture zone. The recent geologic map of the

Monrovia Quadrangle by Thorman (1972), does not however
substantiate this interpretation. However, since the
overall area shows intense weathering, the fracture could
well be missed by surface geologic mapping.

The aeromagnetic data in the area of anomaly 6 is
fairly linear and show several low trough like features
that correlate with the location of anomaly 6. This fact
might be used in the argument for the fracture zone.
interpretation since it is possible that the fracture zone
has been intruded by diabase. Samples collected in this
area, were found to be laterite and paragneisses containing
0.2 and 2 percent monazite and zircon respectively
(Behrendt and Wotorson, 1972). These authors felt that the
monazite and zircon were the cause of the high radiometric
anomaly. Thorman (1972), shows that the area contains
light colored, medium grained granite gneiss. This area
is probably more structurally complex than has been
assumed and only detailed geologic and geophysical mapping

will lead to a better interpretation of this anomaly.

3.

CHAPTER V

RECOMMENDATIONS, SUMMARY AND CONCLUSION

Recommendations

 

Seismic data that have recently been collected off
shore should be incorporated with the offshore
aeromagnetic and gravity data so that sound in-

terpretations can be made concerning the offshore

'structures. Maybe then by understanding these

structures, the true origin of many features that

occur on the geological maps will be found.

Some of the inferred "faults" that occurred con-
stantly in Behrendt's and Wotorson's reports should
be checked by geological ground mapping as their
confirmation might help explain the tectonic
history of Liberia. These faults could also have
economical importance since some mineralization has

occurred along fault zones.

A study of the magnitude and direction of remanent
magnetization of the rocks in Liberia be made as

this would help to explain many of the magnetic

57

58

anomalies. An example of this is the Cape Mount

anomaly.

Summary and Conclusions
The anomalies found in the coastal area of the
Monrovia Quadrangle are caused mainly by faulting,
accompanied at times by the intrusion of diabase
dikes. Where basins have resulted from the
faulting, sediments have accumulated to substantial

thicknesses.

Anomalies found inland in the Monrovia quadrangle
are caused mainly by density contrasts of large

rock bodies.

The east-west radiometric anomaly at the upper
margin of the quadrangle is probably caused by

mineralization along a fracture zone.

In general, the magnetic susceptibility and density
values of the samples did not correlate with the

magnetic and gravity patterns of the quadrangle.

The broad magnetic low and high gravity anomaly at
Cape Mount, is probably the result of the intrusion
of a norite plug into granite gneiss. The mag-
netic pattern would indicate that the intrusion of
the norite took place during a period of magnetic

reversal of the present earth's magnetic field.

59

The area of Liberia is isostatically under-
compensated for at depth, as is seen by the mean
free air value of + 22 milligals for the entire
country. Based on Woollard's curve (Woollard,
1957), a crustal thickness of about 32 km. was

calculated for the region of Liberia.

A dominant northeastern and northwestern trend
pattern is found on the magnetic maps and to a
lesser degree on the radiometric maps. This
pattern is probably tied to the tectonic history

of Liberia.

REFERENCES CITED

REFERENCES CITED

Behrendt, J. C., and Wotorson, C. S. 1972. Tectonic Map
of Liberia Based on Geophysical and Geological
Surveys. Open File Report, U.S.G.S.

, and Wotorson, C. S. 1969. Aeromagnetic and
Gravity Investigations of the Sedimentary Basins
on the Continental Shelf and Coastal Area of
Liberia. West Africa U.S.G.S. Open File Report,
L.G.S. Memo. Report 50.

, and Wotorson, C. S. 1970. Aeromagmetic and
Gravity Investigation of the Coastal Area and
Continental Shelf of Liberia, West Africa, and
Their Relation to Continental Drift. G.S.A.
Bu11., v. 81, pp. 3563-3574.

, and Wotorson, C. S. 1971. Geophysics, v. 36,
pp. 590-604.

, and Wotorson, C. S. 1971a. Aeromagnetic Map of
the Monrovia Quadrangle, Liberia. U.S.G.S. Open
File Report.

, and Wotorson, C. S. 1971b. Total Count Gamma
Radiation Map of the Monrovia Quadrangle, Liberia.
U.S.G.S. Open File Report.

, and Wotorson, C. S. 1971c. Bouguer Anomaly Map
of the Monrovia Quadrangle, Liberia. U.S.G.S.
Open File Report.

, and Wotorson, C. A. 197ld. Aeromagnetic Map of
the Gbanka Quadrangle, Liberia. U.S.G.S. Open
File Report.

Blade, L. V. 1970. Geology of the Bushrod Island-New

Georgia Clay Deposit Near Monrovia. U.S.G.S. Open
File Report, L.G.S. Memo. Report 34.

60

61

Brod, R. J. 1972. Geophysics, v. 37, pp. 701—73.

Dobrin, M. S. 1960. Introduction of Geophysical Pros-
pecting. McGraw-Hill Book Co., Inc. New York.

Grant, F. S., and West, G. F. 1965. Interpretation
Theory in Applied Geophysics. McGraw-Hill Book
Co., Inc. New York.

Hurley, P. M., et a1. 1971. Liberia Age Province and
Adjacent Provinces in Liberia and Sierra Leone.
G.S.A. Bull., v. 82, pp. 3483-3490.

Jakosky, J. J. 1957. Exploration Geophysics. Trija
Publishing Co. California.

Nettleton, L. L. 1940. Geophysical Prospecting for Oil.
McGraw-Hill Book Co., Inc. New York.

Sanderson, D. 1972. Ph.D. Thesis. Michigan State Uni-
versity.

Strangway, D. W. 1967. Mining Geophysics. v. 11 S.E.G.

Thorman, C. H. 1972. Geology of the Monrovia Quadrangle.
In press, U.S.G.S.

White, R. W. 1969. Sedimentary Rocks of the Coast of
Liberia. U.S.G.S. Open File Report, L.G.S. Memo.
Report 39.

. 1970. Reconnaissance Mapping of Weathered
Crystalline Rocks in Liberia. Geological Mining
and Matel. Soc. of Liberia. Bull., v. 4.

, and Leo, G. W. 1969. Geological Reconnaissance
in Western Liberia. L.G.S. Special Paper.

Woollard, G. P. 1959. Journal of Geophysical Research,
v. 64, number 10.

APPENDIX

APPEN DI X

 

 

 

Sample Susceptibility Opaque
# Rock Name Density x 10‘6 %
1 TN95A Granite Gneiss 2.62 4
2 TN95B Granite Gneiss 2.75 1028 .2
3 TN36C Granite Gneiss 2.64 952 .75
4 NRF4 Amphibolite 2.77 2870 4.06
5 NRF3 Amphibolite 2.80 4210
6 NRFl Diabase 2.93 3.55
7 NRG2 Diabase 2.92 3023 3.8
8 NRB4 Diabase 3.04 5793 3.84
9 NRAl Granulite 3.03
10 NRA3 Diabase 3.04 2188
ll NRA4 Gneiss 2.84 3.0
12 TN77G Granite Gneiss 2.49
13 TN75-G Granite Gneiss 2.60 Tr.
14 TN68B Granite Gneiss 2.63 614
15 TNlOOC Granite 2.63 20
16 TNlOOB Granite 2.62
17 NRBl Gabbro 3.07 5099 8.8
18 NRB2 Diabase 3.42* 1.9
19 NRB3 Diabase 3.10* 3877 5.3
20 TN92C11 Granite Gneiss 2.59
21 NRD4 Granite Gneiss 2.58 117 4.3
22 NRD3 Diabase 3.11 4352 6.0
23 NRD2 Hornblendite 3.08*
24 NRC4 Float Ore (Fe) 3.68*
25 NRC4 Iron Ore 4.94
26 NRC4 Iron Ore 5.08
27 TN9rB-3 Granite Gneiss 2.70 528 .5
28 TN94B-1 Chlorite Schist 2.86 Tr.
29 TN94B—2 Chlorite Schist 2.86 Tr.
30 NRCZ Gneiss 2.62 .2c
31 NRCl Gneiss 2.03* .44
32 NRH3 Granite Gneiss 3.52* 1838 Tr.
33 NRIl Granulite 3.20 190 1.84
34 TN92B-3 Granite Gneiss 2.58

62

63

 

 

Map Sample Susceptibility Opaque
# # Rock Name Density x 10"6 %

35 TN91A-3 Chlorite Schist 2.96

36 TN90E-3 Granite Gneiss 2.61

37 TN90E-5 Granite Gneiss 2.72*

38 NRIZ Granulite 3.04 6.4
39 NRI3 Granulite 2.99 147 .35
40 NRI4 Granulite 3.00 193 .66
41 NRIS Diabase 3.05 4104 9.4
42 NRJ3 Gabbro 3.05 5546 8.68
43 NRJ4 Gneiss 2.86 .2c
44 NRJ6 Granulite 3.01

45 NRLl Weathered Quartzite 2.95 756 6.58
46 W1016R Granulite 2.96*

47 WlOlOR Granulite 3.03

48 NRKl Granulite 3.10 126 Tr.
49 NRK2 Gneiss 2.81 146 Tr.
50 NRK3 Granulite 2.97 1678 1.4
51 NRK4 Granulite 3.23 226 3.3
52 NRKS Granulite 3.13

53 TN101A2 Norite 3.00 1728 2.5
54 NRBS Granite Gneiss 2.97* 985 4.4
55 NRF4 Diabase 7.54
56 NRHl Diabase 3.55
57 NRDl Diabase 2.90

 

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MICHIGAN STATE UNIVERSITY LIBRARIES

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TOTAL-COUNT GAMMA RADIATION MAP OF THE MONROVIA
By
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PREPARED BY THE U. S. GEOLOGICAL SURVEY
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