Geology of the Buchanan Quadrangle, 

Liberia 

By RUSSELL G. TYSDAL 

GEOLOGICAL SURVEY BULLETIN 1449 

Prepared in cooperation with the Liberian 

Geological Survey under the sponsorship 

of the Agency for International 

Development~ U.S. Department of State 

This report describes the geology shown. on U.S. 

Geological Survey Miscellaneous Investigations 

Map 1-778-D 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON 1978 
UNITED STATES DEPARTMENT OF THE INTERIOR 

CECIL D. ANDRUS, Secretary 

GEOLOGICAL SURVEY 

W. A. Radlinski, Acting Director 

Library of Congress Cataloging in Publication Data 

Tysdal, Russell G 

Geology of the Buchanan quadrangle, Liberia. 

(Geological Survey bulletin; 1449) 

Bibliography: p. 

Supt. of Docs. no.: I 19.3:1449 

1. Geology-Liberia-Buchanan region. I. Liberia. Geological Survey. 

II. Title. III. Series: United States. Geological Survey. Bulletin; 1449. 

QE75.B9 no. 1449 [QE339.L5] 557.3'08s [556.66'2] 77-608145 

For sale by Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402 

Stock Number 024-001-03064-1 
CONTENTS 

Page 

~bstract --------~-------------------------------------------------- 1 

Introduction -------------------------------------------------------- 1 

Location ------------------~------------------------------------ 2 

Topography ---------------------------------------------------- 2 

Climate -------------------------------------------------------- 2 

~ccess --------------------------------------------------------- 2 

Procedure ------------------------------------------------------ 4 

Previous vvork -------------------------------------------------- 4 ~cknovvledgments ---------------"""'------------------------------ 5 

Geology ------------------------------------------------------------ 5 

~etamorphic rocks --------------------------------------------- 6 

Quartz diorite gneiss unit 2 -------------------------------- 7 

Leucocratic gneiss ----------------------------------------- 7 

~elanocratic gneiss ---------------------------------------- 10 

Composite gneiss unit 2 ----------------------------------- 11 

~igmatite ------------------------------------------------- 12 

Schist ------------------------------------------------------ 12 

Quartzite -------------------------------------------------- 13 ~mphibolite ------------------------------------------------ 13 

Iron-formation --------------------------------------------- 14 

Composite unit z ------------------------------------------- 15 

Igneous rocks --------------------------------------------------- 16 

Grantie ---------------------------------------------------- 16 

Diorite ----------------------------------------------------- 17 

Quartz diorite ---------------------------------------------- 17 

Charnockite ------------------------------------------------ 18 

Diabase ---------------------------------------------------- 18 

Sedimentary rocks --------------------------------------------- 19 

Farmington River Formation ------------------------------ 19 

Edina Sandstone ------------------------------------------- 20 

Beach deposits --------------------------------------------- 22 

Lagoonal deposits ------------------------------------------- 23 

~etamorphism ----------------------------------------------------- 23 

Structure ---------------------------------------------------------- 24 

Foliation and bedding -----------------------+-------------------- 24 

Folds ---------------------------------------------------------- 24 

Faults --------------------------------------------------------- 25 

Northeast-trending faults ---------------------------------- 26 

Northvvest-trending faults ---------------------------------- 26 

Ring structure ------------------------------------------------- 28 

~ge provinces ----------------------------------------------------- 28 

~ineral resources -------------------------------------------------- 29 

References cited ---------------------------------------------------- 30 

III 
IV CONTENTS 

ILLUSTRATIONS 

Page 

FIGURE 1. Index map of Liberia showing the area of Buchanan quadrangle and location of principal towns and rivers ------ 3 

2. Generalized geologic map of the Buchanan quadrangle ____ 8 

3. Structure- and age-province map of Buchanan quadrangle 21 
GEOLOGY OF THE BUCHANAN QUADRANGLE, 

LIBERIA 

By RUSSELL G. TYSDAL 

ABSTRACT 

Crystalline rocks of Precambrian age underlie the Buchanan quadrangle 

and are capped by Cretaceous and younger sedimentary rocks in the coastal 

area. The crystalline rocks, largely of metamorphic origin, include mainly 

gneisses of quartz diorite to granodiorite composition. Less common rock 

types are amphibolite, schist, quartzite, charnockite, and iron-formation. 

Most of the metamorphic rocks are of the amphibolite facies, but some near 

the coast belong to the granulite facies. Igneous rocks are predominantly 

diorite of the Cestos and Sehnkwehn batholiths. Two belts of Jurassic 

diabase dikes form prominent ridges that trend northwest across the quadrangle. Sedimentary rocks are mainly quartz sandstones, unconsolidated 

beach sands, and lagoonal deposits. 

The structure is dominated by northeast- and northwest-trending features. 

The northwest-trending Cestos shear zone is marked by a wide zone of 

mylonite that separates contrasting rock types. It is offset by northwesttrending faults that transect the quadrangle and are, at least locally, marked 

by narrow zones of shearing. The broad northwest-trending Todi shear 

zone near the coast separates granulite-facies metamorphic rock from 

amphibolite-facies rock to the northeast. Prominent folds in the quadrangle 

are broad and open; their wavelengths are measured in kilometers. 

INTRODUCTION 

A cooperative program of reconnaissance study of the geology 

of Liberia was undertaken jointly by the Liberian Geological 

Survey (LGS) and the U.S. Geological Survey (USGS) between 

June 1965 and June 1972. The project was sponsored by the 

Ministry of Lands and Mines of the Republic of Liberia and the 

Agency for International Development, U.S. Department of State. 

Systematic quadrangle mapping of Liberia began in late 1970 

when adequate base maps, prepared as part of the project, became available. The geologic map of the Buchanan quadrangle 

(Tysdal, 1977a) was compiled from data acquired by field map1 
2 GEOLOGY OF THE BUCHANAN QUADRANGLE, LIBERIA 

ping largely done during January-April 1971; from photointerpretation; from data supplied by colleagues in the cooperative program; from data supplied by private company geologists; and 

from interpretation of aeromagnetic, aeroradiometric, and gravity 

data obtained from geophysical surveys flown by Lockwood, 

Kessler, and Bartlett Co. under contract to the Government of 

Liberia. This bulletin, in conjunction with the bulletin on the 

adjoining Juazohn quadrangle (Tysdal, 1978), describes the 

geology of the structurally and metamorphically critical area that 

spans the boundaries of the Liberian, Eburnean, and Pan-African 

age provinces. 

LOCATION 

The Buchanan quadrangle is in the south-central part of Liberia, 

West Africa (fig. 1); its borders are at lat 5° and 6° N. and long 

9°00' and 10°15' W. It is about 7,900 km 2 (about 3,100 mi 2 ) in 

land area and makes up about 8 percent of Liberia. 

TOPOGRAHY 

The southeastern half of the quadrangle is hilly, and local relief 

is about 25 m (82 ft) or less; the land surface slopes gently toward the ocean. The northwestern half of the quadrangle is more 

rugged, consisting of ridges and hills ranging from about 300 to 

500 m (980 to 1,640 ft) above adjacent lowlands. Details, of topography and culture are shown on Miscellaneous Investigations 

Map I-778-A. (U.S. Geological Survey-Liberian Geological Survey, 

1973). 

CLIMATE 

The area receives about 375 em (150 in) of rainfall each year, 

generally from April through October or early November, and 

vegetation is thus a dense rain forest. Streams and rivers are in 

flood stage during the rainy period, and the laterite-surfaced 

roads are at times nearly impassable. The temperature is commonly about 80° to 85°F, though it may be in the upper 70's at 

night and during part ·of the rainy season. The field season is 

limited to the November-through-March dry period. 

ACCESS 

Access by motor vehicle, at the time work was done, was generally limited to laterite-surfaced roads at the eastern and western 

margins of the quadrangle. In the easternmost part, the main 

road extends north from Greenville for about 30 km (about 18 

mi) in the quadrangle, and many short logging roads branch from 
0 

I 

0 

25 50 

I 

25 

I 

INTRODUCTION 

75 100 125 150 KILOMETERS 

I 

50 

II I I 

75 100 MILES 

\ 

'> 

\ 

s 

f 

/ 

IVORY 

COAST 

L "\... CaVnf; 

~ ~,; 

8 

Zwedruo ~ 

Babu

0 

'~, 0 / Dube 

Pyne Town 

Juazohn 

40~------L----------~-------------L----------~-------~ 

FIGURE 1.-Index map of Liberia showing the area of Buchanan quadrangle 

and location of principal towns and rivers. 

it. In the western part of the quadrangle near Buchanan are 

several dirt roads and one paved road; the Liberian-AmericanSwedish Minerals Co. (LAMCO) railroad crosses the quadrangle 

northeastward from Buchanan. 

Rock outcrops are sporadic along the roads because tropical 

weathering has produced thick laterite and saprolite. Many observations were made of saprolite in roadcuts, from which one 

can generally gain structural data and at least a general idea of 

rock composition and texture. Certain rock types, such as quart-
4 GEOLOGY OF THE BUCHANAN QUADRANGLE, LIBERIA 

zite, schist, amphibolite, diabase, iron-formation, and pegmatite, 

can be identified from saprolites quite readily (White, 1971). 

Some areas of the quadrangle were traversed along footpaths, 

particularly by geologists of the Muller Co. and LAMCO, but 

there, also, outcrops are only sporadic. Thus a major program 

was undertaken by the USGS-LGS group to traverse the larger 

rivers where rocks are best exposed and least weathered. Traverses were made by rubber boats along parts of the St. John, 

Timbo, Cestos, and Sehnkwehn Rivers. 

A helicopter was used along segments of some rivers and along 

the coast. Several airstrips in the quadrangle were used in gaining 

access to the interior. However, parts of the quadrangle are uninhabited -and are thus not readily accessible. 

PROCEDURE 

Complete aeromagnetic and aeroradiometric coverage exists for 

Liberia. Extensive use was made of the total-intensity magnetic 

map of the Buchanan quadrangle (Behrendt and Wotorson, 

1974a) to compile figure 2. The radiometric map (Behrendt and 

Wotorson, 1974b) was not nearly as useful. Aerial photographs 

(scale 1 :40,000), taken from 1967 to 1969, are available for most 

of the quadrangle and were used extensively to outline structural 

features. 

Observations plotted in the field on base maps or photographs 

at a scale of 1 :40,000 were transferred in the office to a base map 

at a scale of 1:125,000. All data acquired from photointerpretation, geophysical interpretation, and previous work were also 

plotted on this map. The final geologic map was compiled at the 

1:125,000 scale from these data and was reduced to the 1:250,000 

scale for publication (Tysdal 1977a). 

PREVIOUS WORK 

The Muller Co. made geologic traverses throughout the area 

east of the Cestos River as part of a large mineral exploration 

program encompassing the eastern part of Liberia. They recorded 

many observations from wh1ch 14 unpublished reports and many 

maps were compiled. This work is partly summarized in a report 

by Griethuysen ( 1971). 

The Diamond Mining Corp. of Liberia (DMCL) made reconnaissance studies of heavy minerals of the area north of the 

Sehnkwehn River and east of the Cestos River; this area is 

drained mainly by tributaries of the Sehnkwehn River. Results 
GEOLOGY 5 

of the DMCL heavy-mineral studies are documented only partly.1 2 

In the western part of the quadrangle, LAMCO geologists made 

a reconnaissance study of the rocks in a wide expanse of territory 

along the railroad extending northeastward from Buchanan. The 

work is described by Offerberg and Tremaine (1961). 

SherrrLan ( 1947) made the first compilations and descriptions 

of mining properties in eastern Liberia and noted several localities in the Buchanan quadrangle. Preliminary interpretations of 

the aeromagnetic and aeroradiometric maps of the Buchanan 

quadrangle were made by Behrendt and W otorson ( 197 4a, b) . 

ACKNOWLEDGMENTS 

Many people have contributed to the compilation of the geologic ma.p of the Buchanan quadrangle. Traverses by U.S. Geological Survey personnel, in addition to those of the author, were 

made by George 0. Bachman, Lawrence V. Blade, Robert L. Earhart, Eric R. Force, Philip T. Hayes, Thomas D. Hessin, Sam 

Rosenblum, George C. Simmons, Charles T. Thorman, and Richard 

W. White; Roberts M. Wallace made a preliminary interpretation 

of a few of the Buchanan aerial photographs. Fieldwork by personnel of the Liberian Geological Survey includes that of John 

Carr, David Cooper, and Abraham Cassell, field assistants. 

The hospitality of LAM CO, Tidewater Plantation, West African 

Corp., the Open Bible Standard Mission and National Christian 

Assemblies Missions, and V ANPL Y Logging Co. greatly facilitated logistics and fieldwork. Local officials and townspeople were 

uniformly courteous and helpful in all phases of fieldwork. 

GEOLOGY 

The Buchanan quadrangle forms part of the Guinean shield 

of West Africa and is made up largely of Precambrian igneous 

and metamorphic rocks (fig. 2). Exceptions include mainly diabase 

dikes of Jurassic age, Cretaceous and younger sedimentary rocks 

along the coast, laterite, canga, and young unconsolidated deposits. lVIost of the rocks are gneiss of quartz diorite composition, 

and lesser granodiorite gneiss, schist, amphibolite, iron-formation, 

and dioritic to granitic bodies. Minor amounts of other kinds of 

rocks are also present. 

1 Leuria, Basil, 1966, Quarterly report, December 1, 1965-February 28, 1966 : 

Diamond 1\Iining Corp. of Liberia, unpub. rept., 9 p., 'On file with the Liberian Geological 

Survey, Monrovia, Liberia. .. 2 Leuria, Basil, 1966, Quarterly report, March 1-1\Iay 31, 1966 : Diamond Mining 

Corp. of Liberia, unpub. rept., 6 p., on file with the Liberian Geological Survey, 

Monrovia, Liberia. 
6 GEOLOGY OF THE BUCHANAN QUADRANGLE, LIBERIA 

The classification of igneous and metamorphic rocks is, as far 

as possible, based on general igneous composition as follows: 

Igneous rock Metamorphic rock 

Granitic, undivided Granitic gneiss _______ _ 

Granite ----------------- Granite gneiss _____ _ 

Granodiorite ----------- Grandiorite gneiss __ 

Dioritic, undivided ------- Diorite gneiss _________ _ 

Quartz diorite _________ Quartz diorite gneiss _ 

Diorite ----------------- Diorite gneiss ______ _ 

Percent Percent 

potassium feldspar quartz 

of total feldspar in rock 

>10 

>50 

>10<50 

<10 

<10 

<10 

>10 

>10 

>10 

>10 

<10 

However, an igneous origin is not necessarily implied for the 

metamorphic rocks classified according to this scheme. 

Rock samples were collected from about a third of the outcrops 

observed by the USGS-LGS group, and many of the samples were 

slabbed, etched with hydrofluoric acid, and then stained with 

sodium cobaltinitrite for detection of potassium feldspar. Thus 

the percentage of potassium feldspar was determined fairly accurately. Systematic detailed thin-section studies were not made, 

although thin sections from some samples were examined. 

Rocks that did not fit the above scheme are noted by specific 

compositional names (such as manganese formation) or, where little information is available, by a general name (such as leucocratic 

gneiss). Composite units are designated for areas containing 

combinations of specific rock types that cannot be separated 

appropriately at map scale by use of the available data. The terms 

"leucocratic" and "melanocratic" are used to designate varied 

associations of rocks that are predominantly light colored or 

dark colored, or to designate rock units for which little information other than color index is available. 

Because data came from many sources and were collected by 

many geologists among whom geologic concepts and technical 

terminology varied widely, systematic classification according to 

the above scheme has presented difficulties. Particularly difficult 

was the reclassification of rocks that were named wholly on a 

textural basis. 

METAMORPIDC ROCKS 

By far the bulk of the rocks in the Buchanan quadrangle are 

metamorphic (Tysdal, 1977a). The chronologie order of the 

rock units is unknown, and the units are discussed in this section 

according to rock type. 
GEOLOGY 7 

QUARTZ DIORITE GNEISS UNIT 2 

A unit mapped as quartz diorite gneiss unit 2 is present in the 

eastern part of the quadrangle, east of the Cestos and Todi 

shear zones. The unit underlies an area of some of the lowest 

relief in the quadrangle and has very few outcrops. 3 4 It is mainly 

melanocratic medium- to coarse-grained rock composed of hornblende (20-35 percent), plagioclase (45-70 percent), quartz (8-

20 percent), and locally garnet (0-15 percent). Along the coast, 

biotite-quartz diorite gneiss and granodiorite gneiss are common 

and are locally migmatitic. Thin tabular amphibolite bodies are 

interlayered with the gneiss in the coastal area. Quartzite beds 

were recorded northwest of New Town. MUller Co.4 and Leuria 

(see footnote 1 on p. 5) noted kyanite in stream-sediment 

samples from near Kase Town. 

LEUCOCRA TIC GNEISS 

Leucocratic gneiss is separated into three units in the Buchanan 

quadrangle, mainly on the basis of the different components in 

each unit and on the density of observations for each unit. Each 

is described below. 

Leucocratic gneiss unit, undivided.-Leucocratic gneiss northeast of the Todi shear zone (fig. 2) and northwest of the Cestos 

shear zone, as observed in marginal areas, is composed largely of 

gneisses of quartz diorite composition and lesser granodiorite 

gneiss. The gneisses are medium-to light-colored, medium- and 

coarse-grained rocks containing biotite (5-10 percent) and, locally, 

hornblende. Amphibolite, quartzite, itabirite, schist, and rocks 

of composite unit z (described below) have been mapped separately 

within the unit, and they are probably interlayered in this unit 

elsewhere in the quadrangle. 

The central part of the unit, most of which is inaccessible, 

forms steep ridges that outline broad folds and is characterized 

by many narrow negative magnetic anomalies. The topographic 

pattern is suggestive of quartz-rich rocks, and the magnetic 

anomalies are suggestive of itabirite of low iron content, and 

lesser amphibolite. Aerial photographs show that a foliation is 

present in much of the terrane, and the aeroradiometric proper3 Miiller Company, 1970, Exploration Liberia, 9th progre~>~R report, September-December 

1969: The Hague, Netherlands, Miiller Co., unpub. rept., 12 p., on file with the Liberian 

Geological Survey, Monrovia, Liberia. 4 Miiller Company, 1970, Exploration Liberia, lOth progress report, January-March 

1970 : The Hague, Netherlands, Miiller Co., unpub. rept., 10 p., on file with the Liberian 

Geological Survey, Monrovia, Liberia. 
45' 

30' 

10"00' 

~.?'-

< 

~~ .?--

/0 

45' 30' 

0 

0~ 

~ 15'1- ~ 

0 5 10 15 20 25 30 35 40 KILOMETERS I I I I I I I I I • 

0 S 1 b 115 2b 215 MILES 

15' 

5°00' '-''"'"'UVU,,_ 

Planimetric base generalized from U.S. Geological Survey 

Buchanan Geographic map 1-778-A 

9"00' (X) 

C) 

l1j 

0 

t"4 

0 

~ 

0 

~ 

t-3 

::= 

l1j 

td 

c:: 

0 

= 

~ > z 

£> 

c:: 

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> z 

~ .. l:r:l 

~ td 

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EXPLANATION 

CORRELATION OF MAP UNITS 

TERTIARY 

CRETACEOUS 

JURASSIC 

Stratigraphic succession not implied 

for the following units 

r----.---.----, "" Plutonic 

igneous 

rocks 

Metamorphic 

rocks 

PRECAMBRIAN 

PREPt".RED BY THE U.S. GEOLOGICAL SURVEY AND THE LIBERIAN 

GEOLOGICAL SURVEY UNDER THE JOINT SPONSORSHIP OF THE 

GOVERNMENT OF LIBERIA AND THE AGENCY FOR INTERNATIONAL 

DEVELOPMENT, U.S. DEPARTMENT OF STATE 

DESCRIPTION OF MAP UNITS 

~ Edina Sandstone 

0 Farmington River Formation 

~ Farmington River Formation conglomerate 

IJd>-1 Jurassic dike 

0 Charnockite 

0 Diorite 

~ Quartz diorite 

~Granite 

Q Composite unit z 

0 Iron-formation, oxide facies (itabirite) 

0 Amphibolite 

0 Schist 

~ Composite gneiss unit 2 

~ Melanocratic gneiss 

0 Leucocratic gneiss 

~ Leucocratic gneiss unit 1 

~ Leucocratic gneiss unit 2 

I gndq21 Quartz diorite gneiss unit 2 

Contact 

Fault 

:::::. ...._::: Fault zone or shear zone 

t 

Antiform, showing trace of crestal 

plane and direction of plunge 

Synform, showing trace of trough 

plane and direction of plunge 

FIGURE 2.-Generalized geologic map of the Buchanan quadrangle, Liberia. 

~ 

l%j 

0 

t'4 

0 

~ 

t< 

co 
10 GEOLOGY OF THE BUCHANAN QUADRANGLE, LIBERIA 

ties are suggestive of low potassium content. Behrendt and Wotorson ( 1971) found that a general correlation exists between K20 

content of bedrock and total gamma radiation below 500 cps 

(counts per second). The leucocratic gneiss unit makes up part of 

one of the largest areas in Liberia of low to moderate radioactivity 

(100-250 cps) (Behrendt and Wotorson, 1974b), and thus probably 

contains relatively low concentrations of potassium. 

Leucocratic gneiss unit 1.-Leucocratic gneiss unit 1 southwest of the Todi shear zone in the western part of the quadrangle contains gneisses that are mainly of quartz diorite and 

granodiorite composition and contains many interlayered amphibolite bodies. Quartzite, schist, and minor amounts of rocks 

associated with iron-formation are also present. The unit has 

been examined more extensively than the undivided leucocratic 

gneiss and differs from it in that it (1) contains larger amphibolite bodies and may contain more amphibolite per unit area; (2) 

contains much less iron~formation and associated rocks; (3) 

does not have the abundance of negative magnetic anomalies 

thought to indicate mainly low-grade itabirite deposits; (4) is 

characterized by a foliation that trends northwest instead of 

from northeast to east to northwest; and (5) is found southwest 

of the Todi shear zone. 

Leucocratic gneiss unit 2.-Leucocratic gneiss unit 2 in the 

New Cess River area east of Buchanan is a sillimanite- and 

kyanite-bearing unit. The rock is deeply weathered and the mapped 

areal extent of the unit is based in part on the presence of these 

heavy minerals in saprolites. Sillimanite was observed locally as 

off-white fibrous needles in small knotlike mats or pods a few 

millimeters thick and as intergrown with biotite. The biotite 

gneiss also contains segregated lenticles and pods of quartz and 

feldspar. 

The northernmost and largest area of leucocratic gneiss unit 

2 is on strike with the kyanite-rich gneiss of possible economic 

value of the Mt. Montro area, in the Monrovia quadrangle about 

15 km (9.3 mi) north of Buchanan (Offerberg and Tremaine, 

1961; Stanin and Cooper, 1968; Thorman, 1977). Outcrops and 

saprolite between the two areas do not contain sillimanite or 

kyanite, however. 

MELANOCRA TIC GNEISS 

Melanocratic gneiss crops out mainly southwest of the Todi 

shear zone, although two tiny outcrops were noted along the 

coast in the Bafu Bay area and near Chom Town. This unit is 
GEOLOGY 11 

made up of a sequence of medium- to dark-colored rocks of gabbroic and locally dioritic composition ; interlayered medium- to 

light-colored gneiss largely of quartz diorite composition but 

also of granodiorite composition ; and lesser amphibolite that is 

mapped separately where practical. Quartz diorite to granodiorite 

gneiss makes up more than half the rocks in the northwestern 

part of the unit, but dark rocks predominate in the southeastern 

part. 

The rocks of gabbroic and locally dioritic composition are composed of plagioclase (An35-5o, 45-65 percent), orthopyroxene 

(5-18 percent), clinopyroxene (5-20 percent), hornblende (2-

12 percent), quartz ( <1-20 percent; commonlly about 1 percent), 

potassium feldspar (0-5 percent), and garnet (0-10 percent; 

commonly absent). They are medium- to coarse-grained rocks, a 

uniform yellowish green to dark green, and have a greasy luster. 

Locally, however, they are medium gray and spotted with dark 

clots of mafic minerals. These rocks, which are of the granulite 

facies of metamorphism, apparently form laterally discontinuous 

bodies in the northwestern part of the unit, but southeastward 

they are much more widespread and continuous. rhe foliation is 

generally faint but is more pronounced in the southeastern part 

of the unit. The origin of the granulite rocks is uncertain-they 

may be metasedimentary or metaigneous ; detailed studies of 

them were not undertaken. 

Medium- to light-colored quartz diorite to granodiorite rocks of 

the unit range from fine to coarse grained. They commonly contain a small percentage of biotite and locally contain hornblende 

or are free of mafic minerals. 

COMPOSITE GNEISS UNIT 2 

Composite gneisses are made up of rock types grouped together, because each is individually too small to show at map 

scale (Tysdal, 1977a). The exact location of only a few of the 

rock bodies is known, and the areal extent is best shown by a 

single grouping. Only one composite gneiss unit is mapped in 

the· Buchanan quadrangle. 

Composite gneiss unit 2 is in the southeasternmost part of the 

quadrangle and is a continuation of the graphite belt of the 

Juazohn quadrangle (Tysdal, 1977b). Medium- to dark-colored, 

fine- to coarse-grained biotite-graphite quartz diorite gneiss is the 

chief rock type; granodiorite gneiss is present locally. The graphite is less widely distributed and less abundant than in the 
12 GEOLOGY OF THE BUCHANAN QUADRANGLE, LIBERIA 

Juazohn quadrangle. Amphibolite, quartzite, and abundant pegmatite also characterize the unit, and sillimanite is present in the 

southeasternmost part. Fresh outcrops are sparse in the coastal 

area, except along the beach, but structural data were observed 

in saprolite in many rock cuts. The rocks are isoclinally folded, 

and the folding is reflected in the narrow, linear magnetic pattern 

of the unit. 

MIGMATITE 

The term "migmatite" is applied to rock consisting of a metamorphic (generally mafic) component that is disrupted by a less 

mafic igneous component of pegmatite, aplite, or granite. The 

term is used in areas containing outcrops in which the metamorphic country rock is fragmented and thus may appear to float in 

the younger rock. In the migmatite terminology of Mehnert 

(1968), the rock has agmatic (breccia) structure or schollen 

(raft) structure. In the Buchanan quadrangle, migmatite was 

mapped only in surf-cut outcrops along the coast in the River 

Cess area, mainly because outcrops elsewhere are too small to 

show the gross texture, or because the area of migmatite is too 

small to show at the map scale (Tysdal, 1977a). 

Migmatite in the River Cess area contains gneissic country 

rock of quartz diorite to granodiorite composition with locally 

interlayered amphibolite. The granitic material has separated 

blocks of the country rock into isolated rafts, shows complex flow 

structures, and appears to have soaked into and replaced some of 

the gneiss. In an outcrop about 1.5 km (about 1 mi) south of 

River Cess, the migmatite is cut by several generations of darkgray, fine- to medium-grained dioritic dikes. Several 0.25- to 

1-m- (10- to 40-in-) wide granitic dikes, striking about N. 40° W., 

cut all the other dikes they cross. These grayish-orange finegrained rocks are distinct because they are spotted with round 

black clots of biotite, feldspar, and quartz about 0.5 ·Cm (0.2 in) 

in diameter. 

SCHIST 

Schistose rocks noted in the Buchanan quadrangle are mainly 

biotite-quartz-feldspar schist, mostly in outcrops too small to 

show at map scale (Tysdal, 1977a). The schist band near the 

mouth of the Timbo River is about 3 km (1.9 mi) thick, but it 

is probably folded isoclinally. The rock is fine to medium grained 

and forms poor outcrops except where it becomes coarse grained 

and grades into gneiss. 
GEOLOGY 18 

Biotite-quartz-feldspar schist is also present along the Cestos 

River where much of it is sheared and mylonitic. The mylonite 

precursor may or may not have been schist, but downstream from 

Geni Mountain, schist is present southeast of the mylonite, and 

the same relationship of mylonitic and unsheared (schistose) 

rock also exists in the northwestern corner of the adjacent 

Juazohn quadrangle (Tysdal, 1978). In addition, the fault shown 

along the Cestos River marks a prominent magnetic contrast, 

suggesting a probable change from schist to gneiss. 

The staurolite schist that was noted along the Cestos River, 

southeast of Geni Mountain, is composed of quartz (80-40 percent), biotite (20-35 percent), plagioclase (15-30 percent, staurolite (3-10 percent), and garnet (1-2 percent). Tourmaline ( <1 

percent) was noted in one thin section, and chlorite (penninite) is 

a common alteration product of biotite. The staurolite is porphyroblastic, shows sieve structure, and locally shows helitic structure, 

as does garnet. In one thin section, a zoned relationship is evident; 

the garnet forming a core mineral followed successively outward 

by a thin rim of quartz and a thick rim of staurolite. 

QUARTZITE 

Quartzite, other than in rocks associated with iron-formation, is 

present in resistant isolated layers 1-2 m (3-6 ft) thick, flanked 

by saprolite or laterite. It is fine to medium grained and is poorly 

indurated in the outcrops observed; thus sedimentary structures 

were not noted. The quartzite is commonly light colored, fine to 

medium grained, and nearly pure quartz. Common accessory 

minerals are mica, magnetite, and locally in composite gneiss unit 

2, graphite. A white feldspathic quartzite was noted in one place 

along the coast south of Buchanan. 

AMPHIBOLITE 

The term amphibolite is used for rocks that are of the amphibolite facies of metamorphism and contain about equal amounts of 

plagioclase and hornblende, plus minor amounts of other minerals 

such as quartz, sphene, biotite, garnet, magnetite, and clinopyroxene. 

These rocks form layers a few centimeters thick, interbedded 

with gneiss; discontinuous tabular bodies a few meters to 

hundreds of meters thick ; and irregularly shaped bodies, some of 

which are more than 1 km (0.6 mi) across. Most of the amphibolite bodies are parallel to the foliation of the country rock, but 
14 GEOLOGY OF THE BUCHANAN QUADRANGLE, LIBERIA 

some trend at a high angle to it, as in the Tidewater Plantation 

area 20 km (12 mi) southeast of Buchanan. 

Amphibolite is well exposed at the Nekree Hill quarry north of 

Buchanan, where it includes banded to spotted gneisses. White 

and Leo (1969) found the mafic part to be mainly green hornblende and andesine (An3o-35) with some layers rich in garnet and 

scapolite. Leucocratic layers consist of andesine and as much as 30 

percent quartz. Local clinopyroxene porphyroblasts rimmed with 

hornblende, plus possible pyroxene pseudomorphs in the form of 

finely granoblastic hornblende clusters, suggest retrogression from 

granulite facies assemblages (White and Leo, 1969, p. 10). Crosscutting dikelike zones of leucocratic material probably formed by 

sweating out from the adjacent rock (anatexis). Mafic minerals 

are concentrated at the margin of the leucocratic rock and are 

even more densely packed than in the amphibolitic gneiss country 

rock. In many places, the alinement of dark minerals in the 

gneiss continues faintly through the dikelike leucocratic zone to 

gneiss on the opposite side. These leucocratic zones commonly 

fade into the gneiss by gradual increase in mafic content. 

A negative magnetic anomaly of 100-300 gammas characterizes 

most of the amphibolite bodies, but low-amplitude anomalies exist 

over two of the largest units near Buchanan. 

Amphibolite is resistant to erosion, commonly forming ridges 

and outcrops, and is more prominent on the map (Tysdal, 1977a) 

than it would be if all rock types were equally well exposed. This 

latter point is well demonstrated in the western part of the 

Buchanan quadrangle in the Tidewater Plantation, which is laid 

out in a grid pattern by roads spaced at intervals of 1 km (0.6 

mi) over an area measuring about 3 by 6 km (1.8 by 3.6 mi). 

More than about 80 percent of the relatively fresh rock exposed, 

other than diabase, is amphibolite. Saprolite exposed in roadcuts 

contains very little amphibolite; of the saprolite and the relatively 

fresh amphibolite combined, amphibolite is estimated to make 

up less than about 25 volume percent. If it can be assumed that 

much of the amphibolite present in the area is exposed, then the 

unexposed nonamphibolite rocks and saprolite probably make up 

about 80 to 95 percent of the rocks in the plantation area. 

IRON-FORMATION 

Two kinds of iron-formation are distinguished in the Buchanan 

quadrangle according to the composition of the dominant iron 

mineral-oxide or silicate. The oxide facies of iron-formation is 

discussed here ; the silicate facies is discussed below wth com-
GEOLOGY 15 

posite unit z. Carbonate and sulfide facies iron-formation were not 

found in the quadrangle. 

The oxide facies of iron-formation is mainly itabirite, a metamorphosed rock consisting of interlaminated bands of chert or 

granoblastic quartz and hematite or magnetite. Magnetite itabirites are the most common. The main occurrence of itabirite is 

in the northeasternmost part of the quadrangle in the Gwen Creek 

area, where it forms the backbone of the northeast-trending 

Kaba Mountain and the nearby northwest-trending mountain at 

the northern margin of the quadrangle. Enriched red hematite 

ore was collected atop Kaba Mountain. (Kaba means "red rock" 

in the Bas sa dialect, the main language in the area.) Magnetic 

anomalies associated with the itabirite have amplitudes as large 

as 2,000 gammas. 

Oxide facies iron-formation also crops out about 5 km (3 mi) 

east of River Cess in an isolated hill about 30m (100ft) high and 

about 1 km (0.6 mi) in diameter. This hill is the major topographic feature in the area. Only extensively weathered rock 

consisting of quartz, hematite, and minor magnetite is exposed on 

the hill. It is foliated and sheared, consisting of alternating layers 

(about 0.5 em thick) of granoblastic quartz and hematite grains. 

A magnetite anomaly of about 4,000 gammas amplitude is associated with this hill, but anomalies of much lower amplitude were 

recorded nearby (Behrendt and Wotorson, 1974a). A preliminary 

interpretation of the magnetic data, made by R. W. Bromery of 

the USGS (written commun., 1971), indicates that the magnetic 

mass is complex, and that the exposed magnetic rocks associated 

with the steep-gradient, high-amplitude anomaly may be part of 

an areally larger, buried magnetic mass that extends 3 to 5 km 

(2 to 3 mi) northwest, 1.5 to 2.5 km (0.9 to 1.6 mi) southeast, 

and about 2 km (1.25 mi) southwest. The top of the buried mass 

iR estimated to be 200 to 300 m (650 to 1,000 ft) below the 

surface at its northwestern end and 100 to 150m (325 to 490 ft) 

below the surface at the southeastern end. 

COMPOSITE UNIT Z 

Composite unit z is made up of rocks associated with itabirite; 

most of the rock bodies are individually too small to show at the 

map scale (Tysdal, 1977a). The unit is dominated by the iron silicate facies of iron-formation but also includes quartzite, amphibolite, and locally actinolite, pelitic schist, minor itabirite, and 

leucocratic gneiss. 
16 GEOLOGY OF THE BUCHANAN QUADRANGLE, LIBERIA 

In the Gwen Creek area, the silicate iron-formation includes 

grunerite-magnetite phyllite and grunerite-hornblende-garnetquartz schist. Actinolite schist, associated with quartz-muscovite 

schist, is present in the northernmost part of the area, but hornblende amphibolite is present southward along the river and near 

Kaba Mountain. 

Composite unit z, exposed along the road northeast of Buchanan, 

contains rocks made up of about equal parts quartz and garnet, in 

addition to quartz-grunerite rocks. The quartz is commonly granoblastic in platy or tabular discontinuous segregations parallel to 

foliation but locally forms crosscutting blebs. Grunerite locally 

forms porphyroblasts as much as 3 em (1.2 in) across. These 

quartz-rich rocks are generally massive but have faint layering 

in some places. Quartzite, amphibolite, and associated rocks of 

probable metasedimentary origin are also present. The leucocratic gneiss of this unit, best exposed along the St. John River, 

is medium- to coarse-grained hornblende-bearing rock of quartz 

diorite to granodiorite composition. 

IGNEOUS ROCKS 

Igneous rocks of large areal extent are present in the eastern 

part of the quadrangle, forming dioritic batholiths. Small plutons 

of charnockite, quartz diorite, and granite have been mapped in 

the western part of the quadrangle, and two zones of diabase dikes 

cross the quadrangle. 

GRANITE 

Granite was mapped in two areas of the Buchanan quadrangle, 

near River Cess and along the Timbo River (Tysdal, 1977 a). The 

granite body at the mouth of the Cestos River near River Cess is 

massive light-gray to pink, medium- to coarse-grained hypidiomorphic rock consisting of orthoclase (60-70 percent), plagioclase 

(15-25 percent), quartz (15-20 percent), and biotite (2-4 percent). Near the granite margins, biotite is faintly alined and defines a vague foliation. Contacts with adjacent rocks are sharp at 

the two places where observed, and the pluton interrupts the 

general trend of foliation of the country rock. The granite 

weathers spheroidally, producing rounded boulders as much as 

3 m (10 ft) in diameter. The granite pluton along the Timbo 

River, at about lat 5°45' N. and long 9°45' W., is only about 

200 m (650 ft) across. It consists of light-gray megacrysts of 

orthoclase as much as 2 em (1 in) across in a fine-grained 

groundmass of plagioclase, quartz, biotite, and minor hornblende. 
GEOLOGY 17 

The granite is marked by a positive radiometric anomaly and 

appears to interrupt the trend of the surrounding gneiss. 

DIORITE 

Hornblende diorite makes up the Sehnkwehn and Cestos batholiths (Tysdal, 1977a, b) in the eastern part of the Buchanan 

quadrangle and is a gray-green, medium- to coarse-grained rock 

composed of hornblende (20-40 percent), plagioclase (45-65 percent), quartz (2-12 percent), and potassium feldspar (0-10 percent). The range in quartz content locally straddles the percentage limit (10 percent) separating diorite from quartz diorite, 

but the majority of rocks examined in thin section contain less 

than 8 percent quartz; hence the rocks are diorite. 

Other phases of the batholithic rocks include the large amphibolite body near Kase Town (on the Sehnkwehn River) which appears to have a gradational contact with the batholith and which 

may be a roof pendant. A few aplitic zones were noted along the 

Sehnkwehn River, but these are narrow (a few meters wide) 

linear zones apparently caused by shearing. Neither pegmatites 

nor xenoliths were noted in the batholithic rocks. Topographically 

the batholiths form a nearly featureless surface of low relief. 

Outcrops are sparse ; distances between outcrops along the rivers 

are commonly 0.5 to 1.5 km (0.3 to 1 mi) and locally as much as 

4.5 km (2.8 mi). Rounded boulder like outcrops are typical of 

these massive rocks. 

The magnetic expression is a nearly uniform flat pattern on 

which the limits of the batholiths were largely drawn (Behrendt 

and Wotorson, 1974a). Locally, as along the eastern part of the 

Sehnkwehn River, the batholithic rocks are not characterized 

by this magnetic pattern but have a pattern like that of the nearby 

country rock. 

QUARTZ DIORITE 

Two small plutons, near the mouth of the Timbo River in the 

western part of the quadrangle, are the only quartz diorite intrusions known to exist in the map area. They are composed of 

biotite (2-3 percent), quartz (20-30 percent), and plagioclase 

(65-80 percent). The rock is brown, mediun1 to coarse grained, 

forms abundant rounded outcrops, and is unfoliated except near 

shear zones. These plutons are not characterized by a distinctive 

magnetic or radiometric pattern. 
18 " GEOLOGY OF THE BUCHANAN QUADRANGLE, LIBERIA 

CHARNOCKITE 

Charnockite crops out only in the northwestern part of the 

quadrangle and is part of a larger mass present in the adjacent 

Gbanka quadrangle (Force and Dunbar, 1977). It is dark gray, 

medium- to coarse-grained rock that is commonly rich in large 

brownish-gray feldspar augens, which are locally stretched out to 

elongated lenses (Offerberg and Tremaine, 1961). Along its 

margins, however, the charnockite is fine grained and contains 

garnet (Force and Dunbar, 1977). The charnockite consists mainly of plagioclase (25-58 percent), potassium feldspar (10-48 percent), quartz (11-26 percent), hypersthene (0.3-4 percent), and 

hornblende (3-6 percent) (Sam Rosenblum, oral commun., 1973). 

Other minerals, present locally, include clinopyroxene, biotite, 

garnet, and chlorite. The rock thus ranges in composition from 

hypersthene-quartz diorite to hypersthene granite, but most outcrops are hypersthene-bearing granites; hence the name charnockite. 

Boulders of spheroidally weathered massive rock form the 

typical outcrops. The charnockite is more resistant than the surrounding country rock and is marked by a slight but distinct 

topographic high on aerial photographs. 

DIABASE 

Diabase is found in two separate belts of northwest-trending 

dikes in the Buchanan quadrangle, one near the coast and the 

other in the northeastern part of the quadrangle. It is commonly 

fine grained and ophitic and is composed of labradorite, lesser 

augite, and locally orthopyroxene and minor magnetite and ilmenite. Along the coast immediately south of the harbor ·at 

Buchanan, however, the diabase is coarse grained (gabbroic). It 

consists of about 50 percent plagioclase and 25 to 30 percent 

augite; and quartz ( <5 percent), magnetite (5 to 10 percent), 

and some hornblende that rims the augite. A similar gabbroic 

dike is present 3 to 7 km (1.9 to 4.4 mi) northwestward along the 

motor road from River Cess. 

Diabase dikes form much of the coastline between Buchanan 

and River Cess; one of the better exposures being 18 km (11 mi) 

southeast of Buchanan in a surf-cut outcrop. The diabase is fine 

grained and is largely labradorite with minor augite and magnetite. Three dioritic dikelets, measuring 1 to 5 em (0.4 to 2 in) 

thick, cut the main dike at about 90° to its long axis but do 

not cut the country rock. Thin sections show the dikelets to be 

altered, ·even though the dikelets appear fresh at the outcrop. 
GEOLOGY 19 

The dikelets probably represent a late stage of the magma that 

formed the diabase. The full width of the diabase dike is not 

exposed, but one contact of the dike with gneissic country rock 

is sharp. Xenoliths of gneiss as much as 1 m (3 ft) long are 

caught up in the diabase and are oriented with long axes nearly 

vertical. 

The dikes are mainly vertical or dip steeply south ; they range 

from a few meters to more than 50 m (164ft) in thickness. Because they are resistant to weathering, the dikes commonly form 

prominent linear ridges easily seen on aerial photographs. 

Magnetite in the diabase commonly causes a characteristic narrow linear negative anomaly on the aeromagnetic map of ·the 

quadrangle (Behrendt and Wotorson, 1974a). In areas that have 

not been traversed, but in which a characteristic long narrow 

magnetic anomaly coincides with a characteristic long narrow 

ridge on the aerial photographs, a dike is mapped (Tysdal, 1977a) 

as an observed feature. Field checks have shown this mapping 

technique to be valid. 

Preliminary K-Ar dating of the northWE!St-trending diabase 

dikes of western Liberia yielded ages ranging from Jurassic to 

Devonian (White and Leo, 1969). Subsequent work by Gromme 

and Dalrymple (1972) has shown that dikes that intrude Precambrian crystalline rocks give K-Ar ages of 193 m.y. to 1,213 

m.y. on whole-rock and mineral separates. But K-Ar ages of dikes 

intruding Mesozoic sedimentary rocks near the coast range from 

173 m.y. to 192 m.y. Study of the K-Ar data shows that large and 

differing amounts of excess Ar40 are in the dikes intruded into the 

crystalline rocks and that all the dikes are Early Jurassic. 

Gromme and Dalrymple (1972) stated that the mean paleomagnetic 

directions of 6 dikes intrusive into sedimentary rock are very 

close to those of 19 dikes intrusive into Preeambrian rocks, and 

that the poles of all 25 dikes are in agreement with other Mesozoic paleomagnetic poles from the African Continent. 

SEDIMENTARY ROCKS 

Sedimentary rocks on land are preserved only in the coastal 

area of the Buchanan quadrangle and (on land) form units that 

do not exceed a few meters in thickness. T:tte rocks are mainly 

quartzose sandstone, unconsolidated beach sands, and lagoonal 

deposits. 

FARMINGTON RIVER FORMATION 

The Farmington River Formation crops out along the coast in 

the westernmost part of the quadrangle and is dominated by 
20 GEOLOGY OF THE BUCHANAN QUADRANGLE, LIBERIA 

conglomeratic graywacke but also contains graywacke and a 

few thin shale interbeds. The conglomerate is· most abundant in 

the basal part of the formation and becomes less abundant upward and toward the sea; the decrease in conglomerate is accompanied by a decrease in grain size of the graywacke and an 

increase of shale (White, 1972). The graywacke is poorly sorted 

and consists of coarse angular grains of quartz and feldspar and 

very fine grains of chlorite, sericite, calcite, and accessory grains 

of many different heavy minerals. Clasts of the conglomerate are 

cobble size, are well rounded, and according to a count of 100 

clasts from near Long Reef Point (White, 1972), consist of granitic rock, gneiss, and "granulite" (28 percent), diabase (26 percent), amphibolite (20 percent), Paynesville Sandstone (19 percent-this unit of Paleozoic age does not crop out in the Buchanan 

quadrangle), and quartzite and vein quartz (7 percent). Fragments of gastropod and pelecypod shells and carbonatized plant 

detritus are present locally. Neither the top nor the base of the 

formation is exposed, and bedding was observed only locally. 

In the Buchanan quadrangle, the Farmington River Formation 

is present in the Bassa basin (fig. 3), a structural depression that 

also extends onto the Continental Shelf (White, 1972; Behrendt 

and Wotorson, 1970). Northwest-trending high-angle dip-slip 

faults define the onshore margin of the basin. The thickness of the 

unit is unknown, but Behrendt and Wotorson (1970) believe their 

geophysical data indicate a maximum thickness of 3 to 4 km 

(1.9 to 2.5 mi) of Farmington River or younger rocks in the 

Bassa basin. 

The formation is at least in part Cretaceous, as indicated by 

pollen and spores of probable Albian age obtained from it near 

Roberts International Airport (in Monrovia quadrangle about 30 

km (20 mi) northwest along the coast) (White, 1972). 

EDINA SANDSTONE 

The name Edina Sandstone was applied by White (1972) to a 

few small outcrops of coarse-grained quartz sandstone on islands 

in the lower part of the St. John River near Edina. He described 

it as a locally crossbedded and commonly well sorted sandstone, 

containing well-rounded grains, although granules are present 

locally and grade into quartz-cobble conglomerate in places. Beds 

are typically cemented by clayey material. White measured a 

thickness of about 8 m (26 ft) of the unit near the Mechlin 

River 2.5 km (1.5 mi) upstream from Edina. Other remnantssome consisting of single outcrops-are thinner and are found in 
6"00' 

5°00' 

10°15' 10°00' 

Q~ .. m~~~E'' .... ......... ttlRoberts '--... 

International 't--... 

A1rport u\0 

"*'?.?'> 

< 

"'?4 

.?'> 

.;() 

EXPLANATION 

u --o~ 

Fault or shear zone, dashed where 

approximately located; U, upthrown 

side; D, downthrown side 

0 10 20 30 40 50 KILOMETERS I I I I I I I I I 

0 10 20 30 MILES 

' 

0 

GBANKA (~~ QUADRANGLE 

~44 

"7G'(C' 

()~ 

"'?4' 

9000' 

ZWEDRU 

QUADRANGLE / 

I 

I 

~~/ 

o" 'V. 

I 

I 

I 

I 

JUAZOHN 

QUADRANGLE 

·······p-attern· ······-rrna9netic ..... f"\ine&f .. ··;e9 o 

I" EBURNEAN AGE PROVINCE 

HARPER 

UADRANGLE 

FIGURE 3.-Structure- and age-province map of Buchanan quadrangle, Liberia. 

~:;'} 

t'=j 

0 

t""l 

0 

0 

~ 

~ 

1-' 
22 GEOLOGY OF THE BUCHANAN QUADRANGLE, LIBERIA 

isolated patches along the coast near the mouths of the River Po 

and the Sehnkwehn River, and east of Bafu Bay (Tysdal, 1977a). 

The Edina unconformably overlies older formations, most of 

which are weathered, and is overlain by unconsolidated sediments. 

The age of the Edina is uncertain, although it is younger than 

the Farmington River Formation in which Albian fossils have 

been found. By analogy with sandstone of sinHlar distribution and 

character in Sierra Leone (Pollett, 1951; Reyre, 1966) and Ivory 

Coast (Spengler and Delteil, 1966, p. 108), White (1972) considered the Edina as Tertiary. 

BEACH DEPOSITS 

Unconsolidated quartz sand is present in savannah areas on a 

littoral terrace a few meters above sea level along the coast 

in the Buchanan quadrangle and is composed chiefly of nearly pure 

white quartz sand that overlies brown quartz sand (Tysdal, 

1977a). The limits of most of the unconsolidated quartz sand 

deposits were mapped easily on aerial photographs as the white 

sand of the treeless savannah areas contrasts markedly with the 

adjacent dark vegetation of the rain forest, which is on older 

rocks. The white sand ranges from fine to coarse grained, is subangular to rounded, and forms a veneer as much as 2 m (6.5 ft) 

thick. Similar white sand deposits near Monrovia have been 

studied in some detail by Rosenblum and Srivastava (1971) and 

\Vhite (1972). 

The brown quartzose sand is poorly sorted, fine to coarse 

grained, and forms beach ridges as well as flat-lying deposits. 

Beach ridges and intervening lagoonal deposits between River 

Cess and Greenville were examined by the Miiller Co. (Griethuysen, 1971) in a search for heavy minerals of economic value. 

Auger drilling showed that the ridges average 5 to 8 m (16 to 

26 ft) thick and are underlain by weathered crystalline rock, a 

sandy clay layer, or a dark organic clay layer ;

5 bedding dips seaward at about 5°. Heavy minerals ·constitute about 2 percent 

of the sand in the ridges, are generally dispersed throughout the 

deposit, and are mainly ilmenite, rutile, zircon, monazite, and 

garnet. The heavy minerals are locally concentrated in lenses 

measuring 1 m (3 ft) thick, 15 m (50 ft) across, and 100 m or 

more (325 ft or more) long. Radiometric anomalies caused by 

5 l\Iiiller Company, 1971, Exploration Liberia, 1Rth progress report, NovemberDecember 1970-January-l\Iarch 1971 : The Hague, Netherlands, :'\Iiiller Co., unpub. 

rept., 11 p., on file with the Liberian Geological Survey, Monrovia, Liberia. 
METAMORPHISM 23 

the zircon and monazite mark the position of many of the heavymineral concentrations (Behrendt and Wotorson, 1974b; Sam 

Rosenblum and S. P. Srivastava, written commun., in Behrendt 

and Wotorson, 1971). 

The unconsolidated sand is believed to be of Holocene age. A 

probable minimum age is 1,440±250 m.y., the date yielded by a 

tree stump truncated at the base of the white sand (White, 

1972). White believed that the bulk of the quartz sand is older, 

because soil similar to that in which the tree trunk was found 

overlies brown quartz sand in the Monrovia area. 

LAGOONAL DEPOSITS 

Lagoonal deposits consist mainly of Holocene sand, silt, and 

clay that are being deposited today or were deposited in the 

recent past (Tysdal, 1977a). The deposits are present near the 

mouths of rivers and streams, and because they are marked by 

the presence of mangrove trees, they are readily distinguished on 

aerial photographs. The deposits are probably formed of sediment derived from the sea by tides and trapped by mangrove 

roots. Some of the sediment, however, may be overbank deposits 

from floodwaters. 

METAMORPHISM 

Metamorphic rocks in the Buchanan quadrangle are of the 

amphibolite and pyroxene granulite facies. An exception may be 

the local presence of actinolite noted along Gwen Creek near 

the northern margin of the map (Tysdal, 1977a), in rocks shown 

as amphibolite. Amphibolite facies rocks are present throughout 

most of the quadrangle and range from lower grade amphibolite 

facies rocks (quartz-muscovite-biotite-staurolite schist) through 

upper grade rocks (sillimanite-kyanite-quartzofeldspathic gneiss). 

Additionally, some amphibolite bodies, such as those along the 

road trendi~g north from Greenville, contain ·clinopyroxene, indicating the uppermost grade of the amphibolite facies. 

Granulite facies rocks are present mainly southwest of the 

Todi shear zone and are truncated in and along the northwesttrending part of the shear zone. The southeasternmost margin of 

the melanocratic gneiss unit, which contains the granulite facies 

rocks, is interpreted to be truncated by the shear zone also. Hypersthene, diopside, and locally augite, all commonly associated 

with hornblende, are present in discontinuous outcrops in the 

melanocratic gneiss unit. These granulite-facies rocks are ap-
24 GEOLOGY OF THE BUCHANAN QUADRANGLE, LIBERIA 

parently gradational into rocks of the amphibolite facies. Some 

discontinuous bodies in the Buchanan quadrangle, however, may 

represent metamorphosed hypersthene-bearing igneous rocks and 

thus may have intrusive contacts now obs·cured by subsequent 

metamorphism. 

Most of the rocks within the melanocratic gneiss unit were 

most likely subjected to the same temperature and pressure condit~ons as the pyroxenic rocks but may not have been of a composition suitable to form minerals indicative of the granulite facies. 

Possibly, the pyroxenic rocks simply lacked water to form amphibolite, so pyroxene formed instead. 

STRUCTURE 

Two large structural provinces are evident on the map of the 

Buchanan quadrangle and are separated by the Todi shear zone, 

a major structure (defined by Thorman, 1972) extending northwestward from near Greenville across Liberia and into Sierra 

Leone (fig. 3). A third, but much smaller and less prominent, 

structural province is present in the southeasternmost part of 

the quadrangle. 

Rocks southwest of the Todi shear zone strike northwest; 

those northeast of it range from northeast in strike in the 

eastern part of the quadrangle, to east in the central part, to 

northwest in the westernmost part of the leuococratic gneiss 

unit. Folds observed on aerial photographs in these two structural 

provinces are commonly broad and open. The smaller structural 

province, in the southeasternmost part of the quadrangle, consists of east-trending isoclinally folded rocks that extend east 

into the Juazohn quadrangle (Tysdal, 1977a, b). This province 

(fig. 3) is delimited by a magnetic pattern consisting of alternating long linear magnetic highs and lows (Behrendt and 

Wotorson, 1974a). Rock units within this structural province 

consist of composite gneiss unit 2 and quartz diorite gneiss 

unit 2 (figs. 2 and 3; Tysdal, 1977a). 

FOLIATION AND BEDDING 

Foliations shown on the map reflect compositional layering or 

schistosity, but whether the·se planar elements are actually bedding is unknown. Foliations interpreted from aerial photographs 

are presumed to be parallel to compositional layering, an assumption proved generally valid when checked in the field. 

FOLDS 

Folds shown on the map of the Buchanan quadrangle (Tysdal, 

1977a) are large-scale features mapped wholly or in part through 

the use of aerial photographs and the magnetic data. Folds thus 
STRUCTURE 25 

mapped in the two major structural provinces of the quadrangle are broad, open structures that are many kilometers in 

wavelength. Folds in the southeasternmost part of the quadrangle are long, linear structures about 1-2 km (0.6-1.2 mi) in 

wavelength. 

The best topographic expression of folds is in the leucocratic 

gneiss unit in the north-central to northwest part of the quadrangle. Steeply dipping northeast-trending itabirite and associated rocks of Kaba Mountain define the southeast flank of a syncline; the rocks turn north in the Gbanka quadrangle (Force and 

Dunbar, 1977) to define the northeast end of the syncline. The 

northwest flank is represented by a long narrow ridge. On the 

basis of the magnetic anomaly over the ridge, the topographic 

expression of the ridge, and the fact that the ridge is a continuation of rocks associated with iron-formation in the Gbanka 

quadrangle, the ridge is thought to contain itabirite and associated 

rocks. 

On the basis of aerial photoevaluation, the topography in the 

south-central part of the leucocratic gneiss unit is interpreted to 

reflect a complex synclinal structure. The area is inaccessible and 

no ground data were obtained for it, but the topographic expression is thought to reflect iron-formation and associated quartzrich rocks. This interpretation is based on comparison with areas 

of known iron-formation and associated rocks as observed on 

aerial photographs in the Buchanan, Juazohn, and Zwedru quadrangles. The magnetic pattern, which in general reflects a synclinal structure, indicates that the rocks do not contain a large 

concentration of magnetite. 

The magnetic data show the long linear isoclinal folds in the 

southeastern part of the quadrangle to be 1--2 km (0.6-1.2 mi) 

wide. The contrast of intensity of magnetic highs and lows is 

less than in the Juazohn quadrangle (Behrendt. and Wotorson, 

197 4a; Wotorson and Behrendt, 197 4). This is probably caused 

by the lower angle of dip of the rocks in the Buchanan quadrangle and the absence of magnetite-bearing gneiss. Other magnetic rocks may be in part responsible for the low contrast of 

intensity, although exposures are too few in this coastal area 

to indicate definitely which rock types are present. 

FAULTS 

Faults of two prominent trends are widespread in the Buchanan 

quadrangle. Northeast-trending faults are older and are accompanied by a wide zone of shearing along the Cestos River. North-
26 GEOLOGY OF THE BUCHANAN QUADRANGLE, LIBERIA 

west-trending faults com.monly have a narrow zone of sheared 

rocks and have been intruded by diabase at many places. One 

series of northwest-trending faults, however, defines the Todi 

shear zone, a major structure in the coastal area of western 

Liberia. 

NORTHEAST-TRENDING FAULTS 

A major fault and associated wide zone of shearing, the Cestos 

shear zone, trends diagonally across the quadrangle and controls the course of the Cestos River, for which it is named (Tysdal, 1977a). It is a near-vertical structure that separates melanocratic quartz diorite gneiss and diorite on the east from leucocratic gneiss, iron-formation, and associated rocks on the west. The 

topo&'raphy east of the fault has only minor relief, but the 

topography to the west is generally mountainous. Aerial photographs do not reveal folds east of the fault, but shaw many 

large folds to the west. Indeed, the shear zone truncates some 

of these folds. Magnetic linearity marks the shear zone, and 

contrasting magnetic patterns on opposite sides of the zone reflect the different rock types. Similarly, a radiometric contrast 

exists; the rocks east of the shear zone show less radioactivity. 

All the rocks examined within the zone are sheared, although 

the intensity of shearing differs from place to place. Islands of 

unsheared rock within the zone were not observed, but they could 

possibly be present. To define individual faults within the zone on 

the basis of shearing is not feasible. The fault shown on the map 

(Tysdal, 1977a) is placed about midway within the Cestos shear 

zone; it corresponds to the locus of some of the most strongly 

deformed rocks and represents the line of greatest change in 

magnetic patterns. Most of the rocks in the zone dip steeply; 

vertical dips are common in the zone of most intense mylonitization. 

NORTHWEST-TRENDING FAULTS 

Two systems of faults have northwest trends: one system defines the Todi shear zone near the coast, and the other system is 

younger and is present throughout the quadrangle. 

The Todi shear zone was mapped by Thorman (1972, 1977) in 

the Monrovia quadrangle as a northwest-trending zone of faults 

and sheared rocks separating granulite and amphibolite facies 

metamorphic rocks to the southwest from amphibolite facies 

rocks to the northeast. In the Buchanan quadrangle, long segments of individual faults within the shear zone are defined by 
STRUCTURE 27 

(1) an abrupt change in magnetic pattern, (2) a coincident 

change in radiometric intensity, (3) a strongly alined drainage 

pattern, (4) an abrupt change in strike of rocks, and (5) near 

the Cestos River, abrupt truncation of a prominent northeasttrending ridge. Most of the shear zone is marked by cataclastic 

rocks that range from protomylonite to ultramylonite. The mylonites commonly contain stretched quartz, and are hard, dense, 

and generally more resistant to erosion than unsheared rocks. 

Thus they commonly form outcrops in rivers. Dips of mylonitic 

rocks are commonly vertical or inclined steeply to the southwest. 

Near Bafu Bay, however, the sheared rocks strike northeast, and 

the dips range from 50° SE. to vertical. 

The southeastern end of the T'odi shear zone is not clearly 

understood; hence, neither is the junction of the melanocratic 

gneiss unit and the quartz diorite gneiss unit 2. Nevertheless, the 

short northeast-trending fault along the Sehnkwehn River, near 

the coast, seems to reflect strongly sheared rocks along the trend 

and to reflect the change in strike of the rocks from the general 

southeast trend to a northeast trend. 

Photolineations in the melanocratic gneiss unit immediately 

west of the Sehnkwehn River, as observed independently by myself and the Muller Co. (see footnote 1 on p. 5), reflect the 

curving foliation trends produced by shearing in the rocks along 

the river. East of the river, a second set of photolineations, interpreted by the MUller Co., shows a similar curving trend. These 

lineations may reflect another fault that truncates the melanocratic gneiss unit. 

Only two occurrences of melanocratic gneiss are known beyond 

th,e limit of this proposed fault. One is at the southern margin of 

Bafu Bay, where a sliver of melanocratic gneiss a few meters 

across and many meters long is caught up in a migmatite. The 

second is near Chom Town, where melanocratic gneiss crops 

out on Buto Hill. 

A conspicuous positive Bouguer anomaly of 40-50 mGal parallels the coast of Liberia from Sierra Leone to Ivory Coast, according to Behrendt and Wotorson (1970, 1974c), who correlate 

it with the granulite and other rocks southwest of the Todi 

shear zone. Thorman (1972, 1977) concurs with the correlation 

for the Monrovia quadrangle, and field evidence indicates a similar 

correlation 5n the Buchanan quadrangle as far southeast as Bafu 

Bay. Beyond Bafu Bay, a crustal section fitted to the Bouguer 

anomaly data by Behrendt and Wotors'On ( 197 4c, p. 24) "sug-
28 GEOLOGY OF THE BUCHANAN QUADRANGLE, LIBERIA 

gests that the zone of uplifted granulite may exist offshore 

beneath the continental shelf in the Greenville area." 

Other northwest-trending faults are present throughout much 

of the quadrangle. Many of these can be distinguished on aerial 

photographs by offset ridges and disrupted vegetation patterns. 

Offset magnetic patterns and abrupt truncation of magnetic patterns further delineate some faults and indicate others. Both 

left-lateral and right-lateral displacements (as much as 3 km 

(2 mi)) were noted, but the magi tude of vertical displacements 

is unknown. Field traverses show that a narrow zone of shearing 

is common along many of the faults. Diabase dikes have intruded 

many of them, and most of the dikes in this region may have 

formed in this way. 

The northwest-trending Geni fault offsets the Cestos shear 

zone and adjacent rock units and truncates Geni Mounain, for 

which it is named. On the basis of magnetic patterns, it offsets 

part of the Sehnkwehn batholith and quartz diorite gneiss unit 2. 

Aerial photographs and the magnetic pattern both suggest lateral 

displacement of rock bodies northwest of the Cestos shear zone. 

RING STRUCTURE 

A ring structure was noted in the northeasternmost part of the 

quadrangle, 2 km ( 1.25 mi) west of the eastern boundary of the 

quadrangle and 12 km (7.5 mi) south of the northern boundary. 

The structure, observed only on aerial photographs, has a diameter of about 1 km (0.6 mi). It has concentrically arranged 

ridges separated by streams in an annular pattern, but the ridges 

are also cut by gaps, probably reflecting a subdued radial drainage pattern. The cause of the ring structure and the nature of 

the rock are unknown. 

AGE PROVINCES 

Radiometric age determinations were made by Hurley and 

others (1971) on rocks from Liberia as part of a reconnaissance 

study to outline intracratonic age provinces in several of the more 

ancient regions of the continental crust. Samples from 23 localities in Liberia were dated, mostly by the whole-rock Rb-Sr 

method, but four analyses were done by the K-Ar method (White 

and Leo, 1971) . Three age provinces were recognized: Liberian 

(about 2,700 m.y. old), Eburnean (about 2,000 m.y. old), and 

Pan-African (about 550 m.y. old). In addition, White and Leo 

(1971) showed an area in eastern Liberia that yielded dates 
MINERAL RESOURCES 29 

transitional between those of the Eburnean and Liberian provinces. 

The Pan-African province (fig. 3) is southwest of the Todi 

shear zone, which separates it from other provinces. The one 

rock dated from the Buchanan quadrangle (a leucocratic gneiss 

southeast of Buchanan) was obtained from this province. This 

gneiss yielded a K-Ar age of 510 m.y., obtained on hornblende 

concentrate. White and Leo ( 1971) noted that an Rb-Sr wholerock date of 2,600 m.y. was obtained from an unsheared granulite 

in the Pan-African province in the Monrovia quadrangle (outside the area of fig. 3). This suggests that the granulite-facies 

rocks may be relics of the old crystalline basement, which had 

already been raised to the granulite facies during the Liberian 

thermotectonic event. 

An Eburnean date of 2,070 m.y. was obtained on a rock collected near the Buchanan quadrangle border but in the Harper 

quadrangle (Brock, Chidester, and Baker, 1977) about ·1 km 

(0.6 mi) south of Greenville. The date was obtained by the 

whole-rock Rb-Sr method on a gneiss from composite gneiss unit 2. 

A Liberian age (2,700 m.y.) has been assumed for rocks in 

much of the quadrangle (White and Leo, 1971) ; however, on the 

basis of the projection of age dates southeast along strike from 

dated rocks in the J uazohn quadrangle, a large area of rocks 

east of the Cestos River may have ages transitional between 

Liberian and Eburnean ages. The boundary of Eburnean rocks 

must be in the southeastern part of the quadrangle. 

N.HNERAL RESOURCES 

Heavy minerals of possible economic value are present in beach 

sands along the coast between River Cess and Greenville. These 

deposits, described in the section titled "Beach Deposits," were 

investigated by the Muller Co. (Griethuysen, 1971) and were 

found to contain significant quantities of ilmenite, rutile, zircon, 

and monazite. Griethuysen (1971) reported that within a 0.4-km2 

(0.15-mP) area near Bafu Bay, 100,000 metric tons of heavy 

minerals were indicated in more than 2 million m3 (2.6 million 

yd3 ) of sand. 

Itabirite of possible economic significance is present in the 

Kaba Mountain area (in the northeastern part of the quadrangle) 

and extends northward into the Gbanka quadrangle. The associated magnetic anomalies reach a maximum of about 2,000 gammas but are not nearly as large as those of deposits presently 

mined in Liberia. 
30 GEOLOGY OF THE BUCHANAN QUADRANGLE, LIBERIA 

Diamonds are reportedly being recovered in small quantity 

from a stream draining Buto Hill, near Chom Town, along the· 

coast northwest of Greenville. The occurrence has not been 

investigated. The Diamond Mining Corp. of Liberia (Leuria, 

see footnotes 1 and 2 on p. 5) explored for diamonds in the area 

north of the Sehnkwehn River as far west as the Cestos· River, 

but did not discover any diamonds. 

Several isolated localities of gold in alluvial gravels were reported by Sherman (1947) and Offerberg and Tremaine (1961). 

Sherman (1947) also reported isolated occurrences of other minerals, but none of economic potential. 

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v. 81, no. 12, p. 3563-3574. 

---1971, An aeromagnetic and aeroradioactivity survey of Liberia, West 

Africa: Geophysics, v. 36, no. 3, p. 598-604. 

---1974a, Aeromagnetic map of the Buchanan quadrangle, Liberia: U.S. 

Geol. Survey Misc. Inv. Map I-778-B, scale 1:250,000. 

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Brock, lVL R., Chidester, A. H., and Baker, M. W. G., 1977, Geological map 

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Mehnert, K. R., 1968, Migmatites and the origin of granitic rocks: New 

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Offerberg, Jan, and Tremaine, John, 1961, Report on LAMCO Joint Venture's geological investigations in Liberia between Nimba and lower 

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Caslon Press Boktr. AB, 74 p. 
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Pollett, J. D., 1951, The geology and mineral resources of Sierra Leone: 

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