Transcript UNCONFORMITY

DEPARTMENT OF EARTH SCIENCE
UNIVERSITY OF GHANA
LEVEL 300
GEOL 324: INTRODUCTION TO
GEOLOGY OF GHANA.
2 CREDITS.
Geological Provinces of Ghana.
• On the basis of age data, tectonics and
lithology, Ghana can be divided into five
main geological domains.
• The western Units, found at the eastern
margin of the West African Craton.
• The mobile belt units found at southeastern
part of the country.
• The Voltain sediments at the central part of
the country.
• The Coastal sedimentary basins.
• Tertiary to Recent deposits.
2
The stratigraphic succession of Ghana
Era
Period/Epoch
System/series/information
Caenozoic
Quarternary-Recent
Tertiary-Eocene
Unconsolidated clays and sands of lagoon, delta and
littoral areas.
Partly consolidated red continental deposits of
sandy clay and gravel.
Mesozoic
Cretaceous
Jurassic
Apollonian formation (Upper cretaceous, i.e.
Cenomanian – Campanian)
Amisian
formation
(upper
Jurassic-lower
cretaceous)
Paleozoic
Devonian
Cambrian
Sekondian series (middle Devonian – lower
cretaceous) Accraian series (early or middle
Devonian)
Voltaian system (late Proterozoic to early paleozoic,
i.e. to 300 – 1000 m.y.)
Proterozoic
Upper Precambrian
Buem Units
Togo Units
Dahomeyan Units. Age 610-540Ma
Middle Precambrian
Tarkwaian system (Age unknown, possibly 1650 –
1850 m.y.)
Birimian system (Age uncertain, approximately3
1800 – 2100 m.y.)
Geological Provinces of Ghana
Ghana can conveniently be divided into five geological domains or
provinces on the basis of age, tectonics and lithologic
characteristics of the supracrustal rocks. These are:
(1) The western unit which lies at the eastern margin of the
Precambrian West African Shield or Craton,
(2) The southeastern unit which is at the southeastern part of the
country belonging to the Precambrian Mobile Belt and
(3) The flat lying central unit made up mainly of the sediments of the
Voltaian system
(4) The coastal basins and
(5) Tertiary to Recent deposits
4
Supracrustals: The Birimian and Tarkwaian
The very thick and extensive sequence of metamorphosed sediments and
volcanics that dominate this age province is called the Birimian after the
Birim region in southern Ghana where the rocks were first described in
detail.
Metamorphic grades range from greenschist to almandine-amphibolite
facies in these rocks, which are an important source of diamonds and
manganese ores.
A much smaller and more scattered group of supracrustals, mainly
shallow-water sediments, is called the Tarkwaian (after the town of Tarkwa
in southern Ghana, were they are gold-bearing). Although greenschist to
almandine-amphibolite facies metamorphism is recorded from these rocks,
in many places they are described as being ‘hardly metamorphosed’.
A consensus has not yet been reached about the stratigraphy of the
Birimian and its relationship to the Tarkwaian. Geologists working in
Ghana on the one hand, and Ivory Coast and Upper Volta on the other,
have arrived at different conclusions. It will be necessary to deal with the
two views separately and then to examine reasons for the differences
between them.
5
The Birimian in Ghana
In Ghana the Birimian is sub divided into Lower Birimian, dominated by metasediments,
and Upper Birimian, dominated by greenstone-type metavolcanics.
The lowest part of the succession are primarily phyllites and greywackes. These change
upwards to phyllites and weakly metamorphosed tuffs, greywackes and feldspathic
sandstones. Some of the phyllites contain pyrite, with carbonaceous matter present in most
of them. Silicification is common among the phyllites. Quartzites, calcareous rocks and
conglomerates are rare, but the conglomeratic horizons contain fragments of granitic and
other rocks believed to be derived from older basement.
The Upper Birimian consists chiefly of metamorphosed basaltic and andesitic lavas, now
hornblende-actinolite-schists, calcareous chlorite-schists and amphibolites (the
greenstones). Pillow structures indicating sub-aqueous eruption of the original basaltic lavas
are frequently observed. Minor intrusions of mafic rocks cut the volcanics and there are
small ultramafic bodies in some places. Smaller amounts of rhyolitic and dacitic lavas and
tuffs are also recorded, and subordinate metasediments include phyllite, greywacke, quartzsericite-schists and mica-schists, as well as grits and conglomerates at the base of the
succession.
Bands of gondite (quart-spessartite rock) and manganiferous phyllite occur within the
greenstones. associated with tuffs, silicified argillites (hornstones) and chert. Mn-rich
horizons also occur. Because the rocks are tightly folded and commonly sheared and
fractured, it is not easy to establish stratigraphic successions and estimated thicknesses.
The total thickness of the Birimian in Ghana may be of the order of 10000 to 15000m.
6
The Tarkwaian in Ghana
The Tarkwaian sediments occupy two generally synclinal belts surrounded by Upper
Birimian metavolcanics, about 270 km apart, with smaller occurrences
The sediments are mainly of shallow-water origin, probably fluviatile, and they
contain fragments of Birimian rocks. They could have been deposited in separate
elongate basins as molassic facies derived from erosion of the Birimian, during later
stages of the Eburnian orogeny.
The Tarkwaian appears in general to be less strongly deformed and
metamorphosed than the Birimian. The synclinal structure of the Tarkwaian in the
Tarkwa area is that of open folds having a northeasterly plunge and northwesterly
dipping foliation, the intensity of folding increases to the NW.
In the Bui syncline, Tarkwaian beds have been considerably fractured and more
strongly folded, being overturned in places.
Tarkwaian rocks form the central parts of Birimiam synformal belts, and in the
larger Tarkwa syncline an unconformity is implied by the fact that the Tarkwaian lies
mainly against Upper Birimian metavolcanics in the west, but overlap onto Lower
Birimian elsewhere.
The Birimian and Tarkwaian of Ghana form only a small part of the Proterozoic
domain and are not typical of the whole of it.
7
The Birimian and Tarkwaian in Ivory Coast and Upper Volta
In northeastern Upper Volta (and southwestern Niger), the supracrustal
belts are similar to those in Ghana. In Ivory Coast and Upper Volta, the
greenstone facies (upper Birimian of Ghana) is generally considered to
be either older than or broadly contemporaneous with the predominantly
sedimentary facies (Lower Birimian of Ghana). These interpretations are
based partly on the occurrence of volcanic pebbles in conglomerates of
the sedimentary formations.
Their relationship of the Tarkwaian to the Birimian is less simply defined
than in Ghana, because the two have been interfolded to produce very
complex structures.
As such the Tarkwaian seem to be regarded as lateral facies variations
within the main Birimian sedimentary facies and at palces characterised
by unconformable relationships with underlying rocks.
8
Explanation for the differring Stratigraphies
The different interpretations of the Birimiam stratigraphies in Ghana and the
Francophone territories could be ascribed to the following
1. Outcrops and communications are generally poor throughout much of the
region, and geological mapping is still mainly at reconnaissance level.
2. Foliation of the supracrustals is almost everywhere steep and parallel to
original sedimentary layering, and sedimentary structures that would give
way-up indications are not always easy to find.
3. Structures are often complex and, where different groups of rocks have
been folded together, their relative ages and original relationships are
difficult to unravel.
4. Volcanism was obviously more sporadic and scattered in the west (Ivory
Coast, northern Guinea), and could have broken out at any time. The
more voluminous volcanic activity in the east could have begun earlier in
the north (eastern Upper Volta, southern Niger) than in the south (Ghana).
An explanation along these lines offers a plausible way of reconciling the
contrasted Birimian stratigraphies that have been established in different
parts of the Proterozoic domain.
9
Granitic rocks associated with the Birimain
Most of the granites within the Birimian fall into two main groups:
1. Large syntectonic batholithic or basin type granites known as the Cape
Coast type in Ghana and the Baoule type in Ivory Coast and Upper Volta.
They are generally concordant with regional structures and are often
foliated. Many are two-mica granites, though biotite- and hornblendebearing varieties are also common. Granodioritic compositions
predominate, along with K-rich microcline-phyric adamellites. They are
often migmatitic round their margins.
They were probably derived in part at least by remobilisation of older
Liberian basement, perhaps in part also by granitisation and partial melting
of Birimian metasediments. Pegmatite facies occur throughout these
concordant batholiths, sometimes reaching several meters across, and
microgranite and aplite veins are also common.
2. Smaller discordant and typically unfoliated late-tectonic to post-tectonic or
belt type granites ,known as Dixcove type in Ghana and the Bondoukou
type in Ivory Coast and Upper Volta.
They are less abundant that the older syntectonic granites and have a
wider compositional range: from hornblende- and biotite-beairng granites10to
diorites, monzonites and syenites.
Economic potential in the Lower Proterozoic rocks of West Africa
This Birimain and the Tarkwaian regions are metallogenic province Minerals of
major importance are gold, manganese, diamonds and bauxite. Most of the
mineralisation is in Birimian greenstones and the Tarkwaian rocks, or in soils
and gravels above these formations. Mineralization is structurally controlled,
most deposits lying within or perpendicular to the regional structural grain.
Gold in the Birimian
Most of the primary gold deposits are located along the Lower-Upper Birimian
boundary.
In Ghana, the major primary gold lodes are associated with deep-seated shear
zones partly controlled by local unconformities between Lower Birimian
phyllites and Upper Birimian greenstones.
The country rocks in general comprise metamorphosed carbonaceous and
manganiferous argillites, tuffs and greywackes, along with basic to intermediate
igneous rocks. The primary gold occurs in quartz veins and lenticular reefs and
also in some of the tuffaceous and argillaceous rocks. It is accompanied by
sulphides, especially arsenopyrote but including pyrite and pyrrhotite,
chalcopyrite and bornite, and a little galena and sphalerite.
11
Gold in the Tarkwaian
Sedimentary gold is found in several places in the Banket
conglomerates near the base of the Tarkwaian associated with heavy
minerals, including rutile, zircon and detrital haematitegravel horizons
within the Banket. The gravel layers are extensive. Alluvial (placer)
gold
Modern stream channels near primary and secondary gold districts in
the Birimian and Tarkwaian contain placer gold. The Ofin River
system has been a source of placer gold in Ghana. In addition there
are eluvial deposits, beach sands, terrace deposits and older
Pleistocene stream sediments containing alluvial gold. Many such
occurrences have been prospected and worked in the past and
probably still are.
12
Manganese
In Ghana manganese ores were discovered in 1914 at Nsuta, south-east of
Tarkwa. In addition to metallurgical-grade ore, the deposit contained large
quantities of unique battery-grade ore called nsutite, which is almost pure
MnO2 and could be used in dry cells without processing. There are manganese
ores elsewhere in Ghana, but the Nsuta deposit is by far the largest and is the
only one exploited.
The ore occurs over a range of low hills in the Nsuta area and the manganese
originates mainly from Upper Birimian manganiferous phyllites or their more
highly metamorphosed equivalents, gondites (quartz-spessartite rocks).
Diamond
Alluvial diamonds have been found in Birimian rocks in Ghana. They come
mainly from the Birim field of southeastern Ghana, the largest single diamondproducing area in West Africa. Here the immediate source of the diamonds is a
band of Lower Birimian conglomerates. In the small Bonsa field, about 20 km
south-west of Tarkwa, diamonds occur in conglomerates at the base of the
Tarkwaian.
As the diamonds occur within Birimain and Tarkwaian rocks that are some 2000
Ma old, the source must be even older. No kimberlites have so far been found
in Ghana.
13
THE PAN AFRICAN BELT OF GHANA
Introduction
This occupies the east area of Ghana; and comprise of low-grade
supracrustal belts whose size and general NNE-SSW trend is
similar to that of supracrustals in the WAC. They are confined to a
broad belt stretching to the western half of Nigeria, to the craton
margin, in Benin an southern Togo.
The basement has its last major reactivation in the Pan African. The
rocks have been strongly deformed, being almost everywhere
isoclinally folded with a steep foliation that parallels the trend of the
belts. Metamorphism is generally in the greenschist to amphibolite
facies.
The sediments forming the cover sequence of the basement
Dahomeyan are the deformed and metamorphosed supracrustals of
the Buem and Togo Structural Units. These are thought to lie on
Dahomeyan basement in and near the thrust zone which forms the
margin of the West African craton.
14
Figure 3. Geological map of the Dahomeyide Orogenic Belt. Insert is the West
African Craton (after Attoh, 1998)
15
16
The Buem Structural Units
The Buem Structural Units are the closest to the craton. It is also called called
the Thiele Unit, and forms a band of generally flat country about 15 km across
on average, with scattered small hills. It defines the eastern limit of the Volta
Basin. The rocks constitute a south-eastward dipping sequences dominated by
clastic sediments, mainly sandstones and siltstones, shales and mudstones
being subordinate. There are some massive cherts, limestones, dolomites and
sedimentary ironstones. Conglomeratic horizons are near the base of the
succession. Volcanics interstratified with sediments in the succession include
rocks of both alkaline and calc-alkaline affinities. At least some of them were
erupted under water, for pillow structures are preserved. In many places
throughout the belt, schistose and massive serpentinites, some of them
chromite-bearing, have been tectonically emplaced along thrust planes that cut
the succession. There are also cross-cutting dolerites, which may be of late or
even post-Pan African age.
Rocks of the Buem Formation are largely unmetamorphosed. Deformation is
mainly the result of thrusting, and the overall dip is to the south-east. In Ghana,
Buem Formation is strongly folded into asymmetric overturned structures with
southeasterly dipping axial planes. This interpretation places the volcanics near
the top of the succession and yields estimates for the total thickness of these
largely unmetamorphosed rocks of around 3600 m.
17
THE TOGO STRUCTURAL UNITS
The Togo Units lie immediately to the east of the Buem Formation. It is called
the Akwapimian in Ghana and the Atacorian or Atacora Unit in Togo and
Benin. It occupies an irregular 5-50 km wide strip bordered on the west by
thrust contacts against the Buem Formation, on the east by thrust contacts
with the Dahomeyan basement. It includes the Atacora range in Benin, the
Togo Mountains and the Akwapim range in southern Ghana, where the Buem
Formation wedges out and the Togo Formation defines the eastern boundary
of the Volta Basin. Quartzitic sandstones and quartzites contain conglomeratic
layers, and some quartzites are ferruginous. Phyllites and mica-schists,
including the Kande-Boukombe Series, also have conglomerate horizons.
Marble is recorded north of Boukombe.
Tectonically emplaced slices of basement rocks are mainly gneisses but
include eclogitic and granulite facies rocks of the nearby high-grade
Dahomeyan as well as elongate lenses and pods of serpentinite. The Togo
rocks are more highly metamorphosed than those of the Buem Formation,
and also considerably more deformed. Both metamorphism and deformation
increase towards the south-east. Minor folds are isoclinal; axial planes dip to
the SE. The Togo Formation was probably unconformable on the Dahomeyan,
however, and was brought up from deeper stratigraphic levels by the thrust
faulting, to lie upon the younger Buem rocks. The Togo is correlated with the
Lower Voltaian, partly on the grounds of similar lithologies, both groups of
rocks being dominated by alternations of sandstones and shales (quartzites
and phyllites or schists), with occasional conglomerates and limestones
(marbles).In addition, there is evidence that in southeastern Ghana, where the
Lower Voltaian of the Kwahu Plateau abuts against the Togo Formation of the
18
Akwapim range, there is a progressive increase in intensity of folding and
metamorphism eastwards from Lower Voltaian into Togo Formation rocks.
The basement complex (Dahomeyan Structural Units)
Granulite facies rocks are most abundant close to the margin of the
craton, immediately to the east of the Togo belt, in south-eastern
Ghana and in Togo and Benin.
Gneisses with garnet, pyroxene and scapolite occur among more
ordinary quartzo-feldspathic biotite and hornblende-bearing
varieties. Eclogites (high-pressure garnet-pyroxene rocks
chemically equivalent to basalt) have been recorded from among
large masses of mafic gneisses that include amphibolites and
pyroxenites and contain much garnet.
The high grade rocks are generally considered to have been brought
up from deeper crustal (and upper mantle) levels by the westward
thrust movements that gave rise to the deformation of the Togo belt.
Metamorphism is generally in the amphibolite facies, as indicated
by the occurrence of index minerals such as garnet, sillimanite,
kyanite and staurolite in rocks of suitable composition. Intercalated
among the gneisses and migmatites are numerous supracrustal
relics and they are likely to include remains of supracrustal belts of
the Liberian and Eburnian cycles.
19
THE VOLTA BASIN
There are few detailed geologic descriptions of sediments of the Voltaian Group sequences;
subdivision of the group is difficult due to poor exposure and the lack of laterally persistent
lithological marker beds or fossils.
Generally, the Volta Basin has an overall gently synclinal form. The oldest sediments outcrop
round the margins, the youngest occupy a roughly central position. The group has generally
been divided into three formations, each separated by an unconformity marked by a tillite.
The Lower Voltaian
The Lower Voltaian, the Dapango—Bombouaka Group, is dominated by massive crossbedded feldspathic sandstones. It is responsible for the high ground of the Kwahu Plateau in
the south, at the base of which the uneven pre-Voltaian surface can be seen in places. The
group as a whole is correlated with the Togo Formation on the east of the Volta Basin . The
Lower Voltaian is virtually flat-lying throughout most of the basin, but becomes relatively
intensely folded as the Togo belt is approached.
The Lower Voltaian Formation unconformably overlies the Birimian Supergroup. A radiometric
age of 993 ± 62 Ma from the lower part of the Lower Voltaian Formation gives the
approximate period for the beginning of sedimentation of the group. The deposition
environment for the Lower Voltaian Formation is likely to have been shallow marine or
fluviatile. It reflects a fairly stable tectonic setting throughout the depositional area.
20
The Middle Voltaian
The middle Voltaian known as the Oti Formation in Ghana and
Pendjari Group generally rest with slight angular unconformity on the Lower Voltaian,
and in some places rest directly on the basement. They form much of the low ground
dominated by the Oti Plains of northern Ghana and Togo. The basal conglomerate of
the Pendjari Group is interpreted as a tillite and called in Ghana as the Akroso
Conglomerate. It contains boulders up to more than a metre across, subangular to
rounded, some with striated, polished or pitted surfaces. The tillite is succeeded by a
variety of sediments to including carbonates, often brecciated or slumped, locally baritebearing and partly stromatolitic, along with silexites and silicified argillites.
The rest of the Formation is dominated by shales, siltstones and sandstones. The
Middle Voltaian is generally correlated with the Buem Formation, on account of the
overall lithological similarities between the two groups of rocks.
The Middle Voltaian records a glacial event followed by prolonged marine incursion and
subsidence of the basin. In the eastern parts of its outcrop, adjacent to the Togo belt
(Fig. 9.1), the Pendjari Group has been deformed into generally NNE—SSW trending
asymmetric folds with southeasterly inclined axial planes.
K—Ar dating of glauconite from a borehole core at Tibagona yielded an age of 600±20
Ma.
21
The Upper Voltaian
The Upper Voltaian Formation also known as the The Obosum Formation is
thickest and coarsest in the south-east. The formation is divided into a lower
and an upper units. The lower unit consists mostly of dirty-yellow, fine-grained,
thinly bedded, micaceous feldspathic quartz sandstones with subordinate
argillite intercalations.
The upper unit consists of white to whitish-yellow, massive, fine- to mediumgrained, cross-bedded arkosic and quartzose sandstones. The Upper Voltaian
Formation occurs as scattered outcrops in the central part of the Voltaian
Basin, with an average thickness of about 400 m.
The conglomerates contain pebbles of granite and other igneous rocks, as well
as quartzite fragments, and sedimentary structures show the direction of
transport to have been from the south-east.
The Upper Voltaian Formation constitutes the molasses deposits formed in the
by the erosion of some horizons in the Dahomeyides during the Pan- African
event.
22
THE ACCRAIAN
The Sedimentary rocks of Accra are exposed on the beaches and cliffs and existed in the
Devonian period of over 350 million years ago. The Accraian can be sub-divided into three
formations; with the oldest at the bottom.
1. Upper Sansatone –shale Formation
2. Middle shale Formation
3. Lower sandstone Formation.
Lower Sandstone Formation
The rocks are essentially sandstone with subordinate amounts of grits, breccias and
pebble beds and shales. The sandstones are at places thinly bedded with shale partings,
but generally they are thickly bedded and at places fairly massive with current –bedding
and fossil ripple-marks.
Middle Shale Formation
The rocks of the middle formation are essentially shales, but thin limestones and thin
sandstones may be found. The sandstones become prominent towards the upper part of
the formation. Fossils such as trilobites (extinct Marine Crustaceans), Lamellibranches,
Gastropods and Brachiopods have been collected from the shales. Paleontologists
therefore ascribe the Middle Devonian age to the shales. Small sacle folding is seen at
places.
Upper Sandstone-Shale Formation
The Formation consists of sandstones and shales often interbedded in thin strata; but at23
places the sandstone beomes thicker.
The Sekondi Series
The Sekondi Series consists mainly of sandstones and shales with conglomerates, pebble beds, grits
and mudstones resting with major unconformity on a complex of granites, gneisses and schists.
The general classification based on the occurrence at Sekondi-Takoradi is given below:Sekondi Sandstone.
(b) Upper—Pebbly argillaceous and feispathic sandstones and
conglomerates.
(a) Lower—Massive quartzose sandstones and grits with
subordinate shales and mudstones.
Efia Nkwanta Beds.
(c) Upper—Thin bedded siltstone, shale, shaly sandstone, and some coarse sandstone,
nodules, bands, and lenses of
chert ..
..
..
..
..
..
..
(b) Middle—Friable sandstone, both well bedded and massive,
with interbedded mudstone and shale
..
..
..
(a) Lower—Cross-bedded, soft, fine-grained, pale purple, pink,
grey, green, and cream sandstone ..
..
..
..
with
..
Takoradi Shales.
Black and grey carbonaceous shales, sandy shales, and shaly sandstone, with interbedded grit and
fine-grained sandstone, and with nodules of siderite and pyrite ..
24
Takoradi Sandstone.
(b) Massive and bedded friable ferruginous sandstone with coarse
grained beds, breccia-conglomerate, and interbedded shales
(a) Thin-bedded, brittle, micaceous sandstone with sandy shale and
some clay shale
Elmina Sandstone.
Chocolate and purple felspathic micaceous sandstone, with coarse
sandstone, conglomerate, shale, and mudstone near the base ..
Ajua Shales.
Varved shales, sandy shales, and sandstones containing scattered
boulders and pebbles with a coarse boulder bed at the base
MAJOR UNCONFORMITY
Hornblende-granite of the Dixcove type.
Biotite-granite of the Cape Coast type.
25
1. Ajua Shales
The Ajua Shales, which are the lowest formation of the Sekondi Series, rest directly on the
underlying crystalline rocks. They form a series of isolated outcrops.
The Ajua Shales are well exposed at various places along the coast west of Takoradi.
Typically, the Ajua Shales consist of thin-bedded, black or dark grey shales with
arenaceous laminations and beds of grit, the formation becomes sandy towards the top.
Underlying the typical shales is a basal series, up to 12 or 15 feet thick, of boulder beds,
conglomerates, shales and sandstones. Scattered pebbles and boulders occur throughout
the formation.
This consists of a bed of large boulders of the granite and diorite, rounded or sub-angular,
up to 2 or 3 feet in diameter, and derived from the underlying rocks.
Above the conglomerate lies some 6 feet of thin-bedded, fine-grained, rather shaly
sandstone, greyish yellow in colour, and containing lenses of massive sandstone; and this
is overlain in turn by a further bed, 6 feet thick, of massive, fine-grained, grey sandstone
with very coarsegrained bands. The conglomerate occurs only in the troughs of the
undulating surface; elsewhere the granite is overlain directly by sandstone.
The Ajua Shales are strongly ripple marked.
26
2. Elmina Sandstone
The Elmina Sandstone is, one of the best known and characteristic rocks of the coast
line between Takoradi and Cape Coast. It extends over large tracts of country and
stretches of coast line at Takoradi, Efia, and south of Inchaban, and it is more extensive
at Komenda, and along the coast either side of Elmina.
It is uniform, hard, massive, medium-grained, felspathic sandstone with a characteristic
chocolate or chocolate-purple colour, which is due to the pink felspars and the dark
brown limonitic cement. On the whole it is poorly bedded, well jointed, and strongly
cross-bedded. Towards the base of the formation the rock tends to become coarsergrained, and interbedded with coarse and fine conglomerates, shales, and mudstones.
The coarser conglomerates contain well-rounded pebbles and derived from Upper
Birrimian greenstone, granite, quartz mica-schist, and quartz. The finer conglomerates
are more quartzose and have a more limonitic matrix. At the top of the formation the
sandstone becomes thin-bedded and somewhat shaly
3. Takoradi Sandstone
The base of the Takoradi Sandstone consists of approximately 100 feet of grey shales,
sandy shales, shaly sandstone, and thin-bedded, fine-grained, brittle, micaceous,
cream and pink sandstones. Above the basal shaly beds lies the typical facies of the
Takoradi Sandstone.
Towards the top of the formation the sandstone becomes well bedded and the shale
bands more frequent.
The Takoradi Sandstone, being resistant to weathering, forms hills. The total thickness
of the Takoradi Sandstone is 600 feet.
27
4. Takoradi Shales
The Takoradi Shales and the Takoradi Sandstone belong to the same
general phase of deposition. It is difficult to define exactly the position of the
junction between them. Whereas the lower of the two formations is
predominantly sandstone, the upper one is essentially shaly.
The typical Takoradi Shales are hard, compact, black or very dark grey,
fissile shales or sandy shales, rich in carbonaceous matter. Towards the top
of the formation a series of hard, grey-green grit bands are interbedded with
the shales.
The Takoradi Shales, like the base of the Takoradi Sandstone, are
fossiliferous. The thickness of the Takoradi Shales is estimated to be 650
feet.
5. Takoradi Beds
The term “Takoradi Beds” is used when it is impossible or undesirable to
distinguish between the Takoradi Sandstone and the Takoradi Shales.
There are beds, both shales and sandstones, of which the lithological
characters are very similar in both formations, and it is clear that they all
belong to one general phase of deposition.
The Takoradi Beds is estimated to be approximately 1,200 feet.
28
6. Efia Nkwanta Beds
The Efia Nkwanta Beds occupy an area south of Takoradi and to Cape Coast, together with
blocks at Esamang (near the Elmina by-pass) and Kwapro.
They are divided into a lower, middle, and upper division.
Lower Efia Nkwanta Beds. The type locality of the transition beds between the Takoradi Shales
and the typical Lower Efia Nkwanta Beds. The sandstone of the Lower Efia Nkwanta Beds is a
very fine-grained, soft, cross-bedded, and ripple marked sandstone.
Middle Efia Nkwanta Beds. The sandstones are quartzose and friable, and are not unlike
similar beds in the Takoradi Sandstone, but in colour they are generally a brighter pink or orange
and are not so well graded as the Takoradi Sandstone. The Middle Efia Nkwanta Beds are about
315 feet thick.
Upper Efia Nkwanta Beds. The upper division of the Efia Nkwanta Beds consists of well-bedded,
purple, pink, grey, and green shales and siltstone with some mudstone, fine-grained sandstone,
and a few coarse-grained beds similar in appearance to the Lower Efia Nkwanta Beds but without
cross-bedding.
7. Sekondi Sandstone
It separated the Sekondi Sandstone into an upper and lower division.
Lower Sekondi Sandstone. Detrital chert is a common constituent of both upper and lower
divisions of this formation. The total thickness of the Lower Sekondi Sandstone is approximately
650 feet.
Upper Sekondi Sandstone. The typical rock forming the Upper Sekondi Sandstone is a soft,
argillaceous, feldspathic sandstone with pebbles. The pebbles are generally well rounded and
consist mostly of white quartz with lesser amounts of greenstone, green quartzite, phyllite, and
small fragments of chert. The rock, both pebbly and non-pebbly facies, is poorly graded. 29
THE APPOLONIAN FORMATION
• LOCATION: East along the Atlantic Coast in SW
Ghana and SE Ivory Coast.
• Ghanaian portion referred to as TANO BASIN.
Occupies between the Tano River in the West and
Ankobra in the East. Approximately 116550sq km.
• AGE: Cretacous- Eocene
• ROCKS: Alternating sands, clays and limestone
overlying the Birimian.
• The Nauli limestone is highly fossiliferrous and have
oil shows
30
STRATIGRAPHY
UNIT
• 1.
LITHOLOGY AND THICKNESS
Loose sand, clay and shaly clay, 100-215m
AGE
Recent
• 2.
-----------------------UNCONFORMITY-------------------Foss. Limst. + black shaly clays, 45-120m
Maestri.
• 3.
• 4.
----------------------UNCONFORMITY------------------------Sandstone, minor sh+limst., 610-915m
Camp.
Conglomerate, 23-76m
• 5.
• 6.
-----------------------------UNCONFORMITY-------------Green-gray ss. + minor sh., 300-325m
Albian
Black, carbonaceous shale, 100-450m
• 7.
• 8.
--------------------------UNCONFORMITY--------------------Silts.+igneos and met rk pebbles, 225m
Aptian
Grayish –green ss and sh., over 1200m.
31
KETA BASIN
• LOCATION: EXTREME SE CORNER OF
GHANA
• EXTENT:
375550 sq Km, 220150 sq km
on shore.
• Nearly 50% of the on shore area covered by
Lagoons.
• ROCKS: Mainly siltstones, shales and clays
with layers of fossiliferous limestone. All
covered by sand and gravel. Gentle dips of
about 2o
• AGE:
Devonian to Recent.
• BASEMENT ROCKS: Dahomeyan.
• STRUCTURES: The basin has tectonic Block
bounded by fault system at the northern end.32
SECTION: KETA BASIN
UNIT
THICKNESS LITHOLOGY
AGE
1.
30-60m Sand, Gravel
Recent
----------------------------------------UNCONFORMITY-------------------2. 180m
Fossiliferous Clays
Miocene
--------------------------UNCONFORMITY------------------------------3.
250-700m
Calc. Clays interbedded foss. Eocene
Limest.
4.
5.
120-240m
Bentonitic Clays
Paleocene
45-60m Bluish-gray clays, interbed. Lmst. Maestr.
--------------------------------UNCONFORMITY-------------------------------6.
400-550m. Ferruginous sandstone, shale
Campanian
7.
370m
Grayish-white ss, grav. Muds and sh. Albian
8. 579m
Poorly sorted ss, silst and shales
Aptian
------------------------DOLERITE—70m-----CONFORMITY----9. 610m mic. Shales, silstones, fossiliferous
Devonian.
ECONOMIC POTENTIAL
Hydrocarbon shows have been recorded in two of the four wells drilled33
in the Basin.