Journal of Mining and Qeology, Vol. 35(1), 1999, pp. 23-36 1 1 1  6-2775
© Nigerian Mining and Geosciences Society (NMOS) - Printed in Nigeria
SEDIMENTOLOGY AND DEPOSITIONAL ENVIRONMENT OF AWI FORMATION 
CALABAR FLANK, SOUTHEASTERN NIGERIA
Matthew E. Pit on
Department o f Qeology, University o f Ibadan 
Received 9 June, 1995; Revision accepted 13 July, 1998 
ABSTRACT
A sequence of conglomerates, sandstones, slltstones, claystones carbonaceous shales and mudstones 
which rims the Oban Massif in the Calabar Flank, constitutes the Early Cretaceous (probably Aptian) Awi 
Formation In the southeastern Nigeria. Field studies and laboratory analyses were conducted on these 
sediments to determine their provenance and depositlonal environment.
Field observations show that the sediments are grently dipping (.< 16°) in a southwesterly direction. They 
vary in thickness from thin to very thick beds and are laterally, discontinuous. The pattern of sedimentation 
is cyclic with fining upward sequences. The sediments are textually immature with angular to subangular 
grains dominating, thus indicating short distance transportation.
The sandstones are medium to coarse-grained, poorly sorted, mostly leptokurtic, fine skewed and unimodal 
in distribution. They are subarkoslc with more than 70% quartz which are of Igneous and metamorphic 
origin while the associated claystones contain kaolinite:
Heavy mineral assemblages show the presence of predominantly zircon, tourmaline, rutile, garnet and 
staurolite with a range of 66.7 to 96%. This indicates mature to superrnature sandstones and can be 
attributed to wet climatic conditions. The carbonaceous shales which grade into mudstone In some cases 
are non-fossiiiferous, poorly laminated, pyritized and rich in lignitised wood.
AH these indicate that sediments of Awi Formation were derived from the Oban Massif and deposited in 
environments ranging from channel lag and point bar to flood plain.
INTRODUCTION formation justify its subdivision into two distinct for­
The Calabar Flank, located at the easternmost part mations. Later, Adeleye and Fayose (1978) proposed 
o f the Qulf o f Guinea is that part o f the southern the name Awi Formation to the basal arenaceous 
Nigerian continental margin between the Cameroon part o f the Odukpani Formation. They delineated a 
volcanic trend in the east, the Ikpe platform on the type section and retained the name Odukpani 
west, the Oban Massif and the Calabar hinge line to Formation for the rest o f the succession.
the north and south respectively (Fig. 1) (Nyong and Adeleye and Fayose (1978) described the Awi 
Ramanathan, 1985). Formation as fluviodeltaic, partly fossiliferous, 
The initial rifting o f the southern Nigeria margin folded and cyclothemic, comprising cross bedded 
produced two principal sets' o f faults, trending, NE- sandstones, siltstones, mudstones, .claystones, con­
SW and NW - SE. The former sets o f faults bound glomerates and shales. They correlated it with the 
the Benue depression while the later sets were Early Cretaceous Mamfe Formation o f the Cameroon.
present and active in the area o f the Calabar Flank. Odebode (1982), using microflora dated the Awi 
After the initial rifting episode, the Calabar Flank Formation, Upper Albian - Lower Cenomanian. This 
underwent a somewhat different tectonic and strati­ age assignment is at variance with that o f other work­
graphic developm ent compared to the adjacent ers on the overlying Mfamosing Limestone which 
Anambra and Lower Benue Trough sedimentary ba­ has its age as Middle to Upper Albian (Petters, 1982; 
sins (Murat, 1972). Nyong and Ramanathan, 1985).
The major tectonic elements of the Calabar Flank This paper presents the sedimentology and depo- 
include the Ikang Trough, which was a mobile de­ sitional environment o f Awi Formation from few out­
pression and the Ituk High that was a stable mobile crop locations encountered within the Calabar Flank 
submarine ridge. This is borne out o f sedimentary (Fig. 2). Accessibility to most o f the outcrop loca­
facies distribution in the area (Murat, 1972). tions was ham pered by thick vegetation  that 
Within the Calabar Flank, Reyment (1965) pro­ characterised this area. Some o f the outcrops were 
posed the name Odukpani Formation to a sequence poorly exposed due to extensive weathering and 
comprising o f basal arkosic sandstones and con­ most o f the sampling was affected to a certain ex­
glomerates, limestones in the middle and calcare­ tent by this. However, based on field evidence and 
ous sandstones and alternating limestones and shale laboratory studies which include textural, mineral- 
in the top part. This formation rests unconformably ogical, as well as micropalaeotological evaluations, 
on the basement complex. the provenance and the depositional environment 
Fayose (1978) noted that the lithologic and tec­ o f the sediments were deduced.
tonic structures o f the basal part o f the Odukpani
23
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___ CALABAR FLANK CAMEROON VOLCANICS
Fig. 1. L oca tion  and structural e lem en ts  o f  the C a labar Flank and ad jacen t area  (A fter N yon g and
Ramanathan, 1985).
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STRATIGRAPHY METHODS AND MATERIALS
The lithostratigraphic development in the Calabar The Awi Formation was sampled from four locations 
Flank was controlled by vertical movements of (A to D) within the area o f study (Fig. 2.). Spot 
faulted blocks notably the Ituk High and the Ikang samples were collected from about two feet deep at 
Trough and by associated transgression and regres­ different stratigraphic position in the respective 
sion (Nyong, 1995). outcrops. Most o f the sedimentary structures have 
Different formation names have been proposed been obliterated by weathering and the few encoun­
by several authors for sediments within the Calabar tered were noted, described and photographed.
Flank (Reyment, 1965; Dessau vagi e, 1974, Adeleye 
andFayose, 1978; Fetters, 1982;Pettersetal. 1995). Field Observations
The sedimentary sequence o f the Calabar Flank Locations A: (Km 27, Calabar - Ikom Road)
begins with continental elastics, consisting of a fluvio The stratigraphic sequence here is shown in Fig. 3. 
- deltaic sequence o f cross - bedded sandstones, The overall thickness o f the outcrops is 6 metres 
siltstones, mudstones, conglomerates, claystones and the sedimentary unit unconformably overlies 
and shales. This unit overlies the Precambrian Oban weathered basement made up of granite gneiss. The 
Massif unconformably and constitutes the basal outcrop dips 12°SW and consists o f basal conglo 
part of the Odukpani Formation of Adeleye and merate followed by sandstone, mudstone, siltstone 
Fayose (1978). These elastics (Awi Formation) and capped with highly weathered pebbly sandstone 
belong to Early Cretaceous (probably Aptian) age. unit.
Enhanced subsidence of the faulted blocks results Generally, the sandstones are greenish to 
in the initiation of a series o f marine transgressions. greyish, angular and poorly sorted with large mica 
The earliest o f these transgressions in Middle Albian, flakes. Quartz grains are milky to colourless and 
resulted in the deposition of platform carbonates occasionally smoky. The sandstones display graded 
(Mfamosing Limestones, Petters 1982). Ahardground bedding fining Upward from basal conglomerates 
exposed at the top of the limestone quarry at (Fig. 4). Bedding is discontinuous and lenticular. The 
Mfamosing, marks the break in sedimentation and contact between the Awi Formation and the 
deposition and separates the Mfamosing Limestone weathered granite gneiss is sharp (Fig. 5).
from the thick sequence of black fissile shales with The mudstone bed is about 0,40m thick at this 
minor but frequent intercalation of marls, calcareous location. It is greenish to pinkish, fine grained, silty, 
mudstones and shales above. Petters etal. (1995) plastic and flat bedded. The middle sandstone is 
have given a new name (Ekenkpon Shale formation) capped by siltstone that is fine grained, rippled grey­
to this unit with a type section in Ekenkpon village ish and micaceous. Grains are angular to subangular.
along Calabar - Itu Road. This new name replaces 
Mkalagu/Eke - Aku Formations of earlier workers Location B: This location is at a river channel 
(Petters and Ekweozer, 1982). Deposition o f these accessible off km 26.5 on Calabar-Ikom.road, which 
sediments resulted from a second phase of marine is half a kilometre south o f location A (see Fig. 2). 
transgression which was initiated in Late Albian and The thickness is about 2 metres (Fig. 6) and dips 
continued through to Turonian times with a break 12°SW. The lithology is made up o f sandstones and 
towards the end o f Cenomanian (Nyong and claystones and the base is not exposed. The 
Ramanathan 1985). sandstones are m ilky to brown in colour, 
A thick marl unit with thin dark shale intercala­ unconsolidated with angular grains. They are 
tions overlies the Turonian in the Calabar Flank. This micaceous and poorly sorted. Poorly developed 
unit belongs to the Hew Hetim Marl Formation (Petters cross-bedding with azimuths 200° and 230 (Southedy 
et al., 1995). Santonian and Early Campanian sedi­ and Westerly) have been measured in this location.
ments are not encountered and this interval is re­ The claystones are greyish and plastic. Overlying 
garded as a period of non-deposition and/or erosion this sequence is a pebbly, highly weathered sand­
in the Calabar Flank. The Late Campanian to stone, lateritic unit From the sandstone unit to the 
Maastrichtian sediments are characterized by dark- claystone at the middle indicates a fining upward 
grey, carbonaceous, friable shales with occasional sequence,
thin bands of marlstones and gypsum referred to as 
Hkporo Shales (Reyment, 1965). The Hkporo Shales Location C: (Type section - km 9 South of Awi 
are overlain by Tertiary - Recent sands of continen­ village or km25 Calabar - Ikom Road)
tal origin. These sands represent the Benin For­ This outcrop is well exposed at 9Km south of Awi 
mation (Fig. 2). village or 2Km south o f location A (see Fig. 2). It is
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THICK GEOLOGIC SAMPLE
• l m) LITHOLOGIC DESCRIPTION
DESP0S1T10NAL
SECTION NUMBER ENVIRONMENT
6 O 6 O • • ’•
• . 0 :. 0  • .
O . , f i  
°  • o  ° t WEATHERED, PEB BIV SANDSTONE P. SORTED , FERRUGINOUS
5 R>\-0 .
' ’ . - o ' CHANNEL
> .  °; ,\p LAG
u ' ? *
• t i  o' o 6 o
AS SANDSTONE MED- F. GRAIN WITH CtAYSTONE BAND
POINT BAR
A L . SILT * ONE GREENISH -  GREYISH M ICACEOUS AND F. GRAIN
3
■g.Q CL g o.. A3 SANDSTONE PEGBLY AT BASE
A 2 MUI-STONE, GREENISH-PINKISH, F. GRAIN AND PLASTIC FLOOD PLAIN .
r r . i
2 A1 SANDSTONE RIPPLED, COARSE- MED, GRAIN CROSS-BEDDED
CHANNEL
o '  o  n  o o BASAL CONGLOMERATE LAG
\ s i ' 1
' ^
1 / /l  '  x C' GRANITE GNEISS HIGHLY WEATHERED
v y *
0 * ' \
Fig. 3. Lithologic Succession o f location A (Km 27 Calabar-Ikom Road)
11 I .'!n.r Ur l i  ! i 5» »,r. n il / ; , I
! if - V* '
. 'f -
Fig. 5. Field photograph showing sharp contact 
between Awi Formation and weathered basement.
about 15m thick (Fig. 7) with lot o f facies changes. It been measured in this location. They are micaceous 
dips 15°SW and its base is not exposed. The lithol­ and the bedding is discontinuous and in some parts, 
ogy is made up o f pebbly to very coarse-grained it is iron stained. Some o f the sandstone beds are 
sandstones at the base, mudstones, siltstones, indurated and concretionary. The sandstones were 
claystones and carbonaceous shales at the middle sampled at two levels - the lower and the middle 
and upper parts, respectively. The whole sequence portions.
is cyclical and fines upward. It is folded with E-W The siltstones are greyish to milky, very fine - 
fold axis (Fig. 8). grained and poorly consolidated. They are plastic, 
The sandstones are thickly bedded (Fig. 9), partly micaceous and the beds are laterlly discontinuous. 
weathered, weakly cross-bedded, colour banded, In some cases, the siltstone are sheet-like, rippled, 
poorly sorted and angular. Two sets o f cross bed­ with roots, mottled and colour banded. From the 
ding with azimuths 205° and 208° (southerly) have lower sandstone unit to the middle siltstone indi-
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THICK GEOLOGIC SAMPLE
l m ) NUMBER D E P O S I T  T O N A L  S E C T I O N LITHOLOGIC DESCRIPTION
7 E N V I R O N M E N T
■ A - : * : * - C H A N N E L
W E A T H E R E D ,  P I B B l V  S A N D S T O N E ,  P.  S O R T  E D ,  F E R R U G I N O U S L A G
6 4 F L O O D  P L A I N
1 ^>fsT5|J0NE,CREV,Sh“WM,T,SH/ W 0 ® D  F R A G M E * T P R H S E N X
B 3
l 1 • / • / ' /  /  • S A N D S T O N E , M E D ;  G R A I H  R I P P L E  D ,  P„ S O R T E D
P O I N T  B A R
/  / • / ' /  ' / B  2 S A N D S T O N E ,  M I L K Y - B R O W N I S H  C O A R S E  G R A I  N, R  S O R  T E D
B 1
------------ 0 ____ S A N D S T O N E ,  M 1 L K V - B B 0 W N I S H ,  C O A R S E  C R A I N , C R O S S - B E C D E D
C H A N N E L  L A G
F ig . 6 . L ith o lo g ic  S u c c e s s io n  o f  lo c a t io n  B
cates a fining upward sequence. shales are carbonaceous, pyritized and contain 
The claystones are greenish to milky, well bed­ pieces o f lignitised wood. They are weathered and 
ded, m icaceous, fine grained and plastic. They are poorly laminated. Within the shale units are siltstone 
seen at the upper sequence o f the outcrop. The and daystone interbeds (Fig. 10).
T H I C K G E O L O G I C S A M P L E L I T H O L O G I C  D E S C R I P T I O N D E P 0 S I T 1 0 N A I
( m  ) S E C T I O N N U M B E R E N V I R O N M E N T
15
• * ° -0  • ° -
14 '
WEATHERED PEBBLY SANDSTONE P. SORTED FERRUGINOUS
13 ' • °  ’ *. 
0  , • 0 C H A N N E L  L A G
» C> •
12 . ^  • 0 -°
11 ■
— C I 3 SHALE CA R B . LIG N ITIZ E D  PYRITIZED P. LAMINATED
10 C12 ClAYSTONE GREENISH AND PLA STIC F L O O D  P L A I N•*
*------------------- Ctl SHALE C A R S ,  LI GN1TIZED /YRIT1Z ED  P.LAMINATED
9 C I O  • CLAYSTONE GREENISH V F GRAIN AND PLA STIC
C  9 CLAYSTONt WELL BEDDED .
C  8 CLAYSTONE GREENISH-MILKY PLASTIC
8 Q QJ C 7 SANDSTONE COARSED-MED GRAIN CONCRETIONARY AT TOP
• •
C 6 SILTSTONE RIPPLED, MILKY AND RIPPLED P O I N T  B A R
7 . . • • •
• ! • 9 C  5 c SANDSTONE THICKLY BEDDED COARSE MED GRAIN AND FINE UP
6 9 • • •• 
C  5 b SANDSTONE PINKISH MED-FINE W ITH  CLAY PARTING
5 C  5a SANDSTONE ANG, GRAIN C O A R S E-M ED  P -S O R TED C H A N N E L  L A G
C4 SANDSTONE CROSS BEDDED PEBBLY AT BASE AND FINE UP
/
C 3 SILTSTONE RIPPLED ROOT MOTTLED FINE GRAINED rDUn l1 NKl  T1  BDAADn
3 —  ’ * ~ MU05T0NE W IT H  FACIE CHANGE TO CARS.SH. PYRITIZED
C2 SANDSTONE C O AR SE M E D  GRAIN BROWNISH AND X-BEDDED
2
C H A N N E L  L A G
1 Cl SANDSTONE PE88LY AT BASE W E A TH E R E D  P. SORTED
• • • .•
— 2 - ____________
Fig. 7. lithologic Succession o f Awi Type Section (Location. C)
■f
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I Y OF IBADAN LIBRARY
c
o  .
o
- • o
28
! 1 1 i , ’•
' l ' ’ ' v,' ■
1 1 ' > • «
1 1 , ! ■ ‘ A, i
i •. i '• i .
- l-
■
>, 1 >• ■ b --.
.• ' * .
i!. ’• ’ ■ i i : • . . . i ■ ' . '
f . ■: J "II '■ V • ' .I',,;
y & W  ' . . . .  . , - i
} • i :: ■ I
Fig. 10. Field photograph showing poorly 1 
Fig. 8. Field photography showing fold laminated carbonaceous shales with claystones 
with E-W axis.
___________________ _____________________ and siltstones interbeds.
sandstone is dark, fine grained and micaceous. Apart 
from this sandstone unit, the remaining sequence 
was not sampled because o f extensive weathering 
effect.
Laboratory Studies
Granulometric analysis: Standard method for grain 
size analysis was employed using a set o f sieves ab 
V-2 phi intervals (ASTM) on a Ro-tap shaker for 15 
minutes.
The statistical parameters calculated were based 
on Folk and Ward (1957).
Clay mineralogy: Physical analysis (Plastic Limit and 
Liquid Limit) of the claystones was used for their 
identification as the readily available means. The 
results were superimposed on Bain's (1971) chart 
This location is assessible through Okoyong Usang for classification (Fig. 12). Further analytical studies 
Abasi - Mbarakom footpath off Calabar - Itu Road (see using x-ray diffraction for detailed mineralogical 
Fig. 2). The outcrop occurs along a stream channel assessment of the claystones and limestones within 
as a 'boulder'. The valley wall reveals a sequence this areas is in preparation.
m ade up o f  h igh ly w eathered  sandstones, Heavy mineral studies: Siltstones and sandstones 
claystones and carbonaceous shales (Fig. 11). The samples were analysed for their heavy minerals
T HIC K GEOLOGIC SAMPLE LITHOLOGIC DESCRIPTION
( ! DEPOSITIONm SECTION NUMBER ENVIRONMENT
■o • o ■ • o  d 
■° 0 ■ 0 'o' 0 WEATHERED PEBBLY P. SORTED FERRUGINOUS SANDSTONE CHANNa LAG
2- SHALE CARB .WEATHERED AND POORLY LAMINATED FtOOD PLAIN
Cl AYSTONE GREENISH F. CHAIN AND PLASTK
1- D sandstone dark grey indurated  AND micaceous POINT BAR
Fig. 11. Lithologic succession of location D.
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Thin section petrography); About ten representa­
tive samples o f sandstones and siltstones were se­
lected for thin section petrography. The indurated 
samples were thin sectioned directly but the few 
loosely consolidated ones were impregnated with 
resin before cutting and mounting on slides with 
araldites and Canada balsam. They were examined 
under the flat stage o f a petrological microscope. 
Point count method was used to count each mineral 
and based on this, individual percentages of the 
minerals were computed. Photomicrographs of fea­
tures o f interest were taken. Classification o f the 
sandstones is based on Folk (1974).
Micropalaeotological Analysis: The shale samples 
were subjected to foraminiferal processing using 
anhydrous sodium carbonate and 10% concentrated 
hydrogen peroxide. The sample were washed, dried, 
sieved and then picked for foraminifera. In this study, 
no microforaminifera was recovered.
RESULTS AND INTERPRETATION 
Grain Size Analysis
The Awi sandstones range from 0.32mm to 1.43mm 
and fall between medium to coarse sand. The 
standard deviation (dispersion sorting) values range 
using bromoform (S.G.2.85) as the separating me­ between 0.79 and 1.20 and are predominantly poorly 
dium. The bromoform extracts were rinsed with ac­ sorted. Skewness values are between 0.07 and 0.30 
etone, mounted on slides and studied under a pet­ and are mostly in the fine skewed range with one 
rographic microscope. Their sizes, shapes as well population near symmetrical. Kurtosis values range 
as the percentages of the opaques and non opaques between 0.74 and 1.69. Though leptokurtic type pre­
were estimated. Maturity index (ZTR) (Hubertt, 1962) dominates, it however has very few mesokurtic and 
was estimated for each sample. one platykurticpopulations (Table 1).
Table 1. Calculated Grain size Statistical Parameters for Sandstones
Sample First Graphic Dispersion Incl. Graph. Coefficient Kurtosis
No. Percentile Mean (Mz)___________Sorting (ct,)_________ Skew1 (SKI)_________________________ (KG)________________
B, -1.70 0.32 (Coarse 1.00 (Poorly 0.30 (fine 0.74 (Platykurtic)
sand) sorted) skewed)
-1.80 0.47 (Coarse 1.10 (Poorly 0.16 (Fine 1.34 (Leptokurtic)
sand) sorted) skewed)
B, -1.70 0.87 (Coarse 1.20 (Poorly 0.25 (Fine 1.03 (Mesokurtic)
sand) sorted) skewed)
c , -1.00 1.43 (Medium 1.04 (Poorly 0.07 (Near 1.16 (Leptokurtic)
sand) sorted) symmetrical)
Cm -0.90 0.60 (Coarse 0.79 (Moderately 0.16 (Fine 1.69 (Very Leptokurtic)
sand) sorted) skewed)
C*: -0.75 1.20 (Medium 1.15 (Poorly 0.26 (Fine 1.01 (Mesokurtic)
sand) sorted) skewed)
C7 -1.20 0.87 (Coarse 1.06 (Poorly 0.17 (Fine 1.17 (Leptokurtic)
sand) sorted) skewed)
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Histograms and frequency distributions of the Thin Section Petrography: The result o f petro­
sandstones show strongly unimodal character. Sta­ graphic analysis is showii in Table 2. From here, it 
tistical plotting o f the grain size data based on can be seen that both metamorphic and common 
Friedman (1961) and Folk (1974) show that they fall quartz are present with a total range o f 70 to 80%, 
within river sands and fluvial setting respectively The common quartz show angular to subangular 
(Figs. 13 and 14). The probability ordinate plot (Fig. shapes (Fig. 16). Some of the quartz grains have 
15) contain the upper two segments corresponding equant graihs with distinct straight boundaries (Fig. 
to suspension and saltation loads of Visher (1969). 17). In addition, some of the quartz grains are poly­
crystalline and show strains in different directions 
(Fig. 18).
The feldspars are not fresh and are mainly potas- 
sic type with little plagioclase embedded in the 
matrix. They range from 8 to 13.2%. Rock fragments 
comprise polycrystalline quartz, granite and 
mudclast. The igneous and metamorphic rock frag­
ments are angular to subangular while the mudclasts 
• ^ • are silty and clayey. The range o f the rock fragments 
is 0.1 - 1.3%.
Riv#r *and The matrix is between 2.8 - 7.1 % and is composed 
of kaolinite clays and silt-size feldspars. Most of the 
\ matrix fill the pore spaces and reduce the porosity Beach 
san 4 \ which range from 1 to 8.7%. Iron oxide and calcite 
\ constitute the cement which range from 0.3 to 7.4%.
\
\ From the recalculated framework composition of 
0 ..»2  0-4 0-6 ' 08 V0 1-2 U quartz, feldspars and rock fragments, the sandstone 
t T A N D A M  DC V LA T ION ( M W T I H D ) are therefore classified as subarkose (Folk, 1974) 
(Fig. 19). From the high matrix content (>5%), the 
poor sorting and angular to subangular grains, the 
Figs. 13. Plot of third moment (Skewness) and Awi sandstones are texturallyimmature (Folk, 1951). 
standard deviation for beach and riversands. From the quartz: Feldspar + Rock fragment ratio 
After Friedman, 1961) (Folk, 1974), the sandstones are mineralogically 
submature. ^
Fig. 14. Plot of sorting versus mean size; solid circles show plots of Awi sandstones
(Modified after Folk 1974)
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Table 2. Petrographic Analysis Data
Sample Grain % % % °/o % % % %
Mo size (p.) CQ MQ Feldspar R.F. Mica Matrix <ti Cement
A 1 >20 5 71 9.7 0.3 0.4 6 1 6.6
A3 >20 60 17 8 0.1 - 2.8 6 6.1
A4 >20 62 1 4 8.8 0.3 0.1, 6. 1 8.7 -
C 1 >20 70 8 10 - - 6 - 6.0
C5c >20 15 55 13.2 0.7 1.1 5 3 7.0
C4 >20 64 1 2 12 - 0.4 7 4.3 0.3
C6 >20 1 3 63 9.6 0.2 0.4 - 6.4 7.4
C7 >20 14 66 10 0.5 0.3 5.7 4.3 -
CIO >20 66 10 8.6 0. 1 0.4 3.6 5.2 6.1
D >20 14 58 9 1.3 0.2 7.1 4.4 6.0
CQ = Common Quartz; MQ = Metamorphic Quartz; R.F. = Rock Fragment,- <t> = Porosity
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Fig. 16. Photomicrograph of poorly sorted to Fig. 18. Photomicrograph showing polycrystal­
subangular common quartz grains (Q). line grains with strains in different directions.
Table 3. Data o f Physical Analysis o f Clay Samples
Samples Liquid Plastic Plastic
Nos. Limit Limit Index
S 30 21 10
S 45 20 25
P. 70 36 34
So 55 32 23
55 29 25
Heavy Mineral Petrography
In decreasing order o f abundance, the opaque 
minerals are hematite, leucoxene, ilmenite and 
pyrite. The non opaques are mainly zircon, 
tourmaline, rutile, garnet and staurolite (Table 4).
Fig. 17. Photomicrograph of equant grains with The ZTR index ranges from 66.7 to 96% with the 
straight grain boundaries in different directions ultra stable population increasing in the order of 
abundance, zircon tourmaline and rutile. The range 
Clay Mineralogy indicates mature to supermature sandstones.
Clay mineral studies in the Calabar Flank is still in Pettijohn (1941) has explained that younger sedi­
progress. However, in this study, claystones sampled ments generally contain a greater number o f differ­
from two locations were identified as kaolinite by ent heavy mineral species than the older ones. He 
the superimposition of Atterberge limits (plastic limit observed that heavy minerals o f many Palaeozoic 
and plasticity index) on Bain (1971) Chart (Fig. 12 sediments consist almost entirely o f Zircon, tour­
and Table 3). This method of classification was used maline, rutile and garnet. This he attributed to 
as the readily available means. intrastratal solution by underground water as older 
Several workers have pointed to the fact that ones are more susceptible to this.
weathering of feldspathic rocks in warm humid tropi­ Hoque and Ezepue (197 7 ) ob served  that 
cal climates tend to produce materials essentially sandstones attain higher mineraldgical them textural 
rich in kaolinite (Adeleye, 1978; Ajayi and Agagu, maturity due to intense weathering both at the source 
1981). and during transport.
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Fig. 19. Ternary classification o f sandstones based on framework components.
Table 4. Results of Heavy Mineral Analysis (Point Count)
Sample Opaque Mon-Opaque Zircon Tourmaline Rutile Oamet Staurolitc ZTR Index
no. (0/0) (%) (%) (%) (%) (%) (%) (%)
A3 50 50 30 30 36 2 2 96
B1 85 15 - 26.7 53.3 - • 80
B3 85 15 6.7 26.7 53.3 13.3 - 86.7
C3 87 13 15.4 23.1 53.8 7.7 - 92.3
C5A 84 16 12.5 37.5 37.5 12.5 • 87.5
C5B 90 10 10 50 20 - 20 80.0
C5C 85 15 20 20 26.7 33.3 ■ 66.7
C7 95 5 - 40 40 20 - 80.0
D 95 5 • 40 40 20 - 80.0
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DISCUSSION evidence. The mineralogical maturity was attained 
Provenance due to humid climatic conditions that characterised 
The Awi sandstones are poorly sorted and texturally the area. (Adeleye 1978). Visher (1965) has de­
immature. Such sediments according to Amaral and scribed kaolinite as a clay mineral type more 
Pryor (1977) are deposited under variable current abundant in fluvial sands. This is consistent with 
velocities. Folk(1974) has reported that these kind this study.
o f sediments are usually deposited near to the The Awi sediments display a cyclical fining up­
source. The sandstone are generally leptokurtic and ward sequence which is characteristic of fluvial 
unimodal in distribution which indicate a single environment (Allen, 1965, Visher 1965; Adeleye, 
source. 1974). Okoro and Piw^jide (1990) have associated 
The presence o f both plutonic and metamorphic, cyclic sedimentation and fining upward sequence 
quartz point to a source in basement terrain. The with climatic fluctuation, intermittent tectonic uplift, 
angularity o f the grains particularly among the basin subsidence as well as eustatic sea level 
coarsest fraction suggests nearness to source rocks changes.
which is igneous and/or metamorphic (Krumbein and Beerbow (1964) discarded the invocation of such 
Pettijohn, 1932). The maturity of the heavy minerals external processes as causes of cyclicity particu­
which is a variance with the angular nature of the larly if it is of a local extent. He rather pointed to the 
grains can be associated with humid tropical to and fro migration of a river channel across its flood 
conditions which have depleted the metastable plain coupled with isostatic adjustment of the basin 
components (Adeleye, 1978, Agagu, 1990, pers. as their causes.
comm). Blatt et al. (1980) have described zircon as In the area of study, the sequence beginning with 
the most ubiquitous, non-opaque minerals in silica- pebbly conglomerates which contains mud chips and 
rich rocks. Rutile is unstable in low grade leaf fragments are indicative of channel lag depos­
metamorphic rocks (Force, 1970). Tourmaline is its. Overlying this basal lag is cross bedded coarse 
known to occur in low grade metamorphic and acid to medium grain sandstones that fine upwards to 
igneous rocks. Garnet is found in pegmatites as well siltstone and mudstone. This unit belong to the point 
as in high grade metamorphic rocks and staurolite bar. The occurrence o f lignite remains and pyrites 
is highly diagnostic o f metamorphic source (Folk, in the carbonaceous shales and claystones are in­
1974). The Oban Massif which borders the Awi dicative of flood plain deposits.
Formation to the north has several rock types such With bank caving and accretion, there is bound to 
as granites, d iorites, gneisses, quartzites, be migration in the channel resulting in the 
pegmatites etc. (Rahman et al. 1981) and could be sequence becoming cyclical. In such a setting, 
the source o f the Awi sediments. The predominant conglomerates are deposited in the deeper part of 
direction o f sediment transport is southwards. the channel while the finer siltstones and claystones 
According to Adeleye (1978), Ajayi and Agagu accumulate in flood plains as abandoned channels 
(1981), kaolinite clays are formed as a result of (Le Blanc, 1972). This view is envisaged in this study.
weathering o f feldspathic rocks in warm tropical in The porosity values o f 1 to 8.7% are very low 
climate. A similar setting is envisaged for the when viewed in line with the fact that a freshly de­
formation o f kaolinite found in the Awi Formation. posited sand has porosity values between 40 and 
Whiteman (1981) has pointed out that sandy del­ 45% (Manus and Cogan, 1974). However the low 
taic facies from Gboko merged with coarse sandy values could be attributable to slight compaction 
clastic of the Mamfe Embayment and extended south­ which results in the straining of the quartz. In addi­
wards around the Oban Massif. From this study, the tion, diagenetic alteration of feldspars results in clay 
grains angularity together with the felspar and ma­ matrix which also reduces the porosity.
trix content o f > 5% has suggested near source. In The Awi sediments can thus be said to be cyclic 
essence, sandy deltaic facies from Gboko could not sediments caused by channel switching and depos­
have come a longway with such features retained. ited in a meandening setting.
The Oban Massif is therefore the closest and pos­
sible source of the Awi sediments. SUMMARY AND CONCLUSIONS
The Awi sediments, made up of conglomerates, sand­
Depositional Environment stones, siltstones, m udstones, shales and 
The Awi sandstones which are texturally immature claystones are cyclical with fining upward sequences.
and mineralogically mature were deposited in fluvial The sandstones are texturally immature and indi­
setting. Inference from statistical size distribution cate short distance o f transportation. They are 
and parameters of scatter plots corroborate this subarkosic with the quartz showing contributions
UNIVERSITY OF IBADAN LIBRARY
3S;
from both igneous and metamorphic sources. The inferred that as the Awi elastics were prograding as 
clay mineral is kaolinite and indicates weathering non marine sediments, marine transgression depos­
of feldspathic rocks under warm humid setting. ited the Mfamosing Umestone. Formation on it thus 
Heavy mineral assem blages point to super .matu­ altering the setting in the Calabar Flank. The deposi- 
rity attributed to wet climatic conditions. tional sequence is similar to the Early - Middle Cre­
The basal conglomerates containing mud chips taceous sequence o f eastern Piorth Atlantic conti­
and leaf fragments are indicative o f channel lag nental margin described by Schlee and Jansa (1981).
deposit. The overlying cross bedded coarse - grained 
san d sto n es  with fining .upw ard sequ en ce  to ACKJ^WLEDGEMEmS
siltstones and mudstones are point bar sediments. I am greatly indebted to Dr. Q.I. Unomah who super­
The shales and claystones that are ripped and contain vised this work as part of my M.Sc. project. The anony­
lignites and pyrites are deposits of flood plains. mous reviewers are also acknowledged for their 
From  the fo rego in g , the Awi Form ation is useful criticisms while Prof. A. A. Elueze cannot be 
riverbom e sediments from the nearby Oban Massif* forgotten for his encouragement.
and deposited in a meandering setting. It can be
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