PEBBLE MORPHOMETRIC ANALYSIS OF THE … AWI FORMATION, CALABAR FLANK, SOUTHEAST NIGERIA ESSIEN, NSE...
Transcript of PEBBLE MORPHOMETRIC ANALYSIS OF THE … AWI FORMATION, CALABAR FLANK, SOUTHEAST NIGERIA ESSIEN, NSE...
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PEBBLE MORPHOMETRIC ANALYSIS OF THE BASAL SECTION OF
THE AWI FORMATION, CALABAR FLANK, SOUTHEAST NIGERIA
ESSIEN, NSE UDO; ITAM, ASUKWO ESSIEN; OYAMA, ADAMS A , AND OBIM, V.N DEPARTMENT OF GEOLOGY UNIVERSITY OF CALABAR, CALABAR, NIGERIA
Abstract:A pebble morphometric study was carried out on 400 pebbles samples retrieved from
conglomeritic deposits of the basal section of Awi Formation, Calabar flank, exposed along Calabar-
Oban road, Southeastern Nigeria. The three orthogonal dimensions of Long (L), Intermediate (I) and
Short(S) axes were measured and the various pebbles parameters computed. The computed pebble
morphometric parameters have ranges of values as follow; Flatness Ratio (FR =0.51 – 0.67), Flatness
Index (FI=51% - 67%), Elongation Ratio (ER= 0.70 – 0.83), Maximum Projection Sphericity Index
(MPSI= 0.72 -0.82), and Oblate – Prolate Index (OPI = -1.12-2.48) these values strongly suggest
fluvial deposits. Roundness Index determined based on Sames chart ranges between 43% and 52%
(sub angular to subrounded) and inferred short to long intermediate distance of transport from the
provenance. The diagnostic forms for pebbles of fluvial origin (Compact, Compact Bladed, Compact
Platy and Compact Elongation) of 68% predominate over beach origin (Very Platy, Very Bladed,
Bladed and Platy) of 23%. The bivariate plots of FI versus MPSI and MPSI against OPI, agreed with
this result. The results from the study area favours continental setting of dominantly fluvial origin.
Keywords: Pebble morphometric, Calabar flank, Awi Formation, provenance, fluvial origin.
I. INTRODUCTION
The recent excavations as a result of the construction and expansion of United Cement Company
(Unicem) Southeastern Nigeria (figure 1), exposed good sections of conglomeratic sand/ironstone
unit of the basal unit of the Awi Formation. The pebbly basal unit have been studied and subjected to
pebble morphometric investigation. The study area has not received any geologic attention due to the
fact that it was covered with thick layers of overburden. Pebble morphometric research studies have
has shown that pebble form indices or parameters are good indicators of paleodepositional
environment ( Sames, 1966; Stratten,1974; Nwajide and Hoque,1982; Els,1988; Inyang and Enang,
,2002; Odumodu and Ephraim, 2007a and 2007b ; Odumodu, 2014 ; Itam et al ., 2015 and Itam and
Ugar, 2016) This study will therefore attempt to use the different unweathered quartz pebbles
encountered in the investigated area to interpret the paleodepositional environment of the deposit .The
results obtained will therefore be compared with results obtained in other part of the basin previously
investigated.
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Figure 1: Map of the study area showing location of the samples.
Location of the Study Area
The area of study is located along Calabar - Oban Road (figure 1), Cross River State, Southeastern
Nigeria. The geographic position lies within latitude N05°06ʹ 42 ́ ʹ to N05 10 ́ 05ʹʹ and longitude E008°
30 ʹ 50ʹʹ to E008° 34 ʹ 57ʹʹ and is part of Calabar Flank basin.
II. GEOLOGICAL SETTING
The Calabar Flank is an epirogenic sedimentary basin in southeastern Nigeria (Murat, 1972).The
basin according to Nyong (1995) is bounded by the Oban Massif in the north with the Calabar hinge
line separates the basin from the Niger Delta basin in the south and the Ikpe platform and Cameroon
Volcanic Line delineate it in the west and east respectively (figure 2). The origin of this basin is
associated with the opening of the South Atlantic in the Cretaceous times when the South American
plate drifted away from African plate. The major structural elements within the basin include the Ikang
Trough (graben structure) and Ituk High (horst) which were mobile depression and stable mobile
submarine ridge that influenced the distribution sedimentary of facies (Murat, 1972 and Nyong, 1995).
The stratigraphic succession in the Calabar Flank is shown in Table 1. Sediment thickness is over
3500m with the onlap (or featheredge) of the outcropping units exposed along the fringes of the Oban
Massif basement complex. The Formations are best exposed along Calabar –Ikom road and a
succession consists of five (5) Cretaceous and a Tertiary lithostratigraphic units. Awi Formation is the
oldest basal unit and sits nonconformably on the basement complex of Oban Massif. The Formation
is Aptian in Age (Adeleye and Fayose, 1978).This is overlain by Mfamosing Limestone of Middle-
Upper- Albian age (Petters, 1982) deposited during the first marine transgression in the South Atlantic.
This in turn is succeeded by Late Albian- Cenomanian to Turonian, Ekenkpon Shale. Subsidence on
the faulted blocks of horst and graben allowed wide spread deposition of shales with minor marl and
mudstone intercalation. The New Netim Marl which is Coniacian in age, succeeded the shale unit. The
Santonian period was marked by a major unconformity in the Calabar Flank. Nkporo Shale of Late
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Campanian to Early Maastrichtian (Edet and Nyong, 1994) capped marine transgression and
Cretaceous sedimentation in Calabar Flank .The Tertiary continental sands and gravel of the Benin
Formation completes the sedimentation in the basin (Table 1).
Figure 2: Structural map of the Calabar Flank and adjoining areas.
(Modified after Nyong and Ramanathan, 1985)
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Table 1: Lithostratigraphic correlation between Calabar Flank. Abakaliki Trough,Anambra Basin and
the Middle Benue Trough (Petters et al;2010)
III. Materials and Methods
A total of four hundred (400) unweathered quartz pebbles were collected with fifty (50) pebbles each
from eight (8) different locations (L1-L8). Pebble morphometric measurement using Vernier caliper
after Krumbeins (1941) method was used. The Long (L), Intermediate (I) and Short (S) axes of the
pebbles were measured. Pebble indices such as Flatness Ratio (FR) Flatness Index (FI), Elongation
Ratio (ER), Maximum Projection Sphericity Index (MPSI) and Oblate Prolate Index (OPI) with the
visual estimation of pebble roundness using Roundness Chart of Sames (1966).The results obtained
were compared with those recorded in other localities previously studied.
IV. RESULTS AND DISCUSSION
Field description of the study area
The lithologic description of study area consists of conglomeratic sandstone, pebbly sandstone and
ironstone (figure 3).The conglomeratic sandstone is reddish brown, clast supported to matrix
supported, imbricated conglomerate, with coarse – medium, poorly sorted. The lithologic cross section
of outcrop sample in location 5 (L5) is about 2.0m thick and made up of pebbly sandstone ( figure 3a)
.This unit is characterized by reddish brown, dominantly angular to subrounded clast supported at the
base to matrix support at the top. The top of this unit is texturally characterized by mudstone units.
The sandstone is coarse to medium, with poorly sorted grains. The overall sequence represent fining
upward succeession (graded bed). At locations 6-8 (L6-8) the thickness of the outcrop units in local
quarry ranges from 3.8m to 4.0m (figure 3b) and consists of pebbly sandstone and a thin bed of reddish
brown ironstone. The pebbly sandstone exhibits similar characteristics features as previously
described.
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Thickness Lithology DescriptionDepositionalEnvironment
1.9
m
Flu
via
l -
Ch
ann
el
Dep
osit
Red
dis
h b
row
n,
matr
ix s
up
po
ted
,p
oo
rly
so
rte
d,
co
ars
e -
med
ium
gra
inp
eb
bly
san
dsto
ne
Thickness Lithology Description
Flu
via
l -
B
ea
ch
Deposits
Reddish browncoarse - medium
grain
Reddish browniron stone
0.96m
0.4m
1.4
m
.
Depositional Environment
Reddish brown, matrix suppoted,
poorly sorted, coarse - medium grain
pebblysandstone
Reddish brown, matrix suppoted,
poorly sorted, coarse - medium grain
pebblysandstone
3a
3b Figure 3: Lithostratigraphic cross sections of the mapped area in: (a) location 5, L5 (b)
location 6 – 8, L6 – L8
3b
Figure 3: Lithologic cross sections of outcrop samples from the study area of:
(a) Location 5, L5 (b) Location 8, L8
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Pebble morphometric parameters
The average results of the various computed pebbles morphometric parameters and roundness values
are presented in Tables 2 and 3
Table 2: Average pebble morphometric parameters from the various study locations.
LOCATION L(cm) I(cm) S(cm) FR FI ER L-I/L-S MPSI OP R (%)
L1(n=50) 4.07 3.15 2.39 0.59 59.00 0.78 0.44 0.76 -0.20 44.34
L2(n=50) 4.03 3.11 2.22 0.55 55.38 0.78 0.51 0.73 -0.01 49.42
L3 (n=50) 3.52 2.91 2.29 0.65 64.96 0.83 0.51 0.79 -0.03 50.22
L4(n=50) 3.90 3.21 2.62 0.67 67.40 0.82 0.56 0.82 0.80 49.92
L5 (n=50) 3.67 2.81 1.94 0.54 53.76 0.77 0.49 0.72 -0.11 43.66
L6 (n=50) 3.73 2.93 2.11 0.56 56.32 0.79 0.38 0.74 -1.12 45.18
L7 (n=50) 3.94 3.25 2.37 0.60 59.94 0.82 0.44 0.75 -1.08 52.48
L8 (n=50) 3.47 2.39 1.75 0.51 50.94 0.70 0.63 0.72 2.48 43.40
OVERALL MEAN 3.79 2.97 2.21 0.58 58.46 0.79 0.49 0.75 0.09 47.33
Legend: L=Long axis, I =Intermediate axis, S = Short axis, FR =Flatness Ratio, FI= Flatness Index, ER
=Elongation Ratio, MPSI =Maximum Projection Sphericity Index, OPI = Oblate Prolate Index, R
=Roundness, n = number of samples per location.
Table 3: Characteristic features and paleoenvironmental significant of the various computed parameters.
Pebble
morphometric
parameters
Characteristics Defined limits from
previous studies
Interpretation of
depositional
environment/
processses
Flatness Index (FI) FI consists of over 84% above
fluvial limit and 16 % below
this limit
Lutig, 1962
Beach (<45%)
Fluvial (> 45%)
Predominantly fluvial
Elongation Ratio
(ER)
Over 78% has values between
0.60 -0.90
Hubert,1968
Fluvial(0.6-0.9)
Fluvial processes
Maximum
Projection
Sphericity Index (
MPSI)
85% of the MPSI sampled
occurred above the imaginary
limit (0.66)
Dobkins and Folk, 1970
Beach (< 0.66)
Fluvial (> 0.66)
Predominantly fluvial
Oblate Prolate Index
(OPI)
Characteristics by 69% of the
OPI value plotted above -1.5
limit line and 33% below
Sneed and Folk,1958
Beach ( < -1.5)
Fluvial ( > -1.5)
Dominantly fluvial
with few beach
influences.
Roundness( R) 15% has values below 0.35
,while 35% has values above
0.45
Sames,1966
Fluvial(< .35%)
Littoral(>0.45%)
Mainly littoral/beach
action with little
fluvial
Form Geometry 21%C, 7.25%E, 24%CB,
15%CE,3.5%P, .75%VB,
0.5%VP,17.25%B, 8.75%CP,
VE1%
Sneed and Folk, 1958;
Dobkins. and Folk, 1970
and Gale, 1990.
Fluvial(C,E,CB, CE)
Beach (P,VP,VB,B)
Dominantly fluvial
with few beach
influences.
The interpretation of these results are carried out in conjunction with other works (Sneed and Folk,
1958 ; Lutig 1962; Sames, 1966; Hubert, 1968 ; Dobkins. and Folk,1970 ; Gale, 1990; Odumodu and
Ephraim 2007a and b ; Ogala,et al,.2010; Udo and Mode, 2013; Odumodu,2014; Itam and David,2015;
Itam et al ., 2015 and Ideozu and Ikoro,2015).The result obtained show that the mean Flatness Ratio
(FR) has value of 0.58. The FI ranges from 50.94% to 67.40% with average mean of 58.46% indicating
International Journal of Recent Trends in Engineering & Research (IJRTER) Volume 03, Issue 01; January - 2017 [ISSN: 2455-1457]
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fluvial depositional environment (Lutig, 1962). The mean ER for pebbles evaluated is 0.79 which is
approximately within the 0.60-0.90 range for fluvial setting (Hubert, 1968 and Itam and Inyang, 2015).
The mean value of MPSI in the study area is 0.75 and ranges from 0.72-0.82. All the sampled pebbles
have values above the 0.66 limit (Dobkins and Folk, 1970), that separates fluvial from beach pebbles
(Tables 2 and 3). These values suggest fluvial depositional environment. The OPI ranges from -1.12
to 2.48 with a total mean value of 0.09. These values are greater than -1.5; lower empirical limits
(Dobkins and Folk, 1970), which distinguishes beach dominated pebbles from river. This analysis
shows the dominance of fluvial influence over beach.
From the analysed data the mean roundness values is 47 % (43%-52% implying angular to
subroundness).This infer short/intermediate travel distance of the pebble from the provenance. This
sediments are likely to be sourced from the weathering of the basement rocks of the adjoining Oban
Massif Nigeria and intrabasinal activities (Itam and David, 2015).
The results obtained from the pebble morphometric analysis were further analysed using bivariate
plots. This was carried to further discriminate the paleoenvironment of deposition. Morphometric plots
of FI against MPSI (Stratten, 1974) and MPSI versus OPI (figure 4a and b) of all the pebbles sampled
point to predominantly fluvial influence. Also, the bivariate plot of Roundness (R) against Elongation
ratio (ER) in figure 4c using the upper limits of 35% and 45% Roundness of pebbles shaped in river
and littoral environment of Sames ( 1966 ) and the fluvial limit of Elongation ratio of 0.60 to 0.90 for
pebbles influence by fluvial processes (Hubert,1968 ), this infer fluvial–beach environment.
4a
0
20
40
60
80
100
120
0 0.2 0.4 0.6 0.8 1 1.2
FLA
TNES
S IN
DEX
MPSI
FLUVIAL
BEACH
BEACH FIELD
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4b
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
-10 -8 -6 -4 -2 0 2 4 6 8 10
MP
SI
OPSI
Fluvial
Beach
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4c
Figure 4: Bivariate plots of; (a) FI against MPSI (b) MPSI against OPI (c) R against ER
According to (Sneed and Folk, 1958; Dobkins. and Folk ,1970 and Gale, 1990 ), Compact (C),
Elongation ( E), Compact Bladed (CB) and Compact Elongate (CE) are most indicative of fluvial
action whereas Platy( P), Very Platy ( VP),Very Bladed (VB) and Bladed ( B ) are diagnostic of beach
setting (C), Elongation ( E), Compact Bladed (CB) and Compact Elongate (CE) are most indicative of
fluvial action whereas Platy( P), Very Platy ( VP),Very Bladed (VB) and Bladed ( B ) are diagnostic
of beach setting .The result of the analysed pebbles values of 21%C, 7.25%E, 24%CB, 15%CE,
3.5%P, 1.75%VB, 0.5%VP, 17.25%B, 8.75%CP,and 1%VE (figure 5).These values point to
predominantly fluvial than beach processes in the shaping of the analysed pebbles in the study area.
Comparing this result obtained with those recorded by previous authors (Itam and Inyang, 2015 and
Itam et al; 2015) in the distant location of the Awi Formation, the result is in agreement with
predominantly fluvial over beach processes.
0
20
40
60
80
100
120
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
RO
UN
DN
ESS
ELONGATION RATIO
Hubert , ER limit
Sames % R<35%
Sames % R>45%
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Figure 5: Pie chart showing form names from the study area
Legend: C= Compact, CP =Compact Platy, CB = Compact Bladed, CE = Compact Elongate,
P=Platy, E= Elongation, VP =Very Platy, VB =Very Bladed, VE= Very Elongate
V. CONCLUSION
The pebble morphometric parameters have proven to be good indicators for distinguishing
paleodepositional environments in situations where fossils are lacking especially in a continental
setting such as Awi Formation. The various parameters obtained in the pebble morphometric analysis
together, with the different bivariate scatter plots have shown that the depositional environment of the
basal of the section of the Awi Formation is predominantly influence by fluvial processes with beach
/ littoral influence .The results obtained corroborate with the previous work, carried out on elsewhere
within the Awi Formation
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