Journal of Earth Sciences and Geotechnical Engineering, vol. 6, no.2, 2016, 63-71
ISSN: 1792-9040 (print), 1792-9660 (online)
Scienpress Ltd, 2016
Topography and Particle Size Gradation
Characterization of the Benin Formation in the Lower
Niger Delta, Nigeria
Abam 1 T.K.S, Akpe A. C. 2 and Oba T2
Abstract
The Benin Formation as a significant geologic unit of the Niger delta is well known.
However, its importance in engineering geology, as a veritable source of construction
material and foundation layer to many civil structures is hardly discussed. This study
utilized the records from over 250 geo-referenced boreholes distributed mostly on linear
alignments to the east and west of the Niger delta to generate the surface longitudinal
profile of the Benin Formation in both the west and the eastern sections of the Niger
Delta. In addition, particle size gradation characteristics of the sediments were analyzed to
detect possible trends that reflect the predominant mechanisms of transportation. The
study shows an initial steep gradient from Agbor to Sapele to the west, followed by a
gentler and almost flat profile, with occasional depressions that are infilled with silty clay
sediments, that may be linked to buried river channels. To the east, the longitudinal
profile mirrors the ground surface, with a gradient approximating 1.91x10-4 as a first step
in determining the depth to sand in the study area. The particle sizes within the depths of
civil engineering significance (0-40m), show a tendency of upward fining. The
Uniformity coefficients show some constancy, suggestive of similar environmental and
energy conditions of deposition.
Keywords: Benin Formation, Topography, particle size distribution, uniformity
coefficient
1 Introduction
The geology of the Niger delta is dominated by a threefold lithostratigraphic sequence
comprising an upper sandy Benin formation, an intervening unit of alternating sandstone
and shale named the Agbada formation, and a lower shaly Akata formation (Short and
1
Institute of Geosciences and Space Technology, Rivers State University of Science and
Technology, PMB 5080, Nkpolu-Oroworukwo, Port Harcourt, Nigeria.
2
Groundscan Services Nigeria Limited, 1 Abiye Brown Avenue, Trans-Amadi, PortHarcourt,
Nigeria.
64
Abam T.K.S, Akpe A. C. and Oba T
Stauble 1967). These three units extend across the whole delta and each ranges in age
from early Tertiary to Recent.
Sediments originating from the vast and geologically complex hinterland are dispersed
through the delta by river, tidal, wave, and currents in the continual evolution of the Niger
Delta. According to Allen (1965) Niger Delta growth began during the Late Wisconsin
lowstand of the sea when the rivers entrenched the continental shelf to reach mouths
above submarine canyons at the shelf edge. He identified the oldest stratigraphical unit of
the Late Quaternary deltaic pile as the strand plain sand (Older Sands) representing a
marine transgression into the hinterland. Later, in the Holocene, when sea-level became
relatively stable, there was a regressive advance across these sands giving rise to the
Younger Suite, the uppermost stratigraphical formation in the deltaic lens. The
Lithofacies of this suite are said to grade upward from open shelf clays, through pro-delta
slope layered clays, silts, and sands, to well-bedded sands formed on the delta-front
platform, river mouth bars, and beaches (Allen 1965).
The uppermost part of the Benin Formation has been altered by interglacial transgressions
during the Quaternary period (Oomkens, 1974). The upper layers of the Late Quaternary
Delta are comprised of inter-bedded layers of sand, silt and clay deposited during changes
in sea level (Allen 1970). The most recent post-glacial sea level rise covered the earlier
sediments with deposits of Holocene sediments.
The Benin Formation is better known for its hydraulic properties and groundwater
yielding potential (Amajor 1991; Ophori 2007 ; Nwankwoala and Ngah 2014 ) because of
the predominantly uniform and granular composition, which allows for reasonably high
permeability. Since it pervades across the N/D and it’s relatively on the surface, it is
encountered by most physical infrastructural developments that entail significant
foundation engineering works. This makes the understanding of its geotechnical
properties imperative.
Behind the beach ridges and barrier islands is the tidal mangrove swamps in which
organic-rich sands and silts are being deposited. Cross-stratified river bar sands are also
accumulating in association with top-stratum silts and clays in the delta floodplain
environment. This younger suite overlie the Benin Formation and constitute the
superficial layer on which many civil structures such as houses and towers, roads and
bridges, ancillary structures for the oil and gas sector are built.
The costs and performances of such civil structures usually depend on the thickness and
geotechnical properties, including the competence of the layer as well as proximity to the
Benin Formation. It is in this context that the topography of the Benin Formation becomes
pertinent. The Benin Formation, consisting largely of sandy units, exhibits significantly
high intergranular stresses, moderate to high bearing capacity, lower compressibility and
consequently settlement, moderate to higher pile axial capacity, attributes that make
occurrence of sand on the surface to be profound interest.
Secondly, the sandy unit of the Benin Formation is seen as a source of construction
material, for filling in reclamation projects, as road sub-base and in cases where subgrade
is to be replaced, for sandcrete, shoreline nourishment and as a constituent of asphalt for
road pavements. Where sand is not within economic depth, sourcing of sand either
through barging or other means of transportation can disproportionately escalate project
cost. Lateral or vertical proximity to sand is therefore of paramount significance.
Topography and Particle Size Gradation Characterization of the Benin Formation
65
2 Methodology
Boreholes through three transects running from cawthorne channel near Bonny to
Obrikom (190km), Agbor through Sapele to Omadino and further south to Escravos
(188km) and from Brass through Ubeta to Obrikom were correlated. A total of 250
borehole records were utilized, broken down as follows; 183 for cawthorne channel to
Obrikom and 66 for the Agbor to Escravos section. The elevations of boring locations and
the first occurrences of significant sand units were measured. The topographies of both
ground surface and top of major sand units along these transects/ alignments were
generated to reveal the surface expression of the “Benin Formation”.
Soil samples retrieved from borings at different locations on the alignment were subjected
to particle size gradation analysis using sieves in order to reveal possible vertical trends in
particle sizes and uniformity coefficients.
3 Results and Discussion
The topography of the Western section of the Benin Formation (Agbor to Escravos ) in
comparison to the ground surface is presented in Fig. 1. This section is characterized by a
fairly elevated upstream part with some significant gradient up to the Sapele end, where
changes in ground surface elevation become gradual. The topography of the Benin
Formation (Agbor-Sapele) somewhat reflects the significant slope of the ground surface
and to a large extent almost coincides with the topography of the ground surface, but for
some overburden sediments comprising silty and sandy clay. The thickness of this
overburden is at a minimum at the points of inflexion in the gradient of the ground
surface. The exposure of this section of the Benin Formation to interglacial
transgressions during the Quaternary period (Oomkens, 1974) may be responsible in part
to its alteration and current erosional status.
Changes in elevation of the Benin Formation underlying the coastal estuaries and
mangrove swamps shown in (Fig. 2) and amplified in Fig. (3) are generally within 30m,
the deepest sections usually suspected to be abandoned or buried old river channels.
In some areas, for example, the Abitiye Canal segment, sandy layers “supposed Benin
Formation” are exposed on the surface in the form of a coastal Island, and provide a ready
source suitable construction aggregates.
66
Abam T.K.S, Akpe A. C. and Oba T
120
100
Agbor
TOPOGRAPHY OF GROUND SURFACE AND BENIN FORMATION FROM AGBOR-SAPELE-OMADINO TO ESCRAVOS
0
-20
Escravos
Abiteye Canal
20
Asafama Canal
Obodo Community
60
Elevation
40
(amsl m)
Otumara Creek
80
64 + 000
62+000
63 + 000
60+000
61 + 000
58+950
54+850
58 + 000
52 + 000
50 + 000
51 + 000
48+500
49 + 000
48 + 000
46+000
47 + 000
44+000
43 + 000
42 + 000
41 + 000
40 + 000
37+500
39 + 000
36 + 000
35 + 000
34 + 000
33 + 000
22+775
29+200
18 + 000
17 + 000
16 + 150
-53
-60
-105
-141
-40
Figure 2: Topography of Benin Formation in comparison to the ground surface in the
Western Niger delta
TOPOGRAPHY OF GROUND SURFACE AND BENIN
FORMATION FROM OMADINO TO ESCRAVOS
15
10
5
0
Elevation
-5
(amsl m)
-10
-15
-20
64 + 000
63 + 000
62+000
61 + 000
60+000
58+950
54+850
58 + 000
52 + 000
51 + 000
50 + 000
49 + 000
48+500
48 + 000
47 + 000
46+000
44+000
43 + 000
42 + 000
41 + 000
40 + 000
37+500
39 + 000
36 + 000
35 + 000
34 + 000
33 + 000
29+200
22+775
18 + 000
17 + 000
16 + 150
-25
Figure 3: Amplified section of Topography of Benin Formation within the Estuaries and
Mangrove swamps Western Niger delta
The depth to the Benin Formation in sections of the Eastern Niger Delta (Cawthorne
Channel to Obrikom) closely mirrors the surface topography (Fig. 4). As in the western
section, thicker silty and sandy clay overburdens occur in the marine sections of the
alignment between Cawthorne Channel and Okochiri. These are areas that are still
experiencing sedimentation and are at different stages of consolidation. Unlike in the
western section, the surface of the Benin Formation in the eastern marine section in spite
of some undulations generally dips towards the sea with a slope of about 1.91x10-4.
10
OBRIKOM
OBRIKOM
OKPURUPUALI
67
OKOCHIRI
20
NAFCON
Elevation
(amsl m)
Alakiri
30
Cawthorne Channel
40
OKANSU
50
IGBO-ETCHE
ELELENWO
TOPOGRAPHY OF GROUND SURFACE AND BENIN FORMATION FROM CAWTHORN CHANNEL TO OBRIKOM
AWARA
Topography and Particle Size Gradation Characterization of the Benin Formation
0
-10
-20
1
5
9
13
17
21
25
29
33
37
41
45
49
53
57
61
65
Figure 4: Topography of Benin Formation in comparison to the ground surface in the
Eastern Niger delta
The sand particle gradation for soil samples from different sections are presented in figs. 5
to 7. Particle size distribution curves for borings in the marine segment between Omadino
and Escravos (Fig. 5) show that the sand sizes are mostly fine-medium. As expected,
towards the upstream, between Agbaloke and Agbor, the average sand sizes increase to
medium to coarse classes, with Effective (D10) and Mean (D50) sizes in the neighbourhood
of 0.25mm and 0.8mm respectively and with Uniformity coefficients varying from 3 to
5. As shown in Fig. 6 to 8, there is a tendency for upward fining, but there are occasional
departures from this trend. This is understandable particularly in this case, where there
may be multiple mechanisms of transport.
This upward fining tendency is also manifest in the eastern segment of the Niger delta
although not as pronounced as indicated in a depth profile plot of the effective and mean
particle sizes for borings from Cawthorne channel, Obigbo and Obrikom (Fig. 7 and 8).
The variations in the effective particle size with depth are within narrower margins (0.2 to
0.3 for D10 ) in comparison to the Escravos – Agbor segment. The mean particle size
(D50) on the other hand increases upstream (Fig. 8) as expected, reflecting the
predominant fluvial transport mechanism. The slightly lower D50 and particle size
distribution at the Obrikom end in comparison to Obigbo, which is a more elevated
ground, reflects the influence of the Niger River flood plain. A plot of depth profile for
Uniformity coefficient (Fig. 9) shows a narrow margin of variation around 2, indicating a
somewhat similar energy condition of deposition for the sediments.
68
Abam T.K.S, Akpe A. C. and Oba T
Particle size distibution for BH: 44
Particle size distibution for BH: 32
BS Sieves
63µm
20mm
BS Sieves
600m 2mm
100
90
90
80
80
70
70
60
50
40
30
20
10
0
0.0001
CLAY
0.001
0.01
FINE
0.1
particle size (mm)
COARSE
FINE
SILT
63µm
20mm
6.3mm
percentage passsing (%)
percentage passsing (%)
100
212µm
50mm
1
10
MEDIUM COAR
40
30
20
10
0
CLAY
COB
0.001
0.01
FINE
0.1
particle size (mm)
COARSE
FINE
SILT
Particle size distibution for BH: 29
1
MEDIUM COAR
100
90
80
80
70
70
60
50
40
30
20
10
0
CLAY
0.001
0.01
FINE
0.1
particle size (mm)
COARSE
SILT
FINE
COARSE
GRAVEL
COB
BS Sieves
1
MEDIUM COAR
SAND
63µm
20mm
6.3mm
90
0.0001
FINE
Particle size distibution for BH: 28
600m 2mm
percentage passsing (%)
percentage passsing (%)
100
212µm
50mm
10
SAND
BS Sieves
63µm
20mm
6.3mm
50
COARSE
GRAVEL
600m 2mm
60
0.0001
FINE
SAND
212µm
50mm
10
COARSE
FINE
COB
GRAVEL
212µm
50mm
600m 2mm
6.3mm
60
50
40
30
20
10
0
0.0001
CLAY
0.001
0.01
FINE
0.1
particle size (mm)
COARSE
SILT
FINE
1
MEDIUM COAR
SAND
10
COARSE
FINE
COB
GRAVEL
Figure 5: Comparison of particle size distribution in selected locations
Topography and Particle Size Gradation Characterization of the Benin Formation
Agbaloke D10 (mm)
0
0.2
0.00
2.00
4.00
0
10
Depth 20
30
(m)
40
50
0
10
Dept 20
h (m) 30
40
50
Ekan Ovu D10 (mm)
0
0.2
0.00
2.00
4.00
0
10
Depth 20
(m) 30
40
50
Omutu D10 (mm)
0
0.2
0.00
10.00
0
0.2
0
0.00
0.5
1
Omutu D50 (mm)
0
2
4
0
10
Dept 20
h (m) 30
40
50
Agbor Uniformity Coeff. Cu
0.4
1
0
10
Depth 20
(m)
30
40
50
20.00
0
10
Depth 20
(m) 30
40
50
Agbor D10 (mm)
0.5
Ekan Ovu D50 (mm)
Omutu Uniformity Coeff. Cu
0.4
0
10
Dept 20
h (m) 30
40
50
0
0
10
Dept 20
h (m) 30
40
50
Ekpan Ovu Uniformity Coeff. …
0.4
0
10
Depth 20
(m)
30
40
50
Agbaloke D50 (mm)
Agbaloke Uniformity Coeff. Cu
0.4
69
5.00
Agbor D50 (mm)
10.00
0
0
0
0
Dept 20
h (m) 40
Depth 20
(m)
40
Dept 20
h (m) 40
60
60
60
0.5
1
Figure 6: Variation of effective and mean sizes and Uniformity coefficients with depth at
the upstream sections of Benin Formation
70
Abam T.K.S, Akpe A. C. and Oba T
Effective Particle Size (mm)
0
0.2
Effective Particle Size (mm)
0.4
0
0
0.2
Effective Particle Size (mm)
0.4
0
0
0
5
5
5
10
10
Depth 15
(m) 20
Depth 15
(m) 20
25
25
30
30
25
35
35
30
0.2
0.4
10
Depth
15
(m)
20
Figure 7: Variation of effective size with depth at Cawthorne Channel, Obigbo and
Obrikom respectively
Mean Particle Size (mm)
0.2
0.4
Mean Particle Size (mm)
Mean Particle Size (mm)
0.6
0.2
0
0
0.4
0.2
0.6
0.4
0.6
0
5
5
10
10
Depth 15
(m) 20
Depth 15
(m) 20
25
25
30
30
25
35
35
30
5
10
Depth
15
(m)
20
Figure 8: Variation of Mean size with depth at Cawthorne Channel, Obigbo and Obrikom
respectively
Unif Coeff. Cu
0.00
0
2.00
Unif Coeff. Cu
Unif Coeff. Cu
4.00
0.00
2.00
0.00
4.00
0
0
5
5
5
10
10
Depth 15
(m) 20
10
Depth 15
(m) 20
25
25
Depth
15
(m)
20
30
30
25
35
35
30
2.00
4.00
Figure 9: Variation of Uniformity Coefficient with depth at Cawthorne Channel, Obigbo
and Obrikom respectively
The sediments which form the delta also vary with distance from sediment source. The
more proximal deposition zones, such as the shoreline, are dominated by tidal channel and
coastal barrier sands overlaying earlier (Holocene) marine clays. The delta slope
deposition zone, in water depths of approximately 15m, is characterized by marine sands.
As distance from shore and water depth increase, more distal sediments such as clay are
Topography and Particle Size Gradation Characterization of the Benin Formation
71
deposited in these deeper shelf margin conditions. This association of sediments
represents a typical progradational sequence of delta development.
The Benin Formation is significant for different reasons for oil and gas, hydrogeological
exploration and for geotechnical as well as a veritable source of construction material.
4 Conclusions
Knowledge of the topography of the Benin Formation is important in exploring for sand
as a construction material in an area with limited exposures. The upper sections of the
Benin Formation (Agbor to Sapele) have limited top soil cover consisting of silty and
sandy clay, and are exposed to erosion, accounting for the comparatively steep gradient.
The Benin Formation underlying the estuaries and mangrove swamps has a gentler and
almost flat profile, with occasional depressions that are infilled with silty clay sediments,
that may be linked to buried river channels. In the east, the longitudinal profile of the
Benin Formation has a gradient of approximately 1.91x10-4. The particle sizes within the
depths of civil engineering significance (0-40m), show a tendency of upward fining. The
Uniformity coefficients show some constancy, suggestive of similar energy conditions of
deposition.
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