Geology and Depositional Environment of Areas around Ihe, Anambra Basin, South-Eastern Nigeria

1.2.1 DRAINAGE

Ihe and its environs are sedimentary environment. It consists of a dendritic drainage pattern (Fig. 2) which flow in the northwest-southeast direction. The study area is made up of lots of rivers and spring, the most prominent rivers in the area are Agbudu River, Ogugu River, Amaowele River. Agbudu River occurs at the northwestern part of the area seen mainly at Agbudu village and flows in the N-S direction. Ogugu River is seen in Ogugu and occurs at the southern part of the area exposing an outcrop of shale, it flows in the N-S direction. Amaowele River occurs in Amaowele and it flows in a NE-SW direction. A spring was seen on an outcrop along Agbogugu road, the water was seeping from the contact between two beds of sand.

1.2.2 RELIEF

The study area shows varying elevation, averaging 200m in the north, 370m in the east, 300m in the south and 150m in the east above sea level. The low elevation found in the east can be attributed to the numerous river channels present in the area. The relief map is shown in Fig. 3.

1.2.3 CLIMATE AND VEGETATION

The climatic conditions of the study area is characteristic of tropical hinterland (fig 4). The rainfall lasts for the duration of 7 months or more usually from April- October and sometimes November, the rain decreases towards August which is known as August Break while dry season lasts for 5 months from November-March or less. Rainfall in the area occurs mainly as orographic, frontal and relief rainfall and it is usually accompanied by thunderstorm and strong winds. The temperature and relative humidity of the area is usually high except during harmattan while atmospheric pressure is high about 880-960mmhg at the mountainous regions and 760‑780mmhg in the lowlands (Iloege, 1980).

Ihe and environs fall within the rainforest and savannah vegetation belts (Fig 5). The vegetation within the area is controlled by the topography, drainage, relief; lithology, climate and anthropogenic activities, hence dense rainforest vegetation consisting of very tall trees, climbers and palms occur at the lowlands and deep gullies, while the savannah shrubs and grasses occur at the highlands.

2.2.1 NKPORO GROUP

The Nkporo group, dated as Campano-Maastrichtian, was established as the oldest lithostratigraphic unit of the Anambra Basin (Nwajide, 1990). The group comprises of the Nkporo Formation, Owelli Formation and Enugu Formation (Reyment, 1965). Nwajide and Reijers (1996) suggested that the shale and sandstone members of the group was deposited in a deltaic environment while Reyment and Barber (1956), suggested marine origin based on the occurrence of the ammonite, Libycoceras afikpoense, as well as the pelecypod, Inoceramus, crabs, fish teeth, bryozoans and echinoids.

The Nkporo Formation is exposed at Leru, 72 kilometres along the Enugu-Port Harcourt Expressway, and is described as a coarsening upward deltaic sequence of shales and interbedded sands and shales with occasional thin beds of limestone which were deposited during a short interval of marine transgression (Ladipo et al; 1992). It overlies the Eze-Aku Group of the Southern Benue Trough and is estimated to have a subsurface thickness of about 1km (Agagu and Ekweozor, 1982; Kogbe, 1989).

The The Owelli Formation is composed of medium to coarse grained, predominantly cross‑stratified feldspathic sandstones, deposited in fluviatile subenvironments such as point bars and vegetated flood plains, subjected to incision by occasionally rejuvenated rivers, hence the unit is fluviatile in proximal parts to marine basinward (Agumanu, 1993; Nwajide, 2013). Body fossils recovered from the Owelli Formation include ammonite species such as Libycoceras dandense, and Sphenodiscus sp. which dominate the lower levels (Reyment, 1965), and pelecypods such as Inoceramus which occur mainly in the sandstone/siltstone lenses at the upper levels (Okoro,1995; Uzoegbu et al; 2013).

The Enugu Formation has its type locality in the Enugu municipality at Ugwu Onyeama, near Onitsha road flyover (Nwajide, 2013). The formation is composed of three members namely the Enugu shale and the Otobi and Okpaya sandstone members (Nwajide, 2013). The Enugu shale is composed of carbonaceous shales and coals with the upper half deposited in lower floodplain and swampy environments and overlie the Nkporo Formation (Ladipo et al; 1992). Odunze and Obi (2013) concludes that the shale is composed of carbonaceous shales, laminated mudstone and interlaminated very fine-grained sandstone/siltstone. The carbonaceous shales form the basal portion of the Formation with typical trace fossil assemblages including Teichichnus and Planolites. Sections of the Enugu Shale are characterized by synsedimentary faults and folds (Obi and Okogbue, 2004). Sections are also characterized by thin ubiquitous concretions rarely more than 20cm characterized by sideritic cores and pyritic rims (Odunze and Obi, 2013). Deposition of the sediments of the Enugu/Nkporo Formations reflect a funnel-shaped shallow marine setting that graded into channels and low energy marshes.

2.2.2 COAL MEASURES

The Coal Measures represent an extensive Maastrichtian interval and document a period of non‑marine alternating with shallow marine sedimentation in the Anambra Basin (Nwajide, 2013). It is made up of the Mamu Formation, Ajali Formation and Nsukka Formation.

The Mamu Formation, formerly known as the Lower Coal Measures, overlies the Nkporo Group in most parts of the Anambra Basin (Simpson, 1954; Reyment, 1965). The formation constitutes the lower part of the Enugu cuesta and comprises a succession of sandstones, siltstones, mudstones, coal seams and rare shales (with concretionary siderite and marcasite) (Nwajide, 2013). It was deposited during a regressive period, as the shallow sea that deposited the Nkporo Group gradually receded. The formation was deposited under marshy to possibly marginal marine environment (Nwajide and Reijers, 1996; Akande et al; 2007).

Further regression of the shallow sea led to the deposition of the continental Ajali Formation, also known as the Sandstone Series or False-Bedded Sandstone, which overlies the Mamu Formation (Tattam, 1944; Obi, 2000). The formation consists mainly of poorly to moderately sorted, unconsolidated medium grained sandstones, with isolated fine grained sandstones and well- rounded quartz pebbles. It also consists of minor mudstones and sand/silt/clay heteroliths. It displays a dominant cross-bedding sedimentary structure. It is associated with reactivation surfaces, mud drapes, tidal bundles, backflow ripple channels cut and fills, lateral accretion surfaces, as well as Skolithos and Ophiomorpha ichnogenera (Ladipo et al; 1992). The Ajali Formation marks the maximum regression at a time when the coastline was still concave.

The Nsukka Formation, formerly known as the Upper Coal Measures, conformably overlies the Ajali Formation (Obi, 2000; Nwajide, 2013). It has its type locality at Nadu river, 14km north of Nsukka (Reyment, 1965). The fluvio-deltaic formation consists of variety of sandstones that passes upward into well-bedded blue clays, fine-grained sandstones, and carbonaceous shales with thin bands of limestone (Ladipo, 1986; Obi et al; 2001). It is composed mainly of interbedded shales, siltstones, sands and thin coal seams, which have become laterized in many places where they form resistant capping on mesas and buttes (Uzoegbu et al; 2013). The Danian Nsukka Formation as well as the overlying Imo Formation in the Niger-Delta Basin marks the onset of a marine transgression and a return to marshy conditions in the Anambra Basin (Akande et al; 2015). The Nsukka Formation also marks the end of sedimentation within the Anambra Basin.

3.2.1 RECONNAISANCE

Reconnaissance study was done in December 2021 prior to the main field work, in order to familiarize with the locals and survey the area in general. Approval to carry out field studies was obtained from village heads and local security. Routes to navigate the study area were established and navigated.

3.2.2 GEOLOGIC MAPPING

An elaborate field mapping was carried out in the area of study to describe the lithologies present and establish their stratigraphic relationships. Geologic mapping involves; location and description of outcrops, measurements of bed attitude, and taking note of structures present.

Equipment used during geologic mapping include:

• Global positioning system (GPS) used in taking coordinates (longitude, latitude, elevation).
• Geologic compass used in measuring the attitude of the bed (strike, dip direction, dip amount) and the trends of fractures.
• Geologic hammer used for collection of samples.
• Field notebook used for documentation of observations made on the field.
• Sample bags used to package samples collected from the field.
• Marker and masking tape used in labeling the samples collected for easier identification.

3.3.1 SIEVE ANALYSIS

8 sandstone samples were collected from 2 beds in L1/Ihe, 2 beds in L3/Isuawa, L6/Ihe, 2 beds in L7/Awgu and L10/Abudu. The samples were labeled accordingly and the results of various locations are represented in table 5-12.

PROCEDURES

The samples were sun-dried and disaggregated in the laboratory using a mortar and rubber padded pestle. 50 g of each disaggregated sample was measured using a weighing balance as test portions for sieve analysis. The samples were sun-dried and disaggregated in the laboratory using a mortar and rubber padded pestle. 50 g of each disaggregated sample was measured using a weighing balance as test portions for sieve analysis. These test portions were sieved with a Ro- tap shaker for 20 minutes using a stack of sieves with sieve mesh sizes 0.5 phi apart. The retained sieve fractions were weighed with a weighing balance to an accuracy of 0.1g and recorded. The retained sieve weights were normalized and their percentages and cumulative percentages calculated. Graphical plots of cumulative retained sieve weight percentages against phi scale (i.e. sieve mesh sizes) were made on an arithmetic-log probability sheet using the method proposed by Visher (1969). From the graphs, phi values of certain weight percentages (5%, 16%, 25%, 50%, 84% and 95%) were obtained and used to calculate mean size (MZ), sorting or standard deviation (d), Skewness (Sk1) and kurtosisi (KG) which are the required univariate statistical parameters for sieve analysis as defined by Folk and Ward (1957). These univariate statistical parameters are then used to derive bivariate plots and mulitivariate parameters which are used for further interpretation of depositional environment in addition to the univariate parameters.

SIEVE ANALYSIS STATISTICAL PARAMETERS AND THEIR INDICATIONS

These include: Univariate statistical parameters defined by Folk and Ward (1957), Bivariate plots of Friedman (1961), Moila and Weiser (1968), and mulitivariate statistical parameters, which are derived from a combination of univariate statistical parameters as used by Sahu (1964), to obtain a certain range of values which may indicate a distinct depositional environment out of a range of adjacent environments.

A. Univariate Parameters

The univariate parameters are measures of central tendency defined by Folk and Ward (1957).

They include mean size (MZ), sorting or standard deviation (d), Skewness (Sk1) and kurtosis (KG) These univariate parameters are derived from various combinations of phi values of certain weight percentages (5%, 16%, 25%, 50%, 84% and 95%) which are obtained from the probability-log curve. The values of univariate parameters when obtained are compared to standard ranges of values which add to various interpretations.

- Mean Size

It reflects the overall competency of the transport medium. The mean size of the grains is obtained by taking three phi values on the probability-log curve which correspond to certain weight percentages (16%, 50% and 84%) and finding the average. The obtained mean size is used to classify sand particles into coarse (0- 1), medium (1-2) or fine sand (2-3). Histogram plots may also be made by plotting individual retained weight percentage against phi intervals. Unimodality of grain size may reflect deposition in a unimodal system such as a fluvial environment where variability of flow is low. Bimodality may represent deposition in beach environment where differentiation in sorting is produced by swash and backswash current. Abrupt variations in mean sizes are commonly linked to rapid changes in hydraulic energy usually associated with tidal and wave action.

- Sorting

It is an indicator of the fluctuation in the kinetic energy of the depositing agent. Upward improvement in sorting may reflect a change in environment, it could be a transition from deeper water (upper shore face) to shallower water (fore shore). Fluctuating sorting values often result from differences in water turbulence and variability in current velocity. Sorting classifies grains into very well sorted (<0.35), well sorted (0.35-0.50), moderately sorted (0.50-1.00), poorly sorted (1.00-2.00), very poorly sorted (2.00-4.00) and extremely poorly sorted (>4.00). Poor sorting reflects variable current velocity and turbulence during deposition. Good sorting reflects smooth steady flow.

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- Skewness

It measures the asymmetry of frequency distribution. Skewness of grains may be positive, negative or symmetrical. Negative skewness also known as coarsening skewness implies that the velocity of the depositing agent operated at a higher value than the average velocity for a greater length of time than normal. It occurs in littoral beach, and tidal inlet environment where winnowing action of waves and tide currents are more dominant. Positive skewness also known as fine skewness are typical of sheltered quiet water areas in rivers and deep water where bottom currents of wave base surge does not disturb bottom current sediment. Near symmetrical reflects a broad spectrum of populations present in a sample.

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- Kurtosis

Kurtosis measures the peakedness of a curve. It can be used to tell the degree of sorting at the centre of the curve. It is not an environmentally-diagnostic parameter Flat peaked curves of poorly sorted sediments are described as platykurtic (0.67-0.90), while strongly peaked curves of well sorted sediments are described as leptokurtic (1.11-1.50). Other descriptions include very platykurtic (<0.67), mesokurtic (0.90-1.11), very leptokurtic (1.50-3.00), extremely leptokurtic (>3.00).

B. Bivariate Parameters

These include plots of skewness vs. sorting (Friedman, 1961) and plots of mean size vs. sorting (Moila and Weiser, 1968). Both plots are used to distinguish between river and beach sands.

C. Multivariate Parameters

These refer to a combination of univariate parameters to obtain values that correspond to specified ranges of values which distinguishes beach deposits from shallow marine deposits and fluvial deposits from shallow marine deposits. The multivariate parameters include:

- Y Beach: shallow marine = 15.6534MZ + 67.7091 σ1+ 18.1071SK1 + 18.5043KG

Where Y < 65.3650 indicates beach deposition; and Y > 65.3650 indicates shallow marine deposition.

- Y Shallow marine: fluvial = 0.2852 MZ - 8.7604d- 4.8932 SK1 + 0.0482 KG

Where Y < -7.419 indicates fluvial deposition; and Y > -7.419 indicates shallow marine deposition.

3.3.2 PEBBLE MORPHOMETRY ANALYSIS

50 pebbles were selected based on their surface, form, texture and sphericity. The pebbles collected should be more rounded than angular in order to avoid errors during study. The pebbles used were gotten from the first bed in L1/Ihe. The shapes and classes of the pebbles are then used to classify depositional environment.

PROCEDURES

During the field work, 100 pebbles were collected from L1/IHE and their three axes (long (L), intermediate (I) and short (5)) were measured with a vernier caliper (Dobkins and Folks, 1970). Pebbles with distinct fresh breaks, obvious primary elongation or flatness, lithologic inhomogenity were discarded to assure the true values of the desired parameter. The pebble forms were calculated based on;

1. Maximum Projection Sphericity, (MPS) after Sneed and Folk, 1958

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2. Oblate Prolate index (OPI) after Dobkins and Folk, 1970

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3. Flatness Index (FI) after Lutig, 1962

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Sneed and Folk (1957) provided a sphericity form diagram in which 10 classes are defined. They include; Compact, compact platy, compact bladed, compact elongate, platy, bladed, elongate, very platy, very bladed and very elongate.

Different pebble forms are known to occur more in one environment than the others (Sneed and Folk 1958). The three shape classes diagnostic of beach environment includes; Platy, very platy, and very bladed. While those forms most diagnostic of fluvial environments are; Compact, compact bladed, and compact elongate (Dobkin and Folk 1970).

3.3.3 PALEOCURRENT ANALYSIS

Statistical analysis of paleocurrent data involves estimation of means, variances and hypothesis testing. For crossbedding data, the frequency distribution ranges from 0-360o, the 30o and 45o class interval are commonly the size of the interval depending on variability and number of observations. Rose diagram is a histogram converted to a circular distribution where the modal class indicates the direction towards which the current moved, the midpoint of the modal class (mean vector) indicates the specific direction of flow.

Mean Vector Azimmuth (MVA)

It gives mean direction of the depositional agent which is used to infer regional slope and source direction. It is applied to complete the vector mean, where Σ Sin A and Σ Cos A are the sums of the sines and cosines of individual readings. It could be computed graphically by assigning a unit length to each vector or from the grouped data.

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- Variance

The variance is a measure of the variability of the flow direction. The variability of the paleocurrent pattern is estimated by the simple variance. It can be used to interpret environment of deposition. Variance of less than 2000 shows a fluviatile environment, 2000-6000 indicates fluviatile-deltaic environment, 6000-8000 signifies shallow marine environment and greater than 8000 shows marine environment.

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Where: A i = individual measurements and n = total number of measurements.

- Vector Strength

Vector strength is a measure of the direction and the magnitude of the depositional agent of sediment. High values of vector strength indicate low dispersion and low values indicate high

dispersion.

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4.1.1 OUTCROP DESCRIPTION

Sedimentologic logging was carried out on ten (10) outcrops. These outcrops were generally exposed by road cuts and streams. The Lithologic units include fine- medium- coarse grained sandstone, friable sandstones, shale, and ironstone. Field Descriptions and distributions of the lithologic units encountered in various outcrop locations are discussed below and an outcrop distribution map can be seen at Fig. 31.

Location 1/Ihe

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The outcrop is located at a quarry site along Ihe-Ogugu road. It has a lateral width of 120m, and vertical height of about 19m. The outcrop consists of 5 beds as shown by the stratigraphic profile in Fig. 11. Bed 1 consists of white medium grained lithified sandstone, it has a height of approximately 5m and it has crossbeds (Fig 12B). Bed 2 is made up of white friable fine grained sandstone with height of 7m. Bed 3 consists of reddish brown shale with height of 1m. Bed 4 has white friable fine grained sandstone with height of 1.5m and bed 5 is made up of reddish brown shale with height of 4m.

Location 2/Isuawa

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The outcrop is located at an abandoned quarry siite along isuawa village road heading to Abudu village. It has a lateral width of 200m, height of 10m with erosional base. The outcrop is a hard reddish brown fissile rock (Fig. 13).

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Location 3/Owelli

The Outcrop is located in Owelli village along Ihe-Ogugu road, 50m away from the road. It has a height of 2m and a lateral extent of 10m (Fig. 15). The outcrop consists of three sandstone beds. Bed 1 is a reddish brown, coarse grained sandstone with a height of 0.80m and sparse bioturbation (bivalve boring: Fig. 15B). Bed 2 is a medium grained white sand with height of about 0.60m, and bed 3 is a medium grained white sand with lateral extent of 2m.

Location 4/Owelli

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The outcrop is located in Owelli village, along Owelli-Ogugu road. It has a lateral width of 10m and a height of 2m. the outcrop is a medium grained friable sandstone. Its profile is shown in Fig, 16 and the image of the exposure is shown in Fig. 17.

Location 5/Amoli

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The outcrop is located close to a stream in Amoli village, it has a lateral extent of 5m and a vertical height of about 1m. The outcrop consists of highly weathered reddish brown shale. The profile of the outcrop is shown in Fig. 18 and the images in Fig. 19.

Location 6/Ihe

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The outcrop is located along Ihe road, close to St Michael's Catholic Church. It has a lateral width of 10m and vertical height of 4m. The outcrop is a medium grained yellowish sand, fractured along strike and presence of bioturbations (Fig.20B). The profile of the outcrop is shown in Fig 20, and the picture of the location is shown in Fig. 21.

Location 7/Agbaogugu

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The Outcrop is located along Enugu-Port Hacourt expressway close to Agbaogugu junction. It has a height of 7m and a lateral width of 150m. The Outcrop consists of 4 beds as shown by the stratigraphic profile in Fig. 22. Bed 1 is a medium grained yellow sand with erosional base, there is a presence of ripple laminations (Fig. 23B) and micro kink fold (Fig. 23C) on the bed, the bed has a height of 2.5m. The second bed is a medium grained white sand with parallel laminations and a height of 2.5m. Bed 3 is a medium grained sand with ironstone concretion, it has a height of 1.5m. The final bed consists of a fine grained silty sand with a height of 1m. The image of the outcrop is shown in Fig. 23.

Location 8/Awgu

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The location is a vegetative outcrop located along Enugu-Port Hacourt expressway, it has a lateral extent of 10m and a vertical height of 5m. The outcrop consists of two beds, the first bed is a friable fine grained yellow sand with parallel laminations and erosional base. The second bed consists of very fine grained white sand. The stratigraphic profile and image of the outcrop are shown in Fig, 24 and Fig. 25 respectively.

Location 9/Awono Owelli

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The Outcrop is located at Awono Owelli village, it has a lateral extent of 30m and a height of 5m. The outcrop is a hard reddish brown rock (Fig. 26). similar to the one seen in location 2 and it could be referred to as laterite.

Location 10/Agbudu

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The Outcrop is located at a stream in Agbudu village, it has a height of 6m and a lateral extent of 50m. The outcrop consists of 4m high yellowish brown fine grained sandstone, overlain by a 2m high reddish brown fine grained sandstone. The stratigraphic section of the outcrop can be seen in Fig. 27 and the image in Fig. 28.

Location 11/Ugbonabo

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The outcrop is located along the road in Ugbonabo villge just before the civic centre. It has a height of 1.5m and a lateral extent of 0.75m. The outcrop consists of reddish-brown fissile shale. The stratigraphic section and image of the outcrop can be seen in Fig. 29 and 30 respectively.

4.1.2 LITHOFACIES ANALYSIS

Sedimentary facies may be defined as a deposit of sedimentary rock which can be distinguished from others by its lithology, geometry, sedimentary structures, palaeocurrent pattern and fossils present (Selley, 1985). It can also be defined as sedimentary rocks that reflect the conditions under which they were formed. There are six (6) lithofacies observed in the study area; Crossbedded sandstone facies, Shale facies, Structureless (massive) sandstone facies, Parallel laminated sandstone facies, bioturbated sandstone facies, and Ironstone facies. A facie map is shown in fig. 38.

- CROSSBEDDED SANDSTONE FACIES

The crossbedded sandstone Facies was seen at L1/Ihe. Bed 1 in L1/Ihe is characterized by planarcross-bedded sandstone foresets trending 110SE/290NW with dip of 13o. The bed is medium grained, partially sorted, white in color and has a thickness of 4.5m height. It is overlain by massive fine grained sandstone.

Interpretation

Planar crossbed is formed mainly by migration of large-scale, straight crested ripples and dunes. It forms during lower flow regimes and they show the direction of river flow. Planar crossbed are typical of fluvial environment.

- SHALE FACIES

The shale facies were seen at L4/Owelli, L5/Amoli, and L11/Ugbonabo. They occur as single outcrops and are mostly weathered on the surface. At L4/Owelli, some parts of the shales have been laterized, also at L5/Amoli and L11/Ugbonabo, the shales present are highly weathered. The shales are generally reddish brown with a thickness of 1m at L5/Amoli, 1.5m at L11/Ugbonabo and 2m at L4/Owelli.

Interpretation

Shales represent a variety of settings ranging from deltaic interdistributary bays and muddy marine shelves. This lithofacies could be interpreted to represent deposition from suspension in low-energy settings, beyond the influence of coastal rivers. The fine grained, fissile and laminated nature of the shales infers deposition by suspension sediment fallout in a low energy environment (Thomas et. al., 2002; Bridges, 2006).

- STRUCTURELESS SANDSTONE FACIES

The structureless (massive) sandstone were found at five locations namely; L1/Ihe, L8/Awgu, L10/Agbudu, L3/Owelli. The sandstones were L10/Agbudu, reddish brown at L3/Owelle and white at L1/Ihe, L8/Awgu. Generally, the sandstone ranges from fine grained to medium grained. The bed thickness ranges from 2m at L8/Awgu, L3/Owelli, L10/Agbudu bed 2, 4m at L10/Agbudu bed 1 to 6m at L1/Ihe.

Interpretation

Structureless sandstone occur as a result of rapid deposition through deceleration of high density turbidity due to sediment laden current (Collinson et al., 2006). They are characterized by the absence of stratification, medium to coarse grain which may indicate sediment laden turbidity current (Bouma, 1962; Lowe, 1982 and Mutti, 1992). It could also be favoured by suspensions from flood or gradual aggradations of sediments at steady or near steady flow which deposits fine grained sediments (Johansson et al., 1998).

- PARRALLEL LAMINATED SANDSTONE FACIES

The parallel laminated sandstone facies were found at two locations namely; L7/Agbaogugu and L8/Awgu. At L7/Agbaogugu, it was found in the second bed which is bounded by wavy laminated sandstone below and ironstone above. At L8/Awgu, it was found in the first bed which is overlain by a massive sandstone. The grain size ranges from fine grained at L8/Awgu to medium grained at L7/Agbaogugu. Bed thickness ranges from 2-3m. The wavy laminated sandstone seen in bed 1 of L7/Agbaogugu is yellowish in color with medium grained sandstone. It is 2m thick, and it has micro kink fold.

Interpretation

Lamination is a small-scale sequence of fine layers called laminae, they are usually smaller than beds (< 1cm). Parallel laminated sandstones are attributed to quieter current regimes of low intensity which resulted to migration of straight crested dunes or bars during periods of low water level or waning of channels recording slow sedimentation (Hjellbakk, 1997, Miall, 1996). The ripple laminated bed can be attributed to sands deposited in shoreline beach.

- BIOTURBATED SANDSTONE FACIES

The Facies is characterized by the occurrence of sparse to intense bioturbation. The types of traces seen include bivalve borings and round borings which seem to be recent. The facies were seen at L3/Owelle, and L6/Ihe. The bioturbated sandstone facies seen at L3/Owelle is coarse grained and reddish brown in color with a thickness of 0.75m. Bivalve borings are present in this sandstone, some are overtuned, and they have an infilling of white muddy materials. The bioturbated sandstone facies at L6/Ihe consist of medium grained yellowish sandstone, there are fractures along the strike direction and it has a thickness of 4m. The sandstone is heavily bioturbated with borings which appear to be recent.

Interpretation

Bivalve borings are formed by borings of Gostrochaenolites. They occur in firm, but not lithified sediments in low energy situation and also in shoreline rocks.

IRONSTONE (FERRUGINIZED SANDSTONE) FACIES

This Facies consists of dark brown coloured and consolidated ironstone as well as ferruginous medium grained sandstone. The Ironstone occurred as a single bed of 1.5m thick in L7/Agbaogugu. It is underlain by a medium grained parallel laminated sand and overlain by fine grained silty sand.

Interpretation

Intensive subaerial weathering of rocks may have led to the deposition of ironstone (highly ferrginized sandstone) (Schellman, 1989). Ferruginized sandstones (Ironstone) denote deposition in an unrestricted shallow marine environment with a complete or partial aerobic condition resulting to heamatite type of iron formation. This suggests a post depositional effect due to subaerial exposure and an oxidizing condition.

4.1.3 LITHOFACIES ASSOCIATION AND DEPOSITIONAL ENVIRONMENT

The identified lithofacies (6 in number) documented in the study area were further used to classify their various Facies Associations (FA) based on their lithologies, bioturbation, contacts, surfaces and geometry. Each Facies Association can be linked to a possible depositional environment and conditions. A summary of the facie association is given in table 2 and it was used to create a geologic map (Fig. 52).

- MARINE (FA 1)

FA 1 is dominated by thick dark fissile shale lithofacies of distal settings. The dark shale indicates very rich organic matters while the brown or very light grey shale show little organic matter. It is observed at L4/Owelle, L5/Amoli, and L11/ugbonabo.

Interpretation

FA 1 is interpreted as offshore to distal offshore and distal prodelta settings which is of very low energy. However, Suchy and West (2001) claims that rather than representing maximum transgression, dark shale denotes response to warmer, wetter, greenhouse climate (Suchy and West, 2001).

- SHOREFACE (FA 2)

This facies association is dominated by medium to coarse grained planar crossbedded sandstone facies, medium to coarse grained structureless sandstone facies, and medium to coarse grained parallel laminated sandstone facies. It is observed at L1/Ihe, L3/Owelle, L6/Ihe and L7/Agbaogugu.

Interpretation

FA 2 is interpreted as upper shoreface, which is of high energy. Medium to coarse grained sediments represents a high energy environment and they are mostly tide dominated.

- MOUTH BAR (FA 3)

FA 3 is characterized by fine grained planar crossbedded sandstone facies, fine grained structureless sandstone facies and fine grained parallel laminated sandstone facies. It is observed in L1/Ihe, L8/Awgu and L10/Abudu.

Interpretation

The fine grain parallel laminated sandstone is interpreted as a low energy environment. The fine grained structureless sandstone facie are deposited as channels while being transported slowly by the river.

4.2.1 Paleocurrent Analysis

Paleocurrent values were measured from the planar crossbed seen in bed 1 at L1/Ihe. The bed is a pebbly white medium grained sandstone, with a thickness of 4.5m. The paleocurrent values gotten can be seen in Table 3.

Interpretation

The rose diagram can be said to show a unimodal pattern. The general provenance of the sediments can be seen to be in the NW direction. Mean vector azimuth (MVA) gives the mean direction of the depositional agent used to infer regional slope direction, a value of 79.6o was gotten, the variance which is a measure of the variability of the flow direction, has a value of 758.635o which signifies a fluviatile environment. Vector strength is a measure of the direction and magnitude of the depositional agent of sediment, a vector strength of 0.047533 which indicates a low dispersion of the sediments.

4.2.2 Pebble Morphometry Analysis

50 pebbles were selected based on their surface, form, texture and sphericity. The pebbles measured were gotten from the first bed in L1/Ihe. The shapes and classes of the pebbles are then used to classify depositional environment. The values of the measured pebbles can be seen in appendix 1.

Interpretation

Based on Sneed and Folk classes (Fig. 38, 40), nothing plotted on the compact area, 8% are compact-platy, 8% are compact-bladed, 10% are compact-elongate, 14% are platy, 34% are bladed, 16% are elongate, 2% are very-platy, 4% are very-bladed and 4% are very elongate. From the bivariate plots (Fig. 39), most of the pebbles plotted at the fluvial area more than the beach area. Using both the sphericity form and the bivariate plots, the results show that the sandstone is of fluvial origin.

4.2.3 Sieve Analysis

Eight (8) sandstone samples were collected from the field. The samples were labeled accordingly and the results of various locations can be seen in appendix 2. Different plots for the samples are shown in fig. 41-48. Table 4 shows the values of univariate parameters for the eight samples.

Interpretation

The probability-log plots show three-segment curve types corresponding to traction, saltation and suspension. All the outcrops were three-segment curves corresponding to traction, saltation and suspension with the exception of L10/Agbudu.

The mean size of all the samples at L1/Ihe (bed 1) and L3/Owelli (bed 2) are coarse- grained with mean sizes of 0.949 and 0.911 respectively. The mean size reflects the strength of the transporting agent which corresponds to high-energy. Samples L1/Ihe (bed 2), L3/Owelli (bed 3), L6/Ihe, L7/Agbaogugu (bed 1 and 2) and L10/Agbudu show mean sizes of 1.006-1.904 which are interpreted as medium-grained and are of a lower energy than coarse-grained sands.

All the samples are poorly sorted (1.105-1.959) with the exception of L3/Owelli (bed 2) which is moderately sorted (0.865). Samples L1/Ihe (bed 1), L1/Ihe (bed 2), L3/Owelli (bed 2 and 3), and L7/Agbaogugu (bed 2) are negatively skewed (0.25-0.051) and are interpreted as coarse- symmetrical. L6/Ihe, L7/Agbaogugu (bed 1) and L10/Agbudu are positively skewed (0 . 243‑0.007) and are interpreted as symmetrical-fine.

Based on Kurtosis values, L7/Agbaogugu (bed 2) and L6/Ihe are leptokurtic (1.343-1.463), samples L3/Owelli (bed 2 and 3) and L1/Ihe (bed 1) are mesokurtic (1.066-0.926), while L1/Ihe (bed 2) and L7/Agbaogugu (bed 1) are platykurtic (0.796-0.827) and only L10/Agbudu is interpreted as very platykurtic (0.600).

From the multivariate values in table 5, the Ybeach Vs. shallow marine plot shows shallow marine environment of deposition for all the samples. The Yshallow marine Vs fluvial of all the sandstone samples points to fluvial environment except for L3/Owelli (bed 2) corresponds to shallow marine environment.

4.3.1 Environment of Deposition from Facies Analysis

There are six (6) lithofacies observed within the study and they were grouped into three (3) facie associations which are marine, shoreface and mouth bar. This implies that the sediments were deposited within the marginal marine, shallow marine and marine environment. The environment of deposition of the study area is interpreted as a Fluvial dominated Marginal Marine environment as results gotten from pebble morphometry, paleocurrent analysis and sieve analysis pointing to a fluvial setting, with sieve analysis further narrowing it down to a Marginal marine environment because of the poorly sorted sands present. The Late Campanian-Maastrichtian sediments within Ihe and environs experienced interplay of fluvial process with high sediment influx and low energy which led to the deposition of poorly sorted sediments.

4.3.2 Environment of Deposition from Paleocurrent Analysis

The rose diagram showed a unimodal fan shaped pattern with low variability which indicates a graded environment. The value gotten as variability of the flow direction (Variance) is 758.635o which interprets the sandstone to be of a fluvial setting.

4.3.3 Environment of Deposition from Pebble Morphometry

On the basis of pebble form, the OPI, MPS, and FI fall within the limits of shallow marine. The bivariate plot of MPS vs. FI shows about 64% of the pebbles plot in the fluvial portion of the graph, 30% plot in the beach part of the graph and 6% plot in non-diagnostic portion of the graph. Also, the bivariate plot of OPI vs. MPS shows that 44% plot in the fluvial part of the graph, 16% plot in the beach portion of the graph and 40% plot in the non-diagnostic part of the graph. This interprets the environment as a fluvial dominated environment.

4.3.4 Environment of Deposition from Sieve Analysis

Majority of the probability-log plots show three-segment curve types corresponding to traction, saltation and suspension which may suggest shallow marine environment and two-segment curve types corresponding to traction and saltation was present, which may invariably be related to deposition in fluvial setting.

Bimodality of grain size reflects deposition in a fluvial environment where differentiation in sorting is produced by swash and backwash currents. The sandstones are poorly sorted showing low energy conditions which is characteristic of a fluvial dominated setting in a marginal marine environment. The symmetrically skewed sands suggest that the depositing medium operated at a steady velocity for a long period of time while coarse-skewness may indicate that the velocity of the depositing agent operated at a level higher than normal for a long period of time .

Results from the univariate, bivariate and multivariate parameters show that the sand samples are mainly fluvial sediments with few shallow marine exceptions. The beach interpretation obtained from the YBeach: shallow marine analysis implies that the sediments were deposited in shallow agitated waters.

5.1.1 Laterite

The laterites in the study area are being used majorly as construction materials both at the artisanal and industrial scale because of its hardness and texture. The laterites are locally mined by the villagers with the aid of hammer, shovel and pan. Okogbue (1986) highlighted the importance of laterites in engineering geology, stating that laterites are commonly used for highway construction in eastern Nigeria and that they have performed satisfactorily because of their high strength, low water absorption and their high specific gravity. Bhavan (2012) restricted the use of laterites stating that they cannot withstand high pressure and such it should be used for light structures, partition walls, boundary walls, etc. He stated that laterites as a building stone possesses one important advantage; it is soft when quarried and can be easily cut into blocks and bricks which on exposure to air becomes hard. Laterites can also be used in the cement industry as an additive for lowering the clinkerization temperature and for supplementing aluminous and iron content required in the manufacture of cements.

5.1.2 Sand

This study area has abundant sandstone accumulation. The sands in the area are mined both by quarrying with the aid of caterpillars and excavators and also by crude methods using shovel and pans. Different grain sizees of sands are popular for different reason; Very fine grained to fine grained sands are popularly used for molding huts and local paint. While medium to very coarse sands are used as aggregate in cement block production, they are also used as building material and road construction aggregates.