Microfacies analysis and depositional environment of limestone deposits: Arochukwu – Obotme – Odorikpe axis southeastern Nigeria

Microfacies analysis of limestone deposits from Arochukwu – Obotme – Odoro Ikpe Axis has been carried out complimented with biostratigraphic and sedimentological analysis. Fieldwork and Laboratory techniques employed are standard methods as used in limestone petrography (preparation of the limestone thin sections), sedimentological and biostratigraphic studies. Petrographic analysis results indicate that the limestone is made up of between 80% and 60% allochems while cements make up between 19% and 38% respectively. The allochems consists of shell fragments, whole pelecypod shells, algal grains, diatoms, foraminiferids, whole gastropods shells, bryozoans, ostracods, crinoids and coral fragments in addition to ooids/peloids, quartz grains, intraclasts and phosphate grains that make up the limestone. The cement type is interpreted as sparite and porosity type is interparticle and fracture. Important fossils identified in the limestone samples include Anomalinoides sp, Textularia sp, ostracods, pelecypods, gastropods and corals which occur either as whole skeletal forms or as fragments. Based on the microfacies characteristics, the limestone in the study area are sandy bioclastic packstone, bioclastic packstone and bioclastic wackstone bioclastic wackstone / bioclastic packstone at Locations 5 (Amuvi), (Obotme) – 2, (Asaga) 3 and 4 (Okobo) and sandstone with about 10% shell fragments at Location 2 (Odoro Ikpe), – sandy bioclastic packstone. Two microfacies identified in the study area which correspond to SMF 5 and SMF 9. The environment of deposition is interpreted as normal marine, subtidal to shallow shelf which is consistent with shallow inner neritic to middle neritic interpreted from biostratigraphy (foraminifera and palynomorphs) identified in the limestone and associated shale samples.


Introduction
This research focussed microfacies analysis and the depositional environment of the limestone deposits and associated sediments around Arochukwu-Obotme-Odoro Ikpe area Southeastern Nigeria. The study area Arochukwu and Environs is located within the Afikpo Basin and covers parts of Ikono and Ini Local Government Areas of Akwa Ibom State and Arochukwu Local Government Areas of Abia State -Southeastern, Nigeria. The study area is approximately 348.7 km 2 and located within longitudes 7° 44'E -8° 03'E and latitudes 5° 15 N -5° 28'N ( Figures 1 and 2 1 ). It is accessible by roads through the Ikot Ekpene -Obotme axis and Umuahia -Arochukwu axis including tracks. Benue Trough is characterized by three main sedimentary phases which include the Abakiliki -Benue phase (Aptian -Santonian), the Anambra -Benin phase (Campanian -Mid Eocene) and the Niger Delta Phase (Late Eocene -Pliocene). [2][3][4] Majority of the sedimentary beds have been deposited in a NE -SW whereas others were deposited. The ranges of environment deposition typical of the Lower Benue Trough and within the Afikpo Basin are marine, continental, and transitional represented by the Asu River Group, Eze Aku Group, Nkporo, Mamu, Ajali, Nsukka, Imo, Ameki Formations, Nanka Sands and Benin Formation see Table 1. Ideozu, 1 Ideozu and Ikoro, 5 Ideozu and Solomon, 6 Ideozu and Akatakpo 7 and Ameh et al. 8 have extensively discussed the sedimentology, stratigraphy, structural geology and petroleum geochemistry of the study area.

Technical Paper
Abstract Microfacies analysis of limestone deposits from Arochukwu -Obotme -Odoro Ikpe Axis has been carried out complimented with biostratigraphic and sedimentological analysis. Fieldwork and Laboratory techniques employed are standard methods as used in limestone petrography (preparation of the limestone thin sections), sedimentological and biostratigraphic studies. Petrographic analysis results indicate that the limestone is made up of between 80% and 60% allochems while cements make up between 19% and 38% respectively. The allochems consists of shell fragments, whole pelecypod shells, algal grains, diatoms, foraminiferids, whole gastropods shells, bryozoans, ostracods, crinoids and coral fragments in addition to ooids/peloids, quartz grains, intraclasts and phosphate grains that make up the limestone. The cement type is interpreted as sparite and porosity type is interparticle and fracture. Important fossils identified in the limestone samples include Anomalinoides sp, Textularia sp, ostracods, pelecypods, gastropods and corals which occur either as whole skeletal forms or as fragments. Based on the microfacies characteristics, the limestone in the study area are sandy bioclastic packstone, bioclastic packstone and bioclastic wackstone -bioclastic wackstone / bioclastic packstone at Locations 5 (Amuvi), (Obotme) -2, (Asaga) 3 and 4 (Okobo) and sandstone with about 10% shell fragments at Location 2 (Odoro Ikpe), -sandy bioclastic packstone. Two microfacies identified in the study area which correspond to SMF 5 and SMF 9. The environment of deposition is interpreted as normal marine, subtidal to shallow shelf which is consistent with shallow inner neritic to middle neritic interpreted from biostratigraphy (foraminifera and palynomorphs) identified in the limestone and associated shale samples.

Materials
The materials used during fieldwork include compass/clinometer, GPS, measuring tapes, sample bags and sacks properly labelled, camera, protective clothing and field notebook in addition to topographic map indicating the position of the study area (Ikot Ekpene Sheet 322 on a scale of 1: 50,000).

Methods and Sample/Data Collection
Methodologies applied in this research include fieldwork and laboratory analysis. The approach used during field mapping was to document and describe the outcrops in detail in addition to methods modified from Miall, 9 Tucker 10 and Reijers. 11 The procedures adopted for each outcrop include systematic recording of physical and biological properties of each outcrop, sketching of the outcrop including obvious features, estimating the mineralogical composition and rock type, measurement of bed thickness including strike and dip, taking note of textural features and details on fabric, preliminary field interpretation and establishing correlatable units and collection of representative samples for thin section. The outcrops identified were examined for bedding contacts, bed thickness variations, synsedimentary, sedimentary and biogenic structures as well as syn-and post-depositional structures. Laboratory techniques employed in the preparation of the limestone thin sections are standard methods as used in limestone petrography. The thin sections were labelled appropriately for the petrographic study. The prepared thin sections were studied under plane -and cross-polarized light to identify the constituent minerals of the framework grains in the limestone samples including their abundances. Great care was taken in the procedure adopted because identification of the minerals and framework grains depend on the subtle characteristic interference colours. 12 Representative samples of limestone from the study area were trimmed and polished to the required thickness of 0.03mm, suitable for optical microscopy and photomicrographs of the thin sections were made. Limestone samples were sampled from Nsukka Formation, (forty-six (46) thin sections of limestone) from five outcrops at Odoro Ikpe (Location 1), Obotme (Location 2), Asaga (Location 3), Okobo (Location 4), Amuvi -Arochukwu (Location 5) and were prepared for petrographic studies. The spot sampling method was adopted in collecting data. 13

Sedimentology
The lithostratigraphic units at this location is the Nsukka and Benin Formations -observed in both outcrop and borehole drilled 500 m from the outcrop. From field relations, the Nsukka Formation is unconformably overlain by the Benin Formation and an unconformity was established based on high dip values and biostratigraphy. 1 The borehole penetrated all the beds in the outcrop -Nsukka and Benin Formations. The Nsukka Formation is 4.92m made up of shale and limestone (0.91 -3.40 m) having sharp to gradational contacts. The shale is grey to greenish, massive and fractured in outcrop, in the borehole the shale is grey. Whereas the limestone, is whitish to brownish, and mainly composed of lime mud and sand fractions. The Benin Formation is 53.73 m comprising a sequence of pebble, sand and clay beds ranging from 0.10 -12.33 m. The bedding contacts is gradational to erosional with pebble lags. The Benin Formation is friable, poor to very poorly sorted, angular to well-rounded especially the pebbles, whitish to reddish with sedimentary structures such as planar crossbeds and biogenic structures indicating influence of the marine. The pebble beds are, friable, round to very well rounded, poorly to very poorly sorted and matrix supported (comprising mainly clay, sand and silts), muscovite flakes are common especially in the clayey matrix. The sandy beds are fine to coarse grained sands -clay clasts are common, clayey, and friable and poorly to very well-sorted. Iron stone fragments are common; in addition, the sandy beds are whitish to brownish. The clayey beds have thicknesses, ranging from 0.10 -0.90 m. They are whitish with reddish patches, mottled with bedding contacts -sharp to erosional.

Biofacies Description
The clay beds of the Benin Formation have yielded no diagnostic palynomorphs and foraminifera, they are mostly barren but reworked palynomorphs are observed. The Nsukka Formation at this location, is fossiliferous with diagnostic forms (Foraminifera and palynomorphs). Gastropods, pelecypods and ostracods as observed this justifies the shallow marine environment interpreted for the shale and limestone beds.

Sedimentology
The Nsukka Formation consists of two lithology -shale and limestone at this outcrop. The limestone occurs as a lens within the shale but have now been exposed on the surface by erosion and scattered as boulders.
In addition, some of the boulder -limestone have been affected by chemical weathering. Thickness of the boulders ranges from 0.

Lithologic Description
Depth (m) Lithologic Description

25.38
Limestone, bioclastic grey to whitish with abundant bivalves and gastropod shells.

36.66
Shale, grey to black, fissile with joints and joint sets in all directions. Water from the underlying bed rises to the surface indicating that the bed below is a sandy bed probably Ajali Formation that has been charged with water, issuing out as spring.

Sedimentology
The Nsukka Formation at this outcrop comprises clays, interbedded shale and calcareous reddish sand, whitish to greyish bioclastic limestone and thick grey to black shale. The shale is grey to black, fissile, fossiliferous and fractured. Water flows out through this cracks or fractures indicating that, there is reservoir sand below, probably the Ajali Formation. The limestone is bioclastic with abundant shell fragments, whole gastropod, pelecypod and bivals. The sand lenses within the shale is reddish and calcareous.

Biofacies Description
The outcrop has yielded mainly benthonic foraminifera such as

Sedimentology
The limestone in this outcrop occurs along the Amuvi -Okobo road, the thickness ranges from 10cm to 24cm and rests directly on the shale (Figure 4). The limestone beds Lithologic contact: Limestone / shale. This limestone lenses of the Nsukka Formation outcrops along Okobo road, the thickness ranges from 10cm to 24cm and rests directly on the shale. It is bioclastic with abundant shell fragments greyish to reddish.

Biofacies Description
Foraminifera identified from thin section studies include Valvulinera jacksonensis, Textularia sp, Nonionella cummunis and Eponides psuedoelevatus and gastropod shells. These forms suggest a Maastrichtian to Palaeocene for the limestone and the environment of deposition is interpreted as shallow marine.

Petrographic Analysis results
The results of the petrographic analysis of the limestone samples

Microfacies Analysis
Microfacies of the limestone deposit in the study area, have been identified by the study of thin sections and involve the determination of the composition and textures of the limestone. Microfacies analysis is the aggregate of all the sedimentological and biostratigraphic criteria. Differentiation of microfacies (MF) types form an essential part of facies analysis. Two microfacies -Packstone and Wackstone have been interpreted for the study area.

Packstone Microfacies
The packstone microfacies consist of more than 50% of the grains, in a micritic matrix. 22 It comprises various grains as its components such as skeletal fragments, peloids and ooids. The packstone microfacies in the study area are sandy bioclastic packstone (Location 1) and bioclastic packstone (Locations 3 -5).

Location 1 Sandy Bioclastic Packstone Microfacies
These facies comprise skeletal fragments, peloids, gastropods and abundant sand grains in a micritic matrix. Diagenetic minerals include calcite, silica and hematite. The porosity type is mainly interparticle and intraparticle. See Table 2, Plates 5 and Figure 3.

Wackstone Microfacies
The wackstone microfacies is a carbonate microfacies with more than 10% allochems / grains embedded in a micritic matrix. 22 The faunal assemblage present in this microfacies consists of gastropods, pelecypods and brachiopods. Location 2 is interpreted as bioclastic wackstone.

Bioclastic Wackstone Microfacies
These facies are made up of ostracods, pelecypods, gastropods and shell fragments, leaf fragments, ooids, peloids and intraclasts. Lime mud is very abundant, cement is mainly micritic. Diagenetic minerals include calcite, silica and hematite. Porosity type is mainly intraparticle and fracture. See Table 3, Plates 6 and Figure 4.

Standard Microfacies 5
The bioclastic packstone facieses composed of skeletal fragments which demonstrated faunal diversity, these include whole shells and fragments of pelecypods, brachiopods and ostracods. The nonskeletal fragments comprise ooids, peloids and intraclasts. These facies are deposited in a normal marine, subtidal to shallow shelf environments. 25

Standard Microfacies 9
The bioclastic wackstone facies is composed of skeletal fragments which shows faunal diversity.

Discussion
Petrographic analysis results indicate that the limestone is made up of between 80% and 60% allochems whereas cements make up between 19% and 38% respectively. The limestone is interpreted as bioclastic for the study area based on Dunham's classification scheme. 22 The allochems consists of shell fragments, whole pelecypod shells, algal grains, diatoms, foraminiferids, whole gastropods shells, bryozoans, ostracods, crinoids and coral fragments in addition to ooids/peloids, quartz grains, intraclasts and phosphate grains that make up the composition of the limestone (see Plates 5 -7). The cement type is interpreted as sparite and porosity type is interparticle and fracture. Important fossils identified in the limestone samples which make up the limestone beds include Anomalinoides sp, Textularia sp, ostracods, pelecypods, gastropods and corals. These fossils occur either as whole skeletal forms or as fragments. Based on a chart of common lithofacies symbols used in describing carbonate reservoirs and associated rock types two microfacies have been identified. See Tables 2 -5. The limestone types correspond to SMF 5 and SMF 9. 22,[26][27][28][29] The environment of deposition is interpreted as normal marine, subtidal to shallow shelf. The interpreted environment of deposition is consistent with shallow inner neritic to middle neritic interpreted from biostratigraphy based on foraminifera and palynomorphs identified in the limestone and shale samples from the study area. 1 The limestones in the study have varying compositions -bioclastic wackstone / bioclastic packstone at Locations 5 (Amuvi), (Obotme) -2, (Asaga) 3 and 4 (Okobo) and sandstone with about 10% shell fragments at Location 2 (Odoro Ikpe), -sandy bioclastic packstone. Based on the microfacies characteristics, the limestone in the study area are sandy bioclastic packstone, bioclastic packstone and bioclastic wackstone. Based on Dunham's classification, they are classified as bioclastic packstone and bioclastic wackstone microfacies (Figures 3 -7, Plates 5 -7 and Tables 3 -7). 22 Gastropods, pelecypods, foraminifera, ostracods and algae constitute the major bioclasts while pellets and lithoclasts are the minor non-bioclastic components of the limestone such as ooids, peloids and intraclasts. 14, 15,30 The presence of ostracods, gastropods, pelecypods, coral fragments and leaf fragments strongly supports a shallow marine for the limestone and associated shales in the area combined with biostratigraphy. 1

Conclusion
The results of this research based on data generated from field and petrographic studies, indicates that the limestone beds where deposited in a shallow marine environment. The limestones in the study area are mostly bioclastic with abundant bi-valve shells, gastropods, pelecypods and shell fragments. Based on Dunham's classification, they are classified as bioclastic packstone and bioclastic wackstone and correspond to standard microfacies 5 and 9. 22