GEOCIENCIAS DIC 2010-COLOR.vp Petrographical Study of Ewekoro Carbonate Rocks, in Ibese, South Western Nigeria 1Akinmosin, A., and 2Osinowo, O.O. 1 Department of Geosciences,University of Lagos, Lagos 2 Department of Geology, University of lbadan, lbadan E-mail: waleakinmosin2001@yahoo.com ABSTRACT Limestone deposits can be found in every system of the geologic column since the Precambrian. In classifying this rock, various attempts have been made and notable among them is the classification scheme based on three components: allochems, cements and matrix. Another classification divided limestone on the basis of texture into grainstone, packstone, mudstone, wackestone and boundstone. Texturally, limestone ranges from fine to coarse grained with aragonite and calcite as the most common minerals. This work was aimed at identifying and describing both the allochemical and orthochemical components of the limestone deposits in order to classify and deduce their depositional environment. Eight core samples of limestone deposits representing the Ewekoro Formation of the Dahomey Basin were studied petrographically. The result of the analysis carried out on the samples showed that the Ibese limestone is composed of the following allochems: sponges, echinoids, bivalves, coralline algae, and pelecypod which represent the skeletal content and intraclasts, pellets and ooids which represent the non skeletal grains. The orthochemical constituents include the carbonate mud matrix and the sparry calcite cement. These constituents observed from the slides indicate that the Ibese limestone belongs to the class of wackestone and could have been deposited in a quiet and low energy environment. RESUMEN Los depósitos de Caliza se pueden encontrar en todo sistema de la columna geológica a partir del Precámbrico. Para su clasificación se han hecho varios intentos y entre ellos es notable el sistema de clasificación basado en alo-químicos, cemento y matriz. Otra forma de clasificación es según la textura. Texturalmente, la caliza va del grano fino a grueso con Aragonito y Calcita como los minerales más comunes. Este trabajo permitió identificar y describir los componentes alo-químicos y ortho-químicos de los depósitos de Caliza con el fin de clasificar y deducir su ambiente de deposición. Se estudiaron petrográficamente ocho muestras de núcleos de depósitos de Caliza representativos de la Formación Ewekoro en la Cuenca Dahomey. Los resultados de los análisis mostraron que la caliza Ibese está conformada por los siguientes alo-químicos: esponjas, equinodermos, moluscos, algas coralinas, y pelecípodos e intraclastos, pellets y ooides . Los componentes ortho-químicos incluyen carbonato en la matriz y cemento de calcita espática. Estos componentes observados en las secciones delgadas indican que la caliza Ibese es de la clase Wackosa y podría haber sido depositado en un ambiente tranquilo y de baja energía. Keywords: Carbonate, Depositional, Environment, Petrographic, Allochems, Micrite and Fossiliferous. Palabras clave: carbonato, deposicionales, medio ambiente, petrográfico, aloquímicos, micrita y fosilíferas Record Manuscript received: 10-05-2010 Accepted for publication: 05-12-2010 EARTH SCIENCES RESEARCH JOURNAL Earth Sci. Res. S J. Vol. 14, No. 2 (December, 2010): 187-196Research Groupin Geophysics UNIVERSIDAD NACIONAL DE COLOMBIA Introduction Approximately 20% of the sedimentary rocks is made up of carbonate rocks especially Limestone and dolomite (Chilingar et al. 1972). The Dahomey Basin (Fig.1) is a marginal pull-apart basin (Klemme, 1975) or Margin sag basin (Kingston et al., 1983), which was initiated during the early Cretaceous separation of African and South American lithospheric plates. The eastern flank of this basin is delineated by the Okitipupa high, while the Western flank extends as far as into Togo and Ghana. The Dahomey Basin contains piles of sediments that have been found to be about 3000m and thickens offshorewards. Studies (geology, stratigraphy, textural and geochemical) on the eastern Dahomey Basin have been undergone by previous authors some of which include Jones and Hockey, 1964;Reyment, 1965; Omatsola and Adegoke, 1981 Coker and Ejedawe, 1983; Billman, 1992; Elueze and Nton, 2004 and Akinmosin et al, 2005. In classifying limestone, various attempts have been made, notable among them are the classification scheme of Folk (1959,1962) which was based on three components: allochems, matrix and cements. The classification of Dunham, 1962 divides limestone on the basis of texture into grainstone, packstone, mudstone, wackestone and boundstone. This work is aimed at identifying and describing both the allochemical and orthochemical components of Ibese limestone deposits based on petrographic studies and to deduce the depositional environment. Stratigraphy The area of study covers Ibese, Ogun State, Southwestern Nigeria. It is within the Eastern Dahomey Basin of Nigeria. (Fig. 2). Ibese lies at the southern edge of the Ewekoro depression and is overlooked by the escarpment capped by the Ilaro Formation which forms a prominent feature in the area. Ibese quarry is located along Ilaro-Idiroko road and it lies between latitudes 6 0 9 1 N and 7 0 N and longitudes 3 0 E and 3 0 3 1 E. Ibese quarry is operated by the Dangote cement company. The reviewed work of Omatsola and Adegoke (1981) on the Cretaceous stratigraphy of the Dahomey Basin has recognized three formations belonging to the Abeokuta Group. These are: the Ise Formation, consisting essentially of continental sands, grits and siltstones, overlying the basement complex. Neocomian to Albian age has been assigned to this formation. Overlying the Ise Formation is the Afowo Formation, which consists of course to medium- grained sandstones with variable interbeds of shales, siltstones and clay. The sediments of this formation were deposited in a transitional to marginal marine environment Turonian to Maastritchtian age has been assigned to this formation. The Araromi Formation consists essentially of sand, overlain by dark-grey shales and interbedded limestone and marls occasional lignite bands. The formation conformably overlies the Afowo Formation and Maastrichtian to Paleocene age has been assigned (Omatsola and Adegoke, 1981). Overlying the Abeokuta Group conformably is the Imo group, which comprises of shale,limestone and marls. The two-lithosratigraphic units under this group are: Ewekoro Formation and Akinbo Formation. Adegoke (1977) described the formation as consisting of shaly limestone about12.5m thick which tends to be sandy and divided it into three microfacies. Ogbe (1972) further modified this and proposed a fourth unit. It is Paleocene in age and associated with shallow marine environment due to abundance of coralline algae, gastropods, pelecypods, echinoid fragments and other skeletal debris. Akinbo Formation lies on the Ewekoro 188 Akinmosin, A., and Osinowo, O.O. Figure 1: East-West geological section showing the Dahomey Basin and upper part of the Niger Delta (After Whiteman, 1982). Formation and it comprises of shale, glauconitic rock bank, and gritty sand to pure grey and with little clay. Lenses of limestone from Ewekoro Formation grades literally into the Akinbo shale very close to the base. The base is characterized by the presence of a glauconitic rock. The age of the formation is Paleocene to Eocene. Overlying the Imo group is the Oshoshun Formation. It is a sequence of mostly pale greenish-grey laminated phosphatic marls, light grey white-purple clay with interbeds of sandstones. It also consists of claystone underlain by argillaceous limestone of phosphatic and glauconitic materials in the lower part of the formation. Eocene age has been assigned to this formation (Agagu, 1985). The sedimentation of the Oshoshun Formation was followed by a regression, which deposited the sandstone unit of Ilaro Formation (Kogbe, 1976). The sequence represents mainly coarse sandy estuarine deltaic and continental beds, which show rapid lateral facies change. The coastal plain sands are the youngest sedimentary unit in the eastern Dahomey Basin. It probably overlay the Ilaro Formation unconformably, but convincing evidence as to this is lacking (Jones and Hockey, 1964). It consists of soft, poorly sorted clayey sand and pebbly sands. The age is from Oligocene to Recent. Methodology Samples used for this work are core samples retrieved from a well in Ibese. Eight Samples were selected at an average vertical interval of 1.5m. Part of the field work included thorough optical examination of the samples. Afterwards, the selected samples were clearly labeled and kept in sample bags for laboratory analysis. Prepared slides from the collected samples were closely examined under plane polarized light.The stage of the microscope was rotated continuously to attain different views of the slides. Photomicrographs of each slide were taken under crossed nicols and comparison was made with carbonate photomicrograph catalogue to ascertain the compositional features. Results and discussion Lithology The limestone samples from Ibese quarry are medium grained with colour ranging from light brown to cream. Dark brown stains were also observed on Petrographical Study of Ewekoro Carbonate Rocks, in Ibese, South Western Nigeria 189 Contour Line Dispensary River Egba River Oke 0 1 2km Footpath 3 3.01 3.02 3.03 3 3.01 3.02 3.03 6.95 6.96 6.97 6.98 6.99 7 6.95 6.96 6.97 6.98 6.99 7 Ile Owo Joga Abule Egba Araromi Disp. Sch. Sch School Settlement Major Road Ibese Study Area Figure 2: Location map showing the study area some of the exposed samples which is an indication of oxidation of the iron content. The samples were observed to be highly fossiliferous. Petrographic Description Allochemical constituents, carbonate mud matrix (micrite) and sparry calcite cement are the major components of carbonate rocks, Folk (1959, 1962). The petrographic description of Ibese limestone was based on these three parameters. The term “allochem” is applied to carbonate grains greater than 0.062mm Rich, 1964. Discussion Allochems The non skeletal grains identified from the petrological study are intraclast, pellets and ooids while the skeletal grains represent the fossil content. Skeletal Grains (Fossil Content) For the identification of the fossil content of the Ibese limestone, the following points were noted: shape, internal micro-structure and presence of spines or pores. The following were identified with varying depth: Sponges, echinoderms, pelecypods, bivalves, coralline algae and foraminifera Globorotalia cerroazulensis. Sponges The sponge spicules are composed of silica or calcite. Sponges were identified in Plates 1 and 6 at depths 23m and 40m respectively. They serve as source of silica for the formation of chert nodule and the silicification of the limestone. Echinoderms It was observed that sparite cement crystal had grown syntaxially around the echinoderm fragment identified in the lbese limestone deposit. (See Plate 3a). Bivalves Bivalve shells identified in the deposit consisted of several layers of specific internal microstructure composed of micron-sized crystallites. (See Plate 3b). Non Skeletal Grains Intraclasts These are fragments that have been reworked within the area of deposition thereby forming new sediment. Sizes of identified intraclasts ranged from sand size to pebble or boulder size. Sub-angular or subrounded types were rare and some of them were identified to possess irregular protuberances. Some have complex internal structure and contain fossils, pellets, quartz silt as seen in Plates 1, 2, 5, 7 and 8. Pellets These are rounded, spherical to elliptical or ovoid aggregates of microcrystalline calcite ooze which are devoid of any internal structure. They show a uniformity of shape and size (See Plate 6). Ooids These are particles that show either radical or concentric structure (Plates 2 and 5). Ooid typically form in agitated waters. 190 Akinmosin, A., and Osinowo, O.O. Table 1: Summary table of components of ibese limestone Plate Depth (M) Fossil Intraclast Ooid Pellet Support Sparry Calcite 1 23.0 Sponge Spicules v X v MUD/GRAIN v 2 24.5 Sponge Spicules v v v MUD v 3 34.0 Echinoid Sponge Spicules Bivalves X X X MUD X 4 37.5 Pelecypod X X X GRAIN v 5 38.5 Pelecypod v v v MUD/GRAIN v 6 40.0 Sponge Spicules X X v MUD X 7 47.5 v X v GRAIN v 8 48.5 Globorotalia Cerroazulensis v X v MUD v Carbonate Mud Matrix (Micrite) It is considered as forming very largely by rather rapid chemical or biochemical precipitation in sea water settling to the bottom and at times undergoing some later drifting by weak currents. Micrite was identified in most of the samples studied (Plates 2,3,4,7 and 8) Sparry Calcite Cement This type of calcite generally forms grains or crystals 10 microns or more in diameter and is distinguished from microcrystalline calcite by it’s clarity as well as coarser crystal size. Large coarse grains of sparry calcite which were white and grey in colour were observed to be dominant in Plates 1 and 7. Environmental Interpretation This is based on the fossil content and the dominant support of the Ibese limestone (Table 1). The fossil contents include sponges, echinoids, bivalves, coralline algae, pelecypod which are typical of an open shelf environment. The presence of echinoid indicates deeper waters. The dominant support is mud and muddiness is generally a property of rocks deposited in quiet water and a low energy environment. The absence of shell fragments is indicative of this. Conclusion Petrographic studies of Ibese limestone deposit have shown that the rock is highly fossiliferous with the identified fossils indicating deposition in an open shelf environment. Moreover, the limestone deposit was equally observed to be principally mud supported which is indicative of rocks deposited in quiet water and a low energy environment. From the aforementioned textural characteristics, Ibese limestone deposits can be classified as Wackestone according to Dunham, 1962. References Adegoke O. S. (1977): Stratigraphy and Paleontology of the Ewekoro Formation (Paleocene) of South western Nig. Bulletin American Paleontologist, Vol. 71, No 295, 375pp. Agagu O.A (1985): A Geological Guide Bituminous Sediments in South Western Nigeria. Unpublished Report, Department of Geology University of Ibadan. Akinmosin A., Odewande A.A., and Akintola A.I. (2005): Geochemical Composition and Textural Features of some Carbonate Rocks in Parts of Southwestern Nigeria. Ife Journal of Science, Vol. 7, num. 1. Billman H.G. (1992): Offshore Stratigraphy and Paleontology of Dahomey Embayment, West Africa. Nigerian Association of Petroleum Explorationist Bulletin, Vol.7, No. 2 pp. 121-130. Chilingar, G.V., Mannon R.W. and H.R. Riecke (1972): Oil and Gas production from Carbonate Rocks Ed by G.Larson and G.V. Chilingar pp 408. Elsevier, New York. 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(1976): The Cretaceous and Paleocene Sediments of South Eastern Nigeria in: Kogbe, C. A. (ed) Geol. of Nigeria. Elizabethan Publishing Co. Lagos. Ogbe, F.G.A., (1972): Stratigraphy of Strata Exposed in the Ewekoro Quarry, Saith Western Nigeria. In: African Geology, pp. 305-322. Omatsola M.E. And Adegoke O.S. (1981): Tectonic and Cretaceous Stratigraphy of the Dahomey Basin. Journal of Mining and Geology, Vol. 54 pp 65-87. Reyment R.A. (1965): Aspect of the Geology of Nigeria. Ibadan University Press, 145pp. Rich, M. (1964): Petrographic classification and method of description of carbonate rocks of the Bird Spring Group in Southern Nevada: Journal of Sedimentary Petrology, Vol.34, pp 367-378. Whiteman, A.J. (1982). Nigeria: Its petroleum Geology, Resources and Potentials, vol.2, Graham and Trotman, London. Pp. 268. Petrographical Study of Ewekoro Carbonate Rocks, in Ibese, South Western Nigeria 191 Appendix 1 192 Akinmosin, A., and Osinowo, O.O. Plate 1: Photomicrograph of Ibese limestone (Depth 23m). Plate 2: Photomicrograph of Ibese limestone (Depth 24.5m). Petrographical Study of Ewekoro Carbonate Rocks, in Ibese, South Western Nigeria 193 Plate 3: Photomicrographs of Ibese Limestone (Depth 34.0m) Plate 4: Photomicrograph of Ibese limestone (Depth 37.5m) 194 Akinmosin, A., and Osinowo, O.O. Plate 5: Photomicrograph of Ibese limestone (Depth 38.5m). Plate 6: Photomicrograph of Ibese limestone (Depth 40.0m) Petrographical Study of Ewekoro Carbonate Rocks, in Ibese, South Western Nigeria 195 Plate 7: Photomicrograph of Ibese limestone (Depth 47.5m). Plate 8: Photomicrograph of Ibese limestone (Depth 48.5m) CONTENIDO Using the micro – resistivity method to detect hispanic ancient floors at Nombre de Dios, Panamá Alexis Mojica1, Louis Pastor2, Richard Vanhoeserlande3, María Salamanca Heyman4 127 Contribution of topographic and penetrometric measurements to a site characterization,case of the Kekem Landslide, National Road N°5 (Western Cameroon) Jean Jacques Nguimbous-Kouoh1, 2* and Eliezer Manguelle-Dicoum1, 2 135 Thermal modeling of pluton emplacement and associated contact metamorphism:Parashi stock emplacement in the Serranía de Jarara (Alta Guajira, Colombia) Lina Fernanda Martínez and Carlos A. Zuluaga C.1 145 Potential littoral sediment transport along the coast of South Eastern Coast of India S. Saravanan and N. Chandrasekar 153 X-ray color maps of the zoned garnets from Silgará Formation metamorphic rocks,Santander Massif, Eastern Cordillera (Colombia) Carlos Alberto Ríos R.1, Oscar Mauricio Castellanos A.2, Akira Takasu3 161 Reservoir characterization by multiattribute analysis: The Orito field case Jairo G. Guerrero1,2, Carlos A. Vargas2 and Luis A. Montes2 173 Laboratory testing of the Monotonic behavior of partially saturated sandy soil Noureddine Della1 181 Petrographical Study of Ewekoro Carbonate Rocks, in Ibese, South Western Nigeria 1Akinmosin, A., and 2Osinowo, O.O. 187