GEOCIENCIAS-VOL 13-2 2009.vp EARTH SCIENCES RESEARCH JOURNAL Earth Sci. Res. J. Vol. 13, No. 2 (December 2009): 148-166 GEOLOGY AND DEPOSITIONAL ENVIRONMENT OF CAMPANO-MAASTRICHTIAN SEDIMENTS IN THE ANAMBRA BASIN, SOUTHEASTERN NIGERIA: EVIDENCE FROM FIELD RELATIONSHIP AND SEDIMENTOLOGICAL STUDY Adeigbe, O.C and Salufu, A. E Corresponding address 1&2 Department of Geology, University of Ibadan, Nigeria olukris2001@yahoo.com and seemeomoh@yahoo.com ABSTRACT The study area lies within the Anambra Basin and it is made up of Enugu Shale, Mamu Formation, Ajali Sandstone, and Nsukka Formation. This study aimed at determining the geology and depositional environmental of these formations through field relationship and grain size distribution morphologic studies. The field data shows Enugu Shale as fissile, light grey with extraformational clast which graded into Mamu Formation which is made up of shale, coal and sandy shale. It passes upward into Ajali Sandstone which is characterized by cross beds, Herring- bone structures and Ophiomorpha burrows. The youngest formation within the basin is Nsukka Formation. The granulometric study of Mamu Formation shows fine to medium grains, coarse, medium to fine grain for Mamu and Ajali Formation respectively. The standard deviation indicates poorly sorted. The kurtosis shows leptokurtic, platykurtic to very leptokurtic for both while the skewness values indicate positive and symmetrical in all except for Ajali Sandstone that is nega- tively skewed. The bivariate and the multivariate results reveal shallow marine and fluvial deposits for both Mamu Formation and Ajali Sandstone respectively. The paleocurrent direction of Ajali Sandstone indicates southwest while the provenance is north- east. The fissility of Enugu Shale suggests that it was deposited in low energy environment, distal to proximal lagoon environment. The presence of extraformatonal clast within Enugu Shale indicates fluvial incursion. However, the textural analysis of Mamu Formation suggests a sediment deposited in a low energy environment which favoured deposition of fine to medium size sedi- ments that is, estuary environment. Textural result of Ajali Sandstone in the study area coupled with the field data such as Her- ring-bone structures, and Ophiomorpha burrows, revealed that Ajali Sandstone was deposited in a tidal environment probably littoral environment. While the light grey colour observed in the Nsukka Formation suggests sediments deposited under an ox- idizing marine environment. Key words: Anambra Basin, Geology, depositional environment, Mamu Formation and Ajali Sandstone 148 Manuscript received: 11/08/2009 Accepted for publication: 18/12/2009 ENERO 30-GEOCIENCIAS-VOL 13-2 2009.prn D:\GEOCIENCIAS 13-2 DIC 2009\GEOCIENCIAS-VOL 13-2 2009.vp sÆbado, 30 de enero de 2010 18:16:33 p p p Composite 133 lpi at 45 degrees RESUMEN El área de estudio está en la cuenca Anambra, compuesta de las Arcillas Enugu, la formación Mamu, la Arenisca Ajali, y la formación Nsukka. Este estudio ayudó a determinar la geología y el ambiente de deposición de estas formaciones mediante relaciones de campo y estudios morfológicos de distribución de tamaño de grano. Los datos de campo muestran que las arcillas Enugu son quebradizas, de color gris claro con clastos extraformacionales que gradaron en la formación Mamu la que está compuesta de arcillas, carbón y arcilla arenosa. La Arenisca Ajali está caracterizada por estratificación cruzada, estructuras de espiga y túneles de Ophiomorphas. La formación más joven de la cuenca es la Formación Nsukka. El estudio granulométrico de la Formación Mamu muestra tamaños de grano fino a medio, grueso, y de medio a fino para las Formaciones Mamu y Ajali respectivamente. La desviación estándar indica una pobre clasificación de los granos. La curtosis son leptocúrticas, platycurticas a muy leptocúrticas para ambas formaciones, mientras que los indicadores de asimetría muestran asimetría positiva y simétrica en todos los casos excepto para la Arenisca Ajali que presentó asimetría negativa. Las distribuciones bivariadas y multivariadas indican depósitos someros marinos y fluviales para las Formaciónes Mamu y Arenisca Ajali respectivamente. La dirección de las paleocorrientes de la Arenisca Ajali es suroeste mientras que la procedencia es el noreste. La fragilidad de las arcillas Enugu sugiere que fué depositada en un ambiente de baja energía, en ambiente de laguna distal a proximal. La presencia de clastos extraformacionales indica incursión fluvial. Sin embargo, el análisis de textura de la Formación Mamu sugiere que los sedimentos fueron depositados en un ambiente de baja energía la cual favoreció la depositación de sedimentos de tamaño finos a medio, esto es en ambiente de estuario. Los resultados de textura de la Arenisca Ajali en el área de estudio junto con los datos de campo tal como estructuras de espigaizarra laguna cercana y túneles de Ophiomorphas, revelaron que la Arenisca Ajali fué depositada en ambiente de marea probablemente ambiente de litoral. Mientras el color gris claro observado en la Formación Nsukka sugiere sedimentos depositados en un ambiente marino oxidante. Palabras clave: Cuenca Anambra, Geología, Ambiente deposicional, Formación Mamu y Arenisca Ajali. Introduction The tectonism in Southern Nigeria probably started in Early Cretaceous, with the separation of Africa from South American and opening of the Atlantic. This re- sulted in the development of the Benue Trough which stretched in a NE-SW direction (Fig. 1) and resting un- conformably upon the Pre-Cambrian basement complex (Table 1). It extends from the Gulf of Guinea to the Chad Basin and is thought to have been formed by the Y-shaped (RRR) triple junction ridge system that initiated the breaking and dispersion of the Afro-Brazilian plates in the Early Cretaceous (Kogbe, 1989). After the evolution of the Benue Trough, sediments started depositing into the trough with Asu River Group being the oldest sediment followed by Ezeaku Group, and Awgu Group respectively (Nwajide, 1990). Santonian age marked the stage when the basin experi- enced another phase of tectonic event that involved deforma- tion, folding, faulting and uplift of the Pre-Santonian sediments leading to the formation of Anambra Basin which evolved as a depression to the west of the uplift (Benkhelil, 1987). Anambra Basin is a Cretaceous depo-centre that received Campanian to Tertiary sediments (Nwajide, 1990 and Obi, 2000). The strati- graphic setting of Southern Nigeria comprises sediments of three major sedimentary cycles. The first two cycles belong to the Pre-Santonian sediments while the third cycle belongs to Post-Santonian sediments which are found in the Anambra Ba- sin and Afikpo Syncline (Nwajide, 1990). In Anambra Basin, the strongly folded Albian- Coniacian succession (Pre-Santonian sediments) is over- lain by nearly flat-lying Campanian-Eocene succession (Table 1). The oldest sediment in the Anambra Basin is Nkporo Group (Nwajide, 1990). It was deposited into the basin in Late Campanian, comprising Nkporo Shale, Owelli Sandstone and Enugu Shale (Reyment, 1965 and Obi, 2001). 149 GEOLOGY AND DEPOSITIONAL ENVIRONMENT OF CAMPANO-MAASTRICHTIAN SEDIMENTS IN THE ANAMBRA BASIN, SOUTHEASTERN NIGERIA: EVIDENCE FROM FIELD RELATIONSHIP AND SEDIMENTOLOGICAL STUDY ENERO 30-GEOCIENCIAS-VOL 13-2 2009.prn D:\GEOCIENCIAS 13-2 DIC 2009\GEOCIENCIAS-VOL 13-2 2009.vp sÆbado, 30 de enero de 2010 18:16:34 p p p Composite 133 lpi at 45 degrees 150 ADEIGBE, O.C. AND SALUFU, A. E Figure 1: Tectonic map of Southeastern Nigeria (After Murat, 1972) Table 1: Correlation Chart for Early Cretaceous Tertiary strata in the Southeastern Nigeria (After Nwajide, 1990). Age Abakaliki-Anambra Basin Afikpo Basin m.y 30 Oligocene Ogwashi-Asaba Formation Ogwashi-Asaba Formation 54.9 Eocene Ameki/Nanka Formation/ Nsugbe Sandstone (Ameki Group) Ameki Formation 6.5 Palaeocene Imo Formation Nsukka Formation Imo Formation Nsukka Formation 73 Maastrichtian Ajali Formation Mamu Formation Ajali Formation Mamu Formation 83 87.5 Camparian Npkoro Oweli Formation/Enugu Shale Nkporo Shale/ Afikpo Sandstone Santonian Agbani Sndstone/Awgu Shale Non-deposition/erision 88.5 Coriacian Eze Aku Grupo (incl. Amasiri Sandstone)Turonian Eze Aku Grupo 93 100 Cenomanian- Albian Asu River Group Asu River Group ENERO 30-GEOCIENCIAS-VOL 13-2 2009.prn D:\GEOCIENCIAS 13-2 DIC 2009\GEOCIENCIAS-VOL 13-2 2009.vp sÆbado, 30 de enero de 2010 18:16:34 p p p Composite 133 lpi at 45 degrees Nkporo Group is overlain by Mamu Formation (fig.2). It was deposited in Early Maastrichtian (Kogbe, 1989 and Obi, 2000). It comprises succession of siltstone, shale, coal seam and sandstone (Kogbe, 1989). Ajali sandstone (Maastrichtian) overlies Mamu Formation (Reyment, 1965 and Nwajide, 1990) which is mainly unconsolidated coarse-fine grained, poorly cemented; mudstone and siltstone (Kogbe, 1989). Ajali Sandstone is overlain by diachronous Nsukka Formation (Maastrichtian-Danian) which is also known as the Upper Coal Measure (Reyment, 1965 and Obi, 2000). Imo Shale (Paleocene) overlies Nsukka Formation (Nwajide, 1990). It comprises clayey shale with occasional ironstone and thin sandstone in which carbonized plants remains may occur (Kogbe, 1989). The Eocene stage was characterized by regressive phase that led to deposition of Ameki Group (Obi, 2000). 151 GEOLOGY AND DEPOSITIONAL ENVIRONMENT OF CAMPANO-MAASTRICHTIAN SEDIMENTS IN THE ANAMBRA BASIN, SOUTHEASTERN NIGERIA: EVIDENCE FROM FIELD RELATIONSHIP AND SEDIMENTOLOGICAL STUDY Nsukka Formatio Ajali Sandstone Mamu Formation Nkporo Shale Awgu Shale Eze-Aku Group Odukpani Formation Asu River Group Basement complex LENGEND 0 25 50 Kilometres 1 : 2 500 000 R. Niger Paleocene- eocene R. Bemue 7° E 8° E 7°N 6°N N Figure 2: Regional stratigraphy of southeastern Nigeria (After Nwajide 1990). ENERO 30-GEOCIENCIAS-VOL 13-2 2009.prn D:\GEOCIENCIAS 13-2 DIC 2009\GEOCIENCIAS-VOL 13-2 2009.vp sÆbado, 30 de enero de 2010 18:16:34 p p p Composite 133 lpi at 45 degrees 152 ADEIGBE, O.C. AND SALUFU, A. E Figure 3. Geologic and the cross sectional map of the study area. �M M Age Litholog Sample No Lithologic description of 116.2 4.2 Campanian No Sample Grey fissile shale, strike of N156°/E8°SW, dip direction of 2560/80 112.0 17.0 No Sample Grey fissile shale dip amount of 8°, strike of N156°E/8°SW 80.0 10.0 No Sample Grey fissile shale 70.0 22.0 No Sample Grey fissile shale with normal fault (Throw 0.4 and heave 0.1) 48.0 28.0 No Sample Grey fissile shale with strike of N155°W/8°SE dip direction of 255°/8° 20.0 20.0 No Sample Grey fissile shale with strike of N153°W/9°SE and dip direction of2550/80 Figure 4. Lithologic description of Enugu shale as exposed along Enugu-Onitsha Road. ENERO 30-GEOCIENCIAS-VOL 13-2 2009.prn D:\GEOCIENCIAS 13-2 DIC 2009\GEOCIENCIAS-VOL 13-2 2009.vp sÆbado, 30 de enero de 2010 18:16:35 p p p Composite 133 lpi at 45 degrees Local stratigraphy description In the study area, Enugu Shale is the oldest formation and it is exposed in four different places; Location 1 (Enugu-Onitsha express road), Location 5 (River Ekulu), Location 6 and 7 (Ogbete) (Fig. 3). At the four locations, the outcrops have similar characteristics except for location 1 (Enugu –Onitsha express road) that has the widest exposure (both in vertical and lateral extents). The lateral extent is es- timated to be over 1000m while the vertical extent is 116.2m (Fig.4). Along Enugu-Onitsha expess road (Location1] Enugu Shale has a basal bed It is dipping in the direction of 255°/8°, and trends in direction of S255°/8°SE. of grey fis- sile shale, estimated to be 20m thick (Fig.4). It is dipping in the direction of 255°/8°, and trends in direction of S255°/8°SE. The basal bed passes into another grey fissile shale unit of 28m thick, having similar altitude with the un- derlying bed. The section continued with 22m thick bed of grey fissile shale, characterized by extraformational clast (Fig. 5). This unit is overlain by a faulted (Fig. 6) fissile shale bed. The faulted bed is a normal fault with heave of 0.1m and throw of 0.4m. It is 10m thick, dipping in 245°/8° direction with strikes similar to the underlying bed. The faulted shale bed is overlain by grey shale of about 170m thick, having similar altitude with underlying bed. This sec- tion is terminated by 4.2m thick grey shale. The Mamu Formation overlies Enugu shale. It is ex- posed at two locations; Location 2 (Onyeama coal mine) and Location 7 (Proda) of which Onyeama mine (Location 2) has the widest exposure. At the abandoned Onyeama coal mine in Enugu, the outcrop comprises succession of sand- stone, siltstone, coal seam, heteroliths and shale, which has been grouped into three major facies as follows: (i) Shale facies units. (ii) Coal facies unit (iii) Sandy shale facies unit Shale Facies This facies is the oldest unit of the three, it commences with basal thin bed of light yellow, well laminated sandstone. The 153 GEOLOGY AND DEPOSITIONAL ENVIRONMENT OF CAMPANO-MAASTRICHTIAN SEDIMENTS IN THE ANAMBRA BASIN, SOUTHEASTERN NIGERIA: EVIDENCE FROM FIELD RELATIONSHIP AND SEDIMENTOLOGICAL STUDY Figure 5. Extraformational clast inbedded within Enugu Shale along Enugu-Onitsha road. Figure 6. Faulted Shale unit of Enugu Shale exposed along Enugu-Onitsha road. Figure 8. Burrows of Planolites within Mamu Formation at Onyeama coal mine, Enugu Figure 7. Rippled Sandstone unit of Mamu Formation exposed at Onyeanma coal mine, Enugu. ENERO 30-GEOCIENCIAS-VOL 13-2 2009.prn D:\GEOCIENCIAS 13-2 DIC 2009\GEOCIENCIAS-VOL 13-2 2009.vp sÆbado, 30 de enero de 2010 18:16:37 p p p Composite 133 lpi at 45 degrees basal sandstone is overlain by dark grey horizontal fissile shale bed. The fissile shale graded into fine, well sorted, rip- pled (Fig. 7) light yellow sandstone. It transits into a dark-grey fissile shale bed that dips towards 260°/7° and strikes in N350°W/7°SE trend. It graded into light brown, bioturbated sandstone that has been reworked. It has traces of planolites burrows (Fig. 8). The sandstone bed is overlain by two thin beds of dark grey fissile shale. The last shale bed graded into 2.2m thick moderately consolidated white siltstone that is laminated with traces of plant roots. Coal Facies unit The coal facies unit is about 26m thick with alternation of coal seems, heterolith, sandstone and siltstone (Fig. 9). The basal bed of this facies is a coal seam of 0.1m thick. It passes unto heterolith which is overlain by white, moderately con- solidated sandstone. The sandstone graded into a black coal seam. The facies continued upward with two whitish, lami- nated, rippled siltstone beds of 2m and 3m thick respec- tively. The siltstone makes erosional contact with the overlying medium-fine consolidated flat sandstone bed. The sandstone bed passes into heterolith which is then overlain by 0.2m black coal seam (Fig. 10) that terminates the coal fa- cies unit. Sandy Shale Facies Unit This unit is estimated to be over 130m. It is mainly of sand- stone beds that are light yellow, well sorted-clayey, bioturbated, cross bedded with wave ripples. The sandy shale (Fig. 11) is overlain by 4.2m thick white laminated sandstone (Fig. 12). Ajali Sandstone It is exposed at two stations in the study area; Location 3 (Ngwo) and Location 9 (9th Mile). The widest exposure of this formation is at Ngwo. The vertical exposure is esti- 154 ADEIGBE, O.C. AND SALUFU, A. E �M M Age Litholog Sample No. Lithology 157.7 130.4 Mastrichtian 6 Samples Sequence of sandstone and sandy shale. Brownish red lateratized sandstone and grey iron stone 27.3 20.5 2 Samples Sequence of different strata from sandstone, shale, siltstone, sandy shale and sandstone, coal, and siltstone 6.8 6.7 No Sample Sequence of different strata from sandstone, shale, siltstone,and siltstone 0.5 No Sample Dark grey fissile shale 0.5 3 samples Light yellow well sorted and laminated sandstone Figure 9. Lithologic description of Mamu Formation exposed at Onyeama coal mine, Enugu Figure 10. Coal seam unit of Mamu Formation exposed at Onyeama Coal mine. ENERO 30-GEOCIENCIAS-VOL 13-2 2009.prn D:\GEOCIENCIAS 13-2 DIC 2009\GEOCIENCIAS-VOL 13-2 2009.vp sÆbado, 30 de enero de 2010 18:16:38 p p p Composite 133 lpi at 45 degrees 155 GEOLOGY AND DEPOSITIONAL ENVIRONMENT OF CAMPANO-MAASTRICHTIAN SEDIMENTS IN THE ANAMBRA BASIN, SOUTHEASTERN NIGERIA: EVIDENCE FROM FIELD RELATIONSHIP AND SEDIMENTOLOGICAL STUDY Figure 11. Sandy shale unit of Mamu Formation exposed at Onyeama Coal mine Figure 12. Laminated Sandstone bed of Mamu Formation exposed at Onyeama Coal mine �M M Age Lithology Sample No Lithology description 21.0 10.0 Mastrichtian 1 Sample Grey white coarse, poorly sorted, clayey, unconsolidated, herringbone, crossbreds, with reactivated surface 11.0 9.0 1 Sample White, poorly sorted unconsolidated, clayey, crossbeded fine grained sandstone 2.0 2.0 1 Sample White, poorly sorted unconsolidated, clayey, fine grained sandstone Figure 13. Lithologic description of Ajali Sandstone exposed at Ngwo Figure 14. Herringbone cross bed within Ajali Sandstone exposed at Ngwo, Udi Figure 15. Reactivation surface within Ajali Sandstone exposed at Ngwo, Udi ENERO 30-GEOCIENCIAS-VOL 13-2 2009.prn D:\GEOCIENCIAS 13-2 DIC 2009\GEOCIENCIAS-VOL 13-2 2009.vp sÆbado, 30 de enero de 2010 18:16:40 p p p Composite 133 lpi at 45 degrees mated to be 21m while the lateral extent is estimated as 500m. The section starts with a basal, poorly sorted, poorly cemented unconsolidated white fine grained sandstone bed that is coarsening upward (Fig. 13).Ten meter (10m) thick, grey white poorly sorted, unconsolidated clayey sandstone bed that is characterized by herringbone cross beds (Fig. 14) and reactivation surface (Fig. 15) overlies the basal bed. This unit terminates the section with lateralized over- burden. Nsukka Formation It is exposed in one location within the study area, location 10. It is along Enugu- Eze-agu road. The vertical extent of the section is 4.8m while the lateral extent is 20m. Ferruginous brown, consolidated unit of 2m thick begins this section (Fig. 16). It graded into grey fissile shale. The shale unit trends S153°E/9°NW. It give effervescence in the presence of dil. HCL acid, thus indicating that it is carbona- ceous. The fissile shale passes unto 0.9m thick grey fissile shale bed. The section continues upward to shaly sandstone bed overlying the shale. The lithology transits into a brown, consolidated, highly ferruginised sandstone overlying the sandy shale. The section gets terminated by 3m thick brown- ish red, well consolidated, laminated ferruginised sandstone bed. Nsukka Formation is the youngest formation in the study area. The composite log of the exposed formations in the study area is shown in Fig. 17. This revealed the field spatial rela- tionship within the Anambra Basin, Southwestern, Nigeria. Methodology Fourteen representative samples of unconsolidated sand- stones were collected from the study area. Eleven samples were collected from Mamu Formation at different horizons while three samples were collected from Ajali Sandstone (the unavailability of sandstones horizon/unit within Enugu Formation does not allow for grain size analysis). The sam- ples were later disaggregated and divided into two equal parts of 50gm each. Sieving was done for each sample for 15 minutes on a Ro-tap sieve shaker, using a set of U.S standard sieve at ¼ phi sieve interval. Cumulative curves of the grain size distribution were plotted from the sieve results. The univariate, bivariate, and multivariate parameters were computed from the sieve re- sults after Folk and Ward (1970), Miola and Weiser (1977), and Sahu (1964). The formulae of mean size (Mz), standard deviation (ói),skewness (Ski) and kurtosis (KG), used to 156 ADEIGBE, O.C. AND SALUFU, A. E �M M Age Lithology Sample No Lithology description 4.8 3.0 Maastrichtian - Early Paleocene No Sample Brownish red lateratised sandstone, well Consolidated, laminated sandstone. 4.5 0.2 No Sample Brown sandstone consolidated, ferruginised, laminated sandstone 4.3 0.3 No Sample Grey fissile sandy shale 4.0 3.0 No Sample Grey ironstone 3.7 0.9 No Sample Grey fissile shale 2.8 0.8 No Sample Grey fissile shale with dip amount of 9°, And strike of 153° 2.0 2.0 No Sample Brown ferruginised sandstone Figure 16. Lithologic description of Nsukka Formation exposed at Eze-agu, Enugu ENERO 30-GEOCIENCIAS-VOL 13-2 2009.prn D:\GEOCIENCIAS 13-2 DIC 2009\GEOCIENCIAS-VOL 13-2 2009.vp sÆbado, 30 de enero de 2010 18:16:40 p p p Composite 133 lpi at 45 degrees compute the sandstones that underlie the study area are given below respectively. Mz j � � � � � ( ) / ( ) / ( ) / . � � � � 16 50 84 3 34 16 4 95 5 6 6 (1) Ski � � � � ��� � � ��� � 16 34 2 2 8 16 5 9 2 2 9 5 ( ( ) ( ( ) (2) kG � ( . ( � �� � ���9 5 2 44 7 (3) Multivariate parameter is computed for the sandstones in the study area by: Yu = 0.2852 MZ – 8.760451 – 4.8932ski + 0.0482KG. If the computed values of Yu > - 7.419, it is inter- preted as shallow marine deposits while value of Yu < - 7.419 is interpreted as fluvial deposits (Sahu, 1964). The dip and az- imuth of twenty five foreset of planner cross beds were mea- sured from the cross bedded units of the Ajali Sandstone exposed in the study area. The measurement were carefully taken and recorded. The measurement were analyzed using variable constraints system by calculating the Mean Vector Azimuths (MVA) and variables of the cross bedded units of the Ajali Sandstone (Steinmetz,1962) in order to determine the paleocurrent of the Ajali Sandstone as well the trend of the paleocurrent by constructing rose diagram. Results The probability curves for the samples of Mamu Forma- tion and Ajali Sandstone show that the sandstone units of Mamu Formation were deposited by suspension and saltation (Fig. 18) while that of Ajali Sandstone were de- posited by suspension, saltation, and traction (Fig. 18). The mean size result shows that the sandstone units of Mamu Formation are fine to medium (Table 2). The stan- dard deviation result shows that the sandstone units of Mamu Formation are well sorted to moderately well sorted. The mean size result of Ajali Sandstone shows coarse, medium and fine sandstone. The standard devia- tion result reflects that the sandstone is poorly to moder- ately sorted (Table 2). The Histogram plots of the cumulative weight percent against the phi scale show that the Mamu Formation and Ajali Sandstone are unimodal (Fig. 19). The results of bivariate plots for Mamu formation and Ajali Sandstone in the study area indicate beach and flu- vial deposits (Fig. 20 and Fig. 21) while multivariate re- sults indicate mainly shallow marine and fluvial deposits (Table 3). However, the result of the paleocurrent analysis of the foreset of cross bedded units is well pronounced in the Ajali Sandstone and less pronounced in the Mamu Forma- tion (this may be due to the presence of siltstone, shale and coal in Mamu and complete absence in Ajali sandstone). This revealed that Ajali Sandstone has mean vector azi- muth of 239°0 that is in direction of south west, and its variance is 1,472 (Table 4). However, the rose diagram in- dicates paleocurrent direction toward southwest while provenance is in direction of northwest (Fig. 22). 157 GEOLOGY AND DEPOSITIONAL ENVIRONMENT OF CAMPANO-MAASTRICHTIAN SEDIMENTS IN THE ANAMBRA BASIN, SOUTHEASTERN NIGERIA: EVIDENCE FROM FIELD RELATIONSHIP AND SEDIMENTOLOGICAL STUDY �M M Age Lithology Sample No Formation Lithology description 299.9 4.8 Maast-Paleo No samples Nsukka Formation Sequence of sandstone and shale. Brownish red lateratized sandstone and grey iron stone 294.9 21.0 Maastrichtian 3 samples Ajali Sandstone White sandstone coarsing upward. Herringbone crossbeds and reactivation surface 273.9 157.7 11 samples Mamu Formation Sequence of different strata from sandstone, shale, siltstone, sandy shale and sandstone,coal,siltstone, sandstone, coal and fossiliferous sandstone. 116.2 116.2 Campanian No Samples Enugu Formation Grey fissile shale with normal fault Figure 17. Composite local stratigraphy of the study area ENERO 30-GEOCIENCIAS-VOL 13-2 2009.prn D:\GEOCIENCIAS 13-2 DIC 2009\GEOCIENCIAS-VOL 13-2 2009.vp sÆbado, 30 de enero de 2010 18:16:40 p p p Composite 133 lpi at 45 degrees 158 ADEIGBE, O.C. AND SALUFU, A. E - 1 - 0.5 0 0.5 1 1.5 2 2.5 3 3.5 Mamu 5 Mamu 6 Mamu 7 Figure 18b. Log probability curves for Mamu and Ajali Sandstone that underlie the study area - 1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 Mamu 8 Mamu 9 Mamu 10 Mamu 11 Figure 18c. Log probability curves for Mamu and Ajali Sandstone that underlie the study area - 1 - 0.5 0 0.5 1 1.5 2 2.5 3 3.5 Ajali 1 Ajali 2 Ajali 3 Figure 18d. Log probability curves for Mamu and Ajali Sandstone that underlie the study area - 1 - 0.5 0 0.5 1 1.5 2 2.5 3 3.5 Mamu 1 Mamu 2 Mamu 3 Mamu 4 Figure 18a. Log probability curves for Mamu and Ajali Sandstone that underlie the study area ENERO 30-GEOCIENCIAS-VOL 13-2 2009.prn D:\GEOCIENCIAS 13-2 DIC 2009\GEOCIENCIAS-VOL 13-2 2009.vp sÆbado, 30 de enero de 2010 18:16:42 p p p Composite 133 lpi at 45 degrees 159 GEOLOGY AND DEPOSITIONAL ENVIRONMENT OF CAMPANO-MAASTRICHTIAN SEDIMENTS IN THE ANAMBRA BASIN, SOUTHEASTERN NIGERIA: EVIDENCE FROM FIELD RELATIONSHIP AND SEDIMENTOLOGICAL STUDY -1-0.50.00.5 1 1.5 2 2.5 3 3.5 10 20 30 40 50 60 W t% -1-0.50.00.5 1 1.5 2 2.5 3 3.5 10 20 30 40 50 60 W t% -1 -0.50.00.5 1 1.5 2 2.5 3 3.5 10 20 30 40 50 60 W t% -1-0.50.00.5 1 1.5 2 2.5 3 3.5 10 20 30 40 50 60 W t% Phi-scale Mamu 3 Phi-scale Mamu 1 Phi-scale Phi-scale Mamu 4 Mamu 2 (a) -1 -0.5 0.0 0.5 1 1.5 2 2.5 3 3.5 10 20 30 40 50 60 W t% Mamu 5 -1 -0.5 0.0 0.5 1 1.5 2 2.5 3 3.5 10 20 30 40 50 60 W t% -1 -0.5 0.0 0.5 1 1.5 2 2.5 3 3.5 10 20 30 40 50 60 W t% -1 -0.5 0.0 0.5 1 1.5 2 2.5 3 3.5 10 20 30 40 50 60 W t% Mamu 6 Mamu 7 Mamu 8 Phi-scalePhi-scale Phi-scale Phi-scale (b) -1 -0. 0. 0. 1 1. 2 2. 3 3. 1 2 3 4 5 6 Wt Mamu 9 1 0. 0. 0. 1 1. 2 2. 3 3. 1 2 3 4 5 6 -1 -0. 0. 0. 11. 2 2. 3 3. 1 2 3 4 5 6 Wt Phi-scale Phi-scale Wt Phi-scale Mamu 10 Mamu 11 (c) ENERO 30-GEOCIENCIAS-VOL 13-2 2009.prn D:\GEOCIENCIAS 13-2 DIC 2009\GEOCIENCIAS-VOL 13-2 2009.vp sÆbado, 30 de enero de 2010 18:16:42 p p p Composite 133 lpi at 45 degrees 160 ADEIGBE, O.C. AND SALUFU, A. E -1 -0.5 0.0 0.5 1 1.5 2 2.5 3 3.5 10 20 30 40 50 60 W t% -1 -0.5 0.0 0.5 1 1.5 2 2.5 3 3.5 10 20 30 40 50 60 W t% Ajali 2 -1 -0.5 0.0 0.5 1 1.5 2 2.5 3 3.5 10 20 30 40 50 60 Ajali 3 Phi-scale Phi-scale Phi-scale (d) Ajali 1 Figure19a-d. Histogram plots of the Mamu Formation and Ajali Sandstone that underlie the study area 0.2 0.4 0.7 1.45 2.1 0.6 1.2 1.8 3.0 Fluvial Beach M z * * * * ** * Ajali Sanstone * Mamu Fm. * * * ° ° ° ° Figure 20. Graph of Mz against �i for the sandstone in the study area (After Miola and Waiser, 1977). Sk i 0.90.70.60.40.1 0.5 -1.5 0.0 -0.5 -2.0 0.5 1.0 0.2 0.3 0.8 1.21.11.0 1.3 -1.0 1.5 2.0 * * * * ** * * * * -3 * * ° ° ° ° Fluvial Beach Ajali Sanstone Mamu Fm. Figure 21. Graph of Ski against � for the sandstone of the study area (After Friedman, 1961). ENERO 30-GEOCIENCIAS-VOL 13-2 2009.prn D:\GEOCIENCIAS 13-2 DIC 2009\GEOCIENCIAS-VOL 13-2 2009.vp sÆbado, 30 de enero de 2010 18:16:42 p p p Composite 133 lpi at 45 degrees 161 GEOLOGY AND DEPOSITIONAL ENVIRONMENT OF CAMPANO-MAASTRICHTIAN SEDIMENTS IN THE ANAMBRA BASIN, SOUTHEASTERN NIGERIA: EVIDENCE FROM FIELD RELATIONSHIP AND SEDIMENTOLOGICAL STUDY Table 2. Computed grain size parameters derived from probability curves Sample NO Sorting �1 Values Skewness SKI values Kurtosis KG Values Mean size MZ Values Mamu1 Well sorted 0.47 positively skewed 0.1 leptokurtic 1.2 fine sand 2.3 Mamu2 well sorted 0.5 symmentrical 0.05 leptokurtic 1.2 fine sand 2.3 Mamu3 Moderately well sorted 0.47 positively skewed 0.1 very platykwtic 0.6 fine sand 3 Mamu4 Moderately well sorted 0.7 positively skewed 0.1 platykurtic 0.9 fine sand 2.7 Mamu5 poorly sorted 1.1 positively skewed 0.01 leptokurtic 1.2 fine sand 3 Mamu6 well sorted 0.5 positively skewed 0.01 platykurtic 0.6 fine sand 3 Mamu7 moderately well sorted 0.75 very positively skewed 0.1238 leptokurtic 1.2 1fine sand 2. Mamu8 well sorted 0.45 Symmentrical 0.05 platykwtic 1.2 medium sand 1.3 Mamu9 moderately sorted 0.7 Symmentrical 0.05 very platykwtic 1.5 medium sand 1.2 Mamu10 Poorly sorted 1.1 0.1 positively skewed 0.1 very leptokurtic 1.5 medium sand 1.2 Mamu11 Poorly sorted 1.6 Symmentrical 0.05 very leptokurtic 1.5 medium sand 1.2 Ajali1 poorly sorted 1.1 negatively skewed 0.3 platykwtic 0.6 coarse sand 0.5 Ajali2 moderately- well sorted 0.7 Symmentrical 0.05 very leptokurtic 1.5 medium coarse sand 1.3 Ajali3 Poorly sorted 1.6 Symmentrical -0.1 leptokurtic 1.2 fine sand 3 ENERO 30-GEOCIENCIAS-VOL 13-2 2009.prn D:\GEOCIENCIAS 13-2 DIC 2009\GEOCIENCIAS-VOL 13-2 2009.vp sÆbado, 30 de enero de 2010 18:16:42 p p p Composite 133 lpi at 45 degrees Discussion Depositional environment The fissility and the fine nature (Grain size) of the Enugu Shale as indicated by the field data suggest that Enugu Shale was deposited below the wave base, accumulated in rela- tively low energy environment i.e in a distal to proximal la- goon (Amaral and Pryor, 1974). The presence of extraformational clast within the Enugu Shale indicates that there was fluvial influence during the period of deposition of Enugu Shale within the Anambra Basin (Tucker, 1996). The light grey colour of the shale shows that Enugu Shale was deposited on the surface of the basin where oxidation could take place (Dapple, 1974). The fine medium grained sandstone, siltstone and fissile shale and Planolites burrows that dominates Mamu Forma- tion succession as observed from sieve and field relation- ships suggests that the unit was deposited in an environment where there was little or no existence of tidal or wave action i.e quiet environment where low energy favoured deposi- tion of fine – medium size sediments. The presence of coal beds that alternate the shale, siltstone and sandstone units within Mamu Formation sequence indicates that Mamu For- mation was deposited in estuary environment similar to the observation made by Reyment (1965) and Nwajide and Reijers (1996). The well sorted to moderately well sorted (Table 2) of Mamu Formation is an evidence of quiet envi- ronment with very low energy of deposition. The kurtosis for Mamu Formation revealed leptokurtic to platykurtic (Ta- ble 2) which suggest that Mamu Formation were sourced from more than one source which is similar to the observa- tion made by Akaegbobi and Boboye (1999). The bivariate plots (Figs. 20 and 21) and multivariate (Tables 3 and 4) show that Mamu Formation was deposited in a shallow ma- rine with prevalent of fluvial incursion. Information from the field data shows that Ajali Sand- stone is profusely cross bedded, characterized by Herring- bone structures and Ophiomorpha burrows. The presence of these structures indicates tidal environment with high en- ergy (Tucker, 1996). However, the presence of reactivation surface (Fig. 14) within Ajali Sandstone suggests fluvial ac- 162 ADEIGBE, O.C. AND SALUFU, A. E Table 3. Multivariate results of sandstone of Mamu Formation and Ajali Sandstone in the study area Sample Results Interpretation Ma1 -1.7107 Shallow marine Ma2 -1.7210 Shallow marine Ma3 3.9218 Shallow marine Ma4 -3.8829 Shallow marine Ma5 -9.8211 Shallow marine Ma6 -1.5508 Shallow marine Ma7 -4.0921 Shallow marine Ma8 -1.3619 Shallow marine Ma9 -4.0942 Shallow marine Ma10 -10.6749 Fluvial Ma11 -22.2570 Fluvial Aj1 -11.8965 Fluvial Aj2 -4.0942 Shallow marine Aj3 -21.0239 Fluvial ENERO 30-GEOCIENCIAS-VOL 13-2 2009.prn D:\GEOCIENCIAS 13-2 DIC 2009\GEOCIENCIAS-VOL 13-2 2009.vp sÆbado, 30 de enero de 2010 18:16:42 p p p Composite 133 lpi at 45 degrees 163 GEOLOGY AND DEPOSITIONAL ENVIRONMENT OF CAMPANO-MAASTRICHTIAN SEDIMENTS IN THE ANAMBRA BASIN, SOUTHEASTERN NIGERIA: EVIDENCE FROM FIELD RELATIONSHIP AND SEDIMENTOLOGICAL STUDY Table 4. Paleocurrent analysis of Ajali Sandstone in the study area S/N Azimuth (A) Dip (D) Sin (A) Cos (A) Cos (D) b = COS (A * D) a = Sin (A*D) (AZ- MVA)2 1 255 10 -0.9659 -0.2588 0.9848 -0.2549 -0.9512 256 2. 245 5 -0.9063 -0.4226 0.9962 -0.4210 -0.9029 36 3. 257 10 -0.9744 -0.2250 0.9848 -0.2216 -0.9596 324 4. 257 10 -0.9744 -0.2250 0.9848 -0.2216 -0.9596 324 5. 255 9 -0.9659 -0.2599 0.9877 -0.2556 -0.9540 256 6. 170 8 0.1736 0.9848 0.7903 -0.9752 0.1719 4,761 7. 180 20 0 -1 0.9397 -0.9397 0 3,481 8. 255 12 -0.9659 -0.2588 0.9781 -0.2531 -0.9447 256 9. 270 10 -1 0 0.9848 0 -0.9348 961 10. 230 8 -0.7660 -0.6428 0.9903 -0.6366 -0.7586 81 11. 300 25 -0.8660 0.5 0.9063 0.45315 -0.7849 3,721 12 252 20 -0.9511 -0.3090 0.9397 -0.2904 -0.8937 169 13 253 15 -0.9563 -0.2924 0.9659 -0.2824 -0.9237 196 14 250 18 -0.9397 -0.3420 0.9512 -0.3253 0.8938 121 15 252 30 -0.9511 -0.3090 0.8660 -0.2676 -0.8237 169 16 258 32 -0.9781 -0.2079 0.8480 -0.1763 -0.8294 361 17 252 28 -0.9511 -0.3090 0.8829 -0.2728 -0.8554 169 18 258 26 -0.9781 -0.2079 0.8746 -0.1818 -0.8554 361 19 250 48 -0.9397 -0.3420 0.6691 -0.2288 -0.6288 121 20 144 32 0.5878 -0.8090 0.8480 -0.6860 0.4985 9,025 21 162 28 0.3090 -0.9519 0.8829 -0.8404 0.2728 5,929 22 204 22 -0.4067 -0.9135 0.9272 -0.8470 -0.3771 1,225 23 230 15 -0.7660 -0.6428 0.9659 -0.6209 -0.7399 81 24 233 30 -0.7986 -0.6018 0.8660 -0.5212 -0.6916 36 25 185 19 -0.0872 -0.9962 0.9455 -0.9420 -0.0824 2,916 �-16.04 �-9.5033 �35336 Mean vector Azimuth = Tan-1 (� sen A)/(� Cos A) Where � SMA = –16.04 � COSA = – 9.503 Theredore, MVA = Tan-1 –16.04/–9.5033 = Tan-1 1.6878 = 59 Hence MVA = 59 + 180 = 239° Variance = �(Ai – MVA)2/(N – 1) A1 = azimuth MVA = Computed mean vector azimuth (239°) N = total number of outcome (25) Therefore, Variance = 35,336/(25 – 1) = 35,336/24 = 1,472. ENERO 30-GEOCIENCIAS-VOL 13-2 2009.prn D:\GEOCIENCIAS 13-2 DIC 2009\GEOCIENCIAS-VOL 13-2 2009.vp sÆbado, 30 de enero de 2010 18:16:42 p p p Composite 133 lpi at 45 degrees tion Reading, (1996) and Tucker, (1996). Thus, these field observations show that Ajali Sandstone was deposited under a tidal and fluvial channels interaction i.,e littoral environ- ment. The sieve analysis shows that Ajali Sandstone is coarse, medium to fine grain poorly sorted sandstone (Table 2) which suggests fluctuation in energy of deposition. Also the coarsening upward nature of Ajali Sandstone as indi- cated by the univariate result (Table 2) suggests that the ba- sin was shallowing upward (Dapple, 1974 and Tucker, 1996). This shallowing upward of the basin at the time of de- position of Ajali Sandstone into the Anambra Basin may be as a result of fluvial interference with the marine environ- ment. The paleocurrent analysis result shows that the variance value is 1,472. This value indicates that Ajali Sandstone was deposited in fluvial environment (Selley, 1966). The value of Mean Vector Azimuth (MVA) for Ajali Sandstone indi- cates 239° AZ (Table 4). This value and the rose diagram (Fig. 22) for Ajali Sandstone suggest that the direction of paleocurrent as at the time of deposition, acted in southwest direction and the provenance was northeast. The rose dia- gram reflects unimodal high variability paleocurrent pattern for Ajali Sandstone (Fig. 22). This paleocurrent pattern (Unimodal) suggests sediments deposited in an environment where fluvial currents was prevalent with net long-shore marine transport (Selly, 1966). The direction of provenance indicated that the sediments of Ajali Sandstone were sourced from the Basement Complex of Nigeria probably Obudu hill and or Cameroun Mountains in southeastern part of the country. The presence of carbonaceous fissile shale within the Nsukka Formation as observed from the field data suggests that, it was deposited in shelf to shoreface environment (Akande and Mucke 1993, Obi, 2000). The light grey colour of the shale unit and the ferruginized nature of the sandstone unit of Nsukka Formation as observes from the field study, indicates that Nsukka Formation was deposited in an oxi- dizing environment where adequate oxygen was able to in- teract with the sediments, probably basin surface (Tucker, 1996). Conclusion The study of the ancient environment of the sediments of Anambra Basin has been reconstructed from the field rela- tionships and textural analysis results. Before the Santonian tectonic event Anambra Basin was still in platform stage. However, sequel to Santonian tectonism, the study area opened up as Anambra Basin. Thus, the basin began to com- municate with the Atlantic ocean. There was a rise in the sea level during the Campanian times and the sea transgressed into the land. The incursion of the Atlantic Ocean caused the basin to become deeper and quiescent. Thus, Enugu Shale was deposited into the basin. Gradually, the shoreline started withdrawing seaward from the land. Hence, the basin be- came starved of shale (Enugu Shale) as a result of the grad- ual increase in depositional energy within the basin. This resulted in the emergent of sediments deposited in a coastal environment of fluvial and marine water interaction. Before the transgression of sea into the land, the land was already covered with giant plants and trees. When trans- gression phase set-in, those plants and trees were covered up with water and they began to decompose. Regression phase followed the transgression phase simultaneously and subse- 164 ADEIGBE, O.C. AND SALUFU, A. E N Provenance Paleocurrent direction Figure 22. Rose diagram of Ajali Sandstone within the study area ENERO 30-GEOCIENCIAS-VOL 13-2 2009.prn D:\GEOCIENCIAS 13-2 DIC 2009\GEOCIENCIAS-VOL 13-2 2009.vp sÆbado, 30 de enero de 2010 18:16:42 p p p Composite 133 lpi at 45 degrees quently led to the withdrawal of shoreline seaward such that sediments that were transported from the land were depos- ited and buried the decomposed plant and tree remains in the study area as Mamu Formation.Thus these plant remains were transformed into coal seams that exist in the Mamu Formation. The shoreline continued to withdraw at a rapid rate such that the basin became shallow. As a result of this shal- lowness of the basin at this period paved way to deposition of poorly sorted coarse sandstone into the basin in a littoral environment as Ajali Sandstone. Sedimentation of Ajali Sandstone in the basin lasted till Late Maastrichtian times when the shoreline began to move landward and the sea level began to rise. Thus, Nsukka Formation was deposited in an oxidizing shallow marine environment. Deposition of Nsukka Formation terminated sedimentation in the study area. References Akaegbobi I.M and Boboye O.A. (1999). Textural, Struc- tural Features and Microfossil assemblage Relation- ship as a Delineating criteria for the Stratigraphic boundary between Mamu Formation and Nkporo Shale within the Anambra Basin, Nigeria. NAPE Bull, Vol. pp. 193-206. Akande, S.O. and Mucke, A. (1993). Depositional Environ- ment and Diagenesis of Carbonates at Mamu/Nkporo Formations, Anambra Basin, Southern Nigeria. Journal of African Earth Sciences, vol. 17, pp. 445-456. Amaral, E.J. and Pryor, W.A. (1974). Depositional Environ- ment of St. Petters Sandstone deduce by Textual Analy- sis. Journal Sed. Petrol., Vol. 40, pp. 32-55. Benkhelil, J. (1987). Cretaceous Deformation Magmatism and metamorphism in the Lower Benue Tough, Nigeria. Geol. Journ. Vol. 22, pp. 467-493. Dapple, E.C. (1974). Sandstone types and their Associated Depositional Environments. Journal of Sedimentary Pe- trology. Vol. 4, pp. 695-707. Folk, R.L. (1970). Petrology of Sedimentary Rocks. Hemphill, Austin, Texas 182p. Folk, R.L. and Ward, W.C. (1957). A study in the signifi- cance of grain size parameter. Journal of Petrology. Vol. 37, pp. 327-354. Friedman, G.M. (1961). Distribution between Dune, beach, and River sandstones, J. Sed. Pet., Vol. 37 pp. 327-354. Kogbe, C.A. (1989). The Cretaceous and Paleogene Sedi- ments of Southern Nigeria: In Kogbe C.a. (Ed.), Geol- ogy of Nigeria. Elizabethan Publ. Vo., Lagos, Nigeria. 273-286 pp. Miola, R.J. and Weiser, D. (1977). Textural parameters: An Evaluation. Journal. Sed. Petrology. Vol. 38, pp. 45-53. Murat, R.C. (1972). Stratigraph and Pale geography of Lower Tertiary, Southern Nigeria, in Dessavagie, T.P.J, and Whiteman (Eds.), Afri-geol. University of Ibadan, Nigeria. 425 p. Nwajide, C.S. and Reijers, T.J.A. (1996). Geology of the Southern Anambra Basin. In: Reijers, T.J.A. (Ed), se- lected chapters on Geology, SPDC, Warri, pp. 133-148. Nwajide, C.S. (1990). Cretaceous Sedimentation and Paleogeography of the Central Benue Though. In: Ofoegbu, C.O; (Ed.), The Benue. Tough structure and Evolution International Monograph Series, Braunschweig, pp. 19-38. Obi, C.G. (2000). Depositional Model for the Campanian- Maastrichtian Succession, Anambra Basin, Southeast- ern Nigeria. Ph.D Thesis, University of Nigeria, Nsukka, Nigeria. Reading, H.G. (1996). Sedimentary Environment Processes, Facies and Stratigraphy. Blackwell Scientific Publica- tions, Oxford. 671. p. Reyment, R.A. (1965). Aspect of the Geology of Nigeria. Ibadan University Press. 145p. Sahu, R., (1964). Textual Parameters: An Evaluation of Flu- vial and Shallow Marine Deposits. J Sed. Pet., Vol. 34, pp. 513-520. Selley, R.C. (1966). Paleocurent and sediment Transport in near Shore Sediment of Sincte Basin. Libya. J. Geol. Vol. 75, No. 2, pp. 215-222. Steinmetz, G. (1762). Paleocurrent and Provenance Deter- mination of Sandstone Mustang Island, Texas, J. Sed. Petrol, vol. 30. pp. 753-780. Tucker, M.E. (1996). Sedimentary Rocks in the Field. John Weley and Sons, Chichester, New York, 150 p. 165 GEOLOGY AND DEPOSITIONAL ENVIRONMENT OF CAMPANO-MAASTRICHTIAN SEDIMENTS IN THE ANAMBRA BASIN, SOUTHEASTERN NIGERIA: EVIDENCE FROM FIELD RELATIONSHIP AND SEDIMENTOLOGICAL STUDY ENERO 30-GEOCIENCIAS-VOL 13-2 2009.prn D:\GEOCIENCIAS 13-2 DIC 2009\GEOCIENCIAS-VOL 13-2 2009.vp sÆbado, 30 de enero de 2010 18:16:43 p p p Composite 133 lpi at 45 degrees