Geochemical Appraisal of the Mamu Shales Exposed around Igodor in the Benin Flank of the Anambra Basin, Nigeria

The Auchi area of Edo state which lies within the Benin flank of the Anambra Basin host shaly sediment exposures that have been classified by previous researches as units of the Mamu Formation. This study evaluated samples of this sediment from Igodor near Auchi for its geochemical and mineralogical properties, and interpreted its, depositional environment and geotectonic setting. In order to achieve this, field studies were carried out and representative samples obtained for Inductively Coupled Plasma Mass Spectroscopy (ICP-MS) analyses of major oxides, trace and rare earth elements. X-Ray Diffraction analysis was also carried out to determine the mineralogical composition. Some of the minerals determined were Aragonite and galena. Binary plots, triplots and elemental ratio plots including SiO2/, Al2O3, Th/Sc, Th/Co and La/Sc , Th-Sc-Zr, and the abundance of Cr, Ni were employed to determine the provenance. The concentration of detrital indicators such as SiO2, Al2O3 and TiO2, with averages of 51.95, 25.34 and 1.39 respectively, indicate high detrital influx into the Benin Flank of the Anambra basin. SiO2/Al2O3 ratios of 1.80-2.20, indicate that the shales were made up of pure kaolinite. The Ni and Cr abundance indicated a mafic and felsic provenance for the sediments, however, Th/Sc, Th/Co and La/Sc ratios show that the provenance was predominantly felsic, while the Th-Sc-Zr triplot shows that the depositional setting was passive. DOI: https://dx.doi.org/10.4314/jasem.v24i3.15 Copyright: Copyright © 2020 Okiotor and Ighodaro. This is an open access article distributed under the Creative Commons Attribution License (CCL), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Dates: Received: 16 November 2019; Revised: 11 January 2020; Accepted: 22 February 2020

The studied area around Igodor is located in Etsako East local Government area of Edo state, Benin Flank of the Southern Anambra Basin, Nigeria. It falls within the coordinates N 07 0 27' and E006 0 28'. Here the Benin Flank of the Anambra Basin is exposed in several road cuts revealing both the Mamu and the Ajali Formation sediments. This study was aimed at appraising the geochemical characteristics of these sediments with a view to determining their depositional environment and provenance using sedimentological, mineralogical and geostatistical techniques. The objective is to use these proven and modern techniques to evaluate these sediments and produce a reference resource material.
Geological setting and stratigraphy: The Anambra Basin is part of the Lower Benue trough. The Post-Santonian collapse of the Anambra platform led to the emergence of several parts of the Lower and Middle Benue Basins during the Campanian -Maastrichtian and a shift in the depositional axis of sediments for the third transgressive cycle to the Anambra Basin. These sediments consist of the marine Nkporo / Enugu Formations (lateral equivalents) overlain by the deltaic successions of the Mamu Formation and the marginal marine Ajali Formation in the Anambra Basin. The Upper Cretaceous sediments are overlain by the transgressive Paleocene -Eocene shales, sandstones and siltstones of the proto -Niger Delta in the southern fringes of the Anambra Basin (Akande et al, 2011).

MATERIALS AND METHOD
This research involved Field Mapping, logging and sample collection of exposed road cut sediments in the study area, laboratory studies involving sedimentological, geochemical (ICP-MS) and X-Ray diffraction analyses of collected samples were done. In order to establish the qualitative and quantitative constituents of the major, minor, trace and rare earth constituents of the sediments X-ray diffraction and Inductively Coupled Plasma Mass spectrometry (ICP-MS) analyses techniques were employed.
Sample Collection: A total of Eight (8) fresh samples were collected from the exposed road cut section for this study. Sampling was done on fresh, unweathered surfaces in order to reveal the original, unaltered sedimentological and mineralogical (chemical) properties of the rocks. The samples were kept in sample bags (polythene) and later taken to laboratory for analyses.

RESULTS AND DISCUSION
Resulting from the field study, four (4) lithofacies of sandstone, siltstone, claystone and shales were determined. The results as obtained from Activation Laboratory, Ontario, Canada are presented in Tables 1, 2, 3 and 4. Mamu Formation:  (Amajor, 1987); C = Ezeaku shale (Amajor, 1987); D = Ifon Shale (Ajayi et al, 1989); E = Abu Tator Shales, Egypt Mostafa (2005). *Average composition of sedimentary rocks. (Clark and Washington, 1924and Taylor, 1964, in Asuen, G.O, 1984 (Nwajide and Reijers,1996). Its best exposures on the Benin Flank of the Anambra basin, was found along the road cuts at Igodor at Etsako East Local Government Area of Edo state. The rock units are dominantly shales, siltstones, heteroliths, fine sandstones and claystones (Plate 1).The shales are black and often splintery in intervals up to 2m in thickness. They are interlaminated in most sections with fine sandstones and siltstones. The sandstones are dominantly fine to medium grained. In some horizons, siltstones and fine sandstones are laterally transitional. It is very dark in colour, and alternate with siltstones and fine sands with rootlets, indicative of a coal swampy environment of deposition. These observations also agree with that of Akaegbobi, 1999;Nwajide and Reijers, 1996a. Elemental concentrations in sediments result from the competing influences of provenance, weathering, sorting, and sediment diagenesis (Quinby-Hunt et al., 1991). The major oxides results presented in table 1 form the basis for the mineralogical and geostatistical interpretation of the major oxides. The Trace and rare earth elements were also determined by ICP-MS analysis (See Table 3). These were used to determine the provenance and the tectonic setting of these sediments. The studied shales show enrichment of elements that are chemically resilient and are associated with terrigenous influx, such as SiO2, Al2O3 and TiO2 (See Table 2). These elements can survive throughout intensive chemical weathering and diagenesis (Cullers, 2000). Their concentration in sediments is used as a measure of detrital input. The major constituents of the studied shale samples do not vary greatly from one location to another. The SiO2, Al2O3 and TiO2 tend to form together the main constituents of the studied shales and are normally related to clays. The SiO2, Al2O3 and TiO2 show both strong positive and negative correlation in most of the samples (Table 5). This indicates that the major constituents SiO2, Al2O3 and TiO2 of the studied shale samples are dominantly terrigenous in origin but might have originated from different sources. The average of Al2O3 content in the Mamu shales of the Benin Flank is 25.53 with enrichment factor of 0.97 (Table.2). High concentration of alumina is a good indicator of detrital influx. This result shows high detrital influx as indicated by the high alumina content with enrichment factor of 1.87, supported by the high TiO2 enrichment factor of 1.12 (Table 2). In order to determine the clay type SiO2/Al2O3 ratio was used. According to Felix (1977), SiO2/Al2O3 ratio is used for determining the presence of pure montmorrillonite (2.80 to 3.31) and pure kaolinite (about 1.18) in sediments. It was discovered that the clay type in these shales is of pure Kaolinite therefore roads constructed in this area will last longer than if the clay type was mixed with montmorrilonite. According to Ahmed, 1997, enrichment of Fe2O3 in shales could be attributed to their formation under reducing condition with high input of non-reactive iron to the Basin. Therefore the depositional environment of the Benin Flank of the Anambra Basin with average Fe2O3 content of 3.75% and enrichment factor of 1.11 could be said to be a reducing one. Cr and Ni abundance in siliciclastic sediments is usually considered a useful provenance tool. Wrafter and Graham (1989), stated that high Cr and Ni content are mainly found in ultramafic rock-derived sediments. Therefore it could be said that the provenance of the sediments from the Benin Flank of the Anambra Basin were from mafic and felsic source rocks (Table 7). However, elemental ratio plots show that the sediments where predominantly from a felsic source (Table 8). Th-Sc-Zr plots indicate that the sediments where from passive margin. Clay minerals can be used as stratigraphic markers and environmental indicators. The type of clay minerals found in shale is a function of provenance and diagenetic history. Depositional environment has a considerable influence on the clay mineralogy through early mineral transformations in the basin of deposition, (Russell, 1970). The XRD results reveal that the minerals are predominantly silicate minerals. The presence of a minerals such as Aragonite which is unstable and can be reworked by gravity flows as well as galena ( a lead glance) which probably arrived at the Benin Flank from the Abakaliki Anticlinorium support the detrital influx of these sediments into this region.

Conclusion:
The present study enabled the understanding of the inherent geochemical properties of the Mamu shales exposed at the Benin Flank of the Anambra Basin. From the study, concentration of SiO2, Al2O3 and TiO2 indicate high detrital influx into the Benin Flank of the Anambra basin. SiO2/Al2O3 ratios indicate that the shales were made up of pure kaolinite. The Ni and Cr abundance indicated a mafic and felsic provenance, however, Th/Sc, Th/Co and La/Sc ratios show that the provenance was predominantly felsic, while the Th-Sc-Zr ratio shows that the depositional setting was passive.