Tropical Journal of Science and Technology, 2020, 1(1), 16 - 34 ISSN: 2714-383X (Print) 2714-3848 (Online) Available online at credencepressltd.com Geochemistry of lignite associated shales in Ogwashi-Asaba Formation: Implication for paleoredox, source-area weathering, provenance and tectonic settings 1Osokpor, J. and 2Nkwagu, S. 1&2Department of Earth Sciences, Federal University of Petroleum Resources, Effurun, Nigeria Corresponding author’s e-mail: [email protected]; [email protected], Cell: +2347038912726 Abstract Shale samples obtained from the Obomkpa lignite belt were analysed for geochemical attributes such as major, trace and rare earth elements using XRF, ICP-MS and ICP-OES techniques. Paleoweathering and paleoredox conditions, provenance and tectonic settings were evaluated. High CIA (97.79 – 99.00), CIW (99.92 – 99.93), and MIA (95.58 – 98.00) values indicated intense weathering in the source areas and high detrital input dominated by strong chemical weathering, which led to the formation of clay minerals by hydration and leaching of ions such as Ca⁺2, K⁺ and Na⁺ known to be present in the feldspar minerals. Cross plot of CIA vs SiO2 shows that the mineral matter composing the shale samples have been substantially recycled in the course of weathering. Ratios of redox-sensitive trace elements (V/Cr = 0.92 av., Ni/Co = 2.8 av., and Th/U = 4.14 av.) suggests an oxic paleodepositional environment for the sediments. The strongly positive Ce anomaly (Ce/Ce*) (0.83 – 0.96) also affirms an oxic setting. Trace element ratios (La/Sc, Th/Sc, Th/Co, Th/Cr, and Cr/Th and Eu/Eu*) applied for provenance discrimination and with a range of values: 1.31 – 2.89, 0.86 – 1.04, 0.35 – 1.74, 0.11 – 0.14, 7.23 – 9.52, and 0.52 – 1.38 respectively, for the samples, compared to those of sediments derived from mafic and felsic sources. A felsic provenance was inferred for the shales after due comparison. A plot of of Zr/Sc vs. SiO2 suggested substantial reworking of the sediments. Inferences made by integrating various discrimination plots of major element concentrations (SiO2/Al2O3, TiO2, Na2O+CaO, Al2O3 (Na2O+CaO) vs. Fe2O3+MgO), classified the shales as Active Continental Margin (ACM) sediments. Keywords: Lignite geochemistry, provenance, alteration, paleoredox, paleoweathering 1. Introduction associated with the lignites have not been The Obomkpa area (Fig. 1) is well known exploited and are thus, the centre of focus geologically for exposures of the of this study. sediments of the Ogwashi-Asaba Generally, shales are the most Formation. The Formation, Oligocene- abundant form of clastic sediments in miocene in age, has been studied by sedimentary basins globally (Pettijohn, various authors (Reyment, 1965; Short and 1975). Mineralogical and chemical Stauble, 1967; Okezie and Onuogu, 1971, compositions of terrigeneous sedimentary 1985; Dessauvagie, 1974; Nwajide, 2013). rocks are the products of several variables The lignites of the Ogwashi-Asaba such as provenance, weathering Formation were discovered by the Mineral conditions, sorting during transport, Survey of Nigeria (1908; 1909) and are sedimentation and post-depositional underlain and capped by shales. Within the diagenesis, climate and tectonics formation, the lignites are confined to a (McLennan, 1989; Johnson and Basu, narrow belt of about 10miles wide and 1993; Nesbitt and Young, 1996; Nesbitt et 150miles long trending northwest- al. 1996). In geochemical studies, the southeast from the Niger in the west to the major elements and some selected trace Nigerian-Cameroun frontier, east of elements like Th, Sc, Co, Cr, Zr, Hf, Y, Ni, Calabar (Whiteman, 1982). The shales Co, V with rare earth elements (REEs) and 16 Osokpor, J. and Nkwagu, S.: Geochemistry of lignite associated shales in Ogwashi-Asaba formation: Implication for paleoredox, source-area weathering, provenance and tectonic settings their elemental ratios are used as sensitive including La/Sc, La/Co, Th/Sc and Zr/Cr indicators of the provenance, paleoredox have been established to be good conditions, tectonic setting, discriminators between mafic and felsic paleoweathering conditions of the clastic source rocks (Tijani et al., 2010). Trace sedimentary rocks (Bhatia, 1983; Bhatia elements such as La, Th and Zr are said to and Crook, 1986; Roser and Korsch, 1986, be more concentrated in felsic igneous 1988; McLennan and Taylor, 1991; rocks while Co, Sc and Cr have higher Johnson and Basu, 1993; McLennan et al., concentrations in mafic rocks (Ronov et 1993; Condie, 1993; Nesbitt et al., 1996; al., 1974; Wronkiewicz and Condie, 1987, Fedo et al., 1997) 1990). The major element discrimination The paleoredox condition of diagrams of Bhatia (1983) has commonly sediments is delineated using the ratios of been used to classify the tectonic settings redox-sensitive trace elements which of sedimentary basins as applied in this include V/Cr, Ni/Co and Th/U ratios work (e.g., Armstrong-Altrin et al., 2004), (Jones and Manning, 1994). These ratios although attention is drawn to their reflect depositional environments of subjective usage (Armstrong-Altrin and sediments as anoxic, oxic or dysoxic. The Verma, 2005), as the diagrams are amount of weathering undergone by produced using the concentration of highly sediments is studied using weathering mobile elements including SiO2, Na2O, indices like Chemical Index of Weathering CaO and K2O. These oxides show low (CIW), Chemical Index of Alteration enrichment or depletion of quartz, mica (CIA), and Mineralogical Index of and plagioclase feldspars in intensely Alteration (MIA) (Harnois, 1988; Nesbitt weathered sediments. As opposed the and Young, 1992; Voicu et al., 1997). major oxides, rare earth elements (e.g., La, Although considerable work has Ce, Nd, Gd, Yb, Y), are most suitable for been carried out on the Lignite-bearing the discernment of provenance and Ogwashi-Asaba Formation, especially as it tectonic setting since they have relates to hydrocarbon exploration comparatively low mobility during stratigraphy of the oil-rich Niger Delta sedimentary processes and their short Basin, very little has been achieved in residence times in seawater (Taylor and understanding the paleoredox, McLennan, 1985; Bhatia & Crook, 1986; paleoweathering, provenance and source- Wronkiewicz & Condie, 1987, 1989 and area weathering of the. In this work an 1990). Also, trace elements, because they attempt is made to establish the are quantitatively transferred into paleoredox, paleoweathering, provenance sediments during sedimentary processes, and source-area weathering of the lignite reflect the signatures of the parent bounding shales in the Ogwashi Asaba materials and are thus, also suitable for Formation through inorganic signatures discerning the source rock compositions derived from the sediments, in order to and tectonic environments of sediments improve and add to the geological (Bhatia and Crook, 1986; McLennan, information concerning the formation. 1989; Condie, 1993). Elemental ratios Tropical Journal of Science and Technology, Volume 1 Number 1, 2020 17 Osokpor, J. and Nkwagu, S.: Geochemistry of lignite associated shales in Ogwashi-Asaba formation: Implication for paleoredox, source-area weathering, provenance and tectonic settings Fig 1. Location map of study area showing sample collection points in and around Obomkpa 1.1 Geology and field relation Neogene with an age range of Oligocene- The proximal (northern) Niger Delta is Miocene (Reyment, 1965; Short and characterized by three formational units, Stauble, 1967 and Dessauvagie, 1974) two of which (Imo Shale and Ameki (Fig. 2). Outcrops of the formation Formations) of Neogene age, hence older consists of interbeds of white, blue and than the objective formation in this study pink clays, cross-bedded sands, (Ogwashi-Asaba Formation) (Fig. 2). In carbonaceous mudstones, shales and seams general, the lithostratigraphic units of the of lignite (Okezie and Onuogu, 1971; Niger Delta Basin has been classified into Nwajide, 2013). From studies of pollen two: (i) the outcropping units found data, Okezie and Onuogu (1971, 1985) mainly in the northern depositional belt assigned the lignites to tropical and semi- (Doust and Omatsola, 1990) and (ii) tropical plants consisting mainly of palms. subsurface units, (Reyment, 1965; Frankl An upper floodplain environment of and Condry, 1967; Short and Stauble, deposition was inferred for the formation 1967; Adegoke, 1969; Whiteman, 1982; from the plot of Short and Stauble (1967). Nwajide, 2013) which occur in the central The samples obtained for this study are to distal delta. The outcropping units are from dark shales overlying lignite seams in surface lateral equivalents of the and around Obomkpa town (Fig. 3), a subsurface units (Fig. 2). community renowned for excellent Stratigraphically, the Ogwashi- exposures of the lignite facies of the Asaba Formation straddles the Paleogene- Ogwashi-Asaba Formation. Tropical Journal of Science and Technology, Volume 1 Number 1, 2020 18 SUBSURFACE SURFACE OUTCROPS Niger Delta Oldest known Youngest Southern Benue Oldest known Basin Age known Age Trough Age BASIN Stratigraphy Recent Benin Oligocene Recent Benin Formation Miocene Formation Recent Miocene Ogwashi-Asaba Agbada Formation Oligocene Formation Eocene Eocene Ameki Formation Eocene Delta Niger Imo Shale Paleocene Recent Formation Akata Eocene Eocene Nsukka Formation Formation Ajali Formation Maastrichtian Campanian Mamu Formation Campanian Anambra Anambra Campanian/ Maastrichtian Nkporo Shale Santonian