Textural and geochemical characteristics of the Ajali Sandstone, Anambra Basin, SE Nigeria: Implication for its provenance

Abstract

This study presents the mineralogical, textural and geochemical characteristics of the regional Maastrichtian Ajali Sandstone in Anambra Basin, SE Nigeria. The intent is to highlight possible constraints on the chemical weathering conditions of the source materials on one hand, and to infer the provenance on the other hand. The investigation approach involved field studies and collection of samples from 12 different outcrop locations, followed by laboratory studies involving grain-size analysis (GSA), major and trace elements analyses using the X-ray fluorescence (XRF) method as well as thin section petrography. Field studies show that the sandstones are friable at all locations and range in color from white in freshly cut stone, to reddish brown on weathering. In addition, the sandstone units are cross-bedded and show graded bedding exemplified by fining upward sequence. Textural examination indicates that the sandstones range from fine to medium sands, constituting about 76 to 99% sand fraction, with graphic mean grain size of 0.23 to 0.53 mm. Standard deviation (sorting) ranges from 0.56 to 1.24 Ø and implies moderately well sorted sediments. Inferred from the textural indices, the depo-environmental discrimination of the Ajali Sandstone revealed a fluvial/river system-dominated sedimentary process. The sandstones are quartz arenite with quartz greater than 90% and less than 5% K-feldspar which indicate a predominant basement source as also revealed by the heavy mineral assemblages. In addition, major elemental oxides shows SiO 2 content greater than 96% for the fresh Ajali Sandstone samples with extreme depletion of mobile oxides such as Na 2 O, CaO and the ferromagnesian minerals through weathering and sedimentary processes. Provenance and tectonic setting discrimination using geochemical data and compositional maturity revealed typical felsic igneous-dominated cratonic environment while inter-elemental ratios (such as Zr/Cr, Y/Ni, Th/Sc, La/Sc and La/Co) and ternary plots (e.g. Th-Sc-Zr; La-Th-Sc and Th-Co-Zr) reflect passive continental margin setting for the Ajali Sandstone. Consequently, the source area is constrained to the Precambrian basement rock units of Adamawa-Oban massif areas to the east of the Anambra Basin and the adjacent Abakaliki Anticlinorium.

Petrographic and geochemical study of the Maastrichtian Ajali Sandstone, North Central Nigeria

“…10) for sed i mentary prov e nance, ma jor ity of the sam ples plot within the quartz ose sed i men tary prov e nance with one sam ple ly ing within the fel sic ig ne ous prov e nance, sup port ing the in ter pre -ta tion that the sand stones were de rived from a mixed prov enance of gra nitic rocks and re cy cled sed i ments. Also, the high CIA and CIW val ues of the sam ples sup port the as ser tion that the re cy cled sed i ments were de rived from the up lifted ex trusive and sed i men tary units of the ad ja cent Abakaliki Anticlinorium (Tijani et al, 2010), and the ad ja cent Cam er oon Basement Com plex and Oban Mas sif (Hoque and Ezepue, 1977). Fur ther more, the high Zr/Sc val ues (68-313 ppm) of the quartz arenites in the Ajali Sand stones and low Zr/Sc val ues (44-55 ppm) of the mudstone sam ples are sim i lar to the values of sed i ments de rived from fel sic and mafic base ment (Osae et al, 2006) of the ad ja cent base ment com plex.…”

“…The CIW, CIA and PIA val ues for the Ajali Sand stone vary from 77.1 to 99.8%, 74.1 to 99.2% and 76.6 to 99.4%, re spectively (Ap pen dix 1). The high CIW, CIA and PIA val ues sug gest high de gree of chem i cal weath er ing in the source area from adja cent Cam er oon base ment com plex and Oban Mas sif (Hoque and Ezepue, 1977;Tijani et al, 2010).…”

“…The Maastrichtian Ajali Sand stone (Reyment, 1965) extends from Fugar/Agenebode area in the west along the Udi Hill in the east, and thins south-east of Okigwe in Ni ge ria (Hoque and Ezepue, 1977;Tijani et al, 2010) thereby form ing an overall blan ket-like ge om e try (Ladipo, 1986). It is an ex ten sive stratigraphic unit of the Anambra Ba sin with a max i mum thick ness of 500 to 600 metres in the cen tral part of the ba sin (Simpson, 1954).…”

“…Hoque and Ezepue (1977) also in ter preted the Ajali Sand stone as a fluvio-deltaic de posit and sug gested a der i va tion from the Cam er oon-Adamawa high lands. Tijani et al (2010) stud ied the tex tural and geo chem i cal char ac ter is tics of the Ajali Sand stone and noted that the source area is con strained to the Pre cam brian basement rock units of Adamawa-Oban Mas sif ar eas to the east of the Anambra Ba sin and the ad ja cent Abakaliki Anticlinorium. Banerjee (1979), Amajor (1984) and Ladipo (1986Ladipo ( , 1988 de scribed a num ber of fea tures in dic a tive of a tidal to shal low sub-lit to ral palaeoenvironment: Ophiomorpha and Sko lithos; bi po lar-bi modal cur rent az i muths; and re ac ti va tion sur faces of in ferred tidal or i gin.…”

Sedimentology and Permeability Distribution of the Ajali Sandstone around Odugbo, North Central Nigeria

“…The point -counting method of Ingersoil et al (1984) adopted by Tijani, et al (2010) was used for the different mineral grains quantitative compositional analysis while the modal analysis was by counting more than 300 points per thin section according to Dickson (1970).…”