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

Abstract

The Up per Cre ta ceous Ajali Sand stone is an ex ten sive strati graphic unit of the Anambra Ba sin in south ern Ni ge ria. It consists of fri a ble, white cross-bed ded sand stones ex hib it ing a fin ing up ward se quence. Struc tures such as bioturbation, pla nar and her ring bone cross beds in di cate vari abil ity in depositional en vi ron ment rang ing from fluviatile to deltaic. Petrographic and geo chem i cal com po si tions (ma jor and trace el e ments) of these sand stones have been in ves ti gated to de ter mine their prov e nance, tec tonic set ting and weath er ing con di tions. Fif teen sand stone sam ples ex am ined are rich in quartz but poor in feld spar and lithic frag ments. Tex tur ally, the frame work grains of the sand stones are fine-to me dium-grained, sub-an gu lar to sub-rounded, mod er ately sorted to poorly sorted, pos i tively skewed and leptokurtic. The sand stones are tex tur ally im ma ture as de picted by their sub-an gu lar edges of grains, but min er al og i cally ma ture in terms of high per cent age of quartz. The high chem i cal in dex of al ter ation (CIA) val ues (71.0-99.2%) for these sand stones sug gests that they were de rived from highly weath ered rocks in the source area. The com po si tion of the ma jor ox ides in the sand stones re vealed that SiO 2 (49.1-99.7%), Al 2 O 3 (0.2-30.3%), Fe 2 O 3 (0.4-1.8%) and TiO 2 (0.06-3.2%) were the most abun dant el e ments in all sand stone sam ples in dicat ing a high de tri tal quartz and clay min eral con tent. The Al 2 O 3 /TiO 2 ra tios (1.47 to 12.48), Ti/Zr (6.48-18.63) and Zr/Cr (2.24-22.36) sug gest that the sand stones were de rived from vari able base ment com plex rocks, in clud ing some con tri bu tion from mafic or ultra mafic com po nents. In fer ences from the plots of K 2 O/Na 2 O ver sus SiO 2 in di cated a pas sive mar gin tec tonic set ting for the sand stones. The high load ing of CaO and Na 2 O in more than 50% of the sam ples is in dic a tive of terrigenous input, mainly in the form of car bon ates and sil i cates. These re sults are gen er ally con sis tent with a der i va tion of the sand stones from ad ja cent ig ne ous and meta mor phic base ment com plexes (Cam er oon-Adamawa high lands and Oban Mas sif) while the ex ten sive and blan ket-like ge om e try of the Ajali Sand stone is in dic a tive of an ex cel lent res er voir for ground wa ter in the Anambra Ba sin.

Minerology, geochemical composition and geotechnical properties of termite mound soil

“…As a general rule, stable mineral assemblage have low Fe 2 O 3 /K 2 O values less stable assemblage located close to the sediment source and (Hayasali et al, 1997;Ocheli et al, 2018). The value of Al 2 O 3 /TiO 2 obtained for the studied termite mound soils were in the range 4.68-6.61 supporting a line to passive continential margin (Ogala et al, 2015).…”

Geochemistry of siliciclastic rocks from the Shemshak Group (Upper Triassic–Middle Jurassic), northeastern Alborz, northern Iran: implications for palaeoweathering, provenance, and tectonic setting

“…This quartzolithic petrofacies iden ti fied in sand stones of the Shemshak Group is also re ported in the north of Alborz Range (Zanchi et al, 2009) and Cen tral Iran (Salehi et al, 2017). The dis crim i na tion di a grams of Bhatia (1983) and Roser and Korsch (1986) have been used com monly to de ci pher the tec tonic set ting of an cient sed i men tary bas ins (Nowrouzi et al, 2014;Jafarzadeh et al, 2014;Ogala et al, 2015;Vosoughi Moradi et al, 2016). In this study, we used three di a grams based on ma jor, trace and rare earth el e ments for de ci pher ing the tec tonic set ting of the source area.…”

Geochemistry of rocks (Late Cretaceous) in the Anambra Basin, SE Nigeria: insights into provenance, tectonic setting, and other palaeo-conditions

“…Discrimination diagrams proposed by Bhatia (1983) and Roser and Korsch (1986) have been traditionally and widely used for decades to obtain tectonic information from sediments/sedimentary rocks deposited in basins (e.g. McCann, 1988;Burnet and Quirk, 2001;Gu et al, 2002;Tijani et al, 2010;Ejeh et al, 2015;Ogala et al, 2015;Ejeh, 2016;Zaid, 2016), although their validity has been questioned (e.g. Winchester and Max, 1989;Shao et al, 2001;Armstrong-Altrin and Verma, 2005;Ryan and Williams, 2007;Borges et al, 2008;Armstrong-Altrin, 2009;Guo et al, 2011;Ghosh et al, 2012;and Zaid, 2012).…”

“…Four probable provenances have been suggested based on field evidence, petrography, and geochemistry of outcrop data set (Figure 1): (1) the eastern Precambrian basement complex rocks of Cameroon (Hoque, 1976(Hoque, , 1977Hoque and Ezepue, 1977;Agumanu, 1993); (2) Campano-Maastrichtian detritus derived from the Abakaliki Anticlinorium, sequel to the Santonian uplift (Simpson, 1954;Short and Staȕ ble, 1967;Burke et al, 1972;Nwachukwu, 1972;Murat, 1972;Whiteman, 1982;Amajor, 1987a;Tijani et al, 2010); (3) Sediments derived from the Oban-Adamawa (Adamawa-Sarduana Massif) basement complex (Amajor, 1987a;Agumanu, 1993;Tijani et al, 2010); and (4) Sediments sourced from the Ibadan Craton (West African massif) and the Oban Massif (south-eastern, Nigeria) (Nwajide and Reijers, 1996). Most of the aforementioned provenance studies were focused on the exposed sections of the sedimentary fills of Anambra Basin, especially the Ajali (e.g., Hoque, 1977;Hoque and Ezepue, 1977;Amajor, 1987a,b;Tijani et al, 2010;Ogala et al, 2015), Mamu (e.g., Ogungbesan, and Adedosu, 2020), Owelli sandstone (e.g., Agumanu, 1993), Enugu and Awgu (Overare et al,2020) Formations using directional sedimentary structures or petrographic and/or geochemistry data. However, the tectonic setting of the sedimentary fills of the Anambra basin has been reported to be of passive margin, based on the surface geochemical data (e.g., Tijani et al, 2010;Ogala et al, 2015).…”

“…Most of the aforementioned provenance studies were focused on the exposed sections of the sedimentary fills of Anambra Basin, especially the Ajali (e.g., Hoque, 1977;Hoque and Ezepue, 1977;Amajor, 1987a,b;Tijani et al, 2010;Ogala et al, 2015), Mamu (e.g., Ogungbesan, and Adedosu, 2020), Owelli sandstone (e.g., Agumanu, 1993), Enugu and Awgu (Overare et al,2020) Formations using directional sedimentary structures or petrographic and/or geochemistry data. However, the tectonic setting of the sedimentary fills of the Anambra basin has been reported to be of passive margin, based on the surface geochemical data (e.g., Tijani et al, 2010;Ogala et al, 2015).…”