Sedimentary and petrofacies analyses of the Amasiri Sandstone, southern Benue Trough, Nigeria: Implications for depositional environment and tectonic provenance A.U. Okoro a , E.O. Igwe b, * , C.S. Nwajide c a Department of Geological Sciences, Nnamdi Azikiwe University, Awka, Nigeria b Department of Geology, Ebonyi State University, Abakaliki, Nigeria c Geoscan Technical Services Ltd, Awka, Nigeria article info Article history: Received 4 January 2016 Received in revised form 26 July 2016 Accepted 1 August 2016 Available online 4 August 2016 Keywords: Petrography Lithofacies Provenance and tectonic setting Amasiri Sandstone Southern Benue Trough abstract This study was undertaken to determine the depositional environment, provenance and tectonic setting for the Turonian Amasiri Sandstone, southern Benue Trough, Nigeria, using lithofacies analysis and re- appraisal of petrography of the sandstones. Local stratigraphy and field relationships show a thick suc- cession of shales alternating with elongate/parallel sandstone ridges extending eastwards from Akpoha to Amasiri through Itigidi and Ugep to Apiapum areas. Lithofacies analysis reveals 9 lithofacies suggestive of storm (mass flow) and tidal shelf processes. These include dark grey to black laminated shale/silty mudstones, bioturbated mudstones, coquinoid limestones, very fine-grained bioturbated sandstones with shell hash/debris in places and limestone rip-up clasts, massive and chaotic sandy conglomerate with rip e up clasts, fine to medium-grained, parallel laminated sandstone, hummocky cross-stratified, massive, medium to coarse-grained sandstones, medium to very coarse-grained, planar cross-bedded sandstone, with clay-draped foresets and Ophiomorpha burrows, and coarse-grained trough cross- bedded sandstone. Petrofacies analysis identifies the sandstones as feldspathic and arkosic arenites. Ternary plot of framework mineralogy indicates derivation from an uplifted continental block related to the nearby Oban Massif and Cameroon Basement Complex. © 2016 Elsevier Ltd. All rights reserved. 1. Introduction The Eze-Aku Group in the southern Benue Trough was depos- ited during the extensive Late Cenomanian to Turonian trans- gressive event (Murat, 1972). The group consists of extensive shales interbedded with sandstones, siltstones, limestones and marls (Reyment, 1965; Whiteman, 1982; Nwajide, 2013). The Amasiri Sandstone, Konshisha River Sandstone, Agila Sandstone and Makurdi Sandstone were deposited as sandy lithofacies of the Eze- Aku Group (Whiteman, 1982). The Amasiri Sandstone (Reyment, 1965; Whiteman, 1982) rep- resents the uppermost part of the Eze-Aku Group outcropping on the eastern limb of the Abakaliki Anticlinorium in the southern Benue Trough. It outcrops as parallel ridges trending northeast e southwest, alternating with dark grey to black shales, siltstones and minor limestones. The sandstones have been variously interpreted as tidal/subtidal shallow marine deposits (Banerjee, 1980), storm- dominated shallow shelf deposits (Amajor, 1987), marginal ma- rine sand deposits (Ojoh, 1990), and braided river deposits (Odigi, 2012). The present study aims to contribute to the descriptive detail of the Amasiri Sandstone outcrops within Akpoha, Amasiri, Itigidi, Ugep and Apiapum localities. It will further identify the lithofacies, propose a depositional model, and interpret the prov- enance, paleoclimate and tectonic setting of sedimentary basin. 2. Geologic setting The Benue Trough had been structured into pull-apart sub-ba- sins or grabens generated by sinistral strike-slip displacements inherited from pre-existing transcurrent fault zones in the Pan- African mobile belt (Benkhelil,1989; Nwajide, 2013). It is part of the much larger West and Central African Rift System (Fairhead, 1988; Genik, 1993) which originated during the breakup of the Gondwana supercontinent and the opening of the southern Atlantic and Indian * Corresponding author. E-mail address: ezekieloigwe@yahoo.com (E.O. Igwe). Contents lists available at ScienceDirect Journal of African Earth Sciences journal homepage: www.elsevier.com/locate/jafrearsci http://dx.doi.org/10.1016/j.jafrearsci.2016.08.001 1464-343X/© 2016 Elsevier Ltd. All rights reserved. Journal of African Earth Sciences 123 (2016) 258e271