Abstract

This paper presents one of the most detailed facies analysis of the siliciclastic rocks on the western flank of Anambra basin, Southwestern Nigeria. The study is based on four stratigraphic outcrop sections exposed as road cuts and cliff surface of river channel in Auchi and Fugar environs. The stratigraphic sections were studied in terms of lithology, textural characteristics, sedimentary structures, geobody geometry, paleocurrent signatures and bed forms, allowing the recognition and organization of facies into facies associations in order to interpret the depositional environments. Nine lithofacies were recognized namely, structured to massive conglomerate facies (Gmm), fine- to medium-grained, planar cross bedded, sandstone facies (Sx), thick-bedded, medium-grained, moderately-sorted, sandstone facies with rashes of sub vertical Ophiomorpha burrows (Sxb), medium-grained, moderately-sorted, planar cross laminated, quartzose sandstone facies (Sxl), coarse to medium-grained, massive, sandstone facies (Sm), fine- to coarse-grained, trough cross bedded, quartzose, sandstone facies (St), medium-grained, moderately sorted, herringbone cross stratified, reddish brown sandstone facies (Sxh), fine-grained, moderately sorted, fine-grained, white, hummocky cross stratified sandstone facies (Sh), and medium-grained, moderately sorted, rippled sandstone facies (Sr). The spaciotemporal examination of these facies allowed the identification of five facies associations (FAs). The facies associations are FA 1, FA 2, FA 3, FA 4 and FA 5. FA 1 consists of Gmm, St, Sm and Sx is interpreted as indicating deposition in braided fluvial channel. FA 2 is constituted by Sh, St and Sm and is interpreted as deposits of shoreface. Facies Sxh and Sm together with the occurrence of reactivation surfaces were identified in FA 3 and are interpreted as tidal channel deposition. FA 4 is interpreted as indicating deposition in delta front on account of fan- shaped paleocurrent signature and the presence of facies Sx, Sxl and Sxb. Lastly, FA 5 is comprised of Sx, Sm and Sr and strongly suggests deposition in distributary channel environment. Therefore, sediments in the four sedimentary outcrops in the study area were deposited in fluviodeltaic to shallow marine (shoreface) settings.

Keywords: Anambra basin; Western flank; Facies analysis; Facies association; Fluvio-detaic; Shoreface