SEQUENCE STRATIGRAPHY OF UPPER CRETACEOUS SEDIMENTARY SUCCESSIONS IN THE WESTERN FLANK OF ANAMBRA BASIN, NIGERIA

Abstract

Cretaceous sedimentary outcrops located within Auchi and Agenebode environs were investigated for the purpose of determining the provenance and environment of deposition of their sediments. The investigation involved field studies of the outcrops in four main localities: Apana-Imiegba, Auchi, Ayowgiri and Fugar, using integration of granulometric, petrographic, heavy mineral analysis and biostratigraphy studies. Field study of the road cut exposures in the localities revealed that the rock types found in the study area have been subdivided into three facies based on lithological and sedimentary structures. These facies are: shale (base), clay and ferruginized sandstone. The road cut exposure of sandstone in Fugar locality revealed the rock is across bedded, sandstone sequence is medium grained (1.21) and moderately sorted (0.94), the grains are coarsely-skewed and mesokurtic distributed. That of Auchi locality, exposed by the Orle river valley compose of a sequence of reddish-brown, medium grained (1.34), moderately sorted (1.18), hummocky and herringbone cross-bedded friable sandstone. The grains are mesokurtic and symmetrically distributed with value of -0.05 and 1.32. Petrographic study of representative samples from Apana-Imiegba localities contains 90.2-97% quartz, 1.1-6.5% feldspar, 1.2-5.1% lithic fragments, 1-1.2% cement and 0-1% matrix. The sandstones is classified as quartz-arenites and sublitharenites, with similar assemblages of heavy minerals suggesting derivation mainly from acid igneous rocks, gneisses and older sandstones. The ZTR indices values indicate all the sandstones are mature to supermature with values ranging from 84.2% to 100%. Biostratigraphically, palynological study of Apana-Imiegba shale revealed that the sequence belongs to Foveotriletes margaritae assemblage zone characterized by diagnostic forms such as Buttinia andreevi, Retidiportes magdalenensis, Periretisyncolpites spp, Cingulatisporites ornatus, Auriculiidites sp, Constructipollenites ineffectus, and Monocolpopollenites sphaeroidites that depict Maastrichtian age. Micropaleontological analysis shows that it comprises of benthic arenaceous foraminifera assemblage of Ammobaculties amabensis, Ammobaculites bauchensis, Ammobaculites benuensis, Haplopragmoides talokaense, Haphragmoides saheliense, Haplopragmoides hausa, Ammobaculities sp, Ammobaculites coprolithiformis and Haplophragmoldes sp. that depict Maastrichtian age, an interval that is deduced to fall within the marine incursion period in the Anambra Basin. The presence of Ammobaculities benuensis, Ammobaculites corprolithiformis and Ammobaculite amabensis suggest a coastal swamp, tidal flat to estuarine, delta front to inner neritic environments of deposition. The Cretaceous and Tertiary sedimentary successions are resolved to respective depositional sequence, classified by its systems tracts. The Cretaceous and Tertiary sedimentary successions are composed of depositional sequence, which are divisible into contemporaneous depositional systems. The contemporaneous depositional systems identified are lowstand (LST), transgressive (TST) and highstand (HST) systems tracts. The LST occur as the first depositional system tracts bounded at the base by the sequence boundary with sequence boundary. The TST delinated within the sediments are separated by maximum flooding surfaces from the overlying HST. Sequence boundaries were also recognized at the limits of the HST, except where the HST was not recognized. This implies the sediments of these depositional systems must have suffered intense erosion.