Palynostratigraphy and palaeoenvironment of the Danian sediments from the Nsukka Formation of the Anambra Basin at the vicinity of Ikpankwu, southeastern Nigeria.

Abstract

Detailed investigations of biostratigraphy of the Nsukka Formation in the Ikpankwu area are paramount due to the fluctuation of depositional environment, fossil flora abundance and diversity interchange, and lateral facies change dynamics from the southern to northern sections of this formation. The microflora assemblages, chronostratigraphic age and depositional paleoenvironment of index palynomorphs suggest a Danian age for the studied strata, based on global dinoflagellate cyst events. The persistent abundance of monocolpate assemblages and the biostratigraphic resolution attained, suggest that the studied sediment interval is continuous. The marker species of terrestrial microflora (Constructipollenites ineffectus, Echitriporites trianguliformis, Foveotriletes margaritae, Longapertites group, Proxapertites group, Retidiporites magdalenensis, Scabratriporites annellus, Spinizonocolpites group, Syncolporites marginatus) and dinocysts (Carpatella cornuta, Danea californica, Fibrocysta licia, Kenleyia pachycerata, Spiniferella cornuta subsp. kasira), compared with several reference sections from the tropical to subtropical province, engineered a refined geologic age and paleoenvironment of deposition of the sediments. These dinocysts taxa are global Danian index fossils which are valuable in the identification of earliest Palaeocene sequence of the formation. The depositional paleoenvironment alternately fluctuates from continental setting to marginal marine inner neritic and outer neritic conditions, based on the presence of land-derived miospores and dinocysts. These macroenvironments are the products of effective vacillating depositional environment systems between the lower and upper deltaic plains, oscillating from tidal flat, lagoon, tidal bar to nearshore open marine influenced settings. Lycopodiaceae-Zlivisporis blanensis fern spore with fragile perispore substantiate the autochtonous vegetation and least transport and palynomorph hydrodynamic system inherent in the predominant pollen and spore species and close proximity to fresh water settings visible in medium to coarse grained sandstone units of the middle section. These microenvironment syntheses of the Nsukka Formation revealed a confirmed and pronounced transgressive systems tract (TST), articulated in the shale sample from the Ikpankwu section of the Nsukka Formation, Anambra Basin. The palaeoenvironments and palynostratigraphic dynamics illustrate that overall progradation was trailed by sporadic retrogradation of the delta with a high biostratigraphic resolution system indicative of continuous biological imprints of the terrestrial and marine microflora of the Nsukka Formation during the Danian.