Palynological evidence for the Turonian/Campanian boundary between the Abakaliki and the Anambra Basins, as exposed at Leru along the Enugu-Port Harcourt expressway, southeastern Nigeria

摘要

Quarrying of a dolerite intrusion has exposed a depth of over 7m of a succession of rocks at Leru on the Enugu-Port Harcourt expressway in southeastern Nigeria. The sequence is made up of coarse grained sandstone at base, overlain by fissile, grey shale that is nodular at the middle portion and successively overlain by a very thick greyish fissile shale at the top. Systematic sampling from 6.0 m depth at 0.5 m intervals to the top yielded a rich assemblage of Middle Turoniart dinofiagellate cysts (at 6.0-5.5 m), Campanian pollen and dinofiagellate cysts (at 5.0 - 0.0 m) and a boundary zone of mixing (at 5.0 - 4.5 m) between them. The depth (6.0 - 5.5 m) corresponds to very calcareous and (5.0 -0 m) to non-calcareous beds. Turonian dinofiagellate cysts include Subtilisphaera pirnaensis, S. pontis-mariae, Batiacasphaera sp., Diconodinium sp., Paiaeocystodinium sp., Palaeohystrichophora infusorioides, Leiosphaeridia sp. and Scutkabolus lapidaris. The Late Campanian floras consist of Longapertites marginatus, Monocolpites marginatus, Retibrevitricolpites triangulatus, Distavemisporites simplex, Scabratriporites simpliformis, Senegalinium dubium, Phelodiniutn gaditanum, Paiaeocystodinium australinum and Dinogymnium cf euclaense. The Campanian /Turonian boundary, within which reworked Turonian palynomorphs are mixed with the indigenous Campanian flora, marks the sequence boundary. In the absence of the Coniacian-Santonian Awgu Shale, the Campanian Nkporo Shale oversteps onto the Turonian Eze Aku Shale. The boundary reflects the Low Stand Systems Tract (LST) in the Coniacian-Santonian time and accounts for the absence of Santonian deposits in this basin. The 5.0 m depth could be taken to marks the beginning of Anambra depositional cycle.