Planktonic foraminifera of the Nkalagu Formation type locality (southern Nigeria, Cenomanian–Coniacian): biostratigraphy and palaeoenvironmental interpretation

Abstract

A study on the planktonic foraminifera of the southern Nigerian Nkalagu Formation (Cenomanian–Coniacian) was carried out. Four biostratigraphic zones are proposed for the (?)middle Turonian–Coniacian interval in southern Nigeria: (1) Praeglobotruncana cf. stephani Zone (middle? Turonian); (2) Marginotruncana sigali Zone (late Turonian); (3) Dicarinella primitiva Zone (latest Turonian); and (4) Dicarinella concavata Zone (Coniacian). Based on planktonic/benthonic foraminiferal ratios and environmental index forms, a general deepening of depositional environments is indicated from late Cenomanian–Turonian and Coniacian ages. Upper Cenomanian sediments were deposited on an inner shelf (0–70 m, 0–20% planktonic foraminifera; only one Heterohelix species occurs). During the (?)middle to early late Turonian, an upper bathyal environment of about 600 m water depth is indicated (46–94% planktonic foraminifera, with heterohelicids dominating and a relatively large number of keeled specimens). The middle late to latest Turonian interval is characterized by 20–71% planktonic foraminifera with heterohelicids dominating and very rare keeled specimens, pointing to an upper bathyal depositional environment (ca. 250 m water depth). A (deeper) upper bathyal environment (ca. 600 m water depth), dominated by heterohelicids but with up to 30% hedbergellids during the Coniacian, is indicated by 63–93% planktonic foraminifera with a relatively large number of keeled specimens. In general, an open marine deep-water environment (upper bathyal) is indicated by the (?)middle Turonian–Coniacian planktonic foraminiferal faunas, further influenced by periods of eutrophication or (weak) salinity fluctuations. The (?)middle Turonian and latest late Turonian were time intervals of highest surface productivity in southern Nigeria.