Sequence stratigraphical interpretation of the Paleocene to Miocene (Selandian–Aquitanian) palynofacies framework of the Niger delta basin, southeastern Nigeria

Abstract

Palynofacies analysis of the outcropping Paleogene to early Neogene formations in the Niger Delta provides a snapshot of the palynofacies framework and events related to sequence stratigraphic cycles during the Cenozoic. Five palynofacies groups defined by particulate organic matter were synthesized into four main palynofacies sequence stratigraphic events denoted as A to D, recognized from base to top of the Imo, Nanka and Ogwashi formations. Palynofacies sequence stratigraphic event A in the basal dark grey shales of the Imo Formation is dominated by amorphous organic matter (AOM), opaque particles, and marine palynomorphs, and small numbers of terrestrial microflora suggestive of a transgressive systems tract (TST) for the Selandian section of the formation. High percentages of AOM and opaque particles, paucity of structured phytoclasts, and maximum numbers of dinoflagellate cysts were observed in palynofacies stratigraphic cycle B in the Thanetian section of the Imo Formation, indicating deposits of an early TST. The absence of lath-shaped opaque debris and few equidimensional forms indicated marine depositional origin for the transported opaque debris. A lowstand systems tract (LST) was interpreted for the fining upward sandstone unit separating these TST packages. Palynofacies sequence stratigraphic cycle C, which was identified in the Nanka Formation, was characterized by an increase in terrestrial microflora, structured phytoclasts, and opaque particles, with relative small numbers of dinoflagellate cysts and AOM signifying late highstand systems tract (LHST) for the Ypresian sediments. The attributes of palynofacies stratigraphic cycle D established in the Ogwashi Formation were the maximum numbers of terrestrial palynomorph taxa, structured phytoclasts, opaque debris, abundant AOM and limited amounts of marine species, suggestive of the LST during the Priabonian to Aquitanian. The Niger Delta Basin sediments are interpreted as deposits of the earliest Danian to Thanetian transgression, Ypresian highstand interval and Priabonian to Aquitanian regression interrupted by minor marine transgressions. The palynofacies sequence stratigraphic cycles model demonstrates that an overall basin-wide retrogradation was succeeded by progradation of the delta during the later Cenozoic.