Abstract:
The Anambra Basin of Nigeria, part of the larger West and Central African Rift System (WCARS) across Africa, contains primarily Cretaceous to Palaeocene shallow to marginal marine and freshwater sedimentary deposits. The organic-rich deposits
of the Cretaceous Nkporo and Mamu formations within the Anambra Basin and the Palaeocene Imo Formation of the Niger
Delta Basin constitute important conventional source rocks. Despite its economic significance, research on organic matter characterisation, palynofacies, sea-level fluctuations, palaeoclimate, hydrogeography, basin restriction, palaeobathymetry and the
factors controlling organic matter preservation remains largely undocumented. The here presented new inorganic and organic
geochemical and organic petrography data of the Cretaceous to Palaeocene deposits serve to refine the regional interpretation at
a basinal scale and within the supra-regional context of the WCARS. Geochemical palaeotemperature proxies suggest a warm
and humid tropical palaeoclimate during the Late Cretaceous within the study area. Furthermore, the measured TOC values indicate poor to very good organic content. Palynofacies analysis revealed high abundances of opaque and translucent phytoclasts
and low amounts of palynomorphs with negligible amorphous organic matter (AOM) in the studied mudrocks. Two palynofacies
groups suggest shallow-marine conditions in a proximal shelf setting and a heterolithic oxic basin. The palynofacies of the upper
Imo Formation reveal higher percentages of dinoflagellate cysts during the early highstand phase, with equidimensional, opaque
phytoclasts representing the maximum flooding phase, accompanied by warmer conditions. Additionally, abundant terrestrial
phytoclasts, Deltoidospora spp., Classopollis spp. and geochemical indicators collectively indicate warm tropical climatic conditions consistent with geochemical interpretations. The palaeobathymetry reconstructions suggest a shallow seaway during the
Upper Cretaceous in the Anambra Basin.
