Lithofacies and ichnology of the Late Maastrichtian–Danian Nsukka Formation in the Okigwe area, Anambra Basin, Southeastern Nigeria

Abstract

Sedimentological and ichnological studies of the Late Maastrichtian to Danian Nsukka Formation in the Okigwe area of the Anambra Basin, Southeastern Nigeria were carried out to ascertain the lithofacies and to reconstruct the depositional environment. The lithofacies include concretionary black shale (F1), heterolithic facies (F2), bioturbated mudstone (F3), bioturbated sandstone (F4), ripple-bedded sandstone (F5), cross-bedded sandstone (F6), parallel-bedded sandstone (F7), bioturbated black shale (F8), and bioturbated shelly sandstone (F9). Some of the beds are significantly bioturbated with bioturbation index (BI) ranging from 3 to 5. Five lithofacies associations were distinguished and interpreted as follows: lithofacies association I represents offshore to lower shoreface transition sediments; lithofacies association II is middle to lower shoreface; lithofacies III comprises upper shoreface to foreshore and lagoon/bay sediments; lithofacies association IV is of upper shoreface origin; and lithofacies association V is proximal offshore. The shallow marine setting was dominated by currents, waves and occasional storm. The trace fossil assemblage comprises Ophiomorpha nodosa and Skolithos linearis representing the Skolithos ichnofacies and Thalassinoides suevicus, Teichichnus rectus, Paleophycus herberti and Planolites montanus representing the Cruziana ichnofacies. The six trace fossils described in this study are equally distributed into two ethological categories, domichnia (dwelling burrows) and fodinichnia (feeding burrows). The distribution of the two ethological categories in the sediments within the shallow marine environment was controlled by the energy of the environment, amount of organic detritus, degree of oxygenation and salinity.