ABSTRACT
This project provides results of detailed field sequence stratigraphy, sedimentology and sedimentary structure studies in Enugu, Mamu and Ajali Formations. Thirty-two (32) outcrop sections were mapped: Six (6) in Enugu Formation, eight (8) in Mamu Formation and six (6) in Ajali Formation respectively. Sequence stratigraphic, sedimentologic and biostratigraphic analyses were applied towards the geology of the formations. The field sequence stratigraphic interpretation reveals: two (2) sequence boundaries (SB), one (1) lowstand systems tract, one transgressive systems tract (TST), one (1) maximum flooding surface (MFS) and two (2) highstand systems tract (HST) in the three formations altogether. Thirteen lithofacies were identified in this study: cross-stratified sandstone, bioturbated cross-stratified sandstones, massive/poorly stratified sandstone, bioturbated sandstones, parallel laminated sandstone, wavy laminated sandstone, sand-dominated heterolithics, mud-dominated heterolithics, mudstone and shales, conglomerate/ pebbly sandstone, shelly limestones, oolithic ironstones, coals and lignite. These were grouped into four facies associations namely; Estuarine bayhead delta facies association in lower part of Ajali, central estuarine facies association recognized in Enugu and Mamu Formations, tidally influenced coastal shoreline facies association in Mamu and Ajali Formations, and transgressive shallow marine shelf lithofacies association in Mamu Formation. Sedimentary structures identified are as follows: Incised valley fills, reactivation surfaces, double mud drapes, laminations, inclusions (rip-up clasts), tidal bundles, Phycosiphon, Skolithos, Ophiomorpha ichnogenera and herringbone cross-stratification respectively. Ajali Formation was characteristic of 1st, 2nd and 3rd order cyclicity with an erosional truncation at the base in Location 12.Biostratigraphic microfossil recorded are as follows: Cingulatisporites ornatus, Longapertites sp., Laevigatosporites sp., Buttania andreevi, Trichodium sp., Milfordia sp., Monocolpites marginatus, Longapertites marginatus, Proxapertites Cursus, Botryococcus braunii, Haplophragmoides excavata, Ammobaculites subcretacea, Saccammina complanata, Ammodiscus glabratus, Textularia parvula, Haplophragmoides sp and Trochammina sp. The synthesis of the foregoing results suggest a Late Campanian to Mid-Maastrichtian age in a central estuarine to coastal to shallow marine shelf environments. The data generated in this study if applied in the future will fascilitate the interpretation of the field sequence stratigraphy, sedimentology and sedimentary structure studies of Anambra Basin.
