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Sedimentology and Depositional Environments of the Maastrichtian Mamu Formation, Northern Anambra Basin, Nigeria

Adamu M. Lukman, Rufai Ayuba and Tope S. Alege

The Maastrichtian Mamu Formation is an intracratonic formation within the Anambra Basin and is one of the three upper Cretaceous coal measures. An extensive geological field mapping was carried out on well-exposed lithostratigraphic sections. Successions were measured and their peculiar sedimentological features such as textures, physical and biogenic sedimentary structures and used to characterize the facies, facies variations, facies associations and grain size analysis were documented and used to interpret the depositional environments and suggest a paleogeographic model. Eleven lithofacies identified from the formation were grouped into subtidal channel facies and intertidal flat facies based on association The Mamu Formation is made up of bioturbated cross to massive bedded sandstone, siltstone, dark grey to light grey fissile shale, mudstone and milky white claystone which often displays a fining upward trend. The sandstone facies is coarse to fine-grained, poorly to moderately well sorted, leptokurtic and negatively skewed possibly deposited from fluvial source. Tide generated sedimentary structures such as herringbone cross beddings, tidal bundles, reactivation surfaces, clay drapes and clay flasers suggest tidal dominance over wave process. Typical sedimentary structures displayed by the subtidal channel and intertidal flat facies include herringbone cross-bedding, trough cross bedding, tabular cross-bedding, reactivation surfaces, burrows and clay drapes suggest tidal dominance over wave process. Vertical burrows of Ophiomorpha and Skolithos which belongs to Skolithos ichnofacies typifies littoral/intertidal environment suggests show colonization of only suspension feeders typical of high-energy environment. The multivariate scatter plots of the discriminate functions calculated from grain size data indicate deposition of the sandstones in a predominantly shallow marine/subtidal deposit. The fissility of shale the textural study of the siltstone facies suggests deposition in a low energy environment. However, field and textural studies of the sandstone facies suggest that the sediments were deposited in a relatively high-energy environment with tidal influence. The probability curve suggests deposition by rolling to bottom suspension rolling condition. The bimodal oblique paleocurrent pattern obtained shows that the formation was deposited in environment such as shoreline (beach) or fluvio-deltaic where tidal effect is significant. Paleocurrent studies and textural characteristics indicate that the sediments were from more than one source. Probably a regressive model is proposed for the Mamu Formation. Mamu Formation in the study area is deposited in subtidal channel and intertidal flat environments.