Sedimentology And Stratigraphy Of The Campano Maastrichtian Succession Northeastof Idah Town Northern Anambra Basin

The Campano-Maastrichtian successions in the northern Anambra Basin comprisestheNkporoFormation, Mamu Formation and Ajali Sandstone.An extensive geological fieldwork was carried out, where lithostratigraphic sections were studied and described. Featuressuch as textures, physical and biogenic sedimentary structures (facies variations andassociations) were documented. Some selected representative samples of the sedimentarydepositional facies were also subjected to grain size analysis.The NkporoShale is made upofsandy shale, silty clay, grey mud, bioturbatedsiltstone which graded into Mamu Formationwhich is made up ofcross-massive bedded sandstone with bioturbation structures, siltstone,dark grey to light grey fissile shale, mudstone and milky white claystone which oftendisplays a fining upward trend. It passes upward into Ajali Sandstone which is made up offine to coarse-grained, cross-bedded sandstone and fissile shale with bioturbationstructures.The sandstone facies of the Mamu Formation is coarse to fine-grained, poorly tomoderately sorted, leptokurtic and negatively skewed possibly deposited from fluvial source.The bioturbated lower sandstone beds contain burrows of Ophiomorphawhich belong toskolithosichnofacies that typifies littoral environment while the Ajali Sandstone is pebblycoarse to medium-grained, poorly to moderately sorted, predominantsmesokurtictoplatykurtic orvery platykurtic and negative to near symmetrical.The linear discriminatefunction scatter plots results reveal shallow marine deposits forthe sandstone facies of MamuFormation and AjaliSandstone.The fissility of shale suggests that it was deposited in a low energy environment.However, the textural studies of the Nkporo Formation and the siltstone facies of the MamuFormation suggests the sediments to be deposited in a low energy environment whichfavoured deposition of fine to medium size sediments, that is, deltaplain to prodeltaenvironment. Field and textural studies of the sandstone facies of the Mamu Formationsuggest the sediments to be deposited in a relatively high energy environment with tidalinfluence of transport and deposition with two main facies association, that is, subtidal andintertidal.The cross-beddings, unimodal paleocurrent of Ajali Sandstones also indicatesfluvial or fluvial-dominated deltaic environment while the bimodal oblique paleocurrentpattern obtained for Mamu Formation shows that the formation was deposited inenvironment such as shoreline (beach) or fluvio-deltaic where tidal effect issignificant.Paleocurrent studies and textural characteristics indicate that the paleocurrentdirection is northeast while the provenance is southwest suggesting that the sediments weremainly sourced from the Santonian Okigwe-AbakalikiAnticlinorium and partly from thenearby Oban Massif for the Ajali Sandstone in the study area while the sediments of thesandstone facies of Mamu Formation were sourced from more than one source.The resultsobtained from the textural, structure and statistical analyses revealed that the sediments in the study area were deposited in fluvial, deltafront through deltaplain to prodeltain shallowmarine environment.

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