Petrography and Biostratigraphic Studies of Campano-Maastrichtian Sequences of Anambra Basin Southeastern Nigeria

Anambra Basin located in the South Benue Trough is a Post Santonian synclinal sedimentary fill containing over 5000 m thick of Upper Cretaceous to Recent sediment. The study involved the use of field observation, sedimentological, petrographical and biostratigraphic studies of the sandstones and shale exposed within the study area to establish the depositional environment, textural and compositional maturity, provenance, tectonic settings, age relationship and to understand the stratigraphy of the basin. For this purpose, a total of 13 samples were subjected to heavy mineral and granulometric analyses, 9 samples of were subjected to micropaleontological analysis, and 2 samples of Ajali Sandstone were subjected to thin section analysis.

Field observations show that Enugu Shale is fissile with the presence of extraformational clast, Nkporo Group consists of shale, siltstone, mudstone and ironstone intercalation. Mamu Formation consists of shale, coal, and sandy shale unit, which graded into Ajali sandstone characterized by cross beddings, herringbone structure and ophiomorpha burrows.

The sieve analyses of the Ajali sandstone indicate that they are generally coarse grained, poorly to moderately sorted, mesokurtic to leptokurtic, nearly symmetrical to very coarse skewed. The result of bivariate analysis reveals that Ajali sandstone is fluvial sand, while the multivariate results show that some of the samples of Ajali Sandstone are shallow marine deposit while others are fluvial deposits. The textural result of Ajali Sandstone in the study area coupled with the field data such as Herring-bone structures, and Ophiomorpha burrows, revealed that Ajali Sandstone was deposited in a tidal environment probably littoral environment. The paleocurrent direction of Ajali Sandstone measured from the beddings indicates southwest, while the provenance direction is toward the northeast. The Heavy mineral assemblages indicated from the heavy mineral analysis include: Zircon, Tourmaline, Rutile, Epidote, Garnet, Staurolite, Sillimanite, Chloritoid and Titanite which suggested their provenance to range from Acid igneous and Dynamothermal Metamorphic rock. The ZTR index indicates that the Sandstones were moderately mature mineralogically with a ZTR index that ranges from 11.54 to 58.3%. Thin section studies revealed the Sandstone to be quartz arenite to sublitharenite with more than 82% quartz. From the palynology and foraminiferal studies, it can be inferred that the paleo-environments of Nkporo, Enugu, and Mamu Shale were probably Marginal Marine environment based on the occurrence of forms such as Phelodinium bolonienae, Microforaminiferal wall linings, a large number of Botryococcus braunii, and dominance of arenaceous foraminiferal species. The age of Enugu/Nkporo Shales were suggested to be Early-Late Maastrichtian, the age of Mamu Shales were suggested to be Early Maastrichtian, and based on the occurrence of palynomorphs such as Dinogymnium undulosum, Phelodinium bolonienae and Botryococcus braunii. Based on t he occurrence of foraminiferal such as Haplophragmoides excavate, the age of Enugu shale was suggested to be Coniacian-Maastrichtian.