Depositional Environments and Sequence Stratigraphic Interpretation of the Campano-Maastrichtian Nkporo Shale Group and Mamu Formation Exposures at Leru-Okigwe Axis, Anambra Basin, Southeastern Nigeria.

A total of six (6) lithofacies were distinguished from two (2) outcrop sections of Cretaceous succession of Nkporo and Mamu Formations exposed in a cascading topography on both sides of the Enugu-Port Harcourt expressway in Leru, Anambra Basin. The lithofacies include: Interstratified Calcareous Mudstone Shale Facies (A), Horizontal and Cross Stratified Sandstone Facies (B), Heterolithic Sandstone and Muddy Shale Facies (C), Heterolithic Shale and Siltstone Facies (D), Poorly Stratified Sandstone Facies (E) and Red Sandstone Facies (F). Interpretation of depositional environments using lithofacies stacking pattern, sedimentary structures and biostratigraphic analyses inferred an estuarine/lagoonal to tidal delta to shallow, open marine depositional model for the Nkporo Shale and an estuarine to tidal delta to shallow marine paleodepositional model for the Mamu Formation in the study area. Sequence stratigraphic interpretations established depositional patterns with one complete and one incomplete 3rd order sequences, comprising of Highstand Systems Tracts (HST) and Transgressive Systems Tracts (TST). The packages (Systems Tracts) formed distinct complexes consisting of a number of genetically linked retrogradational, progradational and aggradational deltaic systems. Major architectural facies elements in the packages include: shoreface deposits, channel fills, and prodelta shales in sediment starved areas. Two Type-1 Sequence Boundaries (SB), a condensed section and two chronostratigraphic significant surfaces (Maximum Flooding Surface; MFS) were delineated in the sections. The sequence boundary identified at the 68m level in Leru 1 is a textbook example of an onlap stratal termination pattern in outcrops. The surface provided the boundary between Nkporo Shale and Mamu Formation in the studied sections. Alternation of Highstand Systems Tracts and Transgressive Systems Tract sands and shales, in the studied sections provides a union of reservoir and seal rocks that are essential for hydrocarbon accumulation and stratigraphic trapping.