Ichnological characteristics and variability of Miocene deposits in the Cenozoic Niger Delta: Examples from cores in the Coastal Swamp and Offshore depobelts

Abstract

This work examines ichnological attributes and variability of core samples from four wells (1–4) within the Coastal Swamp and shallow Offshore depobelts in the Niger Delta basin. The cores consist of facies interpreted as mainly shoreface deposits. These deposits are in successions and well-developed. The shoreface successions are divided up into a lower, middle and upper shoreface. The lower shoreface lies within fair weather wave base. The ichnofossils assemblages characteristically represent a diverse and abundant Cruziana ichnofacies. The middle shoreface contains swaly and lesser hummocky cross-stratified sandstones with an ichnofossil assemblage dominated by the Skolithos ichnofacies. The upper shoreface is dominated by planar and trough cross-beddings. Ichnofossils are fairly abundant and consist of deeply penetrating structures of the Skolithos ichnofacies. These facies reveal a wide range of variability, mostly limited to the lower and middle shoreface intervals. This variability appears to be determined by relative storm dominance and the episodic character of associated storm bed deposition. The successions are grouped into three: (1) high energy shorefaces consist entirely of stacked storm beds with moderate bioturbation, (2) moderate energy shorefaces comprises stacked laminated to burrowed intervals, recording recurrent erosive storm bed emplacement, followed by opportunistic colonization, and normally overprinted by resident fair-weather burrowing, and (3) low energy shorefaces ordinarily exhibit intense bioturbation with low preservation of storm beds. Discrimination between lower and middle shoreface deposits is mainly based on the resident fair-weather ichnofossil assemblage; under increased energy settings. In addition, based on storm intensity, relative water depth, storm frequency and shoreline character, the nature of coexistent lower and middle shoreface facies can differ significantly along strike. In some cases, the lower and middle shoreface differentiation was difficult.