A paleogeographic model for the sandstone members of the Imo Shale, south-eastern Nigeria

• Re-interpretation of the paleoenvironment of the sandstone members of the Imo Formation using facies analysis.
• Integrating clay mineralogy and palynology to re-interpret the depositional environments for the mudrocks in the Imo Shale.
• Depositional models and paleogeographic evolution of the Imo Formation were reconstructed.

A new interpretation of the Palaeocene to Eocene strata of SE Nigeria has been developed based on field facies analysis and borehole data from the area. The area is considered to have been a tidally-dominated shelf setting which underwent a series of changes in sea level during the deposition of the Imo Shale. An initial transgression led to the deposition of a widespread marine shale unit that is interpreted as an offshore succession. This was followed by an influx of sands that are texturally mature, coarse to fine-grained that show large-scale cross-stratification with dip angles of between 15° and 25°. These are interpreted as the deposits of large sandwaves on a tidally-dominated shelf. Transport direction determined by the dominant large scale cross beds indicate a north-westerly transport direction. The presence of illite and nontronite clay mineral types in the offshore shales typically suggest marine environment whereas the occurrence of kaolinite which indicates terrestrial influence. A shale bed above the sand wave unit is overlain by wave-ripple cross-laminated sandstone considered to have formed in an upper shoreface setting. An overlying fossiliferous shale with a restricted fauna and shell lag is interpreted as having been formed in a lagoon with restricted circulation. The progradational succession is capped by a return to deeper water facies comprising a mixed carbonate–siliciclastic succession. This includes mudstone with limestone layers, calcareous sandstone with fossiliferous marl beds, fossiliferous and bioturbated sandstone. These beds represent a relative sea level rise on the shelf, accompanied by a decrease in siliciclastic input. This data provides a new conceptual model for the paleogeographic evolution of the sandstone member of the Imo Shale.