Oscillatory ripples, evaluation of ancient wave climates and epierogeny in the Anambra basin and the Afikpo sub-basin, southeastern Nigeria
AbstractThe analyses of geometrical properties of wave ripple marks from the Campanian – Early Maastrichtian sandstone units of the Nkporo and Enugu Formations in the Afikpo Sub-basin and the Anambra Basin respectively,
have permitted a reconstruction of the ancient oceanographic conditions, epierogenic patterns and paleogeographic history of the basins. The Nkporo Sea in the Afikpo Sub-basin was characterized by low bottom water orbital velocity (47.75 cm/s - 51.42 cm/s) and low wave periods (1.36 – 4.35 s). The minimum and maximum estimated wavelengths vary from 1.46m -1.70m, and 26.05m - 29.44m respectively. The wave heights range from 0.41m – 0.47m while the wave power is from 0.66 kw/m – 0.78 kw/m. The bathymetric estimates for the rippled deposits range from 14.82 – 16.62 m, suggesting shallow water depths for the units. The Campanian Sea of the Anambra Basin had low bottom water orbital velocity waves (57.17 – 58.41 cm/s) and low to moderate wave periods (1.63 – 5.90 s). Minimum and maximum wavelengths of 2.10 - 2.19 m wavelengths of 50.74 – 54.23 m were estimated for the ancient sea
respectively. The wave heights varied from 0.61 – 0.64 m while the wave power varied from 2.04 – 2.32kw/m. The paleowater depth varied from 31.35m – 33.65m. These estimated wave climates and water depths indicated that the Campanian – Early Maastrichtian Seas were shallow and marked by low to moderate hydrodynamic energy conditions. The short to intermediate wave periods indicates a 40 – 50 km fetch while the relatively higher wave power in the ancient Anambra Sea indicates a larger and wider basin than the ancient Afikpo Sub-Basin Sea. The epierogenic behavior interpreted from the maximum paleowater depths and shoreface stratigraphic thicknesses confirm dominance of the ancient Anambra Sea by retrogradational base level dynamics while the ancient Afikpo Sea,
with an initial prograding shoreline, later became retrogradational.