Ancient deposition environment, mechanism of deposition and textural attributes of Ajali Formation, western flank of the Anambra Basin, Nigeria

Abstract

The Maastrichtian siliciclastic sedimentary units of the Ajali Formation, Western Flank, Anambra Basin, Nigeria, exposed at Fugar, Ayogwuiri, Auchi, and Uzebba all within the Ajali Formation have been studied for its pebble morphometry characteristics in order to decipher the ancient deposition environment, mechanism of deposition and textural attributes. Outcrop samples were collected for this study. Fifty-one samples were analyzed for grain size parameters, while ten samples were for the pebble morphometric index of sediments at the source area. The grain size analysis results reveal a medium to coarse-grained sands particle, which suggests a moderate to high energy environment of deposition. Also, the skewness and sorting of the sediments show an average of 0.1 and 0.9 phi respectively; this suggests river and or fluviatile sands, further supported by the bivariant-plots for paleoenvironments (Skewness vs. Median Standard. deviation vs. Median). Furthermore, the Coarsest Sand - Median (CM) diagram of the Ajali Formation indicates that the deposition of sediment was by rolling, bottom suspension, and graded suspension. The environment of Ajali Formation is further authenticated by the Pebble morphometric environmental determination chart, with over 70% of the sediments, plotting in the fluvial/river environment, confirming a fluviatile depositional environment.

Keywords

• Ajali formation
• Deposition environment
• Pebble morphometry
• Anambra basin
• Nigeria