Sedimentological interpretation of coastal ditch cuttings: implications for subsurface geology and provenance in Dahomey Basin, Nigeria

Abstract

Understanding sediment provenance and subsurface geology of an area plays an important role in geological exploration and environmental studies. This research characterized subsurface geology and provenance of coastal sediments in some parts of the eastern Dahomey Basin, southwestern Nigeria. One hundred twenty-six ditch cuttings were retrieved from four boreholes drilled along the coastal–near-coast area of the basin. Samples were subjected to sedimentological descriptions, heavy mineral analysis and geochemical analysis (atomic absorption spectrometry (AAS)), using standard laboratory procedures. The presence of stable–ultra-stable heavy minerals and the paucity of unstable minerals characterized the analyzed samples. AAS showed that SiO2 was the dominant oxide in all the 42 representative samples analyzed, with concentration that generally exceeds 90 wt%, thereby complementing ZTR index to ascertain sediment maturity. Careful comparison of the sedimentological data and heavy mineral indices shows several non-correlatable depositional cycles in the wells, while five distinct sequences were revealed from bivariate geochemical plots. Dominance of plutonic provenance, with very minor contributions from metamorphic rocks, continental depositional environment and repeated cycles of long transportation and deposition were inferred. This study demonstrated the usability of ditch cuttings to predict provenance, paleoenvironment and transport history in terrains with paucity of outcrops, core data and fossil-deficient intervals.