Lithofacies characterization and channel development in the outcrops, of Cretaceous sedimentary rocks, eastern Dahomey Basin, southwestern Nigeria

摘要

Outcrop exposures of Cretaceous sedimentary rocks in Dahomey Basin, southwestern Nigeria, allow the recognition of slope related sediments and slope valleys. Detailed outcrop logging permits the delineation of massive to cross bedded loose sand with lateral fades change, normally graded sandstone, clay with injected sand, clayey silt with abundant clasts, channel complexes associated with different lithologies, loose sand, very fine to coarse grained sandstone and conglomerate with clasts of sandstone and siltstone. These lithofacies revealed history of erosion and deposition in various proportions. Four sets of channel development were delineated; the initial valley cut, complex reincision and fill, regional erosion and channel abandonment. These four categories of channel development indicate significant erosion and are possibly related to periods of lowstands. The study shows that the first set of channel infill probably commenced with deposition of turbidites accompanied by marine transgression associated with contemporaneous slumping. This progresses vertically to amalgamated (network of channels) channels filled with diverse lithologic units while the topmost channel is filled with channel lag and conglomerate. Channel infills are composed essentially of sediments derived from turbidites, slumps and debris flows. Two types of architectural elements recognized are single-story channel and multistory channel complexes. The different stages of channel development can be considered in terms of low efficiency and high efficiency flows, which are related to slope equilibrium.