A photogeological study of the fold structure in okemesi area, Nigeria.

The Okemesi fold, situated in southwestern Nigeria, is one of the most identifiable geological structures in the basement complex. It is a manifestation of Late-Precambrian deformation of metasediments which have been heavily fractured and sheared in places. Aerial photographs covering about 150km2 of the fold belt were selected for the study because of their large scale and good resolution for structural delineation. A total of 175 fractures were extracted with a cumulative length of about 155 kilometers. They are in three main classes of azimuths: 011o-020o, 021o-030o and 101o-110o. Results indicate the existence of a series of faults that offset the fold trend. Field observations also show that schists and schistose quartzites display strong foliation and shearing which are indicated by displacements along sub-parallel planes. Faulting in the area probably post-dated the folding episode as indicated by the Itawure Fault which separates the Okemesi fold into double-plunging halves; one towards the north and the other towards the south. These structures are observed to be the main factor controlling the drainage pattern in the area. The area of study displays a series of folds which were possibly generated by overthrusting of low-dipping rocks and subsequent fracturing that resulted in the plunging of the fold axis. Journal of Mining and Geology Vol. 43 (2) 2007: pp. 125-13

Lithofacies characterization and channel development in the outcrops of cretaceous sedimentary rocks, Dahomey basin, southwestern Nigeria.

Outcrop exposures of Cretaceous sedimentary rocks in Benin 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 facies changes, 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 are related to significant erosion and are possibly related to periods of lowstands. The outcrop exposures show that the first set of channel infill probably commenced with deposition of turbidites accompanied by marine transgression associated with contemporaneous slumping. This progress 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 complex. The different stages of channel development can be considered in terms of low efficiency and high efficiency flows, which are related to slope equilibrium. Keywords: Outcrop, slope, channel development, channel architecture, process model, lowstandJournal of Mining and Geology Vol. 43 (2) 2007: pp. 131-14