Abstract
Grain size analysis and sedimentology of beach sediments in Lagos, southwestern Nigeria have been studied. Ten fresh beach sediment samples were randomly collected at a depth of 25 cm - 40 cm along Lagos and Badagry axis. The sediments were analyzed by wet sieving method. Ternary diagrams indicate that the beach sediment is silty. The graphic mean ranges from 1.02ф (medium grained) to 2.21ф (fine grained) with an average of 1.61ф (medium grained). The preponderance of medium grained sediments and lack of coarse sands suggest moderate to high energy conditions of deposition. Graphic standard deviation ranges from 0.01ф - 0.81ф (very well sorted to moderately well sorted), with an average of 0.52ф (moderately well sorted). This is probably due to rapid back and forth flow of the depositing medium. Inclusive graphic skewness ranges from -0.38 to 1.41 (strongly coarse skewed to strongly fine skewed), with an average of 0.41 (strongly fine skewed). The graphic kurtosis values are between 0.44 and 1.37, which indicates very platykurtic to leptokurtic; with an average of 0.78 (platykurtic). This variation suggests that part of the sediment achieved its sorting elsewhere in high-energy environment. Histograms of the sediments exhibit both unimodal and bimodal trends. The arrangement is dominantly asymmetrical with varying modal class, which could be attributed to variation in the energy of the transporting medium. Bivariate plots (simple skewness vs. standard deviation and mean size vs. standard deviation) show the samples plotting mainly in the river sand zone, which suggests that the sediments were initially river sediments before reworking to the present site; other parameters that discriminate between river channel deposits, overbank deposits and overbank-pool deposits show the beach sediments plotting in the river channel zone. Linear discriminant functions analysis indicates a shallow marine beach environment in agitated water, with inputs from aeolian source. The Passega C-M diagram suggests suspension and rolling as the mode of deposition.
Highlights
Analysis of grain size distribution has been widely used by sedimentologists to classify sedimentary environments and elucidate transport dynamics
The preponderance of medium grained sediments and lack of coarse sands suggest moderate to high energy conditions of deposition
Bivariate plots show the samples plotting mainly in the river sand zone, which suggests that the sediments were initially river sediments before reworking to the present site; other parameters that discriminate between river channel deposits, overbank deposits and overbank-pool deposits show the beach sediments plotting in the river channel zone
Summary
Analysis of grain size distribution has been widely used by sedimentologists to classify sedimentary environments and elucidate transport dynamics. Grain size frequency distribution and textural factors may reflect the mode of transportation and depositional history of an area. Several workers have endeavoured to infer depositional environment and hydrodynamics from grain size data [1]-[17]. [18] found that the mean particle size of sand was the most important factor influencing vegetation composition, structure and distribution in the Malindi Bay coastal sand dunes in Kenya. The grain size characteristics in intertidal zone are changing with sediment transport especially disposal and removal of fine sediments. In tide dominated beaches the tidal current can play an important role to change the textural characteristics of beach sediments in an estuarine zone. The objectives of this research are to determine the texture of the beach sediments in the study area; infer the textural attributes of the sediments and the likely source(s) of the sediments and determine the hydrodynamic regime under which the sediments were deposited
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