SEDIMENTOLOGICAL STUDY OF BOTTUM SEDIMENTS: A CASE STUDY OF RIVERS ERO AND ADOGO AJAOKUTA KOGI STATE, NIGERIA

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CHAPTER ONE:

INTRODUCTION

1.1  General Introduction

Fluvial bed sediments is one of the first phenomena that scientists have studied and observed amongst other riverbed properties such as morphology, hydraulic characteristics and ability to alter their section in a short distance. This is most probably due to their direct evidence and ease of observation. Gomez et al. (2001) conducted a notable study on the evolution of ideas in this field, of which one can conclude that the interest that this domain has raised has been shown for a long period of time; the study also includes the first comments on the causes of the processes of reduction of bed material size, mainly by particle abrasion and hydraulic sorting.

Sediment texture refers to the shape, size and three-dimensional arrangement of the particles that make up sediments or sedimentary rocks. Textural characteristics therefore constitute fundamental descriptive measure of sediments and sedimentary rocks. The focus of textural studies is to employ graphical, moments and statistical methods to discriminate between sedimentary successions based on environment of deposition.

Bivariate plots between various statistical grain size parameters have also been successfully used for distinction of such environments. Sahu (1964) has also used Linear Discriminant Functions (LDF) to discover the relationship between variances exhibited by statistical parameters.

The banks of river are inherently unstable due to the erosive power of current. This is particularly so where rivers flows through their own detritus .However, in certain circumstances it is an inherent property of river channels to meander seriously across their flood plain, as water moves across the bend the current velocity increases at the outer bank of the curve and decreases on the inner bank leading a deposition of poorly sorted sediments .River sediments are therefore sensitive to the physical changes of the transporting medium.

Grain size distribution studies of river sediments provide a wealth of information on the intrinsic properties of sediments and their depositional history through graphical or calculated methods used along with other textural properties (Blott and Pye, 2001; and Martins, 2003).

A graphical method to perform a granulo­metric analysis was presented more than half a century ago by Folk & Ward (1957). This al­lowed sedimentologists to calculate approxi­mate grain-size parameters, obtained from graphs by computerized data analytical techniques enabling much more precise calcu­lations of statistical parameters such as the average size of grain, sorting, skewness and kurtosis. These parameters are considered by some earth scien­tists as essential for classification of sedimentary environments.          

A correlation between size parameters and transport processes as well as depositional mechanisms of sediments has been established by exhaustive studies (Folk and Ward, 1957; Mason and Folk, 1958; Friedman, 1961, 1967). Grain size being the most fundamental property of sediment particles, affecting their entrainment, transport and deposition, therefore provides important clues to the sediment provenance, transport history and depositional conditions (e.g. Folk and Ward, 1957; Friedman, 1979; Bui et al., 1990; and Blott and Pye, 2001).

In its broadest scope, provenance analysis includes all inquiry that would aid in reconstructing the lithospheric history of the Earth (Basu, 2003). In sedimentary petrology, the term provenance has been used to encompass all factors related to the production of sediment, with specific reference to the composition of the parent rocks as well as the physiography and climate of the source area from which sediment is derived.

The intent of sedimentary provenance studies is to reconstruct and to interpret the history of sediment from the initial erosion of parent rocks to the final burial of their detritus. The ultimate goal of provenance studies is to deduce the characteristics of source areas from measurements of compositional and textural properties of sediments, supplemented by information from other lines of evidence (Pettijohn et al., 1987).

The major river in the study area is River Ero with River Adogo as its major tributary which is an ephemeral river which dries up during the dry season. The River Ero empties into the river Niger, and the drainage pattern in the area may be said to be dendritic, it has an average channel width of 87m.The direction of flow of drainage in the NW-SE direction. This study therefore attempts to document the similarities between the sedimentological and petrgraphical properties of the sediment samples of Rivers Ero and Adogo.

1.2  Geographical Location and Accessibility

The study area is one of the major town in Ajaokuta Local Government in Kogi State and lies within latitude 07o 34’ 47.4’’ N and longitude 08o 37’ 37.9”E respectively. The area is generally accessible and motor able, the availability of both tarred road and footpaths provide good accessibility to the area and to Riverbed.

Ajaokuta local government area was created from Okene Local Government on 27th august, 1991 and has its headquarters at Agodo. The Local Government is in the central senatorial district of the state and covers a landmass if 1362 square kilometers.

Ajaokuta Local Government Area is bounded to the North East by Lokoja Local Government, Bassa Local Government to the Northwest, Ofu Local Government to the East and South West by Okene and Adavi Local Government Areas respectively. Ajaokuta Local Government has its Headquarters in the town of Egayin in the South of the area at 6o 40’ 11” N,8o48’ 19” E.

The area under study is accessible through roads and foot paths. Majority of the rivers in the study dried up due to lack of rainfall, so we had ample time and opportunity to move through the river bed which would have been difficult if they were not dried up.

SEDIMENTOLOGICAL STUDY OF BOTTUM SEDIMENTS: A CASE STUDY OF RIVERS ERO AND ADOGO AJAOKUTA KOGI STATE, NIGERIA