CHAPTER ONE: 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.
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.
Fig 1.1: Map of study area showing location and accessibility
Inset: Geologic map of Nigeria after (Kogbe, 1976).
The aim of this study is to carry out a comparative sedimentological and petrographic investigation of the stream sediments of Rivers Ero and Adogo.
The project work was done in three stages. The first stage entails detailed geologic mapping of the study area. Fourteen soil samples were obtained at an interval of 200m from the river channels at different locations. And samples were collected from depth of about 20-25cm.
The second stage includes the preparation of samples for laboratory analysis; grains size analysis and petrographic studies. The final stage involves the interpretation and collation of results, and writing up of project.
The study area is characterized by linear settlement with sparse population and is occupied by the Ebira people. The rural areas include Ero, Eroko and Geregu, these are located along the river bank of river Niger. Some of the village and towns in the study area are far apart and linked by footpaths and rivers. The land in the study area is mainly used for agricultural purposes, farming and fishing along River Ero being the major occupation in the area. (Fig.2)
Fig. 1.2: Type of land use in the study area
The climatic condition of the study area fall within the tropical climate region where wet and dry season are prevalent. Average annual rainfall is between 1100mm - 1300mm with temperature as high as 36.7˚C.
The study area occurs within the rainforest belt with ever green vegetation which is characterized by Guinea Savannah type Vegetation and the study area comprises occasionally of stunted trees. (Fig. 1.3)
Fig. 1.3: Guinea savannah type of vegetation in the study area
The study area is characterized by hills ridges of basement complex rocks. The drainage system over the areas of the basement complex is usually marked with the proliferation of many small stream channels. The channels of these smaller streams are dry for many months especially from November to May. The major river in study is river Niger. Others include river Ero and river Adogo, which drains into the river Niger. The drainage pattern of the study is indicated as dentritic, it develops in region underlain by homogenous materials where the subsurface geology have similar resistance to weathering such that there is no apparent control on the flow direction of the tributaries, which later join larger streams or rivers at acute angle.
The significance of grain size parameters of detrital sediments has been well established by Inman (1952), Folk (1966), and Friedman (1961, 1967). From the 1960’s to the 1980’s, a large number of papers on statistical parameters of grain size distributions of recent sedimentary environments, and the distinction between beach, dune and river sands were published using graphic calculation method (Folk and Ward, 1957),or calculated moment measures (Friedman, 1961). More than one hundred sedimentological papers support the relation between grain size parameters and sedimentary environment (e.g. Inman, 1952; Friedman, 1961; Warren, 1974; Martins, 2003).The importance of the grain size as a fundamental property of sedimentary materials, regarding their origin and history, was emphasized by McCave and Syvitski (1991).
Martin (2003) agreed that a large part of information related to sedimentary particles transport and deposition can be obtained from grain-size. Cheetham et.al. (2008) carried out a comparison of grain-size analysis methods for sand-dominated fluvial sediments, he concluded that, the data for sand dominated fluvial sediments gained from the long-established sieve and hydrometer method can be compared with confidence to those obtained by modern studies using laser diffraction techniques.
Furthermore, many attempts have been carried out to differentiate the sediments from widely varying environments by using the statistical parameters and their relationships. Most of these attempts were based on bivariate plots of grain size parameters (Mason and Folk, 1958; Friedman 1961, 1967; Moiola and Weiser, 1968), linear discriminant functions (Sahu, 1964), shape of size frequency curve (Sindowski, 1957; and Visher, 1969) and triangular diagrams (Asseez, 1972). Most of the bivariate plots are designed to differentiate among beach, fluvial and various aeolian sediments. The applied type of bivariate plots depends on the size parameters suggested by Folk and Ward (1957) Moshrif, (1980); and Taj (2003) evaluated the grain size parameters in distinguishing ancient sedimentary environments. The differences in grain size parameters of the sediments generally reflect the transport, erosional and depositional processes that operate in the specific area (Veerayya and Varadachari, 1975). In addition, it may reflect change in energy or water depth in the depositional environments (Amaral and Pryor, 1977).
Petrographic analysis of major framework constituents would not have been possible without the invention of thin-section petrography by H.C. Sorby (1880) who also carried out the first detailed investigation of quartz varieties. Judd (1886) first recognized the influence of climate on the preservation of feldspars. These early studies were expanded by Mackie (1899), who established criteria for the recognition of quartz derived from igneous and metamorphic rocks and the use of feldspars as indicators of contemporaneous climate. Studies by Cayeux (1906, 1929), Goldman (1915), Gilligan (1919) and Dake (1921) illustrate that descriptive thin-section sandstone petrography was already well established at the beginning of the 20th century.
P.D. Krynine and F.J. Pettijohn proposed the first versions of the sandstone classification schemes that are still being used (Klein, 1963; Okada, 1971). Dickinson and Suczek (1979) and Dickinson and Valloni (1980) demonstrated that modal composition of sands is largely controlled by plate tectonics, confirming earlier suggestions of Crook (1974) and Schwab (1975). The appeal of this seemingly straightforward method to infer ancient plate-tectonic settings led to an enormous interest in the study of sandstone framework mineralogy and the rapid buildup of a petrographic database (Breyer, 1983; Valloni, 1985; Dickinson, 1985, 1988)