1.1 General Introduction
Sedimentology is the scientific study of sediments and sedimentary rocks. It involves studying the processes that result in their formation (erosion and weathering), transport, deposition and diagenesis (Gary Nichols, 1999). Sedimentologists apply their understanding of modern processes to interpret geologic history through observation of sedimentary rocks and sedimentary structures. The aim of sedimentolgy is to derive information on the depositional conditions which acted to deposit the rock unit, and the relation of the individual rock units in a basin into a coherent understanding of the evolution of the sedimentary sequences and basins, and thus the Earth’s geologic history as a whole. Sedimentary rocks are crucial part of sedimentary basins whereby they are either consolidated or unconsolidated. Detrital rocks are formed by the sedimentation of minerals and rock fragments that were obtained from the mechanical breakdown and chemical decomposition of pre-existing rocks and transported to a place of deposition from the source area. These sediments later form sedimentary rocks such as siltstone, sandstone, shale or conglomerate after undergoing lithification. Although, most sedimentary rocks bear evidence of their depositional environment, transporting media and original mineralogical composition, such evidences can be reflected by the textural characteristics, sedimentary structures and mineralogical composition.
1.2 Aim and Objectives
This work involves the sedimentological studies of the Maastrichtian sedimentary successions exposed around Okpekpe and Imiegba South Western Anambra Basin, Nigeria.
The Objectives include:
- Detailed geological field mapping of the study area.
- Representative samples were obtained for laboratory analysis such as grain size analysis, thin section petrography and heavy mineral separation results.
- In order to determine the textural characteristics, depositional setting and source area tectonics of the study area.
1.3 Geographical Location and Accessibility
The study area is demarcated by Latitudes 0070 111 N to 0070 131N 3511 and longitudes 0060 251 E to 0060 261 E 5211, which is the South-Western part of the Anambra Basin, the contact between Mamu and Ajali Formation.(Fig 1.1)
Accessibility is through major road and footpath which cuts across Apana, Okpekpe and Imiegba and serves as routes to the exposed outcrops during the mapping exercise.
Fig. 1.1: Location Map of the Study Area
1.4 Settlement and Land Use
Linear and Nucleated settlement pattern were found in the study location and the major occupation of the inhabitants is farming.
The lands in the study areas are used for agricultural purposes.
1.5 Relief and drainage
The outcrops studied have a hilly and undulated topography. The study area is well drained by ephemeral streams and rivers.
1.6 Climate and Vegetation
The two main seasons that dominates the climate in the studied area are the rainy season and the dry season.
Vegetation is part of the rain forest vegetation in Nigeria and it’s typical of a transition belt between the forest and Savannah (grassland) belt.
1.7 Review of Previous Work
Several studies by different geologists have been carried out on the Mamu Formation, this includes the stratigraphic and biostratigraphic studies of the Mamu Formation (Petters, 1987, Ladipo 1988, Adeniran 1991).
Friedman (1961) used grain texture as a paleoenvironmental tool and sedimentary structures such as cross-bedding to establish paleocurrent direction. Sedimentological studies of sandstone unit in the basin have been carried out by some geologist such as Murat (1972), Nwajide (1996) and Reyment (1977) because the Anambra Basin has attracted more attention within the last decades.
Reyment (1964) described the Mamu Formation as a regressive deltaic offlap sequence. Olufemi (1985) studied the subsurface geology of the Mamu Formation in Southwestern section of the Anambra basin, through standard exploitation criteria. Reyment (1965), Hoque and Ezepue (1977) have suggested continental and fluvio-deltaic settings as a regressive phase of a short lived Maastrichtian transgression. Ladipo (1986) argued a tidally influence regime in a shelf or shore-line environment. Amajor (1984), Agagu et al; (1985): Umeji and Nwajide 2007 mainly concentrated on the sedimentology and stratigraphy of the formation at the depo-center along Onitsha-Nsukka axis.. Previous work on the Ajali sandstone by Reyment (1965) indicates that the Ajali sandstone is thickest at the Udi plateau area where it attains over 300m and extends continuously in thin outcrops to the South-East of Okigwe.