ASPECT OF SEDIMENTOLOGICAL EVALUATION OF EXPOSURES OF EASTERN DAHOMEY BASIN, AT IJESHA- IJEBU, SOUTHWESTERN NIGERIA

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TABLE OF CONTENTS

PAGES Table of contents I
Certification IV
Dedication V
Acknowlegements VI
Abstract VII
List of figures VIII
List of tables IX
Chapter one 1
1.0 INTRODUCTION 1
1.1 CLASSIFICATION OF SEDIMENTARY ROCKS 5
1.2 AIMS AND OBJECTIVES OF STUDY 6
1.3 SCOPE OF INVESTIGATION 6
1.4 LOCATION AND ACCESSIBILITY 7
1.5 DRAINAGE 10
1.6 CLIMATE AND VEGETATION 11
1.7 LITERATURE REVIEW 12
Chapter two 13
Regional geological setting and evolution 13
2.0 DAHOMEY BASIN 13
2.1 STRATIGRAPHY AND TECTONIC SETTING OF DAHOMEY BASIN 15
2.1.1 Abeokuta Group 15
2.1.1.1 Ise Formation 16
2.1.1.2 Afowo Formation 16
2.1.1.3 Araromi Formation 16
2.1.2 Imo Group 17
2.1.2.1 Ewekoro Formation 17
2..2.2 Akinbo Formation 17
2.1.3 Oshosun Formation 18
2.1.4 Ilaro Formation 18
2.1.5 The Coastal Plain Sand 18
Chapter three 20
3.0 METHODOLOGY 20
3.1 FIELD INVESTIGATION 20
3.2 LABORATORY TECHNIQUES 22
3.2.1 grain size analysis 22
3.2.1.1 mechanical sieving 22
3.2.1.2 hydrometer test 23
3.2.2 petrographic analysis 26
Chapter four 27
Results and discussion 27
4.0 LITHOFACIES DESCRIPTION AND INTERPRETATION 27
4.0.1 lithofacies of location a 27
4.0.2 lithofacies of location b 30
4.0.3 lithofacies of location c 34
4.1 GRAIN SIZE ANALYSIS 36
4.1 PETROGRAPHY 39
Chapter five 42
Conclusion and recommendations 42
5.0 CONCLUSION 42
5.1 RECOMMENDATION 42
References 43
Appendix 45

ABSTRACT

The study area, Ijesha-Ijebu falls within the Nigeria sector of the Eastern Dahomey Basin. It lies within the contact zone between the south-western basement rocks and the Eastern Dahomey Basin. The principal objective of this project work is to deduce the depositional environment and identify the formations in the study area.

The field and laboratory investigation were carried out during this project work. Nineteen (19) rock samples collected were described for its: grain texture, color, and orientation of grains and identification of the mineralogical composition were carried out. Measurement of strike and dips of cross-beddings, measurement of each bed along the profile, taking of photographs of important sedimentary structures such as burrows, dissolution, and unconformity and logging of the exposure were done on the field. The laboratory investigation involves grain size analysis and petrographic analysis.

The lithofacies recognized are: Clay Unit, Sandstone Unit, and Conglomerate Unit. On the basis of the integrated studies of the grain texture and palaeocurrent direction, it was concluded that the exposure does not have a defined sequence. The result of the studies also suggests that the exposure possesses a fining upward sequence and a fining downward sequence.

The result of the grain size distribution shows that the sediments ranges from Unimodal to Trimodal, poorly sorted to moderately sorted. The textural group ranges from gravelly sand to sandy gravel. The result of the petrographic analysis indicates that the sandstone have high quartz content; the size of the grains ranges from angular to sub- angular. This suggests that the sediments were deposited from a short distance and the environment of deposition is mainly Continental (Fluvial).

LIST OF FIGURES

Figures                                                                                                                                     Pages

1: Topographic map of the study area showing the sample location……………………………………….. 8

2: Accessibility map of the study area………………………………………………………………………………… 9

3: Drainage map of the study area……………………………………………………………………………………. 10

4: Geologic map of eastern Dahomey Basin……………………………………………………………………… 14

5: East-West Geological Section showing the Dahomey Basin and upper part of the

Niger-Delta…………………………………………………………………………………………………………………… 15

6: Collection of samples across the exposure…………………………………………………………………….. 21

7: Grain size analysis at the Engineering laboratory Crawford University, Ogun state……………. 25

8: Lithology log of Profile A…………………………………………………………………………………………… 28

9: Vertical log section of Profile B…………………………………………………………………………………… 31

10: vertical section of Location B showing the sampling points…………………………………………… 32

11: Crossbedding…………………………………………………………………………………………………………… 33

12: Bioturbations…………………………………………………………………………………………………………… 33

13: section of profile C showing the Erosional Gully…………………………………………………………. 34

14: Lithology of Profile C………………………………………………………………………………………………. 35

15: Textural Characteristics of the sediments……………………………………………………………………. 37

16: Plot of Cummulative mass retained against the Particle diameter…………………………………… 38

17: Photomicrograph of sandstone (Sample B3) showing Quartz (Q) and the Matrix

(M)………………………………………………………………………………………………………………………………. 40

18: Photomicrograph of Conglomerate (Sample B7a) showing Quartz (Q)…………………………… 40

19: Photomicrographs  of  (a):  Plane  polarized  light  (b):  crossed  polarized  light  of Sandstone (Sample A1) showing Quartz (Q)…………………………………………………………………………………….. 41

LIST OF TABLES

Tables                                                                                                                                      Pages

Table 1: Classification of sedimentary structures of sedimentary rocks………………………………….. 3

Table 2: Stratigraphic succession in the Dahomey Basin…………………………………………………….. 19

Table 3: Coordinates of Study Profile………………………………………………………………………………. 22

Table 4: Textural classification of the sediments……………………………………………………………….. 36 Table 5: Average Modal Analysis of the sediment samples        

ASPECT OF SEDIMENTOLOGICAL EVALUATION OF EXPOSURES OF EASTERN DAHOMEY BASIN, AT IJESHA- IJEBU, SOUTHWESTERN NIGERIA