Abstract

Structural analysis of joints and fault data obtained from exposures of the Enugu and Mamu formations formed the basis for establishing the paleostress direction and stress field rotation within the study area. Four sets of joints were dominant in the study area. Analysis of two conjugate sets: J1 and J2 separated by a dihedral angle of 40° shows that the maximum principal stress s₁ at the time of formation of these joints was oriented 128° azimuth and 16° plunge. The intermediate stress s₂ was oriented 262° azimuth and 70° plunge while the minimum stress s₃ was oriented 35° azimuth and 16° plunge. The joint sets gave a NW-SE orientation of the maximum principal stress. The joint spacing ranges from 10 - 100 cm in the Enugu Shale to about 300 cm at the top unit of Mamu formation. The abrupt termination of the siltstone/fine sandstone unit against shale within the Enugu shale is an evidence of the deformation of the unit by several normal faults. Two faults F1 and F2 form a conjugate fault pattern. F1 has a listric fault plane dipping NE, a growth index of 1.35 and is flanked by a small-scale rollover structure. F2 is oriented 315° azimuth parallel to one of the joint sets and dipping in the NW direction. The paleostress direction deduced from these faults suggest that the maximum stress (s₁) orientation was 275° and plunge 5°; the intermediate stress (s₂) was orientated 10° azimuth and 40° plunge while the minimum stress (s₃) orientation was 179° azimuth and 52° plunge. This shows a clockwise rotation of the stress fields that formed the joints and the faults respectively. Extrapolation of the results of the orientation analysis of these fractures into the subsurface will enhance the understanding of permeable zone, fluid migration pattern and therefore increase the success rate in the exploration and exploitation of groundwater and even in the management of the mine drainage problems in the Enugu coalfields.

Key words: Kinematics, paleostress direction, principal stress, dihedral angle, faults, joints.