Structural geology of the Neogene Maliau Basin, Sabah

The Maliau Basin (Maliau outlier) is made up of about 7,500 metres thick sandstone and mudstone layers deposited in a deltaic-coastal environment, assigned the Kapilit Formation. The layers at the base of the basin consist mainly of mudstones reaching up to 2,000 metres thick. Near the rim of the basin, thick sandstone layers and coal seams occur. Towards the centre of the basin a series of sandstone-dominated and mudstone-dominated sequences of various thicknesses occur. The deposition took slight unconformity place during the Middle Miocene (10-15 million years ago). The basin sits with slight unconformity on older sedimentary rocks (Tanjong Formation), also comprising of thick layers of sandstone and mudstone. The orientation of bedding generally follows the semi-circular shape of the basin. The dip of bedding varies from 5-10 degrees at the centre to 45-50 degrees at the rim. Subvertical to vertical fractures shows four main fracture orientations, NW-SE, NE-SW, NNW-SSE and WNW-ESE. Faulting is quite rare inside the basin. However, outside the basin, minor normal faults occur trending E-W and NE-SW. A sheared zone occurs at the southeastern part of the basin, possibly due to a major fault, the Lonod Fault. Based on regional and local structures the Maliau Basin is interpreted to have developed initially in an extensional regime, whereby an enormous amount of sediments were deposited in a subsiding basin and later subjected to compression (inversion)