How was the Maliau Basin formed?
(Information was adapted from Technical Assistance Report No 33 by Dr. Felix Tongkol for the Management of Maliau Basin Conservation Area Project)
Evolution (How the Basin Was Formed)
The evolution of the Maliau Basin, together with the other Neogene sedimentary basins was structurally controlled. Deep-seated structures such as faults found within the pre-Neogene rocks are directly or indirectly manifested in the overlying Neogene sediments. The topography and structural trends of the underlying rocks controlled the initial development of the Neogene basins. However, subsequent extension and compression in the region controlled the shape of the basins as we see today. It is envisaged that the early development of the Maliau Basin was closely related to the deposition and deformation of the older underlying sedimentary rocks.
The evolution of the Maliau Basin is traced back from the early Miocene epoch, about 22 million years ago. After the Pliocene epoch, about 5 million years ago, the architecture of the Maliau Basin has not changed, except for the effect of weathering and erosion on its surface. During the early Miocene most of Sabah as we know today was still submerged under the sea. The land mass during this time mostly occur in western Sabah (where the Crocker Range and Trusmadi Range are today) and several islands in east Sabah (where the basement rocks near Lahad Datu occur today). The Pre-Neogene rocks forms a fold-thrust belt trending NE-SW in western Sabah and swinging E-W in northern and eastern Sabah. The fold-thrust belt resulted from compression of the pre-Neogene rocks as the Australia-Pacific Plates collided with the southeastern margin of the Eurasia Plate. The coastline and sedimentary basins possibly follows the regional structural trend of the pre-Neogene rocks. The uplifted pre-Neogene rocks provided sediments for the newly formed basins in western and eastern Sabah.
After the development of the fold-thrust belt, the eastern part of Sabah began to subside and formed a huge elongate basin trending NE-SW where older Neogene sediments began to accumulate. The subsidence of the basin, accompanied by regional normal faults, was probably related to the ongoing NW-SE extension in this region due to the opening of the Sulu Sea Basin located further north. The major NW-SE horizontal fault south of Sabah possibly forms the southeastern boundary of the extension. As the region continued to subside more and more sediments accumulated. In certain unstable areas, such as slopes, large submarine slumps and slides occurred producing the Neogene melange deposits. The relative motion of horizontal faults and normal faults may have initiated the segmentation of the huge elongate basin into several smaller basins.
By early Middle Miocene, between 14-15 million years ago, the eastern part of Sabah was again subjected to NW -SE compression as tectonic plate interaction continued in this region. Active volcanism in eastern Sabah during this time was probably related to this tectonic force. The compression resulted in the gentle folding of the older Neogene and concentric shape of the smaller basins, with the faults acting as their boundaries. At this time, most of eastern Sabah was still under the sea, mostly very shallow waters. The concentric-shaped basins were subsequently filled by younger Neogene sediments continuously until about 9 million years ago when the basins was uplifted above sea level due to continued compression in eastern Sabah. The compression resulted in the gentle folding of the sedimentary layers and at the same time accentuated the concentric shape of the basins, through the reactivation of old faults. The intrusion of Mt. Kinabalu in West Sabah occurred during this time. As a result of the crustal shortening of the region due to the continued tectonic plate collision in this region, the whole of Sabah, including the shallow coastal area we see today, was fully uplifted above sea level by the end of Miocene time, about 5 million years ago. The Maliau Basin and the surrounding areas were probably uplifted to its present height with a slight tilt to the Southeast. Following the uplift, the Maliau Basin and surrounding areas were subjected to intense weathering and erosion that continued up to this day.