The ‘Rajang accretionary prism’ and ‘Lupar Line’ problem of Borneo
Authors: Hutchison Charles S. 
Published: April 2008

The Rajang Group, composed of the Belaga and Lupar Formations of Sarawak and the Embaluh Group and Selangkai Formation (in part) in Kalimantan, had a turbidite sedimentation history from Early Cretaceous to Late Eocene. These rocks generally young northwards. Inliers within the eastern Miri Zone have been mapped as the Kelalan and Mulu Formations. The Rajang Group was compressed into a steeply dipping quartz-veined phyllite-quartzite complex by the Sarawak orogeny and unconformably overlain locally in the north by the Upper Eocene continental to neritic Tatau Formation, extensively in the south by Middle to Upper Eocene basal continental sequences of the Ketungau, Mandai and Melawi basins, and widely in the north by the Upper Oligocene coastal to marine Nyalau Formation. In Sabah and East Kalimantan, the Upper Cretaceous to Upper Eocene Mentarang, Sapulut, Trusmadi, and possibly the East Crocker Formations, also belong to the Rajang Group. The West Crocker Formation demonstrates rapid facies changes into the more shaly Temburong Formation, and was deposited as sandy turbidites throughout the Oligocene. To the south their equivalents are the nearshore Kelabit and Long Bawang Formations. The West Crocker Formation was folded and uplifted in several Miocene pulses, resulting in regional unconformities and igneous events at Mount Kinabalu. The West Crocker Formation has not been metamorphosed, and dips are shallower than in the Belaga and Trusmadi Formations. Its provenance probably was from the uplifted Upper Cretaceous to Eocene Lurah and Kelalan formations of NE Kalimantan and East Sarawak. It is therefore proposed that the West Crocker and Temburong formations be excluded from the Rajang Group. Middle to Late Miocene Crocker Formation uplift and deformation, herein called the Sabah orogeny, was synchronous with spectacular basin inversion throughout the South China Sea and in the Meratus Mountains of Kalimantan. Uplift ceased in the Late Miocene and undeformed post-inversion formations unconformably overlie inverted folded structures.

The Rajang Group flysch-belt may be interpreted as a north-facing accretionary prism. The Schwaner Mountains represent a subduction-related Lower to Upper Cretaceous volcanoplutonic arc. Scattered Eocene volcanism, Miocene Sintang intrusives and Pliocene Metalung volcanics in Central Kalimantan and Sarawak post-date subduction. The Rajang Basin (Danau Sea) rapidly narrowed and by Eocene time subduction was transformed to collision as the Rajang Group was compressed between the Schwaner Mountain Zone and the Luconia-Balingian-Miri block. The Ketungau Basin is in sharp contact with the Rajang Group along the bounding Lupar Fault, which can be traced northwards into the East Natuna region. Palaeocurrents show that the Upper Eocene basal sandstones have a provenance in the metamorphosed Sibu Zone. The Melawi and Mandai basins of Kalimantan also unconformably overlie the flysch-belt. The basins are not forearc and were formed after transformation of the accretionary prism to a landmass formed of a collisional orogenic complex.