The sedimentology and tectonics of the Temburong Formation - deformation of early Cenozoic deltaic sequences in NW Borneo

Abstract

The sedimentology and deformation of the Temburong Formation in East Brunei are described. Original sedimentary structures are still preserved, particularly in the arenaceous sediments and indicators suggest deposition mostly in a shallow water, subsaline embayment or lower alluvial floodplain environment. The style of deformation varies according to lithology and bed thickness and the rocks are metamorphosed to sericite grade. Deformation is thought to have been caused by loading rather than tectonic compression. The timing of the deformation took place probably between the Te l-4 and Te 6 Indonesian Letter Stages, i.e. upper Oligocene to Lower Miocene. No basal conglomerate has been found between the Temburong Formation and the overlying Setap Shale Formation and there is unlikely to be an unconformity separating the two formations although a hiatus between Te 1 _ 4 and Te s has been recognised in major platform carbonate sequences in the Melinau Limestone Formation at Melinau, Batu Gading and Keramit as well as the turbiditic limestone at Selidong. The regional distribution of the Temburong Formation is discussed in relation to the paleogeography of NW Borneo. PREVIOUS WORK The Temburong area was investigated by oil company geologists who recognised sub-metamorphic features in the rocks and apart from an age determination, did little further work, concluding that the rocks were unlikely to contain hydrocarbons. Wilhelm (1926) and Morgan (1932) made traverses of the Temburong river to the top of Bukit Pagon and compiled a regional geological map dividing the area into three tectonic subdivisions comprising, from N to S, the Labu syncline, the Temburong anticline and the Ulu Temburong isoclinal area. A later revision of the stratigraphy of Brunei using aerial photographs and data from Jordi & Wijkhuizen (1955) and Ditzel (1956) was published in Liechti, et ai. (1960) and Wilford (1961). The Temburong area was subdivided into three Formations, the coal-bearing Belait Formation (Middle-Upper Miocene) in the north, the dominantly clayey, highly folded Setap Shale Formation occupying the middle reaches of the Temburong river and the Meligan Formation forming a synformal sequence of arenaceous sediments lying unconformably above the Setap Shale Formation and with an erosional upper boundary. Planktonic foraminifera from the Belalong confluence (Ditzel, 1956) indicated a Lower Miocene age for the Setap Shale. The Meligan Formation was presumed to be Upper Miocene.