Temburong Gravel; Geology and Composition

The Temburong gravel consists mainly of Meligan sandstone (Early Miocene age) which outcrops in the headwaters of the Temburong river between Bukit Tudal and Bukit Lesong. The parent rock is generally a grey, fine-grained sandstone but some coarser sandstones and occasional interbedded conglomerates also occur. The gravel was transported downstream during Pleistocene times and deposited in terraces over a relatively flat river valley floor between Bukit Teluggong and the Pandaruan river in Limbang, Sarawak (Figure 7.5). These terraces contain the bulk of the gravel reserves in the Temburong district. Smaller deposits are found in the flood plain of the Temburong river and in the channel of this river upstream of Kampong Biang.

The main gravel terraces are located to the west of the Temburong river, except near Kampong Batang Duri and Kampong Biang where two small terraces are located to the east. There are three well-defined terraces. Terrace I, at approximately 76 metres above MSL, is the oldest and has an average gravel thickness of 1.8 metres. The youngest terrace III, at 12 metres above MSL, is the most laterally extensive with gravel thickness of 1.5 to 3.8 metres. Scattered remnants of middle terrace II, at 61 metres above MSL, are located to the east of the high terrace. These deposits are small and not commercially attractive.

The terraces have a flat top and relatively steep sides. The top surface is generally level but often dips at a gentle angle to the east. Gravel is invariably exposed on the sides of the terrace. The overburden thickness varies from 0.2 to 2.4 metres and consists of white sand overlain by sandy clay soils. The soils are poor for agriculture and only support a thin primary forest. The gravel lies directly on undulating Setap Shale 'bedrock'. Post-depositional weathering is seen on all the terraces, the higher one to a greater degree than the lower one.

The size of components of the gravel ranges from a few centimetres to boulders with a maximum diameter of 48 centimetres. Approximately 40% of the gravel is between 10 and 18 centimetres in diameter. The proportion of boulders decreases away from the source, with a transition from cobble to pebble gravel (Wentworth scale) towards the north.

The gravel is grey in colour and usually homogenous. The shape of the gravel is largely determined by the joints and bedding planes of the parent material (Meligan sandstone). Bedding traces, if present, are usually parallel to the longest axis of the pebble. Boulders are less symmetrical than the cobbles and pebbles. The composition of boulders, cobbles and pebbles is a fine to medium-grained sandstone. White vein quartz is present in the pebble fraction but is rarely seen in the cobble fraction.

The flood plain of the Temburong river contains relatively thin, discontinuous beds of gravel. They are channel deposits within the river alluvium. Pits and exposures in the river banks show a thicknesses range from 1 to 3 metres. These deposits are composed of much smaller pebbles than the terrace gravel and they are interbedded with clay, detrital shale and peat. The gravel beds in the alluvium have a lensoid shape with a limited lateral extent.

Gravel is found in the river bed of Temburong river upstream from Kampong Biang. This river bed gravel consist of moderately hard boulders and averages 2.4 metres thick although some of the gravel lenses are as much as 6 metres thick.

Tests on the engineering characteristics of the gravels show that they are suitable for most construction (concrete) and civil engineering works. The specific gravity varies between 2.3 and 2.5 and this reflects the mineralogical composition of mostly quartz. The carbonate, sulphate and organic contents are low, 0.01, 0.4 and 1.2% respectively.

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