(Above image is 500x375. Original size is 640x480)
In the next leg of our journey, we shall travel through the soft landscape extending between the Subis and Lambir mountains. The plains and low-rolling hills are the site of extensive oil palm plantations. The regularity of this morphology reflects an underlying geological uniformity.
The inland area is characterised by the widespread extend of the mudstones of the Setap Formation, as can be observed in numerous outcrops along the main road. The apparent monotony of this mudstone morphology can be deceiving. At Ngebol, the oil palm plantations are pierced by a series of active mud volcanoes, which create a surrealistic landscape. The popping sounds of the exploding gas bubbles add to the magic of the site.
To the north, the younger sandstones of the Lambir and Miri Formations give the coastal area its first relief, in the form of spectacular cliffs. At Tanjong Batu, the 30m high cliffs are cut abruptly by horizontal terraces along a striking angular unconformity. At Auban, marine mudstones yield an extraordinarily well preserved fauna of fossil molluscs and crabs, illustrated there by a comprehensive collection of pictures.
Kampong Tengah
This outcrop exposes two contrasting stratigraphic units: the lower part consists of blue fossiliferous marlstones while the overlying unit consists of grey silty mudstones, with rare fossil shells. The sharp bounding surface is a candidate maximum flooding surface, separating inner-middle neritic shelf muds from downlapping prodelta muds.
Tanjong Batu
The cliffs at Tanjung Batu provide a spectacular exposure of the Middle Miocene clastics of the Lambir Formation. Coastal, shallow marine sandstones and mudstones are overlain by some 16m of stacked estuarine sandstones, which form the main body of the cliff. The top of the escarpment is itself sharply truncated, with horizontal-laying terrace deposits resting on the folded Miocene strata. The estuarine sandstones are interpreted as the back-filling of an incised valley and their base is a sequence boundary.
Auban
The coastal exposures north of Tanjung Batu provide a series of continuous outcrops through the Lambir / Miri Formations. At Auban, these shallow marine sand-shale intercalations include a number of marine flooding surfaces. Here, the associated neritic muds are particularly fossiliferous: the high abundance and diversity of the mollucs is made more remarkable by the exceptional state of preservation, as shown in the accompanying pictures.
Mud Volcanoes of Ngebol
The mud volcanoes are located within the oil palm plantations, on the road to Bintulu, near Bekenu Junction. Local people call the place "Ngebol", meaning "seepage". The larger area is called "Setap", a name that has been used to characterise the mudstone formation in the area.
Mud volcanoes consist of mud mounds with crater-like features produced by erupting gases; they are not related to volcanic activity. The popping sounds of the exploding gas bubbles is a characteristic feature of active mud volcanoes. The craters are usually water-filled and frequented by birds. Local people often carry out their bird-hunting activities next to the mud volcanoes. Mud volcanoes are tiny pressure valves at the surface of the earth. They are normally situated above the crest of geological structures (anticlines or diapirs) or zones of structural disturbance. There, the deep-seated, overpressured soft mudstones are usually enriched in water and associated with gases. The gases under pressure make their way to the surface through areas of least resistance. During their rise, the gases mix with the muds and create a slurry. Close to the surface, the gases become more buoyant, rising more rapidly. They expand and erupt at surface in the form of bubbles within the liquefied mud.
The constant upward flow of the slurry and subsequent hardening of the muds creates the mounded nature of the mud volcanoes. These features are transient, being constantly re-shaped by new bubbles and new mud flows. When they grow too high above the ground, they are often abandoned, the vents migrating to areas of less resistance, away from the mound. The viscosity of the mud determines the shape of the mound; in Ngebol, the water-rich muds create low and broad craters, reaching a maximum of one meter in height. The origin of the gases is mostly related to hydrocarbon generation in the subsurface, as evidenced by the frequent oil films seen around the mud volcanoes. The gases consist usually of methane, which can be lit up by a flame.
Mud volcanoes are common in many areas of Northern Sarawak, Brunei and Sabah where the Setap Shales are outcropping. In the Jerudong Limbang area, shallow coreholes have identified various layers of liquid mud of 30-140ft thickness, coinciding with a faulted zone. In the Klias Peninsula, the higher subsurface pressures and the generally higher viscosity of the mudstones have resulted in a more violent type of mud volcanism. There, the manifestations consist of huge explosions often lasting many days, and associated with ignited gas flares. Such big volumes of rocks are expelled during these cycles of activity that whole islands are known to have been created in this way.
