Bulletin of the Geological Society of Malaysia, Volume 36, Dec. 1994, pp. 1-9
1Petroleum Research Institute, PETRONAS, Ulu Kelang, Selangor D.E.
2Geological Survey Department, Kuching, Sarawak
Abstract: The Lupar Line is regarded by many geologists as a major suture which has resulted from plate movements that largely determined the Cretaceous to Paleogene history of northern Borneo. Nevertheless, there remain many uncertainties regarding the relationship of the various belts and rock types that have been distinguished. Most of the many attempts to reconstruct the history of Borneo and the South China Sea involve the incorporation of a model of the nature and timing of events along the Lupar Line. Thus new factual observations in this area are of regional significance.
The gabbro and pillow lavas within the Lupar Formation have been interpreted as faulted-in slices of oceanic crust, and by implication as older than the Lupar. However, an examination of quarries and excavations made in the areas of dams and quarries constructed for the Batang Ai Hydroelectric Project has shown that at least some of the gabbro within the bedded flysch typical of the Lupar Formation is intrusive, and pillow lava interbedded, contrary to the view that they represent older oceanic crust emplaced tectonically as faulted slices within the Lupar. The evidence for this is:
a) Unfaulted contacts of concordant sills of gabbro within the formation, which is thermally metamorphosed for at least 50 m at the contact.
b) Pillow lavas in contact and concordant with the Lupar Formation along an unfaulted contact. At one place both the base and top of a small flow is exposed.
This implies that the lavas are contemporaneous with the Upper Cretaceous Lupar Formation, and at least some of the gabbro is probably also of the same age, although it could be younger, if unrelated to the lavas.
Consideration of this evidence and the results of recent mapping in adjacent parts of Kalimantan to the south, suggests that:
i) the gabbro and pillow lava are not oceanic crust, but intrusive into and extrusive within the Lupar Formation;
ii) the junctions between the Lubok Antu Mélange and the Lupar Formation, and that between the Lupar and Layar Formation, may be major sutures, whereas the Lupar Valley itself may only be a fault zone within a broad mélange belt that extends south beneath the northern rim of the Ketungau Syncline.