The eastern Sundaland margin in the latest Cretaceous to Late Eocene: Sediment provenance and depositional setting of the Kuching and Sibu Zones of Borneo

Abstract

The Kuching Zone in Borneo comprises several large sedimentary basins of Late Cretaceous to Late Eocene age. In West Sarawak the Kayan Basin includes the Upper Cretaceous to Lower Eocene Kayan Group and further east in the Kuching Zone is the Ketungau Basin, consisting of the Middle to Upper Eocene Ketungau Group.The Kayan Group is composed of the Kayan Sandstone and the Penrissen Sandstone. U-Pb detrital zircon ages and heavy minerals from the Kayan Sandstone suggest two major drainage systems: 1) Late Cretaceous to Paleocene rivers supplied sediment with abundant Cretaceous, Permian-Triassic and Precambrian zircons primarily from SW Borneo and East Malaya–Indochina and 2) Paleocene to Early Eocene rivers provided sediment containing almost entirely Cretaceous zircons from the Schwaner granites of SW Borneo. Differences in heavy minerals and zircon ages of the Lower Eocene Penrissen Sandstone support interpretations of an unconformity above Kayan Sandstone.The Ketungau Group is interpreted to be unconformably above the Kayan Group, representing a new basin. The Ngili Sandstone, the oldest formation of the group, contains sediment derived from nearby sources, probably the Triassic Sadong and Kuching Formations. The Middle to Upper Eocene Bako-Mintu Sandstone, Silantek Formation and the probable Upper Eocene Tutoop Sandstone have similar sources to the Kayan Sandstone and have partly reworked the underlying sediments.Field observations, heavy minerals and U-Pb zircon age data identify a large depositional system passing from the Kuching Zone terrestrial setting into Sibu Zone Rajang Group submarine fan deposits. This system was active for at least 25 to 30 Ma and at times the catchment area extended into the Malay Peninsula, possibly Sibumasu. The Kuching Supergroup sediments can be correlated with deep marine Rajang Group sediments based on detrital zircon ages and heavy mineral assemblages. There was some magmatism but the scarcity of contemporaneous zircons and compositional maturity of heavy mineral assemblages indicates that it was very minor. There is no support for subduction magmatism and the new observations are inconsistent with models suggesting the Sibu Zone was the source of sediments for a Kuching Zone forearc basin.