Provenance changes in NW Borneo and the development of the Miocene Baram Delta: implications from detrital zircon geochronology and heavy mineral analysis

The Cenozoic of Borneo consists of thick clastic sedimentary rocks that were deposited in various fluvial-lacustrine, shallow marine and deep-marine settings. From the Eocene onwards, the sedimentary successions play an important economic role. Onshore, Eocene sediments comprise coal seams of commercial interest, while Miocene deposits offshore are important hydrocarbon reservoirs. In principle there are two main sediment sources that have contributed to the successions. The Cretaceous arc-related Schwaner Mountains in the south of the island, and the Triassic tin belt granites from the Malay Peninsula. Characteristic detrital zircon U-Pb ages and heavy mineral signatures reveal a change of these sources through time, related to major tectonic developments, such as the opening of the South China Sea. The Miri Zone in NW Borneo consists of Eocene turbidites of the Rajang Group, unconformably overlain by tidally-influenced delta deposits of the Oligocene Nyalau Formation. The Rajang Group turbidites overall reveal a provenance related to the Schwaner Mountains, while the Nyalau Formation indicates a Malay tin belt source, bypassing southern Borneo. The Miocene deposits of the Baram Delta form the youngest successions of the Miri Zone, and comprise fluvial, tidal, deltaic and shallow marine deposits. They are the analogue for the hydrocarbon reservoir offshore Brunei and northern Sarawak. In contrast to the underlying Nyalau Formation, the Baram successions were sourced directly from uplifted Rajang Group tubidites from the interior of Borneo. Thus, studying compositional variations and provenance characteristics help to understand sources and sedimentary routing pathways.