The Rajang Unconformity: Major provenance change between the Eocene and Miocene sequences in NW Borneo
Abstract
The offshore Sarawak Basin NW of North Sarawak is a major hydrocarbon province in SE Asia. A very thick sedimentary sequence of Oligocene to ?Early Miocene age, named Cycle 1, is an important hydrocarbon source and reservoir. Despite numerous wells the stratigraphy and tectonic history is not very well understood. The Nyalau Formation of onshore North Sarawak is the supposed equivalent of the offshore Cycle 1 sequence. The Nyalau Formation is a thick sedimentary sequence of mainly tidal to deltaic deposits. The formation is dominated by well-bedded sandstone-mudstone alternations and thicker sandstones with abundant bioturbation. The sandstones are predominantly arenaceous. Various lithic fragments and feldspar indicate multiple sources and fresh input from igneous and metamorphic rocks. Interbedded thin limestone beds and marls yielded Early Miocene foraminifera for the upper part of the succession. Zircons separated from the sandstones yielded mainly Cretaceous and Triassic ages. The Triassic is the dominant age population. The Nyalau Formation conformably overlies the Buan Shale and the Tatau Formation, and in places unconformably overlies the Belaga Formation. The Belaga Formation is part of the Rajang Group that represents remnants of a large submarine fan deposited in the Late Cretaceous to Eocene in Central Sarawak. In contrast to the Nyalau Formation, the majority of zircons from the Rajang Group have Cretaceous ages. This marks an important change in provenance at the major unconformity separating the Belaga and Nyalau Formations. This unconformity was previously interpreted as the result of an orogeny in the Late Eocene. However, there is no evidence for a subduction or collision event at this time in Sarawak. We interpret it to mark plate reorganisation in the Middle Eocene and name it the Rajang Unconformity. Borneo is the principal source of Cretaceous zircons which were derived from the Schwaner Mountains and West Sarawak. The dominant Triassic zircon age population in the Nyalau Formation indicates either major input from the Malay Peninsula (Malay-Thai Tin belt) or Indochina (SE Vietnam). It also suggests that Borneo supplied little or no sediment to Sarawak in the Oligocene to Early Miocene.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2017
- Bibcode:
- 2017AGUFMEP21A1829B
- Keywords:
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- 1165 Sedimentary geochronology;
- GEOCHRONOLOGY;
- 1625 Geomorphology and weathering;
- GLOBAL CHANGE;
- 1862 Sediment transport;
- HYDROLOGY;
- 8177 Tectonics and climatic interactions;
- TECTONOPHYSICS