Bulletin of the Geological Society of Malaysia, Volume 81, May 2026, pp. 99 – 115
The history of fluvial to coastal plain sand deposition and the Cycles of Sarawak
Peter Lunt1,2,*, Mohammad Nazrul3, Khairool Anwar Laksamana3
1 Stratos Energy Advisors PLT, C4-5 Riana Green East, Wangsa Maju, 53300 Kuala Lumpur, Malaysia
2 Southeast Clastic and Carbonate Research Laboratory, Universiti Teknologi PETRONAS, 32610 Seri Iskandar, Perak, Malaysia
3 Petroleum Sarawak Berhad (Petros), Wisma Bapa Malaysia, Petra Jaya, 93502 Kuching, Sarawak, Malaysia
*Corresponding author email address: plunt@mac.com
Abstract: A new description of sedimentation onshore Sarawak is given, using the Cycles scheme of empirically observed sequences of deposition. The review clarifies a confused history of terminology and offers a tectono-stratigraphic framework (for the Oligo-Miocene boundary period) that can be used to classify and describe sedimentary geology in a genetic stratigraphy that ties to regional geology. This framework requires a well-constrained biostratigraphy, particularly for the Cycle I to II boundary that can be correlated with the regional Base Miocene Unconformity (BMU). Previous reports have not included this correlation, which is important as it places the geological history of Sarawak in a much wider reconstruction. The nature of the Cycle I to II boundary is an abrupt transgression which, in the area around the Tatau Province in the west, covers an angular unconformity. The second boundary documented here (top Cycle II) is expressed as a rapid rotation in the direction of sediment supply, through almost ninety degrees, associated with a transgression (subsidence) in the west and coeval uplift in the hinterland of Borneo.
The coherent movement of entire sedimentary systems at each Cycle boundary has implications through Walther’s Law that can be tested, for example in continuous and well-sampled well sections just offshore. This approach brings rigour to apparently contradictory prior accounts of the onshore Sarawak Formations and Cycles. As a result, outcropping lithofacies, which are found in each of the three Cycle-related phases, can now be placed in three different palaeogeographic settings reflecting this tectono-stratigraphic development. The most common lithofacies have been given the names Nyalau Formation, Kakus Member and Biban Sandstone Member, and these can now be subdivided with a simple tabulated nomenclature to reflect the episodic stratigraphy (e.g. Nyalau I, II and III etc.).
Keywords: Stratigraphy, Sarawak tectonics, basin development, Sarawak
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Manuscript received 23 December 2023;
Received in revised form 1 November 2024;
Accepted 9 May 2025
Available online 29 May 2026
https://doi.org/10.7186/bgsm81202609
0126-6187; 2637-109X / Published by the Geological Society of Malaysia.
© 2026 by the Author(s). This is an open access article distributed under the terms of the Creative Commons Attribution (CC-BY) License 4.0.
