Bulletin of the Geological Society of Malaysia, Volume 80, November 2025, pp. 5 – 19
A review of the Letter Stages of Sundaland and their relevance to Sarawak geology
Peter Lunt1, *, Xiwu Luan2
1 Stratos Energy Advisors PLT, PL@Stratos-sea.com, C4-5 Riana Green East, Wangsa Maju, 53300 Kuala Lumpur, Malaysia
2 College of Earth Science and Engineering, Shandong University of Science and Technology (SDUST), 590 Qianwangang Rd, Qingdao 266579, China
* Corresponding author email address: plunt@mac.com
Abstract: Almost a century ago, the first subdivision of the Cenozoic (Tertiary) stratigraphy of SE Asia was developed at the Geological Survey of the East Indies (Indonesia). This system eventually became known as the Letter Classification, and its subdivisions were called ‘Letter Stages’. In this paper an overview of the Letter Stages is given, and their continued use in Southeast Asian stratigraphy is justified. Unfortunately, many instances of mis-application of the Letter Stages have held back stratigraphic and geological studies, so the four most problematic boundaries are described, set to a modern time scale, and placed in a local geological context. Since the 1970s, the Letter Stages have been integrated with planktonic microfossil zones and palynology datums (the latter especially relevant at the Oligo-Miocene boundary), thus allowing the correlation of geological change across different depositional facies. Events with different sedimentary characteristics can be shown to be coeval over a wide area, and this geographic variation in geologic change is essential to describe what must have been complex geological transitions. This important information is lost if the Letter Stages are not used or not correctly identified.
Keywords: Biostratigraphy, SE Asia, Cenozoic, time scales, larger foraminifera
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Manuscript received 1 November 2024;
Received in revised form 7 March 2025;
Accepted 24 March 2025
Available online 28 November 2025
https://doi.org/10.7186/bgsm80202502
0126-6187; 2637-109X / Published by the Geological Society of Malaysia.
© 2025 by the Author(s). This is an open access article distributed under the terms of the Creative Commons Attribution (CC-BY) License 4.0.
