Article

Notes on the geology of Sarawak

From Munshipedia, the MBRAS digital historical encyclopedia

Notes on the geology of Sarawak

J. B. Scrivenor, a geologist employed by the Federated Malay States Government, published this paper in 1927 to make generally available the findings of his 1904 fieldwork in Sarawak, which had originally appeared in a now out-of-print government report. The article provides a systematic account of the stratigraphy, lithology, and igneous geology of Upper and Lower Sarawak, with particular attention to the gold-field of Bau and the coal-district of Sadong, and it amplifies the earlier work of J. S. Geikie on gold occurrences in the same region.

Summary

Scrivenor’s primary objective was to document the geological structure of Sarawak in a form accessible to a wider readership than the original government gazette supplement. He organises the discussion around the major lithological units—limestone, marl, shale, sandstone, grit, conglomerate, and igneous rocks—tracing their distribution from the gold-field of Upper Sarawak (Bau, Bidi, Jambusan) through the coal-district of Sadong and the isolated mountains of Matang and Santubong. A significant portion of the paper is devoted to fossil evidence, including a previously unpublished description by G. C. Crick of three Jurassic ammonites (Perisphinctes spp.) collected at Tai Parit, Bau, which Scrivenor publishes here in Crick’s memory. These ammonites, together with the lamellibranch Alectryonia amor and the bryozoan Heteropora stylina previously described by R. B. Newton, anchor the limestone of Upper Sarawak firmly in the Middle Oolite (Jurassic).

The paper also addresses the structural geology of the region, drawing a detailed comparison between the fold patterns of Upper Sarawak and those of the Kinta gold-field in Perak. Scrivenor argues that in both areas a resistant limestone series has been folded into large-amplitude structures while an overlying, less resistant sedimentary series has been puckered into smaller folds and further complicated by solution sinkage. He notes that the sedimentary series in Sarawak is younger than Middle Oolite but that its precise age remains uncertain, with the coal-bearing strata at Sadong being Tertiary (previously regarded as Eocene by Posewitz). The igneous rocks are divided into porphyritic dykes (hypersthene-andesites) that acted as mineralising agents in the gold-field, and larger holocrystalline masses (hypersthene-gabbro and quartz-diorite) at Matang, Sijenjang, and Ta Faw Shak.

Key Findings

  • Three Perisphinctes ammonites (diameters approximately 50, 80, and 150 mm) collected at Tai Parit, Bau, were described by G. C. Crick and indicate Middle Oolite, probably Oxfordian, age for the marl in which they occur (pp. 290–291).
  • The limestone of Upper Sarawak is correlated with the Middle Oolite on the basis of Alectryonia amor (a species restricted in Europe to the Middle Oolite), Heteropora stylina, and Corynella as described by R. B. Newton in 1897 (p. 289).
  • The porphyritic dyke-rocks of the gold-field are identified as hypersthene-andesites (or hypersthene-porphyrites) with a glassy groundmass containing microliths and magnetite, plagioclase phenocrysts with high extinction angles, and ferromagnesian minerals of hypersthene and hornblende; biotite occurs in the Bunkok rock (pp. 292–293).
  • The coal-bearing strata of Sadong are certainly Tertiary, though their precise horizon relative to the shales at Bau remains unproven; Posewitz had previously assigned them to the Eocene (p. 292).
  • A dark calcareous conglomerate in an anticline between Bau and Bidi contains rounded coal pebbles coated and veined with calcite, alongside chert and sandstone clasts (p. 291).
  • The structural comparison with Kinta shows that in both areas the sedimentary series overlies the limestone, is more intensely folded, and has been affected by solution sinkage; Scrivenor rejects the view that the sedimentary series is stratigraphically below the limestone (pp. 293–294).

Conclusion

Scrivenor’s definitive takeaway is that the geology of Sarawak, while structurally analogous to the Kinta gold-field, differs in the age of its constituent series: the limestone is Jurassic rather than Carboniferous, and the overlying sedimentary rocks are younger than Middle Oolite rather than Triassic. He emphasises that the palaeontological evidence, while sufficient to fix the limestone age, remains insufficient to resolve the precise stratigraphic position of the coal-bearing and shale units, and he closes with a practical observation that the oil-bearing strata of Brunei and Miri offer far more instructive models for petroleum prospecting than the sparse and unfavourable indications in the Malay Peninsula.

Context

  • The primary field evidence derives from Scrivenor’s own 1904 itinerary (Kuching, Bau, Bidi, Sadong, Santubong, Matang) and from specimens deposited in the British Museum of Natural History, including the Crick ammonites.
  • The paper’s historiographical contribution lies in making accessible a government report that had been out of print for over two decades, and in publishing Crick’s ammonite description posthumously; it also provides one of the earliest structural comparisons between the Kinta and Sarawak gold-fields.

References