Bulletin of the Geological Society of Malaysia, Volume 11, Dec, 1979, pp. 283 – 313
Department of Geology, University of Malaya, Kuala Lumpur, Malaysia
*alias Daud Abdul Fattah
Abstract: Since over 95% of present tin output in Malaysia and Indonesia is mined from the “alluvium”, and a considerable proportion of future production will derive from deeply buried coastal and submerged offshore placers where accurate target definition based on sound geological concepts is essential to reduce high exploration expenditures, greater emphasis need be given to integrated studies of the Late Cainozoic stratigraphy and the associated placers.
Offshore drilling and seismic data, and investigations of adjacent coastal mines, indicate that discontinuously rising Late Cainozoic eustatic sea levels and accompanying climatic changes have been the major controls on Sundaland sedimentation, facilitating regional lithostratigraphic correlation and subdivision into Sundaland Regolith (Late Miocene to Early Pliocene), Older Sedimentary Cover (Early Pliocene to Early Pleistocene), and Young Alluvium (Late Pleistocene to Holocene).
A tentative time framework for the depositional history and tin placer evolution appears as follows. The Sundaland Regolith, including lateritic soils and primitive eluvial/colluvial (kulit) placers, first developed under a semi-arid climate. Unconformably overlying Older Sedimentary Cover represents a piedmont fan facies (e.g. Boulder Beds) adjacent to bedrock scarps, demarcated downslope from a finer alluvial plain facies of Old Alluvium. Rich piedmont (gugup) placers are exemplified by the Western Boulder Clays in the Kinta Valley. Near the end of the Early Pleistocene, increased precipitation resulted in stream entrenchment forming residual-elutriational (kaksa=karanlg) and braided stream (kaksa and mincan=karang gantong) placers. Following a lateritisation phase, a second braided stream entrenchment resulted in further placer reworking and redeposition downstream as a younger mincan series within the Transitional Unit. Superficial reworking to form beach placers followed during two major transgressions across the Sunda Shelf, responsible for the “Older marine unit” and Younger Sedimentary Cover. Drop of sea level following the first transgression resulted in deep meander incision and placer destruction, and although subsequent fill was generally tin-barren, these young valleys have been deceptive targets for previous unsuccessful offshore exploration ventures.
The search for tin should take account of the fact that the major Southeast Asian sources are piedmont fan, braided stream and residual-elutriational placers formed on the emergent Sundaland continent within Older Sedimentary Cover (and Transitional Unit), and localised on the flanks of positive granite features and in adjacent shallow bedrock troughs. The Indonesian “Tertiary” series have a similarly high potential. Seismic interpretation of stratigraphic “pay horizons” should provide the main basis for future drill target definition and the delineation of offshore placer boundaries.