West Borneo Microplate Younger than Supposed?

Abstract

THAT part of West Kalimantan (Indonesian Borneo) which lies south of the Kapuas River occupies a key position in Southeast Asian geology. It lies along the southern flank of the Cretaceous-Cainozoic “Northwest Borneo Geosyncline”, and has been postulated as the source of the geosynclinal sediments¹. Van Bemmelen² considers this region to be the continental core of Borneo, part of the “Sunda Shield”, and an extension of Continental Asia—a concept I have recently questioned^3,4. Hamilton⁵ has recently postulated that South east Asia is a mosaic of microplates which have been rafted together, and probable Late Mesozoic-Cainozoic subduction zones have been recognized in Sarawak to the north^5,6. Speculation has recently been revived, and controversy joined, as to whether Southeast Asia, and West Borneo in particular, formed part of Gondwanaland^7–11. Constraint in the form j of factual radiometric, palaeontological, and palaeomagnetic data are badly needed, it seems to me, in order that such speculation may lead to a positive advance in knowledge. The purpose of this letter is to record a small addition to this constraint for West Kalimantan, which, for all its importance, is little text known.