The existence of extensive tectonic melange in areas of West Kalimantan previously considered to be stratigraphically cohesive implies the existence of a major suture zone trending WNW just south of Semitau, on the Kapuas River. In the Boyan River area the melange contains blocks of sedimentary rocks, metamorphics of both greenschist and blueschist facies, gabbro and ultramafic rocks. The blocks are usually elongate in the direction of strongly developed pervasive shear surfaces and not markedly deformed internally. They are dispersed in a complex deformed matrix which is argillitic to phyllitic and range in size from less than I cm up to many metres. In addition, exotic rock types forming masses several kilometres across are also inferred to be inclusions in the melange. Greenschist facies metamorphic rocks and lenticular granitic bodies near Semitau are included in the melange, and are not part ofa basement high as previously thought and the Boyan Formation is now recognised as this Late Cretaceous melange zone, and not a coherent Jurassic formation as described by Zeylmans van Emmichoven (1939).
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