Bulletin of the Geological Society of Malaysia, Volume 19, April 1986, pp. 69 – 78
1Bureau of Mineral Resources, Geology and Geophysics, Canberra, Australia
2Geological Research and Development Centre, Bandung, Indonesia.
Abstract: The existence of extensive tectonic mélange 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 mélange 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 1 cm up to many metres. In addition, exotic rock types forming masses several kilometres across are also inferred to be inclusions in the mélange. Greenschist facies metamorphic rocks and lenticular granitic bodies near Semitau are included in the mélange, and are not part of a basement high as previously thought and the Boyan Formation is now recognised as this Late Cretaceous mélange zone, and not a coherent Jurassic formation as described by Zeylmans van Emmichoven (1939).
The boundary zones of the mélange are exposed on both the northern and southern sides, and are probably both gradational in part. In both areas the mélange is bordered by strongly deformed turbidite and mass-flow deposit of the Late Cretaceous Selangkai Formation. Thick olistostromes contain shallow water limestone and sandstone inclusions and the Orbitolina sp. bearing sandstone and conglomerate at Belikai is probably a large slab of shallow-water sedimentary rocks carried into a deep-water basin represented by marl and thin-bedded turbidites. The Selangkai Formation has subsequently undergone polyphase deformation which produced tight folds, tectonic breccias, boudinaged bedding and lenticular shearing. The intensity of deformation increases towards the boundaries of the Boyan mélange.
The Boyan mélange has all the characteristics of a subduction-related structural unit. However, the presence of a subduction zone in this area in the Late Cretaceous poses several tectonic problems. No igneous activity of this age is present in the area, and the presence of rocks of similar age north of the mélange zone, both sedimentary and intrusive, indicate that the suture zone may represent a major deep-seated fault zone related to the beginning of subduction to the north at a later time. Alternatively it may represent the southern boundary of the major subduction mélange of latest Cretaceous and Tertiary age in Sarawak or a short-lived independent subduction event, halted by the collision of Kalimantan with a terrane composed of the basement granites and associated Mesozoic strata of West Sarawak.