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The geology and tectonics of the Bacan Region, eastern Indonesia

Malaihollo, Jeffrey Francis Anthony; (1993) The geology and tectonics of the Bacan Region, eastern Indonesia. Doctoral thesis (Ph.D), UCL (University College London). Green open access

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Bacan is located in the zone of convergence between the Australian, Philippine Sea and Eurasian Plates. This study has established a new stratigraphy for Bacan, based on geological mapping, aerial photography, new biostratigraphic dating, K-Ar and Ar-Ar dating, petrography, mineral and whole rock geochemistry. The oldest rocks in Bacan belong to the Sibela Continental Suite and are of probable Palaeozoic age. The complex includes continental phyllites, schists and gneisses of upper amphibolite-lower granulite facies, with fabrics typical of dynamo-thermal metamorphism. The protoliths are mostly arc-related pelites, derived from the Australian craton and deposited on an active margin. Isotopic dating yields extremely young ages due to interaction with hydrothermal fluids. Juxtaposed against the continental rocks is the mostly unmetamorphosed, arc-related Sibela Ophiolite. It is metamorphosed locally along shear zones to upper amphibolite facies. Isotopic dating yielded a Cretaceous age with an Oligocene-Miocene overprint. This ophiolite was probably derived from the Philippine Sea Plate. The Saleh Complex consists of metabasites and foliated metasedimentary rocks. Arc-related calc-alkaline metabasites have suffered lower greenschist facies metamorphism (~250-360°C, ~4 kb) and may be cogenetic with the Bacan and South Bacan Formations. The metasedimentary rocks may be related to the Sibela Continental Suite. In north Bacan, the oldest formation is the Upper Eocene Bacan Formation which comprises interbedded basic-intermediate arc volcanic and turbiditic volcaniclastic rocks, metamorphosed under conditions transitional between the prehnite-pumpellyite and pumpellyite-actinolite facies (250-330°C, -2 kb). The oldest rocks in south Bacan belong to the Lower Miocene South Bacan Formation; an interbedded arc volcanic and turbidite sequence, metamorphosed to prehnite- pumpellyite facies (~240-330°C, ~2 kb). The Oligocene Tawali Formation is the oldest formation on Kasiruta, consisting of arc lavas and volcaniclastic turbidites, metamorphosed to zeolite facies (~180°C, <2 kb). The Bacan, South Bacan and Tawali Formations represent different parts of an arc, active from Late Eocene until Early Miocene, resulting from northward subduction of the Indo-Australian Plate under the Philippine Sea Plate. There is a major Early Miocene unconformity, due to the collision of Australian continent with the Philippine Sea Plate, above which the Lower-Middle Miocene Ruta Limestone was deposited. These shallow marine limestones include four microfacies: open platform, platform build up (patch reef), tidal bar and foreslope talus. The Ruta Formation was locally interrupted by sudden influxes of volcaniclastic material, forming the Amasing Formation, which contains three facies: shallow marine, shoal or estuarine, and beach deposits. The Upper Miocene-Pleistocene Kaputusan Formation, composed of the Goro-goro, Pacitak and Mandioli Members, rests unconformably above the Ruta and Amasing Formations. The Goro-goro Member consists of arc andesites, with magma diversification achieved mainly by fractionation. It includes four eruption centres, erupting from Late Miocene to Pleistocene, with the oldest in south and the youngest in north Bacan. The Upper Miocene-Upper Pliocene Pacitak Member consists of reworked pyroclastic and volcaniclastic material, deposited in a shallow marine environment. The Upper Miocene-Lower Pliocene Mandioli Member consists of limestones which formed fringing coastal reefs. This formation was produced by eastward subduction of the Molucca Sea Plate. Quaternary arc basalts are related to movement along the Sorong Fault Zone.

Type: Thesis (Doctoral)
Qualification: Ph.D
Title: The geology and tectonics of the Bacan Region, eastern Indonesia
Open access status: An open access version is available from UCL Discovery
Language: English
Additional information: Thesis digitised by ProQuest.
Keywords: Earth sciences; Indonesia
URI: https://discovery.ucl.ac.uk/id/eprint/10100987
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