Roberts, Spencer John; (1993) The foraminiferal biostratigraphy and biofacies of the neogene sediments of the Halmahera region, Ne Indonesia. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The Halmahera Arc is a volcanic arc forming part of the Philippine Sea Plate which is moving westward on the north side of the Sorong Fault Zone. This zone is an area of active deformation separating the Australian Plate from the Pacific region in northeast Indonesia. The arc is a relatively young feature which was initiated in the Late Miocene by the eastward subduction of the Molucca Sea Plate beneath Halmahera. Basins in the vicinity of Halmahera contain up to 5 km of Neogene sedimentary rocks, a reflection of the rapid subsidence in this area. These are now partly exposed on land as the result of Pliocene or younger deformation. The sediments contain abundant and diverse foraminiferal assemblages. Compilation of micropalaeontological and K-Ar data from material collected in the last nine years has produced a revised Neogene stratigraphy which now includes the northwest arm, where three new formations have been introduced. In the southwest arm the stratigraphy of the Weda Group has been redefined. A planktic foraminiferal zonation scheme for the Upper Miocene to Upper Pliocene has been erected for the Halmahera area and seven zones and three subzones have been established. Qualitative and quantitative studies of the benthic foraminifera has produced a palaeobathymetric zonation for the area. Four zones ranging from Inner Neritic to Lower Bathyal were identified. Microfacies analysis of the Subaim Limestone Formation identified six lithofacies and ten sublithofacies. Several models for areas of Miocene carbonate deposition in the Halmahera Region are described. Shallow marine conditions with localised reef development around emergent basement highs characterised the Subaim Formation throughout the Miocene and Early Pliocene. Integration of foraminiferal biostratigraphy, sedimentology and tectonic studies of this region allows palaeogeographic reconstructions of the Halmahera Region for the Neogene. Four principal periods have been identified; Early Miocene, Middle Miocene, Late Miocene to Early Pliocene and Post 3 Ma.

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