Dardji Noeradi

Magmatism in western Indonesia, the trapping of the Sumba Block and the gateways to the east of Sundaland

Abstract The western Sulawesi magmatic belt and the Sunda–Banda arc define the eastern and south-eastern margins of Sundaland, which is part of the relatively stable Eurasian plate. The eastern margin is bounded by the Makassar Strait which separates western Sulawesi from Kalimantan. The initial opening of the Makassar Strait took place in the late Cretaceous–Early Tertiary leading to the opening of the Pacific–Indian Ocean gateway during Neogene time. Recent studies indicate similarities in the late Cretaceous–Paleogene stratigraphic sequence and calc-alkali magmatism between Sumba, south Sulawesi and south-east Kalimantan, suggesting a Sundaland origin for all of these areas. The southward migration of Sumba to the present frontal arc position of the Sunda–Banda arc has occurred since Late Cretaceous–Paleocene time.

Cenozoic magmatism in Kalimantan and its related geodynamic evolution

Abstract The NE–SW Tertiary magmatic belt of central Kalimantan is related to two separate periods of subduction; during the Eocene–Oligocene and Late Oligocene–Miocene. The younger magmatic belt is superimposed upon the earlier belt. This magmatic belt is characterized chiefly by Late Oligocene–Miocene volcanic products, among which limited exposures of the Eocene volcanics have also been mapped by previous investigators. This calc-alkaline magmatic belt has become known as the ‘gold belt’ of Central West Kalimantan on account of a number of discoveries of Neogene epithermal gold mineralization. This mineralization is found in central to proximal volcanic settings and occurred at relatively shallow depths. The earliest known subduction-related magmatism took place in the Eocene–Early Oligocene with the emplacement of calc-alkaline silicic pyroclastics, followed by a period of continental collision. Subsequent subduction-related magmatism continued from Late Oligocene–Pleistocene, during which time the magma evolved from calc-alkaline to potassic calc-alkaline. Plio-Pleistocene magmatism resulted in the formation of basalt flows. The present available K–Ar ages of the Cenozoic volcanics range from 51 to 1 Ma.

Multivariate statistical analysis for characterization of sedimentary facies of Tarakan sub-basin, North Kalimantan

ABSTRACT A study of sediment dynamics has been conducted on the Tarakan sub-basin, North Kalimantan, Indonesia using multivariate analysis. Multivariate statistical techniques can be used to determine sediment with similar characteristics and be a good proxy to recognize sedimentary facies and depositional environment. These methods have been applied to characterize the sedimentary facies at Tarakan sub-basin. A total of 23 samples were taken from several locations on the different depths of 56–2554 m that represent varieties of environment. The study is interpreted using compositional data analysis associated with a grain size trend analysis (GSTA), principal component analysis (PCA), and cluster analysis (CA). The GSTA value showed the dominance of poorly sorted silt indicating that the sediment is mostly deposited in low-energy depositional. The CA and the PCA determined three distinct sedimentary facies: shelf facies, upper bathyal facies, and lower bathyal facies. The facies were grouped as similar sediment and depositional environment. Sedimentological variables applied in the characterization were described to be an important tool for the interpretation of depositional environments – indirectly showing hydrodynamic energy.