The Geology of Indonesia/Stratigraphy/Basement

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Distribution map of basement rocks in Indonesia

'Crystalline Basement'

Crystalline schists is a general term used by R. W. van Bemmelen (1949) which include other lithologies such as gneiss, phylite, slate and marble (Sartono, 1979). This unit is widely distributed in the Indonesian Archipelago, but there is much diversity of opinion as to their age. The crystalline schists certainly do not belong to a distinct geological epoch, neither are they of an archaic age as has been supposed by many earlier authors. Sedimentary deposits of various ages can be altered into crystalline schists by regional metamorphism, aided by contingent processes of diffusion of constituents. Often a succession of alterations can be distinguished under the microscope, giving them a polymetamorphic appearance and enabling us to distinguish between several types of schists (viz. crystalline schists and phyllites) (edited from van Bemmelen, 1970).

In some areas the transition of fossil-bearing strata made possible a direct age determination, while in other areas only the upper limit is fixed by the age of the first transgressive formation. There are also indications of different metamorphic processes / polymetamorphism.

Hereunder some instances are mentioned:

  • Sumatra

The crystalline schists in South Sumatra (Geological Maps of South Sumatra Nos. I, 2, 4 and 5, scale I : 200,000) and North Sumatra (VAN BEMMELEN, 1932 d) are presumably parts of an old basement complex of pre-mesozoic age. Mesozoic sediments of Sumatra in many localities show a phyllitic facies. In the Islands West of Sumatra exposures of schistose pre-tertiary rocks are known. TERPSTRA (1932) mentions phyllites and amphibolites.from Sipura, and at the West coast of Nias crystalline schists crop out at the base of the Paleogene near Sumasuma.

  • Borneo

The schistose and phylitic formations of the smaller islands on the Sunda Shelf are generally of young paleozoic and upper triassic age (VAN BEMMELEN, 1940 d). Crystalline schists of West and Central Borneo are known as Schwaner Block, named after Carl Schwaner. This block is older than Upper Triassic (ZEYLMANS VAN EMMICHOVEN, 1939, p. 21) and partly also older than the Permo-Carboniferous (ZEYLMANS v. E., 1939, pp. 56-58); but also younger formations may have a schistose facies, e.g. the Eocene has obtained in some belts a phyllitic character (TER BRUGGEN, 1935). ZEYLMANS VAN EMMICHOVEN published in 1940 a study, in which he described the schists and gneisses of the Schwaner Mts in Central Borneo, which are considered to be older than the Permo-Carboniferous. The crystalline schists of the Meratus Mountains in SE- Borneo are older than the Alino Formation (? Jurassic acc. to KOOLHOVEN, 1935) which contains detrital matter of the schists.

  • Sulawesi

The polymetamorphic crystalline schists of Sulawesi are older than the Mesozoic or Young Paleozoic (BROUWER, 1941, p. 257-258), but there are also phyllites of mesozoic and eocene age (Tinombo Formation, ? Pompangeo Formation, Maroro Formation). In VAN BEMMELEN, 1949, contributions to the petrology of the crystalline schists of central Sulawesi were made by WILLEMS (1937), EGELER (1946, 1948) and DE ROEVER (1947). In Southeast Celebes and Buton BOTHE (1927) distinguishes a metamorphic (phyllitic) facies of the Mesozoic (Kendari facies) from a less or non-metamorphic facies (Buton facies). In the Northern Moluccas, isolated outcrops of crystalline schists are found on Obi (BROUWER, 1924a, p. 47) and on Bacan (Sibella Mts), which might be metamorphic paleozoic rocks, while in the Sula Islands (BROUWER, 1921 b, 1926) their age could be fixed as pre-Liassic.

  • Banda Arc

In the outer arc of the Southern Moluccas, crystalline schists are widely exposed. In Seram distinction can be made between phyllites which are pre-upper triassic (presumably Young Paleozoic) and older polymetamorphic crystalline schists (VALK, 1945 and, GERMERAAD, 1946). In Leti, transition from epimetamorphic schists to phyllitic rocks with intercalations of permian crinoidal limestones are found (MOLENGRAAFF, 1915). In Timor, crystalline schists occur in an overthrust complex of rocks, associated with ophiolites. These schists are at least pre-young Mesozoic, and partly pre-Permian (BROUWER, 1942 b, p. 364).

  • Java

In Java, crystalline schists have been found in the Loh Ulo region where they are partly cretaceous and partly older (HARLOFF, 1929, 1933), and in the Ciletuh area, where their age is uncertain (at least pre-Eocene).

  • Papua

In New Guinea and the islands off its North coast, crystalline schists and phyllites are found in the pre-tertiary basement complex, but there is little information on their composition and age (The oldest fossil-bearing strata of the Central Mountains are Silurian).

  • Philippines

In the Philippines, no old crystalline schists are definitively known, the oldest formation being, presumably, young Mesozoic (Baruyen cherts and shales).

The oldest pre-young paleozoic and presumably more or less autochthonous outcrops of crystalline schists are found in the central Sunda Land (West and Central Borneo and Lampung Districts of South Sumatra). The crystalline schists in the circum-Sunda Mountain System forms parts of over-thrust complexes and are partly pre-young Paleozoic, partly Young Paleozoic, Mesozoic, and even Eocene. The petrography and facies of these various schists formations will be discussed in the chapter on the regional geology.