Upper Triassic carbonate deposits of Seram (Indonesia): palaeogeographic and geodynamic implications

Abstract

The Upper Triassic limestones of central and western and Seram, were deposited on an extensive carbonate platform, already identified in various localities all around the Banda Sea: Sinta Ridge, central–east Sulawesi, Buru, Misool and the Wombat Plateau (off NW Australia). The Triassic deposits are found in the parautochthonous, as well as in the allochthonous series of Seram; the facies of the two series are of Gondwanian–Australian type in the Parautochthonous and of Laurussian–Asian type in the Allochthonous. The Asinepe Limestone (=Manusela Formation), on which this study is based, has been considered as part of the allochthonous series. The Triassic reefal lithotypes can be divided into four main facies, as follows: (1) the boundstone facies forming the buildup cores; (2) the oncolitic grainstone-rudstone facies, indicating high energy conditions; (3) foraminiferal packstone-grainstone facies, characteristic of moderate to high energy conditions; and (4) the foraminiferal-megalodont mudstone facies, inferred to have accumulated in a quiet lagoonal environment. Forming the buildup cores of about 4 m long, 2 m high; they are separated by depressions filled with reef detrital sands; The Asinepe Limestone was deposited during the Late Triassic (Carnian–Norian to Rhaetian). This range is corroborated by foraminiferal and palynological data, and the coral framebuilders give evidence for a Norian–Rhaetian age. There is a some consensus that Seram and the Island of Buru, located west of Seram, belong to the same tectonic block (Seram-Buru Block). According to geochemical and geodynamic interpretations, the Seram-Buru Block is derived from Irian Jaya or from Papua New Guinea. The palynological results are consistent with reconstructions that place the Seram-Buru Block in a palaeogeographic zone distinct from that of Sulawesi, and therefore from the Kolonodale Block. For these reasons, it is here proposed that, during the Upper Triassic, the Seram-Buru Block and the Kolonodale Block were two separated entities, the former located in a more tropical position than the latter. In addition, the Seram palynological association shows a composite microflora of the warm Onslow and the cold Ipswich-types, suggesting that the Seram microcontinent moved progressively to the north and crossed the boundary between the temperate and the warm subequatorial belts at one point in the Late Triassic–Early Jurassic.