Upper Triassic carbonate-platform facies, Timor-Leste: Foraminiferal indices and regional tectonostratigraphic association

Abstract

Abstract Timor is part of the non-volcanic Outer Banda Arc with chaotic geology in the late Neogene collision zone between Sundaland (Southeast Eurasia) and the Australian continent. Confusion in the distinction between Triassic, Jurassic and Cenozoic shallow-marine carbonate units has led to a lack of appreciation of the widespread extent of Triassic carbonate-platform facies, at least in Timor-Leste. It also resulted in misunderstanding the different tectonostratigraphic/palaeogeographic affinities of the shallow-water limestones. Foraminiferal assemblages are critical in stratigraphic discrimination of these units. This study records for the first time the foraminiferal microfauna of Upper Triassic shallow-water carbonate deposits in Timor. Many of the foraminiferal species are known from North Africa and Europe and date these units to the Carnian–Rhaetian. The Upper Triassic Bandeira Group represents extensive sheet-like carbonate-platform deposits (possibly on horst plateaus as well as shelf areas adjacent land in the intracratonic sea). Contacts between the hard limestone, forming fatus (limestone peaks) and ridges, and friable mud-dominated surrounding units are often obscured under deep tropical soil cover. At all localities the Bandeira Group is closely associated in outcrop with coeval basinal mud-dominated deposits (Babulu and Aitutu groups). With these it forms part of the East Gondwana Interior Rift Association (EGIRA), similar to the classical Dachstein Platform–Hallstatt Basin facies associations found in the European Alps. The basinal units contain turbiditic and debris-slide deposits that include clasts derived from the carbonate platform. Shallow-water carbonate facies attributed to EGIRA, are present throughout the East Gondwana Interior Rift from the Papuan Basin in the north to Exmouth Plateau, part of the Northern Carnarvon Basin, in the south. These were deposited before Gondwanan breakup along this rift system. The Triassic carbonate-platform deposits have a different dominant biogenic mineralogy (aragonite rather than calcite) and a different architecture to the bryozoan-crinoidal carbonate mounds present in the Permian of EGIRA in Timor-Leste. No Lower and Middle Triassic shallow-water carbonate-platform deposits have been recognized in Timor-Leste. The Bandeira Group, confined to the Upper Triassic, belongs to a stratigraphic association (EGIRA; autochthonous) different both in constituent units and outcrop coverage from that of the allochthonous Overthrust Terrane Association to which the Lower Jurassic carbonate-platform deposits (viz. Perdido Group) belong. The Bandeira Group has been confused not only with the Perdido Group but also with other shallow-water limestone units of the Permian and Miocene in some former studies of Timor. The distinction between these units solves a major tectonostratigraphic problem in Timor, and may apply elsewhere along the Outer Banda Arc.