The Ayer Chawan Facies, Jurong Formation, Singapore: Age and observation of syndepositional pyroclastic sedimentation process with possible peperite formation

The Ayer Chawan Facies is one of the volcano-sedimentological facies belonging to the Jurong Formation in Singapore. The geology of the south-western Jurong Island had never been documented before the island was reclaimed in 1998. The present geological, sedimentological and geochronological study of the Ayer Chawan Facies was undertaken as part of an underground rock caverns storage project which was carried out from 2009 to 2014. The Ayer Chawan Facies is composed mainly of grey to black mudstone interbedded with grey siltstone/sandstone. In addition, the pyroclastic rocks of varying thickness are also intercalated with the sedimentary rocks. The pyroclastic rocks yielded an U-Pb zircon age of 240.6 ± 1.2 Ma (Middle Triassic) as determined by Laser Ablation Inductively Couple Plasma Mass Spectrometer (LA ICP-MS). Based on the stratigraphic and sedimentological evidence of the presence of peperitic textures/structures, it is interpreted that the pyroclastic rocks were emplaced into the sediments during deposition which was still in a wet and unconsolidated condition.