Zircon Age dating of the Sajahat Formation From Punggol, Singapore

Pan, Xiaohua (Nanyang Technological University) | Oliver, Grahame John Henderson (Nanyang Technological University) | Chu, Jian (Nanyang Technological University) | Goh, Kok Hun (Infrastructure Design & Engineering, Land Transport Authority) | Wei, Xiaoqian (Infrastructure Design & Engineering, Land Transport Authority) | Kumarasamy, Jeyatharan (Infrastructure Design & Engineering, Land Transport Authority)



The Sajahat Formation is considered to be the oldest rock unit in Singapore. However, the age of deposition is uncertain. According to the Geological Map of Singapore, the Sajahat Formation has been found on Pulau Tekong, Pulau Sajahat and at Punggol Point. However, the occurrence at Punggol has not been confirmed due to the lack of present day outcrops. As part of a site investigation, two boreholes were drilled at Punggol. Hornfelsed quartzite (very similar to that found on Pulau Sajahat) cut by diorite and granodiorite dykes were logged in the core samples. Zircons from these rocks were radiometrically dated using the Laser Ablation ICPMS U-Pb method. The results of the analysis of the detrital zircons indicate that the quartzite was deposited at or later than 337±3 Ma (Early Carboniferous) and before the intrusion of a diorite dyke at 285±1 Ma (Early Permian). A granodiorite dyke was dated at 260±3 Ma (Late Permian). Therefore, the quartzite at Punggol can be confirmed to be the Sajahat Formation of Carboniferous age and is the oldest dated rock in Singapore. The engineering implication of identifying the types of formations is discussed.

1. Introduction

The Sajahat Formation in Singapore is defined as those variably metamorphosed, unfossiliferous, sedimentary rocks comprising quartzite, sandstone, and argillite (Public Works Department 1976, Sharma et al., 1999; Lee and Zhou, 2009; Zhou and Cai, 2011). Previous studies indicate that it is probably the oldest rock unit in Singapore based on outcrops found in Pulau Tekong, Pulau Sajahat and Sajahat Kechil. Lee and Zhou (2009) proposed that the age of deposition of the Sajahat Formation was probably Lower Palaeozoic based on its complex deformation history and multiple intrusion of dykes. However, a Carboniferous to Permian age cannot be ruled out. The Sajahat Formation is very similar to the Mersing Formation in eastern Johor which is assumed to be Carboniferous in age since it is overlain by fossiferous Permian conglomerates with an angular unconformity (Oliver and Gupta, 2017). The deformed Sajahat Formation was considered to predate the undeformed Gombak Norite which has been U-Pb zircon dated by Oliver et al. (2014) at 260±2 Ma (Late Permian). The Sajahat Formation is therefore probably pre-Late Permian in age (Oliver and Gupta, 2017). However, there is no direct evidence of the age of deposition of the Sajahat Formation so far.