Sedimentological characteristics and new detrital zircon SHRIMP U–Pb ages of the Babulu Formation in the Fohorem area, Timor-Leste

Abstract

Sedimentological characteristics and zircon provenance dating of the Babulu Formation in the Fohorem area, Timor-Leste, provide new insights into depositional process, detailed sedimentary environment and the distribution of source rocks in the provenance. Detrital zircon sensitive high-resolution ion microprobe (SHRIMP) U–Pb ages range from Neoarchean to Triassic, with the main age pulses being Paleozoic to Triassic. In addition, the maximum deposition ages based on the youngest major age peak (ca 256–238 Ma) of zircon grains indicate that the basal sedimentation of the Babulu Formation occurred after the early Upper Triassic. The formation consists predominantly of mudstone with minor sandstone, limestone and conglomerate that were deposited in a deep marine environment. These deposits are composed of six lithofacies that can be grouped into three facies associations (FAs) based on the constituent lithofacies and bedding features: basin plain deposits (FA I), distal fringe lobe deposits (FA II) and medial to distal lobe deposits (FA III). The predominance of mudstone (FA I) together with intervening thin-bedded sandstones (FA II) suggest that the paleodepositional environment was a low energy setting with slightly basin-ward input of the distal part of the depositional lobes. Discrete and abrupt occurrences of thick-bedded sandstone (FA III) within the FA I mudstone suggests that sandstone originated from a collapse of upslope sediments rather than a progressive progradation of deltaic turbidites. This combined petrological and geochronological study demonstrates that the Babulu Formation in the Fohorem area of the Timor-Leste was initiated as a submarine lobe system in a relatively deep marine environment during the Upper Triassic and represents the extension of the Gondwana Sequence at the Australian margin.

东帝汶Fohorem地区的沉积学特征和锆石年龄测定，提供了关于该地区沉积过程、沉积环境详情和源岩分布的新认识。碎屑锆石灵敏高分辨率离子探针（SHRIMP）U-Pb年龄范围是从太古代到三叠纪，主要年龄脉冲是古生代至三叠纪。此外，基于锆石颗粒的最年轻的主要年龄峰值（约256-238Ma）而得的最大沉积年龄表明Babulu组的基部沉积发生在晚三叠世早期之后。该组主要由泥岩组成，含有少量的砂岩、石灰岩和砾岩，沉积于深海环境。这些沉积物由六个岩相组成，并可根据岩相和岩层特征分成三个相组合（FAS）：盆地平原沉积物（FA I）、远端边缘突出体沉积物（FA II）和中至远端突出体沉积物（FA III）。泥岩（FA I）为主并夹薄层砂岩（FA II）说明古沉积环境是一种低能量环境，由沉积突出体远端部分稍向盆地方向流入。厚层砂岩（FA III）在泥岩相中偶尔而突然的出现说明砂岩源于上坡沉积物的崩溃，而不是三角洲浊流的逐步进积。这种岩石学和地质年代学的综合研究表明，东帝汶Fohorem地区的Babulu组起始是晚三叠世较深海洋环境中的水下突出体系统，代表冈瓦纳地层在澳大利亚边缘的延伸。

KEYWORDS:

• Timor-Leste
• Fohorem
• Babulu Formation
• sandstone
• lobe system
• deep marine environment
• SHRIMP zircon U–Pb geochronology

Acknowledgments

We thank Prof. Ian S. Williams and Prof. Ron Harris for constructive reviews. This work was supported as a Basic Research Project (GP2011-004; Tectonic evolution of the western Gyeonggi Block and construction of geologic DB system) of the Korea Institute of Geoscience and Mineral Resources (KIGAM), funded by the Ministry of Knowledge Economy, Korea. This study also contributes to the Project ‘Geological Mapping of the Suai District in Timor-Leste.’ The Project was implemented by the Korea International Cooperation Agency (KOICA), a government agency carrying out grant aid programs of the Republic of Korea and the Secretariat of State for Natural Resources (SERN) on behalf of the Democratic Republic of Timor-Leste.