A foraminiferal study on Middle Eocene-Oligocene break-up unconformity in northern Taiwan and its correlation with IODP Site U1435 to constrain the onset event of South China Sea opening

Abstract

Taiwan is situated in the SE Asian continental margin where lower Cenozoic strata are well exposed. Its geological and tectonic setting provides a unique opportunity to study the time constraint of the break-up unconformity relevant to the opening of the South China Sea. A detailed foraminiferal study on four land sections and a deep core across the boundary between the Upper and Lower Wulai Groups in the Hsüehshan Range, northern Taiwan, was conducted. Results show remarkable changes of depositional environment and lithology from the amalgamated fluvial dominant sandstones (>2000m) of Middle Eocene in the Lower Wulai Group to the overlying inner neritic shales and siltstones (>2000m) of Oligocene in the Upper Wulai Group. Integrated sedimentological and foraminiferal data further suggest an occurrence of a depositional hiatus between 32 ∼33 and 40Ma across the boundary between the Kankou Formation of the Upper Wulai Group and the Szeleng Sandstone of the Lower Wulai Group in the northern Hsüehshan Range, as same as the scenario seen in the Western Foothills (25–39Ma) of central Taiwan and the Penghu Basin (25–40Ma) in the Taiwan Strait.This stratigraphic hiatus in the Taiwan mountain belt marks a regional break-up unconformity between the Middle Eocene syn-rift strata and the overlying Oligocene post-rift sequences in the NE South China Sea. The break-up unconformity can be correlated with those commonly recognized in the seismic profiles of the Pearl River Mouth Basin and drilling of the IODP site U1435 (Expedition 349) in the northern shelf-slope of the South China Sea. The upper time limit of the breakup unconformity 32Ma could mark the tectonic change from rifting to depression in the Paleogene Asian continental margin which eventually led to opening of the deep South China Sea in the Early Oligocene (∼33Ma).