Refining stratigraphic ages of Northern Hsuehshan Range in northern Taiwan by detrital zircon U-Pb dating

摘要

Taiwan is located on northern margin of South China Sea, which evolved from rifting basin to passive margin from Paleocene to Miocene. The northern Hsuehshan Range exposes the Eocene to Miocene strata, which offers a unique place to study the rifting basin history. However, the strata ages in the northern Hsuehshan Range are controversial, particularly for the Hsitsun Formation and Szeleng Sandstone that may correlate with a proposed break-up unconformity in the Eocene and Oligocene. Previously, the Hsitstun Formation was considered to be pre-Early to Middle Eocene in age based on the presence of Nummulites junbarensis in the overlying Szeleng Sandstone. In this paper, we present new U-Pb ages of detrital zircon grains from the Hsitsun Formation and Szeleng Sandstone to constrain the depositional ages of these units. In the Hsitsun Formation, ca. 8% of zircon U-Pb grains are Cenozoic in age and have a mean age of 35 Ma indicating that the maximum deposition age of late Eocene for this formation. Combining these results with the ca. 30 Ma age for Paling Formation indicates the Szeleng Sandstone is between 30 - 35 Ma. The new ages determinations suggest that the deposition was continuous with no evidence of a break-up unconformity from Szeleng Sandstone to Paling Formation. Finally, because much of the stratigraphic interpretations and correlations are based on what were interpreted to be "in situ" biostratigraphic markers, the new U-Pb dates argue for recycling and reworking of several key fossil assemblages. This result has important implications for other stratigraphic interpretations both in Taiwan and in other orogcnic systems around the world.