Age of the Tananwan Formation in Northern Taiwan: A Reexamination of the Magnetostratigraphy and Calcareous Nannofossil Biostratigraphy

Chorng-Shern Horng

doi:10.3319/tao.2013.11.05.01(tt)

Abstract

Over the past two decades, the succession strata of the Tananwan Formation exposed horizontally on the Linkou Tableland in northwestern Taiwan has been either dated to a short time interval of 0.9-0.7 Ma or assigned to a vague long period within the Matuyama Epoch covering probably both the Olduvai and Jaramillo normal events. In this study, a reexamination of magnetostratigraphy and calcareous nannofossil biostratigraphy of this formation was carried out. The results show that the strata with stable remanent magnetizations have reversed magnetic polarities only, and the layers with marine sedimentary facies consistently contain specimens of large Gephyrocapsa. These updated results suggest that the Tananwan Formation was deposited within the interval of 1.46-1.24 Ma, corresponding to a short period of reversed magnetic polarity of the Matuyama Epoch.

Highlights

The Tananwan Formation, exposed on the Linkou Tableland in northwestern Taiwan, is characterized by its flat-lying strata consisting of loosely cohered sandstones and mudstones intercalated with conglomerates (Figs. 1a and 2a, c, d)
Because the morphologic evolutional events of Gephyrocapsa are well preserved in the Gutingkeng Formation of the Erhjen River (EJR) section in southwestern Taiwan (Chen et al 1977; Shea and Horng 1999; Horng and Shea 2007), samples from the EJR section straddling the stratigraphic levels from the first appearance datum (FAD) of medium Gepyrocapsa up to the post-FAD of Gephyrocapsa sp. 3 (Fig. 1b) were collected for comparison with calcareous nannofossils in the Tananwan Formation
Low-temperature magnetic measurements of magnetic extracts from the Tananwan Formation show that three magnetic transitions occur near 20, 40, and 115 K upon warming of saturation isothermal remananet magnetization (SIRM) curves (Figs. 3a - b)

Summary

Introduction

The Tananwan Formation, exposed on the Linkou Tableland in northwestern Taiwan, is characterized by its flat-lying strata consisting of loosely cohered sandstones and mudstones intercalated with conglomerates (Figs. 1a and 2a, c, d). Overlying the Tananwan Formation is the Linkou Formation composed of thick or massive conglomerates This lithofacies change was due to uplifting episodes of northern Taiwan during the Plio-Pleistocene Penglai Orogeny. Over the past two decades, methods including electron spin resonance, pollen, magnetostratigraphy and calcareous nannofossil have been used to construct a more detailed chronology of the Tananwan Formation. These studies claimed that the succession strata of the Tananwan Formation may represent a long period in the Matuyama Epoch most likely covering the Olduvai and Jaramillo normal events (Lee et al.2002), or a short time interval between 0.9 and 0.7 Ma (Shih 1991; Tseng et al 1992; Wei et al 2009)

Methods

Results

Conclusion