Plio-Pleistocene Extra-Ordinary Sedimentation Phenomenon in an Active Collision Zone: Southern Taiwan

LEE, TEH-QUEI, and CHORNG-SHERN HORNG, Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan

Magnetostratigraphies of two Plio-Pleistocene sedimentary rock sequences, Tsengwenchi and Chishan sections, located at southwestern part of Taiwan were analyzed. They are situated at the passive margin of an active collision zone where the Philippine Sea plate and the Eurasian continent plate have collided against each other since late Miocene. Lithofacies of these two sections mainly consist of thick fine-grained mudstone with thin alternating shale/sandstone beds.

Magnetostratigraphy of Tsengwenchi section recorded the complete magnetic polarity zonations from upper Gauss to lower Brunhes normal epochs. However, that of Chishan section recorded the consequence from upper Olduvai event to lower Brunhes epoch. The total thickness of the Tsengwenchi section is about 4000 m and that of the Chishan section is about 2000 m. These results reveal that the mean sedimentation rate of the two sequences is higher than 2 mm/yr. Moreover, both sections indicated than an extraordinarily high sedimentation rate has occurred during the Jaramillo event (about 700 m deposited within 70 k.y.; i.e., 1 cm/yr). This phenomenon proposes that subsidence of the deposition basin should have happened during this period. In addition, after that time, a depositional interru tion has been found at the top of the Matuyama epoch.

Magnetic mineralogical analyses indicate that the major magnetic minerals contained by the rock samples are different from time to time: magnetite is found generally in the formations older than Olduvai event; greigite [Fe(3)S(4)] dominates in the rocks that were deposited during the Olduvai and Jaramillo magnetic subchrons; changes from greigite to pyrrhotite were observed in the stratigraphies younger than the Jaramillo event. This trend indicates a reducing sedimentary environment at the studied area in the past.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)