第四紀澎湖水道遷移之探討

Abstract

本研究利用層序地層學的原理，分析震測與井測資料並配合化石定年結果，對比台灣西南部海域的地層。根據化石定年應用到地層的對比，可以確定的年代有CN13b 上界 (1.05 Ma) ，以及更新世與全新世邊界 (0.0117 Ma) ；由此則可依照前人研究的全球海水面變化曲線，去劃分其間的層序年代。結果顯示很可能至少在1.06 Ma時的臺灣海峽中就已經有古澎湖水道出現，並開始不斷的下切與充填 (cut and fill) 。而近陸側的地層持續不斷傾斜與下陷，表示此時期臺灣南部陸地可能已經抬升，且進入前陸盆地的時期，並伴隨著約3.1 mm/yr 的沉積速率。若從震測圖上直接觀察，可以發現至少在0.47 Ma 時，已經有來自臺灣所形成的古曾文溪三角洲往西堆積至剛形成的海峽。另外震測剖面也顯示，澎湖水道的變遷，至少有三期的演化，其中第三期包括現今澎湖水道。而這三期的澎湖水道隨著深度和位置呈現向西移動的趨勢，表示著澎湖水道至少從1.06 Ma 開始以每年28~30 mm/yr 的速度逐步向西移動。這與GPS 觀測到台灣受板塊聚合而造山帶向西移動的速度大致吻合，推測澎湖水道向西的遷移應和臺灣板塊聚合有關。Seismic profiles, well information, and fossil dating records were used to carry out sequence analysis of Quaternary deposit in offshore southern Taiwan in order to delineate the westward migration of Penghu Canyon. The boundary between Pleistocene and Holocene (0.0117 Ma) can be easily identified in the seismic profiles. Other than that, the upper boundary of CN13b (1.05 Ma) can be defined in the well controls. Consequently, seismic sequence boundaries are identified from the seismic profiles, then correlated to the eustatic sea-level curves. The results reveal that Paleo-Penghu Channel had already existed in southwestern Taiwan since at least 1.06 Ma. The continuous foreland subsidence in western Taiwan, due to the arc-continental collision, caused a relatively fast sediment accumulation rate of about 3.1 mm/yr. Furthermore, the Tzeng Wen River deltaic system prograding toward west into the Taiwan Strait started at least since 0.47 Ma. Moreover, it is possible to separate the Paleo-Penghu Channel migration into at least three stages according to the seismic interpretation. The burying depths of Paleo-Penghu Channel decreased from east to west during the migration stages. The westward migration of Penghu Channel started from 1.06 Ma to present with a rate of about 28~30mm/yr in offshore southern Taiwan. This migration rate is consistent with the GPS velocity data of the movement of the Taiwan Central Mountain Range to a stable Asia. Therefore, the Penghu Channel migration has a close connection with the tectonic history of the arc-continental collision in Taiwan.