台灣西南部六重溪層與崁下寮層假整合界面的成因

Abstract

晚上新世以來，蓬萊造山運動（Penglai orogeny）在台灣西南部形成前陸盆地的構造環境。由於前陸盆地呈現快速沉降，因而沉積巨厚的晚上-更新世沉積層。上-更新世以來(300萬年以來)，全球海水面變動頻繁使得西南部的前陸盆地中記錄了完整的氣候變化所造成的數十個第六級的沉積層序。本研究主要針對崁下寮層底部的第一個次層序進行研究，由於此次層序的底部界面是一具有分布廣泛的侵蝕面構造，由震測剖面顯示此地層界面形成落差約數百公尺的不平整侵蝕面，其代表此時期曾經形成大範圍的侵蝕作用。 本研究主要以野外地質調查為主，研究區域北由沄水溪南至曾文溪，長約30公里共完成9處的六重溪層與崁下寮層界面露頭的研究，經岩性、沉積相、層序地層分析，探討此層序的古環境，以及不整合面形成的原因。 根據沉積相的分析，本研究認為此具不整合面的層序界面是因低水位時期的海水面下降時，在曝露陸地，以及海盆中形成下切河谷所造成的侵蝕面構造。由此層序的厚度估算，此時期海水面下降的幅度至少超過105公尺。因此，此本研究認為此時期的海水面變動是受到10萬年週期(歲差)氣候循環變化的效應所造成。

The Penglai orogeny is forming a foreland basin in the southwestern Taiwan since the Pliocene. Because the foreland basin was a highly subsidence, that it was deposited a hugely Plio-Pleistocene sediments. Since the Plio-Pleistocene, the global sea-level changes make the foreland basin for recording lot of the high-frequence sequence stratigraphy of the sixth order glacio-eustatic fluctuation (c. 41ky). Based on the well-done biostratigraphy and magnetostratigraphy of Plio-Pleistocene deposits, the study is focus on the first para-sequence from the bottom of the Kanhsialiao Formation which had been correlated to the oxygen isotope stage 36. For the purpose to understand the absolute eustatic sea-level change during the para-sequence in Taiwan, nine synchronous sections have been done by laterally tracing and depositional environments analysis. Based on the lithofacies analysis, the depositional environments indicated that the deeper offshore located in the south area and shallowing to the north subaerial environment during the lowstand sea-level stage. The coastal line can be interpreted around the Kuohsi area (the Liouchung river) where dominated by tidal-dominated shoreface environment. Indeed, following the sea-level rise epoch (transgression), the Kuohsi section became to the middle offshore and finally shallowing back to the tidal-dominated shoreface environment at the end of the sequence. Since the shoreline environment can be as a marker of the shoreline, the accommodation also can be estimate by the Kuohsi section. To consider the tectonic subsidence, there are still having 105 m to account for the eustatic sea level rise without the decompaction calculation. The result of sea level fluctuation indicates more higher than the reports deduced by the 41ky cycle in the world. Upon to the observation, the sixth-order glacio-eustatic fluctuation (c. 41ky) must be affected by fifth-order glacio-eustatic fluctuation (c. 100ky).