海岸山脈泥岩區一萬三千年來的間歇性侵蝕事件－來自岩芯中移置超微化石的證據

Abstract

研究山脈侵蝕是了解地表地質作用過程中重要的議題，而山前盆地沉積的研究可以作為了解山脈侵蝕歷史的方法之一，但並非盆地中所有的沉積物都能分辨出其來源。中央山脈及海岸山脈(八里灣層，蕃薯寮層)都含有板岩或其碎屑，因此，花東縱谷中具有板岩碎屑的泥質沉積物很難斷定到底是來自中央山脈還是海岸山脈。近年來中央地質調查所在花蓮平原鑽取了數口岩芯，發現這些岩芯一萬多年來的沉積物中含有移置而來的晚中新世至更新世超微化石；本研究進而確認這些超微化石係侵蝕自海岸山脈八里灣層及蕃薯寮層，因此可作為指標，用以探討海岸山脈侵蝕事件及古花蓮溪的歷史。 本研究利用定量化超微化石計數方法，對花蓮平原岩芯及花蓮溪流域泥質沉積物加以分析，共分析了221個樣本，配合中央地質調查所岩芯沉積相分析及花蓮溪流域地形觀察，發現： 1. 花蓮平原岩芯一萬多年來的沉積物中移置而來的超微化石乃源自海岸山脈地層，經花蓮溪攜帶而沉積。晚冰期至全新世早期花蓮溪最北可氾濫至北埔地區。 2. 現生花蓮溪河口沉積物移置超微化石量在颱風或雨季時增多。表示海岸山脈泥岩區的侵蝕量明顯增加，也反映中央山脈的侵蝕量增加所導致的化石被稀釋的現象不彰，凸顯侵蝕作用與岩性的密切關連。 3. 海岸山脈在全新世早期約10. 4ka有一明顯的侵蝕事件，使得北埔井及明義井都留下大量移置超微化石的紀錄。

The erosion of mountain belt is a major topic in studying Earth surface processes, which can be resulted from tectonic uplift or climate change and is often discussed in this tectonic active island. Basin deposits in front of mountain belt provide sedimentary sequence for studying mountain erosion. The sediments in the Longitudinal Valley of Eastern Taiwan come from both Eastern Coastal Range and Central Range. It is difficult to discriminate their sources since both Ranges export slate detritus into the sediments. These several years, many cores were drilled from the Hualien Plain by Central Geology Survey in Taiwan and some reworked nannofossils in the core sediments were found. This study further discerned that the reworked nannofossils were derived from the strata of Coastal Range. The reworked nannofossils are thus taken as an indicator for studying erosion history of the Eastern Coastal Range since late glacial. In order to measure the abundance of reworked nannofossils, analysis in this study was made quantitatively both in the four core sediments of Hualien Plain (Beipu, Minyi,Tsichi and Kuanhua) and the present muddy sediments of the Hualien River. Totally 221 samples were analyzed. The preliminary conclusions are: 1. The Hualien River flooded further north than present, to Beipu area, during late Glacial and early Holocene. 2. After typhoon the abundance of reworked nannofossils in muddy sediments of Hualien River still increased even though Central Range also provides lots of sediments to dilute them. This may indicate the close relationship between erosion rate and lithology. 3. A remarkable erosion- event occurred in the Eastern Coastal Range during early Holocene (about 10.4 ka) which was evidenced by abundant reworked nannofossils in core sediments of Beipu and Minyi of Hualien Plain.