化石保存之構造變形－以苗栗地區奇異掘海膽化石為例

Abstract

西部麓山帶為臺灣地質構造分區之一，可細分為內麓山帶及外麓山帶。內麓山帶以山脈居多，富含中新統地層；外麓山帶則以丘陵、台地為地形特徵，內含上新統、更新統地層。約從六百萬年前蓬萊造山運動開始，西部麓山帶因著構造變動而留下不少斷層與褶皺，這些構造同時也影響到臺灣海膽化石之產狀，因部分樣本受到應力而變形、破裂，導致前人研究古生物形體與生態時必須將之排除。然而若換個角度想，這些樣本可以用於探討其出產地層在構造變動過程中之變化。 本研究以苗栗頭嵙山層香山段出產的奇異掘海膽Scaphechinus mirabilis為研究題材，其露頭集中於出磺坑背斜與三義斷層以西直到海岸邊，北起中港溪，南抵大安溪，分屬外麓山帶。由於化石破裂與變形成因不僅有構造應力和沉積物壓力等因素，若作用在沙錢海膽埋藏弱面時也可能會造成標本破裂。利用X光掃描與偏光顯微鏡觀察變形及破裂化石之內部結構，以虛擬震源機制解標示破裂面錯動方向，藉此分辨兩者的差別。在總計708件研究樣本中，10件用Micro-CT掃描分析，以及16件用於製作化石薄片，一共有28個薄片（包括前人留存的4個薄片）。針對任何可能是奇異掘海膽遺骸成為化石後，因為外界應力擠壓而造成之變形及破裂，並與前人模擬臺灣造山的砂箱實驗結果、車籠埔斷層槽溝做比較，藉機探討變形化石在區域地質構造上的應用潛力。 研究結果可確認部分樣本保持有與化石平行的側向應力造成之剪力破裂，類似斷層誘發之單斜褶皺、箱型褶皺之地質現象，或與周圍高硬度礦物顆粒接觸所產生之壓溶現象，可視為構造應力作用之化石證據，增添相關地質教科書新範例。

The Western Foothills is one of the geologic regions in Taiwan. It can be divided further in the Inner Foothills with mountainous terrains and Miocene strata and the Outer Foothills with hills and terraces and Plio-Pleistocene strata. Faults and folds have been occurring in this region since the Penglai orogeny that began around 6 Ma. These major regional structures have also affected the fossil preservation in Taiwan. Some samples were found to be deformed due to stress. Although they do not contain perfect biologic outlines of the fossil and are often being discarded by amateur collectors, some of them may provide clues about the regional tectonic history. The goal is to study and document the deformed and fractured specimens of the fossil sand dollar Scaphechinus mirabilis from the Xiangshan Member of the Toukoshan Formation in Miaoli County, Taiwan. Fossil occurrences are concentrated on the western sides of the Chuhuangkeng Anticline and the Sanyi Fault, reaching to the coastline; from the Zhonggang River in the north to the Daan River in the south. Fossiliferous outcrops belong to the Outer Foothills. On the other hand, fossil deformations and fractures can be caused not only by either tectonism and/or overburden pressures due to sedimentary loading, fragmentation can occur along the taphonomic weak planes of sand dollar tests. By making thin sections and analyzing some rare specimens with exceptional deformation features under the Micro-CT tomographic imaging, the interior of deformed or fractured specimens can be revealed and reconstructed. Furthermore, virtual focal mechanism is adopted here to illustrate the geometry of the fault plane. Among 708 specimens examined, 10 samples were analyzed further under Micro-CT and 16 samples were sent for making thin sections (a total of 28 thin sections) Any deformation and/or fracture features due to ancient earthquakes are documented in detail. Comparisons with previous works on sandbox models of Taiwan orogeny and Chenlungpu Fault are discussed. Results show that some rare samples with deformation features are comparable to the ones associated with the Chenlungpu Fault in terms of geometry. Fault-triggered structures, such as box folds and monocline, are documented from the sand dollar fossils from Taiwan for the first time. Pressure dissolution due to harder quartz pebbles acting upon calcareous sand dollar tests as a direct indication of region tectonism is also reported. Deformed fossils illustrated are exceptional and should be considered as textbook examples for structure geology in the future.