泥岩為軟弱岩體，在受大地應力作用時會產生與堅硬岩體不同的變形行為。本研究以車瓜林斷層為例，對其進行野外地表地質調查、地質剖面清理以及岩心分析，透過由公釐至公里不同等級的中視尺度觀察，紀錄跨泥質斷層帶的構造變化及岩石特徵。車瓜林斷層位於台灣西南部的泥岩區，斷層上下盤皆為古亭坑層，是一條東北-西南走向且現今活躍的左移逆斷層，總長約22公里。經野外地表地質調查發現，車瓜林斷層自大?亭山至千秋寮一帶可以於野外良好追跡約5公里，斷層上盤為厚層泥岩偶夾薄層至中層砂岩；下盤則多為巨厚泥岩。斷層帶出露寬度約10-30公尺，產狀為黃褐色與深黑色泥質條帶所交織的寬帶，顏色上與斷層帶兩側的灰色泥岩有所差異。然而斷層帶中的細部組構在泥岩表面清理的前後有顯著的差異，因此，本研究選定車瓜林斷層的兩個露頭分別進行地質剖面及鑽井的施作，以剔除斷層帶受地表風化的因子。地質剖面及岩心分析的結果顯示，於斷層露頭上的泥質岩多具鱗片狀裂理，裂面帶有擦痕且破裂面的方向具有一致性，數十公尺的斷層帶中主要有兩種岩體單元：含砂質角礫泥岩及含黑色條帶泥岩。含砂質角礫泥岩中，砂質碎屑呈數公分至公釐的角礫或透鏡狀被泥質基質包夾；泥岩中則出現數公釐寬的密集黑色條帶狀構造並常夾有角礫狀泥質碎屑，且碎屑長軸具方向性的排列。前人對於黑色條帶的成因主要為純粹碎裂作用細粒化所致或同時有黏土礦物相的轉變，本研究所觀察到的黑色條帶常伴有擦痕及雷德剪切構造出現，故推測其為斷層作用下的產物，可作為應變集中的指標及車瓜林斷層活動的證據，亦可藉此觀察到泥岩斷層帶中應變集中的現象。然而本研究在缺少微觀的觀察狀況下，難以確定其完整的變形機制及礦物組構以探討斷層演育，仍需後續的研究使泥岩斷層帶中的構造意義更加完備。;Fault zones tend to have different architectural characteristics in different protoliths. Mudstone, as an incompetent rock, usually deform in a different way compare to competent rocks during faulting. This study focus on the Chegualin active fault which mainly appears in the argillaceous Gutingkeng formation in the southwestern Taiwan. We have conducted surface geology survey, fault zone scraping and core analysis to document fault zones at mesoscopic scale. The Chegualin fault can be well traced for 5 km long from Mt. Dabuting to Cheinchouliao in the field. The fault ranging from 10 to 30 meters wide are seemingly composed of yellowish brown and darkly black intertwined stripes distinguished from the adjacent host rocks in grey. Commonly, massive mudstone occasionally intercalated with thinly to medium bedded sandstones occurs in the hanging wall while very thickly bedded mudstone in the footwall. The result from the outcrop section mapping and core analysis indicate that there are two major rock units in the fault zone, which are sandy breccia and black gouge bands. The black bands are centimeter to millimeter in thickness and contain clasts with prefer orientations. The fault outcrops and rock samples with black bands also show scaly fabric, slickenside and Riedel shear structure. Previous studies have shown that black bands in argillaceous fault zone may be attributed to grain size reduction or clay mineral phase change along with cataclasis. Based on our observation and literature review, we infer that black bands form under the low-rate, progressive deformation of faulting and can be considered as the strain localization indicators. However, it’s difficult to completely determine the deformation mechanism and mineral assemblage because relevant microscopic observations are lacking. A further study at microscopic scale is necessary in order to resolve how a fault zone in argillaceous rock may evolve.
