| 摘要: | 本研究分析臺灣西北部中新統至更新統頁岩之黏土礦物種類與其相對豐度、砂岩之礦物成分，並探討同一岩層於不同地點、不同埋藏深度所經歷之成岩作用，研究樣本取自桃園台地觀音一號井岩心及桃園台地東側之大漢溪、竹坑溪以及三峽溪(以麓山帶稱之)露頭樣本。觀音一號井所取之岩心由淺至深為頭嵙山層至大寮層(250 至2603 公尺，更新世至中新世)，共計30 份樣本；麓山帶從年輕至老之地層為錦水頁岩至木山層(各地層最大埋深估計為2550 至5200 公尺，上新世至中新世)，共計35 份樣本。結果顯示觀音一號井樣本之膨脹性黏土礦物(如膨潤石)含量高，且隨埋藏深度增加，膨脹性和非膨脹性黏土礦物(如伊萊石)含量無明顯改變。麓山帶樣本中，膨潤石、混層伊萊石/膨潤石隨埋藏深度增加而減少；伊萊石則普遍含量豐富。同時，4000 公尺以下之樣本出現少量有序性混層伊萊石/膨潤石。於淺部地層樣本中，伊萊石含量高且和膨脹性黏土並存。本研究另討論伊萊石結晶度變化和顯微分析。麓山帶地層中，最大埋藏深度超過4500 公尺(包含石底層下部及下伏之地層)，伊萊石結晶度範圍為0.28 至0.44 。2θ；埋深小於4500公尺地層中(觀音一號井和麓山帶中石底層上部及其上伏之地層)，伊萊石結晶度較高，範圍為0.26 至0.40 。2θ。由於伊萊石為碎屑性來源為主，透過電子顯微鏡(SEM)的礦物元素分析，觀察到深部地層中部份外來伊萊石置換成混層伊萊石/膨潤石，導致少數樣本之伊萊石結晶度稍低。另外，由元素能量散射分析(EDS)可得，麓山帶中新世地層中，自生性混層伊萊石/膨潤石的元素組成隨埋藏深度增加而變化：鐵、鎂離子減少；矽、鋁離子增加。從膨脹性黏土含量隨埋藏深度減少、有序性混層礦物出現、顯微礦物觀察以及混層礦物之元素組成可得: 觀音一號井與麓山帶之中新統至更新統岩石雖為相同沉積物來源，但因岩石埋深不同(麓山帶地層較深)，導致麓山帶之成岩作用程度較觀音一號井地層高。 We analyzed clay minerals and their relative abundances of Miocene to Pleistocene shales as well as mineral compositions of sandstones sampled from northwestern Taiwan. Results of vertical changes on clay mineral species and abundances yield information on degree of burial diagenesis along with possible rock types in sediment source terrains. Samples were collected from two localities, one is from the core materials of KY‐1 well, locating on the western coast of the Taoyuan Tableland, a series of 30 samples was collected from the Pleistocene Toukoshan Formation to the Miocene Taliao Formation, ranging from 181 to 2603 m in depth. The other is from three outcrop sections along the Dahan River, Tzukeng River and Sanshia River, respectively, in the NW Foothills. We collected 35 samples from the Pliocene Chinshui Shale to the Miocene Mushan Formation with an estimated maximum burial depth ranging in between 2550 to 5200 m deep. Results from X‐ray powder diffraction analysis on collected samples show that KY‐1 samples are rich in expandable clays, mainly smectite and minor mixed‐layer illite/smectite. There is no characteristic vertical change for the relative abundances of expanding clays and non‐expanding clays (e.g., illite) at the KY‐1 well. For the samples from the Foothills, the abundance of expanding clays is high in the upper section and decreases its relative abundance when increasing burial depths. In addition, it appears that the depth range of 4000 to 4500 m reveals an ordering mixed‐layered illite/smectite. We therefore interpret that the above depth range is the depth where significant transformation of smectite to illite occurs. However, illites are generally high in abundance throughout the composite sections of the Foothill. As illite is abundant for all studied samples and it coexists with large amount of expanding clays in rocks experiencing shallow burial, we examined the illite crystallinity in order to understand its origin. For burial depths shallower than 4000 m (i.e. for all formations at the KY‐1 well and Peiliao and younger formations in the foothills), illites show a crystallinity with a Kübler Index of 0.25 to 0.40 。2θ, indicating that most of the illites are sourced from low‐grade metamorphic terrains. For burial depths deeper than 4000 m (i.e. for Shiti and older formations in the Foothills), some of illites show lower crystallinity with a Kübler Index of 0.40 to 0.44 。2θ. Through microscopic and SEM/EDS analysis in the samples collected from the Foothill regions, the chemical compositions changed among the mixed‐layered Illite/smectite, which were iron and magnesium ions decreased and silicon and aluminum ions increased with increasing depth. Moreover, in the deeper burial depth, some of detrital illites were altered to mixed‐layered illite/smectite, which is probably the reason why decreasing illite crystallinity decreases in the deeper depth in the Foothills. These results indicate that the strata in the Foorhill region experienced more diagenetic effects than strata in the KY‐1 well. |
