台灣泥火山沈積物之特性、來源與西南部石灰岩體之隱示

Abstract

摘要 台灣陸地上有許多泥火山，大多坐落於西南部的海岸平原和麓山帶，以及東部海岸山脈南段利吉層泥岩中。由於泥火山沉積物可以紀錄整個泥火山作用的過程且與下覆岩石地層有密不可分的關係，為了區辨沈積物的來源，本研究採集了陸上泥火山沈積物及其附近地層岩樣來進行沈積物粒度、黏土礦物和地球化學的分析，藉由分析結果的比對，以探討泥火山沈積物的可能來源地層。此外，有越來越多的研究結果顯示甲烷源自生性碳酸鹽的沈澱都發生在海底泥火山的表面，因此，究竟有哪些證據支持泥火山的模式，也是本研究的另一研究目的。 研究結果顯示，主要元素的哈克圖中，鋁對其他元素的投影圖呈現線性關係，這可能是受到顆粒效應影響所造成的稀釋結果。經過標準化後，獲得兩個群體的分佈，且各對比於研究區域中之南部和北部，再由主要元素和微量元素的探討，且配合沈積物粒度的比較，推測下段古亭坑層為研究區域南部泥火山沈積物的主要來源，而中崙、關子嶺地區，由於中崙層與鳥嘴層化學性質相近，藉由二氧化矽地質溫度計的計算後，推測沈積物來源可能源自於中崙層。 另外，藉由Rb/Sr、Rb×100/Ti、Al/Ti、Ba/Nb比值增加和Th/La比值降低的變化與板岩岩屑含量增加一致，但西南部所有泥火山沈積物中的元素比值皆低於含板岩岩屑的地層-上部古亭坑層和下部古亭坑層的上部，推測泥火山沈積物主要來自於下部古亭坑層或更老地層，其深度至少深於1000公尺以上，然而也藉由這些比值的變化，推測板岩出露地層的年代為早期更新世。 綜合小崗山的斷層型態、碳同位素、泥岩礫堆積、石灰岩體中的二次化石以及在西南外海的琉球嶼泥火山發育更可當為現今陸上石灰岩體的類比，顯示台灣西南部的石灰岩體應該生長於古構造高區上，而此古構造高區可能就是泥火山或是泥貫入體。

Abstract Mud volcanoes are widely distributed in the southwest foothill, plain and eastern Coastal Range of Taiwan, as well as in many locations around the world. Generally, mud volcanoes are thought to be the expulsion of fluid from overpressurized sediments. Thus, the sediments can record the processes of mud volcanism and intimately associated with the beneath sedimentary sequences. For better understanding about the sediment provenance of mud volcanoes, we have collected the samples from the mud volcanoes and sedimentary sequences in the southwest Taiwan and eastern Lichi Formation to study their particle sizes, clay minerals and geochemistry. In addition, there are growing results that methane-derived authigenic carbonate precipitation occurs on surface mud volcanoes. The aims of this article are to correlate their provenance and to develop a model for origin of coral reefs through summarizing the previous studies and our data. Several results have been drawn from the present study: (1) variation diagrams of major elements show linear trends owing to grain-size effect. After normalization, there are two distinct groups over the southwest Taiwan. One group is located in the northern part while another in the southern part; (2) the lower Gutingkeng Formation in the southern part should be the main source in terms of geochemical and particle size data; (3) the sediments of Chunglung and Kuantzuling hot springs may be originated from Chunglung Formation based on estimation of silica geothermometer and geochemical data; (4) increase in Rb/Sr, Rb×100/Ti, Al/Ti, and Ba/Nb ratios and a decrease in the Th/La ratio of the Kanhsialiao Formation beginning at early Pleistocene are consistent with input of slates in the source region. However, all mud volcano sediments show low values in these ratios and suggest that the source sediments may be derived from more than 1 km depth; (5) fault characteristics, carbon isotope data, mud breccia, reworked fossils, and analogue of Liuchieuyu mud volcano, indicate that the limestones in the southwest Taiwan may develop at local structural highs where mud volcanoes occurred.