臺灣西南部珊瑚礁在矽質碎屑古環境中最初發育機制之探討

Abstract

台灣西南部的更新世珊瑚礁，係分別發育於上新–更新世前陸盆地中，由逆衝斷層與背斜所形成的幾個構造高區之上。本研究係針對大崗山、小崗山、半屏山以及鳳山等地，所觀察到由深海相基底泥岩向上突然轉變為珊瑚礁石灰岩的現象進行探討，期望可由此探討各珊瑚礁的最初發育機制。野外主要調查剖面計17個，同時使用石灰岩礦場中鑽透化石珊瑚礁與基底泥岩交界處的岩心43口，進行其過渡帶的岩石組成與垂直方向的岩相遞變觀察，並且針對必要標本進行穩定碳、氧同位素組成分析。此外，選定甲仙地區四德巷、白雲仙谷以及牛埔等三個剖面，以做為與在大崗山、小崗山、半屏山以及鳳山等地基底泥岩中所見特殊碳酸鹽膠結泥岩產狀的比較，同時也藉此驗證Hayasaka（1932）關於甲仙四德巷與壽山東南側礦場泥岩頂部地層中，所含泥灰岩與大量巨帶蛤化石的記載。綜合觀察結果，總計在10個野外剖面與17口鑽探岩心中辨識出包括塊狀、管狀以及岩礫狀等不同產狀的碳酸鹽膠結泥岩；鑽取岩樣中的149件，其δ13C分析値介於 –53.7 ‰ 至 –10.9 ‰，顯示其碳酸鹽膠結物碳的來源與伴隨甲烷溢出的冷泉作用有關。由於充填白雲石質泥岩礫的漏斗狀構造與大部分的冷泉碳酸鹽岩，均發育或沉積於細粒的矽質碎屑基底泥岩；並且冷泉碳酸鹽岩的分佈相當廣泛，顯示泥岩基底中碳氫化合物逸出事件的發生相當普遍。至於大崗山地區所見岩理緻密並且含有許多大型滿月蛤貝類的塊狀冷泉碳酸鹽岩，則顯示在大崗山珊瑚礁發育之前泥岩基底曾有持續相當長時間甲烷逸出的冷泉作用。綜合岩相觀察結果，建構一個符合台灣西南部更新世珊瑚礁與基底泥岩接觸關係的沉積相模式；其中，基底泥岩頂部的不規則冲蝕面、充填碳酸鹽膠結泥岩礫與碳酸岩煙柱的漏斗狀沉積，以及塊狀冷泉碳酸鹽岩的出露，可能均為區域地質構造活動時期所造成；至於與含化石泥岩同時沉積的礫岩相，可能均為基底泥岩在快速淺化的過程中所沉積，標誌古環境由矽質碎屑（非碳酸鹽）快速轉變為碳酸鹽環境的過程。此一基底泥岩快速淺化與岩相驟變的現象，可能主要是受到地體構造活動的影響，而與更新世前陸盆地中逆衝斷層的向西推進有關。在此基底泥岩快速淺化的過程中，出露的塊狀冷泉碳酸鹽岩成為不穩定地體構造背景中，珊瑚與珊瑚藻等表覆生物在矽質碎屑環境生長的硬底質，可能在礁體的最初發育扮演關鍵性的角色；此種冷泉碳酸鹽岩成為造礁珊瑚與珊瑚藻在矽質碎屑古環境中最初生長硬底質的現象，很可能為世界首例。

Pleistocene scleractinian reefs in southwestern Taiwan developed on several local structural highs that were closely associated with anticlines and thrust faults in a Plio-Pleistocene foreland basin. This study focused on the abrupt facies changes from the underlying terrigenous mudstone of the deep-water facies upward into the reefal limestones of coral reefs, including the Takangshan (TKS), Hsiaokangshan (HKS), Panpingshan (PPS), and Fengshan (FS). To reveal the initial mechanisms of reef development, I examined the lithologies and vertical facies changes of 17 outcrops and 43 borehole cores from quarries of these reefal limestones and performed petrographic and isotopic studies. In addition, 3 outcrops in the Chiahsien area were involved into this study, which including Ssutehsiang, Paiyunhsiangku, and Niupu. Purposes of this fieldwork have two folds. One is to compare the specific occurrences of carbonate-cemented mudstones in TKS, HKS, PPS, FS, and those in the Chiahsien area. The other is to recheck Hayasaka’s descriptions (1932) about the unique occurrences of fossil lucinids bivalves and fossiliferous marls of Chiahsien and the southeastern Shoushan. Various occurrences of dolomitic mudstone were observed from 10 outcrops and in 17 borehole cores, containing massive dolomitic mudstones, carbonate pipes, dolomitic cobbles, and dolomitic pebbles. The δ13C values of 149 samples ranged from –53.7 ‰ to –10.9 ‰, indicating that the carbonate cements of these mudstones were all cold-seep carbonates in origin. Majorities of the cold-seep carbonates and funnel-shaped structure packed with dolomitic cobbles were precipitated and formed within the fine-grained siliciclastic mudstones. The wide occurrences of seep carbonates in the study area suggesting the hydrocarbon seeping occurred extensively. The compactness and association of large lucinid bivalves in massive cold-seep carbonates further indicate pronounced, long-lasting seepage of methane occurred antecedently the development of Takangshan Reef. A schematic model was proposed to illustrate the occurrences of various lithologies and lithofacies associations. The erosional surfaces on siliciclastic mudstones, funnel-shaped structure, and the exhuming of massive cold-seep carbonates exposed might have been occurred concurrently during a tectonic unstable time in SW Taiwan. The deposition of fossiliferous mudstone interfingered with the conglomerate lithofacies, and represented the rapid facies transition from siliciclastic (non-carbonate) into carbonate environment. The root of this rapid facies change was presumed to be tectonic movement, probably related to the westernward thrust migration in the Pleistocene foreland basin. The exposed massive cold-seep carbonates provided substrate for the encrustations of corals as well as coralline algae and might have played a crucial role in the initial development of coral reefs in a siliciclastic paleoenvironment of active tectonic setting. This is probable the first case of the world that cold-seep carbonates acting as initial colonization hardgrounds of hermatypic corals and corallines.