台灣恆春半島中新世樂水砂岩之沉積環境與沉積作用研究;A study on the depositional environment and depositional processes of the Miocene Loshui Sandstone, Hengchun Peninsula, Taiwan

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Title:	台灣恆春半島中新世樂水砂岩之沉積環境與沉積作用研究;A study on the depositional environment and depositional processes of the Miocene Loshui Sandstone, Hengchun Peninsula, Taiwan
Authors:	李健瑀;Li, Jian-Yu
Contributors:	地球科學學系
Keywords:	恆春半島;濁流岩;馬可夫鏈統計分析;岩相;岩相組合
Date:	2020-07-28
Issue Date:	2020-09-02 15:37:12 (UTC+8)
Publisher:	國立中央大學
Abstract:	本研究針對恆春半島東側，沿佳樂水至九棚海岸出露之中新世樂水砂岩，建立總厚度約1,500公尺的詳細地層柱，進行岩相與古沉積環境分析。由岩相組合與重力流事件層堆積，可瞭解沉積樂水砂岩深海扇之重力流流動機制及沉積作用，及古沉積環境演育。本研究共分類出17種岩相，包含7種濁流岩相、8種混合事件層岩相、1種雜亂堆積岩相及1種泥岩相。再透過馬可夫鏈統計分析及卡方檢定，進一步分析出各岩相的變異相。在15種事件層岩相當中，濁流岩岩相比例佔93.6%，遠高於混合事件層岩相比例 (6.4%)。並佐以幾種常見岩相 (如：中厚混合事件層，頂部含岩屑流層) 的沉積機制，指示出古沉積環境位於深海扇近端。由事件層連續變化，可再進一步劃分出9種岩相組合，各自代表不同地形單元，分別由距沉積物供應源頭最近的海底峽谷至深海扇過渡帶，分布至較遠的近端中部深海扇及近端離軸深海扇。透過事件層厚度及岩相組合連續變化，顯現三次向上增厚再減薄的長週期循環，解釋為三次深海扇進積再退積的循環，並且堆積了三期代表距沉積物供應源頭最近的巨厚型事件層岩相。樂水砂岩深海扇堆積於被動大陸邊緣之海底平原，卻能形成厚度至少達1,500公尺厚，並以厚層事件層為主的濁流岩層序。如此巨厚的濁流岩層序，暗示在沉積同時期，盆地納積空間持續增加。前人研究已指出晚期中新世時期 (即為樂水砂岩堆積時期) 外大陸邊緣產生張裂活動，本研究推測樂水砂岩乃堆積於大陸斜坡坡底的張裂盆地中。另外，由樂水砂岩所量得之古水流流向為由南往北，顯示為南高北低的古海床地形變化。此地形變化不同於古地理所顯示的由西北往東南變深的大陸斜坡趨勢。本研究認為樂水砂岩深海扇的局部地形變化可能受控於此張裂盆地邊界正斷層的位態。 ;The Loshui Sandstone, a Miocene turbidite succession with a nominal thickness in 1,500 m, is exposed in the eastern Hengchun Peninsula, Taiwan. We conduct lithofacies and facies association analysis to understand the mechanisms, depositional processes and paleo-environmental evolutions of the event-bed succession. The Markov chain stochastic analysis and Chi-squared test have been performed both to analyze the sequential development of sedimentary structures and lithofacies. Seventeen lithofacies with specific thickness, lithology and internal structure are identified. These lithofacies include seven turbidite facies, eight hybrid event-bed facies, one chaotic slumped-bed facies and one hemipelagic mud facies. Within fifteen event-bed facies, turbidite facies (93.6%) is the major facies type comparing to hybrid event-bed facies (6.4%). The paleoenvironment is interpreted as proximal submarine lobes of a deep-sea fan as judged from flow mechanisms of dominant lithofacies. Nine facies associations, corresponding to various morphological units, ranging from canyon-fan transition to proximal mid-fan lobe and proximal off-axis lobe. Continuous vertical variations of event-bed thickness and facies association show three long-term cycles of thickening then thinning upward trends, denoting three cycles of progradation and retrogradation. The interpreted submarine fan located at the base of continental slope of a passive continental margin. The vertically aggrading turbidite succession is dominated by thickly-bedded event beds, amounting to 1,500 m in thickness. This indicates that the rate of basin subsidence is keeping pace with the rate of sediment supply. We interpret that the late Miocene Loshui Sandstone is accumulated in a rifted basin along the base of continental slope in a passive continental margin. Furthermore, the northward-directed paleocurrents as measured from the Loshui Sandstone demonstrate that the depositional trough of the Loshui submarine fan deepens toward the north. This northward deepening trend is contradicted to the regional physiographical feature of the continental slope, which exhibits deepening toward the southeast. We interpret this local topographic variation for the Loshui submarine fan may be constrained by the attitude of the border fault that forms the rift basin.
Appears in Collections:	[Graduate Institute of Geophysics] Electronic Thesis & Dissertation

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