紅土是林口台地的重要地質特徵之一，常見於台灣北部第四紀環境變遷之討論，惟其特性仍未被充份瞭解，其成因也一直仍有爭議。本研究以野外勘查、Ｘ光粉末繞射、光學及電子顯微岩象分析和主要及微量元素與鍶、釹同位素地球化學等方法綜合探討林口紅土之來源。
林口層由黏土、粉砂粒為主、厚約十公尺之紅土及風化砂岩和變質石英砂岩礫石與砂、泥基質為主之厚層礫石所組成，紅土下層由具有白色斑塊之斑紋帶所組成。紅土層呈現從「禾樂石＋赤鐵礦＋伊萊石」至「高嶺石＋針鐵礦＋伊萊石／膨潤石混層＋蛭石＋三水鋁石」之向上轉變趨勢，反映一個現地風化剖面。然而跨越紅土和礫石層界面之野外產狀及礦物組成及性質之不連續和可淋離（如鎂、鉀）及非遷移性（如鋯、鍶、鈮）元素之異常行為不符合一般風化剖面之特徵，顯示紅土母質並非其下覆之礫岩。紅土之粒徑、組成和涉及細粒伊萊石優勢排列之微組構的存在進一步指示其前身可能以細粒泥砂沉積物為主，而具高溫偏析產狀之鈦磁鐵礦的出現暗示亦含有少量安山岩質凝灰碎屑來源。铷鍶和釤釹同位素訊號之相似性顯示紅土母質與礫岩具有相同之主要沉積物來源。
整體資料顯示林口紅土是由與礫岩同源之細粒泥砂沈積物現地風化化育而成，鐵離子有向下遷移之趨勢，使紅土及其下礫岩層之界面部份有鐵絕對富集之特徵，斑紋帶之形成可能涉及後續地下水面之昇降所造成的元素遷移。

Linkou laterite is one of the most important geological features of Linkou Tableland, often discussed in the context of Quaternary environmental changes in northern Taiwan. This study aims to discuss the origin and formation processes of Linkou laterite based on field investigation, X-ray powder diffraction data, optical and electron microscopic petrography, major and trace element analyses, and Sr and Nd isotope geochemistry.
Linkou Formation consists of a ~10-m thick, clay- and silt-dominated laterite layer with a mottle zone containing irregular white domains in its lower portion, and a thick conglomerate bed composed mainly of metamorphosed quartz sandstone and weathered sandstone conglomerates and sand-mud matrix. An upward mineralogical change from halloysite + hematite + illite to kaolinite + goethite + illite/smectite + vermiculite + gibbsite in the laterite appears to suggest an in-situ weathering section. However, the discontinuity in field occurrence and mineralogical constituents and properties and the abnormal behaviors of leachable (i.e., Mg, K) and relatively immobile (i.e., Zr, Sr, Nb, etc.) elements across the laterite-conglomerate contact are uncharacteristic of common weathering profiles and suggest that the underlying conglomerates is not the laterite’s precursor. The grain size, constituent, and microstructural features involving preferred orientation of fine-grained illite grains further imply that the laterite’s parent material consisted mainly of confine-grained sediments with a minor contribution of andesitic tuffaceous debris including titanomagnetite having exsolution features similar to those in the nearby andesitic rocks. However, the laterite precursor and conglomerates were probably derived from the same principal sediment provenance as indicated by similarities in Rb-Sr and Sm-Nd isotope signals.
The data collectively suggest that the Linkou laterite was formed by in-situ weathering of fine-grained sediments congenerous to the underlying conglomerates, with a downward migration of iron ions giving rise to an absolute enrichment of iron in the vicinity of the contact between the laterite and conglomerate layers. The formation of the mottle zone may have involved subsequent elemental mobilization due to cyclic changes of groundwater level.

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