| Summary: | 碩士 === 國立臺灣大學 === 地質科學研究所 === 106 === Lithostratigraphy, magnetostratigraphy, depositional environment and sandstone compositions of foreland basin deposits in western Taiwan, from late Miocene to Pleistocene strata, have been systematically researched. According to the petrographic studies of foreland basin deposits, sedimentary provenance originated from southeastern China (the Cathaysia Block) in Eocene-Miocene, but progressively switched to Taiwan orogenic belt due to the arc-continental collision since late Miocene (~6.0 Ma). Furthermore, the Hsuehshan Range and the Western Foothills were uplifted and eroded in sequence during Pliocene to Pleistocene. In this study, we focus on detrital zircon U-Pb dating and petrographic analysis of sandstones collected from late Miocene to Pleistocene strata, aiming to understand the origin of sedimentary provenance and the exhumation history of the western flank of Taiwan orogenic belt.
The results of petrography show that sedimentary and low-grade metamorphic lithics fragments gradually increased from late Miocene to Pleistocene in sequence. In addition, the detrital zircon U-Pb age populations of late Miocene-Pleistocene strata are similar to those of Eocene-Miocene strata, suggesting that the synorogenic sediments from late Miocene to Pleistocene were derived from the orogenic belt. Based on the characteristics of U-Pb ages, lithology and metamorphic grade of Eocene-Miocene strata, we classified the provenances into Eocene, Oligocene and Miocene formations. We utilize statistical analysis to quantify the proportion of the exposed provenances of each sample, which can reveal the relation between source and sink thoroughly.
In northern Taiwan, the Hsuehshan Range was uplifted during late Miocene. Foreland basin received the sediments from the Miocene strata of Hsuehshan Range since Pliocene. During early Pleistocene (2.6-2.3 Ma), Oligocene strata of Hsuehshan Range started to expose and had 1-9% proportion among the source area. Subsequently, Oligocene strata widely exposed after 2.3 Ma (with 17-40% proportion among the source area). The drainage system have begun to carry the sediments from Eocene strata of Hsuehshan Range (1-9% proportion among the source area) since late Pleistocene (~1.46 Ma). After that, Eocene strata widely exposed (with 28% proportion of the source area) in the meantime as Linkou formation deposited.
In central Taiwan, the Hsuehshan Range was also uplifted during late Miocene. Foreland basin also received the sediments from the Miocene strata of Hsuehshan Range since Pliocene. During early Pleistocene (2.1-1.0 Ma), Oligocene strata of Hsuehshan Range started to expose and had 5-15% proportion among the source area. Eocene strata of Hsuehshan Range started to widely expose since late Pleistocene (~1.0 Ma) with 46% proportion among the source area. During 1.0-0.7 Ma, the sum of proportion of Eocene and Oligocene strata gradually decreased (from 60% to 23%), in converse, proportion of Miocene strata gradually increased (from 40% to 77%), which suggested that the Miocene strata of Western Foothills were widely eroded. According to the statistical analysis of the upper Toukoshan deposit (~0.7-0.3 Ma), the proportion of the provenances were similar to the framework of mountain nowadays.
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