| Summary: | 碩士 === 國立成功大學 === 地球科學系碩博士班 === 100 === For a better understanding on the sedimentary environment evolution and major flood events occurred since 10,000 BP on the Pingtung and Ilan Plains, ten cores with a total length of 400 m were analyzed. The sedimentation rates were then calculated in an attempt to correlate with global base-level change. Furthermore, the relationship between fluvial stacking pattern, sea-level change and sedimentation rate in these areas were also reconstructed.
Based on the characteristics of core sediments, such as grain size and sedimentary structures, 10 lithofacies (1 gravel facies, 5 sand facies and 4 mud facies) are identified, and can be grouped into 4 facies assemblages. They represent four braided river sub-environments: longitudinal bar, sandy channel, crevasse splay and floodplain.
The lower part of the six Pingtung Plain cores is dominated by floodplain mud, overlain by multistory channel sands and thin floodplain mud layers. In places in the upper core, 1m-thick longitudinal clast-supported gravel beds overlay the floodplain mud, which is interpreted as high-energy flood deposits.
The four Ilan Plain cores consist of amalgamated-clast supported gravel of longitudinal bars. But at the bottom of the Tian Song Pi core, floodplain muds interbed with channel sands, which probably represent low-energy fine-grained deposits during formed channel migration.
Combining results obtained from this study and previous research in the Pingtung Plain, it is suggested that the sediments were transported westward from Central Ridge since 10,000 BP, and there were at least two major flood events since 6,000 BP. In the Ilan Plain, the sediments were transported eastwardly, and three major flood events in the last 5,000 years have been recorded.
Eight wood fragments were collected from mud beds for dating from 4 Pingtung Plain cores: Lu Liao, Fu Tian, Zhu Tian and Mei Her. The age of beds at 20~37 m ranges from 6,000 BP to 7,400 BP, giving an average sedimentation rate of 3~5 m per 1,000 years. There is no age dates in Ilan Plain cores.
The average sedimentation rates decrease both eastward and westward from the central Pingtung Plain, and reach the maximum southward toward the shoreline. The variation in sedimentation rates are possibly resulted from both tectonic and palaeotopographic controls: high accommodation created by an incised valley since the Last Glacial maximum and the development of Pingtung syncline in the central plain, and a southward tipping of the deposition surface.
The sedimentation rates decrease both northward and southward from the central Ilan Plain. It may result from the extra accommodation space generated by an incised valley and the Okinawa Trough. And the eastward increase in sedimentation rate might also be caused by the tilting of the deposition surface in the same direction.
The sedimentation rates between 10,000 BP and 7,000 BP are greater than those from 7,000 BP to the Present in both study areas, which might be related to the sea-level rise after the Last Glacial. The higher sedimentation rate and floodplain-dominated mud with single-storey (isolated) channel sands in the lower part of the cores (10,000 BP to 7,000 BP) suggest deposition during rising sea level and higher rates of generation in accommodation, which in turn is interpreted as a transgressive systems tract.
The upper part of cores (between 7,000 BP and present) is dominated by amalgamated multistorey channel sands. And the lower sedimentation rates in this interval suggest a reduction in accommodation space during sea-level highstand, and probably represent a highstand systems tract.
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