| Summary: | 碩士 === 國立臺灣大學 === 地質學研究所 === 87 === In order to understand the sedimentary environmental changes since the Last Glacial and detailed correlations of the sediments in southern Pingdong Plain, 14 sediment cores have been analyzed. Using core descriptions, observations of recent sedimentary environment, facies and sequence analysis of core sediments, the environmental evolution of southern Pingdong Plain can be constructed.
According to the characteristics of core sediments, such as sedimentary structures, bioturbations, shell and plant fragments, 21 kinds of lithofacies can be recognized. These include 3 kinds of conglomerate facies, 6 kinds of sandstone facies, 3 kinds of interbedded facies and 9 kinds of mudstone facies. From the assemblages of lithofacies, 4 kinds of sedimentary systems and 14 kinds of sedimentary facies can therefore be constructed. According to the water-depth changes delineated from sedimentary facies, one maximum flooding surface (MFSa), one sequence boundary (SB1) and two sequences (SQ0, SQ1) exist in every core. The SB1 formed about 15000 to 18000 years ago, and the MFSa formed about 8500 to 9500 years ago. The sediments in SQ1 deposited due to the rising sea level after the Last Glacial, and the sediments in SQ0 should deposit in earlier times.
In the Last Glacial Maximum (LGM), an incised valley might form in southern Pingdong Plain. With the influence of regional tectonic activities, the valley became a wider and deeper basin. About 12000 to 15000 years ago, estuarine sediments started to occur in some cores to the south. About 10000 to 12000 years ago, a distinct estuary system formed in the basin, including lagoons and bay-head deltas. About 8000 to 10000 years ago, the region of transgression became the largest, and MFSa formed. After 8000 years ago, the speed of transgression slowed down, sediments prograded, and this area has gradually become the present Pingdong Plain.
The sediments of southern Pingdong Plain were affected by sea level change, tectonic activities and climatic changes. Besides, the late Quaternary marine sediments of plain areas around Taiwan show different features because of differences in locations, topographic characteristics and tectonic settings. In the future, we need to develop multi-disciplinary researches in order to understand more delicate environmental changes.
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