| Summary: | 碩士 === 國立臺灣大學 === 地質科學研究所 === 106 === In the northern tip of the active Taiwan Mountain Belt, a Quaternary extensional basin, namely Taipei Basin, has developed atop the deformed orogenic basement as a result of post-orogenic collapse, and comprises 670m siliciclastic sediments infill. This study provides preliminary stratigraphic architecture and facies analysis on 37 cores to display the nature of basin filling and its evolution.
The basin deposits could be divided into five stratigraphical units, which, from the bottom to the top, are lower member of Banchiao Formation (400-180ka), upper member of Banchiao Formation (180-160ka), Wuku Formation (160-30ka), Jingmei Formation (30-23ka), and Songshan Formation (23-0ka). These deposits could also be classified into 11 lithofacies, which represent alluvial fan, braided river, floodplain, lake, inner estuary, central estuary and volcanic ring plain facies.
The lower member of Banchiao Formation exhibits a ribbon-like alluvial-fan gravels intercalating with volcanic deposits in the northwest. The upper member of Banchiao Formation is a planar lacustrine mud layer extending to whole basin. The upper 3 units each appears like a sedimentary wedge that thickens toward the bounding fault and outlet of the basin, except the Jingmei Formation is composed of alluvial-fan gravels whose depocenter is on the southwestern basin margin. Drastic facies change and their stacking pattern imply the fluvial system in this basin was modulated by glacio-eustatic fluctuations since MIS (Marine Isotopic Stage) 5. However, stratigraphic geometry and facies distribution demonstrate the complex interplay among bounded fault, volcanos and inherited paleo-topography, which are predominate controlling factors in this collapse basin.
By integrating Taipei Basin stratigraphy with regional geology, it can be inferred that the basin started accumulating sediment along a paleo-valley inherited from the mountain building stage. As the bounding normal fault formed in consequence of orogenic collapse, its hanging wall slid down and captured paleo- Xindian River into a depression that began to pond gravels as the result of Tatun Volcanoes Group growing in the north at 400ka. Meanwhile, deposits from surrounding volcanoes jammed up the paleo-valley, and eventually dammed up the Taipei Basin, forming a deep tropical lake at 180ka. When the lake levee was broken down at 160ka, basin subsidence and bounded fault expansion enlarged the accommodation space, the basin was occupied by widespread fluvio-estuarine deposits, and its tidal river reached the basin at sea-level highstand of MIS5 and 3 as well. Later, a large amount of sediments was fed from a large captured river in the Last Glacial Age to deposit alluvial fan gravels. When sea level rose since Last Glacial Maximum, the basin was first inundated as a large estuary and then filled up by inner estuarine and fluvial sediments.
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