Late Quaternary landform evolution and sedimentary successions in the Miaoli Tableland, northwestern Taiwan

Abstract

Abstract. Elevated Quaternary sedimentary complexes in the western foreland of the central mountain ranges of Taiwan are called tablelands. Their mostly flat surfaces are deeply incised by fluvial processes. The landforms and the fluvial systems in the Miaoli Tableland are investigated by high-resolution terrain analyses based on different datasets. Sediments are described in 51 outcrops and characterized by grain size composition. The outcrops revealed complete or incomplete sequences of the general scheme from bottom to top: sandy tidal–coastal units overlain by gravel- and cobble-rich fluvial deposits always with a fine-grained silt-rich top cover layer influenced by aeolian deposits. All layers are unconsolidated sediments. Three subtypes of this sequence were identified, with respect to the occurrence of the fluvial deposits. The relation of tectonic and erosional processes including the rework of gravels is discussed. The results reveal a tableland surface much more disaggregated than previously mapped, suggesting that individual tableland segments represent remnants of an inferred palaeotopography. The tableland surfaces have been separated into Sedimentary Highlands (SH-I and SH-II) and Sedimentary Terraces (ST) by geometrical properties. The Alluvial and Coastal Plains (AL) represent broad valley bottoms (“box-shaped valleys”) in the dendritic drainage systems below 150 m and the coastal plains. The landforms and predominantly the sediment sequences are discussed in the context of the existing stratigraphical schemes of the Toukoshan Formation and the so far rarely used Lungkang Formation. The latter is recommended as the stratigraphical term for the refined subdivision of the uppermost part of late Quaternary sediments in the Miaoli Tableland.