| 摘要: | The lateritic gravel tablelands in Taiwan are mostly distributed at the north of Choshuichi and the west of foothill area. It is one of the major terrains between Taipei and Taichung basins. They are spacious and flat in most part. Originally, they are fan-deltas deposited at the foothills dipping seaward during the middle and late Pleistocene.
Following several intermittent uplift movements, these fan-deltas have become a group of terraces, which are composed of gravel beds and covered by lateritic soils. Each of these tablelands differs in altitude, in thickness of soils and gravels, and also in dissection. Some of them retain a large part of the original surface, while the others have been dissected into hill country.
Seven tablelands have been selected, which can be classified into three groups:
(1) less-dissected Taoyuan and Houli tablelands, (2) partly dissected Linkou and Tatu tablelands, and (3) strongly dissected Chutung, Chunan, and Miaoli table- lands. First of all, their altitude, slope, relative height, and dissection degree are measured and analyzed from the maps of 1:25,000 scale maps. The data thus obtained are compared with one another in order to reveal the similarity and the difference in land-forms of these tablelands. Next, the lateritic soils are also analyzed by grain size foi their composition, pattern and spatial difference. Finally, the lateritic soils are discussed in relation to tableland sedimentation environment, tableland stage correlation, degree of dissection, and gravel layer sequence. Eight conclusions are obtained as follows:
(1) The tablelands studied have altitudes between 50-300. The difference in altitude may be interpreted as a result of the times and the locations of its formations, that is, the earlier the period of its formation and the stronger tendency to the southward from its original location, the higher is its altitude. The slopes of most tablelands are between l0°-25°, and the relative altitude between 50-l50m; both values tend to increase positively with the degree of dissection.
(2) All the tablelands are formed under similarly geological and climatic conditions. Thus the defferences in degree of dissection may be attributed to the nature of the area of the river watersheds occupying the tableland and the channel slope. In general, severe dissection is observed at the area of large watershed and steep slope.
(3) Linkou, Chutung, Chunan, and Miaoli tablelands have been severely dissected and are belonging to mature stage. The valley systems have been developed into the fifth in stream order according to Strahler's method, with a valley density of 4-6 km/km^2. Tatu, Houli, and Taoyoun tablelands are less dissected, belonging to young stage, and their valley systems are of the third in stream order, with a valley density below 4 km/km^2.
(4) The lateritic soils of the tablelands are of deposition in origin, instead of residual weatherings from bedrocks. According to their grain size composition, they can be divided into four types: (a) silt, (b) silt clay, (c) silt loam, and (d) loam. The first of which belongs, to marine or lacustrine deposit. The rests belong to river deposits. Their differentiations in grain size are due to the slope of their original surface and the magnitude of the tablelands.
(5) There are two types of lateritic soil profile in the area under investigation: single grain-size type and compound grain-size type. For both the general features of sedimentation are characterized by their grain size that tends to decrease toward the upper part of the profile.
(6) The grain-size type of the lateritic soil and its change of profile are similar within the same tableland surface, except that grain size tends to decrease a little bit toward the lower course direction. Such a grain size charateristic is helpful to explain the correlation of tablelands and to the identification of fault cliffs or erosional cliffs.
(7) That the lateritic soils cover the tableland gravels directly and the grain size alters abruptly indicate the fact that there is a time interval between these two layers, and therefore show an diastemic unconformity relationship.
(8) Grain size of the lateritic soil does not affect the degree of dissection. However, its thickness is an important facter. Thick lateritic soil layer is much slow than that of thin one in dissection.
As mentioned above, from altitude morphometry we can understand the tectonic movement in recently geologic time of Taiwan Island; from dissection morphometry we can understand the geomorphic stage of a region and their relationship with valley density. These are topics necessary to be studied further. The dicussions on the types of lateritic soil grain-size and its relationship with sedimentation environment, tableland correlation, degree of dissection as well as gravel layer sequence are the important and initial part of this study. |
