hree steps of fluvial terraces, T1, T2 and T3 in descending order, are present along the west bank of the Kaoping River (Figs 5 and 6). Correlation of these terraces with those on the Yuehkuang Mountains is difficult because of lack of terrace continuity; hence we use different symbols for both areas. N-S trending terrace risers, often parallel to the Kaoping River, suggest that most terraces were formed by the south-flowing Kaoping River. These terraces are not exactly mapped in the previous studies.
1 is only found at west of Chuliao, southernmost of the study area at about 120 m (inner margin) to 90 m (outer margin) above sea level (Figs. 6 and 7) . Terrace riser, separating from T2, is ca. 20 m high and very clear (profile a-a’ in Fig. 7). T1 is underlain by fluvial gravels, composed of subrounded gravels with few matrixes, at least 13 m thick at Loc.5 (Fig. 7). This terrace can be traced northward for 8 km distance, as dissected narrow terrace remnants of 100-120 m above sea level (Figs. 6 and 7
T2 is the extensive terrace (ca.1 km wide at north and ca. 2.5 km wide at south Figs. 5, 6, and 7）and is developed over 25 km long starting southward from the Chishan Park to the north. Since the inner margin of T2 is N-S trending, parallel to the Kaoping River, terraces, especially T2, could be formed by the Kaoping River. This at there was a relatively stable time to permitting the formation of
glomerate metimes is sometimes difficult because of their similarities in facies. Hence the thick
alluvial lowland to the west. Further detailed fieldworks are required to relief and structural one.
erraces and fault on the w
T
T
).
suggests th
extensive T2, which is absent in southern part of Fig. 5 due to removal by intensive landslide. Distinction between terrace deposits and underlying Lingkou Con
so
ness of terrace deposits is often uncertain. One excellent exposure at south of the Chishan Park (Loc. 6 in Fig. 9) reveals that a clear unconformity between the eastward dipping (25°E) Plistoocene Lingkou Conglomerate and T2 deposits of ca. 5 m thick (Photo A), mainly composed of subrounded fluvial gravels.
Figure 7. Terrace map of the Chuliao area and profile a-a’ across T1 and T2. Eastern margin of T2 is limited by sharply defined erosional cliff, retreated from the original tectonic scarp.
Figure 8. Longitudinal profile of terraces，modern flood plain and the river floor along the Kaoping River, drawn from the photomap of 1:5,000 in scale. Remarkable difference etween the inner margin height and outer margin height of T2 is caused by flexural deformation on the north, and by presence of secondary deposits on the south.
The longitudinal height distribution of terraces is shown in Fig. 8. The inner
ery close to the base of flexural scarp. b-b’
profile is for T3, younger terrace than T2. Notice that the apparent amount of vertical offset of T3 is smaller than T2, suggesting progressive deformation.
margin of T2 ranges from 140 m at Chishan Park (Fig. 9) and 125 m at Shangcholi (Fig. 10), where the inner margin of T2 is sharply defined. Considerable height difference between the outer and inner margin at these two locations are due to strong eastward deformation of T2, as will be discussed in III-2. In the southern area, the inner margin height is higher than the upper stream, reaching 160 m at maximum, which is significantly higher than the height of Shangcholi, located further upstream.
In addition, the inner margin height is rather irregular, and T2 surface dips eastwards considerably, although its outer margin is limited by clear terrace riser from the alluvial lowland. These features suggest that the inner part of original T2 terrace surface, originally created by the Kaoping River, is covered by alluvial fan deposits of various scales, transported by eastward flowing tributaries. A dashed line shown in Fig. 8 is a possible boundary between the original terrace surface and secondary covered fan surface, judging from the different gradient of terrace surfaces and ranges from ca. 115 m to 60 m. The southward gradient of T2, represented by a dashed line, is 40 m/22 km, and larger than 15 m/22 km for the alluvial lowland (Fig. 8).
Figure 9. Detailed contour map of fluvial terraces and their deformation in the Chishan Park area (drown by 20 m grid DEM). Contour map and profiles represent the remarkable eastward warping scarp deforming T2 and T3. Smooth and convex profiles on the eastern margin is very distinctive.
A buried fault (Kaoping River Fault) is estimated v
Terraces lower than T2 are collectively called as T3 (group) and are fragmentary preserv
osits, although the longitudinal profile of T2 is steeper than the modern flood plain or ed at several sites (Figs. 5 and 6). One is at the Chishan Park, where a small T3 with ca.10 m lower height than T2 (Fig. 9) at its inner margin is present along the small east flowing stream, clearly indicating tributary origin. The other is in the southern part of near Chuliao, where relatively wide T3 is present at ca. 50 m in height, separated by ca. 30 m high terrace riser (Fig. 6). The correlation within T3 group is uncertain.
We have no age control from these terraces, underlain by fluvial gravel without prominent sand or clay lense for dating. It is also difficult to find out a direct relationship between the terrace formation and sea level changes, because the terraces of the study area do not continue to any of marine terraces. However, since this area is not so far from the coast (Fig. 1), it may possible to assume some of terraces correlate with sea level change. We tentatively assume that the most extensive and relatively well preserved T2 terrace as a possible correlative with high sea level of MIS 5e, judging from the following reasons: First, it is unlikely to say that T2 can continue to the lowest sea level of MIS 2, morphology of which should be very much steeper than the alluvial lowland and should be found beneath the Holocene alluvial dep
present river floor. Secondly, the formation of an extensive terrace requires some stable time as mentioned before, Then thirdly, the high sea level of MIS 5e usually leave its morphological trace as a prominent and well preserved marine terraces with presence of younger marine terraces, corresponding to MIS 5c or 5a when the uplift rate is high enough to emerge these terraces on land. Thus, as the first approximation, we assume that T2 possibly represent MIS 5e of ca. 125 ka, although we know that the degree of preservation of the terrace of this stage, is very much changeable depending on the original scale and nature of deposits in addition to the height etc. If the above mentioned correlation is the case, higher T1 can be correlated with MIS 7.
In any case, the estimation of formation age of terraces is very tentative, leaving important problems to be solved, supported by dating. Although we are uncertain about the age of terrace, it is quite obvious that T2 is clearly tectonically deformed and provides a positive evidence as the active fault for the Kaoping River Fault.