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| :::Western Central Range Backbone Ridges |
General Strstigraphy
The thick and monotonous Tertiary submetamorphic argillaceous sediments in the Central Range can be divided
into two belts of different stratigraphic division and succession as mentioned before. In the western
Hsuehshan Range belt, the presence of two carbonaceous units and thick sandstone strata has made possible
the subdivision of the argillite-slate sequence into a number of mappable lithostratigraphic units in
northern Taiwan. These submetamorphic rocks in northwestern Taiwan were first named the Wulai Series by
Ichikawa (1929). Although the Wulai Series has been classified into a number of different formational names
in different maps (Chung, 1973), it was originally divided into two groups and six formations (Ichikawa,
1932) as follows:
The type localities of all these formations are in the drainage areas of the Hsintienchi and the Tahanchi.
With certain modifications, these formational names were followed by many subsequent workers in mapping the
areas underlain by the Wulai Series in northern Taiwan and in the Hsuehshan Range belt in general. Four of
these old formational names of the Wulai series are retained and adopted in the legend of the present
geologic map to avoid the introduction of too many new stratigraphic names. The first formational name,
Kueishan Formation, is not well- established in that the definition and stratigraphic description of the
formation are ambiguous. It may represent different stratigraphic units in different geologic maps. The
Kueishan Formation is, therefore, replaced by a more acceptable new stratigraphic name, Aoti Formation, in
the present map. The Tsuku Sandstone is now considered a locally present sandy member of the Tatungshan
Formation, and is found exclusively in the northern part. It is not separated as an individual unit in this
map. Due to facies changes, the above stratigraphic classification is not applicable to the central and
southern parts of the Hsuehshan Range belt and a different stratigraphic system has to be used for those
areas. This will be discussed later.
In the eastern Backbone Range belt, an immense series of argillite and slate or phyllite is exposed without
intervening carbonaceous or thick sandstone members. Based mainly on fossils (foraminifers) discovered in
the slates and with reference to the lithologic characteristics, two broad stratigraphic units are
recognized in the Backbone Range belt as previously stated: the Miocene Lushan Formation and the Eocene
Pilushan Formation. The basis of stratigraphic subdivision is more paleonto-logical than lithological and
may not be entirely consistent with principles of lithostratigraphic classification. This is the only
method, however, that can be applied for stratigraphic classification in this monotonous argillaceous
sequence.
The Lushan Formation represents all the proven Miocene metamorphosed argillaceous sediments exposed in the
southern and eastern parts of the slate series in the Central Range. The Pilushan Formation is composed
mainly of slate and phyllite, but alternating interbeds of hard quartzitic sandstone and slate are more
predominant in the lower part. It is distributed mainly in the higher ridges and southern part of the
Central Range. Indisputable Eocene large foraminifers have been discovered in these rocks.
Within the last ten or more years, detailed geologic investigations have been undertaken in this Tertiary
slate series of the Central Range as a result of geothermal exploration programs and other major engineering
geologic projects. Enhanced knowledge of the stratigraphy and structure of this slate terrain has been
acquired, leading to a more accurate scheme of the stratigraphic sequence and correlation. Table 3 shows the
stratigraphic classification, age and correlation of the different Tertiary formations in the argillite-slate
series in the Central Range based on all available information and data from published and unpublished
sources.
Taible 3. Stratigraphic classification and correlation of the Hsuehshan Range and the Backbone
Range belts in the Central Range, Taiwan |
| Geologic Age |
Western Foothills |
Northern Part of Hsuehshan Range Belt |
Central and Southern Parts of Hsueshan Range Belt |
Backbone Range Belt |
| Miocene |
Nankang Formation
Shihti Formation
Taliao Formation |
Sulo Formation |
|
Lushan Formation |
| Mio-Oligocene |
Mushan Formation |
Aoti Formation |
Likuan Formation (?) |
| Oligocene |
Wuchihshan Formation |
Tatungshan Formation
(Tsuku Sandstone)
Kankou Formation |
Shuichangliu Formation |
Stratigraphic Break
(Unconformity ?) |
| Szeleng Sandstone |
Meichi Sandstone |
Paileng Formation |
| Oligo-Eocene |
|
Hsitsun Formation |
Chiayang Formation |
Pilushan Formation
(Hsinkao Formation) |
| Eocene |
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Tachien Sandstone Shihpachungchi Formation |
The ages of the rock formations in Table 3 are dated largely by paleontologic evidence. However, diagnostic
fossils are often lacking in some of these units and fossil zonation is not always continuous in the strata.
Thus the ages of some formations must be inferred from relative stratigraphic sequence or adjacent fossil-
bearing units; uncertainties in age determination for some units cannot be avoided. Extensive future studies
are needed to modify and review the above stratigraphic table. All the different rock formations listed in
Table 3 will be discussed in more detail in the following paragraphs.
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