Stratigraphy of The Backbone Range Belt
PILUSHAN FORMATION
This formation includes all the undifferentiated Eocene rocks exposed in the Backbone Range belt. The
formation name was proposed for the first time in the second revised edition of the Geologic Map of Taiwan
to replace the old name "Hsinkao Formation" used in the first edition of the map.
The Hsinkao (Niitka) Formation was proposed by K. Tan (1944) for the Eocene slate and phyllite exposed in
the high part of the Central Range. The type locality of the formation is the highest peak of Taiwan,
Yushan. This formation has never been adequately defined and described because the type section cannot be
studied in detail due to inaccessibility. The Hsinkao Formation is composed mainly of dark colored
quartzitic sandstone, limy to marly lenses, and irregular conglomerate bands. Alternating interbeds of slate
and fine quartzitic sandstone are predominant in the lower part. The total thickness and the upper and lower
boundaries of this formation are unknown. Assilina niitakaensis was discovered by Tan at the type locality,
the age of which is Eocene. In later mapping, the Hsinkao Formation was defined to include all the slaty
beds in the Central Range in which Eocene foraminifers were found.
All the undifferentiated Eocene slates in the Backbone Range belt of the Central Range were mapped as
Hsinkao Formation in the first edition of this map. During the present map compilation, the type locality of
the Hsinkao Formation, the Yushan Mountain, has been identified to be underlain by the Chiayang Formation
(or Yushanchushan Formation as discussed before). On the other hand, the type locality Yushan is located in
the Hsuehshan Range belt and not in the Backbone Range belt. Therefore, it is not appropriate now to apply
the name Hsinkao Formation to represent all the undifferentiated Eocene rocks exposed in the Backbone Range
belt. In the editorial meetings, the concensus of opinion agreed to propose the stratigraphic term "Pilushan
Formation" as a substitute for the old term "Hsinkao Formation" to name all the undifferentiated Eocene
rocks in the Backbone Range belt of the Central Range.
The type locality of the Pilushan Formation is the mountain ridge Pilushan located northeast of Tayuling,
which is the highest station on the central cross-island highway. This is also the type area of the
Pilushanian stage proposed by L.S. Chang (1963a). This formation is distributed mostly in the high mountains
near the crest and to the east of the crest zone of the Central Range and these areas are marked largely by
heavy vegetation and precipitous slopes of difficult accessibility. It is thus difficult to study in detail
the lithologic section, stratigraphic sequence and total thickness of this unit. This formation must be
several thousand meters thick by rough estimation.
The Pilushan Formation is composed mainly of slate and phyllite with some thin to thick metamorphosed
sandstone interbeds. A part of the sandstone is calcareous, whereas other parts are arkosic to subarkosic.
The sandstone is fine- to coarse-grained. Lenses of dark reddish volcanic rocks form the other lithologic
component of the Pilushan Formation and serve as an important stratigraphic indicator of this unit. The
protoliths of the metamorphosed volcanic rocks could be diabase or basaltic tuffaceous rocks. Disseminated
limestone or marl layers or lenses are also intercalated in the Pilushan Formation. In places limestone
conglomerate or calcareous sandstone has been found. Conglomerate lenses are also exposed at different
horizons in the Pilushan Formation. The clasts of the conglomerate consist of quartz, schist and
metamorphosed sandstone.
Large foraminifers were discovered in the matrix of the conglomerate and in the limy nodules intercalated in
the slates of the Pilushan Formation. These fossils include Nummulites sp., Assilina formosensis
Hanzawa, Discocyclina sp., and Asterocyclina sp. These faunas were named the
Discocylina/Nummulites zonule by Yabe and Hanzawa (1930) and correspond to the Pilushanian stage of
Chang (1963). The age is probably middle to late Eocene.
The Pilushan Formation is distinguished from other younger slate units in the Central Range by the greater
abundance of sandstone interbeds, the inclusion of more volcanic lenses, the presence of Eocene large
foraminifers, and the intercalation of more limestone or limy conglomerate lenses or layers. All these
conspicuous lithologic and paleontologic features may be of help to recognize and map the distribution of
the Pilushan Formation in the rugged terrain of the Central Range.
L. S. Lee and C. I. Lee (1977) have traced all the fossil localities in the Central Range where Paleogene
large foarminifers have been discovered or reported. They have prepared a report and a map showing the
distribution of these fossil localities, which are of great help in mapping the areal distribution of the
Pilushan Formation in the Central Range. Based on these data, the Pilushan Formation is known to form a
north-south belt oh the crest zone and eastern flank of the Central Range. This belt is 1 to 5 kilometers
wide and begins north at Suao on the northeastern coast. Southward this Eocene slaty belt traverses Tungshan
in Ilan-hsien, Chungyangchienshan, the south slope of Pilushan to Kuanyuan-Pilu on the central
cross-island highway. It extends further southward to Hsinkuan, Lingkuan, Kuaiku, and Kuanshan Pass on the
south cross-island highway, and reaches the northeast side of the town of Liukuei in Kaohsiung-hsien,
Finally it extends to Nantawushan in Pingtung-hsien and ends at the southern extremity Tashulinshan
at the south of Tawushan. The total length of this belt of Pilushan Formation is nearly 250 kilometers. Some
of the large foraminifers in these localities are from the clasts of the conglomerate and the exact age of
the conglomerate must be determined by later studies on the matrix.
In addition to the above-mentioned main Eocene slate belt, another slate belt is exposed on the eastern side
of the Tananao metamorphic complex, west of the Longitudinal Valley between Yuli and Chihpen in
Taitung-hsienhsien. No diagnostic fossil has yet been found in this slate belt. However, this belt is
also mapped as the Eocene Pilushan Formation because abundant sandstone interbeds, a lithologic
characteristic of the Pilushan Formation, have been found in the slates.
It is to be mentioned that all the reported "Cretaceous (?)" slate units in the published literature such as
the Pihou Formation and the Litao Formation of Yen (1956) and the Chulai Formation of Stanley and others
(1981) are all included in the Pilushan Formation in this text and the geologic map because no convincing
evidence of Cretaceous age has yet been found in these units. On the other hand, the slate in these units
can not be differentiated from the slate in the Pilushan Formation. Therefore the Pilushan Formation is
temporarily applied to cover all these questionable "Cretaceous" formations.
The Pilushan Formation is exposed mainly in remote areas of the Central Range. Only a few localities along
the main belt of this formation have been studied in considerable detail. Many local formation names were
proposed, including the Nansuao Formation in Suao on the northeastern coast (Tsan, 1977), the Sanhsinshan
Formation in Tapingshan, Ilan-hsien (Wu, 1976), the Heiyenshan Formation on the central cross-island
highway (C. H. Chen, 1979b), and the Kuaiku Formation on the south cross-island highway (C. T. Lee, 1977).
All these local Eocene units may be correlated to the Pilushan Formation discussed in this text.
The Pilushan Formation unconformably overlies the Tananao metamorphic complex. However, this unconformity
may be largely obliterated by a later fault as previously discussed. In general the Pilushan Formation is in
direct contact with the overlying Miocene Lushan Formation, with no intervening Oligocene strata. Only on
the south cross-island highway, the overlying formation is the Likuan Formation of possible latest Oligocene
age, which will be discussed in the next paragraph. In both cases a marked stratigraphic gap exists between
the Eocene Pilushan Formation and its overlying Miocene or latest Oligocene rocks. This break could be a
Miocene/ Eocene unconformity which will be discussed more fully in the chapter on "Orogenic movements and
stratigraphic breaks".
LIKUAN FORMATION
The distribution of the Likuan Formation is very limited so that this unit cannot be shown on the 1:500,000
geologic map of Taiwan. This formation is thus included into the Pilushan Formation on the map. However, the
Likuan Formation may be the only formation of possible Oligocene age in the Backbone Range belt. Therefore
it is also discussed in this text.
The Likuan Formation was proposed by C. T. Lee (1977) and the type locality Likuan is located in the western
part of the south cross-island highway. The type section of the unit is about 900 kilometers thick and is
divided into five parts. The lithology is represented mainly by dark gray to yellowish gray, fine- to
medium- grained sandstone with interbeds of sandy shale and sandstone. The sandstone is usually laminated
where rich in carbonaceous materials. A part of the sandstone is arkosic or muddy arkosic. A 3-meter
conglomeratic limestone is found at the basal part of the Likuan Formation and is richly fossiliferous. This
limestone serves as the marker bed of the Likuan Formation and has been named as Lepidocyclina
limestone by some workers. Hashimoto and Matsumaro (1975b) discovered Nummulites fossils in the limestone
which are dated as Oligocene Rupelian stage. However Lee (1977) believed that these Nummulites fossils could
be derived or reworked fossils because Globigerinoides of early Miocene age has been found in the lower part
of the Likuan Formation. He dated the Likuan Formation as most probably early Miocene. As the first
appearance of Globigerinoides has been proved to be late Oligocene, the age of the Likuan Formation is
believed to be early Miocene to late Oligocene (T. C. Huang, 1980b). The Likuan Formation may be correlated
to the Taliao Formation and the Mushan Formation (Aoti Formation) in the western foothills and to N4-N5 in
the zonation of foraminifers. It is possible that the lower part of the Likuan Formation may be correlated
to the upper part of the Wuchihshan Formation of Oligocene age. The Likuan Formation has not been found in
other areas except the south cross-island highway, possibly due to lack of mapping data.
The Likuan Formation unconformably overlies the Eocene Kueiku Formation (C. T. Lee, 1977; T. C. Huang,
1980b), which is the equivalent of the Pilushan Formation. This represents the Miocene/Eocene break as
mentioned above. No distinct angular discordance or other evidence has been observed at the contact. Lee
reported that the limestone conglomerate at the basal part of the Likuan Formation is in fault contact with
the underlying Eocene rocks and the unconformity may have been obliterated by the faulting.
LUSHAN FORMATION
The Lushan Formation is a new stratigraphic name proposed in the first edition of the geologic map. It is
composed of argillite and slate of proven early Miocene or early middle Miocene age in the Backbone Range
belt of the Central Range. The type locality of this formation is the Lushan hot spring east of Wushe
(Jenai) in Nantou-hsien. Miocene foraminifers were found in slates and marly nodules in that area,
which extend for an east-west width of at least 14 kilometers on the western slope of the Central Range near
the Nengkao Pass. The faunas in the area were grouped into the Lushanian stage by L.S. Chang (1926b and
1963a) and the age may include Aquitanian, Burdigalian, and earliest Langhian. As the Lushanian is a
time-stratigraphic term defined solely by its faunal content, it is not a cartographic unit for geologic
mapping. In the compilation of this map, a new lithostratigraphic name, Lushan Formation, was proposed to
include the argillite-slate series embodied in the Lushanian stage of Chang. The Lushan Formation consists
largely of black to dark gray argillite, slate, and phyllite with occasional interbeds of dark gray compact
sandstone and disseminated marly nodules. The estimated thickness is several thousand meters. The Lushan
Formation is distributed mainly in the Backbone Range belt and separated from the Hsuehshan Range belt to
the west by a boundary fault, the Lishan fault or the Lanyangchi fault. This formation is exposed in the
north at the mouth of the Lanyangchi stream in Ilan-hsien and extends southward along the crest zone
of the Central Range through Hohuashan, Nengkaoshan to Hsiukuluanshan for a length of approximately 150
kilometers and a width of several to 10 or more kilometers. South of the Yushan Range, the Lushan Formation
is exposed east of the Laonungchi stream southward to the eastern mountains of the Pingtung valley down to
the Hengchun Peninsula. It is exposed also in southeastern Taiwan in the area of Chihpen and Tawu. The
distribution of the Lushan Formation is quite extensive on the map but reliability of the mapping data is
questionable due to lack of detailed information. Further detailed work is required to show more accurately
the distribution of the Lushan Formation in the Central Range.
The foraminiferal fossils in the Lushan Formation are characteristic of the Lushanian stage named by L. S.
Chang (1963a). When the Lushanian was first proposed, these fossils were dated as of early Miocene age.
Later paleontologic studies proved that the age of the Lushanian is mainly middle Miocene and corresponds to
N7-N9 zonation (L. S. Chang, 1976). Recently the study of nannofossils in the Lushan Formation in the
Liukuei area of Kaohsiung-hsien showed that the faunas are in the nannofossil zones NN3-NN5 and may
grade partly into NN2 zone. The age of the Lushan Formation is now considered early to middle Miocene, The
lowest part of the Lushan Formation in the type area yields no fossil remains. Thus the extent of the Lushan
Formation may be more extensive than that of the Lushanian stage. The Lushan Formation is correlated to the
Taliao Formation, the Shihti Formation, and the Nankang Formation in the western foothills, and to the Sulo
Formation in the Hsuehshan Range belt.
In the geothermal exploration program carried out in the Lushan hot spring area, the stratigraphy of this
formation has been quite well understood at the type locality. The Lushan Formation there has been divided
into three members by P. T. Chang (1984a), reaching a total thickness of more than 5,000 meters. The lower
part, named the Chunyang member, is about 3,500 meters thick. This is composed of dark gray slate with
intercalated fine-grained sandstone or siltstone in places. Comparatively thick sandstone beds are found
locally. Near the Wanta Reservoir, several thin basalt lenses are intercalated in this lower member. Most of
the leading fossils of the Lushan Formation were found in this member. The middle member, the Chiufeng
member, is about 700 meters thick. It consists of bluish gray fine-grained metamorphosed sandstone and dark
gray sandy slate. Sandstone is more predominant and contains many irregular quartz veins. The upper member
is named the Kunyang member. The exposed thickness is more than 1,000 meters without reaching the top. This
is composed mainly of well-foliated phyllite with a brilliant silky luster. Thin to thick metamorphosed
sandstone is interbedded in this member in places.
In northern Taiwan, the Lushan Formation in the Chingshui-Tuchang area of Ilan-hsien has been studied
in more detail in a geothermal exploration program. The Lushan Foramtion there is also divided into three
members (Wu, 1976). The middle member of fine-grained metamorphosed sandstone is 400-800 meters thick. Both
the upper and lower members are composed of a thick series of dark gray argillite or slate with local
interbeds of fine-grained metamorphosed sandstone. The lithology is quite monotonous. However, characteristic
Lushanian fossils are very rare in the rocks exposed in this area. Some Miocene foraminifers are reported to
have been found in the slates to the northeast of the geothermal area (personal communication of C. S. Lee)
In southern Taiwan, geologic study in the Laonungchi drainage area and the south cross-island highway led to
the introduction of two local formation names that are the equivalent of the Lushan Formation. These are the
Changshan Formation of Tsan (1964) and the Meishan Formation of C. T. Lee (1977). Lushanian fossils (Chang,
1975 and 1876) and nannofossils (Chi, 1978; T. C. Huang, 1986b) have been found in these two formations. The
rocks are mainly grayish black argillite with local intercalations of thick- to medium-bedded sandstone. The
metamorphic rank of the rocks is rather low. Being dissected by fault in both areas, the total thickness of
the formation is unknown but could be more than 1,000 meters. Southward to Chaochow in Pingtung-hsien,
geologic investigation of the route of the proposed south-link railway revealed that equivalent strata of
the Lushan Formation underlie the study area. A local name, the Yilin Formation, was proposed by Hu and
others for these rocks (1984). This unit is composed largely of a thick sequence of argillite and slate,
containing alternating interbeds of sandstone and slate and grayish black metamorphosed sandstone. Miocene
fossils were found in the argillaceous rocks.
In eastern Taiwan, the strata supposed to be equivalent to the Lushan Formation are exposed in the hilly
areas in the Chihpen-Tawu area in Taitung-hsien. The local name of the unit is called the Chihpen
Formation (Lee and Chang, 1984). The lithologic character of the Lushan (Chihpen) Formation in this area is
quite different from that in the type area because sandstone members are much more abundant in the sections.
The Chihpen Formation is divided into six members, in a total thickness exceeding 5,000 meters. Three of the
members are characterized by gray, light gray to deep gray, tine-grained metamorphosed sandstone, partly
exhibiting a schistose texture. Intercalations of black slate in the sandstone are rather common. Quartz
veins are numerous in the sandstone. The other three members are represented by well-foliated deep gray to
grayish black argillite, slate and phyllite with thin interbeds of sandstone. Only a single fossil locality
with a Miocene fauna has been reported near the southernmost margin of the area. Because of the abundant
sandstone in the formation, some geologists are of the opinion that the Chihpen Formation may include
Oligocene rocks in its lower part, although no fossil evidence has yet been found.
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