Stratigraphy of The Northern Part of The Hsuehshan Range Belt
HSITSUM FORMATION
This is the oldest stratigraphic unit exposed in the Hsuehshan Range belt. The name, Hsitsun Formation, was
first used by Ooe (1931) to represent the lower member of the Wulai Series in northern Taiwan. This formation
is composed of well-foliated dark gray slate and phyllitic slate. Interbeds of dark colored, fine-grained,
hard quartzose sandstone are more abundant in the lower part of the formation. The present outcrops of the
Hsitsun Formation are restricted to a narrow belt extending along the axis of an anticline from the type
locality Hsitsun on the border of Taichung-hsien and Ilan-hsien to the village near Yuanshan on the Ilan
plain. The lower part of the Hsitsun Formation was named the Chungling (Churei) Formation in some early
reports, but this subdivision is entirety unwarranted in small-scale maps. The exposed part of the Hsitsun
Formation in the anticlinal axis at the type locality may attain 600 meters in thickness, but the lower part
of the formation is not exposed. The age of the Hsitsun Formation has not been determined paleontologically
because no diagnostic fossil has ever been found in the rocks. The Hsitsun Formation is referred to as
Oligocene to Eocene because it conformably underlies the Szeleng Sandstone whose age is inferred to be
Oligocene. The exact age of the Hsitsun Formation must be verified by further paleontologic studies and/or
other means. The Hsitsun Formation is lithologically and stratigraphically correlated to the Chiayang
Formation exposed in the central part of the Hsuehshan Range belt.
SZELENG SANDSTONE
The Szeleng Sandstone is an important lithologic marker that is used to subdivide the immense
argillite-slate sequence of the Hsuehshan Range belt into a number of mappable units. The name was first
applied by Ooe (1931) to a thick quartzitic sandstone formation in the Wulai Series of northern Taiwan;
the type locality Szeleng is on the north cross-island highway in Ilan-hsien. This formation is
characterized by thick-bedded, light gray to whitish gray, quartzitic sandstone or quartzite intercalated
with dark gray argillite or slate. The sandstone is dark where it is in association with carbonaceous shale.
The carbonaceous shale often grades into coaly or graphitic lenses laterally, but both the coal and the
graphite are contaminated by clayey impurities and are of no mining value. The sandstone is medium- to
coarse-grained and grades into conglomeratic sandstone locally. It is generally arkosic to subarkosic.
Cross-bedding and ripple marks are common in the sandstone. The type locality Szeleng structurally lies in a
northeast-trending anticline. On the southeast flank of the anticline, the Szeleng Sandstone is about 350
meters thick and is composed dominantly of whitish quartzitic sandstone. The Szeleng Sandstone on the
northwest flank of the anticline is separated into two sandstone members by an intervening argillite-slate
member that is approximately 100 meters thick. The total thickness of the Szeleng Sandstone on this flank
reaches 700 meters or so.
The Szeleng Sandstone is widely exposed in northern and northeastern Taiwan. The northernmost distribution
of this unit is in the upper drainage areas of the Tahanchi, the Peishihchi and the Nanshihchi, mainly as
small exposures. It is well exposed on the eastern part of the Taipei-Ilan highway and the north
cross-island highway. Scattered outcrops of the Szeleng Sandstone are distributed near Toucheng, Chiaochi,
and Yuanahan on the western hillside of the Ilan plain in northeastern Taiwan. Southward to central Taiwan,
the Szeleng Sandstone is replaced by the Paileng (Hakurei) Formation and these two formations may be
correlative units. The Szeleng Sandstone is replaced by the Meichi Sandstone in the eastern part of the
Hsuehshan Range belt. All these units will be discussed later in this chapter. The quartzitic sandstone of
the Szeleng Sandstone is usually devoid of identifiable fossil remains. Only the slaty shale intercalated in
the sandstone yields some molluscan and foraminiferal faunas, and these are not age-diagnostic. Other fossils
reported to have been found in the Szeleng Sandstone are suspect and age of the Szeleng Sandstone remains to
be verified by future studies. At present the Szeleng Sandstone is tentatively ascribed to early Oligocene on
the basis of inferred stratigraphic sequence.
The Szeleng Sandstone is a carbonaceons unit in the Hsuehshan Range belt. This coaly and carbonaceons member
is developed only in the western part of the Hsuehshan Range, indicating closeness to shore toward the west.
Cross-stratification in the sandstone also shows a paleocurrent coming mainly from the west. All the combined
lithologic characters and sedimentary features indicate that this formation was deposited in a near-shore
deltaic to shallow neritic environment, and the sediment was derived mainly from mainland China to the west.
KANKOU FORMATION
All the metamorphosed argillaceous sediments overlying the Szeleng Sandstone in the Hsuehshan Range belt are
classified into two formations in the map legend: the Kankou Formation and the overlying Tatungshan
Formation. Both are old formational names of the Wulai Series proposed by Ichikawa (1930 and 1932). The
Shuichangliu Formation in central Taiwan is considered to be the equivalent of these two formations. The
demarcation between the Kankou Formation and the Tatungshan Formation is arbitrary and subjective.
The Kankou Formation is composed predominantly of black to dark gray shaly sediments which have been either
indurated into argillite or metamorphosed into slate or phyllitic slate. The argillite and slate are compact,
often forming precipitous walls showing distinct pencil structure or woody fracture surfaces. Sandy interbeds
are rare to very rare in the Kankou Formation. Distinction between the Kankou Formation and the overlying
Tatungshan Formation is made on the abundance of sandstone or siltstone interbeds. The Kankou Formation
grades upward into the Tatungshan Formation, in which more sandy or silty interbeds appear in the
argillaceous sequence. No clear-cut boundary between these two units can be clearly defined in areas lacking
the locally intervening Tsuku Sandstone member. Because the occurrence of sandy interbeds varies in
different places, the thickness of the Kankou Formation also varies greatly in different sections, ranging
from 600 to 1,200 meters. Slaty cleavage is well-developed in the southeastern part of the outcrop area, and
generally cuts bedding at an angle. The faunas and geologic age of the Kankou Formation are the same as the
Tatungshan Formation and are discussed below. Foraminifers and calcareous nannofossils are the main organic
remains in both formations. The Hsuanyuan (Kayahara) Shale of some old Japanese reports (Ooe, 1931) is
correlated to the Kankou Formation and is composed mostly of slaty shale.
TATUNGSHAN FORMATION
The Tatungshan Formation is widely distributed in the Hsuehshan Range belt and is exposed in all the leading
fold structures from the northern coast southward to central Taiwan. The lower part of this formation is
composed of dark gray to black argillite closely interbedded with gray fine-grained sandstone and muddy
siltstone. Contacts between the argillite and sandstone or muddy siltstone are rather irregular. The two are
often gradational into each other in beds of 10 centimeters to two meters thick. The muddy siltstone is
compact and resistant to weathering, often forming sharp bluffs along river beds resembling sandstone scarps.
This is the most striking feature of this formation. The upper part of the Tatungshan Formation is
represented largely by dark, compact argillite and sandy shale with fewer interbeds of sandstone or muddy
siltstone. The argillaceous beds are commonly massive when wet but show good fissility and cleavage surface
on drying. Bedding is distinct in some places and indistinct in some other places, but spaced and slaty
cleavage are relatively well developed. The reported thickness of the Tatungshan Formation varies from 800
meters to 2,000 meters and more in different papers as the type section of the formation has not been
accurately measured. Different workers may have had different ideas in defining the upper and lower limit of
the formation. However, reasonable estimates indicate that the total thickness of the Tatungshan Formation
is on the order of 1,500 meters or more.
In the northern part of the Hsuehshan Range belt, thick sandstone beds are more abundant in the lower part
of the Tatungshan Formation. The sandstone is dark gray, muddy, and fine-grained, with thin interbeds of
argillite. Some geologists are inclined to call it siltstone rather than sandstone. This sandstone member
reaches a, total thickness of 200'meters or more and was formerly named the Tsuku Sandstone Formation by
Ichikawa (1932). It serves as a well-defined marker to separate the Tatungshan Formation from the Kankou
Formation below. The Tsuku Sandstone represents only the sandstone-rich part of the lower Tatungshan
Formation and is restricted to the northern part of the outcropping area. Southward from the Wulai area in
northern Taiwan, the thick sandstone member gradually thins out and the Tatungshan Formation is composed
mainly of argillite with muddy sandstone interbeds. Further southward to central Taiwan, the muddy sandstone
interbeds in the Tatungshan Formation gradually diminish and the formation is composed largely of black to
dark gray argillite and slate with minor sandy interbeds. This is where distinction between the Tatungshan
Formation and the Kankou Formation becomes increasingly difficult, and the Shuichangliu Formation has been
proposed in central Taiwan to include these two formations.
Small lenticular to irregular bodies of basaltic rocks and a few lava flows are disseminated at many places
in the Tatungshan Formation. Some volcanic lenses occur also in the Kankou Formation but are much less
abundant. The volcanic rocks are more common in northern Taiwan than in central Taiwan. The volcanic rocks
vary in thickness from less than one meter to several tens of meters. They are usually interbedded at
different stratigraphic levels in the argillite and slate, in general conformity with the foliation of the
enclosing sediments although discordant sheets may also occur. Horizontal extension or length of the volcanic
bodies is uncertain because it is difficult to trace them laterally due to rugged terrain and heavy
vegetation cover. The actual distribution of the volcanic bodies is probably much more extensive than is
shown on the present map. These volcanic effusives are believed to have erupted from various fissures
contemporaneous with deposition of the argillaceous sediments of the Tatungshan Formation.
The Tatungshan Formation, the Kankou Formation, and the correlative Shuichangliu Formation contain a rich
assemblage of faunas, including mollusks, foraminifers, and echinoids. For some time these three formations
have been considered Oligocene based on the foraminiferal fauna in the Tatungshan Formation exposed in
Yushan, which is about 30 kilometers northeast of the type locality Tatungshan. The Yushan faunule
represents the Oligocene Shihtsaoan stage proposed by L.S. Chang (1954). The Oligocene age has also been
supported by the study of nannofossils contained in this formation.
AOTI FORMATION
In the old Japanese literature, two rock units were proposed for the clastic sedimentary rocks conformably
overlying the Tatungshan Formation in the Wulai Series: the Kueishan (Kizan) Formation and the Chuchih
(Kussyaku) Formation. These two formation names have lost status among geologists because their definitions
are ambiguous and often lead to misleading stratigraphic interpretations. A new stratigraphic term, Aoti
Formation, was proposed as the uppermost unit of the Wulai Series that constitutes the argillite-slate
sequence of the Hsuehshan Range belt. The Aoti Formation was named by Yen and Chen (1953) after a fishing
village on the northeastern coast of Taiwan. Unfortunately this stratigraphic term is a misnomer as the
strata exposed at the type locality are mainly the Wentzekeng Formation which is correlated to the
Wuchihshan Formation or the Tatungshan Formation. For this reason some geologists have suggested replacing
the Aoti Formation with a new name, Fulung Formation, because a good section of the Aoti Formation is
exposed near Fulung southeast of the village of Aoti. However, the Aoti Formation has been in general use in
the geologic literature for some time and has been accepted by many workers. The name is still retained in
the legend to avoid creating new stratigraphic names.
The Aoti Formation conformably overlies the Tatungshan Formation but is not composed of the characteristic
sequence of argillite or slate in the Hsuehshan Range belt. It is a carbonaceous or coal-bearing unit
similar to the Szeleng Sandstone, but the rocks are only slightly indurated and are thus considerably less
metamorphosed than all the underlying formations of the Wulai Series. The distribution of the Aoti Formation
is restricted to the northeastern part of the Hsuehshan Range belt; no Aoti Formation is exposed to the
southwest of the Touchienchi stream in Hsinchu-hsien. The two major outcrop areas of the Aoti Formation are
both northeasterly-trending synclinal belts. The first belt begins from Fulung on the coast and extends
southward through Pinglin to Ayushan for a length of 55 kilometers and a width of 5 kilometers. The second
belt is located in the upper tributaries of the Tahanchi stream near Kaokang, Paling, and Hsiuluan. It is
approximately 40 kilometers long and dissected by a dextral fault in the middle. Lesser exposures of the
Aoti Formation are found in a number of strips between Hsintien and Wulai and southwest of Tingshuangchi in
Taipei-hsien.
In field mapping, the Aoti Formation is divided into the lower Makang member and the upper Fangchiao member.
The Makang member is about 370 meters thick and is composed of thin, alternating beds of gray, fine-grained
sandstone and dark gray shale. These clastic rocks are slightly indurated and weakly compacted, showing a
faint foliated texture. The sandstone is partly micaceous and contains sand pipes and ripple marks. In
places it is ferruginous with nodular clay-ironstone concretions. The alternating beds range in thickness
from several centimeters to several meters, although some thick sandstone beds attain a thickness of more
than 10 meters. The exposed Fangchiao member in the type locality is about 350 meters thick and is the
coal-bearing member of the Aoti Formation. It consists mainly of thick-bedded, medium-to fine-grained
sandstone interbedded with dark gray shale or carbonaceous shale. Thin interlaminations of
sandstone-siltstone-shale are also characteristic. The sandstone is grayish white to bluish white, ranging
in bed thickness from 20 centimeters to 5 meters and more. It is compact and well-jointed, with an obscure
banded texture. The sandstone in the upper part is feldspathic and somewhat kaolinized. Two impure coal beds
are found in the upper part of the Fangchiao member, each less .than 20 centimeters thick and generally not
workable. From the study of different sections in various places, the maximum thickness of the Aoti Formation
may reach nearly 1,200 meters.
When the Aoti Formation was studied in the first synclinal belt at the type locality near Fulung, no
positive age indicator was identified among the fossils collected. This formation was assigned to late
Oligocene to early Miocene age because it lies conformably on the Oligocene Tatungshan Formation in which
the Yuhangian fauna of the Shihtsaoan stage of Chang was discovered. The Aoti Formation was also named the
Lowest coal-bearing formation in early stratigraphic study to differentiate it stratigraphically from the
Lower coal-bearing formation (Mushan Fromation) in the western foothills. However, recent studies confirm
that stratigraphically the Aoti Formation is the equivalent of the Mushan Formation (See Table 3), but these
two units are located in differing geologic provinces of Taiwan so they are categorized in two different
stratigraphic systems.
SULO FORMATION
This is a new stratigraphic unit proposed for the first time in this revised geologic map of Taiwan. The
type locality Sulo is a small village in the upper course of the Tahanchi stream in Taoyuan-hsien. This
formation includes all the Miocene rocks conformably overlying the Aoti Formation in the Hsuehshan Range
belt of the Central Range. The age of this unit is early to middle Miocene (N5 to N8). The type section is
exposed in the second synclinal belt formed of the Aoti Formation mentioned before. The outcropping areas
cover the mountain terrain of Sankuang and Hsiuluan in the upper courses of the Tahanchi and the Touchienchi
streams.
The discovery of Miocene rocks in the argillite-slate series of the Central Range was first reported by the
Japanese geologist Y. Oinouye and his team (1928) during their reconnaissance study of the oil geology in
Taiwan. After World War II, Yen and Chiang (1964) studied these rocks again and discovered a belt of Miocene
rocks of 40 kilometers long and nearly 6 kilometers wide exposed in the core of an ENE- trending syncline in
the Paling-Sulo area along the north cross-island highway in the upper reaches of the Tahanchi in
Taoyuan-hsien. The underlying formation is the Tatungshan Formation quite typical of the Hsuehshan Range
belt. They believed these slightly indurated Miocene rocks are exactly the same as those exposed in the
western foothills. They thus mapped them as the Nankang Formation, the Shihti Formation and the Taliao
Formation which are common Miocene litho-units in the western foothills. Due to a discrepancy in
stratigraphic sequence, they interpreted that these rocks are at a fault contact with the underlying
Tatungshan Formation. Subsequent detailed mapping in the area (Tsan, 1976; Tang and Yang, 1976) did not
substantiate the fault hypothesis and the Tatungshan Formation is found conformably underlying the Miocene
strata without any break. Stratigraphic analysis shows that the only formation that lies conformably on the
Tatungshan Formation is the Aoti Formation. Additional lithostratigraphic study proves that the lowest part
of the Miocene rocks in the area concerned resembles closely the Aoti Formation in gross features. Thus the
Miocene rocks in the study area of Yen and Chiang (1964) can be stratigraphically divided into two units:
the Aoti Formation which has already been discussed and the overlying Sulo Formation which is newly proposed
in this revised map and text.
Miocene fossils have been discovered in the Sulo Formation. On the basis of Foraminifera, L. S. Chang (1973)
stated that these Miocene fossils are correlated to those found in the Miocene Hsichih Group in the western
foothills and can be ascribed to N8 zone (partly N7). Later study of the nannofossils by T. C. Huang (1980a)
proved that the nannofossils in the Sulo Formation belong to zone NN4 and also to foraminiferal zone N8. The
age is considered mainly middle Miocene. As the Aoti Formation in the Hsuehshan Range belt is correlated to
the Mushan Formation in the western foothills, the Sulo Formation in the same belt can thus be correlated to
the Tailiao Formation, the Shihti Formation and the Nankang Formation in the western foothills. The age of
the Sulo Formation is then dated as early to middle Miocene.
In a recent paper dealing with the Sulo Formation by Tu and others (1987), the total thickness of the Sulo
Formation at the type locality is estimated to be a little more than 1,000 meters. The underlying strata are
represented by the Aoti Formation and then the Tatungshan Formation. The former is generally barren of
fossils. The basal part of the Sulo Formation is composed of gray argillite or slate about 250 meters thick.
Thin, gray, fine-grained sandstone is locally intercalated in the argillite. The lower part of the Sulo
Formation consists of thick, gray to light gray, fine-to medium-grained sandstone about 120 meters thick. On
top of this thick sandstone is a member of alternating sandstone and shale about 100 meters thick. Scattered
coaly fragments have been found in the shaly rocks. A thick and massive sandstone member of 200 meters thick
lies on top of the alternating beds. The sandstone is gray to bluish gray, fine-to medium-grained, and
partly muddy. Overlying the sandstone is a shale member 120 meters thick. Organic remains are rather
abundant in the shale, which seems to be scarcely metamorphosed. A thick unit of bluish gray calcareous
sandstone about 200 meters thick overlies the shale member. Cross-bedding, ripple marks, and trace fossils
and other sedimentary structures are distinct in the sandstone, which forms the uppermost exposed part of
the Sulo Formation.
The Hsichih Group in the western foothills, the Sulo Formation in the Hsuehshan Range belt, and the Lushan
Formation in the Backbone Range belt are chronologically correlative units because they all contain the same
early to middle Miocene fossils. However, these three units are stratigraphically located in three different
geologic provinces. They differ in lithologic section, in stratigraphic sequence, and in depositional
environment; thus these three units differ lithostratigraphically. This is the main reason to propose a new
stratigraphic name, the Sulo Formation, for the Miocene rocks exposed in the Hsuehshan Range belt as
discussed in this section.
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