Miocene Stratigraphic Units
In the western foothills, Miocene rocks are well represented in northwestern Taiwan where a complete
sequence of the stratigraphic column is exposed. Thus the description of the Miocene stratigraphic units is
based mainly on those exposed in northwestern Taiwan, but the equivalent units in other parts of western
Taiwan will also be discussed. As mentioned before, the Miocene rocks in northern Taiwan are composed of
three depositional cycles, each represented by one coal-bearing formation and one marine unit. These three
cycles are named the Yehliu Group, the Juifang Group, and the Sanhsia Group. All the important rock
formations in these three groups are described in the following paragraphs.
YEHLIU GROUP
This is a lithostratigraphic term proposed in the 1974 geologic map and includes two rock units formed in
the early Miocene sedimentary cycle in western Taiwan. Yehliu is a renowned resort area on the northern
coast between Chilung and Tanshui. The area is noted for the picturesque hoodoo rocks formed of thick-bedded
calcareous sandstone (Hsu, 1964). The two formations in the Yehliu Group are the Mushan Formation and the
overlying Taliao Formation. The former is a coal-bearing unit whose lower part could be of Oligocene age and
the latter is a marine unit. The Yehliu Group is restricted to the northern half of the western foothills.
No outcrop of the Yehliu Group has been found south of the Hsilochi stream in Nantou-hsien, except in
subsurface drill holes. The Yehliu Group may be correlated with the lower part of the Hsichih Group proposed
by Japanese geologists (L.S. Chang, 1953).
MUSHAN FORMATION
The Mushan Formation was proposed by Yen and Chen (1953) to replace the older term "Lower coal-bearing
formation" for the lowest of the three Miocene coal-bearing formations in northern Taiwan, It conformably
overlies the Wuchihshan Formation at a transitional contact. Mushan, the type locality, is a local
geographic name in the city of Chilung. The distribution of this coal-bearing unit is restricted to the
northwestern part of Taiwan. It is well developed in the Chilung area on the northern coast and extends
southward through a large part of Taipei-hsien, Taoyuan-hsien and Hsinchu-hsien to the
northern part of Miaoli-hsien.
The age of the Mushan Formation was previously assigned to the Miocene, but C. Y. Huang (1979) stated that
the Mushan Formation is of Oligocene age. However, T. C. Huang (1979a) and T. Huang (1974) are of the
opinion that the age of the Mushan Formation is Oligocene-Miocene and is correlated to nannofossil NN1 zone.
This is the age of the Mushan Formation adopted in this text.
Lithologically the Mushan Formation is quite similar to the underlying Wuchihshan Formation and
differentiation of these two units is often difficult. In field mapping a thick bed of dark gray shale
exposed in the upper part of the Wuchihshan Formation is considered as a good horizon to indicate the base
of the Mushan Formation (Yen et al., 1953). The shale is approximately 40 to 50 meters thick. Where this
shale marker is obscure or poorly exposed, these two formations are hardly separable and the boundary can
be only arbitrarily determined. Chou (1974) is of the opinion that a bed of coarse-grained orthoquartzite
can be used as the upper limit of the Mushan Formation and the boundary is stratigraphically lower than the
dark gray shale of Yen and others. White orthoquartzite to protoquartzite sandstone is predominant in the
Mushan Formation and the sandstone is commonly cross-bedded. However, sand grains in the Mushan sandstone
are finer and less compacted than those in the Wuchihshan sandstone. Dark gray shale or carbonaceous shale
and interlaminations or interbeds of sandstone and shale are the other characteristic rock types in the
Mushan Formation. The Mushan Formation is a shelf-type carbonaceous unit and contains few marine fossils. In
northernmost Taiwan, a maximum of three workable coal beds is present in the upper part. Each coal bed
ranges from several to 60 centimeters thick but varies greatly in thickness and workability in different
coal fields. Local pinching and swelling of the coal is common. In other parts of northern Taiwan, only one
or two coal beds are minable with an average thickness of 20 to 40 centimeters each. The total thickness of
the Mushan Formation is nearly 650 meters in the measured section along the northern coast, but a thickness
range of 450 to 700 meters is reported in different areas and from subsurface data.
In Taoyuan-hsien and Hsinchu-hsien, the Mushan Formation is composed of alternating light
gray, fine-grained sandstone and dark shale in beds several to scores of centimeters thick. Some sandstone
beds reach over 2 meters thick. The whole formation is 600 meters thick here. The white sandstone that is a
characteristic marker of the Mushan Formation in northern Taiwan is only locally and discontinuously exposed
here and is quite insignificant. Only one poor and impure coal bed is intercalated in the lower part of this
formation, about 10 to 30 centimeters thick. It is of poor quality and of little mining value.
The Mushan Formation in Miaoli-hsien is composed of light gray, fine-grained, compact sandstone and
dark gray 'shale. The sandstone is locally banded and laminated, containing disseminated carbonaceous
fragments. The shale is sandy or carbonaceous, yielding some marine fossils. Whitish gray, medium- or
fine-grained sandstone is occasionally found, reaching several meters thick. Irregular and thin coaly lenses
or layers are also sparsely intercalated in the dark shale but are not workable. All these rocks are
intensely folded and fractured, such that the complete stratigraphic sequence and total thickness cannot be
accurately determined. Good exposures are distributed along the Wenshuichi valley to the east of Shangtao
hot springs. The Mushan Formation in north-central Taiwan (Miaoli-hsien and parts of Hsinchu-hsien)
has been named the Wenshui Formation (Ho and others, 1954; Tsan and others, 1961) to distinguish it from its
white-sandstone dominating coal-bearing counterpart in northern Taiwan. The Wenshui Formation is a marine
unit, in contrast to the near-shore carbonaceous facies of the Mushan Formation.
In some coal fields of northern Taiwan, basaltic tuff and a few lava flows are found in the Mushan Formation
in both surface outcrops and underground (Ho and Lin, 1965). These volcanic rocks occur as lentils, streaks,
or irregular blocks of different sizes and shapes intercalated with the sediments at various horizons. The
thickest tuff body is nearly 200 meters, recorded in a hole drilled at Tawulun. The volcanic activity is
probably contemporaneous with sedimentation of the surrounding strata and is associated with the Kungkuan
volcanic stage, which will be discussed in detail with the next stratigraphic unit, the Taliao Formation.
TALIAO FORMATION
The Taliao Formation is a marine unit directly overlying the coal-bearing Mushan Formation in the early
sedimentary cycle. The formation name was proposed by Ichikawa (1930) after the village of Taliao near the
town of Sanhsia, Taipei-hsien. The distribution of the Tailao Formation is the widest of all the
formations in the Yehliu Group. It extends from the northern coast to the Hsilochi stream in
Nantou-hsien. In northern Taiwan and the type locality, the Taliao Formation is a monotonous sequence
of thick-bedded to massive sandstone alternated with thin to thick beds of shale and silty shale, the
sandstone-shale ratio being 1:1 to 1.5:1. The sandstone is light gray to light bluish gray and fine-grained,
mostly subgraywacke or graywacke with lesser protoquartzite. It usually forms precipitous ledges or bluffs
ranging from several to scores of meters thick. The shale is dark gray and is more prominent in the lower
part. The Taliao Formation yields a rich fauna of foraminifers, mollusks, and echinoids. The dominant fossil
is Ditrupa sp. although it is not a guide fossil of this formation. Concentrated Ditrupa
fossiliferous zones generally form good physical evidence for field recognition of the Taliao Formation.
In northernmost Taiwan, one calcareous sandstone member 50 to 60 meters thick occurs in the middle of the
Taliao Formation. This thick sandstone member often forms salient homoclinal ridges and invariably
constitutes most of the headlands or promontories along the northern coast; the most well known of which is
Yehliu. With this massive sandstone as a middle member, the Taliao Formation is divided into three members
in geologic mapping (Ho and others, 1964). Both the upper and the lower members are composed of thick-bedded
sandstone alternating with dark gray shale and silty shale yielding abundant marine fossils. The thickness
of the Taliao Formation in northernmost Taiwan is 500 to 550 meters. However, it was reported only 350
meters thick at the type locality. Southward from the coastal area, the middle sandstone member gradually
becomes obscure and the three-fold subdivision of the Taliao Formation is no longer applicable. In most
parts of Taipei-hsien and Taoyuan-hsien, the Taliao Formation is represented by alternating
thick sandstone and dark gray shale and the total thickness is reduced to 300 to 400 meters. The
argillaceous sediments in the formation increase progressively toward the south until it finally changes
into a predominantly shaly unit, the Piling Shale, in north-central Taiwan.
In the old Japanese reports and maps, one tuffaceous unit, the Kungkuan Tuff proposed by Ichikawa (1930),
was considered as a regular formation underlying the Taliao Formation and overlying the Mushan Formation in
northern Taiwan. The volcanic rocks in Kungkuan Tuff are composed mainly of basaltic pyroclastic rocks and
subordinately of basaltic lavas and tuffaceous sediments with a few thin clastic limestone lentils. The
thickness of the tuff beds exposed in various places ranges from a few meters to a maximum of 200 meters or
more> In recent studies (Ho and Lin, 1965; Ho, 1969), it has been shown that the various tuff bodies mapped
as "Kungkuan Tuff" in the early Miocene strata of northern Taiwan are not necessarily correlative units or
of the same stratigraphic level. The Kungkuan Tuff cannot be defined as a regular and persistent formation
in the lower Miocene succession. The exposed tuff bodies display no physical continuity and assume countless
shapes. Many of them are too irregular to be treated as geometric entities. These tuff bodies differ widely
in geologic occurrence, lateral extension, stratigraphic position, and thickness in different outcrops. They
occur only as a number of irregular and discontinuous lenses or masses intercalated in either the Taliao
Formation, or the Mushan Formation, although they are more common near the base of the Taliao Formation. In
places the thick tuff lens may cross the formation boundary and extend from the lower Mushan Formation into
the overlying Taliao Formation. In other places the tuff unit is completely missing in both the Mushan
Formation and the Taliao Formation. The occurrence of these tuffaceous bodies manifests that they are the
volcanic products erupted from a number of centers active during the deposition of the Mushan Formation and
the Taliao Formation in early Miocene time. Pyroclastic piles were built up from these centers, containing
minor basaltic lava flows. They transgressed different horizons of the sedimentary succession in different
places. The emission of the tuff spanned a much longer time range in the early Miocene than previously
believed. This volcanism has been called the Kungkuan volcanic stage by Yen (1950 and 1958) and Ho (1969).
The tuffaceous rocks formed in the Kungkuan volcanic stage are distributed sporadically in northern Taiwan,
extending from the northern coast to Chiopanshan in the Tahanchi stream of Taoyuan-hsien. The best
development of the tuff cones is in the Chingshuikeng anticline of Taipei-hsien, where the tuff bed
attains a thickness of more than 200 meters.
PILING SHALE
In north-central Taiwan, especially in the southern part of Hsinchu-hsien and in Miaoli-hsien, the Pilling
Shale is the correlative of the Taliao Formation, The formation name was proposed by Ho and others (1954)
and the type locality Piling, is located in the Nanchuang coal field in Miaoli-hsien. The formation is a
thick sequence of compact dark gray to black shale intercalated with a few sandstone beds in the middle. At
the type locality the upper and lower parts of the Piling Shale are composed of black, compact, poorly
bedded shale with irregular claystone concretions and sandy interbeds. The middle part, about 55 meters
thick, consists of light gray, fine-grained sandstone with thin shaly partings and shale interbeds. The
total thickness of the Piling Shale measures 325 meters at the type locality, but the thickness varies
from 300 to 400 meters in different sections. No tuff or other volcanic rocks have ever been found in the
Piling Shale.
TAKENG FORMATION
In central Taiwan, the Takeng Formation is correlative to the Yehliu Group. The formation name was proposed
by Ho and others (1956) and the type locality, Takeng, is located in the hills of Chungliao-hsiang in
Nantou-hsien, The Takeng Formation conformably overlies the Tsukeng Formation mentioned previously.
The lower part of the Takeng Formation consists of light gray, thick- to medium-bedded sandstone and dark
gray shale, about 200 to 300 meters thick. The upper part is represented by dark gray shale and an
alternation of sandstone and shale. It is 400 to 500 meters thick. Glauconite has been found in the shale
and concentrated glauconitic zones have been found in places.
Marine fossils are rather abundant in the Takeng Formation but detailed biostratigraphic studies are still
lacking at present. In the stratigraphic scheme of Chi (1981), the upper part of the Takeng Formation is
correlated to the Taliao Formation; and the lower part, to the Mushan Formation. As the Mushan Formation is
of Oligocene-Miocene age, T. C. Huang (1979a) placed the upper boundary of the Oligocene in the lower part
of the Takeng Formation in central Taiwan.
JUIFANG GROUP
The Juifang Group is a new stratigraphic term first proposed on this geologic map. .This group of rocks
represents the middle sedimentary cycle including a coal-bearing unit (Shihti Formation) and a marine unit
(Nankang Formation) in the Miocene of northern Taiwan. Juifang is a mining town on the Chilung river
southeast of Chilung city and is an important coal producer in Taiwan. The Juifang Group has a much wider
distribution than the Yehliu Group and extends from the northern coast to the southern part of the western
foothills. Formations correlative to the Juifang Group have also been found in the Hengchun Peninsula at the
southern tip of Taiwan. The Juifang Group and the Yehliu Group discussed in this report were grouped together
and named the Hsichih Group in previous Japanese maps and reports (L.S. Chang, 1953).
SHIHTI FORMATION
The Shihti Formation is the most important of the three Miocene coal-bearing formations in western Taiwan.
It conformably overlies the Taliao Formation and underlies the Nankang Formation. This carbonaceous unit was
first named the Middle coal-bearing formation by Ichikawa (1930). The term Shihti Formation was introduced
later by Yen and Chen (1953). The type locality, Shihti, is located at the headwaters of the Chilung river
in Pingchi-hsiang of Taipei-hsien. The Shihti Formation and the coal beds are best developed in
Chilung city, Taipei-hsien, and Taoyuan-hsien. Several scattered belts of the Shihti
Formation are exposed also in Hsinchu-hsien and Miaoli-hsien to the south. Further southward
to Taichung-hsien and Nantou-hsien, the carbonaceous shelf-type sediments in the Shihti
Formation are gradually transitional into marine rocks, manifested by the disappearance of coal beds and by
the increasing number of marine organisms.
The Shihti Formation in northern Taiwan consists of sandstone, siltstone, shale, and thin coal beds. The
characteristic rock type is a lamination of sandstone, siltstone, and shale represented by irregular bands
of white silt or sand and dark gray shale ranging in thickness from a few millimeters to several
centimeters. These interlaminated beds are characteristic of a coastal tidal flat to lacustrine environment.
They are also characteristic of the other two Miocene coal-bearing formations in western Taiwan: the Mushan
Formation and the Nanchuang Formation. The shale is dark gray or grayish black and often carbonaceous. It
grades into claystone where the fissility disappears. The sandstone is mostly feldspathic, light gray to
white, and fine- to medium-grained. Massive and thick white sandstone in the lower part of the formation
often forms precipitous scarps and constitutes good lithologic markers. Coal beds and layers are abundant
throughout the whole formation. Workable coal beds are found mostly in the upper part, with an average
thickness of 0.3 to 0.6 meter each. They are non-persistent, with local pinches and swells.
In northernmost Taiwan, the Shihti Formation is divided into two members by Ho and others (1964). The lower
Patoutze Member is characterized by three massive and cliff-forming white sandstone beds, each 12-15 meters
thick. This member is 160 meters thick and contains only one workable coal bed, which is near the bottom.
The upper Szechiaoting Member is 150-160 meters thick and is more diversified in lithology. It is composed
of laminations or alternations of sandstone and shale. A total of five workable coal beds have been found in
the upper member, with the main coal bed reaching a maximum thickness of one meter. Except brackish-water
gastropods, the Shihti Formation is poor in marine fossils but rich in plant remains. Cross-bedding, ripple
marks, and other sedimentary features indicating shallow water deposition are common in the sandstone. The
banded lamination, the predominance of arkosic and orthoquartzitic sandstone, the scarcity of marine
organisms, and the presence of coal beds suggest that the Shihti sediments were accumulated on a coastal
plain or tidal flat area sloping toward the sea. They represent transitional sedimentary deposits on the
unstable shelf of a sedimentary basin. A network of delta lakes, marshes, and shifting stream channels is
characteristic of this depositional environment, which is favorable for the growth of peat and other
carbonaceous rocks (Ho, 1966).
Away from the northern coastal area, the white sandstone marker beds in the lower part of the Shihti
Formation become indistinct, and the Patoutze and Szechiaoting Members cannot be separated. In a large part
of Taipei-hsien, Taoyuan-hsien, and Hsinchu-hsien, the Shihti Formation becomes a
single unit composed of dark gray shale and carbonaceous shale, light gray to whitish gray sandstone, and
banded laminations of shale, sand and silt. Although five coal beds are intercalated in the formation, only
one to three coal beds are minable. The thickness of the Shihti Formation increases to 400 to 450 meters.
Sedimentlogical study indicates that the Shihti Formation thickens gradually from the northwest toward the
southeast, the thickest sequence being 600 meters, measured in the Nanchuang coal field of
Miaoli-hsien. The sandstone percentage decreases slightly in this same direction.
In Miaoli-hsien, the Nanchuang coal field is well known for the production of excellent coking coal
from the Shihti Formation and there it consists of a varied sequence of sandstones, shales, sandy shales,
and interlaminations of sandstone and shale. Only one irregular coal bed averaging 25 centimeters thick is
workable. The total thickness of the Shihti Formation in this locality is 500 to 600 meters. To the west of
the Nanchuang coal field, the Shihti Formation is exposed in the core of the Chuhuangkeng anticline, the
important oil-producing structure of Taiwan. Here the Shihti Formation is known as the Chuhuangkeng Formation
by petroleum geologists. Only a small part of the Chuhuangkeng Formation is exposed, represented by thin
alternations of light bluish gray sandstone and dark gray shale. Thin coal lenses have been found, mostly in
the subsurface. The coal is of inferior quality and not minable. The measured subsurface thickness of the
Chuhuangkeng Formation reaches 500 meters and more.br>
No tuff or other volcanic rocks have been found in the Shihti Formation northeast and east of Taipei.
However, volcanic rocks are emplaced in many coal belts included in the Shihti Formation to the southwest
and south of Taipei. These volcanic rocks occur only as small irregular bodies of limited extent. They are
often encountered in the underground mine workings and can seldom be mapped on the surface. This volcanic
activity is named the Chienshih volcanic stage in the present text. The main volcanic rocks are basaltic
tuff and breccia. Lava flows, which are less common, are represented by olivine basalt and olivine
teschenite. These tuffaceous rocks either completely truncate and take the place of the coal beds in the
mines or are mixed with the coal in variable proportions. They have been found also in the country rocks.
They may be largely the product of contemporaneous pyroclastic or lava eruption in the coal-bearing
sediments. The actual mode of occurrence of these tuffaceous rocks is uncertain because many of them can be
observed only in the subsurface adits and are not available to field studies.
The Shihti Formation is a coal-bearing unit deposited in environments that span the transition between
marine and continental. Marine organisms are rare and guide fossils cannot be adequately determined. Coal
and carbonaceous strata serve as the important clue in identifing this carbonaceous unit in field mapping.
However, the coal marker beds gradually disappear south of Miaoli. It is then quite difficult to identify
the Shihti Formation lithostratigraphically further to the south, where the equivalent unit of the Shihti
Formation can be only arbitrarily delineated. As the Tertiary stratigraphy in Taiwan is marked by
intertonguing relationships of the rock units in a vertical sequence, the exact correlation of the Shihti
Formation in central and southern Taiwan is not well defined and may vary between different workers. Based
on the correlation table of Chi (1981), the Shihti Formation is correlated to the lower part of the
Shuilikeng Formation in central Taiwan. The Shuilikeng Formation is discussed in the next section.
NANKANG FORMATION
The marine rocks overlying the Shihti Formation were divided into two stratigraphic units in previous
Japanese geologic maps, the Tsouho Formation below and the Nankang Sandstone above (Ichikawa, 1930).
However, the stratigraphic sections in the type localities of these two formations cannot be compared with
each other. Field differentiation of these two formations is often vague and arbitrary due to lack of a
well-defined boundary that is objective and easily discernible. For this reason an old term, "Upper marine
fossil beds", was proposed by some Japanese geologists as a mappable unit to include both the Tsouho
Formation and the Nankang Sandstone. Other workers called this unit the Nankang-Tsouho Formation, which is
not a proper term because it includes two geographic names in one formation. The Nankang Formation was
proposed and used for field mapping by Ho and others (1964) to replace the "Upper marine fossil beds," which
also is not an acceptable formal stratigraphic name. The Nankang Formation thus defined includes all the
marine strata overlying the Shihti Formation and underlying the Nanchuang Formation, or all the rocks
formerly ascribed to the Tsouho Formation and the Nankang Sandstone. The type locality, Nankang, is a large
town between Taipei and Chilung, and the type section is exposed east of this town.
The Nankang Formation is composed of thick- to thin-bedded, light bluish gray, fine-grained calcareous
sandstone and dark gray shale or siltstone, yielding a rich fauna of foraminifers and mollusks. The
sediments were deposited chiefly in a shallow neritic environment. The sandstone is mainly lithic graywacke
and subgraywacke. Thick and massive, cliff-forming sandstone is prominent in the Nankang Formation of
northern Taiwan. Due to marked facies change and variable sandstone-shale ratio, the subdivision and
nomenclature of the Nankang Formation differ in different parts of Taiwan from north to south so that it is
called locally by different names. The Nankang Formation is divided into two massive sandstone members and
three sandstone-shale members in northern Taiwan. These five members merge into one middle shale member and
two sandstone-dominant members in north-central and central Taiwan. South of the Hsilochi stream, the
sandstone that dominates the Nankang Formation passes into an un differentiated dark gray shale sequence.
The arenaceous constituents in the Nankang Formation and its correlative formations thus decrease gradually
from north to south and argillaceous sediments increase proportionally in the same direction. There is also
an apparent increase in thickness toward the south.
In northernmost Taiwan between Taipei and Chilung, the Nankang Formation has been divided into five members
in the study of coal geology (Ho and others, 1964; M. Y. Hsu, 1964). Two of these members are composed of
massive and cliff- forming sandstone whereof the Nankang Sandstone was originally defined. These five
members are described below in ascending order.
1. Shihjen Member (160-180 m)
Alternating beds of fine-grained light gray to light bluish gray sandstone and dark gray shale ranging in
bed thickness from less than one meter to 10 meters.
2. Nuannuan Sandstone Member (110-150 m)
Light bluish gray, fine-grained, calcareous sandstone, richly fossiliferous in part and often forming sharp
ridges or deep gorges. Dark gray fossiliferous shale beds 1 to 5 meters thick are intercalated in the lower
and middle parts.
3. Tahua Member (80-130 m)
Thin to medium interbeds of dark gray shale or mudstone and light bluish gray, fine-grained sandstone with
banded laminations of sandstone and shale.
4. Hsinliao Sandstone Member (110-150 m)
Massive to thick-bedded, light bluish gray, calcareous, fine-grained sandstone. The sandstone is
cliff-forming and fossiliferous, resembling the Nuannuan Sandstone Member in lithologic character and
topographic expression. Thin gray siltstone and shale are intercalated in the sandstone.
5. Shihfenliao Member (160-220 m)
Thin-bedded, dark gray shale and light bluish gray sandstone in the lower part; alternation of sandstone and
shale in the upper.
The total thickness of the Nankang Formation is 700 to 750 meters in northernmost Taiwan. The lowest member,
the Shihjen Member, can be correlated with the Tsouho Formation of the old reports, and the other four
overlying members may be equivalent to the Nankang Sandstone. However, there are two sandstone members of
similar lithology and thickness in the Nankang Formation, the Nuannuan Member and the Hsinliao Member. Field
geologists often confuse these two sandstone members and indiscriminately put the lower boundary of the
Nankang Sandstone either at the base of the Nuannuan Member or the base of the Hsinliao Member depending on
which member they found in the field first. This results in much stratigraphic confusion in field mapping.
Because the Nankang Sandstone and the Tsouho Formation are both composed of similar marine clastic rocks
intercalated between two coal-bearing units, they are combined into one formation in most later papers. On
the basis of local lithologic variations and sandstone-shale ratio, the Nankang Formation can be further
subdivided into different member categories for detailed mapping in different areas.
One dark lenticular vitric tuff bed is exposed in the lower part of the Shihfenliao Member in Lilao, which
is east of Chinkuashih on the northern coast. This is believed to be a contemporaneous tuffaceous bed. Other
tuffaceous bodies are also disseminated irregularly in the Nankang Formation to the south of Taipei. The
important area is Maohoshen in Chienshih-hsiang, Hsinchu. This volcanic activity is thus ascribed to
the Chienshih volcanic stage as mentioned previously.
To the south and southwest of Taipei, the Nankang Formation cannot be divided into the five members
recognized to the north, because the sandstone members have diverged into many separate sandstone beds with
thick intervening shales. Instead, a distinctive thick fossiliferous shale or siltstone interval in the
middle part serves to subdivide this formation into three members for field mapping and structural analysis.
The upper and lower members are composed of interbedded sandstone and shale, but the sandstone usually
predominates, often in thick beds that form cliffs. These three units locally have different names. They are
considered either of member rank or of formation status depending on the geologists who named them. For a
general description of the stratigraphy, the three units of the Nankang Formation in the Chuhuangkeng oil
field of Miaoli-hsien (Chinese Petroleum Corporation, 1971) are discussed in ascending order.
1. Peiliao Sandstone (300-400 m)
This lower unit is also called the Peiliao Formation in some papers. It is composed of light gray to light
bluish gray, fine- grained sandstone which is partly muddy or calcareous. Bedding is generally not distinct
except where shale in intercalated with the sandstone; bed thickness ranging from one to five meters or
more. The sandstone forms precipitous cliffs and is often dissected by deep gorges. Marine fossils abound in
the upper rather muddy part of this sandstone unit. The sandstone commonly contains intercalations of dark
gray or grayish black shale, which reach several meters thick. Alternations of sandstone and shale are more
predominant in the upper part.
2. Talu Shale (250-340 m)
This middle unit is characterized by gray to dark gray shale, interbedded with a few sandstone or siltstone
lentils. The shale abounds in foraminifers and other marine fossils. Toward the western coastal area, light
gray beds of protoquartzite are thicker and more abundant in the subsurface sections of the Talu Shale. This
sandstone is the most important reservoir in the oil and gas fields of Taiwan. The sandstone gradually thins
eastward to the Miaoli area and is seldom exposed. It wedges out completely before reaching the Pakuali
anticline to the east.
3. Kuanyinshan Sandstone (150-300 m)
The upper member of the Nankang Formation consists dominantly of light bluish gray to light gray,
fine-grained calcareous sandstone, intercalated with dark gray shale and interlaminated sandstone and shale.
One poor coal bed is locally found in the upper part. The sandstone is thick-bedded or massive and
precipitous to ledge-forming. Mollusks and foraminifers are abundant in the calcareous sandstone,
which contains several concentrated zones of Operculina ammonoides for which this unit was called the
"Operculina Sandstone" in old reports.
The three-fold subdivision of the Nankang Formation is applicable to a large part of western Taiwan,
including Taoyuan-hsien, Taichung-hsien, and Nantou-hsien Although lithology and
thickness varies locally in these units, the overall lithology and scheme of stratigraphic classification
remain the same in all these areas. The non-resistant Talu Shale forms a readily recognized middle member in
this three-fold subdivision. The shale usually forms a topographic depression. The rich fossil content is
also characteristic of the Talu Shale. North of the Chuhuangkeng oil field, this middle shale unit is
composed mainly of siltstone and shale in a large part of Hsinchu-hsien. This siltstone unit is 150
to 250 meters thick and has different names in different areas. It is generally considered as a member
within the Nankang Formation. The siltstone is gray to light bluish gray and massive. It grades either into
muddy sandstone or into dark gray mudstone. Exfoliation features are well developed on large or small scales.
The concentrated siltstone is highly calcareous and is composed of quartz and calcite or calcareous shells
in almost equal amounts.
In the Kuansi area of Hsinchu-hsien, irregular limestone beds are intercalated in the lower part of
the Nankang Formation (Peiliao Formation or its equivalents). These beds make up the most important
limestone in the Miocene of northern Taiwan. Several lenticular limestone beds have been discovered, mostly
of irregular extension and grading into calcareous sandstone laterally. Each limestone bed is several to
scores of meters thick and several hundred meters to a few kilometers long. The thickest part may reach 150
meters. Tuffaceous lenses are also found locally associated with the limestone.
In central Taiwan, the equivalent unit of the Nankang Formation is called the Shuilikeng Formation, which
was proposed by Ho and others (1956) during their field mapping in the Nantou coal fields. The type locality
Shuilikeng is a small town on the stream Hsilochi in the southeastern part of Nantou-
hsien
. The
Shuilikeng Formation conformably overlies the Takeng Formation and underlies the Nanchuang Formation. It is
formed mainly of massive to thick-bedded sandstone alternating with dark gray shale. The middle part of this
unit is marked by a fossiliferous shale interval named the Changhukeng Member. This shale member is several
hundred meters thick and serves to subdivide the Shuilikeng Formation into three members as in the Nankang
Formation of northern Taiwan. The upper and lower members are both composed of thick-bedded or massive,
cliff-forming, light gray sandstone with dark shale intercalations. Glauconite is abundant in the middle and
lower parts of the Shuilikeng Formation but is generally absent in the upper member. The fossil content of
the Shuilikeng Formation is comparable to that of the Nankang Formation, but the upper and lower boundaries
of the two formations are probably not precisely equivalent. In the correlation table of Chi (1981), both
the upper and lower limits of the Shuilikeng Formation may extend beyond those of the Nankang Formation of
northern Taiwan. This is the reason for using a different name for the strata in central Taiwan generally
equivalent to the Nankang Formation.
South of the Hsilochi stream, strata correlative to the Nankang Formation are argillaceous beds recognized
only locally. Sandstone occurs only as sparse, thin interbeds. Strata in southern Taiwan correlative to the
Nankang Formation have been named the Tapang Formation or the Sanmin Shale, two quite similar marine units.
The Tapang Formation was named by Tsan and Keng (1962) after the villuge of Tapang in Chiayi-hsien.
It is composed chiefly of dark gray compact shale containing some thin carbonaceous particles. Light gray,
fine-grained sandstone beds, about several to 20 centimeters thick and partly calcareous, are frequent in
the upper part. The strata of the Tapang Formation are often intricately folded and even overturned in
places. The total thickness is unknown, but at least 800 meters are exposed. Recent field studies have
demonstrated that the Tapang Formation in the Kaohsiung area is correlative and continuous with the Lushan
Formation of similar geologic age. The Sanmin Shale was proposed by Chung (1962) for the shaly sediments
exposed in the axial zone of the Hunghuatze anticline in Kaohsiung-hsien north of Chiahsien. This
unit is composed largely of dark gray shale with a few thin muddy sandstone beds, and is richly
fossiliferous. Very thin lenticular coal or coaly shale is found in the lower part of this formation. The
thickness of the Sanmin Shale is at least 800 meters but the base is not exposed. Recent biostratigraphic
studies show that the Sanmin Shale is largely correlative with the Nanchuang Formation in northern Taiwan.
Only the lower part of the Sanmin Shale is possibly equivalent to the Nankang Formation
(see Table 5).
SANHSIA GROUP
This group represents the youngest Miocene sedimentary cycle in western Taiwan. The name of this group was
proposed by Ichikawa (1929) when he first surveyed the coal fields in northern Taiwan. Sanhsia is a large
town southeast of Taipei on the border between Taipei-hsien and Taoyuan-hsien. This group is
made up of a lower coal-bearing formation (Nanchuang Formation) and an upper marine unit (Kueichulin
Formation) in northern Taiwan. Both formations are composed predominantly of thick sandstones. However,
marine argillaceous sediments increase appreciably toward the south with considerable increase in total
stratigraphic thickness. The Sanhsia Group is the most extensively distributed Miocene unit in western
Taiwan. It is especially widespread in southern Taiwan, where the western foothills are underlian largely by
the Sanhsia Group and some overlying Pliocene rocks.
NANCHUANG FORMATION
The Nanchuang Formation has been used by many authors to replace the old term "Upper coal-bearing
formation," which does not carry a geographic name. Ho (Ho and others, 1954) first used this name and called
this unit the Nanchuang coal-bearing formation. The term Nanchuang Formation is now applied chiefly to the
upper Miocene coal-bearing rocks in western Taiwan. The type locality Nanchuang is in the drainage of the
Chungkangchi stream of Miaoli-hsien. The Nanchuang Formation is conformably overlain by the
Kueichulin Formation and underlain by the Nankang Formation. It is reported missing in some wells drilled by
the Chinese Petroleum Corporation on the western coast in central Taiwan. This upper coal-bearing formation
has the widest distribution among the three Miocene carbonaceous units of western Taiwan. It extends from
the north coast southward to the Alishan Range in Chiayi-hsien.
In northern and central Taiwan, the Nanchuang Formation is represented by deltaic deposits laid down in a
mixed continental and marine environment. It contains a number of coal beds, abundant coarse clastic
deposits, and scarce marine fossils. The best developed coal is in north-central Taiwan (Miaoli-hsien
and Hsinchu-hsien)where there are up to five workable coal beds. No important coal beds have been
discovered in the Nanchuang Formation in northern .Taiwan, where coal mining is operated discontinuously on
a small scale. No persistent and minable coal beds are present in the Nanchuang Formation in central Taiwan.
Lenticular coal beds, however, are exposed in the Nanchuang Formation further south in Nantou-hsien
hsien and Chiayi-hsien. Small-scale coal mining was attempted in several southern areas without much
success. South of the Alishan Range, the Nanchuang Formation is characterized mainly by shallow marine
strata with increasing amounts of argillaceous sediment and marine fossils. Several different names have
been used for the Nanchuang Formation due to the facies changes observed.
The Nanchuang Formation was called the Wutu Formation in northernmost Taiwan by Yen and Chen (1953). Wutu is
a small town between Taipei and Chilung. In Wutu the lower part of the Nanchuang Formation is exposed in the
core of a syncline where the section is incomplete and cannot be accepted as the type locality of a unit of
formation rank. Exposures of the Nanchuang Formation are generally small and localized in northern Taiwan
except in one coal belt on the northern coast east of the Chinkuashih gold-copper mine, where a more
complete section is observed. Here the formation consists of massive to thick-bedded, white, medium- grained
sandstone, interbedded with dark gray shale, light bluish gray mudstone, and banded interlaminations of
sandstone, siltstone, and shale. The white sandstone is highly quartzose to subarkosic, and ranges from
fine- to coarse-grained. Two thin and irregular coal beds that are locally of mining value are found in the
lower part. Several thin uneconomic coaly lenses are present in the upper part. The total thickness of the
Nanchuang Formation in northernmost Taiwan is 600 to 700 meters. The upper contact is gradational into the
overlying Tapu Formation.
In the southwestern part of Taipei-hsienand Taoyuan-hsien, the Nanchuang Formation is composed
of interbedded laminations of sandstone, siltstone and shale, grayish white to light gray, fine-grained
sandstone, and dark gray carbonaceous shale. Sandstone beds are several to 20 meters thick, with
disseminated carbonaceous fragments, coal particles, and ferruginous concretions. The upper part of the
Nanchuang Formation is characterized by a white sandstone member, about 100 meters thick, that contains
minor thin-bedded sandstone and shale. The sandstone is medium-grained and poorly compacted, in beds 2 to 5
meters thick. It contains coaly lenses and iron concretions and is often oxidized to brownish red color. The
total thickness of the Nanchuang Formation is 500 to 600 meters. The Nanchuang Formation contains several
thin and irregular to discontinuous coal beds but workable coal is restricted to locally thick lenses.
The Nanchuang Formation in Taoyuan-hsien and Hsinchu-hsien is characterized by numerous
volcanic beds in its lower and middle parts. The volcanic rocks are mainly basaltic tuff and tuff breccia,
with subordinate basaltic flows. Three to five tuff beds or bodies are intercalated in the rocks without any
definite stratigraphic horizon. They extend discontinuously along the strike of the strata or slightly cut
across it. They vary from 2 to 30 meters thick, infrequently to 100 meters or so, and from scores of meters
to nearly 2 kilometers long. These volcanic beds represent pyroclastic piles erupted from scattered centers
in the marginal basin where the Nanchuang sediments were deposited. The volcanic products spread out
laterally from local vents and were incorporated in the surrounding sediments as irregular and discrete
bodies. This extensive volcanic activity has been ascribed to the Chiopanshan volcanic stage. (Yen, 1958;
Tsan, 1961)
The Nanchuang Formation is the most important coal-bearing formation m Hsinchu-hsien and
Miaoli-hsien and is best developed in the Szetoushan coal field of Miaoli-hsien, where it
contains a maximum of 5 workable coal beds. It is composed of white sandstone, dark gray shale, and
interlaminated sandstone, siltstone and shale. The Nanchuang Formation is divided into an upper white
sandstone member and a lower sandstone and shale member, totaling 800 to 900 meters thick (Ho and Keng,
1953). The members are known locally by different names. In the Chuhuangkeng oil field of
Miaoli-hsien, the lower member is named the Tungkang Formation and the upper member, the Shangfuchi
Sandstone. They have been raised into formational rank in this and many other areas.
The lower member is 550 to 750 meters thick, and is composed chiefly of interlaminated white sandstone or
siltstone and dark gray shale, resulting in irregular and lenticular alternating white and dark bands
several millimeters to several centimeters thick. Gray to grayish white, thick-bedded sandstone is commonly
interbedded with the laminated beds. The sandstone ranges in thickness from one to 10 meters or more and
contains abundant shaly fragments and red ferruginous nodules. Several thin coal lenses have been found,
and one more continuous lower coal bed is minable in some coal fields of Miaoli-hsien. The upper
white sandstone member is 60 to 150 meters thick, and consists of white sandstone which is made up of medium
to coarse rounded to subrounded quartz sand grains loosely cemented by clay and sand. The sandstone is
massive to thick-bedded, and commonly forms precipitous scarps. It is interbedded with minor laminations of
sandstone and dark gray shale. The upper member contains 7 to 8 coal horizons generally discontinuous
lenses. At any given locality, only one or two coal beds generally reach a minable thickness of 0.3 meter.
In the Szetoushan coal field, the upper white sandstone member contains three to four persistent and minable
coal beds, each about 0.3 to 0.4 meter thick, with a maximum of 0.8 meter thick.
No workable or persistent coal beds have been found in the Nanchuang Formation south of the Houlungchi stream
in Miaoli-hsien and Taichung-hsien, where this formation is characterized by light gray to
grayish white, fine-grained sandstone and dark gray shale (Ho and Tan, I960). The sandstone is massive to
thick-bedded, with little to no white sandstone characteristic of the Nanchuang Formation elsewhere. Thinly
laminated sandstone, siltstone and shale still form a distinctive lithologic marker in the formation,
interbedded with dark gray shale and irregular coal stringers. Owing to the lack of white sandstone and
workable coal, the Nanchuang Formation cannot be subdivided here, and field recognition of this formation is
difficult due to poorly defined upper and lower boundaries. The general thickness of this formation ranges
from 550 to about 650 meters.
The Nanchuang Formation in Nantou-hsien consists of a variable sequence of sandstone, dark gray
shale, and interlaminations of sandstone and shale. Sandstone is still the predominant rock type. It is
mostly light gray, fine-grained, and thick- bedded. White medium-grained sandstone similar to that in
northern Taiwan is restricted to few beds of medium thickness, found in close proximity to two discontinuous
coal beds, each about 0,2 to 0.3 meter thick. The coal beds are extremely lenticular and vary from
disseminated carbonaceous particles to thin laminations, and to coaly stringers or lenses. All the different
lithologic components in the Nanchuang Formation grade into one another vertically and laterally. The total
thickness is 500 to 600 meters. The Nanchuang Formation in central Taiwan is still a carbonaceous unit
deposited close to the shore. Marine mollusks and foraminifers are more abundant here than toward the north,
indicating more marine influence and a more marine environment toward the south.
The southern end of the coal-bearing Nanchuang Formation is in the Alishan Range in Chiayi-hsien . The
main rock types in the Alishan coal field are light bluish gray thick-bedded sandstone, interlaminated
sandstone and shale, and dark gray shale (L.S. Chang and others, 1960). Several thin beds of white sandstone
represent the lithology characteristic of the Nanchuang Formation in northern Taiwan. The upper part of this
formation contains four coal lentils that vary greatly in thickness and extension. Marine fossils are much
more abundant than in equivalent strata to the north, although broken plant remains are found in
carbonaceous shale close to the coal beds. With the base not exposed, the measured thickness of the
Nanchuang Formation is more than 1,000 meters in the Alishan area.
In southern Taiwan (Tainan-hsien and Kaohsiung-hsien), the coal-bearing Nanchuang Formation is
replaced by marine strata which include, in ascending order, the main part of the Sanmin Shale, the
Hunghuatze Formation, and the Changchihkeng Formation. The Sanmin Shale has been discussed in the previous
section. The Hunghuatze Formation was named by Chung (1962) and the type locality Hunghuatze is located in
the Hunghuatze anticline in Kaohsiung-hsien. This formation is a sequence of gray, fine- to
medium-grained thick-bedded sandstone with interbeds of dark gray shale and gray muddy sandstone. It
reaches 1,200 meters thick. The Changchihkeng Formation was proposed by Ho (1956) when he mapped the
Chutouchi oil field. The type locality Changchihkeng is on the Tapuchi stream in Tainan-hsien. The
Changchihkeng Formation is a thick sequence of dark gray shale and light gray fine-grained sandstone
containing an assemblage of varied marine faunas. The sandstone is dense, compact, and partly calcareous
subgraywacke, locally containing carbonaceous matter and broken plant remains. No white sandstone or coal
beds have been found. The shale is very compact and slightly indurated. Local basaltic tuff lenses reach a
maximum thickness of 20 meters and a maximum length of 2 kilometers. The total thickness of the
Changchihkeng Formation is well over 1,000 meters and may reach 1,600 meters or more toward the south and
the east.
KUEICHULIN FORMATION
The Kueichulin Formation and its equivalents constitute the uppermost Miocene unit in western Taiwan, and a
large part of this formation has been ascribed to the Pliocene by recent paleontologic studies. The
Kueichulin Formation is represented generally by shallow marine sandstone and shale that conformably
overlies the Nanchuang Formation. The formation name was first proposed by Torii and Yoshida (1931) in the
Chuhuangkeng oil field of Miaoli-hsien. In northern Taiwan, the Kueichulin Formation is usually
divided into two sandstone units in geologic mapping. This formation is represented by two
sandstone-dominated units and an intervening shale member in central and south- central Taiwan. In southern
Taiwan, this dominantly sandstone formation grades laterally into a number of shaly and sandy units and
finally into a thick shale series with a subordinate sandstone member south of Chiahsien in the lower course
of the Nantzehsienchi stream in Kaohsiung-hsien. This suggests that the area of subsidence during
deposition of the Kueichulin Formation was gradually deepening from northern Taiwan toward southern Taiwan.
The total thickness of the Kueichulin Formation also increases from 800 meters in the north to 2,000 meters
or more in the south.
In northern Taiwan (Taipei-hsien, Taoyuan-hsien, Hsinchu-hsien, and parts of
Miaoli-hsien), the Kueichulin Formation is generally divided into two units, both composed mainly of
thick-bedded or massive sandstone. The sandstone is fine- grained, light bluish gray, and generally muddy or
clayey. It is composed of protoquartzite, lithic graywacke, and subgraywacke. Sand and clay in the sandstone
are often intimately mixed in various proportions in the sandstone. Dark gray shale interbeds are common in
this formation, in places reaching 10 meters thick, and contain disseminated quartz grains or irregular
patches of quartz aggregates. The two units of the Kueichulin Formation in northern Taiwan are the Tapu
Formation and the overlying Erhchiu Formation. These two units were proposed by Ichikawa (1930) and the type
localities are in Sanhsia (the name of this Group), Taipei-hsien.
The Tapu Formation is composed of thick-bedded, light gray muddy sandstone. The muddy interbeds in the
sandstone yield abundant foraminifers and mollusks. Calcareous sandstone is more predominant in the south,
where dissolution by groundwater has produced shallow caves. The cavernous sandstone landscape in Szetoushan
of Miaoli-hsien is characteristic of the calcareous sandstone. A few lenticular beds of white
sandstone are commonly intercalated in the Tapu Formation. The white sandstone is especially prominent and
abundant on the northern coast, and is locally indistinguishable from the underlying Nanchuang Formation.
The thickness of the Tapu Formation is 300 to 400 meters. The overlying Erhchiu Formation is also composed
of thick-bedded, light bluish gray muddy sandstone. However, alternations of sandstone, siltstone, and shale
are more abundant in this formation. The sandstone is comparatively loosely consolidated and the degree of
cohesion seems to decrease slightly upward. Very little calcareous sandstone and no white sandstone die
found in this formation. These features distinguish the Erhchiu Formation from the Tapu Formation, but the
differences are subtle. The thickness of the Erhchiu Formation is usually 400 to 500 meters but is reported
to be nearly 600 meters at the type locality.
The Kueichulin Formation in the Chuhuangkeng oil field (type locality) is separable into two parts roughly
correlative with the Tapu and Erhchiu Formations. The lower part consists of thick-bedded sandstone
interbedded with dark gray shale. The upper part is composed of alternating muddy sandstone and shale with
some interbeds of grayish white sandstone. South of the type locality, a distinctive shale member over 100
meters thick is exposed in the middle of the Kueichulin Formation, enabling the subdivision of this
formation into three members. In a large part of central Taiwan, the three correlative units of the
Kueichulin Formation are named the Kuantaoshan Sandstone, the Shihliufeng Shale, and the Yutengping
Sandstone in ascending order. The type sections and localities are in Sanyi of Miaoli-hsien and
Fengyuan of Taichung-hsien.
The Kuantaoshan Sandstone was applied by Lin (1953) to a sequence of sandstone and shale exposed in
Kuantaoshan near Sanyi of Miaoli-hsien. At the type locality, the formation is composed of fine- to
medium-grained, light bluish gray sandstone, interbedded with occasional dark gray shale and rare pebbly
bands. The thickness is about 250 to 300 meters but increases to 500 meters toward the south. The sandstone
is massive or thick-bedded, generally cliff-forming. It is muddy and contains shale fragments and thin coaly
stringers. It is calcareous where shells of mollusks are concentrated.
The name Shihliufeng Shale was applied by Chang and Ho (1948) to a shale sequence exposed in the Taan
anticline in Miaoli-hsien. It is composed dominantly of dark gray shale, yielding a large assemblage
of foraminifer, mollusk, and crab faunas. This shale unit forms a useful marker horizon, distinguished by
its subdued landform in outcrop and by its rich fossil content. However, this unit shows significant lateral
variation. The Shihliufeng Shale is best developed in the southern part of Miaoli-hsien and in
Taichung-hsien and wedges out toward the north. It is poorly developed in the Chuhuangkeng oil field,
the type locality of the Kueichulin Formation, and is not mappable further to the north. Near the type
locality, the total thickness of the Shihliufeng Shale varies from 100 to 200 meters but increases to 250
meters or more in southern Taiwan.
The Yutengping Sandstone was first proposed by Lin (1954) as the uppermost division of the Kueichulin
Formation. The Tawu Siltstone of Chang (1955) is an equivalent. The Yutengping is a sandstone unit with
abundant shale interbeds. The sandstone is gray to light gray, fine-grained, thick- to medium-bedded, and
generally forms rows of hogback ridges. It is impure, containing considerable muddy matrix. Thin interbeds
and interlaminations of sandstone and shale are common. Some carbonaceous particles are dispersed in the
sandstone. The total thickness of the Yutengping Sandstone ranges from 250 meters to a maximum of 550
meters.
In the Chiayi-Tainan area in central-south Taiwan, a group of three formations are equivalent to the
Kueichulin Formation. They are, in ascending order, the Tangenshan Formation, the Chunglun Formation, and
the Niaotsui Formation. The Tangenshan Formation is correlated to the Kuantaoshan Sandstone and derives its
name from the Chutouchi oil field (Ho, 1956). It is described in detail below. No significant outcrop of the
Tangenshan Formation is found in south-central Taiwan. The stratigraphic relation between the Tangenshan
Formation and its overlying Chunglun Formation is not clear. Some geologists are of the opinion that the
lower part of the Chunglun Formation could be the lower boundary of the Kueichulin Formation, and that a
Tangenshan Formation equivalent is not present in south-central Taiwan. Both the Chunglun Formation and the
Niaotsui Formation were named by Stach (1957) and their type localities are in the Chunglun anticline in
Chiayi-hsien (old Tungtzechiao oil field). The Chunglun Formation is composed mainly of grayish black shale
and sandy shale with local intercalations of muddy sandstone. The total thickness is approximately 500
meters including both surface and subsurface sections. The Niaotsui Formation is fanned of gray sandstone
and muddy sandstone with minor shale and sandy shale intercalations. The total thickness is 650 meters.
These two formations can be correlated to the Shihliufeng shale member and the Yutengping sandstone member
of the Kueichulin Formation. The top part of the Niaotsui Formation may also be correlated with the basal
part of the Chinshui Shale in northern Taiwan.
In southern Taiwan in the areas of Tainan-hsien and the n3rthern part of Kaohsiung-hsien
, strata correlative to the Kueichulin Formation are divided into four formations (Ho, 1956; S. L. Chang
and Chung, 1957). All four units were first named and described in the Chutouchi oil field in
Tainan-hsien. Currently paleontologists correlate the upper two units, the Maopu and the
Ailiaochiao, with the Yutengping Sandstone of central Taiwan. The third unit, the Yenshuikeng Shale, is
correlated with the Shihliufeng Shale. The lowest Tangenshan Formation is correlated with the Kuantaoshan
Sandstone (see Table 5).
1. Tangenshan Formation
Light bluish gray, fine-grained sandstone is the main rock type of this formation. The sandstone is massive
and irregularly jointed, often forming sharp cliffs and deep ravines. Bedding is rarely discernible. The
sandstone is subgraywacke and graywacke, often muddy and ranging from muddy sandstone to sandy claystone.
The sandstone is usually interbedded with thick dark gray shale and gray sandy shale. The total thickness
of the formation is 450 to 500 meters.
2. Yenshuikeng Shale
This is a thick sequence of dark gray shale with occasional lenticular interbeds of siltstone and sandstone
which are more prominent in the lower part. Due to lack of fissility, the shale is actually claystone or
mudstone in most places. The siltstone is rich in red ferruginous concretions and small calcite veins.
Fossils are abundant in the more calcareous beds. The known thickness of this shale unit varies from 200 to
250 meters.br>
3. Ailiaochiao Formation
The Ailiaochiao Formation consists dominantly of thinly laminated dark gray shale and white sandstone or
siltstone. The lower part of this formation is made up of gray, fine-grained banded sandstone in beds 1 to
2 meters thick. Carbonaceous fragments, sand pipes, and ripple marks have been found in the rocks. The
thickness of this formation ranges from 400 to 500 meters.
4. Maopu Shale
This was formerly ascribed to the upper part of the Ailiaochiao Formation. It is predominantly a shale unit
composed of dark gray shale with thin interbeds of fine-grained sandstone and muddy sandstone. The shale is
slightly banded. This unit is 300 to 400 meters thick.
Further southward to Kaohsiung-hsien, geologists of the Chinese Petroleum Corporation named three
corresponding units of the Kueichulin Formation (Sun, 1965; Chi, 1979b). These are, in ascending order:
1. Wushan Formation (1360 m.)
This unit consists of alternating interbeds of thick-bedded, fine-grained sandstone, grayish black shale,
and sandy shale, with interlaminations of sandstone and shale; the lower part has been cut by faulting.
2. Kantzeliao Formation (680 m.)
A local unconformity possibly separates the Wushan Formation from the overlying Kantzeliao Formation. This
unit is composed of gray shale and sandy shale with local thin sandstone interbeds.
3. Nanshihlun Sandstone (240 m.)
This unit is composed of light gray thick-bedded muddy sandstone with local interbeds of calcareous
sandstone. The Kueichulin Formation was previously considered the youngest Miocene unit in the western
foothills. Recent paleontologic studies, however, prove that the Kueichulin Formation is largely Pliocene
(see Table 4 and Table 5). The boundary between Miocene and Pliocene is now placed in the Shihliufeng
Member of the Kueichulin Formation. The overlying Yutengping Member is Pliocene and the underlying
Kuantaoshan Member is Miocene. Likewise in northern Taiwan, the Erchiu Formation and the upper part of the
Tapu Formation are Pliocene; the lower part of the Tapu Formation is Miocene.
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