Quaternary Stratigraphic Units
The Quaternary deposits in the western foothills include the Pleistocene Toukoshan Formation and its
equivalents, the Tananwan Formation, and younger terrace deposits, raised reefs, and alluvial fills of
various categories.
TOUKOSHAN FORMATION
The Toukoshan Formation was proposed in the Tungshih area of central Taiwan (Torii, 1935; Lin, 1935). The
Toukoshan Formation was called by several different stratigraphic names in the old Japanese reports such as
Tokazan Conglomerate, Tokazan Beds, Tokazan Group, and Tokazan Series. It has been named the Toukoshan
Formation in many recent reports and in this geologic map. Toukoshan is the name of a small hill east of
Fengyuan in Taichung-hsien and is pronounced tokazan by the Japanese. At the type locality, the
formation is classified into three divisions. The lower division is 900 meters thick and consists mainly of
sandstone and shale with pebbly horizons. Fragments of drift wood are seen in the sandstone. Both marine
faunas and mammalian fossils are found in the rocks. The middle division is composed of 50 to 100 meters of
alternating beds of sand, clay, and gravel, containing both fresh-water and marine molluscan faunas. The
upper division of the Toukoshan Formation consists of several hundred meters of massive conglomerate with a
few thin beds of crudely cross-bedded sandstone.
The Toukoshan Formation is of early Pleistocene age. It is widely distributed in the western foothills and
rests conformably on the Pliocene Cholan Formation, with only a minor hiatus reported locally. The main
outcrops are in the rolling hills and high terraces in the western part of the western foothills.
Conglomerate is well developed only in central Taiwan between the Tachiachi and the Hsilochi streams.
Elsewhere the Toukoshan Formation is represented mostly by alternations of sandstone, shale, and mudstone
with thin conglomerate interlayers. Therefore, "Toukoshan Formation" is considered a better stratigraphic
term than "Toukoshan Conglomerate." Different formational names have been proposed for strata equivalent to
the Toukoshan Formation in areas away from the type locality.
In general the Toukoshan Formation can be distinguished into two lithofacies which are gradational to each
other. One is the conglomerate facies and the other is the sandstone and shale facies. In old Japanese
papers, these two facies were separately named the Houyenshan Conglomerate and the Tunghsiao Sandstone.
These represent only two lithofacies within one stratigraphic unit, however, rather than two distinct
formations one above the other in regular sequence. L.S. Chang (1955) later named these divisions the
Houyenshan facies (conglomerate) and the Hsiangshan facies (sandstone and shale) of the Toukoshan Formation,
but "facies" is not a formal stratigraphic term that entitles a geographic prefix. In this geologic map, the
Toukoshan Formation is divided into two facies categories as described above.
The Toukoshan Formation contains early to middle Pleistocene fossils of mammals, fish, mollusks, echinoids
and foraminifers, and is especially rich in molluscan faunas. The mammalian fossils include Stegodon
sp., Rhinoceros sp., Elephas sp., Felis sp., and others. The whole formation is
characterized by a mixed assemblage of marine, brackish, and fresh-water faunas which indicate the
depositional environments vary from place to place. The thick conglomerate, the crude stratification, the
great range in thickness and facies variation, and the mixed faunas indicate that the Toukoshan Formation is
a clastic accumulation laid down near the shore during a regression. The depositional environments
interpreted in different areas vary from fluvial to nearshore and shallow marine.
In central Taiwan and south-central Taiwan, both the conglomerate facies and the sandstone and shale facies
of the Toukoshan Formation are well-developed, the former usually overlying the latter. In the lower part of
the formation, massive, light bluish gray to light gray, fine-grained to silty sandstone is the dominant
lithology. The sandstone is loosely cohered, cross-bedded in part, and contains wood fragments. Bluish gray
to gray shale is often interbedded in the sandstone. Stratification is poorly developed except where shale
interbeds are present. The sandstone also contains thin conglomeratic layers or lentils which are not
limited to any definite horizons. Massive and thick conglomerate is more distinct in the upper part of the
Toukoshan Formation, forming steep walls and parapet-shaped ridges, and reaches several hundred to nearly
1,000 meters thick. The clasts are mostly of sedimentary rock types with 50 percent quartzite and other
well-indurated sandstone, Clasts are rounded to subrounded, ranging from a few centimeters to one meter in
diameter. The matrix is mostly fine sand, with some calcareous and ferruginous cement. Sorting is generally
poor. Thin lenses or beds of sand and silt are abundant in the conglomerate.
The exposed thickness of the Toukoshan Formation is approximately 1,000 to 1,500 meters, each lithologic
member varying in thickness from place to place. The thick conglomerate member is exposed here and there but
the conglomerate often grades laterally into alternating beds of sandstone and shale without definite
pattern. The conglomerate may vary from irregular beds scores of meters thick to a massive member nearly
1,000 meters thick, such as that observed along the streams of the Wuchi and the Hsilochi.
In northern Taiwan, the Toukoshan Formation is characterized mostly by alternating beds of loosely
consolidated sandstone arid shale. Conglomerate is represented by only a few intercalated pebbly or
conglomeratic layers. This unit has been called various stratigraphic names in this area. The Toukoshan
Formation here was called the Yangmei (Yobei) Formation (Makiyama, 1933) and the Tunghsiao (Tusyo) Formation
(Makiyama, 1935) in Hsinchu and Miaoli. These two units were both described as alternating beds of bluish
gray sandstone, siltstone, and shale. Thin beds of conglomerate are intercalated in the sandstone, and the
conglomerate may reach 50 meters thick in the lower part of the Yangmei Formation. In northernmost Taiwan,
the Toukoshan Formation is correlated with the Kuanyinshan (Kannozan) Formation (Makiyama, 1934) which
underlies the Kuanyinshan volcano. This formation is composed of gray sandstone, gray to greenish gray
tuffaceous sandstone, shale, and thin conglomerate interlayers. The sandstone is very loosely consolidated
and the conglomerate looks like a gravel bed. Andesite pebbles have been found in the gravels, indicating
that eruption of the Tatun Volcano Group in northern Taiwan may have taken place during deposition of the
Toukoshan Formation, It is also possible, however, that the Kuanyinshan Formation is correlative to the
Tananwan Formation (Lin, 1963; Ho 1969 a) which will be described later. The steep dip angle of the
Kuanyinshan beds may be the result of tilting during the uplift of the volcano.
PLEISTOCENE FORMATIONS IN SOUTHERN TAIWAN
In central-south Taiwan, strata equivalent to the Toukoshan Formation have been divided into three
formations by geologists of the Chinese Petroleum Corporation. These are, in ascending order, the
Kanhsialiao Formation (Stach, 1957), the Erchungchi Formation (Stach, 1957) and the Liushuang Formation
(Stanley Chang, 1962). The type localities of these three formations are in the drainage of the Tsengwenchi
east of Hsinying in Tainan-hsien. The Kanhsialiao Formation is about 540 to 1,000 meters thick, and
is composed mainly of interbeds of shale and sandstone; sandstone is more prevalent toward the north. The
Erhchungchi Formation is about 440 meters thick, and is composed of alternating beds of shale and fine- to
medium-grained sandstone which contains abundant marine molluscan fossils and wood fragments. The Liushuang
Formation is 1,040 meters thick, and is composed mostly of dark gray to bluish gray mudstone and shale with
thin sand and silt interbeds. A few thick sandstone beds are intercalated locally in the upper and the lower
parts. Both molluscan and foraminiferal fossils are abundant. Wood fragments are also present.
On the west bank of the Laonungchi stream in Kaohsiung-hsien, another conglomeratic unit is exposed near the
village of Liukuei. This was named the Liukuei Formation or the Liukuei Conglomerate (Torn, 1933) and
conformably underlies the Liushuang Formation, It is composed of thick conglomerate, coarse-grained
sandstone, sandy shale, and mudstone. Pebbles in the conglomerate range from a few centimeters to 30
centimeters in diameter, and are composed mostly of sandstone, quartzite, quartz, sandy shale, and minor
volcanic rocks. The pebbles are subrounded and are set in a matrix of sand and silt. Drift wood, clay
ironstone concretions, and coaly fragments are found in the formation. The thickness of the Liukuei
Formation at the type locality is about 300 meters but the formation thickens appreciably southwards. The
Liukuei Formation is bounded by faults on both sides and contains no diagnostic fossils. It is correlated to
the conglomerate member of the Toukoshan Formation on the basis of lithologic character alone. The Liukuei
conglomerate is much more consolidated and cemented than the Toukoshan conglomerate and is also older,
believed to be largely of early Pleistocene age.
In the hilly country of Yuching to the west of the Chutouchi oil field in Tainan-hsien, the name
Yuching Shale has been proposed for a unit equivalent to the Liushuang Formation (Ho, 1956). The Yuching
Shale is composed mainly of mudstone and shale, with fossils of mainly Pleistocene age. In the village of
Nanhua in Tainan-hsien, thick and massive sandstone is exposed at the base of this mudstone series.
This sandstone is cliff-forming, fine-grained, light gray, and loosely cohered. It ranges in thickness from
several to scores of meters but usually wedges out at both ends and is non-persistent. This sandstone member
is locally called the Chinmien Sandstone (Ho, 1956) and is found in Nanhua only. This sandstone marker forms
the base of the Yuching Shale, separating it from the underlying Peiliao Shale.
Limestone reefs are distributed sparsely in the Pleistocene rocks of southwestern Taiwan. The limestone is
whitish gray, massive, and porous, with little or no evidence of stratification. The reef limestone occurs
either as biohermal or biostromal bodies developed in different horizons in the strata. They vary from
several to nearly 100 meters thick and from tens to several thousand meters long. The limestone lentils
often thin out laterally over a short distance and grade into stratified muddy sediments. The common
reef-building organisms include foraminifers, calcareous algae, corals, and mollusks. The important
limestone localities in southern Taiwan are Takangshan, Hsiaokangshan, Panpingshan, and Shoushan.
TANANWAN FORMATION
The Tananwan Formation was first proposed by Makiyama (1934) for the fine-grained, flat-lying marine
sediments exposed in the northwestern part of the Linkou terrace on the northern edge of Taiwan. In this
report, the Tananwan Formation is defined to correspond only with Lin's (1963) Paotoutze facies of the
Tananwan Formation (Ho, 1969a). The Tananwan Formation consists of alternating beds of fine-grained
sandstone, siltstone, shale, and claystone, either medium- to thick-bedded or massive. Cohesion and
compaction of the sediments are generally poor and the rocks are friable and easily disintegrate into sand,
silt, and clay. The color is light gray or light bluish gray when fresh. The sandstone is partly reddish due
to iron oxidation. The clayey beds contain disseminated sand grains and wood fragments. Lenses or irregular
patches of gravel are intercalated in the sediments. Bed thickness ranges from 0.5 to 4 meters but
stratification is often poorly developed. Some thick-bedded, coarse-grained sandstone occurs in the lower
part of this formation, containing pebbles up to 3 centimeters in diameter.
The Tananwan Formation yields abundant foraminifers and marine mollusks of early Pleistocene age. The
exposed thickness of the Tananwan Formation varies locally, reaching a maximum of 170 meters on the
northwestern coast. The total thickness is unknown because the base of the formation is not exposed. The
Tananwan Formation is exposed only in the Linkou terrace and not in other coastal terraces of northwestern
Taiwan. Lithologically the Tananwan Formation is quite similar to the Kuanyinshan Formation or the Toukoshan
Formation in northern Taiwan. However, the Tananwan Formation is always flat-lying or very gently warped
whereas the other two formations often dip at angles of 20° to 70°. This is an important means to
distinguish the Tananwan Formation from the Toukoshan Formation in the field. Angular discordance is
believed to exist between these two formations although this break has not been actually observed due to
limited occurrence of the Tananwan Formation.
LATERITIC TERRACE DEPOSITS AND TERRACE DEPOSITS
The terrace deposits are differentiated into two categories: lateritic and non-lateritic. They are of the
same lithologic characteristics, although one has a red soil veneer on top. Although laterite is generally
used by geologists to designate the red clay overlying the terrace gravels, pedologists are inclined to
called it red soil rather than laterite because, in the definition of soil chemistry, the red soil has not
yet reached the mature stage of lateritization. The terrace deposits (lateritic and non-lateritic) have been
given different stratigraphic names by different workers in various places. These names are the Taishan
gravel facies of the Tananwan Formation, the Linkou Formation, the Tableland Gravel, the Tientzehu
Formation, the Chungli Formation, the Taoyuan Formation and others. These units are not considered exactly
synonymous by Pleistocene geologists and may differ slightly in age. Distinctions are subtle, however, and
no lithologic characteristics distinguish these units. They are possibly distinguished on the evidence of
physiographic features, relative elevations, carbon-14 dating, and obscure depositional breaks.
Lithologically all these formations are composed of non-stratified gravel beds without marked variation in
gross features. They are thus all grouped under the term terrace deposits (lateritic and non-lateritic) in
the compilation of this geologic map.
The lateritic and non-lateritic terrace deposits are widely distributed in the rolling hills and coastal or
river terraces of western Taiwan. They are exposed in the Linkou terrace, the northernmost of a series of
high coastal terraces between Taipei and Hsinchu, the Taoyuan terrace, the Yangmei terrace, the Chungli
terrace, the Hukou terrace, and others. These are all covered by similar gravel beds. Southward the terrace
gravel is distributed in such geographically prominent localities as Houyenshan in Miaoli-hsien,
Tiehchenshan and Tatushan in Taichung-hsien, Pakuashan in Changhua-hsien, Tungting terrace in
Nantou-hsien, Tsukoshan in Yunlin-hsien, and Lingkou terrace in Kaohsiung-hsien. The
terrace deposits extend southward to the Hengchun Peninsula and beyond to the Oluanpi terrace at the
southern tip of Taiwan. These terrace deposits are also distributed in the peripheral uplands bordering the
basinal areas of central Taiwan: the Puli basin, the Yuchih basin, and the Taichung-Nantou basin. In eastern
Taiwan terrace deposits are widely distributed along the Eastern Longitudinal Valley from Hualien to
Taitung. The flat-lying terrace deposits generally overlie the Toukoshan Formation and other older strata
with a sharp angular unconformity, comprising the most distinct unconformity in Taiwan.
In the Linkou terrace, the terrace deposits are named the Linkou Formation, which comprises all the gravels
and the lateritic mantle, including the Taishan gravel facies of the Tananwan Formation as defined by Lin
(1963). The gravel partly intertongues with and partly overlaps marine sediments of the Tananwan Formation
(Ho, 1969a). The marine sediments of the Tananwan Formation and the terrace gravel were first laid down
together in a transitional coastal environment during the early stage of Pleistocene sedimentation,
resulting in subaerial accumulation of thick gravel interbedded with fine marine sediments. The gravel later
overlapped marine sediments as the shoreline prograded eastward.
The terrace deposits are composed largely of unconsolidated gravel with flat- lying sandy or silty lenses,
generally poorly stratified and poorly sorted. Clasts range from several millimeters to more than 2 meters
in diameter. Particles less than 2 millimeters are composed mainly of individual mineral grains, although a
few rock fragments are also present. The gravel is made up of all sorts of rock types that vary from place
to place and with the provenance. In general lithic and quartzitic sandstones are the predominant
constituents of the gravel. The gravel is usually intimately mixed with variable proportions of
sand-silt-clay. In places the deposit is formed principally of the fine clastic fraction (sand and finer)
with sparsely disseminated pebbles. The muddy matrix constitutes the dominant coloring agent of the
gravel deposit, which is generally earthy yellow with a shade of brown due to limonitic contamination. Where
the terrace deposits are lateritic, red matrix clay is developed in the surficial part of the gravel and
grades upward into red lateritic mantle soil. The thickness of the gravel deposits varies in different
terraces, ranging from scores of meters to 200 meters and more. The laterite or red soil forms an upper
veneer ranging in thickness from several to 10 meters.
There have been several suggestions as to the origin of gravel terrace on the coastal area in northwestern
Taiwan, including piedmont accumulation, high coastal terrace sedimentation, and epirogenic movement of the
island. The prevailing view is that these terrace deposits were formed by coalescing of alluvial aprons
rimming the streams on the eastern margins of the present high terraces. Constant shifting of stream
channels and intermittent upheaval of land resulted in the accumulation of thick and extensive gravel
terraces along the coast.
RAISED CORAL REEFS
Raised coral reefs are sparsely distributed in several small areas in northern and eastern Taiwan and are
well developed in southern Taiwan, where reefs surround the Hengchun Peninsula and the islands of Lutao and
Lanhsu. Only the reefs in southern Taiwan are extensive enough to be shown on the present geologic map.
The uplifted reefs form several levels of coastal terraces fringing the main island of Taiwan. The reef
limestone is generally several meters thick, composed mostly of corals and some mollusks. It usually rises
to 2 to 3 meters above the sea level at low tide. The highest coral reef does not exceed 20 meters above sea
level. Boulder beds are found at the basal part of the coral reef, a part of which is composed of reef
breccia.
ALLUVIUM
Alluvial deposits of clay, silt, sand and boulders cover the coastal plains of western Taiwan, the Pingtung
valley, the Ilan plain in northeastern Taiwan, the Eastern Longitudinal Valley and the large basinal areas
of Taiwan. The alluvium forms the flood plains and recent terraces of the larger streams that dissect the
island. The alluvium also includes coastal sand dunes, recent lacustrine and swamp deposits, and cave
deposits in limestone terrain.
Of particular interest is the alluvial Tainan Formation, which mainly underlies the Tainan terrace and
extends eastward to Hsinhua and Kuanmiao. It s composed of silt, clay, sand, and loam, reaching a thickness
of 16 to 36 meters. The sand is partly cross bedded. Abundant foraminifer and mollusk fossils are
characteristic of a variety of depositional environments including lagoonal, estuarine, shallow marine, and
eolian settings. The deposition of the Tainan Formation represents a cycle of marine transgression and
regression on the eastern rim of the present Tainan Plain. The age of the Tainan Formation was considered
Pleistocene in early studies but is now ascribed to middle Holocene on the evidence of C-14 radiometric
dating.
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