Pliocene Stratigraphic Units
The Pliocene rocks in the western foothills include the upper part of the Kueichulin Formation, the Chinshui
Shale and the Cholan Formation, and equivalent units. The Kueichulin Formation has been discussed above, and
is conformably overlain by marine clastic sediments that form the remainder of the Pliocene section reaching
a total thickness of 2,000 to 2,500 meters. The Pliocene rocks form a series of rolling hills to the west of
the higher standing hills of Miocene rocks in the foothills belt. The Pliocene sediments become shalier and
thicker southward from central Taiwan. Lenticular limestone bodies are found locally in the Pliocene rocks
in southern Taiwan. The most notable locality is the Chentoushan limestone in Kuantzeling, Tainan. This
limestone has a maximum thickness of 100 meters but thins out laterally. The Pliocene deposits in
south-central Taiwan have been subdivided into a number of local litho stratigraphic units by geologists of
the Chinese Petroleum Corporation. These units are more or less similar in general lithologic characters and
are differentiated by subtle lithologic variations.
In southern Taiwan the Pliocene rocks are composed of a thick mudstone series which was named the Gutingkeng
(Kutingkeng) Formation by Torii (1932) when he was mapping the oil fields in southern Taiwan. However, the
established stratigraphic sequence of this formation has never been well documented. It is now understood
that this mudstone series actually includes deposits of a wide range of geologic ages, from latest Miocene
to Pleistocene, and reaches several thousand meters thick. The stratigraphy of the Gutingkeng Formation
remains poorly under- stood. In many places in southern Taiwan, the so-called Gutingkeng Formation is really
of Pleistocene age.
CHMSHUI SHALE
The term Chinshui Shale was first used by Ando (1930) in his study of the geology of oilfields in
Miaoli-hsien. The type locality Chinshui is a small village in the center of the Chinshui gas field.
Because of its distinct lithology and characteristic faunas, this shaly unit has become an important marker
for stratigraphic correlation in western Taiwan. It is conformably overlain by the Cholan Formation and
underlain by the Kueichulin Formation. The shale is dark gray, and characterized by well-developed spheroid
exfoliation structures. Lenticular dark gray sandy beds and thin siltstone to mudstone layers are common. In
places the shale is poorly sorted and rich in marly nodules. The maximum thickness of the Chinshui Shale is
about 400 meters in the center of the Chinshui basin. This formation thins out to 100 meters or less to the
east and west. It is about 80 to 100 meters thick in central Taiwan. The Chinshui Shale was deposited in a
shallow to deep neritic environment with abundant marine organisms; common fossils include crabs, gastropods,
pelecypods, foraminifers and corals.
CHOLAN FORMATION
The Cholan Formation was named by Torii (1935) after the town of Cholan in the southern part of
Miaoli-hsien. The upper part of the Cholan Formation may be of Pleistocene age (see Table 5). This
formation is composed of 1,500 to 2,500 meters of sandstone, siltstone, mudstone, and shale in a monotonous
alternating sequence. Regional exposures of these rocks are characterized by cuesta or hogback ridges
resulting from alternating sandy and shaly beds. Facies changes are rapid in this thick clastic sequence,
and no key beds have been found to subdivide this formation into distinct members. The sandstone is light
bluish gray to light gray, fine-grained, and slightly micaceous, ranging generally in bed thickness from
scores of centimeters to two meters. Some sandstone beds are greater than 5 meters thick. The sandstone
varies from graywacke to subgraywacke. A few interbeds of whitish orthoquartzite are also found. The shale
and mudstone are bluish gray to dark gray, generally in beds 20 to 50 centimeters thick, and locally
thicker.
Cross bedding and ripple marks indicate that the Cholan Formation is mostly of shallow water origin. The
sandstone is comparatively loosely consolidated except where it is cemented by carbonate. Massive, light
gray, fine-grained muddy sandstone is prominent in the upper part. The sandstone contains carbonized wood
fragments, coaly particles, and coarse clastic detritus. Conglomeratic lenses and thin layers are
intercalated in the upper part of the Cholan Formation, which grades transitionally into the overlying
Toukoshan Formation, The first appearance of a thick and distinct conglomerate bed in the uppermost Cholan
Formation is stratigraphically important, usually considered to mark the top of the Cholan Formation and the
base of the Toukoshan Formation in field mapping. The Cholan Formation contains a rich faunal assemblage in
various horizons, including echinoids, crabs, mollusks and foraminifers. Slumps, load cast, and
ball-and-pillow structure are observed in the sandstone.
The sandstone content of the Cholan Formation is 40 to 60 percent in northern and central Taiwan between
Taipei and Chiayi. Sand content gradually decreases southward from Chiayi in both abundance and grain size.
Dark gray shale, mudstone, and siltstone become more abundant to the south as the thickness of the Cholan
Formation also increases appreciably. In the Tainan-Kaohsiung area, the sand content in the Cholan Formation
is less than 10 percent and the whole formation is an immense mudstone series correlated to the Gutingkeng
Formation.
Pliocene rocks in the Chiayi area have been separated by the Chinese Petroleum Corporation into the
following two lithostratigraphic units (Stach, 1957).
| Liuchungchi Formation |
Sandy shale and muddy sandstone |
760m |
| Yunshuichi Formation |
Shale and sandy shale with muddy |
450m |
| |
sandstone interbeds and reef |
|
| |
limestone lenses |
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In the Tainan-Kaohsiung area in southern Taiwan, strata equivalent to the Chinshi Shale and the Cholan
Formation are divided into the Chutouchi Formation and the conformably overlying Peiliao Shale, Both units
were proposed by Ho (1956) during his study of the Chutouchi oil field. The Chutouchi Formation is
characterized by three to five thick, lenticular sandstone beds, separated by thick intervals of dark gray
shale or alternating sandstone and shale. The lowest sandstone bed often marks the base of this formation
overlying the Maopu Shale. The sandstone is light gray, clayey, and slightly micaceous. Bedding is usually
indistinct and the sandstone grades imperceptibly into shale or claystone. Sandstone beds are of variable
thickness, commonly ranging in thickness from 10 to 30 meters and locally wedging out altogether. The
thickness of the Chutouchi Formation is also variable, ranging from 500 to 750 meters. The overlying Peiliao
Shale is composed of bluish gray to grayish black shale, which is partly non-fissile mudstone and may
correlate with the Gutingkeng Formation mentioned before. The rocks are slightly banded with intercalations
of interlaminated shale and fine sandstone. The total thickness of the Peiliao Shale is 1,200 meters.
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