General Stratigraphy
INTRODUCTION
The western foothills are composed mainly of littoral to shallow marine Oligocene to Neogene rocks. Chemical
or organic deposits are scarce, represented only by lenticular limestone bodies sparsely intercalated at
various horizons in the rocks. These rocks form a continuous sequence from upper Oligocene into lower
Pleistocene. Except local breaks or diastems, no distinct stratigraphic or structural breaks are known. As a
result of rapid progress in biostratigraphic studies, many old paleontologic ages of the rocks in the
western foothills have been revised or modified. These chronological data are derived mainly from the study
of planktonic foraminifers and calcareous nannofossils in association with limited information from
paleomagetic analysis. A number of fossil zones have been established and have proven to be of
inter-continental correlation value. This significant progress in biostratigraphy is extremely useful in the
study of Cenozoic chronology, correlation and tectonics in Taiwan.
Table 4. Zonation, age and correlation of planktonic fossil of
Cenozoic rocks in the western foothills of Taiwan |
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Table 4 shows the zonation, age and correlation of the planktonic fossils of the Cenozoic rocks in the
western foothills. This table is based mainly on data compiled by T. C. Huang of the Chinese Petroleum
Corporation with only minor modifications. Planktonic foraminiferal zones established by Blow (1969) and
calcareous nannofossil zones proposed by Martini (1971) are used for the faunal codification and zonation in
Table 4. These are the well-known standards generally applied by paleontologists in stratigraphic
classification and age assignment in biostratigraphic studies.
Rapid vertical and lateral lithofacies changes are common in the stratigraphic sequence of Taiwan. In the
western foothills, the marked lithologic variation of Tertiary rocks from north to south necessitates the
establishment of many different formation names for practical field mapping. The result is a large number of
formation names in use in different areas of western Taiwan. Correlation of these formations is based
largely on fossil evidence, but difficulties are encountered where facies fossils are involved, or where the
rocks lack diagnostic fossils, or are barren of organisms. Also the nearly continuous deposition in the
basin produced a thick lithologic succession in which vertical changes are typically transitional or
inter-fingering. Consequently many of the formation boundaries are difficult to define, and identification
of such boundaries is often arbitrary and may vary with different authors. The lack of well established
formation boundaries and other marker beds increases the uncertainty in stratigraphic correlation of the
rocks in the western foothills. Table 5 shows the up-to-date correlation of the upper Cenozoic rocks in
different parts of the western foothills. This table was prepared and compiled on the basis of the data and
correlation charts published in the following four papers.
( Table 5. )
- Huang, T. C. (1980a)Oligocene to Pleistocene calcareous .nannofossil biostratigraphy of the Hsuehshan Range and western foothills in Taiwan: Geol. Paleont, SE Asia, vol. 21, pp. 191-210.
- Tunyow, Huang (1978)Significant new look on the Tertiary stratigraphy of Taiwan: Petroleum Geology of Taiwan, no. 15, pp. 167-179.
- Tunyow, Huang (1981)An attempt for absolute chronology of Neogene in Taiwan and biostratigraphic correlation: Ti-Chih, vol. 3, pp. 135-137.
- Chi, W. R. (1981)Calcareous nannoplankton biostratigraphy and stratigraphic correlation of the Mesozoic and Cenozoic sequences in central, southern and eastern Taiwan, Republic of China: 10th Convention Indonesian Petrol. Assoc., pp. 3-49.
| Table 5. Correlation of Tertiary and Pleistocene rock in the western foothills |
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Although paleontologic and stratigraphic data in Taiwan have been rather well documented in recent years,
the systematic formulation of standard biostratigraphy and chronology has not yet been well established. The
lack of well represented stratotypes of most stratigraphic units, rapid lithofacies changes, and complicated
geologic structures may be the main causes for these deficiencies. From the study of planktonic fossils, it
is straightforward to recognize and subdivide the Tertiary rocks in Taiwan into "series" rank, but it is
difficult to classify them systematically into "stage" status. Therefore, stratigraphic correlation in some
areas or sections listed in Table 5 may not be well founded. They should be constantly revised and updated
as new evidence is available.
The general stratigraphy of the rocks in the western foothills is described in the following paragraphs, and
the characteristics of the different lithostratigraphic units will be described in more detail in the
succeeding sections.
OLIGOCENE ROCKS
The Wuchihshan Formation and the lower part of the Mushan Formation are the two proven Oligocene units in
the western foothills. In the northwestern part, these two formations are both represented by carbonaceous
to coal-bearing whitish sandy rocks and grayish black shaly rocks of nearshore sedimentary facies. Toward
the southeast, the Wuchihshan Formation is gradually dominated by gray to black shale and grades into
shallow marine environment. The Oligocene rocks are exposed mostly in northern Taiwan. The Wuchihshan
Formation has not been found south of the Tahanchi stream in Taoyuan-hsien. Only a small exposure of
Oligocene strata (Tsukeng Formation) is found in an anticlinal core in central Taiwan. However, subsurface
Oligocene rocks have commonly been encountered in wells drilled by the Chinese Petroleum Corporation in
western Taiwan.
As stated by T. C. Huang (1979), the boundary between Oligocene and Miocene in northern Taiwan is placed in
the middle of the Mushan Formation in the western foothills or in the Aoti Formation in the Hsuehshan Range
belt. He is of the opinion that the time of tuff eruption in the Mushan Formation could be the beginning of
the Miocene Epoch in northern Taiwan. In central Taiwan, T. C. Huang has indicated that the boundary between
Oligocene and Miocene could be in the lower part of the Takeng Formation and the underlying Tsukeng
Formation is chiefly Oligocene.
MIOCENE ROCKS
The Miocene rocks are composed mainly of shallow marine to nearshore clastic deposits, conformably overlying
the Oligocene rocks in the western foothills. These rocks consist of a thick succession of alternating
sandstone, siltstone and shale. Carbonate reefs and lenticular tuff bodies are dispersed locally in the
Miocene strata. The total thickness of the Miocene varies throughout the western basin but in general it
increases eastward to a maximum of 5,500 meters near the western flank of the Central Range, The Miocene
sedimentary basin is aymmetric, characterized by a foredeep on the east and a foreland or shelf on the west.
There is a marked thinning of all Miocene units toward the western foreland, forming a clastic wedge
tapering toward the western coastal plain and the Taiwan Strait. The Neogene sequence is 1,400-1,900 meters
thick below the western coastal plain, and only 500 meters thick underneath the Penghu Islands in the Taiwan
Strait.
The Miocene rocks in northern Taiwan comprise three sedimentary cycles, each including one coal-bearing
formation and one marine unit. These three cycles are considered as early, middle, and late Miocene in a
chronological order, although this subdivision may not be fully consistent with the stage boundaries
determined by paleontologic evidence. Each cycle begins with a coal-bearing formation ranging in thickness
from 300 to 700 meters. This carbonaceous facies is succeeded by a marine unit of clastic sediment generally
rich in fossils and 500 to 700 meters thick. These three sedimentary cycles are each given a group name in
the legend for the compilation of the geologic map. They are the lower Yehliu Group, the middle Juifang
Group, and the upper Sanhsia Group. The last Group name is an old stratigraphic term (Ichikawa, 1929), but
the other two. Group names are proposed for the first time in this map.
The three coal-bearing formations are composed principally of white to grayish white orthoquartzitic to
feldspathic sandstone, gray protoquartzite, interlaminated beds of sand-silt-shale, and carbonaceous shale.
Each formation includes one to six workable coal beds, with coal thickness averaging 0.3 to 0.6 meter.
Concretions and thin bands of clay ironstone are found locally in the coal-bearing rocks. The three
intervening marine formations consist mainly of shale, siltstone, and fine- to medium-grained sandstone. The
sandstones are generally ledge-forming, light bluish gray, and calcareous, chiefly subgraywacke and
protoquartzite. The shales and siltstones are dark gray, either massive or thin-bedded, and richly
fossiliferous. These marine formations are each subdivided for field mapping into two or more lithologic
members, each comprising either a thick shale sequence, a massive sandstone series, or a succession of
alternating sandstone and shale. Thick and massive sandstone members that form precipitous cliffs are
generally abundant and prominent in the marine formations of northern Taiwan.
Thin and discontinuous volcanic strata are scattered in the Miocene rocks of northern Taiwan. They occur as
elongate lentils, irregular bodies, or more continuous converging or diverging layers. Basaltic tuff is the
chief component along with a few basaltic lava flows. Small volcanic lenses appear also in the upper Miocene
strata of southern Taiwan.
Limestone is rare in the Miocene of Taiwan. One thin clastic limestone bed 1 to 5 meters thick is locally
associated with the lower Miocene tuff beds. The exposures are discontinuous and extremely localized.
Several thick and bulging reef limestone bodies are intercalated in the middle Miocene rocks in the
foothills of Hsinchu-hsien.
A gradual change in Miocene stratigraphy from northern Taiwan toward central and southern Taiwan is
reflected in lithologic characters as well as in stratigraphic thicknesses. The older Miocene formations
thin and disappear successively to the south. That is, the further south in the Miocene belt, the less
complete is the exposed Miocene sequence, and the younger the age of the basal Miocene unit exposed. The
thickness of individual Miocene units seems to increase gradually toward the south. The three coal-bearing
formations in northern Taiwan give way progressively southward to marine clastic deposits. With the
disappearance of the coal fades, the three sedimentary cycles of northern Taiwan become indistinct toward
central and southern Taiwan, where mainly marine rocks are found in the Miocene column. The other
significant change is that the Miocene rocks are progressively more shaly and thicker from northern Taiwan
toward southern Taiwan. The sandstone members of the three marine formations in northern Taiwan are largely
absent in the south, having passed into thicker shale-dominating units (see Fig. 5).
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Figure 5. North-south facies change in the sedimentary basin of the western foothills, showing variation of
lithology and thickeness of Neogene rock |
Lateral facies changes are common in the Miocene rocks, making it difficult to trace individual beds and
formations. Further difficulties are caused by faulting and folding, which are severe in many places. The
stratigraphic column worked out in one area cannot be applied to another area without significant
modifications. The Miocene is characterized by massive sandstone formations in the north with alternations
of sandstone and shale and thin coal beds. These rocks grade into thick sandstone and shale beds in central
Taiwan, where the carbonaceous facies becomes insignificant. Thicker and dominantly marine shaly rocks in
south-central Taiwan completely lack workable coal beds. Toward the southern part of the western basin, the
entire Miocene column is composed of a continuous sequence of dark gray marine shales, claystones, and
siltstones with only minor sandstones. The shaly rocks in the south are more than 4,000 meters thick in the
middle and upper Miocene sections; the lower Miocene is not exposed in southern Taiwan.
PLIOCENE ROCKS
Pliocene units are distributed in the hills and lowlands west of the exposed Miocene strata in the western
foothills. They are composed mainly of marine clastic deposits attaining a total thickness of 2,000 meters
in the north and 4,000 meters in southern Taiwan. Pliocene rocks are rarely exposed in northernmost Taiwan,
but become gradually abundant south of the Tahanchi stream on the border between Taipei-hsien and
Taoyuan-hsien.
The Pliocene rocks exhibit a lithofacies change from north to south similar to that of the Miocene rocks.
The Pliocene sediments thus become progressively thicker and finer grained toward southern Taiwan. South of
the Hsilochi (Tachoshuichi) stream, the Pliocene can still be divided into a number of local
lithostratigraphic units, represented largely by alternating beds of shale, siltstone, and sandstone with
insignificant conglomerate layers, having a thickness of well over 3,000 meters. Reef limestone occurs in
several horizons in the Pliocene of southern Taiwan.
South of the Tsengwenchi stream, the Pliocene sediments are nearly 4,000 meters thick according to
subsurface data. Most of them are characterized by a spectacular dark gray mudstone series extensively
distributed in Tainan-hsien and Kaohsiung-hsien. This series was called the Kutingkeng
(Gutingkeng) Formation in old literature. The massive mudstone erodes at the surface to a typical badlands
topography and bedding is generally not discernible. Invariably lenticular fine sandstone (graywacke to
subgraywacke) beds are intercalated in the mudstone. The sandstone is light gray, banded, and generally less
than 50 centimeters thick except for some massive sandstone ledges. In a few places, small impure limestone
lenses are intercalated in the mudstone, varying from 2 to 20 meters thick. The limestone generally merges
into calcareous sandstone before completely thinning out. This thick and monotonous mudstone series defies
any attempt at detailed stratigraphic and structural analysis. A considerable part of the mudstone series
(Gutingkeng Formation) in southern Taiwan is of Pleistocene age on the basis of recent paleontologic
studies.
The boundary between Miocene and Pliocene in western Taiwan is now placed at the boundary between
foraminiferal zones N17 and N18 (see Table 4) by paleontologists. This boundary is located between the
Kuantaoshan Sandstone Member and the Shihliufeng Shale Member of the Kueichulin Formation, which was
formerly believed to be mainly late Miocene in age. More precisely, the upper boundary of the Miocene
Epoch should be in the middle of the Shihliufeng Shale Member (Table 4).
QUATERNARY ROCKS
Quaternary deposits in the western foothills are represented largely by Pleistocene mudstone, siltstone,
shale, sandstone and conglomerate, including both marine and continental fades. A large part of the reef
limestone lenses or layers in southern Taiwan have been dated as Pleistocene in recent studies. The
Pleistocene deposits are exposed mainly in the hills and lowlands in the western part of the western
foothills. Recent paleontologic studies have placed the boundary between Pliocene and Pleistocene in the
middle part of the Cholan Formation (see Table 4), which was considered to be mainly Pliocene in older
literature. Recent palynological studies have also established Quaternary stratigraphy and zonation in a
number of sections in Taiwan.
The Toukoshan Formation is the oldest Pleistocene unit in western Taiwan, and is distributed extensively in
the western foothills. The next younger Pleistocene unit is the Tananwan Formation, which is believed to
rest unconformably on the Toukoshan Formation and other older units. All the Pleistocene units younger than
the Tananwan Formation are generally flat-lying, and are unconformable with respect to the older strata
The other Quaternary deposits are mainly surficial veneers and valley or basin fillings consisting of
fluvial, lacustrine, estuarine, or marine sediments. They show varied landforms, such as coastal or river
terraces and alluvial plains. Terrace deposits are extensive in the rolling hills and coastal terraces of
western Taiwan and the Hengchun Peninsula. A uniform sequence of terrace development is recognized in
western Taiwan, remarkably dissected into several terrace levels. Terrace deposits are also present in
several inland basins, the Eastern Longitudinal Valley, and on some offshore islands.
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