Oligocene Stratigraphic Units
The Oligocene stratigraphic units in the western foothills include the Wuchihshan Formation and the
Wentzekeng Formation in northern Taiwan and the Tsukeng Formation in central Taiwan. The two Oligo-Miocene
units, the Mushan Formation in northern Taiwan and the Takeng Formation in central Taiwan, are discussed
below in the section on Miocene stratigraphic units,
WUCHIHSHAN FORMATION AND WENTZEKENG FORMATION
The Wuchihshan Formation is the oldest Tertiary stratigraphic unit exposed in the western foothills. The
formation name Wuchihshan was proposed by Yen and Chen (1953) and the type locality is in a high mountain
range southwest of the city of Chilung. This formation is distributed mostly in the Taipei-Chilung area,
and extends southward as far as the drainage of the Tahanchi stream in Taoyuan-hsien. Massive to
thick-bedded, white, medium- to coarse-grained sandstone is predominant at the type locality. The sandstone
is mostly orthoquartzite or protoquartzite and lesser feldspathic sandstone. A few medium-thick beds of
conglomeratic sandstone occur in the lower and the upper parts, and gray to light bluish gray subgraywacke
is found in the middle part. Dark gray shale and alternations of sandstone and shale are interbedded with
the white sandstone. Coaly to carbonaceous lentils or streaks are scattered in the dark gray shale but are
of no mining value. Glauconitic shale and sandstone are present, mostly in the middle part. The Wuchihshan
Formation is nowhere completely exposed because its lower part is generally dissected by faulting. The
thickest measured section near the type locality at Wuchihshan to Tawulun is approximately 1,200 meters
without reaching the bottom. The exposed thickness of this formation along the northern coast is 900 to
1,000 meters.
The Wuchihshan Formation is a carbonaceous unit of mixed marine and continental origin. Organic remains are
very scarce in the rocks. Recent sediment logical studies of Yuan (1979) on the northern coast of Taiwan
showed that the Wuchilhshan Formation is characterized mainly by fluviatile and transitional
marine-continental facies. Based on the types of sedimentary structures and their sequential arrangement in
the strata as well as changes in gram size, three facies models are recognized by Yuan: point bar, delta,
and turbidite. The Wuchihshan Formation is thus divided into three parts. The lowermost part consists of a
fluvial fining-upward sequence intercalated with strata deposited in lagoonal and beach environments. The
middle part is composed of rocks deposited in deltas or interdeltaic shorelines, over a basal part composed
of turbidite deposits of shallow or deep marine origin. The upper part is a repetition of the middle part
except that its basal part consists of significant shelf mud intercalated with minor shelf sand. Deltaic
coarsening-upward sequences are well developed in this upper part.
Coarse clastic detritus and sand content in the Wuchihshan Formation are more abundant in the north and the
west but gradually decrease toward the southeast where argillaceous sediments increase appreciably. The
gradual increase in argillaceous sediments toward the southeast may reflect a deepening from a continental
shelf environment in the northwest to a slope or shallow marine environment in the southeast during the time
of deposition of the Wuchihshan Formation. Due to this marked change in lithology, the Wuchihshan Formation
was subdivided into two facies by Chou (19 74); a sandstone facies in the northwestern part and a shale
facies in the southeastern part. The boundary between these two facies designations is approximately marked
by a northeast-trending line that extends from Chilung in the northeast through Taipei and Hsintien to
Taoyuan. Chou named the sandstone facies the Tawulun facies and the shale facies the Chingtan facies.
However, "facies" is not a stratigraphic term of any kind. It cannot be prefixed by a geographic name and
used as a formal stratigraphic unit. The Japanese geologist Ichikawa (1930) proposed previously the name
"Chingtan Formation" for the predominantly shaly facies of the Wuchihshan Formation. However, the Chingtan
Formation at the type locality Chingtan is only 200 meters thick due to faulting and no complete section can
be studied. Therefore it cannot be considered as the stratotype for a stratigraphic unit.
In the compilation of this new geologic map, the name Wuchihshan Formation is used to designate only the
Oligocene strata in northern Taiwan that are made up mainly of sandstone facies. The shaly counterpart of
the Wuchihshan Formation is given another name, the Wentzekeng Formation, which was first proposed by Tsan
(1981). A well developed section of the Wentzekeng Formation is exposed at the type locality Wentzekeng,
located on the northeastern coast between Pitou and Aoti. From the study of Tsan (1981), the base of the
Wentzekeng Formation is marked by the Lungtung Sandstone, which is composed of coarse to very coarse
orthoquartzite about 100 meters thick. Thin interbeds of quartzose conglomerate and dark shale are
intercalated in the Lungtung Sandstone, which may be stratigraphically and lithologically compared with the
Szeleng Sandstone in the Hsuehshan Range belt. The Wentzekeng Formation is composed of a thick sequence of
deep gray to black argillite and shale with intercalations of muddy sandstone and siltstone. The lower part
consists of deep gray argillite about 700 meters thick, containing some thin interbeds of muddy sandstone
and siltstone. The upper part is composed of alternating interbeds of argillite and muddy sandstone reaching
430 meters thick.
In the vicinity of Chiopanshan, which is the southern end of the exposed Wuchihshan or Wentzekeng Formation,
the rocks are represented mainly by dark gray shale and siltstone (Tsan, 1962). The upper part consists of
dark gray shale interbedded with minor sandy shale or muddy sandstone. The lower part contains alternating
grayish white sandstone and dark gray shale in beds of several centimeters to 2 meters thick. No
coarse-grained white sandstone has been found. Carbonaceous matter is common in the dark gray shale, and
marine fossils have been reported (Huang and Cheng, 1983). The exposed thickness of the Wentzekeng Formation
in the area is 400 to 500 meters, but the base is not exposed due to faulting.
The Wuchihshan Formation was considered Miocene in early literature. Recent study of the nannofossils by T.
C. Huang (1978; 1979a and b), however, discovered fossils of NP25 zone in the rocks and the age is therefore
late Oligocene. A similar Oligocene age was determined by Huang and Cheng (1983) in the study of contained
foraminifers. Chronologically the Wuchihshan Formation can be correlated to the Oligocene strata in the
Hsuehshan Range belt ranging from the Tatungshan Formation to the Szeleng Sandstone. The Wentzekeng
Formation also contains well preserved fossiliferous zones with foraminifers, mollusks and echinoids. Huang
and Cheng (1983) have indicated that the faunas are equivalent to those of the Oligocene Shihtsaoan stage.
TSUKENG FORMATION
The oldest Tertiary unit in central Taiwan (Taichung-hsien and Nantou-hsien) is the Tsukeng
Formation, which was first named by Ho and others (1956). The age of this formation is now considered to be
Oligocene. The type locality, Tsukeng, is in the rolling hills east of Nantou city. This name is applied to
a mixed sequence of dark gray shale, whitish gray sandstone, and tuffaceous sediments, with a measured
thickness of 250 meters. The base is nowhere exposed. In the exposed column of the Tsukeng Formation, the
lower part is composed of dark gray shale to grayish red shale with some tuffaceous lenses. The middle part
consists of whitish gray and light gray, fine-grained sandstone containing small coaly fragments. Lenticular
sedimentary tuff is found toward the top, which grades laterally into tuffaceous sediments and then
non-tuffaceous sandstone and shale. Most of the tuffaceous rocks are purple or light green. The upper part
is composed of dark gray shale interbedded with gray, fine-grained sandstone. The shale is slightly
tuffaceous. Glauconite is predominant in the whole section, either as distinct layers or as disseminated
particles. Marine fossils are abundant.
Exploratory holes were drilled by the Chinese Petroleum Corporation at the type locality of the Tsukeng
Formation. Chiu (1975) reported that the total thickness of the Tsukeng Formation is 1220 meters including
both surface and subsurface sections. The lower part of the formation consists of light gray to grayish
fine-grained calcareous sandstone with interbeds of grayish black shale. The middle part is characterized
by black shale with fine-grained sandstone interbeds. The rocks are richly glauconitic in places. The upper
part is composed of light gray fine-grained to medium-grained sandstone intercalated with grayish black
shale and tuffaceous shale in red or green colors. The foraminiferal faunas collected from the drill cores
of the Tsukeng Formation indicate that they are equivalent to the faunas in the Oligocene Shihtsaoan stage
(Chiu, 1975). The Tsukeng Formation is thus correlated to the Tatungshan Formation in the Hsuehshan Range
belt.
The Tsukeng Formation of the western foothills and the Shuichangliu Formation (or Tatungshan Formation) of
the Hsuehshan Range may be age-equivalent units, but they differ significantly in lithologic character. The
Tsukeng Formation is dominantly sandstone with an appreciable amount of tuffaceous material, and is not
metamorphosed. The Shuichangliu Formation is composed mostly of metamorphosed shale. Therefore, these two
formations differ markedly in lithostratigraphy and cannot be combined as the same lithostratigraphic
unit.
The most interesting paleontologic problem of the Tsukeng Formation is the discovery of Discocyclina sp.
near the tuffaceous shale in the upper part of the sequence. This fossil is considered to be a guide fossil
of the Eocene. Both Ho and others (1956) and Chiu (1972 and 1975) believed that Discocyclina sp. is a
reworked or derived fossil in the Miocene rocks. Hashimoto and Kurihara (1974) are of the opinion that the
fossil is in situ and the Tsukeng Formation is upper Eocene and possibly at an unconformable contact with
the overlying Miocene rocks. Positive evidence of this structural break, however, has not been given in
their paper.
The study of nannofossils by Chi (1979a) in surface and subsurface samples of the Tsukeng Formation
discovered that the fossil assemblages above and below the Discocyclina-bearing tuffaceous shale are quite
complex and are represented by mixed geologic ages including: Mesozoic (Late Cretaceous), Paleocene, Eocene
and Oligocene, The upper part of the Tsukeng Formation is characterized by late Oligocene fossils, whereas
the middle and lower parts are dominated by Eocene fossils. As Oligocene fossils are rather abundant near
the Discocyclina-bearing, zone, Chi concluded that the age of the Tsukeng Formation should be assigned to
the late Oligocene. All the fossils of earlier ages are believed to be largely derived or reworked
organisms.
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