Stratigraphy
This section deals with only the non-metamorphosed Neogene rocks exposed in the southern part of the
Hengchun Peninsula; the argillite-slate series to the north has been elucidated earlier. Early on,
Japanese geologists named all the Miocene strata exposed on the Hengchun Peninsula the "Hengchun Formation"
(Rokaku and Makiyama, 1934). Later paleontologic study by L. S. Chang (1963b) ascribed the Hengchun
Formation to middle and late Miocene. Recent biostratigraphic studies assigned these Miocene rocks to
foraminiferal zones N12 to N17 (C. Y. Huang, 1984) and nannofossil zones NN6 to NN11 (T. C. Huang,
unpublished report). No nannofossils have been found in the lower part of the exposed Miocene strata, and
Pelletier and Stephan (1986) are of the opinion that the lower part of the sequence may be lower Miocene.
After World War II, more detailed mapping of the Hengchun Peninsula was undertaken by Tsan (1974a and b). He
subdivided the normal Miocene rocks into two lithostratigraphic units. The lower of these, the Changlo
Formation, is characterized by deep gray shale with some sandstone interbeds. The overlying Loshui Formation
is a thick sequence of sandstone, shale and minor conglomerate. Although other formational names have been
proposed by subsequent workers (Pelletier et al., 1986; C. Y. Huang, 1984), Tsan's stratigraphic terminology
is adopted in the compilation of the geologic map and in this text. The definition, lithology and
distribution of these two rock units, however, have been considerably revised and modified on the basis of
extensive work by other geologists.
The thick and extensive Miocene clastic rocks on the Hengchun Peninsula attain an estimated maximum
thickness of 4,000 meters (Pelletier and others, 1986). Pliocene and Pleistocene rocks in small and
scattered outcrops overlie the Miocene strata, possibly unconformably. Marine fossils are not abundant
enough for a continuous zonation of the Miocene sequence. Sedimentary structures characteristic of
turbidities are common (Sung et al., 1986), including well-developed Bouma sequences, flute casts, groove
casts, channeling, graded bedding, slumps, convolute bedding, load casts, parallel laminations and others.
Lithofacies vary much more than in Miocene rocks of the western foothills. From the study of sedimentary
tectonics (Pelletier and others, 1986), the provenance of the Miocene sediments on the Hengchun Peninsula is
to the west or north, on the Asiatic continental margin. Sediment transport was generally from northwest to
southeast. Local changes in paleocurrent directions apparently reflect shifting turbidite fan channels.
CHANGLO FORMATION
The Changlo Formation was named after a small village about 20 kilometers northeast of the city of Hengchun.
The best section of the formation, however, is exposed in the hills east of Shihmen on the Szechungchi
stream. Pelletier and others (1986) subdivided this formation into two parts. The lower part consists mainly
of grayish black shale and siltstone with thin sandstone interbeds of several to scores of centimeters
thick. A channel-filling sandstone of nearly 100 meters thick is found in the middle part of this sequence.
The age may be early Miocene but no fossil evidence has yet been discovered. The upper part is exposed in
Shihmen and to the south of Shihmen, composed of alternating shale and sandstone. The sandstone forms the
high ridges of Hutoushan, Santaishan and Laofoshan which are all distinct reliefs on the Hengchun Peninsula.
The age of this upper part is middle Miocene, NN6 to NN8-9. The rocks contain a large amount of mafic,
ultramafic and acid igneous detritus, lithologically represented by basalt, diabase, gabbro, keratophyre,
plagiogranite and others. These rocks occur as conglomerate clasts, slumped blocks, and grains in green
sandstone beds. The remarkable Shihmen gorge on the Szechungchi stream is cut through a thick conglomeratic
sequence that represents a typical channel-fill deposit, Muller and others (1984) reported that these mafic
rocks are early Oligocene to middle Miocene in age based on the study of associated nannofossils, radiolaria
and foraminifers. They are characteristic of an ophiolite suite and are possibly similar to the ophiolite
blocks enclosed in the Lichi Formation in eastern Taiwan. The ophiolites from both areas may be derived from
the same source, the oceanic crust of the South China Sea.
LOSHUI FORMATION
The type locality of this formation is Chialoshui, a resort area about 16 kilometers east of the city of
Hengchun. This formation is composed of gray sandstone, dark gray shale and alternating interbeds of
sandstone and shale. Lenticular and irregular channelized sandstone is found on the western side of the
peninsla. The age of this formation is possibly late Miocene, NN10 to NN1L The Loshui Formation conformably
overlies the Changlo Formation and is exposed separately on the eastern and western coasts of the Hengchun
Peninsula. On the west, the Loshui Formation is exposed in the area of Lilungshan and Wenchaoshan and also
in the hilly area to the east of the coastal strip from Fengkang to Checheng. The rocks consist of
thick-bedded sandstone with interbeds of siltstone and shale, reaching nearly 2,000 meters thick. Close to
the western coast, channel-fill conglomerate lenses are exposed in the upper part. Clasts in the
conglomerate are composed of sandstone, basalt, diabase and gabbro. To the south of the Szechungchi stream,
the sandstone facies is gradually replaced by shale and siltstone that contain several sandstone-filled
channels. The Loshui Formation on the eastern coast is distributed in the hilly area of Manchou and
Chialoshui. It is composed of bluish gray thick sandstone with interbeds of shale and sandstone, attaining a
total thickness of nearly 1,200 meters. Conglomerate interbeds are scarce on the eastern coast. In
comparison with the western section, the Loshui Formation exposed on the eastern coast may represent a
distal sedimentary facies, farther from the sediment source.
KENTING FORMATION
The Kentmg Formation is another remarkable melange unit more or less similar to the Lichi melange of eastern
Taiwan. Tsan (1974a and b) named this formation after the Kenting Park at the southernmost tip of Taiwan.
The Kenting Formation is distributed mainly in the neighborhood of the Kenting Park and northward along a
strip to Hengchun and Paoli. Erosional remnants that may be blocks of the melange are exposed to the north
in Chienshan on the highway north of the town of Checheng. The Kenting melange consists of a pervasively
sheared and chaotic deep gray muddy to silty matrix that includes numerous exotic blocks of different sizes
and lithologies. The intensely sheared argillaceous matrix breaks up into small curved and polished chips
that are bounded by slickensides or slip surfaces. In some places the matrix encloses numerous small pebbles
of basalt, diabase, gabbro, and keratophyre. These pebbles are very bright due to tectonic polishing, and
are so numerous locally that the rock is best called a pebbly mudstone. Some of these pebbles may be formed
by mechanical breakdown of conglomerate clasts during shearing. In some of the best outcrops, the melange
displays stripes of brown, blue and green, resembling a rude stratification. The enclosed blocks range in
size from less than one centimeter to more than one kilometer across and consist of mainly siltstone and
sandstone, with lesser amounts of interbedded siltstone and sandstone, conglomerate, pillow lava, volcanic
breccia and peridotite. Sandstone blocks display a variety of lithofacies and colors. They commonly show
features characteristic of turbidities such as graded bedding and various sole marks. The conglomerate
blocks are more spectacular due to their size and resistance to weathering. They generally form huge and
precipitous blocks isolated in the muddy matrix and are topographically very conspicuous. The composition of
the conglomerate is quite similar to that found in the Changlo Formation.
A variety of fossils have been found in the Kenting Formation and reported by many paleontologists. A recent
study of the nannofossils by Chi (1982) shows diverse ages. The oldest assemblages are Eocene, others are
Oligocene, but most are late Miocene. As the youngest species indicate a late Miocene age, Chi believes the
age of emplacement of the Renting melange is late Miocene or at the Mio-Pliocene boundary. More recently T.
C. Huang and others (1983) have discovered early Pliocene nannofossils in the Kenting blocks and they are of
the opinion that the age of the Kenting Formation is late Pliocene or younger, possibly correlative to the
Lichi melange of eastern Taiwan.
The origin of the Kenting melange has been interpreted quite differently by different geologists. Two
attempts to reconstruct the history of the melange are given in this section. Page and Lan (1983) stated
that the Kenting melange and its conglomerate blocks contain fragments of oceanic lithosphere, representing
only the upper part of an ophiolite sequence that was tectonically removed from an oceanic suite. Most
probably the ophiolitic rocks came from a now-subducted part of the South China Sea (Biq, 1977). Most of the
ophiolitic components could have been generated by the usual plutonic and extrusive processes, but the
volcanic breccias (basalt and diabase) that contain various admixed rocks would have accumulated in talus
piles at the foot of young fault scarps related to sea-floor spreading. Page and Lan (1983) believed that
the ophiolites in the Kenting melange came from the East Taiwan Ophiolite (Liou et al., 1977) and originated
in the South China Sea. The ophiolitic components of the Kenting melange were transported westward toward
the continent instead of eastward like the components of the Lichi melange in eastern Taiwan. Large-scale
debris flows thus swept westward to form the Kenting melange and were apparently mingled with continental
debris from the Asiatic continental margin in the process of emplacement. These components were probably
incorporated in the accretionary wedge that formed alongside the Luzon Island Arc during the convergence
between the Philippine Sea and Eurasian plates. Uplift of the Kenting melange probably occurred
intermittently as the arc-continent collision progressed.
On the other hand, Pelletier and Stephan (1986) interpreted the Kenting Formation as a tecto-sedimentary
melange created at the front of an overthrust during the main phase of folding on the Hengchun Peninsula. It
is the result of both sedimentary and tectonic processes. The materials came mainly from the normally bedded
Miocene strata. They were sheared and deformed by the advance of a nappe along which Miocene strata from the
east were overthrust toward the west. The Kenting melange was formed at the front so that the normal Miocene
strata are in fault contact with the underlying Kenting melange. They stated that this main tectonic event
is related to the collision of the Luzon Arc and the Asiatic continental margin. From geochemical data, they
interpret that all the basic, ultrabasic and acid igneous pebbles in the Kenting melange are derived from
oceanic crust of the South China Sea, with no component derived from the Asiatic continent. The reworked
magmatic (ophiolite) components in the Miocene strata and their redeposition in the Kenting melange have a
common source in the East Taiwan Ophiolite (Liou et al., 1977). The Kenting melange and the related movement
of the overthrust were formed before the deposition of the Maanshan Formation at the beginning of
continent-arc collision, at about 4 to 5 m.y. ago.
MAANSHAN FORMATION
The normal Miocene strata on the Hengchun Peninsula are capped by three Pliocene to Pleistocene rock units
as reported in previous literature. These are the Maanshan Formation, the Szekou Formation and the Hengchun
Limestone. Current usage includes the Szekou Formation as part of the Maanshan Formation as the two units
are lithostratigraphically almost indistinguishable.
The Szekou Formation is exposed in narrow gullies and deeply eroded hillsides in the vicinity of the city of
Hengchun and the tablelands to the west. The name of the formation was first proposed by Rokaku and Makiyama
(1934). This formation is composed of friable and loosely consolidated siltstone, shale, fine-grained
sandstone, and lenses or irregular layers of gravel. The color is light bluish gray. Bedding is generally
indistinct with bed thickness ranging from 0.2 to 2 meters, The strata are flat lying or slightly tilted.
Gravel lenses are more common in the lower part, with gravel fragments generally 3 to 5 centimeters across.
The Szekou Formation contains a rich assemblage of mollusks and foraminifers. The total thickness may reach
50 meters or more.
Rokaku and Makiyama (1934) and Tsan (1974) reported that the Szekou Formation is conformably overlain by the
Hengchun Limestone. These two formations could intertongue, however, and represent two different facies of
the same age. Wedges of the Hengchun Limestone have been observed within the Szekou Formation in some
sections, and the Szekou Formation was found to overlie the Hengchun Limestone at Tsailuhsia by Cheng and
Huang (1975). Because field mapping indicates that the Hengchun Limestone overlies the Szekou Formation in
some places and is overlain by the Szekou Formation in other places, they apparently interfinger.
Ishizaki (1942) proposed the Maanshan (Mudstone) Formation, distinguishing it from the Szekou Formation on
fossil evidence. He indicated that the Maanshan Formation is slightly older. These two formations are
similar in gross lithology and are considered the same unit by many geologists. The Maanshan Formation is
composed also of alternating fine-grained sandstone, mudstone, and shale. In a recent biostratigraphic
study, Cheng and Huang (1975) pointed out that the Maanshan Formation is of late Pliocene to early
Pleistocene age. They reported that the Maanshan Formation is unconformably covered by the Hengchun
Limestone, which is overlain conformably by the Szekou Formation. The study of nannofossils by Chi (1982)
ascribed the Maanshan Formation to NN5-NN9 in Martini's zonation, the same Plio-Pleistocene age. The
Maanshan Formation and the Szekou Formation are grouped together on the present map. It is likely that the
Maanshan Formation, the Szekou Formation, and the Hengchun Limestone were deposited over more or less the
same time span that may have extended from late Pliocene to early Pleistocene, The Szekou Formation and the
Maanshan Formation are clastic facies and the Hengchun Limestone is a reef limestone. They interfinger or
overlap one another in different places. If the Hengchun Limestone facies is not found between the Maanshan
Formation below and the Szekou Formation above, the two cannot be distinguished lithologically and can only
be mapped as a single formation.
The exposures of the Maanshan Formation are quite limited and are observed only along the highway between
Hengchun and Kenting. Detailed stratigraphic data are, therefore, rather scarce. From the drilling record of
the Taiwan Power Company, the Maanshan Formation is at least 100 meters thick, and the base has not been
penetrated. It is generally believed, that an unconformity separates the Maanshan Formation from the older
normal Miocene strata on the Hengchun Peninsula, but present-day contact between the Maanshan Formation and
the Kenting Formation is interpreted to be a fault.
HENGCHUN LIMESTONE
The Hengchun Limestone was named by Rokaku and Makiyama (1934) after the Hengchun Peninsula. The Hengchun
Limestone crops out southeast of the city of Hengchun; on the terrace east of Pitzetou; and at Maanshan,
Kueitzechio, the Kenting Park, and Oluanpi, Exposures of this limestone are also distributed north to
Shoushan and Fengpitou in Ksiohsiung-hsien. The Hengchun Limestone is an organic reef composed of the
remains of a large variety of organisms: corals, foraminifers, mollusks, and calcareous algae. These
organisms are more abundant in the lower part of the limestone unit. Where the limestone is well-developed,
it is gray to creamy white, either massive and compact or porous. Solution cavities are abundant. The
thickness of the Hengchun Limestone varies from place to place, commonly in the range of several to scores
of meters; the thickest part toward the south may reach 100 meters. The age of the Hengchun Limestone is
middle Pleistocene for the most part. This limestone unit is overlain by the Oluanpi sand and gravel beds in
some places, A reported unconformity between the Hengchun Limestone and the underlying Maanshan Formation
could be a very minor break of intraformational nature as the Hengchun Limestone is found intercalated in
the rocks of the Maanshan Formation in some places, if the latter includes also the Szekou Formation (Cheng
and Huang, 1975). This break could also be an artifact produced by differential compaction.
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