Geologic History
After the Nanao Orogeny, a Tertiary basin was developed on the basement complex of the Tananao Schist. Based
on paleontologic data, marine transgression in the basin began perhaps in early Eocene; and brought in a
thick sequence of argillaceous sediments. The transgression of argillaceous beds continued to early middle
Miocene time, with deposition taking place mostly in outer neritic to upper bathyal environments. These
argillaceous sediments were transformed into an argillite-slate series in the Central Range whose history is
discussed earlier. Subsequently this old sedimentary basin gradually migrated toward the west with the
center of sedimentation also migrating westward. Most of the younger Neogene strata of western foothills
were deposited in a trough to the west of the depositional axis of the argillite-slate series.
The young Tertiary sedimentary trough passes into the western continental shelf, with a marked thinning of
all Neogene units toward the west. Overlapping of strata and local disconformities in the Neogene rocks
overlying the basement on the western shelf zone of the trough in subsidence record local fluctuations in
subsidence. Tertiary sedimentation in the western basin began in middle or late Oligocene time. The early
deposits were fluvial and transitional marine-continental facies in the northwestern part with white
sandstone as the dominant lithology. Depositional environments were gradually more marine toward the
southeastern part where mainly shaly rocks were deposited in a shallow marine setting. This difference in
depositional facies is reflected in the rocks of the Wuchihshan and Wentzekeng Formations. From the study of
the drilling logs of the Chinese Petroleum Corporation in the coastal and terrace areas in western Taiwan,
the upper Cenozoic rocks of the western foothills overlapped basement rocks of different ages. These
basement rocks are mainly of sedimentary origin and can be divided into three age groups: Paleozoic (?),
Cretaceous and Paleocene, and Paleocene/Eocene (Ho, 1982).
As sedimentation proceeded from Oligocene to early Miocene time, relative changes in sea level gave rise to
initial deposition of a carbonaceous non-marine unit and a near-shore marine unit in the basin. This was
followed successively by three rhythmic cycles of sedimentation in northern Taiwan from early Miocene to
late Miocene, each of which records a regressive period of carbonaceous deposition and a transgressive
period of marine deposition. Each depositional cycle begins with a coal-bearing facies, indicating an
emergence of the shelf area and a general regression of the sea. This facies mostly reflects lagoonal,
estuarine, and/or deltaic to tidal-flat environments of deposition with rare marine incursions. The
coal-bearing formation is overlain by transgressive marine strata, mostly reflecting shallow marine
environments.
The Miocene basin deepened toward the south. The coal-bearing shelf-type sediments in the north gradually
gave way to marine sediments toward the south with regional northward transgression of the sea. The extent
of submergence and marine transgression, however, decreased from early Miocene toward late Miocene. Thus,
the younger the coal-bearing formation, the more extensive its distribution toward the south, and the less
it is replaced by marine strata in the basin. Eventually marine transgression covered the whole Miocene
basin south of the Alishan Railway in southern Taiwan with the complete obliteration of shelf-type
carbonaceous rocks. Most of the marine strata are neritic or shallow marine deposits, but may be in part
upper bathyal deposits.
As the Miocene basin gradually deepened from northern Taiwan toward southern Taiwan, the thickness of
clastic sediment increased greatly to the south. The Miocene sediments fine southward, with the southern
part of the basin ac- cumulating thick deposits of clay and silt with only scattered sandy stringers. Thus
the Miocene seaway in western Taiwan was deeper and more open to sea toward the south.
Volcanic eruptions provided basaltic pyroclastic deposits and some lava flows to the Miocene basin,
especially in northwestern Taiwan. The most extensive and voluminous volcanic phase was the Kungkuan
volcanic stage in the early sedimentary cycle. In the middle sedimentary cycle, volcanic activity was
comparatively weak and of very limited distribution (Chienshih volcanic stage). Volcanism (Chiopanshan
volcanic stage) was active again in the late sedimentary cycle when the coal-bearing Nanchuang Formation
was being deposited. This volcanic phase was distributed widely in north-central Taiwan. Central Taiwan is a
region of volcanic quiescence during the Neogene. Miocene volcanism, however, was slightly active in
southern Taiwan where sporadic volcanic eruptions from various centers resulted in the accumulation of small
basaltic pyroclastic piles and lava flows in the middle to upper Miocene sediments.
Carbonate deposition is relatively unimportant in the Miocene depositional history of western Taiwan. An
accretionary carbonate mass was developed during middle Miocene time in the eastern hills of Hsinchu-hsien.
Several small reef masses were localized, probably resulting from a gentle epirogenic movement. An irregular
sea floor shoaled to near-surface conditions, and the local relief was sufficient to allow biohermal reef
growth in the Miocene basin.
From latest Miocene into Pliocene time, marine transgression covered the Neogene basin of western Taiwan,
The young Pliocene rocks are sandy in the north and gradually become shaly toward the south. These Pliocene
sediments were deposited mainly in shallow marine environments with some oceanic influence. The basin
deepened and was open to the ocean toward the south. No volcanic eruptions took place in the western basin
in Pliocene time.
Reefs developed locally around the Pliocene sea basin with a slight regression of the sea water. The reef
limestone kept growing at various stages in different localities from Pliocene to Pleistocene times. Thick
and large organic reefs grew more frequently in Pleistocene time toward the southwestern part of the Neogene
embayment, in southern Taiwan, forming reefal and bioclastic intercalations in the clastic sediments. Coral
reefs are still accumulating around the main island and the subordinate islands of Taiwan.
A thick sequence of predominantly muddy to silty sediment was laid down south of the Tsengwenchi stream in
southern Taiwan. This argillaceous sedimentation began possibly in late Miocene, continued through the
Pliocene, and ended in the Pleistocene. This is the Gutingkeng Formation described previously. These
voluminous argillaceous sediments were most likely deposited in an outer neritic to bathyal environment, in
water depths greater than in other parts of the Neogene basin.
Sedimentological studies (Chou, 1973) of the rocks in the western foothills indicate that the provenance of
the Paleogene and Miocene sediments is to the northwest, the area of the present Taiwan Strait and the China
mainland. However, the Pliocene sediments were derived largely from the submetamorphic terrain of the
Central Range, partly from the sedimentary and metamorphic paleo-highlands in the Taiwan Strait.
The major and most important orogenic movement in the Neogene basin of western Taiwan occurred toward the
end of the Pliocene when a thick conglomerate (Toukoshan Formation) accumulated, mainly in central Taiwan.
This coarse detritus was laid down in a deltaic to fluvial piedmont system, manifesting a rigorous uplift of
the eastern borderland and a gradual retreat of the sea in the Neogene trough. The thick conglomerate graded
laterally north and south into fine clastic sediments that characterize shallow marine to estuarine
environments. Similar coarse conglomerate accumulated further south in the Pingtung valley near Liukuei.
The rapid uplift foreshadowed the coming orogenic paroxysm, which reached its climax in early Pleistocene
time with the gradual emergence of folded mountain chains. The western basin collapsed as the Neogene
sediments were folded and thrust into piles and built into highlands. This Pleistocene diastrophism, the
Penglai Orogeny, is discussed in previous sections. Some recent papers have elaborated this orogenic episode
into a number of phases, and indicated that the early phase of this orogeny may have begun in Miocene time.
The post-orogenic deposits are the laterite-capped tableland gravels which lie unconformably on the eroded
surfaces of deformed Neogene or early Pleistocene rocks, containing fragments of the underlying units.
When the western basin was under Plio-Pleistocene compression and breaking up, or immediately after major
movement, extensive volcanic eruptions were active in the northern extremity of Taiwan and on the offshore
islands. Two important groups of andesitic volcanoes, the Tatun Volcano Group on the west and the Chilung
Volcano Group on the east, were built up by successive eruptions near the northern coast. The volcanic
activity in these two volcano groups may have begun in late Pliocene time and reached its climax in the
Pleistocene Epoch. At about the same time in the Pleistocene, great volumes of basaltic lava rose up quietly
as fissure eruptions through the Pleistocene and Neogene sediments on the Penghu Island in the Taiwan
Strait. The basalt flows form an extensive sheet of plateau basalt on the western shelf of the Neogene
basin. The evolutionary history of the Penghu plateau basalt may have to be changed if the age of volcanism
is proved to be Miocene substantiated by future studies.
During and following the early Pleistocene orogeny, a new cycle of sedimentation began in a new trough on
the west, which is now occupied by the western coastal plain, the Taiwan Strait, and other offshore areas.
These events indicate that the axis of sedimentation of the western depositional trough is migrating
successively westward toward the foreland with passage of geologic time.
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