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Diastrophism and Orogenic Movements
The Oligocene and Neogene rocks were laid down continuously in the western sedimentary basin with periodic
oscillations of sea level. No distinct angular unconformity has ever been observed in the thick Neogene
sequence of western Taiwan. Minor disconformities have been reported, however, especially in the upper
Miocene and lower Pliocene rocks of southern Taiwan. Three discordant levels of contacts were indicated by
Stach (1957), probably resulting from diastrophic control of cyclic sedimentation in the upper Neogene to
lower Pleistocene rocks of southern Taiwan, Sun (1965) reported another break in Neogene deposits in
Kaohsiung-hsien, where Miocene to Pliocene shale overlies a gently tilted Miocene unit toward the southwest.
Other minor or local disturbances in the upper Miocene and lower Pliocene rocks of southern Taiwan have been
mentioned by Schreiber (1962). The absence of the Nanchuang Formation in wells drilled in the Taichung area
(Chou, 1970) may mark another depositional break in the Miocene of central Taiwan.
Tang (1977) also reported an upper Miocene unconformity in the rocks underlying the Chiayi-Yunlin plain. In
the drilling records of the western plain area, different Miocene formations have been found overlapping the
Peikang basement high. In addition, a number of local breaks have also been discovered in the subsurface
Miocene sections. These stratigraphic discontinuities or breaks can only be delineated from the study of
subsurface geology, however, and cannot be shown on surface geologic maps. These local tectonic features may
manifest repeated oscillations of sea level or local uplift and subsidence during the late Cenozoic.
The paroxysm of orogenic disturbance in Taiwan, especially in the western foothills, occurred in
Plio-Pleistocene time, and is called the Penglai Orogeny. Penglai is the old geographic name for Taiwan and
this event is the most important one in Taiwan. This widespread and intense orogeny is ascribed to the
result of interaction between the Eurasian plate and the Philippine Sea plate by the collision of the Luzon
island arc with the Asiatic continent. Detailed discussion is given in the chapter on plate tectonic setting
of Taiwan.
The main phase of this orogenic episode began in western Taiwan near the end of the Pliocene and the
beginning of the Pleistocene when thick conglomerate accumulated in many places in the subsiding basin of
western Taiwan. This is the Toukoshan Formation, which has recently been dated as mainly Pleistocene.
Therefore much of the deformation in the Penglai Orogeny was late in early Pleistocene time. Rapid uplift of
land area yielded a large amount of coarse clastic detritus and foreshadowed the coming orogenic paroxysm
which reached its climax in the middle Pleistocene. During the climax, the western basin was deformed, and
folded and faulted mountain chains gradually emerged. The sediments laid down after the climax are
represented by late Pleistocene lateritic to non-lateritic gravel beds which lie unconformably on the eroded
surface of all deformed older rocks. This angular discordance is the only distinct and conspicuous
unconformity that can be observed in the Cenozoic rocks of Taiwan. The Pleistocene orogeny has not yet ended
as shown by the frequent earthquake tremors and local folding and uplifting of the tableland gravel in some
places. Suppe (1981) believes the present-day orogeny is probably just as intense as that of the early
Pleistocene.
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