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| :::Western Central Range Backbone Ridges |
Geologic History
Possibly in early Eocene time, marine transgression brought clastic argillaceous sediments into the Tertiary
basin overlying the Tananao Schist in the Asian continental margin terrane now elevated into the Central
Range of Taiwan. The basement of the basin is the pre-Tertiary Tananao Schist. Marine transgression in this
basin proceeded from Eocene, Oligocene, to early middle Miocene times. A thick sequence of undifferentiated
argillaceous sediments was deposited in outer neritic to upper bathyal environments during this time. The
sediments attain a great thickness, difficult to estimate because of the complex structures resulting from
later metamorphism and rock deformation.
Toward the western shelf area where the water was shallower, the argillaceous sediments in the basin grade
into and interfinger with thick arenaceous sediments in which coaly and carbonaceous beds were developed.
Two coal-bearing units (Aoti Formation and Szeleng Sandstone) were formed in the western part of the basin,
the present Hsuehshan Range belt. Periods of rhythmic regression and transgression of the sea resulted in
the deposition of these two carbonaceous units alternating with marine argillaceous formations in the
stratigraphic column of the Hsuehshan Range belt. This carbonaceous facies diminishes and disappears toward
the eastern and southern parts of the basin, where mostly deep-water sedimentation took place resulting in
thick, dark gray argillaceous sediments now exposed in the Backbone Range.
During the deposition of the argillaceous sediments in the Tertiary sedimentary basin, one controversial
problem is whether major crustal movements occurred in the geologic evolution of the basin from Eocene to
middle Miocene times. All the argillaceous sediments in the basin are apparently continuous and no distinct
uncon- formity has yet been observed. However, recent deformation and metamorphism may have obliterated and
obscured any early-formed structural breaks or deformation features.
One the basis of recent geologic information, it is likely that continuous sedimentation took place in the
Hsuehshan Range belt in the western part of the basin, close to the land area. From Eocene to middle
Miocene, no marked structural break has been recorded. In the Backbone Range belt in the eastern part of the
basin, however, crustal movements may have caused a break in deposition following accumultion of the Eocene
rocks. In this oceanward part of the basin, Oligocene strata were either not deposited or were later eroded
away. Sedimentation began again following marine transgression in late Oligocene or early Miocene time; such
that Miocene or upper Oligocene rocks unconformably overlie the Eocene strata. T. C. Huang (1980a) stated
that this Oligocene/Eocene unconformity is world wide and is closely related to the overall lowering of sea
level in Oligocene time. This Oligocene hiatus could be related to the N conglomerate mentioned by L. S.
Chang (1972), but no clear discordance of any sort can be delineated in the field.
During sedimentation of the argillaceous sediments, submarine volcanism took place in many local centers of
the basin. This volcanic activity is more widespread in the northwestern part and the south-middle part of
the basin. The submarine eruptions opened fissures and brought up mostly basaltic pyroclastic materials and
some lava flows. They spread out over the floor of the basin and were intercalated in the sediments at many
scattered places. Other varieties of volcanic eruption are also known but less common. A subordinate part of
the volcanic rocks could have erupted during the later orogeny. However, study has not progressed far enough
to distinguish different phases and ages of the volcanic activities in this geologic province. Ernst and
others (1981) stated that the diabasic dikes in the Tananao basement complex could be feeders of the basaltic
volcanoes in the slate series, because no foliation is detected in these diabases. The age could be Early
Tertiary. These basic dikes show evidence of only the most recent metamorphism.
All the Tertiary sediments were affected by the Penglai Orogeny in Plio- Pleistocene time. This orogeny
elevated and transformed the argillaceous and other sediments in the basin into a cordillera, the present
Central Range system. The pre- Tertiary crystalline basement was also brought up and exposed on the eastern
flank of the Central Range. Low-grade metamorphism accompanied this orogeny but no major igneous activity is
known in the Central Range. The shaly rocks were transformed into argillites, slates, and phyllites while
the arenaceous rocks became indurated sandstones and white quartzites. Graphitic shale was derived from
coaly or carbonaceous beds. The degree of metamorphism exhibited by rocks exposed at the surface increases
from the western shelf area toward the eastern axial zone of the mountain range.
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:::© CENTRAL GEOLOGICAL SURVEY, MOEA
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TEL: +886-2-29462793FAX: +886-2-29429291
E-mail:cgs@moeacgs.gov.tw
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