Geologic History
The Coastal Range is the northern extension of the Luzon volcanic arc. This chapter deals mainly with the
geologic evolution of this arc and the associated eastern sedimentary basin. The relation of geologic
evolution and plate tectonics is discussed in the succeeding chapter. The lithologic characters of the
eastern basin indicate a period of turbidity current sedimentation, volcanism, and submarine slumping, which
worked one after the other to produce the structural and stratigraphic features displayed in the Coastal
Range of to-day.
Large-scale submarine andesitic eruption ushered in the earliest recorded volcanic episode in the
development of the Luzon volcanic arc in early Miocene or older times. This volcanism was a result of the
subduction of the Eurasian plate beneath the Philippine Sea plate. Important centers of volcanic activity
are recognized in the Chimei Igneous complex and Lutao and Lanhsu Islands, and are likely distributed
elsewhere throughout this belt. It is tentatively postulated that fine-grained andesite was the oldest lava
emitted in the complex, succeeded by siliceous pyroclastic units. Intrusion and extrusion of porphyritic
andesite ensued, followed closely by diorite intrusion with associated copper mineralization in the Chimei
area and in the offshore islands. Following dioritic intrusion, a second episode of volcanic activity
produced mainly tuff, agglomerate and andesite. This upper volcanic series overlies a certain part of the
earlier copper-bearing volcanic series.
Near the end of the second volcanic episode, large amounts of andesitic fragments and detritus were thrown
out from many widely scattered submarine centers. These ejecta later accumulated in the basin west of the
arc to form the Tuluanshan Formation, probably in middle or late Miocene time. The main centers of
pyroclastic ejection could be located further east of the present Coastal Range. The great thickness and
extensive distribution of the andesitic agglomerate manifest the profuse and widespread outpouring of
volcanic fragments in a subsiding basin. Part of the agglomerate may have been eroded from the basin slopes
and redeposited in deeper basinal areas. Tuffaceous materials were mixed with sandy sediments in the basin
to form tuffaceous sandstone of the Tuluanshan Formation. Toward the end of agglomerate eruption in about
Pliocene time, thin carbonate deposits accumulated locally on the Tuluanshan agglomerate.
After deposition of the Tuluanshan Formation and associated limestone lenses, the basinal area may have
emerged for a short time before early Pliocene transgression. Field evidence of this break is not distinct
or elsewhere consistent, however, and the contact between the Tuluanshan Formation and its overlying marine
unit has been interpreted by different geologists as unconformable, disconformable or continuous.
Beginning possibly as early as later early Pliocene, transgression occurred with rapid subsidence of the
basin. Marine clastic deposition predominated when the borderland of the trough or the newly emerged ridges
in the basinal area were supplying ample detrital materials into the continuously subsiding basin. Vigorous
downwarping of the trough was coupled with rapid sedimentation of clastic rocks from Pliocene to early
Pleistocene times, mainly by turbidity currents. The resulting Takangkou Formation contains more than 3,000
meters of clastic orogenic sediments, composed mainly of shale, lithic sandstone and conglomerate, with a
considerable amount of volcanic constituents and minor limestone lentils. The coarse conglomerate clasts
were furnished mainly by the argillite-slate terrain of the Central Range cordillera west of the trough.
The early-formed sediments of the Takangkou Formation contain a large amount of volcaniclastic detritus
derived from the eastern volcanic arc. In later time, continuous arc-continent collision brought the basinal
area into close contact with the ancient Asiatic continent. An appreciable amount of clastic sediments from
the continental margin then entered the trough of Takangkou sedimentation. Arc volcanic activity did not
stop completely, however, and continuing pyroclastic eruptions produced ash layers in the Takangkou
Formation into the Pleistocene.
Contemporaneous with the deposition of the Takangkou Formation, large- scale submarine mass movement and
slumping took place along the western margin of the basin in late Pliocene time, A large supply of rock
debris in association with weak and lubricant muddy materials slid into the basin along the western slope of
the sea floor. Exotic sedimentary and igneous blocks of different sizes and ages were incorporated into the
muddy matrix during downslope movement, leading to the accumulation of the Lichi melange, the olistostromal
facies of the Takangkou Formation. Submarine sliding may have repeatedly reworked the Lichi sediments. The
highly disturbed and chaotic Lichi muddy mass or melange is the end product of these movements. The mafic
and ultramafic blocks represent an ophiolite suite related to the oceanic crust or upper mantle materials.
Their inferred origin could be the South China Sea east of the old Asiatic continent on the western side of
the sedimentary basin. The South China Sea has been subducted at the latitude of Taiwan, but remains to the
south.
The Penglai Orogeny took place in Plio-Pleistocene time, when the Eurasian continental margin encountered
the subduction zone west of the Luzon Arc. All the Neogene sediments in the eastern basin together with its
volcanic basement were subjected to intense folding and faulting, and uplifted to form the Coastal Range and
its characteristic structural features. Probably before and after the early Pleistocene paroxysm, a great
deal of rock debris was shed eastward into the basin from the Central Range cordillera which was then
actively elevated and bared to the basement complex. This rapid transport of coarse fragments led to the
deposition of a thick conglomerate series, the Pinanshan Conglomerate, at the eastern foot of the southern
cordillera. The coarse alluvium from the west was in direct contact with the Lichi sediments in the east.
This conglomerate formation was strongly deformed in the Pleistocene orogeny.
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