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Introduction
Earlier Geologic Maps of Taiwan
Geographic Setting
General Geology And Geologic Provinces Of Taiwan
Explanation Of Legend And Representation Of Geologic Data
Eastern Central Range
Western Central Range Backbone Ridges
Western Foothills
Eastern Coastal Range
Geology Of The Hengchun Peninsula
Major Geologic Features Of Taiwan
Plate Tectonic Setting
References


:::Eastern Coastal Range
General Description Stratigraphy and Lithology Geologic Structure Geologic History Longitudinal Valley of Eastern Taiwan
Stratigraphy and Lithology
The present-day Coastal Range trends north-south and faces the Pacific Ocean toward the east. On the west it is separated from the Central Range cordillera by a narrow linear valley. The Coastal Range is the most mobile strip of the Taiwan orogenic belts and is of exceptional tectonic interest due to the predominance of chaotic rocks or melange that played an important role in the history of the mountain belt. The Neogene deposits in this eastern basin are mainly marine and partly volcaniclastic, having a combined thickness of 6,000 to 7,000 meters.
Only Neogene rocks have been found in the Coastal Range, and the Neogene sequence and lithofacies here are quite different from those in western Taiwan. There are varying views on the stratigraphic classification of the Neogene rocks in the Coastal Range. Although several stratigraphic classification schemes have been proposed by different workers, the nomenclatorial scheme of T.L. Hsu (1956) has been widely adopted due to its practicability and priority. The general stratigraphic sequence pertinent to the analysis of the tectonic history of the Coastal Range is described in considerable detail in reports by T.L. Hsu (1976b) and Chi et al. (1981). Five lithostratigraphic units (Hsu, 1956) have been distinguished by them with approximate age designation, which can be used as major subdivisions in the stratigraphic analysis and map compilation of the Coastal Range. In an ascending order, these five units are: (1) igneous complex (2) agglomerate series (3) marine clastic formation (4) chaotic melange and (5) conglomerate. The lithology and stratigraphic characteristics of these five units will be discussed in this order in the following sections.

CHIMEI IGNEOUS COMPLEX

One magmatic suite, originated in a volcanic island arc setting, is exposed in the core of the Coastal Range and on two offshore islands in the southeast. The main magmatic body crops out in the neighborhood of the village of Chimei on the lower course of the Hsiukuluanchi river. The Chimei Igneous Complex (Ho, 1969c) covers an area of nearly 22 square kilometers. A small isolated exposure of this igneous complex is also found to the south at Changyuan. The rocks are chiefly andesitic flows and pyroclastic rocks but also include subordinate basaltic andesite, diorite, and many kinds of dike rocks. Porphyritic andesite has also been found. This complex assemblage of varied igneous rock types includes several phases of intrusive and extrusive stages and complex facies relations.
A phase of porphyry copper mineralization with accompanying hydrothermal alteration occurred in this igneous complex. This igneous complex has thus become the target of intensive exploration activities and its geology has been mapped by several teams of Chinese and foreign geologists. Each came out with a different map, however, and differing interpretation of the lithology and igneous history. This magmatic tectonic subunit has thus become a controversial subject in Coastal Range geology. Detailed mapping to date has apparently focused on only a very small part of the entire complex where copper mineralization is known. The large part of this magmatic suite has not been well studied and the complicated geologic framework and history of the igneous complex remains unknown.
A tentative elucidation of the igneous activities in the Chimei Igneous Complex (Ho, 1971) has postulated an initial eruptive phase of fine-grained andesite succeeded by siliceous pyroclastic materials. Intrusion and extrusion of porphyritic andesite ensued. This was followed clearly by one important phase of diorite intrusion with associated copper mineralization. Fine-grained andesite appears to be the most favorable host for the copper minerals. A slight break separates the intrusive diorite phase from a second episode of volcanic activities, which mainly post-date mineralization. The younger volcanic series is composed mainly of tuff and agglomerate, chiefly andesitic and subordinately basaltic. Porphyritic andesite and some strongly silicified quartz-feldspar rocks are also present. The Tuluanshan Formation could be related to this second stage of volcanic activities but the exact relationship is not clear.
K-Ar apparent ages of six whole-rock samples in the Chimei Igneous Complex fall in the range of 9 to 22.2 million years (Ho, 1969c). Northeast of the Hsiukuluanchi river, the four dated samples yield apparent ages scattering from 17 to 22.2 million years. The oldest age of 22.2 m.y. may put an upper limit on the beginning of intrusion and crystallization of the igneous mass. Significantly younger ages, 9 and 14.2 million years, were obtained for the two samples collected southwest of the Hsiukuluanchi, where reduction of radiometric ages may be due to intense metal- lization and metasomatism of the rocks. The total time span of the emplacement of the different rocks in the Chimei Igneous Complex could be quite long. Based on recent K-Ar dating (Richard et al., 1986), volcanic activity in the Chimei Igneous Complex may have begun in early Miocene and ended in early Pliocene time. Two groups of rocks have been distinguished in this Complex. The radiometric ages of the first group of rocks can be divided into three stages. The first stage ranges from 23.6 to 19.7 m.y.; the second stage 15.2 to 13.7 m.y.; and the third from 10.6 to 9 m.y.. Future field studies are needed to interpret the geologic significance of these three stages of ages. The radiometric ages of the second group of rocks are comparatively younger, in the range of 9.2-4.4 m.y. This time span could be related to the eastward subduction of the South China Sea plate beneath the Philippine Sea plate.
The two southeastern offshore islands, Lutao and Lanhsu, are petrographically and tectonically related to the Chimei Igneous Complex.

LUTAO

Lutao is an island off the coast about 34 kilometers southeast of the city of Taitung. This island is underlain mainly by andesitic agglomerate with small scattered exposures of andesite lavas. Lateritic terrace gravel covers the andesite in the central part of the island. Raised and recent coral reefs fringe most of the island. Hornblende-bearing andesite is the dominant rock type both in flows and as clasts in agglomerate. The andesite and agglomerate of Lutao have been dated radiometrically by Richard and others (1986). The age span is 4.3 to 1.8 m.y. (Pliocene). This is younger than the andesite of both the Chimei Igneous Complex and Lanhsu Island. It has been suggested that the magma in Lutao had a different source.

LANHSU

Lanhsu Island is located about 90 kilometers southeast of Taitung and south of Lutao. This island is also made up of andesite lava flows and agglomerate. The lavas are exposed in the central part of the island and are covered by pyroclastic deposits on the periphery. The volcanic highland is surrounded partly by alluvial fans and talus accumulations. Raised coral reefs fringe the island. Hornblende- bearing andesite is the dominant component of the andesite lavas. Porphyry copper mineralization similar to that in the Chimei Igneous Complex occurs in the andesite, which is more or less hydrothermally altered. The agglomerate contains clasts of mainly basalt, but also andesite, gabbro, diorite, and serpentinite. The agglomerate is much darker than the agglomerate exposed in the Coastal Range. Some geologists have reported that the agglomerate is cut by basaltic dikes, mainly of olivine basalt.
Of special interest is the discovery of limestone boulders in a stream bed on the northwestern side of the island. The locality is named Longtao and the limestone contains Aquitanian foraminifers. The stratigraphic relation of the limestone to the volcanic rocks is still uncertain: The limestone could be lenticular boulders brought up by pyroclastic eruption and later embedded in the agglomerate beds. It could also form interlayers in the agglomerate, which also contains tuff and marl horizons. The significance of this critical limestone needs further study. Four samples of andesite and two samples of agglomerate were dated by an Australian Mining Company by K-Ar method (Ho, 1975 and 1982).
Four of the samples yield radiometric ages scattering from 13.4 to 5.76 million years. An age spread from middle Miocene to Pliocene is indicated for these rocks. The age of the volcanic rocks on Lanhsu is generally the same as that of the Chimei Igneous Complex in the Coastal Range. This is corroborated by the fact that porphyry copper deposits have been found both on Lanhsu and in the Chimei Igneous Complex. The volcanic eruptions on Lanhsu and the Coastal Range however, may have extended over a considerable time span in the Neogene time. Recent K-Ar dating (Richard et al., 1986) indicates that the age span of the volcanic rocks on Lanhsu ranges from 5.5-3.7 m.y. (mainly Pliocene), similar to that of the younger group of volcanic rocks in the Coastal range.
Hsiaolanhsu (small Lanhsu), is an islet southeast of Lanhsu and is geologically closely related to it. Like Lanhsu, the islet is composed of agglomerate and andesite lava flows, predominantly of hornblende andesite type. The volcanic eruption on Hsiaolanhsu could be a little later than that on Lanhsu. (Richard et al., 1986)
All the volcanic rocks in this stratigraphic unit are related to the volcanic arc system of the Luzon Arc, the active portion of which is situated south of Taiwan. The age data given above show that the volcanism in the Luzon Arc System becomes progressively younger from the Chimei Complex in the Coastal Range to the southern offshore islands. The general trend of younger volcanism to the south is a striking feature of the Luzon volcanic arc. No active volcano has ever been found in this arc in Taiwan. Active volcanoes are present farther south, however, including Batan and Babuyan Islands between Taiwan and the Philippines and the volcanoes on Luzon Island in the Philippines. The tectonic significance of the volcanic rocks in eastern Taiwan is discussed in the report "Tectonic evolution of Taiwan" (Ho, 1982).

TULUANSHAN FORMATION

The lowest exposed stratigraphic unit overlying the Miocene andesitic igneous core is the Tuluanshan Formation, a volcanic unit associated with or overlying the igneous complex. The Tuluanshan Formation consists largely of agglomerate, tuff, and tuffaceous sediments, apparently related to the underlying andesitic arc volcanism. Minor basalt and dacite are present in addition to the dominant andesite. The estimated thickness of the whole unit is 1,000 to 1,500 meters. Lithologies are quite varied in the Tuluanshan Formation. This formation name was first proposed by Hsu (1956) for a thick agglomerate series exposed in the Tuluanshan ridge, one of the highest peaks in the Coastal Range. This agglomerate formation is exposed over a little less than half the area of the Coastal Range. It is distributed extensively in the middle part but extends to the north and south as well. The Tuluanshan Formation also contains water-sorted andesitic pebbles, representing a sedimentary accumulation of pyroclastic debris rather than a direct product of volcanic eruption.
Discontinuous limestone lenses occur at and near the top of the agglomerate series along the eastern flank of the Coastal Range. The thickness of limestone ranges from less than 1 meter to more than 10 meters but the thickest part may reach 50 meters. This shallow-water limestone yields Pliocene foraminifers (Cheng and Wei, 1983), and reworked or secondary planktonic foraminifers of early to middle Miocene age which were earlier thought to reflect the true age of the limestone (L. S. Chang, 1967). No fossils have been found in the Tuluanshan Formation proper, whose age has to be inferred from overlying and underlying fossil-bearing strata. A recent study of nannofossils, however, shows that the upper part of the Tuluanshan Formation ranges in age from middle to late Miocene, the youngest age being near the Miocene/Pliocene boundary (Chi et al., 1981).

TAKANGKOU FORMATION

The Tuluanshan Formation is overlain by a thick volcaniclastic sequence, the Takangkou Formation, which comprises mostly shale, sandstone and conglomerate, and which contains considerable volcanic detritus. In field mapping this immense clastic series was first divided into two rock associations by Hsu (1956). The lower one is the conglomerate-shale-(graywacke) series and the upper one is the shale-graywacke series. The former was named the Takangkou Formation; the latter, the Chimei Formation which is named after the same locality as the Chimei Igneous Complex. The type localities of both units are in the Hsiukuluanchi drainage area. These two formations are gradational, with no clear-cut boundary, and the distinction between them reflects local facies variations within one sediment mass. Biq (1969) deemed it convenient to combine the two units into one formation which is named the Takangkou Formation. This geologic map follows this usage, which has been widely accepted by most field geologists in Taiwan.
The Takangkou Formation is composed of a stratified marine clastic sequence deposited west of and on the volcanic arc. In places pebbles and boulders of andesite have been found in the basal part of the Takangkou Formation. The bulk of the Takangkou sediments reflects fairly deep-water conditions. The total thickness of the Takangkou Formation is estimated to be approximately 3,000 to 4,000 meters. Dark gray mudstone and siltstone with thin to thick sandy beds are prominent in this unit in the southern part of the Coastal Range. There is also limited occurrence of pebbly mudstone and olistostromal blocks with ophiolitic and sedimentary clasts (containing Miocene fossils) similar to those of the Lichi melange, which will be discussed in the next section (Chi et al., 1981). In the northern Coastal Range, the chief lithology is siltstone and mudstone with occasional thin sandy beds, but thick polymictic conglomerate members are developed in the upper and middle sections, reaching a maximum thickness of several hundred meters in places. The clasts of the conglomerate are chiefly slate and metasandstone similar to the slate terrane of the Central Range. Subordinate amounts of ophiolite and andesite fragments are also present.
The clastic rock in the Takangkou Formation, especially the sandstone in the lower part, is characterized by the predominance of well-developed turbidite sedimentary structures. The Takangkou Formation in the southern part has been called the turbidite formation by Wang and Chen (1966). The sandstone is commonly graded. Convolute bedding, flute casts, load casts, groove casts, and other sole marks are locally very distinct (T.L. Hsu, 1954; F.W. Huang, 1977). Penecontemporaneous slump structures are common in many exposures. These features are characteristic of flysch or turbidite deposition, which is common in orogenic belts around the world. These sedimentary structures are lacking or very poorly developed in clastic rocks of the western Neogene belt. This is the main difference between the two Neogene clastic rock associations that are located in distinctly different tectonic provinces of Taiwan.
Marine microfossils are abundant in the Takangkou sediments but molluscan fossils are very rare. Foraminifers in the shaly sediments range in age from late Miocene to Pliocene but are mostly of Pliocene age according to L. S. Chang (1967, 1968, and 1969). New biostratigraphic studies on nannofossils (Chi et al., 1981) show that the Takangkou Formation ranges from middle Pliocene to middle Pleistocene. The age of the rocks and of the contact with the underlying Tuluanshan Formation varies in different places. The Takangkou Formation on the eastern coast is upper middle Pliocene to upper Pliocene, whereas that in the western part along the Hsiukuluanchi stream is mainly lower Pleistocene. The contact relationship between the Takangkou Formation and the underlying Tuluanshan Formation has been reported to be depositional, disconformable, and unconformable by different geologists. A paleontologic break is at least present between these two formations based on the recent work of Chi and others (1981).
Recent sedimentological study of the sandstones in the Takangkou Formation by Teng (1979 and 1980) indicated two sources of the arenaceous sediments. One is from the volcanic arc, characterized by the abundance of volcanic detritus. The other is from the clastic terrain of ancient Asiatic continent, including also thick conglomerate facies of turbidity current origin. The former is distributed mostly in the lower part of the Takangkou Formation and is named the Fanshuliao Formation by Teng. The latter is more prominent in the upper part of the Takangkou Formation and is named the Paliwan Formation by Tens.

LICHI FORMATION

The Lichi Formation is widely exposed at the southern end of the Coastal Range in a 1 to 3 kilometers wide strip that extends northward along the western side of the Range for nearly 70 kilometers to Loho. This formation was named by Hsu (1956) after the village of Lichi, about 8 kilometers northeast of the city of Taitung. It is composed of a thick series of gray muddy sedimentary rocks containing abundant exotic blocks of various lithotypes and different sizes. The outcrops of the formation form continuous badlands with isolated resistant blocks. The Lichi Formation is a typical melange (Hsu, 1976b) consisting of a heterogeneous mixture of rock materials consisting of a pervasively sheared argillaceous matrix thoroughly mixed with native and exotic tectonic fragments or blocks. No particular mode of origin is implied
The mudstone matrix of the Lichi Formation is chaotic, intricately sheared, and not perceptibly bedded. The best exposure may reveal faint crude layering on a gross scale. The crude layers share the same orientation as the shear surfaces. All the quasi-planar features vary in orientation from one locality to another, but typically they are steeply dipping and strike nearly north-south rather than east-west. Most of the exotic blocks are less than a few meters across, some as small as the size of a bean. Several exceedingly large ones are as much as one square kilometer or more in areal extent and emerge from the badlands as isolated hills. They generally display slickensides and other evidence of shearing, and some are overturned. The total thickness of the Lichi Formation is unknown as the base is nowhere exposed. It is at least 1,061 meters thick based on the log of a stratigraphic hole drilled by the Chinese Petroleum Corporation near Taitung (Meng and Chiang, 1965). It is now recognized that the Lichi melange represents a special lithofacies and its thickness may vary from place to place.
Exotic blocks in the Lichi melange are composed largely of sandstone and ophiolite. Less conspicuous, generally smaller fragments are siltstone, shale, and mudstone, with minor limestone, conglomerate and andesitic agglomerate. One large isolated limestone block was left stranded on the alluvial plain of Taitung, when the enclosing muddy matrix eroded and washed away. Most sandstone blocks are subgraywacke and protoquartzite, and display typical turbidite structures. Blocks of interstratified sandstone and shale are also common.
The most striking exotic components are mafic and ultramafic blocks, which collectively make up the Miocene "East Taiwan Ophiolite" (Liou et al., 1977). Rock types include peridotite, gabbro, serpentinite, diabase, plagiogranite, basalt, and basalt breccia. The largest ophiolite exposure near Kuanshan is 3.5 kilometers long and more than 1 kilometer wide. Similar rocks are found as discrete blocks throughout the melange unit and are especially abundant near the city of Taitung. The stratigraphy, petrology, and paleogeographic history of the East Taiwan Ophiolite in Kuanshan is detailed by Liou et al., (1977); and Suppe et al. (1981). From their studies, the ophiolite stratigraphy is represented by a basal sequence of breccia composed of gabbro, diabase, and peridotite, interpreted to be landslide breccias derived from a transform fault scarp in the oceanic crust. The breccia is capped by pelagic red shale deposited below the calcite compensation depth and an overlying sequence of glassy basaltic pillow lavas and volcanic breccia with intercalated red shale. The preserved stratigraphy may represent the surficial part of the oceanic crust. The basal mafic breccia displays the effects of partial ocean-ridge type metamorphism under greenschist- to rare amphibolite-facies conditions (Liou and Ernst, 1979). Nannofossils discovered in the red shale matrix of the plutonic breccia are lower to middle Miocene (T.C. Huang et al., 1979).
Fossils of mixed origin and ages have been found in both the matrix and the exotic blocks of the Lichi melange. Most of the foraminiferal fossils are Miocene, with Pliocene fossils in only a few localities (L. S. Chang, 1967, 1968, and 1969). New biostratigraphic studies on the basis of calcareous nannofossils (Chi, 1980) show that the Lichi melange contains mixed fossils ranging in age from Oligocene to middle Pliocene. The age of the Lichi Formation is considered late Pliocene by Page and Suppe (1981) and middle to late Pliocene by Chi (l 982).
Page and Suppe (1981) indicated that the Lichi melange was formed by large- scale submarine mass movement and is a type of olistostromal mass. They envisaged that the Lichi melange represents only an olistostromal fades of the Takangkou Formation and is thus either conformable or intertongues with the underlying Takangkou Formation, displaying an inferred northwest source for the melange. The olistostromal materials were derived from the outer non-volcanic arc between the North Luzon Trough and the Manila Trench. This inferred source terrane no longer exists but a major fault zone is inferred to separate the source terrane from the basin of Lichi deposition. This basin is the northern continuation of the North Luzon Trough. The fault zone produced relief and instability for olistostrome formation during the initial stage of arc-continent collision.
Most geologists have now agreed that the raw materials of the melange represent an olistostromal accumulation derived from a west source. The olistostromes were induced by conditions related to continent-arc collision and were assembled in a subduction zone complex. Therefore the melange may have been subjected to post-depositional tectonism. The presence of andesitic exotic blocks in the muddy matrix indicates that island arc materials from the east have also been involved in the complex. Some geologists have emphasized the pervasively sheared nature of the muddy matrix, which manifests the effect of superposed tectonic deformation. The Lichi Formation could thus be interpreted as an olistostromal melange reworked or remolded into a tectonic melange during a subsequent stage of plate collision. The French geologists (Barrier and Muller, 1984; Barrier and Angelier, 1986) call the Lichi melange a tecto-sedimentary melange.

PINANSHAN CONGLOMERATE

The Pinanshan Conglomerate was named by Hsu (1956) for a thick conglomerate exposed mainly on Pinanshan hill northwest of the city of Taitung. This is the youngest unit in the Eastern Coastal Range save possibly the Milun Formation and alluvium. Scattered outcrops of this conglomerate are also distributed along the lower course of the Pinantachi stream for a distance of 7 to 15 kilometers north of the city of Taitung, mostly restricted to the southern end of the Coastal Range. The Pinanshan Conglomerate is crudely bedded and poorly sorted, and is composed largely of cobbles derived from the metamorphic terrain of the Central Range west of the Longitudinal Valley. The cobbles range from 5 to 15 centimeters in diameter, consisting of metamorphosed rocks of several types.
This conglomerate unit is a fluvial piedmont deposit. The age of the conglomerate unit is probably middle or late Pleistocene and the reported thickness varies from 500 to 3,000 meters. It is younger than the Lichi melange and locally rests unconformably on the Lichi Formation, although faulting has locally altered the contact, intertonguing the two units. Metamorphic fragments of the Tananao Schist terrane do not appear in beds older than middle to late Pleistocene, but appear first in the non-marine Pinanshan Conglomerate. This has an important bearing on the timing of plate collision in eastern Taiwan. Most of the conglomerate has been strongly folded, and some of it dips nearly 70o.

MILUN FORMATION

The Milun Formation or the Milun Conglomerate of Hsu (1956) in Hualien is believed to be correlative with the Pinanshan Conglomerate by some geologists. This formation is exposed in Milun, about 2 kilometers north of the city of Hualien in eastern Taiwan. It was called the Milun (Beiron) Conglomerate Formation by Usami (1939) and the Milunpi Conglomerate by Lin (1963). The Milun Formation consists of massive, poorly bedded conglomerate composed of pebbles 10 to 20 centimeters across. Pieces of semi-carbonized wood are embedded in the conglomerate, together with a few sand and sandy shale interlayers. The thickness of the Milun Formation is at least 350 meters in Milun. The conglomerate exposed in the Sopa terrace near Juishui in the Eastern Longitudinal Valley may be equivalent to the Milun Conglomerate. The Milun Formation is moderately disturbed and tilted to a maximum of 30o or so. No fossils have been found in the Milun Formation and the age of the formation can only be approximately determined on the basis of gross lithology and general structural characteristics. Some early workers suggested that the Milun Formation may be correlated to the Toukoshan Formation of western Taiwan. In this map, the Milun Formation is correlated with the younger Tananwan Formation of Pleistocene age. In the Milun area, the Milun Formation is covered unconformably by younger gravel beds which were named the Milun Formation by Lin (1963). Careful distinction must be made between the Milun Formation (conglomerate) shown on the present map and the Milun Formation (gravel beds) proposed by Lin.
The Milun Formation of Lin (1969) can be compared with the Raised Coral Reef in age. It covers the top of the Milun terrace north of Hualien. This unit is composed of basal boulder layers, reef limestones, gravel beds, and sand and clay admixtures, reaching a thickness of 10 meters or so. It rests unconformably on the Pleistocene and older rocks.

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