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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


:::Western Central Range Backbone Ridges
General Geologic Features General Stratigraphy Stratigraphy of The Northern Part of The Hsuehshan Range Belt Stratigraphy of The Central and Southern Parts of The Hsuehshan Range Belt Stratigraphy of The Backbone Range Belt Geologic Structure and Metamorphism Geologic History
General Geologic Features

DISTRIBUTION

The greater part of the Central Range consists of a mighty sequence of indurated metamorphosed argillaceous Tertiary sediments. These rocks form the geologic subprovince immediately west of the pre-Tertiary metamorphic complex. This subprovince covers chiefly the crest zone and the western flank of the Central Range. It extends southward to the Hengchun Peninsula and encircles the southern end of the Tananao Schist crystalline belt to the eastern flank of the Central Range. A narrow rim of slates and phyllites oh the eastern edge of the Central Range from Taitung to Yuli also belongs to this geologic subprovince. For ease of description, the rocks in this subprovince have been named the argillite-slate belt or argillits-slate series of the Central Range (Ho, 1967 a and b).
The argillite-slate series extends from Santiaochio at the northeastern corner of the island southward to Mutanshan on the Hengchun Peninsula for a length of approximately 350 kilometers. The maximum width reaches about 50 kilometers. It forms all the high peaks along the crest zone of the Central Range and the two highest ridges on the west, the Hsuehshan Range and the Yushan Range. In total this argillaceous belt underlies a little less than half of the mountain areas of Taiwan. Although these Tertiary argillaceous deposits .compose the sedimentary cover of the pre-Tertiary crystalline basement, they are well indurated and deformed, and have furnished themselves clastic detritus to the younger formations in the basin further to the west.
Geologic subdivision and distribution of sandstorn facies<br>in the argillite-slate series
Figure 4. Geologic subdivision and distribution of sandstorn facies
in the argillite-slate series

LITHOLOGY AND GEOLOGIC SUBDIVISION

The various rock types in the argillite-slate sequence can be classified into two lithostratigraphic associations, found in two distinct geographic regions. These form two belts in the Tertiary submetamorphic terrain (Fig. 4). The western belt is called the Hsuehshan Range belt, named after Taiwan's second highest ridge which is located approximately in the middle of the belt. The Hsuehshan Range belt is about 200 kilometers long, and averages 20-25 kilometers wide. It begins at Fulung on the northeastern coast, extending southward through Wulai, Hsuehshan, Puli, and the Jihyuehtan (Sun-Moon Lake) to the Yushan Range, which contains the highest peak in Taiwan, This belt continues southward to the upper course of the Laonungchi stream in the Pingtung valley. To the east and south of the Hsuehshan Range belt is the Backbone Range belt, covering all the crest ridges and the southern part of the Central Range. The Hsuehshan Range belt is separated from the western foothills province on the west by the Chuchih boundary fault and from the Backbone Range belt on the east by the Lishan boundary fault.
The great bulk of the metasedimentary rocks are dark gray, well-cleaved argillite, slate, and phyllite or, more generally, indurated to metamorphosed argillaceous rocks. These argillaceous rocks contain abundant small quartz veins. Argillite is more predominant in the western part, whereas slate and phyllite are more abundant toward the east. The argillite is defined as hard mudstone which has suffered only slight recrystallization. With the eastward increase of metamorphic grade, the hard shale grades into slate and then phyllite gradually. The degree of metamorphism of the shaly rocks increases gradually from the western margin of the belt toward the eastern pre-Tertiary basement found in the core zone of the Central Range. There is no definite correlation between geologic age and degree of metamorphism which is controlled largely by proximity of the argillaceous rocks to the center of deformation.
White and gray sandstone constitutes the other important litho-facies in this shaly sequence (see Fig. 4), usually associated with thin and irregular lenses of impure coal or carbonaceous shale. The white sandstone is medium- to coarse-grained but the gray sandstone is mostly fine-grained. The sandstone is either thick and massive or contains interbeds of dark gray indurated shale and slate. Stratigraphic boundaries between sandstone and shale are gradational. Interfingering relationships have been shown on large-scale maps. Limy to marly lenses or nodules are scattered in the slates, mostly in the higher parts of the Central Range. Thin and discontinuous conglomerate beds or lentils occur at several horizons in the eastern and southern parts of the argillite-slate belt. The pebbles are derived partly from the metamorphic basement and partly from the argillite and slate sequence. Disseminated small lenses or pods of volcanic rocks are scattered in the argillaceous rocks. These are mainly basaltic pyroclastic rocks but there are also minor andesite, dolerite, and other pyroclastic products of less mafic composition.

AGE AND FOSSILS

Foraminifers and less abundant mollusks are found in the argillite-slate sequence and calcareous nannofossils have recently been found in the low-rank metamorphic shales. Other common organisms include corals, echinoids, and algae. These fossils are concentrated in local zones, sparsely distributed in the shaly rocks or in the limy lenses. Some fossils have been collected from the matrix of the conglomerate lentils. Well-preserved organisms are scarce in the white quartzitic sandstone. Although low-grade metamorphism has destroyed a part of the fossils, some are sufficiently well-preserved for paleontologic identification. These fossils are distributed in a number of scattered exposures and offer the best evidence for comparative age etermination. Complete faunal zonation is, however, hardly possible because no continuous fossiliferous section is observed and barren stratigraphic sections are quite common.
In the earliest geologic maps and reports, the argillite-slate belt was considered pre-Tertiary because no fossils or other means of dating the rocks were available. The early discovered microfossils are largely foraminifers (Tan, 1937; Tomita and Tan, 1937). They are confined to the marly nodules or limy lenses and the pebbles and matrix of the conglomerates exposed in the higher parts of the Central Range. These fossils include Discocyclina, Nummulites, and Assilina and their age is mainly Eocene (Yabe and Hanzawa, 1930). The undifferentiated argillite-slate sequence was thus inferred to be Eocene by early Japanese workers. After the Restitution of Taiwan, L.S. Chang (1954) first reported the occurrence of Oligocene foraminifers at some localities in northwestern Taiwan where the characteristic Eocene fossils are absent. Additional work revealed more possible Oligocene localities within the argillite-slate sequence, which was then considered to be Paleogene in general. Later extensive studies discovered that the foraminifers collected near the Lushan hot spring in Wushe (Jenai-hsiang) of central Taiwan are of lower Miocene (Aquitanian) age (Chang, 1962b). These Miocene foraminifers were subsequently found m many other areas in the argillite-slate belt, including southern Taiwan, Hengchun Peninsula, northwestern Taiwan, and northeastern Taiwan (Chang, 1976). On the basis of foraminiferal studies, the slate-argillite sequence to date has been assigned an age ranging from Eocene to lower middle Miocene. However, the abundant fossil evidence gives only an improved knowledge of the distribution of rocks of different ages in the argillaceous sequence; it provides little help in clarifying different stratigraphic units that could be used in field mapping.
Two coral species, Astrocoena and Elephantaria, were discovered in one conglomerate bed intercalated in the lower part of the slate sequence in northeastern Taiwan. The age probably latest Cretaceous or early Paleocene. Based on this evidence, the slaty beds below the conglomerate were considered Cretaceous in age by Yen and others (1956) and were named the "Pihou Formation." The conglomerate was named the E conglomerate and considered a basal conglomerate separating the Eocene from the Cretaceous. However, the provenance of the coral clasts in the conglomerate is still unknown. No positive Cretaceous faunas have yet been found in the "Pihou Formation" to validate its age assignment. On the other hand, L.S. Chang (1966) discovered large Eocene foraminifers in the E conglomerate and he believed that at least a part of the E conglomerate must be intraformational conglomerate rather than a basal conglomerate. More recently Miocene planktonic foraminifers have been discovered in the slates that were mapped as "Cretaceous Pihou Formation" in previous maps (L.S. Chang, 1970). Miocene foraminifers were also discovered in the matrix of the E conglomerate (L.S. Chang, 1974). Paleontologic proof of "Cretaceous" in the slate sequence is thus still questionable, and convincing evidence remains to be found. The E conglomerate will be discussed further in the section on orogenic events and stratigraphic breaks in this chapter. In the more than 10 years since publication of the first edition of this text, no fossils from the so-called "Cretaceous Pihou Formation" have been identified as definitely Cretaceous; for this reason, no Cretaceous formation is shown on the present map.
In conclusion, the metamorphic shaly rocks have been assigned progressively younger ages with the progress of paleontologic studies. This shaly sequence was first inferred to be pre-Tertiary on the basis of structural features: later paleontologic work has identified Eocene. Oligocene, and Miocene ages in the argillites and slates, but has still failed to corroborate a Cretaceous age.

LITHOSTRATIGRAPHIC SUBDIVISION

The stratigraphic analysis of this thick argillite-slate sequence is still in progress because lithostratigraphic subdivision of these undifferentiated argillaceous rocks is difficult. This is chiefly because of the monotony in lithology, obscure stratigraphic sequence, restricted occurrence of fossils of different ages, and lack of distinct structural breaks. Detailed stratigraphic subdivision is uncertain without determinable lithologic or structural breaks. The complete stratigraphic succession and total thickness of the argillite-slate sequence cannot be accurately determined because neither its top nor its base can be clearly defined. It must be on the order of several thousand meters thick. Repetition of strata by thrusting and isoclinal folding is common, and strata have been overturned over large areas.
Many different stratigraphic names have been proposed and used for all or parts of the argillite-slate sequence. Generally a single unifying rock-unit name was proposed for the whole sequence due to the continuity of the "undifferentiated" beds, their monotony, and the lack of known stratigraphic breaks. In the early Japanese reports, this entire argillite-slate sequence was named the "Slate Series" and was divided into upper and lower parts. Later, more detailed geologic mapping named the argillites in northwestern Taiwan the Wulai (Urai) Series (Ichikawa, 1929) and the slates in eastern Taiwan the Suao (Suo) Series (Ogasawara, 1933). Both series were further subdivided into groups and then into individual formations. The Suao Series is considered older mostly on inferences drawn 1'rom their lithologic, metamorphic, and structural features. In central Taiwan the slate series has been called the Shuichangliu (Suiyoryu) Formation (Hayasaka and others, 1936) and the Puli (Hori) Formation (Oinouye and others, 1928) in separate areas. It has been named the Chaochow Formation (Rokaku and Makiyama, 1934) on the Hengchun Peninsula and the Changshan Formation (Tsan, 1964) in Kaohsiung-hsien. Most of these stratigraphic names cover part or all of a mighty sequence of argillaceous rocks that is probably of different ages in different places. Details of physical stratigraphy of these units, such as type sections, the nature of top and lower boundaries, total thickness, and stratigraphic correlation, are generally uncertain. No further attempt will be made to review fully the original definitions and usages of the names that have been used for these argillaceous sediments.
Some geologists attempted to classify the rocks merely on the basis of contained fossils or inferred ages. Such stratigraphic units suffer the common drawback of the lack of distinct mapping boundaries and of well-represented lithologic sections. The fossil-dated rocks are also difficult to restore to complete and undisturbed sequences so that stratigraphic relationships still cannot be unraveled.
Although dark gray argillite and slate (phyllite) are the predominant rock types in both the Hsuehshan and the Backbone Range belts, the detailed lithologic composition of the two belts differs significantly and serves as the basis for lithostratigraphic classification. The variation of lithologic sequence in these two belts represents differences in depositional environments and conditions of formation. The rocks in these two belts are therefore represented by two separate systems of stratigraphic nomenclature and map pattern in the map legend. The Hsuehshan Range belt has more carbonaceous units and thick beds of quartzitic sandstone, and nearly no limy lenses. Except a small part of the seemingly non-metamorphosed shales, the shaly sediments are mostly indurated into argillites or mildly metamorphosed into slates. Conglomerate is rare and pyroclastic effusives are more abundant in the northern and central parts of this belt. In the Backbone Range belt, on the other hand, the rocks are comparatively highly metamorphosed. Slate to phyllite is the predominant rock types, containing intercalations of marly or limy nodules, siltstone, sandstone, and conglomerate. In places the slate is intimately interbedded with thin to medium beds of quartzitic sandstone, but thick beds of white quartzite are not present. Pyroclastic rocks are exposed mostly in the middle and southern parts of the Backbone Range belt.
Two distinct lithofacies can be distinguished in the Hsuehshan Range belt, an argillite-slate facies and a carbonaceous sandstone facies (Fig. 4). The carbonaceous sandstone facies is represented by thick- to medium-bedded white or gray sandstone including thin coaly lenses or carbonaceous layers. These thick sandstone beds provide the best lithologic markers for stratigraphic subdivision in the "undifferentiated" argillaceous series. The boundary between the argillite or slate and the sandstone is reasonably satisfactory for mapping and accordingly forms a suitable boundary between different rock-stratigraphic units. On this basis, a number of lithostratigraphic units can be differentiated in the Hsuehshan Range belt for field mapping and stratigraphic analysis. Due to facies change from the north to the south, the stratigraphic systems and nomenclature differ between the northern and central parts of the Hsuehshan Range belt, as fully discussed in the section on "general stratigraphy."
The Backbone Range belt is the least known geologic terrain in Taiwan. Only widely spaced route traverse mapping has been carried out in this belt because of rugged' terrain and difficult accessibility. No carbonaceous sandstones are exposed in the Backbone Range belt, so the stratigraphic scheme of the Hsuehshan Range belt cannot be extended eastward into this belt. An extensive area of the slate sequence is overturned in the northern part of this belt, increasing the difficulty of determining the undisturbed sequence. Although this belt covers more than half of the Tertiary submetamorphic terrain, only two broad stratigraphic units are distinguished: the Miocene Lushan Formation and the Eocene Pilushan Formation. Both formations were proposed for the first time during the compilation of this new geologic map and are composed mainly of slate and phyllite with local intercalations of indurated sandstone. Sandstone interbeds are more abundant in the Pilushan Formation, especially its lower part. Disseminated marly, limy, and conglomeratic lenses are found in widely scattered places in the Pilushan Formation. The stratigraphic and structural relationships of the argillites and slates between the Hsuehshan Range belt and the Backbone Range belt are not clear and are still controversial.
No distinct lithologic break has been found between the Lushan Formation and the Pilushan Formation. The essentially similar argillaceous lithology of these two rock units makes the boundary arbitrarily defined at most places. Distinction between these two formations has to be made largely on fossil evidence and this faunal boundary cannot be applied in field mapping. The distribution of these two formations on the geologic maps is, therefore, quite uncertain and constant revision is needed as additional faunal data become available. No Oligocene faunas have been found in the Backbone Range belt. Questionable late Oligocene rocks have been reported only in the southern part, (C.T. Lee, 1977; T.C. Huang, 1980b) and are of limited distribution. The Miocene and Eocene slate formations could be separated by a stratigraphic gap representing an unconformity. However, no good evidence of an angular discordance has ever been discovered. The presence of conglomerate between these two formations in southern Taiwan was reported by L.S. Chang (1972) and was named the N conglomerate by him. A paleontologic break between Oligocene and Eocene rocks on the south cross-island highway was recently reported by T.C. Huang (1980b). This break could represent an unconformity, possibly correlative with the unconformable surface suggested by Chang's N conglomerate.

TIME-STRATIGRAPHIC (STAGE) SUBDIVISION

Stage is a stratigraphic unit characterized by the fauna the unit contains. Because no faunal units at the stage level are of global extent, different sets of stages are defined for different geologic provinces. The stage name is usually created by adding the suffix "ian" to the name of a locality with good fossiliferous sections. Stages are used loosely as time-stratigraphic units although strictly a "stage" is a biostratigraphic unit, because it is defined entirely by the faunas it contains. A stage is not a rock unit and cannot be used as a cartographic unit for field mapping. Based on the study of foraminifers, L.S. Chang (1962a and 1963a) defined four stages in the argillite-slate belt of the Central Range and summarized below.
Pilushanian stage (Eocene)
Rocks of this stage are distributed in the crest zone, the western flank, and the southeastern flank of the Central Range. The chief lithology is represented by well-cleaved slate or phyllite and gray quartzitic sandstone. The Eocene age is proved by the occurrence of Nummulites sp., Assilina formosensis Hanzawa, Discocyclina sp., and Asterocyclina sp., in the rocks.
Hsuehshan stage (Eocene).
This stage is represented by several different rock units: the Szeleng Sandstone, the Hsitsun Formation, and the Hsinkao Formation. The age of this stage is reported to be early Eocene or older Paleogene; however, diagnostic Eocene fossils have been found only in the Hsinkao Formation. The Hsuehshan stage is considered to be older than the Pilushanian stage which is chiefly middle or late Eocene.
Shihtsaoan stage (Oligocene).
This stage is represented by all the argillite-slate units overlying the Szeleng Sandstone and including the uppermost Aoti Formation. The characteristic smaller foraminifers include Gaudryina hayasakai Chang, Globigerina ampliapertura Bolli, and others.
Lushanian stage (Miocene).
A 14-km wide slate belt near Lushan of Nantou-hsien on the immediate western slope below the crest of the Central Range forms the type section of this stage. The rocks are mostly dark gray slates intercalated with dark gray compact sandstone and small marly nodules. The characteristic fossils include Orbulina suturalis Bronnimann, Globigerinoides bisphericus Todd, and others. These fossils are mostly of early Miocene Aquitanian age or slightly younger. Recent fossil studies indicate that the Lushanian faunas could be mainly middle Miocene, and extend partly into early Miocene. The Lushanian stage is also represented by the slaty beds on the Hengchun Peninsula.


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