Vitrinite re¯ectance as a possible indicator of metamorphic grade and
stratigraphic position of formations: a study of Oligocene metapelites in
NE Taiwan
M.-L. Lin*, C.-K. Lin, M.-J. You
Department of Earth Science, National Taiwan Normal University, 88 Ting Chou Rd., Taipei, Taiwan, R.O.C.
Received 24 August 1999; revised 10 May 2000; accepted 19 June 2000
Abstract
A total of 47 samples of Oligocene metapelites collected from the northeastern (NE) Hsuehshan Range, Taiwan, were studied by measuring vitrinite re¯ectance. The vitrinite re¯ectance of the three studied stratigraphic units, from top to bottom, are as follows: the Tatungshan Formation: R
max mean maximum vitrinite reflectance 1:35±2:76% and Rm mean random vitrinite reflectance
1:18±2:56%;the Tsuku Formation:Rmax2:89±3:51% andRm2:37±3:26%;and the Kankou Formation:Rmax4:06±5:97% andRm
3:04±5:07%:EitherRmaxorRm changes continuously along the studied geologic sections, and the variation serves well as a metamorphic
grade indicator. In the studied area, each stratigraphic formation has its own characteristic vitrinite re¯ectance value range. It is postulated that, in the future, vitrinite re¯ectance can also serve as an indicator of stratigraphic position of formations to aid stratigraphic identi®cation and correlation in geologic mappings.q2001 Elsevier Science Ltd. All rights reserved.
Keywords: Hsuehshan range; Vitrinite re¯ectance; Metapelites; Taiwan
1. Introduction
Accurate geological maps are the basis of all geologic works (Barnes, 1981). As Wallace (1975) stated in his 1974 Jacklin lecture: ªThere is no substitute for the geolo-gical map Ð absolute noneº (p. 11 of Barnes, 1981). However, in a fossil-barren, monotonous thick sequence, assigning a stratum to its genuine chronological position most often poses a real challenge. A good example is the situation found in the northeastern (NE) Hsuehshan Range, Taiwan. The exposed thick argillite-slate sequence is virtually void of fossils, which has therefore caused dif®cul-ties with regard to stratigraphic correlation. Vitrinite is a very common component in metapelite (Bostick, 1971, 1974; Diessel and Of¯er, 1975) and, as such, is very sensi-tive to local stress, tectonic stress, weathering, host rock heat conductivity, duration time and particularly to tempera-ture change (Cook et al., 1972; Stach et al., 1982; Teich-muÈller, 1987; Barker, 1988; Lewan, 1994; Huang, 1996). Vitrinite re¯ectance, therefore, has been employed as a useful indicator to evaluate the metamorphic grade of
meta-pelites (Frey et al., 1980; Kisch, 1980, 1987; Frey, 1987; TeichmuÈller, 1987). The term ªmetamorphismº is used in a very broad sense here and may include late diagenesis, burial metamorphism, and very low- to low-grade meta-morphism as suggested by Frey (1987). In addition, the normal increase of vitrinite re¯ectance with depth observed in borehole pro®les, the Hilt's law, is caused by rising temperature with depth (Stach et al., 1982). Accordingly, its variation is also believed to be closely related to different positions of stratigraphic sequence and/or burial depths of strata (TeichmuÈller and TeichmuÈller, 1979; Lin and Kao, 1996). This makes it possible to use vitrinite re¯ectance as an indicator of stratigraphic position of formations to aid with both stratigraphic identi®cation and correlation in geological mappings.
Taiwan is situated in the southeastern offshore of main-land China (Fig. 1A). The Hsuehshan Range (IVa of Fig. 1B), a Tertiary very low-grade metamorphic terrane of a fold and thrust belt of Taiwan (Chen et al., 1983; Chen and Wang, 1995; Teng, 1990), is composed of a thick argil-lite-slate sequence with minor amounts of meta-sandstone of the Eocene to Middle Miocene age (Ho, 1975, 1988; Teng, 1987; Teng et al., 1988). Although the vitrinite re¯ec-tance of metapelites from the Hsuehshan Range is report-edly capable of categorizing the metamorphic grade of
Journal of Asian Earth Sciences 19 (2001) 223±232
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* Corresponding author. Tel.: 1886-2-2934-7120; fax: 1 886-2-2933-3315.
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metapelites (Tan et al., 1994), the relationships between the vitrinite re¯ectance and geological features of a selected area has not been published before. The geology of the study area, the NE Hsuehshan Range (IVa of Fig. 1B), has been documented by Tang and Yang (1976), Huang and Ho (1989), and Lee et al. (1990). The strata and geologic struc-tures are well exposed along the Taiwan No. 2 and No. 9 Highways (Fig. 2). Low relief and easy accessibility to the outcrops also allow for the collection of representative samples. In the present study, through detailed sampling in the whole area of the NE Hsuehshan Range, it is shown that the vitrinite re¯ectance of metapelites changes continu-ously along every geologic section. Here, a clear example is given to demonstrate that the vitrinite re¯ectance varies systematically in conjunction with the stratigraphic forma-tions and that vitrinite re¯ectance can serve as an indicator of stratigraphic position of formations in the study area.
2. Geotectonic framework and geologic features
The island of Taiwan is located at the boundary between the Philippine Sea Plate and the Eurasia Plate (Fig. 1A). Since the Late Miocene, continuous accretion and oblique
collision between the Luzon Arc and the Eurasia Continen-tal Margin have sequeezed up the ContinenContinen-tal Margin and Island Arc materials to form mountain ranges (Teng, 1987, 1990). The main body of the orogen has reached a steady state with a maximum altitude of almost 4000 m in central Taiwan (Suppe, 1981). The northeastern part of the island is presently subjected to a post-orogenic extension associated with the opening of the Okinawa Trough (Suppe, 1984; Lee and Wang, 1988; Teng, 1995).
According to Ho's classi®cation of the geological provinces of Taiwan (Fig. 1B; Ho, 1975, 1988), the Long-itudinal Valley Fault (VI) marks the suture between the Luzon Arc (VII) and the Eurasia Continent. The basement, a pre-Tertiary metamorphic complex of the east Central Range (V), lies west of the Fault. The Tertiary sedimentary strata of the Backbone Range (IVb), the Hsuehshan Range (IVa) and the Western Foothills (III) were all forged into a series of west-vergent folds and thrusts trending NNE±SSW to NE±SW, representing deformed Continental Margin sediments. The sedimentary sequences underlying the Coastal Plain (II) and the Taiwan Strait (I) sit on the rifted Eurasia Continental Margin, which has not yet been incor-porated into the collisional orogen (Teng, 1987).
The Hsuehshan Range, a Tertiary very low-grade
M.-L. Lin et al. / Journal of Asian Earth Sciences 19 (2001) 223±232 226
metamorphic terrane of the fold and thrust belt of Taiwan (Teng, 1987), is composed of a thick argillite-slate sequence with minor amounts of meta-sandstone of the Eocene to Middle Miocene age (Ho, 1988). The litho-stratigraphy of the NE Hsuehshan Range, from top to bottom, consists of the Tatungshan, the Tsuku and the Kankou Formations as well as the Szeleng Sandstone (Tang and Yang, 1976; Teng
et al., 1988; Huang and Ho, 1989). Their distribution in the study area is shown in Fig. 2. According to Tang and Yang (1976) and Huang and Ho (1989), the Tatungshan Formation (700 m in thickness) is composed of dark gray to black argillite with few interbeds of ®ne-grained sand-stone and muddy siltsand-stone. The Tsuku Formation (300 m in thickness) consists of dark gray, muddy and ®ne-grained
M.-L. Lin et al. / Journal of Asian Earth Sciences 19 (2001) 223±232 227
Table 1
Vitrinite re¯ectance for all samples (Nos. 1±47) from the NE Hsuehshan Range, Taiwan
Sample no. Formationa Number of vitrinite particles measured
Rmaxrange (%) Rmax(%) Rmax 20%(%) Rm(%) Rmin(%)
1 TTS 54 1.05±2.00 1.35 1.66 1.18 0.97
2 TTS 19 1.32±2.03 1.75 1.97 1.54 1.05
3 TTS 45 1.59±3.31 2.38 2.90 2.09 1.50
4 TTS 72 1.71±3.01 2.25 2.64 2.09 1.53
5 TTS 56 1.75±3.05 2.39 2.83 2.00 1.58
6 TTS 41 1.27±2.48 1.97 2.36 1.71 1.30
7 TTS 57 1.92±3.43 2.67 3.11 2.40 1.68
8 TSK 49 2.70±4.18 3.51 3.94 3.04 2.57
9 KK 43 3.29±5.15 4.06 4.73 3.74 2.46
10 KK 98 4.00±5.70 4.82 5.32 3.96 2.82
11 KK 39 4.20±5.70 4.95 5.57 4.11 2.74
12 KK 44 4.01±6.47 5.22 5.82 4.12 3.36
13 KK 34 4.48±6.59 5.42 6.20 3.65 2.12
14 SS 68 3.74±7.05 5.71 6.21 3.78 1.67
15 KK 35 4.67±6.63 5.60 6.33 4.95 3.06
16 KK 28 5.02±6.95 5.97 6.74 5.07 3.06
17 TTS 28 0.80±2.68 1.57 2.34 1.45 1.22
18 TSK 43 2.59±4.49 3.43 4.05 3.02 2.33
19 TSK 61 1.95±4.22 3.21 3.84 2.70 1.93
20b TTS 50 2.04±3.80 2.89 3.41 2.37 1.90
21 KK 55 3.47±5.71 4.41 5.19 3.58 2.48
22 SS 31 4.85±6.48 5.66 6.28 4.54 2.89
23 KK 28 4.45±6.86 5.30 6.12 4.74 2.96
24 TSK 23 2.45±3.97 3.16 3.70 2.78 2.08
25 TTS 39 1.68±3.81 2.73 3.39 2.30 1.59
26 KK 10 4.26±6.95 5.59 6.62 4.64 2.13
27 KK 30 4.15±5.89 5.17 5.69 3.95 2.77
28 KK 50 3.74±5.65 4.62 5.28 3.82 2.22
29 TTS 69 1.91±3.12 2.45 2.82 2.33 2.12
30 TTS 65 1.88±3.72 2.50 3.10 2.37 2.19
31 TTS 80 1.47±2.62 2.14 2.31 2.06 1.85
32 TTS 33 1.81±3.38 2.50 3.00 2.39 2.15
33 TTS 39 1.75±3.36 2.50 2.85 2.37 2.13
34 TTS 29 0.97±2.02 1.42 1.83 1.25 1.05
35 TTS 9 1.13±1.91 1.53 1.90 1.35 1.14
36 TTS 27 0.96±2.38 1.50 2.16 1.35 1.22
37 TTS 31 1.07±2.53 1.60 2.22 1.52 1.38
38 TTS 23 1.32±2.18 1.79 2.06 1.68 1.41
39 TTS 64 1.53±4.28 2.47 3.51 2.33 2.11
40 TTS 75 2.16±4.34 2.76 3.33 2.56 2.29
41 TTS 64 1.91±3.78 2.56 3.08 2.41 2.16
42 TTS 61 1.94±3.10 2.59 2.90 2.43 2.25
43 TSK 54 2.39±4.02 3.20 3.69 2.68 2.00
44 TSK 38 2.74±4.02 3.27 3.72 3.00 2.72
45 TSK 51 2.73±4.28 3.47 4.00 3.26 2.95
46c TSK 16 2.49±3.36 3.20 3.26 2.70 2.41
47 KK 41 3.70±6.30 4.69 5.40 4.43 4.05
a The abbreviated symbols as the same as in Fig. 2.
b The stratigraphy of the sample as TSK in Fig. 2.
sandstone/siltstone with thin beds of argillite. The Kankou Formation (700 m in thickness) is a thick, massive, black to dark gray argillite bed. The Szeleng Sandstone (over 300 m in thickness) is mainly composed of medium- to course-grained quartzitic sandstone intercalated with carbonaceous shale and argillite. The major geologic structures, trending NNE±SSW from west to east, are the Shihtsao Fault, the Pihu Syncline, the Yingzuling Anticline and the Paling Fault (Fig. 2) (Tang and Yang, 1976; Lee et al., 1990). A NW±SE cross-section roughly perpendicular to the struc-tures in the southern part of the study area is also depicted in Fig. 2.
3. Study methods and results
A total of 47 argillaceous rocks were sampled (Fig. 2), among which 24 were taken from the Tatungshan Forma-tion, 8 from the Tsuku FormaForma-tion, 13 from the Kankou Formation and 2 from the Szeleng Sandstone. Most of the rocks exposed in the area are massive and their attitudes of bedding/cleavage are dif®cult to measure. Because weath-ering can affect the accuracy of vitrinite re¯ectance (Lewan, 1994), fresh samples as opposed to oriented ones were chosen in this study.
Coal rank was determined through re¯ectance measure-ments on dispersed vitrinite particles (Fig. 3) using a polished block of random section of whole rock (Barker, 1996), a procedure recommended by the American Society for Testing Materials (ASTM, 1976). A Carl Zeiss Axioplan
Pol microscope equipped with an MPM 200 photometer was employed for the measurements. Working conditions included oil immersion n1:518;monochromatic
polar-ized light (546 nm) and magni®cation £500 with a diaphragm diameter of 0.25 mm (5mm diameter of
measurement). Vitrinite, as shown in Fig. 3, was chosen to measure random vitrinite re¯ectance (Rm), maximum
vitrinite re¯ectance (Rmax) and minimum vitrinite
re¯ec-tance (Rmin). Standards of Gd±Ga±garnet (1.704%), SiC
(7.39%) and sapphire (0.50%) were used for calibration. The precision and accuracy of the measurements are within 0.5 and 3%, respectively. All measurements ofRm,Rmaxand
Rmin for each sample were used to calculate an average
value of mean random vitrinite re¯ectance Rm; mean
maximum vitrinite re¯ectance Rmax; mean of highest
20% ofRmax Rmax20%and mean minimum vitrinite
re¯ec-tance Rmin; respectively (Kilby, 1991). Additionally, a
histogram of theRmaxof each sample was plotted to show
the distribution patterns and was used to evaluate the relia-bility of the measurements as suggested by Hacquebard and Donaldson (1974) and Davis (1978).
The maximum vitrinite re¯ectance range (Rmax range),
Rmax;Rmax20%;RmandRmin for all of the samples in the
study area are listed in Table 1. The ranges of R max;
Rmax20% andRmfor the studied formations are summarized
in Table 2. On the whole, the values of bothR
max;andRm
decrease from southeast to northwest in the study area (Table 1, Fig. 2).
4. Discussion
4.1. Data quality of vitrinite re¯ectance measurements
The range ofRmaxin all of the samples here is rather large
(Table 1). This is not uncommon for clastic rocks during metamorphism, sinceRmaxof vitrinite is likely to have been
disturbed and ampli®ed by local pressure and tectonic stress due to the Plio-Pleistocene arc continent collision from
M.-L. Lin et al. / Journal of Asian Earth Sciences 19 (2001) 223±232 228
Table 2
Range of vitrinite re¯ectance for the studied stratigraphic formations in the NE Hsuehshan Range
Formation R
maxrange (%) Rmrange (%) Rmax20%range (%)
Tatungshan 1.35±2.89 1.18±2.56 1.66±3.51
Tsuku 3.16±3.51 2.68±3.26 3.26±4.05
Kankou 4.06±5.97 3.58±5.07 4.73±6.74
Szeleng 5.66±5.71 3.78±4.54 6.21±6.28
relatively hard adjacent minerals, such as pyrite and quartz (Cook et al., 1972; TeichmuÈller, 1987).
According to Hacquebard and Donaldson (1974) and Davis (1978), a normal distribution in the histogram of
Rmax representing more than 30 measurements (e.g. Fig.
4A) is required to verify whether the correspondingR max;
can be deemed acceptable. For samples with fewer than 30 measurements (Fig. 4B), data are considered reliable as long as theRmaxandRmfrom the same rock sample have a linear
correlation (e.g. Fig. 4C). In the present study, theRmaxand
Rmfor all samples have a correlation coef®cient (r) larger
than 0.85, except for that of sample No. 23 withr0:63:
Because it is practically impossible to measure the trueRmax
value, some approximation is inevitable; in fact, R max 20%
was ®rst postulated by Cook et al. (1972). In this study, a good relation of R
max 20%1:071 Rmax10:329 with r
0:994 (Fig. 5A) strongly supports the hypothesis that either
Rmax 20%or R
max;can represent the degree of
metamorph-ism of rocks in the study area. Besides, other research has suggested thatR
m andRminincrease with rising temperature
and can therefore represent the degree of metamorphism (TeichmuÈller and TeichmuÈller, 1981). As shown in Fig. 5B, a correlation of R
m0:747 Rmax10:407 with r0:959
exists betweenR
m and Rmax: This correlation, however, is
better suited for R
m values below 3.0%. A similar ®nding
was reported in NW Germany (TeichmuÈller and TeichmuÈller, 1981). This indicates that (1) the quality ofR
max:is, evidently,
better than that ofR
m;(2)Rmaxmust be measured whenRmis
above 3.0%; and (3) it is possible to measureR
monly when
Rmis below 3.0%. A smaller correlation r0:738between
RminandR
max is shown in Fig. 5C. Obviously,Rmin is less
reliable. This may have resulted from the fact that the vitrinite particles in the study area were too narrow in width (,15mm;
Fig. 3) to yield good measurements.
4.2. Relationships between vitrinite re¯ectance and geologic features
A close correlation between vitrinite re¯ectance/illite
crystallinity and degree of metamorphism/stratigraphic sequence was repeated for samples collected along the Northern Taiwan E±W Cross-Island Highway, which is located southwest of the present study area (Chen et al., 1994; Lin and Kao, 1996). A similar ®nding is observed in the present study area on the western ¯ank of the Ying-zuling Anticline, where the strata display a younging west-ward trend along the Taiwan No. 9 Highway (Fig. 2). From west to east (samples No. 8 to No. 13), R
max increases
continuously 3:51%!4:06%!4:82%!4:95%!
5:22%!5:42% (Table 1). A second example lies in the
vicinity of Keng-fang, where the strata are younger from southwest to northeast along the Taiwan No. 2 Highway (Paci®c coast) (Fig. 2). From northeast to southwest (samples No. 41 to No. 45), R
max increases gradually 2:56%!2:59%!3:20%!3:27%!3:47%(Table 1).
Another example is seen in the area south of the Shihtsao Fault and north of the Pihu Syncline in the western part of the study area (Fig. 2). The geologic section from south to north (samples No. 29 to No. 32) contains anticline, syncline, then anticline (Fig. 2; Lee et al., 1990). Sample No. 31, the nearest to the syncline axis, is the youngest strata of the four in the section and has the lowestR
max2:14%
as well (Table 1; Fig. 2). With the syncline axis extended westward to the Taiwan No. 9 Highway, sample No. 6 also has a lowR
max 1:97% (Table 1; Fig. 2). Similarly, sample
No. 4 in the vicinity of the syncline axis has a lowerR max
2:25% when compared with No.3 Rmax2:38%and No.5
Rmax2:39%(Table 1; Fig. 2).
These three examples can be further exempli®ed by the correlation betweenR
max andRm: Fig. 5B shows that the
data points are uniformly distributed along a well-de®ned line. On the basis of their R
max and Rm; three separate
clusters of samples belonging, respectively, to the Tatung-shan (700 m in thickness), the Tsuku (300 m in thickness) and the Kankou (700 m in thickness) Formations in descending order are evident (Fig. 5B). It is apparent that the smallerR
maxandRm values correspond to the younger
formation. In other words, the ranges ofR
maxandRmre¯ect M.-L. Lin et al. / Journal of Asian Earth Sciences 19 (2001) 223±232 229
Fig. 5. (A)R
the stratigraphic levels of the studied formations (Fig. 2). It should be noted that the data of the Tsuku Formation, the thinnest stratigraphic formation studied, spread to a much narrower range than do those of the other two formations. It appears that the ranges ofR
max andRmcovered by
indivi-dual clusters are closely related to the thickness of the strati-graphic formation. The same phenomena are found in the plot ofR
maxvs.Rmax 20%(Fig. 5A).
These observations support ®ve features. First, along a section, the strata near the axial part of a syncline are younger, and thus each has a lowerR
maxthan other samples
farther away from the axial part of the syncline; on the other hand, the opposite holds true for an anticline. Secondly, the ranges of R
max and Rm are well associated with a certain
level and thickness of stratigraphic formation. Thirdly, vitri-nite re¯ectance is dictated by the stratigraphic level. Fourthly, the metamorphic grade of the strata in the study area increases with the stratigraphic position, and this is mainly due to non-disturbed deep bury. Finally, meta-morphism seems to have achieved its peak before any fold-ing ever took place. These features clearly demonstrate that the observed variations of vitrinite re¯ectance could largely be accounted for by the Hilt's law (i.e. the normal increase of vitrinite maturity with depth) and that the vitrinite re¯ec-tance is able to serve as an indicator not only of meta-morphic grade but also of stratigraphic position in the present study. Note that because the metamorphic isotherms
are generally not parallel to the stratigraphic horizons, systematic variation of vitrinite re¯ectance along a certain direction parallel to the stratigraphic horizons is not implau-sible. However, such a change, if present, cannot be discerned based on the available data.
Of signi®cance is that the values R
max5:71% and
5.66% (samples No. 14 and No. 22 in Table 1) of the inter-calated carbonaceous metapelites in the oldest Szeleng Sandstone are lower than the highest R
max value
(5.97%; sample No. 16 in Table 1) of the younger Kankou Formation in the studied area (Figs. 2 and 5B). The northwestward decrease in vitrinite re¯ectance across the Paling Fault (Fig. 2) may account for this. However, this kind of reversing trend with depth has also been reported in the Teufelspforte borehole in the Saar District of Germany (TeichmuÈller and TeichmuÈller, 1968). This phenomenon may be considered a result of (1) the occurrence of sand-stones, with higher heat conductivity than mudsand-stones, thereby bringing about a retardation of chemical rank and (2) on top of the sandstones, the presence of mudstones, which has acted as a dam in trapping exces-sive heat and which has therefore enhanced their own chemical rank (Robert, 1985). Alternatively, the reversed trend may be an artifact caused by biased sampling. Only two samples of the Szeleng Sandstone were collected in the present study. More sampling is required to con®rm this point.
M.-L. Lin et al. / Journal of Asian Earth Sciences 19 (2001) 223±232 230
4.3. Vitrinite re¯ectance as an indicator of stratigraphic position
On the basis of the simpli®ed geologic map (Fig. 2), the strata at the site of sample No. 46 were previously assigned to the Kankou Formation (Tang and Yang, 1976; Huang and Ho, 1989). However, owing to the R
max and Rm ranges
(Table 2; Fig. 5B), which re¯ect the metamorphic grade of the studied formations in general, here it is postulated that these strata (R
max3:20% andRm2:70% in Table 1)
most likely belong to the Tsuku Formation and not the Kankou Formation. As shown in Fig. 2, an anticline axis is located about 2 km to the north of the site of sample No. 46. The strata exposed on the northern ¯ank of the anticline along the coast are designated as the Tsuku Formation, whereas no corresponding stratum is shown to the south of the anticline axis (Geologic Map of Tang and Yang, 1976). In light of these considerations, it is suggested that sample No. 46 is in fact in the outcrop belt of the Tsuku Formation on the southern ¯ank of the anticline. Thus, it is believed that a hypothetical boundary, between the Tsuku and the Kankou Formations on the southern ¯ank of the plunging anticline, as shown by the heavy dashed line in Fig. 6, is located to the west of sample No. 46 and to the north of No. 47. This is also consistent with the bedding attitude near study sample No. 46 (see Fig. 6).
Likewise, on the simpli®ed geologic map (Fig. 2) near the boundary between the Tsuku and the Tatungshan Forma-tions, sample No. 20 was taken to be in the Tsuku Formation by Tang and Yang (1976) and Huang and Ho (1989). However, based upon the R
max and Rm values (Fig. 5B),
it is more plausible that the strata at that site (R
max2:89%
andR
m2:37% in Table 1) are in the Tatungshan
Forma-tion instead. Although there is no other circumstantial evidence at the moment, any previous error may have been caused by (1) a mis-identi®cation of the stratigraphic formation in the geologic investigation, or by (2) the use of an inaccurate topographic map in the geologic mapping.
5. Concluding remarks
A total of 47 samples of different stratigraphic formations of the Oligocene metapelites collected from the NE Hsueh-shan Range, Taiwan, were studied by measuring vitrinite re¯ectance R
maxandRm:The results concerning the
rela-tionships between vitrinite re¯ectance and geologic features show that (1) eitherR
max or Rm changes continuously in
geologic sections, and (2) the ranges ofR
maxandRmin each
studied stratigraphic formation are closely associated with the level and thickness of that stratigraphic formation.
Because vitrinite is a very common component in meta-pelite and is very sensitive to temperature, vitrinite re¯ec-tance has usually only been employed as a useful indicator to evaluate the degree of metamorphism. The present study, however, demonstrates that for a restricted area, vitrinite
re¯ectance additionally bears a great potential in being applicable as an indicator of stratigraphic position of forma-tions and as an aid in stratigraphic identi®cation and corre-lation in geologic mappings.
Acknowledgements
The writers wish to express their appreciation to Prof. L.I. Tsai, Institute of Applied Geology, National Central University, Taiwan for providing facilities for polishing blocks as well as his critical comments. They are also very grateful to Prof. P.Y. Chen, Department of Earth Science, National Taiwan Normal University and Prof. T.F. Yui, Institute of Earth Sciences, Academia Sinica, Taiwan, R.O.C. for their helpful comments and critical reviews. This study was partly sponsored by a research grant from the National Science Council of the R.O.C.
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