Directory UMM :Data Elmu:jurnal:J-a:Journal of Asian Earth Science:Vol19.Issue1-2.Feb2001:
Journal of Asian Earth Sciences 19 (2001) 223±232
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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
max mean maximum vitrinite reflectance 1:35±2:76% and R
m mean random vitrinite reflectance
Tatungshan Formation: R
max 4:06±5:97% and R
m
1:18±2:56%; the Tsuku Formation: Rmax 2:89±3:51% and Rm 2:37±3:26%; and the Kankou Formation: R
max or R
m changes continuously along the studied geologic sections, and the variation serves well as a metamorphic
3:04±5:07%: Either R
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. q 2001 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 geological 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®culties 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 sensitive to local stress, tectonic stress, weathering, host rock
heat conductivity, duration time and particularly to temperature change (Cook et al., 1972; Stach et al., 1982; TeichmuÈller, 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* Corresponding author. Tel.: 1886-2-2934-7120; fax: 1886-2-29333315.
E-mail address: [email protected] (M.-L. Lin).
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 metamorphism 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 mainland 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 argillite-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¯ectance of metapelites from the Hsuehshan Range is reportedly capable of categorizing the metamorphic grade of
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224
M.-L. Lin et al. / Journal of Asian Earth Sciences 19 (2001) 223±232
Fig. 1. (A) Plate tectonic con®guration of the Taiwan area. (B) Geological provinces of Taiwan (after Ho, 1975): I, Taiwan Strait; II, Coastal Plain; III, Western Foothills; IVa, Hsuehshan Range; IVb, Backbone
Range; V, Eastern Central Range (Tananao Schist); Va, Tailuko Belt of the Tananao Schist; Vb, Yuli Belt of the Tananao Schist; VI, Longitudinal Valley Fault; VII, Coastal Range (Luzon Arc).
M.-L. Lin et al. / Journal of Asian Earth Sciences 19 (2001) 223±232
225
Fig. 2. Sample locality map of the NE Hsuehshan Range, Taiwan. Simpli®ed geological map as compiled from Tang and Yang (1976), Huang and Ho (1989) and Lee et al. (1990).
226
M.-L. Lin et al. / Journal of Asian Earth Sciences 19 (2001) 223±232
Fig. 3. Photomicrograph of vitrinite (v) particles in metapelites from the NE Hsuehshan Range. Polished surface and oil immersion. The inner length in the
scale rectangle is 20 mm. (A) Sample No. 4 from the Tatungshan Formation; (B) sample No. 44 from the Tsuku Formation; (C) sample No. 10 from the
Kankou Formation; note the abundance of pyrite (p) concretions (white) in the sample; and (D) sample No. 14 from the Szeleng Sandstone.
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 structures 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 continuously along every geologic section. Here, a clear example is
given to demonstrate that the vitrinite re¯ectance varies
systematically in conjunction with the stratigraphic formations 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 Continental Margin have sequeezed up the Continental 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 Longitudinal 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 incorporated into the collisional orogen (Teng, 1987).
The Hsuehshan Range, a Tertiary very low-grade
227
M.-L. Lin et al. / Journal of Asian Earth Sciences 19 (2001) 223±232
Table 1
Vitrinite re¯ectance for all samples (Nos. 1±47) from the NE Hsuehshan Range, Taiwan
Sample no.
Formation a
Number of vitrinite
particles measured
Rmax range (%)
max (%)
R
max 20% (%)
R
m (%)
R
min (%)
R
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20 b
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46 c
47
TTS
TTS
TTS
TTS
TTS
TTS
TTS
TSK
KK
KK
KK
KK
KK
SS
KK
KK
TTS
TSK
TSK
TTS
KK
SS
KK
TSK
TTS
KK
KK
KK
TTS
TTS
TTS
TTS
TTS
TTS
TTS
TTS
TTS
TTS
TTS
TTS
TTS
TTS
TSK
TSK
TSK
TSK
KK
54
19
45
72
56
41
57
49
43
98
39
44
34
68
35
28
28
43
61
50
55
31
28
23
39
10
30
50
69
65
80
33
39
29
9
27
31
23
64
75
64
61
54
38
51
16
41
1.05±2.00
1.32±2.03
1.59±3.31
1.71±3.01
1.75±3.05
1.27±2.48
1.92±3.43
2.70±4.18
3.29±5.15
4.00±5.70
4.20±5.70
4.01±6.47
4.48±6.59
3.74±7.05
4.67±6.63
5.02±6.95
0.80±2.68
2.59±4.49
1.95±4.22
2.04±3.80
3.47±5.71
4.85±6.48
4.45±6.86
2.45±3.97
1.68±3.81
4.26±6.95
4.15±5.89
3.74±5.65
1.91±3.12
1.88±3.72
1.47±2.62
1.81±3.38
1.75±3.36
0.97±2.02
1.13±1.91
0.96±2.38
1.07±2.53
1.32±2.18
1.53±4.28
2.16±4.34
1.91±3.78
1.94±3.10
2.39±4.02
2.74±4.02
2.73±4.28
2.49±3.36
3.70±6.30
1.35
1.75
2.38
2.25
2.39
1.97
2.67
3.51
4.06
4.82
4.95
5.22
5.42
5.71
5.60
5.97
1.57
3.43
3.21
2.89
4.41
5.66
5.30
3.16
2.73
5.59
5.17
4.62
2.45
2.50
2.14
2.50
2.50
1.42
1.53
1.50
1.60
1.79
2.47
2.76
2.56
2.59
3.20
3.27
3.47
3.20
4.69
1.66
1.97
2.90
2.64
2.83
2.36
3.11
3.94
4.73
5.32
5.57
5.82
6.20
6.21
6.33
6.74
2.34
4.05
3.84
3.41
5.19
6.28
6.12
3.70
3.39
6.62
5.69
5.28
2.82
3.10
2.31
3.00
2.85
1.83
1.90
2.16
2.22
2.06
3.51
3.33
3.08
2.90
3.69
3.72
4.00
3.26
5.40
1.18
1.54
2.09
2.09
2.00
1.71
2.40
3.04
3.74
3.96
4.11
4.12
3.65
3.78
4.95
5.07
1.45
3.02
2.70
2.37
3.58
4.54
4.74
2.78
2.30
4.64
3.95
3.82
2.33
2.37
2.06
2.39
2.37
1.25
1.35
1.35
1.52
1.68
2.33
2.56
2.41
2.43
2.68
3.00
3.26
2.70
4.43
0.97
1.05
1.50
1.53
1.58
1.30
1.68
2.57
2.46
2.82
2.74
3.36
2.12
1.67
3.06
3.06
1.22
2.33
1.93
1.90
2.48
2.89
2.96
2.08
1.59
2.13
2.77
2.22
2.12
2.19
1.85
2.15
2.13
1.05
1.14
1.22
1.38
1.41
2.11
2.29
2.16
2.25
2.00
2.72
2.95
2.41
4.05
a
b
c
The abbreviated symbols as the same as in Fig. 2.
The stratigraphy of the sample as TSK in Fig. 2.
The stratigraphy of the sample as KK in Fig. 2.
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 sandstone and muddy siltstone. The Tsuku Formation (300 m in
thickness) consists of dark gray, muddy and ®ne-grained
228
M.-L. Lin et al. / Journal of Asian Earth Sciences 19 (2001) 223±232
Table 2
Range of vitrinite re¯ectance for the studied stratigraphic formations in the
NE Hsuehshan Range
Formation
max range (%)
R
m range (%)
R
max20% range (%)
R
Tatungshan
Tsuku
Kankou
Szeleng
1.35±2.89
3.16±3.51
4.06±5.97
5.66±5.71
1.18±2.56
2.68±3.26
3.58±5.07
3.78±4.54
1.66±3.51
3.26±4.05
4.73±6.74
6.21±6.28
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 coursegrained 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 structures 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 Formation, 8 from the Tsuku Formation, 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 weathering 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 measurements 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 n 1:518; monochromatic polarized light (546 nm) and magni®cation £ 500 with a
diaphragm diameter of 0.25 mm (5 mm 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¯ectance (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 of Rm, Rmax and
Rmin for each sample were used to calculate an average
m ; mean
value of mean random vitrinite re¯ectance R
maximum vitrinite re¯ectance Rmax ; mean of highest
max 20% and mean minimum vitrinite re¯ec20% of Rmax R
tance Rmin ; respectively (Kilby, 1991). Additionally, a
histogram of the Rmax of each sample was plotted to show
the distribution patterns and was used to evaluate the reliability of the measurements as suggested by Hacquebard and
Donaldson (1974) and Davis (1978).
The maximum vitrinite re¯ectance range (Rmax range),
max 20%; R
m and R
min for all of the samples in the
max ; R
R
max ;
study area are listed in Table 1. The ranges of R
Rmax 20% and Rm for the studied formations are summarized
m
max ; and R
in Table 2. On the whole, the values of both R
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 of Rmax in all of the samples here is rather large
(Table 1). This is not uncommon for clastic rocks during
metamorphism, since Rmax of 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
Fig. 4. (A) Histogram of Rmax from sample No. 8. (B) histogram of Rmax from sample No. 37 and (C) diagram of Rm vs. Rmax from sample No. 37 in the NE
Hsuehshan Range.
M.-L. Lin et al. / Journal of Asian Earth Sciences 19 (2001) 223±232
229
max 20% vs. R
max ; (B) R
m vs. R
max ; and (C) R
min vs. R
max for metapelite from the NE Hsuehshan Range. TTS: Tatungshan Formation; TSK:
Fig. 5. (A) R
Tsuku Formation; KK: Kankou Formation; and SS: Szeleng Sandstone.
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.
max ;
4A) is required to verify whether the corresponding R
can be deemed acceptable. For samples with fewer than 30
measurements (Fig. 4B), data are considered reliable as long
as the Rmax and Rm from the same rock sample have a linear
correlation (e.g. Fig. 4C). In the present study, the Rmax and
Rm for all samples have a correlation coef®cient (r) larger
than 0.85, except for that of sample No. 23 with r 0:63:
Because it is practically impossible to measure the true Rmax
max 20%
value, some approximation is inevitable; in fact, R
was ®rst postulated by Cook et al. (1972). In this study, a
max 1 0:329 with r
max 20% 1:071 R
good relation of R
0:994 (Fig. 5A) strongly supports the hypothesis that either
max ; can represent the degree of metamorph max 20% or R
R
ism of rocks in the study area. Besides, other research has
min increase with rising temperature
m and R
suggested that R
and can therefore represent the degree of metamorphism
(TeichmuÈller and TeichmuÈller, 1981). As shown in Fig. 5B,
max 1 0:407 with r 0:959
m 0:747 R
a correlation of R
max : This correlation, however, is
m and R
exists between R
m values below 3.0%. A similar ®nding
better suited for R
was reported in NW Germany (TeichmuÈller and TeichmuÈller,
max : is, evidently,
1981). This indicates that (1) the quality of R
max must be measured when R
m is
m ; (2) R
better than that of R
m only when
above 3.0%; and (3) it is possible to measure R
m is below 3.0%. A smaller correlation r 0:738 between
R
max is shown in Fig. 5C. Obviously, R
min is less
min and R
R
reliable. This may have resulted from the fact that the vitrinite
particles in the study area were too narrow in width (,15 mm;
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 Yingzuling Anticline, where the strata display a younging westward trend along the Taiwan No. 9 Highway (Fig. 2). From
max increases
west to east (samples No. 8 to No. 13), R
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
max increases gradually
(samples No. 41 to No. 45), R
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
max 2:14%
of the four in the section and has the lowest R
as well (Table 1; Fig. 2). With the syncline axis extended
westward to the Taiwan No. 9 Highway, sample No. 6 also
max 1:97% (Table 1; Fig. 2). Similarly, sample
has a low R
max
No. 4 in the vicinity of the syncline axis has a lower R
max 2:38% and No.5
2:25% when compared with No.3 R
max 2:39% (Table 1; Fig. 2).
R
These three examples can be further exempli®ed by the
m : Fig. 5B shows that the
max and R
correlation between R
data points are uniformly distributed along a well-de®ned
m ; three separate
max and R
line. On the basis of their R
clusters of samples belonging, respectively, to the Tatungshan (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
m values correspond to the younger
max and R
the smaller R
m re¯ect
max and R
formation. In other words, the ranges of R
230
M.-L. Lin et al. / Journal of Asian Earth Sciences 19 (2001) 223±232
Fig. 6. Modi®ed geologic map of the Keng-fang area, extracted from Fig. 2. Heavy dashed line represents the hypothetical boundary between the Tsuku and
the Kankou Formations proposed in this study.
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
m covered by indivi max and R
appears that the ranges of R
dual clusters are closely related to the thickness of the stratigraphic formation. The same phenomena are found in the
max 20% (Fig. 5A).
max vs. R
plot of R
These observations support ®ve features. First, along a
section, the strata near the axial part of a syncline are
max than other samples
younger, and thus each has a lower R
farther away from the axial part of the syncline; on the other
hand, the opposite holds true for an anticline. Secondly, the
m are well associated with a certain
max and R
ranges of R
level and thickness of stratigraphic formation. Thirdly, vitrinite 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, metamorphism seems to have achieved its peak before any folding 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¯ectance is able to serve as an indicator not only of metamorphic 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 implausible. However, such a change, if present, cannot be
discerned based on the available data.
max 5:71% and
Of signi®cance is that the values R
5.66% (samples No. 14 and No. 22 in Table 1) of the intercalated carbonaceous metapelites in the oldest Szeleng
max value
Sandstone are lower than the highest R
( 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 sandstones, with higher heat conductivity than mudstones,
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 excessive 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
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
m ranges
max and R
Ho, 1989). However, owing to the R
(Table 2; Fig. 5B), which re¯ect the metamorphic grade
of the studied formations in general, here it is postulated
m 2:70% in Table 1)
max 3:20% and R
that these strata (R
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 Formations, sample No. 20 was taken to be in the Tsuku Formation
by Tang and Yang (1976) and Huang and Ho (1989).
m values (Fig. 5B),
max and R
However, based upon the R
max 2:89%
it is more plausible that the strata at that site (R
and Rm 2:37% in Table 1) are in the Tatungshan Formation 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 Hsuehshan Range, Taiwan, were studied by measuring vitrinite
m : The results concerning the rela max and R
re¯ectance R
tionships between vitrinite re¯ectance and geologic features
m changes continuously in
max or R
show that (1) either R
m in each
max and R
geologic sections, and (2) the ranges of R
studied stratigraphic formation are closely associated with
the level and thickness of that stratigraphic formation.
Because vitrinite is a very common component in metapelite and is very sensitive to temperature, vitrinite re¯ectance 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
231
re¯ectance additionally bears a great potential in being
applicable as an indicator of stratigraphic position of formations and as an aid in stratigraphic identi®cation and correlation 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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www.elsevier.nl/locate/jseaes
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
max mean maximum vitrinite reflectance 1:35±2:76% and R
m mean random vitrinite reflectance
Tatungshan Formation: R
max 4:06±5:97% and R
m
1:18±2:56%; the Tsuku Formation: Rmax 2:89±3:51% and Rm 2:37±3:26%; and the Kankou Formation: R
max or R
m changes continuously along the studied geologic sections, and the variation serves well as a metamorphic
3:04±5:07%: Either R
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. q 2001 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 geological 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®culties 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 sensitive to local stress, tectonic stress, weathering, host rock
heat conductivity, duration time and particularly to temperature change (Cook et al., 1972; Stach et al., 1982; TeichmuÈller, 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* Corresponding author. Tel.: 1886-2-2934-7120; fax: 1886-2-29333315.
E-mail address: [email protected] (M.-L. Lin).
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 metamorphism 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 mainland 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 argillite-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¯ectance of metapelites from the Hsuehshan Range is reportedly capable of categorizing the metamorphic grade of
1367-9120/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved.
PII: S 1367-912 0(00)00034-1
224
M.-L. Lin et al. / Journal of Asian Earth Sciences 19 (2001) 223±232
Fig. 1. (A) Plate tectonic con®guration of the Taiwan area. (B) Geological provinces of Taiwan (after Ho, 1975): I, Taiwan Strait; II, Coastal Plain; III, Western Foothills; IVa, Hsuehshan Range; IVb, Backbone
Range; V, Eastern Central Range (Tananao Schist); Va, Tailuko Belt of the Tananao Schist; Vb, Yuli Belt of the Tananao Schist; VI, Longitudinal Valley Fault; VII, Coastal Range (Luzon Arc).
M.-L. Lin et al. / Journal of Asian Earth Sciences 19 (2001) 223±232
225
Fig. 2. Sample locality map of the NE Hsuehshan Range, Taiwan. Simpli®ed geological map as compiled from Tang and Yang (1976), Huang and Ho (1989) and Lee et al. (1990).
226
M.-L. Lin et al. / Journal of Asian Earth Sciences 19 (2001) 223±232
Fig. 3. Photomicrograph of vitrinite (v) particles in metapelites from the NE Hsuehshan Range. Polished surface and oil immersion. The inner length in the
scale rectangle is 20 mm. (A) Sample No. 4 from the Tatungshan Formation; (B) sample No. 44 from the Tsuku Formation; (C) sample No. 10 from the
Kankou Formation; note the abundance of pyrite (p) concretions (white) in the sample; and (D) sample No. 14 from the Szeleng Sandstone.
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 structures 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 continuously along every geologic section. Here, a clear example is
given to demonstrate that the vitrinite re¯ectance varies
systematically in conjunction with the stratigraphic formations 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 Continental Margin have sequeezed up the Continental 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 Longitudinal 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 incorporated into the collisional orogen (Teng, 1987).
The Hsuehshan Range, a Tertiary very low-grade
227
M.-L. Lin et al. / Journal of Asian Earth Sciences 19 (2001) 223±232
Table 1
Vitrinite re¯ectance for all samples (Nos. 1±47) from the NE Hsuehshan Range, Taiwan
Sample no.
Formation a
Number of vitrinite
particles measured
Rmax range (%)
max (%)
R
max 20% (%)
R
m (%)
R
min (%)
R
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20 b
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46 c
47
TTS
TTS
TTS
TTS
TTS
TTS
TTS
TSK
KK
KK
KK
KK
KK
SS
KK
KK
TTS
TSK
TSK
TTS
KK
SS
KK
TSK
TTS
KK
KK
KK
TTS
TTS
TTS
TTS
TTS
TTS
TTS
TTS
TTS
TTS
TTS
TTS
TTS
TTS
TSK
TSK
TSK
TSK
KK
54
19
45
72
56
41
57
49
43
98
39
44
34
68
35
28
28
43
61
50
55
31
28
23
39
10
30
50
69
65
80
33
39
29
9
27
31
23
64
75
64
61
54
38
51
16
41
1.05±2.00
1.32±2.03
1.59±3.31
1.71±3.01
1.75±3.05
1.27±2.48
1.92±3.43
2.70±4.18
3.29±5.15
4.00±5.70
4.20±5.70
4.01±6.47
4.48±6.59
3.74±7.05
4.67±6.63
5.02±6.95
0.80±2.68
2.59±4.49
1.95±4.22
2.04±3.80
3.47±5.71
4.85±6.48
4.45±6.86
2.45±3.97
1.68±3.81
4.26±6.95
4.15±5.89
3.74±5.65
1.91±3.12
1.88±3.72
1.47±2.62
1.81±3.38
1.75±3.36
0.97±2.02
1.13±1.91
0.96±2.38
1.07±2.53
1.32±2.18
1.53±4.28
2.16±4.34
1.91±3.78
1.94±3.10
2.39±4.02
2.74±4.02
2.73±4.28
2.49±3.36
3.70±6.30
1.35
1.75
2.38
2.25
2.39
1.97
2.67
3.51
4.06
4.82
4.95
5.22
5.42
5.71
5.60
5.97
1.57
3.43
3.21
2.89
4.41
5.66
5.30
3.16
2.73
5.59
5.17
4.62
2.45
2.50
2.14
2.50
2.50
1.42
1.53
1.50
1.60
1.79
2.47
2.76
2.56
2.59
3.20
3.27
3.47
3.20
4.69
1.66
1.97
2.90
2.64
2.83
2.36
3.11
3.94
4.73
5.32
5.57
5.82
6.20
6.21
6.33
6.74
2.34
4.05
3.84
3.41
5.19
6.28
6.12
3.70
3.39
6.62
5.69
5.28
2.82
3.10
2.31
3.00
2.85
1.83
1.90
2.16
2.22
2.06
3.51
3.33
3.08
2.90
3.69
3.72
4.00
3.26
5.40
1.18
1.54
2.09
2.09
2.00
1.71
2.40
3.04
3.74
3.96
4.11
4.12
3.65
3.78
4.95
5.07
1.45
3.02
2.70
2.37
3.58
4.54
4.74
2.78
2.30
4.64
3.95
3.82
2.33
2.37
2.06
2.39
2.37
1.25
1.35
1.35
1.52
1.68
2.33
2.56
2.41
2.43
2.68
3.00
3.26
2.70
4.43
0.97
1.05
1.50
1.53
1.58
1.30
1.68
2.57
2.46
2.82
2.74
3.36
2.12
1.67
3.06
3.06
1.22
2.33
1.93
1.90
2.48
2.89
2.96
2.08
1.59
2.13
2.77
2.22
2.12
2.19
1.85
2.15
2.13
1.05
1.14
1.22
1.38
1.41
2.11
2.29
2.16
2.25
2.00
2.72
2.95
2.41
4.05
a
b
c
The abbreviated symbols as the same as in Fig. 2.
The stratigraphy of the sample as TSK in Fig. 2.
The stratigraphy of the sample as KK in Fig. 2.
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 sandstone and muddy siltstone. The Tsuku Formation (300 m in
thickness) consists of dark gray, muddy and ®ne-grained
228
M.-L. Lin et al. / Journal of Asian Earth Sciences 19 (2001) 223±232
Table 2
Range of vitrinite re¯ectance for the studied stratigraphic formations in the
NE Hsuehshan Range
Formation
max range (%)
R
m range (%)
R
max20% range (%)
R
Tatungshan
Tsuku
Kankou
Szeleng
1.35±2.89
3.16±3.51
4.06±5.97
5.66±5.71
1.18±2.56
2.68±3.26
3.58±5.07
3.78±4.54
1.66±3.51
3.26±4.05
4.73±6.74
6.21±6.28
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 coursegrained 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 structures 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 Formation, 8 from the Tsuku Formation, 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 weathering 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 measurements 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 n 1:518; monochromatic polarized light (546 nm) and magni®cation £ 500 with a
diaphragm diameter of 0.25 mm (5 mm 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¯ectance (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 of Rm, Rmax and
Rmin for each sample were used to calculate an average
m ; mean
value of mean random vitrinite re¯ectance R
maximum vitrinite re¯ectance Rmax ; mean of highest
max 20% and mean minimum vitrinite re¯ec20% of Rmax R
tance Rmin ; respectively (Kilby, 1991). Additionally, a
histogram of the Rmax of each sample was plotted to show
the distribution patterns and was used to evaluate the reliability of the measurements as suggested by Hacquebard and
Donaldson (1974) and Davis (1978).
The maximum vitrinite re¯ectance range (Rmax range),
max 20%; R
m and R
min for all of the samples in the
max ; R
R
max ;
study area are listed in Table 1. The ranges of R
Rmax 20% and Rm for the studied formations are summarized
m
max ; and R
in Table 2. On the whole, the values of both R
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 of Rmax in all of the samples here is rather large
(Table 1). This is not uncommon for clastic rocks during
metamorphism, since Rmax of 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
Fig. 4. (A) Histogram of Rmax from sample No. 8. (B) histogram of Rmax from sample No. 37 and (C) diagram of Rm vs. Rmax from sample No. 37 in the NE
Hsuehshan Range.
M.-L. Lin et al. / Journal of Asian Earth Sciences 19 (2001) 223±232
229
max 20% vs. R
max ; (B) R
m vs. R
max ; and (C) R
min vs. R
max for metapelite from the NE Hsuehshan Range. TTS: Tatungshan Formation; TSK:
Fig. 5. (A) R
Tsuku Formation; KK: Kankou Formation; and SS: Szeleng Sandstone.
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.
max ;
4A) is required to verify whether the corresponding R
can be deemed acceptable. For samples with fewer than 30
measurements (Fig. 4B), data are considered reliable as long
as the Rmax and Rm from the same rock sample have a linear
correlation (e.g. Fig. 4C). In the present study, the Rmax and
Rm for all samples have a correlation coef®cient (r) larger
than 0.85, except for that of sample No. 23 with r 0:63:
Because it is practically impossible to measure the true Rmax
max 20%
value, some approximation is inevitable; in fact, R
was ®rst postulated by Cook et al. (1972). In this study, a
max 1 0:329 with r
max 20% 1:071 R
good relation of R
0:994 (Fig. 5A) strongly supports the hypothesis that either
max ; can represent the degree of metamorph max 20% or R
R
ism of rocks in the study area. Besides, other research has
min increase with rising temperature
m and R
suggested that R
and can therefore represent the degree of metamorphism
(TeichmuÈller and TeichmuÈller, 1981). As shown in Fig. 5B,
max 1 0:407 with r 0:959
m 0:747 R
a correlation of R
max : This correlation, however, is
m and R
exists between R
m values below 3.0%. A similar ®nding
better suited for R
was reported in NW Germany (TeichmuÈller and TeichmuÈller,
max : is, evidently,
1981). This indicates that (1) the quality of R
max must be measured when R
m is
m ; (2) R
better than that of R
m only when
above 3.0%; and (3) it is possible to measure R
m is below 3.0%. A smaller correlation r 0:738 between
R
max is shown in Fig. 5C. Obviously, R
min is less
min and R
R
reliable. This may have resulted from the fact that the vitrinite
particles in the study area were too narrow in width (,15 mm;
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 Yingzuling Anticline, where the strata display a younging westward trend along the Taiwan No. 9 Highway (Fig. 2). From
max increases
west to east (samples No. 8 to No. 13), R
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
max increases gradually
(samples No. 41 to No. 45), R
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
max 2:14%
of the four in the section and has the lowest R
as well (Table 1; Fig. 2). With the syncline axis extended
westward to the Taiwan No. 9 Highway, sample No. 6 also
max 1:97% (Table 1; Fig. 2). Similarly, sample
has a low R
max
No. 4 in the vicinity of the syncline axis has a lower R
max 2:38% and No.5
2:25% when compared with No.3 R
max 2:39% (Table 1; Fig. 2).
R
These three examples can be further exempli®ed by the
m : Fig. 5B shows that the
max and R
correlation between R
data points are uniformly distributed along a well-de®ned
m ; three separate
max and R
line. On the basis of their R
clusters of samples belonging, respectively, to the Tatungshan (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
m values correspond to the younger
max and R
the smaller R
m re¯ect
max and R
formation. In other words, the ranges of R
230
M.-L. Lin et al. / Journal of Asian Earth Sciences 19 (2001) 223±232
Fig. 6. Modi®ed geologic map of the Keng-fang area, extracted from Fig. 2. Heavy dashed line represents the hypothetical boundary between the Tsuku and
the Kankou Formations proposed in this study.
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
m covered by indivi max and R
appears that the ranges of R
dual clusters are closely related to the thickness of the stratigraphic formation. The same phenomena are found in the
max 20% (Fig. 5A).
max vs. R
plot of R
These observations support ®ve features. First, along a
section, the strata near the axial part of a syncline are
max than other samples
younger, and thus each has a lower R
farther away from the axial part of the syncline; on the other
hand, the opposite holds true for an anticline. Secondly, the
m are well associated with a certain
max and R
ranges of R
level and thickness of stratigraphic formation. Thirdly, vitrinite 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, metamorphism seems to have achieved its peak before any folding 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¯ectance is able to serve as an indicator not only of metamorphic 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 implausible. However, such a change, if present, cannot be
discerned based on the available data.
max 5:71% and
Of signi®cance is that the values R
5.66% (samples No. 14 and No. 22 in Table 1) of the intercalated carbonaceous metapelites in the oldest Szeleng
max value
Sandstone are lower than the highest R
( 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 sandstones, with higher heat conductivity than mudstones,
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 excessive 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
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
m ranges
max and R
Ho, 1989). However, owing to the R
(Table 2; Fig. 5B), which re¯ect the metamorphic grade
of the studied formations in general, here it is postulated
m 2:70% in Table 1)
max 3:20% and R
that these strata (R
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 Formations, sample No. 20 was taken to be in the Tsuku Formation
by Tang and Yang (1976) and Huang and Ho (1989).
m values (Fig. 5B),
max and R
However, based upon the R
max 2:89%
it is more plausible that the strata at that site (R
and Rm 2:37% in Table 1) are in the Tatungshan Formation 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 Hsuehshan Range, Taiwan, were studied by measuring vitrinite
m : The results concerning the rela max and R
re¯ectance R
tionships between vitrinite re¯ectance and geologic features
m changes continuously in
max or R
show that (1) either R
m in each
max and R
geologic sections, and (2) the ranges of R
studied stratigraphic formation are closely associated with
the level and thickness of that stratigraphic formation.
Because vitrinite is a very common component in metapelite and is very sensitive to temperature, vitrinite re¯ectance 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
231
re¯ectance additionally bears a great potential in being
applicable as an indicator of stratigraphic position of formations and as an aid in stratigraphic identi®cation and correlation 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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