Graptolite biostratigraphy and its correlation of the

Geosciences Journal

Vol. 11, No. 1, p. 23 − 38, March 2007

Graptolite biostratigraphy and its correlation of the Mungok and Yeonghung formations at Yeongwol area, Korea Hyun Su Cho Jeong Yul Kim* } Department of Earth Science Education, Korea National University of Education, Chungbuk 363-791, Korea ABSTRACT: Based on graptolites from the Mungok and Yeonghung formations at Yeongwol, Korea and recent taxonomy on graptolites, the previous three graptolite zones are refined as the Anisograptus matanensis, Adelograptus cf. tenellus, Callograptus spp. zones, and two zones are firstly proposed for the Tremadocian rocks in the area: the Paradelograptus antiquus and Aorograptus victoriae zones, in ascending order. The Anisograptus matanensis Zone yields Anisograptus matanensis indicating an early Tremadoc age. The Adelograptus cf. tenellus Zone is erected by the occurrence of Adelograptus cf. tenellus and Psigraptus jacksoni which represent lowermost Late Tremadocian in age. The Callograptus spp. Zone yielding plenty of dendroids including Callograptus and Dendrograptus, is tentative but useful to correlate with other rocks in the Sino-Korean Block. The Paradelograptus antiquus Zone is recognized by the occurrence of Paradelograptus antiquus and Clonograptus aureus and is correlated with the middle Late Tremadoc of Yukon, Canada. The Aorograptus victoriae Zone consists mainly of Aorograptus victoriae and subordinately of Kiaerograptus and Clonograptus and corresponds to the eponymous zones elsewhere. The boundary between the Mungok and the Yeonghung formations consists of ribbon rock and grainstone to packstone facies of the Mungok Formation and laminated dolomite, argillaceous limestone, and wackestone to grainstone facies of the Yeonghung Formation and is drawn within the Aorograptus victoriae Zone which indicates middle Late Tremadocian. Key words: graptolite, biostratigraphy, Ordovician, Mungok Formation, Yeonghung Formation, Yeongwol

1. INTRODUCTION Graptolites have been considered as the most useful tool for establishing the biostratigraphy of Ordovician shale sequences around the world (Bulman, 1970). Even though the Cambrian-Ordovician strata, namely the Choseon Supergroup, mainly have been exposed in the middle-east part of the Korean peninsula, the graptolite study has been hardly carried out so far. It is, for the most part, presumably due to the difficulty in collecting graptolites; that is, stratigraphically narrow range of the fauna and lithologically and tectonically unfavorable condition for the faunal preservation. The existence of graptolites from the Yeongwol Group was firstly noted by Kobayashi and Kimura (1942) as fragments of the dendroids with one figure. Recent studies on *Corresponding author: [email protected]

graptolites mainly dealt with the Tremadocian paleogeography (Jin, 2002; Cho, 2003), the types of Psigraptus (Jin, 2002), the development of Psigraptus (Cho, 2003; Kim et al., 2006), and biostratigraphy and correlation (Jin, 2002; Cho, 2003; Kim et al., 2003) based on the occurrences of various graptolites. However, there are still unanswered questions including the expansion of the stratigraphical range of graptolites and its correlation with other continents. This paper aims, based on new proposals and materials, to propose the Late Tremadoc graptolite biostratigraphy of the interval including the upper part of the Mungok Formation and the lower part of the Yeonghung Formation, to define the age of the boundary between these two formations, and to correlate the graptolite biozones of the Yeongwol area with coeval ones elsewhere.

2. GEOLOGIC SETTING AND FOSSIL LOCALITY The Cambrian-Ordovician sedimentary sequence in South Korea, the Choseon Supergroup, exposed in the centraleastern part of the Korean peninsula and consisted predominantly of carbonate with lesser amounts of sandstone and shale, rests on the Precambrian granitic gneiss and is overlain by the Silurian Hoedongri Formation or CarboniferousTriassic sedimentary sequence, the Pyeongan Supergroup (Fig. 1). Kobayashi et al. (1942) first recognized the five types of sequences within the Choseon Supergroup, each with a distinct lithologic succession and geographic distribution: the Duwibong-type, Yeongwol-type, Jeongseon-type, Pyeongchang-type, and Mungyeong-type sequences. Recently Choi (1998a), based on International Stratigraphic Guide (Hedberg, 1976; Salvador, 1994), noted the inappropriateness of this stratigraphic nomenclature of this supergroup and subsequently proposed the Taebaek, Yeongwol, Yongtan, Pyeongchang, and Mungyeong groups to replace the proposal of Kobayashi et al. (1942). The Yeongwol Group in the study area is widely exposed on the western and southwestern side of the Baegunsan Syncline, including most of the western part of Yeongwol, Jecheon, and western region of Danyang area. The group is divided into five formations; namely, the Sambangsan, Machari, Wagok, Mungok, and Yeonghung formations in ascending

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Hyun Su Cho and Jeong Yul Kim

Geologic map showing the distribution of the Choseon Supergroup and Pyeongan Supergroup (modified from GICTR, 1962). 1, Precambrian basement rocks; 2, Okcheon Supergroup (late Precambrian-early Paleozoic); 3, Taebaek Group; 4, YeongwolMungyeong Groups; 5, Yongtan Group; 6, Pyeongchang Group; 7, Pyeongan Supergroup; 8, Daedong Group; 9, Mesozoic granite or igneous rock. Abbreviations: Precam., Precambrian; Unc., Uncertain; Cambro-Ordov., Cambro-Ordovician; Carb., Carboniferrous; Trias., Triassic; Meso., Mesozoic. Fig. 1.

order (Yosimura, 1940; Kobayashi, 1966; Choi, 1998a). The geologic ages of lower three formations are assigned to Cambrian, while the upper two to Ordovician. The lowermost Sambangsan Formation consists exclusively of siliciclastic sediments, whereas the upper four formations are dominantly of carbonates.

2.1. Fossil Locality The Tremadocian graptolite sequence described herein is measured from nine structurally separate sections: the Karaejae, Ugga-gol, Dojang-gol, Ppelchi-gol, Dae-gol, Myeongjon, Namaeri, Dumok, and Namkyo sections (Fig. 2). The Ugga-gol section (37°16'26"N+128°26'45"E; Fig. 3A) is outcropped along the road side at the vicinity of Machari village, which is 15.2 km north from Yeongwol town. This section is ca. 90 m in thickness and the beds strike N15°E and dip 20°NW. Graptolites were collected from three intervals including seven horizons in the upper part of the Mungok Formation. The lower interval, ca. 18-22 m above the base of the measured section, yields Callograptus and Dendorgraptus, the middle interval, ca. 28-35 m above the base of the measured section, yields Dictyonema and Paradelograptus, and the upper interval, ca. 43-62 m above the base of the measured section, yields Aorograptus, Clonograptus, and Kiaerograptus. The Dojang-gol section (37°15'59"N+128°26'50"E; Fig. 3B) is exposed along the foot of the mountain at the vicinity of Machari village, which is 10.5 km north from the Yeongwol town. This section is ca. 20 m in thickness and the beds strike N10°W and dip 70°SW. Graptolites were procured

from two intervals including five horizons in the upper part of the Mungok Formation. The lower interval, ca. 2-6 m above the base of the measured section, yields Callograptus, and the upper interval, ca. 7-12 m above the base of the measured section, yields Paradelograptus, Adelograptus, Clonograptus, Aorograptus and Kiaerograptus. The Karaejae section (37°16'29"N+128°26'41" E; Fig. 3C) is exposed along the unpaved roadside at the vicinity of Machari village, which is 16 km north from Yeongwol town. This section is ca. 40 m in thickness and the beds strike N25°E and dip 43°SE. Graptolites were procured from two intervals including six horizons in the upper part of the Mungok Formation and the lower part of the Yeonghung Formation. The lower interval, ca. 2-7 m above the base of the measured section, yields Aspidograptus, Adelograptus, Aorograptus, Clonograptus, Kiaerograptus and Paradelograptus, and the upper interval, ca. 11-12 m above the base of the measured section, yields Aspidograptus, Kiaerograptus, Araneograptus and Paradelograptus. The Ppelchi-gol section (37°13'06"N+128°23'35"E; Fig. 3D) is exposed along the local road No. 88 at the vicinity of Peil-jae tunnel, which is 12.5 km northwest from Yeongwol town. This section is ca. 50 m in thickness and the beds strike N30°E and dip 42°NW. Graptolites were procured from the three graptolite intervals including six horizons in the upper part of the Mungok Formation. The lower interval, ca. 8-17 m above the base of the measured section, yields Adelograptus, the middle interval, ca. 19-25 m above the base of the measured section, yields Psigraptus, and the upper interval, ca. 43-47 m above the base of the measured section, yields Aspidograptus, Cal-

Graptolite biostratigraphy and its correlation of the Mungok and Yeonghung formations at Yeongwol area, Korea

25

Geologic map of Yeongwol area (modified from GICTR, 1962). 1, the Karae-jae; 2, Ugga-gol; 3, Dojang-gol; 4, Namkyo; 5, Ppelchi-gol; 6, Namaeri; 7, Dumok; 8, Dae-gol; and 9, Myeongjon sections. Fig. 2.

lograptus, Dendrograptus, and Dictyonema. The Dae-gol section (37°10'35" N+128°21' 27"E; Fig. 3E) is exposed along the Jungang railway and the national road No. 38 in the vicinity of Changwon village, which is 10.0 km southwest from Yeongwol town. This section is ca. 40 m in thickness and the beds strike N32°E and dip 40°NW. Graptolites were collected from three intervals including four horizons in the Mungok Formation. The lower interval, ca. 13 m above the base of the measured section, yields Dendrograptus and Adelograptus, the middle interval, ca. 28-34 m above the base of the measured section, yields Dictyonema and Psigraptus, and the upper interval, ca. 34-37 m above the base of the measured section, yields Adelograptus. The

Myeongjon section (37°09'25" N+128°21'38" E; Fig. 3F) is exposed along the national road No. 59 at Myeongjon village, which is 11.5 km southwest from Yeongwol town. This section is ca. 50 m in thickness and the beds strike N20°E and dip 40°NW. Graptolites were collected from three intervals including nine horizons in the upper part of the Mungok Formation. The lower interval, ca. 7.5-12 m above the base of the measured section, yields Dictyonema and Adelograptus, the middle interval, ca. 17.2-30 m above the base of the measured section, yields Callograptus, Desmograptus, Dictyonema, Adelograptus, and Psigraptus, and the upper interval, ca. 35-42 m above the base of the measured section, yields Dictyonema and Adelograptus. The

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Hyun Su Cho and Jeong Yul Kim

Biozonation in synthetical stratigraphic column. A, the Ugga-gol; B, Dojang-gol; C, Karae-jae; D, Ppelchi-gol; E, Dae-gol; F, Myeongjon; G, Namaeri; H, Dumok; I, Namkyo sections. Yh., Yeonghung; Mn., Mungok; Fm., Formation; Ad., Adelograptus; Ao., Aorograptus; Callo., Callograptus; P., Paradelograptus. Fig. 3.

Namaeri section (37°12'15" N+128°25' 10"E; Fig. 3G) is exposed along a riverside path in the vicinity of Yeongwolsamgeori, which is 4.5 km west from Yeongwol town. This section is ca. 64 m in thickness and the beds strike N25°E and dip 43°NW. Graptolites were collected from three intervals including seven horizons in the upper part of the Mungok Formation. The lower interval, ca. 8-17 m above the base of the measured section, yields Dendorgraptus, Dictyonema, and Adelograptus, the middle interval, ca. 25-31 m

above the base of the measured section, yields Psigraptus, and the upper interval, ca. 35 m above the base of the measured section, yields Callograptus. The Dumok section (37°12'30"N+128°25'50"E; Fig. 3H) is exposed along the national road No. 38 in the vicinity of Dumok village, which is 3.5 km west from Yeongwol town. This section is ca. 70 m in thickness and the beds strike N25°E and dip 45°NW. Graptolites were collected from two horizons in the upper part of the Mungok Formation. The lower hori-

Graptolite biostratigraphy and its correlation of the Mungok and Yeonghung formations at Yeongwol area, Korea

Fig. 3.

(Continued)

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Hyun Su Cho and Jeong Yul Kim

zon, ca. 17 m above the base of the measured section, yields Adelograptus, and the upper horizon, ca. 45 m above the base of the measured section, yields Callograptus. The Namkyo section (37°15' 12"N+128°25'42"E; Fig. 3I) is exposed along the national road No. 31 in the vicinity of Mungok village, which 9.5 km north from Yeongwol town. This section is ca. 35 m in thickness and the beds strike N25°E and dip 60°NW. Graptolites were collected from only one horizon in the upper part of the Mungok Formation. The horizon, ca. 2 m above the base of the measured section, yields Psigraptus.

2.2. Lithology around the Mungok-Yeonghung Boundary The Mungok Formation has generally been reported to show four dominant lithofacies: ribbon rock, grainstone to packstone, lime conglomerate, and marlstone to shale facies (Kim, 1999; Kim and Choi, 2000; Fig. 4). In general, five noticeable lithofacies, laminated dolomite, massive black dolomite, argillaceous limestone, wackestone to grainstone, and bioturbated limestone lithofacies have been employed to describe the Yeonghung Formation (Choi and Jeong, 1990; Choi and Woo, 1993; Fig. 4). However, the lithology including the Mugok-Yeonghung boundary is very problematic because of severe dolomitization and soil covering at the most sections. Consequently, there are more or less discrepancies among the researchers to describe the lithology of the upper part of the Mungok Formation and the lower part of the Yeonghung Formation (Choi and Woo, 1993; Kim, 1999; Kim and Choi, 2000). The upper part of the Mungok Formation commonly appears the alternation of four lithofacies anomalously, even though it might be the dominant facies of marlstone to shale. The uppermost part of the Mungok Formation consists of two facies: ribbon rock and grainstone to packstone facies. The lowermost part of the Yeonghung Formation consists mainly of laminated dolomite, argillaceous limestone, and wackestone to grainstone lithofacies. Laminated dolomite facies in the lower part of the Yeonghung Formation frequently shows desiccation and synaerisis crack, and sometimes stromatolite (Fig. 4). The boundary between the Mungok and Yeonghung formations is not fully established yet, but can be examined in six sections: the Karae-jae, Ugga-gol, Dojang-gol, Namkyo, Namaeri and Dumok sections (Fig. 4). The transition from the Mungok to Yeonghung formations is gradational or shows the dolomitization. The Mungok-Yeonghung boundaries of the Namaeri and the Dumok sections show severe dolomitization and soil-covered, and so it is difficult to find fossils including graptolites from the sections. At Namkyo section, the Mungok-Yeonghung boundary is designated as the first occurrence of a 50 cm-thick laminated lime mudstone bed overlying a 2 m-thick grainstone to packstone bed. The Dojang-gol section includes the boundary as the association of 1.2 m-thick reddish gray grainstone to packstone,

4.0 m-thick ribbon rock, and 2.0 m-thick massive dolostone in ascending order of the uppermost of the Mungok Formation, which is overlain by the alternation of the laminated lime mudstone and wackestone to grainstone of the Yeonghung Formation. The interval including the Mungok-Yeonghung boundary, of the Ugga-gol section consists of the alternation of ribbon rock and grainstone to packstone of the uppermost of the Mungok Formation, and the bioturbated limestone and thin laminated lime mudstone of the lowermost of the Yeonghung Formation. And the Karae-jae section shows good exposure of the lithologically continuous sequence including the boundary between the Mungok and Yeonghung formations, and consists of the alternation of a ribbon rock and marlstone to shale beds of the Mungok Formation and thin laminated lime mudstone and wackestone to grainstone of the Yeonghung Formation. As shown in Figure 3 and 4, the interval including the boundary between the Mungok and Yeonghung formations collectively reveals the ribbon rock, marlstone to shale, and grainstone to packstone facies of the Mungok Formation and the thin laminated lime mudstone, bioturbated limestone, and wackestone to grainstone facies of the Yeonghung Formation.

3. GRAPTOLITE BIOSTRATIGRAPHY It has been so far reported three graptolite zones from the Mungok Formation of Yeongwol area: the Anisograptus richardsoni, Adelograptus, and Callograptus curvithecalis zones in ascending order (Jin, 2002; Cho, 2003; Kim et al., 2003). According to new proposals for the graptolite biostratigraphy, the previous graptolite zones are refined: the Anisograptus matanensis, Adelograptus cf. tenellus, and Callograptus spp. zones. Based on the new materials from the study area, the two zones are newly proposed: the Paradelograptus antiquus and Aorograptus victoriae zones in ascending order (Figs. 3 and 4). As shown in Figure 5, collectively 220 materials of Lower Ordovician (Tremadocian) graptolites are procured from the formations: 2 materials of the Anisograptus matanensis Zone, 127 of Adelograptus cf. tenellus Zone, 18 of Callograptus spp. Zone, 18 of Paradelograptus antiquus Zone, and 55 of Aorograptus victoriae Zone (Fig. 5). Among the present materials, Adelograptus cf. tenellus (Linnarrson, 1871), Paradelograptus antiquus (Hall, 1899), and Aorograptus victoriae (Hall, 1899) are the representative species for the international graptolite biozonation. Callograptus spp. such as Callograptus curvithecalis Mu, 1955, Callograptus cf. radiatus (Hopkinson, 1875), and Callograptus cf. hopkinsoni (Bulman, 1934) are useful materials for correlation with those of the Sino-Korean Block.

3.1. The Anisograptus matanensis Zone 3.1.1. Composition

This zone at Yeongwol is recognized by the occurrence of

Graptolite biostratigraphy and its correlation of the Mungok and Yeonghung formations at Yeongwol area, Korea

Fig. 4.

Correlation and zonation of the measured sections. Symbols are same as Figure 3. Numbers are in meter.

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Fig. 5.

Hyun Su Cho and Jeong Yul Kim

Composition of the graptolite zones and its abundance in number.

Graptolite biostratigraphy and its correlation of the Mungok and Yeonghung formations at Yeongwol area, Korea

31

dinopora flabelliformis flabelliformis and R. f. canadensis (Cooper et al., 1998). The Anisograptus matanensis Zone of Yukon, Canada consists mainly of Anisograptus matanensis 3.1.2. Stratigraphic occurrence The base of this zone is so far unknown due to dolomiti- and subordinately of Anisograptus cf. canadensis and Bryozation and its top is drawn at the first appearance datum graptus? sp. (Jackson and Lenz, 2003; Jackson and Norford, (FAD) of Adelograptus cf. tenellus, the representatives of 2004). The A. matanensis Zone established in NewfoundAnisograptus matanensis Ruedemann, 1937.

land and Yukon correlates with the zone of Yeongwol (Fig. 6). A case of Dayangcha, China, where A. matanensis occurred in a very thin interval without associated graptolites (Zhang and Erdtmann, 2004), is similar to that of Yeongwol. Occurrence of only rare dendroids above the A. matanensis horizon of Dayangcha makes difference between them. The assemblage 3.1.3. Remarks The zone originally erected as the Anisograptus richard- with Anisograptus delicatulus and A. compactus reported in soni Zone, but recent studies elsewhere have regarded A. Lancefield of Australia (Cooper and Stewart, 1979) can be richardsoni Bulman, 1941 as a junior synonym of Anisograptus correlated with this zone of Yeongwol. matanensis Ruedemann, 1937 (Cooper et al., 1998; Wang and Wang, 2001; Jackson and Lenz, 2003; Jackson and Norford, 3.2. The Adelograptus cf. tenellus Zone 2004; Zhang and Erdtmann, 2004). A suggestion followed herein, it is reasonable to change the A. richardsoni Zone to 3.2.1. Composition This zone consists mainly of Adelograptus cf. tenellus the A. matanensis Zone. In Newfoundland of Canada, A. matanensis is widespread (Linnarrson, 1871) (Fig. 7E) and subordinately of Calin upper slope and oceanic sequences together with Rhab- lograptus sinicus Mu, 1955, Dendrograptus suni Mu, 1955,

the overlying zone. This zone has been tentatively established under the Adelograptus Zone of the Yeonjeong section at Yeongwol, based on the occurrence of the zonal representatives, Anisograptus matanensis Ruedemann, 1937 (Jin, 2002).

Fig. 6.

Correlation of the Tremadocian graptolite zones of Yeongwol with those of other continents.

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Hyun Su Cho and Jeong Yul Kim

A, Paradelograptus antiquus (Hall, 1899), KNUE-Ad 1004; B, Clonograptus aureus Jackson, 1973, KNUE-Cl 1001a; C, Aspidograptus lotolatzensis (Mu, 1955), KNUE-As 1003a; D, Clonograptus cf. rigidus (Hall, 1858), KNUE-Cl 1003a; E, Adelograptus cf. tenellus (Linnarsson, 1871), KNUE-Ad 1005. Scale of A, 1 mm, B, 5 mm, and C-E, 2 mm.

Fig. 7.

and Psigraptus jacksoni Rickards and Stait, 1984.

3.2.2. Stratigraphic occurrence

Its base at Yeongwol is defined by the FAD of Adelograptus cf. tenellus and the top is the FAD of Callograptus radiatus, which is the base of overlying Callograptus spp. Zone. This zone has been set on many sections at Yeongwol, based on the occurrence of the zonal representatives, Adelograptus cf. tenellus (Jin, 2002; Cho, 2003; Kim et al., 2003; Figs. 3 and 4).

3.2.3. Remarks

The zone originally subdivided into three subzones: the Lower Adelograptus, Psigraptus jacksoni and Upper Adelograptus subzones in ascending order (Jin, 2002; Cho, 2003; Kim et al., 2003). The recent investigations have procured the

zonal representatives Adelograptus cf. tenellus from the entire zonal range of the Yeongwol area. And it is reasonable to substitute the zonal nomenclature of the Adelograptus cf. tenellus Zone for the Adelograptus Zone, using the zonal representatives Adelograptus cf. tenellus. At Dayangcha of China, the Psigraptus Subzone has been raised to zonal status replacing the previous Muenzhigraptus–Psigraptus Zone (Zhang and Erdtmann, 2004). However, at Yukon of Canada, the Adelograptus Zone has been still retained considering a very thin stratigraphical interval of Psigraptus (Jackson and Lenz, 2003). Comparing to NE China, the Yeongwol area shows different graptolite biostratigraphy in the Adelograptus cf. tenellus Zone. The graptolites from Yeongwol are occurred as the adelograptids to the psigraptids in turn, whereas those from Dayangcha and

Graptolite biostratigraphy and its correlation of the Mungok and Yeonghung formations at Yeongwol area, Korea

33

Hebei as the both taxa simultaneously. Considering the regional biostratigraphic difference with NE China, it is reasonable to retain the Adelograptus cf. tenellus Zone at Yeongwol as to be proposed. The Adelograptus cf. tenellus Zone at Yeongwol corresponds to the Adelograptus cf. tenellus Zone at Yukon (Fig. 6), in which Ancoragraptus bulmani Spjeldnaes, 1963, Psigraptus arcticus Jackson, 1967, and Psigraptus lenzi Jackson, 1967 have been reported (Jackson and Lenz, 2003; Jackson and Norford, 2004). It should be correlated to the Psigraptus jacksoni Zone at Dayangcha of China (Zhang and Erdtmann, 2004; Fig. 6) and the Psigraptus Zone of Tasmania and Victoria of Australia (Rickards et al, 1991; Fig. 6), based on the appearance of Adelograptus associated with Psigraptus faunas, discovered in Jilin and Hebei of China (Zhao and Zhang, 1985, 1986; Song, 1993; Zhang and Erdtmann, 2004) and Tasmania and Victoria of Australia (Cooper and Stewart, 1979; Rickards and Stait, 1984, Rickards et al., 1991; Cooper, 1999). Clonograptus sp. from Tasmania of Australia associated with P. jacksoni (Rickards and Stait, 1984) presumably belongs to Adelograptus according to the revision of Clonograptus by Lindholm and Maletz (1989). In this regards, it can be correlated with each other indicating the same age. Even though some researchers (Maletz and Egenhoff, 2001; Zhang and Erdtmann, 2004) extended the zone to Scandinavia, it is hesitated herein to consider the faunal composition especially without psigraptids from Scandinavia (Fig. 6).

3.3. The Callograptus spp. Zone 3.3.1. Composition

This zone consists mainly of Callograptus spp. such as Callograptus curvithecalis Mu, 1955, Callograptus cf. hopkinsoni (Bulman, 1934) (Fig. 8A), Callograptus cf. radiatus (Hopkinson, 1875) (Fig. 8B), and Callograptus cf. catenoides (Lin, 1992) (Fig. 8D), and subordinately of Dendrograptus cf. desmograptoides (Boucˇ ek, 1956).

A, Callograptus hopkinsoni Bulman 1934, KNUE-Ca 1001a; B, Callograptus radiatus (Hopkinson, 1875), KNUE-Ca 1002b; C, Aspidograptus implicatus (Hopkinson, 1875), KNUE-As 1002a; D, Callograptus cf. catenoides (Lin, 1992), KNUE-Ca 1005b; E, Kiaerograptus cf. pritchardi (Hall, 1899), KNUE-Ki 1003; F, Aorograptus victoriae (Hall, 1899), KNUE-Ao 1002. Scale of A, 5 mm and B-F, 2 mm. Fig. 8.

3.3.2. Stratigraphic occurrence

The basis of the zone is defined by the FAD of Cal- interval. It is more reliable to sustain the genus level lograptus radiatus and the upper boundary is drawn at the nomenclature as the Callograptus spp. Zone. The upper FAD of Paradelograptus antiquus, the representatives of part of the Mungok Formation is dominated by shallowthe overlying zone. This zone ranges approximately 18-28 m and 43-50 m from the base of the Ugga-gol and Ppelchi-gol sections of the Mungok Formation, Yeongwol, respectively (Figs. 3 and 4).

3.3.3. Remarks

Jin (2002) tentatively erected this zone as the Callograptus curvithecalis–Dendrograptus Zone because of no grap-

toloids for stratigraphic correlation with other areas. However, the continuity of C. curvithecalis is uncertain and many other species of Callograptus has been procured from this

water limestone and shows extremely rare occurrence of graptolite fauna, which may be resulted from change in depositional setting from fossiliferous deep water to intertidal to supratidal environment. Consequently, this zone is somewhat difficult to recover the graptoloids and sporadically yields the dendroids from the coeval sections at Yeongwol. Wang and Erdtmann (1986, 1987) also tentatively proposed the Callograptus taitzehoensis Zone at the Xiaoyangqio section of Dayangcha, overlain by the AdelograptusClonograptus Zone, and discarded it. Although this zone is a tentative biozone, it is very useful to correlate the coeval

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Hyun Su Cho and Jeong Yul Kim

ones at least within the Sino-Korean block. The Callograptus spp. Zone at Yeongwol corresponds to the lower part of the Aorograptus victoriae Zone of the Yeli Formation, Dayangcha (Fig. 6), China which was erected by Zhang and Erdtmann (2004).

catus (Hopkinson, 1875) (Fig. 8C), Aspidograptus? minor Bulman, 1934, Araneograptus cf. macgillivrayi (Hall, 1897), Clonograptus cf. rigidus (Hall, 1858; Fig. 7D), and Rhabdinopora sp.

3.4. The Paradelograptus antiquus Zone

Its base at Yeongwol is drawn at the FAD of Aorograptus victoriae but its top is uncertain as no succeeding graptoloids younger than this zone have been discovered in this area. The ranges of this zone are approximately 0-20 m, 43-65 m, and 12-22 m from the base of the Karae-jae, Ugga-gol, and Dojang-gol sections of the Mungok Formation at Yeongwol, respectively (Figs. 3 and 4).

3.4.1. Composition

This zone consists mainly of Paradelograptus antiquus (Hall, 1899) (Fig. 7A) and subordinately of Aspidograptus lotolatzensis Mu, 1955 (Fig. 7C), Adelograptus cf. tenellus (Linnarsson, 1871), and Clonograptus aureus Jackson, 1973 (Fig. 7B).

3.4.2. Stratigraphic occurrence

Its base is drawn at the FAD of Paradelograptus antiquus and its top at the FAD of Aorograptus victoriae, the representatives of the overlying zone. This zone are recognized approximately 28-43 m and 0-12 m from the base of the Ugga-gol and Dojang-gol sections of the Mungok Formation at Yeongwol, respectively (Figs. 3 and 4).

3.4.3. Remarks

This zone, originally proposed as a subzone of the Adelograptus victoriae Zone from the Peel River Section at

3.5.2. Stratigraphic occurrence

3.5.3. Remarks

Williams and Stevens (1991) proposed the Aorograptus victoriae Zone as the latest Tremadocian in western New-

foundland, Canada based on the appearance of the eponymous species. The zone at Dayangcha, China yielding A. victoriae and Kiaerograptus corresponds to that of Yeongwol. The zone of Yeongwol should correspond to the Kiaerograptus pritchardi Zone of Yukon, Canada containing a diverse assemblage of graptolites including K. pritchardi, Paradelogratpus antiquus and Aorogratpus victoriae (Fig. 6). Cooper (1999) also erected the Kiaerograptus Zone overlying the Paradelograptus antiquus Zone as one of the global graptolite zones, and Maletz (1999) called it the Kiaerogratpus interval. Based on the graptolite assemblage, the Aorograptus victoriae Zone of Yeongwol corresponds to the upper part of the eponymous zone elsewhere (Fig. 6): the Aorograptus victoriae Zone at the Martin Point South, Green Point, and St Paul’s Inlet sections of the Cow Head Group, western Newfoundland, Canada (Williams and Stevens, 1991), at the Laotougou and Xiaoliaohuangdi sections of the Yehli Formation in Dayangcha, China (Zhang and Erdtmann, 2004), and at the Duo Lake Formation in Yukon, Canada (Jackson and Norford, 2004).

Yukon, Canada (Jackson, 1974), has been raised to zonal status (Jackson and Lenz, 2000). In the Peel River section, the zone comprises of diverse fauna including Adelograptus antiquus, Adelograptus victoriae, and Clonograptus aureus. In the recent research, Jackson and Lenz (2003) considered the zone as could be a subzone of the A. victoriae Zone with the overlying K. pritchardi Subzone, but tentatively sustained the zone due to rare occurrence of A. victoriae. The zone at Yeongwol will be sustained until recovering A. victoriae within the zone. The Paradelograptus antiquus Zone at Yeongwol corresponds to the eponymous zone of the Peel River, Tetlit Creek, and Rock River sections of the Richardson Trough Formation at Yukon, Canada (Jackson, 4. COMPARISON BETWEEN GRAPTOLITES AND 1974; Jackson and Lenz, 2000; Fig. 6). Cooper (1999) pro- OTHER FOSSILS posed the synonymous zone within the global chronozones associated with Paradelograptus antiquus, Aorograptus victo- At the interval including the boundary between the Mungok and Yeonghung formations, fossils have hardly been riae, Adelograptus species, and Paratemnograptus species. obtained except for a few collections from several meters below, or several tens of meters above the boundary. Based 3.5. The Aorograptus victoriae Zone on conodonts, Won and Lee (1977) suggested the boundary could be extended to lower Arenigian, and Lee (1979) cor3.5.1. Composition This zone consists mainly of Aorograptus victoriae (Hall, related the Yeonghung Formation to Llandeilian to Cara1899) (Fig. 8F) and subordinately of Adelograptus cf. tenel- docian. Lee and Lee (1999) have tentatively established the lus (Linnarsson, 1871), Kiaerograptus cf. bulmani (Thomas, conodont biostratigraphy with four informal conodont assem1973), Kiaerograptus cf. hengshanensis Lin, 1981, Kiaerograp- blage zones of the Mungok Formation. Among the zones of tus kiaeri (Monsen, 1925), Kiaerograptus? kutchini Jackson Lee and Lee (1999), Assemblage Zone 4 (= Paracordyloand Lenz, 2003, Kiaerograptus taylori (Hall, 1899), Kiaerograp- dus gracilis Zone) can be related to the boundary (Fig. 9), tus cf. pritchardi (Hall, 1899) (Fig. 8E), Aspidograptus impli- but their study sections did not show the lithological con-

Graptolite biostratigraphy and its correlation of the Mungok and Yeonghung formations at Yeongwol area, Korea

35

Comparison of the graptolites zonal ranges with those of the trilobites (Kim and Choi, 2000) and conodonts (Lee and Lee, 1999). Symbols are same as Figure 3. Abbreviations: Grap., Graptolites; Trilo., Trilobites; Cono,. Conodonts; An., Anisograptus; m., matanensis; P., Paradelograptus; anti., antiquus; Ao., Aorograptus. Fig. 9.

tinuity across the boundary; that is, being placed in about 15 m soil covering, and they are only restricted in the Mungok Formation. According to the conodont data and biozones from Löfgren (1993, 1996), Maletz et al. (1996), and Nicoll et al. (1999), the Paracordylodus gracilis Zone of Lee and Lee (1999) is correlated with the Paracordylodus gracilis Subzone of the Paroistodus proteus Zone and the Hunnegraptus copiosus graptolite zone of Late Tremadocian. Based on the trilobite species obtained from the three stratigraphically separated intervals of the Mungok Formation, Kim and Choi (2000) proposed the trilobite biozones: the Yosimuraspis, Kainella, and Shumardia zones. The Shumardia Zone is a relatively long stratigraphic range, occu-

pying the lower two-thirds of the upper member of the Mungok Formation (Kim and Choi, 2000; Choi et al., 2003; Fig. 9), which is considered as late Tremadocian in age. The early Arenigian trilobite fauna, Kayseraspis sp. cf. laticauda (Kobayashi, 1934) and Asaphopsoides maepoensis Choi, 1998 from the Mungok Formation of Danyang area (Choi, 1998b) and the late Tremodocian to early Arenigian conodont, Paracordylodus gracilis, from the Mungok Formation of Yeongwol area (Lee and Lee, 1999) presumably designate same interval with the A. victoriae Zone of Yeongwol. The difference in age among the studies on three faunas requires more detailed research. Considering the previous studies on the conodonts and

36

Hyun Su Cho and Jeong Yul Kim

trilobites, the boundary between the Mungok and Yeonghung formations has been placed around the boundary between Tremadocian and Arenigian. For a long time, however, the boundary between Tremadocian and Arenigian has been known as the FAD of Tetragraptus approximatus, which is the interval between the Hunnegraptus copiosus and Tetragraptus approximatus zones. In Yeongwol area, the graptolites within this interval have not been recovered yet. As mentioned above, the uppermost Tremadocian graptolite interval at Yeongwol area is so far the Aorograptus victoriae Zone of Late Tremadocian. The boundary between the Mungok and Yeonghung formations at Yeongwol is placed within the Aorograptus victoriae zone of Late Tremadocian (Fig. 9). The boundary between Tremadocian and Arenigian can be traced in the lower part of the Yeonghung Formation (Fig. 9), but it will be come up against the obstacles followed by shallow water lithology and pervasive dolomitization.

5. CONCLUSIONS The previous graptolite zones of the Anisograptus richardsoni, Adelograptus, and Callograptus curvithecalis in ascending order are renamed as the Anisograptus matanensis, Adelograptus cf. tenellus, Callograptus spp. zones

based on the new proposals by many investigations and new materials found from the Mungok and Yeonghung formations in the study area. Two new graptolite biozones, the Paradelograptus antiquus and Aorograptus victoriae zones, are proposed. The boundary between the Mungok and Yeonghung formations is established within the Aorograptus victoriae Zone of middle Late Tremadocian. The boundary between Tremadocian and Arenigian could be drawn on the lower part of the Yeonghung Formation, which placed in somewhat above the middle Late Tremadocian Aorograptus victoriae Zone. It differs from the previous studies on the conodonts and trilobites, which designated the ArenigTremadoc boundary at the upper part of the Mungok Formation or at the boundary between the Mungok and Yeonghung formations, respectively. ACKNOWLEDGMENTS: We thank to two anonymous referees for their invaluable reviews and comments which improved the manuscript.

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