l
FACIES
47
43-56
I
PI. 10-12
5 Figs.
i
1 Tab.
Erlangen 2002
Facies of a Lower Ordovician Carbonate Shelf (Mungok Formation: Taebaeksan Basin, Korea) Dong Hee Kim and Duck K. Choi, Seoul KEYWORDS: LITHOLOGICASSOCIATIONS I)EPOSITIONALFACIES TRILOI'IITKBIOFACIES-TAEt',AEKSAN BASIN (KOle.E,'\) LOWER ORDOVICIAN
Summary The lithologie associations within the Lower Ordoviclan Mungok Formation in Korea define I'dmr , dcpositional facies that formed across a continental margin fringing the Sino-Korean block: these facies represent lagoonal/restricted marine, shoal, inner shelf, and outer shelf environments. The stacking pattern of these facies reveals two systems tracts composed of five deposilional sequences. The lower highstand systems tract consists of the lagoonal/restricted marine and shoal facies, whereas the upper lowstand systems tract comprises, in ascending order, inner shelf, outer shelf, and inner shelf facies. Three trilobite biofacies are recognized in the Mungok Formation: i.e., Yosimuraspis. Kainella, and Shumardia biofacies in ascending order. The Yosimurasl)iS Biofacies is dominated by Yosimuraspis but also contains.l@o~aspLs and Elkanaspis. The predominance of the endemic eponymous taxon suggests a lagoonal/restricted marine en vironment. The nearly monotaxic Kainella Biofacies, which comprises pandemic genera such as KaineIIa and occasionally Leiostegium, may represent a less restricted environment than the Yosimuraspis Biofacies. The Shumardia Biofacies occurs in the marlstone/shale litho facies through relatively thick stratigraphic interval and is dominated by cosmopolitan trilobite taxa with some endemic species. The lithofacies and cosmopolitan trilobite asscmMage of the Shumardia Biofacies indicate that it occupied an outer shelf environment. The vertical succession of lithofacies and trilobite biofacies in the Mungok Formation records ira general a shift from a restricted, shallow water environment to deeper-water environment.
The Cambrian-Ordovician sedimentarb' rocks ol)he Choson Supergroup are well exposed in the Taehaeksan Basin in the mid-eastern part of tile Korean Peninsula (Fig. 1). In the early Paleozoic the Taehaeksan Basin was a shallow marine siliciclastic-carbonate svstcnl with coarsc elastic sediments in the eastern margin of the Taebaeksan Basin and progressively deeper water facies to the west (Yongwol area) (Chough et al.. 2000 and references therein). This siliciclastic-carbonate system persistcd throughout tile Cambrian unlil rapid accumulation off carNmate sediments in the Yongwol area created a broad carbonate shelf across the Taebaeksan Basin in the earliest ()rdovician. The carbonate shelf, which was characterized by low-relief topography ,xith scattered shoals, lagoons, and tidal []als, lasted inlo tile Early and Middle ()rdoviclan (Choi el al., 2001 ). l,ower ()rdoxician strata of the Choson Supergroup in Yongwol area arc assigned to the Mungok Formation (Kobayashi. 1960). During Ihc last decade, the invertebrate fossils (Kobayashi, 196(); Kim et al., 1990: Choi el al.. 1994: Kim and Choi, 1995, 1999.2000a), sedimentology (Paik tat al., 1991 ; Choi et al., 1993), and diagenetic history (Chung ctal., 1993) of the Mungok Formation have been extensively studied. Although, the new stratigraphic and paleontological data significantly improved the understanding of the unit, a detailed picture of its depositional environments and their development with time has not been developed. In this paper, wc analyze the depositional system of the Mungok Formation based on the temporal and spatial distributions of lithofacies and trilobite biofacies to trace the sea-level changes that occurred in this area during the Early Ordovician. 2 GEOI,O(-;IC SETTIN(;
1 INTRODUCTION Biofacies at near continental margins often are associated with specific lithofacies and primarily rctlect tile ecological tolerance of organisms and their taphononfic modification, whereas sedimentary facies are controlled by the geographic, hydrodynamic and tectonic settings of the sedimentary basin (Ludvigsen el al., 1986: Stitt. 1998; Pegel, 2000).
The Cambrian-Ordovician sedimentary rocks in southern Korea, the Choson Supergroup, are widely distribuled in the mid-eastern pan of the Korean peninsula (Fig. I). The unit consists predominantly of carbonates with subordinate sandstone and shale. The Choson Supergroup accumulated on a continental margin-type depression that was contiguous with the cratonic margin of the Sino-Korean block (Chough ctal., 2000; Choi et al., 2001 ). It has been divided into five groups bascd on their unique tithologic succession, faunal contents.
Address: D.H. Kim, D.K. Choi, School of Earth and Environmental Sciences, College of Natural Sciences. Seou/ National University, Seoul 151-742, Korea, E-mail: D.H. Kim (
[email protected]), Duck K. Choi (dkchoi(r
I
44
Fig. 1. Loc',dity maps. (A) Tectonic division of the Korean peninsula and adjacent areas. Abbreviations: l=Imjingang belt, K=Kyonggi massil, N=Nangrim massif, O=Okchon belt,P=Pyongnam basin, Q-D=Qinling-Dabiebelt, S=Sulu belt, T=Taebaeksanbasin.Y=Yongnam massif. (B) Simplified geologic map of the Taebaeksan Basin, mainly showing the distribution of the Choson Supergroup; the open rectangle in the center represents the location of Fig. I C. (C) Index map of Yongwol area showing the locations of measured sections of the Mungok ForT'nation. 1, Machari section; 2, Nodarigol section; 3, Songhwangdong section; 4, Tumok section; 5, Karam-1 section; 6, Karam-II section; 7, Namaeri section; 8, Chommal-I section; 9, Chommal-ll section; and 10, Paeiljae section. Abbreviations: MTF=Machari thrust fault, PTF=Pyongchang thrust fault. and geographic distribution: viz., Taebaek, Yongwol, Yongtan, Pyongchang, and Mungyong ~ o u p s (Kobayashi, 1966; Choi, 1998). The Yongwol Group occupies the western half of the Taebaeksan Basin (Fig. 1B) and comprises the Sambangsan, Machari, Wagok, Mungok, and Yonghung formations in ascending order (cf. Yosimura, 1940). The lower three formations are assigned to the Cambrian and the upper two to the Ordovician. The Mungok Formation, which consists mainly of carbonate with lesser amounts of shale, represents marine environments ranging from supratidal to deep subtidal facies (Paik et al., 1991; Choi et al., 1993). It conformably overlies the Upper Cambrian Wagok Formation, which is a monotonous succession of light gray to gray, massive dolostone. It also grades conformably into the overlying Yonghung Formation, which comprises massive to thickbedded, light to dark gray, fine to medium crystalline dolostone in its lower part, and dark gray, burrow-mottled limestone in the upper part. The Mungok Formation ranges from 140 to 210 meters in thickness and is subdivided into four members based on contrasts in dominant lithofacies: the Karam, Paeiljae, Cbommal, and Tumok members in ascending order (Kim and Choi, 2000b; Fig. 2). The basal Karam Member (29-55 m thick) consists mainly of ribbon rock and grainstone/packstone with intercalation of thin limestone conglomerate beds and chert layers. The Paeiljae Member
(28.6-35 m thick) is a sequence of light gray to gray, massive to crudely-bedded grainstone/packstone which is pervasively dolomitized. The Chommal Member (29.6-50.8 m thick) is characterized by alternation of ribbon rock and limestone conglomerate beds, with a few grainstone/packstone beds in the upper part. The Tumok Member (41.2-77 m thick) comprises ribbon rock, grainstone/packstone, limestone conglomerate, and marlstone/shale.
3 LITHOFACIES This analysis is based on examination of ten measured sections of the Mungok Formation in the northern part of the Yongwol area (Figs. 1 and 2). Four major lithofacies were employed in describing the sections: ribbon rock, grainstone/packstone, limestone conglomerate, and marlstone/ shale lithofacies (Kim and Choi, 2000b). 3.1
Ribbon Rock Facies
The ribbon rock lithofacies is characterized by an alternation of calcareous and argillaceous layers, which produces a conspicuous banded appearance (P1.10/1-4). Each couplet is several millimeters to a few centimeters thick. The light layer is composed of lime mudstone/wackestone to
45
Fig. 2. Correlation of the measured sections of the Mungok Formation in the Yongwol area, showing the stratigraphic positions of mc tubers and depositional facies. Numbers in parenthesis below the cohmms are the same as indicaled in Fig. I C. Stratigraphic horizons of the Iossil collections used for biofacies analysis (Fig. 4) are indicated as arrows. packstone that commonly displays normal grading, cross bedding, and bioturbation, whereas the dark layer is much finer and contains more argillaceous components. Varying proportions of the clastic and carbonate components within the lithofacies resulted in a broad structural spectrum, commonly expressed as planar, lenticular, and nodular to [laser
bedding (Paik el al., 1991: Choi et al., 1993" PI. 10/1-4). Although Paik el al. (1991) interpreted this lithofacies as forming in a storm-infhlenccd high intertidal to shallow subtidal setting. Choi ct al. (1993) assigned it lo a shall{~w to deep ramp setting. D#;imuraspi.,, and Kainella biofacies arc recognized in lhe ribbon rock lithofacies.
46
Fig. 3. Simplified environmental model for the Ordovician carbonate shelf of the Taebaeksan Basin, showing the relative positions of four depositional facies and their typical lithologic associations.
3.2 Grainstone/Packstone Facies The grainstone/packstone lithofacies occurs normally as medium- to thick-bedded and massive to poorly stratified limestone or dolostone (P1.10/5). Grains are peloids, intraclasts, bioclasts, and ooids (P1. 10/6), although the recognition of allochems is usually difficult due to pervasive dolomitization. This facies occasionally contains quartz grains and chert nodules or layers. Cross-lamination and graded bedding are also common in this lithofacies. This lithofacies represents a tide-dominated shoal environment (cf. Paik et al., 1991 ; Choi et al., 1993). No fossils were recovered from this lithofacies.
3.3 Limestone Conglomerate Facies Limestone conglomerates within this lithofacies are clast-supported or matrix-supported, unsorted, and show a wide variety of clast orientation, including chaotic, vaguely imbricated, and stacked edgewise (PI. 10/7-8). The compo-
Plate Fig. 1-4.
Fig. 5. Fig. 6. Fig. 7. Fig. 8.
10
sition of clasts is mostly lime mudstone, and subordinately grainstone, dolostone, and even limestone conglomerate. The composition of clasts in a single bed is also variable, including lime mudstone and grainstone or dolostone (P1. l 0/7). Matrices are composed of dolostone, grainstone, and lime mud. Clasts range in size from a few millimeters to greater than 15 cm in maximum diameter. Most are well rounded, discoid to spherical in shape, but angular to irregularly-shaped clasts are not uncommon. It is often difficult to differentiate limestone conglomerate beds from grainstone/ packstone lithofacies when they are dolomitized. Beds of this lithofacies range normally from 10 cm to 20 cm in thickness, but often amalgamated to reach up to 2.4 m. The thick limestone conglomerate beds maintain their thickness, whereas the thinner ones tend to pinched out laterally. This lithofacies has been frequently interpreted to have formed under storm influence in a supratidal to subtidal setting (Paik et al., 1991), or in a deeper middle ramp to basinal environment (Choi et al., 1993). On the other hand, Chough et al. (2001 ) and Kwon et al. (2002) suggested that most, i f not all,
Representative lithology of the Lower Ordovician Mungok Formation, Yongwol, Korea. Ribbon rock lithofacies composed of an alternation of calcareous and argillaceous layers. The light layer is composed of lime mudstone/wackestone to packstone, whereas the dark layer contains more argillaceous components. This litho facies is commonly expressed as planar, lenticular, and nodular to flaser bedded, based on varying proportions of the clastic and carbonate components. Lens cap is 5 cm in diameter. Grainstone/packstone lithofacies represented by medium- to thick-bedded and massive to poorly stratified dolostone. Hammer is 26 cin long. Photomicrograph of the grainstone/packstone showing vague outlines of allochems. Open nicols. Black bar for scale is 0.2 mm long. Limestone conglomerate lithofacies, showing weakly imbricated and edgewise clast orientation. Lens cap is 5 cm in diameter. Amalgamated limestone conglomerate beds with chaotic clast orientation. Pencil is 10 cm in long.
Plate
10
47
48
of the limestone conglomerates in the Mungok Formation are not a product of storm activity, but are pseudoconglomerates that were formed by diagenetic processes. Thus, little emphasis should be given to the limestone conglomerate facies in interpreting the depositional environment. This lithofacies is barren of fossils. 3.4
Marlstone/Shale Facies
The marlstone/shale lithofacies is greenish gray to dark gray in color and ranges fi'om 20 cm to 2 m in thickness (PI. 11/1). The bed generally starts with greenish gray massive shale and with increasing micritic content grades into crudely bedded marlstone in the upper part, although occasionally the reverse arrangement is also observed. The crudely bedded marlstone frequently grades upward into the ribbon rock lithofacies, which is in turn overlaiq by the limestone conglomerate lithofacies. This lithofacies was deposited in a low-energy deep ramp to basinal environment (Choi et al., 1993). The Shumardia Biofacies occurs in the marlstone/ shale lithofacies.
4 PATTERNS OF FACIES CHANGES 4.1 Depositional Facies
As described above, rocks of the Mungok Formation are divided into four dominant lithofacies: i.e., 1) ribbon rock, 2) grainstone/packstone, 3) limestone conglomerate, and 4) marlstone/shale lithofacies. Based on the association of these lithofacies, four depositional facies are recognized in the Mungok Formation: they are lagoonal/restricted marine, shoal, inner shelf, and outer shelf facies (Figs. 2 and 3). The succession of the depositional facies indicates that the Mungok Formation comprises two systems tracts composed of five depositional sequences. 4.1.1
Lagoonal/Restricted Marine Facies
The lagoonal/restricted marine facies occurs at two levels within the Mungok Formation (Fig. 2), both within the
Plate Fig. 1. Fig. 2. Fig. 3. Fig. 4. Fig. 5. Fig. 6. Fig. 7. Fig. 8.
II
basal Karam Member. The lower one occurs at the basal part of the Mungok Formation, immediately above the light gray to gray massive dolostone of the Wagok Formation. This interval ranges from 4.4 to 17.1 m thick and is made up of several meter-scale, shallowing-upward successions, each of which is a variable association of ribbon rock, limestone conglomerate, or grainstone/packstone facies (Figs. 2 and 3). The Yosimuraspis Biofacies recognized in this interval includes Yosimuraspis. Jujuyaspis, and Elkanaspis (Kim and Choi, 2000a). The upper lagoonal/restricted marine facies measures from 6.9 to 24.2 m in thickness and occurs at the uppermost part of the Karam Member (Fig. 2). The lithology in this interval is primarily ribbon rock with occasional intercalation of thin limestone conglomerate or grainstone/packstone beds (PI. 11/2). Vertical and horizontal burrows, normal grading, and cross-lamination occur more abundantly in this facies (P1. 11/3). 4.1.2 Shoal Facies The shoal facies also occurs at two levels in the Mungok Formation. The lower interval (8.2-20.4 m) occurs in the middle of the Karam Member between the two packages of lagoonal/restricted marine facies (Fig. 2). This interval is dominated by massive to crudely bedded grainstone/packstone with some reddish brown argillaceous seams in the lower part and chert layers or nodules in the upper part. Each grainstone/packstone bed has a sharp lower boundary and often show normal grading, bioturbation, and cross-lamination. Quartz grains are locally common in the lower part and are mostly of subangular to subrounded silt or sand (P1.11/ 4). The chert layer of the upper part is 5-10 cm in thickness and has a good lateral continuity (PI. 11/5). Cherts are generally black but often purple or white, and show distinct parallel lamination. Relict sponge spicules or fragments of trilobites or brachiopods are preserved in the chert. The upper package of shoal facies (28.6-35 m thick), equivalent to the Paeiljae Member, is characterized by a monotonous sequence of light gray to dark gray, massive to crudely bedded dolostone (PI. 10/5), which is thought to be composed mostly of oolites and peloids (PI, 10/6). Burrows,
Representative lithology of the Lower Ordovician Mungok Formation, Yongwol, Korea. Marlstone/shale lithofacies. Lens cap is 5 cm in diameter. Successive occurrence of ribbon rock and grainstone/packstone beds in the upper lagoonal/restricted marine facies. Hammer is 26 cm long. Burrow-mottled ribbon rock beds in the upper lagoonal/restricted marine facies at the Karam section. Pencil for scale is 14 cm long. Photomicrograph of the grainstone/packstone lithofacies with quartz grains in the lower shoal facies. Crossed nicols. Black bar is 0.5 ram. Chert-bearing grainstone/packstone beds in the lower shoal facies. Hammer is 26 cm long. Alternating occurrence of ribbon rock and limestone conglomerate beds in the lower inner shelf facies. Lens cap is 5 cm in diameter. Typical lithologic successions of grainstone/packstone, ribbon rock, limestone conglomerate, and marlstone/ shale beds observed in the outer shelf facies. Hammer is 26 cm long. Dark-colored marlstonc/shale bed underlain by limestone conglomerate and ribbon rock beds in the outer shelf facies. Pencil is 14 cm long.
Plate
11
49
50
Plate
Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig.
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. I1. 12. 13. 14. 15. 16. 17. 18. 19. 2(1. 21. 22. 23. 24.
12
Representative trilobites from the Yosimuraspis, Kainella, and Shumardia biofacies fl'om the Mungok Formation, Yongwol, Korea. The name of taxon is followed by morphology of fossil, fossil locality, repository number, and magnification.
Yosimuraspis vulgaris Kobayashi, 1960, cranidium, Kiwagol section, SNUP 524, X 1.8. Yosimuraspis vulgaris Kobayashi, 1960, pygidium, Kiwagol section, SNUP 527, X 3.7. Jujuyaspis sinensis Zhou in Chen et al., 1980, cranidium, Karam-I section, SNUP 537, X 1.5. Jujuyaspis sinensis Zhou in Chen et al., 1980, pygidium, Kalchijae section, SNUP 513, X 3.4. Elkanaspisjilinensis Qian in Chen et al., 1985, cranidium, Kalchijae section, SNUP 506, X 10. Agnostidae gen. and sp. indeterminate, cephalon, Songhwangdong section, SNUP 473, X 6.1.
Kainella euo, rachis Kobayashi, 1953, cranidium, Chommal-I section, SNUP 454, X 0.9. Kainella eurvrachis Kobayashi, 1953, pygidium, Songhwangdong section, SNUP 471, X 1.3. Leiostegium sp., latex cast of cranidium, Songhwangdong section, SNUP 468, X 1.2. Leiostegium sp., pygidium, Songhwangdong section, SNUP 469, X 2.5. Micragnostus coreanicus Kobayashi, 1960. cephalon, Chungsan section, SNUP 301, X 14. Micragnostus coreanicus Kobayashi, 1960, pygidium. Chungsan section, SNUP 306, X 11.5. Shumardia pellizzarii Kobayashi, 1934, cranidium, Omandong section, SNUP 582, X 18.2. Shumardia pellizzarii Kobayashi, 1934, pygidium, Paeiljae section, SNUP 585, X 20. Hystricurus megalops Kobayashi, 1934, cranidium, Omandong section. SNUP 568, X 5.3. Hystricurus megalops Kobayashi, 1934, pygidium, Karam-I section, SNUP 573, X 8. I. Asat~hellus sp,, cranidium, Chungsan section, SNUP 416, X 3.4. Asaphellus sp., pygidium, Paeiljae section, SNUP 491, X 4.4. Dikelokephalina asiatica Kobayashi, 1934, cranidium, Paeiljae section, SNUP 497, X 8.4. Dikelokephalina asiatica Kobayashi, 1934, pygidium, Paeiljae section, SNUP 545, X 2.0. Apatokephalus hyotan Kobayashi, 1953, cranidium, Omandong section, SNUP 556, X 8.2. Hukasawaia o'lind#qca Kobayashi, 1953, cranidium, Omandong section, SNUP 567, X 21.7. Koraipsis spinus Kobayashi, 1934, cranidium, Paeiljae section, SNUP 483, X 3.4. Koraipsis spim,s Kobayashi, 1934, pygidium, Karam-II section, SNUP 485. X 2.4.
cross-bedding, and lamination are relatively well preserved in places (e.g., Songhwangdong section). No fossils were recovered fi'om this facies. 4.1.3
Inner Shelf Facies
The inner shelf facies occurs primarily in the Chommal Member, which displays a sharp contact with the underlying massive dolostone of the shoal facies and grades upward into the outer shelf facies of the overlying Tumok Member. This interval consists of meter-scale, shallowing-upward successions, which are association of ribbon rock and limestone conglomerate or grainstone/packstone beds (Figs. 2 and 3; PI. 11/6). The limestone conglomerate beds are less frequently intercalated in the middle and upper parts, while instead the grainstone/packstone beds become predominant. The top of the inner shelf facies is marked by the occurrence of grainstone/packstone bed containing a few chert lenses or layers. Relict sponge spicules are preserved as ghost structures in the chert. The Kainella Biofacies was documented in the Chommal Member (inner shelf facies) (Kim and Choi, 1999). A narrow (12.8-15.3 m thick) interval of inner shelf facies is recognized at the uppermost part of the Mungok Formation in the two sections: one is from the Tumok section, where the ribbon rock is associated with reddish grainstone/packstone, and the other from the Namaeri section which comprises ribbon rock, grainstone/packstone, limestone conglomerate, and red shale beds (Fig. 2 and 3). This interval is barren of ff)ssils.
4.1.4 Outer Shelf Facies The outer shelf facies (41.2-77 m thick) occupies most of the Tumok Member (Fig. 2) and comprises ribbon rock, grainstone/packstone, limestone conglomerate, and marlstone/shale (P1.11/7-81. This facies constitutes meter-scale, shallowing-upward cycles. The outer shelf facies is distinguished primarily from the inner shelf facies by the occurrence of greenish gray to dark gray marlstone/shale beds. The lower part of this facies is represented largely by cycles composed of marlstone/shale, ribbon rock, and limestone conglomerate beds in ascending order (Figs. 2 and 3). Occasionally, the arrangement of marlstone/shale, limestone conglomerate, and ribbon rock beds from bottom to top is also observed. The upper part of this facies is characterized by frequent incursions of grainstone/packstone beds, while the ribbon rocks are less commonly observed (Figs. 2 and 3). The outer shelf facies contains the Shumardia Biofacies, which is characterized by cosmopolitan trilobite taxa with some endemic species (Choi et al., 1994). 4.2
Trilobite Biofacies
Biofacies are defined on the relative abundance of genera taken from either a single bed or a short stratigraphic interval (Ludvigsen and Westrop, 1983; Ludvigsen et al., 19861. Trilobite abundance for each collection in this study is based oil smaller numbers of cranidia and pygidia for polymeroids and of cephala and pygidia for agnostoids.
Plate
12
51
Biofacies analysis of the Mungok Formation indicates that three distinct biofacies are easily recognized in the |()Ul'teen collections (Table 1). Because of the strong numerical dominance of the eponymous genera and their stratigraphic separation, these biofacies are readily apparent and do not need to be further analyzed by statistical methods, h should also be mentioned that most of the genera obtained fi-om the Mungok Formation are monospecific. 4.2.1
Yosinmrasl)is Biofacies
The Yosimura.v~is Biofacies has a low diversity and high species dominance of trilobites (Kim and Choi. 2 0 0 0 a
PI. 12). Yo.~imltra.vl~i.~ vtdA, aris Kobavashi, 1960 reF,rcscnt:+, 73-96 percent of the Iolal Iaulla. whereas Yt(/uya,',pi.s ,',ilwn.~i.~ Zhou in Chen et al., 198()and ElkaJta.vl)i.v.iilineJmi,s Qian in Chen ei al.. 1985 together cons/itute 4-27 percent (Fig. 4). The genus Yoximttra.sl~is has been reported from North China (Chen et al.. 1980, 1985: Zhou and Zhang, 1985: Duan cI al.. 1986), South China (Peng, 1990). and Australia (Shergold, 2000). Its association with Ji(/tt.va,sl~i.s and~or Elkana,v~i.s has also been docurnented in North Cl+tina (Zhou and Zhang. 198..5: Duan el al., 1986). The dominance of this endemic gellus su,~.~esls that the Yo.simura.sl)is Biofacies represertts a restricted t+narilm en',iron~llent isolated fl+om the open ocean. However. the sporadic occurrence o1 the pan-
52
demic Jujuyaspis indicates intermittent connections with the open sea. The Yosimuraspis Biofacies occurs in six collections (Table 1), and is confined stratigraphically to the lowest several-meter interval of the Karam Member. Brachiopods are locally abundant at certain horizons and include Obohts, Palaeobolus, Westoniu, and Eoorthis. 4.2.2 Kainella Biofacies The Kainella Biofacies consists predominantly of Kainella euo, rachis Kobayashi, 1953, which makes up 85 to 100 percent of the fauna (Fig. 4; PI. 12). Leiostegium sp. composes as much as 15 percent of the facies, while agnostid gen. and sp. indeterminate is rare. Kainella has been recorded from North America (Lochman, 1965; Pratt, 1988; Dean, 1989), South America (Kobayashi, 1935; Harrington and Kay, 1951; Harrington and Leanza, 1957), and Asia (Kobayashi, 1953; Duan et al., 1986; Kim and Choi, 1995). Similarly Leiostegium is known from China (Zhou and Zhang, 1978, 1985; Chen et al., 1980: Peng, 1984; Chen et al., 1985; Duan et al., 1986; Chen et al., 1988: Lu and Zhou, 1990). Australia (S hergold, 1975; Jell, 1985), South America (Harrington, 1937; Harrington and Leanza, 1957; Pribyl and Vandk, 1980), and North America (Hintz, 1953; Pratt, 1988; Dean, 1989). The abundance of these widespread genera suggests that the depositional environment of the Kainella Biofacies was less restricted than that of the Yosimuraspis Biofacies. This biofacies also has a low diversity and high species dominance, and is confined to the lowermost ribbon rock bed of the Chommal Member. This biofacies is recognized in four collections (Tablel). In addition, brachiopods and machaeridians occur in this biofacies.
percent) and Koraipsis (23 percent); and the upperAsaphellusrich collection is characterized by dominance of AsapheIlus (90 percent). The Asaphellus/Koraipsis-fich collection and Asaphellus-rich collection are only documented from the Karam section and differ fi'om the Shumardia-rich collection by the relative dominance of Asaphellus. This variation can be attributed to either real ecologic differences or taphonomic sorting (e.g. see Westrop, 1986). Cranidia or pygidia of Asaphellus and Koraipsis (commonly exceeding 10 ram) are much larger than those of Shumardia (usually less than 1 mm) in size. In addition, the Asaphellus- and Asaphellus/ Koraipsis-rich collections commonly occur as shell-beds densely packed with numerous skeletal parts. Thus, the Asaphellus- and Asaphellus'/Koraipsis-rich collections are interpreted as taphonomic variants of the Shumardia Biofacies.
5 DISCUSSION
The Mungok Formation was formed in a carbonate shelf environment, while sedimentary features indicating slope- to basinal settings are not identified (Choi et al., 2001). Lithologic associations define four depositional facies: i.e., lagoonal/restricted marine, shoal, inner shelf, and outer shelf enviromnents (Figs. 2 and 3). The vertical successions of these lithofacies reveal two systems tracts composed of five 4th- or 5th-order depositional sequences (cf. Tucker, 1993) (Fig. 5). The lower highstand systems tract (59.0-84.2 m thick) is bounded below by the Wagok-Mungok lithostratigraphic boundary, which is considered as a transgressive surface
4.2.3 Shumardia Biofacies The Shumardia Biofacies occurs mainly in greenish gray to dark gray marlstone/shale beds of the Tumok Member (Fig. 2). This biofacies is characterized by frequent occmTence of cosmopolitan taxa including Micragnostus coreanicus Kobayashi, 1960, Shumardia pellizzarii Kobayashi, 1934, Asaphellus spp., Hvstricurus megalops Kobayashi, 1934, Hystricurus sp., Dikelokephalina asiatica Kobayashi, 1934, and Apatokephalus hyotan Kobayashi, 1953 (Pl. 12). Endemic forlns include Koraipsis spinus Kobayashi, 1934 and Hukasawaia cylindrica Kobayashi, 1953. The Shumardia Biofacies is more diverse than the other biofacies and occurs through a relatively thicker stratigraphic interval. The combined features of lithofacies and trilobite assemblage of the Shumardia Biofacies indicate that this biofacies was developed on outer shelf environments. Apart from trilobites, brachiopods, ostracods, pehnatozoan stems, and fossils of uncertain affinity were recovered commonly from this interval (Kobayashi, 1960: Kim et al., 1990). Three subgroups can be recognized in the Shumardia Biofacies (Fig. 4); the lower Shumardia-rich collections are composed mainly of Shumardia (> 70 percent) with some Apatokephalus or Hystricurus; the middle Asaphellus/ Koraipsis-rich collection is dominated by Asaphellus (48
Table 1. Fossil localities of the Mungok Formation with their geographic coordinates and biofacies. Locality (collection)
Latitude-Longitude
Biofacies
Chorrunal-I
37"12'58"N 128"23'56"E Yosimuraspis
Paeiljac
37~
128~
Yosimuraspis
Kiwagol
37~
128~
Yosimuraspis
Omandong
37"15'04"N 128"25'04"E Yosimuraspis
Kalchijae
37"l 7'03"N I28*26'53"E Yosimuraspis
Karam-I
37~
Chommal-I
37*lY00"N 128*2Y47"E
Kamella
Songhwangdong
37~
128~
Kainella
Nodarigol
37~
128"26'52"E
Kainella
Karam-II
37*17'30"N, 128~
Chungsan
37~
Paeiljac
37"13'45"N, 128023'30"E
Shumardia
Omandong
37~
128"24'55"E
Shumardia
Karam-I
37~17'30"N, 128"25'50"F,
Shumardia
t 28"25'55"E
128~
Yosimuraspis
Kainella Shumardia
53
Fig. 4. Pie diagrams representing the relative abundance of trilobite laxa ill six collections from the gosimurasl)i.s Biofacics, lk)ttr collections from the Kainella Bio(acies, and six colleclions from Ihr Shul~zordir Biofacies. The fossil colleclions, the stcatigraphic locations of which are not indicaled in Fig. 2, represent spot samples, and 'their geographic locations are sht)\vn in Fig. IC. Abbreviations: Yv=Yosimuraspis vulgaris Kobayashi, 1960, Js=Jt(fi,yaslJLS.rb~ep~wsZhou i ll Chen ctal., 1980. E i=l:lkap~asvisjilinezzsis Qian in Chen el al., 1985, Ke=Kainella eHrvraclzi.s Kobayashi, 1953. Ls=l.eiosleo,iz~m sp., Ax=agnostid gen. and sp. indeterminate, Mc=Micragnostus coreanicus Kobayashi, 1960. Sp=Shuma~zlia pelliz,garii Kobayashi, 1934. l-hn=ttyslricurl~,~ megalops Kobayashi, 1934 and Hystricurus sp., Da=Dikelokepholino asiatico Kobayashi, 1934. Ah=Al)atokel~haluv 1~3"otanKobayashi, I g53. As=Aseq)/~ellus spp., Ks=Koraipsis spinus Kobayashi, 1934, llc=Hukasawaia cvlindrica Kohayashi, 1953. by sharp facies change from grainstone/packstone to ribbon rock, and the presence of vuggy porosities widespread in the uppermost part of the W a g o k Formation. This systems tract consists of three 4th- or 5th-order depositional sequences (parasequences) (Fig. 5): the lower parasequence comprising lower lagoonal/restricted marine facies consists of several meter-scale, shallowingu p w a r d c y c l e s ; the s t a c k i n g pattern of the m i d d l e parasequence displays a general shallowing-upward trcnd
from shoal facies to lagotmal/rcstrictcd lnarinc facies; and lhe upper parasequcncc comprises shoal facies, representing a shallowing-upward cycle. These three parasequcnces arc interpreted as a highsland systems tract based on a gradual shallowing-upward trend and the presence o1" sequcnce boundary al the lop of the ~,Jpper par~tscqucnce. The upper lowstand syslems tract (71.2- 123.0 m lhick) comprises the upper hall of the Mungok Formation and consists of two 4th- or 5th-order depositional sequences
54
Fig. 5. Generalized columnar section of the Lower Ordovician Mungok Formation, showing the depositional facies, biofacies, sedimentary systems tract, and depositional sequences. For explanation of lithologic symbols, see Fig. 3. Abbreviations: FS=Flooding surface. (Fig. 5). The sharp discontinuity surface between shoal facies of the lower highstand systems tract and inner shelf facies of the upper lowstand systems tract is regarded as a sequence boundary. Just above this boundary, ribbon rock shows no vertical and horizontal burrows and cosmopolitan taxa such as Kainella and Leiostegium are dominated. This implies that depositional environment of the ribbon rock facies above this boundary is deeper than that below it. The stacking pattern of the inner shelf facies displays a general shallowing-upward cycle from ribbon rock to limestone conglomerate or grainstone/packstone beds. The fifth parasequence is composed of outer shelf facies and inner shelf facies (Fig. 5). Initially, the rate of sea-level rise became accelerated and thus carbonate production failed to keep pace with the sea-level rise. As a consequence, marlstone/shale lithofacies formed in outer shelf environment. With time, the sea-level rise has slowed down, while the accommodation space was filled, with increasing carbonate production, by ribbon rock and grainstone/packstone. The grainstone/packstone lithofacies of the upper inner shelf facies is characterized by massive to poorly stratified dolostone. The inner shelf facies grades upward into the tidal-flat environment of the Yonghung Formation (Yoo and Lee, 1997), suggesting that the upper boundary of the
lowstand systems tract may lie somewhere within the Yonghung Formation. In the Mungok faunas, the association of biofacies changes with changes in lithofacies suggests that they simply reflect lateral, facies-related shifts accommodating the sea-level rise. The depositional environments of the Yosimuraspis, Kainella, and Shumardia biofacies were considered to represent lagoonal/restricted marine, inner shelf, and outer shelf environments, respectively (Fig. 5). The Yosimuraspis and Kainella biofacies are characterized by abundance of nominal taxa, and low species diversity, whereas the Shumardia Biofacies comprises pandemic forms, with relatively high species diversity and moderate species equitability (cf. Ludvigsen and Westrop, 1983).
6 CONCLUSIONS The Early Ordovician Mungok Formation comprises four major lithofacies such as ribbon rock, grainstone/ packstone, limestone conglomerate, and marlstone/shale. Based on these lithofacies associations, four depositional facies are recognized: lagoonal/restricted marine, shoal, inner shelf, and outer shelf environments. The stacking
55
pattern o f these f a c i e s in the M u n g o k F o r m a t i o n represents
two systems tracts consisting of five depositional sequences. The lower highstand systems tract, which is bounded below by a transgressive surface at the boundary between the Wagok and the Mungok formations, is characterized by successive occurrence of lagoonal/restricied marine facies and shoal facies, and comprises three 4th- or 5th-order parasequences. The upper lowstand systems tract consists of inner shelf facies, outer shelf facies, and inner shelf facies in ascending order, and is divided into two 4th- or 5th-order parasequences. Three distinct trilobite biofacics are recognized in the Mungok Formation, re[erred from old to young to the Yosimuraspis, Kainella, and SDumardia biofacies. The gosimuraspis Biofacies consists dominantly of Yosimura.v)is and subordinately of Jujuyaspis and Elkanospi,v. The dominance of endemic genus Yosimuraspis indicates that the Yosimuraspis Biofacies developed in lagoonal/rcstrictcd marine environment, with intermittent connection to the open sea. The Kainella Biofacies is characterized by the occurrence of cosmopolitan taxa Kaitwlla and Leiostegium, and represents an inner shelf setting. The Stmmardia BiGfacies comprises a number of cosmopolitan trilobite taxa such as Micragnostus, Asaptwl lus, SITuma,dia, tlystrict.4ru.v, Apatokephalus, and DikelokepDalino, and indicates an outer shelf environment. ACKNOWLEDGMENTS
We are grateful to J. F. Taylor (Indiana University of Pennsylvania) for his meticulous and constructive cornments on the manuscript. Thanks arc extended to E. F10gelKahler for editorial suggestions, which greatly improved the final version of the manuscript. This siudy was supported by BK 21 Project. This is a contribution to IGCP 410 "The Great Ordovician Biodiversification Event".
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Manuscript received October 10, 2001 Revised mansucript received March 3, 2002
