Sequence stratigraphy of the Middle Cambrian Daegi Formation (Korea), and its bearing on the regional stratigraphic correlation

Sedimentary Geology 191 (2006) 151 – 169 www.elsevier.com/locate/sedgeo

Sequence stratigraphy of the Middle Cambrian Daegi Formation (Korea), and its bearing on the regional stratigraphic correlation Min Sub Sim ⁎, Yong Il Lee School of Earth and Environmental Sciences, Seoul National University, Seoul 151-747, Republic of Korea Received 10 August 2005; received in revised form 7 December 2005; accepted 8 March 2006

Abstract The sediments that comprise the Middle Cambrian Daegi Formation in eastern central Korea are interpreted to have been deposited in a carbonate ramp setting. The Daegi Formation consists of nine lithofacies; argillaceous rock, ribbon rock, oolitic limestone, skeletal limestone, microbial limestone, intraclastic limestone, micritic limestone, bioturbated limestone and crystalline limestone, which are grouped into three facies associations presenting, from base up, outer ramp, grain shoal and inner ramp facies associations. Two depositional sequences can be distinguished in the Daegi Formation. Sequence 1 in the lower part of the Daegi Formation comprises a lower transgressive systems tract (TST) and an upper highstand systems tract (HST). The TST consists of terrigenous deposits with subordinate limestones, while the HST contains various types of limestone. Sequence 2 comprises the upper half of the Daegi Formation and was deposited during a TST. The sequence stratigraphic analysis of the Daegi Formation suggests that the sediments were deposited during one 3rd-order sea-level change, during the middle to late Middle Cambrian. The sequence stratigraphic framework of the Daegi Formation can be excellently correlated with coeval deposits in North China, the Zhangxia and the lower part of the overlying Gushan formations, which belong to the same tectonic block as the Korean Peninsula, the Sino-Korean Block (SKB). The almost identical sequence stratigraphic frameworks in two separate basins suggest that the sequence stratigraphic development on the SKB was mainly influenced by eustatic changes during the Middle Cambrian. © 2006 Elsevier B.V. All rights reserved. Keywords: Cambrian; Carbonate; Korean Peninsula; Ramp; Sequence stratigraphy; Sino-Korean Block

1. Introduction Since the global sea-level model was proposed by Vail et al. (1977), it became widely accepted that sequence stratigraphy offers a unique concept to divide the rock record into chronostratigraphic units, and provides a global framework for geochemical, geochronological, palaeontological and facies analysis. In sequence stratigraphy, however, the major controls on deposition are not only the changes of eustatic sea-level but also ⁎ Corresponding author. Tel.: +82 2 878 0252; fax: +82 2 871 3269. E-mail address: [email protected] (M.S. Sim). 0037-0738/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2006.03.016

local tectonics and variations in sedimentary processes. Carbonate platforms develop in a whole range of geotectonic settings, but generally prefer passive continental margins, where subsidence is generally slow. Hence, a detailed analysis of the sea-level histories recorded in carbonate ramp successions would make it possible to identify and qualify the eustatic component among controls on sequence formation. Once collected in many different basins, it can help to understand global sea-level changes. The present study deals with the carbonate rock succession of the Middle Cambrian Daegi Formation in the Teabaeksan Basin which crops out in the central

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eastern part of the Korean Peninsula (Fig. 1). So far, various interpretations on the depositional environment of the Daegi Formation have been proposed. Yun (1978) proposed that the Daegi Formation comprises repeated transgressive and regressive strata deposited in a shallow-marine environment and that dolomitic limestones in the upper part were deposited in a lagoon environment. Kim and Park (1981) also proposed that the Daegi Formation was deposited in shallow-marine environments near wave base, whereas Park and Han (1986a, 1987a,b) and Park et al. (1987) interpreted that it was deposited in a wide range of depositional settings varying from tidal flats to continental slope. The objectives of this study are to interpret the depositional

environment and sequence stratigraphic framework of the Daegi Formation, and to compare its depositional environments and the sequence framework with those of the coeval successions of the North China platform which belongs to the same tectonic block with the Korean Peninsula. On the basis of these results, eustatic sea-level changes during the middle to late Middle Cambrian will be interpreted and the validity of the global sea-level model will be evaluated. 2. Geological setting The Daegi Formation (Middle Cambrian) consists mostly of carbonate rocks with interbedded siliciclastic

Fig. 1. Simplified geological map of the study area (modified after GICTR, 1962). The studied sections are Seokgaejae, Sangdong, Nammyeon, Imgye and Dogye in clockwise direction.

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rocks and forms part of the lower Palaeozoic Joseon Supergroup in the Taebaeksan Basin located in eastern central Korea (Fig. 1). The Joseon Supergroup unconformably overlies Precambrian granitic gneiss and metasedimentary rocks, and is unconformably overlain by the Carboniferous to Triassic Pyeongan Supergroup. In the study area, the Joseon Supergroup comprises the Jangsan, Myobong, Daegi, Hwajeol, Dongjeom, Dumugol, Makgol, Jigunsan and Duwibong formations with decreasing age (Fig. 2). The lower four formations were deposited during the Cambrian and the upper five were deposited during the Ordovician (Kobayashi, 1966). It has been however proposed that the Jangsan Formation, the lowermost unit, may not belong to the Joseon Supergroup but to the late Proterozoic strata (Kim and Lee, 2003). Tectonically, the lower Palaeozoic, when the Joseon Supergroup was deposited, is regarded as a stable cratonic period (Lee and Lee, 2003), and the occurrence of mature quartzose sandstones in the lowermost (Jangsan Formation) and middle (Dongjeom Formation) parts of the Joseon Supergroup suggests that the basin margin was relatively stable. On a large scale, the part of the Korean Peninsula in which the study area is situated forms part of the Sino-Korean (North China) Block (SKB; Kobayashi, 1966; Reedman and Um, 1975) that formed part of Gondwanaland before breakup (Metcalfe, 1996). Development of extensive carbonate platform deposits in the SKB (Feng et al., 1989; Meng

Fig. 2. Stratigraphic subdivision of the Joseon Supergroup in the study area (modified after Cheong, 1969).

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et al., 1997) indicates that the SKB was a stable setting such as a passive margin or cratonic interior. The Daegi Formation is about 200–300 m thick and is exposed over a large area around Taebaek (Fig. 1). It is composed mainly of milky white to light gray, massive limestone, and oolitic and dolomitic limestone. Limestone beds are generally poorly bedded and mostly pure in composition. However, the lower part of the Daegi Formation usually has a relatively thick shale bed. The Daegi Formation conformably overlies the early Middle Cambrian Myobong Formation which is composed of dark gray to greenish gray slate and shale with intercalations of thin sandstone and limestone beds in the middle part. The Myobong Formation is interpreted to have been deposited up sequence in outer to inner shelf, and to carbonate platform setting (Park et al., 1994). The Daegi Formation is in turn overlain conformably by the Upper Cambrian Hwajeol Formation which is mostly composed of ribbon rock and flat-pebble conglomerate deposited in a relatively deep-marine environment (Park, 1985; Park and Han, 1986b). Trilobite biozones indicate that the age of the Daegi Formation is Middle Cambrian (Kobayashi, 1966). 3. Depositional settings 3.1. Facies analysis Three outcrop sections and four drill cores were used in this study. They are located, from east to west at Seokgaejae, Dogye, Sangdong, Imgye and Nammyeon (Fig. 1). Outcrops and drill cores were measured in detail in terms of lithology, texture and sedimentary structures. The lower boundary of the Daegi Formation was designated by the first appearance of dark gray oolitic limestone and the upper boundary at the top of last oolitic limestone bed. This oolitic limestone represents a characteristic lithofacies of the uppermost part of the Daegi Formation and commonly contains micritized ooids and spar-filled oomoulds. More than 300 fresh rock samples were collected at regular intervals of about 2 m. From these samples, more than 100 thin sections were prepared for petrographic observation and geochemical analysis. Standard staining techniques using Alizarin Red-S and potassium ferricyanide were applied to discriminate between calcite and dolomite, and to test the presence of Fe, respectively. Based on lithology, texture and fossil and grain contents, the Daegi Formation is subdivided into nine lithofacies (Fig. 3) and some of them are locally dolomitized. They are: (1) argillaceous rock, (2) ribbon rock, (3) oolitic limestone, (4) skeletal limestone, (5) microbial limestone, (6) intraclastic limestone, (7)

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Fig. 3. Columnar sections of the Daegi Formation in the study area. Horizontal distance not to scale. Sequence boundary zone (SBZ) is marked by dotted lines. Solid lines are correlation lines using a combination of lithological units and sequence stratigraphic events (upper line: uppermost oolitic limestone, middle line: Renalcis-dominant microbial mat, lower line: transgressive surface at the base of argillaceous rocks).

micritic limestone, (8) bioturbated limestone, and (9) crystalline limestone. The following descriptions and interpretations of these lithofacies are summarized in Table 1. 3.1.1. Argillaceous rock The argillaceous rock lithofacies is about 20 m thick and occurs in the lower part of the Daegi Formation at

the Seokgaejae and Dogye sections (Fig. 3). This lithofacies consists of greenish gray or dark brown mudstone or siltstone. Generally, fossils and bioturbation structures are poor and although rare this lithofacies contains glaucony-bearing beds. Three sublithofacies can be distinguished on the basis of the relative abundance of siltstone and mudstone. The lowermost part of the argillaceous rock lithofacies is thin-laminated

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Table 1 Lithofacies and interpretated depositional environments from the Daegi Formation Lithofacies

Main features

Environments

(1) Argillaceous rock

Poorly fossiliferous, mud-silt alternation, basal scours and crosslamination of the siltstone layers, glaucony-bearing beds Nodular-shaped mudstone or wackestone delineated by clayey to marl flaser, laminated argillaceous part, a few echinoderms and ostracods Planar and trough cross-lamination, shallow-marine cements, partly including some oncoids Discoidal-shaped intraclast, space between clasts are filled with ooids and cements, commonly associated with oolitic limestone

Outer ramp to basin

(2) Ribbon rock (3) Oolitic limestone

(4) Intraclastic limestone (5) Skeletal limestone (5a) Echinoderm–brachiopod Echinoderms and brachiopods, mainly grain-supported, limestone syntaxial overgrowth cement, radiaxial fibrous calcite cement (5b) Trilobite limestone Trilobites and some ostracods, mainly matrix-supported, some Renalcis (6) Microbial limestone (6a) Epiphyton-dominated limestone (6b) Renalcis-dominated limestone (7) Micritic limestone (8) Bioturbated limestone (9) Crystalline limestone

Microbial mound, dendritic textures, shallow-marine cements, mainly associated with grain-dominated limestone facies Microbial mat, clotted structures, mainly associated with micrite-dominated limestone facies Massive but occasionally showing faint peloidal and clotted textures, few trilobites and ostracods, some Renalcis Pervasively bioturbated, bioturbated parts composed of dolomites, few bioclasts and peloids, some Renalcis –

greenish gray siltstone. It overlies the underlying skeletal grainstones with a sharp erosion surface. The middle part, which forms most of this lithofacies, comprises dark brown mud-dominant heterolithic facies containing thin layers of siltstone (2 to 10 mm thick) (Fig. 4A). Small basal scours and cross-lamination are common sedimentary features of the siltstone layers, and the siltstone grades upward into the overlying mudstone. Erosion surfaces are either undulose-concave or essentially flat and low-angle truncation of the underlying mudstone bed is a general feature associated with these surfaces (Fig. 4A). In contrast to the siltstone layers, dark brown mudstones lack any sedimentary structures. The upper part of the argillaceous rock is composed of siltstone containing a few carbonate nodules and shows a gradational contact with the overlying ribbon rock lithofacies. Argillaceous sediments are in general deposited in lowenergy environments. Without any firm evidence, Park et al. (1987a) interpreted that the argillaceous rock lithofacies was deposited in a mudflat environment. However, although rare, the occurrence of glaucony (Fig. 4B) suggests a low sedimentation rate and a depth in the range of 50–500 m (Odin and Fullagar, 1988). This suggests that the argillaceous rock lithofacies was deposited in a low-energy environment, below the photic zone where carbonate production was reduced dramatically

Outer ramp (Aigner, 1985; Torok, 1998) Grain shoal (Hine, 1977; Moshier, 1986) Grain shoal (Dravis, 1979)

Grain shoal to outer ramp (Heckel, 1972; Taylor and Wilson, 2003) Inner ramp (Heckel, 1972; Lefebvre and Fatka, 2003) Grain shoal to outer ramp Inner ramp Inner ramp (Choi and Simo, 1998) Inner ramp –

or absent. The presence of cross-laminated siltstone layers eroding underlying mudstones, however, indicates that the sea floor had been affected by the currents or storm events. In addition, the absence of subaerial exposure features also supports the deposition of argillaceous sediments in a full subtidal environment. Thus, it is interpreted that this lithofacies was deposited in open-marine, outer ramp environments, where occasional storms delivered silty sediments from the shallow platform. 3.1.2. Ribbon rock Ribbon rocks occur in the lower part of the Daegi Formation within the Seokgaejae, Dogye and Sangdong sections. This lithofacies is characterized by cm-thick light gray nodule-shaped limestone delineated by much thinner clayey to marl flasers (Fig. 5A). The ribbon rock mainly consists of lime mudstone to wackestone with a few echinoderm and ostracod fragments. Argillaceous flasers are several millimeters thick, generally parallel to the bedding plane and are composed of various clay minerals such as chlorite and illite. Argillaceous parts are often laminated. Some rounded quartz silt grains are disseminated in this lithofacies. The thickness of the ribbon rocks ranges from 0.5 to 5 m. Generally, the ribbon rocks overlie argillaceous rock gradationally or oolitic grainstone abruptly. This lithofacies grades upward to skeletal limestone.

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deposited is interpreted to be the transition zone between a clastic sedimentation-dominated zone and a carbonate sedimentation-dominated zone.

Fig. 4. (A) Slab photographs of argillaceous rock lithofacies showing thin layers of siltstone grading upward into the overlying mudstone (marked by solid arrows). Small basal scours (marked by open triangles) and cross-lamination (marked by a broken arrow) are observed in the argillaceous rock lithofacies. (B) Photomicrograph showing glauconite in the siltstone layer (G: glauconite, Q: quartz).

The formation of ribbon rocks is often related with diagenesis (e.g., Maurer and Schlager, 2003), but ribbon rocks are generally believed to have been deposited in an outer shelf/ramp environment (Aigner, 1985; Lee and Kim, 1992; Torok, 1998). First of all, the gradual transition from the underlying argillaceous rock lithofacies to this lithofacies supports this interpretation. The presence of lamination in the argillaceous flasers indicates the quiescent phase of sedimentation during the carbonate-free period, which suggests that the argillaceous flasers represent background terrigenous mud sediments that settled from suspension. On the other hand, the intercalation of limestone nodules suggests the periodic deposition of carbonate sediments delivered from the nearby carbonate platform, probably by storminduced offshore currents (e.g., Hips, 1998). Hence, the depositional environment in which the ribbon rocks were

3.1.3. Oolitic limestone Oolitic limestone is well exposed in all studied sites of the Daegi Formation. The thickness of this lithofacies ranges from 0.1 to 1.5 m but locally reaches several meters in the upper middle part of the Daegi Formation. In the lower part, oolitic limestone gradually overlies skeletal limestone lithofacies and is sharply overlain by argillaceous rock or ribbon rock. It occurs at the top of the depositional cycle composed of, from the base up, argillaceous sediments, ribbon rock, skeletal limestone and oolitic limestone. Locally, this lithofacies is found with intraclastic limestone whose intraclasts are composed of oolitic grainstone and shows small-scale crossstratification. Ooids commonly have a spherical shape but they may be broken. The ooids have a unimodal size distribution with an average diameter of about 0.2 mm. Cortex fabrics are not preserved in the ooids while the ooids often show up as spar-filled oomoulds or even are replaced by inclusion-rich sparry calcite or dolomite crystals (Fig. 5B). Some skeletal grains, commonly echinoderms, are also observed in this lithofacies. In addition, the oolitic limestone lithofacies in the lowermost part of the Daegi Formation at the Seokgaejae section includes many oncoids (Fig. 5C). Generally, these oncoids float in the oolitic matrix and their size increases with their abundance. Their size may reach several centimeters (Fig. 5C). Unlike the ooids, these oncoids preserved their concentric lamination made of micrite but do show a marked asymmetry in lamina width. The nuclei of oncoids are formed by fragments of coated grains, skeletal grains and peloidal limestone. In the upper middle part of the Daegi Formation, oolites are generally dolomitized and form a relatively thick bed reaching up to several meters. In all studied sections, the uppermost part of the Daegi Formation is composed of the oolitic limestones which mainly contain micritized ooids and spar-filled oomoulds. The oolitic limestone lithofacies is interpreted to have been deposited in current- or wave-agitated shallow subtidal environments forming ooid shoals (Loreau and Purser, 1973; Hine, 1977; Moshier, 1986). This interpretation is supported by the lack of micrite, the presence of cross-stratification, the stratigraphic relationship with the associated rocks and good sorting of ooids. The presence of broken ooids represents vigorous agitation in the depositional setting and the presence of intraclasts of oolitic grainstone suggests that early lithified ooids were broken and re-sedimented probably due to storm events.

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Fig. 5. Photograph and photomicrographs of the Daegi Formation. (A) Nodular lime mudstone lithofacies. (B) Oolitic limestone replaced by dolomite (D) selectively. (C) Oncoids in the oolitic limestone lithofacies. (D) Brachiopod showing partial replacement by quartz (Q). (E) Skeletal grainstone mainly composed of echinoderm fragments. (F) Skeletal wackestone containing trilobite (Tr) and brachiopod (Br) fragments.

Additionally, the occurrence and morphology of oncoids indicate that part of the grain shoal was under the relatively moderate energy conditions. 3.1.4. Skeletal limestone Skeletal limestones are distributed widely over the Daegi Formation distribution area and texturally they are diverse from wackestone to grainstone. At the Seokgaejae section, grain-supported skeletal limestones mainly occur in the lower part and matrix-supported skeletal limestones occur in the upper part of the Daegi Formation. The grain-supported skeletal limestones mainly contain brachiopod (Fig. 5D) and echinoderm fragments (Fig. 5E). Most echinoderms are pelmatozoans, and their fragments are commonly composed of large single calcite crystals and often doughnut-like columnal fragments. Commonly, this lithofacies contains diage-

netic features such as syntaxial overgrowths and fibrous calcite cements. Some skeletal grains in the grainsupported skeletal limestone are fragmented. In contrast, the skeletal grains of the matrix-supported skeletal limestone are mainly composed of trilobite fragments (Fig. 5F), ostracod fragments, and rare brachiopod. Renalcis is often observed. The thickness of the skeletal limestones varies from several centimeters to 7 m. The grain-supported skeletal limestone overlies ribbon rock and is sharply overlain by ribbon rock or gradually overlain by oolitic limestone. Commonly, matrixsupported skeletal limestone is associated with micritic limestone or bioturbated limestone. Skeletal limestones were probably diversified in various environments on the carbonate platform. Generally, grainsupported skeletal limestone is the characteristic of a highenergy setting (e.g., grain shoals, beaches) enough to

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remove micrite, whereas matrix-supported skeletal limestone is typical of a low-energy setting (e.g., deep-water areas, protected areas). The grain-supported skeletal limestones in the Daegi Formation is interpreted to have been deposited in the grain shoals on the basis of the presence of broken skeletal grains, the relationship with the associated rocks and the open-marine fauna. The abundance of brachiopods and echinoderms reflects a hard substrate with relatively strong sea-water agitation to grant some advantages to suspension feeders (Heckel, 1972; Taylor and Wilson, 2003). The matrix-supported skeletal limestones in the Daegi Formation are interpreted to have been deposited in a shallow part of the carbonate platform, based on abundant micrite contents, the presence of Renalcis suggestive of deposition in the photic zone probably in the platform interior part (Nguyen, 1986) and the stratigraphic or lateral relationship with the associated rocks. This interpretation is also supported by the abundance of trilobites whose lifestyle is relatively unrelated to the degree of sea-water agitation, more favoring muddy substrate than epibenthic suspension feeders such as brachiopods (Heckel, 1972; Lefebvre and Fatka, 2003). Therefore, the skeletal limestone in the Daegi Formation mainly comprises two types: (1) skeletal grainstone deposited in a sand shoal or platform margin setting and (2) skeletal wackestone mainly deposited in a shallow part of the carbonate platform. 3.1.5. Microbial limestone Microbial limestones occur commonly upward of the middle part of the Daegi Formation succession. In the middle part of the Seokgaejae section, microbial limestones occur with a lenticular shape or thick layer (Fig. 6A) of about 1 m thick. It is composed of pure limestone containing less argillaceous materials than enclosing lithologies and is characterized by dark gray mottles in milky white limestone part (Fig. 6A and B). Dark gray mottles are mainly composed of micritic microbial micro-fossils. The major frame builder of most microbial limestones in the middle part of the Seokgaejae section (Fig. 1) is Epiphyton with rare Girvanella and Renalcis, and occurs associated with oolitic limestone lithofacies. In the upper part of the section, however, the major frame builder is Renalcis and the microbial limestone is associated with micritic limestone lithofacies. Well-preserved calcareous algae are normally surrounded by nonferroan calcite cement. Blocky calcites often succeeded synsedimentary fibrous calcite cements. Compared to the well-preserved microbial structures at the Seokgaejae section, microbial preservation at other sections is rather poor due to recrystallization, resulting in aggrading neomorphism to micro-spar.

Microbial limestones mainly consisting of calcareous microbes are distributed widely in Cambrian carbonate platform deposits (Riding and Voronova, 1985) and Epiphyton and Renalcis are common components of Palaeozoic reefs. Commonly, Epiphyton is found in shelf-edge facies (Pfeil and Read, 1980; James and Gravestock, 1990), whereas Renalcis is generally abundant in calm conditions such as the interior of carbonate platforms (Tsien, 1979; Nguyen, 1986; James and Gravestock, 1990). It most likely reflects environmentally influenced characteristics of calcareous microbes, and Pratt (1984) also interpreted that the dendritic form exhibited by Epiphyton is favored in the environments where constant turbulence prevents colonies from beyond a certain size, and clotted form exhibited by Renalcis is favored in the environments where turbulence is relatively low. Hence, on the basis of environmentally influenced characteristics of calcareous microbes and the relationship with the associated rocks, Epiphyton-dominant microbial limestone is interpreted to have been deposited principally in a grain shoal and shallow outer ramp setting and Renalcis-dominant microbial limestone in an inner ramp setting. 3.1.6. Intraclastic limestone The intraclastic limestone lithofacies occurs in the uppermost part of the Sangdong section and in the middle part of the Nammyeon section (Fig. 1). Intraclasts are moderately sorted, and made of various carbonate rocks, mainly grain shoal carbonates such as oolitic grainstone and reworked intraclastic limestone. Ooids in the oolitic grainstone intraclast are truncated at clast margins. The spaces between intraclasts are mainly filled with ooids (Fig. 6E). These intraclastic limestones were sometimes completely dolomitized. Thickness of the intraclastic limestone bed is b 1 m. This lithofacies is commonly associated with oolitic grainstone. Intraclastic limestones could have been deposited in various environments on a shallow-marine carbonate platform (Demicco and Hardie, 1986). Among them, the subtidal deposition of this lithofacies is inferred from the close stratigraphic relationship with grain shoal carbonates and lack of subaerial exposure. The intraclasts are thought to have undergone cementation prior to their formation and transport, indicated by truncated ooid grains at clast margins. In modern cases, subaqueous clasts of cemented grainstones are common on the shelfmargin shoals such as the Bahama Bank (e.g., Dravis, 1979). Hence, intraclastic limestones in the Daegi Formation can be interpreted to be storm deposits formed after early lithification on the sea floor.

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Fig. 6. Photographs (A, B, E, F) and photomicrographs (C, D) of the Daegi Formation. (A) Microbial mounds whose outlines are marked by dotted lines. (B) Dark gray mottles (arrows) in microbial limestone. (C) Dendritic structure of Epiphyton composed of cluster of branching micritic rods. (D) Renalcis composed of grape-like aggregate of micrite spheroids. (E) Intraclastic dolomite (marked by dotted lines) with ooid matrix. (F) Bioturbated limestone containing some clots of Renalcis (arrows).

3.1.7. Micritic limestone Micritic limestone lithofacies mainly occurs in the middle part of the Seokgaejae section. It is composed of micrite and micro-spar with minor fossils, trilobites and ostracods, and minor bioturbation. Micritic limestone is commonly massive, but weakly shows faint peloidal and clotted textures. It often contains relics of unidentified grains which are irregularly shaped and filled with microspar precipitated in the mould. The thickness of micritic limestone ranges from 1 to 3 m and this lithofacies is commonly associated with Renalcis-dominant microbial limestone and other micrite-rich lithofacies. In general, lime mud accumulation suggests the deposition in a quiet subaqueous environment, and which is also supported by the lack of high-energy depositional structures. Low-energy environments are found in deep-

water areas below wave base or in lee of grain shoals on the carbonate platform. In the Daegi Formation, the stratigraphic and lateral relationship with the associated rocks precludes a deep-water interpretation. Therefore, a lowenergy, inner ramp environment protected from waves and storms by sand shoals or barriers is postulated for the deposition of this lithofacies. In addition, thickness, lack of argillaceous materials and scarceness of bioturbation indicate a relative higher carbonate production and accumulation rate (e.g., Choi and Simo, 1998). Although the origin of abundant micrite in this lithofacies is enigmatic, the presence of faint traces of microbe such as clotted texture suggests that parts of them were originated from disaggregation of calcareous microbe. In addition, inorganic precipitation might be another possible source of micrite.

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3.1.8. Bioturbated limestone Bioturbated limestones mainly occur in the upper part of the eastern Daegi Formation. This lithofacies is pervasively bioturbated and the bioturbated parts are represented by dolomites that show a brighter or more reddish color than unbioturbated limestones (Fig. 6F). The bioturbated part is composed of fine to coarse dolomite crystals, and the unbioturbated part consists mainly of micrite or micro-spar with a few bioclasts (i.e., trilobites) and peloids. Locally, the bioturbated limestone contains palaeokarstic features such as dissolution cavities, and some clots of Renalcis (Fig. 6F). The thickness of the bioturbated limestone ranges from 1 to 3 m. It is commonly associated with Renalcis-dominant microbial limestone and matrix-supported skeletal limestone, and is often overlain by the grain shoal carbonates. The abundance of micrite and the presence of trilobite fossils in this lithofacies suggest low-energy and normal marine conditions, respectively. The presence of Renalcis also indicates that bioturbated limestones were deposited in the photic zone, probably in the platform interior part (Nguyen, 1986). In addition, the stratigraphic and lateral relationship with the associated rocks supports an inner ramp interpretation. Therefore, the bioturbated limestone lithofacies is interpreted to have been deposited in an inner platform setting protected from agitating waves, where the sedimentation rate was relatively low.

3.1.9. Crystalline limestone Many carbonate rocks of the Daegi Formation were recrystallized probably by hydrothermal alteration such that the original fabrics and sedimentary structures are not preserved. Crystalline rocks are more abundant in the western part of the Daegi Formation distribution area than in the eastern part, and are mined for high-quality limestones for industrial use. Some crystalline rocks show preferred alignment of calcite crystals, suggestive of recrystallization under significant stress. 3.2. Daegi depositional system The lithofacies present in the Daegi Formation, except the crystalline limestone, are combined into three facies associations, each formed under a particular set of depositional conditions (Fig. 7). Facies association 1 is composed of micritic limestone, bioturbated limestone, Renalcis-dominant microbial limestone and matrix-supported skeletal limestone, which represents deposition in quiet shallow subtidal environments. Facies association 2 is composed of oolitic limestone, grain-supported skeletal limestone, Epiphyton-dominant microbial limestone and intraclastic limestone. This facies association represents deposition in wave- or current-agitated, shallow subtidal environments such as a grain shoal at or near wave base. Facies association 3 is composed of ribbon rock and

Fig. 7. Schematic diagram of three facies associations. Each facies association (FA) was formed under a particular set of lithofacies and depositional conditions.

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argillaceous rock, which is interpreted to have been deposited in a relatively deep subtidal to basinal environment below wave base. This suggests that the sediments composing the Daegi Formation were deposited in shallow-marine environments, except for facies association 3. This interpretation is consistent with the interpretations given in previous studies (Yun, 1978; Kim and Park, 1981; Park and Han, 1986b). So, the depositional system of the Daegi Formation is interpreted to be a ramp setting rather than a rimmed shelf, based on constituent lithofacies, gradational stratigraphic relationships between lithofacies and the absence of significant shelf-edge barriers or reefs (i.e., Burchette and Wright, 1992). The absence of intertidal or supratidal facies suggests that extensive shallow-water facies probably existed over the investigated area. Simplified facies belts distribution on the Daegi ramp is outlined in Fig. 8. 4. Sequence stratigraphy and sea-level change 4.1. Sequence stratigraphic framework Based on the vertical and lateral distributions of the facies associations, a sequence stratigraphic framework

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for the Daegi Formation was established. The construction dynamics of the Daegi carbonate ramp follows the model proposed by Burchette and Wright (1992). The depositional sequence and their boundaries are depicted in Fig. 3. Two 3rd-order depositional sequences are recognized in the Daegi Formation. The lower depositional sequence is well exposed in the lower part of the Seokgaejae, Dogye and Sangdong sections, and the upper depositional sequence is exposed across the entire study area. However, the two sequences are not bounded in the Daegi Formation but also include parts of the both underlying and overlying formations. The composite columnar section (Fig. 9) shows the development of sequences in the Daegi ramp system during the Middle Cambrian. 4.1.1. Sequence boundary Between sequences 1 and 2, a stratigraphic unit is observed showing a out-of-sequence lithofacies change across the different types of lithofacies stacking patterns. Below this interval, middle to outer ramp lithofacies are dominant with a progradational stacking pattern, while above the unit inner ramp lithofacies are dominant and aggradational stacking is more prominent. This sequential change in the Daegi Formation is interpreted to be the

Fig. 8. Schematic reconstruction of the Middle Cambrian Daegi ramp system showing the distribution of the faunal elements and sediment compositions.

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sequence boundary zone rather than a distinct boundary surface, because this interval usually consists of several meters of amalgamated parasequences exhibiting subaerial diagenetic features and it was very difficult to define a single surface due to the lack of distinguishing characteristics (e.g., Montanez and Osleger, 1993). This transition zone is represented by micritic limestone showing many dissolution pipes and the subsequent development of probable terra-rosa-like palaeosols (Fig. 10) suggestive of numerous subaerial exposure events under moist conditions. Such characteristics are well observed in the inner platform areas, whereas the Sangdong section representing the mid-platform shows little evidence for subaerial exposure. 4.1.2. Sequence 1 Sequence 1 is topped by the sequence boundary zone and the lower boundary may be placed in the underlying Myobong Formation. Sequence 1 is composed of a transgressive systems tract (TST) in the lower part and a highstand systems tract (HST) in the upper part and records the drowning of the terrigeneous source and subsequent establishment of the carbonate system. The TST comprises terrigenous deposits with subordinate ribbon rock, skeletal limestone and oolitic limestone, whereas the HST mainly consists of oolitic limestone, with bioclastic, micritic and microbial limestones. The glauconitic horizons in the thick argillaceous rock lithofacies in the lower part of TST suggest that this part of the succession represents the maximum flooding and condensed section of the sequence. Internally, sequence 1 is composed of numerous meter-scale cycles, which consist of ribbon rock at the base, and skeletal wackestone to packstone in the middle, and an oolitic or skeletal grainstone cap (Fig. 11A). In the lower part of this

Fig. 9. Representative columnar section, relative sea-level curve and depositional sequence of the Daegi Formation at the Seokgaejae section. For lithofacies symbols refer to Fig. 3.

Fig. 10. Outcrop photograph of dissolution pipes and terra-rossa-like palaeosols (marked by black arrows).

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composed of a lower more argillaceous part and an upper more calcareous part, which is similar to the meter-scale cycles showing a systematic upward increase in the proportion of calcareous sediments.

Fig. 11. (A) Outcrop photograph of cyclic deposition in sequence 1. (B) Outcrop photograph of the parasequence boundary showing argillaceous rock sharply overlying the oolitic skeletal limestone. Hammer for scale is 25 cm long.

sequence, however, cycles usually contain argillaceous rock lithofacies in the lowermost part (Fig. 11B). The ribbon rock and argillaceous rock in the lower part of the subtidal cycle were deposited in low-energy environments, deeper than wave base, and oolitic grainstone and skeletal packstone or grainstone were deposited in highenergy environments at or around wave base. The upper and lower boundaries of each cycle are relatively sharp (Fig. 11B), whereas internal boundaries between constituting lithofacies in a cycle are gradational. Cycles are asymmetric in that they exhibit progressive shallowing. The outer to middle ramp cycles are generated by progradation of shallow subtidal lithofacies over deeper subtidal part of the ramp, thus resulting in a shallowingupward trend. These meter-scale shallowing-upward cycles (5th-order paraseqences) are arranged into largescale cycle sets (4th-order cycle) up to about 15 m thick, and five of the latter cycles are recognized at the Seokgaejae section in the eastern part of the platform. Broadly, these large-scale cycles (parasequence sets) are

4.1.3. Sequence 2 Sequence 2 is bounded below by the sequence boundary zone and the upper boundary may be placed in the overlying Hwajeol Formation. Sequence 2 in the Daegi Formation comprises a TST (Fig. 3). TST deposits are well exposed in all studied sites of the Daegi Formation, and the maximum thickness is 80 m at the Seokgaejae section. In the lower part of the TST aggrading parasequences are present, while in the upper part retrograding parasequences can be found. In the eastern area, the lower TST is characterized by facies association 1 representing shallowwater protected environments, which is composed of micritic limestone, bioturbated limestone, Renalcis-dominant microbial limestone and matrix-supported skeletal limestone. The eastern part of the lower TST deposit was deposited in a shallow inner ramp setting landward of ooid shoals. These inner ramp skeletal wackestones are characteristic of shallow-water conditions in the photic zone, but in a protected setting where only low-energy waves could interact with the sediment (Gomez-Perez et al., 1998). In contrast to the skeletal wackestones, the micritic limestones with few skeletal grains are interpreted to present a more restricted and lower-energy environment. Contrasting with the eastern area, the lower TST deposit in the western part is mainly composed of thick oolitic grainstone beds, deposited in a grain shoal setting. It comprises an aggrading succession. Hence, the aggradational stacking of similar lithofacies suggests that the sedimentation rate kept up with the rate of sea-level rise. During the deposition of the upper TST, however, the eastern area of the Daegi Formation, which had been covered by protected low-energy platform environments during the deposition of the lower TST, was also characterized by oolitic grainstones like the western part. Subsequently, these oolitic sediments were overlain by the tempestite-dominant lithofacies (thin-bedded turbiditedominant lithofacies of Park and Han, 1985) of the overlying Hwajeol Formation. This suggests that the facies belts were pushed landward while the sedimentation rate could not keep up with the rapid rates of sea-level rise. 5. Eustatic sea-level change: implications for regional stratigraphic correlation The sequence stratigraphic concept is the idealization of unconformity-bounded packages (the sequences) to heighten the understanding of the geological controls on

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the deposition, especially with respect to the sea-level. Furthermore, it was asserted that an accurate sea-level history could be reconstructed from sequence analysis, leading to the concept of a global sea-level model (GSM), which could be applied to the interpretation of continental margin strata worldwide (Vail et al., 1977; Haq et al., 1988). The global sea-level model, however, remains a focus of controversy (Carter et al., 1991; Christie-Blick and Driscoll, 1995; Miall and Miall, 2001), and these debates mainly center on the roles of local geological processes (e.g., local tectonic movement, rate of subsidence, sedimentation rate, compaction, and so on), which also influence sequence geometry and timing of sequence development. Hence, a key concern to interpret the global sea-level model is whether a eustatic signal can be separated from other signals. Ideally, given a succession of sequences in a sedimentary basin, the only way to demonstrate that any of the sequence boundaries reflects global eustatic control is to demonstrate which of them can be correlated with boundaries in other basins that are not tectonically related with one another (Miall, 2005). In general, however, although relative changes in sea-level in the basins are apparent, the passionate contributions to relative sea-level changes between eustatic fluctuations and tectonic pulses are difficult to evaluate. Practically, therefore, tectonically stable basins, such as within cratonic basins, would be good candidates to extract a common eustatic component from a collection of relative sea-level records. As shown in this study, the Middle Cambrian Daegi Formation in the Korean Peninsula was deposited on a low-angle, relatively low-energy carbonate ramp setting with no significant slope break, and consists of two depositional sequences. Likewise, in North China which belongs to the same tectonic block as the Korean Peninsula, thick carbonate sediments were deposited during this period and also contain oolite-dominated ramp deposits (Meng et al., 1996). Such similar carbonate ramp systems distributed widely in different sedimentary basins on the same tectonic block (SKB) suggest that sea-level was relatively high, blocking supply of terrigeneous sediment to both basins and that there was little fault activity on the SKB to form a distinct break of slope. Previous studies have reported that during the Middle Cambrian sedimentary basins in North China and the Korean Peninsula were likely located in a cratonic interior mainly covered with shallow epeiric seas (Meyerhoff et al., 1991; Meng et al., 1997; Wan and Zeng, 2002; Lee and Lee, 2003), which suggests that the basin-forming mechanism was slow thermal subsidence in both regions during this period. In addition, in the absence of tectonic activity, such processes as tectonic

subsidence, terrestrial sediment supply, isostasy and compaction are not expected to generate 3rd- or higherorder sequence boundaries, but can act to modulate, within a limited range, the timing of sequences developed dominantly through eustasy (Christie-Blick, 1991; Reynolds et al., 1991). Therefore, the roles of tectonics in the relative sea-level changes could be considered to be relatively limited in both basins, and they would provide a testable case for identifying the eustatic sea-level changes during this period and a mean for examining the validity of the global sea-level model. In the Middle Cambrian carbonate succession of North China, three 3rd-order sea-level changes are recognized with an overall transgressive trend. Based on the trilobite biozones (Kang, 2004), the Daegi Formation is coeval to the Zhangxia Formation and the lower part of the overlying Gushan Formation in North China (Fig. 12). The Zhangxia Formation is interpreted as a 3rd-order sequence, and the Gushan Formation is interpreted as the transgressive deposit as a whole (Fig. 13). Therefore, the

Fig. 12. Correlation of the biozones of the Daegi Formation with those of North China (Kang, 2004).

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Fig. 13. Simplified columnar section and depositional sequences of the Zhangxia and Gushan formations, North China (modified after Meng et al., 1997).

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stratigraphic boundary between the Zhangxia and Gushan formations corresponds to the sequence boundary (Meng et al., 1997). Based on the trilobite biozones of the Middle Cambrian strata in both Korean and Chinese basins, the

sequence boundary between the Zhangxia and Gushan formations can be correlated with the sequence boundary in the Daegi Formation, which is located between the Amphton zone and the Cyclolorenzella zone (Fig. 14).

Fig. 14. Comparison of middle to late Middle Cambrian relative sea-level changes between the Korean Peninsula (Daegi Formation) (A) and North China (Zhangxia and Gushan formations) (B). Both the Daegi and Zhangxia formations contain four 4th-order cycles after maximum flooding event in the middle Middle Cambrian (see the boxed mark) (modified after Meng et al., 1996, 1997).

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The detailed sea-level curves of the Daegi and Zhangxia formations (Fig. 14) show that four 4th-order cycles are present in both basins after the maximum flooding event in the middle Middle Cambrian. After sequence boundary, the transgressive stage during the deposition of the Gushan Formation in North China agrees with the steady transgressive phase recorded in the upper part of the Daegi Formation and the lower part of the overlying Hwajeol Formation. Lithologically, the oolite-dominated cycles are more common in the lower part of the Gushan Formation and the storm-bed-dominated cycles are more common toward the top (Meng et al., 1997). Probably, the upper part of the Daegi Formation, which shows thick oolite beds, is well correlatable with the lower part of the Gushan Formation, and the lower part of the overlying Hwajeol Formation composed of tempestite-dominated cycles (thin-bedded turbidite-dominated cycles of Park, 1985; Park and Han, 1985) may be correlatable with the upper part of the Gushan Formation, based on the trilobite biozones (Kobayashi, 1966). As a result, the Middle Cambrian Daegi Formation and the coeval deposits in North China show a very similar trend in relative sea-level records. This indicates that these two separated sedimentary basins recorded an almost identical sea-level history with little influence from local factors, suggesting that a eustatic sea-level change influenced the depositional history in both basins. This agrees with the global sealevel curve that at least one 3rd-order eustatic sea-level fall is suggested during the middle to late Middle Cambrian, probably between the Amphton zone and the Cyclolorenzella zone (Kang, 2004). Conclusively, the coevality of sequence boundaries and eustatic sea-level changes recorded in these two separate basins supports the validity of the global sea-level model, and this study contributes to confirm the possibility that sequence stratigraphy can offer a unifying concept to divide the rock record into chronostratigraphic units. But, more documentation needs to be accumulated for the general application of the concept of the global sea-level model.

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Two 3rd-order depositional sequences are distinguished in the Daegi Formation, including parts of the underlying and overlying formations. Sequence 1 in the lower part of the Daegi Formation comprises a TST in the lower part and a HST in the upper part. It consists of five 4th-order cycles. A TST consists of terrigenous deposits with subordinate limestones and a HST mainly consists of various types of limestone. Rare glauconitic horizons in the lower part of the TST indicate that this part of the succession represents the maximum flooding zone. Sequence 2 comprises the upper half of the Daegi Formation and is mainly composed of TST. In the lower part of the TST parasequences stack aggradingly, while parasequences in the upper part stack retrogradingly. The sequence analysis of the Daegi Formation suggests that at least one 3rd-order eustatic sea-level change occurred during the middle to late Middle Cambrian. This sequence stratigraphic framework of the Daegi Formation can be well correlated with that of the coeval Zhangxia and Gushan formations in North China. With the tectonically stable setting of both basins and little apparent influence from other controlling factors such as terrestrial sediment supply, the sequence development on the SKB was mainly influenced by eustatic changes in sea-level during the Middle Cambrian. The results of this study support the validity of the global sea-level model and contribute to the understanding of the global sea-level model for the middle to upper Middle Cambrian. Acknowledgements This study was supported by the Korea Research Foundation Grant (KRF 2004-015-C00588). The authors are grateful to Dr. Y. Kim, K.H. Sur and J.C. Kim for their help in the field. The authors also thank Y. W. Lee, S.K. Hong and Y.J. Joo for their helpful and constructive comments. This paper benefited much from constructive comments from Drs. J. Reijmer, B.W. Sellwood and anonymous reviewers.

6. Conclusions The Middle Cambrian Daegi Formation is interpreted to have been deposited on a low-angle, relatively lowenergy carbonate ramp with no significant break of slope. The Daegi Formation consists of nine lithofacies: argillaceous rock, ribbon rock, oolitic limestone, skeletal limestone, microbial limestone, intraclastic limestone, micritic limestone, bioturbated limestone and crystalline limestone. These lithofacies are grouped into three facies associations; outer ramp, grain shoal and inner ramp facies associations.

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