Early Ordovician reefs from the Taebaek Group, Korea - Springer Link

Geosciences Journal Vol. 17, No. 2, p. 139 − 149, June 2013 DOI 10.1007/s12303-013-0024-0 ⓒ The Association of Korean Geoscience Societies and Springer 2013

Early Ordovician reefs from the Taebaek Group, Korea: constituents, types, and geological implications Suk-Joo Choh Jongsun Hong Ning Sun Sung-Wook Kwon Tae-Yoon Park Jusun Woo Yi Kyun Kwon Dong-Chan Lee Dong-Jin Lee*

} Department of Earth and Environmental Sciences, Korea University, Seoul 136-713, Republic of Korea Department of Earth Sciences and Resources, China University of Geosciences, Beijing 1000083, China Department of Earth and Environmental Sciences, Korea University, Seoul 136-713, Republic of Korea

} Division of Polar Earth-System Sciences, Korea Polar Research Institute, Incheon 406-840, Republic of Korea

Department of Geoenvironmental Sciences, Kongju National University, Gongju 314-701, Republic of Korea Department of Earth-Science Education, Chungbuk National University, Cheongju, 361-763, Republic of Korea Department of Earth and Environmental Sciences, Andong National University, Andong 760-749, Republic of Korea

ABSTRACT: The Early Ordovician (early to middle Floian) bioherms of the Dumugol Formation, Korea, are compiled and their paleoenvironmental and paleogeographic implications are discussed. These reefs are mostly made up of microbialite (stromatolite and thrombolite) and lithistid sponge Archaeoscyphia, with subordinate “receptaculitid” calathids. Three types of reefs are identified based on biotic association and texture: 1) lithistid sponge-microbialite, 2) microbialite (thrombolite) with minor lithistid sponge, and 3) lithistid sponge-microbialite-calathid. The first and third type reefs are surrounded by intraclastic-skeletal packstone to grainstone and overlain by lime mudstone, whereas the second type reefs are surrounded and overlain by bioturbated wackestone and nodulebearing shale. These relationships appear to reflect varying depositional conditions during development of the reefs. The constituents of the Dumugol reefs are roughly comparable to coeval structures of Laurentia and South China with the exception of the absence of incorporated sessile organisms (i.e., Lichenaria, Pulchrilamina, and bryozoan) and delayed arrival (more than 10 myr) of calathids in the Sino-Korean Craton. This temporal disparity of biotic appearance is probably related to differential dispersal rates and patterns of sessile organisms which are largely controlled by the relative position of landmasses, epicontinental seas and major oceans. Further discovery and study of the Early Ordovician reefs from the Sino-Korean Craton will provide crucial information for understanding migration pathways of sessile organisms and paleogeographic reconstruction of the western margin of Gondwana in the Early Paleozoic. Key words: Sino-Korean Craton, South China, paleogeography, Gondwana, Early Ordovician, lithistid sponge, calathid, stromatolite, thrombolite

1. INTRODUCTION The Early Ordovician was a critical juncture in the evolutionary path of the Phanerozoic carbonate reefs which is characterized by the transition from middle to late Cam*Corresponding author: [email protected]

brian microbialite-dominated reefs to Middle to Late Ordovician metazoan-dominant reefs (Wood, 1999; Webby, 2002). The Early Ordovician reefs are well developed in Laurentia and South China, and consist of meter-scale domal bodies composed primarily of microbialite (i.e., stromatolite and thrombolite) with less diverse sessile organisms such as lithistid sponge and “receptaculid” calathid (Wood, 1999; Webby, 2002; Wang et al., 2012) (Table 1). The sessile components in these reefs prelude the commencement of the “Great Ordovician Biodiversification Event” where newly emerging metazoans such as coral, bryozoan, and stromatoporoid locally participated in reef construction (Webby, 2002, 2004). The early Tremadocian reefs of Laurentia are characterized by microbialite with subordinate lithistid sponge and local appearance of primitive tabulate coral Lichenaria (e.g., Pratt and James, 1982, 1989; Bova and Read, 1987). Coeval reefs of South China show the first incorporation of calathids into microbialite-dominated reefs (Wang et al., 2012). In the late Tremadocian, lithistid sponge-microbe reefs with calathids widely developed in Laurentia and South China (e.g., Church, 1974; Pratt and James, 1982; Pratt, 1989; Cañas and Carrera, 1993; de Freitas and Mayr, 1995; Adachi et al., 2009, 2012). The advent of bryozoans in South China resulted in first bryozoan reefs (Zhu et al., 1993; Cuffey et al., 2013) and bryozoan-bearing reefs associated with lithistid sponges, calathids, microbes, and pelmatozoans (e.g., Liu et al., 1997; Li et al., 2004; Adachi et al., 2009, 2012; Xiao et al., 2011; Wang et al., 2012). A recent report noted the first association of a probable primitive stromatoporoid (see Webby, 1986) Pulchrilamina in late Tremadocian lithistid sponge-microbial reefs of South China (Adachi et al., 2012). Most of these late Tremadocian reef components continued to flourish in Floian reefs with more frequent association of Pulchrilamina in Lau-

140 Suk-Joo Choh, Jongsun Hong, Ning Sun, Sung-Wook Kwon, Tae-Yoon Park, Jusun Woo, Yi Kyun Kwon, Dong-Chan Lee, and Dong-Jin Lee

rentia and South China (Toomey and Nitecki, 1979; Pratt and James, 1982; Pohler and James, 1989; Adachi et al., 2012; Wang et al., 2012). In contrast to those in South China, Early Ordovician reefs have been rarely reported from the Sino-Korean Craton (Li and Zhu, 1995; Bai et al., 2010; Xiao et al., 2011; Wang et al., 2012). Much of what is known about the Early Ordovician reefs of the Sino-Korean Craton has been from the Dumugol Formation of the Taebaeksan Basin, Korea (Lee and Choi, 1987; Kim and Lee, 1996, 1998; Lee et al., 2001; Kwon et al., 2003). This study aims to document and assess the potential geological significance of the carbonate buildups of the Dumugol Formation. 2. GEOLOGIC SETTING AND METHOD The Taebaeksan Basin is located at the eastern margin of the Sino-Korean Craton where mixed siliciclastic-carbonate sequences of the Joseon Supergroup (Cambro-Ordovician) are unconformably underlain and overlain by Precambrian basement and the Pyeongan Supergroup (Carboniferous to Triassic), respectively (Fig. 1a) (Chough et al., 2000). Cambro-Ordovician sequences of the Taebaeksan Basin has been assigned to the Taebaek Group (Choi, 1998), and ten formal lithostratigraphic units of the group have been established (Choi and Chough, 2005). The Dumugol Formation crops out in southern Taebaek and eastern Yeongwol areas on east-central Korea (Fig. 1a). The formation measures up to 250 m in thickness and is characterized by alternation of marl and limestone (Kwon and Chough, 2005). Choi et al. (2004) informally subdivided the formation into shale-dominant lower member, middle member composed of cyclic alternations of carbonate- and shale-dominant facies, and carbonate-dominant upper member. This study adopts revised upper boundary of the formation at the first occurrence of massive dolostone which is the former boundary between the lower and middle Makgol Formation (Choi et al., 2004; Choi and Chough, 2005). Three trilobite biozones have been established within the Dumugol Formation: i.e., the Asaphellus, Protopliomerops, and Kayseraspis zones, in ascending order (Kobayashi, 1934; Kim et al., 1991). The lower two biozones range from the middle to late Tremadocian, while the Kayseraspis Zone is considered to represent the lower Floian (Kim et al., 1991). Seo et al. (1994) established four conodont biozones, Chosonodina herfurthi-Rossodus manitouensis, Glyptoconus quadraplicatus, Pracordylodus gracilis, and Triangulodus dumugolensis zones, in ascending order, with the Tremadoc-Floian boundary at the top of the Glyptoconus quadraplicatus Zone. The Dumugol Formation is considered to have been deposited in inner to outer ramp environments with occasional storm influxes (Lee and Choi, 1987; Lee and Kim, 1992; Kim and Lee, 1998; Kwon et al., 2003; Choi et al., 2004; Kwon and Chough, 2005; Kwon et al., 2006).

The study area is located in the vicinity of Taebaek where some previously reported and several new Early Ordovician reefs are discovered (Fig. 1a): six reefs from the middle member of the Dumugol Formation at the Gumunso area (Fig. 1c; G1-2 and G3) (Lee and Choi, 1987; Kim and Lee, 1996, 1998) and more than five reefs from the uppermost Dumugol Formation of the Seokgaejae pass, approximately 9 km southeast from Gumunso (Fig. 1d; SJ) (Lee et al., 2001; Kwon et al., 2003; Choi et al., 2004; Kwon et al., 2006). Although the precise age of the reefs is indeterminable, those from the middle part of the formation are estimated to be near at the Tremadocian-Floian boundary, while those from the upper part may be assignable to the middle Floian (Fig. 4) (e.g., Kobayashi, 1934; Kim et al., 1991; Seo et al., 1994). In this study, we report two additional localities of patch reefs: five reefs in three horizons along the Cheoramcheon streambed about 350 m northeast of the Gumunso reefs (Fig. 1c; C1-2 and C3) and a reef from the uppermost Dumugol Formation near Sangdong, Yeongwol County, approximately 17 km northwest of Gumunso (Fig. 1b; S1). These reef-bearing intervals were described in the field at 1:20 scale based on limestone texture, carbonate grain type, and sedimentary structure (Figs. 1e, f, and g). Samples were selected from the reef core, flank, and surrounding non-reef deposits, respectively. Slabs were etched by 10% HCl to enhance meso-scale observation, and transversely- and longitudinally-cut thin sections were prepared to identify reef constituents and microfacies. Relative proportions of reef components were used to classify reef types (Fig. 3). 3. RESULTS 3.1. Gumunso Area Six patch reefs in three horizons of the middle Dumugol Formation have been reported along the Hwangjicheon riverbed (Lee and Choi, 1987; Kim and Lee, 1996, 1998) (Fig. 1c; G1-2 and G3). Additional five reefs in three horizons were found along the Cheoramcheon streambed (Fig. 1c; C1-2 and C3). The heavily deformed and disturbed nature of the area does not allow the construction of comprehensive columnar section along the Cheoramcheon stream. Judging from the relative stratigraphic positions among the reefs with similar constituents and surrounding facies, the Cheoramcheon reefs are considered to be contemporaneous to those cropping out on the Hwangjicheon streambed. The reefs of both localities are composed of meter-scale lenticular micritic bodies interbedded with well-stratified and differentially compacted surrounding strata (Figs. 1f and 2a, c, d). In the lower two horizons the reefs are 1.3–4.5 m wide and 0.7–1.9 m high. These structures overlie intraclasticskeletal packstone to grainstone, surrounded by intraclastic-

Early Ordovician reefs from the Taebaek Group, Korea

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Fig. 1. (a) Simplified geologic map of the eastern Taebaeksan Basin. Modified after Chough et al. (2000). (b–d) Aerial view of study area and the location of patch reefs. Satellite images are captured from http://map.daum.net. (b) Sangdong area, (c) Gumunso area, and (d) Seokgaejae area. (e–g) Columnar description of patch reefs and adjacent strata. S = shale, M = mudstone, W = wackestone, B = boundstone, P = packstone, G = grainstone, D = dolostone.


Fig. 2. Massive and lenticular Dumugol patch reefs in outcrop. (a) Photograph of G2 sponge-microbial patch reef (SMr) at Gumunso. Note its massive body and surrounding deposits of intraclastic-skeletal packstone to grainstone (P/Gi) and overlying facies of differentially compacted mudstone with dissolution seams (Md). (b) Detailed photograph of rectangle in Figure 2a showing transverse cut of cone-shaped Archaeoscyphia (Ar; arrows and their sketches). Note the occurrence of many such round-shaped fossils which are mostly lithistid sponges. (c) Photograph of C1 sponge-microbial reef (SMr) along the Cheoramcheon streambed east of the Gumunso reefs. (d) Photograph of G3 thrombolitic patch reef (Tr) at Gumunso along the Hwangjichun streambed enclosed by differentially compacted bioturbated wackestone (Wb) and overlain by nodule-bearing shale (Sn). (e) Photograph of S1 sponge-microbial reef (SMr) at the Sangdong. Note patches of partially dolomitized burrow (arrows). (f) Photograph of SJ sponge-microbial-calathid reefs (SMCr) at the Seokgaejae area where the upper three of five reef-bearing intervals are exposed along a forest service road. Marker pen and coin are 14 cm long and 2.3 cm in diameter, respectively. Stratigraphic top is toward the upper part of each photograph.

skeletal packstone to grainstone with lithistid sponge fragments, and overlain by lime mudstone with dissolution seams or limestone-shale alternations (Figs. 1f and 2a, c). They are primarily composed of densely distributed lithistid sponges, subequal stromatolites, and subordinate inver-

tebrate fragments of trilobites, pelmatozoans, inarticulated brachiopods, and nautiloids. Minor burrows and irregular pore spaces (Lee and Choi, 1987) with flat base occur sporadically throughout the reefs. The lithistid sponges have an annulated obconic body up to 3 cm in diameter (Figs.


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Fig. 3. Photomicrographs of the Dumugol boundstone. (a) Cone shaped body with undulatory outline and ladder-like arrangement of spicules (arrows) of Archaeoscyphia (Ls) from the Gumunso G2 lithistid sponge-microbial patch reefs. Note an Archaeoscyphia (Ls1) apparently providing substrate for attachment of another individual (Ls2). (b) Densely distributed Archaeoscyphia (Ls) with shell fragments in the Gumunso G2 lithistid sponge-microbial patch reefs. (c) Upward widening bush-like masses composed of peloids (arrows and dashed lines) reminiscent of Epiphyton-like calcimicrobe forming microbial clots (Mc) in the Gumunso G3 thrombolitic reef. (d) Microbial clots (Mc; arrows) and rare lithistid Archaeoscyphia (Ls) of the Gumunso G3 thrombolitic reef. (e) Well-preserved Archaeoscyphia sponge (Ls) of the Sangdong S1 patch reef. (f) Lithistid sponges (Ls) associated with faint micro-stromatolites (Ms; dotted white lines) of the Sangdong S1 patch reef. Note micro-stromatolite surrounding the outside of lithistid sponges (arrows). (g) Gobletshape calathid (Rc; left) and Archaeoscyphia (Ls) with donut-shaped transverse cut (white arrow) in the Seokgaejae patch reef. Note the holdfast of the calathid (Rch) which appears to be attached (black arrow) to another Archaeoscyphia. (h) Columnar micro-stromatolites (Ms) composed of alternation of darker micrite and light peloidal micrite from the Seokgaejae patch reef.


3a and b) with wall containing regular lattice-like arrangement of spicules (Fig. 3a; see also Finks, 1967); the sponges are provisionally identified as Archaeoscyphia (e.g., Lee et al., 2001; Kwon et al., 2003). They are densely populated in parts and commonly attached to other sponges (Figs. 3a and b). Domal to planar micro-stromatolites and thrombolitic clots are often intergrown with the sponges. Micro-stromatolites commonly encrust the sponges and extend upward as well as outward. Micritic clots composed of aggregated peloidal clumps are commonly attached on the surface of the sponges or patch of shell fragments and grow upward. The upper reef-bearing horizon contains patch reefs about 1.7 m wide and 1.5 m high (Fig. 2d) which is characterized by pervasive centimeter-scale dark micritic clots without lamination. These reefs overlie bioturbated wackestone and overlain by nodule-bearing shale (Fig. 1f; G3). The reefs are dominated by Epiphyton-like calcimicrobes with minor sponges and fragments of trilobites, inarticulated brachiopods and gastropods. The Epiphyton-like calcimicrobe occasionally shows branching series of peloids and comprises upward-widening micritic clumps (Fig. 3c) which are tentatively identified as poorly-preserved Epiphyton (e.g., Hong et al., 2012). The intergrowth of such calcimicrobe thalli resulted in the formation of dark clots and pervasive thrombolitic texture. 3.2. Sangdong Area A domal patch reef of approximately 1.2 m wide by 0.7 m high exposed along a creek in the Sangdong area (Fig. 1b; S1) is probably coeval to those of the uppermost Dumugol reefs in the Seokgaejae pass (Fig. 1d; SJ). It is composed of massive bluish gray micrite surrounded by dolomitic flanking bed (Fig. 2e) and well-bedded bioturbated mudstone to wackestone with intercalated lenticular intraclastic packstone to grainstone (Fig. 1e). This micritic patch reef consists of subequal proportion of lithistid sponge Archaeoscyphia (Figs. 3e and f) and stromatolitic fabric with crinkled laminations (Fig. 3f). Subordinate constituents include fragments of pelmatozoans, trilobites and brachiopods and dolomitized burrows (Fig. 2e). The walls of Archaeoscyphia are variably preserved from well-preserved regularly arranged ladder-like spicule networks to poorly-preserved peloidal micritic fabric with scattered spicules. These sponges are frequently encrusted by stromatolite composed of alternations of dark micritic and light peloidal laminae (e.g., Kazmierczak et al., 1996; Adachi et al., 2009) (Fig. 3f). 3.3. Seokgaejae Area Five meter-scale (1.5–3.0 m wide and 0.8–1.3 m high) patch reefs from five horizons in the upper Dumugol For-

mation occur along a forest service road (Figs. 1g and 2f) in the Seokgaejae area (Kwon et al., 2003). These lenticular reefs with concave base are composed of massive bluish gray micrite. These reefs overlie intraclastic packstone to grainstone, surrounded by well-bedded intraclastic packstone to grainstone with fragments of reef constituents and overlain by bioturbated thinly-bedded dolomitized lime mudstone (Fig. 1g). The sessile component of these reefs are very similar to that of the Sangdong (S1) patch reefs, consisting of lithistid sponge Archaeoscyphia preserved in growth position with characteristic undulatory outline in longitudinal cuts and donut-shape in transverse cuts (Fig. 3g; white arrow). Subordinate occurrence of goblet-shaped calathids with well-developed holdfast and double walls of up to 3 cm in diameter (Church, 1991; Lee, 2000; Lee et al., 2001; Kwon et al., 2003) has not yet been found from other Dumugol reefs (Fig. 3g). The surface of Archaeoscyphia and calathids is commonly encrusted by microbial component. Microbial fabrics include crinkled stromatolitic laminae of dark micrite and peloidal texture, domal to columnar micro-stromatolite (Fig. 3h), and non-laminated peloidal micrite. Other minor constituents are fragments of pelmatozoans, trilobites, ostracods, and gastropods. 3.4. Types of Dumugol Reefs The reefs of the Dumugol Formation occur in two stratigraphic levels: from the middle Dumugol Formation (Gumunso and Cheoramcheon) and upper Dumugol Formation (Sangdong and Seokgaejae) (Fig. 4). Based on the dominant constituents and textures of the boundstone, the reefs can be subdivided into three types: 1) lithistid sponge-microbialite (Fig. 1b; S1, Fig. 1c; C1, C2, G1, and G2), 2) microbialite (thrombolite) with minor lithistid sponges (Fig. 1c; C3 and G3), and 3) lithistid sponge-microbialite-calathid patch reefs (Fig. 1d; SJ). Reefs with thrombolitic microbial clots and subordinate lithistid sponges are surrounded by bioturbated wackestone and overlain by nodule-bearing shale and are only present in the middle Dumugol Formation at the Gumunso and Cheoramcheon areas (Fig. 1c; C3 and G3). Other reefs are characterized by dominant lithistid sponge Archaeoscyphia and stromatolitic fabric with minor clotted thrombolitic fabric, substrate and surrounding facies of intraclastic-skeletal packstone to grainstone and overlying lime mudstone. Lithistid sponge Archaeoscyphia in middle (Fig. 1c; C1, C2, G1, and G2) and upper Dumugol patch reefs (Fig. 1b; S1, and 1d; SJ) show a distinctive size variation which warrants further taxonomic evaluation and analysis of associated sediments. Calathids only occur sparsely in the uppermost Dumugol reefs (Figs. 1d; SJ and 4) which are one of the oldest known occurrence in the Early Ordovician reefs of the Sino-Korean Craton to date (e.g., Li and Zhu, 1995; Lee, 2000; Lee et al., 2001; Kwon et al., 2003).


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Fig. 4. Composite column of the Dumugol Formation and schematic distribution of reef-bearing horizons. Modified from Lee and Choi (1987), Kim and Choi (1996) and Kwon and Chough (2005).


4. DISCUSSION 4.1. Early Ordovician Metazoan Evolution and Reef The Early Ordovician reefs were preceded by a few siliceous sponge-bearing microbial reefs of the middle Cambrian (Cambrian Series 3) to late Cambrian (Furongian) (Hong et al., 2012). Subsequent inception of the “Great Ordovician Biodiversification Event” led to multifold increase of organisms and their diversity which impacted the constituents, ecology, and evolutionary pattern of carbonate reefs (Webby, 2002, 2004). Sessile organisms emerged and incorporated into the early Early Ordovician (Tremadocian) reefs of Laurentia and South China include primitive tabulate coral Lichenaria, “receptaculid” calathids of possible algal affinity, bryozoan, and early stromatoporoid Pulchrilamina. Lichenaria first appeared in early Tremadocian thrombolitic-

Renalcis microbial mounds of the Green Head bioherm complex, Newfoundland (Pratt and James, 1982). Calathidmicrobial reefs of early Tremadocian age from Wentang of South China record the first emergence of calathids in Early Ordovician reefs (Wang et al., 2012). Various types of late Tremadocian bryozoan-bearing reefs (Zhu et al., 1993; Adachi et al., 2012; Wang et al., 2012; Cuffey et al., 2013) are found in Yichang, and the oldest (Tremadocian-Floian) occurrence of Pulchrilamina within lithistid sponge-microbial and lithistid sponge-bryozoan reefs have been reported from Three Gorges and Tongzi of South China (Adachi et al., 2012). Such incorporation of newly emerging sessile organisms into largely microbial- to lithistid sponge-microbial-dominated Early Ordovician reefs resulted in numerous types of metazoanbearing reefs which eventually became the forerunner of metazoan-dominated reefs of the Late Ordovician and onwards (Wood, 1999; Webby, 2002).

Table 1. Selected occurrences of the Early Ordovician reefs from the Laurentia, South China and Sino-Korean paleocontinents Age

Laurentia (and Precordillera) Thrombolite-lithistid sponge-

Pulchrilamina mounds (with bryozoan)

(Pratt and James, 1982; Pohler and James, 1989)

South China

Sino-Korean

Lithistid sponge-bryozoan-calathid reefs with Pulchrilamina; Lithistid sponge-microbial-calathid reefs with Pulchrilamina (Li et al., 2004; Adachi et al., 2012)

Lithistid sponge-microbialcalathid bioherms (Li and Zhu, 1995; Lee et al., 2001; Kwon et al., 2003; this study)

Calathid-microbial reefs

Microbial (thrombolite)lithistid sponge patch reefs (this study)

Lithistid sponge-calathid-

Pulchrilamina-stromatolite (with bryozoan) mounds (Toomey, 1970)

(Webby, 2002)

Floian Calathid-microbial reefs (Webby, 2002)

Sponge-microbial biostromes

Lithistid sponge-calathid-microbial patch reefs (Hintze, 1973; Church, 1974)

(Liu and Zhan, 2009)

Stromatactis-bearing mud mounds

(Pohler and James, 1989) Kilometer-scale microbial buildups with lithistid sponge (de Freitas and Mayr, 1995)

Early Ordovician

late Tremadocian

Stromatolite-lithistid sponge-calathidthrombolite bioherms (Cañas and Carrera, 1993)

Bryozoan-calathid-microbial reefs (Xiao et al., 2011; Wang et al., 2012)

Lithistid sponge-calathid-stromatolite patch reefs (Church, 1974)

Bryozoan reefs (Zhu et al., 1993; Cuffey et al., 2013)

Thrombolite mounds with lithistid sponge

Microbial-lithistid spongecalathid reefs (Li et al., 2004; Xiao et al., 2011; Adachi et al., 2012)

(Pratt and James, 1982; Pratt, 1989)

early Tremadocian

Lithistid sponge-bryozoan-(Pulchrilamina) reefs; Bryozoan-pelmatozoan reefs (Xiao et al., 2011; Adachi et al., 2012)

Thrombolite mounds with Lichenaria (Pratt and James, 1982) Stromatolite-thrombolite bioherms with lithistid sponges

(Bova and Read, 1987)

Calathid-microbial reefs

(Wang et al., 2012)


4.2. Significance of Dumugol Reefs The present study describes the late Early Ordovician (early to middle Floian) Dumugol reefs which represents one of the first compilations of such biogenic structures in the Sino-Korean Craton to date. Previous reports of calathids “Soanites” and Archaeoscyphia from Early Ordovician carbonates of Jilin Province (Guo, 1983), early to middle Floian Archaeoscyphia-Calathium-stromatolite reefs from Hebei Province, northeast China (Li and Zhu, 1995), and Tremadocian to Floian sponge-microbial biostromes from Pingquan, Hubei Province (Liu and Zhan, 2009) suggest the possibility of wider development of Early Ordovician reefs in the Sino-Korean Craton. The differences in substrate and surrounding facies between thrombolitic microbial reefs and lithistid sponge-microbe(calathid) reefs suggest that the depositional environment might be a controlling factor for development of different reef types in the Dumugol Formation. Thrombolitic reefs underlain and surrounded by bioturbated wackestone and overlain by nodule-bearing shale indicate that the reef might have formed under calm subtidal environment. On the other hand, lithistid sponge-microbial reefs overlying intraclastic packstone to grainstone, surrounded by intraclastic packstone to grainstone with fragments of reef constituents and overlain by bioturbated lime mudstone suggest the development of the reefs in higher-energy shallower subtidal environment. A similar spectrum of the Early Ordovician reef development in varying depositional environments has been reported elsewhere (e.g., Pratt and James, 1982; Bova and Read, 1987; de Freitas and Mayr, 1995), and thus the Dumugol reef types and depositional environments warrant further detailed study. The Early Ordovician reefs from the Sino-Korean Craton known to date are composed of various proportions of microbialites (stromatolites and thrombolites), lithistid sponge Archaeoscyphia, calathids and fragments of trilobites, brachiopods and gastropods, and burrows. Such composition appears to fit into the general spectrum of the Early Ordovician reef types reported from Laurentia and South China (Table 1). However, the sessile components of the Dumugol reefs show marked difference from those of Laurentia and South China: Lichenaria, Pulchrilamina, and bryozoans are absent, and apparent delayed appearance of the calathids which first appeared in early Tremadocian reefs of South China and were widespread in lithistid sponge-microbialcalathid-bearing reefs across Laurentia and South China during late Tremadocian. The appearance of the calathids in the Dumugol reefs may have been later than the earliest calathid reefs in South China by 10 myr and at least 5 myr later than the widespread occurrence of such structures in Laurentia and South China. Such marked temporal differences appear to reflect dispersal patterns of the sessile organisms primarily influenced by relative position of the

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continents and epicontinental seas. It is of great interest to reveal the pathway along which the calathids might have dispersed across epicontinental seas surrounding the South China and Sino-Korean cratons. Further studies of the Early Ordovician reefs from other areas of the Sino-Korean Craton might be able to provide clues to resolve timing and dispersal patterns of sessile organisms through shallow epicontinental seas and across major oceans, which could provide useful information for reconstruction of the early Paleozoic paleogeography of the eastern margin of Gondwana. As Webby (2002) pointed out, the current understanding of the full spectrum of the Ordovician reef pattern is still far from being complete, and in particular, research on the Early Ordovician reefs of the Sino-Korean Craton is at its incipient stage, whereas recent discoveries and reports during the last decade significantly improved our understanding of the Early Ordovician reefs of South China (e.g., Adachi et al., 2009, 2012; Xiao et al., 2011; Cuffey et al., 2013). Thus this study demonstrates the urgent need for further discovery and study of the Early Ordovician reefs of the Sino-Korean Craton to understand factors behind the apparent temporal disparity of the Early Ordovician reef building sessile organisms. Consideration of such factors involved in the continued incorporation of certain metazoans in Early Ordovician reefs might also provide a key clue for understanding the rise and fall of reef building organisms throughout the Phanerozoic. 5. CONCLUSION 1. Numerous meter-scale patch reefs occur in the middle and upper parts of the Dumugol Formation, Taebaek Group, Korea. These reefs mainly consist of microbialite (stromatolite and thrombolite), lithistid sponges, and local occurrence of calathids. These Dumugol reefs represent the Early Ordovician reefs in the Sino-Korean Craton and are comparable to coeval reefs of South China and Laurentia. 2. The Dumugol reefs are divided into three types: 1) lithistid sponge-microbialite type from the middle and upper Dumugol Formation, 2) thrombolite with minor lithistid sponge type from the middle Dumugol Formation and 3) lithistid sponge-microbialite-calathid type from the uppermost Dumugol Formation. 3. Thrombolitic reefs are surrounded and overlain by bioturbated wackestone and nodule-bearing shale, and the lithistid sponge-microbe reefs are by intraclastic skeletal packstone to grainstone. The development of different reef types might reflect varying depositional conditions. 4. The main components and depositional conditions of the Dumugol reefs appear to fit the global trend of the late Early Ordovician (Floian), with the exception of incorporated sessile organisms which are markedly different from those of South China and Laurentia. Metazoans such as Lichenaria, Pulchrilamina and bryozoan are absent in the Early


Ordovician reefs of the Sino-Korean Craton and calathids apparently first appeared in the Dumugol reefs ca. 10 myr later than those of the earliest calathid-bearing reefs of South China and up to 5 myr later than extensive development of such structures in South China and Laurentia. 5. The apparent temporal disparity in the appearance of sessile organisms in the Early Ordovician reefs of the SinoKorean Craton probably reflect that the dispersal pattern of sessile organisms may have been influenced by paleogeography of the early Phanerozoic. ACKNOWLEDGMENTS: This study was supported by a grant from Korea Institute of Energy Technology Evaluation and Planning and Ministry of Knowledge and Economy (2011201030006B) to SJC, a grant from Korea Research Foundation (KRF-R1A4007-2010-0011026) to DCL, and Basic Science Research Program through the National Research Foundation of Korea (KRF-2012-005612) and 2012 Academic research Fund of Andong National University to DJL. We appreciate D.K. Choi of SNU for thorough and meticulous review. We also thank J. Park of KU for constructive comments and S.M. Kang, J.R. Oh, M. Lee of KU and K.S. Ko of ANU for their assistance in the field.

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