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Carbonate Breccias in the Lower-Middle Ordovician Maggol Limestone (Taebacksan Basin, South Korea): Implications for Regional Tectonism In-Chang Ryu, Seong-Jae Doh, Seon-Gyu Choi, Seoul KEY'WORDS:CARBONATEBRECCIA- PALEOKARST - TAEBACKSANBAS[N (SOLrI"HKOREA)- MII)DLt-{ORDOVICIAN(MAGGOL LIMESTONE)

Contents Summary 1 Introduction 2 Stratigraphy 3 Maggol Limestone 3.1 Types of cycle 3.1.1 TypeA 3.1.2 Type B 3.1.3 Type C 3.1.4 Type D 3.1.5 Type E 3.2 Meter-scale cycle and cycle set 4 Biostratigraphic considerations 5 Occurrence of carbonate breccias 6 Genesis of carbonate breccias 7 Discussion 8 Conclusions References

SUMMARY Carbonate breccias occur sporadically in the LowerMiddle Ordovician Maggol Limestone exposed in Ihe Taebacksan Basin, South Korea. These carbonate breccias have been previously interpreted as intraformational or fault breccias. Thus, little attention has been focused on tectonic and stratigraphic significance of these breccias. This study, however, indicates that the majority of these breceias are solution-collapse breccias, which are causally linked to paleokarslification. Carbonate facies analysis in conjunction with conodont biostratigraphy suggests that an overall regression toward the top of the Maggol Limestone probably culminated in subaerial exposure of platform carbonates during the early Middle Ordovician. Extensive subaerial exposure of platform carbonates resulted in paleokarst-relamd solution-collapse breccias in the upper Maggol Limestone. This subaerial exposure event is manifested as a major paleokarst unconformity elsewhere beneath the Middle Ordovician sequence, most notably North America and North China. Due to its global extent, the early Middle Ordovician paleokarst unconformity ('the Sauk-Tippecanoe sequence boundary') has been viewed as a product of

second-order eustatic sea level drop during the early Middle Ordovician. Although we recognizes a paleokarst breccia zone in the upper Maggol Limestone beneath the Middle Ordovician sequence, the early Middle Ordovician sequence boundary appears to be a conformable transgressive surface or a drowning unconformity, rather than a major paleokarst uncon fortuity. The paleokarst breccia zone in the upper Maggol Limestone is represented by a thinning-upward stack oi'exposure-capped tidal flat-dominated cycles that are closely associated with multiple occurrences of paleokarst-rclatcd solution-collapse breccias. The palcokarst breccia zone in the upper Maggol Limestone was a likely consequence of repeated high-frequency sea level fluctuations of fourth- and fifth-order superimposed on a second- and third-order eustatic fa]l in sea level that was less than the rate of tectonic subsidence across the platform. It suggests that second- and thirdorder eustatic sea level drop may have been significantly tempered by subslanfial tectonic subsidence near the end of Maggol deposition. The tectonic subsidence in the basin is also evidenced by the occurrence of coeval off-platlorm lowstand siliciclastic quartzite lenses as well as dchris ['low carbonate breccias. With the continued tectonic subsidence, subsequent rise in the custatic cycle caused drowning and deep flooding of carbonate platform, l:orming a conformable transgressive surface or a drowning unconformity on the top of the paleokarst breccia zone. This tectonic implication contrasts nolably with tile slowly subsiding carbonate platform model for the Taebacksan Basin as previously interpreted. Here we propose that the Tacbacksan Basin evolved from a slowly subsiding carbonate platform to a rapidly subsiding intracontinental rit't basin during Ihe early Middle Ordovician. This study also provides a good example that the falling part of the eustatic sea-level cycle may not produce a significant event at all in a rapidly subsiding basin where the rate of eustatic fall always remained lower than the rate of subsidence. 1 INTRODUCTION During the last two decades, there has been a growing awareness of the importance of paleokarsts in ancient carbonate successions (Han'ison and Steincn, 1978, Estcban and

Address: Dr. In-Chang Ryu, Dr . Seong-Jae l)oh. I)r. Seon-Gyu Choi, Department of Earth and En;ironmental Sciences, Korea University, Seoul 136-701, South Korea; e-maih [email protected]

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Fig. 1. Geologic map showing the distribution of the Joseon Supergroup and the Ogcheon Group in the Ogcheon Belt, South Korea. The Ogcheon Belt is composed of two basins: western Ogcheon (I) and eastern Taebacksan (II). Although these two basins appear to be situated in the same tectonic province, there are no slratigraphic and structural affinities between the two basins. The Cambro-Ordovician Joseon Supergroup is mainly distributed in the Taebacksan Basin. Tile study areas include northwestern (Yemi area) and southeastern (Dongjeom area) limbs of the Backunsan Syncline, Taebacksan Basin.

type area (i.e., southern limb of the Backunsan Syncline; Fig. 1) is mainly shallow marine in origin and is subdivided into two groups; the Cambrian Samcheog Group and the Ordovician Sangdong Group (Fig. 2; Cheong, 1969). The Cambrian Samcheog Group includes the Jangsan Quartzite, the Myobong Slate, the Pungchon Limestone, and the Hwajeol Formation (Fig. 2; Cheong, 1969). The overlying Sangdong

Group consists of the Lower and Middle Ordovician strata of the Dongjeom Quartzite, the Dumugol Shale, the Maggol Limestone, the Jigunsan Shale, and the Duwibong Limestone (Fig. 2; Cheong, 1969). Although the Joseon Supergroup appears to be a thick conformable succession of quartzite, shale, and limestone, our field observations reveal that these strata comprise numerous meter-scale shallow-

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Fig. 2. Lithostratigraphic nomenclature of the Cambro-Ordovician Joseon Supergroup, Backunsan Syncline (Cheong, 1969) and its sequence stratigraphic interpretation.

Klappa, 1983; James and Choquette, 1988; Fritz et al.. 1993; Budd et al., 1995; George and Powell, 1997). With the advancement of sequence stratigraphy, the recognition of paleokarsts in the platform carbonates became more crucial for defining the subaerially exposed sequence boundary (Kerans, 1988; Sarg, 1988; Fritz et al., 1993; Montafiez and Osleger, 1993; Elrick, 1996). Eustatic sea level fall is largely responsible to development of the subaerially exposed sequence boundary in ancient carbonate successions, which resulted in paleokarsts beneath the sequence boundary (Kerans, 1988; Sarg, 1988; Fritz et al., 1993: Montafiez and Osleger, 1993; Elrick, 1996). Although subaerial exposure does not necessarily characterize all aspects related to the sequence boundary (e.g., Sarg, 1988), it is significant that the subaerially exposed sequence boundary can show a transition to the conformable transgressive surface when eustatic sea level tall is significantly modified by local and regional subsidence (Schlager, 1999). The Cambro-Ordovician strata in South Korea, the Joseon Supergroup, are widely distributed in the Taebacksan Basin in the eastern part of the NE-SW-trending Ogcheon Belt (Fig. 1). These strata have been considered as a confommble succession recording continuous sedimentation on a slowly subsiding carbonate platfonn (Cheong, 1969). Although a slowly subsiding carbonate platform is more likely to experience widespread subaerial exposure during eustatic sea level fall, widespread development of paleokarst indicati ng a prolonged subaerial exposure has previously not been documented in the platform carbonates of thc Cambro Ordovician Joseon Supergroup. The recent study, however, suggests that the Joseon Supergroup can be divided into a

Cambrian to lower Middle Ordovician succession and an overlying Middle Ordovician succession, separated by a paleokarst breccia zone (Ryu et al., 1997). This paleokarst breccia zone is represented by extensive paleokarst-related solution-collapse breccias in the upper Maggol Limestone beneath the Middle ()rdovician sequence (Fig. 2). Although previous workers have reported the local occurrence ol+ carbonate breccias (i.e., the Yemi Breccia) (Figs. 2 and 3A), these breccias have been sirnply interpreted as intraforrnationat or fault breccias (Kim and Kwon, 1970). Thus, liltle attention has been focused on the stratigraphic and tectonic significance of the carbonate breccias. In this study, we examine the nature o[" the carbonate breccias within the Lower-Middle Ordovician strata; we describe the early Middle Ordovician sequence boundary that lnay be causally linked Io development of the carbonate breccias in the upper Maggol I.imestone: and we evaluate the possible synchronicity of this boundary with custatic sea level l]uctuations to assess the importance of eustacy or lectonics during the early Middle ()rdovician in the Taebacksan Basin, Soud~ Korea (Fig. 2). 2 STRATIGRAPHY More than 1500 m of the Can}bro-Ordovician Joseon Supergroup is preserved in the Taebacksan Basin (Figs. I and 2). The Joscon Supergroup rests unconformably on thc Ptecambrian granite, gneiss, and metasedmlentary rocks, and is overlain by fluvial I;~ deltaic siliciclastic rocks of the Middle CarboniFerous to Lower Triassic Pyeongan Supergroup (Fio. 2; Cheong, 1969). The Joseon Supergroup in the

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ing-upward cycles arranged into four u'ansgressive-regressive packages (Fig. 2). Each transgressive-regressive package is assignable to a second-order supersequence set that ranges in 10 to 100 Ma. In this study, we mainly focus on the interval of the Lower-Middle Ordovician Maggol Limestone that displays an extensive occurrence of carbonate breccias (Fig. 2).

The nodular mudstone may be a distal storm facies deposited on the middle ramp between burrowed wackestone and deeper water shale (Aigner, 1985). Thin skeletal packstone or flat-pebble conglomerate cap is deposited above storm wave base during severe storms (Osleger and Read, 1991). Thus, the type A cycle reflects rapid shallowing upward 3.1.2 Type B Cycle

3 MAGGOL LIMESTONE The Lower-Middle Ordovician Maggol Limestone attains a thickness of over 200 m in the basin and consists mainly of massive to bedded dolomitic limestones with lensshaped dolomites (Cheong, 1969). The formation overlies the storm-influenced, mixed clastics and carbonates of the Lower Ordovician Dumugol Shale (Lee and Kim, 1992), and is overlain by the regionally extensive thin unit of the Middle Ordovician Jigunsan Shale (Figs. 2 and 3). Locally, the Yemi Breccia overlies the Maggol Limestone near the west of Yemi (Figs. 2 and 3A). The Jigunsan Shale is overlain by the storm-influenced open-marine platform succession of the Duwibong Limestone that displays an overall shoaling-upward stack of oolitic and fringing shoals (Figs. 2 and 3; Lee, 1988). Sediments of the Maggol Limestone record bioturbation, flat-pebble conglomerates, stromatolites, thin to thick laminations, ripple marks, evaporite mineral casts, bird-eye structures, and desiccation cracks (Paik, 1985; 1987). Regional studies indicate that the Maggol Limestone was deposited in a tidal flat environment (Paik, 1985; 1987; Woo and Park, 1989). Recently, Woo (1999) described nine lithofacies from this formation based on lithology and sedimentary structures. By considering the depositional environments and the mode of successive facies, Woo (1999) recognized 28 facies associations and subdivided the formation into the lower limestone, middle dolomite, and upper limestone members that show a gradual regression from subtidal through intertidal to supratidal environments towards the top. Although we acknowledge nine lithofacies described by Woo (1999), our detailed facies analysis indicates that the Maggol Limestone mainly consists of fourteen lithofacies that include mudstone, nodular mudstone, burrowed wackestone, flat-pebble conglomerate, dolomite, peloidal packstone, thrombolite-stromatolite bioherm, ooidoncoidal grainstone, intraclast grainstone, ribbon rock, thick laminite, cryptalgal laminite, and carbonate breccia (Fig. 4). Each lithofacies are grouped into five types of carbonate cycles: types A to E (Fig. 5), which is equivalent to a facies association recognized by Woo (1999).

3.1 Types of Carbonate Cycle 3.1.1 Type A Cycle The type A cycle consists of nodular mudstone, burrowed wackestone, skeletal packstone, and flat-pebble conglomerate. Nodular mudstones grade upward into burrowed wackestones abruptly capped by thin skeletal packstone or flat-pebble conglomerate (P1.6/1).

The type B cycle is primarily composed of burrowed wackestone. A thick mottled limestone bed includes the complete cycle. However, this limestone bed is marked by the repetition of couplets that consist of a weakly bioturbated lower part and a pervasively bioturbated upper part (PI. 6/2). Burrows are slightly inclined or near horizontal, but vertical burrows become more abundant toward the cycle top (PI. 6/ 2). Burrows are always replaced by fine- to medium-sized, euhedral dolomites (Woo, 1999). Laterllay discontinuous skeletal packstone laminae are occasionally associated within the cycle, which contain fragments of gastropods, brachiopods, echinoderms, ostracods, and trilobites. It has been observed that infaunal organisms tend to make shallow burrows in subtidal environments, whereas deeper vertical burrows are more common in intertidal environments (Rhoads, 1967). Abundant horizontal or slightly inclined burrows indicate that the cycle was deposited in a subtidal envirormaent below fair-weather wave base under normal marine condition (Rubin and Friedman, 1977). The upward increase of vertical burrows may suggest increasingly shallow condition, perhaps near lower intertidal zone. Laterally discontinuous skeletal packstone laminae are rapidly deposited strom beds that escaped homogenization by burrows (Osleger and Read, 1991). Thus, the type B cycle represents a shallowing upward transition from subtidal to lower intertidal environments, which was occasionally influenced by storm wave reworking. 3.1.3 Type C Cycle The type C cycle is characterized by stratiform thrombolite-stromatolite bioherms (P1.6/3). Many of the bioherms stack on top of one another up to five meters thick. Stacked bioherms exhibit bi-directional and low-angle crossbeddings, and are overlain by ooidal-oncolitic grainstone and ribbon carbonates (P1.6/3). Thin lime-sandstone layers are occasionally interbedded with the stacked bioherms, which show horizontal to low-angle cross-laminations and ripple marks. The thrombolite-stromatolite bioherms are able to establish on stable substrates in a shallow subtidal environment (Osleger and Read, 1991). Stacked bioherms indicate episodic shallowing to lower intertidal depths as well (Aitken, 1967; Kennard and James, 1986). Shallow subtidal and lower intertidal conditions for the bioherms are supported by the bi-directional and low-angle cross-beddings (Dill, 1986). Ooidal-oncolitic grainstones and ribbon carbonates accumulated adjacent to bioherms and formed in shallow subtidal to lower intertidal conditions (Osleger and Read, 1991 ).

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Fig. 3. Detailed geologic maps of the study areas. A) Yemi area (m(~di fled frc~mGeoh~gi cnl I nvesti~ntion Corps of tlac 'raebacksan Rc~gion, 1962). B) Dongjeom area (modified from Paik, 1981). The Jc)seon ,%pergroup generally distributes with a northeast-southwest strike and dip (usually 20~ ~ to northwest in the west of Yemi area, where as with a~l east-west strike and dip (usually 30~'~40 ~) tc~ north in Dongjeom area. Asterisk indicates the occurrence of paleokarst-related scHutic~n-collapse breccias in the upper Maggol l.,imestt~ne. Note the occurrence of coeval off-platform lowstand siliciclastic quartzite lenses and the Yemi Breccia that is ir~tcrprcted as debris flow carbonate breccias.

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Sedimentary structures in the intervening lime-sandstone layers with stacked bioherms also suggest shallowing to lower intertidal environment. Thus, the type C cycle is interpreted to be a shallowing upward transition from shallow subtidal to lower intertidal environments. 3.1.4 Type D Cycle The type D cycle is also characterized by thrombolitestromatolite bioherms. However, the bioherms are laterally discontinuous and domical in shape (Pl. 6/4). The bioherms exhibit non-laminated microstructures, but they typically have stromatolitic laminae outlining the outer surface of the bioherm (PI. 6/4). Each bioherm is surrounded by ribbon carbonates that are, in turn, overlain by laminites (PI. 6/4). Some domical bioherms contain halite and gypsum pseudomorphs. It has been suggested that domical thrombolite-stromatolite bioherms are typical of lower intertidal environment (Logan et al., 1964; Aitken, 1967). Ribbon carbonates also suggest that the bioherms were deposited in lower intertidal conditions (Osleger and Read, 1991). The overlying laminites, however, are mechanically deposited couplets of fine silts and mud drapes laid down on the intertidal flat by storm and tidal currents (Hardie and Ginsburg, 1977). Thus, the type D cycle is interpreted to be a shallowing upward transition from lower intertidal to intertidal flat. Halite and gypsum pseudomorphs within stromatolites indicate a semiarid climate and hypersaline conditions. 3.1.5 Type E Cycle The type E cycle mainly consists of laminites that comprise couplets of fine silts and muds as well as quartz silts. As a cycle, thick laminites grade up into thin laminites without intervening of other lithofacies (P1.6/5). Enterolithic structures are occasionally observed within the laminites (P1.6/ 5). The thick laminites formed in intertidal flat mechanically deposited fine silts and mud drapes laid down on the intertidal flat by storm and tidal currents (Hardie and Ginsburg, 1977). Thin laminites may be cryptalgal laminites that locally formed in the intertidal flat. The quartz silt laminae are interpreted to be eolian origin, blown onto the tidal flat from continent.

3.2 Meter-scale Cycle and Cycle Set The five types of cycle recognized in the Maggol Limestone are decimeter scale (PI. 1). A bundle of the decimeterscale cycle comprises meter-scale shallowing-upward cycles that are the parasequence of sequence stratigraphic terminology in that it is relatively conformable successions of genetically related beds or bed sets bounded by marine flooding surfaces and their correlative surfaces (Van Wagoner et al., 1987). The decimeter- and meter-scale cycles in the Maggol Limestone are therefore regarded as fifth- and fourth-order cycles, respectively. These cycles may have been driven by glacio-eustacy due to a typical 4:1 bundling

of fifth-order cycles within fourth-order cycles (Fig. 5). The 4:1 bundling may manifest the short eccentricity (95-123 ky) to long eccentricity (413 ky) ratio of the Milankovitch astronomical rhythms (Osleger and Read 1991). Fourth-order meter-scale shallowing-upward cycles can be broadly grouped into two main types: subtidal and peritidal (Fig. 5). The subtidal fourth-order cycles are exclusively composed of type A or type B fifth-order cycles, and typify a retrogradational to aggradational stacking of highenergy flat-pebble conglomerate or burrowed wackestone shoals over deeper water shaly facies (Fig. 5). Whereas, the peritidal fourth-order cycles consist of the repetition of types C, D and E fifth-order cycles, and are characterized by a progradational stacking of tidal flat facies over shallow subtidal to intertial biohermal facies (Fig. 5). On the basis of the systematic changes in stacking pattern of meter-scale fourth-order cycles, we recognize five distinct units (I to V) in the middle and upper Maggol Limestone (Fig. 5). Each unit can be classified as a third-order shallowing-upward cycle that is the parasequence set of sequence stratigraphic terminology. The unit I mainly consists of the subtidal fourth-order cycle that was deposited in deep to middle ramp settings with frequent storm activities. As a meter-scale shallowing-upward cycle, a thick basal deeper water shale is overlain by a bundle of the type A cycles (Figs. 4 and 5). Each fourth-order cycle typically ranges from 7 to 8 meters in thickness and stacks upward maintaing relatively consistent thickness (Figs. 4 and 5). With the underlying deeper water shale of the lower Maggol Limestone, the unit 1 may represent an aggradational middle ramp cycle set that was formed during a period of late transgressive to early highstand in sea level (Fig. 5). The unit II includes the middle dolomite member of Woo (1999), which is mainly composed of medium-sized, subhedral to euhedral dolomite crystals (Fig. 4). Sedimentary structures in the middle dolomite member are almost obliterated due to severe dolomitization. However, some relic sedimentary structures such as flat-pebble conglomerates and bioturbation may suggest that the middle ramp to shallow subtidal fourth-order cycles dominate in the middle dolomite member (Fig. 4). Up section, frequent association of these relict sedimentary structures may indicate a gradual shoaling-upward facies transition in the middle dolomite member (Figs. 4 and 5). Overlying the middle dolomite member are thick stacks of peritidal cycles that mainly consist of types C and D cycles (Figs. 4 and 5). Progressive shoaling and progradation is also evidenced by an overall thinning-upward stack of the peritidal cycles (Figs. 4 and 5). Thus, the unit lI can be considered as a progradational cycle set that is interpreted to have been deposited during a period of highstand in sea level (Fig. 5). Episodic subaerial exposure during late highstand may be expressed by the carbonate breccias that cap certain peritidal cycles in the unit 1I (Figs. 4 and 5). A major marine-flooding surface is recognized in the top of the unit II (Fig. 5). The unit III is characterized by three repetitions of the meter-scale shallowing-upward cycle (Fig. 4 and 5). Each meter-scale shallowing-upward cycle ranges from 23 to 27

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meters in thickness and is composed of stacked shallow subtidal-dominated cycles (type B cycle) in the lower portion gradually shoaling up to stacked intertidal-dominated cycles (type C or D cycles). A thickening-upward stacking pattern is discernible in the lower part of the unit (Figs. 4 and 5). Higher in the unit, each meter-scale shallowing-upward cycle stacks upward maintaing relatively consistent thickness (Figs. 4 and 5). Thus, the unit III can be interpreted to represent a retrogradational to aggradational cycle set formed during a period of transgressive to early highstand in sea level (Fig. 5). The overlying unit IV can be subdivided into two parts, the lower and the upper (Figs. 4 and 5). The burrowed wackestone dominated subtidal cycles (type B cycle) are common in the lower part, where as the exposure-capped peritidal cycles (type D and E cycles) are common towards the upper part (Figs. 4 and 5). An upward increase in the ratio of tidal flat cycle (type E cycle) to intertidal cycle (type D cycle) is observed in the upper part of the unit in concert with cycle thinning (Figs. 4 and 5). Increasing dolomitization is also associated with this trend. However, the laminated finesized dolomites in this unit can be differentiated from the massive medium-sized dolomites of the unit II. Thus, the unit IV may represent a progradational cycle set that formed during the highstand in sea level (Fig. 5). The upper part of the unit IV contains similar facies to the upper part of the unit II, namely exposure-capped peritidal cycles (Figs 4 and 5). However, the upper part of the unit IV is dominated more by tidal-flat laminates (e.g., type E cycle) and terrigeneous silts as well as carbonate breccias (Figs. 4 and 5). The succeeding unit V is composed of the type A and B cycles that were deposited in middle ramp and shallow subtidal environment (Figs. 4 and 5). A thin basal shale grades upward into nodular mudstone and thick burrowed wackestone that is abruptly covered by shale without a flatpebble conglomerate cap (P1. 7/1). Up section, the cycle mainly consists of thin basal shale overlain by a bundle of thin nodular mudstone (Fig. 4 and PI. 7/2). Cycle stacking patterns in the unit V, however, do not show a consistent thickness change (Figs. 4 and 5). The lack of consistent cycle thickness change is interpreted to have resulted from the unfilled accommodation, which is common in transgressive deep subtidal deposits (Gianniny and Simo, 1996). Thus, the unit V can be interpreted to represent a retrogradational cycle set that formed during transgression (Fig. 5). The contact of the unit V with the underlying unit IV is a single sharp surface that can be recognized across the entire platform. Locally, lime-sandstone and thin transgressive lag deposit overlies this surface (PI. 7/3). The abrupt landward shift of facies across this surface suggests rapid flooding and an associated increase in accommodation; this contact may be a major marine-flooding surface. 4 BIOSTRATIGRAPHIC CONSIDERATIONS Macrofossils such as trilobites, cephalopods, and brachiopods show that the Maggol Limestone is Lower and Middle Ordovician in age (Kobayashi, 1966). Based on a relatively limited number of cephalopod specimens,

Kobayashi (1966) designated three fossil horizons in the Maggol Limestone: the Manchuroceras, Polydesmia, and Sigmorthoceras horizons in ascending order. The Manchuroceras and Polydesmia horizons in the lower and middle Maggol Limestone may be dated to Lower Ordoviclan, and the Sigmorthoceras horizon in the upper Maggol Limestone is assignable to Middle Ordovician (Kobayashi, 1966). Conodonts have been used to date the Maggol Limestone of the Backunsan Syncline (Lee and Lee, 1971; Lee, 1976; Lee and Lee, 1990). Detailed conodont biostratigraphy in the Dongjeom area indicates that the upper Maggol Limestone contains a late Arenigian to early Darriwilian fauna (Fig. 6; Lee and Lee, 1990). The conodonts of the unit HI include Aurilobodus leptosomatus, Drepanodus altipes,

Histoidella serrata, Paltodus inconstans, Rhipidognathus laiwuensis, R. maggolensis, Scolopodus asperus, S. cotv~utiformis, S. giganteus, and Ulrichodina wisconsiensis (Fig. 6; Lee and Lee, 1990). However, the conodonts of the units IV and V are relatively rare, but include the long range taxa such as Tangshanodus tangshanensis, Acontiodus

viriosus, Belodella rigida, Drepanodus homocurvatus, Scolopodus euspinus, S. eburnus, S. flexilis, S. nogamii, Triangulodus changshanensis (Fig. 6). The overlying Jigunsan Shale and Duwibong Limestone provide a Darriwilian conodont fauna that includes Aurilobodus aurilobus, A.

simplex, A. serratus, Dapsilodus compressus, Drepanodus flexuosus, Drepanoistodus suberectus, Eoplacognathus suecicus, Erraticodon tangshanensis, Panderodus gracilis, and Plectodina onychodonta (Fig. 6; Lee and Lee, 1990). Based on the conodont occurrence, Lee and Lee (1990) established five conodont biozones in the upper Maggol Limestone, the Jigunsan Shale, and the Duwibong Limestone (Fig. 6). The Aurilobodus leptosomatus (unit III) and unnamed biozones (units IV and V) in the upper Maggol Limestone are assignable to North American Mid-continent faunal zones 2, 3 and lower 4 (Fig. 6). Whereas the Eoplacognathus suecius-E, jigunsanensis biozone in the Jigunsan Shale, and the Plectodina onychodonta and Aurilobodus serratus biozones in the Duwibong Limestone correspond to North American Mid-continent faunal zones upper 4 and 5, respectively (Fig. 6). A reappraisal of the conodonts described above and their correlation to more recent conodont zonations (e.g., Miller, 1988; Ross et al., 1993) indicates that the Arenigian and Darriwilian boundary may exist between the unit Ili and the unit IV (Fig. 6). Just below and above this boundary conodonts representing Microzarcodina parva biozone were recently reported by Choi (2000). Hence, all subaerial exposure surfaces above this level are potential candidates for the Sauk-Tippecanoe cratonic sequence boundary of Sloss (1963, 1988), which resulted from a pronounced drop in secondorder eustatic sea level during the early Middle Ordovician. As well, the Lower and Middle Ordovician boundary (the Ibexian and Whiterockian boundary) appears to coincide approximately with the transition from the unit II and the unit III below the Aurilobodus leptosomatus biozone (Fig. 6). Of particular importance is that a marked decrease in conodont abundance records above the boundary between

43

Fig. 5. Simplified columnar section of the middle and upper Maggol Limestone and tile overlying Jigunsan Shale. Two distinctive T-R cycles are recognized in the units l to IV. The overlying unit V is a stack o1 subtidal fourth-ordeF cycles. Each boundary of T-R cycles represents a major marine-flooding surface. Note the 4:1 bundling of fifth-ordcr cycles within totulh-ovder cycle,,,. Abbreviations: MI:S = major marine-flooding surface; Min A Z = Minimum Accommodation Zone: Max AZ = Maximum Accommodation Zone: TST = transgressive systems tract: HST = highstand systems tract.

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, of tile transgressive N'htggol lSlnestonc (the unit V) that overlies tile Sauk-Tippccanoc sequence boundar\. Fig. I. Tile Sauk-Tippccum~c sequcnce boundary bet\~ecn tile unit IN,' and the unit V. This boundary rcptcsents a conlIormablc Iran,,gx+essJxe surface (TS) or a drowning tillt_olliolnlitv. "Fhc overlying unit \,; is composed ~)1the iS,pc i\ and B cycles thai \\crc cleposilcd in middle ralnp :.lnd shallow sublidaI en\ htmnlcnts. The unit IV beneath Ille trail~,glcssi\c %lllac_e ix ciluracteri/ed b v tile lIracttire breccia ( FB i. I falulllcr foF ',tale. I-i7. 2. T x pical outcrop expie,,sion tel the middle to deep ranlll c\clc in the tlpl)crinosl Maggol LimeXlOlle. Tile cxcle consist~ of thin c:.iicareotlS black shale (CBS/