Hwajeol Formation Research Articles

Ribbon rocks revisited: the upper Cambrian (Furongian) Hwajeol Formation, Taebaek Group, Korea

Ribbon rocks revisited: the upper Cambrian (Furongian) Hwajeol Formation, Taebaek Group, Korea

Depositional origin of subtidal meter-scale cyclic successions in the Cambrian Hwajeol Formation

Meter-scale cyclic successions of limestone and marl or mudstone commonly formed in shallow marine subtidal environments during the early Paleozoic. Such alternations have been believed to represent primary processes and/or post-depositional. The cyclic facies changes in association with primary processes have been interpreted as allocyclicity and autocyclicity. The Furongian Hwajeol Formation of the Taebaek Group in mid-eastern Korea is host to such early Paleozoic subtidal cyclic successions, showing limestone- mudstone alternations. Sedimentological and geochemical analyses including diagenetically inert element ratio analyses suggest that the cyclic successions were originated from sedimentary origin rather than differential diagenesis. Furthermore, trace elements analysis reveals that the mud fractions formed under suboxic or anoxic conditions, while the limestone facies accumulated under oxic conditions. The repetitions of oxic and oxygen-depleted conditions can be interpreted to be a result of the repeated deisolation and isolation of the water mass in the semi-closed epicontinental sea partly restricted from the open ocean. Repetitive opening and closing of the sea caused vitalization and deactivation of the carbonate factory in the platforms. Accordingly, this restriction of water-mass can deactivate the carbonate platform on the silled basin. This hypothesis provides a plausible explanation for the formative mechanism of the subtidal cyclic successions with meter-scale alternations of subtidal limestone and gradually upward thickening mudstone beds. The gradually thickening of mudstone beds would reflect an increasingly lengthened period of closure and deoxygenation of the semi-closed epicontinental sea due to gradual decrease in rising rate during low-order transgression and gradually extended duration of higher order relative sea-level stillstand or fall superimposed low-order transgressions.

Late Cambrian missing link in macroborer evolution preserved in intraclasts

Macroborings are known to be rare to absent in middle to late Cambrian successions, the time interval between the end-early Cambrian archaeocyath extinction and the advent of new boring organisms during the mid-late Ordovician (the Ordovician Bioerosion Revolution). The occurrence of macroborings was controlled mainly by hard-substrate availability; archaeocyath reefs and hardgrounds provided synsedimentarily lithified substrates during the early Cambrian and Ordovician, respectively. In this study, we report bioerosion structures from micritic intraclasts of flat-pebble conglomerates in the Hwajeol Formation (Furongian), Taebaeksan Basin, Korea. Two types of macroborings (Trypanites and cf. Gastrochaenolites) are recognized, both of which penetrate micritic clasts and are filled with bioclastic grainstone matrix. Several lines of evidence, including the sharp boundaries and unaltered shapes of macroborings, as well as the occurrence of macroborings penetrating the coating of iron- and/or manganese oxide-coated micritic clasts, indicate that these macroborings formed after the cementation of micritic substrates. The Hwajeol macroborings would have formed on micritic clasts and/or hardgrounds that were eroded and formed flat pebbles. The presence of bioerosion structures within clasts of the Hwajeol flat-pebble conglomerates supports a previous hypothesis that macroborers survived in hardgrounds during the middle to late Cambrian, after the extinction of archaeocyath reefs. In addition, the Hwajeol cf. Gastrochaenolites is the earliest of its kind, implying that there could be more kinds of macroborings hidden within Cambrian flat-pebble conglomerates.

Ontogeny of the two co-occurring middle Furongian (late Cambrian) shumardiid trilobites and the protaspid morphology of shumardiids

AbstractShumardiid trilobites had a small, unique morphology, and formed a key constituent in trilobite faunas during the Cambrian–Ordovician. Because of their unusual morphology, they have been the subject of research s, into various aspects such as their life habit, functional morphology, evolutionary origin and ontogeny. Originally, a flat, adult-like protaspid morphology was suggested for shumardiids, but subsequently a bulbous protaspid morphology interpreted to be associated with metamorphosis was also suggested for this unique trilobite group. This article documents the ontogeny of the two co-occurring shumardiid trilobites, Akoldinioidia latus Park and Kihm and Koldinioidia choii Park and Kihm, from the middle Furongian Hwajeol Formation, Taebaeksan Basin, Korea. Interestingly, protaspides of the two shumardiids have a bulbous morphology. Given the stratigraphic occurrences of the two shumardiids, it can be inferred that commutavi protaspis appeared quite early in the shumardiid evolution. The co-occurrence of the two closely related trilobites is reminiscent of sexual dimorphism, although further evidence is required to prove it. The appearance of metamorphosis-undergoing protaspides in the Furongian shumardiids may have been due to the onset of increasing ecological pressure in the early phase of the Great Ordovician Biodiversification Event.

Reefs in the Early Paleozoic Taebaek Group, Korea: A Review

AbstractVarious early Paleozoic (Cambrian Series 3–Middle Ordovician) reefs are found in the Taebaek Group, eastern Korea, located in the eastern margin of the Sino‐Korean Block. They occur in every carbonate‐dominant lithostratigraphic unit of the group, but their morphology and composition differ markedly. The Daegi Formation (middle Cambrian: Cambrian Series 3) contains siliceous sponge‐Epiphyton reefs formed in a shallow subtidal environment, which is one of the earliest metazoan‐bearing microbial reefs after the archaeocyath extinction. The Hwajeol Formation (upper Cambrian: Furongian) encloses sporadic dendrolites consisting of Angulocellularia, which developed in a relatively deep subtidal environment, representing a rare deeper water example. The onset of the Ordovician radiation resulted in the formation of microbialite–Archaeoscyphia–calathiid patch reefs in shallow subtidal deposits of the Lower Ordovician Dumugol Formation. Subsequent late Early Ordovician relative sea‐level fall established extensive peritidal environments, forming microbial mats and stromatolites of the Lower–Middle Ordovician Makgol Formation. Ensuing Ordovician radiation resulted in one of the earliest metazoan skeletal reefs of the Middle Ordovician Duwibong Formation, constructed by stromatoporoid Cystostroma and bryozoan Nicholsonella, and developed around shallow shoals. These reefs reflect ongoing evolution and sea‐level change during the early Paleozoic, and exemplify a rare glimpse of peri‐Gondwanan records of reef evolution, which warrant detailed investigations and comparison with their counterparts in other regions.

Revision of the conodont zonation of the uppermost Hwajeol Formation (Furongian), Taebaeksan Basin, Korea

The uppermost interval of the Hwajeol Formation, Taebaeksan Basin, has been assigned to the Fryxellodontus inornatus-Monocostodus sevierensis-Semiacontiodus lavadamensis Zone. The main species in the zone are Fryxellodontus inornatus Miller, Hirsutodontus hirsutus Miller, Monocostodus sevierensis (Miller), Semiacontiodus lavadamensis (Miller), S. nogamii Miller, Utahconus utahensis (Miller), and Cordylodus proavus Muller. These species were re-examined to evaluate the biostratigraphy of the Bangteogol and Maesangol sections. Data were added from eleven sections around the Taebaeksan Basin, which were studied previously by the first author. Conodont occurrences above the Cambrooistodus minutus Zone are characterized by a lack of vertical and lateral continuity in sections. Conodont recovery is relatively poor but is sufficient for biostratigraphic assignment. This trend of conodont occurrence seems to be related to sedimentary process, including debris slides and eustatic sea-level fluctuation. Nevertheless, detailed study of the characteristic species of the zone leads to the conclusion that retention of the zone is reasonable. However, it is renamed the M. sevierensis-S. lavadamensis Zone because Fryellodontus inornatus occurs stratigraphically lower than the other two nominate species. The M. sevierensis-S. lavadamensis Zone is correlated with equivalent zones of western USA, Canada, North China, and Australia.

Furongian (late Cambrian) trilobites from the Asioptychaspis subglobosa Zone of the Hwajeol Formation, Korea

Park, T.-Y.S. & Kihm, J.-H., 3.10.2014. Furongian (late Cambrian) trilobites from the Asioptychaspis subglobosa Zone of the Hwajeol Formation, Korea. Alcheringa 39, xxx–xxx. ISSN 0311-5518The Hwajeol Formation of the Taebaek Group, Korea extends from the Jiangshanian Stage to Cambrian Stage 10. The lower part of the formation incorporates the Asioptychaspis Zone, which is closely correlated to the Asioptychaspis–Tsinania Zone of North China. This study documents silicified trilobites from the lower part of the Hwajeol Formation in the Sagundari section. Asioptychaspis subglobosa occurs throughout the interval; hence the biozone is renamed as the Asioptychaspis subglobosa Zone in this study. The Asioptychaspis subglobosa Zone of the Hwajeol Formation yields 14 polymerid species belonging to 14 genera. Of these, 11 species including three new species are documented for the first time in Korea. Among other taxa, the occurrence of Caznaia is stratigraphically significant because two biozones were established in Australia based on representatives of this genus: i.e., the Rhaptagnostus clarki patulus–Caznaia squamosa–Hapsidocare lilyensis and Rhaptagnostus clarki prolatus–Caznaia sectatrix zones in ascending order. Owing to the lack of cosmopolitan trilobites, the international correlation of these Iverian biozones has been equivocal. The occurrence of Caznaia from the Asioptychaspis subglobosa Zone of Korea indicates that the Rhaptagnostus clarki patulus–Caznaia squamosa–Hapsidocare lilyensis and Rhaptagnostus clarki prolatus–Caznaia sectatrix zones of Australia are correlated with the Asioptychaspis subglobosa Zone of Korea and the Asioptychaspis–Tsinania Zone of North China.Tae-Yoon S. Park [typark@kopri.re.kr] and Ji-Hoon Kihm [jhkihm@kopri.re.kr] (corresponding author), Division of Polar Earth-System Sciences, Korea Polar Research Institute, Incheon 406-840, Korea.

Conodonts from the Sesong Slate and Hwajeol Formation (Guzhangian to Furongian) in the Taebaeksan Basin, Korea

AbstractAbundant conodont elements have been recovered from the Hwajeol Formation, to allow five zones to be erected: Proconodontus, Eoconodontus notchpeakensis, Cambrooistodus minutus, Cordylodus proavus, and Fryxellodontus inornatus‐Monocostodus sevierensis‐Semiacontiodus lavadamensis zones, in ascending order. More confident biozones are recognized in the Sesong Slate and lower Hwajeol Formation in the Makgol section, a part of the southern limb of the Baekunsan syncline, Taebaeksan Basin, Korea, especially focusing on the conodont biostratigraphic boundary of two units, and the subdivision potential of the previous “Proconodontus Zone”, lowermost biozone of the Hwajeol Formation. Similarly, only a few conodont elements recovered from upper 14.5 m interval, namely the Furongian portion of the Sesong Slate, in the Makgol section did not allow erection of a biozone. Nevertheless, this part of the unit plus the basal 2.5 m interval of the Hwajeol Formation is characterized by the occurrence of Prooneotodus rotundatus (Druce and Jones), Teridontus nakamurai (Nogami), Phakelodus elongatus (An) and Phakelodus tenuis Müller, This interval marks the early Furongian “Prooneotodus rotundatus Zone”. The rest of the measured section yielded relatively abundant conodonts, so three conodont biozones are proposed, based on the successive appearance of key species: Proconodontus tenuiserratus, Proconodontus posterocostatus, and Proconodontus muelleri zones, in ascending order, and thus allowing subdivision of the previous “Proconodontus Zone”. The four conodont biozones are correlated with the relevant biozones of North and South China, and North America.

Ontogeny of the Ptychaspidid TrilobiteQuadraticephalus elongatusKobayashi, 1935 from the Furongian (Late Cambrian) Hwajeol Formation, Korea

The development of the trilobite pygidium involves both an articulation process at the frontal part and the formation of new segments at the rear end, and hence the development of the meraspid pygidium entails complicated morphological changes. This study deals with the ontogeny of the Furongian (late Cambrian) ptychaspidid trilobite,Quadraticephalus elongatus(Kobayashi, 1935), from the Hwajeol Formation of the Taebaek Group, Taebaeksan Basin, Korea, with a special focus on the segmentation process during the meraspid pygidial development. Compared to the ontogeny of a ptychaspidid trilobite,Asioptychaspis subglobosa(Sun, 1924), which is assumed to be an ancestral species ofQ. elongatus, the convexity of the cranidium ofQ. elongatusincreased in a slower rate; the yoked free cheek ofQ. elongatussplits to form a ventral median suture in a later developmental stage; and, a rim-like ridge, which disappeared in the early holaspid pygidium ofA. subglobosa, was maintained in the late holaspid period ofQ. elongatus. These morphological changes with growth imply that paedomorphosis was involved in the evolution ofQ. elongatus. Eleven stages are recognized for the meraspid pygidial development, which began with an accumulation phase during which the number of segments increased from three to seven, followed by an equilibrium phase with seven segments, and ended up with a depletion phase during which the number of segments within the pygidium decreased to four. During the depletion phase, the pygidial length did not increase or even slightly decreased. The onset of the epimorphic phase, in which the total number of trunk segments does not increase anymore, precedes the onset of the holaspid period, demonstrating that the developmental mode ofQ. elongatusis protomeric.

Furongian conodonts from the Machari Formation in the Gokgeum section, Yeongwol, Korea

This is a conodont biostratigraphic study on the Machari Formation in the Gokgeum section, exposed nearby Mt. Orosan, Hutan-ri, west Yeongwol-gun, Korea. Three biostratigraphic intervals are recognized by euconodont taxa in the Furongian part of the Gokgeum section: i.e., Unnamed, Proconodontus muelleri, and Eoconodontus notchpeakensis zones in ascending order. The Unnamed zone herein is equivalent to the previous “Barren Interzone”, the Proconodontus muelleri Zone is also equivalent to the previous Proconodontus muelleri-Cambrooistodus cambricus Zone, and the Eoconodontus notchpeakensis Zone is a new, uppermost one. Therefore, there are four conodont zones of the Machari Formation known hitherto; i.e., late Middle Cambrian Westergaardodina matsushitai Zone and three Furongian zones including Unnamed zone, Proconodontus muelleri Zone and Eoconodontus notchpeakensis Zone in ascending order. These Machari conodont biozones are correlated with conodont biozonal frameworks in western USA, Canada, Argentina, North and South China, Australia and Hwajeol Formation in the Duwibong area, Korea.

Dikelocephalid trilobites from theEosaukiafauna (Upper Furongian) of the Taebaek Group, Korea

TheEosaukiafauna is proposed for the upper Furongian trilobite assemblage from the interval spanning from the upper part of the Hwajeol Formation to the lowermost part of the Dongjeom Formation in the Taebaek Group, Taebaeksan Basin, Korea. It is characterized by the dominance of dikelocephalid trilobites comprisingEosaukia micropora, E. bella, E. acuta, Mictosaukiacf.M. globosa, andTaebaeksaukia spinatan. gen. n. sp. Taxonomic reappraisal of the genusMictosaukiathat has been employed as an upper Cambrian index taxon in eastern Gondwana reveals that more than half of the species ofMictosaukiabelong inEosaukia.This study clarifies the generic concept ofEosaukia, which provides a more reliable biostratigraphic correlation for the upper Furongian strata in eastern Gondwanan regions. TheEosaukiafauna is correlated with the“Mictosaukia”faunas from the upper Fengshanian of North China, upper Taoyuanian of South China, and upper Payntonian of Australia.

Ontogeny of the Furongian (late Cambrian) remopleuridioid trilobiteHaniwa quadrataKobayashi, 1933 from Korea: implications for trilobite taxonomy

AbstractThe monophyly of the trilobite Order Asaphida has been challenged. The Superfamily Remopleuridioidea was included in the Order Asaphida, based on the ventral median suture and highly bulbous protaspis of the late Furongian–Tremadocian representatives of the group. The remopleuridioid,Haniwa quadrataKobayashi, 1933 from the Furongian (late Cambrian) Hwajeol Formation of Korea, represents a primitive morphology of the Remopleuridioidea. This trilobite does not have a typical globular morphology of asaphoid protaspis, and the free cheeks remained yoked together during the whole of the development. This supports the previous proposition that the Superfamily Remopleuridioidea should be excluded from the Order Asaphida. In addition, the evolution of a highly globular protaspis of the Ordovician remopleuridioid trilobites from the less bulbous protaspis corroborates the possibility of multiple evolutions of a highly globular protaspis. It can be argued that the possession of a highly globular protaspis does not guarantee the membership of the Order Asaphida, and thus the concept of the Order Asaphida should be emended.

Post‐embryonic development of the Furongian (late Cambrian) trilobite Tsinania canens: implications for life mode and phylogeny

The current concept of the order Asaphida was proposed to accommodate some Cambrian and Ordovician trilobite clades that are characterized by the possession of a ventral median suture. The family Tsinaniidae was recently suggested to be a member of the order Asaphida on the basis of its close morphological similarity to Asaphidae. Postembryonic development of the tsinaniid trilobite, Tsinania canens, from the Furongian (late Cambrian) Hwajeol Formation of Korea, reveals that this trilobite had an adult-like protaspis. Notable morphological changes with growth comprise the effacement of dorsal furrows, sudden degeneration of pygidial spines, regression of genal spines, and loss of a triangular rostral plate to form a ventral median suture. Programmed cell death may be responsible for degenerating the pygidial and genal spines during ontogeny. Morphological changes with growth, such as the loss of pygidial spines, modification of pleural tips, and effacement of dorsal furrows, suggest that T. canens changed its life mode during ontogeny from benthic crawling to infaunal. The protaspid morphology and the immature morphology of T. canens retaining genal and pygidial spines suggest that tsinaniids bear a close affinity to leiostegioids of the order Corynexochida. Accordingly, development of a ventral median suture in T. canens demonstrates that the ventral median suture could have evolved polyphyletically, and thus the current concept of the order Asaphida needs to be revised.

Conodonts from the Hwajeol Formation (Upper Cambrian) in the Seokgaejae area, southeast margin of the Taebaeksan Basin

The Cambrian-Ordovician succession crops out well in the Seokgaejae area of the southeastern margin of the Taebaeksan Basin. The Upper Cambrian Hwajeol Formation of the Seokgaejae section was studied to refine the conodont zones previously proposed and to correlate them with other areas. Based on euconodont species, four zones are recognized: a new and unnamed informal lowermost and the Proconodontus, Eoconodontus notchpeakensis and Cambrooistodus minutus zones in ascending order. This zonal framework is basically equivalent to that of southern (Lee and Lee, 1988; Lee, 2001, 2002) and northern (Lee, 1989, 1990, 2004) limbs of Paekunsan syncline. The four zones of Seokgaejae section are correlated with those of other parts of the world, including North China, North America and Australia. The study was unable to refine conodont zonation of the basal and uppermost parts of the formation. This may be related to either sealevel change or structural control on sediment deposition in the section.

Furongian trilobites from theAsioptychaspis andQuadraticephalus zones of the Hwajeol Formation, Taebaeksan Basin, Korea

This paper describes the upper Furongian trilobites from the Hwajeol Formation and, based on these trilobite faunal assemblages, proposes formally two trilobite biozones, theAsioptychaspis andQuadraticephalus zones. A total of 14 species belonging to ten genera are described: i.e.,Micragnostus sp.,Pseudagnostus planulatus, Koldinioidia sp.,Hamashania pulchera, Hamashania sp. cf.H. busiris, Asioptychaspis subglobosa, Changia sp.,Quadraticephalus elongatus, ptychaspidid genus and species indeterminate,Lophosaukia orientalis, Lophosaukia? sp.,Tsinania canens, Tsinania sp., andHaniwa sosanensis. TheAsioptychaspis Zone is characterized by predominance ofAsioptychaspis subglobosa, while the succeedingQuadraticephalus Zone is dominated byQuadraticephalus elongatus, Lophosaukia orientalis, Haniwa sosanensis, andMicragnostus sp. TheAsioptychaspis andQuadraticephalus zones are well correlated with thePtychaspis- Tsinania andChangia (orQuadraticephalus) zones of North China, respectively. These two trilobite assemblages of the Hwajeol Formation support a strong paleogeographical link between the Taebaeksan Basin and North China Platform during the late Furongian.

Late Cambrian conodonts from Cheungsan, Kangwon Province, Korea

Conodonts from 20 samples, collected from two measured sections in the Late Cambrian Hwajeol Formation in the Cheungsan area, Jeongseon-gun, Kangwon Province, Korea, are classified into 27 species and 18 genera by both multielement and form taxonomies. Five conodont biozones are recognized in the formation: Proconodontus, Eoconodontus notchpeakensis, Cambrooistodus minutus, Cordylodus proavus and Fryxellodontus inornatus-Monocostodus sevierensis-Semiacontiodus lavadamensis Zones in ascending stratigraphic order. These zones are correlatable with equivalents in other areas including the Paekunsan syncline of Korea, North China, Iran, Australia and North America.

Conodonts from the Upper Cambrian Sesong and Hwajeol Formations in the Sabuk Area, South Korea

Abstract This paper presents a conodont biostratigraphic study on the Upper Cambrian Sesong and Hwajeol Formations in the Sabuk area, Korea. Two samples near the base of the Sesong Slate at the Pukil section contain lower Upper Cambrian conodonts, comprising Furnishina furnishi, F. kranzae, F. pernical?, F. triangulata, Hertzina elongata, Laiwugnathus doidyxus?, Phakelodus elongatus, Muellerodus? obliquus, Westergaardodina matsushitai and W. moessebergensis. This is the first conodont record of the Upper Cambrian formations recovered yet in the northern limb of the Paekunsan syncline. The faunal assemblage is correlatable with the lower Upper Cambrian W. matsushitai Zone of North China. Four local biozones are recognized in the Hwajeol Formation, i.e. the Proconodontus, Eoconodontus notchpeakensis, Cambrooistodus minutus and Cordylodus proavus Zones in ascending order. This zonal scheme is essentially equivalent to that of the southern limb of the Paekunsan syncline and they are correlatable with zones in other parts of the world, including North America, Australia, Iran and China.