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ISSN : 1225-6692(Print)
ISSN : 2287-4518(Online)
Journal of the Korean earth science society Vol.41 No.4 pp.367-380
DOI : https://doi.org/10.5467/JKESS.2020.41.4.367

Detrital zircon U-Pb ages of the uppermost Jinju Formation in the Natural Monument No. 534 ‘Tracksite of Pterosaurs, Birds, and Dinosaurs in Hotandong, Jinju’, Korea

Yong-Un Chae1, Jong Deock Lim2, Cheong-Bin Kim3, Kyung Soo Kim4, Sujin Ha1, Hyoun Soo Lim1*
1Department of Geological Sciences, Pusan National University, Busan 46241, Korea
2National Research Institute of Cultural Heritage, Daejeon 34122, Korea
3Department of Physics Education, Sunchon National University, Suncheon 57922, Korea
4Department of Science Education, Chinju National University of Education, Jinju 52673, Korea
*Corresponding author: tracker@pusan.ac.kr Tel: +82-51-510-2251
August 13, 2020 August 25, 2020 August 26, 2020

Abstract


LA-MC-ICP-MS zircon U-Pb dating was conducted to constrain the timing of fossil formation and the depositional age of the uppermost Jinju Formation located in Natural Monument No. 534 (Tracksite of Pterosaurs, Birds, and Dinosaurs in Hotandong, Jinju), and 87 Cretaceous, 1 Precambrian, and 2 Jurassic zircons were obtained from 90 valid analytical points. Most Cretaceous zircons were found to have a youngest graphical peak age of ca. 106.5 Ma, suggesting the depositional age of the uppermost Jinju Formation. Based on this study and previous works, the average sedimentation rate of the Jinju Formation was calculated to be approximately 0.17-0.31 mm/year in the Milyang Subbasin, and the Cretaceous zircons of the uppermost Jinju Formation seem to have originated mainly from the western or northwestern parts of the Gyeonggi Massif. Unlike the Nakdong and Hasandong formations of the Sindong Group, most zircons analyzed in the uppermost Jinju Formation were Cretaceous. This suggests that volcanic activity occurred in the area closer to the Gyeongsang Basin due to the roll-back of subducting paleo-Pacific Plates during the Jinju period.



초록


    National Research Foundation of Korea
    2020R1A2C1012522

    Introduction

    In Hotandong area, a large number of pterosaur, bird, and dinosaur footprint fossils were discovered from the Jinju Formation during the development project of the Jinju Innovation City in 2011. Among the discovered footprint fossils, the pterosaur footprints were evaluated to be one of the best in the world in terms of preservation condition, population, density, and diversity. In recognition of those academic value and the importance of other fossils, this site was designated as Natural Monument No. 534 (Tracksite of Pterosaurs, Birds, and Dinosaurs in Hotandong, Jinju) in 2011. In addition, the ‘Jinju Pterosaur Tracks Museum’ has been established to display well-preserved fossils among the excavated specimens and two protective constructions have been installed for on-site preservation of some track fossils. Diverse body and trace fossils have been reported in the Cretaceous Jinju Formation (Table 1, Fig. 1). This formation is also important because the direct outcrop-scale evidence of volcanic activity has been reported for the first time in the Gyeongsang Supergroup (Choi, 1986a;Noh and Park, 1990;Lee and Lim, 2008).

    So far, several researchers proposed the maximum depositional ages of the Jinju Formation based on the detrital zircon U-Pb dating results in different regions (Lee, Y.I. et al,. 2010;Lee, T.-H. et al., 2010;Lee et al., 2017;Lee, T.-H. et al., 2018a). In the previous study, SHRIMP zircon U-Pb dating was performed to investigate the age of the fossils in the Jinju Pterosaur Track Museum. As a result, 3 Barremian, 21 Aptian, and 9 Albian zircons were obtained from the total of 35 zircons (Kim, K.S. et al., 2016). Such age distribution was interpreted as a result of continuous volcanic activity, and among them, ca. 109 Ma, the weighted mean age of 9 Albian zircons, was proposed as the maximum depositional age of the Jinju Formation at the site (Kim, K.S. et al., 2016). Since this age is just the weighted mean age of 9 Albian zircons without enough consideration, however, it seems to be inaccurate, and the depositional age of the Jinju Formation is still in debate.

    In this study, we collected Jinju sandstone samples containing syndepositional volcanogenic zircons at the Natural Monument No. 534 site, and LA-MC-ICP-MS U-Pb dating was carried out to investigate the reliable depositional age and track formational age of the uppermost Jinju Formation. This study is also expected to provide the important information of the potential provenance of Cretaceous zircons and upward-increasing population of Cretaceous zircon in the Sindong Group.

    Study Area and General Geology

    The Jinju Inovation City area including Natural Monument No. 534 is a place where more than 2,000 pterosaur footprints and 78 pterosaur trackways with various shapes and sizes discovered in 6 strata. In addition, this area is a very rare tracksite in the world because the preservation condition of distinct webbed manus imprints, claw marks and phalangeal joint trace of pterosaur is excellent (Fig. 2, Kim, K.S. et al., 2016). It is also paleoecologically important because of the occurrence of various fossils, such as bird footprints (>1,000), dinosaur footprints (>100 theropods, >10 sauropods, >30 ornithopods), shellfishes, carbonized trees, woods, aquatic plants, insects, crocodile footprints and traces of swimming crocodiles (Kim, K.S. et al., 2016;Lockley et al., 2020).

    The Cretaceous Gyeongsang Basin is the largest nonmarine sedimentary basin in the Korean Peninsula, occupying a wide area in the Gyeongsang Province. It is known to have been formed by the oblique subduction of the Paleo-Pacific Plate beneath the Eurasian Plate, but its formation mechanism is not yet clearly recognized. This basin can be subdivided into Yeongyang, Uiseong, and Milyang subbasins from north to south, and the Gyeongsang Supergroup can be divided into the Sindong, Hayang, Yucheon, and the Bulguksa intrusive groups in ascending order based on the intensity of syndepositional magmatism (Chang, 1975, 1977;Choi, 1986b;Chang et al., 2003). The Sindong Group is lithostratigraphically divided into the Nakdong, Hasandong, and Jinju formations from the bottom to the top (Choi, 1986b). The study area is located in the Milyang Subbasin, and belongs to the uppermost Jinju Formation (Fig. 1a). The Jinju Formation is made up mainly of lacustrine dark grey to black mudstones, black shales, and channel sandstones. In the study area, however, tuffaceous sandstones and sandstones containing much calcareous materials are commonly found (Fig. 3, Kim, K.S. et al., 2016).

    In the first study on mollusk in the Jinju Formation, the depositional age of this formation was suggested as a relatively wide period covering the entire Early Cretaceous (Fig. 1c, Kobayashi and Suzuki, 1936). Since then, many efforts have been made to constrain the depositional age of the Jinju Formation more precisely based on various paleontologic studies (Fig. 1c, Choi, 1985;Choi and Park, 1987;Yi et al., 1994;Park and Chang, 1998;Park et al., 2013). Among them, the latest study on insect fossils suggested the depositional age of this formation as Albian (Nam and Kim, 2016).

    As already known, however, the radiometric dating is more precise than paleontologic data. So far, several zircon U-Pb dating results have been reported in the Jinju Formation (Fig. 1b, Lee, Y.I. et al., 2010;Lee, T.-H. et al., 2018a). The first detrital zircon U-Pb dating performed in the Jinju Formation proposed a maximum depositional age of 106.0±1.9Ma using the youngest single grain age method (Lee, Y.I. et al., 2010). However, the youngest single grain age has a weakness in the possibility of Pb loss due to zircon alteration and significant age deviations between individual zircons derived from an igneous rock (e.g. Sato et al., 2016). Therefore, when the maximum depositional age is calculated by the youngest single grain age method, there is the possibility that the outlier appears as the youngest age (Lee, 2016;Lee, T.-H. et al., 2018b). To avoid such problem, a more conservative approach is to use the weighted mean age or concordia age of the youngest clustered analytical spots, and the maximum depositional age of the lower Jinju Formation obtained using this method was suggested as 112.4±1.3Ma (Lee, T.-H et al., 2010).

    Samples and Analytical Method

    Dark gray medium-grained sandstone sample (HT-1) was collected from the Natural Monument No. 534 site in Hotandong, Jinju (Figs. 3c and 3d). It is mainly composed of siliciclastic (mainly quartz and feldspar) and carbonate grains (peloid and ooid), and the intergranular matrix is filled with micrite (Fig. 4). In addition, clotted ooid and pelloid grains, and chert fragments are occasionally observed (Fig. 4a). The size of quartz grains ranges from 100 to 700 μm, but most of them fall in the range between 250 and 300 μm, and quartz grains are characterized by straight extinction with angular to subangular form (Fig. 4).

    Zircon grains were separated using the conventional heavy liquid and magnetic techniques. Then, they were mounted in epoxy resin and the mount was polished to expose the internal sections of the zircons. Before LA-MC-ICP-MS U-Pb analysis, backscattered electron and cathodoluminescence images of the polished zircon surface were obtained using a scanning electron microscope (JEOL6610LV) installed in Korea Basic Science Institute (KBSI). Then, these images were used to observe the internal structures and select the analytical spots (Fig. 5a). For U-Pb age dating, 99 zircons were analyzed using LA-MC-ICP-MS in KBSI (Fig. 5a). And each spot in 91500 standard zircon (1065.4±0.3Ma; Wiedenbeck et al., 1995) per 5 analytical spots in HT-1 zircons was analyzed using a laser beam with 15 μm in diameter. Raw data was processed using the Iolite 2.5 (Paton et al., 2011) and ISOPLOT program (Ludwig, 2008). Analytical spots showing >15% discordant age were excluded from the age calculation in order to increase reliability by eliminating problems of mixing different ages in a single zircon grain and Pb loss (Table 2) (cf. Gehrels et al., 1995). The 206Pb/ 238U and 207Pb/ 206Pb ages were applied, respectively, for zircons younger and older than 1000 Ma.

    Results

    Most zircons from the HT-1 sample show prismatic to short prismatic form, and are approximately 100- 250 m in size (Fig. 5a). In addition, most zircons show distinct growth zoning and relatively constant brightness in CL images (Fig. 5a). All the analyzed Jinju zircons have a Th/U ratio of more than 0.2, except for one Precambrian zircon (HT-1-3) and two Cretaceous zircons (HT-1-46 and 47) (Table 2, Fig. 5b). And 90 valid spots were obtained out of 99 analytical spots, and the U-Pb ages range from the Precambrian to Cretaceous (Table 2). However, except for one Precambrian zircon (1956.4±8.3 Ma) and two Jurassic zircons (170.3±1.2 and 166.7±1.3Ma), the ages of 87 zircons are clearly concentrated in the Cretaceous period (Fig. 5c and d). Among them, the minimum and maximum apparent ages are 100.8±1.1 and 115.0±1.4 Ma, respectively (Table 2). And the youngest graphical peak age and weighted mean age were calculated to be ca. 106.5 and 107.2±0.4Ma (n=82, MSWD=10.6), respectively (Fig. 5d).

    Discussion

    Depositional age of the Natural Monument No. 534 tracksite and average sedimentation rate of the Jinju Formation

    Most zircons, except Precambrian zircons, have Th/ U ratios exceeding 0.2 indicating igneous origin (Hoskin and Schaltegger, 2003;Hartmann and Santos, 2004). In general, the shape, size, and brightness in the CL image of zircons separated from sandstone are diverse if they were derived from various source rocks. However, most zircons analyzed in this study show similar characteristics indicating the dominant source rock (Fig. 5a). The dominance of angular to subangular quartz grains showing parallel extinction, and the concentrated zircon ages (ca. 106.5Ma) in the probability density distribution also support that the most Cretaceous zircons were originated from the syndepositional or nearly syndepositional volcanism (Fig. 5c). Excluding the 5 spots from the t-test, the 206Pb/ 238U weighted mean age using 82 spots was calculated to be 107.2±0.4Ma (n=82, MSWD=10.6) (Fig. 5d). However, the deviation in the ages of the Cretaceous zircons is slightly large. In addition, there are two peaks at about 106.5 and 110Ma in the probability density distribution and their wings overlap each other (Fig. 5c and d). These characteristics suggest the possibility of the mixing with reworked zircons with ca. 110Ma or earlier detrital ones in the main peak. Therefore, it is reasonable to determine the depositional age and fossil-formational age of the study area as ca. 106.5Ma rather than 107.2±0.4Ma. This age, within error range, is in good agreement with the Cheongryongsa Basalt (108.0±2.6Ma; Kim et al., 2011, n=9) in the Chilgok Formation above the Jinju Formation. The maximum depositional age of the Chilgok Formation has been reported to be 108.7 ±0.5 Ma (Lee, T.-H. et al., 2018a, n=11). However, it was calculated through the weighted mean age from only 11 spots out of 170 analytical spots, and there are younger zircons with ca. 105-107Ma, Thus, the depositional age of the Chilgok Formation could be younger by further studies.

    From what has been discussed above, the zircon UPb age of the uppermost Jinju Formation obtained in this study seems to indicate the timing of syndepositional volcanism and the formation age of fossil assemblages. It is appropriate to determine that the tracksite in this study was deposited at ca. 106.5Ma, the Albian period (113-100.5Ma). Considering the depositional age of the uppermost (ca. 106.5Ma) and lowermost (112.4± 1.3 Ma, Lee, T.-H. et al., 2010) Jinju Formation, the depositional period of this formation seems to be about 6 my. Since the thickness of the Jinju Formation in the Milyang Subbasin is about 1,800 m in Jinju area and about 1,000 m in Hapcheon area (Kim and Yun, 1969), the average sedimentation rate can be calculated to be ca. 0.17~0.31 mm/year in the Milyang subbasin.

    Provenance of the Cretaceous zircons in the study area

    In previous studies, the Cretaceous zircons in the Jinju Formation were interpreted to have originated from the west of the Gyeongsang Basin because of the negative εHf(t) values of Cretaceous zircons in the lower Jinju Formation (Lee, 2016). In addition, the spatiotemporal domains of Cretaceous igneous rocks can be divided into four chronological groups (Group I: ca. 119-106 Ma, Group II: ca. 99-87Ma, Group III: ca. 86-82 Ma, Group IV: ca. 76-67 Ma) according to the study on the location of plutonic and volcanic rocks in the Korean Peninsula (Fig. 6, Kim, SW et al., 2016). Among them, the depositional age of the Jinju Formation determined in this study belongs to Group I, and this domain mainly includes the Nanglim Massif, Imjingang Belt, Gyeonggi Massif and Yellow Sea. Additionally, Cretaceous igneous rocks distributed in the Group I area generally tend to rejuvenate from NW to SE direction (Fig. 6, Kim, S.W. et al., 2016). The depositional age of the Jinju Formation is also the later part of Group I, and the age is spatially distributed mainly in the southern or southeastern area of the Group I. Therefore, the Cretaceous zircons of the uppermost Jinju Formation seem to have originated mainly from the western or northwestern part of the Gyeonggi Massif.

    Increase of the Cretaceous zircons over time in the Sindong Group

    According to previous studies, the proportion of Cretaceous zircons in the Nakdong and Hasandong formations is generally lower than that in the Jinju Formation (Lee, Y.I. et al., 2010;Lee, 2016). In addition, the metamorphic and sedimentary rock fragments are dominant in the Nakdong and Hasandong formations, but the volcanic rock fragments derived from syndepositional volcanic activities are commonly found in the upper Jinju Formation (Lee et al., 2015).

    During the Cretaceous, the Korean Peninsula was located in the margin of the Eurasian Plate, and was greatly influenced by igneous activities due to the subduction of the paleo-Pacific Plate. Therefore, the occurrence of syndepositional Cretaceous zircons in the uppermost Jinju Formation is expected to have a spatiotemporal relationship with both the evolution pattern of the subducting sea plate and the evolution of the Sindong Group. According to recent studies on the igneous rocks distributed in the Korean Peninsula, a significant magmatic gap from 160 to 120Ma has been reported in South Korea (Sagong et al., 2005;Kim S.W. et al,. 2012). Furthermore, with the proposal of the new tectonic model, due to shallow subduction of the paleo-Pacific Plate under the Korean Peninsula and its deep subduction under the Northeast China, it was suggested that active igneous activities took place on far northwest region from South Korea during the magmatic gap (Fig. 6b, Kim, S.W. et al., 2012, 2016). Subsequently, the distribution of igneous activities over time also has gradually shifted to the southeastward due to the rollback of the subducting plate. As a result, magmatism resumed on the Korean Peninsula at about 120Ma (Kim, S.W. et al., 2012, 2016). Based on the above results, the upward increase of the Cretaceous zircons in the Sindong Group and the frequent appearance of tuffaceous sandstones in the uppermost Jinju Formation can be interpreted as the effect of the roll-back of the subducting sea plate along with the evolution of the Sindong Group.

    During the Nakdong and Hasandong period, the region of the main igneous activities in the eastern margin of the Eurasian continent was located relatively far from the Gyeongsang Basin (eastern China). However, the igneous activities seem to have occurred near the Gyeongsang Basin in the Jinju period (Fig. 6). The upward increase of volcanic rock fragments in the Sindong Group (Nakdong Formation: 0%, Hasandong Formation: 0-45%, Jinju Formation: 47-78%; Lee et al., 2015) also supports such interpretation. After then, the direct appearance of Cheongryongsa Basalt showing SE paleoflow direction (Jeon and Sohn, 2008) in the Chilgok Formation above the Jinju Formation indicates the continued southeastward shift of the region of igneous activities. At that time, igneous activities seem to have occurred within the Gyeongsang Basin or in adjacent area.

    Conclusion

    The depositional age of the uppermost Jinju Formation in the Milyang Subbasin, Gyeongsang Basin, where the Natural Monument No. 534 is located, was calculated to be ca. 106.5Ma (Albian) based on the zircon U-Pb dating results. Considering the maximum depositional age and the thickness of the Jinju Formation, the average sedimentation rate can be estimated to be approximately 0.17~0.31 mm/year in the Milyang Subbasin, and the Cretaceous zircons of the uppermost Jinju Formation seem to have originated mainly from the western or northwestern part of the Gyeonggi Massif. The upward increase of the Cretaceous zircons in the Sindong Group suggests the continued southeastward shift of the region of igneous activities due to the rollback of the subducting paleo- Pacific plate during the Jinju period.

    Acknowledgment

    This study was funded by the National Research Institute of Cultural Heritage, and the National Research Foundation of Korea (2020R1A2C1012522). This manuscript has benefited much from constructive comments by two anonymous reviewers.

    Figure

    JKESS-41-4-367_F1.gif

    (a) Geologic map of the Gyeongsang Basin. circles: reported fossil-bearing sites in the Jinju Formation, stars: sampling location for the zircon U-Pb dating in the Jinju Formation, (b) Zircon U-Pb dating results conducted in the Jinju Formation, (c) paleontological dating results reported in the Jinju Formation. *(1) Kobayashi and Suzuki (1936: mollusk), (2) Choi (1985: palynomorph), and Choi and Park (1987: palynomorph), (3) Yi et al. (1994: palynomorph), (4) Park et al. (2013: insect), (5) Park and Chang (1998: conchostraca), (6) Nam and Kim (2016: insect).

    JKESS-41-4-367_F2.gif

    Photographs of footprints and trackway fossils found in the study area (from Kim, K.S. et al., 2016). (a) pterosaur manus print with claw traces, (b) webbed manus print of pterosaur, (c) pterosaur pes track with claw traces, (d) theropod footprint, (e) a well-preserved pterosaur trackway.

    JKESS-41-4-367_F3.gif

    Outcrop photographs and columnar section of the study area. (a) The northern slope of the study area showing sedimentary section, (b) The eastern slope of the study area showing the sampled level for dating, (c) The sampling location for dating in the eastern slope, (d) Columnar section showing the level of the fossils and sampled bed (modified from Kim K.S. et al., 2016).

    JKESS-41-4-367_F4.gif

    Thin-section microphotographs of the Jinju sandstone sample (HT-1). Note that it consists mainly of siliciclastic (angular to subangular quartz and feldspar) and carbonate grains (peloid and ooid).

    JKESS-41-4-367_F5.gif

    (a) a SEM-CL image of the analyzed zircons showing the size, shapes, zoning and analyzed spots on the surface, (b) a probability density diagram plotted all valid spots (n=90), (c) a probability density diagram plotted valid spots showing Cretaceous (n=87), (d) Terra-Wasserburg concordia diagram and weighted mean age for 82 spots.

    JKESS-41-4-367_F6.gif

    (a) The distribution of the tectonostratigraphic terranes in the E. Asia and the approximate age distribution of plutonic and volcanic rocks during the Cretaceous period, (b) Inferred cross-sectional views of the tectonic model of the E. Asian continental margin near the Korean Peninsula during the Sindong period, (b1) the Nakdong and Hasandong period, (b2) the Jinju period (modified from Kim S.W. et al., 2012, 2016;Lee et al., 2017).

    Table

    List of fossils found in the Jinju Formation, Gyeongsang Basin, Korea

    LA-MC-ICP-MS detrital zircon U-Pb ages of the uppermost Jinju Formation in the Milyang Subbasin, Gyeongsang Basin

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