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Volume 32 Issue 3
Jun.  2021
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Hanxiao Li, Miaoyan Wang, Muhui Zhang, Paul B. Wignall, Manuel Rigo, Yanlong Chen, Xianlang Wu, Zhumin Ouyang, Baojin Wu, Zhaoyang Yi, Zaitian Zhang, Xulong Lai. First Records of Late Triassic Conodont Fauna and δ13Ccarb from the Dengdengqiao Section, Dangchang County, Gansu Province, Northwestern China. Journal of Earth Science, 2021, 32(3): 646-656. doi: 10.1007/s12583-021-1428-9
Citation: Hanxiao Li, Miaoyan Wang, Muhui Zhang, Paul B. Wignall, Manuel Rigo, Yanlong Chen, Xianlang Wu, Zhumin Ouyang, Baojin Wu, Zhaoyang Yi, Zaitian Zhang, Xulong Lai. First Records of Late Triassic Conodont Fauna and δ13Ccarb from the Dengdengqiao Section, Dangchang County, Gansu Province, Northwestern China. Journal of Earth Science, 2021, 32(3): 646-656. doi: 10.1007/s12583-021-1428-9

First Records of Late Triassic Conodont Fauna and δ13Ccarb from the Dengdengqiao Section, Dangchang County, Gansu Province, Northwestern China

doi: 10.1007/s12583-021-1428-9
More Information
  • Based on a study of 49 conodont and 57 g eochemical samples from the Upper Triassic, carbonate-dominated Dengdengqiao Formation, Qinling Basin, China, the Carnian conodonts and carbon isotope records are first reported. Two genera and four species have been identified amongst 87 conodont elements: Mosherella praebudaensis, Mo. longnanensis sp. nov., Mo. sp., and "Misikella" longidentata. The presence of Mo. praebudaensis indicates that the lower part (bed 2) of the formation is attributable to the Julian (lower Carnian) substage. A radiolarian fauna identified in a previous study belongs to the upper Carnian, but the sampling horizon is unclear. The δ13Ccarb curve shows a~1.8‰ negative excursion beginning from upper part of bed 3, but its stratigraphic location is uncertain. The Dengdengqiao Formation is clearly at least partly of Carnian age but could include younger strata. The abundant calcareous algae at the section is probably due to some transport rather than preserved in site. The unusual ecosystem with rare marine organisms may reflect long-term stressful and unfavorable conditions at Dengdengqiao.
  • Electronic Supplementary Material: Supplementary material (Table S1) is available in the online version of this article at https://doi.org/10.1007/s12583-021-1428-9.
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First Records of Late Triassic Conodont Fauna and δ13Ccarb from the Dengdengqiao Section, Dangchang County, Gansu Province, Northwestern China

doi: 10.1007/s12583-021-1428-9

Abstract: Based on a study of 49 conodont and 57 g eochemical samples from the Upper Triassic, carbonate-dominated Dengdengqiao Formation, Qinling Basin, China, the Carnian conodonts and carbon isotope records are first reported. Two genera and four species have been identified amongst 87 conodont elements: Mosherella praebudaensis, Mo. longnanensis sp. nov., Mo. sp., and "Misikella" longidentata. The presence of Mo. praebudaensis indicates that the lower part (bed 2) of the formation is attributable to the Julian (lower Carnian) substage. A radiolarian fauna identified in a previous study belongs to the upper Carnian, but the sampling horizon is unclear. The δ13Ccarb curve shows a~1.8‰ negative excursion beginning from upper part of bed 3, but its stratigraphic location is uncertain. The Dengdengqiao Formation is clearly at least partly of Carnian age but could include younger strata. The abundant calcareous algae at the section is probably due to some transport rather than preserved in site. The unusual ecosystem with rare marine organisms may reflect long-term stressful and unfavorable conditions at Dengdengqiao.

Electronic Supplementary Material: Supplementary material (Table S1) is available in the online version of this article at https://doi.org/10.1007/s12583-021-1428-9.
Hanxiao Li, Miaoyan Wang, Muhui Zhang, Paul B. Wignall, Manuel Rigo, Yanlong Chen, Xianlang Wu, Zhumin Ouyang, Baojin Wu, Zhaoyang Yi, Zaitian Zhang, Xulong Lai. First Records of Late Triassic Conodont Fauna and δ13Ccarb from the Dengdengqiao Section, Dangchang County, Gansu Province, Northwestern China. Journal of Earth Science, 2021, 32(3): 646-656. doi: 10.1007/s12583-021-1428-9
Citation: Hanxiao Li, Miaoyan Wang, Muhui Zhang, Paul B. Wignall, Manuel Rigo, Yanlong Chen, Xianlang Wu, Zhumin Ouyang, Baojin Wu, Zhaoyang Yi, Zaitian Zhang, Xulong Lai. First Records of Late Triassic Conodont Fauna and δ13Ccarb from the Dengdengqiao Section, Dangchang County, Gansu Province, Northwestern China. Journal of Earth Science, 2021, 32(3): 646-656. doi: 10.1007/s12583-021-1428-9
  • The Late Triassic marine strata widely distributed in the western part of Qinling and adjacent areas record a key period in the evolutionary history of this region. The South China Block and North China Block merged to form the Qinling Orogenic Belt in the Late Triassic (Yang et al., 2016, 2002), resulting in the disappearance of Qinling Basin (e.g., Li et al., 2003; Lai et al., 1995, 1992; Yin et al., 1988). However, few fossils have been reported from the widely distributed Late Triassic strata in this region. Records are restricted to the radiolarian fauna recovered from the Dengdengqiao Section, Dangchang County (southern Gansu Province) (Fig. 1), which was speculated to be of Carnian (early Late Triassic) age (Xu, 1992). In this paper, we report on a conodont fauna from the Dengdengqiao Section and provide the first reliable fossil evidence for Upper Triassic strata in the Qinling region.

    Figure 1.  Present-day location of the Dengdengqiao Section indicated by the red star.

    Late Triassic conodonts have received much less attention compared with those of the Early or Middle Triassic (e.g., Liu et al., 2020; Chen Y et al., 2019; Chen Y L et al., 2019; Li et al., 2019; Yang et al., 2019; Lyu et al., 2018). Conodont biostratigraphy is an important tool for the subdivision and correlation of Late Triassic marine strata (e.g., Rigo et al., 2018, 2007; Zhang et al., 2018a, b; Chen and Lukeneder, 2017) thanks to the good preservational potential, wide distribution and high evolutionary rates of conodonts.

    For Carnian conodonts, their P1 elements can be roughly divided into two categories: segminate and segminiplanate. At present, Carnian conodont zones are mainly based on the segminiplanate elements belonging to Paragondolella, Quadralella, Budurovignathus, Carnepigondolella, Metapolygnathus, Mazzaella, Gladigondolella, Kraussodontus and Acuminatella. Nevertheless, their evolutionary relationships are complex (e.g., Karádi et al., 2020; Chen et al., 2016; Mazza et al., 2012a) and the base index conodont of Carnian still remains controversial. In comparison, the contemporary segminate elements of Mosherella, Neocavitella and "Misikella" have received much less attention. Among them, the genus Mosherella may have significant potential for the subdivision of Carnian strata, and the type species Mosherella newpassensis is a potential auxiliary index fossil for the basal Carnian (Sweet et al., 1971; Mosher, 1968).

    Mosherella has been reported from North America, Europe, Turkey and Malaysia (e.g., Chen and Lukeneder, 2017; Kolar-Jurkovšek and Jurkovšek, 2010; Orchard, 2010; Orchard and Balini, 2007; Orchard and Tozer, 1997; Mosher, 1968) but in South China only segminiplanate conodonts have been widely reported from Carnian strata (Jiang et al., 2019; Zhang et al., 2018a, b, 2017; Shi et al., 2017; Sun et al., 2016). Only a single segminate conodont specimen, attributed to Mosherella, is known from the Guandao Section, Nanpanjiang Basin (the same materials were listed in Lehrmann et al., 2015 and Wang et al., 2005). Here, the discovery of Mosherella in South Qinling increases the occurrence of this genus, and provides us an opportunity to study it in detail.

  • The Dengdengqiao Section is located in the Dangchang County, Gansu Province, northwestern China (Fig. 1, start point GPS 33.844°N, 104.542°E, height 1 597 m a.s.l.). The Dangchang area belongs to the South Qinling belt (Dong et al., 2015), and was situated in the east part of the Qinling Basin in the Late Triassic (Fig. 2).

    Figure 2.  Paleooceanographic reconstruction of Qinling area in early Late Triassic. 1. Center of an aulacogen; 2. terrestrial deposition; 3. oldlands; 4. ocean floor (modified from Lai et al., 1995, 1992).

    The Triassic strata can be subdivided into six formations in Dangchang (Lai, 1992), in ascending order, they are Maresongduo Formation (T1m), Guojiashan Formation (T2gj), Qinyu Formation (T2q), Huashiguan Formation (T2h), Dengdengqiao Formation (T3dd), and Daheba Formation (T3dh). These formations have conformable contacts, except for the Dengdengqiao and Daheba formations which have a tectonic contact (Lai, 1992). The Maresongduo Formation is mainly characterized by thick-bedded dolomite or dolomitic limestone. The Guojiashan Formation mostly consists of thick-bedded micritic limestone and bioclastic limestone with an abundant Anisian fauna including conodonts, bivalves, brachiopods, gastropods and ammonoids. The succeeding Qinyu Formation is composed of shales, siltstone and micritic limestone with banded chert and has a bivalve and conodont fauna indicating a Ladinian age. The overlying Huashiguan Formation is comprised of thin-bedded, carbonaceous slates, calcareous siltstone, shale, micritic limestone and slump breccias and lacks fossils. The studied Dengdengqiao Formation is about 400 m thick and mainly composed of thin to thick-bedded micritic limestone with abundant calcareous algae, radiolarians, and a few foraminifers, brachiopods and ostracods (Figs. 3 and 4). The Daheba Formation, the youngest marine deposits in Dangchang, is mainly represented by thick-bedded calcareous siltstone, silty shale, and thin-bedded micritic limestone, shale and mudstone. Some plant fossil fragments and pollen fossils were documented from this formation (Lai, 1992).

    Figure 3.  Outcrop photographs of Dengdengqiao Formation. (a) Thin-bedded, with a few medium-bedded, micritic limestone of beds 2–3; (b) medium- to thick-bedded micritic limestone of beds 6–10, including a 3 m thick, massive layer (bed 9).

    Figure 4.  Photomicrographs of thin sections from Dengdengqiao Section. (a)–(g) Algae fragments in fossiliferous micrite, (a) arrows indicate quartz grains, from sample DDQ-5, bed 1, (b) from sample DDQ-69.5, bed 3, (c) from sample DDQ-70, bed 3, (d) from sample DDQ-177, bed 7, (e)–(f) from sample DDQ-189, bed 8, (g) from sample DDQ-226, bed 10; (h) fossiliferous micrite, arrow indicates foraminifer, from sample DDQ-194, bed 8; (i) sparse biomicrite, arrow indicates ostracod, from sample DDQ-139.5, bed 3. Scale bar=100 μm in (a) (f) (h) (i), scale bar=200 μm in (b) (d) (e) (g), scale bar=20 μm in (c).

  • In total, 49 conodont samples (about 4–5 kg per sample), 57 geochemical samples and 200 rock samples for thin sections were collected at Dengdengqiao (Fig. 5). The samples are indicated by the abbreviation DDQ and the corresponding meters refer to height above the base of the measured section. Conodont samples were dissolved in a 10% solution of acetic acid. The residues were then separated using an LST-an inorganic heavy liquid (Yuan et al., 2015). Afterwards, the conodonts were collected using a stereoscopic binocular microscope and photographed using a scanning electron microscope (SEM). All specimens are stored, and all experiments were carried out, in the School of Earth Sciences, China University of Geosciences (Wuhan). Most samples were barren. However, 87 conodont elements have been obtained from 9 samples, among which only 2 samples contain P1 elements (15 in total) (Fig. 5, Pl. 1). Two genera and four species were identified, including a new species: Mosherella longnanensis sp. nov. All conodonts are very small, and have very low conodont alteration indexes (CAI=1–2).

    Figure 5.  Conodont distribution and carbon isotopes in the Upper Triassic strata at Dengdengqiao Section, Dangchang, northwestern China.

    Geochemical samples were collected from fresh parts of the rock, avoiding weathered surfaces, veins of calcite and fossils, using an electric drill. The resultant ~2–3 g of powders were sent to the State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences (Wuhan) and analyzed using a MAT-253 mass spectrometer with standard methodology (see Song et al., 2013). Isotopic values are shown as per mil relative to the Vienna Pee Dee belemnite (VPDB) standard and the analytical precision was better than ±0.1‰ and ±0.2‰ for δ13C and δ18O, respectively.

    The rock samples thin sections were made at the Langfang Geological Services Company. Petrographic analyses and photographing were carried out using a polarizing microscope (Zeiss Axioscope A1) in School of Earth Sciences, China University of Geosciences (Wuhan).

  • The geochemical samples from Dengdengqiao are mostly fresh with well-preserved sedimentary textures. The average values of δ13C and δ18O are +4.7‰ and -4.7‰, respectively and show very weak correlation (Fig. 6). In addition, the conodont alteration index (CAI=1–2) is very low, indicating modest burial depths. Based on these observations, we consider that the carbon isotope record at Dengdengqiao Section is likely to be an original signal.

    Figure 6.  Crossplot of δ18Ocarb vs. δ13Ccarb.

    The δ13Ccarb values increase slowly from +4.2‰ at the base of the section to +5.6‰ in the upper part of bed 3 (Fig. 5). This is followed by a gradual decline of values that culminates in a low point at the base of bed 5 of +3.8‰, before values rise again to +4.5‰ in bed 6. A second, sharper negative excursion occurs in bed 7 (values fall to +3.4‰) before values gradually rise to nearly +5‰. Finally, δ13Ccarb values gradually fall again to +4.1‰ in bed 10.

  • The conodonts from the Dengdengqiao Section indicate that the lower part (bed 2) of the section belongs to the Julian (lower Carnian) substage. In a previous study, a radiolarian fauna was recovered from the middle or upper part of Dengdengqiao Formation, but the exact sampling position is unclear (Xu, 1992). The radiolarian fauna includes Acanthosphaera sp., Archaeospongoprunum collare, Betraccium sp., Cenosphaera marginate, C. sp., Flustrella sp., Liosphaeridae gen. indet., Monostylosphaera sinensis, Pantanellium sp., Sepsagon? spiralis, Spongostaurus sp., Stylosphaera sp., Spumellarina spp., Stauracontium sp. These taxa suggested a Carnian (early Late Triassic) age according to Xu (1992). O'Dogherty et al. (2010) summarized the biostratigraphy of Triassic radiolarians, and noted that the genus Betraccium ranged from the late Carnian to Rhaetian, the genus Pantanellium ranged from the late Carnian to Jurassic, and the genus Monostylosphaera only occurred in the Carnian. These ranges suggest the upper part of Dengdengqiao Formation is probably of late Carnian or potentially younger age.

    Correlation using the δ13Ccarb curve is rendered difficult because of the lack of biostratigraphic markers in the middle or upper part of Dengdengqiao Formation. It can be noted that the Carnian carbon isotopic record from Longchang, Guizhou Province (Sun et al., 2016) bears some comparison with the Dengdengqiao values. The Julian 2–early Tuvalian (mid Carnian) interval at Longchang shows a prolonged low point of δ13C values that coincides with the Carnian Humid Episode, a major climatic event, and is similar to that seen in the upper part of bed 3–bed 7 at Dengdengqiao. However, the overall values of δ13Ccarb at the latter location are some 2‰ heavier and confirmation of this correlation requires further age-diagnostic criteria.

  • The Dengdengqiao Formation is composed of dark grey, micritic limestone and a 5 cm thick grey mudstone bed between bed 1 and bed 2 (Figs. 3 and 5). The lower part of the formation (beds 1–4) is mostly thin-bedded with a few medium-bedded layers, and the upper part (beds 5–10) is mainly medium- to thick-bedded but includes a 3 m thick, massive layer (bed 9). Poorly sorted and highly angular quartz grains occur at the base of Dengdengqiao Formation (bed 1), recording a brief clastic input phase (Fig. 4a). Abundant calcareous algae were observed in the whole formation (Fig. 4), and a few foraminifers, brachiopods and ostracods were found in several thin sections.

    In the previous studies, the Dengdengqiao Formation was considered to be a deep basinal facies because of the presence of radiolarians (Lai, 1992; Xu, 1992). Here we note that the general rarity of fossils suggests harsh depositional conditions, perhaps due to low oxygen levels at the seafloor. The presence of a few calcareous algae potentially indicates a shallow water, sunlit environment, but is likely that they were transported from shallow water into the basin. In addition, the foraminifers found in Dengdengqiao Formation, from samples DDQ-25 (bed 2), DDQ-162 (bed 5), DDQ-179 (bed 7), DDQ-194 (bed 8) were rare and only simple, compressed flattened forms (Fig. 4f). These small, likely opportunistic species are interpreted to be indicative of unfavorable environments (e.g., low oxygen levels) (e.g., Bernhard, 1986).

  • Mosherella longnanensis sp. nov. Li and Lai

    Plate 1.1–4

    Figure Plate 1.  1–4. Mosherella longnanensis sp. nov., registration Nos. DDQ 20032.5001–20032.5004; 5–7. Mosherella praebudaensis Chen and Lukeneder, 2017, registration Nos. DDQ 20032.5005–20032.5007; 8. “Misikellalongidentata Kozur and Mock, 1974, registration Nos. DDQ 20212.8001; 9. Mosherella sp., registration Nos. DDQ 20032.5008. a. Lateral view; b. lower view; c. upper view. 1–7, 9 from sample DDQ-32.5, bed 2; 8 from sample DDQ-212.8, bed 10. All are P1 elements. Scale bar=100 μm. All the specimens are preserved at China University of Geosciences, Wuhan.

    Etymology: Dengdengqiao Section, Longnan City, the type locality.

    Holotype: Pl. 1.2, registration No. DDQ20032.5002

    Paratype: Pl. 1.1, registration No. DDQ20032.5001

    Type locality and horizon: Dengdengqiao Section, sample DDQ-32.5 in the upper part of bed 2, Dengdengqiao Formation, Dangchang, northwestern China in Julian, early Carnian, Late Triassic.

    Diagnosis: A segminate P1 element has about 8–9 posteriorly inclined, moderately to largely fused denticles. The basal cavity is expanded, oval or subrounded in shape and is located at the posterior end of the element. The posterior margin of basal cavity is rounded or blunt. The cusp is the largest denticle and it is posteriorly followed by a single tiny one.

    Description: In lateral view, the species is a bladelike P1 element, the length: height ratio is about 2 : 1. It has 8–9 moderately to largely fused denticles. The first one or two denticles on the anterior end are erect, and then are increasingly inclined toward the posterior end of the element. The largest denticle (cusp) is the second one from posterior end, and is wider and higher than other denticles. The terminal denticle on the posterior end is much smaller than the cusp. The lower margin of the basal cavity is straight or slightly downward curved. In the lower view, the basal cavity is slightly to moderately expanded, oval or subrounded in shape, and is posteriorly located with the widest part close to the posterior end. The posterior margin of basal cavity is rounded or blunt. The basal groove develops from a small pit in the basal cavity and extends to the anterior end of the element.

    Remarks: This species can be distinguished from Mo. postkockeli, Mo. newpassensis and Mo. praebudaensis by its rounded or blunt posterior margin of basal cavity, which is similar to middle Carnian Mo.? budaensis. In addition, Mo. longnanensis sp. nov. has a larger, posteriorly located basal cavity compared with Mo. postkockeli and Mo. newpassensis. Also, in contrast with Mo. praebudaensis and Mo.? budaensis, it has longer blade with more denticles, particularly a small denticle next to the cusp on the posterior end. In summary, this species has compounded characteristics of other species in this genus, and may be the key species in the evolutionary lineage.

    The specimen Mosherella sp. (Pl. 1.9) looks like Mo. longnanensis sp. nov. in lateral view, but only has a basal groove in lower view. The basal cavity may be broken due to the poor preservation.

    It is worth noting that this species is assigned to the genus Mosherella rather than Neocavitella, the reasons are as follows. Firstly the denticles are uniformly diverging in Neocavitella (Sudar and Budurov, 1979) but increasingly inclined toward the posterior end of the element in Mosherella, which could be easily differentiated by the denticles on the anterior blade. Secondly the basal cavity of the holotype (Pl. 1.2) is not as large and deep as that in Neocavitella. Since the basal cavity of Mo. praebudaensis ranges from small to large (Chen and Lukeneder, 2017), it is not difficult to distinguish Mosherella and Neocavitella just by the size of basal cavity.

    Materials: 4 specimens

    Stratigraphical distribution and age: Julian, early Carnian, Late Triassic.

  • Through systematic conodont sampling, two genera and four species were identified from the Dengdengqiao Formation for the first time, they are Mosherella praebudanensis, Mo. longnanensis sp. nov., Mo. sp. and "Misikella" longidentata. They indicate that the lower part (bed 2) of the formation is of Julian (early Carnian) age. This discovery, together with the previous record of late Carnian radiolarians in the middle or upper part of the formation, indicates that the formation is of Carnian age with possibly younger strata in the upper part.

    The δ13Ccarb curve shows a ~1.8‰ negative excursion in the middle part of the Dengdengqiao Formation which may correspond to an excursion seen during the Carnian Humid Episode although this association requires further age criteria since there is no key biostratigraphic data in the middle or upper part of the section.

    The rare marine organisms in the formation suggest long-term stressful and unfavorable environment in early Late Triassic of the Qinling region. The presence of calcareous algae at Dangchang was probably transported into the area from shallow water environments.

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