Science in China Ser. D Earth Sciences 2005 Vol.48 No.3 291ü297
291
High-resolution Induan-Olenekian boundary sequence in Chaohu, Anhui Province TONG Jinnan1, Hans J. Hansen2, ZHAO Laishi1 & ZUO Jingxun1 1. Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, China; 2. Geological Institute, Copenhagen University, Oster Voldgade 10, DK-1350 Copenhage, Denmark Correspondence should be addressed to Tong Jinnan (email:
[email protected])
Received October 27, 2004
Abstract The West Pingdingshan Section in Chaohu, Southeast China’s Anhui Province is one of the potential GSSP candidates for the Induan-Olenekian boundary. Extensive study has been done on the Induan-Olenekian boundary strata at the section to meet the requirements of defining the GSSP precisely. This paper focuses on the boundary strata at the section to present a review of the main results achieved in recent years. The Induan-Olenekian boundary strata here cover an interval of about four meters at the West Pingdingshan Section, in which the key indices of defining the boundary and the chief transitional processes of some parameters at the boundary are included. The relationship between the boundaries defined by ammonoids and by conodonts is well clarified at the section. The magnetic polarity analysis shows that the boundary interval belongs to a normal polarity zone. Both the carbonate and organic carbon isotopes compositions experienced a significant conversion in the boundary interval. Keywords: Lower Triassic, Induan-Olenekian boundary, GSSP, candidate, Chaohu of China. DOI: 10.1360/04yd0361
The Lower Triassic of Chaohu has been extensively studied owing to its classic stratigraphic sequence formed in a unique paleogeography, very easy accessibility and comfortable working conditions in the area, and the Induan-Olenekian boundary strata received special attention in recent years to meet the requirements of defining the GSSP of the boundary. The best-studied boundary sequence in Chaohu is at the West Pingdingshan Section, which was recommended as a GSSP candidate for the Induan-Olenekian boundary[1, 2].
clarified in various aspects especially including biostratigraphy and magnetostratigraphy. Recent studies on the Lower Triassic in Chaohu have involved many aspects of the strata and achieved some results valuable to clarify the boundary sequence. This paper is designed to provide some details on the Induan-Olenekian boundary strata based upon the study at the West Pingdingshan Section, which would be of great importance to understanding the boundary sequence and defining the GSSP properly.
There have been three GSSP candidates and a reference section proposed for the Induan-Olenekian boundary to date, but the boundary has not been well defined yet, and the boundary sequence is necessarily
The West Pingdingshan Section is located in the suburb of Chaohu City within two kilometers away from the downtown. Chaohu is a mid-sized city in the middle part of Anhui Province with very good traffic
1
General geology
Copyright by Science in China Press 2005
292
access. It has been authorized to be open to all foreigners since the 1990s. It is only about 180 km north of the Meishan Section, where the GSSPs of the Permian-Triassic boundary and the base of the Changhsingian Stage are situated, and a railway and an expressway directly connect Chaohu with Meishan. The Lower Triassic of Chaohu was deposited in a deep part of carbonate ramp on the Lower Yangtze Block, which was situated in the low-latitude eastern Tethyan archipelago with rich and diversified organisms favoring the definition of the GSSP. The specific paleogeography made for a Lower Triassic sequence composed of the frequent alternations of mudrock and limestone in Chaohu and in particular the mudrock and limestone are roughly balanced in the strata from the Permian-Triassic boundary to the lower Olenekian. The densely alternating beds of mudrock and limestone not only yield a continuous biostratigraphic sequence but also provide possibility for the study of such magnetostratigraphy and chemostratigraphy as well as cyclostratigraphy. Studies in Chaohu have covered most aspects, including regional geology and specific studies on the Triassic. The Lower Triassic sequence has been well defined in lithostratigraphy, (conodont, ammonoid, bivalve) biostratigraphy, sequence stratigraphy, ecostratigraphy, sedimentology, magnetostratigraphy, carbon isotope stratigraphy, etc. The Induan-Olenekian boundary strata have especially studied in details these years to provide a candidate for the GSSP definition[1]. Figure 1 shows some main results achieved recently in the study of the Lower Triassic in Chaohu. But some results have been in preparation for publication, such as magnetostratigraphy, carbon isotope stratigraphy and conodonts. Many new forms especially in conodonts have been discovered in our studies but not formally described yet, so here we use them in code names for the moment. Since Chaohu was in a deep part of the Lower Yangtze carbonate ramp in the Early Triassic, the Lower Triassic is mainly composed of thin-bedded limestone interbedded with mudrock or shale though the sequence becomes dominated by limestone and
Science in China Ser. D Earth Sciences
even thick-bedded limestone in the upper part of the Lower Triassic (middle and upper Olenekian) due to the regional regression with the collision between the ü Lower Yangtze and North China blocks[3 5]. The Induan-Olenekian boundary defined by the base of the ammonoid Flemingites-Euflemingites Zone and the FAD of the conodont Neospathodus waageni is about 40 m above the Permian-Triassic boundary in the area, which is located slightly prior to the top of the second Triassic normal magnetic polarity zone and the peak of the first Triassic positive excursion of carbon isotopes δ 13Ccarb[1, 6](fig. 1). It becomes popular and more acceptable that the GSSP is defined by the First Appearance Datum (FAD) of some key fossil forms. However, the FAD work is generally very laborious and requires many repetitious and constantly refining investigations to establish the lineages of the key fossils and prove the truth of the FADs. The resolution in the boundary the strata had best is in centimeter and microfossils are generally more favorable for the precise definition of the boundary than macrofossils due to their small sizes and abundance to be found in the refined subbeds and very limited samples. The Induan-Olenekian boundary strata are composed of the interbeds of thin-bedded limestone and mudrock or shale at the West Pingdingshan Section. For the sake of the comprehensive and accurate understanding of the boundary sequence the strata are subdivided in centimeter for describing and sampling. In general, the subbeds are based upon the natural beddings but some thicker-bedded beds are even subdivided by thickness (fig. 2). 2 Induan-Olenekian boundary sequence The boundary strata have been sampled by subbed for several times, including macrofossils (bivalves and ammonoids), microfossils (conodonts), magnetic polarity, carbonate (inorganic) and organic carbon and oxygen isotopes, and so on. The most subbeds yield rich macrofossils and microfossils. But bivalves and ammonoids are usually collected more from mudrock though they are occasionally observed in limestone as well. Conodonts are generally much richer in limestone but they are also found in mudrock
High-resolution Induan-Olenekian boundary sequence in Chaohu, Anhui Province
293
Fig. 1. Integrated Lower Triassic sequence in Chaohu, Anhui Province. CH, Changhsingian Stage; DL, Dalong Formation; DMAS, Dongmaashan Formation; M. Tri, Middle Triassic; UP, Upper Permian; N, normal polarity; R, reversal polarity.
294
Science in China Ser. D Earth Sciences
Fig. 2. Excursions of carbon isotopes composition and magnetic parameters and distribution of some conodonts in the Induan-Olenekian boundary strata at the West Pingdingshan Section in Chaohu. Conodonts: 1, ramiform elements; 2, Neospathodus dieneri Type 1; 3, N. dieneri Type 2; 4, N. dieneri Type 3; 5, N. cristagalli; 6, N. waageni n. subsp. A; 7, N. waageni n. subsp. B; 8, N. waageni waageni; 9, N. n. sp. E; 10, N. n. sp. I; 11, N. novahollandiae; 12, Platyvillosus costatus; 13, P. hamadai; 14, total specimens.
High-resolution Induan-Olenekian boundary sequence in Chaohu, Anhui Province
or shale. The magnetic polarity samples were taken only from the harder limestone with a certain thickness. The inorganic carbon isotopes are detected only in the calcite-contained rocks though all the subbeds were sampled and analyzed. Fossils. All the subbeds in the boundary strata have been carefully sampled for several times to reconfirm the occurrence of various forms. The common fossils in the rocks are ammonoids, bivalves and conodonts. They have been found in most subbeds but usually enrich some beds. However, the ammonoids and bivalves are mainly preserved in mold and an exact identification is usually difficult to the ammonoids without a well-preserved suture line. Most ammonoids could be recognized only at a generic level and very few at a specific level[7], such as Prionolobus sp., Gyronites sp., Koninckites lolowensis, Koninckites sp., Flemingites sp., Euflemingites sp., Dieneroceras cf. ovale, Dieneroceras sp., Pseudosageceras tsotengense, Pseudosageceras sp., Paranorites cf. ovalis, Paranorites sp., Preflorianites cf. strongi, and Prospingitoides sp., though hundreds of specimens are found. The bivalves from the boundary strata are very monotonous and only very few species of Eumorphotis are rich in some beds though Claraia and Posidonia are occasionally discovered. Ammonoid and bivalve fossils are mostly from the mudrock or shale but ammonoids are observed in limestone as well. All subbeds also have been analyzed for conodonts including mudrock and shale though conodonts are much richer in limestone beds. Even an easy field laboratory for the acidization of samples was built in a farmhouse at the section to ensure sufficient samples involved and the occurrence of various conodont species proven in all subbeds. The extensive conodont investigation has discovered many new forms but they are to be published. Most conodonts belong to Neospathodus, which is a typical Triassic new comer after the end-Permian mass extinction and contains many important elements in stratigraphy. The common index forms include Neospathodus dieneri, N. cristagalli, N. waageni, and Platyvillosus spp.[8], among which N. waageni has been suggested as the index fossil of the
295
Induan-Olenekian boundary[1, 9]. The data from the Lower Triassic of Chaohu indicate that there are at least three morphotypes recognizable in the N. dieneri group but their FADs are below the boundary strata though all these three types co-exist in this interval. The N. waageni group also includes three types, which are believed different subspecies or even different species1). The first appearance of N. waageni n. subsp. A is in Subbed 24ü16 and N. waageni n. subsp. B is in Subbed 24ü20, while N. waageni waageni has its FAD in Subbed 25ü10. N. waageni n. subsp. A looks a primitive form in the Waageni Group and N. waageni waageni is more featured as the Sweet’s holotype[11]. N. waageni n. subsp. B is characteristic of an elongate blade-type element. The origination and evolution of these three forms are to be studied but the lineage from N. waageni n. subsp. A to N. waageni waageni looks apparent. N. cristagalli is common in the boundary strata but it occurs mainly in the higher part of the boundary interval though some similar specimens are seen in the lower part but it has not observed in the rocks below the boundary strata at the section. Platyvillosus costatus is relatively a general form in the uppermost part of the boundary interval at the section and it is also very common in the strata above the boundary interval. Magnetostratigraphy. The whole sequence from the top of the Permian to the base of the Middle Triassic in Chaohu has been sampled for magnetostratigraphy. The sections from the Permian-Triassic boundary to the lower Olenekian were sampled and detected in 2002 and the result has been briefly reported[1]. The sequence from the Induan-Olenekian boundary strata to the base of the Anisian was sampled and detected in the late 2003. All beds hard enough for drilling were sampled in the boundary strata. The results from the boundary strata are shown in fig. 2 and all data fall into a normal magnetic polarity zone, which is in the upper part of the second main Triassic polarity zone in view of the whole magnetostratigraphic sequence from the Chaohu sections[1](fig. 1). Carbon isotopes. The samples for carbonate car-
1) The type specimens of these three types (subspecies) collected from Chaohu have been illustrated in [10].
296
bon isotopes (δ 13Ccarb) were taken by subbed but a few samples of shale have no CO2 recovered so no signals are detected from the subbeds (fig, 2). In view of the whole Lower Triassic excursion of δ 13Ccarb in Chaohu (fig. 1), the Induan-Olenekian boundary strata are situated in the first Triassic positive zone following a long and gentle increasing throughout the most Induan Stage, but the δ 13Ccarb in this positive zone is mostly shifting around zero. In the boundary interval the lower part has the carbon isotopes composition predominated by negative values, mainly below zero, while the values are mostly over zero in the upper part. The samples for the organic carbon isotopes (δ 13Corg) were taken at intervals of 10 cm. The middle and upper parts of the Induan Stage and the lower part of the Olenekian were also sampled at the same interval. From the whole curve the boundary strata are located in a negative-shift zone and the excursion in this interval shows an interesting transition from a frequently shifting stage in the lower part to a constantly increasing stage in the upper part (fig. 2). Induan-Olenekian boundary. Zakharov[12] proposed that the base of the Olenekian was defined at the FAD of ammonoid Hedenstroemia bosphorensis in South Primorye. However, Hedenstroemia looks a form living mainly in the Boreal Realm and it is rarely found in the vast low-latitude Tethyan and other regions. Zakharov[12] also suggested that the base of the Olenekian might be defined by the appearance of a diverse assemblage: Hedenstroemia, Meekoceras, Juvenites, Pseudoprosphingites, Arctoceras, Flemingites and Euflemingites. It is a very practicable suggestion to correlate the boundary but this assemblage is hardly applied to the precise definition of the GSSP of the boundary since the FADs of these ammonoids varied considerably. At the Tree Kamnya Ravine Section in South Primorye, for instance, Hedenstroemia occurred much earlier than the other forms[12]. In the Tethyan Realm, Flemingites and its relative forms (the Flemingitan[13]) are usually regarded as the index fossil to correlate the boundary, as they are common forms in the realm while Hedenstroemia is quite scarce there[14]. Tong et al.[1,9] proposed conodont Neospathodus waageni as the key fossil to define the Induan-Olenekian
Science in China Ser. D Earth Sciences
boundary and ammonoid Flemingites and Euflemingites as reference fossils. The FAD of Neospathodus waageni is quite close to those of Flemingites and Euflemingites at least in the low-latitude areas including Chaohu. At the West Pingdingshan Section some decided specimens of Flemingites first occur in Subbed 25-1 and those of Euflemingites in Subbed 25-2 while some specimens from Subbeds 24-21 and 24-22 also probably belong to Flemingites and Euflemingites. The Waageni Group from the section includes three types that have their first occurrences in an ascending order: N. waageni n. sp. A in Subbed 24-16, N. waageni n. sp. B in Subbed 24-20 and N. waageni waageni in Subbed 25-10. All forms occurred firstly in a relatively low abundance with only very few specimens discovered, but they became very common in the upper part of the boundary interval (fig. 2) and in the overlying strata. Among the three types in the Waageni Group N. waageni waageni is characterized by the holotype of N. waageni Sweet[11] (1970, p. 260, pl. 1, figs. 11, 12). N. waageni n. subsp. A is characteristic of a straight basal margin in its lateral view and relatively more denticles as well as probably a smaller size[10]. It is thus regarded as a primitive form in the Waageni Group. N. waageni n. subsp. B is different from the other two forms in its elongate unit and an elliptical basal cavity[10]. Both N. waageni n. subsp. B and N. waageni waageni probably derived from N. waageni n. subsp. A, but the evolutionary lineage among the three forms and the origin of the Waageni Group are to be studied. However, the three forms are clearly distinguished and they are even probably different species. The Induan-Olenekian boundary should be at the base of Subbed 24-21 according to the occurrences of Flemingites, Euflemingites and their relatives. The boundary defined by conodonts is at the base of Subbed 24-16, which is 26 cm below the ammonoid boundary, if the definition is based upon the FAD of the N. waageni (s. lat.), i.e. the FAD of the Waageni Group or N. waageni n. subsp. A, or at the base of Subbed 25-10, which is 48 cm above the ammonoid boundary, if based on the FAD of N. waageni (s. str.), i.e. the FAD of N. waageni waageni or N. waageni
High-resolution Induan-Olenekian boundary sequence in Chaohu, Anhui Province
Sweet. The FAD of N. waageni n. subsp. B is in Subbed 24-20 and its base is only 3 cm below the ammonoid boundary. Considering the evolutionary lineage, both N. waageni waageni and N. waageni n. subsp. B can be chosen as the definition of the Induan-Olenekian boundary. The boundary defined by the FAD of the Waageni Group, i.e. the FAD of N. waageni (s. lat.) or N. waageni n. subsp. A, might be more practicable as the Waageni Group, especially N. waageni waageni, has a relatively long stratigraphical range and N. waageni n. subsp. A shows a primitive stage of the group. Of course, if the three forms are regarded as different species, N. waageni n. subsp. B or N. waageni (s. str.) would be also a good marker. 3 Conclusion Chaohu has the best objective conditions for studying the Lower Triassic. The achievements in the Induan-Olenekian boundary strata at the West Pingdingshan Section should have met the requirements of defining the GSSP of the boundary. All the sequences of conodont and ammonoid biostratigraphy, magnetostratigraphy and carbon isotope stratigraphy can be well clarified in the Induan-Olenekian boundary sequence at the section. For a precise definition of the boundary, the conodont Neospathodus waageni Group (all three forms) is a good index. According to the data from the Chaohu section, the boundary clearly falls into a normal magnetic polarity zone and the carbonate carbon isotopes (δ 13Ccarb) evolved in an interval from negative to positive though the positive values are mostly only slightly over zero in the boundary strata. The organic carbon isotopes (δ 13Corg) also show a transition from a frequently shifting to a constantly increasing stage. Acknowledgements This is one of the serial studies of the GeoTurn Group at the China University of Geosciences. The study on the fossils profits from the great helps of Drs. Mike Orchard, Yuri Zakharov and Wu Shunbao, and Prof. Huang Junhua and Li Jinlian assisted in the analysis of carbon isotopes. The work was supported by the National Natural Science Foundation of China (Grant Nos. 40325004, 40232025 and 40072011), the Ministry of Education of China (Grant No. 03033), and the Chinese “973 Program” (Grant No. G2000077705).
297
References 1. Tong, J., Zakharov, Y. D., Orchard, M. J. et al., A candidate of the Induan-Olenekian boundary stratotype in the Tethyan region, Science in China, Series D, 2003, 46: 1182ü1200. [Abstract] [PDF] 2. Tong, J., Zakharov, Y. D., Orchard, M. J. et al., Proposal of Chaohu section as the GSSP candidate of the I/O boundary, Albertiana, 2004, 29: 13ü28. 3. Zhang, G., Zhu, J., Chou, F. et al., Study of the Early and Middle Triassic Lower Yangtze sea basin, Acta Geologica Sinica (in Chinese with English abstract), 1989, 63: 134ü147. 4. Zhu, H., Ge, W., Chen, Y. et al., Early and Middle Triassic sedimentary characters in Lower Yangtze region and the forecast of oil and gas, Selected Papers of Lithofacies and Paleogeography (in Chinese), Beijing: Geological Publishing House, 1992, (8): 19 ü37. 5. Yin, H., Ding, M., Zhang, K. et al., Dongwuan-Indosinian Ecostratigraphy of Yangtze Platform and Its Margins (in Chinese with English abstract), Beijing: Science Press, 1995, 1ü338. 6. Tong, J., Qiu, H., Zhao, L. et al., Lower Triassic inorganic carbon isotope excursion in Chaohu, Anhui Province, China, Journal of China University of Geosciences, 2002, 13(2): 98ü106. 7. Tong, J., Zakharov, Y. D., Wu, S., Early Triassic ammonoid succession in Chaohu, Anhui Province, Acta Palaeontologica Sinica, 2004, 43: 192ü204. 8. Sweet, W. C., Mosher, L. C., Clark, D. L. et al., Conodont biostratigraphy of the Triassic. Geological Society of America Memoir, 1971, No.127: 441ü465. 9. Tong, J., Yin, H., Zhang, J. et al., Proposed new Lower Triassic stages in South China. Science in China, Series D, 2001, 44: 961 ü967. [Abstract] [PDF] 10. Zhao, L., Orchard, M. J., Tong, J., Lower Triassic conodont biostratigraphy and speciation of Neospathodus waageni around the Induan-Olenekian boundary of Chaohu, Anhui Province, China, Albertiana, 2004, 29: 41ü43. 11. Sweet, W. C., Uppermost Permian and Lower Triassic conodonts of the Salt Range and Trans-Indus Ranges, West Pakistan, in Stratigraphic Boundary Problems: Permian and Triassic of West Pakistan (eds. Kummel, B., Teichert, C.), University of Kansas, Department of Geology Special Publication 1970, (4): 207ü275. 12. Zakharov, Y. D., The Induan-Olenekian boundary in the Tethys and Boreal Realm. Annals of Civil Museum of Rovereto, Archaeology, History and Natural Science Section (Annali dei Musei Civici di Rovereto Sezione Archeologia, Storia e Scienze Naturali), 1996, 11: 133ü456. 13. Spath, L. F., The Eo-Triassic invertebrate fauna of East Greenland. Meddlelser om Grønland, 1930, 83: 1ü90. 14. Yin, H., Tong, J., Chinese marine Triassic stages and boundaries of Lower Triassic stages, Earth ScienceüJournal of China University of Geosciences (in Chinese with English abstract), 2002, 27: 490ü497.
