Reconstruction of the mid-Hirnantian palaeotopography in the Upper Yangtze region, South China

Abstract

Reconstruction of the Hirnantian (Late Ordovician) palaeotopography in South China is important for understanding the distribution pattern of the Hirnantian marine depositional environment. In this study, we reconstructed the Hirnantian palaeotopography in the Upper Yangtze region based on the rankings of the palaeo-water depths, which were inferred according to the lithofacies and biofacies characteristics of the sections. Data from 374 Hirnantian sections were collected and standardized through the online Geobiodiversity Database. The Ordinary Kriging interpolation method in the ArcGIS software was applied to create the continuous surface of the palaeo-water depths, i.e. the Hirnantian palaeotopography. Meanwhile, the line transect analysis was used to further observe the terrain changes along two given directions. The reconstructed palaeotopographic map shows a relatively flat and shallow epicontinental sea with three local depressions and a submarine high on the Upper Yangtze region during the Hirnantian. The water depth is mostly less than 60 m and the Yangtze Sea gradually deepens towards the north.

Headnote

Abstract. Reconstruction of the Hirnantian (Late Ordovician) palaeotopography in South China is important for understanding the distribution pattern of the Hirnantian marine depositional environment. In this study, we reconstructed the Hirnantian palaeotopography in the Upper Yangtze region based on the rankings of the palaeo-water depths, which were inferred according to the lithofacies and biofacies characteristics of the sections. Data from 374 Hirnantian sections were collected and standardized through the online Geobiodiversity Database. The Ordinary Kriging interpolation method in the ArcGIS software was applied to create the continuous surface of the palaeo-water depths, i.e. the Hirnantian palaeotopography. Meanwhile, the line transect analysis was used to further observe the terrain changes along two given directions.

The reconstructed palaeotopographic map shows a relatively flat and shallow epicontinental sea with three local depressions and a submarine high on the Upper Yangtze region during the Hirnantian. The water depth is mostly less than 60 m and the Yangtze Sea gradually deepens towards the north.

Key words: palaeotopography, palaeo-water depth, Hirnantian, Upper Yangtze region, South China, ArcGIS.

INTRODUCTION

The Himantian is an important geological time interval in the Late Ordovician, which recorded the end- Ordovician continental glaciation and mass extinction (Sheehan 2001; Saltzman & Young 2005; Finnegan et al. 2011; Melchin et al. 2013). The Hirnantian sediments are widely distributed and well preserved in South China, which has become one of the key study regions about the Himantian geological and biological events. Many aspects of the Himantian strata and fossils in South China have been studied for many decades, including dominant graptolite and brachiopod faunas (Rong 1979; Rong et al. 2002; Chen et al. 2005a; Zhan et al. 2010), the end-Ordovician mass extinction event (Chen et al. 2005b), stratigraphic classifications and correlations (Chen et al. 2000, 2006; Rong et al. 2010), geochemistry (Fan et al. 2009; Zhang et al. 2009; Gorjan et al. 2012), palaeogeography (Mu et al. 1981; Chen et al. 2004; Rong et al. 2011), etc. These previous studies provide a solid foundation and good opportunity to examine the palaeo-water depths and palaeotopographic characteristics in South China during the Hirnantian. Rong & Chen (1987) have briefly reconstructed the facies pattern of the time when the Kuanyinchiao Beds were deposited (basically equal to the mid-Hirnantian) based on the brachiopod assemblages in the form of hand-sketching. In the present study, based on a big collection of the Himantian sections, we use the ArcGIS software to reconstruct the mid-Hirnantian palaeo- topography of South China in a quantitative and three- dimensional way. It is the first time to reconstruct the Himantian marine terrains in South China, which can be used in subsequent analysis such as predicting the palaeo-water depth in unknown regions and their biofacies and lithofacies characteristics, and inferring the patterns of ocean currents in the study area.

In this study, data of 374 Hirnantian sections of the Upper Yangtze region were collected through the Geobiodiversity Database (GBDB, http://www.geobiodiversity.com; Fan et al. 2013). The palaeo-water depths of those sections were ranked according to the lithofacies and biofacies characteristics. Using the ArcGIS software, we created the continuous surface of the mid-Hirnantian palaeotopography in the Upper Yangtze region and then examined its spatial patterns and the characteristics of terrain changes.

DATA AND METHODS

Study region and time interval

The Upper Yangtze region of the South China Palaeo- plate was chosen as the study region. It mainly includes the eastern Sichuan, northern Guizhou, Hubei and north- western Hunan provinces and Chongqing Municipality. The mid-Hirnantian, particularly the time interval in which the sediments of the Kuanyinchiao Beds were deposited, was selected as the study time interval. The strata and fossils of the Upper Yangtze region in this time interval have been intensively studied for decades and provide the necessary data and comprehensive background knowledge for our palaeotopographic reconstruction.

In the mid-Hirnantian, the Upper Yangtze region was covered with several different lithostratigraphic units (Fig. 1). The Kuanyinchiao Beds, mainly grey limestones or mudstones, are widely distributed over most of the Upper Yangtze region (area A in Fig. 2), and yield abundant benthic shelly fauna, including brachiopods (Hirnantia fauna, Rong 1979) and trilobites. In the southwestern Shaanxi Province, the Nanzheng Formation is composed of mudstones, siltstones and interbedded limestones, and yields graptolites and benthic shelly fossils (area B in Fig. 2, Li & Cheng 1988). In the southeastern Sichuan Province, the upper parts of the Tiezufeike and Daduhe formations (area C in Fig. 2, Mu et al. 1979, 1993; Hu 1980) consist of black shales/silty shales and thin-bedded limestones. At the bordering area between the Hunan and Guangxi provinces, the strata assigned to the mid-Hirnantian are parts of the Tianmashan and Tianlingkou formations (area D in Fig. 2, Chen et al. 1981, 2014; Liu & Fu 1984), which are composed of sandstones, siltstones and slates with some graptolites; these strata gradually changed to black shales towards the north (central Hunan Province).

Data set

Based on the analysis of rock composition, texture, structure and fossil characteristics, we ranked the water depths of each deposition area of the Upper Yangtze region (see Table 1).

A total of 374 sections were examined to reconstruct the palaeotopography. All the section data were collected from the GBDB online database. Then each section was assigned a rank value of palaeo-water depth according to the scheme in Table 1.

Palaeotopographic reconstruction

Although some ranks of water depth haven been assigned an estimated water depth (such as 10-30 m for Rank IV, see note for Table 1), the precision of the estimation is quite limited. For this reason, we used the ranks of water depth, instead of the water-depth values, to reconstruct the palaeotopography of the Upper Yangtze region. The palaeotopographic maps (Figs 2, 3) were created based on the ranks of palaeo-water depths by using the interpolation tools in ArcGIS 10. After comparing all the interpolation methods in the ArcGIS software, the Ordinary Kriging method in the Spatial Analyst tools was adopted for reconstructing the palaeo- topography. Parameter settings are as follows: Kriging method is Ordinary Kriging?, Output cell size is ?2000?, Number of points of the search radius settings is ?15?, and the other parameters are set as the default.

Based on the reconstructed palaeotopography, the line transect analysis was adopted to create two topographic profile graphs (Fig. 4), one from Point a to Point b and the other from Point c to Point d (Fig. 2), to observe the terrain changes along these two directions.

RESULTS AND DISCUSSIONS

Based on the reconstructed contour map (Fig. 2) and 3D stereogram (Fig. 3), we can see that, during the mid- Himantian, the Yangtze Sea mainly covered the area of the southeastern Sichuan, northern Guizhou, north- western Hunan and Hubei provinces, and the Chongqing Municipality in the Upper Yangtze region. The water depth did not exceed Rank V in most parts of the Yangtze Sea, which means that the water depth of the Yangtze Sea is mostly less than 60 m. On the whole, the Yangtze Sea gradually deepened towards the north. In the northern Hubei Province, the water depth reached the deepest rank, which may imply an open- sea environment. However, this is still uncertain because detailed stratigraphic records of the Himantian Stage near the northern margin of the South China Palaeoplate are lacking. It is notable that the main part of the Upper Yangtze region appears to be covered by a relatively flat and shallow epicontinental sea, which provided the palaeogeographic setting for the deposition of the very thin Kuanyinchiao Beds with the widely distributed Himantia shelly fauna. The Hunan?Hubei Submarine High (Chen et al. 2004), reached its biggest coverage at that time, partly due to the concentration of continental glaciation around the South Pole (Fan et al. 2011). Meanwhile, three local depressions occurred in the southeastern comer of the Sichuan Province and southern Chongqing Municipality (Area 1 in Fig. 2), mid-western Hunan Province (Area 2 in Fig. 2) and the area north of the Hunan?Hubei Submarine High (Area 3 in Fig. 2).

From the two topographic profile graphs (Fig. 4), we investigated the lateral changes in palaeo-water depths. It is easy to see the terrain changes from Zunyi City to Yichang City (in the northeast) and from Anhua County to Xixiang County (in the north-northwest). It is clear that Zunyi City, Yanhe County, Yichang City, Anhua County and Chengkou County were relatively low-lying regions in the Hirnantian, while Wufeng County and Xixiang County were high regions. From Point a to Point b, a local depression shows first, then the Hunan? Hubei Submarine High and finally an open-sea environ- ment. From Point c to Point d, a deeper local depression shows, then the Submarine High and a depression, and finally an oldland. From these graphs, we can see that the rank of water depth of Point b is V?VI, which means the depth is deeper than 60 m. If we assume that the water depth of Point b is 70 m, we can estimate the average slope of the sea floor of the Upper Yangtze Sea by dividing the differences of the water depth between Point a and Point b by the distance between them. The average slope is 0.004°. Although there existed a sub- marine high at the bordering area of Hunan and Hubei (Figs 3, 4), the average slope of the Upper Yangtze region is obviously smaller than that of the modern continental shelf, which indicates a relatively flat terrain of the Upper Yangtze region. However, the areas in the Upper Yangtze region in different water depths yielded different biotic communities and rock types. If we take the rock fold into account, the distance between points a and b will be larger and the slope between them will become smaller, which means the Yangtze Sea will be more flat.

CONCLUSIONS

The online stratigraphic and palaeontological database provides the opportunity for palaeotopographic recon- stmnetion. In this paper, we estimated the water-depth ranks of 374 sections during the Hirnantian based on their lithofacies and biofacies and reconstructed 2D and 3D palaeotopographic maps using tools in the ArcGIS software.

The reconstructed topographic maps indicated that the Upper Yangtze region was a semi-closed, shallow epicontinental sea during the Hirnantian. It consisted of a small uplift (Hunan?Hubei Submarine High) and several local depressions. The Yangtze Sea gradually deepened towards the north and its water depth is mostly less than 60 m, showing a very gentle slope of the sea floor.

Acknowledgements. We thank Renbin Zhan and Petr ?torch for constructive reviews on an earlier version of this paper, and Xu Chen and Michael J. Melchin for their constructive comments and suggestions. This study was supported by Chinese Academy of Sciences (XDB10010100), National Natural Science Foundation of China (41221001, 41290260, 41272042 and 41202004). This is a contribution to the Geobiodiversity Database project (www.geobiodiversity.com) and IGCP Project 591 ?The Early to Middle Palaeozoic Revolution?.

References

REFERENCES

Bureau of Geology and Mineral Resources of Hunan Province (ed.). 1988. Regional Geology of Hunan Province. Geological Publishing House, Beijing, 721 pp.

Chen, X., Yang, W. R., He, Z. Q. & Wang, S. H. 1981. Ordovician graptolite bearing beds of Xing?an, Guangxi. Journal of Stratigraphy, 5, 36?45 [in Chinese].

Chen, X., Rong, J. Y., Mitchell, C. E., Harper, D. A. T., Fan, J. X., Zhan, R. B., Zhang, Y. D., Li, R. Y. & Wang, Y. 2000. Late Ordovician to earliest Silurian graptolite and brachiopod biozonation from the Yangtze region, South China, with a global correlation. Geological Magazine, 137, 623?650.

Chen, X., Rong, J. Y., Li, Y. & Boucot, A. J. 2004. Facies patterns and geography of the Yangtze region, South China, through the Ordovician and Silurian transition. Palaeogeography, Palaeoclimatology, Palaeoecology, 204, 353?372.

Chen, X., Fan, J. X., Melchin, M. J. & Mitchell, C. E. 2005a. Hirnantian (latest Ordovician) graptolites from the upper Yangtze region, China. Palaeontology, 48, 235?280.

Chen, X., Melchin, M. I, Sheets, H. D., Mitchell, C. E. & Fan, J. X. 2005b. Patterns and processes of latest Ordovician graptolite extinction and recovery based on data from South China. Journal of Paleontology, 79, 841?860.

Chen, X., Rong, J. Y., Fan, J. X., Zhan, R. B., Mitchell, C. E., Elarper, D. A. T., Melchin, M. J, Peng, P. A., Finney, S. C. & Wang, X. F. 2006. The Global boundary Stratotype Section and Point (GSSP) for the base of the Hirnantian Stage (the uppermost of the Ordovician System). Episodes, 29, 183?196.

Chen, X., Fan, J. X., Chen, Q., Tang, L. & Hou, X. D. 2014. Toward a stepwise Kwangsian Orogeny. Science China Earth Science, 57, 379?387.

Cooper, R. A. & Sadler, P. M. 2012. The Ordovician period. In A Geologic Time Scale 2012 (Gradstein, F. M., Ogg, J. G., Smith, A. G. & Ogg, G. M., eds), pp. 489?523. Elsevier, Oxford.

Fan, J. X., Peng, P. A. & Melchin, M. J. 2009. Carbon isotopes and event stratigraphy near the Ordovician and Silurian boundary, Yichang, South China. Palaeo- geography, Palaeoclimatology, Palaeoecology, 276, 160?169.

Fan, J. X., Melchin, M. J., Chen, X., Wang, Y., Zhang, Y. D., Chen, Q., Chi, Z. L. & Chen, F. 2011. Bio stratigraphy and geography of the Ordovician?Silurian Lungmachi black shales in South China. Science China Earth Science, 54, 1854?1863.

Fan, J. X., Chen, Q., Hou, X. D., Miller, A. I., Melchin, M. J., Shen, S. Z., Wu, S. Y., Goldman, D., Mitchell, C. E., Yang, Q., Zhang, Y. D., Zhan, R. B., Wang, J., Leng, Q., Zhang, El. & Zhang, L. N. 2013. Geobiodiversity Database: a comprehensive section-based integration of stratigraphic and paleontological data. Newsletters on Stratigraphy, 46, 111?136.

Finnegan, S., Bergmann, K., Eiler, J. M., Jones, D. S., Fike, D. A., Eisenman, L, Elughes, N. C., Tripati, A. K. & Fischer, W. W. 2011. The magnitude and duration of Late Ordovician?Early Silurian Glaciation. Science, 331, 903?906.

Gorjan, P., Kaiho, K., Fike, D. A. & Chen, X. 2012. Carbon- and sulfur-isotope geochemistry of the Hirnantian (Late Ordovician) Wangjiawan (Riverside) section, South China: global correlation and environmental event interpretation. Palaeogeography, Palaeoclimatology, Palaeoecology, 337?338, 14?22.

Hu, Z. G. 1980. The ?Dalmanitina bed? in the region of the lower reaches of the Dadu River. Journal of Stratigraphy, 4, 29?36 [in Chinese].

Li, J. J. & Cheng, El. J. 1988. Lithological and biotic characteristics of the Nanzheng Formation in the Liang shan area of Elanzhong in Shaanxi province. Geology of Shaanxi, 6, 31?44 [in Chinese, with English abstract].

Liu, Y. R. & Fu, El. Y. 1984. Graptolites from the Wufeng Formation (Upper Ordovician) of Anhua, Elunan. Acta Palaeontologica Sinica, 23, 642?648 [in Chinese, with English abstract].

Melchin, M. J., Mitchell, C. E., Elolmden, C. & ?torch, P. 2013. Environmental changes in the Late Ordovician? early Silurian: review and new insights from black shales and nitrogen isotopes. Geological Society of America Bulletin, 125, 1635?1670.

Mu, E. Z., Zhu, Z. L., Chen, J. Y. & Rong, J. Y. 1979. The Ordovician System of Southwest China. In Carbonate Biostratigraphy of Southwest China (Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, ed.), pp. 108?154. Science Press, Beijing [in Chinese].

Mu, E. Z., Li, J. J., Ge, M. Y, Chen, X., Ni, Y. N. & Lin, Y. K. 1981. Paleogeographic maps of the Late Ordovician in the Central China region and their explanation. Journal of Stratigraphy, 5, 165?170 [in Chinese].

Mu, E. Z., Li, J. J., Ge, M. Y., Chen, X., Lin, Y. K. & Ni, Y. N. 1993. Upper Ordovician Graptolites of Central China Region. Palaeontologia Sinica, New Series B, 29. Science Press, Beijing, 393 pp. [in Chinese, with English summary].

Rong, J. Y. 1979. The Himantia fauna of China with the comments on the Ordovician?Silurian boundary. Acta Stratigraphica Sinica, 3, 1?29 [in Chinese].

Rong, J. Y. 1986. Eco stratigraphy and community analysis of the Late Ordovician and Silurian in southern China. In Selected Papers of the 13th and 14th Annual Convention of the Palaeontological Society of China (Palaeontological Society of China, ed.), pp. 1?24. Anhui Science and Technology Publishing Elouse, Elefei [in Chinese, with English summary].

Rong, J. Y. & Chen, X. 1987. Faunal differentiation, biofacies and lithofacies pattern of Late Ordovician (Ashgillian) in South China. Acta Palaeontologica Sinica, 26, 507?535.

Rong, J. Y., Johnson, M. E. & Yang, X. C. 1984. Early Silurian (Llandovery) sea-level changes in the Upper Yangzi region of central and southwestern China. Acta Palae- ontologica Sinica, 23, 672?697 [in Chinese, with English summary].

Rong, J. Y., Chen, X. & Elarper, D. A. T. 2002. The latest Ordovician Himantia Fauna (Brachiopoda) in time and space. Lethaia, 35, 231?249.

Rong, J. Y., Chen, X., Zhan, R. B., Fan, J. X., Wang, Y., Zhang, Y. D., Li, Y., Huang, B., Wu, R. C. & Wang, G. X. 2010. New observation on Ordovician?Silurian boundary strata of southern Tongzi County, northern Guizhou, southwest China. Journal of Stratigraphy, 34, 337?348 [in Chinese, with English abstract].

Rong, J. Y., Chen, X., Wang, Y., Zhan, R. B., Liu, J. P., Huang, B., Tang, P., Wu, R. C. & Wang, G. X. 2011. Northward expansion of Central Guizhou Oldland through the Ordovician and Silurian transition: evidence and implications. Science China Earth Sciences, 41, 1407?1415 [in Chinese].

Saltzman, M. R. & Young, S. A. 2005. Long-lived glaciation in the Late Ordovician. Geology, 33, 109?112.

Sheehan, P. M. 2001. The Late Ordovician mass extinction. Annual Review of Earth and Planetary Sciences, 29, 331?364.

Zhan, R. B., Liu, J. B., Percival, I. G., Jin, J. S. & Li, G. P. 2010. Biodiversification of Late Ordovician Himantia Fauna on the Upper Yangtze Platform, South China. Science China Earth Sciences, 53, 1800?1810.

Zhang, T. G, Shen, Y. A., Zhan, R. B., Shen, S. Z. & Chen, X. 2009. Large perturbations of the carbon and sulfur cycle associated with the Late Ordovician mass extinction in South China. Geology, 37, 299?302.

Zhang, L. N., Fan, J. X., Chen, Q. & Melchin, M. J. 2014. Geographic dynamics of some major graptolite taxa of the Diplograptina during the Late Ordovician mass extinction in South China. GFF, 136, 327?332.