Paleoecological associations of middle Llandovery (Silurian) corals ...

SCIENCE CHINA Earth Sciences • RESEARCH PAPER •

April 2013 Vol.56 No.4: 640–646 doi: 10.1007/s11430-012-4567-0

Paleoecological associations of middle Llandovery (Silurian) corals from Huaying Mountain, eastern Sichuan Province WANG GuangXu1*, ZHAN RenBin1, DENG ZhanQiu1 & LIU JianBo2 1

State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, China; 2 School of Earth and Space Sciences, Peking University, Beijing 100871, China Received May 9, 2012; acceped September 4, 2012

Corals are comparatively abundant in the upper member of the Baiyun’an Formation (upper Aeronian to lowerest Telychian, Llandovery, Silurian) of Huaying, eastern Sichuan Province, Southwest China. Three coral associations have been recognized, i.e., in ascending order, the Mesofavosites Association, the Dentilasma Association, and the Paraceriaster Association. The Mesofavosites Association occurs at horizons indicating a shallow and high-energy environment. It is dominated by large favositids accompanied by a few broken pieces of halysitids and small solitary rugose corals. The Dentilasma Association has a pretty high coral diversity, dominated by the solitary rugose coral Dentilasma meitanense, together with some subordinative heliolitids and favositids. The sedimentary evidence shows that this association might live in a quiet and nutritive shallow water environment. The Paraceriaster Association yields in the grayish green calcareous mudstone indicating a comparatively deep water environment. Systematically it contains abundant fasciculate Paraceriaster micropora and a few favositids. The distinction among these three coral associations shows that corals could adopt different adaptive strategies under different environments, which provide some new data for the investigation on the ecology of Silurian corals and further study on the “Lower Red Beds”. coral associations, Baiyun’an Formation, Llandovery (Silurian), Huaying Mountain, eastern Sichuan Citation:

Wang G X, Zhan R B, Deng Z Q, et al. Paleoecological associations of middle Llandovery (Silurian) corals from Huaying Mountain, eastern Sichuan Province. Science China: Earth Sciences, 2013, 56: 640–646, doi: 10.1007/s11430-012-4567-0

The middle Llandovery of the Yangtze Platform is characterized by marine red beds known as the “Lower Red Beds (LRB)” (e.g., the Rongxi Formation at some localities). Although it is still controversial about its exact age [1–3], great progress has been achieved with regards to its paleogeographic distribution, variation in thickness, and so on [1, 4]. Paleoecological analysis, especially on the sensitivity of fossils to the environmental change, provides a more important and more reliable tool for the further investigation of LRB. Unfortunately, fossils in these rocks are usually poor [5], and hence little work has been done in this area, *Corresponding author (email: [email protected])

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amongst which the most important case studies include the paleoecological analysis of brachiopods during this interval conducted by Rong [6] and corals from the “Hanchiatien Formation” (namely the Baiyun’an Formation in this Paper) by Zhou [7]. Corals are more sensitive to the environmental change, thus more paleoecological information could be obtained through the analysis on suitable material of corals. However, only the Sifengya Formation from northeastern Yunnan and the Baiyun’an Formation from Huaying Mountain, eastern Sichuan yield comparatively abundant corals during this particular time interval, i.e., late Aeronian to earliest Telychian [8]. The Baiyun’an Formation of Sanbaiti section earth.scichina.com

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(30°8.67′N, 106°42.29′E) is ideal for the paleoecological study because it is easier to access and the corals from this formation are taxonomically more diversified and have more apparent systematic changes with the lithological changes of the rocks.

1 Materials Corals are collected from the upper Baiyun’an Formation at this section layer by layer during two field seasons (one in July, 2009 and the other in August, 2010). Paleoecological analysis given below is based largely on the systematic study of corals [8] and the detailed observation on the preservation of corals and the sedimentary structures of the rocks in the field. The conspicuous changes of coral fauna cannot be attributed to its macroevolution because of its short duration [2–4] and slow evolution of corals in geological time. The changes are also not caused by the biological factors, because no reef develops at horizons bearing corals, although some weak local topographic relief exists. All these favorable factors provide an excellent opportunity to understand how corals adopt different adaptive strategies under different environments. Paleoecological analysis in this study is based largely on the corals, although brachiopods, bryozoans, stromatoporoids, algae, and microfossils like conodonts [9] also exist. Unfortunately, the exposure of the fossiliferous part of the Baiyun’an Formation at this section extends transversely fewer than 100 m, which limits the spatial analysis of the paleocommunities recognized here. In spite of the difficulty in determining whether the fossil groups belong to recurrent assemblages or not, given its ecological significance, “coral association” is used temporarily for coral fossils with autochthonous or para-autochthonous preservation at the same horizon.

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Rock sequence and sedimentary facies

The upper Baiyun’an Formation at Sanbaiti has a series of litho-facies, and the coral faunas from these rocks change accordingly (Figure 1). Three coral associations are recognized. Rock sequence and sedimentary facies of these associations are explained below and more detailed discussion about the paleoecology is given in Section 4. The lower member of the Baiyun’an Formation (Layer 0) is characterized by purplish red mudstone, and the grayish green mudstone gradually increases at the uppermost part of this layer (basal part of Figure 2(c)). Layer 1, marking the beginning of the upper member of this formation, has a lithology of 50 cm thick oolitic limestone (Figure 2(d)). And then bioclastic limestone with average thickness of 25 cm (upper part of Figure 2(c)), 3 m thick fossiliferous cal-

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careous mudstone (Figure 2(e)) and purplish red mudstone with local grayish green mudstone (Figure 2(f)) are observed in Layer 2 in ascending order. Layers 3–5 are characterized by thick purplish red mudstone intercalated with grayish green mudstone and sparse nodular limestone. Coral-bearing Layer 6 is composed mainly of grayish green calcareous mudstone (Figure 3(a)-6). Layer 7 is about 7 m thick grayish yellow argillaceous limestone (Figure 3(a)-71-3), with grayish green calcareous mudstone, which contains rich coral fossils (Figure 3(a)-72). Layer 8, containing corals of low diversity, consists mainly of grayish green calcareous mudstone at its lower part and grayish yellow calcareous mudstone at its upper part (Figure 3(a)-8). Ripple marks (Figure 2(b)) are found from the lower member of the Baiyun’an Formation at Laoliziya section (30°13.09′N, 106°45.36′E), indicating a shallow-water sedimentary environment. Rong et al. [10], Johnson et al. [11] and Chen and Rong [5] also found ripple marks and cross bedding from the equivalent rocks in other areas of South China, all indicating a shallow water environment. The transition within Layer 2 from grayish green mudstone, through oolitic limestone and bioclastic limestone, to fossiliferous calcareous mudstone, shows a gradual deepening of seawater. This trend is also supported by some sedimentary evidence, such as the well-developed laminar algae (Figure 4(a), (b)) and local fossil pieces (Figure 4(c), (d)). Thick alternate purplish red and grayish green mudstone of Layers 3–5 above suggests the seawater was becoming shallower again. Layer 6 underlies the purplish red mudstone and Layer 7 grades from grayish yellow argillaceous limestone in the middle part, both of which contain the Dentilasma Association with highly diversified and in-situ fossils, suggesting warm, shallow seas with rich nutrient sources. Layer 8 grades from the underlying argillaceous limestone and have a lithology of calcareous mudstone, yielding the Paraceriaster Association with dominant rugose ramose Paraceriaster. Although many broken pieces of corals are observed at the horizons, some specimens with little damage may indicate no significant transportation from their living habitats, although they might be transported for a short distance before burial. Generally, all available data obtained from Layer 8 show a comparatively deep and quiet marine environment with possible occasional storms.

3 Coral associations After detailed systematic study on the corals from the upper Baiyun’an Formation, we have differentiated the coral fauna of this formation into three associations based on careful statistics of each collection and the sedimentary characters of their yielding rocks. They are, in ascending order, the Mesofavosites Association, the Dentilasma Association, and

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Figure 1 Stratigraphic column and range chart of corals from the upper Baiyun’an Formation at Sanbaiti section, Huaying Mountain, eastern Sichuan, showing different coral apperances with the changing of lithological facies.

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Figure 2 Horizons of the Mesofavosites Association. (a) Overall rock sequence, numbers indicate the layers in Figure 1; (b) symmetrical ripple mark seen at Laoliziya section nearby, which is not observed at Sanbaiti section; (d) oolitic limestone; (e), (c), (f) showing rocks containing the Mesofavosites Association and their underlying and overlying strata.

the Paraceriaster Association. 3.1

The Mesofavosites Association

This association is composed mainly of favositids such as Mesofavosites chuannanensis and Favosites sp., together with a few fasciculate rugose corals Paraceriaster. Some coral fragments, such as Halysites, Cladopora, Tryplasma,

Aphyllum, and bryozoans are also seen at some horizons (Figure 4(c), (d)). Massive favositids are commonly large in size (e.g., the largest one measuring 25 cm×15 cm) while taking medial growth strategy. Rugose coral Paraceriaster micropora of this association, in contrast to the same constituent of the Paraceriaster Association, presents low offset density and thin internal skeleton structures (Figure 5-1, 2).

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Figure 3 Horizons of the Dentilasma Association and the Paraceriaster Association. (a) Outcrop of upper part of the Baiyun’an Formation at Sanbaiti section, Huaying; numbers indicate layer number in Figure 1, among which 71, 72 and 73 represent lower, middle and upper part of Layer 7 respectively; capital letters represent horizons of the Dentilasma Association (B) and the Paraceriaster Association (C). (b), (c) show field photos of the Dentilasma Association and the Paraceriaster Association respectively.

Figure 4 Sedimentological characteristics of horizons yielding the Mesofavosites Association. (a) Lower part laminar algae and upper part favositids; (b) polished slab of laminar algae; (c) fragments of halysitids; (d) fragments of small solitary rugose corals. Scale bars represent 1 cm.

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Figure 5 Representative corals of the Mesofavosites Association (1, 3) and the Paraceriaster Association (2, 4). 1 and 2, Paraceriaster; 3, Mesofavosites; 4, Paleofavosites. a, Transverse section; b, longitudinal section. Scale bars represent 1 mm.

3.2

The Dentilasma Association

This association, dominated by solitary cystiphyllid coral Dentilasma together with some small- to medium-sized heliolitids, favositids and rugose coral Paraceriaster micropora, exhibits the highest diversity among the three associations recognized in this paper. The dominant Dentilasma, only occurred in this association, grows long (up to 30 cm of some individuals) and thick (some individuals even over 5 cm in diameter). Septa of those accompanying

heliolitids are not developed. Those tabulates and rugose coral Paraceriaster micropora show some similar external and internal features just like the situation occurred in the Mesofavosites Association. 3.3

The Paraceriaster Association

This association is dominated by rugose coral Paraceriaster micropora, with subordinate small-sized tabulate coral colonies. Halysitids and large-sized favositids of the Mesofa-

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vosites Association and the dominant rugose coral Dentilasma of the Dentilasma Association disappeared in this association. Compared with the Mesofavosites Association, both tabulate and rugose corals of this association possess thick skeletons (Figure 5). The diameter of those tabulate coral colonies is normally smaller than 5 cm, and the height of those crusting types is 3 cm at most (Figure 5-4). In addition, from the internal structure of crusting colonies, we find that they adopt peripheral growth strategy.

4 Ecological characteristics of the associations As discussed above, the sedimentology of the rocks yielding the Mesofavosites Association indicates a shallow-water, high-energy environment. The information presented by the members of the association reflects their strategy of adapting to the environment: large favositids have the strongest potential tolerance to the water current. Medial growth strategy is adopted commonly by massive tabulates, and the thin skeleton structures of Paraceriaster contribute to their growing upward fast and, as a result, protect them from sediment fouling [12]. The Dentilasma Association flourishes in a quiet, welloxygenated and nutritive shallow sea. Several kinds of corals compete to grow under this favorable condition, so high diversity and abundance characterize this association, particularly for the dominant rugose coral Dentilasma. The Paraceriaster Association also developed in a quiet environment with low sedimentary rate and comparatively deep water. Growing slowly and expanding laterally, as described above, are the two main characters of this association. And just because of these two features, the coral expansion of this association became possible. Corals need lateral expansion to capture more space, with lights and nutrition rather than to grow too fast with the danger of being smothered.

outside South China are needed to test whether the coevolution between the environmental change and the coral faunal turnover exists in a wider and more significant scale. (2) Evidence from both corals and the yielding rocks indicates a gradual deepening of seawater during the middle Llandovery in Huaying, eastern Sichuan, which leads to the gradual decrease of terrigenous supply from the Chuanzhong Oldland and hence the disappearance of LRB and the appearance of carbonate deposits. Yu Changmin, Rong Jiayu, Wang Xiangdong, Wang Yi from Nanjing Institute of Geology and Palaeontology, Tang Lan from Guangxi University, provide a lot of constructive suggestions and comments on this paper. Cheng Xiting (NIGP) helps with the preparation of thin sections. Huang Bing, Li Guipeng and Liang Yan (Nanjing Institute of Geology and Palaeontology) give the authors much help in the field. This study was supported by National Natural Science Foundation of China (Grant Nos. 40825006, 41221001, 41290260) and State Key Laboratory of Palaeobiology and Stratigraphy. This paper is a contribution to the IGCP Project 591—The Early to Middle Paleozoic Revolution. 1

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5 Concluding remarks Based on systematic study on corals and the sedimentological characters of their yielding rocks of the Baiyun’an Formation at Sanbaiti section, Huaying, eastern Sichuan Province, two main conclusions are summarized below: (1) Three coral associations are recognized from the upper Baiyun’an Formation, and they are, in ascending order, the Mesofavosites Association, the Dentilasma Association, and the Paraceriaster Association. Detailed synecological analysis on these coral associations provides some new data for the investigation on the paleoecology of Silurian corals and further study on LRB. Unfortunately, due to limited outcrops of the Baiyun’an Formation, this case study is within a small geographic area, and more investigations within and


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