Journal of Earth Science  2017, Vol. 8 Issue (4): 614-621   PDF    
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Ediacaran Macrofossils in Shunyang Valley, Sixi, Three Gorges District, Hubei Province, China
Mason Roger1, Yuejie Li1, Kenan Cao1, Long Yu1, Zhen-Bing She1,2    
1. School of Earth Sciences, China University of Geosciences, Wuhan 430074, China;
2. State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan 430074, China
Abstract: Previously undescribed Ediacaran macrofossils are documented in and close to a quarry southwest of Zigui in Shunyang Valley, near Sixi Village, Yangtze Three Gorges region, Hubei Province, China. Discoidal impressions, vendotaenids, and a new branching form occur in bedded carbonates through the entire 235 m thickness of the Dengying Formation. The quarry and nearby outcrops in the stream valley have significant potential for further discoveries of Ediacaran macrofossils.
Keywords: Ediacaran    macrofossil    Dengying Formation    Sixi    
0 INTRODUCTION

The fossil record of the Late Neoproterozoic Era records the evolution and diversification of a host of macroscopic organisms, with strata of Ediacaran (635–541 Ma) age revealing particularly significant fossil assemblages. Several discoveries of Ediacaran macro-and microfossils have been made in China (e.g., Yin et al., 2015; Chen Z et al., 2014; Yuan et al., 2011; Tang et al., 2008; Zhu et al., 2008; Chen J et al., 2004; Xiao et al., 1998; Sun, 1986), and have considerably enhanced scientific understanding of metazoan and algal evolution during the Neoproterozoic to Cambrian interval.

Some of the most prolific Chinese Ediacaran fossils come from the Three Gorges area of South China, which has been the subject of multiple geochemical, stratigraphic, and paleontological studies (e.g., Xin et al., 2016; Cui et al., 2015; Wang et al., 2015; She et al., 2014; Liu P J et al., 2014; Hu et al., 2012; Jiang et al., 2011). In this region, the Ediacaran System consists of the Doushantuo Formation and overlying Dengying Formation, which outcrop at several sites on the limbs of the Huangling anticline (Fig. 1). The Doushantuo Formation is a carbonate and black shale sequence deposited between 635 and 551 Ma (Condon et al., 2005; though see An et al., 2015). Phosphorites in Member II of the Doushantuo Formation have yielded microfossils of algae, acritarchs, and putative metazoans in sections to the southwest at Weng'an (Cunningham et al., 2015; Yin et al., 2007; Xiao et al., 1998). Shales in the Miaohe Member outcropping in the Huangling anticline preserve algal and enigmatic macrofossils as carbonaceous compressions (Xiao et al., 2002), but these have recently been suggested to be time equivalent to the Shibantan Member of the Dengying sFormation on the basis of chemostratigraphic and paleontological considerations (An et al., 2015). The overlying Dengying Formation in the Three Gorges area is characterized by almost exclusive occurrence of carbonates, and is constrained in age to between 551 and 541 Ma (Condon et al., 2005). The Dengying Formation is generally divided into three lithostratigraphic units, in ascending order the Hamajing dolostone, Shibantan limestone, and Baimatuo dolostone members (Zhao et al., 1988). The Shibantan Member is characterized by dark grey, finely laminated bituminous limestones, often referred to as "fetid limestone". The general presence of horizontal bedding and occasional occurrence of low-angle cross stratification and rip-up clasts, along with the absence of dissolution vugs and tepee structures, suggest that the Shibantan limestones were deposited in a subtidal marine environment, between fair weather and storm wave bases (Chen et al., 2014).

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Figure 1. Simplified geological map of the Yangtze Three Gorges area (a) and stratigraphic column of the Dengying Formation in the Shunyang Valley, Sixi. Inset map in (a) shows the location of the studied area. Main fossil horizons are indicated in (b).

Ediacaran fossils have been described in the Dengying Formation at Shibantan in the Xiling Gorge area by the Yangtze River, Hubei Province (Sun, 1986; Ding and Chen, 1981) where they are succeeded by Cambrian strata containing trilobites at an appreciably higher level (Zhao et al., 1988). The site of the 1981 discovery is now submerged beneath the reservoir of the Gezhouba Dam near Yichang (Fig. 1a). Anderson et al. (2011) recorded macrofossils up to about 3 mm long, notably Vendotaenia, preserved as carbonized films on bedding surfaces in calcareous shale in the Shibantan Member at Jiulongwan, on the southeastern limb of the Huangling anticline (Fig. 1a). Most recently, Chen et al. (2014) recorded macrofossils of several soft-bodied taxa at Wuhe, alongside associated trace fossils, acritarchs, and Vendotaenia, while microbial mats and bacterial biomarkers were reported within the Shibantan Member by Duda et al. (2014).

1 DISCOVERY OF A NEW EDIACARAN LOCALITY

In July 2010, a field visit to the Shunyang Valley by Yu Long and Roger Mason discovered unusual sedimentary structures in the Shunyang Valley, a tributary stream of the Sixi River beyond the entrance gate of the Three Gorges Bamboo Forest Park at Sixi Village, 10 km SW of Zigui, Hubei Province (Fig. 2). The structures occur in the Dengying Formation and are likely to be chert nodules in dolomitic limestone that have been weathered into high relief by stream erosion. Although similarities to Ediacaran macrofossil holdfast discs were noted at the time, it remains to be determined whether the nodules formed around organic remains of Ediacaran age. Further investigations of the Shunyang Valley by She, Mason, Li and Cao found that the dolomite bedding surfaces seen in 2010 have been entirely covered by quarry spoil although loose blocks within the stream bed revealed multiple Vendotaenia specimens (Fig. 3), confirming that the locality lies within the Shibantan Member of the Dengying Formation. It appears that Anderson et al. (2011) discovered Vendotaenia in the main Sixi Valley. A previously unreported active stone quarry ~50 m upstream from this locality reveals numerous slabs of limestone dipping at about 40°SW that are also exposed in the stream bed. Exposed bedding plane surfaces revealed numerous circular structures (Fig. 4), some of which we provisionally identify as Ediacaran fossils.

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Figure 2. Chert moulds on a bedding surface in a dolostone layer eroded by a stream. These chert bodies might have formed around frond holdfasts (arrowed as a, b and c) or fragments, or be parts of algal mats, but further investigation is required to confirm this. Scale in cm.
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Figure 3. Vendotaenia preserved as carbonised compressions on a bedding plane in a loose block. Arrows point to some of the more evident Vendotaenia. Note the piece of possible microbial mat surrounded by the rectangle.
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Figure 4. (a) Overview of slab containing a possible holdfast shown by arrow. (b) Close-up of the possible holdfast impression. (c) and (d) Wrinkled surface of limestone. Diameter of coin in (c) and (d) is 20.5 mm.
2 THE SHUNYANG SECTION

Kenan Cao, Yuejie Li and Zhen-Bing She have completed a reconnaissance log of the complete section through the Deng-ying Formation in the Shuyang Valley (Fig. 1b). The base of the Dengying Formation is marked by solution cavities in the natural valley wall. It is succeeded by 13 m of the Hamajing dolomite member. The overlying Shibantan Member is 185 m thick and composed of dark grey, fine-to medium-bedded bituminous limestones which show distinct fine laminations on weathered surfaces and contain Ediacaran fossils at several levels (Fig. 1b). Abundant chert concretions are also present at many horizons. The Baimatuo Member is distinguished from the darker Shibantan Member by its light white colour and comprises medium-to thick-bedded dolostones with a total thickness of 37 m. It contains abundant dissolution vugs in dolostones and is succeeded conformably by dark Cambrian shales (Fig. 1b).

3 PETROGRAPHY

Dengying limestones are characterized by alternating millimeter-scale organic-rich and organic-lean layers (Figs. 5a, 5b). The organic-rich layers are composed of 10–20 μm calcite crystals with abundant interstitial organic matter (Figs. 5c, 5d). Clear calcite crystals 20–30 μm wide, which probably formed by early diagenetic growth, are also present as elongate patches parallel to the primary laminae (Figs. 5a, 5c). The organic-lean layers consist of anhedral blocky calcite crystals 40–70 μm across (Figs. 5e, 5f). Vendotaenia fossils frequently occur as carbonaceous compressions at this locality on exposed bedding planes and as loose blocks, sometimes washed by the flowing stream. In some cases, similar compressions can also be observed on freshly broken bedding planes. Therefore, they are unlikely to be modern organic material filling natural irregularities on bedding planes. They are not easily resolved in bedding normal sections (Figs. 5e, 5f) but can be observed in bedding parallel sections (Figs. 6a6d) and on bedding planes (Fig. 3). The Vendotaenia fossils are present as flattened and folded ribbon-like structures up to a few millimetres long and 100–600 μm in width (Figs. 3, 6a6c). Micro-Raman images confirm the ubiquitous presence of organic matter in Vendotaenia fossils (Figs. 6e and 6f). Their original structure, however, has been modified by recrystallization of calcite (Fig. 6d) and thermal degradation of the organic matter which produced finely disseminated carbonaceous particles (Fig. 6e, arrowed). The presence of broad D-band (disordered carbon) attests to a rather low degree of thermal maturation for the organic matter, with peak metamorphic temperature roughly around 200 ℃ (Kouketsu et al., 2014). Dolomite rhombs are also observed associated with organic matter and calcite (Figs. 6d6e), suggesting diagenetic dolomitization.

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Figure 5. (a) and (b) Limestone of the Shibantan Member showing finely-laminated organic-rich layers intercalated with coarser-grained organic-lean layers. (c), (d) Close-ups of organic rich-layers showing diagenetic calcite spar (arrowed in (c)). (e), (f) Brownish organic-rich structures (arrowed), possibly Vendotaenia. Bedding normal sections under plane polarized light.
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Figure 6. (a)–(d)Vendotaenia-like carbonaceous compressions observed in bedding parallel sections of Shibantan limestones. (d) Close-up view of the fossil in (c), showing brown organic matter with cracks probably caused by recrystallization of calcite. Note also secondary dolomite rhombs (arrowed). (e) Micro-Raman image of organic matter (OM, red), calcite (green), and dolomite (purple) of the marked area in (d). Note the tiny disseminated OM particles (arrowed) in homogeneous OM. (f) Raman spectra of OM in calcite (1), calcite with minor OM (2), dolomite rhombs (3), calcite associated with OM (4), and disseminated tiny particles of OM (arrowed in e). Cal. Calcite; Dol. dolomite; D-band. disordered peak for OM; G-band. graphitic peak for OM.

The limestone displays millimeter-scale crinkled lamination in cross section (Figs. 5a, 5b), which corresponds to wrinkled surfaces on bedding planes (Figs. 4c, 4d). Similar structures in the Shibantan Member from sections near Wuhe have been interpreted as microbial mats (Meyer et al., 2014; Chen et al., 2013), a view with which we concur.

4 EDIACARAN FOSSILS

The predominant Ediacaran fossils are circular impressions on bedding planes. They are difficult to distinguish from inorganic circular features such as sections through circular concretions. We have not yet found any fronds and Chen et al. (2014) only recorded a few frond fragments at a comparable level in the Shibantan Member on the east limb of the Huangling anticline. At Shunyang, some circular structures have internal features that resemble Ediacaran holdfasts (Figs. 2 and 4) and their relations with mm scale lamination of the host carbonate support interpretation as fossils rather than concretions. There are two types of potential Ediacaran discs; a smaller type up to 20 cm across with marked relief and internal structures, and a larger type up to 60 cm across that are a different colour from the dark carbonate host rock (Fig. 7). Large disc-shaped structures occur on successive mm scale beds, shown in cross-section in Fig. 7b. They cover different areas in different beds and thus cannot be sections through post-deposition concretions. We prefer to interpret them as relatively long-lived organic structures that re-colonised successive layers of carbonate mud on the sea bed.

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Figure 7. (a) Discoidal structure on bedding surface. (b) Fracture surface through disc structure normal to (a) showing internal layering within the disc parallel to mm scale lamination of surrounding dolomite.

An additional branching impression, comprised of multiple long, bifurcating strands, is also present on one of the slabs (Fig. 8). The preserved portion of the structure is ~11 cm long and consists of more than 24 wavy, non-aligned grooves that converge at the lower left side of Fig. 8a. The grooves are 0.5–1 mm wide and separated from each other by variable distances up to 2 mm. The width of the strands varies from 13 mm near their broken proximal ends to ca. 78 mm at their distal ends. It is apparent that the impressed filaments bifurcated at their proximal ends and further bifurcation is also visible in their middle parts (arrowed in Fig. 8a). Liu A G et al. (2014) reported centimetre-sized structures of quadrilaterally arranged fibrous bundles of ridges from ca. 560 Ma strata in Newfoundland, Canada and interpreted them as impressions of possible muscle-bearing metazoans. Although the branching strands at Shunyang are of similar size, they differ from the Newfoundland fossils because they occur as grooves rather than ridges and do not show a quadrilateral arrangement. Alternatively the grooves could be microbially induced sedimentary structures (Lan and Chen, 2013). On-going investigations might reveal more occurrences of these structures to allow for taxonomic identification.

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Figure 8. Impression of radiating branching strands, Dengying Formation, Shunyang Valley. Diameter of coin is 20.5 mm. (a) and (b) show the same object photographed under different lighting conditions (c.f., Lan and Chen, 2013).
5 PROTECTION OF THE SIXI FOSSIL SITE

We are documenting the Sixi quarry fossil site at an early stage in our investigative studies hoping to ensure that it is protected from anthropogenic degradation, because other Ediacaran sites in the Yangtze Three Gorges region have been lost or damaged. The fossil locality at Shibantan was lost beneath the reservoir of the Gezhouba Dam, and roadside sections near Jiulongwan have been disfigured by painted labels and holes drilled for paleomagnetic and geochemical research. An outcrop of Nantuo tillite has recently disappeared from the Sixi stream valley during reconstruction of the stream banks, and burial of putative Ediacaran fossils in eroded carbonates at our original Sixi site is described above.

Bedding plane slabs in the Shunyang quarry resemble rock faces in a disused quarry near Woodhouse Eaves, Charnwood Forest, Leicestershire, England, that yielded the holotype of Charnia masoni (Ford, 1958). It was removed to the New Walk Museum, Leicester, for protection from damage (Ford, 2011). The site is protected as a Site of Special Scientific Interest by Natural England, the organisation responsible to the British government for nature conservation. The fossiliferous slabs have been shown in educational TV programmes, and continue to provide new information for paleontological research (e.g., Wilby et al., 2015, 2011). We would like Shunyang to be designated as a protected resource, and its location in the Three Gorges Bamboo Forest Nature Reserve should facilitate this. The present active quarrying at Shunyang provides an opportunity for removal of large blocks containing fossils to museums, as in Charnwood, to be followed by protection of the whole Shunyang Valley section when quarrying ends. The authors of this paper urge the leaders of Hubei Geological Bureau and managers of the Bamboo Forest to designate the Shunyang Valley as a special geological site and protect it for future generations of Chinese geology students and researchers. We are encouraged that Zigui County People's Congress is pressing ahead with legislation to protect field geology sites in the county, following a visit by Deputy Directors Yaoqun Wang and Yu Li in March 2017, guided by Prof. Kunlong Yin from CUG.

We anticipate that further scientific investigation of this site will yield significant paleontological and geological information.

ACKNOWLEDGMENTS


The authors acknowledge with thanks valuable advice from Dr. Alex Liu and the late Dr. Trevor Ford about the Ediacaran fossils. Dr. Liu also helped in drafting the manuscript. Zhen-Bing She acknowledges financial support from the National Natural Science Foundation of China (No. 41272038) and the State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences (No. GBL11610). Micro-Raman imaging was performed at the London Centre for Nanotechnology at University College London and Dominic Papineauis thanked for assistance with the analyses. The final publication is available at Springer via http://dx.doi.org/10.1007/s12583-017-0773-1.


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