A unique condition for early diversification of small shelly fossils in the lowermost Cambrian in Chengjiang, South China: Enrichment of phosphorus in restricted embayments

Gondwana Research 25 (2014) 1139–1152

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A unique condition for early diversification of small shelly fossils in the lowermost Cambrian in Chengjiang, South China: Enrichment of phosphorus in restricted embayments Tomohiko Sato a,⁎, Yukio Isozaki a, Takahiko Hitachi a, Degan Shu b a b

Department of Earth Science and Astronomy, University of Tokyo, Tokyo 153-8902, Japan Early Life Institute, Northwest University, Xi'an 710069, China

a r t i c l e

i n f o

Article history: Received 31 August 2012 Received in revised form 14 July 2013 Accepted 18 July 2013 Available online 29 July 2013 Keywords: Phosphorite Small shelly fossil Cambrian Explosion Chengjiang South China

a b s t r a c t In the earliest Cambrian the major diversification of small shelly fossils (SSFs) was the first episode of the socalled Cambrian Explosion. In order to clarify the background environmental conditions of this event, we examined the lowermost Cambrian strata with bedded phosphorites in the Chengjiang area, South China. The lowermost Cambrian (the Zhongyicun Mb of the Zhujiaqing Fm) in eastern Yunnan is composed of bedded phosphorites and phosphatic limestones with diverse SSFs. We investigated 3 sections within the Chengjiang area at Hongjiachong, Maotianshan, and Xiaolantian. Detailed lithostratigraphic analysis of outcrops and drill cores at Hongjiachong indicates that the Zhongyicun Mb consists of 5 distinct units, A to E in ascending order. The presence of 15 genera of SSFs in 20 horizons shows that the Zhongyicun Mb yields two distinct SSF assemblages of Fortunian age (earliest Cambrian; 541–529 Ma); i.e. the first assemblage with simple-shape SSFs (Anabarites, Protohertzina) from basal Unit A, and the second assemblage with various molluscan shells (Paracarinachites, Ocruranus–Eohalobia) from Units C–E. As well as abundant phosphate grains, all SSFs occur as clastic grains, suggesting that phosphorite was primarily formed in extreme shallow-water settings, as were the small shelly animals. We established that the first occurrence of the second SSF assemblage is at least 5 m lower (ca. 1–2 myr earlier in age) than previously reported in Chengjiang, and we speculate that the major diversification in SSF assemblage likely occurred during the Fortunian, at least before ca. 534 Ma. Judging from the rift-related tectonic setting and relevant paleogeography of western South China, we further speculate that the Zhongyicun Mb was primarily deposited in restricted embayments in the Kangdian rift basin, and that the rapid SSF diversification in the Fortunian occurred in a unique setting in highly phosphorus-rich seawater. © 2013 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction The Ediacaran to Cambrian period of time represents one of the most important intervals in the history of Earth's life, because it includes the prelude to the so-called Cambrian Explosion event (e.g., Maruyama et al., 2013. Santosh et al., 2013; Zhang et al., 2013, and references therein). Although various possible causes have been proposed for the Cambrian Explosion, such as the rise of oxygen (Runnegar, 1982), the increase of nutrients (Brasier, 1992), the genomic evolution (Peterson and Butterfield, 2005), the evolution of a predator–prey system (Parker, 2003; Porter, 2011) etc., the ultimate cause and processes of the diversification of animals have not been well explained. Nonetheless, the major diversification of small shelly fossils (SSFs) during the earliest Cambrian (e.g. Li et al., 2007; Steiner et al., 2007; Maloof et al., 2010) represents the actual beginning of the Cambrian Explosion that was in the middle of global phosphorite deposition (e.g. Cook and Shergold, 1984, 1986; Cook, 1992; Shields et al., 2000; Shen et al., 2010). Very thick phosphorites ⁎ Corresponding author. Tel.: +81 354546618; fax: +81 354658244. E-mail address: [email protected] (T. Sato).

that were deposited in particular in South China in the Early Cambrian yield abundant SSFs; therefore, phosphate enrichment in seawater likely played an important role in the initial radiation of the early skeletal animals and the phosphorite deposition (e.g. Brasier, 1992). Analogy with modern phosphorite accumulation off the western coasts of Africa and South America suggests that the Early Cambrian phosphorite formation (phosphogenesis) was primarily caused by coastal upwelling of phosphorus-rich deep-water (e.g. Brasier, 1992). Li (1986) described the regional distribution and petrographic characteristics of the Early Cambrian phosphorites in South China, and suggested that the phosphorites were deposited in shallow areas on the western margin of the Yangtze Platform, a suitable environment for the precipitation of phosphorus from sea water; however, the detailed mechanism and the precise setting of the phosphogenesis is not yet known. Steiner et al. (2007) distinguished five SSF zones in the Lower Cambrian of South China; i.e. in ascending order, the Anabarites trisulcatus–Protohertzina anabarica Assemblage Zone (Zone 1), Paragloborilus subglobosa–Purella squamulosa Assemblage Zone (Zone 2), Watsonella crosbyi Assemblage Zone (Zone 3), poorly fossiliferrous interzone (Zone 4), and Sinosachites flabelliformis–Tannuolina

1342-937X/$ – see front matter © 2013 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.gr.2013.07.010

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zhangwentangi Assemblage Zone (Zone 5). The first assemblage is uniquely composed of simple-shaped fossils, such as Anabarites sp. and Protohertzina sp., although most of the SSF elements are still enigmatic in taxonomy. In contrast, the second assemblage is composed of more complex cap-shaped fossils, almost half of which are reasonably identified as molluscs, such as Purella sp. and Paracarinachites sp. This assemblage change from the first to the second marked a remarkable increase

in the number of genera from 19 to 140 (Li et al., 2007). This rapid appearance of new taxa was described as the first major diversification in the earliest Cambrian (e.g. Qian, 1989, 1999; Qian and Bengtson, 1989; Qian et al., 2001; Li et al., 2007; Steiner et al., 2007). By compiling the appearance times of SSFs from Morocco, Siberia, Mongolia, and China, Maloof et al. (2010) concluded that this major diversification occurred at ca. 534 Ma during the Terreneuvian (earliest Cambrian;

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Fig. 1. Locality and paleogeographic settings of the Chengjiang area in eastern Yunnan, South China. Upper right: Paleo-depositional setting of the Yangtze Platform in the Early Cambrian (modified from Li, 1986; Zhu et al., 2003). Upper left: close-up index map of eastern Yunnan with the representative Early Cambrian sections. Lower: lithostratigraphic correlation between the Hongjiachong, Meishucun, Laolin, and Xiaotan sections in eastern Yunnan. DB: Daibu Mb, ZYC: Zhongyicun Mb, DH: Dahai Mb, SYT: Shiyantou Fm, *U–Pb tuff age (Zhu et al., 2009), **stage boundary ages (Peng et al., 2012).

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Peng et al., 2012). As this event was no doubt the first and the largest diversification in the SSF's history, a more detailed explanation is needed for its direct cause and relevant environmental conditions. In order to identify the background environmental changes relevant to the SSF diversification and the phosphorite deposition, we analyzed the stratigraphy and lithofacies of the SSF-rich Lower Cambrian strata in eastern Yunnan. The present study investigates particularly the lowermost Cambrian at three well-exposed sections in the Chengjiang area, i.e., Hongjiachong, Maotianshan, and Xiaolantian sections (Fig. 1). In addition to conventional field mapping of all the surface exposures, we made a 120 m-deep drill at a site next to an abandoned quarry at Hongjiachong. For detailed petrographic descriptions, we prepared nearly 300 polished slabs and over 400 thin sections of the rock samples both from the outcrops and drill cores. In addition to conventional analysis of bioclast composition and grain size distribution under the microscope, we analyzed with SEM-EDS the chemical composition of clastic grains in the phosphorites. The aim of this paper is to report the detailed stratigraphy and sedimentary textures of the lowermost Cambrian phosphorites in the Chengjiang area, and to discuss their depositional setting with respect to the habitat of the earliest Cambrian small shelly animals. 2. Geologic setting During the late Ediacaran to Early Cambrian, South China was located as an isolated continental block at a low latitude (Li et al., 2008). A thick pile of the Ediacaran–Cambrian strata was deposited in the shallow epeiric sea throughout the southwestern half of South China. The Lower Cambrian phosphorites are extensively exposed in South China, particularly in the southwest of the Yangtze Platform (Li, 1986). The most fossiliferrous Lower Cambrian phosphorites are in Yunnan, Sichuan, and S. Shanxi. In eastern Yunnan the deposition of phosphorite and dolomite was mainly in the Kangdian basin that was surrounded by the Dianzhong paleo-land to the west and by the Niushoushan paleo-land to the southeast (Fig. 1; Li, 1986; Zhu et al., 2001). The depth of the basin became greater to the east, therefore, the eastern Lower Cambrian strata are generally thinner. The Chengjiang area on the north of Fuxian Lake is well known for its Early Cambrian Chengjiang fauna with exceptionally well-preserved soft-bodied animal fossils (e.g., Shu, 2008). Its depositional site was located within the Kangdian basin next to the Dianzhong paleo-land. The Lower Cambrian phosphorites in the Chengjiang area mainly consist of phosphatic carbonate of an inner-shelf facies (Siegmund, 1997). According to the stratigraphic framework of Zhu et al. (2001), the uppermost Ediacaran and Lower Cambrian in eastern Yunnan consist of 4 formations, the Dengying, Zhujiaqing, Shiyantou, and Yu'anshan Fms, in ascending order; the Chengjiang fauna occurs in the Yu'anshan Fm. This stratigraphic subdivision and nomenclature are used in this article. We studied the Lower Cambrian phosphorites of the Zhongyicun Fm at three sections in abandoned quarries, i.e., Hongjiachong, Maotianshan, and Xiaolantian (Fig. 2). We mainly focused on the Hongjiachong section (24°37′N, 102°56′E) where the lowermost Cambrian phosphorites are the most extensively exposed. The site of the scientific drill core is located next to the large outcrop of an abandoned quarry at Hongjiachong. 3. Lithostratigraphy The total thickness of the uppermost Ediacaran and Lower Cambrian at Hongjiachong is ca. 120 m. The homoclinal beds strike N–S and dip 20–30°E (Fig. 2). At Hongjiachong the lowermost Cambrian Zhujiaqing Fm consists of the Daibu Mb (~45 m; dolomite, siliceous mudstone), the Zhongyicun Mb (~40 m; phosphorite), and the Dahai Mb (~1–2 m; dolomite) (Zhu et al, 2001). The overlying Shiyantou Fm consists of a beige siltstone (~65 m) with glauconitic sandstone and black shale (~12 m) at the base (Luo et al., 1984). The 120 m-deep drill at

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Hongjiachong penetrated these Lower Cambrian rocks from the Shiyantou Fm down to the Daibu Mb (Fig. 2). Fig. 3 shows the lithostratigraphic columns of the Zhujiaqing Fm at Hongjiachong. The Daibu Mb is mainly composed of bedded dolostones, calcareous mudstones, and gray siliceous dolostones. Although its base is not exposed, the total thickness reaches at least 40 m. The Zhongyicun Mb is composed of bedded phosphorites and interbedded phosphoritelimestones. The base of the member is marked by the first occurrence of a thick phosphorite bed, whereas its top is covered by a white dolostone of the Dahai Mb. The Dahai Mb at Hongjiachong is represented solely by a 1 m-thick bed of dolostone. The Shiyantou Fm is mainly composed of black mudstones with a glauconite-bearing phosphatic conglomerate (ca. 60 cm-thick) at its base. On the basis of its lithologic characteristics, we further subdivided by lithology the Zhongyicun Mb at Hongjiachong into 5 units (Figs. 3–5), i.e., from bottom to top, Unit A (22 m, bedded dark gray phosphorite), Unit B (2.5 m, phosphorite/limestone alteration), Unit C (4.5 m, phosphorite/limestone alteration), Unit D (8 m, thin-bedded phosphorite), and Unit E (3 m, phosphate-rich, thin-bedded dark gray phosphorite). The ratio between phosphatic beds and limestone beds differs from unit to unit. Units B and C are lighter gray and rather carbonate-rich and phosphate-poor, compared with Units A, D and E. In each unit, phosphorite beds have more detrital phosphate grains than neighboring limestone beds (Fig. 6). More than 200 thin sections were examined for the petrography of the 5 units that were analyzed for their chemical composition by SEM-EDS (JEOL JSM-6060LV + Oxford INCA X-sight at the University of Tokyo, Komaba). Unit A is composed of dark gray, ca. 10 cm-thick bedded phosphorite. Bedding planes are planar and parallel, slightly wavy in part. In each bed, there is no lamination or grading. Phosphorite beds are enriched in well-rounded clastic grains of phosphorite (phosphoclasts) of ca. 100 μm in diameter (brown color in photomicrograph; Fig. 6) with minor detrital grains of quartz (siliciclasts) within a calcite matrix (colorless and transparent). In contrast, the limestone beds are composed of less detrital phosphate and quartz grains within a calcite matrix (Fig. 6). The SSFs occur in a coarse-grained phosphorite with phosphate nodules (ca. 1 cm) in the lowermost part of Unit A. Most of the SSFs are less than 1 mm, the interior cavity of some of which is filled with dolomite and silicate within a phosphate matrix (Fig. 7). Unit B is composed of an alternation of dark gray phosphorite beds (ca. 10 cm thick) and light gray limestone beds (ca. 5 cm thick). There are 12 pairs of phosphorite and limestone beds in Unit B. The boundary surfaces between the phosphorite and limestone beds are planer and parallel (Fig. 6). The phosphorite beds contain thin bands of dark gray phosphate (ca. 200 μm-thick). Unit C comprises an alternation of dark gray phosphorite beds (ca. 15 cm thick) and light gray limestone beds (ca. 5 cm thick). Unit C contains more phosphorite than Unit B. There are 20 pairs of phosphorite and limestone beds in Unit C. At the base of Unit C, there is a unique intercalated ca. 20 cm-thick sandstone bed, which obliquely overlies Unit B (Fig. 4), indicating the presence of a minor hiatus. In the outcrops this sandstone bed is distinct from under- and over-lying phosphorites. The sandstone bed contains relatively large (up to 2 mm in diameter) clastic grains of phosphorite in a sparitic calcite matrix together with abundant SSF bioclasts. Unit D is an alternation of dark gray, relatively thick-bedded phosphorite (ca. 20–30 cm) and light gray limestone (ca. 5 cm). The phosphorites and limestones of Unit D show a higher contrast in color (dark gray vs. light gray) than those of Unit C (Fig. 5). The phosphorite beds are mainly composed of ca. 50 μm-sized well-rounded phosphoclasts in a calcite matrix. Limestones are composed of well-rounded phosphoclasts (100–200 μm) in a calcite matrix; a few beds (e.g. H102204 and H102111) contain SSF bioclasts. Unit E is composed of relatively thin-bedded (5 cm in average) dark gray phosphorite beds. A slump structure (ca. 30 cm-thick) occurs in the middle of Unit E (Fig. 5), suggesting the presence of a significant

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slope near the depositional site. The phosphorites are composed of well-rounded phosphoclasts in a calcite matrix. SSF bioclasts and ooids (ca. 100 μm) are abundant in the uppermost part (ca. 2 m-thick) of Unit E. Phosphorites in Unit E contain more dark organic matter than those in other units.

The five units of Zhongyicun phosphorite beds at Hongjiachong all contain phosphoritic clastic grains. Within each unit, the size of the phosphoclasts is relatively constant even between phosphorite and limestone beds; however, their maximum size varies considerably throughout the stratigraphic column. Fig. 8 shows the measured

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Fig. 3. General stratigraphic column of the Upper Ediacaran–Lower Cambrian sequence at Hongjiachong with an enlarged part of the upper Zhongyicun Mb. The Zhongyicun Mb is subdivided into 5 distinct units; i.e., Units A–E, in ascending order. Note the marker sandstone at the base of Unit C.

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Fig. 4. Outcrop views of the Zhongyicun Mb at Hongjiachong. 1: The middle part of the Zhongyicun Mb showing the unit subdivision. Note that the boundary between Unit B and Unit C shows cross bedding in overlying sandstone bed and Unit B. 2: Close-up photo of slump structure in Unit E. 3: Close-up photo of the boundary between Units B and C. Note that phosphorite beds of the unit are truncated by the basal sandstone of Unit C.

maximum sizes of phosphoclasts/siliciclasts from 40 horizons of Units A–E. In Units A, B and C, only the sandstone (H102902) at the base of Unit C contains N 2 mm-sized phosphoclasts. This sandstone bed consists of relatively coarser grains, containing numerous SSFs of over mm. A sandy bed in Unit D (H102112) also contains mm-sized phosphoclasts. Units D–E contain large phosphoclasts up to 2 mm, much larger than those from Units A–C.

4. SSF zonation and age of the phosphorites SSFs were extracted from 20 horizons out of 60 and processed with 10% acetic acid from the phosphorite beds at Hongjiachong. As illustrated in Figs. 8 and 9, 15 SSF genera were identified. SSFs occur abundantly particularly in Unit E of the Zhongyicun Mb (H101603, H101702, H102111, H102112, H102901, and HONG91–112); however, the overlying Dahai Mb and Shiyantou Fm at Hongjiachong are barren of fossils including SSF. The lowest horizon of the SSF occurrence at Hongjiachong was recognized at the base of the Zhongyicun Mb (H104913 and H105002). The SSF assemblage from the lowermost part of Unit A includes Anabarites sp. (annelid), Conotheca sp. (hyoliths), Protohertzina sp. (chaetognath spine), Siphogonuchites sp. (coeloscleritophora sclerite), Spinulitheca sp. (hyolith) and Spirellus sp. (cyanobacteria). The middle-upper part of Unit A is devoid of SSFs. While no SSFs occur in Unit B, Unit C yields SSFs solely from the basal sandstone bed. This unique sandstone contains the following SSFs; i.e., Ocruranus sp. (polyplacophora), Eohalobia sp. (polyplacophora), Paracarinachites sp. (polyplacophora), Purella sp. (monoplacophora), Hexaconnularia sp. (scyphozoa) and other cap-shaped fossils. The uppermost parts of Units D and E contain abundant SSFs of many genera; mainly Ocruranus sp., Eohalobia sp. and Paracarinachites sp.

The SSFs from the Hongjiachong section can be clearly sub-divided into two distinct assemblages; one is represented by Anabarites sp., Conotheca sp., Protohertzina sp., and Siphogonuchites sp., and the other by Eohalobia sp., and Paracarinachites sp. These two assemblages do not overlap stratigraphically at Hongjiachong. In comparison with recently proposed SSF stratigraphy, the two assemblages correspond to the first and the second assemblages described by Steiner et al. (2007), respectively; therefore, the age of the Zhongyicun Mb is constrained to be Fortunian within the Terreneuvian of the Early Cambrian (541–529 Ma; Peng et al., 2012). At Hongjiachong, the boundary between the first and the second assemblages has not been well constrained; however, we tentatively set it at the sandstone bed at the base of Unit C. 5. Discussion On the basis of the above detailed stratigraphy and lithofacies of the lowermost Cambrian strata in eastern Yunnan, South China, we now discuss the ambient conditions of the first and the major SSF diversification; in particular, the depositional setting of the unique lowermost Cambrian phosphorite beds, the use of the mid-phosphorite sandstone as a marker in regional correlations, and the precise timing of the major SSF diversification. Finally, we try to identify the possible geological setting of the massive phosphogenesis in connection with the SSF diversification. 5.1. Depositional setting of the phosphorites The most significant feature we obtained from microscopic observations is the fact that all phosphorite beds are mostly composed of clastic phosphorite grains embedded in a calcite matrix (Figs. 6, 7). It is also

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Fig. 5. Close-up photos of each unit; thin bedded phosphorite of Unit A, alternation of phosphorite and limestone beds of Unit B, alternation of phosphorite and limestone beds of Unit C, thick bedded phosphorite of Unit D, and thin bedded phosphorite of Unit E.

noteworthy that phosphorite beds include smaller amounts of quartzofeldspathic terrigenous grains, and that all SSFs occur as individual clastic grains in a calcite matrix. These features clearly suggest that the origin of phosphate mineral was not in the depositional site of the bedded phosphorite but somewhere else, probably in much shallower water than the limestone. The clastic grains of phosphate, SSFs, and quartz were likely transported into a relatively deeper site along a slope, and were deposited in bedded limestones. The depositional site of the limestones was in a relatively deeper part of the slope, as suggested by the presence of slump structures (Fig. 4); nonetheless it was still in a shallower part of a basin. All the relevant data mentioned above suggest that the primary site of the phosphogenesis, i.e., the phosphate factory, was likely in very shallow environments of the basin from which the abundant clastic phosphate grains were transported into a relatively deeper setting. Units A, D and E are mainly composed of phosphorite beds, whereas Units B and C are nearly half occupied by limestone. The phosphate content is generally higher and the grain size is generally larger in the former than the latter. This tendency likely reflects the difference in the depth of deposition; i.e. Unit A, D, and E were deposited in a relatively shallower site than Units B and C. The maximum size of clastic grains of phosphate and quartz in bedded phosphorites likely reflects the proximity to a shoreline. Therefore, the stratigraphic trends shown in Fig. 8, i.e., fining upwards from Unit A to Unit B and coarsening upwards from Unit C to Unit D, also support the above interpretation. The depositional setting of the lowermost Cambrian in eastern Yunnan, at least in the Chengjiang area, likely experienced alternating deepening and shallowing during the bedded phosphorite deposition. This further suggests that the habitats of the Early Cambrian small shelly animals were also in a relatively or even extremely shallower part of the basin.

The one and only exception to the above is the unique sandstone bed of basal Unit C, which clearly truncates the bedding of the topmost Unit B with an erosional surface (Fig. 4). The deposition of sandstone is extremely rare in Units B and C that were deposited in relatively deeper settings. The maximum grain size of the sandstone (2 mm diameter) is abnormal in relatively fine-grained rocks of Units B and C. Judging from these facts, we interpret the sandstone as a turbidite bed that accidentally reached the deeper part of basin. The sole occurrence of SSFs from this sandstone within Units B and C also agrees with this interpretation. All the SSFs from the Zhongyicun Mb at Hongjiachong are phosphatized and occur as clastic grains in the same manner as the phosphate grains. The petrographic observation of the SSFs suggests that original calcite shells were secondarily replaced by phosphate (Fig. 7). For example, the interior of a phosphatized SSF shell is mainly filled with phosphate mud containing some detrital quartz and dolomite grains (Fig. 7). Similar features were reported from other SSF cases from Siberia and Australia (Bengtson, 1992; Porter, 2004, 2010). The filling-up of cavities by phosphatic mud with quartz and dolomite detrital grains likely occurred during early diagenesis at the primary phosphorite depositional setting, clearly before their final transportation into the deeper depositional setting of bedded phosphorites. In short, we can conclude that the earliest Cambrian “phosphorite factory” in eastern Yunnan was likely located in extremely shallow parts of the basin, at least separated significantly from the depositional site of bedded limestone. Generally speaking, phosphogenesis requires a very high concentration of phosphorus in seawater by certain mechanisms, such as winnowing and reworking of phosphorus-rich rocks or sediments. Previous researchers considered that the primary phosphorite precipitation occurred in a subtidal shelf associated with deep-water upwelling, associated with shoreward transport and entrapment of phosphorus,

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Fig. 6. Slab and thin-section views of alternating phosphorite and limestone beds from Unit C (H102804) and Unit D (H102105) in the Zhongyicun Mb of the Zhujiaqing Fm in the Hongjiachong section. In the slab, the dark gray parts are phosphorite grainstone, and the light gray part is phosphatic limestone wackestone. Both types are composed of major amounts of clastic phosphate grains and dolomite grains with calcite cements.

and in-situ deposition of phosphate beds (e.g. Cook and Shergold, 1984). The above observations/discussion suggest, however, that phosphate beds were formed in two steps; primary precipitation in seawater extremely enriched in phosphorus, and secondary delivery/deposition in deeper depositional settings. In order to clarify the phosphogenesis, therefore, we need to identify the setting of the primary precipitation rather than the depositional site of bedded phosphorites. 5.2. Mid-phosphorite marker sandstone As noted above, the sandstone of basal Unit C at Hongjiachong is quite unique in the interval of fine-grained sediments of Units B and C. This uniqueness, in turn, suggests its utility in lithostratigraphic correlations of the Lower Cambrian strata in eastern Yunnan, South China. This sandstone bed also occurs in two other sections in Chengjiang, i.e. Maotianshan, and Xiaolantian (Fig. 2), suggesting that this bed

was a distinct marker within the Chengjiang area. Frequent mining of phosphorite ore has taken place in the Chengjiang area as well as in many other parts in eastern Yunnan including the Meishucun area. Traditionally, the main target of the phosphorite ore was two-fold; the “lower phosphorite” and “upper phosphorite” separated by the “white (unprofitable) dolomite” in the lowermost Cambrian in eastern Yunnan (e.g. Luo et al., 1984; Zhu et al., 2001), and so it was in the Chengjiang area. The marker sandstone bed occurs at the base of Unit C, namely in the middle of the “dolomite” between the two ore horizons. It is noteworthy that this sandstone horizon likely corresponds to the turning point from the deepening trend in the Lower Zhongyicun Mb to the shallowing one in the upper half of the member. We can widen the significance of the sandstone bed to a much broader context by correlating the Lower Cambrian basins throughout the southwestern part of South China. For example, Li et al. (2009) reported a “mudstone” in more or less the same horizon in the lowermost

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Fig. 7. Photomicrographs of phosphorites (Unit A, D, and E) in the Zhongyicun Mb at Hongjiachong. SEM-EDS elemental mapping images (P, Si, Mg, Ca) of a calcite-cemented phosphatic grainstone (Unit D). SSFs have phosphatic shells with phosphate internal mold containing detrital quartz and dolomite grains (ph, phosphate; dl, dolomite; qz, quartz). Small detrital phosphate grains in a calcite matrix (Unit A). Phosphatized SSFs and phosphate clastics in a phosphatic matrix (Unit E).

Cambrian in the Laolin section in northeastern Yunnan (Fig. 10). Although this bed is much finer-grained, it represents the one and only terrigenous clastic unit within the fine-grained Zhongyicun carbonate/ phosphorite succession at Laolin. A similar clastic bed in the same horizon was reported elsewhere in eastern Yunnan; e.g. the black mudstone bed in the mid Zhongyicun Mb in the Xiaotan section (Li et al., 2013) and the coarse-grained clastic bed of “Unit 7” in the well-known Meishucun section (Luo et al., 1984). The thickness of the Zhongyicun Mb is generally greater in northeastern Yunnan (Li, 1986) than in eastern Yunnan, in a remarkable contrast with thinner sections of near-shore facies, as in the Xianfeng section (Lei, 1986). Accordingly, the phosphorite beds in northeastern Yunnan are regarded to have been deposited in deeper parts of the basin with respect to those in the south (Li et al., 2009; Li et al., 2013). What is noteworthy is the occurrence of the marker clastic bed constantly in the middle horizon of the Zhongyicun Mb regardless of total thickness (Fig. 10). The occurrence of the second SSF assemblage is restricted solely to the upper half of the Zhongyicun Mb from and above this marker bed and never detected from the lower half. This also supports the nearly contemporaneous deposition of a single clastic bed throughout the basin. 5.3. Timing of the SSF diversification From the Zhongyicun Mb at Hongjiachong, two distinct SSF assemblages were discriminated; i.e. Anabarites trisulcatus–Protohertzina anabarica Assemblage and Paragloborilus subglobosa–Purella squamulosa

Assemblage. It is noteworthy that these two assemblages are considerably different from each other; i.e. the first assemblage consists solely of simple forms without molluscs, whereas the second contains more complex forms including molluscs, such as Paracarinachites sp. and Ocruranus–Eohalobia group (benthic polyplacophoran). The previous study in the Chengjiang area (Jiang and Chen, 2008) also reported almost the identical two assemblages, although the precise range chart of SSFs was not illustrated in columnar sections. At any rate, this assemblage change from the first to the second represents the first and the most significant diversification event in the SSF history (Li et al., 2007; Steiner et al., 2007), and it occurred during the deposition of the lowermost Cambrian Zhongyicun Mb in eastern Yunnan. Due to a 20 m-thick barren interval in the lower half of the Zhongyicun Mb (Fig. 8), the two SSF assemblages do not co-occur at Hongjiachong, and the precise boundary horizon between the two is not well constrained. According to the previous report (Jiang and Chen, 2008), the first assemblage occurred in the lower Zhongyicun Mb (corresponding to Units A, B and C), but the second was in the upper half (corresponding to Units D and E). Thus the first appearance of the second assemblage was recognized at the boundary between Units C and D. This study, however, detected the second SSF assemblage at the base of Unit C, at least 5 m below the previously assigned horizon. Although we still cannot pinpoint the first appearance datum of the second assemblage, this event, in other words the first major SSF diversification, no doubt occurred much earlier (probably ca. 1–2 myr) than previously recognized.

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Fig. 8. Simplified stratigraphic column showing the schematic lithofacies change, the maximum size of phosphate and silicate clastic grains, and the representative SSF occurrence in the Zhongyicun Mb of the Zhujiaqing Fm at Hongjiachong. Units A, D and E are more phosphatic than Units B–C. In the middle of the Zhongyicun Mb, the sandstone at the base of Unit C exceptionally contains mm-sized phosphoclasts. Note that the faunal transition from SSF Assemblage 1 (Anabarites trisulcatus–Protohertzina anabarica) to SSF Assemblage 2 (Paragloborilus subglobosus–Purella squamulosa) lies between the lowermost part of Unit A and the top of Unit B.

At Hongjiachong, the marker sandstone of basal Unit C is the lowermost horizon of the second assemblage, and all SSFs are included as detrital grains (Fig. 7). The turbidite origin of the sandstone implies

A

B

C

that the SSFs were transported from much shallower environments. These conditions suggest that small shelly animals thrived in extremely shallow-water environments, and that animals of the second assemblage

D

E

500 µm

F

200 µm

500 µm 100 µm

500 µm

G

1000 µm

H

K I

200 µm 200 µm

L

J

200 µm

200 µm

200 µm

200 µm

Fig. 9. Early Cambrian SSFs from the Zhongyicun Mb of the Zhujiaqing Fm at Hongjiachong. (A) Chaetognath spine Protohertzina sp. (B) Hyolith Conotheca sp. (C) Coeloscleritophora sclerite Siphogonuchites sp. (D) Problematic tubular fossil Spinulitheca sp. (E) Scyphozoan Hexaconnularia sp. (F) Cyanobacteria Spirellus sp. (G–J) Polyplacophora Paracarinachites sp. (K) Polyplacophora fossil Eohalobia sp. (L) Polyplacophora Ocruranus sp. Fossils A–E occur from H105002, Unit A, and fossils G–L occur from H102901, Unit C.

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Meishucun

Hongjiachong

(Li et al., 2009; Li et al., 2013)

Assemblage 2 * 535.2±1.7 Ma

small shelly fossils 2nd assemblage 1st assemblage

sandstone Lower phosphorite

10 m

Stage 2

SYT ZYC DH

Terreneuvian

Laolin, Xiaotan

Upper phosphorite

DB

Fortunian

541.0 Ma**

Ediacaran

** 529.0 Ma

Cambrian

(Luo et al., 1984; (This study) Brasier et al., 1990)

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Fig. 10. Schematic correlation of the Zhongyicun Mb in eastern Yunnan and the stratigraphic distribution of the first and second SSF assemblages. Note that the sandstone in the middle horizon of the Zhongyicun Mb serves as a marker bed for lithostratigraphic correlations in eastern Yunnan. This sandstone bed apparently records the first appearance of the second SSF assemblage at the Hongjiachong (this study) and Meishucun sections (Luo et al., 1984), and is roughly calibrated to be ca. 534 Ma (Fortunian). Although the second SSF assemblage has not been found below this horizon, the existence of a hiatus at the base of the sandstone suggests a slightly earlier timing for the main diversification of small shelly animals. (*U–Pb tuff age by Zhu et al., 2009; **stage boundary ages by Peng et al., 2012).

may have possibly appeared earlier than the sandstone deposition. Further study is needed for more detailed SSF stratigraphy of the lower half of the Zhongyicun Mb, in particular that of Unit B. Recently, age assignment has improved considerably for the Early Cambrian (Peng et al., 2012); however, further refinement of the earliest Cambrian is not high enough in resolution. According to Peng et al. (2012), the ages of the base and top of the Zhongyicun Mb is assigned at 541.0 Ma (the PC–C boundary) and 529.0 Ma (the base of the Cambrian Stage 2; the appearance of Watsonella crosbyi). In the Meishucun section the GSSP of the basal Cambrian in South China, a tuff bed in “Unit 5” at the mid-Zhongyicun Mb, yielded zircons with a U–Pb age of 535.2 ± 1.7 Ma (Zhu et al., 2009). On the basis of the above stratigraphic correlation (Fig. 10), the boundary between Units B and C is roughly calibrated at around ca. 534 Ma. As discussed above, the diverse SSFs of the second assemblage are regarded to have appeared earlier than the deposition of the marker sandstone, thus the major diversification event started already before ca. 534 Ma, i.e. during in the Fortunian. This timing is much earlier than the previous estimate of Maloof et al. (2010) who assumed that the mid-Nemakit–Daldynian age was between 534 and 530 Ma. 5.4. Phosphogenesis in the Kangdian rift basin The above discussion concluded that the habitats of small shelly animals were in a shallow-water environment closely associated with the primary depositional site of the lowermost Cambrian phosphorite. The detailed mechanisms of phosphate precipitation and/or phosphatization of possibly calcite shells were not yet clearly explained; nonetheless at the end of the discussion, we speculated that the phosphorites might have been deposited in a unique shallow-water environment, such as a small-scale restricted embayment along a shoreline, rather than on a shelf margin at the upwelling front as previously imagined,

and that the SSF diversification possibly occurred in such extreme conditions (Fig. 11). The Lower Cambrian phosphorite of the Zhongyicun Mb in eastern Yunnan was deposited in shallow seas in the Kangdian Basin on the Yangtze Platform (Fig. 1). The Ediacaran extensional tectonics in southwestern South China generated several rift-related basins in eastern Yunnan to develop complicated topographic features in a shallow shelf (Li, 1986). This brought a significant variation in thickness, lithofacies, and biofacies of the Zhongyicun Mb. It is noteworthy that the occurrence of Lower Cambrian phosphorites in South China was not ubiquitous but restricted to the southwestern part. For example, even in shallow shelf domains, the Lower Cambrian in the Three Gorges area lacks thick phosphorite beds (e.g. Sawaki et al., 2008). Previous studies suggested that the source of phosphorus in the Early Cambrian was deep-sea water welled up along a shelf edge (e.g. Cook and Shergold, 1984; Brasier, 1992) in analogy with modern offshore margins of Chile and Peru. The upwelling origin of phosphorus, however, is unlikely in the case of the Lower Cambrian phosphorites in eastern Yunnan, because their depositional setting on an inner shelf (Fig. 1) is totally different from the outer shelf margin with upwelling. In addition, the total amount of the Lower Cambrian phosphorites was the largest in the Earth's history (ca. 4000 million tons; Notholt and Sheldon, 1986). Any modern analogue of marine phosphorite deposition, such as upwelling zones off Peru and Namibia (Föllmi, 1996), is not comparable. The above relations suggest that the site of the Lower Cambrian phosphorite deposition was not an outer shelf margin as previously imagined, and that the source of abundant phosphorus was not upwelling deep-sea water. In general, it is difficult to make unique seawater composition highly enriched particularly in phosphorus in an open marine setting because of the huge volume of diluting seawater. The burial of phosphate in sediments also requires certain redox conditions of seawater (e.g. Ingall and

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Fig. 11. Schematic diagram of the reconstructed depositional setting of the Zhongyicun phosphorite in eastern Yunnan. Above: a simplified paleogeographic reconstruction of the Kangdian rift basin in western South China, below: a possible setting of the primary phosphorite deposition and habitat of small shelly animals in restricted embayments along the margin of the rift basin with unique seawater composition. Note that the occurrence of the lowermost Cambrian phosphorite beds (the Zhongyicun Mb in eastern Yunnan) was restricted to the Neoproterozoic Kangdian rift basin in western South China (above), along which plume-related intrusive igneous suites occurred (Li et al., 2003). In general, plume-derived magma is alkaline and enriched in phosphorus, and so are the volcanic parts, such as kimberlite and carbonatite. Their surface exposure/weathering likely provides huge amounts of phosphorus into the neighboring rift basins, as is observed in modern East Africa. The unique concentration of phosphorus is inevitable for the deposition of a large amount of phosphorites, without connection to vast seawater in open oceans. As a possible candidate for the site of primary phosphorite deposition in eastern Yunnan, small-scale restricted embayments along the basin margin are speculated (below). Although any practical example has not been identified yet in eastern Yunnan, probably owing to the erosion-vulnerable nature of ephemeral basins, small-scale restricted embayments, with a provenance from weathered alkaline volcanics, likely accommodated the unique condition for the primary phosphorite deposition. As well as the intrusion of alkaline magmas, the development of these embayments might be controlled by rift-related fault systems. The detrital grains of phosphorites were secondarily transported into deeper parts of the basin to be deposited as bedded phosphorite and interbedded phosphorite–limestone of the Zhongyicun Mb. The detrital nature of all SSFs in the Zhongyicun Mb positively suggests that the Fortunian SSF diversification occurred not in an open marine environment, but likely in restricted embayments with unique phosphorusenriched seawater.

Jahnke, 1997; Goldhammer et al., 2010). In order to produce unique seawater highly-enriched in phosphorus enough to accumulate phosphate, what needs to be prepared is a particular depositional environment clearly separated from the vast seawater of an open ocean. Although we still do not have any direct field evidence, we suggest that a smallscale restricted basin, such as a lagoon-like embayment along a shoreline, is a possible setting for the primary phosphorus concentration and precipitation. As supplementary information, phosphorite deposition in shallow restricted embayments was also suggested for the early Middle Cambrian phosphorite of the Georgina Basin in northern Australia on the basis of petrology and geochemistry (e.g. Howard and Hough, 1979). Besides the upwelling of deep-sea water, the origin of a huge amount of phosphorus is expected in igneous rocks exposed on land, in particular those of alkaline composition. Along the Kangdian basin in western South

China (Fig. 1), Neoproterozoic plume-related alkaline igneous rocks are present, including granites, granodiorites, tonalites, diorites, gabbros, mafic dykes, and small ultramafic bodies (Li et al., 2003). It is noteworthy, therefore, that the occurrence of the Lower Cambrian phosphorite is limited to the Kangdian basin. This N–S trending basin was developed along a Neoproterozoic rift system that was born at ca. 820–750 Ma in association with the breakup of the supercontinent Rodinia (e.g. Li et al., 2003, 2008; Wang and Li, 2003). Continental rifting is generated by mantle plume activity, in particular, by the impingement of a plume head at the bottom of the lithosphere beneath the supercontinent. Because of the unique geochemistry of mantle-derived magma, plume-related, mostly alkaline, igneous complexes tend to have higher contents of phosphorus (e.g., Woolley and Kempe, 1989; Bell and Simonetti, 1996). The large variety of alkaline

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volcanic rocks includes carbonatite and kimberlite, and some of their surface-weathered exposures are excavated for phosphorite; e.g. the phosphorite mines along the Great Rift Valley in East Africa (e.g. the Minjingu Phosphate Mine; Banzi et al., 2000). Likewise along the margins of the Early Cambrian Kangdian basin, phosphorus-rich alkaline magma suites have evolved (Li et al., 2003) by the weathering/erosion of volcanic rocks, which might supply a large amount of phosphorus into the neighboring rift basins. In order to accumulate highly concentrated phosphorus, small-scale near-shore embayments likely played an important role to avoid dilution by a large quantity of seawater from an open ocean (Fig. 11). At present, however, we have information solely on the plumerelated plutonic rocks but not on volcanic ones. We still cannot identify the putative small embayment of the primary phosphorite deposition. Lack of any practical example is probably owing to the erosionvulnerable nature of ephemeral small-scale restricted embayments with a sedimentary provenance from weathered alkaline volcanics. Nonetheless, the combined assumption of the exposure/erosion of alkaline volcanics with a high content of phosphorus and the development of small-scale lagoon-like embayments in a shoreline of the Kangdian basin is a new possible explanation for the unique phosphogenesis in eastern Yunnan in the Early Cambrian. This speculation for the origin of the huge deposition of Lower Cambrian phosphorites in eastern Yunnan is in remarkable contrast to the previous ideas of deep-sea water upwelling. Moreover, this alternative idea potentially requires re-consideration on the birthplace of small shelly animals and the origin of phosphatic biomineralization, as eastern Yunnan appears like the hotspot of the Early Cambrian “explosive” evolution. We need to seek more convincing lines of evidence to prove or disprove this speculative and challenging interpretation.

4. The deposition of the primary phosphorite likely occurred in restricted embayments along the shoreline under unique seawater composition without connection to an open ocean. 5. The diversification of small shelly animals also likely occurred in such extremely shallow embayments. 6. The source of phosphorus may have been rift-related magmatic rocks in the Kangdian basin in western South China. Plume-derived alkaline volcanics and their surface weathering likely provided a unique concentration of phosphorus to restricted embayments along the basin margins.

6. Conclusions

References

This study analyzed the detailed lithostratigraphy and lithofacies of the phosphate-bearing lowermost Cambrian Zhongyicun Mb of the Zhujiaqing Fm in the Chengjiang area in eastern Yunnan, South China. We obtained the following new results. 1. The Zhongyicun Mb at Hongjiachong is lithologically subdivided into Units A–E in ascending order. Units A, D and E are more phosphatic than Units B–C. 2. A distinct 20 cm-thick sandstone at the base of Unit C represents a unique coarse-grained bed within the phosphate-dominant Zhongyicun Mb. This sandstone is obliquely overlying the top of Unit B. 3. All phosphorites occur as clastic grains, as well as SSFs, within a calcareous matrix. 4. Two distinct SSF assemblages were obtained; Unit A yields simple forms such as Anabarites sp., Conotheca sp., Protohertzina sp. and Siphogonuchites sp., Unit B is barren of SSF, and Units C–E yield mollusk shells such as Eohalobia sp. and Paracarinachites sp. 5. The first appearance of the second SSF assemblage was at the base of Unit C, at least 5 m below the previously reported horizon. This timing is much earlier than previously recognized for ca. 1–2 myr On the basis of these new findings, possible settings of the peculiar phosphorite deposition and rapid SSF diversification in the earliest Cambrian in South China are speculated as follows. 1. Primary phosphorites were likely deposited in extremely shallow settings along the earliest Cambrian basin margins in eastern Yunnan. Their clastic grains were secondarily transported into deeper water to be deposited with carbonates. 2. The unique sandstone at the base of Unit C can be used as a marker bed in lithostratigraphic correlations in eastern Yunnan. 3. The major SSF diversification likely occurred during the Fortunian (the earliest Cambrian at least before ca. 534 Ma).

Acknowledgement Masafumi Saitoh, Daisuke Kofukuda, Takumi Futamori (The University of Tokyo), Miyuki Tahata (Tokyo Institute of Technology), Jian Han, Xiaoyong Yao (Northwest University), and Junfeng Guo (Chang'an University) provided great assistance in the field. Tsuyoshi Komiya (The University of Tokyo), Shigenori Maruyama, and Yusuke Sawaki (Tokyo Institute of Technology) gave us many constructive comments. Brian Windley (University of Leicester) corrected our English writing. Three anonymous reviewers helped us to improve the manuscript. This study was supported by a Grant-in-Aid from the Japan Society for the Promotion of Science (No. 11J07691 to T. S. and No. 20224012 to Y. I.), the Global COE program “Earth to Earths” (Tokyo Institute of Technology and The University of Tokyo), the Natural Science Foundation of China (NSFC grant 41272019), the Ph.D. Programs of the Foundation of the Ministry of Education of China (20116101130002), “111 project” (No. P201102007), and the MOST Special Fund from the State Key Laboratory of Continental Dynamics, Northwest University.

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