Environmental setting of the Cambrian Terreneuvian rocks from the southwestern Yangtze Platform, South China

Journal Pre-proof Environmental setting of the Cambrian Terreneuvian rocks from the southwestern Yangtze Platform, South China Xiaojuan Sun, Christoph Heubeck, Michael Steiner, Ben Yang PII:

S0031-0182(19)30275-5

DOI:

https://doi.org/10.1016/j.palaeo.2019.109424

Reference:

PALAEO 109424

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Palaeogeography, Palaeoclimatology, Palaeoecology

Received Date: 25 March 2019 Revised Date:

23 October 2019

Accepted Date: 23 October 2019

Please cite this article as: Sun, X., Heubeck, C., Steiner, M., Yang, B., Environmental setting of the Cambrian Terreneuvian rocks from the southwestern Yangtze Platform, South China, Palaeogeography, Palaeoclimatology, Palaeoecology, https://doi.org/10.1016/j.palaeo.2019.109424. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

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Environmental setting of the Cambrian Terreneuvian rocks from the

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southwestern Yangtze Platform, South China

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Xiaojuan Suna,b, Christoph Heubecka,c, Michael Steinera, Ben Yangd

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a

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Germany

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b

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Palaeontology, Chinese Academy of Sciences, Nanjing 210008, China

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c

Department of Geosciences, Universität Jena, Burgweg 11, 07749 Jena, Germany

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d

Institute of Geology, Chinese Academy of Geological Sciences, Baiwanzhuang Street 26, 100037

Institute of Geological Sciences, Freie Universität Berlin, Malteserstraße 74-100, 12249 Berlin,

State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and

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Beijing, China

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Abstract

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Terreneuvian strata from the western Yangtze Platform host diverse assemblages of

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small shelly fossils (SSFs), which are key for understanding the Cambrian Bioradiation Event

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and have been the objects of numerous geochemical studies to understand environmental

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factors during Fortunian and Cambrian Age 2. The region has been proposed to host the

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vacant stratotype of Cambrian Stage 2. However, a comprehensive study of paleogeography

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and evolution of sedimentary facies for the Terreneuvian is still lacking. Here we present a

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detailed sedimentological and petrographic study of six stratigraphic sections from the

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western Yangtze Platform. Eleven depositional environments (facies associations) are

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proposed based on the characterization of twenty-five facies. We further reconstruct

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palaeogeographic model based on the evolution of depositional environments and discuss the

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formation of phosphorites. The Zhujiaqing Formation deposited diachronously on the

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Ediacaran strata, and facies associations range from tidal flats, shoal, back-shoal to semi-

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restricted and energetic subtidal. As sedimentation continued, protected shallow subtidal

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became widespread. Evidence presented from the Zhujiaqing Formation indicates that strata

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in the laolin section are more continuous than those on other parts of the Yangtze Platform. -1-

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However, stratigraphic condensation and the protected and partly restricted environment

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should be taken into consideration when discussing a proposed global GSSP “Laolinian Stage”

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(Cambrian Stage 2). In-situ phosphates formed in various environments, which argues against

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the previous assumption that primary phosphate formed only in isolated embayments.

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Concentration of granular phosphorites occurred in shoal areas by winnowing and reworking

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of in-situ phosphate and phosphatized skeletons, while in the intra-platform basin, repeated

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alternations of syndepositional phosphogenesis, current reworking, and amalgamation of

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storm-generated phosphatic event beds contributed to the concentration of granular

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phosphorites.

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Keywords: Sedimentary condensation, Carbonate platform, Phosphogenesis, Zhujiaqing

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Formation, Cambrian Stage 2, Cambrian biomineralization

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1. Introduction

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The Ediacaran - Cambrian (E-C) transition globally marks a major turning point in

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Earth’s history (Brasier and Lindsay, 2001). This includes but is not confined to the breakup

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of Rodinia and the subsequent assembly of some of its constituent cratons into Gondwana

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(Deynoux et al., 2006; Li et al., 2008; Meert and Lieberman, 2008;), several “phosphorite

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giant” deposits (Cook and Shergold, 1986; Cook, 1992; Álvaro et al., 2016), and most

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importantly, the major radiation of metazoans (McCall, 2006; Marshall, 2006; Erwin et al.,

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2011). A worldwide early Cambrian small shelly fossils (SSFs) diversification represents the

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beginning of Cambrian metazoan diversification (Steiner et al., 2007; Maloof et al., 2010;

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Landing et al., 2013). This event is well preserved in the Zhujiaqing Formation from the

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Yangtze Platform, comprising a chert-phosphorite-carbonate sedimentary sequence. However,

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despite intense stratigraphic studies (Luo et al., 1982; Qian, 1989; Qian et al., 2002; Li et al.,

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2004; Steiner et al., 2007; Yang et al., 2014, 2016) and the interpretations of geochemical

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signatures reported from the Zhujiaqing formation (Brasier et al., 1990; Zhou et al., 1997;

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Shen and Schidlowski, 2000; Li et al., 2009, 2013; Shields et al., 2001; Cremonese et al.,

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2013; Bowyer et al., 2017), detailed sedimentary facies and paleogeographic reconstruction is

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lacking. This hinders our understanding of the depositional context coinciding with the SSFs

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bio-radiation, and also hinders the adequate interpretation of the paleobiological and

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geochemical changes in the rock record.

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The widespread occurrence of phosphorites across the E – C boundary interval has for

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many years been related to the explosion of SSFs, but the connection has remained enigmatic

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(Brasier and Lindsay, 2001). Researchers highlight the taphonomic effects of phosphorus-rich

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waters rather than any evolutionary effects (Brasier, 1992; Creveling et al., 2014). Based on a

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study in the Chengjiang area of eastern Yunnan, assumption was proposed by Sato et al.

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(2014) that primary phosphate formed in unique small isolated embayments along the

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shoreline, and the diversification of SSFs occurred in such unique embayments. This is in

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sharp contrast to the former conclusion that P production was offshore and phosphorite was

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transported toward the shallower environment (Siegmund, 1995). However, such putative

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embayments along the shoreline lack any direct field evidence, and the temporal- and spatial-

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related rocks were not investigated. Besides, Landing et al. (2013, 2016) proposed that the L4

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carbon isotope excursion peak (Li et al., 2009) above the first appearance datum (FAD) of

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Watsonella crosbyi, and within the Zhujiaqing Formation at Laolin in eastern Yunnan, would

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best define a Cambrian Stage 2 GSSP, tentatively named “Laolinian Stage”. This study

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provides the necessary sedimentological data allowing a more holistic review of the proposal.

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Former paleogeographic and lithofacies studies about the lowermost Cambrian strata

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in Eastern Yunnan mainly focused on the mineralogy and concentration mechanisms of the

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phosphorite mines in the Zhongyicun member of the Zhujiaqing Formation (Ge et al., 1983;

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Xu et al., 1984; Lei et al., 1986; Zeng et al., 1987; Huang et al., 1990; He et al., 1989)

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without placing sections in a regional process-oriented context with detailed facies analysis.

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Herein, six profiles in eastern Yunnan have been investigated in detail from outcrop-scale to

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thin-section petrography. These sections are well exposed along active mines, roadcuts and

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valleys, and a general time framework has been biostratigraphically constrained (Yang et al.,

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2014). The present study aims to provide new information on sedimentary facies evolution of

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the Zhujiaqing Formation, and to reconstruct the depositional setting of the Terreneuvian

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strata from the southwestern Yangtze Platform.

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2. Geological setting

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2.1. Paleogeography of the Yangtze Platform and eastern Yunnan across the E-C transition

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The Yangtze Platform developed since the late Neoproterozoic on the southeastern

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margin of the Yangtze Block at low latitude. Its paleogeography and tectonic framework had

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a major influence on its stratigraphic architecture. The platform area was internally structured

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by numerous fault-bounded shelf basins during the early Ediacaran, along its margins, slope

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and basinal-facies sediments were deposited (Zhu et al., 2003; Jiang et al., 2011). The

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uppermost Ediacaran in shallow-water facies of the Yangtze Platform is the carbonate-

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dominated Dengying Formation (ca. 551-542 Ma; Zhu et al., 2007). The Dengying Formation

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has a varying thickness between 90 and 800 m across the platform (Steiner et al., 2007).

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Several paleokarst surfaces in the Dengying Formation indicate frequent subaerial exposures

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(Xue et al., 1992; Siegmund and Erdtmann, 1994; Shan et al., 2017). The contact of the

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Dengying Formation with overlying Cambrian strata is a widespread unconformity almost

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everywhere on the Yangtze Platform (Zhu et al., 2003). The lowermost Cambrian strata are

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characterized by predominant carbonate and mudstone/shale on the northwestern and

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southeastern part of the Yangtze Block, respectively. A “transitional zone” characterized by

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interbedded shale and carbonate is located in northeastern Guizhou, southern Hubei and

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northern Hunan provinces (Fig. 1A). Thickness of lower Cambrian strata increases from the

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deep-water setting (e.g., in southern Hunan, eastern Guizhou) towards the shallow-platform

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facies (e.g., in eastern Yunnan, southern Sichuan and northwestern Guizhou). Aside from the

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sections in the Three Gorges region, strata in eastern Yunnan preserve the most complete and

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diverse stratigraphic record of the E - C transition in South China.

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Eastern Yunnan is located on the western part of the Yangtze Platform. Its

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sedimentation was controlled by the north-south-trending Kangdian rift basin since the

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Neoproterozoic; the rifting continued into the early Ediacaran (Wang and Li, 2003) and then

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passed into thermal subsidence. A widespread epeiric sea occupied the Yangtze Platform

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during the late Ediacaran time. At the beginning of the Cambrian, the Zhujiaqing Formation

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was deposited in the Kangdian Basin, which was surrounded by an island chain to the west

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and by the Niushoushan paleo-island to the southeast (Fig. 1B). However, unlike the southern

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Sichuan province to its north, which was constantly a very shallow flat-topped carbonate

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platform, detailed paleogeography of the Zhujiaqing Formation is complicated. Based on the

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studies of phosphorite deposits in the Zhongyicun Member of the Zhujiaqing Formation, a

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phosphorite-rich belt (Fig. 1B) occurs in the central part of the Kangdian Basin. In detail,

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supratidal-intertidal environments dominate the western part near the paleo-islands chain, and

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intertidal-subtidal environments dominate the east in the Kangdian Basin (Luo et al., 1982;

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Ge et al., 1983; Chen et al., 1985; Luo et al., 1991). However, three depositional “sags” that

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accommodated variable thicknesses of the Zhongyicun Member are apparent (Fig. 1C).

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2.2. Ediacaran - Cambrian stratigraphy in eastern Yunnan

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In eastern Yunnan, the upper Ediacaran Dengying Formation consists of dolomitized

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shallow marine to peritidal carbonate and quartzose siltstone (Zhu et al., 2003).

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Cambrian Zhujiaqing Formation overlies the Dengying Formation unconformably (Fig. 2). It

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ranges from a few tens of m to more than 200 m in thickness. The FAD of Trichophycus

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pedum (T. pedum), the index fossil of the E-C boundary, occurs in eastern Yunnan slightly

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delayed compared to siliciclastic platforms due to facies restriction (Weber et al., 2007).

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The

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Regional stratigraphic correlation is mainly based on SSFs biostratigraphy (Luo et al., 1982;

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Qian et al., 1996, 2002; Zhu et al., 2001; Li et al., 2004; Steiner et al., 2007; Yang et al.,

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2014). Three SSF zones for intra-platform and inter-platform correlations have been

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established. To correlate unfossiliferous strata, multi-proxy chemostratigraphic studies (Zhou

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et al., 1997; Li et al., 2009, 2013; Cremonese et al., 2013) were undertaken. The δ13Ccarb data

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from the Zhujiaqing Formation show generally increasing δ13Ccarb values with prominent

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excursions (Zhu et al., 2006; Shields-Zhou et al., 2013). Large excursions combined with bio-

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stratigraphically correlated units enable the general correlation of the Zhujiaqing Formation

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with strata in Morocco and Siberia (Li et al., 2013; Landing et al., 2013; Landing and

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Kouchinsky, 2016), but detailed stratigraphic correlations of some isotopic excursions are

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still equivocal.

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The Zhujiaqing Formation is subdivided into three members: The basal Daibu

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Member (0 to 60 m thick) comprises uniform, thin- to medium-bedded dark grey chert and

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siliceous/argillaceous dolostone with interbedded siltstone. It grades upwards into the

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Zhongyicun Member (10 to 80 m thick), which is lithologically varied and characterized by

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several mineable phosphorite beds. SSFs of Zone I (Anabarites trisulcatus-Protohertzina

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anabarica Assemblage Zone) and Zone II (Paragloborilus subglobosus-Purella squamulosa

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Assemblage Zone) in the Zhongyicun member show an upward-increasing diversity. The

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calcareous Dahai Member consists of dolostone and limestone with a varied thickness: 1m at

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Meishucun section, 40 m at Laolin, and 60 m at Xiaotan section (Li et al., 2013; Cremonese

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et al., 2013). The lower part of the Dahai Member comprises whitish, medium- to thick-

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bedded dolostone; its upper part comprises grey, thin- to thick-bedded dolostone and

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dolomitic limestone with abundant SSFs of Zone III (Watsonella crosbyi Assemblage Zone).

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The overlying Shiyantou Formation consists mainly of siltstone. At the upper part of the

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Shiyantou Formation, the Sinosachites flabelliformis-Tannuolina zhangwentangi Assemblage

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Zone occurs in thin-bedded and lenticular limestones. The overlying Yuanshan Formation

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consists of black shale and carbonaceous siltstone. Its lower part contains the earliest record

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of trilobites in China (Steiner et al., 2001). The current international stratigraphy divides the

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lowest Cambrian series, the “Terreneuvian” (Landing et al., 2007; Peng et al., 2012), into the

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Fortunian Stage and the overlying, unnamed “Cambrian Stage 2”. The top of the

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Terreneuvian Series (and the top of Cambrian Stage 2) has not been defined yet but is

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expected to be close to the FAD of Gondwanan trilobites.

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3. Methods and materials

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We measured and sampled six well-exposed stratigraphic sections with particular

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emphasis on lithology, sedimentary structures and bedding surfaces in the field. These

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sections include the Laolin (26 16′44.1″N, 103 13′25.1″E), Zhujiaqing (26 18′18.87″N, 103

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13′16.07″E), Lishuping (26 。 22′38.1″N, 103 。 12′51.7″E), Xianfeng (25 。 31′24.0″N, 103 。

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4′11.1″E), Mingyihe (24 46′7.14″N, 102 28′30.30″E) and Meishucun (24 50′35.25″N, 102

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23′02.62″E) section, listed from the north to south of the northeastern Yunnan region.

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Rocks in the Zhujiaqing Formation are composed of calcite, dolomite and francolite

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besides smaller proportions of clay, chert, silt-sized quartz and other minerals. The herein

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applied terminology of rock types is as follows: The term phosphorite is commonly defined

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as a rock unit contains >18% weight percent P2O5 (Pufahl, 2010), however, for practical use

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in field and microscopic observation, rock containing >40% phosphate particles or

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microsphorites is defined here as phosphorite, while for rocks containing <40% and >10%

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phosphate particles or microsphorites, “phosphatic rock” is applied. Phosphorite textures are

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classified according to Trappe (1998). Folk’s (1962) classification is used to denote grain and

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matrix type, and Dunham’s (1962) classification for depositional texture for both carbonates

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and phosphorites. Facies have been defined by a combination of both micro- and macro-

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studies and refer basically to a series of genetically related depositional events. We have used -7-

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descriptive terms to name facies, and the interpretation of facies is based on comparison with

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studies on modern and ancient environments. Facies association defined broad environmental

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complexes.

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Hand samples were polished for detailed sedimentary study. Thin sections were

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petrographically characterized using standard transmitted-light microscopy and scanning

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electron microscopy (SEM) using a ZEISS supon VP operating in energy dispersive X-ray

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analysis mode at the Freie Universität Berlin. Selected polished thin sections were carbon-

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coated and examined by ‘hot cathode’ cathodoluminescence (CL) microscope (Type HC3-

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LM) for diagenetic features at the Museum für Naturkunde Berlin. The acceleration voltage

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of the electron beam was 14 keV and the beam current ranged between 0.1 and 0.3 mA.

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4. Results

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4.1. Lithostratigraphic members at investigated outcrops

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Lithologic characteristics of the studied sections are shown in the stratigraphic and

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sedimentary logs (Fig. 3). The contact of the Zhujiaqing Formation with the underlying

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Dengying Formation is unconformable. The top of the Dengying Formation at Xianfeng,

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Meishucun, and Mingyihe show dissolution cavities with corroded bedrock surface (Fig. 4A),

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and paleorelief ranging from 2cm (e.g., at Mingyihe) to 50cm (e.g., at Meishucun) are present.

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The strata above show wedging and onlap especially at Meishucun. These may indicate

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subaerial exposure and surface-developed paleokarstification. The cavities are filled with

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phosphorus conglomerate and sandstone. The Daibu Member is preserved in Laolin,

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Zhujiaqing and Lishuping sections. It is characterized by predominantly siliceous beds,

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argillaceous dolostone and shaly siltstone. The Daibu Member is in sharp and parallel contact

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with the underlying Dengying Formation and gradational contact with the overlying

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Zhongyicun Member, which is phosphorus-rich, and contains multiple beds of economic

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phosphorites. Sedimentary structures and lithologies of the Zhongyicun Member varied

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across the region. In the southern sections, e.g., near Meishucun, the Zhongyicun Member

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consists mainly of medium- to thick-bedded granular phosphorite (Fig. 4B) containing

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various grain types and stromatolites. A bedded-shale unit separates the phosphorite into the

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upper and lower ones, and zircons extracted from a bentonite layer in the shale have been

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dated multiple times (e.g., 538.2±1.5 Ma by Jenkins et al., 2002; 539.4 ± 2.5 Ma by

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Compston et al., 2008; 536.5 ± 2.5 Ma by Sawaki et al., 2008). At Laolin, however, this

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member is thicker, and mainly consists of thin-, medium- and thick-bedded phosphatic

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limestone/dolostone, silty dolostone, and phosphorites (Fig. 4C) with various laminated

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sedimentary structures. The Dahai Member is defined here by the appearance of

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whitish/brownish medium- to thick-bedded dolostone. Microbial laminations are abundant. A

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sharp facies change is shown across the contact of the lower and upper Dahai Member,

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especially at Lishuping, where up to 50 cm paleo-relief (Fig. 4D) and siltstone and

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conglomerate beds above are observed (Figs. 7D, 7E; section 4.2.11). The upper Dahai

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Member, which is absent at Meishucun and Mingyihe, is mainly characterized by rhythmic

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medium- and thick-bedded, slightly nodular dolostone and dolomitic limestone with

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interbedded calcareous shale. The contact of the Dahai Member with the overlying Shiyantou

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Formation is parallel (Fig. 4E). The Shiyantou Formation mainly consists of dark gray

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siltstone, which implies the cessation of the carbonate factory and the establishing of a

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siliciclastic shelf on the Yangtze Platform. At the studied sections, the basal Shiyantou

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Formation contains dm- to m-thick interbedded glauconitic sandstone, thin-bedded or

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concretionary phosphorite and argillaceous shale, all confirming low sedimentation rates.

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4.2. Facies analysis and environmental interpretation

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Twenty-five facies were recognized based on observations on outcrops, polished slabs

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and petrographic thin sections. Eleven facies associations were summarized representing

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different depositional environments (Figs. 5, 6, 7, 8 and 9). The main petrography,

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sedimentary structures and early diagenetic features of the described facies are listed in Table

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1.

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4.2.1. Semi-restricted subtidal facies association (F1)

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Facies association F1 only occurs in the Daibu Member. It consists of thin-bedded,

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argillaceous dolostone with interbedded shaly siltstone (F1a) and carbonaceous dolomitic

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chert (F1b) (Fig. 5A, Table 1). Horizontal, wavy and discontinuous laminations exist in the

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argillaceous dolostone, where laminated euhedral to subhedral crystalline dolomudstone and

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dolowackstone appears. Some wavy lamina-sets show symmetrical forms, and therefore were

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interpreted as wave-ripple laminations. Fine-grained phosclasts dominate the dolowackstone,

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and dark carbon-rich materials distributed around the clasts as well as in between dolomite

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crystals (Fig. 8A). The dolomudstone also contains abundant black carbon-rich materials. The

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dark grey F1b facies shows thin, laterally continuous planar and parallel-sided bedding. It

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frequently interbedded with F1a. No small shelly fossils or trace fossils have been found in

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this facies association.

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The depositional environment of this facies association is interpreted as semi-

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restricted subtidal, probably around the average storm-wave base. The presence of wavy

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laminae and wave-ripple laminations indicate that sedimentation was influenced by episodic

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storms (Li and Schieber, 2018). Thinly interbedded shaly siltstone suggest episodic terrestrial

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input, possibly relate to climatic driven siliciclastic influx (Tucker, 2003; Salad et al., 2015).

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The black carbon-rich material might originally be organic compounds. The richness of

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organic compounds reflects high primary productivity and minimal clastic dilution, possibly

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enhanced by seasonal stratification. The scarcity of bioturbation indicates limited infaunal

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activity, which is consistent with possible seasonal stratified conditions.

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4.2.2. Tempestite-dominated subtidal facies association (F2)

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Facies association F2 occurs in the lower part of the Zhongyicun Member at Laolin

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(Fig. 3). F2 is organized in repetitive thin beds of heterolithic mud- and packstone (F2a) and

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dm-thick beds of homogenous packstone (F2b) (Fig. 5B, Table 1). Between these beds, black

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siltstones of about 5-10 cm thick are common. Bioturbation is low so that the original

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lamination is largely preserved. However, internal sedimentary structures within laminae are

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not obvious. The packstone laminae are irregular, discontinuous and sharp based with gutter

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cast, micro-scours and black rip-up clasts at the bottom (Fig. 5C). The packstone in F2a is

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composed of very fine-sand sized grains including peloids (dolomite and phosphate in

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minerology), carbonaceous grains (possibly fecal pellets), phosoncolites and SSFs (Fig. 8B).

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Both the peloids and SSFs show abundant phosphate “envelopes” (Fig. 8C). Grains are

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cemented by fine-crystalline calcite. The mudstone in F2a is dark gray and dark brownish

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argillaceous dolostone. In F2b, the grain types and cements are similar as those in F2a.

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Locally, mm-thick mudstone bands displaying erosive upper surface present in F2b.

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Facies association F2 represents multiple tempestites deposited in a low energy

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subtidal background. Carbonate mud deposited out of suspension forming a muddy seafloor.

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The very fine-grained packstone and mudstone couplets can be existed in both tempestites

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and fine-grained turbidites (Stow and Bowen, 1980). However, the irregular laminae, various

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erosive features and rip-up clasts are more typical in tempestites (Flügel, 2004). Facies F2b

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has the same grain compositions as those in F2a, besides, the mudstone bands in F2b have

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erosive upper surface. These may indicate that F2b is resulted from multiple tempestites

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amalgamation. The dark gray color of the mudstone in F2a and the interbedded black

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siltstone may indicate a semi-restricted subtidal setting during storm-free periods or may also

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resulted from high bio-productivity.

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4.2.3. Energetic subtidal facies association (F3)

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Facies association F3 is present in the middle Zhongyicun Member of northern

276

Yunnan (Fig. 3, Table 1). It consists of three facies including massive silty dolostone

277

intercalated with calcareous siltstone (F3a), silty-dolomitic-intraclastic phosphorite (F3b) and

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rhythmic dolomitic phosphorite (F3c). The proportion of siltstone, dolostone and phosphate

279

varies among sections, but the phosphate content increases up-section. The lower part of F3

280

consists of thin- to medium-bedded F3a intercalated with F3b (5 to 10 cm thick, Fig. 5D).

281

The dark gray massive silty dolostone in F3a contains subhedral dolomite, quartz silt,

282

uniformly dispersed pyrite, and minor organic compounds and phosphate micro-concretions.

283

Conglomerate beds showing normal grading and rip-up clasts occur in F3a and were

284

succeeded by structure-less silty dolostone (Fig. 5E). Intercalated thin calcareous siltstone

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beds in F3a contain small lenticular phosclast packstone and black micro-concretions of iron-

286

oxidize. The F3b is wack- and packstone in texture. Abundant erosive features are observed

287

at the base of coarse-grained packstone in F3b. Sub-angular phosclasts displaying low degree

288

of corrosion and amalgamation. In the thin bed of F3b, the coarse-grained packstone

289

gradually changed upward to very fine-grained packstone and wackstone, which show wavy

290

laminations and wave ripples (Fig. 5F). The thin-bedded F3c is a 10m-thick unit in the

291

uppermost part of F3, its dark gray color is due to the high proportion of phosphate and

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organic matter (Fig. 5G). In situ phosphogenesis is represented by thin mudstone phosphate

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(or named microsphorite) in the dolomudstone layers (Fig. 8D). Phosphatic dolostone

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repeatedly alternated with phosclast packstone in F3c, where scours and gradation are

295

common. Some bioclast-rich packstone lenses occur in F3c. These lenses are mostly

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gradational, coarse grained and contain many erosive surfaces (Fig. 5H, Table 1). SSFs in

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these lenses are highly diversified and show various degrees of phosphatization (Fig. 8E),

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reflecting variations in their preservation stages during their pre-depositional history.

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299

Features described in F3 indicate significant changes in magnitude and frequency of

300

depositional events. In the lower portion of the F3, distinct sedimentary features are rare in

301

silty dolostone, which may indicate a high relative sea level during deposition. Conglomerate

302

beds in F3a represent storm-induced density flow, and the lenticular packstone in calcareous

303

siltstone may be caused by storm-induced currents. In F3b, coarse-grained packstone with

304

erosive base and gradual contact with rippled very fine-grained pakstone may represent

305

tempestite, of which the top is affected by waves. In F3c, the interlaminations of mudstone

306

phosphate with phosclast packstone indicate that in situ phosphogenesis occurred near the

307

sediment-water interface, and the phosphate could be easily eroded by storms or strong

308

currents and redeposited nearby. This would require a quick phosphogenesis and frequent

309

reworking. The bioclast-rich packstone lenses in F3c represent amalgamated proximal storm

310

events (Flügel, 2004) because these packstone lenses are relatively thick, coarse-grained, and

311

contain composite grain types and many erosive surfaces. These also indicate a stratigraphic

312

condensation (Decreasing of the sedimentation rate rather than accumulation rate; Gómez and

313

Fernández-López, 1994). Based on the above interpretations, the F3 thus represents energetic

314

subtidal environment characterized by storm-induced density flows and currents, alternating

315

deposition and erosion, early phosphogenesis and amalgamation of tempesites.

316

4.2.4. Shoal barrier and back-barrier facies association (F4)

317

Facies association F4 is present in the Zhongyicun Member at Xianfeng (Figs. 3, 5I).

318

It comprises three facies including conglomeratic dolostone (F4a), interbedded phosphorite

319

and dolostone (F4b), and oncolitic, bioclastic, phosclastic packstone (F4c). In F4a, the

320

dolostone is laminated mudstone (Table 1). Phosphate pebbles occur as thin and poorly sorted

321

conglomerate beds or lenses in the dolostone, and phosphatic SSFs, phosphate crusts and

322

phosphate concretions are observed (Fig. 5J). F4b consists of fine grained phosclast

323

grainstone and dolomudstone. Wavy and lenticular sedimentary structures and erosive

- 13 -

324

features are common 5K). Gypsum pseudomorphs occasionally occur in the dolomudstone

325

beds. F4c is packstone and rudstone in texture. Grains composed of sand-sized phosclasts,

326

highly diversified phosphatic SSFs, and sand- to pebble-sized oncoids. Phosphate crusts

327

occur on the irregular surface of the packstone laminae. In situ breccias locally occur in F4c

328

related to subaerial exposure. Besides, bedrock with small paleorelief, corroded surface and

329

bedrock clasts may indicate a small scale paleokarst (Fig. 5L).

330

The laminated dolomudstone beds in F4a indicate a low energy protected lagoon

331

environment. The phosphate crusts in F4a indicate that phosphogenesis happened in situ, and

332

phosphate pebbles in F4a are sourced from ambient indurated mudstone phosphate. The

333

combination of phosclast grainstone with dolomudstone in F4b indicates a moderate

334

energetic environment, possibly a sand shoal, interacted with low energy environment. The

335

existence of gypsum pseudomorphs in the dolomudstone of F4b indicates restriction of the

336

low energy water body. All these suggest that F4b formed a marginal sector of a sand shoal

337

that bordered a restricted lagoon. F4c are locally associated with exposure features. Thus, the

338

F4 represents a shoal barrier and back-barrier (protected) environment.

339

4.2.5. Shoal complex facies association (F5)

340

Facies association F5 is distributed at Meishucun and Mingyihe (Figs. 3, 4A, 6A). F5

341

consists of microbial, stromatolitic and granular phosphorites (F5a), and phosphatic dolostone

342

(F5b). Parallel and cross beddings are observed in thin-, medium- to thick-bedded F5a (Table

343

1). Scours are common. Phosphatized microbial mats and stromatolites are abundant in the

344

lower part (Figs. 6B, 6C, 8F). Some of them are reworked and deposited as thin and

345

discontinuous conglomerate. Granular phosphorites (Fig. 6D) in F5a show moderate sorting

346

and variable grain types (Table 1, Fig. 8G). Phosphatic SSFs always occur in shell-

347

concentrated beds, especially at Mingyihe (Fig. 3). Trace fossils were found from several

348

horizons in the F5a and Treptichnus pedum was founded from the upper phosphorite just

- 14 -

349

above the gray shale (F6) at Meishucun (Weber et al., 2007; Zhu et al., 2001). The cement of

350

phosphorites in F5a consists of dolospar, crystalline apatite or silica. A phosphate-quartz

351

pebble bed occurs in F5a containing phosphatic SSFs of Zone I and Zone II (Yang et al.,

352

2014, Fig. 3). F5b only occurs in the upper part of F5 at Meishucun (Fig. 6E). The F5b is

353

characterized by wavy bedding/laminations and numerous erosional surfaces and channels.

354

The dolostone is mud- and floatstone in texture and contains reworked phosphate grains and

355

phosphate crusts (Fig. 6F). In the upper part of F5b, occasional cm-scale desiccation cracks

356

are observed.

357

Facies association F5 suggests fluctuating energy caused by waves, currents and

358

minor water level changes. Parallel and cross-bedded, microbial, stromatolitic and granular

359

phosphorite (F5a) represents a shoal depositional environment that frequently affected by

360

waves and currents. The variable cement mineralogy is likely due to micro-geochemical

361

conditions in the soft sediment, in turn in part a function of local paleo-bathymetry, and the

362

complex diagenetic history of the phosphorite. The amalgamation of variable grain types in

363

the phosphorites and the co-occurrence of SSFs of two zones in a conglomerate bed of F5a all

364

indicate sediment reworking and hiatal concentration. Sedimentary structures in F5b suggest

365

a low energy subtidal to intertidal lagoon environment. Thus, we interpret the depositional

366

environment of F5 as a shoal complex with its upper part gradually shoaling.

367

4.2.6. Protected siliciclastic shallow subtidal facies association (F6)

368

A local distinct facies association exists at Meishucun (Fig. 3, Table 1). The rock

369

consists mainly of flaky laminated gray shale. The gray shale occasionally contains tabular

370

laminae that consist of detrital sand-sized glauconite, phosphate, pyrite and barite grains and

371

some SSFs (Fig. 6G). These tabular laminae with erosive bases, normal grading and without

372

evidence of wave reworking atop occur in the lower part of the gray shale succession, but

- 15 -

373

seldom occur in its upper part. Bioturbation is present in gray shale but the intensity is low

374

(Fig. 6G).

375

This facies association represents a protected siliciclastic shallow subtidal. The pale

376

gray color of the shale and the existence of bioturbation indicate an oxygenated sea floor.

377

Graded tabular laminae of detrital sandstone with erosive bases and without evidence of wave

378

reworking atop presumably represent occasional distal storm deposits.

379

4.2.7. Tidal sand flat facies association (F7)

380

Quartz-rich dolarenite locally occurs in the Zhongyicun Member at Mingyihe (Fig.

381

6H, Table 1). It shows bimodal cross bedding, wavy and flaser bedding. The dolarenite is

382

recrystallized to a coarse interlocking fabric. Well rounded, fine- to medium-grained quartz is

383

unevenly distributed.

384

The grain supported fabric of the quartz-rich dolarenite indicates a well-agitated

385

shallow-water environment. The bimodal cross bedding, wavy and flaser bedding are typical

386

sedimentary structures of tidal influence. Thus, a tidal sand flat depositional environment is

387

suggested.

388

4.2.8. Subtidal to intertidal mixed flat facies association (F8)

389

Interbedded or laminated, phospeloidal grainstone and dololutite are present in the

390

upper part of Zhongyicun Member at Mingyihe. Gray phospeloidal grainstone beds alternate

391

with off-white, laminated/bedded dololutite. Convolute bedding occurs in the lower part (Fig.

392

6I). Infaunal activity is indicated by horizontal trace fossils (Fig. 6J). The bases of the

393

phospeloidal grainstone layers are slightly erosive. The grains are densely packed and mainly

394

consist of very-fine-grained phospeloids, quartz and doloclasts (Table 1, Fig. 8H). Grains are

395

cemented by dolomite. In the upper part of F8, the amount of dololutite increases, and wavy

396

and lenticular laminations are dominant.

- 16 -

397

Features in this facies association point to a shallow subtidal to intertidal mixed flat

398

depositional environment. Tidal influence is indicated by the rhythmical alternation of

399

laminated, wavy and lenticular grainstone with dololutite. The millimeter- to centimeter-scale

400

wavy and lenticular sedimentary structures are interpreted as tidal bedding (Pratt and James,

401

1986; Demicco and Hardie, 1994), which formed due to declining tidal current energy and

402

the resulting change in sand to mud ratio. Convolute bedding reflects the indurated state of

403

the sediment at shallow subtidal. The horizontal traces indicate an oxygenated seafloor. They

404

may have formed during slack-water periods in small and shallow tidal pools.

405

4.2.9. Subtidal to intertidal phosphatic mudflat facies association (F9)

406

At Laolin, facies association F9 is represented by phosclast-rich dolostone (F9a, Fig.

407

6K) intercalated with burrowed dolostone (F9b) (Fig. 6L, Table 1). The medium to thick-

408

bedded F9a is wack- and packstone in texture, allochems include phosintra-clasts and

409

siliceous sponge spicules (Fig. 9A). The matrix consists of quartz silt and carbonate mud,

410

which was dolomitized at a later stage. Lenticular phosintra-clast packstone with erosive base

411

locally occur (Fig. 6M). Occasionally, desiccation cracks are present in F9a (Fig. 6N). Small-

412

scale oblique laminations are dominant in the phosclast wackstone, and skip marks are

413

commonly observed on the bedding plane (Fig. 6O). Phosphate occurs in the form of

414

intraclasts and micro-concretions (Fig. 9B). Phosphate concretions can coalesce laterally and

415

form dark phosphate crusts 1 to 3 cm thick (Fig. 6N). At Lishuping, the facies are also

416

characterized by F9a and F9b, only with more exposure features such as desiccation cracks

417

and in situ breccias. At Zhujiaqing section, a 45-cm-thick dark gray silty argillaceous

418

mudstone (F9c) occurs. It is thinly bedded and contains phosphate grains and disseminated

419

pyrites. It also occurs at the same stratigraphic interval as a 0.8-m-thick unit in the Nizheqing

420

section (about 1 km north of Zhujiaqing, Qian et al., 1996).

- 17 -

421

Features described above point to a subtidal to intertidal phosphatic mudflat facies

422

association. The common erosional features and oblique laminations indicate constant current

423

influence. Exposure is indicated by desiccation cracks and brecciated intervals. Phosphate

424

crusts indicate in-situ phosphogenesis. The frequent occurrence of primary phosphate and

425

phosintraclasts in F9 implies frequent reworking. The intensive burrows and hardgrounds in

426

F9b indicate a low net sedimentation. The low carbonate content, dark gray color, and

427

disseminated pyrite in F9c point to a setting with significant terrestrial influx in a low energy

428

possibly dysoxic subtidal environment.

429

4.2.10. Intertidal to supratidal mudflat facies association (F10)

430

Facies association F10 occurs in the lower part of the Dahai Member at all

431

investigated sections. It corresponds to whitish/brownish, tabular-planar, laminated dolostone

432

(10a) and doloclast rudstone (F10b). Many of the thin laminae in the dolostone are probably

433

indicative of bacterial mats and hemispherical microbial laminae formed by cyanobacteria

434

(Fig.7A). Fenestral fabrics are observed. Some microbial laminae are distorted and cracked

435

due to shrinkage. Some microbial laminae are preferentially silicified forming laminated

436

chert bands. Sparse phosphate and quartz sand grains occur within dolomite beds. Commonly,

437

up to cm-thick doloclast rudstone interbedded with laminated dolostone are observed (Figs.

438

7B, 9C, Table 1). Thin microsphorite layers, which are associated with phosclast wackstone

439

occasionally occur in F10a (Fig. 7C). SSFs are rare in this facies association.

440

This facies association represents deposition on a shallow intertidal to supratidal

441

mudflat. Sediments were periodically flooded during storms or high tides to maintain growth

442

of microbial mats and transport carbonate mud. This is indicated by the remnants of

443

microbial lamination, fenestral fabrics and cracks. Dolomitization via early diagenetic

444

replacement of micrite is commonly found associated with tidal-flat environment (Ramail,

445

2008; Baldermann et al., 2015). The up to cm-thick doloclast rudstone represents storm event

- 18 -

446

of reworked intra-clasts deposited on tidal flat. Sparse quartz and phosphate sand grains were

447

possibly blown onto the tidal-flat by wind. Early diagenetic chert has been repeatedly

448

observed in ancient supratidal sediments (Pratt, 2010) although not common on modern tidal

449

flats.

450

4.2.11. Protected shallow subtidal facies association (F11)

451

The F11 occurs in the upper Dahai Member in Xianfeng, Laolin, Zhujiaqing and

452

Lishuping and consists of two main facies and three local facies (Fig. 3, Table 1). The three

453

local facies occur at the base of this facies association, which is named here as calcareous

454

siltstone and conglomerate (F11a) at Lishuping, medium-bedded bioclastic packstone (F11b)

455

at Laolin, and conglomeratic, phosphatic dolostone (F11c) at Xianfeng. The two main facies

456

include argillaceous, fine- to medium-crystalline dolostone intercalated with calcareous shale

457

(FA11d), and argillaceous, microcrystalline limestone intercalated with calcareous shale

458

(FA11e). F11a is 15-20 cm in thickness. The graded conglomerate bed is underlain and

459

overlain by calcareous siltstones with foliations (Fig. 7D). It consists of well-rounded,

460

medium-sorted, and gradational quartz and phosphate pebbles (Fig. 7E), and carbonate mud

461

and silt matrix. F11b is 40 cm thick at Laolin (Fig. 7F). It is packstone in texture (Figs. 9D

462

and 9E) and the top of F11b contains sandstone lenses and abundant burrows (Fig. 7F). F11c

463

is a 0.5 m thick, thin-bedded unit above the whitish dolostone of F10 at Xianfeng (Fig. 7G).

464

Laminated and partly bioturbated dolomitic limestone contains reworked phosphate grains,

465

hardground clasts, phosphatic SSFs and phosphate crusts (Fig. 7H). Both F11a and F11b are

466

overlain by thick calcareous shale that contains carbonate nodules (Figs. 3, 7D, 7F). This unit

467

is the basal part of F11d, and the rest of F11d is dominant by gray thin- to thick-bedded

468

crystalline dolostone. Intercalations of calcareous shale are a few cm thick. Parallel to wavy

469

and slightly nodular bedding planes are present (Fig. 7I). Black cm-thick microsphorite layers

470

occasionally occur with phosphate rip-up clasts in the dolostone, but their lateral continuity

- 19 -

471

does not exceed a few centimeters (Fig. 7J). F11d also contains minor proportions of

472

randomly distributed phosphatic SSFs and some are marginally burrowed (Fig. 9F).

473

Cosmopolitan molluscs Watsonella crosbyi occurs in F11d. Many phosphatic SSFs and

474

phosintraclasts are partly replaced by dolomite crystals (Fig. 9G). Bioturbation is minor (0-

475

30%). Laminations and dispersed pyrite crystals are observed at some beds. F11e retains the

476

identical sedimentary structures, grain components and fossil content as in F11d. The

477

limestone in F11e consists of micro-spars (Fig. 9H). Thin layers of dolomitic limestone

478

commonly occur, especially where lamination is dominant. F11e consists of less calcareous

479

shale intercalations than F11d. However, at Laolin, the top 0.7m of F11e is calcareous shale

480

that contains carbonate nodules and phosphate pebbles (Figs. 7K, 7L).

481

Facies F11a represents a protected subtidal environment dominated by detritus input

482

and temporal low carbonate productivity. Composition of the pebbles is different from the

483

surrounding rocks, indicating a distal origin of these pebbles. Thus, the conglomerate may

484

represent an event deposit in the protected subtidal zone. The packstone texture and the

485

common SSFs in F11b point to a moderate energy subtidal setting. Bioturbations in the

486

sandstone lenses in F11b indicate an oxygenated seafloor. In F11c, the abundant phosphatic

487

hardground clasts and phosphate crusts indicate a low sedimentation rate and frequent

488

reworking. The laminated and only slightly bioturbated mudstones represent a protected

489

subtidal background. The dominant mudstone texture in F11d and F11e indicates a low

490

energy protected environment. The SSFs are phosphatized both on the shells and in the inner

491

cavity fills. When the SSFs were exposed to the pore water horizon under anoxic, ferruginous

492

redox regime with the alignment of high phosphorus saturation in the pore water, replacement

493

of original shell material by apatite occurred. Phosphatization replaced the shell and the shell

494

cavity fillings and finally forming phosphatic steinkerns (Creveling et al., 2014; Dattilo et al.,

495

2019). Burrows occurred on the phosphatic shell substrate when the fossils are not buried

- 20 -

496

deeply (Fig. 9F). If the phosphatic SSFs were neither winnowed nor buried out of the

497

phosphogenesis window, phosphatization continued, and formed laterally continuous

498

phosphate crusts. Subsequent reworking produced phosphate pebbles (Fig. 7L). A similar

499

history of early diagenesis, reworking and burial processes is discussed by Brasier et al.

500

(1979) for the “shelly limestone facies” in the lower Cambrian Home Farm Member of

501

central England. Considering the dominant mudstone texture and the appearance of

502

cosmopolitan molluscus such as the Watsonella crosbyi (Brasier et al., 1996; Steiner and

503

Ergaliev, 2011; Kouchinsky et al., 2017), the depositional environment of F11d and F11e is

504

interpreted to be a protected shallow subtidal setting with minor episodic terrestrial influx.

505

However, redox conditions might have been fluctuated, which is indicated by the variable

506

intensity of bioturbation, lamination and pyrite formation in the different parts of the rocks.

507

5. Discussion

508

5.1 Depositional framework of the Zhujiaqing Formation

509

Deposition of the Zhujiaqing Formation occurred in a shallow semi-restricted marine

510

basin (Kangdian basin) within a larger epeiric basin (Li, 1986; Chen et al., 1987). The

511

Kangdian basin was connected to the open ocean via seaways from the southeast and possibly

512

south (Xu et al., 1984; Ge et al., 1983; Chen et al., 1987). Characterization of the twenty-five

513

facies above (Table 1) has permitted to construct a facies model for the lowermost Cambrian

514

chert-phosphate-carbonate sequence in northeastern Yunnan. Each facies association

515

developed in a specific paleogeographic setting, representing environments from tidal flats,

516

shoal complex, shoal barrier and back-barrier to intra-platform basin (Fig. 10A). The tidal

517

flats are represented by sand tidal flats (F7), mixed tidal flats (F8) and mud tidal flats (F9,

518

F10). Shallow lagoon (back-barrier) deposits always occur as thin interlayers in the shoal

519

complex or within back-barrier such as facies F5b, F4a and dolomudstone layers in F4b. The

520

shoal deposits (F5a, F4c) in the shoal complex and shoal barrier display intensive reworking

- 21 -

521

and a high density of hiatus implicated by composite grain types and SSFs, frequent erosive

522

surfaces, and sediment amalgamation and concentration (Sections 4.2.4 and 4.2.5). The intra-

523

platform basin consists of facies associations F1, F2, and F3 representing a shallowing

524

upward subtidal with increased stratigraphic condensation (Section 4.2.3). The upper

525

Zhujiaqing Formation displays another facies distribution pattern as shown in figure 10B.

526

Five facies described in Section 4.2.11 suggest an extended protected shallow subtidal (F11)

527

that mainly consists of carbonate mudstone with diversified phosphatic SSFs and

528

phosintraclasts.

529

5.2. Platform evolution of the Zhujiaqing Formation

530

A spatial and temporal arrangement of the facies among the studied sections is shown

531

in Fig. 11 based on the above facies analysis and supported by a detailed biostratigraphic

532

framework (Yang et al., 2014). The Daibu Member characterized by semi-restricted subtidal

533

(F1) that mostly deposited around storm wave base only present in the northern sections

534

(Laolin, Zhujiaqing, and Lishuping) of the studied area. It was originally considered that its

535

time equivalent strata exist in the Meishucun section as tidal flat deposits, which was named

536

Xiaowaitoushan Member (the top 10.8m of the Dengying Formation; Luo et al., 1984).

537

However, later investigation at Wangjiawan (Fig. 1C) and the discovery of disconformity and

538

possible paleokarstification at the top of the Xiaowaitoushan Member support a preference

539

that the Xiaowaitoushan Member is below the Daibu Member (He., 1989; Qian et al., 1996).

540

Even though the surface-related paleokarstification is observed in our study, it is still possible

541

that during the initial deposition of the Daibu Member, shallow water tidal flat

542

(Xiaowaitoushan Member) deposited at the subaqueous uplift of the Meishucun area.

543

Subsequently, because of the continued uplift, sedimentation was interrupted at Meishucun

544

area, but meanwhile the Daibu Member remained its deposition in the northern area (Zhang

545

et al., 1997; Fig. 11).

- 22 -

546

Lateral heterogeneity of facies associations characterized the Zhongyicun Member,

547

while the Dahai Member is characterized by laterally continuous distribution of facies

548

associations. Depositional environments in the Zhongyicun member range from tidal flats,

549

shoal complex, shoal barrier and back-barrier at Meishucun, Mingyihe and Xianfeng to

550

energetic subtidal setting with frequent reworking above storm wave base at Laolin,

551

Zhujiaqing and Lishuping. As the sedimentation continues, intertidal to supratidal dolostone

552

(F10) finally deposited (Fig. 11). At Laolin, F10 occurs in the upper subzone of the SSFs

553

Zone II (Yang et al., 2014), however, because of the rarity of SSFs in F10 at Xianfeng and

554

Meishucun, it is unclear whether the F10 is deposited earlier or coevally compared to the F10

555

in Laolin. Thus, it is uncertain whether F10 formed through progradation, which is common

556

on Holocene tidal flats, or by aggradation, which is rare in Holocene examples but had been

557

inferred from Cambrian strata (Koerschner and Read, 1989). Anyhow, these indicate that

558

strata correlation in Ishikawa et al. (2008) between the Dahai Member at Meishucun (F10)

559

with the strata at upper Dahai Member in Laolin (F11) (L4, Fig.11) is incorrect. Besides, the

560

upper Dahai Member is dominated by a protected shallow subtidal carbonate mudstone

561

containing phosphatic SSFs from Zone III. This Zone is also preserved at Xiaotan further

562

north and Deze further east (Li et al., 2004; Steiner et al., 2007; Fig. 1C), but is missing at

563

Meishucun and Mingyihe.

564

In comparison with other coeval early Cambrian carbonate successions such as those

565

in Morocco and Siberia (Maloof et al., 2010; Kouchinsky et al., 2017), The Zhujiaqing

566

Formation is thinner even in the thickest Laolin section at the studied area. However, this is

567

mainly due to subaqueous reworking and stratigraphic condensation rather than subaerial

568

erosion. Facies analysis at Laolin manifests continuous subaqueous deposition and gradual

569

facies change from F1 to F10 (Fig. 11). Even across the so called “local unconformity”

570

(Landing and Kouchinsky, 2016; Yang et al., 2014; The top of F10 in this paper), only facies

- 23 -

571

change from tidal mudflat (F10) to moderate energy subtidal (F11b) occurs. Fossils above

572

and below this boundary belong to the upper Subzone of Zone II (Fig. 11), which indicates

573

that there is little time missing across this contact. Stratigraphic condensation at Laolin thus is

574

reflected by frequent reworking and re-sedimentation, especially in F3 and F9 (Sections 4.2.3

575

and 4.2.9), which may due to a limited accommodation space at the intra-platform basin.

576

A positive δ13Ccarb excursion occurs at Laolin (L4 in Fig. 11) above the FAD of

577

Watsonella crosbyi in the upper Dahai Member (F11). Preliminary in Siberia, the

578

stratigraphic interval between the peak of a positive δ13Ccarb excursion I´ and the FAD of

579

Watsonella crosbyi was defined as the “Fortunian-Cambrian Stage 2 transitional beds” in the

580

Anabar Uplift of Siberia Platform (Kouchinsky et al., 2017). These beds can be correlated

581

with the stratigraphic interval between the L4 excursion and the FAD of Watsonella crosbyi

582

at Laolin. The L4 peak at Laolin has been proposed as index for the vacant Cambrian Stage 2

583

(Landing et al., 2013), but must be seen with scrutiny because of the protected and partly

584

restricted depositional environment described above (Section 4.2.11).

585

5.3. Phosphogenesis and phosphorite deposition in the Kangdian Basin

586

An important phenomenon emerging from facies reconstruction is the distribution of

587

primary phosphate in the Zhujiaqing Formation. In-situ phosphogenesis is observed in

588

shallow subtidal (F3c and F11), shallow lagoons (F5b, F4a and F9) and tidal mudflat (F10) as

589

phosphate crusts, and in shoals where it occurs as cements, phosphatic stromatolites and

590

microbial mats, and phosphate-coatings (F5a, F4c). In-situ phosphogenesis also occurs as

591

phosphate envelopes on shell substrates, which were subsequently transported as allochems

592

(e.g., in F2). Phosphatic “steinkerns” (Dattilo et al., 2019) in F11 also embody localized in-

593

situ phosphogenesis immediately following the deposition of shells. The ubiquitous

594

occurrence of phosphogenesis in various shallow-water environments is in sharp contrast to

595

the assumption that primary phosphogenesis only occurred in unique isolated embayments

- 24 -

596

along a shoreline (Sato et al., 2014) It differs from the modern and Cenozoic phosphogenic

597

environment in much deeper shelf settings where coastal upwelling took place (e.g., Garrison

598

and Kastner, 1990). Indeed, phosphogenesis and phosphorite deposition in shallow marine to

599

near coast environments characterize many early Cambrian phosphorite deposits worldwide.

600

These places include the Indian Himalaya (Tal Fm.; Mazumdar and Banerjee, 2001),

601

Australia (Georgina Basin, Gowers Fm.; Southgate, 1986, 1988), and Kazakhstan (Chulaktau

602

Fm.; Heubeck et al., 2013).

603

The widespread phosphogenesis in shallow water deposits in the Zhujiaqing

604

Formation indicates sufficient phosphorus supply and ideal geochemical condition for

605

phosphogenesis. Petrographic analysis in F3 shows that high proportion of organic matter and

606

dispersed pyrite accumulated in the carbonates. Because of the energetic conditions described

607

in most parts of F3, a restricted and stratified basin model is not likely. We assume that a high

608

bioproductivity produced abundant organic matters on the sea floor, and decomposing of

609

these organic matter could consume a lot of oxygen at the sea bottom and in the pore space.

610

Organic-bound phosphorus could have been liberated by bacterial recycling. Conditions in

611

the shoals, back-shoal and tidal flats appear to have been different because these settings have

612

low potential to accumulate large amounts of organic matter on the seafloor, but on the other

613

hand, stromatolites and microbial mats were widespread in these areas, and can possibly

614

create micro-environments for the storage, release and concentration of phosphate to promote

615

in situ precipitation (Caird et al., 2017). Additional input from other P sources, such as

616

continental weathering (Sato et al., 2014), cannot be excluded but requires evidences about

617

the source rock and plausible means of transportation. It should also be noted that phosphorus

618

adsorbed on Fe-Mn oxides may act as a major but overlooked source of P in many Ediacaran-

619

Cambrian phosphorites (Creveling et al., 2013).

- 25 -

620

Granular phosphorites are widely distributed in the Zhongyicun Member at the

621

investigated sections. However, long-distance transportation is not supported by this study.

622

Concentration of economic phosphorites mainly took place in shoals by winnowing and

623

multiple reworking of primary phosphate and phosphatized skeletons. In the intra-platform

624

basin, concentration took place by repeated alternations of syndepositional phosphogenesis,

625

frequent reworking, and amalgamation of storm-generated phosphatic event beds. Such

626

concentration mechanisms are comparable to those documented from the Permian Phosphoria

627

Formation of a relatively autochthonous origin for the phosphate grains with, at most, local

628

winnowing and reworking (Hiatt and Budd, 2001).

629

6. Conclusions

630

The Zhujiaqing Formation of the Cambrian Yangtze Platform (South China) was

631

deposited in a shallow semi-restricted marine basin within a larger epeiric basin. Even though

632

located at the shallow shelf, our detailed analysis of 25 facies shows that the paleogeography

633

was initially characterized by an uneven seafloor with heterolithic facies, and then developed

634

into a relatively flat seafloor.

635

The succession begins in the northern part of the study area with semi-restricted

636

subtidal facies association, which is lacking in the southern region. Subsequently, silty

637

dolostone and dolomitic phosphorite of an energetic subtidal environment that locally

638

includes tempestites follow in the northern region, while in the southern region, granular

639

phosphorites and muddy dolostones of shoals, back-shoal and tidal flats developed. Further

640

up section, depositional environments become peritidal mudflat. Finally, carbonate mudstone

641

and calcareous siltstone of a protected shallow subtidal environment follow.

642

Evidence presented from the Zhujiaqing Formation indicates that strata in the laolin

643

section are more continuous than those on other parts of the Yangtze Platform. However,

- 26 -

644

stratigraphic condensation and the protected and partly restricted environment makes the use

645

of the isotopic markers such as L4 peak for the definition of Cambrian Stage 2 problematic.

646

In situ phosphogenesis is widespread in various environments in the studied region,

647

such as lagoon, tidal flat and shoal complex. This argues against the previous assumption that

648

primary phosphate formed only in unique isolated embayments. It is also different from the

649

classical coastal upwelling model. We propose that a high bioproductivity and bacterial

650

recycling on the southwestern Yangtze Platform provided the main source of phosphorus.

651

The spatial distribution of granular phosphorites are confined, no long-distance

652

transportation is evidenced by this study, however, local multiple winnowing and reworking

653

at shoal and its surrounding area not only lead to the concentration of granular phosphorite

654

but also to the accumulation of numerous hiati. Whereas in intra-platform basin settings,

655

concentration of phosphorites is mainly by repeated alternations of syndepositional

656

phosphogenesis, current and wave reworking, and amalgamation of storm-generated

657

phosphatic event beds.

658

Acknowledgements

659

This work was funded by the German Research Foundation (DFG Research Group

660

FOR 736 “The Precambrian-Cambrian Biosphere Revolution: Insights from Chinese

661

Microcontinents”; subproject He2418/4-2) to C. Heubeck, and the National Natural Science

662

Foundation of China Grant (grant number 41672029) to M.Y. Zhu (Nanjing Institute of

663

Geology and Palaeonotology, CAS). We are grateful to Dr. F.C. Zhao (Nanjing Institute of

664

Geology and Palaeonotology, CAS) and S.S. Zhang for field work support. We thank K.

665

Born (Museum für Naturkunde Berlin), A. Giribaldi (FU Berlin) and J. Evers (FU Berlin) for

666

technical assistance. We thank the Members of FOR736 for helpful scientific discussions,

667

especially Dr. B. Weber (FU Berlin) for providing us trace fossils feedback and Prof. J.M.

- 27 -

668

Zhang (CAS) for stimulating discussions. We are grateful to Dr. Huan Cui and an anonymous

669

reviewer for their constructive reviews.

670

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Xue, Y.S., Tang, T.F., Yu, C.L., 1992. Paleokarst Cave phosphorites of the Upper Sinian Dengying Formation in Southern China. Acta Sedimentologica Sinica 10, 145-153. Xu, S.R., Wang, C.W., Wang, E.Y., 1984. Some problems on the geology of lower cambrian phosphorite deposits in Yunnan. Geological Review 30, 477-488.

923

Yang, B., Steiner, M., Li, G.X., Keupp, H., 2014. Terreneuvian small shelly faunas of east

924

Yunnan (South China) and their biostratigraphic implications. Palaeogeography,

925

Palaeoclimatology, Palaeoecology 398, 28-58.

926

Yang, B., Steiner, M., Zhu, M. Y., Li, G.X., Liu, J.N., Liu, P.J., 2016. Transitional Ediacaran

927

– Cambrian small skeletal fossil assemblages from South China and Kazakhstan:

928

Implications for chronostratigraphy and metazoan evolution. Precambrian Research

929

285, 202-215.

930

Yang, C., Li, X.H., Zhu, M.Y., Condon, D.J., 2017. SIMS U-Pb zircon geochronological

931

constraints on upper Ediacaran stratigraphic correlations, South China. Geological

932

Magazine 154, 1202-1216.

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933 934

Zeng, Y.F., Yang, W.D., 1987. Mechanism of enrichment of Kunyang and Haikou phosphorite deposits, Yunnan China. Acta Sedimentologica Sinica 5, 19-28.

935

Zhang, J.M., Li, G.X. & Zhou, C.M. 1997. Deposits of the volcanic eruption event from the

936

basal Lower Cambrian phosphatic sequence in eastern Yunnan and their significance.

937

Journal of stratigraphy, 21, 91-100. (In Chinese with English Abstr.)

938

Zhou, C.M., Zhang, J.M., Li, G.X., Yu, Z.Z., 1997. Carbon and oxygen isotopic record of the

939

early Cambrian from the Xiaotan Section, Yunnan, South China. Scientia Geologica

940

Sinica 32, 201-211.

941 942

Zhu, M., Li, G., Zhang, J., 2001. Early Cambrian stratigraphy of east Yunnan, southwestern china: a synthesis. Acta Palaeontologica Sinica 40, 4-39.

943

Zhu, M., Zhang, J., Steiner, M., Yang, A., Li, G., Erdtmann, B., 2003. Sinian- Cambrian

944

stratigraphic framework for shallow to deep-water environment of the Yangtze

945

Platform: an integrated approach. Progress in Natural Science 13(12), 951-960.

946

Zhu, M.Y., Babcock, L.E., Peng, S.C., 2006. Advances in Cambrian stratigraphy and

947

paleontology: Integrating correlation techniques, paleobiology, taphonomy and

948

paleoenvironmental reconstruction. Palaeoworld 15, 217-222.

949 950

Zhu, M., Zhang, J., Yang, A., 2007, Integrated Ediacaran (Sinian) chronostratigraphy of South China. Palaeogeography, Palaeoclimatology, Palaeoecology 254, 7-61.

951

Figure captions

952

Fig. 1. A) Generalized paleogeography of the Yangtze microcontinent during the early -

953

middle Meishucunian. (Refer to Fig. 2 for a detailed geochronologic sequence). B)

954

Generalized sedimentary facies and palaeogeography of the Yangtze micro-continent during

955

the deposition of the Zhongyicun Member of the Zhujianjing Formation. Modified after Li

956

(1986) and Chen et al. (1985, 1987). C) The extent of Zhongyicun Member in eastern

957

Yunnan, and the locations of the sites mentioned in the text. Modified after Luo et al. (1982).

- 39 -

958

Fig. 2. Generalized stratigraphic columns of the lower Cambrian succession in eastern

959

Yunnan

960

Fig. 3. Stratigraphic and sedimentological logs of measured sections. SSFs assemblage zones

961

of Meishucun, Xianfeng, Laolin and Lishuping are after Yang et al., 2014; SSFs assemblage

962

zones of Zhujiaqing are after Qian et al., 1996.

963

Fig. 4. Lithostratigraphic members and important bedding contact at selected sections. White

964

triangles point to stratigraphic-up. A) Surface (subaerial) karst at the top of the Dengying

965

Formation: Black phosclast conglomerate of the overlying Zhujiaqing Formation fills the

966

paleokarst cavity. The scale bar is 3 cm in total. B) Gray medium- to thick-bedded

967

phosphorite (between arrows) in the Zhongyicun Member at Meishucun Phosphorite Mine. C)

968

Thin- to thick-bedded phosphatic dolostone, silty dolostone, and phosphorite in the

969

Zhongyicun Member (below the dashed line) overlain by the dolostone in the Dahai Member

970

at Laolin. D) Erosional contact of the lower and upper Dahai Member (dashed line) at

971

Lishuping. E) Parallel contact (dashed line) of Shiyantou Formation silty shale to underlying

972

Dahai Member nodular limestone at Laolin.

973

Fig. 5. Outcrop and hand sample images from facies associations F1 to F4. A) Semi-

974

restricted subtidal (F1) represented by alternating argillaceous dolostone with interbedded

975

siltstone (F1a) and carbonaceous dolomitic chert (F1b). Note the laterally continuous, planar

976

and parallel-sided bedding contact. B) Very thin- to thick-bedded, tempestite-dominated

977

subtidal (F2). The yellow star indicates the location of the sample shown in Fig. 5C. C)

978

Heterolithic mud- and packstone (F2a). Note the irregular laminae and gutter cast (e.g., black

979

arrow). Sample lal43 from Laolin. D) Thin- to medium-bedded dolostone and phosphorite of

980

energetic subtidal (F3). E) Grading density flow succeeded by dark gray silty dolostone

981

(arrow) in F3a, sample lal38 from Laolin. F) Silty-dolomitic-intraclastic phosphorite (F3b),

- 40 -

982

note the erosive surface (e.g., black arrow) at the bedding contact, and wavy laminations (e.g.,

983

white arrow) in the very-fine-grained phosclast packstone. Sample lal35 from Laolin. G)

984

Thin-bedded dark gray rhythmical dolomitic phosphorite (F3c, below the dashed line). The

985

yellow star indicates the location of the sample shown in Fig. 5H. H) Bioclast-rich packstone

986

in F3c. The diversified phosphatic SSFs occur together with black angular phosintraclasts

987

(e.g., black arrow). Sample lal31 from Laolin. I) Shoal barrier and back-barrier (F4) at

988

Xianfeng section. J) Phosphate flat-pebbles (upper left) and phosphate concretions (lower

989

part) in the conglomeratic dolostone (F4a). Sample xif1/4 from Xianfeng. K) Interbedded

990

phosphorite and dolostone (F4b). The whitish laminae represent dolomudstone, and the black

991

laminae represent phosclast grainstone. Sample xif1/7 from Xianfeng. L) Micro-karstification

992

(upper part) in the oncolitic, bioclastic, phosclastic packstone (F4c). Note the irregular

993

surface of the bedrock and the dolostone gravel (black arrows) as a result of dissolution. The

994

phosphate crusts occur on the irregular surface of packstone (e.g., white arrows). Sample

995

xif1/12 from Xianfeng.

996

Fig. 6. Outcrop and hand sample images of facies associations F5 to F9. A) Subaerial

997

unconformity (dashed line) between yellowish peritidal dolostone of the Dengying Formation

998

and the overlying SSFs-bearing grey phosphorite of the Zhujiaqing Formation at Meishucun

999

section. B) Winkled phosphatic bio-mat surface (also called “elephant skin” structure, e.g.,

1000

arrows) from the Mingyihe section. C) Phosphatic stromatolites growing on top of the

1001

dolostone. D) Granular phosphorite (F5a) at Meishucun. E) Phosphatic dolostone (F5b) at

1002

Meishucun (between arrows). F) Reworked phosphate grains and phosphate crust in the

1003

dolostone of F5b (e.g., arrows). G) Gray shale at Meishucun, tabular beds of detritus

1004

sandstone locally occur. Note the bioturbation (e.g., white arrows) at the base of the

1005

sandstone beds. H) Quartz-rich dolarenite at Mingyihe. I) Interbedded or laminated

1006

phospeloidal grainstone (grey) and dololutite (offwhite). Note the convolute bedding in the - 41 -

1007

center (arrow). J) Horizontal traces in F8 at Mingyihe. K) Medium- to thick-bedded

1008

phosclast-rich dolostone (F9a), bedding planes are parallel to erosive (e,g., arrow). L) Upper

1009

image: polished hand-sample of burrowed dolostone (F9b). Note several generations of

1010

hardgrounds, burrows and lithoclasts. Sample 11-259 from Laolin. Lower image: Line

1011

drawing of the hand-sample above. M) Close-up view of F9a. Note the several erosive

1012

surfaces (e.g., arrows). N) Desiccation cracks in F9a (e.g., arrows). O) Skip marks (e.g.,

1013

white arrows) on top of the bedding plane in F9a.

1014

Fig. 7. Outcrop and hand sample images of facies associations F10 and F11. A). Lateral

1015

linked stromatolitic hemispheroids in thick- to medium-bedded laminated dolostone in F10a.

1016

B) Intra-formational doloclast rudstone (between arrows) in F10b. Sample xif1/17 from

1017

Xianfeng. C) Bundles of alternating dolomite and phosphorite laminae overlain by phos-

1018

intraclast rudstone. D) Thin-bedded calcareous siltstone and conglomerate (F11a; between

1019

arrows). E) Conglomerate bed in F11a showing normal grading and pebbles composed of

1020

well-rounded quartz (white) and phosphate (gray). F) Medium-bedded bioclastic packstone

1021

(F11b; between the dashed lines). Note the vertical burrows in sandstone lenses (lower right

1022

inset; Sample 11-251 from Laolin). G) Thinly bedded dolostone of the upper Dahai Member

1023

(above the dashed line) at Xianfeng. H) Conglomeratic, phosphatic dolostone (F11c). Note

1024

the hardground clasts (e.g., white arrows) and phosphate crusts (blue arrows) indicating early

1025

phosphogenesis and frequent reworking. Sample xif1/19 from Xianfeng. I) Argillaceous, fine

1026

to medium crystalline dolostone intercalated with calcareous shale (F11d) showing parallel to

1027

wavy and slightly nodule bedding plane at Zhujiaqing. J) Microsphorite layers (e.g., arrows)

1028

occur together with phosintraclasts and phosphatic SSFs in F11d. Sample 11-246 from Laolin.

1029

K) Nodule limestone at the upper part of F11e at Laolin. L) Black phosphate pebbles in F11e.

1030

Sample 11-230 from Laolin.

- 42 -

1031

Fig. 8. Thin-section photomicrographs of representative facies from F1 to F8. A)

1032

Argillaceous dolostone in F1a. The vertical increase of dolomite crystal size may indicate an

1033

original texture change from mudstone to wackstone. Note the fine-grained phosclasts in the

1034

wackstone laminae. The opaque rims (arrows) around phosclasts consist of carbon-rich

1035

material (sample 11-275 from Laolin). B) Packstone and mudstone laminations in F2a. Thin

1036

section was stained with Alizarin red S, differentiating between dolomite (unstained) and

1037

calcite (stained red). Framework grains include carbon-rich opaque clasts (e.g., a), phos-

1038

peloid (e.g., b), dolo-peloid (e.g., c) and SSFs with phosphate envelope (e.g., orange arrow)

1039

(sample lal42 from Laolin). C) “Phosphate envelope” (arrow) of SSFs in F2b. (sample lal44

1040

from Laolin). D) In situ phosphogenesis (arrows) in the rhythmical dolomitic phosphorite

1041

(F3c) (sample lal33 from Laolin). E) Bioclast-rich packstone in F3c. SSFs show various

1042

degrees of phosphatization (e.g., blue arrows). The doloclasts are marginally micro-bored

1043

(e.g., yellow arrows). The rock is cemented by dolospar (sample lal262 from Laolin). F)

1044

Phosphatic stromatolites in F5. Well-rounded quartz grains fill interstices (sample 11-715

1045

from Meishucun) G) Phosphatic ooids and phos-peloids in phosclast grainstone of F5. Note

1046

the isopachous apatite cement (e.g., white arrow) around phos-peloids in the phosphatic

1047

compound ooid. All the grains are cemented by two generations of chalcedony, the first

1048

generation is isopachous (e.g., blue arrow) and the second generation is the brownish pore-

1049

filling (sample Mei3 from Meishucun). H) Laminated phospeloidal grainstone (2) and

1050

dololutite (1) in F8. The grainstone consists of very fine-grained quartz (a), phos-peloids (b)

1051

and minor doloclasts (c). Note the slightly erosive contact at the base of the sandstone lamina.

1052

(sample An 10 from Mingyihe).

1053

Fig. 9. Thin-section photomicrographs of representative facies from F9 to F11. A) Phosclast

1054

wackstone from F9a. The matrix is dolomitized and phosphatized. Sponge spicules

1055

commonly distributed (e.g., arrows). Rectangle at upper right indicates the area shown in Fig. - 43 -

1056

9B. (sample 11-259 from Laolin). B) X-ray element maps of Ca, Mg, P and Si in the matrix

1057

of F9a showing phosphate concretions grow around siliceous nucleus (e.g., arrow). C) Intra-

1058

formational doloclast rudstone in F10 (sample 11-255 from Laolin). D-E) Fully recrystallized

1059

bioclastic packstone under plane polarized light (D) and cathodoluminescence (E). The black

1060

grains in E are the relicts of phosphatic SSFs (sample 11-252 from Laolin). F) Burrowed

1061

phosphatized SSFs in F11d (sample 11-244 from Laolin). G) Phosphatized SSFs in F11 were

1062

partly affected by dolomitization during burial (e.g., black arrow). The phosclast in the lower

1063

part is mudstone phosphate. Abundant sponge spicules exist in the phosclast (e.g., white

1064

arrow) (sample 11-244 from Laolin). H) Partly dolomitized sparitic limestone, a common

1065

microfacies in F11. Calcite spars are stained red while dolomite remains colorless (sample

1066

11-230 from Laolin).

1067

Fig. 10. Facies model of the Zhujiaqing Formation showing principal distribution of facies

1068

and facies associations. Legends please refer to Figure 3. A) Platform interior with laterally

1069

variable facies associations. B) Platform interior with laterally homogenous but protected

1070

facies association.

1071

Fig. 11. Facies architecture in the bio- and chemostratigraphic framework of the Zhujiaqing

1072

Formation across a south to north transect at Eastern Yunnan. The δ13Ccarb data of Meishucun

1073

are after Brasier et al., 1990; the δ13Ccarb data of Laolin are after Li et al., 2009. SSFs

1074

assemblage zones of Meishucun, Xianfeng, Laolin and Lishuping are after Yang et al., 2014;

1075

SSFs assemblage zones of Zhujiaqing are after Qian et al., 1996. Numbers 1 and 2 in the red

1076

circles represent two different correlations of the top Dengying Formation (the discarded

1077

Xiaowaitoushan Member) at Meishucun, Mingyihe and Xianfeng with F1. Number 1

1078

represents that F1 deposited above the Xiaowaitoushan Member, while number 2 represents

1079

that the Xiaowaitoushan Member equals to the lower part of F1, and the upper part of F1 is

- 44 -

1080

missing in the southern areas because of erosion. For detailed interpretation please refer to

1081

the text in section 5.2.

1082

Table caption

1083

Table 1. Summary of facies associations, with texture and main components, characteristic

1084

sedimentologic and diagenetic features, and environmental interpretation of each facies.

- 45 -

Facies

Texture and main components

F1 - Semi-restricted subtidal facies association Argillaceous dolostone with Planar, parallel, thin-bedded, dark grey euhedral to subhedral interbedded shaly siltstone (F1a) dolomites, contain phosclasts and black carbonaceous materials, and locally interbedded with mm- to cm-thick shaly siltstone Carbonaceous dolomitic chert Tabular, laterally continuous cm-thick laminated dark grey chert, (F1b) with variable amount of black organic matter and dolomite rhombs F2 -Temprestite-dominated subtidle facies association Heterolithic mud- and packstone Cm-thick, dark gray mudstone and well-sorted, very fine-grained (F2a) packstone. Grain types include phosphate and dolomite peloids (<0.2 mm, with phosphate envelope), carbonaceous grains, SSFs and phosoncolites. Cement is fine crystalline calcite. Siltstone intercalations Homogenous packstone (F2b) Dm-thick, fine-grained packstone. Grain types are similar as those in F2a, mudstone strips F3 – Energetic subtidal facies association Massive silty dolostone Thin- to medium-bedded silty dolostone containing dolomite, quartz intercalated with calcareous silt, pyrite, and minor organic compounds, cm-thick calcareous siltstone (F3a) siltstone, occasional graded conglomerate Silty-dolomitic-intraclastic Cm-thick (maximum 10 cm) grey phosphorite, consists of phosclast phosphorite (F3b) wack and packstone with dolomite and quartz silt matrix Rhythmical dolomitic phosphorite Thin-bedded phosphorite, mud- and packstone in texture, consists (F3c) of phosintraclasts. Lenses of bioclast-rich packstone locally occur, composite grain types include phosclasts, carbonate clasts and phosoncoids. The matrix/cement is crystallized to dolospars F4 – Shoal Barrier and back-barrier facies association Conglomeratic dolostone (F4a) Thin- to medium-bedded dolomudstone containing phosphate conglomerate lenses or thin beds. In-situ phosphogenesis is also observed in the form of concretions and crusts. Interbedded phosphorite and Thin-bedded to mm- laminated dolomudstone and phosclast dolostone (F4b) grainstone

Characteristic sedimentary structures and

Environmental interpretation of

early diagenetic features

facies

Wavy laminations, wave ripples, horizontal laminations Horizontal laminations, early chertification

Semi-restricted subtidal, above or around the storm wave base with occasional terrestrial input Low energy subtidal

Wavy and lenticular laminations, gutter cast, micro-scar, phosphate envelopes

Low energy subtidal with frequent tempestites

Massive sedimentary structure, erosive top surface of mudstone strips

Amalgamated tempestites

Massive texture and pyrites in the silty dolostone, erosive features

Subtidal above storm wave base, with occasional tempestites

Normal grading, gutter cast, wave ripples

High energy subtidal

Hardgrounds, scours, normal grading, lamination, phosphogenesis

Energetic subtidal with early phosphogenesis, frequent reworking and amalgamation of tempestites

Erosional surfaces, laminations, phosphogenesis

Protected lagoon

Wavy and lenticular bedding/lamination, weak bioturbation and gypsumpseudomorphs in the dolomudstone

Sand shoal margin that boarded a restricted lagoon

Oncolitic, bioclastic, phosclastic Dm-thick, variable grain components including phosclasts, oncoids packstone (F4c) and Phosphatic SSFs, carbonate cement/matrix F5 – Shoal complex facies association Microbial, stromatolitic and Thin-, medium- to thick-bedded, consists of stromatolites, microbial granular phosphorites (F5a) mats, phosphatic ooids, phosoncolites, quartz, doloclasts, phosclasts and phosphatic SSFs. Cements are variable including apatite, dolospar and silica. Phosphatic dolostone (F5b) Medium-bedded, consists of dolomudstone, phosclasts and phosphatic SSFs F6 – Protected siliciclastic subtidal facies association Gray shale (F6) Light-coloured fissile shale, Tabular laminae consist of silt- to sand-sized glauconite, phosphate, pyrite and barite grains F7 – Tidal sand flat facies association Quartz-rich dolarenite (F7) Thin- to medium-bedded, well-sorted, sand-sized quartz and dolostone clasts F8 – Subtidal to intertidal mix flat facies association Interbedded or laminated, Off-white dololutite, grey phospeloidal grainstone, grains composed phospeloidal grainstone and of very well-sorted, very fine grained phos-peloids, and minor dololutite (F8) proportion of doloclasts and quartz. Grains are cemented by dolomite F9 – Subtidal to intertidal phosphatic mudflat facies association Phosclast-rich dolostone (F9a) Medium to thick-bedded, Phosclast wack- and packstone, dolomitic and siliceous matrix, phosphate micro-concretions, sponge spicules and phosphatic SSFs Burrowed dolostone (F9b) Silty argillaceous mudstone (F9c)

Cm-thick beds, carbonate mudstone, phosclasts, carbonate pebbles, sandstone clasts Thinly bedded, consists of argillaceous mud, disseminated pyrites and phosphate grains

F10 – Intertidal to supratidal mudflat facies association Laminated dolostone (F10a) Medium to thick-bedded, laminated, microbial dolomite and crystalline dolomite, sparse quartz and phosphate sand grains, occasional mm-thick microsphorite layers

Burrows, phosphate crusts, desiccation cracks, miro-karst

Sand shoal, partly emerged

Parallel and cross bedding, erosive scars, phosphatic microbial mats and stromatolites

Shoal complex

Wavy bedding/lamination, scours, phosphate crusts, desiccation cracks

Low energy subtidal to intertidal lagoon

Flaky lamination, normal grading, weak bioturbation

Protected subtidal receiving significant terrestrial influx

Bimodal cross bedding, wavy and flaser bedding, intense recrystallization

Tidal sand flat

Planar and parallel bedding surface, Convolute bedding, flaser to wavy and lenticular lamination, horizontal trace fossils

Shallow subtidal to intertidal mixed tidal flat

Planar, parallel bedding, phosphate crusts, desiccation cracks, oblique lamination, phosphogenesis

Peritidal mudflat with constant current influence

Multiple hardgrounds, burrows

Condensed horizon

Flaky lamination

Low energy subtidal (possibly dysoxic) with significant terrestrial supply

Algal laminites, low relief domal stromatolites ( 0.5-1 cm) with microbial laminites continuous between domes,

Intertidal or supratidal mudflat

Doloclast rudstone (F10b)

Cm-thick, sub-angular intraclast dolostone, dolomite cement

F11 - Protected shallow subtidal facies association Calcareous siltstone and Thin-bedded siltstone and conglomerate, the conglomerate consists conglomerate (F11a) of well-rounded quartz and phosphate pebbles Medium-bedded bioclastic packstone (F11b) Conglomeratic, phosphatic dolostone (F11c) Argillaceous, fine- to mediumcrystalline dolostone intercalated with calcarous shale (11d) Argillaceous microcrystalline limestone intercalated with calcareous shale (11e)

chert lenses, fenestral fabric and in situ brecciation Normal grading or disordered, commonly interbedded with F10a Normal grading in the conglomerate, flaky lamination in the siltstone

Well-sorted, very fine grained phosphatic SSFs and peloids, sandstone lenses Thinly bedded, dolomitic mudstone, hardground clasts, phosphate pebbles, phosphate crusts and phosphatic SSFs Medium- to thin bedded, grey fine- to medium-crystalline dolostone, phosphatic SSFs, phosphate crusts and phos-intraclasts, and calcareous shale (cm to dm thick)

Undistinguishable due to intense recrystallization Lamination, partly bioturbation, multiple deposition, hardground, phosphogenesis Wavy, sub-parallel bedding, nodular bedding, lamination and minor bioturbation (0-30%)

Medium to thin-bedded, grey micro-crystalline limestone, dolomite rhomb, phosphatized shell fragments, phosphate crusts and phos-intraclasts, and calcareous shale (cm thick)

Wavy to nodular bedding, lamination and minor bioturbation

Storm deposit on tidal flat or channel Protected subtidal dominant by detritus input and low carbonate productivity Moderate energy shallow subtidal Protected subtidal, slow sedimentation rate Protected shallow subtidal, with episodic terrestrial influx

Protected shallow subtidal with minor terrestrial influx

Highlights New description of 25 sedimentary facies of Terreneuvian strata from south China We grouped 11 facies associations ranging from peritidal to intra-platform basin Facies restriction makes using isotopic marker to define Cambrian Stage 2 a problem Phosphogenesis occurs in various settings instead of only in isolated embayment High bioproductivity provided the main source of phosphorus

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: